geofisica colombiana n° 6 pp 62-65 diciembre de 2002 bogota, d,c, issn 0121 2974 solucion a un problema estatico en un area con topografia abrupta usando una velocidad de reemplazamiento variable ------------------' luis antonio castillo lopez profesor asociado, universidad nacional de colombia, curso posgrado en geofisica e-mail: ic as tillo gacie n c ia s run a l.e d u co carlos pedraza analista senior, petroseis t tc a. resumen teniendo en cuenta que para poder obtener una buena imagen del subsuelo, es necesario que las soluciones estaticas sean las mas acertadas posibles en la etapa inicial del procesamiento, y no gastar tiempo en ensayos con procesos mas sofisticados, sin tener resuelto el problema estatico, se propone implantar un metodo de procesamiento usando un modelo de velocidad de reemplazamiento variable en el calculo de la soluci6n estatica, se pretende explicar que el valor de la velocidad de reemplazamiento influye notoriamente en la imagen de la estructura, la cual puede verse deformada en gran rnagnitud. tarnbien puede ser cuestionado c6mo ha sido asignado este valor de manera constante en los proyectos, dando mas importancia al amarre con areas cercanas que a la soluci6n misma del problema estatico. se mostrara un estudio en que se emplea una serie de datos reales procesados a partir de un modelo de velocidad de reemplazamiento variable y otra con un modelo de velocidad con stante, las que -al ser comparadas y anal izadasdan como resultado grandes contrastes en las imageries obtenidas. este metodo se ha ido desarrollando durante el procesamiento de informaci6n de las areas con variaciones severas de topografia en colombia. en el caso del area del piedemonte llanero, usando el paquete interactivo promax, algoritmo gauss-seidel. palabras clave: refraccion, superficie irregular, correcciones estaticas, proceso abstract a problem in the reflection seismology is the precise determination offield statics correction that have to be applied to the reflection data in order to eliminate effects of eathering layer. normally these corrections are calculated by shallow reflection surveys. the waves refractated at the weathering layers are in the reflection seismograms and constitute part of the called "first breaks". these are waves where the source-receiver ray trajectories are located within the weathering layer and partly along its bottom. the arrival times through the weathering layer needed to computed the field statics corrections. in this work, a simple and efficient method is described to obtain field statics using only field seismograms recorded by the cop configuration. as input data ones considered: a) the surface topography ofthe seismic line, b) the first breaks and c) the geometry data. the method consists of determining from a starting velocity model, this model has a layer constructed taking into average thickness a velocity for the area. the method propose to seismic processing, was tested on raw register an a seismic line with abrupt topography in a colombia area. the obtained corrections with variable velocity were better than those computed with a constant velocity model. the statics solutions are necessary to obtain a good subsurface image. but in the initial processing sequence it is necessary, and do not spend time in surveys with other process, that do not resolve the static problem. in this paper a processing using static model calculate with variable velocity is proposed. this method have been development in the processing data in areas with irregular surface. for instance, piedemonte l1anero. the software calculation is promax, algorithm gauss-sediel. keywords: refraction, irregular surface, static corrections, seismic processing 62 geofislca colombiana, 6, diciembre [)e 2002 solucion a un problema estatico en un area con topocrafia abrupta usando una velocldad de reemplazamiento variable introduccion los datos sismicos son mostrados como si fueran adquiridos (disparados y registrados) sobre una superficie plana (figura i), sin mostrar los cam bios por elevacion existentes en el area que se va a prospectar. esta hipotetica superficie plana es conocida como el datum sismica. para reemplazar las capas cercanas de la superficie, entre la superficie del terreno y el datum, con la intencion de remover los efectos de tiempo relacionado a diferencias en la elevacion de la superficie. reflexion / refraccion de ondas sismlcas fijente sismica capa2 figura i. cuadro esquernatico de un par fuente receptor y la trayectoria de rayo, que incide sobre un medio. la velocidad de la segunda capa es cornunmente usada para ia velocidad de remplazamiento. este procesarniento "rule of thum b" se basa enla suposicion que la segunda capa representa el promedio de velocidad entre la superficie y el datum. tambien se asume que con esta velocidad pueden ser removidos adecuadamente los efectos de estructuras en tiempo, causados poi' cam bios en la elevacion de superficie. se debe tener en cuenta que este procedimiento, efectuando un promedio de velocidad en un modelo donde la estructura es compleja (caso particular de muchas estructuras encontradas a 10 largo de la geologia colombiana) da como resultado una estructura 0 imagen deform ada de la misma. en esta presentacion no se pretende discutir el procedimiento anterior, sino dar otra alternativa a una rnejor aproximacion 0 interpretacion de un modelo. marco teo rico un gran problema encontrado en prospeccion sismica de reflex ion es la determinacion precisa de las correcciones estaticas que deben ser aplicadas a los datos para eliminar los efectos de la topograffa en la zona de baja velocidad (zb v). las ondas refractadas en la base de la zbv, generalmente representadas en los sismogramas de reflexion como primeros arribos, son ondas cuyas trayectorias de los ragedfislca cdldmbiana, 6, diciembre de 2002 yos, desd la fuente hasta el receptor, se localizan predominantemente a 10 largo de la zbv y de la ba se, estas sefiales deben contener la informacion necesaria para el calculo de los tiempos de transite de las trayectorias verticales de las ondas sismicas a 10 largo de aquella zona y para el calculo de las correcciones estaticas. en este trabajo se presenta un metodo simple y eficiente de calculo de las correcciones estaticas de campo, a traves del uso exclusivo de datos de reflexion sismica registrados con la tecnica cop ("common depth point"). ei concepto clave que enmarca la refraccion sismica se basa en la ley de snell (figura i), la establece que todo rayo al atravesar un medic entre dos formaciones de velocidades diferentes: ~=seni v2 senr <i) los radios de los sen os del angulo incidente (i) y del refractado (i') son iguales a la razon de las velocidades de las dos formaciones (vi, v2). uno de los motivos para aplicar las correcciones estaticas es asegurar una buena imagen en el proceso sismico. las anornalias estaticas pueden producir estructuras falsas en la seccion sismica (longitudes de onda espaciales mas grandes que la longitud del tendido). las refracciones estaticas son efectivas para corregir anornalias de longitudes de onda espaciales mas largas. adernas, los rnetodos de refraccion perm iten determinar el espesor y las velocidades de las capas cercanas a la superficie al efectuar anal isis de primeros arribos sobre los registros de campo (figura 2). l ---..j figura 2. registro bruto que resalta en la parte superior los primeros arribos y su picado (manual 0 automatico). metodologia este rnetodo consiste en efectuar un modelamiento de la velocidad de reernplazamiento, usando la variacion de velocidades calculadas para el primer refractor y no como una velocidad promedio. ei rnetodo fue probado en un tramo de una linea sismica 20, con una topogratia y eventos superficiales abr\lptos, y los resultados muestran las diferencias entre los metodos. 63 castillo et al. es importante recalcar que los datos sismicos superficiales deben ser preservados si es posible para evitar cambios de estaticas antes de apilar. cuando la topografia es severa, esto es mejor efectuado al hacer un "shift" de proceso (datum flotante) que sigue la topografia. cuando no es efectuado la correccion por refracci6n se distorsionan las reflexiones (hiperbolas) y las refracciones no son corregidas (tilander et ai, 1995). ejemplo en las figuras 3 y 4, se muestra un registro de campo al cual fue aplicado la correcci6n por refracci6n, donde pueden ser observados los cambios en los arribos vistos en cada traza, viendose afectados tanto las refracciones y reflexiones. en este proceso las velocidades de reemplazamiento juegan un papel importante, con 10 cual un modelo de velocidades variable puede ser considerado como una mejor aproximaci6n a la geologia. figura 4. ampliaci6n de la zona afectada por la irregularidad del relieve en el registro sismico ya rnostrado. figura 3. registro sismico que muestra los efectos de la topografia en la estructura de los datos. en los resultados de las secciones apiladas con diferentes modelos de velocidades, puede observarse que los resultados son bien diferentes, con 10 cual se deja a consideraci6n el uso en la utilizaci6n de diferentes velocidades. segun la interpretaci6n efectuada por el interprete el modelo con una velocidad de reemplazamiento variable es mucho mas aproximado a la realidad (figuras 5 y 6). 64 figura 5. secci6n apilada con velocidad de reemplazamiento constante. conclusiones datos adquiridos en una variedad de terrenos incluyen la cordillera oriental, los cuales presentan rocas expuestas en superficie y zonas montaftosas con pendientes fuertes. rocas de alta velocidad en la superficie pueden causar desacople disparo-receptor y malos alineamientos debido a deterrninaci6n errada de estaticas, geofislca colombiana, 6, diciembre de 2002 solucion a un problema estatlco en un area con topografia abrupta usando una velocidad de reemplazamiento variable figura 6. secci6n apilada can velocidad de reemplazamiento variable. ei uso de un modelo de velocidades no muy aproximado puede dar como resultado una imagen distorsionada del subsuelo, con 10 cual se recalca el uso de un modelo de velocidades mas aproximado. geofisica colombiana. 6. diciembre de 2002 agradecimientos agradecemos especialmente al grupo de geofisica de petroseis ltda. en bogota, por su colaboracion en cuanto a equipos, experiencia y conocimientos, al igual que a la universidad nacional quien junto a petroseis ltda., hacen parte del desarrollo y evolucian de la tecnologia en procesamiento sismico en colombia. bibliografia sheriff, r. e., & geldart, l., p. 1991, exploracion sismologica. volumen ii. procesamiento e interpretacion de datos. noriega editores. tilander, n. g. & mitchel, r. g. 1995, processing of seismic data from overthrust areas in latin america. chevron overseas petroleum, inc. sail ramon (california), usa. yilmaz, 0., 1987, seismic data processing. society exploration geophysicists. tulsa (ok), usa. 65 geofisica colombiana pp. 5-b diciembre de 2002 bogota, d.c. issn 0121 2974n° 6 efecto de la rugosidad del terreno en las secciones sismicas: modelamiento por elementos finitos robinson quintana', sandra cespedes" alfredo ghisays', gabriel perez2, luis montes' resumen las zonas de piedemonte se caracterizan pol'las capas someras de baja velocidad y poria topografia rugosa. en esos estratos se produce un ruido coherente liamado backscattering que apantalla las ondas que provienen de los retlectores profundos, esta sefial se observa en las secciones sismicas del piedemonte colornbiano, oscureciendo la imagen del subsuelo. para atenuar esta sefial, se presenta en este articulo un metoda basado en la prediccion de dicho ruido y su posterior remocion de los registros. para ello se simula la respuesta de la capa somera mediante el modelamiento nurnerico en elementos finitos de la propagacion de la onda acustica en un modelo del subsuelo. posteriormente, esta respuesta se sustrae traza poi'traza del registro sintetico, logrando una atenuacion sustancial del ruido en el disparo. se presentan resultados obtenidos de la simulacion con datos sinteticos, palabras clave: piedemonte, ruidd, registro sismicd, modelamiento numerico, simulacion abstract the foothills areas are characterized by low velocity shallow layers and rugged topopgraphy, inside these stratas a coherent noise named back scattering is produced, that noise overlaps the waves comming from the deeper reflectors. in seismic sections of the colombian foothills this signal is observed, obscuring the subsurface image. a method to attenuate this signal is presented in this paper, the approach is based in the prediction of this noise and the posterior remotion from the seismic records, the shallow layer response is simulated through the numerical modeling in finite elements of the acustic wave propagation in a subsurface model. lately, the shallow layer response is subtracted trace by trace from the synthetic record, getting a high noise's attenuation in the shot. we present results obtained by synthetic data modeling. keywdrds: foothills, noise, seismic record, numerical modeling, simulation introduccion en zonas de topografia rugosa con cambios laterales de velocidad en la capa somera, se produce un ruido llamado backscattering, que distorsiona y oscurece la informacion que viene de las capas mas profundas del subsuelo, donde poi'10 general se halla el objetivo. las irregularidades de la superficie se comportan como puntos difractores y las reflexiones, en esa superficie libre, se manifiestan como patrones coherentes en todo el registro. se ha intentado remover su presencia par aplicacion de correcciones estaticas (berryhill, 1979), pero estas s610funcionan cuando la distancia entre la fuente y el receptor es pequefia y la elevacion moderada (bevc, 1996). otra estrategia usada es continual' el campo de onda hasta un niveillamado datum, sin embargo, el ruido permanece en la seccion resultante. grupo de investigacion en geoflsica, facultad de ciencias, universidad nacional de colombia, bogota. geofisico senior. empresa colombiana de penoieos (ecopetrol), bogota. profesor asoctaco. departamento de geociencias, facunad de cienclas, universidad nacional de colombia, bogota. e-mail: imontes@ciencias.unal.edu.co geofislca colombiana, 6, diciembre de 2002 metoda en general, la geometria y distribucion de velocidades en la capa somera se pueden estimar de una seccion apilada. la ecuacion de onda se resuelve mediante elementos finitos, modelando asi la respuesta de la capa somera y, en particular, el efecto debido a la topografia. se presenta una estrategia para e1iminar este ruido antes de cualquier paso del procesamiento sismico, incluida la migracion. se hizo un primer modelo 2d de la capa somera con superficie superior irregular. ei modelo se dividio en elementos finitos cuadrados. en uno de los nodos de la grilla, se coloco la fuente en tiernpo cero (en este caso una ondicula de ricker); luego, un program a desarrollado en c++ bajo diffpack (langtangen, 1996) calcula ei campo de onda en todos los elementos para cada intervalo de muestreo. como resultado se tiene un primer sismograma sintetico, un segundo modelo consta de la capa somera sobre un medio semiinfinito; nuevamente se resuelve la ecuacion de onda en el nuevo modelo, y se obtiene un segundo sismograma. restando estos sismogramas obtenemos la respuesta debida, unicamente de la capa mas profunda, que es la zona de interes. 5 quintana et al. solucion de ecuacion de onda por elementos finitos la ecuacion de onda acustica en elementos finitos en su forma matricial, (segerlind, 1984), se expresa par: [m]{ii} + [k]{u} = {f} donde [m] es lamatriz de masa, [k] la de rigidez, {j} es el vector de fuerza y {u} el campo de perturbacion de la onda en cada elemento del media. la solucion de la ecuacion de onda expresada en (i) por elementos finitos es: donde u' ,u'-i yui-2 representan el campo de onda en cada elemento del media estimado en los instantes actual ((), anterior (1-1) y tras anterior (1-2), respectivamente. en una region donde se atenue la energia de la onda, la ecuacion (i) se rnodifica mediante la introduce ion de un termino que incorpara ese efecto. ese coeficiente de amortiguacion se expresa mediante la matriz de amortiguamienta. finalmente la expresion queda: [m] {ii}+ [c ]{u}+ [k ]{u}={f}, ecuacion cuya solucion viene dada por la expresion u' ={r'[m]+ 0.5[c]} -i. [r'[m ]{2u'-' u'-2} + 0.5[c ]u'-2 t[k ]u'-i + {f}!] (4) matematicarnente es muy complicado determinar la matriz de arnortiguacion c, ya que se desconoce la matriz de viscosidad; por tanto, se formula la hipotesis de que la solucion amortiguada sea una cornbinacion lineal de la solucian no amortiguada y, en consecuencia, la matriz [c] sea una cornbinacion linea: de las matrices de rigidez y de masa. la estabilidad de la solucian mediante elementos finitos requiere que las dimensiones de la grilla del modelo satisfagan las denominadas condiciones de courant, en las cuales se establece una relacion entre las dimensiones de la grilla, ax=~y, el intervalo de muestreo st, y la velocidad del medio, v, en tal elemento, dadas por: resultados y discusion se construyo un modelo sencillo de la capa somera con una superficie irregular por medio de una grilla en dos dimensiones, (figura 1). ei rnodelo consta de 27 322 nodos y 36 867 elementos. las dimensiones de grilla cuadrada que garantizan cumplir las condicio6 (i) y e j (2) x·eje figura 1. modelo 20 con velocidades de intervalo en profundidad. (3) nes de courant son 7 m de lado. a la capa somera se le asigno una velocidad de 1000 m/s y a la subyacente 2500 mls. v (2500m)!:it (0.002s) s.jd.5 ax la fuente representada en una ondicula de ricker, con frecuencia de 40 hz, se coloco en el elemento 96 del modelo, activandola en el tiempo cero. ei tiempo total de la simulacion es 0.358 segundos. se ejecuto el program a en el primer modelo tomando (5) figura 2. instantanea del tiempo 258 ms, muestra el ruido generado y la onda sabre la primera interfase. geofislca colombiana, 6, oiciembre de 2002 efecto de la ruoosidad del terreno en las secciones sismicas, modelamiento por elementos finitos figura 3. instantanea del tiempo 258 ms con los valores de amplitud. muestras en el tiempo cada 2 ms. ei resultado presentado en la instantanea de la figura 2 corresponde a la respuesta de la capa somera. observe las anomalias y multiples causadas por la capa somera. como se puede observar en la figura 3, las amplitudes en la zona superficial alcanzan valores que estan entre 2.83 x 10.9y 5.0 i x 10'9, en contraste con las amplitudes del frente de onda que solo alcanzan valores comprendidos entre -2.62 x 10,9y-i.53 x 10'9, debido al ruido generado por las multiples y la dispersion. un segundo modelo fue generado a partir del primero ..para ello se adiciono, debajo del primer modelo, una tercera capa profunda con una velocidad de 3000 m/s. una interfase quebrada separa este figura 4. en el tiempo 358 ms la onda penetra en el tercer medio, se observan las difracciones en la capa somera. geofislca colombiana. 6. oiciembre de 2002 figura 5. la instantanea muestra s610 las reflexiones de la capa mas profunda y el ruido somera atenuado. nuevo medio de la segunda capa (figura i). ei program a se ejecuto sobre este segundo modelo. del resultado de esta simulacion se selecciono una instantanea, la cual se muestra en la figura 4. se observa una complejidad de retlexiones, entre las que se incluye el efecto de la capa somera sobre la informacion del retlector profundo quebrado. restando de la segunda simulacion el resultado de la primera, se obtiene la respuesta, debida unicamente de la capa mas profunda, que es nuestro interes. la figura 5 muestra el resultado de esta sustraccion. se ha eliminado toda serial asociada a la capa somera. por otro lado, la informacion del reflector profundo quebrado queda realzado, aun can ruido facilmente eliminable por los procesos convencionales. conclusi0i'ies en los model os sinteticos, se ve que la metodologia de eliminacion del ruido por el metodo de elementos finitos para la solucion de la ecuacion de onda es apro iada para atenuar el ruido causado por las irregularidades de la topografia. se requiere conocer la geometria y las velocidades de las capas someras para obtener registros semejantes a la realidad. se propone el metodo de elementos finitos para simular los ruidos causados por la capa de baja velocidad y su futura aplicacion para remover estos ruidos de datos reales. ei software desarrollado y el hecho de usar la ecuacion de onda permiten su aplicacion a model os 3d. 7 quintana et al. agradecimientos los autores agradecen a la universidad nacional de colombia, al instituto colombiano para el avance de la ciencia y la tecnologia "francisco jose de caldas" (colciencias) y aj instituto colombiano del petroleo (icp-ecopetrol) por la colaboracion y ej apoyo para ilevar a buen termino la presente investigacion. este articulo es resultado del proyecto modelamiento de fenomenos de dispersion sismica asociado a ja topografia y a las capas someras del subsuelo, cofinanciado por colciencias mediante contrato no. rc 271-2000. 8 bibliografia berryhill, j. r. 1979. wave equation datuming. geophysics 44(8): 1329-1344. bevc, d. 1996. flooding the topography: wave equation datuming ofland data with rugged acquisition topography. geophysics 62(5): 1558-1569. langtangen, h. p. 1996. efficient element solution a/the linear wave equation in dif./jjack. the diffpack version 1.4. report series. sinteff. university of oslo. segerund,1. 1984. applied finite element analysis. john wiley & sons. zienklewlcz, o. 1992. el metoda de los elementos jinitos. vol. 2. mcgraw-hili. geofisica colombiana. 6, oiciembre oe 2002 slowness surface calculation for different media using the symbolic mathematics language maple® earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004) : 63 67 manuscript received july 2004. 63 paper accepted october 2004. slowness surface calculation for different media using the symbolic mathematics language maple® miguel duarte1, carlos piedrahita1, trino salinas1, hernando altamar 2 and karen pachano 2 1 instituto colombiano de petróleos , ecopetrol. piedecuesta – colombia. e-mail: mudarte@ecopetrol.com.co; tsalinas@ecopetrol.com.co. 2 universidad industrial de santander -uis. bucaramangacolombia abstract starting from the equation in different media, we obtain the different type of waves that can exists in such media. the evaluation of the eigenvalues and eigenvectors let us construct the slowness surfaces. in general complex calculations case have to be made. in this work, routines were implemented in the symbolic language maple® and the slowness surfaces were plotted. this work is relevant for the modelling of equivalent media that simulates natural fractured reservoirs, like those common in the colombian foothills. it is important to understand the seismic response of this reservoirs for exploration of this areas. key words: christoffel matrices, slowness vector, cubic symmetry, hexagonal symmetry, isotropic, symmetry, slowness surfaces, eigenvalues, eigenvectors resumen partiendo de la ecuación en diferentes medios, obtenemos los diferentes tipos de ondas que pueden existir en tales medios. la evaluación de los valores propios y los vectores propios permite construir las superficies de lentitud. en general, es necesario hacer complejos casos de cálculos. en este trabajo se implementaron algunas rutinas en el lenguaje simbólico maple® y se señalaron las superficies de lentitud este trabajo es pertinente para el modelado de medios equivalentes que simulan los depósitos fracturados naturales, como los que aparecen en las zonas de piedemonte colombianos. es importante entender la reacción sísmica de estos depósitos para la exploración de estas áreas. palabras clave: isotropía, matriz de christoffel, simetría cúbica, simetría hexagonal, superficies de lentitud, valores propios, vectores propios, vector de lentitud. © 2004 esrj -unibiblos. miguel duarte et al. methodology this work was developed entirely in the field of the linear theory of elasticity. for homogeneous media the solution of the general elastodynamic equation reduces to the solution of the christoffel matrix eigenvalues problem. the christoffel equation is solved numerically using the symbolic mathematics programming language maple®. the phase velocity is obtained as a function of the anisotropic parameters and the propagation direction. the phase velocity 1/v is used to obtain the slowness, which is plotted using the standard plotting maple function in polar coordinates. the resulting graphic shows the relation between the slowness vector and the propagation angle. formulation of the problem and results theory research – fundamentals the deformation laws for a linear elastic body are determined by the linear momentum equation (1), the constitutive equation or the hooke’s law (2) and the small strain – displacement linear relation (3) (sen, 2002), (krishnaswamy, 2004), (pšenčík, 1994): 2 ij i j uu x t τ τ ρ ρ ∂ ∂ ∇ ⋅ = ↔ = ∂ ∂ && (1), : ij ijkl klc cτ ε τ ε= ↔ = (2), ( )1 1 2 2 k l kl l k u uu u x x ε ε ⎛ ⎞∂ ∂ = ∇ + ∇ ↔ = +⎜ ⎟∂ ∂⎝ ⎠ (3). where: τ is second order stress tensor, ρ is density u is displacement vector c is fourth order elasticity tensor or stiffness tensor ε is second order linearized finite strain tensor or tensor of small strain , ,i j kx x x – cartesian coordinates. strain and stress tensor are symmetric. this assumption leads us to a contracted form of the above equations, the voigt notation. equation (3) defines 6 different equations for only three components of the displacement vector ui, therefore some other conditions (4) need to be imposed over the small strain components in order to guarantee the existence of the displacement field (mase, 1970). 2 22 2 0 0ij jmkm ik k m i j j i kx x x x x x x x ε εε εε ∂ ∂∂ ∂ ∇× ×∇= ↔ + − − = ∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ (4) thus, combining (1), (2), (3) and (4) the most general form of the elastodynamic equation is obtained: 2 2 k ij j l u uc i x x t ρ ⎛ ⎞∂ ∂∂ =⎜ ⎟∂ ∂ ∂⎝ ⎠ (5) note that the stiffness tensor reduces to a 6x6 symmetric matrix c i j k lc ij due to the symmetry of the stress and strain tensors. the condition (5) gives a simpler strain – displacement relation. the stiffness matrix can be obtained by combination of coordinate transformations corresponding to the material lattice symmetry properties. in the most general case the stiffness matrix has got 21 independent elastic constants. this is the monoclinic material. hexagonal symmetry, for example, has got 5 different elastic constants. this system has got one of the axes of symmetry such that a rotation around this axis does not change the stiffness matrix. for homogeneous materials in which elastic constants and density are constant, (5) reduces to (6): 2 2 2 k i ij j l u uc x x t ρ∂ ∂ = ∂ ∂ ∂ (6) first consider harmonic plane waves moving through homogeneous anisotropic medium. then the solution must be sought in the next form: ( ) ( ) ( ) ( ), , i iik vt l xik vt l x i i iu x t ue u x t u e− −− −= ↔ = (7) where: u is the displacement amplitude vector, integration constant, i is an imaginary unit, 64 2k π λ = is the wave number, λ is wavelength, v is phase velocity, t is time, l is unit vector along the propagation direction, x is position vector. replacing the displacement vector (7) in the elastodynamic equation (6) and applying the fourier transform, we get: 2 2 0ij jk kl i ll c l u u k ρω − = (8) where: ijl is director cosines matrix of the propagation vector, 2 vkv πω λ = = – angular frequency. the equation (8) can be rewritten as: ( )2 0ik ik kv uδ ργ − = (9) where: il ij jk kll c lγ = ⋅ ⋅ – christoffel matrix (10) the slowness 1p v = is obtained by solving the next equation for the three values of v2. ( 2det 0ik ik vδ ργ − =) (11) therefore, three types of plane harmonic waves along the propagation direction l are obtained, with three different phase velocities v(i). the corresponding displacement vectors are mutually orthogonal; they are also called polarization vectors. the vectors ( )i ju l and can be parallel, in that case the corresponding wave to that displacement direction is called pure longitudinal wave. perpendicular directions of displacement correspond to pure shear waves. ( )i ju body of the programming routine this routine plots the wave surfaces for all cases of anisotropy > restart: with(linearalgebra): unprotect(gamma,gamma): stiffness matrices stiffness matrix in the voigt form > c:=matrix(6,6,shape=symmetric): now the user must choose the anisotropy case: isotropic case (2 elastic constants) > c[1,1]:=lambda+2*mu: c[2,2]:=lambda+2*mu: c[3,3]:=lambda+2*mu: c[4,4]:=mu: c[5,5]:=mu: c[6,6]:=mu: c[1,2]:=lambda: c[1,3]:=lambda: c[2,3]:=lambda: evalm(c); cubic anisotropy (3 elastic constants) > c[1,1]:=c11: c[2,2]:=c11: c[3,3]:=c11: c[4,4]:=c44: c[5,5]:=c44: c[6,6]:=c44: c[1,2]:=c12: c[1,3]:=c12: c[2,3]:=c12: evalm(c); hexagonal anisotropy, vti (5 elastic constants) > c[1,1]:=c11: c[2,2]:=c11: c[3,3]:=c33: c[4,4]:=c44: c[5,5]:=c44: c[6,6]:=c66: c[1,2]:=c11-2*c66: c[1,3]:=c13: c[2,3]:=c13: eval(c); orthorhombic anisotropy (9 elastic constants) > c[1,1]:=c11: c[2,2]:=c22: c[3,3]:=c33: c[4,4]:=c44: c[5,5]:=c55: c[6,6]:=c66: c[1,2]:=c12: c[1,3]:=c13: c[2,3]:=c23: evalm(c); monoclinic anisotropy (13 elastic constants) 65 miguel duarte et al. 66 > c[1,1]:=c11: c[1,2]:=c12: c[1,3]:=c13: c[1,4]:=c14: c[1,5]:=c15: c[1,6]:=c16: c[2,2]:=c22: c[2,3]:=c23: c[2,4]:=c24: c[2,5]:=c25: c[2,6]:=c26: c[3,3]:=c33: c[3,4]:=c34: c[3,5]:=c35: c[3,6]:=c36: c[4,4]:=c44: c[4,5]:=c45: c[4,6]:=c46: c[5,5]:=c55: c[5,6]:=c56: c[6,6]:=c66: evalm(c); ray director cosines matrix > l:=matrix(6,3): l[1,1]:=l[1]: l[2,2]:=l[2]: l[3,3]:=l[3]: l[4,2]:=l[3]: l[4,3]:=l[2]: l[5,1]:=l[3]: l[5,3]:=l[1]: l[6,1]:=l[2]: l[6,2]:=l[1]: evalm(l); christoffel matrix > gamma:=transpose(l).c.l; eigenvalues and eigenvectors the elastic constants should be declared according to the chosen anisotropy case: consts:=c11=16.57*10^10,c12=6.39*10^10,c44 =7.956*10^10,rho=2332: (cubic anisotropy) > consts:=c11=3.46e10,c13=1.07e10,c33 =2.84e10,c44=8.36e9,c66=1.26e10,rho=2500: (vti elastic constants) >dir_l:=l[1]=sin(theta),l[2]=0,l[3]=cos(theta): >ch:=characteristicmatrix(subs(dir_l,gamma), k); > d:=determinant(ch); > factor(d); > k:=[solve(d,k)]; > map(simplify,%,trig); > a:=subs(k=k[3],ch); > a:=map(simplify,subs(k=k[3],ch),trig); > solve({a[1,1]*u1+a[1,2]*u2=0},{u1}); > simplify(%,trig); > solve({a[2,1]*u1+a[2,2]*u2=0},{u1}); > simplify(%,trig); > rank(a); > eg:=map(simplify,columnspace(a)); > plot([subs(consts,dir_l,sqrt(rho/k[1])),subs(consts ,dir_l,sqrt(rho/k[2])),subs(consts,dir_l,sqrt(rho/k[ 3]))],theta=0..2*pi,color=[red,blue,green],coords= polar); > plot([subs(consts,dir_l,sqrt(k[1]/rho)),subs(consts ,dir_l,sqrt(k[2]/rho)),subs(consts,dir_l,sqrt(k[3]/r ho))],theta=0..2*pi,color=[red,blue,green],coords= polar); > phi_1:=theta+arctan(diff(sqrt(k[1]/rho),theta)/sqrt (k[1]/rho)); > v_1:=sqrt(sqrt(k[1]/rho)^2+diff(sqrt(k[1]/rho),the ta)^2); > phi_2:=theta+arctan(diff(sqrt(k[2]/rho),theta)/sqrt (k[2]/rho)): > v_2:=sqrt(sqrt(k[2]/rho)^2+diff(sqrt(k[2]/rho),the ta)^2): > phi_3:=theta+arctan(diff(sqrt(k[3]/rho),theta)/sqrt (k[3]/rho)): > v_3:=sqrt(sqrt(k[3]/rho)^2+diff(sqrt(k[3]/rho),the ta)^2): > plot({[subs(consts,dir_l,v_1),subs(consts,dir_l,phi _1),theta=0..2*pi],[subs(consts,dir_l,v_2),subs(co nsts,dir_l,phi_2),theta=0..2*pi],[subs(consts,dir_l, v_3),subs(consts,dir_l,phi_3),theta=0..2*pi]},colo r=[red,blue,green],coords=polar); > plot({subs(consts,dir_l,sqrt(k[3]/rho)),[subs(const s,dir_l,v_3),subs(consts,dir_l,phi_3),theta=0..pi/2] },theta=0..pi/2,color=[red,blue],coords=polar); in the anterior routine the term vti means vertical and transversally isotropic media. routine work examples this program plots the three wave surfaces for any anisotropic material: slowness surface, phase velocity surface and group velocity surface. the program was tested using the next stiffness matrix for the vti case: 10 3 3.4 0.9 1.07 0 0 6 0.94 1.07 0 0 0 4 3.46 1.07 0 0 0 1.07 2.84 0 0 0 10 , 2500 0 0 0.836 0 0 0 0 0 0.836 0 0 0 0 0 0 1.26 kgc pa m ρ ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ = ⋅⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠ ij = figure 1. stiffness matrix for sandstone (thomsen, 2000) conclusions the theoretical study of anisotropic media is important for studying the effect of different type of media on wave propagation. particularly the effect that fractures, typical of colombian foothills. we expect to continue our study of this concepts and the development of computational tools that simulate wave propagation in general transversal inhomogeneous media. figure 2. slowness surfaces for sandstone. horizontal axis x3. vertical axis x1. slowness given in s/m. figure 3. slowness surfaces for sandstone. horizontal axis x3. vertical axis x1. phase velocity given in m/s. acknowledgments the authors wish to thank the support received from the colombian institute of petroleum (icp), the research division of ecopetrol, in particular the research unit uin, which had let accessed laboratory data as well as information related to anisotropy. references sen, mrinal. (2002) lectures: “seismic wave propagation in anisotropic media”. the university of texas at austin. krishnaswamy, sridhar. (2004) “acousto-optical methods of materials characterization”. northwestern university. pšenčík, ivan. (1994) “introduction to seismic methods”. pppg/ufba, el salvador. mase, george e. (1970) “theory and problems of continuum mechanics”. schaum’s outline series, mcgraw-hill thomsen, leon. (2000) “weak elastic anisotropy”. applied seismic anisotropy: theory, background, and field studies. geophysics reprint series no. 20, seg. 67 geoflslca colomblana n2 1, octubre de 1992 31 programa recursivo en pascal para determinacion de zonas en una secuencia, por el metodo de analisis de varianza luis a. montes profesor asistente depto de geociencias-facultad de ciencias-universidad nacional de colombia montes l.a: programa recursivo en pascal para determinacion de zonas en una secuencia, por el metodo de analisis de varianza. geofis. colomb. 1:31-36, 1992. issn 0121-2974 resumen los datos qeoloqicos cornunmente forman una secuencia, como registros electricos de pozo, pudiendo dividirse en segmentos hornoqeneos. las zonas pueden identificarse por rnetodos de analisis de varianza. ei programa contiene un procedimiento recursivo que evalua un coeficiente de zonacion entre varies posibles arreglos de datos, seleccionando el mayor valor para seleccionar ellimite entre los dos nuevos segmentos. este proceso se repite en cada zona hasta que el tarnano del segmento alcance un limite. en un registro electrico se identificaron 21 contactos. ei programa pude ser usado en otras areas de la geologia. abstract geologic data commonly form a sequence, for example well logs, and they may be subdivided into relative uniform intervals corresponding to stratigraphic units. distinct zones within ordered data sets can be identified by analysis of variance method. the computer program is a recursive procedure in which arrangements into two contiguous zones are considered at each stage. in each proposed boundary a zonation index is calculated, and is selected the highest one to stablish two different zones. the procedure is repeated for each zone until number of data in segments reach a limit. in a digitized electric log, it was possible to identify 21 boundaries. the program is general and can be used in others branch of geology. 1. introduccion muchos datos geologicos se presentan como secuencias de datos. en elias es posible reconocer intervalos con caracteristicas comunes 0 zonas de litologia constante. este proceso de dividir una secuencia en segmentos relativamente unifo rmes se denomina zonackin. las distintas zonas dentro de la secuencia pueden identificarse por el metodo estadistico de analisis de varianza. la busqueda de estes "contactos" implica una tecnica recursiva, en la cual en la secuencia se evalua un indice, que permite determinar dos segmentos, en los cuales nuevamente se realiza el proceso de determinacion del indice y la posterior seqmentacion. ei proceso termina cuando los segmentos alcanzan un limite. ei metodo se implanta a traves de un programa escrito en lenguaje pascal, y se ensaya en una secuencia de un registro electrico de un pozo en la localidad de madrid, c/marca. 32 montes: programa para determlnar zonas en una secuencla 2. metodo de zonacion ei rnetodo de analisis de varianza examina las diferencias entre las medias de dos poblaciones cuyos elementos contribuyen a traves de un modelo lineal. ei modelo plantea que a toda serial contribuyen tres facto res: x(ij) = i.l + ali) + e(ij) donde i.l ; media de toda la poblaci6n.a~.>: es el efecto de i" zona particular. e i]) : ~or aleatono asociado con cada o ervaaon. ei rnetodo se basa en dos mediciones diferentes e independientes de la varianza. la sum a de cuadrados dentro de la secuencia total se calcula de acuerdo a la siguiente fonnula: x;j = i-punto en j-segmento ~ = media de j-segmento nj = # de puntos en j-segmento m = # de segmentos la suma de cuadrados entre segmentos se calcula como: sse = i;m (5< x\2 / (m-1) }=i i' [2] x = media total de la secuencia x(ij) = i-qbservaci6n en j-segmento. xu) = media del j-segmento. no) = numero de observaciones en j-segmento. m = numero total de segmentos. las fonnulas representan una medida de la varianza alrededor de la media total x de la secuencia. ei coeficiente de zonacion se calcula como: r = 1 ssw/sse 3. procedimiento [3] ai principio la secuencia total se divide en dos segm~tos, una parte inicial corta que constituya al pnmer segmento y la parte rootanta ei segundo segmento. ~ ~ntinuaci6n. se calculan ssw, sse y el indlce de zonaclon r en cada segmento, segun las fonnulas 1, 2 y 3. ellimite entre los dos segmentos se muava a 10 largo de la secuancia a posiciones sucooivas, de acuerdo a un valor da paso da avance definido, por cada movimiento del limite sa calculan nueva~enta los valores de ssw, sse y r. ai terrninar de barrer la secuencia se obtienen un conjunto de indices de zonaci6n; uno por cada posicion de ei limite. s": ~usca dentro de ese conjunto el valor de r maximo. y sa escoge dicha posici6n como la primera zona de contacto. luego cada nuevo segmento genarado se particiona, consloeranoose cada segmento como un bloque complatamante distinto, iterando el procedimiento, para consequir-nuevas zonas de contacto en cada segmento. ei proceso continua hasta que la secuencia se divida an seqrnentos que no pueden superar una longitud minima. 4. aplicacion [1] ei programa se ensay6 con un registro electrico de pozo digitalizado manualmente (tabla 1)' situ ado en la localidad de madrid (cundinamarca). para esta registro se utiliz6 una sonda corta nonnal de 20 em y los valores se muestraron a profundidades mayo res de 330 m. ei programa sa cre6 con el uso del compilador hp pascal-9000 y sa ajecut6 an un microcomputador hewlett-packard 9000, del departamento de geociencias de la universidad nacional. para la implantaci6n de esta tecnica se utiliz6 el lenguaje pa.scal, por la facilidad que presenta este lenquaje de .liamado recursivo de rutinas; una segunda razon es el hecho de ser pascal un lenguaje muy conocido y de tacil aprendizaje. su utilizaci6n permiti6 determinar 21 contactos en la secuencia del registro (ver tabla 2.), los cuales coinciden intuitivamente con las inflexiones de la curva del registro (fig.1 y 2). ai observar ia gratica del registro de pozo, se puede observar la bondad de ajuste del rnetodo y del programa en la detenninaci6n de las zonas ~a . ~ntacto que .encierran segmentos htologlcamente homogeneos siendo estos de diferentes espesoroo. ' ~i co~portamiento . del programa asia influenclado por el longltud del segmento inicial y por ei valor del paso de avance al recorrer la secuencia, !}enerando resultados diferentoo para valoroo distlntos de ootos panimetros. 5. conclusiones ei uso de programas de computador an la determinaci6n de estructuras geol6gicas es cada dia mas importante. ' se utiliz6 un programa para la identificaci6n de zonas !itoi6gical"!'.enta hom~g{meas, y se pudo datermlnar su utlhdad al aphcar al metodo an al analisis da un registro electrico. sa puedan procesar secuencias da infonnaci6n axtensas an periodos da tiampo muy cortos al tiempo para analizar 100 datos la tomo a' la ~aq';jina aproxi":l~damanta 20 segundos, incluldo la genaraclon de la salida da resultados. la tecnica de zonacion sa utiliza en otras areas da la geologia, qua contamplan analisis de secuencias da datos. geofislc8 colomblana ~ 1, octubre de 1992 33 , ~_1 1 ij--t----+--+-h i iii ---_.-'_.-~.~~~---.-~-~ --~~ i-"-::~:::':"-. _ ..:..! i i i-t':' _._.. figura 1. registro electricc de pozo. sensibilidad: 10 divisiones = 500 nm; profundidad: cada division = 0.9434 m los segmentos indican unidades litoloqlcas homoqeneas. ei tope de la columna es 332.6 m y la base 423.3 m. --_.=-!!. ~._--~._-~w. ~ '~, -r-r-' ---t--~~--fa ~; ~ _ i ,i . . (: ~i ,,/it .. ---1--'· i -i~i i j .~'''''''+-=",-+---.,; i figura 2. [ccntinuacion fig.1). registro electrtco de pozo. sensibilidad: 10 divisiones = 500 nm; protundidad: cada division = 0.9434 in los segmentos indican unidades litologicas homcqeneas. ei tope de la columna es 425 my la base 519 m. 34 montes: programa para determinar zonas en una secuencia tabla 1. resistividades a distintas profundidades. los valores se muestrearon a intervalos iguales. prof resist prof resist prof resist prof resist 332.6 133.33 378.9 293.33 425.2 186.67 471.5 200.00 334.4 133.33 380.7 220.00 427.0 186.67 473.3 240.00 336.3 166.67 382.6 133.33 428.9 213.33 475.2 240.00 338.1 100.0 384.4 306.67 430.7 126.67 477.0 220.00 340.0 133.33 386.3 333.33 432.6 126.67 478.9 253.33 341.9 100.00 388.1 266.67 434.4 126.67 480.7 220.00 343.7 186.67 390.0 173.33 436.3 126.67 482.6 266.67 345.6 86.67 391.9 300.00 438.1 133.33 484.4 160.00 347.4 86.67 393.7 313.33 440.0 333.33 486.3 120.00 349.3 160.00 395.6 313.33 441.9 353.33 488.1 120.00 351.1 80.00 397.4 186.67 443.7 353.33 490.0 133.33 353.0 66.67 399.3 166.67 445.6 333.33 491.9 200.00 354.8 66.67 401.1 166.67 447.4 333.33 493.7 306.67 356.7 106.67 403.0 146.67 449.3 313.33 495.6 300.00 358.5 133.33 404.8 166.67 451.1 280.00 497.4 266.67 360.4 133.33 406.7 193.33 453.0 266.67 499.3 200.00 362.2 166.67 408.5 200.00 454.8 200.00 501.1 200.00 364.1 253.33 410.4 200.00 456.7 226.67 503.0 200.00 365.9 353.33 412.2 400.00 458.5 333.33 504.8 320.00 367.8 333.33 414.1 546.67 460.4 266.67 506.7 240.00 369.6 186.67 415.9 500.00 462.2 233.33 508.5 233.33 371.5 213.33 417.8 573.33 464.1 366.67 510.4 220.00 373.3 286.67 419.6 466.67 465.9 333.33 512.2 233.33 375.2 286.67 421.5 253.33 467.8 300.00 514.1 233.33 377.0 300.00 423.3 233.33 469.6 266.67 515.9 246.67 , programa para zonacion de secuencias de datos 1 programa para zonacion de secuencias de datos consta de un modulo principal y dos funciones } x : arreglo que contiene datos a procesar } meant: valor medio de la secuencia total } { datos es ei nombre del archivo de entrada } program lone(data,input, output); var data: file of real; x : array [1 ..256] of real; k : integer; meant: real; j --------} funcion que calcula la media del segmento definido liminf1 : cota inferior del segmento a promediar } limsup1 : cota superior del segmento } x: datos a procesar, definidos entre liminf1 y limsup1 } function mean(liminf1, limsup1 : integer) : real; var i : integer; suma : real; begin suma :=0.0; for i := liminf1 to limsup1 do sum a := suma + x~l; rnean.esuma/tllmsup t liminf1); end; {======================================================================} geofislca colombians n2 1, octubre de 1992 35 mean 1 : media del primer segmento } mean2 : media del segundo segmento } liminf, lirnsup : iimites del segmento a analizar y dividir }} r : coeficiente a evaluar, para determinacion de limites pos : indicador de r evaluado } inc: longitud de paso de avance } delta: indicador de posicion de maximo r procedure border(liminf, lirnsup : integer); var mean1, mean2, ssw, ssb, rmax, r : real; pes, inc, i, delta: integer; begin if (lirnsup liminf) > 8 then begin pas := 0; inc := 4; rmax:= 0.0; while (iimsup-liminf-inc > 4) do begin ssw := 0; mean1 := mean(liminf, liminf+inc); rneanz := mean(liminf+inc+ 1, lirnsup): for i := liminf to liminf+inc do ssw := ssw + sqr(x[i]-mean 1); for i := liminf+inc+ 1 to lirnsup do ssw := ssw + sqr(x[i]-mean2); ssw := ssw/(limsup-liminf-4); ssb := sqr(mean 1-meant)+sqr(mean2-meant); pos := pos + 1; if ssb > 0 then begin r := t-tssw/ssb): if r > rmax then begin rmax.er: delta.epos: end' end' ' inc := 'inc +2; end' border(liminf+delta+ 1, limsup); bordertliminf, lirninf -delta): if rmax > 0 then writeln(liminf+delta,' ',rmax:4:3); end; end; {=========================================================================} begin { •• inicio del principal·· } reset(data,'datos') ; k '=0' while' (not eof(dala)) do begin k := k + 1; read(data, x[k]); end' c1ose(data); meant:=mean(1,k); border(1,k); end. { •• fin del principal •• } 36 montes: programa para determinar zonas en una secuencia cota superior del segmento tabla 2. muestra el tope superior del segmento y el coeficiente r r profundidad (m) 88 84 74 72 70 68 63 61 59 54 48 46 44 42 32 30 23 19 12 10 7 0.323 0.223 0.573 0.785 0.807 0.462 0.802 0.879 0.423 0.081 0.790 0.940 0.918 0.361 0.155 0.221 0.268 0.148 0.442 0.484 0.389 493.6 486.3 467.8 465.9 460.4 456.7 447.4 443.7 440.0 430.7 419.6 415.9 4122 4085 3900 3863 373.3 3659 353.0 349.3 343.7 , bibliografia davis, j. c. (1986): statistics and data analysis in geology. 211pp. john wiley & sons co, newyork. gill, d. (1970): application of statistical zonation method to reservoir evaluation and digitized-log analysis. the am. assoc. of petrol. geologists bulletin. 54 (5):719-729. miller, r. l. & j.s. kahn. (1962): statistical analysis in the geological sciences. john wiley & sons co. tucker, a.b. (1987): programming languages. pp.18-66. mcgraw-hili inc. new york. geoflslca colomblana n2 1, octubre de 1992 31 programa recursivo en pascal para determinacion de zonas en una secuencia, por el metodo de analisis de varianza luis a. montes profesor asistente depto de geociencias-facultad de ciencias-universidad nacional de colombia montes l.a: programa recursivo en pascal para determinacion de zonas en una secuencia, por el metodo de analisis de varianza. geofis. colomb. 1:31-36, 1992. issn 0121-2974 resumen los datos qeoloqicos cornunmente forman una secuencia, como registros electricos de pozo, pudiendo dividirse en segmentos hornoqeneos. las zonas pueden identificarse por rnetodos de analisis de varianza. ei programa contiene un procedimiento recursivo que evalua un coeficiente de zonacion entre varies posibles arreglos de datos, seleccionando el mayor valor para seleccionar ellimite entre los dos nuevos segmentos. este proceso se repite en cada zona hasta que el tarnano del segmento alcance un limite. en un registro electrico se identificaron 21 contactos. ei programa pude ser usado en otras areas de la geologia. abstract geologic data commonly form a sequence, for example well logs, and they may be subdivided into relative uniform intervals corresponding to stratigraphic units. distinct zones within ordered data sets can be identified by analysis of variance method. the computer program is a recursive procedure in which arrangements into two contiguous zones are considered at each stage. in each proposed boundary a zonation index is calculated, and is selected the highest one to stablish two different zones. the procedure is repeated for each zone until number of data in segments reach a limit. in a digitized electric log, it was possible to identify 21 boundaries. the program is general and can be used in others branch of geology. 1. introduccion muchos datos geologicos se presentan como secuencias de datos. en elias es posible reconocer intervalos con caracteristicas comunes 0 zonas de litologia constante. este proceso de dividir una secuencia en segmentos relativamente unifo rmes se denomina zonackin. las distintas zonas dentro de la secuencia pueden identificarse por el metodo estadistico de analisis de varianza. la busqueda de estes "contactos" implica una tecnica recursiva, en la cual en la secuencia se evalua un indice, que permite determinar dos segmentos, en los cuales nuevamente se realiza el proceso de determinacion del indice y la posterior seqmentacion. ei proceso termina cuando los segmentos alcanzan un limite. ei metodo se implanta a traves de un programa escrito en lenguaje pascal, y se ensaya en una secuencia de un registro electrico de un pozo en la localidad de madrid, c/marca. 32 montes: programa para determlnar zonas en una secuencla 2. metodo de zonacion ei rnetodo de analisis de varianza examina las diferencias entre las medias de dos poblaciones cuyos elementos contribuyen a traves de un modelo lineal. ei modelo plantea que a toda serial contribuyen tres facto res: x(ij) = i.l + ali) + e(ij) donde i.l ; media de toda la poblaci6n.a~.>: es el efecto de i" zona particular. e i]) : ~or aleatono asociado con cada o ervaaon. ei rnetodo se basa en dos mediciones diferentes e independientes de la varianza. la sum a de cuadrados dentro de la secuencia total se calcula de acuerdo a la siguiente fonnula: x;j = i-punto en j-segmento ~ = media de j-segmento nj = # de puntos en j-segmento m = # de segmentos la suma de cuadrados entre segmentos se calcula como: sse = i;m (5< x\2 / (m-1) }=i i' [2] x = media total de la secuencia x(ij) = i-qbservaci6n en j-segmento. xu) = media del j-segmento. no) = numero de observaciones en j-segmento. m = numero total de segmentos. las fonnulas representan una medida de la varianza alrededor de la media total x de la secuencia. ei coeficiente de zonacion se calcula como: r = 1 ssw/sse 3. procedimiento [3] ai principio la secuencia total se divide en dos segm~tos, una parte inicial corta que constituya al pnmer segmento y la parte rootanta ei segundo segmento. ~ ~ntinuaci6n. se calculan ssw, sse y el indlce de zonaclon r en cada segmento, segun las fonnulas 1, 2 y 3. ellimite entre los dos segmentos se muava a 10 largo de la secuancia a posiciones sucooivas, de acuerdo a un valor da paso da avance definido, por cada movimiento del limite sa calculan nueva~enta los valores de ssw, sse y r. ai terrninar de barrer la secuencia se obtienen un conjunto de indices de zonaci6n; uno por cada posicion de ei limite. s": ~usca dentro de ese conjunto el valor de r maximo. y sa escoge dicha posici6n como la primera zona de contacto. luego cada nuevo segmento genarado se particiona, consloeranoose cada segmento como un bloque complatamante distinto, iterando el procedimiento, para consequir-nuevas zonas de contacto en cada segmento. ei proceso continua hasta que la secuencia se divida an seqrnentos que no pueden superar una longitud minima. 4. aplicacion [1] ei programa se ensay6 con un registro electrico de pozo digitalizado manualmente (tabla 1)' situ ado en la localidad de madrid (cundinamarca). para esta registro se utiliz6 una sonda corta nonnal de 20 em y los valores se muestraron a profundidades mayo res de 330 m. ei programa sa cre6 con el uso del compilador hp pascal-9000 y sa ajecut6 an un microcomputador hewlett-packard 9000, del departamento de geociencias de la universidad nacional. para la implantaci6n de esta tecnica se utiliz6 el lenguaje pa.scal, por la facilidad que presenta este lenquaje de .liamado recursivo de rutinas; una segunda razon es el hecho de ser pascal un lenguaje muy conocido y de tacil aprendizaje. su utilizaci6n permiti6 determinar 21 contactos en la secuencia del registro (ver tabla 2.), los cuales coinciden intuitivamente con las inflexiones de la curva del registro (fig.1 y 2). ai observar ia gratica del registro de pozo, se puede observar la bondad de ajuste del rnetodo y del programa en la detenninaci6n de las zonas ~a . ~ntacto que .encierran segmentos htologlcamente homogeneos siendo estos de diferentes espesoroo. ' ~i co~portamiento . del programa asia influenclado por el longltud del segmento inicial y por ei valor del paso de avance al recorrer la secuencia, !}enerando resultados diferentoo para valoroo distlntos de ootos panimetros. 5. conclusiones ei uso de programas de computador an la determinaci6n de estructuras geol6gicas es cada dia mas importante. ' se utiliz6 un programa para la identificaci6n de zonas !itoi6gical"!'.enta hom~g{meas, y se pudo datermlnar su utlhdad al aphcar al metodo an al analisis da un registro electrico. sa puedan procesar secuencias da infonnaci6n axtensas an periodos da tiampo muy cortos al tiempo para analizar 100 datos la tomo a' la ~aq';jina aproxi":l~damanta 20 segundos, incluldo la genaraclon de la salida da resultados. la tecnica de zonacion sa utiliza en otras areas da la geologia, qua contamplan analisis de secuencias da datos. geofislc8 colomblana ~ 1, octubre de 1992 33 , ~_1 1 ij--t----+--+-h i iii ---_.-'_.-~.~~~---.-~-~ --~~ i-"-::~:::':"-. _ ..:..! i i i-t':' _._.. figura 1. registro electricc de pozo. sensibilidad: 10 divisiones = 500 nm; profundidad: cada division = 0.9434 m los segmentos indican unidades litoloqlcas homoqeneas. ei tope de la columna es 332.6 m y la base 423.3 m. --_.=-!!. ~._--~._-~w. ~ '~, -r-r-' ---t--~~--fa ~; ~ _ i ,i . . (: ~i ,,/it .. ---1--'· i -i~i i j .~'''''''+-=",-+---.,; i figura 2. [ccntinuacion fig.1). registro electrtco de pozo. sensibilidad: 10 divisiones = 500 nm; protundidad: cada division = 0.9434 in los segmentos indican unidades litologicas homcqeneas. ei tope de la columna es 425 my la base 519 m. 34 montes: programa para determinar zonas en una secuencia tabla 1. resistividades a distintas profundidades. los valores se muestrearon a intervalos iguales. prof resist prof resist prof resist prof resist 332.6 133.33 378.9 293.33 425.2 186.67 471.5 200.00 334.4 133.33 380.7 220.00 427.0 186.67 473.3 240.00 336.3 166.67 382.6 133.33 428.9 213.33 475.2 240.00 338.1 100.0 384.4 306.67 430.7 126.67 477.0 220.00 340.0 133.33 386.3 333.33 432.6 126.67 478.9 253.33 341.9 100.00 388.1 266.67 434.4 126.67 480.7 220.00 343.7 186.67 390.0 173.33 436.3 126.67 482.6 266.67 345.6 86.67 391.9 300.00 438.1 133.33 484.4 160.00 347.4 86.67 393.7 313.33 440.0 333.33 486.3 120.00 349.3 160.00 395.6 313.33 441.9 353.33 488.1 120.00 351.1 80.00 397.4 186.67 443.7 353.33 490.0 133.33 353.0 66.67 399.3 166.67 445.6 333.33 491.9 200.00 354.8 66.67 401.1 166.67 447.4 333.33 493.7 306.67 356.7 106.67 403.0 146.67 449.3 313.33 495.6 300.00 358.5 133.33 404.8 166.67 451.1 280.00 497.4 266.67 360.4 133.33 406.7 193.33 453.0 266.67 499.3 200.00 362.2 166.67 408.5 200.00 454.8 200.00 501.1 200.00 364.1 253.33 410.4 200.00 456.7 226.67 503.0 200.00 365.9 353.33 412.2 400.00 458.5 333.33 504.8 320.00 367.8 333.33 414.1 546.67 460.4 266.67 506.7 240.00 369.6 186.67 415.9 500.00 462.2 233.33 508.5 233.33 371.5 213.33 417.8 573.33 464.1 366.67 510.4 220.00 373.3 286.67 419.6 466.67 465.9 333.33 512.2 233.33 375.2 286.67 421.5 253.33 467.8 300.00 514.1 233.33 377.0 300.00 423.3 233.33 469.6 266.67 515.9 246.67 , programa para zonacion de secuencias de datos 1 programa para zonacion de secuencias de datos consta de un modulo principal y dos funciones } x : arreglo que contiene datos a procesar } meant: valor medio de la secuencia total } { datos es ei nombre del archivo de entrada } program lone(data,input, output); var data: file of real; x : array [1 ..256] of real; k : integer; meant: real; j --------} funcion que calcula la media del segmento definido liminf1 : cota inferior del segmento a promediar } limsup1 : cota superior del segmento } x: datos a procesar, definidos entre liminf1 y limsup1 } function mean(liminf1, limsup1 : integer) : real; var i : integer; suma : real; begin suma :=0.0; for i := liminf1 to limsup1 do sum a := suma + x~l; rnean.esuma/tllmsup t liminf1); end; {======================================================================} geofislca colombians n2 1, octubre de 1992 35 mean 1 : media del primer segmento } mean2 : media del segundo segmento } liminf, lirnsup : iimites del segmento a analizar y dividir }} r : coeficiente a evaluar, para determinacion de limites pos : indicador de r evaluado } inc: longitud de paso de avance } delta: indicador de posicion de maximo r procedure border(liminf, lirnsup : integer); var mean1, mean2, ssw, ssb, rmax, r : real; pes, inc, i, delta: integer; begin if (lirnsup liminf) > 8 then begin pas := 0; inc := 4; rmax:= 0.0; while (iimsup-liminf-inc > 4) do begin ssw := 0; mean1 := mean(liminf, liminf+inc); rneanz := mean(liminf+inc+ 1, lirnsup): for i := liminf to liminf+inc do ssw := ssw + sqr(x[i]-mean 1); for i := liminf+inc+ 1 to lirnsup do ssw := ssw + sqr(x[i]-mean2); ssw := ssw/(limsup-liminf-4); ssb := sqr(mean 1-meant)+sqr(mean2-meant); pos := pos + 1; if ssb > 0 then begin r := t-tssw/ssb): if r > rmax then begin rmax.er: delta.epos: end' end' ' inc := 'inc +2; end' border(liminf+delta+ 1, limsup); bordertliminf, lirninf -delta): if rmax > 0 then writeln(liminf+delta,' ',rmax:4:3); end; end; {=========================================================================} begin { •• inicio del principal·· } reset(data,'datos') ; k '=0' while' (not eof(dala)) do begin k := k + 1; read(data, x[k]); end' c1ose(data); meant:=mean(1,k); border(1,k); end. { •• fin del principal •• } 36 montes: programa para determinar zonas en una secuencia cota superior del segmento tabla 2. muestra el tope superior del segmento y el coeficiente r r profundidad (m) 88 84 74 72 70 68 63 61 59 54 48 46 44 42 32 30 23 19 12 10 7 0.323 0.223 0.573 0.785 0.807 0.462 0.802 0.879 0.423 0.081 0.790 0.940 0.918 0.361 0.155 0.221 0.268 0.148 0.442 0.484 0.389 493.6 486.3 467.8 465.9 460.4 456.7 447.4 443.7 440.0 430.7 419.6 415.9 4122 4085 3900 3863 373.3 3659 353.0 349.3 343.7 , bibliografia davis, j. c. (1986): statistics and data analysis in geology. 211pp. john wiley & sons co, newyork. gill, d. (1970): application of statistical zonation method to reservoir evaluation and digitized-log analysis. the am. assoc. of petrol. geologists bulletin. 54 (5):719-729. miller, r. l. & j.s. kahn. (1962): statistical analysis in the geological sciences. john wiley & sons co. tucker, a.b. (1987): programming languages. pp.18-66. mcgraw-hili inc. new york. montes and cárdenas.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 91-103 stacking on common reflection surface with multiparameter traveltime luis a. montes v.1 and andrés cárdenas contreras2 (1) luis a. montes v.: geosciences department, universidad nacional de colombia, bogotá, d.c. colombia. lamontesv@unal.edu.co (2) andrés cárdenas contreras: engineering faculty. universidad distrital francisco josé de caldas, bogotá, d.c. colombia. acardenas@udistrital.edu.co abstract commonly seismic images are displayed in time domain because the model in depth can be known only in well logs. to produce seismic sections, pre and post stack processing approaches use time or depth velocity models whereas the common reflection method does not, instead it requires a set of parameters established for the first layer. a set of synthetic data of an anticline model, with sources and receivers placed on a flat topography, was used to observe the performance of this method. as result, a better reflector recovering compared against conventional processing sequence was observed. the procedure was extended to real data, using a dataset acquired on a zone characterized by mild topography and quiet environment reflectors in the eastern colombia planes, observing an enhanced and a better continuity of the reflectors in the crs stacked section. keywords: seismic section, crs, multiparameter traveltime, coherence analysis. resumen las imágenes sísmicas son comunes en dominio del tiempo debido a que el modelo en profundidad se conoce solamente en registros de pozo; para producir las secciones sísmicas los enfoques de procesamiento antes y después de apilado usan modelos de velocidad en tiempo o profundidad no así el método de superficie común de reflexión que requiere de un conjunto de parámetros establecidos para la primera capa. aplicada a datos sintéticos de un anticlinal con fuentes y detectores colocados sobre una superficie plana, el método exhibió un buen desempeño desplegando los reflectores más claramente que los suministrados por el procesamiento convencional. el procedimiento se aplicó a una línea sísmica adquirida en los llanos orientales colombianos, en una zona caracterizada por suave topografía y reflectores de ambientes tranquilos, se observó un realce y mayor continuidad en los reflectores. palabras claves: sección sísmica, crs, tiempo de tránsito multiparámetro, análisis de coherencia. 91 earth sciences research journal manuscript received february 16 2006. accepted for publication december 12 2006. introduction recently a new approach has been developed to estimate a more approximate traveltime function through geometrical optics approximation. the approach is based in the common reflection surface (crs) method, supported on fictitious normal wave and normal incident point wave used to describe zero offset ray propagation (tygel et al., 1997). the traveltime is expressed in terms of the emergence angle of this ray and the wave front curvatures of the two waves. the traveltime approximation was defined for 3d media and related with the kinematic attributes of the wave field. the method has been extended to study how offset affects amplitude by obtaining analytical curves (biloti et al., 2002) to estimate velocity in depth (ortega et al. 2003) and static corrections (grossfeld et al, 2001). the crs method uses a set of multi-covering data acquired on a flat surface without assumption of horizontal parallel planar interfaces. in case of non planar surfaces, the dataset must be continued to flat datum before stacking process. crs traveltime function figure 1 displays a v0 velocity layer over a semi infinite v1 velocity medium separated by a curved interface. the ray perpendicular to the reflector in j point (named normal ray) emerges with an angle β at xo point spending t0 to travel from xo to j and back to xo (zero offset traveltime). over the flat surface, the couple source-detector images the curved reflector through the ray that follows the trajectory s-j-g (called paraxial ray); hence, the traveltime function may be estimated dividing the path length of the ray by v0 speed. in figure 1, two wave fronts are shown where the hypothetical n wave is generated by an exploding reflector with curvature radii rn and the nip wave is generated by a point source in r with curvature radii rnip. both measured in the vicinity of the central ray. for the coordinate system fixed to the central ray, the square of the traveltime for the paraxial ray is approximated by tygel et al. (1997): stacking on common reflection surface with multiparameter traveltime. figure. 1. model for an arbitrary reflector and crs parameters 92 . (1) here, the midpoint and the half offset are pair coordinates defining a new vector space with β, kn, and knip crs parameters. the last two parameters represent the wavefront curvature of both n wave and nip wave respectively. the crs process flowchart contains the same steps presented in a conventional sequence up to nmo stack. this step is performed applying the move-out formula expressed in the multiparameter traveltime of the equation 1, providing a better zo section of crs parameters during the coherence analysis of the multiple shots and receivers data. before using multiparameter montes and cárdenas., esrj vol. 10, no. 2. december 2006 figure. 1. anticline model displaying the rays reflecting on the interfaces for the symmetrical array. figure. 3. shot against receiver position distribution with all gather configurations. the pair (x, h) defines the new vector space. 93 traveltime it is necessary to be familiar with the near surface velocity model (e.g., to provide t0 and v0 values). model an anticline model of 5 km wide and 3 km depth with three layers over a semi infinite medium was build (figure 2), and a synthetic dataset with 75 records with 40 m source interval and 20 m group interval was generated. a split spread geometry with 100 receivers and 25 maximum fold was used, where h varies from 5 to 505 m increasing 10 m and x ranges from 505 m to 4455 m increasing 10 m. the map of shots against receivers coordinate distribution is displayed in figure 3 with all gather configurations (e.g., common shot (cs), common receiver (cr), common mid point (cmp) and common offset (co)). the new pair coordinates, half-offset and mid point, are linearly independent and define a new vector space, where the traveltime function as defined in equation 1 is established. conventional stacking assumes that reflections are represented by hyperbolic traveltime and reflectors by points. the crs approach proposes that the recorded data has to be associated with common reflection surfaces, instead of common reflection points. hence to stack the data, it may be focused to a common reflection surface using multiples shots and receivers, allowing the inclusion of more traces in the procedure. in figure 4 the surfaces represent the traveltime functions in x-h domain for three reflectors in the anticline model, recalling that for each point on reflector there is an associated crs surface in this space. first interface parameters estimation to solve the traveltime function, for the first layer, implies to know the parameters β, kn, and knip, whereas v0 distribution can be established using depth hole and up-hole time of the shots usually available in seismic surveys, and t0 values from zo sections. through three different configurations the expression 1 may be simplified as explained below. in common mid point setting, where the fixed point coincides with the middle point xm = 0, equation 1 is reduced to figure. 4. traveltime functions expressed in x-h domain associated to the anticline interfaces. 94 stacking on common reflection surface with multiparameter traveltime. , (2) with q = knipcos2β. on the other hand, in zero offset configuration h=0, equation 1 becomes: (3) and finally in common shot configuration, where x = h, equation 1 is simplified to . (4) where .u=kn+knip the equations 2, 3, and 4 are linearly independent with 3 unknown parameters β, rn, and rnip which means that this system equation is solvable, and the simplified flow diagram shown in figure 5 explains the procedure used to estimate them. in the first step, the parameter q = knipcos2β is estimated in the multi-coverage data sorted in cmp configuration, according to equation 2. the β and kn values are iteratively approximated, using kn = 0 as initial setting in expression 3, and using this β value to know kn, and so on. finally, the third parameter is calculated through . in figure 6, the behavior of the q parameter for the first interface is plotted in vector space x-h. according to biloti et al. (2001) this parameter may become imaginary depending on knip values, but in this case is always real. the traveltime for a horizontal flat reflector is , and when it is compared with equation 2, allows to define the parameter which relates stacking velocity with crs operators. figure 7 shows the behavior of emergence figure. 5. the flow diagram explains the steps followed to obtain the three parameters. 95 montes and cárdenas., esrj vol. 10, no. 2. december 2006 figure. 6. combined parameter q for the first interface in the x-h domain. figure. 7. the emergence angle β of rays normal to the first interface in the new vector space. 96 stacking on common reflection surface with multiparameter traveltime. angle β for rays normal to the first interface. its values range from -28º to 28º, being zero at the anticline’s apex. although the kn surface may set any real value, in this case is always positive as shown in figure 8 where it ranges between 2.4510 e004 and 5.2995e-004. due to its small value, its contribution to equation 1 is too low compared with the other parameters. in figure 9, the kn behavior is depicted with values ranging from 0.0016e-004 to 9.7585e-004, recalling that by definition knip = 0 corresponds to a flat reflector. figure. 8.the kn curve in terms of the new pair coordinates. figure. 9.the knip parameter for the first interface in the x-h domain. 97 montes and cárdenas., esrj vol. 10, no. 2. december 2006 figure. 10. comparison between nmo and crs corrected cmp gather figure. 11. stacked section of the anticline model provided by nmo operator. 98 stacking on common reflection surface with multiparameter traveltime. figure. 12. former crs stacked section before applying multiparametric travel time corrections figure. 13. final anticline stacking by means of crs operator. 99 montes and cárdenas., esrj vol. 10, no. 2. december 2006 figure. 14. stacked section provided by conventional seismic processing sequence figure. 15. stacked section obtained through crs approach.. 100 stacking on common reflection surface with multiparameter traveltime. synthetic data figure 10 shows at the left a cdp gather, at the middle the same gather after nmo correction, and at the right the same gather after crs correction. in the cmp domain, the two traveltime expressions are equivalent; therefore the two corrected gathers look similar, indicating the performance of crs correction. figure 11 shows the nmo stacked sections with the three interfaces clearly displayed as expected. the figure 12 shows the former crs stacked section with the reflectors correctly positioned in time, but with a blurred image. this effect decreases in depth. at this stage of the crs process, the travel time correction has not been applied. the figure 13 shows the final stacked section provided by means of crs operator, the events were enhanced and displaced to their correct position in time. comparing the images provided by nmo and crs techniques, a similar coherence is observed in the three reflectors, indicating the robust performance of crs stacking method. real data tested on synthetic data, the next step was to apply the technique to real data using a high quality seismic dataset acquired on the eastern colombian land plains. the dataset was processed using a conventional sequence and a final stacked section through nmo correction was obtained (see figure 14). following the same methodology explained for the anticline model, the parameters associated to the first layer were estimated and fed to the algorithm to provide the crs stacked section displayed in figure 15. according to the theory, in case of parallel plane interfaces, the normal emergence angle becomes zero and also kn ≅ 0, because the interfaces in the zone are considered flat. in order to get comparable images, the dataset was continued to a flat datum and the same residual static corrections were applied on both seismic processing sequences. as result, the deep reflectors in crs stacked section are enhanced and a better continuity observed. the zones between 1200 – 1620 ms for both stacked images were zoomed and displayed in figures 16 and 17 in order to remark their differences. as can be observed, the crs section increased the coherence in reflectors defining a better lateral figure. 16. 1200-1620 ms interval image of crs stacked section. 101 montes and cárdenas., esrj vol. 10, no. 2. december 2006 figure. 17. 1200-1620 ms interval image of nmo stacked section. continuity. conclusions the anticline model studied shows the detailed behavior of the three parameters provided by crs stacking. these results provide valuable information to be considered when the crs operator technique is applied to real data. the common reflection surface corresponds to an image built through the stacking of a number of seismic traces from different cmp gathers, in which the sources and receivers are close to the image point, and are corrected through the simulation of a zo section. this is acceptable because in the xm-h–t domain, traces are organized in such a way that it is possible to obtain a better approximation and quality in the seismic data processed thanks to the parameters triplet. this method requires no knowledge of an underground model. it only requires the surficial velocity in the vicinity of stacked trace location and it can produce the simulation of a very precise zo. in special cases where the topography is rather abrupt, it will be useful to define a more accurate geological model. the crs stacking method furnishes additional information through the three kinematic stacking parameters, making possible to define the interfaces, since their location, depth and curvature may be determined. the emergence angle and the nip wave front curvature are the two most routinely used parameters to permit reverse tracing of rays. the crs method guarantees that through analytical expressions based on the spherical representation of wave fronts, overtime correction is described for a given sourcereceiver pair with respect to the trace of a zo image. determining the triplet of parameters requires larger computational effort but it provides a better zo sections with enhanced and more continuous reflectors as displayed in crs stacked sections of real data. the obtained results encourage crs research in an effort to improve the performance of the algorithm to apply this technique to any seismic line. 102 stacking on common reflection surface with multiparameter traveltime. acknowledgements the authors wish to thank the geophysics group of the universidad nacional de colombiabogota and its department of geosciences for the support given to the crs research project. also, we express our grateful to halliburton inc for the provided software used along the research and the anonymous reviewers who contribute with their suggestions. references biloti r., santos, l., and tygel m. (2001). multiparametric traveltimes: estimation and inversion. ph.d. thesis, state university of campinas, brazil (in portuguese). grossfeld v. (2001). correction of topographic effect in seismic data using multiparameter traveltime formulas. master thesis, state university of campinas, brazil (in portuguese). hubral p. and krey, t. (1980). interval velocities from seismic reflection traveltime measurements. seg. ortega, c. and montes, l. (2003). depth velocity model estimated through the diffraction function: a real data application. geofísica colombiana. 61-64. (in spanish). tygel m., hubral, p., and schleicher, j. (1997). eigenwave based multiparameter traveltime expansions. ann. int. mtg. seg. expanded abstracts, 1770-1773. vieth k. (2001). kinematic wavefield attributes in seismic imaging. ph.d. thesis, karlsruhe university, germany. 103 montes and cárdenas., esrj vol. 10, no. 2. december 2006 geofis. colomb. santafe de bogota d.c issn • 0121·2974 las geociencias en la filatelia juan manuel moreno murillo profesor asistente departamento de geociencias-facultad de ciencias-universidad nacional de colombia moreno j.m: las geociencias en la filatelia. geofis. colomb. 2:51-70, 1993. issn 0121·2974 la filatelia ha sido conocida desde antario como el "pasatiempo de los reyes y el rey de los pasatiempos", manteniendo y aumentando su popularidad, aun luego de cumplir 153 aries desde la introducci6n del sello de correos. la academia de la lengua espanola define la filatelia como el arte 0 ciencia que consiste en estudiar y coleccionar sellos de correos 0 estampillas postales y materiales afines. sin embargo, el correo es anterior a las estampillas pero estas, fueron la causa por la cual el correo se populariz6. la palabra filatelia esta compuesta de dos vocablos griegos: phi/os que significa amante 0 amigo y, ate/ia que se deriva de ateles y que significa peg ado de antemano, es decir sello. las estampillas (sellos de correo), son pequerias vinetas que los gobiernos emiten para ser utilizadas en el franqueo de las cartas, por 10 cual tienen un valor representativo en la moneda del pars que 10 emite. luego de cumplir su funci6n, entran en el campo de la historia y de la ciencia a traves de la filatelia. ei que colecciona estampillas satisface el conocimiento cada dla, pues hay un interes especial en la busqueda, un tanto indefinible, por las caracterfsticas de ella, que es una obra de arte en miniatura que sirve para unir al hombre, para lievar sus mensajes, su comunicaci6n y la imagen de un pais sin fronteras. a consecuencia de estas caracterfsticas propias, las estampillas son utilizadas adernas como medio de difusi6n, pues ilegan a cualquier lugar del mundo, exhibiendo costumbres, riquezas, ciencia y cultura del pais emisor. por ello, la cual la fi/atelia se ha convertido en el mas popular pasatiempo. la filatelia es ciencia, por que es uno de los estudios mas serios y profundos, debido a 10 imposible del dominio internacional del filatelista. si aun es diffcil el conocimiento cabal de las estampillas de un pais, distinguiendo bien las originales, las clasicas, los errores, la edici6n, las tecnicas, cantidades, etc; mas aun 10 sera, con relacion a las colecciones ternaticas como en este caso de las geociencias, tema que a nivel nacional es practicarnente desconocido 52 moreno: las geociencias en la filatelia nivel gran filatelicamente, perc que a internacional tiene una representaci6n. sin embargo, su obtenci6n se facilita a traves de cataloqos internacionales, canje y relaci6n con otros filatelistas. ei coleccionar sellos es sin duda el pasatiempo cientffico mas difundido, y sequn los historiadores, se remonta a los mismos dlas en que apareci6 el primer sello de correos en el mundo (1 0 6 de mayo de 1840, sequn varios autores). ei arte 0 ciencia de coleccionar estampillas se aumenta, por que su valor cultural y didactico es unico. adernas, en muchos casos como el que aquf se presenta, la ternatica en geociencias y otros t6picos, permiten difundir y comunicar conocimientos de manera practice y sencilla. la colecci6n de estampillas presentada, tanto de colombia como del resto del mundo constituye una ternatica, que aporta informaci6n adicional de interes especifico en geociencias, donde se quiere resaltar la calidad y variedad de emisiones en los diferentes t6picos que difunden a nivel internacional, colecciones de minerales, f6siles, variedad de potenciales enerqeticos nacionales, y/o eventos conmemorativos relacionados con las geociencias. la ausencia de emisiones nacionales en esta ternatica es notoria, comparativamente con otros parses del mundo, hay poca edici6n de sellos dedicados a las geociencias y la mayorfa han side emitidos dentro de la divisi6n historica-filatelica nacional. es asf que podemos mencionar las estampillas antiguas (1919-1939); las estampillas modernas (1940-1958) y en menor proporci6n las del poscentenario (1959 a la fecha). ei mayor interes de las emisiones se nota hacia las riquezas naturales, potencial de hidrocarburos y algunos minerales estrateqicos. desde el punta de vista de sitios de interes, relacionados a eventos 0 fen6menos geol6gicos, se destacan las unicas ediciones alusivas a los volcanes galeras y nevado del ruiz, al parque de los nevados y, en 1986, la emisi6n conmemorativa por el desastre de armero. en relaci6n con la geofisica, se destaca la emisi6n conmemorativa al ano geoffsico internacional, evento que fue reconocido a nivel filatellco por varios parses. desde el punta de vista de la astronomfa se reconoce el observatorio astron6mico nacional y una emisi6n conmemorativa a nicolai copernico. de las escuelas que ofrecen carreras en geociencias en el pais, filatelicamente solo se ha reconocido la universidad nacional y en especial la escuela de minas de medellin al cumplir su primer centenario de fundaci6n. con respecto al resto del mundo, se presentan algunas emisiones de diferentes parses de los cuales se ha podido obtener series completas 0 sellos individuales relacionados con las geociencias, aunque no de todos, si se hace de algunas de sus emisiones representativas, ya que algunas son de diffcil consecuci6n. en general son notorias las grandes emisiones relacionadas con temas como la mineralogia, paleontologfa (con entasts en dinosaurios), la geoffsica y en energfas convencionales. se destacan las emisiones de minerales y su empleo como piedras preciosas y semipreciosas principal mente en parses africanos y de asia. las geociencias en la filatelia, son el reflejo cultural y cientifico de los pueblos, ya que a traves de ella, los pafses participan al mundo parte de sus acontecimientos, de su historia, sus riquezas y sus personajes. las estampillas, luego de cumplir su finalidad como el pago del porte postal, entran en el campo de la ciencia e historia como afici6n e investigaci6n a traves de la filatelia. dado que son muchos los temas importantes para tener un homenaje postal, que representen una imagen real y positiva del pais, y aunque es cierto que la labor de la .selecci6n de motivos para las emisiones es muy diffcil y esta es adernas escasa, es necesario que de una u otra manera las comunidades cientfficas, eventos de interes nacional e internacional, museos colombianos, colecciones de universidades e instituciones relacionadas con las geociencias, sean tenidas en cuentas para una emisi6n postal mas representativa, junto con la partici aci6n activa de las comunidades geocientfficas nacionales. geofisica colombiana no.2, abril 1993 colombia 53 escuelas de geologia en la filatelia universidad nacional de colombia -19681367-j()i.7 . u\i\h!si~\1l \\qo'<.\1 bogota 100 anos de fundaci6n -1987mineralogia medellin -1932riquezas naturales esmeraldas -1937--1935wateflow & sons ltd. li t eq raf i a nal. -1932wateflow & sons ltd. -1972colombia nul resello oficial -1989thomas de la rue 54 moreno: las geociencias en la filatelia colombia 1932 "ineralosia rlouezas naturales oro -1935waterlow & sons ltd. -1932american bank sobreporte aereo -1938litografia noll. -1939-1937resello -1934resello 4q centenario cartagena allerican bank -1944-1956departamento de narino productor de oro litografia "loll. minas d. oro de placer geofisica colombian a no.2, abril 1993 colombia 55 hidrocarburos -1932riquezas naturales sobre oorte aereo -1932-1934water low & sons american bank -1939-1944american bank nacionalizacion -1937resello -1944columbian bank -1938li t oc raf i a nal. -1951-1958departamento de norte de santander -1956litografia nal. petroleras del catatumbo resello aereo 56 moreno: las geociencias en la filatelia colombia mineralogia riquezas naturales platino halita 1932 -1956minas de platina depa~tamento del chaco -1956-1990-1952salinas de zidaqui~a minas de manau~e -1956-1952-1986hierro y carbon -1937sob~eca~ga -1958resello -1954ace~ia5 paz del rio confe~encia side~u~gica el ce~~ej6n geofisica colombiana no.2, abril 1993 57 geofisica estados unidos 6uatei'iala' sismologia la tierra peru ecuador ano geofisicb 1957 1958 nac iones uni das fteteorol06ia y cliftatol06ia brasil meteor-osel salvador jiiiiij . . .• iii -~ •. '~~. ~~ . j.:li j. climatologia -1985moreno: las geociencias en la filatelia58 colombia geofisica y astrono"ia ano geofisico internacional1959 a~te de dete~minar altitudes -1949--1974--1946vulcanologia volcan 6aleras -1954-19~9parque los volcan nevada del ruiz -1954-19~9ar.ero -1985geofisica colombian a no.2, abril 1993 alemania 59 geofisica aolicaciones -1980paleontologia flora fosil cristalografia slstemas crlstallnos liechetenstein (mineralogia -1989-) fauna fosil galena cuarl0 60 moreno: las geociencias en la filatelia geofisica meteorologia oceanografia a~o geofisico internacional (1957-1959) hungria f0f!, , ,iliac_hit po&\~q'iq , mongolia a~o del sol tranouilo -1964 1965geoflsica colombian a no.2, abril 1993 61 magyar posta (hungr .ia) paleontologia y mineralogia pez fosil cuplea hunga~ica cobre 1969 amonitas reineckia c~assicdstata cua~zd amatista f6sil vegetal tortuga cupr-ita calcita y esfalerita 62 moreno: las geociencias en la filatelia brasil estados unidos plineralogia belgica ," oil \\1l8on1doos 101{;lia petl'"oleo cal'"b6n cuba hiel'"l'"o brasil hiel'"l'"o -1975texico chile cobl'"e niquel venezuela -hierrocobre geofisica colombiana no.2, abril 1993 63 mozambique rlineralogia 1979 corye'os·isn mocrmbique cuarzo berild maoneti t a 1urmalina esdinelas fluod t a 64 moreno: las geociencias en la filatelia kenya mineralogia minerales y piedras preciosas yeso 11.....'" .. amazonita 1977 t,.-ona • galena xil6palo , cu a r zo amatlsta tu r na li n a .i.&j~;,;.'{i ~~, , co,.-ind6n azul be,.-ilo azul fluol'"ita cu ar z o agata g,.-anate rodolita col'"ind6n roj a geofisica colombiana no.2, abril 1993 65 union sovietica mineralogia 1963 piedras semipreciosas de los urales tooacio esmeralda cuarzo jasoe rodonitd cudl"zo amatista -1971colecci6n de joyas en el fondo nacional de diamantes malaauita concresional zafiro y diamantes amatista con diamantes diamantes en flor diamante sin lalla diamantes en rosa 66 moreno: las geociencias en la filatelia portugal mineralogia pirita , minas de hie,.-,.-o 1977 ma,.-moles mineria de uranio recursos naturales eneraia hid,.-ica geotermia energia e6lica hidroca,.-buros energia 50la,.geofisica colombiana no.2, abril 1993 67 polonia paleontologia fauna f6sil -1965saurios y reptiles eras paleozoica y mezosoica cuadrupedo ter-r-es tre .iur-ae ico pelicosaurio herbivoro permico (± 200 m.a) plesioaaurio acuatico jurasico(±190 m.a) dinosaurio terrestre jurasico (± 200 id.a) zt340 ~ i dinosaurio terreatre cretacico (± 80 m.a) depredador acuatico permico (± 200 m.a) 68 moreno: las geociencias en la filatelia cuba paleo .....tologia fauna f6sil -1985reptiles y sau~os era mesozolca reptil volado~ con pica sin dientes c~etacico sude~io~ (± 80 m.d) sdu~io reptil te~~est~e ju~asico-(± 200 m.d) bipedo te~~est~e he~bivo~ c~etacico i. (± 120 m.d. cudd~upedo te~~est~e ju~aslco (± 200 m.d) cuba..:.. sdu~io con un cue~no c~etacico (±140 m.a) sau~io terrestre bipedo cretacico s.(± 80 m.a) 5aurio terrestre carnivora cretacico (± 140 m.a) geofisica colombiana no.2, abril 1993 69 riquezas naturales islandia hidr-ocdr-bur-os austria amon i t a nueva guinea decddd hidr-016gicd ecuador petr-61eo portugal (azores) ener-gid geoter-micd vulcanologia union sovietica 196570 moreno: las geociencias en la filatelia cuba paleontologia fauna f6sil -1987reptiles y sauros coomos hhit pelicosaur-us euoplocephalus nicaragua paleontologia styr-acosaur-us fauna f6sil -1987reptiles y sauros, nicaragua,m_ .." lit 10!!.!? t· ~., .. tr-icer-atops mammuthus nicaragua"'" m, lit 15!!.!? nicaragua"''''''' lit 15!!.!? dimetr-odon nicaragua'"'' ,,.,111:30" dinichthys uintater-ium pter-anodon vargas et al.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 5-19 geoinformation density: a criterion on anh block negotiation carlos a. vargas j.1,2 – armando zamora r. 1 – andres pardo 1,3 1 agencia nacional de hidrocarburos – anh 2 departamento de geociencias, universidad nacional de colombia – sede bogotá 3 departamento de geología, universidad de caldas corresponding author: carlos a. vargas j., e-mail: cavargasj@unal.edu.co abstract based on the general information included within the anh exploration and production information service, the information distribution densities of the colombian territory were determined. these densities are referred in seismic length per square kilometer and amounts in magnetic-gravimetric and geochemical information data or drill-hole length per sq. km. a probabilistic distribution was assessed along the density distribution and cost distribution for each variable. the variables, as information layers, were cross-referenced in order to define relative weights that assess information with respect to the presence or absence of information in the treated area. this procedure could be regarded as a methodological support proposal for area negotiation of the oil industry. a better approach to the quandary should include: new input of data into the system, a division of the territory in smaller areas adjusting to complex geometries, and considering the particular market conditions for each basin. nevertheless, it is apparent that the obtained results favor the consolidation of a conceptual framework that at medium term will allow a conscious approach towards block negotiation between the petroleum industry and the anh. key words: density of information, oil exploration, negotiation, colombia. resumen la clasificación automática de señales sísmicas se ha llevado a cabo típicamente sobre representaciones el uso de la información de carácter general incluida dentro del servicio informativo de la exploración y de la producción de anh permitió la determinación de densidades de distribución de información del territorio colombiano. estas densidades se refieren a longitud de sísmica por el kilómetro cuadrado, cantidades de datos de información magnético-gravimétrica y geoquímica, así como longitud de perforación por kilómetro cuadrado. una distribución probabilística fue entonces estimada para la distribución de la densidad y de la distribución de costos para cada variable. las variables como capas de información fueron pesadas relativamente para determinar la presencia o 5 earth sciences research journal manuscript received march 13 2007. accepted for publication june 20 2007. ausencia de la información en el área tratada. este procedimiento se podría mirar como una propuesta metodológica para la ayuda en la negociación de áreas de interés en la industria del petróleo. un acercamiento mejor al problema debería incluir: nueva entrada de datos en el sistema; una división del territorio en áreas más pequeñas que se ajusten a las geometrías complejas; y considerar las condiciones de mercado particulares en cada cuenca. sin embargo, es evidente que los resultados obtenidos favorecen la consolidación de un marco conceptual que en el mediano plazo permitirá un acercamiento consciente hacia la negociación de bloques de interés exploratorio para la industria petrolera y la anh. palabras claves: densidad de información, exploración de petróleo, negociación, colombia. introduction a negotiation is a communication framework where the assessed object considers an interaction between value estimations of each participating transaction party. this practice may become subjective, as each participant may imprint a differential value based on its own cognizance. evidently, he who has the utmost knowledge of the object will have a better position to assess the cost benefit relationship and go appropriately further in the negotiation. the strategy is to set up a large sum of negotiation criteria to establish block allocation mechanisms within the practice of exploratory promotion developed by the anh. one of these criteria may be supported on the hypothesis of geoinformation density, that is, “the possibility that information distributed in a specific area may be considered as typical and a criterion to quantify its relative investment”. accepting this statement as starting point, it can be demonstrated that geological, geophysical, geochemical, and in general all geospatial information, may be conceived as an objective tool for block appraisement. in this line of thought, the goal of this paper is guided towards defining a conceptual tool that will allow supplementing block negotiation undertaken by the anh within its mission outline. methodological aspects based on the general information included within the anh exploration and production information service epis such as longitude and location of several seismic programs, gravimetric, magnetometric, and geochemical location coverage along with well distribution performed during the exploratory history of colombia, information distribution densities have been determined from territory area discretization (e.g., 25km x 25km). these densities are referred in seismic length per square kilometer and magnetic-gravimetric and geochemical information data amounts or drillhole length per square kilometer. once the information densities have been established for the area, the probabilistic distribution is assessed along with the inferred relative weighing factors to the density distribution and cost distribution for each variable. the variables, as information layers, have been cross-referenced in order to define relative weights that assess information weighters with respect to the presence or absence of information in each treated area. figure 1 depicts a synthetic outline establishing the procedures followed on this paper. data it could be stated that a number of the geospatial variables analyzed show strong contrasts with respect to their distribution in colombia. figure 2 shows the location of seismic information depicting a dense coverage towards the eastern plains (llanos orientales) basin including the eastern mountain range foothills, putumayo and magdalena valley (high, medium, and low), pacific off-shore coast (central and south region), atlantic off-shore coast, and sinú-san jacinto and cesar-ranchería basins. in contrast, low information densities are seen towards the orinoquía and amazon regions, north of the department of chocó, and along the mountain range axis. a similar situation is seen with the geoinformation density: a criterion on anh block negotiation 6 vargas et al., esrj vol. 11, no. 1. june 2007 7 magnetic and gravimetric information, well distribution, and geochemical controls (figures 3, 4, and 5). even though the epis database has information voids in some periods and there are time differences in data entry in the same database, probably as a testimony of the change of administration, its current structure and contents remains representative and may guarantee that the database is suitable to assess the exploratory tendencies and ensure an appropriate criterion for block negotiation. analysis and results distribution function to understand how spatial distribution of geoinformation density could be analyzed, it is necessary to begin from an analysis of the distribution function that better suits the observations. for instance, seismic density registered in colombia could be used as a representative variable for this procedure. figure 6 shows the distribution of density frequencies and the best fit representing two models of statistical distribution: normal and gamma. for the first event, a strong unbalance is seen, suggesting that the distribution could not comply with a normal distribution process. usually, offer and demand processes follow normal distributions. in this event, although it is a similar scenario for area allocation, it follows a gamma pattern fit, specifically an exponential distribution (ayyub and mccuen, 2003). exponential distributions are useful to assess processes through time (blaesid and granfeldt, 2003). however, this condition is not reflected in our issue because this is a data set encompassing the whole observation period, and illustrates a scenario where the main biasing frequency almost dominates the distribution below the mode and in turn allows values greater than frequency. a checkmark for this distribution may be seen in the simple and cumulative probability curves (figures 7 and 8). the behavior is similar to “club admittance”, in other words there is a minimum fee to be a member, but any sum above the minimum is also admissible and very exceptionally members below the fee will be presented. this is situation fundamentally imposed by the petroleum industry, which possibly will promote increases in admission quotas when the business tends to be more attractive. in effect, for a greater observation time and under the same energy requirements in colombia, it could be expected that the seismic information density as a criterion for block allocation (admission quota) tends to be greater. table 1 shows the parameters for each distribution. table 1. statistical parameters for types of distributions applied to seismic data density per unit area. a similar situation is seen in distribution functions for information densities associated to the amount of magneto-gravimetric points and drill-hole length (figures 9 and 10). for well data (m/km2), this index depicts in certain degree, the inversion rate executed to know a given area. likewise, information density related with the number of geochemical stations should take into account the dissimilarity in the level of tests for each station (toc, pyrolysis, organic petrography, sulphur content, metal content, rock extracción, liquid gas chromatography, and biomarkers). this information is conditioned to the existence of an analysis sequence protocol as a function of the results obtained from certain initial testing such as toc and pyrolysis. therefore, its representability only grants a partial nature (figure 11). a representation of these variables in the colombian territory is shown in density information maps associated with the analyzed variables (figures 12, 13, 14, and 15). table 2 summarizes the exponential distribution parameters converted into relative weights, describing all the observed information. normal exponential domain median 0.387563 0.387563 variance 0.219149 0.150205 generally, empirical adjustments with confidence levels can be seen, widely reproducing the behavior for the amount of data for the observed period. estimation of a relative weighting factor for area negotiation the exponential probability distribution function and probability estimation have been defined as (blaesid and granfeldt, 2003): (1), (2), where is the arithmetic mean and x the discrete value of the distribution of the estimated probability. for all previously analyzed information densities, it is necessary to shorten the equation (2) to a relative weight factor that promotes the generation of new knowledge in areas where data is scarce: (3), the weighting factor ( ) is a tool that relatively contemplates initiatives directed towards new surveys under different block contracting employed by anh. according to equation (3), areas with plenty of information will be penalized with low weighting factors and viceversa. obviously, upon promoting strategies encouraging enforcement of new scientific and technological procedures on areas of interest, the need to estimate other weighting factors with the equation (3) is a must. information layer weighting as can be expected, the relative sum of related weights with all variables, allows having an approach to the spatial distribution of information density. although average costs for each variable are very different and entail another relative weighting to market averages. table 3 shows colombian seismic kilometer average costs, unitary values for each magneticgravimetric and geochemical point and linear drill-hole foot (with more frequent analyses), as developed approximate amounts. it could be seen that seismic is the most expensive exploratory variable, apart from being the most widely used. likewise, the technical link between seismics and wells is noticeable, being these two representable variables of areas where the major exploration investments have been executed.therefore, superimposing the drill-hole density map on top of the seismic density information map, or at least one of these two variables, may be a criterion enough to estimate a map of information density relative to the weighting providing support for block negotiation processes. nevertheless, equation 4 is an empirical approach that assumes the superimposition of all variables taken into consideration and weighting the relative cost of each variable. the estimates seen in equation (4) enabled performing a weighted geoinformation density map (figure 16). this map highlights the areas where the effect of new exploratory information has less effect. possibly, the current setting may vary as new data is added to the information foundation that engineered it. according to figure 16, there are areas where no exploratory efforts have been undertaken as those analyzed, namely those in the colombian eastern border, the borders with venezuela and brazil, in the serranía of san lucas, the junction of the pastos, the sierra nevada of santa marta, and the lower cauca river valley. this state of 8 geoinformation density: a criterion on anh block negotiation (4), 9 inform ation d ensity form ula seism ic (km /km 2) m agnetic-gravim etric stations (am ount/km 2) d rill-hole length (ft/km 2) g eochem ical stations (am ount/km 2) table 2. exponential distribution param eters converted into relative w eights, describing all the observed inform ation vargas et al., esrj vol. 11, no. 1. june 2007 figure 1. general aspects on the methodology used in this work. density weight data for four information layers were weighted to obtain one layer of useful relative weight geoinformation in support of block negotiation processes. figure 2. distribution of seismic information associated to all hydrocarbon exploration projects in colombia. geoinformation density: a criterion on anh block negotiation 10 figure 3. distribution of the information associated to potential fields (aerial and terrestrial aeromagnetometry and gravimetry) product of several exploration mining and energy projects in colombia. figure 4. distribution of wells associated to several hydrocarbon exploration projects in colombia. 11 vargas et al., esrj vol. 11, no. 1. june 2007 figure 5. distribution of geochemical stations related to several hydrocarbon exploration projects in colombia. geoinformation density: a criterion on anh block negotiation figure 6. seismic data density relative frequency distribution and best fit representing two models of statistical: normal and gamma (exponential). 12 figure 7. cumulative probability curves for seismic density information fitted to typical normal and exponential distribution curves. figure 8. simple probability curve for seismic information density fitted into normal and exponential distribution typical curves. 13 vargas et al., esrj vol. 11, no. 1. june 2007 figure 9. simple probability curve for magnetometric and gravimetric information density adjusted to typical normal and exponential distribution curves. figure 10. simple probability curve for drill-hole length information density adjusted to typical normal and exponential distribution curves. geoinformation density: a criterion on anh block negotiation 14 figure 12. distribution map for information densities for the colombian territory. figure 11. simple probability curve for geochemical stations density information adjusted to typical normal and exponential distribution curves. 15 vargas et al., esrj vol. 11, no. 1. june 2007 figure 13.distribution map for information densities for drill-hole lengths for the colombian territory. figure 14. distribution map for gravimetric and magnetometric information densities for the colombian territory. geoinformation density: a criterion on anh block negotiation 16 figure 15. distribution map for geochemical station information densities for the colombian territory. figure 16. colombian territory distribution map for geochemical station information densities. 17 vargas et al., esrj vol. 11, no. 1. june 2007 18 affairs can be explained in view of the current geological knowledge. these areas do not seem to be favored by their inherent geological conditions and possibly have no mineral-energy potential of interest. in contrast, some areas have vast amounts of information density, evidenced by the widespread of exploratory activity such as the eastern plains, putumayo, and the magdalena mid-valley. on the other hand, some belts and areas with high information density are suddenly interrupted making it uncertain to know the potential continuity of the prospects that have not been adequately analyzed as the northern part of the pacific coast and towards the north of putumayo. although it is necessary to further analyze the information densities of these areas, it is also clear that the socio-environmental conditions of these areas are intricate and have hindered the possible development of potential prospects. conclusions the development of the present knowledge management assignment based on the epis database should be regarded as a methodological support proposal for area negotiation whose quantitative results rise above the preliminary results. a better approach to the dilemma should include: new input of data into the system as a consequence of an exhaustive search of data to complement the current petroleum knowledge of the country. the discretization of the territory in smaller areas adjusted to complex geometry, showing the current conditions for block negotiation. 1. 2. an approach of the problem considering the particular market conditions for each basin. data entry that establishes new information layers allowing to improve the relative weighting of geoinformation such as seismic information reprocessing and dating. breaking up information in basic data that allows to establish information layers with differing weights (e.g., differentiation of stratigraphic wells into different diameters and considering supplementary surveys such as electrical registry and petrophysical testing). nevertheless, it is apparent that the obtained results favor the consolidation of a conceptual framework that at medium term will allow a conscious approach towards block negotiation between the petroleum industry and the anh. acknowledgments to the staff at schlumberger, particularly to edgar medina and liliana salcedo, for enduring two months of my intense requirements over the available epis system information. references ayyub, b.m. and mccuen, r.h. (2003). probability, statistics, and reliability for engineers and scientists. new york: chapman & hall/crc press. balakrishnan, n. and basu, a. p. (1996). the exponential distribution: theory, methods, and applications. new york: gordon and breach. blaesild, p. and granfeldt, j. (2003). statistics 3. 4. 5. exploratory variable unit cost range mode (approx.) amount performed (2006) seismic (linear km) u$12000 – u$40000 u$18000 277.329 magneto-gravimetric station u$1.5 – u$80 u$15 1.155.624 geochemical station u$25 – u$1500 u$80 27.655 drilling (linear foot) u$350 – u$890 u$390 53.890.746 table 3. commercial values and amounts performed up to date for the exploratory variables used for this study. this data was approximated from anh existent information. geoinformation density: a criterion on anh block negotiation with applications in biology and geology. new york: chapman & hall/crc press. spiegel, m. r. (1992). theory and problems of probability and statistics. new york: mcgrawhill, p. 119. 19 vargas et al., esrj vol. 11, no. 1. june 2007 geofisica colomblana ~ 1, octubre de 1992 . 5 el alumbramiento de una ciencia luis a. briceno profesor asociado depto de geociencias-facultad de ciencias-universidad nacional de colombia briceno l.a.: el a l umbr arn ie nt.o de una ciencia. geofls. colomb. 1:5-8, 1992. issn 0121-2974 "mientras el hombre no se haya explicado el secreto del universo, no tiene derecho de ester satisfecho" alumbrar significa lienar de luz y claridad 0 paner luz en alqun lugar. alumbrar es dar vista al ciego y tarnbien conceder feliz parto, puede en ultirnaa significar tomarse del vino. para hablar del nacimiento e historia de una ciencia hay necesidad de saber que hace y para qua sirve esa ciencia. ' definir la geoffsica y engendrar definiciones no as facil y acudir al diccionario no siempre es sabio, haqanlo y encontraran nueve palabras muertas en ei tiempo. construiremos una definicion que salga del trabajo y que sea valida par un periodo igual al de la validez del mismo. la geofisica la he vivido y entendido como la ciencia que tiene co mo objetivo at modelamiento dinarnico de nuestro globa, 0 parte de el, a partir de la observacion y analisia de parametres fisicos. modelamiento dinamico que quiere door: sus caracteristicas qeornetricas, con variaciones tempo ra les , y \a naturaleza de las fuerzas que las originan, es door ei modele ha de ser eepacio-temporal. jules renard actualmente, para que una disciplina que trabaja con hechos, sobre un marco teo rico , praduciendo nuevos conocimientos, pueda constituirse en ciencia, ha de tener: un nombre un objeto c1aramente definido publicaciones permanentes una metodologia de referencia una asociacion 0 grupo de interesados trascender barreras geograticas adernas debe ser enseiiada en centres de educaoion superior. porque la geofisica posee estas caracteristicas, se ha convertido en ciencia universal, independiente de la geologia, que aparta ei objeto y de la fisica, de la cual toma su metodologia. pera especificamente, i,de que temas trata la geofisica? ei problema se puede considerar historlcarnente: quienes han tratado de diferenciar el aspecto fisico y medible de la tierra, han creado nombres y divisiones (udias & mezcua, 1986): en ei siglo xvii kircher [hotandes) llama a esta area de las ciencias naturales: geoscopia; 6 brlceilo: ei alumbramlento de una elencla a.g. wemer en 1750 utiliza el tennino geognosia; humboldt a quien se puede considerar como uno de los padres de la geoffsica utilizo el termino de "geograffa fisica"; f. neunann, el de fisica de la tierra. metherie a finales del siglo xviii usa el termlno de teoria de la tierra: saigey, a comienzos del xix, el de fisica del globo; en inglaterra en 1838, w. hopkins habla de la geologia fisica para referirse al aspecto fisico rnaternatioo de la geologia; en 1863, w. kertz en alemania, utiliza forrnalmente el termino de geofisica y, g. neumayer en 1871 define la geofisica como el conocimiento de las relaciones fisicas de la tierra. ya establecioo el tennino de geoffsica, gutemberg en 1929 establece una de sus primeras divisiones en capitulos de rnecanica, qravitackin, electricidad, magnetismo, opfica, calor y composioion de la materia. la asociacion que promueve el estudio de todos los problemas que se refieren a la figura de la tierra, la fisica del globo, los oceanos, la atmosfera, iniciando, facilitando y coordinando las investigaciones de tooos los problemas de la geodesia y la geofisica que requieren coopeeaclon internacional, es la union intemacional de geodesia y geofisica (iugg), fundada en 1911 (bates et ai., 1982), la cual esta compuesta por siete asociaciones intemacionales: de. geodesia (lag), de sismologia y fisica del interior de la tierra (iaspei). de meteorologia y fisica de la atmosfera (iamap), de geomagnetismo y aeronomia (iaga), de ciencias fisicas del oceano (iapso), de vulcanologia y quimica del interior de la tierra (iavcei), de hidrologia cientifica (lash). aparte de las secciones aceptadas por la iugg, la union geofisica americana (agu), organizada en 1919, adiciona las de paleomagnetismo, tecto no fisica, p1anetologia y fisica solar e interplanetaria. no es facil hacer clasificaciones e inclusiones de la ciencia bajo nombres integrales, pues como se senala en brlceflo & mojica (1990), cada vez hay mayor atomizacion de las ciencias. vale la pena senalar que howell (1962) seiiala que a veces se incluye en ia geofisica otros cuatro sectores que son: glaciologia, geotermometria, geocosmogonia y geocronologia. en ia practica, es dificil establecer fronteras tematicas entre areas como la astronomia, astrofisica, ffsica solar e interplanetaria, planetologia. meteorologia, afmosferica. igual cosa sucede, con la oceanografia fisica, fisica de este modo nuestras definiciones 0 clasificaciones son temporales, mientras son superadas por la dinamica investigativa y es prudente por 10 tanto hablar de las ciencias geofisicas, sabiendo que algun dia hablaremos de las ciencias magnetometricaso gravimetricas. la geofisica se consolida como ciencia entre 1863 y 1911; pero "hechos geofisicos", los realiza el hombre desde antes de la era cristiana. con el riesgo de ser incompletos y no hacer suficiente justicia a quienes han trabajado por enriquecer datos y conocimientos fisicos de nuestro p1aneta, haremos un recuento de estos: aristoteles (324-382 a.c.) : compila su "meteorologica", compendia del cual menos de la mitad esta dedicado a los aspectos atrnosfericos y 10 restante a tern as de oceanografia, astronomia y meteoritos; opinando acerca del origen de los terremotos. los primeros sismoscopios se ie atribuyen a chang heng en china, quien en 132 a.c., pudo indicar no solo la ocurrencia de sismos sino tarnbien su direccion. aparte de algunos trabajos esencialmente geograticos (ptolomeo, 90-168 d.c. p.ej.), durante varios siglos de dominio religioso, no hayaporte significativo de hechos cientificos. 8610 con el renacimiento europeo y hombres como leonardo da vinci (14521519) se reinicia la discueion y la experimentaclon cientifica, que incluye la fisica de la tierra. leonardo trabaja sobre el campo gravitacional terrestre y la propaqaoion y reflexion de ondas. durante los tres siglos siguientes a da vinci (1500-1800), la geofisica se desarrolla paralelamente con las ciencias fisicas y matematicas. podemos sintetizar este desarrollo historico con los siguientes datos: geor!;lius agricola (1494-1555), aleman. autondad en mineria, publico en 1546, "de re metalica", quizas el primer trabajo que trataba de la prospeccion minera. william gilbert (1540-1603). en inglaterra, establece las bases de la electricidad y el magnetismo. observo los efectos de inclinacion y declinacion magnetica. francis bacon (1561-1526). explic6 la naturaleza de las fuerzas gravitacional y magnetica y recalc6 la necesidad de experimentacion fisica. sugirio origenes comunes para america y africa. galileo galilei (1564-1642). desarrollo la forrnulacion para ei movimiento pendular. invento un termometro (1593). geofislca colomblana ~ 1, octubre de 1992 . 7 rene descartes (1596-1650). estableciola geometria analitica. publico" meteorology", sobre vientos, nubes y precipitaciones; midio indices de refraccion para explicar el arco iris. evangelista torricelli (1608-1647). con la invenoion del barornetro de mercurio demostro ia existencia de la presion atmcsferica, en 1643. christian huygens (1629-1695). en su ''tratado de la luz" en 1693, explico el comportamiento de las ondas al incidir en una interfase. isaac newton (1642-1727). cre6 y orqanizo el calcu]o. enuncio las leyes baslcas del movimiento y explioo las verdaderas causas de las mareas en su "principia" (1669). pierre bouguer (1698-1758). compare el efecto gravitacional de masas locales montaiiosas con las regionales de la tierra utilizando, en 1740, mediciones pendulares. benjamin franklin (1706-1790). estudio los relarnpaqos y recornendo el usa del pararrayos. basado en medidas de corriente y temperatura publico un mapa de la corriente del golfo (gulf stream). john michell (1724-1793). profesor de geologia de la universidad de cambridge, publico importantes articulos sobre las causas de 10s terremotos (1760) y describio un rnetooo para medidas del campo gravitacional mediante "la balanza de torsion" (1777). pierre laplace (1749-1827). utiliza sus desarrollos matematicos para mostrar porque los f1uioosse desplazan a 10 largo de isobaras sobre esferas en rotacion, similar al trabajo de coriolis en 1835. ei siglo xix se inicia entonees con una serie de hechos geofisicos aislados; porque, a pesar de haber cientificos brillantes, la geofisica para su consolidacion requeria de un so porte tecnologico en cuatro areas esenciales, a saber: instrumentacion, transpo rte, comunicacion y, procesamiento. con excepcion del telegrafo, inventado por s. morse en 1837, los desarrollos en comunicacion, procesamiento e instrumentacion, en general, solo se darian en el siglo xx. mientras tanto, en 1845, robert mallet (ir1andes 1810-1885) experimenta en la medicion de velocidades de ondas sismicas mediante ei usa de fuentes explosivas. en el campo gravitacional, el siglo xix se caracteriza por la iniciacion del mapeo e interpretacion del campo gravitacional terrestre; en este trabajo fue pionero ei hunqaro roland von eotvost (1848-1919), quien en 1890 construyo la primera balanza de torsion. un hecho de importancia global de este siglo fue el establecimiento de redes para observacion de datos fisicos de la tierra. por recomendacion del aleman friedrich von humboldt se establecieron estaciones maqneticas en muchas partes del globo durante la decada de 1830. uno de los principales colaboraoores de humboldt fue karl f. gauss (1777 -1855) quien invento el magneto metro bifilar e inicio mediciones exactas a traves de toda europa. las redes elsmokiqicas se iniciaron despues de 1880 cuando un grupo de investigadores britanicos: gray, ewing y milne construyeron el primer eisrnometro en tokio. este sisrnornetro era esencialmente mecanico , solo hasta 1906 el ruso b. b. galitzin desarrollo los sismcqratos electrornaqneticos. en america, el primer stsmoqrato fue instalado en berkeley (california) en 1887. en 1895, john milne propuso en londres una red mundial de estaciones sismicas y en 1899 estaba en funcionamiento una red de 27 estaciones que operaban en todos los continentes. aparte de estos hechos cientificos en la geofisica, durante el siglo xix paralelamente se consolidaba la ciencia especialmente en alemania con los siguientes hechos: a) publicacion en 1887 del primer texto geofisico: handbuch der geophisyk por s. gunther. b) creacion en 1893 por g. gerland de la primera revista especializada: beitrage zur geophisyk. c) establecimiento de la primera catedra de geofisica en 1898, en la universidad de gottingen y que ocupo emil wiechert. se puede decir que la ciencia geofisica nace entonces con la alborada del siglo xx, consolidanoose con la creacion en 1906 de la seismological society of america (ssa), en 1911 de la international union of geodesy and geophysics (iugg) y en 1919 de la american geophysical union (agu). la geofisica tendria en nuestro siglo que afrontar el reto de ayudar al hombre a explicar los secretos de nuestro planeta, y bien que 10 ha logrado. en una entrega especial de la revista scientific american, del primer semestre de 1991, dadicada a "celebrar la pasion de la humanidad por la comprension"; esto es, a resaltar los 5 temas que en ei milenio y mas especificamente en este siglo, marcaron los hitos de la ciencia y la tecnologia, se seiiala "ia revolucion en geofisica" como uno de e1los. 8 briceno: ei alumbramiento de una ciencia bibliografiaciertamente hemos alcanzado logros en el conocimiento de la estructura y cornpo sicion del planela tierra, sabemos como se mueve su corteza, conocemos bastante acerca de las causas aisrnovolcanicas, y podemos deteclar con gran detalle la presencia de petroleo en la parte mas extema de su corteza. habra necesidad de otro espacio posterior en la revisla para resaltar el desarrollo, estado actual y limilaciones de la geofisica a nivel mundial. no podemos sin embargo dejar de introducimos en 10 que ha sucedido con el alumbramiento de la geofisica en colombia. briceno & mojica (1990) reconocen la presencia de mediciones geofisicas en colombia con anterioridad al siglo xx, pero seoatan que solo hasta despues de 1920, comienzan a aplicarse las tecnicas de prospeccion geofisica para hidrocarburos en nuestro territorio, por parte de empresas extranjeras. la sismologia instrumental nace en colombia, a finales de la decada del 30, en el instituto geofisico de los andes colombianos. ello, gracias al esfuerzo del padre jesus emilio ramirez qui en con la colaboracion del doctor luis guillermo duran publicarian en 1957 el primer texto nacional, sobre prospecoion geofisica. en briceno & mojica (op. cit.) s9 senala ei papel fundamental que jugaron instituciones gubernamentales como el ingeominas, ei instituto geogratico agustin codazzi y ecopetrol, en la fundarnentaclon de la geofisica nacional. sin embargo, a pesar de ser esta ciencia un producto de la accion interdisciplinar. i y la colaboracion interinstitucional e internacional, en colombia fue considerada en sus inicios como apendice complernentario de otras ciencias 0 tecnicas. hasla 1980, nuestra geofisica no poseia una asociacion ni una publicacion especializada y propia. consideramos entonces que con la fundacion doe la academia colombiana de ciencias geofisicas y ei inicio de esta publicacion podamos colaborar con "el alumbramiento de la geofisica en colombia". bates, c., t. saskell & a.b. rice. (1982): geophysics in the affairs of man. 492 p. pergamon press l.tda. briceno, l.a. & j. mojica. (1990): ei estado de desarrollo y de insercion social de la geologia y la geofisica en colombia. mision de ciencia y tecnologia, la oontorrnacion decomunidades cientificas en colombia. vol 3, tomo 1:281-334. ministerio de educion nacional, departamento nacional de planeacion y fonade, santafe de bogota. charon, j. (1967): de la fisica al hombre. 262p. ediciones guadarrama, madrid. galil eo , s. (1914): two new science. 300p. dover publications, inc. new york. translate by h. crew and alfonso de salvio. gannon, g. (1971): biografia de la fisica. 262p. salvat editores s.a, alianza editores s.a. navarra. howell, b.f. (1962): lntro duccion a la geofisica. 435p. ediciones omega, barcelona. tellford, w.m., l.p. seldart, r.e. sheriff & d.a. keys. (1976): applied geophysics. 860p. cambridge university press. ramirez, j. & l.g. duran. (1957): prospeccion geofisica. 254p. universidad nacional de colombia, santale de bogota. udlas, a.& j. mezcua. (1986): fundamentos de geofisica. 419p. alhambra universidad, madrid. geoffs. colomb. .santafe de bogota d.c issn 0121-2974 estudio geomagnetico en el volcan galeras luz amalia ordonez b. ffsica msc. geoffsica u.n. ord6iiez l. a.: estudio geomagnetico en el volcan galeras. geoffs. colomb. 2:3-8, 1993. issn 01212974 resumen se presenta un resumen de la metodologfa y los resultados de los modelos bidimensionales de anomalfa rnaqnetica de campo total y estimativos de la profundidad de la isoterma de curie. ei estudio se realiz6 a partir del analisis, en el dominio de la frecuencia, de datos geomagneti'cos tomados en inmediaciones del volcan galeras, en el mes de julio de 1989, epoca de reactivaci6n del mismo. abstract geomagnetic data takes in the galeras volcano, during its reactivation in july 1989, allows to present a bidimensional model of the total field magnetic anomaly. an approach to the curie isothermal is also presented. methodology and results has been worked out in the frequency domain. 1. introduccion en areas volcanicas se aplican ampliamente los estudios geomagneticos, para el seguimiento temporal de la actividad volcanica, como uno de los metodos de predicci6n de erupciones. la base que posibilita su aplicaci6n es el hecho de que la temperatura geotermica sobre y en regiones cercanas a los volcanes, cambia de acuerdo al estado de actividad particular, generando alteraciones de las propiedades maqnsticas de las rocas, que pueden ser detectadas en la variaci6n de la intensidad del campo geornaqnatico. ei estudio realizado en el galeras, se presenta como un aporte al reconocimiento de su estructura, mediante la presentaci6n de los modelos bidimensionales de fuentes individuales de las anomalfas mas relevantes por su intensidad y determinaci6n de la profundidad de la corteza litol6gica maqnstlca, profundidad de la isoterma de curie. por otra parte, como inicio de posibles trabajos futuros que permitan hacer seguimientos temporales. en este trabajo se utilizaron fundamentalmente las tecnicas del analisis espectral, con el fin de resolver: a) algunas operaciones en el dominio de la frecuencia: filtros de alta frecuencia y reducci6n al polo norte rnaqnetico, para extraer las anomalfas generadas por fuentes superficiales y para facilitar los mecanismos de interpretaci6n, respectival'nente; b) mediante el anallsis de los espectros de potencia reducidos al polo, estimar los pararnetros de las fuentes individuales, como aproximaci6n a la soluci6n del problema directo de la teorla de campos de potencial; c) estimar la profundidad de la isoterma de curie. 2. informacion se realizaron 175 estaciones localizadas sobre perfiles independientes circundantes y radiales al crater principal (galeras), de acuerdo a las condiciones de acceso terrestre, a intervalos aproximados de 500 m, entre puntas de observaci6n consecutivos, cubriendo un area aproximada de 100 km2 (fig.1); se establecieron cinco estaciones principales y como estaciones secundarias se tomaron los puntas de cruce de perfiles independientes. la incertidumbre total de las mediciones es de 16 gammas, equivalente a un 0.05% del promedio de la intensidad de los valores de campo medidos. de la comparaci6n relativa de estos perfiles, se escogieron aquellos mas relevantes por mostrar fuertes contrastes de anornalla, picos maxirnos negativos del orden de mil gammas y se analizaron en el dominio de la frecuencia, para obtener modelos bidimensionales de las fuentes que los generan. 4 ord6i\ez: estudio geomagneticoen el volcan galeras 6jj o .0 • 0 ~le f lor t d 0 .. o o o o o l o. . .... .... .. ..... je so n ~ a ~~o i ~'b0 ze~le~s o • .. o • -.... :.o o . '0 '.o o o~pos to'... 0. 0 c.or . *ai to toe 0 .. .. , .. .' . o.... 61 3 -+-r-,~.,..,--..,.--r--r-r""'-iti-r-r""""rrt..--rrrt-rr-r-rn 948 978953 958 963 968 973 kilometros figura 1. distribuci6n de estaclones. coordenadaagausslanas. galeras,crater principal. la magnitud de las anomalfas (fig.2) corresponde al perfil extendido que circunda el edificio volcanico. los puntos de maxima anomalfa correspondientes geogn3ficamente a las vecindades de "las cuadras" y "santa rosa", concuerdan con la ubicaci6n de maxima anomalfas de otros perfiles y, en consecuencia, son seleccionados sectores .del perfil para establecer los modelos de fuente bidimensional e individual. la intensidad de picos de anomalla negativa se explica por el hecho de que las rocas del volcan galeras son predominantemente andesitas oroqenicas (cepeda, 1985). estas rocas presentan un valor medio de susceptibilidad maqnetica alta [0.135 unidades c.g.s., medida en laboratorio (dobrin, 1960)] y magnetismo remanente practicarnente despreciable (logachev & zajarov, 1978). una representaci6n bidimensional de anomalfa de campo total se muestra en la fig.3, en la que se pueden apreciar dos zonas de anomalfa negativa: una localizada alrededor de las coordenadas 974;631, que por su ubicaci6n se interpreta como expresi6n del crater principal y la otra alrededor de 960;631, que podrla corresponder a la presencia de un crater secundario, no apreciable en la superficie. 3. analisis espectral la interpretaci6n de los datos geomagneticos es util para contribuir a estudios geol6gicos regionales. sin embargo, en algunos casos se obtienen anomalfas muy claras, que pueden ser separadas de los efectos de fuentes vecinas y que siendo simples en apariencia, pueden ser modeladas como fuentes individuales y los metodos de anafisis utilizados para infbrir sobre sus formas y dimensiones. geofisica colomblana n° 2, abril de 1993 1'1'1'1 '1 (j) « l l« l') -1'10'1 l: s c odros s't.e . roso -2'100 9010 40 kilometros figura 2. perfil circundando el edificio volcanlco. las cuadras-sta rosa: puntos de maxima anomalia. para el analisis cuantitativo, ante la imposibilidad de obtener relaciones analiticas que describan la anomalfa generada por fuentes reales, se adoptan modelos de forma geomatrica regular que pueden representar estructuras geol6gicas. pero, las relaciones analiticas aun para estas formas simples involucran funciones no lineales que dificultan su soluci6n. ei analisis espectral, reduce el problema a la soluci6n de ecuaciones lineales y es esta la raz6n por la cual se uliliza en el analisis e interpretaci6n de datos geomagnalicos. la descomposici6n espectral, transformada de fourier para un conjunto de datos discrete y finito, haciendo consideraciones estadfsticas adecuadas, es equivalente a la transformada discreta de fourier. para la determinaci6n de los coeficientes se utiliza en muchos casos practices. en particular en aste trabajo, el algoritmo denominado transformada rapida de fourier, cuya eficiencia es sustenlada en el tratamiento de datos geotrsicos por varios autores, entre otros davis (1973), kanasewich & agarwal (1970), kanasewich (1981). figura 3. anomalla magn6t1ca de campo total. 6 ord6i'\ez: estudio geomagn6tleo en el volean galeras la importancia de la descomposici6n espectral, radica en el hecho de definir funciones que representen un hecho ffsico representativo del proceso. los coeficientes de la expansion representan la importancia relativa de cad a terrnino 0 arm6nico y de estos es posible obtener el espectro de potencia, como expresi6n de la energfa contenida en un intervalo y cuyas caracterfstica dependen del proceso mismo. para un proceso arm6nico y estacionario, representado por un conjunto de datos discrete y finito, el espectro de potencia "e" es discreto, y ssta dado por: con ai y b, coeficientes de la expansi6n y sumatoria extendida de 1 a n, siendo n el nurnero de arm6nicos representative para el proceso (pryestley, 1981). 4. espectro de potencia dique finito del anal isis de perfiles independientes pueden obtenerse modelos de fuente bidimensionales. un modelo bidimensional que puede representar unidades geol6gicas reales es el modele de dique, cuyas caracteristicas geometricas fundamentales son: el espesor, su extensi6n en profundidad y el angulo de inclinaci6n con respecto a la direcci6n de la linea 0 perfil de observaci6n. para estudios maqnetometricos, el dique es caracterizado por el valor de su susceptibilidad rnaqnatica y el angulo con respecto a la direcci6n del campo inductor, orientado con respecto al polo norte maqnetico. para el analisis de perfiles cuyo modelo de fuente puede representarse por un dique vertical finito y cuyo campo inducido ssta orientado fundamentalmente en la misma direcci6n del campo inductor (esto es el magnetismo remanente resulta despreciable frente al magnetismo inducido). el espectro de potencia reducido al polo norte magnetico ha side deducido analilicamente y puede darse por la relaci6n (pedersen. 1978): e(k)= 4m2exp(-2kh){sen(ka)/ka}{1-exp(-kt)} (2) m = susceptibilidad magnetica/unidad de volumen, dada en unidades c.g.s. k = 2 nil. numero de onda dado en rad/unidad de longitud, l = longitud del perfil, n= 1,2.3 ... h, t = profundidad tope superior e inferior, a = espesor del dique. 5. analisis cuantitativo del espectro de potencia tomando el logaritmo natural a la relaci6n (2), los termlnos de la expresi6n pasan a ser sumandos independientes. representando funciones en tsrminoe de los para metros fundamentales de cuyas respectivas soluciones es posible determinar los parametres de la fuente, haciendo las siguientes aproximaciones: 1) el termino fundamental del decaimiento exponencial del espectro, para ios primeros arm6nicos, define la profundidad del tope superior de la fuente. asl, si m es la pendiente del decaimiento y con h profundidad del tope superior: (1) m = 2h [3] 2) el efecto combinado de la profundidad del tope superiory la extensi6n vertical. introduce un pica maximo, para los primeros armonicos. que decae rapidarnente y de la determinaci6n del nurnero de onda k, correspondiente al pica maximo, se obtiene la profundidad del tope inferior t, en la forma: i<c = log (h/t)/(h-t) [4] en este punta es necesario tener en cuenta que la maxima profundidad discernible en el espectro, esta determinada por la longitud del perfil v, en consecuencia, no siempre aparece el pica maximo. en este caso, la profundidad del tope inferior queda indeterminada y sola mente es posible deducir que se encuentra a una profundidad mayor que u2. 3) determinando dos arm6nicos consecutivos (k1 y k2) para los cuales el espectro es igual 0 aproximadamente igual a cero, como unica soluci6n no trivial de (2), se tiene el espesor del dique, en la forma: (5) 4) tomando el limite (p) cuando k tiende a cero, en (2), se define la susceptibilidad magnetica m en la forma: m = 2/po (6) en esta forma se estimaron los parametros fundamentales para los modelos de dique vertical finito reducido al polo norte magnetico. en perfiles independientes. cuyo patr6n representa clara mente la posibilidad de modelar fuentes individuales. co~ . los paramet~os as! obtenidos y ?pli~lzandolos, rrye.dlante proceso de ajuste iteratlvo en el domlnlo del espacio. se definieron los mejores valores para los para metros de la fuente. presentados en la tabla 1 y en las correspondientes graficas de las figs.4-5. geofislca colombiana n° 2, abril de 1993 7 tabla 1 perfil h a m d las cuadras • 0.5 2.0 0.8 -0.12 90 0.7 2.0 0.9 -0.12 132 sta. rosa· 1.4 3.3 1.28 -0.12 90 1.6 3.3 1.28 -0.12 90 parametros de 105 modelos de fuente individuales: h= profundidad tope superior en km; t= profundidad tope inferior en km; a= espesor en km; m= susceptibilidad (c.g.s); d= angulo con respecto al campo inductor en grados .• valores obtenidos medianle el analisis de espectros de polencia, •• ajustados en el dominio del espacio. (f) « l l« l') -212j(21(il -.-.-.1 i i i i i i i -7 3 kilometros figura 4. modelo perfil correspond/ente a las cuadras * * 200 (f) « l l« d 800 -+-,-,.......-r-r---r-.,-,.......-r-r---r---r -9 2 kilometros figura 5. perfil clrcundando el edlflcio volc6nlco. las cuadras-sta rosa. 6. profundidad de la isoterma de curie toda fuente de anomalia maqnetica liene extensi6n vertical iimitada, si no por otra raz6n, por el hecho de que las rocas pierden sus caracterfsticas maqnsticas a la profundidad donde se alcanza la temperatura de curie. esta profundidad se manifiesta en el espectro de potencia, por la presencia de un pico maximo para los primeros arm6nicos que decae rapidarnente, cuando la longitud del perfil alcanza la profundidad del tope inferior de la fuente. la frecuencia, wmax., correspondiente a este pico maximo esta inversamente relacionada con la extensi6n en profundidad de la fuente "d" mediante la relaci6n [6] dada por boler (1978), relaci6n previamente probada para su aplicaci6n, obteniendo desviaci6n menor del 10% en los modelos analizados, constituidos por conjuntos de diques de para metros dados. d = 1iwmax [7] en el galeras, se analizaron los espectros de potencia reducidos al polo, correspondientes a cuatro perfiles independientes representando: toda el area estudiada (global), areas norte, sur y centro de la misma (figs.6-9). los valores obtenidos se muestran en la tabla 2, que refleja valores relalivamente superficiales, si se tiene en cuenta que este en general se encuentra entre 20 y 30 km de profundidad, hecho que se explica por el regimen tsrrnlco del area, ocasionado fundamentalmente por la actividad del volcan, en cuyo crater es de esperar temperaturas del orden de los 750"c, temperatura de fusi6n de las lavas (leet & judson, 1968) que supera la temperatura de curie para rocas andesfticas, predominantes en el galeras, que es del orden de los 500°c. (delgado, 1983). las diferencias correspondientes a las zonas establecidas, concuerdan con la proximidad de los perfiles al crater principal. tabla 2 region norte sur centro global profunoidad de curie (km) 1.6±10% ~-5.8 -o.9±10% -5.8±10% profundidad dada con respecto al nivel del mar, siendo negativo por debajo de este. ei espectro correspondienle a la regi6n sur, no muestra el pico maximo (profundidad indeterminada). 8 ord6i\ez: estudio geomagnetico en el volcan galeras 3.50 <{ « 19.00 t---l t---l u u z z w w f1.50 f-a a g.g.o 200 numero de ond/) figura 6. espectro de patencia. perfil representando toda la regi6n. 4 <{ t---l u z w f-a g.9.00 o 200 numero de onda figura 7. espectro de patencia. perfil representando la regi6n norte. 7. conclusiones la realizaci6n de estudios maqnetometricos. en particular, y geoflsicos, en general, contribuye efectivamente en el establecimiento de modelos de las estructuras volcanicas, como contraste entre intrusiones rnaqmaticas recientes y rocas consolidadas, como se aprecia claramente en los perfiles trazados en el galeras. ei anal isis espectral de datos de campos de potencial es confiable y util como mecanisme de interpretaci6n cuantitativa. serra pertinente dar continuidad al trabajo retomando los puntas de observaci6n, para dar inicio a estudios de seguimiento temporal y realizar un cubrimiento de informaci6n bidimensional, que permita establecer modelos tridimensionales, que daran una mejor aproximaci6n en el establecimiento de modelos adecuados. o 100 numero de onda figura 8. espectro de potencia. perfil representando la regi6n central. <{ t---l u z w f-a q.. 0.5 e 50 100 numero de onda figura 9. espectro de potencia. perfil representando la regi6n sur referencias boler f.m ~1978l: aeroma~netic measurements, 'i!agnetic b~~~6'ng';fr]t~e~~ ~~~a 1!>~~g~i~t~?etd~i~ea~t~hee. davis g.j. (1973): statistic and data analysis in geology. john wiley & sons, inc., u.s.a. delgado r, (19~3): magnetometrla. editorial pueblo y educacion. ciudad de fa habana, cuba. dobrin m.b. (1960): introduction to geophysical prospecting. mc. graw hili, u.s.a. kanasewlch e.r. & r.g. a~arwal. (1970): analysis of combinedpi:~~~ys~g~2w:~?8~~~71~eldin wave number domain. kanasewlch e.r. (1981): time secuence .analysis in geophysics. the university of alberta press, winnipeg. leel ed..& s, judson 1196~\: fundamentos de geologia iisica. oitonal l.imusa. mexico. loga~hev a. & v. zejerov. 11978\: exploraci6n rnaqnettca. reverte sa, barcelona, espai1'a. peders,en l.b. (1978~: a statim' analx3is of pot1£cial fields ~~':~~3~95~rtjcalc rcular cy inder a d a dike. eophysics prles~~a~e~ijw~vi ~~,c5~~~~~il!l~ri~~d time series, vol 1. geofisica colornbiana pp 70-75 bogota, d.c.n° 6 diciernbre de 2002 issn 0121 2974 modelamiento sismica utilizando teoria de rayos en medios transversalmente lsotroplcos: teoria y ejem p_lo-=-s-=---_ carlos pieorahita, ta,no salinas y saul guevara' resumen en el presente trabajo hacernos un resumen de las propiedades basicas de un medio rransversalrnente isotropico (ti). mostramos las propiedades basicas delmedio utilizando conceptos de la rnecanica del continuo y las propiedades del trazado de rayos en dichos medics. posteriormente presentamos diferentes rnodelos elementales con un medio ti y el respectivo modelarnienro sismico utilizando el paquete comercial norsar2d. finalmenre considerarnos un modele de un area de interes en la exploracion sismica, localizado en la g ografla nacional, que esra siendo evaluada par ecopetrol, la empresa estatal colornbiana. palabras clave: medios anisotropicos, medids transversalmente isdtropicos, parametros de thomsen, trazamientd de rayds, metodo del frente de onda, angulo de simetria abstract in this paper, we show the basic properties of transvcrsal isotropic medium (ti), using the continuous mechanic concepts and .hc way tracing technique. later, the norsar2 software models several responses in tl models. finally, we considered the model of an area, that is being evaluated by ecopctrol. keywdrds: anisotropic media, transversaly isotropic media, thomsen's parameters, ray tracing, wave front method, angle of symmetry introduccion en el presente articulo presentamos las conceptos basicos de un medio anisotr6pico, en particular para un rnedio transversalrnente isotr6pico (mtl). este ultimo tipo de medics tienen aplicaci6n en el modelamiento de regiones que poseen fracturas y en la construecion de imagenes en zonas complejas. este caso se presenta potencialrnente en muchas areas de colombia. teoria de anisotropia se presenta una descripci6n de la anisotropia, con base en los articulos de thomsen (1986), y daley and hron (1977). un rnedio elastico anisotr6pico es un medio general donde la propagaci6n depende de la direcci6n. el efecto anisotr6pico se manifiesta en diferentes situaciones; par ejemplo, medias fracturados, minerales anisotr6picos localizados en forma aleataria en las rocas a una serie de capas delgadas (muy finas respecto a la longitud de onda utilizadas) isotr6picas. ernall: cpiedrah@ecopetrol.com,co, tsalinas@ecupetrol.com,co, sguevar@ecopetrol.com.co, grupo de geofisica, modelarniento del subsuelo, instituto colornbiano del pelroleo, ecopetrol, 70 si consideramos un medio elastico lineal general, la ley de hooke generalizada puede ser escrita como: (1) donde (j ij "" tensor de esfuerzo c ijkl "" tensor de constantes elasticas eij "" tensor de deformacion por las condiciones de simetria del esfuerzo y la deformacion, se puede demostrar que el tensor de constantes elasticas po see sirnetrias de forma que c ijkl = cklij = c ijlk' para todo i, j,k,l (2) esto genera un total de, a 10 sumo, 36 elasticas diferentes. par tanto, se puede representar el tensor de constantes ehisticas utilizando la liamada notaci6n de voigt, la cual consiste en representar pares de indices de la siguiente manera 1 i ~ 1,22 ~ 2,33 ~ 3,23 = 32 ~ 4, (3) 13=31~5,12=21~6. gedfislca colombiana, 6, diciembre de 2002 modelamiento sismico utlllzando teoria de rayos en medlos transversalmente isotropicos, teoria y ejemplos de esta forma podemos representar el tensor de constantes elasticas como una matriz cuadrada cmn' donde ai, n = 1,2, 3,4, 5, 6. de esta forma la ley de hooke puede ser escrita como si ademas suponemos que el solido en consideracion po see una funcion de energia, es decir, c33 (c33 2c44) (cj3 2c44) 0 0 0 (c33 2c44) c33 (cj3 2c44) 0 0 0 (c33 2c44) (c33 -2c44) c33 0 0 0 c =mn 0 0 0 c33 0 0 (6) 0 0 0 0 cd 0 0 0 0 0 0 c33 i i u = 2c ijkleijekl = 2cmnemen' (5) entonces la matriz de constantes elasticas sera simetrica y tendra, a 10 sumo, 6 + 36-6 = 21 constantes diferentes. este seria el tipo mas 2 general de solido. este tipo de modelos son demasiado complejos para ser utilizados en la practica, en el modelamiento y el procesamiento sismico solo se utilizan casos particulares y simplificados del casu generico. mediosisotropicos los model os elasticos mas simples son los ilamados isotropicos. para esta situacion el medio esta caracterizado par una matriz caracterizada completamente por dos constantes elasticas, una posible descripcion de un medio isotropico es donde cjj = a + ~ y c44 = ~ son las constantes independientes que aparecen en 1a matriz en funcion de los ilamados pararnetros de lame. como es bien conocido en la literatura sismologica, se puede probar que existen dos tipos de soluciones para estas ecuaciones: las ilamadas ondas pylas ilamadas ondas s. en este casu la velocidad de propagacion de los dos tipos de soluciones son iguales en todas las direcciones y son iguales a (7) respectivamente. a estas soluciones se puede ilegar utilizando la aproxirnacion de la optica geornetrica de orden cera para la solucion, 0 sea suponer que (8) geofisiga colombiana, 6, oigiembre de 2002 (4) donde cil (cii 2coo) clj 0 0 0 (cil 2c,,) cil cij 0 0 0 cn cij cd 0 0 0 c mn = 0 0 0 c44 0 0 (9) 0 0 0 0 c44 0 0 0 0 0 0 coo a,(x) '" funcion amplitud l(x) '" funcion de tiernpo de transite se debe recordar que estas soluciones son obtenidas suponiendo que se esta trabajando en las altas frecuencias, 0 sea que la longitud de onda es bastante rnenor que los detalles caracteristicos del modelo a considerar. medios transversalmente isotropicos (mti) un nivel de complejidad mayor en un medio se obtiene cuando consideramos la ilamada simetria hexagonal 0 medios transversalmente isotropicos (mti). en este casu tenemos un eje de sirnetria azimutal respecto a un eje escogido. esta generalizacion introduce tres (3) constantes adicionales. para este casu la matriz de coeficientes elasticos es igual a donde se considera el eje z el eje de sirnetria, comparando las ecuaciones 6 y 9, se observa que el casu isotropico es un casu degenerado del casu transversal mente isotropico si hacemos los siguientes reemplazos (10) utilizando esta matriz de coeficientes ilegamos a la ecuacion de onda para medios transversal mente isotropicos, la cual posee tres tipos de soluciones (daley and hron, 1977; geoltrain, 1988) ilamadas cuasi-longitudinales qp, cuasi-transversa qsh y cuasi-transversa qsv. seguirernos denominando estas soluciones sirnplemente como ondas p, sh y sv, respectivamente. para estos frentes de onda las velocidades de fase cumplen las siguientes formulas, analogas ala ecuacion 7: pv;,(e)=h cd +c44 +(cii-c33)sen2 e+ d(e)); pv~v(e)=~[ c33 +c44 +(cii-clj)sen2 e-d(e)); pv~fi (e) =c66 sen' e+c44 cos 2 e, (i i) donde p es la densidad, 71 piedrahita et al. 8 es el angulo entre el frente de onda y el eje vertical (eje de simetria), (12) d(8) = {(c33 c44)2 + 2 [(c13 + c44)2 (c33 c44) (cii + c33 2c44) sen' 8] + i + [(cii + c33 2c44)2 4 (c13 c44)2] sen" 8}"2 estas ecuaciones son bastante complejas algebraicamente, como puede observarse en particular en la ecuacion 12. a partir del trabajo de thomsen (1986) se define la llamada anisotropia debil como un caso especial de anisotropia, de modo que unos pararnetros dados poseen un valor muy pequefio en este tipo de situaciones. este autor muestra que este modelo se ajusta a muchos casos que se presentan en las rocas terrestres. con este fin thomsen define cinco constantes, tres de ellas adimensionales. cuando estos pararnetros adimensionales tienden acero, el modelo anisotropico tiende a comportarse como un modelo isotropico: cii -c'j . di . i r i .£=---'-, parametro a imenslona. epresenta e porcentaje 2c33 de diferencia entre la velocidad p horizontal y vertical. y c66 -c44, parametro adimensional. representa el porcentaje 2c44 de diferencia entre la velocidad de la onda s horizontal y vertical. (cl' +c44)' -(c, -c44y8' ", parametro adimensional. este pa2c33 (c33 +c44) rametro no tiene un sentido fisico obvio. ao =~c33/p, velocidad de propagacion de las ondas pverticales. po =~c44/p, velocidad de propagacion de las ondas p verticales. con estos nuevos parametros y haciendo aproximaciones de taylor de primer orden, las velocidades de fase se simplifican a las siguientes expresiones: ! vp(8)=ao (i+ ': 8cos' 8+ esen' 8), vsv(8)=~o[l+ ~i(e-8)sen28cos28j vsh(8)=~o (1 + ysen' 8) (13) trazamiento de rayos e.nmedios anisotropicos utilizando los pararnetros de thomsen como entrada a un modelo sismico, podemos trazar rayos en un modelo para medios transversalmente anisotropicos integrando las ecuaciones del trazado de rayos. en este tipo de modelos el trazado de rayos no esta determinado unicamente por los parametres de thomsen (en caso de anisotropia debil), sino tambien par la orientacion de los ejes de simetria de los materiales. estos a su vez estan determinados por la orientacion de trenes de fractura, por ejemplo. 72 en este trabajo se utiliza el metodo de construccion del frente de onda a traves del software norsar2d2 • ei metodo de construecion del frente de onda propaga el campo de onda en forma total, en vez de construir cada rayo en forma individual. ei frente de onda es representado como una malla triangular con un rayo localizado en cada nodo y se utiliza trazamiento dinarnico de rayos para propagar los rayos entre pasos de tiempo sucesivos. ei muestreo espacial del frente de onda se mantiene utilizando interpolacion de nuevos rayos entre los nodos a medida que la distancia entre los rayos y la diferencia angular entre sus tangentes excede un criterio (vinje et al., 1999 y halladay, 2001). ejemplos simples de modelamiento sismico a continuacion se presentan varios ejemplos basicos de modelamiento donde se compara la diferencia que produce la variacion de los parametres de thomsen en el trazado de rayos, con un modelo isotropico geometricamente equivalente. este modelo consta de dos reflectores, uno horizontal y otro buzante. el objetivo de este experimento es observar las diferencias entre ese mismo modelo cuando los materiales son isotropicos y anisotropicos, en particular, se analiza la anisotropia en dos casos: cuando el angulo de simetria de todos los bloques es 0 grados y cuando el angulo de simetria es +40 grados, respectivamente. ambos modelos anisotropicos son iguales en los parametres que los definen, excepto en los angulos de simetria de los materiales. en la figura 1se observa las velocidades de la onda p para todos los model os. en las figuras 2 y 3 aparecen las salidas graficas del norsar2d, que representan los parametros £ y 8, de los modelos anisotropicos. en las figuras 4, 5 y 6 se observan los disparos para los tres modelos: isotropicos, anisotropico con angulo de simetria de 0 grados y anisotropico con angulo de simetria de +40 grados, !'ilti gao"",,1ry proporty pid"" oflliunl< :2,0... 1:::1 figura 1. modelo de velocidades del modelo (ondas p). 2 norsar2d es una marca reuisfrada de la empresa norsar de naruega. correo eleciromco: www.norsar.cam. geofislca colomb lana. 6, oiciembre de 2002 mooelamiento sismico utlllzando teoria oe rayos en meoios transversalmente isotropicos, teoria y ejemplos figura 2. parametro epsilon del modelo. buncl'"/model model plots options o,l,sl:. .. nce <1< .... ) 6.0 8.0 10.0 th",,","cn p':"'",,'nct",.. oell'" """"'l""" 0.02~ h.v:~~'splinc .... 1 horizont3l -,i~ figura 3. pararnetro della del modelo. = ii figura 4. disparo en el modelo isotropico. samples options x (k:m) figura 5. disparo del modelo anisotropico, angulo de simetria igual a 0 grados. geofisica colombiana. 6, diciembre de 2002 respectivamente. en los modelos anisotropicos, el tiempo de transito es menor para un evento dado; adem as la variacion del angulo de simetria produce sismogramas diferentes, como se corrobora en la interseccion de la segunda hiperbola con la linea de tiempo de 4.0 s. es decir, se intersecan en puntos diferentes. 3amp1es optlons i'·x (ion). i 12.0413 i) tl~):~ figura 6. disparo del modelo anisotropico, angulo de simetria igual a +40 grados. distance {km) 0.0 2.5 5.0 7.5 10.0 12.5 15.0 0.0 1.0 3.0 ___ 5.0 7.0 p'-ve l oclt.g (km/s) 3.5 3.6 3.7 3.83.2 3.3 3.4 figura 7. modelo de velocidades (ondas pl. mejoramiento de la imagen sismica: aplicacion en la busqueda de hidrocarburos a continuacion se presenta un modelo de cierta complejidad geo16gica debido a una falla inversa de gran desplazamiento. esta situaci6n es atractiva desde el punto de vista de petr6leos porque es de gran prospectividad exploratoria. este rnodelo muestra unidades de shale con comportamiento anisotropico que por efecto de la falla rotan hasta un angulo de 30 grados sobre capas de menor velocidad. se observa que al considerar erroneamente que el medio anisotr6pico es isotropico, las interfaces tienen un corrimiento lateral y vertical con respecto a su verdadera posicion. este tipo de aproximaciones tiene un gran impacto en la cali dad de las imagenes que se obtienen. 73 piedrahita et al. en seguida se presentan dos modelos anisotropicos con parametros de thomsen iguales a e = 0 = o.l y s = 0 = 0.2, aniso 1 y anisoz, respectivamente. en la figura 8 aparecen los angulos de simetria de los bloques, en ambos modelos en la figura 8. posicion (km) 8 10 12 144 0.5 1.0 ~'.5 o ~20 '"i= 2.5 distance (kr'l) 0.0 2.5 5.0 7.5 10.0 12.5 15.'')"eg::ft::[?':' -::: 7.{1 3.0 3.5 4.0-'-------------------------.j s!:dmmetr8 azls angle (degrees).figura /0. modelo anisotropico, parametres iguales a epsilon = delta = 0.1. 10,0 15,0 20.0 25.0 figura 8. angulos de simetria del modelo para los materiales tl. posicion (km) 8 104 12 14 16 en la figura 9 se observa una seccion 20 isotropica. en las figuras loy ii se observan las secciones 20 de cad a modelo anisotropico, respectivamente. en la figura 12 se compara el modelo isotropico con elmodelo anisoz. se confirman menores tiempos de transite 0 velocidades aparentes mayores para el modelo anisoz. esto genera las diferencias entre los horizontes en ambos modelos, como se observa en la figura 12. para elmodelo aniso i las diferencias con el modelo isotropico son menores, 10 cual confirma el sentido fisico de los parametres de thomsen, que al ser mas pequefios produce, una similitud mayor con el modelo isotropico, 0.5 1.0 e15 o ~20 '"i= 2.5 30 3.5 4.0-"--------------------------.j figura /1. modelo anisotropico, pararnetros iguales a epsilon = delta ~ 0.2. posicion (km) 8 10 posicion (km) 8 10 4 12 14 16 4 12 14 16 4.01-'----------------------_----.j figura /2. superposicion de datos sinteticos del modelo isotropico y del modelo anisotropico aniso 2. se obsrva que estos ultirnos tienen un corrimiento horizontal y vertical.figura 9. secci6n zo de! modelo isotropico. como las presentadas en el piedemonte lianero colombiano, las cuales han marcado la pauta en la exploracion en los ultirnos 15 afios, seria de impacto realizar modelamiento y procesamiento teniendo en cuenta efectos anisotropicos en los materiales que componen los modelos sismicos, conclusiones a partir de los diferentes experimentos computacionales, se concluye que la que la no consideracion del fenomeno de anisotropia produce un efecto importante en la localizacion de las estructuras en las imagenes sismicas. para el caso de estructuras complejas 74 geofislca colom8iana, 6, oiciembre de 2002 modelamiento sismico utillzando teoria de rayos en medlos transversalmente isothopicos, teoria y ejemplos agradecimientos agradecemos el apoyo del lnstiruto colombiano del petr6leo para el apoyo en el estudio de los temas considerados, como tam bien a la empresa norsar par sll apoyo en las dudas encontradas en el rnanejo del software norsar2d. bibliografia alkhalifah, t. 1996. seismic processing in transversely isotropic media. cwp-236. center for wave phenomena, csm. daley, p.f .. and hron, f. 1977. "reflection and transmission coefficients for transversely isotropic media", bulletin of the seismological society a/america. 67(3): 661-675. dellinger, l.a. 1991. anisotropic seismic wave propagation. sep-69. stanford exploration project. geofislca colombiana. 6, diciembre de 2002 geol train, s. 1988. propagation of elastic waves in transversely isotropic media. cwp-074r. center for wave phenomena. csm. hallada y, i. anne-louise. 200 i. seismic modelling of lateral ramp structures. tesis de maestria. universidad de calgary. newrtck, r.t.; la wton, d.c., and spra rr, d.a. 2000. velocity issues in the colombian foothills. research report. vol. 6. fold-fault research project. universidad de calgary. alberta y queen's, calgary. thomsen, l. 1986. "weak elastic anisotropy. geophysics, 51: 1954-1966. vinje, v.; astebol, k.; iversen, e., and g.loystdal, h. 1999. "3-d ray modeling by wavefront construction in open models", geophysics, 64: 1912-1919. winterstetn, d.f. 1990. velocity anisotropy terminology for geophysicists. geophysics 55: 1070-1088. 75 nota editorial con un gran esfuerzo conjunto de investigadores nacionales se ofrece este nuevo numero de geofisica colombiana, con la esperanza de que satisfaga el interes de la comunidad geocientffica y con la firrne convicci6n que redundara en un desarrollo prolffico de trabajos en la geoffsica y en su aplicaci6n a la soluci6n de los problemas que la industria y el pais demandan. estamos alentando a la nueva generaci6n de cientificos de la tierra a mostrar sus trabajos y producciones, quienes de una manera tesonera y patri6tica ie siguen apostando a este pais que tanto necesita de sus nuevas generaciones y del compromiso de las mismas en su destino hist6rico. ese compromiso manifiesto de las nuevas generaciones se evidencia cuando en esta edici6n se plasman los trabajos de dos egresados del programa de magister en geoffsica de la universidad nacional de colombia. son ellos pedro vega y felix montano, quienes hacen su contribuci6n en instrumentaci6n sismol6gica en tiempo real y al analisis de parametres geoffsicos en el piedemonte, respectivamente. de la misma manera, el estudio de variaci6n solar acometido por rosa ladino, yaqueline figueredo y emna valdez expresa la panicipaci6n de nuevas geocientfficas. grato es tarnbien reconocer la intervenci6n en esta publicaci6n de tres nuevos docentes vinculados al posgrado en geofisica: en primer lugar el profesor luis castillo hace un aporte a la soluci6n del problema de migraci6n en medios con gradientes de velocidad; la profesora sandra sanchez presenta su estudio de sondeos electromagneticos y su aplicaci6n en la soluci6n del problema inverso; finalmente, los diversos tratados sobre transmisi6n y procesarniento de seiiales en sismologia puestos a disposici6n por el profesor carlos vargas, un grupo de colaboradores de distintas instituciones, carla perez, augusto capera, julio cuenca de la unam, j. florez y j. g6mez. otras entidades muestran sus nuevos resultados a traves de investigadores como elkin salcedo y maria bermudez en lngeorninas, hugo monsalve en la universidad del quindio, jaime le6n y sergio rinc6n en la universidad nacional, sede manizales. nuevamente registramos la continua participacion en la construcci6n de conocimiento de instituciones como ingeominas, universidad del quindio, universidad aut6noma de mexico, universidad del valle, universidad distrital «francisco jose de caldas» y universidad nacional de colombia a traves de sus distintos prograrnas y sedes. esta resefia es un reconocirniento al esfuerzo que tales entidades asumen diariamente y un aliciente al mismo, por 10 que el estado debe revertir en ellas recursos que garanticen la continuaci6n perrnanente de esa tarea. son estos resultados obtenidos por estos nuevos autores de nuestra revista 10 que nos hace mantener la fe y la confianza en un futuro prornetedor para las generaciones venideras, y 10que nos sostiene con el indeclinable animo de participar en esta tarea mancomunada de la producci6n y continuidad de esta, su revista geoftsica colombiana. finalmente, un agradecimiento especial a la universidad nacional de colombia por su apoyo con stante a esta publicaci6n. geofis. colomb. santafli de bogota d.c issn • 0121·2974 la temperatura del interior de la tierra luis a. briceno g. profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia jesus a. eslava r. profesor titular departamento de geociencias-facultad de ciencias-universidad nacional de colombia briceno l. a & j. eslava: la temperatura del interior de la tierra. geofis. colomb. 2:25-30, 1993. issn 0121-2974 resumen ei problema de definir la temperatura del interior de la tierra, se analiza a partir de las observaciones del f1ujotermico y la aceptaci6n de procesos de convecci6n en el manto. anallsis comparativos de perfiles termiccs, adlabaticos y de puntos de fusi6n, permiten obtener un perfil 6ptimo de distribuci6n de temperaturas, que adernas satisface la observaci6n sismica de un nucleo externo ifquido. abstract the problem of defining the earth interior temperature is analized from heat flow observations and the acceptance of convection processes in the mantle. comparative analyses of thermal, adiabatic, and melting point profiles, allow to obtain the optimum temperature profile, wich fits the seismic model of a liquid external core. 1. introduccion todos sabemos, de una u otra forma, que la dorada "es mas caliente" que bogota y que las temperaturas promedio son mayores en el ecuador que en los polos. sin embargo, la forma de variaci6n y el orden de magnitud de la temperatura en el interior de la tierra no son tan bien conocidos, a pesar de tener unas ideas generales. por ejemplo, las fuentes de aguas termales y las erupciones de lava nos muestran un f1ujo de calor desde el interior y hacia la superficie de la tierra. la fisica elemental nos ensefia que hay transferencia neta de calor desde los cuerpos con mayor temperatura hacia los cuerpos con temperaturas mas bajas. las preguntas de i,quien produce el calor?, i,donde? y i,c6mo?, tendran que ser respondidas usando toda c1ase de medidas y deducciones cientfficas. 2. generalidades ei calor se mide en muchas unidades, siendo las mas usadas la caloria (cal) y el joule (1 joul=4,186 cal), obviamente puede tarnblen expresarse en cualquier otra unidad de energfa. cuando se hace referencia al calor y/o la temperatura de la tierra, es mas util usar el concepto de "f1ujo de calor", que es la cantidad de calor por unidad de tiempo que atraviesa un area unitaria dada. en geotermologia se usan la 26 briceno & eslava: la temperatura del interior de la tierra unidad de f1ujode calor (hfu = 1jlcal/cm2/s) y el rnwatt/rn" (10·3joul/m2/s). algunas cifras pueden i1ustrarel regimen tsrrnico en la superficie de la tierra. ella recibe energia procedente del sol y del interior del planeta; cerca del 40% de la energia solar es reflejada nuevamente bacia el espacio, el 16% es retenido por la atm6sfera y el 44%, restante, es absorbido por las superficies iiquidas y s61idas de la tierra (caimi, 1979). la energia total que liega a la tierra proveniente del sol es del orden de 1,7x1017 watt (w), la cual, despuss de distribuirse y de irradiarse deja un promedio de f1ujo de calor en la superficie de 3,36x105 mw/m2 (8,000 hfu). por otra parte, el calor que el interior de la tierra transfiere hacia el exterior por conducci6n, tiene un promedio global de 61 mw/m2 , cerca de 3x10'2 w en todo el planeta, 10 cual equivale a 5.000 a 6.000 veces menos energia que la suministrada por el sol. las fuentes internas del calor de la tierra, las constituyen el material fundido del nucleo terrestre y el material radiactivo. una de las posibilidades de que la tierra tenga un interior fundido podria ser analizada a partir de la energia potencial de las particulas que formaron la tierra, mediante la alta presi6n por acumulaci6n de masa, aunada a la energia de decaimiento radiactivo. ei calor de la tierra se ha mantenido debido a la pobre conductividad termlca de la mayoria de las rocas de la corteza y a la alta opacidad a la radiaci6n de calor. de mayor interes para nosotros son las fuentes internas de calor, perc es imposible en una simple medici6n separar las contribuciones externas de las internas. afortunadamente, los efectos de calentamiento externos son peri6dicos y la mayoria de ellos tiene periodos que son relativamente cortos en relaci6n con el tiempo gastado por la conducci6n del calor a cualquier profundidad. la mayoria de los materiales de la corteza son pobres conductores del calor, de modo tal que las f1uctuacionesperi6dicas de la temperatura en las capas superficiales de la tierra se reducen en amplitud y se retardan en fase con la profundidad. la "profundidad de entrada" (profundidad a la cual, la amplilud de la onda se reduce en un 99%) de la onda diaria es de 0,5-1 m en las superficies s61idasde la tierra y hasta 30-50 m en las superficies liquidas, con variaciones muy grandes y f1uctuantessequn el tipo de superficie: ooga 300g en las s61idasy practicamente nula (oog a 0,5°g) en las iiquidas. en las capas externas s61idas la temperatura maxima se da aproximadamente una hora despuss del mediodia y una 0 2 horas antes que la temperatura maxima del aire; la temperatura minima se presenta a la salida del solo un poco antes. en las superficies liquidas es dificil definir temperatura maxima y temperatura minima, por ser tan pequeno el rango de variaci6n de la temperatura diaria. por su parte, en las capas profundas de los sectores s6lidos, las temperaturas maxima y minima presentan un desfase de 12 horas con respecto a sus similares de la zona superficial, en los sectores iiquidos este fen6meno es insignificante. la profundidad de entrada de la onda anual es de 7 a 10m en las superficies s6lidas y hasta 300-400 m en las liquidas y con apreciable movimiento, con variaciones muy grandes y f1uctuantesen superficies s6lidas (0-500c)y muy pequefias en las iiquidas (6-10°c). en las capas externas de las superficies s61idasla onda anual de la temperatura presenta los valores mas altos y mas bajos en los solsticios 0 entre 15 a 30 dias despuss: en las superficies iiquidas, esto se da 2 0 3 meses despuss del solsticio (casi en el equinoccio). en las capas profundas de los cuerpos s61idos las temperaturas maxima y minima presentan un desfase de casi 6 meses en relaci6n con los respectivos extremos de la temperatura superficial; al contrario, en los cuerpos iiquidos los extremos son sirnultanaos con los de las capas superficiales, porque el proceso turbulento origina esa ocurrencia slrnultanea. una oscilaci6n termica to.sen wt, impuesta en la superfcie plana (z=o)de un medio semiinfinito de difusividad (,,), penetra como una onda decreciente representada por: t(z)=to.exp(-{"[w/2"n .z).sen(wt-{"[w/2" n.z) [1] donde: t(z) = temperatura a la profundidad z, to = temperatura en z = 0, w = frecuencia angular de la oscilaci6n tj = difusividad termlca, v[2"w] = v = velocidad de penetraci6n, "[2"/w] = z· = longitud de atenuaci6n. para" = 1,2x10·sm2s·1(tfpica derocas igneas) y w = 2xlo"'s" (r = 10' arios), z = 350 m. de este modo, si se pueden hacer medidas del f1ujode calor a profundidades a las cuales las f1uctuaciones tsrmlcas superficiales sean amortiguadas y de magnitud despreciable, entonces sera posible obtener buenos valores del f1ujode calor hacia afuera, debido a fuentes internas de la tierra. sin entrar en detalles tecnicos. se puede afirmar que las lecturas del f1ujo de calor se hacen generalmente en minas profundas 0 pozos, alcanzando profundidades hasta de dos kil6metros. se observa que el f1ujo de calor, iiberado a traves de un area a, es proporcional al gradiente de temperatura: (1/a)(dq/dt) = q = kvt (= k.{dtldz}) [2] a = area, q = calor, q = f1ujocal6rico, k = conductividad termica 0 cal6rica, caracteristica de cada material, vt = gradiente vertical de temperatura. si la entrada de calor a un medio es constante a partir de un tiempo cero, su temperatura aumentara con el tiempo. si el material es relativamente buen conductor del calor (alto k). entonces el calor se transporta rapidamente y la geofisica colombiana n° 2, abril de 1993 27 3. regimen termico en el mantotemperatura aumenta gradualmente. por el· contrario, si el material es pobremente conductivo (bajo k), la temperatura aumentara mas rapidarnente. la mayoria de las rocas tienen conductividad tsrmlca del orden de 2,5 w/m.oc, mientras que para los sedimentos es 0,7 :5: k:5: 1,0 w/m.oc y es, ademas, directamente proporcional al porcentaje de agua. gran cantidad de medidas de gradientes termicos se realizaron en la parte continental de la corteza y se encontraron rangos de variaci6n entre 10°c a 50°c/km y f1ujos de calor promedio de 61,1 mw/m2• como una de las fuentes principales de energia tsrrnlca eran los elementos radiactivos, principal mente uranio (u), torio (th) y potasio (k), mas abundantes en la corteza continental granitica, se esperaba que los valores de f1ujo de calor en la corteza oceanica fueran mas bajos; un gramo de uranio produce alrededor de 0,71 callano y uno de torio 0,20 cal/ario (hurley, 1985). no obstante, excluyendo zonas an6malas (p.ej. de expansi6n), los promedios ocsanicos (61,5 mw/m2) eran practicamente iguales a los continentales. las explicaciones de este hecho no se pueden dar en terrninos de compensaci6n de materiales radiactivos entre corteza y manto superior, 10 cual implicaria que el manto oceanico fuera mas caliente que el continental y, por eso, menos denso, fen6meno este que no se observa en la forma del geoide. las explicaciones deberan contemplar, en cualquier caso, balances dlnarnicos, en concordancia con los hechos de la tect6nica de placas. basados en anal isis geoquimicos, existe la convicci6n de que por 10 menos dos terceras partes del f1ujo de calor en la corteza continental se originan en la corteza misma, con no mas de 15-20 mw/m2 viniendo del manto. por otra parte, casi todo el f1ujo de calor oceanica debera provenir del manto. siendo asi que las causas del f1ujo de calor son diferentes y que las areas continentales requieren de 10 a 20 veces mas tiempo que las oceanicas para alcanzar el equilibrio, podrla ser que la igualdad de valores es coincidencial. ei mayor espesor de la corteza continental puede ser una necesidad tsrmlca, para dar una transferencia de calor mas lenta y lograr un estado de equilibrio. es 16gico suponer, y asl 10 demuestra la sismica, que la temperatura del manto debe ser cercana a la de fusi6n parcial, man ten ida as! por convecci6n desde la regi6n inferior, mas liquida. un valor razonable en esta frontera es 1.300°c, marcando esta interfase el ~imite inferior de la litosfera, que es la unidad geol6gica que participa en la dinamica de las placas. en la base de la corteza (",35 km) la temperatura es 600°c, 10 cual implica un tj.t adicional de 700°c. si consideramos: q = 15 mw/m2 y k = 2,5 w/moc entonces, de la ecuaci6n [2]. t:"z '" 117 km. de donde la litosfera tend ria un espesor (termico) de 117 + 35 km. si la perdida de calor de la tierra fuera debida solamente a conducci6n 'i si la conductividad tsrrnlca k, 0 difusividad t) (t)= klp.c, expresa la capacidad de un cuerpo para perder su calor interne por conducci6n, p=densidad y c=calor especifico), de las partes profundas fueran similares a los valores de laboratorio para rocas igneas (k = 2,5 w/moc; t) = 1,2x1o,6 m2/s); entonces, el tiempo de enfriamiento ('16rmioo) de una capa de espesor d (distancia recorrida), seria: [3] y el interior de la tierra aun no sabria que el exterior es frio; al reemplazar en [3], , = 4,5x1 09 aries, d = 400 km. fen6meno importante a altas temperaturas es la transferencia de calor por radiaci6n. para un cuerpo gris en el cual la opacidad (e) es independiente de la longitud de onda (a) y la temperatura (t), la conductividad radiativa (k,) es tal que: [4] en donde, t) es el indice de refracci6n yo' es la constante de boltzmann (5,67x108 w/mok4 ). sin embargo, parece que e tarnblen crece, disminuyendo considerablemente kr (stacey, 1977). los is6topos de vida corta, ahora extinguidos, aumentaron considerablemente la temprana generaci6n de calor, de modo tal que, aun cuando la tierra hubiese comenzado completamente fria (dificilmente), el calor liberado habrla side suficiente para traer el manto cerca de la fusi6n en no mas de 108 aries, estableciendose luego un fen6meno de convecci6n y manteniendo un regimen de temperatura interna mas 0 menos estacionario, independiente del f1ujo de calor. la disminuci6n de calor disminuiria la rata de convecci6n, sin afectar demasiado el perfil de temperatura, el cual debsra satisfacer s610 dos condiciones: 1) no debsra ser menor que el perfil adiabatico: 2) los materiales, al menos en parte de la tierra, deben estar 10 suficientemente calientes y por eso suficientemente "blandos" para permitir el f1ujo convectivo. la situaci6n es auto-estabilizada porque la magnitud de la viscosidad del manto es una funci6n de la temperatura. la disminuci6n de la convecci6n hara que la tierra se caliente ligeramente, tornandose mas "bland a", y permitiendo as! que la convecci6n se acelere nuevamente. si el f1ujo de calor en cualquier momento disminuye a un punta en el cual no puede haber convecci6n, entonces habra enfriamiento por conducci6n, perc este proceso es tan lento que el perfil de temperatura 28 briceno & eslava: la temperatura del interior de la tierra psrmanecera en el punta de inestabilidad convectiva durante billones de mos. la necesidad de que el gradiente termico no sea menor que el adlabatico se puede ver considerando un medio hornoqeneo con un gradiente vertical de temperatura (t aumenta con la profundidad z) en el cual un elemento particular experimenta un pequerio desplazamiento vertical (hacia arriba, z disminuye), sin permilirsele ninguna transferencia de calor por contacto con su vecindad. debido a la descompresi6n adiabatica, su temperatura bajara, si el elemento se encuentra ahora a la misma temperatura que el nuevo medio, se dice que el gradiente de temperatura del medio es exactamente adiabatico, esto es, que corresponde al gradiente de presion para cambios adiabaticos, si el gradiente es manor, el elemento se sncontrara entonces mas frlo y dense que el medio circundante y tsndera a hundirse nuevamente; el medio es entonces estable, sin tendencia a la convecci6n. por otra parte, si el gradiente de temperatura es mayor (mas pronunciado), el elemento estara mas caliente, por 10 tanto menos dense que su vecindad y el empuje tendera a elevarlo mas. ei medio sera entonces inestable y podra entrar en convecci6n espontaneamente. entre mayor sea el gradiente de temperatura, mas fuerte sera la convecci6n, 10 cual tiene el efecto de transferir calor hacia arriba, reduciendo el gradiente de temperatura hacia el adiabatico (cesando la convecci6n). en un sistema flurdo grande con lenta convecci6n, en particular el nucleo exterior, parece que el gradiente de temperatura debera mantenerse muy cercano al valor adiabatico. pero en el caso del manto, se debera considerar tambien la segunda condici6n basica que debera satisfacer el perfil de temperatura: el material debe ser 10 suficientemente "blando" para permilir lentas deformaciones bajo tensiones de corte moderadas. de este modo, y para que haya convecci6n en el manto, debera tener este un perfil de temperatura cercano a su punta de fusi6n, el cual es mayor que el gradiente adiamtico. los perfiles adiabaticos se oblienen mediante ecuaciones termodinamicas, por ejemplo: (dt/dz)adiabalico= y.t.g/<!j [5] y = parametro de gruneisen = uk/pc. = uk ipc u = coeficiente de expansi6n volumetrica; ks~ k, = incomprensibilidades isotermica y adiabatica; c., cp = calores especfficos a volumen y presi6n constante; p = densidad. t = temperatura, 9 = aceleraci6n pravitacional, <!j = v 2 4/3(vs ) vp, \ts = velocidades de ondas dilatacionales y de cizalla. 4. regimen termico en el nucleo estudios del dinamo geomagnetico consideran que la convecci6n en el nucleo externo es quizas la mejor posibilidad de fuerza motriz. esto implica que por 10 rnenos allf el gradiente de temperatura debe exceder el valor adiabatico, teniendo este ultimo como una buena aproximaci6n y el cual se calcula mediante la ecuaci6n anterior: (dt/dz}z=2900 km = 0,71°/km debido a su conductividad electrica, el nucleo es mejor conductor tsrmlco que el manto. la conductividad termica es dominada por /a contribuci6n electr6nica (ke). k = ke = l.~t/pe)= 25 w/m.oc l = 2,45x10 w/"c2= constante de wiedemannfranz; t = temperatura; pe = resistividad electrica. obteniendose asr que: q = k.(dt/dz) = 18 mw/m2 do/dt = a.q '" 2,7x10'2 w a", 1,5x10'4 m2= area externa del nucleo este serra el calor transmitido desde el nucleo hacia el manto por conducci6n y debido a su gradiente de temperatura adiabatico. sin embargo, este calor adiabatico no puede hacer trabajo externo util y la fuente que alimenta el dinamo estara constituida por una componente adicional de f1ujode calor por convecci6n, que puede liegar a ser el doble de la anterior: '" 5x10'2 w. ouizas la unica fuente plausible de tanto calor podrra ser el potasio radiactivo, que estarfa distribufdo en todo el nucleo 0 en el nucleo externo unicamente, con mayor opci6n de la ultima posibilidad. calculos de la cantidad de calor en el nucleo, necesaria para la convecci6n permiten estab/ecer que el gradiente de temperatura en el nucleo externo debe ser adiabalico, excepto en un pequeno range interior, donde es menor el gradiente, prevaleciendo la difusividad. sin embargo, si el nucleo interior contiene potasio, /a pequena zona difusiva desaparecera. la solidificaci6n del nuc/eo interior y la suposici6n de que, al igual que el externo, esta compuesto en gran parte de hierro, permiten hacer cojeturas acerca de la temperatura de la frontera del nucleo interne y luego extrapolarlo. serra calcular la temperatura de solidificaci6n del hierro 0 mezclas de hierro a las condiciones de presi6n apmpiadas. sin embargo el fen6meno de fusi6n no esta completamente entendido y su extrapolaci6n a curvas de presi6n y puntos de fusi6n en el nucleo han conducido a calculos bien diferentes (higgins & kennedy, 1971; boschi, 1974; jacobs, 1973). se cree actualmente que el azufre es un compo.nentede! nucleo y esto reduce el punta de fusl6n del hierro. los valore1 obtenidos de esta manera dan 3.090ok en la interfase nucleomanto y 4.2500k en el nucleo interno. geofisica colombiana n° 2, abril de 1993 5. diferenciacion del nucleo 29 gualquier teo ria de la historia tsrrnica de la tierra debe explicar la presencia del nucleo interno s61ido y el externo liquido. si alguna vez estuvo completamente fundida, (,c6mo se logr6 el presente estado. jacobs (1973) presenta la siguiente explicaci6n suponiendo un nucleo ferroso y un manto rico en silicatos. en el limite manto-nucleo existira una discontinuidad en la curva de punta de fusi6n vs. profundidad, a pesar de que la temperatura es continua. las representaciones de la discontinuidad pod ran ser: gaso i): punto de fusi6n en el nucleo siempre mayor que en el manto (fig.1). es imposible, pues la temperatura real dsbera estar por debajo del punta de fusi6n en el manto y por encima en el nucleo y mantenerse continua en la frontera. gaso t): guando la tierra se enfriaba a partir de un es ado de fusi6n, el gradiente de temperatura pudo haber sido esencialmente adiabatico, obteniendose fuertes corrientes de convecci6n y un enfriamiento rapido de la superficie. la solidificaci6n comenzaria a aquella profundidad a la cual la curva que representa la temperatura adlabatica interceptara la curva de punta de fusi6n (fig.2). nuctec r-r-eprofundidod i l figura 1. punto de fusi6n·profundidad. caso i). la solidificaci6n habria comenzado entonces en el centro de la tierra y no en la .frontera mantonucleo. ei nucleo s6lido creci6 hasta que la adlabatica intercept6 la curva de punta de fusi6n en dos partes. en el enfriamiento posterior, el manto se solidifica de abajo hacia arriba. ei ifquido entre a y b qued6 atrapado. ei manto se enfriarfa a una rata relativamente rapida, dejando la capa liquida practicarnente a su temperatura original, aislado por encima por el rapldo engrosamiento de una capa de silicatos y por debajo por el nucieo interno, ya solidificado. monto nllcl.o profundldod, ~ figura 2. punto de fusi6n·profundidad. caso ii). gaso iii: siguiendo un anal isis similar, se o en na que el nucleo deberia ser liquido en su totalidad (fig.3). si la tierra hubiese tenido un origen frio y nunca hubiera estado completamente fundida, los casos ii) y iii) serian posibles, pero habria otros problemas aun mayores. la fig.4 sintetiza el regimen tsrrntco, en la interfase manto-nucleo. ,; ,; monto hllcl,o / adiabo/icol-.~ profundidod. i' figura 3. punto de fusi6n.profundidad. caso iii). i m~cl.od.l;""do i ""clio lohdo...~,. _.-._.-. / /-:---'-'-'-. _.-,.;~_._.»>: _.-;::::"1 ,// ..... -;.-"// : -: /' 1// _._.-'-'-'-'_._ ptrl,'ldeadiobol'ccli __ p,rfildtp.f p.rf'l~leml>t'ftlt"ro pro/"ndldod,z figura 4. regimen termlco en manto-nucleo sobre las consideraciones tsrmtcas de corteza, tanto oceanica como continental, manto y nucleo, se puede hacer una sintesis grafica del perfil de temperatura en el interior de la tierra, como se muestra en la fig.5 30 briceno & eslava: la temperatura del interior de la tierra 5000 3. -perfildetemperoturo -perfil de p.f 4000 o... z o....a.. e•i2000 1.000 , 1000 2000 3000 4000 5000 profundidod en k (m ) figura 5. temperatura·profundidad (modificada de stacey, 1977) 1000 bibliografia boschi, e. (1974): melting of iron. geophys. j., 38,3276. caimi, e.a. (1979): la energfa radiante en la atm6sfera. 62pp. eudeba, buenos aires. higgins, s.h. & s. kennedy. (1971): the adiabatic gradient and the melting point gradient in the core of the earth. j. geophysics rev. 76, 1870. hurley, p. (1985): lque edad tiene la tierra? trad. j.m.calvelo. eudeba, buenos aires. jacobs, j.a. (1973): the earth's core and geomagnetism, 1st ed. 168pp. pergamon press. stacey, f.d. (1977): physics of the earth. 414pp. john wiley, new york. microsoft word articulo alexander caneva corregido331marzo.doc earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004) : 3 9 manuscript received may 2004. paper accepted october 2004. 3 using the fractal dimension of earthquake distributions and the slope of the recurrence curve to forecast earthquakes in colombia alexander caneva 1 and vladimir smirnov 2 e-mail address: alexander.caneva@uan.edu.co 1 research center, university antonio nariño, bogotá, colombia – geophysics – 2 earth physics division, faculty of physics, lomonosov moscow state university, moscow, russia abstract an estimation and analysis of temporal and spatial variations (on the surface, and depending on depth) of the parameters of the seismic regime (fractal dimension of the earthquakes distribution d and slope of the seismic recurrence curve b) in colombia are carried out, considering the fractal properties of seismicity. the variations of the difference (b – d/a), in time are analyzed (where a is the exponent of the power law e ~ la which establishes the relation between the energy e of the earthquake and the size l of its focus). this difference describes the deviation of the geophysical medium from a “stable” state for intervals of time in which strong earthquakes occur. the possibilities to use the variations of the parameters of seismic regime to forecast earthquakes are discussed. key words: fractal dimension, earthquake prediction, seismic recurrence, seismic regime. resumen se lleva a cabo la estimación y el análisis de las variaciones en el tiempo y en el espacio (en superficie y en función de la profundidad) de los parámetros del régimen sísmico (dimensión fractal de la distribución de sismos d y la pendiente del gráfico de recurrencia sísmica b) de colombia considerando las propiedades fractales de la sismicidad. se analizan las variaciones de la diferencia (b – d/a) en tiempo (donde a es el exponente de la expresión e ~ la , que vincula la energía e del sismo con el tamaño l de su foco). esta diferencia describe una desviación que experimenta el medio geofísico con respecto a un estado estable en los intervalos de tiempo en que ocurren terremotos fuertes. se analizan las posibilidades de utilizar las variaciones de los parámetros de régimen sísmico para pronosticar sismos fuertes. palabras clave: dimensión fractal, predicción de terremotos, recurrencia sísmica, régimen sísmico.  2004 esrj unibiblos. alexander caneva and vladimir smirnov 4 introduction the term seismic regime is understood as an assemblage of earthquakes distributed in space and time. the quantitative characteristics of the regime are the statistical parameters of this assemblage (smirnov, 1995). it is known that earthquakes are distributed according to their energies, in space, and time in an irregular way (pisarenko, 1989). the distribution of earthquakes according to their energies is given by the gutenberg-richter law, which represents the distribution of the number n of earthquakes with respect to their energies e as a power law: n ~ eb (aki & richards, 1980). according to this law the number of small events in a given region of space is greater than the number of strong events. on the other hand, the distribution of defects n, according to their sizes l in a certain region of space also follows a power law: n ~ l-d (aki, 1981; fukao and furumoto, 1985). the heterogeneity of the assemblage of earthquakes corresponds to their self-similar, hierarchic (fractal) structure (keilis-borok et al. 1989). the fractal character of the spatial and temporal heterogeneity of seismicity corresponds to some general properties of the seismic regime (rykunov et al. 1987), and these fractal properties together with the law of recurrence are the basis for understanding the self-similarity of the seismic process (sadovsky & pisarenko, 1991). considering that a fractal object is heterogeneous on all scales (crownover, 1995), it is indispensable to reformulate the seismic recurrence law paying attention to the fractal properties of earthquakes assemblage. in the same way, we have to connect the spatial, temporal and energetic statistical properties of the seismicity one to each other. these properties are the expression of a physical process: the evolution of the lithosphere (smirnov, 1995). the quantitative estimation of the parameters defining the seismic regime allows clarifying their features. let us analyze these aspects in more detail. the seismic regime parameters slope of the curve of recurrence b the characteristic of self-similarity of seismic process in the energetic aspect is the slope of graphic of recurrence. this graphic shows the relation, existing between the numbers n of earthquakes in a region having different values of energy e: ( )1 2 1 2 1 +−       = b e e n n (1) where ni is the number of events with energy between (ei, ei+de), b is the slope of the graphic of recurrence. from equation (1): ( ) akkbn loglog 0 +−−= , (2) where ek log= is called the earthquakes energetic class (pisarenko, 1989), and n is the number of events in the range k+dk. in the classic formulation (aki & richards, 1980) the law of seismic recurrence expresses the relation between the number of earthquakes n and the energy of those earthquakes e. this law characterizes the probability of occurrence of earthquakes with certain energy. in this case it is assumed that the earthquakes are distributed uniformly in space and time, so the classic law of seismic recurrence reflects just the energetic properties of seismicity without considering the earthquakes distribution in space and time. fractal dimension d of the earthquakes spatial distribution the fractal dimension d can be understood as a measure of geometric self-similarity. it determinates the relationship between the numbers of non-empty cells ni (cells which contain at least one element of the assemblage) of different size li inside the region of space (fukao & furumoto, 1985): d l l n n −       = 2 1 2 1 . (3) the number of non-empty cells of size l satisfies: ( ) dlln −= (4) whereas the density of events in a euclidean space of dimension r is defined as: rl n=µ , (5) being n the average of events in cells of size l. if there are altogether m events distributed among n cells in a certain region of space then: n m n = (6) and from equation (4) rdl −≈µ . when the events are distributed uniformly rd = and µ = constant. if the assemblage of events has a fractal structure then d < r and al −≈µ , where 0>−=α dr , indicating that with the reduction of the cell’s size the density of events increases. so, the density estimation over events according to equation (5) loses its sense for a using the fractal dimension of earthquake distribution and the slope of the recurrence curve to forecast earthquakes in colombia fractal assemblage since its value depends on the cell size l. generalized law of seismic recurrence a previous work (smirnov, 1995) tried to reconcile the classical definition about seismic regime parameters (in terms of b and d – values) with the fractal features of seismicity. in an attempt to formalize certain statistical characteristics of seismic recurrence, (smirnov, 1995) smirnov introduced the so-called generalized law of seismic recurrence. the analytical expressions for the seismic regime parameters were derived from the following aspects: after an earthquake had occurred in a certain region of space, there is an interval time and a certain rank of energy values such that no further earthquake with the same energy can occurs during this period. assume that the size of the region is given by: αλ= 0lr , (7) where l0 is a linear size of the focus of the earthquake (characteristic size of the destroyed region of the physical medium defining the scale of the event) (rice, 1982); λ and α are constants. physically, r is the size of the region that had distended as a result of the earthquake. assume that the time of prohibition of new earthquakes in region r is: βθ=τ 0l , (8) where θ and β are constants. the seismogenic region (the assemblage of seismic focuses, for epicenters as for hypocenters) has a fractal spatial structure (aki, 1981). if we divide a region of size l into smaller regions (for example spheres) of ∆ size, the number of such sub-regions that contain earthquakes becomes: ( ) d l nn       ∆ =∆= , (9) where the constant d is the fractal dimension of the seismogenic region (crownover, 1995). note that the sismogenic region can be related to a system of faults, which also has fractal properties (ulomov, 1993). the energy of the elastic earthquake waves depends on the size of the focus according to a power law: ale 0ε= . (10) this formula is based on an empirical relation between the energy of the earthquake e and the linear size of focus l0 (kasahara, 1981). from these assumption it follows: in a region of space of size l, during a time τ, a number n of earthquakes can occur, each one with focus of size l0, such that the regions where these stresses are released as a result of earthquakes, do not intersect each other. the number of these non-intersecting regions, or the number of distinct earthquakes n, is defined by the fractal geometry of the seismicity of this region. from the anterior postulates we may formulate a new law of seismic recurrence, which takes into consideration the fractal properties of seismicity. covering the studied region with spheres of size ( )0lr=∆ (that do not intersect each other), and taking the equation (9) ( )dr ln = . if during the time interval τ, on the average, n earthquakes with linear dimensions l0 take place, then, during a time interval t, on the average, t/τ times more events will occur. therefore in the region of size l, during a time interval t, ( )drl tn τ = earthquakes will be observed with linear l0 focal size. from equation (7) and equation (8), and considering that ( )ael 1 0 ε= , the earthquakes number becomes ( ) ( ) a d a dd e tl n β+α−β+α ε      θ       λ = . from this expression we obtain the generalized law of seismic recurrence that considers the fractal properties of the seismicity: btldbkn +++−= logloglog (11) where ek log= , ( ) a db β+α= and θ−λ−ε= logloglog dbb from equation (11), analytical expressions are obtained for the parameters of the seismic regime: tl e n b , lg lg       ∂ ∂−= (12) and te l n d , lg lg       ∂ ∂= (13) where the subscripts l (size of the region), t (the interval of time) and e (the energy of the earthquakes) indicate the constancy of parameters. a similar to the equation (11) expression was analyzed and proposed (chelidze, 1993) based on results presented in papers (mazhkenov, 1989; smalley et al. 1987). the fractal seismicity properties, besides the power law of seismic recurrence, assure the self-similarity of seismic processes. from the generalized law of seismic recurrence: alexander caneva and vladimir smirnov 6 n ∼ db le ⋅− ∼ dab ll 00 ⋅− ∼ ( )abdl − 0 . (14) the worldwide average values of the parameters of the generalized law of seismic recurrence are, respectively: 5.0≈b and 5.1≈d (aki, 1981; hirata, 1989), whereas the value commonly accepted of a is near 3 (kasahara, 1981), therefore the value of the exponent in formula (14) is 0≈− abd , then we have: abd ≈ . (15) this empirical expression (first proposed by keilis-borok et al. 1989) should not be understood like a correlation between the b and d values. this expression describes the relation between the average values (taking over sufficiently long intervals of time) of these parameters. several references where the seismic activity after a main shock is analyzed (e.g. hirata, 1989) point out immediately after the main shock the deviation with respect to the theoretical relation d ≈ ab reaches the maximum value. this means, d ≠ ab when the geophysical medium is far from its stationary (background, common, normal) level of seismicity. with the attenuation of aftershocks, the difference ( )abd − returns again to its average value near zero. there is also published information about variations of the same type in case of induced seismicity in dams (smirnov et al. 1994). in this case, the distance of the magnitude |d ab| is maximal during the period of dam filling. it seems that the relation d ≈ ab depends on a certain “stable” state of the geophysical medium. as a result of a strong earthquake on a smaller scale in comparison with the characteristic scale of the global rupture, the “stable” dynamic state loses its original balance, which leads to a redistribution of tensions in that region forming, after a certain time, a new “stable” scheme. this way, it seems that the deviation from the relation d ≈ ab is a measure of the (seismic) stability of the geophysical medium. estimating the seismic regime parameters estimating the b – value the maximum likelihood estimation (hudson, 1964) of the parameters in the law of seismic recurrence was described in (sadovsky & pisarenko, 1991) expressing this law mathematically as follows: akbn jj loglog +∆−= , where j = r, r+1,...,r+n, nj, is the average of the number of earthquakes in the interval ( ) ( )       +∆+−∆+ 2 1 02 1 0 10,10 jkkjkk , normalized at the interval t in study; ( )ek log= , which corresponds to the parameter of seismic activity a; ∆k is the variation of the energetic class for the construction of the histogram. the maximum likelihood estimates of b and a is found from the system of equations: ( ) ( )btamk btam ϕ′+∆ =ϕ− 1 0 10lg 0 (16) where: ( ) ( ) ∑ ∑ ∑ ∑ + = ∆− = ∆− = + = ∆−ϕ′ =ϕ = = nr rj kb n rj kb n rj j nr rj j jkb b jmm mm j 1010log 10 1 0 (17) and {mj} is the histogram of distribution of earthquakes with respect to energy (number of earthquakes in the j cell). this way, the empirical data enter into the equations (12) in form of the parameters: m0 and m1. estimating the d – value to estimate the fractal dimension of an assemblage of earthquakes we used the correlation dimension: ( ) l lc d l log log lim 0→ −= , with l the linear size of the cells into which the euclidian space is divided. c(l) is the integral of correlation: ( ) ( )lrrnlc ji ≤−= , where n is the number of pairs of events separated by a distance not greater than l (feder, 1988). for empirical data, we cannot take the limit for l → 0. to solve this problem we use a scaling approach, which consists of selecting a rank of l values where the relation between ( )clog and ( )llog is close to linear. the region of scaling is limited below by the accuracy of the data, and above by the size of the studied region or by real variations of the assemblage structure (nonlinearities of the curve ( ) ( )( )lfc loglog = . such nonlinearities indicate nontrivial changes with size of the fractal dimension. estimating fractal dimension of the earthquakes assemblage hypocenters was carried out by constructing the correlation integral histogram; then using the fractal dimension of earthquake distribution and the slope of the recurrence curve to forecast earthquakes in colombia based on itthe function: ( ) ( )( )lfc loglog = was constructed; the region of scaling was identified; and the value of d was calculated by regression. table 1. colombian seismic regime parameters values region b σb d σd colombia 0.59 0.02 1.55 0.03 colombia (shallow) 0.56 0.02 1.53 0.03 colombia (deep) 0.57 0.03 1.61 0.03 results the catalogue of earthquakes in colombia was provided by the: institute of geoscientific, miningenvironmental and nuclear investigation and information of colombia, ingeominas (responsible for the national seismological network of colombia) (ingeominas, 1999). the catalogue contained 17221 records since 1993 to 1999. after eliminating double (repeated) records, records with erroneous format, and aftershocks the number of records diminished to 10898. the estimation of the parameters of seismic regime was carried out from this “clean” catalogue. estimating the b and d– values for colombia the estimation of b and d values was carried out using specially designed software. table 1 shows the results for the entire colombian territory, including a differentiation for shallow and deep seismicity. figure 1. variations of b-value � , d-value ο and in dashed line (b-d/a) for earthquakes as function of time for the entire territory of colombia. the symbol indicates strongest earthquakes. when these curves are analyzed it is difficult to identify some regularity except the counter-phase variation of parameters in the intervals 1996-1997 and 1998-1999. the b-value is related to the existing proportion between the number of weak and strong events an increase in b–value corresponds to a rise in the number of weak events and a decrease in the number of strong events, while a reduction of b–value indicates a rise in the number of strong events and a decrease in weak events. the d–value characterizes the level of spatial events homogeneity. the less the value of the fractal dimension, the more located are the events; that is a decreasing of d corresponds to a grouping of the events. figure 2. figure 1. variations of b-value � , d-value ο and in dashed line (b-d/a) for earthquakes as function of time for the entire territory of colombia. the symbol indicates strongest earthquakes. figure 1 depicts the variations of the curve b d/a, which is equivalent to the expression (ba d) that corresponds to a stable state of the medium. in intervals of time in which strong earthquakes appear, no peculiar |b d/a| variations are observed, possibly due to the heterogeneity of events when considered over the whole colombian territory (as the type of seismicity, whether of subduction, volcanic, types are different for distinct regions in the country). to reduce the degree of heterogeneity in depth we separately considered the events with h < 100 km (figure 2). in this case the counter-phase variation of the b and d – values is observed for almost all the time intervals. but this behavior, as well as a greater deviation of |b d/a| from zero, can be observed starting at 1996. after reaching the maximum at the end of 1997, the deviation tends again to zero. exactly in this interval of time there occurred the strongest events with mb = 5.8 and h = 0 km in the region of urabá in northwest of colombia (this region is characterized by shallow seismicity). alexander caneva and vladimir smirnov 8 for deep seismicity no particular behavior was observed. although in this case the coincidence of the presence of a strong earthquake and the deviation of the difference |b d/a| from zero is evident. this unique example is not enough for considering this deviation as a prognostic element. it is important to note that there are several strong earthquakes at the starting time of the study and one more at the end (figure 2). however, as for statistical analysis it is not appropriate to evaluate the initial and final parts of a catalogue, we ignored those events and considered as relevant only one event, which took place in the middle of the analyzed period. this way, the relevance for earthquake prognosis of the value of the |b d/a| difference could not be checked in the case of colombia. in a paper (smirnov, 1995), the deviation from zero of the |b d/a| difference is described, in relation to catastrophic earthquakes with ms ~ 7 – 8, whereas the events considered as strong in this work are only ms ~ 5 6. figure 3 shows the variations of the b–value in space (on the surface). in the region near 75º western longitude and 5º north latitude, an “island” of diminished values of b can be observed. figure 3. distribution of b-value on the surface for the entire territory of colombia this is a region of low seismicity located in the valley of magdalena river. when studying the b-value behavior for deep and for shallow seismicity, respectively, we must take into account that most earthquakes with h > 100 km take place in the seismically active zone called “nest of bucaramanga” (near 73º western longitude and 5º north latitude) while the seismicity in the rest of the country is mostly shallow. this way the distribution of the values of b on the surface for shallow seismicity does not change in relation to figure 3. figure 4 shows the variations of the average values of b and d as a function of the depth for the entire colombian territory. it is interesting to note the increase of b and the decrease of d starting at around of 100 km depths, related to the seismicity in the “nest of bucaramanga”. the decrease of d– value indicates an increase of the spatial earthquakes concentration at this depth, matching with the characteristics of a nest (region) of deep seismic activity. these counter-phase variations with the depth, starting at 100 km, are possibly related to specific conditions of pressure and temperature at those depths (particularly in the “nest of bucaramanga” that, to a great extent, defines the deep seismicity in colombia (caneva, 2000)). figure 4. variations of b-value ο and d-values � in function of depth for the entire territory of colombia. using the fractal dimension of earthquake distribution and the slope of the recurrence curve to forecast earthquakes in colombia conclusions based on the catalogue of earthquakes of colombia we estimated the parameters of its seismic regime. the average values of the recurrence slope graph b were obtained in general for colombia (b ≈ 0.6) and separately for the shallow and for deep seismicity. the analysis of the b – values variations in time and in space (both on surface and with depth) has also been accomplished. the average fractal dimension value d of epicenters for colombian earthquakes was found (d ≈ 1.6). counter-phase variations of the b and d – values in time and with depth were observed. the variations of the (b – d/a) difference were analyzed. this difference corresponds to a deviation from a stable state of the geophysical medium, in such intervals of time in which strong earthquakes occur. the absence of significant correlation does not allow, yet, using the variations of the (b – d/a) difference to forecast earthquakes in colombia. references aki, k. (1981). a probabilistic synthesis of precursory phenomena. earthquake prediction. amer. geophys union. washington. p. 556 -574. aki, k. and richards, p. (1980). quantitative seismology. theory and methods. w. h. freeman. caneva, a. (2000) fractal properties of seismicity of colombia. ph.d. thesis. msu, moscow. in russian. chelidze, t. (1993). fractal damage mechanics of geomaterials. terra nova 5, pp. 421 – 437. crownower r. (1995). introduction to fractals and chaos. jones and barlett publishers.. feder, j. (1989). fractals. plenum press, new york. fukao,y., furumoto, m. (1985). hierarchy in earthquake size distribution. phys. earth planet. inter. v. 37, pp.149 – 168. hirata,t. (1989). a correlation between the b-value and fractal dimension of earthquakes. j. of geophys. res. 94. b6. pp. 7507 7514. hudson, d. j. (1964). statistics. lectures on elementary statistics and probability. geneva. 2. ingeominas. red sismológica nacional de colombia. ingeominas.http://www.ingeomin.gov.co. bogota, colombia. kasahara, k. (1981). earthquake mechanics. cambridge university press. 264 p. keilis-borok, v. i., kosobokov v. g. & mazhkenov v. g. (1989). similarity in spatial distribution of seismicity. vychislitelnaya seismologia. v. 22. pp. 28 – 40. in russian. mazhkenov, s. a. (1989). diagnostics of times of increased probability of earthquakes in northern tjan-shan. ph.d. thesis. moscow. in russian. pisarenko, v. f. (1989). the law of recurrence of earthquakes. .diskretnye svoistva geofizicheskoi sredy (pod red. sadovskogo m. a.). м.: nauka. in russian. rice, j. (1982). mechanics of earthquake focus. m. mir. rykunov, l. n., smirnov v. b., starovoyt y. o. & chubarova, o. s. (1987). self-similarity of seismic emition on time. dokl. an sssr. v. 297. no 6. pp. 1337 – 1341. in russian. sadovskii, m. a. & pisarenko v. f. (1991). some peculiarities of seismic process in block media. м.: nauka. 96 p. in russian. smalley, r. f., chatelain h. -l., turcotte d. l. & prevot r. (1987) a fractal approach to the clustering of earthquakes. bull. seismic. soc. am. 77. pp. 1368 – 1382. smirnov, b. v. (1995). recurrence of earthquakes and parameters of seismic regime. vulkanologia and seismologia. no 3. pp. 59 – 70. in russian. smirnov, v. b., cherepantsev a. s. & mirzoev v. k. (1994). fractal properties of induced seismicity in the region of the dam nurekskoe. induced seismicity. м.: nauka. p. 138 – 147. in russian ulomov, v. i. (1993). global order of the seismicgeo-dynamic structures and some peculiarities of the seismic zoning and forecasting of earthquakes. seismicity and seismic zoning of northern euro-asia. pod red. ulomova v. i. м.: ifz ran.pp. 24 – 44. in russian. geociencias dic 2010-color.vp using the micro – resistivity method to detect hispanic ancient floors at nombre de dios, panamá alexis mojica1, louis pastor2, richard vanhoeserlande3, maría salamanca heyman4 1 laboratorio de ingeniería aplicada, centro experimental de ingeniería, universidad tecnológica de panamá e-mail: alexis.mojica@utp.ac.pa. 2 laboratoire sisyphe unité mixte de recherche 7619, université pierre et marie curie, france, coopération géophysique pour l’amérique central. e-mail louis.pastor@upmc.fr 3 service proffesionnalisation, ufr de physique, université paris diderot paris 7, france, coopération géophysique pour l’amérique centrale. e-mail: vanhoe@paris7.jussieu.fr 4 department of anthropology, the college of william and mary, usa. e-mail: mafesalam@hotmail.com abstract this article describes the application of the method of electric prospection to the detection of floors of hispanic type associated with the site of nombre de dios. this site represents an important and ancient spanish settlement founded during the time of the european conquest in the american continent around the year 1510. the electric prospection developed in the site was conformed by a mapping of apparent resistivity of a small area (10 x 10) m obtained through an electrode arrangement pole pole type and developed during the dry time. the results obtained in the interpolation process for the obtaining of the apparent resistivity map of the subsoil showed a group of electric anomalies with high values, which went associated to a ground system of pebbles built during the period. the areas with low values of apparent resistivity were associated to the sedimentary material that characterizes the region. later on, the excavation works carried out on one of the electric strong anomalies agreed with the results of this geophysical prospection; a probe of 0.5 x 0.5 x 0.2 m revealed the superior part of a floor of pebbles; and this colonial structure rests under a wet layer of superficial sedimentary material. below the hispanic floors, the moisture turned out to be bigger. later to the excavations, and with the objective of generating a synthetic map of electrical resistivity with the same characteristics observed in the map of values of measured resistivity, it was intended a 3d geoelectric model incorporating the main characteristics of the floor of pebbles detected during the excavation, as well as the surrounding materials. resumen este artículo describe la aplicación del método de prospección eléctrica para la detección de un piso de tipo hispánico asociado con el lugar denominado “nombre de dios”. este sitio representa una importante y antigua colonia española fundada en la época de la conquista europea en el continente americano alrededor del año 1510. la prospección eléctrica desplegada en el sitio estuvo conformada por un mapeo de resistividad aparente en un área pequeña (10 x 10) m obtenida a través de un arreglo de electrodos polo polo y se desarrolló durante la época de verano. los resultados obtenidos en el proceso de interpolación para la obtención del mapa de resistividad del subsuelo mostraron un grupo de anomalías geoeléctricas con valores altos, las cuales fueron asociadas con pisos de guijarros construidos durante este período. las áreas con bajos valores de resistividad aparente se asociaron a los materiales sedimentarios que caracterizan la región. más tarde, los trabajos de excavación llevados a cabo en una de las zonas con anomalías eléctricas fuertes concordaron con los resultados de esta prospección geofísica, un sondeo de 0,5 x 0,5 x 0,2 m reveló la parte superior de un nivel de guijarros, y esta estructura colonial descasaba sobre una capa húmeda de material sedimentario superficial. por debajo de los pisos hispánicos, la humedad resultó ser más elevada. posterior a las excavaciones, y con el objetivo de generar un mapa sintético de resistividad eléctrica con las mismas características observadas en el mapa de valores de resistividad medida, se desarrolló un modelo geoeléctrico 3d incorporando las principales características de piso de piedra y de los materiales del entorno detectados durante el excavación. research groupin geophysics universidad nacional de colombia earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 127-134 keywords: electric prospection, apparent resistivity, apparent resistivity mapping, floor of pebbles, nombre de dios, 3d modeling. palabras clave: prospección eléctrica, resistividad aparente, mapeo aparente resistividad, guijarros,nombre de dios, modelo 3d. record manuscript received: 15/05/2010 accepted for publication: 03/11/2010 introduction the city of nombre de dios was one of the first european settlements founded in the atlantic coast of the isthmus of panama and in american mainland and for this reason, it is considered as one of the oldest hispanic sites. this city was founded in the year of 1510 by diego de nicuesa and its importance resided on its belonging to the port of the atlantic that connected spain with many colonies of south america. after this port was abandoned in 1597 because of the attack of the english pirates, nombre de dios was forgotten for four centuries, fact that helped it to preserve part of its archaeological remains (salamanca, 2007). after these events, the city is occupied and nowadays it is consolidated as one of the main section called santa isabel’s district, province of colón (north of panama). with the passing of time the population went growing and some places lie below certain structures for what it was necessary to carry out an archeological evaluation of the place; and based on this, a multi-disciplinary team accomplished certain scientific activities that represent the electric prospection. this arises from the fact that the study of the physical properties of the subsoil, through measurements realized in the surface of it can give referent lights to the presence and or absence of archeological buried features. these archeological elements are characterized for presenting physical properties (electric, magnetic, thermic or electromagnetic) different to the way that surrounds them, which make them potentially detectable. these techniques of exploration of the subsoil have been successfully applied in some archeological sites of colonial type in the american continent. (pastor et al, 2001; lascano et al, 2003; mojica et al, 2004; caballero et al, 2004; chávez et al, 2005; mojica, 2007a, 2007b; mojica and garcés, 2008; mojica et al, 2009). these techniques, which are categorized as non-destructive, play a very important role in the process of the extended description of an archeological site. a geophysical prospection based in the electric method was developed in the southwest part of the actual settlement of nombre de dios, with the principal objective to detect pavements or floors characteristic of the period of the spanish conquest. site description and historical setting nombre de dios is located to 23 km east of portobelo, at the atlantic coast of the isthmus of panama (figure 1). the place, repose on a sedimentary formation of the secondary period in which limestones and tuffs become present from the ocu formation. few kilometers from the site, it is encountered the río hato formation of the quaternary, which is characterized by the existence of conglomerates, sandstones, lutites, tuffs and non-consolidated sandstones. related to the historical context, mentioning that christopher columbus in his fourth voyage was searching a new route toward the asiatic continent, and he went over the caribbean coast of the isthmus of panama and found (in 1503) in its central zone, the first spanish village on the continent called santa maría de belén, which time later was going to be destroyed by the natives of the region. after that, numerous expeditions were carried out in the all region with the objective to find deposits of precious metals. at a later time the spain in the desire to colonize mainland decides to divide the explored zone into two governments: nueva andalucía and castilla de oro. the government of castilla de oro was ranged from cabo gracias a dios (located in the actual nicaragua) to the boundary with the government of nueva andalucía which is located in the middle of uraba’s gulf (actual colombia). in the year of 1510, it was founded in castilla de oro’s government, a port in the caribbean coast of panama which was named nombre de dios; later on this port played a very important role in the commerce between spain and the originating wealth of panama city, as a result of the conquest process of south america. geophysical test principles and field procedures the use of the techniques of geophysical prospection in archaeological explorations is a topic that has been strongly compiled in a utter literature and as example of it we can find the works of scollar et al. (1990), dabas et al. (1998), milsom (2003), campana and piro (2009) among others. one of the most important geophysical techniques used in archaeology is the electric 128 alexis mojica, louis pastor, richard vanhoeserlande, maría salamanca heyman figure 1.geographical location of nombre de dios and simplified geologic map of its surroundings. prospection (tabbagh, 1992; dabas et al. 1998); which is used to measure differences in the electric resistivity of the subsoil. this physical parameter of the subsoil constitutes the base of this type of explorations, and the same one is affected by the differences in the porosity of the grounds, its content of water, its chemical nature and its thermodynamic properties (nover, 2005). the device of electric prospection used in this study is composed of a source of ac power, which is connected to two metallic electrodes (a and b) inserted in the ground. when a certain current intensity i (in ma) flows through the subsoil another couple of electrodes (m and n) register the difference of electric potential v (in mv) generated in the subsoil. thanks to the data of v, i and a parameter k (in meter) that depends of the geometry of the four electrodes aligned in the surface it is possible to determine the distribution of the electric apparent resistivity (�ap) in the subsoil through the following equation: � �ap k v i � 2 (1) for effects of mapping this physical parameter, it is necessary to maintain fixed the distance among these electrodes, displacing them all equally through the separate profiles. the exploration depth doesn’t vary and it is determined through the separation among the electrodes. with the idea of obtaining an image at level of pixels on the variations of the electric resistivity of the ground for a constant depth, it was used a pole-pole electrode configuration, which is composed by a movil part that carries wooden electrodes with the couple of a and m electrodes separated in a distance of 0.50 m; the other couple of electrodes (b and n) is located far from the prospecting area (see figure 2). the operation of measuring values of the difference of electric potential throughout a profile is repeated in other parallel profiles and separated to each other in a fixed distance covering in this way, the surface of interest and keeping constant the electrical current i (5 ma in this case). this methodology has been used for different authors (hesse, 1994; dabas et al. 2000; caballero et al. 2004; matias et al. 2006; mojica 2007a; tonkov, 2008). the nature of the superficial ground (in this case sedimentary) the low level of contamination and the superficial archaeological evidence (ceramic remains and metallic materials characteristic of the period) constituted the necessary conditions for the development of an electrical prospection. however, elements like the density of the flora and some topographical aspects were the factors that influenced in the election of the area whose dimension of its surface was of (10 x 10) m (see figure 3). the measurements of the difference of electric potential were carried out throughout profiles of 10 m of longitude separated to a distance of 0.50 m among them, being obtained a total of 441 registrations. there were used in this study, electrodes of stainless steel of 30 cm of longitude by 5 cm diameter. the used resistivimeter consisted of a source of ac power of 50 v of output and a maximum current electricity of 5 ma. geophysical results, interpretation and archaeological excavation the result of the interpolation on a 0.25 m regular grid obtained in this study is shown in figure 4(a). the apparent electrical resistivity values range is extended between 90 and 360 ohm.m. this result shows two strong anomalies (a and b) in dark tonality with values of apparent resistivity greater than 180 ohm.m, which can be associated to cultural material. it also shows up a conducting area with anomalies in light gray tonality, with values of apparent electrical resistivity below 180 ohm.m which can be associated to the sedimentary material characteristic of the place. the dotted lines in the map correspond to the possible limits of these strong anomalies. based on this result, an archaeological excavation of (0.5 x 0.5 x 0.2) m of dimension that was developed in the central coordinates x = 9 m and y = 4 m (figure 4 (b)). the result obtained in the excavation revealed the existence of a superficial wet layer of 0.20 m of thickness constituted by sedimentary material (organic material) characteristic of the dry station. then it was detected a floor of pebbles very similar to the ones found in the hispanic site of old panama, located in the pacific coast of the isthmus of panama. the thickness average of this layer oscillates among the 0.10 m. the two layers lie on the same sedimentary material, but with a strong moisture level. using the micro – resistivity method to detect hispanic ancient floors at nombre de dios, panamá 129 figure 2. general scheme of measurement processes and acquisition of data in field through a pole pole electrode configuration. 130 alexis mojica, louis pastor, richard vanhoeserlande, maría salamanca heyman figure 3. map of the nombre de dios community and interest area. figure 4. (a) map of apparent electrical resistivity of the prospected zone and final interpretation, and (b) result of the archaeological excavation with dimensions (0.5 x 0.5 x 0.2) m in the place of nombre de dios. a posteriori 3d forward modelling with the objective of deepening in the results obtained in the electric and archaeological prospections, it was intended the reconstruction of a synthetic map of apparent resistivity based on the dimensions of the prospected area, the electrode configuration used (pole-pole in this case,) the dimensions and geoelectric characteristics of the structures contained in the subsoil (superficial layer with organic material, floor of pebbles and sedimentary ground, characteristic of the site). the response to the 3d model of electrical resistivity applied in this work was calculated through a numerical approximation of finite difference. this method is used to solve the distribution of the electric potential v due to a punctual source of electric flow in the surface of a half-space that possesses a three-dimensional arbitrary distribution of electrical resistivity (dey and morrison, 1979). these potential and electric current are related to the electrical resistivity of the structures by the equation: ��� � � � � �� � � � � � 1 � �� � � � ( , , ) ( , , ) ( ) ( x y z v x y z t x x ys y z zs s) ( )� � (2) in this equation (xs, ys, zs) correspond to the reference of the punctual source of electric flow, ã is the load density in a point of the cartesian space (x, y, z), � is the electric resistivity and � represents the dirac delta function. the solution of v (x, y, z) is obtained by deriving the difference equations of (2) through a discretization of the (x, y, z) space over which the problem is to be solved (dey and morrison, 1979). the space (that represents the subsoil) is discretized through a mesh of rectangular prisms whose nodes are separated from irregular forms in the directions x, y and z, and indexed for (j, k, l) for each direction. in each one of these nodes (where the electric potential v is unknown) the equation (2) is integrated under a corresponding elementary volume (v) and it is obtained: � �� � � � � � 1 �( , , ) ( , , ) ( , , ) ( x y z v x y z dx dy dz i x y zj k l s s s v j k l, , ) ��� (3) thanks to the transformation of the integral volume of the left member of the equation (3) to an integral surface through green’s theorem; a separated equation is obtained for a single node inside the mesh. this equation possesses bound functions to: (i) the geometry of the mesh and the value of the electrical resistivity (by default) of each rectangular prism, and (ii) the unknown value of the electric potential (v). if they take into account the differential equations of all the nodes of the analyzed mesh, it is obtained a matrix representation in the way: [c][v] = [i] (4) in this equation c represents a matrix related to the geometry and the distribution of the electrical resistivity in the mesh, which corresponds to well-known terms, v is another matrix that contains the unknown values of the electric potential in all the nodes, and i represents a matrix related with the well-known terms of intensity of electric current. the program used in this study was res3dmod (version 2.0, developed by m.h. loke) and it is based on the works of dey and morrison (1979). in the 3d electrical resistivity model proposed in this study, there were established 4 levels composed by rectangular prisms whose characteristic are shown in table 1. the values of electrical resistivity assigned to the rectangular prisms of the proposed model were chosen according to the standard guide of using the direct current resistivity method for subsurface investigation (designation d 6431 – 99). figure 5(a) shows the three-dimensional representation of the group of structures and figure 5 (b) a plan view of the same structures extended among 0.20 and 0.30 m of depth. the application of the program generated a synthetic dataset of apparent resistivity to different depths with 5% of noise (given in terms of the separation of the voltage of electrodes and electricity in the pole-pole setting); from the generated data, there were extracted those corresponding to the first level of depth for all the profiles, that is to say for the distance am = 0.5 m and after that, the next was to map this information using the same procedure than the one carried out with the data of apparent electrical resistivity measure. the result of this process is shown in figure 6 which is very similar to the one obtained in figure 4 (a), which means that the distribution of using the micro – resistivity method to detect hispanic ancient floors at nombre de dios, panamá 131 figure 5. (a) three-dimensional model of prisms that represent the grounds of pebbles (2500 ohm.m) and the other elements (sedimentary materials of 300 ohm.m between 0 and 0.20 m, and 90 ohm.m to deeper), and (b) plan view of the prisms model used in this study for a range of depth extended between 0.20 and 0.30 m. the rectangular prisms of figure 5, constitutes a good model for the representation and study of the electric anomalies associated to colonial floors. discussion the results of the electric prospection developed in the site of nombre de dios, have revealed the existence of strong electric anomalies of the subsoil with values above 180 ohm.m and that they are associated to a floors of pebbles system, which were very common in the colonial period. the range of apparent resistivity values obtained for the superficial layer is associated to the sedimentary material of the secondary period that is also composed of organic material. in this same aspect, the synthetic map generated by the three-dimensional modeling through the discretization of the subsoil for finite difference, possesses similar characteristic to the map of apparent electrical resistivity measued of figure 4 (a), with a first layer of rectangular homogeneous prisms of 0.20 m of thickness and 300 ohm.m of resistivity; representing to the superficial sedimentary layer. the second layer (located between 0.20 and 0.30 m of depth) is more complex and it reveals a group of prisms with 3 different values of electrical resistivity: the rectangular prisms of 2500 ohm.m that represent the system of floors of boulders, the prisms of 300 ohm.m associated to sedimentary floor with organic components, and those of 90 ohm.m with the same characteristics that the floor type described previously, but with certain level of moisture. below these two levels, it shows up a group of homogeneous prisms that are extended until 1 m of depth and of 90 ohm.m of electrical resistivity. conclusions even the prospected area doesn’t possess a significant extension, the results obtained in it demonstrates in first place, a good lateral resolution referring to electric mapping with electric anomalies that define the limits of floors of pebbles characteristic of the foundation period of the old city. thanks to this geophysical intervention it became evident the existence of archaeological features of hispanic type, being demonstrated in this way, the historical importance of the site. in this same aspect the three-dimensional model of rectangular prisms proposed in this work, constitutes a good model for the representation of old floors and the surrounding subsoil. based on the previously exposed thing, it becomes necessary a more extensive exploration to detect other archaeological features of this site, so that the life forms of the first european in the isthmus of panama can be understood. references caballero, o., mojica, a. and martín-rincón, j. (2004). prospecciones geofísicas y arqueológicas para la recuperación de la traza urbana de panamá la vieja: el caso de la calle santo domingo, geofísica. 60, 22-43. campana, s. and piro, s. (2009). seeing the unseen-geophysical and landscape archaeology, taylor and francis group, united kingdom, 331 pp. chávez, r. e., cámara, m. e., ponce, r. and argote, d. (2005). use of geophysical methods in urban archaeological prospection: the basílica de nuestra señora de la salud, patzcuaro, mexico, geoarchaeology. 20, no. 5, 505-519. dabas, m., delétang, h., ferdière, a., jung, c. and zimmermann, w.h. (1998). la prospection – collection archéologiques, errance, paris, france, 223 pp. dabas, m., hesse, a. and tabbagh, j. (2000). experimental resistivity survey al wroxeter archaeological site with a fast and light recording device. archaeological prospection. 7, no. 2, 107-118. dey, a. and morrison, h. f. (1979). resistivity modeling for arbitrarily shaped three – dimensional structures. geophysics. 44, no. 4, 753-780. hesse, a. (1994). a new light device for automatic recording of apparent resistivity on pole – pole configuration. 29th archaeometry symposium, ankara, turkey, may, 9-14. lascano, e., osella, a., de la vega, m., buscaglia, s., senatore, x. and lanata, j. l. (2003). geophysical prospection at floridablanca archaeological site, san julian bay, argentina, archaeological prospection. 10, no. 3, 175-192. matias, h.c., monteiro, f.a., rodrigues, f.e., machado, c. and luzio, r. (2006). detection of graves using the micro – resistivity method, annals of geophysics. 46, no. 6, 1235-1244. milsom, j. (2003). field geophysics, john wiley & sons ltd, united kingdom, 232 pp. mojica, a., pastor, l. and caraballo, m. (2004). características geomagnéticas del subsuelo en el sector noreste del conjunto monumental de panamá viejo, tecnociencia. 6, no. 1, 49-60. mojica, a. (2007a): application des méthodes géophysiques à la détection des sites précolombiens dans la région centrale de panama et paramétrisation géoarchéologique dans le site hispanique de panama viejo, ph.d. thesis, sisyphe laboratory, university of paris 6, paris, france. mojica, a. (2007b). prospecciones eléctricas de rasgos hispánicos en el sector central del sitio arqueológico de panamá viejo, arqueología del área intermedia. 7, 123-135. 132 alexis mojica, louis pastor, richard vanhoeserlande, maría salamanca heyman figure 6. synthetic horizontal map of apparent resistivity obtained from the prisms model presented in figure 5. mojica, a. and garcés, a. (2008). electrical prospection applied to detect buried colonial remains at las monjas de la concepción convent, old panama archaeological site, international journal of south american archaeology. 3, 29-35. mojica, a., chichaco, e., navarro, m., pastor, l. and vanhoeserlande, r. (2009). magnetic investigation of cultural features in the west zone of the old panama archaeological site (central america), international journal of south american archaeology, 5, 59-66. nover, g. (2005). electrical properties of crustal and mantle rocks a review of laboratory measurements and their explanation, surveys in geophysics. 26, 593-651. pastor, l., vanhoeserlande, r., florsch, n., florsch, i., toral, j., gonzález, j., lezcano, m. and mojica, a. (2001). prospección arqueogeofísica en panamá la vieja: presentación de casos, arqueología de panamá la vieja, avances de investigación, época colonial. 1, 43-61. salamanca, m. (2007). arqueología histórica en el puerto colonial español de nombre de dios, panamá. technical report, department of anthropology, center for archaeological research, college of william and mary. scollar, i., tabbagh, a., hesse, a. and herzog, i. (1990). archaeological prospecting and remote sensing, cambridge university press, cambridge, united kingdom, 696 pp. tabbagh, a. (1992). méthodes géophysiques appliqués à la prospection archéologique, mém. soc. géol. france. 161, 9-15. tonkov, n. (2008). geophysical survey of the thracian site at halka bunar locality in the area of the chirpan heights. geoarchaeology and archaeomineralogy, proceedings of the international conference, 2930 sofia, bulgaria, october, 329-332. acknowledgment the authors want to extend their gratefulness to the universidad tecnológica de panamá and college of william and mary for the offered support. to the embassy of france in panama and their cultural service, and to cccac. to mrs. angelica deglass, president of the quinquennium committee of the city of nombre de dios, and to mr. guido olmos for all the facilities and permissions he offered to the development of this experience. to the authorities of nombre de dios for their support, in this interesting project. using the micro – resistivity method to detect hispanic ancient floors at nombre de dios, panamá 133 articulo 7.indd earth sciences research journal geological engineering earth sci. res. sj. vol. 15, no. 2 (december, 2011): 129 136 an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran mehdi torabi kaveh and mojtaba heidari department of geology, faculty of science, bu-ali sina university, mahdieh ave., 65175-38695 hamedan, iran. e-mail: mehditorabikaveh@yahoo.com, heidarim_enggeol@yahoo.com abstract this paper describes the results of engineering geological investigations and rock mechanics studies carried out at the proposed chamshir dam site. it is proposed that a 155 m high solid concrete gravity-arc dam be built across the zuhreh river to the southeast of the city of gachsaran in south-western iran. the dam and its associated structures are mainly located on the mishan formation. analysis consisted of rock mass classification and a kinematic analysis of the dam foundation’s rock masses. the studies were carried out in the field and the laboratory. the field studies included geological mapping, intensive discontinuity surveying, core drilling and sampling for laboratory testing. rock mass classifications were made in line with rmr and dmr classification for the dam foundation. dam foundation analysis regarding stability using dmr classification and kinematic analysis indicated that the left abutment’s rock foundation (area 2) was unstable for planar, wedge and toppling failure modes. resumen este articulo describe los resultados de una investigación de ingeniería geológica y estudios de mecánica de roca que se llevo a cabo en el lugar propuesto para le represa chamshir. se propone una presa de 155m de altura, de arco gravitacional en concreto de solido, la cua debe ser construida a través del rio zuhreh al sureste de la ciudad de gachsaran en el suroeste de irán. la presa y su estructura asociada son localizadas principalmente sobre la formación mishan. el análisis consistió en la clasificación del macizo rocoso y un análisis cinemático de la fundación de la masa rocosa de la presa. los estudios se llevaron a cabo en campo y laboratorio. los estudios de campo incluyeron cartografía geológica, un estudio intensivo de discontinuidad, perforación de núcleo y toma de muestras para pruebas de laboratorio. la clasificación de la masa rocosa se realizo de acuerdo con la clasificación rmr y dmr para la fundación de la presa. el análisis de basamento rocoso de la presa en relación a la estabilidad usando la clasificación dmr y el análisis cinemático indico que el estribo izquierdo del basamento (área 2) es inestable para tipos de fallo planares y de cuña. palabras claves: el rio zuhreh, sitio presa chamshir, fundación rocosa, clasificación dmr, análisis cinemático. keywords: sochagota the zuhreh river, chamshir dam site, rock foundation, dmr classification, kinematic analysis. record manuscript received: 28/05/2011 accepted for publications: 30/11/2011 introduction the most important advantage of a favourable rock mass classification is that it has parameters describing most of a rock mass’s engineering characteristics for providing base input data for engineering design purposes. rock quality designation (rqd, deere 1964) and rock mass rating (rmr, bieniawski 1989) are two of the most commonly used numerically-expressed rock mass classification systems. several researchers have referred to rmr as being a useful tool for describing rock mass foundations (di salvo, 1982; van schalkwyk, 1982; marcello et al., 1991; hemmen, 2002; ramamurthy, 2004). some difficulties are involved in using rmr for dam foundation studies, such as very doubtful water pressure consideration, there are no good rules for quantifying the adjusting factor for joint orientation and watering changes introduce changes in properties concerning the rock mass and the joints. guidelines have only been offered regarding general stability against horizontal sliding, which is important but is not a very common problem. dam mass rating (dmr, romana, 2004) has been proposed as an adaptation of rmr, giving tentative guidelines for several practical aspects of dam engineering and for dam foundation appraisal in preliminary studies taking account of the effects of rock mass anisotropy and water saturation. the chamshir dam site on the zuhreh river is located in south-western iran, about 20 km southeast of the city of gachsaran (50° 52’ 36” e and 30° 10’ 59” n, figure 1). the dam is now being studied and has been designed as a 155 meter high concrete gravity-arc dam; its useful reservoir volume is 1.8 milliard cubic meters (figures 1 and 2). exceptional topographical, hydrological and geological circumstances regarding the river mehdi torabi kaveh and mojtaba heidari130 in the chamshir gorge has led to the site being proposed as a suitable option for dam construction, concerning available national resources use (i.e. water storage for irrigation projects). so far some researchers have studied the rock mass conditions of the chamshir dam site (e.g., gharouni-nik, 2008; torabi-kaveh et al., 2010). this paper explains the engineering geological assessment involved in the safe design of the proposed chamshir dam site. such geotechnical investigation has been carried out at the project site and in the laboratory. various laboratory tests and detailed discontinuity surveying were performed to assess rock mass characteristics. the chamshir dam site rock mass was studied using rqd, rmr and dmr classification and kinematic analysis more accurately assessed the dam foundation. geological setting geological factors play a major role in designing and constructing a dam (ichikawa, 1999) as they control the nature of geological formations and also provide the needed materials for construction. many cases have occurred throughout the world where dam foundation rock mass conditions were not sufficiently known and the cost of construction and treatment greatly exceeded the original budget. according to the 1:100,000 geological map of ghachsaran (setodehnia and o.b. perry, 1966) (figure 1), the geological formations in the study area, from oldest to youngest, are gachsaran (early miocene), mishan (early-middle miocene), aghajari (miocene-late pliocene), bakhtiari (late pliocene-pleistocene) and alluvial sediments. the mishan formation (an isocline) along with the gachsaran formation cover the western part of the dam reservoir and dam site. this formation has two different facies; the first consists of biohermy limestone and forms a great lens within a second facies which consists of alternating marl and limestone layers. the zuhreh river has created the long and narrow chamshir gorge by erosion of biohermy limestone, thereby making it a suitable location for dam construction (figure 2). the chamshir dam reservoir is located on the gachsaran, mishan and aghajari formations; the gachsaran formation’s stratigraphy sequence in the study area is similar to that of the khuzestan area (tehran-sahab and parab-fars consulting engineering companies, 1997). this formation has 7 members: the oldest member is 40m thick consisting of alternating thick anhydrite, limestone and shale layers. the second member is a 115m thick salt layer, with anhydrite alternating with thin limestone layers. the third member is a 347m thick anhydrite layer with salt. the fourth member consists of a 290m thick salt layer with marl, gray limestone and anhydrite. the fifth member is 342m thick red and gray marl with alternating layers of gypsum. the sixth member is figure 1. geological map of the study area (modified from the gachsaran geological map, 1:100,000, iranian oil operating companies (iooc), 1966). an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 131 258m thick, having alternating layers of anhydrite (or gypsum), salt, red marl and limestone. the seventh member is the youngest member, being 139m thick and having alternating gypsum, gray marl and limestone layers. it should be mentioned that sulphate layers outcrop as gypsum on the surface and as anhydrite at deeper levels. the gachsaran formation covers most parts of the projected dam reservoir (figure 1). according to field observations, members 5, 6 and 7 of the gachsaran formation would be in contact with the dam reservoir in this area and only member 7 outcrops downstream of the dam. the aghajari formation would form a small part of the reservoir at its south-western corner and consists of sandstone, siltstone, conglomerate, gypsum and marl. young and old terraces are also present along the banks of the zuhreh river (figure 1), consisting of coarse grained gypsum particles and fine grained silt and sand sediment. the study area is in the zagros folded area or external zagros (stocklin, 1968) and simply folded belt (berberian, 1995). zagros folding compressional tectonic forces have created some faults and thrust faults having a nw-se trend in the study area; the dezh soleyman thrust (dst), murd thrust and chamshir fault area are the most important ones (figure 1). the role of the dst in the study area is important according to field observations. the gachsaran formation is uplifted along this fault from deeper parts to the surface. it has dissected some parts of the mishan formation in the north-eastern branch of the chamshir syncline and has consequently thrust the gachsaran formation over the mishan formation. the extensive tectonic pressure of the dst created the important chamshir fault area, this being the source of several springs throughout this area and the dst. the zuhreh river’s deviation from its direct pathway into the chamshir fault area could also provide reasonable evidence of tectonic activity in the study area. materials and methods engineering geological investigations and rock mechanics studies include discontinuity surveying, core drilling, in situ and laboratory testing. quantitative description of discontinuity (i.e. orientation, spacing, persistence, roughness, aperture and filling materials) were determined in situ by exposure logging according to the international society for rock mechanics’ (isrm) standards (1981). laboratory tests were carried out on the core samples to quantify the physical and geomechanical properties of intact rocks at the dam site. site investigation the dam site was investigated in two stages; the site was geologically studied and mapped in detail. thirty-four boreholes were drilled (1,578 m), 8 of them pertaining to the dam site. six boreholes were drilled (565m total depth) during the first stage (1999 to 2000); the second stage was carried out between 2008 and 2009 when 2 boreholes were drilled (246 m total depth). two locations have been have been considered for field studies regarding the proposed areas for constructing a dam in the chamshir gorge (figure 3). five hundred discontinuities were measured (250 on both the left and right abutments). four dominant discontinuity sets were identified on the left (area 1) and right (area 1 and 2) abutments of the proposed dam site (tables 1 and 2). five dominant discontinuity sets were figure 2. the chamshir dam site (chamshir gorge). mehdi torabi kaveh and mojtaba heidari132 table 1. the left and right abutments’ discontinuity characteristics (area 1). table 2. the left and right abutments’ discontinuity characteristics (area 2). plungetrendaverage dip (°)average dip direction (°) type of discontinuitylocation 7735713177bedding right abutment 1414378323j1 1032779147j2 1429876118fault set 7930611126bedding left abutment 1213678316j1 1232676146j2 1730573125fault set plungetrendaverage dip (°) average dip direction (°) type of discontinuitylocation 7735013170bedding right abutment 1214478324j1 1132079140j2 1430576125fault set 782791199bedding left abutment 1414878328j1 1332676146j2 1312073300fault set 1 929881118fault set 2 figure 3. satellite image of chamshir dam site (http://www.google.com/earth/index.html). an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 133 identified (table 2) for the left abutment (area 2). a quantitative description of discontinuity in two areas included type and orientation in the left and right abutments (tables 1 and 2). results and discussion rock mass quality rqd and rmr were also used for obtaining the exposed rocks’ engineering properties within the dam foundation. the data were collected from the dam site. the rqd values were determined by examining drill cores and joint frequency (table 3). the table shows that the left and right abutments’ rqd values were excellent for the projected dam construction. table 4 gives the rmr values, rock unit quality being classified as very good. dmr classification dmrsta (related to dam stability against sliding) value was: dmrsta = rmrbd + cf × rsta where rmrbd (basic dry rmr) resulted from adding the rmr’s first four parameters plus a water rating of 15 and rsta was the dam stability adjustment factor. regarding hoek-brown criteria, hoek has advocated the use of a “dry rmr” obtained with the maximum rating for water, simultaneously introducing real pore pressures into the computations (hoek et al., 2002). rmrbd was obtained by adding the first four parameters of rmr plus 15: 1) compressive strength, tested in water conditions similar to future ones, i.e. saturated when the rock is going to be saturated and having the same ph as that water; 2) rock mass rqd; 3) significant governing joints’ spacing (s); 4) significant governing joints’ conditions (s); and 5) water rating (wr), always 15 (as if dry). the rsta (adjustment factor for dam stability) was obtained (table 5). the danger of sliding became reduced when the significant joint’s dip direction was not almost parallel to the dam’s downstream-upstream axis due to the geometrical difficulties involved in sliding. such effect could be taken into account by multiplying dam stability adjusting factor rating rsta by a geometric correction factor (cf): table 3. the right and left abutments’ rqd values. table 4. the chamshir dam foundation’s rmr classification table 5. dam stability rsta adjustment factors, according to joint orientation; ds dip downstream/us dip upstream/a any dip (romana, 2003a). rqd value place obtained from joint frequencyobtained from cores 10090-100right abundant 10090-100left abundant left abutment rock massright abutment rock mass situation rateconditionsrateconditionsdescription 4.025-504.025-50compressive strength (mpa) 20.090-10020.090-100rqd (%) 20.0>220.0>2joint spacing (m) 25.0sum of five parameters25.0sum of five parametersdiscontinuity condition 15.0dry15.0drywater flow 84.084.0rmrbd rate 0favourable0favourablejoint orientation rate 84.0very good84.0very goodrmr type of dam vf f fa u vu very favourable favourable fair unfavourable very unfavourable fill others 10-30 ds 0-10 a gravity 10-60 ds 30-60 us 60-90 a 10-30 us 0-10 a arch 30-60 ds 10-30 ds 30-60 us 60-90 a 10-30 us 0-10 a rsta 0 -2 -2 -15 -25 (1) mehdi torabi kaveh and mojtaba heidari134 cf = [1 – sin (αd – αj)] 2 (αd > αj) cf = [1 – sin (αj – αd)] 2 (αd < αj) where αd was dam axis upstream-downstream direction and αj was the dip direction of the significant governing joint. dam foundation status dmrsta was calculated (table 6). the chamshir dam will built on mishan limestone and marl rock units. the valley walls at the dam site are steep, having 80°– 90° slopes on the left abutment and 75°–90° on the right abutment. the valley runs nw-se (310°). dip direction is 40° ne for the left abundant and 220° sw for the right abundant. dmr classification for the chamshir dam’s foundation (for areas 1 and 2) is shown in tables 4 and 5. the results obtained from dmr classification (tables 7 and 8) indicated that the left abutment (area 2) was instable; the results of this classification were compatible with field conditions. kinematic analysis kinematic refers to the motion of bodies without referring to the forces causing them to move (goodman, 1989). kinematic analysis is very useful for investigating possible rock mass failure modes and determining maximum safe slope angle (mssa). many studies have determined slope failure modes (markland, 1972; goodman, 1976; goodman and shi, 1985; matherson, 1988) and evaluated slope stability (özsan and akin, 2002; aksoy and ercanoglu, 2007; kulatilake et al., 2011) using a stereo(2) (3) table 6. the degree of dam safety regarding sliding (romana, 2004). table 7. the dmr classification of the right abutment. table 8. the dmr classification of the left abutment.. degree of safetyrock mass rate no primary concerndmrsta > 60 concern60>dmrsta>30 serious concern30>dmrsta graphic projection technique. kinematic analysis was used for the study area to estimate the mssa regarding the three basic failure modes: plane sliding, wedge sliding and toppling. the aforementioned kinematic analysis was performed for left abutment slopes (area 2) at the dam site using dominant discontinuity sets. kinematic analysis (table 9 and figure 4) results indicated that joint inclination was the most important parameter affecting rock mass instability. the analysis revealed possible wedge, planar and toppling failures in the left abutment (area 2). conclusions the concrete chamshir dam will be located on the limestone and marl rocks of the mishan formation. good rock mass quality was indicated for these rocks; however, according to dmr and kinematic analysis, most parts of the dam foundation (except the left abutment, area 2) were safe, being rated low-risk in terms of instability occurrence and magnitude. it is therefore recommended that slope failure should be constantly monitored. despite rqd and rmr values showing favourable condition for the dam abutments, the dmr classification provided more accurate assessment and was more reliable, i.e. considerable correlation between such classification and the kinematic analysis. acknowledgments the study was financed by the mahab ghodss consulting engineering company. the authors are grateful to dr. hamid reza zarei for supplying the authors with the dam site’s practical test data and mr. jason garry for editing the paper in english and mr. mirmohammad miri for translating it. references aksoy, h. and ercanoglu, m. (2007). fuzzified kinematic analysis of discontinuity-controlled rock slope instabilities, eng geol. 89, 209–19. right abutment situation area 2area 1 fault setj2j1beddingfault setj2j1beddingtype of discontinuity 0.430.438.82×10-43.9×10-41.841.840.341.7×10-2cf -7-7-7-15-7-7-7-15rsta 84rmrbd 78.0255.70dmrsta left abutment situation area 2area 1 fault set 2fault set 1j2j1beddingfault setj2j1beddingtype of discontinuity 1.842.380.163.072.220.430.161.645.48×10-4cf -7-7-7-7-15-7-7-7-15rsta 84rmrbd 0.0068.40dmrsta an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 135 table 9. kinematic analysis regarding sliding in the left abutment (area 2). figure 4. kinematic conditions for the left abutment (area 2), 1: bedding, 2: j1, 3: j2, 4: fault set ι, 5: fault set π, 6: slope face orientation of intersection lineswedge sliding results plunge (deg.)trend (deg.)inters. linemaximum safe angle slope face dip direction sliding along joint sets 8.8 9.1 4.2 4.1 4.2 75.9 51.7 44.3 75.9 5.1 55.8 58.1 29.0 28.6 57.0 322.8 39.6 223.0 168.8 28.8 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 90 90 90 90 90 76* 68** 90 90 90 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 toppling failureplanar failure maximum safe angle (deg.) failure along joint set slope face dip direction maximum safe angle (deg.) failure along joint set slope face dip direction 90 90 53**** 90 90 1 2 3 4 5 340.0 340.0 340.0 340.0 340.0 90 76*** 90 90 90 1 2 3 4 5 340.0 340.0 340.0 340.0 340.0 * potential wedge failures along the intersection lines for joint set (1) with fault set 1 were possible if slope angle exceeded 76 degrees. ** potential wedge failures along the intersection lines for joint set (1) with fault set 2 were possible if slope angle exceeded 68 degrees. *** potential planar failure along faults was possible if slope angle exceeded 76 degree. **** potential toppling failure due to the orientation of joint set 2 was possible if slope angle exceeded 53 degrees. analysis must be carried out for faults sited close to the slope face in the vicinity of such faults. mehdi torabi kaveh and mojtaba heidari136 berberian, m. (1995). master “blind” thrust faults hidden under the zagros folds: active basement tectonics and surface morphotectonics, tectonophysics. 241, 193-224. bianewski, z. t. (1989). engineering rock mass classification, wiley, chichester, 251 pp. deere, d. u. (1964). technical description of cores for engineering purposes, rock. mech. eng. geol. 1, 1722. di salvo, c. a. (1982). geomechanics classification of the rock mass at segunda angostura dam, 14th icold, rio de janeiro, q53 r30. gharouni-nik, m. (2008). comparison of the results of fdt and plt in determining deformability modulus of the rock mass in the chamshir dam site in iran, the 3rd international conference on site characterization, taipei, taiwan, 1-4 april. goodman, r. e. (1976). methods of geological engineering in discontinuous rocks, san francisco, west publishing. goodman, r. e. (1989). introduction to rock mechanics, 2nd ed. new york, wiley. goodman, r. e. and shi, g. h. (1985). block theory and its application to rock engineering, new jersey, prentice-hall. hemmen. (2002). paris dam, internet site for dmr dams foundations, http:// www.stmr.es. hoek, e., carranza-torres, e. and corkum, b. (2002). hoek-brown failure criterium-2002 edition, narms, toronto. http://www.google.com/earth/index.html ichikawa, k. (1999). geological investigation of dams, proc. of 2nd asian symposium on engineering geology and the environment, malaysian national group, bangi, malaysia, 1-44–1-57. isrm. (1981). rock characterization, testing and monitoring, in: brown, e.t. (ed.), isrm suggested methods, pergamon press, oxford, 211pp. kliche, c. a. (1999). rock slope stability sme, littleton, co. kulatilake, p. h. s. w., wang, l., tang, h. and liang, y. (2011). evaluation of rock slope stability for yujian river dam site by kinematic and block theory analyses, computers and geotechnics. 38, 846–860. marcello, a., eusepi g., olivero, s. and di bacco, r. (1991). ravanasella dam on difficult foundation, 17th icold, vienna, q 66 r 21. markland, j. t. (1972). a useful technique for estimating the stability of rock slopes when the rigid wedge sliding type of failure is expected, imp, coll, rock mech, res, rep, 19, 10. matherson, g. d. (1988). the collection and use of field discontinuity data in rock slope design, q j eng geol. 22, 19–30. özsan, a. and akin, m. (2002). engineering geological assessment of the proposed urus dam, turkey, eng geol. 66, 271–81. ramamurthy, t. (2004). a geo-engineering classification for rocks and rock masses, international journal of rock mechanics & mining sciences. 41, 89–101. romana, m. (2003a). dmr (dam mass rating). an adaptation of rmr geomechanics classification for use in dams foundations, int. cong. on rock mechanics, (technology roadmap for rock mechanics) south african inst, of min, and meta, 977-980. romana, m. (2004). dmr (an adaptation of rmr), a new geomechanics classification for use in dams foundations, 9th congresso luso de geotecnia, aveiro, 12pp. setudehnia, a. and ob-perry, g. t. (1966). geological map of gachsaran, iranian oil operating companies (iooc), 1,100,000. stocklin, j. (1968). structural history and tectonics of iran: a review, aapg bull. 52, 1229-1258. tehran sahab and parab fars consulting engineering companies. (1997). engineering geology report concerning the chamshir dam site (in persian), chamshir dam project, teheran, iran. torabi-kaveh, m., heidari, m., zarei, h. r. and ghiasi, m. b. (2010). engineering geological investigation of the foundation of the chamshir dam site by use of dmr classification (iran), 7th international symposium on eastern mediterranean geology, university of çukurova, adana, turkey, 18‐22 october. van schalkwyk. (1982). geology and selection of the type of dam in south africa, 14th icold, río de janeiro, q51. r44. untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 30-39 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india srinivasamoorthy k1, sarma vs2, vasantavigar m1, vijayaraghavan k1, chidambaram s1 and rajivganthi r1 1department of earth sciences, annamalai university, annamalainagar – 608 002 2national geophysical research institute, hyderabad -500606 (council of scientific & industrial research, new delhi) corresponding author email: moorthy_ks@yahoo.com abstract an attempt was made to identify the extent of pollution in the aquifer matrix of tirupur, a highly industrialized zone of tamilnadu state, south india. electrical imaging techniques were adopted with a syscal pro-96 system, for measuring apparent resistivity values using different electrodes separation. the first profile conducted at valipalayam recorded a resistivity range of <10 ù m at a depth of 8 m, which indicates contamination of top soil due to discharge of effluents. an increase in resistivity >45.5 ùm was observed at a depth of 27 to 47 m indicating the possibility of contamination. the second profile conducted at pethichettipuram indicates source of contamination at left end corner with a drop in resistivity <46.5 ù m at a depth of 7.91 m. a drop in resistivity <21.6 ù m was also observed at a depth of 11.5 m indicating a contaminated zone in deeper regolith. the third survey conducted in palayakadu indicates contamination of regolith at a depth of 0 to 20 m with a resistivity less than 40 ùm. the fourth survey at chellapuram indicates contamination of overburden with resistivity >11.5 ùm, to a depth of about 10 m. five imaging surveys conducted across the contaminated sites reveals that shallower regoliths are highly contaminated and deeper aquifers are free from contamination except a few locations. key words: industrial zone, resistivity, tomography, 2d model, regolith, contamination. 30 manuscript receiver: january 05th, 2009. accepted for publication: may 05th, 2009. resumen se intentó determinar el alcance de la contaminación en la matriz acuífera de tirupur, una zona altamente industrializada del estado de tamilnadu, al sur de la india. se usaron sondeos eléctricos para obtención de imágenes con un sistema syscal pro-96, para medir los valores de resistividad aparente con diferentes separaciones de electrodos. el primer perfil realizado en valipalayam registró un rango de resistividad <10 ù m, a una profundidad de 8 metros, lo cual indica la contaminación de la parte superior del suelo debido a la descarga de efluentes. un aumento de la resistividad > 45,5 ùm se observó a una profundidad de 27 a 47 m, indicando la posibilidad de contaminación. el segundo perfil realizado en pethichettipuram indica la fuente de contaminación en el extremo izquierdo con una caída de la resistividad <46,5 ù m, a una profundidad de 7,91 m. un descenso en la resistividad <21,6 ù m también se observó a una profundidad de 11,5 m, indicando una zona contaminada en lo más profundo. el tercer sondeo realizado en palayakadu revela una contaminación a una profundidad desde 0 a 20 m con una resistividad inferior a 40 ùm. el cuarto perfil en chellapuram indica una contaminación de los terrenos de resistividad >11,5ùm, a una profundidad de unos 10 m. cinco estudios de imágenes realizados a través de los sitios contaminados revelan que los acuíferos superficiales están altamente contaminados y los acuíferos más profundos están libres de contaminación, a excepción de unos pocos lugares. palabras clave: zona industrial, resistividad, tomografía, 2d modelo, contaminación. introduction the purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface as these measurements help to estimate the true resistivity of the subsurface. the true resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. in many engineering and environmental studies, the subsurface geology is very complex and the resistivity can change rapidly over short distances (keller and frischknecht 1966; daniels and alberty 1966). resistivity values have a much larger range compared to other physical quantities mapped by other geophysical methods. 2-d and 3-d electrical surveys are the practically commercial techniques used with the recent development of multi-electrode resistivity surveying instruments (griffiths et al. 1990) with aid of computer inversion softwares (loke, 2004). igneous and metamorphic rocks typically have high resistivity values but vary on the degree of fracturing, and the percentage of the fractures filled with ground water. the resistivity of ground water approximately varies from 10 to 100 ù m depending on the concentration of dissolved salts present. the low resistivity (about 0.2 ù m) in groundwater is mainly due to the presence of industrial contaminant metals such as fe, cu, pb and zn along with leaching of cations and anions like potassium, chloride, sodium, bicarbonate, silicates etc., from rock sources and manmade influences greatly reduce the resistivity of ground water to less than 1 ù m even at fairly low concentrations (pathak and venkatesshwar, 2001) electrical imaging techniques new developments in recent years is the use of electrical imaging surveys, where the resistivity changes in vertical direction, as well as in horizontal direction along the survey line, to map areas with complex geology (griffiths and barker, 1993). these surveys are usually carried out using 25 or more electrodes connected to a multi-core cable which is attached to an electronic switching unit, connected to a lap top computer with an electronic switching unit to automatically select the relevant four electrodes along a 31 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india straight line. electrical imaging will provide information about distinct subsurface boundaries and conditions, which can indicate soil or bedrock lithology variations (edwards, 1977). from the measured field data, simulated depth sections are constructed (apparao and sarma, 1981 1983) with over lapping data levels. to plot the data from a 2-d imaging survey, the simulated section contouring method is normally used due to its ease in pictorial representation with different arrays for mapping the same region, which gives rise to very different contour shapes in the simulated section plot. the syscal pro-96 system is capable of measuring apparent resistivity values with different electrode configurations for wenner electrode arrangement. resistivity simulated sections was prepared using the apparent resistivity values and interpretation were carried out using res2dinv software (loke, 1997). the output is presented in the form of subsurface images which is a useful system for electrical resistivity tomography (ert). concept the concept consists of using multi core cables, which contain arrangement of cables and electrodes one takeout every 5m with 64 electrodes. the measuring unit includes relays, which automatically carries out the sequence of readings introduced in its internal memory. the system takes readings for many combinations of transmission and reception pairs so as to achieve a mixed profiling and sounding pairs (abem, 2004). the total length of the cable is equal to the spacing of electrodes which determines the depth of investigations (ron barker et.al, 2001). the final depth of the investigations of a lund imagine survey depends on, geometry of cables (type of array, number of electrodes, spacing between electrodes and number of segments) and the measurement of signal by the equipment, namely the amplitudes of the signal, existing noise, power specifications of the equipment and its ability of filtering the noise through the stacking process (loke, 2001). methodology a known amount of current (i) is pumped to energize the subsurface using current electrodes and the response is measured on the ground surface in the form of voltage (v) through potential electrodes. resistance (v/i) is calculated and further, the apparent resistivity is computed by the formula ña = k× (v/i), where k is the geometrical factor which depends upon the type of configuration that is used. in the case of profiling, the apparent resistivity is obtained along a line using an appropriate electrode arrangement. from the resistivity plot the anomalous zones are identified without any depth estimation. in the case of sounding, apparent resistivity is obtained at a particular place (identified through profiling) by increasing electrode separations and the data is interpreted in terms of layer parameters quantitatively. the present proposed technique is the wenner array which has the strongest signal strength for a comprehensive subsurface picture. forward modeling program the free program, res2dmod.exe, is a 2-d forward modeling program which calculates the apparent resistivity pseudo section for a user defined 2-d subsurface model. the program helps to choose the finite-difference (dey and morrison, 1979a) or finite-element (silvester and ferrari, 1990) method to calculate the apparent resistivity values. the res2dmod.exe program indicates the contours in the simulated section produced by the different arrays over the same structure. study area tirupur is located 50 km east in coimbatore district of tamilnadu state in south india at latitudes and longitudes 11.18° n 77.25° e with a total extent of 27 sq.km (figure 1). tirupur an important trade center of india gained recognition as the leading source of knitted garments, casual wear and sportswear. geology of the study area is entirely composed of precambrian shield area called as the indian peninsu32 srinivasamoorthy k, sarma vs, vasantavigar m, vijayaraghavan k, chidambaram s and rajivganthi r lar complex with wide range of igneous and metamorphic rocks. the most common rock type in the study area is gneiss. gneiss is a generic term for a large variety of metamorphic rocks, and can have both sedimentary and igneous origin. the gneiss found in the tirupur region is of high metamorphic grade and is mainly of the biotite type, but quartzo-feldspatic gneiss is also found. these rocks are thought to have been formed during the archaean time period, approximately 3.8 to 2.5 billion years ago, which means that they are among the world’s oldest rocks (gustavsson et al., 1970), (sankararaaj et al., 2002). thestudy area is an undulating plain with gentle slopes. the elevation ranges between 225 33 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india figure 1. study area with imaging locations. and about 300 m (kristina furn, 1998). the climatic conditions in the study area are “semiarid” with a mean annual temperature of 29.4°c. (ssluo, 1998). the annual average rainfall in the study area is 527.2 mm. hydrogeology of the study area groundwater occurs in two different aquifers, one shallow aquifer formed by the weathered zone and the other deeper aquifers connected to the fracture system in the rock with vertical and horizontal fractures. these fractured zones extend down as 200 m or more. the transmissivity of the shallow aquifers varies from 11.4 to 51.0 m2/day, with an average of about 30.7 m2/day. the hydraulic conductivities vary from 2.9 to 20.0 m/d with an average of 9.2 m/d. the average thickness of the shallow aquifer is 3.7 m. the average specific yield is 4.0×10–2 (cgwb, 1993). the vertical fracture zones constitute an unconfined aquifer, demarcated as the most potential aquifer in the area, in which groundwater development is focused. transmissivity in the vertically fractured aquifer vary from 52.1 to 497.0 m2/day. the hydraulic conductivity is 1.8 m/day with a range of 0.9 to 3.4 m/day. the aquifer ranges from approximately 45.0 to 142.3 m below ground level with an average thickness of 97.3 m (table 1). in horizontal fracture aquifer the transmissivity is 52.8 m2/day with a hydraulic conductivity of 0.5 m/day. the horizontally fractured layer depth ranges from 28.8 to 129.5 m below ground level (cgwb, 1993). the groundwater table more or less follows the topography, but with a smaller slope than the surface. the hydraulic gradient is approximately 2.8 m/km with a steeper slope of about 6.5 m/km towards the river noyil indicating the groundwater flow along east west direction. groundwater is the main source of water supply in the study area although presence of river noyyal which dries during summer season. the study area has been demarcated for heavy exploitation of groundwater used mainly for industrial purposes (senthilnathan and azeez, 1999). at present 9000 knitting, processing and manufacturing units consume nearly 86 mld (million liter per day) of water, but 90 mld of water used are discharged as effluents containing a variety of dyes and chemicals like acids, salts, wetting agents, soaps and oil, leading to contamination of the ground and surface water and soil in and around the study area (rajaguru and subbram, 2000). the pollution load calculated from pollution control board, 2008 was as follows: total dissolved solids 23.54 lakh tonnes, chloride 13.11 lakh tonnes, sulphate 1.25 lakh tonnes, total suspended solids (tss) 0.97 lakh tones, chemical oxygen demand 0.90 lakh tonnes, biological oxygen demand 0.29 lakh tonnes and oil & grease 0.01 lakh tones (jacob thomson, 1998). hence an attempt was made to get further information on the extent of groundwater contamination by using ert studies. results and discussion the present study was made to delineate the pollutant zone through ert method. in tirupur surveys, the imaging system was used with single cable with 25 takeouts at 5 m interval by syscol pro 96 instrument by adopting wenner array. electrodes were 0.5 m long and made of stainless steel; they were planted to 34 srinivasamoorthy k, sarma vs, vasantavigar m, vijayaraghavan k, chidambaram s and rajivganthi r table 1. aquifer parameters of the study area physical parameter (unit) hydraulic conductivity (m/day) thickness (m) storativity porosity (%) shallow 9.2 3.7 4.0* 10-2 4.5 vertical 1.8 97.3 5.2* 10-3 8.0 a depth of 0.4 m. each electrode was watered to ensure good contact with the ground. a total of four image lines were measured with the images varying in length from 0 to 50 m parallel to the river noyyal. this effectively gave a maximum depth of imaging of 20 m. the first survey was conducted at valipalayam 1km from river noyyal (figure 2). the profile shows a low resistivity zone ranging from 10 to 100 ù m, indicating presence of highly weathered rock materials. the basic concept of electrical resistivity method is to demarcate higher resistivity zones within the low electrical resistivity rocks at the sub surface. this is because; the very low resistivity is an indicator of highly weathered rock material. the regolith with a resistivity range of <10 ù m is found at a depth of 8 m indicating the contamination of top soil due to the discharge of effluents. the weathered and fractured zones were identified at a depth of 27 to 47 m with increase in resistivity from 46 to 95.5 m indicating that the deeper layers are exposed to groundwater contamination in the absence of clay materials (ron barker and others, 2001). the second survey was conducted near pethichettipuram just 1km away from the noyyal river. the survey indicates source of contamination at left end corner with a drop in resistivity by <46.5 ù m at a depth of 7.91 m (figure 3). the same trend was also noted at a depth of 11.5 m with a drop in resistivity by <21.6 ù m indicating the contaminated zone at deeper regoliths. this is supported by a groundwater sample collected in a dug well to a depth of 15m showing higher tds value >3,500 ppm. the fractured and massive rocks revealed higher resistivity varying from 46.7 to 2200 ù m indicating the non polluted nature of deeper formations. a thin subsurface layer with low resistivity extends to deeper area in the middle of the profile, followed by a thick layer with high resistivity (resistivity greater than 1000 ù m) acting as a basin at deeper depth (apparao and sarma, 1993). this basin is more dominant at the last quarter of the traverse where it is closer to the surface. this high resistivity layer could represent the bedrock, which is competent without any structures. from the survey result a geological division is observed between the lower and high resistivity materials. in the central part of the profile a very low resistivity zone exist, indicating existence of an aquifer within the profile, based on its lower resistivity in relation to the background resistivity. the third survey was conducted in palayakadu (figure 4), indicating extensive contamination of regolith from ground surface to 20 m as the resistivity of the regolith drops less than 40 ùm. bed rock resistivity was higher >1058 ù m indicating their massiveness at shallow depth. the profile produced three different resistivity of layered rock which is clearly defined by their different resistivity layers at various depths as follows; a thin subsurface layer with comparatively low resistivity values ranging from 10 to 46.7 ùm at an depth of about 15 m. this layer of low resistivity is typical of weathered rock materials of the underlying rocks in the area. an intermediate resistivity layer ranging from 101 to 218 ù m at the intermediate depth zone, which could be represented as weathered to moderately weathered rock material. this depth layer between the depth ranges (15-40 m) has a comparatively moderate resistivity from 150 to 1018 ù m. a thick layer with a comparatively high resistivity (>218 ùm) is also observed below the layers of low to intermediate resistivity rocks. this relatively high resistivity rock layer represents the presence of fresh rock material with no structural patterns like fractures and joints, as good indications for aquifers (kelley, 1976) due to their sheared nature represented by the wavy pattern. the shallow depth layers could also be interpreted to be layers of different rock materials. the resistivity layers of the different rock materials in the range (180 to 1080 ù m) and its contact with the fresh rock observed at a depth range of about 55 m was also accounted. the fourth survey conducted at chellapuram is shown in (figure 5). the profile indicates that the area is underlined by varying high resistivity rock materials by sharp changes in their electrical resistivity values. the high resistivities observed are typical of fresh granite rocks. the general structural trend observed is as follows; an overburden with low resistivity to a 35 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india 36 srinivasamoorthy k, sarma vs, vasantavigar m, vijayaraghavan k, chidambaram s and rajivganthi r 2 d valipalayam .bin ps.2 0.0 80.0 160.0 240.0 n. 2.60 8.53 14.3 20.1 25.8 31.6 37.0 42.5 47.9 regolith weathered and fractured zone mesured apparent resistivity pseudosection 0.0 80.0 160.0 240.0 n.ps.2 2.60 8.53 14.3 20.1 25.8 31.6 37.0 42.5 47.9 0.0 80.0 160.0 240.0 n. depth 1.25 6.38 12.4 19.8 28.7 33.8 39.4 45.6 calculated apparent resistivity pseudosection iteration 3 rhs error 3.7% inverse model resistivity section 0.197 0.325 0.536 0.884 1.46 2.41 3.97 6.55 resistivity in ohn.n unit electrode spacing 5.00 n. figure 2. resistivity imaging at valipalayam. 37 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india 0.0 32.0 64.0 96.0 128.0 m. 0.0 32.0 64.0 96.0 128.0 m. 0.0 32.0 64.0 96.0 128.0 m. pethuchettipuram-l1 pethuchettipuram-l2 pethuchettipuram-l3 depth teration 5 rms error = 25% 0.500 3.70 7.91 11.5 15.8 21.0 inverse model resistivity section 10.0 21.6 46.7 101 218 471 1018 2200 resistivity in ohm.m unit electrode spacing 2.00 m unit electrode spacing 2.00 m depth teration 5 rms error = 1.46% 10.0 21.6 46.7 101 218 471 1018 2200 resistivity in ohm.m depth teration 5 rms error = 2.0% 0.500 3.70 7.91 11.5 15.8 21.0 0.500 3.70 7.91 11.5 15.8 21.0 unit electrode spacing 2.00 m 10.0 21.6 46.7 101 218 471 1018 2200 resistivity in ohm.m inverse model resistivity section inverse model resistivity section contaminated zone fractured and massive rock aquifer zone figure 3. resistivity imaging at pethuchettipuram. 38 srinivasamoorthy k, sarma vs, vasantavigar m, vijayaraghavan k, chidambaram s and rajivganthi r ws-07palayakadu 0.0 80.0 160.0 240.0 320.0 400.0 m. contaminated zone maximize hard rock 2.60 10.5 18.2 25.8 33.5 41.2 48.8 56.5 2.60 10.5 18.2 25.8 33.5 41.2 48.8 56.5 1.25 9.26 19.0 28.7 39.4 52.4 0.0 80.0 160.0 240.0 320.0 400.0 m. 0.0 80.0 160.0 240.0 320.0 400.0 m. depth inverse model resistivity section mesured apparent resistivity pseudosection calculated apparent resistivity pseudosection iteration 5 abs error 35.4% ps.2 ps.2 10.0 21.6 46.7 101 218 471 1018 2200 resistivity in ohn.m unit electrode spacing 5.00 m. figure 4. resistivity imaging at palayakadu. depth of about 29.3m with a resistivity range of (11to 45 ù m). this layer is due to the presence of weathered materials with greater risk of contamination (urish, 1983). an intermediate zone with resistivity range from 663 to 3328 ù m at a depth of 21.1 to 38.7 m is noted. a very high resistivity zone (125418 ù m) is observed at a depth of 55 m indicates fresh rock material without any structural pattern like folds and faults. resistivity distributions observed at greater depth are high when compared with low resistivity values observed at the surface. the profile therefore could be interpreted as the different layering of weathering with low resistivity values on the surface and higher resistivity values are confined to the fresh electrical resistant rock materials. conclusion an attempt was made in tirupur a highly industrialized zone to determine the extent of pollution in aquifers by using ert techniques. a total of 4 profiles were conducted more or less 1 km and parallel to river noyyal. the profile at valipalayam indicates contamination of top soil by a drop in resistivity by < 10 ù m at a depth of 8 m. the weathered and fractured zones identified at depth of 27 to 47 m with greater resistivity range indicate deeper layers without any groundwater contamination. the second profile conducted near pethichettipuram indicates sources of contamination at a depth of 7.91 m with a drop in resistivity <46.5 ù m. another drop in resistivity at a depth of 11.5 m indicates the contaminated nature of the deeper regolith. the fractured and massive rocks follow the same trend like that of the first location with greater resistivity. the third survey conducted in palayakadu indicates greater contamination of regolith at a depth of 0 to 20 m with a resistivity less than 40 ùm. bed rock resistivity was higher >1058 ù m indicating their massiveness present at shallow depth. the fourth survey conducted at chellapuram indicates high resistivity rock materials which are layered and boundaries of the different resistivity materials are clearly defined by the sharp change in electrical resistivity. the high resistivity observed is typical of fresh granite rocks. from a total of four profiles the first, third and fourth showed top 10 to 25 m of regolith has resistivity of less than 10 ù m, with top 5 m having a resistivity of less than 10 ùm indicating soil with greater contamination. the second profile has shown low resistivity at pockets at shallower depth and resistivity of above 100 ùm is not contaminated. none of the five images measured across the contaminated sites show any strong lateral change in resistivity and it must be admitted that similar information could be obtained with resistivity sounding. a few soundings over the 39 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india top soil weathered and fractured zone massive hard rock unit electrode spacing 5.00 m.resistivity in ohm.m 11.6 43.3 165 622 2344 883 33284 125418 inverse model resistivity section iteration 5 rms error = 4.8%depth 0.0 80.0 160 240 320 m. 13 10.8 21.7 29.3 38.7 50.6 65.3 figure 5. resistivity imaging at chellapuram. area can indicate likely sites for low resistivity regolith and heavy contamination. acknowledgements corresponding author thank the indian national science academy (insa) for the visiting fellowship award and the director, national geophysical research institute (ngri) for permission to work under dr.v.s.sarma. the author acknowledges the reviewer for his help to improve the ms in the present form. references abem, (2004). instruction manual, terrameter sas 4000/sas 1000. apparao, a. and sarma, v.s. (1981). a modified pseudo depth section as a tool in resistivity and ip prospecting, geophysical research bulletin, ngri, india, v. 19, 187-208. apparao, a. sarma, v.s. (1983). the concept of depth of detection and depth of investigation in resistivity and ip, geophysical research bulletin, ngri, india, no. 21, 51-61. apparao, a. subrahmanya sarma, v. sivasarama sastry, r. and srinivas, g.s. (1993). a catalogue of modified pseudodepth sections with different electrode arrays over buried conductive targets, ngri, hyderabad (india), 52 pp. barker, r. d. venkateswara, r.t. and thangarajan, m. (2001). delineation of contaminant zone through electrical resistivity imagine technique. current science journal. 81. no. 3, 277-283. central groundwater board (cgwb) (1993). groundwater resources and development prospects in coimbatore district, tamil nadu, central groundwater board, 30. daniels, f. and alberty, r.a. (1966). occam’s inversion to generate smooth, two dimensional models form magneto telluric data, geophysics. 55, 1613-1624. dey, a. and morrison, h.f. (1979a). resistivity modeling for arbitrary shaped twodimensional structures. geophysical prospecting. 27, 1020-1036. edwards, l.s. (1977). a modified pseudo section for resistivity and induced-polarization. geophysics. 42, 1020-1036. griffiths, d.h. and barker, r.d. (1993). two-dimensional resistivity imaging and modeling in areas of complex geology, applied geophysics. 29, 211-226. gustavsson, y., jacks, g., nilsson, l.y. and taranger, b. (1970). water resources and water supply in coimbatore district, tamil nadu, india, department of land improvement and drainage school of surveying, royal institute of technology, stockholm. jacob thomson (1998). impact of industries on the ground water quality of tiruppur and its ethical implications, ph.d. thesis, dept. of zoology, university of madras, chennai. keller, g.v. and frischknecht, f.c. (1966). electrical methods in geophysical prospecting, pergamon press, oxford. kelly w.e. (1976). geoelectric sounding for delineating groundwater contamination, ground water, 14, 6-10. kristina furn, (2004). effect of dyeing and bleaching industries on the area around the orathapalayam dam in southern india, uptec w04033, issn 1401-5765, tryckt hos institutionen för geovetenskaper, uppsala universitet, uppsala, pp 99. loke, m.h. (1997). electrical imaging surveys for environmental and engineering studies – a practical guide to 2d and 3d survey, penang, malaysia, universiti sains malaysia, unpublished short training course lecture notes. 40 srinivasamoorthy k, sarma vs, vasantavigar m, vijayaraghavan k, chidambaram s and rajivganthi r loke, m.h. (1997). res2dinv ver. 3.3 for windows 3.1, 95, and nt, advanced geosciences, inc, pp. 66. loke, m.h. (2004). rapid2-d resistivity and ip inversion using the least-square method; manual for res2dinv, version 3.54, p53. pathak b. and venkatesshwar rao,(2001). geoelectrical investigation to locate groundwater resources for urban development: a case study in parts of greater guwahati urban area, assam, geophysics, xxii-1:41-46. public works department (pwd) (2003). statement showing the hydraulic particulars of p.w.d. tanks, sub-division irrigation, coimbatore. raja guru, p and subbram v (2000), ground water quality in tiruppur, environmental awareness on quality management of irrigation water, edited by lakshmanaperumalsamy and krishnaraj, department of environmental sciences, bharathiyar university, coimbatore. sankaraaj, l., subramanian, t.p., siddhamalai, a. & farooque ahmed, n. (2002). quality of soil and water for agriculture in noyyal river basin, tamil nadu. joint director of agriculture (research), soil survey and land use organization, tamil nadu department of agriculture, coimbatore. senthilnathan, s and azeez, p.a. (1999). influence of dyeing and bleaching industries on ground water of tirupur, tamil nadu, india. bull environ contam toxicol, 62:330-335. silvestre, p.p. and ferrari, r.l. (1990). finite elements for electrical engineers, 2nd edition, cambridge university press. ssluo (soil survey & land use organisation) (1998). soil atlas coimbatore district, department of agriculture tamil nadu, coimbatore. urish, d.w. (1983).the practical application of surface electrical resistivity to detection of ground water pollution. groundwater, 21:144-152. 41 electrical imaging techniques for groundwater pollution studies: a case study from tamil nadu state, south india carlosvargaset al.indd 99 new geological and geophysical contributions in the section ibagué – armenia, central cordillera colombia carlos a. vargas1, andreas kammer1, mireya valdes2, carlos e. rodríguez1, alexander caneva3, jhon j. sánchez4, eliana arias5, carlos a. cortes6, héctor mora4 1 departamento de geociencias, universidad nacional de colombia – sede bogotá , colombia email: cavargasj@unal.edu.co 2 instituto colombiano del petroleo –icp, bucaramanga, colombia 3 universidad antonio nariño, dirección nacional de investigaciones, bogotá, colombia 4 red sismológica nacional de colombia, ingeominas, bogotá, colombia 5 incorporated research institutions for seismology – iris, washington dc, usa 6 universidad de manizales, facultad de ingeniería, manizales -colombia resumen ha sido interpretada la sección geológica entre las ciudades de ibagué y armenia basada en observaciones geofísicas y estructurales en busca de establecer el estilo estructural dominante y la configuración del basamento en profundidad. inicialmente se realizaron mediciones gravimétricas y magnetométricas levantadas cada 500 m entre estas ciudades, las cuales permitieron soportar el desarrollo del experimento sísmico “armenia-quake”. el alcance del experimento sísmico permitió detectar reflectores hasta aproximadamente 17 km de profundidad en un área de influencia de aproximadamente 25 km. la interpretación integral de la información geológica y geofísica permite concluir que el complejo cajamarca define una estructura sinclinal amplia cerca al municipio de cajamarca. partiendo de dicha información se pudo reconocer la presencia de un basamento compuesto por cuñas de rocas basálticas y sedimentarias asociadas a las formaciones quebradagrande y arquía. igualmente se reconoce la existencia de una gran flexión de la paleo-placa oceánica relacionada a la formación barroso, la cual se encuentra limitada al occidente por la falla cauca-almaguer. la estructura en flexión y la evolución compresiva del estado de esfuerzos en esta región de américa del sur podrían ser las responsables del depósito y posterior plegamiento de la formación cinta de piedra que constituye en gran medida la serranía de santa bárbara. palabras clave: armenia-quake, experimento de sísmica activa, modelamiento geofísico, serpentinitas y terremotos. manuscript received august 2005 paper accepted september 2005 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 99-109 100 carlos a. vargas, andreas kammer, mireya valdes, carlos e. rodríguez alexander caneva, jhon j. sánchez, eliana arias, carlos a. cortes, héctor mora abstract based on geophysical and structural observations a geologic section between the cities of ibagué and armenia (colombia) has been interpreted to establish the dominant structural style and the basement configuration at depth. gravimetric and magnetic measurement were initially taken each 500 m along these cities, which helped in the development of the “armenian-quake” seismic experiment. the experiment illuminated reflectors down to approximately 17 km deep in an influence area of approximately 25 km. the complete interpretation of the geological and geophysical information allows concluding that the cajamarca complex defines a wide synclinal near to cajamarca town. with this information initially it was possible to recognize the presence of a basement compound for wedges of basaltic and sedimentary rocks associated to the quebradagrande and arquía formations. then was recognized the existence of a great flexion of the oceanic paleo-plate related to the barroso formation, which is limited at west by the cauca-almaguer fault. the flexion structure and the evolution of the stress in this region of south america could be responsible for the deposit and later folding of the “cinta de piedra formation” that constitutes the santa bárbara range. key words: armenia-quake, seismic active experiment, geophysical modelling, serpentinites and earthquakes. © 2005 esrj unibiblos. as the town of montenegro. this section represents one of the best outcrops in the central cordillera of colombia where rocks of diverse composition and age can be observed. generally, the stratigraphic base displays metamorphic rocks from the cajamarca complex varying between amphibolic green schists, graphitic schists, phyllites and locally marble whose protolith age has been assigned to the early palaeozoic (figure 2). the pericos fault brought these rocks into contact to the east with the ibagué batholith, an intrusive body of granodiorites of jurassic age, and at to the west the san jerónimo fault brought into contact the cajamarca complex with meta-sedimentary rocks of marine origin that are interspersed with metabasalts from the quebradagrande complex of cretaceous age (maya and gonzález, 1995). the quebradagrande complex formed a tectonic wedge limited by the silvia-pijao fault to the west. to the western side of the silvia-pijao fault some meta-igneous rocks of continental origin and formally denominated the arquía complex become visible (gonzález and núñez, 1991), which in turn are limited to the west by the cauca-almaguer fault. again to the west of the latter appear meta-sedimentary and meta-basalt rocks of oceanic origin of cretaceous age and denominated barroso formation (maya and gonzález, 1995). the barroso formation and the quebradagrande complex are parts of the geological province associated to the same subduction event. introduction the tectonic configuration in the northern andes has been interpreted as the product of at least three orogenic events resulting from the accretion of oceanic crust in the western margin of nw south america (duke-caro, 1990; freymueller et al., 1993). petrological and geochemical evidences of this process have been reported along the section that runs along the road between the cities of armenia and ibagué (figure 1). according to these authors, the protolith that constitutes the lands in the central cordillera of colombia was originated from intra-oceanic rocks and sediments of continental margins that have undergone processes of metamorphism. all these rocks evolved into an orogenic regime associated to intrusive and extrusive bodies that emerged throughout fractures affecting the crust in the region. base on geophysical and structural measurements we defined the relation at depth and the current seismo-tectonic activity that could have triggered the january 25, 1999, mw=6.2 armenia earthquake (monsalve and vargas, 2002). geological framework new observations have allowed to complement the geologic mapping by nelson (1962) and restrepopace (1992) between the city of ibagué and the town of calarcá,and to extend the mapping as far 101 new geological and geophysical contributions in the section ibagué armenia, central cordillera colombia geological and geophysical observations data initially, we performed a lithological and structural survey in the section ibagué calarcá – montenegro. measurements every 500 m were taken and allowed the design of magnetometric and gravimetric profiles supporting the controlled source seismic experiment that we called “armenia quake”. this experiment was developed during june, 2004, detonating four explosive charges of indugel ap+ at the fayat quarry near calarcá. the charges ranked between 75 and 117 kg. in this seismic experiment 36 seismographs reftek-125 and triaxial geophones (f n = 4.5 hz) from the iris-passcal consortium were placed in 95 recording locations. additionally, roughly 2000 hypocentral solutions reported by the seismological observatory of quindío –osqwere used to support the geological and geophysical interpretations. figure 1. geophysical profiles between ibagué and armenia cities along the main road (black line). figure 2. geologic map of the study area. continuous red lines are mapped faults. dashed red lines are main lineaments. volcaniclastic unites (quaternary) cajamarca complex quebradagrande complex ibagué batholith sodimentary formations (terciary) barroso formation arquia complex armenia earthquake (mw=6.2;25-01-99) 102 carlos a. vargas, andreas kammer, mireya valdes, carlos e. rodríguez alexander caneva, jhon j. sánchez, eliana arias, carlos a. cortes, héctor mora method preliminary interpretation of lithological and structural information suggests geological continuity at depth for some structures that are present in the region. further processing using the talwani method (talwani, 1959), of gravimetric and magnetometric data along an iterative adjust allowed to find a geological model that agrees with field observations. although the seismic experiment was designed to map a 45 km-deep section, the location of recording stations in urban areas with high noise levels made difficult quality results beyond 17 km depth. the selected seismic records were processed with a fourth order bandwidth filter (5 – 40 hz), properly edited and stacked to assemble a section, and then corrected by linear move out (lmo). the high topographical contrasts, the lateral variations of velocity and the complex geometry of the geological structures impeded an efficient application of normal move out (nmo) and migration techniques. the section was used to improve the geologic pattern and the corresponding gravimetric and magnetometric models until a model that agreed with the observations was found. results the adjustments of the geologic model with the geophysical information allowed the detection of a subduction structure probably affected by the prevalent orogenic processes from the mesozoic age in the nw andes of south america (figure 3a). it is possible to appreciate that the gravimetric and magnetometric information based on the physical characteristics of the rocks in this area (table 1), is well adjusted to the proposed geologic model. table 1. density and magnetic susceptibility values assigned to the rocks of the different geological units in the study area. geological formation lithology density (g/cm3) magnetic susceptibility (emu) 1. ibagué batholith granodiorite rocks 2.65 0.00030 2. cajamarca complex amphibolic green schists, graphitic schists, phyllites and marbles 2.75 0.00010 3. barroso formation basalts and tuffes 2.95 0.00250 4. subduction complex (arquía compex) serpentinites 3.00 0.00440 5. subduction complex (arquía compex) amphibolites 3.00 0.00011 6. quebradagrande complex basalts and tuffes 2.90 0.00900 7. oceanic paleo-plate peridotites 3.15 0.01000 8. continental crust amphibolites 2.80 0.00550 in this scenario, the cajamarca complex appears to be related with a synclinal structure limited by the pericos and san jerónimo faults, and immersed in a fractured continental crust. the structures associated to the paleo-subduction process have been transformed into wedges to form the quebradagrande complex and the barroso formation. probably the lithologies and structures located between the san jerónimo and silvia-pijao faults correspond to the limit area where an old subduction process happened. indeed, the presence of serpentine belts evidences the exhumation in the area where the old oceanic plate associated to the barroso formation penetrated. the seismic section (figures 3b and 3c) highlights the reflectors that define the wedge of the quebradagrande complex to depths beyond 10 km. on the other hand, the distribution of aftershocks related to armenia earthquake and reported by the seismological observatory of quindío (osq) suggests that the rupture process for this earthquake could be associated to weak planes in the old subduction zone (figure 4). therefore, the silvia – pijao fault may play a relevant role for the seismotectonic regime in region. the rupture process in the area, however, could be related to the same mechanisms responsible for the presence of serpentinite rocks with weak planes developed along the fibrous structure and parallel to the silvia pijao fault. 103 new geological and geophysical contributions in the section ibagué armenia, central cordillera colombia (a) (b) (c) figure 3. geophysical interpretation of the ibagué – armenia section. (a) density and magnetic susceptibility models. (b) seismic profile processed with lmo. distance in meters from calarcá town. (c) superposition of seismic profile onto geological model. 1. granodiorites: ibagué batholithe 2. metamorphites: (cajamarca complex) 3. basalts and tuffes (barroso formation) 4. serpentinites (subduction complex arquia complex) 5. amphibolites (subduction complex rquiacomplex) 6. basalts and tuffes (quebradagrande complex) 7. peridotites (oceanic paleo-plate) 8. amphibolites (continental crust) 104 carlos a. vargas, andreas kammer, mireya valdes, carlos e. rodríguez alexander caneva, jhon j. sánchez, eliana arias, carlos a. cortes, héctor mora figure 4. aftershocks (crosses) of the armenia earthquake (white star) superposed on the geologic section. the hypocentral solutions were reported by the osq. interpretation of the geologic section ibagué armenia this section exposes in short distances lithologies of different tectonic blocks that became the basement of the central cordillera (from east to west). this basement is composed of a metamorphic complex of low pressure temperature (p/t) conditions formed form rocks of the cajamarca complex, and rocks of high p/t ratio associated to gneiss and amphibolites of tierradentro (mccourt, 1984). both complexes are separated by the pericos fault. to the east of this fault the metamorphic basement of high p/t shows remnants of a palaeozoic cover (metasedimentary rocks of santa teresa). the ibagué batholith intruded along this contact during a magmatic event during the triassic – jurassic limit. these two rocky units form a landscape characterized by a moderate relief and a dendritic drainage with a trend nw-se (perpendicular to the direction of these units) that stands out. the cajamarca complex defines a wide synclinal near to the town of cajamarca, in this place the rocks are heavily metamorphosed and folded. our profile, however, does not have enough resolution to show the folds that are well documented in other works (nelson, 1962; restrepo-pace, 1992). near to the “la línea” sector (figure 1) this metamorphic sequence dips moderatelly toward east, defining a wide monoclinal (figure 3a). in this place appears the “la línea gneiss”, an intrusive body that constitutes a high to medium p/t ratio transition between basements. the landscape conformed by the cajamarca complex is dissected and is characterized by a rough topography. the drainage is segmented, with several families of alignments standing out. again in this zone there are notable perpendicular (nw-se) and parallel (sw-ne) alignments to the general direction. other alignments show trends n-s and e-w. as a very specific feature, this tectonic block is affected by an anomalously silicic volcanism that is best represented by cerro machín volcano. this volcano is formed for a complex of domes surrounded by thick sequences of piroclastic flow deposits. the crater has a diameter of 2.4 km and is occupied by dacitic domes with fumarolic activity (cepeda et al., 1995; cepeda and cortés, 1999). this volcano shows an activity restricted to the holocene (about 10.000 years) that is highly explosive of plinian type. the eruptions generated important piroclastic flows, which constitute continuous deposits in a diameter of 15 km from the volcanic amphitheater. between 105 new geological and geophysical contributions in the section ibagué armenia, central cordillera colombia the city of ibagué and the high parts of the toche, amaine and bermejón rivers seven piroclastic flows inserted in layers of ash whose ages vary among 45.000 to 800 years a.p. have been identified (méndez, 2001). near to “la línea”, the san jerónimo fault (also termed the aranzazu-manizales fault) is the limit between the metamorphic basement of the central cordillera and a tectonic complex that becomes a big flake roughly 5 km thick, composed of basaltic and piroclastic rocks of mid-cretaceous age belonging to the quebradagrande complex. these rocks are on the metamorphic basement (cajamarca complex) and are characterized by a marked dipping toward east and by multiple fractures. the intense fragile deformation, the inclination of the beds and the marked topographical relief of the western flank of the central cordillera makes this unit susceptible to the erosion, rock falls and big landslides. to the west of the quebradagrande complex appears the arquía complex, which constitutes the rock basement of the city of armenia and the town of calarcá. because of the strong inclination toward the east, the arquía complex belongs structurally to the quebradagrande complex. it is composed of basement flakes with thickness of less than 2 km that are inserted with serpentines. the basement is composed predominantly of hornblendic schists. to the east the quebradagrande complex is intruded by an elongated granodiorite body (the córdoba stock). the contact of this body with the quebradagrande complex is the silvia-pijao fault, with an area of deformation of several hundreds of meters wide. in the western area, the rocky basement is composed by basaltic rocks of the amaime formation that show a bigger flexion to the west of the cauca-almaguer fault and are dipping east. in the western foothill of santa bárbara range the rocks are affected by the quebradanueva fault that folded a paleogene sedimentary sequence called the “cinta de piedra” formation (mccourt et al., 1984). this sedimentary unit formed a barrier for the drainage that runs from the central cordillera and its existence explains how the volcanoclastic flows came from machín and tolima volcanoes and deposited its materials on the actual quindío glacis. “la línea” structures to the east of the san jerónimo fault the rocks of the central cordillera are dipping consistently toward the east. the dip of the sedimentary rocks of abejorral formation (gonzález and núñez, 1991) is affected by a set of normal faults that causes a sequence to be repeated (the “fault system effect”). figure 5 shows these fringes east of the campanario fault (a local name for the san jerónimo fault). the “fault system effect” is observed in a structural section some kilometres south from “la linea” cutting the cental cordillera in a domino-like fashion (figure 6). the spacing between faults varies between 1 to 2 km, and their continuity suggests a cut toward “la línea tunnel”. the “la linea” tunnel promises to be the most important civil work to improve the transportation system in the central region of colombia, thus considerations about the fault system are relevant. by extrapolating these faults as far as the bermejón river valley makes evident that at least four (five?) of these faults are cutting the route planned for this important ground transportation structure (figure 7). the prolongation of the geologic profile 15 kilometers west from our lithologic, structural, and geophysical observations, suggests that the cauca-almaguer fault could be the structure that facilitated the elastic energy dissipation related to the flexion process of the oceanic paleo-plate that was originally subducted (figure 8). it is possible that the continental rocks of tertiary age (“cinta de piedra” formation) allowed the deposition of sedimentary basins with n-s orientation that are visible today along the fault. 106 carlos a. vargas, andreas kammer, mireya valdes, carlos e. rodríguez alexander caneva, jhon j. sánchez, eliana arias, carlos a. cortes, héctor mora figure 5. geologic map of the region south of armenia city. figure 6. geologic section at south of the town of calarcá, where it is possible to see a set of antitetic faults that cut the central cordillera. legend alluvial fans upper eocene to recent continental deposits paleogene to neogene fluvial deposits middle eocene intrusive and protusive complex córdoba stocks (quarte diorites, gabbros) chlorite-actinolite schists, serpentinites quebradagrande group meta basalts and pyroclastics rocks with sedimentary layers basal non-volcanic sediments amaime group basalts, diabases, pyroclastics rocks, sedimentary layers abejorral formation siltstones, shales, pyroclastics rocks cajamarca group reverse fault anticline syncline 107 new geological and geophysical contributions in the section ibagué armenia, central cordillera colombia figure 7. topographic map of the region near “la línea” tunnel (line with open circles). at least four (five?)normal faults are cutting the route designed for this transport structure. figure 8. geologic section (interpreted and extended) west of the study area. 108 carlos a. vargas, andreas kammer, mireya valdes, carlos e. rodríguez alexander caneva, jhon j. sánchez, eliana arias, carlos a. cortes, héctor mora references • barrero, d. & vesga, c. j., 1976. mapa geológico del cuadrángulo k-9 armero y mitad sur del cuadrángulo j-9 la dorada. escala 1:100.000. ingeominas. bogotá. • cepeda, h., méndez, r., murcia, l. a. núñez, a. & monsalve, m.l., 1995. volcán cerro machín, departamento del tolima, colombia: pasado, presente y futuro. ingeominas, informe interno. • cepeda, h. & cortés, g. p. (1999). el cerro machín, un volcán holocénico explosivo en el centro de colombia. iv reunión técnica de la comisión de geofísica, departamento de geodesia y topografía, tucumánargentina. • duque-caro, h. (1990). the choco block in the northwestern corner of south america: structural, tectonostratigraphic, and paleogeographic implications. j. sout am. earth sci., 3, 71-84 p. • freymueller, j.t., kellog, j.n. & vega, v. (1993). plate motions in the north andean region. j. geophys. res. 98: 21853-21863. • gonzález, h. & núñez, a. (1991). mapa geológico generalizado del departamento del quindío, escala 1:100.000, memoria explicativa, 42 p., ingeominas, bogotá. • kammer, a., 1990. estructuras y deformaciones de los sedimentos cretáceos de la formación abejorral, departamento de antioquia, cordillera central. boletín de geología 34: 33-44, bucaramanga. • maya, m. & gonzález, h. (1995). unidades litodémicas de la cordillera central de colombia. ingeominas. boletín geológico. vol 35, 2 –3, 43-57. • mccourt, w. (1984). the geology of the central cordillera in the department of valle del cauca, quindío and nw tolima (sheets 243, 261, 280 and 300) ingeominas, cali, 49 p., inédito. • mc court, w.j., aspden, j.a. & brook, m. (1984). new geological and geochronological data from the colombian andes: continental growth by multiple accretion. j. geol. soc. london, 141 (5), 831 – 845. • mendez, r. a. (2001). informe sobre la geología y estratigrafía de flujos piroclásticos asociados al vcm. ingeominas, informe interno, inédito. manizales. 35 p. • monsalve, h. & vargas, c. a. (2002). el sismo de armenia, colombia (mw=6.2) del 25 de enero de 1999. un análisis telesísmico de ondas de cuerpo, observaciones de campo y aspectos sismotectónicos. revista geofísica – ipgh, 57:21-57. conclusion geologic and geophysical data used in this study suggest that the cajamarca complex defines a wide synclinal near the town of cajamarca. its structural configuration allows the assemblage to the west of a basement composed by wedges of basaltic and sedimentary rocks of the quebradagrande and arquía formations. new structural evidence supported by gravimetric, magnetrometric and seismic data suggests the existence of a bigger flexion of the barroso formation west of the cauca-almaguer fault which is dipping to the east. the flexion could be a factor for the deposition and folding of the “cinta de piedra” formation in the “santa bárbara” range. this sedimentary unit formed a barrier for the drainages that run from the central cordillera and its existence explain how the volcanoclastic flows came down from machín and tolima volcanoes and deposited their materials on the recent quindío glacis. acknowledgments we wish to thank sincerely many people and institutions that made this work possible. thanks to professors l. h. ochoa, l. castillo, l.a. montes and l.a ordoñez and their students of the graduate programs of geophysics and geotechnics and the undergraduate programs of geology and civil engineering of universidad nacional de colombia. much field work efforts were advanced by students of civil and electronic engineering departments of universidad del quindío under the direction of m. gómez, j. p. durán and c. correa. many thanks to corporación autónoma regional del quindío (crq), colciencias, and alcaldía de armenia for the financial support and particularly to a. lopezreina, a. duque and c. alvarez for their logistic and administrative help. we recognize the invaluable help in the field by ingeominas (manizales) especially to c. carvajal and m. londoño. thanks to iris-passcal inc. (bob greschke), universidad antonio nariño (dirección nacional de investigaciones) and l. sanchez of the german institute for geodesic research. 109 new geological and geophysical contributions in the section ibagué armenia, central cordillera colombia • nelson, h.w. (1962). contribución al conocimiento de la cordillera central de colombia sección entre ibagué y armenia. informe no. 1000. bol. geol.. vol. x (1-3): 161-202. ingeominas, bogotá. • núñez, a. & gonzález, h. (1991). memorias mapa geológico generalizado del departamento del quindío. ingeominas, bogotá. • restrepo-pace, p. (1992). petrotectonic characterization of the central andean terrane, colombia. j. south am. earth sci., 5, 97-116 p. • talwani, m. (1959). rapid gravity computations for two dimensional bodies with application to the mendocino submarine fracture zone., journal geophysical research., p 49-59. an aeromagnetic reconnaissance study is presented to delineate the subsurface structure and tectonic setting at the fayoum-cairo district, which experienced a damaging earthquake on october 12, 1992 of magnitude (m = 5.7). analysis of aeromagnetic and seismicity data demonstrate three significant tectonic faults with trending to the ne-sw, nw-se and e-w. the basement is uplifted in the northern and central parts with a depth of 1.3 km, and deepening in the southern part with a depth of 2.5 km. this is a seismically active zone and historically has experienced damaging earthquakes. in 1847, a damaging earthquake with maximum epicentral intensity (modified mercalli intensity (mmi) = vii) was located there. on the eastern side, earthquake sources are well recognized at different locations. these sources created events of moderate size magnitude m < 5. the focal mechanisms of the major events from these sources are generally strike-slip with normal component. the focal mechanism of the earthquake on october 12, 1992 is normal fault type with strike trends in the nw-se direction. these fault plane solutions are consistent with the tectonic trends derived from the aeromagnetic data mentioned above and suggest that the new tectonics of northeast africa is predominant. este estudio presenta una exploración aeromagnética para delimitar la estructura subsuperficial y el marco tectónico del distrito de fayoum-cairo, que sufrió un terremoto el 12 de octubre de 1992 de magnitud m=5.7. los análisis de datos sismicidad y aeromagnéticos señalan tres fallas tectónicas significativas con tendencias ne-so, no-se y e-o. el subsuelo se elevó en el norte y en el centro a una profundidad de 1,3 kilómetros, y se hundió en el sur con una profundidad de 2,5 kilómetros. esta es una zona sísmicamente activa que en su historia ha tenido terremotos dañinos. en 1847 tuvo lugar un terremoto con intensidad epicentral máxima (escala sismológica de mercalli) de vii. en el lado este se estudiaron las fuentes de terremotos en diferentes partes. estas fuentes crearon eventos de magnitud moderada m<5. los mecanismos focales de los eventos principales en estas fuentes son generalmente fallas de desgarre con componente normal. el mecanismo focal del terremoto del 12 de octubre de 1992 es una falla tipo normal con fuertes movimientos no-se. las soluciones de estas fallas son consistentes con las tendencias tectónicas de los datos aeromagnéticos antes mencionados y sugieren que la nueva tectónica del noreste de áfrica es predominante. analysis of aeromagnetic data for interpretation of seismicity at fayoum-cairo area, egypt ahmed khalil1,2, mostafa toni3, awad hassoup1 and khamis mansour1,2 1 national research institute of astronomy and geophysics, helwan, egypt 2 laboratory of exploration geophysics, earth resource engineering department, faculty of engineering, kyushu university, fukuoka, japan 3 geology dept., faculty of science, helwan university, cairo, egypt earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 7 13 abstract resumen key words: seismicity, aeromagnetic data, structural trends, fayoum-cairo, egypt. palabras clave: sismicidad, datos aeromagnéticos, tendencias estructurales, fayum-el cairo, egipto. record manuscript received: 30/01/2013 accepted for publication: 23/05/2014 electrical and magnetic methods 8 ahmed khalil, mostafa toni, awad hassoup and khamis mansour introduction studying the structure of fayoum-cairo area has become significant as the basement and its fraction played the main role in the seismic activity constructing the geological framework of southern egypt. the magnetic method is believed to be efficient in this respect (hassoup, et al., 2009), so the magnetic exploration has been preformed aiming to find out the subsurface structures set up at the dahshour area. this area lies between the latitudes 28°8 and 30°00 n and longitudes 30° 4 and 32° 00 e in egypt (fig. 1). it is generally characterized by surface sedimentary rocks that can physiographically be divided into three main units: the qarun lake, the plains of ancient terraces, and the plateau surface with irregular scarped faces. the qarun-lake and its surrounding cultivated lands cover the major part of the fayoum depression. there exist beach terraces of different levels and widths around the qarun-lake which extend till the scarpment of qattrani hill (353 m). this area is structurally controlled by a set of normal faults having long periods of growth and some of these faults manifest strike-slip movements. deep drilling in the northern part of the western desert has exhibited large numbers of swells and basins (said, 1990). meshref (1990) states that the basement rocks in the western desert of egypt have been affected by the oldest e-w and ene trending faults which in turn intersect with the younger nw and nnw trending faults. these two fault systems have large vertical and horizontal displacements. the dahshourqattrani area is particularly affected by the nw and e-w-trending faults. the nw-trending fault, called the qattrani fault, has a throw of about 350 m. however, the e-w trending fault, called the g. sheeb fault, has a throw between 150 and 200 m. the oligocene basalt occupies these two faults. figure 1: study area map and the 1992 dahshour earthquake. on the basis of geophysical data (i.e., seismic and gravity data), abu el-ata (1990), outlined three structural heights and two low: 1the abu roash high that strikes in the n-ne-s-sw and e-new-sw directions. 2el-sagha height which is oriented into nw-se directions. 3the el-faras-el-fayoum height, which is oriented in e-ne-wsw and n-nw-s-sw directions. ghazala (2001) concluded that four significant tectonic zones characterize this area: the graben of nile valley, the uplift at the east nile valley, the ginidi basin and the kattaniya uplift. the study of the subsurface structure of the fayoum-cairo area is significant to understand relationship between the basement, its fraction and seismic activities at the area. the dahshour area is highly deformed with fault systems into different directions (said 1990). the most predominant trend is the nw-se followed by the e-w trend. according to (naim, et al., 1993), the area is mainly affected by the nw-se fault trend in the gulf of suez represented by the qattrani fault of normal type, and the mediterranean e-w trend represented by the gabal el-sheeb fault (fig. 2). figure 2: surface and subsurface faults around dahshour area (developed by moustafa and takenaka, 2009; data derived from egsma, 1981). w.d.: western desert, e.d.: eastern desert seismicity the area under investigation experienced a moderate sized earthquake on august 7, 1847 (fayoum earthquake) with epicentral intensity mmi = vii (maamoun et al., 1984). on the basis of instrumental records, this area is characterized by moderate seismicity of m < 6 (fig. 3a). earthquakes are distributed in a large area, which extends from dahshour (on the western side of the river nile to the red sea area and the large-magnitude recurrence is quite big (maamoun et al., 1984; abou elenean, 1997). the earthquake distributions were developed on the basis of records of helwan observatory (1900~1997) and the recent established egyptian national seismological network (ensn) from 1997 to 2012. the fault plane solution of the 1992 earthquake main shock and spatial distribution of its aftershocks imply that this event is characterized by normal faulting with a slight strike -slip component (el-hadidy, 1993; hussein et al., 1998). abou elenean (1997), using the epicentral distribution, seismicity level, and the similarity of focal mechanisms, considered the area of dahshour as a seismogenic zone. tectonically, the faults of this area tend into e-w, parallel to the mediterranean trend, or n-s parallel to the gulf of suez trend. the nw-se and e-w directions correspond with the surface features, which appeared immediately after the occurrence of 1992 earthquake (japanese expert team, 1993). the majority of earthquakes occurred along the welldefined surface faults with e-w to w-nw orientation as shown in (fig. 2). the epicenters and focal mechanisms of the earthquake on october 12, 1992 and three other events recently occurred around the area, are shown in (fig. 3b). three of these seismic events (june 29, 2000, local magnitude scale (ml) = 4; july 07, 2005, ml = 4.2; and october 30, 2007, ml = 3.7) occurred at the east of the dahshour area inside the suez-cairo shear zone (abou elenean et al., 2009). focal mechanisms of all these events are normal faults with strike-slip movements. cairo 9analysis of aeromagnetic data for interpretation of seismicity at fayoum-cairo area, egypt figure 3a: seismicity map of dahshour area showing epicenters of seismic events from 1900-2012. figure 3b: epicenters and focal mechanisms of seismic events around dahshour area (abou elenean et al., 2009). the earthquake on october 12, 1992 (body wave magnitude (mb) =5.8, epicentral intensity mmi = viii), occurred in dahshour. it caused tremendous damage performing major disaster in the nile delta area. it was noticed in all over egypt, from alexandria to aswan, and even in some parts of surrounding countries (hussein et. al., 1996; abd el-aziz, 2008). the intensity distribution map of this event, according to the modified mercalli intensity scale (mmi), is shown in (fig. 4); intensity was estimated (mmi = vi-vii) in cairo, giza, and fayoum, where maximum level of damage was reported. the report of japanese expert team, (1993) estimated that about 8300 dwellings were destroyed, 561 people were killed and 6500 were injured; an official investigation revealed (japanese expert team, 1993) that 1343 schools were damaged beyond any repair, 2544 had required major repair and had required repair or maintenance; the economic losses caused by this earthquake were estimated at more than us$ 35 million dollars. the earthquake shaking caused liquefaction of the nile deposits at many sites near the epicenter ( khater, 1992; thenhaus et al., 1993). recent estimations of seismic hazard (riad et al., 2000; moharram et al., 2008; el-hadidy, 2012) show that for 10% probability of exceedence in 50 years (a return period of 475 years), thus the area of study is characterized by moderate level of seismic hazard (pga=50-100 cm/s2) (fig. 5). figure 4: map of intensity distribution of the earthquake on october 12, 1992 (thenhaus et al., 1993) shows agricultural lands as light grey areas and urban regions as dark grey area. figure 5: mean peak ground acceleration (cm/sec²) in egypt with 10% probability of exceedance in 50 year (475 year return period) (el-hadidy, 2012). 10 ahmed khalil, mostafa toni, awad hassoup and khamis mansour aeromagnetic data generally, the qualitative interpretation for the magnetic maps, aims to get a clear view of the subsurface structures and an estimation of the relative depth of magnetic anomalies sources. it deals with the description of anomalies, especially their symmetry, strike, extension, width, amplitude and gradients (nettelton, 1976). the aeromagnetic data is provided by the western digital company (1983). these data were corrected prior to our working with them. different processing methods are applied to the aeromagnetic data. the processing in this study was preliminary carried out by reduction of the magnetic northern pole (rtp), in order to overcome the undesired distortion of the shapes, sizes and locations of the magnetic anomalies; an average magnetic inclination of 41° was used for the area, to eliminate the obliquity of the inclination of the earth’s magnetic field. the filters technique (regional, residual and band pass filters) were used in the interpretation and delineation of shallow and deep subsurface structures of the studied area. the quantitative interpretation has been used to determine the depths of shallow subsurface structures (faults and dykes), basaltic intrusions, as well as the basement complex of the considered area; the radially averaged power spectrum is the used method for interpretations. the analysis and processing of the aeromagnetic data were done through specialized computer program geosoft: oasis montaj version 6.4.2, (geosoft, 2007). the aeromagnetic map of total intensity (fig. 6) exhibits some positive (red colored) and negative (blue colored) anomalies, and reveals a prominent positive feature, as a circular shape, located at the center of the map area. this anomaly of major positive amplitude is called agnes (red colored); the north part of the map is occupied by a magnetic belt majorly positive, divided into two positive closures and some positive small noises. the first anomaly tends nearly into the ne-sw with maximum positive amplitude at the northwestern corner; the second one is irregularly shaped at the northeastern part, with maximum positive amplitude of moderate positive anomalies are included between the foregoing positive closures, especially around agnes anomaly; however, the southern part of the map area reveals major negative magnetic belts. the first one is nearly a ne-sw negative magnetic anomaly extending from the southeastern to the southwestern of the map (blue colored); this negative belt exhibits more than six local closures with negative amplitudes and these anomalies also demonstrated condensed noise in their contour lines. the southwestern corner of this belt is occupied by an elongated negative closure with low frequency and low amplitude (blue colored); moreover, the nw-se moderately positive magnetic belt, lies between the two negative anomalies and the moderately steep magnetic gradient delimits the previous southern negative magnetic belt and the northern positive magnetic belt. figure 6: aeromagnetic map of anomalies in nt (compiled by the western digital company, 1983). aeromagnetic data processing reduction to the magnetic north pole. a reduction-to-the pole (rtp) transformation is typically applied to the magnetic data of total intensity to minimize the polarity effects (blakely, 1996). rtp is a filtering technique used to align the peaks and gradients of magnetic anomalies directly over their sources. in this study, the aeromagnetic anomaly data about total intensity reduce the magnetic pole (rtp) (fig.7) through geosoft: oasis montaj version 6.4.2 (geosoft, 2007). this map (fig.7) reflects the northward shift in the positions of the inherited magnetic anomalies, due to the elimination of the inclination and their declination of the magnetic field at this area; likewise, the sizes of the anomalies become larger and centered to some extent, over their respective causative bodies. in addition, the magnetic gradients became more intensive and steep, as well as the anomalies reliefs increase, giving rise to a higher resolution in the lithologic and structural inferences encountered. several zones with high and low magnetic values are present; the magnetic highs are separated from magnetic lows by steep gradients. the elongations of magnetic contour lines and their gradients at center of the area indicate that it is structurally-controlled by faults having major axes in nwse and e-w directions. the high magnetic anomalies in the northwestern side can be attributed to the occurrence of basic intrusions of the subsurface with high magnetic contents; the axes of the fault as well as the directions of magnetic anomalies are trend to the nw-se and ne-sw in the northwestern, central and the southern parts. a further research of the rtp aeromagnetic map (fig. 7) includes several local anomalies in the central, southwestern and northeastern parts; these anomalies have different reliefs, polarities and shapes. the general magnetic trends of these regions are almost nw-se, nesw and e-w. figure 7: rtp aeromagnetic map of anomalies in nt. digital filtering frequency filtering represents a major component of magnetic data processing. as a rule, digital filters are used for signal enhancement that is to remove unwanted noises and enhance desired signals. the nature of “noises” and “signals” varies from case to case. the filtering technique in this study was performed using a cut-off frequency that ranges between 0.08 cycle/ unit data and 2.0 cycle/ unit data. the map after high-pass filtered (fig. 8) elucidates high-frequency and short-wavelength spot-like magnetic anomalies, which are inferred as residual components. the map (residual) with high-pass component clearly shows several clusters of positive and negative magnetic anomalies, with higher resolution than those on rtp map. the local variations in both frequency and amplitude of these anomalies may be due to the difference in their compositions and/or the relative depths of 11analysis of aeromagnetic data for interpretation of seismicity at fayoum-cairo area, egypt their sources. the major trends (e-w, nw-se, n-nw-sse and ne-sw) are distinguished for the near-surface structures. this indicates that the trend of prominent faults (the trend on the rtp aeromagnetic map) extended in the subsurface up to shallow depths. moreover, the random orientation of small scale anomalies reflecting that the shallow subsurface has been affected by different stress regimes of the neo-tectonics that may have not affected deep – seated rocks. figure 8: aeromagnetic map after high-pass filter in nt. the anomaly map with low-pass magnetic filter (fig. 9) shows a pattern of gradual rotation of magnetic trends from ne-sw and nw-se towards e-w trend. the low-pass filtered anomalies give view about the subsurface depth, thus the gradual rotation of trends implies that suggested stress regimes (ne-sw and ne-sw) were contained in shallow depths, where the e-w trend dominates only in the deepest part. the deep-seated zone seems to be affected by an e-w stress trend. also the map (regional) with low-pass magnetic component presents positive and negative magnetic anomalies with deep-seated high amplitude. the deep zone seems to be affected by the e w stress trend. figure 9: aeromagnetic map with low-pass filter in nt. the anomaly map with band-pass magnetic filter (fig. 10) shows that still persist well-defined trends of anomalies on the rtp aeromagnetic map. however, some regional anomalies that do not appear to be related to a subsurface structure, are most probably a result of regional variations in the magnetization or magnetic susceptibility of the rocks at medieval depths. the band magnetic filtered anomaly map shows that the well defined trends of anomalies in the aeromagnetic map still persist. figure 10: aeromagnetic map with band–pass filter in nt. analytical 3d signal method the analytical signal method is a powerful technique to evaluate buried structures which cause significant linear magnetic anomalies, such as fault zones, steps and dykes (saadi et al., 2008). the map with analytical signal (fig. 11) was calculated through geosoft: oasis montaj software version 6.4.2 (geosoft, 2007). this map mostly reveals a number of elongated high anomalies tending to e-w, which is characterized by increasing the small undulations in their contour lines. these anomalies are separated by a lot of local elements of high relief, small area extensions and elongated shapes. this map (fig.11) could be directly used to obtain the corresponding value of the expected source bodies, in respect to the level of observations and it also displays several circular and linear maxima closures. figure 11: signal analysis on aeromagnetic map with rtp in nt/km2. 12 ahmed khalil, mostafa toni, awad hassoup and khamis mansour analysis of power spectrum transformation the method of radial average power spectrum is used to determine the depths of volcanic intrusions, depths of the basement complex and the subsurface geological structures. several authors, such as bhattacharya (1965), spector and grant (1970), garcia and ness (1994), maurizio et al., (1998), explain the spectral analysis technique based on the analysis of the magnetic data using the fourier transform. for this study, the fast fourier transform (fft) was applied on the aeromagnetic data with rtp, to calculate the energy spectrum. as a result, a two-dimensional power spectrum curve was obtained (fig. 12) on which two main average levels (interfaces) with depths of 0.5km and1.8km below the measuring level (for the aeromagnetic map with rtp) were revealed for the deep seated components and also the surface magnetic components respectively. the depth estimations on the aeromagnetic map with rtp indicated that, the depth on the top of the basement complex lies at 1.3 km, while the depth to the basement intrusion is at depth of 2.5 km, below the measuring level. figure 12: radial average power spectrum and the resultant depth estimation on the aeromagnetic map of anomalies with rtp. structural trends analysis the fayoum–cairo district lies in the south western of cairo, egypt which has been affected by several damaging earthquakes and magnetic data have been analyzed to provide new information about the tectonic setting and subsurface structures. structural trend analysis techniques are frequently used in various fields of geology and geophysics for defining structural problems. in fact the distinctness with which faults appear on the magnetic map depends principally on the existence and the strength of magnetic contrast in the body rocks involved. the aeromagnetic map of anomalies with rtp and the residual anomalies map were interpreted to determine the common structural trends affecting the study area. the azimuth and length of each detected lineament on different maps probably represent the different lengths and directions of faults and/or contacts. these structure systems were statistically analyzed and plotted in the form of rose diagrams, as shown in (fig.13). examination of these diagrams indicates three predominant structural trends which have varying intensities and lengths. the predominant tectonic trends are the n-w, n-e, and e-w, which produce affectation as deduced from the magnetic point of view, by the other hand, other minor structural trends on the rose diagrams such as those: n-s, n-ne, and e-ne are less significant in this area. figure 13: rose diagram showing structural trends systems analyzed and plotted. conclusion this study is devoted to the analysis of aeromagnetic data for seismicity interpretation at dahshour area, egypt, in order to detect major structural elements on subsurface which affect both, the sedimentary section and the underlying basement complex. close examination of the different anomalies through the aeromagnetic map with rtp, reveals the dominance of negative magnetic anomalies in southeastern. also, the differences in magnetic mitigation between each two adjacent magnetic highs and lows, suggest a comparable variation of composition of the subsurface rocks: these anomalies may be attributed to the occurrence of basic intrusions in the subsurface which have high magnetic content at different depths. the analysis of filtered magnetic maps is characterized by the presence of prominent trends in central part, with nw–se and ne–sw directions; these faults may be responsible of the earthquakes occurred. the basement is uplifted in northern and central parts with a depth of 1.3 km, and becoming deeper in southern with a 2.5 km depth. earthquakes have been occurred in this faulting area with high correlation between seismicity distribution and the inverted tectonic trends: this suggests that the n-nw-s-se directions on the two nodal planes are fault planes (fig. 3b). in contrast, the intensity distribution map does not show correlation with the inverted tectonic trends from the aeromagnetic data presented here where the isoseismals elongate into north-south direction. this is mainly due to the surface soil formation as site effect (i. e., the recent alluvium deposits elongate in the north-south trend, covering the nile river and its valley). these formation controls much of the intensity distribution map (fig. 5). north east 0 90 180 270 13analysis of aeromagnetic data for interpretation of seismicity at fayoum-cairo area, egypt references abd el-aziz, k. 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(1990). the geology of egypt. rotterdam brookfield: a. a. balkema press, the netherlands. 734 pp. spector, a., grant, f. s.(1970):statistical models for interpreting aeromagnetic data. geophysics, 35, pp. 293–302. thenhaus, p. c., sharp, r. v., celebi, m., ibrahim, a. b. k., van de pol, h. (1993). reconnaissance report on the 12 october 1992 dahshour, egypt, earthquake. united states geological service (usgs) survey open file report, pp. 93-181. using neuronal networks to determine site response from coda waves: application in armenia, colombia earth sciences research journal earth sci. res. j. vol 8 nº 1 (dec. 2004): 52 55 manuscript received july 2004 paper accepted octubre 2004 52 ossodas, a portable digital system for seismological signal acquisition pedro riascos1, andres racines2, jhon caicedo 3, jorge mejia4 & hans meyer 5. e-mail address: pepar@osso.org.co observatorio sismológico del sur occidente – osso/uv universidad del valle, facultad de ingeniería corporación osso §www:http://www.osso.org.co e-mail:corposso@osso.org.co tel : 57(2)6821078, 57(2)6827662 fax: 57(2)6821078, 57(2)3313418 cali colombia sudamérica abstract this paper presents the concept and design of a digital system to record seismological signals. the hardware and software designed has been interfaced to the pc 104 bus architecture in order to achieve a modular system to acquire, process, record and transmit seismic data by telemetry. the modularity of this system offers independence technology from the manufacturers of this kind of equipments both in hardware and software, also because the system was developed under the linux operating system gnu, which allows easy update of hardware; the software is an open platform developed in c, which allows the user to configure the system to operate online and updating according to needs. in brief, we propose in this paper a system design option for seismological instrumentation, which presents significant advantages also for other regions and countries with high seismicity and needs of earthquake monitoring, but have serious limitations in acquiring and maintaining commercial systems imported from developed countries. key words: seismology, instruments, ossodas, pc104, linux gnu, spread spectrum resumen el artículo presenta el diseño de un sistema digital para registrar señales sísmicas. el hardware y software diseñado son adicionados a la arquitectura de bus pc/104 con el propósito de tener un sistema modular para adquirir, procesar, almacenar y transmitir datos sísmicos por telemetría. la modularidad de este sistema ofrece independencia tecnológica con los fabricantes de estos equipos tanto en hardware como en software, ya que el sistema fue desarrollado bajo una plataforma linux gnu que permite al usuario configurarlo o modificarlo de acuerdo con sus necesidades en conclusión, proponemos una alternativa de sistema para la instrumentación sismológica, el cual es ventajoso para aquellas regiones con alto grado de sismisidad, necesidad de equipos para instrumentación sismológica, severas limitaciones para adquirir y mantener sistemas importados de países desarrollados. palabras clave: sismología, ossodas, instrumentación, pc104, linux gnu, espectro disperso © 2004 esrj unibiblos. ossodas, a portable digital system for seismological signal acquisition 53 introduction presently there is an increasing number of portable field system for seismic data acquisition available on the market, with high dynamic range, high data recording capacity and with ethernet and radio connectivity; these gave many seismological observatories the opportunity to improve the observation of seismicity, including observatories in colombia, but with the disadvantage of high costs for maintenance, repair and updating, as well as dependency of a single provider, (i.e., the manufacturers abroad). on the other hand, commercially available seismological instrumentation typically is expensive, rather specialized, predominantly built with proprietary hard and soft components, and very difficult to keep in a good condition. besides overcoming these disadvantages, it is also our aim to further our capacities in scientific instruments design and construction. taking this into account, it is necessary to develop our own seismology instrumentation equipment, with state-of-the-art design but at very low manufacturing, operation and maintenance costs, and transparent to the user, so that they could easily be adapted to specific needs, but also looking for the possibility of upgrading it with the lowest possible expenses. system description the digital acquisition system has been provided with six analog input channels available for instrumentation, which can be connected to two sensors (i.e., a tri-axial seismometer or 3components accelerometer). the signals are conditioned with a programmable gain amplifier and low pass filtered to 250 hz in order to avoid aliasing effects. a 16-bit a/d converter is used to sample the amplified and filtered signals. the digitized data is processed with a digital filter developed for a wide range of frequencies, programmable by the user [1]. the processed data are stored locally and can be accessed online by radio through a serial or ethernet interface. in applications where online acquisition is not required (i.e., the equipment is a standalone station), the data is recorded on a hard disk and downloaded periodically. the user can access remotely, initialize and check the operation of the system, load and observe the acquired data and update the software and recording parameters used for acquisition. the system uses a 40 gb hard disk, which allows one month of continuous recording at 1000 samples/second using 3 channels, or eight months of continuous recording at 125 samples/second. in applications where data transmission (radio, internet, etc.) is used, the hard disk will not store until its maximum capacity because there is a stored option to erase data once have been transferred a high volume of data by telemetry into a data collected station. hardware description the acquisition system includes a signal conditioning stage, a power card to supply ±12 and 5 vdc to the signal conditioning stage, an internet card for communication with an external network, an 8 serial port card to connect the gps system and a spread-spectrum two-way radio, a 16bit a/d converter, and an interconnection card to access the external devices. the signal conditioning stage was designed with a programmable gain amplifier, to amplify the signal supplied by the sensor with a gain range from 1 to 8000 (i.e., 80 db), and input impedance of 10 e 10 ohm || 6 pf. to avoid aliasing effects during the digitisations; a 6th order butter worth low-pass analog filter with a cut-off frequency of 250 hz was added tthis stage. the 16 bits a/d converter has 8 differential or 16 single-ended inputs, with a maximum sampling rate of 100 ksps, 96 db of resolution, and programmable gains of 1, 2, 4 and 8. the maximum input voltage of the converter is ± 10 vdc for a linear operation. figure 1 shows the block diagram of the system as well as the signal conditioning stage. pedro riascos et al. 54 figure 1: block diagram of the ossodas acquisition system the operating system the acquisition system was developed under the linux operating system gnu, which is distributed according to the open source model where the programmers are free to adapt the source code for custom purposes. the actual configuration uses a cpu with the following features [2]: • host platforms supported: redhad linux 7.3 • processor: 2x x 86 • clock speed: 100 mhz • memory: 32mb sdram sodimm module • power supply: atx, 250watt • dev104+, bus extender board (pc/104+ to 2-slot isa and 2-slot pci backplane) • mz104+, pc/104 board with dual ethernet additionally, the programs to acquire, record, and transmit the signals are written in c under linux and they are included in the following modules: • automatic data acquisition • gps configuration • data processing and storing • network configuration • time synchronization features and applications the system designed has the following features: • real-time acquisition of seismic signals • remote monitoring • gps time synchronization • two 3-channel analog inputs • 16-bit digital output • rs232/ethernet connectivity • telemetry access (spread spectrum) • removable hard disk data storage • 176 db full dynamic range • 80 db user selectable analog amplifier • 96 db digital output • programmable sample rate up to 1000 sps • weather sealed housing (for hostile environments) • rapid deployment by unskilled personnel the ossodas system can be used in applications such as: • measurement of environmental noise • seismic signal monitoring • seismic profiling • bridge and building monitoring • passive seismic monitoring (oil fields) • quarry blast vibration control • strong motion seismology technical specification at full operation, the system requires 1.8 a, which could be supplied with a 12 v, 100ah battery and two solar panels of 80w. the system is enclosed in a hermetically sealed pelican pfv case to operate under outdoor conditions. all connectors for accessories of the system are circular plastic connectors amp. the sensor used with the system is the electronic lennartz le3d-light which has a natural frequency of 1 hz and bandwidth of 80 hz [3], although the user can use sensors from different suppliers. figure 2 shows the ossodas layout and table 1 shows the ossodas physical characteristics. ossodas, a portable digital system for seismological signal acquisition table 1:ossodas physical char conclusions and futu a low cost digital system acquisition has been developed acquiring, process, record seismological data by telem propose a system design optio instrumentation, which pr advantages for other regions high seismicity and needs of ear as well as serious limitations maintaining imported commerci the system reliability was chec that is, all of the measured valu the experimental results by val stages that were verified conditioning, the acquisition c and the digital signal processing the future work will be focus digitally configurable analog user to select the gain directly and design of analog and digital figure 2: ossodas layout 55 acteristics re work for seismic signal . the system allows and transmitting etry. in brief, we n for seismological esents significant and countries with thquake monitoring, in acquiring, and al systems. ked stage by stage, es correspond with idated systems. the are: the signal ard, time accuracy, . ed on the design of gains, allowing the from the software, filters using programmable circuits [4]. this technology allows configuring the system in a wide range of frequencies, from 1 to 1000 hz. also, the future work will aim at decreasing the power required by the system in order to operate at low current. this is possible due to the availability of solid-state storage (i.e., disk on chip and compact flash card), which allows sharing the operating systems and acquisition process with the hard disks during the operation of the system. references diamond system corporation, 2002. “rugged embedded solutions for harsh environments and mobile applications”. s. w smith, (1999) “the scientist and engineer guide to digital signal processing”. california technical publishing san diego california, second edition. “le-xd geophone family”, document number 9900003, lennartz electronic, email: info@lennartzelectronic.de “programmable analog circuits”, ispac handbook, lattice semiconductor corporation, january 2000. e. freaking, (1997). “digital signal processing in communication systems”, chapman & hall, new york, ny. f. scherbaum. (1994) “basic concepts in digital signal processing for seismologists”, springerverlag. geofisica colombiana n° 6 pp 62-65 diciembre de 2002 bogota, d,c, issn 0121 2974 solucion a un problema estatico en un area con topografia abrupta usando una velocidad de reemplazamiento variable ------------------' luis antonio castillo lopez profesor asociado, universidad nacional de colombia, curso posgrado en geofisica e-mail: ic as tillo gacie n c ia s run a l.e d u co carlos pedraza analista senior, petroseis t tc a. resumen teniendo en cuenta que para poder obtener una buena imagen del subsuelo, es necesario que las soluciones estaticas sean las mas acertadas posibles en la etapa inicial del procesamiento, y no gastar tiempo en ensayos con procesos mas sofisticados, sin tener resuelto el problema estatico, se propone implantar un metodo de procesamiento usando un modelo de velocidad de reemplazamiento variable en el calculo de la soluci6n estatica, se pretende explicar que el valor de la velocidad de reemplazamiento influye notoriamente en la imagen de la estructura, la cual puede verse deformada en gran rnagnitud. tarnbien puede ser cuestionado c6mo ha sido asignado este valor de manera constante en los proyectos, dando mas importancia al amarre con areas cercanas que a la soluci6n misma del problema estatico. se mostrara un estudio en que se emplea una serie de datos reales procesados a partir de un modelo de velocidad de reemplazamiento variable y otra con un modelo de velocidad con stante, las que -al ser comparadas y anal izadasdan como resultado grandes contrastes en las imageries obtenidas. este metodo se ha ido desarrollando durante el procesamiento de informaci6n de las areas con variaciones severas de topografia en colombia. en el caso del area del piedemonte llanero, usando el paquete interactivo promax, algoritmo gauss-seidel. palabras clave: refraccion, superficie irregular, correcciones estaticas, proceso abstract a problem in the reflection seismology is the precise determination offield statics correction that have to be applied to the reflection data in order to eliminate effects of eathering layer. normally these corrections are calculated by shallow reflection surveys. the waves refractated at the weathering layers are in the reflection seismograms and constitute part of the called "first breaks". these are waves where the source-receiver ray trajectories are located within the weathering layer and partly along its bottom. the arrival times through the weathering layer needed to computed the field statics corrections. in this work, a simple and efficient method is described to obtain field statics using only field seismograms recorded by the cop configuration. as input data ones considered: a) the surface topography ofthe seismic line, b) the first breaks and c) the geometry data. the method consists of determining from a starting velocity model, this model has a layer constructed taking into average thickness a velocity for the area. the method propose to seismic processing, was tested on raw register an a seismic line with abrupt topography in a colombia area. the obtained corrections with variable velocity were better than those computed with a constant velocity model. the statics solutions are necessary to obtain a good subsurface image. but in the initial processing sequence it is necessary, and do not spend time in surveys with other process, that do not resolve the static problem. in this paper a processing using static model calculate with variable velocity is proposed. this method have been development in the processing data in areas with irregular surface. for instance, piedemonte l1anero. the software calculation is promax, algorithm gauss-sediel. keywords: refraction, irregular surface, static corrections, seismic processing 62 geofislca colombiana, 6, diciembre [)e 2002 solucion a un problema estatico en un area con topocrafia abrupta usando una velocldad de reemplazamiento variable introduccion los datos sismicos son mostrados como si fueran adquiridos (disparados y registrados) sobre una superficie plana (figura i), sin mostrar los cam bios por elevacion existentes en el area que se va a prospectar. esta hipotetica superficie plana es conocida como el datum sismica. para reemplazar las capas cercanas de la superficie, entre la superficie del terreno y el datum, con la intencion de remover los efectos de tiempo relacionado a diferencias en la elevacion de la superficie. reflexion / refraccion de ondas sismlcas fijente sismica capa2 figura i. cuadro esquernatico de un par fuente receptor y la trayectoria de rayo, que incide sobre un medio. la velocidad de la segunda capa es cornunmente usada para ia velocidad de remplazamiento. este procesarniento "rule of thum b" se basa enla suposicion que la segunda capa representa el promedio de velocidad entre la superficie y el datum. tambien se asume que con esta velocidad pueden ser removidos adecuadamente los efectos de estructuras en tiempo, causados poi' cam bios en la elevacion de superficie. se debe tener en cuenta que este procedimiento, efectuando un promedio de velocidad en un modelo donde la estructura es compleja (caso particular de muchas estructuras encontradas a 10 largo de la geologia colombiana) da como resultado una estructura 0 imagen deform ada de la misma. en esta presentacion no se pretende discutir el procedimiento anterior, sino dar otra alternativa a una rnejor aproximacion 0 interpretacion de un modelo. marco teo rico un gran problema encontrado en prospeccion sismica de reflex ion es la determinacion precisa de las correcciones estaticas que deben ser aplicadas a los datos para eliminar los efectos de la topograffa en la zona de baja velocidad (zb v). las ondas refractadas en la base de la zbv, generalmente representadas en los sismogramas de reflexion como primeros arribos, son ondas cuyas trayectorias de los ragedfislca cdldmbiana, 6, diciembre de 2002 yos, desd la fuente hasta el receptor, se localizan predominantemente a 10 largo de la zbv y de la ba se, estas sefiales deben contener la informacion necesaria para el calculo de los tiempos de transite de las trayectorias verticales de las ondas sismicas a 10 largo de aquella zona y para el calculo de las correcciones estaticas. en este trabajo se presenta un metodo simple y eficiente de calculo de las correcciones estaticas de campo, a traves del uso exclusivo de datos de reflexion sismica registrados con la tecnica cop ("common depth point"). ei concepto clave que enmarca la refraccion sismica se basa en la ley de snell (figura i), la establece que todo rayo al atravesar un medic entre dos formaciones de velocidades diferentes: ~=seni v2 senr <i) los radios de los sen os del angulo incidente (i) y del refractado (i') son iguales a la razon de las velocidades de las dos formaciones (vi, v2). uno de los motivos para aplicar las correcciones estaticas es asegurar una buena imagen en el proceso sismico. las anornalias estaticas pueden producir estructuras falsas en la seccion sismica (longitudes de onda espaciales mas grandes que la longitud del tendido). las refracciones estaticas son efectivas para corregir anornalias de longitudes de onda espaciales mas largas. adernas, los rnetodos de refraccion perm iten determinar el espesor y las velocidades de las capas cercanas a la superficie al efectuar anal isis de primeros arribos sobre los registros de campo (figura 2). l ---..j figura 2. registro bruto que resalta en la parte superior los primeros arribos y su picado (manual 0 automatico). metodologia este rnetodo consiste en efectuar un modelamiento de la velocidad de reernplazamiento, usando la variacion de velocidades calculadas para el primer refractor y no como una velocidad promedio. ei rnetodo fue probado en un tramo de una linea sismica 20, con una topogratia y eventos superficiales abr\lptos, y los resultados muestran las diferencias entre los metodos. 63 castillo et al. es importante recalcar que los datos sismicos superficiales deben ser preservados si es posible para evitar cambios de estaticas antes de apilar. cuando la topografia es severa, esto es mejor efectuado al hacer un "shift" de proceso (datum flotante) que sigue la topografia. cuando no es efectuado la correccion por refracci6n se distorsionan las reflexiones (hiperbolas) y las refracciones no son corregidas (tilander et ai, 1995). ejemplo en las figuras 3 y 4, se muestra un registro de campo al cual fue aplicado la correcci6n por refracci6n, donde pueden ser observados los cambios en los arribos vistos en cada traza, viendose afectados tanto las refracciones y reflexiones. en este proceso las velocidades de reemplazamiento juegan un papel importante, con 10 cual un modelo de velocidades variable puede ser considerado como una mejor aproximaci6n a la geologia. figura 4. ampliaci6n de la zona afectada por la irregularidad del relieve en el registro sismico ya rnostrado. figura 3. registro sismico que muestra los efectos de la topografia en la estructura de los datos. en los resultados de las secciones apiladas con diferentes modelos de velocidades, puede observarse que los resultados son bien diferentes, con 10 cual se deja a consideraci6n el uso en la utilizaci6n de diferentes velocidades. segun la interpretaci6n efectuada por el interprete el modelo con una velocidad de reemplazamiento variable es mucho mas aproximado a la realidad (figuras 5 y 6). 64 figura 5. secci6n apilada con velocidad de reemplazamiento constante. conclusiones datos adquiridos en una variedad de terrenos incluyen la cordillera oriental, los cuales presentan rocas expuestas en superficie y zonas montaftosas con pendientes fuertes. rocas de alta velocidad en la superficie pueden causar desacople disparo-receptor y malos alineamientos debido a deterrninaci6n errada de estaticas, geofislca colombiana, 6, diciembre de 2002 solucion a un problema estatlco en un area con topografia abrupta usando una velocidad de reemplazamiento variable figura 6. secci6n apilada can velocidad de reemplazamiento variable. ei uso de un modelo de velocidades no muy aproximado puede dar como resultado una imagen distorsionada del subsuelo, con 10 cual se recalca el uso de un modelo de velocidades mas aproximado. geofisica colombiana. 6. diciembre de 2002 agradecimientos agradecemos especialmente al grupo de geofisica de petroseis ltda. en bogota, por su colaboracion en cuanto a equipos, experiencia y conocimientos, al igual que a la universidad nacional quien junto a petroseis ltda., hacen parte del desarrollo y evolucian de la tecnologia en procesamiento sismico en colombia. bibliografia sheriff, r. e., & geldart, l., p. 1991, exploracion sismologica. volumen ii. procesamiento e interpretacion de datos. noriega editores. tilander, n. g. & mitchel, r. g. 1995, processing of seismic data from overthrust areas in latin america. chevron overseas petroleum, inc. sail ramon (california), usa. yilmaz, 0., 1987, seismic data processing. society exploration geophysicists. tulsa (ok), usa. 65 untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 54-63 the maximum effect of deep lakes on temperature profiles – determination of the geothermal gradient v. t. balobaev1, i. m. kutasov2 and l. v. eppelbaum3 1permafrost institute, siberian branch of the russian academy of sciences, yakutsk 677018, russia 2pajarito enterprises, 640 alta vista, suite 124, santa fe, new mexico 87505, usa 3dept. of geophysics and planetary sciences, raymond and beverly sackler faculty of exact sciences, tel aviv university, ramat aviv 69978, tel aviv, israel corresponding author, e-mail: levap@post.tau.ac.il; fax: +972 3 6409282 abstract understanding the climate change processes on the basis of geothermal observations in boreholes is an important and at the same time high-intricate problem. many non-climatic effects could cause changes in ground surface temperatures. in this study we investigate the effects of deep lakes on the borehole temperature profiles observed within or in the vicinity of the lakes. we propose a method based on utilization of laplace equation with nonuniform boundary conditions. the proposed method makes possible to estimate the maximum effect of deep lakes (here the term “deep lake” means that long term mean annual temperature of bottom sediments can be considered as a constant value) on the borehole temperature profiles. this method also allows one to estimate an accuracy of the determination of the geothermal gradient. key words: geothermal gradient, reduced temperature, laplace equation, lake resumen el entendimiento de los procesos de cambio climático basado en las observaciones geotérmicas en pozos es importante y a la vez un problema intrincadamente complejo. muchos efectos no climáticos podrían causar cambios en las temperaturas de la superficie terrestre. en este estudio investigamos el efecto de los lagos profundos sobre los perfiles de temperatura registrados en pozos, al interior o en las inmediaciones de los lagos. proponemos un método basado en el uso de la ecuación de laplace con condiciones de frontera no uniforme. el método hace posible estimar el máximo efecto de los lagos profundos (el término “lago profundo” significa que 54 manuscript received: january 01th, 2009. accepted for publication: february 27th, 2009. la temperatura media anual de los sedimentos del fondo, evaluada sobre un largo período, puede ser considerada como un valor constante) sobre los perfiles de temperatura en los pozos. este método también permite estimar la precisión en la determinación del gradiente geotérmico. palabras clave: gradiente geotérmico, temperatura reducida, ecuación laplace, lago. i. introduction at present many efforts are made to determine the trends in ground surface temperature history (gsth) from geothermal surveys. in this case accurate subsurface temperature measurements are needed to solve this inverse problem � estimation of the unknown time dependent ground surface temperature (gst). the variations of the gst during the long term climate changes resulted in disturbance (anomalies) of the temperature field of geological formations. thus, the gsth data could be evaluated by analyzing the present precise temperature-depth profiles. the effect of surface temperature variations in the past on the temperature field of formations is widely discussed in the literature (e.g., lachenbruch and marshall, 1986; beltrami et al., 1992; shen and beck, 1992; bodri and cermak, 1995; harris and chapman, 1995; huang et al., 1996; guillou-frottier et al., 1998; huang and pollack, 1998; huang et al., 2000; pollack and huang, 2000; majorowicz and safanda, 2005; eppelbaum et al., 2006; hamza et al., 2007; hopcroft et al., 2007; rath and mottaghy, 2007; gonz´alez-rouco et al., 2008; kooi, 2008). ii. previous investigations: some research background earlier the forward calculation approach (fca) was used for the analysis and interpretation of borehole temperatures in terms of the gsth (eppelbaum et al., 2006). three groups based on the geographical proximity were formed. fifteen borehole temperature profiles from europe (5), asia (4) and north america (6) were selected (huang and pollack, 1998; www.geo.lsa.umich. edu/~climate). the objective of this study was the estimation of the warming rates in the 20th century by the fca method and comparing with those obtained by the few parameter estimation (fpe) technique (huang et al., 1996; huang and pollack, 1998). it was reasonable to assume that for close spaced boreholes, the values of the warming rates obtained by the two inversion methods, should vary in narrow limits. the results of inversions (fca) have shown that for boreholes in north america the current warming rates vary in the 0.412.45 k/100a range. the wide range for the warming rate of 0.33-2.48 k/100a was also determined for boreholes in europe. interesting results were obtained for four boreholes in asia (china) (eppelbaum et al., 2006). in this case the warming rate varies in relatively narrow limits (1.16-1.59 k/100a.). the warming rate estimated by the fpe technique (huang and pollack, 1998) varied in wide ranges: 0.38-2.49 k/100a (north america); 0.21-3.75 k/100a (europe), and 0.30-2.53 k/100a (asia). thus, we can conclude that for boreholes in north america and europe both approaches provide practically the same ranges of warming rates. for asian boreholes the fca approach gives a more consistent (narrow) range of warming rates (1.16-1.59 k/100a). the results of temperature inversion by both techniques indicate that probably some of non-climatic effects (vertical and horizontal water flows, steep topography, lakes, vertical variation in heat flow, lateral thermal conductivity contrasts, thermal conductivity anisotropy, deforestation, forest fires, mining, wetland drainage, agricultural development, urbanization, etc.) may have perturbed the borehole temperature profiles. influence of these factors has been studying by many authors (e.g., carslaw and jaeger, 1959; lachenbruch, 1965; kappelmeyer and haenel, 1974; blackwell et al., 1980; majorowicz and skinner, 1997; guillou-frottier et 55 the maximum effect of deep lakes on temperature profiles – determination of the geothermal gradient al., 1998; lewis and wang, 1998; kohl, 1999; safanda, 1999; pollack and huang, 2000; cermak and bodri, 2001; gosselin and mareschal, 2003; gruber et al., 2004; bodri and cermak, 2005; mottaghy et al., 2005; nitoiu and beltrami, 2005; allen et al., 2006; taniguchi, 2006; chouinard and mareschal, 2007; hamza et al., 2007; safanda et al., 2007). at the same time exact calculation of all these factors is a complex physical-mathematical problem, which obviously will be completely solved in a future by the method of successive approximations. the temperature regime of sedimentary formations is influenced by many environmental and geological factors (local relief, sedimentation, erosion, lateral conductivity contrasts, underground water movement), past climate, and by the heat flow from the earth’s interior – terrestrial heat flow. most of temperature surveys are conducted in boreholes. in many cases the drilling sites of boreholes are located within or outside of deep lakes (we employ the term “deep lake” to designate that long term mean annual temperature of bottom sediments could be considered as a constant value). the objective of this study is to evaluate to what extent the proximity of deep lakes can affect the temperature profiles of wellbores. in 1974 balobayev and shastkevich published results of their analytical study which can be used to determine the configuration of the steady temperature field of formations beneath the lakes of an arbitrary contour (balobayev and shastkevich, 1974). taking into account that this publication is not easily accessible to researchers, we present below a brief summation of the results of this study (balobayev and shastkevich, 1974). authors assumed that the lake existed for an infinitely long period of time. in this case the solution of laplace equation with nonuniform boundary conditions can be used to describe the steady temperature field of formations beneath the lakes and estimate the maximum effect (due to assumption that the lake existed for an infinitely long period of time) of lakes on borehole temperature profiles. iii. climate reconstruction methods: some typical disturbances and restrictions we should note that all climate reconstruction methods are based on one-dimensional heat conduction equation. it is assumed that a uniform boundary condition is applied on a plane surface, the formation is a laterally homogeneous medium, and the thermal properties can depend only on a depth. for this reasons any subsurface temperature variations arising from conditions that depart from that theoretical model have the potential to be incorrectly interpreted as a climate change signature (pollack and huang, 2000). to demonstrate the well selection procedures we briefly present two examples. in the study conducted by guillou-frottier et al. (1998), only 10 from 57 temperature profiles were selected for inversion of past ground surface temperatures. as was mentioned by nitoiu and beltrami (2005) from over 10,000 borehole temperature logs worldwide (the international heat flow commission global geothermal data set), only about 10% of these data are currently used for climate studies because a number of known non-climatic energy perturbations are superimposed on the climatic signal. therefore, an extreme caution should be used in selection of temperature-depth profiles for inferring the ground surface temperature histories. the following criteria were considered in rejecting boreholes from the study: steep topography, proximity of lakes, water circulation, instrumental problems, other identifiable terrain effects (such as heat refraction, permafrost effects), and recent changes in surface conditions (clearing of trees). for most of the boreholes that were discarded, the shallowest part of the temperature profile is perturbed. as was mentioned by co-authors these perturbations are often similar to the perturbations due to changes in surface temperature. if the terrain conditions had not been considered, warming would have been inferred for 25 boreholes. ten boreholes show apparent cooling, and only one shows no difference. to screen out borehole temperature data from eastern brazil with 56 v. t. balobaev, i. m. kutasov and l. v. eppelbaum indications of possible perturbations arising from non-climatic effects, the following quality assurance conditions (we basically agree with these criteria) were imposed (hamza et al., 2007): 1. the borehole is sufficiently deep that the lower section of the temperature-depth profile allows a reliable determination of the geothermal gradient, presumably free of the effects of recent climate changes. order of magnitude calculations indicate that surface temperature changes of the last centuries would penetrate to depths of nearly 150 m, 2. the time elapsed between cessation of drilling and the temperature log is at least an order of magnitude large compared to the duration of drilling, 3. the temperature-depth profile is free from the presence of any significant non-linear features in the bottom parts of the borehole, usually indicative of advection heat transfer by fluid movements, either in the surrounding formation or in the borehole itself, 4. the elevation changes at the site and in the vicinity of the borehole are relatively small so that the topographic perturbation of the subsurface temperature field at shallow depths is not significant, and 5. the lithologic sequences encountered in the borehole, have relatively uniform thermal properties, and are of sufficiently large thickness that the gradient changes related to variations in thermal properties do not lead to systematic errors in the procedure employed for extracting the climate related signal. out of a total of 129 temperature logs only 17 were found to satisfy the above set of quality assurance conditions (hamza et al., 2007). corrections can be applied, for example, to correct borehole temperature profiles for the effect of topography (lachenbruch, 1965; blackwell et al., 1980; safanda, 1994, 1999). however, this is rarely done because the amplitude of the climatic signals is often smaller than the uncertainty on these corrections (chouinard and mareschal, 2007). safanda et al. (2007) presented interesting results of repeated temperature logs from czech, slovenian and portuguese borehole climate observatories within a time span of 8-20 years. the repeated logs revealed subsurface warming in all the boreholes amounting to 0.2-0.6 oc below 20 m depth. the warming rate of 0.05 oc/yr. at the czech observatory (located in a park within the campus of the geophysical institute in prague) was estimated. this warming rate is two times more than the simulated value (using the surface air temperature as a forcing function). it was assumed that subsurface temperature at the station is influenced by new structure built within the campus of the geophysical institute within the last 10-20 years and/or by other components of infrastructure built 40-50 years ago. the authors (safanda et al., 2007) conducted a quantitative analysis of these effects by solving numerically the heat conduction equation in a 3d geothermal model of the borehole site. it was found out that the mentioned anthropogenic structures influence the temperature in the borehole quite strongly. nitoiu and beltrami (2005) attempted to correct borehole temperature data for the effects of deforestation. the authors simulated the ground surface temperature changes following deforestation by using a combined power exponential function describing the organic matter decay and recovery of the forest floor after a clear-cut (covington, 1981). the presented examples demonstrate that application of this correction could allow incorporate many borehole data into the borehole climatology database (nitoiu and beltrami, 2005). iv. working equations let’s assume that the well site is located within or outside of a deep lake. as it was mentioned above, we consider the long-term mean annual temperature of bottom sediments as a constant value (for the first time this problem was shortly outlined in balobaev et al. (2008)). we will assume that z = 0 is the vertical 57 the maximum effect of deep lakes on temperature profiles – determination of the geothermal gradient coordinate of the lake’s bottom. the temperature regime of geological formations in this area (within and outside of the lake) is subjected to the thermal influence of the lake. the extent of this influence depends mainly on the lake’s dimensions, on the current depth, the distance from the lake, and on the difference between the long term mean annual temperature of bottom sediments and the long term mean annual temperature of surrounding lake formations (at z = 0). we will assume that the lake existed for an infinitely long period of time. the following designations will be used below: ñ,ö, z are cylindrical coordinates (ñ is the distance from the z axis, ö describes the angle from the positive xz-plane to the point, and z is depth); tis is the long term mean annual temperature of bottom sediments; and tot is the long term mean annual temperature of surrounding lake formations at z = 0. firstly, let consider a lake of an arbitrary contour (figure 1). the laplace equation for the semi-infinite solid area is � � � � � � � � 2 2 2 2 2 2 2 1 1 0 t r t t t z� � � � � � � � (1) the boundary conditions are t (ñ, ö, z = 0) = tis within the lake area. t (ñ, ö, z = 0) = tot outside the lake area t (ñ = �, ö, z) = tot + �z where � is the regional (outside the lake area) geothermal gradient. the solution of laplace equation is possible by division of an arbitrary contour lake into sectors. however, the solution is expressed through a complex poisson integral and fairly elaborate and time-consuming computations are needed (balobayev and shastkevich, 1974). let’s ñmax be the maximum value of the set ñ1, ñ2, …., ñn. by introducing a safety factor (the maximum thermal effect of the lake on temperature profiles) we can assume that the lake has a circular shape with a radius ri = ñmax. now the laplace equation and boundary conditions are � � � � � � 2 2 2 2 1 0 t r t t z� � � � � (2) t (�, z = 0) = tis � � ri t (�, öz = 0) = tot � £ ri t (ñ = �, z) = tot + �z the solution eq. (2) is (balobayev and shastkevich, 1974): t (ñ, z) = tot + �z + m(tis – tot (3) m z a a k a k( , ) [ ( , ) ( , )]p � � � � �1 1 2 1 2 3 2 2 (4) a z z r a z r zi i 1 2 2 2 2 2 2 2 � � � � � � � � ( ) ; � � � � (5) a z r z r r i 3 2 2 2 2 1 2 2 22 2 � � � � � � � �� � � � � � � � � ; ; (6) r z k r z r i i 2 2 2 2 2 2 4 � � � � � � � � ; ( ) (7) where � �( , ) ( , ) 1 2 2 2k kand are the complete elliptical integrals of the third order (abramowitz and stegun, 1965). for the center of the island (� = 0) m z z z r ( ) � � � 1 2 1 2 (8) the temperature gradient for the well drilled at the center of the lake (ñ = 0) can be determined from eqs. (3) and (8). 58 v. t. balobaev, i. m. kutasov and l. v. eppelbaum figure 1. division of an arbitrary contour lake into sectors (after balobayev and shastkevich, 1974). � �* ( , ) ( ) ( ) / � � � � � � � � � � � � t z z t t r z ris ot i i 0 2 2 2 3 2 (9) �� � � �� � �� � � � � � � �* ( ) ( ) /is ot i i t r z r 2 2 2 3 2 (10) for distances ñ > 0 from eq. (3) we obtain � � �* ( , ) ( )� � � � dt z dz t dm dzis ot � (11) or �� � �� � � �* ( )t t dm dzis ot (12) introducing the reduced temperatures, tr(�, z) from eq. (3), we can write t z t z t z m t tr ot is ot( , ) ( , ) ( )� �� � � � �� (13) v. example of calculations let’s consider a 30 m deep lake with a radius of ri = 100 m and tis = 10 oc. the regional geothermal gradient is � = 0.0300 oc/m and tot = 20 oc. the drilling site of a 3000m wellbore is located at a distance of 150m from the center of the lake (table 1). 59 the maximum effect of deep lakes on temperature profiles – determination of the geothermal gradient table 1. the functions dm(z,0)/dz and m(z, ñ) z, m -dm/dz, 10-61/m, ñ = 0 the function m(z, ñ) distance from the center of the lake (ñ), m 0 50 100 150 200 300 400 20 50 100 150 200 250 300 400 500 600 700 800 900 1000 1100 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 9429 7155 3536 1707 894.4 512.3 316.2 142.7 75.43 44.43 28.28 19.08 13.47 9.852 7.421 5.727 3.617 2.427 1.707 1.245 .9362 .7215 .5677 .4547 .3698 .8039 .5528 .2929 .1679 .1056 .0715 .0513 .0299 .0194 .0136 .0100 .0077 .0061 .0050 .0041 .0034 .0025 .0019 .0015 .0012 .0010 .0009 .0007 .0006 .0006 .7612 .4937 .2606 .1538 .0991 .0683 .0496 .0292 .0119 .0135 .0100 .0077 .0061 .0049 .0041 .0034 .0025 .0019 .0015 .0012 .0010 .0009 .0007 .0006 .0006 .3828 .2815 .1787 .1189 .0827 .0598 .0449 .0275 .0184 .0131 .0098 .0076 .0060 .0049 .0041 .0034 .0025 .0019 .0015 .0012 .0010 .0009 .0007 .0006 .0006 .0525 .0950 .0985 .0805 .0628 .0488 .0384 .0249 .0172 .0125 .0094 .0073 .0059 .0048 .0040 .0034 .0025 .0019 .0015 .0012 .0010 .0009 .0007 .0006 .0006 .0168 .0370 .0518 .0512 .0450 .0379 .0315 .0219 .0157 .0117 .0090 .0071 .0057 .0047 .0039 .0033 .0025 .0019 .0015 .0012 .0010 .0009 .0007 .0006 .0006 .0042 .0100 .0174 .0212 .0222 .0215 .0198 .0159 .0125 .0099 .0079 .0064 .0052 .0044 .0037 .0032 .0024 .0018 .0015 .0012 .0010 .0008 .0007 .0006 .0006 .0017 .0091 .0076 .0101 .0116 .0122 .0122 .0111 .0095 .0080 .0066 .0056 .0047 .0040 .0034 .0030 .0023 .0018 .0014 .0012 .0010 .0008 .0007 .0006 .0005 what are the magnitudes of the formation temperature perturbations (expressed through the reduced temperatures) caused by the lake? the results of calculations after eqs. (4) and (8) are presented in table 1 and figure 2. we have to note that bottom of the lake has a coordinate z = 0 and because of this the actual depth is z* = z + 30 m. in our case tis – tot = –10 oc and the lake has a cooling effect on the temperature profiles. the values of tr(ñ, z) are decreasing with depth and practically can be neglected for radial distances of 550-600 m from the center of the lake (figure 2). let’s now assume that we have to determine the maximum values of tr(ñ, z) and ä��ñ, z) for the 300-500 m section of the wellbore. the values of tr(ñ, z) can be estimated directly from eq. (13) and table 1: tr(150,300)= –10oc·0.0384 = –0.384 oc; tr(150,500)= –10oc·0.0172 = –0.172 oc to determine the values of �� (ñ, z) for this case we suggest to approximate the function m by a quadratic polynomial (table 2, eq. (14)). m a a z a z*� � �0 1 2 2 (14) dm dz a a z * � �1 22 (15) a a a0 1 3 2 601126 0 3341 10 0 2872 10� �� � � �� �. . . then the values of ���ñ, z) can be determined from eqs. (12) and (15): ��(150,300) = 1,6174•10-3 oc/m; ��(150,500) = 0.4684•10-3 oc/m for the 300-500m section of the well the values of tr(ñ, z) and ��(ñ, z) are presented in figure 3. in our example the value of the regional geothermal gradient (�) is 0.03 °c/m. thus, the accuracy of the determined geothermal gradient is somewhere between 60 v. t. balobaev, i. m. kutasov and l. v. eppelbaum figure 3. the reduced temperature and the value of �� versus depth for ñ = 150 m. figure 2. the reduced temperatures versus radial distance for three depths. table 2. the approximation of the function m by a quadratic polynomial (eq. (14)) z, m m m* (mm*)/m ·100, % 300 320 340 360 380 400 420 440 460 480 500 0.03842 0.03505 0.03207 0.02941 0.02705 0.02494 0.02304 0.02135 0.01982 0.01844 0.01720 0.03822 0.03510 0.03221 0.02955 0.02712 0.02492 0.02295 0.02121 0.01970 0.01842 0.01737 0.51 -0.15 -0.45 -0.49 -0.27 0.07 0.38 0.65 0.60 0.12 -0.97 the average squared deviation = 0.50% (0.4684·10-3/0.03)·100% = 1.56% and (1.6174·10-3/0.03) )·100% = 5.39% at the same time for a wellbore located at the center of the lake the values tr and �� are maximal (table 1, eqs. (12) and (13)): tr(0,300) = –10oc·0.0513 = –0.513 °c; tr(0,500)= –10oc·0.01942 = –0.194 °c, ��(0,300) = 3.16•10-3 °c/m ��(0,500) = 0.754•10-3 °c/m a commercially available software, maple 7 (waterloo maple, 2001), was utilized to compute the function m(ñ, z). vi. conclusions it is shown that borehole paleoclimate investigations are complicated by many disturbing factors, exact calculation of which is a complex physical-mathematical problem. proposed method allows to estimate the maximum effect of deep lakes on the borehole temperature profiles observed within and outside of the lakes. authors assumed that the lake existed for an infinitely long period of time. in this case the solution of laplace equation with nonuniform boundary conditions can be used to describe the steady temperature field of geological formations beneath the lakes and estimate the maximum effect (due to assumption that the lake existed for an infinitely long period of time) of lakes on the borehole temperature profiles. a numerical example to estimate the effect was explained in detail. presented example of calculations testifies to what extent the proximity of a deep lake affects the borehole temperature profiles. an accuracy of the determination of the geothermal gradient is also estimated. acknowledgement the authors thank two anonymous reviewers and prof. luis a. montes v, editor-in-chief of the earth sciences research journal, for their useful comments and suggestions. references abramowitz, m., and i. stegun (1965). handbook of mathematical functions, dover publications inc., n. y. allen, m. r., n. p. gillett, j. a. kettleborough, g. hegerl, r. schnur, p.a. stott, g. boer, c. covey, t. l. delworth, g. s. jones, j. f. b. mitchell, and t. p. barnett (2006). quantifying anthropogenic influence on recent near-surface temperature change, survey of geophysics, 27, 491-544. balobaev, v. t., i. m. kutasov, and l. v. eppelbaum (2008). borehole paleoclimatology – the effect of deep lakes and “heat islands” on temperature profiles, climate of the past, no.4, 1-18. balobayev, v. t., and yu. g. shastkevich (1974). the estimation of the talik zones configuration and the steady temperature field of rocks beneath the lakes of arbitrary contour, in: lakes of the siberia cryolithozone (in russian), nauka publ., novosibirsk, 116-127. beltrami, h., a. m. jessop, and j.c. mareschal (1992). ground temperature histories in eastern and central canada from geothermal measurements: evidence of climate change, palaeogeogr. palaeoclimatol. palaeoecol. 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(2006). anthropogenic effects on subsurface temperature in bangkok, climate of the past discussions, 2, 832-846. waterloo maple (2001). maple 7 learning guide, waterloo maple inc., waterloo, canada. 63 the maximum effect of deep lakes on temperature profiles – determination of the geothermal gradient untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 74-81 magnetic and palaeomagnetic studies as an aid in deciphering groundwater flow: a case study from deccan traps s.k.g. krishnamacharyulu1 and g.v.s. poornachandra rao2 1school of earth sciences, srtm university, nanded -431 606, maharashtra, india 2national geophysical research institute, hyderabad 500 007, andhra pradesh, india email: skgkchary@gmail.com, gvsprao@rediffmail.com abstract this paper reports the utility of palaeomagnetic investigations first time in the ground water studies. the magnetic anomalies along with palaeomagnetic studies are used to locate barriers obstructing groundwater flow in a basaltic terrain. magnetic anomalies in an area of around 4 sq. km in the university campus, nanded, maharashtra, india brought out two distinct anomaly closures that can be attributed to bodies with different directions of magnetizations. palaeomagnetic investigations reveal that the basalts are almost uniformly magnetized, asserting that the anomalies come form the subsurface and the anomalous sources are attributed to weathering of basal basaltic flows. the difference between the measured palaeomagnetic directions of the surface rocks and the observed magnetic anomalies, in corroboration with local geomorphologic conditions and the nature of the basaltic flows in thin basaltic terrains suggest that subsurface weathering plays a major role in the ground water movement. this paper presents a novel idea on the utilization of the palaeomagnetic studies coupled with ground magnetic anomalies in deciphering groundwater flow. key words: ground water, palaeomagnetic, magnetic anomalies resumen este artículo reporta por primera vez, la utilidad de las investigaciones paleomagnéticas en el estudio de aguas subterráneas. las anomalías magnéticas junto con estudios paleomagnéticos se usan para localizar barreras que obstruyen el flujo de aguas subterráneas en terrenos basálticos. las anomalías magnéticas observadas en un área de 4 km2 del campus de la universidad strm en nanded maharashtra, india, indicaron dos cierres anómalos atribuidos a sendos cuerpos con diferentes direcciones de magnetización. las investigaciones paleomagnéticas 74 manuscript received: january 02th, 2009. accepted for publication: june 10th, 2009. revelaron que los basaltos fueron magnetizados casi uniformemente, lo cual indica que las anomalías provienen del subsuelo y que las fuentes anómalas se atribuyen a flujos basálticos basales meteorizados. la diferencia entre las direcciones paleomagnéticas medidas en el afloramiento y las anomalías magnéticas observadas, corroboradas con condiciones geomorfológicas locales y con la naturaleza de los flujos basálticos en laminas delgadas, sugieren que la meteorización del subsuelo juega un papel clave en el movimiento de aguas subterráneas. este artículo presenta una idea novedosa para el uso de estudios paleomagnéticos conjuntamente con anomalías magnéticas para descifrar flujos en aguas subterráneas. palabras claves: aguas subterráneas, paleomagnetismo, anomalías magnéticas. introduction the utility of magnetic method in ground water studies, although limited, to trace the floor of the basins or locating the structures that obstruct or control the groundwater flow is well known. however, palaeomagnetic investigations with unchallenged contributions to calculate the pole positions, to construct the history of the earth’s magnetic field and to decipher the plate motions were never sought after in groundwater investigations. in an aimed finding the clues for anomalous quantities of groundwater in a localized area, the palaeomagnetic data is found to be of utmost value to supplement the magnetic anomalies in identifying the barriers obstructing the groundwater flow. this paper presents the case study of this investigation, where the palaeomagnetic studies are utilized probably for the first time in ground water studies. geology and location of the area figure 1 presents location and general geology of the area. the area lies in the toposheet no. 56 e/8 of survey of india (190 061n & 770 171e) on the banks of river godavari. the area is covered with deccan lava flows with exposures of massive and columnar-jointed basalt, which believed to be erupted during cretaceous-eocene period over the precambrian basement rocks. in general, this area comprises of three flows (kumar and naik, 1990; bhaskar, 1990). approximately 60m thick flow i is of compound pahoehoe type and highly weathered, exhibiting fragmentary tops at some places. this flow i is overlain by columnar aa type flow ii. the present area is exposed with many columnar basalts. flows iii, is a massive basalt exposed in limited places as hillocks. in this area, the elevated places are typically covered by massive basalts, while low relief areas are covered by columnar basalts. table i shows the general stratigraphic sequence of the region and the present study area represents flow ii with exposures of columnar basalts. mostly the ground water in this region is in the fractures and vesicular basalts. the water bearing capacity of the vesicular basalt largely depends on the size and percentage of vesicles, density and degree of interconnection. generally pahoehoe flows exhibit good porosity and permeability with uniform vesicles. in the massive compact basalts the weathering, fractures, joints, shearing leads to secondary porosity. the pretrappean topography of this area was irregular and the lower flows are abutting against the hillocks of the pre-trappean period (raychaudhuri et al., 1984). foote (1876) reported rude massive blocks at basal basaltic flows formed due to weathering. according to him the massive blocks are common striking features in thin basaltic layers particularly in the margin areas of the deccan basalts and decomposition of the trap rocks under certain circumstances gives rise to eminently clayey products containing large percentage of iron. several e-w joints/fractures were identified from photo-interpretation, showing the curvilinear nature on the ground. the godavari river in this region presents a corrugated shape with u turns and right angle bends the central ground water depart75 magnetic and palaeomagnetic studies as an aid in deciphering groundwater flow: a case study from deccan traps ment identified three sets of lineaments in this area in nw-se, n-s and almost e-w directions. (fig.1; cgwb aap report, 1996). the elevation map of the area is presented in figure 2. there is a geomorphic denuded feature in approximately e-w direction (shown by arrow in figure 2), flanked by columnar massive basalts to the north and south, corresponding to flow ii. a perennial lake forming cal de sac to this denuded feature can be seen on the eastern side (figure 2). hydrogeologically this is supposed to be a highly favourable condition for the groundwater. however, the existing bore wells (shown as star symbol in figure 2) in this area established only poor supply of groundwater. the bore wells went up to a depth of 76 s.k.g. krishnamacharyulu and g.v.s. poornachandra rao alluvium deccan trap lineaments study area 19 060 1 77 170 1 figure 1: general geology and location map of the area (after cgwd aap report, 1996) 77 magnetic and palaeomagnetic studies as an aid in deciphering groundwater flow: a case study from deccan traps table i: general stratigraphic sequence of the area covered under toposheet no. 56e/8 (after kumar and naik, 1990; bhaskar, 1990) elevation (msl) (m) flow no. type of flow characters flow iii compound, mixed (61m) forming hilly terrain 405 m red bole/intertrappeans flow ii aa flow (5-47m) highly columnar —355 m red bole flow i compound/mixed (60m) massive fine grained, vesicular and highly weathered 0 200 400 600 800 1000 1200 1400 1600 distance x (m) 0 200 400 600 800 1000 1200 1400 1600 1800 d is ta n c e y (m ) lake figure 2: elevation contour map of the study area (arrow shows the denuded geomorphic feature, dashed lines show the interpreted subsurface blocks and star symbol show the locations of failed bore wells. around 70 m. some open wells dug up to a depth of around 10-15 m within the top flow produce some water. this paper is an attempt to find out the probable cause for the obstruction of the ground water in the subsurface formations. magnetic survey and anomalies: to investigate the anomalous behaviour of the bore wells on the ew denuded area, a magnetic survey was conducted covering an area of approximately 4 sq.km. figure 3 shows the observed total field magnetic anomaly map with a station spacing of 200m. the magnetometer used in this study is a proton precession magnetometer of eg & g geometrics, canada, with a reading accuracy of 1 gamma. the magnetometer readings were corrected for the earth’s normal field and diurnal variation by making repeated measurements at a chosen base station the magnetic anomaly map shows two conspicuous pairs of positive and negative magnetic anomaly contour clusters. these two anomalous zones show almost a similar range of anomalies, with -1400 nt to 2500 nt on the northern anomalous zone and -2200 nt to 1600 nt on the southern zone; but they exhibit different trends, one in almost n-s direction and the other in e-w direction. since the interest is to 78 s.k.g. krishnamacharyulu and g.v.s. poornachandra rao 0 200 400 600 800 1000 1200 1400 1600 1800 distance x (m) 0 200 400 600 800 1000 1200 1400 1600 1800 a a b b d is ta n c e y (m ) figure 3: observed total field magnetic anomaly map of the university campus, nanded. (continuous lines indicate positive anomalies and dashed lines indicate the negative anomalies. the anomalies are expressed in nt and distances in meters. aa and bb are interpreted profiles). have the order of the depths, two-dimensional modeling is attempted though the anomaly pattern suggests a three-dimensional nature. two profiles aa and bb, one from each anomalous zone for quantitative interpretation. various models tried using the algorithm developed by radhakrishna murthy (1998) and found that the dyke model is best-fitted model for these profiles. it is observed that for this model the objective function is minimum and the fit between observed and calculated anomalies is good. the interpretation revealed that the anomalous bodies are situated at depths of 37 m and 40 m respectively under aa and bb. while the intensity of magnetization needed for both is almost the same, the directions of magnetization differ: the values being -74 and 172 degrees respectively. the lava flows of same age are expected to show same magnetization and hence these values are attributed to subsurface features altered probably due to weathering. further the calculated depth values interestingly correspond to the base of the top flow and clearly rule out the possibility of the anomalies coming form the magnetization changes within the top flow. to further rule out of this possibility, twenty two oriented rock samples from the surface were collected from the exposed rock units covering both the anomalous zones, and were subjected to palaeomagnetic and rock magnetic studies. rock magnetism and palaeomagnetism cylindrical specimens of standard dimension were drilled from the oriented samples and their natural remnant magnetism (nrm) was measured using a spinner magnetometer (model dsm-2, schonstedt, usa) at ngri, hyderabad, india. nrm vectors of these samples exhibit some scatter (fig. 4a) with downward pointing inclination in the se quadrant. however, the specimens in each sample reveal good grouping. the sample mean nrm intensity (jn) varies between 0.74 and 41.07 a/m. the susceptibility of these samples is very uniform varying over a limited range of 9.68 to 23.5 x 10-3 si units. stability of the remnant vector in these samples has been examined by subjecting the samples to af and thermal demagnetization on a pilot basis. a peak field of 20-30 mt has been applied to these samples to recover the characteristic remnant magnetization (chrm) vectors in these samples (figure 4b). when the samples were subjected to thermal demagnetization on a pilot basis up to 580 °c at intervals of 50 to 100 °c, the remnant vector in them revealed very stable nature. the samples revealed a blocking temperature of 500 °c where the chrm can be recovered, indicating the presence of titanomagnetite as the carrier of remnant magnetization in these rocks. typical demagnetization characteristics in case of some specimens to af and thermal fields are shown in figure 4c and 4d respectively. the specimen remnant vectors obtained after laboratory af demagnetization have been averaged to obtain the sample mean chrm vectors using fisher’s (1953) method. sample mean chrm improved as result of demagnetization by removal of viscous magnetization acquired by these rocks with the precision parameter (k) increasing from 4.64 to 25.74. the mean chrm vector for all the specimens together is dm = 148.1 and im = + 43.5 (k = 25.74, á95 = 5.88) and are presented in figure 4 b. the chrm vectors of samples from the two anomalous zones, the northern and southern zones in the vicinity of aa and bb are grouped separately and derived the mean remnant magnetic vectors from each anomalous zone. the northern zone showed a mean remnant magnetic direction of dm = 138.1 and im = + 42.24 (k = 29.21, á95 = 7.81) and the southern zone showed a mean remnant magnetic direction of dm = 158.3 and im = + 43.9 (k = 33.72, á95 = 7.27). thus the nrm directions of both the zones are practically the same. discussion and conclusion the inversion of observed magnetic anomaly suggests that two different bodies responsible for the anomalies are differently magnetized in direction, one practically horizontally and the other vertically. since the palaeomagnetic directions of the rocks from the two anomalous zones are almost the same, 79 magnetic and palaeomagnetic studies as an aid in deciphering groundwater flow: a case study from deccan traps and the surface rocks exhibit uniform susceptibility, the cause of the magnetic anomalies need to be expected to lie in the subsurface. in the peripheral areas of deccan basalts there are reports of rude block formations in the basal flows, which in turn forming iron rich clay bodies due to weathering. (foote, 1876). therefore, the magnetic anomalies can be attributed to the two different iron rich clay bodies formed at the base of the flow i. these clay bodies might be acting as local barriers in the subsurface and may be obstructing the flow of ground water, resulting poor ground water conditions in the area down below the lake. 80 s.k.g. krishnamacharyulu and g.v.s. poornachandra rao (a) (b) © (d) figure 4: palaeomagnetic results of the study area: (a) & (b) stereographic plot of sample mean vectors of nrm level and chrm level. (c) and (d) typical af and thermal demagnetization characteristics of specimens from the study area. acknowledgements we thank the director, national geophysical research institute (ngri), hyderabad and prof. i. v. radhakrishna murthy, andhra university, visakhapatnam for their encouragement. we acknowledge the financial support by dst, new delhi (16/165/2002). references bhaskar, b. (1990) geology in part of nanded district of maharashtra, geological survey of india records, v.123, part 6, pp. 83-85. cgwd annual action program report (1996), groundwater resources & potential development of nanded district, maharashtra, central ground water department, central region, govt. of india, nagpur. fisher, r.a. (1953) dispersion on a sphere, proc. roy. soc. london, a 217, pp. 295-315. foote, r.b. (1876) the deccan trap and associated formations, mem. geol. survey of india no.12, pp. 171-199., which reprinted in geol. survey of india special publication no.38., annals of deccan trap study and bibliography on deccan traps (1996) pp.43-65. kumar d. and naik, k..k. (1990) geology in parts of nanded district of maharashtra, geological survey of india records, v.123, part 6, pub. by govt. of india, pp. 81-83. radhakrishna murthy, i.v. (1998) gravity and magnetic interpretation in exploration geophysics, memoir no.40, geological society of india, pp. 363. raychaudhuri, j.k., saxena, n.p., sinha, a.k. and sen, d. (1984) deccan trap flows in parts of nanded and yeotmal districts, maharashtra, geological survey of india special publication no.14, pp. 6-8. 81 magnetic and palaeomagnetic studies as an aid in deciphering groundwater flow: a case study from deccan traps de souza et al.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 45-55 the camaquã basin modelling : tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data thais de souza kazmierczak1, luis a. castillo2, farid chemale jr3, sérgio florêncio de souza3, ernesto gómez londoño4. 1 professora universidade federal do rio grande do sul. e-mail: thais.kazmierczak@ufrgs.br; thaissk@cpovo.net 2 professor, postgrado de geofisica, departamento de geociencias, universidad nacional de colombia, bogotá. e-mail: lacastillol@unal.edu.co, lacastillol@yahoo.com.br, lacastillol@ufrgs.br 3 professor universidade federal do rio grande do sul. e-mail: farid.chemale@ufrgs.br, sergio. florencio@ufrgs.br 4 ingeominas e-mail: egomez@ingeominas.gov.co. abstract a tectonostratigraphic study of the camaquã basin, formed by neoproterozoic to eopaleozoic volcano-sedimentary units, is herein presented. based on lineaments obtained through landsat tm images and geophysical methods, four structural domains were recognized. the depositional and deformational sequences of the guaritas and the bom jardim formations were identified on well logs, where the fluvial deltaic system and brittle to brittle-ductile tectonic evidence is preserved. differential compaction of sedimentary rocks was recognized using geophysical information (electrical (well log) and potential methods (gravimetry)). the sedimentary package presents variable thickness, increasing to the ne portion of the basin, and a compartmentalization in echelon subbasins, delimited by nw-trending structures is observed. a 3d schematic model of the camaquã basin was built based on the geophysical and satellite image data integrated to the digital terrane model. key words: camaquã basin, geophysics, remote sensing, lithostratigraphy, well logs resumen a continuación es presentado un análisis tectonoestratigráfico de la cuenca camaquã. esta cuenca presenta una secuencia volcano-sedimentaria del neoproterozoico al eoproterozoico. basado en lineamentos obtenidos de análisis de imágenes landsat tm y análisis geofísico, donde son reconocidos cuatro dominios estructurales. las secuencias deposicional y deformacional de las formaciones guaritas y bom jardim, son descritas a partir de registros de pozo y la gravimetría, 45 earth sciences research journal manuscript received may 17 2006. accepted for publication may 14 2007. introduction areas located to the southern of brazil, with alterated rock, are very difficult to map due to poor outcrop exposure. to get more subsurface information, other tools have to be used. geophysical techniques and remote sensing, combined with drill core information, were applied to obtain a 3d schematic interpretation of the camaquã basin. the geophysical mapping, spectral analyses, and well log information will help to provide a model of the basin. study area the camaquã basin area is located on the sulriograndense shield (figure 1), southern brazil and limited (i) to the north by phanerozoic units and the vila nova belt (700-850 ma), (ii) to the east, by the tijucas belt (850-540 ma) and dom feliciano magmatic arc (650-540 ma), and (iii) to the south by santa maria chico granulitic complex (paleoproterozoic), phanerozoic rocks and the tijucas belt. the camaquã basin is formed by volcano-sedimentary sequences (620 and 500 ma), during the late stages of the neoproterozoic to eopaleozoic brasiliano-panafrican cycle (paim et al., 2000). the camaquã basin is characterized by at least four major volcano-sedimentary sequences that are recognized showing (figure 2): magmatism of calc-alkaline, shoshonitic to alkaline affinities, and a siliciclastic sedimentary infill of shallow marine, alluvial-fluvial, deltaic and lacustrine deposits. the basin is affected by deformational events of brittle to brittle-ductile conditions and the development of complexes. remote sensing satellite images were used for quantifying and drawing the main structures (crosta, 1993). the area was studied by assessing the relationships between length, azimuth, and density lineaments into a structural model (figure 3). image processing includes data conversion, image generation (smoothing and filtering) and sampling. then, a principal component analysis (pca) was realized to highlight landforms and drainage. the processing of the image applies principal component analyses and image combination of color visual transformation. the transformation (linear) maximizes the variance to convert to a new coordinates system. this process produces a new set of images. the pc1 band was of 3-4-7 composition use in grey scale for highlight of the structural features and vectorization (vieira, 1998). 1959 lineaments were demarcated by considering: azimuth, distance, continuity, and position into the coordinate system. geophysical methods geophysical data (gravimetric survey and well log profiles) provide information about subsurface geology and thickness. gravimetrical methods gravimetrical methods include: acquisition, processing, integration with other studies and interpretation to obtain profiles, and a bouguer map (figures 4 and 5). acquired data permit to characterize areas of major lineaments, being demarcated the canguaçu the camaquã basin modelling: tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data 46 comprendiendo depósitos deltaicos y tectónica dúctil y frágil. variaciones en profundidad y los análisis de los registros sugieren compactación diferencial en la roca, a partir de la información geofísica (métodos eléctricos y potenciales). en el análisis de imágenes fueron consideradas características con la misma composición, brillo y transformada. el último paso incluye filtros que ayudan a analizar los lineamentos estructurales y el modelo de la cuenca. se observaron diferentes espesores sedimentarios, una división en diferentes sub-cuencas, y un incremento de espesor al noreste. es presentado un modelo esquemático 3d de la cuenca de camaquã basado en la geofísica de pozos e imágenes de satélite, integrados al modelo digital del terreno. palabras clave: cuenca de camaquã, geofísica, sensores remotos, litoestratigrafía, registros de pozo. kazmierczak et al., esrj vol. 11, no. 1. june 2007 47 figure 1. location map of the sul-rio grandense shield. ridge, the central gravimetry anomaly, and the structural gravimetry lineaments (e.g., principal structures domains associated to the central gravimetry anomaly). four structural domains were delimited: (i) northwest lineaments (paraná basin sediments), (ii) northeast domain parallel to caçapava suture; (iii) a third domain with north, northeast and northwest orientations, with a predominant north direction; and (iv) northeast and northwest domains occurring in equal proportions, and influenced by the ibaré lineament and caçapava lineament. local gravimetrical profiles divide the camaquã basin in a south region, along with a basement, and the paraná basin showing the possible density relationships between different sediments of godwana with low density (0 and +1), and the sediments of the guaritas formation in the camaquã basin. granitic rocks, with anomalous values of -4, were also correlated with volcanic rocks and the bom jardin alloformation (+2 and +3,5). gravimetrical values in the basement are high (+12). well log profiles the well log profiles have permitted to: (i) individualize the depositional sequences in a part of the basin, (ii) correlate sedimentary packages, and (iii) complement the litho-stratigraphic units within the regional geology. to identify the geometry and thickness, spontaneous potential (sp) and ray gamma (rg) registers from the wells cqp-01-rs (1200m), cqp-02-rs (1200m) and cqp-03-rs (1250m) were described. the consideration of lithological, electrofacies description, and deformation degree characteristics allows proposing two depositional types of sequences: depositional sequences (sq) and deformational sequences (sd). well description the description of well logs allows the recognizance of eight electrofacies (table 1). the electrofacies association permits to obtain depositional and deformational sequences according to the influence of tectonic events, being sd1, sd2, and sd3 deformational systems affected by those events. the electrofacies 48 figure 2. stratigraphy of the camaquã basin (paim et al., 2000). the camaquã basin modelling: tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data 49 figure 3. lineaments obtained from image interpretation and borehole location (upper) and principal component map interpretation in grey scalelandsat tm 222/081(lower). kazmierczak et al., esrj vol. 11, no. 1. june 2007 50 associations without deformation are described by sq1 and sq2. the cqp-01-rs well (figure 5) located at the center of the study area was divided into four sequences and limited by two discontinuity surfaces highlighting different tectonostratigraphic episodes: (i) the upper portion, described from sp, presents two depositional sequences: sq1 and sd2 in the limit between guaritas and bom jardim formations; (ii) in the half part (480810m), sp and gr log coarsening up cycles have been identified and grouped in a deformational sequence sd1 (a1, f1, and v1 electrofacies association). this interval is characterized by andesites belonging to the bom jardim formation and limited by an unconformity; (iii) the last well log measure (810m to 1200m), interpreted like serrated signature in the gr log, represents the depositional system seq1 (a1 and s1 electrofacies association). the cqp-02 rs well (figure 5) is located at the west of study area. in the top interval (0420m) only the gr log was recorded. the log is characterized by deformational (sd1 and sd2) and depositional (sq2) sequences, including fining upward cycles belonging to the bom jardim allogroup. three intervals could be described: (i) conglomeratic electrofacies (c1 and a2) on the upper and intermediate intervals (55m150m and 170m-300m) ; (ii) this interval (300 to 800m) is characterized by the presence of a deformational sequence (breaches, friction plans, and deformational structures). it is associated with tectonic events. (iii) in the final part (850m to the bottom of the drill core), the values of the gr curve change from a positive pattern to an irregular and serrate pattern. the cqp-03-rs well log (1250m), displays three dominant intervals characterized by high values (figure 5): (i) in the first interval, from (0 to 450m), the sp record varies from 25 to 40cps; (ii) in the second interval (600m-950m), sp values go up to 60 cps with a great deflection at 950m (gr and sp); (iii) the lower interval has a more homogeneous sp behavior with a slight fining-upward to siltstones. the a1, s1, f1, and f2 facies in this well belong to the bom jardim formation and correspond to deformational facies associations (sd1 and sd2). interpretation of the electrical and potential geophysical methods and remote sensing geophysical methods permit to obtain gravimetrical, density, and physical information. lithostratigraphic bodies and units delimitation could be done by integrating products such as remote sensing studies, along with correlation of ancient geophysical survey information (crosta, 1993). the geology of the camaquã basin, integrated into the image interpretation (structural domains), provides structural information and a model of the basin. four structural domains could be identified by correlating tectonic and stratigraphic events of the camaquã basin. areas of larger concentrations were characterized in the lineament maps, emphasizing on those areas closer to the main structures of the region (figure 5). the wells analyzed include units from the guaritas and the bom jardim formations. the cqp-01-rs well presents a discontinuity between the guaritas (alluvial deposits) and the bom jardim formations (deltaic deposits). deltaic and alluvial deposits of the bom jardim formation can be observed in the cqp-02rs well, whereas the cqp-03-rs well only comprises deltaic deposits of the same formation. evidence of tectonic events with brittle to brittleductile deformations (frictions planes and deformational structures) is described in the cpq-02-rs and cpq-03-rs wells. discussion and conclusions the tectono-stratigraphic features, obtained by the integration of satellite images (landsat tm) and the gravimetric data, suggests that the camaquã basin modelling: tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data 51 figure 4. gravimetrical map and location of the survey on the sul-rio grandense shield. figure 5. principal domains and lineaments in the sul-rio grandense shield. kazmierczak et al., esrj vol. 11, no. 1. june 2007 52 the ne-elongated camaquã basin is formed by blocks compartmentalized by nw-lineaments. there is an increase of the thickness to the ne portion of the basin, where the guaritas alloformation depocenter is delimited by the ne-trending master fault (located to the east of the cb). most tectonic features of the basin such as nne and nw-trending structures, arose during the latest stages of the brasilian cycle when the basin was formed. these structures were reactived by younger tectonic events through the mesocenozoic (e.g., those related with the south-american and african fragmentation). the tectonic processes in the neoproterozoic were dominated by ductile-brittle to brittle behavior during the formation of the camaquã basin. nw-trending lineaments in the camaquã basin trought the ne-trending structures were also observed. the nw structures, delimiting different compartments of the camaquã basin (sub-basins) (figure 6), were distributed in figure 6. schematic model of the camaquã basin. the camaquã basin modelling: tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data 53 echelon with thickness of the sedimentary package increasing to the ne. structural compartments were identified from the variation of preferential direction of lineaments taken from landsat tm images analyzed through preferential lineaments and gravimetrical contrast values. well logs describe the guaritas and bom jardim alloformations, showing tectonic events with brittle and brittle-ductil deformations establishing deformational and depositional sequences. a change of density with depth within the well log evidences the presence of two alloformations. local and regional gravimetrical profiles limited basement gravimetric anomalies along with the increasing of sediment thickness to the ne in the compartmentalized camaquã basin. computational tools, complementing gravimetrical analysis and geophysical profiles, make possible the technical integration and edition of schematic relationships that demarcated structural limits of the basin, allowing a general figure 7. correlation between cqp-01-rs, cqp-02-rs, and cqp-03-rs wells. kazmierczak et al., esrj vol. 11, no. 1. june 2007 54 ta bl e 1. e le ct ro fa ci es w el l l og (s p an d g r ) a nd li th ol og ic al d es cr ip tio n. the camaquã basin modelling: tectonostratigraphy through integration of geophysical (gravimetry, well log) and remote sensing data visualization. references crain, e. r. (2003). petrophysical handbook stratigraphic analysis part 1. depositional environment, alberta, canadá,. cap. xiii. crosta, a. p. (1993). processamento digital de imagens de sensoramento remoto. ed. rev. – campinas, sp: ig/ unicamp,. 170 p. paim, p. s. g., chemale jr, f., and lopes, r. c. (2000). bacia do camaquã. in: holz, m., de ros, l. f. (eds). geología do rio grande do sul. porto alegre, centro de investigação do gondwana. universidade federal do rio grande do sul, p. 231-274. vieira, m. n. (1998). vetorização e análise de tendências de cartas de lineamentos geológicos.. 97 f. dissertação (mestrado em ciências da computação), universidade federal de minas gerais, belo horizonte. 1998 walker, r. g. and james, n. p. (1992). facies models: response to sea level change. st. john’s: geological association of canada. 509 p. 55 kazmierczak et al., esrj vol. 11, no. 1. june 2007 enero%2031-geociencias-vol%2012-2%20dic-en%20baja[1].pdf earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 194-212 intraseasonal variability of rainfall over northern south america and caribbean region j.d. pabón1 and j. dorado2 1 oficina 317, edificio 212 (aulas de ciencias humanas). department of geography, national university of colombia. e-mail address: jdpabonc@unal.edu.co 2 posgraduate program in meteorology, department of geosciences, national university of colombia abstract based on decadal (amounts for each ten days) precipitation data from meteorological stations situated in northern south america and caribbean region, a decadal precipitation index (dpi) was calculated in order to study the intraseasonal variability (isv) of regional rainfall. the spectral analysis of dpi allows to identify signals with 20-25, 30, 40 and 50-60 days period. according to the analysis of their spatial distribution these signals are well defined over the caribbean island and coastal sector such as in some sectors of the andean region; the 60-days signal is presented only over caribbean region and in some places in the pacific sector; in the eastern lowlands of orinoco and amazon basin these signals are not clearly expressed. exploring the relationship between regional isv and madden-julian oscillation correlation analysis was made. due to the presence of signals different of 30-60 days, the correlation coefficients were very low. considering this situation, high frequency smoothing was applied to dpi time series; after that, a relative correlation was detected between smoothed dpi and madden-julian index (mji). keywords: intraseasonal variability, madden-julian oscillation, rainfall. resumen con base en datos de precipitación decadal (acumulados de diez días) provenientes de estaciones meteorológicas localizadas en el norte de suramérica y en el caribe, se calculó un índice de precipitación decadal (ipd) para estudiar la variabilidad intraestacional (vis) de la precipitación de ésta región. el análisis espectral del ipdmuestra señales con períodos de 20-25, 30, 40 y 50-60 días. de acuerdo con el análisis de la 194 manuscript received: october 10th, 2008. accepted for publication: november 22th, 2008. distribución espacial, estas señales están bien definidas sobre las islas del caribe y la zona costera, así como en algunos sectores de la región andina; la señal de 60 días se presenta únicamente en el caribe y sobre algunos lugares del pacífico; en las tierras bajas de las cuencas del orinoco y amazonas las señales no se expresan claramente. se analiza también la relación entre lavis regional de la precipitación y la oscilación demadden-julian. debido a la presencia de señales diferentes a las de 30-60 días en la precipitación, los coeficientes de correlación obtenidos son muy bajos. por esto, se realizó la suavización de las altas frecuencias en las series del ipd y se calcularon nuevamente los coeficientes de correlación del ipd con el índice madden-julian, después de lo cual hubo un notorio aumento de los coeficientes de correlación. palabras clave: variabilidad intrestacional, oscilación maden-julian, lluvia. 1. introduction extreme phases of climate variability bring to different regions warm or cold periods, rainy (more precipitation than normal or more frequent heavy rainfall events) or dry conditions, and so. this variability impacts in several ways ecosystems and economic systems of the countries around the world, producing in some cases disasters. in the climate systemmany processes generate this variability. for example, the tropical pacific phenomena el niño (warm condition) and la niña (cold conditions) are the cause of 2-7 years time scale oscillations of climatic variables known as enso cycle (philander, 1990; hastenrath, 1996; see also enso bibliography in coaps, 2006). in addition to the enso cycle, signals such as quasi-biennial component (ropelewski et al., 1992; meehl, 1997; baldwin et al., 2001), and fluctuations in the period interval of 20-90 days called intraseasonal oscillations (knutson & weickman, 1987; bantzer & wallace, 1996; nogués-paegle et al., 2000; krishnamurti & shukla, 2000; goswami& mohan, 2001; bond & vecchi, 2003; krishnamurti & shukla, 2007) have been identified. today the most studied signal of climate variability is that caused by enso. there are many works related to the effects of enso in monthly precipitation of different regions in the world (ropelewski et al., 1986; ropelewski & halpert, 1987; pabón & montealegre, 1992; peel et al., 2002; poveda, 2004; and many others). currently, seasonal climate prediction schemes are based on the knowledge about particularities of enso cycle in a given region, however, because they do not include other modes of climate variability, prediction fails frequently, especially in month-to-month range and less (see for example hendon et al., 2000; jones & schemm, 2000; jones et al., 2004c). a source of fails in prediction in month-to-month range is associated to the no inclusion of intraseasonal variations in the schemes. in fact, the phases of intraseasonal fluctuations activate and deactivate rainfall for periods of a couple of weeks lasting or forwarding the beginning or end of rainy season, or breaking it. the rainy phase of intraseasonal variability also activates heavy precipitation events and related to them disasters (flashfloods, landslides, etc). due to practical value to improve subseasonal predictability (waliser et al. 2003; webster & hoyos, 2004), the interest on intraseasonal modes of climate variability has been increasing in last decade and many efforts have been doing to study this variability especially the associated tomadden-julian oscillation (madden& julian, 1994), the dominant mode in intraseasonal climate variability. several authors have been studied the intraseasonal variability (hereafter isv) in precipitation for different geographical regions of the world. krishnamurti & shukla, (2000, 2007), for example, found modes with 45 and 20 days period in precipitation in india. wang et al. (1996) explored isv of precipitation in china finding 12, 21 and 43 days period. analysis was made also for afintraseasonal variability of rainfall over northern south america and caribbean region 195 rica (janicot & sultan, 2001; mathews, 2004) and signals over 10-25 and 25-60 days period were found in convection and precipitation in the western region (sultan et al., 2003; mounier & janicot, 2004); satatistically significant spectral peaks over 15 and 40 days period were found for sahel precipitation (janicot & sultan 2001). jones et al. (2004a) using outgoing long wave radiation data developed a climatology for tropical intraseasonal convective anomalies. also, ye & cho (2001), analyzed precipitation data for united states, and found 24 and 37 days signals. isv of convection and precipitation for different regions of south america has been studied by garreaud (2000), petersen et al. (2002), misra (2005). exploring the causes of isv of precipitation many researchers have been paying special attention to its relationship to madden-julian oscillation (mjo), because themjo is the dominant mode of tropical isv. thus, bantzer & wallace (1996) analyzed temperature and precipitation data using satellite data and found a 40-50 days component, close to mjo period. liebman et al. (1994) investigated the relationship between tropical cyclones of the indian and western pacific oceans and the mjo and found that cyclones preferentially occur during the convective phase of the oscillation; but they noted, however, that the increase in cyclone activity during active periods of convection is not restricted to mjo activity and concluded that the last does not influence tropical cyclones in a unique fashion (this situation may be due to the existence of other modes of isv). a similar analysis was done by maloney & hartman (2000a,b) for hurricanes of eastern north pacific and gulf of mexico (information on caribbean is also included). kayano & kousky (1999) studied the mjo in the global tropics using pentad-means for the 1979-1995 period computed for 200and 850-hpa zonal winds, 200-hpa velocity potential, 500-hpa geopotential height and pressure vertical velocity, 925-hpa temperature and specific humidity, slp and total precipitable water (pw); they found in all variables an eastward traveling large-scale oscillatory regime with a period of approximately 45 days. in the other hand, jones et al. (2004b) using pentadal precipitation data based on global precipitation climatology project (gpcp) confirmed that over indian ocean, indonesia, western pacific, eastern south america, western north america, northeast africa, the middle east, and eastern china, extremes precipitation events increases with the presence of active (convective) phase of mjo. barlow et al. (2005) analyzing daily precipitation for southwest asia found that this variable is modulated by mjo activity in the eastern indian ocean, with strength comparable to the interannual variability. bond & vechi (2003) found a relationship between mjo and precipitation of oregon and washington states. isvwas detected in convective processes over amazon region by petersen et al. (2002). the climate variability for northern south america and caribbean region has been studied mainly in interannual scale (hastenrath, 1976; pabón & montealegre, 1992; enfield, 1996; alfaro et al., 1998; enfield & alfaro, 1999; montealegre & 196 j.d. pabón and j. dorado figure 1. northern south america and caribbean region and distribution of meteorological stations used for analysis (the number corresponds to station listed in table 1). intraseasonal variability of rainfall over northern south america and caribbean region 197 table 1. list of meteorological stations used in the study. id station name latitud longitude altitude (m.s.n.m.) country 1 montego bay 18,50 –77,91 1 jamaica 2 3 las américas aeropuerto san juan intl 18,43 18,41 –69,66 –66,00 18 19 dominican rep. puerto rico 4 5 barahona kingston 18,20 17,93 –71,10 –76,78 3 3 dominican rep. jamaica 6 aeropuerto el embrujo 13,37 –81,35 1 colombia 7 aeropuerto sesquicentenario 12,58 –81,72 1 colombia 8 aeropuerto queen beatrix 12,50 –70,01 18 aruba 9 10 aeropuerto hato aeropuerto flamingo 12,20 12,15 –68,96 –68,28 9 6 netherl antills netherl antills 11 12 manaure coro 11,77 11,41 –72,45 –69,66 1 16 colombia venezuela 13 14 matitas esc agr carraipia 11,25 11,22 –73,05 –72,37 20 118 colombia colombia 15 apto. simón bolívar 11,13 –74,23 4 colombia 16 san lorenzo 11,07 –74,03 4 colombia 17 aeropuerto ernesto cortissoz 10,88 –75,30 14 colombia 18 19 maiquetía apto. s. bolívar aeropuerto rafael núñez 10,60 10,45 –66,98 –75,52 43 2 venezuela colombia 20 21 22 23 maturín san bernardo d. aeropuerto rafael barvo lorica 9,75 9,37 9,33 9,27 –63,18 –75,97 –75,26 –75,82 65 22 166 30 venezuela colombia colombia colombia 24 aeropuerto las flores 9,07 –73,98 34 colombia 25 tocumen 9,05 –79,36 45 panamá 26 guanare 9,01 –69,73 163 venezuela 27 28 marcos a. gelabert el salado 8,98 8,92 –79,55 –75,58 13 40 panamá colombia 29 30 31 32 david aeropuerto camilo daza san fernando de apore aeropuerto arauca 8,40 7,93 7,90 7,06 –82,41 –72,52 –67,41 –71,22 29 250 47 128 panamá colombia venezuela colombia 33 aeropuerto otu 7,03 –75,20 630 colombia 34 aeropuerto yariguíes 7,02 –73,80 126 colombia 198 j.d. pabón and j. dorado id station name latitud longitude altitude (m.s.n.m.) country 35 el cucharo 6,87 –73,37 975 colombia 36 tulio ospina 6,53 –75,92 1438 colombia 37 aeropuerto olaya herrera 6,37 –75,97 1490 colombia 38 panamericana 6,22 –77,40 4 colombia 39 aeropuerto puerto carreño 6,18 –67,48 50 colombia 40 41 tuparros boca torno surbata bonza 6,05 5,82 –67,52 –73,07 250 2485 colombia colombia 42 43 u.p.t.c. isla del santuario 5,57 5,47 –73,22 –73,73 2690 2580 colombia colombia 44 aeropuerto matecaña 5,37 –77,08 53 colombia 45 aeropuerto el carano 5,12 –77,08 53 colombia 46 la camelia 5,08 –76,45 1670 colombia 47 48 aeropuerto la nubia aeropuerto el edén 5,03 4,78 –75,47 –76,28 20,80 1204 colombia colombia 49 50 tibaitatá las gaviotas 4,70 4,55 –74,22 –70,93 2543 171 colombia colombia 51 52 centro ad. la unión el salto 4,53 4,47 –76,05 –74,77 920 450 colombia colombia 53 aeropuerto santiago villa 4,28 –75,33 266 colombia 54 aeropuerto el dorado 4,12 –74,15 2547 colombia 55 aeropuerto farfán 4,02 –76,37 955 colombia 56 57 palmira ica aeropuerto buenaventura 3,87 3,85 –76,53 –76,97 975 14 colombia colombia 58 59 60 61 aeropuerto benito salas aeropuerto alfonso bonilla vista hermosa bonanza 3,62 3,55 3,03 2,95 –75,50 –76,38 –73,73 –77,88 439 961 325 10 colombia colombia colombia colombia 62 aeropuerto g. l. valencia 2,43 –76,58 1730 colombia 63 la sierra 2,37 –77,37 1870 colombia 64 la macarena 2,30 –73,13 350 colombia 65 66 paleterá aeropuerto antonio nariño 2,08 1,67 –76,05 –77,47 2900 1796 colombia colombia 67 68 69 70 aeropuerto san luis aeropuerto artunduaga obonuco valparaiso 1,42 1,36 1,30 1,15 –78,13 –75,32 –77,05 –75,42 2961 244 2710 270 colombia colombia colombia colombia pabon, 2000; giannini et al., 2000; chang & stephenson, 2000; chang & taylor, 2002; taylor et al., 2002; poveda, 2004, nobre et al., 2006), especially the associated to enso, with purpose to improve seasonal-interannual climate prediction. the isv of precipitation have been less studied, however some attempts have been carried out by poveda et al. (2002), who analyzed the daily cycle of precipitation of colombianandes and found a significant relationship betweenmjo and daily precipitation, such as between mjo and amplitude of daily cycle. pabón (2007), explored isv using decadal (ten days amounts) precipitation data for different regions of colombia and found 20-25 and 50-70 days periodical components; searching the relationship of the analyzed times series withmjo it was found a low correspondence because the presence of other mode of isv different to mjo mode. these works show the evidences of isvmodes in climate of the region. taking in account the state of knowledge about the isv and the regional importance for improving climate prediction and to strength the disaster prevention systems, especially in the component related to heavy rainfall, this paper try to analyze in more detail the characteristics of isv of precipitation in northern south america and caribbean region. 2. data and methodology for the analysis in the current study as basic data was used daily precipitation for the 1978-2004 period from meteorological stations distributed over northern south america and caribbean region as showed in figure 1 (the 75 are listed in table 1). selection of the meteorological stations was done considering criteria as representativeness of a given region, length of record period at least 20 years and minimal gaps in data series. considering the complex topography over colombian territory that generates a rich climate diversity, it was necessary to include a relatively high density of the network for this region. an initial check was carried out in order to test the quality of data. after this quality control a decadal (amounts for each ten days period) precipitation series were organized and an index (dpi) was calculated using the equation: dpi p p p = s (1) (or simply, the z-score of decadal precipitation) where: p – decadal precipitation; multianual precipitation average for respective decade; standard deviation for the series of a given decade (time sequences of first decades, or second decades of the year and so on). to facilitate the presentation of analysis and results the report was organized for five sectors of the region: islands in caribbean sea region, continental plain lowland of the caribbean coastal zone, mountainous (andean) region, eastern lowlands of orinoco-amazon basin, and pacific sector. to identify signals of isv of precipitation spectral analysis (wilks, 1995) was applied to time series intraseasonal variability of rainfall over northern south america and caribbean region 199 id station name latitud longitude altitude (m.s.n.m.) country 71 villagarzón 1,05 –76,60 440 colombia 72 aeropuerto mariscal sucre –0,13 –78,48 28,11 ecuador 73 manta –0,95 –80,68 13 ecuador 74 aeropuerto gral. m. serrano –3,25 –69,93 84 colombia 75 aeropuerto vásquez cobo –4,16 –69,93 84 colombia 200 j.d. pabón and j. dorado estación san juan (puerto rico) -2 -1 0 1 2 years (in decadal resolution) estación manaure (colombia) -2 -1 0 1 2 d estación apto la nubia (colombia) -2 -1 0 1 2 estación maturin (venezuela) -2 -1 0 1 2 2 d estación apto buenaventura (colombia) -2 -1 0 1 2 d 2 0 0 4 1 9 9 6 1 9 9 7 1 9 9 8 1 9 9 9 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 2 0 0 4 1 9 9 6 1 9 9 7 1 9 9 9 1 9 9 8 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 d e c a d a l p re c ip it a ti o n in d e x years (in decadal resolution) d e c a d a l p re c ip it a ti o n in d e x 2 0 0 4 1 9 9 6 1 9 9 7 1 9 9 9 1 9 9 8 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 years (in decadal resolution) years (in decadal resolution) years (in decadal resolution) d e c a d a l p re c ip it a ti o n in d e x d e c a d a l p re c ip it a ti o n in d e x d e c a d a l p re c ip it a ti o n in d e x 2 0 0 4 1 9 9 6 1 9 9 7 1 9 9 9 1 9 9 8 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 2 0 0 4 1 9 9 6 1 9 9 7 1 9 9 8 2 0 0 0 2 0 0 1 2 0 0 2 figure 2: intraseasonal fluctuations of dpi. intraseasonal variability of rainfall over northern south america and caribbean region 201 figure 3. spectral density for dpi of stations located on the islands (left) and coastal sector (right) of caribbean region. 202 j.d. pabón and j. dorado cont. figure 3. of dpi using commercial software that calculates the spectral density. considering that madden-julian oscillation induces the most outstanding signal of isv in the tropics, an attempt to associate the regional isv of precipitation with mjo was done; therefore the madden-julian index (mji) for 120 and 40°w was compared with dpi series. for that, dpi was smoothed using moving averages to filtrate high frequency modes. mji data was taken from noaa/ncep/cpc web page (see http://www.cpc.ncep.noaa.gov/products/precip/cwlink/daily_mjo_index/details.shtml) finally, correlation coefficients formji and original and smoothed dpi were calculated. 3. analysis and discussion figure 2 shows the 3-points (30 days) moving averages of dpi for five stations (one for each delimited sector) in the region; this presentation visualizes the intraseasonal fluctuations of dpi. the mji for 120 and 40°w is also presented to compare with dpi series. the lines that correspond to mji over both 120 and 40°w has similar fluctuations with a noticeable delay caused by the eastward propagation of mjo, however in 1997-1998 (during strong el niño event) this concordance was disrupted. it is possible to observe also that during el niño events (1997-1998 and 2002-2003 in the analyzed period) the mji tends to have the lowest values, while during cold events la niña (1996, 1999-2000, and 2003-2004) the highest values are presented. at first glance, in the figure 2 it is possible to observe too, that some extreme events of dpi are synchronic in different regions in spite they differ by their magnitude. comparing dpi and mji evolution is not possible to identify any pattern of correlation because in some periods they are in the same phase, and in times are in counter phase. however, a correspondence may be detected for extreme rainfall events: maximums values of dpi are associated with minimum of mji; at least the most outstanding rainfall (heavy rainfall) events are associated to low values of mji. the figures 3-5 present spectra of several stations of the regions analyzed in this paper (due to limitation of space, is not possible to present the spectra for all 78 stations). also, it is necessary to consider that in these graphics the periods less than 2 decades (20 days) are not observed, because 2 decades coincide with 0.5 frequency (niquist frequency), under which is not possible to represent the spectrum. the analysis of figure 3 (left side) shows that for east and central caribbean region the graphics are similar: all spectra have peaks at 20, 30, 45 and 60 days period. in the western sector ( aeropuerto el embrujo–providence island, and aeropuerto sesquicentenario – san andres island) these peaks are not marked and just the 20, 30 and 60 days period are slightly noticeable. in several spectra peaks are presented even over 80-90-days period, however this interval approaches to seasonal scale. the spectra over caribbean coastal sector (figure 3, right) show also peaks at 20, 30, and 45-days period, but the signal over 60-days period is very weak or is missing. in the mountainous sector of the region (figure 4) there are some places were spectra did not presented an outstanding signal or the signals are weak (airport la nubia, airport a nariño and obonuco in pasto, colombia), however many of them (airport camilo daza – cúcuta, uptc and eldorado, also in colombia) have the 20, 30, and 45-days periods are well defined the figure 4 (right side) shows spectra for meteorological stations of the eastern lowlands of orinoco and amazon basin. these spectra in the sector of intraseasonal frequencies are very noisy and it is difficult to identify clearly defined peaks at a given period. the same situation occurs in several places in the pacific sector (figure 5), however there are points like tocumen (panama), panamericana and bonanza (colombia) and manta (ecuador) where peaks outstand over 20-25, 30 and 45-days period. also, a intraseasonal variability of rainfall over northern south america and caribbean region 203 204 j.d. pabón and j. dorado figure 4. same as figure 3, but for the stations located on andean (left) and eastern plain (right) zone region. intraseasonal variability of rainfall over northern south america and caribbean region 205 cont. figure 4. marked peak appears over the interval of 50-60-days period in panamericana and buenaventura (colombia) such as in manta (ecuador). summarizing in a whole the region, the signals 20-25, 30and 45-days period are the most noticeable of the isv of precipitation. a 50-60-days sig206 j.d. pabón and j. dorado figure 5. same as in figure 3, but for the stations of pacific zone. intraseasonal variability of rainfall over northern south america and caribbean region 207 f ig u ra 6 .s pa ti al di st ri bu ti on of sp ec tr al de ns it y fo r 20 -, 30 -, 45 (t op ,f or m le ft to ri gh t) ,5 5,6 0an d 90 -d ay s (b ot to m ) . 208 j.d. pabón and j. dorado f ig u re 7 .s pa ti al di st ri bu ti on of co rr el at io n co ef fi ci en ts be tw ee n m ji ov er 12 0° w (t op ) an d 40 °w (b ot to m ) an d th e d p i or ig in al se ri es (l ef t) ,i ts 3po in ts sm oo th ed d p i (c en te r) ,a nd 5po in ts sm oo th ed d p i (r ig ht ). nal appears clearly in the caribbean region, but is not important in others regions. the maps presented in figure 6 show the spatial distribution of spectral density of 20-, 30-, and 45-days signals. it is possible to conclude that in spite the 20-days signal is observed in all region, the major spectral density is observed over western caribbean and southwestern sector (ecuador and pacific ocean). for 30and 45-days signals there is a similar distribution. the relationship between dpi and mji was explored calculating the correlation coefficients. these coefficients are very low when the correlation is calculated for original (not smoothed) dpi time series, and increase as the high frequency modes are smoothing by moving averages. this fact suggests that the high frequency of isv of regional precipitation is controlled by processes different from mjo. the spatial distribution of correlation coefficients are presented in figure 7. 4. conclusions the analysis made above shows that in the isv of precipitation over northern south america and caribbean region there are signals with 20, 30, 45 and 60-days period. the three first are persistently observed in all the zones of the analyzed region, while the last is observed only in both the caribbean islands and some places of pacific sector. searching the relationship between intraseasonal variability of regional precipitation and madden-julian oscillation it did not find a defined association pattern and even the correlation coefficients betweenmji and dpiwere very low; however, was established that the heavy rainfall events are associated with low values of mji. the low values of correlation coefficients and their increasing with smoothing of dpi suggest that the regional isv is controlled not only by mjo. it is necessary to explore the nature of high frequency (20-days, for example) modes. acknowledgments this paper partially reports the results of the research project “analysis of colombian climate variability generated by processes different from el niño-la niña-southern oscillation cycle” supported by both colciencias (colombian research support agency; grant no. 1118-05-16900, rc 178-2004) and research division of national university of colombia (project numbers dib-20100004448 (805253) and dib-20101004929). references alfaro, e., cid, l. & enfield, d. 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(2000): algunos algoritmos para filtros digitales en el procesamiento de informacion electrornaqnetica. geofis. colomb. 4:37-42. issn 0121-2974. santa fe de bogota, d.c. colombia resumen ei proceso de anal isis de senates en prospeccion electrornaqnetica, ha side trabajado desde diferentes puntos de vista y varios autores han planteado diferentes metodologias. ei objeto de este articulo es plantear algunos algoritmos para filtros digitales basad os en el anal isis de fourier. en el trabajo se muestran las facilidades del uso del dominio de la frecuencia en el anal isis de senates en prospeccion electrornaqnetica. abstract the signal processing in electromagnetic prospecting have been worked from several points of view and several authors there is expounding several methodologies. the object of this paper is to expound some algorithms for digital filters based on the analysis of fourier. in the work the facilities of the use of the frequency domain in the signal processing are shown. 1. introduccion ei problema de la lnduccion electrornaqnetica ha sido tratado por varios autores, maxwell en 1892; price en 1949 planteo por primera vez las ecuaciones basicas en prospeccion electromagnetica; bullard & parker en 1970; bailey en 1977; green & weaver en 1978; dawson & weaver en 1983; park en 1985; robertson en 1987; smith & west en 1987: zhan & oldenburg en 1999 entre otros, han planteado modelos y metodologias para el analisis de la informacion concerniente al rnetodo de induccion electrornaqnetica. la implernentacion de fiitros en el dominio de la frecuencia que algunos autores, de los antes citados, han planteado desde diversos puntos de vista, es el objetivo de este documento, el cual se aborda en torno a la metodologia del analisis de fourier. ei uso de filtros en el dominio de la frecuencia, permite manejar facilmente la informacion y plantear soluciones al problema de inversion. con este objetivo se retoman algunos algoritmos y se plantean otros, que pueden ser utilizados sequn las necesidades en el proceso de anal isis de senates en prospeccion electrornaqnetica. 2. fundamentos fisicos en un estrato delgado asimilable a una placa de espesor d(x,y), conductividad a(x, y) y resistencia superficial r(x,y) (ver figura), en presencia de un campo maqnetico variable en el tiempo, se induce una corriente electrica ~ superficial con densidad k (x, y) y un campo rnaqnetico secundario. smith & west (1987) definen conductancia (8) como: 8 = ad y resistencia superficial, como: 1r(x, y) = a(x, y)d(x, y) [1] la densidad de corriente superficial con la componente ---> vertical del campo electrico e r . sequn la ley de ohm, se pueden relacionar de la siguiente manera: ~ ~ ~ et k(x, y)= j d = r(x,y) [2] la intensidad del campo rnaqnetico total fuera del cuerpo considerado, se pede expresar en funcion del campo primario y el campo secunda rio asociado al campo inducido en el medio. [3] \ " i ; ! hp!~------~\---....-----.....-~--....j.-...-. ,'/ hs " l----l " t~.! (1 "i \ 'r---~...---' + (.:. condiciones de frontera: a) existe continuidad en el campo rnaqnetico primario: [4] b) la componente normal del campo secundario es continua: ~ n. (h~ [5] c) la componente tangencial es igual en magnitud, perc de signa contra rio en las caras del estrato: ~ ~ n x (h~+ h:) = 0 [6] n es un vector unitario normal a la superficie de la placa. ~ hs+ campo rnaqnetico secundario sobre la placa. ~ hs_: campo rnaqnetico secundario bajo la placa. asumiendo que no hay efectos rnaqneticos producidos por corrientes de desplazamiento y que la permeabilidad maqnetica es constante, se plantean las leyes de ampere, faraday y gauss de la siguiente forma: ---> --->' vxh=j [7] ---> ---> ah v xe = -fl-at [8] ---> v.h =0 [9] ei campo rnaqnetico variable en el tiempo y sus componentes, satisfacen la ecuaci6n de laplace y la ley de gauss fuera de la placa considerada, 10 cual hace posible que el campo rnaqnetico y su transformada de fourier se expresen mediante las ecuaciones [10] (bath, 1974). [10] ei analisis de seriales en geofisica se realiza en forma discreta, por 10tanto la transfarmada directa e inversa del campo maqnetico se presenta en terrninos de las siguientes series arm6nicas, ~ -t e-i(kmner + cot) [11] en dande: hjkt: representa la funci6n espacia-temporal campo rnaqnetico. hmnw: representa los coeficientes de fourier del campo rnaqnetico, ~ ,.. ,..r = x ex + yey : vector de posici6n. ~ kmn = km ex + ~ €.y : vector de onda . ~ ~ kmn • r = km x + ~ y kmn : magnitud del vector de onda. z : determina el plano de observaci6n. o :s; m,j s m 1 o :s; m, k s n 1 m: numero de observaciones en la direcci6n "x''. n: numero de observaciones en la direcci6n "y". t: periodo en el tiempo. co : frecuencia angular de serial. t: variable en el dominio tiempo. la expresi6n [12] representa la comunicaci6n entre el dominic espacio-temporal y dominic de la frecuencia usando la transformada directa y la transformada inversa de fourier. [12] con base en estos planteamientos flsico-maternaticos. se desarrollan algunos algoritmos, que permiten operar facilrnente en los dos dominios, simplificando procesos de calculo e implementando criterios de interpretaci6n. 3. derivadas del campo magnetico a partir de la definici6n del campo rnaqnetico hjkt presentada en la ecuaci6n [11], se calculan sus derivadas horizontales, su derivada vertical y su derivada con respecto al tiempo. ai realizar cualquiera de estes procesos, la serie arm6nica conserva su estructura; solamente se presentan cambios en el terrnino que corresponde a los coeficientes de fourier. ei algoritmo empleado para realizar el calculo en cada uno de los dos dominios se muestra en los siguientes esquemas. los subindices m, n, j, k ... , que controlan el proceso de calculo, corresponden a los planteados inicialmente en la ecuaci6n [11]. derivada horizontal en la direcci6n del eje "x" (bath, 1974): .....('-----> ikm hmnw [13] derivada horizontal en la direcci6n del eje "y": [14] derivada vertical en la direcci6n del eje "z" : -..(--;------>. kmn hmnw [15] derivada con respecto a la variable ''t'' : -<'--------->. ico hmnw [16] estos algoritmos, son utiles tanto en procesos particulares como en conjunto para obtener otros resultados, ajustandose siempre a las propiedades de la transformada de fourier. 4. componentes del campo magnetico las derivadas verticales de las componentes horizontales del campo rnaqnetico se pueden expresar en funci6n de las derivadas horizontales de su componente vertical. en este desarrollo se emplean las componentes del campo rnaqnetico secundario, pero igualmente es valido para el campo rnaqnetico primario y para el campo rnaqnetico total. la derivada vertical de la componente horizontal h~ del campo rnaqnetico, es igual a la derivada en la direcci6n "x" de la componente vertical (ver apendice a). a hs a hs__ x z az ax [17] en el dominio de la frecuencia la ecuaci6n [17] se expresa: kmn h~ mnw = i km hi rnnro [18] hs . km hs x mnw = i kmn ''z mnw [19] 10 cual permite esquematizar el proceso de la siguiente forma: h~ i km hi mnw kmn [20].....(:--------=-> la derivada vertical de la componente horizontal ~ del campo maqnetico, es igual a fa derivada en la direcci6n "y" de la componente vertical hi ,(ver apendice a). a~ _ ahi az &y [21] yen el dominic de la frecuencia, kmn ~ mnw = i ~ hi mnw [22] hs . ~ hs •y mnw = i kmn ' 'z mnw [23] .....('-----_---.:...-> i ~ ·himnw kmn [24] smith & west (1987), plantean un filtro semejante al anterior a partir de la transformada de hilbert. 5.problemainverso la metodologia que permite deierminar los parametres resistividad y espesor, considera la componente normal del campo rnaqnetico en el espacio libre fuera de la placa y la componente normal de la ley de faraday. [25] ~ '" (n ~e) a(n .h)n· y x = -il -'-------'--at [26] aey aex ahz--=-il-[27]ax &y at usando la ecuaci6n [2] y aplicando la condici6n de frontera [5] a la ecuaci6n [7], se encuentran los siguientes valores para las componentes horizontales del campo electrico y de la densidad de corriente superficial. (smith & west, 1987): ey = rky [28] ky = 2h~ ai reemplazar estos valores en la componente normal obtenida a partir de la ley de faraday [27], se obtiene una ecuaci6n diferencial que representa el proceso de inducci6n en la lamina, en terrninos de la densidad superficial y sus derivadas horizontales, las componentes y derivadas del campo secundario, la derivada vertical y la derivada con respecto al tiempo de la componente vertical del campo rnaqnetico. (price, 1949). (smith & west, 1987). [29] si se considera un medio hornoqeneo e isotr6pico, la ecuaci6n anterior se reduce considerablemente: r ahi+ _ il ahz ------az 2 at [30] esta ecuaci6n se puede lievar al dominio de la frecuencia, aplicando la transformada de fourier a cada uno de sus terrninos. r kmn hi mnw = ~ i w [ hf mnw hi mnw] [31] hi mnw icrdllw i-.p = (2kmn + icrdllw)''z mnw [32] considerando la ecuaci6n [31] y la ecuaci6n [25] en el dominio de la frecuencia: hz mnw = hf mnw + hi mnw' se encuentran los coeficientes de fourier de la componente vertical del campo rnaqnetico total en terrnino de los parametres od y los coeficientes de fourier de la componente vertical del campo primario: hz mnw = hf mnw + hi mnw [33] hz mnw hf mnw 2(kmn + iadwo) 2kmn + icduo [34) en terrninos generales. es posible solucionar el problema inverso utilizando el rnetodo de minimos cuadrados; tomando como modelo el planteado en la ecuaci6n [32) 0 en la [34), dependiendo de los datos considerados, hz mnw hi mnwo hf mnro hf mnw ei ajuste se realiza en el domino de la frecuencia, con el fin de encontrar los mejores valores de ad . de tal manera que la suma de las desviaciones cuadraticas entre los datos observados y los calculados por el modele sean minima. con base en el muestreo de campo electrornaqnetico y el modele elegido, se obtiene un sistema de ecuaciones simultaneas cuya soluci6n se obtiene utilizando el siguiente proceso iterativo: [35) en donde: j-g: matriz inversa generalizada del jacobiano del sistema. d: matriz de valores observados, en este caso los coeficientes de fourier. m : matriz que contiene las primeras aproximaciones de los parametres. m(m) : modelo evaluado con los para metros m· m+l: matriz que contiene nuevos parametres evaluados a partir de los parametres m, de los datos d y el jacobiano del sistema evaluado tam bien con los para metros m . ei sistema computacional entrega los parametres, encontradolos a partir de los efectos rnaqneticos producidos en el proceso de inducci6n. si se expresa la ecuaci6n [34] en el dominio espaciotemporal, el problema es ahora solucionar la ecuaci6n diferencial: [36] 41 la soluci6n de esta ecuacion, no es tan inmediata como el planteado anteriormente. 6. programas y proceso de calculo ei proceso de calculo se ha implementado en matlab 5.2. perc es posible usar otras formas de proceso y lenguajes, que tam bien presentan muy buenas herramientas y facilidades como fortran 99. c y c++. entre las herramientas utiles se encuentra la subrutina harm. que permite realizar analisis de fourier en tres dimensiones. esta disponible y de uso publico en internet. conclusiones ei analisis de fourier es una herramienta muy eficiente en el proceso de senates discretas en prospecci6n electrornaqnetica, ya que permite plantear en forma sencilla filtros y procesos. usando como informaci6n los coeficientes de fourier (dominio de la frecuencia) de las funciones observadas en el dominio espacio-temporal, se puede plantear y dar soluci6n al problema inverso en electromagnetismo. referencias bibliograficas bailey, r. c. (1977): electromagnetic induction over the edge of aperfectly conducting ocean: the hpolarization case: geophys. j. roy. astr. soc. 48:385392. bath, m. (1974): spectral analysis in geophysics. elsevier scientific publishing company. amsterdam oxfordnew york. bullard, e. c., & r.l. parker. (1970): electromagnetic induction in the oceans, in maxwell. a. e., ed the sea. 4, pt 1: wiley intercience, 695-730. dawson, t. w., & j.t. weaver. (1979): three dimensional induction in a nonuniform thin sheet at the surface of a uniformly conducting earth: geophys. j. roy. astr. sos. 73:83-107. green, v. r., & j.t. weaver. (1978): twodimensional induction in a thin sheetof variable integrated conductivity at the surface of a uniformly conducting earth: geophys j. roy. atr. soc. 55:721-736. maxwell, j.c. (1892): a treatice on electricity and magnetism (3rd ed): clarendon press. park, s. k. (1985): distortion of magnetotelluric sounding curves by three dimensional estructure.s: geopysics. 50: 785-797. price, a. 1. (1949): the induction of electric currents in non-uniform thin sheets and shells: quart. j. mech. and appl. math, iii pt 3,283-310. robertson, r. c., (1987): the electromagnetic response of thin sheets buried in a uniformly conducting halfspace: geophysics 52: 108-117. smith, r. s. & g.f. west. (1987): electromagnetic induction in an inhomogeneus conductive thin sheet: geophysics. 52 (12):1677-1688. zhang, z. & w. oldenburg. (1999): simultaneous reconstruction of 1-0 susceptibility and conductivity from electromagnetic data. geophysics 64 (1):33-47. apendice a relacion entre componentes del campo magnetico ei campo maqnetico esta asociado a una funci6n escalar de potencial a, tal que: ~ h = -va [1a] ~ la magnitud del vector h , se expresa como la derivada en la direcci6n del campo rnaqnetico total de la funci6n de potencia. ah = --(a)al [2a] ~ v(h) = a h al [3a] analizando las componentes de la ecuaci6n anterior en forma individual se tiene: ~ ah ahz -az al [4a] ~ ah ahx ax al [sa] ah fly ~ahy al [6a] ran." "en donde = a ex + fj ey + y ez es un vector unitario en la direcci6n y sentido del campo maqnetico total. a r a a a-0= evo=[a-o+p-o+y-oal ax fly az a ei operador ( ) en el dominio de la frecuencia seal expresa como: a al 0 en el dominio de la frecuencia las ecuaciones [4a], [sa], [6a] se expresan, como: kmn h mnw = qmn hz mnw ik qm h mnw mn hxmnw ik q hn h mnw mn y mnw [7a] [8a] [9a] de las ecuaciones [4a] a [9a] se establecen las siguientes relaciones: kmn hx mnw = i km hz mnw equivalente a: ahs z en el dominio espacio-tiempo. ax y la ecuaci6n: kmn hy mnw = i ~ hz mnw equivalente a: a~ ahs_____ z_ az fly untitled earth sciences research journal earth sci. res. j. vol. 13, no.1 (june 2009): 16-29 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti departamento de evolución de cuencas, facultad de ciencias. iguá 4225 c.p. 11400, montevideo, uruguay corresponding author: rossana muzio, rossana@fcien.edu.uy abstract this work presents new results of a detailed geological and structural investigation focusing the easternmost uruguayan mesozoic magmatic occurrences related to the south atlantic opening. lithological descriptions, their stratigraphic relationships and complimentary lithochemical characterizations carried out in the san miguel region (east uruguay) are presented. three volcanic/sub-volcanic units have been recognized. the felsic volcanic association is composed by rhyolitic dacitic flows, mainly with porphyritic textures and sub-alkaline nature and related pyroclastic rocks. the felsic sub-volcanic association is characterized by granophyres of about 25 km2 of exposed area, crosscut by mafic and felsic dykes. finally, a mafic association has been identified characterized by dykes and a small intrusion of gabbroic composition. all these units are mesozoic in age (130 – 127 ma) and according to their chemical nature they correspond to sub-alkaline to weak peralkaline magmas. key words: petrography, geochemistry, mesozoic, uruguay. resumen este trabajo presenta nuevos resultados geológico-estructurales y geoquímicos relacionados con el magmatismo mesozoico del extremo este de uruguay (sierra de san miguel, departamento de rocha), asociados a la apertura del océano atlántico sur. fueron reconocidas para la región estudiada tres unidades volcánicas/subvolcánicas. la asociación volcánica félsica está compuesta por derrames riolíticos – dacíticos, con texturas dominantemente porfiríticas y naturaleza química subalcalina. asociadas a estas riolitas ocurren 16 manuscript receiver: january 12th, 2009. accepted for publication: march 30th, 2009. niveles de rocas piroclásticas. la asociación subvolcánica félsica está compuesta por granófiros con una extensión superficial de aproximadamente 25 km2, cortados por diques máficos y félsicos. fue identificada también una asociación básica compuesta por diques y un cuerpo de gabro. todas estas unidades son de edad mesozoico (130 – 127 ma) y de acuerdo a su carácter químico corresponden a magmas de naturaleza subalcalina a débilmente peralcalina. palabras clave: petrografía, geoquímica, mesozoico, uruguay. introduction the uruguayan geology is underlain by precambrian terrains comprising a wide range of metamorphic rocks and granitic intrusions. five sedimentary basins rest on the cratonic basement, one palaeozoic and the other four mesozoic in age. mesozoic magmatism tectonically related to the breakup of gondwana is well represented in the uruguayan extension of the paraná basin and in the rift-type basins of santa lucía and laguna merin (fig.1). recently, in the offshore punta del este basin have been also recognized relicts of this magmatic event (ucha et al., 2003). the paraná magmatic province (pmp) is one of the largest continental magmatic provinces in the world (peate, 1997). although the main outcrop areas are exposed in brazil, some extensions are well developed in argentina, paraguay and uruguay. furthermore, uruguay has a privileged position because of the presence of the southernmost extension of these magmatic exposures. during the early cretaceous, a strong volcanic activity made up of basaltic and rhyolitic rocks occurred in south eastern uruguay along the border of the paraná basin. particularly, these exposures are well represented in rift type basins located in the eastern portion of uruguay (laguna merín basin) and comprise from basic to acidic units on which this paper is concerned. several petrologic works have been carried out in the south eastern portion of uruguay, with particular emphasis in the felsic volcanic/sub-volcanic units (muzio, 2000; kirstein et al., 2000; kirstein et al., 2001; muzio et al., 2002; lustrino et al. 2003; lustrino et al. 2005). as main results can be pointed out that the petrologic and isotopic features exhibited by these units differ completely from their temporal correlatives of the paraná province (palmas and chapecó rhyolites, according to bellieni et al. 1986). recent studies performed by muzio et al. (2004) and muzio et al. (2005) in the san miguel region, allowed the identification of a gabbroic body which 17 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region cuenca de paraná b r a z i l a r g e n t i n a sedimentary basins a paraná basin b santa lucía basin c laguna merín basin crystalline basement 54º00´ 31º 00´ a b c exposed areas of mesozoic magmatism jaguarão n 0 100 km salam (santa lucía aiguá merín) lineament montevideo sierra san miguel -35º00’ figure 1. structural framework and location of the area, modified after ucha et al. (2003). has been mapped at semi-detailed scale (fig. 2). this is the first basic intrusive body that has been recognized out-cropping in the southern portion of the laguna merin basin, closed to an important gravimetric anomaly (+90 mga; reytmair, 2001). according to rossello et al. (2000), this gravimetric anomaly results from the development or reactivation of deep parallel fractures, trending n70° to e-w, which affected the crust during gondwana fragmentation. these fractures, recognized through strong structural, magnetic and gravimetric lineaments, became mantle feeding structures that allowed the emplacement of basic/mafic rocks which caused the excess of gravity in the region (rossello et al, 2000; veroslavsky et al., 2002). the aim of this paper is to present the petrologic and structural characterization of the units exposed in the sierra san miguel region, corresponding to the southernmost border of the laguna merín basin (east uruguay), with particular emphasis in the gabbroic intrusion. geologic setting and petrography the sierra san miguel complex is located in the department of rocha (east uruguay), 340 km eastwards montevideo, the capital of the country. it represents an important geomorphologic feature, of about 25 km length and regional trend ew within the wetlands and quaternary sediments of the southern extreme of the laguna merin basin (fig. 2). the structural framework controlling the development and evolution of the santa lucía and laguna merin basins is underlain by the salam (santa lucia – aiguá – merin lineament, after rossello et al.; 2000). this structural feature constitutes a transtensive tectonic corridor of the strike-slip faults (ca. 450 km), cross-cutting the río de la plata craton. as it has been described by rossello et al. (2001), either two pull-apart depocentres (santa lucia and laguna merín basins) at its extremes and the emplacement of mesozoic volcanic/sub-volcanic rocks have been controlled by this structure. according to these authors, the salam compounds two main tectonic phases: (i) a jurassic to early cretaceous extensional stage, represented by intraplate magmatism synchronous with the pmp, and (ii) an aptian – albian dextral transtensive stage. in this context, the tectonic development of the sierra san miguel complex is controlled by the aiguá india muerta – chuy fault, one of the components of the salam lineament that conditioned the opening and extensional development of the laguna merín basin (rossello et al., 2000). the pioneer geological works in the sierra san miguel correspond to walther (1927), caorsi & goñi (1958) and bossi & fernández (1963) and it has been described as a volcanic/sub-volcanic association composed by granophyres and, subordinated rhyolites. these lithologies were later integrated to the arequita formation by bossi (1966). the only available radiometric data by k/ar systematic (whole rock analyses) yielded ages between 125 – 130 ma (bossi, 1969). after recent geological studies developed by muzio et al. (2004), three petrographic associations have been identified, each-one related to different tectomagmatic stages (fig.2). unit 1 – felsic sub-volcanic association (fsa) the fsa is located mainly in the east and central portions of the sierra san miguel which occurs as an irregular body along an ew to ne axis. it is composed by strongly fractured granophyric rocks, cross-cut by sub-vertical basic and acidic/felsic dykes with structural trend n65º and ew respectively. the granophyres are progressively covered by rhyolitic lava flows. the fracture system (sub-horizontal and vertical unfilled fractures) outlining a roughly polyhedral pattern can also be observed. these granophyres are mineralogical and petrographycally homogeneous. the mineral association of these rocks is essentially anhydrous. they are medium to fine grain-sized, with equigranular and occasionally porphyritic hypidiomorphic textures (up to 10% of plagioclase phenocrysts). it can be observed the following mineralogical assemblage: quartz alkali feldspar intergrowths (micrographic/granophyric texture) and 18 rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti 19 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region e st er o de lap un ta n eg ra del tigre c añ ad a rin có n bravo sa n m ig ue l w et la nd s vi g ia h il l r od eo v ie jo cr ee k c ar bo n er a h il l s an m ig ue l sa n m ig ue l f or t sa ra nd íc re ek 18 de ju lio s ar an dí b r a z il 70 0 70 2 70 4 70 6 70 8 5 3 º4 1 ’ 3 3 º3 2 ’ 71 0 71 2 70 2 70 4 70 6 70 8 71 0 71 2 70 0 69 8 69 6 69 4 69 2 69 0 68 8 69 8 69 6 69 4 68 8 69 0 69 2 27 2 27 0 26 8 26 6 26 4 27 2 27 0 26 8 26 6 27 0 lo s d if u n to s h il l o m bú h il l 1 9 4 km . 1 km . 2 km . 3 km . 0 1 n lo ca l r ou te r oa d / l an e c re ek s in te rn at io na l b or de r vi lla ge h ills r hy ol ite s g ab br o g ra no ph yr es fa ul ts / f ra ct ur es c en oz oi c de po si ts c re ek c re ek cre ek u r u g u ay brazil argentina st ud ie d ar ea m on te vi de o de la pl at a ri ve r atla nti coc ea n f ig u re 2. g eo lo gi ca ls ke tc h of th e s ie rr a s an m ig ue lr eg io n, af te r m uz io et al .( 20 04 ). partially altered clinopyroxene (augite) often exhibiting uralitization processes. apatite and opaque minerals are frequent as accessory minerals. in the central-north part of the area, the granophyres are hardly affected by faulting with trends n70º. also, a synmagmatic foliation n250º 260º can be observed. this structural trend is coincident with the emplacement of mafic and felsic dykes. unit 2 – mafic association (ma) two different types of basic rocks have been recognized, according to their geological setting. the first type corresponds to basic dykes which vertically cross-cut the granophyres. as it was mentioned previously, they are porphyritic basic dykes with structural trend n65-70, 75º. they have porphyritic texture with plagioclase phenocrystals (up to 10 %) in a subophitic to intersertal groundmass. the mineral assemblage is composed by plagioclase, orthopyroxene and clinopyroxene (augite), opaque minerals and apatite. the second type of basic rocks is represented by an intrusive body – named as san miguel gabbro (smg). the main outcrop area is located around the central-north region of the sierra san miguel (s33º43’; w53º38’), and it can be described as a sub-horizontal intrusion cross-cutting the granophyric rocks. it is strongly fractured with sub-horizontal and sub-vertical joints. typical spherical structures due to alteration processes, ranging in diameter from 40 to 80 cm, can be observed. according to the iugs classification criteria (le maitre, 1989) this intrusive body can be described as a leucocratic gabbro/dolerite with massive structure, with equigranular and subophitic texture, medium to coarse grain sized, composed by plagioclase (an55-45), altered olivine, orthopyroxene, clinopyroxene (augite), opaque minerals and apatite. occasionally amphibole as deuteric alteration – is present. other small gabbroic intrusions appear as wedges or secondary dykes; all injected through the granophyres. euhedral to subhedral plagioclase crystals surrounded by micrographic/ granophyric intergrowths are present. this petrographic feature is also observed as an interstitial granophyric intergrowth, and occurs plagioclase and clinopyroxene crystals with intergranular arrangement, in association with k-feldspar and quartz. according to shelley (1992) this kind of texture is consistent with residual crystallization under eutectic conditions and corresponds either with the emplacement of shallow intrusions or with fast degassing processes. unit 3 – felsic volcanic association (fva) they correspond mainly to pinkish grey colored rhyolites and compound at least three lava flows softly dipping to the north. they are distributed along the north, northwestern and western region of the sierra san miguel. these lithologies are grouped in the arequita formation (bossi, 1966) and can be separated, according to their petrographic and structural features in: a) rhyolitic lavas, occasionally with porphyritic textures (approximately 10 to 15% of alkali feldspar and/or quartz phenocrysts), with vesicular levels filled by calcite, zeolite and quartz. most of these vesicles are stretched according to flow directions such as n170º to ew. microscopically, spherulitic texture and flow structures formed during the cooling of the rhyolitic lava can be observed. some of the porphyritic varieties present abundant glass phenocrysts, round shaped and brown to dark black colors. some volcaniclastic facies in association with the rhyolites have been found and according to mcphie et al. (1993) descriptive criteria they correspond to volcanic lithic breccias and autoclastic breccias (fig. 3). b) rhyolitic dykes with porphyritic texture trending n60-70º and cross-cutting all the lithologies are present in the study area. they present quartz and k-feldspar phenocrysts in an aphyric matrix composed by 20 rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti quartz, k-feldspar (orthoclase?) and clinopyroxene. their stratigraphic relationship with the mafic dykes was not observed. lithogeochemistry chemical composition of ten samples (one granophyre, three gabbros, one mafic dyke, three felsic dykes and two rhyolites) of the sierra de san miguel complex was analyzed. major and trace elements were obtained by icp – aes techniques whereas ree by using icp – ms, performed at acme laboratories inc., vancouver, canada. the analytical data are listed in table i and illustrated in fig 3-7. they were plotted in different classification and variation diagrams and they were also compared with similar rocks performed by kirstein et al. (2001) and lustrino et al. (2005) in order to evaluate the data for the region. the sio2 variation diagrams (fig.4) show no extremely high contents in the felsic volcanic samples. however, they present high contents of l.o.i. (>3%). the analyzed samples have sio2 ranges from 71.1 to 48.49 wt.%; tio2 from 1.48 to 0.58 wt.%; al2o3 from 16.22 to 11.6 wt.%; fe2o3 from 14.2 to 5.01 wt.%; mgo from 9.11 to 0.23 wt.%; cao from 12.04 to 0.82 wt%; na2o from 3.33 to 2.19 wt.% and k2o from 4.89 to 0.2 wt.%. when plotted against sio2, smooth trends are showed. tio2, al2o3, fe2o3, mgo and cao decrease with increasing sio2 content. na2o does not show a trend at all, while k2o increases towards the rhyolitic terms. furthermore, na2o and k2o present an inverse behavior when compared with lustrino et al. (2005) data. trace element contents present a short range when they are compared with published data (lustrino et al., 2005; kirstein et al., 2000). rb varies from 184.4 to 5.4; zr from 387.1 to 58; y from 76.6 to 22.2; nb from 24 to 3.7; ba from 721.1 to 111 and sr from 207.8 to 85.4 (fig. 5, table i). the ratios between incompatible elements (fig. 6) vary within narrow ranges (zr/nb= 15.03±1.05; zr/y=4.11±1.54; y/nb= 4.39 ± 1.72). these values are very similar to the treinta y tres magma type pointed out by turner et al. (1999); kirstein et al. (2000) and lustrino et al. (2005). the rock samples have been classified using the r1-r2 diagram (de la roche et al., 1980) and tas diagrams (le maitre et al., 1989) (fig. 7a and 7b). two samples of the plutonic rocks plot in the gabbro norite field of the r1-r2 diagram. the volcanic rocks plot mainly in the rhyolitic and dacitic fields and one sample in the basaltic-andesitic field. all samples show a sub-alkaline affinity according to irvine and baragar (1971) diagram (fig. 7c). however, the analyzed data are not enough to establish any petrogenetic link among the samples. the agpaitic index calculated for the samples (a.i. = molar (na+k)/al) ranges from 0.24 to 0.74. the rhyolites range from 0.71 to 0.74; dacites present values of 0.66 and andesites around 0.58. the lower values correspond to the gabbro – norite samples. none of the samples show a peralkaline character as described for the arequita formation by muzio et al. (2002). hence, they have been classified on the basis 21 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region figure 3. lithic breccias and autoclastic breccias associated with the felsic volcanics in sierra san miguel region. 22 rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti 40 50 60 70 80 0.0 1.6 3.2 4.8 6.4 8.0 sio2 k 2o 40 50 60 70 80 2 3 4 sio2 n a o 2 40 50 60 70 80 0 1 2 sio2 t io 2 40 50 60 70 80 0 1 2 3 4 5 6 7 8 9 10 sio2 m g o 40 50 60 70 80 0 10 20 sio2 c a o 40 50 60 70 80 10 11 12 13 14 15 16 17 18 19 20 sio2 a l o 2 3 40 50 60 70 80 0 10 20 sio2 f e 2 o 3 t figure 4. major and minor element diagrams for samples from the san miguel region. symbols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. 23 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region table 1. geochemical data of selected samples from the sierra san miguel region. samples sm50-1 sm50-2 sm50-3 sm008-7 sm008-8 sm008-9 sm51 sm13 sm27 sm29-1 sio2 (wt.%) 65,01 64,24 65,1 58,71 48,98 48,49 56,41 69,72 71,1 69,58 tio2 0,88 0,92 0,87 0,89 0,58 0,59 1,48 0,76 0,63 0,7 al2o3 12,58 12,38 12,38 13,99 16,22 16,13 12,67 12,74 11,6 12,81 fe2o3 5,87 6,43 5,66 8,89 9,53 9,8 14,2 6,15 5,01 5,66 mno 0,14 0,14 0,13 0,17 0,15 0,15 0,2 0,11 0,16 0,13 mgo 1,3 1,44 1,28 3,28 9,11 8,95 2,85 0,3 0,67 0,23 cao 3,95 4,18 3,99 6,44 11,78 12,04 6,28 1,71 1,75 0,82 na2o 3,33 3,13 2,97 3,18 2,25 2,19 2,79 3,24 2,42 2,28 k2o 2,77 2,76 3,12 2,42 0,2 0,25 2,54 3,81 4,22 4,89 p2o5 0,26 0,23 0,24 0,28 0,09 0,11 0,19 0,2 0,17 0,16 cr2o3 0,004 0,005 0,015 0,013 0,053 0,068 0,019 0,015 0,041 0,008 loi % 3,8 3,9 4 1,5 1 1,2 0,3 1,1 2,1 2,6 tot/c % 0,03 0,02 0,03 0,03 0,03 0,05 0,02 0,05 0,05 0,04 tot/s % 0,01 <.01 <.01 <.01 0,01 0,06 0,01 0,01 0,01 0,01 sum % 99,89 99,76 99,76 99,77 99,98 100 99,95 99,86 99,89 99,88 rb (ppm) 135,8 127,3 142,5 89,9 7,3 5,4 88,8 150,7 158,8 184,5 ba 625 599,2 583,9 458,1 111 112,6 365,4 721,1 654,6 712,6 sr 136,8 146,3 146,3 207,8 133,3 139,1 123,1 125,9 92,8 85,4 zr 301,1 276,4 289,6 237,1 72,2 58 189,4 387,1 293,5 327,9 y 57,6 56 56,9 56,4 22,2 22,6 50,2 76,6 64,4 58,1 ga 15,9 16,1 15,8 19,4 14 14,5 19,7 18 15,9 17 hf 8,6 8,1 8,5 7,2 2,2 1,8 5,7 11 8,7 9,3 nb 21,6 19,5 20,8 16,6 4,6 3,7 13,4 24 18,7 20,4 ta 1,4 1,4 1,5 1,2 0,4 0,3 1 1,7 1,4 1,5 th 16 14,2 15,4 7,4 1,8 1,2 8,1 14,2 14,4 14,7 u 2,5 2,3 2,6 1,4 0,4 0,3 1,3 1,7 2,5 2,7 ni 29,7 18,8 67,5 31,5 110,5 118,1 65,2 43,9 152,1 39 la 38,8 37,5 40,1 28,7 7,3 7,3 23,1 46,6 42,4 40,7 ce 96,4 91,8 99,4 68,7 16,9 16,1 54,2 107,2 99,6 102,2 pr 10,71 10,06 10,77 8,41 2,24 2,18 6,47 13,1 11,38 10,17 nd 44,3 41,3 43,6 36,8 10,2 9,9 27,3 54,8 45 41 of the alumina saturation index (a.s.i. = molar al/ (na+k+ca)) yielding values from 0.63 to 1.24. most of the samples present a metaluminous character (a.s.i. <1), and only three of them present a weak peraluminous character (a.s.i.>1). the samples present moderate light to heavy ree fractionation in chondrite normalized plot [(la/yb) í = 5.02 to 2.25]. in one hand, the rhyolites and the acidic to basic dykes have very strong negative eu anomalies (eu/eu* = 0.23 to 0.19), and a flat to slightly concave pattern of hree. on the other hand, the gabbro – norite samples show a flat pattern when ree normalized against chondrite c1 (la/yb) n = 5) and the highest values for eu/eu* (from 0.26 to 0.28) yielding less marked negative eu anomalies (fig. 8a). the rocks analyzed do not show typical patterns in the primitive mantle normalized spidergram when compared with the early cretaceous santa lucía type and aiguá type magmas (turner et al., 1999; kirstein et al., 2000). negative peaks of ba, nb, sr and pb are present. only the rhyolitic samples do not show pb troughs (fig. 8b). the plutonic rocks show almost a flat pattern in primitive mantle normalized diagram. enrichment in ba, th and u is observed. discussion and conclusions field relationships, petrography and geochemistry indicate a bimodal character for san miguel complex and reflect the general character of extensional magmatism. this complex is associated with a regional fault system responsible for the opening and extensional development of the laguna merín basin. 24 rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti samples sm50-1 sm50-2 sm50-3 sm008-7 sm008-8 sm008-9 sm51 sm13 sm27 sm29-1 sm 9,1 8,9 9 8,5 2,7 2,5 6,1 11,7 10,3 8,6 eu 1,87 1,86 1,79 2,26 0,77 0,79 1,55 2,35 2,02 1,84 gd 9,35 9,08 9,41 9,03 3,08 3,24 7,4 11,97 10,31 9,06 tb 1,8 1,7 1,75 1,78 0,64 0,66 1,41 2,36 2,01 1,81 dy 9,85 9,57 9,84 9,68 3,58 3,92 8,07 12,87 10,92 10,08 ho 1,94 1,95 2,05 1,95 0,75 0,81 1,76 2,53 2,16 1,99 er 6,08 5,95 6,18 5,91 2,32 2,37 5,25 7,59 6,55 6,06 tm 0,98 0,91 0,92 0,87 0,36 0,36 0,82 1,16 0,96 0,94 yb 5,86 5,72 5,83 5,69 2,33 2,19 5 7,09 6,17 5,82 lu 0,87 0,85 0,88 0,85 0,35 0,33 0,75 1,06 0,94 0,84 zr/nb 13,94 14,17 13,92 14,28 15,70 15,68 14,13 16,13 15,70 16,07 zr/y 5,23 4,94 5,09 4,20 3,25 2,57 3,77 5,05 4,56 5,64 y/nb 2,67 0,46 0,49 0,38 0,10 0,09 0,47 0,39 0,54 0,56 (la/yb)n 4,75 4,70 4,93 3,62 2,25 2,39 3,31 4,71 4,93 5,02 eu/eu* 0,20 0,21 0,19 0,26 0,27 0,28 0,23 0,20 0,20 0,21 rb/sr 0,99 0,87 0,97 0,43 0,05 0,04 0,72 1,20 1,71 2,16 agpaitic index 0,673 0,657 0,667 0,561 0,241 0,24 0,579 0,742 0,736 0,705 a.s.index 0,82 0,801 0,813 0,728 0,643 0,629 0,684 1,034 1,007 1,239 25 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region thus, the sierra san miguel morphological feature shows a clear correspondence with the main faults of the basin and the aiguá – india muerta – chuy lineament, trending n60º which is also discordant with other regional structures of the punta del este basement. the studied area shows structural concordance between the conspicuous foliation (n60º – 70º) exposed in the granophyres and the brittle deformation trends (e-w and n70º) affecting all the mesozoic rocks, including the regional trend of the sierra san miguel the stratigraphic array observed for the sub-volcanic/volcanic lithologies (gabbro, granophyres and rhyolites) shows that the fault system had important vertical components synchronous with the magmatic events in order to formulate a close spatial-temporal relation between these rocks. the san miguel gabbro should be linked with the tectomagmatic event responsible of the opening of the laguna merín basin 0 100 200 300 400 20 30 40 50 60 70 80 zr y 0 100 200 300 400 0 10 20 30 zr n b 0 100 200 300 100 200 300 400 500 600 700 800 sr b a 45 50 55 60 65 70 75 80 0 20 40 60 80 100 120 140 160 180 200 sio2 n i 0 5 10 15 20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 th t h /n b figure 5. trace element diagrams for samples from the san miguel region. symbols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. 26 rossana muzio, elena peel, ethel morales, gerardo veroslavsky, bruno conti 0 100 200 300 400 0 10 20 30 zr n b (a) 0 100 200 300 400 20 30 40 50 60 70 80 zr y (b) 0.001 0.01 0.1 1 10 40 50 60 70 80 rhyolite rhyodacite-dacite andesite tran sub-ab bas-trach-neph com/pan trachyte phonolite zr/tio2*0.0001 s io 2 ( c) figure 6. trace element diagrams and trace element ratio diagram for samples from the san miguel region. symbols: � felsic dykes;� felsic lavas; � gabbros; � basic dyke; � granophyre. 16 14 12 10 8 6 4 2 0 40 50 60 70 80 sio2 n a o + k o 2 2 (b) (c ) 35 40 45 50 55 60 65 70 75 80 85 0 2 4 6 8 10 12 14 16 18 20 alkaline subalkaline sio2 n a 2 o + k 2 o gabbronorite olivine gabbro alkali gabbro syenogabbro syenodiorite essexite nepheline syenite syenite quartz syenite alkali granite syenogranite monzogranite granodiorite diorite gabbro tonalite quartz monzonite monzonite monzonitediorite m on zo ga bb ro 400 400 800 800 1200 1200 1600 1600 2000 2000 2200 2400 2800 0 0 200 600 1000 1400 1800 r1 r 2 (a) figure 7. classification diagrams. (a) multicationic classification diagram (de la roche et al. 1980) ; (b) tas classification diagram (le maitre et al. 1989); (c) tas classification diagram (irvine and baragar, 1971). symbols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. that also allowed the generation of a great depocentre filled by basic lavas named puerto gómez formation (bossi, 1966), based on the geological and structural framework. these evidences allow us to point out a continuous reactivation of the basement which conditioned the faulting and jointing style that affected all the mesozoic rocks. the structural trends presented by the dykes and the spatial arrange exposed by faults and fractures, most of them concordant with ancient planes of weakness along the basement, remark the presence of dilatants situations in agreement with the dextral sense of displacement proposed for the fault system cebollatí – merín and aiguá – india muerta – chuy. this structural framework, according to rossello et al. (2000) determines the development of the salam lineament and was the main mechanism responsible for the genesis and tectomagmatic evolution of the laguna merín basin. the spatial distribution, type and volume of magmatism related to the laguna merín basin, allow us to suggest that it represents a magmatic pull-apart type depocentre, sub parallel to the atlantic margin and developed in a pre-structured cratonic domain. according to this, the volcanic rocks exposed in the eastern region of uruguay would represent different magmatic pulses synchronous with the tectonic reactivation of the basement. this would explain the coexistence of intrusive/shallow and effusive terms in a relative short time of emplacement. as a result of the drill campaign performed by private founds near lascano city (www.ume.com.uy), the rocks described correspond to gabbros/dolerites and granites (granophyres), providing us valuable information close to the sierra san miguel region. the lascano geophysical anomaly has been interpreted as being created by gabbro to granitic (granophyres) intrusive rocks underlying a package of magnetic basalts and felsic volcanic rocks. whatever be the trigger responsible for the genesis of this gravimetric anomaly, its development can be related to the tectonic evolution of the salam lineament during the early stages of the south atlantic opening (rossello et al., 2007). according to the geological background and field relationships among the different units, the basic and acidic magmatism are considered mesozoic in age and related with the magmatic events of continental scale predecessors of the opening of the south atlantic ocean (lustrino et al. 2003). further chemical analyses should be carried out in order to explain petrogenetic correlations between these magmas and the main magma types recognized by other authors. acknowledgments the authors are grateful to the comisión sectorial de investigación científica for the financial support to 27 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region s a m p le /p ri m it iv e m a n tl e s a m p le /c 1 c h o n d ri te 1 10 100 500 la ce pr nd sm eu gd tb dy ho er tm yb lu (a) 0.0001 0.001 0.01 0.1 1 10 100 1000 cs rb ba th u nb ta la ce pb pr sr nd sm zr hf eu gd tb dy y ho er tm yb lu (b) figure 8. 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(1927). contribución al conocimiento de las rocas basálticas de la formación de gondwana en la sud-américa. boletín del instituto de geología y perforaciones nº 9, montevideo, 43 p. winchester, j.a. and floyd, p.a. (1977). geochemical discrimination of different magma series and their differentiation products using immobile elements. chemical geology 20, 325 – 343. www. ume.com.uy , uruguay mineral explorations inc. website. 29 mesozoic magmatism in east uruguay: petrological constraints related to the sierra san miguel region geociencias dic 2010-color.vp x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) carlos alberto ríos r.1, oscar mauricio castellanos a.2, akira takasu3 1 escuela de geología, universidad industrial de santander, bucaramanga, colombia 2 programa de geología, universidad de pamplona, colombia 3 geoscience department, shimane university, japan corresponding author: carlos alberto ríos reyes, escuela de geología, universidad industrial de santander, bucaramanga, colombia. tel. 57 7 6343457; fax. 57 7 6343457; e-mail: carios@uis.edu.co abstract the metamorphic rocks of the lower paleozoic silgará formation of the santander massif, eastern cordillera (colombia), were affected by a barrovian-type metamorphism under low to high temperature and medium pressure conditions. these rocks contain garnet porphyroblasts, which show several kinds of chemical zoning patterns. the garnet grains behave as closed systems with respect to the rock matrix. most of the observed zoning patterns are due to gradual changes in physicochemical conditions during growth. however, some garnet grains show complex zoning patterns during multiple deformation and metamorphic events. resumen las rocas metamórficas de la formación silgará del paleozoico inferior del macizo de santander, cordillera oriental (colombia), fueron afectadas por un metamor?smo tipo barroviano bajo condiciones de temperatura baja a alta y presión intermedia. estas rocas contienen porfidoblastos de granate, los cuales muestran varios tipos de patrones de zonación química. los granos de granate se comportan como sistemas cerrados con respecto a la matriz de la roca. la mayoría de los patrones de zonación observados son debidos a cambios graduales en las condiciones fisicoquímicas durante el crecimiento. sin embargo, algunos granos de granate muestran patrones de zonación complejos durante múltiples eventos de deformación y metamorfismo. introduction chemical zoning in metamorphic rocks has been widely studied since it preserves a chemical record of its growth history in rocks of very different chemical compositions and over a wide spectrum of metamorphic conditions. garnet often shows a distinct chemical zoning and a broad range of variability in terms of significative end-members of the solid solution, providing data to constrain the p-t-t evolution of metamorphism in orogenic belts (florence and spear, 1993). previous studies reveal that the growth zoning of garnet is controlled by several processes: (1) diffusion-controlled growth (chernoff and carlson, 1997; spear and daniel, 2001; meth and carlson, 2005), (2) differences in the degree of achievement of equilibrium for different elements (i.e., partial equilibrium) and heterogeneities in rock composition (i.e., local equilibrium) (meth and carlson, 2005), and (3) effect of multiple nuclei (daniel and spear, 1998; spiess et al., 2001). all these processes occur during crystal growth. diffusional reequilibration is the only post-growth process that leads to zoning in minerals (schwandt et al., 1996; fraser et al., 2000; ríos et al., 2008). garnet zoning may also be affected by fluid-infiltration (open systems) and deformation (erambert and austrheim, 1993; skelton et al., 2002), and these processes may be coupled for deformation changes grain size and reorganizes grain boundaries, affecting rates and pathways for fluid-mediated diffusion (kim, 2006). williams (1994) suggested a metamorphic/deformation-controlled porphyroblast growth model where the growth-rate changes are controlled by microstructure development that is manifested by compositional and textural zoning. garnet zoning patterns can be symmetrical or asymmetrical (hickmott and spear, 1992). duebendorfer and frost (1988) suggested that an original zoning pattern can be modified and truncated by development of schistosity due to selective dissolution during shearing, and kim (2006) earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 161-172research groupin geophysics universidad nacional de colombia keywords: metamorphic rocks, silgará formation, santander massif, zoned garnets, physicochemical conditions palabras clave: rocas metamórficas, formación silgará, macizo de santander, granates zonados, condiciones fisicoquímicas. record manuscript received: 01/04/2010 accepted for publication: 20/11/2010 indicated that zoning in garnet porphyroblasts can be modified by the development of surrounding foliations and by associated preferential dissolution and precipitation effects. therefore, truncated/overgrown zoning patterns may provide crucial information to elucidate the growth-deformation history of garnet and the mechanism by which they develop in (poly)metamorphic rocks. garnets of metapelitic rocks from the silgará formation of the santander massif provide an opportunity to study the nature of crystal growth, taking into account that they are chemically heterogeneous and retain a record of their growth history, and chemically heterogeneous and retain a record of their growth history, and contain microfabrics that aid in relating periods of garnet growth/dissolution to periods of deformation. several types of garnet chemical zoning have been reported in these rocks, which, in addition to microfabrics, were used for reconstruction of p-t-deformation paths during orogenesis (ríos, 1999; ríos and takasu, 1999; castellanos, 2001; ríos et al., 2003a, 2003b, 2008; garcía et al., 2005; castellanos et al., 2004, 2008). this paper focuses on the relationship between microfabrics and chemical zoning patterns of garnets of the metamorphic rocks of the silgará formation. geological setting the santander massif (eastern cordillera of colombia) comprises an early paleozoic metamorphic complex composed of the following geological units: bucaramanga gneiss complex, silgará formation and orthogneiss, which are intruded by several igneous bodies, most of them of triassic-jurassic age (goldsmith et al., 1971; boinet et al., 1985; dörr et al., 1995). figure 1 shows a generalized geological map of the santander massif. the metamorphic history of this massif is important for interpretation of the geologic and tectonic evolution of the northwestern continental margin of south america (ríos et al., 2008). the rocks of interest in this study are garnet-bearing pelites of the lower paleozoic silgará formation, which has been studied in detail by ríos and co-workers (ríos, 1999; ríos and takasu, 1999; castellanos, 2001; ríos et al., 2003a, 2003b, 2008; garcía et al., 2005; castellanos et al., 2004, 2008). the silgará formation was affected by caledonian regional metamorphism under lowto high-temperature and medium-pressure conditions (400-700 oc and 4.0-7.5 kbar), with the distinction of the biotite, garnet, staurolite-kyanite and sillimanite zones (ríos et al., 2003a, castellanos et al., 2008). well-exposed sections of this metamorphic unit crop out at the santander massif (figure 1), which is long established as classic area for the study of rock metamorphism and deformation caused by continental collision during the caledonian orogeny (ríos et al., 2008). analytical procedures x-ray maps were collected using the jeol 8800 electron probe microanalyzer of the geoscience department at the shimane university (japan). the analytical conditions were as follows: 15 kv accelerating voltage, 25-75 na beam current, 40-80 msec/pixel dwell time. full details on garnet analyses and continuous traverses for elemental distribution were presented in previous studies (ríos, 1999; castellanos, 2001; ríos et al., 2003a; garcía et al., 2005; castellanos et al., 2008). chemical zoning maps in this study, we reveal a number of important observations regarding the major element zoning and its correlation with microfabrics in garnet-bearing 162 carlos alberto ríos r., oscar mauricio castellanos a., akira takasu figure 1. generalized geological map of the santander massif modi?ed after ward et al. (1973), showing the regional distribution of metamorphic rocks of the silgará formation, showing locations of the investigated garnet-bearing pelites (white stars). pelites of the silgará formation at the santander massif. representative analyses of garnet are given by ríos (1999) and castellanos (2001). garnet is almandine rich with minor pyrope, grossular and spessartine. most garnet grains exhibit typical growth zoning patterns, except for the sillimanite zone sample, which shows a diffusion dominated zoning profile. garnet in amphibole-bearing assemblages is richer in grossular than that in amphibole-free assemblages. x-ray color maps reveal that mn and mg zoning is likely related with to smooth changes in p-t conditions during prograde and retrograde metamorphism and, hence, the distribution of these elements are controlled by local equilibrium. however, zoning in ca exhibits variable trends. in some cases, ca and mn-mg zoning patterns are spatially related, suggesting that the main process that controlled the distribution of ca is equilibrium. in most cases, however, it is generally unrelated to zoning patterns in mn and mg, in agreement with observations by spear and daniel (1998), suggesting additional processes. in these cases, we interpret the distribution of ca as a result of the complex interplay of equilibrium and the diffusive transport of ca from the rock matrix to the growing grains of garnet. a discussion of examples of chemical zoning in garnet, which are illustrated in figures 2-13, is presented below. garnet zone as shown in figure 2 (sample pcm-441), the most striking characteristic of garnet is the high mn concentration, which from core to rim varies from 52,2 to 24,9 mol% spessartine. mg increases from core to rim (3,9 to 9,7 mol%). garnet has a low-ca core (7,2 mol%) with an inflection midway (13,9 mol%), developing a pseudohexagonal band, between core and rim, decreasing towards the rim (7,8 mol%). according to spear (1993), a change in chemical zoning character from growth zoning to diffusion zoning by progressive homogenization is attributed to diffusion with increasing metamorphic grade. garnet-staurolite zone as shown in figures 3-4, ca and mn-mg zoning patterns are spatially related. garnet from sample pcm-361 (figure 3) is strongly zoned in mn, which decreases from 20,1 mol% in the core to 1,6 mol% in the rim. mg increases from core to rim (6,4 to 10,8 mol%). ca decreases from core (5,0 mol%) towards a low-ca annulus (1,8 mol%) with a sharp pentagonal outline, developing a slight discontinuity in the zoning midway between core and rim; then increases towards the rim (5,5 mol%). at the lower end of the garnet grain, the low-ca annulus is truncated against the matrix (biotite), suggesting dissolution during foliation development, although this feature is not observed in the opposite upper end of the garnet grain. the qualitative line scan through the interior of the grain suggests that zoning in ca is smooth whatever the process that formed this zoning pattern, the consequences for garnet zoning were mild. the high ca / low mn rim zone contains apatite, monazite and ilmenite aligned parallel to the margins of the garnet, whereas the euhedral low-ca annulus within the garnet corresponds to a change in mineral inclusion abundance, but does not correspond to a change in the mineral inclusion assemblage itself. the low-ca annuli should be assumed as a reaction boundary, x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) 163 figure 2. x-ray maps of mg, mn, and ca showing compositional zoning of the smallest garnet porphyroblast in sample pcm-441, which is a pelitic schist from the garnet zone characterized by a mineral assemblage of muscovite + quartz + plagioclase + garnet ± biotite, with minor k-feldspar, tourmaline, apatite, zircon, epidote, calcite and fe-ti oxides. figure 3. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-361, which is a pelitic schist from the garnet-staurolite zone characterized by the peak metamorphic assemblage of quartz + plagioclase + muscovite + biotite + garnet + staurolite, with minor phases such as ilmenite, apatite, monazite, xenotime, zircon and rutile. numerous inclusions of ilmenite define different patterns of distribution. garnet diameter = 3.25 mm. warm colors correspond to higher concentrations. which is a function of p-t-x. if the variation in ca is related to episodic reaction (consumption) of ca-bearing minerals the nature of inclusion may change. garnet from sample pcm-516 (figure 4) shows a decrease in mn from core (22,5 mol%) to rim (15,3 mol%). note a small reversal in zoning near the rim. mg increase from core to rim (12,0 to 16,1 mol%). mg and mn distributions show strongly negative correlation each other, whereas ca distribution is not. ca decreases outwards (5,3 to 3,6 mol%) and reaches a low-ca annulus with sharp pentagonal outline at mid-region (3,6 mole %); then increases towards the rim (6,3 mol%). it is well known that changes in the reactant and product assemblages occur at different times and at different sites during the reaction history of garnet crystals of different size and, therefore, their effects they cannot only be the result of events affecting the entire rock, such as a change in pressure, temperature or fluid composition. these effects reflect kinetic factors that cause elements (notably ca) to fail achieving full chemical equilibrium during garnet growth (i.e., partial equilibrirum, ríos et al., 2008). we consider that inclusion-free rims and inclusion-rich cores, supported by the occurrence of low-ca annuli lacking inclusions in garnet (e.g., pcm-361, figure 3, and pcm-516, figure 4), cannot be assumed as representing variable growth rates. the problem is diffusion of a given element (ca), and garnet can grow (high or low growth rates) with or without ca (i.e., even if diffusion of ca is hampered, fe-mn-mg garnet can grown fast/slow). texturally, although garnet outlines in these samples are slightly rounded, it is possible to observe that their rims were basically parallel to the euhedral low-ca annuli and that the majority of garnet consumption occurred at the corners or intersections of growth crystal faces of garnet. the abrupt variations in ca distribution from core to rim are probably due to the consumption of ca-enriched mineral phases in garnet-producing reactions, developing zoning patterns with euhedral low-ca annuli parallel to the garnet outlines. according to chernoff and carlson (1997), this is typical of growth zoning and suggests that very little volume diffusion took place after growth, which is not a surprise, taking into account that the diffusion coefficient of ca in garnet is smaller than that of mg-mn-fe. oscillatory zoned grains display a concentric rhythmic layering with distinct composition. it is frequently recorded from metasomatic ?uid-dominated environments and mineralized hydrothermal systems such as skarn deposits (smith et al., 2004). oscillatory zoning has been reported in calc-silicate rocks (e.g., pcm-514, figure 5). in these rocks, garnet shows a complex oscillatory zoning in ca, fluctuating between 17,0 and 23,2 mol%, opposite to the trend displayed by mg (8,9 to 11,2 mol%) and mn (19,4 to 22,6 mol%) zoning. mg and mn show a negative correlation. note at least two high-ca annuli with sharp hexagonal outlines. the variable grossular content may be related to reactions involving other calcic phases. a change in mineral assemblage may account for some of the oscillatory zoning trends observed, but, to explain this type of complex zoning, appearance/disappearance of ca-rich phases is needed. alternatively, other process (variation in fluid composition and/or infiltration of fluid) controlling the fluctuations in the availability of elements must be involved. the ca-rich bands in garnet may correlate with the breakdown of epidote, plagioclase and/or calcic amphibole 164 carlos alberto ríos r., oscar mauricio castellanos a., akira takasu figure 4. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-516, which is a pelitic schist from the garnet-staurolite zone characterized by a mineral assemblage of quartz + plagioclase + k-feldspar + muscovite + biotite + garnet, with minor ilmenite and calcite. garnet diameter = 1.20 mm. warm colors correspond to higher concentrations. figure 5. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-514, which is a calc-silicate rock from the garnet-staurolite zone characterized by a mineral assemblage of quartz + plagioclase + k-feldspar + garnet + ca-amphibole, with minor epidote, calcite, biotite, ilmenite, rutile and magnetite. garnet diameter = 0.65 mm. warm colors correspond to higher concentrations. or with the presence of a ca-rich fluid; garnet growth continued with decreasing grossular content after each of these events, with ca fractionating into garnet, plagioclase and/or calcic amphibole (ríos et al., 2008). a complex oscillatory zoning as described here has a controversial origin and it is difficult to explain it by a cyclic addition and loss of mineral phase(s) from the chemical system. abrupt compositional shifts are interpreted to re?ect sudden changes in the parameters controlling garnet growth as a result of episodic in?ections in the p–t path (garcía-casco et al., 2002) or changes in the garnet-producing reactions (jamtveit and anderson, 1992). garnet usually shows a normal zoning with mn content decreasing from core to rim, although a minimum mn content near rim sometimes is observed, revealing a reversal zoning, which in many cases reflects post-peak resorption and reequilibration during cooling by elemental diffusion during retrograde metamorphism. as shown in figure 6 (e.g., sample pcm-420), garnet exhibits growth zoning: from core to near rim, there is an increase in mg (from 3,1 to 12,0 mol%) and a decrease in mn (from 43,2 to 5,6 mol%) and in ca (from 18,0 to 7,6 mol%). note the mn distribution showing a small reversal zoning at rim. reversal in mn (from 5,6 to 9,7 mol%) is accompanied by reversal in mg (from 12,0 to 10,6 mol%). concentrations of mg and mn in the core of the smaller garnet (upper left side) correspond with near-rim compositions in the larger crystal, although ca composition is not similarly systematic. however, even if ca in the core of the grain does not reach similar concentration as in the core of the larger grain, it should be systematic because ca increases in the core. this can be explained due to the fact that the garnet was not cut through the core or that the core of the smaller grain started growth after the core of the larger grain. a more complex chemical zoning is observed in sample pcm-618 (figures 7 and 8), which reveals amoeba-like and sector zoning in garnet. amoeba-like zoned garnet (figure 7) shows a similar chemical zoning as that reported by daniel and spear (1998) for mn, which reveals multiple nuclei formed simultaneously in the core region, with nuclei expanding by growth in amoeba-shape forms along preexisting mineral grain boundaries. however, it is not clear that there are distinct nuclei taking into account the ca distribution in garnet. therefore, the zoning pattern of the core suggests dissolution, diffusion modification and overgrowth of a single grain at intermediate stages of metamorphism (i.e., before the outer mg-rich rim was formed). to identify various nuclei, district concentric zoning about them should have developed until the crystals merge into a single grain with continuous overgrown bands. the distribution of mn, with a localized maximum, suggests a single nucleus. note the strong negative correlation between mg and ca, with mg-poor and ca-rich regions in the core compared to adjacent regions. garnet shows normal zoning up to mantle, with a reversal chemical zoning from mantle to rim, revealing a retrograde metamorphism. sector-zoned garnet occurs in graphite-bearing pelites (e.g., pcm-618, figure 8) and involves the preferential inclusion of graphite and quartz with a crystallographic control. garnet displays variable patterns of zoning. it reveals that distribution of elements follows a radial (sector) trend, but in other cases distribution follows patchy and concentric trends. note the strongly correlated distributions of mg and ca, with mg-poor and ca-rich regions compared to adjacent garnet. mg from core (9,9 mol%) to rim (11,4 mol%), accompanied by a decrease of mn (from 1,8 to 0,7 mol%) and ca (from 8,2 to 7,94 mol%). as mentioned above, a localized maximum in mn suggests a single nucleous for the garnet grain. the formation of sector-zoned garnets generally is related to rapid increase of temperature during metamorphism, which that may be due to enhanced heat flow related to an extensional event associated with coeval magmatism and thermal metamorphism and/or maybe due to strong strain in high temperature shear zones as suggested by kleinschmidt et al. (2008). castellanos et al. (2004) discuss in detail the occurrence and growth history of chemically sector-zoned garnets. garnet zoning patterns can change due to dissolution, solution transfer, and diffusional modification (spear, 1993). however, garnet zoning may have been influenced not only by metamorphic processes but also by deformation. additional examples of resorption are illustrated in figures 9 and 10. meth and carlson (2005) suggest that mn reversal zoning may have resulted from partial disequilibrium at millimeter scale. the core, represented by a high mn concentration, is not situated in the geometrical center of the grain, which according to ríos et al. (2008) suggests either asymmetrical growth or that a significant amount of resorption has taken place. mn is not concentric about individual parts of the garnet, but rather is zoned in irregular, amoeba-like shapes, a pattern that reflects fast growth along grain boundary surfaces and slower dissolution and replacement of quartz inclusions (spear and daniel, 1999). garnet in sample pcm-47 (figure 9) shows normal zoning through the core to the inner rim, but in the outer rim the chemical zoning is reversed, revealing a retrograde metamorphism. from core to outer rim, there is an increase in mg (from 1,8 to 3,1 mol%) and a decrease in mn (from 4,5 to 1,5 mol%) and in ca (from 3,4 to 2,7 mol%). garnet in sample pcm-523 (figure 10) shows normal zoning from core to rim. it displays an increase in mg (from 9,0 to 13,1 mol%) and a decrease in mn (from 32,2 to 20,9 mol%). ca shows an irregular distribution (from 12,6 to 17,7 mol%). garnet is interpreted as broken by dissolution. an alternative explanation is that the grain was affected by a process of replacement by chlorite, as indicated by the high mg areas adjacent to the lower garnet rim. the chlorite grains seem to be larger than those in the matrix and not fully oriented along the main foliation, suggesting a replacement x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) 165 figure 6. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-420, which is a quartz-feldespatic pelitic rock from the garnet-staurolite zone characterized by a mineral assemblage of quartz + plagioclase + k-feldspar + biotite + garnet, with minor ilmenite and magnetite. garnet diameter = 1.75 mm. warm colors correspond to higher concentrations. of garnet by retrogression(+fluid infiltration). the (apparent) lack of orientation of chlorite suggests that the process took place with little, if any, deformation. zoning patterns of garnet reflect alteration due to dissolution/resorption induced by deformation. a truncation of chemical zoning is not only an evidence of tectonic dissolution in progressive shear zones but also an excellent indication of subsequent garnet resorption. truncation has to do with dissolution, but tectonic dissolution is not demonstrated in this study. static replacement (as indicated by biotite or quartz) seems to have been important in the development of the truncation. anomalies in the chemical zoning have occurred adjacent to boundaries of the textural zones and within the zone as suggested by vollbrecht et al. (2006). data reported by rios (1999) and castellanos (2001) revealed that the compositions of the garnet at the textural boundaries are different for different traverses, which can be attributed to dissolution. according to vollbrecht et al. (2006), this supports the suggestion that zoning reversals within garnet porphyroblast and asymmetrical zoning patterns can be explained by (1) preferential dissolution and precipitation under partial/local chemical disequilibrium and (2) changes of bulk shortening direction. sillimanite zone as shown in figure 11 (sample pcm-953), garnet shows normal zoning, which is broken by dissolution. garnet shows normal zoning up to mantle, with a reversal chemical zoning from mantle to rim, revealing a diffusion modification during retrogression. there is an increase in mg (from 2,0 to 3,1 mol%) and in ca (from 3,0 to 3,1 mol%) from core to mantle, with mg and ca decreasing to 2,6 and 1,7 mol%, respectively, from mantle to rim. mn decreases from core (6,3 mol%) to mantle (2,1 mol%), increasing from mantle (2,1 mol%) 166 carlos alberto ríos r., oscar mauricio castellanos a., akira takasu figure 7. x-ray maps of mg, mn, and ca showing compositional zoning of amoeba-like zoned garnet in sample pcm-618, which is a pelitic schist from the garnet-staurolite zone. garnet shows patch and amoeba-like shapes of the ca zoning. garnet diameter = 2.50 mm. warm colors correspond to higher concentrations. x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) 167 figure 8. x-ray maps of mg, mn, and ca showing compositional zoning of sector-zoned garnet in sample pcm-618, which is a pelitic schist from the garnet-staurolite zone. garnet diameter = 2.25 mm. warm colors correspond to higher concentrations. figure 9. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-47, which is a pelitic schist from the garnet-staurolite zone. garnet diameter = 1.40 mm. zoning is mild. it seems that diffusion at near peak conditions relaxed the original growth pattern. warm colors correspond to higher concentrations. to rim (7,0 mol%). chlorite clearly replaces garnet. hence, it seems that the dissolution (replacement) process took place during retrogression(+fluid infiltration). the (apparent) lack of orientation of chlorite suggests that the process took place with little, if any, deformation. this textural evidence also suggests that it was related to cooling as demonstrated by the high mn rim (produced either by diffusion modification). on the other hand, it is clear that ca behaves independently and does not mimic mg and mn zoning but it should be taken into account that the variation in ca in the core-mantle region is smooth. garnet resorption would be expected to produce an irregular, embayed garnet margin and an overgrowth on such irregular boundary should produce a subhedral to anhedral low-ca annulus, as observed in the studied garnets (e.g., pcm-473, figure 12). garnet exhibits reverse zoning, with a decrease in mg (from 11,0 to 6,0 mol%) and increase in mn (from 6,9 to 9,3 mol%) from core to rim. composition is more homogenous in the interior of the crystal and ca content increases slightly within the outer core (4,6-7,0 mol%), where it reaches a maximum, then decreases to 4,5 mol% at rim, developing a low-ca annulus. a sharp decrease of grossular content from core to the mid-region has been interpreted by menard and spear (1993) as produced by resorption of garnet during production of staurolite, which is unlikely because the garnet core is euhedral. chemical zoning does not follow the shape of the marked embayments (filled by quartz, biotite and ilmenite), indicating that it is broken by dissolution. the garnet grain was probably dissolved (and replaced), but tectonic-drive dissolution is not demonstrated. the classical theory of pressure-driven dissolution predicts dissolution at the interfaces (mechanical discontinuities) orthogonal to the larger main principal stress and precipitation at the interfaces parallel to the smaller main principal stress. this is not clearly seen in the above images. an alternative is that it breaks apart in different pieces during non-coaxial deformation. in this case, the growth zoning is broken, but dissolution does not necessarily take place. after breaking apart, the pieces may experience further growth or they may experience retrogression (dissolution). it also suggests resorption or partial breakdown of the garnet (spear et al., 1995) during the p-t decreasing probably linked to the exhumation of the rock. however, embayments along the garnet grain originally interpreted as a result of a resorption process, can be also interpreted as growth features, resulting from pinning of garnet grain boundaries adjacent to quartz (pyle and spear, 1999). we suggests, however, that garnet affected by progressive shear displays tectonic dissolution features, as revealed by the chemical zoning of garnet, which is abruptly truncated against the main metamorphic foliation of the rock. as previously stated, it seems that dissolution of garnet is related with replacement under (apparently) static conditions during retrogression rather than with tectonic dissolution. figure 13 (sample pcm-971) shows a pattern of quartz inclusions in garnet that tends to form a sigmoidal arrangement or snowball structure, which is known as synkinematic growth structure (e.g., schoneveld, 1977). microstructure of garnet shows different features between inner and outer parts of the grain. the inner part contains a sigmoidal arrangement of inclusions whereas the outer part has not inclusions. garnet exhibits normal zoning, with mn decreasing from 5,8 to 0,4 mol% and mg increasing from 1,0 to 2,9 mol% from core to rim. ca also decreases from core (4,8 mol%) to rim 168 carlos alberto ríos r., oscar mauricio castellanos a., akira takasu figure 10. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-523, which is a pelitic schist from the garnet-staurolite zone sillimanite zone. garnet diameter = 2.00 mm. warm colors correspond to higher concentrations. figure 11. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-953, which is a pelitic schist from the sillimanite zone. garnet diameter = 2.00 mm. warm colors correspond to higher concentrations. (1,8 mol%). therefore, it is characterized by a gradual decrease of mn and ca from core to rim, counterbalanced by a simultaneous increase of mg. the strong correlation in the zoning of mn, ca and mg suggests that these elements achieved local equilibrium during garnet growth. however, some authors (e.g., ikeda et al., 2002) suggest to study in detail the microstructure, crystallographic orientation, chemical composition and three-dimensional shape of sigmoidal garnet in order to establish evidences that can be used to discuss its origin, and in particular to distinguish between rotational versus non-rotational models. chemical zoning of garnet from metapelitic rocks of the silgará formation can be summarized as follows: (1) garnet is almost pure almandine end member, with very low content of mg, mn and ca. in addition, only a slight increase of mn was observed at the core; (2) garnets exhibits normal zoning with increasing mg components from core to rim and decreasing mn and ca components from core to rim, suggesting prograde metamorphism; (3) garnet exhibits normal zoning through to the inner rim, but in the outer rim the chemical zoning is reversed, reflecting the effects of retrogression, (4) sector-zoned garnet, with very different models of zoning; (5) amoeba-like zoned garnet; (6) anomalies in the chemical zoning at the textural boundaries reveals not only a modification by metamorphic processes but also deformation. patterns can deviate from normal growth zoning in garnet (e.g., euhedral bands concentric about the garnet core, patches or spiral to curving patterns). although the zoning in all elements is broadly concentric, detailed examination of the x-ray maps reveals a more complex pattern as occurring in sector zoned garnet, where a strong negative correlation between mg and ca is observed. chemical zoning of garnet is generally asymmetric and core compositions do not always coincide with the geometric center of the garnet. zoning patterns of garnet reflect alteration due to dissolution/resorption induced by deformation. a truncation of chemical zoning is not only an evidence of tectonic dissolution in progressive shear zones but also an excellent indication of subsequent garnet resorption. conclusions x-ray color maps of garnet in pelitic rocks of the silgará formation reveal complex patterns of nucleation and growth that probably were strongly affected by processes of dissolution, solution transfer (non demonstrated) and diffusion modification (during retrograde metamorphism). however, even if dissolution occurred, simultaneous precipitation is not clear. mg and mn is interpreted to reflect equilibrium with the rock matrix, whereas ca appears to be controlled by diffusive transport between garnet and rock matrix, which however cannot be demonstrated here largely because ca-bearing minerals have not been described in detail and their reaction history (fluxes) not analyzed. mn reversal zoning is the result of partial reequilibration at the millimeter scale during retrogression, and oscillatory zoning of ca appears to have been generated from slow intergranular diffusion in local chemical heterogeneities in the distribution of nutrients (ca-bearing phases). inclusion-free rims and inclusion-rich cores likely resulted from variable growth rates, which is supported by the occurrence of garnet showing low-ca annuli lacking inclusions and high-mn inclusion-rich cores, which is not demonstrated here. therefore, lowand high-ca concentrations can be explained by ca-bearing reacting phases rather than by growth rate, even if ca-bearing phases do not react, taking into account that growth rate has to do with overstepping of equilibrium reaction boundaries. the lowand/or high-ca annuli should be assumed as reaction boundaries (a function of p-t-x) during garnet growth history, and truncation of the annuli is x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) 169 figure 12. x-ray maps of mg, mn, and ca showing compositional zoning of the garnet porphyroblast in sample pcm-473, which is a pelitic schist from the staurolite-kyanite zone characterized by a mineral assemblage of quartz + plagioclase + muscovite + biotite + garnet + staurolite, with minor sillimanite, ilmenite and magnetite. garnet diameter = 2.50 mm. warm colors correspond to higher concentrations. not only an evidence of tectonic dissolution in progressive shear zones but also an excellent indication of subsequent garnet dissolution/resorption. anomalies in the chemical zoning at the textural boundaries are different for different traverses, revealing not only a modification by a diffusion process but also the influence of microfabrics (non demonstrated) that promoted a dissolution/resorption process. acknowledgements this research was supported by the japan-idb scholarship program latin america and the caribbean and the universidad industrial de santander for funding authors. we have bene?ted from research facilities provided by the geoscience department of shimane university (japan). we are indebted to this institution for allowing us the use of the electron probe microanalyzer. we are also indebted to dr. a. tsunogae for his helpful comments, discussion, and assistance with the electron microprobe data acquisition. authors also thank to anonymous referees for their critical and insightful reading of the manuscript. we are most grateful to the above-named people and institutions for support. references castellanos, o., chemical composition of the rock-forming minerals in the silgará formation and p-t conditions in the mutiscua area, santander massif, eastern cordillera, colombia. unpublished master thesis, shimane university, matsue (japan), 2001, 146 pp. castellanos, o., ríos, c., takasu, a., chemically sector-zoned garnets in the metapelitic rocks of the silgará formation in the central santander massif, colombian andes: occurrence and growth history, boletín de geología (universidad industrial de santander), 26, 2004, pp. 91-18. castellanos, o., ríos, c., takasu, a., a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes, earth sci. res. j. 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metamorph. geol., 18, 2000, pp. 665-681. garcía-casco, a., torres-roldán, r.l., millán, g., monié, p., schneider, j., oscillatory zoning in eclogitic garnet and amphibole, northern serpentinite melange, cuba: a record of tectonic instability during subduction?, j. met. geol., 20, 2002, pp. 581–598. garcía, c., ríos, c., castellanos, o., medium-pressure metamorphism in the central santander massif, eastern cordillera, colombian andes: constraints for a collision model, boletín de geología, 27, 2005, pp. 43-68. 170 carlos alberto ríos r., oscar mauricio castellanos a., akira takasu figure 13. x-ray maps of mg, mn, and ca showing compositional zoning of the largest garnet porphyroblast in sample pcm-971, which is a pelitic schist from the sillimanite zone. garnet diameter = 8.00 mm. warm colors correspond to higher concentrations. hickmott, d., spear, f.s., majorand trace-element zoning in garnet from calcareous pelites in the nw shelburne falls quadrangle, massachusetts: garnet growth 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colombian andes, j. south amer. earth sci. 16, 2003a, pp. 133-154. ríos, c., gelvez, j., marquez, r., kynetics of the nucleation and growth garnet in the silgará formation metapelitic rocks, southwestern santander massif, boletín de geología, 25, 2003b, pp. 23-38. ríos, c.a., castellanos, o.m., takasu, a., a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia, earth sci. res. j., 12, 2008, pp. 7-30. skelton, a., annersten, h., valley, j., äo18 and yttrium zoning in garnet: time markers for fluid flow? j. metamorph. geol., 20, 2002, pp. 457-466. smith, m.p., henderson, p., jeffries, t.e.r., long j., williams, c.t., the rare earth elements and uranium in garnet from the beinn an dubhaich aureole, skye, scottland, uk: constraints on processes in a dynamic hydrothermal system, j. petrol., 45, 2004, pp. 457-484. spear, f., metamorphic phase equilibria and pressure-temperature-time paths. mineralogical society of america, washington, dc, 1993. spear, f.s, kohn, m.j, paetzold, s., petrology of the regional sillimanite zone, west-central new hampshire, u.s.a., with implications for the development of inverted isograds, am. mineral., 80, 1995, pp. 361-376. spear, f., daniel, c., three-dimensional imaging of garnet porphyroblast sizes and chemical zoning: nucleation and growth history in the garnet zone, geol. mater. res., 1, 1998, pp. 1-42. spear, f.s., daniel, c.g., diffusion control of garnet growth, harpswell neck, maine, usa, j. metamorph. geol., 19, 2001, pp. 179-195. spiess, r., peruzzo, l., prior, d.j., wheeler, j., development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation-driven rotations, j. metamorph. geol., 19, 2001, pp. 269-290. schoneveld, c., a study of some typical inclusion patterns in strongly paracrystallinerotated garnets. tectonophys., 39, 1977, pp. 453-471. schwandt, c.s., papike, j.j., shearer, ch.k., trace element zoning in pelitic garnet of the black hills, south dakota, am. mineral., 81, 1996, pp. 1195-1207. vollbrecht, a., pawlowski, j., leiss, b., heinrichs, t., seidel, m., kronz, a., ductile deformation of garnet in mylonitic gneisses from the münchberg massif (germany), tectonophysics, 427, 2006, pp. 153-170. williams, m.l., sigmoidal inclusion trails, punctuated fabric development and interactions between metamorphism and deformation, j. metamorph. geol., 12, 1994, pp. 1-21. whitney, d., ghent, e., prograde reactions and garnet zoning reversals in staurolite schist. british columbia: significance for thermobarometric interpretations, j. metamorph. geol., 11, 1993, pp. 779-788. x-ray color maps of the zoned garnets from silgará formation metamorphic rocks, santander massif, eastern cordillera (colombia) 171 saad bakkali.indd 123 analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco) saad bakkali earth sciences department faculty of sciences and techniques, abdelmalek essaadi university, tangier, morocco email: saad.bakkali@menara.ma resumen en la gran cuenca de oulad abdoun en marruecos se inició la explotación de un nuevo depósito de fosfatos para reemplazar al de grand daoui que se encuentra agotado. existen inclusiones estériles de caliche que dificultan la extracción de las rocas fosfatadas y que son difíciles de detectar. la resistividad de los caliches excede un valor de 200 ω.m contra 80 a 150 ω.m para la roca fosfatada. se llevó a cabo un proyecto de prospección eléctrica con un equipo schlumberger sobre una extensión de 50 hectáreas. se obtuvieron modelos del perfil geológico mediante análisis de gradiente horizontal del mapa de los “distorsiones” de fosfatos. se logró localizar modelar la localizacion de las inclusiones de caliche y cubicar las reservas de fosfatos de manera más confiable. palabras clave: prospección geofísica, resistividad, fosfatos, gradiente horizontal, marruecos. abstract in the great oulad abdoun basin exploitation of a new phosphate deposit was begun after the grand daoui horizon was exhausted. inclusions of sterile hardpan—so-called “disturbances” —are hard to detect and interfere with phosphate extraction. their resistivity is above 200 ω.m as against 80 to 150 ω.m for the phosphate-rich mineral. a schlumberger resistivity survey over an area of 50 hectares was carried out. models of the geology were successfully obtained from the analysis of the horizontal-gradient map of “disturbances”. a new field procedure was tested to estimate the depth of disturbances. phosphate reserves were better known. key words: geophysical surveys, resistivity, phosphate, horizontal-gradient, morocco. © 2005 esrj unibiblos. manuscript received august 2005 paper accepted september 2005 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 123-131 124 saad bakkali introduction morocco is a major producer of phosphate, with an annual output of 19 million tons and reserves in excess of 35 billion cubic meters. this represents more than 75% of world reserves. resistivity surveys have been successfully used in the oulad abdoun phosphate basin in khouribga province (figure 1), which is about 120 km south of casablanca. the present survey was carried out in the sidi chennane deposit which a part of oulad abdoun basin, extending over some 800,000 hectares. the sidi chennane deposit is sedimentary stratiform and contains several distinct phosphate-bearing layers. these layers are found in contact with alternating layers of calcareous and argillaceous hardpan. in this field, extraction was begun after grand daoui deposit was exhausted. however, the new deposit contains many inclusions and lenses of extremely tough hardpan locally known as “dérangements” or disturbances, found throughout the phosphate-bearing sequence. the hardpan pockets are normally detected only at the time of drilling. they interfere with field operations and introduce a severe bias in the estimates of phosphate reserves. direct exploration methods such as well logging or surface geology are not particularly effective. however, the chemical changes which are detectable at the hardpan/phosphate rock interface produce an important resistivity contrast. other factors such as changes in lithofacies and clay content and consistence appear to account for some additional resistivity difference. it was found that normal phosphate-bearing rock has a resistivity of 80 to 150 ω.m while the hardpan typically features resistivity values of between 200 and 1000 ω.m. a pilot resistivity survey was performed over an area of 50 hectares. the objective of this experiment was to try and map and constrain the anomalous regions corresponding to hardpan. a resistivity map was expected to allow the electrical resistivity signals to be imaged in 3d. we used a schlumberger array with a span of 120 m designed to reach a depth of 40 m. the so-called disturbances appear at random so that the apparent resistivity map may be used by the operating personnel as a kind of radar to plan the sequence of field operations. we use traditional analytic analysis by horizontal-gradient to circumscribe the extension in-depth of the effects of the random disturbances, or any other resistivity anomalies that may be present. overview of the area of study the sidi chennane phosphate deposit is within the oulad abdoun basin about 33 km south-east of khouribga (figure 2). its boundaries are : west, meridian 372500 (lambert), south, meridian 22800 (lambert), east, highway rp22, and north, the outcrops of the basement of the phosphate-rock sequence. the climate of the phosphate plateau is essentially arid. rainfall is from november to may and is usually below 400 mm. vegetation is of sparse dwarf palm trees. rural population subsists on cattle ranching and seasonal agriculture in small villages, or douars. ground water is increasingly scarce. scattered wells depend on an aquifer in the turonian limestones at depths of 100 m or more, which is sealed by the senonian marls. figure 1. location of the area of study. 125 analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco) figure 2. main phosphate basins in morocco. geology the phosphate mineral was deposited over a long time window from maestrichtian (late cretaceous, about 80 ma), to lutetian (early eocene, 40 ma). however, deposition was irregular. some layers are missing. oulad abdoun basin occupies most of the phosphate plateau which is bounded toward the north by red outcrops of pre-cenomanian sediments forming an extension of the south edge of the central massif. the western boundary is the rhamna range, the beni amir plain is to the south and the upper atlas of beni mellal extends to the east. the geology of the study area is well understood (see figure 3 for stratigraphy). figure 3. general outline of the stratigraphy. the geologic sect ion rests unconformably on paleozoic schists and quartzites. the basement is well located and the sedimentary cover is fairly thick (kchikach and hiyane, 1991). the uppermost formations of the maes t r ich t ian and eocene contain the phosphate-bearing strata which are 30 to 50 m thick. the earlier deposits, i.e. the lower 5 to 28 m, are clayey phosphates of maestrichtian age. the upper 20 to 30 m are less homogeneous. they are layered phosphate marls and sandstones with some limestones of eocene age. below the phosphate-bearing strata one finds up to 70 m of senonian marls and limestone marls; 20 to 60 m of turonian limestones; a cenomanian formation of al ternately gypsum marls and limestone marls; and finally 10 to 60 m of red marls and mudstones of pre-cenomanian age. the disturbances may be differentiated by size of the pocket or inclusion, type of material, hardness, clay content, or type of contact with the phosphate rock. two main types of disturbances are found. the first type is found throughout the mineral deposit : it appears to be a random mixture of limestones, marls, clays, cherts and low-grade phosphate with large amounts of cherty limestone. the second type is highly disturbed and lacks any dominant facies. it appears as an accumulation of low-grade phosphate limestone blocks with large nodules of chert, marl, some fragments of chert and phosphate rock. the latter type forms inclusions of 10 to more than 150 m and is the most abundant during mining operations (figure 4). 126 saad bakkali figure 4. adverse effects of « disturbances » on mining operations. the study area was selected for its representativity and the resistivity profiles were designed to contain both disturbed and enriched areas (kchikach and hiyane, 1991). the sections were also calibrated by using vertical electrical soundings. high values of apparent resistivity were encountered due to the presence of near-vertical faulting between areas of contrasting resistivity, and fault zones which may contain more or less highly conducting fault gouge. the gouge may contain gravel pockets or alluvial material in a clay matrix (kchikach et al., 2002). such anomalous sections are also classified as disturbances. apparent resistivity values in these profiles locally exceeded 200 ω.m. in order to locate and define the anomalous areas or disturbances, an electric current of intensity i was passed between electrodes a and b, and the voltage drop ∆v was measured between the potential electrodes m and n. the apparent resistivity is found from ρ app = k (∆v/i), where k is the geometric constant of the instrument which depends only on the distance between electrodes (gasmi et al., 2004). thus the ratio between i and ∆v yields the resistivity of the terrain. our schlumberger profiles set required the electrodes to be aligned and equidistant from their central point o so that mn<<ab (bakkali and bouyalaoui, 2004). the lateral inhomogeneities of the ground can be investigated by means of the apparent resistivity obtained from the survey. as the surface extension of the layers is displayed we may infer the presence or absence of any disturbances as well as any facies variations. our resistivity measurements were performed by means of a syscal2 resistivity meter by these pockets are found both in the underlying formation and in the upper members of the phosphate sequence, and as a result there are strong resistivity contrasts between the disturbances and the normal phosphate-bearing rock. these contrasts were confirmed in the test runs (figure 5). geophysical prospection could thus be based on prior evidence from field data. figure 5. a resistivity traverse over three « disturbances ». field procedures resistivity is an excellent parameter and marker for distinguishing between different types and degree of alteration of rocks. resistivity surveys have long been successfully used by geophysicists and engineering geologists and the procedures are well established. 127 analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco) brgm instruments using a rectangular array of 20 m x 5 m. in order to reach a mean depth of exploration of 40 m we carried out 51 traverses at a spacing of 20 m. there were 101 stations at 5 m distance for every traverse, which makes 5151 stations altogether in the survey (bakkali and bahi, 2005). methodology interpretation of resistivity anomalies is the process of inferring information on the position and composition of a target mineral body in the underground. in the present case the targets were essentially the inclusions called perturbations. the amplitude of an anomaly may be assumed to be proportional to the volume of a target body and to the resistivity contrast with the mother lode. if the body has the same resistivity as the mother lode no anomaly will be detected. apparent resistivity measurements are obtained from a harmonic potential v which fulfils laplace’s equation ∆v=0 in the surrounding space external to the body. the potential gradient falls off as 1/r2 (blakely, 1995). as a first approximation, the scalar potential in the neighborhood of the perturbations and within the sources complies with the equation ∆v=-2ρi δ(r), where δ(.) is the dirac delta, ρ is the resistivity in the anomalous region and i is the current at a point electrode in an elastic halfspace, i.e. the topographic surface of the study area (dey and morrinson, 1979). the apparent resistivity map which one obtains from such a survey is actually a map of discrete potentials on the free surface, and any major singularity in the apparent resistivities due to the presence of a perturbation will be due to the crossing from a “normal” into a “perturbed” area or vice versa. in other words, the apparent resistivity map may be considered a map of scalar potential differences assumed to be harmonic everywhere except over the perturbed areas. thus one assumes that the potential difference data may be processed by fourier analysis without any terrain correction (cooper, 2004). horizontal-gradient and downward prolongation process figure 6 shows the map of resistivity anomalies corresponding to “disturbed” phosphate zones. figure 7 is the corresponding map of the “disturbed” zones. this procedure enables us to define the sources. the standard procedure was basically fourier filters which enhance some features of the anomaly map at the expense of other features, thus enabling us to connect the mapped surface anomalies to depth (bakkali, 2006). figure 6. a map of apparent resistivity anomalies of the area of study. 128 saad bakkali figure 7. a map of the corresponding “disturbed” zones of the resistivity anomalies map. (figure 8) was calculated starting from a plan obtained by an adjustment by the method of least squares of a grid of 5 cells out of 5 cells centered on the point to determine. we note the good correlation between “disturbed” phosphate zones and the horizontal-gradient intensity) the downward prolongation of the horizontalgradient apparent resistivity map reveals the extension in-depth of the contrasting densities. the operator of prolongation in the spectral domain is e z u v2 2 2+r where is the depth of prolongation and o the frequency defined as u v2 2= +o ] g , where u and v are the frequencies in the eastern and northern directions (gunn, 1975). we have applied this process to the map of the horizontal-gradient apparent resistivity to estimate the extension in-depth of the contrasting of density between the disturbances and the normal phosphate-bearing rock. this singular procedure will enable us to map the extension of anomalic zones in-depth (bakkali, 2006). the data are imaged symmetrically in the space domain in order to minimize edge effects on the transformed map. the second-derivative operation is equivalent to high-pass filtering. it is useful for interpretation as it refines the response of the geophysical instrument, and it produces sharper contours. as derivation is sensitive to noise (cooper and cowan, 2003), this procedure enhances the expression of any structures and discontinuities. the magnitude of the horizontal-gradient is usually estimated by finite difference methods from values measured at gridded points on the apparent resistivity anomaly map. the magnitude is determined using the following equations: , , , x y x x y y x y app app app 2 2 2 2 2 2 2 2 = +t t t l ^ ^ ^ h h h with , x x y ax2 , ,app i j i j1 1 2 2 = -t t t+ p ^ h and , y x y ay2 , ,app i j i j1 1 2 2 = -t t t+ p ^ h where x is the eastern coordinate and y the northern coordinate. ,i jt is the pseudo-apparent resistivity defined at grid point (i,j). grid intervals in the x-direction and y-direction are axp and ayp respectively. maxima horizontal-gradient may be occur immediately over steep or vertical boundaries separating rock masses of contrasting densities. on the horizontal-gradient apparent resistivity map, lines drawn along ridges formed by enclosed high horizontal-gradient magnitudes correspond to these boundaries. the horizontal-gradient represents in this case an indicator of the level of variation of the contrast of density between the disturbances and the normal phosphate-bearing rock. the value of the intensity of the horizontal-gradient anomalies 129 analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco) otherwise some spurious high frequency effects may appear around the edges. once the horizontal-gradient resistivity map was filtered in the frequency domain the downwardprolonged corresponding map at a depth 5 to 50 m (figure 9) was restored back to the space domain by inverse fourier transformation. downward prolongation was carried out in steps of 10 meters which enabled us to sample the entire sequence of level of the contrasting of density between the disturbances and the normal phosphate-bearing rock. results and conclusions the resistivity map as obtained by the above procedure in the study area provided a direct image for an interpretation of the resistivity survey. we were able to identify the anomalies which turned out to be strongly correlated with the disturbances. we found that the disturbances as detected from surface measurements were distributed apparently at random as confirmed by the various maps of the downward prolongation of horizontal-gradient apparent resistivity intensity. these maps represent an effective indicator of the intensity level of “disturbance” in depth. the anomalous areas were neatly outlined. as mentioned above, the disturbances were outlined in depth down to 50 m. the contrasting of density between the disturbances and the normal phosphatebearing rock from 5 to 50 m depth attest that the disturbances are contrasted in-depth comparatively on the surface. the high intensities of the horizontal-gradient over anomalous areas are interpreted as reflecting the fact that the pockets of disturbed material tend to become more consolidated at depth. we find that the analytic prolongation procedure of the horizontal-gradient helps to better constrain the location of anomalous areas on the surface. the overall effect is that of scanning the anomalous bodies. we have described an analytical procedure to identify the anomalies of a specific problem in the phosphate mining industry. the results proved satisfying. data processing procedures as horizontal-gradient were found to be consistently useful and the corresponding prolonged maps may be used as auxiliary tools for decision making under field conditions. it was found that the maps of horizontal-gradient at different depths were particular useful to the surveyors for improving and constraining their estimates of phosphate reserves in the deposit. figure 8. horizontal-gradient apparent resistivity intensity map in ω.m-1 of figure 6 (contrast density level between the disturbances and the normal phosphate-bearing rock corresponds to contrast of nuances of gray color. 130 saad bakkali figure 9. downward prolongation of horizontal-gradient apparent resistivity intensity maps . 131 analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco) references • bakkali, s. 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foundation using dmr classification and kinematic analysis, southwest of iran mehdi torabi kaveh and mojtaba heidari department of geology, faculty of science, bu-ali sina university, mahdieh ave., 65175-38695 hamedan, iran. e-mail: mehditorabikaveh@yahoo.com, heidarim_enggeol@yahoo.com abstract this paper describes the results of engineering geological investigations and rock mechanics studies carried out at the proposed chamshir dam site. it is proposed that a 155 m high solid concrete gravity-arc dam be built across the zuhreh river to the southeast of the city of gachsaran in south-western iran. the dam and its associated structures are mainly located on the mishan formation. analysis consisted of rock mass classification and a kinematic analysis of the dam foundation’s rock masses. the studies were carried out in the field and the laboratory. the field studies included geological mapping, intensive discontinuity surveying, core drilling and sampling for laboratory testing. rock mass classifications were made in line with rmr and dmr classification for the dam foundation. dam foundation analysis regarding stability using dmr classification and kinematic analysis indicated that the left abutment’s rock foundation (area 2) was unstable for planar, wedge and toppling failure modes. resumen este articulo describe los resultados de una investigación de ingeniería geológica y estudios de mecánica de roca que se llevo a cabo en el lugar propuesto para le represa chamshir. se propone una presa de 155m de altura, de arco gravitacional en concreto de solido, debe ser construida a través del rio zuhreh al sureste de la ciudad de gachsaran en el suroeste de irán. la presa y su estructura asociada son localizados principalmente sobre la formación mishan. el análisis consistió en la clasificación del macizo rocoso y un análisis cinemático de la fundación de la masa rocosa de la presa. los estudios se llevaron a cabo en campo y laboratorio. los estudios de campo incluyeron cartografía geológica, un estudio intensivo de discontinuidad, perforación de núcleo y toma de muestras para pruebas de laboratorio. la clasificación de la masa rocosa se realizo de acuerdo con la clasificación rmr y dmr para la fundación de la presa. el análisis de basamento rocoso de la presa en relación a la estabilidad usando la clasificación dmr y el análisis cinemático indico que el estribo izquierdo del basamento (área 2) es inestable para tipos de fallo planares y de cuña. palabras claves: el rio zuhreh, sitio presa chamshir, fundación rocosa, clasificación dmr, análisis cinemático. keywords: sochagota the zuhreh river, chamshir dam site, rock foundation, dmr classification, kinematic analysis. record manuscript received: 28/05/2011 accepted for publications: 30/11/2011 introduction the most important advantage of a favourable rock mass classification is that it has parameters describing most of a rock mass’s engineering characteristics for providing base input data for engineering design purposes. rock quality designation (rqd, deere 1964) and rock mass rating (rmr, bieniawski 1989) are two of the most commonly used numerically-expressed rock mass classification systems. several researchers have referred to rmr as being a useful tool for describing rock mass foundations (di salvo, 1982; van schalkwyk, 1982; marcello et al., 1991; hemmen, 2002; ramamurthy, 2004). some difficulties are involved in using rmr for dam foundation studies, such as very doubtful water pressure consideration, there are no good rules for quantifying the adjusting factor for joint orientation and watering changes introduce changes in properties concerning the rock mass and the joints. guidelines have only been offered regarding general stability against horizontal sliding, which is important but is not a very common problem. dam mass rating (dmr, romana, 2004) has been proposed as an adaptation of rmr, giving tentative guidelines for several practical aspects of dam engineering and for dam foundation appraisal in preliminary studies taking account of the effects of rock mass anisotropy and water saturation. the chamshir dam site on the zuhreh river is located in south-western iran, about 20 km southeast of the city of gachsaran (50° 52’ 36” e and 30° 10’ 59” n, figure 1). the dam is now being studied and has been designed as a 155 meter high concrete gravity-arc dam; its useful reservoir volume is 1.8 milliard cubic meters (figures 1 and 2). exceptional topographical, hydrological and geological circumstances regarding the river mehdi torabi kaveh and mojtaba heidari130 in the chamshir gorge has led to the site being proposed as a suitable option for dam construction, concerning available national resources use (i.e. water storage for irrigation projects). so far some researchers have studied the rock mass conditions of the chamshir dam site (e.g., gharouni-nik, 2008; torabi-kaveh et al., 2010). this paper explains the engineering geological assessment involved in the safe design of the proposed chamshir dam site. such geotechnical investigation has been carried out at the project site and in the laboratory. various laboratory tests and detailed discontinuity surveying were performed to assess rock mass characteristics. the chamshir dam site rock mass was studied using rqd, rmr and dmr classification and kinematic analysis more accurately assessed the dam foundation. geological setting geological factors play a major role in designing and constructing a dam (ichikawa, 1999) as they control the nature of geological formations and also provide the needed materials for construction. many cases have occurred throughout the world where dam foundation rock mass conditions were not sufficiently known and the cost of construction and treatment greatly exceeded the original budget. according to the 1:100,000 geological map of ghachsaran (setodehnia and o.b. perry, 1966) (figure 1), the geological formations in the study area, from oldest to youngest, are gachsaran (early miocene), mishan (early-middle miocene), aghajari (miocene-late pliocene), bakhtiari (late pliocene-pleistocene) and alluvial sediments. the mishan formation (an isocline) along with the gachsaran formation cover the western part of the dam reservoir and dam site. this formation has two different facies; the first consists of biohermy limestone and forms a great lens within a second facies which consists of alternating marl and limestone layers. the zuhreh river has created the long and narrow chamshir gorge by erosion of biohermy limestone, thereby making it a suitable location for dam construction (figure 2). the chamshir dam reservoir is located on the gachsaran, mishan and aghajari formations; the gachsaran formation’s stratigraphy sequence in the study area is similar to that of the khuzestan area (tehran-sahab and parab-fars consulting engineering companies, 1997). this formation has 7 members: the oldest member is 40m thick consisting of alternating thick anhydrite, limestone and shale layers. the second member is a 115m thick salt layer, with anhydrite alternating with thin limestone layers. the third member is a 347m thick anhydrite layer with salt. the fourth member consists of a 290m thick salt layer with marl, gray limestone and anhydrite. the fifth member is 342m thick red and gray marl with alternating layers of gypsum. the sixth member is figure 1. geological map of the study area (modified from the gachsaran geological map, 1:100,000, iranian oil operating companies (iooc), 1966). an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 131 258m thick, having alternating layers of anhydrite (or gypsum), salt, red marl and limestone. the seventh member is the youngest member, being 139m thick and having alternating gypsum, gray marl and limestone layers. it should be mentioned that sulphate layers outcrop as gypsum on the surface and as anhydrite at deeper levels. the gachsaran formation covers most parts of the projected dam reservoir (figure 1). according to field observations, members 5, 6 and 7 of the gachsaran formation would be in contact with the dam reservoir in this area and only member 7 outcrops downstream of the dam. the aghajari formation would form a small part of the reservoir at its south-western corner and consists of sandstone, siltstone, conglomerate, gypsum and marl. young and old terraces are also present along the banks of the zuhreh river (figure 1), consisting of coarse grained gypsum particles and fine grained silt and sand sediment. the study area is in the zagros folded area or external zagros (stocklin, 1968) and simply folded belt (berberian, 1995). zagros folding compressional tectonic forces have created some faults and thrust faults having a nw-se trend in the study area; the dezh soleyman thrust (dst), murd thrust and chamshir fault area are the most important ones (figure 1). the role of the dst in the study area is important according to field observations. the gachsaran formation is uplifted along this fault from deeper parts to the surface. it has dissected some parts of the mishan formation in the north-eastern branch of the chamshir syncline and has consequently thrust the gachsaran formation over the mishan formation. the extensive tectonic pressure of the dst created the important chamshir fault area, this being the source of several springs throughout this area and the dst. the zuhreh river’s deviation from its direct pathway into the chamshir fault area could also provide reasonable evidence of tectonic activity in the study area. materials and methods engineering geological investigations and rock mechanics studies include discontinuity surveying, core drilling, in situ and laboratory testing. quantitative description of discontinuity (i.e. orientation, spacing, persistence, roughness, aperture and filling materials) were determined in situ by exposure logging according to the international society for rock mechanics’ (isrm) standards (1981). laboratory tests were carried out on the core samples to quantify the physical and geomechanical properties of intact rocks at the dam site. site investigation the dam site was investigated in two stages; the site was geologically studied and mapped in detail. thirty-four boreholes were drilled (1,578 m), 8 of them pertaining to the dam site. six boreholes were drilled (565m total depth) during the first stage (1999 to 2000); the second stage was carried out between 2008 and 2009 when 2 boreholes were drilled (246 m total depth). two locations have been have been considered for field studies regarding the proposed areas for constructing a dam in the chamshir gorge (figure 3). five hundred discontinuities were measured (250 on both the left and right abutments). four dominant discontinuity sets were identified on the left (area 1) and right (area 1 and 2) abutments of the proposed dam site (tables 1 and 2). five dominant discontinuity sets were figure 2. the chamshir dam site (chamshir gorge). mehdi torabi kaveh and mojtaba heidari132 table 1. the left and right abutments’ discontinuity characteristics (area 1). table 2. the left and right abutments’ discontinuity characteristics (area 2). plungetrendaverage dip (°)average dip direction (°) type of discontinuitylocation 7735713177bedding right abutment 1414378323j1 1032779147j2 1429876118fault set 7930611126bedding left abutment 1213678316j1 1232676146j2 1730573125fault set plungetrendaverage dip (°) average dip direction (°) type of discontinuitylocation 7735013170bedding right abutment 1214478324j1 1132079140j2 1430576125fault set 782791199bedding left abutment 1414878328j1 1332676146j2 1312073300fault set 1 929881118fault set 2 figure 3. satellite image of chamshir dam site (http://www.google.com/earth/index.html). an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 133 identified (table 2) for the left abutment (area 2). a quantitative description of discontinuity in two areas included type and orientation in the left and right abutments (tables 1 and 2). results and discussion rock mass quality rqd and rmr were also used for obtaining the exposed rocks’ engineering properties within the dam foundation. the data were collected from the dam site. the rqd values were determined by examining drill cores and joint frequency (table 3). the table shows that the left and right abutments’ rqd values were excellent for the projected dam construction. table 4 gives the rmr values, rock unit quality being classified as very good. dmr classification dmrsta (related to dam stability against sliding) value was: dmrsta = rmrbd + cf × rsta where rmrbd (basic dry rmr) resulted from adding the rmr’s first four parameters plus a water rating of 15 and rsta was the dam stability adjustment factor. regarding hoek-brown criteria, hoek has advocated the use of a “dry rmr” obtained with the maximum rating for water, simultaneously introducing real pore pressures into the computations (hoek et al., 2002). rmrbd was obtained by adding the first four parameters of rmr plus 15: 1) compressive strength, tested in water conditions similar to future ones, i.e. saturated when the rock is going to be saturated and having the same ph as that water; 2) rock mass rqd; 3) significant governing joints’ spacing (s); 4) significant governing joints’ conditions (s); and 5) water rating (wr), always 15 (as if dry). the rsta (adjustment factor for dam stability) was obtained (table 5). the danger of sliding became reduced when the significant joint’s dip direction was not almost parallel to the dam’s downstream-upstream axis due to the geometrical difficulties involved in sliding. such effect could be taken into account by multiplying dam stability adjusting factor rating rsta by a geometric correction factor (cf): table 3. the right and left abutments’ rqd values. table 4. the chamshir dam foundation’s rmr classification table 5. dam stability rsta adjustment factors, according to joint orientation; ds dip downstream/us dip upstream/a any dip (romana, 2003a). rqd value place obtained from joint frequencyobtained from cores 10090-100right abundant 10090-100left abundant left abutment rock massright abutment rock mass situation rateconditionsrateconditionsdescription 4.025-504.025-50compressive strength (mpa) 20.090-10020.090-100rqd (%) 20.0>220.0>2joint spacing (m) 25.0sum of five parameters25.0sum of five parametersdiscontinuity condition 15.0dry15.0drywater flow 84.084.0rmrbd rate 0favourable0favourablejoint orientation rate 84.0very good84.0very goodrmr type of dam vf f fa u vu very favourable favourable fair unfavourable very unfavourable fill others 10-30 ds 0-10 a gravity 10-60 ds 30-60 us 60-90 a 10-30 us 0-10 a arch 30-60 ds 10-30 ds 30-60 us 60-90 a 10-30 us 0-10 a rsta 0 -2 -2 -15 -25 (1) mehdi torabi kaveh and mojtaba heidari134 cf = [1 – sin (αd – αj)] 2 (αd > αj) cf = [1 – sin (αj – αd)] 2 (αd < αj) where αd was dam axis upstream-downstream direction and αj was the dip direction of the significant governing joint. dam foundation status dmrsta was calculated (table 6). the chamshir dam will built on mishan limestone and marl rock units. the valley walls at the dam site are steep, having 80°– 90° slopes on the left abutment and 75°–90° on the right abutment. the valley runs nw-se (310°). dip direction is 40° ne for the left abundant and 220° sw for the right abundant. dmr classification for the chamshir dam’s foundation (for areas 1 and 2) is shown in tables 4 and 5. the results obtained from dmr classification (tables 7 and 8) indicated that the left abutment (area 2) was instable; the results of this classification were compatible with field conditions. kinematic analysis kinematic refers to the motion of bodies without referring to the forces causing them to move (goodman, 1989). kinematic analysis is very useful for investigating possible rock mass failure modes and determining maximum safe slope angle (mssa). many studies have determined slope failure modes (markland, 1972; goodman, 1976; goodman and shi, 1985; matherson, 1988) and evaluated slope stability (özsan and akin, 2002; aksoy and ercanoglu, 2007; kulatilake et al., 2011) using a stereo(2) (3) table 6. the degree of dam safety regarding sliding (romana, 2004). table 7. the dmr classification of the right abutment. table 8. the dmr classification of the left abutment.. degree of safetyrock mass rate no primary concerndmrsta > 60 concern60>dmrsta>30 serious concern30>dmrsta graphic projection technique. kinematic analysis was used for the study area to estimate the mssa regarding the three basic failure modes: plane sliding, wedge sliding and toppling. the aforementioned kinematic analysis was performed for left abutment slopes (area 2) at the dam site using dominant discontinuity sets. kinematic analysis (table 9 and figure 4) results indicated that joint inclination was the most important parameter affecting rock mass instability. the analysis revealed possible wedge, planar and toppling failures in the left abutment (area 2). conclusions the concrete chamshir dam will be located on the limestone and marl rocks of the mishan formation. good rock mass quality was indicated for these rocks; however, according to dmr and kinematic analysis, most parts of the dam foundation (except the left abutment, area 2) were safe, being rated low-risk in terms of instability occurrence and magnitude. it is therefore recommended that slope failure should be constantly monitored. despite rqd and rmr values showing favourable condition for the dam abutments, the dmr classification provided more accurate assessment and was more reliable, i.e. considerable correlation between such classification and the kinematic analysis. acknowledgments the study was financed by the mahab ghodss consulting engineering company. the authors are grateful to dr. hamid reza zarei for supplying the authors with the dam site’s practical test data and mr. jason garry for editing the paper in english and mr. mirmohammad miri for translating it. references aksoy, h. and ercanoglu, m. (2007). fuzzified kinematic analysis of discontinuity-controlled rock slope instabilities, eng geol. 89, 209–19. right abutment situation area 2area 1 fault setj2j1beddingfault setj2j1beddingtype of discontinuity 0.430.438.82×10-43.9×10-41.841.840.341.7×10-2cf -7-7-7-15-7-7-7-15rsta 84rmrbd 78.0255.70dmrsta left abutment situation area 2area 1 fault set 2fault set 1j2j1beddingfault setj2j1beddingtype of discontinuity 1.842.380.163.072.220.430.161.645.48×10-4cf -7-7-7-7-15-7-7-7-15rsta 84rmrbd 0.0068.40dmrsta an engineering geological appraisal of the chamshir dam foundation using dmr classification and kinematic analysis, southwest of iran 135 table 9. kinematic analysis regarding sliding in the left abutment (area 2). figure 4. kinematic conditions for the left abutment (area 2), 1: bedding, 2: j1, 3: j2, 4: fault set ι, 5: fault set π, 6: slope face orientation of intersection lineswedge sliding results plunge (deg.)trend (deg.)inters. linemaximum safe angle slope face dip direction sliding along joint sets 8.8 9.1 4.2 4.1 4.2 75.9 51.7 44.3 75.9 5.1 55.8 58.1 29.0 28.6 57.0 322.8 39.6 223.0 168.8 28.8 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 90 90 90 90 90 76* 68** 90 90 90 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 340.0 1-2 1-3 1-4 1-5 2-3 2-4 2-5 3-4 3-5 4-5 toppling failureplanar failure maximum safe angle (deg.) failure along joint set slope face dip direction maximum safe angle (deg.) failure along joint set slope face dip direction 90 90 53**** 90 90 1 2 3 4 5 340.0 340.0 340.0 340.0 340.0 90 76*** 90 90 90 1 2 3 4 5 340.0 340.0 340.0 340.0 340.0 * potential wedge failures along the intersection lines for joint set (1) with fault set 1 were possible if slope angle exceeded 76 degrees. ** potential wedge failures along the intersection lines for joint set (1) with fault set 2 were possible if slope angle exceeded 68 degrees. *** potential planar failure along faults was possible if slope angle exceeded 76 degree. **** potential toppling failure due to the orientation of joint set 2 was possible if slope angle exceeded 53 degrees. analysis must be carried out for faults sited close to the slope face in the vicinity of such faults. mehdi torabi kaveh and mojtaba heidari136 berberian, m. 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(2004). dmr (an adaptation of rmr), a new geomechanics classification for use in dams foundations, 9th congresso luso de geotecnia, aveiro, 12pp. setudehnia, a. and ob-perry, g. t. (1966). geological map of gachsaran, iranian oil operating companies (iooc), 1,100,000. stocklin, j. (1968). structural history and tectonics of iran: a review, aapg bull. 52, 1229-1258. tehran sahab and parab fars consulting engineering companies. (1997). engineering geology report concerning the chamshir dam site (in persian), chamshir dam project, teheran, iran. torabi-kaveh, m., heidari, m., zarei, h. r. and ghiasi, m. b. (2010). engineering geological investigation of the foundation of the chamshir dam site by use of dmr classification (iran), 7th international symposium on eastern mediterranean geology, university of çukurova, adana, turkey, 18‐22 october. van schalkwyk. (1982). geology and selection of the type of dam in south africa, 14th icold, río de janeiro, q51. r44. contenido efecto de la rugosidad del terreno en las secciones sismicas: modelamiento por elementos finitos robinson quintana, sandra cespedes, alfredo ghisays, gabriel perez y luis montes vides 5 amenaza sismica por metoda determlntsta en el trianqulo de fallas valera-bocono-oca en el occidente de venezuela 9 raymi a. castilla, frank a. audermard m. y jose antonio rodriguez cambios temporales del valor de b: lposibles correlaciones con la actividad hidrotermal del volcan tungurahua? c. indira molina p., mario c. ruiz 17 monitoreo de emisiones de gas radon vinculado a la actividad microsismica en la zona de sampacho, cordoba, argentina virginia grosso, romina daga, guillermo sagripanti y diego villalba 2a dispersion sismica por presencia de capas someras de alta velocidad: modelamiento con diferencias finitas 2s sandra cespedes, robinson quintana, alfredo ghisays, gabriel perez y luis montes analisis espacial v temporal de q-coda en el piedemonte l1anero (colombia) juan p. duran, carlos a. vargas y luis a. briceno 33 determinacion de mecanismos focales para temblores pequeiios 0 moderados: region del caribe nororiental victor a. huerfano moreno 40 resultados del uso de geodesia satelital para estudios geodinamicos en colombia 43 hector mora-paez, robert trenkamp, james kellogg, jeff freymueller y milton ordonez-villota t.eves de atenuaclon de la intensidad macrosismica en colombia augusto antonio gomez capera y elkin de jesus salcedo hurtado 53 soluclon a un problema estatlco en un area con topografia abrupta usando una velocidad de reemplazamiento variable luis antonio castillo lopez y carlos pedraza 62 optlmizacion en la soluclon del problema inverso en geofisica usando el algoritmo de entrenamtento supervisado de levenberg-marquardt yaqueline figueredo baez y ramiro leon diaz campos 66 modelamiento slsmtco utilizando teo ria de raves en medios transversalmente tsotroptcos: teo ria v ejemplos carlos piedrahita. trino salinas y saul guevara 70 geociencias 15-1 julio 2011.vp tunnelling in urban areas by epb machines: technical evaluation of the system marilena cardu, pierpaolo oreste politecnico di torino, land, environment and geo-engineering department, italy igag-cnr, torino, italy e-mail: marilena.cardu@polito.it abstract the paper refers to the methods adopted for building a high-speed railway tunnel system between bologna and firenze (italy), focusing attention on the bologna node which represents the heart of the system, connecting the high speed network’s main lines. the project includes 9 tunnels, accounting for 73 km of the 78 km route crossing below the apennines. the paper pays attention to the main aspects to be taken into consideration for correctly choosing the tunnel boring machinery (tbm) to be used in urban areas. the fundamental point in analysing technical aspects regarding an earth pressure balance (epb) machine concerned storing the main excavation parameter values; having collected and organised such data, statistical methods were used for processing it, the instantaneous velocities attained were empirically estimated and idle times were evaluated. the evaluation was made by calculating excavation specific energies (during different excavation phases) to find a satisfactory correlation with the type of ground crossed. interesting results have been found by comparison with other excavation parameters; in particular, a better understanding of an earth pressure balance shield’s working phases has been reached thanks to an experimental study conducted during the construction of tunnels for a high-speed railway system in italy. the paper contains details collected regarding the operation of two different epb machines. resumen este artículo se refiere a los métodos utilizados para la construcción de túneles para un sistema de trenes de alta velocidad entre bologna y firenze (italia), el punto de interés está sobre el nodo de bologna, como el corazón de sistema, conectando las líneas principales de la red de alta velocidad. el proyecto incluye nueve túneles, con 73 de los 78 km cruzando por debajo de los apeninos. este artículo presenta los principales aspectos a tener en consideración para la correcta selección de máquinas tuneladoras (tbm) utilizadas en las áreas urbanas. el fundamento en el análisis de los aspectos técnicos consiste en un balance de presión de tierra (epb) de la máquina relacionado a los principales parámetros en la excavación; una vez recogido y organizado los datos, se realizaron análisis estadísticos, se estimaron las velocidades empíricamente y evaluaron los tiempos de espera. la evaluación fue realizada para el cálculo de las energías específicas de excavación (en diferentes fases de perforación) para encontrar una correlación satisfactoria con el tipo de terreno atravesado. los resultados obtenidos son interesantes en comparación con parámetros de otras excavaciones; en particular, una mejor compresión en el balance de la presión de tierra en cada fase de trabajo, ha sido descrita gracias a un estudio experimental realizado durante la construcción de túneles para un sistema de trenes de alta velocidad en italia. este artículo contiene información detallada recogida de la operación de dos maquinas epb diferentes. introduction research instruments’ evolution in the field of tunnel construction has contributed towards the improvement of excavation technologies and planning management, almost during the last 30 years. excavation techniques’ evolution has also been stimulated by growing urbanisation and the assimilation of the “underground” concept as a space resource, becoming more and more exploitable from various points of view. one of developed countries’ major needs nowadays concerns mobility. many projects have been promoted during recent years to improve interconnections between european countries. this has involved the problem of tunnelling a great part of the work (barla g., 1994). the best european example highlighting such need has been the eurotunnel project which involved constructing three underwater tunnels linking france and the uk. another important project which is still in execution is the high speed railway system; it will research groupin geophysics universidad nacional de colombia earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 5-11 keywords: earth pressure balance (epb), specific energy, excavation parameter, tunnel, tunnelling. palabras clave: balance presión tierra (epb), energía especifica, parámetro excavación, túnel. record manuscript received: 13/01/2011 accepted for publication: 15/05/2011 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:07 p p p composite 133 lpi at 45 degrees provide a more efficient and faster connection amongst european countries. a critical point has to be resolved when carrying out this work as rail layouts have to cross heavily urbanised areas (barla g., 2000; bilotta e. et al., 2002). mechanised shields offer a series of advantages and work can be carried out without having to support the roof. the equipment’s technical characteristics do not represent a basis for effective evaluation because two mechanically identical machines can behave in completely different ways, depending on soil characteristics. it is thus very difficult to make a reliable and unambiguous evaluation of one machine’s best performance compared to that of another (bebendererde s., 1995; bovetti b., 2003). the only way to resolve this point is to evaluate the equipment’s working parameters during excavation. the most important parameter concerns excavation specific energy which is often used for determining a machine’s performance (altindag r., 2003; copur h. et al., 2003; cardu m. et al., 2006; tardaguila i. et al., 2007; acaroglu o. et al., 2008; exadaktylos g. et al., 2008). defining the excavation specific energy concept excavation specific energy (es) represents the amount of energy (expressed in mj) needed to excavate a unit volume of ground (rostami j.,ozdemir l., 1993; friant j.e., ozdemir l., 1993). this may be given by: e w t v s tot� � (1) where wtot is total power, t is excavation time and v is excavated volume. the shield’s feed motion and the head’s rotation speed must be taken into account when evaluating energy consumption due to the excavation of a unit volume of ground. the general formula for calculating total excavation specific energy is given by the sum of these two contributions. feed motion contribution the parameters for calculating a machine’s feed motion are: • the thrust exerted by hydraulic jacks s • a machine’s advancing speed v • excavation time t. tunnel diameter must be known, as this is used to calculate a tunnel’s cross-section. the power needed for a machine’s advance (wadv) can then be obtained by using the following expression: w s v adv � � (2) the rate of excavated volume per time unit is: v a v t� � �( ) (3) where a is a tunnel’s cross-section and product v�t represents advance mad in time t. equation (1) can thus be written for feed motion specific energy contribution (es adv), as: e w t v s v t a v t s a s adv adv� � � � � � � � (4) rotation specific energy the data needed for calculating rotation are: • the torque necessary to rotate head c • the head’s rotation speed ù • tbm advance speed v. rotation power (wrot) is given by: w crot � �� (5) excavation specific energy consumed by head rotation (es rot) is thus: e w t v c t a v t c a v s rot rot� � � � � � � � � � � � (6) total specific energy this represents a machine’s behaviour in terms of non-uniform progression. equation (7) is obtained by substituting (1) in equations (4) and (6) (teale r., 1965): e e e s a c a v s tot s adv s rot. . .� � � � � � � (7) it should be noticed that total excavation specific energy is expressed by a formula which does not include time as a variable; it can thus be considered a feature characterising excavation in the stretch which has been calculated from the machine’s parameters. moreover, it can be observed that energy, as would be expected, grows with thrust, torque and rotation speed; excavation difficulty increases as these three factors also increase. excavation specific energy obviously decreases as progression speed increases. analysis of field performance data and discussion the following considerations were based on data collected when high-speed tunnels in the bologna node excavation were being cut by two identical epb tbms (cicala t., 2003; guidarelli d., 2005). table 1 gives the machines’ main characteristics. the first machine (epb1) worked at advanced chainage (about 1,500 m) compared to the other one (epb2). it is very important when analysing data to take into account the kind of ground in which work is being done. excavation specific energy shown in the following graphs has been represented as a function of route chainage to facilitate correlation with different types of ground identified by geological characterisation. epb1 es is represented in figure 1; it varies from a minimum value of around 15 mj/m3 to a maximum of 40 mj/m3. several types of ground are present in this range, particularly: • clay, from 1,500 m to 1,800 m chainage • moist sand, from 1,800 m to 2,100 m • dry sand, from 2,100 m to 2,400 m • dry gravel, from 2,400 m to 2,700 m. a negative es gradient was envisaged from 1,500 m to 2,100 m because of a machine’s progressive adaptation to local ground conditions. crossing from clay to sand around 1,800 m chainage was confirmed by decreasing es (a machine’s adaptation to lithological crossing). sands were initially on the crown and their presence increased as excavation progressed, making excavation conditions become worse, thereby stressing the need for a machine to become suitable to different kinds of ground. this was not observed on contact between 6 marilena cardu, pierpaolo oreste agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:07 p p p composite 133 lpi at 45 degrees moist and dry sands (around 2,100 m chainage) where a clear increase in es was envisaged. dry sands, being more compact than moist sands, would require a greater effort to be removed. sampling was not regular from 2,100 m to 2,350 m chainage due to some of the machine’s sensors malfunctioning; only a net increase in energy was identifiable (but it was impossible to establish whether such evaluation was completely reliable). sampling worked regularly again from 2,350 m. this stretch crossed gravels and contact was not abrupt. table 1: specifications for the epb lovat rme370se used in the bologna node excavation diameter 9.4 m epb length + back–up 180 m epb weight + back–up ~990 t theoretical maximum progression speed 8 cm/min minimum bend radius 250 m total thrust capacity 10,197 t total power 5,100 kw torque (standard conditions) 1,022 t·m at 1.97 rpm maximum torque 2,043 t·m at 0.98 rpm peak torque 2,452 t·m standard cutting power 2,700 kw analogous considerations could be made for epb2 excavation. figure 2 shows that es values were lower than those for epb1; this clearly stood out by comparing both machines’ performances at the same chainage (figure 3). unfortunately, the data for epb2 was only available for chainage between 1,500 m and 2,050 m. the phenomenon could have been due to stress release in the ground; the first tunnel (cut by epb1) was excavated between gravels and sands, leading to gradual adaptation by the machine, confirmed by a negative es. gradient. the original stress distribution of the ground was modified as the second tunnel (cut by epb2) was excavated in close proximity to the first one (less than one diameter), resulting in reduced es. the decrease in es was evaluated as being about 30%. this result could also have been related to reduced ground strength due to epb1 tunnel excavation; however, no data was available regarding the mechanical characteristics of the ground measured on site during the excavation to confirm this hypothesis. table 2 shows the main ranges of es values obtained in different kinds of ground for both epb1 and epb2. tunnelling in urban areas by epb machines: technical evaluation of the system 7 figure 1. epb1 excavation specific energy figure 2. epb2 excavation specific energy figure 3. comparing epb1 and epb2excavation specific energies agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:08 p p p composite 133 lpi at 45 degrees table 2. ranges of excavation specific energy values in different kinds of soil type of soil excavation specific energy (mj/m3) clay 25-35 moist sand 25-30 dry sand 32-38 dry gravel 15-25 excavation specific energy compared to progression speed progression speed is one of the most important parameters when analysing the excavation cycle (innaurato n., 1990; kovàri k., 2002). figure 4 compares the es with epb1 instantaneous progression speed at different chainages. according to equation (7), progression speed was inversely related to es, which was made evident by the plot (increased velocity coincided with decreased es). this was fully in line with the pertinent literature (graham p.c., 1976; nielsen b., odzemir l., 1993; o’rourke j.e. et al., 1994; rostami j., 1997; thuro k., plinninger r.j., 2003; gong q.m., zhao j., 2009). the two graphs’ normalised gradients were similar, apart from the opposite sign. this was expected because the change in speed obtained from the pushing jacks’ extension represented the effect of lithological variation (lovat r. et al., 2001). increased energy and decreased speed indicated greater resistance to excavation by the ground. excavation specific energy compared to torque figure 5 compares es to torque at different chainages for epb1. a direct correlation was apparent, as expected from equation (7), where the term directly proportional to torque prevailed. the correlation was confirmed by the plot and the gradient was similar for both variables. since estot was mostly dependent on rotational specific energy (specific energy due to thrust is smaller and is usually ignored) and torque was a function of the cutting force acting on the cutting tools, then such direct correlation would be expected (nielsen b., odzemir l., 1993; us army corps of engineers, 1997; boniface a., 2000; gong q.m. et al., 2007). excavation specific energy compared to rotation speed figure 6 shows the roughly constant value for rotation speed at different chainages compared to es variations for epb1. rotation speed and torque were roughly proportional to es and, as rotation speed was kept practically constant, energy was only affected by torque changes during excavation. es variation associated with rotation speed constancy showed that the latter was the externally controlled variable. an operator could set a constant level of rotation speed to maintain constant tool wear, while torque would change to cope with resistance to excavation produced by ground changes. on the other hand, if rotation speed were increased, then instantaneous penetration would decrease, and thus both the cutting force acting on the tools and torque would decrease. 8 marilena cardu, pierpaolo oreste figure 4. comparing epb1 excavation specific energies with instantaneous progression speed figure 5. comparing epb1excavation specific energies to torque figure 6. comparing excavation specific energies and rotation speed agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:09 p p p composite 133 lpi at 45 degrees excavation specific energy compared to total thrust figure 7 compares es to thrust at different chainages for epb1. according to equation (7), total specific thrust (thrust per unit cross-section) coincided with specific advance energy and was added to specific rotation energy, to give total es. thrust (as rotation speed) could be controlled by an operator and total thrust plot was scarcely correlated to total es plot, apart from some excavation specific energy peak having some counterpart in thrust peaks, probably representing attempts by an operator to raise progression speed in difficult ground (lovat r. et al., 2001; mair n., 1997). figure 8 gives specific thrust energy, specific rotation energy and total es (thrust energy was a small fraction of total energy). energy values were calculated statistically by the moving average method, assuming 60 m progression steps. figure 8 shows that rotation specific energy was comparable to total es throughout the chainage. only rotation specific energies’ contribution would thus be considered in the following. suggested evaluation methodology rotation speed (as shown in figure 6) made a very low contribution since it was kept almost constant by the operator. the most important parameters to be considered when calculating excavation specific energies were thus torque (which is a function of rotation speed, as in equation (5)) and progression speed (marcheselli p.p. et al., 1995; schmalzbauer s., 1984). knowledge of progression speed is determinant during mechanised full-section tunnel excavation to evaluate a machine’s performance in real time or at the end of a shift and to improve (if necessary) its performance regarding operator intervention related to inspectable parameters. obviously, if progression speeds were known (at least regarding a meaningful sample of excavated length), it would become possible to optimise a machine’s work and increase overall excavation time. it has been previously stated how little influence jack thrust has on evaluating total es. equation (7) can thus be simplified as: e c a v s � � � � (8) equation (8) shows that energy mainly depended on two parameters (torque and progression speed) by assuming constant rotation speed ù. the following equation was obtained by multiplying excavation specific energy by tunnel cross-section a: e a c v s � � � � � � (9) á was expressed in mj/m. equation (9) showed good correlation between es and coefficient á. the example in the graph shown in figure 9 referred to a 35 mj/m3es value, supplying a 2,360 mj/m á coefficient. therefore, if: � � � � � c v tcos (9’) then: c v� � �� � (10) tunnelling in urban areas by epb machines: technical evaluation of the system 9 figure 7. comparing epb1 excavation specific energies to thrust figure 8. comparing epb1excavation specific energies separately due to rotation and progression speeds figure 9. correlation between excavation specific energy and á coefficient agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:09 p p p composite 133 lpi at 45 degrees hence: v c � �� � (11) equation (11) shows that torque and progression speed were proportional dimensions, if rotation speed were considered as a constant. maximum torque is the most important parameter to be considered when making a decision during tunnel design stage for evaluating the most suitable type of machine. maximum torque data was provided by the tbm manufacturer. the graph in figure 10 gives an example of progression speed evaluated from torque data for epb1. assuming 2,500 t.m average torque at tbm cutter head and 1 rpm rotation speed, then a 1.05 mm/s net progression speed would be obtained as a reliable average value. analysis of a machine’s work performance can be gone into in more depth, leading to obtaining more accurate predictions. tbm work cyclically, through a sequence of well-defined elementary steps, requiring precisely known time intervals. each cycle consists of an excavation stage (with a machine advancing through the thrust of the jacks), a support erection stage (where a new lining ring is installed) and idle time devoted to maintenance. the sum of lining erection times and maintenance time represents the inactive time between successive ring installations and is repeated at each ring. at the end of a shift, therefore, elapsed excavation time is much shorter than total elapsed time. the ratio between the actual excavation time and total time, given by: �� t t exc total (12) represents a machine’s exploitation coefficient (machine utilisation time). when ç and instantaneous progression speed are known, a machine’s daily (24 hours) progression can be calculated as: v t v day total inst� � � �� 3 6. (13) where vday is daily advance rate (m/d), ttotal is total working time (hours/day) and vinst is the net advance rate (mm/s). exploitation (machine utilisation time) coefficient ç is obviously not known a priori. in the given example this has been calculated by solving the equation for ç, daily progression speed being known as it can be obtained from measured daily advance. however, this only gives a machine’s utilisation time factor; every contractor should predict this factor based on experience, a particular project site and conditions prior to construction activity. instantaneous progression speed can be obtained by averaging the data stored in a machine’s databank files for the 24-hour period being considered. this would then give: �� � � v t v day total inst 3 6. (14) the average progression speed calculated for both epb1 and epb2 was 0.99 mm/s; a 0.157 exploitation coefficient was then found from these values, meaning that 15.7% of total 24 hours/day working time had been spent in pure excavation. total time was assumed to be 24 hours and, from the exploitation coefficient, a 3.8 hours/day excavation time was thus calculated. total inactive time thus amounted to 20.2 hours/day; this was a very high value, showing the need for optimising working stages. by dividing the 20.2 hours by the number of rings, it was found that the time elapsed was 2.2 hours between the end of the erection of a ring and the start of the erection of the next one. part of this time was spent on assembling the ring, which takes about 40 min, and the remaining part was consumed in ordinary maintenance, including time spent waiting for the hardening of the mortar that connects the segments of the ring and maybe some other breakdowns. the results are shown in table 3. table 3. mean daily advancement rate values obtained with 9 rings/day for epb1 mean daily progression, m 13.5 mean instantaneous speed, mm/s 0.99 working hours in a day 24 exploitation coefficient ç 0.157 pure excavation time, hours 3.8 total inactive time, hours 20.2 time to assemble a ring, min 40 maintenance time, hours 14.2 mean maintenance time/ring, hours 1.58 conclusions comparing the most important excavation parameters led to interesting results for better understanding of earth pressure balance shields’ working phases. in particular, the following considerations should be highlighted: • excavation specific energy es did not appreciably depend upon thrust; rotation velocity too, being constant during drive, did not seem to substantially influence evaluation of excavation specific energy; • excavation specific energy es was directly correlated to torque; 10 marilena cardu, pierpaolo oreste figure 10. correlation between torque and progression speed agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees • ground stress release appreciably modified excavation specific energy values. in this case, this was the excavation of a second tunnel by machine epb2 at a distance of less than one diameter from the first tunnel which had already been built by epb1, occurring with a 30% es reduction; • excavation specific energy es varied widely with ground characteristics: a large variation in excavation specific energy ranges was in fact observed as a function of the subsoil encountered, from 25-35 mj/m3 for clay, to 15-25 mj/m3 for dry gravel, to 32-38 mj/m3 for dry sand; and • exploitation coefficient ç was low and varied from 0.10 to 0.20 for the project being analysed. progression speed was one of the main parameters to be considered during excavation. regarding this point, an interesting method could be suggested, namely automatic continuous diagraphy (dac). through an appropriate scale factor it enables estimating a given machine’s daily progression speed (progression speed having already been defined) to establish the necessary torque c to be applied by a tunnelling machine. references barla, g. (1994): “scavo di gallerie in prossimità della superficie”. v ciclo di conferenze di meccanica e ingegneria delle rocce (mir). torino. barla, g. (2000): “lo scavo meccanizzato delle gallerie”. viii ciclo di conferenze di meccanica e ingegneria delle rocce (mir). torino. bilotta, e., g. russo and c. viggiani (2002): “cedimenti indotti da gallerie superficiali in ambiente urbano”. xxi convegno nazionale di geotecnica: opere geotecniche in ambiente urbano. l’aquila. bebendererde, s. (1995): “advanced techniques for epb-tbm operation”. tunnel et ouvrages souterrains, 128. bovetti b. (2003): “studio della subsidenza indotta dallo scavo meccanizzato della metropolitana di torino attraverso modellazione numerica e back-analysis”. tesi di laurea, politecnico di torino, torino. altindag r. 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(1997): “principio di funzionamento di uno scudo ss ed epb”. gallerie e grandi opere in sotterraneo, 55. marcheselli, p.p. and a. nicola (1995): “criteri di scelta di macchine di scavo per la realizzazione di tunnel. il caso particolare del passante ferroviario di milano”. giornata di studio sullo scavo meccanizzato integrale delle gallerie: progettazione integrata e criteri di scelta delle macchine. naz. congr. sig. roma. schmalzbauer, s. (1984): “scavi di gallerie in terreni soffici con gli scudi”. gallerie e grandi opere in sotterraneo, 19. tunnelling in urban areas by epb machines: technical evaluation of the system 11 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 140-147 mapping high-angle basement faults in the middle benue trough, nigeria from gravity inversion surface g.c. onyedim1, k.d. alagoa2, i.o. adedokun2, a.a. aderogba2 and c. ovuru2 1 department of physics, obafemi awolowo university, ile_ife, osun state, nigeria 2 department of geology and physics, niger delta university, amassoma, wilberforce island, bayelsa state, nigeria e-mail: gonyedim@oauife.edu.ng abstract high-angle faults appear as vertical off-sets on a topographic surface including that derived through the inversion of gravity data. in this study of the middle benue trough, nigeria, gravity data for the area was bandpass filtered and inverted to generate a topographic surface map of the basement. furthermore, a topographic slope surface map was produced by calculating the horizontal gradient magnitude over the surface. an automatic method was used to scan the horizontal gradient magnitude grid to identify maxima whose loci were mapped as high-angle faults in the basement. three major ne-sw trending faults of regional dimension were identified. there are also shorter but significant nw-se trending faults. the ne-sw and nw-se sets are interpreted as conjugate pairs within the fracture system of the nigerian basement complex. in places, segments of these faults represent bounding faults enclosing the four major sub-basins which were identified around bashar, mutum biyu, wukari and lafia. key words: gravity anomalies, inversion, bandpass filter, topographic surface, basement, high-angle faults. resumen ángulo mayor de las fallas aparece como salida vertical en una superficie topográfica que incluye la derivada mediante el cambio de los datos de gravedad. en este estudio de la depresión intermedia benue, nigeria, los datos de gravedad para el área fue filtrado de arriba hacia abajo por el paso de banda para generar un mapa topográfico de la superficie del basamento. además, un mapa de superficie topográfico de la pendiente fue producido por el cálculo de la magnitud del gradiente horizontal sobre la superficie. se uso un método automático para explorar la magnitud máxima de la gradiente horizontal de la red para identificar los loci graficados en mapas que muestran las fallas con ángulos mayores en el basamento. 140 manuscript received: 30/07/2009 accepted for publication: 15/12/2009 tres ne-sw tendencias principales de la falla de la dimensión regional fueron identificado. allí están las tendencias de las fallas nw-se más cortas pero significativa. se establece que el ne-sw y nw-se son interpretados como pares conjugados en el sistema de la fractura del basamento complejo de nigeria. en algunos lugares, los segmentos de estas faltas están representan por coordenadas encerradas por cuatro principales sub-cuenca que son identificadas alrededor de bashar, mutum biu, wukari and lafia. palabras clave: anomalías de la gravedad, inversión, filtro de paso de banda, superficie topográfica, subsuelo, fallas de ángulo mayor. introduction the benue trough is part of a long stretch of the central african rift system and one of about seven inland sedimentary basins in nigeria. at present, oil exploration and production activities are concentrated mainly in the niger delta located at the southern end of the trough. few exploratory wells have been drilled in the benue trough by multinationals but they were soon abandoned as unproductive. however research results on the sedimentary rocks in parts of the trough show that they are capable of generating and expelling oil mapping high-angle basement faults in the middle benue trough, nigeria from gravity inversion surface 141 figure 1. generalized geological map of the benue trough and adjacent areas. lbt: lower benue trough; mbt: middle benue trough; ubt: upper benue trough. 1, precambrian; 2, jurassic “younger granites”; 3, cretaceous; 4, post-cretaceous sediments; 5, cenozoic-recent basalts. the study area is enclosed with thick lines (after zaborski, 1998). and/or gas due to their high organic productivity (nwachukwu, 1985 and obaje et al., 2004). furthermore, considering that the oil and gas industry now uses advanced technology to aid in the search for resources, drilling and extraction, and allows for the development of resources that were not previously economically viable, there is currently renewed intense geological and geophysical research interest in the exploration for oil and gas in the other inland basins especially anambra, benue trough and its northern extension the chad basin. together, if proven, these new sources of oil and gas will replace or complement production from existing wells as they decline, and help to ensure adequate oil and gas supplies to meet future demands. prior to the comparatively very expensive exercise of acquisition and analysis of seismic data in the exploration for oil and gas in sedimentary basins it is common practice to conduct preliminary investigations using, for example, potential field (gravity/magnetic) data. in sedimentary basins, basement faults are important structurally because they can influence and hence determine the overall basin architecture, tectonic history and control mineralization sites, oil and gas traps and groundwater flow patterns. due to the blanket of sedimentary rocks whose thickness can be considerable in places, the basement faults cannot be mapped directly using conventional field methods. in this circumstance, this study was aimed at utilizing an indirect method to map faults and associated structures in the basement through the analysis and interpretation of gravity data. the study area the study area, covering a region of 12,100 km2 and shown in fig.1, is located in the benue trough, nigeria. it is bounded by latitudes 7o 00’n and 9o 30’n and longitudes 8o 30’e and 11o 00’e. the trough is a ne-sw trending sedimentary basin, with an approximate length of about 800 km and bounded on its southwestern and northeastern ends by the niger delta and the chad basin respectively. the area, whose northeastern and southwestern boundaries are marked by large dotted lines in fig.1, is referred to as the middle benue trough (mbt) (cratchley and jones, 1965). the area was chosen for study because, according to nwachukwu (1985), the mbt is considered to be the most prospective area within the trough because depths to the mature zones are moderate (2-4 km). however, compared to other parts of the trough, the mbt is relatively poorly mapped, especially in its northeastern part. dessauvagie (1969) conducted a photogeological study of the area southeast of bashar. offodile (1976) prepared maps for the area around lafia-keana-awe region and nwajide (1985) mapped the area around makurdi. stratigraphically, the cretaceous sedimentary succession (beginning from the oldest to the youngest) consists of the asu river group, the awe, keana, eze-aku, awgu and lafia formations. nwachukwu (1985) suggested that exploration targets in the basin include both the sub-santonian and supersantonian sediments, with the eze-aku group, awgu shale, and nkporo shale being more prospective than the stratigraphically lower asu river group. no post-cretaceous sediments occur in the middle benue trough. volcanic activity was also relatively minor in scale and was concentrated in its southern part. a notable occurrence is the phonolithic trachyte of wase rock described by wright (1989). basalt flows occur around awe and dolerite sills, of cenozoic age have been encountered within the lafia formation (obaje et al, 1994). the pre-mesozoic rocks, which form the basement, consist mainly of granulitic gneisses, migmatites, older granites, younger granites, porphyries and rhyolites, migmatites and gneisses. materials and methods the gravity data for the area were compiled from the bouguer gravity map published by ajayi and ajakaiye (1981). the map was digitized at 1 km spacing and the resulting data was gridded using the program minc based on the minimum curvature procedure (webring, 1981). a shaded relief image of the gravity anomalies is shown in fig. 142 g.c. onyedim, k.d. alagoa, i.o. adedokun, a.a. aderogba and c. ovuru figure 2: bouguer gravity anomaly map of the study area (compiled from ajayi and ajakaiye, 1981). 2. in conformity with most features of the trough, the general trend of the gravity anomalies is northeast-southwest. the magnitude of the anomalies ranges between -62 mgal and 16 mgal. as with previous gravity studies by cratchley and jones (1965) and adighije (1979) the gravity map is characterized by a ne-sw trending regional positive anomaly flanked by linear negative anomalies. justification for the method on a topographic map or digital elevation model (dem), faults have distinctive expressions which include the linear alignment of areas of steep slope, apparently down-dropped or uplifted topography. onyedim and ogunkoya (2002) observed that linear features which appear on topographic maps may be found to mark sites of vertical movements, tilting or horizontal displacements and therefore are characteristic of parts of the crust that have been subjected to block-faulting. eliason (1992) proposed that topographic lows can indicate geological fractures because near-surface planar segments of fracture zones locally control the orientation of aligned topographic lows by influencing erosion processes. in general, gravity anomalies are sensitive to the vertical changes in density at contacts where the beds are displaced vertically due to faulting. as the angle of the contact of density contrast between the two rock types (or the dip of the fault) decreases, the resulting gravity signal becomes broader and flat and therefore less definable. for this reason, shallowly and moderately dipping contacts in rocks of differing densities are essentially ‘invisible’ to gravity and would not produce significant features in the gravity inversion surface (gravis). therefore, the premise of the methodology used in this study is that topographic features showing significant vertical offset on a gravis can be interpreted as high-angle faults. in this study of the middle benue trough, nigeria, the bandpass filtered gravity map was inverted to produce a topographic surface map of the basement. points of maximum slope on the gravis were determined and where such points form prominent linear features, they have been interpreted as high-angle faults in the basement. filtering of gravity data gravity anomalies in the study area, as show in fig.2, are the sum effect of the density variations at various depths and of different magnitudes. in order to produce a map of the basement topography by inversion of gravity data, it was necessary to determine first the anomalies due primarily to the density contrasts in the basement rocks. the standard approach is to apply a bandpass filter which removes the high frequency (short wavelength) components associated with shallow features; thereby leaving the low frequency (long wavelength) anomalies of interest. the application of a bandpass filter requires knowledge of the cut-off wavelength. rather than estimate these by trial and error, the matched filtering procedure described by phillips (1997) was adopted in mapping high-angle basement faults in the middle benue trough, nigeria from gravity inversion surface 143 figure 3. the power spectrum of the gravity data. the linear segments corresponding to the density layers used to model the observed spectrum and to compute depths are demarcated by short horizontal lines. figure 4. an image of the bandpass filtered gravity field showing anomalies due to basement sources. the cutoff wavelengths are those shown for layer 3 in table.1. this study. the computed log averaged radial power spectrum of the gravity data is shown in fig. 3. a characteristic feature of the power spectrum curve is the rapid drop in energy at the low frequency region, which is indicative of response to deep-seated sources and a gentler decline at the high frequency end caused by near surface sources of density contrast (negi et al., 1983). based on the changes in slope the power spectrum curve for the area shows that there are three major density layers overlying a half-space. these are marked as layers i, ii, ii and a half-space in fig. 3. bandpass filters were designed for the density layer models whose depths were also computed from the slopes of regression lines fitted to the observed segments in the power spectrum curve. the bandpass filter wavelengths and depths computed for the model density layers are shown in table 1. table 1. bandpass wavelengths and depths determined for the density layers from the power spectrum of the gravity data. layer no. bandpass wavelength range (km) depth (km) 1 1.0 1.38 0.32 2 1.38 8.90 1.07 3 8.90 80.0 5.91 4 80.0 infinity 35.62 considering that the depth of 5.91 km determined for layer iii is comparable to the values determined for the basement in parts of the trough by other workers (benkhelil, 1988, ofoegbu, 1984 and saugy, 1987), the corresponding cutoff wavelengths for the layer were used to compute the gravity anomalies associated with the basement rocks. the resulting image of the bandpass filtered gravity field due to sources of density contrast in the basement is shown in fig. 4. subsequent analyses were performed on the bandpass filtered gravity dataset. inversion of gravity data and basement fault interpretation the bandpass filtered gravity data was inverted using the 3d inversion method described by cordell and henderson (1968). noting that the depths computed from a power spectrum curve are essentially mean depths to the top of an en144 g.c. onyedim, k.d. alagoa, i.o. adedokun, a.a. aderogba and c. ovuru (a) (b) figure 5. the inverted topographic surface of the basement is shown as (a) shaded relief image and (b) 3d perspective plot. figure 6. an image of the slopes surface determined by computing the horizontal gradient magnitude of the inversion surface. grayscale bar shows slope values. semble of sources (naidu, 1968; spector and grant, 1970) and a value of 5.91 km calculated for the depth to the basement, the modeling was constrained by specifying 6 km as the depth to the middle reference surface of the density layer with a density contrast of -0.35 gm-cm-3. fairhead and okereke (1990) used a density contrast of -4.0 gm-cm-3. the basement topographic surface (bts) map generated from this analysis is shown as a shaded relief image in fig. 5a and as a 3d perspective plot in fig. 5b. to interpret faults on the bts, the interpreter starts by conducting a lineament analysis, which involves the identification of linear zones of maximum gradient. thus, the interpreter needs to plot the bts in such a way that such zones representing offsets in the surface can be identified easily. four plotting options are available: shaded relief view (fig. 5a), profiles or cross-sections, 3d perspective view (fig. 5b) and a plan view showing points on the bts that locally define its maximum slope. the maximum slope option was used in this study because of the subjective nature of the first three options. accordingly, a grid of horizontal gradient magnitude (hgm), which defines the slopes, was computed from the bts grid. this is plotted in fig. 6. furthermore, a subroutine of the program boundary described by phillips (1997) was used to scan the hgm grid for maximum slope points, which are plotted as crosses in fig. 7. discussion of results the inverted surface images in figs. 5a and 5b show that the basement surface is composed of juxtaposed sub-basins and horsts. the prominent sub-basins are those located southeast of bashar, around mutum biu and wukari and south of lafia. they are part of the sub-basins identified by their morphostructural anomalies (onyedim et al, 2004a) and from the basement isobath map determined from aeromagnetic and gravity data (benkhelil, 1988, 1989). the 3d perspective plot in fig. 5b reveals that the sub-basins located south of bashar and lafia are connected and appear to form a continuous n-e trending depression spanning the entire length along the northwestern boundary of the mbt. the horsts form a central n-e trending belt stretching from the southwestern corner through awe and up to muri in the northeastern corner. the main product for fault interpretation is the map of maximum slope points shown in fig. 6. here, the map of maximum slope points was superposed as a transparency over the hgm image to ascertain that the maximum points indeed fall along the ridges of the hgm surface. the correspondence between the two is quite remarkable. therefore, faults were interpreted on this image by highlighting linear features, which show as collinear or slightly curvilinear arrangement of maximum slope points. the aligned maxima, which are interpreted as high-angle faults, are shown with thick lines in fig. 7 while the sub-basins are labeled sb1, sb2, sb3 and sb4 respectively. a linear feature can be considered structurally significant if it extends over a distance of about 4 km or more (onyedim and ocan, 2001). in some cases, short but aligned segments may also satisfy this criterion. based on this, the major faults in the area are those labeled aa’, bb’ and cc’ in fig. 7. fault aa’ consists of segments whose strike alternate between ne-sw and approximately e-w to wnw-ese. they coincide approximately with the boundary of the cretaceous sediments and the precambrian basement. fault bb’ is a better defined linear feature. in places, fault aa’ and bb’ form the boundaries to the sub-basins sb1 and sb4. the continuity of the fault cc’ is broken around wukari by a nw-se trending fault segment, which forms part of the nearly circular feature enclosing sub-basins sb3. part of the fault cc’ links the wukari sub-basin sb3 with the mutum biu sub-basin sb2. the region between the faults bb’ and cc’ is dominated by swarms of smaller faults trending mainly nw-se. according to onyedim et al (2004b) the nw-se trending mapping high-angle basement faults in the middle benue trough, nigeria from gravity inversion surface 145 figure 7. points of maximum slope on the gravity inversion surface are marked by crosses on the slope surface image. aligned linear and curvilinear sets of points are interpreted as lineaments (thick lines) representing faults in the basement. grayscale bar shows slope values. faults form a conjugate pair with the ne-sw trending faults and are referred to collectively as riedel shear faults. according to kogbe (1983), ananaba (1983) and oluyide (1988), the ne-sw and nw-se fracture systems predate the rifting of the benue and niger troughs and can be interpreted as significant features of the tectonic framework of the nigerian basement complex. conclusions the fault pattern in the basement beneath a sedimentary basin can be unraveled by analyzing the topographic surface of the basement obtained by inverting gravity data. this study has demonstrated the effectiveness of the method involving the production of an inversion surface for the bandpass filtered data and identifying faults as the maximum slopes on this surface. application of the method to the gravity data of the middle benue trough, nigeria showed that the major faults in the basement trend ne-sw and nw-se. in places, segments of these faults form bounding faults to sub-basins in the area. acknowledgements the paper was developed and written with the co-authors when the corresponding author was on a sabbatical leave at the niger delta university, amassoma, bayelsa state. we wish to thank the authorities of the university for the provision of computer facilities and the usgs for use of their potential field software for gravity data processing and interpretation. references adighije, c.i. 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(1989). volcanic rocks in nigeria, kogbe, c.a (editor), geology of nigeria, rock view (nigeria) ltd, jos, nigeria. 125-173. zaborski, p.m. (1998). a review of the cretaceous system in nigeria. africa geoscience review, 5, (4), 385-483. mapping high-angle basement faults in the middle benue trough, nigeria from gravity inversion surface 147 geoflslca colomblana ~ 1, octubre de 1992 . 9 aplicacion del analisis de fourier para describir la variacion diurna de elementos climaticos jesus eslava profesor titular depto de geociencias-facultad de ciencias-universidad nacional de colombia ramiro diaz profesor asociado depto de geociencias-facultad de ciencias-universidad nacional de colombia eslava, j. & r. dlaz: apllcacion del anal isis de fourier para describir la variacion diurna de elementos clirnatico s. geolis. colomb. 1:9-20, 1992. issn 0121-2974 resumen en este trabajo, con base en los valores medios horarios de la presion, temperatura y humedad relativa del aire, determinados an la estacion observatorio meteorologico nacional, ubicada en la ciudad universitaria de santale de bogota, y aplicandoles una metodologia que incluye un analisis de fourier, se obtiene un modelo rnatematico que no solo representa la variacion diurna de los valores de esos elementos, sino que tambien posibilita pronosticar con buena exactitud los valores horarios de ese elernento, para periodos de seis y doce horas, con base en ei promedio de las 24 horas anteriores. abstract a time model for pressure, temperature and air relative humidity, has been established for the data of the national meteorological observatory station located at the campus of the national university in santafe de bogota. to get this model, fourier analysis has been applied to the hourlu mean values of the before mentioned parameters. the results provide not only diurnal variations, but allow the forecasting of the parameters for periods of six and twelve hours, from the average of the last 24 hours. 1. introduccion la finalidad de este trabajo es la de definir un modelo rnaternatico que permita d'eterminar y cuantificar la evolucion de los elementos rneteoroloqicos a traves del tiempo. trabajos previos -sobre ei tern a fueron desarrollados por eslava & dlaz (1991a,b,c), en particular aqui se caracteriza un modelo, mediante ei analisis de fourier, en terrnino s de los para metros de frecuencia y longitud de onda de las senales de presion, temperatura y humedad relativa del aire (media horaria) definidas para la estacion observatorio meteorologico nacional. en el modelo se utilizan los valores medios que se determinan para cualquier periodo continuo de 24 horas, esos valores se corrigan para adecuarlos a los que se determinarian si se utilizaran los 24 valores horarios. previos unos comentarios sobre el proceso de rnodernizacion de la meteorologia y la aplicacion de modelos rnaternaticos para simular parte 0 la totalidad de las propiedades fisicas de la atmosfera, se da una descripcion de la metodologia utilizada y el analisis de los datos, se determina la forma como se pronostican los valores horarios de la presion, temperatura y 10 eslava & diaz: analisis fourier descr. variac. e1em. clim. humedad relativa del aire y se incluyen ejemplos de aplicacion del modelo. 2. generalidades sobre modelos matematicos y meteorologia una de las particularidades caracteristicas de la meteorologia moderna es la de implementar un amplio e impetuoso proceso de utilizacion de los rnetodos matematicos y de los computadores en todos los asuntos aplicados y te6ricos. ei proceso de modemizacion de la meteorologia ernpezo a comienzos del presente siglo, pero su impulso mas fuerte 10 loqro despues de la aparicion y el rapido perfeccionamiento de las computadoras. las enormes posibilidades de aplicacio n de los modelos rnaternaticos. como meto do de conocimiento de las leyes del universe real y su aplicacion en la actividad practica de la humanidad, muy lejos de ser agotadas, permiten ampliar las investigaciones y desarrollar trabajos tecnico s y cientificos dirigidos a encontrar solucion a todos los asuntos relacionados directa 0 indirectamente con la meteorologia, en particular, 0 con la ciencia, en general. estas posibilidades estan relacionadas con la construocion de modelos matematicos que simulen desde cada una de las caracteristicas de cada uno de los elementos involucrados con la atmosfera, hasta supermodelos que permitan pronosticar no solo el proximo estado del tiempo sino el clima mismo. los modelos matematicos posibilitan sustituir la investigacion de un objeto 0 elemento real "no matemalico" (un fenomeno de la naturaleza, una o varias caracteristicas de uno 0 varios elementos meteorologicos, etc.) por la solucion de un problema matematico y analizar esa realidad utilizando todo el aparato matematico universal que no esta condicionado 0 no depende de la naturaleza· real del objeto 0 elemento. despues de elaborar el correspondiente modele matematico, con 10 cual se formaliza ei elemento, que destaca y describe, con ayuda de ecuaciones, sus rasgos y propiedades esenciales, se puede realizar un analisis cuantitativo detallado y se puede predecir como se comportara el elemento bajo diferentes condiciones. la utilizacion de metodos numencos en computadores amplio el numero de asuntos a ser estudiados mediante modelos matematicos. actualmente, al construir la mayor parte de modelos matematicos, no es necesario recurrir a simplificaciones 0 elias cada dia son menos; basicamente 10 fundamental as tomar en consideracion las particularidades sustanciales del e1emento y reflejarlas en el modelo. de todas formas siempre surge el problema de la calidad del modelo hipotetico y la decision sobre si son 0 no correctas las suposiciones iniciales. la solucion del problema de la aplicabilidad de un modele no es estrictamente matematica y, por 10 mismo, no puede resolverse por metodo s maternatioos. aqui ei criterio fundamental es la experirnentacion, la practica. en el sentido mas amplio de la palabra, que permite comparar diferentes modelos y seleccionar el que reproduzca las caracteristicas del elemento estudiado, dentro de los limites de la precision requerida y que sea, simultineamente, el mas simple. cada una de las propiedades 0 caracteristicas del sistema atmosferico, interactua con una 0 todas las propiedades 0 caracteristicas restantes y, ademas, 10 hace en un completo rango de escalas temporales y espaciales, que abarcan desde los tenornenos de muy pequeria escala, baja amplitud y corto periodo, (milimetricos y de muy corta ouraclon) hasta aquellos que cubren toda la tierra e inclusive el universo mismo y duran muchos arios (alta amplitud y largo periodo). puesto que las propiedades fisicas que definen el estado y evolucion del tiempo y el c1ima son muy complejas y variadas es imposible, en el estado actual de la ciencia, concebir un modelo que defina a la atmosfera en forma completa. por 10 anterior, es necesario concebir modelos unitarios y/o parciales que consideren toda 0 una parte de una 0 varias de esas propiedades. la comunidad cientifica internacional, en general, y la meteorologica, en particular, ie asignan un importante papel al modelado numerico del sistema atmosferico. en este sentido coinciden todos los investigadores, cuyas ideas y propuestas son resumidas muy adecuadamente por nunez (1987); de este autor y para resaltar la importancia del modelado matematico en el desarrollo de la meteorologia, se hace a continuacion, una sintesis adaptada. los procesos del sistema atmosferico pueden representarse en terminos de un conjunto de ecuaciones dinamicas y termodinamicas junto con las ecuaciones apropiadas de estado y las leyes de conservacion para constituyentes como ei agua, el anhidrido carbonico y el ozono en ei aire y la sal en los oceanos. esas ecuaciones, oblliamente, contienen los diferentes procesos que determinan los cambios en temperatura, velocidad, densidad y presion y se agregan, ademas, otros procesos relevantes como la evaporacion, condensacion, precipitacion, radiacion y la transferencia de calor y cantidad de movimiento por adveccion, conveccion y turbulencia, como as; tambien los procesos quimicos y biologicos. tanto en el modelado matematico del estado del tiempo como en el del clima, se usan los mismos principios fisicos, pero en el del clima se introduce un numero mayor de aproximaciones fisicas y numericas, a causa de la limitada capacidad actual para observar el sistema c1imatico y computar su comportan"ento. geoffslca colombians n9 1, octubre de 1992 . 11 nul\ez (1987) igualmente concluye que los modelos tarnbien pueden usarse con cierta confianza para investigar la reacci6n del clima ante las imaginables perturbaciones naturales y las provocadas por ei hombre, y aaber si esos cambios pueden distinguirse de las fluctuaciones climatioae naturales. obviamente, para lograr establecer esa distinci6n, es necesario determinar muy bien 10 relacionado con las f1uctuaciones naturales, las cuales se intenta conocer y cuantificar con base en modelos que usan los datos observados, tal y como es el caso del presente trabajo. aun cuando a nivel de la regi6n tropicalecuatorial y a nivel local, no se han logrado avances significativos en ei modelado numerico del tiempo ni del clima, ya se han logrado importantes progresos en el pron6stico nurnerico del estado del tiempo a nivel mundial y en la simulaci6n de los campos globales observados de presi6n, vientos, temperatura y liuvias, junta mente con cambics estacionales y con particularidades regionales importantes tales como los monzones. para finalizar esta secci6n, podemos resumir senalando que una de las etapas criticas en la investigaci6n basica de las propiedades de los elementos meteorol6gicos y su distribuci6n temporal y espacial es la de la construccion y anal isis de modelos matematicos que correspondan muy bien con cada una de las propiedades y particularidades de los elementos. si se quiere liegar a desarrollar una teoria satisfactoria sobre el tiempo y ei clima sera necesaria, obviamente, ia uni6n de todos los modelos que tiendan a definir la estructura de la atm6sfera ylo la variabilidad espacial y temporal de cada uno de los componentes del sistema atmosterico: esa uni6n ofrecera una base para la major comprensi6n de los diferentes procesos meteorol6gicos y poslbilitara la implementaci6n de modelos integrados que simulen todos los procesos atrnostericos relevantes y que perrnitan pronosticar adecuada y oportunamente ei estado del tiempo y el del dima. en el sentido anterior, cada una de las investigaciones te6ricas que determinen un modele sobre alguna de las caracteristicas de un e1emento de la atmosfera, se constituye en un aporte pequeno perc necesario para la solucion futura del modele integrado. 3. metodologia utilizada lo que se acostumbra en general, y en particular en meteorologfa, es tratar, por medio de regresiones, de acomodar .un modelo maternatico preestablecido (lineal, logaritmico, exponencial, etc.) a un conjunto de datos reales de un muestreo, "sin tener en cuenta los contenidos de frecuencias y longitudes de onda de las senales. mediante ei analisis de fourier (vease cooley et al., 1970), se posibilita la determinacion de un modelo rnaternatico no preestablecido y en terminos de los parametros de frecuencia y longitud de onda ya mencionados y que son los determinantes verdaderos en los analiais de variabilidad espacio-temporal de los elementos meteorol6gicos. las lunciones de presion, temperatura, humedad del aire, etc., son funciones temporales y espaciales continuas. un muestreo en ei tiempo de cualquiera de elias (fl')' se puede considerar como ia convolucion de (x) con la funci6n delta de dirac, cuyo resultado es una funci6n discreta y peri6dica. fj = f(x). l l)(x-na) = n="""oo = l f(x-na) = l fijd-na) n=-oo n="""oo [1] en el intervalo 0 :5j :5 ni. los parametres n, 0 y no representan el numero de intervalos de muestreo, ei intervalo de muestreo y la longitud del registro (iongitud de onda fundamental). ei procesamiento cuantitativo de datos en meteorologia, se inicia representando la informacion en terrruno s de una serie arm6nica, la cual permite un tratamiento maternatico adecuado. para muestreos en una dimension la longitud del registro es: l = a = no ei vector de onda esta dado por: k = k".e, k = (21tn/l).e, o:5n:5n-l y ei vector posicion: r = x.e, r= jd.e, o:5j:5n-l cada observaci6n se puede representar en terminos de la serie arm6nica: n-l -4-4 fj = i cn.eik ' [2] n-o i=j(-l) la suma de las desviaciones cuadraticas con respecto a la serie inicial n-l n-l i (i f\2 = i 1)2 j-<j i v j-o 12 eslava & diaz: analisis fourier descr. variac. e1em. clim. es minima, siampre y cuando los coeficientes c correspondan a los coeficientes discretos de f~'urier n-1 c = (1/n). l f,..e;20nj/ n n ;-'0 n·1 cn= (tin). l fj.e"" j-o las transformadas de fourier se denotan abreviadamente asi: f « » c i n esto significa que se puede pasar del dominic del tiam po , al dominio de la frecuencia y viceversa, usando la transformada directa y la transformada inversa de fourier. debido a la longitud fin ita de la serial en el dominio del tiam po , se hace necesario establecer hmites en el dominic de la frecuencia, los cuales dependen de su longitud y de su intervalo de muestreo. la longitud del registro define el limite inferior: u i = 1/l y el intervalo de muestreo define el limite superior liamado de frecuencia de nyquist un=y2d de las dos condiciones anteriores se astablece que el intsrvalo de frecuencia en el espectro as: 1/lsusy2d y el intervalo entre dos frecuencias consecutivas esta dado por: ~u = 1/l liamado tambien co-intervalo de nyquist. en la practlca, para iograr una optima rescluckin, es necesario tomar la longitud del registro tan grande como sea posible. los datos de muastreo fj son reales y los resultados que se desea obtener tam bien deben ser reales; por 10 tanto, al expresartos en terminos de una serie de fourier, se debe tener en cuenta la siguiente propiedad: fj es real y par «=.. cn es real y par [5] en general fj no cumple con esta condicion pero, usando los datos de muastreo, es posible construir una nueva funcion f " en el intervalo o sis 2n-1, que tenga simetria par con respecto al ultimo punta de la serie haciendo: (ver fig.1) [3] • f, i i d i i i i i i ix r i r r r r -2 0 1 2 n 2n·' figura 1. funclcsn f, construlda a partir de los datos de muestreo, con slmetrla par [4] [6] 2n·1 ~~ c = (1/2n). l f,.e"" [7] n j-o se puede construir tarnbien la tuncion par, con respecto al origen de la misma, tomando: f, = f~ en esta nueva funcion 2n, 0 y 2nd representan respectivamente: el nurnero de intervalos, el intervalo de muestreo y la longitud de la nueva funoon par. construida la tunclon par y haciendo: 2cn = a, las ecuaciones [6] y [7] se transforman en las ecuaciones [8] y [9] que son la base te6rica para la determinacion 0 oonstruoolon de modelos que representen las seriales en el tiempo y ia frecuencia de los diferentas elementos meteorologicos. ei espectro de potencia permite identificar las componentes en frecuencia de una senal y separartas posteriormente en el dominic del tiampo. n-1 f,=(aj2)+ l an.cos(k.r) + (aj2).cos(ltl) [8j n.1 . n-1 ~=(21n)[f.+2 l f,.cos(k.r) + fn·cos(ltn)] [9j 1-1 ei coeficiente a. de fourier, se define para cada mes, como: 24 a. = 2.(1/24). 1: fj .. 1 [10j y corresponde a los valores que aparecen en las tablas, como numero de onda cero. en todos los casos: ei subindice n significa el valor del punto 24 (hora 24 0 cero) , n es un punto cualquiera dentro de los limites senalados, fo valor del e1emento ala hora cero (24 horas), f, valor horas 1 a 23, fj valor horas 1 a 24, fn valor a la hora 24 (0 cero). geofislca colomblana n2 1, octubre de 1992 4. analisis de los datos 13 los datos utilizados corresponden a los valores medios para cada hora, de cada mes y afio, de presion (tabla 1), de temperatura (tabla 7) y de humedad relativa del aire (tabla 13), determinados por eslava (1990, 1991, 1992) para la estacion observatorio meteorologico nacional, ubicada en ei extremo ne de la ciudad universitaria de santafe de bogota, en predios dellnstituto geogratico agustin codazzi (igac). para definir esos valores se utilizaron los valores horarios medidos en ese sitio, durante ei periodo 1941-1960, previa una verificacion, homoqeneizacion y procesamiento segun los rnetooos rneteorolcqicos y estadisticos mas adecuados y cornunrnente usados en estos casos. 4.1 presion atmosferica ei coeficiente a. de fourier, se define, para este caso y para cada mes, como: 24 a. = 2.(1124). 1: pi ;'1 [11] y corresponde a los valores que aparecen en la tabla 2, como numero de onda cero. 4.1.1 reproducclon de la seiial mediante la serle de fourier la senal p. (ecuacion (1)) se puede reproducir mediante pi (ecuacion [7]), con desviaciones minimas y se pueden lograr correlaciones que tienen un coeficiente de 0.9999. con esto se hace muy confiable ei rnetooo utilizado. como se senalo antes ei coeficiente a. corresponde al doble del promedio diario determinado para cada mes con base en 24 observaciones horarias. por su parte, a" 8:1, a3, .... ,8:1., definen la presencia de frecuencias en orden ascendente en ei espectro y, por 10 tanto, determinan la forma de la serial, mientras que a, solamente ubica un desplazamiento de la sena a partir del origen. los valores que aparecen en la tabla 2, son coeficientes de fourier obtenidos al aplicar la transformada de fourier (ecuacion (9)) a los datos de la tabla 1. 4.1.2 pronostlco de valores horarlos mediante la ecuacion [9], aplicando la serie de fourier, se calculan los coeficientes de fourier, para ei mes que corresponda, (tabla 2) y se evalua la funcion, usando ei promedio de las 24 horas anteriores, en cualquier punta (hora) aplicando la ecuacion [8]. se pueden determinar, con buena precision, los valores horarios de presion atmosferica para las 6 horas siguientes si ei ultimo dato utilizado, 0 sea ei mas reciente corresponde a las 07 0 13 horas, 0 para las 12 horas siguientes si el ultimo dato utilizado corresponde a las 19 horas. en caso de no disponer del valor promedio determinado con base en las 24 horas anteriores, se puede usar el promedio determinado por cualquiera de las formulas altemativas que existen, previa aplicacion de una correccion para convertirlo en promedio de 24 horas. puesto que en colombia se utiliza generalmente la formula: [12] entonces, ei primer coeficiente de fourier sera: [13] pd = presion atrnosferica media diaria, pol = presion atrnosterica a las 07 horas, este dato puede ser ei cronokiqicarnente anterior 0 posterior al de las 13 0 19 horas que se utilice en la formula, p'3 = presion a las 13 horas, este dato puede ser ei croncloqicarnente anterior 0 posterior al de las 07 0 19 horas que se utilice en la formula, p,. = presion a las 19 horas, este dato puede ser ei cronolcqlcarnente anterior 0 posterior al de las 07 0 13 horas que se uti lice en la formula, ap = diferencia media mensual entre la presion media diaria determinada con base en las observaciones de las 07, 13 y 19 horas, y la determinada con base en 24 horas. ei ap que se utiliza en este trabajo es el determinado por eslava (1990b, 1991 b,d) y que se relaciona para cada mes en la tabla 3. un ejemplo de la aplicacion del modelo al pronostico se presenta en las tablas 4 a 6 y fig.2, en las cuales con los datos de presion atrnosterica medidos a las 07, 13 y 19 horas en el observatorio meteorologico nacional ei 6 y 7 de octubre de 1960 (tornados de igac, 1961), se pronostican los valores horarios para periodos de 12 horas, en ei primer caso, y de 6 horas en ei segundo y tercer caso. las desviaciones (a) entre 10 obtenido aplicando el modelo y 10 medido son minimas (ia mas alta no supera 1 (un) hpa), el analisis muestra que ei modelo tiene lendencia a modificar la amplitud de la curva de variacion real. a pesar de que las desviaciones son minimas, las pruebas muestran que es posible identificar las variables que originan esas desviaciones y diseiiar un filtro que las corrija. 14 eslava & ofaz: an'lisls fourier descr. variac. "em. cllm. tabla 1. presl6n atmo8f6rlca media horarla (hpa), en santa" de bogotlll hora ene feb mar abr may jun jul ago sep oct nov dic ana 01 751.9 752.1 752.2 752.5 752.7 753.0 753.3 753.3 753.0 752.2 751.5 751.3 752.4 02 751.5 751.7 751.8 751.9 752.3 752.6 752.9 752.9 752.6 751.8 751.3 750.9 752.0 03 751.3 751.4 751.5 751.7 752.1 752.5 752.7 752.6 752.3 751.7 751.1 750.7 751.8 04 751.4 751.4 751.7 751.8 752.1 752.5 752.6 752.6 752.3 751.7 751.1 750.7 751.8 05 751.7 751.8 751.8 752.1 752.3 752.6 752.7 752.7 752.6 752.1 751.5 751.1 752.1 06 752.1 752.2 752.2 752.5 752.6 752.9 753.0 753.0 753.0 752.5 751.9 751.5 752.4 07 752.7 752.6 752.7 753.0 753.1 753.4 753.5 753.5 753.4 753.0 752.5 752.2 753.0 08 753.0 753.0 753.1 753.4 753.5 753.7 753.8 753.9 753.8 753.4 752.9 752.5 753.3 09 753.1 753.3 753.4 753.7 753.7 753.8 753.9 754.1 754.1 753.5 752.9 752.6 753.5 10 753.0 753.1 753.3 753.5 753.5 753.5 753.8 753.9 753.9 753.4 752.7 752.3 753.3 11 752.5 752.6 752.7 753.1 753.3 753.4 753.7 753.7 753.4 752.9 752.2 751.9 753.0 12 751.9 752.1 752.2 752.6 752.7 753.0 753.3 753.3 752.9 752.2 751.4 751.3 752.4 13 751.1 751.4 751.4 751.8 751.9 752.7 752.7 752.3 752.1 751.3 750.7 750.6 751.7 14 750.6 750.9 750.9 751.1 751.4 751.9 752.2 752.1 751.5 750.7 750.1 750.1 751.1 15 750.3 750.5 750.3 750.6 750.9 751.4 751.8 751.5 751.0 750.2 749.8 749.8 750.7 16 750.3 750.2 750.2 750.5 750.7 751.3 751.5 751.4 750.7 750.2 749.8 749.8 750.6 17 750.6 750.3 750.5 750.7 751.0 751.4 751.8 751.5 751.1 750.5 750.1 750.1 750.8 18 751.0 750.9 750.9 751.3 751.5 751.9 752.1 751.9 751.5 751.0 750.5 750.3 751.2 19 751.4 751.4 751.7 751.9 752.2 752.6 752.6 752.5 752.2 751.7 751.3 751.0 751.9 20 752.1 752.1 752.1 752.5 752.7 753.0 753.1 753.1 752.7 752.2 751.8 751.5 752.4 21 752.7 752.6 752.6 753.1 753.1 753.4 753.7 753.7 753.3 752.7 752.3 751.9 752.9 22 752.9 752.9 753.0 753.3 753.4 753.7 754.1 754.1 753.7 753.0 752.5 752.2 753.2 23 752.7 752.9 753.0 753.3 753.4 753.7 754.1 753.9 753.5 752.9. 752.3 752.1 753.1 24 752.5 752.6 752.7 753.0 753.1 753.4 753.8 753.8 753.3 752.5 751.9 751.8 752.9 m 751.8 751.9 752.0 752.3 752.5 752.8 753.0 753.0 752.7 752.1 751.5 751.3 752.2 tabla 2. coeflclentes de fourier, obtenld08 al apllcar la transformadb de fourier (ecuacl6n 9) a los promedlos horari08 de la presl6n atmosterlca en santa" de bogota (tabla 1) # ene feb mar abr may jun jut ago sep oct nov dic allo 0 1503.72 1503.85 1503.85 1504.60 1504.95 1505.63 1506.08 1505.96 1505.42 1504.12 1504.69 1502.54 1504.48 1 0.02 0.07 0.07 0.03 0.07 0.03 -0.05 0.01 0.25 0.21 -0.39 0.04 0.07 2 0.39 0.37 0.37 0.37 0.39 0.35 0.38 0.43 0.49 0.41 -0.88 0.33 0.38 3 -1.05 -1.10 -1.10 -1.15 ·1.02 -0.89 -0.90 -1.00 ·1.09 -1.15 0.28 -1.02 ·1.05 4 -0.24 -0.11 -0.11 -0.15 -0.12 -0.06 0.04 -0.01 -0.12 -0.27 0.07 -0.26 -0.14 5 0.46 0.47 0.47 0.54 0.52 0.43 0.42 0.48 0.49 0.54 -1.06 0.43 0.48 6 0.05 0.01 0.01 0.00 0.01 -0.06 -0.01 0,01 -0.05 0.02 0.62 0.07 0.02 7 0.14 0.17 0.17 0.11 0.09 0.11 0.13 0.15 0.05 0.10 1.28 0.17 0.12 8 -0.02 0.02 0.02 0.03 0.03 0.08 0.02 0.00 -0.06 0.00 -1.26 0.02 0.02 io' 0.09 0.04 0.04 0.07 0.04 0.04 0.09 0.06 -0.02 0.08 -0.21 0.08 0.06 0.02 0.03 0.03 0.00 -0.01 -0.06 -0.03 0.01 -0.06 0.00 1.32 0,01 0,01 11 0.00 0.08 0.08 0.05 0.04 0.02 0.02 0.01 0.04 0.04 -0.54 0.01 0.02 12 0.04 0.03 0.03 0.05 -0.01 0.01 0.00 0.01 0.05 0.00 -0.83 0.01 0_01 13 -0.01 0.02 0.02 0.04 0.03 0.05 0.01 0.02 0.04 0.02 1.00 0.04 0.03 14 0.05 0.02 0.02 0.03 0.04 0.01 0.05 0.07 0.09 0.02 0.15 0.02 0.03 15 0.02 -0.02 -0.02 0.00 0.02 0.00 0.02 0.00 0.06 0.01 -0.90 0.02 0.01 16 0.02 -0.01 -0.01 0.02 0.00 0.05 0.00 0.00 0.10 0.02 0.44 0.02 0.03 17 0.02 0.03 0.03 0.04 -0.01 -0.01 0.00 -0,01 0.08 -0.02 0.45 -0.02 -0.01 18 0.02 0.02 0.02 0.02 0.04 0.01 0.04 0.06 0.11 0.02 -0.59 0.04 0.02 19 -0.01 -0.01 -0.01 0.02 0.01 0.03 0.00 0.02 0.03 0.00 -0.04 -0.01 0.01 20 0.00 0.02 q.02 0.01 0.00 003 0.01 -0.04 -0.02 0.00 0.42 -0.01 -0.01 21 0.01 -0,01 -0.01 0.00 0.00 -0.01 0.01 0.00 0.00 0.02 -0.17 0.01 0.00 22 0.02 0.03 0.03 0.04 0.02 0.00 0.01 0.04 -0.05 0.05 -0.12 0,01 0.01 23 0.01 0.01 0,01 0.01 0.01 0.01 0.00 0.02 -0.08 0.01 0.09 0,01 0.02 tabla 3 diterencla media mensual y.p en hpa) entre la presi6n atmosterica media dlaria determlnada con base en 24 observaclones [11 y con observaclones de las 07, 13 y 19 horas [2] mes presion media [1) [2j ap [1]-[21 enero 751.8 751.7 0.1 febrero 751.9 751.8 0.1 marzo 752.0 751.9 0.1 abril 752.3 752.2 0.1 mayo 752.5 752.4 0.1 junia 752.8 752.9 -0.1 julio 753.0 752.9 0.1 agosto 753.0 752.8 0.2 septie 752.7 752.6 0.1 oclubr 752.1 752.0 0.1 noviem 751.5 751.5 0.0~~~ -l~~ ~~~ ~~__ mo 752.2 752.2 0.0 tabla 4 valores medio8 horarios de presl6n atmo8terlca (+750 hpa) en santate de bogota, medldos 1m) y proncencados (p) (6 y 7 de octubre de 1960) apllcando ei modelo \dia 6 hora m 6-10-60 7-10-60 p map m a 01 02 03 04 05 06 07 13 19 20 21 22 23 24 1.7 1.3 1.2 1.2 1.6 2.0 2.5 0.0 -0.4 -0.3 -0.2 -0.1 -0.3 -0.6 1.7 1.7 1.5 1.4 1.7 2.3 3.12.5 0.6 1.4 1.7 2.2 2.5 2.4 2.0 2.1 2.6 3.1 2.9 2.9 -0.4 -0.• -0.6 -0.5 -0.9 geoffslca colombiana n!! 1, octubre de 1992 15 tabla 5. valoree medios horarios de presi6n atmosterica (hpal en san late de bogota. medidos (m) y prollosticadc?s (pi (6',1 7 de octubre de 1960) aphcando modelo idia 6 7 7-10-{)o hora m m p m .. 07 753.1 08 753.1 753.4 -0.3 09 753.2 753.1 0.1 10 753.1 752.9 0.4 11 752.6 752.2 0.4 12 751.9 751.1 0.8 13 750.6 751.0 750.6 0.4 19 751.4 tabla 6. valoree medios horarlos de presi6n atmosterica (hpal en sanlafe de bogota. medidos (m) y prcncsbcedce (pi (6 y 7 de octubre de 1960) aplicando er modelo idia 6 7 7-10-00 hora m m pm .. 07 753.1 13 750.6 14 750.4 750.2 0.2 15 749.9 749.4 0.5 16 749.9 749.3 0.6 17 750.2 749.7 0.5 18 750.7 750.2 0.5 19 751.4 751.4 750.9 0.5 bp. 1((lll -1 '----------='--------------20-1 2j-1i u-ii a3-11u-8 01-7 02-7 03-7 04-7 06-7 oe..v 07-7 hcr-a-c dta _ pron08'f'icada e3 wbd[])a _ d1prr8nc(a fillura 2. valores de presi6n atmosterica medidos (ii y 7 de octubre-1960).::t. prenosncaooe con el modelo (tabla 4) 4.2 temperatura del alre ei coeficiente a, de fourier. se define. para este caso y para cada meso como: 24 a, = 2.(1/24).1: ti i-1 (14) los resultados estan consignados en la tabla 8 y corresponden al numero de onda cero. 4.2.1 reproduccion de la serial mediante la serie de fourier ai igual que 10 senalaoo para la presion atmosferica, la serial t; (ecuaeion [1)) se puede reproducir mediante t, (ecuacion [7)) con desviaciones minimas y se pueden lograr correlaciones que tienen coeficiente de 0.9999. ei coeficiente a es el doble del promedio diario de cada meso eon 24 observaciones horarias. por su parte. a" a2 ...... 8,•• definen la presencia de frecuencias en orden ascendente en el espectro y. por 10 tanto. determinan la forma de la senal, mientras que a, solamente ubica un desplazamiento de la senal a partir del origen. en la tabla 8. se muestran los coeficientes de fourier obtenidos al aplicar la transformada de fourier (ecuacion [9)) a los datos de la tabla 7. 4.2.2 pronostico de valores horarios usando el promedio de 24 horas anteriores y aplicando la serie de fourier: mediante la ecuacion [9) se caleulan los coefieientes. para el mes que corresponda, (tabla 8) y se define la funcion en eualquier punto (hora) en la serie mediante la ecuaclon [8). se determinan con buena precision los valores horarios de temperatura para periodos de 6 0 12 horas, sequn los datos de entrada. si no se dispone del promedio de las 24 horas anteriores. se usa el determinado por eualquiera de las formulas atternativas, previa aplicaclon de una correccion para eonvertirlo en promedio de 24 horas. en colombia se utiliza generalmente la formula: td=(t07+ t'3+2.t,.)/4 [15] entonces, el primer eoefieiente de fourier sera: a, = 2·[td + at] (16) ei at que se utiliza en este trabajo es el determinado por eslava (1990a. 1991a). se relaciona para eada mes en la tabla 9. td = temperatura del aire media diana, t07 = temperatura del aire a las 07 horas, este dato puede ser el cronoloqicarnente anterior 0 posterior al de las 13 0 19 horas que se utiliee en la formula. t" = temperatura a las 13 horas, este dato puede ser el anterior 0 posterior al de las 07 0 19 horas que se utiliee en la formula. t,. = temperatura a las 19 horas, este dato puede ser el anterior 0 posterior al de las 07 0 13 horas que se uti lice en la formula. at = difereneia media mensual entre la temperatura media diaria basada en las 07. 13 y 19 horas, y la temperatura media diaria determinada con base en 24 horas. un ejemplo de la aplicacion del modele al pronosfico se presenta en las tablas 10-12 y fig.3. en las euales con los datos de temperatura del observatorio meteorol6gico naeional del 6 y 7-0ct-1960 (tomados de igac. 1961). se pronostican los valores horarios para periodos de 12, 6 y 6 horas. respectivamente. 16 eslava & diaz: anallsls fourier descr. variac. e1em. cllm. tabla 7. temperatura media horaria del aire, en santafe de bogota, rc) hora ene feb mar abr may jun jul ago sep oct nov die ano 01 9.5 9.6 10.6 10.7 10.9 10.5 9.9 9.9 9.5 10.2 10.2 10.2 10.1 02 9.0 9.2 10.2 10.4 10.5 10.1 9.6 9.5 9.0 9.8 9.8 9.7 9.7 03 8.6 8.7 9.8 10.0 10.2 9.8 9.3 9.2 8.7 9.5 9.5 9.3 9.4 04 8.2 8.4 9.6 9.8 10.0 9.5 9.0 8.9 8.5 9.2 9.3 9.1 9.1 05 7.8 8.1 9.3 9.7 9.8 9.3 8.8 8.7 8.3 8.9 9.0 8.7 8.9 06 7.6 7.7 9.1 9.6 9.8 9.3 8.8 8.5 8.1 8.9 8.9 8.6 8.7 07 8.5 8.5 10.2 11.1 11.5 10.8 10.5 10.3 10.1 10.6 10.7 10.2 10.2 08 11.8 11.8 12.9 13.6 13.8 13.3 12.7 12.7 12.8 13.3 13.4 12.7 12.9 09 14.3 14.3 15.2 15.5 15.4 14.7 14.4 14.4 14.9 15.1 15.4 14.9 14.9 10 16.5 16.4 16.8 16.5 16.4 15.8 15.4 15.6 16.2 16.5 16.9 16.8 16.3 11 17.9 18,2 17.9 17.7 17.1 16.6 16.1 16.5 16.9 17.3 17.8 17.9 17.3 12 18.7 19.1 18.6 18.0 17.6 17.1 16.6 16.9 17.4 17.6 18.3 18.6 17.9 13 19.1 19.4 19.2 18.5 18.1 17.6 17.1 17.5 17.8 17.6 18.4 18.7 18.2 14 18.9 18.9 19.0 18.2 18.0 17.6 17.2 17.4 17.9 17.2 17.8 18.5 18.0 15 18.6 18.5 18.6 17.8 17.7 17.3 17.1 17.3 17.8 16.9 17.2 17.8 17.7 16 17.6 17.7 17.9 17.1 17.3 16.9 16.7 16.8 17.2 16.3 16.5 16.9 17.1 17 16.5 16.6 16.9 16.3 16.4 16.0 16.0 16.0 16.4 15.6 15.4 15.8 16.2 18 15.1 15.4 15.5 15.1 15.3 15.0 14.9 14.9 15.0 14.4 14.4 14.7 15.0 19 13.6 14.1 14.2 13.8 14.2 13.9 13.7 13.7 13.8 13.4 13.4 13.7 13.8 20 13.0 13.3 13.5 13.2 13.5 13.2 12.9 12.9 13.0 12.8 12.9 12.9 13.1 21 12.1 12.3 12.8 12.5 12.6 12.3 12.0 11.9 12.0 12.0 12.0 12.1 12.2 22 11.4 11.5 12.3 12.0 12.1 11.8 11.3 11.4 11.2 11.4 11.6 11.5 11.6 23 10.7 10.8 11.8 11.5 11.8 11.3 10.8 10.8 10.6 . 11.0 11.2 11.1 11.1 24 10.1 10.2 11.1 11.2 11.4 10.8 10.3 10.4 9.8 10.6 10.7 10.7 10.6 m 13.1 13.3 13.9 13.7 13.8 13.4 13.0 13.0 13.0 13.2 13.4 13.4 13.3 tabla 8. coeficientee de fourier, obtenidos al aplicar la transfonnada de fourier (ecuacl6n 9) a los promedios horarios de la temperatura del aire en santafe de bogota (tabla 7) #ond ene feb mar abr may jun jul ago sep oct nov die ano 0 26.26 26.58 27.77 27.50 27.64 26.72 25.94 26.03 26.09 26.38 26.75 26.78 26.67 1 -1.81 -1.84 -1.50 -1.11 -1.16 -1.22 -1.31 -1.38 -1.48 -1.09 -1.07 -1.31 -1.36 2 -4.95 -4.94 -4.38 -4.04 -3.81 -3.79 -3.87 -4.02 -4.53 -3.99 -4.22 -4.45 -4.25 3 0.71 0.76 0.56 0.27 0.34 0.46 0.57 0.53 0.54 0.12 0.02 0.29 0.44 4 1.44 1.43 1.20 0.96 0.77 0.75 0.66 0.80 0.77 0.89 1.06 1.25 0.99 5 0.81 0.89 0.61 0.54 0.46 0.50 0.48 0.53 0.66 0.73 0.78 0.75 0.65 6 0.27 0.19 0.27 0.35 0.42 0.38 0.44 0.43 0.47 0.44 0.33 0.27 0.37 7 0.02 -0.10 0.08 0.11 0.15 0.17 0.16 0.13 0.19 0.07 0.07 0.02 0.07 8 -0.32 -0.25 -0.28 -0.16 -0.12 -0.12 -0.15 -0.15 -0.25 -0.20 -0.17 -0.15 -0.20 9 -0.14 -0.11 -0:11 -0.11 -0.14 -0.13 -0.14 -0.15 -0.18 -0.11 -0.15 -0.14 -0.14 10 -0.04 -0.05 -0.10 -0.11 -0.13 -0.13 -0.12 -0.12 -0.11 -0.11 -0.12 -0.06 -0.10 11 0.02 0.01 0.04 -0.03 -0.03 -0.03 -0.03 -0.03 -0.01 -0.05 -0.04 0.01 -0.02 12 0.14 0.15 0.15 0.13 0.08 0.07 0.08 0.07 0.10 0.06 0.07 0.05 0.10 13 0.09 0.16 0.09 0.08 0.10 0.10 0.06 0.09 0.11 0.11 0.13 0.08 0.11 14 0.06 0.08 0.03 0.08 0.03 0.06 0.04 0.07 0.06 0.04 0.04 0.07 0.05 15 0.02 0.00 0.03 0.05 0.07 0.06 0.05 0.06 0.08 0.08 0.06 0.06 0.04 16 -0.05 -0.05 0.00 -0.03 0.02 0.01 0.00 0.03 0.01 -0.01 0.03 0.03 -0.01 17 -0.03 -0.06 -0.03 -0.05 -0.02 -0.06 -0.01 -0.04 -0.02 -0.02 -0.02 0.00 -0.03 18 0.00 -0.02 0.00 -0.01 -0.01 -0.02 -0.01 -0.03 0.01 -0.01 -0.02 0.00 -0,01 19 -0.02 0.00 -0.02 -0.01 -0.02 -0.01 -0.04 -0.05 -0.06 -0.04 -0.04 -0.02 -0.03 20 0.12 0.10 0.06 0.05 0.04 0.06 0.02 0.05 0.02 0.05 0.05 0.01 0.05 21 0.05 0.02 0.00 -0.03 -0.02 0.00 -0.02 0.00 0.00 0.02 -0.01 0.01 0.01 22 0.01 0.00 0.00 -0.04 0.02 0.03 0.01 -0.01 0.03 0.02 0.02 0.04 0.02 23 -0.01 -0.02 -0.01 0.03 0.02 0.02 0.02 0.05 0.01 -0.02 0.03 0.00 0.02 tabla 9 diterencla media mensualj:t) entre la tabla 10 temperatura media diarla (oc~ eterminada con valoree medlos horarlos de t~eratura del alre base en 24 observaciones [ y la temperatura en santafe de bogota m idos (m) ~ media diaria determinaaa con base en pronosticados (p) (6 y 7 de octubre de 9601 ob8ervaclones de las 07, 13 y 19 horas (2) aphcando el modelo mes tem med ..t hora 6 06-10-60 07-10-60 rn [2j [1]-{2] m p m .. p m .. enero 13.1 13.7 -0.6 01 9.4 7.7 1.7 febrero 13.3 14.0 -0.7 02 9.0 7.4 1.6 marzo 13.9 14.4 -0.5 03 8.7 6.4 2.3 abril 13.7 14.3 -0.6 04 8.4 6.2 2.2 mayo 13.8 14.5 -0.7 05 8.1 5.3 2.8 junio 13.4 14.0 -0.6 06 8.1 5.8 2.3 julio 13.0 13.8 -0.8 07 6.2 9.8 7.2 2.6 agosto 13.0 13.8 -0.8 13 21.0 s~tiem 13.0 13.9 -0.9 19 12.4 oubre 13.2 13.8 -0.6 20 12.0 12.2 -0.2 noviemb 13.4 14.0 -0.6 21 11.2 12.6 -1.4 diciemb 13.4 14.1 -<].7 22 10.6 11.1 -0.5 ano 13.3 14.0 -0.7 23 10.2 10.3 -0.1 24 9.8 9.4 0.4 geoflslca colomblana ~ 1, octubre de 1992 17 4.3 humedad relatlva del airetabla 11. valores medios horarios de temperatura del aire ·c en santafe de bogota, medtdos 1m) y pronostlcados (p) (6 y 7 de octubre de 1900) aphcando el modele idla 6 7 7-10-60 hora m m p m a 07 7.2 08 12.8 13.6 -0.8 09 14.6 14.6 0.0 10 16.0 16.8 -0.8 11 16.8 18.4 -1.6 12 17.1 18.9 -1.8 13 21.0 17.1 18.7 -1.6 19 12.4 tabla 12. valores medios horarios de te.mperatura del aire rci en santafe de bogota, medldos 1m) y pronostlcados (p) (6 y 7 de octubre de 19601 aphcando el modele idla 6 7 7-10-60 hera m m p m a 07 7.2 13 18.7 -1.914 16.1 18.0 15 15.8 17.7 -1.9 16 15.2 17.0 -1.8 17 14.5 18.4 -3.9 18 13.3 15.6 -2.3 19 12.4 12.3 14.6 -2.3 las desviaciones (a) entre los valores obtenidos aplicando ei modelo y los medidos son relativamente altas (en algunos cases cercanas a 4°c), dependen de condil?iones rneteorolcqicas propias de cada dia, en particular de la amplitud de la temperatura y/o de la presencia 0 no de nubosidad. los ensayos indican que e~ posible lievar estas desviaciones a valores rrurumos mediante la aplicacion de un filtro.que involucre las variabl~ que las ocasio nan. esos ensayos tarnbien muestran que las desviaciones gr~ndes l;lueden estar indicando errores en la sene onglnal de datos 0 la presencia de una situacion meteorologica particular 0 no usual; en este sentido el modelo puede ser usado como un control de calidad de la informacion. 'c " ~ 20-6 21-1122-e 23-8 u-li 01-7 02-7 03-7 04-7 05-7 0'1-7 07-7 hora-dfa -epronos'flcada ----*wbdwa fi9.ur. 3. valores de temperatura del aire medidos ii) y 7 de octubre-19601..y pronostlcados con el modele it abla 101 ei coeficiente a de fourier. se define como: 24 a = 2.(1124). i: ui [17] o i-1 los valores resultantes estan consignados en la tabla 14. en el nurnero de onda cero. 4.3.1 reproduccl6n de la seilal mediante la serie de fourier tarnbieo la serial u. (ecuacion [1]) se reproduce mediant~ u, (ecuadon (7). con desviaciones minimas y correlaciones con r = 0.9999. como se senalo antes ei coeficiente a, corresponde al doble del promedio diario determinado para cada mes con base en 24 observaciones horarias. por su parte, a" <1:>, a3,....•<i:>.' definen la presencia de frecuencias en orden ascendente en ei espectro y, por 10 tanto, determinan la forma de la senal, mientras que.a, solamente ubica un desplazamiento de la sena a partir del origen. los valores que aparecen en la tabla 14. son los coeficientes de fourier obtenidos al aplicar la transformada de fourier (ecuacion [9]) a los datos de la tabla 13. 4.3.2 pron6stico de val ores horarios usando el promedio de 24 horas anteriores y aplicando la serie de fourier con la ecuacion (9) se obtienen los coeficientes de fourier (tabla 14) y se determinan, con relativa buena precision, los valores horarios de humedad para periodos de 6 y 12 horas, calculando el valor de la funcion en cualquier punto (hora) en la sene mediante la eouacion (8). en caso de no disponer del valor promedio determinado con base en 24 horas anteriores, se puede usar ~ promedio d~finido par cualquiera de las formulas alternatlvas. previa correccion para convertirlo en media de 24 horas. en colombia se utiliza generalmente la formula: ud = (uo7+u,3+u19)/3 [18] el primer coeficiente de fourier sera: ao = 2,[ud + au) [19] ud, u07' u'3' u,. = humedad relativa media diaria, a las 07, 13 y 19 horas. ,.u = diferencia media mensual entre la humedad relativa media diaria determinada con base en las observaciones de las 07, 13 y 19 horas. y la determinada con base en 24 horas. ei au que se utiliza en este trabajo (tabla 15) fue determinado por eslava (1991c, 1992). un ejemplo de la aplicacion del modelo al pronostico se presenta en las tablas 16 a 18 y fig.4, en las cuales con los datos de humedad relativa del aire medidos en el observatono meteorologico nacional el 6 y 7 de octubre de 1960 a las 07, 13 y 19 horas (tornados de igac,1961), se pronostican los valores horarios para periodos de 12.6 y 6 horas. 18 eslava & diaz: anallsls fourier descr. variac. e1em.cum. tabla 13. humedad relativa media horarla del a1re (%), en santaf6 de bogota hora ene feb mar abr may jun jul ago sep oct nov die ailo 01 88 88 87 87 90 87 88 87 90 92 92 90 89 02 89 88 88 90 90 91 89 89 90 93 93 90 90 03 90 89 89 91 91 91 90 90 91 94 94 91 91 04 91 90 90 91 92 92 91 91 92 94 94 92 92 05 92 91 90 91 92 92 92 92 92 95 94 92 92 06 92 92 90 91 92 93 92 92 92 95 93 91 92 07 89 90 89 89 89 87 87 87 87 91 90 88 89 08 80 80 79 80 79 78 77 78 77 80 79 80 79 09 67 65 67 69 70 69 68 67 65 68 68 67 68 10 57 58 58 63 65 63 62 61 58 62 61 59 61 11 51 52· 54 80 62 80 59 57 55 58 58 54 57 12 48 49 51 59 61 58 58 55 ~ 58 58 52 55 13 48 50 51 59 60 57 55 54 60 ·58 54 55 14 50 52 54 60 60 57 55 54 53 63 61 56 56 15 53 55 58 62 62 58 55 55 54 65 66 60 59 16 58 58 61 65 64 60 56 56 56 68 70 65 61 17 62 62 64 69 67 63 59 59 59 71 73 69 65 18 69 68 70 74 72 68 63 64 65 76 77 75 70 19 75 73 74 79 77 73 68 70 71 81 81 79 75 20 79 76 77 82 81 76 73 74 75 64 84 82 79 21 82 79 79 85 83 79 77 77 79 87 86 85 82 22 84 82 81 86 85 82 80 80 82 89 88 86 84 23 86 84 83 87 87 86 83 83 85 90 90 87 86 24 87 86 85 88 88 87 86 85 87 91 91 88 87 m 74 73 74 77 77 75 73 73 73 79 79 76 76 tabla 14. coeficientes de fourier h obtenidos al aplicar la transformada de fourier jecuaci6n 9) a los promedios horarios de umedad relativa del aire en santaf6 de bogota abla 1) #ond ene feb mar abr may jun jul ago sep ocl nov die ailo 0 147.21 146.33 147.33 154.79 154.83 150.58 146.67 146.33 146.62 158.71 158.04 152.58 151.08 1 4.14 4.83 4.41 2.63 3.72 5.30 6.57 6.36 5.57 2.49 2.07 2.47 4.28 2 20.65 19.25 17.76 16.05 15.79 16.96 17.49 17.79 19.54 17.51 17.60 18.45 17.83 3 -1.59 -1.63 -0.72 -0.53 -1.26 -2.10 -3.21 -2.70 -2.09 0.63 1.39 0.57 -1.17 4 -6.65 -5.83 -5.58 -4.54 -3.64 -3.25 -2.83 -3.64 -3.67 -4.46 -4.64 -5.91 -4.64 5 -2.83 -3.52 -3.31 -2.18 -2.01 -2.53 -2.63 -2.65 -2.80 -3.42 -3.60 -2.95 -2.67 6 -0.69 -0.68 -0.37 -1.32 -1.52 -1.40 -1.66 -1.68 -1.78 -1.72 -0.75 -0.62 -1.26 7 0.92 0.98 0.85 0.43 0.10 -0.22 0.02 0.12 0.29 0.86 0.67 0.83 0.57 8 1.25 1.58 1.25 1.21 1.13 1.00 1.00 1.21 1.46 1.38 0.83 1.04 1.08 9 0.15 0.43 0.11 0.36 0.50 0.48 0.43 0.26 0.46 0.44 0.41 0.21 0.36 1o~ -0.11 0.00 0.02 -0.17 0.27 0.07 0.43 0.21 0.09 0.10 0.22 • -0.22 0.07 11 -0.31 -0.23 -0.45 -0.63 -0.28 -0.01 -0.13 -0.13 -0.20 -0.16 -0.39 -0.39 -0.33 12 -0.46 -0.75 -0.83 -0.62 -0.58 -0.67 -0.33 -0.58 -0.54 -0.54 -0.71 -0.25 -0.58 13 0.05 -0.30 -0.11 -0.33 -0.06 -0.16 -0.33 -0.09 -0.07 -0.37 -0.14 -0.04 -0.18 14 0.05 0.23 0.32 0.00 -0.07 -0.34 -0.11 -0.10 0.07 -0.07 -0.04 0.23 0.04 15 0.04 0.30 0.15 0.03 0.11 -0.18 0.13 -0.01 0.09 0.03 0.15 0.17 0.13 16 0.08 0.33 0.08 0.04 0.21 -0.17 -0.08 -0.04 0.12 0.21 0.17 0.21 0.08 17 -0.07 -0.02 0.14 -0.05 0.26 -0.04 0.02 -0.07 0.04 -0.04 0.02 0.01 0.16 18 -0.22 -0.32 -0.13 -0.26 0.02 -0.10 -0.01 -0.15 -0.13 -0.19 -0.16 -0.38 -0.08 19 0.09 -0.07 -0.02 -0.10 -0.11 0.04 0.09 -0.07 0.21 -0.03 -0.06 0.09 -0.11 20 -0.01 0.08 -0.09 -0.21 -0.03 -0.08 -0.09 -0.03 0.08 -0.13 -0.02 0.16 -0.02 21 -0.1 0.23 '-0.20 -0.03 -0.02 -0.20 -0.01 0.12 0.05 0.07 -0.12 0.05 0.01 22 0.07 0.03 -0.10 -0.05 0.01 -0.19 -0.14 -0.07 -0.04 0.13 -0.10 0.04 -0.11 23 0.01 0.00 0.15 -0.12 0.06 -0.38 0.05 -0.14 -0.05 0.02 0.11 -0.01 -0.05 ei anaftsls de los resultados obtenidos al aplicar el modelo muestra claramente que se pueden tabla 15. dlferencla media mensual entre ladetectar posibles errores en la serie original 0 humedad relativa media diaria (%) determinada que las desviaciones grandes pueden estar con base en 24 datos (1) y 3 datos (2) indicando la presencia de una situaci6n meteorol6gica no usual; por e110, el modelo humedad media au mes (1) (2) [1]-{2] puede utilizarse para efectuar un control de calidad de los datos. enero 74 71 3 las desviaciones (6) entre los valores obtenidos febrero 73 71 2 marzo 74 72 2 aplicando ei modelo y los valores medidos se abril 77 76 1 consideran, en algunos casos, relativamente mayo 77 75 2 altas; elias dependen de condiciones junio 75 72 3 julio 73 70 3 meteorol6gicas muy particulares condicionadas agosto 73 70 3 por la amplitud de la temperatura y la ausencia septiem 73 70 3 octubre 79 77 2 o presencia de nubosidad. noviemb 79 76 3 los ensayos ya efectuados indican que el diciemb 76 74 2 ario 76 73 3 modelo tiene una tendencia a modificar la amplitud de la curva y que es posible lievar estas desviaciones a valores minimos mediante la aplicaci6n de un filtro que involucre las variables que ocasio nan esas desviaciones. geofislca colombians n2 1, octubre de 1992 19 tabla 16 valoree medlo8 horarios de humedad relativa del aire ('y.) en santafe de bogota, medidos iml y prono8ticado8 (p) (6 y 7 de octubre de 1900) aphcando el modele \dia 6 6-10-00 7-10-00 hora m p m • p m • 01 96 95 1 02 97 98 -1 03 98 99 -1 04 98 100 -2 05 99 100 -1 06 99 100 -1 07 100 95 96 -1 13 42 19 92 20 88 94 -6 21 91 79 12 22 93 92 1 23 94 97 -3 24 95 98 -3 tabla 17 valores medios horarios de humedad relativa del aire ('yo) en santafe de bogota, medidos (ml y pronosticados (p) (6, 7 y 8 de oclubre de 1!f60) aplicando el modelo \dia 6 7 7-10-60 hora m m p m • 07 96 08 79 69 10 09 67 72 -5 10 61 62 -1 11 57 53 4 12 57 55 2 13 42 59 53 6 19 92 tabla 18 valores medtcs horarios de humedad relativa del aire ('yo) en santafe de bocreta, medidos (mj r prono8ticados(p) (6 y 7 e octubre de 19 0 aphcando el modele \dia 6 7 7-10-60 hora m m p m • 07 96 13 53 14 66 60 6 15 68 62 6 16 71 62 9 17 74 57 17 18 79 64 15 19 92 84 67 17 •• sa ., ,.l.l_-'-------'-_-'-------'--_'-----'--~'-------"-----c:'--=-__'________'_ 20-6 21~ 22-6 23-6 2(-8 oj-7 02-7 03-7 0-4-7 05-7 08-7 07-7 hora-dfa ---+-pronost1cada. -&wbdlda figura 4. valores de humed.cl "...tiva del aire medidos (6 y 7 de octubre-19601 y pronosticados con el modele (tab a 16) 5. conclusiones los modelos definidos adernas de presentar una excelenle correlacion con los dalos de enlrada, se aplicaron como pronoslicador a varies periodos y se encontro, en la mayor parte de los cases, una muy buena concordancia entre los valores generados y los observados. los resullados oblenidos son muy promisorios pues se trato de una aplicacion preliminar de la melodologia, que aun puede mejorarse medianle el manejo del procedimienlo esladislico que delerrnina los valores medics de los elemenlos meteoroloqicos, idenlificando las variables que generan las modificaciones en la amplilud de la senal y la utilizacion de fillros que conlamplen las variables que originan esas diferencias y las disminuyan a un grade aceptable, previa definicion de las variables que inlervienen y la forma en que 10 hacen. 20 eslava & diaz: analisis fourier descr. variac. e1em. clim. referencias cooley, j.w., lewis, p.a. & welch, p.o. (1970): the fast fourier transform algorithm: programming considerations in the calculation of sine, cosine and laplace transforms. journal sound vib. 12 (3):315337. u.s.a. eslava, j. (1990a): caracteristicas de la variacion diurnade la temperatura del aire en bogota. memorias del xi congreso colombiano de geografia. pp.222-237. acoge, monteria. -(1990b): caracteristicas de la variacion diurna de la presion atrno sferica en bogota. memorias del xi congreso colombiano de geografia. pp.238-249. acoge, monteria. (1991a): variacidn temporal de la temperatura del aire en bogota. rev. acad. colomb. cienc. 18 (68):65-74. santale de bogota. -(1991b): variacion temporal de la presion atmosferlca en colombia. colombia geogrmica 17 (1): 41-109. igac, santafe de bogota. -(1991c): caracteristicas de la variacion diurna de la humedad relativa del aire en santale de bogota. memorias i congreso nal de ing. geogratica. acig, santafe de bogota. -(1991d): variacion temporal de la presion atrnoaterica en bogota. rev. acad. colomb. cienc. 18 (69): 175-181. santale de bogota. -(1992): variacion temporal de la humedad relativa del aire en santafe de bogota. rev. acad. colomb. cienc. 18 (70):333-344. santate de bogota. eslava, j. & diaz, r. (1991a): modelo matematico para describir la variacion horaria de la presion atrnosferica en santate de bogota. memorias i congreso nal. de ing. ge<igrafos. acig, santafe de bogota. -(1991b): modelo rnaternatico para describir la variation horaria de la temperatura del aire en santate de bogota. memorias i congreso nal. de ing. geog. acig, santafe de bogota. -(1991c): modelo rnatematico para describir la variacion horaria de la humedad relativa del aire en santafe de bogota. memorias i congreso nal. de ing. geog. acig, santale de bogota. instituto geograflco "agustin codazzi" (igag). (1961): anales del observatorio meteorologico nacional-ciudad universitaria 1960. 215pp. igac, santale de bogota. nunez, m.n. (1987): clima: evolucion y futuro. boletin informativo techint nl! 247: 3-43. techint, buenos aires. articulo 1.indd earth sciences research journal surface geophysics earth sci. res. sj. vol. 15, no. 2 (december, 2011): 81 88 application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina armando luis imhof1, manuel sánchez2, carlos calvo3 and adriana martín4 1 instituto geofísico sismológico volponi / facultad de ciencias exactas físicas y naturales. unsj. ignacio de la roza y meglioli. rivadavia. c.p. 5400, san juan, república argentina. fax: (54) 264 4234980. tel.: 54 264 4945015. e-mail: aimhof@unsj.edu.ar 2 instituto de mecánica aplicada. facultad de ingeniería. unsj. e-mail: msanchez@unsj.edu.ar 3 departamento de matemática aplicada. facultad de ingeniería. unsj. e-mail: ccalvo@unsj.edu.ar 4 departamento de informática. facultad de cs. exactas, fcas y naturales. unsj abstract a geophysical survey involving seismic refraction tomography (srt) for mapping ‘p’ waves was carried out in sierra santa clara, san juan province, argentina in july 2009. the purpose of the geophysical survey was to determine the degree of fracturing and the rigidity of the rock mass through which it is planned to build a 290 m long road tunnel traversing the mountain almost perpendicular to the axis thereof, at around 100 m depth from the summit. several difficulties arose from the operational point of view which made it almost impossible to conduct fieldwork in normal circumstances. firstly, the topography had almost 45º slopes and 100 m research depths which would have involved having had to use explosives to generate seismic waves reaching sensors which had sufficient signal-to-noise ratio for distinguishing them. legal restrictions regarding the use of explosives on the one hand and insufficient power when using hammer blows on the other made it necessary to design and build a gas-powered gun to achieve the minimum energy (2 kj) required for detecting seismic signals. secondly, using conventional interpretation methods involving layered models was inoperable in such geological structures; seismic tomography methods were thus used which make use of the velocity gradient concept (both lateral and in-depth). this allowed mapping subsurface velocity variations in the form of velocity contour lines. the methodology used with the new seismic waves’ source generator, as well as srt application in this type of geological structure, demonstrated that satisfactory results could be obtained for this kind of geophysical study for geotechnical purposes. resumen se llevó a cabo un estudio geofísico de tomografía sísmica por refracción (srt) con ondas compresionales en la sierra de santa clara, provincia de san juan, república argentina en julio de 2009. el propósito del estudio geofísico fue determinar las variaciones de velocidad de ondas p en el interior del macizo rocoso a fin de inferir grado de fracturación y rigidez del mismo, donde a través del cual se proyecta construir un túnel vial de 290 m de longitud, que atravesará la montaña de forma casi perpendicular al eje de la misma, a 100 m de profundidad desde la cumbre. varias dificultades se presentaron desde el punto de vista operativo, que tornaron altamente compleja la realización del trabajo de campo en circunstancias normales. en primer lugar las condiciones topográficas, con pendientes de 45º y profundidades de investigación de 100 m, que implicaba tener que utilizar explosivos para generar ondas sísmicas que alcanzaran los sensores con suficiente relación señal-ruido para distinguirlas. restricciones legales respecto al uso de explosivos, por una parte, y la potencia insuficiente en caso de uso de golpe de maza por otra, hicieron necesario el diseño y construcción de un cañón impulsado por gas a fin de generar la energía mínima (2 kj) necesaria para la detección de las señales sísmicas. en segundo lugar, no fue posible la utilización de los métodos convencionales de interpretación que emplean modelos de capas, inoperable en este tipo de estructuras geológicas; por ello resultó imprescinpalabras claves: tomografía sísmica – sísmica refractiva – ondas de compresión – procesamiento de señales. keywords: seismic tomography, p wave, seismic refraction, compressional waves, signal processing. record manuscript received: 15/12/2010 accepted for publications: 30/11/2011 armando luis imhof, manuel sánchez, carlos calvo and adriana martín82 dible utilizar el método de tomografía sísmica que hace uso del concepto de gradientes de velocidades, tanto laterales como en profundidad. ello posibilitó mapear las variaciones de velocidad en subsuelo en forma de curvas de nivel. la metodología empleada con el nuevo sistema de generación de ondas sísmicas, así como la aplicación en este tipo de estructuras geológicas de la srt, demostraron la posibilidad de obtener resultados satisfactorios para este tipo de estudios geofísicos con fines geotécnicos. introduction conventional refraction inversion methods use a layered model approach. the subsurface is divided into a number of continuous constant velocity layers having velocities and thicknesses which are modified through interactive forward modeling in an effort to match traveltimes determined from the field data. several interpretation methods have been based on this premise (see among others jakosky, 1940; heiland, 1940; palmer, 1986; dobrin, 1975). these conventional methods require sections of traveltime curves to be mapped to refractors, a task that can be difficult in several geological situations. for example, the presence of extremely hard topographic conditions in mountains surely implies that there can be large and sudden changes in the shape of the bedrock. there may also be localised features such as voids that contradict the assumption of continuous constant velocity layers. the progress of computer technology has simplified calculus procedures, thereby leading to a great development in geophysical data processing methodologies, particularly for application to soil studies regarding civil engineering infrastructure. the amazing increase in microcomputer processing velocity during the last few years, added to their huge storage capability, has enabled the development of algorithms from different theoretical backgrounds, giving birth to refraction tomography (schuster et al, 1993; sethian, 2007; zhang et al., 1998). refraction tomography unlike conventional refraction methods, seismic refraction tomography (srt) does not require that the model be broken into continuous layers having constant velocity. instead, the model is made up of a large number of small constant velocity grid cells or nodes. inversion is performed by an automated procedure involving raytracing through an initial model and comparing the modelled traveltimes to the field data, and adjusting the model grid-by grid to match the modeled traveltimes to these. this process is iteratively repeated until a preset number of iterations has been reached. because there is no assumption of continuous constant velocity layers, srt can model localised velocity anomalies. figure 1. study zone. application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 83 another difficulty related to traditional seismic refraction methods for data interpretation is that they are only applicable to relatively gentle topography, as mentioned in previous paragraphs. several difficulties appear regarding geophysical studies in mountain areas, namely: • the terrain is extremely rugged; and • such material has great lateral variation, making layered model theory inapplicable. survey area location and general geology the study area was located in calingasta, sarmiento, in the southern sector of the province of san juan, argentina, and the district of las heras in the north of the province of mendoza and about 150 km southwards of the capital city of san juan, on the eastern slope of the front range and western edge of the pre-cordillera at an average height of 2,500 m above sea level (masl) (figure 1). the region’s climate can be described as semi-arid to arid, being of the mesothermal type. the present study was conducted in a mountainous area having an ancient marine environment, with the presence of high-angle folding sedimentary rocks, general lithology consisting of sandstones interbedded with siltstones. the area is characterised by the presence of two major mountain bodies, the west one belonging to the front range and the eastern one belonging to the pre-cordillera with a central mountain body, corresponding to the central and western pre-cordillera subprovinces (figure 2) the pedernal-los pozos hills form an outstanding feature of the eastern mountain range, extending 25 km nne-ssw and 8 km w-e. they are mainly made up of paleozoic age calcareous rocks, being more than 2,000 masl high. figure 2 shows the prevalent geological formations in the study area. the main formations are as follows: • the marquesado group (bordonaro, 1980), which includes la laja, zonda and la flecha formations and is composed of limestone, dolomitic limestone, marl, shale and chert; • the mariño formation, made up of sandstones and tuffs presenting strongly structured (high tilt) intercalated materials; and • the uspallata triassic formation, which is composed of medium to coarse-grained olive green metasandstone alternating with olive green shale (cortés et al., 1997). there is abundant folding and fracturing (jointing). the formations dip at a high angle towards the nw, where there are greater shale levels. basic theory refraction interpretation methods leads to seismic velocity determination (typically calculated from the first break pick of the p-wave) as part of the interpretation results. travel time inversion is nonlinear because a perturbation in the velocity field results in changes in the ray paths through it. linearised inversions involve local linearisation of such nonlinear problem. this makes them initial-model dependent. a poor choice of starting model could cause an inversion to converge to an incorrect solution at a local minimum, due to most algorithms looking for a solution close to the starting model. obviously, this hampers interpretation. global optimisation several investigators have developed algorithms that search for global minimum, instead of local ones. the algorithm used in this work for seismic tomography data interpretation used this type of scheme; it is known figure 2. geologic map. sector of exploration (from gonzález díaz y fauqué, 1993). armando luis imhof, manuel sánchez, carlos calvo and adriana martín84 as simulated annealing optimisation and it was developed by kirkpatrick et al., (1983) and bohachevsky et al., (1986). although the original theory was developed for statistical mechanics related to temperature, it soon became adapted to geophysical problems (e.g. chunduru et al., 1996). it follows the basic procedure given below: 1. travel times were computed through an initial model. there are several ways of doing this, basically grouped into two methods: straight or curved ray theory (cerveny, 2001) or numerically resolving the wave equation with some constraints, such us the eikonal equation (vidale, 1988). the latter option has two main advantages: it accounts for curved rays from all types of primary arrivals (i.e. direct ones, refractions or diffractions) and a ray tracing algorithm does not need to be used. there are several schemes for the numerical solution of the wave equation, most being grouped into two methods: finite differences and finite elements methods. here, a fast finite difference algorithm was used. the domain was divided into rectangular blocks or cells having a lateral extension generally half the distance between geophones and depth was defined by the interpreter. 2. the least-square error, e0, was calculated among the measured times (tmeas) and the first theoretical predicted ones (tpred). the leastsquare error was defined for any iteration it, which is named the objective function, as: where n is the number of observations and j denotes each observation. 2. the velocity model was then altered by repeating step 1 and the new least-squares error e1 was computed. 3. the new model was unconditionally accepted if its error was small (e1 <= e0 ), and conditionally accepted with probability: when e1>e0 . in the above equation, δe = e0 e1, t was temperature, q was an empirical parameter and emin global minimum objective function value (ideally, equal to zero). conditional acceptance of models having larger least square errors enabled the algorithm to escape from local minima in its search for the global minimum. (emin-e1)q made the probability of accepting a detrimental step towards local minima tend to zero as the inversion approaches the global minimum. 4. the procedure was repeated until the optimisation converged. the software performed thousands of iterations to obtain the best matches between: i) actual arrival times and distances between sources and receivers; and ii) modelled, calculated times based on ray paths through the finitedifference mesh. each finite-difference array element forming part of ray paths was characterised with a velocity value throughout the optimised interpretation. the extension of the area being researched was the part of the finite-difference mesh having optimised velocities. the interpretation results were typically presented in colour bar form, different colours representing different velocities. interpretation results could also be converted into spreadsheet files for further analysis. methodology figure 3 a-c shows a top view and cross-section, as well as an overview of the study area where source points t1 to t3 were located at the coordinates shown in table 1. figure 3. a) aerial view (schematic) of study zone with indication of seismic profiles, shotpoints (ti) and projected tunnel axis location. (b) lateral view (slice) showing geophone and shots’ spread. (c) photograph of the same sector. (a) (b) (c) (1) (2) application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 85 figure 3-b shows that the trace of the projected tunnel is about 100 m deep from the apex of the mountain, which made logistical tasks become very difficult. a 24-digital-channel seismic device was configured from 12 physical-channel recording equipment having a three pointed shot: a central one (t2) and two at both ends of the profile (t1 and t3). figure 3.b represents the seismic survey location along the hillside. due to the seismograph having 12 channels, shooting had to be repeated at each emission point (in addition to particular shotpoint’s corresponding stacks), moving the line of geophones each time to cover all 24 channels. the total layout length was 280 m with 5 m to 15 m spacing between stations (figure 4a). figure 4b shows the distribution of in-depth information content (coverage) regarding the rays with this three-shot device according to the topography in question. source wave system a new system for generating body waves (p and s), called tamper (imhof, 2009), was designed and implemented with the aim of generating waves at the corresponding shooting points (t1-t3, figure 3 a-b and 4). normally, a 5 kg to 10 kg hammer is used as p wave source, making it possible to generate power up to 0.2 kj (fernandez, 2000). this power scarcely allows reaching 20 m research depths, which was insufficient for this study. another option was to use explosives, which was then discarded as the work also required s wave generation for other kinds of measurements (not described here, see imhof, op.cit.). ordinary seismic wave generation systems use various energy sources, such as elastomeric bands (damper), gunpowder or compressed air buffalo-gun type (pullan et al., 1987), to name but a few. the excellent articles written by miller et al., (1986) and herbst et al., (1998) have compared several of them (see also figure 5). these systems are not very portable, and basically they only allow wave trains to be generated (since they emit vertically). the tamper works like a cannon loaded with nitrogen gas at high pressure (150 bars) as a source of energy that fires a 3 kg bullet at high velocity. the release of energy is variable and depends on the bullet’s trajectory length and gas volume in the chamber, being able to reach up to 20 kj. it can be placed horizontally to generate sh waves, as well. figure 5.b shows an image of this device. shotpoint geographical coordinates gauss-kruger latitude longitude x y t1 s32º08’07,1” 69º00’19,3’’ 6445378,60 2499494,14 t2 s32º08’05,0’’ 69º00’23,1’’ 6445443,28 2499394,53 t3 s32º08’03,0’’ 69º00’26,6’’ 6445504,88 2499302,79 figure 5. amount of energy necessary related to depth of investigation for seismic sources (from herbst et al, 1998). (a) table 1. seismic refraction shotpoint coordinates. (b) figure 4. (a) 24 channels refraction seismic survey array and distances among stations. (b) ray coverage for refraction tomography. armando luis imhof, manuel sánchez, carlos calvo and adriana martín86 figure 6. tamper. vertical-horizontal seismic wave generation system. vertical implementation. figure 7. (a) seismic record ensambled (from t1). (b) time-dist representation (t1 to t3). 2.5 kj were enough for this work, which allowed reaching a 100 m depth research (herbst et al., 1998) (figure 5.a) with a seismic device like that shown in figure 6. results time-distance graphs (dromochrones) six 500 msec. recording time records were obtained for p-waves (see figure 7.a; a typical record for t1). contiguous records for the same shot were assembled in pairs to build one of the 24 channels which allowed observing consistency among the first arrivals, thus facilitating choosing them. the tasks of extracting information from records and transferring it to the time distance graphs were very complicated due to the poor quality of the seismic signals, resulting from prevailing weather conditions (wind) in the study area combined with the terrain’s extreme unevenness which hampered geophone deployment. pre-processing records in seg-1 format were transformed into ascii with seg1to-ascii software prepared by the working group; each signal was then processed separately with bandpass filters. the whole record was reconstructed to enable choosing them more accurately and achieve a more adjusted dromochrone. filtering algorithms (fourier transforms) were prepared in mathcad and matlab platforms. after pre-processing and choosing all records, data were represented in the form of time-distance graphs (dromochrones; figure 7.b). (a) (b) application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 87 input information required by the seismic modelling software consisted of the device’s geometry (figure 4.a) and the first arrivals (i.e. dromochrones). the grid size for finite difference modelling depended on the tomogram’s desired resolution; the highest was used here, so minimum grid size was considered, applying the expressions (pullammanappallil et al, 1994): hx = 0.375 . receiver spacing hz = 0.1875 . receiver spacing figure 8 shows contour lines that represented boundaries between compressional wave velocity gradients. discussion the area being investigated consists of coarse-grained sandstones intermixed with dark shale (the latter most noticeable on the nw side), having bz = 65º nw and az = 100º. the rock is extremely fractured (altered). regarding shale, it should be noted that it has a wide range of seismic wave velocity variation, according to the degree of fracturing. for example, velocity can reach up to 1,500 m/s when weathered, increasing significantly up to 5,000 m/s if not having undergone external alteration. the seismically surveyed area could be divided into two sections: the first one from t1 position up to about 120 m nw where the se mountain side is made up of coarse-grained sandstones which, although somewhat fractured, are nevertheless less altered from around 15 m deep downwards. the situation changed towards the nw side where the presence of altered black shale caused lower vp values. nevertheless, at projected tunnel depths, velocities and hence material strength tended to increase (about 1,600 m/s in the extreme nw; figure 8, under t3). the interblended shales were not very thick. conclusions geophysical refraction tomography was an important tool for evaluating rock rigidity, adding extra information regarding lithological characteristics. p velocity variation was determined at depths greater than 100 m in the survey area. this is the first time it has been done in argentina, bearing the depth of investigation plus hard topographical conditions and refraction tomography in mind. the tamper system would appear to be a valuable tool for countless applications in the future, since it allows p and s waves to be generated at greater depths than the traditional hammer (its maximum range not having been studied yet) without the risk of using explosives. moreover, its portability allows it to be carried into high mountain terrain, allowing the complete characterisation of rock masses, something which could not be performed by means of surface geophysical methods only, having to employ cross-hole studies which require expensive drilling and also provide data having much more bounded distances. acknowledgements we would like to thank juan marcet, director of “escuela de ingeniería de caminos de montaña” at the universidad nacional de san juan and his team for their tremendous cooperation in logistical tasks and positive attitude towards developing new geophysical methods, as well as their valuable contributions so that this daunting task could be carried out. figure 8. seismic refraction tomography profile. (3) armando luis imhof, manuel sánchez, carlos calvo and adriana martín88 we would also like to thank manuel sánchez phd, who was in charge of designing the tamper emission system, for his continued willingness to cooperate with the project. references bohachevsky, i. o.; johnson, m. e. and stein, m. l. (1986). generalized simulated annealing for function optimization, technometrics, 28, 209-217 bordonaro, o.l. (1980). el cámbrico en la quebrada de zonda, provincia de san juan, revista de la asociación geológica argentina. 35, 26-40 cerveny, v. (2001) seismic ray theory. cambridge university press, 700pp. chunduru, r. k.; sen m. k. and stoffa, p. l. (1996) 2-d resistivity inversion using spline parameterization and simulatd annealing. geophysics, 61, 151-161.  cortés, j; gonzález bonorino, g; kourkharsky, m; brodtkorb, a y pereyra, f. 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(1993). wavepath eikonal traveltime inversion: theory. geophysics, 58, 1314-1323. sethian, j. a. (2007). level set methods and fast marching methods: evolving interfaces in computational geometry, fluid mechanics, computer vision, and materials science. cambridge university press, 367 pp. isbn 0521645573 vidale, j. e. (1988). finite difference calculation of travel times, bulletin of the seismological society of america, 78, 2062-2076. zhang, j. & toksoz, m.n. (1998). nonlinear refraction traveltime tomography, geophysics, 63, no.5, 1726-1737. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 61 65 using conodont elements to distinguish permian-triassic boundary disconformity near haftad gholleh, central iran mostafa yousefirad1, somayeh ghanbari2 and mahnaz parvanehnejad shirazi2 1department of geology, faculty of earth sciences, payam-e-noor university, iran 2department of geology, faculty of earth sciences, payame noor university, shiraz, iran email: mostafa.yousefirad@yahoo.com mobile: 00989125700730 paleontology the present study focuses on the stratigraphy of the permian-triassic boundary in the haftad gholleh area in eastern arak and north of mahallat,located in central iran. this boundary of erosional unconformity or disconformity and as in other iranian sequences related to the permian period, contains dolomitic limestone and shale and sandstone disconformably located below the triassic sequence. a detailed measured stratigraphic section has provided conodonts from the permian–triassic boundary (ptb) sequences in the area being studied. three conodont areas have been recognied which place the ptb in this section by precise biostratigraphy. one of the most upper permian sequences belongs to the guadalupian period. the triassic sequence consists of vermiculate limestone layers with coloured shale inter beds belonging to the early triassic period; unconformity at the permian-triassic boundary therefore represents a hiatus of about 10 million years. el presente estudio se enfoca en la estatigrafía del límite pérmico-triásico en el área de haftad gholleh, al este de arak y al norte de mahallat, centro de irán. este límite de discordancia erosiva, al igual que en otras secuencias iraníes relacionadas al período pérmico, contiene calizas dolomías, esquistos y areniscas discordantes localizadas debajo de la secuencia triásica. en una medida estratigráfica detallada del área de estudio se encontraron conodontas del límite pérmico-triásico (ptb, por sus siglas en inglés). se reconocieron tres áreas de conodontas, lo que ubica esta sección del ptb como bioestatigráfica precisa. una de las secuencias superiores del pérmico pertenece al período guadalupiense. la secuencia triásica consiste en calizas con capas vermiculitas coloreadas de esquisto que pertenecen al período triásico temprano. la discordancia del límite pérmico triásico, entonces, representa un lapso cercano a los 10 millones de años. key words: conodont, haftad gholleh, permian-triassic, stratigraphy. palabras clave: conodont, haftad gholleh, permiantriassic, stratigraphy. record manuscript received: 08/10/2013 accepted for publication: 02/04/2013 abstract resumen introduction the end of the palaeozoic period and the beginning of the mesozoic saw the greatest of all extinctions in the phanerozoic causing mass mortality in the marine environment (up to 96% of species, 83% of genera and 55% of families) (sepkoski, 1990; raup, 1991) and a nearly equally large loss of life on land (erwin, 1993). important p/t boundary sections are located in iran (nw, ne and central iran, alborz region, belonging to the iranian plate, and the zagros fold belt, belonging to the arabian plate), oman, southern china, japan, pakistan, afghanistan, turkey, greenland, austria, italy, south africa and antarctica. the permian sea’s last regression period occurred at the end of this system, which is why the upper boundary of the permian strata in iran is commonly discontinuous but has a type of parallel unconformity (disconformity). nevertheless, evidence of continuous sedimentation from the permian to triassic been reported in many regions of iran (julfa, kandovan, amol, shahreza and abadeh). the recommended, but somewhat controversial, chronostratigraphical permian scale is summarised here, according to jin et al., (1997) and wardlaw (1999), but correlated with leven’s alternative classification (leven, 1992; kotlyar and pronina, 1995). the names midian (eventually abadehian) for capitanian, dzhulfian for wuchiapingian and dorashamian for changhsingian have been preferred concerning iran’s stratigraphy (fig. 1). due a lack of macroscopic and microscopic fossils, especially foraminifera, some uncertainties in conodonts’ biostratigraphy have not been studied and clarified. this research thus attempts to study conodonts, the figure 1. permian-triassic boundary in different areas of iran (aghanabati, 2004) mostafa yousefirad, somayeh ghanbari and mahnaz parvanehnejad shirazi disconformity interval and the permian-triassic boundary’s paleoecology in the area being studied. 2. geological setting the haftad gholleh area is a small part of central iran, located between volcanic belts at urmia kerman sanandaj sirjan. the soltanieh dolomites, zagun, lalun, mila and permian sequence deposits in the region are known as palaeozoic facies. soltanieh grey dolomite is overlain by a green to red shale formation containing little sand and mica, becoming converted locally into fine-grained sandstone. shale thickness varies from 100 to 200 metres (zagun formation). overlying the shale are about 50 metres of dense, hard lightcoloured quartzite sandstone which has been assigned to the lalun formation. this formation is overlain by light to dark dolomite containing coral, sand and crinoids, having less than 100 m thickness.the dolomite gradually changes from bottom to top from light grey to light green limestone, bearing trilobite fragments; such dolomite and limestone are assigned to the mila formation. this different kind of facies (permian) is located directly on older formations. no ordovician to carboniferous sequences thus occurred in the area being studied. permian deposits are mainly dolomite. in some locations, the base consists of lalun sandstone. the base of the permian deposits in some parts is calcareous shale associated with red sandstone and conglomerate. permian lithology mainly consists of thick bedded dolomite (light grey) (fig. 2). lithostratigraphy the upper permian sedimentary sequence in the area being studied included limestone continuing with dolomite; limestone has been assigned to the jamal formation. all deposits were isoclinal, indicating marine transgression in the studied area (permian). one sequence, having several meters of shale and sandstone, lacking fossils, had such adistinctive colour that it could be applied as a key bed. there were several metres of thin to medium-bedded claraia vermicular limestone inter-bedded with coloured shale above the key bed; this belonged to the early triassic, based on a study of conodont fragments. it has thus been concluded that early triassic deposits in the study area consisted of two parts: clastic (lower part) and carbonate (upper part), generally being equivalent to the shotori formation. the carbonate part included dolomite inter-bedded with coloured shale. there were few fossils in the sequence. based on the stratigraphic situation, lithological similarities and lithocorrelation, it has been assigned to the anisian age shotori formation (figs. 3 and 4). biostratigraphy apart from the studied section’s base part (related to the permian rock sequence) having few benthic foraminifera, other layers in the studied rock sequence were very poor in terms of fossil content (foraminifera). biological biozoning studies were thus based on conodont elements in the studied section’s complete rock sequence. elements in the studied section were biozoned according to standard kozur biozonation (1977). conodont elements known from permian time would be as follows: hindeodus minutes, streptognathodus barskovi, sweetognathus whitei, neostreptognathodus pequopensis, hindeodus excavates and merrillina divergens. conodont elements known to be related to the triassic age would be as follows: prioniodella prioniodellides, hindeodella multihamata, hindeodella nevadensis, lonchodina nevadensis, hindeodella triassica, ligonodina triassica, diplodella lautissima, pachycladina oblique, hadrodontina anceps, figure 2. the studied area’s geological setting ellisonia triassica, hibardella subsymmertica, parachirognathus geiseri, prioniodella decrescens, enantiognathus bitortus, chirognathus sp., xaniognathus sp., and anchiognathudos sp. (fig. 5). pachycladina oblique (staesch), posterior view, x90, early triassic (early triassic) 1 sweetognathus whitei (rhodes), lateral view, x80, middle permian (guadalupian) 2 hindeodella nevadensis (muller), lateral view, x80, early triassic (early triassic) 3 hindeodus minutes (ellison), lateral view, x80, upper permian 4 hindeodella multihamata (huckriede), lateral view, x95, middle triassic (anisian) 5 chirognathus sp., lateral view, x80, early triassic (scythian) 6 neostreptognathodus pequopensis (behnken), outer view, x80, upper permian 7 ligonodina triassica (muller), lateral view, x80, early triassic (early triassic) 8 enantiognathus bitortus (bender), outer view, x120, early triassic (early triassic) 9 10 hindeodella triassica (muller), lateral view, x80, middle permian lower triassic 11 hindeodus excavatus (behnken), lateral view, x150, middle permian (guadalupian) 12 streptognathus barskovi (kozur), lateral view, x80, middle-upper permian 13 xaniognathus sp., lateral view, x80, lower triassic 14 prioniodella prioniodellides (tatge), lateral view, x120, middle triassic (anisian) 15 lonchodina nevadensis (muller), lateral view, x110, early triassic (early triassic) 16 merrillina divergens (bender & stopple), lateral view, x80, (guadalupian) 17 parachirognathus geiseri (clarck), lateral view, x95, (early triassic anisian) 18 ellisonia triassica (muller), lateral view, x80, early triassic (early triassic). 19 following precise paleontological studies, including conodont recognition and taxonomy regarding samples from the studied section, each fossil’s biozone was identified on the table showing the extent of fossils, based on their presence in each rock sample. two assemblage zones and one acrozone for permian deposits and two assemblage zones for triassic deposits were identified based on the emergence, decline and expanding stratigraphic accumulation index for conodonts having known elements. 4.1. neostreptognathodus pequopensis hindeodus minutes assemblage zone the start of this base’s biozone was consistent with the occurrence of neostreptognathodus pequopensis conodont genus and species and its end with the occurrence of merrillina divergens conodont genus and species. conodont elements contained in this biozone were assigned to the following: hindeodus minutes, hideodus excavatus, sweetognathus whitei, hindeodella triassic and neostreptognathodus pequopensis.figure 3. studied section near haftad gholleh using conodont elements to distinguish permian triassic boundary disconformity near haftad gholleh, central iran figure 4. stratigraphic column of the section being studied near haftad gholleh biozone age was artin-skian, based on the standard kozur biozoning (1977), proposed as being equal to the neostreptognathodus pequopensis biozone. 4.2. merrillina divergens acrozone the base of this biozone started with merrillina divergens and terminated with m. divergens. conodont elements contained in this biozone were identified as follows: merrillina divergens, hindeodella triassic, hideodus excavates. this biozone’s age, according to conodonts, was guadalupian. according to standard kozur biozonation (1977) this zone was the biozone equivalent of merrillina divergens. 4.3. lonchodina nevadensis ellisonia triassica assemblage zone the base of this biozone index was the presence of lonchodina nevadensis and ellisonia triassica and the top of the biozone consisted of pachycladina oblique and hadrontina anceps. conodonts from elements contained in the biozone were assigned to the following: prioniodella prioniodellides, hindeodella multihamat, hindeodella nevadensis, lonchodina nevadensis, hindeodella triassica, ligonodina triassica, diplodella lautissima, pachycladina oblique, hadrodontina anceps, ellisonia triassica, hibardella subsymmertica, parachirognathus geiseri, prioniodella decrescens, enantiognathus bitortus, chirognathus sp. and xaniognathus sp. according to conodonts, this biozone’s age was scythian. based on kozur’s standard biozones (1997), this biozone was the equivalent of the isarcicella isarcica biozone. 4.4. hindeodella multihamata prioniodella prioniodellides assemblage zone this biozone included conodonts such as diplodella lautissima, prioniodella decrescens, hindeodella multihamata, prioniodella prioniodellides, hindeodella triassica and xaniognathus sp. the identified conodonts indicated that it was anisian (fig. 5). conclusions the permian-triassic boundary located near haftad gholleh is an erosional unconformity or disconformity, as in other parts of iran, where figure 5. conodont elements in the studied area mostafa yousefirad, somayeh ghanbari and mahnaz parvanehnejad shirazi figure 6. distribution and biozonation of conodonts from the permian-triassic boundary of the area being studied permian sequences contain dolomitic limestone, shale and sandstone situated below the triassic sequence at a disconformity. based on emergence, decline and expansion of index conodonts, one assemblage zone and one acrozone (permian) and two assemblage zones (triassic) were recognised as follows: neostreptognathodus pequopensis-hindeodus minutes assemblage zone (artinskian); merrillina divergens acrozone (guadalupian); lonchodina nevadensis and ellisonia triassica assemblage zone (early triassic); and hindeodella multihamata, prioniodella prioniodellides assemblage zone) anisian). the age of the uppermost permian sequence thus belonged to the guadalupian era. the triassic sequence consisted of vermiculate limestone layers with coloured shale inter-beds that belonging to the scythian era. the conodont fauna indicated that the permian-triassic sequence from the studied area was deposited in a warm shallow sea having normal salinity. most conodonts in the studied area were euryhaline (i.e. able to tolerate a wide range of salinity). red clastic sediments at the base of the lower triassic indicated that an erosion period started after sedimentation in the midpermian and has been dominant on land in the region. disconformity thus occurred at the permian-triassic boundary in the studied area. acknowledgements this research was supported by arak payame noor university. i would like to thank dr hadi ghaffari and dr mohammad hashem rezai for support and dr babazadeh for reviewing the manuscript and providing pertinent comments. references aghanabati, s.a. (2004). geology of iran, geological survey of iran publisher, tehran, iran, 183 pp. erwin, d.h. (1993). the great paleozoic crisis: life and death in the permian. the perspectives in paleobiolology and earth history, columbia university press, new york, usa, 18 pp. jin, y. wardlaw, b.r., glenister, b.f. kotlyar, g.v. (1997). permian chronostratigraphic subdivisions, episodes, 20, no. 1, 10–15. kodyar, g.v. pronina, g.p. (1995). murgabian and midian stages of the tethyan realm, permophiles, 27, no. 2, 23-26. kozur, h. (1977). beitrage zur stratigraphie des perm. teil i: probleme der abgrenzung und gliederung des perms, freiberger forschungsh, 319, no. 6, 79–121. leven, e.ya. (1992). problems of tethyan permian stratigraphy. international geology review, 34, no. 10, 976-985. raup, d.m. (1991). a kill curve for phanerozoic marine species, paleobiology, 17, no. 1, 37–48. sepkoski, j.jr. (1990). the taxonomic structure of periodic extinction, in: global catastrophes in earth history, sharptonv.l., ward p.d., (eds.), geological society of america special paper, 247, 33–44. using conodont elements to distinguish permian triassic boundary disconformity near haftad gholleh, central iran gecttsic a colombiana pp. 9-16 diciembre de 2002 bogota, d.c. issn 0121 2974 amenaza sismica por metoda determinista en el triangulo de fallas valera-bocon6-0ca en el occidente de venezuela raymi a. castilla, frank a. audermard m. y jose antonio rodriguez departamento de ciencias de la tierra, funvisis, ei llanito, caracas, venezuela, rcastilla@funvisis org.ve resumen ei desarrollo de multiples proyectos de geologia de fallas activas en venezuela ha permitido que el pais cuente hoy dia con una base de datos completa y de gran cali dad, usada como complemento a la informacion instrumental en los calculos de amenaza sismica del pais con metodos probabilistas, en el presente trabajo se utiliza esta informacion geologica disponible para realizar calculos de amenaza con metodos deterministas para luego comparar los resultados y proponer la irnplantacion de metodologias mixtas que permitan aprovechar mejor los datos, tanto en la ventana instrumental como en la geologica. sobre la base de disponibilidad de informacion publicada y de complejidad en la geometria de las mayores estructuras, se ha escogido una zona del occidente del pais limitada por las fallas de valera, bocono y oca. se utilizan sismos maximos creibles de cada falla, distancias rninimas a la superficie de ruptura y las leyes de atenuacion necesarios para crear un marco multiescenario combinando todas las estructuras que puedan, potencial mente, generar amen aza. la zona de estudio, con un area aproximada de 45.000 km2 que incluye tres capitales de estado y una actividad industrial importante, es cubierta con una red espaciada cada 5 kilometres para el calculo de los val ores de aceleracion. los resultados se muestran en mapas con areas de isoaceleraciones donde se puede observar un control marcado de la geometria de las fallas activas sobre los val ores de aceleracion esperados, por encima de la sismicidad asociada a cada una de elias. a difercncia de los metodos de uso mas extendido, el calculo por metodos deterministas no permite la visualizacion de probabilidades de no excedencia, pero permite analizar escenarios de amenaza asociada a fallas que presenten poca sismicidad en la ventana instrumental. palabras clave: amenaza sismica, probabilista, determinista, valor b, bocono, valera, oca, venezuela abstract the development of several studies regarding active faults geology has allowed us to build a complete database wich is used together with the instrumental catalog, in the probabilistic seismic hazard assesment (psha). the geologic data is used here to develop a deterministic seismic hazard assesment (dsha) and then the results are compared with former psha results to propose hybrid approaches that lead us to make good use of all the available information. maximum credible earthquakes and minimum actual distances to the rupture surfaces are used with the attenuation relationships to create a scenario describing the hazard in terms of the maximum acceleration. these strong ground motion values are the result of the maximum earthquake that could be triggered by the faults in the area. the whole zone, including three main cities and important economic activities, is covered in this study by a grid with points every 5 km. the. results are shown in acceleration maps, where the maximum values follow the active faults trends. when the return period of the maximum credible earthquake is included in the analysis this correlation changes and a more likely scenario is created. the pros and cons of both approaches were analized in terms of the available data for venezuela. this has shown that more efforts must be done in the treatment and processing of the data and in the design and application of better methodologies as well. key words: seismic hazard, probabilistic, deterministic, b-value, bdcdno, valera, oca, venezuela metodologia se ha realizado un analisis del catalogo sismico nacional en terminos del valor b y de la relaci6n gutenberg-richter para evaluar las limitaciones que pudieran estar asociadas a la utilizaci6n de dicha base de datos en los calculos de amenaza sismica que se realizan en el pais. dicho analisis se ha concentrado en las variaciones de estos geofislca cdlombiana, 6, diciembre de 2002 9 castilla et al. pararnetros a traves del tiempo y en la consideracion que dichas variaciones son el producto de cambios en la instrumentacion, y no de cambios naturales en el comportamiento de la sismicidad del pais. se realizaron tambien dos ensayos de calculo de amenaza sismica por metodos deterministas, los cuales han permitido, de manera similar al caso anterior, exponer las ventajas y desventajas, asi como las limitaciones que el metodo impone. para este fin se ha tornado la informacion de geologia de fallas activas hasta ahora generada en el pais, en terminos de sismos maximos creibles con sus respectivos periodos de retorno, la ubicacion de todas las fallas y su segmentacion (audemard et al., 2000). se utilize la metodologia clasica (figura i) para el enfoque determinista (krinitzsky, 199 ; reiter, 1990 y otros) con la diferencia que, en el presente ensayo, se toman en cuenta multiples puntos para generar una malla y mapas de isoaceleraciones. en ambos casos se ha escogido una reticula de 5 km de espaciamiento, y para todos los puntos se ha calculado la aceleracion que produce cada falla al desencadenar su sismo maximo probable. usando como parametro de distancia, la minima entre el punto y la falla, dichos val ores de aceleracion son comparados entre si para escoger el maximo y asignarlo al punto. ~nle3 si~ ~'1 paso i paso) • u , '0 y, ~ >0 " 0 y,'s ~ y = .~ ~ 0 yn::e r, paso 4 paso 3 figura 1. etapas basicas del anal isis de amenaza sismica por medio del enfoque probabilista. modificado de smit (2002). para considerar el periodo de retorno de cada uno de los sismos maximos probables, se multiplico la aceleracion por la frecuencia anual de dicho evento. de esta manera se obtiene un valor considerado adimensional, utilizado solo para escoger el valor de aceleracion maxima que muy probablemente afectaria a cada punta. 10 metodo probabilista y el catalogo sismico nacional en venezuela, la sismicidad puede ser considerada moderada cuando se ie compara dentro de un contexto mundial (romero et al., 1999). ei 72,4% del catalogo instrumental, desde 1910 hasta hoy, corresponde a sismos con magnitudes 3<mb<4.5 (figura 2). la ocurrencia de sismos fuertes (>6.0) en estos 92 afios, solo constituye un 0,46% del total del catalogo. no obstante, al amp liar la ventana a observaciones historicas (desde 1530 hasta hoy), se obtiene un registro de 16 sismos destructores con perdidas materiales y de vidas humanas. sismicidad venezuela (1910-2002) 1200~ 1000f-400-4 s 6 magnitudes figura 2. distribuci6nde sismicidad instrumental en venezuela, segun lamagnitud. la figura 3 muestra la distribucion de los epicentros en el catalogo de sismicidad, discriminados segun su magnitud y la profundidad hipocentral, respectivamente. en las mencionadas figuras se puede observar el marcado control que ejerce la presencia de las falias mas importantes en la concentracion de la mayoria de los epicentros para eventos registrados en el catalogo. ei catalogo instrumental puede ser dividido en tres periodos que reflejan, en diversos sentidos, las distintas etapas evolutivas de la red sismologica nacional. el primer periodo, comprendido entre 190 i y 1982, durante el cual son puestos en funcionamiento distintos sismografos para monitoreo. para el momenta de la creacion de funvisis, luego del sismo de caracas de 1967, existian 8 estaciones sisrnologicas con operadores en sitio, pertenecientes a distintos organismos (rendon, 1997). el segundo periodo comienza en 1981 cuando se inicia el proyecto resvac, que permitiria la incorporacion de las estaciones convencionales existentes a una red con 10 estaciones telemetricas de sensores de periodo corto distribuidas en la region central. ei arreglo resultante de este proyecto, junto con otras redes de caracter regeofislca colombiana, 6. oiciembre oe 2002 amenaza sismica por metodo determinista en el triangulo de fallas valera.bocono·oca en el occidente de venezuela a) b) figura 3. a) distribuci6n espacial del catalogo de sismicidad en venezuela con epicentros discriminados segun la magnitud. azul = mb<5.0; verde = 5<mb<6.9; rojo = mb>7. b) distribucion espacial del catalogo de sisrnicidad en venezuela con epicentros discriminados segun su profundidad. amarillo = somera; naranja = intermedia; negro = progional implantadas por otros organism os, ha permitido el procesamiento de un numero importante de eventos, 10 cual representa un mejoramiento sustancial en el catalogo sismico nacional (rend6n, op. cit.). ei comienzo del tercer periodo puede ubicarse en el ana 1999, cuando se pone en marcha el proyecto de modemizaci6n de la red sismo16gica naciona!. dicho proyecto, todavia en fase de ejecucion, contempla la instalaci6n de sensores de tres componentes del tipo banda ancha. todos estos cambios en la configuraci6n y densidad de la red nacional han tenido repercusiones importantes en el catalogo sismico, la figura 4 refleja dichos cambios enla frecuencia de registro por ano. cambios importantes se pueden observar despues de 1980, cuando la resvac entra en funcionamiento, y poco antes del afio 2000, cuando comienza la modemizaci6n de la red naciona!. sismicidad venezuela (1910 2002) 1200 ,--,----,--,-~,__,_--,--,----,_-,-__,__, 1000 800 200 1910 1920 1930 1940 1950 figura 4.registro de actividad sismica anual. geoflslca colombiana, 6, oiciembre oe 2002 otros picos importantes pueden ser observados en asociaci6n con eventos sismicos. se puede observar un aumento considerable de la frecuencia en el afio 1967 asociado al sismo de caracas y, en 1989, asociado a la tormenta sismica que tuvo lugar en falc6n oriental. los cambios antes descritos en la red sismol6gica nacional, al afectar la frecuencia y el nivel de detecci6n de los multiples eventos que suceden cada ano, produjeron tam bien cambios en la informaci6n que se extrae del catalogo, para calcular arnenazas. se puede observar, de distintas maneras, la influencia que ejercen estos cambios del catalogo disponible al realizar el procesamiento de los datos. la figura 5 muestra la relacionnes gutenberg-richter (b-lines) para venezuela en las etapas previa a la instalaci6n de resvac y durante el funcionamiento de la misma, respectivamente. 4 magnitud re laci6n de recurrencia 10' ,--~-~-----_---~-_-'~ logn-a,344-1,15m o 1983·1998 1910·1979 i ~ 10' z 0/) 0 oj 10' 10.2 10'" 0 logn-4,451.0.84h.1' figura 5. relaci6n gutenberg-richter para la primera y segunda etapa descritas anteriormente. 11 castilla et al. ambas relaciones se ajustan bien a los datos del catalogo entre las magnitudes 4 y 6. por encima de este intervalo, el ajuste es mas bien deficiente y las frecuencias son subestimadas. adernas de este iimitado intervalo donde el ajuste puede considerarse confiable, las diferencias que presentan ambas relaciones entre si es otra caracteristica poco satisfactoria. por ejemplo, para un sismo de magnitud 7.5, cada una de las relaciones propone un periodo de retorno de 72 y 191 afios, respecti vamente. la figura 6 permite observar distintos valores b calculados para el catalogo de sismicidad del pais. cada valor es calculado para to do el periodo comprendido entre 1910 y el afio indicado en el eje de las abscisas. ei comportamiento de la curva permite observar, de manera similar a las anteriores, una diferenciacion entre los dos periodos de registro. en este ejemplo, el cambio en el comportamiento de la grafica se encuentra desplazado con respecto al afio de la incorporacion de la nueva red debido al caracter acumulativo de la curva, 10 que no permite el retlejo instantaneo de las variaciones. a partir del periodo 1980i985, la curva tiende a la convergencia alrededor del valor 0,9. aunque dicha convergencia parece indicar una estabilizacion del valor b, 10 que refleja el mejoramiento de la red sismologica nacional, se debe seguir prestando atencion a los primeros afios del catalogo, pues estes influyen de manera significativa en los estudios realizados tomando en cuenta que dicho catalogo es muy completo. cuando el termino b se calcula de modo similar para period os anuales (figura 7), se observa no solo una variacion importante en el valor del mismo en terminos absolutos, con una variacion de 1,4 unidades entre el minimo y maximo de la curva, sino tambien el reflejo de los cambios en la configuracion de la red como un cambio en la frecuencia de los picos en la curva. ei aumento en el valor b medio, luego de la instalacion de la red resvac (de 0,94 a 1,05), refleja el cambio de umbral de deteccion, 10 cual permite registrar en el catalogo mayor cantidad de sismos de menor magnitud. valor b cal cui ado para el catalogo desde 19i0 0.951------::------------------0.9 -c 0.85r--(f--+----------:r"-------:-------ci 'oil 0.8r----\--------:;;/"--------------> 0.75 r----\---rjf'''---------------0.7r----t--j------------------j 0.65r----~.-----------------j 0.6';";----;::-:-:-_--:--:-:-:----:-=-:-_=-:_-:-':--_---'-_----l_----' __ '-----.i 19551960196519701975198019851990199520002005 alios figura 6. variacion del valor b en period os cuya duracion se incrementa a razon de cuatro afios. 12 variaci6n anual del valor b 1970 1975 1980 1985 1990 1995 2000 afio figura 7. variacion del valor b calculado para periodos anuales. todo 10 antes descrito permite observar un hecho logico esperado: existe una correlacion directa entre los cam bios en la configuracion de la red nacional y los cambios en las caracteristicas del catalogo de sismicidad del pais. de esta manera, cualquier estudio que implique el procesamiento y uso del catalogo sismologico nacional (entre ell os el calculo de amenaza sismica), debe pasar necesariamente por una revision minuciosa del mismo, aplicando criterios logicos y coherentes que permitan obtener resultados cada vez mas apegados a la realidad sismica del pais. de igual manera, la metodologia de calculo de amenaza por metodos probabilistas, cuando es aplicada en su forma basica, presenta serias limitaciones que no deben ser menospreciadas. en krinitzsky (1993; 1995), entre otros, se han desarrollado extensas discusiones acerca de estas limitaciones. la relacion gutenberg-richter, en su concepcion original, describe de manera eficiente la sismicidad de un area extensa. cuando se realiza un analisis de amenaza, se supone que esta relacion regional describe de manera satisfactoria el comportamiento de fallas individuales. esta asuncion elimina la-nocion segun la cual, en un momenta dado, cada falla, 0 incluso cada segmento de una misma falla, puede encontrarse en momentos distintos dentro de su propio cicio sismico. la accion de uniformar los val ores b de toda una region impide la posibilidad de observar singularidades inherentes a cada falla, que en definitiva pudieran ser controles importantes para el comportamiento sismogenico de la misma. diversos ensayos han sido realizados en la busqueda de modelos y metodologias que reflejen de manera mas efectiva estas propiedades, entre los que podemos nombrar los realizados para venezuela por sobiesiak et al. (200 i) y grosser et al. (2001). geofislca colombiana, 6, diciembre de 2002 amenaza sismica por metodo determinista en el triangulo de fallas valera.bocon6·oca en el occidente de venezuela tabla i. tabla resumen con lascaracteristicas mas importantes de las 13fallas incluidas en ia zona de estudio. modificada deaudemard etal. (2000). nombre de la sentido de movimiento edad del ultimo tasa de movimiento sismo maximo periodo de estructura (mavor/menor) movimiento (mm/afio) creible retorno (afios) oca dextral <15 ka 045-20 74 4295±1020 ancon dextral <15 ka 16 74 1900 camare-paraiso dextral/lnversa probablemente <15 ka ?20 7 i 670 --dextralln ormal probablemente <15 ka ?30 67 390 socremo inversa/dextral <16ma <20 67 390 urumaco (oeste) dextral <16 ma 005 66 14000 urumaco (este) dextral <15 ka 005 64 1200015000 rio seco dextral <15 ka 035 70 1500 valera sinestral <15 ka 10 69 2000 bocono (anzoateguidextral <15 ka 5 7,5 400 barquisimeto) bocono (cabudaredextralllnversa <15 ka 1-3 7,5 700-2000 moron) flexura frontal reverse <1,6 ma 0,5 7,0 500-750 surandina la victoria-guacamaya dextral <1,6ma 0,6 7,0 2000 ei metoda determinista. ensayo en el occidente del pais ei triangulo de fallas de valera-bocono-oca, que incluye territorio de varios estados, (merida, zulia, trujillo, lara, falcon y portuguesa), presenta una complejidad estructural y cinematica que merece especial atencion. se encuentra limitada por tres fall as de movimiento transcurrente, de velocidades distintas, 10 que sugiere la existencia de complejidades cinematicas intemas y extemas. dichos iimites los constituyen las fallas de oca-ancon, bocono en sus segmentos de mucuchies-anzoategui, anzoategui-barquisimeto y cabudare-moron, y valera en sus dos segmentos (valera y rio momboy) (audemard et al., 2000). con el fin de lograr una mejor y mas completa cobertura de la mall a dentro de la zona del triangulo, el area de estudio se amplio, incluyendo un total de 13. fallas activas, potenciales generadoras de sismos en la region. la figura 8 muestra la distribucion de la sismicidad en la zona de estudio, evidenciando la ocurrencia de eventos dispersos, no necesariamente asociados de manera directa a las estructuras mayores. la relacion de recurrencia para la region presenta caracteristicas similares a la calculada para la completitud del catalogo, el intervalo con comportamiento lineal es corto y por encima de magnitud 6.0 el ajuste de los datos con la lineal es deficiente. se han generado mapas de amenaza sismica usando la informacion disponible de geologia de fallas activas. todos los datos usados, son el resultado de estudios de paleosismologia, y en general de neotectonica en el area escogida. datos como el sismo maximo creible, su periodo de retorno y la geometria detallada de las trazas de geoflslca colombiana, 6, oiciembre de 2002 cada una de las fallas de la region fueron usados para calcular,junto con leyes de atenuacion, las aceleraciones maximas en roca esperadas en cada punto. claramente se puede observar en los mapas, un control importante que ejerce la geometria de las estructuras consideradas en las tendencias de las zonas con mayores aceleraciones. en ei caso de la figura 9 se tomaron en cuenta solo las magnitudes de los sismos maximos creibles asignados a cada segmento de falla (audemard, . 2000). la aceleracion asignada a cada punto es la maxima posible, producida por un sismo de magnitud maxima creible para un segmento especifico de falla. de esta manera, tal valor solo depende de la magnitud y la distancia minima entre el punto y la falla. el valor maximo de aceleracion en este mapa alcanza los 0,46g, mientras que el minimo es de o,06g. de esta manera es facil entender que los valores maximos de aceleracion se encuen!ren alrededor de las fallas de oca, ancon y bocono asociados a magnitudes 7.4 y 7.5 respectivamente. mientras que los val ores minimos son asignados a los puntos mas distantes de las estructuras activas (ej: centro del triangulo), la figura 9 representa entonces, segun los datos geologicos, el peor escenario posible con las caracteristicas que presentan las falias de la zona. cada valor asignado a cada punta es equivalente a la maxima aceleracion esperada en roca para ese sitio, segun la ley de atenuacion usada. con el fin de plantear un escenario en algo mas real se incorporo al calculo el periodo de retorno de cada sismo maximo creible, tambien calculado a partir de datos geologicos. 13 castilla et al, • ••• •. "', \, $ ,••..... .. \•• ••• •.... .... '. rclacion de recurrencia zona de estudio 10' ,------.--~-~--~-~--~--,.__, 10' log n: 4.226 ·o.976m o 10' 0 0 0 0 0 a z ~o -' 10° 10' 10' 10' 10" 0~-~--=-2----:-3----":4----:5'---------:6:-------=----.j magnitud bl figura 8, caracteristicas de la sismicidad en la zona del triangulo, a)ubicaci6n de epicentros reportados en el catalogo. b) relaci6n de recurrencia correspondiente, la figura 10 muestra el resultado de la incorporacion de este dato adicional. en este caso la aceleracion es calculada del mismo modo que en el anterior, 10 que varia es el criterio de escogencia del valor. una vez calculada las distintas aceleraciones que para un punto pudieran producir los distintos sisrnos, se multiplican por la frecuencia anual del sismo correspondiente. se obtiene asi un valor adimensional cuyo maximo es usado para escoger la aceleracion a asignar. de esta rnanera, se obtiene un mapa que permite observar dentro de la zona, las aceleraciones que de manera mas probable lion acslsraclonss • 0,4 • 0,3o 0,2 • 0,15 • 0,05 figura 9. mapa de aceleraciones calculado con los sismos maxirnos creibles para cada falla. 14 pueden afectar a un punto dado dentro de un escenario de sismos maximos creibles. los cambios mas importantes se observan en el area alrededor de la falla de valera, la victoria y bocono en su segmento cabudare-moron con periodos de retorno de 2000 afios para las dos primeras y 1000 afios para la ultima. mientras que las zonas con menos variaciones corresponden a las mas cercanas a fallas con periodos de retorno menores como bocono en su segmento anzoategui-barquisimeto (400 afios) y la flexura frontal surandina (625 aceleraciones • 0,4 • 0,3o 0,2 • 0,15 • 0,05 figura 10. mapa de aceleraciones cal cui ado con los sismos rnaximos creibles y el pedodo de retorno para cada falla. geoflslca colombiana, 6, oiciembre oe 2002 amenaza sismica por metodo determinista en el triangulo de fallas valera.bocono·oca en el occldente de venezuela afios), de igual modo fallas cap aces de producir grandes sismos, que se encuentren lejos de otras estructuras importantes mantienen su influencia en sus zonas aledafias, del mismo modo que para el enfoque probabilista, los rnetodos deterministas presentan multiples limitaciones para el calculo de amenaza. en este tipo de anal isis, la calidad de la informacion e interpretacion geologica surgen como limitantes importantes, de hecho, en un mismo mapa pudieran coexistir datos de distinta calidad y procedencia, resultando asi en valores menos confiables. la dificultad de contar con sismos de magnitud media, pues su presencia en el registro geologico es usualmente enmascarada por sismos mayores, constituye un hecho que limita esta metodologia al uso de sismos maxim os, resultando en valores de movimiento del terreno altos. dichos valores altos de movimiento del terreno, pueden representar una sobreestimacion de la amenaza para estructuras de corta vida util 0 de poca importancia en el manejo del riesgo. debido a esto, los mapas aqui generados son de utilidad cuando se calcula la amenaza para obras de ingenieria de gran importancia en don de se deben considerar sismos con largos periodos de retorno. la estrecha asociacion de los val ores de movimiento del terreno con el tamafio del sismo y la fall a que los produjo, permite acotar dichos valores en un nivel superior. es decir, mientras el enfoque probabilista permite realizar mapas incrementando las magnitudes tanto como el analista 10 desee, la metodologia aqui usada impone un "techo" mediante la magnitud maxima que es capaz de-producir cada accidente tectonico, ei problema de la perdida de nocion temporal dentro del cicio sismico puede ser subsanado cuando se posee un estimado de la fecha del ultimo sismo equivalente al maximo creible, de 10 contrario esta limitante sigue estando presente en esta metodologia tal y como 10 esta en el enfoque probabilista. una ventaja importante que presenta esta metodologia es la posibilidad de asociar cad a valor de aceleracion con los parametres fuente y del sismo que 10produce. esto permitiria en un paso subsiguiente conocer en mas detalle el contenido de frecuencia responsable de la excitacion del terreno. conclusiones los ensayos con enfoques deterministas aqui realizados han demostrado su utilidad para amp liar la vision de la amenaza y acotar los valores segun las limitaciones naturales que imponen las mismas estructuras genreadoras de sismos. ei uso de esta metodoiogia en regiones con bases de datos geologicas completas y de buena calidad, en el cmapo de las fallas activas, permite generar resultados que son de gran unlidad como complemento en enfoques integrales. no obstante, el igual que la metodologia que ooohace uso de las probabilidades, los enfoques detenninistas han mostrado diversas desventajas. en el ensayo realizado se comprobo que debido al caracter netamente geologico de los datos utilizados, la disponibilidad geofislca colombiana, 6, oiciembre de 2002 y calidad de los mismos puede cambiar de una falla a otra. la limitacion que 10 anterior impone radica en que para la realizacion del anal isis es necesario contar con la uniformidad de todos los parametros en todas las fallas. es decir, todas y cad a una de las estructuras consideradas como potenciales generadoras de sismos, deben contar con estudios de geologia de fallas activas que hayan determinado sismos maxirnos creibles, periodos de retorno y edad del ultimo sismo, todos con niveles de confiabilidad similar. a medida que mas y mejores datos se hacen disponibles, este tipo de mapas pueden ser mejorados de manera significativa, incorporando criterios adicionales como el estado de esfuerzos alrededor de las fall as, que permitan plantear escenarios mas reales, usando igualmente datos con origenes distintos a los estudios geologicos. ei analisis del catalogo sismico nacional en terminos del valor b y de la relacion gutenberg-richter ha permitido la identificacion de diversas variaciones en el mismo, asociadas a los distintos cambios realizados en la instrumentacion de la red nacional y no a un cambio real de la sismicidad del pais. si dichas variaciones son trasladadas a los calculos de amenaza sismica sin un exam en critico de sus efectos, los resultados obtenidos pudieran representar s610 un reflejo muy aproximado de la realidad. los mapas asi generados pueden ser de utilidad para la construccion de obras de interes general 0 para el manejo de emergencias. no obstante, la aplicacion de estos resultados en estructuras vitales 0 que impliquen un bajo nivel de aceptacion al riesgo constituiria un hecho preocupante desde todo pun to de vista. los analisis aqui realizados permiten visualizar la evolucion de la red sisrnologica nacional y como sus modificaciones han influido en el catalogo sismico nacional. en este sentido, los resultados aqui obtenidos son una muestra de los avances que en esa materia se han hecho. se hace absolutamente necesario adoptar metodologias mixtas que incorporen otros parametres. esto permitira que los estimados de amenaza sismica arrojen resultados verdaderamente dinarnicos que reflejen de manera mas eficiente las propiedades fisicas que controlan la ocurrencia de sismos y su cambio en el tiempo. referencias audemard,f. a., m. n. machette,j. w. cox, r. l.dart & k. m. haller, 2000, map and database of quaternary faults in venezuela and its offshore regions, open-file report 00-018 usgs. grosser, h., a. rietbrock, m. baumbach, g. romero, j. rojas, 200 i, seismic tomography in the aftershock region of the cariaco earthquake, 1997, eos. trans. agu, 82(47), fall meet. suppl., abstract s32e-03. krinitzsky, e. l., 1993, earthquake probability in engineering-part 2: earthquake recurrence and limitations of gu15 castilla et al. temberg-richter b-values for the engineering of critical structures. the third richards h. jahns distinguished lecture in engineering geology, engineering geology, v.36, pp. i-52. kr!nitzsky, e. l., 1995, deterministic versus probabilistic seismic hazard analysis for critical structures, engineering geology, vao, pp. 1-7. reiter, l., 1990, earthquake hazard analysis. issues and insights, new york, columbia university press, 254 pp. rendon, h., 1997, red sismica de venezuela, en aa vv grases, j., diseno sismorresistente, especificaciones y criterios empleados en venezuela. caracas, 662 pp. 16 romero, g., j. rojas, m. baumbach, h. grosser, m. sobiesiak y grupo de trabajo resica 97, 1999, distribuci6n en profundidad de la actividad sismica y su relaci6n sismotectonica, vi congo venez. sism. e ing. sismica., merida, venezuela. smit, p, 2002, seismic hazard and risk assesment, lecture notes certificat d'etudes des risques geologiques 2002. sobiesiak, m., c. palme, j. choy, m. t. morandi, m. campo, c. granado, 200 i, seismological parameters in the northern andes, venezuela, eos. trans. agu, 82(47), fall meet. suppi., abstract si2b-0599. geofislca colombiana, 6. oiciembre oe 2002 nota editorial la constancia y la perseverancia no se aprenden, se logran con el trabajo y en geoffsica este nos abruma.en nuestras instituciones no hay a veces suficientes alicientes; sin embargo, el ejemplo que personas 0 grupos nacionales e internacionales nos brindan nos incentiva a seguir. ei trabajo geoffsico continua y no podemos rezagarnos un momento, 0 la historia nos dejara. a nivel nacionalla actividad academica y los proyectos en geoffsica siguen avanzando. ei ingeominas, el igac, ecopetrol, el osso, entreotros, desarrollan proyectos relacionados con slsrnica, gravimetrfa, magnetometrfa y sismologfa principalmente; aplicados a la prospecci6n, al entendimiento de la tect6nica basica y a la prevenci6n de desastres. ei compromiso nacional de establecimiento de programas de posgrado en geoffsica y en general en las diversas areas de las ciencias de la tierra, se hace cada vez mas urgente, en este aspecto celebramos la apertura de la especializaci6n en meteorologfa en la facultad de ciencias de la universidad nacional. han surgido nuevas empresas de servicios geoffsicos y otras se han acabado. las aguas subterraneas, la soluci6n de problemas de ingenierfa y de minerfa e incluso ambientales, han aumentado la demanda de esfuerzos muchas veces personales que debemos estimular y abrirles una puerta para la discusi6n y anal isis de problemas especfficos. a nivel internacional la adquisici6n de informaci6n se ha desarrollado aceleradamente y se trabaja en problemas de interpretaci6n, especial mente en slsrnica 3-d, tomograffa, geof6nia y modelamiento nurnsrtco de problemas electromagneticos. ei mar, la atm6sfera y el interior profundo con sometidos permanentemente al anal isis cuidadoso de la geoffsica. nos complace colaborar una vez mas con el desarrollo de la geoffsica nacional, en esta oportunidad geoffsica colombiana presenta 10 artfculos para su discusi6n. ellos tienen que ver con diversos temas que van desde la teorfa en aspectos ffsico-matematicos en el trabajo de la ffsica dora bernal relacionado con un curso de mecanica newtoniana, hasta la presentaci6n de los ultirnos valores de gravedad absoluta medidos en el pais que presentan los ingenieros martfnez, florez y sanchez del igac. hay 2 trabajos sobre sismologfa y 2 mas sobre manejo de informacion meteorol6gica. se completa el nurnero con un trabajo sobre prospecci6n geoelectrica, otro sobre anornalias maqneticas, uno mas sobre un modele barico de geoide para colombia y otro sobre calculo de parametres elasticos a partir de datos sfsmicos. de este modo creemos presentar un corto pero amplio abanico de 10 que en colombia se esta trabajando en las areas geoffsicas. su colaboraci6n en la discusion y anallsis, sigue siendo para "geoffsica colombiana" un aporte y una necesidad. , lópez et al.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 105-116 geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations jesús m. lópez k1., jesús moreira2, and josé gandarillas3. (1) geophysics and astronomy institute. department of geo-environment studies. calle ave. 212 no. 2906, entre 29 y 31, la lisa, cp 17100, ciudad de la habana, cuba. kramer@iga.cu (2) geology and paleontology institute. vía blanca y línea del ferrocarril, san miguel del padrón, cuba. moreira@igp.minbas.cu, moreira@gms.minbas.cu. (3) geophysics and astronomy institute. department of applied informatics. calle ave. 212 no. 2906, entre 29 y 31, la lisa, cp 17100, ciudad de la habana, cuba. pepegh@iga.cu abstract this paper describes the results from a study of the florencia gold-telluride deposit in central cuba, including mineralogical, petrographical, microprobe and chemical analysis. valuable information is provided for the exploration, mining and processing of gold ores from other nearby deposits with similar characteristics. results highlight changes in the mineralogical composition of the ores between the north and south sectors of the deposit, as reflected in metallurgical concentrates after beneficiation and flotation of samples from these sectors. it is shown that gold deposits of the cretaceous volcanic arc of cuba largely consist of native gold, telluride and pyrite, where arsenopyrite is almost absent. traces of lead, zinc and cadmium are present in the periphery of the main ore zones. key words: volcanic arch, cretaceous, mineralogy, electronic microprobe, petrography, gold, telluride, florencia, cuba. 105 earth sciences research journal manuscript received march 9 2006. accepted for publication december 4 2006. introduction despite the long history of mining of auriferous resources in cuba, the technique-productive and technological information was not conserved for different reasons. during several centuries the spanish settlers, and later on the small companies, developed underground or rudimentary mining in the holguín, guaimaro-jobabo, placetas and isla de la juventud areas, which were interrupted by different economical and historical factors. the florencia deposit is located in the e area of the country, in the guaimaro municipality, camagüey. it is known from the last century as a gold mine district and there are several reports from that period describing its mining and infrastructure. in the nineties, the international economic association macdonald mines ltd and geominera s.a. carried out an intense exploration program in florencia and in the surroundings of jobabo area (fig. 1), including maclama and golden hill located to the south. this exploration program led to a pre-feasibility study. works were stopped and no inversion took place because of low prices of gold. the results exposed in this paper include the underground explorations that were developed during the years 70-80 as a cooperation program between the cuban and u.s.s.r government. this information was taken from field works such as drillings and gallery samples, as well as available literature of the florencia district. it was also obtained important laboratory information such as petrographical, microprobe and chemical analysis. the principle aim of this study was to determine the mineral assemblage of gold ore deposits and the way in which precious gold and silver elements occur. the mining of these deposits may cause potentially adverse environmental impacts and some corrective measures are presented in order to mitigate them. materials and methods data were provided by the oficina nacional de recursos minerales from important areas with manifestations of gold located in the cretaceous volcanic arch and information published by foreign companies that carried out works of geological prospecting in different areas of the country. 15 thin sections, 35 polished sections, and 10 fire-essay analysis were also analysed. the results highlight the mineralogical differences between the products of the technological samples obtained in the north and south areas. the presence of minerals of the telluride group and the arsenopyrite absence in the ores, playing the pyrite an important part, is a characteristic feature of the auriferous sites located in the volcanic cretaceous arch. geological setting the cuban tectonic zonation is very distinctive. a continental scarp extends along the northern margin of the island and the narrow ophiolitic belt belonging to the ancient oceanic crust located to the south. the volcanic – sedimentary deposits complex, which represents the formation of an island arc, is developed along the central and southern parts of the territory. metamorphic rocks from the continental mass are well know in western and south-eastern cuba. during the aptian to campanian, a geodynamic volcanic island arc regime was established in the caribbean. outcrops can be observed along the camagüey province. in the central region of the country, the los pasos formation contains several deposits and prospects of the vms (kuroko) type with appreciable contents of cu, zn, and subordinated ag, au, cd and pb. several high anomaly areas have been described with presence of auriferous mineralization of epithermal type in the rocks of the cretaceous volcanic arch of the eastern part in the country, forming the ciego de avila camagüey las tunas and holguín belt (lópez et al., 1990; primelles et al., 1998; primelles et al., 2003; simon et al. (1999; capote, 2004). the camagüey region, is the most important because of its vast number of deposits, and constitutes a district of epithermal deposits. according to díaz de villalvilla et al. (2003), the nature of the rocks of the volcanic arc in camagüey allows to correlate them with those of the dominican republic (fig. 1). there are several types of mineral deposits in central cuba including high sulphide (golden hill) and low sulphide systems (beatriz, florencia), cu-mo-au porphyry (palo seco y la purísima) and au-ag-zn skarns (la purísima) deposits (primelles et al., 1998; primelles et al., 2003; simon et al., 1999). these authors considered the gold mineralization in the territory as an intrusive-related model with intrusive rocks and other representatives of the rocks of the arch, and state that epithermal deposits of the camagüey district are not related genetically with the plutonic bodies of the cretaceous geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 106 lópez et al., esrj vol. 10, no. 2. december 2006 fig. 1. central cuba deposit location. volcanic arch. simon et al. (1999) suggest that there is a relationship between the mineralization and the rhyolites of the sierra formation. this paper underlines the mineralogical characteristics of these deposits by pyrite presence as the main mineral, either disseminated or as sulphur; the absence of arsenopyrite and the occurrence of a variety of minerals such as gold, silver, lead, bismuth and nickel of the telluride’s group: petzite, calaverite, silvanite, hessite, bolinskite, shtiutzite, melonite, and altaite, as well as the presence of gold of high purity and electrum (torres et al., 2005; lópez, 1988; lópez et al., 1998; bortnikov et al., 1988). the geochemistry of halos of cu, ag, pb, zn and mo are characteristic for this type of location. el pilar epithermal prospect is located in the western part of cuba (fig.1), about 24 kms from ciego de avila city. beatriz is the better-studied deposit, located to the north of the guaimaro area (simon et al., 1999). the second in importance is florencia located also in the guaimaro area. golden hill is located to the south of las tunas, and it is representative of sulphate, exposing an area of oxidation. the jobabo municipality area exhibits several groups of small auriferous manifestations such as georgina, iron hill, and maclama. golden hill (little golden hill, three hill and big golden hill sectors), florencia and maclama are located close to the guaimaro village. they are located in a smooth relief and the presence of an oxidation zone makes them feasible for open pit mining using the heap leaching method or using small facilities as vat leaching. regardless of beneficiation-mining process used, it is necessary to be cautious because toxic elements are present, and there are water reservoirs and rivers crossing the region. mineralization the mineralization studied in the florencia deposit has a structural control that extends the body laterally up to 1 km, with thickness of 3040 m. the well-known mineral bodies are four: three in the north area and one in the south. they have forms of long narrow veins, with at least three quartz generations. the quartz types 107 are sterile milk-like quartz, darker quartz -where mineralization is foundedand typical quartz filled open veins with calcite carbonates. gas – liquid inclusions were studied by bortnikov et al. (1988), paded mainly for quartz and carbonates (fig.2). the florencia host rocks are traquibasalts and cretaceous (albian-turonian) andesite basalts. the volcanic rocks are cut by the granodiorite stock and quartz-diorites belonging to the guaimaro stock, and also by a few younger diorites and granodiorites dikes. in the limits of the mineral field, there are two isolated areas denominated north and south, which differ in their mineralogical composition. they are represented mainly by vein – disseminated sulphurous mineralizations. the north area is divided in three bodies with some ramifications, and the dominant mineralization is pyrite-chalcopyrite (fig 3). the south area is lens-shaped with pb-zn-ag-cu poly-metallic mineralization, being better developed the pbzn-ag poly-metallic mineralization. the mineral bodies of the north and south area have been explored using perforation wells up to 170 m depth and 300 m of strike, and underground mining. fig. 2. florencia’s mineral bodies’ district map (shestokanova, 1967). geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 108 fig. 3. small vein located in the second level of the south wall. the main ore is also indicated in the right part of the photo. mineralogical characteristics near 30 minerals are recognized in the veins at the florencia deposit. by-product minerals include different quartz types and textures such as crustiform, banded and cockade with different colours, in occasions grey with open cavities, typical of crack fills. starting from the thermodynamic analysis of the mineral associations in the system au-ag-te (bortnikov et al., 1989), the formation temperatures of the deposit varied between 70 and 280 °c. the main economic components of some veins are gold and silver. the native gold is the major metallic mineral in the ores. it is observed in polished sections in grains of different sizes. table i shows the results obtained by the author related to the count of the grains of gold with different sizes observed in 100 polished sections. in table ii, the study of the mineral composition of the ores demonstrated that the finest gold of high purity is present, as well as gold and silver in form of telluride’s calaverite (aute2), silvanite (au ag te4), petzite (agaute2), hessite (agte), shtiutzite and bolinskite (agbite2). the main concentrations of gold are found in native gold and petzite, broadly distributed among tellurides, and in silver concentrates, mixed with native gold, hessite, and petzite. these data show how gold and silver are represented in the ores, being the tellurides important in their composition. comparing north and south ore sectors, it is observed a smaller development of the sphalerite and galena in the north. here the pyrite contains inclusions of native gold sizes mm number of grains 0, 00 n – 0,015 16 0,020 – 0,040 6 0, 040 – 0,080 4 0,080 – 0,160 2 table. i. grains of gold counting lópez et al., esrj vol. 10, no. 2. december 2006 109 fig. 4. hessite and altaite in galenite. increase 200 x. fig. 5. vein of native gold and petsite in pyrite ii. increase 200 x. geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 110 table. 2. electron microprobe analyses (mass. %) and segregation of gold tellurides (calaverite and petzite) (fig 3, 4, and 5).the analyses of the pyrite mono-fractions with high contents of gold (64 up to 450 g/t), underline the significant role of the pyrite as concentrator of gold in this sector. in the south sector, the galena and the sphalerite are diffused, the native gold prevails in form of small inclusions in the pyrite; and inside the group of tellurides, the altaite and silver tellurides are diffused. results. there are two mineral forms present in florencia deposit ores: gold and silver. the presence of high purity gold and silver tellurides, evidences that a quantitative balance of the interrelations between these two types of ores is needed. during exploration works in 1990, the camagüey geology company took two technological samples of 4 tons each one: technological sample-3 (mt3) in the north sector and technological sample4 (mt-4) in the south sector. the analyses were carried out by the centre of investigations for the mining metallurgic industry executing the investigations for beneficiation by flotation and gravitation enrichment methods. the authors studied the flotation concentrate and gravitation of the mt-4 sample and flotation of the mt-3 sample. the analysis of the three concentrates of the technological mt-3 and mt-4 samples were carried out by the moscow institute of geology and geochemistry of the minerals in order to know the contents of au, ag, te and bi. portions of the samples were dissolved in concentrated hno3 and heated for one hour, and the remaining product, was solved in regal water. the determination of au, ag, te, and bi was done in the concentrate samples and in the break-up samples (after the concentrated hno3 treatment) and later all samples were tested using atomic absorption spectroscopy methods. the analysis was carried out with the contents of no. mineral au ag pb bi te s total 1 petzite 24.80 41.62 33.43 100.05 2 petzite 25.95 40.36 32.61 98.92 3 petzite 49.36 34.62 102.38 4 petzite 11.07 51.72 33.22 98.01 5 hessite 10.01 54.12 36.66 100.79 6 hessite 1.43 59.10 0.45 39.40 100.38 7 hessite 0.60 59.70 0.44 0.27 39.16 100.17 8 hessite 0.12 59.81 1.41 0.10 38.34 99.78 9 hessite 60.00 0.90 38.58 99.48 10 hessite 1.54 60.54 0.37 36.25 100.70 11 shtiutzite 54.70 0.20 42.47 97.39 12 calaverite 41.18 0.47 57.27 98.92 13 calaverite 43.21 56.86 102.09 14 bolinskite 16.67 0.36 36.02 42.59 0.83 98.71 15 bolinskite 13.39 0.10 37.57 43.57 1.13 98.79 16 bolinskite 18.31 35.16 46.14 99.61 17 tsumoite 61.65 37.31 98.66 18 raklidzite 1.61 14.33 37.53 45.52 0.16 99.45 19 altaite 0.18 0.90 57.65 1.23 39.25 98.21 20 altaite 0.60 58.26 1.15 38.92 98.93 21 altaite 0.24 1.09 58.66 0.32 37.03 97.34 lópez et al., esrj vol. 10, no. 2. december 2006 111 these elements in the levels from 1-10 g/t after their concentration. the results are shown in the table iii highlighting the differences between the contents in the elements analysed in the mt-3 and mt4 samples. the content of gold in mt-3 is four times the content in mt-4, as well as the silver and telluride concentrations, in both concentrates are similar but bigger than gold concentration. the relationship ag/au, in the concentrate mt 4 is 5.1 (gravitation) and 4.4 in flotation, and 1.35 in mt-3. the most important quantitative evaluation of precious metals is obtained starting from the analysis of the minerals of the telluride groups in the concentrate and in the solutions after its treatment with hno3. the quantity of gold that passes to the solution used to find telluride forms in the ores is 10.4 11.5 -% for mt-4 and 7.1%. for mt-3 . the silver, contrary to the gold, is in telluride form in more than 95% .it passes to the breaks-up with the hno3 treatment of the concentrate. it is important to highlight some differences in the composition of the concentrates obtained in mt-3 and mt-4 technological samples shown in the spectral analysis: 1. the biggest pb and zn contents in mt-4 concentrate reflect relative bigger sphalerite and galena quantities in the ores of the south sector. 2. the biggest quantities in sphalerite are related with biggest contents of cadmium (0.15%) in the mt-4 flotation concentrate. 3. it is important to note the cobalt high contents in the concentrate (table iv) and high co/ni relationship values (from 10 up to 20). the content of cobalt increases twice in the mt3 concentrate and it reaches up to 0.11%. this is conditioned by the quantities of pyrite in this concentrate and it is also confirmed by the pyrite mono-fraction results where the cobalt content varies from 0.15% to 0.40%. the data demonstrate that future technological research should take into account the presence of minerals from the telluride group in ores related to precious metals (table v), the silver and the presence of small inclusions of native gold and gold telluride in pyrite. geological factors with potential environmental effects some epithermal gold deposits have been defined and studied in the volcanic cretaceous arch (e.g., little golden hill, big golden hill, three hill, florence, and jacinto). there are other important such as el pilar-san nicolás, la purisima, tres antenas, corral de rojas, and la mina. the presence of several types of mineral deposits genetically distinctive in these formations are characterized by pyrite presence as main mineral, table. iii. chemical analyses results of the concentrate phases, solutions, and break-up remains. concentrates analysis g/t element au ag te bi mt–4 gravitation concentrates 35 179 188 44 break-up 4.0 172 162 42 mt–4 flotation break-up remains 32 7.2 18 5 concentrates 39 172 178 44 break-up 4.1 165 160 42 mt–3 flotation break-up remains 35 5.0 15 5 concentrates 140 190 220 26 break-up 10 182 200 25 break-up remains 128 7.5 21 5 geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 112 table. iv. r esults of the spectral quantitative analysis of the concentrate. c oncentrates e l e m e n t s a ug/t a gg/t zn% c d% m n% pb% b i% c u% c o% n i% sb% a s% sn% m t-4 zone south g ravitation 39.8 100.0 1.9 0.055 0.031 1.0 0.007 0.32 0.047 0.0043 0.090 0.056 m t-4 zone south flotation 33.0 84.0 5.0 0.150 0.090 1.0 0.83 0.50 0.0050 0.040 0.050 0.005 m t-3 zone n orth flotation 122.0 90.0 1.2 0.012 0.027 0.058 1.50 0.110 0.0050 0.022 0.050 0.002 lópez et al., esrj vol. 10, no. 2. december 2006 113 either disseminated or accompanied by quartz in veins. the almost absence of arsenopirite in the ores and the presence of a variety of minerals such as gold, silver, lead, bismuth and nickel belonging to the telluride group is an important common feature to keep in mind during future mining operations. the existence of at least three small deposits with advanced studies and the data presented here, as well as the high prices of gold and silver at present time, permitted to start the mining process in golden hill, florencia and beatriz, and defined the corresponding environmental studies in order to take the systematic measures to avoid or decrease damage as possible. • size of the deposit: they are small and located near each other. they stand out as small elevations on the surface. typical oxidation zones reach 10-20 m depth. ores are formed by pyrite as the main mineral, quartz, carbonates, and mineral loamy product of the wall – rock alteration. the reserves are approximately 5 10 tons of gold. the veins usually have high gold contents and erratic distributions could be present. small mining operations are needed to open pit facilities and benefit chord in the studied resources. • the mobility of the metals and the acid drainage product of the water circulation and the pyrite presence during the mining activity (open pit), could be limited directly by the presence of carbonates in the mineral bodies and associated with the wall rocks. • the oxidation of these sulphurs could generate acid drainage, which can potentially affect the superficial drainages of the area of influence in the mine and consequently enter in the food-chain. some detailed studies are proposed about risks in the area (e.g., the use of some types of biotechnological barriers such as bacteria or mushrooms that neutralises the environmental contamination during the exploitation works in the area). du bray (2004) recollected samples of stream sediment using the open-pit method and heap leaching showing results of < 5 to 12 silver ppm, 0.04 to 0.4 ppm of gold, < 5ppm cadmium, 18 to 81 ppm cooper, 1.4 to 4.2% of iron 200 to 2.020 ppm, manganese, 7 to 197 ppm lead, and 70 to 1.131 ppm zinc. the existence of at least 3 small deposits with advanced studies and the data presented here, as well as the high prices of gold and silver at present time, permitted to start the mining process in golden hill, florencia and beatriz, and defined the corresponding environmental studies in order to take the systematic measures to avoid or decrease damage as possible. table. v. te determination in ores by hidruros generation methods (north area). level 0. mineral body # samples content te g/t 1 5 91 1 7 87.5 1 8 36.6 5 14 13.4 1 19 12.2 3 21 24 1 22 40.6 4 28 2.2 geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 114 conclusions. 1. the presence of telluride group minerals and the absence of arsenopyrite in the cretaceous volcanic arch ores, being the pyrite the most significant, represent a characteristic feature for the auriferous locations. 2. the florencia ore deposit has important mineralogical differences between the two main areas: north and south. 3. the highest pb and zn contents in the mt-4 concentrate reflect the relative bigger quantity of sphalerite and galena in a small part of the south sector. 4. the biggest quantities of sphalerite are related to the biggest contents of cadmium (0.15%) in the mt-4 flotation concentrate. 5. the environmental impact caused by open pit and the pyrite presence (acid drainage mining), and the beneficiation methods used will give tellurides an inadequate final destination as residuals from the mining or during the mining processes. recommendations. optimal operation conditions should be studied more in detail at a proper scale for the industrial recovery of the gold and silver associated to tellurides, as well as the potentially environmental impacts for that process. references bortnikov, n. s., lópez, j. m., guenkin, a. d., krapiva, l. y. and santa-cruz, m. (1988). paragenesis of gold and silver tellurides in the florencia deposit. cuba. international geology review. 30. 294-306. capote, c. m. (2004). ‘análisis estructural sin-arco: una herramienta imprescindible en la evaluación metalogénica de cuba centrooriental’. in: estudios sobre los arcos volcánicos de cuba. centro nacional de información geológica. i.g.p. cd. cuba. 3342. díaz de villalvilla, l., milia, i., santa-cruz, m. and aguirre, g. (2003). ‘formación los pasos: geología, geoquímica y su comparación con el caribe’. in: estudios sobre los arcos volcánicos de cuba. centro nacional de información geológica. i.g.p. cd. cuba. 54-61. du bray, e. (2004). preliminary compilation of descriptive geoenvironmental mineral deposit models.u.s geological survey. open file report. 95 – 831. escobar, e. (1994). mineralización de metales básicos y preciosos, asociados al arco volcánico cretácico de la región ciego de ávila -camagüeylas tunas. in: congreso cubano de geología y minería. ii congreso de geología y minería. programas y resúmenes. santiago de cuba: [s.n.], p. 96. lópez, j. m., moreira, j., pantaleon, g. j., lavandero, r. m., montano, j. and cruz–martín, j. (1998). tipos mineralógicos de algunos yacimientos auríferos de cuba. iii congreso cubano de geología y minería (geomin 98). geología y minería 98. la habana. 371-374. lópez, j. m. (1990). informe del tema 40109. evaluación pronóstico de la mineralización aurífera de cuba, para el mapa metalogénico pronóstico, a escala 1:500 000 instituto de geología y paleontología. archivo igp. archivo onrm. poznaikin v., morales alberto., cañete carlos. lópez, j. m. (1988). composición sustancial y asociaciones parageneticas de los yacimientos hidrotermales auríferos de cuba. referat. moscú. 1-27. primelles, l., alvarado, b. and torres, m. (2003). ‘campo mineral maclama. breve caracterización geológica y mineralógica de la mineralización oro-telurídica. tipos esperados de depósitos’. in: estudios sobre los arcos volcánicos de cuba. cd. centro nacional de información geológica. i.g.p. cuba. 33-42. primelles, l., barroso, a., lugo, r. and escobar, e. (1998). geología y metalogenia del campo mineral guaimaro. enfoque actual. iii congreso cubano de geología y minería (geomin 98). la habana. 390-393. lópez et al., esrj vol. 10, no. 2. december 2006 115 simon, g., kessler, s. e., russell, n., hall, c. m., bell, d. and piñero, e. (1999). epithermal gold mineralization in an old volcanic arc: the jacinto deposit, camagüey district, cuba. economic geology. 94. 487-506. torres, m., sastroputro, s., ulloa, m., pérez, m., de la nuez, d., santa-cruz, m. and alvarado, b. (2005). petrología, mineralogía y alteración hidrotermal de la manifestación de cu porfídico del sector palo seco. camagüey, cuba. i convención cubana de ciencias de la tierra. geociencias 2005. s.c.g. 141-150. zhestokanova v, (1997). informe de los trabajos de búsqueda y exploración realizados en los años 1964-65 en los yacimientos auríferos de la zona de guaimaro en la provincia de camagüey. c.n.f.g, cuba. geology of the florencia gold – telluride deposit (camagüey, cuba) and some metallurgical considerations. 116 geoffs. colomb. santaf' de bogota d.c. issn ·0121·2974 variacion horaria de la humedad relativa del aire en el observatorio meteorologico nacionalsantafe de bogota-colombia jesus a. eslava r. profesor titular departamento de geociencias-facultad de ciencias-universidad nacional de colombia eslava, j.: variaci6n horaria de la humedad relativa del aire en el observatorio meteorol6gico nacionalsantafe de bogota-colombia. geofis. colomb. 2:13-20, 1993. issn 0121-2974 resumen se determinan los valores medios horarios y la variaci6n de la humedad relativa del aire para cada mes y el ano, con base en las observaciones horarias realizadas en la estaci6n observatorio meteorol6gico nacional-ciudad universitaria de santafe de bogota, durante el periodo 1941-1960. se establecen los valores extremos y sus amplitudes y se comparan los resultados obtenidos al aplicar algunas de las f6rmulas usadas para el calculo de los valores medios diarios, con los que se obtienen al establecer un promedio aritmetico de 24 observaciones horarias. abstract midle hourly values and dayly variation of the air relative humity for each month and yearly are determinated during the 1941-1960 period, according with hourly observations done on observatorio meteorol6gico nacional-ciudad universitaria-santafe de bogota. maximum values and its amplitudes are determinated, and using some formulas, were determinate the dayly midle values, and those results, are compared with the obteined from a arithmetic average of 24 observations each day. 1.introduccion ei agua se encuentra presente, en cantidades casi siempre grandes, en cualquier parte de la primera capa de la atm6sfera. sin embargo, por presentarse en forma de vapor la mayor parte del tiempo ella es invisible. de vez en cuando, ese vapor condensa y forma nubes que se encargan de mostrar la existencia de esa agua. las nubes, son entonces, no sola mente la expresi6n visible de la existencia de agua en la atmosfera, sino el mas visible de los elementos de la atm6sfera. interesa la consideraci6n de la humedad del aire en sf misma, en forma de vapor, no s610 porque es origen de las precipitaciones, sino a causa de la infinidad de consecuencias, entre elias las biol6gicas, que originan sus variaciones temporales. sin conocer las caracterfsticas de esas vanacrones es imposible, por ejemplo, comprender la influencia de la temperatura sobre los diferentes organismos: un pequsfio descenso o ascenso de la temperatura, cuando el aire esta hurnedo, produce cambios apreciables en la sensaci6n de confortabilidad de los hombres, animales y plantas; por el contrario, las fuertes bajas de temperatura de los inviernos en siberia y las altas elevaciones de la misma en el sahara, son relativamente iaciles de soportar 0 controlar gracias al descenso de la humedad relativa que acompafia a ambos fen6menos. en trabajos anteriores de eslava (1990a,b; 1991a,c), en los cuales se determinan las caracterfsticas de la variaci6n diurna de la presi6n atmosfsrica y temperatura del aire en santafs de bogota, se menciona que, en colombia, los anafisls de las variaciones diurnas de los diferentes elementos meteorol6gicos son, 14 eslava, j: variaci6n horaria humedad relativa obsv.met.nal. como casi todos los estudios referentes a la meteorologfa, muy escasos, ineditos 0 inexistentes. en el caso de las variaciones diurnas de la humedad relativa del aire, esta situaci6n es cntica y no se conocen ni siquiera breves comentarios. en este trabajo se recopila la informaci6n horaria, publicada, sobre humedad relativa del aire en santafe de bogota y, una vez, verificada, procesada y homogeneizada, se definen y analizan los valores horarios medios, con base en las observaciones realizadas en la estaci6n observatorio meteorol6gico nacional durante el perfodo 1941-1960 y, las humedades relativas maximas y minimas para cada mes y el ario, tarnbien se aplican algunas de las f6rmulas usadas para el calculo de los valores medios diarios y se comparan esos resultados con los que se obtienen al establecer un promedio aritrnetico de las 24 observaciones horarias. 2. generalidades puesto que el vapor de agua es el componente mas variable y, en muchos aspectos, el mas importante de la atm6sfera, varios autores han estudiado sus caracterfsticas, sus variaciones, su importancia, influencia y consecuencias 0 efectos sobre el tiempo, el clima y la vida, en general. con base en los anatisls de esos autores, entre ellos riehl (1954,1979), de martonne (1963), gordon (1965), petterssen (1968), mazzeo et al. (1972), de fina & ravelo (1973), lowry (1973), retallack (1973a,b) y mertins (1976), se efectuan las siguientes definiciones y comentarios sobre la humedad relativa, el vapor de agua y la humedad atrnosferica que el representa. en el aire siempre se encuentra vapor de agua, que sale de la superficie y entra en la atm6sfera, en forma de vapor, por diferentes procesos: a) evaporaci6n y transpiraci6n que se dan en los mares, rlos, lagos, suelo, vegetaci6n, etc.; b) surgencias sspontaneas: geiseres, fuentes minerales y termales, etc.; c) diversas combustiones que se realizan para producir calor y fuerza motriz: madera, gas de alumbrado, hidrocarburos, humos de las turbinas, de las locomotoras, etc. posteriormente, al condensarse, ese vapor de agua, origina las nubes y regresa a la tierra en forma de precipitaci6n, completando asi el cicio hidrol6gico y dando forma en todo este proceso al tiempo y los climas de la tierra. ei agua contenida en el aire es, por 10tanto, constantemente renovada por los ingresos de nuevas masas de vapor aportadas por los procesos ya seiialados, principalmente los de evaporaci6n y transpiraci6n que se dan en la superficie de la tierra. 2.1 importancia ei hecho de que exista mayor 0 menor cantidad de ~apor de agua en la atm6sfera, es decir que el arre sea mas 0 menos humedo, tiene muchos e importantes efectos no s610relacionados con el tiempo y el clima sino con los diferentes asuntos humanos. una relaci6n sucinta y s6~uramente muy incompleta de esos efectos 0 aplicaciones puede ser la siguiente: a) ei aire humedo se calienta mas que el aire seco bajo la acci6n directa de los rayos solares; b) el aire humedo es mucho mejor absorbente de la radiaci6n terrestre; c) el vapor de agua en el aire, conjuntamente con otros parametres meteorol6gicos, tales como la velocidad del viento y bajas temperaturas en perfodos de inversi6n termlca radiativa, contribuye al deterioro de ciertos objetos por acci6n de los contaminantes. se han encontrado valores crlticos de la humedad relativa que al ser excedidos producen un aumento rapido en la velocidad del proceso corrosivo. por ejemplo, el aluminio, en presencia de atm6sferas contaminadas con s02' tiene una humedad relativa crftica de 80%; d) el vapor de agua interviene en la formaci6n de nieblas de acido sulfurlco que danan los tejidos animales; e) el vapor de agua favorece la reducci6n de la visibilidad; f) el vapor de agua influye en la sensibilidad de las plantas en presencia de contaminantes filot6xicos; g) la sensaci6n de bienestar que experimentan los humanos, animales y plantas, depende, particularmente, de la cantidad de vapor de agua que contiene el aire: si el aire es seco, la piel puede transpirar, con 10 que se consigue refrescarla: perc si el aire se encuentra saturado, no se produce ninguna transpiraci6n y se experimenta una sensaci6n de incomodidad que afecta los diferentes procesos externos e internos de los cuerpos; h) la sequedad del aire hace soportables las temperaturas mas extremas: las bajas temperaturas de -40°c de los inviernos siberianos y las muy altas del sahara son faciles de resistir gracias al descenso de la humedad relativa que las acornpafia: i) en aire humedo, un debil descenso 0 una pequeiia elevaci6n de la temperatura producen una impresi6n muy sensible tanto sobre el hombre como sobre los animales y plantas; j) la entrada 0 salida de vapor de agua en la atm6sfera implica no s610 un cambio en la cantidad de masa atmosferica, sino un cambio en la energfa disponible y la presencia de procesos de transferencia de energia que influyen en el estado del tiempo y en el clima de un lugar; k) la salida de vapor de agua de la atm6sfera implica tambien un proceso de limpieza de la atm6sfera, puesto que al condensar el vapor de agua y precipitar hacia la superficie terrestre arrastra muchos de los contaminantes existentes en la atm6sfera; i) un bajo contenido de agua en la atm6sfera permite el rapido enfriamiento del aire y puede posibilitar la formaci6n de heladas; por el contrano un alto contenido de agua irnpide la presencia de esa danina situaci6n 0, por 10 menos, suavlza sus efectos. geofislca colombiana n° 2, abril de 1993 15 2.2 definiciones se define como humedad del aire 0 humedad atrnosferica a el contenido de agua en la atm6sfera. ei vapor de agua contenido en la atm6sfera es objeto de dos clases de medidas: a) se puede apreciar el peso del vapor de agua (my) contenido en una unidad de volumen de aire (v), asto es la "humedad absoluta": ha = mjv [1] 0, el peso del vapor de agua presente en el aire (my en gramos) por la masa de aire humsdo que 10contiene (my + md, en kg) que es la "humedad especffica": [2] b) se puede intentar saber en que medida seria el aire capaz de absorber todavia una cierla cantidad de agua, se trata de la "humedad relativa" (u) que es entonces una relaci6n (porcentual) entre la humedad real y la de saturaci6n para la misma temperatura y presi6n. la humedad atmosferica tarnblen puede expresarse con la ayuda de otros parametres, entre ellos los mas usados son la presi6n 0 tensi6n del vapor (e) y la raz6n de mezcla (r). 2.2.1 tensi6n del vapor (e) la tensi6n del vapor es la presion que tendrfa el vapor de agua si abarcara al s610 el volumen ocupado por el aire hurnsdo. se puede igualmente decir que es la presi6n parcial del vapor de agua en la mezcla con el aire seco, que constituye el aire hurnedo. esta presi6n, por supuesto tiene una relaci6n directa con la cantidad de masa de vapor de agua presente en el aire. la cantidad de vapor que puede, en un volumen determinado a una temperatura dada, permanecer en estado gaseoso, es limitada. cuando se alcanza este limite, se dice que el vapor es saturante 0 que el aire esta saturado; si liegan nuevas cantidades de vapor de agua a ese aire, habra condensaci6n. la presi6n del vapor, en estas condiciones, es la tensi6n de vapor saturante (es) que s610 depende de la temperatura del aire (t): para el equilibrio de condensacion (agua-vapor) log es= -4,9283.log t 2937,4.(1ft) + 22,5518 para el equilibrio de sublimaci6n (hielo-vapor) log es = 9,5553 (2667ft) 2.2.2 raz6n de mezcla (r) la raz6n de mezcla del aire es el cociente de la masa de vapor de agua (m.) por la masa de aire seco (md) con el cual este vapor esta asociado. en la practica se expresa en milasima 0, si se prefiere, en gramos de vapor de agua por kilogramo de aire seco: [5] cuando el aire ssta saturado, el valor que toma la raz6n de mezcla se llama raz6n de mezcla de saturaci6n (rs) 0 raz6n de mezcla saturante. 2.2.3 humedad relativa (u) la humedad relativa es la forma mas facil de expresar hasta que medida el aire se encuentra pr6ximo 0 no a la saturaci6n (0 a la condensaci6n), ella es la relaci6n entre la masa de vapor de agua real mente presente en la unidad de aire y la masa de vapor necesaria para la saturaci6n del aire a la misma temperatura 0, 10 que es equivalente, el cociente de la tensi6n de vapor de una muestra de aire por la tensi6n de vapor de la rmsrna muestra df' aire saturado (a la misma presi6n y temperatura): u = e/es [6] ei cociente debe multiplicarse por 100 si se desea expresar la humedad relativa en porcentaje, 10que es usual. cuando el vapor no es saturants. la masa de vapor de agua presente en el aire es mas 0 menos proporcional a la tensi6n de vapor y se puede obtener un valor aproximado de la humedad relativa por la f6rmula: [7] cuando el aire no esta saturado, e < es' la humedad relativa es inferior a 100%; para aire saturado, es igual a 100%, puesto que e = es' de todos los conceptos anteriores, se puede concluir que conociendo uno 0 dos valores de ciertos elementos meteorol6gicos puede determinarse facilrnente la humedad atmosfsrica a traves de uno 0 de todos los parametres que suelen utilizarse para expresarla. algunas de las f6rmulas mas usadas para determinar esos parametres que expresan la humedad, son las siguientes: u = [e/e,}.100 '" [r/r,}.100 [8] [9] [10] [11] [12] [13] [14] [15] r", e/e, [3] r = e.e![p-e} '" e.e/p r = q/[1-q} '" q=r/[1+r} [4] r, = e.e,l[p-e,} '" e.e,lp r, = q,l[1-q,} '" q, =r,/[1 +r,} e = p.r/[e+r} '" p.r/e ~ = 0622 p = presi6n atmosfarica 16 eslava, j: variaci6n horaria humedad relativa obsv.met.nal. resulta entonces que s610 conociendo la humedad relativa (objeto de este trabajo) y la temperatura del aire (ver eslava 1990a, 1991a), puede determinarse cualquiera de los otros indicadores que expresan la humedad del aire. la presi6n atrnosferica para cada sitio puede determinarse conociendo s610 su altitud y ubicaci6n geografica, sepun 10establece eslava (1990c). 3. informacion utilizada los datos utilizados en este trabajo corresponden a los valores medios para cada hera, de cada mes y aiio (tabla 1) determinados para la estaci6n observatorio meteorol6gico nacional, ubicada en el extrema ne de la ciudad universitaria de santafe de bogota, en zona aledaiia al instituto geografico agustrn codazzi. la descripci6n de la ubicaci6n del observatorio y de las areas aledaiias se encuentra en igac( 1961), reproducida por eslava (1990a,b;1991 a,c), los valores horarios de la humedad relativa del aire se determinaron con un (1) higr6grafo "instrument corporation" de registro diario, dos (2) higr6grafos "fuess" de registro semanal y dos (2) psicr6metros "fuess". para definir los valores medios horarios (tabla 1) y los rnaximos y minimos (tabla 2) se utilizaron los valores horarios medidos en ese sitio, durante el perfodo 1941-1960, previa una verificaci6n, homogeneizaci6n y proceso sequn los metodos meteorol6gico-estadisticos mas adecuados y cornunrnsnts usados en estos casos. esos valores tarnbien fueron verificados y homogeneizados, en su oportunidad, y se publicaron en las siguientes referencias: servicio meteorol6gico nacional (1950), ministerio de agricultura divisi6n de investigaci6n (19521958) e instituto geogratico "agustrn codazzi"(igac) (1959-1961). para el analisis de las caracteristicas de la variaci6n diurna de la humedad relativa, al igual que con 10sucedido con los valores horarios de presi6n y temperatura del aire (eslava 1990a,b;1991a.c), s610se utiliz6 el periodo 1941 a 1960, puesto que durante ese intervalo el observatorio tuvo la ubicaci6n mas representativa y estable de toda su historia y porque, acernas, son los unicos datos publicados al respecto. del ano 1961 a 1968 aun cuando el igac, entidad responsable del observatorio en esa epoca, continu6 con las mediciones horarias, esos datos no se han publicado y la informaci6n horaria original no se ha verificado, procesado, ni homogeneizado y, por 10tanto, su utilizaci6n directa podria introducir serios errores en los anatlsis: del ano 1969 en adelante el servicio colombiano de meteorologia e hidrologia, scmh, (actualmente instituto colombiano de h.idrologia, meteorologia y adecuaci6n de t!erras, himat), se encarg6 de la operaci6n de dlcha. estaci6n y suspendi6 la observaci6n horana. por otra parte, el crecimiento y desarrollo del igac, en cuyos predios aun funciona la estaci6n, ocasion6 que el observatorio meteorol6gico nacional se trasladara al costado norte del edificio del igac y se encuentre, actualmente, rodeado de un parqueadero asfaltado, 10que, obviamente, ie ha quitado toda representatividad, a la nueva informaci6n que suministra, desde el punta de vista de los analisis meteorol6gicos que tiendan a determinar el regimen normal y natural. 4. variac ion horaria de la humedad relativa del aire la humedad del aire, como ya se mencion6, es un elemento meteorol6gico muy importante en la determinaci6n de los diferentes estados de la atm6sfera, no solamente por su influencia sobre la precipitaci6n sino por sus efectos biol6gicos. su valor se encuentra sujeto a rapidas fluctuaciones bajo la influencia, especialmente, de la direcci6n del viento, por el aporte 0 supresi6n de humedad que puede realizar este; por esa raz6n, los datos medios diarios, mensuales 0 anuales de la humedad relativa, y los mapas que los indican, se han considerado (equivocadamente) de poco valor y han sido poco usados, aunque algunas de las consecuencias de la humedad (nubosidad y precipitaci6n) figuran representadas en todos los estudios meteorol6gicos. un analisis de las caracteristicas de la variaci6n de la humedad relativa durante el dfa arroja resultados interesantes que posibilitan una utilizaci6n de los datos mas acorde con la importancia que ellos representan. se comprueba la ocurrencia de s610un periodo con un valor maximo y uno con un valor minima diario de humedad relativa en bogota. eso coincide en terminos generales con 10 que sucede en la regi6n tropical y es similar a la de la temperatura, pero con la diferencia de que cuando ocurre la maxima temperatura se presenta la minima humedad relativa y cuando se da la minima temperatura sucede la maxima humedad relativa. ei maximo medio horario se registra, en general, durante todo el periodo entre las 04 hl (hora local) y las 06 hl 0 dentro de esas horas (excepto en noviembre que ocurre de las 03 hl a las 05 hl); a nivel anual este maximo ocurre a las 05 hl y a las 06 hl, 10cual indica que el maximo valor ocurre, en promedio, alrededor de las 05:30 hl. los minimos medios horarios ocurren dentro del periodo de las 12 hl a las 14 hl, excepto en octubre que ocurren.a las 11 hl y a las 12 hl; por esto puede conslderarss que el minima valor se presenta a las 12:30 hl 0 13:30 hl sspun el mes, a nivel anual ocurre -en promedioa las 12:30 hl. de i? tabla. 2, se concluye que el rango 0 amplitud maxima absoluta de la variaci6n de la humedad oscila entre 97% y 71% y la amplitud media entre 73% y 57%. geofisicacolombiana n° 2,abrilde 1993 17 tabla 1 humedad relativamedia horariadelaire,en santafede bogota, (%) hor ene feb mar abr may jun jul ago sep oct nov dic ano rango 01 88 88 87 87 90 87 88 87 90 92 92 90 89 5 02 89 88 88 90 90 91 89 89 90 93 93 90 90 5 03 90 89 89 91 91 91 90 90 91 94 94 91 91 5 04 91 90 90 91 92 92 91 91 92 94 94 92 92 4 05 92 91 90 91 92 92 92 92 92 95 94 92 92 5 06 92 92 90 91 92 93 92 92 92 95 93 91 92 3 07 89 90 89 89 89 87 87 87 87 91 90 88 89 4 08 80 80 79 80 79 78 77 78 77 80 79 80 79 3 09 67 65 67 69 70 69 68 67 65 68 68 67 68 5 10 57 58 58 63 65 63 62 61 58 62 61 59 61 6 11 51 52 54 60 62 60 59 57 55 58 58 54 57 11 12 48 49 51 59 61 58 56 55 53 58 56 52 55 13 13 48 50 51 59 60 57 55 54 53 60 58 54 55 12 14 50 52 54 60 60 57 55 54 53 63 61 5 56 13 15 53 55 58 62 62 58 55 55 54 65 66 60 59 13 16 58 58 61 65 64 60 56 56 56 68 70 65 61 12 17 62 62 64 69 67 63 59 59 59 71 73 69 65 12 18 69 68 70 74 72 68 63 64 65 76 77 75 70 14 19 75 73 74 79 77 73 68 70 71 81 81 79 75 13 20 79 76 77 82 81 76 73 74 75 84 84 82 79 11 21 82 79 79 85 83 79 77 77 79 87 86 85 82 10 22 84 82 81 86 85 82 80 80 82 89 88 86 84 9 23 86 84 83 87 87 86 83 83 85 90 90 87 86 7 24 87 86 85 88 88 87 86 85 87 91 91 88 87 6 m 74 73 74 77 77 75 73 73 73 79 79 76 76 6 tabla 2 valoresmaximos y minimos de humedad relativadelaireen santafede bogota (%) mes maxima maxima media minima minima amplitud max absolu media media absolu absolu media enero 100 97 74 26 14 86 71 febrero 100 97 73 24 3 97 73 marzo 100 96 74 28 13 87 68 abril 100 97' 77 37 27 73 60 mayo 100 96 77 39 26 74 57 junio 100 96 75 38 29 71 58 julio 100 96 73 36 19 81 60 agosto 100 96 73 36 27 73 60 septiem 100 97 73 33 23 77 64 octubre 100 97 79 36 24 76 61 noviemb 100 96 79 36 27 73 60 diciemb 100 96 76 29 14 86 67 ano 100 96 76 33 3 97 63 18 eslava, j: variaci6n horaria humedad relativa obsv.met.nal. ei valor mas alto 0 maximo absoluto de 100%, ocurre aproximadamente en 63 dias de cada 100 (cerca de 20 dias por mes); valores maxirnos diarios entre 95% y 99% se dan en 32 dlas de cada 100 (cerca de 10 por mes) y s610 5 dias de cada 100 (aproximadamente uno por mes) se presentan valores rnaximos inferiores a 95%. por otra parte, el valor mas bajo que ha ocurrido tiene una probabilidad de ocurrencia de 5 aries por cada 100; ese valor fue de 3% y se registr6 en febrero de 1948 (ver ministerio de agricultura divisi6n de investigaci6n: anales del observatorio meteorol6gico nacional 1948). otras caracteristicas generales que presenta la variaci6n diurna de la humedad relativa en santafe de bogota, pueden resumirse ast: antes de la salida del sol, las humedades relativas son altas (superiores a 90%) con, a su vez, los valores mas altos en octubre y noviembre (mesas liuviosos) y los mas bajos en marzo (comparativamente seco); entre las 06 hl y las 12 hl, la humedad relativa desciende rapldarnente y a las 12 hl ya es de 55%, en promedio anual, a esta hora los valores mas bajos ocurren en enero (48%) y febrero (49%) y los mas altos en mayo (60%); alrededor de las 13 hl, se presenta la rnlnirna humedad relativa media horaria, en promedio anual de 55%, con el valor medio mas bajo en enero (48%) y el mas alto en mayo (60%); a partir de las 14 hl se presenta un ascenso gradual, un poco menos fuerte que el descenso de la manana y hasta las 04 hl 0 05 hl donde ocurre el maximo medio que se mantiene hasta i:is 06 hl; para una misma hora, los meses en los cuales e.e dan los valores maximos y mlnirnos cambia 5egun sea el periodo del dla: entre las 07 hl y 14 hl el maximo para cada hora ocurre en mayo y, durante el resto del dia ocurre en octubre y noviembre; los minimos tienen mas cambios que los rnaxirnos y se dan entre las 09 hl y 15 hl, mayormente, en enero, entre las 01 hl y 06 hl en marzo y abril, entre las 16 hl y 01 hl y 07 hl a 09 hl en julio y/o agosto y/o septiembre; • en la tarde (12 hl-19 hl) ocurren los mayores cambios durante el ano, para cada una de esas horas el rango 0 amplitud anual es de 12% a 14% (diferencia, para cada hora, entre la mas alta y la mas baja humedad relativa media mensual); al contrario, las mas bajas amplitudes (3%-6%) se dan en las horas de la mad rug ada yen la manana (01 hl-10 hl); • a las 06 hl y 08 hl se presenta la menor variaci6n durante el ano de los valores de humedad relativa media horaria, y a las 14 hl ocurre la mayor variaci6n. las figs.1-5 muestran, como ejemplo, la variaci6n diurna de la humedad en los solsticios uunio y diciembre) y equinoccios (marzo y septiembre) y a nivel anual; la fig.6, por su parte, muestra la variaci6n para cada hora de la amplitud de los valores medios mensuales. por otra parte, al compare' el valor medio de la humedad relativa determinado con base en 24 valores horarios (ecuaci6n [1], tabla 3) y las que se establecen aplicando dos de las f6rmulas que se han usado y se usan frecuentemente (ecuaciones [2] y [3], tabla 3) se encuentra que la f6rmula que mas sa ajusta es la [3] que es la que actualmente ssta en uso generalizado. u (') 100 80 80 "'u--'----'___''___'_'____"__'_~'_'__-'--'----'___"___'_'____"__'_-'-:-'----'-'--'i 2 a 0( s 6 7 8 9 1011 12 ta 14 is 16 17 ,/\ la 20 21222:\ 24 figura 1. variaci6n horaria del valor medlo de la humedad relativa del aire en santafe de bogot~, u en %. marzo u(') 100 80 80 40 l..l-'____"__'_~'_'___'_-'----'___"___'_'____"_~,__l........l._--i-~~ 1 2 3 0( 5 6 7 8 9 to ii 12 13 14 15 16 17 hi 192021 22 23 24 , ~ figura 2. variaci6n horaria de la humedad relativa del aire en santafe de bogot~ d.c., u en %. junio u(%) 100 80 60 40 i_l.-..l-.j.--l. ! , , , ! , i i ! i ! ! ! ! ! ! i , , i 2 3 4 5 6 7 b 9 10 11 12 13 t4 15 16 17 16 19 20 21 22 23 2. figura3. variacl6n horarla de la humedad relativa en santafe de bogot~ d.c., u en %. septiembre geofisica colombiana n° 2, abril de 1993 19 tabla 3 valores medios diarios de humedad relativa del aire en bogota (%j, calculados con diferentes ecuaciones mes humedades relativas medias desviaciones [1] [2] [3] [2]-[1] [3]-[1] enero 74 69 71 -5 -3 febrero 73 69 71 -4 -2 marzo 74 70 72 -4 -2 abril 77 74 76 -3 -1 mayo 77 74 75 -3 -2 junio 75 71 72 -4 -3 julio 73 68 70 -5 -3 agosto 73 68 70 -5 -3 septiembre 73 68 70 -5 -3 octubre 79 76 77 -3 -2 noviembre 79 75 76 -4 -3 diciembre 76 72 74 -4 -2 ano 76 71 73 -5 -3 [1] u=[luj/24 valores calculados con datos tabla 1. [2] u=[u07+u'3+u1a]/3 valores calculados con datos tabla 1. [3] u=[u07+u,3+u19]/3 valores calculados con datos tabla 1, uj = humedad relativa media horaria, i = 1 a 24. u07' u'3' u1a, u'9 = humedad relativa media a las 07, 13, 8 y 19 horas, respectivamente. u(:t) ,()() eo eo 40 .l! i i i ! i----l.-l-' , ! i i ! , , i , , i , , i , ? '\ .. '" f; '1 a a 10 11 12 1:'\ 14 15 iii 17 ir iii 20 2\ 222324 figura 4. variaci6n horaria de la humedad relativa en santafe de bogota, u en %. diciembre u«) i()() ::' .0 ::: ~l! ~j~ ::. •0 .:: i': i u 40 ::: .:: ':: 20 !ll ill.. i:: .. ...... hi ::: .. .. ... ... g. ... :,: ... .. i:: ::: ... ::: ... ::: .. ... ... ::. l:: ~!l::1 ::: ... ... ... :: ':. .. i:: ... m.. ... ... ... ::: ... ::: :: .. ... ... :: ::: .. iii ::: i:: .., .. ... m i::... ...... ur 1 2. 3 4 s 6 7 8 9 to 11 1213 14 15 16 17 ir 192021222324 figura 5. variaci6n horaria de la humedad relativa en santafe de bogota, u en %. valor anual au(%)i. 10 1 2. :3 , 5 " 7 6 9 10 11 12 13 l' 15 18 17 16 19 20 21 2223 z.f figura 6. variaci6n horaria interanual de la amplitud (au%) de la humedad relativa del aire en bogota d.c. 5. conclusiones ei analisis de los datos de humedad relativa media horaria muestra que la variaci6n diurna de ese elemento meteorol6gico en santafe de bogota, a travss del afio, es bastante homoqsnea y presenta un s610 valor maximo y un s610 valor mfnimo por dia; esa variaci6n coincide con 10que sucede en la zona tropical y mas especificamente en la ecuatorial. la diferencia entre el promedio mensual' mas alto y el mas bajo no es muy grande (a nivel medio anual es de 6%, con un maximo medio de 14% y un minimo medio de 3%). sin embargo, la diferencia entre la maxima y minima humedad relativa en un dia critico puede ascender a 90% o mas, regularmente es un valor cercano a 60%. los valores maximos diarios de humedad relativa del aire, mas comunes, oscilan entre 95% y 100%; en 95 dias de cada 100 puede presentarse una humedad relativa con un valor igual 0 superior a 95% y s610 5 dfas de cada 20 eslava, j: variaci6n horaria humedad relativa obsv.met.nal. 100 ocurre un valor maximo diario que no supera el 95%. ei maximo se presenta alrededor de las 05:30 hl y el minimo a las 13:00 hl. ei valor mas bajo de humedad relativa que se ha registrado es de 3% en febrero de 1948, coincidiendo con la mas baja temperatura (5.2"c) que se ha medido en santafe de bogota. ei valor medio de la humedad relativa, calculado con base en 24 observaciones diarias, es de 76% y cambia muy poco a traves del afio (entre 79% y 73%). aun cuando las humedades relativas medias mensuales son muy semejantes, se alcanza a detectar que en los meses mas liuviosos (octubre y noviembre con 79%) la humedad relativa es mas alta que en los meses menos liuviosos (abril y mayo con 77%) y, obviamente, los meses mas secos presentan las menores humedades relativas. se comprueba que la ecuaci6n utilizada actualmente para definir los valores medios diarios de humedad relativa, basada en el promedio aritrnetico de los valores de las 07, 13 y 19 hl, se ajusta bastante bien con la que utiliza el promedio de 24 observaciones horarias, s610presenta una desviaci6n media de -3%. en terminos generales la variaci6n de la humedad relativa es inversa a la de la temperatura del aire y, por las relaciones que hay entre elias, se puede decir que la humedad absoluta varia, casi como la temperatura del aire, en sentido inverso a la de la humedad relativa. referencias bibliograficas de fina, a. & a. ravelo. (1973): climatologia y fenologia agricolas. 282pp. eudeba, buenos aires. eslava, j. (1990a): caracteristicas de la variaci6n diurna de la temperatura del aire en bogota. memorias del xi gongreso colombiano de geografia. pp.222-237. acoge, monteria. ••••• (1990b): caracteristicas de la variaci6n diurna de la presion atrnosferica en bogota. memorias del xi congreso colombiano de geograffa. pp.238-249. acoge, monterfa. ---(1990c): modelos para determinar la presion atrnosferica media mensual y anual en colombia. colombia geoqrafica xvi(2):7-92. igac, bogota. (1991a): variaci6n temporal de la temperatura del aire en bogota. rev. acad. colomb. cienc. 18(68): 65-74. santafe de bogota. ----(1991b): variaci6n espacial de la presi6n atmosferlca en colombia. colombia geografica xvii(1): 5-39. igac, santafe de bogota. ----(1991c): variaci6n temporal de la presi6n atmosferica en colombia. colombia geoqrafica xvii(1):41-109.igag, santafs de bogota. instituto geogratico "agustin codazzi" (igag). 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(1952-1958): anales del observatorio meteorol6gico nacional-ciudad universitaria 1944 a 1957. santafe de bogota. petterssen, s. (1968): introducci6n a la meteorologia. 429pp. espasa-calpe, madrid. retallack, b.j. (1973a): compendio de apuntes para la formaci6n de personal meteorol6gico de la clase iv, vol.i, ciencias de la tierra. publicaci6n omm n° 266. 219pp. omm, ginebra. ----(1973b): compendio de apuntes para la formaci6n de personal meteorol6gico de la clase iv, vol. ii, meteorologia. publicaci6n omm n° 266. 357pp. omm, ginebra. riehl, h. (1954): tropical meteorology. 392pp. macgraw-hill, new york. ----(1979): climate and weather in the tropics. 612pp. academic press, london. servicio meteorologico nacional (1950): anales del observatorio meteorol6gico nacional-ciudad universitaria 1941 a 1943. 226pp. bogota. geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 119-133 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria o.a. okunlola, o.c. adeigbe and o.o.oluwatoke department of geology, university of ibadan nigeria gbengaokunlola@yahoo.co.uk abstract amphibole and quartz schist which occur in association with migmatite gneiss, granitic gneiss and pan african older granite bodies around ibadan area, southwestern nigeria, were studied with a view to elucidate their compositional characteristics and their evolution. mineralogical determinations from optical studies show a high proportion of granular quartz and accessory muscovite in the quartz schist. the amphibole schist on the other hand comprises mainly dark colored bands of hornblende with subordinate tremolite, chlorite and minor amounts of plagioclase and quartz. chemical analysis of the samples obtained by inductively coupled plasma mass spectrometry (icpms) instrumentation method, involving major and trace elements reveal the silicieous nature of the quartz schist and and the amphibole .samples of both rock units are also marked by relatively elevated contents of ba, zr, rb, la and ce and zr. in addition, variation plots using na2o, al2o, k2o suggests arenaceous sedimentary ancestry for the quartz schist and an igneous ancestry most probably mafic extrusive volcanics for the amphibole schist. provenance indicators, such as ba, in the quartz schists suggest derivation of this sedimentary protolith from the weathering of largely granitic rocks. similarity of the amphibole schist progenitors with subalkaline basaltic andesite is also implied by the na2o+k2o versus sio2 bivariate plot, while the na2o+k2o-fe2o3 (t)-mgo ternary plot reveal their calc-alkaline affinity. tectonically, the quartz schists evolved within the passive margin environment, whereas the mgo-fe2o3-al2o3 ternary plots reveal an ocean island basalt tectonic evolution for the amphibole schist. key words: amphibole, quartz, schist, schistose rocks, ibadan-nigeria, petrogenetic resumen anfíboles y esquistos de cuarzo que ocurren en asociación con gneiss migmatiticas, gneiss de granito y cuerpos antiguos de granito pan africano alrededor del área de ibadan, a sudoeste de nigeria, fueron estudiados con el fin de aclarar las características composicionales y su evolución. 119 manuscript received: 13/05/2009 accepted for publication: 10/12/2009 paginas 29-30.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp martes, 09 de febrero de 2010 18:32:42 p p p composite 133 lpi at 45 degrees determinaciones mineralógicas a partir de estudios ópticos muestran una alta proporción de cuarzo granular y moscovita accesorios en el esquistos de cuarzo. por otro lado, los esquistos anfíboliticos incluyen bandas de color oscuro principalmente de hornblenda con tremolita, clorito y de pequeñas cantidades de plagioclasa y cuarzo. el análisis químico de las muestras obtenidas por el método acoplamiento inductivo de espectrometría de masas (icpms), revela la naturaleza de silicieous de los esquistos de cuarzo y los anfíboliticos. muestras de ambas unidades de rocas también están marcados por el contenido relativamente elevado de ba, zr, rb, la y ce y zr. además, la variación de las gráficas na2o, al2o, k2o sugiere arenas sedimentarias antiguas para el esquisto de cuarzo y una ígnea antigua muy probablemente mafica para los esquistos anfíboliticos. los indicadores de procedencia, tales como ba, en los esquistos de cuarzo sugieren derivación de protolito sedimentario desde la erosión de las rocas graníticas en gran medida. la similaridad entre los progenitores de esquisto anfíboliticos con andesita basáltica subalkaline también implican na2o + k2o versus sio2, por la variación de la gráfica na2o + k2o-fe2o3 (t)mgo revelan su afinidad calco-alcalinas. tectónicamente, los esquistos de cuarzo evolucionaron en un ambiente de margen pasivo, mientras que el gráfico mgo-fe2o3-al2o3 parcela ternario revelan una evolución tectónica basalto en la isla de océano para los esquistos anfíboliticos. palabras clave: anfíboles, cuarzo, esquistos, schistose rocks, ibadan-nigeria, petrogenetic. introduction the nigerian basement complex which occurs within the neoproterozoic to early paleozoic, pan-african (ca 0.6 ga.) province east of the west african craton is loosely classified into three principal subdivisions. these are the ancient migmatite-gneiss–quartzite complex, the schist belts and the pan-african (ca 600 ma.) granitic series (elueze and okunlola, 2003). the migmatite gneiss suites are mainly ca 2.8 to 2.0 ga. ages. most authors on the nigerian basement complex subscribe to the view that the rocks of these suites comprise largely a sedimentary series with associated minor igneous rocks which has been variably altered by metamorphic, migmatic and granitic processes (oyawoye, 1972; rahaman, 1988; okunlola, 2005). a number of north-south trending proterozoic schist belts occur conspicuously within the western part of nigeria with few in the eastern parts and show distinctive petrological and structural features. fourteen of such belts have been delineated (rahaman, 1976; odeyemi, 1977; ekwueme and shing1987; elueze, 1991; okunlola, 2001; elueze and okunlola, 2003). belts in the southwest include iseyin, igarra, egbe ,isanlu, ifeilesha,, lokoja -jakura, and toto-gadabuike. as at now however, there is no complete agreement on location, delimitation, geological nomenclature and geodynamic setting of the nigerian schist belt. these schistose rocks occurrences around ibadan constitute the southern extension of the n-s trending iseyin oyan belt. this study hence aims at elucidating the compositional features and hence petrogenetic affinity of these schistose rocks of ibadan area. this is expected to further lead to the understanding of the geodynamic evolution of the schist belts and the precambrian of nigeria. lithological association and petrography in the study area, the schistose rocks are associated with an almost north~ south trending unit of the migmatite gneiss complex. the contacts are tectonic in places with prominent slickenside features and cataclastites. these units were later intruded by pan-african granitic rocks of adammellitic and pegmatitic varieties. these schistose rocks are mainly quartz schist and amphibole schist. the quartz schist form prominent features especially west of ajibode and sango area (fig. 1). their outcrop pattern indicates possibly a refolded fold (fig. 1). they are usually fine grained and form extensive north -south trending bodies. in places, they are characterized by preponderance of quartz rubbles and usually whitish grey in color. but in areas of prominent iron staining as a result of thick overlying lateritic regolith they are usually brownish gray in color. they are the most extensive of the schistose rocks studied. under the microscope, the rock unit is composed mainly of quartz and muscovite (fig. 3) quartz occurs as cloudy anhedral grains with characteristic weak birefringence, prominent undulose extinction and low relief while muscovite occurs as tiny elongate plates with poikilitic crystals of quartz. the amphibole schists on the other hand occur in places as greenish, fissile bodies sometimes weathered or as lenticular and sometimes ovoid shaped bodies comprising of indistinct dark colored bands of greenish black amphibole, light colored bands of plagioclase and minor quartz. thin section study reveals that the amphiboles are mainly hornblende, tremolite and chlorite. 120 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke paginas 29-30.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp martes, 09 de febrero de 2010 18:32:42 p p p composite 133 lpi at 45 degrees 121 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria 140140140140 figure 1. geological map of study area. table 1. modal composition of the quartz schist 1 2 3 4 5 6 7 quartz (%) 92 94 93 90 89.5 91 92 muscovite(%) 7 3 5 8 8 8 5 microcline (% 1 2 2 1 1 1 2 magnetite(%) 0.3 0.1 zircon trace trace trace trace trace trace trace rutile trace trace trace trace trace trace trace enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:21 p p p composite 133 lpi at 45 degrees 122 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke q m h bar scale = 2 mm figure 2. photomicrograph of amphibole schist of ibadan area in transmitted light showing. fine grained anhedral quartz (q), irregularly oriented hornblende (h) and interstitial mica m) table 2. modal composition of the amphibole schist. (%). 1 2 3 4 5 6 7 hornblende 69 67 66 66 67 69 67 plagioclase 10 9 12 14 9 10 10 biotite 7 6 6 5.5 5 5 7 quartz 8 9 8.5 9 10 9 9 chlorite 0.6 0.5 0.5 0.5 0.5 0.5 0.5 pyroxene 5 5 5 4 3.5 4 6 tremolite _ 3 _ _ 3.5 2.5 _ total 99.6 99.5 98 99 98.5 100 99.5 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:22 p p p composite 133 lpi at 45 degrees (fig. 2) quartz and plagioclase occur in minor proportions with accessory biotite and zircon. hornblende occurs as small xenoblastic crystals, greenish in color while tremolite grains tend to be long, prismatic and usually pleichroic from pale green to dark green. quartz occurs as small anhedral grains with wavy extinction. the modal composition of the quartz schist and amphibole schist are shown in tables 1and 2. geochemical features samples and analytical techniques seven unaltered and fresh samples each of quartz schist and amphibole schist were analyzed for both major oxide and trace element concentrations at the activation laboratories, canada, using the inductively coupled plasma analytical technique. analytical results are presented in tables 3 and 4. discussion of results average sio2 value in the quartz schist is high (95%). this is comparable to values obtained for most quartz schist samples of the nigerian metasedimentary belt (okonkwo, 1992; elueze and okunlola, 2003; okunlola, et.al., 2006). conversely, the average sio2 values in the amphibole schist (53.5%) is expectedly much lower and also lower than those of the average values of schistose amphibolites of nigeria (ajayi, 1980; elueze, 1985; elueze and okunlola, 2003b). fe2o3 values in the samples of the quartz schist range from 0.33%0.62% with a mean of 0.42% while in the amphibole schists they range from 9.63%-9.76% with a mean concentration of 9.7%. the comparatively lower fe2o3 values in the quartz schist are due to the almost total absence of melanocratic minerals. the values for the amphibole schist are comparable to those of the schistose amphibolite varieties of burum metabasalts while it is lower than those of the 123 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria bar scale = 2 mm m q figure 3. photomicrograph of quartz schist of ibadan area in transmitted light showing coarse anhedral grains of quartz (q)and tiny interstitial muscovite. (m) pagina 33.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp martes, 09 de febrero de 2010 18:33:12 p p p composite 133 lpi at 45 degrees 124 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke table 3. major element (%) and trace element (ppm) compositions of samples of quartz schist from ibadan area. r1 r2 r3 r4 r5 r6 r7 sio2 94.46 94.07 95.95 95.7 96.01 94.72 94.41 al2o3 1.47 2.24 1.32 1.13 1.52 1.38 1.72 fe2o3(t) 0.33 0.38 0.35 0.62 0.41 0.39 0.46 mno 0.006 0.004 0.005 0.004 0.005 0.004 0.005 mgo 0.06 0.05 0.05 0.04 0.05 0.04 0.06 cao 0.91 0.24 0.95 0.73 0.88 0.74 0.5 na2o 0.11 0.11 0.13 0.08 0.15 0.09 0.083 k2o 0.36 0.35 0.3 0.26 0.35 0.302 0.3 tio2 0.067 0.224 0.105 0.123 0.23 0.09 0.07 p2o5 0.04 0.04 0.04 0.04 0.04 0.04 0.04 loi 1.268 0.875 1.191 0.963 0.566 0.906 1.364 total 99.09 98.59 100.4 99.7 100.211 98.702 99.012 ba 84 131 98 53 94 79 102 ce 13.2 67.8 15.6 104 50.15 22.7 77.6 co 1 1 1 1 1 1 1 cr 20 20 60 20 20 20 20 cu 10 30 10 10 10 10 10 ga 2 2 2 2 2 2 2 la 9.1 23.8 8.8 40.4 20.53 15.93 25.12 nb 2 4 2 2 2 2 4 nd 5.1 27.3 6.3 46.2 21.23 12.72 29.73 ni 20 20 20 20 20 20 20 pb 13 14 9 9 12 9 13 rb 12 15 10 7 15 9 12 sn 2 1 1 1 2 1 1 sr 19 31 20 14 21 18 24 th 2.3 8.7 2.3 9.4 5.7 8.3 3.01 v 8 9 7 9 9 8 9 y 3 24 3 23 22 23 20 zn 320 160 280 310 270 310 220 zr 122 710 182 184 300 145 454 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees banded and massive varieties of the same area (elueze and okunlola, 2003). the average k2o, na2o and cao values are greater in the amphibole schist (0.62%, 2.67% and 13.0% respectively) compared to those of the quartz schists (0.32%, 0.11% and 0.71% respectively). these k2o and na2o values for the amphibole schist are higher compared with most of the schistose amphibolite samples of nigeria which are of tholeiitic affinity. however, for the quartz schist they are within limits for metasedimentary rocks (okonkwo and winchester, 1992.) the average values of mgo and tio2 for the quartz schist are low (both less than 0.06%) similarly, the average value for these oxides for amphibole schist at 7.17% and 0.29% respectively are also lower than those of tholeiitic basalts of nigeria and those of the average values for holleindain archean metabasalts (jahn et.al, 1974). average value for the quartz schist is 0.13% while that of the amphibole schist is 0.28%. provenance of the protolith and tectonic setting the na2o/al2o3 against k2o/al2o3 plot (fig. 4) (garrels and mckenzie, 1971) shows the entire quartz schist samples plot in the sedimentary/metasedimentary field while the amphibole schists plot in the igneous /metaigneous field. this 125 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria 0 0.05 0.1 0.15 0.2 0.25 0.3 0 0.1 0.2 0.3 quartz schists amphibole schists sedimentary/metasedimentary igneous k o\ai o2 2 3 n a o \a io 2 2 3 figure 4. na2o/al2o3 vs. k2o/al2o3 plot for quartz schist and amphibole schist of ibadan (after garrels and mckenzie, 1971). fransician greywacke 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 20 40 60 80 100 cao (wt%) s io (w t% ) 2 figure 5. sio2-cao plot for quartz schists of ibadan. (field of fransician greywacke after brown et al., 1979). symbols as in figure 2. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees 126 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke table 4. major element (%) and trace element (ppm) compositions of samples of amphibole schist from ibadan area. r8 r9 r10 r11 r12 r13 r14 sio2 53.2 53.81 53.29 52.98 53.75 53.53 53.99 al2o3 10.89 13.44 12.11 12.39 11.02 12.09 12.22 fe2o3(t) 9.76 9.63 9.75 9.66 9.71 9.62 9.74 mno 0.179 0.168 0.169 0.174 0.177 0.169 0.179 mgo 7.07 7.26 7.3 7.11 7.17 7.22 7.03 cao 14.74 11.29 12.98 14.57 13.29 11.36 12.89 na2o 2.04 3.3 3.1 2.54 2.53 3.06 2.16 k2o 0.85 0.39 0.62 0.44 0.79 0.83 0.42 tio2 0.262 0.305 0.283 0.307 0.287 0.278 0.263 p2o5 0.06 0.06 0.06 0.06 0.06 0.06 0.06 loi 1.314 0.778 0.636 0.199 1.046 0.649 1.283 total 100.4 100.4 100.298 100.43 99.83 98.866 100.235 ba 93 27 60 56 87 33 64 ce 24.2 10.8 18 25 10.5 22 12 co 53 50 52 49 50 59 48 cr 860 710 790 850 700 860 730 cu 40 50 50 40 40 60 50 ga 10 11 10 10 12 11 10 la 9 4 7 6 9 5 5 nb 1 1 1 1 1 1 1 nd 10 6 8 10 6 8 7 ni 250 180 220 195 230 198 232 pb 13 5 6 9 12 8 10 rb 13 4 9 4 11 10 8 sn 2 1 1 1 1 2 1 sr 111 91 101 95 106 103 100 th 4.4 0.7 2.8 4.0 0.9 3.1 1.9 v 144 207 189 203 158 177 150 y 9 11 13 9 13 9 7 zn 310 110 110 205 302 253 180 zr 105 33 81 45 101 39 79 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees suggests clear distinct genesis for the protoliths for these rock units. the provenance of sedimentary rocks is usually inferred from the framework constituents of the rocks (pettijohn, 1975; potter, 1978; dickinson and vallani, 1980; okonkwo, 2005) from the relatively low content of ba, rb and sr in samples of the quartz schist (table 3) there is a strong possibility of a sedimentary source depleted in arkosic composition, although the contribution on of a felsic source is not ruled out (van de kamp and leake, 1986; okonkwo and winchester, 1996; okonkwo, 2005). this is further supported by the plot of samples outside and above the field of franscisian greywacke on the sio2 versus cao diagram of brown et al, 1979. (fig. 5). this shows that the protolith is not of a greywacke type. the plot of log (fe2o3/k2o) versus log (sio2/al2o3) diagram (herron, 1988) (fig. 6), shows most of the quartz schist plot in the 127 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria quartz arenitesubarkose sublitharenite arkose wacke shale fe-shale fe-sand 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 litharenite log(sio \ai o )2 2 3 l o g (f e o \k o ) 2 2 2 3 figure 6. log (fe2o3/k2o) vs. log (sio2/al2o3) for the quartz schist of ibadan (after herron, 1988). symbols as in figure 2. 40 50 60 70 80 0 2 4 6 8 10 12 14 (wt%)sio2 basaltic trachyandesite basalt basaltic andesite andesite trachyte phonolite tephriphonolite phonotephrite tephrite trachy andesite foidite trachybasalt picro basalt dacite n a o + k (w t% ) 2 2o figure 7. na2o+k2o vs. sio2 (total alkali vs. silica,) plot for the amphibole schist of ibadan (after le maitre et al., 1989). symbols as in figure 2. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees quartz sandstone field thus defining a quartz sandstone protolith for the samples. for the amphibole schist, the total alkali versus silica plot of le maitre et.al. (1989) (fig. 7) reveal samples plotting in the basaltic andesite field indicating a protolith intermediate between basalt and andesite in composition. the bold dash lines in figure 7 shows that the amphibole schist all plots in the subalkaline series (macdonald, 1968). the na2o+k2o-fe2o3(t)-mgo ternary plot (fig. 8) after kuno (1968) further discriminates the samples as having a calc alkaline affinity, this is further confirmed by the k2o versus 128 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke calc alkaline series tholeiitic series mgona2o+k o2 fe o2 3total figure 8. ternary plot of molecular proportions na2o+k2o-fe2o3total-mgo for the amphibole schist (after kuno, 1968). symbols as in figure 4.8. 40 45 50 55 60 65 70 75 80 0 1 2 3 4 5 k o (w t% ) 2 sio (wt%)2 high-k (calcalkaline) series calc-alkaline series low-k (tholeiitic) series figure 9. plot of k2o vs. sio2 for the amphibole schist (after le maitre et al., 1989) symbols as in figure 4.8. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees 129 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria oceanic island arcpassive margin continental island arc active continental margin -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 d is cr im in an t f u n ct io n 2 discriminant function 1 figure 10. a discriminant functional plot for the quartz schist, showing the tectonic fields, (after, bhatia, 1983). symbols as in figure 4.8. discriminant function 1= -0.0447 sio2 – 0.972 tio2 + 0.008 al2o3 – 0.267 fe2o3 + 0.208 feo – 3.082mno + 0.140mgo + 0.195 cao + 0.719 na2o – 0.032k2o + 7.510 p2o5 + 0.303 discriminant function 2= -0.421 sio2 + 1.988 tio2 – o.526 al2o3 – 0.551 fe2o3 – 1.610 feo + 2.720 mno+ 0.881 mgo -0.907 cao – 0.177 na2o – 1.840 k2o + 7.244 p2o5 + 43.57 oi morb iac c sci mgo ai o2 3 fe o2 3 figure 11. ternary plot of al2o3-mgo-fe2o3 molecular proportions of the amphibole schist. diagram is after pearce et al., 1977. symbols as in figure 4.8. c continental iac island arc and active continental margin morb mid-oceanic ridge basalt oi ocean island sci spreading centre island enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:23 p p p composite 133 lpi at 45 degrees 130 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke table 5. chemical index of alteration (cia) % of quartz schist and amphibole schist. sample number cia (%) rock name r1 52 quartz schist r2 76 quartz schist r3 49 quartz schist r4 51 quartz schist r5 52 quartz schist r6 55 quartz schist r7 66 quartz schist average 57 sample cia (%) rock name r8 38 amphibole schist r9 47 amphibole schist r10 42 amphibole schist r11 41 amphibole schist r12 40 amphibole schist r13 44 amphibole schist r14 44 amphibole schist average 42 table 6. index of compositional variability icv % of quartz schist and amphibole schist of ibadan area. sample designation icv rock name r1 1.25 quartz schist r2 0.60 quartz schist r3 1.42 quartz schist r4 1.64 quartz schist r5 1.36 quartz schist r6 1.20 quartz schist r7 0.86 quartz schist average 1.20 sample designation icv rock name r8 3.19 amphibole schist r9 2.39 amphibole schist r10 2.81 amphibole schist r11 2.79 amphibole schist r12 3.07 amphibole schist r13 2.68 amphibole schist r14 2.66 amphibole schist average 2.80 plagioclase al o2 3 k o2cao+na o2 k-feldspar smectite average archean mudstone average ibadan quartz schist average ibadan amphibole schist av shale post archean upper crust kaolinite illite figure 12. ternary plot of molecular proportions al2o3-(a)-cao+na2o(cn)-k2o(k) for the rocks (after mclennan et al., 1995). upper crust composition and average post archaean shale from taylor and mclennan, 1985. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:24 p p p composite 133 lpi at 45 degrees sio2 plot (fig. 9) after le maitre et al (1989).this shows that the amphibole schist of ibadan area have a distinct petrogenetic character from most of the amphibolitic rocks of the basement complex of nigeria which are mostly tholeiitic with archean metabasalt affinity (ajayi et.al., 1980; elueze, 1980; okonkwo, 1992; okunlola and elueze, 2003; okunlola et.al., 2006) tectonically, the quartz schist samples plots as passive margin rocks on a discriminant function diagram after bhatia (1983). (fig. 10) the amphibole schists samples on the other hand, plot in the ocean island field on the mgo-fe2o3 (t)-al2o3 ternary plot after pearce et al (1977) (fig. 11). source rock weathering and sediment maturity of the protolith the chemical index of alteration (cia) (nesbitt and young 1982) defined as cia= [al2o3/(al2o3 + cao + na2o + k2o)] *100 has been used to determine the degree of weathering of the protolith of the rocks in the study area. the quartz schist samples have an average cia value of 57% (table 5) whereas those of the amphibole schist record an average value of 42%. this indicates a relatively moderate weathering of the source rocks. in comparison, quartz schists from igarra area have an average cia value of 53.9% (okeke and meju, 1985) and those of jakura area (elueze and okunlola, 2003) have an average value of 62.1%. therefore the protoliths of the ibadan quartz schists show fairly higher degree of weathering than those of the igarra schist and conversely a lower degree than those of the jakura schists. the differential weathering degree is further confirmed by the ternary plot of molecular proportions: al2o3 (a) cao + na2o (cn) k2o (k) (fig. 10) after mclennan et al. (1985), the quartz schists plot between the post-achaean shale and the upper crust compositions but very close to the average archaean mudstone (above the plagioclase – k-feldspar mid-line) indicating still a moderately strong weathering of the rocks. the amphibole schists on the contrary plot below the plagioclase – k-feldspar mid-line, indicating a lower degree of weathering relative to the quartz schists. from the index of compositional variability (icv) = (fe2o3 + k2o + na2o + cao + mgo + tio2)/ al2o3 (wt %) (cox and lowe, 1995) which measures the abundance of alumina relative to other major constituents of the rock except sio2, average values of 1.20 and 2.80 for the quartz schists and for the amphibole schists respectively were calculated (table 6). normally, compositionally immature pelitic rocks will have high icv, whereas mature pelitic rocks with very little non-silicate clay minerals will possess low values (<0.6) (elueze and okunlola, 2003). the value for the quartz schist show moderate to high compositional maturity which is a reflection of the moderate intensity of weathering of the source rocks for the sediments prior to metamorphism. mature to moderately mature pelitic metasediments (such as quartz schist) are characteristics of relatively stable cratonic environments (weaver, 1989; elueze and okunlola, 2003). conclusions petrographic and chemical data earlier described strongly indicates a sedimentary protolith, probably quartz sandstone, for the quartz schists. the quartz sandstone was derived from the moderate to fairly intense weathering of granitic rocks. quartz sandstones are generally regarded as formed from recycled sediments or from materials derived from weathering under low relief conditions as well as under low rates of sedimentation. such conditions are obtained in stable cratonic environment like the passive margin. it has been shown that first cycle quartz schists can be formed under a unique combination of tectonic and climatic conditions. these include prolonged transport involving slow rate of sedimentation or prolonged alluvial storage with low relief and severe tropical weathering conditions (weaver, 1989). mature pelitic rocks may also be produced by very strong chemical weathering of first cycle materials. the quartz schist of ibadan area are believed to be metamorphosed products of moderate to slightly intensive weathered felsic source rocks, subjected to prolonged transport, sorting and deposition under stable cratonic environment. on the other hand, an igneous protolith, viz basaltic andesite is indicated for the amphibole schists. these metasedimentary rock is a metamorphosed extrusive mafic volcanic of calc alkaline affinity distinct from what has been deduced for amphibolitic rocks of other parts of the schist belt of nigeria .they may have evolved from a deep crustal source and emplaced in a domain of active sedimentation. acknowledgements this study has benefited from the numerous assistances of colleagues, relatives and students. these contributions are duly acknowledged. 131 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:24 p p p composite 133 lpi at 45 degrees references ajayi, t.r. (1980) on the origin and geochemistry of amphibolites of ife-ilesha area, southwestern nigeria. journ. min. and geol, 17:170-195 ajibade, a.c. 1972 provisional classification and correlation of the schists belt northwestern nigeria. in geology of nigeria, kogbe c.a (ed.). elizabethan pub. co., lagos, pp 85-90. bhatia, m.r. 1983 plate tectonics and geochemical composition of sandstones. j. geol. 91, pp 611-627. brown, e.h., babcock, r.s. and clarke, m.d., 1979 of geology of precambrian rocks grand canyon. prec. res. pp 219-141. caby, r.m. 1989 precambrian terranes of benin, nigeria and northeast brazil and the late south atlantic fit. geological society of america. special paper 230; pp 145-158. cox, r. and lowe, d.r. 1995 controls on sediment composition on a regional scale. conceptual view: j. sed. res v.65; pp1-12 dickinson, w.r. and valloni, r., 1980 plate settings and provenance of sands in modern ocean basins. geology 8, pp 82-86. ekweme b.n., and shing,r.,1987 occurrence, geochemistry and geochronology of mafic-ultramafic rock in the obudu plateau, south eastern nigeria in srivasta, r.k. and chadta, r., (eds)magmatism in relation to diverse tectonic settings pp42-57. elueze, a.a. 1980 dynamic metamorphism and oxidation of amphibolites of tegina, area, northwestern nigeria. prec.res: vol. 14, pp.368-379 elueze, a.a. 1991 rift system for proterozoic schist belt in nigeria. tectonophysics; vol. 209 pp 167-169 elueze, a.a. and okunlola, o.a. 2003 petrochemical and petrogenetic characteristics of metasedimentary rocks of lokoja-jakura schist belt, central nigeria in elueze a. a (ed.), journal of mining and geology vol. 39(1), pp 21-27. garrels, r.m. and mckenzie, f.f 1971 evolution of sedimentary rocks. wm norton and co., new york, 394p. herron, m.m.,1988 geochemical classification of terrigenous sands and shales from core log data. j. sed. petrol., 58; pp 820-839. kuno, h., 1968 differentiation of basalt magmas. in: hess h.h. and poldervaart (eds.), basalts: the poldervaart treatise on rocks of basaltic composition. vol. 2, interscience, new york, pp 623-688. le maitre, r.w., bateman. p., dudek, a.m., keller, j., lameyre, le bas m.j., sabine, p.a., schmid, r., sorensen, h., streckensen, a., wolley, a.r., and zanettin, b.,1989. classification of igneous rocks and glossary of terms. blackwell, oxford. 240p macdonald, g.a. 1968 composition and origin of hawaiian lavas. (coats, r.r, hay, r.l., and anderson, c.a. eds., studies in volcanology: a memoir in honour of howell williams. geol. soc. amer. mem 116, pp 477-522. mclennan, s.m., hemming, s.r., taylor, s.r., and erickson, k.a., 1985 early proterozoic crustal evolution: geochemical and nd, pb isotopic evidence from metasedimentary rocks, southnorthern america. geochimica et cocmochimica, acta 59; pp 1153-1127 nesbitt, h.w. and young g.m. 1982 early proterozoic climates and plate motions inferred from major element chemistry of lutites. nature 199, pp 715-717. odeyemi, i.b.,1977. the basement rocks of bendel state of nigeria. unpublished phd. thesis. university of ibadan, nigeria. okonkwo, c.t 1992 structural geology of basement rocks of jebba area, nigeria. journal of mining and geology, vol 28 (2) pp 203-209. okeke, p.o. and meju, m.a. 1985 chemical evidence for the sedimentary origin of igarra supracrustal rocks s.w. nigeria. journal of mining and geology 22, pp 97-104 okonkwo,c.t., 2005 geochemistry of quartzites and quartz mica schists in jebba area, south western. nigeria journal of mining and geology vol. 41(2), pp 163-173. okonkwo, c.t. and winchester, j.a, 1996 geochemistry and geotectonic setting of precambrian amphibolites and granitic gneisses in the jebba area, southwestern nigeria. j. min. geol. 32(1), pp11-18. okunlola, o.a. (2001) geological and compositional investigation of precambrian marble bodies and associated 132 o.a. okunlola, o.c. adeigbe and o.o.oluwatoke enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:24 p p p composite 133 lpi at 45 degrees rocks in the burum and jakura areas, central nigeria. phd thesis, university of ibadan, nigeria. 193p. okunlola, o.a. (2005) mettalogeny of precambrian pegmatites of nigeria. mineral wealth 143/41 p 42-45. okunlola, o.a. akintola, a.i. and egbeyemi, r.o., 2006 compositional features and petrogenetic affinity of precambrian amphibolitic schists of sepeteri area, southwestern nigeria.global journal of geological sciences 4.2:199-207. oyawoye, m.o. 1972 the basement complex of nigeria, in, dessauvagie t.f.j. and whiteman a.j. (eds.) africa geology, university of ibadan., pp 66-82. pearce, t.m., gorman, b.e. and birkett, t.c. (1977) the relationship between major element chemistry and tectonic environment of basic and intermediate volcanic rocks. earth. planet. sci. lett., 36, pp 121-132. of the northwestern pacific margin pettijohn, t.j. 1957 sedimentary rocks. harper and brothers, new york, 718pp potter, p.e. 1978 petrology and chemistry of modern big river sands. j. geol; 86, pp 423449. rahaman, m.a., (1976) review of the basement geology of southwestern nigeria. in; kogbe, c.a. ed) geology of nigeria, elizabethan publication company, lagos van de kamp, p.c. and leake, b.e., 1986 petrography and geochemistry of feldspathic and mafic sediments. trans. roy. soc. edin. earth sci. pp 411-499 weaver, c.e 1989 clays, muds and shales. elsevier, amsterdam, 820p. 133 compositional and petrogenetic features of schistose rocks of ibadan area, southwestern nigeria enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:24 p p p composite 133 lpi at 45 degrees geociencias 15-1 julio 2011.vp tunnelling in urban areas by epb machines: technical evaluation of the system marilena cardu, pierpaolo oreste politecnico di torino, land, environment and geo-engineering department, italy igag-cnr, torino, italy e-mail: marilena.cardu@polito.it abstract the paper refers to the methods adopted for building a high-speed railway tunnel system between bologna and firenze (italy), focusing attention on the bologna node which represents the heart of the system, connecting the high speed network’s main lines. the project includes 9 tunnels, accounting for 73 km of the 78 km route crossing below the apennines. the paper pays attention to the main aspects to be taken into consideration for correctly choosing the tunnel boring machinery (tbm) to be used in urban areas. the fundamental point in analysing technical aspects regarding an earth pressure balance (epb) machine concerned storing the main excavation parameter values; having collected and organised such data, statistical methods were used for processing it, the instantaneous velocities attained were empirically estimated and idle times were evaluated. the evaluation was made by calculating excavation specific energies (during different excavation phases) to find a satisfactory correlation with the type of ground crossed. interesting results have been found by comparison with other excavation parameters; in particular, a better understanding of an earth pressure balance shield’s working phases has been reached thanks to an experimental study conducted during the construction of tunnels for a high-speed railway system in italy. the paper contains details collected regarding the operation of two different epb machines. resumen este artículo se refiere a los métodos utilizados para la construcción de túneles para un sistema de trenes de alta velocidad entre bologna y firenze (italia), el punto de interés está sobre el nodo de bologna, como el corazón de sistema, conectando las líneas principales de la red de alta velocidad. el proyecto incluye nueve túneles, con 73 de los 78 km cruzando por debajo de los apeninos. este artículo presenta los principales aspectos a tener en consideración para la correcta selección de máquinas tuneladoras (tbm) utilizadas en las áreas urbanas. el fundamento en el análisis de los aspectos técnicos consiste en un balance de presión de tierra (epb) de la máquina relacionado a los principales parámetros en la excavación; una vez recogido y organizado los datos, se realizaron análisis estadísticos, se estimaron las velocidades empíricamente y evaluaron los tiempos de espera. la evaluación fue realizada para el cálculo de las energías específicas de excavación (en diferentes fases de perforación) para encontrar una correlación satisfactoria con el tipo de terreno atravesado. los resultados obtenidos son interesantes en comparación con parámetros de otras excavaciones; en particular, una mejor compresión en el balance de la presión de tierra en cada fase de trabajo, ha sido descrita gracias a un estudio experimental realizado durante la construcción de túneles para un sistema de trenes de alta velocidad en italia. este artículo contiene información detallada recogida de la operación de dos maquinas epb diferentes. introduction research instruments’ evolution in the field of tunnel construction has contributed towards the improvement of excavation technologies and planning management, almost during the last 30 years. excavation techniques’ evolution has also been stimulated by growing urbanisation and the assimilation of the “underground” concept as a space resource, becoming more and more exploitable from various points of view. one of developed countries’ major needs nowadays concerns mobility. many projects have been promoted during recent years to improve interconnections between european countries. this has involved the problem of tunnelling a great part of the work (barla g., 1994). the best european example highlighting such need has been the eurotunnel project which involved constructing three underwater tunnels linking france and the uk. another important project which is still in execution is the high speed railway system; it will research groupin geophysics universidad nacional de colombia earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 5-11 keywords: earth pressure balance (epb), specific energy, excavation parameter, tunnel, tunnelling. palabras clave: balance presión tierra (epb), energía especifica, parámetro excavación, túnel. record manuscript received: 13/01/2011 accepted for publication: 15/05/2011 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:07 p p p composite 133 lpi at 45 degrees provide a more efficient and faster connection amongst european countries. a critical point has to be resolved when carrying out this work as rail layouts have to cross heavily urbanised areas (barla g., 2000; bilotta e. et al., 2002). mechanised shields offer a series of advantages and work can be carried out without having to support the roof. the equipment’s technical characteristics do not represent a basis for effective evaluation because two mechanically identical machines can behave in completely different ways, depending on soil characteristics. it is thus very difficult to make a reliable and unambiguous evaluation of one machine’s best performance compared to that of another (bebendererde s., 1995; bovetti b., 2003). the only way to resolve this point is to evaluate the equipment’s working parameters during excavation. the most important parameter concerns excavation specific energy which is often used for determining a machine’s performance (altindag r., 2003; copur h. et al., 2003; cardu m. et al., 2006; tardaguila i. et al., 2007; acaroglu o. et al., 2008; exadaktylos g. et al., 2008). defining the excavation specific energy concept excavation specific energy (es) represents the amount of energy (expressed in mj) needed to excavate a unit volume of ground (rostami j.,ozdemir l., 1993; friant j.e., ozdemir l., 1993). this may be given by: e w t v s tot� � (1) where wtot is total power, t is excavation time and v is excavated volume. the shield’s feed motion and the head’s rotation speed must be taken into account when evaluating energy consumption due to the excavation of a unit volume of ground. the general formula for calculating total excavation specific energy is given by the sum of these two contributions. feed motion contribution the parameters for calculating a machine’s feed motion are: • the thrust exerted by hydraulic jacks s • a machine’s advancing speed v • excavation time t. tunnel diameter must be known, as this is used to calculate a tunnel’s cross-section. the power needed for a machine’s advance (wadv) can then be obtained by using the following expression: w s v adv � � (2) the rate of excavated volume per time unit is: v a v t� � �( ) (3) where a is a tunnel’s cross-section and product v�t represents advance mad in time t. equation (1) can thus be written for feed motion specific energy contribution (es adv), as: e w t v s v t a v t s a s adv adv� � � � � � � � (4) rotation specific energy the data needed for calculating rotation are: • the torque necessary to rotate head c • the head’s rotation speed ù • tbm advance speed v. rotation power (wrot) is given by: w crot � �� (5) excavation specific energy consumed by head rotation (es rot) is thus: e w t v c t a v t c a v s rot rot� � � � � � � � � � � � (6) total specific energy this represents a machine’s behaviour in terms of non-uniform progression. equation (7) is obtained by substituting (1) in equations (4) and (6) (teale r., 1965): e e e s a c a v s tot s adv s rot. . .� � � � � � � (7) it should be noticed that total excavation specific energy is expressed by a formula which does not include time as a variable; it can thus be considered a feature characterising excavation in the stretch which has been calculated from the machine’s parameters. moreover, it can be observed that energy, as would be expected, grows with thrust, torque and rotation speed; excavation difficulty increases as these three factors also increase. excavation specific energy obviously decreases as progression speed increases. analysis of field performance data and discussion the following considerations were based on data collected when high-speed tunnels in the bologna node excavation were being cut by two identical epb tbms (cicala t., 2003; guidarelli d., 2005). table 1 gives the machines’ main characteristics. the first machine (epb1) worked at advanced chainage (about 1,500 m) compared to the other one (epb2). it is very important when analysing data to take into account the kind of ground in which work is being done. excavation specific energy shown in the following graphs has been represented as a function of route chainage to facilitate correlation with different types of ground identified by geological characterisation. epb1 es is represented in figure 1; it varies from a minimum value of around 15 mj/m3 to a maximum of 40 mj/m3. several types of ground are present in this range, particularly: • clay, from 1,500 m to 1,800 m chainage • moist sand, from 1,800 m to 2,100 m • dry sand, from 2,100 m to 2,400 m • dry gravel, from 2,400 m to 2,700 m. a negative es gradient was envisaged from 1,500 m to 2,100 m because of a machine’s progressive adaptation to local ground conditions. crossing from clay to sand around 1,800 m chainage was confirmed by decreasing es (a machine’s adaptation to lithological crossing). sands were initially on the crown and their presence increased as excavation progressed, making excavation conditions become worse, thereby stressing the need for a machine to become suitable to different kinds of ground. this was not observed on contact between 6 marilena cardu, pierpaolo oreste agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:07 p p p composite 133 lpi at 45 degrees moist and dry sands (around 2,100 m chainage) where a clear increase in es was envisaged. dry sands, being more compact than moist sands, would require a greater effort to be removed. sampling was not regular from 2,100 m to 2,350 m chainage due to some of the machine’s sensors malfunctioning; only a net increase in energy was identifiable (but it was impossible to establish whether such evaluation was completely reliable). sampling worked regularly again from 2,350 m. this stretch crossed gravels and contact was not abrupt. table 1: specifications for the epb lovat rme370se used in the bologna node excavation diameter 9.4 m epb length + back–up 180 m epb weight + back–up ~990 t theoretical maximum progression speed 8 cm/min minimum bend radius 250 m total thrust capacity 10,197 t total power 5,100 kw torque (standard conditions) 1,022 t·m at 1.97 rpm maximum torque 2,043 t·m at 0.98 rpm peak torque 2,452 t·m standard cutting power 2,700 kw analogous considerations could be made for epb2 excavation. figure 2 shows that es values were lower than those for epb1; this clearly stood out by comparing both machines’ performances at the same chainage (figure 3). unfortunately, the data for epb2 was only available for chainage between 1,500 m and 2,050 m. the phenomenon could have been due to stress release in the ground; the first tunnel (cut by epb1) was excavated between gravels and sands, leading to gradual adaptation by the machine, confirmed by a negative es. gradient. the original stress distribution of the ground was modified as the second tunnel (cut by epb2) was excavated in close proximity to the first one (less than one diameter), resulting in reduced es. the decrease in es was evaluated as being about 30%. this result could also have been related to reduced ground strength due to epb1 tunnel excavation; however, no data was available regarding the mechanical characteristics of the ground measured on site during the excavation to confirm this hypothesis. table 2 shows the main ranges of es values obtained in different kinds of ground for both epb1 and epb2. tunnelling in urban areas by epb machines: technical evaluation of the system 7 figure 1. epb1 excavation specific energy figure 2. epb2 excavation specific energy figure 3. comparing epb1 and epb2excavation specific energies agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:08 p p p composite 133 lpi at 45 degrees table 2. ranges of excavation specific energy values in different kinds of soil type of soil excavation specific energy (mj/m3) clay 25-35 moist sand 25-30 dry sand 32-38 dry gravel 15-25 excavation specific energy compared to progression speed progression speed is one of the most important parameters when analysing the excavation cycle (innaurato n., 1990; kovàri k., 2002). figure 4 compares the es with epb1 instantaneous progression speed at different chainages. according to equation (7), progression speed was inversely related to es, which was made evident by the plot (increased velocity coincided with decreased es). this was fully in line with the pertinent literature (graham p.c., 1976; nielsen b., odzemir l., 1993; o’rourke j.e. et al., 1994; rostami j., 1997; thuro k., plinninger r.j., 2003; gong q.m., zhao j., 2009). the two graphs’ normalised gradients were similar, apart from the opposite sign. this was expected because the change in speed obtained from the pushing jacks’ extension represented the effect of lithological variation (lovat r. et al., 2001). increased energy and decreased speed indicated greater resistance to excavation by the ground. excavation specific energy compared to torque figure 5 compares es to torque at different chainages for epb1. a direct correlation was apparent, as expected from equation (7), where the term directly proportional to torque prevailed. the correlation was confirmed by the plot and the gradient was similar for both variables. since estot was mostly dependent on rotational specific energy (specific energy due to thrust is smaller and is usually ignored) and torque was a function of the cutting force acting on the cutting tools, then such direct correlation would be expected (nielsen b., odzemir l., 1993; us army corps of engineers, 1997; boniface a., 2000; gong q.m. et al., 2007). excavation specific energy compared to rotation speed figure 6 shows the roughly constant value for rotation speed at different chainages compared to es variations for epb1. rotation speed and torque were roughly proportional to es and, as rotation speed was kept practically constant, energy was only affected by torque changes during excavation. es variation associated with rotation speed constancy showed that the latter was the externally controlled variable. an operator could set a constant level of rotation speed to maintain constant tool wear, while torque would change to cope with resistance to excavation produced by ground changes. on the other hand, if rotation speed were increased, then instantaneous penetration would decrease, and thus both the cutting force acting on the tools and torque would decrease. 8 marilena cardu, pierpaolo oreste figure 4. comparing epb1 excavation specific energies with instantaneous progression speed figure 5. comparing epb1excavation specific energies to torque figure 6. comparing excavation specific energies and rotation speed agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:09 p p p composite 133 lpi at 45 degrees excavation specific energy compared to total thrust figure 7 compares es to thrust at different chainages for epb1. according to equation (7), total specific thrust (thrust per unit cross-section) coincided with specific advance energy and was added to specific rotation energy, to give total es. thrust (as rotation speed) could be controlled by an operator and total thrust plot was scarcely correlated to total es plot, apart from some excavation specific energy peak having some counterpart in thrust peaks, probably representing attempts by an operator to raise progression speed in difficult ground (lovat r. et al., 2001; mair n., 1997). figure 8 gives specific thrust energy, specific rotation energy and total es (thrust energy was a small fraction of total energy). energy values were calculated statistically by the moving average method, assuming 60 m progression steps. figure 8 shows that rotation specific energy was comparable to total es throughout the chainage. only rotation specific energies’ contribution would thus be considered in the following. suggested evaluation methodology rotation speed (as shown in figure 6) made a very low contribution since it was kept almost constant by the operator. the most important parameters to be considered when calculating excavation specific energies were thus torque (which is a function of rotation speed, as in equation (5)) and progression speed (marcheselli p.p. et al., 1995; schmalzbauer s., 1984). knowledge of progression speed is determinant during mechanised full-section tunnel excavation to evaluate a machine’s performance in real time or at the end of a shift and to improve (if necessary) its performance regarding operator intervention related to inspectable parameters. obviously, if progression speeds were known (at least regarding a meaningful sample of excavated length), it would become possible to optimise a machine’s work and increase overall excavation time. it has been previously stated how little influence jack thrust has on evaluating total es. equation (7) can thus be simplified as: e c a v s � � � � (8) equation (8) shows that energy mainly depended on two parameters (torque and progression speed) by assuming constant rotation speed ù. the following equation was obtained by multiplying excavation specific energy by tunnel cross-section a: e a c v s � � � � � � (9) á was expressed in mj/m. equation (9) showed good correlation between es and coefficient á. the example in the graph shown in figure 9 referred to a 35 mj/m3es value, supplying a 2,360 mj/m á coefficient. therefore, if: � � � � � c v tcos (9’) then: c v� � �� � (10) tunnelling in urban areas by epb machines: technical evaluation of the system 9 figure 7. comparing epb1 excavation specific energies to thrust figure 8. comparing epb1excavation specific energies separately due to rotation and progression speeds figure 9. correlation between excavation specific energy and á coefficient agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:09 p p p composite 133 lpi at 45 degrees hence: v c � �� � (11) equation (11) shows that torque and progression speed were proportional dimensions, if rotation speed were considered as a constant. maximum torque is the most important parameter to be considered when making a decision during tunnel design stage for evaluating the most suitable type of machine. maximum torque data was provided by the tbm manufacturer. the graph in figure 10 gives an example of progression speed evaluated from torque data for epb1. assuming 2,500 t.m average torque at tbm cutter head and 1 rpm rotation speed, then a 1.05 mm/s net progression speed would be obtained as a reliable average value. analysis of a machine’s work performance can be gone into in more depth, leading to obtaining more accurate predictions. tbm work cyclically, through a sequence of well-defined elementary steps, requiring precisely known time intervals. each cycle consists of an excavation stage (with a machine advancing through the thrust of the jacks), a support erection stage (where a new lining ring is installed) and idle time devoted to maintenance. the sum of lining erection times and maintenance time represents the inactive time between successive ring installations and is repeated at each ring. at the end of a shift, therefore, elapsed excavation time is much shorter than total elapsed time. the ratio between the actual excavation time and total time, given by: �� t t exc total (12) represents a machine’s exploitation coefficient (machine utilisation time). when ç and instantaneous progression speed are known, a machine’s daily (24 hours) progression can be calculated as: v t v day total inst� � � �� 3 6. (13) where vday is daily advance rate (m/d), ttotal is total working time (hours/day) and vinst is the net advance rate (mm/s). exploitation (machine utilisation time) coefficient ç is obviously not known a priori. in the given example this has been calculated by solving the equation for ç, daily progression speed being known as it can be obtained from measured daily advance. however, this only gives a machine’s utilisation time factor; every contractor should predict this factor based on experience, a particular project site and conditions prior to construction activity. instantaneous progression speed can be obtained by averaging the data stored in a machine’s databank files for the 24-hour period being considered. this would then give: �� � � v t v day total inst 3 6. (14) the average progression speed calculated for both epb1 and epb2 was 0.99 mm/s; a 0.157 exploitation coefficient was then found from these values, meaning that 15.7% of total 24 hours/day working time had been spent in pure excavation. total time was assumed to be 24 hours and, from the exploitation coefficient, a 3.8 hours/day excavation time was thus calculated. total inactive time thus amounted to 20.2 hours/day; this was a very high value, showing the need for optimising working stages. by dividing the 20.2 hours by the number of rings, it was found that the time elapsed was 2.2 hours between the end of the erection of a ring and the start of the erection of the next one. part of this time was spent on assembling the ring, which takes about 40 min, and the remaining part was consumed in ordinary maintenance, including time spent waiting for the hardening of the mortar that connects the segments of the ring and maybe some other breakdowns. the results are shown in table 3. table 3. mean daily advancement rate values obtained with 9 rings/day for epb1 mean daily progression, m 13.5 mean instantaneous speed, mm/s 0.99 working hours in a day 24 exploitation coefficient ç 0.157 pure excavation time, hours 3.8 total inactive time, hours 20.2 time to assemble a ring, min 40 maintenance time, hours 14.2 mean maintenance time/ring, hours 1.58 conclusions comparing the most important excavation parameters led to interesting results for better understanding of earth pressure balance shields’ working phases. in particular, the following considerations should be highlighted: • excavation specific energy es did not appreciably depend upon thrust; rotation velocity too, being constant during drive, did not seem to substantially influence evaluation of excavation specific energy; • excavation specific energy es was directly correlated to torque; 10 marilena cardu, pierpaolo oreste figure 10. correlation between torque and progression speed agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees • ground stress release appreciably modified excavation specific energy values. in this case, this was the excavation of a second tunnel by machine epb2 at a distance of less than one diameter from the first tunnel which had already been built by epb1, occurring with a 30% es reduction; • excavation specific energy es varied widely with ground characteristics: a large variation in excavation specific energy ranges was in fact observed as a function of the subsoil encountered, from 25-35 mj/m3 for clay, to 15-25 mj/m3 for dry gravel, to 32-38 mj/m3 for dry sand; and • exploitation coefficient ç was low and varied from 0.10 to 0.20 for the project being analysed. progression speed was one of the main parameters to be considered during excavation. regarding this point, an interesting method could be suggested, namely automatic continuous diagraphy (dac). through an appropriate scale factor it enables estimating a given machine’s daily progression speed (progression speed having already been defined) to establish the necessary torque c to be applied by a tunnelling machine. references barla, g. (1994): “scavo di gallerie in prossimità della superficie”. v ciclo di conferenze di meccanica e ingegneria delle rocce (mir). torino. barla, g. (2000): “lo scavo meccanizzato delle gallerie”. viii ciclo di conferenze di meccanica e ingegneria delle rocce (mir). torino. bilotta, e., g. russo and c. viggiani (2002): “cedimenti indotti da gallerie superficiali in ambiente urbano”. xxi convegno nazionale di geotecnica: opere geotecniche in ambiente urbano. l’aquila. bebendererde, s. (1995): “advanced techniques for epb-tbm operation”. tunnel et ouvrages souterrains, 128. bovetti b. (2003): “studio della subsidenza indotta dallo scavo meccanizzato della metropolitana di torino attraverso modellazione numerica e back-analysis”. tesi di laurea, politecnico di torino, torino. altindag r. 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(2009): “in situ tbm penetration tests and rock mass boreability analysis in hard rock tunnels”. tunnell undergr space techn 2007; 22, 303–16. us army corps of engineers (1997): “engineering and design—tunnels and shafts in rock”. cecw-eg engineer manual. appendixc: tunnel boring machine performance concepts and performance prediction. boniface a. (2000): “tunnel boring machine performance in basalts of the lesotho formation”. tunnell undergr space tech. 15, 49–54. lovat, r. and m. giorelli (2001): “design criteria for two state-of-the-art large diameter epb tbms, passante ferroviario di bologna tunnel project, italy”. proc. world tunnel congress aites-ita. mair, n. (1997): “principio di funzionamento di uno scudo ss ed epb”. gallerie e grandi opere in sotterraneo, 55. marcheselli, p.p. and a. nicola (1995): “criteri di scelta di macchine di scavo per la realizzazione di tunnel. il caso particolare del passante ferroviario di milano”. giornata di studio sullo scavo meccanizzato integrale delle gallerie: progettazione integrata e criteri di scelta delle macchine. naz. congr. sig. roma. schmalzbauer, s. (1984): “scavi di gallerie in terreni soffici con gli scudi”. gallerie e grandi opere in sotterraneo, 19. tunnelling in urban areas by epb machines: technical evaluation of the system 11 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees geofisica colombiana n° 6 pp 24-27 diciembre de 2002 bogota, d.c. issn 0121 2974 monitoreo de emisiones de gas radon vinculado a la actividad microsismica en la zona de sampacho, cordoba, argentina virginia grosso, romina daga, guillermo sagripanti y diego villalba departamento de geologia. universidad nacional de rio cuarto. agencia postal 3. cp x5804zab rio cuarto. cordoba. arg entina. email: mvgrosso@exa.unrc.edu.ar resumen en el sur de la provincia de cordoba, region central de la republica argentina, se encuentra la zona pleistoseista sampacho, donde se ubica la falla activa las lagunas, que probablemente genero la crisis sismica del lode junio de 1934, cuando ocurrieron dos terremotos destructivos de m5.5 y m6.0 grados. las modificaciones en las emisiones del gas radon (rn 222) y los aumentos en la ocurrencia de microsismos son considerados geoindicadores precursores en areas bajo amenaza sismica. en la etapa presismica de terremotos ocurridos en japon, estados unidos, italia, mexico y la antigua union sovietica, se han detectado anomalias en los niveles de ernision de este gas, como asi tam bien incremento de la actividad microsismica. el objetivo del presente trabajo es monitorear las emisiones de gas radon para establecer su vinculo con la actividad microsismica en proximidades de una seccion de la falla activa las lagunas, por medio de la aplicacion de la tecnica del track-etch, e intentar establecer un geoindicador precursor de sismos para la misma. como conclusion, se comprueba que existe vinculacion entre los cambios en la ernision del gas radon y la actividad sismica local, asi como entre la variacion del nivel de ernision y el tamaiio de los sismos leves superficiales, que han ocurrido en el periodo de registro. palabras clave: falla; terremoto, radiacion alfa; track etch abstract in the south of cordoba province, argentine, the pleistocene zone ofsampacho is located, where the active fault las lagunas is situated. this probably generated the seismic crisis of 10th june 1934, with the occurrence of two earthquakes ofm5.5 and m6. modifications in radon gas emissions (rn 222) and increases in the microseismic occurrence are considered as precursor geoindicators in areas under seismic hazard. anomalies in this gas emission levels, as well as an increase in the microseismic activity were detected in the pre-seismic stage of the earthquakes that occurred in japan, usa, italy, mexico and the former soviet union. the objective of the present paper is to monitor the radon gas emissions in order to establish its relation to the microseismic activity in an area nearby a section of the active fault las lagunas, by means of the track-etch technique and to try to establish a seismic precursor geoindicator for it. as a conclusion, it was proved that there exist relations between the changes in the radon gas emission and the local seismic activity, and between the variation of emission level and the size of superficial light earthquakes, that have occurred in the record period. key words: fault, earthquake, alpha-radiation, track-etch ubicacion ei area de estudio se ubica en el sector so del departamento rio cuarto, al sur de la provincia de c6rdoba, regi6n central de la republica argentina, entre los 64° 30' y 65° 00' de longitud oeste y 33° 30' y 34° 00' de latitud sur aproximadarnente, (figura 1). introduccion la regi6n de estudio se encuentra en el area sismotect6nica de las sierras de c6rdoba y san luis, aproximadamente a 650 km hacia el este de la zona de subducci6n (fosa chilena). se piensa que la deformaci6n ne6gena en la precordillera y las sierras pampeanas tiene 24 geofislca colombiana, 6. oiciembre de 2002 monitoreo de emisiones de gas radon vinculado a la actividad micros is mica en la zona de sampacho, cordoba, argentina iu:pubu ca a rgenti na provincia de cordoba r<--i---45° 65° 64' 63° 62° 35 ~ area de estudio figura i. ubicaci6n area de estudio. coincidencia con el desarrollo de una losa plana de subduccion, alrededor de 10 a 15 ma, aunque la fase principal de movimiento es asignada al plioceno-pleistoceno, su continuaci6n en el holoceno es evidenciada por la sismicidad, que se concentra en la latitud 33s, (en costa, 1996) y en la ocurrencia de terremotos destructivos de junio de 1934, en sampacho, y mayo de 1936 en san martin (san luis), al se de las sierras pampeanas. en la zona pleistoseista sampacho, se identifican numerosas falias geologicas, entre elias la falla activa denominada las lagunas. esta se ubica a 15 km al oeste de la localidad de sampacho con un rumbo de 0350 0400 n y caracter inverso con plano buzante al se. presenta una escarpa recti linea de aproximadamente 24 km de longitud, alcanzando en algunas secciones 9 m de altura en materiales loessicos, que genera una importante barrera at escurrimiento superficial, evidenciada por la presencia de una serie de lagunas alineadas, como suco, seca, dumbull, goyo y chachaue (sagripanti et al.,2001). la reactivacion de esta estructura probablemente genero la crisis sismica dell 0 de junio de 1934, con la ocurrencia de dos terremotos de m5.5 y m6.0, e intensidad epicentral de ix grados (mm), considerados dos de los mas destructivos ocurridos en la zona de sismicidad reducida de argentina. ademas de los terremotos historicos destructores anteriormente mencionados, la sismicidad superficial en la region es frecuente, ya que en ella han ocurrido numerosos movimientos que superan valores de m4.0, y una actividad microsismica que, en promedio, supera geofislca colombiana, 6, oiciembre oe 2002 los 9 eventos par afio (sagripanti et al., 1998). ei inventario de pararnetros sismicos recopilado de los eventos ocurridos desde el 1826 hasta la actualidad indica que estos son mas de 130. a partir de un operativo sismol6gico movil, se determin6 que la profundidad de la capa sismoactiva tiene entre 12 y 15 km (mingorance, 1991). las fluctuaciones en algunos parametres fisicos de las rocas pueden utilizarse como geoindicadores precursares en areas bajo amenaza, como modificaciones de la velocidad de la onda sismica p, deformaciones y basculamiento de la superficie, variacion de la resistividad electrica de los materiales, aumento en la ocurrencia de microsismos y modificaciones en las emisiones del gas radon (bolt, 1981). como ejemplo de esto ultimo, podemos citar los terremotos de tangshan en 1976 (m7.8); 1rpinia, ltalia en 1980 (m6.6), izu-oshima-kinkai, japon, en 1977 (m7.0); mexico en 1985 (ms8.1) y kobe, japon en 1995 (ms 7.2), en el que, con anterioridad al evento principal, se registr6 un aumento de las emisiones 20 veces mayor que los val ores normales. se estima, a partir del estudio geol6gico-estructural de la zona, de los antecedentes de sismicidad historica y actual, y de las evidencias paleosisrnologicas y neotectonicas, que esta posee un importante potencial sismogenico. por ello se escogi6 una seccion de la falla activa las lagunas para desarrollar los estudios del presente trabajo, cuyo objetivo es monitarear las emisiones de gas radon para establecer si tienen vinculacion con la actividad microsismica generada en proximidades de esta seccion, e in ten tar establecer un geoindicador precursor de sismos para la misma. metodologia la metodologia utilizada para la medicion de las emisiones de gas radon es la tecnica del track-etch, la cual consiste basicarnente en el registro, par medio de detectores (peliculas de nitrocelulosa sensibles a la radiacion alfa, tipo lr-115), de las trazas radiactivas emitidas por la desintegraci6n del gas radon, las particulas alfa emitidas se difunden y transportan a traves de suelos parosos y fracturas de las rocas, dejando a su paso trazas en los detectores colocados a una profundidad de aproximadamente 65 em (sagripanti et ai., 1998). ei tiempo de exposicion es 30 dias aproximadamente, luego del cuallos films impactados son sometidos a un bombardeo quimico para aumentar el tamafio de las trazas. el conteo de las mismas se efectua con un microscopio optico y contador de puntos, calculando el promedio de impactos, el cual se expresa en nurnero de impactos por mnr' por dia, para tal fin se estableci6 una red monitora cuya disposici6n establece un perfil transversal a la traza de la seccion en estudio, consta de cuatro estaciones ubicadas en la base de la escarpa, en los bloques elevado y hundido, la cuarta en la parte central del bloque elevado que opera como estacion de control, para obtener el regis25 grosso et al. tro de un sector mas estable. se denominan fll2, fll3, fll4 y fllc, respectivamente, figura 2. n ® 0.5 0 1km referencias _____ 560 gurva de nivel directriz 0 lagunas humedal camino vecinal arroyos fll2 red monitorao figura 2. ubicacion de la red monitora. ei periodo de monitoreo fue entre los meses de mayo de 200 i a julio de 2002, durante ei cual se tomaron catorce muestreos. ei retabla i. planilla resumen gistro de la actividad sismica local ocurrida durante el mismo se lievo a cabo por medio de un inventario de parametros macrosismicos e instrumentales. los datos obtenidos del conteo, numero y distribucion temporal de los eventos ocurridos fueron tratados y representados graficamente para analizarlos e interpretarlos posteriormente. resultados en la planilla resumen (tabla i), se presentan los datos de los registros de impactos generados por la radiacion alfa, en el periodo en estudio. la actividad microsismica superficial en la zona durante este periodo fue 4 eventos, de los cuales los dos primeros alcanzaron m3. i (05/01/02) ym3.2 (20/02/02), y los restantes 14/07/02 y 20/07/02 se estima fueron de m<2.0, ya que no fueron registrados con el sismografo del inpres (instituto nacional de prevencion sismica), emplazado en la zona de estudio, el cual esta calibrado para registrar eventos de magnitudes mayores que esta. sobre la base de los datos obtenidos de los conteos se elaboro un grafico que representa las variaciones en las emisiones de radon en el periodo de muestreo y la ocurrencia de los eventos registrados (figura 3). discusion y conclusiones del analisis e interpretacion de los resultados se observe un aumento en los valores de ernision del gas radon y una posterior disminucion, previa a la ocurrencia de los eventos sismicos, 10 cual evidenciaria la vinculacion que existe entre la emision y la ocurrencia de microsismos. se estima que la amplitud de esta variacion estaria relacionada con el tamafio de los sismos, ya que las emisiones se triplican antes de la ocurrencia de los eventos de m3.1 y m3.2, yno alcanzan a duplicarse antes de un evento de m<2. acerca de la efectividad que podria tener el metodo de monitoreo de las emisiones de gas radon como sistema de alerta temprana de sismos, se conciuye, en forma preliminar, que los resultados obtenidos son alentadores para la seccion de la falla las lagunas, con sismos leves superficiales. por esta razon se considera necesario con tar con un mayor periodo de monitoreo de las emisiones y sus relaciones con la ocurrencia y el tamafio de los eventos, para establecer un sistema de alerta temprana en la seccion de la falla activa en estudio. estaciones fll2 flu fll4 fllc periodo de registro (imp/mm'/dia) (imp/mm'/dia) (imp/mmvdia) (imp/mm'/dia) mayo/ol junio/ol 2,289 3,085 2,300 junio/ol julio/ol 2,348 2,100 1,868 1,897 julio/ol agosto/ol 2,034 2,163 1,403 1,846 ag./o 1 sept./o 1 2,863 4,734 3,620 3,326, sept./o 1 oct./o i 5,169 5,541 5,772 4,994 oct./o 1 nov./o 1 6,168 5,500 5,332 1,941 nov./o 1 dic./o i 4,985 5,469 4,069 2,354 dic./o 1 enero/02 4,665 3,976 3,354 2,978 enero/o 1 febrero/02 4,81 3,821 3,945 3,852 febrero/02 marzo/02 3,371 2,496 2,389 1,857 marzo/02 abri 1102 2,918 2,544 3,109 1,735 abrill02 mayo/02 3,029 2,82 2,557 1,686 mayo/02 julio/02 2,931 3,414 3,083 1,509 julio/02 julio/02 2,547 2,237 2,395 1,947 26 geofislca colombiana, 6, oiciembre de 2002 monitoreo de emisiones de gas radon vinculado a la actividad micros is mica en la zona de sampacho, cordoba, argentina (,.5 ..,--------------------------, _6.0 ~ 5.5 ;;5.0 s 4.5 .§ 4.0 c.35 t~:i, ~~ .. :6 1.5 ~\~:~ 0.0 '--"'-----"'--'iju---j periodo de registro +--~u2 fu.j .a fua __ cl»<'lwi... 0: sismos ocurridos figura 3. emisi6n de gas rad6n vs. tiempo. agradecimientos al program a geoindicadores para la evaluacion y el monitoreo ambiental, al proyecto calidad ambiental. evaluacion y monitoreo de riesgos naturales, financiado por sec y t-unrc, y al proyecto pautas para la planificacion y gestion de los recursos hidricos del departamento rio cuarto a partir del analisis geoambiental de su estado actual y su tendencia evolutiva, financiado por la agencia cordoba ciencia, por su apoyo al desarrollo de este trabajo. bibliografia bolt, a. b. 1981. terremotos. ed. reverte. costa, c. y c. vita finc!. 1996. late holocene faulting in the southeast sierras pampeanas of argentina. geofislca colombiana, 6, diciembre de 2002 mingorance f. 1991. analisis y evaluacion del tectonismo reciente asociado a la falla sampacho, cordoba, argentina central. revista geofisica 35, ppi3-39. sagripanti g.; origlia d. y campanella 0.1998. sismicidad en una region de intraplaca. departamento rio cuarto. cordoba. argentina. aetas. conferencia internacional sistemas modernos de preparaci6n y respuesta ante riesgos sismicos, volcanicos y tsunamis. santiago. chile. sagrjpanti, g. y h. sch1avo. 1998. metodo de track-etch y su aplicacion en el monitoreo de fal!as sismogeneradoras en la region de sampacho, departamento rio cuarto, cordoba. aetas. segunda reunion nacional de geologia ambiental y ordenacion del territorio, san salvador de jujuy. sagripanti, g., costa, c., origlla, d. y schiavo, h. 2001. avances en la caracterizacion del peligro sismico en el departamento rio cuarto, cordoba, argentina. iv congreso de geologia y mineria de la sociedad cubana de geologia, geomin 200 i. cuba. aetas. cd. 27 articulo 6.indd earth sciences research journal seismic processing earth sci. res. sj. vol. 15, no. 2 (december, 2011): 121 -128 thin-layer detection using spectral inversion and a genetic algorithm kelyn paola castaño1, germán ojeda2 and luis montes1 1 universidad nacional de colombia, facultad de ciencias, departamento de geociencias, sede bogotá, colombia. e-mail: kpcastanog@unal.edu.co, lamontesv@unal.edu.co 2 ecopetrol s.a. bogotá, colombia. e-mail: g.ojeda@ecopetrol.com.co abstract spectral inversion using a genetic algorithm (ga) as an optimisation approach was used for increasing the seismic resolution of a particular dataset; by contrast with the conjugate gradient method, a ga does not require a good starting model but rather a search space. the method discriminated layers thinner than λ/8 when tested on synthetic and log data. when applied to a seismic dataset concerning the barco formation in the catatumbo basin, colombia, spectral inversion led to recovering information from seismic data contributing towards the vertical identification of geological features such as thin distributary channels deposited in a deltaic environment having a tidal influence. the results revealed that a ga outperformed traditional minimisation schemes. resumen la inversion spectral con algoritmo genetico como estrategia de optimizacion ha sido utilizado para incrementar la resolucion de un volumen sismico, contrario al metodo del gradiente conjugado el ga no require un buen modelo inicial sino un espacio de busqueda. en datos sinteticos y registros de pozo el metodo discrimino capas mas delgadas que λ/8. al usarse a un volumen de la formacion barco en la cuenca del catatumbo, colombia, la inversion spectral recupero informacion que contribuyo a la identificacion de delgados canales distributarios depositados en un mabiente transicional con influencia de mareas. el resultado hallo que esta estrategia es mas eficiente y efectiva que los procedimeintos convencionales de minimizacion. palabras claves: algoritmos genetico, inversion spectral, inversion sismica, formacion barco, cuenca catatumbo, resolucion sísmica. keywords: genetic algorithm, spectral inversion, seismic inversion, barco formation, catatumbo basin, seismic resolution. record manuscript received: 30/04/2011 accepted for publications: 10/12/2011 introduction the reliable estimation of thickness during exploration activities can help to delineate prospects despite a lack of well information; it may help in obtaining more accurate volumetric hydrocarbon production calculations and in planning drilling involving lower risk. vertical seismic resolution is the key to extracting detailed stratigraphy which can distinguish two close seismic events associated with geological events. important oil reservoirs often have layers having a thickness which is below seismic resolution. the transitional palaeocene barco formation in the catatumbo basin consists of varying thickness sand wedges and, as it has several producing oil fields, there is great interest in mapping thin layers within it for drilling future wells. according to the rayleigh criterion, layers thinner than λ/8 cannot be resolved by seismic imaging (kallweit and wood, 1982; widess, 1973); tuning-thickness analysis based on this model has been used as a thickness mapping method for several decades now. amplitude and frequency variations through layers having changing thickness was used as a tool for extracting stratigraphic details (partyka et al., 1999), spectral decomposition using discrete fourier transform (dft) becoming a powerful technique for mapping thickness and geological discontinuity. a set of seismic attributes based on the spectral analysis of seismic reflection was drawn up to map changes in thin reservoirs (marfurt and kirlin, 2001). spectral decomposition has been successfully used for enhancing stratigraphic features. portniaguine and castagna (2004) introduced a method for spectrally decomposing a seismic trace, solving the inverse problem by the conjugate gradient to minimise the objective function; this technique is computationally time-consuming due to the method’s iterative nature. spectral inversion uses interference patterns for identifying and characterising layers (partyka, 2005); other results have shown that inverting seismic data through spectral decomposition provides reflectivity profiles which are then used for estimating layer thickness (puryear and castagna, 2008). seismic data’s vertical resolution has been increased by using the spectral inversion method, examining the thin-bed tuning response in the frequency domain. using a genetic algorithm (ga) meant that spectral inversion gave a global solution by minimising the objective function. the method was tested on synthetic and log data and applied to seismic data for interpreting thin distributary channels deposited in a deltaic environment having a tidal influence. kelyn paola castaño, germán ojeda and luis montes122 (b) geological setting the barco formation consists of interbedded fine-grained sandstone with mudstone; it was deposited in a deltaic environment having a tidal influence (nuñez and saavedra, 2006). thin sequences of bioturbated fine-grained sandstone are cross bedded with fine-grained sandstone and gray mudstone. some coal horizons are present, mainly at the top of this unit. the formation’s thickness varies from 500 feet to 700 feet, running northeast to southeast of the basin. two folds have been interpreted in the area (ojeda et al., 2009); a syncline has been located in the western part by the 3d seismic programme, followed by an anticline structure in the eastern part consisting of two domes. a main fault and secondary fault to the east of the former separate these two domes’ structures (figure 1a). wells p33, p34 and p35 reach the top of the barco formation on the south dome. this formation is divided into four depositional cycles, each representing a maximum flooding surface and hence stratigraphic limits. figure 1b shows x-line 183 where the barco formation and the four depositional cycles link wells p34 and p35. theory an appropriate seismic attribute must be directly sensitive to geological features or reservoir properties, allowing a geologist to interpret a (a) figure 1. (a) top: structural contour map of the barco formation (depth in feet); the prospect is of an anticline formed by two domes. (b) the 183 x-line shows the two domes and wells p34 and p35 reaching the top of the south dome and the four depositional cycles of the barco formation. thin-layer detection using spectral inversion and a genetic algorithm 123 geological structure and its particular environment. coherence integrates information contained in adjacent traces to extract information which may not be easily recognised on time scale maps; high coherence thus indicates lateral lithological continuity whereas abrupt changes may suggest faults and fractures (chopra & marfurt, 2007). fault-to-fault coherence is used to enhance stratigraphic and structural discontinuity, discerning faults and channel geometry. root mean square (rms) amplitude may be applied to recognise seismic anomalies and squaring amplitude values within an analysis window can enhance important amplitudes above noise level. on other hand, combination of attributes has been used to characterize reservoirs (guerrero et al, 2010) and has allowed identify channels in transitional environment with tidal influence. spectral inversion due to the widess model occurring in nature as the exception not the rule, spectral inversion is based on the fact that an impulse pair of reflectors can be decomposed in odd and even parts (castagna, 2004; chopra et al., 2006). seismic resolution is increased because the odd part constructively interferes when thickness becomes thinner. the process differs from conventional methods because inversion is driven by geological knowledge and is based on local frequency spectrum characteristics obtained from spectral decomposition techniques (marfurt and kirlin, 2001). inversion is defined in this method (puryear and castagna, 2008) through an amplitude spectrum’s constant periodicity for a layer of given thickness; it takes advantage of the fact that spacing between spectral peaks and notches is precisely the inverse of layer thickness in the time domain (partyka et al., 1999). when an analysis point lies at the centre of a layer (in the time domain), the impulse pair may be expressed by: here, and are the top and base reflection coefficients and t is layer thickness. the equation’s fourier transform gives the real and imaginary parts: where ƒ is frequency, ro and re are the odd and even component of the coefficient pair. as demonstrated by puryear and castagna (2008) there is a cost function o(t,k) given by: where g(f ) is amplitude regarding frequency and . equation 4 is solvable after evaluating o(t, k) at each frequency and finding the global minimum in a k-t model space. ro is determined by and re from .time sample at t1 top of reflector r1 is given by g(f ) is the complex spectrum for the reflection coefficient pair. extended to multiple layers and assuming seismograms to be the convolution between r(t) and a known wavelet w(t), the spectral decomposition of a seismic trace s(t) within time window length tw can be expressed as: a too short window affects frequency resolution and a too long window deteriorates time resolution. when the wavelet spectrum is known, r(t) and t(t) are estimated by optimising the function: here, fh and fl are high cut-off and low cut-off frequency, αe and αo are weighting functions. the αo/αe ratio is adjusted according an acceptable trade-off between noise and resolution and αo >> αe in the case of the widess model. the best solution is achieved when objective function o (t, re, ro, t), given by equation (6), is minimised over the frequency rank, supplying model parameters re, ro and t. genetic algorithm unlike local optimisation approaches, overall ones attempt to find the misfit function’s global minimum, most of them being stochastic by nature. regarding real data, it can never be known whether an achieved solution is the optimal one; however, even starting with poor initial data, achieved solutions give good results.a genetic algorithm (ga) is widely used in numerical analysis to find accurate solutions for optimising problems because it is one of the best ways to solve a problem about which little is known, thereby creating a relatively quick high-quality solution to a particular problem. a ga tends to thrive in an environment where there are many solutions and in which the search space is uneven and has many hills and valleys. a ga is based on analogies regarding biological evolution (simulated evolution). it uses a population of abstract representations (genome / genotype) of candidate solutions (phenotypes) resolving an optimisation problem. it starts from a randomly-generated population of individuals evolving through successive generations. each individual’s fit in a particular population is evaluated; multiple individuals are stochastically selected (based on their fit) and modified (recombined and randomly mutated) to form a new population. the new population is then used for the algorithm’s next iteration. the algorithm usually ends with the maximum number of iterations (generations) or satisfactory adjustment of the target population has been achieved. a ga review is beyond the scope of this document but one treatise is recommended (glover at al., 2003. chapter 3, pp55) method spectral inversion was first tested on a semi-infinite five-layer model (figure 2, left-hand side) where a 4 ms sample rate synthetic seismogram was produced by convolving the model’s reflectivity series with a 30 hz ricker wavelet (figure 2, right-hand side). the method involved using a 128 ms window centred at 64 ms and applying a 128 m gaussian-tapered fourier transform. it was assumed that αe = αo, i.e. an equal even/odd contribution to the reflection coefficient pair. the selected seismic data covered a 1 km2-area sub-volume from a post-stacked 3d data survey of the catatumbo basin containing 1,271 (1) (2) (3) (4) (5) (6) (8) (7) kelyn paola castaño, germán ojeda and luis montes124 traces. the dataset regarding the top and the bottom of the barco formation was located at the top of the south dome field concerning an oil producing area considered to be stratigraphically complex. good quality borehole data regarding the field was available. data volume was retrodeformed to flatten the top of the barco formation to avoid structural strain effects appearing in the trace due to trace inversion. seismic data quality and vertical seismic resolution were assessed to establish interpretation scope during the initial stage. a 71 m wavelength having 18 m vertical resolution was provided at 40 hz dominant volume frequency together with 2,850 m/s average speed estimated by sonic logs. discussion of results synthetic model figure 3 shows the cost function pattern calculated for a couple of layers where inversion-estimated values were close to optimum values, having lower than 6 ms estimation errors. an additional minimum may be observed at the top left of figure 3a where layer 1 thickness varied from 30 to 60 ms and layer 2 was thinner than 5 ms. figures 3b and 3c contain other minimums where layer 3 thickness vanished; therefore, the layers had to be constrained to non-zero thickness to avoid such border effect. the synthetic seismogram in figure 2 provided an accurate model when it was inverted, considering the 2 ms seismic resolution and the tiny difference between the expected and obtained reflection coefficients. the foregoing was reinforced by comparing the data’s spectra amplitude before and after inversion, depicting a similar pattern and small power decay (figure 4). by contrast with the conjugate gradient optimisation method tested by puryear and castagna (2008), a ga does not require a good starting model and only needs a defined search space containing a particular solution. a conjugated gradient matrix can be analytically formulated, leading to rapid numerical evaluation; the cost of computing its derivatives makes a ga a faster alternative. a ga introduced performance improvement here by doing a fast exhaustive search and finding a good solution quickly. t(ms) rc 40 -0.15 10 0.18 55 -0.12 10 0.18 figure 2. left-hand side, a 5-layer model’s reflectivity values; centre, the reflectivity series; right, the synthetic seismogram provided by convolution with a 30 hz ricker wavelet. figure 3. the optimum point (red star) was close to that estimated by inversion (white star), having lower than 10 ms estimation errors. (a) (b) (c) thin-layer detection using spectral inversion and a genetic algorithm 125 t(ms) rc 36 -0.12 11 0.12 54 -0.07 8 0.19 figure 4. left-hand side, inverted reflectivity series; right-hand side, the spectra before and after becoming inverted. figure 5. in-line 163 shown in bold regarding the area containing the three wells. (a) some stacked traces close to well p33, (b) the lithic column with gr curve used for discriminating shale and sand units, (c) the same traces after inversion. (a) (b) (c) figure 6. in-line 63, above the original stacked data and below the inverted sections (increased resolution). channel features can be observed in the two zoomed-in boxes. (a) (b) (c) (d) kelyn paola castaño, germán ojeda and luis montes126 real data ga was used with spectral inversion with regarding the sub-volume having 1,271 seismic traces and 128 ms window length. the stacked time section was tied to the well and the stacked traces in the volume were inverted. figure 5 shows stacked traces near the well before (figure 5a) and after inversion (figure 5c) and the lithic column (figure 5b) with the tied reflectors associated with sand (yellow) and shale (grey) packages interpreted from well logs. the inverted reflectors discriminated the top and bottom of sand units whereas the reflectors in original data were associated with the thicker layers having a 10 ms uncertainty time position. spectral inversion increased vertical resolution discriminating layers within the predicted resolution limit (6 ms) and a well-defined sand-shale sequence was thus determined. the inversion did not recover the top of the sand package at the bottom of the section, probably due to a border effect regarding the restricted window. stacked in-line 63 is shown in figure 6, before (top) and after inversion (bottom), including time resolution and the number of reflectors having lateral continuity. some discontinuous reflectors in the inverted section’s left zoomed-in box, shown by an arrow around 50 ms, suggested an aggradational stack of layers inside a channel which was not visible before inversion. the reflector’s geometrical pattern inside the right-hand zoomed-in box suggested the base of a channel around 60 ms; two fluvial seismic patters were observed in the 47 ms time-slice shown later on in figure 7. inverted seismic images revealed vertical detail comparable with well information sharply contrasting with the horizontal seismic resolution imposed by the original acquisition; the 20 ms in-line and the 40 ms x-line separations were not modified by the inversion. rms amplitude and seismic volume coherence were calculated before and after inversion. the rms amplitude 47 ms time-slice is shown in figure 7a, a box limiting the zoomed-in area. rms amplitude 17 ms time-slice and inverted data coherence are shown in figures 7b and 7c, respectively, where the rms image shows an n-s sinuous continuity pattern to the east, interpreted as being a small meandering distributary channel which was consistent with the icp palaeoenvironmental interpretation (2006) and supported by core analysis of well p33 drilled in the channel. another sinuous pattern observed to the west was interpreted as a mefigure 7. (a) 47 ms time-slice rms amplitude of complete original seismic volume, (b) rms amplitude of original selected sub-volume, (c) rms amplitude of inverted data showing sinuous patterns, (d) coherency did not reveal structural patterns and (e) interpretation of two sinuous geometry channels having an n-s trend (arrows). (a) (b) (d) (c) (e) thin-layer detection using spectral inversion and a genetic algorithm 127 andering channel; by contrast, these patterns were not observed in rms amplitude before inversion (figure 7b), probably due to the tuning thickness effect. 17 ms time-slice coherence (figure 7d) did not reveal structural seismic patterns as being faults or dipping reflectors, thereby agreeing with the absence of important structures in the sub-volume. increased horizontal resolution would lead to facies identification supporting the above interpretation; however, there were artefacts and noise levels revealing limitations in horizontal resolution which were related to bin size and footprint effect. conclusions thickness constraints in spectral inversion should be used as input for more accurate model-based reflectivity inversion. a ga minimisation algorithm performed well, achieving greater accuracy and faster execution. thickness inversion from real data clearly showed layering below seismic resolution and above 6 ms, i.e. our data’s predicted resolution limit. inversion, however, failed to capture the top of a sand package at the bottom of the section, suggesting that there was a border effect in the analysis window. lower than seismic resolution sand and shale packages can be resolved through spectral inversion and correlating them with stratigraphic information derived from well logs. the inversion method had the necessary power for resolving the vertical top and bottom reflectors associated with thin layers. rms amplitude of inverted data indicated meandering sinuous geometries which were possibly related to small distributary channels on a tidal plain. however, horizontal resolution should be improved to identify facies association thereby leading to a conclusive result. time-slice coherency did not reveal any evidence related to structural features, this being consistent with the absence of important structures in the selected sub-volume. this methodology has provided information making a significant contribution to the vertical identification of geological features such as thin distributary channels in a flat tidal environment. however, it is recommended that future studies should be carried out aimed at solving problems related to seismic horizontal resolution. acknowledgements the authors would like to thank william agudelo, carlos piedrahita and jorge monsegny from the instituto colombiano del petróleo (icp) for their support and contributions and also dr. jonh castagna for discussion regarding the concepts behind spectral inversion. financial support and data permission was provided by ecopetrol s.a. this paper has resulted from an msc thesis in geophysics written by kelyn castaño at the universidad nacional de colombia. references castagna, j. p. 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(1998). interpretation of incised valleys using new 3d seismic techniques: a case history using spectral decomposition and coherency. the leading edge (17), 1294-1298. portniaguine, o., and castagna, j. p. (2004). inverse spectral decomposition. 74th annual international meeting,seg, expanded abstracts, 1786-1789. puryear, c. i., and castagna, j. p. (2008). layer-thickness determination and stratigraphic interpretation using spectral inversion: theory and application. geophysics, 73 (2), 37-48. sen, m., and stoffa, p. l. (1992). multilayer avo inversion using genetic algorithms. proceedings of the seg/eaeg summer research workshop (6), 581-589. widess, m. b. (1973). how thin is a thin bed? geophysics, 38 (6), 11761180. geofislca colomblana ~ 1, octubre de 1992 57 reinyeccion: una herramienta en geologia ambiental juan manuel moreno profesor asistente depto de geociencias-facultad de ciencias-universidad nacional de colombia moreno, j.m: reinyecci6n: una herramienta en geologia ambienta!. geoffs. colomb. 1:57-64, 1992 issn 0121-2974 resumen la aplicaci6n de tecnicas de reinyecci6n para residuos provenientes de las actividades petroliferas, dentro de estratos confinados en ei subsuelo , permitirian dar un mejor manejo am biental donde quiera que un pozo es perforado y disminuiran las improntas dejadas por el deterioro en ei ambiente en los terrenos adecuados para las piscinas almacenadoras, tanto de residuos como de lodos de perforaci6n, generando beneficios tanto econ6micos como de impacto arnbiental. abstract reinjection technology of wastes derived from oil drilling wells in confined layers into the subsuriace will give a better environment management wherever a well is drilling, minimizing the iootprint on disturbed areas designed to muds and wastes reserve pits generating environmental and cost benefits. 1. introduccion mediante estudios multidisciplinarios realizados por equipos conformados por diferentes especialistas (ge610gos, geofisicos, hidroge6logos. ingenieros de petr61eos, de producci6n, quimicos, computaci6n y otros), es posible la busqueda y selecci6n de formaciones geol6gicas aptas para la reinyecci6n de residuos provenientes de las perforaciones petroliferas con ell fin de dar un mejor manejo ambiental a los campos, en las etapas de exploraci6n y explotacion y minimizar, tanto los riesgos que generan la disposici6n de los desechos en las piscinas, como tarnbieo las huellas dejadas en las areas aledarias a los pozos luego que la aclividad exploratoria ha cesaoo, produciendo beneficios econornicos y de protecci6n ambiental eliminando, al maximo, las piscinas reservorias y disminuyendo asi el area utilizada por estas en el habitat durante el desarrollo de un campo petro iifero. la inyecci6n de f1uidos en yacimientos petroliferos ha side utilizada mas frecuentemente para ell desplazamiento de hidrocarburos y mantenimiento de presi6n y aotua eficientemente para aumentar el recobro de petr6leo. ei agua y ei gas han sido los fluidos mas usados para tales fines. la tecnologia de la inyeccion de agua 0 gas se ha desarrollado a tal punto que los procesos han side bien estudiados en el labo rato rio , matematicamente y en el campo y son constantemente mejorados. los parametres que determinan la respuesta de los yacimientos petroliferos a estos procesos han side bien definidos. la inyecci6n mediante pozos, de liquidos residuales como salmueras, aguas residuales industriales y liquidos radiaclivos, se realiza dia a dia con mas precisi6n, cuando se dispone de horizontes profundos en formaciones qeoloqicas con caracteristicas sedimentol6gicas, estructurales, hidrogeologicas e hidrogeoquimicas adecuadas, resultando asi un sistema de evacuaci6n economica en determinados casos, ya que existen problemas 58 moreno: relnyecci6n, una herramienta en geologia amblental de seguridad en espeeial cuando se han inyectado productos toxlcos 0 radioactivos. los desechos de pertoracion, lodos y fragmentos de roca se generan donde quiera que un pozo petrolero para aceite 0 gas es perforado. tradicionalmente estos residuos son ubicados en el fonda de los mares, cuando la expioracion se realiza costa afuera, 0 cuando se desarrollan en superfieie, en las conocidas piscinas de reserva junto a las plataformas de pertoracion: estas piseinas presentan generalmente tres prineipales desventajas: 1) ocupan una gran cantidad de espaeio; 2) permanente mantenimiento de las piscinas, y de los fluidos que en elias reposan, y un constante monitoreo para prevenir la contaminacion por metales, sales y otros contaminantes y, 3) la toma de decisiones adecuadas cuando eventual 0 definitivamente es necesario cerrar las piscinas reservorias y la ubicacio n de los residuos de una manera sana ambientalmente y a un alto costo para las industrias. reducir y manejar apropiadamente los desechos derivados de las perforaciones petroliferas ha sido una mela importante para la industria petro iera y en la actualidad b. p. exploration (alaska) ha desarrollado, principal mente en tierras permanentemente heladas (permafrost), innovados metooo s tanto en la produccio n de hidrocarburos, manejo de lodos y de los residuos, con el fin de dar una acertada respuesta a las regulaciones ambientales de ley en los sitios donde se desarrollan sus campos. dichas innovaeiones incluyen investigaciones en rnetodos para minimizar sus residuos y mejorar su manejo, implementando nuevas tecnlcas para eliminar el tradicional uso de piscinas para la ubicacion de los residuos. la nueva tecnologia desarrollada representa el mas significante cambio en el manejo de los residuos de perforacion, desde ei comienzo de un estudio de exploracion hasta la explotacion y desarrollo de un campo (b.p. exploration, 1991). esta tecnologia basicarnente consta de dos etapas: la primera de separacion, seleccion, lavado y molida de los fragmentos y residuos de rocas provenientes de la pertoracion, de tal manera que parte de ellos puedan ser utilizados como fuentes de material para relleno u obras civiles y evitar ia alteracion de zonas cercanas al campo; en la segunda etapa los residuos iiquidos de esta primera fase de seleocion son mezclados, junto con los demas desechos de lodos y olras sustaneias, para ser reinyectados en el subsuelo en formaciones geologicas que actuen como acuiferos confinados adecuados y no utilizables para 10 obteneion de agua de consumo y que permitan el almacenamiento de grandes volumenes de iiquidos residuales durante mucho tiempo. 2. inyeccion de fluidos para recobro los yaeimientos petroliferos contienen mezclas de petro leo , gas y agua sometidos a presion. las fuerzas de empuje para mover el petroleo las suministran el agua connata (de torrnacion) y el gas en solucion como consecuencia de su expansion, producto de la reducclon de presion. la produccion de petroleo produce una reduccion de la presion en los yacimientos a menos que sean reemplazados por agua 0 gas en aquellos que posean acuiferos 10 suficientemente actives y grandes. cuando la presion ha declinado y la tasa de produccion no es economica. la inyeccion de fluidos ha sido utilizada como un rnetodo convencional de recobro para aumenlar la produccion de petroleo en un yacimiento. los procesos de recobro pueden subdividirse en metodos convencionales y metooos no convencionales. entre los primeros (recuperacion secundaria). se incluyen aquellos que se consideran tecnica y econornlcamente probados como son la inyecclon de agua, gas (no miscible) e inyecclon de vapor ciclico. los rnetodos de recobro no convencionales (recuperacion terciaria) 0 de alta tecnologia incluyen los procesos de recobro adicional que no son tan econornicos para la recuperacion del petroleo y dentro de los cuales se pueden mencionar la inyeccion continua de vapor, combustion "in situ", inyeccion de co2 miscible, surfactantes-polimeros, soluciones de polimeros e inveccion de hidrocarburos miscibles y calentamiento electrico (urribarri, 1979; ferrer, 1979; ferrer & becerra, 1979). varios proyectos piloto y algunos en gran escala usando tecnicas no convencionales, se etectuan en el mundo, permitiendo asi recuperar gran parte del petroleo remanente. 3. inyeccion de uquidos residuales la inyecclon en niveles profundos del subsuelo de iiquidos residuales como salmueras, aguas residuales industriales y iiquidos radioactivos es un tema que han tratado ampliamente custodio & diaz (1976) y otros autores, donde incluyen gran parte de las caracteristicas minimas que debe poseer un acuifero que pueda ser utilizado como deposito de residuos durante largo tiempo, asi como una sucinta reseiia historica de los primeros ensayos de inyeccion profunda de salmueras residuales. poco despues de 1930 se inicia la inyeccion de salmueras de la minera del petroleo en texas (usa). solo despues de 1950 se inicio la inyeccion profunda de residuales de la industria qui mica y nuclear en los estados unidos. en otras partes del mundo se realizan tambit'm, perc el exito de muchos proyectos de inyeccion profunda radica en utilizar un medio karstico 0 muy fisurado y medios porosos y permeables para que la colmatacion por solidos en suspension, coloides 0 precipilados sea lenta (custodio & diaz, 1976). la utilizacion de medios poco permeables 0 afectados tectonicamente, lieva a resultados frecuentemente poco satisfactorios. geofislca colomblana ~ 1, octubre de 1992 59 4. residuos de perforaciones petroleras la exploracion para la produccion de aceite y gas es una operacion industrial que inevitablementegenera desechos y mientras que los paises donde se desarrollan trabajos de axplotacion poseen sus propias regulaciones ambienlales, las industrias aseguran un propio manejo de los desechos de perforacion. es importante recordar que la gran mayoria de los desechos son simplemente no peligrosos. los lodos de pertoracion, fragmentos de roca y el agua de produceion son sustancias naturales, con la adicion de pequenas cantidades de aditivos quimicos (fig.1). dentro de los principales residuos de pertoracicn se pueden mencionar, los siguientes: 1) lodos de perloraci6n en la pertoraclon a rotaci6n juegan un papel importante los lodos que son ftuidos de mezclas de arcilla cuya composici6n predominante son los filosilieatos, bentonita, barita, y otras subslancias naturales usadas para cumplir con los siguientes objetivos: transportar los fragmentos de roca a la superlicie, lubricar y refrigerar la broea de perloraci6n, controlar las extremadamente altas presiones que se encuentran cuando un pozo de petr61eo 0 gas es perlorado miles de pies bajo la superlicie terrestre y mantener las paredes del pozo; sin lodos de pertoracion seria imposible perlorar de una manera segura. la barita en forma natural, como sulfato de bario, se usa como un agente de peso para aumentar la densidad de los lodos y ayudar a controlar la presi6n de los f1uidos ejercida por las formaciones con gas 0 aceite. olro aspecto destacable en los lodos de perloraci6n es la viscosidad, ya que esla es importante para la capacidad de arrastre de los detritos de roea dentro del lodo. ademas de estos, deben tenerse en cuenta el ph, y el contenido de arenas. otras sustancias usadas en los lodos de perloraci6n incluyen polimeros identicos a los usados para aumertar la densidad de otras sustancias. los lignosulionatos, derivados de cortezas de arboles, son usados en los sistemas de lodos como adelgazantes. en el pasado, cantidades traza de cromo fueron usadas en lodos ligno-suifonatados pero han sido eliminados a medida que la industria ha liegado a eslar segura de su potencial toxicidad. otros aditivos en los lodos incluyen soda caustica (hidr6xido de sodio) para controlar el ph de los lodos y lam bien es controlado el crecimiento bacteriano. otros lodos son preparados con base en hidrocarburos, con el fin de mantener estables las presiones del pozo. durante la perforaci6n de intervalos productores de aceite, eantidades trazas de hidrocarbonos tambioo enlran en el sistema de lodos; sin embargo, las cantidades de estos son menores a causa de las presiones positivas ejercidas por los lodos para eliminar la producci6n de hidrocarburos durante la perforaci6n. 2) fragmenlos de ~erforaci6n son slmplementeos fragmenlos de suelos y roca que son removidos a medida que la broea penetra en los estratos generando el orificio que liegara a ser el pozo pro ductor. generalmente los pozos perlorados comienzan con diarnetros mayores a las diez pulgadas y se reduce a medida que se liega a las formaciones potencialmente productoras. durante el desarrollo de esta fase pueden ser encontrados materiales de todo lipo de qranulornetnas y litologias 10 cual va ha influir en el volumen de fragmentos gruesos, extraidos durante la pertoracion. 3) aqua de producci6n emerge asoclada con el aceite y debe ser separada antes de que este sea enviado a traves de los oleoductos a los cenlros de refinacion. esla separacion ocurre en eslaciones y centros de flujos donde se separa con fines de ser reinyectada dentro de los reservorios en los procesos que ayudan a oblener recobros adicionales de petr6leo. en los acuiferos confjnados pueden exislir aguas cuyas caracterislicas quimicas y la presencia de materiales s61idos disueltos no permilen su utilizaci6n como agua potable, y puede entonces usarse este acuifero como horizonte almacenador de residuos, siempre y cuando cumpla con los normas eslablecidas. olros desechos asociados incluyen cualquier otro componente que emerja asociado con el aceile 0 gas de producci6n y dentro de los cuales podemos mencionar: suelos conlaminados, fragmentos de la tuberia de recubrimiento, esquirlas metalicas de los lanques y unidades de tratamiento de los desechos, aguas conlaminadas provenientes de los lanques almacenadores y camiones cislemas asociados con desechos varios y, ocasionalmente maleriales radiactivos que ocurren naluralmente durante las aclividades de perloraci6n. grandas cambios han ocurrido a nivel mundial en ei manejo de los residuos asociados a la actividad pelrolera desde que las leyes reguladoras para el manejo de los desechos s6lidos, han entrado en vigencia como para el caso especifico del estado de alaska, donde esta siendo implementada la tecnologia de reinyecci6n mitigando asi el impacto generado por las perloraciones (b.p exploration, 1991). generalmenle los desechos asociados son tralados de dos maneras: fos iiquidos son inyeclados en acuiferos apropiados a traves de pozos de inyecci6n y. los materiales solidos, previamente lavados y descontaminados, son reubicados en la superficie como material de rellenos. en la actualidad, la ubicaci6n de los lodos y fragmentos de raca en las piscinas ha permilido la disposici6n de algunos de sus componentes relalivamente benignos en ei ambiente. si las piscinas no fueran manejadas apropiadamente el volumen de lodos y fragmentos podrian sobrepasar cerca de los diez mil barriles por pozo, raz6n por la cual desde que se desarrollan campos con cientos de pozos en producci6n, exploraci6n y planeaci6n, el manejo de los sistemas de lodo y residuos as rsalmente importanle. 60 moreno: relnyecclon, una herramlenta en geologia amblental fuente material ubicacion looosyfragmentos lodos agua de produccion ~~~--------_i residuos asociados -, ..... "', -", \. -, \, pvzo cit produccion materiales reactivos ocasionale5acuifero util gravas para relleno lavadoymolida aceite reciclado acuifero util aceite recuperado gravas de relleno gravaspara relleno aceite reciclado acuifero util figura 1. principales fuentes, tipo de residuos y su destinacl6n; se destacan los aculferos utiles para reinyecci6n. (modificado de b.p. exploration, 1991) 5. sistema de lodos para la reinyeccion asociado a la plataforma de perforacion se encuentra instalado el sistema de lodos usado en la pertoracion para los lines anteriormente establecidos (fig.2). dentro de este sistema de lodos se encuentran centrifugas, desarenadores, mesas vibratorias (rumbas), las cuales permiten realizar una primera separacion de los fragmentos solldos tipo grava y arenas, para que los lodos sean reciclados y reutilizarlos cientos de veces mientras el pozo as perforado, siempre y cuando se conserven todas sus caracteristicas fisicas adecuadas a la pertoracion. esta traccion f1uida continua su proceso y parte de el, asociado con las aguas de produccion y dernas componentes quimicos pueden ser utilizados como lodos para la reinyeccion dentro de acuiferos conlinados (fig.2). 6. tecnica para la reinyeccion de residuos en general, los pozos en produccion incluyen gas, agua y aceites los cuales son enviados via oleoductos hacia las baterias donde son separados. ei crudo as entonces enviado desde las astacionas de bombeo hacia los centros de refinacion. transportados por grandas distancias a traves de los poliduclos. ei gas removido del petroleo as enviado por gasoductos 0 inyectado en ei reservorio para labores de recuperacion secundaria. una gran parte del agua de tormacion es inyectada tarnbien para ei recobro secundario y el resto se utilizara en las labores de reinyeccion de rasiduos. realmente los lodos de pertoracion y solldos removidos provenientes de ellos en el proceso de reciclaje, deben ser ubicados en alqun lugar y tradicionalmente son dispuestos en piscinas construidas para tal fin con paredes levantadas con gravas y recubiertas generalmente con geotextiles. sin embargo, se prasentan problemas con fugas que requieren un adecuado tratamiento ambiental, que incluyen obras y monitoreo constante de acuerdo a las leyes reguladoras de control ambiental. las tecnicas convencionalas contemplan el relleno de las piscinas utilizadas durante el proceso de perforacion, las cuales seran cubiertas con suelos y revegetacion nativa. un constante monitoreo de estas areas permite asegurar que los desechos se encuentran seguramente encapsulados y que no se presentaran contaminacionas posteriores. la nueva tecnologia desarrollada recientemente incluye la implementacion de sofisticados sistemas de pulverizacion, lava do y molienda. geoffslca colombians ~ 1, octubre de 1992 61 o >~ o,.... e c:•e o o ~ (11 10 0 !t. ,~~ it q:~:~ -, 0 df ' -,,po :. 0 '0 c t 0 /0 " > / 0 / 0 ...j / .~ 0-==\l->, " .....0 ~ , • 0 c 0 q.. ~0 '" c't"t ~i i .. i ii .. :;; q, ~ k>i ii ; ie, [ ~'i i .. 0 0'0 i i ~.. c ,'ii '0 l0 i'll c: 'u" " , q. ~i.....0 k:> c) 0 0 c,.>~ 0. ' .. 0 " ". >i :• ... i . " e'0 u'. u .!.'::... ',0.'';~: "e ' ... 0..." ' .. '0';; .. i .' .. :a:: ..... i ", i ·0'0 ~ ; .g •0 "~..•a:: c: '0 u .. u .0• e ".. a:: vj .. o >~ 0' •... 3•~ q. ;.. o o.lo o :. 00 ·.."... ';; ~ •'0 o u h•:2 ..0 on -0 ~ "e-o· 0'0~. -o '0 o ...j c: '0.~ •>c:'.a:: 00 ~:; ~'i' '.. ' .. :;.': u ": .2· . i 'u '.' i i· .u·. o '·0 col • i .. ~."=1" '. ~ :: . i . '0.' ii~i :'.::.:.~.~ ~": 1,1:.':;':.:' i ··:'·.~o~:·· ,.:>':"'!""""......,~......j.~... i ·z', . ~ ii' :,::.:::.;.:: i \'.'::a: --i: r:: ii :::'::>~: iii:.':}: i 1··,.1:·· ': , . : i .'.: '. iii':' ::...:....: .'.: ,i:: ;':,' 11 .' .. '. : .. ' i i'::,',.:...... . .. . figure 2. slstem. de eep«ecl6n de lodos, grav.s y reslduos. los reslduos son mezcledoa pa.'4 su ublcacl6n en el 8cutfero, por reinyeccl6n (b.p.1991) 62 moreno: relnyeccl6n, una herramlenta en geologia amblental se realizara entonces ia remocion del contenido solido de las piscinas para ser lavados, molidos y seleccionados para que los residuos solidos finos se pulverizen y mezclen con la fraccion liquida para ser ubicados dentro del acuifero confinado previamente seleccionado para reinveccion a profundidades bajo ei subsuelo. las gravas iimpias puede ser usadas para rellenos y material de oonsfrucoon limitando asi la necesidad de alterar otras areas para obtener este tipo de materiales (fig.2). la mayor innovaeion del sistema implicaria la eliminacion total de las piscinas, implementando en ia plataforma una nueva planta transportable que consta de los molinos, lavadoras, tamizadores y seleccionadoras adecuadas para que los residuos sohdoe se separen durante el proceso de perforacion y los residuos liquidos sean reinyectados en los acuiferos confinados. 7. equipo requerido para el desarrollo de estas nuevas tecnologias en campos petro iifero s donde sea recomendable la aplicacion del proceso de reinyeccion. se contara con gran parte del equipo necesario para hacerlo, puesto que se utilizaria la misma plataforma de pertoracion para el proceso de reinveccion de los residuos. en la plataforma podernos mencionar ei sistema de inyeccicn de lodos que se ernplean con los fines expuestos anteriormente y que servira para la dleposicion de los desechos en ei acuifero seleccionado . lmplernentacion de la planta rnovit de lavado y molienda para lograr obtener una mezcla adecuada que junto con los desechos iiquidos y las aguas residuales de los lavados del material tipo grava, pueda ser reinyectado en ei subsuelo. se utilizaran las infraestructuras ya establecidas en los pozos exploratorios 0 reutilizando pozos abandonados dentro del mismo campo 0 en cercanias de ei, 0 en su defecto diseiiar nuevos pozos exclusivamente para este proceso (fig.3). 8. definicion de estratos confinados la definicion de los acuiferos confinados se realiza en profundidades de alrededor de 3.000 a 5.000 pies. dichos estratos con las caracteristicas de un acuifero confinado deberan ser totalmente permeables y estar ubicado entrecapas impermeables (fig.3), donde los residuos permanezcan por mucho tiempo y de esta forma estaran dispuestos efectivamente en zonas aisladas del medio sin perturbaciones ambientales. 8.1 aspectos geol6glcos e hidrol6glcos en un campo que se desee realizar reinyeccion profunda de residuos se definira, de acuerdo a custodio & diaz (1976), un nivel que debe cumplir con las siguientes caracteristicas: 1) que posea suficiente porosidad, permeabilidad, espesor y extension superficial como para permitir caudales de inyeccion y votumenee almacenables suficientes. 2) que se irate de un acuifero confinado que contenga agua 0 minerales sin valor economico y sea tal que ei techo y la base sea 10 suficienternente impermeable como para que no se produzcan fugas perjudiciales hacia los acuiferos superiores con aguas 0 minerales de lnteres eoonornico. 3) que at acuifero este confinado en toda su extension y, en caso de no serlo, que ei agua que se desplace y expulse a consecuencia de la reinyeccion no tenga caracteristicas perjudiciales. 4) que ei flujo natural no haga aflorar los residuos inyectados. 5) que no existan fallas y zonas de fractura que afecten al nivel confinante (figal. ptr,o ... io'n lavoda, f'o' .... nto1 dt roca pa'aatlltn05 capa peaioc["8le rolth"cion paoductora figura 3. sistema de perforacl6n, y tratamlento de iodosj residuos en sup-erficie y su'ublcaci6n en subsuelo (modlficado de b.p. 1991) en general, los acuiferos cautivos profundos con aguas salobres 0 saladas son aptos como almacenadores, pero debe tenerse en cuenta el interes futuro para esas aguas con ei fin de obtener agua dulce por desalinizacion u obtener iones menores quimicamente. un nivel poroso no es necesariamente apto para reinyeccion, pues puede tener permeabilidad demasiado baja 0 incluso nula para efeclos practicos, de tal manera que se debe siempre tener en cuenta la porosidad eficaz. geofisica colombiana n2 1, octubre de 1992 63 ei almacenamiento de residuos puede ser real por alguno 0 por una cornbinacion de los tres siguientes mecanismos: 1) desplazamiento del agua existente, haciendota aflorar en alglin lugar. siendo el almacenamiento funcion del volumen y de la porosidad. 2) oompresion del f1uido existente. 3) expansion del acuifero por aumento de •• • i c1 .. : ..... _ iilrl----------l ---------:. :..:.~.;,;~.~ ..:-; , ··.·::.:~.i~.•.-·.'.:-.::~ _-_-_c~ ~!...-_-=_-= figura 4. rlesgos de contaminaci6n de aculferos de agua dulce a traves de fall as 0 fracturas en las cap-as sellos del acuttero para reinyecci6n (modiflcado de custodio & dlaz, 1976) otros parametres de importancia y que son tema de invesfiqacion por parte del equipo multidisciplinario, corresponden a los aspectos quimicos de los residuos, extension superficial del liquido inyeclado, ei caudal de inveccion e incremento de presion, presion maxima de inyeccion, exploracion de acuiferos profundos, sondeos de reconocimiento y por ultimo la inslalacion del pozo de inyeccion 0 la adecuacion del pozo existente. con ei fin de definir los estratos optimos para realizar la inyeccion de los residuos es necesario desarrollar una rnetodologia adecuada de acuerdo a cada area especifica de los especialistas participantes en la definicion, y disponer de toda la informacion correspondiente y, si es necesario, tener las facilidades para obtenena. 8.2 informacion basica para desarrollar ei trabajo multidisciplinario y definir los estratos confinados de acuerdo a las especificaciones anteriores se requerira de: a) geologia en detalle, tanto del subsuelo como de superficie. b) analisis de geologia estructural del campo. c) lineas sisrnicas del area seleccionada para la reinyeccion, lanto en dos (20) como en tres (3d) dimensiones. d) perfiles de pozo existentes del campo en estudio. e) mapas geologicos, y ~e contorno~ para los diferentes horizontes poslbles de utlhzar como almacenadores. f) datos quimicos de los lodos y residuos. g) toda la informacion requerida en la parte ingenieril del proceso. 8.3 anausla con la informacion baslca disponible, el equipo de investigadores procedera de acuerdo a cada topico, a analizar las posibilidades de aplicar el proceso. desde ei punta de vista geologico y geofisico se realizara la interpretacion de las iineas sismicas, con ei fin de definir los estratos confinados con las caracteristicas correspondientes a partir de la definicion de horizontes de reftexion y con la ayuda de los perfiles de pozo evaluar las formaciones seleccionadas desde ei punto de vista de litologia, porosidad, permeabilidad, resistividad, agua de torrnacion, contenido de f1uidos etc. ei usa de la sismica tridimensional hace parte integral del metooo para la pro duccion de irnaqenes del subsuelo ya que permite, en cases favorables, la obtencion de informacion delallada de la litologia, porosidad, f1uidos y estructuras que benefician notablemente la definicion de acuiferos para reinyeccion. es de vilal importancia realizar el respectivo analisis estructural para la deteccion de estructuras, posibles fall as y fracturas tanto superficial corno en ei subsuelo, que permitieran la salida de los residues tanto en superficie, 0 como para que sirvan de via de comunicacion con acuiferos utiles y se produzcan entonces contarninaciones no deseadas (figa). deben realizarse las respectivas co rrelaciones, entre las informaciones de los diferentes pozos y la sismica, con el fin de definir tridimensionalmente la geometria de los horizontes seleccionados tanto horizontalmente (area) como verticalmente (espesores) , para cubicar la capacidad almacenadora. ei uso de modernos y sofisticados rnodelos computarizados en todas las disciplinas involucradas, complemenlan la tecnologia para lievar a cabo un buen manejo de los residuos. 9. beneficios ambient ales de la reinveccion ei reciclaje y reinyeccion de los lodos y residuos de perforacion en acuiferos confinados represenla un significante beneficio ambiental para las areas donde se desarrollan campos petroliferos. dentro de estos se podrian senalar en orden de prioridades, que no habria contacto entre los residuos y el ambiente durante la generacion, manejo 0 ubicacion. en segundo lugar el tamano areal utilizado por la plataforma de exploracion puede ser reducido grandemente si las piscinas reservorias no son tan grandes y corno resultado, menor cantidad del ecosistema sera alterado por su construccion y por la ubicacion de los residuos. la reduccion del numero y tamano de las piscinas reduce el area natural afectada por rellenos de gravas 0 explanaciones, tanto para 64 moreno: relnyeccl6n, una herramlenta en geologia amblental la ubicaci6n de la plataiorma, como en las vias de acceso y en construcci6n de poliductos para el transporte de los hidrocarburos. avances en el desarrollo de plataformas mas sofisticadas y que dispondrian de menor area para su instalacion, tambien contribuye a disminuir las huellas dejadas por la actividad exploratoria. 10. beneficios economicos los costos en la construcci6n, mantenimiento y sellamiento de las piscinas reservorias son significativamente mas altos que el costa por la implementaci6n de la p1anta m6vil con separadores, iavadoras y molinos para los residuos, con el fin de reahzar la reinyecci6n de los residuos al mismo tiempo que se realiza la perloraci6n (b.p. exploration, 1991). 11. conclusiones en un proceso de reinyecci6n de residuos f1uidos de perforaci6n, el conocimiento de la roca del yacimiento, de los acuiferos confinados y de los ftuidos que con1ienen, es un aspecto fundamental para su planificaci6n, desarrollo y control. los estudios de goologos, geofisicos, hidroge6logos e ingenieros de yacimientos, quimicos, de computaci6n y otros son basicos para determinar ubicaci6n, formas, propiedades, y continuidad de las rocas en los yacimientos y en los acuiferos confinados, asi como las propiedades y distribuci6n de los f1uidos contenidos en ellos. todos los rnetodos de recobro que emplean tecnicas de inyecci6n de ftuidos en campos petroliferos se aplican a las formaciones geol6gicas productoras, mientras que la reinyecci6n de los residuos de perloraci6n se realiza en formaciones acuiferas confinadas, diferentes al nivel ptoductor y a menores profundidades, teniendo en cuenta las caracteristicas 6ptimas necesarias para realizarlo. estas nuevas tecnologias, en el manejo de los residuos de perloraci6n, puede ser aplicada en muchas partes del mundo donde las limitaci6n para la ubicaci6n de los residaos, tanto en superficie como costa afuera, son necesarios para proteger el medio ambiente. cuando la producci6n de residuos s61idos predomina dentro de fragmentos tamalio gravas, estos pueden ser utilizados para obras como rellenos, base para carreteras y otras plataformas dentro del mismo campo y demas obras civiles que puedan utilizar este tipo de materiales; ademas, ia reducci6n en at numero de piscinas y el diserio de plataformas mas adecuadas, hace que las cantidades de material usado como base para las obras civiles, el cual muchas veces es lievado desde grandes distancias, sean menores. la puesta en practica de las tecnicas de reinyecci6n reduce notablemente el area alterada que debe ser rehabilitada con vegetaci6n, para restaurar de nuevo el habitat adecuado para las diferentes especies nativas de las regiones afectadas. referencias b.p. exploration (alaska) inc. (1991): innovative waste management in the oil and gas industry. b.p. exploration's drill cuttings washer and grinder. 12 pp. environment & regulatory affairs. anchorage, alaska. custodio, e., & m. diaz. (1976): calidad del a9ua subterranea. en custodio & llamas: hidroloqia subterranea. 1! ed., tomo ii. ediciones omega, barcelona. ferrer, g. j. (1978): potencial de los rnetodos no convencionales para el recobro de petr6leo. revista agempet 10:39-54. medellin. ferrer, g. j. & m. becerra. (1979): estado actual de la recuperaci6n termica en venezuela. revista petr61eo y tecnologia, iii (6):17-27. maracaibo, venezuela. rojas, g. & o. mirth. (1979): reseria de investigaciones sobre recobro. revista petr61eo y tecnologia, iii (3):16-20. maracaibo, venezuela. urrlbarrl, o. (1979): analisis de las caidas de presi6n y perdidas de calor en los pozos inyectores de vapor del pro.yecto m~. revista petr61eo y tecnologia,lii (6):7-14. maracaibo, venezuela. geoffsica colombiana pp 60-64 [u n l o de 2002 bogota 0 c issn 0121 2974n° 5 adquj stcton, registro y tr ansmtalon en tiempo real de seiiales stsmotoqlcas bajo tcplip carlos a. vargas-jimenez', g. castellanos', j. a. florez', j. p. gomez' resumen se presenta el desarrollo de un sistema de monitoreo de variables sismicas, que incluye la adquisici6n y adecuaci6n de 16canales simultaneos, el preproceso, la grabaci6n y la transmisi6n sobre redes ip en tiempo real. ei sistema esta orientado al sistema windows, para 10 cual se emplearon librerfas dinarnicas orientadas al acceso de dispositivos en tiempo real. asi mismo, el acceso de datos al pc se realiza vfa dma, para implementar el proceso multiple de sefiales en tiempo real. la arquitectura de la red ip de monitoreo sfsmico es cliente/servidor, en la cual se tome como servidor un pc dotado de una tarjeta de adquisicion de datos, encargado de realizar la conversion analogo-digital de las senales, almacenar en archivos las que correspondan a eventos sfsmicos y, al mismo tiempo, atender las solicitudes de los diferentes clientes. el software de cliente permite a los usuarios visualizar en tiempo real las senales que el servidor esta adquiriendo y realizar un procesamiento basico de las senales registradas en el servidor. palabras clave: aoquisicion de datos, tcp/ip, tiempo real abstract the constant evolution of network technologies has made possible the development of applications facilitating the real time access to the information. this way, it is possible the implementation of remote monitoring systems that using pcs achieves efficient transmission of signals through computer networks. in this work, the procedures system design and implementation procedures are explain in client/server environment that, based on tcp/lp, transmits in real time 16 seismological signals over lan network and the internet. the server is a pc endowed with an acquisition data card and it is the one in charge of carrying out the signal analog/digital conversion, to store in files those that correspond to seismic events and, at the same time, to assist the applications of the different clients. the client's software allows the users to view in real time the signals that the server is acquiring and to carry out a basic processing to the signals that have been registered in the server. key words data acquisition, tcp/ip, real time introduccion ei auge de los pc, y especial mente de internet, en los ultirnos adosha permitido el desarrollo de sistemas que monitorean en tiempo real sefiales que representan variables ffsicas a un bajo cos to. en este caso se trata de sefiales stsmicas capturadas por un geofono, el cual genera un voltaje de acuerdo con la velocidad del movimiento del suelo. estos sensores se encuentran en diferentes ubicaciones geograficas, por 10 que se hace necesario utilizar enlaces telernetricos para centralizar la informaci6n. una vez agrupadas las sefiales, en el servidor se efectua la conversi6n analogo-digital. en este artfculo se desarrolla un sistema de monitoreo sfsmico que contempla la adquisici6n, el preproceso, el registro de seriales sfsmicas y su posterior envfo y acceso a traves de redes de ip. la transmisi6n de las sefiales se realiza bajo el protocolo tcp/ip, logrando que el software opere en intranets e internet sin necesidad de modificaciones en su estructura. asf mismo se detalla el funcionamiento de los algoritmos de procesamiento digital de sefiales implementados en el software de cliente para dar herrarnientas a los sisrnologos que estudian las seiiales adquiridas remotamente en el servidor. 'universidad nacional de colombia, departamento de geociencias, grupo de geotislca, bogota. i universidad nacional de colombia, grupo control & procesamiento digital de senates. 60 adquisicion de senales sismologicas las sefiales sismol6gicas son consideradas como no estacionarias, y su ancho de banda esta entre 0.001 y 50 hz, por 10 que una frecuencia de conversi6n de 100 muestras por segundo por canal es considerada suficiente, restricci6n facilmente superable para el desarrollo de sistemas de proceso en tiempo real, incluida su transmisi6n. en general, hay tres formas de transferir los datos del dispositivo de adquisici6n a la memoria en la arquitectura del pc (hazzah, 1997): por consultas 0 polling, por interrupciones y por acceso directo a memoria (dma, direct memory access). de estas tres tecnicas, la adquisici6n por dma es la que ofrece mayores ventajas para aplicaciones que deben garantizar el muestreo de una 0 varias sefiaies a una frecuencia constante, ei dma permite que dispositivos externos transfieran informaci6n directamente desde 0 hacia la memoria del sistema, sin la intervenci6n del procesador, mediante su respectivo contralador, el cual se apodera temporajmente del manejo de los buses (de datos, direcciones y control) del microprocesador, y transfiere los datos directamente del puerto 0 de la memoria, a localidades de memoria especfficas, 0 viceversa. ei dma tiene como prop6sito principalliberar ala unidad central de procesarniento (cpu) de la carga de atender a algunos control adores de dispositivos, opci6n que no se tiene en los casos de adquisici6n por coilsultas 0 por interrupciones. el dma acnia como una unidad de procesamiento secundaria, toma el control de los buses de datos, direcciones y geoflslca colombiana, 5, diciembae de 200-\ adquisiciqn registro y transmisiqn en tiempo real de senales sismolqgicas bajo tcp/ip control, se encarga de capturar el ruimero de muestras con que fue programado y los almacenan a partir de la posici6n especificada (garcia de celis, 1999). de esta manera, dma es un metodo que proporciona el multiproceso por hardware. la tarjeta de adquisici6n de datos que se conecta al pc que desempefia el papel de servidor debe cumplir los siguientes requerimientos: conversi6n simultanea de 16 canales, resoluci6n de 12 bits, frecuencia de conversi6n superior a loomps y posibilidad de realizar transferencias dma. en el mercado se encuentran disponibles diferentes model os de tarjetas que cumplen con estos requerimientos. en el caso particular, el software de servidor fue desarrollado utilizando una tarjeta national instruments modelo pci mia 16e4 (national instruments, 1997). sin embargo, el manejo de dispositivos desde el sistema windows a 32 bits no es directo, y es necesario el empleo de dll tdinamic linked library) especializadas. en particular se emplea el vxd (virtual device driver), que tiene acceso irrestricto a todos los dispositivos de hardware, examina libremente las estructuras de datos del sistema operativo y puede acceder cualquier direcci6n de memoria (hazzah, 1997). asi, la vxd puede manejar interrupciones y realizar transferencias por dma. para la tarjeta escogida, se proporcionan las vxd en forma de api (application programming interface), que se compone de una librerfa de rutinas que permite programar la tarjeta y configurar parametros como la frecuencia de conversion, el rango de las sefiales de entrada y el manejo del buffer, entre otros. ellenguaje de programaci6n utilizado fue el labview 6i, el cua! es un lenguaje de programaci6n grafico orientado a la instrumentaci6n (national instruments, 2(00). desde la aplicacion hecha en labview se accesa el hardware a traves de la api mencionada. las funciones contenidas en las dll de la api permiten realizar transferencias dma. el metodo como se transfieren los datos de la tarjeta de adquisici6n a la memoria es definitivo para poder garantizar una frecuencia de conversi6n constante. pol'ser windows a 32 bits un sistema operativo de multitarea apropiativa, no se puede garantizar la ejecuci6n de una rutina de tiempo real critico (hard real-time), como es en este caso la conversi6n aid a una frecuencia constante. este requerimiento se logra realizando la tarea de tiempo real cntico por hardware, es decir que, una vez programada la tarjeta de adquisici6n, esta realiza aut6nomamente las conversionesaid, transfiriendo los datos a un buffer dma. debido a que la tarjeta es para bus pci, se puede manejar busmaster dma, que no tiene tantas restricciones como las transferencias dma para busisa (hazzah, 1997).con bus-master dma se pueden manejar apuntadores a direcciones pol' encirna de las 16mb,y ademas el buffer no tiene que serconsecutivo (scatter-gather dma). una vez cumplidos los requerirnientos de tiempo rea!, la aplicaci6n que se ejecuta en el servidor toma las muestras del buffer, que han sido puestas en este, en virtud de la transferencia busmasterdma. para el caso de las sefiales sismicas es necesario contar con informacion de un cierto mimero de muestras anteriores a la declaraci6n del evento. esto se debe a que los algoritmos que declaran el evento, activan el comando de almacenamiento varias muestras despues del inicio del sismo, cuando se ha superado un umbral de sensibilidad elegido por el usuario. se recomienda un pre-evento de alrededor de 30 s, que puede variar dependiendo del tipo de estudios que se realicen en la red sismica. la duraci6n exacta del pre-evento es especificada por el usuario una vez se inicia el roceso de conversi6n aid; de esta forma, si se considera que se muestrea la sefial 100 veces por segundo y se define un pre-evento de 30 segundos, entonces el buffer tiene una longitud de 3000 palabras por canal, ya que se necesitan dos bytes (una palabra), para almacenar cada muestra convertida a una resolucion de 12 bits. la figura i muestra la sefial digitalizada en el servidor, expresada en cuentas. para una resoluci6n de 12 bits y sefial bipolar, se tiene un rango que va desde -2048 a 2048 cuentas. una cuenta equivale a 2.44 my, considerando ul rango para la sefial analoga de -5 a 5 voltios. 1400 96f·'4'··--.... .... ...,..'i"""i'-----....-----....----------~...,..------2llol .. " '. " "." d'cci·d-!=dd.-ir"=h\---ir.h=7r<~irihi;__i==it_-1 '211-rv\/ll\!\iti\f~7c1iui'll .. -illfigura i. ejemplo de seiiales adquiridas en el scrvidor registro de eventos sismicos en el servidor una red sismica 0 incluso una sola estaci6n sismica que opera continuamente a ciertas frecuencias y con una alta resoluci6n, es este caso 12 bits, produce una cantidad enorme de datos que son a menudo diffciles de guardar y analizar. ei objetivo de los algoritmos de disparo es detectar las sefiales sismicas tipicas (terremotos, fuentes control ad as geofislca colombiana, 5, diciembre de 2001 . i i i' 1de senales sismicas, explosiones nucleares, sefiales subterraneas, etc.), en las cuales aparece ruido sismico constantemente. una vez se detecta un supuesto evento sismico, el algoritmo guarda todas las sefiales entrantes y s610 se detiene despues de que se declara el fin del evento, 0 despues de un tiempo predeterminado. los algoritmos automaticos de disparo son relativamente ineficaces cuando se comparan 61 con la experiencia humana, asi, estes pueden generar archivos falsamente activados por supuestas sefiales sismicas 0, por el contrario, no alrnacenaran pequefias sefiales detectadas como ruido sismico; por tales motivos la sensibilidad del filtro estara dada por las necesidades de la red sismica. en el servidor se utiliza el algoritmo de disparo sta/lta (short term average/long term average), que realiza procesos de filtraci6n de las sefiales sismicas. ei sta mide la amplitud del instante de la sefial sismica, mientras el lta cuida del promedio actual de la amplitud del ruido sismico. la ecuaci6n en diferencia que permite describe el sta/lta es (lee, 1989): 1 k-i yen) = -x(n)+-' yen-i) k k donde x representa la entrada, y el promedio computado, k es el tamario del sta 0 del lta, n representa el periodo de tiempo actual y n-i el perfodo de tiempo previo. la ecuaci6n en diferencia (i) implementa un filtro pasabajos normalizado, donde la frecuencia de corte es control ada por k. un k grande representa una pequefia frecuencia de corte y un gran tiempo de promedio. para determinar k si se ha especificado la frecuencia de corte, se debe resolver la siguiente ecuaci6n (lee, 1989): a2 + (2· cos(w· t) 4)a + 1= 0 donde a = ilk, w es la frecuencia de corte en radianes y t es el periodo de muestreo. para aplicar el sta/lta, se calcula la amplitud absol uta de cada dato de la serial entrante. luego, se calcula el promedio de amplitudes absolutas en ambas ventanas. en el pr6ximo paso se calcula una proporci6n de ambos valores de sta/lta; esta proporci6n se compara continuamente a un valor seleccionado de umbral; si la proporci6n excede este umbral, se dec lara un evento. esto no necesariamente significa que la red empieza a grabar sefiales sismicas, puesto que real mente en el sistema se tienen varias estaciones. todas las redes sismicas y la mayorfa de los registradores sismicos tienen un mecanismo de disparo que define cuantos y que canales tienen que estar en un estado activado antes de que el instrumento 0 la red empiecen a grabar datos. despues de que una serial sismica termina gradualmente, se desactiva el disparo. esto pas a cuando la proporcion de sta/lta actual cae debajo de otro para metro que entra el usuario. ei sta/ lta de desactivaci6n debe tener un nivel de umbral mas bajo (0 raramente igual) que los sta/lta que activan el nivel de umbraj. ademas de los datos adquiridos durante la activaciondel disparo, las redes sisrnicas y los registradores sismicos agregan una cierta cantidad de datos sismicos al even to archivado que corresponden al pre-evento de tiempo (pem), y despues de que el estado activo termina, el sistema tam bien agrega un post-evento de tiempo (pet), en la figura 2 se presenta una serial sismica y los calculos de sta y de lta. en la segunda grafica se observa un crecimiento lento pero constante del lta y una variaci6n del sta que sigue de una forma rapida los cam bios en la sefial; de esta forma se declara el evento cuando la raz6n sta/lta supera el umbral predefinido. 62 vargas £t al. (i) l.~t~~v];~(it~,~a,~,/\,/~<~j ;orcr-~-~---'-v--""",,--~~------, \ st,ola..t,olr:;,lu (1119ger~liye / __ :et il" ,-._ _ .111 fl'\1 ..... j'" '"o'-'-"~~~'"'---'---'--~----"-'=="-"'-'=~d 5 c ·go'----io~~;o-~,o-----"'(l--'~t--e~o--70~.-j[o figura 2. ejemplos de promedios sta/lta y activacion de disparos transmision en un entorno de red en tiempo real ei auge de internet y las intranets ha convertido a tcp/ip en el protocolo de comunicaciones mas ampliamente usado en las redes de computadores a nivel mundial. por esta raz6n todos los sistemas que utilicen redes de computadores como medio de transmisi6n estan tendiendo a utilizar tcp/ip como protocolo de comunicaci6n (parker, 1997), los sistemas de instrumentaci6n no son la excepci6n. de esta forma, los usuarios pueden tomar datos ai tiempo que son adquiridos en el servidor, accesando la informaci6n en tiempo real, sujeto a la velocidad de la red. en el sistema desarrollado para el presente trabajo, el servidor debe estar en capacidad de transmitir 16 sefiales digitalizadas con una resoluci6n de 12 bits y a 100 muestras por segundo, 10que deterrnina que se debe estar transmitiendo como minima 25.6kbps. esta velocidad de transmisi6n determina la velocidad minima necesaria, pero no considera los bits que agrega el protocolo de transmisi6n de datos. otra consideraci6n de disefio es que el servidor debe estar en capacidad de atender peticiones simultaneas de diferentes clientes conectados a la red. clientes en labview programa de adquisici6n servidor data socket figura 3. modelo clienre/servidor geofislca colombiana, 5, oiciembae de 2001 adquisicion. registro y transmision en tiempo real de senales sismologicas bajo tcp/ip en la figura 3 se ilustra el esquema de conexi6n entre el proceso de adquisici6n en el servidor y los clientes. esta conexi6n se basa en la utilizaci6n de sockets, los cuales son puntos finales de enlaces de comunicaciones entre procesos. es decir que cada circuito que entre y sale de la capa tcp se identifica de manera unica por una combinaci6n de mimeros, 0 sockets. estos mimeros son la direcci6n ip de la rnaquina y el mimero de puerto usado por el software tcp, haciendo posible que un proceso hable con otro proceso a traves de la red, basado por completo en el mirnero de socket. la tecnologia datasockets provee al programador una api para transmisi6n de sefiales a traves de una red, utilizando url (uniform resource locator). cada url identifica explicitamente un iinico proceso en una maquina especifica, por ejemplo una url puede ser dstp:11 200.30.41.75/datos. ei modelo de red cliente/servidor esta constituido par aplicaciones cliente que requieren los servicios que proporciona la maquina servidor. la aplicaci6n de servidor se dedica a escuchar las direcciones conocidas (puertos de servicio) para el requerimiento de servicios. cuando una petici6n tiene lugar, el proceso servidor despierta de su estado latente y atiende la direcci6n solicitada ejecutando las acciones especificas que sean procedentes. en esta aplicaci6n se trabaja con 16 sefiales, par 10que se decidi6 construir una trama propia con estas sefiales y agregarle un corto encabezado, que contiene el nurnero de canales enviados y otra infarmaci6n necesaria para que el cliente interprete cada sefial sismica. esta unica trama se escribe a un socket, que previamente ha sido abierto. resultados v conclusiones ei resultado final es una aplicaci6n en ambiente windows que adquiere hasta 16 sefiales sfsmicas, las procesa y registra acorde a los resultados del algoritmo de sta/lta. ei sistema de adquisici6n fue probado satisfactoriamente con las sefiales que entrega la red sismica regional eje cafetero, en las instalaciones de ingeominas-manizales, en colombia. esta red sismica esta dotada de registradores electromecanicos que gene ran impresiones continuas de todas las estaciones. de esta manera se puede efectuar una comparaci6n visual entre los archivos generados por el sistema de adquisici6n digital y los registros el pape!. se observ6 que los sismos emergentes que crecen lentamente no son siempre detectados por el algoritmo stailta. par esa raz6n fue implementado un algoritmo de detecci6n mas simple, basado en un umbra 1de amplitud. asf, el usuario puede elegir la declaraci6n de evento par stai lta 0 par umbra!. la figura 4 muestra las sefiales recibidas en un cliente conectado al servidar a traves de una red lan a iombps. cinco canales indican sefiales de ruido sismico y uno de los canales registra una sinusoidal de prueba entre ada por un generador de sefiales. '~~mij\1v\f'~pplft~~,lgf1lf01'r~ffim~\f , i' , j " i i f ' __ ' r\7\j\/\j\l\f\ ~~~~vt~\m~~ l' i t i i i t it figura 4. recepci6n de senates en el cliente para el usuario, el proceso de conexi6n es total mente trasparente. una vez se cuente con la infraestructura de red, s610 es necesario conocer la direcci6n ip del servidor y el nombre del proceso. las herramientas de software implementadas que dispone el cliente se agrupan en tres categonas: preprocesamiento en el dominio del geoflslca colombiana, 5, diciembre de 2001 tiempo, analisis en el dominio de la frecuencia y aplicaci6n de filtros. el programa permite aplicar las siguientes funciones de ventana a sismos que se cargan desde archivos: hanning, triangular, blackman, hamming, blackman exacta, coseno tapered y blackman-harris. asi mismo puede utilizar filtros chebyshev, butterworth y elipticos. es63 vargas et al tos filtros permiten variar sus frecuencia de corte, la clase: pasabajos, pasaaltos, pasabanda 0 rechazabanda y el orden del mismo. en el dominio de la frecuencia se pueden realizar fft, auto-espectro de potencia, el espectro cruzado de potencia y la transformada de hilbert. la figura 5 detail a las herramientas de procesamiento digital de sefiales que posee el software de cliente. r =i 0>1,,0:<1, 9 r.. 1!!!il :;t.mil :;;;i!!j l!] ~"',-'lw1l ~lj q il =:l"'~-ilf",....~::.,.,. --m~l figura 5. procesamiento digital de senates efectuado par el software de cliente se observe tam bien una mejora sustancial del desernpefio de las rutinas de procesamiento en tiempo real, cuando se trabaja con mimeros expresados en formato entero, ya que una operacion entre mimeros expresados en coma flotante consume mas tiempo de ejecuci6n de la cpu que la misma operacion reajizada sobre nurneros enteros. de igual forma se anota que almacenar los datos adquiridos en farrnato binario presenta mas ventajas que el almacenamiento en ascii. almacenar en formato binario es mas rapido que en ascii: esta vent aja es importante debido a la caracteristica de tiempo real de la aplicacion. por otra parte, los archivos binarios son de menor tarnafio y no se pierde precision al realizar la conversion a ascn. 64 referencias bibliograficas aki, k., richards, p. 1980: quantitative seismology theory and methods, w.h. freeman and company. cadzow,j. a. 1973: discrete time systems, prentice hall, englewood cliffs, n. y. f. harvey.1994: dma fundamentals on various pc platforms, national instruments. garcia de celis, c. 1999: ei universo digital del ibm pc, cuarta edici6n. hazzah, k. 1997: writing windows vxds and device drivers, r&d books. nationalinstruments.1997: pc i-mid e series user manual multifunction i/o boards for pci bus computers. lee, w. h. k. 1989: toolbox for seismic data acquisition, procesing and analysis, international association of seismology and physics of the earth's interior. national instruments,2000: labview user manual national instruments, lab view version 6i-internet-ready measurement applications. parker,t. 1997: teach yourself tcp/ip in 14 days, segunda edici6n, prentice hall. proakis,j., manolakis,d. 1998: tratamiento digital de sefiales, tercera edici6n, prentice hall. taylor,f, mellott, j. 1998: hands-on digital signal procesing, mcgraw-hill. tmkoczy,a. 1999: understanding and setting the stailta trigger algorithm and associated parameters, http://www.seismo.com/msop geofisica colombiana, 5, diciembre de 2001 articulo 6.indd earth sciences research journal seismic processing earth sci. res. sj. vol. 15, no. 2 (december, 2011): 121 -128 thin-layer detection using spectral inversion and a genetic algorithm kelyn paola castaño1, germán ojeda2 and luis montes1 1 universidad nacional de colombia, facultad de ciencias, departamento de geociencias, sede bogotá, colombia. e-mail: kpcastanog@unal.edu.co, lamontesv@unal.edu.co 2 ecopetrol s.a. bogotá, colombia. e-mail: g.ojeda@ecopetrol.com.co abstract spectral inversion using a genetic algorithm (ga) as an optimisation approach was used for increasing the seismic resolution of a particular dataset; by contrast with the conjugate gradient method, a ga does not require a good starting model but rather a search space. the method discriminated layers thinner than λ/8 when tested on synthetic and log data. when applied to a seismic dataset concerning the barco formation in the catatumbo basin, colombia, spectral inversion led to recovering information from seismic data contributing towards the vertical identification of geological features such as thin distributary channels deposited in a deltaic environment having a tidal influence. the results revealed that a ga outperformed traditional minimisation schemes. resumen la inversión espectral, con algoritmo genético como estrategia de optimización, se utilizó para incrementar la resolución de un volumen sísmico. contrario al método del gradiente conjugado el algoritmo genético no requiere un buen modelo inicial sino un espacio de búsqueda. usado en datos sintéticos y registros de pozo el método discriminó capas más delgadas que λ/8. al usarse en un volumen de la formación barco en la cuenca del catatumbo, colombia, la inversión espectral recuperó información que contribuyó a la identificación de delgados canales distributarios depositados en un ambiente transicional con influencia de mareas. el resultado mostró que esta estrategia es más eficiente y efectiva que los procedimientos convencionales de minimización. palabras claves: algoritmos genetico, inversion spectral, inversion sismica, formacion barco, cuenca catatumbo, resolucion sísmica. keywords: genetic algorithm, spectral inversion, seismic inversion, barco formation, catatumbo basin, seismic resolution. record manuscript received: 30/04/2011 accepted for publications: 10/12/2011 introduction the reliable estimation of thickness during exploration activities can help to delineate prospects despite a lack of well information; it may help in obtaining more accurate volumetric hydrocarbon production calculations and in planning drilling involving lower risk. vertical seismic resolution is the key to extracting detailed stratigraphy which can distinguish two close seismic events associated with geological events. important oil reservoirs often have layers having a thickness which is below seismic resolution. the transitional palaeocene barco formation in the catatumbo basin consists of varying thickness sand wedges and, as it has several producing oil fields, there is great interest in mapping thin layers within it for drilling future wells. according to the rayleigh criterion, layers thinner than λ/8 cannot be resolved by seismic imaging (kallweit and wood, 1982; widess, 1973); tuning-thickness analysis based on this model has been used as a thickness mapping method for several decades now. amplitude and frequency variations through layers having changing thickness was used as a tool for extracting stratigraphic details (partyka et al., 1999), spectral decomposition using discrete fourier transform (dft) becoming a powerful technique for mapping thickness and geological discontinuity. a set of seismic attributes based on the spectral analysis of seismic reflection was drawn up to map changes in thin reservoirs (marfurt and kirlin, 2001). spectral decomposition has been successfully used for enhancing stratigraphic features. portniaguine and castagna (2004) introduced a method for spectrally decomposing a seismic trace, solving the inverse problem by the conjugate gradient to minimise the objective function; this technique is computationally time-consuming due to the method’s iterative nature. spectral inversion uses interference patterns for identifying and characterising layers (partyka, 2005); other results have shown that inverting seismic data through spectral decomposition provides reflectivity profiles which are then used for estimating layer thickness (puryear and castagna, 2008). seismic data’s vertical resolution has been increased by using the spectral inversion method, examining the thin-bed tuning response in the frequency domain. using a genetic algorithm (ga) meant that spectral inversion gave a global solution by minimising the objective function. the method was tested on synthetic and log data and applied to seismic data for interpreting thin distributary channels deposited in a deltaic environment having a tidal influence. kelyn paola castaño, germán ojeda and luis montes122 (b) geological setting the barco formation consists of interbedded fine-grained sandstone with mudstone; it was deposited in a deltaic environment having a tidal influence (nuñez and saavedra, 2006). thin sequences of bioturbated fine-grained sandstone are cross bedded with fine-grained sandstone and gray mudstone. some coal horizons are present, mainly at the top of this unit. the formation’s thickness varies from 500 feet to 700 feet, running northeast to southeast of the basin. two folds have been interpreted in the area (ojeda et al., 2009); a syncline has been located in the western part by the 3d seismic programme, followed by an anticline structure in the eastern part consisting of two domes. a main fault and secondary fault to the east of the former separate these two domes’ structures (figure 1a). wells p33, p34 and p35 reach the top of the barco formation on the south dome. this formation is divided into four depositional cycles, each representing a maximum flooding surface and hence stratigraphic limits. figure 1b shows x-line 183 where the barco formation and the four depositional cycles link wells p34 and p35. theory an appropriate seismic attribute must be directly sensitive to geological features or reservoir properties, allowing a geologist to interpret a (a) figure 1. (a) top: structural contour map of the barco formation (depth in feet); the prospect is of an anticline formed by two domes. (b) the 183 x-line shows the two domes and wells p34 and p35 reaching the top of the south dome and the four depositional cycles of the barco formation. thin-layer detection using spectral inversion and a genetic algorithm 123 geological structure and its particular environment. coherence integrates information contained in adjacent traces to extract information which may not be easily recognised on time scale maps; high coherence thus indicates lateral lithological continuity whereas abrupt changes may suggest faults and fractures (chopra & marfurt, 2007). fault-to-fault coherence is used to enhance stratigraphic and structural discontinuity, discerning faults and channel geometry. root mean square (rms) amplitude may be applied to recognise seismic anomalies and squaring amplitude values within an analysis window can enhance important amplitudes above noise level. on other hand, combination of attributes has been used to characterize reservoirs (guerrero et al, 2010) and has allowed identify channels in transitional environment with tidal influence. spectral inversion due to the widess model occurring in nature as the exception not the rule, spectral inversion is based on the fact that an impulse pair of reflectors can be decomposed in odd and even parts (castagna, 2004; chopra et al., 2006). seismic resolution is increased because the odd part constructively interferes when thickness becomes thinner. the process differs from conventional methods because inversion is driven by geological knowledge and is based on local frequency spectrum characteristics obtained from spectral decomposition techniques (marfurt and kirlin, 2001). inversion is defined in this method (puryear and castagna, 2008) through an amplitude spectrum’s constant periodicity for a layer of given thickness; it takes advantage of the fact that spacing between spectral peaks and notches is precisely the inverse of layer thickness in the time domain (partyka et al., 1999). when an analysis point lies at the centre of a layer (in the time domain), the impulse pair may be expressed by: here, and are the top and base reflection coefficients and t is layer thickness. the equation’s fourier transform gives the real and imaginary parts: where ƒ is frequency, ro and re are the odd and even component of the coefficient pair. as demonstrated by puryear and castagna (2008) there is a cost function o(t,k) given by: where g(f ) is amplitude regarding frequency and . equation 4 is solvable after evaluating o(t, k) at each frequency and finding the global minimum in a k-t model space. ro is determined by and re from .time sample at t1 top of reflector r1 is given by g(f ) is the complex spectrum for the reflection coefficient pair. extended to multiple layers and assuming seismograms to be the convolution between r(t) and a known wavelet w(t), the spectral decomposition of a seismic trace s(t) within time window length tw can be expressed as: a too short window affects frequency resolution and a too long window deteriorates time resolution. when the wavelet spectrum is known, r(t) and t(t) are estimated by optimising the function: here, fh and fl are high cut-off and low cut-off frequency, αe and αo are weighting functions. the αo/αe ratio is adjusted according an acceptable trade-off between noise and resolution and αo >> αe in the case of the widess model. the best solution is achieved when objective function o (t, re, ro, t), given by equation (6), is minimised over the frequency rank, supplying model parameters re, ro and t. genetic algorithm unlike local optimisation approaches, overall ones attempt to find the misfit function’s global minimum, most of them being stochastic by nature. regarding real data, it can never be known whether an achieved solution is the optimal one; however, even starting with poor initial data, achieved solutions give good results.a genetic algorithm (ga) is widely used in numerical analysis to find accurate solutions for optimising problems because it is one of the best ways to solve a problem about which little is known, thereby creating a relatively quick high-quality solution to a particular problem. a ga tends to thrive in an environment where there are many solutions and in which the search space is uneven and has many hills and valleys. a ga is based on analogies regarding biological evolution (simulated evolution). it uses a population of abstract representations (genome / genotype) of candidate solutions (phenotypes) resolving an optimisation problem. it starts from a randomly-generated population of individuals evolving through successive generations. each individual’s fit in a particular population is evaluated; multiple individuals are stochastically selected (based on their fit) and modified (recombined and randomly mutated) to form a new population. the new population is then used for the algorithm’s next iteration. the algorithm usually ends with the maximum number of iterations (generations) or satisfactory adjustment of the target population has been achieved. a ga review is beyond the scope of this document but one treatise is recommended (glover at al., 2003. chapter 3, pp55) method spectral inversion was first tested on a semi-infinite five-layer model (figure 2, left-hand side) where a 4 ms sample rate synthetic seismogram was produced by convolving the model’s reflectivity series with a 30 hz ricker wavelet (figure 2, right-hand side). the method involved using a 128 ms window centred at 64 ms and applying a 128 m gaussian-tapered fourier transform. it was assumed that αe = αo, i.e. an equal even/odd contribution to the reflection coefficient pair. the selected seismic data covered a 1 km2-area sub-volume from a post-stacked 3d data survey of the catatumbo basin containing 1,271 (1) (2) (3) (4) (5) (6) (8) (7) kelyn paola castaño, germán ojeda and luis montes124 traces. the dataset regarding the top and the bottom of the barco formation was located at the top of the south dome field concerning an oil producing area considered to be stratigraphically complex. good quality borehole data regarding the field was available. data volume was retrodeformed to flatten the top of the barco formation to avoid structural strain effects appearing in the trace due to trace inversion. seismic data quality and vertical seismic resolution were assessed to establish interpretation scope during the initial stage. a 71 m wavelength having 18 m vertical resolution was provided at 40 hz dominant volume frequency together with 2,850 m/s average speed estimated by sonic logs. discussion of results synthetic model figure 3 shows the cost function pattern calculated for a couple of layers where inversion-estimated values were close to optimum values, having lower than 6 ms estimation errors. an additional minimum may be observed at the top left of figure 3a where layer 1 thickness varied from 30 to 60 ms and layer 2 was thinner than 5 ms. figures 3b and 3c contain other minimums where layer 3 thickness vanished; therefore, the layers had to be constrained to non-zero thickness to avoid such border effect. the synthetic seismogram in figure 2 provided an accurate model when it was inverted, considering the 2 ms seismic resolution and the tiny difference between the expected and obtained reflection coefficients. the foregoing was reinforced by comparing the data’s spectra amplitude before and after inversion, depicting a similar pattern and small power decay (figure 4). by contrast with the conjugate gradient optimisation method tested by puryear and castagna (2008), a ga does not require a good starting model and only needs a defined search space containing a particular solution. a conjugated gradient matrix can be analytically formulated, leading to rapid numerical evaluation; the cost of computing its derivatives makes a ga a faster alternative. a ga introduced performance improvement here by doing a fast exhaustive search and finding a good solution quickly. t(ms) rc 40 -0.15 10 0.18 55 -0.12 10 0.18 figure 2. left-hand side, a 5-layer model’s reflectivity values; centre, the reflectivity series; right, the synthetic seismogram provided by convolution with a 30 hz ricker wavelet. figure 3. the optimum point (red star) was close to that estimated by inversion (white star), having lower than 10 ms estimation errors. (a) (b) (c) thin-layer detection using spectral inversion and a genetic algorithm 125 t(ms) rc 36 -0.12 11 0.12 54 -0.07 8 0.19 figure 4. left-hand side, inverted reflectivity series; right-hand side, the spectra before and after becoming inverted. figure 5. in-line 163 shown in bold regarding the area containing the three wells. (a) some stacked traces close to well p33, (b) the lithic column with gr curve used for discriminating shale and sand units, (c) the same traces after inversion. (a) (b) (c) figure 6. in-line 63, above the original stacked data and below the inverted sections (increased resolution). channel features can be observed in the two zoomed-in boxes. (a) (b) (c) (d) kelyn paola castaño, germán ojeda and luis montes126 real data ga was used with spectral inversion with regarding the sub-volume having 1,271 seismic traces and 128 ms window length. the stacked time section was tied to the well and the stacked traces in the volume were inverted. figure 5 shows stacked traces near the well before (figure 5a) and after inversion (figure 5c) and the stratigraphic column (figure 5b) with the tied reflectors associated with sand (yellow) and shale (grey) packages interpreted from well logs. the inverted reflectors discriminated the top and bottom of sand units whereas the reflectors in original data were associated with the thicker layers having a 10 ms uncertainty time position. spectral inversion increased vertical resolution discriminating layers within the predicted resolution limit (6 ms) and a well-defined sand-shale sequence was thus determined. the inversion did not recover the top of the sand package at the bottom of the section, probably due to a border effect regarding the restricted window. stacked in-line 63 is shown in figure 6, before (top) and after inversion (bottom), including time resolution and the number of reflectors having lateral continuity. some discontinuous reflectors in the inverted section’s left zoomed-in box, shown by an arrow around 50 ms, suggested an aggradational stack of layers inside a channel which was not visible before inversion. the reflector’s geometrical pattern inside the right-hand zoomed-in box suggested the base of a channel around 60 ms; two fluvial seismic patters were observed in the 47 ms time-slice shown later on in figure 7. inverted seismic images revealed vertical detail comparable with well information sharply contrasting with the horizontal seismic resolution imposed by the original acquisition; the 20 ms in-line and the 40 ms x-line separations were not modified by the inversion. rms amplitude and seismic volume coherence were calculated before and after inversion. the rms amplitude 47 ms time-slice is shown in figure 7a, a box limiting the zoomed-in area. rms amplitude 17 ms time-slice and inverted data coherence are shown in figures 7b and 7c, respectively, where the rms image shows an n-s sinuous continuity pattern to the east, interpreted as being a small meandering distributary channel which was consistent with the icp palaeoenvironmental interpretation (2006) and supported by core analysis of well p33 drilled in the channel. another sinuous pattern observed to the west was interpreted as a mefigure 7. (a) 47 ms time-slice rms amplitude of complete original seismic volume, (b) rms amplitude of original selected sub-volume, (c) rms amplitude of inverted data showing sinuous patterns, (d) coherency did not reveal structural patterns and (e) interpretation of two sinuous geometry channels having an n-s trend (arrows). (a) (b) (d) (c) (e) thin-layer detection using spectral inversion and a genetic algorithm 127 andering channel; by contrast, these patterns were not observed in rms amplitude before inversion (figure 7b), probably due to the tuning thickness effect. 17 ms time-slice coherence (figure 7d) did not reveal structural seismic patterns as being faults or dipping reflectors, thereby agreeing with the absence of important structures in the sub-volume. increased horizontal resolution would lead to facies identification supporting the above interpretation; however, there were artefacts and noise levels revealing limitations in horizontal resolution which were related to bin size and footprint effect. conclusions thickness constraints in spectral inversion should be used as input for more accurate model-based reflectivity inversion. a ga minimisation algorithm performed well, achieving greater accuracy and faster execution. thickness inversion from real data clearly showed layering below seismic resolution and above 6 ms, i.e. our data’s predicted resolution limit. inversion, however, failed to capture the top of a sand package at the bottom of the section, suggesting that there was a border effect in the analysis window. lower than seismic resolution sand and shale packages can be resolved through spectral inversion and correlating them with stratigraphic information derived from well logs. the inversion method had the necessary power for resolving the vertical top and bottom reflectors associated with thin layers. rms amplitude of inverted data indicated meandering sinuous geometries which were possibly related to small distributary channels on a tidal plain. however, horizontal resolution should be improved to identify facies association thereby leading to a conclusive result. time-slice coherency did not reveal any evidence related to structural features, this being consistent with the absence of important structures in the selected sub-volume. this methodology has provided information making a significant contribution to the vertical identification of geological features such as thin distributary channels in a flat tidal environment. however, it is recommended that future studies should be carried out aimed at solving problems related to seismic horizontal resolution. acknowledgements the authors would like to thank william agudelo, carlos piedrahita and jorge monsegny from the instituto colombiano del petróleo (icp) for their support and contributions and also dr. jonh castagna for discussion regarding the concepts behind spectral inversion. financial support and data permission was provided by ecopetrol s.a. this paper has resulted from an msc thesis in geophysics written by kelyn castaño at the universidad nacional de colombia. references castagna, j. p. (2004). spectral decomposition and high resolution reflectivity inversion. ann. seg meeting, tulsa, oklahoma. chopra, s., castagna, j. p., and portniaguine, o. (2006). seismic resolution and thin-bed reflectivity inversion. canadian society of exploration geophysicists recorder, 19–25. chopra, s., and marfurt, k. j. (2007). seismic attributes for prospect identification and reservoir characterization. (s. j. hill, ed.) tulsa, oklahoma, usa: seg eage. glover f. and kochenberger g., (2003). handbook of metaheuristics. international series in operational research and management series. international academic publishers. norwell, mass. chapter 3. genetic algorithms. pp. 55-82. guerrero j., vargas, c. and montes, l. (2010). reservoir characterization by multiattribute analysis: the orito field case. earth sci. res. j. (14), 173-180. kallweit, r. s., and wood, l. c. (1982). the limits of resolution of zerophase. geophysics (47), 1035–1046. icp. (2006). evaluación integrada de yacimientos campos área lisama, área llanito y sardinata. santander. piedecuesta: ecopetrol s.a. nuñez, m., y saavedra, j. l. (2006). definición de un modelo estático para las formaciones barco y catatumbo, campo sardinata, cuenca catatumbo, colombia. bucaramanga: universidad industrial de santander, facultad de ingenierías fisicoquímicas, escuela de geología. marfurt, k. j., and kirlin, r. l. (2001). narrow-band spectral analysis and thin-bed tuning. geophysics , 66 (4), 1274–1283. ojeda, g. y., garcía, p. a., rubiano, j. l., and gómez, f. h. (2009). multiattribute-based net sand estimation in transitional reservoirs: barco formation, sardinata field, colombia. seg expanded abstracts, 3530-3533. partyka, g. a., gridley, j. a., and lópez, j. a. (1999). interpretational aspects of spectral decomposition in reservoir characterization. the leading edge (18), 353-360. partyka, g. a. (2005). spectral decomposition: seg distinguished lecture. (seg, editor) http://ce.seg.org/dl/spring2005/partykaabstract.shtml (2006). peyton, l. r., bottjer, r., and partyka, g. (1998). interpretation of incised valleys using new 3d seismic techniques: a case history using spectral decomposition and coherency. the leading edge (17), 1294-1298. portniaguine, o., and castagna, j. p. (2004). inverse spectral decomposition. 74th annual international meeting,seg, expanded abstracts, 1786-1789. puryear, c. i., and castagna, j. p. (2008). layer-thickness determination and stratigraphic interpretation using spectral inversion: theory and application. geophysics, 73 (2), 37-48. sen, m., and stoffa, p. l. (1992). multilayer avo inversion using genetic algorithms. proceedings of the seg/eaeg summer research workshop (6), 581-589. widess, m. b. (1973). how thin is a thin bed? geophysics, 38 (6), 1176-1180. geofisica colombian a n° 6 pp. 28-32 diciembre de 2002 bogota, d.c. issn 0121 2974 dispersion sismica por presencia de capas someras de alta velocidad: modelamiento con diferencias finitas sandracespedes',robinsonquintana" alfredoghisays',gabrielperez'y luis montes' resumen mediante el modelamiento con diferencias finitas, se estudia el efecto de la dispersion sismica por la presencia de capas someras de alta velocidad, con contrastes y espesores variables, y su influencia en la informacion sismica. a diferencia de los metodos convencionales de eliminacion de ruido de base fundamentalmente maternatica, por ejemplo: f-k, radon, se aplica un esquema deterministico basado en el modelamiento de propagacion de ondas que reproduce las seiiales no deseadas (ernst et al., 1998; blonk et al., 1995; blonk and hermann, 1994). ei entendimiento detallado del fenorneno que genera estas seiiales permite discernir 10 que se considera ruido y, por tanto, definir estrategias para su atenuacion (lamer et al., 1983). por la presencia de capas someras de alta velocidad, gran parte de la energia sismica original se dispersa en estas capas y solo una pequeiia fraccion se transmite al interior de la tierra, generando ruido coherente que enmascara la informacion proveniente de los reflectores mas profundos (leslie and evans, 1999). se diseno un modelo 20 y se obtuvieron varios sismogramas sinteticos, la sirnulacion muestra el mecanismo de dispersion relacionado con la inversion de velocidad en las capas mas someras, 10 que permite caracterizar su influencia sobre los registros sismicos. la informacion no deseada (ruido) es sustraida de la que se considera informacion. los resultados obtenidos en este trabajo se circunscriben al analisis de datos sinteticos. palabras clave. dispersion, capas someras de alta velocidad, modelamiento abstract through finite difference modelling, we studied the effects of seismic scattering due near surface high velocity layers, with varied contrasts and thickness, and their influence on seismic data. despite conventional methods to eliminate noise mainly based in a mathematical approach, i.e., f-k analysis, radon transform, we apply a deterministic scheme based on the modelling of wave propagation, generating the unwanted signals (ernst et al., 1998; blonk et al., 1995; blonk and hermann, 1994). the detailed understanding of the backscattering phenomenon allows to discern what can be considered noise and therefore to define attenuation' s strategies (lamer et al., 1983). in presence of near surface high velocity layers, part of the original seismic energy is scattered in these layers and only a smaller fraction is transmitted to the earth' s interior, generating coherent noise that masks the information comming from the deeper reflectors (leslie and evans, 1999). a 20 model was designed to obtain synthetic seismograms. the simulation shows the scattering mechanism related with velocity inversions in the near surface layers, allowing to characterize their influence on seismic records. the unwanted signal is subtracted from that considered information. the results obtained in this work are bounded to the analysis of synthetic data. keywords: scattering, near surface high velocity layers, modeling introduccion en una secuencia sedimentaria normal, la densidad de las rocas aumenta gradualmente con ia profundidad, por 10 cuai la velocidad de propagacion de las ondas sismicas tambien se incrementa, esto pergrupo de investigaci6n en geoffsica, facuttad de ciencias, universidad nacional de colombia, bogota. geofisico senior. empresa colombiana de petr61eos (ecopetrol), bogota, gaperez@ecopetrol.corn.co profesor asociado. departamento de geociencias, facuttad de ciencias, universidad naclonal de colombia, bogota, imontes@ciencias.unal.edu.co 28 mite que las ondas se reflejen al encontrar contrastes de impedancia acustica, siendo asi registradas en superficie. en muchas zonas, la exploracion sismica en tierra se ve afectada por la presencia de capas someras de alta velocidad, ya que elias introducen distorsiones sobre los datos sismicos, enmascarando la informacion proveniente de los horizontes reflectores mas profundos, la obtencion de imagenes sismicas en este tipo de zonas es ardua por ia poca penetracion de las ondas acusticas en las capas someras de alta velocidad y por la presencia de ruido dificil de atenuar (purnell, 1992). geofislca colombiana, 6, oiciembre oe 2002 dispersion sismica por presencia de capas someras de alta velocldad, modelamiento con diferencias finitas ei problema de penetracion de la onda acustica en las capas de alta velocidad se acentua cuando estas yacen sobre materiales de baja velocidad, ya que en esta situacion se p esentan perdidas de energia que se caracterizan por la generacion de ondas guiadas en la capa de baja velocidad, resultando una propagacion horizontal dominante de la energia sismica (leslie and evans, 1999). actualmente existen varias tecnicas para reducir los efectos de las distorsiones introducidas en la informacion sismica por fenornenos complejos de propagacion. usualmente las ondas guiadas directas, como el ground roll (ondas de rayleigh), pueden ser suprimidas razonablemente bien durante la adquisicion 0 el procesamiento, por ejemplo, con patrones de geofonos. cuando el ground roll es dispersivo y proviene de direcciones diferentes a la in-line, los arreglos de geofonos no son suficientes para atenuar esta c1ase de ruido. una forma de compensar los efectos de las capas someras sobre los datos sismicos es aplicar correcciones estaticas a un nivel, 10 que es aceptable si la energia viaja verticalmente. cuando los cambios en elevacion y velocidad son muy grandes, las correcciones estaticas no son suficientes y es necesario tener en cuenta las trayectorias de propagacion de las ondas (bevc, 1996). aunque la continuacion hacia arriba de la ecuacion de onda (wave equation datuming) produce informacion de mejor calidad, los fenomenos de superficie relacionados con dispersion y multiples permanecen en el registro. otros metodos, de base fundamentalmente matematica.tratan de establecer filtros para remover la sefial no deseada en otro dominio. estes se hacen mediante transformaciones, como los filtros f-k, radon, etc. en muchos casos no es posible remover el campo dispersado completamente, ya que es dificil separar la sefiai de interes del ruido. los rnetodos basados en la polarizacion se usan para discernir entre las reflexiones de los horizontes profundos y las ondas dispersadas en las capas someras. estos metodos necesitan informacion con multiples componentes, la cual no esta disponible aun en muchas zonas. adicionalmente el estado de polarizacion de las ondas dispersadas en las capas someras puede ser tambien irregular (ernst, et al., 1998). ~~cientemente se han desarrollado aproximaciones basadas en la idea de que es posible obtener imagenes de las dispersiones causadas por las inhomogeneidades de las capas someras. ei metodo se basa en que el campo de onda dispersado puede ser considerado como una selial deterministica que puede ser modelada por medio de la teoria de onda (ernst, et ai., 1998). algunos de estos algoritmos (algoritmos de enfoque de imagen 0 "imaging") dependen del grado de entendimiento de los fenomenos que se presentan cuando la onda se propaga a traves del medio. por eso la precision con que se pueda simular el medio es de suma importancia porque permite, mediante la comparacion con datos sinteticos (obtenidos mediante modelamiento numerico), establecer comparaciones con los datos reales e interpretarlos (perez, 1997). geofislca colombiana, 6, oiciembre de 2002 modelamiento con diferencias finitas se desarrollo un esquema para estudiar los fenomenos de dispersion sismica que ocurren en presencia de capas someras de alta velocidad y su influencia en la informaci6n sismica. ei estudio se baso en la simulacion de la propagacion de ondas en modelos bidimensionales, donde se varian el espesor y las velocidades de las capas someras. para esta simulaci6n se utiliz6 la ecuacion de onda para un fluido isotropico, no viscoso y no homogeneo (donde los campos de velocidad y densidad son funciones de la posicion), que formulada en terminos de la presion acustica es: lel? 2[a(la?j a(la?] a(la?j]--=c -+-+-+f(x,y,z) (i) p elt ax p ax 0' p 0' az p az donde x, y, z son las coordenadas de posicion, p = p (x, y, z) es el campo de presion.p =p(x,y,x)es la densidad, c = c (x, y, z) es la velocidad de propagacion de la onda y f(x, y, z) es la funcion de la fuente 0 de excitacion del campo de onda (berkhout, 1985). para la formulacion bidimensional de la ecuacion (i) en una solucion explicita en diferencias finitas, de cuarto orden en espacio y de segundo orden en tiempo, se reemplazan los operadores diferenciales por cocientes de diferencias, obteniendose la siguiente expresi6n: p"+] =' 2p"' _ p"~] + ,u z ,[ °f\.tz _ i '.} '.} i.: p,,} ',} 12ax2 "p,,} (,6(p"+p" ,)-(p" ,+p" ,)-30p"''\'v: 1+1,) i-i,) 1+2,) 1-2.j i,).rr f\.tz (_1_ _ 1 jfo(p" + p" ) _ (p" ,+ r: )'\. 8ilx2 .. \? i+l,) 1-1,) i+2,j i-2,) ppo+],} ph,j i1t2 i (, n n n n -vrv rvn '\. 1211 2 --: , \! 6(p;,j+] + p;,j-]) (p;,j+2 + p;.j'z) ~op;,j r: y po,} f\.t2 (1 i jfo(" p") (" ,,)j811 2 --',' \? p;,)+] + ;,j-] p,j+2 + p;,j'z} ~ + y p;,j+] po.}.] j,"' f\.t 2 11y ',} ax (2) donde !'1.t ,~,!'1y son los interval?s de muestreo en tiempo y espacio en las direcciones x-y, respectivamente; pd denota el valor del campo de presion en la posicion (i, j) de la grilla en el n-paso de tiempo, y u7,j y p;,j yi:; son los valores de la velocidad de propagacion, densidad y funci6n de excitaci6n, respectivamente. la ecuacion (2) permite encontrar los valores del campo en un paso de tiempo dado, recursivamente a partir de los valores obtenidos en los dos pasos anteriores, y de los valores de las propiedades del medio (velocidad y densidad) y la funcion de excitacion en cada punto de la grilla (perez, 1997). para garantizar la estabilidad de la soluci6n de (2), el modelo debe satisfacer la condicion (alford et ai., 1974): 29 cespedes et al. 8 urnax en donde 111 es el intervalo de muestreo en tiempo, i1x es el tamafio de la celda y um., es el valor maximo de la velocidad de propagaci6n de las ondas en el modelo. resultados y discusiones en el presente trabajo se utilizaron dos modelos bidimensionales para estudiar los fen6menos de dispersi6n sismica en presencia de capas someras de alta velocidad. ei primer modelo utilizado (figura i) se caracteriza por contener tres capas: la somera de alta velocidad y de espesor variable, separada por una interfase irregular de una capa de velocidad mas baja; una interfase plana a una profundidad de 250 m limita un medio semiinfmito con alta velocidad. distancia (m) ~ -0 -6 :;-':;;';;:')0(, 'i5 c 2 "e, modelo de velocidades figura 1. modelo de velocidad con tres capas. la capa de baja velocidad se encuentra entre dos capas de alta velocidad. ei segundo modelo (figura 2) s610 tiene dos capas; se diferencia del primero en que la capa somera de alta velocidad yace sobre un medio semiinfinito de baja velocidad. distancia (m) figura 2. modelo de velocidad con una capa de alta velocidad que yace sobre una capa semiinfinita de baja velocidad. 30 (3) para el modelamiento se utiliz6 una grilla cuadrada con celdas espaciadas cada 5 m en am bas direcciones. teniendo en cuenta el criterio de estabilidad, se utiliz6 un m de 0.5 ms y un tiempo de registro de 0.5 s para cada prueba. en la simulaci6n, la fuente de energia sismica, ondicula de ricker, se localiz6 en el centro del borde superior del modelo; y los receptores, en cada una de las celdas sobre el mismo borde. lnstantanea (t=0.25) figura 3. instantanea para el t = 0.2 s. muestra las ondas reflejada y refractada. la figura 3 muestra la propagaci6n de la onda a traves del primer modelo para el tiempo 200 ms. cuando el frente de onda choca contra la primera interfase, gran parte de la energia primaria es reflejada hacia la superficie, debido al gran contraste de impedancia acustica, la poca energia que se transmite al segundo medio se propaga en esta capa de baja velocidad hasta alcanzar la base de esta capa (interfase plana a 250 m), donde se refleja nuevamente. cuando la reflexi6n del horizonte plano horizontal alcanza la interfase irregular, parte de [a energia se refleja y parte se refracta hacia la capa somera de alta velocidad, propagandose hasta alcanzar la superficie. como los contrastes de impedancia acustica, tanto allimite superior como al inferior, de la capa de baja l)irtaihi;l(ln) figura 4. lnstantanea para t= 0.3 s. muestra la onda guiada en la capa de baja velocidad. geofislca colombiana, 6. diciembre de 2002 dispersion sismica por presencia de capas someras de alta velocldad, modelamiento con diferencias finltas velocidad son grandes, gran parte de la energia sismica queda atrapada en esta capa, comportandose como una guia de onda, como se observa en la figura 4. la simulacion en ei primermodelo genera un sismograma sintetico (figura 5), en el que se observa que las multiples, generadas por reflex ion en la interfase irregular, enmascaran la sefial de las reflexiones mas profundas. ademas, se observa que la forma irregular de la capa somera de alta velocidad distorsiona los tiempos de arribo de las reflexiones mas profundas. distancia (m) ..~.,~ :--j,~,,~.i!:. sismograma registrado figura 5. sismograma sintetico generado para el modelo de la figura i con age. ei metodo para eliminar las sefiales que traslapan la informacion de los horizontes reflectores mas profundos consta de dos etapas: en la primera se simula la propagacion de la perturbacion en el segundo modelo, en la cual se obtiene unicamente la respuesta de la capa somera de alta velocidad. en la segunda, la respuesta obtenida de la sirnulacion en el segundo modelo se sustrae de la respuesta lograda en . el primer modelo (figura 5). la simulacion con el segundo modelo se ejecuto utilizando los mismos parametres del modelo inicial. como resultado se obtuvo un sismogama sintetico (figura 6) que contiene las dispersiones y multiples generadas por la capa somera de alta velocidad. de acuerdo con el metodo planteado, al sismograma de la figura 5 se ie resto -traza a trazael sismograma de la figura 6. la figura 7 muestra el sismograma obtenido despues de sustraer la sefial no deseada; este muestra mejor continuidad en el reflector profundo. en la figura 7 se nota que las dispersiones y las' multiples causadas por la capa de alta velocidad han sido removidas, pero pennanecen las reflexiones de los horizontes mas profundos. los tiempos de arribo de estas reflexiones presentan distorsiones como consecuencia de la geometria de la cap a somera de alta velocidad, distorsiones que pueden ser corregidas por procesamiento convencional, como correcciones estaticas a un nivel. geofislca colombiana, 6, diciembre de 2002 sism ogram a de l a sella) no desead a figura 6. sismograma sintetico generado para el segundo modelo. con agc. distancia (m) o,""oe ~, :.0-.. j inil? zt i i tl' i i 'i ~: ... ,i/. 1\il.ljllb~e i~h!' tl ' w~c 111!.lllllll i ~ , ,~ j i ( "'1, )" n~t? r f ~ ,"j ~ ri , , pi pi' r .;, ",1m i! ii i ii : ! ! : r ~ : , i i figura 7, sismograma obtenido con el rnetodo propuesto, se observa la continuidad del reflector plano. conclusiones. ei presente trabajo muestra una aplicacion del modelamiento numerico para estudiar los efectos de dispersion sismica por presencia de capas someras de alta velocidad, aplicando la tecnica de diferencias finitas. ei concepto basico del metodo es la estimaci6n del campo de onda dispersado para luego sustraer este de la in formacion sismica. se muestra la importancia del modelamiento numerico en la obtencion de imageries que pennitan comprender los diferentes fenomenos que afectan la calidad de la informacion sismica. ei hecho de que se utilice la teoria de onda para describir este tipo de feno31 cespedes et al. menos hace que el metoda pueda ser aplicado en la investigacion de areas con alta complejidad, en la que otros metodos han sido menos efectivos. una buena definicion de las propiedades de las capas someras permite la obtencion de mejores resultados. esto implica que para aplicar este rnetodo sobre informacion real se debe tener un buen conocimiento de las caracteristicas fisicas de las capas someras. con este metodo se logro separar la informacion de interes (reflexiones de los horizontes profundos) de la serial no deseada, producida par capas someras de alta velocidad. se espera que al aplicar esta metodologia antes de una secuencia de procesamiento, se puedan obtener imagenes sfsmicas de mejor calidad. agradecimientos los auto res agradecen a la universidad nacional de colombia, al instituto colombiano para el avance de la ciencia y la tecnologia "francisco jose de caldas" (colciencias) y al instituto colombiano del petroleo (icp-ecopetrol) par la colaboracion y el apoyo para llevar a buen termino la presente investigacion. este articulo es resultado del proyecto modelamiento de fenomenos de dispersion sismica asociado a la topografia y a las capas someras del subsuelo, cofinanciado par colciencias mediante contrato no. rc 271-2000. 32 bibliografia alford,r. m., kelly,k. r., and boore,d. m. 1974. accuracy of finite difference modeling of the acoustic wave equation. geophysics 39: 834-842. berkhout,a. 1. 1985. seismic migration: imaging of acoustic energy by wave field extrapolation. elsevier. beve, d. 1996. flooding the topography: wave equation datuming of land data with rugged acquisition topography. geophysics 62 (5): 1558-1569. blonk,b., and herman,g. c. 1994. inverse scattering of surface waves: a new look at surface consistency. geophysics 59 (6):963-972. blonk,b., herman,g. c., and drljkoningen,g. g. 1995. an elastodinamic inverse scattering method for removing scattered surface waves from field data. geophysics 60, (6): 1897-1905. erns,f., herman,g. c., and blonk,b. 1998. acquisition/processingreduction of near surface scattering effects in seismic data. the leading edge 17 (6): 759-764. larner,k., chambers,r., yank, m., lynn, w., and wai, w. 1983. coherent noise in marine seismic data. geophysics 48: 869-874. leslie,d. m. and evans,b. j. 1999. 69th ann. internal. mtg., soc. expl. geophy. perez,g. 1997. the quality of the surfaces consistency assumption in residual statics stimation. m. sc. thesis. colorado school of mines. purnell,g. 1992. imaging beneath a high velocity layer using converted waves. geophysics 57 (11): 1444-1452. geofislca colombiana. 6. diciembre de 2002 determinacion de nuevas estaciones absolutas de gravedad en colombia william alberto martinez diaz • juan alberto florez munoz' laura marlene sanchez rodriguez' • instituto geografico "agustfn codazzi" martinez, w.a., j.fl6rez & l.sanchez: determinaci6n de nuevas estaciones absolutas de gravedad. geoffs. colomb. 3:81·87, 1995. issn 0121-2974 resumen durante el mes de mayo de 1995 se establecieron en colombia tres nuevas estaciones con valor absoluto de la gravedad utilizando el gravrmetro axis fg-5. estas mediciones fueron realizadas por defense mapping agency (d. m.a.) de los estados unidos y ellnsttuto geogratico "agustfn codazzi" (igac), como aporte a la actualizaoion de la red gravimetrica nacional y a la deflnlelon del modelo geoidal local. abstract three new absolute gravity stations were established in colombia in may 1995. a gravity meter axis fg·5 was used. these measurements were conducted by the defense mapping agency (d.m.a) with the collaboration of the igac. they will allow to actualize the national gravity net and to help in defining the local geoidal model. 1. introduccion en el presente ano, el instituto geografico "agustin codazzi" ha trabajado en el desarrollo del plan de actualizaci6n de la red gravirnetrica nacional (igac, 1995). para este prop6sito la oficina d.ma en colombia desplaz6 al pais un grupo de operadores, el gravimetro absolute, un equipo de posicionamiento satelitario y tres gravrmetros relativos lacoste & romberg (g-115, g-125 y g-4). este proceso obedece, principalmente, a la necesidad de incrementar la precisi6n del datum gravimetrico en colombia, asi como proveer al usuario de informaci6n actualizada y confiable como referencia a los levantamientos e investigaciones en geociencias que se ejecutan en el pars. la obtenci6n de datos gravimetricos absolutos permite al igac determinar con mayor precisi6n el modele de geoide local necesario, de una parte, para la obtenci6n de alturas ortometncas a partir de posicionamiento satelitario y, de otra, para mejorar el conocimiento de la evoluci6n de la dinamica terrestre en el territorio colombiano. 2. gravrmetria absoluta 2.1 esquema general del equipo en general, la gravimetrra absoluta tiene por finalidad la determinaci6n del valor de la aceleraci6n de la gravedad por medio de la medici6n de las cantidades distancia y tiempo (torge, 1989). . actualmente, el metoda empleado en este tipo de gravimetrraes la interferometrra optlca, principio de operaci6n de los qravfrnetros absolutos desde la decada de los 60. ei gravrmetro empleado para las ultirnas mediciones en colombia es el gravrmetro absoluto fg-5. ei nombre de este instrumento se deriva de la expresi6n "faller gravimeter 5th generation". en este, se mantiene el principio basico de medici6n simultansa de tiempos y distancias per medios intsrterometricos. no obstante, el aparato presenta un cambio tecnol6gico notable en la configuraci6n de sus componentes, asi como en la geometrra del conjunto, dado que este dispone el mecanisme compensador -superspringalirreado verticalmente con ta carnara de vacio en donde la masa de prueba es iiberada en catda libre. esta disposici6n hace que los efectos de inclinaci6n se disminuyan en relaci6n con los modelos predecesores tipo jila. las caracteristicas mas notables del equipo son (axis instruments company, 1993): desempeno: exactitud: ± 2x10'8 m/s2 precision: ± 1x10.8 m/s2 tiempo de rnedicion: menor de dos horas rango dinarnlco: mundial rango de temperatura: 15-25°c rasgos del instrumento: controlador de la toma automatica de datos (pc 486) procesamiento y almacenamiento de datos en tiempo real rata de medicion de diez segundos super resorte de 60 s. de periodo. medicion lnterterornstnca en linea vertical optica de colirnacion para alineacion vertical masa de prueba rodeada por una recamara de arrastre cart servocontrolado para slevacion y calda de la masa de prueba laser de helio-neon de frecuencia estabilizada referencia de tiempo de rubidio bomba de vaclo turbomolecular bomba de vacto lonico. las componentes del equipo se pueden sintetizar en cuatro grupos principales (fig.1): camara devacio 1-----1e--------+ haz de prueba q..._+--l-----. fotodiodo sistema de enfoque, corte y recomblnacion del laser super resorte figura 1. componentes principales del gravimetro absoluto fig. 5 camara de vaclo. es un contenedor cilfndrico de aluminio, en el cual se hace vacio medio, que aloja el ensamble de la masa de prueba, su prisma reflector y el mecanismo de desplazamiento de dicha masa. en su base existe una ventana a traves de la cual inc ide el laser proveniente de la base del interterometro y sale, una vez es reflejada por el prisma. super-resorte. este mecanismo contiene un segundo prisma reflector, alineado vertical mente con el primero y suspendido de un resorte cuyo periodo (controlado electronicamente) es t=60 s. la finalidad del super-resorte es contrarestar los movimientos verticales de la corteza, reduciendo la dispersion generada en las caldas por cambios en la geometria del sistema. base del lnterterometro. conjunto ubicado entre la carnara de vaclo y el super-resorte, en ella se genera una luz laser estabilizada de hello-neon. la base contiene el laser y la optica necesaria para enfocar, dirigir y separar la luz en dos rayos, uno hacia la carnara y otro hacia el super-resorte. esos rayor, una vez reflejados por los respectivos prismas, son recombinados para ser enviados como haces de interferencia a un fotodiodo ubicado en la caja de etectronica. • caja de electronica. en este sistema ocurre la rnedicion. la serial proveniente de la base del mterterometro es convertida, al pasar por un fotodiodo, en pulsos til que son reducidos en un factor de 4000 y son analizados junto con los pulsos provenientes de un oscilador de rubidio (reducidos en un factor de 2000) mediante un sistema contador. as! son enviados al computador en donde el software calcula el valor de la aceleracion (g) relacionando las magnitudes tiempo (t) y distancia (a). 2.2 obtenci6n de registros con base en la informacion presentada por axis instruments (actual mente micro-g solutions inc.) la obtencion de registros ocurre como se indica a continuaclon. ei instrumento puede operar en el modo de cafda libre 0 en el modo de ianzarniento de la masa de prueba. en el modo de caida, una sefial proveniente del computador hace que el controlador del sistema inicie el proceso. ei carro que soporta a la masa de prueba y el ensamble del prisma reflector se acelera a mas de 1g, de modo que la masa es liberada y comienza a caer. un sistema controlador evita que el carro toque la masa durante la catoa, a la vez que ie permite recuperarla en la base de la carnara de vaclo y elevarla nuevamente. la luz reflejada por el prisma ensamblado a la masa de prueba genera un haz de interferencia por cad a y2 de a (a=633 nm.) durante el movimiento del prisma. dado que la masa se acelera, los haces ocurren cad a vez mas cercanos en el tiempo. la serial que sale del fotodiodo (avalanche photo diode apd) es una onda senoidal cuya frecuencia es proporcional a la velocidad del objeto en caida. durante el curso de la medici on se genera cerca de un rnlllon de haces de interferencia. un elemento discriminador transforma las sefiales del apd en pulses de tipo til (fig.2). estos son escalados, es decir, divididos por un factor definido por el usuario (general mente 4000) mediante un contador de escala. un contador universal de intervalos de tiempo (utlc) mide el intervalo de tiempo ocurrido entre cad a pulso escalado. ei programa fit se encarga de ajustar cada par de tiempo y distancia a una trayectoria parabolica para determinarel valor de g. cuando el carro y la masa de prueba han descendido pasando el punta de rscuperacion de la masa, el controlador ordena al "cart" reducir su aceleracion y comenzar a detenerse. la masa es asl recibida y desplazada lentar iente hacia arriba a su posicion de reposo, quedando iista para un nuevo experimento, el proceso se reinicia. la secuencia toma cerca de cinco segundos y se puede repetir seis veces por minuto. onda recombinada senal "fringe" fotodiodo ----+-+-+---i----1r-+-+-+-+-+-+-+-t-h-+-+-+-t-1f----+ -+:+::.m1 contador universal ~t :::' de intervalos de --------;------t: tiempo --------!-------!,..-.----+--------;------:~--~---+ chip comparador pulsos ttl pulsos "fringe" escalados juu~iu~uuui..jui..llmjl!m pulsos del reloj t t~i ~ ~ t ~: iacumulador de valores escalados de la senal de l----~~~~~~~~~~~~~~~~~----icomputadori iacumulador de valores escalados de la senal de figura 2. obtenci6n de registros en el gravimetro absoluto fig.5 3. estaciones gravimetricas las mediciones se realizaron en el lapso de una seman a por cada estaci6n, en las ciudades de bogota, honda y cartagena. de esta forma, se estableci6 un circuito de calibraci6n instrumental (bogota-honda) y se defini6 una estaci6n para cubrimiento de la zona norte de colombia (cartagena). estas mediciones se realizaron veinticuatro anos despues de las lievadas a cabo por el doctor james e. faller y su equipo en 1971 en la universidad naclonal-boqota (faller, 1971, y hammond & faller, 1971). los valores de gravedad obtenidos durante el proyecto sstan referidos al nivel del piso, dado que la magnitud del gradiente vertical de la gravedad para cad a sitio se incorpor6 como dato al software del fg-5. en el area inmediatamente cercana a cada estaci6n absoluta se ha establecido una estaci6n excsntnca. ei nurnero de mediciones realizadas en cad a estaci6n es de 4800 (48 sets, cada uno de 100 cafdas por hora). la figura 3. muestra la salida final de datos obtenida para la estaci6n bogota. en la parte superior cad a rectanqulo representa un set y cada barra indica el error en microgales de cada uno. este es el diagrama de barras de error. en la parte inferior izquierda aparece el histograma que resume la distribuci6n de los 4800 datos, cada divisi6n de la escala horizontal corresponde a 10ilgales; en la parte inferior derecha se observa el valor medio de la gravedad para las 4800 medidas y la desviaci6n standar de los 48 sets y la de las 4800 observaciones. adicionalmente aparecen correcciones obtenidas con base en los sensores del equipo y su procesamiento en el software. los resultados obtenidos se muestran en la tabla 1. tabla 1 valores de gravedad y posici6n (wgs-84) de las estaciones absolutas i estacion i latitud i longitud i altura i gravedad i precision i bogota 9801-95 4.620112 285.936949 2612.48 977,374.487 ± 2.031 j,1gal honda 9802-95 5.202412 280.265443 293.85 977,928.090 ± 14.900 j,1gal cartagena 9803-95 10.41970 284.474323 160.00 978,142.215 ± 33.110 j,1gal 48 hours since 1995: 12522:00:00 1525 10 bogota preliminary value 2 high ::~-r 10 ilgal/dfv set gravity mean: 9.773744871 set std dev (j.1gal): 2.031 mean std dev (j.1gal): 12.13 drift: -.219 ± .512 fjgal/day number of sets: 48 100 drops/set i set 48 of 48 speed of light: -10.7 effective heigth: -19.5 datum z transfer: 300.6 @ .00 em laser correction: .000 barometric prenure: 2.82 polar motion: -1.43 ocean loading: .000 status: sequence complete 05/07/95 1995:127 21:52:16 -10 n = 4800 5 low i figura 3. salida final de datos en el gravimetro absoluto fig.s 3.1 descripci6n bogota 9801-95/bogota r1 (fig.4) 5tun6metroa ~p'--d u 0iil1.~ 1--------sbagob1ir1 do figura 4. ubicaci6n estaci6n con valor absoluto de gravedad • bogota bogota 9801-95 1) ei punto esta ubicado en la estaci6n sismol6gica "jose rafael goberna s.j." del instituto geofisico de los andesuniversidad javeriana. 2) para acceder a la estaci6n, se parte de la entrada al colegio san bartolome la merced en la carrera 5 no.34-00, intersecci6n de la carrera 5 con calle 34. en la entrada al colegio. se avanza en direcci6n norte (a la izquierda) y se sigue la via interna ascendente 600 m. durante el trayecto se alcanzan los edificios del colegio, hasta liegar al campo deportivo y en el costado noreste del campus a la estaci6n sismol6gica. los ultirnos 50 m. de recorrido son sobre un carreteable en hierba. la construcclon es la mas grande localizada sobre este camino. 3) la estaci6n esta localizada dentro de la edificaci6n (ia cual esta incrustada en la ladera) en el cuarto no.2, el cual es el sal6n central y el mas amplio. las distancias a los muros del sal6n son: ai muro suroeste en direcci6n noreste 1.07 m, al muro noroeste en direcci6n sureste 1.34 m, al costado noroeste de la pilastra en direcci6n noroeste 1.20 m, en el centro de una incrustaci6n de concreto de 0.8xo.8 m. 4) la estaci6n esta marcada por un disco de bronce estampado "dma gravity base 9801-95 may 19951gac bogota". bogota r1 los parratos 1 y 2 son los mismos de la anterior. 3) la estaci6n es exterior, ubicada en el costado noroeste de la edificaci6n. a 1,1 m. en direcci6n noreste del centro de la puerta de acceso, a 0,30 m. del muro noroeste de la construcci6n. esta es una incrustaci6n en el piso (de baldosa) junto a un pequeno antejardln. 4) la estaci6n ssta marcada por un disco de aluminio estampado "instituto geografico np-a2e1, 1954". honda 9802-95/honda r1 c a, r e r a 10 --------------carrera 9 quebra:g figura 5. ubicaci6n estaci6n con valor absoluto de gravedad • honda honda 9802-95 1) la estaci6n se ubica en la zona hist6rica de la ciudad de honda, cerca a la confluencia del rio magdalena y la quebrada seca. 2) a partir de la catedral alto del rosario se avanza hacia el este sobre la calle 10 a la intersecci6n con la carrera 12 a, y de alii, aproximadamente a 150 m, a la carrera 11. se dobla a la izquierda (norte) se avanza 10m hasta alcanzar la calle 10 (at costado derecho). se dobla a la derecha (este) y se recorren aproximadamente 75 m en descenso hasta la carrera 10 a. se gira a la derecha (sur) y se recorren aproximadamente 25 m hasta la bifurcaci6n de la via. se toma el brazo izquierdo (calle 9) y se avanza en sentido sureste aproximadamente 75 m hasta la carrera 10 (al costado izquierdo). se gira a la izquierda (noreste) sobre la carrera 10 al comienzo del puente sobre la quebrada seca. de auf, se recorren aproximadamente 40 mala izquierda hasta el museo del rio magdalena. 3) la estacion se halla en el laboratorio de fosiles cerca al centro de la construccion del museo. las distancias dentro del cuarto son: al muro noroeste en direccion sureste 1.12 m, al costado noreste del meson en concreto 2.05 m, al noreste del ventanal suroeste 1.03 m. 4) la estacion esta marcada por un disco de bronce grabado "dma gravity base honda 9802·951gac mayo 1995". honda r1 los parratos 1 y 2 son los mismos de la anterior. 3) la estacion excentrica honda r1 se encuentra en el extremo noroeste del pasillo frontal del museo, justo dentro de la reja de acceso. si no es posible entrar al museo, se puede observar desde el exterior de la reja como una extension del pasillo. las distancias correspondientes son: al muro noreste del pasillo en direccion suroeste 1.96 m, al muro suroeste del edificio en direcclon noreste 0.70 m, a la reja de acceso 0.12 m. 4) la estacion esta marcada por un disco de bronce estampado "dma gravity base honda r1 igac mayo 1995". cartagena 9803-95/cartagena r1 cru,@c,rt"''''r'~ __ plitqueadero entrada o pczo ~ puillo (plitio ~ is telecom • a03 ·9s cuarto de equlpo, ~~------.l-~ei ie",n figura 6. ubicaci6n estaci6n con valor absoluto de gravedad cartagena cartagena 9803 ·95 1) la estacion se localiza aproximadamente a 2.5 km al oeste del centro de cartagena. es la cima del cerro la popa. 2) ei punto se halla en el cuarto de equipos al costado sur de la edificacion de telecom, el cual se halla enseguida (al norte) del monasterio de la popa. 3) las distancias dentro del cuarto son: al muro norte en direccion sur 1.05 m, al muro este en direccion oeste 2.05 m, al muro oeste en direccion este 3.40 m. 4) la estaclon esta marcada por un disco de bronce estampado "dma gravity base c/gena 9803-95 igac mayo 1995". cartagena r1 ei parrato 1 es el mismo, que la anterior. 2) la estacion se localiza en el costado este del primer escalon del monumento hexagonal de aproximadamente 15 m de altura. ei monumento es una cruz en concreto en la zona de parqueo al norte del monasterio aproximadamente a 35 m. de la entrada. 3) la posicion de la sstacion es: al extremo norte del escalon en dirsccion sur 1.60 m, sobre el piso de asfalto 0.37 m. 4) la estacion esta marcada con un disco de bronce estampado "dma gravity base c/gena r1 igac mayo 1995". conclusiones como resultado de este trabajo, colombia cuenta con un nuevo datum gravimetrico, referencia para las mediciones de la red gravimetrica nacional, asl como de los levantamientos gravimetricos hechos en el pais. la utilizacion de las estaciones bogota y honda como circuito de calibracion instrumental (con estaciones intermedias en alban y villeta) proporciona un standar gravimetrico de alta precision, asl como un rango amplio de' gravedad en una distancia relativamente corta y a 10 largo de una via en buen estado de transitabilidad. la existencia de estos nuevos valores en el territorio colombia no permite referir a ellos la informacion gravimetrica existente y de este modo incrementar la calidad de los resultados obtenidos en el modelamiento del geoide local. es posible vincular las estaciones colombianas con las existentes en la republica de venezuela y de esta forma contribuir a la resolucion y ajuste del sistema red latinoamericana de normalizacion de la gravedad 1995relang-95 (mcconnell et ai., 1995) . agradecimientos se agradece la colaboraclon prestada por defense mapping agency, tanto a su oficina de enlace en colombia en cabeza del doctor arturo camacho, como a la oficina de saint louis missouri; al doctor walter spita, gravimetrista de dma, por sus aportes en cuanto a la seleccion de los sitios y criterios para el procesamiento posterior de la informacion; al equipo de observadores, geodestas keith krauterbluth, david stizza y leonard weingarth; al instituto geohsico de los andes-universidad javeriana, por la ayuda brindaba para efectuar las mediciones en la estacion sismol6gica "jose rafael goberna s.j"; al museo del rio magdalena por permitir la' realizacion del trabajo en el laboratorio de f6siles del museo y a la empresa nacional de telecomunicaciones telecom-bolivar, por facilitar la utllizaclon de su estacion de comunicaciones en el cerro la popa-cartagena. referencias bibliograficas axis instruments company. 1993: fg-5 absolute gravimeter. axis. faller, james e. 1971: bogota-colombia: final reporton absolute gravity determination. interamerican geodestic survey. hammond, james a. & j.e.faller. 1971: results of absolute gravity determinations at a number of different sites. j.g.a. vo1.76,no.32. instituto geografico "agustin codazzi". 1995: actualizaci6n de la red gravirnetrica nacional de primer orden. santate de bogota. mcconnell, r.k., i.escobar, l.sanchez, m.i.pastorino, m.araneda, m.avendano & j.l.telleria. 1995: progreso reciente en el desarrollo de la red latinoamericana de gravedad 1995. iii reunion tecnica de la cornision de geoffsica, instituto panamericano de geograffa e historia. mexico. torge, w. 1989: absolute gravimetry in its operational phase-some results and problems. 6th international symposium "geodesy and physics of the earth", proceedings part ii, pp.1-13. akademie der wissenschaften der ddr, postdam. archivo05.indd 41 manuscript received february 27, 2006. earth sciences research journal earth sci. res. j. vol. 10, no. 1 (jun. 2006): 41-51 application of a heuristic method for the estimation of s-wave velocity structure andrés josé alfaro castillo ciees, bogotá, colombia. e-mail: alfaro@ciees.org abstract the assessment of local site effects is one of the most important subjects in engineering seismology. in order to perform an assessment, it is necessary to determine the s-wave velocity structure of the site. additionally, in some basins, it is very important to know the deep sedimentary structure, due to the amplification phenomena of low frequency waves. there are several techniques to achieve this purpose; probably the most inexpensive technique is using the vertical component of microtremors measured with an array of seismographs. the phase velocity of rayleigh waves is inverted to an s-wave velocity (vs) profile using optimization techniques. most of the time, least square methods have been applied in the inversion. recently, heuristic methods have also been used for the estimation of the s-wave velocity structure from microtremor. in this study seven arrays of microtremors in the city of tsukuba city were performed, located to the ne edge of kanto basin, in order to estimate the deep s-wave velocity structure. the spatial autocorrelation method spac was used to determine phase velocity dispersion curves in the frequency range from 0.3-2.5 hz. the determination of vs profiles reached a depth of 750 m. two methods were used to estimate the swave velocity structure: inversion method and a heuristic method via the combination of downhill simplex algorithm with a very fast simulated annealing method. comparisons with vs from the existent results from ps-logging tests at the center of the array showed the reliability of the heuristic method. key words: heuristic method, annealing method, inversion, kanto basin, tsukuba, spac, shear wave velocity. resumen la evaluación de los efectos locales es una de las labores más importantes en la ingeniería sismológica. con el fin de realizar una evaluación es necesario determinar la estructura de velocidades de ondas s del sitio. adicionalmente, en algunas cuencas, es importante conocer la estructura de los sedimentos profundos, debido al fenómeno de amplificación de ondas de baja frecuencia. existen varias técnicas para lograr este propósito, probablemente la menos costosa es el uso de la componente vertical de los microtemblores registrados mediante un arreglo de sismógrafos. la velocidad de fase de las ondas rayleigh se invierte para estimar un perfil de velocidades de ondas s (vs) usando técnicas de optimización. en la mayoría de los casos se ha aplicado el método de los mínimos cuadrados en la inversión. recientemente, los métodos heurísticos también han sido utilizados para la estimación de la estructura de velocidad de las ondas a partir de microtemblores. 42 andrés josé alfaro castillo en este estudio se desplegaron siete arreglos para microtemblores en la ciudad de tsukuba (japón), ubicada en la parte nororiental de la cuenca de kanto, con el fin de determinar la estructura profunda de velocidad de las ondas s. para determinar las curvas de dispersión de velocidad de fase en el rango de frecuencias 0.3-2.5 hz se utilizó el método de la autocorrelación espacial spac. la determinación de los perfiles de vs alcanzó una profundidad de 750 m. se utilizaron dos métodos para estimar la estructura de velocidad de las ondas s: un método de inversión y un método heurístico vía la combinación del método downhill simplex algorithm con el método very fast simulated annealing. las comparaciones de la estructura de velocidades vs con los resultados existentes de pruebas ps de registros de pozo en el centro del arreglo demostraron la confiabilidad del método heurístico. palabras clave: método heurístico, método de annealing, inversión, cuenca de kanto, tsukuba, spac, velocidad de cizalla. © 2006 esrj unibiblos microtremors techniques have been accepted during the last decades as a really good tool for reconnaissance and research of both shallow and deep soil structures (alfaro, 2005a, 2005b; alfaro and yokoi, 2005; alfaro 2006). there are several techniques using microtremors, during the last years, however horizontal vertical spectral ratio techniques (hvsr) have been used all over the world after nakamura’s classical paper (1989) to determine the soil’s predominant periods and dynamical classification soil (alfaro et al. ,2001; bhattarai, 2005 ); hvsr has prompted several discussions due to the lack of robustness in the theory (horike et al., 2001), arai and tokimatsu (2000, 2004) however, developed a technique that allows the determination of vs structure by means of inversion of hvsr. they developed a complete formulation assembling surface waves to achieve the inversion. in this research, however a microtremor array technique was used, because it has a robust theory and it has been used all over the world, mainly in japan. the method uses the microtremor records obtained at stations deployed in a triangular array. the measurements are taken simultaneously at all stations, which are operating for a short duration of time. the analysis of the microtremor records is performed through the spatial autocorrelation coefficient method (spac method) introduced by aki (1957, 1965) and established by okada (2003). it is important to mention another technique for arrays analysis, frequency-wavenumber spectrum method (f-k), developed by capon (1969) and introduction the geometry of the subsoil structure, the soil types and the variation of their properties with depth, along with the lateral discontinuities and the surface topography can produce large amplifications of ground motion and increase the damage during destructive earthquakes. for this reason the accurate knowledge of the geometry and the vs structure of alluvial–diluvial deposits and the basement are very important in microzonation studies. the vs structure is usually determined in the field by using conventional seismic prospecting techniques (reflection, refraction, boreholes) and in the laboratory through dynamic tests on soil samples. the use of conventional seismic exploration methods presents some difficulties when the deep sedimentary structure needs to be determined. for example, in reflection and refraction surveys, the use of artificial sources such as explosives or vibrators is necessary, practices that sometimes are not easy in urban areas. furthermore, the dimensions of the required arrays are large according to the desired penetration depth and therefore it is difficult to be deployed in populated areas. additionally, the cost of large scale deep geophysical prospecting is high, and for this reason, in most cases during site effect studies, the depth of the seismic basement is limited to a layer with vs larger than 400 m/s (engineering bedrock) and not the real very deep underground reflector of the incident waves (vs > 3000 m/s, seismic bedrock). additionally, the cost for implementing a deep borehole is also high and the results are valid only for a single site. 43 application of a heuristic method for the estimation of s-wave velocity structure applied to microtremors by horike (1985) and okada (2003). the f-k method is used to estimate the dispersion curve of rayleigh waves and the velocity structure. the weakness of this method is the need of simultaneous measurement with several stations. the spac method is based on the theory of the stationary random functions, according to which, microtremor is considered as a stationary stochastic process both temporally and spatially. in this study, microtremor measurements were performed at one site, representative from the geological point of view, in the city of tsukuba, where information of a deep borehole of 1300 meters is available (hayashi, 2005; hayashi et al., 2005). the practical aim of this study is to estimate a vs profile, especially for depths larger than 500 m, reaching bedrock depth. to examine the efficiency and the accuracy of the method the results are compared with the borehole vs profile at the site. theoretical background conventional spac method aki (1957, 1965) presented a theoretical background for estimating phase velocities by means of the spac method. for this research yokoi (2005a, 2005b, and 2005c) developed a set of computer codes following okada (2003) and morikawa et al. (2004) in order to obtain the s-wave velocity structure. the phase velocity at frequency ω will be obtained as the argument of the bessel function. in this study we used a software b_fit developed by yokoi (2005a) to find the optimum value for phase velocity. that program uses the levenberg-marquardt method and some subroutines from press et al. (2003). to perform the inversion two programs were used: disp_sma1 (yokoi, 2005b) and surf96 (herrmann and ammon, 2004). disp_sma1 is a program to obtain the optimum underground velocity structure for a given dispersion curve of rayleigh waves based on the downhill simplex method combined with the simulated annealing approach. simulated annealing method the simulated annealing method is based on the idea of thermodynamics in which a metal melt reaches to a low-energy state through gradual decrease in temperature (metropolis et al., 1953). after that kirkpartrick et al., (1983) applied the idea to optimization problems with an analogy between thermodynamics and optimization as shown in table 1. the misfit to be minimized during inversion corresponds to energy in thermodynamics, and parameter changes due to variations of material state. this move of parameters is controlled by cooling schedule of the system with decrease temperature. table 1. analogy between thermodynamics and optimization thermodynamics optimization material state possible model energy objective function change of material state move to neighbor model temperature control parameter freezing state heuristic solution the algorithm of the simulated annealing method is shown in figure 1. first, a cooling schedule and an initial temperature are defined, t 0 . search areas for all the unknown parameters are also defined before calculation. then, an initial model, m 0 , is randomly generated within the defined parameter spaces. the misfit, e (m 0 ) for the initial model is calculated using equation (1). ∑ = − = n j j j o j c t tctc n me 1 )( )()(1 )( σ (1) where cc(t j ) and co(t j ) are calculated and initial phase velocities at a period of t j . σ(t j ) and n are the standard deviation and number of observations. next, a random perturbation is added to the initial model to generate a neighbor solution, m 1 . the misfit order is calculated again, e(m 1 ) for the neighbor model. if the difference of the misfits of the two models, δe = [e(m 1 )-e(m 0 )], is negative, m 1 becomes the present model. if the difference is positive (m 1 is the worse model), m 1 is still chosen as the present model with a probability p = exp(δe/t). because of the temperature-dependent probability, a model with high misfit is frequently 44 andrés josé alfaro castillo chosen at high temperature. at low temperature, a not suitable model is not often selected and only a good model becomes the present model. after these processes have been repeated for all the parameters at predetermined times, the temperature is decreased according to the cooling schedule. the present model can be modified to the neighbor model near the global solution by repeating the above processes. although, the simulated annealing method is one of various local search methods using a perturbation of the model parameters, the simulated annealing method works as a global search method at high temperature, because it allows an incremental change on the misfit surface. however, works as a local search method at low temperature. this feature is different from pure random search methods, such as the monte carlo search method. there are several algorithms used in the simulated annealing method. in this paper the very fast simulated annealing ((vfsa) ingber, 1989) is used. in the very fast simulated annealing, a perturbation is generated using equation (2). )( minmax01 ii i ii mmymm −+= (2) where y i is defined in (3)         −    +−= − 1 1 1)5.0sgn( 12 iu k k ii t tuy (3) y i is generate from a ui from the uniform distribution [ ]1,0uui ∈ the cooling schedule is based on (4) )exp(0 a k cktt −= (4) where c and a are constants, in this study to=1.0, a=0.06 and c=1.3 to achieve a fastest convergence. figure 1. algorithm for s-wave velocity structure assessment using the simulated annealing method (yamanaka, 2004). downhill simplex method the downhill simplex method developed by nelder and mead (1965) method, requires only function evaluations, not derivatives. according with press et al. (2003) the method is not very efficient in terms of the number of function evaluations that it requires. however, the downhill simplex method may frequently be the best method to use if the figure of merit is “get something working quickly” for a initial model (m 0 ) and temp t i = t o calculation of e (m 0 ) generation of neighbor model m 1 = m 0 + μδm calculation of e (m 1 ) calculation of δ e = e (m 1 ) e (m 0 ) yes no e < = 0 ? save m1 as new model with prob. of exp (δ e/ti) save m 1 as new model (m 0 = m 1 ) a a no no yes yes iteration at ti? decrease temperature number of temp. ? near global model m 0 45 application of a heuristic method for the estimation of s-wave velocity structure problem whose computational burden is small. in this study some subroutines from press et al. (2003) were used. observation of microtremors the data of microtremors were recorded in the north-eastern part of the kanto basin, in the city of tsukuba (japan), where the depth to bedrock was found to be about 600 m with a deep borehole. observations were performed on july 27 and 28 2005; additional measurements were done on august 4 and 16 to improved some data sets of non acceptable quality. observations included seven arrays: one with 29 m radius (largest side of 50 m); two with 115 m radius (largest side of 200 m); two with 290 m radius (largest side of 500 m) and two with 520 m radius (within largest side 900 m). the geometries of the arrays are shown in figures 2 and 3. hereafter, these configurations are called r and b arrays. the smallest arrays (50 m side) and the middle array (200 m side) are shown in figure 2. there were simultaneous observations at four sites, with vertical-component velocity type seismographs (vse12-cc manufactured by tokyo sokushin ltd.), with a natural period of to = 10 s, also the short period velocity seismometers (hs-1, manufactured by oyo-geospace) with 2 hz natural frequency. the microtremors were recorded by digital recorders mcseis-mt (manufactured by oyo corporation) with a resolution of 24 bits for a/d conversion, finally converted with an analog band pass filter, which was from 0.1 s to 5.0s, the sampling frequency was 100 hz. when applying the spac method it is only necessary to gather the vertical component to obtain rayleigh waves and avoid any kind of interference due to the presence of love waves, making the analysis easier (aki, 1957). the records were synchronized with the time code generated by global position system (gps) clocks. three sets of data on 200 m array, 10 sets of data with 500 m array (total duration 2 hours) and 10 sets of data with 900 m array (total duration 2 hours) were gathered. to ensure to check the stability of environmental conditions and instruments, data was gathered for all sensors in a single place. after this, stability analysis sensors were deployed in the field, starting with the smallest array. on the other hand, data quality depends on the amount of interferences from anthropogenic sources like vehicles; in this study 500 m arrays recorded the highest interference due to the location of b5 and r5 stations, close to an avenue with intense traffic, particularly trucks. an example of waveforms can be observed in figure 4, which corresponds to microtremors recorded with a 900 m array. using the spac method it is possible to assess spac coefficients, as shown in figure 5, which are functions of distance and frequency. for low frequencies, spac coefficients have maximum values. the next step in the analysis is to fit a zero-order bessel function of the first kind for every frequency, this procedure implies the verification of the values that could be used for fitting. figure 6 shows some examples of fitting bessel functions. for certain frequencies it is possible to use data from various distances, in the case of low frequencies, however, it is possible to use only data from the largest array. the b_fit program by yokoi (2005a) was used. by means of inversion it is possible to assess the dispersion curve; examples are shown in figure 7. figure 2. location of the observation sites. the observation sites are located in the northern part of the kanto basin, japan, in tsukuba city, where the depth of bedrock is known to be about 600 m by a deep borehole (hayashi et al., 2005). arrays dimensions are 50 m side x 200 m side. 46 andrés josé alfaro castillo figure 3. b8 is the center of the b arrays and r4 is the center of the r arrays. b5-b6 length is 500m. r1-r2 length is 900m. observations were carried out during workdays. note that b5 and r5 are located in an avenue. figure 4. an example of the microtremor records for 900 m array; stations r4, r1, r2 and r3 respectively. each trace indicates vertical ground velocity component. 4 3 2 1 0 -1 -2 -3 -4 4 3 2 1 0 -1 -2 -3 -4 4 3 2 1 0 -1 -2 -3 -4 4 3 2 1 0 -1 -2 -3 -4 2000 4000 6000 8000 10000 12000 14000 16000 180000 2000 4000 6000 8000 10000 12000 14000 16000 180000 2000 4000 6000 8000 10000 12000 14000 16000 180000 2000 4000 6000 8000 10000 12000 14000 16000 180000 v el oc ity m ki ne s samples 47 application of a heuristic method for the estimation of s-wave velocity structure figure 5. spac coefficients obtained from simultaneous observations, 200 m array. figure 6. an example of fitting zero-order bessel functions of the first kind for several frequencies (0.30 hz, 0.49, 1.53 and 2.50 hz). “*” = data from 50 m array, “+” = data from 200 m array, “x” = data from 500 m array. 1 0.5 0 -0.5 -1 0 1 2 3 4 5 sp a c c oe ffi ci en ts frequency (hz) 48 andrés josé alfaro castillo determination of vs structure the phase velocity was inverted to vs structure using two methods: the least squares method (herrmann and ammon, 2004) and the combination of downhill simplex algorithm with very fast simulated annealing method (ingber, 1989; yokoi, 2005c). the misfit to be minimized during inversion corresponds to energy in thermodynamics, and parameter change does to move of material state. this move of parameters is controlled by cooling schedule of the system with temperature decrease. figure 7 shows dispersions curves obtained with sets of 50 m data array plus r200 m data arrays plus sets of r900 m array. figure 8 shows one example of results using surf96 (herrmann and ammon, 2004), and figure 9 shows the results obtained using: the very fast simulated annealing method, which are in good agreement with existing ps logging and borehole data, and the results derived by herrmann and ammon (2004).. results using both methods are suitable, however one advantage of the heuristic method via the combination of downhill simplex algorithm with very fast simulated annealing method is the possibility of assessing thickness and vs, meanwhile in herrmann and ammon (2004) it is necessary to fix either the thickness or the vs of each layer. advantages of the surfer96 software include the calculation of: standard error (km/s); mean residual (km/s); average residual (km/s) and percent of signal power fit in percentage. these results indicate that the combination of downhill simplex algorithm with very fast simulated annealing method is a promising tool in phase velocity inversion. probably, this is also true for other inversion problems in engineering seismology. discussion and conclusions among several geophysical prospecting methods, passive methods have the advantage they do not need either artificial sources that could disturb people or expensive drilling. judging from the waveforms obtained, mainly for station r5 and b5 in 500 m arrays, they recommend avoid large avenues or roads, because waveforms could include interferences and results could be not suitable. also, it is advisable perform the measurements during hours with minimum interferences due to punctual loads, like heavy trucks, or follow the recommendation of apostolidis et al. (2004) to locate the stations at least 50 m far from avenues or roads. the most significant advantage, of estimation of shear wave velocity structure, using arrays of long period microtremors, is that the method allows for the reliable determination of vs profiles down to large depths (about 600 m) with relatively small apertures of the deployed arrays (900 m). this is significant for accurate soil response studies in large cities, where open free spaces suitable for deployment of large conventional arrays are difficult to find and high energy sources cannot be easily used. . acknowledgements the author gratefully acknowledges the opportunity and the support given by the japan international cooperation agency jica, the international institute of seismology and earthquake engineering iisee. also thank to dr. toshiaki yokoi, for his permanent support during the research, to mr. koichi hayashi and mr. kunio aoike from oyo corporation whom cooperation during field measurement was priceless. most of the figures were draw using gnuplot (williams and kelly, 2004). 49 application of a heuristic method for the estimation of s-wave velocity structure figure 7. phase velocities of rayleigh waves obtained from vertical component of microtremors. the figure shows the comparison of three sets of 200 m dispersion curve with 2sspac. figure 8. vs soil structure using surf96 (herrmann and ammon, 2004). the 900 m arrays allowed observing periods as long as 3 s. 50 andrés josé alfaro castillo references • aki, k. 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(2005a). combination of downhill simplex algorithm with very fast simulated annealing method an effective cooling schedule for inversion of surface wave’s dispersion curve. proc. fall meeting of seismological society of japan. s16-08010854. • yokoi, t. (2005b). b_fit. program for fitting the spatial auto-correlation coefficient determined from observed data to bessel function and determine the dispersion curve. international institute of seismology and earthquake engineering. • yokoi, t. (2005c). disp_sma1 program to obtain the optimum underground velocity structure for the given dispersion curve of rayleigh wave based on the down hill simplex method combined with the simulated annealing approach. international institute of seismology and earthquake engineering. earth sci. res. j. vol. 11, no. 2 (december 2007): 97-107 mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications awni t. batayneh. department of geology, king saud university , po box 2455 , riyadh 11451 , saudi arabia corresponding author. tel.: ++966-56-8086395. e-mail address: awni_batayneh@yahoo.com (a. t. batayneh) abstract during 2006, geoelectrical measurements using the vertical electrical sounding (ves) method were conducted on the eastern red sea coast in jordan, using the syscal-r2 resistivity instrument. the objectives of the study were (i) to evaluate the possibility of mapping of quaternary sediments medium in areas where little is known about the subsurface geology and to infer shallow geological structure from the electrical interpretation, and (ii) to identify formations that may present fresh aquifer waters, and subsequently to estimate the relationship between groundwater resources and geological structures. data collected at 47 locations were interpreted first with curve matching techniques, using theoretically calculated master curves, in conjunction with the auxiliary curves. the initial earth models were second checked and reinterpreted using a 1-d inversion program (i.e., resix-ip) in order to obtain final earth models. the final layer parameters (thicknesses and resistivities) were then pieced together along survey lines to make electrical cross sections. resistivity measurements show a dominant trend of decreasing resistivity (thus increasing salinity) with depth and westward toward the red sea. accordingly, three zones with different resistivity values were detected, corresponding to three different bearing formations: (i) a water-bearing formation in the west containing red sea saltwater; (ii) a transition zone of clay and clayey sand thick formation; (iii) stratas saturated with fresh groundwater in the east disturbed by the presence of clay and clayey sand horizons. deep borehole (131 m) drilled in the northwestern part of the study area for groundwater investigation, has confirmed the findings of the resistivity survey. key words: red sea coast; geoelectrical measurements; geology; hydrogeology; jordan. resumen durante 2006, mediciones geoeléctricas que usaban el método eléctrico vertical de sondeo (evs) fueron realizadas en la costa oriental del mar rojo en jordania, usando el instrumento de resistividad syscal-r2. los objetivos del estudio fueron (i) evaluar la posibilidad de cartografiar los sedimentos cuaternarios en áreas donde poco se sabe sobre la geología subterránea y deducir 97 earth sciences research journal manuscript received july 8 2007. accepted for publication december 10 2007. estructuras geológicas someras a partir de interpretación eléctrica, y (ii) identificar formaciones que pueden presentar acuíferos, y posteriormente estimar la relación entre recursos de agua subterránea y estructuras geológicas. los datos recolectados en 47 sitios fueron interpretados primero con técnicas para emparejar las curvas, usando curvas principales teóricamente calculadas, conjuntamente con las curvas auxiliares. los modelos iniciales terrestres fueron comprobados y reinterpretados en segundo lugar usando un programa de inversión 1-d (i.e., resix-ip) para obtener modelos finales terrestres. los parámetros finales de la capa (espesores y resistividades) entonces fueron ensamblados a lo largo de líneas de medición para hacer secciones transversales eléctricas. las mediciones de resistividad demuestran una tendencia dominante de disminución de la resistividad (además incremento de la salinidad) con la profundidad y hacia el oeste del mar rojo. por consiguiente, tres zonas con diversos valores de resistividad fueron detectadas, correspondiendo a tres diferentes formaciones portadoras: (i) una formación acuífera en el oeste que contiene el agua salada del mar rojo; (ii) una zona de transición entre arcilla y arena gruesa arcillosa; y (iii) estratos saturados con agua subterránea fresca en el este con presencia de arcilla y horizontes arcillosos arenosos. pozos (131 m) perforados en la parte noroccidental del área del estudio para la investigación de agua subterránea, han confirmado los resultados. palabras claves: costa del mar rojo; mediciones geoeléctricas; geología; hidrogeología; jordan. introduction the problem of the salination of groundwater aquifers arises in coastal areas, where the excessive pumping of unconfined coastal aquifers by water wells leads to the intrusion of sea water. this negative effect of human activity has been recorded in many areas of the world. hence, this problem is likely to arise in areas like jordan that has poor water resources (low precipitation and high evapotranspiration) and has mismanagement of water resources (e.g., batayneh, 2006; batayneh and qassas, 2006). jordan is considered as one of the ten poorest countries in water in the world. an arid climate, high natural growth rates, and forced migrations have conspired to push available water resources to the limit. annual rainfall in jordan ranges from 600 mm in the northwestern highlands to less than 100 mm in the eastern and southern regions. it is estimated that 80.6% of jordan receives less than 100 mm of rainfall per year (salameh and bannayan, 1993). assuming that the average rainfall in this area is 70 mm, dry areas in jordan receive 5 billion m3 a year. most of this water flows in small drainage basins or wadis to end up in playas (qa’s), and ultimately are lost to evaporation. due to the scarcity of boreholes in the east area of the red sea coast in jordan which could provide information on the configuration of the different water bodies, vertical electrical sounding (ves) survey, utilizing a schlumberger array configuration, syscal-r2 resistivity instrument (iris instruments, france), were performed on 47 sites for several purposes: (1) verification of the presence of the different water-bearing formations and estimation of their depth and thickness; (2) finding the relationship between the resistivity variations and the different configurations of the water-bearing formations; and (3) mapping the water table in the shallow coastal aquifer and selecting new location(s) for drilling. data from a single shallow (33 m) borehole (k1, fig. 1) drilled by the jordan phosphate mines company showed saline water (tds > 30,000 mg/l) at 32 m deep, and a 131 m deep borehole (k3028, fig. 1) drilled by the water authority of jordan for groundwater investigation encountered saline water (tds > 27,000 mg/l) at 127.5 m deep. the data were analyzed and used to correlate the results of the geoelectrical surveys (see section field measurements and methods of interpretation). the k1 shallow borehole penetrated alternating bands of gravel and sand down to a depth of about 4 m which is underlain by a unit composed by approximately 25 m clay and clayey sand sediments. the third unit corresponds to the sand/sandy clay containing saline water (saturated). data from mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications 98 batayneh. esrj vol. 11, no. 2. december 2007 99 the k3028 deep borehole shows that three units of sediments were found. the upper unit has approximately a thickness 36.5 m and consists of medium to coarse grained size gravel and sand. the underlying 91 m thick unit is mainly composed by clay and clayish sand. the third unit is composed by sand/sandy-clay sediments containing saline water (saturated). study area the area under study is approximately located at 30 km to the south of aqaba city in the southwestern part of jordan. it is limited to the south by the jordanian-saudi border and to the west by the red sea (fig. 1). the study area is located in an extremely arid environment with an annual average precipitation of 70 mm. rainfall generally occurs during the winter months (november to january). however, there are years where the rainfall is absent; while in other years, ephemeral floods of short duration may occur. the climate of the region is very hot in summer (april to august) with temperatures exceeding 38 ºc in summer (april to august). the area under investigation was included in the 1:50,000 national geological mapping project carried out by the geological mapping division of the natural resources authority of jordan (al khatib, 1987). this map shows that metamorphic rocks of the late proterozoic aqaba complex dominate the eastern side of the study area (fig. 1). it varies in composition from monzogranite to alkali feldspar granite. the coastal plain between the metamorphic rocks and the red sea consists mainly of quaternary continental sediments. these constitute clastics (clay, sand, and gravel) deposited in fan deltas, with some intercalations of lacustrine sediments (clay, gypsum, and aragonite) of pleistocene age (al khatib, 1987). the coastal plain area (fig. 1) is approximately 7 km long and 9 km wide and is accessible from a modern highway joining jordan with saudi arabia. the alluvial shallow aquifer is the primary source of water for domestic, municipal, and industrial use in the region. the recharge to this aquifer takes place either along the elevated areas in the east and northeast sides, or due to local surface water infiltrations. field measurements and methods of interpretations figure 1. location of ves sites (black triangle) in relation to geology. also shown is the location of two profile lines a-a’ and b-b’ and two boreholes. the elevation contours are in meters. the inset map shows the location of the investigated area on jordan. palestine grid are used. surface resistivity methods have been used for groundwater research for many years. earth resistivities are related to important geologic parameters of the subsurface including types of rocks and soils, porosity, and degree of saturation (keller and frischknecht, 1966). it was shown by parasnis (1956 and 1966) that the electrical resistivity of rocks and minerals, except for massive sulfides and graphite, vary in a wide range between 1 to 107 ohm-m, whereas coastal aquifers that are prone to saline water are identified by relatively low resistivity values. thus, saltwater can be easily distinguished from almost any combination of lithological types. resistivity methods are used to map the freshwater-saltwater interface and for studying conductive bodies of hydrogeological interest (e.g., zohdy and jackson, 1969; ayers, 1989; barongo and palacky, 1991; khair and skokan, 1998; gnanasundar and elango, 1999; mukhtar et al., 2000; batayneh, 2006). in general, the resistivity method involves measuring the electrical resistivity of earth materials by introducing an electrical current into the ground and monitoring the potential field developed by the current. the most commonly used electrode configuration for geoelectrical soundings, which was used in this field survey, is the schlumberger array. four electrodes (two current a and b and two potential m and n) are placed along a straight line on the land surface such that the outside (current) electrode distance (ab) is equal to or greater than five times the inside (potential) electrode distance (mn). vertical sounding, in schlumberger array, were performed by keeping the electrode array centered over a field station while increasing the spacing between the current electrodes, thus increasing the depth of investigation. the potential difference (δv) and the electrical current (i) are measured for electrode spacing and the apparent resistivity (ρa) is calculated by the equation: where is the geometrical factor that depends on the electrode arrangement for the schlumberger array. a total of 47 ves stations were established across the study area. the data were collected using a syscal-r2 resistivity instrument (iris instrument, france). the layout of the survey stations is superimposed on the geological map in figure 1. the locations of the ves sites were considerably restricted by logistical difficulties. the presence of narrow valleys and topography prevented a wider coverage. the maximum ab/2 spacing of the schlumberger array ranged from 15 m to 900 m. the separation of the current electrodes was = 3, 4, 6, 8, 10, 12, 16, 20, 24, 30, 40, 50, 60, 80, 100, 150, 200, 250, 300, 350, 400, 500, 600, 800, 1000, 1200, 1400, 1600, and 1800 m. the potential electrode separation was = 1, 10, 20 and 40 m. the increase of the potential electrode separation mn allowed that readings from the same current electrode spread ab with the previous and expanded mn were taken. the sounding curves were subjected to a preliminary interpretation using the partial curve matching technique by zohdy (1965), and orellana and mooney (1966). based on this preliminary interpretation, initial estimates of the resistivities and thickness (layer parameters) of the various geoelectric layers were obtained. in a second analysis method, the layer parameters derived from the graphical curve matching was then used to interpret the sounding data in terms of the final layer parameters through a 1-d inversion technique (e.g., resix-ip, interpex limited, golden, co., usa). inversion analyses of the sounding curves have been made with an average fitting error of about 5%. quantitative interpretation of geoelectrical sounding curves is complicated due to the well known principle of equivalence (van overmeeren, 1989). data from the k3028 borehole (fig. 1) was used to minimize the choice of equivalent 100 mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications    1m-ohm ι δv κρa   2 mn an.am πκ  101 batayneh. esrj vol. 11, no. 2. december 2007 models by fixing thicknesses and depths to certain levels, and allowing the adjustment of resistivity. correlation between ves stations 4 and 5 and lithology from the k3028 borehole was performed (fig. 2) in order to determine the electrical characteristics of the rock units with depth. based on the lithological log from the k3028 borehole, the geological interpretation of the geoelectrical model for ves 4 and ves 5 (fig. 2) is: (i) a resistive layer (500-1700 ohm-m) with variable thickness and moisture content at the surface, which consists of a mixture of gravel, silt, and sand; (ii) a thick clay and clayish sand layer having resistivity values of 30 to 40 ohm-m; and (iii) a saturated sandy layer with saline water having resistivity values of 1 to 10 ohm-m. field results analysis of ves curves due to the distinctive characteristics features in the field of the apparent resistivity curves, the ves stations show four types of curves: type i, type ii, type iii, and type iv (figure 3). these types were defined in terms of the number of geoelectrical layers and their respective resistivity relationships. among the four types of curves, type i and type ii curves show similar shape of field curves with layer resistivities decreasing with depth such that ρ1>ρ2>ρ3. such curve behaviour undoubtedly proves the presence of a low-resistivity layer at the bottom of the section. of the 47 field curves, 19 field curves at ves stations 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 20, 21, 22, 28, 29, 30, 38, and 39 were classified as type i (fig. 3a), and 15 of the field curves at ves stations 12, 13, 18, 19, 24, 25, 26, 27, 36, 37, 43, 44, 45, 46, and 47 were classified as type ii (fig. 3b). eight of the field curves at ves stations 15, 16, 17, 31, 32, 33, 34, and 35 were classified as type iii and reflect the presence of five geoelectric layers where the layers resistivity relationship is ρ1>ρ2<ρ3>ρ4<ρ5 (fig. 3c). the remaining five field curves at ves stations 14, 23, 40, 41, and 42 made in the eastern and southeastern parts of the study area were classified as type iv and reflect the presence of three geoelectric layers where the layers resistivity relationship is ρ1>ρ2<ρ3 (fig. 3d). it is to be noted that the apparent resistivity values representing the surface layer in the four types of field curves vary considerably at different ves locations, mainly due to the fact that resistivity depends on the soil moisture content. the analysis of each type of curve in relation to groundwater in the study area is discussed in detail in the following sections. type i the type i curve (fig. 3a) generally shows: (1) a relatively thin surface layer of coarse grained loose sand, gravel, and sand dune existing below the ground surface having apparent resistivity of 300 to 1800 ohm-m, followed by (2) the presence of sand, clay, sandy clay, and gravel of varying grain size (fine to medium) of 50300 ohm-m apparent resistivity; and (3) a lowresistivity third layer with apparent resistivity < 12 ohm-m, indicating saline water. the type i curves were obtained from soundings measured in the western side of the study area (fig. 1). type ii the type ii curve (fig. 3b) is composed by (1) a thin layer of medium to coarse grained sand and gravel (surface layer) with an apparent resistivity of about 250 to 1300 ohm-m, (2) a second fine to medium grain size sand and gravel layer (100500 ohm-m), and (3) a relatively low-resistivity third layer (20-100 ohm-m) indicating a clay and clayish sand formation. the fields curves of type ii are obtained in the north-northeast trending narrow zone located to the east of the type i curve (fig. 1). type iii the type iii curves (fig. 3c) were obtained from soundings taken from profiles ii and iv. apparent resistivity values of these curves are relatively low, when compared to the other three types. the type iii field curves describe qualitatively a model composed of five layers where the layer resistivity relationship is ρ1>ρ2<ρ3>ρ4<ρ5. 102 mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications figure 2. correlation between sounding curves and borehole well lithology. figure 3. apparent resistivity curves 103 from the top to the bottom, these are: (1) a sandy, clayish sand, and gravel of fine grained size having apparent resistivity of 60 to 250 ohm-m, followed by (2) the presence of clay and clayish sand formation (40-80 ohm-m), a (3) saturated sand with good groundwater quality (100 to 200 ohm-m), (4) a clay and clayish sand formation (40-80 ohm-m), and (5) a sandy formation saturated with fresh groundwater (80200 ohm-m). type iv the type iv sounding curves (fig. 3d) describe qualitatively a model composed by (1) a relatively thin surface layer with an apparent resistivity of 200 to 1300 ohm-m of medium to coarse grained material with a variable moisture content, followed by (2) the presence of a relatively low-resistivity second layer (4080 ohm-m), indicating a clay and clayish sand formation, and (3) a sandy formation saturated with fresh groundwater (100-200 ohm-m). the apparent resistivity values increase with depth from 100 to 200 ohm-m depending on the saturated condition. figure 4 is a typical sounding data plot and bestfit model for four sounding data corresponding to the four types of field curves (i, ii, iii, and iv; fig. 3). it is of interest to note that the soundings of fig. 4a were derived from the western part (site 1, type i); fig. 4b is based on sounding (site 37, type ii); fig. 4c is based on sounding (site 33, type iii); fig. 4d is based on sounding in the southeastern (site 41, type iv) side. the best-fit models are derived along geoelectrical soundings points with lower and upper bound models (1-5% error). to the left of fig. 4, it is shown the schlumberger apparent resistivity curve with data (points) superimposed on the best match 1-d inversion (solid line). to the left of fig. 4, the interpreted results in terms of resistivity and depth together with the allowable of equivalence (dashed lines) are shown. electrical cross sections the results from the 1-d inversion of the geoelectrical soundings were compiled and plotted along two typical profiles: a-a’ and b-b’ (fig. 1). in these sections, the thickness of the subsurface layers and their geological structures are given. two interpreted resistivity vs. depth cross sections are shown in figs. 5 and 6. cross section a-a’ (fig. 5) represents the behaviour of the northern side of the studied area. the distribution of the resistivity along the geoelectrical profile indicates the presence of two zones with different properties: the first zone is from sounding point 1 to 11 corresponding to type i field curves and the second is from 12 to 13 representing type ii field curves. as seen in fig. 5, the first zone shows resistivity values in the range 400-1800 ohm-m at the surface, which are considered caused by coarse grained sand, gravel, and sand dune at 3-50 m depth. in general, the layer thickness increases with increasing distance from the coast, followed by the presence of a relatively conductive second clay and clayish sand layer, ranging from 20 to 100 ohm-m, at shallower depths and thickening landwards. the top of this layer is above mean sea level (msl), by several hundred meters at sites 7, 8, 9, 10, and 11 (fig. 5), followed by conductive layer with resistivity values of 1 to 11 ohm-m corresponding to saline water. the depth of this conductor is closed to msl at coastal sites and show increasing depth with increasing distance from the coast. in the second zone shown in fig. 5 (sites 12 and 13), the range is from 400-600 ohm-m and 5 to 15 m thick at the surface, which are considered caused by sand and gravel of varying grain size, followed by the presence of sand and sandy clay having resistivity values of 200 to 300 ohm-m and 10 to 30 m thick, followed by the presence of a relatively conductive third clay and clayish sand layer that has a resitivity value of 40 ohm-m at shallower depths. cross section b-b’ (fig. 6) represents the behaviour of the southern part of the studied area. the cross section shows three different zones with different properties: the first zone is from sounding point 39 to 38 (type i field curves), while the second zone from 37 to 36 (type ii field curves) and the third zone from 35 to 31 (type iii field curves). the first zone shows resistivity values in the range 500-600 ohm-m at the surface, considered to have been caused batayneh. esrj vol. 11, no. 2. december 2007 104 mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications by coarse grained sand, gravel and sand dune at 10-20 m depth, followed by the presence of a relatively conductive second clay and clayish sand layer, ranging from 30 to 90 ohm-m, at shallower depths and thickening landwards, followed by conductive layer with resistivities of 2 to 3 ohm-m corresponding to saline water. in the second zone (fig. 6, sites 37 and 36), the range is from 200-600 ohm-m and 40 to 50 m thick at the surface, considered to have been caused by sand and gravel of varying grain size (fine to medium), followed by the presence of sand and sandy clay having resistivity values of 80 to 100 ohm-m and about 100 m thick, followed by the presence of a relatively conductive third clay and clayish sand layer that has a 30 ohm-m resistivity value. in the third zone (fig. 6, sites 35, 34, 33, 32 and 31), the main features of the derived structure, from the surface downward, may be summarized as follows: (1) a relatively thin surface layer of fine to medium sand, gravel, and sand clay that has a 100-300 ohm-m resistivity value and 5-15 m thick, followed by (2) the presence of clay and clayish sand having resistivity values of 50 to 90 ohm-m and 30 to 40 m thick, followed by (3) saturated sand with good groundwater quality having resistivity values of 100 to 270 ohm-m and 40 to 80 m thick; (4) a clay and clayish sand formation (50-80 ohm-m resistivity values and 40-90 m thick), and (5) a sandy formation saturated with fresh groundwater (100-350 ohm-m). as seen from fig. 6, the cross section is characterized by low to moderate resistivity values at the surface when compared with the cross section (fig. 5). this decrease in resistivity values from about several hundred ohm meters on the surface along profile a-a’ to less than 650 ohm-m on the surface along profile b-b’ can be explained by more homogenous gravel and clayish sand nature of the pleistocene sediments in the southern parts than the northern parts. geological and hydrogeological discussion as seen before, the geoelectrical stratification of the study area comprises three to five units of high-, relatively high, relatively low-, and low-resistivity values from the surface down. the western part of the study area (ves sites 1-11, profile a-a’, fig. 5) can be easily interpreted directly in terms of geological and hydrogeological structures: a thin resistive coarse grained sand and gravel underlain by some tens of meters thick strata of fine to medium grain sand, gravel, clay, and clayish sand, followed by saltwater. as indicated by the cross section of profile b-b’ (fig. 6), the basic structure beneath the central part of the study area (ves sites 3736 profile b-b’, fig. 6) comprises three units of moderate high, relatively low, and low resistivity values from the surface down. the surface resistive layer represents dry sand and gravel. the second layer, marked by resistivities of some tens of ohm meters could indicate the presence of sand and sandy clay. the low resistivities values (~30 ohm-m) in the third layer may be related to the presence of clay and clayish sand layer. in addition, the eastern part of the study area (ves sites 35-31, profile b-b’, fig. 6) comprises a succession of clay and clayish sand with fresh groundwater. conclusions the use of ves surveying technique was proved useful to map the subsurface of structurally complex area where little geologic information is available due to lack of drillholes. ves surveys can also be used to locate bodies of groundwater and zones with anomalous electrical properties. the apparent resistivities measured on the study area can be explained by resistivity distributions involving four extensive electrically distinctive zones (i-iv, fig. 7). the study area can then be subdivided in four compartments (fig. 7). electrical differences between the four zones cannot be explained by water content, as different geological contexts have to be considered. as a result, two different hydrogeological behaviours can be distinguished: (i) a saline groundwater coastal aquifer in the western part, and (ii) a fresh groundwater aquifer in the eastern part. an important general result of this study is the existence of different coastal hydrogeological zones directly linked to geological subsurface structures, despite a monotonic surface cover 105 batayneh. esrj vol. 11, no. 2. december 2007 figure 4. typical electrical resistivity sounding data and best-fit three to five-layer model interpretations corresponding to the four types of field curves. (a) site 1 (type i field curves); (b) site 37 (type ii field curves); (c) site 33 (type iii field curves); and (d) site 41 (type iv field curves) figure 5. composite electrical cross section inferred from geoelectrical sounding data along a-a’ profile (see fig. 1 for profile location). resistivities are in ohm-m 106 mapping subsurface formations on the eastern red sea coast in jordan using geoelectrical techniques: geological and hydrogeological implications figure 6. composite electrical cross section inferred from geoelectrical sounding data along b-b’ profile (see fig. 1 for profile location; see fig. 4 for legend). resistivities are in ohm-m figure 7. map of the four electrical distinct zones i-iv in relation to location of the ves sites and geology. the elevation contours are in meters. palestine grid are used 107 batayneh. esrj vol. 11, no. 2. december 2007 the surface alluvial and wadi sediments. the geological structures of coastal areas within an active plate may significantly control the groundwater behaviour and knowledge of the tectonic history of the region; therefore, it is necessary to deal with its hydrogeology. the second important result is that the area is characterized by a thick clay and poorly permeable substratum that occurs in the central part. this result provides new ideas about the hydrogeology of the area. the results obtained are in a good agreement with the borehole data available. acknowledgments the author wishes to acknowledge the support received by the natural resources authority of jordan, iris syscal-r2 resistivity equipment. facilities provided by the geophysics division of the natural resources authority of jordan are acknowledged. references al khatib, f., 1987. the geology of jabal al mubarak & al yamaniyya. map sheet nos. 3048 iv & 2948 i. geological mapping division, natural resources authority, amman, jordan. ayers, j., 1989. conjunctive use of geophysical and geological data in the study of an alluvial aquifer. ground water 27: 625-632. barongo, j., and palacky, g., 1991. investigations of electrical properties of weathered layers in the yala area, western kenya, using resistivity soundings. geophysics 56: 133-138. batayneh, a., 2006. use of electrical resistivity methods for detecting subsurface fresh and saline water and delineating their interfacial configuration: a case study of the eastern dead sea coastal aquifers, jordan. hydrogeology journal 14: 1277-1283. batayneh, a., and qassas, h., 2006. changes in quality of groundwater with seasonal fluctuations: an example from ghor safi area, southern dead sea coastal aquifers, jordan. journal of environmental sciences 18: 263-269. gnanasundar d, elango l (1999) groundwater quality assessment of a coastal aquifer using geoelectrical techniques. journal of environmental hydrology 7: paper 2 keller, g., and frischknecht, f., 1966. electrical methods in geophysical prospecting, pergamon, ny. khair, k., and skokan, c., 1998. a model study of the effect of salination on groundwater resistivity. journal of environmental geophysics 2: 223-231. mukhtar, a., sulaiman, w., ibrahim, s., abdul latif, p., and hanafi, m., 2000. detection of groundwater pollution using resistivity imaging at seri petaling landfill, malaysia. journal of environmental hydrology 8: paper 3. orellana, e., and mooney, h., 1966. master tables and curves for vertical electrical sounding over layered structures. interciencia, madrid. parasnis, d., 1956. the electrical resistivity of some sulphide and oxide minerals and their ores. geophysical prospecting 4: 249-279. parasnis, d., 1966. mining geophysics. elsevier science, amsterdam. salameh, e., and bannayan, h., 1993. water resources of jordan, present status and future potentials. fredrich eberet stiftung, amman. van overmeeren, r., 1989. aquifer boundaries explored by geoelectrical measurements in the coastal plain of yemen. a case study of equivalence. geophysics 54: 38-48. zohdy, a., 1965. the auxiliary point method of electrical sounding interpretation, and its relationship to the dar zarrouk parameters. geophysics 30: 644-660. zohdy, a., and jackson, d., 1969. application of deep electrical soundings for groundwater exploration in hawaii. geophysics 34: 584-600. geociencias-vol 12-1 sep.vp earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 62-71 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) saad bakkali1 and mahacine amrani2 1earth sciences department geosciences & environment group faculty of sciences and techniques, abdelmalek essaadi university, tangier, morocco saad.bakkali@menara.ma 2engineering process department faculty of sciences and techniques, abdelmalek essaadi university, tangier, morocco amrani.mahacine@menara.ma abstract wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals. since then, significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields. in geophysics, the power of wavelets for analysis of non stationary processes that contain multiscale features, detection of singularities, analysis of transient phenomena, fractal and multifractal processes, and signal compression is now being exploited for the study of several processes including resistivity surveys. the present paper deals with denoising moroccan phosphate “disturbances” resistivity data? map using the haar wavelet mother transform method. the results show a significant suppression of noise and a very good smoothing and recovery of resistivity anomalies. key words: resistivity, phosphate, disturbance, haar, wavelet, sidi chennane, morocco. resumen la transformada wavelet tuvo sus orígenes a inicios de los 80’s en el análisis de señales sísmicas, que debido a avances matemáticos significativos han permitido su aplicación a diversos campos. la energía de la ondícula; usada en el análisis de procesos no estacionarios con rasgos de múltiples escalas, detecciones de singularidades, análisis de fenómenos transientes, procesos fractales y multifractales, y compresión de señales, es aplicada a diferentes procesos incluyendo sondeos de resistividad. este artículo muestra la atenuación del ruido en el mapa 62 manuscript received april 7, 2008. accepted for publication june 2, 2008. de perturbaciones de resistividad en los fosfatos marroquíes mediante el uso de la ondícula haar en la transformada wavelet. los resultados indican una atenuación significativa del ruido, un buen suavizado y recuperación de las anomalías de resistividad. palabras clave: resistividad, fosfato, perturbaciones, haar, ondícula, sidi chennane, marrueco. introduction geophysical data are often contaminated with noise and artifacts coming from various sources. the presence of noise in data distorts the characteristics of the geophysical signal resulting in poor quality of any subsequent processing. consequently the first step in any processing of such geophysical data is the “cleaning up” of the noise in a way that preserves the signal sharp variations. wavelet transforms are relatively recent developments that have fascinated the scientific, engineering, and mathematics community with their versatile applicability. for geophysical processes, in particular, tools that offer the ability to examine the variability of a process at different scales are especially important. wavelet analysis is an emerging field of applied mathematics that has provided new tools and algorithms for solving such problems as are encountered in fault diagnosis, modelling, identification, and control and optimization (kumar et al., 1994). the theory has acquired the status of a unifying theory underlying many of the methods used in physics and signal processing. the decision as to which representation (expansion) to use for a signal, for example wavelet expansion versus fourier or spline expansion depends on the purpose of the analysis. wavelets have become increasingly popular for analyzing data in the geosciences. wavelets re-express data collected over a time span or spatial region such that variations over temporal/spatial scales are summarized in wavelet coefficients. individual coefficients depend upon both a scale and a temporal/spatial location, so wavelets are ideal for analyzing geo-systems with interacting scales (riedi, 1998). so, the wavelet transform filtering method has become a powerful signal and image processing tool which has found applications in many scientific areas. this method is a widely used technique that is applicable to the filtering geophysical data (kumar et al., 1997). the present paper deals with denoising moroccan phosphate “disturbances” resistivity data map using the haar mother wavelet transform method. (include reference) the results show a high significant suppression of the noise and a very good smoothing and recovery of the resistivity anomalies signal. so the haar wavelet mother transform processing is thought to be a good method to geophysical anomaly filtering and optimizing estimation of phosphate reserves. the geophysical context resistivity is an excellent parameter and marker for distinguishing between different types and degree of alteration of rocks. resistivity surveys have long been successfully used by geophysicists and engineering geologists and the procedures are well established. (include reference) the study area is the oulad abdoun phosphate basin which contain the sidi chennane deposit. the sidi chennane deposit is sedimentary and contains several distinct phosphate-bearing layers. these layers are found in contact with alternating layers of calcareous and argillaceous hardpan. however, the new deposit contains many inclusions or lenses of extremely tough hardpan locally known as “derangements” or “disturbances” (figure 1), found throughout the phosphate-bearing sequence (kchikach et al., 2002). the hardpan pockets are normally detected only at the time of drilling. direct exploration methods such as well logging or surface geology are not particularly effective they interfere with field operations and introduce a severe bias in the estimates of phosphate reserves (figure 2). 63 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) the study area was selected for its representativity and the resistivity profiles were designed to contain both disturbed and enriched areas. the sections were calibrated by using vertical electrical soundings. high values of apparent resistivity were encountered due to the presence of near-vertical faulting between areas of contrasting resistivity, and fault zones which may contain more or less highly conducting fault gouge. the gouge may contain gravel pockets or alluvial material in a clay matrix. such anomalous sections are also classified as disturbances. apparent resistivity values in these profiles locally exceeded 200 �?m. (bakkali, 2005; bakkali et al., 2006). the apparent resistivity map (figure 3) obtained from a further survey was considered in fact a map of discrete potentials on the free surface, and any major singularity in the apparent resistivities due to the presence of a perturbation will be due to the crossing from a “normal” into a “perturbed” area or vice versa. in other words, the apparent resistivity map may be considered a map of scalar potential differences assumed to be harmonic everywhere except over the perturbed areas. interpretation of resistivity anomalies is the process of extracting information on the position and composition of a target mineral body in the ground. in the present case the targets were the inclusions called perturbations. the amplitude of an anomaly may be assumed to be proportional to the volume of a target body and to the resistivity contrast with the mother lode. if the body has the same resistivity as the mother lode no anomaly will be detected. thus assumed in fact and in first approach that the resistivity anomalies would be representative of the local density contrast between the disturbances and the mother lode. level disturbance of the anomalous zones is proportionnal to resistivity intensity (figure 4). (bakkali, 2005; bakkali, 2006). 2006 (1)? 2006 (2)? the wavelet analysis approach the wavelet transform is a time-frequency decomposition which links a time (or space) domain function to its time-scale wavelet domain representation. the concept of scale is broadly related to frequency. small scales relate to short duration, high frequency features and correspondingly, large scales relate to long duration, low frequency features (daubechies, 1990). wavelets are functions that satisfy certain mathematical requirements and are used in representing data or other function. in the signal analysis framework, the wavelet transform of the time (or 64 saad bakkali and mahacine amrani * phosphate “disturbance”. space) varying signal depends on the scale that is related to frequency and time (or space) (daubechies et al., 1992). the 2d wavelet method provides information on many more resolution than the former method. it is a powerful tool particularly suitable in denoising, filtering and analyzing problems and potential singularities in geophysical context (foufoula-geogiou et al., 1994) (grossmann et al., 1989). moreover this property is crucial for performing an efficient denoising resistivity anomaly map of the moroccan phosphate deposit “disturbances”. theorical review traditionally, fourier transform has been used to process stationary signals acquired by computers. in this way, the representative spectrum of frequencies is obtained from the time series produced during ac65 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) figure 2: (a) location of the studied area in the sedimentary basin of ouled abdoun. (b) section showing the disruption of the exploitation caused by disturbances. (c) stratigraphical log of the phosphatic series of sidi chennane: (1) hercynian massif; (2) phosphatic areas; (3) marls; (4) phosphatic; marls; (5) phosphatic layer; (6) limestones; (7) phosphatic limestone; (8) discontinuous silex bed; (9) silex nodule; (10) dérangement formed exclusively of silicified limestone; (11) dérangement constituted of a blend of limestone blocks, marls and clays; (12) dérangement limit; (13) roads. quisition of the signal by the computer. for non stationary signals, typical of engineering processes, the existing methodologies have not been fully developed. windowed fourier transform, also called short-tine? fourier transform, was first applied using a gaussian type window (walker, 1997). for a given signal f(t), a conventionaly defined signal g(t – t0) is applied to a window of time that moves along with the original signal, forming a new family of functions: fg(t0,t) = f(t)g(t – t0). functions formed this way are centred on and have a duration defined by the characteristic time window of the function g(t). windowed fourier transform is thus defined as: � �f w t f t g t t e dtg fwt, ( ) ( )0 0� � � (1) this transform is calculated for all t0 values and it gives a representation of the signal f(t) in the time frequency domain. if a space function f(x) instead of a time signal, is considered, a representation is given in the spacefrequency domain (meyer, 1993). however as a windowed fourier transform represents a signal by the sum of it sine and cosine functions, it restricts the flexibility of the function g(t – t0) or g(x – x0) making a characterization of a signal and simultaneous location of its high frequency and low frequency components difficult in the time-frequency domain or the space-frequency domain. wavelets transform were developed to overcome this deficiency of windowed fourier transform in representing non-stationary signals. wavelets transform is obtained from a signal by dilatation-contraction and by the translation of a special wavelet within the time or space domain. the expansion of this signal into wavelets thus permits the signal’s local transient behaviour to be captured, while the sine and the cosines can only capture the overall behaviour of the signal as they always oscillate indefinitely (walker, 1997). 66 saad bakkali and mahacine amrani figure 3: a map of resistivity anomalies for ab=120 m using shepard’s method for resistivity data excentrility? signal analysis and the haar wavelet mother in the fourier analysis, every periodic function having a period of 2 and an integrable square is generated by an overlay of exponential complexes, wn(x) = ejnx, n = 9, � 1, � 2, �obtained by dilations of the function w(x) = ejx : wn(x) = w(nx). extending the idea to space for � integrable square functions, the following is defined : � �a b a t b a a b r a, , , ,� � � �� � � �� � � 1 0 (2) the function � is called a mother wavelet, where a is the scale factor and b is the translation parameter. the family of simpler wavelets, which will be adopted in the present work, is that the haar wavelet : � � � ( ) , ( ) , ( ) [ , ] x if x x if x x if x � � � � � � � � � 1 0 1 2 1 1 2 1 0 01 for the one-dimensional no stationary function f(x) that decrease to zero when x � , the following assumption is normally adopted : � �p q px x q p , ( ) ( )� 2 22 (3) the scale factor of 2-pq is called the localization or dyatic translation and k is the translation index associated with the localization, where p and q � � (meyer, 1982) proved that wavelet thus defined are orthogonal, i.e., � � � �p q l m p l q m, , , ,� � for p q l m z, , � �where is equal to the scalar product and � refer to the delta function of dirac. thus the function f(x) can be rewritten as follows : f x c xp q p q qp ( ) ( ), ,� � � � � �� � (4) the values of the constant cp,q are obtained by wavelet transform in its discrete form. then is expanded into a series of wavelets with their coefficients obtained from c f f x x dxp q p q p q, , ,( ) ( )� � � � �� � (5) 67 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) figure 4: a map of the disturbed noisy phosphate zones corresponding to figure 3. the wavelet transform can also be calculated using special filters called quadrature mirror filters (mallat, 1989). they are defined as a low-pass filter, associated with the coarser scale, and a high-pass filter to characterize the details of the signal. the signal f(x) then is described as: f x c x d xp q p q p q o q qp pq ( ) ( ) ( ), , , ,� � ��� � 0 0 0 � � (6) where c f x x dxp q p q0 0, ,( ) ( )� � � � (7) and d f x x dxp q p q, ,( ) ( )� � � � (8) in the expansion of f(x) by equation 6, the first term represents the approximation of the signal and the second the signal details, filtered by the approximation. the function �p0,q(x) is denominated a scale function or father wavelet, and it is responsible for obtaining the approximation of the signal, while the mother wavelets, �p,q are responsible for the generation of the details filtered by the approximation (polikar, 1999) (wickerhauser, 1994). for the family of haar wavelets, the scale function is �p q x if x 0 1 01, ( ) [ , ]� � and�p q x if x 0 0 01, ( ) [ , ]� � . the mother wavelets, responsible for the details in the haar family, are expressed as: � p q p px if q x q p , ( ) ,� � � � � � � � � � 2 2 2 1 2 2 (9) � p q p px if q x q p , ( ) , ( )� � � � � � � � � � � 2 2 1 2 2 12 (10) and �p,q(x) = 0, otherwise. the processing data the resistivity data base is a compilation of 51 traverses at a spacing of 20 m. there were 101 stations at 5 m distance for every traverse, which makes 5151 stations all together in the resistivity survey. we choose the haar wavelet basis for its smoothness and compact support (torrence et al., 1998). we calculated the magnitude square of the haar wavelet transform coefficients (rioul et al., 1991) using origin pro 8 routine (origin pro, 2007) for each resistivity traverse (figure 5). then we deferred all the results to built a 2d wavelet spectrum regular maps which represent in fact filtering and denoising map of the phosphate deposit “disturbances”. since a major potential application of wavelets is in image processing, the 2d wavelet transform is a necessity to be applied as a detector and analyser of singularities like edges, contours or corners (ucan et al., 2000). (tsivourakipapafotiou et al., 2005). results & conclusions figure 6 represents an indicator of the level of variation of the contrast of density between the disturbances and the normal phosphate-bearing rock. the surface modeling of resistivity anomalies is obtained by autosignal routine from our apparent resistivity survey transformed data obtained using the haar wavelet mother response. these procedure enables us to define the surface phosphate disturbed zones. the haar wavelet analysis surface of phosphate deposit disturbance zones modeling as obtained by the above procedure in the study area provided a direct image for an interpretation of the resistivity survey. these method enable us to identify the anomalies area which turned out to be strongly correlated with the disturbances. the use of magnitude square of the haar mother wavelet transform represent an effective filtering method which makes it possible to attenuate considerably the noise represented by the minor dispersed and random disturbances. the overall effect is that of scanning and denoising the anomalous bodies. comparatively to classical approaches used in filtering and denoising the same geophysical data map (bakkali, 2007), the advantage of the haar wavelet transform method is doesn’t introduce significant distorsion to the shape of the original resistivity signal . the haar wavelet output of the apparent resistivity which correspond to the wavelet output of the anomalous phoshate deposit map obtained from such a technical tool represent the crossing dominate area from a “normal” into a “perturbed” area or vice versa. moreover the level of disturbance is very clearly 68 saad bakkali and mahacine amrani shown. the proposed filtering and denoising method using haar wavelet transform tends to give a real estimation of the surface of the phosphate deposit “disturbances” zones with a significant suppression of the noise. the level disturbance resulting from such method is also more defined in all the disturbed zones. we have described an analytical procedure to analyze the anomalies of a specific problem in the phosphate mining industry. the results proved satisfying. data processing procedures as the haar wavelet mother response transform of resistivity data map was found to be consistently useful and the corresponding map may be used as auxiliary tools for decision making under field conditions. 69 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) figure 6: wavelet output of the phosphate deposit “disturbances” map given in figure 4. 350 300 200 150 250 100 50 350 300 200 150 250 100 50 0 100 200 300 400 500 distance in meters r e s is ti v it y in o h m .m r e s is ti v it y in o h m .m disturbed zone phosphate zone phosphate zone b. 350 300 200 150 250 100 50 0 100 200 300 400 500 350 300 200 150 250 100 50 distance in meters r e s is ti v it y in o h m .m r e s is ti v it y in o h m .m disturbed zone phosphate zone disturbed zone n o is e n o is e a. figure 5: example of real resistivity traverse data of the survey (a) and the corresponding haar wavelet transform denoised output (b). references bakkali, s., (2005). analysis of phosphate deposit “disturbances” using the horizontal-gradient responses of resistivity data (oulad abdoun, morocco), earth sci. res. j. vol.9, n°2, p123-131. bakkali, s., (2006). a resistivity survey of phosphate deposits containing hardpan pockets in oulad abdoun, morocco, geofisica internacional, 45 (1), p73-82. bakkali, s. and l. bahi., (2006). cartographie des «dérangements» de séries phosphatées par mesures de résistivités électriques, journal des sciences pour l’ingénieur, j.s.p.i., 6, p1-10. bakkali, s., (2007). enhancement of edges of sidi chennane phosphate “disturbances” using sunshading responses of resistivity data, russian geology and geophysics, rgg, 48 (9), 775–781. daubechies, i., (1992). ten lectures on wavelets, 357 pp., soc. for ind. and appl. math., philadelphia, pa. daubechies, i., mallat, s. and a. s. willsky., (1992). introduction to the special issue on wavelet transforms and multiresolution signal analysis, ieee trans. inf. theory, 38(2), 529–531. daubechies, i., (1990). the wavelet transform, time-frequency localization and signal analysis, ieee trans. inf. theory, 36(5), 961–1005. foufoula-georgiou, e., and p. kumar., (1994). wavelets in geophysics, academic, san diego, calif., 384 pp. grossmann, a., kronland-martinet, r. and j. morlet., (1989). reading and understanding continuous wavelet transforms, in wavelets:time-frequency methods and phase space, edited by j. combes, a. grossmann and p. tchamitchian, 2–20, springer-verlag, new york. kchikach, a., jaffal, m., aifa, t. and l. bahi., (2002): cartographie de corps stériles sous couverture quaternaire par méthode de résistivités électriques dans le gisement phosphaté de sidi chennane (maroc). comptes rendus. geosciences, 334, 379-386. kumar, p., and e. foufoula-georgiou., (1994). wavelet analysis in geophysics: an introduction, in wavelets in geophysics, edited by e. foufoula-georgiou and p. kumar, academic, san diego, calif. 1–43. kumar, p. and e. foufoula-georgiou., (1997). wavelet analysis for geophysical applications, agu, reviews of geophysics, 35, 4, 385–412. mallat, m., (1989). a theory for multiresolution signal decomposition; the wavelet representation, ieee trans. on pami, 11(7), 674-693. meyer, y., (1992). wavelets and operators, cambridge univ. press, new york. meyer, y., (1993). review of an introduction to wavelets by c. k. chui and ten lectures on wavelets by i. daubechies, bull. am. math. soc., 28(2), 350–360. origin pro 8 (2007). about origin pro 8 sro,v8.0724 software, copyright 1991-2007, origin lab corporation, www.originlab.com. polikar, p., (1999). the engineer’s ultimate guide to wavelet analysis , the wavelet tutorial, http:/engineering. rowan.edu/~polikar. riedi, r.h., (1998). multifractals and wavelets: a potential tool in geophysics, seg meeting 1998, technical session on novel transforms, new orleans, la, september 1998. rioul, o., and m. vetterli., (1991). wavelets and signal processing, ieee signal process. mag, 8(4), 14–38. systat (2002). about autosignal v1.6 software, copyrignt 2002 aisn software inc. torrence, ch. and g.p. compo., (1998). a practical guide to wavelet analysis, bull. am. meteorol. soc., 79, 61-78. tsivouraki-papafotiou, b., tsokas, g.n. and p. tsurlos. (2005). wavelet denoising of magnetic 70 saad bakkali and mahacine amrani prospecting data, journal. balkan geophysical society, vol 8, 2, 28-36. ucan, o.n., seker, s., abora a.m and a. ozmen, a., (2000). separation of magnetic fields in geophysical studies using a 2-d multiresolution wavelet analysis approach. journal. balkan geophysical society, vol 3, 3, 53-58. walker, j.s., (1997). fourier analysis and wavelet analysis, notices of the ams, vol 44, 6, 658-670. wickerhauser, m. v., (1994). adapted wavelet analysis from theory to software, 486 pp. 71 denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco) letter from editor a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombiacarlos alberto ríos reyes, oscar mauricio castellanos alarcón and akira takasu 7 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombialorena rayo and carlos a. zuluaga 31 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeriaakintola, o.f. and adekeye, j.i.d. 44 denoising resistivity phosphate “disturbances” using haar mother wavelettransform (sidi chennane, morocco)saad bakkali and mahacine amrani 62 petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”ríos, c.a., castellanos, o.m., gómez s.i. and ávila, g.a. 72 seismic velocity determination in gravel and sands using piezocrystalsarmando luis imhof and juan carlos santamarina 107 geofisica colombiana n° 6 pp.33-39 diciembre de 2002 bogota, d.c. issn 0121 2974 analisis espacial y temporal de q-coda en el piedemonte llanero (colombia) juan p. duran', carlos a. vargas' y luis a. briceno' resumen la variacion espacial y temporal del pararnetro de atenuacion q-coda en un sector del piedemonte l1anero es estudiada usando las ondas coda. la estirnacion de la atenuacion de la amplitud de la coda se calcula por medio del modelo de dispersion isotropica simple de sato, a partir de sismogramas de sismos que ocurrieron en la region entre septiembre de 1993 a junio 2002. siguiendo la metodologia empleada por aki y chouet (1975), se analizaron los sismogramas en cuatro band as centradas a 1.5, 3, 6 y 12 hz con sus respectivos anchos de banda de 1,2,4 y 8 hz. por regresion linear se calculo el valor de qc a i hz. a partir de la expresion propuesta por puli para modelar las amplitudes de las ondas coda por medio de elipses, se arnpiio esta expresion para calcular la variacion espacial del qc-l, para un medio tridimensional (tomografia anelastica). y por medio de inversion por minimos cuadrados se calculo el valor de atenuacion para cada celda del modelo tridimensional. los resultados se correlacionaron con el marco tectonico regional encontrando que cuando la atenuacion aumenta con la profundidad, corresponde a zonas con tectonica de piel gruesa y por el contrario, cuando la atenuacion disminuye con la profundidad, corresponde a zonas con tectonica de piel delgada. por otro lado, la variac ion temporal de los minimos valores de qc (maximas atenuaciones) se correlaciono cualitativamente con sismos de magnitud mayor de 3.5 y, adernas, se cuantifico la distribucion temporal de qc par medio de la dimension fractal, mostrando que la agrupacion de los valores minimos de qc aumenta antes de la ocurrencia de un evento principal y decrece su agrupacion despues de la ocurrencia del even to principal. palabras clave dndas co'oa, oc, atenuacion, dimension fractal, pieoemonte abstract the space-time variation of the parameter of attenuation q-coda in a sector of the piedemonte l1anero was studied using the coda waves. the estimation ofthe attenuation of the amplitude of coda is calculated by means of the model of simple isotropic dispersion of sa to, from earthquakes seismograms happened in the region from september 1993 to june 2002. following the methodology used by aki and chouet (1975), they analyzed seismograms in four centered bands to 1.5,3,6 and 12 hertz with its corresponding bandwidths of 1, 2, 4 and 8 hertz. and by regression to linear calculated the value of qc to i hertz. from the expression proposed by puli to model the amplitude of the waves coda by means of ellipses, this expression was extended to calculate the space variation ofthe qc-i, (anelastic tomography). and by inverse square minimums, the value of attenuation for each cell of the three-dimensional model was calculated. their results were correlated with the regional tectonic frame showing that when the attenuation increases by depth, it corresponds to zones with tectonics of thick skin and, on the contrary, where the attenuation diminishes by depth, it corresponds to zones with tectonics of thin skin. on the other hand, the temporary variation of minimum values ofqc (maximum attenuations) was correlated to qualitatively earthquakes of magnitude greater to 3,5 and, in addition, the temporary distribution ofqc was quantified by dimension fractal showing that the grouping of the minimum values ofqc increases before ocurring a main event and its grouping decreases after occurring the main event. keywords: coda suaves, oa, attenuation, fractal dimension, foothills estudiante de la maestna de geotlsica, universidad nacional de colombia. profesor del departamento de geociencias, universidad nacional de colombia. ibriceno@ciencias.unal.edu.co introduccion un mecanisme para determinar las condiciones de atenuacion de una zona, son las ondas coda, que son ondas retrodispersadas, resultado de la suma de ondas prim arias dispersadas por la interaccion con dispersores 0 heterogeneidades y por 10 tanto su geofislca cdlombiana, 6, diciembre de 2002 33 duran et al. atenuaci6n en la amplitud se debe principalmente a las condiciones tect6nicas de la regi6n donde se registr6 el sismo, mas que a la misma trayectoria recorrida por la onda des de la fuente hasta la estaci6n sismol6gica. desde el punto de vista temporal, la atenuaci6n de las ondas coda es igualmente interesante, debido a la correlaci6n que existe entre la variaci6n de qc-i y la sismicidad. en general, se ha observado un decrecimiento de valor de qc (aumento en la atenuaci6n del medio) antes del evento principal 0 de la erupci6n y posterior al evento, un aumento en el valor de qc (disminuci6n de la atenuaci6n del medio). la variaci6n de la atenuaci6n se traduce en un cambio del regimen de esfuerzos; por tanto, la variaci6n de qc puede ser usado como un indicador de los cambios temporales de esfuerzos en una area. en este estudio se gener6 un modelo tridimensional de atenuaciones (tomografia anelastica) en un sector del piedemonte llanero entre los departamentos de meta y cundinamarca, entre la latitud 3.50 a 5.30 norte y longitud 74.60 a 72.30 oeste. la variaci6n espacial de la atenuaci6n se correlacion6 con ei marco tect6nico regional. ademas, se analiz6 la correlaci6n entre evoluci6n en el tiempo de qc con la ocurrencia de sismos de magnitud mayor de 3.5 y su distribuci6n temporal por medio de la dimensi6n fractal. marco tectonico y geologico regional a 10 largo de la margen oriental de la cordillera oriental se prolonga una amplia zona de fallas de tipo compresional conocido como sistema del piedemonte llanero 0 sistema frontal de la cordillera oriental, asociado a la reactivaci6n de antiguas fallas originadas durante una fase extensiva de las edades jurasica y cretacea (lngeominas, 1998). algunos autores consideran este sistema como limite entre la placa suramericana y ei bloque norte de los andes (paris, et al., 2000). _ la regi6n se caracteriza por presentar alta actividad sismica, con sismos de todo tipo de magnitud y de profundidades focales hasta unos 50 0 60 km, asociables a la actividad de esta zona de fallamiento. adernas, el analisis de los principales rasgos geomorfol6gicos en abanicos cuatemarios seiialan una alta actividad neotect6nica, 10 cual indica que el proceso orogenico en la cordillera no ha cesado (ingeominas, 1998). ei sector sur de la regi6n se caracteriza por roc as del basamento metam6rfico andino y del paleozoico superior, expuestas en las zonas extern as contra la cuenca de los llanos, principalmente en una serie de altos 0 domos con direcci6n general paralela al rumbo estructural de la cordillera y que definen su culminaci6n estructural, ei sector centro y norte se caracteriza por pliegues que se presentan en trenes mucho menos espaciados que los altos estructurales del sector sur y, por tanto, por un mayor acortamiento de la cobertera cretacica en esta zona (mora y kammer, 1999). los domos del flanco oriental, que definen la culminaci6n estructural de la cordillera, muestran una asimetria variable, que en 34 un caso extremo esta dada por la presencia de una falla inversa en su borde oriental, que levanta un anti forme del bloque occidental. un ejemplo de tal situacion se tiene en el domo de chingaza (mora y kammer, 1999). en cuanto a los pliegues formados en la cobertera cretacica y terciaria del interior de la cordillera, se han propuesto modelos de una tect6nica epidermica, cuyas caracteristicas fundamentales se asumen como validas para toda la cordillera (mora y kammer, 1999). sin embargo, kammer y mojica (1996) sefialan que la dificultad principal de aplicar cualquier modelo de escamaci6n en la cordillera oriental radica en que no existe un contraste ideal entre el bas amento cristalino fragil y poco susceptible a plegarse y una cobertera sedimentaria con propiedades mecanicas muy distintas, separados entre si por un horizonte ductil de deslizamiento. por tanto, el 1110deja que prop one un despegue restringido a un horizonte particular debe ser abandonado en las areas donde exista una deformaci6n entre el sustrato precretacico y su cobertera (mora y kammer, 1999). datos se seleccionaron 505 sismogramas para un periodo comprendido de septiembre de 1993 a junio 2002. su selecci6n, producto de una extensa depuraci6n de registros tuvo criterios como la claridad de arribos de las fases p y s, y que la longitud de la coda garantizara como minimo 50 seg. los eventos seleccionados presentaron rangos de magnitud 1 < m < 5.5 y profundidades focales entre 0 y 30 km. la mayoria de los epicentros se localizaron a 10 largo del sistema de fall as del piedemonte llanero. a partir de los datos seleccionados, se escogi6 una area para hacer la inversi6n de datos, (figura i): '. / / "//r j /. .. ~+" .. s: *, <. • figura i. localizacion de los epicentras de los sismas utilizadas para calcular la tornografla de qc. geofislca colombiana, 6. diciembre de 2002 anausis espacial y temporal de q·coda en el piedemonte llanero (colombia) metodologia analisis espacial de qc para calcular el valor de qc se utiliz6 el mode 10 de dispersi6n isotr6pica simple de sato que asume un medio homogeneo con velocidad y densidad constante, donde las fluctuaciones son debidas a dispersores homogeneos distribuidos aleatoriamente; por ejemplo, fracturas, fall as, contrastes de densidad y an om alias de velocidad. de acuerdo con el modelo de sato, y asumiendo que las ondas coda s610 tienen ondas s dispersadas, la densidad de energia dispersada de la onda coda (es), por la n-esirna heterogeneidad, sera igual a es(r,t if) = w04(f~go k(a );-n/tlq rtr donde r es la distancia hipocentral; t es el lapso de tiempo medido desde el tiempo de origen del sismo; wo es la energia total radiada desde la fuente; go es el coeficiente total de dispersi6n; k(a) = (jla) in [(a + 1)/(a i)], (a> i); a = tits; ts es el tiempo de viaje de la onda directa s. linealizando la ecuaci6n (1), obtenemos inl(as)2 k(ti ts)t in[ 1 jw (t-ts) ac j nocrvt qc entonces el valor de qc se obtiene a partir de una simple regresion linear sobre sismogramas filtrados en unos rangos frecuencia seleccionados. siguiendo la metodologia empleada por aki y chouet (1975), se analizaron los sismogramas en cuatro bandas centradas en 1.5,3,6 y 12 hz con sus respectivos anchos de banda de 1,2,4 y 8 hz. por regresi6n linear se calcul6 el valor de qc a 1 hz. ademas, los valores de qc se calcularon para diferentes tiempos con el fin de rnuestrear la atenuaci6n de las ondas coda a diferentes rangos de distancia. regionalizacion de qc para estimar la variacion espacial de la atenuaci6n de las ondas coda (qc") en un medio tridimensional, se amplio la expresi6n propuesta por puli para modelar las amplitudes de las ondas coda por medio de elipses, a una expresion elipsoidal de la forma don de res la distancia entre la fuente y la estacion, v es la velocidad de la onda s, x, y son las coordenadas en superficie y t es el tiempo total que las ondas coda necesitan para recorrer la regi6n comprendida entre la estacion y el epicentro a velocidad constante con un coeficiente de atenuacion q~i,y es igual a tt=2t + v s 2 geofislca colombiana. 6, diciembre de 2002 tv es la ventana de coda 0 sea el tiempo usado para calcular el valor de qc' si una onda gasta un tiempo t, para recorrer una regi6n con un coeficiente de atenuaci6n q~/, el resultado efectivo de q~ies relacionado al camino de atenuaci6n por la relaci6n (vargas, 2000): (5) para un medio tridimensional, la expresi6n (5) se expresa como un hiperplano de la forma (i) (6) que relaciona el valor de q~ide cada elipsoide con la fracci6n de tiempo (t/t) respecto al tiempo total que las ondas coda necesitan para recorrer la regi6n (en este caso una celda de 3 dimensiones) con un valor de atenuaci6n q;ipara un evento registrado en una estaci6n. la anterior expresi6n se puede expresar en la forma ax = y, siendo a, x y y (2) a= t,iik ["k t" + ...+ 1+ l+ ...+ i1f11p t; 1; 1; (7) 1x= + ...+ + ...+o; a.; e., (8) (9) para determinar el valor de atenuaci6n en cada celda, se utiliz6 la inversion por minimos cuadrados amortiguados, don de el factor de peso es igual a una matriz diagonal de peso (02 ) que consiste en el error estimado en una observaci6n (e.) normalizado por el error medio (e") (3) analisis temporal de qc para estimar la variacion temporal de qc, inicialmente se estudi6 la distribuci6n temporal de los valores qc antes y despues de grandes eventos sismicos (magnitud mayor de 3.5). se espera que los minimos valores de qc (maximas atenuaciones) ocurran antes del evento principal, y que despues del even to principal (minimas atenuaciones) los altos ocurran. si esta relaci6n se cumple, confirmariamos la idea de que para este sector podemos interpretar los cam bios temporales de qc como cambios del estado de esfuerzos en la corteza. adernas de esta aproximaci6n cualitativa al problema de la variacion temporal de la atenuaci6n, se utiliz6 la dimensi6n fractal con el objetivo de cuantificar el agrupamiento temporal de los minimos valores de qc y correlacionar la variaci6n temporal del agrupamiento de qc con la ocurrencia de grandes sismos. (4) 35 duran et al. vinciguerra (200 i) sefiala que un rnetodo sencillo para calcular la dimension fractal (d) es el metoda de la integral de correlacion, si nr<r es el nurnero de pares de eventos separados por una distancia temporal menor que r, la integral de correlacion de una distribucion de n sismos se define como 2nc(r)= r«r n(n -1) (10) (ii) linealizando la ecuaci6n ii, se obtiene 10gc(r) ""d logr (12) por tanto, la dimension fractal (d) puede ser estimada a partir del mejor ajuste lineal de log c(r) vs. log r<r. si los eventos estan concentrados 0 agrupados en un espacio de tiempo muy pequefio 0 = o. £1 caso opuesto ocurre para distribuciones periodicas donde los eventos estan distribuidos igualmente en el tiempo; en este caso no habra agrupacion, y 0 = 1. sin embargo, para intervalos de muestreo temporal muy cortos, 0 sera igual a o y, por el contrario, para intervalos muy largos d tendera a i (latora, rapisarda y vinciguera, 1998). se calculo la dimension fractal para una ventana rnovil de 90 dias, y se asume que el 0 calculado correspond fa al tiempo del ultimo evento en la ventana. resultados y anausis de los resultados varia cion espacial de q;' los valores de atenuacion cal cui ados a partir de los sismogramas fueron filtrados e interpolados por el metodo de minima curvatura para suavizar la respuesta e intentar buscar la tendencia, obviando las anomalias. a partir de la inversion de las atenuaciones calculadas de los sismogramas, se creo un modelo tridimensional de atenuaciones (qc·l) para profundidades menores de 20 km. £1 tarnafio optimo se estimo de los val ores de la matriz de resolucion del modelo y del residual de la inversion. se analizaron tres casos extremos, con diferentes tamafios de grilla. ver tabla 2 y figura 2. tabla 2 parametros para la seleccion del tamano optimo de celda no. de tamaiio de la celda max. valor divisiones (kin) mat. de res. residual 5x6xl0 50.61 x 47.91 x 2.85 0.8 0.0733 8x9x 10 31.6 x 31.0x 2.84 0.6 0.0753 15x 16x 10 16.87 x 18.04 x 2.85 0.28 0.0775 36 a) b) c) figura 2. matriz de resoluci6n para una tomografia de atenuaci6n de a) 5 x 6 x 10 divisiones; b) 8 x 9 x 10 divisiones; c) is xl6 x 10 divisiones. ai analizar las graficas, se aprecia que el residual no varia significativamente entre la inversion en cada una de las grillas; por el contrario, el valor de la matriz de resolucion disminuye al aumentar el nurnero de celdas en la grilla. de acuerdo con menke (1989), cuanto mas cercano sea el valor de la matriz de resolucion a la unidad, el parametro calculado (en nuestro caso, la atenuacion en cad a celda) estara mas cerca del pararnetro real. por 10tanto, entre menor sea el numero de celdas mejor seria la respuesta, pero esto sacrificaria el detalle en el modelo. por tanto se decidio que el tamafio optimo de las celdas es de 31.6 x 31.0 x 2.84 km, donde el maximo valor de la matriz de resolucion es mas 0 menos 0.6, y el numero de eeldas en cada capa permite analizar en detalle la variacion espacial. geofislca colombiana, 6, oiciembre oe 2002 analisis espacial y temporal de q·coda en el piedemonte llanero (colombia) a) perfil no 1 b) ~ .. perfil no 2 perfil no 3 d) figura 3 a) atenuaci6n a 1.5 ian; b) perfil 1-1'; c) perfil 2-2'; d) perfil 3-3'. los resultados de ja variacion espacial qc-i, a partir de los sismogramas (figura 3), muestra una franja de anomalias de atenuacion con sentido sw ne, con una flexion hacia el se en la parte media, marcando cuatro anomalias de atenuacion de sur a norte, y una quinta anomalia en al parte inferior de la figura (sefialadas como a, b, c, dye, respectivamente). las anomalias no s610 varian lateralmente sino tam bien en profundidad. los val ores promedio de la atenuacion de las anomalias a y c, muestran un aumento en su atenuacion a medida que se profundiza de 0.026 a 0.030 y de 0.014 a 0.055, respectivamente. ademas, a medida que se prafundiza, la anomalia a se "desplaza" hacia el norte. por el contrario, las anomalias b y d disminuyen su valor de atenuacion con la profundidad. la anomalia b disminuye su valor de atenuacion promedio de 0.026 a 0.020 y se "desplaza" hacia el oeste. la anomalia d disminuye su atenuacion hasta el punto que a 19 km de profundidad desaparece la anomalia. a diferencia de las cuatro anomalias anteriores, la atenuacion de la anomalia e permanece mas 0 menos constante con un valor de 0.025. de acuerdo con el marco geologico y tectonico regional, la anomalia de atenuacion e se correlaciono con las rocas 'del basamento metamorfico andino y del paleozoico superior. la diferencia en la variacion de la atenuacion de las anomalias a y b plantea un problema que podria ser explicado si asurnimos que geofislca colomb lana. 6. oiciembre oe 2002 donde la atenuacion disminuye con la profundidad, la deforrnacion sola implico la cobertera cretacica (tectonica de piel del gada); por el contrario, donde la atenuacion aumento, con la profundidad, la deformacion implico la cobertera cretacica y el basamento precretacico (tect6nica de piel gruesa), conforme con el planteamiento de mora y kammer (1999). sin embargo, esto plantea otro problema: (,c6mo explicar que la anomalia a y b, que son adyacentes y que en superficie parecen estar controladas por la falla de guiacaramo, tengan dos tendencias tectonicas tan diferentes? de acuerdo con los resultados del trabajo de velasquez (2002), este problema se puede responder si asumimos la existencia de dos basamentos que se comportan mecanicamente diferentes: uno que absorbio la deformacion y se plego junto con la cobertera cretacica y otro mas competente que no se deformo y sirvio como rampa de despegue para la cobertera cretacica. otra solucion al problema la aporta el trabajo rowan y linares (2000), que demuestra, a partir de la interpretacion de datos sismicos, que la falla agua clara 0 guiacaramo se profundiza hacia el norte, sacando rocas mas antiguas al norte, mientras al sur s610 afecta las rocas masjovenes. por tanto, esto explicaria por que la anomalia a aumenta su atenuacion con la profundidad, mientras la anomalia b no la aumenta, ai igual que la anomalia a y b, la anomalia c y d se diferencian por su variacion de la atenuaci6n en prafundidad. esto puede ser explicad por una deformacion homogenea de basamento y cobertera en la anomalia c, mientras que la anomalia d marcaria una zona donde el basamento no se deform a y sirve como despegue de la cobertera. es importante sefialar que la ubicacion geografica de la anomalia c coincide con el area del domo de chingaza, donde mora y kammer (1999) sefialan que hay una concordancia entre las deformaciones y el plegamiento que afecta al basamento precretaceo y la cobertera cretacica. variac/on temporal de qc entre enero 1995 hastajunio del 2002, se correlacionaron los cambios en los valores minimos de qc (rnaximas atenuaciones) con la ocurrencia de eventos de magnitud mayor de 3.5. las variaciones mas drasticas de la tendencia temporal de qc son descritas a continuacion (figura 4). a partir de enero de 1995, los valores qc aumentan hasta un valor maximo, para luego volver a disminuir hasta octubre de 1996 (ver a, en la figura 4). el aumento del valor de qc coincidio con la ocurrencia de tres eventos de magnitud mayor de 4.5; la disminucion de qc, con la ocurrencia de eventos men ores de 3.7. entre marzo de 1997 hasta abril de 1999, la tendencia de los valores de qc fue disminuir; sin embargo, el sismo de magnitud mayor de 4.5, en febrero de 1998, marco un pequefio aumento en el valor de qc (ver b, en la figura 4). ei valor de qc vuelve a aumentar hasta octubre del 2000. sin embargo, esta tendencia puede dividirse en dos: antes y despues de 37 dunan et al. 3:;0 ::: °olm~~~~~~~oo~1~1~lml~l~lboo1~i~w~1160=~ffio~~~woo~iw l~~o(d!;os) figura 4. variacion temporal de qc vs ocurrencia de sismos de magnitud mayor a 3.5. la ocurrencia de dos sismos de magnitud mayor de 4.5 en junio de 1999 (ver c, en la figura 4). desde febrero de 200 i hasta junio de 2002, hay una tendencia a aumentar el valor de qc (ver d, en la figura 4). aunque se aprecia una correlaci6n entre los sismos de grandes magnitudes (mayor de 3.5) y los mfnimos valores qc (menores de 170), todavfa no podemos concluir nada acerca de la evoluci6n de la atenuaci6n en el tiempo ni caracterizarla. debido a esto, se dec idi6 calcular la dimensi6n fractal para la distribuci6n temporal y determinar si la distribuci6n en el tiempo es aleatoria, peri6dica 0 agrupada y, en ultimo caso, si hay agrupaci6n de qc, y determinar si estan relacionados con los eventos de mayor magnitud. el analisis de la dimensi6n fractal d para un rango de qc menores de 170 con ventana m6vil de 90 dias y un r<r: 30,60 y90 dias, mostr6 (ver figuras 5a, 5b y 5c). las variaciones relativas de d, muestran una tendencia de la disminuci6n de la dimensi6n fractal antes de la ocurrencia de un gran evento; por tanto, los valores de qc se agrupan en el tiempo. despues de la ocurrencia del evento, la dimensi6n fractal vuelve a aumentar su valor, 0 sea que la distribuci6n .de qc en el tiempo es aleatoria. de acuerdo con los resultados obtenidos con la correlaci6n cualitativa entre qc y los sismos de gran magnitud,junto con el analisis de la variaci6n de la dimensi6n fractal en tiempo, se interpret6 la disminuci6n de qc (aumento en la atenuaci6n) y su agrupamiento en el tiempo como cam bios en la estructura heterogenea de la corteza (en nuestro caso fracturas y fallas) y por tanto un aumento en el estado de esfuerzos. ei aumento de qc (disminuci6n de la atenuaci6n) y el aumento en la dimensi6n fractal se interpreta como una liberaci6n de esfuerzos. 38 ,+ 'l' "r":' .o z • e 3.5'3./ 3.74 4-.5 ~ 5 co loll) ,''''''''(0''') "" .':,,..: i a ::: i'i'.".::: 'n'" :::w::i~ ~~i~=,"oo o ~ :: 0., :: [t7'fl'• 31·'· ~...., 9 • 4,~ 0 figura 5. analisis de la evolucion temporal de d, con una ventana movil de 90 dias y a) r<r: 30, b) r<r: 60 dias y c) r<r: 90 dias. conclusiones a partir de los resultados del trabajo se lleg6 ados grandes conclusiones: lnicialmente se esperaba que la variaci6n espacial de la atenuaci6n (qc-l) fuera un reflejo del fracturamiento de las rocas; sin embargo, para esta zona, la variaci6n de qc-i marca el contraste entre geofislca colombiana, 6, diciembre de 2002 anallsis espacial y temporal de q·coda en el piedemonte llanero (colombia) dos tipos de deformacion: homogenea e inhomogenea. en el primer caso, existe concordancia entre las deformaciones y el plegamiento entre la cobertera cretacica y un basamento precretacico (tect6nica de escamaci6n gruesa), y que se expresa en un aumento de la atenuaci6n con la profundidad. otra deformaci6n de tipo de escamaci6n delgada, donde la deformaci6n s610 afecta la cobertera cretacica, se presenta como una atenuaci6n que disminuye con la profundidad. por tanto, la atenuaci6n de qc-l permite discriminar las zonas con mayor anisotropia en la zona. la variaci6n temporal de qc representa el cambio temporal del estado de esfuerzos de [a corteza y, por tanto, debe esperarse que la atenuaci6n aumente antes de la ocurrencia de un sismo de gran magnitud, que para esta zona correspondi6 a sismos de magnitud mayor de 3.5. despues del evento principal, al disminuir los esfuerzos, la atenuaci6n disminuye. bibliografia aki, k., and chouet, b. 1975. origin of coda waves: source, atennuation, and scattering effects. journal of geophysical research 80: 3322 3342. ingeominas. 1998. estudio general de amenaza sismica de colombia. segunda edicion. bogota. kammer, a. y mojica, j. 1996. una cornparacion de la tectonica del basamento de las cordilleras central y oriental. geologia colombiana, (20): 93-106. kammer, a. 1999. observaciones acerca de un origen transpresivo de la cordillera oriental. geologia colombiana (24): 29-47. geofislca colombiana. o. diciembre de 2002 latora, v.; rapisarda, a. y vinciguerra, s. 1998. a fractal approach to the temporal distribution of microseismicity at low eastern of mt. etna during 1989-1994. physics on the earth and planetary interiors (109): 115-127. menke, william. 1989. geophysical data analysis: discrete inverse theory. academic press inc. mora, a. y kammer, a. 1999. cornparacion de los estilos estructurales en la seccion entre bogota y los farallones de medina, cordillera oriental de colombia. geologia colombiana (24): 55-82. paris, g.; machette, m. n.; dart, r. and haller, k. m. 2000. map and database of quaternary faults and folds in colombia and its offshore regions. open-file report 00-0284. u.s. department of the interior. u.s. geological survey. rowan, m. g. y lrnares, r. 2000. fold evolution matrices and axial surface analysis offault-bend folds: aplicattion to the medina aticline, eastern cordillera, colombia. aapg, 84(6): 741-763. vargas, c. a. 2000. atenuacion de ondas coda en el territorio colombiano. ingeominas, observatorio vulcanol6gico y sismologico de manizales. velasquez, a. 2002. modelamiento geofisico cortical, por medio de metodos de campos potenciales, con base en un modelo estructural del piedemonte llanero, colombia. trabajo de grado dpto. de geociencias. universidad nacional de colombia. vinciguerra, s. 200 i. deep earthquakes as a precursor of the september-october 1989 flank eruption at mt. etna volcano (italy). benfield greig hazard research centre, department of geological sciences, university college london, gower street, london. 39 articulo 4.indd earth sciences research journal meteorology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 109 114 detecting low frequency cycles in rainfall series from colombian coffee-growing area by using descriptive methods andrés j peña q.1, álvaro jaramillo r.2 and maría j paternina q.3 1 disc. de agroclimatología. centro nacional de investigaciones de café, cenicafe. km 4 vía antigua chinchiná-manizales. e-mail: andres.pena@cafedecolombia.com 2 disc. de agroclimatología. centro nacional de investigaciones de café, cenicafe. km 4 vía antigua chinchiná-manizales. e-mail: alvaro.jaramillo@cafedecolombia.com 3 estudiante de especialización en estadística aplicada. fundación universitaria los libertadores. bogotá. e-mail: mjpaterninaq@unal.edu.co abstract descriptive statistical methods were used for improving climatic variability scenarios regarding rainfall using time series from five representative pluviometric stations (miguel valencia, naranjal, cenicafé, la bella and paraguaicito); such scenarios are used to make decisions regarding coffee-growing. the purpose was to find signs of cyclic behaviour besides those associated with el niño southern oscillation (enso) which happens every 3.5 to 4.5 years. signals were found of decadal to interdecadal cycle (10 to 30 years), modulating known enso effects related to periodic changes in the pacific ocean and solar activity. resumen con el objeto de mejorar la generación de escenarios de variabilidad climática, que son utilizados para tomar decisiones en el cultivo del café, se exploran en forma descriptiva las series temporales de lluvia medidas en cinco estaciones pluviométricas representativas de la zona cafetera colombiana (miguel valencia, naranjal, la bella y paraguaicito) para encontrar señales de comportamiento cíclico, además del asociado con la oscilación del sur el niño (enso), que es de 3.5 a 4.5 años. se encontraron señales de ciclos de periodo decadal a interdecadal (10 a 30 años) que modulan los efectos ya conocidos del enso y que están asociados a cambios periódicos en el océano pacífico y la actividad solar. palabras claves: el niño oscilación del sur (enos), oscilación decadal del pacífico (odp), series de tiempo, semivariograma. keywords: el niño oscilación del sur (enos), oscilación decadal del pacífico (odp), series de tiempo, semivariograma. record manuscript received: 20/04/2011 accepted for publications: 05/12/2011 introduction climate cycle detection is an ancient topic; our ancestors thought that our life was governed by the seasons’ rhythms; it would thus be logical to think that it would be the same in the long-term (burroughs, 2004). one can now speak about climate cycles being associated with changes in the sun (climate engine) and other cycles related to the ocean (climatic regulator). the most studied cycles from the former group are the 11-year cycle associated with sunspots and the 22-year cycle related to changes in the sun’s magnetic field (burroughs, 2004). the most important cycle in the second group is the 3to 4-year cycle related to el niño southern oscillation (enso). mesa et al., (1997) found that hydrology and rainfall in colombia are usually affected by cycles related to changes in the tropical pacific ocean’s surface temperature than by any other cycle. enso’s local climatic effects have a defined pattern; rainy years are associated with la niña (negative phase) and dry years with el niño (positive phase), as described by guzmán & baldión (1997 and 1999). nevertheless, some very dry or rainy years do not correspond to la niña or el niño, i.e. dryness or wetness intensity are not associated with oceanic niño index (oni) intensity (figure 1). some authors have found that cycles having longer periods than enso could modulate this phenomenon’s effect on climate; several authors have found cycles having a 10-40 year rainfall series period since the earliest part of the 20th century. the most important work in colombia has shown 16-year (peña, 1982) and 35-year (poveda et al., 2002; poveda, 2004) related to special phases in solar motion (landscheidt, 2001) and pacific decadal oscillation (pdo) peaks which could be affecting enso’s effects on climate (verdon & franks, 2006). the purpose of this paper was therefore to assess rainfall time series at five locations having an annual dataset going back more than 46 years in the colombian coffee-growing area (a rain-fed crop system) to detect other cycles, apart from enso, which could be used as predictable patterns for annual rainfall forecasting. andrés j peña q., álvaro jaramillo r. and maría j paternina q.110 simple methodological approximations based on moving averages and semivariograms were used as an alternative to periodograms, which have been shown to have problems when used for determining the presence of cycles and oscillations (hernandez, 1999), and the autocorrelation function whose results would not have been appropriate in exploratory analysis because correlation level significance would not have been high and the aim was to identify signals not to quantify them. moving averages with burroughs filter (eq.1) were used as in the mathematical expression shown for (eq.2). using running means or moving averages was common practice in climatology; although this was a powerful tool for reducing short-term fluctuations in meteorological time series, it became unused because unweighted moving averages could be inappropriate for explaining periodicity (lewis, 1960). therefore, burroughs (1978) proposed moving averages with filters to improve the smoothness of meteorological series. function (eq.1) is known as a seven-year triangular filter and its great advantage lies in effectively suppressing high frequency elements. kutzbach & bryson (1974) explained the importance of non-high frequency components in a climatological time series. weighted moving averages can show cyclical time series’ behaviour. however, the semivariance function was also used here for improving results. this function is used in geostatistics to evaluate spatial correlation (structural analysis); it was used here as a temporal correlation estimator. this function was thus estimated with the experimental semivariogram calculated according to wackernagel (1995), as shown in (eq.3). figure 1. the relationship between oceanic niño index (oni) and rainfall anomalies at the cenicafé pluviometric station. there was an inverse correlation; however, some dry (wet) years did not correlate with el niño (la niña) and oni mean intensity did not correspond to rainfall intensity. figure 2. rain gauge locations and inter-annual (monthly) patterns. time series longitudes are shown beside monthly pattern profiles; i.e. paraguaicito has had a historical dataset dating back from 1963 up to 2008, while miguel valencia has a historical data set dating from 1956 up to 2008 data and methodology five rain gauges were chosen having historical annual data sets going back a long way; they formed part of the colombian federation of coffee growers’ meteorological network. the rain gauges were located in the coffee-growing area (c.g. zone), between 4°24´ n (paraguaicito station) and 5°36´ n (miguel valencia station) at different heights above sea level (in meters, masl) (figure 2). = 2 (4 / ) 16 2( / ) �� = 1 16 ( 3 +2 2 +3 1 +4 +3 +1 +2 +2 + 3)� (1) (2) detecting low frequency cycles in rainfall series from colombian coffee-growing area by using descriptive methods 111 where z(x) is the value in x time, z(x+h) is a value separated by h time from the previous value and n is the pair’s number separated by this time (and it is not constant). semivariograms were drawn using a semivariance/ sample variance ratio on the y-axis to have the same scale at all locations. as opposed to a geostatistical approach, where the aim is to fit a model to the experimental semivariogram to be interpolated, semivariograms were used in this work to detect signs of cyclical behaviour, i.e. it was used as a descriptive tool. semivariance function could thus be inversely interpreted regarding the correlation function; when there is high semivariance there is a negative correlation, when semivariance is close to one there is no correlation, whereas there is positive correlation if there is low semivariance. thus when there is cyclical behaviour, a semivariogram having high to low semivariance oscillations is expected when distance (time, in this case) becomes increased. figure 3. moving averages for annual rainfall time series. there was a cyclic pattern; a dry period followed a wet period at the five stations analysed here. figure 4. semivariogram for time series. minimum semivariance / sample variance ratio value denotes signs of cyclic behaviour in the period shown on the y axis. (3) andrés j peña q., álvaro jaramillo r. and maría j paternina q.112 figure 5. hypothetical time series generated with the experimental semivariogram illustrated in figure 4 (left-hand column). the right-hand column gives an experimental semivariogram for each series. 11-year period signal (a), 21-year period signal (b), 33-year period signal (c) and reconstructed series representing mean annual rainfall pattern in the area (d). (a) (b) (c) (d) detecting low frequency cycles in rainfall series from colombian coffee-growing area by using descriptive methods 113 figure 6. reconstructed and running mean for a time series for the cenicafe and la bella locations. discussion and conclusions figure 3 shows the annual rainfall’s cyclic pattern recorded at five stations. consecutive wet years alternated with consecutive dry years; the wettest periods in the area were 1971-1977 and 1996-2002 and the driest periods were 1977-1983 and 1989-1995. there was an exceptional period having greater annual rainfall in the longer time series: 1951-1957. overall, 12 to 18 years between crests (rainy years) were noticed in weighted time series. in spite of the fact that there were more “la niña” months (and more intense cooling in the pacific ocean) between 1971 and 1977 than between 1996 and 2002, the latter period was rainier regarding 1971–1977. it was not related to enso effects, at least not completely because when “la niña” occurred there was greater rainfall in the area (guzman & baldión, 1999). similar results were obtained when the driest periods were analysed; there were less “el niño” months between 1977 and 1983 than between 1989 and 1995; however, the dry period was longer after 1977. figure 4 shows signs of periodical variability in annual rainfall time series. three signals (decadal, bi-decadal and interdecadal) occurred at almost all stations which other authors have associated with sunspots (burroughs, 2004; hiremath, 2006), solar magnetic cycles (almeida, 2004) and ocean phenomena such as pdo and north atlantic oscillation (nao) (poveda, 2004; tourre et al., 2001). three hypothetical time series (n=65) were calculated for each signal (11, 21 and 33 years), having values between -1 and 1 to obtain a reconstructed series by summing representing mean annual rainfall pattern in the area (figure 5). the reconstructed series representing mean annual rainfall pattern (figure 5d) was compared to the smoothed time series (running means with burroughs filter) for la bella and cenicafé locations. a relationship was found between running mean series and bit-lagged reconstructed series (figure 6). the results showed that signs of low frequency cycles could be detected in annual rainfall series by using simple statistical methods related to a description of variability and that there were relevant cycles in rainfall figure 7. semivariogram for sea surface temperature time series in the equatorial pacific ocean. andrés j peña q., álvaro jaramillo r. and maría j paternina q.114 series which could have been modulating enso effects. this occurred because sea surface temperature time series’ semivariance function (equatorial pacific ocean) was related to enso phenomenon, showing signs of cycles having oneand three-decade periods as rainfall series (figure 7), whereas a two-decade cycle would mostly be related to an 18.5 year cycle associated with the pdo’s moon-tidal cycle (currie, 1991; yasuda, 2009). references almeida, a., gusev, a., mello, m., martin, i., pugacheva, g., pankov, v., spjeldvik, w. and schuch, n. (2004). geofísica internacional. 43, no. 2, 271-279. burroughs, w. (1978). on running means and meteorological cycles, weather. 33, no. 3, 101–109. burroughs, w. (2004). weather cycles: real or imaginary?, cambridge university press, cambridge, uk, 317 p. currie, r. g. (1991), luni-solar 18.6-year signal in tree-rings from argentina and chile, pure and applied geophysics. 137, no. 3, 281– 300. guzman, o. and baldión, v. (1997). el evento cálido del pacífico en la zona cafetera colombiana, cenicafé. 48, no. 3, 141-155. guzman, o. and baldión, v. (1999). influencia del evento frío del pacífico en la zona cafetera colombiana, cenicafé. 50, no. 3, 222-237. hernandez, g. (1999). time series, periodograms and significance, journal of geophysical research. 104, no. a5, 10355-10368. hiremath, k. (2006). the influence of solar activity on the rainfall over india: cycle-to-cycle variations, journal of astrophysics and astronomy. 27, no. 2-3, 367-372. kutzbach, j. and bryson, r. (1974). variance spectrum of holocene climatic fluctuations in the north-atlantic sector, journal of atmospheric science. 30, no. 8, 1958-1963. landscheidt, t. (2001). trends in pacific decadal oscillation subjected to solar forcing, internet site of the institute for research in cycles of solar activity, http://www.john-daly/theodor/pdotrend.htm (last accessed november 2010). lewis, p. (1960). the use of moving averages in the analysis of time series, weather. 15, no. 4, 121–126. mesa, o., g. poveda and f. carvajal. (1997). introducción al clima de colombia, universidad nacional de colombia, medellín, colombia, 390 p. peña, d. (1982). algunas características de la precipitación en palmira (colombia), turrialba. 32, no. 3, 219-228. poveda, g., velez, j., mesa, o., hoyos, c., mejía, f., barco, o. and correa, p. (2002). influencia de fenómenos macroclimáticos sobre el ciclo anual de la hidrología colombiana: cuantificación lineal, no lineal y percentiles probabilísticos, meteorología colombiana. 6, 121-130. poveda, g. (2004). la hidroclimatología de colombia: una síntesis desde la escala inter-decadal hasta la escala diurna, revista academia colombiana de ciencias. 28, no. 107, 201-222. tourre, y., rajagopalan, b., kushnir, y. and barlow, m. (2001). patterns of coherent decadal and interdecadal climate signals in the pacific basin during the 20th century, geophysical research letters. 28, no. 10, 2069-2072. verdon, d. and franks, s. (2006). long-term behaviour of enso: interactions with the pdo over the past 400 years inferred from paleoclimate records. geophysical research letters, 33, l06712, doi:10.1029/2005gl025052. wackernagel. h. (1995). multivariate geostatistics. an introduction with applications, springer-verlag, berlín, germany, 389 p. yasuda, i. (2009). the 18.6-year period moon-tidal cycle in pacific decadal oscillation reconstructed from tree-rings in western north america. geophysical research letters, 33, l05605, doi:10.1029/2008gl036880. geofisica colombiana pp 65-72 bogota 0 cn° 5 dic l e rn b r e de 2001 issn 0121 2974 observacion y anatisis de la varlacton de parametros geofisicos de un pozo ubicado en el piedemonte lianero felix humberto montano mejia universidad nacional de colombia, bogota resumen este articulo presenta el desarrollo de instrumentaci6n especializada que se diseii6 y se puso en funcionamiento con el fin de real izar la observacion y el anal isis de la variaci6n de parametres geofisicos de un pozo ubicado en el piedemonte llanero. la instrumentaci6n esta compuesta por una estaci6n remota para el registro multipararnetro y una estaci6n local en la cual se analizan los registros bajados de la estaci6n remota mediante la interrogaci6n peri6dica. palabras clave: prospeccion geofisica, tomografia electrica, piedemonte, parametros geofisicos abstract this paper shows geophysical characterization near a shallow well located in the llano university (villavicencio), which was conveniently selected because it's piedemonte l1anero proximity, zone characterized by a very active fault system. the specialized instrumentation development is also described that was designed and started up to carry out a telemetry observation using a multiparameter remote station located near the well under observation and one local station in the national university of colombia (bogota) from where geophysical parameters variations an analysis are permanently monitored. key words: geophysical prospecting, electrical tomography, piedemonte, geophysical parameters introduccion dado que colombia esta localizada en un ambiente sismotectonico de gran actividad y con alta frecuencia de sismos, existe la necesidad de contribuir con las observaciones de la actividad sismica enfocada en los fenomenos premonitorios. este proyecto de investigacion considero el desarrollo de instrumentacion especializada que permita el registro permanente de parametres geoffsicos de un pozo somero y su entomo, la interrogacion desde una estacion local y el analisis de los resultados. localizacion del area de estudio ei pozo de observacion se localiza a unos 10 km al se de la ciudad de villavicencio, en predios de la universidad del llano, 50 m al este de los laboratorios de ingenierfa electronica. sus coordenadas geograficas son: latitud: 4°.04.485' norte (941.800 planas origen bogota) longitud: 73°.35.061' oeste (['055.400 planas origen bogota) la altura es de 369 m sobre el nivel del mar. se encuentra localizado sobre un terreno plano, en una zona donde la pluviosidad promedio anual alcanza 2700 mm. marco geologico la zona preferencial para el inicio de las observaciones se ubica en la ciudad de villavicencio, proxima al borde oriental de la cordillera oriental de colombia, en los departamentos de meta, cundinarnarca y casanare, conocida como piedemonte lianero. es una zona caracterizada por un geofisica colombiana, 5, diciembre de 2001 sistema de fallas muy activo, denominado sistema de falla del borde lianero, que incluye en su parte central las fallas de altamira, acacias, guayuriba, colepato, cumaral, tesalia, lengupa, guaicaramo, servita, santa marfa y yopal, entre otras (paris y romero, 1994; ingeorninas, 1998; ingeominas, upes, 1999; sarriaa, 1990). estas fallas son en su mayorfa de tipo inverso y de empuje (page, 1986), con una componente destral apreciable, y definen el iimite entre rocas paleozoicas y cretacicas de la cordillera y los estratos menos perturbados del terciario y cuatemario que infrayacen en los llanos orientales, al este de la cordillera. ei area local donde se encuentra el pozo corresponde a terrazas cuaternarias de origen aluvial, y por tanto, de cornposicion areno gravosa, seguramente asociadas a los rfos ocoa y guayuriba. entorno geofisico se intent a hacer una breve descripcion tanto de la naturaleza dinamica de tipo regional que caracteriza el area, como de algunas caracterfsticas ffsicas locales en y alrededor del pozo. sismotectonica ei area de estudio esta caracterizada por una sismicidad relativamente superficial « 50 km), que genera movimientos por esfuerzos, principal mente compresivos entre 0.1 y 1.1 cm/afio que pueden generar sismos de magnitud cercana a ms = 7 (vergara et al., 1995). campos gravitacional y magnetico los valores absolutos de los campos gravitacional y magnetico proporcionan una referencia regional sobre ubicacion global y caracteristicas regionales del area. estos val ores cambian en el tiempo y puede ser mas (nil encontrar val ores relativos 0 anomalias que dan interferencias sobre 65 felix humberto montano mejia distribuci6n local de densidades (caso gravimetrico) 0 susceptibilidades magneticas (caso magnetico): a la vez se pueden proporcionar ideas sobre estructuras generales. ei valor del campo gravitacional, g, cercano a predios de la universidad de llano es de 977852 mgal (i gal = i cmls ), para 1998, mientras que el campo magnetico en junio de 2000 era del orden de 31800 nt (nano teslas). la anomalia de bouguer en el area cercana a villavicencio es del orden de -100 mgal, indicando una profundidad del orden de 40 km al manto y localmente dominada por sedimentos de comparativarnente baja densidad. caracterizacion electrica los rnetodos electricos utilizados en este trabajo -sondeos electricos verticales, sev, tomograffa de resistividad electrica, ter, y potenciales espontaneos, spproporcionan informaci6n de mayor detalle y caracterizan particularmente el sitio, pudiendose iniciar una observaci6n de sus valores en el tiempo, para asociarlos con cambios fisicos del subsuelo. ubicacion de sondeos y tomografias la figura i muestra la ubicaci6n de los estudios electricos y las tomograffas realizadas en la universidad del llano. figura i. ubicaci6n de sev y tornograffas caracterizacion con sondeos electricos verticales tabla 1. localizaci6n de sev sev latilud longitud descripcum 1 4°04.485' 73°35061 ' pozo de observaci6n 2 4°04.329' 73°35.010' cancha de futbol 3 4°.04.319 73°35057 veterinaria 4 4°04.270 73.35.101 elec:tr6nica 5 4°04.424' 73°35.287' malia externa 6 4°04.518' 73°35.110' via hacia el ideam 66 las siguientes figuras muestran los resultados de los sev, con su respectiva interpretaci6n. ......., l '.: ~ -r::.1----+-o---..._:~ /' i~.l"k" 'w • '\, figura 2. sev 1 polo de observaci6n. r.m.s.: 2.13% tabla 2. [nterpretacion pozo de observacron n p h d profundidad 1 1491 075 075 -075 2 5965 4536 5286 -5286 3 3240 2195 27236 -27.236 4 659.8 con n p h d r.m.s. cap a mimero resistividad de la capa en ohmios-metro. espesor de la capa en metros (m) profundidad de la base de la capa en metros rafz cuadratica media, de desviaci6n entre los resultados del modele y los datos obtenidos. r.m.s. i 2mm l(n) -nf ioi , , ~ ,v ''\ ~ i,m <, /' \ '4c.~, .. i ' " , figura 3. sev 2 cancha de futbol. r.m.s.: 4.1 % gt o r t sica colombiana, 5, diciembae de 2001 observacion y anallsis de la variacion de parametros geofisicos de un pozo ubicado en el piedemonte llanero tabla 3 interpretaci6n canci-la de futbol n p h d profundidad 1 1914 0.75 0.75 -0.75 2 646 199 2.74 -2.74 3 16016 633 9.07 -9.07 4 714 figura 4. sev 3 veterinaria .r.ms. 6.32% tabla 4. interpretacion veterinaria n p h d profundidad 1 857 09 09 -0.9 2 395 0998 1.9 -1.9 3 7674 21 4 -4 4 244 ...../,"""c_, __... --1--.···· -'+------. ------a'" 1 ldc 1 10 figura 5. sev electronica. r.m.s .. 4.31% geoff sica colombiana, 5, qiciembre de 2001 tabla 5 interpretacion electronica n p h d profundidad 1 191 1.26 1.26 -1.26 2 3888 219 232 -232 3 459 1000c , , ________________ l • ... _ , ,, ,, ,, ,. ,. , , . -·--------o-~---~----·-----~------/v ...~:~: __ l00:c~;; __;;~, ~----=o,£----+--->-----+-~'~ir----+:--------j ,.-~, ,-------,--------._,---------mj/ 10010 figura 6. sev 5 exterior unillano. r.m.s .. 4.52% tabla 6. interpretacion exterior unillano n p h d profundidad 1 763 2.18 218 -2.185 2 6484 2.76 4.94 -4.942 3 530 .. --------r""'"":'0000 -~--,--. --~:---------_._-----------1~ ._---,abr. 100, 10 figura 7. sev exterior ideam. r.m.s .. 4.6% 67 felix humberto montano mejia tabla 7 interpretaci6n exterior ioeai-." n p h d profundidad 1 625 1.17 1.17 -1.17 2 195 0.907 2.08 -2.08 3 1107 126 14.7 -1468 2180 8.71 234 -2339 477 los sev muestran que en los primeros 75 m dominan valores de resistividad electrica que varfan entre 150 y 5000 ohm-metro. estos valores son tipicos de sedimentos areno gravosos, sin contenidos metali cos y mas 0 menos saturados. por observaci6n directa de resistividades en diferentes litologfas en el area, se ha establecido la siguiente correlaci6n de resistividades con iitologfas: tabla 8. resumen de propiedades electricas del subsuelo aango de aango de litologia resistividad profundidad probable ohm-m m 190 1900 08 -13 efectos superficiales 195-750 1.2 25 arenas mas 0 menos saturadas > 1000 10 25 gravas y conglomerados < 100 arcillas 68 tomografia la naturaleza del subsuelo y su relativa continuidad se puede apreciar mejor, con la suposicion de no continuidad latera] de las unidades, en la que se apoya la ter; quizas, esta es la herramienta que permitira un mejor seguimiento de las propiedades ftsicas del subsuelo. la siguientes figuras muestran dos secciones realizadas, una con centro en el pozo con direccion n40e y otra localizada a una distancia en linea recta de 200 m en cercanias a la rectorfa de la universidad. reclorla ps.z 00 1.0/-~-'--'--"--'--'--'----'--'~'--~-'---'--'--'---'---'--'-~--'~'--"--~-'--'--'--"---'---'-~~---''--'--''---'--~-'---l--i 4.2 8.0 11.9 160 48.032.0 64.0 measured apparent resistivity pseudosection, ps.z 0.0 1.0 i-':::::~~~~;;s;;;~~~;;;~;;;~':'~-;;~~;-"--"--'--::-"'='--i 4.2 .--8.0 11.9 16.0 32.0 48.0 64.0 calculated apparent resistivity pssudnssction depth iteration 3 rms error = 11.8 % 0.0 16.0 o.5 r-'---'---'---'---'---'---'---'---'---'---'--"---'---'--"--"--"--"--"--"--"--"--"--'--'--'----'--'--'----~'--'----~~~~~____\. 4.3 8.7 32.0 48.0 64.0 15.5 inverse model resistivity section_____ c:::j_i1iiilii __ c:::j _ 163 284 498 870 1522 2662 4655 8141 resistivity in ohm.m unit electrode spacing 2.0 m. figura 8. tomograffa rectoria m. m. m. geofislca colombiana, 5, qiciembre de 2001 observaci0n y analisis de la variaci0n de parametros geofisicos de un pozo ubicado en el piedemonte llanero ps.z 00 16.0 :~ i'8.0 11.9 measured apparent resistivity pseudosection 320 caseta 48.0 m ps.z 0.0 1.0 r~~ ~ ~iii;;::~1 4.2 8.0 11.9 15.5 16.0 32.0 480 m.64.0 calculated apparent resistivity pseudosection depth iteration 3 rms error = 12.9 % 00 16.0 32.0 48.0 64.0 m.0.51~~~. ~ ~ ~~~~~ 4.3 8.7 inverse model resistivity section_____ c::::j c::::j_gi.i'2:l _ 236 386 631 1033 1688 2761 4515 7384 resistivity in ohm.m unit electrode spacing 2.0 m. figura 9. tomograffa pozo de observaci6n como se aprecia en las secciones, dominan los valores altos de resistividad electrica (sedimentos gruesos), como ya se habia inferido a partir de los sey. potenciales espontaneos dentro de la caracterizaci6n electrica de sitio, la otra herramienta utilizada fue la medici6n de potenciales espontaneos, conocidos como sp. con esta herramienta se miden diferencias de potencial electrico entre dos puntos cualesquiera de la superficie generados por efectos electroqufmicos 0 electrocineticos, como hip6tesis inicial, se esperarfa que un medio sometido a esfuerzos diferenciales genera un patr6n de estos sp y cuya variaci6n temporal estuviera asociada a variaci6n de esfuerzos, se realizaron medidas de sp en un area cercana ai pozo, la separaci6n de electrodos fue de 5 m y se hizo a 10 largo de los radios, con separaciones de 30°. las lecturas se reajizaron sobre un circulo de radio maximo de 50 m. en la figura 10 se muestran los resultados. geoflslca colombiana, 5, dic!embre de 2001 figura 10. medici6n de potencial espontaneo 6! felix humilerto montano mejia los valores observados de sp se obtuvieron con electrodos polarizables, y muestran patrones que, en principio, coinciden con la tendencia de las geoformas; los valores rnaximos estan alineados en una direcci6n aproximada n40e y los mfnimos son perpendiculares a los arueriores (vease figura ii). los valores minimos son del orden de 10 my (milivoltios) y los rnaximos, un orden de magnitud mayor (100 my). 2::5 '-_ '.figura 1j. comportamienros porenciales esponraneos con electrodos polarizables "_, dado que se trata de observaciones iniciales realizadas con el fin de :."::;estimar la rnagnirud, es prernaturo atribuir a fen6menos compresionales. ~.:es posible que esta manifestaci6n electrica se deb a a ruidos antropicos u ~~ptros efectos min desconocidos; sin embargo, se recomienda en el futuro :j c.observar atentamente las variaciones en arnplitud y los patrones de com~portamiento. tambien se recomienda repetir las pruebas utilizando elect:~odos no polarizables (acopjados a un amplificador de instrumentaci6n), ~~on el fin de eliminar los efectos de polarizaci6n de los electrodos. disenoinstrumental la parte basica del presente proyecto comprende el disefio y la caracterizaci6n de un sistema de adquisicion y proceso de informacion geoffsica, seleccionada de acuerdo con experiencias intemaciona1es (asteriadis, figura /2. folo de la estacion remota 70 figura /3. foldde la estacion local 1999, 2000). ei instrumental esta compuesto de una estaci6n para ser instal ada en el sitio de observaci6n (estaci6n remota) y de otra de recepci6n y analisis en ellaboratorio de procesos (estaci6n local). las figuras 12 y 13 y muestran las dos estaciones. estacion remota se trata de una aplicaci6n de software y hardware, disefiada para funcionar sobre un computador personal, que proporciona comunicaci6n con los dispositivos de campo (principal mente sen sores), almacena la informaci6n registrada en archivos pianos, y tambien permite la supervisi6n de la evoluci6n de sefiales en el sitio de interes. estacion local la constituye la unidad de interrogaci6n y proceso (uip), ubicada en el laboratorio de posgrado de geoffsica, en la universidad nacional de colombia. se reduce basicarnente a un computador con plataforma windows, algunas herrarnientas de office con software de comu• nicaciones y de instrumentaci6n. esta estacion tiene las siguientes funciones: interfase del usuario con la estaci6n remota. interrogar la estaci6n remota, bajar los archivos generados por las diferentes variables. transferir archivos y aplicativos 0 modificaciones a la estaci6n remota con el fin de mejorar 0 adecuar los procesos. manejar la informaci6n y procesarla. resultadosinstrumentales a continuaci6n se presentan algunos ejemplos del software desarrollado para la estaci6n local y remota. estacion remota la siguiente figura corresponde al desarrollo del instrumento virtual que permite al usuario en el sitio 0 de manera remota observar cada una de las variables escogidas para la observaci6n y el registro. geofisica colombiana, 5. diciembre de 200"1 oijservacion y analisis de la variacion de parametros geofisicos de un pozo ubicado e" el pie demonte lla'jero figura /4. intcgraci6n de senates en la estaci6n remota estacion local dado que se requiere leer y analizar la informacion recogida en terreno por la estacion remota, se diseiiaron varios instrumentos virtuales que permiten leer los archivos de forma secuencial tal como se genera ron pero a la velocidad que elija el analista. las rutinas contemplan la posibilidad de leer los registros parcial 0 total mente y aplicarles diferentes procesos de analisis. a continuacion se presentan algunos ejemplos de los programas de analisis de datos. software de e n e l i s i: de datos y ajuste polinomial en la figura is aparece el instrumento virtual de analisis de datos y ajuste polinomial. este aplicativo generalmente debe ejecutarse en la estacion local con el fin de tomar cualquier archivo de los registrados en la estacion remota y reproducirlos mediante este instrumento, el cual ie permite al analista observar los val ores rninimo, maximo, promedio, la violacion a limites de alarm a y la curva de tendencia. figura /5. lnstrumento virtual de analisis de datos y ajuste polinomial geofisica colombiana, 5, diciembre de 200-1 ei instrumento cuenta con ventanas para la configuracion del ruimero de datos poi'muestrear, el grado del polinomio de ajuste, la localizacion de los datos, etc. software de e n e l is i s de datos filtro butterworth en la figura 16 se presenta el instrumento virtual que ie perrnite al usuario reproducir los datos registrados en la estacion remota y pasarlos poi' un filtro de butterworth. el instrumento tiene la posibilidad de configurarle los parametres del ruirnero de seiiales poi' muestrear, el orden del filtro, la frecuencia de cone, el tipo de filtro (pasa altos, pasa bajos, etc.). presenta como resultados en pantalla la seiial lefda, el variograma de los datos elegidos, la seiial filtrada, el valor maximo, minimo y el promedio de la seiiallefda. figura 16. instrumento virtual filtro de butterworth resultados generales, conclusiones v recomendaciones resul tados desde el punto de vista geoffsico, se realize una caracterizacion preliminar en el entorno de un pozo ubicado en la universidad del llano pudiendose iniciar una observacion de sus val ores en el tiempo, para asociarlos a cam bios ffsicos del subsuelo. se inicio una base de datos con medidas de sp, ter y sev, para su control y observacion temporal en un area de actividad tectonica. se disefio, construyo y caracterizo una estacion multipararnetro de acceso remota para observaciones sismologicas. se implernento y adecuo la estacion local para el analisis de la informacion recogida poria estacion remota. se irnplemento un sistema de comunicacion entre la estacion local y la remota, con el cual se puede interrogar desde la esracion local la estacion multipara metro, descargar en la estacion local la informacion registrada en tiempo pasado u observar el componamiento en tiempo real. desde la estaci on local hacia la remota se puede hacer transferencia de archivos y modificaciones de programas, etc. 71 felix humberto montano mejia se prob6la configuraci6n de la estacion remota desde la estacion local. se desarrollo, utilizando un programa para la implementacion de instrumentos virtuales, el software para la adquisici6n, interpretacion y analisis de datos. se planreo un merodo para la rnedicion de sp (potenciales espontaneos), el cual permite esrimar errores en la medicion, conclusiones la entrega de las estaciones se constituye en un aporte instrumental que permite la observacion y el analisis sistematico de fenomenos premonitorios de actividad sismica en el piedemonte lianero. a la universidad nacional de colombia se ie facilita promover actividades de investigacion en el campo de la sisrnotectonica mediante el desarrollo de nuevos proyectos encaminados a la obtenci6n de informacion de parametres premonitorios de sismicidad. ej sitio elegido para dar inicio a las observaciones preliminares es estrategicamente conveniente, entre otras razones por su proximidad con el piedemonte llanero, la infraestructura disponible, la cooperacion interinstitucional, etc. se realizaron registros de algunas variables en la zona de estudio. sin embargo, estos datos son insuficientes para determinar patrones de comportamiento; solo se han utilizado en la puesta en marcha y en el acondicionamiento de la estacion remota. recomendaciones • continuar con las investigaciones de fen6menos premonitorios de actividad sismica utilizando la infraestructura generada como resultado del desarrollo de este proyecto. estudiar los potenciales espontaneos en diferentes zonas proxirnas al piedemonte llanero, observar las tendencias y evaluar su comportamiento como fenomeno premonitorio de sismicidad. dado que la estacion remota posee 16 entradas analogi cas, implementar otras sefiales como ernanacion de radon, sefiales metereologicas, etc. se recomienda que la universidad nacional de colombia y la universidad del llano fortalezcan el convenio interinstitucional con el cual se garantiza la continuidad de las investigaciones de fenornenos premonitorios. poner en rnarcha de manera definitiva de las estaciones y el monitoreo y analisis continuo de la informacion. entregar a la universidad nacional de colombia el disefio de estaciones remoras y una metodologfa de interrogacion con el fin de implementar una red de estaciones. 72 referencias bibliograficas asteriadis, g. & arabelos, d. 1999: "equidistance ordering of large elu1hquakes in the aegean and surrounding areas". survey review. asteriadis, g., arabelos, d., contadakis,m. e., & others. 1999: the use of outlier detecting method in time series of continuous daily measurements of underground water level and temperature in earthquake prediction of investigation. internal report university aristotelic oftessalonik. asteriadis, g. & contadakis, m. e. 2000: recent results of the research for preseismic phenomena on the underground water and temperature in pieria, nothern greece. briceno, l. a. 200 i: observaci6n y analisis de la variaci6n de parametres geofisicos en un pozo en el piedernonte llanero. ingeomtnas, upes. 1997: microzonificaci6n sismica de santa fe de bogota. ingeomtnas. 1998: estudio general de amenaza sismica de colombia. nationalinstruments. 1999: measurement and automation catalogue. nationalinstruments.2000: labview basico i manual del curso. nationalinstruments. 2000: labview basico ii manual del curso. page, w. d. 1986: geologia y sismicidad del nordeste de colombia. isa, integral, woodward & clyde consultants, 281 pp. paris, g. y1. romero. 1994: fallas activas en colombia. bol. geol. vol. 34, nos. 2-3. ingeominas, pp. 5-25. sarrla, a. 1990: ingeniena sismica, ediciones uniandes-ecoe. vergara, h, taboada, a., romero, 1., paris, g. ye castro. 1995: neotect6nica del borde ilanero: resultados preliminares. memorias del serninario de sismotect6nica del borde ilanero. j. valdiri, editor. ingeominas, 115 pp. eofisica colombiana, 5, diciembre de 2001 hugomonsalve.indd 85 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 85-98 seismological observatory at quindio university (o.s.q.): a firm long-term step toward resolving regional seismicity monsalve jaramillo hugo1,2, correa montaño isabel cristina1,2, and valencia mina william1 1 ceifi, quimbaya research group quindío university 2 seismological observatory at quindío university osq. e-mail: hugom@uniquindio.edu.co resumen el eje cafetero (zona central de colombia, cultivada con café), y en particular, el departamento del quindío es constantemente golpeada por sismos de diferentes magnitudes. los sismos son el resultado de la compleja tectónica regional debido a la interacción entre las placa nazca, caribe y sur américa. la convergencia de estas placas es responsable de la formación de los andes. por estas razones, el osq (observatorio sismológico del quindío) e ingeominas, como las entidades estatales encargadas de monitorear la actividad sísmica, han de contribuir, a si como los centros regionales de investigación, al desarrollo de los estudios sismológicos y el análisis de la actividad tectónica local, para así obtener nuevos conocimientos a cerca de los desastres naturales que amenazan a las comunidades en la región y entender mejor la estructura interna de la corteza en la región del eje cafetero. la combinación de diferentes modelos de velocidad para la región del eje cafetero ha permitido la definición de elementos clave para tener una mejor percepción de la estructura de la corteza. en consecuencia, la determinación de un modelo de velocidades mejorado para la región cafetera ha ayudado a localizar mejor los sismos y a determinar con mayor precisión los espesores de la corteza y la estructura del manto superior entre los terrenos adyacentes que fueron generados en diferentes períodos geológicos. la magnitud ml fue determinada para los campos cercanos e intermedios y usando el modelo presentado en este articulo, el problema del rango dinámico de saturación de los sismógrafos durante sismos relevantes en campo cercano es solucionado. la magnitud local de algunos sismos superficiales fue calculada usando sismos con distancias epicentrales entre 10km y 300km asi como las magnitudes de algunos sismos profundos con distancias hipocentrales entre 100 km y 600 km. la evaluación del potencial sísmico en la región del eje cafetero empieza con el registro de la sismicidad reciente en chocó y en la parte norte del departamento del valle, y está basada en el análisis comparativo entre esta sismicidad y la registrada antes de la ocurrencia de los sismos mas importantes de los últimos 42 años. finalmente, considerando las dos fuentes de subducción, un sismo cuya magnitud de momento sísmico entre 6.7 y 7.3 podría esperarse en la próxima década, con una probabilidad de 87%. palabras clave: quindío, sismicidad, instrumentación. (manuscript received may 2005, paper accepted october 2005) 86 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina abstract the coffee belt (the central coffee-growing zone in colombia), and more particularly, the state of quindio is constantly hit by earthquakes of different magnitudes. earthquakes result from the complex regional tectonics, brought about by interaction between the nazca, caribbean, and south american plates. the convergence of these plates is responsible for the formation of the andes. for these reasons, the osq (sesimological observatory of quindio, from its acronym in spanish) and ingeominas, as government offices in charge of monitoring earthquake activity, have to contribute, along with research centers of the region, to the development of seismological studies and analysis of local tectonic activity in order to obtain new knowledge regarding natural disasters which are threat to the communities in region and be able to better understand the internal crustal structure in the coffee belt region. the combination of different velocity models for the coffee belt region has allowed the definition of key elements to have better insight about the structure of the crust. consequently, determining an improved new velocity model for the colombian coffee-growing region has helped to a better location of earthquakes and to a more accurate determination of differences in crustal thickness and the structure of the upper mantle between adjacent terrains that were built during different geological periods. the ml magnitude was determined for the near and intermediate fields, and, using the model presented in this article, the problem of dynamic range saturation of seismographs during relevant earthquakes in near field is overcome. the local magnitude of some shallow earthquakes was calculated using shocks with epicentral distances ranging between 10km and 300km as well as the magnitudes of some deep earthquakes with hypocentral distances ranging between 100km and 600km. seismic potential assessment in the coffee belt region begins with recent seismicity recorded in chocó and the northern part of valle (a state in colombia) and is based on comparative analysis between this seismicity and the one recorded before the occurrence of the most important earthquakes in the last 42 years. finally, taking into consideration the two subduction sources, an earthquake with seismic moment magnitude ranging between 6.7 and 7.3 could be expected within the next decade, having a probability of 87%. key words: quindío, sismicity, instrumentation. © 2005 ersj unibiblos introduction as a result of the emergency caused by the january 25, 1999, mw 6.2 quindio earthquake, quindio university and ingeominas worked jointly in the installation and operation of a temporary seismograph network, assigning this responsibility to the studies and research center of the engineering faculty (ceifi) which became, since february 1999, the seismological observatory of quindio university, using telemetry instruments, seismographs, and accelerographs borrowed from ingeominas in order to monitor the aftershocks. currently, there are eight accelerographs in operation and six telemetered seismograph stations (table 1), five of which are located in armenia and one in manizales (rodeo, which belongs to manizales volcanological and seismological observatory ovsm). since may 1999, when the observatory began operations, and up to now, the o.s.q. has been in charge of carrying out seismic monitoring activities in quindio associated to active faults in the area, however, other local and relevant worldwide seismic activity is also detected. in terms of knowing regional seismicity, the information collected by the local network has been of vital importance in understanding the local geodynamic processes related to urban planning and risk management activities in the region, identifying seismic sources, activity that ceifi has been conducting. in addition, the information that is analyzed and provided by ceifi is, and has been, of 87 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity great support to different government officials involved in civil protection and disaster prevention activities in order to carry out programs of information diffusion and disaster management. table 1. osq station network. type statión departament city/town latitude longitude accelerographic (quindío university) uniquindío quindío armenia 4.506° -75.624° accelerographic (accelerographic (agreement with colombian national accelerographs network– rnac)) calarcá quindío calarcá 4.549° -75.618° poscosecha quindío armenia 4.506° -75.624° génova quindío génova 4.219° -75.775° dagua valle dagua 3.662° -76.708° calima valle calima-darien 3.884° -76.580° trujillo valle trujillo 4.228° -76.334° toche valle toche 3.650° -76.090° seismological guayaquil quindío planadas 4.460° -75.625° la mina * quindío pijao 4.377° -75.727° rodeo tolima machin volcano 4.490° -75.392° caicedonia valle caicedonia 4.320° -75.814° riber * valle ribera alta 4.519° -75.889° modin * valle cartago 4.670° -75.875° (* not operating permanently) a seismograph network is a useful tool to know the inner structure of the earth, earthquake waves propagation patterns, earthquake-source mechanisms, as well as urban seismic microzoning and hazard assessment of a specific region. for this purpose, it is necessary to have qualified staff that is trained in processing and analyzing data, capable of improving their knowledge of seismic phenomena and generating new hypothesis regarding seismic behavior of the studied region. the installation of an observatory in a region enables gathering of valuable information to conduct studies and increase the knowledge of local seismicity as well as studies about the regional crustal structure. the observatory becomes thus a frim long-term step to help resolve regional seismicity osq goals given the fact that quindio department and the coffee belt region as well as other zones in the country are usually threatened by earthquakes, the seismological observatory has as top priority goal of offering logistic support to the research center for the analysis of tectonic activity in the state of quindio for which constant monitoring, recording, and broadcasting activities are being carried out 365 days a year. in order to generate new knowledge about the natural hazards to which the region is constantly exposed, research projects are developed. these projects allow knowing, assessing, and improving social development conditions and subsoil conditions to provide early advise on disaster prevention and the determination of potential areas for future urban development of engineering works. regional tectonic setting the state of quindio has geological units of oceanic and continental origin which date back to paleozoic and mesozoic ages (arquia complex, quebradagrande complex, ultrabasic rock fragments, and cordoba igneous complex) which are intruded into by igneous units from the late mesozoic and overlaid by tertiary sedimentary sequences. these units, which comprise the regional geological basement, are separated by big regional faulting and are covered by a big fluviovocanic unit of quaternary age, quindio and pereira fans, and by local quaternary units. the general geological and tectonic schemes are presented in figure 1 (espinosa a. 2005). 88 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina from the tectonic point of view, the state of quindio sits on the romeral fault system. the three main faults of the system, cauca–almaguer, silvia–pijao, and san jerónimo, go across the region with strike component n15e. the importance of those faults through the geological history of the zone has been unquestionable, but its activity during the quaternary has been inconsistent (espinosa a. 2005). seismic activity in this zone locates between isodepths of 80-160km within the wadati-benioff zone of nazca plate, which subducts along the colombian northwest (monsalve, 1998), and also along numerous crustal faults. out of these faults, the most noticeable are cordoba, navarco, silvia-pijao, buenavista, el salado, cauca-almaguer, armenia, salento, montenegro, palestina, and cauca-patía faults which are active faults with probable magnitudes ranging between 6.1 and 6.9 mw (guzmán, 1998). taking into consideration all this tectonic complexity, the seismological observatory at quindio university (osq) contributes to the development of projects that generate new knowledge about the hazards that threaten the region, in order to improve plans and processes regarding both disaster mitigation and disaster prevention. figure 1. geological scheme of quindío department. (espinosa , 2005). 89 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity seismicity of quindío departament 1999 2005 colombia is located in one of the most active seismic regions of the world dubbed “the pacific ring of fire”. colombia’s tectonics is complex because of the convergence of three plates: nazca, south american, and caribbean plates. these plates interact with one another originating compression, tension, and shear stresses within the region, which in time cause accumulation of energy. this interaction has originated three mountain ranges: the western, the central, and the eastern ranges, being the central one the oldest of all, and the eastern one the youngest. the most active zones in terms of seismic activity are located throughout the andean region of the country, along which the mountain ranges are settled; all this forms thus a complex morphological setting that is currently object of research. quindío department is located in the andean region of colombia, more exactly, to the western flank of the central mountain range. its boundaries are defined by risaralda department to the north, tolima department to the east, and valle del cauca to the southwest. for the geological complexities previously mentioned and the critical location, it is obvious that the coffee belt region is prone to significant shaking from earthquakes. quindío department is one of the most seismically active areas in colombia. this was confirmed by the occurrence of the january 25th quindio earthquake (1999), which took place at 18:19:16 ut (13:19:16 hl), with a magnitude mw=6.2 and a depth of 18 km. a mw 5.5 aftershock occurred at 22:40:12 ut (17:40:12 hl) the same day. the main shock ruptured along a left strike slip with normal component fault surface. this faulting located at shallow depth in the continental crust. its rupture length was 11.7 km and 10.6 km width; this yields a total rupture area of 124 km2. 391 well-identified and well-located aftershocks were used to constrain the rupture area. the aftershocks were recorded between depths of 9 and 20 km (monsalve y vargas 2002). this seismic event was the largest shallow earthquake instrumentally-recorded ever in the coffee belt region. its epicenter was located near cordoba (a municipality of quindio department) 17km from armenia. the death toll was 1,185; about 8,523 were injured, and more than 100.000 constructions were affected in 28 municipalities of three states (quindío, risaralda, caldas, tolima y valle) (monsalve et al, 2002). between february 1999, and august 2005, the osq has recorded 13.607 events (table 2). not all of them have been located due to the little coverage of the network. the events (figure 4), which have been located with the velocity model mentioned above, have yielded magnitudes ml ranging between 0.04 and 6.2, all of them having shallow depths. (for further information about the earthquakes location, see the location catalog on the osq web page: http:// www.uniquindio.edu.co menú investigaciones, ceifi, grupos, o.s.q ) between may 1st, 1999 and march 31st, 2000, 4,256 aftershocks were recorded. this produced a maximum daily rate of 58 events (october 21st) and an average of 15 events/day. we point out that during june 1999, a maximum number of 56 events took place and the minimum were registered during december 1999 march 2000, yielding an average of 14 and 13 earthquakes respectively (figure 3). in general, the occurrence of aftershocks continues with a fast exponential decrease rate as a function of time. the seismicity during 1999 remained at high levels most of the year as a result of the numerous aftershocks generated by january 25th earthquake (1999). during the year 2000 seismicity was low, except during april 10th 11th where 108 and 115 earthquakes/day were registered. because of their magnitude, the following events were remarkable during the year 2000 catalog: february 4th m l = 3.8 (córdoba fault) february 17th m l = 3.6 (armenia fault) february 19th m l = 3.6 (córdoba fault) march 11th m l = 4.0 (armenia fault) april 10th m l = 3.7 (córdoba fault) october 31st m l = 3.5 (armenia fault) 90 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina table 2. 1999 – 2005 earthquake catalog osq. 1999 seismicity 2000 seismicity 2001 seismicity 2002 seismicity month number of events month number of events month number of events month number of events january 2338 january 199 january 39 january 45 february february 237 february 236 february 30 march march 188 march 65 march 45 april april 547 april 186 april 100 may 478 may 139 may 378 may 128 june 637 june 53 june 302 june 17 july 645 july 281 july 217 july 24 august 541 august 250 august 159 august 49 september 362 september 218 september 181 september 51 october 470 october 234 october 106 october 28 november 260 november 205 november 123 november 26 december 227 december 15 december 150 december 32 2003seismicity 2004seismicity 2005seismicity month number of events month number of events month number of events january 34 january 12 january 19 february 30 february 26 february 123 march 24 march 21 march 107 april 25 april 77 april 67 may 29 may 29 may 49 june 38 june 24 june 331 july 17 july 46 july 225 august 25 august 12 august 221 september 15 september 14 october 27 october 101 november 43 november 255 december 9 december 12 91 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity figure 2. number of local earthquakes registered by the osq, from 1999 to august 2005. figure 3. number of daily events between may 1st, 1999 and march 31st, 2000. maximum and minimum seismicity levels can be observed. seismicity during 2001 had its highest level during the period february may. the events occurred on january 16th 09:56 pm (local time), and march 2nd 06:46 and 06:47 (local time), were felt in pijao and felt in the southern part of armenia, respectively. the first of them had a ml magnitude of 3.1 and was located 8.3km southwest of armenia. during 2002 and 2003 seismic activity remained very low, with mild increase from may to june 2002, and june and november 2003. the three first months of 2004, seismicity levels were very low with a subtle increase in april with increases in activity on the 19th and on the 20th during which 76 local earthquakes whose magnitudes 92 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina ranged 0.6 and 2.9 ml. two of which were felt in armenia and calarcá. the first, april 19th at 04:04 local time (09:04ut), had a m l magnitude of 2.9. the second, same day (19:19 ut), reached a magnitude 2.3 m l. the following days seismic activity remained normal until september, where a significant increase in activity arose, with an important event on november 15th at 4:06 am local time with magnitude 6.7 in richter scale and epicenter located in the pacific ocean, 50.7 km to the southwest of bajo baudó (choco). until 2005, seismicity continues to be stable and it is worth mentioning the occurrence of an earthquake (june 22nd, 2005) at 03:07am local time (08:07 ut), with magnitude of 4.5 m l , 12.92km deep. this event was strongly felt in the whole quindío and in some municipalities of risaralda state. there were no casualties or material losses. more than 336 aftershocks were reported until june 30th, all of them having magnitudes noticeably inferior to the main shock. the aftershock occurred the same day at 09:25 am local time, with a magnitude m l of 2.9 which was felt in some towns near calarcá. this seismicity is associated to the tectonically active area of quindio department around cordoba municipality and is directly related to the activity that originated the january 25th earthquake in armenia in 1999. figure 4. events located by the osq since 1999 to 2005. six events with magnitudes m l equal to or greater than 4.0, from 1999 to 2005 are shown in figure 5. the osq recorded four of these six local earthquakes; the other two are relate to the seismic activity occurred on january 25th, 1999 and were located by the colombian national seismograph network (rsnc, from its acronym is spanish) in view of the fact that then, quindio university had not fully set up the seismological observatory and started its activities. 93 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity figure 5. location of earthquakes with magnitudes m l greater than or equal to 4.0. table 3. localización sismos con magnitudes mayores o iguales a 4.0. year month day local time latitude longitude depth magnitude located by 1999 01 25 13:19 4.432 on -75.703 ow shallow (less than 30 km.) mw= 6.2 rsnc 1999 01 25 17:40 4.408 on -75.732 ow shallow (less than 30 km.) mw= 5.6 rsnc 1999 02 21 00:27 4.482 on -75.689 ow shallow 12 km. m l = 4.07 osq 1999 02 25 10:39 4.381 on -75.664 ow shallow 15.95 km. mw= 4.35 osq 1999 10 13 08:10 4.324 on -75.680 ow shallow 6.67 km. m l = 4.12 osq 2005 06 22 03:07 4,433 on -75,684 ow shallow 12,92 m l = 4.5 osq model for the location of seismic events in order to locate the earthquakes registered by the osq, the phase information recorded by different stations of ovsm and osso was required, and the model developed by (espinosa and monsalve, 2000) was used. this model is a one-dimensional velocity model derived for the coffee belt region using seismic tomography and well-located earthquakes between 4ºn and 6ºn within the coffee belt region with the software hypo71 and the non-linear ray inversion method velest. these earthquakes were later relocated using the joint hypocentral determination (jhd), (dewey, 1971) in order to verify the quality of the model generated for this zone (espinosa y monsalve 2002). the data used for velocity structure inversion included 566 earthquakes suitable for a good model (out of 1,337). the quality of the located events was 94 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina high: rms= 0.057 s, 12km deep, average magnitude of 1.25 and a gap of 158º. from a seismological point of view, a good velocity structure model is important to obtain location of events, as well as reliable source parameters to understand ground shaking during strong ground motion the results obtained with the one-dimensional model show a uniform distribution of velocities. the final result of the inversion using an initial model shows a velocity model of flat layers with a good resolution of superficial layers (0-100km depth). however, for deeper layers the resolution is not really good, because most earthquakes located at shallow levels and there are not many seismic rays that cross those regions (espinosa and monsalve 2002). considering the results of different inversions, we model the continental crust for the coffee belt region as consisting of six flat layers where the moho is situated, from 30 to 35km deep. the wave velocities up to 100km depth, are: 6.0km/s at depths of 6km, 7.0km/s at depths of 24km, 7.4km/s for a 22km-thick layer, 7.8km/s for a 14km-thick layer, and finally, a zone with velocities of 8km/s (figure6). table 4. depths, layer, initial and final velocity ranges and final model. the velocity range is based on four output models. the final model is defined as the lowest average rms (espinosa and monsalve 2002). layer depth range of initial velocities range of final velocities final velocity model 1 5.1 4.0-5.50 4.68-5.53 5.2 2 -6 5.5-6.10 5.51-6.78 6.1 3 -30 6.1-7.00 6.41-7.40 7.0 4 -52 6.5-7.50 7.40 7.4 5 -66 7.8-8.00 7.82 7.8 6 -100 8.0-9.00 8.00 8.0 this study allowed to establish a velocity model for the crust below the zone as well as to understand better the tectonic features of the region, which is important to have more accurate knowledge of the crustal structure in the colombian coffee belt and therefore to improve local earthquake hazard assessment studies. figure 6. final velocity models (blue line: average model (final)). (espinosa and monsalve 2002). calculation of local magnitude (m l ) using strong motion accelerograms besides earthquake location, it is really important to obtain the local magnitude for strong earthquakes using the accelerograms. for this purpose, the method proposed by barragán (2003) is used. this method was developed processing the most important time history records of shallow events (39 events) and events of intermediate depth, (40 events) that have occurred in the central region, los santanderes region, and the western region of the country which were provided by the colombian national accelerograph network (rnac), from 1994 to 2001. the accelerograph records are used as an acceleration input to the equation of motion of the wood-anderson torsion seismograph to produce synthetic seismograms which is then read in the standard manner thus permitting an easy and dependable calculation of the local magnitude ml of important events (barragán 2003). the colombian national accelerographs network (rnac) reported a magnitude m l 6.2 for quindio 95 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity earthquake (january 25th, 1999), and with this method, a magnitude m l 6.22 was calculated using the complete procedure and records from 23 accelerographic stations; and a magnitude m l of 6.10 using a preliminary procedure with records of 26 stations (barragán l. f. 2003) which confirms the reliability and simplicity of the method. to have a summary of the process to calculate ml with the method proposed by barragán (2003), let us start by the data that is collected during normal processing activities; it includes instrumental and baseline correction of the records as well as their separation into three orthogonal components: vertical, east-west, and north-south. using the software degtra for windows version 1.4.7, maximum registered amplitudes in cm/s2 are obtained from the horizontal components (eastwest and north-south) and the formula (barragán, 2003) used for shallow or intermediate earthquakes is applied as follows: • for shallow earthquakes an empirical expression is proposed in order to directly determine local magnitude m l using horizontal ground accelerations m l = log 10 a ace + 1,091ln δ 0,1443 where a ace is the maximum amplitude in cm/s2 (from zero to maximum) registered by an accelerograph set on the bedrock for a particular earthquake at a specified distance and δ is the epicentral distance. • for intermediate depth earthquakes between 70 and 300km deep, another empirical expression for the direct calculation of local magnitude m l using horizontal ground acceleration m l = log 10 a ace + 1,059ln δ 0,1136 where a ace is the maximum amplitude in cm/s2 (from zero to maximum) registered by an accelerograph set on the bedrock for a particular earthquake at a specified distance and δ is the hypocentral distance in km. these equations are only applicable to horizontal strong motion components of accelerograph records. in this way, the local magnitude of earthquakes in the near field is calculated, using strong motion accelerograms and applying this to the regional geotectonic setting. earthquake potential in the coffe belt region earthquake potential was assessed based on the seismicity registered in northern valle and chocó. comparative analysis were done between this seismicity and the one that took place before the occurrence of the most important historical earthquakes that have affected the region during the last 42 years. all this information about the seismicity, registered from 1961 to 2003, was compiled based on the minimum magnitude threshold of ms> 4.4. this refers to seismicity that has been experienced in the region (figure 7) which, because of the lack of a national seismological network at the time, it was necessary to resort to the usgs-neic catalog. with this catalog, the comparative analysis was performed. this analysis comprised the seismicity that took place before the occurrence of the most important earthquakes that have struck the coffee belt region during the last 42 years. through this analysis, it was possible to find a time span (in years) of events recurrence that will repeat in the different sources any time within the next decade. knowing this, and the type of faulting mechanisms associated to the different zones, it is possible to obtain better insight of the earthquake potential in the region (monsalve y ospina 2004). according to the observation of these recurrence patterns an earthquake of magnitude (m w ) ranging from 6.5 to 7.2 could be expected at the subduction zone h=120km between filandia and circasia which would occur the next decade with a probability of 57% and intensities up to vii (ems-92 intensity scale). similarly, an earthquake of magnitude (m w ) ranging from 6.7 to 7.3 in the subducton zone h=60 km, within risaralda and chocó states would occur the next decade with a probability of 51% and intensities up to viii or greater in ems-92 scale. finally, considering the two subduction zones altogether, an earthquake of magnitude (m w ) from 6.7 to 7.3 could be expected the next decade with a probability of 87%. 96 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina figure 7. magnitude distribution for seismicity occurred between january 1960 and november 2003. important earthquakes dates during the last 42 years in the coffee belt region are in blue (monsalve and ospina 2004). in armenia the panorama of the last hundred years is quite clear. it can be observed that earthquakes with intensities equal to or greater than vii have a recurrence of about 20 years; there is a return period between 12 and 13 years for events with intensities equal to or greater than vi. it can also be observed that with recurrence of at least 5 years, earthquakes that may cause important damage have struck and will probably strike the region (monsalve y ospina 2004). figure 8. recurrence of historical events with i o (msk) > vi for armenia. modified by espinosa (1996). 97 seismological observatory at quindío university (o.s.q.): a firm long-term step toward resolving regional seismicity figure 9. earthquakes with seismic moment magnitudes 5.0< mw < 7.2 in the colombian northwestern corner (monsalve 1998). historically, there are several earthquakes occurring at deep and intermediate subduction sources (60220km deep). among them, we draw attention to the earthquakes that are remarkable because of their destructive potential. first, the one occurred on december 20th 1961 (mb=6.3, 176 km. deep), july 30th, 1.962 (mw=6.7, 108 km. deep), november 23rd, 1979 (mb=6.4, 108 km. deep), and february 8th, 1995 (mw=6.6, 100 km. deep); which have directly affected quindío and risaralda department (monsalve, ospina 2004). in figure 9, earthquakes with seismic moment magnitude 5.0< mw < 7.2 in northwestern colombian are shown (monsalve 1998). conclusions in earthquake-prone zones, such as the coffee belt region, it is of paramount importance to know and understand the response of structures to the high seismic activity and resulting hazard that this region faces. this can be achieved by specialized and detailed analyses which can lead to the improvement of earthquake hazard assessment studies in the region. all the studies carried out in the coffee belt region have provided people with knowledge and understanding geological and tectonic characteristics in the zone, thus making it possible to take a step further in the knowledge of crustal structure underlying this region. the answer to many seismological questions may be furnished by the installation of this local network. through the development of different projects that have been conducted since the osq started operations, new knowledge regarding the natural hazards that the coffee belt region is exposed to has been generated. these projects have allowed us to evaluate, and improve the social development conditions, and determine regional subsoil conditions to provide early control in terms of disaster prevention and the determination of potential areas for future urban development of engineering works. 98 hugo monsalve jaramillo, isabel cristina correa montaño and william valencia mina references • barragán, l.f. (2003). determinación de la magnitud local m l a partir de acelerogramas de movimiento fuerte. bachelor thesis, civil engineering. quindío university. • dewey, j.w. (1971), seismicity studies with the method of join hypocenter determination: ph. d thesis, university california berkeley. • espinosa, a., 1996. sismicidad histórica del área pereira-dos quebradas-santa rosa de cabal. informe final sismicidad histórica, seismic risk mitigation project pereirados quebradas and santa rosa de cabal. carder, pereira, 40 p., inédito. • espinosa, a., (2005). un modelo del cuaternario del quindío y risaralda. journal of the colombian academy of physical, natural, and exact sciences, bogotá. • espinosa-ospina, l. and monsalve-jaramillo, h. (2002). determinación de un modelo unidimensional de velocidades para el eje cafetero colombiano utilizando tomografía sismica. bachelor thesis, civil engineering. quindío university. • ingeominas (1999). terremoto del quindío (enero 25 de 1999). technical-scientific report. bogotá. • mc court, w.j., aspden, j.a., brook, m., (1984a). new geological and geochronological data from the colombian andes: continental growth by multiple accretion. j. geol. soc. london, 141 (5), 831 – 845. • monsalve, h., 1998. geometría de la subducción de la placa nazca en el noroeste de colombia: implicaciones tectónicas y sísmicas, master thesis, instituto de geofísica, universidad nacional autónoma de méxico, 106 p. • monsalve-jaramillo, h. and ospina-ostios, l.m. (2004). potencial sísmico de la zona de subducción en el quindio y risaralda (colombian coffee belt region). • monsalve-jaramillo, h. and vargas-jiménez, c.a. (2002). el sismo de armenia (25-01-99, mw=6.2): análisis telesísmico y características de las ondas de cuerpo. journal ipgh, no 57, 21-56. • monsalve, h., castaño, p., díaz, j., fernández, a., pérez, a., calero, l. and pinzón, g (2002). la reconstrucción del quindío lecturas desde la academia. quindío university. armenia – colombia. • parís, g., and marulanda, n., (1975). mapa geológico de la parte occidental de la plancha 387 – bolívar, scale 1:100.000. ingeominas – popayán, inédito. • trenkamp, r., mora, h. and salcedo, e. (2004). possible rapid intrablock strain accumulation rates near cali, colombia determined from gps measurements (19962003). first latin american congress of seismology. armenia (colombia). • vargas, c.a. and carvajal, c.a. (1998). estudio de refracción sísmica de armenia-quindio. journal red sismológica regional del eje cafetero viejo caldas y tolima. 3:16-23. issn:0123-9074. geociencias 15-1 julio 2011.vp communication regarding the geology and vertebrate palaeontology of the adamantina formation (bauru group, upper cretaceous) in campina verde, minas gerais state, brazil emerson ferreira de oliveira, caio césar rangel, diego sullivan de jesus alves, filipi silva limonta, adelino carvalho, carlos roberto a. candeiro laboratório de geologia da universidade federal de uberlândia campus pontal, ituiutaba-mg, brasil. e-mail author: oliveira_geoscience@yahoo.com.br abstract several geological events occurred on the surface of the earth with the division of the megacontinent gondwana, including volcanic activity involving extensive lava flows, thus locally and regionally altering the surface’s geology. lava flows in the serra geral formation predominantly consist of volcanic lava fissure-derived basalt. sedimentation upon extrusive igneous rocks occurred after this volcanic event, leading to the deposition of sediments from three formations (adamantina, uberaba, and marília) collectively comprising the upper cretaceous bauru group, beginning about 83 million years ago. this was a period when large animals inhabited the planet, including the study area; the adamantina formation is the largest and oldest. the city of campina verde is located there and is where most studies were carried out; the region is known as fossil field. resumen varios eventos geológicos ocurren sobre la superficie de la tierra con la división del megacontinente de gondwana, incluyendo actividades volcánicas con extensos flujos lava, tanto local como regionalmente, que alteraron la geología de superficie. los flujos de lava en la formación serra geral consisten en su mayoría de lava volcánica fisurada derivada del basalto. la sedimentación sobre las rocas ígneas extrusivas, ocurrió después de este evento volcánico, depositando los sedimentos de tres formaciones (adamantina, uberaba, y marília) que comprenden el grupo bauru del cretáceo superior, y que inicio hace 83 millones de años aproximadamente. en este periodo habitaban grandes animales en el planeta, incluyendo el área de estudio; la formación adamantina es la más grande y antigua. la ciudad de campina verde, localizada en esta zona, es donde la mayoría de los estudios se llevaron a cabo y es conocida como un campo de fósiles. introduction the bauru basin in southern brazil and northeastern paraguay is related to tectonic and volcanic activity as part of gondwana during the cretaceous period. the basin covers about 370,000 km² in southern and central brazil. fernandes and coimbra (1996) identified the caiuá group (goio êre, rio paraná and santo anastácio formations) and the bauru group (admantina, uberaba and marília formations). the bauru group’s three sedimentary formations were deposited by wind and water, beginning 83 million years ago. these formations crop out in the states of goiás, mato grosso, mato grosso do sul, minas gerais (mg), paraná and são paulo and north-eastern paraguay, covering an area of 370,000 km²; sedimentation occurred in a hot, semi-arid climate, the main event occurring during the upper cretaceous period (brito, 2001). the bauru group’s three formations (in ascending order) are the adamantina, the uberaba and the marília formations. the uberaba formation is only found in the area surrounding the city of uberaba, mg, while the other two are found in every occurrence of the bauru group. this report will emphasise the adamantina formation which was studied in the triângulo mineiro region, specifically near the city of campina verde, mg. the bauru group’s rich fossil locations from the upper cretaceous period are found in the west of mg in central brazil; this is the earliest geological period represented in this area. triângulo mineiro palaeontological knowledge has been developing at pontal do triângulo mineiro (in the campina verde and monte alegre de minas area) since late 1930 and more recently in carneirinho, gurinhatã, iturama and prata. earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 39-43research groupin geophysics universidad nacional de colombia keywords: bauru group, stratification, fossil, sediment, campina verde. palabras clave: grupo bauru, estratificación, fósil, sedimento, campina verde record manuscript received: 30/11/2010 accepted for publication: 25/05/2011 recent discoveries have filled in gaps in our knowledge, yet much work still remains to be done. von huene (1934) recovered fragmentary fossil sauropod dinosaur remains while conducting palaeontological reconnaissance in campina verde in 1910; barcelos (1984) recognised the deposits in which they were found as being adamantina formation (fernandes and coimbra). researchers from faculdade de ciências integradas do pontal palaeontological group (facip) visited the field area and collected a number of isolated invertebrate and vertebrate fossil specimens. only a single paper describing dinosaur remains came from these studies (candeiro, 2007). methodology the studies were conducted in campina verde during 2009 by a group of researchers from the universidade federal de uberlândia’s geology laboratory (labgeol) in pontal with the support of the municipal government. the methodology involved using several research publications on work in campina verde regarding the region’s geology and palaeontology, as well as direct observation of specimens housed in the labgeol collection. the arrangements proposed by fernandes and coimbra (1996) and dias brito et al., (2001) were followed for defining the bauru group’s stratigraphic units. four strategic fossil localities were chosen during the fieldwork based on information from local farmers to facilitate the labgeol paleontological group’s excavations and research; the first two localities (p1 and p 2, figure 1) are located where road 364 is being paved at a great rate of advance. the third locality (p 3, figure 1) is at the crossroads of road 364 with mgt 484, mgt, leading to gurinhatã, mg; 364 continues towards the states of goiás and mato grosso (it is also called the são paulo – cuiabá road). the fourth locality (p 4, figure 1) is at the margin of road 364 on a farm, where fossil remains of a crocodile were taken to the universidade de são paulo (usp) in ribeirão preto in early 2008. excavation materials, such as geologic hammers, tips, spatulas and brushes, were used at these locations. paper boxes and plastic bags were used for the fossil remains to preserve specimens; another field method involved using hydrochloric acid on the rocks to test whether calcite existed and whether there could have been a fossil in the rock in the case of reaction. a pen was also used to record the collections selected and define gps coordinates in the camp book. geological setting barcelos (1984), described the adamantina formation near the city of uberlândia in triângulo mineiro and goiás state. the first outcrops appear in the city of monte alegre de minas and continue in the direction of the city of caçu, goiás state, always beingon top of hills at around 50 to 100 m above the base formed by the sandstone in this formation. between monte alegre de minas and são simão (goiás state), passing through ituiutaba, barcelos (1984) described the adamantina formation sediments as variegated bimodal coarse grained and relatively well-rounded sandstone,with a sparse matrix of fine sandy silt and clay. there are also carbonate nodules and layers. the sandstone has a massive aspect but it is rich in sedimentary structures; these are represented by layers having sandy communication regarding the geology and vertebrate palaeontology of the adamantina formation (bauru group, upper cretaceous) in campina verde, minas gerais state, brazil 40 figure1: geographic map highlighting the locations of study. prepared by oliveira, e. f.; rangel, c. c. stratification across small, deformed and broken layers of sandstone and coarser sandstone carbonate pseudo-nodules. sediments dominate towards the extreme west of the triângulo mineiro, beyond the area around veríssimo which is in close contact with the uberaba formation, extending towards prata, monte alegre de minas, campina verde, iturama and santa vitória. the adamantina formation represents the bauru group’s greatest extension, having a catchment area greater than that of the other formations (uberaba and marília); the adamantina formation covers a large area of the triângulo mineiro. according to fernandes and coimbra (1996), the adamantina formation consists of a combination of very fine-grained sandstone and fine pink to brown silty mudstones, with brown intercalation; this is usual in massive stratification or plane parallel bedding, alternating with medium and small cross-bedding. the unit often displays other hydrodynamic structures such as ripple marks, cross-lamination thrusting, features cutting and filling gaps with intraformal conglomerates with argillaceous silt intraclast grading features at the top. the adamantina formation consists of thin and very thin sand and silty mudstone beds with cross stratification and clay lenses (interbeds). these beds range from massive to plane-parallel strata alternating with cross-stratification layers containing small to medium pebbles (fernandes and coimbra, 1996). very red sediments have been found in the adamantina formation in the campina verde region but there are also dark grey fluvial sediments. concerning sedimentary structures, the adamantina formation has horizontal stratification with fine sediment, coarse sandstone and conglomerate inter beds, with layers of clay and pebbles (figure 2). the adamantina formation’s rocks have not changed much since deposition, especially their reddish, sometimes brown tones, so it is easy to differentiate them from other kinds of rocks. extensive bioturbation in this formation demonstrates that this region had rich invertebrate fauna living in extensive communities. another factor making campina verde an important site for paleontological research is the frequent discovery of fossil remains, such as a recently discovered reptilian, a predecessor of a crocodyliform (candeiro et al., 2010). vertebrate palaeontology palaeontontological fieldwork carried out near campina verde in 2009 resulted in the discovery of isolated elements representing several invertebrate 41 emerson ferreira de oliveira, caio césar rangel, diego sullivan de jesus alves, filipi silva limonta, adelino carvalho, carlos roberto a. candeiro figure 2: geological map of the triângulo mineiro region, minas gerais state, brazil (modified from fernandes and coimbra, 1996) fossil and vertebrate taxa (figure 3). although generic level identification is impossible, due to the specimens’ fragmentary nature, the faunal list includes crocodyliforms (crocodylomorpha indet.), dinosaurs (sauropoda, theropoda indet., abelisauroidea), other vertebrates (vertebrata indet.) and possible molluscs (gastropoda indet.). the theropods are represented by abelisauroidea teeth which have labial and lingual faces which are more convex in cross-section near the anterior edge and flatter posteriorly; the lingual face is slightly concave proximodistally and the cross-sections are exactly as in the abelisauroidea. crocodilian eggs, previously unreported from campina verde, are represented by several elements. numerous isolated mesoeucrocodilian teeth and bones indicate the presence of mesoeucrocodylia indet. adamantina formation palaeontology in the triângulo mineiro region is rich in fossil materials and marks by cretaceous period animals. the adamantina formation has a remarkable faunal record of essentially vertebrate taxa; dinosaur remains are the most representative ones recorded to date (candeiro, , 2010). according to dias-brito et al., (2001) the adamantina formation’s fossil sediments are primarily represented by vertebrates (remains of crocodylomorphs, dinosaurs, turtles, frogs, fish, snakes), plants (charophyta) and molluscs (conchostraceans and ostracods). the adamantina and marília formations account for most fossil records related to the bauru group and they are where dinosaur fossil records are most extensive for this period in brazil. saurischian dinosaur records in the states of goiás, mato grosso, minas gerais and são paulo include sauropods (long-necked herbivorous dinosaurs) and theropods (carnivorous dinosaurs). this region, located in extreme west of mg, represents a promising area and certainly much more research can still be carried out in the campina verde region and, with the collaboration of the labgeol palaeontological group’s services contracted by the municipality, a new point of palaeontological or archaeological research can be created in the triângulo mineiro (i.e. another palaeontologyarea which has been studied since 1940 is already situated in the uberaba region, called peirópolis). concluding remarks the bauru group is composed of sedimentary rocks containing abundant fossils; these rocks differin their modes of origin there by allowing the group to be divided into three formations: the adamantina, marília and uberaba. each of these units has its own peculiarities and characteristic colour and all have been deposited by water or wind. the adamantina formation has reddish coloured sandstone so it is easy to differentiate these rocks from other units; however, colour cannot always be assigned as a method for differentiating formations, as red-coloured rocks can also be founding the marília formation. other classification is required, such as grain-size thickness. calcite was observed in the rocks in the sediments at the study sites and they had bioturbation marks on them. the fossil fragments found were difficult to identify because they were damaged; however this is a good indication that the campina verde area promises to produce important new paleontological localities. this evidence already shows on going activity in that particular environment there by increasing interest in new, more detailed studies to improve scientific results achieved to date. although there are reports regarding some fossils discovered in campina verde, studies of the region are still few; new and more detailed research is necessary to allow better descriptions of geological and paleontological aspects. acknowledgments we would like to thank prof. gerli carrotti (new york) for reviewing earlier drafts of the manuscript and its grammar. the manuscript has been substantially improved after taking the comments made by two anonymous referee into account. this work was part of an undergraduate research project supported by a fellow ship from conselho nacional de desenvolvimento científico e tecnológico (cnpq) awarded to e. oliveira and produtividade em pesquisa/cnpq to r. candeiro. references barboza, e. m., maizatto, j. r. and castro p. t. a. (1993).evidências de tectonismo ativo na sedimentação de rochas do grupo bauru, cretáceo da bacia do paraná, no triângulo mineiro, mg, acta geológica leopoldensia. 16, n°. 38, 97-112. barcelos, j. u. (1993). geologia regional e estratigrafia cretácica do triângulomineiro, revista sociedade e natureza. 5, 9-24. barcelos, j. u. (1984). reconstrução paleogeográfica da sedimentação do grupo bauru baseadanasuaredefiniçãoestratigráficaparcialemterritóriopaulista e no estudo preliminar fora do estado de são paulo, thesis of doctor, instituto de geociências e ciênciasexatas, universidade federal paulista, rio claro, brasil. brito, i. m. (2001).geologia histórica, uberlândia edufu, 413. candeiro, c. r. a. (2007). cretaceous biota of the triângulo mineiro region (brazil): a review of recent finds, estudos geológicos. 63, nº. 1, 65-73. candeiro, c. r. a.(2010) record of the late cretaceous south american genus aeolosaurus (sauropoda, titanosauria): paleogeographical implications. estudios geológicos (madri), 66 243-253. candeiro, c. r. a., abranches, c. t., abrantes, e. a., avilla, l. s., martins, v. c., moreira, a. l., torres, s. r. and bergqvist, l. p. (2004). dinosaurs remains from western são paulo state, brazil (bauru basin, adamantina formation, upper cretaceus), journal of south american earth sciences. 18, 01-10. communication regarding the geology and vertebrate palaeontology of the adamantina formation (bauru group, upper cretaceous) in campina verde, minas gerais state, brazil 42 figure 3: late cretaceous fossils of adamantina formation a – tooth abelisauroidea; b – vertebra of the crocodilomorpha; c, indeterminate bone of crocodilomorpha. candeiro, c. r. a., carvalho, a. a., oliveira, e. f., pereira, c. t., alves, d. s. j., rangel, c. c., alves, e. j. f., alves, m. c., limonta, f. s., ramalho, f. l. and almeida, l. h. c. (2010). new records of vertebrate fossils in the adamantina formation (bauru basin, upper cretaceous) at campina verde, state of minas gerais, brazil, 10th congreso argentino de paleontología y bioestratigrafía 7th congreso latinoamericano de paleontologia, la plata, argentina, septiembre, 142-143. candeiro, c. r. a., marinho, t. s. and oliveira, e. s. (2004). distribuição geográfica dos dinossauros da bacia bauru (cretáceo superior), revista sociedade e natureza.16, nº. 30, 33-55. dias-brito, d., musacchio, e. a., castro,j. c., maranhão, m. s. a. s., suárez, j. m. and rodrigues, r. (2001). grupo bauru: uma unidade continental do cretáceo no brasil – concepções baseadas em dados micropaleontológicos, isotrópicos e estratigráficos, revue paleobio. 20, nº. 1, 245 – 304. etchebehere, m. l c., saad, a. r. and resende, a. c. (1993). reavaliação do potencial do grupo bauru para evaporitos e salmoras continentais, geociências. 12, nº. 2, 333-352. fernandes, l. a. and coimbra, a. m. (1996). a bacia bauru (cretáceo superior brasil), anais da academia brasileira de ciências. 68, nº. 2, 195-205. fernandes, l. m. and coimbra, a. m. (1996). estratigrafia y ambientes deposicionales de la cuenca bauru (cretáceo superior, brasil), acta geológica hispânica. 30, nº. 4, 11-30. oliveira, l. a. and campos, j. e. g. (2003). seqüência conglomerática do membro araguari – grupo bauru – norte do triângulo mineiro, geociências. 22, nº. 1, 43-51. 43 emerson ferreira de oliveira, caio césar rangel, diego sullivan de jesus alves, filipi silva limonta, adelino carvalho, carlos roberto a. candeiro geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 134-139 crs seismic processing of a geological complex area eduardo jiménez1, carlos a. vargas2, 3 and luis a. montes3 1 geoseimic ltda. e-mail: eduardo.jimenez@cable.net.co 2 institute for geophysics, university of texas 3 departamento de geociencias, universidad nacional de colombia, bogotá. e-mail: lamontesv@unal.edu.co abstract we applied the nmo and crs (common reflector surface) approaches to a complex geological area in order to compare their performances for obtaining enhanced images. unlike nmo, crs does not depend on a previous time velocity model and uses a hyperbolic equation to estimate 2d travel times through three parameters (normal ray emergence angle, nip and n wavefront curvatures). to obtain the image a solution provided by coherence analysis algorithm was used. a low quality colombian seismic line acquired in middle magdalena basin was used, where a foothill geological area is characterized by a thrusting fault. the crs provided an enhanced image which allowed a new geological interpretation that is best constrained with other regional observations. key words: common reflection surface (crs) stack, zero-offset (zo), normal moveout correction (nmo), common mid point (cmp) resumen el propósito de esta investigación es comparar las técnicas de superficie común de reflexión (crs) y normal move out (nmo) en su desempeño en zonas geológicamente complejas, procurando imágenes sísmicas de mejor calidad. a diferencia del nmo, el crs no depende de un modelo previo de velocidad y usa una ecuación hiperbólica de tiempos de viaje 2d dependiente de tres parámetros (ángulo de emergencia del rayo normal, curvatura de la onda de punto de incidencia normal y curvatura de la onda normal). para obtener la imagen se usó una solución provista por un algoritmo de análisis de coherencia. se usó una línea sísmica de baja calidad adquirida en la cuenca del valle medio del magdalena colombia, en una zona de pie de monte caracterizada por una falla de cabalgamiento. el crs suministró una imagen mejorada que permitió una nueva interpretación geológica que se ajusta mejora con otras observaciones regionales. palabras clave: superficie común de reflexión crs, zero-offset (zo), corrección por sobre tiempo normal (nmo), punto medio (cmp) 134 manuscript received: 10/06/2009 accepted for publication: 26/10/2009 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:24 p p p composite 133 lpi at 45 degrees introduction the crs method has been considered an attractive stacking method to provide improved zero offset sections (trappe et al., 2001; hertweck et al., 2007). in colombian a known example with the crs applied to complex areas does not exist, although there is a reported paper with crs applied to a mild topography and quiet tectonically zone with relevant results (cárdenas and montes, 2006). in complex tectonically areas seismic tests are hampered by several factors that often lead to low quality seismic images with ravel reflectors, due to poor signal/noise ratio or weak seismic signal focusing. in order to compare nmo and crs performances in complex zones, a low quality seismic line of the middle magdalena basin was used. the area includes a thrust fault in the foothill zone. instead of stack velocities crs used parameters picked automatically by a coherence analysis algorithm (birgin et al., 1999). as result, an enhanced crs image allowed a new geological interpretation. crs method the crs stack is a theoretically well established method (jager et al., 2001; mann et al., 1999; tygel et al., 1997). it considers layers separated by curved reflectors whose reflection comes from a reflecting segment. a second order approximation of transmitted and reflected travelime rays in seismic system was developed (bortfeld, 1989). a multicovering seismic data set is acquired over a set of homogeneous and isotropic layers, with arbitrary velocities separated by smooth interfaces. the seismic system is defined by a zero offset ray, called central ray, which incidences normally on the reflector. in a second order approximation around central point (x0) the travel time of the srg ray is approximated by the hyperbolic function: t x h t x x t k x xn 2 0 0 0 2 0 2 0 2 2 ( , ) sin ( ) cos [ ( � � � � � � � � � � � � � � 0 2 2) ]� k hnip (1) according figure 1, x = (g + s)/2 is the common midpoint, h = (g – s)/2) is half offset, �0 is the near surface velocity and � and t0 are angle of emergence and travel time of the central ray. knip is the wavefront curvature of a hypothetical wave with the source located at point nip kn is the wavefront curvatures of an exploding reflector segment around point nip. a more deep and complete theoretical development about crs can be revised in others references (bortfeld, 1989; tygel et al., 1997) procedure the nmo stacked section was obtained by a current seismic processing sequence in promax®. due to crs does not require a previous known velocity model the velocity analysis and stacking steps were replaced by searching and optimizing local and global crs parameters through coherence analysis solution until obtain an optimized stacked section. this equivalent step was done using mpt® (a numerica´s software trade mark). after stacking, the nmo and crs images were filtered and enhanced applying the same promax sequences. a flow diagram of both sequences is shown in figure 2. search of crs parameters this non conventional procedure replaces the nmo analysis and stacking currently used in seismic processing sequences. the whole procedure is explained in the following sequence of steps with a visual description in the flow chart of figure 3, as was defined by muller (muller, 1999) the first step is to estimate vnmo from cmp gathers at point x=x0, this reduces equation 1 to: t h t h cmp nmo 2 0 2 2 2 4 ( )� � � (2) 135 crs seismic processing of a geological complex area figure 1. the central ray is perpendicular to the reflector ó and emerges at x0 with an � angle. the nip and n waves reach the surface at point x0. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:25 p p p composite 133 lpi at 45 degrees this step is an nmo velocity analysis and stacking which provides an initial simulated zo section (zero-offset). in the second step � is searched. for that, the reflectors in anterior zo section are considered locally flat, i.e. case of small apertures (kn =0), that simplifies equation 1: t x h t x x( , ) sin ( )� � � 0 2 0 0 0 � � (3) in the third step � is known and with the equation in zero offset configuration, kn value is estimated from zo section. finally the knip is calculated using the � parameter and the expression k t nip nmo � 2 0 0 2 � � �cos (4) geological setting geologically the area has been deformed by thrusting associated to compressive tectonic events. in the area a monocline dips to the east and the basement outcrops at west. the cretaceous rocks are below a gently dip eastward tertiary sequence, as seen in figure 4. different structural styles are observed: a first thrust with east vergence, a second one with west vergence and a lineation and ne-sw strike slip fault system. a low to high dip thrust fault emerges in surface with a high inclination. this fault causes the repetition of the cretaceous sequence which was observed in the well sited at west. a second fault inferred from seismic section, allows the elevation of basement over the footwall block below the fault. a tertiary detachment fault interpreted in the seismic section was observed in the cartographic recognition of the area. 136 g.c. onyedim, k.d. alagoa, i.o. adedokun, a.a. aderogba and c. ovuru figure 2. the crs sequence differs from nmo sequence in velocity analysis and stacking steps where instead, searching and optimization of crs parameters are used. figure 3. flow charts with step sequence to obtain final crs stack section. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:26 p p p composite 133 lpi at 45 degrees discussion and results an e-w acquired seismic line was selected to be processed by the two methods in order to compare their performances in producing the seismic images. at east side of the thrust fault a well allowed to identify the stratigraphic units in the seismic section. this seismic line was previously processed using a conventional flow of processing to obtain a pstm section, shown in figure 5. in that figure is observed two different behaviors at left and right of the thrust fault. a set of clear and continuous reflector with gentle dips can be seen from 2200 ms to 400 ms at right of the fault, whereas al right is almost impossible to identify a reflector. the seismic line was properly processed applying a conventional sequence flow with a careful selection of parameters, as result a new stacked section was obtained, providing also a more reliable nmo velocity model to be used as starting model in crs method. the stacked section is displayed in the figure 6, where is evident the enhancement of the image compared with that other in figure 5. after the crs processing a better image is showed the stacked section included in figure 7. in figure 6 the reflectors appear weaker, strong and less coherent can be seen with a better continuity, with low noise and besides that the image owns more seismic events in the shallow part. compared with nmo stack section the crs is a more enhanced and allow to constraints the monoclonal structure descripted in the geological setting section. to compare the quality of the images a s/n content evaluation was made. first the noise was isolated from the signal in the image through band pass filters and then respective spectral analysis´s were done. the results are depicted in figure 9, at top for nmo image and at bottom crs image. comparing the figures 9a and 9b the random noise level is reduced from 27.8% in nmo image to 17.9 % in crs image. it represents a significant improvement in signal to noise ratio and in consequence a rising in the seismic resolution. a post-stack crs migrated section was interpreted using the software geographix (landmark graphics co®). the change in the quality of the image permitted a new geological interpretation matching the seismic section with the 137 crs seismic processing of a geological complex area figure 4. geological section of the zone with several structural styles observed and a well at west, from an interpreted stacked section (internal report). figure 5. in the previous pstm section cdps 2100-2460, clear reflectors are observed from cdp 2380 to the right but blurred at left. figure 6. in the new nmo stack section cdps 2100-2460 the reflectors are observed along the whole section, including at left of cdp 2380. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:28 p p p composite 133 lpi at 45 degrees lithological units identified in the stratigraphic column and the borehole logs (figure 8). a schematic geological section was extracted from the interpretation and depicted in the figure 10. the difference among the geological interpretation gotten in figure 3 and the interpreted in figure 10 is evident. this new interpretation is due to the enhanced quality of the new obtained crs image. conclusions a low quality colombian seismic line acquired in middle magdalena basin was processed using the nmo and the crs techniques looking for an enhanced of the image on a thrusting fault. the crs provided an improved image which allowed a new geological interpretation that is best constrained with other regional observations. our observations suggest that the crs technique improves the imaging of complex areas and constitutes an alternative in seismic 138 g.c. onyedim, k.d. alagoa, i.o. adedokun, a.a. aderogba and c. ovuru figure 7. the crs stack section cdps 2100-2460 displays an enhanced image with better continued and more clear reflectors along the section. figure 8. nmo at top and crs at bottom images with respective spectral analysis show reduction in the level of random noise. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:28 p p p composite 133 lpi at 45 degrees processing. crs-2d method generated seismic sections of good quality without knowledge of subsurface velocity model, essential in expensive process like pre-stack migrations. performance of crs methods in thrusting zone, allowed a new interpretation and geologic model. futures works should emphasize the crs technology in 3-d data and use the information given by crs, attributes in tomography inversion, avo, migration and coherent cube analysis. acknowledgements the authors express grateful to landmark graphics co. latin america to provide promax and geographix software and numerica ltda to permit the use of mpt (multiparmetric processing tool) software implemented with crs. bibliography birgin, e., biloti, r., tygel, m. and santos, l. restricted optimization: a clue to a fast and accurate implementation of the common reflection surface stack method. journal of applied geophysics. vol, 42, issues 3-4. p 143-155. bortfeld, r., 1989, geometrical ray theory: rays and travel times in seismic systems (second-order approximations of the travel times: geophysics, vol 54 no. 3 p 342-349. cardenas, a., and montes, l. 2006, stacking on common reflection surface with multiparameter traveltime. vol. 10, no. 2, earth sci. res. j. p 91-103. hubral, p., 1983, computing true amplitude reflections in a laterally inhomogeneous earth: geophysics, 48 no. 8, p 1051 – 1062. hertweck, t., schleicher, j., mann, j. 2007. data stacking beyond cmp: the leading edge. v 26, no.7. p818-827. jager, r., mann, j., hocht, g., hubral p. 2001. common reflection surface stack: image and attributes. geophysics vol 66 no. 1, p 97 – 109. mann, j., jager, r., muller, t., hocht, g., and hubral, p., 1999, common-reflectionsurface-stack a real data example: journal of applied geophysics, 42, 301 – 318. neidel, n., and taner, m., 1971, semblance and other coherency measures for multichannel data: geophysics, 36, 482–497. muller, t., 1999, the common reflection surface stack surface stack method – seismic imaging without explicit knowledge of the velocity model: ph. d. thesis, university of karlsruhe. tygel, m., muller, t., hubral, p., and schleicher, j., 1997, eigenwave base multiparameter traveltime expansion: 67th annual internat. mtg. soc. expl, geophys 1770 – 1773. vieth, k., 2001, kinematic wavefield attributes in seismic imaging. ph.d. thesis, university at karlsruhe. trappe, h., gierse, g. and pruessmann, j., 2001. case studies show potential of common surface stack structural resolution in time domain beyond the conventional nmo/dmo stack. first break, 625633. special topic. nov. 139 crs seismic processing of a geological complex area figure 9. a new seismic interpretation resulted from the crs image. well 1 fault a fault b well 1 fault a fault b figure 10. a new schematic geological section provided by the interpretation of the crs image. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:29 p p p composite 133 lpi at 45 degrees geociencias-vol 14-1 2010.vp 7 earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 7-16 determination of effective elastic thickness of the colombian andes using satellite-derived gravity data remy a. galán1, iván f. casallas1 1universidad distrital francisco josé de caldas, facultad de ingeniería. kr 7 40-53. bogotá. colombia geophysicalthings@gmail.com abstract gravity anomaly values derived from global geopotential models (calculated from the champ and grace satellite missions), are compared with free air ground gravity data to find the best representation of surface data. using these values and topographical heights extracted from digital topography models, we applied the isostatic response function (admittance) to a collection of profiles, to find an average of elastic thickness for the colombian andes. key words: isostasy, elastic thickness, satellite, admittance, flexure, gravity, topography. resumen se extraen valores de anomalía de gravedad de aire libre derivadas de modelos geopotenciales globales, (calculados de las misiones satelitales champ y grace) los cuales son comparados con datos de gravedad terrestre para encontrar entre estos modelos la mejor representación de los datos de superficie. usando estos valores de anomalía y valores de alturas topográficas extraídos de un modelo de topografía digital, se aplica la función de respuesta isostática (admitancia) a un conjunto de perfiles, para hallar un promedio del espesor elástico de los andes colombianos. palabras clave: isostasia, espesor elástico, satélite, admitancia, flexura, gravedad, topografía. introduction this paper essentially makes use of gravity anomalies and topographical heights to obtain an average of the effective elastic thickness (te) for the colombian andes. the gravity anomalies are derived from data obtained from satellite missions, which are a recent technology that is revolutionizing the world of the geosciences. to get this purpose, geophysical and geodetics concepts are used. in this way, obtaining the value of elastic thickness becomes a technical task, under the principle of a methodological application (admittance analysis), but in the scientific world the debate about the selection of the method to find the te even continuous open; under this context, two schools of thought can be find: the school that accepts admittance analysis developed by manuscript received: 04/01/2010 accepted for publication: 18/03/2010 dorman and lewis (dorman and lewis, 1970) as the function that shows the better fit between topography and gravity functions and argues that coherence analysis function overestimates the values of te, and in the same way, the coherence’s school (forsyth, 1985) argues that admittance analysis does not take into account of subsurface loads in computes of te, and for this reason this latest subestimates te value. the authors with this work do not pretend to take sides in this debate, because this work seeks to be a further contribution to the knowledge of the colombian andes, as well as to contribute to the understanding of the new technologies such as satellital geophysics. isostatic model isostatic models can be classified into two categories: local and regional models. in the isostatic local model, compensation occurs directly beneath the load, which is supported by materials which have a behavior similar to liquids and do not have rigidity. in the isostatic compensation regional model, the load is supported by a material that presents a certain degree of rigidity and hence their behavior is similar to an elastic plate that bends to support the load. among the isostatic local models can found the hypothesis proposed by airy and pratt, whereas the most representative isostatic regional model is proposed by vening meinesz. the concept of lithospheric elastic behavior is developed inside the context of regional isostatic model, in this way the present work takes the theoretical frame of vening meinesz model. gravity data the analytical representation of the ground gravity field is one of the main aims of geodesy; this is work carried out through analysis of different measurements on the earth’s surface (values of gravity, topographical heights, etc.). this analysis leads to the formulation of the equation �v � 0 known also as boundary value problem which is treated in the branch of physical geodesy under the topic of potential theory. the representation of this phenomenon is more understandable when a reference figure that represents the earth is taken. geodesy takes several reference surfaces of representation of the earth’s shape, a physical shape known as geoid and a geometrical-mathematical shape known as ellipsoid; the geoid is the equipotential surface most similar to the sea level mean at rest and it is represented by w = w0. this potential w0 is called real gravity potential, and with the normal gravity potential u0 generated by ellipsoidal surface, make up the theoretical basis of mathematical boundary value problem. the difference between these two potentials is known as perturbation potential or just as potential difference t: t = w u (1) when atmospheric attraction disregards, t is harmonic outside earth and satisfies laplace’s equation: �t � 0 (2) these parameters allow obtaining several functions of the gravity field (gravity anomalies, geoid undulations, vertical deflections, etc.) through its relationship with the anomalous potential t, whose series development is according to (heiskanen and moritz 1967, torge 2001): t r gm r a n nm n n m n ( , ( )) (cos )� � �� � � � � � � � 2 0 (3) analysis of perturbations of artificial satellite orbits has made the largest contribution to global determination of the long-wavelength components of the earth’s gravitational field. the resulting global geopotential models (ggm), are usually provided as truncated series of spherical harmonics. due to several limiting factors these satellite-only ggms are of a limited spherical harmonics degree (typically 20-30), hence spatial resolution.(featherstone w. e., 2003). among the largest list of available global geopotential models (ggm), three models with several properties have been selected to achieve the purpose of obtaining a surface of gravity anomaly that represents the gravity function in admittance analysis. obtaining gravity surface for colombian andes the global geopotential models offer a uniform coverage of the study area, so it is possible to obtain the required term in the gravity field for a particular study. data from ground gravity do not have a uniform distribution, although the generation of maps of gravity anomaly is possible thanks to the applications of several interpolation methods. the selection of the final model that best represents the field of ground gravity, takes place through the analysis of the correlation parameters between the map of ground gravity anomaly and each of the different maps resulting from the models. to achieve the purpose of obtaining a gravity field representation for colombian andes that best fits with ground gravity data, it is necessary to obtain data from sampling 8 remy a. galán, iván f. casallas grids of free air gravity anomalies from global geopotential models (ggm), with a spacing resolution of approximately 0.5 degrees or 30 arcmin, and subsequently to obtain the anomaly maps through interpolation method. the available data of ground gravity anomaly are the third order gravity network of igac showed in the figure 1. the ggm derived gravity anomalies can be computed from spherical harmonics coefficients to degree nmax using: �g gm r n a r nmggm n m n n n � � � � � � �� ( ) (cos ) max 1 02 � (4) 9 determination of effective elastic thickness of the colombian andes using satellite-derived gravity data figure 1: available free air ground gravity data, source: división de geodesia, instituto geográfico agustíýn codazzi ( igac). the selection of the final model that best represents the earth gravity field is done by analyzing the parameters of correlation between the ground gravity map and each of the maps resulting from different models. table 1 shows the obtained correlations. according to data in the table 1, there is a high level of correlation existing between teg4 and ggm02 models, while the correlation between these models and eigen-cg03c model is the lowest. on the other hand, the same table shows the correlation between ggm models and ground gravity data maps and suggests that the average of correlation which only is up to 55 percent. this is the result from low and non uniform ground gravity data coverage over colombia’s continental territory (i.e. colombian amazonia). finally, this suggests that the model that best represent the terrestrial gravity field in colombian continental crust is eigen-cg03c (figure 2) with a correlation coefficient of 0.58921. effective elastic thickness determination on large time scales the earth’s lithosphere exhibit a regional behavior, thus it tends to experiment flexure, due to the loads. it can be assumed that the lithosphere presents the behavior of a filter which removes large amplitudes, i.e., short wavelengths associated with local isostasy models allow the pass of small amplitudes, or long wavelengths are associated with flexural models (watts, 2001). in this filter, load h1(x) produces a deflection y1(x) and load h2(x) produces a deflection y2(x) then load h1(x) + h2(x) produce a flexure y1(x) + y2(x). the filters that have this kind of behavior are called linear space invariant (lsi) and are characterized because when they are subjected to periodic loads its output is also periodic (watts, 2001). in the earth’s internal structure the part which supports the deflection is called effective elastic thickness (te). “this is defined as the thickness of the crust that behaves elastically and that supports some or all topographical load”. (burov and diament, 1995). it lies on a fluid asthenosphere; therefore the largest value of elastic thickness increments the capacity of the lithosphere to support topographical loads without having deflection. airy’s model represents a special case in which the value of te is null. to calculate the effective elastic thickness, there are several methods that are most based on spectral and spatial relationships between topography and gravity, which are obtained through the use of maps or profiles. the approach to be adopted in this work is to use a technique within the framework of spectral methods, using information from profiles. admittance analysis admittance analysis or also known as isostatic response function was developed by leroy m. dorman and brian t. r. lewis in the year 1970. the gravitational admittance “is the wave number parameter that modifies the topography so as to produce gravity anomaly”. (watts, 2001). this allows “expresses the dependence of the gravity anomaly on topography as a function of several physical parameters (plate thickness, plate rigidity, density distribution) and wavelength”. (billen, 2001). admittance function z(k) is defined as: z k g k h k ( ) ( ) ( ) � (5) where k is wavenumber, g(k) and h(k) are fourier transforms of gravity and topography respectively. cross spectrum c(k) is given by: 10 remy a. galán, iván f. casallas table 1: correlation matrix between ggm and ground gravity. correlation matrix ground ggm02c teg4 eigen-cg03c ground 1.00000 0.57244 0.58255 0.58921 ggm02c 0.57244 1.00000 0.96028 0.93157 teg4 0.58255 0.96028 1.00000 0.89357 eigen-cg03c 0.58921 0.93157 0.89357 1.0000 11 determination of effective elastic thickness of the colombian andes using satellite-derived gravity data figure 2: eigen-cg03c free air gravity anomaly map. eigen models (from 01 to 04) are derived from champ and grace observations and present two versions, accompanied by a s only has the satellite component (n = 120) and the accompanying by a c has two components (satellite and terrestrial), this last includes the same terrestrial data contained in the model egm96 and presents until n = 360; these models are maintained and updated by the gfz (geoforschungszentrum). c k n g k h kr r r n ( ) ( ) ( )� � � 1 1 (6) here * denotes conjugate complex and n is the total number of profiles employed. the power spectrum of topography e(k) is given as: e k n h k h kr r r n ( ) ( ) ( )� � � 1 1 (7) finally, observed admittance average zobs(k) is computed from: z k c k e k obs ( ) ( ) ( ) � (8) the observed admittance curve is compared with a set of theoretical admittance curves for several values of effective elastic thickness. the final value of te is obtained through the selection of the lowest mean square error between the observed admittance curve and each one of the theoretical curves. theoretical admittance is defined as: z k g kt a theo c c( ) exp( ) � � �� � � �2 1 � � (9) with: a dk g m c � � � 1 4 ( )� � (10) and d (flexural rigidity): d ete� � 3 212 1( )� (11) where: k � 2� � wavenumber g n m kg � � �6 67 10 11 2 2 . universal gravity constant e pa� 1011 young modulus � � 0 25. poisson rate g gr cm � 9 8 3 . gravity acceleration �c gr cm �2 8 3 . average crustal density �m gr cm � 3 3 3 . average mantle density tc = 33 km contrast density layer depth (average crustal thickness) te effective elastic thickness the mean square error is calculated using: rms pi ri ni n � � � ( )2 1 (12) here: p projected value. r observed value. n sample size to obtain the observed admittance curve, data were used from the free air gravity anomaly map produced from the model eigen-cg03c (figure 2) and the digital terrain model constructed with data from srtm (shuttle radar topography mission) show in figure 3. a set of seven (7) profiles was drawn, both in the map of free air anomaly, as in the digital terrain model, crossing the andes on a perpendicular path and covers it homogeneously, figure 3. profiles were sampled each 1800 m taking the ratio of dtm spatial resolution and free air gravity anomaly map resolution. to employ the fourier analysis this profiles should complete a number of 2n data, where n = 8 because the total distance of the profiles have a range between 400 and 700 km approximately. theoretical admittance curves were computed for te values of 10, 15, 20, 25 and 30 km, in the same way observed admittance curve and its square medium errors were computed too. the results are showed in figure 4 and table 2. 12 remy a. galán, iván f. casallas table 2: admittance mean square errors. mean square errors te=10 km te=15 km te=20 km te=25 km te=30 km teobs 0.0002084 0.0000705 0.0000505 0.0001410 0.0002977 13 determination of effective elastic thickness of the colombian andes using satellite-derived gravity data figure 3: dtm and profiles used in admittance analysis. the profiles were used to extract both gravity and topographical data from maps. in conclusion according to table 2, the average of effective elastic thickness for colombian andes is 20 km. taking into account this value, theoretical admittance curve and observed admittance values with its standard deviations are represented in figure 5. discussion effective elastic thickness computes studies made in andes mountain belt inside colombia, have employed several techniques and also have obtained several values. some of this works were made for south american continental surface and some others specifically in colombian andes employing different methods. kellogg et al(1995) used flexural models 2d employing a fractured plate, obtaining a value of effective elastic thickness between 20 and 25 km below volcanic arc and 60 km for mountain belt placed at north of volcanic arc. quintero (1998) used admittance method with profiles employing bouguer anomaly and topographical heights, in the same 14 remy a. galán, iván f. casallas figure 4: theoretical admittance curves for te = 10, 15, 20, 25 and 30 km, and observed admittance curve. all the curves were calculated from topographical heights and free air gravity anomalies. figure 5: theoretical admittance curve (dashed curve) for te = 20 km and observed admittance values (solid circle) with its respected standard deviations (vertical bars). way used flexural modeling using a continuous plate; in west and central mountain belt found an elastic thickness value of 10 km, while eastern mountain belt does not have elastic support, this means that it has a null value of elastic thickness. londoño (2004) also employed flexural modeling with continuous plate finding for putumayo basin, an elastic thickness value between 20 and 40 km. recently cerón et al (2007) found for plato basin an elastic thickness value of 27 km, this value was computed from flexural analysis with continuous plate. finally, stewart and watts (1997) through continuous plate modeling obtained a value of te for colombian andes of 45 km with 40 km bias. in south america have been conducted several studies, with the main aim of obtaining elastic thickness variations maps, although there are works that provide specific values. ojeda (2000) made his research for andes north region; this region includes colombian east cordillera and sierra del mérida in venezuela. to find the value of elastic thickness was used coherence technique employing hanning and multitaper spectral estimators, obtaining an average value of 30 km. montavani et al (2001) get a spatial variations map using empirical correlations between tidal gravity anomalies and elastic thickness values, finding for colombian andes a value between 69 and 79 km. into recent studies and that have employed satellite-derived data too, there are two works tassara et al (2007) and perez-gusinye et al (2007). tassara et al (2007) has used topographical heights and eigen-cg03c model data to derive bouguer anomalies, which were applied coherence method using continuous wavelets transform’s morlet family, getting an elastic thickness structure map for all south america, in colombian andes found a 40 km value with 15 km bias. perez-gusinye et al (2007) has used topographical heights and eigen-cg03c model data and also ground gravity data, which were applied coherence technique employing multitaper spectral estimators, obtaining the elastic thickness value between 25 and 30 km. in the field of studies that employ spectral methods to find elastic thickness value, all of these make use of coherence technique with bouguer anomalies and several estimators like hanning, multitaper o more recently like wavelet morlet family. starting with this approach, the obtained value of 20 km, in comparison with ojeda (2000) and perez-gusinye (2007) works, is low; although with tassara’s research, there is more agreement, because if the lowest limit value of this study is taken, it will be 25 km of te. finally making a comparison with montavani et al (2001) study, it is noticeable that the value that it gets is markedly high, but is important to highlight that perez-gusinye et al (2007) argue that this values cannot be compared with studies that make use of gravity anomalies and topographical height, because while the tidal loads have a short duration, the elastic thickness responses to a deformation process that only can be measured in geological time. the average value of elastic thickness for the colombian andes obtained in this work, it means, 20 km comparing it with other studies, suggests that this one is similar to the values obtained for the putumayo basin and the volcanic arc. now, comparing this work with quintero’s (1998) study, a value of 20 km is equivalent to twice the obtained value for this author, but, the 10 km value is the lowest inside the totally of known studies. if the same confrontation is made with the obtained value from plato basin study, 20 km value is low too, in the same way that occurs with steward and watts (1997) in which this value is only a half. conclusions the te value obtained in this work is relatively low with the most studies, like it could be expected. the main cause is the kind of method that was used, because other techniques like coherence suggest that admittance gets an underestimation or lower limit of the elastic thickness true value, argument that can be debated, because researchers that have employed admittance argue that processes as erosion and sedimentation tends to thin the topographical surface, and in this way the response of crust is reduced in presence of subsurface loads. although 20 km value is low in relation to other works, it is totally congruent with the results that have to be expected in admittance studies, that make use of free air anomalies, because in studies like mckenzie and fairhead (1997), it has been found that for continental lithosphere the elastic thickness value, with some exceptions, do not exceed 25 km, since, it is usually close to the seismogenic thickness value, this argument is still in discussion. acknowledgments we thank laura sánchez (dgf münchen), miguel ávila (universidad distrital) and very especially to andrés tassara (universidad de concepción) for their suggestions, collaboration, and encouragement in the elaboration of this project. 15 determination of effective elastic thickness of the colombian andes using satellite-derived gravity data references billen, m. i. (2001). i. seafloor morphology of the osbourn trough and karmadec trench and ii. multiscale dynamics subductions zones, ph.d. thesis, california institute of technology, pasadena, u.s.a.. burov, e. b., and diament, m. (1995). the effective elastic thickness (te) of the continental lithosphere: what does it really mean?, journal of geophysical research. 100, no. b3, 3895 3904. cerón j. f., kellogg, j. n. and ojeda, g. y. (2007). basement configuration of the northwestern south american-caribbean margin from recent geophysical data, ct & f -ciencia, tecnología y futuro, 3, no. 3, 25 49. dorman, l. t. and lewis, b. t. r. (1970). experimental isostasy: 1. theory of the determination of the earth’s isostatic response to a concentrated load, journal of geophysical research, 75, no. 17, 3357 3365. featherstone, w. e. (2003). improvement to long-wavelength australian gravity anomalies expected from champ, grace and goce dedicated satellite gravimetry missions, exploration geophysics, no. 34, 69 79. forsyth, d. (1985). subsurface loading and estimates of the flexural rigidities of continental lithosphere, journal of geophysical research, 90, no. b14, 12,623 12,632. heiskanen, w. a., and moritz, h. (1967). physical geodesy, w. h. freeman and company, san francisco, 392 pp. kellogg j., salvador, m. and ojeda, g. (1995). lithospheric structure of the colombian andes: flexure and crustal shortening, eos transactions, 76, 374. londoño, j. (2004). foreland basins: lithospheric flexure, plate strength and regional stratigraphy, ph.d. thesis, louisiana state university, baton rouge, u.s.a. mantovani, m. s. m., de freitas, s. r. c. and shukowsky, w. (2001). tidal gravity anomalies as a tool to measure rheological properties of the continental lithosphere: application to the south american plate, journal of south american earth sciences, 14, 1 14. mckenzie, d. and fairhead, d. (1997). estimates of the effective elastic thickness of the continental lithosphere from bouguer and free air gravity anomalies, journal of geophysical research, 102, no. b12, 27,523 27,552. ojeda, g. y. (2000). analysis of flexural isostasy of the northern andes, ph.d. thesis, florida international university, miami, u.s.a.. pérez, gusinyé m., lowry, a. r. and watts, a. b. (2007). effective elastic thickness of south america and its implications for intracontinental deformation, geochemistry geophysics geosystems, 8, no. 5, 1 22. quintero, camacho w. (1998). interpretación de la estructura cortical de los andes colombianos a partir de las redes magnética y gravimétrica del igac, tesis de maestría, universidad nacional autónoma de méxico, méxico d.f., méxico. smith, w. h. f., and wessel, p. (1990). gridding with continuous curvature splines in tension, geophysics, 55(3), 293 305. stewart, j. and watts, a. b. (1997). gravity anomalies and spatial variations of flexural rigidity at mountain ranges, journal of geophysical research, 102, no. b3, 5327 5352. tassara, a., swain, c., hackney, r. and kirby, j. (2007). elastic thickness structure of south america estimated using wavelets and satellite-derived gravity data, earth and planetary science letters, 253, 17 36. torge, w. (2001). geodesy, third edition, walter de gruyter, berlin, 416 pp. watts, a. b. (2001). isostasy and flexure of the lithosphere, first edition, cambridge university press, cambridge, 458 pp. wessel, p. and smith, w. h. f. (1998). new improve version of generic mapping tools released, eos trans agu, 79(47),579. 16 remy a. galán, iván f. casallas modelamiento geoelectrico y gravimetrico de un sector del suroeste de la sabana de bogota luis alberto briceno guarupe profesor asociado, departamento de geociencias-facultad de ciencias-universidad nacional de colombia irene arango haupt ge610ga, carrera de geologia-facultad de ciencias-universidad nacional de colombia briceno. l. a. & i. arango (2000): modelamiento qeoelectrico y qravirnetrico de un sector del suroeste de la sabana de bogota. geofis. colomb. 4:43-60. issn 0121-2974. santa fe de bogota, d.c. colombia resumen se realiz6 un modelamiento geol6gico del subsuelo en una zona ubicada en inmediaciones del municipio de soacha, entre las coordenadas n 997.000 a 1'000.000 y e 985.000 a 988.000. se determin6, con base en mediciones qravirnetricas y qeoelectricas. controladas con la observaci6n geol6gica de afloramientos y el registro de perforaci6n del pozo terreros ii, que la cuenca se profundiza en sentido se-nw, siendo el grupo guadalupe la base de la sucesi6n litol6gica considerada. ei grupo guadalupe se identifica por presentar resistividades entre 300 y 380 ohm-m y se profundiza desde 60 m al se hasta 240 m al nw. sobre esta unidad se interpretan, siguiendo la misma tendencia de profundizaci6n, las arcillolitas de la fm. guaduas (espesor promedio de 60 m), una capa delgada de areniscas terciarias (15 m de espesor promedio) y rocas terciarias y cuaternarias no consolidadas sin diferenciar. ei mapa de anomalia bouguer simple muestra variaciones entre -142 y -149 mgal, con una tendencia regional de disminuci6n en direcci6n se a nw, coincidiendo en direcci6n y magnitud con valores previamente calculados para esta zona. a partir del mapa de anomalia residual (con un rango de valores de 12.4 a 15.5 mgal), que se utiliz6 para el modelamiento, se determin6 la presencia de un minimo qravimetrico que coincide con la posici6n y sentido de la profundizaci6n de la cuenca y limita al occidente con una zona de fuerte gradiente de anornalia. 10 que permite proponer la continuidad en el subsuelo de la falla de sucre, con una direcci6n aproximada n45°w, que concuerda con 10 esperado a partir de la revision geol6gica preliminar. abstract a subsurface geological modeling was carried out near the soacha area. based on both gravimetric and geoelectric measurements, the examination of surface geological data and the terreros ii well log, it is proposed that the basin deepens from southeast toward northwest, being the guadalupe group the bottom of the lithologic studied sequence. high resistivity values ranging from 300 to 380 ohm-m characterizes the guadalupe group, which is found at 60 m in the southeast and deepens to 240 m in the northwest of the area. above this rock unit, following the same deeping tendence, should appear the guaduas formation (about 60 m thick), a thin belt of tertiary sandstones (about 15 m thick) and non-diferentiated tertiary-quaternary unconsolidated sediments. simple bouguer gravity anomaly map shows variations between -142 to -149 mgal, with a regional decreasing from se to nw that coincides in amplitude and direction with previous values for this zone. the residual anomaly map (ranging from 12.4 to 15.5 mgals), that was used for modeling, shows a relative gravity minimum which matches the trend of deeping and position of the basin and it borders a strong gradient anomaly, suggesting the continuity of the sucre fault in the subsurface, with a n45°w direction. 1. introduccion ei conocimiento del subsuelo de la sabana de bogota es importante dentro del panorama geol6gico del pais por la implicaci6n social y econ6mica, aparte del aspecto tecnico y cientifico. dicho conocimiento contribuye, por una parte a la microzonificaci6n sismica, adernas es la base del planearniento de obras civiles y ayuda, finalmente, a la definici6n de acuiferos subterraneos. por ser un terreno elativamente plano, la adquisici6n de informaci6n geol6gica esta muy restringida y se requiere de la perforaci6n de pozos 0 de la implementaci6n de herramientas geofisicas para obtener un modelamiento real del subsuelo. con el prop6sito de complementar el conocimiento actual de la geologia de la sabana de bogota, en el presente trabajo se escogi6 un sector al sur-occidente de santa fe de bogota, en inmediaciones del municipio de soacha (fig.1), con el fin de realizar en la zona mediciones gravimetricas y geoelectricas, tomando como control en la interpretaci6n de los datos adquiridos, el registro de perforaci6n de un pozo para aguas, ubicado dentro del area y la geologia conocida de los alrededores. por medio de la geofisica se intent6 determinar la continuidad en el subsuelo de las estructuras y unidades aflorantes y, asi mismo, establecer la geometria de las unidades que conforman la cuenca. ei modele geol6gico presentado servira en el futuro como punta de amarre en estudios de zonas cercanas, en donde no se presenten afloramientos y la informaci6n geofisica 0 de pozo sea escasa 0 nula. 2. marco geologico general la sabana de bogota es una altiplanicie situada en la cordillera oriental de colombia entre los 4°30' y los 5 °30' norte y los 74° y 74°30' oeste, es una de las mayores cuencas de su tipo, casi completamente cerrada. se encuentra rodeada por cerros, a una altura aproximada de 2550-2650 m sobre el nivel del mar y abarca un area de 1400 km2 . ei relleno de la sabana de bogota esta constituido por sedimentos terciarios y cuaternarios depositados sobre rocas cretacicas que tambien conforman los cerros adyacentes. la geologia general del area de estudio, que se presenta, descrita a continuaci6n, es producto de una compilaci6n biblioqrafica y posterior revisi6n de la misma en campo, apoyada por un analisis fotogeol6gico. la geologia general se sintetiza en el mapa geol6gico anexo (fig.2). la relaci6n estratiqrafica de las unidades descritas se encuentra detallada en la columna presentada por arango (1998). 2.1 estratigrafia a continuaci6n se presenta una breve descripci6n de las unidades aflorantes en el area de estudio, asl como de las que se espera encontrar en el subsuelo. son formaciones geol6gicas pertenecientes a los periodos cretacico, terciario y cuaternario. cret3cjco: formaci6n chipaque (kvc)-cenomaniano-coniaciano-: en la zona se encuentra conformando el nucleo del anticlinal de cheba. forma parte del grupo villeta, suprayace la fm. une e infrayace la fm. arenisca dura. su contacto superior en el area de estudio se presenta de manera concordante y transicional. litol6gicamente esta conformada por tres segmentos (renzoni. 1962): un nivel inferior constituido par arenitas de grano muy fino con delgadas intercalaciones de arcillolitas; un segmento intermedio conformado por calizas arenaceas fosiliferas con intercalaciones de arcillolitas, y un segmento superior constituido por arcillolitas negras intercaladas con delgadas capas de calizas y areniscas. en el area de estudio aflora el segmento superior, caracterizado por presentar lodolitas de colores gris oscuro a negro, carbonosas, fisiles, con laminaci6n fina paralela en capas y bancos de 10 cm hasta 1 m hacia el tope y con un aumento de espesor, hacia la base, de 1.5 hasta 2 m. presenta intercalaciones de 20 cm hasta 1.5 m de areniscas de grana fino, amarillentas a cafes, con matriz arcillosa y presencia de f6siles (escamas de peces). no se presentan intercalaciones calcareas en el sector. ei espesor de la formaci6n medido en el cerro de cheba fue de 69 m (angel et al., 1988). grupo guadalupe: fm. arenisca dura (kgd) -coniaciano-: es la base del grupo. en la zona de estudio se encuentra aflorando, en ambos f1ancos del anticlinal de cheba, suprayaciendo transicionalmente a la fm. chipaque. se encuentra fallada en ambos f1ancos del anticlinal, al oriente por la falla de sucre, que delimita el contacto con la fm. arenisca de labor y al occidente con una falla que la pone en contacto con la misma formaci6n. n a ~{ j .~ . ~ ~ . . ~~~ figura 1. mapa de localizacion del area de estudio litol6gicamente esta constituida por arenitas finas (algunas veces alcanzan a ser finas a medias), blancas a gris amarillentas (por alteraci6n), bien seleccionadas, duras y macizas, ocasionalmente rnicaceas (1-2%), en capas gruesas (de 1.5 a 10m de espesor). presenta intercalaciones delgadas de arcillolitas lodosas grises, lodolitas arenosas y liditas; capas de 10 a 40 cm que hacia la parte mas superior de la unidad pueden alcanzar espesores mayores, de hasta 5 m. las capas de areniscas predominan en toda la sucesi6n expuesta en la zona. presenta una estratificaci6n principalmente planoparalela, aunque entre algunos bancos se observa laminaci6n fina ondulosa (no es cornun). esta fuertemente compactada por cementa siliceo principalmente (vergara & rodriguez, 1996). ei espesor de la unidad, medido al occidente del eje del anticlinal de cheba, alcanza los 258 m (angel et.al., op.cit.) fm. plaeners (kgp) -santoniano-: no aflora en la zona estudiada. suprayace a la fm. arenisca dura e infrayace a la fm. arenisca de labor (normalmente sus contactos inferior y superior son concordantes y transicionales). litol6gicamente, en los afloramientos cercanos a la zona de estudio, esta constituido por una sucesi6n de arcillolitas y limolitas siliceas con fractura conc6idea y partici6n en romboedros, muy compactas, de colores c1aros intercaladas con bancos de arcillolitas y lodolitas arenosas gris a gris verdoso, fisiles, blandas, conformando bancos de 1.5 hasta 10m de espesor. tarnbien se presentan capas de arenisca de grana muy fino en bancos de estratificaci6n gruesa (hasta 3 m) separadas por finas capas de arcillollitas de color blanco amarillento, muy porosas (por disoluci6n de f6siles). en la quebrada ei molino, al sur del area de estudio tiene un espesor de 63 m. (hernandez & rincon, 1990). eseala 1:20.000 convencionf5 fkch i formacion chipaque ~ formacion arenisca dura ~ formaci6n arenisca de labor i k-tg i formaci6n guaduas cuatemario aluvialjcoluvial curvas de nivel datos estructurales trazo de falla observado trazo de falla cubierto trazo de falla inferido anticlinal invertido represa terreros areas construidas figura 2. mapa geologico fm. arenisca de labor (kgl)-campaniano: en la zona de estudio se encuentra en el f1anco occidental del anticlinal de cheba, mediante contacto fallado con la formaci6n arenisca dura (en posici6n inversa) y en los alrededores de las antiguas canteras terreros, frente a la represa terreros, en el f1anco oriental del anticlinal de cheba, buzando entre 10 y 40 grados hacia el ne y cubierta en su gran mayoria por un espeso coluvi6n. se caracteriza por presentar bancos gruesos de liditas, muy similares a las presentes en la fm. plaeners, intercalados con bancos tarnbien gruesos de areniscas. de base a techo la unidad esta compuesta por areniscas grises blancuzcas a blanco amarillento, de grana fino hasta muy fino, intercaladas con capas de arcillolitas blancas qrisaceas, mas frecuentes y espesas hacia la base de la unidad, conforman bancos de entre 15 cm y 1.5 m. en la base de la unidad se observa fractura en romboedros. en la parte intermedia, la unidad se compone de intercalaciones de areniscas arcillosas blancas y amarillentas, de tamario variable (de muy fino a grueso, tamario medio predominante), porosas por disoluci6n de f6siles y ocasionalmente micaceas, y lodolitas siliceas gris verdosas, micaceas, laminadas. en la parte mas superior de la unidad se presentan capas de areniscas arcillosas blancas amarillentas a gris claras, con tamario de grana variable, desde grueso hasta grava fina, formando capas gruesas que muy comunrnente se acurian 0 toman forma lenticular; las areniscas se intercalan con capas delgadas de arcillolitas grises. hernandez & rincon, op.cit., ie asignaron un espesor de 50 m, medido en la quebrada ei molino. ai sur de las antiguas canteras terreros la unidad alcanza 150 a 200 m de espesor (julivert, 1963). fm. arenisca tierna (kgt) -maastrichtiano inferior-: no aflora en la zona de estudio, perc si en cercanias de la misma, al oriente de la represa terreros (entre la falla terreros y una falla satelite), en el valle del rio soacha (al occidente de la zona) y al sur, en las canteras de la vereda pantoja, donde no se observa el tope. litoloqicarnente esta constituida por arenitas con tarnario de grana variable, con predominio de grana grueso a muy grueso hasta ligeramente conqlomeratico, en potentes bancos separados por finos paquetes de arcillolitas y limolitas oscuras interestratificadas. hacia la base se encuentran bancos gruesos de areniscas friables, arcillosas (de 5 a 10% de matriz), color cafe amarillento a blanco, de grana grueso a muy grueso, con mezcla ocasional de grava fina y esporadicarnente grano fino a muy fino, los cuales se intercalan con capas mas delgadas de limolitas y arcillolitas que hacia el techo forman laminas gruesas. terciario: fm. guaduas (tg) -maastrichtiano superior-paleoceno: suprayace a la fm. arenisca tierna e infrayace a la fm. arenisca del cacho. puede dividirse informalmente en tres segmentos (hubach , 1957): conjunto inferior: constituido por arcillolitas gris oscuras con algunos fosiles en la base (amonitas y bivalvos aplastados), que permiten asignar a la base de la forrnacion una edad maastrichtiano (dietrich, ct. hubach, op.cit.). conjunto medio: conformado por dos niveles de areniscas de grana fino a medio y a veces de grueso a conqlorneratico (arenisca la guia de aproximadamente 30 rn, ct. aalto, (1971), hacia la base y arenisca lajosa en la parte superior). entre ambos niveles arenosos se encuentran capas de arcillolitas gris oscuras que contienen mantos de carb6n explotables (este segmento constituye la parte productiva de la formaci6n). en los horizontes arcillosos abundan los restos de plantas. angel et al., op.cit., proponen, al sur, un espesor de 88 m para este segmento intermedio arenoso. conjunto superior: presenta capas de arcillolitas varicoloreadas (rojizas, azulosas, verdosas y moradas) entre las que se encuentran mantos de carb6n no explotables y bancos de areniscas de grana medio grueso a grueso. tiene un espesor promedio que oscila entre 700 a 1000 m. su ambiente de dep6sito se interpreta como marino transicional (hubach, op.cit.). en el area de estudio aflora la arenisca la guia y las arcillolitas suprayacentes (parte inferior y media del 47 conjunto intermedio), en posici6n normal. la arenisca la guia aflora en el costado oriental del area, buzando alrededor de 25° hacia el nw y las arcillolitas suprayacenles siguen la misma direcci6n y aparecen en contacto fallado con la fm. arenisca de labor, por acci6n de la falla de terreros. fm. arenisca del cacho (tc)-paleoceno-: hubach, op.cit., dividi6 la unidad en tres coniuntos, dos arenosos (infracacho y supracacho) y uno lutitico, intermedio entre los dos. los conjuntos arenosos estan conformados por areniscas de grana medio a grueso hasta conqlorneraticas, mal seleccionadas. hacia el sur de la sabana aparece su afloramiento tipico en la quebrada de san cristobal (hubach, op.cit.), donde alcanza un espesor de 100 metros. fm. bogota (tb) -paleoceno-eoceno inferior: esta constituida por arcillolitas y limolitas abigarradas (moradas rojizas a grises) intercaladas con capas de 2 a 20 m de espesor de areniscas amarillas a rojizas, lodosas (10 a 20%), de grana muy fino a medio. su contacto con la suprayacente fm. regadera es discordante. sequn julivert, op.cit, tiene un espesor de 2000 m. fm. la regadera (tr) -oligoceno-: nombrada por hubach, op.cit., como un miembro de la arenisca del cacho. julivert, op.cit., la eleva al range de forrnacion. constituida por areniscas cuarzosas de grana fino a muy grueso, lodosas (10 a 20%), generalmente friables, con selecci6n regular a mala, cemenladas por 6xido de hierro, color blanco amarillento. presenta frecuentes intercalaciones de laminas arcillolilas grises a violetas-rojizas y conglomerados finos. tiene un espesor aproximado de 320 m. fm. usme (tu) -mioceno-: establecida por hubach, op.cit.. en 1963 julivert la dividio en dos: parte basal, lutitica predominantemente, con intercalaciones arenosas finas y un espesor de 50 m, y parte superior, constituida por intercalaciones de areniscas gruesas y conglomerados de grana fino, con un espesor de 75 m. presenta un contacto concordante con la fm. regadera en el f1anco occidental del sinclinal de usme y un contacto discordante con la misma en el f1anco oriental del sinclinal. cuaternario: fm. tilata (tt) -plioceno-pleistoceno-: establecida por scheibe, 1934. producto del dep6sito de sedimentos lacustres, paludales y marginales distribuidos en la sabana de bogota (hubach, op.cit.). 48 esta compuesta por dep6sitos sin consolidar de arenas de grana fino a grueso hasta conglomerados, limos arcillosos y arcillas rojizas y grises, cenizas, restos de plantas y capas caoliniticas. es muy vulnerable a la acci6n de agentes mete6ricos (presenta gran cantidad de surcos y carcavas). en algunos sectores se presentan restos de mastodontes. en la zona de estudio se observa su presencia en los alrededores de la represa de terreros, don de se encuentran bloques gruesos de areniscas tipicas de la parte inferior del grupo guadalupe, embebidos en una matriz arcillo-terrosa y cubiertos por un paleosuelo negro de 70 cm a 1 m de espesor, que posiblemente contiene un nivel de cenizas volcanicas (comunicaci6n directa del ge610go mauricio diaz s.). fm. sabana -pleistoceno-: constituida por sedimentos producto del relleno lacustre de la sabana de bogota, intercalados con sedimentos menores aluviales y de pantano: arcillolitas, arenas y gravas, producto del dep6sito lagunar: lodolitas y turbas, dep6sitos de pantano, y arcillas de inundaci6n, arenas y cascajos, correspondientes a los depositos fluviales (hubach, op.cit.). en su parte inferior dominan los sedimentos arenosos y en la parte superior los arcillosos. su espesor supera los 400 m (duenas, 1978) dep6sitos coluviales, (qci) : coluviones de matriz arenosa, con bloques angulares de arenita de diferente tarnano y limos y arcillas del valle aluvial de la antigua quebrada terreros., dep6sitos aluviales (qal) gravas, cantos y bloques redondeados en matriz arenosa y arcillosa estan cubriendo parte de la zona plana del area de estudio. 2.2 geologia estructural ei sector sur-occidental de la sabana de bogota, donde se encuentra ubicada la zona de estudio, tiene una alta complejidad tect6nica, caracterizada por la presencia de fallas inversas con direcci6n nw, cubiertas por dep6sitos cuaternarios los pliegues son estrechos y sin orientaci6n preferencial. las zonas mas fracturadas, deterrninadas por conteo de diaclasas en una misma direcci6n, sequn angel et.al. (1988), son las zonas que se extienden a 10 largo de fallas mayores, como es el caso de las dos fallas de importancia presentes en el sector, y la zona ubicada al norte de la represa de terreros, don de la fm. guaduas se encuentra afectada de manera muy con stante por diaclasas asociadas a fallas de pequeria y mediana escala (abundantes en el sector). las principales estructuras geol6gicas presentes en el area son: falla de sucre: se evidencia por el fuerte diaclasamiento que se presenta en la zona rnontanosa, a 10 largo de un sector que fotogeol6gicamente se distingue como un lineamiento marcado. presenta su bloque occidental levantado, el cual esta conformado por rocas de la fm. arenisca dura, con pianos de buzamiento invertidos y las pone en contacto con las arenitas y liditas de la fm. arenisca de labor, tam bien invertidas. corta el flanco oriental del anticlinal de cheba. es de tipo inverso, con rumbo n300w (direcci6n del lineamiento fotogeol6gico) e inclinaci6n de 80° hacia el sw (sequn caro & garcia, 1988). su continuidad bajo el dep6sito cuaternario no es clara. angel et al., op.cit, proponen que es cortada por la falla de terreros y desplazada al occidente una distancia promedio de 200 m. falla de terreros: de tipo inverso, se encuentra cubierta por coluviones 0 por la fm. tilata, perc es evidente su trazo, ya que pone en contacto a la formaci6n guaduas con la fm. arenisca de labor en la parte norte de la represa de terreros y posiblemente es la responsable de la inversi6n de los buzamientos y del grade de fracturamiento de las rocas de la fm. guaduas, observado en las canteras del costado derecho, al norte de la represa de terreros. presenta su mayor desplazamiento hacia el nw de la zona de estudio y su influencia disminuye hasta desaparecer. se inclina al occidente con un plano variable entre 45° y 80° (angel et al., op.cit.). anticlinal de cheba: estructura alargada que presenta ambos f1ancos invertidos. en su nucleo aflora el segmento superior de la fm. chipaque. esta limitado por las fallas de sucre al oriente y del sinclinal del rio soacha, fuera de la zona de estudio, al occidente. hacia el sur, fuera del area estudiada, presenta un desplazamiento de su eje debido a la acci6n de una serie de fallas transversales. tiene un claro cierre periclinal truncado hacia el norte, posiblemente debido a la acci6n de la falla de terreros. 3. gravimetria para la adquisici6n de los datos se utiliz6 un gravimetro worden previa mente calibrado (arango, op.cit). la base topcqrafica fue levantada con nivel de precisi6n (± 0.1 m) debido a que los puntos qeodesicos cercanos al area de estudio, establecidos por el instituto geoqrafico agustin codazzi, fueron destruidos, se hizo necesario el establecimiento de bases qravimetricas de tercer orden que sirvieran como bases de cierre y permi ieran hacer las mediciones requeridas en la zona. las bases auxiliares establecidas fueron gun1 y gun2 (ver mapa de ubicaci6n de estaciones qravirnetricas y �~ric;enq&j~~ng9:modelamiento geoelectrico gravimetrico sector sabana bogota sevs, fig.3), para las cuales se determinaron los siguientes valores de gravedad (sistema de referencia de 1924): gun1=977404.23 mgal gun2=977405.00 mgal la precisi6n en el valor de gravedad de las bases auxiliares es del orden de una centesima de mgal. las estaciones se establecieron en perfiles cuyo maximo tiempo de cierre fue de 3y:zhoras y se utilizaron cierres simples del tipo abcdefa, donde la base a es una de las bases qravirnetricas de tercer orden (gun1 0 gun2). ei valor de la gravedad observada se obtuvo mediante la aplicaci6n de la siguiente f6rmula: goss = lo (0.089)+coer + cma+ cci+ gsa donde lo es la lectura del dial, coeres la correcci6n por deriva instrumental, cma es la correcci6n de mareas terrestres, cci es la correcci6n del error de cierre referido a la base inicial del cicio y gsa es el valor de gravedad real en la base auxiliar. ei datum utilizado para todos los calculos fue el nivel del mar. los valores de gravedad observada obtenidos varian entre 977406.0 a 9774023 mgal. los factores involucrados en la adquisici6n de los datos, tales como las mediciones topoqraficas, la deriva instrumental y el efecto de mareas, fueron corregidos y el maximo error que introducen es de 0.05 mgal. sin embargo, debido a que los datos fueron adquiridos en zona urbana, la introducci6n de ruido al momenta de la lectura de los datos result6 inevitable, a pesar de que se tuvieron especiales cuidados al realizar las mediciones, por 10 que en algunos perfiles el error de cierre alcanz6 los 0.2 mgal. tomando en cuenta que este error es producto de la acumulaci6n de errores pequerios a 10 largo del perfil, se corrigi6 repartiendolo en partes iguales en las estaciones del mismo. una vez obtenida la gravedad observada se procedi6 a realizar el calculo de la gravedad te6rica y de las correcciones de aire libre y bouguer, con el fin de obtener la anomalia simple de bouguer, necesaria para el posterior calculo de la anomalia residual. la f6rmula aplicada en el calculo de la anomalia simple de bouguer fue abs = gobsgteo+ ca donde gobs es la gravedad observada, gteo es la gravedad te6rica y caes la correcci6n de altura. la gravedad te6rica empleada en el presente trabajo corresponde a la referida a la red internacional de estandarizaci6n gravimetnca (igsn) de 1924 y es funci6n de la latitud (<p)' 2gteo(~)= 978049 (1+0.0052884sen <p + 0.0000059 sen4 <p) (mgal) la correcci6n de altura corresponde la siguiente f6rmula donde cal corresponde a la correcci6n de aire libre y cboes la correcci6n bouguer (0 de rnasas). la correcci6n de aire libre esta definida por cal= 0.3086 h donde h es la altura del punta de medida con respecto al datum (que en este caso corresponde al nivel del mar) la correcci6n bouguer corresponde a la siguiente f6rmula: cbo= 0.04188 p h don de pes la densidad. en este trabajo no se realize el calculo de la anomalia total de bouguer debido a que el efecto topoqrafico que producirian las elevaciones circunvecinas seria maximo del orden de 0.2 mgal en las estaciones cercanas a las mayores elevaciones topograficas. decreciendo suavemente hasta 0.1 mgal en las mas lejanas. 10 que implicaria variaciones relativas de maximo 0 1 mgal. para determinar el valor de densidad para los calculos de anomalia bouguer, se tome como base el analisis realizado por calvache & munoz, 1984, para la zona de la sabana de bogota y la cordillera oriental la secuencia litol6gica propuesta por estos auto res coincide con la informacion geol6gica de superficie y de pozo disponible para la zona de estudio. las densidades que proponen se ilustran en la siguiente tabla: itipo de roca i densidad (g/cm3) i precarnbricas 27 metam6rficas del paleozoico inferior 26-265 sedimentarias del paleozoico superior 24-2.6 sedimentarias del cretacico 2.3-245 sedimentarias del terciario 2.2-23 a a n l1 a sevls + sev14 tll1 + se~\t1l a + ...... a .. .. ........ .... ..a.... ...... .... a..a.. a' 6. a .. a a.a .... .. '. a .. a' a la principal herramienta que emplearon para definir esas densidades fueron datos de velocidad de intervalo (a partir de lineas sismicas), adicionalmente utilizaron las tablas de ludwing et ai., 1970 (ct. calvache & munoz, op.cit.) y complementaron con informacion de pozo. .. a" a".".... ••.. ... 6....... .. a.... .. • .. . a...... a"..•....• a , • a .... .......... a a.. • •.. a .. ..• .. a .... a • a.. a a .. •• .. a a . . .. .. .. .... . .. .. . .. 6. • sevt2j13.. + t.. a.... a sev17 .. + sev20 .. sevs+ ~1~+18 + sev7 • +sev3 .. + .. sev16 + se;4 sev~ sev8 +. +sevl a+ • sev9a+ • .. .. sev6 + • • • .... • a .... a •pozo • terreros it • .. a .. a ~52oo 985700 986200 986700 987200 figura 3. mapa de localizacion de sev y estaciones gravimetricas los valores propuestos por esos autores se confrontaron con los esperados a partir del analisis de las litologias conocidas por medio del registro de perforacion del pozo terreros ii, resultando muy acordes con las densidades esperadas. adicionalmente se estimo una densidad promedio de 2.0 g/cm3 , para los sedimentos cuaternarios. de esta manera se obtuvo una densidad promedio total de la cuenca de 2.4 g/cm3 (ver mapa de anomalia bouguer, fig. 4). como el interes del estudio es la determinacion del limite entre las rocas cretacicas y los sedimentos suprayacentes y la definicion de la forma y continuidad de las estructuras en el subsuelo, la anomalia que se modelo fue la anomalia residual (debida a efectos mas superficiales y de extension local). n a 985500 986000 986500 987000 987500 figura 4. mapa de anomalia bouger simple sobre los valores de la anomalia bouguer se realize un filtrado matricial, obteniendo los valores de anomalia regional, los cuales, al sustraerlos de la anomalia inicial, permiten determinar los valores de la anornalia residual, que en el caso estudiado presentan un range de variaci6n de 3.6 mgal (ver mapa de anomalia residual, fig.5). a partir de la informaci6n que se observa en el mapa de anomalia residual se escogieron dos cortes significativos, que pasaran por zonas gravimetricas contrastantes, a-a' y b-b' (ubicados en la fig.5). la informaci6n topoqrafica y qravimetrica disponible de esas secciones se utilize en la elaboraci6n de dos modelos qravimetricos bidimensionales que se integraron posteriormente para concretar un modele qravimetrico general para toda la zona de estudio los datos de anomalia residual se modelaron tomando en cuenta su altura sobre el nivel del mar y su distancia, y se establecieron una serie de bloques con sus respectivas profundidades y densidades. con este fin se utilize un programa que calcula el efecto sobre la componente vertical de la fuerza de gravedad que causarian los bloques propuestos sobre cada uno de los puntos de observaci6n. ei programa aplica el rnetodo de talwani (talwani, et al .. 1959). ibliouca e t ll la u iv~isidad iiacto'" geofisicacolombiana n°4, marzo del 2000 ii n a 999500 999000 998500 998000 985000 9&5500 986000 9&6500 9&75009&7000 figura 6. mapa de gradiente de anomalia residual tomando en cuenta que aparentemente la accion de esta falla solo liega hasta la fm. guaduas (al menos en su parte basal -arcillosa-), se podria sustentar la hipotesis de una falla terciaria (paleoceno temprano"). la propuesta de continuar el trazo de la falla de sucre en el subsuelo se apoya en el analisis de los mapas qravimetricos. en especial del mapa de anomalia residual (fig.5). para el establecimiento del modelo qravimetrico propuesto, se tuvieron en cuenta los datos adquiridos a partir de los sevs realizados en el sector de los cuales se escogieron los mas cercanos a los cortes qravimetricos propuestos y se ilustraron en las qraficas correspondientes. 5.3. geoelectrica a partir de la interpretacion de los sevs realizados (ver qraficas de sevs 1 a 20), se elaboraron 8 secciones qeoelectricas con el fin de visualizar la continuidad de las unidades interpretadas para cada uno de ellos a continuacion se presentan y discuten las cuatro mas representativas. la seccion qeoelectrica i (fig.9), realizada con los sondeos 1, 2 y 16, fue amarrada a traves del sev1 con la informacion del pozo terreros ii (fig 3) permite interpretar el techo del grupo guadalupe (con resistividades que oscilan entre 370 y 390 ohm-m) superficializandose desde 73 hasta 62 m hacia el se. suprayaciendo al grupo guadalupe se encuentra un potente paquete arcilloso (alrededor de 40 m en su parte mas espesa), con resistividades entre 28 y 33 ohm-m, interpretado como arcillolitas de la fm guaduas, que a su vez parecen superficializarse hacia el se. 55lanlientogeoi;u~ctricogravimetrico sector sabana bogota 16.0 15.2 ::, " 14.4 c..ia c c' 00 .: e 13.6 ..-, .: <.: 12.8 :' v anomalia residual 12.0 2600 0 460 920 p = 2.0 gjcm3 ta 2520 2440 e ~ rjj s 2360 2280 p=1.85 gfcm3 »-> p= 2.06 gf c", 1380 1840 2300 p =2.28gl em3 exageraci6n vertical = 3.6 a' c=j suelo c=j relleno t qt. d arenisca terciaria d fro. guaduas d grupoguadajupe [b) sev 15 p= 2.5 gjcm3 metros figura 7. corte gravimetrico aa' por encima de las arcillolitas se encuentra una capa de resistividad variable entre 100 y 145 ohrn-rn con un espesor de entre 23 y 6 rn, interpretada como un banco de areniscas con matriz lodosa, que se asume como terciaria (pod ria corresponder a alqun nivel arenoso de la fm guaduas 0 a areniscas de las formaciones cacho, regadera 0 bogota); en el sector, esa arenisca parece estarse acuiiando hacia el se. por encima de esas unidades se encuentra un paquete de materiales con resistividades variables entre 26 (arcillas) y 120 ohm-m (arenas), que se interpretan como pertenecientes al relleno cuaternario 0 a terciarios sin consolidar y en la parte mas somera a suelos. la secci6n qeoelectrica ii (fig.10) esta construida con base en los sondeos 13, 17, 18, y 16. mueslra el techo del grupo guadalupe, caracterizado por presentar resistividades entre 360 y 390 ohrn-rn, a 103 m de profundidad al norte, somerizandose hasta los 62 m al sur. por encima de este grupo se presenta un espeso conjunto arcilloso (con un espesor mayor al norte -70 my haciendose mas delgado hacia el sur -40 rn-) con resistividades de entre 24 y 29 ohm-rn, interpretado como parte de la fm. guaduas. por encima de este paquete se encuentra una unidad arenosa, can resistividades de entre 100 y 172 ohm-m y espesor de entre 10 y 15 m, interpretada como de edad terciaria. suprayaciendo, estas areniscas, se encuentran sedimentos no consolidados compuestos por litologias variables desde arcillas (8 ohrn-rn) hasta arenas (116 ohm-m) que constituyen el relleno cuaternario y terciario sin diferenciar y el suelo; estas ultirnas litologias tienen en conjunto un espesor de entre 18 y 20 m. la secci6n qeoelectrica iii (fig .11), realizada con base en los sondeos electricos 6, 5, 4, 1, 8 y 9, muestra una somerizaci6n del tope del grupo guadalupe (identificado por presentar resistividades de entre 352 y 372 ohm-m) de norte a sur y un adelgazamiento de los sedimentos suprayacentes en la misma direcci6n (identificados como arcillolitas de la fm. guaduas, con resistividades de entre 13 y 33 ohm-m y un espesor maximo de 82 m y minimo de 26 m). por encima de las arcillolitas de la fm. guaduas se identifica una capa arenosa (resistividades entre 103 y 152 ohrn-rn, espesor promedio de 10m) que se profundiza hacia el sur, interpretada como arenisca terciaria. 1156 geofisica colombiana n°4, marzo del 2000 ii 16.0f ir r. -:~ ,>2 j , , c " () r r; '~, " r. -, j '-''-'v 14.4 j ,, 10 ,j " .: j '0 e 13.6 '"' () ,.) 0 o, r128 .' ojc vu j anomalia residual 120 .~o 576 1152 1728 2304 28802~1 t,1 suelo ib p=1.85g/~ 7 19 p=228g!crr? ~'" b 0 relleno t qt.255(} i 25001 p= 2.0 gjcm3 0 arenisca terdaria '--....... 0 fm. guaduas e p=2.06g/~ 0 grupo guadalupe ~ ttl 2450 [12j sev19e p= 2.5 gjcm3 exageraci6n 2400 vertical = 7.34 "l"'<:rl metros figura 8. corte gravimetrico bb' suprayaciendo las capas descritas, desde superficie y hasta 17 m de profundidad, se presenta una sucesion litotoqica constituida por material no consolidado terciario y cuaternario sin diferenciar y en ijl parte mas superior por suelo, las resistividades que. caracterizan estos sedimentos oscilan entre 122 (sedimentos arenosos) hasta 6 ohm-m (sedimentos arcillosos) la seccion qeoelectrica iv (fig, 12) fue realizada con los sondeos 15, 14, 10 y 11 (fig 3) tiene una orientacion nw-se presenta el tope del grupo guadalupe (resistividades de entre 356 y 378 ohm-m) a 189 m de profundidad al nw y 153 m al se, siendo la mayor profundidad evidenciada por medio de los sondeos electricos realizados sobre esta unidad se presentan arcillolitas de la fm guaduas (de alrededor de 13 ohmm de resistividad y un espesor promedio de 65 m), que se somerizan hacia el se, estas ultirnas se encuentran suprayacidas por areniscas terciarias (con resistividades de entre 82 y 132 ohm-m y un espesor promedio de 15 m), que se profundizan hacia el nw y encima de las cuales se encuentra el relleno terciario-cuaternario no consolidado y suelo que se profundiza de se a nw desde 51 hasta 110m (presenta resistividades variables entre 100 -arenas y material de relleno artificialy 35 ohm-m -arcillas saturadas-). 5.4. establecirniento del modelo geoelectrico con base en el analisis de las secciones qeoetectricas, se propone la siguiente sucesion lito-estratiqrafica en el subsuelo • grupo guadalupe, profundizandose de se a nw desde 62 hasta 189 m, con resistividades de '" 300 a 390 ohrn-rn. • arcillolitas de la fm guaduas, con resistividades promedio entre 20 y 30 ohm-m y espesor promedio de 60 m (variando desde 26 hasta 80 m), profundizandose en el mismo sentido que el grupo guadalupe, • arenisca terciaria. con resistividades entre 82 y 175 ohm-m y espesor promedio de 15 m. correlacionable con alguno de los miembros arenosos de la fm guaduas 0 eventualmente parte de las formaciones cacho 0 regadera iibriceno & arango: modelamiento geoelectrico gravimetrico sector sabana bogota 0 10 p 20 r 30 0 f 40 u 50 n d 60 d 70 a 80 d (m) 90 100 no escala horizontal escala vertical 1:5000 1:1000 convenci0nfs ~ 1~1ij; l2j ~ ejj ~ up grupo guadalupe arcillolitas de fm, cuaduas arenisca terdaria arciilas t--qt areniscas t--qt limost-qt sev 16 profundidad de la unidad 376ohm-m figura 9. seccion geoelectrica i • depositos terciario-cuaternarios sin diferenciar y suelos, no consolidados: caracterizados por presentar resistividades muy disimiles y espesor irregular. 5.5. modelo geologico propuesto compilando la informacion geofisica resultante de las mediciones de gravedad y los sevs. integrada con la informacion geologica disponible: se plantea un modele geologico general para el area de estudio. ei grupo guadalupe se encuentra en la base de la cuenca, profundizandose hacia el nw del area estudiada. hasta aproximadamente 240 rn suprayaciendo a este ultimo se encuentran arcillolitas de la fm. guaduas, can un espesor promedio de 60 m, seguidas par una arenisca terciaria, que podria corresponder a las formaciones cacho 0 regadera 0 alguna de los miembros arenosos de la fm. guaduas arenisca la guia 0 arenisca lajosapor encima de esas unidades consolidadas se encuentra una sucesion de materiales blandos tales como areruscas, limos y arcillas que se interpretan como rellenos terciariocuaternarios sin diferenciar y suelos de acuerdo con las observaciones de campo e inteqracion con la informacion qravirnetrica, se propone la continuidad de la falla de sucre en el subsuelo, con una direccion aproximada n45°w, que esta afectando a las unidades del grupo guadalupe y disminuye su influencia en las capas de la fm guaduas esta falla, de edad terciaria (paleocene"). es cortada por la falla de terreros y posiblemente deflectada por la accion de la misma su inclinacion es de aproximadamente 700 hacia elsw la estructura anticlinal de cheba, que presentn l111 cabeceo hacia el nor-occidente, parece svidenciarse de manera muy suave en el subsuelo, limitada por la falla de sucre (igual que ocurre en superficie) ai oriente de la falla se observa una leve tendencia sinclinal y anticlinal, posiblemente asociadas al mismo evento tectonico que dio origen a la estructura anteriormente descrita. 1158 geofisica colombiana n°4, marzo del 2000 ii 0 10 p 20 r 30 0 40 u 50 n d 60 70 d a 80 d 90 (m) 100 110 13 16 sev 16 profundidad de fa unidad 373ohm-m escala horizontal escala vertical ]:5000 1:1000 ~~~~:j i~;l figura 10. seccion geoelectrica ii o 10 p 20 r 0 30 40 u n d d 7ij a d (m) convencionfs escele horizontaj esc-ala vertical 1:5000 101000 grupo guadajupe r=:-::l~ [2j 1~i'al 356ohm-m aitill""t-ol ~ s.vs a reruscas t-olarcillolites de fm. cuaduas profundidad de la unidad limost'-qtarenisca terciario figura 11. seccion geoelectrica iii 59~~i'1ig.o:m.ooelamiei'1it.o . electrico grayim,etricols.,ec,tqrs~~~~a/~ogotra. 15 ~ l;ffl~ [2] ~ cjj 6 uf convencionfs 0 10 20 30 40 so p 60 r 0 70 f 80 u n 90 d d a d (m) 1 grupe guadalupe arcillolitas de fm. g uad uas arerusca terriaria arcill ee t -qt arenisc as t-qt limost-qt sev 16 profundidad de la unidad 3s8 olun-m eecele horizontal escala vertical 1:5000 1:1000 figura 12. seccion geoelectrica iv lellotee.. e n't ... "'lot. u.i~~liidd acio. 60 geofisicacolombiananq4;:marzo del 2000 conclusiones los resultados obtenidos con base en el analisis de mapas gravimetricos, secciones gravimetricas y geoelectricas, informacion del registro de perforacion del pozo terreros " e informacion geologica de superficie, se presentan a continuacion: • los valores de anomalia bouguer en la zona oscilan entre -149.5 y -143.8 mga. • la anomalia regional disminuye de se a nw, con valores que van desde -159 hasta -162.2 mgal. • la cuenca presenta su mayor profundizacion en la parte central y nor-occidental del area, donde el techo del grupo guadalupe (basamento" de la cuenca) alcanza 240 m, 10 cual se evidencia por la presencia de un minimo gravimetrico relativo muy marcado elongado en sentido n45°w (con valores de entre 12.4 y 13.2 mgal) y por el analisis de las secciones qeoelectncas y gravimetricas. • se propone que la profundizacion de la cuenca es resultado de la accion de la falla inversa de sucre, la cual se extiende bajo el subsuelo, con un senti do n45°w y levanta las rocas de su f1anco occidental sobre las del f1anco oriental, situacion evidenciada en los cortes qravimetricos a-a' y b-b', en el mapa de anomalia residual y en el mapa geologico • la sucesion iitoestratigratica del subsuelo esta conformada de base a techo asi: grupo guadalupe, caracterizado por presentar resistividades entre 300 y 390 ohm-m y densidad de 2.5 g/cm3; arcillolitas de la fm. guaduas, con resistividades de entre 20 y 30 ohmrn, espesor promedio de 60 m y densidad de 206 g/cm3 ; areniscas terciarias, con resistividades entre 82 y 172 ohm-m, espesor promedio de 15 rn y densidad de 2.28 g/cm3 ; en la parte mas superior 'de la sucesion se presentan depositos terciario-cuaternarios sin diferenciar, suelos y relleno artificial, no consolidados, litoloqicarnente conformados por areniscas, limos y arcillas, los cuales se caracterizan por presentar resistividades muy disimiles, espesor irregular y una den sid ad promedio de 2.0 g/cm3 para los primeros y de 1.85 g/cm para los suelos y el relleno artificial. referencias bibliograficas aalto, r. (1971) petrografia de las areniscas de la seccion estratiqrafica de bogota. universidad nacional de colombia, geolog. colomb. 87-36 bogota. angel, c.e., et al. (1988) esludio hidrcqeoloqico de las zonas de soacha y ciudad bolivar. ingeominas, informe 2085, 83 p. y anexos, bogota. arango, h. (1998): modelamiento geologico con base en medidas geoelectricas y gravitacionales de una zona localizada al sur-occidente de santa fe de bogota, en inmediaciones del municipio de soacha. trabajo de grado, carrera de geologia, universidad nacional de colombia, bogota, 60pp calvache, j. & f. munoz. (1984): modelo de la corteza terrestre en un area aledaria al municipio de cumaral, departamento del meta. universidad nacional de colombia, tesis 116pp. y anexos, biblioteca del dpto de geociencias, bogota. caro, p. & j. garcia. (1988) zonificacion geotecnica del distrito especial de bogota. ingeominas, informe 2081, 203pp, bogota. duenas, h. (1978): variaciones clirnaticas del pleistoceno superior y del holoceno en la sabana de bogota. ingeominas, informe 1764. bogota. hernandez, j. & a. rincon. (1990): estudio geologico y qeotecnico general del embalse de soacha. universidad nacional de colombia, tesis, 11opp y anexos, biblioteca del dpto. de geociencias, bogota. hubach, e., (1957): estratigrafia de la saban a de bogota y sus alrededores. instituto geologico nacional, boletin geologico v (2):94-112 bogota. ingeominas. (1985): mapa gravirnetrico de anomalias simples de bouguer. escala 1:1'000.000, elaborado por a. bermudez, r acosta y m. garzon. julivert, m. (1963): los rasgos tectonicos de la region de la sabana de bogota y los mecanismos de forrnacion de las estructuras universidad industrial de santander, boletin geologico 13-14: 5-104 y mapas, bucaramanga. renzoni, g. (1962): apuntes acerca de la litologia y tectonica de la zona este y sureste de bogota. servicio geologico nacional, boletin geologico x (1-3). scheibe (1934): estudios geoloqicos de la cordillera oriental. servicio geologico nacional. estudios geoloqicos y paleontoloqicos de la cordillera oriental de colombia, parte 1, p. 1-58. bogota. talwani, m. j. l., et al. (1959): rapid gravity computations for two dimensional bodies with application to the mendocine submarine fracture zone. journal of geophysical research 64 (1):49-59 vergara, l. & g. rodriguez. (1996): consideraciones sobre la petrografia y diagenesis de los grupos guadalupe (cordillera oriental) y palmichal (piedemonte l1anero) universidad nacional de colombia, geol. colomb. 21 :41-63. bogota. archivo01.indd 7 manuscript received april 25, 2006 abstract the study of seismic loading on buildings and structures caused by technical seismicity has intensified due to increase in loads. loads are of interest not only for existing structures but they must be checked as part of an entire documentation project for new relevant buildings or for planned building reconstruction. the regions of ostrava and karviná were selected for study. the main reasons of our choice are the simultaneous occurrence of different types of technical seismicity: mining induced seismicity, traffic vibrations, industrial activity effects, etc., and the existence of suitable structures for experimental measurement in this area. individual buildings require different approaches for evaluation of their seismic load according to actual czech technical standards. the most suitable set for initial studies will be a group of selected buildings and structures meeting the most rigorous criteria for judging the seismic loading. reliable measurements from analyzed buildings and structures are available for experimental work in the area. the number of places for experimental measurements will decrease after comparation with information of actual seismic loads caused by certain type of technical seismicity in this area. the result of this evaluation will be the so-called maps of clashes of opinions, which should help to choose the measurement area and suitable structures. key words: technical seismicity, mining induced seismicity, seismic loading, building load. resumen el estudio de cargas sísmicas en edificaciones y estructuras causadas por sismicidad técnica se ha intensificado debido al incremento en las cargas. estas cargas son de interés no sólo para estructuras existentes sino que deberían ser revisadas como parte de un proyecto completo de documentación para construcciones nuevas e importantes o para planes de reconstrucción. para el estudio fueron seleccionadas las regiones de ostrava y karviná. las principales razones para nuestra selección son la ocurrencia simultánea de diferentes tipos de sismicidad técnica: sismicidad inducida contribution to evaluation of technical seismicity effect on buildings case study markéta lednická1, 2 , barbara luňáčková1 , zdeněk kaláb1, 2 , eva hrubešová1 and robert kořínek1 1 všb – technical university of ostrava, faculty of civil engineering, l. podeste 1875, ostrava poruba, cz-708 33, czech republic. 2 institute of geonics of the academy of sciences of the cr, studentska 1768, cz-708 00 ostrava poruba, czech republic. corresponding author: z. kaláb, e-mail: kalab@ugn.cas.cz earth sciences research journal earth sci. res. j. vol. 10, no. 1 (jun. 2006): 7-14 8 markéta lednická, barbara luňáčková, zdeněk kaláb, eva hrubešová and robert kořínek introduction the evaluation of seismic loading of buildings and structures is a very important topic. the main aim of this contribution is the presentation of a methodology for elaboration of the so-called “maps of clashes of opinions”. in this case, confrontation of information from different branches will create a new look of a solved problem. seismic loading and parameters of buildings, both in close relation with selected place and time, are the basic input data. these maps of selected areas enable us to determine values of seismic loading for specific places and/or to detect probability of risk for evaluated buildings and structures caused by given seismic vibrations. the knowledge of effecting seismic loadings, (i.e. type of seismic events or vibrations and their intensities) is now an integral part of all project documentations. this is necessary not only for new buildings but also for extensive reconstruction of existing structures. usually, values of velocity are used as the main criterion but values of motions, accelerations or response spectra are also applied. all reasonable input parameters that are influential in the behavior of buildings and structures, including intensity of seismic effects on the surface (e.g. kaláb, 2004) need to be taken into account when maps of clash of opinions are to be prepared. in this contribution and discussion of the selection of these parameters and their inclusion into generating maps are discussed. as an example, we selected the karviná region. this area exhibits intensive seismic activity induced by mining activities (kaláb & knejzlík, 2002, konečný et al., 2003, holub, 2000), annually about 40,000 seismic events are recorded (from 100 to 500 seismic events have ml>1). lot of sources of technical seismicity are also presented here, e.g. industrial seismicity or vibrations generated by traffic. additionally, many interesting buildings and structures, including historical buildings and cultural monuments, are present in the karviná area. methodology the structure for developing the maps discussed is a design implementing gis technology. our first ideas for using of gis in the solution of environment problems were presented formerly (čápová 1997, čápová & kaláb 2002). first, we define the main terms that will be used here during the elaboration of maps of clashes of opinions. the main categories are (see also figure 1): • input theme. • thematic layer. • parameters of thematic layer. por minería, vibraciones por tráfico vehicular, efectos de la actividad industrial, etc., y la existencia de estructuras adecuadas para la medición experimental en el área. los edificios individuales requieren diferentes aproximaciones para la evaluación de la carga por sismicidad, en concordancia con los estándares actuales de la república checa. el conjunto más apropiado para los estudios iniciales será un grupo de edificios y estructuras seleccionadas que cumplen con los criterios más rigurosos para la evaluación de la carga sísmica. medidas confiables de edificios y estructuras analizadas están disponibles para el trabajo experimental en el área. el número de lugares para las medidas experimentales disminuirá luego de la comparación con la información referente a la carga sísmica real causada por cierto tipo de sismicidad técnica en esta área. los resultados de esta evaluación serán los denominados mapas de opiniones controversiales, los cuales deberán ayudar a elegir el área de medición y las estructuras adecuadas. palabras clave: sismicidad técnica, sismicidad inducida por minería, carga sísmica, carga de edificaciones. © 2006 esrj unibiblos 9 contribution to evaluation of technical seismicity effect on buildings case study �������� ����� �� ���� �� �������� �������� ����� �� ������� ������� �������� ����� �� ��������� ��� ���������� ��������� �������� �� ������� ������� ��������� ������� ������� ����� �� ����� ������� ��������� �������� �� ����� ������� ��������� �������� �� ����� ������� � ��������� ������� ������� ����� ������� �������� �� ��������� �������� �� ������� �������� ��������� three input themes will be included into our methodology for elaboration of maps of clashes of opinions in the topic presented here – seismic loading of buildings and structures in a given region: 1) selected region, 2) seismic loadings, and 3) building objects. selected thematic layers with their characteristic parameters will be related to each of these three input themes. the thematic layers will be prepared in the form of map layers and they will be confronted with each other. a sketch of cross interference of all three input themes and cross-fade of thematic layers is presented in figures 2 and 3. as is possible to see in figure 3, subsets, which will be determined by specifi c behaviors and conditions (e.g. buildings or places), will be excluded from the basic set of input data as a result of the cross interference of individual thematic layers. figure 1. sketch of main terms used and their relationships. ���������� �� �������� ����� �������� ����� ��� �������� ����� ��� � �������� ����� ��� ��� ���� �������� �������� ���������� �� �������� ����� ���������� �� �������� ����� ��� �� ����� �� �������� ���� ����� �������� ���������� �� �������� ����� �������� ����� ��� ��� ���� �������� �������� ���������� �� �������� ����� ������� ������� �������� ������ ��� ���������� ���� �� �������� figure 2. basic sketch of interactions among input themes. figure 3. example of cross interference of selected thematic layers. 10 markéta lednická, barbara luňáčková, zdeněk kaláb, eva hrubešová and robert kořínek elaboration of the individual thematic layers will be gradual depending on the available information quantity and complexity of data for the various themes and on fastidiousness during their preparation into the form of map layers. it is possible to compile maps of clashes of opinions at different scales: • national • regional • local • detailed three input themes will be also the foundation for compilation of maps at different scales; however, individual the thematic layers will be different depending on the scale. it means that certain simplifi cation (or exclusion of thematic layers with very detailed information) must be accepted for maps with high values of ratio scales. due to complexity of the presented methodology at present we are only preparing maps with regional scale. therefore, in this contribution a methodology for selection of thematic layers for regional maps will be presented. maps with information about seismic loading of a given area depending on local conditions (geological, hydrogeological, etc.) and on buildings affected by seismic loading will be the main result of our elaboration (figure 4). one basic output can then be data about expected natural seismic loading, its distribution in the studied area, and eventually values of technical seismicity (isolines of velocities). this information can be used for assessing the location of a new building, fi rst for buildings that need special conditions (e.g. technology industries sensitive to vibrations). the information about endangered structures and buildings that are sensitive to this type of dynamic loading will be the next possible outputs. endangerment of structures is favored in specifi c conditions, such as unfavorable geological conditions, and increased surface seismic effect due to local geology. figure 4. possible forms of utilization of map of clash of opinions. ������� places with expected increased seismic loading – assessment of sites for new buildings. less resistant structures affected by seismic loading – need for response check. values of expected seismic loading for selected objects –retrofitting– parameters of input themes and their thematic layers area of interest the area of interest is an output theme that encloses the region under study. of course, the region parameters have direct infl uence on both input themes – seismic loading and engineered constructions and objects. the selection of an area of interest can be made based on: • occurrence of sources of seismic vibrations. • request for individuals or organizations. • occurrences of structures those are less resistant against loading – signifi cant buildings, endangered buildings, or other type of structures. the size of the selected region is related to the accuracy of processing. this means that if the selected region is small then obtained outputs will be more detailed. on the contrary, large regions will demand certain simplifications during the elaboration of individual themes. this is necessary 11 contribution to evaluation of technical seismicity effect on buildings case study because of the large volumes of input data, complex and subsequent processing. confrontation between seismic loading and infrastructure will result in the preparation of maps of clash of opinions. most of parameters are closely bounded in space and time, therefore, the selection of parameters for thematic layers is very important. some of these parameters can influence the effect of seismicity on the surface: effects can be enhanced or diminished; some others can influence buildings (foundation conditions, hydrogeological situation, etc.). the methodology of creation of thematic layers for the area of interest will not be described in detail. our presumption is that these thematic layers are available at present. in czech republic, many of the themes were prepared by the czech geological survey – geofond in prague using gis technology (čápová, 2005). thematic layers of area of interest: • geology (local – depending on selected scale). • local tectonics. • depth to watertable. • accumulation of water on surface. • thickness of sedimentary layers. • deformation of surface due to underground mining. • undermined regions. • landslides and other dynamic events. • behavior of rock medium (rheology). constructional objects and structures the methodology of elaboration of constructional objects and structures thematic layers will be described in detail because these thematic layers are not currently available and it is necessary to create them. the proposed maps must first confront buildings and structures with seismic loading. this methodology of processing of construction objects will be based on demands of current czech technical standards for evaluation of seismic loading of objects. now, there are standards for evaluation of natural seismicity (csn 73 0036 or eurocode 8) or technical seismicity (csn 73 0040). when evaluating the structured response during natural or technical seismicity we classify the buildings and structures according to their resistance class and/or their class of socio-economic significance (czech technical standards 73 0031, 73 0036, 73 0040). the classification of objects into classes enables us to explore strength according to applied seismic loading. this classification will be the base for classification of structures in a given area and the subsequent creation of individual thematic layers. there are six classes of resistance named from a to f. class a includes unstable buildings, historical buildings, and buildings with extensive plastic décor, archaeological structures, and others. class b is represented by common brick buildings, structure with ground area up to 200 m2 with a maximum of three stories. class c contains good reinforced structures for panels, large brick buildings, stone bridges etc. class d includes steel and concrete body structures, wooden and half-timbered houses, monolithic tank tower and others. class e consists of concrete-steel buildings, steel constructions and steel towers; the most resistant structures, for example hold-out structures, represent class f. as follows from the classification presented above, the most rigorous postulates are determined for the class a: historical buildings and buildings that do not meet construction criteria. building age is one of criterion that will have significant influence on the seismic response of structures affected by seismic loading. the main reasons for this are changes of behavior -strength properties of construction materials in time. analysis of individual building ages in the area of interest will be necessary before creation of an adequate thematic layer. certain subsets of these historical buildings will include cultural monuments. for many areas these lists are available. the reminder classes, b to f, will be divided depending on constructional and material behaviors that can constitute other thematic layers. the standards mentioned above divide structures into four classes according to their socio-economic significance. these classes are named u, i, ii and iii. individual classes respectively comprise structures and buildings with extraordinary, great, medium, and wee economic or social significance. wherever we need classes with the most rigorous requirements, we will classify the studied structures into the u and i classes. these structures and buildings are of vital importance and damage to them could endanger the lives and health of many people or cause great cultural and physical damage. class u structures 12 markéta lednická, barbara luňáčková, zdeněk kaláb, eva hrubešová and robert kořínek include significant traffic structures, important bridges, dams, and other significant structures. class i is represented by traffic constructions such as bridges, tunnels, highways, first and second-class roads, aerodrome routes, water service structures, headstocks, engine-rooms of winding engines, buildings with principal ventilators for gassy mines, structures for blast furnaces, high chimneys and antenna masts, tanks and reservoirs for chemical and oil products, and main structures of power stations. this class also includes buildings of civic amenities with high capacity to accord the people and lack evacuation routes, cinemas, theatres, hospitals, schools, supermarkets, waiting rooms, station halls, dedans, museums, government archives, observation towers and others. thematic layers of constructional objects and structures: • ages of buildings and structures. • cultural monuments. • technology construction: monolithic structures, framed structure buildings, towers, etc. • used materials: brick, timber, steel, steel-concrete structures. • economic and social significance: objects in classes u or i. seismic loadings this input theme is the main term incoming into the creation of maps of clash of opinions. the main outputs will be simultaneously, information about general seismic loading of the area under study, i.e. types of loading and their intensity from these maps. the individual types of seismic loadings will be prepared as maps of thematic layers and than they will be confronted both with thematic layers of the area of interest, i.e. possible changes of intensity of seismic effect on surface, and with parameters of thematic layers of constructional objects and structures. some of the thematic layers will be closely dependent on time due to the quick variability nature of some types of seismic vibrations. because of variability perhaps it will be necessary to elaborate for each type of seismic loading an individual thematic layer. generally, the seismic loading is divided into two groups: natural seismic loading and loading generated by technical seismicity. detailed information about the first group is available at present. it is possible to use maps that are included in the czech technical standards (see above); a first option is a map with isolines of intensity degrees (msk-64), the second one are values for proposed acceleration of foundations. however, it will be necessary to elaborate thematic layers that represent individual types of vibrations generated by technical seismicity. we start from a basic classification of technical seismicity and its intensity. now, we propose this basic subdivision for technical seismicity: • induced seismicity: – mining induced seismicity. – seismicity generated near high reservoirs. – seismicity generated by pumping or impression of medium. – weight of a big dumping hopper. • seismic events generated by an artificial source: – traffic. – quarry blasts. – pulsations of water jet. – industrial activities. as stated above, the seismic loading caused by technical seismicity is strictly connected with self sources: industrial activities, vibration of engines, traffic of highways, roads, railways, and blasting operations, quarries, and induced seismicity, mining areas, dams. this fact will be used in the methodology of preparation of thematic layers of seismic loadings. therefore, when we start with the idea to elaborate a thematic layer with certain type of technical seismicity, a first task will be to determine the source of vibrations including their intensity. the next task will be determining the impact of this source; it will be necessary to verify the seismic effect of a given source to document its influence on buildings and structures. some of these layers can only be prepared after experimental measurements in a given area e.g. velocity isolines on surface generated by mining induced seismicity. themes of seismic loading: • intensity of natural earthquakes. • proposed acceleration of foundations. • sources of induced seismicity – undermined areas (actual or abandoned mines), reservoirs. 13 contribution to evaluation of technical seismicity effect on buildings case study • isolines of maximum velocity on surface (mininginduced seismicity). • sources of technical seismicity – intensity, range impact. conclusion in this proposal of a methodology for creation of maps of clash of opinions is presented. a proper elaboration of maps under discussion will be a stepby step procedure depending on availability of input data and based on fastidiousness of their inclusion into a particular layer. input provocation for creation of these maps results from the necessity of selection of representative buildings for completion of experimental measurements to obtain their seismic response. a sketch of proposed system on a regional level is shown on figure 5. a simple example of sequence of maps on a regional level in the karvina area is shown in figure 6. here, in the upper left a generalized estate map with demarcation of mining claims exploited for a long time; to the upper right part there is a map with demarcation of mining claims exploited for a long time completed by isolines of seismic loading by mining induced seismicity (summary information for 2004 in mm.s-1); in lower left part there is a map completed by main roads that are the main connection between big cities; and in lower right part there is a signed collision between densely populated areas and higher seismic risk. acknowledgements this paper has been achieved with the fi nancial support of the ministry of education, youth and sports of the czech republic, project no. 1m6840770001. figure 5. sketch of proposed system on regional level for elaboration of map of clash of opinions. local geology area of interest depth to watertable accumulation of surface waters kumulace povrchov�ch vod ��� �� ����� �� �������� seismic loading constructional objects and structures thickness of sedimentary layers deformation of surface due mining undermined areas landslides and other dynamic events sesuvy a dynamické jevy behaviors of rock massif ages of structures and buildings cultural monuments construction technology used construction material economic and social significance intensity of natural earthquakes proposed acceleration of foundations isolines of maximum velocity on surface sources of induced seismicity sources of technical seismicity 14 markéta lednická, barbara luňáčková, zdeněk kaláb, eva hrubešová and robert kořínek figure 6. example of sequence of regional level in karviná area. • kaláb, z., and j. knejzlík (2002). systematic measurement and preliminary evaluation of seismic vibrations provoked by mining induced seismicity in karviná area, publs. inst. geophys. pol. acad. sc. m-24, no. 340, 95-103. • konečný, p., v. velička, r. šňupárek, g. takla, and j. ptáček (2003). rockbursts in the period of mining activity reduction in ostrava – karviná coalfield, 10th congress of the isrm technology roadmap for rock mechanics, johanesburg, south african, september, 665-668. • czech technical standard čsn 73 0036 (1973). seismic loads of buildings, czech republic. • czech technical standard čsn 73 0040 (1996). loads of technical structures by technical seismicity, czech republic. • czech technical standard čsn en 1998–1 eurocode 8 (1998). design of structures for earthquake resistance, czech republic. references • čápová, d. (1997). mge tools used to help protect czech environment, global links, intergraph’s international mapping/gis and civil letters. 3, no. 6. • čápová, d. (2005). internet site for data on geological and geophysical information system of czech geological survey – geofond, http://caag.geofyzika. cz/cz/geofond_caag_soubory/frame.htm (in czech) (last accessed august 2005). • čápová, d. and z. kaláb (2002). gis project to monitor induced seismic activity in areas of abandoned underground mines, publs. inst. geophys. pol. acad. sc. m-24, no. 340, 121-129. • holub, k. (2000). the influence of temporary mining interruption on induced seismicity, acta montana. series a, 118, no.16, 61-70. • kaláb, z. (2004). impact of seismicity on surface in mining affected areas: general description, acta geodyn. geomater. 1, no. 133, 35-39. articulo 2.indd earth sciences research journal volcanology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 89 100 zeolitisation of neogene sedimentary and pyroclastic rocks exposed in paipa (boyacá), in the colombian andes: simulating their natural formation conditions fredy alberto quintero ortíz, johel torres valenzuela and carlos alberto ríos reyes escuela de geología, universidad industrial de santander, bucaramanga, colombia. e-mail: carios@uis.edu.co abstract the present study concerns the synthesis of zeolites from neogene sedimentary and pyroclastic rocks exposed around the sochagota lake, paipa (boyacá). two synthesis methods were used: conventional hydrothermal treatment and alkaline fusion followed by hydrothermal reaction. both raw materials and synthesised zeolytic products were characterised by x-ray diffraction (xrd), fourier transform infrared spectroscopy (ftir) and nuclear magnetic resonance (nmr). several zeolytic phases were synthesised, including faujasite (fau), phillipsite (phi) and sodalite (sod). the results showed that the alkaline fusion approach was more efficient regarding hydrothermal conversion of the raw materials than conventional hydrothermal treatment, taking into account that zeolytic products having a higher degree of crystallinity and few impurities were obtained in this way. this study was aimed at applying experimental mineralogy to a laboratory simulation of the geological conditions in which zeolites can occur as the basis for defining criteria for exploring natural zeolites in colombia, with prospects for the profitable exploitation of these mineral resources in different parts of colombia. resumen el presente estudio se refiere a la síntesis de zeolitas a partir de rocas sedimentarias y piroclásticas del neógeno que afloran alrededor del lago sochagota, paipa (boyacá). dos métodos diferentes de síntesis se utilizaron: (1) tratamiento hidrotérmico convencional y (2) fusión alcalina seguida por reacción hidrotérmica. tanto las materias primas, como los productos zeolíticos sintetizados se caracterizaron por difracción de rayos x (drx), espectroscopia infrarroja con transformada de fourier (irtf) y resonancia magnética nuclear (rmn). diferentes fases zeolíticas fueron sintetizadas, las cuales incluyen faujasita (fau), filipsita (phi) y sodalita (sod). los resultados muestran que la incorporación de la fusión alcalina fue más eficiente en la conversión hidrotérmica de las materias primas que el tratamiento hidrotérmico convencional, teniendo en cuenta que los productos zeolíticos mostraron mayor grado de cristalinidad y baja cantidad de impurezas. la contribución de este estudio fue aplicar la mineralogía experimental para simular a escala de laboratorio las condiciones geológicas y composicionales que pueden relacionarse con la formación de zeolitas en un contexto geológico específico, aspectos relevantes para definir criterios de exploración de zeolitas naturales en colombia con perspectivas de explotación rentable de estos recursos minerales en diferentes regiones del país. palabras claves: lago sochagota, paipa, síntesis, reacción hidrotérmica, zeolitas, mineralogía experimental, condiciones naturales de formación. keywords: sochagota lake, paipa, synthesis, hydrothermal reaction, zeolites, experimental mineralogy, natural formation conditions. record manuscript received: 10/04/2011 accepted for publications: 25/11/2011 introduction zeolites are crystalline aluminosilicates having open 3d framework structures consisting of sio4 and alo4 tetrahedra linked to each other by oxygen atoms to form regular intracrystalline cages and channels having molecular dimensions. the lattice’s negative charge is neutralised by a positive charge from cations located within the material’s pores; they also contain water and/or other molecules within their pores. zeolites were first described from vugs and fissures in basaltic flows in the mideighteenth century and they are now known to be widespread in a variety of geological environments. zeolites may occur naturally as authigenic minerals formed from volcanic glass and various rock-forming minerals by interaction with aqueous solution or fluid in many geochemical environments (breck, 1974; mumpton, 1977; gottardi and galli, 1985; hay, 1986; boles, 1988). more zeolite species occur in vesicles and fissures in basic volcanic rocks than in any other geologic setting. however, zeolites from sedimentary rocks in particular, represent the most important occurrences, both in terms of areal extent of deposits and abundance of certain zeolite species (langella et al., 2001). others may be obtained from laboratory synthesis for specific commercial uses and for improving the understanding of their geological occurrence. zeolites have unique porous properties fredy alberto ortíz q., johel torres valenzuela and carlos alberto ríos r.90 and are used in various applications in petrochemical cracking, ionexchange (water softening and purification), separation and the removal of gases and solvents; other applications would include agriculture, animal husbandry and construction. many experimental studies concerning zeolite synthesis have been conducted to clarify the geological conditions regarding zeolite formation, using different starting materials in varied chemical and temperature conditions (breck, 1974; gottardi and galli, 1985). zeolites can be readily synthesised in hydrothermal conditions and a lot of information is readily available concerning synthetic zeolites and chemical resolution for zeolite synthesis (hawkins, 1981). the chemical preparation of synthetic zeolites from silica and alumina is expensive; zeolite researchers are thus seeking cheaper raw materials for zeolite synthesis to reduce costs, including clay minerals such as kaolinite (breck, 1974; barrer et al., 1974; boukadir et al., 2002), halloysite (klimkiewicz and drąg, 2004), illite, smectite, interstratified illite-smectite (baccouche et al., 1998), montmorillonite (cañizares et al., 2000) and bentonite (boukadir et al., 2002; ruiz et al., 1997), natural zeolites (kang and kazuhiko, 1997; watanabe et al., 2005; covarrubias et al., 2006), volcanic glasses (breck, 1974; barrer, 1982; vitarelli et al., 1983; colella et al., 1985; moirou et al., 2000), diatomite (anderson et al., 2005; holmes et al., 2001), high silica bauxite (puerto and benito, 1996), oil shale (shawabkeh, 2004) and natural clinker (ríos and williams, 2008; ríos et al., 2008; sandoval et al., 2009). the present study reports the laboratory simulation of the hydrothermal alteration of neogene sedimentary and pyroclastic rocks exposed around the sochagota lake (paipa, boyacá) to zeolites to understand the geological conditions controlling their formation. colombia has many volcanic areas; however, unlike other regions worldwide, there are no reports about the occurrence of natural zeolites, except in cretaceous volcanic rocks from the timba basaltic flows and sills and barroso-amaime complex (nivia, 1989; nivia et al., 2006). the geological similarity between several colombian regions, such as paipa, and those reported in figure 1. the study area’s geographical location and generalised geological map of the south of paipa (boyacá), modified from velandia and cepeda (2004), showing the geological raw materials’ origin. zeolitisation of neogene sedimentary and pyroclastic rocks exposed in paipa (boyacá), in the colombian andes: simulating their natural formation conditions 91 other parts of the world (taylor & surdam, 1981; ghiara et al., 1999; manassero et al., 2000; weisenberger & spürgin, 2009) would suggest that colombia has zeolite deposits; such similarities concern tectonic setting, type of volcanism and deposits and the occurrence of explosive rhyolitic and trachytic volcanic rocks having alkaline to calcoalkaline affinity. zeolites as products of hydrothermal crystallisation are generally known from active geothermal systems associated with volcanic rocks. pardo et al., (2005, and references therein) have reported the occurrence of a geothermal system in paipa. however, zeolite deposits of economic interest have not been discovered in colombia and no related research has been carried out until now. this study identifies such lack as being the main gap in knowledge which may hinder our ability to fully understand how natural zeolites form. likewise, few studies deal with zeolite synthesis from geomaterials. the main purpose of the present work is to contribute towards understanding zeolitisation of sedimentary and pyroclastic rocks in paipa (boyacá) in the colombian andes and the formation of zeolite deposits in colombia. laboratory simulations were thus used to transform them in hydrothermal conditions to better understand and explain the possibilities of conversion in natural geological environments. one of the most exciting developments of this work focused on the hydrothermal zeolitisation of sedimentary and pyroclastic rocks. developing future work on zeolite synthesis could be very important for colombia and zeolite researchers in designing materials in many areas of technology for specific environmental applications, particularly referring to using zeolites in removing contaminants and toxic components from the environment. experimental procedure materials and chemical reagents neogene sedimentary and pyroclastic rocks outcropping around the sochagota lake in paipa formed the starting materials for our experiments (figure 1). the tilatá formation (ngqt) out crops in the lowlands near the sochagota lake; it consists of fineto coarse-grained quartz sandstones, interbedded with siltstones, claystones and sandy conglomerates. pardo et al., (2005) have identified trachyandesite to alkaline rhyolitic ignimbrites, pyroclastic surge, block and ash flow deposits and volcaniclastic deposits, mapped as ngqv and related to plio-pleistocene, highly explosive volcanism. several aspects regarding the occurrence of these rocks are illustrated in figure 2. the geological raw materials were prepared prior to synthesis, using the following steps: rough crushing with a retsch jaw crusher bb 200 to ~ 2mm, milling with a retsch rm100 mortar grinder mill to clay particle size and sieving with a 200 mesh ro-tap sieve shaker; 63 µm particles were selected for zeolite synthesis. 99% analytical grade sodium hydroxide pellets (aldrich chemical company, inc.) and distilled water were used in all experiments. zeolite synthesis the synthesis of zeolytic materials was investigated by using conventional hydrothermal synthesis and alkaline fusion prior to hydrothermal synthesis, taking into account that direct treatment of the raw materials using alkaline solutions could be quite time-consuming, energy-intensive and/or environmentally-unfriendly, and could be overcome by using the alkaline fusion approach. during classic hydrothermal synthesis, calculated amounts (0.96 g and 2.97 g) of naoh pellets were added to distilled water in plastic reaction beakers (250 ml) to prepare naoh solutions at different concentrations (1 and 3 m). the raw materials were then added to the alkaline solutions in an 8 ml/g alkaline solution/raw material ratio. an alkaline fusion step was incorporated in the second method prior to the hydrothermal treatment to enhance the hydrothermal conditions for zeolite synthesis, considering the large amounts of aluminosilicates which can be dissolved by using this method. first, 6.20 g of raw material and 7.44 g of naoh (pellets) in raw material/alkaline activator (1/1.2 weight ratio) were dry mixed in a mortar grinder mill for 30 min and the resultant mixture was fused at 600°c for 1 h. the fused product was ground in an agate mortar and then 4.40 g of the product was dissolved in 21.50 ml distilled water (1/4.9 ratio) in plastic reaction beakers (250 ml) with constant stirring to form the amorphous precursors. the progressive addition of reagents in both methods involved stirring them until their dissolution homogenised the reaction gels. figure 2. field photographs of neogene sedimentary and pyroclastic rocks around the sochagota lake. (a) sochagota lake from the south. (b) pyroclastic flow deposit. (c)-(d) poorly sorted sandstones. (e)-(f ) sandy to clay deposits (a) sochagota lake (d) (b) (e) (c) (f ) fredy alberto ortíz q., johel torres valenzuela and carlos alberto ríos r.92 the amount of reagents used for preparing the hydrogels was based on previous experimental work developed by ríos et al., (ríos et al., 2007; ríos and williams, 2008, 2010; sandoval et al., 2009; ríos et al., 2008, 2009a, 2009b, 2010a, 2010b, 2011). hydrogels were aged in static conditions for 24 h. the hydrothermal reaction was carried out in static conditions, transferring the hydrogels to 65 ml teflon (polytetrafluoroethylene ptfe) bottles (cowie technology ltd) for preparations heated at the required temperature (80°c and 100°c) for different reaction times (24, 48, 96 h). the reactors were removed from the oven at the scheduled times and quenched in cold water to stop the reaction; hydrogel ph was measured before and after hydrothermal treatment. the reaction mixtures were then filtered and washed with distilled water to remove excess alkali. the samples were oven-dried at 80°c overnight and then the dried samples were weighed and kept in plastic bags for characterisation. characterising the raw materials and synthesised zeolites powder x-ray diffraction patterns for the raw materials and synthesised products were recorded on a philips pw1710 diffractometer operating in bragg–brentano geometry with cu-kα radiation (40 kv and 40 ma) and secondary monochromation. data was collected in the 3–50° 2θ range (0.02° step size). the crystalline patterns were compared with the standard line patterns from the powder diffraction file database supplied by the international centre for diffraction data (icdd), with the help of joint committee on powder diffraction standards (jcpds) files for inorganic compounds. the solid phases’ morphological structure was examined by scanning electron microscopy (zeiss evo50) and the mineral phases’ chemical composition was studied using the edxs mode in the following analytical conditions: i probe 1 na, eht=20.00 kv, 100 µa beam current, signal a=se1, wd=8.0 mm. fourier transform infrared (ft-ir) figure 3. hand-specimen photographs of the raw geomaterials. (a)-(b) very fineto fine-grained sandstones. (c) volcanic tuff with the pumice lapilli clasts within the fine ash matrix. figure 4. starting materials’ x-ray diffraction patterns. (a) (b) (c) spectra were collected between 4,000 and 400 cm−1 on a bruker tensor 27 ft-ir spectrometer in kbr media for investigating zeolite framework vibrations. a couple of milligrams of sample were mixed with about 200 mg of kbr powder. the mixture was pressed into a tablet which wass then used in the analysis. results and discussion starting materials a description of the macroscopic characteristics of the raw geomaterials selected for zeolite synthesis (figure 3) is now presented. briefly, the fjpz11 sample had a weathered white, fissile and compact aspect with no stratification. it was classified as very fineto fine-grained sandstone in which 5%-25% of the sand grains were composed of feldspar, being white, muddy, matrix-supported as a result of kaolin forming after feldspar. the fjpz24 sample was also very fineto fine-grained sandstone having a high feldspar content, being white, muddy, matrix-supported (due to kaolinitisation of feldspar). the fjpz50 sample was classified as a volcanic tuff, a pyroclastic geomaterial formed as fragmental products of volcanic eruptions, having feldspar phenocrystals, in a white, vesicular, volcanic glass matrix. it consisted of very poorly-sorted pumice fragments and very angular lithics (2-64 mm range), chaotically distributed in a white, vesicular, volcanic glass and porous matrix. the x-ray powder diffraction patterns for the starting materials are shown in figure 4, revealing high-intensity reflections attributed to quartz and kaolinite, although they also showed the presence of albite, crystoballite and tridymite. the tuff was characterised by the occurrence of an amorphous aluminosilicate (see the broad hump at 2θ = 17.5-20.5°, having a maximum at 2θ = ~22o). it also had a strong 26.6° diffraction peak at 2θ, which was attributed to quartz. synthesised zeolites x-ray diffraction zeotypes were identified by comparing the diffraction pattern to the international centre for diffraction data’s powder diffraction file (pdf) database. our data indicated the formation of phillipsite (pdf 000-511497), sodalite (pdf 010-88-1190) and faujasite (pdf 000-39-1380). hydrothermal transformation of geomaterials in 1 m naoh solutions. the xrd patterns in figure 5 show the progressive synthesis of zeolite by the conventional hydrothermal method, using solutions of 1 m naoh. this revealed a very small reduction of the starting materials’ characteristic peaks and the appearance of weak reflection peak intensity corresponding to traces of phillipsite-type zeolite, indicating low activation efficiency. the fjpz11 sample had constant peaks corresponding to quartz and kaolinite, but phillipsite appeared (after 24 hours) with increasing reaction time. the only phase formed for the fjpz24 sample was phillipsite after 96 hours. zeolitisation of neogene sedimentary and pyroclastic rocks exposed in paipa (boyacá), in the colombian andes: simulating their natural formation conditions 93 figure 5. synthesis products’ x-ray diffraction patterns after activating raw materials in 1m naoh solutions. figure 6. synthesis products’ x-ray diffraction patterns after activating raw materials in 3m naoh solutions. (a) (a) (b) (b) (c) (c) the activation of the fjpz50 sample was unsuccessful, as no zeolite phase became formed, and only a decrease in crystoballite peak intensity and increase in characteristic quartz peaks were observed, although the latter decreased after 96 hours. hydrothermal transformation of geomaterials in 3 m naoh solutions. the xrd pattern in figure 6 shows that increasing naoh solution concentration favoured the formation of sodaliteand phillipsite-type zeolites, with a progressive decrease in peak intensity for the phases related to the minerals present in the starting materials. the products resulting from the synthesis always had starting materials’ relict phases. a progressive kaolinite dissolution was observed for the fjpz11 sample between 6 and 24 hours of reaction, disappearing after 96 hours. the characteristic quartz fredy alberto ortíz q., johel torres valenzuela and carlos alberto ríos r.94 previous case, except for the occurrence of sodalite having weak reflection peaks. using the fjpz50 sample as starting material, a progressive loss of its amorphous character turned it into a synthesis product characterised by the appearance of relative poorly crystalline phillipsite only after 96 hours. in this case, an increase in characteristic quartz peak intensity and the presence of other relictic phases such as albite was observed in the products so obtained. transformation of geomaterials by alkaline fusion followed by hydrothermal treatment. zeolite synthesis by this method revealed that incorporating an alkaline fusion step promoted the dry reaction between the mineral phases present in the raw geomaterials and the alkaline activator. as shown in figure 7, there was an almost complete transformation of the starting materials into high purity faujasite-type zeolite and phillipsite, although relictic phases (quartz and albite) of the starting materials still remained in the synthesis products. the alkaline-fused products were sodium silicate and aluminate salts. newly-formed compounds dissolved in water more readily than the mineral phases in the starting materials, which had low dissolution speed, some of them occurring as relict phases in the synthesis products. the activation of the fjpz11 sample produced a rapid dissolution of the alkaline-fused products only after 6 hours reaction, accompanied by a gradual decrease in the intensity of characteristic quartz peaks which persisted in the products so obtained. moreover, the occurrence of faujasite was recorded after 24 hours, showing an increase in the intensity of characteristic peaks between 24 and 96 hours. the activation of the fjpz24 sample produced similar results to those observed in the previous case, except for the fact that faujasite had less intense peaks. the transformation of the tuff (fjpz50 sample) was characterised by the loss of the starting material’s amorphous nature, with progressive dissolution of the alkalinefused products, except for the gradual increase of characteristic quartz peak intensity, with the occurrence of phillipsite along with quartz and albite in the synthesis products. phillipsite was formed after 24 hours, showing an increase in its characteristic peak intensity with the reaction time. scanning electron microscopy sem images of the morphology of the synthesised zeolytic products obtained after activating the starting materials are shown in figure 8, revealing a change in the morphology of the original surface of raw geomaterial particles (blocky morphology, not shown here) to the modified product’s far rougher surface. the zeolytic products obtained were characterised by the occurrence of phillipsite, sodalite and faujasite. figure 8a illustrates complex aggregates of large clusters of radiating tetragonal prisms of phillipsite. sodalite occurs as bladed crystals having a ‘‘cotton-ball”-like morphology (figure 8b). the typical morphology of well-developed octahedral faujasite crystals displaying interpenetration-twinning is shown in figure 8c. fourier transform infrared spectroscopy based on the results of the x-ray diffraction analysis, the fjpz24 sample was selected to confirm its transformation during synthesis in hydrothermal conditions and the presence of an aluminosilicate framework in the zeolites, which was shown in the ftir spectra (figure 9). the starting material’s characteristic peaks became less pronounced in some regions, resulting in their disappearance, although sometimes they only became weakened in intensity. characteristic mineral phase vibrations present in both the starting material and synthesis products were observed in the present study; however, in other cases, some vibration bands moved toward higher or lower frequencies. the bands at 3,659.22, 3,620.68 and 3,456.36 cm-1 corresponded to oxygen-hydrogen bond vibration frequencies belonging to the water molecules retained in the starting material. kaolinite’s characteristic vibrations were observed at 1,115.34, 1,029.56 figure 7. synthesis products’ x-ray diffraction patterns after activating raw materials using the alkaline fusion approach. (a) (b) (c) peak had decreased intensity, indicating this mineral’s stability during hydrothermal treatment. moreover, the progressive dissolution of starting material’s mineral phases was accompanied by the appearance of sodalite after 6 hours. a progressive increase in the intensity of peaks characteristic of this zeolytic phase was observed between 6 and 96 hours. activation of the fjpz24 sample, produced similar results to those observed in the zeolitisation of neogene sedimentary and pyroclastic rocks exposed in paipa (boyacá), in the colombian andes: simulating their natural formation conditions 95 (surface o-h bond bending), 917.14 (internal bending o-h bond), 669.49 and 540.04 cm-1. the mid infrared silicate spectra in the 1,200400 cm–1 range were classified into four characteristic bands of around 1,000, 780, 695 and 450 cm–1 regarding the standard quartz spectra (saikia et al., 2008). according to saikia et al., (2008), among these four characteristic peak regions, a peak at 695 cm–1 is unique to crystalline materials, which however was not observed in the sample analysed here. the 1,029.56 (asymmetric t-o stretching vibrations) and 788.68 (symmetric t-o stretching vibrations) cm-1 bands were characteristic for quartz. new vibration bands appeared concurrently with the gradual disappearance of the starting material’s characteristic bands, which may have been attributed to zeolytic materials. zeolites frameworks vibrations give rise to typical bands in mid and far infrared spectra. a distinction was made between to4/2 tetrahedra external and internal vibrations (with, for example, t = si or al). the original assignments for the main ir bands (flaningen et al., 1971) were as follows: the 1,250-950 cm-1 region was attributed to the tetrahedral asymmetric stretching zone’s inner bonds and the asymmetric stretching of external bonds between tetrahedral zones. the band at 995.79 cm-1 corresponded to asymmetric stretching in faujasite. sodalite occurrence was confirmed by the bands at 991.08 and 1,012.31 cm-1. there was no evidence of ir absorption in the 820-750 cm-1 range assigned by flaningen et al., (1971) to the stretching vibrations of external bonds between tetrahedra. there was a poorly defined band at 682.51 cm-1 in the 750-650 cm-1 spectral zone which could be attributed to symmetric stretching in faujasite. there were two well-defined bands at 709.81 and 690 cm-1, which were in good agreement with the bands for sodalite reported by flaningen et al., (1971). the bands in the 650–500 cm-1 region were related to the presence of double rings (d4r and d6r) in the zeolytic materials’ framework; the band at 557.42 cm-1 corresponded to the hexagonal double ring vibration which is attributed to faujasite. there was no evidence of bands corresponding to sodalite, which is characteristic for this zeotype, taking into account that it does not have a double-ring framework like faujasite. there was no evidence of ir absorption in the 500-450 cm-1 region related to o-t-o bond deformation vibrations (flanfigure 8. sem images of representative synthetic zeolites. (a) complex aggregates of large clusters of radiating tetragonal phillipsite prisms. (b) bladed sodalite crystals. (c) well-developed octahedral faujasite. figure 9. ftir spectra of unreacted raw material (on the top) and representative synthesis products. parameter wave number (cm-1) double ring 557 nw asymmetric stretching 710 wsh 728 mw 795 mwsh 991 sb 996 sb symmetric stretching 691 s 690 wsh 682 mwb t-o bending ~ 450 s pore opening s = strong; ms = medium strong; m = medium; mw = medium weak; sh =shoulder; b = broad; w = weak table 1. infrared spectral data for the synthesis products. (a) (b) (c) ingen et al., 1971), except for a vibration band around 450 cm-1. there was no evidence of the well-defined symmetric bands reported for sodalite in previous studies (flaningen et al., 1971). table 1 shows the ir spectral data for the synthesised zeolites. fredy alberto ortíz q., johel torres valenzuela and carlos alberto ríos r.96 29si and 27al nuclear magnetic resonance figure 10 shows the nmr spectra for starting material 29si and 27al (fjpz24 sample) and synthesised products. the starting material’s 29si spectrum (figure 10a) had two well-resolved signals at -91.13 and -107.23 ppm attributed to the existence of two different silicon sites and revealed the presence of kaolinite and quartz, respectively, as reported in previous studies (barron et al., 1983; thompson and barron, 1987; hayashi et al., 1992; letaief et al., 2006). activated starting material’s chemical shift values for 29si spectra are illustrated in figures 10b-10d, indicating the occurrence of different silicon sites for the zeolytic phases so obtained, and relics of kaolinite and quartz. the 27al spectrum (figure 10a) consisted of a simple resonance at 1.23 ppm, which was assigned to octahedrally coordinated al in kaolinite and quartz. the 27al spectra for the treated geomaterial (figures 10b-10d) had resonance signals at 1.23 ppm, attributed to octahedral al (corresponding to relictic phases), and at 57.86 and 60.79 ppm, attributed to tetrahedral al. the experimental method used in zeolite synthesis played an important role in determining the zeotype to be formed. an alkaline fusion step was carried out prior to the hydrothermal treatment because it plays an important role in enhancing the hydrothermal conditions for zeolite synthesis. according to ríos & williams (2008), synthesis chemistry becomes subject to perturbations caused by the presence of impurities in the starting materials, which may remain insoluble during crystallisation and figure 10. 29si and 27al nmr spectra of the (a) unreacted raw material (fjpz24 sample) and representative synthesis products after hydrothermal treatment (b) and (c) in 1 and 3m naoh solutions, respectively, and (d) alkaline fusion followed by hydrothermal reaction. (a) (b) (c) (d) zeolitisation of neogene sedimentary and pyroclastic rocks exposed in paipa (boyacá), in the colombian andes: simulating their natural formation conditions 97 make undesired species nucleate, thereby developing mixtures of different zeolytic materials coexisting in the synthesis products, and the dominant crystalline phase depending on formation conditions. simulating geological conditions for zeolite formation experimental mineralogy was used for simulating zeolite formation conditions in a specific geological context such as paipa, where explosive rhyolitic and trachytic volcanic rocks having alkaline and calcoalkaline affinity occur, to ascertain in which conditions and from what compounds their formation may occur. most of the exploitable natural zeolite deposits in the world are found in initially glass-rich, clastic formations of volcanic origin, frequently being associated with acid volcanism. according to gottardi (1989), these deposits form as a result of the slow percolation of meteoric water through volcanic tuff beds, becoming more alkaline and saline with depth, thus raising its ph. this leads to zonal variation of water chemistry reacting with the host rock, which is expressed in the mineral product’s vertical zonation. thus, the volcanic glass is altered to clay minerals at the top and different types of zeolites form below. authigenic alkali feldspars crystallise at greater depth. the same vertical zonation could be established in the laboratory by transforming the proposed starting materials and simulating hydrothermal conditions similar to those found in this geological environment. zeolites can be readily synthesised in a few hours in hydrothermal conditions and there is a lot of information regarding zeolite synthesis solution chemistry (hawkins, 1981). however, a major problem concerns zeolite synthesis at high temperatures and pressures regarding the formation of zeolites in nature (sand, 1980). according to hawkins (1981), this can be addressed if zeolites typical of low-temperature conditions are synthesised from natural reactants in chemical conditions similar to those in which natural minerals are thought to be formed. the synthesis of zeolytic materials from zeolite-absent rocks demonstrates that such simulation may reproduce the overall geological conditions in which these mineral phases may form in nature. the transformation of aluminosilicate geomaterials in zeolytic phases can be explained by two main stages: the dissolution of raw geomaterials’ aluminosilicate phases or their alkaline-fused product releasing si and al and zeolite crystallisation. an attempt was first made to destroy the structure of the aluminosilicates present in the starting materials by separating the silicon tetrahedra and aluminium octahedra which finally re-crystallised in several zeolite frameworks. geomaterials’ transformation in zeolites reflects mineralogical, textural and chemical responses to the starting materials in hydrothermal conditions. undoubtedly, the mineralogical composition of the aluminosilicate-rich starting material played an important role, as well as the presence of elements other than si and al, which promoted the production of different types of zeolites and undesirable mineral phases. very low fe and ca content can control synthesis, taking into account that febearing minerals can have inert behaviour and ca-bearing phases can act as a zeolite synthesis inhibitor (juan et al., 2007). the glass phase is also very important during zeolitisation because it easily dissolves into the alkaline solution compared to crystalline phases like quartz or feldspar, which generally persist in synthesis products. although the initial mineralogical composition influences hydrothermal reaction product mineralogy, its effect will be minor compared to the effect produced by factors such as solution permeability, temperature and composition (ph). however, the major input of chemical constituents into aqueous systems comes from mineral dissolution, and, therefore, knowledge of mineral solubility and their mode and rate of dissolution is essential for interpreting water-rock interaction and the transport of dissolved solids in water systems (stefánsson, 2001). zeolite formation by hydrothermal weathering varies according to the nature and the origin of the weathered rocks. in nature, the origin of zeolites is related to the hydrothermal transformation (hydrolysis) of glass to smectite, prior to zeolite formation, which was not reported in the study area probably related to the requirements for large-scale zeolitisation, such as a high percentage of glassy particles, high permeability and favourable hydrological conditions (manassero et al., 2000). on the other hand, during rock-solution interaction, differences in the reaction products, crystallisation sequences, and reaction kinetics seem to be related to several intensive variables such as interaction time, hydrological system (closed or open), temperature, ph, contact solution chemical composition and starting materials (e.g. barth-wirsching and holler, 1989). the reasons are diverse and related to aspects such as volcanic processes, as well as other less understood ones, including the relationship between the zeolitisation of pyroclastic rocks and the formation of natural zeolites. the intention was to produce synthetic zeolites in a short time and at the lowest possible temperature, which is very important for geochemists whose aim is to reproduce synthetic zeolite formation in the laboratory in hydrothermal conditions and obtain better understanding of natural zeolite formation in a specific geological context. the alteration of neogene sedimentary and pyroclastic rocks exposed in the paipa area was thus evaluated by alkaline activation at low temperatures and autogenous pressure using two different methods, which are described below. this approach was also interesting because it involved hydrothermal alteration of primary minerals or rocks into zeolites, although transformation was generally slow and the synthetic zeolites crystallised with other phases. therefore, zeolite synthesis in the laboratory requires the verification of the geological conditions involved in the formation of this group of minerals. colombia has a number of volcanic areas within which natural zeolites may be found. however, it was evident from this study that it is not enough to just determine geological similarity and pyroclastic volcanic regions between colombia and other parts of the world where zeolites have been reported but also to determine areas where volcanic rocks have reacted with alkaline groundwater. there is thus a connection between the type of lithology and type of site where natural zeolites should be sought. exploration strategies different descriptive and genetic geological models have been applied to the exploration of mineral deposits which are based on the geological setting, host rock lithology, form of the deposit, economic minerals, the mineralogy of ore and host rocks and geochemical and geophysical characteristics common to a number of similar mineral deposits presumed to have been formed by the same genetic process. this study was intended to define exploration criteria for natural zeolites in volcanic terrain like paipa (colombia). recent studies (e.g., taboada et al., 2000) have proposed that volcanic rocks occuring in paipa originated from the caribbean plate’s subduction. pardo et al., (2005) have identified trachyandesitic to alkaline rhyolitic ignimbrites, pyroclastic surge, block and ash flow deposits and volcaniclastic deposits in this area which are related to plio-pleistocene, highly explosive volcanism. however, it is also common that extensive lacustrine deposits represented  by intercalated  tuffs, siltstones, diatomites and sandstones occur in such geological setting (brathwaite, 2003, 2006). diatomite may occur down or up  stratigraphically from lacustrine tuffs (brathwaite, 2006) and has been reported in several parts of colombia, although there is no report of its appearance in the study area. the main zeolite occurrences are in tuffs and volcaniclastic rocks in lakes or deep sea basins (hay and sheppard, 2001), although they are also common in low temperature hydrothermal alteration zones in geothermal and epithermal systems (utada, 2001). according to alfaro et al., (2010) the paipa volcano has an active hydrothermal system as inferred from surface manifestations and an inactive volcano-magmatic system. the paipa-iza fault fredy alberto ortíz q., johel torres valenzuela and carlos alberto ríos r.98 is interpreted as a basement structure related to a previous extensional tectonic phase which became reactivated during andean uplift, preserving the nature of open breaks facilitating hydrothermal fluid circulation. the zeolite deposits are located on or close to faults, indicating that the faults acted as feeders for heated alkali chloride water that caused the deposition of the zeolite minerals (brathwaite, 2006). there is thus a spatial association with geothermal systems and faults in the volcanic area of paipa to provide a source and conduit for heated alkaline solutions which react with vitric tuffs to form the zeolites. hydrothermal eruption breccias, sinter overlying zeolytic tuffs and hydrothermal adularia in permeable fault zones are indicators of hydrothermal activity that may have formed zeolites, as suggested by brathwaite (2006). no evidence has been presented in this study to support the occurrence of all these requirements, meaning that they cannot be used as exploration guides. according to brathwaite (2006), geophysical signatures have provided useful information on zeolite deposits which can be associated with broad magnetic lows within which there are coincident gravity and resistivity lows. the mineralogy of hydrothermal alteration is one of the most useful geological exploration criteria for natural zeolites. a start can be made in what would be a geological curiosity using different analytical techniques, such as x-ray diffraction and scanning electron microscopy. however, using microthermometry of fluid inclusions potentially provides key information on the genesis of most ore deposits and cannot be replaced by other techniques as a guide for mineral exploration (camprubí, 2010). the following exploration criteria for zeolite deposits in a geological setting such as paipa are thus suggested: • lacustrine vitric tuff host rocks provide large volumes of reactive volcanic glass; • diatomite occurring down or up stratigraphically from lacustrine tuffs; • a spatial association with geothermal systems and faults to provide a source and conduit for heated alkaline solutions; • geophysical signatures having coincident gravity and resistivity lows within broad magnetic lows representing hydrothermal alteration zones; • mineralogy of the hydrothermal alteration; and • mineralising fluids. geology undoubtedly plays a very important role in natural zeolite exploration. however, different zeotypes and their favourable geologic settings and controls must be understood and detected on a regional or local scale. on the other hand, the application of several analytical techniques would provide valuable information to be integrated with geology. according to robert et al., (2007), a successful strategy should emphasise detecting geological features typical of favourable settings and hydrothermal manifestations of deposits, such as alteration and mineralisation. conclusions zeotypes like phillipsite, sodalite and faujasite zeolitisation having very few impurities (quartz and feldspars) were successfully synthesised from neogene sedimentary and pyroclastic rocks outcropping in paipa (boyacá) in the colombian andes. the experimental method used in the synthesis played an important role in determining the zeotype to be formed. the alkaline fusion approach was more effective than the conventional hydrothermal treatment in transforming the starting materials. zeolite synthesis chemistry was subject to perturbations caused by the presence of impurities in the starting material, which remained insoluble during crystallisation and made undesired species nucleate, thereby developing mixtures of different zeolytic materials coexisting in the synthesis products. further studies should be carried out in well-optimised experimental conditions using different alkaline solutions to compare the effect of the activating agent on starting materials’ transformation and to better understand mineral alteration in hydrothermal conditions. simulation conditions to be used in zeolite synthesis should thus reproduce the origin of natural zeolites as a process related to the hydrothermal transformation of glass to smectite, prior to the formation of zeolites, which could not be proved in this study. it is therefore very important to consider differences in reaction products in the rock-solution interaction, as well as crystallisation sequence and reaction kinetics, which seem to be related to several intensive variables. this research provides valuable information for defining geological exploration criteria in geological settings similar to those in which zeolites have been reported in other parts of the world to focus studies concerning the exploration of natural zeolites in colombia. acknowledgments the authors gratefully acknowledge the universidad industrial de santander for financially supporting the fieldwork. this study has benefited from research facilities provided by the following institutions: universidad industrial de santander, university of wolverhampton, university of durham and instituto zuliano de investigaciones tecnológicas. we would like to thank mr. miguel ramos, mrs. barbara hodson, mr. alejandro torres and dr. david apperley for assistance with xrd, sem, ftir and nmr data acquisition, respectively. 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en ocasiones los hacen extensivos a los problemas digestivos 0 respiratorios, por inhalaci6n de gases 0 cenizas digeridas; en algunos casos a la reducci6n de la visibilidad, la importancia del movimiento del aire en el trans porte de las cenizas y de la liuvia en los posibles f1ujos de escombros. no es facil encontrar referencias cuantitativas (a veces ni siquiera cualitativas) que muestren la modificaci6n de los valores de los elementos meteorol6gicos ocasionada por la introducci6n en la atm6sfera de s61idos, ifquidos y/o gases durante un perfodo de actividad volcanica, este es el primero de una serie de arttculos que intentaran mostrar, cuantitativa y/o cualitativamente sequn las posibilidades, el efecto que la relativamente pequeria erupci6n del volcan nevado del ruiz del 13-11-85, ocasion6 en los valores de algunos de los elementos meteorol6gicos basicos, tanto en la capa de aire superficial como en las capas de mayor altitud. aca, se analiza la reducci6n de la radiaci6n solar entrante al sistema tierraatm6sfera, a traves del estudio de la cantidad de horas diarias de brillo solar (insolaci6n) que ocurrieron durante el perfodo 11-16-nov-1985 (antes, en y despues de la erupci6n). los datos recopilados en las estaciones meteorol6gicas se extractaron de los archivos de el himaty la federaci6n nacional de cafeteros. los datos originales, verificados y adecuados sequn las necesidades especfficas del trabajo y los resultados de los procesos realizados, se encuentran parcialmente consignados en la tabla y figuras involucradas en el trabajo. 38 eslava: efectos erupcl6n del vnr sobre la atm6sfera. caso: insolaci6n 2. generalidades las cenizas y gases expulsados por el volcan nevado del ruiz formaron capas, -en la atmosfera, de diferentes espesores y densidades que cubrieron rapidamente toda la zona ubicada al noroccidente del tolima, desde los comienzos de la actividad y posteriormente a la mayor parte del pais; ello impidi6 u obstaculiz6 el paso de la energfa solar en el resto de esa tarde y dlas siguientes. utilizando los datos meteorol6gicos disponibles se infiere que se ocasionaron cambios temporales en los valores de los elementos meteorol6gicos que comenzaron desde el dla anterior por los gases y cenizas emitidas con anterioridad y que por efecto de los movimientos locales y generales del aire se habfan acumulado en sectores ubicados al w y nw del volcan, ei gran nurnsro de partfculas que se introdujeron en la atmosfera por la erupci6n del volcan nevado del ruiz el 13-11-85, se difundio en la atm6sfera formando, principalmente, el penacho de cenizas y la nube gaseosa en forma de arco, descritas por eslava (1985) y krueger (1985, citado en smithsonian institution, 1985), respectivamente. sin embargo, las partlculas s61idas y gaseosas se hicieron presentes en casi toda colombia y ocasionaron una reducci6n notoria en la cantidad de horas de sol por dfa que normalmente ocurren en esa epoca del ana; con mayor intensidad, obviamente, en las zonas cercanas al volcan y en las ubicadas por debajo del penacho de cenizas. a su vez, la disminuci6n de las horas de brillo solar, origino una correlacionada reduccion de la radiacion solar entrante en el sistema tierraatm6sfera en cada sitio y una disminucion (no exactamente correlacionada, sino modificada por los factores ffsicos de cada sitio) de las temperaturas medias del aire, con un aumento de las mfnimas y una disminuci6n de las maximas. esas desiguales variaciones de temperatura, en union con el aporte de vapor de agua y nucleos de condensaci6n (ceniza), afectaron la distribuci6n de la humedad y los de mas elementos meteorol6gicos, entre ellos el regimen temporal y espacial de las liuvias. las magnitudes de algunas de esas alteraciones fueron pequenas y, muy seguramente, no perceptibles por los metodos de medici6n disponibles. esas innumerables variaciones temporales y espaciales estuvieron sometidas a factores extern os entre los cuales el factor prima rio fue la radiaci6n solar y sus variaciones; dependieron basicamente de la interrelaci6n que se da entre los elementos meteorologicos y entre ellos y el conjunto de procesos zonales de convecci6n termica y dinamica, de la influencia extensiva de los diferentes sistemas meteorol6gicos, como la ltc, de la presencia de movimientos del aire determinados 0 condicionados por la geometrfa tierra-sol, la latitud, la altitud, la configuraci6n del relieve, el suelo, la vegetaci6n, la hora del dfa 0 noche en la cuailleg6 a cada sitio la masa principal de partfculas, etc. los impactos generales entonces se modificaron por el medio ambiente local y/o regional. todos esos formadores, condicionantes 0 agentes que producen 0 modifican el estado del tiempo, ocasionaron una gran variedad de alteraciones. las caracterfsticas generales de los cambios en uno de los mas importantes elementos meteorol6gicos, se presenta aca. 3. efectos sobre la insolacion (brillo solar) como ya se mencion6, el efecto prima rio del gran nurnero de partlculas introducidas en la atm6sfera por la erupci6n del volcan nevado del ruiz el 13 de noviembre de 1985 fue el de obstaculizar la entrada de radiaci6n solar en la troposfera y superficie terrestre, disminuyendo el nurnero de horas de sol (brillo solar) y la cantidad de energfa que normal mente se recibe en cada sltio por dfa. la intensidad de la disminuci6n fue variable, en espacio y tiempo, dependiendo en cada lugar del espesor de las capas de ceniza y gases, del dia y la hora en que esas partlculas liegaban a cada sitio y de los muy variados factores que influyeron (iatitud, altitud, configuraci6n del relieve, suelo, vegetaci6n, distancia al volcan, movimiento general y local del aire, etc.). los datos (tabla 1), muestran los valores de insolaci6n para el periodo comprendido entre el 11 y el 16 de noviembre de 1985 (antes, en y despues de la erupci6n) y el promedio para todo el mes de noviembre. las diferencias entre esos valores indican los cambios que normalmente ocurren, en unos dias y sitios, por efecto de los sistemas meteorol6gicos normal mente actuantes y en otros dfas y sitios por la uni6n de los efectos meteorol6gicos normalmente actuantes y por los gases y cenizas inyectados en la atm6sfera por el volcan nevado del ruiz (antes, en y despues de la erupci6n). a manera descriptiva y con el prop6sito de poder efeetuar las eomparaciones neeesarias, se muestra en la fig.1 un anal isis de la distribuci6n espacial promedia de la insolaci6n durante el mes de noviembre de 1985. esa distribuei6n es aproximadamente la que en forma tfpiea se darfa si s610 se presentaran los efeetos de los sistemas meteorol6gieos normal mente actuantes. se eoncluye, en forma general, que se pueden eonsiderar (para este mes y elemento) dos grandes zonas: a) una zona que eubre la regi6n del pacifico, la regi6n andina y la regi6n del catatumbo; en esta zona los sistemas meteorol6gieos aetuantes (por ej., la itc) obstaeulizan el paso de la radiaei6n solar y log ran que s610 se den menos de 5 horas de brillo solar por dfa y; b) una zona eomprendida por la mayor parte de la orinoquia, amazonia y la regi6n del caribe; en esta los sistemas meteorol6gieos actuantes no obstaeulizan mayormente la entrada de radiaci6n solar y oeurren mas de 5 horas de insolaei6n por dfa/promedio. la fig.2 i1ustra la variaei6n de la insolaci6n, del 11 al 16-nov-1985, en 12 silios localizados en areas eubiertas per el penaeho de eenizas; en ella se sintetiza el e/deto de la eeniza y gases, que origin6 una clara disminuei6n de la insolaci6n durante el perfodo crftieo del 13 a116. geofisica colombiana n° 2, abril de 1993 39 to· to· ... i ._.........1-...-../.1 c./ \. .-:'> ......._ .....'t _,j' ',__ -,-, i . ',) ,) k_.~ \..~' . '\ (.j'.a..! '<; !»< !r. \ r'''/ \~.'v'. ". brasil ..r--...rj (') \s'·'\..\..h 5 ,..---...... 1 peru :~. ~ 5 .. o· '0 .. .. .. o· to· ",. to· .. . f"gura 1.•sone"as (horas-<leclmos) que indican la insolacion (brillo solar) media dlarla de noylembre de 1985 40 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n tabla 1. insolael6n (brilio solar en horas y deelm0t) re~lstrada en estaelones meteorol6",leas ubleadas en colombia; antes, en y despues de la erupei6n del ole n nevado del rulz del13 de novlernbre de 1985 estacion valores diarios med diferencias nombre-municipio 11 12 . 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m amazonas araracuara-araracuara 4.8 4.6 0.2 1.1 0.4 0.2 5.4 -0.6 -0.8 -5.2 -4.3 -5.0 -5.2 apto.vasquez cobo-leticia 5.6 0.0 2.4 0.0 1.5 1.2 2.9 2.7 -2.9 -0.5 -2.9 -1.4 ·1.7 antioquia urrao-urrao 5.6 0.0 2.4 0.0 1.5 1.2 2.9 2.7 -2.9 -0.5 -2.9 ·1.4 -1.7 canas gordas-canas gordas 6.9 0.0 3.9 0.0 2.9 0.1 4.3 2.6 -4.3 ·0.4 -4.3 -1.4 -4.2 musinga-frontino 6.6 0.0 2.1 0.0 2.1 0.1 3.6 3.0 -3.6 -1.5 -3.6 -1.5 -3.5 villarteaga-muta ta 1.7 0.0 3.1 0.0 4.6 1.3 3.3 -1.6 -3.3 -0.2 -3.3 1.3 -2.0 uniban-apartado 3.7 0.0 5.7 0.3 4.0 2.6 4.4 ·0.7 -4.4 1.3 -4.1 ·0.4 -1.8 riogrande-apartado 1.2 0.2 4.5 2.8 4.0 -2.8 -3.8 0.5 -1.2 tulenapa-chigorodo 3.5 0.0 4.1 0.1 4.3 4.1 ·0.6 -4.1 0.0 -4.0 0.2 apto.los cedros-apartado 3.8 0.0 4.5 0.4 4.8 4.0 4.8 1.0 -4.8 -0.3 -4.4 0.0 -0.8 el mellito-neclocli 6.5 0.0 5.0 1.4 7.9 3.9 4.8 1.7 -4.8 0.2 -3.4 3.1 -0.9 la selva-rionegro 8.0 0.0 1.5 0.0 1.7 0.7 4.0 4.0 -4.0 -25 -4.0 -2.3 -3.3 gja.exp.el nus-san roque 6.9 1.1 3.6 0.2 3.7 2.4 4.8 2.1 -3.7 -1.2 -4.6 -1.1 -2.4 corrientes-san vicente 1.3 9.2 10.0 4.7 4.9 4.3 -3.0 4.9 5.7 -0.2 0.6 las violetas-san rafael 7.6 1.2 1.6 0.0 4.5 1.4 4.1 3.5 -29 -2.5 -4.1 0.4 -2.7 san carlos-san carlos 7.0 0.3 1.8 0.0 2.8 0.5 3.7 3.3 -3.4 -1.9 -3.7 -0.9 -3.2 apto.pto.berrio-pto.berrio 7.7 2.0 6.0 0.1 5.9 5.6 6.2 1.5 -4.2 ·0.2 ·6.1 -0.3 -0.6 apto.otu-remedios 1.5 5.4 0.7 1.6 5.6 6.1 5.1 -36 0.3 -4.4 -3.5 0.5 1.0 gja.la nacional-tamesis 7.3 0.0 2.8 0.0 3.1 1.2 5.0 2.3 -5.0 -2.2 -5.0 -1.9 -3.8 hda.montenegro-valparaiso 8.2 0.1 3.0 0.0 4.1 2.0 5.4 2.8 -5.3 -2.4 -5.4 -1.3 -3.4 mesopotamia-la union 7.1 0.0 0.3 0.0 0.2 2.0 3.9 3.2 -3.9 -3.6 -3.9 -3.7 -1.9 ita andes-andes 8.1 0.0 3.6 0.0 3.9 3.1 4.7 3.4 -4.7 -1.1 -4.7 -0.8 -1.6 el rosario-venecia 8.1 0.0 2.2 0.0 2.4 2.2 4.7 3.4 -4.7 -2.5 -4.7 -2.3 -2.5 la plata-venecia 7.0 0.0 4.6 0.0 4.3 2.0 5.4 1.6 -5.4 -0.8 -5.4 -1.1 -3.4 concordia-concordia 4.6 0.0 0.5 0.0 1.0 0.4 2.1 2.5 -2.1 -1.6 -2.1 -1.1 -1.7 hda.cotove-s.f de antioquia 8.0 0.1 1.5 0.0 2.2 0.6 4.8 3.2 -4.7 -3.3 -4.8 ·2.6 -4.2 hda.piunti-buritica 3.5 0.0 19 0.7 0.0 0.0 3.2 0.3 -3.2 ·1.3 -25 -3.2 -3.2 sta.isabel-sta.isabel 5.8 1.5 4.0 0.8 1.7 3.1 4.1 1.7 -2.6 -0.1 -3.3 -2.4 -1.0 apto.olaya herrera-medellin 6.0 0.1 1.5 0.0 3.2 1.2 4.2 1.8 -4.1 ·2.7 -4.2 -1.0 -3.0 tullo ospin"'-bello 6.4 0.0 1.2 2.0 0.7 4.5 1.9 -4.5 -3.3 -2.5 -3.8 aragon-santa rosa de os os 6.5 0.0 0.1 0.0 2.5 0.1 3.9 2.6 -3.9 -3.8 -3.9 -1.4 -3.8 guayabito-santo domingo 7.0 1.0 2.8 0.0 3.8 0.8 4.7 2.3 -3.7 -1.9 ·4.7 -0.9 -3.9 el vivero-guarne 4.2 0.0 0.0 0.0 2.0 0.3 2.3 1.9 -2.3 -2.3 -2.3 -0.3 -2.0 la salada-caldas 6.0 0.0 0.7 0.0 5.6 0.9 3.8 2.2 -3.8 -31 -3.8 18 -2.9 anori-anori 3.6 0.4 4.9 0.5 0.8 2.3 3.6 0.0 -32 1.3 3.1 -28 -1.3 arauca sara vena-sara vena 4.2 2.3 3.5 3.1 3.8 0.0 4.7 -0.5 -2.4 -1.2 -1.6 -0.9 -4.7 apto.arauca-arauca 9.2 9.8 10.7 4.1 7.8 0.0 6.8 2.4 3.0 3.9 -2.7 1.0 -6.8 atlantico repelon-repelon 9.6 8.8 2.2 5.6 4.5 7.0 6.8 2.8 2.0 -4.6 -1.2 -2.3 0.2 normal mana ti-mana ti 9.4 8.3 2.3 4.0 3.6 5.0 6.6 2.8 1.7 -4.3 -2.6 ·3.0 -1.6 el limon-mana ti 9.2 7.8 20 4.0 5.1 6.6 6.6 2.6 1.2 -46 -2.6 -1.5 0.0 gja.sta.lucia-santa lucia 8.2 7.0 2.6 4.8 3.5 8.0 6.1 2.1 0.9 -3.5 -1.3 -2.6 1.9 apto.e.cortissoz-soledad 10.6 9.3 5.5 5.8 70 9.2 7.2 3.4 2.1 ·1.7 -1.4 -0.2 2.0 base naval-barranquilla 9.0 2.9 4.4 6.2 8.2 6.6 2.4 -3.7 -2.2 -0.4 1.6 las florez-barranquilla 9.5 9.2 4.1 5.4 7.1 7.8 6.9 2.6 2.3 -2.8 -1.5 0.2 0.9 bolivar galerazamba-santa catalina 10.3 7.4 0.8 3.3 7.6 9.8 7.4 2.9 0.0·6.6 -4.1 0.2 2.4 apto.rafael nunez-cartagena 10.6 9.8 1.7 4.3 8.0 10.4 7.7 2.9 2.1 -6.0 -3.4 0.3 2.7 escuela naval-cartagena 10.1 8.7 2.4 6.7 10.3 7.5 2.6 1.2 -5.1 -0.8 2.8 islas del rosario-cartagena 10.2 6.1 1.5 4.0 3.7 9.5 7.2 3.0 -1.1 -5.7 -3.2 ·3.5 2.3 sta.rosa de simiti-simiti 6.7 4.8 3.0 2.1 1.2 5.0 5.4 1.3 -06 -2.4 -3.3 -4.2 -0.4 canelos-simiti 6.1 3.3 2.7 0.4 15 4.2 19 -0.9 -1.5 -38 -2.7 carmen de bolivar-c. de b. 8.2 6.3 2.7 4.6 6.7 9.1 6.3 1.9 0.0 -3.6 ·1.7 0.4 2.8 presa arroyo grande-m.la baja 8.6 3.7 1.9 4.8 7.5 89 6.1 2.5 -2.4 -4.2 -1.3 1.4 2.8 nueva florida-maria la baja 9.3 5.4 2.3 6.8 7.8 6.7 6.5 2.8 -1.1 -4.2 0.3 1.3 0.2 san pablo-maria la baja 9.7 5.9 2.1 6.7 4.4 5.9 6.3 3.4 -0.4 -4.2 0.4 ·1.9 -0.4 boyaca esclusa tolon-chiquinquira 8.5 6.7 1.8 0.0 4.5 1.2 3.3 5.2 3.4 -1.5 ·3.3 1.2 ·2.1 villa carmen-samaca 7.5 1.8 0.2 3.5 6.9 5.3 2.2 -3.5 -5.1 -1.8 1.6 gachaneca-samaca 9.8 4.8 1.7 1.1 3.2 3.8 4.4 5.4 0.4 -2.7 -3.3 -1.2 -0.6 villa de leyva-villa de leyva 6.6 6.2 2.2 0.0 3.0 2.5 4.1 2.5 2.1 -1.9 -4.1 -1.1 -1.6 la sierra-duitama 4.4 1.8 0.0 1.0 5.4 3.8 0.6 -2.0 -3.8 -2.8 1.6 surbata bonza-duitama 5.9 7.6 3.3 0.0 0.0 0.0 4.8 1.1 2.8 -1.5 -4.8 -4.8 -4.8 univ.pedag.tec de col-tunja 8.3 6.8 2.0 1.1 3.9 3.6 4.8 3.5 2.0 -2.8 -3.7 -0.9 -1.2 belencito-nobsa 8.0 8.1 0.0 2.8 2.0 4.2 3.8 3.9 -4.2 -1.4 -2.2 tunguavita·paipa 5.8 6.0 2.6 0.6 3.5 0.8 4.6 1.2 1.4 -2.0 -4.0 -1.1 -3.8 san rafael-tibasosa .5.9 6.8 2.2 0.0 3.3 2.5 4.1 1.8 2.7 -1.9 -4.1 -08 -1.6 sierra nevada cocuy-guican 0.6 1.1 32 -2.6 -2.1 chiscas-chiscas 6.3 6.8 3.9 0.2 3.1 3.8 4.9 1.4 1.9 -1.0 -4.7 -1.8 -1.1 boavita-boavita 6.6 6.2 4.5 0.2 5.7 2.3 5.1 1.5 1.1 -0.6 -4.9 0.6 -2.8 aptoalleras.c.-sogamoso 10.2 8.6 2.5 0.0 0.0 1.6 5.0 5.2 3.6 -2.5 -5.0 -5.0 -3.4 nuevo colon-nuevo colon 9.3 5.7 3.2 0.4 2.7 4.0 5.1 4.2 0.6 -1.9 -4.7 -2.4 -1.1 sutatenza-sutatenza 6.3 3.7 5.9 0.0 2.3 0.9 4.7 1.6 -1.0 1.2 -4.7 -2.4 -3.8 rondon-rondon 6.2 4.0 4.4 0.0 2.2 0.4 4.2 2.0 ·0.2 0.2 -4.2 -2.0 -3.8 el tunel-cuitiva 9.5 7.9 2.5 3.9 4.6 0.9 5.4 4.1 2.5 -2.9 ·1.5 -0.8 -4.5 caldas samana-samana 2.5 0.1 0.8 0.1 0.3 0.3 2.2 0.3 -2.1 -1.4 -2.1 -1.9 ·1.9 santa helena-marquetalia 6.0 2.4 0.1 0.8 1.7 2.1 3.7 2.3 -1.3 -3.6 -3.1 -2.0 -1.6 naranjal-chinchina 7.2 4.0 5.2 0.0 4.0 3.0 5.0 2.2 ·1.0 0.2 -5.0 -1.0 ·2.0 cenicafe-chinchina 7.5 4.5 4.9 5.3 2.2 -0.8 -0.4 fac.agronomia-manizales 6.1 3.9 3.0 0.0 3.5 23 43 1.8 -0.4 -1.3 -4.3 -0.8 -2.0 santagueda f.-palestina 6.9 4.8 5.5 0.0 4.8 5.2 5.8 1.1 -1.0 -0.3 -5.8 -1.0 ·0.6 apto.la nubia-manizales 4.8 4.4 1.4 0.0 2.3 1.0 4.7 0.1 -0.3 -3.3 -4.7 -2.4 -3.7 granja luker-palestina 6.7 4.4 5.3 0.0 4.0 4.6 5.0 1.7 -0.6 0.3 ·5.0 -1.0 -0.4 -------------------------------------------continua geofisica colombiana n° 2, abril de 1993 41 tabla 1 {conllnuacl6nl' insolacl6n (brlllo solar en horas y declmo~ reglslrada en eslaclones meleorol6glcas ublcadas en colombia; an es, en y despues de la erupcl6n del volc'n evado del rulz del 13 de novlembre lie 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14·m 15-m 16-m las brisas-villamaria 1,5 0.7 2.2 8.2 4,2 2,7 ·3,5 ·2.0 4.0 rafael escobar-supia 7.4 1.0 4.0 0,0 4.2 2.4 5.0 2.4 -4.0 ·1.0 ·5.0 -a.8 ·2.6 san fellx·salamina 4,6 3.0 0.6 0.0 0.3 0.4 3.6 1.0 -a.6 ·3.0 -3.6 -3.3 ·3.2 caqueta apto.g.artunduaga·florencia 6.4 7.6 3.3 0.0 4.4 2.1 5.7 0.7 1.9 -2.4 -5.7 ·1.3 ·3.6 macagual-florencia 5,8 7.5 3.5 0.0 5.5 0,8 5.9 -0.1 1.6 -2.4 -5.9 ·0.4 -5.1 valparaiso-florencia 8,6 2.5 0,0 3.6 0.1 6.5 2.1 -4.0 -6.5 ·2.9 -6.4 la mono-belen de los andaquies 6.4 8.3 4.1 4.5 0.1 5.8 0.6 2.5 -1.7 ·1.3 -5.7 san vicente del caguan= 4,9 10.6 3.2 0,0 1.9 3,9 5.9 -1.0 4.7 -2.7 ·5.9 -4.0 -2.0 maguare-doncello 6.3 4,0 0.0 1.9 07 5.4 0,9 .1.4 -5.4 -3.5 -4.7 puerto rico-puerto rico 5.8 7,0 1,5 0.0 5.9 0.0 4.9 0.9 2.1 -3.4 -4.9 1.0 -4.9 casanare aguazul-aguazul 9.2 5,6 3,8 0,7 3.9 2.7 5,6 3.6 0.0 ·1.8 -4.9 ·1.7 -2.9 apto.yopal-yopal 10.2 5.2 5,7 1.4 3.5 1.5 5.2 5.0 0.0 0.5 ·3.8 ·1.7 -3.7 modulos-orocue 8,1 4.9 9,5 0,3 4.0 0.0 6.4 1.7 -1.5 3.1 ·6.4 ·2.4 -6.4 cauca santa leticia-purace 7.9 6.4 0.4 0.9 0.0 3.9 4.0 2.5 -3.5 -3.0 ·3.9 paletara-coconuco 8,3 2.9 1.5 0.0 3.3 4,1 4.0 4,3 ·1.1 -2.5 -4.0 -0.7 0.1 gabriel lopez-totoro 5,0 2,5 0.0 0,0 2.0 3,1 3.6 1.4 ·1.1 ·3.6 -3.6 -1.6 ·0.5 venta de cajibio-cajibio 29 1,9 0.0 0.1 2.1 3,1 3.3 ·0.4 -1.4 -3,3 -3.2 -1.2 ·0.2 apto.machangara-popayan 4.2 2.1 0.8 0.0 4.2 1.1 4.5 -03 -2.4 -3.7 -4.5 -0.3 -3.4 la florida-popayan 5.3 3,4 0.9 0.0 2.1 3.5 4.8 0,5 ·1.4 -3.9 -4.8 -2.7 -1.3 manuel mejia·el tambo 8,1 5.4 3.4 0.9 5,6 5.5 5.6 25 -0.2 ·2.2 -4.7 0.0 ·0.1 ing.cauca-miranda 7,0 3.6 0.8 0.0 3.8 3.9 5.0 2,0 ·1.4 -4.2 -5.0 -1.2 -1.1 miranda-miranda 4,7 2.4 0.3 0.0 2.7 1.9 4.4 0,3 -2.0 -4.1 -4.4 -1.7 -2.5 valencia-san sebastian 46 2,1 0.2 0.0 0.8 2,2 3.6 1.0 -1.5 -3.4 ·3.6 -2.8 -1.4 la fonda-el bordo 6.8 6,8 1.2 0.2 5.3 5.6 6.3 0.5 0.5 -5,1 ·6.1 ·1.0 -0.7 bolivar-bolivar 5.6 0,9 1.7 0.1 1.7 2.3 5.0 0.6 -4.1 -3.3 -4.9 -3.3 ·2.7 los milagros-bolivar 5,2 4.2 0.1 0,0 2.7 6.6 5,1 0,1 -0.9 -5.0 ·5.1 ·2.4 1.5 mercaderes-mercaderes 7.3 4.5 2.1 0.0 6.1 4.4 5.9 1.4 -1.4 -3.8 ·5.9 0.2 -1.5 la sierra-la sierra 3,0 0.8 0.0 1.5 1.1 4.3 -1.3 ·3.5 -4.3 -2.8 -3.2 bonanza·guapi 2,9 1,1 2.3 0.0 2,3 2,9 0.0 -1.8 ·0.6 -2.9 ·0.6 cesar la mata-la gloria 8,0 3,9 6.6 1.8 1.5 1,8 4.1 3.9 -02 2.5 -2.3 -2.6 -2.3 chiriguana-chiriguana 8,5 9.0 3.5 9,2 7.8 0.7 1.2 -4,3 1.4 la gloria-la gloria 93 5.6 8.7 5.0 2.8 7,0 6.6 2.7 ·1.0 2.1 -1.6 -3.8 0.4 s,j. de oriente-robles-la paz 7,3 8.0 5.4 6.7 6.3 6.1 5.9 1.4 2.1 ·0.5 0.8 0.4 0.2 motilonia codazzi-codazzi 9,3 8.3 6.7 5.7 5,3 9.6 6,7 2.6 1.6 0.0 ·1.0 -1.4 2.9 socomba-becerril 8.7 8,2 5.0 7.1 6.0 2.7 2.2 -1.0 1.1 villarosa·valledupar 7.8 7,9 5.6 5.4 5.9 8.3 6.5 1.3 1.4 -0.9 -1,1 ·0.6 1.8 el callao-valledupar 9.5 8,6 7.4 6.0 6,2 8.8 7.5 2.0 1.1 -0.1 ·1.5 ·1.3 1.3 guaymaral·valledupar 100 6.3 5.8 3.6 9.2 7.5 2.5 -1.2 ·1.7 ·3.9 1.7 pueblo bello-pueblo bello 8.1 7.3 4.1 3.7 6,2 6.1 6.1 2.0 1.2 -2.0 ·2.4 0.1 0.0 choco la mansa-carmen de atrato 6,5 0,0 0.6 0.0 1.6 1.1 3.1 3.4 -3,1 ·2.5 -3.1 -1.5 -2.0 apto.el carano-quibdo 1.4 0,0 0.8 0.0 1.1 1.2 3.2 -1.8 -3.2 ·2.4 -3.2 -2.1 ·2.0 la teresita·riosucio 3,0 0.0 0.0 2.4 0.0 1.4 3.3 ·0.3 -3,3 -3.3 -0.9 -3.3 -1.9 san j. del palmar-s,j,del pal 2,2 0.0 0.0 0.0 0.3 0.3 1.6 0.6 -1,6 ·1,6 ·1.6 -1.3 -1.3 apto.condoto·condoto 0,0 0.0 0.4 0.0 2.1 1.9 2,9 -2.9 -2.9 -2.5 ·2.9 -0.8 -1.0 noanama·istmina 0.4 0.0 0.0 0.0 0.0 1.2 2,1 -1.7 ·2.1 -2.1 -2.1 ·2.1 -0.9 panamericana-bahia solano 0.1 0.0 2.8 0.7 0.0 2,3 ·2.2 -2.3 0.5 -1.6 -2.3 cordoba puerto nuevo-tierralta 6,5 0.3 6.2 4.2 3.2 4.9 1.6 -4,6 1.3 ·0.7 -1.7 turipana-cerete 8.1 3.0 5.2 1.8 7.2 8,5 6.0 2.1 -3.0 -0.8 -4,2 1.2 2,5 univ.de cordoba-monteria 8.3 3.2 7.1 2.9 7.3 6.5 6.7 1.6 -3.5 0.4 -3.8 0.6 ·0.2 la doctrina·lorica 9,5 3.0 4.3 3.0 9.4 7.7 6.2 3.3 ·3.2 -1.9 ·3.2 3.2 1.5 ayapel-ayapel 8.9 2.0 4.7 1.9 5.4 4,7 5.4 3.5 -3.4 ·0,7 -3.5 0.0 -0.7 planeta rica-planeta rica 8.7 4.2 2.0 4.2 7.2 5.5 3.2 -1.3 -3.5 -1.3 1.7 cundinamarca alberto williamsontibacuy 7.7 3,8 2.7 1.8 1.7 2.4 4.1 3.6 -0.3 ·1.4 -2.3 ·2.4 -1.7 pandi-pandi 8.6 2.8 4,2 0.0 1.6 2,5 5.2 3.4 ·2.4 -1,0 ·5.2 -3.6 ·2.7 pasca·pasca 3.6 6.1 1,8 3.7 0.5 1.2 3.4 0.2 2.7 ·1,6 0.3 ·2.9 -2.2 tibaitata·mosquera 5.7 3.8 4.5 0.0 2.4 1.1 4.6 1.1 ·0.8 -0.1 -4.6 -2.2 ·3.5 las mercedes·anapoima 5.8 4.1 3.5 0.0 4.6 2.7 4,9 0.9 -0.8 -1.4 -4.9 ·0.3 -2.2 granja san jorge-soacha 5.8 1.6 2.2 0.0 0,5 1.2 3.5 2.3 -1.9 ·1.3 -3.5 ·3,0 -2.3 ~ilos-choconta 8.3 52 3.0 0.0 3.8 5.2 5.0 3.3 0.2 -2.0 ·5,0 -1.2 0.2 base aerea madrid-madrid 6.3 2,2 4.8 0.0 2,7 1.3 4.1 2.2 .1.9 0.7 -4.1 -1.4 ·2,8 misiones-mesitas del colegio 7.7 1.3 2.1 0.0 0.9 0.3 3.4 3.3 ·2.1 -2.3 -3.4 ·2.5 -3.1 mesitas de sta.ines·cachipay 6.0 1,7 1.7 0.1 3,8 1.5 4.2 1.8 ·2.5 -2.5 -4.1 ·0.4 -2.7 apto.palanquero-puerto salgar4.7 2.6 3,5 0.0 2.1 4.1 5.1 ·0.4 ·2.5 -1.6 ·5.1 -3.0 -1.0 yacopiyacopi 5.4 4.6 4.3 0.0 1.2 0.5 4.8 0.6 ·0.2 -0,5 -4.8 -3.6 -4.3 montelibano-yacopi 6.3 3.4 4.6 0.0 1.2 0.2 4.4 1.9 -1.0 0.2 -4.4 -3.2 -4.2 san cayetano-san cayetano 6,8 4.3 1.4 0.4 0.4 0.1 3.3 3.5 1.0 ·1.9 -2,9 ·2.9 -3.2 isla del santuario-fuquene 7.1 6.8 4.1 0.3 7.0 3.4 5.5 1.6 1.3 -1.4 -5.2 1.5 -2,1 el japon-medina 3.7 1.8 5.7 0.2 2.4 0.9 3.8 ·0,1 ·2.0 1.9 -3.6 ·1.4 ·2.9 gacheta-gacheta 4.8 2.1 4.4 0.2 3.6 3.3 4.5 0,3 -2.4 ·0.1 -4,3 -0.9 -1.2 guajira 6.8 0.8 0.7 -4.4 0.2ma titas-riohacha 7.6 7.5 2.4 7.0 6.4 -0.4 la paulina-fonseca 9.2 9.5 8.9 8.4 8,8 9,5 8.0 1.2 1.5 0.9 0.4 0.8 1.5 manaljre-manaure 10.2 8.6 7,2 4.6 8,2 8.4 7.6 2.6 1.0 -0,4 -3.0 0.6 0.8 apto.maicao-maicao 9.8 9,2 6.0 6.7 4,7 8,2 6.8 3.0 2.4 -0.8 -0.1 -2.1 1.4 nazareth-uribia 8.0 6,9 6.5 5,9 3,6 2,8 5.7 2.3 1.2 0.8 0.2 -2.1 -2.9 esc.agr.carraipia·maicao 7.0 7.8 4.8 5.6 5.4 8.2 6.0 1.0 1.8 ·1.2 ·0.4 -0.6 2.2 urumita·urumita 7,3 6.4 5.7 6.0 5.5 6.6 0.7 -0.2 -0.9 ·0.6 .1.1 guaviare 5.0 -5.0 0.8 -0.6 -4.7 -4,6 san j. de guaviare-sn j.de g, 0.0 5.8 4.4 0.3 0.4 0.5 -4.5 continua 42 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n tabla 1 (contlnuac'6nl' inaolacl6n (brilio solar en horas r declmo~ reglatrada en eataclonea meteorol6glcaa ublcadas en colombia; an aa, an y deapuea da la arupcl6n dal volc'n avado del rulz del 13 da novlambra da 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m huila sevilla-pitalito 6.7 7.6 3.5 0.2 4.3 0.6 4.4 2.3 3.2 ·0.9 -4.2 ·0.1 ·3.8 parque arq.·san agustin 7.7 7.2 3.1 0.4 5.1 0.1 5.1 2.6 2.1 ·2.0 -4.7 0.0 ·5.0 altamira-altamira 8.8 6.9 4.8 0.0 5.3 1.6 5.5 3.3 1.4 -0.7 ·5.5 ·0.2 ·3.9 resina-guadalupe 3.2 5.7 0.8 0.2 7.0 1.0 3.6 ·0.4 2.1 ·2.8 ·3.4 3.4 ·2.6 cueva los guacharos-acevedo 3.9 7.1 3.8 0.0 2.8 0.5 3.3 0.6 3.8 0.5 ·3.3 ·0.5 ·2.8 esc.agr.la plata-la plata 8.2 8.2 3.5 0.0 4.2 2.4 4.8 3.4 3.4 ·1.3 -4.8 ·0.6 ·2.4 jorge villamil-gigante 4.3 5.9 2.4 0.0 3.1 0.7 4.0 0.3 1.9 ·1.6 -4.0 ·0.9 ·3.3 iquira-iquira 6.4 7.3 2.1 0.1 4.4 2.2 4.7 1.7 2.6 ·2.6 -4.6 ·0.3 ·2.5 el juncal·palermo 7.0 10.0 4.5 0.0 6.4 3.0 6.3 0.7 3.7 ·1.8 ·6.3 0.1 ·3.3 algeciras·algeciras 7.5 7.3 2.7 0.1 3.3 2.8 4.6 2.9 2.7 ·1.9 -4.5 ·1.3 ·1.8 apto.benito salas-neiva 8.3 10.2 3.4 0.0 6.2 1.9 5.5 2.8 4.7 ·2.1 • ·5.5 0.7 ·3.6 palacio vegalarga·neiva 7.1 9.2 0.0 0.0 2.1 1.7 4.1 3.0 5.1 -4.1 -4.1 ·2.0 ·2.4 villaranza-neiva 7.8 9.3 4.3 0.0 4.8 3.3 5.4 2.4 3.9 ·1.1 ·5.4 ·0.6 ·2.1 santa maria-santa maria 7.3 6.1 1.2 0.0 1.7 1.1 3.9 3.4 2.2 ·2.7 ·3.9 ·2.2 ·2.8 san alfonso-villavieja 8.7 6.0 0.5 0.0 6.9 0.4 4.6 4.1 1.4 -4.1 -4.6 2.3 -4.2 la legiosa-colombia 7.3 8.0 4.8 0.0 6.2 1.4 4.7 2.6 3.3 0.1 -4.7 1.5 ·3.3 magdalena la ye-cienaga 10.5 9.3 3.4 5.5 6.2 9.5 7.2 3.3 2.1 ·3.8 ·1.7 ·1.0 2.3 apto.simon bolivar-sta marta 10.5 10.1 4.9 6.0 7.8 9.6 7.9 2.6 2.2 ·3.0 ·1.9 ·0.1 1.7 san lorenzo-santa marta 4.9 2.1 0.6 0.3 0.6 1.0 2.0 2.9 0.1 ·1.4 ·1.7 ·1.0 ·1.0 parque tayrona·santa marta 8.0 6.9 5.3 5.5 6.9 6.8 6.3 1.7 0.6 ·1.0 ·0.8 0.6 0.5 apto.las flores-el banco 9.7 7.6 7.0 6.2 1.4 7.1 6.2 3.5 1.4 0.8 0.0 -4.8 0.9 los alamos-san sebastian 10.0 6.9 5.7 4.3 5.5 7.5 6.4 3.6 0.5 ·0.7 ·1.9 ·0.9 1.1 algarrobo-fundacion 7.8 8.2 3.1 3.9 6.2 7.5 6.0 1.8 2.2 ·29 ·2.1 0.2 1.5 zacapa-araca taca 9.3 8.6 3.3 5.6 3.7 9.3 6.2 3.1 2.4 ·2.9 ·0.6 ·2.5 3.1 patuca-cienaga 9.6 8.7 2.1 5.7 5.2 7.8 6.4 3.2 2.3 -4.3 ·0.7 ·1.2 1.4 prado sevilla-cienaga 9.7 9.1 3.7 6.4 5.2 6.7 6.6 3.1 2.5 ·2.9 ·0.2 ·1.4 0.1 meta la macarena·la macarena 0.8 6.9 0.9 0.0 1.8 1.1 4.6 ·3.8 2.3 ·3.7 ·4.6 ·2.8 ·3.5 vista hermosa-vista hermosa 4.8 5.3 0.8 0.0 2.6 2.7 4.4 0.4 0.9 ·3.6 -4.4 ·1.8 ·1.7 la holanda-granada 4.7 7.5 6.9 0.1 4.7 2.0 4.8 ·0.1 2.7 2.1 -4.7 ·0.1 ·2.8 mesetas·mesetas 3.9 8.2 3.5 0.0 5.8 2.0 3.9 0.0 4.3 ·0.4 ·3.9 1.9 ·1.9 carimagua-puerto gaitan 8.9 2.2 8.9 6.7 2.2 -4.5 2.2 san carlos de guaroa·s.c.de g. 5.7 7.0 8.1 1.0 1.8 1.5 5.3 0.4 1.7 2.8 -4.3 ·3.5 ·3.8 la l1bertad-villavicencio 6.2 6.4 6.2 0.8 4.0 0.9 4.5 1.7 1.9 1.7 ·3.7 ·0.5 ·3.6 pachaquiaro·puerto lopez 7.7 7.4 0.9 2.8 5.8 5.3 2.4 2.1 -4.4 ·2.5 0.5 univ.del llano-villavicencio 4.6 6.1 0.2 4.0 3.8 4.5 0.1 16 -4.3 ·0.5 ·0.7 hda.la cabana-cumaral 4.7 4.9 5.3 0.6 2.9 1.1 4.2 0.5 0.7 ·1.1 ·3.6 ·1.3 ·3.1 barranca de upia-cumaral 4.2 2.9 4.7 0.0 2.0 0.4 4.1 0.1 ·1.2 0.6 -4.1 ·2.1 ·3.7 narind monopamba-puerres 2.1 2.8 2.0 0.7 3.6 2.1 4.3 ·2.2 ·1.5 ·2.3 ·3.6 ·0.7 ·2.2 el encano-pasto 3.9 1.1 0.6 0.0 1.4 2.5 4.3 ·0.4 ·3.2 ·3.6 -4.3 ·3.9 ·1.8 gja.el mira·tumaco 0.2 0.4 0.4 3.5 0.1 0.5 2.4 ·2.2 ·2.0 ·2.0 1.1 ·2.3 ·1.9 ricaurte·ricaurte 0.6 1.1 0.0 0.0 3.2 1.1 3.3 ·2.7 ·2.2 ·3.3 ·3.3 ·0.1 ·2.2 apto.la florida-tumaco 1.3 0.0 0.1 4.9 4.9 2.2 4.1 ·2.8 -4.1 -4.0 0.8 0.8 ·1.9 la union·la union 3.6 2.4 1.3 0.0 1.0 5.3 5.6 ·2.0 ·3.2 -4.3 ·5.6 ·4.6 ·0.3 obonuco-pasto 4.5 3.4 0.6 0.0 1.3 2.6 4.3 0.2 ·0.9 ·3.7 -4.3 ·3.0 ·1.7 apto.antonio narino-pasto 5.5 3.3 0.9 0.0 1.8 1.3 5.6 ·0.1 ·2.3 -4.7 ·5.6 ·3.8 -4.3 apto.san luis·ipiales 5.7 2.1 0.2 0.6 2.5 5.8 5.2 0.5 ·3.1 ·5.0 ·4.6 ·2.7 0.6 el paraiso·tuquerres 6.5 1.3 0.4 0.0 1.7 6.2 5.0 1.5 ·3.7 -4.6 ·5.0 ·3.3 1.2 bombona·consaca 5.7 3.0 0.4 0.0 4.4 5.9 ·0.2 ·2.9 ·5.5 ·5.9 ·1.5 botana·pasto 6.6 3.9 0.0 0.0 1.1 6.6 5.3 1.3 ·1.4 ·5.3 ·5.3 -4.2 1.3 ospina perez-consaca 7.1 4.0 1.6 0.0 1.6 3.6 5.8 1.3 ·1.8 -4.2 ·5.8 -4.2 ·2.2 barbacoas-barbacoas 0.6 0.1 0.3 1.0 0.5 1.9 3.1 ·2.5 ·3.0 ·2.8 ·2.1 ·2.6 ·1.2 norte de santander apto.camilo daza-cucuta 5.0 2.2 5.3 3.6 4.2 5.1 4.7 0.3 ·2.5 0.6 ·1.1 ·0.5 0.4 iser-pamplona 5.5 6.3 5.0 1.6 2.9 1.7 3.3 2.2 3.0 1.7 ·1.7 ·0.4 -1.6 santa isabel-cucuta 2.3 4.7 0.8 2.7 1.2 3.1 3.8 ·1.5 0.9 ·30 ·1.1 ·2.6 ·0.7 blonay-chinacota 3.4 2.6 4.0 1.5 2.7 1.0 2.4 1.0 0.2 1.6 ·0.9 0.3 ·1.4 carmen de tonchala·cucuta 1.1 1.8 4.1 1.7 1.7 2.9 3.5 ·2.4 ·1.7 0.6 ·1.8 ·1.8 ·0.6 risaralda-zulia 0.9 0.2 0.7 0.8 0.0 0.8 0.1 -o.e ·0.1 0.0 ·0.8 cinera villa olga-cucuta 10.1 4.0 8.4 5.4 0.6 0.0 3.6 6.5 0.4 4.8 1.8 ·3.0 ·3.6 francisco romero-salazar 2.1 1.0 0.7 1.2 1.6 4.6 2.8 ·0.7 ·1.8 ·2.1 ·1.6 ·1.2 1.8 tlbu-tibu 3.7 2.8 1.8 3.1 0.6 ·0.3 ·1.3 apto.aguas claras-ocana 2.7 3.4 5.1 2.8 1.9 3.4 4.5 ·1.8 ·1.1 0.6 ·1.7 ·2.6 .1.1 centro adm.abrego·abrego 7.3 5.0 2.2 3.8 2.0 3.0 4.6 2.7 0.4 ·2.4 ·0.8 ·2.6 ·1.6 la playa-la playa 0.0 3.6 1.9 0.0 0.0 1.6 3.6 ·3.6 0.0 ·1.7 ·3.6 ·3.6 ·2.0 esc.agrcachira·cachira 6.8 4.9 3.5 0.5 1.9 4.5 4.0 2.8 0.9 ·0.5 ·3.5 ·2.1 0.5 silos-silos 7,.6 5.3 3.3 0.3 2.5 1.5 3.0 4.6 2.3 0.3 ·2.7 ·0.5 ·1.5 putumayd villa garzon·villa amazonica 5.3 5.5 4.2 0.0 3.3 0.5 5.1 0.2 0.4 ·0.9 ·5.1 ·1.8 -4.6 acueducto mocoa-mccoa 6.4 5.7 3.7 2.5 0.0 5.0 1.4 0.7 ·1.3 ·2.5 ·5.0 sibundoy-sibundoy 7.2 2.5 2.6 0.0 2.7 0.2 4.6 2.6 ·2.1 ·2.0 ·4.6 ·1.9 -4.4 michoacan-sibundoy 3.8 2.3 0.3 0.0 1.3 0.0 3.7 0.1 ·1.4 ·3.4 ·3.7 ·2.4 ·3.7 pto leguizamq.pto leguizamo 1.1 5.9 0.4 0.0 5.8 -4.7 0.1 ·5.4 -5.8 quindid maracay-quimbaya 5.6 3.8 2.2 0.0 3.8 3.0 4.8 0.8 ·1.0 ·2.6 ·4.8 ·1.0 ·1.8 la bella·calarca 4.8 2.0 1.6 0.0 2.1 2.9 4.0 0.8 ·2.0 ·2.4 ·4.0 ·1.9 ·1.1 paraguaycito-buenavista 6.9 3.3 3.8 0.0 3.8 3.7 5.0 1.9 ·1.7 ·1.2 ·5.0 ·1.2 ·1.3 el sena-armenia 2.5 1.4 1.7 0.0 1.2 0.5 3.4 ·0.9 ·2.0 ·1.7 ·3.4 ·2.2 ·2.9 risaralda la joya-pereira 5.2 4.7 4.0 0.0 5.0 6.0 5.2 0.0 ·0.5 ·1.2 ·5.2 ·0.2 0.8 apto.ma tecana-pereira 5.2 3.5 5.5 0.0 5.5 3.6 5.0 0.2 ·1.5 0.5 ·5.0 0.5 ·1.4 el jazmin-santa rosa 4.9 2.9 1.9 0.0 1.7 3.3 3.7 1.2 ·0.8 ·1.8 ·3.7 ·2.0 ·0.4 el cedral-pereira 2.4 2.0 2.5 0.0 1.2 1.0 2.5 ·0.1 ·0.5 0.0 ·2.5 ·1.3 ·1.5 la bohemia-pereira 6.4 5.0 3.3 0.0 4.7 5.2 5.6 0.8 ·0.6 ·2.3 ·5.6 ·0.9 ·0.4 continua geofislca colomblana n° 2, abril de 1993 43 tabla 1 (continuacl6nt. insolacl6n (brlllo solar en horal y diclmo~ reglltrada en eltaclonel meteorol6glcal ublcadal en colombia; an el, en y delpuil de la erupcl6n del volcen evado del rulz del 13 de novlembre de 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m veracruz-sta.rosa de cabal 3.4 3.4 3.7 0.0 1.3 2.8 3.7 -0.3 -0.3 0.0 -3.7 -2.4 ·0.9 la camella-santuario 4.4 0.9 0.4 1.7 7.3 2.9 4.0 0.4 ·3.1 ·3.6 -2.3 3.3 -1.1 pueblo rico-pueblo rico 3.8 0.0 0.6 0.0 0.4 1.0 3.2 0.6 ·3.2 ·2.6 -3.2 -2.8 ·2.2 apto.sesquicentenar-s.andres san andres y prdvidencia 9.3 8.2 3.7 7.7 9.6 10.1 5.6 3.7 2.6 -1.9 2.1 4.0 4.5 apto.providencia-providencia 9.8 5.7 6.7 7.6 8.8 9.5 6.0 3.8 -0.3 0.7 1.6 3.5 3.5 santafe de bogota d.c eldorado didactica 5.7 3.5 4.3 0.0 2.9 0.7 4.3 1.4 ·0.8 0.0 -4.3 -1.4 ·3.6 vivero venado de oro 4.5 1.9 0.0 0.0 5.3 2.8 1.7 -0.9 -2.8 ·2.8 2.5 apto.eldorado pista 1-2 5.5 4.1 4.6 0.0 3.9 0.4 4.8 0.7 -0.7 -0.2 -4.8 -0.9 -4.4 santander campo capote-velez 5.7 2.4 4.0 0.1 6.2 3.0 4.4 1.3 -2.0 ·0.4 -4.3 1.8 ·1.4 albania-albania 5.4 1.2 0.3 0.0 2.2 0.1 3.4 2.0 ·2.2 -3.1 ·3.4 -1.2 ·3.3 cimitarra-cimitarra 7.9 2.1 5.3 0.4 7.2 2.5 5.8 2.1 ·3.7 -0.5 -5.4 1.4 ·3.3 apto.yariguies-barrancabermej 6.5 3.1 3.6 0.3 8.6 6.8 5.9 0.6 -2.8 -2.3 -5.6 3.0 0.9 villa leiva-sabana de torres 7.3 5.6 4.2 0.0 7.0 3.2 4.8 2.5 0.8 -0.6 -4.8 2.2 -1.6 univ.ind.sant.-bucaramanga 2.7 2.4 1.0 0.0 2.7 6.0 2.7 0.0 -0.3 -1.7 -2.7 0.0 3.3 apto.palonegro-lebrija 4.4 3.2 2.3 0.2 2.5 4.2 3.3 1.1 -0.1 -1.0 ·2.1 -0.8 0.9 provincia-sabana de torres 7.3 6.3 2.1 0.0 2.9 8.7 5.4 1.9 0.9 -3.3 ·5.4 -2.5 3.3 cachiri-surata 4.6 4.9 3.6 0.7 1.7 3.2 3.5 1.1 1.4 0.1 ·2.8 .1.8 -0.3 granja velez-velez 5.8 2.6 3.7 0.0 4.0 4.5 4.7 1.1 ·2.1 ·1.0 -4.7 -0.7 ·0.2 el cucharo-pinchote 0.7 3.7 7.9 8.4 6.4 ·5.7 ·2.7 1.5 2.0 esc.agr.mogotes-mogotes 5.2 4.0 3.7 0.2 4.1 5.9 4.9 0.3 -0.9 ·1.2 -4.7 -0.8 1.0 charala-charala 7.0 6.0 5.2 1.3 4.8 5.4 5.3 1.7 0.7 -0.1 -4.0 ·0.5 0.1 capitanejo-capitanejo 7.5 7.3 6.0 1.0 7.0 7.9 6.2 1.3 1.1 -0.2 -5.2 0.8 1.7 granja t1naga-cerrito 7.1 7.0 4.5 1.2 0.6 3.8 4.1 3.0 2.9 0.4 ·2.9 -3.5 -0.3 zapatoca-zapatoca 5.9 2.5 2.3 1.1 1.7 0.0 4.0 1.9 -1.5 ·1.7 ·2.9 -2.3 -4.0 aguas blancas-san vicente 0.6 2.2 1.9 0.0 1.3 2.7 2.5 -1.9 -0.3 -0.6 -2.5 -1.2 0.2 berlin-tona 6.8 1.0 5.4 1.8 4.4 4.6 2.2 -3.6 0.8 ·2.8 ·0.2 sucre majagual-majagual 7.1 4.8 6.0 1.4 4.7 8.6 5.0 2.1 -0.2 1.0 ·3.6 -0.3 3.6 tolima anchique-natagaima 9.1 6.2 4.6 0.0 5.8 3.3 5.7 3.4 0.5 -1.1 ·5.7 0.1 -2.4 la montana-dolores 5.5 6.8 3.7 4.1 7.0 5.8 5.0 0.5 1.8 -1.3 ·0.9 2.0 0.8 hda.madron-dolores 6.3 6.8 3.3 0.0 5.3 3.4 4.7 1.6 2.1 -1.4 -4.7 0.6 -1.3 luis bustamante-vlllarica 2.7 2.9 1.1 0.0 0.0 0.0 2.0 0.7 0.9 -0.9 -2.0 -2.0 -2.0 lozania-purificacion 8.7 6.6 6.2 0.0 4.2 3.0 5.3 3.4 1.3 0.9 -5.3 ·1.1 -2.3 nataima-espinal 10.7 5.4 3.0 0.0 6.9 0.7 5.3 5.4 0.1 -2.3 ·5.3 1.6 -4.6 guamo-guamo 10.0 6.8 3.4 0.0 6.7 2.3 5.5 4.5 1.3 -2.2 -5.5 1.2 -3.2 apto.santiago vila-flandes 9.8 5.5 5.4 0.0 5.6 4.2 5.8 4.0 -0.3 -0.4 ·5.8 -0.2 -1.6 chapeton-ibague 3.1 3.4 0.2 1.0 0.1 2.3 4.2 ·1.1 -0.8 -4.0 ·3.2 -4.1 -1.9 cajamarca·cajamarca 8.7 5.1 2.1 5.1 0.2 0.0 4.2 4.5 0.9 -2.1 -0.9 -4.0 -4.2 hato de opia perales-ibague 9.1 6.4 1.2 0.0 4.8 1.6 4.9 4.2 1.5 -3.7 -4.9 ·0.1 -3.3 apto.perales-ibague 8.5 5.9 3.6 1.6 4.0 0.1 4.5 4.0 1.4 -0.9 -2.9 ·0.5 -4.4 el sal to-ambalema 8.5 4.2 6.0 0.0 33 3.3 5.4 3.1 ·1.2 0.6 -5.4 -2.1 ·2.1 villa hermosa-villa hermosa 6.2 2.1 2.5 0.0 2.6 0.6 3.3 2.9 -1.2 -0.8 ·3.3 -0.7 ·2.7 la trinidad-l1bano 7.8 2.8 0.6 0.0 1.1 1.4 3.1 4.7 -0.3 -2.5 ·3.1 -2.0 -1.7 la quinta-lerida 8.9 5.3 8.5 0.0 3.1 4.3 5.9 3.0 -0.6 2.6 -5.9 ·2.8 ·1.6 apto.planadas-planadas 8.5 4.5 2.8 0.0 1.8 0.9 3.8 4.7 0.7 ·1.0 -3.8 -2.0 ·2.9 gja.demostracion-chaparral 9.0 4.9 3.5 0.0 4.6 0.3 4.7 4.3 0.2 -1.2 -4.7 -0.1 -4.4 mesa de pole-ataco 6.2 4.1 4.3 0.0 2.2 0.1 3.7 2.5 0.4 0.6 ·3.7 -1.5 -3.6 quinta san antonio-s.antonio 8.2 2.7 1.2 0.0 3.8 0.6 4.5 3.7 -1.8 -3.3 -4.5 ·0.7 ·3.9 el limon-chaparral 6.9 5.3 1.6 0.0 3.1 0.8 3.6 3.3 1.7 -2.0 -3.6 ·0.5 -2.8 la esperanza-honda 6.9 4.2 3.5 0.0 1.4 3.9 5.3 1.6 -1.1 ·1.8 -5.3 -3.9 ·1.4 valle del cauca potrerito-jamundi 7.8 4.5 0.8 0.0 6.7 3.8 5.2 2.6 -0.7 -4.4 -5.2 1.5 ·1.4 univ.del valle-cali 8.2 2.7 1.2 0.0 4.9 4.2 5.0 3.2 -2.3 ·3.8 -5.0 ·0.1 -0.8 palmira ica-palmira 8.8 3.4 2.4 0.0 3.8 4.9 5.1 3.7 -1.7 ·2.7 -5.1 ·1.3 -0.2 aptoabonllla a.-palmira 9.4 4.9 1.9 0.0 5.5 5.5 5.2 4.2 -0.3 ·3.3 -5.5 0.0 0.3 ~~~~a~'f~l'b~~g~laria 9.1 3.3 2.2 0.0 4.8 5.5 5.6 3.5 ·2.3 -3.4 ·5.6 -0.8 -0.1 7.0 3.5 1.4 0.0 3.6 4.7 5.1 1.9 ·1.6 -3.7 ·5.1 -1.5 -0.4 manuel mallarino-trujlllo 5.1 0.3 0.4 0.0 2.3 4.8 4.0 1.1 -3.7 ·3.6 -4.0 ·1.7 0.8 tenerife-cerrito 5.1 2.4 0.0 0.0 20 4.1 4.2 0.9 .1.8 -4.2 -4.2 ·2.2 ·0.1 el vinculo-buga 7.3 1.5 1.5 0.0 4.8 5.3 5.4 1.9 -3.9 ·3.9 -5.4 -0.6 -0.1 heraclio uribe-sevilla 2.9 3.3 1.2 0.0 0.3 2.1 3.1 -0.2 0.2 -1.9 ·3.1 ·2.8 ·1.0 tierrablanca-roldanlllo 7.2 2.3 0.1 0.0 4.7 6.7 4.9 2.3 -2.6 -4.8 -4.9 ·0.2 1.8 cent.adm.la union-la union 6.5 2.8 0.4 0.0 2.4 5.9 4.7 1.8 -1.9 -4.3 -4.7 ·2.3 1.2 arturo gomez-alcala 5.6 2.8 3.9 0.0 5.6 3.2 5.0 0.6 ·2.2 -1.1 ·5.0 0.6 ·1.8 cum barco-sevilla 6.1 1.9 0.0 0.0 1.3 2.6 3.5 2.6 ·1.6 -3.5 ·3.5 -2.2 -0.9 colpuertos-buenaventura 3.1 0.2 0.0 0.0 3.4 1.6 3.4 -0.3 ·3.2 ·3.4 ·3.4 0.0 -1.8 julio fernandez-restrepo 5.6 3.2 0.0 0.2 2.1 0.0 3.3 2.3 -0.1 ·3.3 ·3.1 ·1.2 ·3.3 argelia el recreo-argelia 3.9 1.5 0.5 0.0 1.5 1.9 3.3 0.6 ·1.8 -2.8 ·3.3 ·1.8 -1.4 subestacion alban-el cairo 3.6 0.2 0.8 0.0 1.4 2.4 3.5 0.1 -3.3 ·2.7 -3.5 ·2.1 ·1.1 la mision-buenaventura 0.0 0.0 0.0 0.0 0.3 1.6 1.7 -1.7 ·1.7 -1.7 ·1.7 -1.4 -0.1 vaupes apto.mitu-mitu 5.8 4.5 2.0 0.2 4.1 1.2 5.1 0.7 -0.6 ·3.1 -4.9 ·1.0 -3.9 vichada las gaviotas-san jose de ocune 6.3 1.6 7.7 0.9 4.0 0.2 5.5 0.8 -3.9 2.2 -4.6 -1.5 -5.3 apto.pto carreno-pto carreno 9.2 7.8 9.3 6.1 1.4 1.5 6.0 3.2 1.8 3.3 0.1 -4.6 -4.5 med mes = valor medio diario en noviembre de 1985 diferencias: ej: 11-m = valor del dfa 11 menos valor medio diario en noviembre de 1985 (med mes) 44 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n morall/ella 10.0 d.d • dia11 ~dla.l6 _ dla12 o dlale lit] via 1s • dia 1+ a ¥idia nov figura 2. insolaci6n (hora-deetme) registrada del 11-16 de nov-1985 y promedia en nov-1985, en 12 poblaciones ubicadas en areas cubiertas por el penacho de cenizas: chinchina-caldas (chin), palestina-caldas (pale), espinal-tolima (espi), ambalema-tolima (amba), pandi-cundinamarca (pan d), puerto salgar-cundinamarca (salg), nuevo col6n-boyaca (nuev), sutatenza-boyaca (suta), yopal-casanare (yopa), salazar-n.de santander (sala), sabana de torres-santander (saba) y saravena-arauca (sara). dla a dla, la situaci6n durante el perfodo 11 a 16 de noviembre-1985. puede sintetizarse asi: 11-nov-85. la dlstnbuci6n del numero de horas de brillo solar, en este dla, si bien no es igual al promedio para el mes de noviembre (fig.1), se considera muy cercana a 10que normalmente puede ocurrir en un dla cualquiera: los valores en algunas areas son superiores y en otras inferiores a los promedios. en la regi6n andina, llanos orientales y regi6n del caribe, la tendencia fue la de superar los promedios hasta en 2 horas (en algunas pequeiias areas hasta en 3 horas) y en la amazonia y algunas areas de los departamentos de choc6 y nariiio la tendencia muestra valores inferiores al promedio en 1 y. en algunos casos, hasta 2 horas. esas diferencias. analizadas porcentualmente (fig.3), muestran que en la mitad dei pals se presentaron valores que representan entre el 80% y el 120% respecto al promedio. 10 cual puede considerarse normal dadas las diferentes caracterlsticas fisiograficas. aumentos superiores al 20% ocurrieron en la regi6n del caribe, algunos sectores de los departamentos de arauca, casanare y una parte de la regi6n andina (en mayor % al sur de antioquia, boyaca, tolima y norte del huila); descensos superiores a120%se presentaron en la mayor parte de la amazonia, sw del departamento del choc6 y w de nariiio. 12-nov-85. en este dfa, la distribuci6n de la insolaci6n aparentemente coincide con las caracterrsticas generales que muestran los promedios (fig.1): una zona con valores relativamente bajos que cubre la regi6n del catatumbo, la regi6:l andina, la regi6n del pacifico y otra zona con valores altos que abarca la regi6n del caribe y la mayor parte de la orinoquia y amazonia. sin embargo. los valores de la primera zona son mas bajos y los de la segunda zona son mas altos que los promedios. se encuentran dos importantes situaciones, cada una de elias con areas crrticas especlficas (fig.4): a) un notorio aumento de 2 hasta mas de 3.5 horas en la mayor parte de huila, tolima, suroccidente del meta, la mayor parte de caqueta y el centro de boyaca; b) una extraordinaria. mas que notoria, disminucion de la insolaci6n diaria en toda la regi6n andina, norte de la regi6n del pacifico. suroccidente de la regi6n del caribe y algunas areaspequerias como leticia y ifmites entre meta y vichada. esta disminuci6n fue de tal magnitud que en casi toda el area cubierta por los departamentos de antioquia y choc6 la insolaci6n fue 0.0 horas (cero.cero), 0 muy cerca a cero, durante este dla. lo anterior, aparentemente, fue ocasionado por una reactivaci6n 0 intensificaci6n de la itc y la acci6n de gases y fina ceniza (altamente concentradas en esa zona) que transportadas por los vientos generales y locales (principalmente estos ultirnos) y de altitudes inferiores a 7.000 m, procedlan de las emisiones. pequeiias perc constantes, que se estaban dando en el volcan nevado del ruiz desde los dlas previos. la ceniza y gases, como nucleos de condensaci6ncongelaci6n, originaron 'lubes de gran espesor e impulsaron la intensificaci6n de la itc. 13-nov-85. la dlstnbuci6n de la insolaci6n en este dfa presenta variaciones sustanciales con respecto al promedio. en todo el pals se present6 una notable disrninucion del brillo solar, con sectores crlticos ubicados en caldas. norte del tolima, santander, boyaca, amazonas, nariiio (fig.5). la forma y ubicacion de esta zona con disrninucion en ef brillo solar es muy similar a la nube de gases detectada el 14 de noviernbre y que fuera reseiiada por krueger (en smithsonian institution, 1985). esto sugiere que la capa de gases, formada e intensificandose desde los dras anteriores, fue en gran parte responsable de la disminuci6n de la insolaci6n ocurrida en los dras 12 y 13 de noviembre. unicamente en guajira. cesar, norte de bolivar, arauca, vichada, guainla y oriente del meta se conservaron las caracterlsticas generales de presentar mas de 5 horas de insolaci6n per dra, aun cuando en la orinoquia se presentaron aumentos hasta de 2.5 horas (40%) y, al contrario, en la parte correspondiente a la regi6n del caribe ocurrieron disminuciones de hasta 2 horas (20%). 14-nov-85. en todo el territorio colombiano, a excepci6n de la guajira y el norte del cesar, se registraron valores de insolaci6n inferiores a 5 horas/dla y una notable disminuci6n con respecto al promedio de noviembre1985. esa disminuci6n fue superior a 3 horas, excepto en la regi6n del caribe, regi6n del catatumbo, norte de arauca y nororiente del vichada, donde la disminuci6n oscil6 entre 1 y 3 horas, en general. porcentualmente la insolaci6n de este dfa represent6, en la mayor parte del pals, menos del 20% de 10 normal; en algunas pequeiias areas, relativamente, represent6 entre el 20% y el 100% (por ej., la regi6n del caribe. la regi6n del catatumbo, norte de arauca, nororiente del vichada. litoral pacffico en nariiio y norte del choc6 (fig.6). 15-nov-85. la insolacl6n continu6 siendo inferior al promedio en la mayor parte del pars, aun cuando en un grado menos fuerte que los dras anteriores. valores de brillo solar superiores a 5 horas/dfa s610 se presentaron en la regi6n del caribe y algunas otras pequeiias areas (por ej., arauca y occidente de santander). excepto en algunas areas de c6rdoba, santander y huila, en el resto del pals hubo disminuci6n en el numero de horas de sol cc'nrespecto al promedio. esa disminuci6n tuvo valores de 1.5 a 2 horas. en los casos mas fuertes la insolaci6n represent6 s610el 30-40% de 10normal (fig.?). geofislca colomblana n° 2, abril de 1993 45 to' to' escala .' ..' '0 ., .' -----..:.-130 \ o' ../.r--""""......\ 'i ) ",/ g.\. o· peru ·0 o· ,.' to' ./nulo.g ....... figura 3. isohelias que seiialan, en %, la diferencla entre la insolacl6n del 11-nov-1985 y la media dlarla de nov-1985 46 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n ?o' ..',,' s·-.......r\ ..) l s c s c a o so '00 "o~ •. .. venezuela .' cgo brasil peru .. iluld.6.l.*. ,.' ,,' ..' 10 .' .' .' cfigura 4. isohelias que seilalan, en "10, la diferencia entre la insolaci6n del 12-nov-1985 y la media diaria de nov-1985 geoffsica colombiana n° 2, abril de 1993 47 to· to· e $ca l a o· \ 120 \ .././"".-"'\ .......\, \ ,/.-c, o· ecuador o,.., 40 peru ... '0 o· .. .. t • j""-1).g.ljiiii. to· ,.. figura 5. isohelias que seilalan, en %, la diferencia entre la insolaci6n del 13-nov-1985 y la media diaria de nov-1985 48 eslava: efeetos erupci6n del vnr sobre la atm6sfera. caso: insolaei6n to" 0" ." 0" l.==---...... __ to· peru t." escal" o so '00 venezuela .." '0 0" .. <,"\' 30 ',\ r-. " ,"l''\..,..... \.. \. _.} \ . \ \ brasil 20 h" 12" to",.. figura6. isohelias que seilalan, en %, la diferenela entre la insolaei6n del 14-nov-1985 y la media diaria de nov-1985 geofisica colombiana n° 2, abril de 1993 49 ... 7'· 70· escai.a o '0 '00 rso r». ," venezuela o· cbrasil ··l.,/1iulo.:i.l,in. peru 72· 70·71· 71· ii· '0 i· .. o· figura 7. isohelias que sei'lalan, en %, la diferencia entre la insolaci6n del 15-noy·1985 y la media diaria de -r noy·1985 50 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n 16·nov·85. en este dla el efecto de disminuci6n de la insolaci6n fue menos intense que los dlas anteriores y la situaci6n empez6 a normalizarse. los valores estuvieron cerca de 10 normal (100-120%) en la regi6n del caribe y sector central de santander, areas en las cuales tam bien los valores fueron superiores a 5 horas/dia. en el resto del pals el brillo solar fue menos intense que 10 normal, con un mayor efecto de disminuci6n (70-80%) en choc6, antioquia y algunos sectores de huila, tolima, caldas, cundinamarca, boyaca y las regiones de orinoquia y amazonia. conclusiones los analisis efectuados permitieron establecer las siguientes caracterlsticas generales relativas al comportamiento de los elementos meteorol6gicos frente a la inyecci6n en la atm6sfera de productos s61idos, ifquidos y gaseosos efectuada por el volcan nevado del ruiz. las partlculas s6lidas y gaseosas se hicieron presentes en casi toda colombia y forma ron capas en la atm6sfera, de diferentes espesores y densidades; ello impidi6 u obstaculiz6 el paso de la energfa solar y ocasion6 cambios temporales en los valores de los elementos meteorol6gicos, en el aire superfcial y a diferentes altitudes. esos cambios comenzaron desde el dla anterior, por efecto de los gases y cenizas emitidas con anterioridad y que por los movimientos locales y generales del aire se habfan acumulado en sectores ubicados al w y nw del volcan. en general, los efectos, perduraron hasta el dla 16. se ocasiono una reducci6n notoria en el numero de horas de sol por dla, durante el perfodo del12 al16 de noviernbre, comparando con las que en promedio ocurrieron en todo el mes de noviembre de 1985. este hecho, a su vez, origin6 una reducci6n de la radiaci6n solar entrante en el sistema tierra-atm6sfera y, a nivel del aire ubicado en la superfcie terrestre, una disminuci6n de las temperaturas medias del aire, con un aumento de las temperaturas minimas y una disminuci6n de las rnaxirnas. esas variaciones, en uni6n con el aporte de vapor de agua y nucleos de condensaci6n (cenizal afectaron la distribuci6n de la humedad del aire' y de los de mas elementos meteorol6gicos, eiltre eum: el regimen temporal y espacial de las ii'jvias. las magnitudes de algunas de esas alteraciones fueron p&queiias y, muy seguramente, no perceptibles por ios metod os de medici6n dis;jooibles. la distribuci6n de la inso'aci(~n en el perfodo 12 a 16 de noviembre presenta variaciones· sustanciales con respecto al promedlo. en todo el pals se present6 una notable disminuclon del orillo solar; la forma y ubicaci6n de las areas con dism;nuci6n en el brillo solar es muy similar a la nube de gases, esto sugiere que la capa de gases fue en gran parte responsable de la disminuci6n de la insolaci6n ocurrida en los dlas 12 y 13 de noviembre. ei dia 12-nov-1985 ocurri6 una extraordinaria, mas que notoria, disminuci6n de la insolaci6n diaria en toda la regi6n andina, norte de la regi6n del pacifico, suroccidente de la regi6n del caribe y algunas areas pequeiias como leticia y iimites entre meta y vichada; esta disminuci6n fue de tal magnitud que en casi toda el area cubierta por los departamentos de antioquia y choc6 la insolaci6n fue de 0.0 horas (cero.cero), 0 muy cerca a cero. ei dla 14, por ej., en todo el territorio colombiano, a excepci6n de la guajira y el norte del cesar, se registraron valores de inso/aci6n inferiores a 5 horas/dla y una notable disminuci6n con respecto al promedio de noviembre-1985; esa disminuci6n fue superior a 3 horas, con algunas excepciones donde la disminuci6n oscil6 entre 1 y 3 horas. porcentualmente la insolaci6n de este dia represent6, en la mayor parte del pals, menos del 20% de 10 normal. uno de los efectos colaterales, entre los muchos que se originaron, fue el ocurrido en la regi6n del cocuy, departamento de boyaca: las cenizas convirtieron el sector occidental de la sierra nevada del cocuy en una superfcie oscura en la que la nieve presentaba un color grisaceo; este cambio intempestivo en la composici6n y color de la cubierta de sus cumbres modific6, aumentando, sustancialmente la capacidad de absorber la energla solar y, por 10 tanto, de la aptitud para efectuar el proceso de iicuefacci6n 0 deshielo, hecho que paulatinamente ocurri6. las condiciones generales resumidas en las conclusiones anteriores no fueron uniformes para cada dia ni para cada regi6n 0 zona, ya que se detectaron variaciones locales que diferian de la situaci6n general establecida. esas innumerables variaciones temporales y espaciales estuvieron sometidas a factores externos entre los cuales el factor prima rio fue la radiaci6n solar y sus variaciones; ademas, la complejidad de los procesos atrnosfericos y su continua y temporal modificaci6n dependieron basicarnente de la interrelaci6n que se da entre los elementos meteorol6gicos y entre ellos y el conjunto de procesos zonales de convecci6n terrnica y dinarnica, de la infiuencia extensiva de los diferentes sistemas meteorol6gicos (por ej., la ltc), de la presencia de movimientos del aire determinados 0 condicionados par la geometrfa tierra-sol, la latitud, la altitud, la configuraci6n del relieve, el suelo, la vegetaci6n, la hera del dia 0 noche en la cual lieg6 a cada sitio la masa principal de partlculas, etc. todo 10 anterior justifica el hecho de que las caracteristicas atmosfericas no presentaran una uniformidad en los cambios mencionados. los impactos generales entonces se modificaron por el medio ambiente local y/o regional. bibliografia eslava, j. 1985. consideraciones sobre los aspectos meteorol6gicos y efectos relacionados con la dispersi6n de fragmentos y gases emitidos por el volcan arenas del nevado del ruiz el 13 de noviembre de 1985. geol.colomb. 14:165-173. santafe de bogota. federacion nacional de cafeteros. 1986. anuario meteorol6gico 1985. 360pp. cenicafe. chinchina, caldas. instituto colombiano de hidrologla, meteorologia y adecuacion de tierras (himat). 1991. informaci6n meteorol6gica de 1985 existente en los archivos de himat (sistema de informaci6n hidrometeoroi6gica). santafe de bogota. smithsonian institution. 1985. ruiz volcano, colombia. sean bt. letin 10 (10):2-4, 25-35. washington. geociencias-vol 12-1 sep.vp earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 107-117 seismic velocity determination in gravel and sands using piezocrystals armando luis imhof1 and juan carlos santamarina2 1 instituto geofísico sismológico volponi / facultad de ciencias exactas físicas y naturales. ignacio de la roza y meglioli. rivadavia. c.p. 5400, san juan, república argentina. fax: (54) 264 4234980. tel.: 54 264 4945015. e-mail: aimhof@unsj.edu.ar. 2 center for applied geomaterials research. atlanta, georgia. usa. http://geosystems.gatech.edu postal address: instituto geofísico sismológico volponi / ruta 12, km. 17. c.p. 5413 rivadavia san juan argentina abstract the exact determination of seismic waves’ propagation velocities has great importance in the geotechnics due to from that it is possible to determine, among other parameters, the dynamic ones: elasticity e, rigidity g, poisson !, compressibility b; as well as to reach a knowledge on the stress-strain behavior for the studied soil samples. the seismic waves transmission considered in tests at laboratory scale carried out in the present work is a phenomenon that produces very small deformation, and so doesn’t disturb the material. this allows to apply the results in a more general scale to study the behavior of soils in situ and to predict their answer to stress. with the purpose to study the response of particulate material subjected to seismic excitements at small scale, samples of gravels and sands were successively introduced in an odometric cell, exciting them with impulsive signals and registering the corresponding seismograms through general purpose piezoelectric transducers embedded in ends of the cell. the distance source-receiver was interval increased, which enabled, from the corresponding regression straight lines, to calculate in precise form the propagation velocities (for p waves). the tests were carried out in samples of dry alluvial soil with three different grain sizes. the respective frequency spectra of the signals were determined for two packing modes: loose and compact, what added information on the medium characteristics. 107 manuscript received april 18, 2008. accepted for publication june 3, 2008. the results showed that general purpose piezocrystals can be adapted for design and construction of a complete low cost acquisition system that brings great resolution in time, facilitating as consequence very precise calculations of the transmission velocities that take in a later stage to reliable determination of dynamic parameters of the soil. keywords: piezocrystals, seismic waves, compressional waves, signal processing. resumen la determinación precisa de velocidades de propagación de ondas sísmicas reviste gran importancia en la geotecnia debido que a partir de las mismas es posible determinar, entre otros parámetros, los dinámicos: elasticidad e, rigidez g, poisson �, compresibilidad b; así como alcanzar un conocimiento sobre el comportamiento tenso-deformacional de las muestras de suelo estudiadas. la propagación de las ondas sísmicas considerada en los ensayos de laboratorio llevados a cabo en el presente trabajo es un fenómeno que produce ínfima deformación, por lo que no disturba el material. ello permite aplicar los resultados en una escala más general para estudiar el comportamiento de los suelos in situ y predecir su respuesta a esfuerzos. con el propósito de estudiar la respuesta de materiales particulados sometidos a excitaciones sísmicas a pequeña escala, se introdujeron sucesivamente muestras de gravas y arenas en una celda odométrica, excitándolas luego con señales impulsivas y registrándose los correspondientes sismogramas a través de transductores piezoeléctricos de uso general adosados en extremos de la celda. si incrementó por intervalos la distancia emisor-receptor lo que posibilitó, a partir del trazado de las rectas de regresión correspondientes, calcular en forma precisa las velocidades de propagación (ondas p). los ensayos se llevaron a cabo en muestras de suelo aluvional seco con tres tipos diferentes de granulometría. se determinaron los respectivos espectros de frecuencia de las señales para dos estados de compactación considerados, lo que permitió aportar información sobre las características del medio en estado suelto y compacto. los resultados mostraron que los piezocristales de uso general pueden ser adaptados para diseño y construcción de transductores que funcionen tanto como emisores como receptores de ondas y que permitan obtener registros con gran resolución en tiempo, posibilitando como consecuencia de ello cálculos muy precisos de las velocidades de propagación que lleven en una etapa posterior a determinación confiable de parámetros dinámicos del suelo. palabras clave: ondas sísmicas, piezocristales, ondas compresionalesm, procesamiento de señales. introduction this investigation presents the development and implementation of geophysical techniques based in transmission of elastic waves using low cost instruments developed for these work. the techniques were applied for sand and gravels samples at laboratory scale. the precise determination of seismic wave velocities has great importance in civil engineering because from that it is possible to determine the dynamic modulus (e.g. rigidity g and compressiblility or bulk, b). see for example, the following expression (sheriff, 1994; sheriff et al, 1995): 108 armando luis imhof and juan carlos santamarina v b g p � � 4 3 " (1) which express compressional wave velocity (vp) related to bulk (b) and rigidity (g) dynamic modules, and density (") of the medium traversed by the wave. the fact to consider particulate media (i.e. not continuum) adds some level of complexity to equation (1), which transforms in (lee, 2003): v b g n s b s b n b p soils sk sk w a g � � � � � � � � � � � � � � � � � � � � #4 3 1 1 $ % % & ' ( ( � ( )1 1 n nsg w" " (2) where s is the degree of humidity of the soil sample; " density, n porosity, bsk bw bg bulk modules for skeleton (solid phase), water and grain respectively, and vp the propagation velocity of p waves. seismic waves propagation in these tests is a small deformation phenomenon, so does not disturbs the material involved. thereforre, it is possible to extrapolate the methodology to field to study gravel and sands behaviour in situ and predict their response to stress at major scale. the performance of geophysical tests in granular media at laboratory scale, particularly of coarse grain (e.g. alluvial gravels), faces difficulties related to the transducer-medium coupling, to the type of sources, the selection of transducers for the reception, the inherent dispersion (i.e. velocity variation with frequency) and the selective attenuation of frequencies with regard to the size of the particles, among others. test assembly main characteristics of emission-reception transducers general purpose piezocrystals (disk cell type; apci, 2000) of 20mm of diameter were used as sources and receivers of seismic waves, figure 1(a) . their reversible characteristic allows them to work converting electric energy in mechanical and vice versa so they can be implemented as emitters and receivers. an electric impulse makes vibrate the crystal mainly to its resonance frequency (wells, 1977), which is inversely proportional to the thickness of the piezoelectric material (being approximately a half of the wavelength, )/2). besides, the bandwidth of the registered signal also possesses an inverse relationship with the duration of the excitement (lee, 2003). the signal created by the signal generator (pulser) is not identical to that verily emitted by the transducer (that is a mechanical one), since it also depends on the proper characteristics of the medium. therefore it is important to carry out a monitoring of the real transmitted waveform. with that purpose it was implemented the system represented in figure 1 (b). the true signal emitted by the piezocrystal (pzt1 for brevity) is monitored by the digital storage oscil109 seismic velocity determination in gravel and sands using piezocrystals figure 1. (a) disk cells. (b) transducers connection to pulser and dso for real transmitted wave monitoring. loscope (dso) by means of other piezoelectric (pzt2), stuck to it. figure 2 presents on one hand the real form of the output pulse; and for other the frequency content of it. it’ll be studied what happens to the last as the waves transverse the medium under study. system assembly figure 3 shows the cylindrical steel cell where the samples were introduced, and the outline of system connection is visualized. as signal generator a pulser was built, which generated impulsive excitements of short duration (10*s) in periodic form (each 10 ms) and with high amplitudes (range 40 400v). connected to the pzt receiver, an operational amplifier fet tl084 was used of up to 80 db gain, fed with 12 vcc, with 100hz highpass filter. as seismograph, a 60mhz digital real time oscilloscope tektronix tds 210 of two channels and one additional for external trigger was used, with sampling rate up to 1 gsample/sec. the instrument possess an rs232 interface. a basic algorithm was designed and implemented to transfer data to pc, for later signal processing. as wave emitters e and receivers r, general purpose piezoelectric ceramics of 4.1khz of resonance frequency transducers were used, direct arrivals to be able to calculate the transmission velocities, it is necessary the determination of the first arrivals accurately (direct waves). therefore in confined media it is essential to know the types of possible arrivals in order to eliminate the undue ones (i.e. the no-direct ones). imhof (2007) studied several types of wave propagation inside cylindrical test steel probes: direct, refracted, reflected and transmitted through the walls of the cell, verifying that: (a) the waves transmitted by the steel walls will arrive much faster that the other ones, due to the high propagation velocity there (vsteel = 6000m/s; santamarina et al, 2001); (b) for distances h>0.13m and diameters += 0.125m the wall refracted waves will reach r before the other types in all the cases ; (c) although the wall reflected 110 armando luis imhof and juan carlos santamarina figure 2. output signal from pulser. (a) real output wave. (b) spectral density. waves don’t arrive before the direct ones in any case, however as h increases the arrival times for both will approach to each other. this will imply interference in the complete registration of the signal. studying the directivity of this type of transducers, imhof (2007) demonstrated that, for distances h greater than 0.13m (figure 3) the wall refractions were not detected due to the fact that the critical angles were inside the area of crystal directivity shade, which attenuated them almost in 70-80%. at last, to avoid detection of waves that spread through the cell walls, so the base as the cover were isolated with rubber o-rings, that produced uncoupling of waves. methodology tests were carried out using dry samples of sands (s) and gravels (g) (sp and gp related to unified soil classification system, uscs, by american society of testing materials, astm) with three different sizes (#20 #50; #8 #20 and #2 #8, astm) and two packing modes: loose (a) and dense (c). table 1 summarizes sample characteristics which brought to six complete tests. at first velocities were determined, starting from a sample of sand (sp) with grain sizes among 0.297mm (#50) and 0.841mm (#20), repeating the determinations later for the other ones. the sample was introduced in the test cell (figure 3) until reaching the distance h = 70 mm. in the base a couple of pzt (source e, pzt1 and trigger to channel 1, dso, pzt2) was mounted, covering then the material with a steel cap with the receiving, r transducer stuck in its central part with an epoxi elastic mastic, and connected to channel 2, dso through the amplification system. after obtaining the first record the cap was removed and more sample material was added, until the next distance h, repeating the acquisition. in total there were six recordings for each sample and packing mode, for distances h=70, 90, 110, 130, 150, and 170mm. all the records were transmitted to pc through rs232 interface. the signal to noise (s/r) relationship was improved using stacking with the purpose to cancel or minimize the noises randomly and so enhancing the incoming signal. the number of stacks commonly used was 128, but on occasions, mainly for the #2-#8 samples, reached 1024. the sample was placed in two successive packing forms: loose a, poured with tablespoon taking care that the grains remained with the greatest volume of holes; and dense c compressing it in mechanical form in order to minimize the poral space without breaking the grains. 111 seismic velocity determination in gravel and sands using piezocrystals pulser dso amplifier (60 db) e r sample trigger cap (steel) h figure 3. cillindric cell for tests and connection scheme. table 1. volumetric form and shallow texture of soil samples used. (courtesy: instituto de materiales y suelos, fi-unsj) size particle diameter esfericity texture #20 #50 0.841 mm-0.297 mm 0.8 subrounded #8 #20 2.38 mm-0.841 mm 0.7 subangled to subrounded #2 #8 10 mm-2.38 mm 0.8 subrounded to rounded results the signals quality diminishes as the size of the particles grows, (e.g. figure 4 (a) vs figure 4 (c)). particularly it was not necessary to perform stacking to get the records for figure 4 (a)-(b), while the number of stacks to obtain the records shown in figures 4 (c)-(d) were of 128. finally to get legible records for sample #2-#8 (not shown) the number of stcaks ascended to 1024. the explanation of this phenomenon will be elucidated later when analyzing the frequency spectra for every signal. after the first arrivals’ picking for all signals, distance and time data were recorded in matrix form. linear regression was calculated for each one of them and then represented graphically together with corresponding travel times versus h distances (figure 5). the inverse of regression line slope constitutes the propagation velocity. 112 armando luis imhof and juan carlos santamarina 0 0.00125 0.00188 0.0025 0 0.5 p waves. sample #20-#50c 0 6.25 10� -4 0.00125 0.00188 0.0025 0 0.2 0.4 0.6 p waves. sample #20-#50a time [sec] 0 0.00125 0.00188 0.0025 0.05 0 0.05 0.1 p waves. sample #8-#20a 0 0.00125 0.00188 0.0025 0 0.1 0.2 0.3 p waves. sample #8-#20c (a) 6.25 10� -4 (b) 6.25 10� -4 time [sec] (a) (b) 6.25 10� -4 h=70 mmh=70 mm h=90 mmh=90 mm h=110 mmh=110 mm h=130 mmh=130 mm h=150 mmh=150 mm h=170 mmh=170 mm time [sec] time [sec] figure 4. compressional waves for increasing source-receiver distances (h). (a)-(b): sample #20-#50 (a and c); (c)-(d): sample #8-#20 (a and c). 113 seismic velocity determination in gravel and sands using piezocrystals 0 0.05 0.1 0.15 0.2 0 0.001 0.0015 measured fitting measured fitting linear regression.#20-#50 a e-r distance [m] t ra v e l t im e [s e c ] 0 0.05 0.1 0.15 0.2 0 0.001 0.0015 measured fitting measured fitting linear regression.#20-#50 c e-r distance [m] t ra v e l t im e [s e c ] 0 0.04 0.08 0.12 0.16 0.2 0 0.001 0.0015 measured fitting measured fitting linear regression.#8-#20 a e-r distance [m] t ra v e l t im e [s e c ] 0 0.04 0.08 0.12 0.16 0.2 0 0.001 0.0015 measured fitting measured fitting linear regression.#8-#20 c e-r distance [m] t ra v e l t im e [s e c ] 0.05 0.07 0.09 0.11 0.13 0.15 0 0.001 0.0015 measured fitting measured fitting linear regression.#2-#8 a e-r distance [m] t ra v e l t im e [s e c ] 0.05 0.07 0.09 0.11 0.13 0.15 0 0.001 0.0015 measured fitting measured fitting linear regression.#2-#8 c e-r distance [m] t ra v e l t im e [s e c ] 5 10� -4 5 10� -4 5 10� -4 5 10� -4 5 10� -4 5 10� -4 figure 5. linear regression. determination of propagation velocity (p wave) in samples #20-#50 (a-b); #8-#20 (c-d) y #2-#8 (e-f). (a y c). applying fast fourier transform (fft), the power spectra were evaluated for each one of the registered signals and represented in figure 6. finally, figure 7 shows the frequency content variation with distance h for the different cases under study. 114 armando luis imhof and juan carlos santamarina 70 mm. 90 mm. 110 mm. 130 mm. 150 mm. 170 mm. 0 6250 signal spectra sample #20-#50 c frequency [hz] 0 6250 1.25 10� 4 signal spectra sample #20-#50 a frequency [hz] p o w e r s p e c tr a signal spectra sample #8-#20 csignal spectra sample #8-#20 a signal spectra samples #2-#8 csignal spectra sample #2-#8 a p o w e r s p e c tr a 1.87 10� 4 2.5 10� 4 1.25 10� 4 1.87 10� 4 2.5 10� 4 (a) (b) p o w e r s p e c tr a 0 6250 frequency [hz] 0 6250 1.25 10� 4 frequency [hz] 1.87 10� 4 2.5 10� 4 1.25 10� 4 1.87 10� 4 2.5 10� 4 © (d) frequency [hz] 0 5000 1 10� 4 frequency [hz] 1.5 10� 4 2 10� 4 © (d) 0 5000 1 10� 4 1.5 10� 4 2 10� 4 figure 6. power spectra for increasing h. samples #20-#50; #8-#20 y #2-#8 (a and c). discussion compressional wave propagation the quality of the signals got worse as particle size increased. on one hand the fact to augment the particle size decremented teh contact area between the sample and the tyranbsducer (i.e. transducer-sample coupling). on the other the typical wavelength calculated for the samples (see below power spectra of signals) approaches to particles size, appearing the diffraction phenomenon (potts & santamarina, 1993). this turned critic in the case of sample #2-#8. in general it was observed that the measured travel times presented minimum dispersion, being adjusted very well to a straight line, because the material is considered homogeneous and isotropic (at laboratory small scale). the propagation velocities in all cases are lower than those of wave in air: 343 m/s to 20 ºc, 1atm (carmichael, 1982,1989; weast, 1988; guéguen & 115 seismic velocity determination in gravel and sands using piezocrystals 60 80 100 120 140 160 180 2000 3750 5500 7250 9000 frecuency response #20-#50 a y c h [mm] f re c u e n c y [h z ] 60 80 100 120 140 160 180 2000 3375 4750 6125 7500 frecuency response #8-#20 a y c h [mm] f re c u e n c y [h z ] 60 80 100 120 140 160 180 1000 2750 4500 6250 8000 sample a sample c sample a sample c frecuency response #2-#8 a y c h [mm] f re c u e n c y [h z ] figure 7. comparative diagram showing frequency response versus source-receiver distances. samples #20-#50; #8-#20 y #2-#8 (a and c). palciauskas, 1994), fluctuating in the range 150 215 m/s. it was verified that the propagation velocities in loose media are always lower than those in compacted samples. this appears as a logical result because the material with smaller volume of holes presents a more similar behavior to the solid (rock), that possesses greater propagation velocity that the wave in the air. the vp also diminishes as the grain size increases (compare figure 5 a,c,e). regarding sample #2-#8, it is very difficult to detect the first arrivals accurately. the explanation of this phenomenon is analized in the following section. power spectra of signals. the frequency content of the signals for the loose state was lower than that for the compact one, probably because the material in the last case resembles more to a continuous medium (compare in figure 6 the couples: (a)-(b) for sample #20-#50; (c)-(d) for sample #8-#20 and (e)-(f) for sample #2-#8). the maximum frequency content was obtained for the sample #20-#50 (a and c); being located in the range 4000hz (a) 8000hz (c), decreasing as the particle size increased to the interval 2500hz (a) 6000hz (c) for sample #8-#20 and rounding 2500hz fot sample #2-#8. therefore, as the particle size increases, decreases the sample frequency response, being accentuated the filtering of higher ones (low-pass filter). in the case of the sample #2-#8 a particular phenomenon takes place. according to the spectrum displayed at figure 6 (c)-(d), the average wave frequency is about f~4000 hz (h = 0.07m). considering a velocity of propagation vp~140 m/s: wavelength vp f m s s m mm� � � � �) 140 40001 0035 35 / / . . also, the maximum amplitude values take place for smaller frequencies as h increases and therefore � also grows. the grain size for sample #2-#8 is among 2.38mm and 10mm; therefore ) rounds into the order of particles size. this implies the beginning of diffraction phenomenon that will hinder the detection of first arrivals due to scattering (potts et al, 1993; santamarina et al, 2005). figure 7 shows frequency response versus distance h for the samples #20-#50; #8-#20 and #2-#8 a and c, showing that the bandwith is greater in c that in a packing modes. besides, the frequency content decreases as h increases, logical result due that earth responds as a low-pass filter to seismic waves. only the sample #2-#8 presents irregular behavior, due to the described phenomenon. conclusions general purpose piezocrystals (disk cells) can be adapted for the construction of transducers to generate and detect seismic p waves at laboratory scale using soil samples (gravel and sands), in the range of frequencies that allows an appropriate balance between resolution and penetration. the propagation velocities calculated for the three samples in loose and compacted media increase in the second case; the typical frequency contents increase too. compact soil samples filter less the high frequencies than do the loose ones. the spectrograms amplitude peaks moves toward lower values of frequency as the grain size increases. finally, it was proved the efficiency of these type of piezocrystals to perform high resolutive and low cost laboratory experiments. references american piezo com international (2000) piezoelectric ceramics: principles and applications. apci. carmichael, r.s. (1982) handbook of physical properties of rocks. crc press, boca raton, 345 pp. 116 armando luis imhof and juan carlos santamarina carmichael, r.s. (1989) crc practical handbook of physical properties of rocks and minerals. crc press, 741 pp. guéguen, y. & palciauskas, v. (1994) introduction to the physics of rocks. princeton university press. princeton; 294 pp. imhof, a. l. (2007) “caracterización de arenas y gravas con ondas elásticas: tomografía sísmica en cross hole”. ph.d. thesis. universidad nacional de cuyo. mendoza argentina. lee, j.s. (2003) high resolution geophysical techniques for small-scale soil model testing. ph.d. thesis. georgia institute of technology. atlanta. usa. potts, b.d. & santamarina, j.c. (1993) geotechnical tomography: the effects of diffraction geotechnical testing journal. 16, nº1, 510-517. santamarina, j.c. & fratta, d.. (2005) discrete signals and inverse problems. an introduction for engineers and scientists. ed. wiley & sons.uk. santamarina, j.c.; klein, k.a. & fam, m.a. (2001) soils and waves. wiley & sons. ltd. england. sheriff, r.e. & geldard, l.p. (1995) exploration seismology. 2nd edition. cambridge university press; ny. 592 pp. sheriff, r.e. (1994) encyclopedic dictionary of exploration geophysics. geophysical reference series. society of exploration geophysics (seg). tulsa, ok. usa. weast, r.c. (1988) crc handbook of chemistry and physics. 1st student edition. crc press, boca raton. wells, p.n.t. (1977) biomedical ultrasonics. academic press. new york, 635 pp. 117 seismic velocity determination in gravel and sands using piezocrystals letter from editor a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombiacarlos alberto ríos reyes, oscar mauricio castellanos alarcón and akira takasu 7 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombialorena rayo and carlos a. zuluaga 31 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeriaakintola, o.f. and adekeye, j.i.d. 44 denoising resistivity phosphate “disturbances” using haar mother wavelettransform (sidi chennane, morocco)saad bakkali and mahacine amrani 62 petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”ríos, c.a., castellanos, o.m., gómez s.i. and ávila, g.a. 72 seismic velocity determination in gravel and sands using piezocrystalsarmando luis imhof and juan carlos santamarina 107 geociencias dic 2010-color.vp thermal modeling of pluton emplacement and associated contact metamorphism: parashi stock emplacement in the serranía de jarara (alta guajira, colombia) lina fernanda martínez and carlos a. zuluaga c.1 1 department of geological sciences, universidad nacional de colombia, edif. manuel ancizar, of. 301, ciudad universitaria, bogotá, colombia. e-mail: cazuluaga@unal.edu.co abstract in the northernmost portion of the serrania de jarara (alta guajira, colombia), low medium grade metamorphic rocks from the etpana metamorphic suite were thermally affected by emplacement of a small calc-alkaline intrusion (parashi stock). detailed petrographic analysis in collected rock samples across the ne and nw plutonic contacts show occurrences of textural and mineralogical changes in the country rock fabric that evidence contact metamorphism overprinting regional metamorphism of the etpana suite. these changes include growth of andalusite (chiastolite), calcic clinopyroxene and amphibole porphyroblast crosscutting sn+1 metamorphic foliation. hornblende-plagioclase barometry (ca. 3.1 kbar) and cooling models for the stock show maximum time temperature evolution in the country rock at the interpreted depth of intrusion (ca. 11 km) and help to evaluate the behavior of the country rock with the changing local geotherm. resumen en el norte de la serrania de jarara (alta guajira, colombia), rocas metamórficas de grado bajo – medio asociadas al conjunto metamórfico de etpana fueron térmicamente afectadas por el emplazamiento de una pequeña intrusion calco-alcalina (stock de parashi). un análisis petrográfico de muestras de rocas colectadas a lo largo de los contactos ne y nw del pluton muestran cambios mineralógicos y texturales en la roca caja que evidencia metamorfismo de contacto superpuesto al metamorfismo regional del conjunto etpana. estos cambios incluyen el crecimiento de porfiroblastos de andalucita (quiastolita), clinopiroxeno cálcico y de anfíboles cortando la dirección de foliación sn+1. cálculos barometricos, usando la asociación hornblenda-plagioclasa (aprox. 3,1 kbar), y modelos de enfriamiento del stock muestran una evolución de las temperaturas maximas en la roca caja a la profundidad de emplazamiento interpretada (ca. 11 km) y ayudan a evaluar el comportamiento de la roca caja con los cambios que ocurrieron en la geoterma local. introduction many of the geologic events that affected northern south america since the late proterozoic are almost completely recorded in rocks of the region known as alta guajira, in colombia (zuluaga et al., 2009). precambrian and paleozoic collisions are recorded by a granulitic gneiss of grenvillian age (cordani et al., 2005, cardona et al., 2006) and by a paleozoic gneiss that is related either to the taconic orogeny or the alleghenian orogeny (cardona et al., 2006; zuluaga et al., 2009). jurassic continental rifting and development of a passive continental margin and subsequent cretaceous-tertiary compressive events have also left traces in the alta guajira geologic record (zuluaga et al., 2009). the present tectonic configuration of the alta guajira is mostly due to interactions of the caribbean, pacific and south american tectonics plates during late mesozoic and cenozoic times (james, 2000; zuluaga et al., 2009). the focus of this paper is to study overprinting contact metamorphism related to a small calc-alkaline intrusion in low to medium grade regional metamorphic rocks and to model the thermal effects of pluton emplacement at depths of less than 12 km. the nw portion of the serrania de jarara (alta guajira, colombia) offers a unique opportunity of such study thanks to the well-preserved surface geology of the area and its excellent outcrops. in the serrania de jarara, a quartzdioritic cenozoic pluton intrudes a late cretaceous (?) mélange with metasedimentary matrix and mafic ultramafic blocks. especially, our research addresses the emplacement conditions of the pluton in the low – medium grade metamorphic country rock that produced a contact metamorphic aureole around the pluton. earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 145-152 keywords: parashi stock, etpana metamorphic suite, alta guajira, thermal models. palabras clave: parashi stock, conjunto metamorfico de etpana, alta guajira, modelos térmicos. record manuscript received: 15/03/2010 accepted for publication: 29/09/2010 research groupin geophysics universidad nacional de colombia sampling and methodology two areas (nw and ne of the stock) were selected for systematic rock sampling along 2 km sections, starting at the pluton contact. sampling was accompanied by a detailed structural description including data collection from joints, cleavage, foliation, and folds’ axes. nearly 100 samples were collected, but only 20 representative samples were selected for petrographic analyses in thin section and four samples were selected for microprobe analyses (mo-2ivd-5, mo-2ivd-12, ag-2ivd-3, gm-2ivd-58). the petrographic analyses of chosen samples consisted in textural description and mineral identification. on one specific sample, with evidence of contact metamorphism, microstructures were described in more detail in order to characterize those linked to contact metamorphism. microprobe analyses followed petrographic interpretation. core and rim compositions were determined for 3 amphibole grains and 3 plagioclase grains in a sample from the pluton (sample gm-2ivd-58). microprobe analyses were carried out with the jeol 8600 probe microanalizer hosted at the university of alabama. the quantitative mineral analyses were performed using a probe of 1 �m beam diameter, 20 na beam current, 15 kv accelerating voltage and counting times of 30 to 45 msec in each peak. structural formulae for amphibole and plagioclase were calculated following the methodology of robinson et al. (1982) and amphibole compositional classification followed leake et al. (1997, 2004). hb-pl thermobarometry, to determine emplacement pressure and temperature, followed the methodology of hammarstrom and zen (1986) and hollister et al. (1987) for crystallization pressure (total al in hornblende) and the holland and blundy (1994) hornblende – plagioclase thermometer for crystallization temperature. a cooling two-dimensional model for the pluton was obtained with the heat 3d program (wohletz and heiken, 1992; wohletz et al., 1999) freely available from the los alamos national laboratory geodynamics kware heat 3d web page (http://geodynamics.lanl.gov/wohletz/heat.htm). this computer program is based in the solution of the heat transfer equation in multiple dimensions with non-linear elements of rock heterogeneity and spatially and temporally variable convection. this program uses a numerical solution for the heat conduction equation: � � � � � � � � t t k t x t y t z � � � � � � � � � 2 2 2 2 2 2 , which overcomes the problems faced when trying to solve analytically the equation. in the above equation � is the thermal diffusivity, t is the time, and t is the temperature. the program allows to quantify heat flow around a magma body and to understand changes in the local geothermal gradient that resulted in contact metamorphism. the model predictions for maximum and minimum temperatures reached in the country rock were used to evaluate microstructures that indicate contact metamorphism overprinting regional metamorphism. geological setting the serrania de jarara is located in the central portion of the alta guajira, colombia (fig. 1), as a small slightly mountainous northwest-trending elongated area. lithologically, this zone is characterized by low medium grade metamorphic rocks (etpana metamorphic suite) and a pluton (parashi stock), surrounded by tertiary sedimentary rocks (siamana and uitpa formations) and quaternary deposits. the first known works about the geology of the guajira peninsula are those of renz (1959), bürlg (1960) and rollins (1960). however, lookwood (1965) was the first to present a detailed study of the metamorphic rocks and the intrusive body present in the serrania de jarara. this author grouped the metamorphic rocks and coined the term etpana formation. in this paper, we adopt the name etpana metamorphic suite (zuluaga et al., 2008) referring to the metamorphic rocks exposed in the northern sector of the serrania de jarara and serrania de macuira and consisting of a large lithologic variety such as quartzite, quartz-schist, graphite schist, talctremolite schist and phyllite associated with blocks of serpentinites, gabbros, and blueschist. lookwood (1965) argued that the protolith for these rocks correspond to a thick sequence of shales and quartz arenites with small quantities of tuffs, deposited in a deep basin. the parashi stock is an intrusive body of quartzdioritic to granodioritic composition with aplitic and dacitic associated dikes. the name parashi pluton was proposed by renz (1959) and redefined by lookwood (1965) as parashi quartzdiorite, who characterized the body as a mesozonal intrusion with a narrow contact metamorphic aureole in the country rock and reported a hornblende k/ar age of 48 ± 4 ma in a quartzdiorite sample from the pluton. this middle eocene cooling age is confirmed by hornblende and biotite k/ar ages in the range 45 – 47 ma reported in cardona et al. (2007). this pluton probably belongs to a paleogene magmatic event recorded at the boundary between the caribbean plate and the south american plate. for example, intrusives of tholeitic affinity, and interpreted as related to island arc tectonic setting, have been reported at the south end of the caribbean plate in venezuela, tobago, talamana and virgin islands (lidiak and jolly, 1996). similar bodies of dioritic to granodioritic composition and tertiary age have also been recognized in panama, costa rica, jamaica, haití, puerto rico and virgin islands (lidiak and jolly, 1996). the rocks of the etpana metamorphic suite were accretioned to south america and partially exhumed during late cretaceous – early tertiary caribbean plate convergence with the south american plate in a tectonic event known as the caribbean episode (james, 2000). the age of the unit has been interpreted to be late cretaceous according to stratigraphic relations with surrounding rocks, including the intrusive episode of the parashi stock during the middle eocene. the mentioned crystalline lithologies are partially covered towards the borders of the serrania by tertiary and quaternary sedimentary units. west, south and east of the serrania de jarara, the siamana formation, a 430 m thick oligocene marine sequence of very fine and fine grained, calcareous and fosiliferous sandstones, lies non-conformably over the etpana metamorphic suite. towards the north, the uitpa formation, an oligocene to lower miocene (rollins, 1960) 150 m thick marine sequence of very fine grained sandstones locally fosilifeorus interlayered with biosparites, mudstones and siltstones, overlies partially the siamana formation in angular uncorfomity and partially lies non-conformable on top of the crystalline rocks of the serrania de jarara. additionally, a great variety of recent deposits of different environments exist in the alta guajira, towards the north of the serrania de jarara it is common to find intramareal dark brown calcareous mud in flat horizons and alluvial fans with abundant coarse grained quartz pebbles. parashi stock the parashi stock is located in the northernmost portion of the serrania de jarara. this is a small elongated intrusive of approximately 56 km2 with the long axis oriented n 45º e (fig. 1). lithologic varieties observed in the stock include a compositional range from tonalites to monzogranite, with enclaves of melanocratic diorites (fig. 2), aplitic dikes (mainly towards the contact) and andesite-dacite dikes intruding the stock and the etpana metamorphic suite. a contact metamorphic aureole is observed around the pluton, in the west margin, the width of the aureole is about 50 m. a sericite-chlorite146 lina fernanda martínez and carlos a. zuluaga c. thermal modeling of pluton emplacement and associated contact metamorphism: parashi stock emplacement in the serranía de jarara (alta guajira, colombia) 147 figure 1. generalized geologic map of the serrania de jarara showing sample localities mentioned in the text. figure 2. outcrop characteristics of the etpana metamorphic suite (left) and the parashi stock (right). note the compositional variation in fine layers and the folding in the etpana suite. note also the enclaves within the tonalite. hammer and chisel give an idea of the scale in the photographs. quartz-feldspar phyllite was metamorphosed to a coarse-grained biotite-quartzoligoclase hornfels. the aureole is larger in the north and east margins (up to 75 m) where metasedimentary calcareous rocks were metamorphosed to diopside-plagioclase-calcite-garnet-quartz hornfels (lookwood, 1965). structurally, the pluton is characterized in map view by its high aspect ratio and by deflection of regional markers (host rock structures), this deflection is most notable in a narrow structural aureole. other structural characteristics of the pluton are a poorly developed magmatic fabric and the presence of a few stoped blocks observed near the contact with variable dimensions form tens of meters to centimeters. tonalite is the most common lithology in the stock; however, variations to granodiorite and monzogranite are observed locally. the most common mineral assemblage is plagioclase + quartz + hornblende + biotite with titanite, zircon, apatite and pyrite as accessory minerals (fig. 3). euhedral to subhedral zoned oligoclase to andesine plagioclase (30% to 40%) and anhedral quartz (20% to 30%), in small pods, are the primary constituents of the rock. the most abundant mafic mineral is subhedral to anhedral olive green to green brown magnesio-hornblende (10% to 30%), which is often accompanied by anhedral olive green to green brown biotite (8% to 10%). inclusions in hornblende are abundant and consist mainly of titanite, zircon and locally biotite. hornblende and plagioclase analyses for sample gm-2ivd-58a are shown in table 1. diorite enclaves are observed commonly within tonalite towards the contacts of the stock. they are fine grained and composed mainly of plagioclase + hornblende + biotite ± quartz. abundant dikes are observed intruding the stock and the etpana metamorphic suite in a n-s array. aplitic dikes occur in the boundaries of the stock as gray-pink fine-grained aplites and are composed primarily by quartz (61%) and alkali feldspar (24%) with minor plagioclase (14%). andesite-dacite dikes have a porphyritic microstructure and contains phenocrysts of zoned plagioclase, quartz, zoned hornblende and biotite. note that the relationship between the stock and the dikes is still unknown. etpana metamorphic suite this metamorphic unit consists mainly of a metasedimentary matrix with embedded mafic and ultramafic blocks. the matrix is variably composed of quartzites, mica schist, graphite schist ad tremolite schist (fig. 2). foliated quartzites appear like discontinuous lenses within the suite and are observed commonly folded. they are composed of quartz with minor hornblende, biotite, garnet, and plagioclase. mica schist have a paragenesis of quartz + muscovite + biotite with minor garnet, chloritoid and plagioclase, and locally andalusite porphyroblasts cutting the sn+1 regional metamorphic foliation (fig. 3). retrograde chlorite is common in quartzite and mica schist. tremolite schist consists mainly of tremolite-actinolite + calcic clinopyroxene + talc with minor plagioclase. porphyroblasts of tremolite are also observed crosscutting the sn+1 metamorphic foliation (fig. 3). blocks of serpentinite, gabbro, and blue schist are observed throughout the unit, and eclogite blocks are suspected to be present. serpentinite consists essentially of antigorite locally with relict pyroxene, minor magnetite, crisotile and carbonate veins. gabbros are medium grained equigranular with cataclastic microstructure near fault zones (milonites). they are composed of plagioclase + pyroxene + amphibole olivine epidote. blocks of glaucophane schist are found within serpentinite bodies and are composed of clinozoicite + glaucophane + chlorite + sodic plagioclase, minor titanite and calcite. boulders of eclogite and other high pressure rocks present in a tertiary conglomerate are suspected to be originated from the suite. quartzites and tremolite schist are the most common lithologies near the contact with the parashi stock and commonly appear in interlayered white and black millimetrical bands (mo-2ivd-5). although it is possible to observe local silicification in the outcrops near the contact, the regional metamorphic foliation is still present in rocks from the contact aureole and the overprinting contact metamorphism is mainly evident in thin section. the presence of tremolite, calcic clinopyroxene and plagioclase porphyroblasts cross cutting sn+1 regional metamorphic foliation within the tremolitic schist (samples mo-2ivd-5, fig. 3) and porphyroblasts of andalusite also cross cutting sn+1 in graphite-biotite schist (samples ag-2ivd-3 y mo-2ivd-15, fig. 3) support field observations of the contact metamorphic aureole around the pluton. pluton emplacement emplacement depth and emplacement process in calc-alkaline intrusives, the al content in calcic amphibole in the presence of an appropriated buffer assemblage can be used to determine 148 lina fernanda martínez and carlos a. zuluaga c. table 1. electron probe micro-analyses of amphibole and plagioclase grains for sample gm-2ivd-58a (parashi stock). anf(1) pl(1) anf(2) pl(2) n(3) 14 13 9 7 sio2 47.69 59.05 48.03 59.05 tio2 1.03 0.94 al2o3 7.59 26.11 7.27 26.11 feo 14.49 14.71 mgo 13.09 13.35 mno 0.55 0.60 cao 11.69 6.94 11.84 6.91 na2o 0.87 7.28 0.84 7.31 k2o 0.61 0.22 0.58 0.26 si 7.02 2.64 7.05 2.64 ti 0.11 0.11 al 1.32 1.38 1.29 1.38 fe2+ 1.78 0 1.76 0 mg 2.87 0 2.89 0 mn 0.07 0.07 ca 1.85 0.33 1.84 0.33 na 0.25 0.63 0.26 0.63 k 0.11 0.01 0.11 0.01 (1) near-rim compositions for mineral pair 1 (2) near-rim compositions for mineral pair 2 (3) number of averaged analyses pressure of crystallization because of a linear relationship between crystallization pressure and al content. several calibrations use this empirical relationship to calculate emplacement pressure (e.g., hammarstrom and zen, 1986; hollister et al., 1987; johnson and rutherford, 1989; schmidt, 1992; anderson and smith, 1995). in this work, the experimental calibration of anderson and smith (1995) based on the previous calibrations of schmidt (1992) and johnson and rutherford (1989) was chosen because it considers the temperature effect on the estimated pressure. the crystallization temperature was calculated using the holland and blundy (1994) hornblende-plagioclase thermometry, were stable hornblende and plagioclase coexist in calc-alkaline rocks. the hornblende-plagioclase thermometry actually consist of two reactions, the a thermometer uses the edenite-tremolite reaction (edenite + 4 quartz = tremolite + albite, in quartz rich rocks) and the b thermometer uses the edenita-richterite reaction (edenite + albite = richterite + anorthite) (holland and blundy, 1994). thermometry and barometry results for two amphibole-plagioclase mineral pairs (mineral analyses shown in table 1) using the anderson and smith (1995) calibration and temperatures from holland and blundy (1994) calibration gives a crystallization pressure of 3.1 ± 0.6 kbar with temperatures in the range 670 – 692 ºc. the calculated pressure (3.1 0.6 kbar) indicates upper level crustal emplacement in agreement with field evidence, as microstructural observations points mostly to elastic deformation (e.g., mixtures of lithologies in the aureole and deflection of regional markers). however, viscous flow is not completely disregarded because of the presence of boudinage. the emplacement process of this pluton will fit better a description of emplacement-ascent driven by buoyancy and regional stress as a viscoelastic diapir in the sense of miller and paterson (1999), where the rheologic behavior of the country rock varies both temporally and spatially from brittle to ductile. the form of the pluton also points to a viscoelastic diapir interpretation since diapirs tend to have high aspect ratios (weinberg & podladchikov, 1994) in contrast with dyke-fed intrusions that are expected to be typically sheet-like, with a low aspect ratio (e.g., mccaffrey & petford, 1997, cruden & mccaffrey, 2001). cooling model it must be emphasized that the conductive cooling model used here only applies to static, or motionless, magma bodies and do not consider heat transfer facilitated by fluid circulation. nonetheless, it is useful as a valuable “base-line” against which to evaluate thermal histories of more dynamic advective and convective systems where movement of liquids facilitates heat transfer. the cooling model attempts to estimate the highest temperature reached in the country rock at the emplacement depth. using a pressure near the value calculated above (3 kbar) and tonalite average density of 2,8 gr/cm3, the emplacement depth for the intrusive is ca. 11 km. the assumed stock geometry for thermal modeling is that of a cylindrical body of 9 km diameter and 9 km height. since there are not available data for the extension of the stock with depth, the height from the stock was assumed. it was also assumed that the stock was emplaced at 3 km below the surface; this implies that the shallowest part of the stock was probably in a zone with brittle to elastic deformation; however, this assumption has not implications in model results since this emplacement depth could be any value between the assumed above and the current erosion level as inferred from thermobarometry. other model assumptions are: magma temperature of ca. 690 ºc (silicic magma), magma thermal conductivity of 3.0 w/m k, magma heat capacity of 1800 j/kg k (murase and mcbirney, 1973; clauser and huenges, 1995; waples and waples, 2004), an initial local geothermal gradient inside the magma body of 20 ºc/km, a near surface geothermal gradient of 25 ºc/km, a closed simple conductive cooling magmatic system, and host rock consisting of pelitic schist with density of 2770 kg/m3, conductivity of 2.5 w/m k, and heat capacity of 1150 j/kg k (murase and mcbirney, 1973; clauser and huenges, 1995; waples and waples, 2004). figure 4 illustrates model states after the intrusion with pelitic country rock at 0, 5 x 103, 10 x 103, 15 x 103, 20 x 103 and 30 x 103 years after the intrusion. in the initial stage, just after the intrusion, the magma is at an initial temperature of 690 ºc. thermal evolution at 10 x 103 years from the intrusive event shows a cooling magma and an anomalous high temperature geotherm in the country rock. the temperature of the country rock increased up to 461 ºc at 11 km depth and 100 m from the contact. in the subsequent 20 x 103 and 30 x 103 years after the intrusion, the temperature profile in the country rock does not show significant changes. at 40 x 103 years after intrusion, the temperature of more than half of the magma body had decreased to less than 600 ºc. in the country rock, the horizontal thermal profile show that the maximum temperature at 4 x 104 years is 467 ºc, which represents the contact metamorphic peak and indicates that the temperature in the inner aureole increased beyond the peak at 3 x 104 years, this contact metamorphic peak last from the time the maximum temperature is reached to 8 x 104 years. more than 80% of the magma has crystallized at 10 x 104 years after the intrusion and temperature at 11 km depth in the inner aureole has dropped to 463 ºc. the temperature into the country rock fell to 428 ºc at 3 x 105 years after the intrusion. when semipelitic schist and serpentinite country rocks are considered as the country rock instead of pelitic schist, the disturbance of the local geotherm behaves differently. with a semipelitic country rock, contact metamorphic peak is reached earlier than in a thermal modeling of pluton emplacement and associated contact metamorphism: parashi stock emplacement in the serranía de jarara (alta guajira, colombia) 149 figure 3. photomicrographs of the andalusite-mica schist (left) and tremolite schist (center) from the etpana metamorphic suite and tonalite (right) from the parashi stock. note the crosscutting relations between porphyroblasts of andalusite and tremolite with the regional metamorphic foliation of the etpana suite. pelitic country rock; the maximum temperature of 422 ºc is reached at 2 x 104 years and this disturbance extends trough 3 x 104 years. a serpentinite country rock will reach the maximum temperature of 475 ºc at 5 x 104, and the disturbance will last up to 10 x 104 years. calculated latent heat in the model shows that during the first 1 x 104 years only 35% of the magma body has crystallized; subsequently, at 5 x 104 years 69% of the body has crystallized, and at 10 x 104 years 88% of the body has crystallized. this indicates that the crystallization is faster during the initial stages after emplacement, and that the complete crystallization of magma body occurs at about 12 x 104 years after intrusion. discussion and conclusions the thermal behavior in the contact aureole shows that the thermal peak is not reached at the same time in all areas around the pluton, while rocks in the inner aureole start cooling after its thermal peak those further out of the contact are still below their thermal peak. in the model, the country rock at 1 km from the stock reaches its maximum temperature (ca. 465 ºc) at 1 x 104 years after the intrusion; this is the thermal metamorphic peak of the contact metamorphism. rocks 10 km away from the contact reach their maximum temperature at 15 x 104 years after the intrusion. the assemblage andalusite + biotite (sample mo-2ivd-15) occurs in a wide field at low pressure; however, the absence of cordierite restrict temperatures below the first cordierite producing reaction (mg-chlorite + al2sio5 = mg-cordierite + h2o), i.e., below 500 ºc. this is in agreement with results from the thermal model of pluton emplacement where modeled temperatures in a pelitic host rocks gives a maximum temperature of ca. 465 ºc. thermal modeling, following the observed geological features and with reasonable assumptions for physical and chemical magma and country rock properties, helps to understand the geologic history of intrusive – country rock system and has to be used only as an approximation to the real behavior of the system. model predicts that, at a depth of 11 km, temperatures above 400 ºc that could produce a metamorphic contact aureole in the country rock are only reached in a narrow zone of less than 200 m around the contact and that the peak temperature is reached soon after the intrusion (ca. 465 ºc at 1 x 104 years after the intrusion, see fig. 4). 150 lina fernanda martínez and carlos a. zuluaga c. figure 4. thermal modeling results of a run with a pelitic country rock. the red lines in the figures represent the contact of the intrusive with the country rock. the color bar the bottom represents the temperature scale. acknowledgements this work was developed within the mapping project of the alta guajira region, colombia, with funding from ingeominas and universidad nacional de colombia. the authors thank ingeominas for their financial and logistic support during fieldwork in the serrania de jarara. the department of geological sciences (universidad nacional de colombia) allowed the use of petrographic microscopes and other equipment. we also thank to dr. harold stowell (department of geological sciences, university of alabama) for his assistance with the acquisition of microprobe analyses. the manuscript was improved following comments and suggestions from two anonymous reviewers. references anderson, j.l. & smith, d.r., 1995. the effects of temperature and fo2 on the al-in-hornblende barometer. american mineralogist, 80, 549-559. bürgl, h., 1960. geología de la península de la guajira: serv. geol. nac. (colombia), bol. geol., 6, 129-168. cardona, a; cordani, u, macdonald, w., 2006. tectonic correlations of pre-mesozoic crust from northern termination of the colombian andes, caribbean region. journal of south american earth sciences 21. 337-354. cardona, a., weber, m., wilson, r., cordani, u., muñoz, c. m., & paniagua, f., 2007. evolución tectono-magmática de las rocas máficas-ultramáficas del cabo de la vela y el stock de parashi, península de la guajira: registro de la evolución orogénica cretácica-eocena del norte de suramérica y el caribe. xi congreso colombiano de geologia, bucaramanga. 6p. clauser, c., & huenges, e., 1995. thermal conductivity of rocks and minerals. in: t. j. ahrens (ed.), rock physics and phase relations a handbook of physical constants, agu reference shelf, vol. 3, p. 105-126, american geophysical union, washington. cordani, u.g., cardona, a., jiménez, d.m., liu, d., nutman, a.p., 2005. geochronology of proterozoic basement inliers in the colombian andes: tectonic history of remnants of a fragmented grenville belt. from: vaughan, a.r.m., leat, p.t. pankhurst, r.j. (eds), terrane processes at the margins of gondwana. geological society, london, special publications, 246, 329-346. cruden, a. r. & mccaffrey k. j. w., 2001. growth of plutons by floor subsidence: implications for rates of emplacement, intrusion spacing and melt-extraction mechanisms, physics and chemistry of the earth, part a: solid earth and geodesy, 26, 303-315. hammarstrom j.m., & zen, e., 1986. aluminum in hornblende; an empirical igneous geobarometer. american mineralogist, 71, 1297-1313. holland, t. & blundy, j., 1994. non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. contributions to mineralogy and petrology, 116, 433-447. hollister, l.s. grissom, g.c., peters, e.k., stowell, h.h., & sisson, v.b., 1987. confirmation of the empirical correlation of al in hornblende with pressure of solidification of calc-alkaline plutons. american mineralogist, 72, 231-239. james, k.h., 2000. the venezuelan hydrocarbon habitat, part 1, tectonics, structure, palaeogeography and source rocks. journal of petroleum geology, 23, 5-53. johnson, m.c. & rutherford m.j., 1989. experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera (california) volcanic rocks, geology,17, 837-841. leake, b.e., woolley, a.r., arps, c.e.s., birch, w.d., gilbert, m.c., grice, j.d., hawthorne, f.c., kato, a., kisch, h.j., krivovichev, v.g., linthout, k., laird, j., mandarino, j.a., maresch, w.v., nickel, e.h., rock, n.m.s., schumacher, j.c., smith, d.c., stephenson, n.c.n., ungaretti, l., whittaker, e.j.w., and youshi, g., 1997. nomenclature of amphiboles: report of the subcommittee on amphiboles of the international mineralogical association, commission on new minerals and mineral names. the canadian mineralogist, 35, 219-246. leake, b.e., woolley, a.r., birch, w.d., burke, e.a.j., ferraris, g.,grice, j.d., hawthorne, f.c., kisch, h.j., krivovichev, v.g., schumacher, j.c., stephenson, n.c.n., and whittaker, e.j.w., 2004. nomenclature of amphiboles: additions and revisions to the international mineralogical association’s amphibole nomenclature, american mineralogist, 89, 883–887. lidiak, e.g., & jolly, w.t., 1996. circum-caribbean granitoids: characteristics and origin. international geological reviews, 38, 1098–1133. lockwood, j. p., 1965. geology of the serrania de jarara area. guajira peninsula, colombia. ph.d. thesis, princeton univ.167 p. new jersey. mccaffrey k.j.w. & petford, n., 1997. are granitic intrusions scale invariant?. journal of the geological society london, 154, 1–4. miller r.b., & paterson s.r.,1999. in defense of magmatic diapirs. journal of structural geology, 21, 1161-1173. murase t., & mcbirney a.r., 1973. properties of some common igneous rocks and their melts at high temperatures. geological society of america bulletin, 84, 3563-3592. renz, o., 1959. geología de la parte sureste de la península de la guajira. iii congreso geologico venezolano, boletín de geología, publicación especial, ministerio de minas e hidrocarburos., 3, 317-347. robinson p., schumaclier j.c., spear f.s., laird, j., klein, c., evans, b.w., & doolan, b.l., 1982. phase relations of metamorphic amphiboles: natural occurrences and theory. in: veblen dr, ribbe ph (eds) amphiboles: petrology and experimental thermal modeling of pluton emplacement and associated contact metamorphism: parashi stock emplacement in the serranía de jarara (alta guajira, colombia) 151 400 0 100 200 300 500 20 100806040 time (1000 years) te m p er at u re (c ) figure 5. peak temperature reached at ca. 50 m from the contact at 11 km depth in the models shown in figure 4. phase relations. reviews in mineralogy 9b, mineralogical society of america, washington, dc, 1-227. rollins, j., 1960. stratigraphy and structure of the goajira unpublished ph.d. thesis, dept. geol., univ. of nebraska, lincoln, nebraska (revised version in press, univ. of nebraska press). schmidt, m.w., 1992. amphibole composition in tonalite as a function of pressure: an experimental calibration of the ai-in-hornblende barometer. contributions to mineralogy and petrology, 110, 304-310. waples, d. & waples, j., 2004. a review and evaluation of specific heat capacities of rocks, minerals, and subsurface fluids. part 1: minerals and nonporous rocks. natural resources research, 13, 97-122. weinberg, r. f. & podladchikov, y. 1994. diapiric ascent of magmas through power-law crust and mantle. journal of geophysical research, 99, 9543-9559. wohletz, k., heiken, g., 1992. volcanology and geothermal energy. university of california press, berkeley, ca, 432 pp. wohletz, k., civetta, l., & orsi, g., 1999. thermal evolution of the phlegraean magmatic system. journal of volcanology and geothermal energy, 91, 381-414. zuluaga, c. ochoa, a., muñoz, c., guerrero, n., martinez, a., medina, p., pinilla, a., rios, p., rodriguez, p., salazar, e., zapata, v. 2009. cartografía e historia geológica de la alta guajira, implicaciones en la búsqueda de recursos minerales, acuerdo especifico 030 de 2006. ingeominas-universidad nacional de colombia, sede bogotá, 536 p. 152 lina fernanda martínez and carlos a. zuluaga c. geociencias-vol 13-2 2009.vp 97 earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 97-107 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential orlando hernández1,2, ralph r.b. von frese2,3, khurama, s.1,4 1 dept. of geosciences, universidad nacional de colombia, bogotá, d.c., colombia ohernandezp@unal.edu.co 2 school of earth sciences & byrd polar research center, the ohio state university, columbus, oh 43210, usa 3 laboratory for space geodesy & remote sensing research, the ohio state university, columbus, oh 43210, usa 4 ingeominas. bogotá, d.c., colombia abstract a prominent positive free-air gravity anomaly mapped over a roughly 50-km diameter basin is consistent with a mascon centered on (4°30`n, -69°15`w) in the vichada department, colombia, south america. the inferred impact crater is nearly one third the size of the chicxulub crater. it must have formed recently, in the last 30 m.a because it controls the path of the vichada river. the impact crater structure has been partially eroded and is almost completely covered by vegetation. no antipodal relationship has been established. the proposed impact appears to have contributed to the development of mineral deposits of economic interest. the impact shock waves extensively thinned and disrupted the precambrian cratonic crust. a thick sedimentary cover, dense vegetation and erosional processes greatly limits direct geological testing of the inferred impact basin. ground follow up gravity and magnetic surveys are recommended for confirming the regional free air gravity anomalies. key words: impact crater, vichada, gravity, magnetic. resumen una anomalía gravimétrica positiva de aire libre ha sido mapeada sobre una cuenca de aproximadamente 50 km de diámetro, la cual coincide con la ubicación de una concentración de masa o mascon, centrada en las coordenadas 4o30`n, -69o15`, en el departamento de vichada, colombia, suramérica. la estructura de impacto de meteorito inferida corresponde a una tercera parte del tamaño de la estructura de impacto de chicxulub. esta estructura debe haberse formado en los últimos 30 m.a. debido a que controla la trayectoria del río. la estructura de impacto ha sido parcialmente erosionada y está casi completamente cubierta por una densa vegetación. no se han establecido relaciones anti polares. el impacto de meteorito propuesto pudo haber contribuido al desarrollo de depósitos minerales de interés económico. las ondas de choque han adelgazado y distorsionado intensamente la corteza cratónica precámbrica. una delgada secuencia sedimentaria, la vegetación densa y los procesos erosivos limitan considerablemente la verificación de la estructura mediante geología de superficie. se recomienda realizar estudios de gravimetría y magnetometría terrestre para confirmar la ocurrencia de las anomalías gravimétricas regionales asociadas a la estructura. manuscript received: 11/06/2009 accepted for publication: 20/11/2009 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:13 p p p composite 133 lpi at 45 degrees palabras clave: estructura de impacto, vichada, gravimetría, magnetometría. introduction the vichada structure is located in the southern region of the vichada department, colombia (4º30’n – 69º15’w), having a multiple ring structure with inner and outer ring diameters of 30 km and 50 km, respectively (figure 1).the vichada structure was initially proposed by rocca (2004) who recognized a possible large impact crater structure based on the remote sensing interpretation of color landsat satellite images of the area, with a resolution of 200m, obtained from the nasa’s john c. stennis space center, usa (figure 2, rocca, 2004). this area is part of the vichada plain and it is partially covered by quaternary deposits and vegetation. the central core consists of a ring about 30 km. in diameter which surrounds a central depression of circular shape and 20 km. in diameter. in this innermost region, there is a basin, the relief is quite smooth and it is the deepest part of the structure. the central basin is covered by dense vegetation and it is surrounded by 2 concentric rings of low hills of few meters each that have been intensively eroded. the outermost ring has 50 km in diameter and in the south, the vichada river flows around it in a perfect semi-circle path following the external limits of this outer ring of hills. the outer ring also controls some local drainage systems in the north and northwest. the flow of the rivers around these features is anomalous. the geology of the area is limited exposed and is represented by tertiary sandstone and claystone formations, covered by quaternary deposits (de la espriella et al., 1990). the crystalline basement is formed by the precambrian parguazan granite, outcropping along the vichada and orinoco rivers far away to the east of the studied area. the hidden geology includes metasedimentary and granitic rocks with an extensive sedimentary cover. the sedimentary 98 orlando hernández, ralph r.b. von frese and khurama, s. figure 1: geographic location of the vichada structure in eastern colombia, northwestern south america. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:13 p p p composite 133 lpi at 45 degrees units are composed by a heterogeneous sequence of conglomerates, sandstones and clays. they are dated oligocene to pliocene and they cover the precambrian crystalline basement rocks. the structure was forwarded modeled for gravity and magnetic signatures that were compared with observed gravity and magnetic profiles (khurama, 2007). this forward modeling eliminates the possibility of the presence of other geological causative bodies such as igneous intrusions, maars, diatremes or sedimentary structures, leaving the model of an impact crater as the most probable phenomenon that could generate the circular structure. rocca (2004) suggested that the vichada structure is probably older than 30 ma. there is not additional available geochronological data to support the age of the structure. the vichada flat plain is located east of the andean mountains. this part of northwestern south america has poorly been studied for crustal properties. the lack of airborne and terrestrial geophysical data and poorly known geology has been recognized in global databases such as the crust 2.0 model (bassin et al., 2000) that assignees crustal thickness to the guiana craton taken data from other cratonic areas of the world. extending the hunt for impact basins into south america is problematic because the jungle and quaternary deposits cover more than 95% the crust and the remote environment greatly limits field studies. thus, our understanding of the geological and geophysical properties of the crust is very poor. however, egm96 spherical harmonic earth gravity model to degree and order 360, egm96 data (lemoine, et al., 1998) provides important details of the gravity field that can further our understanding of the crustal features. spectral correlation theory was used to analyze the co-registered faga and tge for their anomaly correlations. specifically, the fourier transforms t and f of tge and faga, respectively, were used to obtain their correlation spectrum (von frese et al., 1997a, kim, et al., 2000) given by cc k k f k t k t k f k ( ) cos( ) re ( ) ( ) ( ) ( ) � � � � � � � � where cc(k) is the correlation coefficient between the kth wavenumber components f(k) and t(k), and denotes taking the real parts of the wavenumber components. usually, 99 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential figure 2: tm image of the vichada structure showing the vichada river, the inner and out ring structures of 30 km and 50 km diameter, respectively (modified from rocca, 2004) enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:14 p p p composite 133 lpi at 45 degrees cc(k) is evaluated from the cosine of the phase difference between the two kth wavenumber components. using the correlation spectrum between faga and tge, spectral correlation filters were designed to extract terrain-correlated free-air gravity signals. those wavenumber components showing intermediate to high positive (ccp(k) > 0.3) and negative (ccn(k) < 0.3) correlations were identified. the cut off values for the correlation filter were determined to minimize correlative features between the terrain-decorrelated free air and compensating terrain gravity components. in100 orlando hernández, ralph r.b. von frese and khurama, s. terrain correlated free air gravity anomalies tdfaga mgals tcfaga mgals terrain decorrelated free air gravity anomalies corrected terrain gravity anomalies moho km ctge mgals depth to moho (km) figure 3. gravity anomaly maps including: a) terrain correlated free air gravity anomalies, tcfaga b) terrain decorrelated free air gravity anomalies, tdfaga c) compensated terrain gravity effects ctge and d) depth to moho, calculated at 20 km altitude. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:15 p p p composite 133 lpi at 45 degrees versely transforming positively and negatively correlated free-air wavenumber components according to the selected cut off values yielded the terrain-correlated free air gravity anomalies (tcfaga) in figure 3.a. the residual terrain-decorrelated free-air gravity anomalies (tdfaga) in figure 3.b were calculated by subtracting tcfaga from faga, so that faga = tcfaga + tdfaga tcfaga are explained by anomalies associated with the topography while tdfaga include the gravity effects of sources within the crust (i.e., local bodies) and the subcrust. as a result, figure 3 shows a) the terrain correlated free air gravity anomalies (tcfaga), b) terrain de-correlated free air gravity anomalies (tdfaga), c) the compensated terrain gravity effects (ctge) and d) inversely modeled depth to moho, respectively, calculated at 20 km from the earth gravity model 1996 (hernández, et al., 2008) that is constrained by 30-arc minute averages of available surface gravity values and gravity estimates from orbital variations of satellites operating at altitudes generally higher than 700 km (lemoine et al., 1998). tcfaga and tdfaga have been obtained by spectrally correlated terrain gravity effects (tge) and free air gravity anomalies (faga). similarly, fctge and fdtge were calculated. tcfaga detail is particularly significant over the vichada structure. the terrestrial egm96 anomaly estimates in figure 3 essentially reflect the lower order effects from gravity observations located some ± 20 km. the egm-96 results include the prominent positive anomaly feature that overlies the vichada structure basin in figures 1 and 2 centered on (4º30’n – 69º15’w) in the vichada plain. it consists of a circular central positive anomaly flanked by a negative ring anomaly that in isometric view looks like a mexican sombrero. the sombrero anomaly is commonly observed over giant impact basins on the moon and mars, where it is attributed to the effects of uncompensated concentrations of mass produced within the crust from the impact (figure 4; von frese et al, 2009). the central positive free-air gravity anomaly results from the subjacent concentration of enhanced mass or ‘mascon’ of uncompensated components of 101 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential orientale basin, moon isidis basin, mars wlic orientale, moon isidis basin, mars wlic, antarctica figure 4: terrain gravity effects (left), free air gravity anomalies (center) and depth to moho (right) of giant impact craters including the orientale basin in the moon (up), the isidis basin in mars (middle) and the wilkes land impact crater in antarctica (bottom; von frese et. al., 2009). the vichada structure has similar features in gravity anomaly maps. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:15 p p p composite 133 lpi at 45 degrees any basaltic fill in the crater and the mantle plug intruded into the crust when it recoiled from meteorite impact. the strength of the lithosphere presumably supports the mascon against mass-equilibrating isostatic forces to produce the central gravity maximum. lithospheric strength also maintains the isostatically under-compensated thickened crust around the mantle plug to produce the flanking ringed free-air minimum. in the sections below, we quantitatively model the crustal properties of the inferred vichada impact crater from the egm-96 and terrain data. we also consider the implications of the inferred impact for the development potential economic deposits. gravity anomaly modeling figure 3 provides more detailed views of the egm-96 free-air gravity anomalies at 20-km altitude superposed on the terrain topography of the vichada study area. gravity anomaly estimates were derived from the egm-96 gravity model evaluated to spherical harmonic degree and order 90 to emphasize the more robust qualities of the egm-96 measurements. the higher frequency components, for example, are relatively poorly determined according to the model’s covariance properties. and at lower altitudes, the anomaly estimates have dramatically increasing noise levels, as well as decreasing reliability due to the lack of terrestrial gravity observations (von frese et al., 1999) and the lack of uniqueness of the gravity modeling (von frese et al., 2005). tcfaga and tge were normalized to facilitate recognizing the anomaly correlations between them (von frese et al., 1997). to enhance the visual perception of the anomaly correlations, the following transformation was applied: zi (x) = � � � �z xi x x z � � � � � � � where �z and represent the values of the mean and standard deviation, respectively, of the signal x. the expression in parentheses standardizes the xi coefficients to zero mean and unit standard deviation and is dimensionless. however, the values for mean and standard deviation of of the normalized signal z can be specified by the user to facilitate the visual analysis. local favorability indices (merriam and sneath, 1966) were used to highlight the various anomaly correlations in the normalized tcfaga and fctge (figure 5). positively correlated features were mapped out by summed local favorability indexes (slfi) obtained by: slfi zi x z z zi y z z i � � ( ( ) ) ( ( ) )� � � � where z(x) and z(y) were the normalized tcfaga and tge coefficients, respectively. the peak-to-peak correlations between the two data sets were mapped out by positive slfii (figure 5a), whereas trough-to-trough correlations were mapped out by the negative coefficients of slfii (figure 5b). the slfi coefficients brought out the positively or directly correlated features, while suppressing the negatively and null correlated features between tcfaga and tge. to enhance the perception of inversely correlated features obtained from wavenumbers with negative correlation coefficients, the normalized and scaled datasets were subtracted cell by cell for differenced local favorability indexes (dlfi) by: dlfi zi x z z zi y z z i � � ( ( ) ) ( ( ) )� � � � positive features in tcfaga which were correlative with negative tge features (peak-to-trough) were mapped out by positive dlfi in figure 5c, whereas negative tcfaga that were correlative with positive tge were mapped out by negative dlfi in figure 5d. the dlfi coefficients emphasized the inversely correlated features, while suppressing the positively and null correlated features between tcfaga and tge. with an exception of figure 5b, it is clear how the slfi and dlfi products show local favorability indices that enhance the vichada impact structure. the mascon gravity anomaly in figure 6 consists of the well-defined relatively positive 26 mgal anomaly overlying the inferred impact basin. to evaluate the inferred mascon’s first order crustal properties, moho relief was determined with gravity effects that fully nullified the gravity effects of the terrain. then, this moho relief was adjusted to lower levels until it nullified the egm-96 mascon free-air gravity anomaly. thus, the mascon was interpreted underlying the basin as the uncompensated component of the mantle plug given by the difference between the two moho levels. the modeling, although not unique, accounts for the independently observed terrain and gravity data of the study region. the spherical coordinate gravity effects of the terrain were modeled at 20 km altitude using gauss-legendre quadrature integration (von frese et al., 1981; hernandez, 2006) and a density of 2.85 gm/cm3 for the components of the terrain. the altitude of 20 km was chosen for modeling these effects to limit local terrain density errors (leftwich et al., 2005). then, moho variations were modeled from the in102 orlando hernández, ralph r.b. von frese and khurama, s. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:15 p p p composite 133 lpi at 45 degrees 103 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential tcfaga (peak) to fctge (peak) slfi peak to peak slfi troughto trough tcfaga (trough) to fctge (trough) tcfaga (peak) to fctge (trough) dlfi t eakrough to p dlfi peak to trough tcfaga (trough) to fctge (peak) figure 5: local favorability indices including a) slfi tcfaga peak to fctge peak, b) slfi tcfaga trough to fctge trough c) dlfi tcfaga peak to fctge trough and d) dlfi tcfaga trough to fctge peak. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:16 p p p composite 133 lpi at 45 degrees version of complete bouguer anomalies obtained by subtracting the terrain gravity effects from zero free-air anomalies. for this modeling, a horizontal density contrast of 0.4 g/cm3 was assumed between mantle and crust fully compensated the basin’s crust across the moho. figure 6 summarizes the modeling results on a profile along e-w for an inverse modeling using the utilities from gm-sys. the geological model shows the inferred moho estimates that characterize the mantle plug beneath the impact basin as being some 30-km across and 5-km thick. these spatial dimensions are probably minimal because erosion, sedimentation, and viscous relaxation of the crust have muted the topography. however, the overlying positive egm-96 free-air anomaly suggests that the crustal model may not be completely compensated. therefore, the inferred moho was adjusted to obtain the equilibrium moho with gravity effects that effectively nullify the egm-96 free-air anomaly. discussion the crustal modeling suggests that the mantle component in figures 3d and 6 can account for the mascon of the inferred vichada impact crater. the diameter d1 � 50 km of its central basin ring estimated from the topography is nearly one third times the 180-km central basin diameter of chicxulub impact crater. large multi-ring basins of the moon and other terrestrial planets appear to follow the dn n� ( )2 1 d1 ring spacing rule for the diameter dn of the nth-ring (e.g., spudis, 2005). by this rule, the vichada impact crater roughly intersects the vichada river as shown in figures 1 and 2. this result raises the interesting possibility that meteorite-impacted crust influenced the formation of mineral and hydrocarbon deposits of economic interest in the area, as proposed for the sudbury impact crater and associated mineral deposits in ontario, canada (wilson, 1989). the most expedient and viable test of the inferred vichada structure mascon is to explore for confirming anomalies with airborne gravity and magnetic surveys. impacts such as chicxulub are also known to modify crustal magnetization significantly. satellite magnetic observations mostly characterize the thinned crust of the study region as positively magnetized consistent with the effects of post-impact igneous modifications of the crust. clearly, modern aeromagnetic surveying can provide another important test of the crustal properties of vichada impact crater. economic implications resource-forming processes can be linked to approximately 25% of all terrestrial impact structures world-wide, either as 104 orlando hernández, ralph r.b. von frese and khurama, s. c:\vichada\crater gravity observed calculatedg ra vi ty (m g al s) d ep th (k m ) 40 -10 -60 2 22 42 crust (2.2 gm/cc) mantle (3.25 gm/cc) distance (km) mascon -10 165 340 c:\vichada\masconvw gravity observed calculated g ra vi ty (m g al s) d ep th (k m ) 40 -10 -60 32 36 40 crust (2.85 gm/cc) mantle (3.25 gm/cc) distance (km) mascon -10 165 340 c:\vichada\cratervw gravity observed calculated g ra vi ty (m g al s) d ep th (k m ) 40 -10 -60 -0.1 -0.05 0 distance (km) outer ring -10 165 340 crust (2.85 gm/cc) inner ring figure 6: inversely modeled tcfaga at 20 km, oriented e-w along the vichada structure, enhancing the topography (upper right) and the mascon (lower right) or mass concentration of the mantle plug into the lower crust. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:16 p p p composite 133 lpi at 45 degrees a direct result of the impact event or through subsequent modification of the target rocks. meteoroid impacts are catastrophic events that release enormous amounts of energy that can heat, displace and metamorphose the target rocks instantaneously. the extreme geologic environment and processes that result from an impact event can result in the creation, or preservation of, a range of economic resources. grieve and masiatis (1994) predict that resource-forming processes can be linked to approximately 25% of all terrestrial impact structures, either as a direct result of the impact event or through subsequent modification of the target rocks. the sudbury impact event was one of the largest ever recorded on the earth, with an estimated original crater diameter in excess of 200 km. the impact event is predicted to have excavated a hole in the crust 20 km deep and created a melt sheet 60 km wide and 2.5 km deep (masaitis, 1989). during the early stages of cooling of this melt, the sudbury igneous complex, sulphide-rich liquids exsolved and settled to the bottom of the sheet to produce the 50 or more known ni, cu and pge deposits of the sudbury camp. extreme shock pressures, in excess of 35 gpa, near the point of impact may create impact diamonds by the direct modification of the atomic structure of graphite or coal within the target rocks. while rich concentrations of diamond may be found in impact melt rocks produced from a graphite-bearing target rock, eg 1.5 mt at 9.13 carats/t at zapadnaja, ukraine (grieve and masaitis, 1994), these solely consist of fine-grained aggregates suitable only for industrial purposes. the most significant example of a progenetic relationship of an economic resource with an impact crater is between the witwatersrand goldfields and the vredefort structure where many of the mining districts are likely to have been preserved from erosion by the downwards displacement by several kilometres of the target rocks within the structure surrounding the vredefort dome. hydrocarbon accumulations are one of most common economic resources associated with impact craters, with almost half of the twenty-six craters formed in north american basins associated with commercially exploited fields. a number of geologic effects resulting from a meteorite impact directly affect the ability of the target rocks to generate and accumulate hydrocarbons. the most important effect is the creation of thick layers of brecciated and faulted rock, both within and outside the crater, that can form ideal reservoir rocks that can produce at high rates of flow. these can then be “capped” by a layer of fine grained fallout that can produce a natural seal for the reservoir. structural traps may also be produced faulting in the collapsed terraces of a complex crater. heat transferred into the ground by the impact and stored in the deformed rocks of the central uplift and the surrounding melt and breccia can interact with ground or meteoric waters to produce local hydrothermal systems. for example, fluid inclusion studies of the 24 km diameter haughton crater in canada suggest that a hydrothermal system with temperatures between 200ºc and 600ºc existed for several thousand years following the impact event. except for the origin of the melt, these hydrothermal systems are analogous to the convection cells set up by igneous intrusions and have the potential to remobilize metal and form epithermal deposits. several small vein-hosted deposits have been found within, or close to, the central uplift of several impact craters, including historically mined deposits totaling 300kt grading 3% pb, 1.5% zn near the 55 km diameter siljan structure in sweden (grieve and masiatis, 1994). very large impact craters, which can maintain hydrothermal activity for around a million years, can produce their own exhalative systems in the overlying sedimentary rocks. two such deposits, with a combined resource of over 6mt @ 4.4% zn, 1.4% cu and 1.2% pb, are formed in the sedimentary rocks of sudbury basin that overlie the thick sequence of impact breccia and melt. perhaps more important than the transient hydrothermal cells driven by heat generated by the impact event, deformation of the target rocks will have a long term effect on the local hydrogeology around the impact site. the same deformed rocks that form hydrocarbon reservoirs can also be excellent aquifers. craters can also naturally fill with water to form large lakes in wet climates. when the mouth of the river draining from the 70 km diameter manicouagan structure was dammed it produced a reservoir with a capacity of approximately 142 trillion litres of water. outflow from this reservoir powers two hydroelectric power stations with a combined capacity of 2500 mw per annum. the impact crater itself can form a third order basin where the deposition of sediments may be greatly different from the surrounding area. if a crater-lake is formed in a dry climate, this can provide an excellent location the formation of thick evaporite deposits. massive oil shale deposits are found within several craters in the former ussr, containing a potential resource of close to a billion barrels of oil. the vichada structure can be one of the seven hidden impact crater structures that may be for each known outcropping impact craters that have generated associated mineral deposits. conclusions the occurrence of an impact in the vichada plain, in northwestern south america is consistent with the regionally disrupted and thinned crust of the vichada structure which includes a local 50km diameter depression beneath a 105 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:16 p p p composite 133 lpi at 45 degrees well-defined egm-96 free-air gravity maximum. the crater’s terrain gravity effects accommodate an underlying roughly 30-km wide, 5-km thick mantle plug produced by crustal rebound from meteorite impact. however, the inferred mascon signal in the egm-96 gravity data suggests that no more than about 70% of the mantle plug’s mass may be isostatically compensated at the moho. the crustal ring structure of the vichada impact crater intersects the vichada river thus the impact pre-dated and influenced the development of the vichada river and the morphological features around its inner and outer rings. the impact scenario also helps explain the crustal thinning on the studied area. impact craters have generally been poorly regarded as targets or economic resources despite many global examples confirming their prospectivity. the greatest chance of exploration success lies in targeting craters formed in sedimentary basins for hydrocarbons; almost 50% of such structures in north america are associated with oil or gas reserves. in the quest to meet the world’s growing energy needs, an understanding of the role of hydrocarbons in meteorite impact structures may become increasingly important. this can only happen with a clearer understanding of impact structures in general and the conditions necessary to form petroleum systems within them. identifying impact structures that meet these conditions is difficult, but the results of such efforts hold the possibility of significant economic returns. the vichada impact crater is supported from landsat-tm images and interpreted egm-96 data. this 50kmdiameter impact crater is the biggest in south america. this structure is an interesting prospect for mineral and/or hydrocarbon exploration. it has a multiple ring structure covered by a sedimentary sequence of varying thickness from 200m to 1600m. the vichada impact crater affected rocks from the precambrian guiana granitic craton and tertiary sedimentary units. a combined em and magnetic airborne survey is recommended to resolve additional crustal and basin properties and evaluate its economic potential acknowledgements elements of this research were produced with support from the geophysical group at the universidad nacional de colombia, fulbright commission, anglogold ashanti and colfuturo. references bassin, c., laske, g., masters, g. 2000, the current limits of resolution for surface wave tomography in north america, eos trans agu, 81, f897. de la espriella, r., flórez, r., galvis, j., gonzález, c.f., mariño, j. & pinto, h. 1990. geología regional del norte de la comisaría del vichada. geología colombiana, (17), 93-106. grieve, r.a.f. and masaitis, v.l. 1994. the economic potential of terrestrial impact craters: internat. geol. rev., 36, 105-151. hernandez, o. 2006 tectonic analysis of northwestern south america from integrated satellite, airborne and surface potential field anomalies, phd dissertation, shool of earth sciences, the ohio state university, pp 198. hernández o., von frese r., khurama s. 2007. geophysical evidence of an impact crater in northwestern south america”, 2007joint assembly agu, acapulco. may 2007 khurama, s., 2007 caracterizacion geológica y geofisica de la estructura del rio vichada, inspeccion de palmarito, departamento de vichada, msc thesis, geosciences department, universidad nacional de colombia, pp 164. grieve, r.a.f. and masaitis, v.l. 1994. the economic potential of terrestrial impact craters: internat. geol. rev., 36, 105-151. kim, j.w. von frese, r.r.b., rae, k.h. 2000. crustal modeling from spectrally correlated free-air and terrain gravity data a case study of ohio. geophysics, 65, 4, 1057-1069. leftwich, t.e. von frese, r.r.b., potts, l.v., kim, h.r. roman, d.r., taylor, p.t., barton, m. 2005, crustal modeling of the north atlantic from spectrally correlated free-air and terrain gravity, j. geodynamics, 40 (2005) 23-50. lemoine, f.g., chinn, d.s., cox, s.m., factor, j.k. kenyon, s.c. , klosko, s.m., luthcke, s.b., olson, t.r., pavlis, e.c., pavlis, n.k., rapp, r.h., smith, w.h.f., sandwell, d.t. 1994, bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry. j. geoph. res, 99, 21803-21824. 106 orlando hernández, ralph r.b. von frese and khurama, s. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:17 p p p composite 133 lpi at 45 degrees masaitis, v.l., 1989. the economic geology of impact craters: internat. geol. rev., 31, 922-933. merriam, d. f. and sneath, p. h. a. 1966, quantitative comparison of contour maps: j. geophysical research, 7, 1105-1115 rocca, m.c.l. 2004 rio vichada: a possible 50 km wide impact structure in colombia, south america, 67th annual meteoritical society meeting 2004. spudis, p.d., 2005, the geology of multi-ring impact basins, cambridge university press, new york, ny, 277 p. von frese, r.r.b, hinze, w.j., braile, l.w., luca a.j. 1981, spherical earth gravity and magnetic anomaly modeling by gauss legendre quadrature integration,, j. geophys., 49, 234-242. von frese, r.r.b., jones, m., kim, j.w., kim, j.h. 1997, analysis of anomaly correlations, geophysics, 62, 1, 342-351. von frese, r. r .b., kim, h. r., tan, l., kim, j. w., taylor, p. t., purucker, m. e., alsdorf, d. e. and raymond, c. a. 1999. satellite magnetic anomalies of the antarctic crust. annali de geofisica, 42, 2, 309-326. von frese, r. r. b., l. v. potts, s. b. wells, t. e. leftwich, h. r. kim, j. w. kim, a. v. golynsky, o. hernandez, and l. r. gaya-pique´, 2009. grace gravity evidence for an impact basin in wilkes land, antarctica, geochem. geophys. geosyst., 10, q02014, doi:10.1029/2008gc002149. wilson, m. 1989, igneous petrology, winchester, unwyn hyman, 466 pp. 107 geophysical evidence for an impact crater in vichada, northwestern south america and its economic potential enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:17 p p p composite 133 lpi at 45 degrees articulo 3.indd earth sciences research journal hydrogeology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 101 108 hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india k. brindha and l. elango department of geology, anna university, chennai600 025, india. corresponding author. e-mail: elango34@hotmail.com, elango@annauniv.edu abstract hydrochemical study was carried out in madhuranthakam located near chennai in tamil nadu, india with an objective of understanding the suitability of local groundwater quality for domestic and irrigation purposes. twenty groundwater samples were collected in february 2002 and analysed for physical and chemical parameters. groundwater in this area was found to be within the desirable bureau of indian standards and world health organisation limits for drinking water. ca-hco3 was the dominant groundwater type. groundwater in this area was assessed for irrigation purposes on the basis of sodium percentage (na%), magnesium hazard (mh), residual sodium carbonate (rsc), sodium absorption ratio (sar), permeability index (pi) and united states department of agriculture (usda) classification. most of the groundwater samples were suitable for irrigation, except in a few locations (15%) based on mh. overall the groundwater quality was suitable for drinking and domestic purposes and permissible for irrigation activities. resumen un estudio hidroquímico fue llevado en la localidad madhuranthakam cerca chennai en tamil nadu, india, con el propósito de evaluar la calidad de las aguas subterráneas locales para usos domésticos y fines de riego. veinte muestras de agua subterránea fueron recogidas y analizadas en términos de parámetros físicos y químicos en febrero 2002. las aguas subterráneas en esta área fueron encontradas aptas como potables según los limites sugeridos por el bureau estándares de india y los límites de la organización mundial de la salud para el agua potable. ca-hco3 es dominante en las aguas subterráneas. las aguas subterráneas en esta área fueron asignada para propósitos de riego sobre la base del porcentaje de sodio (na%), magnesio peligroso (mh), carbonato sodio residual (rsc), proporción de absorción de sodio (sar), índice permeabilidad (pi) y la clasificación del departamento de agricultura de los estados unidos (usda). la mayoría de las muestras de las aguas subterráneas son aptas para el riego, excepto en unas pocas ubicaciones (15%), basadas sobre mh. en general, la calidad del agua subterránea fue apta para el consumo, uso doméstico y permisible para las actividades de riego. palabras claves: agua subterraneas, madhuranthakam, calidad de agua potable, calidad de agua de riego, tamil nadu, india. . keywords: groundwater, madhuranthakam, drinking water quality, irrigation water quality, tamil nadu, india. record manuscript received: 10/03/2011 accepted for publications: 24/11/2011 introduction groundwater is a precious source of fresh water, being the most distributed form on the earth, excluding the polar icecaps and glaciers. groundwater studies are gaining more importance in the present day as it is used for almost all purposes such as domestic, industrial and agricultural activities in most parts of the world. improper management of this replenishable resource may lead to groundwater contamination and scarcity. when some ions and minerals are present beyond the permissible limit, they become unsuitable for drinking and irrigation purposes, which may be due to natural and also anthropogenic causes. groundwater quality has been given lot of importance and studied worldwide (lahermo and backman, 1999; omo-irabor et al., 2008; baalousha, 2010). several regions in india have encountered degradation in groundwater quality too, due to rapid urbanisation and an exponential increase in population (ramesh and elango, 2005; brindha and elango, 2010; brindha et al., 2011). the present study was carried out with the aim of understanding groundwater quality and its suitability for drinking and irrigation purposes in madhuranthakam, located near chennai in tamil nadu, india. groundwater is the major source for domestic and irrigation practices in this area. there has been an increase in the demand for groundwater due to the growth of the local population. groundwater is usually put to direct use in rural areas without proper monitoring and treatment. groundwater may also become contaminated by the agrochemical products used for irrigation. the groundwater quality in the nearby regions, namely chennai, kancheepuram and chengalpet, k. brindha and l. elango102 has been studied earlier (elango and manickam, 1987; ramesh, 1999; rajmohan et al., 2000; elango et al., 2003a; elango et al., 2003b; kumaresan and riyazuddin, 2006). however, no studies have been carried out in the madhuranthakam region pertaining to groundwater quality. the suitability of groundwater for domestic and irrigation purposes thus had to be determined based on the presence of major ions in the groundwater of this region. the present study, which was carried out in 2002, will serve as baseline data for comparing future groundwater quality. study area the study area is situated 76 km south of chennai city and forms a part of madhuranthakam taluk, kancheepuram district, tamil nadu, india (figure 1). most of the annual rainfall of 1,206 mm is received from october to december and the rest during the southwest monsoon season from june to september. climatic condition in the study area varies from 39oc to 40oc during summer (april to june) and 20oc to 24oc during winter (october to december). the study area is surrounded by a number of tanks which are used for drinking and agricultural purposes. this area is intensively cultivated by pumping groundwater from dug wells as well as surface water resources. geologically this area is characterised by charnockites of precambrian era. outcrops of these rocks are found in many parts of the study area. sandy clay and clayey sand overlay the charnockites with thickness ranging from 1.5 to 5m. groundwater occurs under unconfined condition mainly in the weathered and fractured part of this charnockites in this area. figure 1. location of study area and sampling wells. parameter unit minimum maximum mean ph 7.2 8.2 7.69 ec µs/cm 200 1900 807.6 calcium mg/l 11.84 93.9 52.21 magnesium mg/l 3.65 132.46 24.37 sodium mg/l 27.59 82.76 50.35 potassium mg/l 0 2.74 1.17 carbonate mg/l 6 54.01 28.5 bicarbonate mg/l 30.51 256.27 136.9 chloride mg/l 20.35 32.41 21.91 sulphate mg/l 30.48 44.89 35.64 table 1. statistical summary of physicochemical parameters. hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 103 sampling and analysis a well inventory survey was carried out in february 2002 to obtain background information about the well population, lithology, well use etc. forty-seven wells were investigated and 20 wells (figure 1) were chosen as representative wells for groundwater sampling based on electrical conductivity (ec). the groundwater samples were collected in clean 500 ml polyethylene bottles. the sampling bottles were soaked in 1:1 diluted hcl solution for 24 hours and washed twice with distilled water before sampling. they were washed again in the field with groundwater sample filtrates. the groundwater samples were collected from bore wells after pumping the water for about 10 minutes. field parameters such as ec and ph were measured in the field using portable digital meters. samples were analysed for major ions in the laboratory using the standard recommended methods (apha, 1998). sulphate concentration in the groundwater samples was analysed using a uv visible spectrophotometer. sodium and potassium content was determined by using a flame photometer and calcium, magnesium, chloride, carbonate and bicarbonate by titration technique. total ions measurement precision was checked by calculating the ion balance error (ibe). the ibe was within ±10%. total dissolved solids (tds) were calculated by using the formula: tds (mg/l) = ec (µs/cm) x 0.64; total hardness (th) was calculated by using: th = 2.497ca + 4.115mg in mg/l. results and discussion table 1 shows the maximum, minimum and mean concentrations of various parameters. the order of cation dominance was ca2+>na+>mg2+>k+ and hco3 ->so4 2->co3 2->clfor anions. the general chemical nature of groundwater can be understood by plotting major cation and anion concentrations on a piper trilinear diagram (piper, 1944). ca-hco3 was the major water type dominant in this area (figure 2). the next dominant water type was mixed ca-na-hco3. drinking water quality groundwater used for domestic purposes, such as drinking and cooking, should be free from toxic chemicals and pathogens. domestic water quality indicates that a particular parameter at a given concentration may be suitable for the human body beyond which it is unsuitable. the concentration of various ions in the groundwater samples was compared with bureau of indian standards (bis, 2003) and world health organisation (who, 1993) standards, which are given in table 2 wherein all the groundwater samples were found to be within the suitable limits. the groundwater in this area was thus seen to be fit for domestic consumption, based on the major ion analysis carried out in this study. the groundwater samples were classified regarding tds and th (tables 3, 4 and 5). most of the groundwater was fresh and suitable for drinking purposes based on tds (freeze and cherry, 1979; davis and deweist, 1966). groundwater in this study area varies from soft to very hard (sawyer and mccarty, 1967). irrigation water quality good quality irrigation water is essential for achieving maximum crop productivity. groundwater suitability for irrigation purpose in this study area was assessed using sodium percentage (na%), magnesium hazard (mh), residual sodium carbonate (rsc), sodium absorption ratio (sar), permeability index (pi) and united states department of agriculture (usda) classification. groundwater in most of the study area was found to be suitable for irrigation. irrigation water having high ec content will affect root area and water flow. groundwater in this area was grouped according to the guidelines figure 2. groundwater type. k. brindha and l. elango104 parameter bis (2003) who(1993) percentage of samples above maximum allowable limit highest desirable limit maximum permissible limit highest desirable limit maximum permissible limit ph 6.5– 8.5 6.5 – 9.2 6.5 – 8.5 6.5 – 9.2 nil th (mg/l) 300 600 100 w500 nil calcium (mg/l) 75 200 75 200 nil magnesium (mg/l) 30 100 50 150 nil chloride (mg/l) 250 1000 200 600 nil sulphate (mg/l) 150 400 200 400 nil sodium (mg/l) 200 nil table 2. comparison of groundwater samples with bis and who standards. table 3. freeze and cherry (1979) classification of groundwater based on tds (mg/l). table 4. david and deweist (1966) classification of groundwater based on tds (mg/l). table 5. sawyer and mccarty (1967) classification of groundwater based on th (mg/l). table 6. suitability for irrigation based on usda classification. tds (mg/l) water type number of samples percentage <1,000 fresh 19 95 1,000 10,000 brackish 1 5 10,000 1,00,000 saline nil nil >1,00,000 brine nil nil tds (mg/l) classification number of samples percentage <500 desirable for drinking 11 55 500 1,000 permissible for drinking 8 40 1,000 3,000 useful for irrigation 1 5 >3,000 unfit for drinking and irrigation nil nil total hardness (mg/l) type of water number of samples percentage <75 soft 2 10 75 150 moderately high 7 35 150 300 hard 7 35 >300 very hard 4 20 ec (µs/cm) salinity class percentage of samples remark on quality <250 c1 5 excellent or low 250-750 c2 45 good or medium 750-2250 c3 50 permissible or high 2250-5000 c4 nil unsuitable or very high hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 105 established by the united states salinity laboratory (freeze and cherry, 1979) based on ec (table 6). this showed that groundwater in this area was permissible for irrigation. sodium is an important parameter for irrigation water and is denoted as na% which was calculated from the formula given in equation 1 (wilcox, 1955) and all concentrations were expressed in meq/l. the na% in this area is given in table 7. groundwater was suitable for irrigation in 60% of the samples while 40% were permissible to doubtful. ec and na% are plotted in figure 3 which showed that most of the groundwater samples were good for agriculture. the concentration of bicarbonate and carbonate higher than calcium and magnesium will influence the suitability of water for irrigation purposes. the rsc value was computed using the following formula (equation 2) where ions were expressed in meq/l. rsc = (co3 2+ hco3 -) – (ca2+ + mg2+) however, regarding rsc, all samples fall within the safe category for irrigation. sar is another important parameter for determining the desirability of irrigation water. the sar values were calculated by using equation 3 (richards, 1954), where all the concentrations were given in meq/l. all the groundwater samples collected from this area were excellent on the basis of sar. water used for irrigation can be classified into four typesc1, c2, c3 and c4 based on salinity hazard and s1, s2, s3 and s4 based on sodium hazard. figure 4 shows the plot of groundwater samples grouped on the above basis. most groundwater samples fall under c1s1 and c2s1 which are suitable for irrigation. comparatively few samples were grouped under c3s1 (which is permissible for irrigation). thus the groundwater in this area was seen to be suitable for irrigation based on salinity hazard and sodium hazard. mh for irrigating water was calculated using the formula mh = mg/ (ca + mg) x 100, where concentrations were given in meq/l (szabolcs and darab, 1964). mh above 50 is considered to be unsuitable for irrigation. groundwater had an mh above 50 in 15% of the samples, thus not being fit for irrigation. table 7. suitability for irrigation based on sodium percent. % na suitability for irrigation number of samples percentage <20 excellent 5 25 20-40 good 7 35 40-60 permissible 6 30 60-80 doubtful 2 10 >80 unsuitable nil nil (1) (2) (3) )( 100)(% 22 ++++ ++ +++ + = knamgca xknana 2 22 ++ + + = mgca nasar figure 3. suitability of irrigation water, based on ec and sodium percent. k. brindha and l. elango106 the suitability of groundwater for irrigation based on pi was calculated using equation 4, where concentrations were in meq/l. class i and class ii waters are considered to be good and suitable for irrigation while class iii water is unsuitable for irrigation (doneen, 1964). figure 5 shows that one groundwater sample was not suitable for irrigation based on pi whereas the rest of the samples were good. in general, the groundwater fall within the permissible category for irrigational use, except for a few locations where it was unsuitable based on mh. conclusion groundwater quality of an area must be studied to understand its suitability for domestic and irrigation purposes. all the groundwater samples collected from the madhuranthakam area, tamil nadu, india, showed that the major ions fall within the permissible range. the dominant groundwater type was ca-hco3. based on tds, 95% of the groundwater was fresh and permissible for drinking. the groundwater varied from soft to very hard on the basis of th. the suitability of groundwater for irrigation was assessed from na%, ec, rsc, sar, mh, pi and usda classification. the groundwater in this area was seen to be good and suitable for drinking and domestic purposes. however, the groundwater was unsuitfigure 4. irrigation water salinity and alkalinity hazard. figure 5. classification of irrigation water based on permeability index. )( 1003 +++ + ++ + = namgca xhcona pi (4) hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 107 able for irrigation in a few places, based on mh. overall, groundwater in the madhuranthakam area remains usable. the present groundwater quality status must be maintained by taking precautionary measures such as rainwater harvesting, less use of chemical fertilisers and the ongoing monitoring of groundwater quality in this region. acknowledgements the authors would like to thank the department of science and technology-funds for improvement in science and technology (dst-fist) (grant no. sr/fst/esi-106/2010) and the university grants commission-special assistance programme (ugc-sap) (grant no. ugc drs ii f.550/10/drs/2007(sap-1)) for providing financial support which helped in creating facilities to carry out this work. references apha (1998). 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(1955). classification and use of irrigation waters, usda, circular 969, washington dc, usa. geoffslca colombiana nr 1, octubre de 1992 una solucion al problema inverso en geomagnetismo ramiro l. diaz profesor asociado depto de geociencias-facultad de ciencias-universidad nacional de colombia 53 diaz, r.l.: una solución al problema inverso en geomagnetismo . geofís. colomb. 1:53-56 , 1992. issn 0121-2974 resumen un terna importante en geofísica, es el problema inverso en geomagnetismo. en este trabajo se propone un método para reso lverlo, que consiste en hallar la solución de un sistema móvil de ecuaciones no lineales a lu largo de un perfil magnético. conociendo los datos de anomalías magnéticas y por medio de un programa qu e ejecuta un proceso iterativo eficiente, se determina la estructura bidimensional bajo el perfil muestreado . abstract an ínteresting aspect in geophysics is the inverse problem in geomagnetism. this paper pro poses a method for so lving it. so, th e objective is lo solve a mobi l system of non linear equations along a magnetic profile . from knowed magnetic anoma lies and using an efficient iterative process , it is possible lo find a bidimensional structure below a sample pro file . 1. introduccion encontrar el modelo que mejor interprete la estructura geológica a través de sus manifestaciones magnéticas, equivale a decir que se ha solucionado el problema inverso en geomagnetismo. se prop_one un método para encontrar una solución al problema inverso a partir del modelo propuesto por campbell & hines ( 1981) . el método consiste en solucionar un sistema móvil de ecuaciones no lineales a lo largo de un perfil magnético, en lo posible perpendicular a la estructura. es un proceso iterativo que se repite hasta lograr el modelo que cumpla con las condiciones requeridas (robert & gex, 1985). la aplicación del método a un perfil magnético permite hacer un análisis de la convergencia y de las ventajas que presenta. 2. consideraciones anauticas campbell & hines (1981), proponen el siguiente modelo en interpretación magnética: n : número de vértices del polígono al cual se aproxim a la secc1on transversal de la fuente magnética t-. t;c: campo magnético total calcu lado a partir de la estructura magnetizada m = 2 x to (1 -cos2 (1) cos2(s)] factor que determina las propiedades magnet1cas de la estructura geológica 54 dfaz: una solución al problema inverso en geomagnetlsmo x : contraste de susceptibilidad magnética t 0 : campo geomagnético (campo magnético inductor) 1 : inclinación magnética s : azimut de la estructura pk (zk,z¡..1) = sen d[(c-8) sen(u-0) +r cos(u-0)]: factor geométrico del modelo d = tan·1 [(2:¡.. , -zk)/(xk+, -xk)] c = (ro'2) -tan·1 (2:¡../xk.,) 8 = (1t/2) -tan·1 (z/xk) u= tan·1 [tan(l)/ tan(s)l,...-..----,.-. r = ln [ v(z2k. 1 + x2k. 1)/ {(2:¡.2 + xk2)] el modelo propuesto por campbell & hines (1981 ), permite calcular la anomalía magnética de la estructura bidimensional sobre un perfil determinado conociendo la posición y geometría de la fuente magnética. el problema así planteado, se denom ina problema d i recto en geomagnetismo. se utiliza el modelo mencionado para dar solución al problema inverso; es decir, conociendo los datos de anomalías magnéticas en un perfil, se pretende determinar la estructura bidimensional. si se tienen n muestras de campo magnético total y se desea encontrar la profundidad de la interfase magnética bajo cada punto de muestreo, se debe pensar en un sistema de n ecuaciones con n incógnitas, lo cual no es conveniente cuando n es muy grande (velleman & hoagiling, 1981 ). para solucionar el problema se propone un sistema móvil de tres ecuaciones que deben recorrer todo el perfil. este proceso se debe repetir hasta que las desviaciones del campo calculado (t:. t;c), con respecto al campo observado en terreno (t:. td.o) , se hagan mínimas. se consi era un sistema de tres ecuaciones q j, o;..,. q . , evaluadas en las estaciones i, 1+ 1, i+2, ~el perfil magnético con tres incógnitas z¡. zi+p z;.,;>· en el recorrido del sistema por el perfil cada punto interviene tres veces en la solución del sistema. cada una de las ecuaciones del sistema se expande en términos de una serie de taylor con el fin de encontrar la relación de recurrencia que se emplea en el proceso iterativo (bruce, 1981) asf: o;= 0°; + (80°/8zj [z; zoj + [80° /8z;. 1] [z;.1 -zo;.1] + (80° /8z;.2] [z;.2 -z 0 ;.2] + o, 0;.1 = 0°;.1 + [80°;./8zj [z;z oj + (80°¡j8z;.1) (z;. 1 -z 0 ;. 1) + (80°;./8z;.2] [z;.2 -zo;.2] + o, 0 ;.2 = 0 °;.2 + [80°;./8zj [z; z oj + (80°¡j8z;. 1) (z;.1 -z o;. 1) + [80 °;j8z;.2] [z;.2 -z 0 ;.2] +0, [3] en donde, z; : profundidad real de la estructura zo; : estimador de z; o; : función evaluada en (z;. z;.1, z;.2) 0 °; : función evaluada en (z"¡, zo;.,, zoi+2) 80 ° /8z; : derivada de la función o;. evaluada en z"¡, zo;.,. z0 ;.2 a, : residuo en la serie de taylor. los valores o;. 0 ;.1• 0 ;.2 y 0, deben tender a cero a medida que el campo calculado se aproxima al valor del campo observado ; es decir, el modelo obtenido representa la estructura geológica. bajo estas condiciones el sistema [3] se puede expresar de la siguiente forma: 0 °; = [8qo /8zj [z0 ; zj + [80° / 8z;. 1] [z0 ;. 1 -z;. 1] + [80 ° /8z;.:j [z0 ;.2 -z;.2) 0°;.1 = (80°;./8zj [z 0 ; zj + [80°;./8z;.1) [z0 ;. 1 -z;. 1) + [80 ";. ,18z;.2) [z 0 ;. 2 -z;.2l 0°;.2 = [80 °;./8zj [z0 ; zj + [80°;j8z;. 1) (z"¡., -z;.1] + [80°;./8z;.2) [z0 ;.2 -z,. 2) [4] geoffslca colombiana ~ 1, octubre de 1992 a o. 1 de modo que: a o = j (zoz) z = zoj' l ao [5] z : matriz cuyos elementos son los valores que más se aproximan a los z¡ reales zo : matriz que contiene los estimadores de z; ao : matriz de desviaciones j : jacobiano del sistema el proceso se realiza utilizando un programa cuyo algoritmo se plantea en la ecuación de recurrencia [5]. se inicia con una primera aproximación a la estructura geologica y se ajusta el modelo, mediante aproximaciones sucesivas, hasta obtener la estructura que mejor satisface el muestreo magnético del campo total. en el programa se usó una tolerancia de 1 0.1 1 para los 0 ;. 3. convergencia y exactitud del meto do se considera el perfil magnético cuyos datos se consignan en la tabla 1, con origen en el punto a. observando los resultados de los cálculos suministrados por el programa (tablas 2 a 5), se infiere que, a partir de una burda aproximación, en más o menos cinco iteraciones se consigue un modelo ajustado a la estructura geológica, lo cual garantiza su eficiencia. los puntos 1 y 8 son de amarre en el modelo. la topografía se mide como una altura sobre el nivel del mar y los valores de z son profundidades bajo el nivel del mar. 55 tabla 1 datos del perfil magnético estac anomalia(y) di<t.horiz(m) topogr(m) 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 a 235.7 b -33.4 e -44. 1 o -111 .0 e -64.6 f -66.8 g -52.4 h 288.8 0.0 400.0 1000.0 1600.0 2200.0 3000.0 3800.0 4800.0 tabla 2 datos de la 1 iteración z;' (m) ll t;" (y) o; (y) 0. 1 870.0 -45.6 6.8 860.0 -62. 1 4.7 830.0 -62.9 1.6 800.0 -110.7 0.2 840.0 -43.1 0.9 850.0 -25.1 8 .2 0. 1 tabla 3 datos de la 11 iteración z;' (m) llt;" (y) o; (y) 0.1 927.5 -54.2 -1 .8 1408.4 -66.7 0.1 1162.2 -64.5 0.1 764.0 110.7 0.2 115.3 -44. 1 0.0 495.9 -33.4 0.0 0.1 0. 1 50.0 40.0 0. 1 30.0 60. 1 70.1 0. 1 z; (m) 0. 1 927.5 1408.4 1162.2 764 .0 775.3 495.9 0.1 z; (m) 0. 1 917.6 1328. 0 1106.1 690.8 758.7 500.5 0. 1 56 díaz: una solución al problema inverso en geomagnetísmo z;' (m) 1 0.1 2 917.6 3 1328.0 4 1106. 1 5 690.8 6 756.7 7 500.5 8 0.1 tabla 4 4. conclusiones datos de la 111 iteración !l t;" (y) -53.4 -66.8 -64.6 -111 .0 -44. 1 -33.4 tabla 5 o; (y) -1 . 1 0.0 0.0 0.0 0.0 0.0 z; (m) 0.1 900.6 1297.3 1106.7 690.3 753.3 499.1 0.1 en este trabajo se presenta una solución al problema inverso en geomagnetismo. el algoritmo que se emplea ofrece eficiencia y facilidad para manejar grandes cantidades de información, con un método de análisis numérico sencillo. referencias datos de la v iteración cai!ipbell, d.l. & hines. (1981 ): manua l of geophysical hand calculator programs . seg, usa. z;' (m) 1 0.1 2 900.7 -52.4 3 1296.4 -66.8 4 1105.5 -64.6 5 690.4 -111 .0 6 753.2 -44.1 7 499.0 -33.4 8 0.1 , o; (y) -0.1 0.0 0.0 0.0 0.0 -0.1 z; (m) 0.1 900.5 1296.4 1105.4 690.4 753.2 499.0 0.1 bruce, e. (1981 ): introducción al análisis numérico con fortran . univ. ped. tec . colomb., tunja. robert, a. & p. gex. (1985) : inter_prétation geophysique rapide . logiciel basic pour micro-<>rdinateur. bulletin de l'lnstitut de géophysique, n26. université de lausanne. velleman, p.f. & d.c. hoaglling. (1981) : applications basics and computin_g of exploratory data analysis. duxbúry fyress, boston , massachuset1s. anal/sis de sensibil/dad en la estimacion de la funcion caracteristica de punta de hamilton de la sismica geometrica luis alfredo montes vides profesor asociado, departamento de geociencias universidad nacional de colombia montes, l (2000): analisis de sensibilidad en la estirnacion de la funcicin caracteristica de punto de hamilton de la sismica geometrica. geofis. colomb. 4:31-36. issn 0121-2974 santa fe de bogota, d.c. colombia resumen usando la aproxirnacion de segundo orden de los tiempos de transite en modelos sismicos constituidos por capas hornoqeneas e isotr6picas separadas por interfaces suaves, los tiernpos de los eventos de reflexi6n registrados en vecindad del rayo central pueden estimarse a partir de una funci6n hiperb61ica que corresponde a la funci6n caracteristica de punto de hamilton, la cual no es conocida nurnericarnente en la optica geometrica. en la sismica, esta se puede determinar a traves de una regresi6n lineal entre las coordenadas fuente-detector y los tiempos de transite de por 10 menos nueve rayos apropiadamente seleccionados y registrados en configuraciones de cubrirniento multiple. un anal isis de sensibilidad permite conocer la estabilidad y robustez del rnetodo de regresi6n que estima la funci6n caracteristica de los tiempos de transite. abstract using the second-order approximations of the traveltimes in a seismic model which consists of a pile of homogeneous layers separated by smooth interfaces, the traveltimes of reflection events registered in the vicinity of the central ray can be estimated from an hyperbolic function that corresponds to hamilton's point characteristic in optics, where it is not numerically known. in seismic, it can be determined through a linear regression between the source-detector coordinates and the traveltimes of at least nine rays properly selected and registered in multiple fold configurations. a sensibility analysis allows to know the stability and robustness of the linear regression method to estimate the characteristic traveltimes function. 1. introduccion las leyes generales para representar propiedades globales de los rayos transmitidos y reflejados en un sistema sismico fueron establecidas usando las ideas de la optica qeometrica (bortfeld, 1989). en un modelo de la tierra constituido por capas hornoqeneas e isotropicas separadas por interfaces suaves con velocidades y densidades arbitrarias, el sistema queda limitado por la superficie anterior considerada plana (de adquisici6n), y la posterior arbitrariamente curvada (de reflexi6n). ei tiempo de transite de cualquier rayo cercano a un rave de referencia denominado rayo central se expresa por una sencilla funci6n rnatricial, que es equivalente a la funci6n caracteristica de punta de hamilton de la optica. maternaticamente, la teoria de aproximacion paraxial es la contra parte sismica de la 6ptica qausstana. problemas, tales como fenornenos de enfocamiento (bortfeld & kemper, 1991) y miqracion de amplitudes (bortfeld & kiehn, 1992) han sido tratados exitosamente sin conocer la estructura detallada del sistema sismico. la teoria provee formulas mas simples y compactas, deseables en problemas de inversion. ei proposito de este articulo es evaluar el proceso que estima la funci6n que suministra el tiempo de un evento reflexion asociado a una superficie del sistema, y analizar como los errores cornetidos en la identificacion de los primeros arribos puede influir en el estimative de dicha funcion: error que se transfiere a los resultados de cualquier proceso de inversion que haga uso de esta teoria. 2. ravos v tiempos de trans ito el sistema se considera compuesto de capas homoqeneas e isotropicas separadas por interfaces suaves, esta limitado por la superficie sobre la cual se posicionan las fuentes y los geofonos liamada superficie anterior, y por la superficie donde tiene lugar la reflexion, denominada superficie posterior. en la aproxirnacion geometrica 0 paraxial, un rayo transmitido que incida normalmente sobre el reflector se usa como rave central, a este se referencian los rayos vecinos 0 paraxiales. ei sistema de coordenadas se fija al rayo central, haciendo coincidir el origen del sistema cartesiano con el punto donde el rayo intersecta la interface, asi el plano xyes tangente a la superficie anterior en el punta de partida del rayo central. el plano xy' es tangente al punto de interseccion del rayo central con la superficie posterior. con la ayuda de estos sistemas cualquier rayo paraxial puede representarse por los vectores posicion y lentitud (slowness) proyectados sobre los pianos tangentes (xy ,xy') en las superficies anterior (x, p) y posterior (x' , p'), ver fig.1. para diferenciar los vectores asociados al rayo central de aquellos asociados a los rayos paraxiales, la pareja de vectores asociados al rayo central se escribe con el suscrito "0", asi en la superficie anterior la pareja sera (x 0 ' po)' en la aproximacion de segundo orden de los tiempos de transite las parejas de vectores en la superficie de reflexion se relacionan con los vectores de la superficie anterior a traves de la ecuacion: [x'] [ao bo][ x j p' = co do p-po (1) con ao,bo,cr,,12o matrices [acobianas 2x2. un rayo trazado desde x en la superficies anterior a x' en la posterior y de all[ hasta x" sobre la anterior representa una onda que atravesando el sistema es reflejada con un tiempo de transite: i(x,y) =~ +po(x+y)+~(x+y)d~ico~(x+y) +~(y -x)s;;'ao~(y -x) (2) donde 7;, es el tiempo de transite del rayo central de ida y vueita (two way time). ei tiempo de transito zero-offset se obtiene cuando x = x' . superficie posterior figura 1. sistema sismico mostrando el ravo central con los sistema cartesianos definidos en las superficies anterior v posterior. el ravo paraxial atraviesa el sistema hast a el reflector. la ecuacion (2) corresponde a la caracteristica de punta de hamilton en optica, liamada asi por dar el tiempo de propaqacion entre dos puntos. aun usada en el disefio de instrumentos opticos la funcion caracteristica de punto de hamilton no se conoce explicita ni numencamente. sin embargo en sismologia el sistema es la tierra y fa forma de la funcion se expresa en (2), pudiendo estirnarse nurnericarnente. las matrices b~jao,/2~jf:o son sirnetricas, en consecuencia la funcion (2) posee nueve incognitas: seis en b~i ao,d~ico, una en 7;, y dos en po' utilizando al menos nueve tiempos de transite apropiadamente registrados las incognitas de fa expresion (2) pueden resolverse y por tanto estimar numericamente la funcion caracteristica de los tiempos de transite. 3. configuracion v tiempo de arribo para estimar la funcion caracteristica se requiere de por 10 menos nueve mediciones (picking) de tiempos de arribo de las ondas reflejadas, ya que la ecuacion (2) posee nueve incognitas. para simplificar las ecuaciones denotaremos los parametros as;: una configuraci6n con disparo en el origen del sistema cartesiano da lugar a una familia de trazos fuente comun (common-shot). ai situar los geofonos en las coordenadas (x gi , y gi) yal usar la ecuaci6n (2) se tiene un sistema de ecuaciones que matricialmente tiene la forma: ix y 2 x y y2 2 x y 2g9 g9 xk9 1'9 g9 1'9 xg9 g9 g9 yg9 ei sistema presenta tres pares de columnas iguales 0 linea/mente dependientes, por tanto no admite soluci6n. este resultado indica que es imposible conocer la funci6n caracteristica usando una familia de trazos con fuente comun. un analisis similar en trazos obtenidos en receptor comun crp /leva a un sistema con seis grados de libertad, imposibilitando conocer los parametres a partir de una familia de trazos crp. ai considerar trazos registrados en configuraci6n cmp se obtiene el sistema de ecuaciones: = por el hecho de contar con seis grados de libertad se concluye que en un unico arreglo cmp no es posible conocer la funci6n caracteristica. -p -q u~ por ultimo en una familia de trazos common-offset nuevamente se obtiene un sistema con seis grados de libertad encontrando /a misma imposibi/idad hallada en los tres casos anteriores. lo anterior permite afirmar que para determinar /a funci6n caracteristica se requiere utilizar tiempos de arribo registrados en distintas configuraciones. 4. aproximacion hiperbolica en la exploraci6n sismica se sabe hace tiempo que en medios con capas planas y horizontales, los tiempos de transite de las reflexiones aproximadamente verticales se ajustan mejor por funciones hiperb61icas que por funciones de tipo parab61icas (ursin, 1982). ai considerar el cuadrado de la funci6n de tiempos de transite dada en (2) y despreciando los terrninos mayores a la segunda potencia se obtiene: t(x,t"l =[j: -2])0 -~(x+xjf +2j:g(x+xjd~'co~(x+~)+~(~ -x)-b,;l&i(~ -x)] (3: la ecuaci6n (3) representa los tiempos de transite en la aproximaci6n hiperb6lica. ei conjunto de n tiempos registrados en distintas configuraciones da lugar a un sistema matricial de n ecuaciones. 5. analisis de sensibilidad la estabilidad se define como la propiedad que posee un estimador central a ser insensible a la presencia de pequefios errores aleatorios en los datos, al suministrar informaci6n "a priori" sobre el modelo se puede garantizar la estabilidad en /a soluci6n. la robustez, indica la sensibilidad del pararnetro a un numero reducido de errores apreciables en los datos. estimar la funci6n caracteristica usando tiempos de transito de eventos de reflexi6n es un problema de regresi6n lineal de un sistema de ecuaciones sobredeterminado que se expresa de la forma: minimizar (4) la soluci6n para (1) seran los va/ores de tfl que generen el menor error if , que se obtiene a traves de la matriz inversa generalizada, ver apendice a. para estudiar la sensibilidad en la determinaci6n de la funci6n caracteristica, la ecuaci6n (3) se perturba introduciendo errores aleatorios en los tiempos de transite. estas secuencias de nurneros aleatorios se generan con una distribuci6n normal con media fl = 0 y desviaci6n estandar 0" * 0 . se lievaron a cabo nueve ensayos, con gaussianas de 0", ={2,4, ...,18ms}, el valor mayor representa el tiempo usado por un frente de onda con frecuencia de 100 hz para viajar una longitud de onda a a una velocidad de 1000 m/s. con este intervalo se indica que el error se relaciona con la incertidumbre para identificar en una longitud de onda el tiempo de arribo (picking). con fuentes y deteclores en configuraci6n cmp la funci6n caracteristica viene dada por la expresi6n: t(o, x) = to + xux, (5) donde u = b~i a 0 y to se pueden estimar a traves del metoda de la inversa generalizada. por otro lade cuando fuente y detector estan en configuraci6n zero-offset la funci6n caracleristica de tiempos de transite toma la forma: lex) = to + j\x + xvx (6) donde e =d~jco y pose obtienen usando la matriz de moore-penrose. ver apendice a. el tiempo para una configuraci6n zero-offset se puede aproximar al tiempo de arribo al detector mas proximo a la fuente. ligeras perturbaciones se introdujeron en los tiempos de transite del sistema de ecuaciones (3) y se estudi6 como los errores en esos tiempos de arribo influyen en los parametres, para ello se cre6 un programa en matlab 4.0 que calcula la funci6n caracteristica exacta a partir de tiempos de transite obtenidos de un modelo de capas plano horizontales, utilizando el programa de trazamiento de rayos seis88 desarrollado por cerveny et al. (1980) en la universidad de charles en praga. posteriormente se adicionaron a los tiempos de arribo cadenas de errores estocasticos generados por una funci6n de distribuci6n normal, con media cera y 0", * 0; los resultados arrojados en el proceso se muestran en la tabla 1. tabla 1 a(s) ()(s) p, r, ~i 1~2 vn vi1 v12 vn 0 0.5 0 0 1 0 1 0 0 0 0.002 0.4997 -0.0006 -0.0009 0.9937 0.0109 0.9922 0.0011 0.001 0.0021 0.004 0.4993 -0,0011 -0.0017 0.9874 0.0219 0,9844 0.0026 0.0011 0.0041 0.006 0.499 -0.0015 -0.0026 0,981 0.033 0.9766 0.0045 0.0002 0.0062 0.008 0.4987 -0.002 -0.0034 0.9746 0.0443 0.9687 0.0068 -0.0016 0.0084 0.01 0.4984 -0.0024 -0.0042 0,9681 0,0556 0.9608 0.0095 -0.0043 0.0106 0.012 0.4981 0.0027 -0.005 0.9615 0.067 0,9528 0.0126 -0.008 0.0128 0.14 0.4978 -0.003 -0.0057 0.9548 0,0785 0.9448 0,0161 -0.0126 0.015 0.16 0.4975' -0.0033 -0,0065 0,9481 0.0901 0.9368 0.0199 -0,0181 0.0173 0.18 0.4973 -0.0035 -0,0072 0.9413 0,1019 0.9287 0.0242 -0.0246 0.0196 los resultados muestran que en el caso extrema de maximo nivel de ruido en los tiempos de arriba (0"=18ms), los parametros j3;,iao (u) y to se obtuvieron con errores bajos (menores a 3%), indicando la buena estabilidad de los parametres. ai perturbar a nivel maximo los tiempos en zero-offset se hall6 que el po estimado no se desvi6 significativamente de su valor verdadero, mientras que d;,i£ () se calcul6 con un error de 10%. para disminuir ese nivel de incertidumbre los tiempos zero-offset deben ser calculados a partir del ajuste de datos en configuraci6n cmp. en general los resultados indican que a mayor error en estimar los tiempos (picking) mayor error en la determinaci6n de los parametres. conclusiones en el presente trabajo se muestra el procedimiento para estimar la funci6n caracteristica de los tiempos de transite en la aproximaci6n hiperb61icausando para ello un conjunto de tiempos de reflexi6n registrados en cobertura multiple. un anallsis de sensibilidad sobre el procedimiento de estimaci6n de la funci6n caracteristica indica que se pueden conocer los parametres de esta funci6n con un pequeno margen de error, el cual esta ligado a la calidad de los datos colectados, la funci6n estimada representa los tiernpos de eventos de reflexion en la vecindad del rayo central en el rango de offsets donde es valida la correcci6n nmo. agradecimientos este trabajo es un resultado parcial obtenido durante mis estudios conducentes a ph.d. en la universidad federal de para en brasil, bajo la direcci6n de los doctores walter sollner y peter hubral a quienes manifiesto un gran agradecimiento por sus valiosas ideas y apoyo durante el desarrollo del traba]o. as! como en la sustentaci6n del mismo. referencias bibliograficas arthanari, t. s. & y. dodge. (1981): mathematical programming in statistics. j, wiley and sons. bortfeld, r. (1989): geometrical ray theory: rays and traveltimes in seismic systems. geophysics 54:342-349. bortfeld, r., & r.m. kemper. (1991): geometrical ray theory: line foci and point foci in the anterior surface of seismic systems: geophysics 56:806-811. bortfeld, r.. & m. kiehn. (1992): reflection amplitudes and migration amplitudes, geophys. prospect. 40:873884. cerveny, v., l. klimes & v. psencik. (1988): complete seismic-ray tracing in 3-d structures. d. doornbos ed. seismological algorithms. academic press. dantzig, g. b. (1963): linear programming and extensions. princeton university press. menke, w. (1973): geophysical data analysis: discrete inverse theory. academic press. ursin, b, (1982): quadratic wavefront and traveltime approximations in homogeneous layered media with curved interfaces. geophysics, 47:1012-1021. apendicea solucion del problema inverso sea el sistema algebraico lineal de n-inc6gnitas (mt, m2 , ... , mn) relacionadas a traves del sistema acoplado de r-ecuaciones dado por: gttmt + gt2m2 + + gtnmn = d, g21ml + g22m2 + + g2nmn = d2 (a-1) donde gil y b, tienen valores conocidos, cuando el numero de ecuaciones es mayor que el nurnero de inc6gnitas el problema se dice que es sobredeterminado. matricialmente el sistema de ecuaci6nes a1 se representa asi: (a-2) la soluci6n consiste en estimar los parametres del modelos dado par m'" i de modo que los datos previsibles a pre se aproximen 10 mejor posible a los datos observados a ohs . sus diferencias se expresan por el vector de error if = a obs j pre, siendo la mejor solucion aquella que genere el menor error medido a traves de la norma del error iil( if )l!. las normas mas usadas son la de error absoluto minimo y la de error cuadratico medio minimo definidas respectivamente como: (if)11= 2::)&, y (----""-~~ (if)ii=/''i& 2 "jl...a ' para cada norma se supone que los datos se comportan sequn un tipo de estadistica, as! para la norma 2 los datos obedecen a una distribuci6n de gauss y en este caso la soluci6n al sistema a-2 viene dado par el metoda de la matriz inversa generajizada de moore-penrose (menke, 1987): (a-3) el superindice t indica traspuesta. suponiendo que existe la matriz inversa (0' arl se obtiene: (a-4) en el caso de la norma 1, para el sistema a-2 la soluci6n se establece a partir de su representaci6n como un problema de programaci6n lineal (arthanari & dodge, 1981). la representaci6n sequn la teoria de investigaci6n de operaciones viene dado par: minimizar 2:: sujeto a: om if = a , (a-5) el cual fue resuelto par el denominado rnetodo simplex 0 simplex revisado de la programaci6n lineal (dantzig, 1963). generalmente los datos contienen ruido que causa error al estimar los parametros del modelo, si los datos tiene asociada una funci6n de distribuci6n caracterizada par una matriz de covarianza dada por [cov(ffi)]=mam , donde _m es la matriz inversa generalizada de moorepenrose. si los datos no estan correlacionados y tienen una varianza a~ entonces la solucion que genera el menor error cuadratico tendra una matriz de covarianza dada por: que resulta tinalmente en: (a-6) la expresion a-6 indica que fa matriz de covarianza de los parametres esta controlada por la rnatriz de covarianza de los datos y por la matriz inversa generalizada. la introduccion de informacion "a priori" garantiza la obtencion de la solucion en el problema de minimos cuadrados, esa informacion es una restriccion sobre los valores que tomaran los parametres, esto se presenta como el caso donde un parametro debe tomar un valor constante conocido 0 multiplicado de una matriz de pesos ponderados, dando un mayor valor a aquellos que presentan mayor estabilidad. en el caso particular de ajuste a una parabola la ecuacion a-2 se expresa en forma matricial asl: (a-4) ei calculo de ff1 por minimos cuadrados de a-4 segun la inversa generalizada a-4, sera: geofis. colomb. . santafe de bogota d.c. issn • 0121·2974 fractales y series de datos geofisicos luis alfredo montes vides profesor asistente departamento de geociencias-facultad de ciencias-universidad nacional de colombia montes l. a: fractales y series de datos geofisicos. geofis. colomb. 2:9-12, 1993. issn 0121-2974 resumen la geometria de fractales ha surgido como una herramienta potencialmente util para la caracterizaci6n de datos en geofisica. comunmente, los datos geofisicos conforman series de tiempo, que exhiben un comportamiento aleatorio 0 variaci6n a corto y a largo plazo. un ejemplo tfpico son los registros anuales de temperatura. la traza de un registro es una curva con una dimensi6n fractal d, caracterizada por un exponente h. en el presente trabajo se utiliza el metoda de analisis de rango en cambios de sscata, creado por h. e. hurst, para determinar la dimensi6n fractal de una serie de datos geofisicos, y su medida de auto-afinidad. abstract there is a new geometry which provides a potentially tool for the characterization of geophysical data: the fractal geometry. generally, geophysical data consist of records in time or data series, for example yearly records of temperature, and they show a random behavior or variation on both a short and a long-term time scale. the trace of a record is a curve with a fractal dimension d, and it is characterized by an exponent h. in this paper, the hurt's rescaled range analysis method is used to determine the fractal dimension of a geophysical data serie d and h, his self-affinity measure. 1. introduccion en los ultirnos 10 aries se ha reconocido la importancia de la geometria de fractales en las ciencias naturales, como apropiada para describir las estructuras presentes en la naturaleza. la geometrfa fractal suministra una descripci6n y modelo matematico para la mayorfa de las formas aparentemente compleja halladas en la naturaleza. su propiedad mas importante es la ausencia de cualquier caracteristica de longitud 0 tiempo. esto quiere decir que las funciones de distribuci6n de probabilidad de los fractales, adecuadamente normalizadas, pueden expresarse de manera invariante bajo cambios de escala. ei interes desde el punto de vista de la geoffsica se ha manifestado con ensayos en distintos campos por distintos autores (burrough, 1984; unwin, 1989; jones et al., 1989; hewett, 1986). burrough estim6 la dimensi6n fractal de datos geoffsicos, basado en anal isis de densidad de espectros de frecuencia. una caracterfstica fundamental de los datos geofisicos es el comportamiento estadfstico fuertemente no-gaussiano. jones y sus colegas, afinman que existe una correlaci6n entre los datos geofisicos y las propiedades fractales de sus series de tiempo. en este articulo se utiliza la ley empirica de analisis de rangos de escala planteado por hurst, para determinar la dimensi6n fractal de una secci6n en un registro electrico de pozo, partiendo del calculo empfrico del exponente de hurst (h) como medida del grade de autoafinidad en el mismo y la relaci6n con su dimensi6n fractal. 2. fundamentos teoricos la definici6n formal del concepto de fractal fue planteada por el creador de la geometrfa fractal, benoit mandelbrot, en los siguientes terminos: "un fractal es un conjunto para el cual la dimensi6n hausdorf-besicovith estrictamente 10 montes: fractales y series de datos geofisicos excede la dimensi6n topoi6gica"; y de una. manera menos formal como: "la forma hecha de partes similares de algun modo". un elemento 1-dimensional como un segmento de una linea posee una propiedad de similitud a escala, pudiendo dividirse en n partes identlcas al original, cada una un factor de escala 1/n. un objeto 2-dimensional puede descomponerse en n partes iguales al original con un factor de escala de (1/n)'i2. a su vez un cubo se descompone en n pequeiios cubos con factor de escala (1/n)113. en conclusi6n un objeto d-dimensional puede dividirse en n pequeiias partes, cada una similar a la primera y reducida en un factor de escala r = (n)1/d despejando 0 se obtiene la dimensi6n fractal delobjeto. o = 10g(n) / log(r) a diferencia de la dimensi6n euclidiana, la dimensi6n fractal es estrictamente no entera, bajo magnificaci6n, un segmento de la figura principal luce parecida perc no identica a la primera. las formas que tienen un comportamiento marcadamente aleatorio, se repiten estadisticamente unicamente cuando las dos direcciones (t y f[td se magnifican a escalas diferentes. si t se magnifica en un factor r(r*t), entonces f[t] se magnifica en un factor rh(rh*f). esta escala no uniforme es conocida como autoafinidad y esta ligada fuertemente al concepto de dimensi6,n fractal. la dimensi6n fractal de una forma auto-affn, se relaciona con el para metro de escala h mediante la ecuaci6n: 0= 2 h [1] 3. metodo de analisis de rangos bajo cambios de escala ei metoda estadistico de anallsis de rango bajo cambios de escala, fue inventado por h.e. hurst en 1950 (hurst, 1951; hurst et at, 1965). para su entendimiento se considera el siguiente problema: se debe estimar la capacidad de almacenamiento de una represa bas ado en registros observados de descargas de agua, el valor ideal del volumen de la represa es aquel que nunca permita que esta se rebose 0 que se vacie completamente. en un tiempo t recibe una descarga de agua &(t), liberando en un perfodo de tiempo t la cantidad promedio <&(t»,. ei f1ujo promedio 0 el volumen recibido en un periodo de t aiios es: [2] ese volumen promedio recibido debe ser igual al volumen liberado en ese mismo periodo. a su vez el volumen neto acumulado 0 la diferencia entre el volumen recibido y el liberado en un periodo r es: [3] la diferencia entre el maximo y minima f1ujo acumulado x es el rango r, el cual mide la capacidad de almacenamiento para mantener la descarga media y nunca liegar al vaciado complete. r('r) = max x(t,-r) min x(t,-r) l s t s r l s t s r [4] donde t es un entero y t es el periodo considerado, la desviaci6n estandar se estima de las observaciones asf: i=t s = { (1ft) ~1 {e(t)-<e(t», }2 r [5] investigando muchos fen6menos naturales, hurst encontr6 que la relaci6n rls para tales fen6menos se describe muy bien por la relaci6n: r/s = (-r/2)h [6] que es la liamada ley empfrica de hurst claramente r depende del perlodo t en consideraci6n y se espera que aumente al incrementarse t, es decir al cambiar la escala de tlsmpos. este concepto de anallsis de rango bajo cambios de escala coincide con el concepto de auto-afinidad de un fractal cuando se buscan partes similares al total al realizar cam bios de escala 0 magnificaci6n. 4. proceso de datos en el presente trabajo se tom6 un registro electrico dlgitalizado. para garantizar la calidad de los datos se tomaron aquellos que estaban a una profundidad mayor de 100 m y hasta un limite de 480 m como cota inferior. dicho registro pertenece a una zona de rocas sedimentarias, cuya secuencia refleja el comportamiento del clima en perfodos geol6gicos lejanos. los valores que representan la resistividad de las rocas a diferentes profundidades, estan consignados en la tabla 1. se consideraron 10 periodos diferentes, el primero correspondia a los 10 m iniciales de la secuencia (a partir de los 100 m), para los siguientes se increment6 el valor del perfodo (20 m para el segundo, etc) hasta abarcar la totalidad de la secuencia 380 m (480 m tope de la secuencia). en cada perfodo se calcul6 el valor medio de la resistill 'dad, [2], y el acumulado [3). se observ6 que este valor fuera estrictamente mayor que cero (0). geofisica colombiana n° 2, abril de 1993 11 a continuaci6n se determin6 el valor del range r [4], asf como la desviaci6n sstandar alrededor de la resistividad media, en cada perfodo. luego para cada perlodo se encontr6 el cociente entre el rango (r) y la desviaci6n sstandar (s). finalmente se hizo una regresi6n lineal entre el cociente r1s y el perfodo en una escala loglog, para determinar el exponente h [6] y la dimensi6n fractal del registro [1]. ei resultado de los calculos se muestra en la tabla 2. tabla 1 valores de resistividad, muestreados en el registro a intervalos de 1m, el primero corresponde al techo de la secuencia y el ultimo al tope inferior 29 29 30 33 41 60 75 85 105 116 124 134 131 129 130 135 144 158 166 171 173 177 178 161 141 149 143 129 122 122 109 106 110 118 125 156 178 191 195 203 211 215 207 195 158 143 136 120 123 122 121 116 114 113 112 113 113 110 106 103 103 107 110 114 115 99 83 71 65 58 54 55 52 56 86 97 111 115 106 95 87 84 89 80 74 65 65 57 61 62 67 66 70 72 85 93 95 135 132 121 114 90 69 61 57 46 46 48 48 52 62 82 104 131 166 202 190 161 135 102 93 94 87 92 80 76 67 64 63 62 64 61 49 42 35 33 31 31 31 32 33 33 34 35 41 50 50 62 53 70 53 55 43 44 43 38 45 36 51 34 52 34 40 34 36 35 37 36 36 37 38 37 42 40 45 41 54 42 60 43 67 41 83 26 93 27 95 27 116 27 150 28 195 29 218 33 247 37 222 39 167 35 139 34 117 35 114 36 124 37 144 40 170 40 190 42 205 49 209 55 213 62 210 64 217 59 215 57 241 63 248 63 248 56 218 53 216 50 221 51 212 48 168 48 130 112 46 71 121 47 63 143 53 57 183 75 58 131 110 55 115 157 52 95 185 52 67 181 53 59 181 55 57 187 58 63 189 64 63 190 82 63 188 113 62 188 128 64 187 133 65 178 137 66 175 115 68 165 97 70 151 86 75 140 83 77 126 84 88 120 86 97 109 100 115 111 120 144 106 155 195 109 146 248 121 128 248 122 125 248 121 125 248 117 130 248 117 108 248 150 109 248 169 116 145 162 121 98 158 118 78 144 121 93 140 94 126 75 132 63 126 57 118 57 111 60 108 56 121 48 127 41 113 41 107 43 106 43 102 43 93 • techo tope superior, •• base tope inferior 5. discusion de resultados un analisis de regresi6n lineal efectuado entre el 10g(r1s) y el log(t) arroj6 los siguientes resultados: se hall6 que el exponente de hurst era de 0.854 con un coeficiente de correlaci6n de 0.997, indicando que la dimensi6n fractal del registro electrico era de 1.146 (fig.1). este resultado coincide con los obtenidos por hewett (hewett at ai., 1986) en el calculo de la dimensi6n fractal por el rnstodo de anatsls de frecuencia, de la porosidad medida en un registro de pozo distinto (1.100 pies), y para el cual el exponente de hurst fue h = 0.855. los datos de la tabla 1 y la fig.1, muestran que la resistividad a 10 largo del registro fluctua fuertemente; sin embargo, la fig.2 indica un comportamiento persistente, reafirmado con el valor del exponente de hurst (h = 0.853), indiscutiblemente mayor que 0.5 (valor correspondiente a una serie de datos completamente aleatoria), 10 cual muestra una tendencia en el proceso que dio su origen. una raz6n por la cual se puede explicar porqus estas secuencias de datos geoffsicos se comportan de acuerdo a una estadfstica de hurst, es que una secuencia sedimentaria refleja el comportamiento del clima en tiempos geol6gicos lejanos y los sedimentos que formaron las rocas actuales fueron transportados por corrientes de agua que erodaron las rocas antiguas, y estos f1ujos se comportan adecuadamente con la estadfstica de hurst. se observa que un registro electrico como serie de datos geoffsicos presenta un comportamiento fractal, el cual puede relacionarse con el proceso que 10gener6, tales como la erosion y el clima de epocas geol6gicas lejanas. la geometrfa fractal permite observar un orden en las formas naturales de los datos geoffsicos, que a primera vista no se observan. tabla 2 muestra los valores de r, s, (r/s), log(t), log(r/s) y los resultados de la regresi6n lineal entre log(r/s) y log(t) r s log(t) log(r1s)r1s 151.9000 31.49937 477.2500 47.96236 722.6500 46.79476 1283.075 43.84825 1750.500 44.36271 2537.938 47.25159 3493.441 47.65990 5496.175 59.98748 5755.375 59.61874 5927.324 56.07979 4.822319 9.950512 15.44297 29.26171 38.78257 53.71115 73.29937 91.62203 96.53634 105.6945 2.302585 2.995732 3.688879 4.382027 4.787492 5.075174 5.393628 5.634790 5.768321 5.940171 1.573255 2.297624 2.737154 3.376280 3.657971 3.983621 4.294552 4.517672 4.569919 4.660552 regression output: constant -0.35835 std err of y est 0.059437 r squared 0.99721 no. of observations 10 degrees of freedom 8 x coefficient(s) 0.0853886 std err of coef. 0.01597 12 monte.: fractale. y .erle. de datos geofi.ico. '"n .. -0o -0. -0 c ::j '+-ea.. 200 400 (ohm-m) figura 1. registro electrico de pozo. cota superior: 100 m (profundidad) o resistividad o n o"loq. -l'"ci 2.0 3.0 4.0 5.0 6.0 log (periodo) figura 2. curva de regresi6n. datos de la tabla 2. y = 0.8538x • 0.358. coeficiente de determinaci6n = 0.9972 bibliografia burrough. p.a. (1981): fractal dimensi6n of landscapes and other enviromental data. nature 294 (5838). burrough. p.a. (1984): the application of fractals to geophysical phenomena: institute of mathematics and its applications 20 (3-4). feder, j. (1988): fractals. plenum press. new york. heinz ...o. peltgen & p.h. ritcher. (1987): the beauty of r-ractals. springer-verlag, new york. heinz, 0., peitgen et al. (1988): the science of fractal images. spnnger-verlag, new york. hewett, l.a. (1986): fractal distributions of reservoirs hetereoqenity and their influence on fluid transport. technical conference of the society of petroleum engineers. new orleans. hurst et al. (1965): long-term storage. an experimental sfudy. constable, london. jones, j.g. et al. (1989): fractal properties of computer-generated and natural geophysical data. computers & geosciences 15 (2):227-235. mahdelbo~ b. (1982): fractals: form, chance and dimension. w.h. freeman, san francisco. unwin, d. (1989): fractals and the geosciences. computers & geosciences 15 (2). pergamon pres, oxford. erikajaramillo.indd 73 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 73-84 application of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia erika cristina jaramillo giraldo1, ximena vega manchola1 and hugo monsalve jaramillo1 1centro de estudios e investigaciones de la facultad de ingeniería (research center at university of quindío)ceifi, quimbaya group e-mail: hugom@uniquindio.edu.co e-mail: manchol@hotmail.com. resumen se describe la metodología usada para desarrollar campañas geofísicas de resistividad y gravimetría aplicadas al subsuelo. estos son métodos totalmente indirectos cuyo propósito es determinar los contrastes de densidad y resistividad en el subsuelo, para relacionarlas con los diferentes tipos de suelos en las áreas de estudio, localizadas al sur y norte de la ciudad de armenia-quindío (puerto espejo y universidad del quindío respectivamente), para determinar la geometría de perfiles longitudinales basados en pozos geotécnicos existentes los cuales solo proveen información unidimensional. la importancia del subsuelo es analizada usando métodos eléctricos de resistividad y gravimetría en proyectos de ingeniería civil. los factores más importantes que influencian los cambios en la resistividad eléctrica y valores gravimétricos para diferentes materiales geológicos también son analizados, así como los rangos de variación de estos valores, dependiendo de las condiciones climáticas y la formación de capas en cada área de estudio. para la tomografía eléctrica, se usa el arreglo electródico wenner-schlumberger debido a su alta resolución vertical y horizontal. para el método gravimétrico, se usa la técnica microgravimétrica porque permite mayor detalle en capas superficiales del subsuelo. los resultados obtenidos son presentados en perfiles eléctricos y gravimétricos en puerto espejo y la universidad del quindío, los cuales fueron útiles como parámetros de calibración de los rangos de resistividad y gravimetría para los tipos de suelos encontrados en estas áreas de colombia, dependiendo también de los factores de humedad y temperatura. el análisis y discusión de los resultados es lo que finalmente permite obtener conclusiones, importante para la escogencia de métodos de exploración geofísica como una alternativa para familiarizarse con las estructuras localizadas en el subsuelo. palabras clave: gravimetría, tomografía eléctrica, perfiles estratigráficos, abanico volcaniclástico, escarpe, armenia (colombia). abstract the methodology used to develop resistivity and gravimetry geophysical surveying applied to the sub-surface is described. these are totally indirect methods whose purpose is to determine the density and resistivity (manuscript received may 2005, paper accepted october 2005) 74 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo contrasts in the sub-surface, in order to relate them with the different types of soils in the areas of study, located south and north in armenia-quindío (puerto espejo and university of quindío respectively), to determine the geometry of longitudinal profiles based on existing geotechnical boreholes which only provide unidimensional information.the importance of sub-surface is analyzed, using electric resistivity and gravimetry methods in civil engineering projects. the most important factors influencing the changes in electric resistivity and gravimetry values for differerent geological materials are also analyzed, as well as the ranges of variation of these values, depending on climate conditions and formation of layers in each area of study. for the electric tomography, the electrodic array wenner-schlumberger is used due to its high vertical and horizontal resolution. for the gravimetric method, the microgravimetry technic is used because this allows better detail in shallow layers of the sub-surface. results obtained are presented in electric and gravimetric profiles in puerto espejo and university of quindío, which are useful as calibration parameters of resistivity and gravity ranges for the types of soils found in these areas in colombia, depending also on the humidity and temperature factors. the analysis and discussion of results is what finally permits to obtain conclusions, important for choosing the methods of geophysical exploration as an alternative to get familiar with the structures located in the sub-surface. key words: gravimetry, electric tomography, stratigrafic profiles, volcaniclastic fan, scarp, armenia (colombia). © 2005 esrj unibiblos introduction the city of armenia, colombia has been the location of several studies (ingeominas, 1999, espinosa, 2000, monsalve y vargas, 2002) as a result of the most recent earhquake in quindío departament (25-01-1999, mw=6.2). for this reason, scientists and investigators have tried to improve general knowledge in geophysical and geological aspects in quindío departament. the most recent interest for the understanding of the sub-surface in armenia has been the geophysical and structural modeling of urban area and surroundings in armenia project, which includes the present study, where the physical methods of gravimetry and electric tomograpy are used, to obtain stratigraphic profiles. the geophysical surveying methods, among which we can mention the electric resistivity and gravimetry, measure a series of physical properties of objects or structures in the sub-surface, that make them different from the surrounding site. the exploration of these contrasts between the properties of diverse materials and the encasing rocks is common in the exploitation of natural resources (petroleum, underground water, minerals etc) and in prospection of support for civil engineering, just like the surveying for the study of conditions for dams or buildings foundations, or the level of soil saturation. the objective of geolectric and gravimetric prospection is to know the dimensions and depth of structures in the sub-surface, taking as reference the resistivity and gravity data, obtained in the site using different techniques. when geoelectric and gravimetric data are correctly analyzed with modern methods and using the appropriate mathematical and computational tools, they provide a cartography of gravity of the sub-surface, which permits to have a clearer idea of geometric characteristics of every layer below. the methodoly used to obtain geophysical imaging of sub-surface using gravimetry and electric tomography applied to university of quindío and puerto espejo, is described in general. in the same way, we present the results obtained from the the analyses and discussions are presented regarding these results. the present study finally considers the electric tomography and gravimetry as tools, having a high potential in the survey of sub-surface, allowing to obtain stratigraphic profiles in two dimensions that represent in a more exact way the real geometry of the layers. this is done in a better form than with currently available geotechnical methods. 75 aplication of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia location the area under study is specifically located at university of quindío and puerto espejo, located northwest and southwest in armenia-quindío –colombia respectively, areas that have relatively flat topographies. inside each of these areas there is a geotechnical borehole, drilled during the seismic microzonification of armenia-quindío project (ais, 1999). figure 1 shows the location of the two zones under study and the soils that support them. figure 2 shows the geotechnical boreholes in them. the geographycal coordinates of gravimetric and tomographic profiles and the geotechnical boreholes are shown in chart 1. chart 1. geographical coordinates of final and initial stations for every gravimetric profile of geotechnical boreholes (p4: puerto espejo and p6: university of quindío) and the midpoints of every tomographic profile location puerto espejo university of quindío gravimetry n (m) 4° 31’ 5.65’’ 4° 31’ 10.02’’ 4° 33’ 14.93’’ – 4° 33’ 23.12’’ e (m) 72° 42’ 44.36’’ 72° 42’ 29.40’’ 72° 39’ 43.75’’ – 72° 39’ 30.85’’ electric tomography pto. medio n 4º 31.296’ 4º 33.466’ w 75º 42.849’ 75º 39.903’ geotechnical boreholes n (m) 4° 31’ 7.22’’ 4° 33’ 22.25’’ e (m) 72° 42’ 43.19’’ 72° 39’ 36.04’’ figure 1. location of tomographic and gravimetric profiles at university of quindío and in puerto espejo (modified from ingeominas, 1999). 76 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo figure 2. boreholes p4 and p6 corresponding to puerto espejo and university of quindío. geological setting the outcrops in the study area are geological formations belonging to cretaceous period (arquía complex), terciary period (fluvio volcanic and volcanic deposits) and quaternary period (quindío fan). these are, residual and saprofit deposits, coluvial deposits, pyroclastic fall deposits, alluvial sediments, and recent fluviotorrential sediments of quindío river. the area of interest is located on a large volcanosedimentary deposit, formed by alternation of pyroclastic flow deposits, laharic and fluviotorrential, alluvial and glacial materials in smaller proportion, mostly covered by volcanic tuffs (ash, , lapilli and volcanic dust), that covered a paleo-topography, generating round and low grade hills, which are the geographical shapes that characterize the region. the materials forming the big volcanic-sedimentary deposit come from the central cordillera, located east and northeast of quindío. these deposits are mostly products of volcanic activity originated at nevado del quindío, santa isabel, and cerro santa rosa volcanoes (gonzález and nuñez, 1991). espinosa (in press), assigns this unit the name of quindío fan and makes a morphological difference with others, concluding that its thickness is not constant, reffering to an increase from the apex, associated with distance to volcanic centers. espinosa’s work defines the terracing (see figure 3) of quindío and pereira fans as a series of descending levels from the apex to the distal part, for this reason, the quindío fan is divided in three sub-groups named as proximal, intermediate and distal fans, so that the closest fan to the apex is encased in an older unit. in the same form, the degree of incision in the oldest units (closest to the apex) is much bigger than that of recent units due to its bigger exposition to alluvial erosion. figure 3. formation of fans, note the terracing between the most recent units (4 and 3) in respect to the oldest (1 and 2) (espinosa, in press). 77 aplication of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia methodology figure 4 shows the methodology used in gravimetric surveying. the objective of the study was to determine the contrast of densities and geometry (orientation and aproximate thickness) of sub-surface layers for the zones of puerto espejo and university of quindío. its reach permitted to classify the study as local type, for this reason the technique used was micro-gravimetry relative gravity testing was done every 10m along profiles of about 500m in longitude, using a worden gravimeter. the cycles were done in periods of less than 2 hours and by the same surveyor, seeking to minimize the level of human error. gravimetric profiles figure 4. flow diagram for the construction of gravimetric profiles. tomographic profiles figure 5. flow diagrams for the construction of tomographic profiles. 78 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo the bouguer anomaly data obtained, along with known geologic information of the region, were processed using a computer program for inversion of gravimetric data (gm-sys v.3.6), from which a geological model of the zones was obtained. figure 5 shows the methodology used in tomographic exploration. the objective of the study was to determine the contrast in resistivity in the subsurface layers in two dimensions, confirming the presence of materials associated to those found in geotechnical and geological studies for the zone. the method used to determine the stratigraphic profiles was the electric resistivity, since the objective was to find the geometry of the layers trough the contrast in their resistivities. the method of electric tomography was applied , using the wennerschlumberguer array due to the fact that bi-dimensional profiles were needed and for the necessity of obtaining complementation parameters to regional geotechnical and geological studies which are unidimensional. this array provides a good resolution in both vertical and horizontal directions. for the area of university of quindio, a 180m profile was made (see figure 1), with electrode spacing of 5 meters, reaching a 38.7 meter deep research , while in puerto espejo the profile reached 144 meters, with a spacing between electrodes of 4 meters and a 31 meter depth. the data of apparent resistivity obtained in the processing of the information were used to find the values of real resistivity using a program specialized in inversion (res2dinv, v. 3.48). results gravimetry measured bouguer anomaly values in the range of -33,23 to -22,29 mgal in puerto espejo and 57,79 a 65,27 mgal at university of quindío, resulted in the following models:. puerto espejo zone the profile shown in figure 3 is extended along a gravimetric line with a longitude of 543,68 meters, heights between 1327,52 and 1349,16 meters above sea level and the values for bouguer anomalies increase to the north of the profile as shown in figure 6. figure 6. model of densities distributions in puerto espejo and its anomaly adjustment (above). 79 aplication of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia the geometry of the layers was stablished, obtainig values in density between 1.5 and 1.75 g/cm3, increasing with depth. four layers relatively horizontal for the profile were obtained (see figure 6.), which show an increase in thickness of the fan to the south, presenting a lightly dipping structure. the deposits found in this regions were associated to la florida fan, which belongs to the group of distals fans. the first layer, with a density of 1.5 g/cm3 shows uniformity in thickness (17 m. aproximately) as shown in geotechnical borehole (see figure 2) associated to volcanic ash. for this profile, the presence of arquia complex is observed below 100m depth approximately, in the north direction of the profile and shows a tendency to deepen in the south direction, due to accumulation of quaternary deposits. university of quindío the profile shown in figure 5 is extended along a gravimetric line with a longitude of 581,06 meters, heights between 1511.67 and 1544,725 meters above sea level, presenting bouguer anomalies in the range of 57,79 to 65,27 mgal, from north to south respectively, as shown in figure 7. figure 7. model of densities distributions at university of quindío and its respective anomalie adjustment(above). the values for density obtained are in the range 1.5 to 1.75 g/cm3 increasing as depth increases. five relatively horizontal layers were also obtained (see figure 7), these show an increase in the thickness of the layers toward south, showing a slightly dipping surface. the deposits found in this sector were associated to circasia fan, which belongs to the intermediate fan group. the first layer presenting a density of 1.5 g/cm3 shows a variation in depth from 11 to 24 m. confirming the thickness of volcanic ashes (21 m.) indicated in geotechnical boreholes p6 (see figure 2), increasing thickness toward north. the presence of arquia complex is after the 200m deep. aproximately in the north side, with tendency to deepen to the south direction of the profile, due to accumulation of quaternary deposits. 80 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo electric tomography a model with real distribution of resistivity in the layers of the sub-surface was obtained for every zone under study, which is represented by the variation in colors. after analyzing tomographic profiles with the corresponding mechanical boreholes, the resistivity ranges for every type of soil were computed. the variables involved were: natural humidity, unitary weight, bearing capacity, swave speed and rqd (r=?, q=?, d=?). puerto espejo the correlation of data obtained in mechanical boreholes p4 and the resistivity values found, allowed to obtain a definitive model for puerto espejo, which reflects a great influence of natural humidity in the values of resistivity in the upper layers. this can be observed in the graph obtained during the analysis (figure 8), while in lower layers, the dominant factor is the degree of cementation and meteorization. figure 8. variation in resistivity with natural humidity. although the variation of resistivity with natural humidity is not linear, there is a tendency in the resistivity, to decrease while humidity increases. a model was obtained in which 7 areas were found (see figure 9) every one with special characteristics as described: figure 9. areas differenciation according to resistivity and natural humidity ranges for tomographic profile in puerto espejo. 81 aplication of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia area 1 presents the lowest humidity condition, being reflected in the largest resistivity range, which is, between 4500 and 9500 ohm-m and is associated to lime sandstones. area 2 exhibits an intermediate humidity condition and resistivity values between 2200 and 4500 ω.m., which is associated to sandy limestone and lime clays. area 3 exhibits the highest humidity conditions, as reflected by the smallest resistivity range, this is, between 335 and 2200 ω.m. and is associated to sandy limestone and lime clays. area 4 shows a high humidity condition and certain degree of cementation. due to the position of this area inside the layer of the volcanic ashes, cementation is dominant over humidity, which is reflected in an increase in the resistivity range (4500 a 9500 ω.m.), associated to cemented sandy limestone. below the volcanic ashes layer, there is a lithological material stratum which shows a constrast with the alter(?) and that corresponds to residual soil with resistivity values ranging between 9500 and 26300 ω.m., the aproximate thickness is 8 m. (see figure 2). finally, a layer presenting the highest resistivity values in the whole tomographic profile (from 26300 ω.m. and aheah), which is associated to volcanic flow after 26 m in depth (see figure 2). university of quindío the correlation of data obtained in borehole p6 along with the resistivity values found, determined the defínitive model for university of quindío, where a high influence of natural humidity is observed in resistivity values for upper layers, from which we produced the graph represented in fig. 10. on the other hand, it was possible to note that the dominant factor in lower layers is the degree of cementation and meteorization, same as in the model found for puerto espejo. figure 10. variation of resistivity with natural humidity. for this sector there is a linear variation of resistivity with the humidity, presenting an inverse variation of resistivity values with natural humidity. we can see here that the atypical values belonging to residual soil and volcanic flow do not make part of the chart, since these deposits are deeper, and there is no significant influence of natural humidity, but there is a high influence of cementation and metheorization. 82 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo this way, a definitive model was found, with seven defined areas (see figure 11) as described: area 1 presents the lowest humidity conditions, being this reflected in the highest resistivity values, with a wide variation range (from 12000 to 47030 ω.m.), the superior and inferior limits of resistivity values for this area are high, due to the high percentage of compactation, since it is located below a parking lot. this material is associated to lime sandstone. zona 2 presents a midterm to high humidity condition and resistivity values between 2000 and 7315 ohm-m, wich is associated to sandy limestone and lime clays. area 3 presents the highest humidity condition, being this reflected in the lowest resistivity range (from 449 to 2000 ω.m.). it is associated to sandy limestone and lime clays. area 4 presents a midterm humidity condition and certain degree of cementation. due to the position of this area inside the volcanic ashes layer, there is a marked tendency of cementation upon the humidity condition, which is reflected in an increase in the figure 11. differenciation of areas according to resistivity ranges and natural humidity for a tomographic profile at university of quindio. resistivity range (7,315 to 18,548 ω.m.), associated to cemented lime clays. below the volcanic ashes layer there is a stratum of lithic material which contrasts with the latter, and corresponds to residual soil. here, values of resistivity ranging between 18,548 and 47,030 ω.m. are observed. the aproximate thickness is 8 m. (see figure 2). finally, a layer with the highest resistivity values in the tomographic profile is observed. (after the 47030 ω.m.), which is associated to volcanic flow and is presented aproximately after 26 m in depth. (see figure 2). analysis and results discussion gravimetry taking into account the terraced topography of the area, the gravimetric models of puerto espejo and the university of quindío show the tendency of terracing of quaternary deposits like the arquia complex, reflecting a lightly dipping structure. it is noticeable that arquia complex at university of quindío is presented at a bigger depth (? dificil entender el significado de la expresion) than in puerto espejo, possibly due to the distribution of quaternary deposits. in puerto espejo, these deposits are less antique in comparisson to those at university of quindío, allowing to find the arquia complex closer to the surface. for both sectors there is a 83 aplication of gravimetry and electric tomography methods to obtain stratigrafic profiles: case study at university of quindío and puerto espejo area, armenia-colombia tendency in the layers to increase their thickness toward the south direction of the profiles, reflecting a slightly dipping surface, indincating the form how the deposits have been accumulated. the micro gravimetry method allowed to analyze the layers of the sub-surface in both sectors in a more detailed way, although superficial. the comparison between the different profiles found however, with the existing ones, was only established to a maximum depth of 40 meters aproximately, since this was the biggest depth found in geotechnical boreholes so as in the tomographic survey. electric tomography it is observed that the range of values in resistivity of the sub-surface at university of quindío is bigger with respect to that of puerto espejo, possibly due to soil characteristics and age of the deposits, those at university of quindío are older, which is reflected in the cementation range, one of the most influencing factors in the increase of resistivity. the age of shallow deposits analyzed at university of quindio is relflected in a higher degreee of compactation, which has a direct influence in the increase of resistivity ranges. both models show a dominant tendency of natural humidity in the variation of resistivity values in the volcanic ashes layer, allowing to diffrenciate permeable soil from those non permeable, while in the lower layers (residual soil and volcanic flow) the dominant factor is the degree of cementation and meteorization. the electric tomography method allowed to view the transition between the different layers of the soil. (volcanic ashes, residual soil and volcanic flow), this is evidenced by the strong contrast in the resistivity values mapped. the anomalies found in every profile demonstrates the effectivity of the electric resistivity method in the survey of underground structures since it is possible to observe a good resolution of geometry and depth, providing civil engineers with an reliable alternative in the analysis of foundations, piping, ducts, or underground structures. a limitation in the electric tomography works applied to civil engineering is the necessity to be carried out on a natural surface (soil) (??) conclusions -the ranges of apparent resistivity for certain types of geologycal material and minerals found reported by various authors differ with the values found in the present study, we suggest this is because in those refferences the conditions under which the resistivity values were obtained are not considered. moreover, these studies have been made in other countries were the composition of the layers of the sub-surface may be different to those of this geographycal region. -analyzing the tomographic profile, according to the geotechnical data from the area under study, its is observed that in the layers of volcanic ashes, (upper layers) the dominant factor in the variation of the resistivity is the natural humidity, while in the lower layers like residual soil and volcanic flow the metheorization degree is dominant. the electric tomography method is useful as a tool of sub-surface surveying at shallow levels and the construction of stratigraphic profiles, an appropriate calibration of the ranges of resistivity is necessary however for the soils of the municipality and for different seasons of the year in order to consider climate factors. it is useful to know the tendencies of the subsurface structures using the electric tomography in civil enginnering, and more specifically in geotechnics, because the level of uncertainty in excavations is reduced for the foundation. with this, the number of tests is also reduced as well as the cost of construction projects. the gravimetric method is of great help for geological characterization studies as well as for the seismic microzoning, because it permits to identify the more dense layers of the sub-surface associated to rock formation such as arquía complex and diffrenciate them from the less dense like quindío fan. due to the difference in characteristics between soils at university of quindío and puerto espejo, there is a variation in the ranges of resistivity, being reduced in puerto espejo. at university of quindío, the resistivity ranges obtained for the same type of soil in puerto espejo were wider. it can be observed that the thickness of the volcanicsedimentary deposits, corresponding to densities between 1.5 and 1.75 g/cm3, is strongly related to the location of each area under study in respect to volcanic centers that originated them. for this reason, 84 erika cristina jaramillo giraldo, ximena vega manchola and hugo monsalve jaramillo the gravimetric profile of puerto espejo presents smaller thickness for those deposits, since this area is located at a distal fan (la florida), while the thickness determined for deposits below university of quindío is larger because they are located in a midterm fan (circasia), being consequently older. references • ais, asociación colombiana de ingeniería sísmica. 1999. estudio de microzonificación sísmica para orientar la reconstrucción de armenia – quindío. informe técnico-científico. bogota. • burger h. r., 1992. exploration geophysics of shallow subsurface, prentice-hall. 489p. englewood cliffs-new jersey. • chica a., 1998. geofisica aplicada –magnetometría y gravimetría-. cuadernos de ingeniería geológica. no. 5. 125 p. medellín, colombia. • duyoner, m. y aguirre, y. (19997) identificación y caracterización macroscópica de la secuencia de tefras de la ciudad de armenia (crq). • espinosa, a., 2000. mapa preliminar de fallas activas del quindío, zona central. informe final proyecto 152, universidad del quindío, armenia, 70 p., inédito. • espinosa, a. (en prensa). un modelo del cuaternario del quindío y risaralda. revista de la academia colombiana de ciencias exactas, físicas y naturales. bogotá. • espitia, m. y caro, p. capítulo 1 geología local y tectónica en ingeominas, (1999) vol ii. • gm-sys, gravity and magnetics modeling system. versión no. 3.60. mike gemperle, gerry connard, milt sagen, and myungmun bae northwest geophysical associates, inc. • loke, m. h. 1999. electrical imaging survey for enviromental and engineerimg studies: a practical guide to 2-d and 3-d surveys. malasia. p 58. • monsalve-jaramillo, h. y vargas-jiménez, c.a. 2002. el sismo de armenia (25-01-99, mw=6.2): análisis telesísmico y características de las ondas de cuerpo. revista ipgh, no 57, 21-56. • núñez, a. y gonzález, h. 1991. memorias mapa geológico generalizado del departamento del quindío. ingeominas, bogotá. geociencias 15-1 julio 2011.vp field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts carlos a. vargas1, paul mann2 and carlos borrero3 1 department of geosciences, universidad nacional de colombia, bogotá 2 institute for geophysics, university of texas, austin 3 department of earth sciences, universidad de caldas, manizales abstract central cordillera of colombia near to the nevado del ruiz volcano, is a seismically active area above the subducting slab of the nazca plate and deforming upper crust of the andes. buildings in the region require strengthening against the effects of both types of hazards: earthquakes and volcanoes. during these trips, we will discuss the 13 november, 1985, nevado del ruiz volcano eruption that destroyed the armero city with about 22,000 deceases, as well the 25 january, 1999, armenia earthquake (m 6.2) that killed about 2000 people and injured 4000 largely as the result of older, poorly constructed buildings. the economic impact of the quake on the region was significant with about 8000 coffee farms either completely or partially destroyed and over 13,000 structures either partially damaged or completely destroyed. both phenomena are associated to a large-transversal structure termed caldas tear, which is controlling strong motion earthquakes and the quaternary volcanism of the northern andes. part 1: tectonic setting of the field trip areas from sedimentary basins to the upper mantle the field trip area represents an important break between a steeper dipping slab to the south associated with the active andean volcanic arc and a shallower dipping slab to the north that is not associated with an active volcanic arc. in the text below, we describe two regional profiles on both sides of this break in the arc and subduction structure based on an attenuation tomographic study of subducted slabs beneath colombia to depths of 275 km (figure 1). both of these sections are supported by gravity and magnetic models. our results indicate a major, 240-km-long tear in the nazca slab at latitude 5.4°n that separates a 30-40°-dipping southern area with arc volcanism and a zone of shallow 20-30°-dipping subduction adjacent to the more buoyant panama arc or "indenter" (figure 1, inset map). this east-west slab tear is collinear with the extinct east-west sandra spreading ridge on the unsubducted nazca plate which we postulate may have formed the zone of weakness along which the tear formed in the subducted slab (figure 1, inset map). the northern edge of the panama arc merges without apparent tearing of the caribbean slab (figure 2). the bucaramanga nest occurs on the downdip continuation of the panama arc and may reflect ongoing breakoff of oceanic crust from the panama arc block that has become impeded in the zone of shallow subduction. on this trip, we will examine crustal faults and volcanic products that align along the linear surface trace of the proposed caldas tear. we will also discuss the earthquake and volcanic hazards associated with this complex, arc-continent tectonic setting that includes the superimposed hazards from both subduction earthquakes and earthquakes produced during shallow crustal deformation. tectonic setting of the field trip area gps data provides a tectonic framework for understanding the collision of the panama arc with northern south america (freymueller et al., 1993; calais et al., 2009) (figure 1; inset map). vectors in western colombia show a marked decrease in velocities consistent with collision near the panama-colombia border (adamek et al., 1988; duque-caro, 1990). the east-west direction of vectors shows that east-west shortening is constant over large areas of western colombia (corredor, 2003). vectors on the maracaibo block of colombia and venezuela show a more northward direction consistent with the tectonic escape of the maracaibo block into the southern caribbean (kellogg and bonini, 1982) (figure 1 inset map). unlike the uniform velocity field in the upper crust known from gps, the subducted slabs beneath colombia change dip rapidly from an area of normal oceanic subduction at a dip angle of 30-40° between latitudes 3.0-5.4°n in southern colombia (figure 3) and to an area of shallow subduction with dip angles of 20-30° in the area north of 5.4°n, adjacent to the panama arc (figure 2). the change in dip is abrupt and suggests a right-lateral e-w trending shear zone (hereinafter called the caldas tear) associated with a slab tear process that can be traced over a distance of 240 km as shown by the eastward displacement of the zone of intermediate depth seismicity at latitude 5.4°n and beneath the western, central and eastern cordilleras (figure 1, inset map). south of the latitude of the caldas tear fault, the volcanic chain of the andean range is active and starts the transition to inactive volcano occurrences with quaternary ages (ruiz volcanic complex) (figure 1, inset map). the adakitic composition of these volcanoes (borrero et al., 2009) and the presence earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 47-74research groupin geophysics universidad nacional de colombia agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:19 p p p composite 133 lpi at 45 degrees of other inactive volcanic bodies to the north of the caldas tear area, in the paipa iza volcanic complex (eastern cordillera), with pliocene-pleistocene ages (pardo et al, 2005) are consistent with melting processes of subducted slabs occurring on both sides of the caldas tear and perhaps using it as a pathway to the surface (figure 1, inset map). earthquake tomography has been used in the past for imaging this region (van der hilst and mann, 1994; taboada et al., 2000; vargas et al., 2007) but differences in their respective resolutions have led to varying proposals for the slab geometry at depth and these interpretations have not been supported by other geophysical data. we present here an integrated study designed to analyze the interaction between the caribbean and nazca plates in the northwestern south america and panama area that is mainly based on the many earthquakes occurring in the region. coda-wave attenuation for imaging plate interactions in northwestern south america we have used the decay rate of the coda amplitudes (q c 1 coda attenuation) of 1674 regional earthquakes located by 17 short-period instruments (t=1 sec) of the colombian national seismological network cnsn (figure 1). hypocentral solutions were calculated using the hypocenter (lienert and hayskov, 1995) program and the velocity model used by the cnsn (ojeda and 48 carlos a. vargas, paul mann and carlos borrero figure 1. tectonic setting of colombia and panama. map shows principal plate boundaries, fault systems and neotectonic faults, as well lineaments (dashed lines) and other smaller faults (thin lines). dots represent hypocentral distribution of the approximately 68,000 earthquakes 1.0 = ml > 6.7 located by the cnsn between 1993 and 2008. color scale, continuously varying, indicates the depth of events. arrows represent station velocity gps vectors relative to stable south america. chep and bogo denote the gps’s stations used as reference to estimate the onset of the panama-arc and south american plate collision. other gps stations in the panama-arc collision area are manz, rion, bucm, mont and cart. the northern and southern profiles are tomographic sections described in this study. key to abbreviations: smm, santa marta massive; choco block, cb; western cordillera, wc; central cordillera, cc; eastern cordillera, ec; perija range, pr; guajira basin, gb; llanos foreland basin, lb; middle magdalena valley basin, mmvb; romeral fault system, rfs; santa marta-bucaramanga fault, smbf; palestina fault, pf; cimitarra fault, cf; mulato-getudo fault, mgf; honda fault, hf; salinas fault system, sfs; garrapatas fault, gf; llanos fault system, lfs; ibague fault, if; sandra rift, sr; bucaramanga nest, bn; cauca nest, cn; paipa iza volcanic complex, pivc; ruiz volcanic complex, rvc. yellow stars correspond to the tauramena earthquake (jan. 19th, 1995, mw=6.5), armenia earthquake (jan. 25th, 1999, mw=6.2) and quetame earthquake (may 24th, 2008, ml=5.7). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:19 p p p composite 133 lpi at 45 degrees havskov, 2001). attenuation estimations were done using the single scattering theory (aki and chouet, 1975; hayskov et al., 1989) in frequency band 1-3 (2 ± 1) hz and a coda-wave time window of 20 seconds which insures the detection of regional structures. additionally, a 3d spatial inversion was completed for establishing a deterministic characterization of the spatial heterogeneity in the lithosphere as an alternative technique for traditional tomographic measurements (o'doherty et al, 1997; lacruz et al, 2009). q c 1 is a useful parameter for examining the acoustic contrast between the upper mantle and the lithosphere because seismic attenuation is a physical parameter closely related to the thermal state of the volume sampled by the coda-waves. interpretation of the attenuation tomography sections were supported by gravity and magnetic modeling of regional data (sandwell and smith, 1997; maus et al., 2007); geothermal observations were derived from oil wells (vargas et al, 2009) and geologic transects were compiled with surficial maps and representative seismic lines (lopez, 2004). the tomographic sections extended from the north (figure 2) and south (figure 3) of the panama arc and indenter to as far east as the llanos foreland basin. these lines cross many interesting tectonic features including: 1) different geomorphological expressions of the northern andean mountains; 2) the two main earthquake nests of colombia (bucaramanga and cauca); and 3) the main active fault systems of colombia (romeral fault zone, santa marta bucaramanga fault system, llanos fault system, garrapatas fault and ibague fault). flat subduction of the caribbean plate and coexistence with the bucaramanga nest: evidence of a tear propagation process tomographic data show the presence of an oceanic crust with thicknesses greater than 20 km related to the caribbean plateau (kerr et al., 1998) with a shallow-subduction angle of $ 15° over a distance of ~100 km that extends from the south caribbean deformed belt to the romeral fault zone (figure 2). beneath colombia, the dip of the subducted caribbean slab steepens to 20-30°. its likely that the interaction of the caribbean slab with continental crust promotes its subduction at higher angles to intermediate and deep depths (~160 km). the seismicity of the bucaramanga nest is tightly concentrated in the caribbean slab at depths of 130 to160 km (zarifi, 2006). there are at least two sets of events forming this pattern and focusing at depths of 80 km and 110 km. focal mechanisms of the bucaramanga nest for events reported by neic (1977-2009) with mb �4.0 suggest a complex process of fracturing that includes both reverse and strike-slip events. we relate this swarm to a complex tearing process linked to updip shallow subduction of the panama indenter and the initiation of slab breakoff at depth (cortes and angelier, 2005). we have compiled data to show the temporal trend of the swarm using the seismicity located by the cnsn between 1993 to 2009. a gradual displacement of events northward and eastward may be due to a tearing in subducting the caribbean plate, that is slowly propagating to the northeast. the paipa iza volcanic complex dated as 1.9-2.5 ma (pardo et al., 2005) is also a possible consequence of the slab tear mechanism, with the nascent tear providing a path for ascending asthenospheric material. although there is not enough geochemical knowledge to prove that ascending magma is a direct consequence of slab tear processes, the low ratio of sr87/sr86 (0.705) and the presence of emplaced xenoliths of metamorphic rocks support this interpretation (jaramillo, 2010). normal subduction of the nazca plate plate and presence of the cauca nest: lengthening the caldas tear? the tomographically-imaged nazca slab is 15 to 22 km in thickness and shows a constant subduction angle of 30-40° to a depth >150 km beneath the active volcanic line which are underlain by high attenuation anomalies all consistent with the normal melting range of subducted oceanic crust (figure 3). high attenuation anomalies and the presence of shallow to intermediate seismicity around the romeral fault zone suggest a strong structural control on magmatic activity. a large low-attenuation anomaly corresponds with the low geothermal gradient observations between the colombia trench and the central cordillera and this anomaly may represent volcanic and sedimentary material accreted to the western cordillera and baudo range (kerr and tarney, 2005) (figure 1). the accretion of this thick sediment package has been proposed to have forced a jump in subduction from along the romeral fault suture in the central cordillera to the present offshore colombia trench (cediel et al., 2003). two additional attenuation anomalies include: 1) a prominent low anomaly located beneath the eastern cordillera and the middle magdalena valley basins and bounded by the llanos fault system to the east and the jurassic igneous bodies of the central cordillera in the west (sarmiento-rojas et al., 2006; gómez et al., 2005 ; branquet et al., 2002) ; field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 49 figure 2. tomographic section along the northern transect. coda-wave attenuation anomalies supporting the dip changes in the subducted slab. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:20 p p p composite 133 lpi at 45 degrees and 2) a low anomaly beneath the llanos foreland basin, which has not been previously documented but may represent the continuation of a deeply buried jurassic rift collinear with the espino and apure-mantecal rifts known in venezuela (ostos et al., 2005). seismicity of the cauca nest is more distributed in depth (70 150 km depth range) and its active evolution seems even more complex than the bucaramanga nest. its n-s trend is limited to the north by earthquakes with focal mechanisms ranging from pure gravitational collapse in the north to strike-slip events parallel to the caldas tear fault (figure 1). at the southern border of colombia (latitude 2.5n) there is a shift to a new pattern of intermediate and deep seismicity associated with flat subduction of the broad area of active volcanoes in southern colombia and northern ecuador (gutscher et al., 2000). linking the panama-arc collision with areas of volcanism and seismic nests differential displacements between the south american plate and the panama-arc based on gps observations suggest a trend of decreasing displacement towards the east (trenkamp et al., 2002) (e.g. motions at sites of bogo vs. sites at mzal, rion, bucm, mont and cart). assuming the bogo station as a reference point south of the caldas tear, and the chep station as the reference point on the panama arc-indenter, we estimate ~24 mm/y of effective differential displacement. given the hypothesis of lateral homogeneity of the crust, constant displacement rate and a seismic offset along caldas tear of ~240 km, we infer that the onset of the panama-south america collision began ~10 ma (late miocene). geologic field observations in the panama-arc (coates et al., 2004) suggest that the age for initiation of collision of the panama-arc against south america occurred between 12.8 to 7.1 ma, which is consistent with our inferred 10 ma age for initiation of the caldas tear. the sandra rift on the subducting nazca plate is collinear with the caldas tear and is thought to have formed the original weakness that would cause the tear in the subducted slab. the age of the sandra rift is 9-12 ma (lonsdale, 2005). although there is no evidence of recent activity along the sandra rift due to lack of instrumentation in the pacific offshore of colombia and panama, recent earthquakes and adakite volcanism along the caldas tear during last ~4 ma, are suggesting that this lineament corresponds to a deep-fault-system that allows to move magmatic fluids and generating high geothermal anomalies around it (vargas et al., 2009). likewise, the caldas tear may be responsible of producing large earthquakes along the trend, including the recent strong motion activity associated to the tauramena earthquake (jan. 19th, 1995, mw = 6.5). two types of focal mechanisms have been proposed for this event, one of them suggest an ew-oriented right lateral movement (dimate et al., 2003). similarly seismic activity in the area of the armenia earthquak (jan. 25th, 1999, mw = 6.2) shows east-west alignment of aftershocks with the caldas tear. moreover, this long right-lateral-weakness-lineament does not seem to work for the quetame earthquake (may 24th, 2008, ml = 5.7) whose focal mechanism is left-lateral; however the ct may play an important role for controlling the propagation of n-s trending faults (like this event or any related to the llanos fault system) as has been suggested in the southwest colombia margin where transverse faults reduce coupling between adjacent segments (collot et al., 2004). we removed the crustal earthquakes from the database of events located by the cnsn between 1993 and 2009 in order to illuminate the upper surface of the subducted slabs beneath colombia (figure 4). the 3d model of this surface images the caldas tear and flat-slab subduction geometry related to the presence of the panama indenter (ramos and folguera, 2009). a corollary of our model for the panama indenter and the formation of the caldas tear is the eastward indentation of geologic features located north of the caldas tear. inspection of the map of quaternary faults and folds of colombia (paris et al., 2000) suggests that some branches of the romeral fault system south of the caldas tear show right-lateral offset from parallel faults including the palestina, cimitarra, mulato-getudo, honda or bituima faults to the north. other faults with sw-ne trend such as if and gf could be part of a transfer zone associated with the caldas tear offset (figure 1). summary of major points for discussion during this trip we combine all observations to propose a regional interpretation of active faults and earthquakes: 1. a buoyant panama arc acts as an eastward-moving indenter that is ramming into the northern andes. indentation produces the distinctive surface fault pattern of v-shaped strike-slip faults along with widening of the northern andes by inversion of preexisting mesozoic foreland basins and widening and deepening of the llanos foreland basin as a farfield effect (figure 1). 50 carlos a. vargas, paul mann and carlos borrero figure 3. tomographic section along the southern transect. coda-wave attenuation anomalies showing the subduction geometry of the nazca plate. transect crosses the southern part of the panama arc/indentor and the cauca earthquake nest. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:20 p p p composite 133 lpi at 45 degrees 2. the indenter point of the panama arc is aligned with the area of shallowest subduction beneath northwestern south america (figure 4). 3. the southern edge of the panama indenter is sharply defined by the caldas slab tear along latitude ~5.4°n (figure 4). the caldas slab tear is collinear with the ~9 ma, extinct, east-west oriented sandra spreading ridge on the unsubducted nazca plate which we postulate may have formed the original zone of lithospheric weakness along which the caldas tear formed in the subducted nazca slab (figure 1, inset map). 4. the northern edge of the panama arc merges without apparent tearing northward into the caribbean slab that subducts beneath northwest colombia (figure 4) 5. the bucaramanga swarm occurs on the downdip continuation of the slab that has preceded the panama arc into the subduction zone and may reflect and active breakoff of an oceanic slab that has become impeded as it enters the zone of shallow subduction. 6. the surface trace of the caldas tear corresponds with the occurrence of strong motion earthquakes in the crust including some several, shallow events in past decades. part 2: field guide to the tectonic setting and stratigraphy of the nevado del ruiz volcano introduction the subduction of the nazca oceanic plate beneath the northwestern margin of continental south america is responsible for the generation of plio-quaternary line of stratovolcanos and associated belt of magmatism along the northern volcanic zone province of colombia (nvz of bourdon et al., 2003). the nevado del ruiz volcano (nrv) is located in the northernmost end of the nvz and is part of the previously-named ruiz-tolima volcanic complex (herd, 1982) (figure 5). research on the nrv has been focused on its recent activity including its voluminous november 13, 1985, eruption products along with its associated volcanic hazards. some authors including thouret et al. (1990), vatin-perignon et al. (1990), and schaefer (1995), have discussed a stratigraphic model for the nrv using different geomorphic, petrological and geochemical approaches. for the purpose of this fieldtrip, we define the post-1.8 ma activity of the nrv according to three main stages: • the “ancestral ruiz” which includes the stratovolcano construction and a small caldera collapse over the period of 1.8 to 1.0 ma. • the “older ruiz” which includes stratovolcano construction and a small caldera collapse from 0.8 to 0.2 ma. • “present ruiz” which includes composite lava domes during the last 0.15 ma. the present nrv is composed of andesitic lava flows intercalated with pyroclastic deposits. borrero et al. (2009) reinterpreted the former ancestral ruiz stage as defined by thouret et al. (1990) and proposed the hypothesis that the magmatism of the ancestral ruiz volcano must have widened its extent to include several monogenetic vents located around the periphery of the “present ruiz”. the distribution of these vents was structurally controlled field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 51 figure 4 top surface of subducted slabs beneath northwestern south america estimated from hypocentral distribution of about 68,000 local earthquakes located by the colombian seismic network (cnsn). a 3d surface was defined by earthquakes and constrains the presence of subducted slabs beneath northwestern south america. tan lines outline the panama indentor including the caldas tear fault to the south. red triangles show active volcanos formed as a result of slab melting. the caldas tear forms a major break in the subduction zone and is collinear with the extinct sandra ridge on the subducting nazca plate. the sandra ridge may have acted as a penetrative zone of lithospheric weakness that was reactivated as the caldas tear fault once the plate was subducted beneath northern south america. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:20 p p p composite 133 lpi at 45 degrees by the villa maría-termales fault zone located to the south-east of the city of manizales (gonzález and jaramillo, 2002). these authors correlate the phases of magmatic activity with the geochemical data and during this trip we will outline a model for the tectonic controls during each of these stages. during this trip, we will tour this region of spectacular scenery and examine evidence for both the constructional and destructive stages of the nevado del ruiz volcano (nrv). these stages are based on a series of outcrops located mainly along the southeastern and northern edges of the nevado del ruiz volcano. during stops 1 to 3 we will examine the main tectonic features that controlled the distribution of the volcanic products during the ancestral stage of the nrv. during stops 4 to 10, we will examine the stratigraphy of the nrv in classic field trip localities that include: 1) the last 11,000 years b.p of tephrostratigraphy related to the main explosive activity of the nrv; 2) different types of pyroclastic, density-current deposits; 3) typical horseshoe-shaped amphitheatres and their associated debris avalanche deposits formed during the older ruiz stages; and 4) lava domes and lava flows related to la piraña stage. we will discuss the subduction and tectonic controls on volcanism in the nrv during all the stops along with seismic and volcanic hazards to the local population. tectonic setting of the nrv the northern volcanic zone (nvz) of colombia is defined by a narrow volcanic arc located above the benioff zone that overlies the steeply subducting slab dipping about 45º to the east (ojeda and havskov, 2001) (figure 5). the nazca plate descends below the north andes at a rate of 56 mm/yr (trenkamp et al, 2002). information on crustal thicknesses in the region surrounding nrv is limited, but existing gravity data indicate that the crust is 40 to 50 km thick along the topographic axis of the cordillera central of colombia. despite these data, schaefer (1995) found evidence for a thin crust (< 35 km) and/or an anomalously dense crust underlying the nrv (figure 5). 52 carlos a. vargas, paul mann and carlos borrero figure 5. location map showing the distribution of active volcanic chains in the northernmost part of the northern volcanic zone in colombia. the green shading highlights the ruiz tolima volcanic complex in the central cordillera, colombia, which includes the nevado del ruiz volcano (nrv) visited on this trip. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:21 p p p composite 133 lpi at 45 degrees field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 53 figure 6. stratigraphic model of nevado del ruiz volcano modified after thouret et al. (1990), lescinsky (1990), young (1991) and schaefer (1995). the piraña stage is proposed by borrero et al. (in prep.). figure 7. geological map of the nrv showing its location at the intersection of the villa maría-termales and palestina faults. map modified from toro et al. (2010) with radiometric ages from thouret et al. (1990). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:22 p p p composite 133 lpi at 45 degrees 54 carlos a. vargas, paul mann and carlos borrero figure 8. %vp-tomograms estimates constructed using first p-arrivals from local earthquakes near the nrv (vargas et al., 2000). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:23 p p p composite 133 lpi at 45 degrees nevado del ruiz volcano nevado del ruiz volcano (nrv) is an ice-covered stratovolcano located in the middle central cordillera of the colombian andes (4º 53'-43'n, 75º 19'-21' w, 5321 meters above sea level), to a distance of 28 km southeast of manizales, the capital of caldas department) (figure 5). in general, the nrv is a compound volcanic edifice, which encompasses a cluster of lava domes, two small parasitic edifices (la piraña and la olleta), and four u-shaped amphitheatres produced by flank collapse and fault activity. based on the measured ice thicknesses and interpolated cross-sections, thouret et al. (2007) estimated the nrv glacial surface area as 25 km2 before and 21.3km2 after the 13 november, 1985 eruption of nrv. the ice cap includes seven glaciers, and the ice cap volume was estimated at 0.57± 0.2 km3 in 2000. by 2003, the nevado del ruiz ice cap had lost as much as 52% of its area and roughly 30% of its volume (thouret et al., 2007). nrv is one of the northernmost, active volcanoes in the andean chain of south america. nrv is approximately 150 km above the benioff zone of the subducting nazca plate (james and murcia, 1984) and is one of the northernmost stratovolcanoes of the ruiz-tolima volcanic complex (figure 5). nrv is part of an arcuate line of nine volcanoes, which form the crest of the central cordillera. hall and wood (1985) identify this aligned, volcanic group as the northernmost segment in their proposed volcano-tectonic segmentation of the nvz. the nrv is intersected by two fault systems: the n20ºe-striking palestina right-lateral, strike-slip fault system and the n75ºw-striking villa maría-termales normal fault system (londoño and sudo, 2002). thouret et al. (1990) made detailed studies of the quaternary eruptive history of nrv and proposed a general stratigraphy formed during three main stages: ancestral, older, and present ruiz (figure 6). most authors who have studied the nrv all agree that the occurrence of a previous event associated with the preruiz lavas correlates with a destructive phase recorded by volcaniclastic deposits surrounding the nrv area. lescinsky (1990) and young (1991) compiled and added new geochronological data especially on recent volcanic events. schaefer (1995) established a main age for the rio claro ignimbrite (formed about 100 ka ago) that defined the limit between the older and present ruiz. recently, borrero et al. (in prep.) dated the lavas from the piraña cone in order to define the piraña stage that forms part of the older ruiz stage (figure 6). the nrv overlies paleozoic metamorphic rocks (cajamarca complex), mesozoic sedimentary and volcanic units (quebradagrande-alao complex), and cretaceous and paleocene intrusive bodies (figure 5). the nrv is located at the intersection of a major right-lateral strike-slip fault (palestina fault) and a major normal fault (villa maría-termales fault (figure 7). both intersecting faults have been active throughout the quaternary (herd, 1982; thouret et al., 1990). villa maría-termales fault and the nrv the direct relationships between volcanism and tectonics has remained controversial for many years. the increased recognition of close spatial relationships between faults and volcanic centers as in the case of the nrv has led to construction of models that attempt to explain the observed patterns of regional deformation as well as the distribution, evolution and morphostructure of the volcanic centers. borrero et al. (2009) suggest that there is a close interaction between tectonics and volcanism in the nrv area, as shown not only by the active faults around the volcano, but also by recent seismicity (figure 7). the villa maría-termales fault was carefully mapped by gonzález and jaramillo (2002), based on its straight alignments of numerous geomorphic features including longitudinal valleys, linear depressions, fault scarps, trenches, sag ponds, shutter ridges, saddles, offset features and tilted quaternary deposits (figure 7). these authors also divided the fault zone into seven fault segments based on variations in physiography and the length of the fault traces as defined by structural barriers composed of north and northeast-striking faults. these fault segments define blocks or wedges each related with relatively higher amounts of seismicity. tomographic images of %vp using first p-arrivals from local earthquakes in the nrv, are used to identify tectonic patterns linked to magmatic activity (vargas et al., 2000). figure 8 shows %vp-tomograms at different depths, that can be used to identify vp anomalies at northern of the nrv. it is interesting that vp contrasts at subcrustal depths follow the same trend of the villa maría-termales fault. this surficial, crustal fault has been proposed by vargas and mann (part 2 of this fieldguide) as the surficial expression of the caldas slab tear, a deep and fundamental fault that separates subducted slabs of different dips (figure 1, inset map). itinerary of fieldtrip stops on day 1: route: departing estelar recinto del pensamiento hotel, we will travel east to the la enea neighborhood for our first stop. from there we will travel west to east on the manizales-fresno highway to the la esperanza rest stop (fig.ure 1). while we are in the second stop, we will spend some time to acclimate ourselves to the higher elevations. please let the leaders know if you feel any intense discomfort. then we will travel to the south-southeast along the la esperanzabrisas dirt road to the third stop, the el arbolito rest point, and allow some time for altitude acclimatization (figure 10). if we are lucky and have clear weather, we will have a wonderful view from here of the nevado del ruiz volcano. from here we will travel to the east on the brisas-murillo dirt road visiting the outcrops of the stops 4 to 10 and examine the stratigraphy of the nrv in classic outcrops localities that will include: 1) the last 14.000 yr. b.p tephrostratigraphy related to the main explosive activity; 2) different pyroclastic density current deposits; 3) typical horseshoe-shaped amphitheatres and their associated debris avalanche deposits, and; 4) lava domes and lava flows. stop 1. la enea neighborhood a model of the "ruiz ancestral" stage (2 to 1 ma) at the nevado del ruiz volcano was defined by toro et al. (2010). the extension of the lava remnants of this stage covered an area of about 200 km2 and were mapped around the northeast-north-northwest sectors of the present edifice of the volcano (figure 7). the ruiz ancestral stage was divided in two stages by borrero et al. (2009), with the first stage corresponding to the ancestral lavas and domes, that define a volcanic cluster aligned with the villamaría-termales fault located mainly to the southeast of manizales in the gallinazo area (figure 11), these domes and flows form a series of aligned hills including gallinazo, amazonas, sabinas, la oliva and the la negra domes and lava flows. a second cluster includes lavas located below the present nevado del ruiz area. at this stop we will examine an alignment of the small “volcanoes” defined by lava domes and lava flows that follow the linear trace of the villa maría termales fault zone in the gallinazo area. field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 55 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:23 p p p composite 133 lpi at 45 degrees the volcanic and sub-volcanic rocks of andesitic and dacitic composition in this area are characteristic of adakitic rocks (toro et al., 2008) (figure 12). both, volcanic and sub-volcanic rocks show high sio2 concentrations (63,87-70,15%), al2o3 (14,18-16,83%), low y (11,20-27 ppm) and yb concentrations (0,94-1,93 ppm); strong enrichment in light rare earth elements (lree) and highly incompatible elements (rb, ba), except for sr which presents low contents and negative anomaly of nb-ta, a characteristic that also distinguishes the calcalkaline magmas. geochemical pattern of lree and multi-elements show a strong fractionation la/yb > 8 with typically low contents of yb� 1.8 ppm , y� 18 ppm. the ruiz tolima volcanic chain in the area of the ancestral ruiz stage domes and lavas show adakitic-like signatures. these rocks are located above 110-160 km above the subduction zone using iso-depths from pedraza et al. (2007) and are about 170 km east of the colombian oceanic trench where the nazca plate is subducting eastward beneath the south american continent. the narrow volcanic arc is defined by a single line of active volcanoes. a comparison with the quaternary ecuadorian volcanoes that also form part of the northern volcanic zone was made by toro et al. (2008) who concluded that a flat slab or small-angle slab conditions to 4,5° n latitude during the late pliocene-early pleistocene could have permitted the formation of the adakite-like rocks. additionally, the post-ruiz ancestral volcanic activity does not exhibit typical adakitic signatures (cf. vatin-perignon et al., 1990 and schaefer, 1995), which means a change in the subduction dynamics to this latitude has occurred since 1 ma. this change probably did not allow continous melting of oceanic crust as is occurring in ecuador. stop 2. la esperanza after 30 -35 minutes driving since stop 1 and weather permitting we will have spectacular views of the u-shaped quebrada chinchiná, which is an example of a deepened valley glacier on the side of the ancestral ruiz edifice. the feature probably represents the glacier stade corresponding to oxygen isotope stage 3, a time when glaciers were still expanding at elevations of 3,000 ± 100 meters above sea level during the lgm (last glacial maximum) ca. 28,000-21,000 yrs b.p. glaciers shrank considerably from 21000 yr b.p. because of greatly reduced precipitation. this is one of the six pleistocene cold stades separated by warmer interstades. in this stop, we observe an outcrop of ancestral ruiz stage andesite lava flows along the main road (figure 13). whole-rock geochemical data of andesite, dacite and minor basaltic-andesite samples from the different outcrops of the ancestral ruiz stage located under and around the present nevado del ruiz edifice share calcalkaline affinities with medium to high k contents (figure 14). the trace elements range variation compared with sio2 suggests a fractional crystallization mechanism. the trace elements pattern is typical of a volcanic arc with sr, k, rb, ba and th 56 carlos a. vargas, paul mann and carlos borrero figure 9. location of departure site in manizales and locations of stops 1 to 3. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:23 p p p composite 133 lpi at 45 degrees enrichments and ta-nb impoverishment. the ree diagram shows a smooth slope with typical lree enrichment (toro et al., 2010). stop 3. el arbolito (mirador cumanday) this stop located at 3800 meters above sea level is the best viewing point to observe the present ruiz edifice composed mainly of lavas and lava domes covered partially by the retreating flat ice cap at the top of the volcano (figure 15). from this viewpoint it is possible to observe a striking contrast between the > 60 m thick ice field, forming and blanketing the summit plateau, and the steep northern plateau margin covered by < 30 m of hard ice (ie, the gualí-molinos glacier). aditionally, the equilibrium line altitude (ela) of this sector is between 5200 and 5100 meters above sea level. stop 4. río gualí at this stop we observe some of the main features of the 13 november, 1985, vnr eruption, when lahars initiated at the headwaters of the gualí river (figure 16). once the avalanches and meltwater floods transformed the debris flows, they remained coherent for up to 104 km in the case of the gualí river. at this point the debris flow mixed with a relatively larger river system (río magdalena). a brief summary of the events of the 13 november 1985 eruption includes the following events: black ash columns erupted from the volcano at approximately 3:00 pm local time, and were likely produced by a strong phreatic explosion. between 5:00 and 7:00 pm, the ash stopped falling. the storm's heavy rain and constant thunder in the evening may have overpowered the noise of the volcano, and with no systematic warning efforts, the residents of armero were completely unaware of the continuing activity at ruiz. at 9:09 pm, on november 13, 1985, nrv, a small volume of mixed andesite and dacite magma was ejected as pumice fall and pyroclastic density currents during the plinian eruption that pumped tephra into the atmosphere for more than 30 km (20 mi) a commercial flight witnessed this eruption. the total mass of the erupted material (including magma) was 35 million metric tons (naranjo et al., 2006) only three percent of the amount erupted from mount st. helens in 1980 (krueger et al., 1990). the eruption reached 3 on the volcanic explosivity index. the mass of the ejected sulfur dioxide was about 700,000 metric tons, or about two percent of the mass of the erupted solid material, making the eruption unusually sulfur rich (krueger et al., 1990). the 1985 eruption lasted only 20 to 90 minutes but reduced the area of the ice cap by 16% from 25 km2 to 20.8 km2 (thouret, 1990). the corresponding volume of snow and ice loss was estimated to be 6 x 107 m3, or field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 57 figure 10. location stops 3 to 10, following the dirt road la esperanza-murillo. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:24 p p p composite 133 lpi at 45 degrees about 9% of the pre-eruption total. an additional 25% of the ice cap area was fractured and/or destabilized (thouret et al., 2007). the catastrophic lahars triggered by the 13 november, 1985, eruption of the ice-clad nrv demonstrate that the interaction of hot pyroclastic deposits with snow and ice could release 30-50 million m3 of meltwater in 30-90 minutes. the 1985 eruption caused a 16% loss in area and a 9% loss in volume of snow, firn and ice. turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0.5-1.6mms-1 (thouret et al., 2007). pyroclastic flows and surges left the ice surface abraded, scoured and gullied on the nereidas glacier to the west, the gualí and azufrado glaciers to the north, the molinos glacier to the northwest and the lagunillas glacier to the east. the complex sequence of pyroclastic flows and surges erupted by nevado del ruiz volcano on 13 november, 1985, interacted with snow and ice on the summit ice cap to trigger catastrophic lahars, which killed more than 23,000 people living at or beyond the base of the volcano. the rapid transfer of heat from the hot eruptive products to about 10 km2 of the snowpack, combined with seismic shaking, produced large volumes of meltwater that flowed downslope, liquefied some of the new volcanic deposits, and generated avalanches of saturated snow, ice and rock debris within minutes of the 21:09 (local time) eruption. about 2 × 107 m3 of water was discharged into the upper reaches of the molinos, nereidas, gualí, azufrado and lagunillas valleys, where rapid entrainment of valley-fill sediment transformed the dilute flows and 58 carlos a. vargas, paul mann and carlos borrero figure 11. panoramic view from the acasa steel recycling plant near manizales showing alignments of the amazonas, sabinas y la negra, and gallinazo domes,a ll of which follow the linear villa maría -termales fault zone. (photo credit: ayala, l.f.). figure 12. (a) tas diagram. (b) mgo vs. sio2 harker diagram showing the compositions of volcanic rocks southeast of manizales (both figures from toro et al., 2008). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:25 p p p composite 133 lpi at 45 degrees avalanches to debris flows (pierson et al., 1990). computed mean velocities of the lahars at peak flow ranged up to 17 m s-1. flows were rapid in the steep, narrow upper canyons and slowed with distance away from the volcano as flow depth and channel slope diminished. computed peak discharges ranged up to 48,000 m3 s-1 and were greatest in reaches 10 to 20 km downstream from the summit. a total of about 9 × 107 m3 of lahar slurry was transported to depositional areas up to 104 km from the source area. initial volumes of individual lahars increased up to 4 times with distance away from the summit. the sedimentology and stratigraphy of the lahar deposits provide compelling evidence that: 1) multiple initial meltwater pulses tended to coalesce into single flood waves; 2) lahars remained fully developed debris flows until they reached confluences with othr major rivers; and 3) debris-flow slurry composition and rheology varied to produce gradationally density-stratified flows (pierson et al., 1990). stop 5. quebrada la marcada during the last 11,000 years, 12 eruptive periods probably each one with multiple explosive events have been identified (herd, 1982). processes included tephra fallout, pyroclastic density currents, multiple slope failures with debris avalanches and lahars, and interaction with the ice cap (thouret et al., 1990). in this stop on the upper flanks of nrv outcrops the tephra deposits related mainly with the late pleistocene holocene plinian to sub-plinian activity in nrv (~ 11,000 yr. b.p. to present). the outcrop located to the left side of the quebrada la marcada field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 59 figure 13. outcrop of andesite lava flows of ancestral ruiz stage. figure 14. (a) tas diagram (b) aracnogram of the ancestral ruiz stage volcanic rocks. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:25 p p p composite 133 lpi at 45 degrees shows multi-story deposits of the nrv and cerro bravo volcano in their last 3200 yr of explosive activity. in the middle of the outcrop there is a pyroclstic flow deposit (correa & penna, 2001, figure 17) this deposit called r-5 by herd (1982) is limited by a 1275 ± 50 yr b.p. lower paleosoil and a 1190 ± 120 yr b.p. upper paleosoil (ages from lescinski, 1990). in the field, we will discuss various ideas for the origin of this deposit. stop 6. la bodega in this spot at the top of the outcrops the ash fall deposits described from stop 5 these deposits overlie channelized debris flows and a sequence of layered volcanic deposits. the latter units correspond to diluted pyroclastic density currents (figure 18). taking into account the work of scolamacchia and macias (2005), these deposits correspond to a base surge formed during complex eruptions with interaction of magma and external water. the eruptions produced deposits with a radial distribution around the vent with a dispersion of several kms (sohn, 1996 in scolamacchia and macias, 2005). broad lenticularity and low-angle cross-stratification observed in this outcrop likely will reflect the outward propagation of horizontal density currents from the phreatic eruptive vent. this is an example of how external water could mix with magma in cool environments. stop 7. río azufrado quebrada la hedionda a horseshoe-shaped amphitheater opens out onto the azufrado riverla hedionda creek (figure 19) (7.5 km long, 2.5 km wide and 1.4 km deep, thouret et al. 1990). the amphitheatre cut deeply into the ancestral ruiz and older ruiz edificies in the uppermost north-northeast flank of the present ruiz immediately below the rim of the arenas crater. the glacier tongues, restricted to the amphitheatre, eventually disappeared because they were steep (>35°), thin (<30 m) and were fractured and destabilized during the 13 november, 1985, eruption (thouret et al., 2007). in addition, these glaciers were covered by thin tephra layers, which decreased the albedo therefore promoting ice melting. lópez and williams (1993) suggest that the extent and locations of hydrothermal discharges at nrv may result from the interactions between hydrothermal fluids and the volcanic edifice. rock dissolution and hydrothermal mineral alteration, combined with physical triggers such as earthquakes, can produce volcanic collapse (figure 20). hot springs and flow paths through the palestina fault were also contributed to the collapse at the northern face of the nrv. it should be noted that catastrophic volcanic collapse, without precursory magmatic activity, is characteristic of many volcanic disasters. stop 8. quebrada la hedionda in the stratigraphic section next to the right margin of the quebrada la hedionda (figure 21), we can observe from base to top: 1) a lower debris avalanche deposits (at the road level, see the truck for scale); 2) in the middle a sequence of pyroclastic density currents (the layered part of the outcrop); and 3) an upper debris-avalanche deposit. the lower, thicker debris avalanche deposit shows a hummocky topography and is composed of brightly-colored, hydrothermally-altered, unconsolidated debris. large blocks up to 10 m across often showing jigsaw cracks, are mixed with a brightly colored, coarse to silty, clay-rich, hydrothermally-altered matrix. this deposit overlies a lava flow (figure 23) and with no intervening pyroclastic deposit. thouret et al. 1990 proposed that this deposit belongs to the la hedionda eruptive stage formed between 3300 and 3100 yr. b.p. 60 carlos a. vargas, paul mann and carlos borrero figure 15. northern face of nrv showing the gualímolinos glacier. the boundary between the older ruiz edifice and present ruiz edifice is located in the area beneath the snow cover. photo: intranet.ingeominas.gov.co/.../430px-ruiz03.jpg agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:26 p p p composite 133 lpi at 45 degrees this lower debris avalanche deposit is covered by 1 to 3 m thick sequence of pyroclastic density currents, at the base ash fall deposits composed by medium to fine ash layers interbedded with lithic lapilli and millimeter-size pumice lapilli layers. these tephra are overlain by thin cross-bedded lapilli layers (0.1 to 0.2 m thick) related to a diluted pyroclastic density current. the top unit resembles a dense pyroclastic density current, slightly weathered and with high organic matter content. thouret et al. (1990) dated a charcoal from this deposit: 3100 ± 70 yr. b.p. the 10 to 15 m thick upper avalanche deposit conformably overlies the middle sequence of pyroclastic layers. it is similar to the lower debris avalanche deposit, although its surface is less chaotic and hummocky and have a distinctive yellowish-orange hydrothermally altered matrix, and thouret et al. (1990) proposed a volume of about 0.5 km. stop 9. la piraña area in this stop we will obverse the la piraña parasitic cone (figure 22), which could be a separate volcano itself, that produce the lava flows that is cut by the road in this point. recently, borrero et al. (in prep.) dated these lavas as 146.4 ± 21.0 and 239.5 ± 21.5 ka, and proposed the piraña stage, a volcanic period field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 61 figure 16. view of deep gualí river valley about 3 km downstream from the arenas crater of nrv showing extensive scour of channel bed with bedrock exposed in many places. scoured material was incorporated into the lahar. note small farm buildings for scale. figure 17. holocene tephra fall with interstratified paleosoils. the middle unit contains a 1275±50 yr b.p. pyroclastic flow deposit (pumice block and ash flow, correa & penna, 2001). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:27 p p p composite 133 lpi at 45 degrees framed within the older ruiz stage. the whole body of geochemical data is found in toro et al. (2010) stop 10. río lagunillas in this stop, we visit the lagunillas valley and at the top the ice cap, the lagunillas glacier. the glacier is below the horseshoe-shaped scar of rockslide debris avalanche deposits, the trough and glaciated valley, and high lateral moraines of middle pleniglacial age (> 28,000 yr. b.p.). additionally, the 15 january, 1995, debris flow deposit is shown by mudlines and splash marks in both sides of the valley. in the headwaters of the lagunillas valley, a 2.5 km long, 1 km wide, and 400 m deep horseshoe-shape amphitheater is a conspicuous geomorphic feature related to another sector collapse at the nrv. this feature appears smaller and narrower than the azufrado river amphitheater and it does not show any obvious tectonic control (figure 23). 62 carlos a. vargas, paul mann and carlos borrero figure 18. view of the top of the 11,000 yr b.p. ash fall deposits interstratified with paleosols overlying a channelized debris flow and a sequence of wet surges. the age of the surge deposits is unknown. figure 19. the horseshoe-shaped amphitheatre of azufrado river produced by collapse of the northern face of vnr approximately 3100 years b.p. (thouret et al., 1990). t he headwaters is located near the active arenas crater with the small fumarole. this geomorphic feature resembles the mt st helens volcano (us) following its may, 1980, eruption. photo credit: usgs (janda, 1985). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:28 p p p composite 133 lpi at 45 degrees the head-valley scar and hummocky topography in the trough floor suggests that repeated retrogressive landslides have contributed to amphitheatre formation along with glacial erosion. landslides are favored by hydrothermal alteration of lava flows of the older ruiz edifice into the head of the lagunillas valley. a recent example of a non-volcanic event is a debris flow of ~550,000 m3 including ice blocks (figure 23) that travelled 6 km down the rio lagunillas valley on 15 january, 1995 (guarnizo and ramírez, 1996). iceand rockslides were observed in the head-valley cliffs in april, 1994, but the triggering factor may have been an earthquake as the flow was first reported 15 min after a long tremor. lópez and williams (1993) have stressed the role of hydrothermal weathering and fluids in destabilizing volcanic edifices and therefore, the lagunillas head catchment remains the area most prone to lahar generation because of glacial thickness. appendix i corresponds to the nevado del ruiz volcano official hazard volcanic map. this map will be referred to during the trip. part 3: field guide to the romeral fault system, colombia: a suture zone between continental and accreted oceanic crust introduction western colombia was built by subduction/accretion mechanisms including the addition of oceanic crustal fragments to the continental margin of the northern south american plate. one accretionary event produced by the collision of south america with buoyant oceanic plateaus that arrived to the continental margin atop the farallon plate is particularly well documented in the rocks that outcrop to the west of the cauca-patia fault. this fault zone marks the boundary between pre-accretion crustal elements and the caribbean-colombian igneous province made of upper cretaceous oceanic plateau fragments (kerr et al. 1997, 2002, 2004; vallejo et al. 2006; alibon et al. 2008). this plate collision event produced profound deformation both in the pre-accretionary crustal elements and in the portions of the caribbean-colombian igneous province accreted to the continental margin. geologic setting of the field trip area the highly deformed rocks of the pre-accretionary crustal elements have been mapped by several authors using different approaches (cf. attached map of cediel, 2010, for the romeral suture zone; map 1). in order to maintain an official nomenclature, we have included in this field guide a fragment of the last version of the geologic map of colombia (modified from gómez et al. 2007; map 2, figure 24), which separated this area into three regional belts named, from west to east, the arquía (np?-ma3), quebradagrande (k1-vcm) and cajamarca (np?ca?-mev) complexes (sensu maya & gonzalez, 1996). the arquia complex forms a discrete belt that separate the caribbean-colombian igneous province (ultramaphic, gabbro, basic volcanic [k2-vm4] and sedimentary (mylonitized) rocks [k2k6-mds4]) to the west from lower cretaceous ultramafic, gabbro (k1-pm), intermediate and basic field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 63 figure 20. close-up of the quebrada la hedionda to the right. the valley of this creek was dissected by the >3100 yr. b.p. debris avalanche deposits. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:29 p p p composite 133 lpi at 45 degrees volcanic and sedimentary rocks of the quebradagrande complex to the east. the arquia complex is composed of metamorphic rocks, mainly greenschists and amphibolites and along the belt located between the cretaceous belts of mafic rocks there are also outcrop gneisses (p-pf, chinchiná, manizales) and meta-sedimentary quartzose clastic rocks (sinifaná meta-sediments). both the arquía schists and the meta-sedimentary rocks are intruded by triassic undeformed granitoids (amagá, cambumbia and pueblito plutons). in turn, the lower cretaceous quebradagrande complex is bounded to the east by the cajamarca complex, composed of metamorphic rocks, including black schists, amphibolites and, gneisses. even though the caribbean-colombian igneous province is separated from the quebradagrande complex by the arquía complex and despite their differences in age, lithology and geochemical characteristics quebradagrande volcanic rocks display supra-subduction zone signature (nivia et al., 2006). for this reason, these belts were frequently miscorrelated. the correlation problems arise because the geological mapping of the country used the name romeral fault for all structures that separate metamorphic rocks (either the cajamarca or arquia complexes) to the east from sedimentary, basic volcanic, gabbroic and ultramafic rocks located to the west that are included in both belts of oceanic rocks the caribbean-colombian igneous province and the quebradagrande complex. this interpretation had created a great deal of confusion and so, in order to solve this problem, maya and gonzalez (1996) proposed for the structures that separated these belts, a new nomenclature that did not use the term romeral fault. in this proposal, the cauca-almaguer fault marks the boundary between the caribbean-colombian igneous province and the 64 carlos a. vargas, paul mann and carlos borrero figure 21. stratigraphic section of quebrada la hedionda, lower debris avalanche deposit (lda) that overlies a lava flow of the older ruiz edifice, in the middle, pyroclastic density current deposits (pdc) and atop, the upper debris avalanche deposit (uda). figure 22. panoramic view of the la piraña parasitic cone, located nne of the present nrv edifice agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:31 p p p composite 133 lpi at 45 degrees arquia complex; the silvia-pijao fault separates the arquia complex from the quebradagrande complex and the san jeronimo fault separates the latter from the metamorphic rocks of the cajamarca complex located to the east (map 2). some geologists suggest that the romeral fault name could still be used as the romeral fault system to include the three main structural boundaries. it has to be stressed, however, that the boundary between continental and accreted oceanic crust, produced by the collision of the continental plate and the oceanic plateau, corresponds to the cauca-patia fault. fieldtrip itinerary day 2: itinerary: we will travel south along the main roadway between manizales and medellin, where we will see outcrops of several cenozoic and mesozoic units (figure 24). the first stop will be in the san peregrino neighborhood. after 30 minutes of discussion we will travel south on the roadway manizales-pereira to the boqueron stop. we will spend some time observing the landscape and discussing the abanico del quindio fan, regional north-south fault trends and miocene to recent west-northwest trends. then we will travel to the northwest to belalcazar town for a third stop under the christ sculpture. there we will have discussion about accretion vs. obduction along western of colombia and have our lunch. after lunch we will travel south to visit stop 4 on the roadway pereira cali and examine the cenozoic tectonics of the miravalles syncline. finally, we will drive to salento town near to the epicentral area of the armenia earthquake (january 25, 1999). trip preparations: this trip will not involve strenuous hiking, but only short walks away from the bus. sturdy hiking boots with good ankle support are required. weather could be cool but might also grow warm (20-30°c). layered clothing should include a water-proof jacket. hat, sunglasses, sunscreen and field knapsack are recommended. stop 1. san peregrino along the roadway between manizales to pereira there are outcrops of the quebradagrande and arquia complexes, as well neogene units covering the paleogene, mesozoic and paleozoic units. near to the san peregrino station it is possible to observe several geomorphologic features associated with the silvia-pijao fault. active tectonics related to this fault have generated steep slopes that have led to instabilities in the roadway (figure 25). during this stop we will discuss about the tectonic evolution of northwestern south america, its regional stratigraphy, as well as differences between the romeral fault and the romeral suture zone. stop 2. boqueron this overlook allows us to examine geomorphological features of the cauca almaguer fault, as well the otun fault of possible miocene age. far on the horizon, its possible to see the quindio fan (figure 26), a volcanic-clastic unit that covers several active fault segments of the silvia pijao fault. one segment, the navarco fault, has been interpreted as a blind fault that produced the armenia earthquake (m 6.2, jan. 25, 1999). however, new evidence suggests a more complex fault process was responsible for this event and other seismic activity in the region. stop 3. belalcazar at this stop we will observe larger tectonic structures formed by the accretion of oceanic crust to the western margin of colombia, as well as evidence for active tectonics along north-south-trending river valleys. this stop shows the geomorphologic expression of accretion associated to the mesozoic subduction on the western margin of the south american plate (figure 27). this accretion has been recognized in colombia by the presence of jurassic (ibague [t3j-pi]; field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 65 figure 23. lagunillas valley showing the foreground with 15 january, 1995, debris flow deposits. the horseshoe-shaped scar morphology is modified by lateral pleniglacial age-moraines observed mainly on the left side of the photo. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:32 p p p composite 133 lpi at 45 degrees mocoa, algeciras batholiths) and cretaceous (antioquia batholith) granitoid rocks that intrude the cajamarca complex east of the central cordillera. in addition, the quebradagrande complex has been proposed to belong to the set of rocks formed within intracontinental marginal basins that have been proposed for the late jurassic to early cretaceous extending from tierra del fuego to mexico through chile, peru and ecuador (nivia et al. 2006). åberg et al. (1984) and aguirre (1987) proposed that these basins might have formed by ensialic expansion or subsidence mechanisms promoted by the roll-back of the subducting lithosphere on this active convergent margin. rifting led to rupturing of the continental margin leaving metamorphic rocks cajamarca and arquia complexes on both sides of the basin. collapse or closure of this basin might coincided with opening of the south atlantic as has been suggested for the basins in southern chile (dalziel, 1981) or might also be due to the collision of the plateau that sliced and piled up crustal elements of the active continental margin into a highly deformed complex. it has been proposed that the arrival and accretion of the caribbean-colombian igneous province to the continental margin choked the subduction zone located along the western end of the arquia complex. choking accommodated the movement of the oceanic plate towards the east and induced a jump of the subduction zone towards the west to the present-day colombia-ecuador trench. this trench marks then the western boundary of the accreted materials, including the choco-panama block. the initiation of this new subduction zone had consequences both at depth and in the surface. in the former environment, the new trench gave rise to several pulses of magmatism that has been migrating towards the interior of the continent since the generation of the santa cecilia and la equis volcanic sequences and the mande batholith, during the eocene up to the position of the active to-date andean volcanoes in the central cordillera (n2-vi, q-vi, q-vc, q-p). at the same time on the surface, crustal shortening due to the accretion process led to deformation accompanied by crustal thickening and consequent isostatic rebound that favored erosion during the early eocene, an episode that can be recognized all over the country by the lack of the corresponding sedimentary record of this age. stop 4. miravalles syncline this panoramic stop is a good site for discussion of the effects of the isostatic equilibrium between late eocene and early miocene when a series of forearc basins developed. the subsiding environment of these basins changed from marine to essentially continental (e.g. the miravalles syncline, figure 28) with important supplies of volcanic and volcanoclastic components produced by the erosion of products of contemporaneous volcanism. late miocene reorientation of deformational stresses usually ascribed to the readjustment of pacific oceanic plates that lead to the generation of the cocos plate produced a compressional phase of the andean orogen that favored deformation of forearc basins. stop 5. salento and the epicentral area of the armenia earthquake near armenia city is salento town, a small village located at 15 km nne from the epicentral area of the armenia earthquake (m6.2, jan, 25th, 1999, depth=16 km). the panaroma from the salento stop allows to us to interpret the interaction between n-s and wnw faults formed along the san jeronimo fault and caldas tear, respectively. the present-day configuration of the colombian region especially in the andean zone is related to the interaction of the cocos, nazca, caribbean, south american plates and more recently to the panama-arc indenter (late miocene). this style of tectonics favors the generation of deep earthquakes, but also the occurrence of superficial strong-motion events. in fact, before 1999 earthquake, the regional historic earthquake record in the “eje cafetero” region (area of armenia, manizales and pereira) suggested that the viejo caldas seismic nest (80-100 km depth with preferential direction n-s) was the main source of the seismic hazard in the area (figure 29). however, the armenia earthquake demonstrated that superficial activity of the n-s fault trend (vargas et al., 2008), associated with blind faults of the romeral fault system, or even related to the caldas tear trend. these fault trends may contribute to the high seismic hazard for cities like armenia, manizales, pereira, cali, medellín and bogotá. references cited åberg, g., aguirre, l., levi, v. & nystrom, j. o. 1984. spreading-subsidence and generation of ensialic marginal basins: an example from the early cretaceous of central chile, in kokelaar, b.p., howells, m.f. & roach, r.a. (eds.), volcanic processes in marginal basins: geological society of london special publication 16, 185-193. adamek, s., c. frohlich, and w. d. pennington, 1988, seismicity of the caribbean-nazca boundary: constraints on microplate tectonics of the panama region: journal of geophysical research, v. 93, p. 2053-2075. aguirre, l., 1987. andean modelling: geology today, 3, 47-48. aki, k., and b. chouet, 1975, origin of coda waves: source, attenuation and scattering effects: journal of geophysical reseach, v. 80, p. 615-631. allibon, j., monjoie, p., lapierre, h., jaillard, e., bussy, f., bosch, d. and senebier, f. 2008, the contribution of the young cretaceous caribbean oceanic plateau to the genesis of late cretaceous arc magmatism in the cordillera occidental of ecuador. journal of south america earth sciences, 26, 355-368. borrero, c., toro, l.m., alvarán, m. and castillo, h., 2009, geochemistry and tectonic controls of the effusive activity related with the ancestral nevado del ruiz volcano, colombia. geofísica internacional, 48: 149-169. bourdon, e., eissen, j.p., gutscher, m.a., monzier, m., hall, m. and cotton, j., 2003, magmatic response to early aseismic ridge subduction: the ecuadorian margin case (south america), earth and plan. sci. lett., v. 205, 123-138. branquet, y., a. cheilletz, p. cobbold, p. baby, b. laumonier, and g. giuliani, 2002, andean deformation and rift inversion, eastern edge of cordillera oriental (guateque-medina area), colombia: journal of south american earth sciences, v. 15, p. 391-407. burke, k. , 1988, tectonic evolution of the caribbean, annual review of earth & planetary sciences, v. 16, 201-230. calais, e., and mann, p., 2009, a combined gps velocity field for the caribbean plate and its margins: american geophysical union, fall meeting 2009, abstract #g33b-0657. cediel, f., shaw, r. p. & caceres, c. tectonic assembly of the northern andean block, in c. bartolini, r. t. buffler, and j. blickwede, eds., the circum-gulf of mexico and the caribbean: hydrocarbon habitats, basin formation, and plate tectonics, aapg memoir 79, 815848 (2003). coates, a., l. collins, m.-p. aubry, and w. berggren, 2004, the geology of the darien, panama, and the late miocene-pliocene collision of the panama arc with northwestern south america: geological society of america, v. 116, p. 1327-1344. collot, j.y., marcaillou, b., sage, f., michaud, f., agudelo, w., charvis, p.,graindorge, d., gutscher, m.a. and spence, g. d., 2004, are rupture zone limits of great subduction earthquakes controlled by upper plate structures? evidence from multichannel seismic reflection data acquired across the northern ecuadorsouthwest colombia margin, j. geophys. res. 109, b11103, doi:10.1029/2004jb003060. correa, i. and penna, a, 2001. evaluación de un depósito de origen piroclástico en el volcan nevado del ruiz datado en 1.275±50 años (in spanish). unpublished bachelor's thesis geology department universidad de caldas. 66 p. 66 carlos a. vargas, paul mann and carlos borrero agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:32 p p p composite 133 lpi at 45 degrees corredor, f., 2003, seismic strain rates and distributed continental deformation in the northern andes and three-dimensional seismotectonics of northwestern south america: tectonophysics, v. 372, p. 147-166. cortes, m., and j. angelier, 2005, current states of stress in the northern andes as indicated by focal mechanisms of earthquakes: tectonophysics, v. 403, p. 29-58. dalziel, i. w. d., 1981. back-arc extension in the southern andes: a review and critical reappraisal: philosophical transactions of the royal society of london, 300, 319-335. duque-caro, h., 1990, the choco block in the northwestern corner of south america: structural, tectonostratigraphic, and paleogeographic implications: journal of south american earth sciences, v. 3, p. 71-84. estrada w., correa, f., naranjo, j. (2009). el deslizamiento de san peregrino. xii congreso colombiano de geología. 7-11 septiembre de 2009. paipa boyacá, colombia. freymueller, j. t., kellogg, j. n., vega, v., 1993, plate motions in the north andean region: journal of geophysical research, v. 98, p. 21853-21863. field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 67 figure 24. location of stops 1 to 5. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:32 p p p composite 133 lpi at 45 degrees 68 carlos a. vargas, paul mann and carlos borrero figure 25. geomorphologic expression of the silvia pijao fault in san peregrino. active tectonics generates instabilities along the roadway connecting manizales and medellin. one lane of the road has been damaged and a new bridge was necessary to keep the road open. modified from estrada et al. (2009). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:34 p p p composite 133 lpi at 45 degrees field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 69 figure 26. landscape viewed from the boqueron stop. a. the silvia pijao fault and the otun fault can be observed from this overview. b. steep topographic gradient from boqueron to dosquebradas town and pereira city has been addressed by the construction of this helicoidal bridge-tunnel structure. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:35 p p p composite 133 lpi at 45 degrees 70 carlos a. vargas, paul mann and carlos borrero figure 27. panoramic view from the belalcazar stop. the building is located on rocks of the caribbean-colombian igneous province (ultramafic rocks, gabbros, basic volcanic rocks [k2-vm4]). far in the horizon appear sequences of oceanic sedimentary mylonitized rocks (k2k6-mds4) that confirm an accretionary belt parallel to the western coast of colombia. figure 28. panoramic view of the miravalles syncline stop along the roadway connecting pereira and cali. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:36 p p p composite 133 lpi at 45 degrees field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 71 figure 29. panoramic view from overlook near salento. n-s and wnw lineaments associated with the san jeronimo fault and the caldas tear respectively provide evidence for active tectonics affecting the area (ie, triangular facets). figure 30. historical record of strong motion events in the "eje cafetero" region. taken from ingeominas webpage: www.ingeominas.gov.co agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:37 p p p composite 133 lpi at 45 degrees gómez, j., nivia, a., montes, n.e., jiménez, d.m., tejada, m.l., sepúlveda, m.j., osorio, j.a., gaona, t., diederix, h., uribe, h. & mora, m. 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(1991). eruption dynamics and petrology of the most recent eruptions of nevado del ruiz colombia, south america, unpublished msc. thesis, louisiana state university, 121 p. zarifi, z., 2006, unusual subduction zones: case studies in colombia and iran, university of bergen. field guides for excursions to the nevado del ruiz volcano and to the romeral fault system (colombia), in the frame of the neotectonics of arc-continent collision concepts 73 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:37 p p p composite 133 lpi at 45 degrees 74 carlos a. vargas, paul mann and carlos borrero a p p e n d ix i. n ev ad o de lr ui zv ol ca no o ffi ci al ha za rd vo lc an ic m ap so ur ce :i nt ra ne t.i ng eo m in as .g ov .co /m an iz al es /m ap a_ de _a m en az a_ v ol ca n_ r ui z.. jp g agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:39 p p p composite 133 lpi at 45 degrees geociencias 15-1 julio 2011.vp an appraisal of the geologic structure beneath the ikogosi warm spring in south-western nigeria using integrated surface geophysical methods ojo, j. s.1; olorunfemi, m. o.2; and falebita, d. e.3 1 department of applied geophysics, federal university of technology, akure, nigeria; e-mail: profjsojo@yahoo.com 2department of geology, obafemi awolowo university, ile-ife, nigeria; 2348037192169, e-mail: mlorunfe@yahoo.co.uk 3department of geology, obafemi awolowo university, ile-ife, nigeria; 2347032987836, e-mail: delefale@oauife.edu.ng; deleebenezer@yahoo.com abstract an integrated surface geophysical investigation involving resistivity and magnetic methods was carried out in the immediate vicinity of the ikogosi warm spring situated in south-western nigeria with a view to delineating its subsurface geological sequence and evaluating the structural setting beneath the warm spring. total field magnetic measurements and vertical electrical sounding (ves) data were acquired along five n-s traverses. magnetic and ves data interpretation involved inverse modelling. the inverse magnetic models delineated fractured quartzite/faulted areas within fresh massive quartzite at varying depths and beneath all traverses. the geoelectrical sections developed from ves interpretation results also delineated a subsurface sequence consisting of a topsoil/weathered layer, fresh quartzite, fractured/faulted quartzite and fresh quartzite bedrock. it was deduced that the fractured/faulted quartzite may have acted as conduit for the movement of warm groundwater from profound depths to the surface while the spring outlet was located on a geological interface (lineament). resumen una investigación geofísica de superficie a partir de métodos resistivos y magnéticos fue realizada en las inmediaciones de fuentes termales de ikogosi en el suroeste de nigeria, para delimitar la secuencia geológica y evaluar la estructura por debajo de fuentes termales. las mediciones de campo magnético total y los sondeos eléctricos verticales (sev) se tomaron a lo largo de cinco líneas transversas en dirección n-s. se incluyó una modelación inversa en la interpretación de los datos magnéticos y resistivos (sev). los modelos magnéticos inversos identificaron áreas de cuarcita fracturada y fallada dentro de cuarcitas masivas frescas a diferentes profundidades por debajo de todas las transversas. las secciones geoeléctricas se desarrollaron a partir de la interpretación de sevs, que también identificaron una secuencia de suelo/capa meteorizada, cuarcita fresca, cuarcita fracturada y unos estratos de cuarcita poco alterada. se concluyo que la cuarcita fracturada pudo actuar como conducto para el flujo de las aguas subterráneas cálidas desde grandes profundidades a la superficie, mientras que la fuente de salida fue localizada en una interfaz geológica (lineamiento). introduction the ikogosi warm spring is located in ikogosi, south-western nigeria (figure1). the spring is a low enthalpy system, its temperature being around 36 oc (oladipo et al., 2005). the geothermal system discharges a virtually constant volume of water all year round. it is a popular national tourist attraction. previous studies of the warm spring have been limited to geological and geochemical investigations (adegbuyi et al., 1996 and ajayi et al., 1996). the current article therefore attempts to characterize the subsurface geological layers and structures beneath this geothermal system using integrated electrical and magnetic methods. the study area’s geological setting the ikogosi area is underlain by south-western nigeria’s basement complex rocks. the area is situated in the pre-drift mobile belt east of the west african and sao luis cratons and north-east of the congo craton which were affected by the pan-african orogeny (c. 600 ma) (fig. 1). the entire belt lies in the reactivated region which resulted from plate collision between the west african craton and the active pharusian continental margin (burke and dewey, 1972; black et al., 1979; caby et al., 1981). the earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 27-34research groupin geophysics universidad nacional de colombia keywords: magnetic, resistivity, warm spring, ikogosi, nigeria. palabras clave: magnético, resistividad, fuentes termales, ikogosi, nigeria record manuscript received: 10/11/2010 accepted for publication: 05/06/2011 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:14 p p p composite 133 lpi at 45 degrees basement complex rocks can be classified (rahaman, 1976, 1988) into the following major groups: (i) migmatite gneiss-quartzite complex; (ii) slightly migmatised to non-migmatised metasedimentary and metaigneous rocks; and (iii) members of the older granite suite. the study area is underlain by a group of slightly migmatised to non-migmatised para-schists and meta-igneous rocks. this group contains rocks which have been previously described as being younger or newer metasediments: the effon psammite formation and the associated epidiorite schist and amphibolite complex. the effon psammite formation (hubbard et al., 1966; de swardt, 1953; dempster, 1967; caby and boesse, 2001) comprises quartzites, quartz schists and granulites which occur largely east of ilesha and run for nearly 180 km in a nne-ssw direction. there are three varieties of quartzite in the study area (fig. 2); these include massive quartzite, fissile quartzite and mica schist/quartz schist. geological features such as faults and shear zones are concealed; however fractures can be identified on a few outcrops along river valleys and on hill tops/slopes. the study area consists of rugged terrain with undulating hills and thick vegetation. the topographical elevation determined from topographic map varies from less than 473 m in the valleys to 549 m on the hills. experimental methods the geophysical investigation was carried out within the warm spring’s immediate catchment area; it involved using electrical resistivity and magnetic methods. the electrical resistivity method involved using the vertical electrical sounding (ves) technique while horizontal profiling was adopted for the magnetic method. five n-s traverses were established, ranging in length from 500 to 600 m. the traverses’ orientation was approximately normal to the suspected e-w-trending regional fault (figure 2); traverse-traverse separation was 100 m. the magnetic method the magnetic method measures variations in the earth’s magnetic field caused by changes in the subsurface’s geological structure or by differences in near-surface rocks’ magnetic properties. when a magnetic field is applied to a material it responds by becoming magnetised (m); such magnetisation is a measurement of magnetic moment per unit volume of material. the field applied to the material is called the applied field (h) and is the total field that would be present if the field were applied to a vacuum. this applied field (h) is related to magnetic induction (b) which is the total flux of magnetic field lines through a cross-sectional area of the material, considering both lines of force from the applied field and from the material’s magnetisation. b, h and m are related by equation (1) in s.i. units. b h m� �� 0 ( ) (1) the constant µo is the permeability of free space (4ð x 10-7 hm-1) which is the ratio of b/h measured in a vacuum. magnetic susceptibility (ê) is another important parameter demonstrating the type of magnetic material and the strength of that type of magnetic effect, such as magnetic permeability, ì. the relationship between magnetic induction b, magnetising force h and susceptibility k, as given by reynolds (reynolds, 1997), is: b h k� �� 0 1( ) (2) where b is in tesla, ìo is free space permeability, h is given in amperes/metres and ê is dimensionless in si units. magnetic measurement in the survey area the magnetic measurements in the survey area were made with a gsm-8 proton precession magnetometer; the equipment measures the earth’s total magnetic field in gamma (nanotesla). magnetic profiling was carried out along 28 ojo, j. s.; olorunfemi, m. o.; and falebita, d. e. figure 1: schematic map of the regional geology of north, west and central africa showing the cratons and mobile belts (modified after black and liegeois. 1993) agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:14 p p p composite 133 lpi at 45 degrees the five traverses at 10 m intervals to achieve better resolution of near-surface geological features, such as faults and fracture zones; 234 magnetic stations were occupied. the observed magnetic field data was corrected for diurnal variations and offset by subtracting base station regional magnetic field from magnetic field measurements taken along the traverse at synchronised times. the corrected magnetic data were used for preparing the residual magnetic an appraisal of the geologic structure beneath the ikogosi warm spring in south-western nigeria using integrated surface geophysical methods 29 figure 2: the local geological map for the area around the ikogosi warm spring showing the survey location (modified after adegbuyi, et al., 1996) figure 3: magnetic maps showing (a) the magnetic anomaly (nt) and (b) the horizontal gradient of the total field (nt/m). the traverse lines are labelled t1 to t5 while the anomalous zones and suspected fault zones are marked with broken lines agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:15 p p p composite 133 lpi at 45 degrees map (figure 3a). a low-pass filter was subsequently applied to the data-set to remove the high frequency near-surface noise effect. a horizontal gradient map (figure 3b) was produced from the filtered magnetic data. a horizontal gradient is a two-component vector, having magnitude and direction. the gradient is normally calculated as the root-sum-square of magnetic anomaly data xand yderivative components (hansen et al., 2005). the method requires the magnetic field’s two first order horizontal derivatives. if t (x, y) is the magnetic field and the field’s horizontal derivatives are ( / / )� � � �t x and t y , then horizontal gradient z (x, y) is given by the equation: z x y t x t y ( , )� � � � � � � � � � � � � � � � � � � � 2 2 (3) producing a horizontal gradient magnetic map is a data enhancement and edge detection technique, enhancing magnetic anomalies associated with faults and other structural discontinuities (hood and teskey, 1989; aboud et al., 2005). lyatsky et al., (1992) and thurston and brown (1994) have provided a detailed discussion of horizontal gradient computation methods. low/negative magnetic peak profiles represent typical anomalous signatures in low-latitude magnetic regions (around the equator where nigeria is located) (parasnis, 1986; fieberg, 2002). correlating these ‘lows’ from profile-to-profile may delineate a fault line/fault zone or magnetic mineral-bearing fault zone. the magnetic data along all the profiles (t1 – t5) was quantitatively interpreted by a technique involving 2.5-d inversion using winglink software (2004 version). several models were tried until a geologically plausible magnetic inversion model was achieved and an approximately fitting model was adopted, having 0.0008 x 10-4 si susceptibility for quartzite host rock and 1.9576 x 10-4 si susceptibility for the anomalous zone; figure 4 shows the interpretation models. since quartzites are metamorphic rocks having relatively low magnetic susceptibility, magnetic anomalies were assumed to have resulted from magnetic materials contained within the fractured quartzite/faulted zones relative to the fresh massive quartzite bedrock. the electrical method the electrical resistivity method involves determining subsurface resistivity distribution by taking ground surface measurements. this involves 30 ojo, j. s.; olorunfemi, m. o.; and falebita, d. e. figure 4: magnetic inversion modelled along the traverses agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:15 p p p composite 133 lpi at 45 degrees passing electrical current (i) into the ground by means of two electrodes; potential difference (äv) is measured between another pair of potential electrodes. what is actually measured, either in the laboratory or the field, is apparent resistivity given by the following equation: � a v g� � 1 (4) where ña is apparent resistivity and g is the geometric factor taking the electrode array’s geometric spread into account. the vertical electrical sounding (ves) technique (zonge et al., 2005) was adopted in the present study; it involves the schlumberger array, this being one of the most commonly used array systems which is mainly used for resistivity sounding in a layered environment where it provides excellent vertical resolution. electrode array investigation depth is 0.125ab (where ab is total current electrode separation) (roy and apparao, 1971). the schlumberger array uses a four-electrode system, two current electrodes (c1 and c2) and two potential electrodes (p1 and p2). apparent resistivity (ña) values were calculated from the following equation: � ! a rl l � 2 2 (5) where r is ground resistance, l is half current-electrode spacing (ab/2), l is half the potential-electrode spacing and ð is a constant (3.142 or 22/7) electrical measurements in the survey area twenty-two vertical electrical soundings (ves) were taken within the study area. a resistivity meter (abem terrameter, sas 300c) was used for taking resistivity measurements. most ves stations were located at 100 m intervals along the traverses used for taking the magnetic measurements (fig. 3), except where topography was inhibited. the station interval was required to achieve good spatial distribution and aid accurate preparation of the subsurface’s geological sequence. the schlumberger array was gradually expanded regarding a fixed centre, electrode spacing (ab/2) varying from 1 to 225 m and having 450 m maximum spread length. ves data are presented as curves of apparent resistivity against ab/2 (figure. 5). the observed apparent resistivity data were inverted to a true geological model (figure 5) of the subsurface using winglink software. the inversion technique finds a resistivity model which approximates measured data within the limit of data errors and is in agreement with an area’s known geology (matias et al., 1994). ves results’ interpretation was used for producing the geoelectric sections (fig. 6); a resistivity map (fig. 7) was drawn for 100 m (ab/2) electrode spacing. an appraisal of the geologic structure beneath the ikogosi warm spring in south-western nigeria using integrated surface geophysical methods 31 figure 5: representative plots of apparent resistivity against ab/2 and interpreted geoelectrical models agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:15 p p p composite 133 lpi at 45 degrees results and discussion the magnetic survey figure 3 shows the magnetic anomaly and total field computed horizontal gradient maps. two anomalous zones were delineated at the southern part of the survey area, across t2 and t3 and on the north-eastern part, across t4 and t5 (figure 3a). magnetic anomaly amplitude for these two identified anomalous zones ranged between -5.3 and -127.8 nt. the horizontal gradient map (figure 3b) partitioned the survey area into two broad anomalous zones. the western flank was characterised by relatively low amplitude (0.1 nt/m–0.7 nt/m) while the eastern flank had relatively high amplitude, ranging from 0.7 nt/m–11.6 nt/m. the area having high horizontal gradients correlated to intensely fractured/faulted quartzite block (figure 4), having an approximately north-easterly trend. the width of the block varied from 150 m to greater than 290 m. the spring outlet was located at the interface between the low and high horizontal gradient zones. the inverse magnetic models (figure 4) identified fractured quartzite/faulted zones at depths of about 100 m below t1, 40 m below t2, 45 m below t3, especially around the spring location, and less than 30 m below t4 and t5. 32 ojo, j. s.; olorunfemi, m. o.; and falebita, d. e. figure 6: the geoelectrical sections along the traverses showing the massive/fresh quartzite, transitional and the fractured/faulted quartzite zones separated by broken lines agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:16 p p p composite 133 lpi at 45 degrees the resistivity survey ves data the typical ves curves and interpreted geoelectric models are displayed in figure 5. the geoelectrical sections (figure 6) delineated four subsurface layers composed of topsoil/weathered layers, fresh quartzite, fractured/faulted quartzite and the fresh quartzite bedrock. the topsoil/weathered material was composed of silt, clayey sand and sand; its thickness ranged between 0.3 and 25.3 m while resistivity ranged from 57 to 4,245 ùm. this horizon rested on a variably thick (0.9–67.8 m) first fresh quartzitic basement, resistivity ranging from 1,702-15,960 ùm. this fresh quartzite acted as confining layer above fractured quartzite in several places, resistivity ranging from 80 to 2,720 ùm. the fractured quartzite constituted the main aquifer unit, having sub-artesian/artesian characteristics. its delineated thicknesses varied from 38.1 to 189.2 m. this aquifer unit had large storage capacity, having been delineated beneath all the ves stations. fresh quartzite constituted the bedrock and had 4,969–99,435 ùm resistivity. the depth to the top of the fresh quartzite ranged from 50.9 to 227.7 m. an appraisal of the geologic structure beneath the ikogosi warm spring in south-western nigeria using integrated surface geophysical methods 33 figure 7: apparent resistivity map for ab/2 equals 100 m showing the massive/fresh quartzite and the fractured/faulted quartzite zones separated by broken lines agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:16 p p p composite 133 lpi at 45 degrees apparent resistivity map figure 7 is an apparent resistivity map produced from the ves data for 100 m half current-electrode separation (ab/2). the map displays high resistivity (2,494–7,407 ?m) (typical of fresh massive quartzite) on the western flank and relatively low resistivity (263–1,896 ?m) (characteristic of fractured quartzite) on the eastern flank. the spring outlet was located at the interface between both zones. map relief was similar to that of the horizontal magnetic gradient map (figure 3b) where the area having a relatively low horizontal gradient correlated with the high resistivity area and the area having relatively high gradient correlated with the low resistivity area. the interface between the low and high horizontal gradients on the magnetic map significantly correlated with the interface between the high and low resistivity area on the resistivity map (compare figures. 3b and 7). conclusions magnetic and vertical electrical sounding data have been used to study the structures beneath the ikogosi warm spring located in ekiti state, south-western nigeria. the magnetic data identified the fractured quartzite/faulted zone as a result of the magnetic minerals contained in it while the subsurface areas were delineated due to variations in resistivity values related to the degree of fracturing and quartzite saturation. the low resistivity areas corresponded to the fractured quartzite/faulted zone delineated by magnetic inversion. it was suspected that the fractured/faulted quartzite may have acted as conduits for the movement of warm groundwater from profound depth to the surface while the spring outlet was located on a geological interface (lineament). acknowledgements the authors wish to acknowledge geotech consultancy services ltd, nigeria, for providing the data and appreciate the use of the winglink software for this study. references aboud, e., goussev, s., hassan, h., supriyanto, s., and ushijima, k., 2005. horizontal gradient and band-pass filter of aeromagnetic data image the subsurface structure; 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radlometrica y geoelectrica de depositos esmeraldiferos luis hernan ochoa gutierrez grupo de investiqacton en geoffsica, departamento de geociencias, universidad nacional de colombia: bogota. ihochoag@unal.edu.co resumen se realizo la caracterizacion geofisica de los depositos esmeraldiferos en el municipio de chivor y zonas de interes en el municipio de macanal, ubicados en el departamento de boyaca-colombia, mediante los metodos de radiometria, emanometria, magneto metria, geoelectric a y electromagnetismo. se desarrollo la exploracion geofisica de las areas preseleccionadas en la exploracion geoquirnica llevada a cabo por cardozo y leal (2000) en el trabajo de grado "evaluacion geologica y geoquimica de nuevas areas potencialmente esmeraldiferas en el sector de macanal (departamento de boyaca)", se dividio el trabajo en dos fases. en la primera fase se utilizaron metodos radiometricos y magnetometricos que ofrecen mayor rendimiento en campo, y por tanto, logran un mayor cubrimiento en el menor tiempo posible. en la segunda fase se emplearon metodos de electromagnetismo y resistividad electrica en zonas de interes preseleccionadas en la fase 1. estos ultimos metodos son mas lentos pero permiten una mejor resolucion. en ambas fases se recorrieron tanto las zonas de mineria activa como las de presencia de anomalias geoquimicas sin explotacion minera actual. los resultados obtenidos mediante la exploracion geofisica fueron comparados con las anomalias geoquimicas y con el modelo geologico de la zona, encontrando una muy buena coincidencia entre todos los model os. con base en los resultados obtenidos en la fase ii, sc recomendaron los sitios de interes para la exploracion directa mediante apiques, perforaciones 0 tuneles, con el fin de determinar si las anomalias encontradas son productivas 0 no, las cuales hacen parte de las recomendaciones de actividades futuras por realizar en el area de estudio. palabras clave: esmeraldas, geofisica, prospeccion, chivor, macanal, colombia abstract this paper presents the results of the geophysical exploration of the emerald deposits in the town ofchivor and interesting areas in the town of mac anal, located in the department of'boyaca (colombia), using a combination of radiometric, emanometric, magnetometric, geoelectric and electromagnetic methods. the geophysical exploration was made in the areas selected by the geochemical exploration done by cardozo and leal (2000) in their research (cardozo et al., 2000). the field work was divided in two steps. in the first, radiometric and magnetometric methods allowing fost field data acquisition. in the second, electromagnetic and resistivity methods were applied. these methods are slower but have higher resolution than the first one, the two steps were applied both in the active mines areas and in the probably productive zones were the mineralization are covered by colluvial deposits defined by geochemical anomalies. the obtained results with the geophysical exploration were compared with the geochemical and geological models with a high correlation between all them. a direct exploration zones were recommended, based on the results of phase 2, by direct drilling, tunnels and open pit works in order to determine if the geophysical anomalies are productive or not. thisjobs has to be make in the future as part of mining prospects develop. keywords: emerald, geophysics, prospecting, chivor, macanal. colombia manuscrito recibido para evaiuacon el 22 de junio de 2003. articulo aceptado para publicaci6n por el comte ednorial el 30 de octubre de 2003. geofislca colombiana, 7, diciembre de 2003 13 luis hernan ochoa gunerrez introduccion las minas han side encontradas en su mayoria por la exposicion de la zona de metasomatismo (albitizada), ocasionada por fenomenos naturales aleatorios 0 por acciones antropicas tales como construeciones de vias. una opcion para encontrar zonas de rnineralizacion la brinda la geoquimica, mediante la cual es posible seleccionar zonas de anomalias de sodio (na), calcio (ca) y potasio (k). sin embargo, hasta el momenta no se ha desarrollado un program a de investigacion mas alla de la geoquimica, salvo kutterink (1975) quien realizo una prospeccion de esmeraldas empleando metodos magneticos, y las investigaciones lievadas a cabo por ingeominas y naciones unidas (1976).' en este trabajo se describen los metcdos geofisicos utilizados en la evaluacion geofisica de zonas potencialmente esmeraldiferas en el municipio de macanal, departamento de boyaca-colombia, realizados por la universidad nacional de colombia, bajo el marco del convenio con minercol, durante los meses de febrero a diciembre de 2000. localizacion el area general de estudio se encuentra localizada en el territorio colombiano, municipio de macanal.ilsoyaca), figura i. itil., "" -figura j. localizaci6n area de estudio. en este municipio se seleccionaron mediante una exploracion geoquimica realizada por cardozo y leal (2000), de la universidad nacional de colombia, unas areas de interes en las cuales se emplearon los metodos geofisicos de prospeccion. sin embargo, la prospeccion geofisica no se limite unicamente a estas areas, con el fin de establecer anomalias que no pudieron ser detectadas debido a iimitaciones en el muestreo geoquimico, en parte por la ausencia de afloramientos de roca en una buena parte de la region. 14 asociaciones mineralogicas con base en la observacion directa de areas productivas, se infiere la presencia de contrastes de propiedades fisicas entre los sitios mineralizados y la roca encajante, susceptibles de ser medidos par metodos geofisicos. esto indica que existe la posibilidad de la deteccion de zonas potencial mente productoras, por el contraste de propiedades fisicas de los materiales. los depositos de esmeraldas estan intimamente relacionados con la presencia de pirita asociada a zonas productivas de esmeraldas en las minas de chivor y otras (giuliani, 1995), de manera que es posible detectar anomalias asociadas a este mineral. los depositos de esmeraldas colombianos no tienen una conexion con fenornenos magmaticos, sino que pueden ser considerados como depositos de pirita alojados en venas de carbonatos con la esmeralda como mineral accesorio (giuliani, 1996). es posible detectar zonas de mineralizacion mediante la medieion de propiedades fisicas del subsuelo y relacionarlas con las zonas de mineralizacion, se realizaron muestreos adicionales de geoquimica para integrar las respuestas geofisicas y geoquimicas de los sitios anornalos asociados a la presencia de depositos de esmeraldas. estudio magnetometrico se ubicaron en total cinco bases de magneto metria en todo el municipio, para medir la variac ion diuma del campo magnetico con estaciones separadas cada 10m; se lievaron a cabo 7.320 mediciones, de las cuales se seleccionaron 5.580 de campo total (en nanoteslas o nt), distribuidas en 13 lineas 0 secciones, asi como 2.200 de gradiente vertical. como se puede observar en la figura 2, se obtuvo un modelo con franjas muy definidas de anomaiias magneticas que sugieren la presencia de tendencias estructurales relacionadas a fallas en zonas centrales de pliegues que pueden estar mineralizadas en direccion n35e en promedio. en el mapa geologico se obs~rva como en la cartografia de campo se mapearon dichos plegamientos en la zona sw del mapa de anomalias, sobre la zona del embalse de la esmeralda y algunos al sur del alto la punta. se hallo que las anomalias magneticas positivas coinciden bien con 10 observado en la cartografia geologica. estas estructuras no se pudieron continuar cartografiando en campo total mente hacia la parte ne de la zona porque se encontraban cubiertas por coluviones. sin embargo, la magnetometria perrnitio proyectar el trazado de las estructuras a 10 largo de la zona, aun en sectores en donde estan enmascaradas por los depositos cuatemarios, debido a las anomalias observadas y a la correlacion de estas con las observaciones estructurales de campo. estudio radiometrico se estudio la radiactividad natural emitida por todos aquellos materiales terrestres que dentro de su composicion contienen elementos geofislca colombiana, 7, oiciembre of 2003 evaluacl6n magnetometrica, radiometrica y geoelectrica de dep6sitos esmeraldiferos que emiten radiaciones gamma. los perfiles se trazaron transversales al rumbo de las estructuras. como se puede observar en la figura 3, se definieron dos zonas de anomalias donde resalta la curva iso-radiometrica de 80 cps, que establece claramente una separaci6n de zonas que sugiere la presencia de dos regiones con un comportamiento geologico diferente. por un lado, se tienen las anomalias mayores a 80 cps, donde se establece la presencia de afloramientos de roca con mayores valores de radiactividad producidos probablemente por la presencia de potasio (k) en las lutitas de macanal. los val ores menores de radiometria corresponden a zonas de lutitas cubiertas por coluviones que enmascaran la radiactividad de las lutitas, sefialando de manera evidente la presencia de dichos coluviones. las gam as de val ores obtenidos sugieren los diferentes espesores de coluvion que se pueden encontrar en la region. sin embargo, la radiometria es un metodo geofisico que no tiene penetracion; por tanto, es una excelente herramienta para apoyar la cartografia geologica, ya que los contrastes entre val ores obtenidos deben ser explicados a partir de la distribucion de las litologias aflorantes. las anomalias radiometricas mayores a 80 cps coinciden de buena manera con la presencia de afloramientos de las lutitas de macanal, confirmada con las observaciones de campo en los lugares cartografiados. se observa que las zonas de coluvion (parte central) coinciden con valores bajos de radiometria. de esta manera podemos determinar las zonas donde se tiene la mayor cercania de las lutitas, para su posterior evaluacion economica a la hora de determinar la explotaci6n de los depositos. la cartografia de campo solamente nos informa de los sitios con presencia de depositos cuatemarios, pero la radiometria no ayuda a definir mejor estos cuerpos. es importante anotar que existe una muy buena correlacion entre los sitios de mineria activa, con los sitios de anomalia radiornetrica mayor a 80 cps. en la zona sw encontramos la mina de quebrada negra, con mineria act iva en este momento, que coincide muy bien con el tren de anomalias mayores. podemos observar hacia la parte ne del area dos zonas de mineria, una de yeso (inactiva en el momento) y otra de esmeraldas, que forman parte de otra zona de anomalia radiornetrica mayor de 80 cps. estudio geolectrico se realizaron sondeos electricos verticales (sev) con los cuales se determinaron las profundidades de las unidades de interes, y a la vez se estimo el espesor de la cobertura asociada por 10 general a depositos cuatemarios. de esta manera se escogieron los sitios para la exploraci6n mas detallada mediante perfiles electricos con el metodo dipolo-dipolo, y asi se modelaron las posibles variaciones laterales entre zonas mineralizadas y roca encajante. con el fin de confirmar la relacion que existe entre las anomalias radiometricas y las profundidades de los coluviones, se geofislca colombiana, 7, diciembre de 2003 figura 2. resultados de la interpolacion de las anomalias magneticas, figura 3. resultados de la interpolacion de las anomalias radiometricas. realizaron seven las zonas de interes definidas mediante. con estos sondeos se determinaron los espesores de tres unidades basicas: coluvion, lutitas de macanal poco alteradas y lutitas de macanal con alto grado de alteraci6n. estas unidades fueron establecidas asumiendo que las zonas de mayor interes, que corresponden a valores de menor resistividad, estan asociadas a zonas de alteracion arcillolitica y presentan posiblemente gran diseminaci6n de sulfuros, en mayor medida que otros sitios poco alterados 0 menos mineralizados donde los valores de resistividad tienden a ser mucho mayores, sin ilegar a tener valores tan altos como los que se pueden presentar en depositos cuatemarios. las caracteristicas de resistividad de cada una de las unidades fueron: coluvi6n mayores a 200 ohm-m, lutitas de macanal poco alteradas entre 80 y 200 ohm-m, lutitas de mac anal muy alteradas menores a 80 ohm-m. en la figura 4 tenemos los sondeos sev-08 y sev-07. estos sondeos muestran resistividades caracteristicas de las tres unidades principales de la zona. tenemos en superficie resistividades muy altas, caracteristicas de coluvi6n con espesor de 7m. a continuacion tenemos resistividades del orden de 100 ohm-m correspondientes a lutitas de macanal poco alteradas, que 15 luis hernan ochoa gutierrez v-« r:=:::r0 m 7m i l i'_.~':: .....-.80m ~~ 100 m 32 m c=j coluvion c::j lutrtas de macahal ~ zohadealteracion sev-08 ... v..,.••• ¥~l•• ii.i-f.~.,. •• _ ... noce~of 8oyeoe a::li.nuth: '6-' pr<l 7m 11m ";:~:-.--r~\ \ l_ \ ........................ \ _ . c::a coluvioh c:=:j lutrtas de macahal 1'~ zohade alteracion 1'01"'1 mln.,..col ltd" eve t uec lor, gec r te tc e sev-07 ... v.,...d" !tout! 8 ... .,1ii oomlch heoer,oj 8~yeoo .......... " ....... , n..... "3eiwg\lrq' 1 figura 4. sev coluvi6n lulitas zona de alteracion, posiblemente no tengan una mineralizaci6n importante con un espesor entre 25 y 6 m, y final mente unas resistividades muy bajas entre 0.3 y 30 ohm-m, que indican la presencia de zonas con un mayor grado de alteraci6n y que probablemente se encuentran mineralizadas. esto indica que los fluidos hidrotermales no liegaron a alterar completamente las lutitas de macanal y quedaron hasta cierta profundidad. las zonas de alteraci6n en estos sitios son mas dificiles de detectar 0 encontrar de manera directa debido a que se encuentran cubiertas tanto por coluvi6n como por lutitas inalteradas. esto confirma la importancia de utilizar metodos geofisicos en la prospeccion de esmeraldas. el perfil dd-05 se realiz6 en el sitio del sondeo sev -21, correspondiente a la mina de quebrada negra, directamente sobre zona de alteraci6n. en la figura 5 se puede ver el resultado del perfilaje. en esta figura se puede observar claramente que las resistividades obtenidas para zona de alteraci6n corresponden a valores menores a 80 ohm-m. ei perfil esta localizado totalmente sobre zona de alteraci6n. se presenta 16 depth 1~t~ra!ioil3rt.1sellor:619% ~o ~.o 1::£1 m~;~.~1iiiiiiiiiiiiii"""'~187 127 170 inverse t-tode! reslslmly sec lion----~~~~-~~~~----009 0.23 059 1.51 3.87 9.89 25 3 bj 0 reslsll'ollflnohmm tjnneleruooe $pacmg 100 m c:::j collman e::j llitltas de macanal t:3 zona de al teracion figura 5. resistividades obtenidas e interpretacion perfil 00-5. mina quebrada negra. una distribuci6n de resistividades bajas, que sugieren dos zonas con diferentes niveles de alteracion, en general se puede decir que la parte izquierda de la zona presenta mayor alteraci6n y, por tanto, mayor posibilidad de mineralizacion, 10que hace mas interesante esta parte de la mina. conclusiones de acuerdo con los resultados obtenidos en el presente estudio, se pueden encontrar algunos indicadores de la efectividad y uso de los metodos propuestos. se definieron dos tipos de metodos geofisicos de acuerdo con la velocidad de aplicacion y cubrimiento. estos metodos son los de cobertura rapida y los de cobertura lenta. los de cobertura rapida, cuyos rendimientos oscilan entre 8 y 10 km de perfilaje diarios, se utilizan para caracterizar con mayor rapidez y cubrimiento las zonas de estudio, con el fin de establecer la localizacion de zonas de interes, a un costa menor, para luego emplear metodos mas lentos y detallados. tal es el caso de la magnetometria que se ernpleo para la ubicaci6n agil de las zonas de corredores de interes asociadas con alineamientos de fallas donde se produjeron las mineralizaciones que posteriormente fueron cubiertas por coluviones y depositos recientes. tambien se encuentran en esta clasificacion los metodos radiometricos, que se emplearon para obtener una caracterizacion geologica de superficie de manera agil y eficiente, y como complemento a la cartografia geologica de la zona. los metodos de cobertura lenta, cuyos sondeos son puntuales y se realizan en el orden de 4 a 6 sondeos por dia, se emplearon para detallar con mayor claridad las zonas de interes definidas con los metodos iniciales. estos metodos empleados fueron geoelectrica, electromagnetismo y emanometria. los geoflslca colombiana, 7, oiciembr[ oe 2003 evaluacl6n magnetometrica, radiometrica y geoelectric a de dep6s1tos esmerawiferos dos primeros sirvieron para confirmar y ubi car de manera detallada los sitios de posible rnineralizacion para posterior desarrollo de mineria. o en profundidad, entre 0 y 100 m, se detectaron, mediante perfilaje y electromagnetismo, zonas de muy baja resistividad, menores a 80 ohm-m, que son de in teres para esmeraldas, no solamente en los sitios previamente seleccionados mediante la geoquimica, sino tambien fuera de ellos y cubiertos con depositos recientes. • los valores de resistividad calculados y menores a 10 ohm-m se relacionan con presencia de material semiconductor en el subsuelo. tales caracteristicas las presentan los sulfuros, y en particular la pirita, presente en cantidades abundantes en las zonas de afloramiento y en las zonas de mina alrededor de las zonas albitizadas. se encontraron diferencias entre las resistividades obtenidas mediante geoelectrica y electromagnetismo, 10 que sugiere presencia de fen omen os de polarizacion, ocasionados por la presencia de sulfuros. se propone la utilizacion de metodos de polarizacion inducida, que manejan frecuencias intermedias, comparadas con las de los metodos empleados en la investigacion, • los valores de mayor intensidad magnetic a, (mas de 32.360 nt) se-relacionan con la presencia de minerales con mayor susceptibilidad magnetica en el subsuelo. esta caracteristica la presenta la pirita, dentro de las zonas de interes mencionadas. las anomalias encontradas son del orden de +/170 nt asociadas con los trenes de rnineralizacion encontrados. • los val ores de radon (mayores a 25 cps) se asocian con zonas de fractura 0 falla. estos valores son bajos y permiten concluir que las estructuras asociadas con la presencia de las mineralizaciones correspond en a plegamientos fallados, donde la presencia de gas radon asociado con fallas profundas no es posible. • los valores anomalos de radiactividad (mayores a 200 cps) se relacionan con la presencia de minerales radiactivos asociados con zonas de mineralizacion, que estan aflorando. el valor de 80 cps de radiometria se tomo como limite de afloramiento de lutitas de macanal (valores mayores a 80 cps) y zonas cubiertas por depositos recientes (menores a 80 cps). la cartografia geologica, realizada conjuntamente con la exploracion geofisica, permite inferir la continuidad, bajo el deposito de ladera 0 coluvion, de tres corredores de fallas de tipo normal y con rumbo aproximado n50e que se pueden seguir desde el departamento de cundinamarca, en las minas ei toro, la guala y buenos aires, hasta la vereda quebrada honda, pasando por la vereda quebrada negra, en el municipio de macanal (boyaca). no obstante, existen much as areas cubiertas por el coluvion en donde se recomienda que el seguimiento debe ser realizado mediante la aplicacion de metodos geofisicos. en los sectores aledaiios a esas fallas se encuentran evidencias de mineralizaciones tales como calcita, geofisica colombiana, 7, oiciembre oe 2003 pi rita, yeso y algunas otras como verdacho y morralla que corresponden a berilos 0 esmeraldas de baja calidad, que generalmente estan asociadas con la presencia de esmeraldas. con los metodos geofisicos aplicados, se identificaron tres corredores que se pueden correlacionar con sistemas de falla; adicionalmente se encontraron anomalias rnagneticas de interes en las secciones realizadas entre: o cuemavaca la punta o q. negra q. honda q. minas q. negra mediania la mayoria de estas anomalias se encuentran bajo el coluvion, el cual tiene un espesor variable entre 2 y 35 metros, de acuerdo con las mediciones geoelectric as hechas mediante sev. las areas seleccionadas como potencialmente esmeraldiferas tienen en cornun: la presencia de rasgos estructurales alineados en una direcci6n aproximada de n50e, desde la quebrada negra hasta el alto de muceiio o resistividades menores a i ohm-m. o baja radiactividad (menor a 60 cps) debido a la alteracion de zonas potasicas a zonas albitizadas, yalta radiactividad por la presencia de minerales radiactivos asociados. esta ultima de manera excepcional y eventual. • alta concentracion de sodio (30,52%) concentracion anornala de radon (mayor a 30). constituye aproximadamente el 2% del area del estudio. en el mapa de la figura 3, denominado 03-zonas de interes, se resume la localizacion de los sitios en donde se recomienda realizar apiques 0 perforaciones en zonas dichas con el fin de atravesar el coluvi6n y encontrar la zona de mineralizacion, las dimensiones de 15 a 20 ern de ancho y 65 metros de longitud de las venas mineralizadas comparadas con las dimensiones de las zonas albitizadas 0 de alteracion que pueden tener varios metros sugiere que no es factible detectar las venas con precision, justificando la exploracion de las estructuras favorables (falias) y las zonas de alteracion como primera fase prospectiva, debido a que para la escala del presente trabajo no se pretende detectar venas de dimensiones muy pequeiias, sino simplemente las manifestaciones de las zonas de mineralizacion, la presencia de pirita, incluso en venas de zonas improductivas, hace que las anomalias encontradas originadas por la presencia de esta no sean suficientes para la deteccion especffica de las esmeraldas. la geofisica debe permitimos encontrar las vetas, pero la exploracion directa final mente establecera su capacidad productora. a pesar de no formar parte de la presente investigacion, si se pudieron establecer directamente en campo los efectos nocivos sobre el medio ambiente que tienen practicas tales como desmorres y tambres. se recomienda evitar el uso de dichas tecnicas 17 luishernanochoagutierrez en las labores de exploraci6n directa y explotaci6n, cambiando dichas practicas por otras tales como apiques 0 tuneles, los efectos nocivos son, entre otros, el deterioro de corrientes de agua, el efecto dafiino para el paisaje, el cambio de condiciones ambientales de los ecosistemas de la zona. la desprotecci6n de laderas origina deslizamientos de tierra, los dep6sitos no consolidados de escombros son muy inestables y pueden ocasionar deslizamientos de estos materiales, con consecuencias tales como perdidas de vidas y perdidas materiales 0 represamiento de corrientes de agua que pueden originar avalanchas de lodos y generaci6n de suelo esteril poco apto para actividades agricolas. tarnbien se pueden tener perjuicios desde el punto de vista econ6mico al depositar los materiales removidos sobre zonas potencialmente prospectivas, sin ningun control. bibliografia branquet, y., laumonier, b., cheilletz, a., giuliani, g. 1999. emeralds in the eastern cordillera of colombia: two tectonics settings for one mineralization. geology, vol. 25, no.7, pp. 597-600. banks, d. a., yardley, b. w. d., cheilletz, a., giuliani, g., rueda, f. 1995. chemistry and source of the high temperature brines in the colombian emerald deposits. mineral deposits, pasava, kribek, zak (eds.), balkema, rotterdam, pp. 557-560. cardozo, c. leal, h. 2000. estudio geologico y geoquimico de un area potencial mente esmeraldifera al ne de macanal (boyaca), cinturon esmeraldifero oriental colombiano. tesis de grado. universidad nacional de colombia. cheilletz, a., giuliani, g. 1997. ei doble origen de las esmeraldas. traduccion de carlos m. arrieta. ___ . 1995. the formation of emeralds in a sedimentary basin: the key role of alkaline brines and thermo chemical sulphate reduction. european union of geosciences. 8, strasbourg, 9 -12 april, terra nova no. 7, p. 203. ___ , rueda, f., feraud, g., france, c., lanord. 1995. the genesis of colombian emerald deposits: an unique example of beryllium mineralization developed in a black shale environment. mineral deposits, pasava, kribek, zak (eds). balkerna, rotterdam, pp. 943-946. 18 . 1995. the genesis of colombian emeralds: a restatement. mineralium deposita no. 31, pp. 359-364. cornelius, s., hulburt, j. r., cornelis, k. 1992. manual de mineralogia de dana, tercera edicion, editorial reverte s.a. edwards, r., atkinson, k. 1985. ore deposits geology, ed. chapman and hall. forero, h. 1987. esmeraldas. recursos minerales de colombia. torno ii, segunda edicion, publicaciones geologicas especiales, ingeominas, pp. 567-605. giuliani, g., cheilletz, a., arboleda, c., carrillo, v., rueda, f., baker, h. 1995. an evaporitic origin of the parent brines of colombian emeralds: fluid inclusion and sulphur isotope evidence. europ. j. mineral, vol. 7,pp. 151-165. giuliani, g., cheilletz, a., sheppard, s. m. f., arboleda, c. 1993. geochemistry of the emerald deposits of colombia. giuliani, g., cheilletz, a., carrillo, l., rueda, f., arboleda, c., sheppard, s. 1993. geoquimica y origen de los depositos de esmeraldas en colombia. giuliani, g., silva, d., couto, p. 1990. origin of emerald deposits of brazil. mineralium deposita sranger verlag. giuliani, g., cheilletz, g., france-lanord, c., rueda, f. 1996. the role of organic matter in high temperature hydrothermal regimes. third isag, st. malo (france), 17-19/09/1996, pp. 683-686. kutterink. 1975. report on exploration geophysics at the emerald mines near gachala, cundinamarca. ingeominas. mineralco s. a. 1996. genesis de los depositos de esmeraldas en colombia e impacto ambiental de las explotaciones esmeraldiferas. bogota, ministerio de minas y energia, p. 1-21. morse, j. g., rana, m. d. h., morse, l. 1982. radon mapping as indicators of subsurface oil and gas. oil & gas journal, pp. 227-246. romero, f., hernandez, o. 1999. caracteristicas mineralogicas e inclusiones fluidas de las esmeraldas del municipio de san antonio de yacopi, cundinamarca, colombia. geologia colombiana. no. 24, pp. 149-158. geoflslca colombiana. 7. oiciembrc de 2d03 universidad national de colombia-bogo!a 1111111111111111111111111111111111 iii770115790371361 nota editorial cada nuevo dia que estemos rnuertos, dificultara mas nuestro renacer. fue un verdadero reto seguir y esperamos que este haya sido nuestro ultimo renacer; geofisica colombiana ha vuelto, con cam bios y con la intenci6n de perdurar, adem as de servir. el nurnero cuatro de nuestra revista aparece ligado a la universidad nacional de colombia, facultad de ciencias, financiado por el programa de posgrado en geofisica, respaldado por el grupo de geofisica (escalafonado por colciencias como grupo de investigaci6n en 1999), con el prop6sito de colaborar en la difusi6n de la investigaci6n y aplicaci6n de temas conexos con la geofisica, a nivel nacional e internacional, pero en particular apoyando el trabajo que la maestrfa de nuestro programa habra de realizar, canalizando esfuerzos para contribuir al desarrollo de las geociencias nacionales. esperamos abrirnos internacionalmente y eso exige mayor esfuerzo, para ofrecer constancia en la periodicidad y madurez y profundidad en los contenidos. nuestro cornite editorial, se ha renovado y especializado, buscando adernas jurados an6nimos dentro de los profesionales mas capacitados, para que revisen y aprueben los diferentes articulos. estos arbitros, diferentes del comite editorial, qarantizaran la competitividad y relevancia internacional. es claro que en los diferentes campos de las geociencias, necesitamos validar nuestros resultados ante la comunidad cientifica internacional. la aplicaci6n de rnetodos no se puede hacer mecanicamente en diferentes entornos geofisicos y esas caracterizaciones deben ser compartidas y sometidas a discusi6n, no puede caber aqui el menor marco de parroquialismo. desde nuestra anterior edici6n hasta la fecha han sucedido, en el ambito nacional, diversos hechos que deben conducir a reflexi6n y a acciones en el campo de la geofisica. por una parte, la posibilidad real de que si no se explora por hidrocarburos en mayor cantidad y calidad, pasaremos pronto a ser importadores de un recurso que poseemos, perc que no se ha sabido manejar; esto junto con la tendencia alcista del precio del petr6leo, hacen que la prospecci6n adecuada sea una urgencia basica, en donde se requiere capacitar a muy alto nivel a profesionales con interes y capacidad, a mediano plazo habra que pensar en la creaci6n de la carrera. por otra parte, el evento sismico del 25 de enero de 1999 ayud6 a mostrar el poco trabajo cientifico en el area de sismotect6nica que se viene desarrollando en el pais. estos dos hechos, junto con el creciente interes por aplicar metodos eficientes en el analisis ambiental de nuestro planeta, asi como un correcto manejo de las politicas de exploraci6n hidrica y minera, caracterizan el momenta en que se aprueba la reestructuraci6n del posgrado de geofisica, paralelamente con el reconocimiento oficial de la labor del grupo de geofisica. ei compromiso es capital y la funci6n de geofisica colombiana, no puede ser inferior a la necesidad de nuestra sociedad. en el presente numero, se abordan temas variados que tienen que ver tanto con procesos de informaci6n y analisis de datos, como con adquisici6n e interpretaci6n de los mismos. en el primer caso se ubican los trabajos de los profesores montes y diaz. ei profesor montes presenta un primer trabajo sobre la incertidumbre en la estimaci6n de la funci6n de tiernpos de transite para eventos de reflexi6n. en un segundo trabajo muestra una tecnica para migrar en tiempo, eventos de reflexi6n de secciones sismicas apiladas sin necesidad de conocer el campo de velocidades. en el articulo sobre filtros digitales en el procesamiento electrornaqnetico, se muestra una metodologia basad a en la transformada de fourier y en el articulo sobre operadores diferenciales, se presentan herramientas utiles en el analisis de la informaci6n en geofisica. es positivo el hecho de dar cab ida en este numero a una muestra del trabajo permanente que en cuba han venido desarrollando las doctoras valcarce-ortega y gonzalez-espinosa. en variados aspectos de la hidrogeofisica. ei articulo de hurtado y l.euro, constituye un interesante punta de discusi6n sobre un modelo de corteza en colombia a partir de anomalias de bouguer. la aplicaci6n de la fisica atrnosferica, muestra en el articulo de montoya y eslava su aplicaci6n en el caso de las tormentas electricas en la sabana de bogota y, siguiendo con corrientes electricas, san in y briceno muestran la posibilidad de aplicar metodos de alta resoluci6n mediante cortes verticales con la nueva tecnica de la tomografia. finalmente el articulo de briceno y arango aporta un modelo geofisico de un sector de la sabana de bogota, que merece su discusi6n y ampliaci6n a otros sectores. ei ultimo articulo de la revista es un resumen del porque y para que de la maestria en geofisica, constituye la ventana del patrocinador, que queda abierta, como siempre para nuevas contribuciones y para que nos hagan liegar sus criticas y comentarios. geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 166-182 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt abdel-lattif. a.1 and mohamed el kashouty2 1 national research institute of astronomy and geophysics, helwan, cairo, egypt. 2 cairo university, faculty of science, geology department abstract in darb el arbaein, the groundwater is the only water resources. the aquifer system starts from paleozoic-mesozoic to upper cretaceous sandstone rocks. they overlay the basement rocks and the aquifer is confined. in the present research, the performance of the statistical analyses to classify groundwater samples depending on their chemical characters has been tested. the hydrogeological and hydrogeochemical data of 92 groundwater samples was obtained from the garpad authority in northern, central, and southern darb el arbaein. a robust classification scheme for partitioning groundwater chemistry into homogeneous groups was an important tool for the characterization of nubian sandstone aquifer. we test the performance of the many available graphical and statistical methodologies used to classify water samples. r-mode, q-mode, correlation analysis, and principal component analysis were investigated. all the methods were discussed and compared as to their ability to cluster, ease of use, and ease of interpretation. the correlation investigation clarifies the relationship among the lithology, hydrogeology, and anthropogenic. factor investigation revealed three factors namely; the evaporation process-agricultural impact-lithogenic dissolution, the hydrogeological characteristics of the aquifer system, and the surface meteoric water that recharge the aquifer system. two main clusters that subdivided into four sub clusters were identified in groundwater system based on hydrogeological and hydrogeochemical data. they reflect the impact of geomedia, hydrogeology, geographic position, and agricultural wastewater. the groundwater is undersaturated with respect to most selected minerals. the groundwater was supersaturated with respect to iron minerals in northern and southern darb el arbaein. the partial pressure of co2 of the groundwater versus saturation index of calcite shows the gradual change in pco2 from atmospheric to the present aquifer pressures. keywords: groundwater samples, groundwater chemistry, multivariate analysis, aquachem, egypt, darb el-arbaein. resumen en darb el arbaein, el agua subterránea es el único recurso hídrico. el sistema acuífero confinado incluye rocas del paleozoico-mesozoico hasta areniscas del cretácico superior sobre el basamento. en esta investigación se probó el desempeño del análisis estadístico para clasificar las muestras de agua subterránea en función de sus características químicas. datos hidrogeológicos e hidrogeoquímicos de 92 muestras de agua subterránea fueron obtenidas en el norte, centro, y sur de darb el arbaein. se usó un esquema de clasificación robusto para separar químicamente en grupos homogéneos las aguas subterráneas del acuífero de la arenisca de nubia. se probaron los rendimientos de varias metodologías graficas y estadísticas 166 manuscript received: 8/10/2009 accepted for publication: 20/12/2009 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:43 p p p composite 133 lpi at 45 degrees para clasificar las muestras de agua, entre ellas análisis de correlación, modo r, modo q y componentes principales. las correlaciones aclaran las relaciones entre la litología, la hidrogeología y la influencia antropogénica. la investigación reveló tres factores a saber: evaporación asociada a procesos agrícolas e impacto de disolución litogénica, las características hidrogeológicas del acuífero, y la superficie meteorizada por donde se recarga el acuífero. se identificaron dos grupos principales subdivididos en cuatro subgrupos que reflejan el impacto de la geología, hidrogeología, posición geográfica, y las aguas residuales agrícolas. el agua subterránea es insaturada con respecto a la mayoría de los minerales seleccionados pero sobresaturada con respecto a los minerales de hierro en el norte y el sur de darb el arbaein. la presión parcial del agua subterránea en relación al índice de saturación de calcita muestra un cambio gradual en pco2 de la presión atmosférica a la presente en el acuífero. palabras clave: muestras de las agua subterránea, química de las aguas subterráneas, análisis multivariado, aquachem, egipto, darb el-arbaein. 1. introduction the groundwater represents a resource for multiple uses (general authority for rehabilitation projects and agricultural development, garpad, 1998-2001). they irrigate 12000 feddans (1 feddan ˜ 1.038 acres) and help 16 villages for farmers along about 400 km from paris town towards the egypt–sudan border. the pumping wells are located in three parts, the northern, the central, and the southern part of darb elarbaein (fig. 1). the northern part extends 90 km to the south from paris town and has an area of 90 km2. the central part extends for 80 km to the south of the northern part and has 120 km2 in area. the southern part extends for 200 km to the south of the central part and has 170 km2 in area. the darb-el arbaein is located in an arid region, where the absolute maximumair temperature are 48.6 oc in may and minimum -2 oc in february. the annual rainfall is less than 1.1 mm, the total monthly rainfall is 0.3 mm. the annual evaporation is about 172 mm, and the maximum evaporation rate in june is about 21.32 mm. the northern, middle, and southern darb el arbaein aquifer were treated statistically in order to clarify the impact of lithogenic and anthropogenic factors. the saturation indices and partial pressure of co2 was estimated by aquachem to delineate the change in pressure from the atmospheric to the current conditions. the statistical analyses and aquachem (phreeqc) were used to delineate the impact of lithogenic and anthropogenic sources on aquifer system. aquachem (phreeqc version 2) is a computer program written in the c programming language that is designed to perform a wide variety of low-temperature aqueous geochemical calculations. 1.1 geomorphology and geological setting darb elarbaein is subdivided into three geomorphologic units, the southern naklai-sheb peneplain; the western atmur peneplain; and a plateau surface (issawi 1971). geologically, the exposed rocks range from pre-cambrian to quaternary sediments (conoco 1989). the 167 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt figure 1. location map of darb el arbaein. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:43 p p p composite 133 lpi at 45 degrees lithostratigraphic successions are divided into seven units, from base to top (conoco 1989, fathy et al., 2001, korany et al., 2002, ghazal, 2002 and el-gammal, 2004): 1) precambrian basement; 2) paleozoic-mesozoic sandstone; 3) lower cretaceous; 4) upper cretaceous; 5) paleocene; 6) eocene; and 7) quaternary. the nubian sandstone is classified into shab, qusier, and taref members (issawi 1973). the kisieba formation is equivalent to both shab and qusier members and taref is equivalent to taref member (conoco 1989). the lithostratigraphic sequence in the northern, central, and southern part of darb el-arbaein is shown in table 1. the sedimentary cover increases far in the northern part (more than 100 m), while it increases to the southern part of the northern darb-el arbaein. there is a little thickness variation in the southern darb el arbaein. the dakhla shale represents the capping aquifer only in the central darb el-arbaein. the darb el-arbaein area is related structurally to the red sea and southwestern regions (egsma 1987a and b). issawi (1971) has identified the faults in e-w, ne-sw and nw-se and three anticlines (bir kiseiba, rage, and shirshir). the uparching of basement rocks is encountered (fathy et al., 2002). 1.2 hydrogeological conditions 92 wells were designed by garpad in the period from 1998 to 2001, with depths from 130 to 535 m. they penetrated the nubian sandstone successions (paleozoic–upper cretaceous), which overlay pre-cambrian basement rocks. the generalized hydrogeological section indicates that the area is dissected by 18 normally -exposed faults and concealed faults (fathy et al., 2002). the central area bordered by gabal abu rayan el-bahari from the north and the kiseiba anticline from the south. the natural recharge of the aquifer system in the western desert is thought to be of about 1.5 billion m3/y (ambroggi, 1966), while groundwater flow from libya to egypt is about 3.782 million m3/y (ezzat and abu atta, 1974). the groundwater believed to have accumulated and preserved in the nubian sandstone aquifer since 10000 y bc or even earlier as revealed by c14 and h3 iso168 abdel-lattif. a. and mohamed el kashouty table 1. lithostratigraphic sequence in the darb el-arbaein (garpad et al., 2001). age location formation max. thickness average, m 1northern darb el arbaein upper cretaceous campanian-maestrichtian quseir 73 – 112 turonian-santonian taref 101 – 202 paleozoic-mesozoic sandstone layer 7 – 45 pre-cambrian ash basement base is not reached 2central darb el arbaein upper cretaceous maestrichtian – early paleocene dakhla 161 – 285 campanian-maestrichtian kiseiba 73 – 112 turonian-santonian taref 75 – 116 paleozoicmesozoic sandstone layer 7 – 64 pre-cambrian ash basement base is not reached 3southern darb el arbaein upper cretaceous campanian-maestrichtian kiseiba 44 – 150 turonian-santonian taref 56 80 pre-cambrian ash basement base is not reached enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:44 p p p composite 133 lpi at 45 degrees topes (german water group, 1977). the stored water in nubian sandstone is mainly fossilized water and ranges from 20000 to 40000 y (shata et al., 1962 and shata 1982). 1.2.1 northern darb el arbaein the aquifer is composed of sandstone of qusier and taref formations and paleozoic-mesozoic sandstone layers. the upper-most layer of qusier formation act as confining layer. the aquifer thickness increases to the northern part. the aquifer is underlain by basement rocks. the latter is become deeper northward indicating the influence of concealed faults (nw-se) and the increase of sedimentary cover to the north (fathy et al., 2002). the groundwater flows to the eastern, northern, and northeastern part (fig. 2a). the hydraulic conductivity (fig. 2b) and transmissivity (fig. 2c) increases to the northern, northwestern, and southwestern part of the northern darb el arbaein. the well efficiency ranges from 61 to 71% at discharge 150 m3/h. the northern darb el arbaein is dissected by normal faults (mostly nw-se). these faults increase the sedimentation to the north and northeast. the vertical displacement of formations along fault plane makes juxtaposition of permeable against impermeable layers. it influences aquifer thickness, precipitation/dissolution, and groundwater salinity. such displacement diminishes the hydrogeological characteristics of the aquifer system towards the south and south-east. the hydrogeophysical cross sections (a-a’), (b-b’), and (c-c’) (fig. 3) revealed that the boreholes fully penetrated the nubian sandstone successions (paleozoic-upper cretaceous). the latter overlie the pre-cambrian basement rocks (el gammal 2004). the nubian sandstone succession is built up of qusier fm., taref fm., and paleozoic-mesozoic sandstone layer. the sediments gets up finest upward, the taref fm. is more permeable than the qusier fm. 1.2.2 central darb el arbaein the groundwater flows from the northern and northwestern part towards the southeastern part (fig. 4a). the dakhla shale thickness increases in the southern part (fig. 4b) of the central darb el arbaein. the hydraulic gradient has changed in the studied area, attributed to structural patterns and lithological changes. the water level dropped about 30 m (average) in 2005 compared in 2000, because of high pumping, increase in wells, and low recharge. the effective porosity of sand layers ranges from 21 to 30%, transmissivity ranges from 280 to 540 m2/d, and storativity ranges from 0.00011 to 0.0009. the aquifer facies (nubia facies) are represented by the paleozoicmesozoic sandstone layers and upper cretaceous rock units (taref and kiseiba formations) (fig. 5) (el sayed 2005). the basement rocks form the base of the aquifer, while the dakhla formation acts as impermeable. the thickness of this aquifer ranges between 200 and 230 m. 1.2.3 southern darb el arbaein the aquifer is built of kiseiba and taref formations of the upper cretaceous (fathy et al., 2001). the upper -most layer of kiseiba formation act as confining layer. the aquifer thickness increase in the southern part, attributed to high concentration of faults to the north. the aquifer is composed of coarse grained sandstone at the base that changes gradu169 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt apiezometric level, m groundwater flow concealed faultto ea st ow ei na t 2704000 2705000 2706000 867000 868000 869000 870000 871000 872000 873000 2697000 2698000 2699000 2700000 2701000 2702000 2703000 71 2704000 2705000 2706000 867000 868000 869000 870000 871000 872000 873000 2697000 2698000 2699000 2700000 2701000 2702000 2703000 ctransmissivity, m2/d 867000 868000 869000 870000 871000 872000 873000 2704000 2705000 2706000 2697000 2698000 2699000 2700000 2701000 2702000 2703000 bhydraulic conductivity, m/d figure 2. (a) piezometric level, (b) hydraulic conductivity, and (c) transmissivity of the nubian sandstone aquifer in northern darb el-arbaein. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:44 p p p composite 133 lpi at 45 degrees ally to fine and medium upward. it directly overlies the basement rocks. the groundwater flow is in the northeastern part (fig. 6a). the capping thickness increases to the northwestern part (fig. 6b). the hydraulic conductivity (fig. 6c) increases in the southern, northern, western, and centraleastern part. the transmissivity (fig. 6d) increases to the southern and southwestern part. the well efficiency at pumping rate 150 m3/h ranged from 84 to 86 %. the geophysical cross section (a-a’) runs n-s direction (fig. 7a). the total aquifer thickness ranged from 70 to 138 m and increases due the southern direction of southern darb el arbaein. the graben and horst were interpreted in the area (fig. 7). the taref and overlying kiseiba fms. are hydraulically interconnected through the faults (el gammal 2004). the geophysical cross section (b-b’) runs w-se direction (fig. 7b). the aquifer thickness increases due the central part, attributed to structural elements. the aquifer is hydraulically interconnected with fault planes. 170 abdel-lattif. a. and mohamed el kashouty 0 60 -60 -120 -180 -240 c c' 0 60 -60 -120 -180 b b' 500 m 0 60 -60 -120 -180 500 m quseir formation taref formation paleozoic-mesozoic sandstone basement concealed fault potentiometric level a a' d ep th ,m d ep th ,m d ep th , m 500 m figure 3. hydrogeophysical cross sections in northern darb el arbaein. the directions were shown in fig. 11. 841000 842000 843000 844000 845000 846000 847000 848000 2652000 2653000 2654000 2655000 2656000 2657000 2658000 to east oweinat to kharga bdakhla shale thickness, m 841000 842000 843000 844000 845000 846000 847000 848000 2652000 2653000 2654000 2655000 2656000 2657000 2658000 apiezometric level, m to kharga to east oweinat groundwater flow figure 4. (a) the groundwater flow and (b) dakhla shale thickness of the nubian sandstone aquifer in central darb el arbaein. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:45 p p p composite 133 lpi at 45 degrees 2. materials and methods the obtained hydro-chemical and hydro-geological data (2002-2004), after garpad organization, were analyzed using statistical and aquachem (phreeqc). 3. multivariate statistical analyses 3.1 introduction the chemical composition of groundwater is controlled by different factors such as precipitation, mineralogy of the watersheds and the aquifer lithology, the climate, the topography, and land use application especially agricultural and industrial activities. the groundwater samples from the area could potentially represent a variety of water types providing an opportunity to test the performance of many of the available graphical, hydrogeochemical and hydrogeological programs, and statistical techniques. the use of the major ions as natural tracers (back, 1966) has become a very common method to delineate flow paths in aquifers. generally, the approach is to divide the samples into hydrochemical facies that is, groups of samples with similar chemical characteristics that can then be correlated with location. the spatial variability observed in the composition of the natural tracers can provide insight into aquifer heterogeneity and connectivity as well as the physical and chemical processes controlling water chemistry. several commonly used graphical methods and multivariate statistical techniques are available including pie, stiff, q-mode, r-mode hierarchical (hca) (judd, 1980; rummel, 1970; berry, 1995; guler et al., 2002) and principal component analysis (pca). for understanding the chemistry of water samples, the multivariate statistical analyses were used (drever, 1997) with assumptions of alther (1979). the hierarchical cluster analysis (hca) analyses (statsoft, inc. 1995) were used to classify the samples into distinct hydrochemical groups based on their similarity, using z scores (ward 1963). in order to make clear the other factors affecting the concentration and distribution of major ions and hydrogeological parameters here, multivariate statistics (pearson coefficient analysis and principle component analysis) were adopted. principle component analysis is one of the analytical tools used to assess element behavior in aquatic system (shine et al. 1995; 171 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt 200 180 160 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200 -220 -240 -260 -280 -300 -320 (a) ne 21 14 recent deposits recent deposits 11 6 (a) sw g re u p d l ev el (m ) dakla fm. dakla fm. k is ei ba f m k is ei ba f m tarfa fm ta rf a f mpaleozoicmezozoic basement paleozoicmezozoic b as em en t 0 1 2 3 4 h. scale paleozoic-mezozoic basement tarfa fm kiseiba fm legend surface deposits shale sand clay fault sandy shale shaly sand shale & clay basement piezometric level d ak la f m . k is ei ba f m ta rf a f m figure 5. subsurface geologic cross section of central darb el arbaein. its direction was in fig. 11 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:45 p p p composite 133 lpi at 45 degrees ruiz 2001; liu et al. 2003; zhou et al. 2004), and it is applied to detect the hidden structure of the data set, trying to explain the influence of latent factors on the data distribution (einax and soldt 1999; simeonov et al. 2000). 3.2 correlation analysis the ground elevation is weakly inversely correlated with water depth, well loss (0.5-0.53), delineate the impact of topography on the well loss. the ground elevation is strongly negatively correlated with k, suggesting that the latter has a lithogenic source rather than anthropogenic. the hydro-geological characteristics of the aquifer system improved in the topographic and potentiometric head highs. the ground elevation is correlated with so4; indicating anthropogenic origin, especially agricultural activity. also the dissolution of evaporite minerals contributes to so4 content in groundwater. water depth is negatively correlated with potentiometric 172 abdel-lattif. a. and mohamed el kashouty 770000 772000 774000 776000 778000 780000 782000 784000 786000 788000 to kharga to tushka to sudan b-capping thickness 770000 772000 774000 776000 778000 780000 782000 784000 786000 788000 2454000 2456000 2458000 2460000 2462000 2464000 2466000 2468000 2470000 2472000 2474000 to kharga to sudan apiezometric level, m to tushka groundwater flow concealed fault 43 770000 772000 774000 776000 778000 780000 782000 784000 786000 788000 2454000 2456000 2458000 2460000 2462000 2464000 2466000 2468000 2470000 2472000 2474000 to kharga to tushka to sudan chydraulic conductivity, m/d 770000 772000 774000 776000 778000 780000 782000 784000 786000 788000 2454000 2456000 2458000 2460000 2462000 2464000 2466000 2468000 2470000 2472000 2474000 2454000 2456000 2458000 2460000 2462000 2464000 2466000 2468000 2470000 2472000 2474000 to kharga to tushka to sudan dtransmissivity, m2/d figure 6. (a) groundwater flow, (b) capping thickness, (c) hydraulic conductivity, and (d) transmissivity of the nubian sandstone aquifer in southern darb el-arbaein. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:46 p p p composite 133 lpi at 45 degrees head, electrical conductivity, total dissolved solids (tds), na, so4, and cl, clarify the impact of agricultural activity infiltrated downward, dissolution of geomedia and evaporation rate that contributed to these ions in groundwater system and depends upon water depth. water depth is weakly correlated with k, confirming the lithogenic source for latter. potentiometric head is negatively correlated with well loss, the latter decreases in the highest potentiometric head in the aquifer system. therefore, the contribution of well loss to drawdown is minimal in the highest potentiometric heads. the latter is weakly correlated with so4, indicate different sources may lithogenic (evaporite, shale, and clay) and anthropogenic (mainly fertilizers). the potentiometric head is strongly correlated with transmissivity, hydraulic conductivity, and discharge rate, caused by dissolution of geo-media that increases with potentiometric heads. therefore, the aquifer permeability and transmissivity was enhanced in the hydraulic head highs. hydraulic conductivity is weakly negatively correlated with drawdown, k, and well loss, showing the impact of geology (hydraulic conductivity) on these parameters. the drawdown increases with decreasing of hydraulic conductivity of the aquifer system and k is liberated by dissolution. hydraulic conductivity is strongly correlated with discharge rate. this is attributed to good yield and should be in high hydraulic conductivity areas. the aquifer system is heterogeneous so such discharge rate distribution depends on the hydraulic conductivity values. drawdown is weakly correlated with well loss; showing that the well loss contributed little to drawdown. drawdown is strongly correlated with formation loss; such correlation confirms that drawdown is mainly influenced by a change in hydraulic conductivity of the formation (formation loss). drawdown in wells is composed mainly of two components, the formation loss and the well loss. electrical conductivity is weakly negatively correlated with k, reflect that the latter is partially derived from lithogenic, i.e. from dissolution of clay and shale sediments. electrical conductivity is strongly correlated with tds, ca, na, so4, and cl, indicating two main processes that responsible for these ions concentrations, evaporation and agricultural wastewater. electrical conductivity is weakly correlated with mg and hco3, clarifying that the last two are partially lithogenic and partially anthropogenic. ca is weakly correlated with k, reflect two sources for both may anthropogenic for former and lithogenic for latter. ca is weakly correlated with na, so4, and cl, these ions may derive partially from evaporation and partially from agricultural wastewater and dissolution. cl is weakly correlated with hco3, attributed partially to evaporation and partially to infiltration of wastewaters. mg is weakly correlated with na, so4, and cl indicating that different sources for these ions may be lithogenic and anthropogenic. na and k are negatively correlated, due to different sources. na is weakly and strongly correlated with hco3 and so4 and cl, respectively. it is attributed to partially infiltration of surface mete173 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt 90 15 165 360 m b b' concealed fault kiseiba formation taref formation basement b 90 15 165 a a' 666 m a figure 7. geophysical cross sections in northern darb el arbaein. its directions were shown in fig. 11 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:46 p p p composite 133 lpi at 45 degrees oric water and mainly evaporation and agricultural wastewaters, respectively. k is weakly negatively correlated with so4, cl, and discharge rate, indicate different sources. on the other hand, k weakly correlated with well loss, reflect the influence of lithology on well loss. hco3 is weakly correlated with cl, attributed to the lithogenic and anthropogenic sources. so4 is strongly correlated with cl, demonstrating that both ions are indeed highly correlated in waters from agricultural areas and also the dissolution of evaporate minerals. 3.3 factor identification r-mode (interrelationships among variables) and q-mode (interrelationships among cases) applications can provide a certain amount of insight into the structure of the data matrix (davis, 1986). r-mode from the similarity matrix, the principal component and the distribution of variables were determined. fig. 8 shows the principal components, weights of the components, and associated variance explained in the physio-chemical data and communalities. however the variables have high communalities and every variable is represented in the model application of this method. for the nubian sandstone aquifer, the spatial distribution of variables based on the three components is presented in fig. 8. the three factors explain 77% of the variance with eigenvalues higher than 1.8. the variance percentage and the eigenvalues for each factor are shown in fig. 8. f1 is positively loaded with ec, tds, ca, mg, na, hco3, so4, cl, and formation loss. such factor represents evaporation process, agricultural impact, and partially lithogenic dissolution. the presence of formation loss in f1 confirm the precipitation/or dissolution processes enhance the aquifer permeability and therefore transmissivity. f1 is negatively loaded with water depth and k, indicating that the latter derived from lithogenic source especially clay and shale dissolution. the negatively loaded of water depth also confirms the impact of anthropogenic sources and evaporation process and influenced by water depth. f2 is positively loaded with ground elevation, hydraulic head, hydraulic conductivity, and discharge; clarifying the hydrogeological characteristics of the aquifer system included together in one factor. f2 is negatively loaded with drawdown, k, formation loss, and well loss, reflecting the different trends in these parameters with hydrogeological characteristics. the k included in f2, confirm its lithogenic source. the increase in discharge rate impact on well efficiency (formation loss and well loss), and drawdown and influence on future outcomes. f3 is negatively loaded with ph and hco3, clarifying the different surface meteoric water that recharge the aquifer, from regional freshwater from the surrounding aquifers, infiltration water, dissolute water, and evaporation. 3.4 cluster analysis the dendrogram investigation, based on hydrogeological and hydrogeochemical data of nubian sandstone aquifer (fig. 9), indicates two main clusters (i and ii). cluster i subdivided into two sub clusters 1 and 2. sub cluster 1 subdivided into two groups (a and b) and five independent cases. group a include eleven samples 34, 44, 46, 47, 48, 49, 50, 51, 52, 65, and 66 (northern part of southern darb elarbaein). group a is characterized by the high hydraulic head and low mg content (fig. 10a and b). group b contains fourteen samples 56, 57, 58, 59, 60, 61, 62, 63, 64, 67, 69, 70, 71, and 72 (northeastern and southwestern part of southern darb el arbaein). it is distinguished by the highest ground elevation, hydraulic conductivity, and transmissivity (fig. 10a). the southern darb el arbaein was contained in the two groups a and b (fig. 11). group a and b have the highest hydraulic head among the groups (fig. 10a). the independent cases 53, 54, and 55 were characterized by high ground elevation, hydraulic conductivity and transmissivity (fig. 10c). independent cases 45 and 68 were distinguished by lowest ph and lowest drawdown, respectively (fig. 10c and d). the previous independent cases were included in sub cluster 1. sub cluster 2 subdivided into two groups (c and d) and five independent cases. group c was represented by ten samples 17, 18, 20, 35, 36, 37, 38, 39, 40, and 41 (northeastern part of field 2 of central darb el arbaein). it has high tds concentration and highest ca content (fig. 10a). group d is composed of eleven samples 21, 22, 23, 24, 25, 26, 27, 28, 31, 32, and 42 (eastern part of field 2 of central darb elarbaein). it is characterized by the highest tds and major ion contents among the groups (fig. 10a). field 2 of central darb el arbaein was included in the two groups c and d (fig. 11). the independent cases 19, 29, 30, 33, and 34 (sub cluster 2) were distinguished by the highest total dissolved solids all over the area (fig. 10d). the last samples was included in the field 2 of central darb el arbaein (fig. 11), they reflect the lithogenic and anthropogenic impact. cluster ii subdivided into two sub clusters (3 and 4). sub cluster 3 includes two independent cases 86 and 87, which has the lowest k concentration and the lowest hydraulic conductivity (fig. 10c and d). sub cluster 4 is subdivided into six groups (e to j) and one independent case. group e contains four samples 1, 12, 15 (field 1 of central darb elarbaein), and 91(northern darb el arbaein). it is characterized by the lowest tds concentration and very high transmissivity (fig. 10a and b). group f was represented by 174 abdel-lattif. a. and mohamed el kashouty enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:47 p p p composite 133 lpi at 45 degrees 175 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt 1 0.5 0 -0.5 -1 factor 3 % of variance = 9 eigenvalue = 1.8 g round elevation w ater dept p otentiom etric he t ransm issivity h idraulic conduc’ d raw dow n e c t d s c a m g n a k h c o 3 s o 4 c l f orm ation loss w ell loss d ischarge s torativity p h 1 0.5 0 -0.5 -1 factor 2 % of variance = 28 eigenvalue = 5.7 g round elevation w ater dept p otentiom etric he t ransm issivity h idraulic conduc’ d raw dow n e c t d s c a m g n a k h c o 3 s o 4 c l f orm ation loss w ell loss d ischarge s torativity p h 1.5 1 0.5 0 -0.5 -1 factor 1 % of variance = 40 eigenvalue = 7.9 g round elevation w ater dept p otentiom etric he t ransm issivity h idraulic conduc’ d raw dow n p h e c t d s c a m g n a k h c o 3 s o 4 c l f orm ation loss w ell loss d ischarge s torativity figure 8. principle component analysis of the nubian sandstone aquifer. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:47 p p p composite 133 lpi at 45 degrees 176 abdel-lattif. a. and mohamed el kashouty cluster i cluster ii sub cluster 1 sub cluster 2 sub cluster 3 sub cluster 4 a i.c. b i.c. c i.c. d i.c. e f g h i j i.c. i.c. figure 9. dendrogram investigation of the hydrogeological and hydrogeochemical characteristics of nubian sandstone aquifer. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:47 p p p composite 133 lpi at 45 degrees 177 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt figure 10. mean hydrogeological and hydrogeochemical values of the clusters a-j and independent cases of the nubian sandstone aquifer. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:48 p p p composite 133 lpi at 45 degrees 178 abdel-lattif. a. and mohamed el kashouty 800000 850000 2500000 2550000 2600000 2650000 2700000 867000 868000 869000 870000 871000 872000 873000 2697000 2698000 2699000 2700000 2701000 2702000 2703000 2704000 2705000 2706000 841000 842000 843000 844000 845000 846000 847000 848000 2652000 2653000 2654000 2655000 2656000 2657000 2658000 770000 772000 774000 776000 778000 780000 782000 784000 786000 788000 2454000 2456000 2458000 2460000 2462000 2464000 2466000 2468000 2470000 2472000 2474000 field 1 northern area middle area southern area 0 50 km field 2 group a group b group c group d group e group f group g group h group i group j independent case darb arb aein figure 11. areal distribution of clusters identified by hca of the northern, central, and southern darb el arbaein. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:48 p p p composite 133 lpi at 45 degrees six samples 77, 78, 80, 81, 82, and 89 (northern darb el arbaein). it has the lowest hydraulic conductivity and transmissivity and highest drawdowns (fig. 10a). group g include three samples 2, 10 (field 1 of central darb elarbaein) and 83 (northern darb el arbaein). it is characterized by high transmissivity (fig. 10a). group h contains five samples 4, 5, 6, 7, and 9 (field 1 of central darb elarbaein). it is characterized by very low hydraulic head (fig. 10a). group i was represented by four samples 73, 74 (southern part of southern darb elarbaein) 76, and 79 (northern darb elarbaein). it is characterized by the highest k content (fig. 10b), may attributed to aquitard diffusion. group j include eleven samples 3, 8, 11, 13, 14, 16 (field 1 of central darb elarbaein), 75, 85, 88, 90, and 92 (northern darb el arbaein). group j is characterized by the highest water depth and very low tds concentration (fig. 10a and b). the independent case 84 was characterized by lowest cl concentration and very high transmissivity (fig. 10c and d). the pie, radial, and stiff methods produce a single diagram for each sample. it is not practical to produce and nominally sort all samples figures, so the choice of similarity would be based on evaluation of the analyst, which is highly subjective; herein it is the multivariate statistical analysis. 4. saturation indices and partial pressure estimation calculation of a mineral saturation index (si) is a convenient method of representing the equilibrium condition of a solution with respect to a mineral. saturation index (si) = log iap/kmineral k is the solubility product defines as the solubility constant and calculated by the law of mass action (appello and postma 1976). here, si = 0, if the mineral is in equilibrium with the solution; si<1, if the mineral is under-saturated and si>1; if the mineral is over-saturated. by using the saturation index approach, it is possible to predict the reactive mineralogy of the subsurface from the groundwater data without collecting samples of the solid phase and analyzing the mineralogy. the saturation indices of selected minerals are illustrated in fig. 12, the groundwater in all is undersaturated with respect to these minerals; delineate the low concentration of these minerals in the geomedia. the groundwater was supersaturated with respect to iron miner179 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt figure 12. saturation indices of the minerals identified by phreeqc of the nubian sandstone aquifer. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:49 p p p composite 133 lpi at 45 degrees als in northern and southern darb el arbaein (fig. 12); reflect the presence of iron concentration in the sandstone aquifer. the partial pressure of the groundwater versus saturation index of calcite is depicted in (fig. 13) to show the gradual change in pco2 from atmospheric to the present aquifer pressures. 5. conclusions and recommendations the geology, the hydrogeology, the dissolution, and the recharge influence the aquifer quality. the graphical and statistical techniques reflected valuable and rapidly accessible information about the chemical composition of groundwater samples. the correlation investigation indicated the agricultural impact on groundwater systems. the factor analysis distinguished into three factors that explain the hydrogeochemical processes in aquifer systems. the dendrogram investigation revealed two main clusters, which subdivided into four sub clusters, depending on the water chemistry and hydrogeological conditions. the groundwater was under-saturated with respect to carbonate minerals and others except the iron minerals in northern and southern darb el arbaein, it was supersaturated. 6. acknowledgement the authors are greatly thanks the garpad organization for data supply. references alther, a. 1979. a simplified statistical sequence applied to routine water quality analysis: a case history. ground water v17, 556-561. ambroggi, r. 1966. water under the sahara: scientific american, v 214, no 5. appello c. and postma d., (1976). geochemistry, groundwater and pollution. a.a. balkema, rotterdam, netherlands, 536p. back, w. 1966. techniques for mapping of hydro chemical facies. us geol surv porf pap 380-382. 180 abdel-lattif. a. and mohamed el kashouty figure 13. total co2 vs saturation index of calcite of the nubian sandstone aquifer. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:49 p p p composite 133 lpi at 45 degrees berry, k. 1995. spatial reasoning for effective gis. gis world books, fort collins, colardo. bierschenk, w. 1964. determining well efficiency by multiple step drawdown test. publication 64. international association of scientific hydrology. continental oil company (conoco, 1989). geologic map of egypt, scale 1; 500000. davis, j.c. 1986. statistics and data analysis in geology: john wiley and sons, ny, tornto-singapore. drever, i. 1997. the geochemistry of natural waters, 3rd edn. prentice-hall, upper saddle river, nj. el gammal, n. 2004. hydrogeological studies in darb el arbaein area, south egypt. master science, geology department., cairo university, 151p. el sayed, e. 2005. groundwater conditions, nubia sandstone aquifer, south of gebel abu rayan el baharai, darb el arbaein area, western desert, egypt. 4th intern conf on the geo of africa, v 1, pp.57-78. einax, j. and soldt, u. 1999. geostatistical and multivariate statistical methods foe the assessment of polluted soils-merits and limitations. chemomet intell lab syst v46, 79-91. ezzat, m., abu atta, a. 1974. exploration of groundwater in el wadi el gedid project area (new valley), part ii, hydrogeological conditions. dakhla – kharaga area; ministry of agriculture and land reclamation, cairo. fathy r., hefnawy m. and abdel hamid a. 2001. contributions to the hydrogeology aspects of the groundwater aquifer in southern part of darb el arbaein area, southern portion of western desert, egypt. al azhar bull sci v12 no2, 175-194. fathy, r., el nagaty, m., atef, a. and el gammal, n. 2002. contribution of the hydrogeological and hydrochemical charateteristics of nubian sandstone aquifer in darb el arbaein, southwestern desert, egypt. al azhar bull sci v13 no2; 69-100. general authority for rehabilitation projects and agricultural development (garpad), 98/2001. lithologic logs and geophysical logs of water wells, darb el-arbaein area, southwestern desert, egypt. general survey of egypt, (egmsa), 1987a. geology and geomorphology of the egyptian component transitional sandstone project; report to groundwater research institute, egypt. general survey of egypt (egmsa), 1987b. geophysical investigation of the egyptians component transitional sandstone project; report to groundwater research institute, egypt. pp 425-447. german water group, 1977. hydrogeological study of groundwater resources in the kufra area; german water engineers, gb, v 5. ghazal, a. 2002. hydrogeological studies in darb el arbaein area, southwestern desert, egypt. master science, geology department cairo university, 167p. guler, g., geophrey, d., and mccray, j. 2002. evaluation of graphical and multivariate statistical methods for classification of water chemistry data. hydrogeology journal, 10: 455-474. issawi, b. 1971. geology of darb el arbaein, western desert, egypt. ann g s e, cairo, v 46; 120p. issawi, b. 1973. geology of southeastern corner of western desert. ann g s e , cairo, v viii. judd, g. 1980. the use of cluster analysis in the derivation of geotechnical classification. bull assoc. eng. geol. v17, 193-211. korany, e., fathy, g. and el nagaty, m. 2002. contributions to the hydrogeology of nubian sandstone aquifer in the middle part of darb el arbaein, southwestern desert, egypt. sedimentaology of egypt, issn 1110-2527, j of the sedimentaological society of egypt, v 10; 119-143. liu, x, li, x., shen, z., wang, d., wai, o, and li, y. 2003. multivariate statistical study of heavy metal enrichmnmet in sediments of the pearl river estuary. environ pollut 121: 377-388. ruiz, f. 2001. trace metal in estuarine sediments from the southwestern spanish coast. mar pollut bull 42 (6): 1843-1857. rummel, j. 1970. applied factor analysis, northwestern university press, evanston. shata, a., knetsch, g. and el shazly, m., (1962). the geology, origin, and age of the groundwater supplies in some desert areas of uar: bull inst desert, el matariya, cairo, v xii no 2: 61-124. shata, a. 1982. hydrogeology of the great nubian sandstone basin; quaternary of engineering, london, v 15; 127-133. 181 statistical investigation of the groundwater system in darb el-arbaein, southwestern desert, egypt enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:50 p p p composite 133 lpi at 45 degrees shine, j., ika, r, and ford, t. 1995. multivariate statistical examination of spatial and temporal patterns of heavy metal contamination in new bedford harbour marine sediments. environ sci technol 29: 1781-1788. simeonov, v., massart, d., andreev, g., and tsakovski, s. 2000. assessment of metal pollution based on multivariate ststistical modeling of hot spots sediments from the black sea. chemosphere v41, 1411-1417. statsoft, inc. 1995. statistica for windows (computer program manual), vo. 3, statsoft, inc., tulsa, ok. ward, h. 1963. hierarchical grouping to optimize an objective function. j am stat assoc 69, pp. 236-244. zou h., peng, x, and pan, j. 2004. distribution, source, and enrichment of some chemical elements in sediments of the pearl river estuary. china continental shelf res v24, 1857-1875. 182 abdel-lattif. a. and mohamed el kashouty enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:50 p p p composite 133 lpi at 45 degrees archivo02.indd 15 manuscript received may 4, 2006. inferences on the geometry of the nazca plate in northwestern perú based on data collected by a local seismograph network hernando tavera1, renzo vilca2 and gloria marín3 1 dirección de sismología, instituto geofísico del perú 2 centro nacional de datos geofísicos, instituto geofísico del perú 3 oficina regional de chiclayo (perú), instituto geofísico del perú corresponding author: hernando tavera, e-mail: hjtavera@geo.igp.gob.pe abstract in central and southern peru, local and teleseismic data allow having insights on the wadati-benioff zone (wbz) geometry. in the region of northern perú, only rough estimates of the wbz geometry have been proposed, and were obtained using teleseismic data due to the reduced number of m b ≥4.0-magnitude earthquakes available. the installation of a local seismic network in the nw border of northern perú, allowed the recording of a large number of earthquakes with magnitudes ranging from 1.8 to 4.1 m l , and maximum depths of 120 km., distributed over the area where the occurrence of earthquakes with magnitudes > 4.0 is scarce. in a vertical section, oriented n70˚e earthquakes are distributed along a 10° dipping plane from the trench. landwards from the coastline, the dip angle increases to 28°. these results allow a high resolution definition of the nazca plate geometry in nw perú. focal mechanisms computed for 22 earthquakes located between 50 and 120 km depth beneath the network suggest the development of deformation processes in direction parallel to the convergence nazca plate direction. key words: nazca plate, subduction, double seismic zone, perú. resumen en la región centro y sur de perú, la geometría de la zona wadati-benioff (zwb) ha sido estudiada con el uso de datos locales y telesísmicos; mientras que, para la región norte, solo se han realizado análisis utilizando datos telesísmicos debido al escaso número de sismos con magnitudes m b ≥4.0. la instalación de una red sismográfica en el borde nw de la región norte de perú, ha permitido detectar un importante número de sismos con magnitudes m l entre 1.8 y 4.1, y profundidades máximas hasta 120 km, todos distribuidos sobre el área en donde la ocurrencia de sismos con magnitudes m l mayores a 4.0 es mínima. en una sección vertical orientada n70°e, los sismos se distribuyen en profundidad, desde la fosa siguiendo una pendiente de 10° y a partir de la línea de costa hacia el continente, el ángulo de inclinación aumenta a 28°. estos resultados permiten definir con alta resolución la geometría de la placa de nazca en el borde nw de la región norte de perú. los mecanismos focales calculados para 22 sismos ubicados por debajo de earth sciences research journal earth sci. res. j. vol. 10, no. 1 (jun. 2006): 15-24 16 hernando tavera, renzo vilca and gloria marin introduction the segment of the andes, located west of perú, is the result of a subduction process of the nazca (np) plate beneath the south american (sa) plate. in general, any major changes produced in the tectonics of the cordillera are somehow related to comparable changes in the wadati-benioff (wbz) zone geometry, (kausel & lomnitz, 1968 and lindo, 1993). in perú, recent studies on the wbz geometry show the existence of a trench-normal subduction in the southern region with the presence of active volcanic chains and a sub-horizontal subduction in the northern and central regions with absence of volcanic activity (stauder, 1975; barazangi & isacks, 1979; cahill & isacks, 1992; tavera & buforn, 2001; bernal & tavera, 2002). these studies used teleseismic data drawn from the neic, and isc catalogues, and recently from engdahl catalogue (engdahl et al., 1997). these catalogues, generally include m b >4.0-magnitude earthquakes, and if more detail on the subduction zone geometry is intended, the use of local (δ<100 km) and regional (100<δ<1000 km) data is necessary. in perú, studies on the wbz geometry using data from local seismic networks have been carried out only in the central and southern regions, several 1d and 3-d models have been proposed (hasegawa & sacks, 1981; grange, 1984; schneider & sacks, 1987; rodríguez & tavera, 1991; cahill & isacks, 1992; lindo, 1993 and tavera & buforn, 2001). in general, it is accepted that in the southern region, the nazca plate continuously subducts with a 30° dip angle down to about 300 km depth; while in the central region, the plate subducts with an initial 10° dip angle from the trench to the coastline, increasing to a 30° dip to a depth of 120 km, and then becoming almost horizontal up to distances of 500 km from the trench. in the case of the northern perú, no studies on the wbz have been carried out using local data, and up to date, the best results are those obtained from the teleseismic data (stauder, 1975; tavera & buforn, 2001). therefore, it has been suggested that the characteristics of the subducting plate are similar to those observed for the central region. in figure 1a, we show the seismicity map for the northern region of perú, with data from the engdahl catalogue (19641999, m b >4.5) and from the instituto geofísico del perú (2000-2003, m b >4.5). according to figure 1a, shallow to intermediate earthquakes form two main groups: 1) the first group locates in front of the coast line where earthquakes probably occur along the subduction interface and 2) the second group located roughly beneath the city of moyobamba, where the fault geometry is poorly known. in the centre of the study area beneath the cordillera (third group, figure1a), almost no earthquakes are observed, with only a few reported, and shallow and intermediate earthquakes located within the local network or near the trace of the recodo fault. in a vertical section with orientation n70°e, parallel to the direction of plate convergence, it can be observed that earthquakes are distributed in depth forming two groups (figure 1b), one is spatially associated with the subducting nazca plate, and the other is formed by events probably occurring inside the overriding sa plate. the first group is likely associated with the plate interface and follows an apparent 20° dip angle from the trench towards the coastline. the absence of earthquakes is evident from 150 km to about 550 km distance (figure 1b). farther landward, the earthquakes form a nearly horizontal plane at 120 km depth up to 700 km distance. in figure 1b, the dashed lines indicate the seismicity trend in the region. this low earthquake activity beneath the cordillera has been observed and discussed by several authors (stauder, 1975; barazangi & isacks, 1979; cahill & isacks, 1992; tavera & buforn, 2001), and is interpreted as consistent with the existence of a seismic gap. la red entre 50 y 120 km de profundidad, sugieren el desarrollo de diversos procesos de acomodación de movimiento en dirección paralela a la dirección convergencia de la placa de nazca. palabras clave: placa de nazca, subducción, doble zona sísmica, perú. © 2006 esrj unibiblos 17 inferences on the geometry of the nazca plate in northwestern perú based on data collected by a local seismograph network f ig ur e 1. ( a) s pa tia l d is tr ib ut io n of th e ea rt hq ua ke s in th e n or th er n re gi on o f pe rú a cc or di ng to th e e ng da hl a nd i g p ca ta lo gu es ( 19 64 -2 00 3, m ≥4 .5 ; e ar th qu ak es w er e pl ot te d di ff er en t sy m bo ls a cc or di ng to th ei r hy po ce nt ra l d ep th s: c ir cl es f or s ha llo w -f oc us e ar th qu ak es ( 060 k m ), a nd s qu ar es f or in te rm ed ia te -f oc us e ar th qu ak es ( 61 -3 50 km ). b la ck tr ia ng le s m ar k th e lo ca tio n of th e st at io ns in th e lo ca l s ei sm ic n et w or k in th e n or th w es te rn b or de r of th e n or th er n re gi on o f pe ru . t he f au lt di st ri bu tio n (d as he d lin es ) co rr es po nd s to m ac ha re e t a l., ( 20 03 ). 18 hernando tavera, renzo vilca and gloria marin unlike the central and southern regions, limited information for the northern region of perú, has not allowed the characteristization of its shallow and intermediate seismicity, nor the wbz geometry to be accurately defined. this study analyses the spatial distribution of the earthquakes in a long nw perú during the 1996-2001 period. all these events were recorded by a local seismograph network composed of 8 short-period stations. the data reveal the presence of microseismicity and help to better define the wbz geometry. additionally, simple focal mechanisms were used to characterize the deformation pattern at the intermediate-depth levels. data and methodology since 1996, a local seismograph network composed of 8 short-period stations (figure 1a) have been operating nw of the northern region of perú. all these stations are equipped with kinemetrics ss-1 vertical component seismometers with a sismalp system 16-bit digital recorder (frechet & thouvenot, 2004). time control is carried out with a goes satellite and the signal is transmitted in real time, from each station, via a rf (uhf and vhf) telemetry system to a recording station located in the city of chiclayo. the hypocentral parameters for all earthquakes were obtained using a modified version of the computer program hypoinverse (klein, 1978) using a velocity ratio of 1.75±0.1 calculated with a total of 2014 p and s-wave arrival times. considering that for the northern region of perú there is no published information on velocity models, we used the 1-d model proposed by lindo (1993) for the microseismicity study of the andean zone central of peru because the wbz geometry is very similar (table 1). in the hypocentral calculation, each event is located with different trial depths that range between 0 and 250 km with a 2 km interval, minimizing the dependence of the final hypocenter on the initial trial solution. the magnitude for each earthquake has been calculated with the record duration by using the relation proposed by ccallo et al. (2002) for the short-period stations of the red sísmica nacional del perú. figure 1. (b). a-b vertical cross-section showing the distribution of the seismicity with depth in figure 1a direction of section is n70°e; i.e., perpendicular to the trench. the dashed line indicates the seismicity trend. table 1. velocity model used to locate hypocenters of the detected events, this model is based on results of lindo (1993) for the central andes. depth range (km) p -ware velocity (km/s) 0-15 5.8 15-30 6.2 30-50 6.8 > 50 8.0 19 inferences on the geometry of the nazca plate in northwestern perú based on data collected by a local seismograph network during a 6-year period (1996-2001), the local seismograph network recorded a total of 1593 earthquakes with s-p time difference, equal to or lower than 30 seconds, all of them detected with more than 5 stations (5 pand s-phases, respectively). in general, uncertainties of 0.1 s and 0.25 s have been estimated for the p and s arrival times, respectively. after calculating the hypocentral parameters for all earthquakes, those with rms travel time residual <0.2 and horizontal (erh) and vertical (erz) formally-computed errors < 2.5 km were selected. this database with the best located earthquakes has 547 events with (m l ) magnitudes ranging between 1.8 and 4.1; most of them (~90%) recorded by 6 or more stations. to evaluate the accuracy of the solutions obtained and analyze possible changes in epicentral coordinates and focal depths calculations were carried systematically varying and the values of vp/vs and the velocity models. in the worst case, changes of hypocentral were lower than 5 km, but in most cases those variations were smaller than 3 km. such small variations are due to the special care taken to identify the p and s arrival times and to the accurate calculation of the poisson relation based on the wadati curve. results and discussion distribution of hypocenters the spatial distribution of the 547 selected earthquakes is shown in figure 2, all of them are distributed within the study area and within coverage of the local seismograph network. three groups of earthquakes can be distinguished in figure 2: the first one is related to shallow-focus earthquakes (circles) distributed between the trench and the coastline. a second group of shallow-focus earthquakes is onshore mainly over the trace of the recodo fault, a large fault in this region whose geometry details are not known. the third group is composed by intermediate-focus earthquakes (squares), distributed within and around the coverage of the local seismograph network. these earthquakes are mostly grouped around the four stations located north of the study area, and in general, cover all the area which in previous studies was thought to lack intermediate-depth earthquakes (figure 1a). in vertical sections oriented n70°e (figure 3a), it could be observed that earthquakes are distributed following a mean alignment along a 10° dipping plane between the trench and the coastline, in agreement with the results of tavera & buforn (2001) obtained using teleseismic data. approximately starting at the coastline, the dip increases to 28° with earthquakes reaching a depth of 120 km. unlike previous studies carried out in this region using teleseismic data (figures 1a and 1b), the distribution of the earthquakes detected by our local network accurately defines the wbz geometry. at distances larger than 400 km east of the central andes the hypocentral parameters are not well constrained, and are assumed to be distributed nearly horizontally at 120 km-depth, as observed in figure 1b (dashed line). the shallow-focus earthquakes located under the local seismograph network could correspond to deformations in the upper crust, since they usually have depths lower than 25 km. eastwards of the local seismograph network, an important group of earthquakes, with depths reaching 40 km suggests the development of significant shallow deformation possibly associated to the recodo fault (figure 3a). at depth, these earthquakes do not show a clear trend; therefore, a detailed study of the recodo fault seismicity with a larger network and better coverage is required. six vertical sections oriented n70°e are shown in figure 3b (see inset of figure 2). it can be seen that the distribution of earthquakes does not show any significant lateral variations, suggesting that in the study area the nazca plate presents the same geometry. focal mechanisms the focal mechanisms of 22 earthquakes were calculated by using p-wave first-motion polarities. for each of these events, at least 5 impulsive p-wave polarity readings were selected. these earthquakes are distributed within the area of coverage of the seismograph network and represent a good sample of the intermediate-focus earthquakes in this study. due to the small number of polarities, at least one of the nodal planes could not be constrained and for this reason we used a priori knowledge of the expected stresses to help constrain the nodal planes. in western perú usually the principal stresses obtained from focal mechanisms of large earthquakes (m b ≥5.0) 20 hernando tavera, renzo vilca and gloria marin f ig ur e 2. s pa tia l d is tr ib ut io n of s ei sm ic a ct iv ity r ec or de d by th e lo ca l n et w or k du ri ng th e 19 96 -2 00 1 pe ri od . c ir cl es c or re sp on d to s ha llo w -f oc us e ar th qu ak es ( 060 k m ), a nd s qu ar es to in te rm ed ia te -f oc us e ar th qu ak es ( 61 -3 50 k m ). t he in se t i nd ic at es th e di re ct io n of th e si x se is m ic p ro fil es d es cr ib ed in f ig ur e 3b . c om pu te d fo ca l m ec ha ni sm s ar e pr oj ec te d ov er th e lo w er h em is ph er e an d nu m be re d ac co rd in g to f ig ur e 3c a nd a pp en di x. b la ck a nd w hi te q ua dr an ts c or re sp on d to c om pr es si on s, a nd d ila tio ns , r es pe ct iv el y. 21 inferences on the geometry of the nazca plate in northwestern perú based on data collected by a local seismograph network f ig ur e 3. ( a) . v er tic al c ro ss -s ec tio n pe rp en di cu la r to th e tr en ch ( n 70 °e ) of th e se is m ic ity s ho w n in f ig ur e 2. t he d as he d lin e in di ca te s th e ge ne ra l t re nd o f se is m ic ity a cc or di ng to fi gu re 1 a. ( b) . v er tic al s ec tio ns o f se is m ic ity f ro m n or th to s ou th a lo ng th e di re ct io ns s ho w in g th e in se t o f fi gu re 2 . ( c) v er tic al s ec tio n of th e 22 e ar th qu ak es w ith c al cu la te d fo ca l m ec ha ni sm s nu m be re d ac co rd in g to f ig ur e 2 an d a pp en di x. t he e nc lo se d st er eo gr ap hi c pr oj ec tio n sh ow s th e di st ri bu tio n p an d t a xe s fo r th e 22 e ar th qu ak es . 22 hernando tavera, renzo vilca and gloria marin conclusions in a subduction zone, the spatial distribution of hypocenters and the characteristics of stresses, provide important information about geometry and local or regional deformation processes. for the nw border of the northern region of peru, the use of a local seismograph network during a 6-year period are oriented in ne-sw direction, parallel to plate convergence (stauder, 1975; cahill & isacks, 1992; tavera & buforn, 2001). the nodal planes were chosen so that the p (or t) axes were close to the n70°e direction (appendix 1). according to figure 2, the solutions obtained for the focal mechanisms are of inverse type (8 solutions) and normal type (14 solutions) suggesting that in the study area an heterogeneous stress field has developed, while results obtained from earthquakes with larger magnitudes show only mechanisms indicative of normal faulting. figure 4a shows the case of event 5 (see figure 1a) for which the polarity of the p wave is well defined at all stations. figure 4b displays the p waveforms recorded at five stations corresponding to events of comparable depth (see figure 3c) and shows a clear change in polarity of the p wave. those events were recorded by the same digital stations and have similar azimuths and take-off angles. figure 3b shows the depth distribution of the 22 earthquakes along with their focal mechanisms, showing the complexity of the deformation processes inside the plate, with the extensional-mechanisms events occurring above the compressional ones. this feature of a “double wbz” has been observed in other subduction zones as japan (fujita & kanamori, 1981; igarashi et al., 2001) and chile (comte & suarez, 1994; comte et al., 1999; rietbrock & waldhauser, 2004) with a double zone of seismic events separated by roughly 10 to 25 km. in this study, because of the limited number of focal mechanisms available and to the little separation between the earthquakes with compressive and extensive processes, the presence of a double seismic zone could not be confirmed. in general, the two families of focal mechanisms can be clearly separated as in figure 3c. for both groups, the t and p axes are sub-horizontal and oriented along the ne-sw direction. the origin of such a complex pattern of deformation is poorly understood; peacock (2001) and wang (2002) suggested that tensional deformation could have its origin in dehydration processes in the subducting plate. at intermediatedepth levels, water is released by dehydration of ecoglite causing an increase in its density and further decrease of its volume inducing the stretching of the oceanic crust and possibly producing earthquakes (kirby et al., 1996). shortening may be induced by figure 4. (a) inferred focal mechanism for event 5. note that the polarity data are close to the null axis and that a clear change of the polarity of p wave is observed. (b) set of p waveform at stations pcu, pma, mta, mpa, pch used to compute focal mechanisms of events of comparable depth. note the change of polarity of the p wave, down for tensional events (numbered 2, 15, 18, 11, 5 in figures 2 and 3) and up for the compressive ones (numbered 16, 12, 4, 14, 21 in figures 2 and 3). the stresses produced in the volume of the plate that controls its curvature. in this simple model it is the bending of the plate due to the subduction process that induces tensional (up-dip) and compressional (down-dip) stresses. 23 inferences on the geometry of the nazca plate in northwestern perú based on data collected by a local seismograph network (1996-2001) enabled the identification of shallow or intermediate seismicity in an area where earthquakes of m b >4.0 earthquakes are rare. the data suggest a predominance of intermediate-focus earthquakes in the centre of the study area (beneath the andes cordillera). at depth, earthquakes help to define the geometry of the subducting plate, which exhibits a 10° dip angle between trench and coastline, and 28° dip angle onshore down to 120 km depth. beneath the andes, the nazca plate is horizontal and earthquakes have a mean depth of 120 km as suggested previously based on scarce teleseismic data (stauder, 1977; tavera & buforn, 2001). the focal mechanisms for intermediate depth earthquakes beneath the local seismograph network suggest the development of complex deformation processes that could be attributed to small fracturing caused by stresses induced by the flexure of the subducting plate or alternatively dehydration processes. acknowledgements we would like to thank the geophysical networks team of the igp that kept operational the local seismic network in the northern region of peru. this work has benefited from the critical reviews of marcelo assumpcao, elisa buforn, diana comte, luis quintanar, hugo perfettini, natalia ruppert and anonymous reviewer. this is igp contribution n°022-cndg-igp/2004. appendix focal mechanisms obtained from the p-wave polarities for the 22 earthquakes described in the text (lower hemisphere projections). at the bottom of each solution appears the reference number used in the text. black and white circles correspond to compressions and dilatations, respectively. 24 hernando tavera, renzo vilca and gloria marin references • barazangi, m. and b. isacks (1979). subduction of the nazca plate beneath perú: evidence from spatial distribution of earthquakes, geophys. j. r. astr. soc. 57, 537-555. • bernal, i., and h. tavera (2002). geodinámica, sismicidad y energía sísmica en perú, monografía cdngsismología, instituto geofísico del perú, lima, perú, 70 pp. (in spanish). • cahill, t., and b. isacks (1992). seismicity and shape of the subducted nazca plate, jour. geophys. res. 97, 17503-17529. • ccallo, f., h. tavera, and i. valdivia (2002). la magnitud local y regional de terremotos ocurridos en perú a partir de la onda lg y la duración de su registro, boletín de la sociedad geológica del perú, 94, 61-69. 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(in spanish). • schneider, j., and s. sacks (1987). stress in the contorted nazca plate beneath southern peru from local earthquakes, j. geophys. res. 92, 13887-13902. • stauder, w. (1975). subduction of the nazca plate under peru as evidenced by focal mechanisms and by seismicity, j. geophys. res. 80, 1058-1064. • tavera, h., and e. buforn (2001). source mechanism of earthquakes in perú, journal of seismology, 5, 519540. • wang, k. (2002). unbending combined with dehydration embrittlment as a cause for double and triple seismic zones. geophys. res. lett. 29 (18), 1889, doi:10.1029/ 2002gl015441. geolisica colombiana pp 40-45 o r c l e m b r e de 2001 bogota 0 c. issn 0121 2974n° 5 sondeos electromagneticos inductivos: teoria, metodo y apttcaclon sandra esperanza sanchez s. departamento de geociencias maestria en geofisica universidad nacional de colombia luis alberto briceno g. profesor asociado departamento de geociencias facultad de ciencias universidad nacional de colombia resumen se estudia la teorfa electrornagnetica para encontrar la respuesta de un medio conductor homcgeneo con una intrusi6n conductora bidimensional, para el caso de una fuente unidimensional (alambre largo) y una bobina receptora. se desarroll6 un program a para encontrar las caracterfsticas de las partes real e imaginaria de la senal que dan informacion acerca de parametres como profundidad, dimensiones de la intrusi6n y contraste de conductividad. se muestra adernas la teorfa para la construccion de un modelo a escala en el que las medidas tomadas sean representativas de las tom adas en campo real. palabras clave: sonded electromagnetico, campos inducidos, prospeccion geoelectrica abstract in this paper we investigate the electromagnetic response of a two-dimensional target buried in a half space conducting earth. the electromagnetic source is an infinite wire. we develop a numerical program to find information about parameters like deep, conductivity contrast and target's dimensions. this information is obtained through the imaginary and real part of the response signal. furthermore, we show the theory of the scaling modeling in electromagnetic sounding to compare the theoretical results with laboratory measurements. key words: electrcmagnetic exploring. induced fields. geqelectaical paospicting introduccion siempre que un cuerpo conductor se halla en presencia de un campo electrornagnetico, se producen en su superficie corrientes de remolino 0 corrientes de foucault (landau y lifshitz, 1975), que son el resultado de aplicar la ley de induccion a cuerpos extensos; si el cuerpo conductor esta enterrado, estas corrientes seran de gran ayuda para su deteccion ya que elias generan campos electrornagneticos que pueden ser medidos en la superficie y ofrecen informacion acerca de las caracteristicas electricas y magneticas de dicho cuerpo. en la figura 1 se muestra de manera cualitativa el campo producido por una bobina (primario), el campo producido por la intrusion conductora (secundario) y como la suma de estos dos campos (resultante) es captada par la bob ina receptora. sin embargo, en / / /,,," ,,('--'" figura i. diagrarna generalizado de prospeccion electrornagnetica. las ifneas continuas representan el campo prirnario y las punteadas el campo secundario 40 muchos casos la respuesta de la intrusion se ve afectada par laconductividad del medio encajante que apantalla tanto el campo primario como el secundario. en este articulo se estudia el efecto de este apantallamiento tomando como base el trabajo de hohmann (hohmann y stanley, 1988), y se muestran los componentes del campo que ofrecen mayor informacion acerca de las caracterfsticas del cuerpo enterrado. fundamentosteoricos ei problema particular que se propone trabajar es calcular el campo secundario producido por un cuerpo bidimensional de conductividad 0 enterrado en un medio hornogeneo de conductividad 0* en presencia de un campo primario producido por un alambre infinito en el cual circula una corriente alterna (figura 2). se parte de las ecuaciones de maxwell, las cuales, en el dominic del tiempo estan expresadas asf: ab vxe+-=oat ad at o p vxh v b v d donde e es el campo electrico, b es la induccion magnetica, h es el campo rnagnetico, d representa el campo de desplazarniento electrico.j es la densidad de corriente y pes la densidad de carga. geof(sica colombiana, 5, diciembpe de 2001 sondeos electromagneticos inductivos, teoria, metodo y aplicacion 1f~/i1:o ..... ......n0.5 /~ .><, / -, <, / <, <, ........i ini··.. __ . i 2 3 -l .5 (i 7 8 9 10 ii:.:/i/:u -0.5 ----iiji// ........__ / --/ -. / +05 re figura 2. componentes x y z del campo magnetico producido per un hila infinite de corriente en un semiespacio conductor mediante el tratamiento de estas ecuaciones y usando la aproximaci6n cuasiestatica (corell ana, 1974), en la cual se considera que las conientes de desplazamiento son pequeiias comparadas con las de conducci6n, y que por tanto pueden despreciarse, se obtienen las ecuaciones de helmholtz (hohmann y stanley, 1988) tanto para los campos como para el potencial vectorial. v'2e + ee = 0 v'2h+eh=o(l) v'2a+ea=o donde k es conocida como la con stante de propagaci6n 0 numero de ondas por radian. las ecuaciones (l) son ecuaciones de difusi6n y tienen soluciones de la forma: ax neik'e-im b .r -iyne ik: e-im ex iwne ikz e iwi donde n es una con stante por definir. estas expresiones muestran que el vector potencial, al igual que los campos y por tanto la densidad de coniente, presentan una variaci6n sinusoidal en el tiempo, y decrecen exponencialmente con la profundidad. este decrecimiento depende del factor de atenuaci6n, el cual, dentro de la aproximaci6n cuasiestatica, es: geofisica colombiana, 5, diciembre de 2001 donde cd es la frecuencia, j.t la permeabilidad rnagnetica y p la resistividad. la profundidad a la cuallos campos han decrecido hasta e' de su valor en superficie se denomina profundidad de penetraci6n 0 profundidad piel, y esta definida asf: 1 [wj.1 j-ii5: __ _ u b 2p es muy cornun en geoffsica hacer uso de los potenciales de shelkunoff, a y f, mediante los cuales los campos electrico y magnetico pueden expresarse como: h v x a e -v x f para facilitar las labores de calculo, suelen utilizarse los modos de propagaci6n transversal elecrrico (h ),en el cual el vector campo electrico es normal al plano de incidencia, y transversal magnetico (tm), en el cual el vector campo electrico esta en el plano de incidencia. en 10 que sigue se trabajara en uno de estos dos modos, segun sea conveniente. problemas tales como el campo producido por una fuente lineal en el vacfo y el campo producido por la misma fuente en presencia de un semiespacio conductor homogeneo se pueden resolver directamente. para la fuente lineal en el vacfo se tienen las siguientes expresiones p "a los campos (orellana, 1974): e(x, z ) = iojj.11 [( 2 2 )ii juv-~ «, ik x + z h(x,z) in [j( z x j k, ikr u x u z 2n r r donde r = (x 2 + z 2 ) ii , ko yk, son las funciones modificadas de bessel de segunda especie de orden cero y uno, respectivamente (sanchez, 1999). n6tese que en la superficie (z = 0) el campo h posee iinicarnente componente z, y adem as en la aproximaci6n cuasiestatica k/ikr)=liiki; y por consiguiente se tiene que en la superficie el campo decae con el inverso de la distancia lateral a la fuente y esta desfasado -900 ( con respecto a la corriente que circula por ei alambre. para el alambre largo sobre un medio conductor hornogeneo los campos e y h son: e; (x, z ) 201 j 1 e"a' cas(ax)dan 0 uo + u, 1 = uo--f e l/oz cas( ax) dan 0 uo + u, 201 f ae"o~ sen(ax)da2n o u 0 + u, 41 hx (x, z ) h~(x,z) i donde u" = (k ,; k" 2 )2, i es la corriente que circula por el alambre, z 0 la impedancia del aire y a=k,; estas ecuaciones estan expresadas en forma integral y por consiguiente es diffcil visualizar su contenido. en la figura 3 se muestran los dos componentes del campo rnagnetico. y z figura 3. cuerpo conductor sumergido en un semiespacio conductor. la ifnea de corriente esta en la direccion y cuando una intrusi6n conductora esta sumergida en el medio conductor, esta produce un campo electrico liamado dispersado, que viene dado por: e d = f is j d (x i , z i )g (x, z ; x i , z') . dx i d: i donde jd es la coniente dispersada debido al contraste de conductividad: j d = (a 2 a i )e y y g es la funci6n de green adecuada para este problema, es decir, el campo producido por un hila de corriente infinito y su hila imagen. la integral se extiende a la zona donde jd #00 sea sobre la cara de la lamina: c(x, z ; x', z') iojj.1 0 ---x 2n f~[ u u , ] cos a(x x ')i 0 e -"1 (z+z) + e -", (,-z') dao ul + uo uj con el fill de realizar las integrales .por un rnetodo numerico, se dividi61a lamina en n celdas iguales y sobre cada una el campo electrico se considera constante. asf el campo electrico queda: e i (xm ' r.;y n + l (a a. "fiy (xiii' yiii)' n=l 42 sanchez et al. donde g, ygp son, respectivamente, la parte dispersada y primaria de la funci6n de green y ~ es la longitud de la celda. esta ecuaci6n se puede escribir en forma matricial: ze = e p con: e e n-2 en_i en y: {z}"1i1 8 "ill (a a * )g ill" g"'11 x' z zlwx' dz'+'111 }t. ~2gs(xii1 -x,,;zil1 +z,,) una vez solucionado este sistema de ecuaciones, se conoce el campo en cada celda y par tanto el campo total: e; (x, z) = e~ (x, z ) + l'. 2i, (a o , )e"g(x, z: x"' z,,) y los componentes del campo rnagnetico se siguen inmediatamente: hx(x,z)= iwj1 0 de,(x,z) ----; h _ (x, z) dz . dey (x, z) dx resultados y conclusiones en las figuras 4 a ii se muestra c6mo varia la parte real e imaginaria de los componentes del campo magnetico en funci6n de la profundidad, altura y contraste de conductividad de la intrusi6n. en las graficas que se presentan en esta secci6n se muestran las componentes vertical (hz) y horizontal (hx) del campo magnetico dispersado por la intrusi6n. en cada una de elias se vera la influencia de factores tales como la profundidad, la altura de la intrusi6n y el contraste de conductividad entre el medio conductor y el cuerpo enterrado. antes de cualquier otro calculo, se analiz6 la convergencia del metodo respecto al numero de celdas en que se divide la intrusi6n; para tal fin se dividi6 una placa de 30 metros de ancho por 90 metros de alto en geoff sica colombiana, 5, diciembrr de 2001 sondeos electromagneticos inductivos, teoria. metodo y aplicacion o 0 0 add dod o 0 0 0 0 0 0 o 0 0 0 -10 h"50m h"100m h"200m ~ -20.g ~ -9 ~ -30 3l .£q -40 .... ..... . . ...... ... ·... . . -50 200 300 400 500 600 700 800 900 recorrido (rn) figura 4. fase del campo rnagnetico horizontal en funcion de la altura de la intrusion. t = 10m. contraste = 103 , d = 50m · d=20m 4.ox10-4 · 0 d=50m· d=100m·d=200m~ d=2000m 30x10-4 ·e ~ ·0 ~ 20x10-4 0 0 •ro 0 •~ 0 0 • ··1.0x10-4 • ·0~. . ~ • i ii iii ii .····· ii ii ii ii .. ... ii ii ii .... ..... 00 200 300 400 500 600 700 800 900 recorrido (m) figura 6. parte real del campo magnetico horizontal en funci6n de la profundidad de la intrusi6n. t = 10m. contraste = 10j, h = 100m 3, 12 y 27 celdas, y se graficaron las partes real y la fase para cada caso (ahem, 1973). se encontr6 que con un numero superior a 12 celdas se obtiene una convergencia aceptable para la parte real, mientras que la fase no present6 cambios significativos. para 10que sigue se analiz6 una intrusi6n de 10 metros de ancho y se tomaron celdas de 10 metros cuadrados de area. efectos de la al tura de la intrusion la amplitud de los campos hx y hz ofrece poca informaci6n ace rca de la altura de la intrusi6n, debido a la atenuaci6n que sufren estos con la profundidad; sin embargo, las curvas para la fase en ambos campos presentan picos considerables a medida que la altura aumenta; esto lleva a pensar que posiblemente estas cui-vassean utiles para determinar la altura de las intrusiones (figuras 4 y 5). sin embargo, como se ve mas adelante (figuras 8 y 9), un efecto similar se tiene cuando la progeofisica colombiana, 5, diciembre de 2001 u · h=20m 0 h=50m -10 · h=100m.. · h"200m. ~ -20en 0 -0~ 0-9 -30 . ~ . • . • i-40 . q) ..en ...ll1 -50 -60 200 300 400 500 600 700 800 900 recorrido (m) figura 5. fase del campo magnetico vertical en funci6n de la altura de la intrusion. t = 10m, contraste = 10j. d = 50m . d=10m 2.0x10-4 0 d=20m. d=50m 0 d=100m~ d=200m 1.0x10-4 ..~. e 0 . g . 0 0 0 ~ s ~ 0 0 0 ~ ~ ~ ~~~ ~~ ~ 0 0.0 ~ ~~n ~ ~~ ~~ ~ ~ ~ 0 0 0 0 0 0 e ~ ..~ a::: -1.0x10-4 . . -2.0x10-4 200 300 400 500 600 700 800 900 recorrido (m) figura 7. parte real del campo magnetico vertical en funci6n de la profundidad de la intrusion. t = 10m, contraste = 10" h = 100m fundidad a la que se encuentra el cuerpo es muy pequefia. se concluye, por tanto, que la altura de la intrusi6n no se puede determinar mediante este metodo. efectos del contraste de conductividad ei contraste de conductividad puede apreciarse mejor en la parte real del campo hz (figura 10). para hz la pendiente a los costados de la intrusi6n es mayor a medida que el contraste aumenta. hx (figllra i i) no permite estlldiar el efecto del contraste de conductividad, ya que estas curvas presentan la misma forma para todos los casos. efecto de la profundidad la pendiente de hx a los lados de la mueslj' (figllra 6) y la pendiente de hz aniba de la inljusi6n (figura 7) dan informaci6n sobre la profundidad del cuerpo, puesto que a mayores pendientes, menores profundidades. 43 sanchez et al oy-----------------;::====il -5 -10 -15 -20 -25 '§' -30 ~ -35 oj -40 ::::: -45 x ~ -50 ql -55 ~ -60 -65 -70 -75 ~ ~ ~ t y • • • • • • • • • • • • • • • -80 +-~-r_~__,-~_,-~_r---,~-,--~____j 200 . . • o::20m o d=50m • oo100m o o=200m ~ oo2000m. ..... . o 0 . o 0 0 0 0 0 .. o odd 000 ...... ..... ......... <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> 300 400 500 600 700 800 900 recorrido (m) figura 8. fase del campo rnagnetico horizontal en funci6n de la profundidad de 1a intrusi6n. t ~ 10m, contrasre ~ 10" h ~ 100m e ~ 15x10,4 f 1.0x10 ,4 ~ o contraste 1000e • contrasle 1000 • contraste 100 [] contraste 10 25x10a 2.0x10,4 5.0x10,5 000. o 0 o.o+-~-.._~-r_~__,-~-,--~--r-~_,~-_i 200 300 500 600 700 800 900 recorrido (m) 400 figura 10. parle real del campo magnetico horizontal en funci6n del contrasie de conductividad. t ~ 10m, d ~ 50m, h ~ 100m cabe anotar, sin embargo, que las fases mostradas en las figuras no son las medidas en prospecci6n, ya que la fase que se mide en campo depende del rnetodo utilizado para tal fin,asi, por ejernplo, para el turam (orellana, 1974) las medidas tomadas son las diferencias de fase entre dos posiciones consecutivas de las bobinas receptoras, en tanto que en otros rnetodos se mide la diferencia de fase con respecto al campo prirnario. en todas las figuras que se muestran se ha graficado el valor absoluto del campo dispersado hx, y para todos los casas la intrusi6n esta ubicada a 600 m de la fuente. modelo a escala comprobar experimental mente 10 que se ha asumido en los items anteriores es bastante diffcil debido, principal mente, a las grandes longitudes que se manejan (",300 m a 3000 m). para tal fin se propone la construcci6n de un modelo reducido can base en la invarianza de esca44 -15 -20 -25 -30 '§' -35 '0 ~ -40 oj ::::: -45 ~-50 ~ -55 -60 -65 -70 ~.~. o 0 0 .~.~ • d=10m o o=20m • o=50m o o=100m o o=200m~. ~. .~.~ 200 700 800 900 recorrido (m) 300 400 500 600 figura 9. fase del campo magnetico vertical en funci6n de la profundidad de la intrusi6n. t ~ 10m, contraste ~ 103 , h ~ 100m 2.0x10,4 i ~ co ~ -10x10,4 -2.0x10,4 1.0x10,4 !~.... • contraste 10000 contraste 1000 contraste 100 'v contraste 10 0,0 200 400 500 600 700 800 900300 recorrido (m) figura /1. parte real del campo rnagnetico vertical en funci6n del contraste de conductividad. t ~ 10m, d ~ 50m, h ~ 100m la de las ecuaciones de maxwell (abem, 1973). en general, las longitudes, los campos y el tiempo deben ser escalados: x px' y py' z pz' e (x, y, z, t ) ae l (x' , y' , z' , t') h (x, y, z, t ) = bh l (x' , y' , z' , t 1 ) t f ct' f' c geof(sica colomblana, 5, diciembre de 2001 sondeos electromagneticos inductivos, teoria. metodo y aplicacion a partir de esto se tiene la relaci6n de escala para la permeabilidad magnetica, conductividad y permitividad electrica: pb ji'=-ji ea pa (j'=-(j b y pa e'= -e be y si se considera que los materiales utilizados no son ferromagneticos, 11=11',se tiene: pb= 1 ae pb a e y: 2 (jl=l(j e y para el voltaje la corriente y la impedancia: v'= e v p2b ap i' = i ae z'= ~z p una vez conocidas las relaciones entre los distintos factores de escalamiento, se deben buscar las magnitudes de campo real para el problema especffico poi' estudiar. en nuestro caso, debido a que el campo puede ser generado por un alambre recto y largo con sus extremos puestos en tierra 0 un lado de un bucle rectangular extenso con dimensiones entre 200 y 800 metros, la caja en la cual vaya el material que simule el terreno debe tener un ancho de 20 a 80 centimetros con el fin de facilitar su ubicaci6n en el laboratorio. portal motivo el parametro de escalamiento p debe ser, aproximadamente de 103. las lineas a 10largo de las cuales se realiza la prospecci6n suelen llegar hasta un kil6metro; por este motivo, y teniendo en cuenta el factor pya encontrado, se deduce que ellargo de la caja debe ser de un metro. la profundidad del recipiente debe tomarse de acuerdo con la conductividad del material utilizado para simular el semiespacio, puesto que esta debe ser igual 0 un poco mayor que la profundidad de penetraci6n. para conocerla se espera entonces conocer los valores de resistividad y frecuencia utilizados. al escalar la conductividad y la frecuencia hay que tener en cuenta que tarnbien para el modelo de laboratorio debe ser valida la aproximaci6n cuasiestatica (es decir que ecd«cr, y cd no debe ser superior a 105 hz); portal raz6n, si para el trabajo de campo se utiliza una frecuencia de 800 hz y se quiere utilizar en el modelo una frecuencia de 105 khz, el factor de escala para el tiempo sena c = 125; con este geofisica colombiana, 5. diciem8re de 2001 dato y con el conocimiento de p, se puede ahora encontrar el factor de esc ala para la conductividad: p2 se sabe que a' = -(j ; poi' esta raz6n se tiene que la c conductividad utilizada en el modelo debe ser a' = 8 * 103 cr. ahora, si 10 que se quiere simular es una mena de pirita cuya conductividad esta entre 100 y 6000 s.m, se debe utilizar en el modelo un blanco de aluminio, que tiene una conductividad de 4* 107 s.m, u otro material de conductividad parecida. si se busca un contraste de conductividad entre el blanco y el medio conductor del orden 106, es necesario que el material que simule el terreno tenga una conductividad del orden de los .m. esta conductividad se puede lograr con soluciones de agua con naci, a un porcentaje del 5% del peso del agua. esta proporci6n representa para el terreno real una conductividad aproximada de 1.25*10·3s.m, el cual es un valor tipico en prospecci6n. con estos datos obtenidos para la conductividad y para la frecuencia se obtiene una profundidad de penetraci6n de 50 centimetros; portal raz6n la profundidad del tanque puede escogerse de unos 70 centfmetros. debido a que los diametros de las bobinas utilizadas en campo son generalmente menores a un metro (entre 10 em y 50 em), las bobinas del modelo deberian tener un diametro del orden de unos cuantos rnilimetros (debido al factor de escala utilizado). referencias bibliograficas abem, 1973: geophysics and electronics, atlas copco. ahb 30-05. hohmann,g. w, 1971: geophysics, v.36, 101. hohmann, g. w, stanley, h. w 1988: electromagnetic methods in applied geophysics, editado poi'nabighian, m. n., society of exploration geophysicists, tulsa. landau,l. d., lifshitz, e. m. 1975: electrodinamica de los medios continuos, barcelona, reverte. orellana,e. 1974: prospecci6n geoelectrica, paraninfo, barcelona. sanchez, s. e., 1999: sondeos electromagneticos inductivos: teorfa, metodo y aplicacion. tesis de grado. depanamento de ffsica, facultad de ciencias, universidad nacional de colombia. 45 geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 44-62 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt magdy a. atya1, olga a. khachay2, aiman abdel latif1, oleg y. khachay2, gad m. el-qady1 and ayman i. taha1 1 national research institute of astronomy and geophysics, helwan, cairo, egypt. 2 institute of geophysics ud ras, ural’s state university, russia. abstract the geothermal potentiality in egypt has a minor significance in the aspects of the egyptians life, while the hydraulic and hydrocarbon resources are more convenient. however, some other applications for the geothermal activity such as direct warming, pools, and physiotherapy make the research for geothermal as requested. in the present work, two locations with rather good geothermal potentiality will be studied; these are hammam faraun and abu swiera (water temperature is about 70 °c; at sinai peninsula). the contribution of the geophysical techniques to evaluate such potentiality could be considered, as its capability to identify the reservoir characteristics and its implementation is acceptable. therefore, a geophysical survey program has been conducted in terms of seventeen vertical electrical soundings (ves) and two wide profiles of control source electromagnetic (csem) forward step at hammam faraun and two wide profiles of csem at abu swiera. the geophysical techniques yield information on the spatial distribution of electrical conductivity, which is the most sensitive parameter to fluids in the rocks. the analysis of the geophysical data, together with the field and geochemical studies lead to the conclusion that, the thermal water in the subsurface formations might be considered as the preferred cause of the high conductivity in the subsurface on/close to the boarder of tectonically active regions, particularly, where the anomalous conductivity is correlated with high heat flow and other geophysical and geological parameters. key words: hammam faraun, abu swiera, vertical electrical soundings, control source electromagnetic, conductivity of subsurface. resumen la potencialidad de la energía geotérmica en egipto tiene una importancia menor en los aspectos de la vida de los egipcios, comparado con los recursos hidráulicos y de hidrocarburos. sin embargo, algunas aplicaciones de la actividad geotérmica, como el calentamiento directo, las piscinas y la fisioterapia, hacen de la investigación de la energía geotérmica necesario. en el presente trabajo, dos localidades con potencialidad geotérmica bastante buena serán estudiados; éstos son faraun y abu hammam swiera (la temperatura del agua es de aproximadamente 70 °c, en la península del sinaí). 44 manuscript received: 03/02/2010 accepted for publication: 14/04/2010 debe considerarse la contribución de las técnicas geofísicas para evaluar el potencial así como la capacidad para identificar las características del reservorio, y su aplicación. un programa de prospección geofísica se realizó por medio de diecisiete sondeos eléctricos verticales (sev) y dos de gran perfil con fuente electromagnética controlada (csem) en hammam faraun y dos grandes perfiles csem en abu swiera. las técnicas geofísicas muestran información sobre la distribución espacial de la conductividad eléctrica, que es el parámetro más sensible a los fluidos en las rocas. el análisis de los datos geofísicos, junto con el campo y los estudios geo-químicos llevan a la conclusión de que, el agua termal en las formaciones del subsuelo podría ser considerada como la causa de la alta conductividad en el subsuelo o cerca del borde de la regiones tectónicamente activas, en particular, donde la conductividad anómala se correlaciona con el alto flujo de calor y otros parámetros geofísicos y geológicos. palabras clave: hammam faraun, abu swiera, sondeos eléctricos verticales, control de código fuente electromagnética, conductividad de superficie. 1. introduction two sites have been selected to study the geothermal occurrences in the central part of sinai peninsula; hamam faraun and abu swiera close to the gulf of suez (figure 1). as shown, the studied areas are located on the eastern side of the gulf of suez region which represents one of the most prolific and prospective oil provinces in egypt. the area includes a number of oil fields such as assel oil field. hamam faraun lies at 29° 11´ 55.18``n and 32° 57´ 18.64´e and the elevation is about 9 m above sea level, while abu swiera area lies 29° 32´ 47.44``n and 32° 48´ 38.60´e and the elevation is about 44 m above sea level. to spot more light on hamam faraun and abu swiera hot springs, we carried out vertical soundings (ves) and an approach to utilize planshet control source electromagnetic (csem) wide-profiles crossing the geothermal active fields. the main aim of this study is to analyze the geophysical, geochemical, and also the geological feed back in order to define the significant tectonic patterns, which are responsible for the structural development of the geological units at the area of study. the study resulted in understanding of the hydro-geothermal regime at hammam faraun and abu swiera through the evaluation of the subsurface imaging and the capability for investment projects. 2. geology and tectonics of central sinai peninsula the central part of sinai province represents a tectonic specific belt put it as one of the most prolific and prospective oil provinces in egypt. in the early tertiary period (oligocene miocene), where the opening of the red sea rift, some volcanic activity took place. in western and central sinai, there are a number of basaltic bodies mostly of doleritic dikes, sills and plugs; flows are known near to abu zenima and hammam faraun (meneisy, 1990). the major geological structural feature of the study area is a well-defined fault block oriented nnw-sse, which tilts strongly eastward on its western side. also there is a fault escarpment overlooking directly the gulf of suez and rising about 300 m above the gulf (el-shinnawi, and sultan, 1973). hamam faraun area is one of more vigorous topography because of the presence of several local mountain areas (gebel hammam faraun and gebel tall) of altitudes varies from 50 to 480 m above sea level. the shallow geological succession in hamam faraun area (fig. 2) could be, mainly, distinguished into the following formation (said, 1962): 1. post-pliocene composed of sand and conglomerate of about 50 m thickness. 2. pliocene rocks, differentiated into sand, sandy limestone, and lagoonal gypsum deposits of thickness about 50 m. 3. miocene and oligocene rocks which composed of gypsum, sandy marl and conglomerate with total thickness of about 70 m. 4. eocene rocks of total thickness 100m, which can be differentiated into, upper eocene of limestone and sandstone, middle eocene of shale and limestone, and lower eocene of chalk and flinty limestone. 5. upper cretaceous rocks which are composed of shale with minor limestone. as for abu swiera hot spring, from the geological point of view, the area is a triangular excavation represents the synclinal trough to the southwest of el-halal anticline massif. 45 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt the head of this triangle is directed northward while its base measures a length of about 25 km facing gebel kharim in the south. the length of this plain attains about 50 km. its ground elevation decreases northward from 300m at its base to about 190 m at its apex. the surface of these plains is dotted by numerous mesas and buttes, which formed of early eocene limestone with cherty bands. this surface is dominated by washed, dark angular to sub-angular cherty and limestone fragments. locally, these plains are covered by active sand dunes, which form several lines of isolated branches. the area is controlled by three different tectonic provinces related to northern part of the african plate (said, 1962). these are the northern egypt fold belt, ne-sw (syrian arc system) and the suez rift which is younger than north egypt fold belt, is dominated by nw to nnw oriented normal faults. 3. hydrothermal regime at the study sites at hammam faraun, the spring issues thermal water at the foot of the gebel from fractured karstifed dolomitic eocene limestone formation. the spring seepages were estimated to 880 m3/d (elramly, 1966), 3000 m3/d (himida and diab, 1976). these seepages are drained through minor channels mixed with the gulf water. the thermal seepages are tested from hammam faraun cave. h2s gas is present with its characteristic odor. the thermal water seepage and flow toward the gulf water. issar et al. (1971) proposed a thermal flow mechanism in which a sequence of aquifers is interconnected by means of a system of faults and fractures zones, resulting in the occurrence of springs at the surface. the contribution of various aquifers to the total outflow of spring is regulated 46 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha mediterenean sea bahariya oasis oasis oasis oasis dakhla kharga g u l f o f s u z e red sea eastern desert western desert farafra cairo 29 00 ` 00 `31 33 00 ` 3500 `27 00 `25 00 ` 31 00 29 00 27 00 25 00 qattara depresssion ayun musa ain sukhna 12 9 6 13 10 11 8 15 167 5 4 3 1 2 14 scale :0 40 80 100 km. legend:hot wells hot springs 32° 42’ 17” 32° 58’ 09.78” 29° 39’ 27.57” ras sidr abou swira 29° 11’ 29.40” gulf of suez scale hammam faraun 0 13.5 27 km 32° 58’ 09.78” n w e s figure 1: location map of the studied area. 47 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt ras sudr ras matarmah legend gulf of suez ras malab abu aggag fm. duwi fm. esna fm. g. el-ahmar fm. gharandal group matulla fm. ras malaab group sabkha deposits sudr fm. tertiany alkali olivine basalt thebes group/egma fm. wadi deposits alluvial deposits scale 0 7,000 14,000 28,000 meters figure 2: geologic map of the studied area. by their relative and absolute potentials. all the pressure in these formation caves depleted with time, an eocene aquifer located at higher level began to flow and continues to do so to the present day. elrefeai (1992) and el-kiki et al. (1992) carried out a cluster analysis of the hydrochemical variables for wells and springs tapping the deep aquifers in the gulf of suez region. the produced dendrogram differentiate the eocene aquifer from the underlying (nubia aquifers) and the overlying miocene one. in this dendrogram the hammam faraun spring was designated as a member in the cluster of eocene aquifer in the region.the underlying esna shale is considered as the aquiclude, which separates the eocene beds from the underlying aquifer (nubia). however, the interconnection through the faults and fracture zone should not be completely disregarded. issar et al. (1971) conceived an evaluation history of the formation waters in the gulf of suez region which the eocene aquifer is a product of mixing, through fracture system, between ancient seawater and pleistocene meteoric water. unfortunately, no more work related to abu swiera hot spring. 4. approach to use the control source electromagnetic (csem) concept of csem the demand to the enhanced geophysical technique and device, in addition to the precise analytical interpretation of the geophysical data, is the resolution of the geophysical complex research, especially by the absence of priory information about the researched place. the device for use the planshet method of electromagnetic induction (hachay o. a., 1997a, hachay o. a. and bodin, 1997b, hachay o. a. et al., 1999, and 2000, hachay o. a., 2002 ) in the frequency domain was developed by chelovechkov a.i. the method was adapted to map and monitor the high complicated geological mediums, to determine the structural factors and criteria of stability of the rock massif in the mine subsurface. the field observation and the way of interpretation make the new technology differ from other known earlier methods of field raying or tomography (hachay o. a. and novgorodova e. n., 1997c, hachay o. a. et al., 1999, and 2000, and lau k.h., and cheng p. 1977). the concept to research the 3d geoelectric medium is based on interpreting the alternating electromagnetic field in the frame of a block-layered isotropic medium with inclusions over three stages (hachay 1997a, and 2002); in the first stage, the geoelectric parameters of the horizontal block-layered medium, which includes the block heterogeneities, are defined, in the second stage, a geometrical model of the different local heterogeneities or groups inside the block-layered medium is constructed based on the data of local geoelectrical heterogeneities, while in the third stage, the surfaces of the searched heterogeneities could be calculated in account of the physical parameters of the anomalous objects. obtainable field data for practical realization of that conception, the system of observation for alternating electromagnetic field with use of vertical magnetic dipole was elaborated. such local source of excitation and regular net of observations allows realizing overlapping by different angles of observation directions. as incoming data for interpretation, modules of three components of magnetic field are used. for the case on surface observations the data are measured on the earth’s surface at the set of distances between the source and receiver as a function of frequencies. the measurements of the module of three components of the magnetic field (vertical ��z� and two horizontal: one directed to the source �hr� and second perpendicular to that direction �hö�) are provided in the frame of planshet for the fixed net with fixed step and fixed length of the planshet’s side. in the frame of profile observations the planshet become to a band or a line and the length of the band or the line is a base of observations or an array. for the variant of the wide profile (band) the source of excitation is located at the beginning of the array on the profile, which is parallel to the measured profile. we shall call that a wide array. it moves systematically with a fixed step of meters. for the variant of a usual profile the source is located on the measuring profile and the moving of the oscillator is similar. for the variant of a planshet survey the source is located into the center of the planshet using the fixed net of observation. then the planshet array moves systematically with overlapping usually on the half of the planshet. processing and interpretation of csem data for each array and fixed frequency ù two interpretation parameters are defined: ñeff (r)=ùr2(�hz�/�hr�)/ð ä(r) =(�hö�/�hr�)100%. where ñeff (r) is the apparent resistivity, r is the distance between the source and the point of observation, ù is the frequency f multiplied by 2ð, �hz� is the module of the vertical 48 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha magnetic component, �hr� is the horizontal component of magnetic field oriented to the source, ð = 3.14, ä(r) is the parameter of geoelectrical heterogeneity, and �hö� is the second horizontal component of magnetic field perpendicular to the direction of the source. that data are the information base for the further interpretation. on the first stage we define the geoelectrical parameters of the 1-d section for each array and each frequency after the preliminary filtration of the data: ñeff (r): ä(r)<a where a is a level of data filtration to the area of inverse problem operator for one-dimensional medium (hachay o. a., 1997a). the interpretation is made in a frame of n-layered model for each array and planshet location. after that each point of the planshet is associated with one and only column of layers thicknesses and corresponding fixed column with resistance of the medium in that layers. gathering information of all planshets together we obtain a many-valued function of each point – distribution of thicknesses and resistances of the medium layers. then we calculate the average value for these distributions for each point of the observation set. thus we obtain the unique distribution of thicknesses of horizontal layers and resistances, which corresponds to the medium model as a cylinder with vertical axis and with a rectangle at the bottom and with a point of observation located in its center. thus we change over layered model to a block-layered model. then, gathering the values of thicknesses and resistances for all points of observation, located on one and the same profile we obtain the file of an average cross-section along the profile. the next step is combining the neighboring blocks with close-range values of resistance to one block. that operation is made according to the fixed scale of resistance. the second stage of interpretation is used to define the geometrical characteristics of conductive inclusions and their equivalent moments, which are proportional to the ratio of the conductivity difference in the host rock and in the inclusion to the conductivity in the host rock. here the approximation principle is used for alternating electromagnetic fields. the initial model of the inclusion is a current line of fixed length. that approximation construction is used for fitting of the average parameter of geoelectrical heterogeneity, which is calculated as an average value of ä in each point of the profile, located in each point of the profile (hachay o. a., 2002). 5. hammam faraun site hamam faraun area is characterized by several hot springs lying at the foot of gebel h. faraun along, a more or less, straight line extending about one km along the gulf of suez coast. the fault forms a steep cliff bordering the gulf of suez. two groups of thermal springs are found. the northern group (70 �c) issues on land while the southern group emerges beneath the surface of the gulf’s water. the cliff above the spring is composed of dolomite. a geological examination of the area (moon and sadek, 1923) indicates a gradual progressive change from dolomite through dolomitic limestone to limestone as one proceeds from the cliff inland. the heat source for these springs is probably derived from high heat flow and deep circulation controlled by faults associated with the opening of the red sea and gulf of suez rifts. it is supposed that the spring’s water is presumably a mixture of brine and water of pleistocene contained in the nubian sandstone aquifer which infiltrated the outcrops on the highland of sinai and emerged along the fault lines bordering sinai peninsula (magaritz & issar, 1973). 6.1 geophysical measurements 6.1.1 vertical electrical soundings (ves); at the area around hammam faraun, seventeen vertical electrical sounding (ves) have been surveyed to ab/2 up to 1000 m in successive steps. the ves locations have been chosen on the basis of the accessibility and applicability of the schlumberger method (figure 3). the apparent resistivity curves have been inverted into one dimension model using the least square method to give a multi layered resistivity model whose response fits the measured sounding curve. zohdy’s method has been applied to the observed data. the output of zohdy’s method has been used as a starting model in the marquardt’s method to get the best fit for the observed data. figure (4) shows the resistivity distribution in the study area. a general outlook to the interpretation of the veses curves reveals that, the number of the interpreted layers is varying from five to seven layers through the study area. the true resistivity of these layers is varying from 0.2 to over 3000 ohm-m, while the thickness varies from 0.9 to 291 m. el-qady et al, (2000) had conducted the 2d inversion for the same data set using an algorithm based on the abic (akaike bayesian information criterion, akaike, 1980) algorithm to obtain convergence to a solution with the optimum smooth49 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt ness using a finite element calculation mesh. the integrated 2-d geoelectric cross section is illustrated in figure 5. the analytical reread of the site via resistivity distribution shows the relation between the hot springs place and the extensional cave inside gebel faraun, while this place describes a resistive strata setting forming the massive base and extending inward the mountain, where the cave shows higher temperature as man goes more inside. then it is supposed that the hot water comes through the fractures connected to this base. 5.1.2. csem data the observation system at hammam faraun includes the measurements along two wide profiles (fig.3); a longitudinal profile on the road band between the mountain and the shoreline of the red sea, and a traversal profile. the longitudinal wide profile is of 400 m length and the band width of the profile is 20 m, the distance between the observations points is 25 m, and between the source locations is also 25 m. the outlet of thermal hot waters is located on the profile between the 4th and 5th pickets of the observation points. the traversal wide profile begins at the sea shoreline traversing the road band and climbing the mountain margins, its length is 95 m and the band width of the profile is 10 to 15 m, the observations are carried out on 5 m separated points, while the distance between the sources of excitation is 15 m. the transversal profile crosses the longitudinal wide profile at the pickets 17th and 18th. the distribution of the average parameter of the geoelectrical heterogeneity at referenced frequencies for both the longitudinal and traversal wide profiles is shown in figure (6a and b) respectively. on figure (6a), for the frequency 5.08 khz and starting from the 7th picket of observation, local anomalies and raising of the background component of the parameters could be clearly noticed, the largest values of the average parameter of geoelectrical heterogeneity are recognized for the frequency 2.54 khz on the pickets 13 and 15. the values of the traversal wide profile (fig.6b) are significantly lower in comparison with the longitudinal wide profile, which testifies the more homogeneous structure of the massif in the transversal direction. approximately to the middle of the profile (picket 11) there are anomalies in the distribution of the parameter on the frequency 0.635 khz, for the second part we see the increasing of its values for higher frequencies, what can testify the more homogeneous structure for that places on the deeper depths. 50 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha gulf of suez topography (100 m) hot spring water well sinai mang. company ves’s location & number csem profile figure 3: location map for the geophysical survey at hammam faraun area figure (7) shows the geoelectrical cross sections with blocked structure and distribution of crack zones in the massif at the three controlled excitation frequencies. on the sections, the contact of the block structure on the pickets 3 and 17 could be sharply seen. the visual inspection and the geological survey show that zones are looking from the surface as faulted and cracked zones. the geoelectrical structure of the block from picket 3 to 17, beginning from the depth 92 m practically does not change its characteristics. it testifies that for increasing the resolution for deeper areas we must increase the base of observation and use lower frequencies with the same step of observation. approximately up to 72 m on the blocked sections, the thermal outlet is situated together with cracked and very conductive rocks, around the 11th picket of observation, the cracked zones reach about 40 m depth. on the section representing the frequency 2.54 khz (fig. 7b) and at the pickets 2–6, the block of rocks with low resistance, which are linked with the zones of outlet and location of high hot thermal water is well mapped. on the same section we see a near surface structure (up to 12 m), which is linked with the outlet of the vertical fault zone, which is linked with the contact. the results of observation on the frequency 0.635 khz testify and increase that conclusion. thus the provided electromagnetic induction observations on the three frequencies give the information about the geometry of the thermal reservoir and also about the degree of rocks faulting, of which consists the block. the last information 51 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt gulf of suez gulf of suez gulf of suez gulf of suez figure 4. resistivity contour maps at different values of ab/2 at h. faraun area. 52 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha figure 5: integrated 2-d geoelectric cross section at hamam faraun hot spring area (el-qady et al., 2000). 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 �, % 2 4 6 8 10 12 14 16 5.08 khz 2.54 khz .635 khz (a) the longitudinal wide profile n figure 6: distribution of the average parameter of geoelectrical heterogeneities along the longitudinal and the traversal (a) and (b) respectively wide profiles (fig. 3). �, % 100 90 80 70 60 50 40 30 20 10 0 0 2 4 6 8 10 12 14 16 18 20 5.08 khz 2.54 khz .635 khz (b) the traversal wide profile n 53 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt 1 3 5 7 9 11 13 15 17 -152 -132 -112 -92 -72 -52 -32 -12 0 50 100 150 200 250 300 350 400 pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more -152 -132 -112 -92 -72 -52 -32 -12 0 50 100 150 200 250 300 350 400 1 3 5 7 9 11 13 15 17pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 1 3 5 7 9 11 13 15 17 -152 -132 -112 -92 -72 -52 -32 -12 0 50 100 150 200 250 300 350 400 pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more (c) 0.635khz. (b) 2.54 khz. (a) 5.08 khz. figure 7. geoelectrical cross-sections along the longitudinal wide profile in hammam faraun. 54 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha 0 2 4 6 8 10 12 14 16 18 20 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 0 2 4 6 8 10 12 14 16 18 20 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 0 2 4 6 8 10 12 14 16 18 20 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 1000 5000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more(c) 0.635 khz (b) 2.54 khz (a) 5.08 khz figure 8. geoelectrical cross-sections along the traversal wide profile in hammam faraun. will be more significant if we shall provide additional cycles of observation for identification of the stability state of the reservoir. figure (8) represents the detailed geoelectrical cross-sections, which had been constructed for the data of the traversal wide profile for the three frequencies. that section reflects the geoelectrical characteristics in the vicinity of the contact with the block of the first wide profile. from the depth 52 m the cross sections for all frequencies show the same block structure: on the 9th picket, on the depth 92–112 m we see a contact in the high resistance bottom of the section. the conductive rocks are situated averagely up to the depth 30 m, and the crack zones of weak intensity up to the depth 12 m. 6. abu swiera site at this site we carried out csem survey only. the electromagnetic observations measured around the thermal sources outlet has been provided using the method of area planshet survey: in which, the sources of excitation has been located on a middle profile between two receiving profiles (observation lines) forming an area planshet. the distance between the excitation source profile and the observation profiles is 15 m, while the points of observation have been situated with a step 5 m. the length of the observation profiles reached 100 m using two controlled source frequencies are 5.08 khz and 0.635 khz (fig. 9). the distribution of the average parameter of the geoelectrical heterogeneities along 55 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt figure 9. location map of the area planshet wide profiles around the thermal water source at abu swiera, sinai peninsula, egypt. line 1 and line 2 within the area planshet wide profiles is shown in figure (10). the geoelectrical cross-sections are shown in figure (11) as (a) and (b) are for line 1 and (c) and (d) are for line 2. the results testify that the search of the source of the hydrothermal system and the inlet channels to their outlet on the surface is a very complicated structural, manly ranged problem and demands methods with high resolution and high comprehensiveness. so for the band 30 m for the ob56 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha 100 90 80 70 60 50 40 30 20 10 0 �, % 5.08 khz 2.54 khz .635 khz 2 4 6 8 10 12 14 16 18 20 line 1 �, % 350 300 250 200 150 100 50 0 2 4 6 8 10 12 14 16 18 20 5.08 khz 2.54 khz .635 khz line 2 figure 10. distribution of the average parameter of the geoelectrical heterogeneities along line 1 and line 2 in the area planshet wide profiles around abu swiera hot spring, sinai peninsula, egypt. 1 3 5 7 9 11 13 15 17 19 21 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m 0 20 50 90 150 200 300 400 50010005000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 1 3 5 7 9 11 13 15 17 19 21 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 50010005000 or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 1 3 5 7 9 11 13 15 17 19 21 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 5001000 5000or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 1 3 5 7 9 11 13 15 17 19 21 -152 -132 -112 -92 -72 -52 -32 -12 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 10005000or more (om"m) mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more (b) line 1 0.635 khz (a) line 1 5.08 khz (a) line 2 5.08 khz (b) line 2 0.635 khz figure 11. geoelectrical cross-sections; (a) and (b) represent line 1, while (c) and (d) represent line 2 in the area planshet observation system at abu swiera, sinai peninsula, egypt. tained geoelectrical cross sections we see the difference in the blocked structure, beginning from the depth 52 m. similar differences are in the upper levels of the sections and in the crack zones location. if we take into account, that the place of the outlet location of the source of the hot water was known, the picket 2 on the source profile, so we can formulate recommendations for using that method together with other geophysical and geochemical methods for solution of the problem for searching the state of the geothermal system. 7. geochemical analysis this study has been done to estimate the variation of the chemical composition of the geothermal waters and to determine the subsurface temperature of the water reservoirs in these regions. chemical geothermometers are applied to the hydrochemical data. also, the hydro-geochemical characteristics of the collected thermal water from hot springs are taken in consideration. 7.1. material and methods two groups of geothermal water samples have been collected from hammam faraun and abu swiera hot springs. their locations are illustrated in fig. (1). it is found that the total dissolved salts content range from 6053.49 to 12687.1 ppm. the samples of thermal water were stored in polyethylene bottles. the electrical conductivity (ec), temperature and ph values of the geothermal water samples were measured after taken from the field directly. the chemical anal57 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt figure 12. piper diagram for the major cations and anions for abu swiera and hammam faraun in egypt. io n e xc ha ng e m ixi ng lin e r ev er se io n e xc ha ng e figure 13. durov diagram for the major cations and anions for the abu swiera and hammam faraun in egypt. n a + k cl + so hco + co3 3 4 groundwater 0 0 ocean 50 40 30 20 10 0 0 10 20 30 40 50 1 2 seaw water-groundwater mixing-line in te ra ct io n w ith c o 2 c a + m g 50 figure 14. langelier –ludwig diagram for the abu swiera and hammam faraun in egypt. sea water rain water 1 2 concentration (meq/l) 1000. 100. 10. 1. mg ca na+k cl so hco4 3 figure 15. schoellar diagram for the major cations and anions for the abu swiera and hammam faraun in egypt. ysis of underground water samples were carried out in laboratory of the ministry of agricultural in cairo, to calculate the values of na, k, mg, ca, cl, so4 and hco3 ( mg/l), results are listed in table(1). the results have been evaluated by applying the aquachem (calmbach, 1997) and wateq (truesdell and jones 1974) programs. 7.2 classification of water chemistry data graphical and statistical methods were used to classify the water sample into groups. piper, durov, schoeller and langelierludwing, and qmode cluster analysis were applied as follows; piper diagram (1944) that modified by langguh (1966) shows that samples numbers 1 and 2 are alkaline water with prevailing sulfate chloride (fig.12), durov digram (llyod and heathcoat, 1985): figure (13) shows the sample numbers 1 and 2 have cl and na dominant frequently indicate the end point of water, langelier –ludwing diagram (fytikas et al, 1989): shows that the samples are na-so4 water types(fig.14), and schoellar diagram (1962) applied on the collected samples and shows that the samples numbers 1 and 2 are similar to each other reflect the same aquifer characters (fig.15). when correlation with gulf of suez and rain water , we fiend that sample numbers 1 and 2 related to rain water and affected by gulf of suez water with different ranges. 7.3 cluster analysis the cluster analysis has been applied using the statistical package (spss 13).the euclidean distance as similarity measurement together with word `s method for linkage has been used to produce grouping. a dendrogram of q-mode cluster analysis (fig.16) shows two groups, the first contain the sample number 1 and rain water have similarity at distance cluster 1 and sample number 2 at distance cluster 2.5.the second group contain gulf of suez water. this coincidence the affect of rain water is more than the gulf of suez water. 7.4 principal component analysis (pca) the principal component analysis (pca) has been extensively applied to different the samples to be compared and give finer prints to identifying the origin of the chemical origin (fig. 17). pca results declare the contribution degree of variation in the data of the study area. 60.201 % 1st pc 35.186% for 2nd pc and 4.613 %for 3rd pc of data set. the 1st pc is strong positive distribution of the loading values for tds, cl, so4 and hco3 and moderate positive loading on ph, mg and ca but negative with k. these ions have dominant contribution to the explanation of the pc1. the strong positive terms are due to dissolution of salts of the geological components. the factor 2 is ca enrichment factor. the presence of dissolved carbonate might enhance or suppress adsorption and forming precipitates. factor 3 has strong positive loading with ca, mg and so4, moderate with tds, na and small with cl. the negative loading with k and hco3, suggesting that two have the same source of fresh water resulting from dissolution by the rain water. 58 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha dendrogram using ward method rescaled distance cluster combine case 0 5 10 15 20 25 label num +– – – –+– – – –+– – – –+– – – –+– – – –+ 1 rainwater 3 = 2 sea water 4 figure 16. q-mode analysis of groundwater samples. table 1: fresh water and sea water percentages in water from study area calculated by means of aquachem (calmbach, 1997). sample no. water sample name fresh water % sea water % 1 abuo sira 90 10 2 hammam pharoun 80 20 3 rain water 100 0 4 gulf of suez 0 100 59 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt table 2. subsurface temperature values in hot springs by geochemical thermometers. sample no. calculated the groundwater temperature by using different geothermometers methods in degree °c k/mg giggenbach et al 1983 na/k fournier & truesdell 1973 na/k truesdell 1976 na/k fournier & porter 1979 na/k fournier 1979 na-k-ca fournier 1979 chalcedony fournier 1977 quartz fournier 1977 quartz fournier 1977 na/li kharaka 1989 1 41 214 216 262 239 206 62 93 95 79 2 48 110 119 175 160 163 62 93 95 60 1.6 e9 1.4 e9 1.2 e9 1.0 e9 8.0 e8 6.0 e8 4.0 e8 2.0 e8 0.0 e0 e ig en va lu e 1 2 3 4 5 6 7 8 9 10 11 component number scree plot component plot in rotated space component 1 c om po ne nt 2 component 3 0 0.5 1.0 1.5 2.0 1.2 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 1.2 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 ph tw a c on d td s n a k m g c a c l s o 4 h c o 3 tw a c on d td s n a k m g c a c l s o 4 h c o 3 1 1 series 2 tw a c on d td s n a k m g c a c l s o 4 h c o 3 series 3 figure 17. factor analysis of the groundwater chemistry analysis. table 1 shows the percentage of gulf of suez and rain water calculated by means of aquachem (calmbach, 1997) by optimizing cl and tds mass balances. the water samples numbers 1 and 2 having 90 % and 80% from rain water respectively. these indicate that sample no.2 is affected by gulf of suez water than sample no.1. then, the salinity of hammam faraun is higher than that of abu swiera spring. the acquired salinity of hammam faraun is probably, resulted from their mixing between the eocene aquifer (similar to the abu swiera water) and the presented seawater due to its location at the seashore or to the product of rock-water interaction. cl-so4-hco3 ternary diagram (fig.18); according to giggenbach, 1988, and giggenbach and goguel, 1989) shows that sample no.1 mature waters while sample 2 lie on the line between chloride water and volcanic water. na-k-mg diagram (giggenbach, 1988) (fig.19) is used to differentiat between immature, partially equilibrated and full equilibrated water. it predicts the temperature of reservoir and the relationship of equilibrium of the water and host rock. the sample no.2 lies on the line between immature water and partially equilibrated water. it lies between temperature lines 160 and 180 °c as sample no.1 lies between lines 240-260 °c. in this study, temperature of the hydrothermal aquifer has been evaluated by using geothermometers methods through aquachem program (calmbach, 1997) and tabulated in table (2). 8. discusión and conclusión two sites have been selected to study the geothermal occurrences in central part of sinai peninsula; hamam faraun and abu swiera close to the gulf of suez. the geophysical measurements on both sites showed that the uppermost 40 meters are cracked low resistive medium connected to a massive resistive base rock. this base rock could be the container of thermal water or at least adjacent to the source of heating. practically, this can be evidenced by the extensional cave in gebel faraun heart, where the more inside in lateral and downward directions results the raise in temperature, which might be interpreted as being closer to the heat source. now it becames an objective need of developing a complex direction of research, which must ensure the theoretical base of exploration and operation of geothermal deposits. that direction must include all key problems, which are linked with their developing such as: 1. developing of geological models of geothermal deposits on the base of detailed geological mapping of volcanic magmatic complexes, which are sources of heat feeding of hydrothermal systems and revealing of the hydrothermal reservoirs structure. 2. providing and interpretation of the geophysical data on the base of models, which are adequate to the geometry of the geothermal reservoirs in the volume, research of the headrace, along what moves the heat 60 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha steam heated water chloride waters sulphate waters bicarbonate waters na ci m ature lw ater peripheral water vo lc an ic w at er figure 18. relative cl,so4 and hco3 contents of abu swiera and hammam faraun in egypt. 20 40 60 80 100 20 40 60 80 100 fully equilibrated waters na 001+ immature waters partially equilibrated waters k*.01 sqr(mg) figure 19. graphical evaluation of the water –rock equilibration temperature after giggenbach, 1988; giggenbach and coralles, 1992 using relative na, k and mg concentrations (mg/kg) of abu swiera and hammam faraun in egyp flow. in that case, a very informative method (control source electromagnetic induction frequency – geometrical research) which is introduced in this paper shows the capability to provide the adequate resolution to analyze and probably solve the geothermal problems. 3. the development models of geophysical and geochemical character, which are used for revealing of the flow picture and estimation of the dynamical characteristics of the heat flow on the base of the geophysical monitoring data. the geochemical analysis to the two samples groups showed that the tds is ranging from 6053.49 to 12687.1 ppm, the water chemistry was classified as alkaline water with prevailing sulfate chloride (piper diagram), the existence of cl and na as dominant frequently indicates the end point of water (durov diagram) and a relation to rain water affected with the gulf water was concluded. the interpretation of the factor analysis model is matching well with all results of sholler analysis and aquachem program as the thermal water were a mixing with meteoric water explain the hydrochemical variation of the thermal water within the eocene aquifer. the present study concludes that the two sites are excellent places for investment in thermal water utilization just for aqua thermal pools, thermotherapy, and thermal basins. 9. reference akaike, h. 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(1974). wateq, a computer program for calculating chemical equilibria of natural waters, jour. research, u.s.geol. surv, 2, 233-248. 62 magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha geociencias dic 2010-color.vp potential littoral sediment transport along the coast of south eastern coast of india s. saravanan and n. chandrasekar centre for geotechnology manonmaniam sundaranar university tirunelveli – 627 012, tamilnadu, india. e-mail: geosaravanan2000@yahoo.co.in abstract in the nearshore zone of present study area, the movement of sand alongshore is due to the action of waves and currents. the sediment movement along the shoreline of study area is mainly governed by the forces associated with the incoming waves and the availability of sediments within the area. the present investigation has been made as an attempt to appreciate the sediment movement in relation to wave activity along the coast. the longshore sediment transport is higher in the northerly direction as compared to southerly direction. the normal condition is for and to be moved annually or more frequently in the shallows and on the beach. waves and wave-driven currents cause longshore drift of sand along the beach and offshore. the net erosive nature of the study area (except kanyakumari) from march to october is due to the prevalence of high waves from s and se directions. resumen en la zona próxima a la costa del área de estudio, el movimiento de la arena a lo largo de la costa es debido a la acción de las olas y las corrientes. el movimiento del sedimento a lo largo de la línea de costa del área de estudio es principalmente gobernado por fuerzas asociadas a las olas entrantes a la costa y la disponibilidad de sedimentos en la zona. esta investigación fue realizada para estudiar el movimiento del sedimento en relación a la actividad de las olas a lo largo de la costa. la dirección del transporte de sedimento a lo largo de la costa es mayor hacia el norte que hacia el sur. las condiciones normales para el movimiento anual del sedimento se encuentran en aguas someras y en la playa. las olas y la corriente de oleaje generan una deriva litoral de las arenas a lo largo de la playa y costa afuera. la erosión natural en el área de estudio (exepto kanyakumari) entre marzo y octubre es debido a la prevalencia del gran oleaje desde la dirección sur y sureste. introduction introduction the knowledge about sediment movement, on a long-term basis, in the nearshore environment is a pre-requisite for the design of shore protection structures and for navigational purposes. it is understood that this movement of sediment, particularly in the surf zone, is governed by complex interplay of waves and the nature of prevailing sediments in addition to morphology of beaches. broadly, the alongshore component of wave energy together with the intense turbulence associated with breaking waves determine the sediment motion quantitatively. investigations on sediment drift based on empirical relationship developed from quantitative estimates of littoral flows through wave refraction studies and field and laboratory experiments on the nature of movement of sediment in the vicinity of natural barriers have been made along various coastlines of the world (inman et.al.,1969; komar and inman, 1970; walton and chiu,1979). along the coastline of india such quantitative determinations have been reported at different localities (prasannakumar, 1985; reddy et.al., 1984; sarma and reddy, 1988; rajamanickam et.al., 1986; chandramohan, 1988, chandrasekar, 1992; chandramohan and nayak, 1991; angusamy, et.al., 1998; sanjeev et.al., 1997; bodapathi, 2000; sanil kumar et.al., 2000, chandrasekar et.al., 2001). the sediment movement along the shoreline of study area is mainly governed by the forces associated with the incoming waves and the availability of sediments within the area. the present investigation has been made as an attempt to appreciate the sediment movement in relation to wave activity and beach morphodynamics along the coast. sharaf and mahar (1997) have examined the sediment transport along the nile delta coast egypt. the budget of the transported sediments along the nile delta coast show that, the area between abu quir and rashid and that between ras el bar and portsaid are characterised by accretion while those between rashid and burullas and between burullus and ras el bar are characterised by erosion. ciavola et.al., (1997) have carried out field measurements of long shore sand transport and control processes on a steep mesotidal beach in earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 153-160research groupin geophysics universidad nacional de colombia keywords: beach, sediment, littoral sediment transport, india. palabras clave: playa, sedimento, transporte sedimento litoral, india. record manuscript received: 05-07-2010 accepted for publication: 30-11-2010 portugal. they have concluded that improvement should be needed in the understanding of the sedimentary behaviour of steep reflective beaches, where, because of the short surf zone, wave energy dissipation takes place over small distances, leading to relatively high alongshore sediment transport rates. wang et.al., (1998) have evaluated the total long shore sediment transport rate in the surf zone by using field measurements and empirical predictions. giosan et.al., (1999) have discussed in detail about the long shore sediment transport pattern along the romanian danube delta coast. they have pointed out that net long shore sediment transport is controlled by the offshore bathymetry and by obstacles, which obstruct that surf zone along the coast. in the indian coast, various investigations pertaining to different aspects of oceanographic and morphological features have been carried out by a number of researchers. veerayya and pankajakshan (1988) have reported long shore sediment transport pattern along the manglore coast. chandramohan (1988) have discussed and estimated the rate of longshore sediment transport along the indian coast. jena (1997) has reported the littoral drift sources and sinks along the indian coast. hanamgond (1993) have explained about the sediment movement on aligadde beach, uttara kannada west coast of india and stated that the beach morphology undergoes cyclic seasonal changes in response to the changing wing and wave climate. sanil kumar et.al., (2000) have studied the long shore currents and beach morphodynamics along kannirajapuram coast, tamilnadu. chandramohan and nayak (1991) have accounted the wave induced longshore sediment transport at different places along indian coast. study area the study area is located along the southern coast of tamilnadu state, india (fig. 1). it extends over a distance of 32.78 km and the southeast coastline borders the bay of bengal. the backshore of the beach is limited by urban infrastructures. the mean sea level (msl) is 2m above the 0m depth chart datum (cd). the local mean tidal range is 0.5m and the maximum tidal level above cd is 1m. the net longshore transport is directed to the north. the drainage pattern along the study area is controlled by minor streams like palaiyar, namiyar, hanuman nadhi and seasonal streams like nilapparai channel and puttanar channel (fig. 2). cliffs are along the kanyakumari coast which projects towards the indian ocean forming a promontory. coastal areas of rasthakadu and kuttapuli have sandy beaches and some areas are rocky in nature. sand mining is also actively pursued along the coast. saravanan et.al (2009) and chandrasekar et.al (2006) are evaluated the post-tsunami assessment and classification of tsunami hazard along the southern coast of india. angusamy (1998) made a panoramic classification of the beaches between mandapam and kanyakumari, tamilnadu based upon the beach composition, beach gradient and beach configuration. moni (1972); prasannakumar and murthy (1987) chandramohan and nayak (1991); rajamanickam et. al., (1986) veerayya and pankajashan (1988) and gujar (1996) have indicated from their studies along west coast of india that the predominant direction of waves in the area are from sw, wsw, w and wnw with periods ranging from 6 to 12 seconds. anbarasu (1994); chandrasekar (1992); angusamy et.al., (1998), chandrasekar et. al., (2001) have discussed about the wave refraction pattern and its role in the redistribution of sediments along the east coast of india. saravanan and chandrasekar (2010a,b) have analysed the monthly and seasonal variation of beach and the distribution of various assemblage of heavy mineral present along the south eastern cost of india. shoreline configuration in the study region, shoreline configuration is appeared to have been controlled and influenced by the predominant monsoonal wind directions of both ne and sw. waves and longshore currents have also played an important role for shaping the shoreline. the shoreline of the study region displays a varying trend in e w, n s, ne sw and nne ssw directions (fig. 3). ne -sw direction this part of the coast is higher than that of any other areas in gulf of mannar with undulating sand hills and the hinterland mostly of reddish soil. in due south, from periyathalai to vatakkottai the ne sw trend is represented by minor zigzag patterns at few places. n -s direction 154 s. saravanan and n. chandrasekar figure1. study area map. potential littoral sediment transport along the coast of south eastern coast of india 155 table1. monthly data of long shore current (v) along the study area (m/s) station a m j j a s o n d j f m tur 0.30 0.12 -0.10 -0.06 -0.28 -0.09 -0.07 -0.04 0.06 -0.13 0.06 0.08 alt 0.31 -0.10 -0.09 -0.07 -0.21 -0.08 -0.07 -0.01 0.03 -0.11 0.04 0.04 mnp 0.39 0.02 -0.07 -0.04 -0.1 -0.07 -0.3 -0.4 0.05 -0.07 0.01 0.16 ovr 0.48 0.01 -0.16 -0.01 -0.18 -0.06 -0.36 -0.39 -0.43 -0.09 0.14 0.29 peru 0.42 0.04 -0.20 -0.13 -0.26 -0.10 -0.29 -0.14 -0.34 -0.06 0.12 0.18 kut 0.40 0.14 -0.21 -0.33 -0.24 -0.24 -0.30 -0.33 -0.33 -0.10 0.12 0.18 nav 0.44 0.13 -0.20 -0.32 -0.22 -0.22 -0.30 -0.22 -0.32 -0.16 0.11 0.20 kup 0.38 0.02 -0.23 -0.20 -0.23 -0.31 -0.26 -0.16 -0.32 -0.04 0.16 0.16 vat 0.50 0.03 -0.05 -0.10 -0.01 -0.21 -0.31 -0.16 -0.30 -0.08 0.10 0.16 kan 0.34 0.14 -0.04 -0.20 -0.07 -0.20 -0.30 -0.15 -0.30 -0.17 0.10 0.16 (-) northerly direction (+) southerly direction tur – thiruchendur; alt – alanthalai; mnp – manappad; ovr – ovari; peru – perumanal; kut – kuttapuli; nav – navaladi; vat – vattakottai; kan – kanyakumari figure 2. coastal geomorphology of the study area figure 3. coastline configuration along the study area. the other direction of configuration is in n-s direction from vembar tuticorin. otherwise the n-s configuration is seen in the study area as a very narrow strip to fill up the gap developed between the two different configurations of nne ssw and ne sw in places like kanyakumari, tuticorin and navaladi. nne ssw direction the next prominent configuration is from tuticorin manappad in the direction of nne ssw. the river mouth of thamirabharani is indented with numerous creeks with a chain of backwaters running parallel to the coast. thiruchendur is a low rocky headland projecting into the sea where, well developed marine terraces are exposed. ne sw direction from vattakkottai kanyakumari, once again the coastal configuration reverts back to ew direction and finally enjoins with west coast by showing a trend of n-s direction for a smaller distance. various natural processes are found to prevail in the coastal segment between vattakottai and kanyakumari. in short, major part of the study area is aligned in the direction of ne-sw. it is worth to note that economically viable heavy minerals are found to get segregated only in this configuration. waves, currents and tides as the waves, currents and tides contribute significantly in the redistribution of the sediments, supplied to the depositional basin, the rate of these oceanographic physical agents is discussed with sediment distribution and littoral transport in the study area. waves are found to provide necessary energy for the movement of water and sediments within the nearshore zone. they govern the evolution of beaches that act as buffer zones for the impinging wave energy. the shape and size of these deposits, in turn, give rise to change in the incoming wave energy through changes in the nature of the wave breaking. when one considers the changes of this dynamic zone over a long time scale, the dominating effect of waves in deciding the orientation of this near elastic and energy absorbing zone becomes evident. currents are monsoonal in character. the currents are variable with respect to monsoonal climate. during sw monsoon (may – october) the currents are branching towards the ne from south easterly direction that flows across the entrance of gulf of mannar. in ne monsoon (november to february), the flow is southward which implies mainly south westerly flow of currents across the gulf of mannar. the monthly variation of longshore currents is presented in the table.1. it was less during the sw monsoon (june to october) and fair weather (march to may) conditions. its direction is northerly during fair weather and sw monsoon seasons whereas towards southerly during ne monsoon period. wind generated surface waves are the principal source of energy input into the littoral zone. they are responsible for the erosion of the coast and for the formation of depositional beach features. a notion on wave action is a fundamental mandatory to have an insight into the processes that takes place on beaches. in the present area of investigation wave parameters are controlled by the change in monsoonal cycle. waves approach the coast in se, s and ne directions with wave period from 3 to 10 seconds (bodapati, 2000). the breaking wave height observed in the study area varies from 0.05 to 0.85 m during sw monsoon period and that of 0.10 to 0.60 m during ne monsoon period. the fair weather attests to the tune of 0.05 to 0.50 m of breaking wave height (table. 2 & photo.1). at kuttankuli, the breaker zone width ranges from 16 to 20 m with maximum during july, october and november (20 m). kanyakumari depicts a variation of the surf zone width from 15 to 20 m with highest during the months of february, may, november and december (20 m). at vattakottai, the surf zone width shows a fluctuation from 15 to 20 m with a rise up to 20 m during may to july and october to december periods. at thiruchendur, the surf zone width varies from 14 to 18 m and with a mount during march and october. at periyathalai, one may able to observe the variation of surf zone width from 14 to 20 m with a maximum during the months of january, april and august (20 m). ovari attests a variation in surf zone width from 12 to 18 m with a highest during november (18 m). in the case of kanyakumari, the surf zone width varies from 11 to 14 m with a peak during the months of june and september (14m). material methods the littoral zone is the most dynamic environment of the coast and one in which constant mobility of sediment is observed. the movement of material in this zone depends mainly on three factors: the nature of material available for transport (size and density), orientation and other features of the coast and the angle of wave approach (swift, 1976; king, 1972). littoral transport plays a major role in the development of certain shoreline features like spits and bars, and is causing considerable coastal erosion and accretion (king, 1974). the proper understanding of the monthly/seasonal littoral transport trend is important for the efficient coastal zone management and development. the longshore sediment transport rate is usually estimated based on empirical equation relating the longshore energy flux in the breaker zone to the longshore transport rate. several discussions have already appeared on the selection of suitable equations for estimation of the longshore sediment transport (graft and overeem, 1979; willis, 1980). using the monthly average littoral environmental parameters collected at different locations of the study area, the longshore transport rates are estimated. the deepwater wave data for the study region is extracted from the wave atlas for the indian coast published by national institute of oceanography, goa (chandramohan et.al., 1990). the following walton and bruno (1989) equation is used to estimate the longshore sediment transport rate. q p h w vc v v g b f � 1290 0 78 5 2 0. ( / ) ( / )� where q = longshore sediment transport rate in m3/year, p = mass density of the sea water = 1025 kg/m3, g = acceleration due to gravity = 9.81 m/s2 cf = the friction coefficient � 0.01, hb = breaking wave height in metre, w = surf zone width in metre, v = measured longshore current velocity in m/s, v/vo = theoretical dimensionless longshore current velocity ˜ 0.4 the monthly longshore sediment transport rates estimated based on the monthly observations on breaking wave height (photo.1), surf zone width and the longshore currents. the monthly volume of longshore sediment transport rates and directions are estimated for the coast at kuttankuli, vattakottai, thiruchendur, alanthalai, manappad, periyathalai, ovari and kanyakumari (table 3). in general, the sediment transport is northerly during march to october and southerly during november to february. the longshore sediment 156 s. saravanan and n. chandrasekar transport is higher in the northerly direction as compared to southerly direction at all locations except kanyakumari (fig. 4). this occurs because the rocky outcrops shelter the kanyakumari beach and manmade features such as harbour across the surf zone would act as a barrier and sand would be deposited on the updrift side of such barrier. the littoral drift quantities are higher from may to october, due to heavy breakers associated with the sw monsoon season. the annual net sediment transport rate is northerly, kuttankuli (87.75 x 103m3/yr), kanyakumari (194.1 x 103m3/yr), vattakottai (83.14 x 103m3/yr), thiruchendur (52.23 x 103m3/yr), alanthalai (243 x 103m3/yr). at kanyakumari, the annual net sediment transport rate is southerly, which is 22.03 x 103m3/yr (table.4). the quantity of littoral drift varies considerably from station to station, depending upon the wave and beach conditions. the maximum northerly annual net drift is observed at alanthalai. this may be due to the complete exposure of the beaches to wave attack throughout the year. at kanyakumari, a net southerly drift is perceptible. the net southerly drift estimated in the kanyakumari location indicates that the growth of shoreline near harbour takes place at the potential littoral sediment transport along the coast of south eastern coast of india 157 table 2. monthly data of breaking wave height (wh) along the study area (m) (2005) station a m j j a s o n d j f m tur 0.30 0.40 0.25 0.25 0.35 0.60 0.20 0.45 0.60 0.25 0.35 0.20 alt 0.40 0.45 0.40 0.25 0.30 0.70 0.30 0.60 0.80 0.30 0.45 0.30 mnp 0.40 0.20 0.25 0.20 0.25 0.35 0.85 0.50 0.55 0.35 0.55 0.50 ovr 0.50 0.20 0.20 0.50 0.30 0.45 0.70 0.45 0.60 0.45 0.50 0.50 peru 0.50 0.35 0.15 0.20 0.30 0.20 0.45 0.20 0.55 0.45 0.35 0.45 kut 0.50 0.50 0.25 0.20 0.20 0.15 0.50 0.20 0.25 0.40 0.55 0.55 nav 0.40 0.45 0.30 0.15 0.40 0.30 0.55 0.40 0.35 0.45 0.445 0.60 kup 0.30 0.40 0.20 0.15 0.25 0.10 0.60 0.30 0.45 0.25 0.40 0.40 vat 0.25 0.15 0.05 0.25 0.10 0.25 0.20 0.20 0.20 0.05 0.15 0.20 kan 0.20 0.20 0.25 0.20 0.15 0.30 0.35 0.35 0.15 0.25 0.20 0.20 tur – thiruchendur; alt – alanthalai; mnp – manappad; ovr – ovari; peru – perumanal; kut – kuttapuli; nav – navaladi; vat – vattakottai; kan – kanyakumari photo 1. wave height measurement during the filed. expense of the sediments eroded from those areas situated north of kanyakumari. the material drifting towards the south from north is blocked at the harbour entrance and gets either deposited or transported to other reaches depending on the flow conditions. this depositional pattern has also been confirmed from the wave refraction analysis as well as littoral drift cell study. the study indicates that in general, the entire stretch between kanyakumari and tiruchendur experiences a sediment transport rate of less than 1.1 x 106m3/yr. the coast between chennai and paradeep experiences a gross sediment transport rate of more than 2 x 106m3/year (sundar and sarma, 1992). subsequently, jena (1997) and prabakar rao (2002) have projected the gross sediment transport rate of southern tamilnadu coast as 0.6 x 106m3/yr and 0.7 x 106m3/yr respectively. the increment in gross sediment transport is noticed during the present investigation as compared to earlier studies (jena, 1997; prabakar rao, 2002). this is mainly due to the slight increase in the approaching wave angles to the coast during the study period. again, the seasonal cyclonic depression and storm anomaly must have also contributed appreciably for the rise in sediment volume during the course of investigation. result and discussion in the nearshore zone of present study area, the movement of sand alongshore is due to the action of waves and currents. the direction of littoral drift is from south to north during the period of march to october when the waves are between s and se and from north to south during the period november to february when the wave directions are between e and ene. the seasonal changes in the direction of littoral drift with the sw and ne monsoons cause cyclic variations of the beach morphology along the coast under investigation. the rocky outcrops scattered across the littoral zone cause reversal of beach cycles at different stations along the coast. for example at kanyakumari (october) during sw monsoon, the southerly movement of sediment is observed. the net littoral drift at all stations is generally from south to north with the exception of kanyakumari station, where the net drift is southwards. this occurs because the northerly littoral drift during sw monsoon is very small (maximum of 41.31x 103 m3/month), and a net annual southerly drift is recorded. the net erosive nature of the study area (except kanyakumari) from march to october is due to the prevalence of high waves from s and se directions. the maximum net annual northerly drift is recorded at alanthalai (243 x 103m3/year). this is mainly controlled by the offshore bathymetry and by the obstacles that impede the surf zone along the coast. above all, overwash and 158 s. saravanan and n. chandrasekar figure 4. littoral sediment movement identified along se india. table 3. longshore sediment transport rate (q) (103 cubic metre/month) (2005) beach j f m a m j j a s o n d tur 201.25 74.14 -44.48 -62.91 -43.21 -38.13 -143.00 -59.42 -69.9 -38.13 66.41 104.86 alt 186.42 55.76 -47.66 -40.67 -88.55 -65.91 -139.82 -83.89 -112.7 -76.26 47.19 127.11 mnp 96.6 5.93 -43.85 -22.24 -42.63 -27.91 -123.92 -35.59 -129.75 -9.53 54.23 54.23 pet 132.4 3.81 -23.83 -5.29 -0.85 -38.92 -62.39 -33.9 -47.66 -3.6 14.3 25.42 ova 95.33 25.2 -8.47 -42.37 -10.01 -397.2 -85.79 -52.83 -25.42 -22.96 21.18 25.42 kut 111.64 -40.51 -30.5 -16.68 -56.72 -48.3 -11.12 -5.4 7.63 -17.48 9.53 10.17 vat 200.19 11.86 -23.83 24.78 -82.61 -21.18 -73.72 -23.73 -158.46 -17.16 44.49 85.8 kan 70.22 1.27 -1.27 -2.33 -0.95 -1.48 -41.31 -22.24 -14.83 1.59 6.67 26.69 onshore-offshore sediment transport are the possible mechanisms that control the drifting of sand. a complete study of wave dynamics is imperative at this instant which includes the measurement across the swash and surf zones of local sediment transport wave height, wave energy conditions, longshore current, grain size, etc., to comprehend and describe the swash processes precisely. also the time scale of each study should be commensurate with that of the duration of the directional wave event that drives the transport. further, emphasis has to be placed on formulating computer models to conjecture the performance of sediment movement for the development and management of coastal zone. conclusions the east coast of india experiences two phases of stormy conditions during south-west and north-east monsoons, the south tamilnadu coast of the of india has comparatively less sediment transport due to the presence of shallow palk bay, gulf of mannaar and the sri-lanka island. they significantly control the longshore sediment transport along the south tamilnadu coast particularly form kanyakumari to tiruchandur region. the reversal trend in the direction of sediment transport between mandapam and kanyakumari due to change in the coastal configuration, deposition and the formation of numerous spits along this coast. but the fluvial activities are negligible in the region. the formations of spits are prominent during the receding of south west monsoon. moreover, the offshore islands present along this distinct coastal tract could protect the beach from the direct attack of waves, which in turn results low energy condition. the normal condition is for and to be moved annually or more frequently in the shallows and on the beach. waves and wave-driven currents cause longshore drift of sand along the beach and offshore. a comprehensive study of major direct and theoretical measurement of longshore drift analysis using different beach parameters should have to be done. so, the study would be very helpful to investigate the littoral drift of sediments and beach morphological changes along the study area. acknowledgement the authors are thankful to dr. bhoop singh, advisor, nrdms, department of science and technology, new delhi and department of science and technology, new delhi for providing the financial assistance under nrdms scheme (es/11/526/2000). references anbarasu, k. (1994) geomorphology of the northern tamilnadu coast using remote sensing techniques unpubli. ph.d. thesis, bharathidasan university, tiruchirapalli,184p. angusamy, n., udayaganesan, p. and rajamanickam, g.v. (1998): wave refraction pattern and its role in the redistribution of sediment along southern coast of tamilnadu, india. ind. j. mar. sci., vo. 27, pp. 173 – 178. bodapati (2000) sediment transport and exchange around rameswaram island between gulf of mannar and palk bay. unpubl. ph. d. thesis, berhampur univ., berhampur, 194 p. chandramohan, p. (1988) longshore sediment transport model with particular reference to the indian coast. unpubl. ph.d. thesis, iit, madras, 210 p. chandramohan,p., sanil kumar, v. and anand, m. (1990) wave atlas for the indian coast. ocea. engg. div., nio, goa, 312 p. chandramohan, p. and nayak. bu. (1991) longshore sediment transport along the indian coast. ind. j. mar. sci., vol. 20, no.2, pp.110-114. chandrasekar, n. (1992) placer mineral exploration along the central tamilnadu coast. unpublished ph.d. thesis, madurai kamaraj university, madurai, 293 p. chandrasekar, n., anil cherian., rajamanickam, m. and rajamanickam, g.v. (2001): influence of garnet sand mining on beach sediment dynamics between periyathalai and navaladi coast, tamilnadu. j. ind. assn. sed., vol.21, no.1, pp.223-233. chandrasekar.n, s.saravanan, j.loveson, m. rajamanickam and g.v. rajamanickam (2006). classification of tsunami hazard along the southern coast of india: an initiative to safeguard the coastal environment from similar debacle. science of tsunami hazards, vol.24, no.1, pp.3-23. graff, v.d. and overeem, v.j. (1979) evaluation of sediment transport formulae in coastal engineering practice. coast. engg., vol.3, pp. 1-32. hanamgond (1993) morphological and sedimentological studies within kavada bay and belekeri bay beaches, uttara kannada district, karnatak, west coast, india. unpublished ph.d. thesis, karnatak university, dharward, india, 265 p. inman, d.l., komar, p.d. and bowen, a.j. (1969) longshore transport of sand. proce. 11th cof. coast engg. asce vol.1, pp. 298-306. jena, b.k. (1997) studies on littoral drift sources and sinks along the indian coast. unpublished. ph.d. thesis, berhampur univ.,berhampur, india, moni, n.s. (1972) systematic study of coastal erosion and defence work in the southwest coast of india. proc. of the 13th coast engg., conf., washington, d.l. vol.2, pp. 411-420. prabakar rao. (2002) sediment transport and exchange around rameswaram island between gulf of mannar and palk bay. umpubl. ph.d.thesis, berhampur univ., berahampur, 200p. potential littoral sediment transport along the coast of south eastern coast of india 159 table 4. seasonal and annual sediment transport rate (q) (103 cubic metre/month) (2005) beach south north net annual gross tur 446.67 -499.21 -52.53 945.88 alt 416.48 -659.47 -242.99 1076 mnp 210.99 -435.44 -224.44 646.44 pet 175.93 -195.029 -19.09 370.96 ova 167.14 -287.579 -120.43 454.72 kut 138.97 -226.72 -87.75 365.7 vat 342.34 -425.49 -83.14 767.83 kan 106.45 -84.42 22.03 190.87 prasannakumar, s. (1985) studies on sediment transport in the surf zone along certain beaches of kerala. unpubl. ph.d. thesis, cochin, univ. of sci. and tech., cochin, 110 p. prasannakumar, s., murthy, c.s. (1987) response of barrier beaches along the southwest coast of india to monsoonal forcing. j. coast. res., vol.3, pp. 343-358. rajamanickam, g.v., vethamony, p. and gujar, a.r. (1986) effect of waves in the redistribution of sediments along the konkan coast. proc. natl. acad. sci., vol.95, no.2, pp.237-244. reddy, b.s.r. sarma, k.g.s. kumar, k.h. (1984) beach changes during normal and cyclonic periods along the visakhapatnam coast. ind. j. mar. sci., vol.14, pp. 210-216. sanil kumar, v., chandramohan, p., ashok kumar, k. gowtham, r. and pednekar, p. (2000) longshore currents and sediment transport along kannirajapuram coast, tamilnadu, india. j. coast. res., vol.16, no.2, pp.247-254. sanjeev, r., chandramohan, p. josanto, v. and sankaranarayanan, v.n. (1997) studies on sediment transport along kerala coast, southwest coast of india. ind. j. mar. sci., vol.26, no.1, pp.11-15. saravanan. s., n.chandrasekar, c. hentry, m. rajamanickam, j. loveson immanuel and p. siva subramanian (2009). post-tsunami assessment in the coastal region between kanyakumari and ovari, tamil nadu – a case study. earth science frontiers, 16(6): 129–137. s. saravanan and chandrasekar (2010a). monthly and seasonal variation in beach profile along the coast of tiruchendur and kanyakumari, tamilnadu, india. journal of iberian geology, 36(1): 39-54. s. saravanan and n. chandrasekar (2010b). distribution and assemblages of heavy mineral suites, se india. international journal of oceans and oceanography, vol.4, no.1, pp 1-15. sarma, k.g.s. and reddy, b.s.r. (1988). longshore sediment transport near visakkapatnam port, india. oce. and shor. mang., vol.11, pp.113-127. sundar, v. and sarma, s.s. (1992) sediment transport rate and its distribution across surf zone off gopalpur port, east coast of india. ind.j. mar.sci., vol.21, no.2, pp.102-106. veerayya, m. and pankajakshan, t. (1988) variability in wave refraction and resultant rearshore current patterns: exposed versus sheltered beaches along north karnataka, west coast of india. ind. j. mar. sci., vol.17, no.2, pp. 102-110. walton, t.l. and chiui, t.y. (1979) littoral sand transport on beaches. tech. rep. tr-041, dept. of coast and oceano, eng., univ. of florida, gainesville, fl 32611. walton, t.l., (jr.) and bruno, r.o. (1989) longshore transport at a detached breakwater, phase-ii. j. coast. res., vol.65, no.9, pp. 667-668. shore protection manual (1975, 1984). u.s.army coastal engineering research centre, dept. of army crops. engrs., 43 p. willis, d.h. (1980) evaluation of sediment transport formulae in coastal engineering practical discussion. coast engg., vol.4, pp. 177-181. 160 s. saravanan and n. chandrasekar enero%2031-geociencias-vol%2012-2%20dic-en%20baja[1].pdf earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 125-155 a new approach on the tectonometamorphic mechanisms associatedwith p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes castellanos a. óscar m.1, ríos r. carlos a.2 and takasu akira3 1 programa de geología, universidad de pamplona, colombia. ocastellanos@unipamplona.edu.co 2 escuela de geología, universidad industrial de santander, colombia. 3 geosciences department, shimane university, matsue, japan. abstract metamorphic pressure and temperature trajectories are generally used as tools to interpret the tectonic history of deformed belts of rocks that record past activity along active plate margins. the pre-devonian metamorphic sequence of the silgará formation forms part of the ancient basement of the central santander massif, including pelitic schists, siliceous schists and intercalation of amphibolites accompanied by marble. this metasedimentary unit has been affected by barrovian typemetamorphismunder middle-pressure/high-temperature conditions. this paper presents on the relationship of tectonic processes to pressure–temperature paths development. detailed petrologic analysis of pelites and related rocks has been used to constrain the p-t history of this region in the eastern colombian cordillera. mineral assemblages and geothermobarometric calculations indicate metamorphic temperatures and pressures of 400-600 °c and 4.0-6.5 kbar, respectively. key words: central santander massif, silgará formation, barrovian type metamorphism, pelitic rocks, petrologic analysis, geotermobarometry. resumen las trayectorias de presión y temperatura de metamorfismo generalmente son usadas como herramientas para interpretar la historia tectónica de cinturones de rocas deformadas que registran antigua actividad a lo largo de bordes de placa activos. la secuencia pre-devónica de la formación silgará forma parte del antiguo basamento que aflora en la región central del macizo de santander, incluyendo esquistos pelíticos, esquistos silíceos con intercalación de anfibolitas acompañadas por mármol. esta secuencia metasedimentaria ha sido afectada por un 125 manuscript receiver: may 30th, 2008. accepted for publication: august 24th, 2008 metamorfismo tipo barroviense en condiciones de presión intermedia / temperatura alta. este artículo especula sobre la relación entre los procesos tectónicos y el desarrollo de trayectorias de p-t. un análisis petrológico detallado de las pelitas y rocas relacionadas ha sido usado para deducir la historia de p-t de esta región. las paragénesis minerales y los cálculos geotermobarométricos indican temperaturas y presiones metamórficas de 400-600 °c y 4.0-6.5 kbar, respectivamente. palabras clave: región central del macizo de santander, formación silgará, metamorfismo tipo barroviense, rocas pelíticas, análisis petrológico, geotermobarometría. introduction the geological evolution of the colombian andes (figure 1) has been interpreted as a compositemargin made up of successively accreted terranes and oceanic island arc sequences from paleozoic to miocene ages (weber et al., 2002), and the most important events took place during devonian-carboniferous and cretaceous times (restrepo and toussaint, 1988). the colombian andes are divided into three mountain ranges, which from east to west comprise the eastern, central and western cordilleras, showing different tectonic histories and configurations. according to cooper et al. (1995), the first two correspond to modern and ancient fold-thrust belts, respectively, and the last one represents an accreted arc. the eastern cordillera splits by the southern termination of the left-lateral strike-slip santa marta bucaramanga fault and the nw–se trending santander massif into two branches that form the perijá range in the west and the mérida andes in the east. ward et al. (1973) did a detailed study on the regional geology of the santander massif. the santander massif exposes its metamorphic basement, which is composed by deformed and metamorphosed rocks that have been tectono-stratigraphically divided into three metamorphic units (bucaramanga gneiss complex, silgará formation and orthogneiss) and cut by intrusive bodies the emplacements of which record caledonian to jurassic magmatic events (goldsmith et al., 1971; banks et al., 1985; boinet et al., 1985; dörr et al., 1995; restrepo-pace, 1995; ordoñez, 2003; ordóñez and mantilla, 2005). the bucaramanga gneiss complex is a sequence of high-grade metamorphic rocks (up to the upper amphibolite facies), which is mainly composed of pelitic gneisses with minor amounts of amphibolites and orthogneisses, and in part migmatitic. the silgará formation is a metamorphic sequence of lowand middle-grade (greenchist and lower amphibolite facies), which consists of meta-siliceous rocks with intercalations of orthoamphibolites intruded by granitoids and gabbros. a gneissose granite (orthogneiss unit) intrudes into the bucaramanga gneiss complex and a wide zone of migmatite is developed around it. the orthogneiss unit generally shows concordant foliation and lineation with those of neighboring metamorphic rocks of the bucaramanga gneiss complex the silgará formation. according to restrepo-pace et al. (1997), the k-ar hornblende ages of 945±140 ma from hornblende gneiss (goldsmith et al., 1971) and rb-sr whole-rock of 680±140 ma from biotite gneiss (goldsmith et al., 1971) 40ar/39ar hornblende of 668±9 ma and 574±8 ma from hornblende gneisses (restrepo-pace, 1995; restrepo-pace et al., 1997), suggests a nickeriense tectonometamorphic event. a tectono-metamorhic event during the early to middle ordovicic affected the pelitic protolith of the silgará formation and a series of syn-tectonic granites, according to ages rb-sr whole-rock of 450±80 ma from gneiss and k-ar hornblende of 413±30 ma from metadiorite (goldsmith et al., 1971), u-pb zircon of 417 ma and 392 ma (banks et al., 1985) and u-pb zircon of 477±16 ma restrepo-pace, 1995). this event has been associated with the caparonensis orogenic cycle in the mérida andes (gonzález de juana et al., 1950), which was accompanied by several syn-tectonic in126 óscar m. castellanos a., carlos a. ríos r. and akira takasu trusions with ages between 460 and 480 ma (restrepo-pace, 1995), and may be correlated with the caledonian orogeny, which was characterized by folding, metamorphism and uplift. the basement of the santander massif is unconformably overlain by unmetamorphosed rocks of the middle devonian floresta formation. the lack of regional metamorphism in the devonian sedimentary units suggests that the last metamorphic event in the eastern colombian andes occurred before the beginning of deposition of the upper emsian beds (forero, 1990). silurian fossils [brachiopod aenigmastrophia sp. 127 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes figure 1. map of northern south america showing major structural features. 1: oca el pilar fault system; 2: sierra nevada de santa marta; 3: santa marta bucaramanga fault system; 4: perijá range; 5: mérida andes; 6: eastern cordillera; 7: central cordillera; 8: western cordillera; 9: baudo range; 10: macarena range; 11: guaycaramo fault system. the santander massif and the study area are indicated by a shadow area and by a white rectangle, respectively. (ludlowvian)] are present in low-grade metamorphic rocks of the santander massif near guaca (forero, 1990). the basement of the santander massif is unconformably overlied by unmetamorphosed rocks of the middle devonian floresta formation. carboniferous and permian clastic to calcareous sediments unconformably overlay this unit. due to pre-cretaceous erosional events, these paleozoic sequences are restricted in their occurrence (kammer, 1993). at the end of the late paleozoic orogenic event, calc-alkaline batholiths (santander plutonic complex) were intruded during the triassic-jurassic time. several k-ar ages on biotite from plutons and batholiths that crop out in the santander massif reflect a magmatic event during the late triassic early jurassic. the regional isograds are not parallel to the outline of the intrusive rocks. the orthogneisses exhibit concordant relationships with the high-grade metamorphic rocks, and may be interpreted as syn-tectonic intrusives emplaced at close to the peak metamorphism. post-paleozoic sediments, which overlay the upper paleozoic marine sediments, include the bocas and jordán formations. the latter consists of fine-grained red beds with interlayered volcanic rocks and intrusive clasts, a is discordantly overlaid by the red bed sequence of the girón formation of jurassic age. cretaceous sedimentary rocks occur around the santander massif and are preserved as erosional relicts. the structural setting of the santander massif is dominated by complex faulting, and the most conspicuous fault is the active northwest-trending linear left-lateral strike-slip bucaramanga santa marta fault, which appears to have had a long history (goldsmith et al., 1971). in the last 20 years, the metamorphic rocks of the santander massif have been the focus of attention of an increasing number of studies (e.g., restrepo-pace, 1995; restrepo-pace et al., 1997; schäfer et al., 1998; garcía y ríos, 1997, 1998; garcía y castellanos, 1998; campos, 1999; garcía y ríos, 1999; montenegro y barragán, 1999; ríos, 1999, 2001; ríos and takasu, 1999, 2000; ríos and garcía, 2001a, 2001b; garcía y campos, 2000; castellanos, 2001; mantilla et al., 2001; 2002; 2003; gélvez and márquez, 2002; ríos et al., 2003a, 2003b; cardona, 2003; castellanos et al., 2004; garcía et al, 2005; ríos, 2005; gómez y ávila, 2006), which have contributed to its geological knowledge and pointed out the importance of this ancient massif as a key for understanding the complex geotectonic evolution of the northwestern continental margin of south america. these studies define a regional metamorphism of middle to high grade that reaches the upper amphibolite facies and varies according to the region in the massif. in this paper, we present petrological data for the silgará formation metapelitic rocks that crop out at the central santander massif (csm). we examine metamorphic textures and mineral assemblages to establish the stages of evolution of the metamorphism, use phase diagram analysis and p–t grid to study the sequence of mineral assemblages and reaction history, and apply the gtb program of spear and kohn (2001) to estimate metamorphic p–t conditions and define the p–t paths experienced by the silgará formation pelitic rocks. however, further studies should integrate this information with both previously published data and structural and geochronological data in order to improve our knowledge of this metamorphic unit and constraint on its tectono-metamorphic evolution in the geological context of the colombian andes. geological setting in the csm, two distinct geological domains constitute the crystalline basement (figure 2): the deformed and metamorphosed rocks of the bucaramanga gneiss complex, silgará formation and orthogneiss and the igneous succession developed after (1) syn-orogenic magmatismwith alkaline affinity during the paleozoic and (2) post-orogenic magmatism with calc-alkaline affinity during triassic-jurassic (goldsmith et al., 1971; banks et al., 1985; boinet et al., 1985; dörr et al., 1995; restrepo-pace, 1995; ordoñez, 2003; ordóñez and mantilla, 2005). the rocks of interest in this study are referred to as the lower paleozoic metamorphic sequence of the silgará formation that crops out in two n-s trending strips and is interrupted by the presence of dykes and sills of orthoamphibolites with banded 128 óscar m. castellanos a., carlos a. ríos r. and akira takasu to gabbroic structures. the silgará formation is mainly composed of metapelitic rocks with minor intercalated psammitic, semipelitic, metabasic and metacarbonate rocks (figure 3), which were affected by a caledonian regional metamorphism that producedmetamorphic zonation ranging from the biotite zone through the garnet and staurolite-kyanite zones up to the sillimanite zone of the typical barrovian zonal scheme under lowto high-temperature and medium-pressure conditions. gómez y avila (2006) carried out a study of the metamorphic sequence cropping out along the valegra stream, making emphasis on the metacarbonate and associated rocks that occur in the contact between marble and polite layers. a broad spectrum of physical conditions, varying from greenschist facies to amphibolite fa129 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes x = 1’317.000 x = 1’304.000 y = 1 ’1 2 7 .0 0 0 x = 1’290.000 y = 1 ’1 4 9 .5 0 0 y = 1 ’1 3 2 .5 0 0 x = 1’290.500 y = 1 ’1 4 7 .0 0 0 mesozoic-cenozoic sedimentary rocks triassic-jurassic intrusive rocks orthogneiss silgará formation bucaramanga gneiss complex migmatization isograd lithological contact fault legend figure 2. generalized geological map of the csmmodified after ward et al., (1970) and garcía et al., (2005), showing the study area as well as the sampling localities of metapelitic rocks of the silgará formation. 130 óscar m. castellanos a., carlos a. ríos r. and akira takasu a b c d e f 5.0 cm 2.0 cm 2.0 cm grt bt sil figure 3. field and hand-specimen photographs from the silgará formation metamorphic rocks at the csm. (a) outcrop of marble quarry in which marble and amphibolite are interlayered with pelitic rocks. (b) metacarbonate rocks (pure and impure marbles, carbonate-silicate rocks, calc-silicate rocks and carbonate-bearing silicate rocks), which form a very complex group within the metamorphic sequence. (c) garnet-bearing pelitic schists displaying a pervasively schistosity, which represent the most common type of metamorphic rock. (d) recumbent fold (with a subhorizontal axial plane strikes n40ºw and dipping 25ºne) in marble interlayered with pelitic schists. (e) polished hand specimen of a pelitic schist (sample pcm-971), showing schistose structure and the largest garnet porphyroblasts in the study area, alternating with non-continuous quartz-rich bands. (f) polished hand specimen of a pelite (pcm-940), showing a schistose and microfolding (tight structure) with a millimeter-scale compositional banding of alternating and non-continuous biotite and sillimanite bands. cies, have been attributed to the investigated metamorphic sequence. recent studies (campos, 1999; garcía y ríos, 1999; montenegro y barragán, 1999; garcía y campos, 2000; castellanos, 2001; castellanos et al., 2004; garcía et al., 2005) reveal a complex metamorphic and deformational history for this geological unit. castellanos (2001) determined the p-t conditions for the high-t and medium-p facies series terrane at the csm. recently, garcía et al (2005) defined a clockwise p-t path followed by the metamorphic basement in this region. however, regarding to the tectono-metamorphic evolution of the silgará formation, few studies have been done so far. here, we re-examine the metapelitic rocks of this metamorphic unit because they are highly sensitive to changes in pressure and temperature conditions and preserve textural, structural and mineralogical evidence that provide important clues to constrain the tectono-metamorphic evolution of the silgará formation at the csm, because they preserve mineral assemblages from different tectonic events that are appropriate for determining the metamorphic p-t conditions. these rocks show overprinting relationships allowing the recognition of three main deformation events (d1, d2, and d3) at the csm, and during the last stage they were affected by an extensive retrograde metamorphism. the first deformation phase d1may be associated to crustal shortening and thickening, developing s1. the d1 deformational fabrics sometimes are poorly preserved due to overprinting and transposition by subsequent deformational events. however, evidence for this event, which is characterized by the mineral assemblage quartz + ilmenite + graphite ± muscovite, includes both inclusion trails in garnet or staurolite porphryroblasts and differentiated layering in hinge zones, defined by alternating phillosilicateand quartz-feldspathic-rich layers. deformation d2 as a result of heating and thrusting developed a dominant schistosity (s2), which is the regional foliation formed at the same time with the first-generation of folds and thrusts, and represented by the mineral assemblage quartz + muscovite + biotite + ilmenite + garnet + staurolite + kyanite ± andalusite ± plagioclase. regional-scale ductile shear zones with a mylonitic foliation parallel to the regional foliation (s2) are associated with d2. d3 has also resulted in the folding or crenulation of s2 and the development of a weak spaced cleavage (s3), which is represented by the mineral assemblage quartz + sillimanite + biotite + garnet. in general, later structures such as flat-lying crenulations, small chevron folds and kink bands overprint the main foliation = d3?. the most recent deformation is observed in all lithologies and has produced the development of tectonites, which could be associated to younger structures such as faulting. a final movement along this tectonic discontinuity is post-d3. this phase of deformation may be corresponding to the last uplift of the eastern cordillera the middle miocene. petrography and mineral chemistry to constrain the p-t conditions of metamorphism of the silgará formation at the csm, 60 samples of metapelitic rocks were petrographycally studied. metapelites display a well-developed schistosity defined by the preferred orientation of muscovite and biotite flakes. the schistosity is also due to the alignment of phylosilicate flakes and crystals of opaque minerals or graphite, sometimes along with aggregates of aluminosilicates, mainly kyanite and sillimanite (fibrolite). biotite is not only present as small flakes underlying the planar structure but also as lozenge-shaped crystals which represent early porphyroblasts and their asymmetry is sometimes different to useful to determine sense of shear. a last generation of biotite appears as undeformed crystals with their cleavage perpendicular or randomly oriented to the schistosity and can be interpreted as postkinematic. quartz and plagioclase form granoblastic-rich fabrics between the mica-rich fabrics. garnets commonly occur as euhedral to subhedral porphyroblasts; their cores are frequently rich in small inclusions of quartz, plagioclase, opaque minerals and graphite, and their rims are poorer in inclusions. in some cases, inclusion patterns define an internal foliation (si) an external foliation (se) = (s2)?, producing sigmoidal inclusions. staurolite occurs as porphyroblasts elongated in the 131 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes foliation, usually showing sigmoidal patterns of inclusions that indicate a synkinematic crystallization, and as porphyroblasts oriented across the foliation that indicate post-kinematic crystallization. both generations of staurolite are rich in quartz and opaque mineral inclusions, although graphite also occurs. accessory mineral phases include opaque minerals, graphite, epidote, apatite and zircon. small scale folding of the schistosity planes has produced a crenulation cleavage, accompanied by segregation of quartz into horizontal layers. these rocks frequently contain evidence of retrograde metamorphism, involving hydration processes, including partial replacement of garnet, biotite and staurolite by chlorite, staurolite by white mica and plagioclase and staurolite by sericite. figures 4 and 5 illustrate the main petrographical aspects of the investigated silgará formation pelites. mineral abbreviations are after kretz (1983). mineral compositions were analyzed using a jeol jxa 8800m electron microprobe analyzer at the geosciences department of shimane university (japan), under the following analytical conditions: accelerating voltage 15 kv, specimen current 2.5 x 10-8 a, correction method after bence and albee (1968). representative analyses of mineral phases are given in tables 1 to 6. muscovite shows a subidioblastic character, and is usually colorless. in general, it shows straight and serrated boundaries and develops a directional lepidoblastic fabric. it is associated with biotite and chlorite. the first type of muscovite occurs as aggregates in the lepidoblastic bands defining the main schistosity of the rock. in some cases bands are deformed following the shape of porphyroblasts of garnet, plagioclase, staurolite or aluminosilicates. it is sometimes replaced by fibrolite. the second type of muscovite occurs as porphyroblasts or within the lepidoblastic bands, and contains inclusions of opaque minerals, quartz and enclosed relicts of staurolite suggesting retrograde metamorphism. the third type of muscovite, occurs as random distributed and oriented inclusions in plagioclase, staurolite, quartz and garnet. the mineral chemistry of muscovite is depicted in table 1 and illustrated in figure 6. the si content is 5.94-6.39 p.f.u. based on 22 anhydrous oxygens (deer et al., 1982). the compositions of muscovite, xna=na/(na+k) varies from 0.02 to 0.36. muscovites from the mineral assemblages st+grt and ky+and+sil shown two groups, one with higher xna than the average from the other samples. biotite shows a subidioblastic character, occurs as aggregates in thin and thick bands, and also as individual grains of different sizes, and as small inclusions in garnet, staurolite and plagioclase. in some cases porphyroblasts are parallel or completely perpendicular to se, suggesting post-tectonic growth for the second case. it shows invariably brown in color, a mottled appearance, straight and serrated boundaries and pleochroism x: dark brown, y: pale brown, z: brown. sometimes biotite is partial to totally replaced by chlorite. the mineral chemistry of biotite is depicted in table 2. the fe/(fe+mg) ratio of biotite ranges from 0.37 to 0.79 and aliv varies from 2.50 to 2.83. biotite from samples pcm-18, pcm-49 and pcm-610 shown fe/(fe+mg) ratios ranging from 0.24 to 0.30 and aliv varies from 2.27 to 2.58, corresponding to phlogopite component. ti content varies from 0.93-3.31. however, no relation was found between content of ti in biotite and grade of metamorphism. chlorite is considered to be a retrograde phase after biotite, garnet and amphibole. si content is 5.05-5.95 p.f.u. based on 28 anhydrous oxygens, and xmg varies between 0.30 and 0.74. garnet occurs as porphyroblasts with idioblastic to xenoblastic character. it is also possible to distinguish two types of garnets according to their texture: poikiloblastic and sector zoned garnets. they sometimes show a trail of inclusions of quartz, graphite and opaque minerals almost perpendicular to the se of the rock, and most are partially included in staurolite porphyroblasts. some garnets are sub-rounded to rounded and show a rotational trail of inclusions of quartz or s-shaped spiral structure. garnets rarely show concordant internal schistosity trails of inclusions of quartz and opaque minerals. in addition, poikiloblastic garnets in some samples show elongated shape suggesting dendritic character possibly due to high rate of growth. the mineral chemistry 132 óscar m. castellanos a., carlos a. ríos r. and akira takasu 133 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes t ab le 1. r ep re se nt at iv e ch em ic al s co m po si tii on s of m us co vi te fr om pe lit ic r oc ks of th e si lg ar á f or m at io n at th e c sm . l it h o lo g y p s p s p s p s p s p s p s p s p s p s p s p s p s p s p s p s s a m p le n o . p c m -1 8 p c m -2 0 p c m -2 1 p c m -4 7 p c m -4 9 p c m -6 1 0 p c m -6 1 8 p c m -6 5 1 p c m -7 0 0 p c m -7 0 1 p c m -7 0 2 p c m -8 8 5 p c m -8 9 3 p c m -8 9 4 p c m -8 9 7 p c m -9 4 0 a n a ly si s n o . 7 1 4 4 6 1 7 2 2 8 5 3 3 4 4 4 w e ig h t % in c . b t m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix s io 2 4 5 .0 9 4 5 .7 7 4 7 .4 1 4 5 .4 3 4 7 .8 1 4 5 .6 9 4 7 .6 9 4 5 .2 2 4 5 .4 8 4 5 .0 7 4 5 .2 3 4 5 .3 6 4 4 .9 1 4 6 .3 4 4 4 .6 1 4 5 .4 3 t io 3 0 .5 8 0 .2 3 0 .3 7 0 .3 2 0 .3 1 0 .6 5 0 .7 0 0 .7 9 0 .2 2 0 .3 7 0 .3 9 0 .4 3 0 .6 8 0 .9 6 0 .9 2 0 .9 2 a l 2 o 3 3 4 .6 6 3 5 .3 4 3 4 .2 9 3 5 .0 9 3 3 .0 1 3 4 .2 7 3 5 .8 2 3 4 .8 0 3 5 .8 7 3 6 .0 0 3 5 .6 9 3 2 .0 7 3 5 .9 2 3 5 .7 4 3 4 .0 5 3 6 .4 0 f e o * 2 .9 5 2 .4 3 2 .9 8 0 .7 5 2 .7 7 3 .4 2 1 .0 6 1 .1 7 0 .6 8 0 .5 5 0 .8 1 4 .0 5 1 .1 3 1 .6 5 2 .3 4 1 .2 6 m n o 0 .0 4 0 .0 0 0 .0 1 0 .0 2 0 .0 1 0 .0 1 0 .0 0 0 .0 1 0 .0 4 0 .0 0 0 .0 2 0 .0 0 0 .0 0 0 .0 1 0 .0 6 0 .0 0 m g o 0 .7 1 0 .5 7 0 .9 7 0 .4 9 0 .9 2 1 .1 2 0 .5 8 0 .6 2 0 .2 3 0 .1 7 0 .3 3 0 .5 9 0 .5 8 0 .8 2 0 .7 1 0 .8 1 n a 2 o 1 .1 7 2 .3 6 0 .9 1 1 .5 4 0 .5 7 0 .1 6 0 .8 2 0 .8 6 2 .3 1 2 .3 9 2 .2 5 0 .3 1 0 .5 7 0 .4 9 0 .4 3 0 .4 2 k 2 o 1 0 .3 3 7 .4 9 9 .3 1 9 .2 8 9 .0 6 1 0 .2 3 9 .3 4 1 0 .4 2 8 .3 0 8 .0 9 8 .2 9 1 0 .5 2 1 0 .0 4 1 0 .8 2 1 0 .1 8 1 0 .5 8 t o ta l 9 5 .5 3 9 4 .2 2 9 6 .2 7 9 2 .9 9 9 4 .5 0 9 5 .6 0 9 6 .0 5 9 3 .8 9 9 3 .2 0 9 3 .0 9 9 3 .0 9 9 3 .3 5 9 3 .8 7 9 6 .8 2 9 3 .3 3 9 5 .8 7 c a ti o n s p e r 2 2 o xy g e n s s i 6 .0 6 4 6 .1 2 7 6 .2 5 4 6 .1 6 0 6 .3 9 4 6 .1 2 2 6 .2 3 4 6 .1 2 1 6 .1 2 9 6 .0 9 8 6 .1 0 9 6 .2 6 2 6 .0 5 6 6 .0 9 5 6 .1 0 4 6 .0 1 9 t i 0 .0 5 9 0 .0 2 3 0 .0 3 6 0 .0 3 3 0 .0 3 1 0 .0 6 6 0 .0 6 9 0 .0 8 0 0 .0 2 2 0 .0 3 8 0 .0 4 0 0 .0 4 5 0 .0 6 9 0 .0 9 5 0 .0 9 5 0 .0 9 1 a l 5 .4 9 3 5 .5 7 6 5 .3 3 0 5 .6 0 8 5 .2 0 2 5 .4 1 1 5 .5 1 8 5 .5 5 1 5 .6 9 6 5 .7 3 9 5 .6 8 0 5 .2 1 8 5 .7 0 8 5 .5 3 9 5 .4 8 9 5 .6 8 3 f e 2 + 0 .3 3 1 0 .2 7 2 0 .3 2 8 0 .0 8 5 0 .3 1 0 0 .3 8 4 0 .1 1 6 0 .1 3 3 0 .0 7 6 0 .0 6 3 0 .0 9 1 0 .4 6 8 0 .1 2 8 0 .1 8 2 0 .2 6 7 0 .1 3 9 m n 0 .0 0 5 0 .0 0 0 0 .0 0 1 0 .0 0 2 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 1 0 .0 0 5 0 .0 0 0 0 .0 0 3 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 7 0 .0 0 0 m g 0 .1 4 1 0 .1 1 3 0 .1 9 1 0 .0 9 8 0 .1 8 4 0 .2 2 4 0 .1 1 3 0 .1 2 5 0 .0 4 7 0 .0 3 4 0 .0 6 6 0 .1 2 1 0 .1 1 6 0 .1 6 1 0 .1 4 5 0 .1 5 9 n a 0 .3 0 6 0 .6 1 3 0 .2 3 2 0 .4 0 4 0 .1 4 7 0 .0 4 1 0 .2 0 9 0 .2 2 6 0 .6 0 2 0 .6 2 8 0 .5 9 0 0 .0 8 3 0 .1 4 9 0 .1 2 6 0 .1 1 5 0 .1 0 8 k 1 .7 7 1 1 .2 7 9 1 .5 6 7 1 .6 0 6 1 .5 4 5 1 .7 4 8 1 .5 5 7 1 .7 9 9 1 .4 2 6 1 .3 9 6 1 .4 2 9 1 .8 5 2 1 .7 2 7 1 .8 1 4 1 .7 7 6 1 .7 8 8 t o ta l 1 4 .1 7 0 1 4 .0 0 6 1 3 .9 4 3 1 4 .0 0 4 1 3 .8 2 0 1 4 .0 0 2 1 3 .8 1 9 1 4 .0 3 6 1 4 .0 1 2 1 4 .0 0 4 1 4 .0 1 8 1 4 .0 5 2 1 3 .9 5 7 1 4 .1 0 2 1 4 .0 0 2 1 3 .9 9 5 * t ot al f e as f eo . 134 óscar m. castellanos a., carlos a. ríos r. and akira takasu t ab le 2. r ep re se nt at iv e c he m ic al s co m po si tio ns of bi ot ite fr om pe le tic r oc ks of th e si lg ar á f om at io n at th e c sm . l it h o lo g y p s p s p s p s p s p s p s p s p s p s p s p s p s p s p s p s s a m p le n o . p c m -1 8 p c m -2 0 p c m -2 1 p c m -2 8 p c m -4 9 p c m -6 1 0 p c m -6 1 8 p c m -6 5 1 p c m -8 8 5 p c m -8 9 3 p c m -8 9 4 p c m -8 9 7 p c m -9 4 0 p c m -9 5 3 p c m -9 7 1 p c m -9 7 2 a n a ly si s n o . 1 1 1 1 1 2 1 0 1 1 1 1 1 1 1 4 1 w e ig h t % in c . b t m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix m a tr ix s io 2 3 6 .9 5 3 6 .9 8 3 6 .5 7 3 5 .7 0 3 9 .6 7 3 8 .2 0 3 5 .0 0 3 4 .9 2 3 3 .1 0 3 3 .8 5 3 4 .7 7 3 4 .1 9 3 4 .8 4 3 4 .7 4 3 4 .2 9 3 3 .6 1 t io 3 1 .0 8 1 .3 7 1 .2 4 1 .4 1 0 .9 3 0 .9 6 2 .5 1 2 .0 8 1 .9 5 2 .9 1 2 .8 8 3 .0 8 3 .0 1 3 .0 7 2 .7 1 2 .9 8 a l 2 o 3 1 8 .9 2 1 8 .1 9 1 9 .4 8 1 8 .5 6 1 8 .7 3 2 0 .4 9 1 8 .6 3 1 9 .3 5 1 7 .8 1 1 9 .6 1 1 8 .9 4 1 8 .0 5 1 9 .7 5 1 9 .4 0 1 9 .2 5 1 9 .6 2 f e o * 1 0 .7 8 1 4 .7 5 1 5 .1 6 1 8 .3 5 1 1 .8 4 1 0 .5 2 2 0 .7 3 2 0 .1 9 2 7 .5 7 2 1 .0 5 1 8 .6 5 2 2 .4 5 2 0 .0 1 1 9 .5 7 2 1 .1 7 2 1 .3 0 m n o 0 .4 0 0 .0 2 0 .1 8 0 .0 0 0 .2 7 0 .1 6 0 .0 1 0 .0 3 0 .2 3 0 .1 9 0 .3 1 0 .3 1 0 .2 2 0 .3 0 0 .1 1 0 .3 1 m g o 1 6 .7 9 1 4 .0 4 1 3 .2 1 1 1 .9 1 1 5 .2 6 1 8 .5 5 8 .0 9 9 .3 3 4 .2 1 7 .8 7 9 .2 0 7 .9 3 8 .6 2 8 .1 6 7 .2 2 7 .9 8 n a 2 o 0 .4 7 0 .5 7 0 .4 1 0 .4 7 0 .3 7 0 .1 3 0 .4 6 0 .3 8 0 .1 7 0 .4 0 0 .3 6 0 .4 9 0 .3 9 0 .3 4 0 .3 5 0 .4 2 k 2 o 8 .7 5 9 .1 5 8 .3 6 8 .8 6 8 .1 8 6 .6 7 8 .9 5 9 .3 9 9 .7 5 9 .5 3 1 0 .2 5 1 0 .0 8 9 .6 7 9 .6 1 9 .1 9 9 .7 1 t o ta l 9 4 .2 0 9 5 .0 7 9 4 .6 1 9 5 .3 0 9 5 .3 3 9 5 .7 4 9 4 .3 8 9 5 .7 1 9 4 .7 9 9 5 .4 8 9 5 .3 6 9 6 .5 7 9 6 .5 2 9 5 .2 3 9 4 .2 9 9 6 .0 4 c a ti o n s p e r 2 2 o xy g e n s s i 5 .4 3 8 5 .5 0 2 5 .4 4 7 5 .3 9 5 5 .7 2 7 5 .4 1 8 5 .4 1 2 5 .3 2 1 5 .3 3 4 5 .2 1 5 5 .3 1 9 5 .2 7 1 5 .2 7 0 5 .3 1 9 5 .3 3 4 5 .1 6 7 t i 0 .1 1 9 0 .1 5 3 0 .1 3 8 0 .1 6 0 0 .1 0 1 0 .1 0 2 0 .2 9 2 0 .2 3 8 0 .2 3 6 0 .3 3 7 0 .3 3 2 0 .3 5 7 0 .3 4 2 0 .3 5 4 0 .3 1 7 0 .3 4 5 a l 3 .2 8 2 3 .1 9 0 3 .4 2 0 3 .3 0 6 3 .1 8 7 3 .4 2 5 3 .3 9 6 3 .4 7 4 3 .3 8 3 3 .5 6 2 3 .4 1 4 3 .2 8 0 3 .5 2 0 3 .5 0 1 3 .5 2 8 3 .5 5 6 f e 2 + 1 .3 2 7 1 .8 3 5 1 .8 8 8 2 .3 1 9 1 .4 2 9 1 .2 4 8 2 .6 8 0 2 .5 7 2 3 .7 1 5 2 .7 1 2 2 .3 8 5 2 .8 9 5 2 .5 3 1 2 .5 0 5 2 .7 5 3 2 .7 3 9 m n 0 .0 4 9 0 .0 0 3 0 .0 2 3 0 .0 0 0 0 .0 3 3 0 .0 1 9 0 .0 0 1 0 .0 0 4 0 .0 3 1 0 .0 2 4 0 .0 4 0 0 .0 4 0 0 .0 2 8 0 .0 3 9 0 .0 1 4 0 .0 4 0 m g 3 .6 8 4 3 .1 1 5 2 .9 3 3 2 .6 8 3 3 .2 8 4 3 .9 2 2 1 .8 6 6 2 .1 1 9 1 .0 1 0 1 .8 0 7 2 .0 9 7 1 .8 2 3 1 .9 4 3 1 .8 6 2 1 .6 7 4 1 .8 3 0 n a 0 .1 3 4 0 .1 6 4 0 .1 1 9 0 .1 3 8 0 .1 0 4 0 .0 3 7 0 .1 3 9 0 .1 1 2 0 .0 5 3 0 .1 2 0 0 .1 0 7 0 .1 4 7 0 .1 1 3 0 .1 0 1 0 .1 0 6 1 .1 2 4 k 1 .6 4 2 1 .7 3 6 1 .5 8 8 1 .7 0 8 1 .5 0 6 1 .2 0 6 1 .7 6 5 1 .8 2 5 2 .0 0 3 1 .8 7 3 1 .9 9 9 1 .9 8 2 1 .8 6 6 1 .8 7 7 1 .8 2 2 1 .9 0 5 t o ta l 1 5 .6 8 7 1 5 .6 9 9 1 5 .5 5 8 1 5 .7 1 4 1 5 .3 8 2 1 5 .3 8 9 1 5 .5 5 0 1 5 .6 7 1 1 5 .7 6 6 1 5 .6 6 0 1 5 .6 9 5 1 5 .7 9 5 1 5 .6 1 6 1 5 .5 6 3 1 5 .5 4 9 1 5 .7 2 1 * t ot al f e as f eo . 135 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes t ab le 3. r ep re se nt at iv e c he m ic al s co m po si tio ns of bi ot ite fr om pe le tic r oc ks of th e si lg ar á f om at io n at th e c sm . l it h o lo g y p s p s p s p s p s p s p s s a m p le n o . p c m -4 7 p c m -6 1 8 p c m -6 5 1 p c m -7 0 0 p c m -7 0 1 p c m -7 0 2 p c m -9 5 3 a n a ly si s n o . 3 1 2 1 p 2 p 5 s z 6 s z 1 4 1 3 1 7 9 1 6 1 7 1 1 8 w e ig h t % m a n tl e ri m c o re ri m ri m c o re c o re ri m c o re ri m c o re ri m c o re ri m c o re m a n tl e s io 2 3 6 .4 4 3 7 .1 0 3 7 .0 6 3 7 .3 1 3 6 .8 4 3 6 .5 8 3 7 .7 4 3 7 .5 1 3 6 .0 7 3 6 .1 0 3 6 .6 8 3 6 .5 2 3 6 .7 0 3 7 .0 6 3 7 .3 1 3 7 .2 8 t io 2 0 .1 4 0 .0 2 0 .0 1 0 .0 0 0 .0 0 0 .0 1 0 .0 4 0 .0 2 0 .0 2 0 .0 4 0 .0 3 0 .0 4 0 .0 0 0 .0 0 0 .0 0 0 .0 2 a l 2 o 3 2 0 .2 3 2 0 .5 7 2 1 .0 2 2 1 .2 2 2 0 .6 8 2 0 .6 2 2 1 .0 1 2 1 .1 1 1 9 .9 5 2 0 .1 6 2 0 .3 6 2 0 .4 5 2 0 .4 6 2 0 .7 3 2 1 .0 3 2 1 .0 8 f e o * 3 3 .6 9 3 5 .6 6 3 6 .2 8 3 6 .2 8 3 4 .7 5 3 5 .2 3 3 4 .2 9 3 4 .7 2 4 1 .0 6 4 1 .5 5 3 8 .0 8 3 8 .8 3 3 7 .0 9 3 9 .1 3 3 1 .2 4 3 4 .6 7 m n o 4 .0 4 1 .4 9 0 .7 9 0 .3 3 0 .1 0 0 .9 9 1 .0 7 0 .1 6 0 .0 3 0 .0 2 0 .8 8 0 .3 1 1 .6 2 0 .1 9 6 .0 7 2 .4 6 m g o 1 .8 8 3 .0 6 2 .5 3 2 .8 9 2 .6 9 1 .7 6 2 .6 9 3 .0 1 1 .3 7 1 .3 9 1 .5 7 1 .8 2 1 .7 2 2 .4 9 1 .9 6 3 .0 5 c a o 3 .4 6 2 .6 6 2 .9 2 2 .8 1 3 .3 1 3 .5 9 4 .0 9 3 .3 5 0 .3 7 0 .3 1 1 .5 8 1 .0 9 1 .6 2 0 .7 7 2 .9 7 1 .9 9 n a 2 o 0 .0 1 0 .0 1 0 .0 0 0 .0 5 0 .0 4 0 .0 0 0 .0 0 0 .0 2 0 .0 7 0 .0 1 0 .0 1 0 .0 9 0 .0 0 0 .0 5 0 .1 1 0 .0 0 k 2 o 0 .0 1 0 .0 5 0 .0 5 0 .0 1 0 .0 1 0 .0 3 0 .0 3 0 .0 5 0 .0 1 0 .0 5 0 .0 5 0 .0 4 0 .0 4 0 .0 5 0 .0 4 0 .0 2 c r 2 o 3 0 .0 0 0 .0 0 0 .0 0 0 .0 1 0 .0 1 0 .0 2 0 .0 3 0 .0 1 0 .0 1 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 2 0 .0 1 0 .0 0 t o ta l 9 9 .9 0 1 0 0 .6 0 1 0 0 .7 6 1 0 0 .9 1 9 8 .4 4 9 8 .8 2 1 0 0 .9 9 9 9 .9 5 9 8 .9 6 9 9 .6 3 9 9 .2 4 9 9 .1 5 9 9 .2 5 1 0 0 .4 8 1 0 0 .7 4 1 0 0 .5 8 c a ti o n s p e r 1 2 o s i 2 .9 7 5 2 .9 8 7 2 .9 7 9 2 .9 8 3 3 .0 0 9 2 .9 9 8 3 .0 0 6 3 .0 0 8 3 .0 0 0 2 .9 8 7 3 .0 1 5 3 .0 0 4 3 .0 1 1 3 .0 0 1 3 .0 0 2 2 .9 9 0 t i 0 -0 0 9 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 3 0 .0 0 1 0 .0 0 1 0 .0 0 2 0 .0 0 2 0 .0 0 2 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 a l 1 .9 4 7 1 .5 1 1 .9 9 2 2 .0 0 0 1 .9 9 0 1 .9 9 2 1 .9 7 1 1 .9 9 5 1 .9 5 6 1 .9 6 5 1 .9 7 2 1 .9 8 3 1 .9 7 9 1 .9 7 9 1 .9 9 3 1 .9 9 2 f e 3 + 0 .0 8 5 0 .0 7 4 0 .0 4 9 0 .0 3 4 0 .0 0 0 0 .0 1 1 0 .0 1 2 0 .0 0 0 0 .0 4 1 0 .0 5 7 0 .0 0 0 0 .0 0 4 0 .0 0 0 0 .0 2 0 0 .0 0 3 0 .0 2 5 f e 2 + 2 .2 1 6 2 .3 2 6 2 .3 9 6 2 .3 9 2 2 .3 7 3 2 .4 0 3 2 .2 7 2 2 .3 2 9 2 .8 1 5 2 .8 1 8 2 .6 1 7 2 .6 6 8 2 .5 4 5 2 .6 2 9 2 .0 9 8 2 .3 0 0 m n 0 .2 7 9 0 .1 0 2 0 .0 5 4 0 .0 2 2 0 .0 0 7 0 .0 6 9 0 .0 7 2 0 .0 1 1 0 .0 0 2 0 .0 0 1 0 .0 6 1 0 .0 2 2 0 .1 1 2 0 .0 1 3 0 .4 1 4 0 .1 6 7 m g 0 .2 2 9 0 .3 6 7 0 .3 0 3 0 .3 4 5 0 .3 2 7 0 .2 1 5 0 .3 1 9 0 .3 6 0 0 .1 7 0 0 .1 7 1 0 .1 9 2 0 .2 2 3 0 .2 1 0 0 .3 0 1 0 .2 3 5 0 .3 6 5 c a 0 .3 0 3 0 .2 2 9 0 .2 5 1 0 .2 4 1 0 .2 9 0 0 .3 1 5 0 .3 4 9 0 .2 8 7 0 .0 3 3 0 .0 2 7 0 .1 3 9 0 .0 9 6 0 .1 4 2 0 .0 6 6 0 .2 5 6 0 .1 7 1 n a 0 .0 0 4 0 .0 0 3 0 .0 0 0 0 .0 1 6 0 .0 1 4 0 .0 0 0 0 .0 0 0 0 .0 0 5 0 .0 2 3 0 .0 0 4 0 .0 0 2 0 .0 2 9 0 .0 0 1 0 .0 1 5 0 .0 3 6 0 .0 0 0 k 0 .0 0 3 0 .0 1 0 0 .0 1 1 0 .0 0 2 0 .0 0 3 0 .0 0 6 0 .0 0 7 0 .0 1 0 0 .0 0 3 0 .0 1 1 0 .0 1 0 0 .0 0 9 0 .0 0 9 0 .0 1 1 0 .0 0 9 0 .0 0 4 c r 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 0 2 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 t o ta l 8 .0 4 9 8 .0 5 0 8 .0 3 6 8 .0 3 5 8 .0 1 3 8 .0 1 1 8 .0 1 2 8 .0 0 7 8 .0 4 6 8 .0 4 4 8 .0 1 0 8 .0 3 9 8 .0 0 9 8 .0 3 6 8 .0 4 6 8 .0 1 7 * t ot al f e as f eo 2o 3 136 óscar m. castellanos a., carlos a. ríos r. and akira takasu t ab le 4. r ep re se nt at iv e c he m ic al s co m po si tio ns of st au ro lit e fr om pe le tic r oc ks of th e si lg ar á f om at io n at th e c sm . l it h o lo g y p s p s p s p s p s p s p s p s p s p s p s p s p s s a m p le n o . p c m -2 0 p c m -2 1 p c m -4 7 p c m -4 9 p c m -6 1 8 p c m -6 5 1 p c m -7 0 0 p c m -7 0 1 p c m -7 0 2 p c m -9 0 5 p c m -9 0 7 p c m -9 4 0 p c m -9 7 1 a n a ly si s n o . 8 1 7 3 4 8 2 1 8 8 7 4 5 4 w e ig h t % s io 2 2 6 .9 5 2 7 .9 4 2 7 .8 6 2 7 .7 4 2 6 .5 5 2 7 .5 6 2 6 .4 4 2 6 .9 7 2 8 .6 2 2 7 .3 1 2 7 .1 1 2 6 .5 4 2 6 .6 8 t io 2 0 .4 6 0 .4 6 0 .5 2 0 .4 0 0 .5 2 0 .6 3 0 .6 4 0 .7 0 0 .5 5 0 .5 8 0 .5 2 0 .8 1 0 .6 0 a l 2 o 3 5 4 .4 4 5 2 .8 1 5 3 .1 7 5 2 .7 7 5 4 .4 8 5 2 .6 8 5 3 .6 4 5 3 .7 8 5 1 .8 5 5 2 .9 7 5 3 .3 3 5 3 .7 6 5 3 .1 7 f e o * 1 3 .4 3 1 2 .7 9 1 3 .8 0 9 .2 2 1 3 .1 9 1 4 .5 0 1 3 .9 4 1 4 .7 6 1 4 .9 7 1 3 .7 9 1 3 .8 4 1 3 .1 6 1 4 .0 5 m n o 0 .0 2 0 .5 0 0 .0 5 1 .2 4 0 .0 1 0 .0 5 0 .0 1 0 .0 3 0 .0 3 0 .2 9 0 .4 1 0 .6 4 0 .1 5 m g o 2 .4 2 2 .2 7 1 .8 1 2 .3 9 1 .2 1 1 .5 5 0 .9 2 1 .5 2 1 .4 5 1 .9 4 1 .8 9 1 .5 4 1 .2 8 c a o 0 .0 3 0 .0 0 0 .0 0 0 .0 2 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 2 0 .0 2 0 .0 0 0 .0 1 0 .0 0 n a 2 o 0 .0 2 0 .0 0 0 .0 2 0 .1 4 0 .0 8 0 .0 1 0 .0 1 0 .0 0 0 .0 0 0 .0 3 0 .0 0 0 .0 7 0 .0 7 k 2 o 0 .0 1 0 .0 3 0 .0 3 0 .0 3 0 .0 4 0 .0 2 0 .0 1 0 .0 6 0 .0 3 0 .0 3 0 .0 3 0 .0 6 0 .0 4 z n o 0 .0 8 0 .6 1 0 .1 5 3 .7 0 1 .3 4 0 .5 0 0 .6 2 0 .0 5 0 .1 2 0 .0 5 0 .3 0 1 .1 6 1 .3 6 c r 2 o 3 0 .0 1 0 .0 4 0 .0 4 0 .0 3 0 .0 4 0 .0 8 0 .0 4 0 .0 5 0 .0 5 0 .0 6 0 .0 4 0 .0 5 0 .0 0 t o ta l 9 7 .8 7 9 7 .4 4 9 7 .4 4 9 7 .6 7 9 7 .4 7 9 7 .5 8 9 6 .2 7 9 7 .9 2 9 7 .6 7 9 7 .0 7 9 7 .4 7 9 7 .8 1 9 7 .3 9 c a ti o n s p e r 2 3 o xy g e n s s i 3 .7 3 8 3 .8 9 6 3 .8 8 4 3 .8 7 6 3 .7 2 0 3 .8 6 2 3 .7 4 8 3 .7 6 3 3 .9 9 9 3 .8 3 1 3 .7 9 5 3 .7 1 7 3 .7 6 1 t i 0 .0 4 8 0 .0 4 8 0 .0 5 4 0 .0 4 2 0 .0 5 5 0 .0 6 6 0 .0 6 9 0 .0 7 3 0 .0 5 7 0 .0 6 1 0 .0 5 5 0 .0 8 6 0 .0 6 4 a l 8 .9 0 0 8 .6 7 9 8 .7 3 7 8 .6 8 9 8 .9 9 5 8 .6 9 9 8 .9 6 3 8 .8 4 4 8 .5 3 9 8 .7 5 7 8 .7 9 7 8 .8 7 3 8 .8 3 5 f e 3 + f e 2 + 1 .5 5 8 1 .4 9 1 1 .6 0 9 1 .0 7 7 1 .5 4 5 1 .6 9 8 1 .6 5 3 1 .7 2 2 1 .7 4 9 1 .6 1 7 1 .6 2 0 1 .5 4 2 1 .6 5 7 m n 0 .0 0 2 0 .0 5 9 0 .0 0 6 0 .1 4 6 0 .0 0 1 0 .0 0 5 0 .0 0 1 0 .0 0 4 0 .0 0 3 0 .0 3 5 0 .0 4 8 0 .0 7 6 0 .0 1 7 m g 0 .5 0 0 0 .4 7 1 0 .3 7 7 0 .4 9 8 0 .2 5 3 0 .3 2 5 0 .1 9 4 0 .3 1 6 0 .3 0 1 0 .4 0 6 0 .3 9 4 0 .3 2 2 0 .2 6 8 c a 0 .0 0 4 0 .0 0 0 0 .0 0 0 0 .0 0 3 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 3 0 .0 0 3 0 .0 0 0 0 .0 0 1 0 .0 0 0 n a 0 .0 0 5 0 .0 0 0 0 .0 0 5 0 .0 3 9 0 .0 2 1 0 .0 0 2 0 .0 0 3 0 .0 0 0 0 .0 0 0 0 .0 0 8 0 .0 0 1 0 .0 1 9 0 .0 1 9 k 0 .0 0 2 0 .0 0 5 0 .0 0 5 0 .0 0 5 0 .0 0 1 0 .0 0 4 0 .0 0 1 0 .0 1 1 0 .0 0 5 0 .0 0 5 0 .0 0 5 0 .0 1 1 0 .0 0 7 z n 0 .0 0 8 0 .0 6 2 0 .0 1 5 0 .3 8 2 0 .1 3 9 0 .0 5 1 0 .0 6 5 0 .0 0 5 0 .0 1 2 0 .0 0 5 0 .0 3 1 0 .1 2 0 1 .1 4 1 c r 0 .0 0 1 0 .0 0 5 0 .0 0 5 0 .0 0 3 0 .0 0 4 0 .0 0 9 0 .0 0 5 0 .0 0 6 0 .0 0 5 0 .0 0 6 0 .0 0 5 0 .0 0 6 0 .0 0 0 t o ta l 1 4 .7 6 7 1 4 .7 1 7 1 4 .6 9 6 1 4 .7 5 9 1 4 .7 4 0 1 4 .7 2 2 1 4 .7 0 2 1 4 .7 4 4 1 4 .6 7 4 1 4 .7 3 3 1 4 .7 5 2 1 4 .7 7 2 1 4 7 7 1 * t ot al f e as f eo 137 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes t ab le 5. r ep re se nt at iv e c he m ic al s co m po si tio ns of a l 2 si o 5 po ly m or ph s fr om pe le tic r oc ks of th e s ilg ar á f om at io n at th e c sm . l it h o lo g y p s (a n d ) p s (a n d ) p s (a n d ) p s (a n d ) p s (k y) p s (k y) p s (k y) p s (k y) p s (s il ) p s (s il ) p s (s il ) p s (s il ) s a m p le n o . p c m -1 8 p c m -2 0 p c m -2 1 p c m -4 9 p c m -1 8 p c m -2 0 p c m -2 1 p c m -4 9 p c m -2 0 p c m -9 5 3 p c m -9 7 2 p c m -9 8 1 a n a ly si s n o . 1 3 1 0 1 1 8 1 6 1 1 7 7 8 3 w e ig h t % f ib ro lit e f ib ro lit e p ri sm a ti c p ri sm a ti c s io 2 3 6 .4 1 3 6 .5 0 3 6 .6 3 3 6 .7 8 3 6 .5 6 3 6 .5 9 3 6 .8 2 3 6 .5 9 3 6 .3 4 3 5 .8 1 3 5 .4 8 3 6 .4 9 t io 2 0 .0 0 0 .0 4 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 3 0 .0 0 0 .0 0 0 .0 3 0 .0 6 a l 2 o 3 6 3 .8 0 6 1 .8 6 6 2 .8 3 6 1 .9 3 6 3 .2 5 6 1 .7 0 6 2 .6 3 6 1 .3 0 6 3 .2 6 6 4 .0 5 6 3 .8 4 6 2 .1 8 f e o * 0 .6 6 0 .8 8 0 .8 2 0 .8 0 0 .9 6 0 .7 3 0 .7 6 0 .8 5 0 .3 3 0 .3 3 0 .1 4 0 .5 9 m n o 0 .0 2 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 2 0 .0 0 0 .0 0 0 .0 0 0 .0 4 0 .0 4 0 .0 0 m g o 0 .0 1 0 .0 1 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 2 0 .0 1 0 .0 0 0 .0 0 0 .0 3 0 .1 9 c a o 0 .0 0 0 .0 1 0 .0 0 0 .0 4 0 .0 2 0 .0 1 0 .0 2 0 .0 2 0 .0 0 0 .0 0 0 .0 1 0 .0 5 n a 2 o 0 .0 1 0 .0 0 0 .0 0 0 .0 1 0 .0 3 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 1 0 .0 2 k 2 o 0 .0 4 0 .0 4 0 .0 1 0 .0 5 0 .0 2 0 .0 5 0 .0 1 0 .0 3 0 .0 4 0 .0 3 0 .0 3 0 .3 0 c r 2 o 3 0 .0 4 0 .0 5 0 .0 3 0 .0 4 0 .0 2 0 .0 6 0 .0 2 0 .0 0 0 .0 3 0 .0 3 0 .0 0 0 .0 3 t o ta l 1 1 0 .9 8 9 9 .3 8 1 0 0 .3 3 9 9 .6 4 1 0 0 .8 7 9 9 .1 6 1 0 0 .2 7 9 8 .8 1 1 0 0 .0 0 1 0 0 .2 8 9 9 .5 9 9 9 .9 0 c a ti o n s p e r 5 o s i 0 .9 7 7 0 .9 9 6 0 .9 9 0 1 .0 0 0 0 .9 8 4 1 .0 0 0 0 .9 9 5 1 .0 0 3 0 .9 8 3 0 .9 6 7 0 .9 6 4 0 .9 9 1 t i 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 a l 2 .0 1 8 1 .9 8 9 2 .0 0 1 1 .9 8 5 2 .0 0 6 1 .9 8 7 1 .9 9 4 1 .9 8 0 2 .0 1 7 2 .0 3 8 2 .0 4 4 1 .9 9 1 f e 2 + 0 .0 1 5 0 .0 2 0 0 .0 1 9 0 .0 1 8 0 .0 2 2 0 .0 1 7 0 .0 1 7 0 .0 1 9 0 .0 0 8 0 .0 0 7 0 .0 0 3 0 .0 1 3 m n 0 .0 0 0 0 .0 0 0 0 .0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 0 m g 0 .0 0 1 0 .0 0 0 0 .0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 8 c a 0 .0 0 0 0 .0 0 0 0 .0 0 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 2 n a 0 .0 0 1 0 .0 0 0 0 .0 0 0 .0 0 0 0 .0 0 2 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 0 0 .0 0 1 7 k 0 .0 0 1 0 .0 0 1 0 .0 0 0 .0 0 2 0 .0 0 1 0 .0 0 2 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 1 0 c r 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 1 t o ta l 3 .0 1 4 3 .0 0 9 3 .0 1 0 3 .0 0 8 3 .0 1 4 3 .0 0 7 3 .0 0 8 3 .0 0 5 3 .0 0 9 3 .0 1 4 3 .0 1 5 3 .0 1 7 * t ot al f e as f eo 138 óscar m. castellanos a., carlos a. ríos r. and akira takasu t ab le 6. r ep re se nt at iv e c he m ic al s co m po si tio ns of pl ag io cl as e fr om pe le tic r oc ks of th e s ilg ar á f om at io n at th e c sm . l it h o lo g y p s p s p s p s p s p s p s p s p s s a m p le n o . p c m -2 0 p c m -2 1 p c m -4 7 p c m -4 9 p c m -6 1 8 p c m -6 5 1 p c m -7 0 1 p c m -7 0 2 p c m -9 5 3 a n a ly si s n o . 1 1 3 1 3 13 1 1 29 w e ig h t % co re ri m ri m co re in c. g rt m an tle ri m ri m co re s io 2 65 .6 5 52 .6 0 62 .0 4 58 .3 3 58 .3 1 60 .0 8 67 .2 3 67 .6 8 63 .5 8 t io 2 0. 00 0. 00 0. 02 0. 00 0. 00 0. 00 0. 02 0. 00 0. 00 a l 2 o 3 21 .9 9 29 .9 1 23 .1 8 26 .3 9 25 .2 8 24 .9 5 19 .6 6 19 .8 0 21 .9 5 f e o * 0. 00 0. 06 0. 05 0. 00 0. 28 0. 04 0. 00 0. 03 0. 05 m n o 0. 01 0. 02 0. 00 0. 00 0. 00 0. 00 0. 01 0. 00 0. 00 m g o 0. 00 0. 00 0. 01 0. 00 0. 01 0. 00 0. 01 0. 00 0. 00 c a o 2. 01 12 .2 2 4. 79 8. 53 7. 03 7. 08 0. 55 0. 74 3. 14 n a 2 o 10 .2 6 4. 38 9. 05 6. 78 7. 62 7. 53 11 .5 2 10 .3 7 9. 97 k 2 o 0. 11 0. 09 0. 14 0. 09 0. 15 0. 13 0. 12 0. 10 0. 36 t o ta l 10 0. 02 99 .2 8 99 .2 7 10 0. 12 99 .2 8 99 .8 1 99 .1 2 98 .7 1 99 .0 6 c a ti o n s p e r 8 o s i 2. 88 0 2. 39 7 2. 77 1 2. 60 6 2. 65 1 2. 68 1 2. 97 0 2. 98 7 2. 83 7 t i 0. 00 0 0. 00 0 0. 00 1 0. 00 0 0. 00 0 0. 00 0 0. 00 1 0. 00 0 0. 00 0 a l 1. 13 7 1. 60 7 1. 22 0 1. 38 9 1. 34 1 1. 31 2 1. 02 3 1. 03 0 1. 15 5 f e 2 + 0. 00 0 0. 00 2 0. 00 2 0. 00 0 0. 01 1 0. 00 1 0. 00 0 0. 00 1 0. 00 2 m n 0. 00 0 0. 00 1 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 m g 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 0 0. 00 1 0. 00 0 0. 00 0 c a 0. 09 4 0. 59 7 0. 22 9 0. 40 8 0. 33 9 0. 33 9 0. 02 6 0. 03 5 0. 15 0 n a 0. 87 2 0. 38 7 0. 78 3 0. 58 7 0. 66 5 0. 65 1 0. 98 7 0. 88 7 0. 86 2 k 0. 00 6 0. 00 5 0. 00 8 0. 00 5 0. 00 9 0. 00 7 0. 00 7 0. 00 5 0. 02 1 t o ta l 4. 99 0 4. 99 5 5. 01 4 4. 99 6 5. 01 6 4. 99 2 5. 01 5 4. 94 5 5. 02 * t ot al f e as f eo 139 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes a b c d e f si grt se grt s2 250 mm 250 mm grt s2 250 mm se grt s1 1 mm grt s2 3 mm grt grt s2 500 mm figure 4. petrographic features of the garnet-bearing pelitic rocks of the silgará formation at the csm. (a) garnet showing a pattern of inclusions of quartz, graphite and opaque minerals perpendicular to the main foliation of a graphite-rich micaschist; ppl, sample pcm-47. (b) garnet showing a pattern of inclusions of quartz and opaque minerals concordant to the main foliation of a micaschist with staurolite; ppl, sample pcm-651. (c) garnet displaying a pattern of dendritic growth (very high growth velocity), with quartz inclusions concordant to the main foliation of a quartz micaschist; ppl, sample pcm-700. (d) snow-ball garnet, showing sigmoidal inclusions of quartz in a micaschist with fibrollitic sillimanite; ppl, sample pcm-971. (e) optically sector-zoned garnet porphyroblasts in a quartz micaschist showing compositional micaand quartz-rich bands; xpl (left) – ppl (right), sample pcm-618. (f) sector-zoned garnet with radial quartz inclusions in quartz micaschist with staurolite; xpl, sample pcm-702. of garnet is shown in table 3. mgo contents in the garnets in the pelitic schists vary between 5.64 and 15.03, the content of feo varies between 68.91 and 93.38, the content of mno varies between 0.05 and 15.76 and the content of cao varies between 1.00 and 10.92. chemical zoning of garnet from the silgará pelitic schists is illustrated in mg-mn-fe and ca-mn-(fe+mg) triangular diagrams (figure 7) and x-ray elementmaps (figure 8), which is summarized as follows: (1) garnet is almost pure almandine end member, with very low content of mg, mn and ca. in addition, only a slight increase ofmnwas observed at the core (e.g., pcm-700); (2) garnets exhibits normal zoning with increasing mg and fe components from core to rim and decreasing mn and ca components from core to rim, suggesting prograde metamorphism 140 óscar m. castellanos a., carlos a. ríos r. and akira takasu a b c d st 3 mm st ky 250 mm1 mm 1 mm st and ky bt sil bt sil figure 5. petrographic features of the aluminosilicate phase-bearing pelitic rocks of the silgará formation at the csm. (a) staurolite porphyroblasts displaying penetration twinning and containing inclusions of quartz, graphite and opaque minerals discordantly with the external schistosity of the rock; xpl (left) – ppl (right), sample pcm-702. (b) paragenesis staurolite-kyanite. kyanite contains discordant quartz inclusions and staurolite shows rotational character; ppl, sample pcm-21. (c) paragenesis staurolite-kyanite-andalusite; ppl, sample pcm-21. (d) aggregates of fibrolitic sillimanite after biotite (left, ppl, sample pcm-981) and prismatic sillimanite (right, ppl, sample pcm-972). e.g., pcm-651, pcm-701, pcm-971); (3) garnet exhibits normal zoning through to the inner rim, but in the outer rim the chemical zoning is reversed, reflecting the effects of retrograde metamorphism (e.g., pcm-47, pcm-953); (4) sector-zoned garnet, with very different models of zoning even in thin section-scale (e.g., pcm-618). in some cases, distribution of elements follows a radial (sector) trend pcm-618, but in other cases distribution follows a patchy or concentric trend pcm-618, pcm-701 and pcm-702. staurolite shows pleochroism (x: dark yellow, y: light yellow, z: pale yellow), and occurs as porphyroblasts, from idioblastic to relictic character. it usually contains abundant inclusions of quartz and minor inclusions of graphite and opaque minerals. these inclusions are usually randomly oriented, but in some cases may be concordant or discordant with the main schistosity of the rock. rims of staurolite are straight or corroded by quartz and shapes range from rounded to elongated and locally overgrowth the main schistosity of the rock, suggesting a rotational and deformational event. staurolite is partly replaced by white mica and chlorite at the rims and along cracks. in some samples staurolite exhibits replacement at the rims by fibrolite. themineral chemistry of staurolite is shown in table 4. figure 9 illustrates color maps of staurolite, which exhibits a high-fe content (xfe = 0.68-0.89), but does not show any zonation in fe and mg (figure 9a and 9b). figure 9c shows a typical staurolite-kyanite grade pelite (pcm-905). sample pcm-49 has the highest zno content (3.70 weight %) and homogeneous zn distribution. al2sio5 polymorphs. kyanite occurs as discrete crystals, with porphyroblastic and poikiloblastic character. it is usually colorless and is closely associ141 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes 0.50 0.40 0.30 0.20 0.10 0.00 n a /( n a + k ) 5.90 6.00 6.10 6.20 6.30 6.40 6.50 si figure 6. chemical compositions of muscovite from pelitic rocks of the silgará formation at the csm expressed in a na/(na+k) vs si diagram; black, open and grey rombs indicate the mineral assemblages sil±st±grt, st+and±ky and st+grt, respectively; the paragenesis and+ky+sil is indicated by open triangles ated with sillimanite (fibrolite) and andalusite. andalusite occurs as discrete crystals with porphyroblastic and poikiloblastic character. it is usually very light pink or colourless and occurs as rectangular or rounded crystals. sillimanite occurs as fibrolite, and the fibrous aggregate are folded. usually it is interlayered, replacing biotite and white mica. it is also found at the rim of staurolite as a replacement. very small fibers occur as inclusions in quartz, plagioclase and micas. the mineral chemistry of al2sio5 polymorphs is shown in table 5. in general, kyanite, andalusite and sillimanite do not display any variation in chemical composition. however, fe in sillimanite is lower than fe in andalusite and kyanite. plagioclase occurs in the granoblastic bands together with quartz, and shows a range of grain sizes, irregular and variable grain shapes, curved and irregular boundaries. a second type of plagioclase shows a xenoblastic to subidioblastic character, occasionally idioblastic character, is twinned or untwined, and occurs also as porphyroblasts in augen structure, usually concordant with se, although sometimes is possible to find rotated porphyroblasts suggesting pre or syn-tectonic growth. random and oriented inclusions of quartz, white mica, biotite and opaque minerals also occur. these two types of plagioclase exhibit a moderate replacement by sericite. a third type of plagioclase shows a xenoblastic character, variable to rounded shapes and small sizes, and occurs as inclusions generally in garnet. the mineral chemistry of plagioclase is shown in table 6. the anorthite content of plagioclase in pelitic schists varies from 0.03 to 0.61. there are no clear relationships between the anorthite contents and the mineral assemblages. the highest xan (0.61) was found in sample pcm-21 and the lowest (0.03) in sample pcm-701. plagioclase from sample pcm-953 exhibits compositional ca zoning from core (an15.34) to rim (an13.55), concordantly with the decrease of ca in garnet from core to rim in the same sample. accesory phases. epidote occurs as discrete crystal or aggregates of small and elongated prismatic crystals with subidioblastic to xenoblastic character. distribution and orientation are random. it 142 óscar m. castellanos a., carlos a. ríos r. and akira takasu mg 30 20 10 30 20 10fe ca fe+mg 70 80 90 70 80 90 mn mg fe rim core core rim mn mn cn fe + mg mn pcm-47 pcm-618(1) pcm-618(2) pcm-618(3) pcm-651 pcm-700 pcm-701 pcm-953 pcm-971 figure 7. chemical compositions of garnet from metapelites of the silgará formation at the csm expressed in mg-mn-fe and ca-mn-(fe+mg) diagrams. 143 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes a b c d e f c a 5 0 0 u m c a 5 0 0 u m c a 1 m m c a 5 0 0 u m c a 5 0 0 u m c a 1 m m figure 8. x-ray elementmaps for ca of garnet porphyroblasts frommetapelites of the silgará formation at the csm. (a) pcm-618, sector-zoned garnet with patch zoning. (b) pcm-618, sector-zoned garnet with radial zoning. (c) pcm-702, euhedral garnet with a hexagonal concentric zoning. (d) pcm-701, subhedral garnet with radial and concentric zoning. (e) pcm-971, sigmoidal garnet showing an irregular concentric zoning. (f) pcm-971, sigmoidal garnet showing an irregular concentric zoning. shows a pleochroism (x: colorless, y: light yellow, z: light green), and some crystals exhibit optical zoning. tourmaline occurs as discrete grains of subidioblastic and prismatic character, green to violet, and exhibits typical optical zoning and pleochroism. it is randomly oriented. it occurs in the matrix and as inclusions in major minerals. apatite occurs as discrete and very small grains. it is subidioblastic and prismatic in shape and occurs in the matrix and as inclusions in quartz, plagioclase and micas. zircon occurs in the matrix with random orientation as very small discrete grains. it is elliptical in shape and shows typical birefringence colors. opaque minerals occur as discrete grains that show very different shapes and orientations both as a matrix phase and as inclusions in almost all major minerals. rutile occurs as discrete randomly-oriented xenoblastic crystals and as matrix and inclusion phases. it is dark-yellow in color and has black haloes at the rims. ilmenite forms discrete crystals with tabular habit, black in color, homogeneous distribution, and concordant to discordant orientation. graphite appears as aggregates of very-fine flaky crystals, black in color, giving to the rock a clouded appearance. metamorphism determination of metamorphic grade of the pelitic sequence of the silgará formation at the csm is based on the experimentally defined stability of index minerals as well as on their chemical composition. mineral assemblages, their relationships, structural features and mode of occurrence as well as common local alternation of lithological members with gradual transition suggest the existence of a volcanoclastic sedimentary sequence made of pelitic rocks with intercalations of semipelitic, psammitic, basic and carbonaceous rocks. field and petrological studies show that the metamorphism of the silgará formation is characterized by the presence of typical index minerals of barrovian type metamorphism (miyashiro, 1994). on the basis of mineral assemblages in pelitic rocks, the silgará formation can be divided into a barrovian-type facies series of biotite, garnet, staurolite-kyanite and sillimanite zones. a separation between the kyanite and staurolite zones has not been done in this study in spite of the occurrence of ductile shear zones with hydrothermal kyanite-bearing quartz veins. all pelitic schists contain the mineral assemblage quartz + plagioclase + biotite + muscovite, which is associated with sillimanite in the southwestern sector of the study area, where schists with garnet (pcm-953) and garnet + staurolite (pcm-971) assemblages also occur. mineral assemblages containing aluminosilicates are characteristic in the northern part of the area. the three al2sio5 polymorphs were observed in each one of the samples pcm-18, pcm-20 and pcm-21. the mineral assemblage quartz + plagioclase + biotite + muscovite + garnet + staurolite is distributed in the northern part 144 óscar m. castellanos a., carlos a. ríos r. and akira takasu a b c fe 500 um mg 500 um al 200 um st st st ms ky figure 9. x-ray element maps of staurolite from metapelites of the silgará formation at the csm. (a) and (b) pcm-701, staurolite porphyroblats developing penetrating twinning and a nearly homogeneous distribution of fe and mg. (c) staurolite associated to kyainite and muscovite, showing a different al concentration. of the study area. the coexistence of biotite and muscovite, accompanied by quartz and plagioclase may be generated by a reaction at p-t conditions slightly below 400 °c and 4 kbar. the presence of garnet coexisting with biotite takes place below 500 °c. this idea is consistent with that of spear (1993) where below 500 °c garnet occurs only in rocks with high ca + mn contents. for the mineral assemblage quartz + plagioclase + biotite + muscovite + garnet + staurolite, the reaction grt + chl = st + bt reaches p-t temperatures 550 °c and pressure ranges from 4 to 6 kbar (spear, 1993). the first appearance of kyanite can be related to its intergrowth with staurolite, which is due to the close relationship between the structures of these minerals. therefore, the mineral assemblage quartz + plagioclase + biotite + muscovite + staurolite + kyanite may establish the highest pressure range 6-7 kbar at about 600°c for the silgará formation at the csm. resorption and replacement of kyanite by sillimanite suggests a probable decrease in pressure (close to 600°c). andalusite is present in the mineral assemblages qtz + pl + bt + ms + (ky + and) or + (ky + and + st) or + (st + and). the reaction kyanite = andalusite occurs at the retrograde stage of metamorphism during a decrease of both p-t or only p. according to goldsmith et al. (1971) the metamorphism of the silgará formation may have occurred during the 410-450 ma event. the metapelitic sequence is not always complete, because in some cases the lower-grade metamorphic zones do not occur. a complex tectonic history probably played a very important role in breaking the metamorphic zoning and therefore some apparent isograds may correspond to faults. on the other hand, the authors consider that is still unclear how the late emplacement of orthogneiss masses affected the metapelitic sequence, as well as the occurrence of crustal granitoid rocks that are generally form to contact aureoles, taking into account that in sectors such as la laguna, textural evidence may indicate the occurrence of hornfelsic rocks and mineral assemblages that overimprinted to early mineral phases and fabrics. in the contact between the silgará formation and the bucaramanga gneiss complex, the contact is often defined by the biotite–sillimanite isograd (ward et al., 1973; restrepo-pace et al., 1997), though this differentiation method seems to imply a common metamorphism for both units (ordoñezcarmona et al., 2006). geothermobarometry figures 10 and 11 summarize the estimated p–t conditions of evolution for the silgará formation metapelitic rocks at the csm. the thermobarometric estimates were done in domains of samples showing textural and chemical evidence of equilibrium among the phases under consideration. mineral compositions were used with the computer program gtb (spear & kohn, 2001), which incorporate most published geothermobarometers, to obtain equilibrium pressures and temperatures from garnet bearing assemblages. various assemblages were used to calculate pressures and the garnet-biotite assemblage was used to calculate temperatures. temperatures and pressures for 3 representative samples (pcm-618, pcm-953 and pcm-971) were calculated using the garnet-biotite geothermometer and the garnet-plagioclasebiotite-muscovite, gasp and garnet-moscovite-biotite-sial2o5 geobarometers. sample pcm-618 consists of sector-zoned and poikiloblastic porphyroblasts of garnet in a granolepidoblastic matrix of quartz, plagioclase, biotite and muscovite. plagioclase also occurs as inclusion in poikiloblastic garnet. sample pcm-953 consists of porphyroblastic garnet in a granolepidoblastic matrix of quartz, plagioclase, muscovite, biotite and sillimanite. sample pcm-971 consists of porphyroblastic garnets in a granolepidoblastic matrix of quartz, biotite and muscovite. chemical compositions were taken from the rim of different sectors of the garnets, which show moderate to strong chemical zoning, and from the rims of plagioclase, biotite and muscovite crystals, which do not show strong or systematic chemical zoning. all iron in biotite and garnet was assumed to be fe2+. metamorphic temperatures were estimated using fe-mg exchange among minerals according to: (1) ganguly & saxena (1984), (2) perchuk & lavrent’eva (1984), (3) indares & martignole (1985), (4) ferry & spear with berman (1990), (5) 145 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes 146 óscar m. castellanos a., carlos a. ríos r. and akira takasu ky and sil ky and sil ky and sil ky and sil t(°c) t(°c) t(°c) t(°c) p (k b a r) p , (k b a r) p (k b a r) p , (k b a r) figure 10. p-t estimates using different calibrations of geothermobarometers from gtb program by spear and kohn (2001): grt-bt thermometer and (a) grt-pl-bt-ms barometer from sample pcm-618, sector-zoned garnet, (b) grt-pl-bt-ms barometer from sample pcm-618, poikiloblastic garnets, (c) grt-pl-bt-ms and gasp barometers from sample pcm-953, (d) grt-ms-bt-al2sio5 barometer from sample pcm-971. triple point for aluminosilicates after holdaway (1971). ky p (k b a r) and sil t(°c) pcm-618, sector-zoned garnet grt-bt thermometer grt-pl-bt-ms barometer pcm-618, poikiloblastic garnet grt-bt thermometer grt-pl-bt-ms barometer pcm-953 grt-bt thermometer grt-pl-bt-ms barometer pcm-953 grt-bt thermometer gasp barometer pcm-971 grt-bt thermometer grt-ms-bt-al o barometer2 5 figure 11. p-t diagram showing polygons of total ranges of calculated temperatures and pressures from pelites of the silgará formation at the csm, using different calibrations of geothermobarometers from gtb program by spear and kohn (2001). triple point for aluminosilicates after holdaway (1971). patino douce et al. (1993), (6) holdaway et al. (1997), (7) gessmann et al., (1997), (8) kleemann & reinhardt (1994). metamorphic pressures were estimated as follows: for samples pcm-618 and pcm-953 using the grt-pla-bt-ms barometer of (a) ghent & stout (1981) fe end member, (b) hodges & crowley (1985), (c) powell & holland (1984) with hodges & spear (1982), (d) hoisch (1990) fe end member, (e) hoisch (1990) mg end member; for sample pcm-953 using the gasp barometer of (a) newton & haselton (1981), (b) hodges & spear (1982), (c) ganguly & saxena (1984), (d) hodges & crowley (1985); for sample pcm-971 using the grt-ms-bt-al2o5 barometer of (a) hodges & crowley (1985), (b) holdaway et al., (1988). according to geothermobarometric calculations reported in this study, temperature and pressure conditions for this area are 400-600 °c and 4.0-6.5 kbars, respectively. *references as listed in the gtb geothermobarometry software of spear & kohn (2001). reaction history and p-t paths figure 12 illustrates the p-t trajectories postulated in this study for the silgará formation pelites at the csm from mineral assemblages, their textural relations and mineral compositions, which can be compared with the p-t path proposed by garcía et al., (2005) for these rocks and two different model p-t paths by spear (1993) followed by low-al pelites for typical regions have undergone crustal thickening or contact metamorphism. the p-t conditions estimated by geothermobarometry were verified by constructing a petrogenetic grid that constrains themineral stability fields and reactions known with various 147 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes 10 8 6 4 2 400 500 600 700 800 grt producing reaction in rocks with mno and cao p re s s u re (k b a r) emplacement of orthogneiss ky sil co m pr es io na l r eg im e ky and and sil st+mx+qtz+an grt+and+bt+h o2 exhumation s t+ m s+ q tz + a n g rt + s il+ b t+ h o 2 extensional regime b t+ m s+ q tz+ h o2 (k f5 + c h ) g rt+ b t+ q tz + h o2 m s + c h l g rt+ m s + c h l s t+ b t+ q tz (2 )+ h o2 m s + c h l k y + b t+ a b + h o2 s t+ m s+ q tz+ a n g rt+ k y+ b t+ h o2 b tin g rt -i n temperature (°c) figure 12. petrogenetic grid illustrating mineral reactions in pelites as a function of pressure and temperature. model of metamorphic p-t evolution followed by the silgará formation pelites at the csm (blackand grey-thick arrows), compared with the p-t path proponed by garcía et al., (2005) indicated by a black arrow, and two p-t paths (spear, 1993) for typical regions that have undergone barrovian facies metamorphism (thin continuous-black arrow) or contact (thin dotted-black arrow) metamorphism. the al2sio5 triple point after holdaway (1971). the narrow grey areas correspond to the biotiteand garnet-in reaction after yardley (1989), and the wide grey area shows the stability field of staurolite after bucher & frey (1994). degrees of certainty, which can be used to decipher the reaction history of the investigated rocks. the patha1-a10 starts out at c. 350°c and 2 kbar. however, the p-t paths b and c at the one proposed by garcía et al. (2005) correspond to the p-t evolution of garnet-bearing pelites that reached the staurolite-kyanite or sillimanite conditions, and probably start out at the same conditions (c. 480-500 °c and 2 kbar), but follow slightly different trajectories. our model can be summarized as follows: a given volume of rock experienced gradual crustal shortening and thickening, accompanied by an increment of pressure and temperature conditions, as a result of burial until reaching the maximum pressure condition (ppeak) is kached. the thickened crust is uplifted slowly and erosion tends to restore the original crust, which is characterized by decompression, with a continued increasing temperature until to reach the maximum temperature condition (tpeak). then, the volume of crust experienced progressive crustal uplifting, accompanied by erosion, rehydration, cooling and decompression. biotite-in reaction. ata1, the initial assemblage in low-al pelites was probably chlorite + k-feldspar (+ muscovite + quartz), from which biotite formed at 380 oc and 3.8 kbar (a2) by the continuous reaction k-feldspar + chlorite = biotite + muscovite + + quartz + h2o. garnet-in reaction. the garnet isograd reaction (a3) is strongly dependent on bulk rock composition and specially mno and cao content (spear, 1993). in mnoand cao-rich rocks, garnet may appear at temperatures below 450 °c whereas in mnoand cao-poor rocks, garnet may not appear to well above 500 °c (spear, 1993). therefore, at a4 garnet is probably present because of themno content of most pelites. no discontinuous reaction has been encountered up to » 530 oc (a5), when garnet was produced by the continuous reaction muscovite + chlorite = garnet + biotite + + quartz + h2o, which explains a simultaneous decrease in muscovite and chlorite and increase in biotite contents within the garnet zone, compared with the lowest-grade pelites from the biotite zone. the garnet zone is characterized by the mineral assemblage garnet + chlorite + botite + muscovite + quartz, which is stable over an interval of metamorphic grade above the garnet-in isograd. staurolite-in reaction. at the staurolite isograd, the first appearance of staurolite may be related to biotite-producing and chloriteand muscovite-consuming reactions, because the staurolite zone is marked by a strong increase in the modal abundance of biotite at the expense of chlorite and muscovite. textural evidence shows that staurolite may be produced by a garnet-consuming discontinuous reaction, such as garnet + muscovite + chlorite = staurolite + + biotite + quartz + plagioclase + h2o, a6 (at 610 oc). when this reaction has finished some more staurolite may be produced by a garnet-absent continuous reaction involving the remaining phases. kyanite-in reaction. the staurolite zone corresponds to buffering along this reaction to the intersection where chlorite, biotite, garnet, staurolite and kyanite coexist (lang and rice, 1985). the kyanite + staurolite association suggests that kyanite and staurolite crystallized from an appropriate bulk composition in the appropriate p/t range (ôahin and erkan, 1994). according to lang and rice (1985), the net reaction at the intersection is approximated by the transition zone reaction chlorite + muscovite = kyanite + biotite + + albite + h2o (a7), which explains the probable kyanite-forming reaction developed through a chlorite-consuming reaction. however, kyanite may also be formed during a garnet-consuming reaction such as garnet + muscovite = kyanite + biotite + quartz (spear, 1993) the pelites from the structurally upper level of the kyanite zone are characterized by the mineral as148 óscar m. castellanos a., carlos a. ríos r. and akira takasu semblage kyanite ± staurolite + biotite + muscovite + plagioclase + quartz. with increasing temperature, staurolite becomes unstable and reacts to produce garnet, kyanite and biotite. staurolite dissapears at the kyanite isograd as a result of the discontinuous reaction staurolite + muscovite + quartz + anorthite = garnet + kyanite + biotite + h2o, a8 (» 700oc) sillimanite-in reaction. according to zhao & cawood (1999) the following sillimanite-producing reactions may be considered: chlorite + muscovite = sillimanite + biotite + + quartz + h2o garnet + muscovite = sillimanite + biotite + quartz staurolite + muscovite + quartz = sillimanite + + biotite + h2o staurolite + quartz = sillimanite + garnet + h2o kyanite = sillimanite textural evidence from the silgará formation pelites suggests that the production of sillimanite includes the appearance of fibrolitic sillimanite-biotite intergrowths with the consumption of chlorite and muscovite or pseudomorphs after garnet porphyroblasts, fibrolite at the expense of staurolite porphyroblasts, with or without the production of garnet, relict kyanite within muscovite crystals, and the polymorphic reaction kyanite = sillimanite (a9). chemical zoning and sillimanite inclusions in muscovite and quartz may indicate decrease in pressure after the formation of sillimanite during staurolite-consuming reactions (increasing temperature). after a near isothermal decompression, the final assemblage will be garnet + staurolite + sillimanite + biotite (+ muscovite + quartz), a10. up to this point, the silgará formation pelites have undergone medium-p/high-t conditions of barrovian facies metamorphism, corresponding to the ky-sil series of miyashiro (1994), following a clockwise pressure-temperature path, which is characteristic for the metamorphic evolution of most orogenic belts (brown, 1993). the final event described in this study is a retrograde metamorphic event, characterized by a decrease in both temperature and pressure parameters. the retrogressive reactions are related to partial replacement of garnet by chlorite and/or biotite along cracks, heavily corroded staurolite enclosed by sericite and chlorite, replacement of biotite by chlorite or ilmenite exsolution in biotite. however, the retrograde trajectory in the silgará formation pelites is not well constrained and can only be inferred from mineral assemblages belonging to greenschist facies. the coexistence of kyanite-sillimanite-andalusite in staurolite-bearing rocks, mineral assemblages and textural observations indicate a sequence of occurrence: first coexistence of kyanite and staurolite, followed by breakdown of kyanite and with presence of sillimanite, and lastly a retrograde metamorphism with the occurrence of andalusite after kyanite. however, the presence of the three al2sio5 polymorphs in some samples suggests a more complex p-t metamorphic evolution. therefore, we propose the occurrence of alternative p-t paths recorded by the silgará pelites from the garnet (b) and staurolite-kyanite (c) metamorphic zones. the alternative p-t paths take into account strong textural evidence supporting the first appearance of kyanite “kyanite-in isograd” by the polymorphic reaction and = ky. however, the p-t trajectory b corresponds to pelites evolved from the garnet to the staurolite-kyanite stability fields, without production of sillimanite, and the p-t trajectoryc closest to the triple point should be considered with the following reaction sequence andalusite ® kyanite ® sillimanite based on pelites that reached the sillimanite conditions. discussion a main characteristic of orogens is a complex deformation history involving various tectonic processes such as burial of crustal slices followed by exhumation of deep metamorphic rocks by nappe stacking, syn-orogenic to post-orogenic extension and tectonic denudation (duguet et al., 2007). therefore, the re149 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes construction of a p-t path is sometimes difficult to constrain. however, this problem can be overcome when the metamorphic series contain garnet-bearing metapelitic rocks such as those of the silgará formation, which underwent metamorphism with a peak at the upper amphibolite facies conditions, being affected during the last stage of deformation by the emplacement of syn-tectonic intrusions (orthogneiss). campos (1999) reported that the occurrence of veins mainly composed of quartz with minor calcite in metasomatic zones of the silgará formation metacabonate rocks indicates a removilization of sio2 and cao due to the action of late magmatic fluids associated to the emplacement of the orthogneiss protolith, which circulated along tectonic discontinuities. additional evidence on the external fluid contribution derived from syn-metamorphic intrusions corresponds to the development of a h2o-rich diopside reaction zone in the presence or absence of scapolite in the metasomatic rocks of the silgará formation. finally, a thermal event overprinted to the regional metamorphism of the silgará formation which the authors propose as due to the emplacement of calc-alkaline granitoids of triassic-jurassic age, that are generally form to contact aureoles, and for example in sectors such as la laguna, textural evidence indicates the occurrence of poikiloblastic andalusite in hornfelsic rocks, with mineral assemblages overprinting early mineral phases and fabrics. during a late triassic-early jurassic intrusive-related regional metamorphic episode, high-t and low-p conditions of the santander massif prevailed (restrepo-pace, 1995). the reconstruction of the p-t paths experienced by this metamorphic unit is a crucial point to decipher the tectonic setting of the deformation events and any proposed model should take into account thermobarometric data. however, the geodynamics and tectonic setting of the santander massif are still matter of debate. p–t paths garcía et al. (2005) already documented p-t conditions for the silgará formation at the csm (6.1-8.5 kbar and 616-784 °c). our results are consistent with a two-stage tectonothermal evolution under high-grade conditions: (1) an early compressional phase of deformation that led to upper amphibolite facies barrovian-type metamorphism and (2) a subsequent extensional phase of deformation that led to near-isothermal decompression and the emplacement of orthogneiss masses. a final evidence of stage of near-isobaric cooling to greenschist facies conditions is widespread. both prograde and retrograde metamorphic evolution may be constrained and characterised by initial crustal shortening and thickening associated with thrusting, subsequent slow exhumation and final cooling. however, due to the preservation of prograde mineral assemblages and critical reaction textures, a more complex series of p-t paths can be inferred. the tectono-metamorphic history of the silgará formation metapelites is characterised by clockwise p-t trajectories (figure 9). the inferred p-t paths are interpreted to result from one geological event, but during different stages of deformation. however, this assumption must be tested by geochronological data. tectonic implications based in both petrological evidence and the inferred p-t paths for the silgará formation pelites, we postulate two possible tectonic scenarios in which the metamorphism of this geological unit might have taken place: (1) medium-grade metamorphism related to collision tectonics and subsequent decompression from extensional collapse of the tectonically thickened crust, probably accompanied by the emplacement of syn-metamorphic intrusions (orthogneiss) and (2) overprinted thermal metamorphism related to the emplacement of calc-alkaline granitoids of triassic-jurassic age. the shape and direction of the clockwise p-t paths inferred in the current study support these interpretations. subsequent decompression and uplift of the pelites, initially under medium-pressure conditions, can be related to an extensional collapse of the thickened crust, where the emplacement of orthogneiss masses provided further heat input into the pelites during their last stage of deformation. later cooling of the pelites can be related to cooling of the orthogneiss masses. restrepo-pace (1995) report a u/pb zircon age of 480 ma for the 150 óscar m. castellanos a., carlos a. ríos r. and akira takasu emplacement of syn-tectonic intrusives suggesting a magmatic event in the late ordovician or early silurian. therefore, the age of the orthogneiss emplacement must be similar to that of the silgará formation, which can be explained only if the emplacement and metamorphism processes of orthogneiss masses occurred at the beginning of the last stage of deformation that affected the silgará formation pelites. uplift and exhumation the thickened crust represented by the medium-pressure, high temperature metapelites of the silgará formation was uplifted slowly and erosion led to restore the original crust, which is characterized by decompression. during this stage the emplacement of orthogneiss masses probably occurred. however, as initial uplift rates are slow compared with heat transport rates, these metapelites were affected by a continusly increasing temperature during uplift until they reached the thermal peak, after which these rocks experienced cooling, accompanied by retrograde metamorphism documented by generally incomplete rehydration reactions and least not-hydrated mineral phases have been preserved as relicts. the uplift of deeply buried rocks in subduction or collision zones can be explained by the buoyancy of crustal rocks at depth (von blanckenburg&davies, 1995; ernst et al., 1997): (1) the buoyancy of tectonically thickened crust when material is removed from the upper crust by erosion or faulting or (2) break-off of the subducting slab, thus removing slab pull and allowing the subducting crust to return buoyantly to the surface. exhumation of the silgará formation is represented by the return toward the earth’s surface of the deeply buried pelitic rocks, a process that is generally driven either by erosion or low-angle normal faulting or both; ductile extension of the crust or lithosphere is slow and cannot account for more than a small fraction of exhumation (ring et al., 1999). conclusions the sequence of mineral assemblages, metamorphic reaction history and estimated clockwise p–t path for the silgará formation at the csm let us to constrain its tectono-metamorphic evolution, which is characterized by initial crustal shortening and thickening associated with thrusting, subsequent slow exhumation and final cooling, which is characteristic during continental collision (brown, 1993). petrographic observation and thermobarometric data indicate that the investigated pelites underwent regional prograde metamorphism and the metamorphic zones equilibrated at significantly different peak pressures and temperatures along the kyanite geotherm. according to the metamorphic reactions and the p-t evolution model proposed in this study, the first mineral assemblage observed was biotite + muscovite + quartz, which probably formed near 425°c and 3.8 kbar. the coexistence of garnet in mineral assemblageswith biotite occurs approximately at 475°c and 4.7 kbar. conditions for the first appearance of staurolite are close to those for the first appearance of kyanite, reaching probably the peak of pressure, 625°c and 7.5 kbar and 650°c and 8.0 kbar, respectively. kyanite breakdown into sillimanite and the peak temperature were estimated at around 680°c and 8.0 kbar, which however do not correspond to the peak temperature (740°c and 7.0 kbar). the silgará formation pelites exhibit a retrograde metamorphism with the occurrence of andalusite and accompanied by reverse zoning at the outer rim of garnets, and chloritization and sericitization processes. calculations for sample pcm-953 were carried out using data from the contact between the rims of garnet and biotite, yielding a temperature range of 497-583°c according to different calibrations of the garnet-biotite pair. this temperature could be interpreted as the peak of the retrograde metamorphic event. there is a strong evidence on the occurrence of an overprinted termal event to the regional metamorphism. the uncertainties of the p-t data are related to the variability of mineral compositions within the studied rocks and to the lack of control of thermodynamic parameters. however, a general trend toward medium pressures at low temperatures of metamorphism is clearly shown by the observed mineral associations. the p-t paths appear to favour models involving the delamination of lithospheric mantle as a factor con151 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes tributing to the change from compressional to extensional tectonics at the csm. medium-pressure pelitic rocks of the silgará formation were probably formed during a collision, with a buried crust reaching pressures up to 6.5 kbar, equivalent to 27 km depth, estimated for the kyanite-bearing pelites. near-isothermal decompression commonly follows the crustal thickening and initially buried rocks were rapidly exhumed as the crust was thinned by structural collapse, extension and erosion. therefore, the metamorphic evidence indicates such a buried crust as a consequence of its tectonic transport during a continental collision is the formation of middle-pressure kyanite-bearing pelites, followed by recrystallisation of lower pressure assemblages within a clockwise p-t. however, to confirm this hypothesis new field and petrological studies should be carried out and geochronological studies are necessary to constrain the p-t-t paths of the caledonian silgará pelites. this work formed part of the msc thesis carried out by o. castellanos at shimane university (japan). the authors are indebted to universidad industrial de santander and instituto colombiano para el desarrollo de la ciencia y la tecnología “francisco josé de caldas” colciencias through grant no. 1102-05-083-95 financially supported fieldwork (research project: “metamorphism and associated metallogeny of the santandermassif, eastern cordillera, colombianandes”).we thank to department of geoscience of shimane university. the authors are most grateful to the above-named people and institutions for support. references banks, p., vargas, r., rodríguez, g.i., shagam, r. 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(1999). tectonothermal evolution of the mayuan assemblage in the cathaysia block: implications for neoproterozoic collision-related assembly of the south china craton. american journal of science, vol. 299, pp. 309-339. 155 a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes articulo 5.indd earth sciences research journal archaeology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 115 120 quaternary dating by electron spin resonance (esr) applied to human tooth enamel eduar carvajal1, luis montes2 and ovidio a. almanza1 1 departamento de física, universidad nacional de colombia, cra 30 nº 45-03, bogotá 2 departamento de geociencias, universidad nacional de colombia. cra 30 nº 45-03, bogotá. corresponding author: ovidio almanza m, tel. (57 1) 3165000 ext 13048; fax (57 1) 3165635. e-mail oaalmanzam@unal.edu.co abstract this paper presents the results obtained from using electron paramagnetic resonance (epr) to analyse tooth enamel found at the aguazuque archaeological site (cundinamarca, colombia), located on the savannah near bogota at 4° 37’ north and 74°17’ west. it was presumed that the tooth enamel came from a collective burial consisting of 23 people, involving men, women and children. the tooth enamel was irradiated with gamma rays and the resulting free radicals were measured using an electron spin resonance (esr) x-band spectrometer to obtain a signal intensity compared to absorbed doses curve. fitting this curve allowed the mean archaeological dose accumulated in the enamel during the period that it was buried to be estimated, giving a 2.10 ± 0.14 gy value. rosy software was used for estimating age, giving a mean 3,256 ± 190y before present (bp) age. these results highlight epr’s potential when using the quaternary ancient ruins dating technique in colombia and its use with other kinds of samples like stalagmites, calcite, mollusc shells and reefs. resumen en este trabajo se presentan los resultados obtenidos por datación mediante el uso de resonancia paramagnética electrónica en muestras de esmalte dental provenientes del sitio arqueológico aguazuque, ubicado en la sabana de bogotá en coordenadas 4° 37’ de latitud norte y 74°17’ de longitud oeste de greenwich. se presume que el esmalte dental proviene de un entierro colectivo compuesto de 23 individuos entre mujeres, hombres y niños. el esmalte dental fue irradiado con rayos gamma y los radicales libres producidos fueron medidos usando un espectrómetro de resonancia del spin electrónico (esr), banda x, con el fin de realizar una curva de intensidad de la señal versus la dosis absorbida por el esmalte. el ajuste de esta curva permitió obtener la dosis media arqueológica acumulada en el esmalte (da), durante el periodo de enterramiento de la muestra. el valor obtenido fue de 2.10 ± 0.14 gy. para estimar la edad de la muestra se usó el software rosy, obteniendo una edad media de las muestras de esmalte de 3,256 ± 190y bp. los resultados obtenidos muestran el potencial uso de la técnica epr como herramienta de datación de muestras cuaternarias en colombia y abre la posibilidad de usarla en otro tipo de muestras como estalagmitas, calcitas, conchas de moluscos y corales. palabras claves: epr, esmalte dental, datación arqueológica. keywords: epr/esr, tooth enamel, colombia, archaeological dating. record manuscript received: 11/05/2011 accepted for publications: 30/11/2011 introduction electron spin resonance (esr) analysis has recently become an alternative c14 and thermo-luminescence dating method which can be applied to a variety of problems in geology, archaeology and paleoanthropology (renfrew c et al., 1992; jonas 1997; walker 2005; grun 1989). esr spectroscopy is the only method for detecting, identifying and quantifying free radicals (halliwell & gutteridge 2007; punchard & kelly 1996). for instance, this method is used for estimating the free radical level and paramagnetic centres produced in some materials by ionising radiation. the some free radicals’ long mean life (above 100 my) make it possible to reveal them in materials as old as 2 my without the risk of overcoming the sample’s saturation limits. esr allows some materials to be dated because free radical concentration is a measurement of the total radiation dose absorbed by the sample during the time it was exposed to radiation and hence their burial time (jonas 1977). esr has been used for dating the formation and more recent re-crystallisation of three types of gypsum samples: massive, bedded and fracture filling gypsum; in this case, the estimated upper miocene–pliocene age agreed with that soil’s stratigraphy (ülkü 2004). molodkov (2001) used esr to date eight terrestrial shell samples from treugolnaya (triangular) cave (northern caucasus), ages ranging from 583,000 to 393,000y old. shigueo et al., (2003) dated a calcite formation found on a rock wall painting at toca da bastiana, rock shelter in serra da capivara national park, piaui, brazil, obtaining an age of 35,000 to 43,000y, indicating that humans lived there before 35,000 years ago (this being the date for the first humans to arrive in brazil). otheduar carvajal, luis montes and ovidio a. almanza116 er papers dating stalagmites, calcite, mollusc shells and reefs are renfrew et al., (1992), jonas (1997), walker (2005) and grun (1989). the present study was aimed at using the esr method for dating tooth enamel from the aguazuque archaeological site in soacha, colombia. theoretical aspect the aguazuque archaeological site (cundinamarca, colombia) is located on the savannah near bogota at 40 37’ north and 74°17’ west (figure 1). the human tooth used in this work was extracted from a skeleton (labelled ag-i) taken from a collective burial site where 23 people were found arranged in a foetal position; men, women and children were identified. the burial site was circular shaped (4.5 m diameter) and as burnt bones and lacerated skeletons were found outside the circle, partially painted red and black, it was deduced that cannibal practices were common by the group responsible for the burial (correal 1990). archaeological research concerning the early and middle holocene period has led to figure 1. location of aguazuque archaeological site (cundinamarca, colombia, 40 37' north and 74°17' west). (taken from groot, a.m., 1992). figure 2. energy levels of an electron placed in a magnetic field. the arrow shows the transitions induced by microwave radiation. reconstructing the way of life for human groups in the region, thereby showing a transition from hunters and gatherers to agricultural societies (groot 1992). a way of life based on hunting several sized mammals has been recorded during a lapse of 11,000 to 5,000 years before today, with rock shelters providing favourites housing places, such as the archaeological sites of el abra, tequendama and nemocón 4 in cundinamarca (correal 1979). sites such as chía i (ardila 1984) and aguazuque (correal 1990) have been investigated and the aforementioned research has led to establishing changes in the form of settlement and adaptation mechanisms (groot, 1992). electron spin resonance (esr) consists of the resonant absorption of electromagnetic energy during electron-spin transitions. a static magnetic field should be applied to resolve different electron-spin levels. unpaired electrons from free radicals have spin equal to ½. there are two magnetic levels (+½ and –½) in a magnetic field, both having different energy (figure 2); the transition between these two levels is possible in the following resonance condition: hν=gβh where ν is resonance frequency, h is plank’s constant, g is the g-factor (which is a constant = 2 for ½ spin), β is the bohr magneton which is an electron magnetic dipole moment and h is the magnetic field. an important conclusion derived from this formula is the linear dependence between the applied magnetic field and resonance frequency. the most frequently used microwave energies drop in what is called the x band. this is because it is a good compromise between sensitivity, sample size and water content effects; ν ≈ 9.8 ghz and h ≈ 350 mt for the x band. esr dating and dosimetry are based on the fact that ionising radiation produces paramagnetic centres having long lifetimes in a number of materials. such centres’ concentration in a given sample is a measure of the total radiation dose to which a particular sample has been exposed. this effect can be used to determine the length of time of such exposure and hence, in many cases, a sample’s burial time (jonas 1997). esr has been used with tooth enamel for determining the age of palaeolithic archaeological sites, mainly because tooth enamel is ubiquitous in these contexts and because it allows dating beyond the 14c dating time range (40–45 ka max.). it is thus also very useful in important sites regarding human evolution studies, sites having palaeontological interest within a quaternary time frame and because many sites lack the volcanic materials needed for k – ar and 40ar/39ar dating (rink 1997). (1) quaternary dating by electron spin resonance (esr) applied to human tooth enamel 117 the aim is to determine the accumulated dose (da) to which a sample has been exposed whilst being buried; the exposure time is then determined by comparing da to the average dose rate over such period: t= da (d) however, no significant difference in the accuracy of dose reconstruction has yet been found using different calibration methods (wieser et al., 2000). each sample’s radiation sensitivity is individually calibrated for dose assessment by the additive dose method; each sample is incrementally irradiated with at least 14 additional doses up to 100 gy or more (avoiding the sample becoming saturated). a linear regression analysis is applied to the esr measurements at each dose increment and the original measurement (laboratory-un-irradiated sample). the original da in the sample is obtained from the negative intercept of the regression line with the dose axis; is estimated as being the total radiation provided by all radioactive elements present in a sample and soil and also cosmic radiation (ikeya 1993; jonas 1997; walker 2005; grun 1989). materials and methods the dentine was initially removed from teeth using a dental drill and water cooling. the enamel was then placed in a 30% naoh solution for one day to disinfect it and separate any remaining dentine (iaea report 2002); 1.4 mm mean enamel thickness was obtained for ag-i. the sample was air-dried at room temperature for three days; a dental drill was used to strip around 50 ± 5 μm from the inside and outside of the enamel surface to ensure that alpha radiation had no effect. the molar enamel powder was divided into nine aliquots. each aliquot was placed inside a plastic capsule for irradiation using 60c standard radiation equipment at room temperature and 8gy/min mean dose rate; the doses ranged from 0 to 740 gy. after irradiation, the esr signal was measured with a bruker esp300 x-band spectrometer. the g  and g  signals’ peak to peak height was used for calculating amplitude. the spectrometer parameters used were: 336 mt central field, 0.02 t scan range, 0.1 m t amplitude modulation, 100 khz modulation frequency, 0.328 s time constant and 20 mw microwave power. the aliquots were then irradiated with additional doses and the same process was repeated until getting 9 data points from esr signal amplitude as a function of γ-ray dose. soil samples were also collected from the aguazuque site and sent for u, th and k content analysis by gamma spectrometry. radiometric analysis is widely used for determining natural radioisotopes in geological samples by means of spectroscopic methods and, especially, for quantifying the uranium, thorium and potassium present in samples. these samples were analysed at ingeominas’ nuclear technology group’s environmental radiometrics’ laboratory which has a low-level gamma spectrometry system with 15 cm lead shielding and high-resolution gehp hyperpure germanium detector (canberra model gc2019), having 1.89% absolute detection efficiency for 661.6 kev. results and discussion figure 3 shows the esr spectra for enamel sample ag-i having an irradiation dose ranging from 54 to 324 gy. a linear model was used for fitting esr signal intensity to dose, as has been done by other authors (baffa et al., 2000; hefne et al., 2002; kinoshita et al., 2001a, 2008b; mascarenhas et al., 1982); very good agreement with linear fit was shown (figure 4). the archaeological da obtained using this method was da = 2.10 ± 0.14 gy. rosy software (brennan et al., 1999) was used for calculating the enamel sample’s age, based on the given da, cosmic radiation dose rate (determined at 251 ± 15 μgy/year), u, th and k content obtained from figure 3. esr spectra of tooth enamel ag-i in a range of 54 to 324 gy. (2) eduar carvajal, luis montes and ovidio a. almanza118 soil sample analysis and enamel (1.4 mm ag-i thickness and 5 mm dentine thickness) in sample. soil water content was 12.1%. the software’s default values were used for the remaining parameters. the cosmic radiation dose rate was obtained using the equation for luminescence and esr dating (prescott & hutton 1994). finding the dose rate to convert da into an age is a crucial step after da has been found. rosy software allows age to be calculated from an enamel sample by taking da due to radioisotopes from adjacent layers into account. the software computes age based on three uranium absorption models (linear, early uptake and a combination of both). table 1 shows the sample concentration (ppm) 238u 232th 40k aguazuque 1.86 ± 0.19 3.72 ± 0.37 1.66 ± 0.09 alpha beta gamma cosmic gamma+cosmic dentine 0.00 0.00 enamel 0.00 0.00 cementum 0.00 0.00 sediment 0.00 46.55 347.31 uranium 0.00 34.54 188.50 thorium 0.00 12.01 158.81 potassium 0.00 0.00 total 0.00 46.55 347.31 251.00 598.31 (alpha+beta) dose rate (µgy/y) = 46.55 total dose rate (µgy/y) = 644.86 table 1. uranium, thorium and potassium concentration as determined by gamma spectrometry. table 2. dose rate obtained from radioisotope concentration, local cosmic radiation and total dose rate. alpha radiation from sediment was not used in the calculation, as about 50 μm was removed from the outer side of the tooth. figure 4. signal versus dose growth showing the linear fitting used. archaeological dose for ag-i was ad = 2.91 ± 0.14 gy; correlation coefficient was 0.9937. soil analysis results. the present work did not determine u, th and k concentration in enamel and dentine as their percentages have been negligible for calculating age in previous work, especially in dates for the holocene period (kinoshita et al., 2008; hefne et al., 2002). typical soil density, enamel and dentine values were used for calculating age (2.00, 3.00 and 2.82 g/cm3, respectively). the layer of enamel stripped off from each side by acid etching was about 50±5 mm; this prevented a possible contribution towards the dose having been made by alpha particles deposited in the sample. using these values and the radioisotope concentrations listed in table 1, rosy software was used for obtaining the alpha, beta and gamma radiation dose rate contribution for each radioisotope in sediment (the results are listed in table 2). these results were the same for any uranium absorption model. considering a 251 ± 15 μgy/year dose rate for cosmic radiation, a 3,256 ± 190y before present (bp) age was thus found for the ag-i sample. it should be noted that rosy software calculates these ages using three uranium accumulation models: linear, early and a combination of both (ikeya, 1993; grun, 1989; fattibene et al., 2010). the way that uranium accumulates in a tooth is established by these models; accumulation increases at the same rate as time in the linear model and accumulation occurs in a short period of time in the early model, relative to a tooth’s age and then remains constant after this short period of time. the tooth’s age was the same when calculated by these three methods; this result was consistent if taken into account that the accumulation of radioactive elements inside the tooth was negligible. conclusions age determined at many countries’ archaeological sites is the result of radiocarbon (14c) analysis of particular items found around the burials and not direct dating of individuals found there. the results regarding either physical or geological phenomena may not be chronologically equal, hence the importance of having techniques like esr which can directly quaternary dating by electron spin resonance (esr) applied to human tooth enamel 119 and absolutely determine the age of individuals of interest. the age of a human burial site found in colombia was determined in this work from a single tooth, a mean age of 3,256 ± 190y bp being found. this result agreed with stratigraphic analysis at sites providing soil ages ranging from 2,800 to 3,500y bp for 50 and 80 cm depths (correal, 1990). esr dating accuracy was obtained relatively straightforwardly, using just a few grams of tooth enamel, thereby highlighting some of the advantages and characteristics of the esr dating method. the results so obtained have contributed towards joining links in the long chain of colombian and american history (where there is still much to do, due to the rich pre-hispanic and colonial past). this paper thus represents a pioneering effort aimed at promoting the dating of our ancestors. acknowledgments we would like to thank the dib, universidad nacional de colombia, for providing financial support (project 8003348), m. peña from ingeominas for soil sediment analysis, the universidad nacional de colombia’s physical anthropology laboratory and its director, j.v. rodriguez, for providing facilities and allowing the use of his samples, as well as am groot (archaeologist) for her support during this research. references a.f. borgonovea, a. kinoshitaa,b, f. chena, p. nicoluccia, o. baffaa, (2007): energy dependence of different materials in esr dosimetry for clinical x-ray 10mv beam. radiation measurements, 42: 1227– 1232. anatoly molodkov; (2001): esr dating evidence for early man at a lower palaeolithic cave-site in the northern caucasus as derived from terrestrial mollusc shells; quaternary science reviews 20: 1051 -1055 ardila, g.i, (1984): chía, un sitio precerámico en la sabana de bogotá. fundación de investigaciones arqueológicas nacionales. banco de la república. bogotá d.c. baffa, o., brunetti, a., karmann, i., neto, c.m.d., (2000): esr dating of a toxodon tooth from a brazilian karstic cave. appl. rad. isot., 52: 1345-1349. brennan, b.j., rink, w.j., rule, e.m., schwarcz, h.p., prestwich, w.v., (1999): the rosy esr dating program, ancient. tl. 17: 45-53. correal, g., (1979): investigaciones arqueológicas en abrigos rocosos de nemocón y sueva. fundación de investigaciones arqueológicas nacionales. banco de la república. bogotá d.c. correal, g., (1990): aguazuque : evidencias de cazadores, recolectores y plantadores en la altiplanicie de la cordillera oriental. fundación de investigaciones arqueológicas nacionales, banco de la república. bogotá d.c. groot, a.m., (1992): checua. una secuencia cultural entre 8500 y 3000 años antes del presente. fundación de investigaciones arqueológicas nacionales. banco de la república. bogotá. grün, r., (2006): direct dating of human fossils. yearb. phys. anthropol. 49: 2-48. halliwell b. and gutteridge j. 2007. free radicals in biology and medicine. oxford university press, usa. hefne, j., yamani, a., al-dayel, o., ikeya, m., al-osaimi, s., (2002): esr dating of tooth from pre-islamic sitin saudi arabia. advances in esr applications. 18: 119-121. iaea report., (2002): use of electron paramagnetic resonance dosimetry with tooth enamel for retrospective dose assessment. report of a coordinated research project. iaea-tecdoc-1331. vienna. ikeya, m., (1993): new applications of electron spin resonance: dating, dosimetry and microscopy, world scientific publishing company, singapore. jonas, m., (1997): concepts and methods of esr dating. radiat. meas. 27: 943-973. kinoshita a., braga f.j.h.n., graeff c.f.o., baffa o., (2001): esr dosimetry of 89srand 153sm-in bone. appl. rad. isot. 54: 269-274. kinoshita, a., figueiredo, a.m.g., felice, g.d., lage, m.c.s.m., guidon, n. baffa, o., (2008): electron spin resonance dating of human teeth from toca da santa shelter of são raimundo nonato, piauí, brazil. nucl. instrum. meth b. 266: 635-639. mascarenhas s., baffa filho o., ikeya m., (1982): electron spin resonance dating of human bones from brazilian shell-mounds (sambaquis). am. j. phys. anthropol. 59, 413-417. prescott, j.r., hutton, j.t., (1994): cosmic ray contributions to doserates for luminescense and esr dating: large depths and long-term time variations. radiat. meas. 23, 497-500. punchard n. and kelly f. 1996. free radicals: a practical approach. oxford university press, usa. renfrew, c. y bahn, p. arqueología. teorías, métodos y práctica (1992): ediciones akal s.a. pp 107147, rink, w.j., (1997): electron spin resonance (esr) dating and esr applications in quaternary science and archaeometry. radiat. meas. 27: 975-1025. shigueo watanabe, walter elias feria ayta and henrique hamaguchi; (2003): some evidence of a date of first humans to arrive in brazil. journal of archaeological science 30: 351–354. ülkü ulusoy; (2004): esr studies of anatolian gypsum; spectrochimica acta part a 60: 1359–1365. walker m. (2005): quaternary dating methods. john wiley & sons ltd, the atrium, southern gate, chichester. chapters 1, 2, 3 and 4. wieser, a., mehta, k., amira, s., aragno, d., bercea, s., brik, a.,bugai, a., callens f., chumak v., ciesielski b., debuyst r., dubovsky s., duliu o.g., fattibene p., haskell e.h., hayes r.b., ignatiev e.a., ivannikov a., kirillov v., kleschenko e., nakamura n., nather m., nowak j., onori s., pass b., pivovarov s., romanyukha a., scherbina o., shames a.i., sholom, s., skvortsov v., stepanenko v., tikounov d.d., toyoda s. (2000): the second international intercomparison on epr tooth dosimetry. radiat. meas, 32, 549–557. earth sci. res. j. vol. 11, no. 2 (december 2007): 139-154 hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan adrian h. gallardo 1,2,*, atsunao marui 1 1aist, geological survey of japan, quintessa japan, yokohama 220-6007 2hytec co. yodogawa-ku, miyahara 2-11-9, osaka 532-0003, japan *corresponding author. a. h. gallardo. e-mail: ad.gallardo@aist.go.jp tel/fax: +81-29-861-3684 abstract hydraulic characteristics of sediments were investigated at j-parc for the purpose of site characterization in relation with the construction of japan’s largest proton-accelerator. a total of 340 samples extracted from 9 exploratory wells were examined by standard laboratory tests and complemented with statistical analyses to quantitatively determine the main terrain attributes. two main hydro-geological units were recognized, although a number of embedded layers defined a multilevel aquifer. grain-size distribution derived from sieve analysis and the coefficient of uniformity showed that soils are poorly sorted. on the other hand, hydraulic conductivity was measured by a number of parameters such as a log-normal distribution. conductivity was also predicted by empirical formulas, yielding values up to three orders of magnitude higher. discrepancies were explained in terms of soil anisotropy and intrinsic differences in the calculation methods. based on the shepherd’s approach, a power relationship between permeability and grain size was found at 2 wells. hydraulic conductivity was also correlated to porosity. however, this interdependence was not systematic and therefore, properties at many parts of the profile were considered to be randomly distributed. finally, logs of electrical conductivity suggested that variations of soil hydraulic properties can be associated to changes in water quality. in spite of the remaining uncertainties, results yielded from the study are useful to better understand the numerical modelling of the subsurface system in the site. key words: drill core, soil properties, heterogeneity, site characterization, proton accelerator resumen las características hidráulicas de los sedimentos fueron investigadas en j-parc con el proposito de caracterizar sitios relacionados con la construcción del acelerador de protones más grande de japón. un total de 340 muestras extraídas a partir de 9 pozos exploratorios fueron examinadas a partir de pruebas de laboratorio estándar y complementadas con análisis estadísticos para determinar cuantitativamente las cualidades principales del terreno. dos unidades hidrogeológicas principales fueron reconocidas, aunque un número de capas encajadas definieron un acuífero de niveles múltiples. 139 earth sciences research journal manuscript received june 30 2007. accepted for publication november 20 2007. la distribución del tamaño del grano derivada de análisis de tamices y del coeficiente de uniformidad demostró que los suelos están mal clasificados. por otra parte, la conductividad hidráulica fue medida a partir de un número de parámetros tales como una distribución logarítmica normal. la conductividad también fue predicha por medio de fórmulas empíricas, mostrando valores de hasta tres órdenes de magnitud. las discrepancias fueron explicadas en términos de la anisotropía del suelo y diferencias intrínsecas en los métodos de cálculo. basados en la aproximación de shepherd, se encontró una fuerte relación entre la permeabilidad y el tamaño de grano en 2 pozos. la conductividad hidráulica también fue correlacionada con la porosidad. sin embargo, esta interdependencia no era sistemática y por lo tanto, propiedades en muchas partes del perfil eran consideradas como distribuidas aleatoriamente. finalmente, los registros de la conductividad eléctrica sugirieron que las variaciones de las propiedades hidráulicas del suelo se puedan asociar a los cambios en la calidad del agua. a pesar de las incertidumbres restantes, los resultados del estudio son útiles para entender mejorar el modelamiento numérico del sistema subterraneo en el sitio. palabras claves: nucleos, propiedades del suelo, heterogeneidad, caracterización del sitio, acelerador de protones. 1. introduction the construction of japan`s largest proton accelerator has been initiated within j-parc (japan proton accelerator research complex) at the coasts of tokaimura, about 100 km from tokyo. in this context, hydraulic conditions of the main sedimentary units were investigated as a pre-requisite to simulate the flow conditions at the site. groundwater is the primary pathway by which radionuclides can migrate from the underground to the biosphere (fairhurst, 2004), so understanding the flow and the type and characteristics of the geologic medium plays a fundamental role to assess the safety of the accelerator. soils are heterogeneous systems with regions more or less favorable to flow distributed spatially in intricate patterns (giménez et al., 1999). lithofacies usually show high spatial variability and as a result, the associated hydrologic and physical properties can be of appreciable different character. as the problem of adequately modeling subsurface becomes more difficult with increasing heterogeneity, determinations based on a large amount of samples have great importance to draw reliable conclusions. an extensive analysis of drill cores appears to be a powerful way to delineate sedimentary structures and to define the formation properties. impervious materials are the most favorable for radio-nuclides isolation and consequently, determination of hydraulic conductivity becomes of paramount importance. pumping and slug tests are frequently used to measure it in porous media; however, the analysis of textural parameters is a comparatively cost-effective and quick technique which has also the advantage of providing information at a more detailed scale. in this line of thought, saturated hydraulic conductivity can be approximated either from laboratory measurements or by empirical formulas derived from the size of the granular sediments (vukovic and soro 1992). moreover, shepherd (1989) derived a power function between permeability and particle size which does not depend on the temperature of the medium, constituting thus an alternative to other techniques. this approach was also followed to predict the hydraulic conductivity of soils at j-parc. porosity changes will influence permeability and hence the possible transport of the radio-nuclides (de craen et al., 2004). for soils of similar origin, the relationship of permeability to porosity can hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan 140 gallardo and marui. esrj vol. 11, no. 2. december 2007 141 be strong (ahuja et al., 1989); therefore, the possible interdependence of these properties was also examined. summing up, the present work presents the results obtained from nine exploratory boreholes within j-parc, evaluating the sedimentary sequence and investigating the main properties of the soils to effectively characterize the hydrogeology of the site. the study attempted to find the existence of meaningful relationships among parameters, providing background information relevant for predictions of groundwater flow in the area. 2. site location and geological setting the j-parc complex is located at the tokaimura, ibaraki prefecture, which is approximately 100 km northeast of tokyo. the investigated wells are essentially concentrated around the proton accelerator, a circular area of about 500 m of diameter close to the mouth of the shinkawa river (fig. 1). the regional geology has been extensively studied by sakamoto et al. (1972). these authors identified the basement of the area as a marine sequence of turbidite and slump deposits from the late cretaceous. the strata reach up to 1,500 m of thickness. above it, the neogene tertiary is occasionally exposed on sea and terrace cliffs. the taga formation from the late miocene outcrops along the shinkawa river and was reached by drilling at about 50 m below the ground. this unit consists mainly of sandy siltstones with abundant mollusk fossils. intercalated tuffs would correspond to sub-aqueous pyroclastic flows. their thickness ranges from nearly 10 m at the inland wells to a minimum of 1 m by the seaside. the miwa formation is in discordance and constitutes the base of the pleistocene. its lower member reaches a maximum thickness of 30 m, and is composed of gravels and mud that filled the valleys at the early phase of the last interglacial transgression. the upper end of the formation consists of sands and pebbly sands deposited at the maximum stage of the transgression. its thickness ranges between 15 and 20 m. in addition, a buried channel with numerous levels of marine and fluvial terraces has been observed in the upper levels of the formation, right below the accelerator site (gallardo, 2006). overlying the miwa formation, the fluvial gravels of the nukada terrace were deposited in unconformity during a regressive period. further regression was responsible of the formation of subsequent terraces (takai and tsuchi, 1963). recent alluvium is present along the streams, while a belt of sand dunes extends along the figure 1. location of the research area and boreholes shoreline. 3. methodology 3.1 monitoring wells and coring nine exploratory wells were drilled in and around the accelerator ring to get basic information about field conditions. soil specimens were recovered continuously at intervals of 1 m by the hybrid boring method (takeda and komiya, 2006). this technique was originally designed for subsurface exploration in damns but is currently being tested in other underground works in japan. as in standard methods, sampling consists in driving a hard bit into the desired depth and coring the material directly into a double steel tube with an internal diameter of 68 mm. however, the success of the hybrid boring relies in the circulation fluid. a dry-foam based on air bubbles originates a fluid that improves considerably the cores recovery even in non-cohesive sediments. soil recovery was close to 100% up to depths of 200 m. after retrieving, cores were wrapped in humid polyethylene to prevent desiccation, and finally packaged in sealed wooden-boxes. while all cores were numbered, logged, and photographed, a subset of 340 samples was used for the physical analyses. at individual wells, the number of analyzed samples increased systematically with the length of the column, bringing more confidence for comparison purposes. 3.2 soils physical analysis the grain-size distribution in soils was determined by sieve analysis for particles above 63 μm, and by an analyzer shimadzu sa-cp3l for finer sizes. the coefficient of uniformity (cu) was derived from the formula: 10 60 d dcu = (1) where d60 and d10 represent the diameters corresponding to the percent finer of 60% and 10% respectively. for conductivity measurements, a 100 cc can was driven into the cores avoiding fracturing or shrinkage. it was left overnight in water to reach saturation, weighted, and finally tested by the falling head method. the results were corrected to a water temperature of 15 ºc in accordance with standard methods. after the analysis, samples were oven-dried at 105 ºc during 24 h, re-weighted, and total porosity (n) was calculated by the relation: s w v vn 100 = (2) where n: total porosity of the sediment (as percentage); vw: volume of water filling the sample voids; and vs: total volume of the sample. the weight of the water removed was converted to volume considering its density as 1 g/cm3. although it is quite difficult to completely saturate a sample (freeze and cherry 1979), this simple method produced quick and satisfactory results. hydraulic conductivity was also calculated from the grain-size distribution. the method of creager et al. (1945) provided a straightforward relation between the effective grain-diameter of 20% weight content (d20) and conductivity. kasenow (2002) presented a detailed compilation of empirical formulas for hydraulic conductivity determination. based on the effective grain diameters of the sediments 142 hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan 143 involved, the equations of slichter and sauerbrei were considered the most appropriate for the analysis. the sauerbrei formula is applicable when the grain-size diameter is no greater than 0.5 mm. it can be written as 17 2 2 3 )1( d n n v gk z tb       − = (3) while the slichter formula can be applied for an effective grain diameter (de) ranging between 0.01 and 5 mm. it takes the form of: ( ) 2 10)( dn v gk s jb= (4) in eqs. (3) and (4), k, hydraulic conductivity (l/t); g, gravitational constant (l/t2); ν, kinematic viscosity for a given temperature (l2/t); n, porosity (unitless); βz and βs, constants; τ correction for temperature; j (n) porosity function; and d, effective grain diameter (l). 4. results and discussion 4.1 hydrological setting the top of the hydrostratigraphic sequence is characterized by a complex of sand and conglomerates with a thickness that varies from 13 to about 55 m (fig. 2). the uppermost 10-13 m of the unit contains basement pebbles and rests of shells attributed to beach and fluvial deposits. the water table is found at an average of 6 m below the ground, sustaining a great number of residential wells used for irrigation and domestic needs. the successive fluctuations of the sea level over geologic times have enhanced the intercalation of thin horizons of silts and clays that cause local confinement at specific sites. siltstones of the taga formation constitute the bottom of the unconfined aquifer. there is some presence of interbedded tuffs and sandy lenses with a thickness ranging from a few cm to nearly 5 m. a somewhat different stratigraphic scheme is seen at well 0402 where fine sandstones occur throughout the borehole to a depth of 200 m. the hydraulic contact between both quaternary and tertiary sandstones might suggest the existence of an irregular unconfined aquifer extending along the coastal plain. 4.2 physical properties grain size is a fundamental physical parameter of sediments and, as such, is a useful descriptive property (friedman et al., 1992). a frequency distribution of the whole population showed that the sediments tend to be unimodal, with a mode of ɸ 2.2 (fine sands), and a negative skewness toward very fine sands (fig. 3). for a better interpretation, soils were discriminated between tertiary and quaternary origin. results indicated that the shallow fluvial and beach deposits have a relatively larger mode, in the order of ɸ 1.1 (medium sand), whereas there are no significant differences in the values and distribution for the rest of the strata. differences in grain size between shallow and deep sediments might be associated with variations in the depositional environments. fluvial sediments generally have coarser grains than units deposited in coastal environments, and their mean and maximum grain size commonly reflects the average energy of the depositional medium (boggs jr., 1987). particles over 2 mm normally consist of shell fragments and basement-derived pebbles transported under conditions of high flow. although simple, the two distinguished sections of the profile supported the distinction between terrigenous deposits near the surface, and gallardo and marui. esrj vol. 11, no. 2. december 2007 144 figure 2. simplified stratigraphy based on the drill cores figure 3. grain sizes frequency distribution obtained by sieve analysis. units: phi (φ) hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan 145 transitional to marine sediments at depth. the degree of sorting of a soil can be estimated based on the range of its standard deviation. following the formula of folk and ward (1957), the calculated standard deviation varied from 2 to 3.5 ɸ, which correspond to very poorly to extremely poorly sorted sediments (folk, 1974). a single parameter may not be adequate to distinguish sediments from different wells; therefore, the coefficient of uniformity (cu) was also calculated. soils having a coefficient smaller than about 2 would be considered uniform (lambe, 1951). such a threshold was exceeded in several orders by the vast majority of the measured samples, demonstrating the good gradation (or poor sorting) of the sediments concerned (fig. 4). in general, there were no substantial differences in the arithmetic mean of cu among boreholes with exception of the well 0403, which showed a mean more than twice the value obtained for the rest of the samples. three groups with different sorting could be qualitatively defined when evaluating the range of cu at each well. as expected, shallow wells presented the best (although still poor) sorting, as they usually intercepted a limited number of formations also associated to higher homogeneity in their sedimentary attributes. the size range reflects the shortterm random variation of the fluid forces involved in sediment transport (bridge, 1981). therefore, it can be assumed that even though shallow sediments would have been deposited in an environment with sufficient energy to transport a wide range of particle sizes, they could have undergone important reworking especially by tides. finer particles may have been removed by the return flow moving up the front of a beach, leaving behind a population of relatively well-sorted, coarser grains (boggs jr., 1987). no meaningful relationship between sorting and geology could be found for wells 0402 and 15-1, while wells 0401 and 0403 presented the widest grain-size distribution throughout the site. in effect, the deeper the well the higher the non-uniformity, as longer soil profiles included more layers and sedimentary horizons, resulting consequently in larger deviations of the cu. 4.3 hydraulic conductivity saturated hydraulic conductivity (ks) is arguably one of the most important hydrologic properties of soils (mbagwu, 1995). conductivity values ranged from 4.5 x 10-2 to 1.3 x 10-6 cm/sec, with an isolated sample in the order of 10-7 cm/sec within a 1.5 m horizon of fine sands and organic silts at well no. 1. the mean value exhibited an irregular pattern along the profile, except for wells 0405 and tokai where ks decreased with depth (fig. 5). hydraulic conductivity at the upper horizons is two to four times higher than values near the wells bottom, in coincidence with the presence of 20-25 m gravel-filled paleochannels. in addition, roots and root channels related to the original vegetation covering the site could promote water movement through the topsoil layers. variances calculated for the ln(ks) at each well ranged between σ2 = 1.44 and 7.24 confirming that the variability of the hydraulic parameter within the soil columns is rather distinct. the distribution of permeability in porous media has been usually reported to approximate a log-normal distribution (freeze, 1975; sudicky, 1986). a plot for the entire data set shows that hydraulic conductivity in j-parc is close to this distribution (fig. 6). this statement was validated by the shapiro-wilk test (one sample vs. normal) at the 1% significance level. the test did not reject the null hypothesis of normality (w: 0.99) and gallardo and marui. esrj vol. 11, no. 2. december 2007 146 consequently, the sample can be considered to follow a log-normal distribution. to reduce the probability of correlation, the statistical distribution should ideally be evaluated on a random subsample of the data set (turcke and kueper 1996). again, the log-normality of ks was evaluated by the shapiro-wilk test for a random set of 100 samples (w: 0.96), confirming that the assumed distribution is correct. values derived from core samples represent ks in the vertical direction. however, it is widely known that in heterogeneous aquifers the permeability tends to be different along each of the principal directions. differences were examined introducing the ratio of horizontal (kh; from empirical formulas) to vertical conductivity (kv; from direct measurements). in general, empirical formulas yielded higher values of ks than direct measurements, with differences sometimes as much as three orders of magnitude. the slichter formula gave the lowest differences, with a mean kh/kv ratio of 8.8 (table 1). this mean ratio applied to most of the stratigraphic columns, defining soils of relatively small anisotropy, with some occasional and random discrepancies presumably caused by local heterogeneities in the mineral content or pore distribution. on the other hand, conductivity calculated by the creager and the sauerbrei approaches showed a poorer agreement with core measurements. although, a mean kh/kv ratio of 24.8 and 29 respectively is still fairly consistent, considering differences between the conductivity tensors may range from 1 to 42 (muskat 1937; bear 1972). conductivity showed substantial variations along depth, with differences of more than two orders of magnitude respect kv measurements, and kh calculated by the slichter solution. the greatest values of kh were found on the upper 50 m of the sequence, between 70 and 130 m, and at about 160 m. it is reasonable to accept larger variations in conductivity at shallow depths, where unconsolidated fluvial sediments exist. these kind of deposits have been described to be strongly heterogeneous (jussel et al., 1994), and would be also affected by surface desiccation and human activities, which promote degradation of the upper horizons and enhance the anisotropy of the soils (regalado and muñoz, 2004). in contrast, there was no reliable relationship to substantiate the differences in conductivity results at deeper soils. it is possible that sediments that appear to be relatively homogeneous are in fact affected by micro-scale bedding and microscopic textural variations, which imprint the profile its anisotropic characteristics. with respect to the empirical formulas, values derived from the slichter equation were nearly three times smaller than results obtained by the methods of creager and sauerbrei. the origin of the differences should be searched within the formulas themselves. grain size is the fundamental independent parameter that controls hydraulic conductivity in unconsolidated sediments (pryor, 1973; morin, 2006) but empirical formulas were elaborated introducing different relations and therefore, it is not unusual they yield divergent results. since each formula bases its calculations on a different effective grain-diameter, more emphasis is made on a determined size fraction, which indefectibly leads to discrepancies in the conductivity distribution. thus, defining the reliability of the determinations remains problematic. although empirical formulas gave just approximate values of hydraulic conductivity and results must be interpreted with caution, they are consistent with the hypothesis of an anisotropic medium with the major conductivity tensor oriented along the horizontal plane, and the minor direction along the vertical. hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan 147 figure 4. distribution of the coefficient of uniformity (cu) for the whole dataset figure 5. profiles of hydraulic conductivity at some of the wells. variance in parenthesis. gallardo and marui. esrj vol. 11, no. 2. december 2007 148 figure 6. distribution of hydraulic conductivity for the entire dataset figure 7. electrical conductivity near the freshwater/seawater interface. hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan 149 as summarized by shepherd (1989), the correlation of permeability and grain size results in a power expression of the form y = axb (5) where (y) is permeability when plotted against grain size (x), the coefficient a is the value of y at x = 1, and b is the slope of the line that is fitted to the data. in contrast to other formulas, this equation does not depend on the temperature of the medium (vrbka et al., 1999). in an attempt to improve predictions, hydraulic conductivity measured at each well was plotted against grain size to find an empirical correlation between both parameters. the best fit was calculated by the least-squares regression. from the nine sets of data only two yielded acceptable values of r-squared in coincidence with the coarse-grain dominated shallow wells (table 2). the calculated slopes were in agreement with other reports (stearns, 1929, rose and smith, 1957) although they were somewhat low, which may be an indication of texturally immature sediments and miscellaneous samples from different environmental populations (shepherd, 1989). no significant relation was seen for the rest of the datasets, possibly due to the mixture of distinct grain sizes. however, a strong correlation has been reported for a subset of samples at well 4-1 (sankoh consultants, 2004) suggesting the theoretical expectations might be confirmed within certain facies of the soil profile. the hydraulic conductivity of granular deposits is also dependant upon porosity (morin, 2006). therefore, the existence of any trend was investigated for each well dataset by the pearson correlation-coefficient. the null hypothesis (there is no relationship) was rejected at the 1% level of significance at three wells, and at the 2% level for one more borehole (table 3). in all cases, the relationship was negative, suggesting that the permeability of the soil would decrease with an increase in the fraction of fines. despite that it is reasonable to have a negative correlation under certain grain-size distributions and packing arrangements (beard and weyl, 1973; morin, 2006), it was not possible to find a consistent pattern all over the vertical cross section. measurements displayed variable curves, and while the relationship was visible at some points, there seemed to be no arrangement at other depths. it is also feasible that migration of colloidal particles could significantly affect the permeability of the porous medium at depth (khilar et al., 1983). in effect, the diameter of colloidal matter has been found to increase with depth in association with the higher groundwater salinity (düker and ledin, 1998). this phenomenon could contribute to clog the aquifers causing an important reduction in permeability. changes are likely to be accompanied by reductions in porosity as well, which may explain the occasional deviations in the negative correlation. it can be concluded that there may be numerous causes for the changes in conductivity in the area, and while the negative trend between conductivity and porosity is significant at many units of the vertical profile, this correlation cannot be generalized to the whole sequence because it is not systematic. the variability of soil properties is so complex that only parts of the structure can be described in a deterministic way, while the rest probably have to be seen as random realizations (heuvelink and webster, 2001; herbst et al., 2005). 4.4 profiles of electrical conductivity gallardo and marui. esrj vol. 11, no. 2. december 2007 changes in the physical properties of soils may modify the rates and chemical composition of water entering the well. figure 7 shows the logs of electrical conductivity (ec) for two wells near the seawater/freshwater interface (marui, 2005). values increased with depth, with a marked change in concentrations at 100 and 160 m in well 0402, and 170 m for well 0401. shallow waters would relate to freshwaters of low ec, while high conductance at depth would be associated either with fossil waters or seawater intrusion. although not shown here, data collected from other wells indicated the lowest salinities occur in the upper 40 m below the ground, where alluvial deposits of higher hydraulic conductivity promote a rapid inflow of meteoric waters and the existence of a freshwater aquifer. in this way, a preferential path for submarine groundwater discharge has been reported at a depth of 30 m (itoh et al., 2006). geology may also explain the changes in ec at a depth of 100 m in well 0402, where an increase in ions concentration coincided with the intercalation of a thin layer of tuffs. unlike the upper aquifer, flow would occur in the opposite direction, toward the land. thus, the change in water chemistry would reflect an increase in na+ and clconcentrations associated with seawater intrusion. at other depths, water quality compared less favorably with physical parameters. density-driven flow can be expected to occur along preferential flow paths surrounded by stagnant zones (schincariol, 1998). however, mixing of water inside wells could have altered the quality of water at different depths. it is possible that seawater flowing through favorable sediments sank due to higher density when entering the wells, which would explain why peaks of salinity occur scattered in the profile. on the other hand, the effect of the wells screen may be another factor obscuring changes in conductance as a function of hydraulic properties. in effect, waters from different depths may enter long-screened wells, so it is not possible to know if poor-quality water is from deeper or shallower sources (izbicki et al., 2005). thus, even though ec logs can be considered a quick and inexpensive tool to delineate major changes of hydraulic properties, the method can not be preferred over direct measurements of soil physical parameters, at least without a multilevelwell system screened at short intervals all over the profile. 5. summary and conclusions drill cores from 9 wells were analyzed to a depth of 200 m at the coastal area of j-parc, and the main lithological facies and hydraulic properties of the sedimentary sequence determined. deposits are strongly heterogeneous, although two main hydrogeological units could be recognized: quaternary sand and gravels constitute an unconfined aquifer with a thickness up to 55 m at the top of the profile, while silty sediments from the tertiary define the lower part of the section. embedded throughout the sequence, there are a number of horizontal layers with variable thickness and composition which define a multi-aquifer system. as expected, the high variability in the sedimentary structure and the inevitable mixing of samples with different origins was associated with important irregularities in the parameters distribution. basic properties as hydraulic conductivity, porosity, and grain-size presented complex patterns difficult to be distinguished by laboratory determinations alone and therefore, results were complemented with statistical approaches to provide a more rigorous interpretation of the field conditions. since soil properties not always 150 hydraulic characteristics of sedimentary deposits at the j-parc proton-accelerator, japan table 1. ratio of hydraulic conductivity calculated by empirical formulas (kh) in relation to core measurements (kv) for 317 samples table 2. power relation between grain size and hydraulic conductivity for three of the wells table 3. correlation of porosity to hydraulic conductivity for the analyzed wells 151 gallardo and marui. esrj vol. 11, no. 2. december 2007 presented a systematic relationship, parts of the spatial structure would better be explained as random distributions. the hydraulic conductivity was described by a lognormal distribution. results from empirical calculations and laboratory tests differed in up to three orders of magnitude, leading to the assumption that conductivity in the horizontal direction is appreciably higher than in the vertical. the approach of shepherd was also applied to improve predictions. the regressions provide a simple relationship between grain size and conductivity although good estimates were limited to just two wells, probably in coincidence with texturally-immature sediments. a negative trend between porosity and hydraulic conductivity was inferred at four wells. it is intuitive to think about a decrease of permeability with an increase in the fraction of fines at depth however, this relationship was not constant and therefore, it is likely that other processes as colloidal migration, surface weathering or soil compaction may be responsible for the changes in the parameters distribution. acknowledgements the authors are very grateful to dr. roger morin, usgs, for providing essential material to accomplish this work. special thanks are extended to dr. diego tanaka, school of system and information engineering, university of tsukuba, for his advises on statistics. finally, the authors want to thank the anonymous reviewers, whose comments helped improve the manuscript. references ahuja, l.r., cassel, d.k., bruce, r.r., and b.b. barnes (1989). evaluation of spatial distribution of hydraulic conductivity using effective porosity data. soil science 148 (6), 404-411. aimrun w., amin, m.s.m., and s.m. eltaib (2004). effective porosity of paddy soils as an estimation of its saturated hydraulic conductivity. geoderma 121, 197-203. bear, j (1972). dynamics of fluids in porous media. dover pubs. new york, 784 pp. beard, d.c., and weyl (1973). influence of texture on porosity and permeability of unconsolidated sand. am assoc petrol geol bull 57 (2), 349-369. boggs jr, s (1987). principles of sedimentology and stratigraphy. merril publishing co. usa. 800 pp. bridge, j.s (1981). hydraulic interpretation of grain-size distributions using a physical model for bedload transport. j sed petrol 51, 1109-1124. 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karhunen-loève transform to suppress ground roll in seismic data ernesto gómez londoño1, luís castillo lópez2 and thaís de souza kazmierczak2 1 ingeominas bogotá, recursos del subsuelo, e-mail: egomez@ingeominas.gov.co 2 universidade federal do rio grande do sul brazil. e-mail: luis.castillo@ufrgs.br; e-mail: thais.kazmierczak@ufrgs.br resumen en este estudio se implementó y modificó el algoritmo de sacchi (2002), basado en la transformada karhunen-loève (k-l), como una técnica para la supresión de ground roll, mostrando que aunque la amplitud del ground roll es mayor que la amplitud de la señal al usar la transformada k-l se tiene éxito en suprimirlo sin causar distorsión en las señales de reflexión, obteniendo mejores resultados frente a técnicas convencionales de remoción: f-k, paso alto y pasa banda. la transformada k-l ha sido utilizada en el campo de la detección y procesamiento de imágenes (levy and linderbaurn, 2000), en el reconocimiento de caras, iris y huellas dactilares como factores biométricos de identidad. una sección sísmica es una imagen del subsuelo por tanto esta transformada puede ser usada en el procesamiento sísmico, porque remueve partes correlacionadas espacialmente y proveyendo una imagen clara y coherente. el algoritmo se aplicó a registros generados con martillo, thumper y fuentes explosivas. se hizo un procesamiento convencional hasta el apilado, remplazando los filtros por le uso de la transformada k-l. los resultados muestran que la transformada k-l hace una mejor recuperación de la amplitud de los reflectores, eliminando refracciones que causan eventos someros irreales e incrementando la coherencia lateral de los eventos sísmicos, facilitando la interpretación geológica. palabras clave: amplitud, coherencia, filtro, onda de tierra, ruido. abstract the sacchi’s algorithm (2002) based on the karhunen-loève (k-l) transform was modified and implemented to suppress ground roll without distortion of the reflection signals, it provided better results than conventional techniques for noise removal like f-k, high-pass and band pass filters. the k-l transform is well known in other fields as image processing (levy and linderbaurn, 2000), face, iris and fingerprint identification. a seismic section is an image of subsurface where the k-l can be useful in seismic processing because spatially uncorrelated signals can be removed providing a clear and coherent image. the algorithm was applied to seismic data generated with hammer, thumper and explosive sources. conventional processing flows were used, but one replaced filters with k-l transform, providing stacked sections. the earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 139-147 140 ernesto gómez londoño, luis castillo lópez and thaís de souza kazmierczak k-l transform recovers better the reflector amplitudes when compared with others filters, also it removes refractions that cause unreal shallow events and increases the lateral coherence of seismic events showing a more interpretable geology. key words: amplitude, coherent, filter, ground roll, noise. © 2005 esrj unibiblos introduction the ground roll is (scales and snider, 1998) generated in shallow layers and is a typical rayleigh wave. it is characterized by low velocity and frequency and high amplitude, being considered a coherent noise in seismic exploration (d'agosto et al., 2002). the kl algorithm transforms the seismic data in a reasonable number of independent eigen functions that represents the most important signal characteristics. it has been considered a very advisable tool in the processing and analysis of reflection data (al-yahya, 1991). the conventional filtering techniques: f-k, high-pass and bandpass are applied in the frequency domain but in the case of ground roll amplitude stronger than reflection signals, f-k filters cause serious distortion of the signal (karsh and bayrak, 2004). high pass filter is based on the low frequency of the ground roll, with the disadvantage that also eliminates the low-frequency content of reflection signals. band pass filter depends on the amount of overlaps between the noise and the signal, therefore when the frequency bands concentrated the energy in each band without overlap, the signal/noise ratio will not change (yilmaz, 1987). the k-l transform separates the ground roll from the reflector signal, permitting to subtract it from seismic data. its application to real data showed that the ground roll suppression increased lateral coherence of seismic events in a stacked section. theoretical background the surface waves in the raw register in figure 1 travel with velocities ranging from 100 to 1000 m/s and with frequencies around 10 hertz and lower than those of reflections and refractions. note how the near surface effects distort travel time curve at the right flank of reflection "a". figure1. different waves in a raw shot gather. the karhunen-loève transform it makes a principal component analysis that is a mathematical way of determining that linear transformation of a sample of points in l-dimensional space which exhibits the properties of the sample most clearly along the coordinate axes. along the new axes, the sample variance are extremes and uncorrelated, see figure 2. using a cutoff on the spread along each axis, a sample may be reduced in its dimensionality. this way it can be used to transform independent coordinates into significant and independent ones. figure 2. k-l transformation principal eigenvector 141 using the karhunen-loève transform to suppress ground roll in seismic data k-l transform implementation the seismic traces x i (t) correspond to the rows of the named data matrix x n*m , n is the number of traces in the gather and m the number of gathers. the zero-lag covariance matrix *n nc is (jones and levy, 1987): x x* * * t n n n m m nc (1) this expression can be decomposed as .v a v* * * *n n n p n n p n tc (2) in equation 2 the columns of the matrix v n*p are the eigenvectors *n nc and a n*n is a diagonal matrix with the eigen values on the diagonal. these eigen values are ordered in descending sequence along the principal diagonal of the matrix a n*n , v *n p t is the corresponding transpose matrix. the principal component of the data can be written by: m v x* * *p m p n t n m (3) if we form the matrix m’ p*m by selecting the upper m rows of the matrix m p*m , m is related with the order of the filter or principal row number, placing zeros in the remaining n-m rows, n is related with the cut of the filter, the ground roll is assumed to be represented by the matrix product between the eigenvectors matrix v n*p and the matrix m’ p*m , that include the principal components in agreement with the following expression: x v m* ' * * ' n m n p p m (4) finally, the ground roll x’ n*m is subtracted from the data matrix x n*m , obtaining the gather without ground roll x x x* * * ' n m n m n m0 (5) xo is a n*m matrix that represents the filtered seismic gather. as mentioned before x n*m is the input data matrix and x’ n*m is a matrix that represents the ground roll extracted from the input data. results the capacity of kl algorithm to suppress ground roll at gather level is evaluated using shot gathers with dispersive waves of high amplitude, obtained from shallow and dip seismic surveys using 3 types of source were used: hammer, dynamite and thumper. hammer the 102.data seismic gather (figure 3) is a record corresponding to the un-01 seismic line. note in the red square like the ground roll diminishes before (left) and after (right) kl algorithm application; the gather is much less noisy in the first 100 ms, with order of the filter: 2 and cut of the filter: 1. figure 4 shows the interactive spectral analysis for seismic record no 12 of un-01, before on top and after on bottom the application of the k-l transform. the k-l filtering gives good results because it compresses more the amplitude spectrum. additionally, the maximum amplitude of the raw power spectrum with the k-l is lower into the red circle (10000) compared with one of the same gather without this filter in the black circle (4,5 e + 07), indicating high amplitude concentrations in the spectrum; corresponding to the ground roll suppressed with k-l. the record with k-l displays seismic events with coherence and continuity. 142 ernesto gómez londoño, luis castillo lópez and thaís de souza kazmierczak figure 4. interactive spectral analysis of seismic data acquired in the un-01 seismic line before (top) and after (bottom) the application of the new kl filter. figure 3. a) seismic gather ‘102.data’ with content of ground roll of high amplitude, acquired with a symmetric spread. b) the ground roll extracted using the kl algorithm, with order of the filter: 2 and cut of the filter: 1. c) the gather after the subtraction of the ground roll. 143 using the karhunen-loève transform to suppress ground roll in seismic data explosive source the shot gather in figure 5, was generated using dynamite, it displays hyperbolic moveout, coherent noise masking the reflection signals. the analyses with f-k, high-pass and the kl filters gave the better results for the last one. the band-pass filter is used where the seismic trace contains low-frequency noise, such as ground roll and high-frequency ambient noise. the seismic reflection energy usually is confined to a bandwidth between 10 to 70 hertz, with dominant frequency of 15 hz. comparison between band-pass [15-30-55-70] (figure 6), and kl filter gave a better result for the kl, the ground roll cone has been suppressed and the lateral coherence of deeper seismic events increased. figure 5. common shot gather ‘1.segy’. comparison between a) f-k filter with the e3 test, b) high-pass filter with 30 hz and c) kl filter. note distortion in the reflection signals and noise presence with the f-k filtering; noise content with high-pass filtering; in contrast, better resolution and increasing the lateral coherence until of deeper seismic events applying the k-l transform. figure 6. common shot gather ‘2.segy’. comparison with filtering capacity between a) band-pass [15 – 30 – 55 70] filter and b) kl filter. note presence of remain ground roll with high amplitude once has been applied the band-pass filter, in contrast to the lower content of noise and increasing the seismic events lateral coherence applying the k-l transform. "a" corresponds to the ground roll cone. 144 ernesto gómez londoño, luis castillo lópez and thaís de souza kazmierczak thumper figure 7 shows an example, of the shot gather 481 with high content of ground roll. the seismic line rm-01, was acquired using a thumper source. a substantial increasing in the lateral coherence of the seismic events below the 200 ms is remarked. figure 8 shows the amplitude spectrum of the shot gather 481 before and after the kl filter application, a significant reduction of the high amplitude corresponding to ground roll is seen. figure 7. a) shot gather ‘481.segy’ containing high amplitude ground roll acquired with split-spread geometry. b) the ground roll extracted using the k-l transform, with a filter order 24 and principal rows 12. c) gather after the subtraction of the ground roll. “a” is the ground roll cone. figure 8. shot gather ‘481.segy’ amplitude spectrum before (left) and after (right) the application of the new kl filter. note in the red circle the existence of high amplitude and lower dips in the spectrum due to the great content of ground roll waves, which diminish with the k-l transform. shallow survey the 2d seismic data was acquired in october-2004 with a split spread configuration in the campus of the national university of colombia, with a geode seismic recorder with 24 channels for high resolution acquisition, the source consist in a hammer (8 kg) and geophones with vertical component. 145 using the karhunen-loève transform to suppress ground roll in seismic data the processing of the un-01 seismic line provided stacked sections shown in figure 9, on top conventional and on bottom with k-l. the section on bottom presents better results, increases lateral coherence and gives a clearer image. on the other hand, the kl suppresses unreal shallow events (indicated by the arrow), due to refractions that are observed with the conventional processing. figure 9. un-01 seismic line with conventional processing until the stacking before (top) and after (bottom) the application of the new kl filter. although a substantial amount of coherent noise has been filtered out, the general waveform character has been maintained. note that the section with the k-l displays one more a clearer and coherent image. deep survey this 2d data was acquired using a thumper in split spread configuration at southern of the middle magdalena valley, involving tertiary sediments from the gualanday group. the stacked section in rm-01 in figure 10 was obtained with conventional processing on bottom and with application of the k-l transform on top. the top image presents a great rank of amplitude (-3 to 3) compared with the conventional stacked on bottom (-2 to 2) as observed into the red circles; indicating a better amplitude recovering in reflectors with k-l use, although the processing sequence did not include fx decon. besides the kl shows coherence and it does not present the whitening effect due to the conventional processing flow (figure 10. red square, bottom). by the other hand, high dips (black arrow), are observed in conventional process image which are not true because the direction of seismic line was n-s, parallels to the beds strikes of the tertiary sediments with very smooth dips; in consequence they must be horizontals in the seismic section, as it is observed in the k-l image in agreement with the subsurface geology. conclusions the algorithm karhunen-loève is computationally economic and efficient to filter seismic data, providing coherent information of subsurface image. the interactive spectral analysis showed more concentrations of high amplitude due to the ground roll, and it verifies that k-l algorithm suppresses the ground roll waves. also it is verified than the amplitude of the ground roll is stronger than the reflection signals. by using k-l algorithm to extract the ground roll it suggests a form to suppress ground roll, without distorting the reflection signals. additionally, using k-l filter increases the lateral coherence of seismic events, in comparison with 146 ernesto gómez londoño, luis castillo lópez and thaís de souza kazmierczak the three conventional methods of suppression: filters f k, high-pass and bandpass. the algorithm was tested with raw gather containing ground roll (high amplitude) obtained from shallow and depth seismic land surveys with different source (hammer, dynamite and thumper. a lateral coherence increase is observed in stacked sections. the k-l use recovers the amplitude of the deep reflectors furnishing a resultant section more approximated to real geology. figure 10. rm-01 seismic line with conventional processing until the stacking before (bottom) and after (top) the application of the new kl filter. although a substantial amount of coherent noise has been filtered out, the general waveform character has be� conventional stacked that are suppressed by the kl. into the red square on the top the image enhancement of the seismic events with the kl is appraised. references • al-husseini, m., glover j. and barley b., 1981. dispersion patterns of the ground roll in eastern saudi arabia, geophysics, vol 46, p. 121-137. • al-yahya, k.m., 1991. application of the partial karhunen-loève transform to suppress random noise in seismic sections: geophys. prosp., 39, p. 77-93. • d’agosto, c., marfurt, k. and steven, j., 2002. modeling and removal of ground roll from horizontal component of c-waves. seg. • jones, i. f., and levy, s., 1987. signal – to – noise enhancement in multichannel seismic data processing: geophys. prosp., 35, p. 12-32. • karsh, h. and bayrak, y., 2004. using the wiener – levinson algorithm to suppress ground roll. journal of applied geophysics 55, p 187-197. • levy, a. and linderbaurn, m., 2000. sequential karhunen-loève basis extraction and its application to images, ieee transactions on image processing, vol 9, no 8, p. 1371-1374. 147 using the karhunen-loève transform to suppress ground roll in seismic data • nesvijski, e. g., 2000. on a possibility of rayleigh transformed sub-surface waves propagation. center of technology, federal university of santa maria, brasil. • sacchi, m.d., 2002. karhunen loève (kl) filtering of seismic data algorithm. signal analysis and imaging group (saig), department of physics, alberta university. • scales, j. and snider, r., 1998. ¿what is noise?: geophysics, vol 63, p. 1122-1124. • yilmaz, o., 1987. seismic data processing: society of exploration geophysicists, tulsa, p. 9-79. • zywicki, d., 1999. traditional seismic surface wave tests. georgia tech research institute (gtri), p. 11.34. geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 148-166 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study adeigbe, o.c and salufu, a. e corresponding address 1&2 department of geology, university of ibadan, nigeria olukris2001@yahoo.com and seemeomoh@yahoo.com abstract the study area lies within the anambra basin and it is made up of enugu shale, mamu formation, ajali sandstone, and nsukka formation. this study aimed at determining the geology and depositional environmental of these formations through field relationship and grain size distribution morphologic studies. the field data shows enugu shale as fissile, light grey with extraformational clast which graded into mamu formation which is made up of shale, coal and sandy shale. it passes upward into ajali sandstone which is characterized by cross beds, herringbone structures and ophiomorpha burrows. the youngest formation within the basin is nsukka formation. the granulometric study of mamu formation shows fine to medium grains, coarse, medium to fine grain for mamu and ajali formation respectively. the standard deviation indicates poorly sorted. the kurtosis shows leptokurtic, platykurtic to very leptokurtic for both while the skewness values indicate positive and symmetrical in all except for ajali sandstone that is negatively skewed. the bivariate and the multivariate results reveal shallow marine and fluvial deposits for both mamu formation and ajali sandstone respectively. the paleocurrent direction of ajali sandstone indicates southwest while the provenance is northeast. the fissility of enugu shale suggests that it was deposited in low energy environment, distal to proximal lagoon environment. the presence of extraformatonal clast within enugu shale indicates fluvial incursion. however, the textural analysis of mamu formation suggests a sediment deposited in a low energy environment which favoured deposition of fine to medium size sediments that is, estuary environment. textural result of ajali sandstone in the study area coupled with the field data such as herring-bone structures, and ophiomorpha burrows, revealed that ajali sandstone was deposited in a tidal environment probably littoral environment. while the light grey colour observed in the nsukka formation suggests sediments deposited under an oxidizing marine environment. key words: anambra basin, geology, depositional environment, mamu formation and ajali sandstone 148 manuscript received: 11/08/2009 accepted for publication: 18/12/2009 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:33 p p p composite 133 lpi at 45 degrees resumen el área de estudio está en la cuenca anambra, compuesta de las arcillas enugu, la formación mamu, la arenisca ajali, y la formación nsukka. este estudio ayudó a determinar la geología y el ambiente de deposición de estas formaciones mediante relaciones de campo y estudios morfológicos de distribución de tamaño de grano. los datos de campo muestran que las arcillas enugu son quebradizas, de color gris claro con clastos extraformacionales que gradaron en la formación mamu la que está compuesta de arcillas, carbón y arcilla arenosa. la arenisca ajali está caracterizada por estratificación cruzada, estructuras de espiga y túneles de ophiomorphas. la formación más joven de la cuenca es la formación nsukka. el estudio granulométrico de la formación mamu muestra tamaños de grano fino a medio, grueso, y de medio a fino para las formaciones mamu y ajali respectivamente. la desviación estándar indica una pobre clasificación de los granos. la curtosis son leptocúrticas, platycurticas a muy leptocúrticas para ambas formaciones, mientras que los indicadores de asimetría muestran asimetría positiva y simétrica en todos los casos excepto para la arenisca ajali que presentó asimetría negativa. las distribuciones bivariadas y multivariadas indican depósitos someros marinos y fluviales para las formaciónes mamu y arenisca ajali respectivamente. la dirección de las paleocorrientes de la arenisca ajali es suroeste mientras que la procedencia es el noreste. la fragilidad de las arcillas enugu sugiere que fué depositada en un ambiente de baja energía, en ambiente de laguna distal a proximal. la presencia de clastos extraformacionales indica incursión fluvial. sin embargo, el análisis de textura de la formación mamu sugiere que los sedimentos fueron depositados en un ambiente de baja energía la cual favoreció la depositación de sedimentos de tamaño finos a medio, esto es en ambiente de estuario. los resultados de textura de la arenisca ajali en el área de estudio junto con los datos de campo tal como estructuras de espigaizarra laguna cercana y túneles de ophiomorphas, revelaron que la arenisca ajali fué depositada en ambiente de marea probablemente ambiente de litoral. mientras el color gris claro observado en la formación nsukka sugiere sedimentos depositados en un ambiente marino oxidante. palabras clave: cuenca anambra, geología, ambiente deposicional, formación mamu y arenisca ajali. introduction the tectonism in southern nigeria probably started in early cretaceous, with the separation of africa from south american and opening of the atlantic. this resulted in the development of the benue trough which stretched in a ne-sw direction (fig. 1) and resting unconformably upon the pre-cambrian basement complex (table 1). it extends from the gulf of guinea to the chad basin and is thought to have been formed by the y-shaped (rrr) triple junction ridge system that initiated the breaking and dispersion of the afro-brazilian plates in the early cretaceous (kogbe, 1989). after the evolution of the benue trough, sediments started depositing into the trough with asu river group being the oldest sediment followed by ezeaku group, and awgu group respectively (nwajide, 1990). santonian age marked the stage when the basin experienced another phase of tectonic event that involved deformation, folding, faulting and uplift of the pre-santonian sediments leading to the formation of anambra basin which evolved as a depression to the west of the uplift (benkhelil, 1987). anambra basin is a cretaceous depo-centre that received campanian to tertiary sediments (nwajide, 1990 and obi, 2000). the stratigraphic setting of southern nigeria comprises sediments of three major sedimentary cycles. the first two cycles belong to the pre-santonian sediments while the third cycle belongs to post-santonian sediments which are found in the anambra basin and afikpo syncline (nwajide, 1990). in anambra basin, the strongly folded albianconiacian succession (pre-santonian sediments) is overlain by nearly flat-lying campanian-eocene succession (table 1). the oldest sediment in the anambra basin is nkporo group (nwajide, 1990). it was deposited into the basin in late campanian, comprising nkporo shale, owelli sandstone and enugu shale (reyment, 1965 and obi, 2001). 149 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:34 p p p composite 133 lpi at 45 degrees 150 adeigbe, o.c. and salufu, a. e figure 1: tectonic map of southeastern nigeria (after murat, 1972) table 1: correlation chart for early cretaceous tertiary strata in the southeastern nigeria (after nwajide, 1990). age abakaliki-anambra basin afikpo basin m.y 30 oligocene ogwashi-asaba formation ogwashi-asaba formation 54.9 eocene ameki/nanka formation/ nsugbe sandstone (ameki group) ameki formation 6.5 palaeocene imo formation nsukka formation imo formation nsukka formation 73 maastrichtian ajali formation mamu formation ajali formation mamu formation 83 87.5 camparian npkoro oweli formation/enugu shale nkporo shale/ afikpo sandstone santonian agbani sndstone/awgu shale non-deposition/erision 88.5 coriacian eze aku grupo (incl. amasiri sandstone)turonian eze aku grupo 93 100 cenomanianalbian asu river group asu river group enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:34 p p p composite 133 lpi at 45 degrees nkporo group is overlain by mamu formation (fig.2). it was deposited in early maastrichtian (kogbe, 1989 and obi, 2000). it comprises succession of siltstone, shale, coal seam and sandstone (kogbe, 1989). ajali sandstone (maastrichtian) overlies mamu formation (reyment, 1965 and nwajide, 1990) which is mainly unconsolidated coarse-fine grained, poorly cemented; mudstone and siltstone (kogbe, 1989). ajali sandstone is overlain by diachronous nsukka formation (maastrichtian-danian) which is also known as the upper coal measure (reyment, 1965 and obi, 2000). imo shale (paleocene) overlies nsukka formation (nwajide, 1990). it comprises clayey shale with occasional ironstone and thin sandstone in which carbonized plants remains may occur (kogbe, 1989). the eocene stage was characterized by regressive phase that led to deposition of ameki group (obi, 2000). 151 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study nsukka formatio ajali sandstone mamu formation nkporo shale awgu shale eze-aku group odukpani formation asu river group basement complex lengend 0 25 50 kilometres 1 : 2 500 000 r. niger paleoceneeocene r. bemue 7° e 8° e 7°n 6°n n figure 2: regional stratigraphy of southeastern nigeria (after nwajide 1990). enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:34 p p p composite 133 lpi at 45 degrees 152 adeigbe, o.c. and salufu, a. e figure 3. geologic and the cross sectional map of the study area. �m m age litholog sample no lithologic description of 116.2 4.2 campanian no sample grey fissile shale, strike of n156°/e8°sw, dip direction of 2560/80 112.0 17.0 no sample grey fissile shale dip amount of 8°, strike of n156°e/8°sw 80.0 10.0 no sample grey fissile shale 70.0 22.0 no sample grey fissile shale with normal fault (throw 0.4 and heave 0.1) 48.0 28.0 no sample grey fissile shale with strike of n155°w/8°se dip direction of 255°/8° 20.0 20.0 no sample grey fissile shale with strike of n153°w/9°se and dip direction of2550/80 figure 4. lithologic description of enugu shale as exposed along enugu-onitsha road. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:35 p p p composite 133 lpi at 45 degrees local stratigraphy description in the study area, enugu shale is the oldest formation and it is exposed in four different places; location 1 (enugu-onitsha express road), location 5 (river ekulu), location 6 and 7 (ogbete) (fig. 3). at the four locations, the outcrops have similar characteristics except for location 1 (enugu –onitsha express road) that has the widest exposure (both in vertical and lateral extents). the lateral extent is estimated to be over 1000m while the vertical extent is 116.2m (fig.4). along enugu-onitsha expess road (location1] enugu shale has a basal bed it is dipping in the direction of 255°/8°, and trends in direction of s255°/8°se. of grey fissile shale, estimated to be 20m thick (fig.4). it is dipping in the direction of 255°/8°, and trends in direction of s255°/8°se. the basal bed passes into another grey fissile shale unit of 28m thick, having similar altitude with the underlying bed. the section continued with 22m thick bed of grey fissile shale, characterized by extraformational clast (fig. 5). this unit is overlain by a faulted (fig. 6) fissile shale bed. the faulted bed is a normal fault with heave of 0.1m and throw of 0.4m. it is 10m thick, dipping in 245°/8° direction with strikes similar to the underlying bed. the faulted shale bed is overlain by grey shale of about 170m thick, having similar altitude with underlying bed. this section is terminated by 4.2m thick grey shale. the mamu formation overlies enugu shale. it is exposed at two locations; location 2 (onyeama coal mine) and location 7 (proda) of which onyeama mine (location 2) has the widest exposure. at the abandoned onyeama coal mine in enugu, the outcrop comprises succession of sandstone, siltstone, coal seam, heteroliths and shale, which has been grouped into three major facies as follows: (i) shale facies units. (ii) coal facies unit (iii) sandy shale facies unit shale facies this facies is the oldest unit of the three, it commences with basal thin bed of light yellow, well laminated sandstone. the 153 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study figure 5. extraformational clast inbedded within enugu shale along enugu-onitsha road. figure 6. faulted shale unit of enugu shale exposed along enugu-onitsha road. figure 8. burrows of planolites within mamu formation at onyeama coal mine, enugu figure 7. rippled sandstone unit of mamu formation exposed at onyeanma coal mine, enugu. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:37 p p p composite 133 lpi at 45 degrees basal sandstone is overlain by dark grey horizontal fissile shale bed. the fissile shale graded into fine, well sorted, rippled (fig. 7) light yellow sandstone. it transits into a dark-grey fissile shale bed that dips towards 260°/7° and strikes in n350°w/7°se trend. it graded into light brown, bioturbated sandstone that has been reworked. it has traces of planolites burrows (fig. 8). the sandstone bed is overlain by two thin beds of dark grey fissile shale. the last shale bed graded into 2.2m thick moderately consolidated white siltstone that is laminated with traces of plant roots. coal facies unit the coal facies unit is about 26m thick with alternation of coal seems, heterolith, sandstone and siltstone (fig. 9). the basal bed of this facies is a coal seam of 0.1m thick. it passes unto heterolith which is overlain by white, moderately consolidated sandstone. the sandstone graded into a black coal seam. the facies continued upward with two whitish, laminated, rippled siltstone beds of 2m and 3m thick respectively. the siltstone makes erosional contact with the overlying medium-fine consolidated flat sandstone bed. the sandstone bed passes into heterolith which is then overlain by 0.2m black coal seam (fig. 10) that terminates the coal facies unit. sandy shale facies unit this unit is estimated to be over 130m. it is mainly of sandstone beds that are light yellow, well sorted-clayey, bioturbated, cross bedded with wave ripples. the sandy shale (fig. 11) is overlain by 4.2m thick white laminated sandstone (fig. 12). ajali sandstone it is exposed at two stations in the study area; location 3 (ngwo) and location 9 (9th mile). the widest exposure of this formation is at ngwo. the vertical exposure is esti154 adeigbe, o.c. and salufu, a. e �m m age litholog sample no. lithology 157.7 130.4 mastrichtian 6 samples sequence of sandstone and sandy shale. brownish red lateratized sandstone and grey iron stone 27.3 20.5 2 samples sequence of different strata from sandstone, shale, siltstone, sandy shale and sandstone, coal, and siltstone 6.8 6.7 no sample sequence of different strata from sandstone, shale, siltstone,and siltstone 0.5 no sample dark grey fissile shale 0.5 3 samples light yellow well sorted and laminated sandstone figure 9. lithologic description of mamu formation exposed at onyeama coal mine, enugu figure 10. coal seam unit of mamu formation exposed at onyeama coal mine. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:38 p p p composite 133 lpi at 45 degrees 155 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study figure 11. sandy shale unit of mamu formation exposed at onyeama coal mine figure 12. laminated sandstone bed of mamu formation exposed at onyeama coal mine �m m age lithology sample no lithology description 21.0 10.0 mastrichtian 1 sample grey white coarse, poorly sorted, clayey, unconsolidated, herringbone, crossbreds, with reactivated surface 11.0 9.0 1 sample white, poorly sorted unconsolidated, clayey, crossbeded fine grained sandstone 2.0 2.0 1 sample white, poorly sorted unconsolidated, clayey, fine grained sandstone figure 13. lithologic description of ajali sandstone exposed at ngwo figure 14. herringbone cross bed within ajali sandstone exposed at ngwo, udi figure 15. reactivation surface within ajali sandstone exposed at ngwo, udi enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:40 p p p composite 133 lpi at 45 degrees mated to be 21m while the lateral extent is estimated as 500m. the section starts with a basal, poorly sorted, poorly cemented unconsolidated white fine grained sandstone bed that is coarsening upward (fig. 13).ten meter (10m) thick, grey white poorly sorted, unconsolidated clayey sandstone bed that is characterized by herringbone cross beds (fig. 14) and reactivation surface (fig. 15) overlies the basal bed. this unit terminates the section with lateralized overburden. nsukka formation it is exposed in one location within the study area, location 10. it is along enugueze-agu road. the vertical extent of the section is 4.8m while the lateral extent is 20m. ferruginous brown, consolidated unit of 2m thick begins this section (fig. 16). it graded into grey fissile shale. the shale unit trends s153°e/9°nw. it give effervescence in the presence of dil. hcl acid, thus indicating that it is carbonaceous. the fissile shale passes unto 0.9m thick grey fissile shale bed. the section continues upward to shaly sandstone bed overlying the shale. the lithology transits into a brown, consolidated, highly ferruginised sandstone overlying the sandy shale. the section gets terminated by 3m thick brownish red, well consolidated, laminated ferruginised sandstone bed. nsukka formation is the youngest formation in the study area. the composite log of the exposed formations in the study area is shown in fig. 17. this revealed the field spatial relationship within the anambra basin, southwestern, nigeria. methodology fourteen representative samples of unconsolidated sandstones were collected from the study area. eleven samples were collected from mamu formation at different horizons while three samples were collected from ajali sandstone (the unavailability of sandstones horizon/unit within enugu formation does not allow for grain size analysis). the samples were later disaggregated and divided into two equal parts of 50gm each. sieving was done for each sample for 15 minutes on a ro-tap sieve shaker, using a set of u.s standard sieve at ¼ phi sieve interval. cumulative curves of the grain size distribution were plotted from the sieve results. the univariate, bivariate, and multivariate parameters were computed from the sieve results after folk and ward (1970), miola and weiser (1977), and sahu (1964). the formulae of mean size (mz), standard deviation (ói),skewness (ski) and kurtosis (kg), used to 156 adeigbe, o.c. and salufu, a. e �m m age lithology sample no lithology description 4.8 3.0 maastrichtian early paleocene no sample brownish red lateratised sandstone, well consolidated, laminated sandstone. 4.5 0.2 no sample brown sandstone consolidated, ferruginised, laminated sandstone 4.3 0.3 no sample grey fissile sandy shale 4.0 3.0 no sample grey ironstone 3.7 0.9 no sample grey fissile shale 2.8 0.8 no sample grey fissile shale with dip amount of 9°, and strike of 153° 2.0 2.0 no sample brown ferruginised sandstone figure 16. lithologic description of nsukka formation exposed at eze-agu, enugu enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:40 p p p composite 133 lpi at 45 degrees compute the sandstones that underlie the study area are given below respectively. mz j � � � � � ( ) / ( ) / ( ) / . � � � � 16 50 84 3 34 16 4 95 5 6 6 (1) ski � � � � ��� � � ��� � 16 34 2 2 8 16 5 9 2 2 9 5 ( ( ) ( ( ) (2) kg � ( . ( � �� � ���9 5 2 44 7 (3) multivariate parameter is computed for the sandstones in the study area by: yu = 0.2852 mz – 8.760451 – 4.8932ski + 0.0482kg. if the computed values of yu > 7.419, it is interpreted as shallow marine deposits while value of yu < 7.419 is interpreted as fluvial deposits (sahu, 1964). the dip and azimuth of twenty five foreset of planner cross beds were measured from the cross bedded units of the ajali sandstone exposed in the study area. the measurement were carefully taken and recorded. the measurement were analyzed using variable constraints system by calculating the mean vector azimuths (mva) and variables of the cross bedded units of the ajali sandstone (steinmetz,1962) in order to determine the paleocurrent of the ajali sandstone as well the trend of the paleocurrent by constructing rose diagram. results the probability curves for the samples of mamu formation and ajali sandstone show that the sandstone units of mamu formation were deposited by suspension and saltation (fig. 18) while that of ajali sandstone were deposited by suspension, saltation, and traction (fig. 18). the mean size result shows that the sandstone units of mamu formation are fine to medium (table 2). the standard deviation result shows that the sandstone units of mamu formation are well sorted to moderately well sorted. the mean size result of ajali sandstone shows coarse, medium and fine sandstone. the standard deviation result reflects that the sandstone is poorly to moderately sorted (table 2). the histogram plots of the cumulative weight percent against the phi scale show that the mamu formation and ajali sandstone are unimodal (fig. 19). the results of bivariate plots for mamu formation and ajali sandstone in the study area indicate beach and fluvial deposits (fig. 20 and fig. 21) while multivariate results indicate mainly shallow marine and fluvial deposits (table 3). however, the result of the paleocurrent analysis of the foreset of cross bedded units is well pronounced in the ajali sandstone and less pronounced in the mamu formation (this may be due to the presence of siltstone, shale and coal in mamu and complete absence in ajali sandstone). this revealed that ajali sandstone has mean vector azimuth of 239°0 that is in direction of south west, and its variance is 1,472 (table 4). however, the rose diagram indicates paleocurrent direction toward southwest while provenance is in direction of northwest (fig. 22). 157 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study �m m age lithology sample no formation lithology description 299.9 4.8 maast-paleo no samples nsukka formation sequence of sandstone and shale. brownish red lateratized sandstone and grey iron stone 294.9 21.0 maastrichtian 3 samples ajali sandstone white sandstone coarsing upward. herringbone crossbeds and reactivation surface 273.9 157.7 11 samples mamu formation sequence of different strata from sandstone, shale, siltstone, sandy shale and sandstone,coal,siltstone, sandstone, coal and fossiliferous sandstone. 116.2 116.2 campanian no samples enugu formation grey fissile shale with normal fault figure 17. composite local stratigraphy of the study area enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:40 p p p composite 133 lpi at 45 degrees 158 adeigbe, o.c. and salufu, a. e 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 mamu 5 mamu 6 mamu 7 figure 18b. log probability curves for mamu and ajali sandstone that underlie the study area 1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 mamu 8 mamu 9 mamu 10 mamu 11 figure 18c. log probability curves for mamu and ajali sandstone that underlie the study area 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 ajali 1 ajali 2 ajali 3 figure 18d. log probability curves for mamu and ajali sandstone that underlie the study area 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 mamu 1 mamu 2 mamu 3 mamu 4 figure 18a. log probability curves for mamu and ajali sandstone that underlie the study area enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees 159 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1 -0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1-0.50.00.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% phi-scale mamu 3 phi-scale mamu 1 phi-scale phi-scale mamu 4 mamu 2 (a) -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% mamu 5 -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% mamu 6 mamu 7 mamu 8 phi-scalephi-scale phi-scale phi-scale (b) -1 -0. 0. 0. 1 1. 2 2. 3 3. 1 2 3 4 5 6 wt mamu 9 1 0. 0. 0. 1 1. 2 2. 3 3. 1 2 3 4 5 6 -1 -0. 0. 0. 11. 2 2. 3 3. 1 2 3 4 5 6 wt phi-scale phi-scale wt phi-scale mamu 10 mamu 11 (c) enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees 160 adeigbe, o.c. and salufu, a. e -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 w t% ajali 2 -1 -0.5 0.0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 ajali 3 phi-scale phi-scale phi-scale (d) ajali 1 figure19a-d. histogram plots of the mamu formation and ajali sandstone that underlie the study area 0.2 0.4 0.7 1.45 2.1 0.6 1.2 1.8 3.0 fluvial beach m z * * * * ** * ajali sanstone * mamu fm. * * * ° ° ° ° figure 20. graph of mz against �i for the sandstone in the study area (after miola and waiser, 1977). sk i 0.90.70.60.40.1 0.5 -1.5 0.0 -0.5 -2.0 0.5 1.0 0.2 0.3 0.8 1.21.11.0 1.3 -1.0 1.5 2.0 * * * * ** * * * * -3 * * ° ° ° ° fluvial beach ajali sanstone mamu fm. figure 21. graph of ski against � for the sandstone of the study area (after friedman, 1961). enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees 161 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study table 2. computed grain size parameters derived from probability curves sample no sorting �1 values skewness ski values kurtosis kg values mean size mz values mamu1 well sorted 0.47 positively skewed 0.1 leptokurtic 1.2 fine sand 2.3 mamu2 well sorted 0.5 symmentrical 0.05 leptokurtic 1.2 fine sand 2.3 mamu3 moderately well sorted 0.47 positively skewed 0.1 very platykwtic 0.6 fine sand 3 mamu4 moderately well sorted 0.7 positively skewed 0.1 platykurtic 0.9 fine sand 2.7 mamu5 poorly sorted 1.1 positively skewed 0.01 leptokurtic 1.2 fine sand 3 mamu6 well sorted 0.5 positively skewed 0.01 platykurtic 0.6 fine sand 3 mamu7 moderately well sorted 0.75 very positively skewed 0.1238 leptokurtic 1.2 1fine sand 2. mamu8 well sorted 0.45 symmentrical 0.05 platykwtic 1.2 medium sand 1.3 mamu9 moderately sorted 0.7 symmentrical 0.05 very platykwtic 1.5 medium sand 1.2 mamu10 poorly sorted 1.1 0.1 positively skewed 0.1 very leptokurtic 1.5 medium sand 1.2 mamu11 poorly sorted 1.6 symmentrical 0.05 very leptokurtic 1.5 medium sand 1.2 ajali1 poorly sorted 1.1 negatively skewed 0.3 platykwtic 0.6 coarse sand 0.5 ajali2 moderatelywell sorted 0.7 symmentrical 0.05 very leptokurtic 1.5 medium coarse sand 1.3 ajali3 poorly sorted 1.6 symmentrical -0.1 leptokurtic 1.2 fine sand 3 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees discussion depositional environment the fissility and the fine nature (grain size) of the enugu shale as indicated by the field data suggest that enugu shale was deposited below the wave base, accumulated in relatively low energy environment i.e in a distal to proximal lagoon (amaral and pryor, 1974). the presence of extraformational clast within the enugu shale indicates that there was fluvial influence during the period of deposition of enugu shale within the anambra basin (tucker, 1996). the light grey colour of the shale shows that enugu shale was deposited on the surface of the basin where oxidation could take place (dapple, 1974). the fine medium grained sandstone, siltstone and fissile shale and planolites burrows that dominates mamu formation succession as observed from sieve and field relationships suggests that the unit was deposited in an environment where there was little or no existence of tidal or wave action i.e quiet environment where low energy favoured deposition of fine – medium size sediments. the presence of coal beds that alternate the shale, siltstone and sandstone units within mamu formation sequence indicates that mamu formation was deposited in estuary environment similar to the observation made by reyment (1965) and nwajide and reijers (1996). the well sorted to moderately well sorted (table 2) of mamu formation is an evidence of quiet environment with very low energy of deposition. the kurtosis for mamu formation revealed leptokurtic to platykurtic (table 2) which suggest that mamu formation were sourced from more than one source which is similar to the observation made by akaegbobi and boboye (1999). the bivariate plots (figs. 20 and 21) and multivariate (tables 3 and 4) show that mamu formation was deposited in a shallow marine with prevalent of fluvial incursion. information from the field data shows that ajali sandstone is profusely cross bedded, characterized by herringbone structures and ophiomorpha burrows. the presence of these structures indicates tidal environment with high energy (tucker, 1996). however, the presence of reactivation surface (fig. 14) within ajali sandstone suggests fluvial ac162 adeigbe, o.c. and salufu, a. e table 3. multivariate results of sandstone of mamu formation and ajali sandstone in the study area sample results interpretation ma1 -1.7107 shallow marine ma2 -1.7210 shallow marine ma3 3.9218 shallow marine ma4 -3.8829 shallow marine ma5 -9.8211 shallow marine ma6 -1.5508 shallow marine ma7 -4.0921 shallow marine ma8 -1.3619 shallow marine ma9 -4.0942 shallow marine ma10 -10.6749 fluvial ma11 -22.2570 fluvial aj1 -11.8965 fluvial aj2 -4.0942 shallow marine aj3 -21.0239 fluvial enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees 163 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study table 4. paleocurrent analysis of ajali sandstone in the study area s/n azimuth (a) dip (d) sin (a) cos (a) cos (d) b = cos (a * d) a = sin (a*d) (azmva)2 1 255 10 -0.9659 -0.2588 0.9848 -0.2549 -0.9512 256 2. 245 5 -0.9063 -0.4226 0.9962 -0.4210 -0.9029 36 3. 257 10 -0.9744 -0.2250 0.9848 -0.2216 -0.9596 324 4. 257 10 -0.9744 -0.2250 0.9848 -0.2216 -0.9596 324 5. 255 9 -0.9659 -0.2599 0.9877 -0.2556 -0.9540 256 6. 170 8 0.1736 0.9848 0.7903 -0.9752 0.1719 4,761 7. 180 20 0 -1 0.9397 -0.9397 0 3,481 8. 255 12 -0.9659 -0.2588 0.9781 -0.2531 -0.9447 256 9. 270 10 -1 0 0.9848 0 -0.9348 961 10. 230 8 -0.7660 -0.6428 0.9903 -0.6366 -0.7586 81 11. 300 25 -0.8660 0.5 0.9063 0.45315 -0.7849 3,721 12 252 20 -0.9511 -0.3090 0.9397 -0.2904 -0.8937 169 13 253 15 -0.9563 -0.2924 0.9659 -0.2824 -0.9237 196 14 250 18 -0.9397 -0.3420 0.9512 -0.3253 0.8938 121 15 252 30 -0.9511 -0.3090 0.8660 -0.2676 -0.8237 169 16 258 32 -0.9781 -0.2079 0.8480 -0.1763 -0.8294 361 17 252 28 -0.9511 -0.3090 0.8829 -0.2728 -0.8554 169 18 258 26 -0.9781 -0.2079 0.8746 -0.1818 -0.8554 361 19 250 48 -0.9397 -0.3420 0.6691 -0.2288 -0.6288 121 20 144 32 0.5878 -0.8090 0.8480 -0.6860 0.4985 9,025 21 162 28 0.3090 -0.9519 0.8829 -0.8404 0.2728 5,929 22 204 22 -0.4067 -0.9135 0.9272 -0.8470 -0.3771 1,225 23 230 15 -0.7660 -0.6428 0.9659 -0.6209 -0.7399 81 24 233 30 -0.7986 -0.6018 0.8660 -0.5212 -0.6916 36 25 185 19 -0.0872 -0.9962 0.9455 -0.9420 -0.0824 2,916 �-16.04 �-9.5033 �35336 mean vector azimuth = tan-1 (� sen a)/(� cos a) where � sma = –16.04 � cosa = – 9.503 theredore, mva = tan-1 –16.04/–9.5033 = tan-1 1.6878 = 59 hence mva = 59 + 180 = 239° variance = �(ai – mva)2/(n – 1) a1 = azimuth mva = computed mean vector azimuth (239°) n = total number of outcome (25) therefore, variance = 35,336/(25 – 1) = 35,336/24 = 1,472. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees tion reading, (1996) and tucker, (1996). thus, these field observations show that ajali sandstone was deposited under a tidal and fluvial channels interaction i.,e littoral environment. the sieve analysis shows that ajali sandstone is coarse, medium to fine grain poorly sorted sandstone (table 2) which suggests fluctuation in energy of deposition. also the coarsening upward nature of ajali sandstone as indicated by the univariate result (table 2) suggests that the basin was shallowing upward (dapple, 1974 and tucker, 1996). this shallowing upward of the basin at the time of deposition of ajali sandstone into the anambra basin may be as a result of fluvial interference with the marine environment. the paleocurrent analysis result shows that the variance value is 1,472. this value indicates that ajali sandstone was deposited in fluvial environment (selley, 1966). the value of mean vector azimuth (mva) for ajali sandstone indicates 239° az (table 4). this value and the rose diagram (fig. 22) for ajali sandstone suggest that the direction of paleocurrent as at the time of deposition, acted in southwest direction and the provenance was northeast. the rose diagram reflects unimodal high variability paleocurrent pattern for ajali sandstone (fig. 22). this paleocurrent pattern (unimodal) suggests sediments deposited in an environment where fluvial currents was prevalent with net long-shore marine transport (selly, 1966). the direction of provenance indicated that the sediments of ajali sandstone were sourced from the basement complex of nigeria probably obudu hill and or cameroun mountains in southeastern part of the country. the presence of carbonaceous fissile shale within the nsukka formation as observed from the field data suggests that, it was deposited in shelf to shoreface environment (akande and mucke 1993, obi, 2000). the light grey colour of the shale unit and the ferruginized nature of the sandstone unit of nsukka formation as observes from the field study, indicates that nsukka formation was deposited in an oxidizing environment where adequate oxygen was able to interact with the sediments, probably basin surface (tucker, 1996). conclusion the study of the ancient environment of the sediments of anambra basin has been reconstructed from the field relationships and textural analysis results. before the santonian tectonic event anambra basin was still in platform stage. however, sequel to santonian tectonism, the study area opened up as anambra basin. thus, the basin began to communicate with the atlantic ocean. there was a rise in the sea level during the campanian times and the sea transgressed into the land. the incursion of the atlantic ocean caused the basin to become deeper and quiescent. thus, enugu shale was deposited into the basin. gradually, the shoreline started withdrawing seaward from the land. hence, the basin became starved of shale (enugu shale) as a result of the gradual increase in depositional energy within the basin. this resulted in the emergent of sediments deposited in a coastal environment of fluvial and marine water interaction. before the transgression of sea into the land, the land was already covered with giant plants and trees. when transgression phase set-in, those plants and trees were covered up with water and they began to decompose. regression phase followed the transgression phase simultaneously and subse164 adeigbe, o.c. and salufu, a. e n provenance paleocurrent direction figure 22. rose diagram of ajali sandstone within the study area enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:42 p p p composite 133 lpi at 45 degrees quently led to the withdrawal of shoreline seaward such that sediments that were transported from the land were deposited and buried the decomposed plant and tree remains in the study area as mamu formation.thus these plant remains were transformed into coal seams that exist in the mamu formation. the shoreline continued to withdraw at a rapid rate such that the basin became shallow. as a result of this shallowness of the basin at this period paved way to deposition of poorly sorted coarse sandstone into the basin in a littoral environment as ajali sandstone. sedimentation of ajali sandstone in the basin lasted till late maastrichtian times when the shoreline began to move landward and the sea level began to rise. thus, nsukka formation was deposited in an oxidizing shallow marine environment. deposition of nsukka formation terminated sedimentation in the study area. references akaegbobi i.m and boboye o.a. (1999). textural, structural features and microfossil assemblage relationship as a delineating criteria for the stratigraphic boundary between mamu formation and nkporo shale within the anambra basin, nigeria. nape bull, vol. pp. 193-206. akande, s.o. and mucke, a. (1993). depositional environment and diagenesis of carbonates at mamu/nkporo formations, anambra basin, southern nigeria. journal of african earth sciences, vol. 17, pp. 445-456. amaral, e.j. and pryor, w.a. (1974). depositional environment of st. petters sandstone deduce by textual analysis. journal sed. petrol., vol. 40, pp. 32-55. benkhelil, j. (1987). cretaceous deformation magmatism and metamorphism in the lower benue tough, nigeria. geol. journ. vol. 22, pp. 467-493. dapple, e.c. (1974). sandstone types and their associated depositional environments. journal of sedimentary petrology. vol. 4, pp. 695-707. folk, r.l. (1970). petrology of sedimentary rocks. hemphill, austin, texas 182p. folk, r.l. and ward, w.c. (1957). a study in the significance of grain size parameter. journal of petrology. vol. 37, pp. 327-354. friedman, g.m. (1961). distribution between dune, beach, and river sandstones, j. sed. pet., vol. 37 pp. 327-354. kogbe, c.a. (1989). the cretaceous and paleogene sediments of southern nigeria: in kogbe c.a. (ed.), geology of nigeria. elizabethan publ. vo., lagos, nigeria. 273-286 pp. miola, r.j. and weiser, d. (1977). textural parameters: an evaluation. journal. sed. petrology. vol. 38, pp. 45-53. murat, r.c. (1972). stratigraph and pale geography of lower tertiary, southern nigeria, in dessavagie, t.p.j, and whiteman (eds.), afri-geol. university of ibadan, nigeria. 425 p. nwajide, c.s. and reijers, t.j.a. (1996). geology of the southern anambra basin. in: reijers, t.j.a. (ed), selected chapters on geology, spdc, warri, pp. 133-148. nwajide, c.s. (1990). cretaceous sedimentation and paleogeography of the central benue though. in: ofoegbu, c.o; (ed.), the benue. tough structure and evolution international monograph series, braunschweig, pp. 19-38. obi, c.g. (2000). depositional model for the campanianmaastrichtian succession, anambra basin, southeastern nigeria. ph.d thesis, university of nigeria, nsukka, nigeria. reading, h.g. (1996). sedimentary environment processes, facies and stratigraphy. blackwell scientific publications, oxford. 671. p. reyment, r.a. (1965). aspect of the geology of nigeria. ibadan university press. 145p. sahu, r., (1964). textual parameters: an evaluation of fluvial and shallow marine deposits. j sed. pet., vol. 34, pp. 513-520. selley, r.c. (1966). paleocurent and sediment transport in near shore sediment of sincte basin. libya. j. geol. vol. 75, no. 2, pp. 215-222. steinmetz, g. (1762). paleocurrent and provenance determination of sandstone mustang island, texas, j. sed. petrol, vol. 30. pp. 753-780. tucker, m.e. (1996). sedimentary rocks in the field. john weley and sons, chichester, new york, 150 p. 165 geology and depositional environment of campano-maastrichtian sediments in the anambra basin, southeastern nigeria: evidence from field relationship and sedimentological study enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:43 p p p composite 133 lpi at 45 degrees estudio geoelectrico en el valle alto del rio patia rosalbina perez ceron instituto de investigaciones en geociencias, minerra y qurmica-ingeominas perez, r. (1995): estudio geoelectnco en el valle alto del rro patfa. geoffs. colomb. 3:27-35. issn 0121-2974 resumen la prospecci6n electrica en el valle alto del rro patla, permiti6 definir una secuencia tfpica constituida de techo a base por: sedimentos recientes con resistividades que varfan entre 20 y 400 ohm-m, areniscas de grana fino a medio con resistividades entre 20 y 70 ohm-m, limolitas a areniscas de grana fino, con resistividades entre 8 y 20 ohm-m y un basamento arcilloso con resistividades menores de 20 ohm-m. se presentan dos tipos de aculleros: libres, asociaoos con arenas y gravas de abanicos y terrazas aluviales (resistividades entre 70 y 400 ohm-m y espesores entre 25 y 75 m), y semiconfinados conformados por unidades arenaceas del terciario (resistividades entre 20 y 30 ohm-m, con espesores entre 35 y 150 m). la calidad del agua es buena con bajo contenido de sales para los primeros acufferos, siendo un poco salobre para los segundos. por otro lado el estudio geoelectrico mostr6 la presencia de una serie de discontinuidades electricas, asociadas con lineamientos y tallas geol6gicas, algunas de las cuales fueron cartografiadas en el estudio geol6gico del area. en general la geoelectrica mostr6 una tect6nica de bloques que afecta las rocas cretaclcas y terciarias, y seguramente los sedimentos cuaternarios. abstract the resistivity survey carried out at the upper valley of the patfa river, provided good information to define the sedimentary sequence of the area. a general sequence from top to base is described as follows: recent deposits with resistivity values between 20 and 400 ohrn-rn, fine to medium grain, sandstones with resistivity values between 20 and 70 ohrn-m, fine grain sandstones and silstones with resistivity values between 8 and 20 ohrn-rn, and clayli basement, with resistivity less than 20 ohm-m. there are two kinds of aquifers; free, related to sands and gravels from alluvial fans and terraces (resistivity values between 70 and 400 ohrn-rn and thickness between 25 and 75), semi-confined related to tertiary sandstones (resistivities values between 20 and 30 ohm-m, thickness between 35 and 150 m). the water quality of the upper aquifers is good enough with small amounts of salt content. saline water is found at the lower aquifer. the resistivity method was useful to locate geological discontinuities; some of them found with surface expression associated with lineaments and faults. some of these geolollical features have been mapped in previous geological works on the area. in qeneral the interpretation based on the resistivity results shows a block tectonics environment affecting cretaceous and tertiary rocks and probably the quaternary deposit. 1_introduccion con la celebracion del convenio de cooperacion entre la oorporaclon para la reconstrucci6n y desarrollo del departamento del cauca (crc) e ingeominas, se dio comienzo al proyecto "lnvestigaci6n hidrogeol6gica en el valle alto del rfo patfa", con el objetivo de abastecer de agua de buena calidad y en cantidad suficiente para solucionar las necesidades tanto de consumo humane en las diferentes poblaciones existentes en el area, como de riego para la ganaderfa y aqrlcuttura de la regi6n. una de las etapas del proyecto fuela prospecci6n electrica, en la cual se levantaron 60 sondeos electricos verticales (sev), con ab/2 entre 200 y 900 m, distribuidos en toda el area de estudio, con mayor enfasis alrededor de los caserfos y poblaciones. con las observaciones geoelectricas se pretendfa encontrar y definir unidades acufferas que pudieran ser captadas por pozos ubicados estrateqlcarnente para el mejor aprovechamiento del recurso hfdrico. 1.1 localizaci6n ei area de estudio esta ubicada al sur del departamento del cauca, sobre el valle alto del rio patla, entre las poblaciones de ei bordo al ne, la fonda al nw, galindez al se y guadualito al sw, limitada al w y e por la cordillera occidental y central respectivamente. la principal via de acceso la constituye la carretera panamericana, a la cual convergen diferentes carreteras que comunican con los distintos municipios y caserlos de la zona (fig.1) 1.2 geologfa y geomorfologfa ei valle alto del rfo patla, esta constituido por una secuencia vulcano-sedimentaria terciaria y sedimentos recientes en depositos aluviales que conforman abanicos, terrazas y dep6sitos de coluvion, sobre un basamento cretaclco, (torres et al., 1991) (fig.2). este ultimo esta constituldo por rocas diamsicas y conforma el limite w del valle en las estribaciones de la cordillera occidental. x.:.!!.800q9 0 0 76°w0 t t» «. 80 0 0 0 oj) '-' 0 ; ; i cja ,j~\ valle cj\ ~ toll maq,"?' ?~:-~a -. '\.-.../tv"?' ,on acj j'...j -j ;..." .., 4j~ ~ d 0 "0 cauca c; 0a., ~ 4j -j -j.... 0 ~a 0 huila x.zao.ooo z on narino -, q~ cp -~'~ -, ), \ x'640000 ) i ~caqueta--putumayo _ area de estudio julio / 91 fig. 1 de mapa indice carretero ponamericona aut or: mo. patricia torres oibujo: guido arcol q 3pkm esc ala figura 1. ubicaci6n del area de estudio ei terciario esta conformado por tres grandes unidades de base a techo: a) forrnaclon mosquera (tem): constituida por una secuencia de areniscas y conglomerados cuarzosos intercalados con arcillolitas ferruginosas y capas de carbon; b) medio terciario (mt): conformado por limolitas intercaladas con areniscas de grano fino en su parte superior, paquetes de limolitas, arcillolitas en su parte media y superior; esta unidad da geoformas angulosas y recortadas; c) formacion galeon (tqg): se encuentra en contacto fallado con la anterior unidad, litoloqicarnente esta conform ada por aglomerados polimigticos y grawacas. presenta una morfologfa de cerros islas cortados, con alturas hasta de 850 m. ei cuaternario ssta conformado por cuatro tipos de depositos: de lianura aluvial (qal), de terraza aluvial (ot), coluviales (qc) y depositos de abanico aluvial (qca). existen dos abanicos en la zona: abanico de galindez constituido por niveles de arenas pumfticas y arenas cuarzo-teldespaticas de grana fino a grueso y abanico de olaya, constituido por gravas en matriz arcillosa. tectonicarnente el area se encuentra afectada por dos sistemas de falla: a) sistema de falla de romeral: comprende los lineamientos y fallas hasta el e del rfo patfa; geneticamente establece el limite occidental de la corteza continental en el occidente colombiano (perez, 1980). asociadas a este sistema se encuentran las fallas de potrerillos, de tipo inverso y que pone en contacto rocas del medio terciario inferior con rocas vulcano-sedimentarias terciarias y cuaternarias. falla de chaguarguico, pone en contacto rocas del medio terciario medio con la parte superior del mismo conjunto. b) sistema del patfa, se ubica al w del rfo patfa. constituido por fracturas tensionales originadas en la zona de flexion de la placa oceanica durante el cretaceo superior (perez op. cit.) (mica en el area. este sistema se encuentra en su mayor parte cubierto por sedimentos vulcano-sedimentarios recientes. 2. prospeccion geolectrica 2.1 fundamento te6rico la resistividad electnca, p, de rocas y sedimentos del subsuelo, se define como la resistencia que estos oponen al f1ujo de la corriente, l,. la resistividad depende entonces de las caracterfsticas de los materiales (composicion, tamafio de grano, selsccion, madurez, cornpactacion, etc.), del tipo de fluido presente en ellos y de los procesos de contacto entre ifquidos, gases y esqueleto solido. la relacion entre estos facto res y la litologfa propiamente dicha se expresa con los conceptos de porosidad y permeabilidad. de esta manera la resistividad depende de la porosidad, permeabilidad y geometrfa de las diferentes unidades litologicas. dependiendo de los objetivos existen diferentes tecnicas de mediclon. en el presente caso se utilizo la tecnlca de sondeo electrico vertical (sev), con el arreglo simetrico de schlumberguer, en el cual se establece un campo electrico entre los electrodos a y b y se mide la diferencia de potencial entre los electrodos m y n, como se muestra en el siguiente esquema: arreglo simemco de schlumberguer con el metoda de resistividad electrica, en su tecnica de sondeo electrico vertical (sev), se busca medir los cam bios de resistividad con la profundidad. estos cambios se pueden equiparar con cambios de litologfa y/o tipo de material y fluidos. en general, el potencial medido en superficie entre dos electrodos puntuales m y n, se puede expresar en tuncion de resistividades y espesores. por 10 tanto, con la medida de parametres electricos en la superficie es posible deducir las caracterfsticas iitologicas del subsuelo. por otro lado la profundidad de investiqacion depende de la distancia entre a y b; asf, a mayor ssparacion entre a y b, mayor profundidad de tnvestipacion (lakuboyskii & liajoy, 1980; astier, 1975). como en todos los rnetodos en los cuales se estudia un campo potencial, existen multiplicidad de soluclones, 10cual dificulta la interpretacion final. sin embargo, con un control adecuado con informacion geologica, de superficie y pozos, informacion geoffsica, registros, etc., es posible afirmar que las soluciones proporcionadas por el metodo estan dentro de un alto grade de confiabilidad. 2.2 hip6tesis de trabajo la reahzaclon de sondeos electricos verticales en el valle alto del rfo patfa puede proporcionar muy buenos resultados teniendo en cuenta que existen unidades que, que por sus caracterfsticas litologicas y composicion rnineraloqica, pueden presentar contrastes de resistividad, apreciables. se esperan valores muy altos para las rocas diabasicas del oretaceo, valores medios a altos para sedimentos y rocas arenaceas de la secuencia terciaria-cuaternaria y valores medios a bajos para sedimentos y rocas limo-arcillosas del cuaternario y terciario, los cuales convenientemente interpretados perrnitiran definir las diferentes unidades y encontrar las que ofrezcan las mejores posibilidades como acufferos. 2.3 toma de datos de campo los puntospara la localizacion de cad a sev, se ubicaron sobre la base geologica (torres, et ai., 1991), teniendo en cuenta aquellos sitios en donde la informacion de superficie era escasa. por otro lado se trato de conformar perfiles, que facilitaran la correlacion, algunos sev se ubicaron encima 0 muy cerca a afloramientos de las roeas antiguas con el fin de patronar estas. en las zonas en donde se suponfa que el relleno reeiente era mas grueso se apliearon ab/2 mayores, iiegando a ser hasta de 900 m; en total se levantaron 60 sev distribuidos en las siguientes planchas : 363-iv-d = 6 387-i-a = 2 364-iii-c ,. 18 . 386-ii-b = 28 386-11-0 = 6 2.4 procesamiento e interpretaci6n los datos de resistividad aparente, p vs ab/2, fueron procesados mediante el programa ves-4.2-s de la tno, que se basa en la soluci6n del problema directo, el cual para un modele dado calcula la curva de resistividad respectiva. esta curva en un proceso iterativo, en el que se fueron variando los parametres del modele (resistividad aparente ylo profundidad), se compar6 con la curva obtenida en campo, hasta obtener un modelo cuya curva fue semejante a la curva de campo. ei modele final se pudo establecer complementando datos electricos con informacion geol6gica del area. 3. caracteristicas geoelectricas generales en el valle alto del rio patfa, se pudieron distinguir tres grupos de reslstividades: a) altas, de 100 a 400 ohm-m, se asocian especial mente a los sedimentos del abanico de galindez y depositos de terrazas compuestos por arenas y gravas semisaturadas; b) medias, entre 20 y 100 ohm-m, se asocian con arenas y/o areniscas de grana medio a fino y limolitas semisaturadas; c) bajas, menores de 20 ohm-m, asociados con sedimentos y rocas arcillosas. 3.1 secciones geoelectricas con el objetivo de analizar la continuidad de las unidades en diferentes direcciones se realizaron secciones en sentido transversal y longitudinal. en las secciones transversales se ve una buena continuidad de las unidades, excepto por las discontinuidades 1 y 7 que al parecer estan asociadas con los dos sistemas de fallas regionales ya descritos y que afectan las rocas crstacicas y terciarias (figs.2-7). en las secciones longitudinales (fig.2, 8-11), la correlacion de las unidades electricas es muy diffcil. se evidencia la presencia de varias discontinuidades, que desplazan las unidades verticalmente, especialmente en las secciones f y g, en donde se aprecia una fuerte tectontca que afecta toda la secuencia terciaria. la secclon h (fig.10), muestra valores altos despues de cierta profundidad, posiblemente indicando la presencia de las rocas diabastcas de la cordillera por debajo de una cubierta cuaternaria. en la secci6n i (fig. 11), se ve la continuidad del abanico de galindez (unidad ta), el cual se extiende hacia el nw, chocando con fa cordillera occidental. en general, se pudo establecer que existe una secci6n tfpica que consta de techo a base, de un paquete con resistividades altas, entre 100 y 400 ohm-m, que corresponderia a los sedimentos mas recientes (arenas y gravas), un paquete con resistividades medias a bajas, entre 20 y 100 ohm-m, que muestran intercalaciones de areniscas de grano medio a fino y limolitas y un basamento con resistividades bajas, menores de 20 ohm-m, correspondientes a arcillolitas. esta secuencia es desplazada, por las diferentes discontinuidades electncas asociadas a los sistemas de fallas que estan afectando la zona, como se ve en las secciones transversales y longitudinales. con v e ii c i 0 he s ml ......,~,.... id..... ,., 10_ ... 1 loo r,,,,,,,, ... 0"'". 10 lioo"'g ... i,i,t,,'o .... "" '5'''''''.'' oi.ouieo'hl .....uon .. ' .. s e c ctce groelect"t'_ l.a' guaouauto / h estlfeclio ~ ." .. " •• 1, •••••• ", .... "." wj """"" .. cd 0"""'0.,,10.60'01'<1"" .!f!is ..... elf,,,,,. ~"ii .. i figura 3. secci6n geoelectrica a-a' lelll .",,,,,, h ...... ft •• , ... ,.... ~l ~.,,",..... 0i0oi" [ill col,.",." ....", .",11,.. j( \00 .... g,.oio,if". rstuoc .. o~{.h"'i.l.£ofi .. n. secelon geoelecuio ii-e' (}!j l,... c"'o<lo , ... 'i<if'" @:tilt, ... ".. g)d'''''''''''ooii>e'''''''''' [it:! .",1010'., figura 4. secci6n geoelectrica b-b' s[~"l t ••_£ikl,i<'~lfll<"([il] 'i.,o(d" ••" ... r.." •• 1 .... ~~..~ [i]q[i, .. , .... , •• i"""'l,1 fsn"'o "'~lioico(jo<l_" ""-" .... 5£ccio" geoelectric" c-c' figura 5. secci6n geoelktrica c-c' '~'i~1-. i "i-----l:~ ,__ ~ coo ii! _-i i m _._-1 --...... ~~i ;:0 -.'--1il1i convenelones m:ii ',t_ ma,e,llo1 ~ .. r••"'cufi"olo ........ "'tv,d4n 'lecion geot l fc'hllea d· 0' it u!i,',-ltd"" .. r..... 1::=---"""':-----1 "1·1 figura 6. secci6n geoelectrica 0-0' i i s q.:-,g ~ ~ sef"'5 ][ ~:; ::.---,-:--------m ~_~o i__ o~ -> i i ,/ rr i i ~--~i /" y cd ...--------1 -f:' 1i-1':1------ace ee' 500 co'velleioil£s ~ grayoiq,ul'iollql\lrodu ~afc"lol,'i .. o. ~ ll"o~i",olflljo'ilcl. ~',cillolitol [l[j "u,iui ~"" wlli/rodol ututio m,_nolog"ofiuyau.f dl\. ... "1 seccion gfoelect"ica it 1i.".""dod oh .. ", _ co"'ociog.o,ltci,ico -"_ili. lit... i~_ 1:..,_-'-' fi,.? figura 7. secci6n geoelectric a e-e' i "f' "fu "f" "t." 'i" ;"f" ulldl<l ">diio<ol:olooco,"fc __ lle on. .. '" s[ctioil gfoflect.,c. ldii,'tuoiii'lf_" figura 8. secci6n geoelectrica longitudinal f-f' 'i:~r:...,._. .~... figura 9. secci6n geoelectrica longitudinal g-g' ~ r" '1:='; .! i i . i~ .>---... i i i·n i_,_,n_ -, l "_' _i"-_ 1_" 1-1_ "_m_ '"0 i i -ii -i i i'm i i 1 i _1-_ 1 1 i i i cd @ 7,-0 [.'i.llli-.....oi.o<oioo,.,,'--'-( on po'" section 6eoelectric-. longltuoin-'l h-il" figura 10. secci6n geoelectrica longitudinal h-h' s£\/.~l sfy·ll s£y, s£y'" s£y-lfl t t t t t 1~:~~:--fi _ /"n i • i"~i~ 1/;---,f:~il l, 0 <b (_o(ofl,,",,-uon ...... section geoelectric-. longltuoi.-'l i-i' :;~;:..:.-~ figura 11. secci6n geoelectrica longitudinal 1-1' mapa de isorresistividad a 10m de profundidad 1008500 726000f-------t-----,(&!,l6iffy'-+-----___.j 726000 721000 f-------t--t---+---_""'i'--------i721000 715000 1--c/---=-l.----,rl'=3c-f-7"''''i-+---f---f_----_i 716000 -f___1f-'-~""",,:'-----+-"'----'r---i711000 721000 716000 r--\c---'-==y-=~-r'''t-7ff+'i_+h_--f__----__i 711000 1-+-411& 993500 998500 1003500 711000 993500 998500 1003500 l__---'~_---l-l _ _'____ -'__ l_ __ l_ _ __\__ _' 706000 \008500 ascale 1:3429 e=-="3 ~ mapa de isopacas del cuaternario 988500 993500 998500 1003500 f-7"~-==,.l--+------ckooh,+,;fi_+-___1f_----_i 726000 mapa de isorresistividad a 100 m de profundidad 988500 998500 1003500 1008500 figura 12. mapas de: (a) isorresislividad-10 m de prof.; (b) isorresislividad-100 m de prof.; (e) isopaeas cuatemario; (d) prof. basamento geoelectrico scale 1:3429~~ (c) 993500 716000 726000 r-------t-------\;g.s~rns---_t---=t====71 726000 721000 f__-----+----:>+l--?~'l-"=-__ f__----_i 721000 711000 >-\-----/.--\--l,..+-+----?--~___.j--------1 711000 706000 "'--_-l __ --'----'-_ _'____ -'-__ l__"--l_~"_' __ _' 706000 988500 993500 998500 100.3500 1008500 scale 13429~ ( b) mapa de p"ofundidad del basamento geoelectrico 988500 1008500993500 7\1000 ,"",c\\\cff~~+\-\c{---___.j--------1 711000 -'----l __ -l __ -'-~_.lc=~_'_ _ _.j 706000 1008500998500 1003500 scale 13429~~ ( d) 3.2 unidades geoelectricas y correlaci6n litol6gica en la tabla 1 se describen cada una de las secciones con las unidades, resistividades (p), profundidad (h), espesor (h) y correlaci6n litol6gica (ver secciones). tabla 1 secciones con las unidades, resistividades (p), profundidad (h), espesor (h) y correlaci6n litol6gica secci6n unidad p h h l1tologia a-a la 110-400 20-60 20-60 ii 30-33 150-170 120-150 iii 12-18 120 70 iv 30-38 150-250 120 b-b v la 6-10 3-7 16 lal ii 8-18 28 5-20 37 iii iva 15 63 70 53 iv 24-32 135-220 90-210 c-c v lal 20-27 5-13 6-18 ii 1500 5 ii lib iii iiib iv 40 18 15-19 6 20-30 110 50 110-180 160 150-280 97 30 60-90 110 50-160 d-d lal 7 13 ii 50-160 9-25 9-25 ii 22-37 45-70 36-47 e-e iii iv v lal it ii 9-30 13 50-80 9-30 3 37-50 8-11 17 9 6-15 350 7-22 >40 350 iii iv v 1.5-5 16 8 140 400 6-10 arenas y gravas semisaluradas (abanico de galindez) areniscas finas a medias saturadas limolilas areniscas finas a medias saturadas basamenlo arcilloso arenas y gravas en matriz arcillosa (abanico de otaya) limos y arcillas areniscas finas a medias saluradas limolilas areniscas gruesas saturadas arenfscas finas a medias saturadas arcillolilas arcillas y limos (depositos aluviales) gravas secas (en terraza aluvial) areniscas medias saturadas limolilas limolilas arcillolitas areniscas finas a medias saluradas arcillas (de lianura de inundaci6n) arenas y gravas saturadas limclitas a areniscas finas saturadas limolitas areniscas finas saturadas arcillolilas limos y arcillas gravas saturadas arcillolilas y iimolilas saturadas arcillolilas limolilas arcillolilas 5-20 35 43 52 20-27 5 13 >100 250 60 260 3.3 mapas a partir de los valores obtenidos de resistividad y profundidad de los diferentes sev, se realizaron 4 mapas (fig.12): a) mapa de isorresistividad del cuaternario realizado a una profundidad de 10m y muestra tres sectores: el sector sw, sobre el abanico de galindez. las resistividades mas altas se prf3sentan hacia la parte distal y disminuyen hacia su parte terminal; el sector central presenta resistividades bajas, menores de 30 ohm-m; en el sector ne, las resistividades son medias. en este mapa se observa c1aramente la distribuci6n de los sedimentos del abanico de galindez (fig.12a). b) mapa de isorresistividad del terciario, se realiz6 a 100 m de profundidad. los valores de resistividad en este mapa en general son bajos. hacia el sw se presenta un sector de altos valores, los cuales podrian ser causados por las rocas crstaclcas de la cordillera occidental que se internan en el valle a esta profundidad (fig.12b). c) mapa de espesor del cuaternario, en este mapa se observa el mayor espesor del cuaternario en el abanico de galindez, en donde alcanza hasta 70 m; en el resto del area el cuaternario no sobrepasa los 30 m. se observa una serie de huecos, posiblemente ya existentes en el terciario (fig.12c). d) profundidad del basamento: muestra una serie de cuencas alineadas ne-sw, 10 cual estaria relacionada con el fallamiento en esta direcci6n. en la parte superior se aprecian tres cuencas con alineamiento diferente, que indicaria fallamiento en esta direcci6n y no visible en la superficie. la profundidad es muy variable, pero en general el basamento se superficializa hacia el ne (fig.12d). 4. interpretacion del modelo geoelectrico 4.1 discontinuidades geoelectricas-lineamienlos geol6gicos con el estudio qeoelectrico se pudieron ubicar varias discontinuidades, algunas de las cuales parecen corresponder a fall as y lineamientos cartografiados en superficie a partir de totoqeoloqla (torres, 1991). en total se detectaron 7 discontinuidades (ver secciones): la 1 y la 7, se han asociado con la falla del patfa al w y la falla potrerillos al e; las otras discontinuidades se asocian con iineamientos con direcci6n general nw/se (fig.2). ei modele geoelectrico obtenido en el valle alto del rro patfa, muestra una tect6nica de bloques muy fuerte especial mente en direcci6n e-w, la cual esta afectando especial mente las rocas cretacicas y la secuencia terciaria. los sedimentos cuaternarios se depositan sobre una geomorfologia terciaria de cerros levantados y valles cortados. la secuencia terciaria ha side afectada por tectonismo continuo, siendo muy dificil realizar una correlaci6n de unidades en todo el valle (secciones g, h, i). ai nw del hfo patfa la secuencia se presenta muy arcillosa, con valores de resistividad muy bajos (secciones c, e y g). esto estaria indicando un levantamiento de la secuencia relacionado con la falla de romeral. hacia el sw (seccion a y h), la secuencia es mucho mas arenosa, 10 que estaria sugiriendo un enterramiento hacia este sector. 4.2 interpretacion hidrogeologica los acuiferos en el valle del rro patfa son de dos tipos: libres y semiconfinados. los primeros estan relacionados con arenas y gravas de depositos de terrazas marginales, cuyas resistividades varian entre 50 y 350 ohm-m y con arenas y gravas del abanico de galindez. alii las resistivid,!des varian entre 70 y 400 ohm-m. los ultimos, estan asociados con la secuencia terciaria y corresponden a paquetes de a eniscas de grana fino a medio con resistividades entre 20 y 70 ohm-m. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 7 -12 reference crust-mantle density contrast beneath antarctica based on the vening meinesz-moritz isostatic inverse problem and crust2.0 seismic model robert tenzer1 and mohammad bagherbandi2,3 1 institute of geodesy and geophysics, school of geodesy and geomatics, wuhan university, 129 luoyu road, wuhan, china 2 division of geodesy and geoinformatics, royal institute of technology (kth), se-10044 stockholm, sweden 3 department of industrial development, it and land management university of gävle, se-80176 gävle, sweden geophysics the crust-mantle (moho) density contrast beneath antarctica was estimated based on solving the vening meinesz-moritz isostatic problem and using constraining information from a seismic global crustal model (crust2.0). the solution was found by applying a least-squares adjustment by elements method. global geopotential model (goco02s), global topographic/bathymetric model (dtm2006.0), ice-thickness data for antarctica (assembled by the bedmap project) and global crustal model (crust2.0) were used for computing isostatic gravity anomalies. since crust2.0 data for crustal structures under antarctica are not accurate (due to a lack of seismic data in this part of the world), moho density contrast was determined relative to a reference homogenous crustal model having 2,670 kg/m3 constant density. estimated values of moho density contrast were between 160 and 682 kg/m3. the spatial distribution of moho density contrast resembled major features of the antarctic’s continental and surrounding oceanic tectonic plate configuration; maxima exceeding 500 kg/m3 were found throughout the central part of east antarctica, with an extension beneath the transantarctic mountain range. moho density contrast in west antarctica decreased to 400-500 kg/m3, except for local maxima up to ~ 550 kg/m3 in the central antarctic peninsula. el contraste de densidad de la discontinuidad de mohorovičić (moho) debajo de la antártida fue estimado con base en la solución del problema isostático vening meinesz-moritz y a partir de datos obtenidos con el modelo sísmico de la corteza global (crust2.0). la solución se encontró a través de un ajuste al método de mínimos cuadrados por el método de elementos. el modelo geopotencial global (goco02s), el modelo topográfico/ batimétrico (dtm2006.0), los datos de espesor del hielo para la antártida (reunidos por el proyecto bedmap) y el modelo sísmico de corteza global (crust2.0) fueron utilizados para calcular las anomalías gravitatorias isostáticas. ya que los datos de crust2.0 para las estructuras de la corteza en la antártida no son exactos (debido a la falta de información sísmica para esta parte del planeta), el contraste de densidad de la discontinuidad de mohorovičić fue determinado a partir de un modelo de corteza homogéneo que tiene una densidad constante de 2,670 kg/m3. los valores estimados del contraste de densidad de la moho se encontraron entre 160 y 682kg/ m3. la distribución espacial del contraste de densidad de la moho exhibe mayores rasgos en la configuración de la plancha tectónica de la antártida continental y su alrededor oceánico. el valor máximo encontrado excede los 500 kg/m3 y se ubica en la parte este continental, con extensión en las montañas transantárticas. el contraste de densidad de la moho (zona de transición entre la corteza y el manto terrestre) en el oeste de la antártida osciló entre 400-500 kg/m3, excepto para la máxima local de ~ 550 kg/m3, en el centro de la península antártida. key words: antarctica, crust, gravity, isostasy, mantle, moho interface. palabras clave: antártida, corteza terrestre, manto terrestre, gravedad, isostasia, discontinuidad de mohorovičić record manuscript received: 23/10/2012 accepted for publication: 04/04/2013 introduction current knowledge about the lithospheric structure beneath antarctica is limited due to a low spatial coverage of high-quality seismic data. seismic studies by kogan (1972) and ito and ikami (1986) were based on localised controlled source seismic experiments. passive seismic studies, based on earthquakes occurring mostly outside the antarctic tectonic plate (due to a lack of intra-plate seismicity within the antarctic plate (okal, 1981), still represent the primary source of information. studies based on an analysis of surface wave velocity can be found in abstract resumen robert tenzer and mohammad bagherbandi evison et al., (1960), kovach and press (1961), bentley and ostenso (1962), dewart and toksoz (1965), adams (1971), knopoff and vane (1978), rouland et al., (1985), forsyth et al., (1987), roult et al., (1994) and bannister et al., (2003). seismic receiver function analysis has been carried out by reading (2006), lawrence et al., (2006) and winberry and anandakrishnan (2004). ritzwoller et al., (2001) used the simultaneous inversion of broadband group velocity measurements to compile a seismic model of the crust and upper mantle beneath antarctica and surrounding oceans. some studies have been based on airborne gravity surveys, for instance by studinger et al., (2004, 2006). llubes et al., (2003) used champ satellite gravity data for estimating crust thickness in antarctica. more recently, block et al., (2009) have estimated crust thickness in antarctica using grace gravity data. despite some authors having investigated antarctic crust thickness, studies addressing lithosphere density structure and density interface in this part of the world are rare. this study was thus aimed at investigating moho density contrast beneath antarctic continental and surrounding oceanic crustal structures. since large areas of antarctica are not yet sufficiently covered by seismic surveys, current knowledge about crust density structure is not complete and accurate. a simple model of 2,670 kg/m3 homogenous crust reference density was thus adopted and moho density contrast was estimated regarding such crustal density. this involved applying the method recently developed by sjöberg (2009) and sjöberg and bagherbandi (2011). isostatic model the generic expression of solving the vening meinesz-moritz isostatic problem was formulated in the following form (sjöberg and bagherbandi, 2011): ( ) ( ) ( )ig r k , ,s d g r φ ′ ′− ∆ρ ω ψ ω = ∆ ω∫∫  where g = 6.674×10-11 m3 kg-1 s-2 is newton’s gravitational constant, r = 6371×103 m earth’s mean radius, ρ∆ moho density contrast, ig∆  (approximate) isostatic gravity anomaly (cf. sjöberg, 2009), k the integral kernel function, and λφφ ′′′=ω′ ddd cos the infinitesimal surface element on the unit sphere. the 3-d position was defined in the spherical coordinates system ( )ω,r , where r is the spherical radius and ( )λφ,=ω denotes the spherical direction with spherical latitude φ and longitude λ . the full spatial angle is denoted as . integral kernel k in equation (1) was defined for parameters ψ and s , where ψ is the spherical distance between observation and (running) integration points ( )ω,r and ( )ω′′,r , and s a ratio function of the moho depth t and earth’s mean radius r, i.e. r/11 ts −=−= τ . the spectral representation of k was given by (sjöberg, 2009): ( ) ( ) ( )∑ ∞ = +− + += 0 n 3 cosp1 3 1,k n ns n ns ψψ where np is the legendre polynomial of degree n for the argument of cosine of the spherical distance ψ . the isostatic gravity anomaly ig∆ computed at position ( )ω,r was defined as follows (cf. vening meinesz, 1931): ( ) ( ) ( ) 0,,, cbi =ω+ω∆=ω∆ rgrgrg where bg∆ is the refined bouguer gravity anomaly, and cg the gravitational attraction of isostatic compensation masses (see also bjerhammar, 1962, chap. 14, eqn. 5; moritz, 1990). the spectral representation of isostatic gravity anomaly ig∆  was defined in terms of spherical harmonics of degree n and order m. the numerical coefficients were generated from spherical harmonic coefficients of gravity anomaly after applying the bouguer gravity reduction term , which was defined by the coefficients of global topographic/bathymetric (density) spherical functions . the coefficients were defined as (sjöberg, 2009): where is the (fully-normalised) surface spherical harmonic function of degree n and order m . the density distribution function equals on land and ocean density contrast is defined as: ; where c is wareference crust density and mean saltwater density. ice and sediment density contrasts were defined relative to reference crust density c. nominal (zero-degree) compensation attraction c 0 ~g stipulated at the sphere of radius r was computed as from (sjöberg, 2009): where , and 0t and are the adopted nominal mean values of moho depth and density contrast, respectively. estimation principle least-squares analysis was used for estimating t and ρ∆ . the linearised observation equation involved expanding the integral term on the lefthand side of equation (1) into a taylor series. the subsequent substitution of the first two terms in the series for 3+ns from equation (3) to equation (1) yielded (sjöberg and bagherbandi, 2011): from equation (5), the linearised observation equation for product ρ∆t was found in the following form: approximation of term in equation (7) by yielded the linearised observation equation for ρ∆ in the following form: least-squares analysis combined the estimated product of t and ρ∆ with a priori values t and κ of such parameters for obtaining improved estimates of t and ρ∆ . the system of observation equations was given in the following vector-matrix form: where is the vector of residuals. parameter vector x and observation vector l read: (1) (2) (3) , , , (4) (5) (7) (8) (6) , , . . . (9) (10) , , elements 1l , 2l and 3l of the observation vector l were formed, respectively, by observables ρ∆t , ρ∆ and t . parameter vector x was defined for unknown corrections and κd to a priori (initial) values of t and ρ∆ . the solution of normal equations was found by solving: covariance matrix xq ˆ of the estimated parameters was computed from: 12 0ˆ −= nqx σ where unit weight variance 2 0σ reads was: ( )xalql ˆ1t2 0 −= −σ data acquisition the computation of isostatic gravity anomalies over the study area of antarctica required the application of gravity corrections due to rough topography, continental ice sheet and variable geological structure (mainly large sediment deposits). the three largest mountain ranges on the antarctic continent are the transantarctic mountains, the west and east antarctica ranges. the transantarctic mountains is formed by a mountain range extending, with some interruptions, across the continent from cape adare in northern victoria land to coats land. these mountains serve as the natural geographical division between east and west antarctica. absolute maxima of topographic gravity correction in this part of the world reach ~400 mgal (tenzer et al., 2009). approximately 98% of the land in antarctica is covered by the continental ice sheet, thickness exceeding 2 km, while only about 0.32% of the land is ice-free. maximum ice thickness is 4,776 m. the ice mass thus represents a significant contribution to the gravity field. another significant gravity field contribution along the continental shelf of antarctica is due to the presence of large sedimentary basins. additional large sedimentary deposits exist inland of antarctica (see studinger et al., 2003; bamber et al., 2006). tenzer et al., (2010, 2012) have demonstrated that ice and sediment stripping corrections to gravity data in antarctica reach about 300 mgal and 60 mgal, respectively. the global geopotential model (goco02s), the global topographic/ bathymetric model (dtm2006.0), ice-thickness data for antarctica (assembled by the bedmap project; lythe et al., 2001) and sediment density and thickness data from the global crustal model (crust2.0) were used in this study to compute isostatic gravity anomalies over the study area of antarctica bounded by parallel 60 arc-deg of southern geographical latitude. all gravity computations were realized on a 2×2 arc-deg surface grid. coefficients from the combined grace and goce satellite global geopotential model goco02s (goiginger et al., 2011) with a spectral resolution complete to degree 180 spherical harmonics were used for producing gravity anomalies. refined bouguer gravity anomalies were obfig. 2 refined bouguer gravity anomalies computed on a 2×2 arc-deg surface grid with a spectral resolution complete to degree 180 of spherical harmonics. values are in miligals. fig. 3 crust-mantle density contrast computed on a 2×2 arc-deg grid using a combined least-squares method. values are in kg/m3. fig. 1 the earth’s solid topography generated on a 2×2 arc-deg grid using dtm2006.0 coefficients complete to degree/order of 180. values are in km. reference crust-mantle density contrast beneath antarctica based on the vening meinesz-moritz isostatic inverse problem and crust2.0 seismic model (11) (13) (12) , . , tained after applying bouguer gravity reductions to goco02s gravity anomalies. the spherical bouguer gravity reduction was computed using coefficients from the global topographic/bathymetric model dtm2006.0 (pavlis et al., 2007) complete to spherical harmonic degree 180. the average density of upper continental crust 2,670 kg/m3 (hinze, 2003) was adopted as reference crust density. the ocean density contrast 1,643 kg/m3 corresponds to mean seawater density 1,027 kg/m3, updated 5×5 arc-min ice-thickness data for antarctica assembled by the bedmap project were used to compute the ice (density contrast) stripping gravity correction. the ice stripping gravity correction was computed with a spectral resolution complete to degree/order 180. the ice density contrast 1,753 kg/ m3 corresponds to glacial ice density 2,670 kg/m3 (cutnell and kenneth, 1995). crust2.0 sediment thickness and density data (on 2×2 arc-deg grid) were used for computing the sediment (density contrast) stripping gravity correction with a spectral resolution complete to degree/order 90. crust2.0 (bassin et al., 2000) was compiled and administered by the us geological survey and the institute for geophysics and planetary physics at the university of california. crust2.0 is an upgrade of crust5.1 (mooney et al., 1998). the earth’s solid topography (i.e. topographical heights onshore and bathymetric depths offshore) generated on a 2×2 arc-deg grid using the dtm2006.0 coefficients complete to degree/order of 180 is shown in figure 1. maximum topographical heights over the study area reach 4.1 km and maximum bathymetric depths are 5.7 km. regional map of refined bouguer gravity anomalies is shown in figure 2, values were between -457 and 419 mgal (mean = -43 mgal; standard deviation = 286 mgal). gravity maxima were found over areas of deepest oceans, and toand gravity minima were located throughout the central part of east antarctica. results the combined least-squares method was used for a simultaneous estimation of moho depth and density contrast. the solution was obtained by solving the system of normal equations, which was defined in equation (11). the observation vector l in equation (10) was formed by three observation types ρ∆= tl1 (eqn. 7), ρ∆=2l (eqn. 8), and stl =3 . the initial values of moho depths st were taken from crust2.0 the variance-covariance matrix q in the least-squares estimation model (in eqn. 11) was computed from (cf. sjöberg and bagherabndi, 2011):           = 2 3 2 2 2 1 2 1 2 1 00 0/ 0/ σ σσ σσ t t q where 1σ and 3σ are the standard errors of ρ∆t and t , respectively, and ( ) 422 3 22 1 2 2 // ttt ρσσσ ∆+= . the standard error 1σ of ρ∆t was computed using the following expression: where 0γ is normal gravity, , and error degree potential coefficients. since crust2.0 moho depths were not provided with a standard error model, relative uncertainties (i.e. standard errors 3σ ) of crust2.0 moho depths of 20% were assumed in forming the matrix q. estimated moho density contrast taken relative to 2,670 kg/m3 crust density are shown in figure 3. the density contrast, was found to be between 160 and 682 kg/m3 (mean = 477 kg/m3, standard deviation = 128 kg/m3). estimated moho density contrast was compared with crust2.0. crust2.0 moho density contrast was computed from crust2.0 upper mantle densities relative to 2,670 kg/m3 crust density. crust2.0 moho density contrast is shown in figure 4. differences between both moho density contrasts are shown in figures 5. whereas large variations of moho density contrast were estimated using the combined least-squares approach (fig. 2), the range of crust2.0 values was within 580 and 730 kg/m3 (fig. 4). such large discrepancies (shown in fig. 5) were likely caused by a low quality of crust2.0 upper mantle density, especially under oceanic crust. crust2.0 upper mantle density under oceanic crust was significantly overestimated. crust2.0 moho density contrast under the antarctic continental crust had a good agreement with our estimates, differences mostly being within ±100 kg/m3. fig. 4 crust2.0 moho density contrast on a 2×2 arc-deg grid. values are in kg/m3. fig. 5 differences between crust2.0 and newly estimated moho density contrast on a 2×2 arc-deg grid. values are in kg/m3. robert tenzer and mohammad bagherbandi , , (14) (15) discussion moho density contrast differs significantly in east and west antarctica (see fig. 3). moho density contrast maxima are situated throughout the central part of east antarctica with the extension under the transantarctic mountain range. moho density contrast there exceeded 500 kg/m3. moho density contrast beneath west antarctica was less pronounced. a typical range was between 400 and 500 kg/m3, except for local maxima in the central part of antarctic peninsula, where values reached ~550 kg/m3. ritzwoller et al., (2001) and block et al., (2009) have estimated that average crust thickness in the west antarctica was ~27 km. average crust thickness of ~40 km was estimated for east antarctica, with maximum crust thickness being ~45 km. these large differences could be explained by different tectonic and geological compositions of west and east antarctica. dalziel and elliot (1982) proposed that east antarctica is a single tectonic block formed predominantly by craton. several smaller crust fragments forming west antarctica have been moved and further reconfigured with respect to east antarctica and each other. ritzwoller et al., (2001) and morelli and danesi (2004) have analysed shear wave velocities over the whole of antarctica. they found significant differences in mantle structure characterised by a low velocity in the west antarctica while a high velocity was detected in the mantle of east antarctica, with a transition occurring below the transantarctic mountains. unlike most of similar mountain ranges, this mountain range was formed in the absence of collision tectonic forces (ten brink et al., 1997). several theories have been proposed explaining possible mechanisms for their formation. models have included thermal buoyancy from an underlying positive temperature anomaly in upper mantle (ten brink et al., 1997), thicker crust giving the origin to an isostatically buoyant load (studinger et al., 2004) or possible collapse of a high-standing plateau with subsequent uplift and denudation (bialas et al., 2007). the latest models based on integrated geophysical analysis assume that multiple mechanisms have contributed to the uplift of the transantarctic mountains (cf. studinger et al., 2004; lawrence et al., 2006). despite the central part of east antarctica being characterised by large variation in continental icesheet thickness and rough bedrock topography of basins and mountain ranges including sub-glacial mountains, significant variations in moho density contrast were almost absent (see fig. 3). prevailing homogenous moho density contrast there could be explained by a more uniform geological structure than that of west antarctica. the location of the largest local density contrast anomalies correspond to the gamburtsev subglacial mountains, aurora basin and dronning maud land. block et al., (2009) have reported decreasing crust thickness on the margins of east antarctica due to rifting events that separated antarctica from the other gondwana landmasses, with the most pronounced signature occurring on the coast of dronning maud land. this trend was not clearly seen in spatial pattern of moho density contrast; instead, large moho density contrast variations across the east antarctic continental margin could be explained by significantly different geological composition between continental and oceanic crusts, involving typically thick and less dense continental crust compared to thin and heavier oceanic crust. distinctive features in the map of moho density contrast geographically correspond to the west antarctic rift in west antarctica;moho density contrast there being typically 400 to 450 kg/m3. similar density contrast was found beneath the ross embayment. a possible explanation for these small values in moho density contrast could be provided by geological structure dominated by volcanic activity occurring since (at least) the early cenozoic. holocene volcanism continues in the ross embayment (kiele et al., 1983; blankenship et al., 1993; behrendt et al., 1994). according to behrendt et al., (1994) the main rifting phase occurred between 105 and 85 ma but episodic extension continued into the cenozoic. extension within the rift system has left most of west antarctica below sea level, with the exception of marie byrd land and parts of the antarctic peninsula. it is assumed that this represents remains of a continuously propagating rift that started during the jurassic period when africa separated from east antarctica and proceeded clockwise to its present location in the ross embayment and west antarctica. the almost complete absence of recent seismic activity suggests that there might not be any undergoing active extension of the rift zone (cande et al., 2000). local maxima of the moho density contrast exceeding ~500 kg/m3 in west antarctica were found beneath marie byrd land. the lithospheric structure there is characterised by topographic doming due to localised hot spot activity (hole and lemasurier, 1994; winberry and anandakrishnan, 2004). moho density contrast was typically less than ~350 kg/m3 beneath oceanic crust; minima were found along the pacific antarctic mid-oceanic ridge. moho density contrast there was estimated to be as low as 160 kg/ m3, corresponding to 2,830 kg/m3 density for the youngest oceanic lithosphere (müller et al., 2008). similar density values have also been found between antarctic and south-american continental tectonic plates, which were separated by a newly-formed oceanic lithosphere. conclusions spatial variations of moho density contrast resembled major features of the antarctic tectonic plate configuration and its geological composition. density contrast maxima were found throughout the central part of east antarctica with the extension under the transantarctic mountain range. moho density contrast there often exceeded ~500 kg/m3, maxima being 682 kg/m3. density contrast maxima were found under mountainous ranges and areas covered by the largest continental ice sheet. these locations are also characterised by the largest thickness of the antarctic continental crust, moho depths there reached ~40 km. according to our estimation, moho density contrast beneath west antarctica was typically 400 to 500 kg/m3, except for local maxima (~550 kg/m3) in the central part of antarctic peninsula. local minima (400-450 kg/m3) beneath the west antarctic rift and the ross embayment are likely attributed to volcanic compositions throughout this divergent tectonic zone. the largest variations of moho density contrast were found across the east antarctic continental margin. moho density contrast beneath the oceanic crust was typically less than ~350 kg/m3. the extreme minima were detected along the pacific antarctic mid-oceanic ridge. references adams, r.d. 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(2012). spectral expressions for modelling the gravitational field of the earth’s crust density structure. studia geophys. geodaet.; doi: 10.1007/s11200-011-0023-7 vening meinesz, f. a. (1931). une nouvelle méthode pour la réduction isostatique régionale de l’intensité de la pesanteur. bull. geod. 29, 33-51. winberry, j. p. and anandakrishnan, s. (2004). crustal structure of the west antarctic rift system and marie byrd land hotspot. geology, 32, 11, 977-980. robert tenzer and mohammad bagherbandi software engineering’s contribution to earthquake engineering gloria inés gonzález-lópez1, félix fernando gonzález-navarro2, mario ordaz-schroeder3 and abigail zamora-hernández4 1 universidad autónoma de baja california (uabc). email: giglzlzy@yahoo.com.mx 2 uabc, department of computing and informatics. email: fernando.gonzalez@uabc.edu.mx 3 universidad autónoma de méxico (unam), seismic engineering area. email: mordazs@iingen.unam.mx 4 uabc. email: azamorah@yahoo.com untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 40-53 rio grande cone tectono-stratigraphic model – brazil: seismic sequences castillo., l.l.a¹, kazmierczak, t. de s.², and chemale., jr., f.³ ¹ profesor, curso de geofísica, departamento de geociencias – universidad nacional de colômbia bogota. ² schlumberger ltd, brazil. ³ professor, instituto de geociêcias, universidade federal do rio grande do sul, porto alegre – brazil. corresponding author email: lacastillol@unal.edu.co, thaissk@povo.net and farid.chemale@ufrgs.br resumen análisis sísmicos integrados con técnicas de interpretación, procesamiento, información de velocidades y datos geofísicos permiten modelar diferentes rasgos de tipo tectónico, estructural o geomorfológico. este trabajo se constituye en una primera aproximación a un modelo tridimensional de un rasgo geomorfológico ubicado en el fondo oceánico. el modelo inicial comprende una configuración bidimensional de una grilla interpretada a partir de secciones sísmicas llevadas a un volumen tridimensional, esto mediante la concepción de una aproximación dos y medio dimensional. además de un cambio de configuración, se hizo un análisis de atributos sobre las imágenes de las secciones y se determinaron las características presentes desde un área de la plataforma, hasta el offshore del sudeste brasilero. el rasgo geomorfológico a tratar es conocido como el cono de rio grande, el cual pudo ser caracterizando con sus principales estructuras tectónicas y estratigráficas, mediante la cartografía y modelamiento tridimensional del subsuelo, con la integración geológica, geofísica y geomorfológica. la geomorfología del cono, se ve influenciada por procesos gravitacionales, corrientes de fondo, estructuras de tipo tectónico y composición litológica, como sus principales controladores que están presentes a partir del neógeno. así, a partir de la interpretación sísmica pueden ser extractados diferentes estructuras tectónicas, sedimentarias y geoformas, por ejemplo presencia de fallas, pliegues, canales, levee naturales, contornitos, etc. palabras clave: sismoestratigrafía, modelamiento, tectonoestratigrafía, interpretación geofísica. 42 manuscript receiver: february 03th, 2009. accepted for publication: june 01th, 2009. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:47 p p p composite 133 lpi at 45 degrees abstract the seismic analyses integrated to interpretation techniques, processing, velocity model and geophysical data have permitted modeling different tectonic, stratigraphy or geomorphology features. this paper is the first approach to geomorphological features for a three dimensional depth model. the initial model comprises a seismic section grid with bidimensional configuration conversed to three dimensional subvolumes. these include a two and half dimensional model. also, an attribute analyses in image section is necessary to determine the most relevant characteristics. the geomorphologic feature corresponds to the named rio grande cone, which is characterized by principal tectonic and stratigraphic structures. that is why mapping and subsurface three dimensional modeling, with geology, geophysical and geomorphologic integration is necessary. the cone geomorphology is characterized by gravitational processes, bottom currents, tectonic structures and lithological compositions which are the principal factors that have been influencing it since neogene. then, the seismic interpretation could extract different tectonic, sedimentary structures and geoforms, for instance, faults, folds, channels, natural levee, contourites, etc. key words: seismic stratigraphy, modeling, tectono stratigraphy, geophysical interpretation. introduction seismic stratigraphy method has improved since development of acquisition, processing and interpretation processes. they include loading, processing, visualization and modeling tools that have allowed the manipulation of format data, screen and the creation of a high quality model. data could be transformed to different configurations like 2d, 2.5d and 3d. the first concepts related to seismostratigraphy interpretation refer to geometry and stratal terminations (mitchum et al. 1994).in the brazil’s southeast area, the rio grande cone has been influenced by tectonic episodes since the cretaceous until the recent. two and half dimensional seismic interpretation on some sections along the surface and the subsurface show the presence of some features located on shelf. they include a fault system, geoforms and folds that influence the slope, proximal and distal shelf and the sea floor. the tectono-stratigraphyc analysis make the description of some geological aspects in subsurface possible. as a consequence, a geological model was obtained by means of the seismic information and a velocity function within seismic interpretation (sarta, 2004). the geological mapping was obtained by means of the seismic stratigraphic interpretation on the rio grande cone . this interpretation allowed the division of the deep sedimentary package into several sequences and a driven-model from a sequential stratigraphy classification could be made. to define seismic units (chronoestratigraphic unit referred as sequences), the first step was to define the unconformities and then recognize the unit according to the facies, the seismic expression and the attributes analyses. 43 rio grande cone tectono-stratigraphic model – brazil: seismic sequences figure 1. rio grande cone location within the pelotas basin – brazil (modified from google earth). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:48 p p p composite 133 lpi at 45 degrees geomorphology, geophysics and computational disciplines are powerful tools that could be integrated in order to obtain a 3d approach by using a driven-model method. this model allows the generation of a more realistic feature of the subsurface geomorphology. in this preliminary research a 3d model obtained from seismic interpretation in the rio grande cone will be documented. location rio de la plata and rio grande cone were formed during the maastrichtian/danian atlantic transgression in south america. the continental shelf and the depression are flooded by the sea. they are typical examples of geoforms can be seen on the uruguay and brazilian platforms. the rio grande cone (rgc) is a relevant geoform that extends 28900 km² on the brazilian offshore (figure 1). it is a subsurface landscape semicircular-shaped noticeable feature that comprises a transition from shelf to slopes with a high amount of shale supply. its source is the highlands and the brazilian craton. seismic data in this analysis, the data corresponds to 3577km of seismic lines that were acquired from brazil’s anp, and other old printed sections (fontana, 1996; porto, 2007). structural and straigraphy models were generated by means of software modules: petrel 2008, matlab (v. 7.0.1), voxler, seismic unix and open source seismic interpretation software (opendtect v3.03e). the dominant frequency seismic information could be obtained from attributes analyses, i.e. an instantaneous frequency map, which exhibits a 30-35hz dominant frequency and 90 µs/ft slowness in the area. a 4ms sample rate, 5 to 10s record length and 28ms windows time analyses are comprised. in order to establish the wavelength, geophysical parameters on pelotas basin whose average velocity is 3050 m/s and frequency of 30hz need to be considered. the expression for wavelength is given by: ë= v/f. (1) then, ë = 3048m/s / 40 hz = 76,2m, by the ë/4 => 76,2 m/4 = 19,05m => 62,5 feet. these calculations determined that the obtained thickness for a minimum layer is between 20 to 100 feet (7 to 30m), otherwise it is not tuned. the seismic resolution limit in section data establishes what can and cannot be seen on subsurface. attribute analysis all information extracted from seismic data is known as attribute and the interpretation depends on the combination of attributes, the quality of data, and the interpreter’s experience. the attribute is used to highlight subsurface features in order to delimit horizons, zones, seismic facies, and geometries by increasing the ability to define structural models and stratigraphic analysis. the advantage of seismic data is the multiplicity, areal extension and depth penetration as it permits obtaining information from seismic attributes (taner et al. 1977, 1979). seismic attributes are calculated from at least two trace input and provide information about lateral variations in set data. an attribute is not restricted to structural or stratigraphic analysis; it has been used to estimate petrophysic properties and geomorphological elements by combining them with well data. historical development of seismic tools and techniques improve the execution of the different stages of seismostratigraphic analysis. seismic stratigraphy and attribute analysis could be considered as tools for geomorphological, geological and geophysical modeling (figure 2). seismic attributes application seismic attributes were introduced in the seventy’s first to display seismic data, then to obtain different derived seismic measures, and finally turned into an analytical tool to predict lithological characterization. although a direct relationship between the attributes and the geological characteristics of the 44 castillo., l.l.a, kazmierczak, t. de s., and chemale., jr., f. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:48 p p p composite 133 lpi at 45 degrees earth has not been established, almost all analyses describe several uses of seismic attributes i.e. classes’ classification discriminator. recently, the seismic attribute application includes surfaces, horizon, geomorphology mapping, sequence stratigraphic interpretation, modeling subsurface and geobody information. they all have been accepted as tools to interpret old strata and processes through the use of modern survey (posamentier and kolla, 2003). then seismic geomorphology is an evolving field, building on the historic approaches of seismic stratigraphy, sequential stratigraphy, fluvial geomorphology and three dimensional modeling. seismic attributes are used in structural, stratigraphy and geomorphologycal interpretation (figure 3). an example of this is the improvement of image resolution thanks to the sophistication of computational systems that allow determining reservoir properties and their lateral continuity. figure 3. dip seismic line display of seismic attribute analysis and seismic stratigraphy interpretation 45 rio grande cone tectono-stratigraphic model – brazil: seismic sequences adquisition processing time line interpretation 1960 1970 1980 1990 2000 now 1 d 2 d 2.5 d 3 d 4 d c o n f i g u r a t i o n cmp configuration air gun marine digital recording digital seismic processing bright spot technology digital migration comercial seismic attribute comercial in colors color display 3 d shot vertical seismic profile introduction to seismic attribute concept (anstey) energetic crises dip moveout geophysicist stratigraphy avo analysis attribute proliferation seismic attributes expansionocean deep cable seismic attributes analysis texture attributes analysis of multi-attributes attributes and horizons classes synthetic seismogram workstation complex traces analyses trace complex seismic stratigraphy seismic inversion analyses horint attributes dip, zimur 3d autotracking attributes respose 3d seismic (brown) subsalt imgen seismic stratigraphy sequences attribute analysis attribute classification spectral decomposition coerence attribute introduction neural network geomorfología sísmica volumentric attributes spectral inversion figure 2. historical development of the seismic method: acquisition, processing, interpretation and modeling (adapted from liner, 2008; chopra and marfurt, 2005 and friedman, 1998). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:48 p p p composite 133 lpi at 45 degrees in the seismic images processing, local seismic attributes analyses (lsaa) are being used to measure seismic signal characteristics of neighboring points. this technique has been applied in different steps of multicomponent seismic image acquisition (fomel, 2007). in this paper attributes were used to delineate structural (dip/azimute, ant-tracking, dip deviation, local structural dip, structural smoothing, variance, etc), stratigraphy (iso-frequence component, local flatness, acoustic impedance, etc) and geomorphologic (gradient, coherence, strata slices attributes, horizon slice attributes etc) features (figures 2 and 3).coherence attribute measures the similarity of a trace with its neighbors and display discontinuities, faults and channels (marfurt et al. 1998). this criterion guides the interpreter in the establishment of seismic reflection attributes and seismic facies patterns. instantaneous amplitude and frequency, which are the most frequently used, show changes in lateral continuous reflections. some attribute analyses were applied to seismic lines belonging to the brazilian offshore. the most important features are the lineaments and fractures found by means of local flat analyses and relative acoustic impedance that showed normal faults influencing the sedimentary package into the rio grande cone. root meters square permits delineating strong reflectors that could be correlated and interpreted by 46 castillo., l.l.a, kazmierczak, t. de s., and chemale., jr., f. structural stratigraphicseismic attributes sequencial stratigraphy interpretation local flat rms windows 9 dip seismic line acoustic impedance rms windows 3 schematic sequence smoothisofrequence figure 3. dip seismic line display of seismic attribute analysis and seismic stratigraphy interpretation julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:50 p p p composite 133 lpi at 45 degrees a sequence stratigraphic model, and delineated by geomorphological elements like channels. seismostratigraphy and sequence stratigraphy of the rio grande cone rio grande cone is located in a passive margin basin characterized by several progradational systems supplied by fluvial and cratonic sediments influenced by sea level fluctuations. some geological features have been reported by authors like alves (1977); fontana (1996) and porto (2007). seismic section and attributes permit identifying structural, sedimentology, and geomorphology features and they allow determining structural and the stratigraphic sequences elements. tectonic structures description the southeast of the brazilian offshore is composed by sequences influenced by rift faults causing a half-graben configuration on the basal sequence. the rio grande cone comprises pos-rift sequences. the post-rift stage starts during aptian, in which the adiastrophic tectonic developed, influencing basal sequences, i.e., the aptian-albian sequence. above it more than 3000 m of sediments belonging to rio grande cone were laid (fontana, 1996). structural sequences are influenced by tectonic and sedimentary structures. relevant structures correspond to active system faults until pleistocene -late wisconsin (alves, 1977). from a structural point of view, fault systems are the most relevant factor that affects clastic sequences. the first domain contains a faults system, including listric faults, thrust and their detachment planes. those structures are characterized by normal faults that were originated by distensive stress generating displaced blocks located on the proximal area (northeast-southwest trends and vergence to the northwest). there are some small faults on the cone’s distal section which structural style changes to inverse faults. this style is located to the end of the section (figure 4). some seismic section permit identifying gravitational deposits associated to fault systems, generating deformational and tectonic structures. studies have 47 rio grande cone tectono-stratigraphic model – brazil: seismic sequences figure 4. three-dimensional structural model of the rio grande cone, brazilian southeast. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:51 p p p composite 133 lpi at 45 degrees determined that some lineaments and faults were originated by fluids decompactation, and have influenced on the generation of fluid escape structures visible on the sea bottom’s surface (fontana, 1996; porto, 2007). the rio grande cone constitutes a huge sedimentary package characterized by the result of channel system morphology, sediment waves and contourities that have been influenced on the upper sequences by a distensional fault system that originated normal faults to the north and inverse faults to the south. stratigraphic sequence in the rio grande cone the seismic stratigraphic interpretation of the pelotas basin includes at least sixteen sequences (butler, 1970; fontana, 1996 and porto, 2007). the cone’s area has geoforms, internal structures and velocity models that can be described with geophysical methods, for instance seismic refraction. the refraction method revealed along to rio grande do sul and the uruguay’s continental margin a 1.8 km/s wedge, and a velocity model for the sedimentary sequence (alves, 1977). the wedge sedimentation lasted from middle miocene to pleistocene. stages of deposition and erosion were caused by eustatic sea level fluctuations, originating the development of four sedimentary sequences. the depocenters distribution suggests a source migration towards the continent, maybe due to marine transgression (alves, 1977). a different interpretation for the pelotas basin and florianopolis shelf established eight sedimentary sequences using seismic reflection (gonçalves et al. 1979). martins (1983) described the rio grande cone as a sedimentary-originated deep sea feature supplied by rio grande do sul highlands, with progradational deposition and gravitational processes (turbidities and another flux) modeled by bottom current. some researchs reports hydrate gas in the pelotas basin, i.e., fontana (1989), rosa et al. (2006). 48 castillo., l.l.a, kazmierczak, t. de s., and chemale., jr., f. figure 5. seismic dip line (di) interpretation of surface (su: unconformity; mrs: maximum regressive and mfs: maximum flooding surfaces) sequences (hnr: high normal regressive, lnr: lowstand normal regressive ). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:52 p p p composite 133 lpi at 45 degrees the generated trasgressive (t), highstand (hnr) and lowstand regressive (lnr) could be identified on seismic sections (figure 5). the shelf edge suffered erosion resulting in the truncation of the prograding sediments. the eroded sediments fed it via suspension or gravitational processes. this feeding was four times greater than holocene rates. seismo-stratigraphy and sequence stratigraphy description in order to describe the evolution of the rio grande cone, it is important to describe its tectono-stratigraphy. the cone comprises several continental and post-rift (drift) sequences. the middle and upper sequences belong to the rio grande cone (figures 5 and 6). the middle sequences correspond to onlap transgressive sequences and downlap basinward. all sequences permit identifying montiforms with bidirectional downlaps that represent the lowstand fan unit deposited on the basin’s deeper area. this is the first lowstand stage in which a rapid decrease on eustatic curve inflection occurs. few continuous reflections and variable amplitudes are found on paleoslopes. middle sequences with erosion surfaces changed from cretaceous to tertiary. geological mapping considerations comprise sequences that have been influenced by different structural and 49 rio grande cone tectono-stratigraphic model – brazil: seismic sequences figure 6. sea level variations (haq et al, 1987) and cronostratigraphy interpretation correlation in the rio grande cone (alves, 1977; fontana, 1996; porto, 2007 and this paper). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:07 p p p composite 133 lpi at 45 degrees stratigraphic styles dominated by facies variations that were caused by a high quantity of sediments supply. the upper sequences that belong to the rio grande cone are characterized by changes from retrogradational to progradational sequences. transgressive (t) and regressive features (lnr) are the result of sea level fluctuations that occurred from aptian. in the holocene transgression (t), the southeast continental margin has not received any significant quantity of terrigenous supply. two processes are still active: widespread pelagic and sedimentation geostrophic. contour current activity developed along lower continental rise. bottom-currents deposits result from upslope flowing processes, while gravity deposits result from downslope process. recent drift sequences allow identifying slope features deposited by bottom-currents, slump and gravity deposits process (alves, 1977). due to the instability of the upper sequences originated by steep slopes, the seismic has established slumps geometric characteristics that lead to a progradational system. the sediment supply in a decline plane influenced by a higher sea level produces a mass flow deposit. the highstand (hnr) is constant from paleocene to middle eocene and oligocene, through regressive intermittent cycles that finished with an oligocene regression (this is shown by hard layering, as result of paleoshelf). there are regional terraces and an erosive surface covered by deltaic events until upper pleistocene on rio de la plata, (martins, 1983; martins et al. 1990). they have similar geometries to the rio grande cone. sedimentary effects in the paleocene cycles could be evident in quiescence tectonic causing stability. in the final cycle, as result of andean tectonics, a regional tilt from west south american took place and 50 castillo., l.l.a, kazmierczak, t. de s., and chemale., jr., f. figure 7. 3d model obtained from seismic stratigraphic interpretation showing paleoshelf and several stratigraphic sequences and highlights the tectono stratigraphic features of the rio grande cone. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:09 p p p composite 133 lpi at 45 degrees caused the beginning of the sea level decrease and the deposition of progradantional sequences. this event could be coincident with haq curve (figure 6). rio grande modeling and mappingthe boundaries and horizon picking obtained in 2,5-d configuration permitted constructing the three-dimensional subsurface image of the rio grande geoform (figure 7). seismic stratigraphic analyses and basal sequence mapping permit identifying platforms with regressive progradational sediments. the sequences are comprised by sequential systems in a marginal sag type since late cretaceous. these correspond to the pelotas basin sequences. the semicircular-shaped plan-view morphology extends to the southeast and strike to the north. the thick recent sequences are crossed by faults systems and by the presence of geophysics anomalies, for instance a reflector that simulates the bottom surface (bsr). this reflector is an anomaly generated by the high impedances caused by hydrate gas presence. below 500 ms, a system faults extends cutting all upper sequences. near offset or proximal sector it includes some clinoform set which has been separated by internal downlap surfaces and are not faulted. visualization and 3d modeling has allowed defining fault systems geometries (figure 7), showing details of normal faults. this process includes fault propagation and could be interpreted as a polygonal faults system due to contraction factors and early fluid expulsion during possible burial, where compaction acts in different directions on clay rich sediments.the rio grande cone progradational layering produce continuous offlap, evidencing features like the continental slope, the marginal cone, progradational systems, and the upper interval that constitute recent continental shelf (urien et al. 2003). the last stage during the inferior holocene sedimentary dispersion included erosive process through submarine sedimentation. eustatic changes are less evident during the neogene. the highstand and lowstand alternation could be aged miocene and pliocene. they are associated to eustatic episodes with sea level changes that are controlled by andean orogenic pulses. conclusionthe feature named rio grande cone could be defined as a huge semicircular shape geological body with a thick package of sediments. it is mainly a shale geoform. the area is influenced by complex structures that affect the whole sedimentary package. the rio grande cone is overlying an older inferior and a middle sedimentary sequence that constitute the pelotas basin sequences. the cone comprises the basin’s younger sequence which is the upper sequence.the three dimensional analysis comprises an academic research of seismic data applied to the modeling and visualization for interactive understanding. this will permit a paleogeomorpholgy reconstruction of rio grande cone. a methodology and a typical sequence of seismic interpretation are given in this paper. the purpose is to show the importance of visualization, attribute analysis and modeling, especially in sequential stratigraphy. it is important to emphasize the use of different computational technologies as tools for data interpretation, visualization, and the recovery of information for the subsurface image by means of a velocity model. acknowledgments this article was written for the doctorate program research in the federal university of rio grande do sul (ufrgs porto alegre, brazil) sponsored by national university of colombia. thanks to national petroleum agency for providing some of the seismic lines used in this paper. thanks to schlumberger for the software contribution, especially to the technical support for petrel’s (module and tools 2008), processing, visualization and seismic interpretation with the geophysical integration data. references al-husseini, m., glover j. and barley b. 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(2004). implementación de un software de pre proceso y análisis de datos sísmicos seismic un. geociencias – universidad nacional de colombia. 163 p. taner, m. t., and sheriff, r. e. (1977). applications of amplitude, frequency and other attributes to stratigraphic and hydrocarbon determination: american association petroleum geologist, memoir 26, 301-327. taner, m. t., kochler, f., sheriff, r. e. (1979). complex seismic trace analyses. gephysics, 44, no. 6, 1041-1063. urien, c. m., martins, l. r. and martins, i.r. (2003). paleoplataformas e programação deltaica do neógeno na margem continental do uruguai e norte a argentina. gravel n. 1. 40 46. porto alegre brasil. yilmaz, o. (1987). seismic data processing: society of exploration geophysicists, tulsa, p. 9-79. zywicki, d., (1999). traditional seismic surface wave tests. georgia tech research institute (gtri), p. 11.34 53 rio grande cone tectono-stratigraphic model – brazil: seismic sequences julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:10 p p p composite 133 lpi at 45 degrees geofisica colombiana pp 40-42 diciembre de 2002 bogota, d.c. issn 0121 2974n° 6 determinacion de mecanismos focales para temblores ~ueiios 0 moderados: region del caribe nororiental victor a huerfano moreno red sismica de puerto rico, universidad de puerto rico,recinto de ma y a q ue z , geologia resumen la determinacion delmecanismo focal de un terremoto contribuyc considerablemente al entendimiento del comportamiento tcctonico de la zona de gcneracion. acrualmentc tenemos las soluciones globalcs cmt (centroid moment tensor) y cl usgs (united states geological survey). la metodologia que utilizan esta basada en el uso de periodos largos para determinar las caracteristicas ocurridas durante el proceso de ruptura; ademas, permiten detcrminar el valor de la energia liberada. los catalogos mundiales se actualizan constanternente para terremotos moderados mayores (> 5.0/5.2, mb). ei objetivo de este trabajo es la aplicacion a eventos locales del metoda de analisis de la amplitud, espectro y polaridades (aspo,1. zahradnick, 200 i) para calcular los para metros focalcs de temblores moderados 0 pequeiios para una red sismica tipica: unas pocas estaciones de banda anchajunto con una red mas densa de sismornetros de periodo corto. se presentan las soluciones focales para dos cventos ocurridos en puerto ricoel primero, en la zona suroeste con magnitud de 3.6 mi (2002/09/0602: 19:48); cl segundo, en la zona sureste con magnitud 3.5 mi (2002/09/07 04:0 1:06). palabras clave: tectonica del caribe, mecanismo focal, catalogo sismico abstract earthquake focal solutions can contribute considerably to the understanding of active tectonics. at present, the centroid moment tensor (cmt) method is one of the most robust and reliable ways for computing focal mechanisms. this method uses long period seismograms to reconstruct the average character of the entire fracturing process of an earthquake, and calculates the energy release. the cmt catalog is routinely update for moderate/strong world wide quakes with magnitudes above 5.0/5.2. the objective of this abstract is to present the amplitude spectra and polarities or aspo method (1. zahradnik, 2001) to calculate the focal parameters from weak events based on available nce instrumental setup: few broadband (bb) stations within a more dense shortperiod network. the method can be summarized as follows: (i) all stations are used to compute the epicenter; (2) the complete records from three-component bb stations arc selected; (3) only amplitude spectras are inverted; (4) feww firts-motion reliable polarities are used as a constraint for the inversion and (5) the green's function with includes near field affects and interference waves is calculated by the discrete wavenumber method. aspo determines five parameters: focal depth, scalar moment, strke, dip and rake, and works below the corner frequency. keywords carl bean tectonics, focal mechanism, seismic catalog introduccion basandonos en los catalogos y la documentacion historica (preinstrumental) as! como en la infonnaci6n recolectada instrumentalmente, podemos concluir que la esquina nororiental de la placa del caribe (enopc) se com porta como una zona sismicamente activa. ei potencial sismica y el transfondo tectonico de las fallas mayores de la enopc (por ejemplo, la trinchera de puerto rico, ei canon de la mona y el pasaje de anegada) ha sido reconocido y estudiado por diversos grupos cientificos. en los ultimos dos siglos, en esta zona se han registrado los terremotos mayores de las islas virgenes (7.4 mb) y del cafion de la mona (7.3 mb). tambien se ha demostrado que zonas adyacentes tienen potencial de generacion de eventos destructivos y maremotos. 40 en la enopc, los temblores moderados y pequefios son particulannente importantes debido a que constituyen la mayorfa de la actividad sismica y estan distribuidos en toda la region. en la red sismica de puerto rico (rspr), aunque se mantiene un catalogo local 0 regional de datos parametricos, no se calculan rutinariamente las soluciones focales. este tipo de anal isis a distancias locales 0 regionales es mucho mas complicado que para aquellos eventos telesismicos (iejanos) debido ala fuerte contribucion de los efectos de la corteza y las heterogeneidades de la parte superior del manto, 10cual aparece retlejado en los sismogramas. modelar este tipo de eventos es imposible utilizando las tecnicas usadas por las redes globales. estudios propuestos sugieren que se pueden analizar los mecanismos gedfislca colombiana, 6. diciembre de 2002 determinacion de mecanismos focales para temblores pequenos 0 moderados, region del carl be nororiental focales compuestos para un selecto grupo de eventos; sin embargo, si esa aproximacion se basa en registros tom ados solo en instrumentos de periodo corto, podemos correr el riesgo de no estar considerando toda la informacion de la fuente, 10 que puede inducir incertidumbre en los resultados. area de estudio y sismicidad la enopc es una zona sismicarnente activa. en puerto rico, el promedio de sismicidad oscila entre 2 y 1sismos diarios con unos muy notorios eventos de enjambres tipicos en las zonas sismicas del sombrero, 19°n, y los canones de la mona e islas yirgenes. la tectonica de esta region esta rnarcada por la convergencia de las placas mayores de norteamerica y del caribe; la primera en proceso de subduccion a una razan de entre loy 20 mm/afio en la trinchera de puerto rico, y la segunda entre 1 y 3 mm/afio en la trinchera de muertos. existen zonas de extension, como los canones de la mona, islas virgenes y el pasaje de anegada; zonas de movimientos laterales en la zona de fallas de los 19°n y en la parte mas occidental de la trinchera de puerto rico; defonnaciones verticales en la zona sismica del sombrero (ten brink, 2002) y en el interior del suroeste de puerto rico. ei catalogo sismico de la rspr (prsn, pagina electronica) mantiene informacion parametrica desde 1987, con mas de 10.000 soluciones (figura i), que reflejan el grado de actividad para esta parte de la enopc; adem as, se mantienen las series de tiempo en formato digital para estaciones de periodo corto desde 1992. mas recientemente (200 i), los temblores se registran en estaciones de banda ancha. sin embargo, soluciones focales solo han sido posibles mediante e: analisis de las polaridades del primer arribo (onda p) y, en algunos casos, la razon de las amplitudes p, sh y sy (huerfano, 1994). soluciones para eventos registrados por las redes mundiales solo reportan 36 mecanismos desde 1976 (cmt, pagina electronica), 10 que representa una minima parte de la actividad real. figura i.muestra la intensa actividad sismica registrada por la rspr. geofislca colombiana, 6, diciembre de 2002 preparacion de los datos si el temblor ocurre en el punto focal, las ondas sismicas viajan por la corteza hasta ilegar a la estacion, don de son registradas segun la capacidad del sismometro, la serial que finalmente se obtiene 5(1', t) mediante el sistema de adquisicion de datos incluye la informacion de la fuente f(r, t), la interrelacion de las ondas elasticas con la corteza c (i', t) y la parte correspondiente al instrumento 1(i', t), tal que: (1) si la respuesta instrumental es conocida mediante las tab las de calibracion y el conjunto de polos y ceros, y se tiene un modelo reaiista de la velocidad de propagacion de las ondas de cuerpo, al menos en las capas superiores, la ecuacion (i) depende solo de !a funcion de la fuente. ei problema puede ser dividido como sigue: i\.t ~ imin.:cmt,usgs s (;::, i) => 2. t ~ 20seg.~ imin.:rnt,aspo 3. t :s; 20seg.:aspo se analizaron dos eventos sisrnicos que fueron reportados como sentidos en puertorico (tabla i). las sefiales de estos temblores se registraron en cinco de las estaciones bb de la rspr. mtp, cpo, cby, agp y mgp, equipadas con 30seg. guralps 3-esp y 40t. tambien se utilize el registro de la estacion sjg que tiene un 120 seg. sts-i (iris, pagina electronica), se aplico la correccion instrumental para todas las senates, rnanteniendolas frecuencias en un rango de 0.1 hz 2hz y se rotaron las cornponentes (x, y, z) ~ (r, t, z). para continuar con el analisis, se requiere una localizacion epicentral preliminar, se deben leer cuidadosamente las polaridades de aquellas estaciones don de sean indudables y se debe disponer de un modelo detallado de velocidad en la corteza. ei espectro observado se com para con los espectros numericos, los cuales se determinan mediante el rnetodo de discretizacion del numero de onda (bouchon, 1981) usando el modelo de velocidades para puerto rico (huerfano, 1994). la diferencia se determina paso a paso mediante la norma l 1*, que indica: error (2) n la suma se ejecuta sobre todas las estaciones (n), sobre todas las componentes y sobre todas las frecuencias utilizadas en la inversion. tabla 1 parametros focales obtenldos. la profundidad del evento es dada por la inversion. centroid'.' loculjzacion plano de falla desplazamiento z» no f'eeha mi lan. longi. (km) azi. hl'('. des. azi. pi. t« 2002/09/07 3.5 18.079 -66.074 294 60 168 120 10 2" 2002/09/06 3.6 18.135 -66.808 10 280 82 96 152 80 41 victor a. huerfano moreno resultados este rnetodo permite determinar cinco parametres: la profundidad focal, elmomento escalar y la geometria de la falla (azimuth, angu10 de hechado y angulo de desgarre). en la tabla i, se detallan los resultados de la inversion, que incluye la profundidad focal, correspondiente a la que reporto el minimo error. en la figura 2 se muestran los resultados de este estudio, adem as de los resultados para la zona local de puerto rico, reportados par el grupo de harvard (cmt) y poi' mendoza (2002), para el caso de las soluciones, utilizando rmt. los mecanismos focales obtenidos coinciden con estudios previos de la tectonica local. para el caso del evento (i "), se sabe que existe en esa zona un movimiento casi lateral en direccion ono-ese, 10 cual i dica que este sismo esta relacionado con este tipo de deformacion: azimuth del vector desplazamiento = 1200 con una inclinacion de 10°, para un plano de falla 024°n. para el caso del evento (2"), en esta zona se presentan patrones complejos que van desde desgarres laterales (gspr.fz) hasta movimientos con un fuerte componente vertical en la esquina suroeste de puerto rico (huerfano et al., 200 1). este ultimo es el posible causante del evento modelado. se tiene un angulo e62°s para el vector de desplazamiento con una componente vertical inclinada de 80°. 21.0 20.5 20.0 19.5 19.0 18.5 18.0 17.5 17.0 i6.5 160 -70 -69 figura2.mecanismos focales obtenidos en este estudio. 42 conclusiones dos eventos pequefios fueron simulados usando una red local de estaciones de banda ancha. ei rnetodo aspo, tambien conocido como lmt (local moment tensor) se aplico en la inversion de los espectros para las sefiales completas. los resultados obtenidos poi' este estudio coinciden con estudios previos de la tectonica de las zonas de generacion. en general, del rnetodo podemos concluir: i. es un metodo rapido para determinar los parametres focales: profundidad, momento escalar y la geometria (azimuth, hechado y desgarre). 2. es una herramienta util en aquellas regiones donde la distribucion de estaciones sismicas consta de unos pocos sismometros bb junto con una red mas densa de sp. 3. en condiciones favorables, pennite solucionar eventos de magnitud mi ::> 3.5. 4. sirve para la implementacion de un catalogo local local de soluciones focales, en adicion a los catalogos mundiales. 5. provee informacion valiosa para un mejor entendimiento de la tectonica local con base en los registros bb. bibliografia bouchon, m. 1981. a simple meted to calculate green's functions for eslastic layered media. bull. seism. soc. am. 71: 959-971. centroid moment tensor (cmt). pagina electronica: www.seismology.harvard.edu huerfano, y., and bataille k. 1994. crustal structure and stress regime near puerto rico. eos suplements. agu. huerfano, y.; hillebrandt, c., and baez-sanchez g. (2001). microseismic activity reveals two stress regimes in southwesterm puerto rico. submited to: tectonophysics. huerfano, y. 2002. earthquake local sources in the puerto rico region: puerto rico tsunami mitigation and warning program. prepared for: university of puerto rico. mendoza, c. 2002. earthquake sources in the caribbean region: puerto rico tsunami mitigation and warning program. prepared for: university of puerto rico. red sismica de puerto rico. pagina electr6nica: rmsismo.uprm.edu ten brink, u.; martin j. 1.; gurrola, h.; dillon, w., and huerfano y. 2002. dynamic topographic in the puerto rico trench. submited to: jrl. zahradnik, j.; jansky, 1., and papatsimpa, n. 2001. focal mechanism of week earthquakes from amplitude spectra and polatities (aspo). pure and appl. geophys., 158: 647-665. geofislca colombiana, 6, oiciembre de 2002 medida de propiedades dinamicas de rocas "in situ" y computo de parametros elastomecanicos luis alberto briceno guarupe profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia ramiro leon diaz campos profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia brlcei'io l.a. & r. draz (1995): medida de propiedades dinamicas de rocas "in situ" y computo de parametres elastomecanicos. geoffs. colomb. 3:73-79. issn 0121-2974 resumen se presentan sinteticamente los fundamentos de la teorla etastomecanica, con definiciones y principales sistemas de unidades. se discuten algunos de los problemas de adquisici6n de datos de ondas p y s y se muestran los resultados encontrados para el caso de unas arenlscas terciarias con velocidades de 3023 m/s y 1772 m/s para las ondas p y s, las que permiten deducir valores de m6dulos elasticos, trpicos de rocas sedimentarias de buena resistencia mecanica, asf p.ej. el m6dulo de young encontrado fue de 2.11 x1 0 dinaslcm y la raz6n de poisson de 0.23. finalmente se plantean relaciones empfricas de los parametres, en funci6n de las velocidades y se dan unos valores de los coeficientes de estas ecuaciones, los cuales se proponen con el ifn de ser enriquecidos y regionalizados en areas particulares. abstract the basic elastomechanic theory is presented briefly. some of the problems in data acquisition are pointed out and results for a terciary sandstone are shown. values of 3023 m/s and 1772 m/s for vp and vs conduces to young's modulus of 2.11x10 dinaslcm and poisson's ratio of 0.23. empirical relations are calculated for e, k, ~ as function of vp and vs, the found coeficients are proposed to be enriched with future studies. 1. introduccion ei planeta tierra, sus rocas que la conforman, 0 los suelos que la cubren en gran parte, se comportan paramuchos casos de interes como medios etasncos. los geoffsicos e ingenieros necesitan, por 10 tanto, cuantificar el grade de elasticldad, para optimizar sus modelos, cualquiera que sea la aplicaci6n a desarrollar. dentro de este marco, hemos realizado algunos trabajos geotrsicos. esencialmente sismica, para colaborar con la ingenierfa en el calculo de parametres etastornecanlcos "in shu". ei usc de otros mstodos como la geoelectrica pueden ayudar para ver la continuidad de las unidades 0 su posible anisotropfa. se muestran algunos ejemplos de medidas realizadas sobre areniscas de la formaci6n guadalupe, por 10 menos en tres sitios diferentes. nos ha parecido conveniente presentar inicialmente un sinteticomarco te6rico acercade las definiciones y relaciones de parametres, asi como de los sistemas de unidades utilizados. en la parte de adquisici6n recalcamos algunos de los problemas baslcos que se encuentran en el campo y en 10 referente al analisis de los resultados hemos establecido algunas relacionesempiricas que podrian servir de guia para su enriquecimiento en trabajos posteriores. 2. los parametros elastomecanicos un medio elasnco es aquel que se deforma linealmente ante la aplicaci6n de esfuerzos externos, perc que al cesar estos, recupera su estado inicial. operacionalmente este hecho se define mediante la liamada ley de hooke, que en su forma completa y compacta se expresa mediante la siguiente ecuaci6n tensorial (udj'as & mezcua, 1986): donde: 't; = tensor esfuerzo; ekl = tensor deformaci6n; ejikl = constante de proporcionalidad (tensor de cuarto rango); i,j,k = subindices enteros (1,2,3) para un sistema ortogonal. en su forma mas general la constante de proporcionalidad tiene 36 elementos diferentes, perc debido a la simetria de esfuerzos y deformaciones, se reducen a 21. para un medio isotr6pico, los elementos de la constante se reducen ados terminos independientes, y la rslacion de esfuerzos y deformaciones se transforma en: donde a y ij. se conocen como los coeficientes de lame. si el medio es hornoqeneo, son constantes. ei coeficiente ij. tarnbien se define como m6dulo de cizallamiento 0 rigidez y define la detorrnacion angular que experimenta un medio al aplicarle esfuerzos tangenciales: ei coeficiente a no liene un significado ffsico di recto , perc se puede relacionar con el coeficie te de compresibilidad, k, denominado a veces de bulk, mediante la ecuaci6n: 2j. = k -~ 3 ei coeficiente de compresibilidad, se define a su vez como la presion hldrostatica dividida por el cambio un ita rio de volumen que se produce: pk .. -fj. v/v ei modulo de young se tjefine como la relacion entre un esfuerzo normal y su deformaci6n asociada: til e= 811 en termlnos de a y ij.: e .. j.!(3j. + 2.1..) (j. + 1-1) finalmente se define la relacion de poisson, o, como la relaci6n entre la elongaci6n y la contracci6n en dos direcciones perpendiculares, esto es: o [8] [1] que en tsrrninos de a y ij. se puede expresar como: j. 0=---2(j. + ~) [9] de este modo, para un medio elastico, las relaciones entre esfuerzos y deformaciones se pueden expresar en terminos de a y ij., perc es quizas mas cornun utilizar el m6dulo de young e, la raz6n de poisson o y el m6dulo de compresibilidad k, junto con ij.. a partir de 'las defhiciones operativas para los m6dulos elasticos, se puede ver, respecto a sus dirnensiones que: [2] • e, k, a y ij. se expresan en unidades de presi6n; cr es adimensional. todas las constantes son positivas; e> k> i > = ij.. las siguientes son relaciones utiles entre diferentes sistemas de medidas: [3] 1 dina/ern" = 0.1 nswton/rn" = 1.020 x 10-6 kg-f/cm2 1 megabar = 1012 dina/em" 1 pascal = 1 pa = 1 ntim2 [4] la forma como se propagan las deformaciones de volumen y forma de un medio etastlco, ante fa acci6n de esfuerzos tanto norm ales a las superficies del medio, como tangenciales a elias, se encuentra aplicando la segunda ley de newton a un elemento diferencial de volumen, tomando una densidad constante y aplicando las relaciones de elasticidad de la ley de hooke. despues de perturbado el sistema y si se considera que las fuerzas exteriores no persisten, se obtiene como ecuaci6n del movimiento de las perturbaciones, la liamada ecuaci6n de navier: [5] [10] [6] donde u es el vector desplazamiento y p la densidad del medio. ai aplicar el operador divergencia a la ecuaci6n [10], se obtiene que: [11] [7] donde e = e" + e22 + e33 = (ou/ox,) + (ou!ox2) + (ou!ox3) es el cambio fraccional de volumen e = t1vn, y: � +21j« ,. p la ecuaci6n [11] es la ecuaci6n de una onda, por 10 tanto, los cam bios de volumen, producidos por esfuerzos normales, se propagan como una onda con una velocidad a, dada por la ecuaci6n [12]. si se aplica el rotacional a la ecuaci6n [10] y se remplaza .:l.u = w, se obtiene la expresi6n: donde: tambisn [13] es una ecuaci6n de onda y expresa el hecho de que las deformaciones angulares, 0 de cizallamiento, producidas por esfuerzos tangenciales en un medio elastico, se propagan como una onda, con una velocidad b, dada por la ecuaci6n [14]. es posible mostrar que para un medio elastico, hornoqeneo e infinito solo existen estos dos tipos de onda, con velocidades a y b, las cuales se denominan ondas de cuerpo. las ondas de deformaci6n volurnetrica, tienen una velocidad mayor que las transversales 0 de cizallamiento, a > b. las primeras se denominan tarnbien ondas p (por ondas de presi6n 0 primarias) y a las otras ondas s (por ondas de cizallamiento -shear-, 0 secundarias) y sus velocidades se notaran como vp y vs respectivamente. las expresiones [12] y [14] permiten expresar los parametres elasticos en funci6n de vp y vs, si se conoce la densidad. ~~-~2 p • o ~-v: p. ~. (1 + c). (1 2ex)e ,. --'--------(1 ex) k,. e (1 20) [18] a partir de este hecho, se han calc.ulado los pa.rametros para diferentes tipos de rocas y matenales de la tierra. se observa que: 1. 0 ~ cr ~ 0.5 con cr = 0.05 para roc as muy duras y cr= 0.45 para material blando de poca consolidaci6n. 2. para iiquidos, j.1 = 0 y o = 0.5 [12] 3. para la mayorfa de rocas de la corteza, e, k y j.1 sstan entre 0.2 y 1.2 megabars. [13] ai hablar de medios elasticos, se supone que la unica disminuci6n de la energfa se realiza por geometrfa. sin embargo, siempre se produce absorci6n por fricci6n interna, fen6meno no elastico que se expresa mediante el coeficiente de disipaci6n especffica q, cuyo inverso asociado a la fricci6n interna de los medios, representa la fraccci6n de energfa disipada durante un perfodo de cada onda: q" = _1_ ~e zn e [19] [14] este tipo de disipaci6n se puede inferir un poco cualitativamente como desviaciones de valores promedios. dutta (1984), ha calculado parametres anelasticos como el rqo (rock quality designation) y la frecuencia de fracturas, a partir de analisis de curvas de regresi6n. ei fen6meno de disipaci6n 0 perdidas no elasticas es dispersive, esto es, depende de la frecuencia de la onda, notandose que a mayores frecuencias hay mayor absorci6n (tellford et. al., 1976). 3. adquisicion de informacion [15] ei calculo de parametres elasticos (young, compresibilidad, rigidez y raz6n de poisson) puede realizarse en el campo ("in situ"), a partir de ensayos sfsmicos, para determinar las velocidades p y s de las ondas en los distintos medios, utilizando normal mente las fases oirecta y refractada. calculadas las velocidades, se emplean las relaciones de la teorfa elastica, expuesta en el numeral anterior, para deducir los parametres de tnteres. cuando el trabajo se realiza a campo abierto y con unidades del subsuelo mas 0 menos paralelas, el problema operativo e interpretativo se reduce considerablemente, pues la diferenciaci6n de ondas p y s se facilita con la utilizaci6n de ge6fonos que respondan a movimientos verticales (ondas p) y ge6fonos que respondan a movimientos transversales (ondas s). generalmente las ondas s tienen mayor amplitud. sin embargo, es muy posible que ambos tipos de detectores reciban tanto ondas p como s y adernas pueden liegar ondas reflejadas y ondas superficiales como las de raleigh y love. un criterio que ayuda a diferenciar las fases de intsres es que vs no puede ser mayor que 0.7 vp. conviene adernas, realizar mas de un disparo con fuentes y arreglos diferentes en los extremos del tendido, con distancias sp-ge6fono cercano (offset), del orden mfnimo de 3 0, donde 0 es la distancia entre ge6fonos y en el centro del tendido, con offset igual a 0/2. cuando el trabajo hay que realizarlo en tuneles 0 galerfas, aumentan los problemas debido principal mente a tres factores: [16] (17) 1. casi siempre hay agua en el piso y, por 10 tanto, se dificulta la ubicaci6n tanto del punta de disparo, como de los detectores. esto obliga a su colocaci6n sobre superficies inclinadas; asf el objetivo de los dos tipos de detectores es mas complementaria que exclusiva. 2. lo mas cornun, es que dentro del tunel haya roca dura, que impide un faci! enterramiento de ge6fonos, 10 que obliga al uso de cementos de secado rapido (enriquecidos con sika), para soportar el ge6fono. esto genera interfases artificiales, con una cierta cantidad de ruido. 3. la iimitaci6n de espacio (direcci6n del tunel), obliga a realizar los tendidos casi siempre en direcciones estructurales (geol6gicamente hablando) constantes, 10 que puede restringir los resultados y conclusiones. 4. analisis de resultados la tabla 1 muestra los valorespromedio, para 57 muestras, con sus desviaciones estandar, maxirnos y mfnimos para vp, vs, raz6n de poisson (r.p) y m6dulos de young (m.y), rigidez (m.r) y bulk (m.b). en general, los valores son tfpicos de rocas de buena calidad mecanica. aparte de estos resultados, y siguiendo un analisis similar al de munoz & araneda (1980) se han realizado ajustes para obtener f6rmulas empfricas que sinteticen el comportamiento de los medios estudiados y faciliten su aplicaci6n. tabla 1 vlu.orj:s piloilkdio, para 57 muj:stras colli sus dj:sviacionzs j:stahdail, ilaxdlos y mfndios para vp, v«, razbn dj: poisson (r.p) y n6dulos dj: young (n.y), rigidj:z (n.r) y bulk (n,b) vp/vs vp vs m/s m/s r.p m.young m.bul< m.rigidol!l.b/m,r dina/cm2 dina/em1 dina/cm2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 1,91 1,73 1,83 1,79 1, 61 1,50 1,59 1, 58 1,49 1,69 1,58 1,68 1, 63 1,80 1,79 1,67 1,67 1,77 1,71 1,86 1,81 1,56 1, 79 1,82 1,96 1,67 1,78 1,90 1,69 1,74 1,66 1,76 1,66 1,76 1,83 1,84 1,65 1,65 1,68 1,66 1,77 1,77 1, 57 1,66 1,75 1,56 1,63 1,73 1,77 1,61 1,60 1,71 1,71 1,77 1,80 1,65 1,75 2600 1362 2600 1503 2609 1429 2625 1467 2639 1635 2647 1765 2653 1667 2689 1698 2740 1835 2745 1628 2757 1740 2771 1650 2791 1714 2800 1556 2809 1572 2813 1682 2857 1707 2857 1615 2857 1667 2857 1539 2885 1596 2893 1855 2895 1618 2903 1593 2903 1484 2906 1737 2909 1633 2936 1546 2963 1758 2963 1702 3016 1815 3030 1724 3051 1837 3125 1775 3143 1719 3158 1714 3161 1920 3167 1919 3200 1908 3208 1932 3226 1818 3227 1821 3235 2056 3286 1983 3289 1879 3333 2143 3333 2039 3333 1932 3358 1901 3362 2087 3382 2110 3411 1996 3419 1994 3443 1944 3500 1944 3514 2131 3534 2017 0,31 0,25 0,29 0,27 0,19 0,10 0,17 0,17 0,09 0,23 0,17 0,23 0,20 0,28 0,27 0,22 0,22 0,27 0,24 0,30 0,28 0,15 0,27 0,28 0,32 0,22 0,27 0,31 • 0,23 0,25 0,22 0,26 0,22 0,26 0,29 0,29 0,21 0,21 0,22 0,22 0,27 0,27 0,16 0,21 0,26 0,15 0,20 0,25 0,26 0,19 0,18 0,24 0,24 0,27 0,28 0,21 0,26 1,31e+ll 1,520+11 1,420+11 1,480+11 1,72e+ll 1,850+11 1,76e+l1 1,820+11 1,9ge+l1 1,76e+ll 1,91e+l1 1,800+11 1,90e+l1 1,67e+ll 1,700+11 1,87e+ll 1,920"'11 1,780+11 1,860+11 1,660+11 1,760+11 2,14e+ll 1, 800+11 1,760+11 1,57e+ll 1,990+11 1,830+11 1,6ge+l1 2,ose+l1 1,96e+ll 2,160+11 2,020+11 2,210+11 2,lse+ll ~ ose+ll 2,050+11 2,400+11 2,41e+ll 2,41e+ll 2,45e+ll 2,260+11 2,27e+l1 2,65e+l1 2,.58e+ll 2,40e+l1 ·2,850+11 2,70e+ll 2,510+11 2,47e+l1 2,7ge+ll 2,840+11 2,67e+l1 2,67e+ll 2,580+11 2,610+11 2,970+11 2,760+11 1,160+11 1,ole+ll 1,lde+ll l,dge+ll 9,17e+l0 7,71e+lo 9,000+10 9,140+10 8,15e+lo l,d8e+ll 9,62e+l0 i,oge+!1 1,04e+ll 1,25e+ll l,24e+ll l,l2e+1l l,lse+ll 1,270+11 l,2de+ll 1,35e+ll 1,330+11 1,d2e+ll 1,320+11 1, 360+11 1,48e+l1 1,lge+ll 1,33e+l1 1,47e+ll 1,26e+ll 1,33e+ll 1,27e+ll 1,41e+ll 1,300+11 1,500+11 1,60e+1l 1,63e+l1 1,37e+ll 1,380+11 1,45e';'11 1,43e+l1 1,62e+11 1,620+11 l,30e+l1 1,50e+11 l,65e+l1 1, 350+11 1,500+11 1,66e+11 1,74e+l1 1,48e+ll 1,490+11 l,71e+ll 1,720+11 1,84e+11 1,95e+11 1,70e+ll l,9le+ll 2,31 1,66 2,00 1,87 1,27 0,92 1,20 1,17 0,90 1,51 1,18 1,49 1,32 1,91 1,86 1,46 1,47 1,80 1,61 2.12 1,93 1,10 1,87 1,99 2,50 1, 47 1,84 2,27 1,51 1,70 1,43 1,76 1,43 1,77 2,01 2,06 1,>8 1,39 1,48 1,42 1,81 1,81 1,14 1,41 1,73 1,09 1,34 1,64 1,79 1,26 1,24 1,59 1,61 1, 80 1,91 1,38 1,74 5,ole+lo 6,10e+lo 5,5le+lo 5,8le+i0 7,22e+10 8,41e+lo 7,500+10 7,7ge+lo 9,090+10 7,160+10 8,17e+l0 7,35e+lo 7,930+10 6,53e+lo 6, 67e.;.10 7,64e+l0 7,87e+lo 7,04e+lo 7,50e+lo 6,390+10 6,880+10 9,290+10 7,07e+lo 6,85e+10 5,94e+lo 8,140+10 7,20e+l0 6,460+10 8,350+10 7,82e+1d 8,890+10 8,030+10 9,110+10 8,51e+lo 7,98e+l0 7,930+10 9,950+10 9,940+10 9,830+10 1,d1e+ll 8,930+10 8,950+10 1,14e+11 l,dge+11 9,53e+lo 1, 240+11 1, 120+11 1, ole+l1 9,760+10 1,ise+11 1,200+11 l,dse+l1 1,070+11 1,020+11 1,020+11 1,230+11 1,100+11 p st mx mn 1, 340+11 2,73e+lo 1, 95e';'11 7,710+10 8,580+10 1,820+10 1,24e+l1 5,ole+lo 1, 61 0,34 2,5 0,9 1,71 3023 1772 0,23 0,10 272 189 0,05 1,96 3534 2143 0,32 1,49 2600 1362 0,09 2, 11e';'11 4,160+10 2,97e+11 1,31e+i1 _ promedio; st desviaci6n standar; !-1x maximo; mn minimo las figs.1 a 3 muestran los coeficientes e, k y u.en funci6n de vp. en todos los casos el mejor ajuste se logra con una ecuaci6n de la forma: y = a xb, en donde y es el respectivo paramstro, x = vp, a y b son los coeficientes de ajuste. los siguientes son los resultados obtenidos: para e: a = 17993.5 k: a = 9632.9 il: a = 8952.3 b = 2.02956 b = 2.05111 b = 2.00405 de estas figuras se puede ver que el valor de b es muy cercano a 2.00 y la dispersi6n alrededor de ese valor te6rico promedio podrfa considerarse como una indicaci6n de la falta de homogeneidad del paramstro sobre la muestra considerada. por otra parte, el valor de a es un indicador de la magnitud del paramstro: los valores comprueban que en general e > k > u. la fig.4 de il contra vs, es igual que en los casos anteriores una parabola, con una mfnima dispersi6n, debido a la f6rmula empleada para su calculo, en este caso b = 2.00101. en la fig.5, se compara la relaci6n de los coeficientes de compresibilidadlrigidez, con la de velocidades vpl>ls. finalmente, la fig.6 es otra parabola alrededor del eje x, que debe tender asint6ticamente al valor de (j = 0.5. las dos ultimas graficas deben tener un mfnimo en las ordenadas, para vs = 0.7 vp. 5. conclusiones las medidas sfsmicas realizadas sobre areniscas de la fm. guadalupe, han permitido establecer unos valores promedio de velocidades de ondas p y s, 10 mismo que para los principales parametres elastomecanicos, dados en la tabla 1, que corresponden a rocas sedimentarias de buena calidad mscanica. se han establecido relaciones empfricas de los m6dulos de young, compresibilidad, rigidez y raz6n de poisson, en tsrminos de las velocidades vp y vs, medibles "in situ". los coeficientes de estas relaciones se plantean, como proceso metodol6gico, para ser mejorados con mas datos y resultados, que ojala puedan ser aplicados a localidades especfficas. bibliografia dutts, n.p. (1984): "seismic refraction method to stydy the foundation rock of a dam". geophysical prospecting, 32, pp 1103-1110. munoz, m. & m. araneda. (1980): "propiedades ffsicas en rocas de mina chuquicamata" tralka, vol.1, no.2,119-142. u. de chile. tellford, w.m., l.p. geldart, r.e. sheriff & d.a. keys. (1976); applied geophysics. cambridge university press. udiss, a. & j. mezcua. (1986: fundamentos de geoflsica. ed. alhambra sa primera ed. madrid. 3.lie-011 .. .... .... .. .. 2.se_011 e q '""• .. ..• 2•.lie-011 .5 0 o ~ l.se-011 .... ~ .. i l.lie-011 vp"dlno8/cm2m.de younga 17993.5 s.lie-011l 2600 2600 2700 2800 2900 3000 3100 3200 3300 3~ 3s00 vp ( m/e> figura 1. relaci6n entre vp y el modulo de young, para areniscas del guadalupe. 2.lie-011 '" 1.se-011 e -q .. -, • ..•.5 0 .. l.lie-1l11 ~ '".. ~ ..-0 ~ s.lie-1l11l ~ a 9632.9 vp 2.86111dtnell8/om2 0.lie-000-+-------,--..----r,-------,-,------'i------,!--2900 3100 3300 3s00 vp (m/.j 2700 figura 2. relaci6n entre vp y el modulo de bulk, para areniscas del guadalupe. .. * * ne 1.21::-011 * *0-, *~ "c 0 8.oc-010 n ~ § *~ *~ * i 4.oc-010 m.de rigldez = 8962.3 vp"dtnoe/om2 0.oc-ooe 2500 2600 2700 21300 2900 j000 j100 j200 jj00 j400 j500 j600 vp (m/o) figura 3. relaci6n entre vp y el modulo de rigrdez, para areniscas del guadalupe. 1.:£_011 , ne l.oc-011 0-,. e c: 0 7.5e-010 n w b o ~ ~ i 5.oc-010 m.de rigiocz 26801.3 v. '."'0' dlnoo/cm2 2. 5e-0' 0 -+-----.,,------,-----,-----,----,-----...----....,--------.----.,------, 1300 figura 4. relaci6n entre vs y el modulo de rlgrdez, para areniscas del guadalupe. 3.00n ~ 2.50o t;: ** o ~ 2.00~ ~ ~ 1.50o ~ ** * '" • * **'"",ill * '"• • '" .. *•• * ..• • ••, .00* * 0.50 -r--ii---r-i----,i---~ i----,----,--,---,------r---,---,-----, 1.40 1.45 1.50 1.55 1.60 1.~5 1.~0 1.~5 1.~0 1.~ 1.~ 1.~ 2.~ ( vp/v.l figura 5. relaci6n entre (vpns) y el (modulo de bulk/modulo de rigidez), para areniscas del guadalupe. 0.36 0.30 s (f) 0.26 (f)..... 1[ ~ ~ 0.20 ..... 5 ~ 0.15 0.10 •• • 0.06 +---,---,----,------,,------,---,...--1--,--1-1--1--1 1.40 1.45 1.60 1.65 1.70 ( vp/v.l 1.50 1.55 -figura 6. (vpns) en funci6n de la relaci6n de poisson, para areniscas del guadalupe. un modelo geodesico para colombia william alberto martinez diaz instituto geografico "agustin codazzi", unidad de geofisica german a. rocha a. universidad distrital "francisco jose de caldas"·facultad de ing.catastral laura marlene sanchez rodriguez instituto geoqraflco "agustin codazzi", unidad de geofisica martinez, w.a., g.a. rocha & l.m. sanchez (1995): un modelo geodesico para colombia. geoffs. colomb. 3:59-66. issn 0121-2974 resumen la informaci6n gravimetrica procesada esta compilada en el mapa de anomalia bouguer simple de colombia, este de panama y aereas marinas adyacentes, publicado por "the geological society of america inc." en 1989. a partir de los valores de la anomalfa lsostatica (aproximada), resultante de la diferencia entre la anomalfa bouguer simple observada y la misma, procesada por medio de la transformada de fourier bidimensional, se aplic6 el modelo ffsico-matemattco de stokes, con el cual se determinan las ondulaciones geoidales relacionando la gravedad normal, el potencial an6malo y las anomalfas de la gravedad; tales ondulaciones se referenciaron al elipsoide wgs-84. abstract the processed gravimetric information is compiled in the simple bouguer gravity anomaly map of colombia, eastern panama and adyacent marine areas, published in 1989 by the geological society of america inc. based on isostatic anomaly values (approximated) obtained for difference between the observed simple bouguer anomaly values and itself, processed by means of the bidimensional fourier transform, the physical-mathematical stokes model was applied. with this model can be possible to determine the geoid undulations, relating the normal gravity, the anomaly potential and the gravity anomalies; these undulations are based on the wgs-84 ellipsoid. 1. formula de stokes ei desarrollo del modelo geoidal esta basado en la ley de gravitaci6n universal, la teorfa del potencial gravitacional, los modelos te6ricos que. representan el comportamiento terrestre y sus desviaciones con respecto al comportamiento real de la tierra. relacionando el potencial anornalo (tij) con la gravedad normal (r) sobre la superficie terrestre se obtienen las ondulaciones geoidales (n). la integral de stokes permite determinar el valor del potencial anornalo resolviendo la ecuaci6n fundamental de la geodesia ffsica: donde: t;: 15r;j15n: es el potencial anomalo 0 de perturbaclon. es el gradiente anornalo (derivada parcial de t en la direccion normal. es la gravedad normal. es la anomalfa de la gravedad empleada en el modelo. r: dg: ecuaci6n que constituye el tercer problema de valor limite en la teorfa del potencial (heiskanen & vening meinesz, 1958; pick et al., 1973). ei potencial anornalo (heiskanen & moritz, 1967) se define como: [2] [1] donde: r: es el radio de la esfera con igual volumen al del elipsoide terrestre. este radio equivale al radio medio de la tierra. do: denota un elemento de superficie sobre la esfera terrestre. ijf: distancia esterica entre el punta de calculo (4), r) y el elemento de superficie ubicado en w, r'), s(ijf): es la funcion de stokes. s(iji) = _1_ 2sen~ + 15cosiji sen~ 2 2 3cos[ln(sen~ + sen2-.!.»)2 2 [3] mediante el teorema de bruns se obtienen las ondulaciones geoidales: t n =..:....!l; r r j'n = -t:j.g s(ijr) do 41tr a es posible sustituir en esta ecuacion la gravedad teorica (f) por un valor medio de gravedad en la superficie terrestre, sin cometer errores apreciables. asf: r j'n = -t:j.g s(1jj) do 41t g a 2. manejo de la informacion gravimetrica la informacion gravimetrica procesada se encuentra recopilada en el mapa "simple bouguer gravity anomaly map of colombia, eastern panama, and adjacent marine areas" (edicion de 1989) (kellogg et.al., 1983) (fig. 1). las fuentes de este mapa son: datos de mott-smith (1941), instituto geoqrafico "agustin codazzi" (1941-1969,1991), exploracion california (1958), nunez (1967), harding,woollard, monges, oldham, case (1971 y levantamiento trans-andino), quintero (1977), kellogg-bermudez (1978). mapas de: van buskirk (1948), case et al. (1971, 1973), case-macdonald (1973), case (1974), bermudez (1978), bonini et al. (1980), bermudez-acosta (1978), kellogg (1980), hayes (1966), bowin (1976), briceno (1978). en esta aproxirnacion a la forma y dimensiones del geoide en colombia se realize la conversion del datum internacional de 1971 sobre el cual esta referenciado el mapa fuente al wgs-84 (world geodetic system-1984) mediante la transtorrnacion abreviada de molodenskii (d.m.a., 1987) 3. anomalia utilizada en el modelo la anomalfa mas apropiada para el calculo del geoide a travss de la f6rmula de stokes es la lsostatica (heiskanen & moritz, 1967), puesto que los valores de esta oscilan en torno acero, su variaci6n es suave y es independiente de la topograffa, facilitando por ello la interpolaci6n y extrapolacion en zonas donde no hay. cubrimiento gravimetrico. por otra parte, la compensaci6n isostatlca ocurre a nivel regional, garantizando que un valor de anomalfa pueda asumirse caracterfstico de un area considerable. la reduccion isostatica desplaza paralelamente las masas topoqrancas, condensandolas sobre la superfice geoidal, ademas tiene .en cuenta el efecto de las masas de compensaci6n y genera deformaciones en las superficies equipotenciales menores que las de la reduccion bouguer, ocasionando un efecto indirecto inferior a las ondulaciones geoidales (pick et ai., 1973; torge,1983). 3.1 calculo de la anomalfa isostatica partiendo de la anornalfa bouguer simple que asume valores altamente positivos en la corteza oceanica y fuertemente negativos en la continental y habiendo eliminado la influencia de las masas topograticas, se deduce que la variaci6n en los valores de anomalfa obedece a la presencia, de otras masas no tenidas en cuenta que son, en terminos regionales, masas de compensacion. ai presentarse la condicion de equilibrio isostatico de la corteza la diferencia entre la anomalfa bouguer observada y la regularizacionde esta por medio de un modelo matematico debe ser cero; en caso contrario la condicion de equilibrio isostatico no esta plenamente satisfecha. de acuerdo con tsuboi (1983), esta diferencia es la anomalfa lsostatica. la forma mas apropiada de obtener la anomalfa bouguer calculada es a traves del analisis discrete de fourier. los datos de anomalfa bouguer simple contenidos en el mapa pueden representarse mediante la serie armonica: [4] [5] donde: cgmn: son los coeficientes de fourier calculados a traves de la transformada directa y equivalen a: cg =_1_", '" t:j.g e-/limn" ezkmn [6] mn mn7.tr /k kmn: es el vector de onda r: es el vector posicion m,n: corresponden al nurnero de puntos muestra en las direcciones x e y. para el procesamiento de los datos se establece una grilla de 64x64 (mxn)con intervalos de 0.25° que corresponden a la dimension de la grilla y al intervalo de muestreo respectivamente. en las zonas sin informacion gravimetrica se asume la anomalfa bouguer igual a cero para evitar el efecto gibbs (dlaz, 1983). en la eliminaclon de las frecuencias altas se utiliza un filtro de paso bajo (dlaz, 1983) que permite obtener senates con frecuencias bajas, longitudes de onda y perfodos largos. en la seleccion de la funci6n filtrada se procure elegir aquella en la cual se eliminan los cambios locales en la anornalla, sin filtrar en exceso la serial, de modo que no obedezca a condiciones continentales que resultan bastante generalizadas para el area en estudio. en la serial filtrada (fig. 2) se eliminan las anomalfas con frecuencias altas, no se observan cambios locales de gravedad y la tendencia no es excesivamente regioncll la tuncion filtrada evidencia en forma general los procesos geotect6nicos que se manifiestan en las variaciones gravimetricas del pafs, proporcionando valores residuales (anomalfa isostatica) representativos para zonas amplias, asf como longitudes de onda largas, perfodos amplios y frecuencias bajas, que son caracterfsticos de la anomalfa isostatica. 3.2 anomalla isostatica .,' la tendencia general de la anomalfa isostatica en colombia (fig. 3) es similar a la de la anomalfa bouguer simple, su valor maximo es de +13 mgal sobre la sierra nevada de santa marta, y el mfnimo 8s de -18 mgal en el . departamento del huila. la magnitud de la anomalfas decrece rapidamente en sentido woeen la zona del pacffico y sobre las cordilleras (5 mgal en 28 km). en la region andina se presentan los valores mfnimos en los departamentos de cauca y huila(-18 mgal), ademas del alargamiento de las curvas en sentido sw-ne en la direcci6n de las cordilleras presentando un gradiente estrecho hacia el sur (5 mgal en 22 km) el cual se ensancha en el magdalena medio (5 mgal en 56 km). sobre el area selvatlca y los llanos orientales, el gradiente es bajo ancho con cam bios suaves de gravedad (5 mgal en 140 km). se destacan como rasgos particulares de mayor importancia los valores altos en la sierra nevada de santa marta (sobre +10 mgal), valores altos negativos en la zona andina (-15 mgal) y costa pacffica (-13 mgal) debido a procesos geotectonicos. 4. determinacion de las ondulaciones geoidales la f6rmula de stokes (ec. [4]) se plantea en terminos de una integral sobre toda la superficie terrestre. ai estudiar un area especffica es necesario sustituir tal integral por una sumatoria de elementos finitos, los cuales se obtienen subdividiendo el area de interes en compartimientos pequefios (k). sa utili zan compartimientos rectangulares formados por la intersecci6n de meridianos y paralelos definidos por un sistema fijo de coordenadas. tarnbisn se requiere la determinaci6n de anomalfas medias en cada elemento de area. tomando la ec. [4], para un compartimiento se tiene: n "" .b: [ ag s(1\1) do 41t g 0 y, para todos los compartimientos: r j'n"" -l ag s(lh) do 41t g k 0 asf: donde: la funci6n de stokes cuantifica el efecto de las anomalias gravimetricas-en este caso la anomalia lsostaticasobre la ondutacion geoidal en el punta de calculo, en otras palabras, actua como una funci6n ponderadora. esta funci6n presenta inconvenientes en la cercania del punta de calculo, ya que alii tiende al infinit~; para r.esolver este problema se definen dos zonas de influencia, una interna, la mas proxima al punta de calculo, y una externa, en torno a la anterior. la contribuci6n total estara dada por (heiskanen & moritz, 1967): n = n, + n. [9] la zona interna se asume plana y su dimensi6n aumenta hasta donde el error en que se incurre por este supuesto deje de ser despreciable. en este trabajo la distancia esferica que define dicha zona es de 0.5° equivalente a 55.3 km referida al elipsoide wgs-84. la contribuci6n de esta zona es: [10] es la distancia plana en metros entre el punta de calculo y el compartimiento (k). la zona externa esta delimitada por la distancia esterica 15.75° a la cual se encuentran los valores de anomalia mas distantes; estos son ponderados por la funci6n modificada de stokes (ec.[11]), que resulta de la norrnallzacion de funci6n original (ec.[3]) (pick et ai., 1973) yes valida para longitudes menores de 25° (heiskanen & vening meinesz, 1958): [11] la funci6n modificada de stokes presenta variaciones pequefias dentro de cada compartimiento permitiendo ~omar el valor correspondiente al centro como el representative. asf, se tiene: la integral final corresponde al area del compartimiento. por tanto: [7] [12] [8] finalmente, sustituyendo las ecs. [10] y [12] en las ecs. [7] y [9) se tiene la contribuci6n total de las anomalias en la ondulaci6n geoidal en cualquier punta: anteriormente se expuso que (g) y (r) corresponden a valores medios de gravedad y radio terrestre respectivamente; sin embargo, se pueden calcular para cad a punta de estudio. asi, el valor de (g) estara dado por la gravedad teorlca (sobre el elipsoide wgs 84) en el punta de calculo y el radio (r) sera su radio geocentrico (ramirez, 1980). 12.25 1'1.26 10.25 9.25 8.25 7.25 6.25 0 :::j i5.25h i<{ .j 4.25 3.25 2.25 1.25 0.25 -0.75 -1.75 -2.75 lcngitud ~ ~ ~ ~ s s s ~ ~ ~ ~ ,~ ~; s s ~. . . .<sl 0co t'-. >.(j lj) "t" ("') (\j ... ~ (). co "-c (0 (j) r-, r-, '" r-, r-, r-, r-.. 1'-. r-, <, '.(1 -o >d 'c, i i i i i i i i i i i i i i figura 1. anomalia de bouguer simple (kellog et ai., 1983) 12. ~~5 11.~~5 10. ~~5 9. ~~5 8. ~~5 7. ~~5 6. ~~5 0 ::j f5. ~~5h f<i: ...j 4. ~~5 3. ~~5 2. ~~5 1.~~5 0.25 -0.75 -1.75 -2.75 l.cngitud s :~ ~ 8 ~ 8 ~ s s 8 8 ~ ~ 2 ~ s. ..... q o· oj r-, -.0 u) -r m (\j .... & 0'co r-, -o ld (ii r-, r-, 1"r-, r-, r-, ro. ror-, r-, .o '-c1 ...0 -il i i i i i i i i i i i i i i i figura 2. anomalia de bouguer filtrada �iti 12,25 11.25 10.25 9.25 8.25 7.25 6.25 0 :::j i5.25~ i--c -j 4.25 3.25 2.25 1.25 0.25 -0.76 -1.76 -2.75 jx'.jgi tlc s s ~ ~ s ~ ~ ~ ~ ~ ~ ~ s ~ ~ ~. . . c:....... ~ oco r-, '0£) ij) '<l"' t1 (\j ~ (s) o:d r-, -o ([) r-, '" r-, r-, r-, r-, r-, r-, '" '" -o '0(1 -o -o i i i i i i i i i i i i i i i i i i i i i i i i . i i i i . i it i i i i i i i , i i i i mtt r i i i i . i i ii, i i i i ~tt figura 3. anomalfa isostlitica 12.25 11.25 10.26 9.25 8.26 7.25 6.25 0 :j i6.26h i-c ..j 4.25 3.26 2.25 1.25 0.25 -0.75 -1.76 -2.75 _cngitlc ~ ~ ~ ~ s s 8 ~: ~ ~ s ~ 8 s 8. . . .:e 0cd i"-o ii) ..,.. f1 (\i ~ 0 c' id r.... 10 i"r-, r-, r-, '" t-.. l"r-, r-, i"..:j -o ..0 i i i i i i i i i i i i i i i .-0 -o i figura 4. ei geoide en colombia (m) 5. el geoide en colombia la figura del geoide (fig. 4) (martinez et at, 1993) mantiene en general la tendencia regional de la anomalfa bouguer simple a travss del pais. este hecho se verifica con el coeficiente promedio de correlaclon cruzada con sobrecubrimiento del 100% (davis, 1973) entre el mapa de ondulaciones geoidales y el de anomalia bouguer simple, estimado en 0.81, proporcionando una incertidumbre de ±19% en las alturas geoidales obtenidas. la longitud de orida minima esta al norte del macizo colombiano en la zona de desprendimiento de las cordilleras central y oriental, lugar donde aparecen las ondulaciones maxirnas negativas (-15m). la longitud de onda predominante oscila entre 500-600 km; los valores maxirnos positivos cercanos a +15m se aprecian en el oceano pacifico sobre la longitud 79° w y latitud 5° n, regi6n donde los valores cambian rapldarnente en sentido woe hasta -5m en ta costa del choc6 (1m de ondulaci6n en 53 km). sobre la sierra nevada de santa marta se observa la transicion de -5 a +5m de ondulaci6n a traves de 68 km woe, mientras que en las zonas marinas los valores oscilan en torno a a m. en el sur de la region andina el gradiente entre curvas es alto, en sentido woe los vafores varian entre -15 y 5 m (1 m de ondulacion en. 7.3 km). hacia el norte entre los departamentos de boyaca y norte de santander los cambios son suaves en torno a -5 m, el gradiente es de 1 tn de ondulacion en 23 km. sobre el area de lianuras y selvas predornina el valor de am para la ondulacion geoidal, excepto en el departamento del caqueta que contiene valores cercanos a +5 m. 6. conclusiones se construyo numericamente el mapa de anomalia lsostatica, que provee informaci6n sobre el comportamiento tectonico e isostatico de la corteza. ei rnetodo empleado plantea una formulaci6n alterna de dicha anomalia y sugiere desarrollar nuevos trabajos sobre la misma en forma tal que se confronte su veracidad. ei comportamiento de los contornos de anomalia lsostatica y ondulaciones geoidales permite concluir que una vez obtenida la magnitud de dicha anomalfa se tiene una nocion bastante aproximada de la forma del geoide, independientemente de la superficie de referencia empleada. los valores rnaxtmos y minimos obtenidos para las ondulaciones geoidales corresponden al intervalo de estos para colombia en el geoide wgs-84. lo anterior sugiere que el modelo establecido para la aproxirnacion del geoide se ajusta al modelo mundial. ei modelo geoidal final permite observar el comportamiento del geoide wgs-84 (wh,te et at, 1990) detalladamente sobre el area de estudio. con respecto al geoide rnundial, las curvas obtenidas presentan deformaciones frente a sus equivalentes, como consecuencia de la diferencia entre los volurnenss de informacion procesados y la escala del trabajo. los resultados obtenidos contribuyen a mejorar la precision de las mediciones realizadas mediante el sistema gps. siendo el datum wgs-84 el mas reciente y por tanto el mas avanzado, es necesario que a el se refieran los trabajos geodesicos y geofisicos en el pais. bibliografia davis, j. c. 1973. data analysis in geology. 549pp. wiley, u.s.a. diaz, r. 1983 tratamiento cuantitativo de datos aerornaqneticos. santiago, 1983: il. tesis (magister en ciencias). universidad de chile. departamento de geologia y geofisica. --1992. apuntes de geofisica: analisis de fourier en geofisica.' 7pp. universidad nacional, bogota. d.m.a. 1984. technical report. part two: supplement to departament of defense. world geodetic system 1984. en: parameters, formulas and graphics for the practical application of wgs-84. pp.601-610. heiskanen, w. a. & h. moritz. 1967. geodesia fisica. 395p. san francisco: freeman. heiskanen, w. a. & f.a. vening meinesz. 1958. the earth and its gravity field. 470pp. mcgraw-hili, n~w york. kellog, j., v. godley, c~ ropain & a. bermudez. 1983. gravity anomalies and tectonic evolution of northwestern south america. memory of the x caribbean geological conference. ingeominas, bogota. . martinez, w., g. rocha & l. sanchez, 1993. primera aproximaci6n .a la figura de geoide·. en . colombia. universidad distrifal "francisco jose de caldas",'facultad de ingenieria, prograina de ingenieria catastral y geodesia. santate de bogota. " pick, m., j. picha & v, vyskocil. 1973. theory of the earth's gravity field. 538pp. elsevier, praqa .. ramirez, r. 1980. geodesia geornetrica. 12pp. universidad distrital, bogota. .. torge, w. 1983. geodesia. primera edicion. 297 p. diana, mexico. tsuboi, c. 1983. gravity. 254pp. allen & unwin, london. . vanicex, p. 1971. physical geodesy (preliminar copy). 176pp. university of new brunswick, new brunswick. white, h., l. decker, m. kumar & defense mapping agency. 1984. sistema geodesico mundial-1984. un marco de referencia mundial, moderno y preciso. revista cartoqrafica. no. 58 (julio/diciembre, 1990); p 65-84. instituto panamericano de geograffa e historia. zakatov, p. s. 1981. curso de geodesia superior. 633pp. mir, moscu. khaksar and maghfouri.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 57-79 paleontological study of the echinoderms in the qom formation (central iran) keyvan khaksar islamic azad university, qom, iran. k1khaksar@yahoo.com iraj maghfouri moghadam lorestan university, khorramabad, iran. irajmmms@yahoo.co.uk abstract the qom formation was formed in the oligo-miocene during the final sea transgression in central iran (figure 1). the best outcrop is located in the vicinities of the qom city, approximately 130 km at the south of tehran. in general, the great heights of the zone are the result of intense tectonic activities. these heights have a number of faults and folds. echinoderms are one of the most important and numerous fossil groups present in the qom formation and confirm the relationship of this environment with free waters. in the present investigation more than 100 prepared samples were studied and 17 species were identified, scanned and classified. these fossils are more abundant in the upper part of the a member, which illustrates the abundance and diversity in c1 and c3 sub-members belonging to the c member. to classify these samples, classical and up-to-date methods were used. however, the systematic schemes were used more frequently (moore, 1966; 1969-1971). besides these studies, the other concomitant microfossils in the formation were investigated simultaneously to estimate the accurate age of them. it is concluded that the study of oligo-miocene echinoderms present in the qom formation is essential and important because, at the same time, the central iran sea had a communicative role between the indo-pacific ocean and the mediterranean sea. key words: central iran, oligo-miocene, limestones, echinoderms. resumen la formación qom se formó durante el oligoceno-mioceno durante la transgresión final del mar en irán central (figura 1). el mejor afloramiento se encuentra localizado en los alrededores de la ciudad de qom, aproximadamente a 130 km al sur de teherán. en general, los altos pronunciados de la zona son el resultado de la intensa actividad tectónica. estos altos poseen un gran número de fallas y pliegues. los equinodermos son los fósiles más importantes y numerosos que se encuentran en la formación qom y confirman la relación de este ambiente con el agua. en la presente investigación más de 100 muestras fueron preparadas y estudiadas, y de las mismas se identificaron y clasificaron 57 earth sciences research journal manuscript received december 15 2006. accepted for publication june 10 2007. introduction the marine qom formation was deposited in the oligo-miocene and is the result of the last transgression of the sea in central iran. the formation contains five members containing limestone interstratified with marlstone and deposited during three sedimentary cycles. the mean thickness of this formation is approximately 900 meters in the vicinities of the qom city. this formation consists of the following five members (figure 2): a member the a member has an average thickness of 55 meters and the thickness of its beds varies between 2 and 200 cm. this member contains several parts starting from the base: limestones with muddy texture without fossils. limestones (packstone) containing benthonic foraminifera. limestones (grainstone) containing oolits and bioclasts. limestones (packstone) with foraminifers, bryozoans, red algae, gastropods, bivalves, and echinoderms. sandstones with glauconite containing remnants of complete echinoderms, bryozoans, and sedimentary structures such as cross-bedding. b member it is composed by 225 meters of thick layers of sandy marlstones and sandstones intercalations (with cross-bedding) and containing glauconite, which is the cause of the green colour of marlstones. the fossils commonly observed • • • • • in this part are benthonic and planktonic foraminifers. c member the c member has an average thickness of 370 meters and has the following sub-members: c1: composed by marlstones interstratified with limestones. the thickness of the bed is between 10 and 200 cm. the limestones have the following texture starting from the base: packstone with abundant fossil assemblages of echinoderms, bivalves, bryozoans, red algae, gastropods, and corals. boundstone containing bryozoans, corals, and red algae. c2: c2 illustrates the termination of the primary sedimentary cycle. it contains the following textures: mudstones with plenty of organic materials. layers of gypsum. c3: this sub-member is composed by limestones. the thickness of the bed is between 5 and 300 cm and composed by the following textures starting from the base: grainstone containing oolits, bioclasts, and sedimentary structures such as crossbedding. packstone and boundstone with echinoderms, corals, bryozoans, and red algae. packstone with shell fragments of bivalves and foraminifers. c4: composed by marlstones containing foraminifers, corals, and internal moulds of gastropods. catapsydrax stainforthi bolli, loeblich, and tappan are also found in this • • • • • • • paleontological study of the echinoderms in the qom formation (central iran) 58 17 especies. esto fósiles son más abundantes en la parte superior del miembro a, el cual ilustra la abundancia y diversidad en los sub-miembros c1 y c3 pertenecientes al miembro c. estas muestras fueron clasificadas por medio de los métodos recientes y clásicos. sin embargo, los esquemas sistemáticos fueron usados con más frecuencia (moore, 1966; 1969-1971). aparte de estos estudios, los otros microfósiles concomitantes en la formación fueron investigados simultáneamente para estimar su edad exacta. se concluyo que el estudio de los equinodermos del oligoceno-mioceno presentes en la formación qom es esencial e importante porque, al mismo tiempo, el mar de irán central permitía la comunicaión entre el océano indo-pacífico y el mar mediterráneo. palabras clave: irán central, oligo-mioceno, calizas, equinodermos. khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 59 figure 1. distribution of oligo miocene marine sediments in central iran.. sub-member belonging to burdigalian. d member this member illustrates the qom formation’s second termination cycle and consists of 22 meters thick layers of gypsum. e member it is composed by 230 meters of thick layers of marlstone intercalated with limestone. the fossils commonly observed are: foraminifers, red algae, gastropods, bivalves, and bryozoans, with following textures (limestone) from the base: packstone with bryozoans, red algae, gastropods, and corals boundstone containing stromatolites, corals, and bivalves grainstone containing bivalves, red algae, and benthonic foraminifers. • • • biostratigraphy because of macro and microfossils of the qom formation such as globorotalia (turborotalia) opima opima bolli present in the member b and catapsydrax stainforthi present in the c member. this formation has been known chronologically. figure 3 shows the vertical distribution of echinoderms. the existence of some echinoderms, echinodiscus balestrai and clipeaster folium michelin, in the lower part of the formation gives it an age of medium-superior oligocene. the decrease in the number of echinoderms towards the upper part of the formation is due to the great migration of these species towards 60 figure 2. stratigrafic section of the qom formation. paleontological study of the echinoderms in the qom formation (central iran) 61 figure 3. vertical distribution of echinoderms in the sediments of the qom formation. the north in burdigalian sup. because earth’s temperature was generally high in burdidaliense (demarcq, 1984). however, the temperature in burdigalian sup. rose even further (flower and kennet, 1994). taphonomy in several places of central iran, the layers are steeps because of folding and tectonic activities, and it takes a long time to identify top and bottom sedimentation layers. in these cases, the presence of echinoderms is one of the best criteria to recognize the polarity and also reflects a calm sedimentary environment. sometimes, it could be observed that the layers, exclusively composed by this genus of echinoderms (scutella and clypeaster), are already erosive. this can be seen in sandstones of the upper member a and sub-member c1. these layers demonstrate turbulent movements at the bottom of the sea. this secondary echinoderm accumulation illustrates the periodic sea storm, which causes erosion of fine sediments and echinoderm fragment accumulation (figure 4). due to the irregular base of the layers, each bed could be originated from remaining sediments (lag) of turbulent sea beds, demonstrating a transgressive phenomenon in the sedimentary basin. geological setting the qom province is part of the central iran zone, in which sub-parallel mountain ranges and plains have general northwest-southeast trend. khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 62 figure 4. formation of beds made of scutella fossils. paleontological study of the echinoderms in the qom formation (central iran) the north-western plain is the terminal part of the qom-ardekan depression, divided into two parts by the small hills of the koshk-e-nosrat heights located to the north of the howz-esoltan lake. the central parts of the province have hillocky topography, while the western and southwestern parts are mountain regions belonging to the urumiyeh-bazman volcanic belt, with some summits up to 3000 meters. the oldest known rocks of this province are of eocene age, which are younger volcanic rocks exposed in western parts of this province and belong to the urumiyeh-bazman volcanic belt. these magmatic rocks can be divided into three zones: ashtian-naragh, tafresh and saveh. the tafresh member has a central position with respect to ashtian-naragh (south) and saveh (north) members. this part is characterized by an important sedimentation and subsidence while the saveh member is characterized by important upper eocene magmatic activity, and the ashtian-naragh member is featured by its high volcanic activities of the neogene. orogenic movements in the late eocene-early oligocene resulted in the creation of lagoonal to continental sedimentary regime characterized by detritic-evaporitic sediments of the lower red formation. local and lateral facies changes are all related to inherited local changes of topography. lagoonal evaporites of rather considerable thickness represent high rate of subsidence in this continental environment (tehrani, 1989). oligo-miocene carbonates of the qom formation represent marine transgressions in this province with marly-calcareous sedimentation. during this period volcanic activities continued locally, represented by analcite bearing basanite and andesites. in the late miocene, thick lagoonalcontinental deposits of upper red formation were replaced by the marine qom formation with two distinct neogene volcanic activities (bozorgnia, 1965). pliocene sequences of qom formation have various facies. they are comprised by three volcanic-subvolcanic, volcanic-sedimentary and detritical units. plutonic igneous bodies, which intrude into the urumiyeh-bazman volcanic rock, are other characteristical feature of the qom province. these intrusive bodies have definitively a conspicuous relation with fracture zones. the intrusive bodies are either extensive or small. the qom province is still an active tectonic environment materialized by active seismicity. the late movements, with general direction of north-northeast compression, configure the actual structural feature of this province. paleogeography considering the results of the present study and those of the previous works in the oligomiocene outcrops in different regions of iran and their environments, it is attempted here to reconstruct the paleogeography of this region. as a result of the tectonic movements of the upper cretaceous and the rise of northern region of the alborz (tehrani, 1989), two different sedimentary basins have been formed: one in the north of the alborz (pentocaspiana basin) that is situated in the present zone of the caspian sea, 63 and the other is located in the southern part of the same mountain range (in central iran). in central iran, the eocene and lower oligocene are characterized by continental sediments (lower red formation). the basal contact of the transgressive deposits of oligo-miocene age with the l.r.f. is marked in many zones by an angular discordance that indicates the influence of the tectonics movements before the marine transgression (bozorgnia, 1965). during the oligocene (chattian) a great channel connecting the mediterranean and the dominion indian-pacific through the south of iran was formed, separating africa and euro-asia. in the miocene (burdigalian), rotations and vertical and horizontal movements of the lut block and arabian plate, and the impact between the latter with turkey, caused the interruption of connection between the mediterranean and the indian ocean during the burdigalian (steininger and rogl, 1984). the sea invaded a part of turkey (adana and karaman), where the outcrops are formed generally by limestones with lithothamnium. in syria, the deposits of the burdigalian have high contents in pecten, clypeaster, and macroforaminifers. the extension of these deposits covers a large area of syria and from mesopotamia to iraq, and from there to the persian gulf. these indicate that a sea passage existed between the mediterranean and the indian ocean. this channel was interrupted in the upper burdigalian because of the orogenic movements that affected the mediterranean coasts and iraq. in the northern zone of the mediterranean, the rzehakia (oncophora, bivalvia) marine series, containing molluscs of the burdigalian, are extended from switzerland to the aral sea. they are attributed to shallow basins, which are isolated and interconnected by a partial connection with the open sea and influenced by continental environments. the langhian is characterized by great geodynamical instability and a sediment substitution of the shallow marine condition by continental sediments in the arabian plate and mesopotamia (buchbinder and gvirtzman, 1976). the last marine regression took place during the langhian in central iran. paleoclimatology skeletal and non skeletal sediment particles were studied and could be summarized as follows: skeletal particles the large number of fauna such as corals and macroforaminifers in upper parts of the c and e members indicate high temperatures. macroforaminifers in the qom formation such as operculina, miogypsina, miogypsinoidi, lepidocyclina, heterostegina, amphistegina, and spiroclypeus are representative of warm sea environments (boltovskoy and wright, 1976; allasinaz, 1983; demarcq, 1984; lauriat et al., 1993). the presence of rotalia viennoti and borelis in limestones illustrates tropical climates (delanoe et al., 1976; lauriat et al., 1993). the presence of red algae (lithophylum, mesophyllum, sporolithonn, and lithothamnium) also represents tropical climate (bosence, 1983). macrofossils such as scutella, clypeaster, echinolampas, amphiope, and maretia represent warm waters. maretia is an indicator of tropical climates, as observed in the indian and pacific domains (llompart, 1983; lauriat et al., 1993). the presence of bio-constructions and hermatipic corals in several layers (e.g., upper layers of the c1 submember) confirms that the temperature ranged from 18o to 30oc (minnery et al., 1985). the presence of bivalves such as amusium and spondylus is representative of warm sea environments (demarcq, 1979; turek et al., 1988). non-skeletal particles these particles such as oolits indicate warm and dry climates belonging to sea environments where evaporation exceeds rainfall (reijers et al., 1983; zeng et al., 1983). oolits are formed at temperatures above 15oc and minimum salt content of 36 percent (lees, 1975). it seems that after the cold lower-oligocene, the temperature in the superior-oligocene increased, reaching its optimal temperature in the burdigalian. then, the temperature suffers khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 64 paleontological study of the echinoderms in the qom formation (central iran) a sudden decrease in the middle miocene (demarcq and pouyet, 1990). in conclusion, it could be understood that the sediments of the qom formation were formed in tropical and subtropical environments. conclusions firstly, the presence of echinoderms in the qom formation reveals a passage to the sea. the presence of echinoderms in the qom formation is one of the best criteria to recognize polarity and also reflects a calm sedimentary environment. the presence of the echinodiscus balestrai oppen and clipeaster folium michelin in the base of formation verifies the age of middle and upper oligocene in these sediments. the recognized genus in this formation represents warm waters. the most important and abundant echinoderms are scutella and clypeaster belonging to the coastal regions and their morphologies indicate energetic environments. as mentioned in the a member, especially for the upper sandstones and the c1 submember, there are layers containing echinoderms. this concentration may relate to storms and turbulence in the sea bed. occasionally and concurrent with the formation of layers of scutella, the content of glauconite increased in the sandstones. this event corresponds to an increase of the depth of the sea and illustrates a transgressive case. in the upper part (b member), the sediments changed to sandy marls containing planktonic foraminifers and glauconite as a result of the transgression. their abundance of echinoderms and variation decreased in upper parts of the formation. this could be attributed to higher temperature in the upper burdigalian, resulting in their migration from central iran. after the sudden drop of temperature in the lower oligocene, the global temperature began to increase. the burdigalian had a warm climate and the temperature increased even further in the upper burdigalian. • • • • • • • • • the presence of corals and foraminifer fossils represents warm climates. systematic class echinodermata subclass euechinoidea superorder gnathostomata order clypeasteroida suborder clypeasterina family clypeasteridae genus clypeaster lamarck, 1801 clypeaster aff. scillae desmoulins pl.1, fig.1 1837 clypeaster scillae desmoulins, p.64. 1901 clypeaster crassicostatus, airaghi, p.35, pl.ii/5, iv/1 1958 clypeaster scillae, smedile, p.35, pl.iii/3 1966 clypeaster scillae, moore, p.462 1984 clypeaster scillae, demarcq, pl.xii/1 material 9 samples distribution miocene inferior of almost all studied profiles. clypeaster aff. folium michelin pl.3, fig.2 1859 clypeaster folium michelin, pl.xx/2 1915 clypeaster martini cottreau, p.98, pl.xi/1-4 1920 clypeaster marginatus forteau, vol.2؛ 1958 clypeaster folium, smedile, p.32, pl.xi/3 material 1 sample distribution oligocene of dochah clypeaster biarritzensis cott. var. trotteri gregory pl.6, fig.2 1891 clypeaster biarritzensis cotteau, ii, p.229 1911 clypeaster biarritzensis var. trotteri gregory. p.662, pl.xlvii/1 1913 clypeaster biarritzensis var. trotteri, fabiani y stefanini, p.78 1921 clypeaster biarritzensis var. trotteri, stefan. p.126, pl.xvii/7 material 1 sample distribution oligocene of shurab. • 65 suborder scutellina superfamily scutellidea family scutellidae genus scutella lamarck, 1816 scutella subrotundus (leske) pl.2, fig.1 1841 echinodiscus subrotundus leske (1778) en agassiz, p.5 1917 ? lambertiella checchiarispoli, p.57 1966 scutella subrotunda, moore, p.477, fig.366/1 1981 scutella subrotunda, kalantari, pl.61/1-4 material 9 samples distribution oligocene and miocene inferior of almost all the profiles. family astriclypeidae genus echinodiscus leske, 1778 echinodiscus balestrai oppenh pl.3, fig.3 1939 echinodiscus balestrai, stefanini, p.127 material 2 samples distribution oligocene of bichareh and dochah. genus amphiope agassiz, 1840 amphiope bioculata (desmoulins) pl.3, fig.4 1815 scutella bifora var.3 lamarck, p.10, n7؛ 1837 scutella bioculata var. a tipus desmoulins, p.232, n23؛ 1906 amphiope bioculata, lambert, p.50 1948 amphiope bioculata, mortensen, p.413, fig.243 1966 amphiope bioculata, moore, p.489, fig.374/1 1983 amphiope bioculato, liompart, p.70, pl.1-3 material 1 sample distribution burdigalian of eidajti. superorder atelostomata order cassiduloida family echinolampadidae genus echinolampas gray, 1825 echinolampas cfr. vilanovae cotteau pl.4, fig.1 1890 echinolampas vilanovae cotteau, p.73, pl.ix/1-5 1981 echinolampas cfr. vilanovae, kalantari, pl.63/1-2 material 14 samples distribution burdigalian the kamar kuh, navab, eidajti, bichareh. echinolampas (macrolampas) discus desor pl.5, fig.1 1858 echinolampas discus desor, synopsis, p.307 1877 echinolampas discus, dames, echiniden. p.43, pl.iii/1 1919 echinolampas discus, stefanini, ii, p.17 1935 echinolampas (macrolampas) discus, venzo, p.230, pl.xix/2,3 material 8 samples distribution miocene inferior of the central part of the basin. superorder echinacea order echinoida family echinidae genus psammechinus agassiz y desor, 1846 psammechinus affinis fuchs pl.7, fig.2 1972 psammechinus affinis, naini. p.223, fig.77,78 1981 psammechinus affinis, kalantari, pl.63/3-5 material 3 samples distribution burdigalian of shurab and bichareh. order phymosomatoida family phymosomatidae genus micropsis cotteau, 1856 micropsis aff. tremadesi cotteau pl.7, fig.3 1890 micropsis tremadesi cotteau, p.96, pl.xv/3-6 material 2 samples distribution aquitanian of eidajti and shurab. superorder atelostomata order spatangoida suborder micrasterina family spatangidae khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 paleontological study of the echinoderms in the qom formation (central iran) genus spatangus gray, 1825 spatangus corsicus desor pl.7, fig.1 1869 spatangus corsicus, taramelli, p.2176, pl.ix/1-2 1877 spatangus corsicus, cotteau in locard. p.333, fig.1-3 1885 spatangus hemiornatus mazzetti y pantanelli, p.62, pl.i/3 1901 spatangus corsicus, airaghi. p.215 1919 spatangus corsicus, stefanini. p.139, pl.xiv/6 1967 spatangus corsicus, menesini, p.153, pl.ii/6 material 4 samples distribution burdigalian of nardaghi. genus maretia gray, 1855 maretia aff. aragonensis cotteau pl.4, fig.2 1988 maretia aragonensis, gomez, p.640, pl.31/6 material 2 samples distribution miocene inferior of bichareh and do baradar. suborder micrasterina family loveniidae genus breynia desor, 1847 breynia aff. australasiae (leach) pl.8, fig.2 1815 spatangus australasiae, leach, p.68, pl.82 1858 breynia crux-andra, desor, p.408 1891 breynia australasiae, ramsay, ll, p.37,55 1951 breynia australasiae, reitzel, p.63 1946 breynia australasiae, clark, p.381 1951 breynia australasiae, mortensen, ll, p.132, pl.x,xi,xii 1966 breynia australasiae, moore, p.613, fig.499/2 material 3 samples distribution aquitanian of do baradar, kamar kuh and bichareh. genus lovenia (vascoaster) lambert, 1915 lovenia (vascoaster) sulcatus (haime) pl.8, fig.1 1853 breynia sulcatus haime, p.216 1951 lovenia (sarsella) sulcata, mortensen, p.95, fig.44/a 1966 lovenia (vascoaster) sulcatus, moore, p.613, fig.498/3 material 4 samples distribution burdigalian of do baradar and nardaghi. suborder hemiasterina family schizasteridae genus schizaster agassiz, 1836 schizaster cfr. beloutschistanensis d`archiac y haime (1853) pl.6, fig.1 1981 schizaster cfr. beloutschistanensis, kalantari, pl.65/13-15 material 12 samples distribution burdigalian of nardaghi, navab, bichareh. schizaster aff. vilanovae cotteau pl.5, fig.2 1890 schizaster vilanovae cotteau, p.38, pl.iv/10-13 material 8 samples distribution aquitanian of do baradar, nardaghi, bichareh and jorabad. subclass cidaroidea order cidaroida family cidaridae genus prionocidaris agassiz, 1863 prionocidaris sismondai (mayer) pl.6, fig.3 1907-8 prionocidaris sismondai mayer, v.34, p.142 1966 prionocidaris sismondai, moore, p, 330, fig.247/1h material 1 unit (incomplete). distribution burdigalian of jorabad. plates plate 1 1clypeaster aff. scillae desmoulins. aquitanian, do baradar. c1.2. 1a: apical view. 1b: ventral view 1c: lateral view. plate 2 1scutella subrotundus (leske). aquitanian, do baradar. 3a: c1.8, apical view 3b: c1.11, 66 basal view 3c: c1.11, apical view. plate 3 1scutella sp. aquitanian, eidajti. c1.2, 1a: apical view 1b: oral view. 2clypeaster aff. folium michelin. oligocene, dochah. a.4. apical view. 3echinodiscus balestrai oppenh. oligocene, dochah. a.5, 2a: apical view 2b: lateral view. 4amphiope bioculata (desmoulins). burdigalian, eidajti. c3.19, apical view. plate 4 1echinolampas cfr. vilanovae cotteau. burdigalian, kamar kuh. 5a: c3.a8, apical view 5b: c3.a8, lateral view 5c: c3.a10, apical view. 2maretia aff. aragonensis cotteau. lower miocene, bichareh. c1.3, 6a: apical view 6b: lateral view. plate 5 1echinolampas (macrolampas) discus desor. aquitanian, eidajti. c1.15, 1a: apical view 1b: lateral view. 2schizaster aff. vilanovae cotteau. aquitanian, jorabad. c1.14, 4a: apical view 4b: lateral view. plate 6 1schizaster cfr. beloutschistanensis d`archiac y haime. burdigalian, navab. 2a: c3.4, apical view 2b: c3.5, apical view 2c: c3.4, posterior view. 2clypeaster biarritzensis cott. var. trotteri gregory. aquitanian, shurab. a.3, apical view. 3prionocidaris sismondai (mayer). burdigalian, jorabad. e.6, fragment, lateral view. plate 7 1spatangus corsicus desor. burdigalian, nardaghi. 3a: c4.12, apical view 3b: c4.12, lateral view 3c: c4.13, apical view. 2psammechinus affinis fuchs. burdigalian, shurab. c3.4, 3a: oral view 3b: lateral view. 3micropsis aff. tremadesi cotteau. aquitanian, eidajti. c1.13, 4a: oral view 4b: lateral view. plate 8 1lovenia (vascoaster) sulcatus (haime). burdigalian, 1a: c3.4, do baradar, apical view 1b: c3.14, nardaghi, apical view 1c: c3.14, nardaghi, lateral view. 2breynia aff. australasiae (leach). aquitanian, kamar kuh. c1.19, 2a: apical view 2b: lateral view. plate 9 1individuals of the clipeaster and echinolampas in life position (sub-member c1), eidajti. 67 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 paleontological study of the echinoderms in the qom formation (central iran) 68 69 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 paleontological study of the echinoderms in the qom formation (central iran) 70 71 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 paleontological study of the echinoderms in the qom formation (central iran) 72 73 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 paleontological study of the echinoderms in the qom formation (central iran) 74 75 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 references agassiz, a. 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(1939). molluschi del giuralias della somalia, parte ii. 103-268, tav. xiiixxvii [xiv-xxviii]. palaeontographia italica. steininger, f.f., and rogl, f. (1984). paleogeography and palinspastic reconstrution of the neogene of the mediterranean and paratethys. in dixon, j.e and robertson, a.h.f. (eds.): the geological evolution of the eastern mediterranean, 659-668. springer-verlag, berlin. taramelli, t. (1868-1869). sopra alcuni echinidi cretacei e terziaru del friull. atti real. ist. veneto se. lett. arti, (3), 14, 2140-2178, 2 pl. tehrani, j.j. (1989). stratigraphy of iran and type sections of formations. universidad de teheran, 1977, 342 pages. turek, v., marek, j., and benes, j. (1988). la gran enciclopedia de los fosiles. artia, praga, 520 pages. venzo, s. (1935) i:fossili del neogene trentino, veronese e bresciano, ii: cefalopodi, gasteropodi, scafopodi, echinidi e celenterati— conclusioni. palaeontographia italica. 35. 201255. pl.17-19. zeng, y.f., lee, n.h., and huang, y.z. (1983). sedimentary characteristics of oolitic carbonates from the jialingjiang formation (lower triassic), south sichuan basin, china. in peryt, t.m. (ed.): coated grains. 176-187, springerverlag, berlin. 79 khaksar and maghfouri, esrj vol. 11, no. 1. june 2007 geotisica colombian a pp 46-52 diciembre de 2001 bogota d.c. issn 0121 ·2974n° 5 estacion digital sismologica portatil sobre telefonia movl] celular e internet carlos a. vargas-jimenez', jaime leon hincapie2 , sergio rincon boter02 resumen se propone la implernentacion de una esracion de campo conformada pol'un sistema de acondicionamiento y adquisicion de seriales de baja frecuencia sobre un computador portatil. esta estaci6n se encarga de generar eventos en diferentes modos y los publica en la red de telefonfa m6vil celular e lnrernet medianre paginas activas; su configuraci6n se realiza direcramente en la estaci6n 0 remotamente mediante sockets tcp-ip. ei software de la estacion permite la detecci6n de eventos sfsmicos empleando el algoritmo sta/lta, asf como la captura poi'perdida de rango, captura peri6dica y captura continua de los canales. el sistema de acondicionamiento y adquisicion de sefiales se diseno como un sistema embebido y orientado poi'com andos con base de tiempo real y memoria de almacenamiento propios. palabras clave: estacion sismologica, cdpd, aoquisicion de datos, transmision de datos abstract it was developed a portable station with a low frequency signal conditioning and acquisition system for seismic intentions. this station record evenrs in differenr modes and publish them in the cellular mobile telephone network via active pages. its setup is done directly in the station or remotely via sockets tcp-ip. the station software allows detection of seismic events using the sta/lta algorithm, as well as threshold detection, periodic capture and conrinuous channels capture. the conditioning and acquisition system was designed as an embedded command driven system with its own real time clock and storage memory. key words: seismological station, cdpd, data acquisitlon, data transmission introduccion las estaciones de campo permiten la captura de sefiales provenientes de sensores para registrar sismos, ni veles de agua en nos, temperatura ambiente, precipitaci6n y muchas otras variables, las cuales son transmitidas para su posterior analisis. en general, la mayorfa de las estaciones para prop6sitos de monitoreo sismol6gico estan basadas en la telemetrfa conven9ional y las aut6nomas (lee, 1989). en el primer caso, la exigencia de condiciones aceptables de transmisi6n no siempre es realizable (ausencia de linea de vista, costos elevados de retransmision, etc.), mientras el segundo tipo, exige el desplazamiento peri6dico de personas para la obtenci6n de la informaci6n. en el presente trabajo se propone una estaci6n portatil con conexi6n a la red de telefonfa m6vij celular. la estacion adquiere sefiales sismol6gicas para su posterior proceso y analisis, incjuido el almacenamiento, y generando eventos que pueden ser solicitados rernotamente. sistema embebido de adquisicion la captura de eventos sfsmicos se realiza par medio de un arreglo triaxial de ge6fonos, es decir, sensores que registran la velocidad del movimiento de la tierra en sus tres componentes e, n, z (este, norte, z). las sefiales sfsmicas analogas, asf como otras provenientes de universidad nacional de colombia. departamento de geociencias, maestria en geolisica, bogota. e-mail: cvarg as@ciencias.unal.edu.co universidad nacional de colombia. sede manizales. 46 laptop with cdpdmodem -i~~//.~~'"''' '-a=~:::gj=~ ~ i ~d=-----------, / analog signals / from sensors i embe doed i acquisition system figura i. estaci6n portatil • .~ljq, ciip m ~tn cioolllcdi'd i ..---..<...,r···-v....·"/'{' cop!) '. ... ( cellular telephoner~ network amps· ...., ~ tdma • figura 2. conexi6n en red de la esracion poriatil geoftsica colombiana, 5. diciembre de 2001 estacion digital sismologica portatil sobre telefonia movil celular e internet sensores de temperatura, inc li n ac ion , etc., son capturadas, acondicionadas y convertidas en seiiales digitales para su posterior proceso en el pc, en el cual se tiene el sistema de adquisicion embebido (sae), encargado de responder a las solicitudes recibidas mediante el puerto serial rs232. ei sistema sae esta compuesto de unidades de acondicionamiento y captura de seiial, proceso central (microcontrolador), memoria de almacenamiento, base de tiempo, interfaz serial y visualizador. -. //liquid crystal '\h) '\ dsplay ) ~" ~i~marsf:r~ rv' l.~y ~nramj ( real time (/ \ clock.~ , control process u0 ! cornands j~t / /, i' //signal cond~ioningadc~/ < ==:ji voltage .............................................................. / ., .. references ./', •.' ...... analog signals from sensors figura 3. diagrama funcional del sistema de adquisici6n ernbebido unload de control de proceso para el control e integracion de la adquisicion, a1macenamiento, visualizacion, base de tiempo y comunicacion se requiere un controlador con mediana velocidad de procesamiento, bajo consumo de energia, puertos para la conexion de perifericos e/s, interrupciones extemas, temporizadores y uart para la cornunicacion serial. para el sistema se escogio el microcontrolador atmel at89c55 debido a sus caracterfsticas: compatibilidad con el juego de instrucciones intel 805x que perrnite el uso de compiladores para la programacion en lenguaje c, memoria de programa tipo flash de hasta 20k, bajo cos to y facil adquisicion. adernas, el bus de direcciones del microcontrolador permite asignar posiciones de memoria a diferentes dispositivos, de igual forma como se hace en un pc. ei bloque de direcciones els (entrada/salida) de cada dispositivo se asigna de acuerdo con la cantidad de registros necesarios para su funcionamiento. una vez asignadas las direcciones para cada dispositivo, se debe generar la base para su activacion. en el diseiio la seleccion de dispositivo de acuerdo con el bloque de memoria de acceso actual se realiza mediante dispositivos logicos programables (pld). es necesario anotar que debido a que la arquitectura del microcontrolador maneja un bus compartido para el byte bajo del bus de datos y direcciones, se debe retener la parte baja del bus de direcciones una vez se genera acceso a los dispositivos mapeados en memoria, acondicionamiento y adquisicion las senales de mayor frecuencia por adquirir provienen de los geofonos con componentes espectrales hasta los 30 hz, pat'a los cuales la frecuencia de muestreo seleccionada es fs= 100hz. la resolucion digital escogida es de 16 bits, que gatantiza la adquisicion de seiiales sfsmicas usualmente limitadas a un geofisica colombiana, 5, diciembre de 2001 rango dinarnico de 60 db (lee, 1989) para eventos naturales, y hasta 154db en sismologfa de banda ancha para eventos muy debiles con perfodos de i a 300 s (lemaire, 1997). laadquisicion triaxial se debe realizarde forma paralela en los tres sensores, puesto que de esta rnanera no se introducen retardos entre las seiiales para cada eje. en este caso se empleo el convertidor sigma-delta acondicionador de seiial (ad7714) que tiene un nivel de ruido muy bajo. ei convertidor acondiciona la sefial de entrada (prearnplificacion, filtracion y digitalizacion), las entradas del sistema de adquisicion se irnplementaron como seiiales en configuracion diferencial, para las cuales es posible seleccionar entre fonnato unipolar 0 bipolar, segun el tipo de adquisicion necesaria. la funcion nominal de respuesta en frecuencia del filtro pasabajos en el convertidor ad (kester 200 i) esta dada por: i h (f ) i = i-.!.. x sen ( n .7[ . 1/ is) 1 3 n sen (7[ .1/ is) las referencias de voltaje son necesarias para proveer a cada convertidor ad un nivel de potencial muy estable en temperatura y con muy bajo nivel de ruido, que en este caso corresponde al ad780an, por su estabilidad buena de temperatura (3 ppmf'c), bajo nivel de ruido (loon y/ohz), bajo costa y una patilla de ni vel de voltaje proporcional para sensar la temperatura de la tarjeta. almacenamiento y reloj de tiempo real el aim: cenamiento de los datos debe ser del tipo volatil, con tiempo de acceso del orden de centenas de ns, con capacidad superior 0 igual a la cantidad disponible de direcciones del bus de datos de la unidad de control de proceso y bajo costo. para el efecto se utilize el integrado bq4842 que posee memoria ram estatica no volatil con capacidad de almacenamiento de hasta 10 aiios en ausencia de alimentacion, muy bajo consumo de energia, muy rapido tiempo de acceso, reloj en tiempo real, oscilador intemo y registros de calibracion de la hora para compensacion de desajuste por temperatura. comunicacion serial la informacion capturada por el sistema debe ser transmitida a un pc, el cual porta el software encargado de realizar el almacenamiento en la base de datos y el procesamiento. la velocidad de comunicacion entre la tarjeta sae y el pc seleccionado es de 19200bps para cumplir con los requerimientos de procesamiento en tiempo real. visualizacion ei sistema de adquisicion requiere visualizar el estado actual del modo de trabajo y detalles de captura, especialmente cuando se trabaja en el modo autonomo (captura periodica de los canales). para la implementacion de la visualizacion en el sistema se escogio el modulo de cristal iiquido de 16 caracteres x 4 lfneas, el cual emplea el controlador hitachi h044 780. implementacion se utilizo el cad protei para realizar el diseiio de la tarjeta sae, es decir, el diagrama esquernatico y la tarjeta de circuito impreso. en la figura 4 se detalla la conexion de los dispositivos empleados en sae. ei sistema es alimentado por una baterfa recargable de 12 y, 38 ah regulada a 5 v por el circuito, la cual pro vee suficiente corriente para permitir la alimenracion continuada por mas de 500 horas. 47 vargas et al. figura 4. diagrama esquernatico de la tarjeta sae ei nivej de ruido rms de la tarjeta se obtuvo a partir de la captura en modo uno de 500 muestras para el canal uno en el convertidor ad tres, modo bipolar y ganancia uno. la entrada fue con ectad a a una resistencia igual a 390 ohm, ja cual es la impedancia (parte resistiva) vista en el ge6fono empleado cuando la masa se encuentra anclada. ei paso minimo en esta configuraci6n es 76,29 ~ v, yel ruido obtenido es 41~ v como se ve en la figura 5, definiendo la resoluci6n efectiva (analog devices, 1998) como la relaci6n entre la maxima escala de entrada (2 x vref x gain) y el ruido rms de salida, se tiene: 2 x 2.5 x 1 re self = log 2 ( 0, 000041 ) 16,89bits 48 ei ran go dinarnico se define como la relaci6n de entrada maxima y entrada minima cuantificable por el ade. para este caso se tiene un ruido de cuantificaci6n menor al ruido rms, por 10 cual la entrada minima cuantificable es 41 ~ v, yel ran go dinamico es 5v dr = 20 x log ( ) = 101,72db 0.00004lv ei ruido del sistema se debe al ruido de cuantificaci6n y al ruido del dispositivo, asf como a la diferencia de potencial entre las tierras y el ruido inducido por la 16gica digital en la linea de alimentaci6n. ei disefio del circuito impreso es deterrninante en ei resultado de la adquisici6n, y por tal motivo se diseii6 el pcb con pianos de tierra analoga y geofisrca colombiana, 5, diciembre de 2001 estaci6n digital sismol6gica portatil sobre telefonia m6vil celular e internet ac estimate (mvrms) 10.0-41 dc estimate (mv) fo.1 08mode 1 event gain 'l , 8ipoiaradl ch 3 0,100 0,000 .::r-----tr---j~-t_-_+-___rli_-+-_+___,_-i_-+-__+--t_-_++____ii_-+-__+--_n____._+__r____li_-+-it 'e . ~ -0,100 c \mi\~n\i~vij11 ~ -0,200 ~--+-_+-f-+--+-__+--+---++--i--+--+--+--+---+--+--_+--+--+---+--+--+ e; : <, -0,300 c--+-__+-+-+--+-__+--+---+--i:--+---+--+--+---+--+---+---t--_+---j--t---o..qooo~o6' 0.25"6)50 0,75",',00",.25",.50",.75"2):11)"2.25"2.50"2.75"ioo i2s·'3.s0"i7s' 4.00"4.25 .q_50"~.75· s}jo times f'crwe r spectruf"y'l spectrum type glpower (vrms .....2) i \/v'indow ~ih8.nning ~11ii '¥'91 i plot 1 ............. .£lo'1.-. iii,-e t 0.00.'0 5.'0 10,0 15,0 20,0 25,0 30,0 35,0 ~o,o ~5,0 50,0 time -+igli 'i"' i i pia' ° /"-j figura 5. ruido y espectro de potencia de la tarjeta con entrada ov digital independientes con mas del 75% de su area dedicada a elias y con bajo acoplamiento capacitivo, pistas de alimentaci6n anchas, doble cara y vias en estafio plomo, como el rango dinamico de los ge6fonos no sera mayor a 60 db (lee, 1989) para eventos sfsmicos naturales y el rango dinarnico del sistema se encuentra en 10 i, 72db, no fue necesario realizar un disefio mas complejo para los pianos de tierra ni hacer acoples 6pticos entre la 16gica digital en el microcontrolador y los convertidores, ei error de offset es de 1081lv 10 que equivale a un error cercano a 2 veces el valor lsb(76, 11v), es posible minimizar este error si se programaran los registros de calibraci6n en los convertidores, para visualizar la respuesta del filtro hasta el primer notch (100hz) se captur6 una sefial senoidal con frecuencia variable de 5 a 105 hz en incrementos de 5 hz manteniendo con stante la amplitud. y se muestra a continuaci6n el valor en db del voltaje rms obtenido teniendo como referencia el potencial rms a 5 hz, 65 10585·15 25 ~5 --5 ~ ~-·-1-0 \ -..1-5 \ 20 \ -25 \ ~;)0 \ ----il5 \ 40 v-45 50 frequency hz figura 6. respuesta en frecuencia del filtro en convertidores ad geof(sica colombiana, 5, diciembre de 2001 puesto que el reloj de tiempo real tiene una precisi6n de 20 segundos por dia, es necesario calibrarlo peri6dicamente con una base de tiempo de mayor confiabilidad, siendo la mas empleada a ni vel mundial el utc (coordinated universal time) proporcionado por diferentes agencias tales como el nist (national institute of standards and technology), el observatorio naval y ellnternational bureau of weights and measures in paris, entre otras. ei utc se puede obtener a traves del sistema de posicionamiento global (gps) 0 mediante internet via los protocol os daytime (rfc-867), time (rfc-868) y network time (rfc-1305). el sistema de adquisici6n recibe actualizaciones peri6dicas de tiempo cuando el usuario las realiza localmente 0 remotamente. como se ve en la figura i, sae se conecta a un computador portatil, el cual posee conexi6n a internet, permitiendo actualizar el reloj en el computador con el utc mediante program as como el proporcionado porel nist internet time service (national institute of standards and technology, 2001). ai actualizarel reloj de sae con el reloj del computador se sincroniza el sistema de adquisici6n con la hora ute. este rnetodo permite una precisi6n en el sistema de hasta i s con respecto al utc debido a los retardos de paquetes en la red, y al tiempo de ejecuci6n de los comandos de ajuste de hora en la tarjeta sae, pero debido a que en el sistema no se requiere la interacci6n de varias estaciones para la toma de decisiones, entonces este metodo es valido. en caso de ser necesaria la sincronizaci6n de multiples estaciones portatiles, sera preciso implementar la integraci6n del gps con sae para aumentar la precisi6n de tiempo con orden de ms, para 10cual se dejaron libres en el disefio los pines pl.2 y pi.3 para realizar la conexi6n al gps de forma serial usando un segundo uart. programacion del sae ei microcontrolador se encarga de integrar todos los dispositivos y, espera la solicitud de tareas por pane del computador, las cuales pueden ser de configuraci6n, informaci6n 0 de modo. por ejemplo, es posible ajustar 0 consultar la ganancia y el modo (unipolar, bipolar) para cada canal, asicomo la fecha y hora en el reloj de tiempo real, la temperatura de la tarjeta, y adernas se pueden solicitar los modos de trabajo de acuerdo con el tipo de adquisici6n (perdida de ran go, captura de ge6fonos, captura peri6dica de los canales, 49 captura continua de n rnuestras). tarnbien es posible ejecutar una prueba de integridad de la memoria y la batena en el reloj de tiempo real. en el desarrollo del software se emple6 el c como lenguaje de programaci6n para el microconrrolador, y el abel para los dispositivos 16gicos programables. la estructura del programa del rnicrocontrolador esta basada en apuntadores a funciones. cada funci6n representa en el sistema una tarea de configuraci6n, informaci6n 0 modo de trabajo. en la interrupci6n serial se encuentra el administrador de tareas, que se encarga de separar los argumentos, validar la solicitud y ajustar el apuntador de funciones para ejecutar el comando. ei corto tiempo de ejecuci6n para realizar la interpretaci6n de comandos comparado con el periodo de muestreo y la naturaleza de la aplicaci6n penni ten el uso de este modele satisfactoriamente evitando la implementaci6n de un rnicleo (kernel) preerntitivo ni multitarea (labrosse, 2000), pues s610 se ejecutara un modo de trabajo al mismo tiempo y se ie asigna la misma prioridad a todas las tareas. task manager / mode tasks idle task figura 7. administrador de tareas en sea la cantidad de comandos implementada fue de 16, y una vez ingresados se procede a su interpretaci6n y ejecuci6n; si no se pudiera ejecutar la solicitud por errores en los argumentos 0 porque ya esta ejecutandose otra tarea, entonces se reporta el error via el puerto serial cuando la disponibilidad lopermita. los datos almacenados en la memoria exterria permanecen aun en ausencia de alimentaci6n; estos datos incluyen la informaci6n de configuraci6n de ganancia y modo en los canales de entrada de los convertidores. esta informaci6n es verificada al generar la reiniciaci6n en el sistema, la cual se genera extemamente mediante solicitud en la linea serial 0 al conectar la alimentaci6n de la tarjeta. la sincronizaci6n se puede hacer desde el servidor (consola) 0 remotamente desde la aplicaci6n del usuario (c1iente). software para el pc el pc se encarga de integrar el sistema de adquisici6n, la base de datos, la transmisi6n de eventos y la configuraci6n remota. con este objetivo, se desarroll6 un program a servidor para el sistema operati vo windows que envfa comandos al sistema de adquisici6n, interpreta las solicitudes del usuario y almacena los eventos en la base de datos. 50 vargas et al portable station laptop --------------------,/-embedded acquisition system sae server application rs232 ·······u··· analog signals from sensors -'-,-------------------------------------figura 8. software en el pc servidor este programa permite la edici6n de los usuarios que ejecutaran solicitudes remotas, asf como los lugares donde se podra ubicar la estaci6n. los eventos capturados se visualizan en esta aplicaci6n 0 en la aplicaci6n del cliente. la aplicaci6n servidora se program6 en microsoft visual basic y el enlace a la base de datos se realiz6 en microsoft access haciendo uso del motor jet. figura 9. programa sea server 'tifue l(tzt.270.m. " dent 25/07/01 figura 10. captura en modo i de ge6fono de componente vertical z con ganancia 8 la aplicaci6n de servidor permite cuatro modos de captura. ei modo 0, el cual esta optimizado para la detecci6n de sismos empleando el algoritmo sta-lta (short term average -iong term average) (basseville y benbeniste, 1986) para 10 cual es necesario conectar dos ge6fonos horizontales y un ge6fono vertical en configuraci6n enz. ei algoritmo sta-lta es un tiltro pasabajos que promedia dos ventanas (a largo plazo y a corto plazo) con los iiltirnos datos capturados y calcula la relaci6n entre los mismos; este promedio indica la relaci6n de densidad de energfa que sera muy cercana a la unidad cuando no ha ocurrido una variacion importante en la serial del sensor. cuando se genera un evento, la ventana a corto plazo aumenta su densidad de energfa y se hace comparativamente geoflslca colombiana, 5, diciembre de 2001 estacion digital sismologica portatil sobre telefonia movil celular e internet mas grande que la de la ventana a largo plazo. los valores de tiempo de las ventanas indican la frecuencia de corte del filtro normalizado, el cual se puede definir como: sta = sta + (abs(jj-32768}-sta) kl lta = lta + (abs(f;-32768}-lta) k2 donde f i es la muestra lefda actualmente, sta es el promedio aproximado de la ventana a corto plazo y lta es el promedio aproximado de la ventana a largo plaza. ei valor 32768 es el offset de la muestra capturada a 16 bits. k, y is son el ruirnero de muestras de la ventana sta y lta, respectivamente. las ecuaciones anteriores pueden ser reescritas como: yen) = (1/ k) * x(n) + ((k 1) / k) * yen 1) donde x representa la entrada, y el promedio caiculado, y k es ks 0 k 2 . ei indice n representa el in stante de tiempo actual y n-l es el instante de tiempo previo. ei usuario puede configurar los val ores de relaci6n que activaran el inicio de evento (enthrhld) al igual que la relaci6n que indica la fmalizaci6n del mismo (disthrhld), siendo esta inferior a la relaci6n de activaci6n. tambien se debe almacenar una ventana que contenga n muestras anteriores al evento (pem) y otra con n muestras posteriores ala detecci6n de finalizaci6n (pet). aunque el tamafio de las ventanas ltay sta depende del contenido espectral de la sefial sismica, estos valores oscilan generalmente entre 30-60 s y 0.5-3 s para la ventanas lta y sta, respectivamente. , ;-pil&i sftsmileloil1 ir-pet~, i i i, ill,j ~ !~~~~ i v: i i, , i i i i figura j j. preevento y postevento en sefial sismica ei procesamiento de la sefial usando el algoritmo sta-lta permite disminuir grandemente el espacio necesario en disco para el almacenarniento, pero no garantiza que se capturan todos los eventos ni que los capturados realmente son eventos sismicos, pero es una buena aproximaci6n. ei modo i se emplea para adquirir un detenninado mimero de muestras de la sefial en cualquier canal; el modo 2 genera un evento cuando se detecta la perdida de un rango rmnimo 0 maximo de valores previamente establecidos para un canal, y el modo 3 realiza una captura peri6dica de 6 canales cada n segundos de acuerdo con la ganancia y modo configurados previamente y no requiere de la conexi6n del computador portatil cuando no es necesario consultar el even to hasta que se termina el modo de captura. las paginas activas fueron programadas en php (personal home page) y realizan la consulta y descarga remota de eventos de la base de datos; el enlace se reaiiza tambien mediante odbc en php. este modelo permite la busqueda poi' fechas de captura en los eventos, modo de captura, usuario que reajiz61a solicitud de captura y localizacion de la estaci6n. las solicitudes de descarga e informaci6n de los eventos se reciben poi' medio de html en un browserv se procesan para general' una sentencia sql, la cual se ejecuta en el servidor para ser retornados al usuario los resultados en geofisica colombiana, 5, diciembre de 2001 forma de tabla, en la cual se genera un hipervinculo poi'cada evento. este hipervinculo genera una nueva pagina con informaci6n ampliada sobre el evento y perrnite la descarga del archivo con la captura en formato ascii para los encabezados y formato binario para las muestras leidas, software para el cliente ei usuario puede verificar los eventos generados en la estaci6n poi'medio de un browser para html. para configurar la estaci6n 0 iniciar modos de captura es necesario que el usuario posea la aplicaci6n cliente. figura j 2. visualizaci6n de eventos en internet la aplicaci6n cliente se program6 en microsoft visual basic y su estructura es igual ala aplicaci6n servidora, con la diferencia de que los datos no son obtenidos desde la base de datos, sino desde los archivos descargados de las paginas activas usando el browser. figura 13. configuraci6n remora mediante el programa sea cliente para disminuir los costos del sistema se puede migrar la aplicaci6n actual para el servidor empleando asistentes personales digitales pda y hi pc (handheld pc) con slot pcmcia tipo ii para conectar el modem copd. estes pueden ser programados en la versi6n de microsoft visual basic para windows embedded. 51 vargas et al. transmision de datos para los sistemas de captura en tiernpo real y transrnision posterior de los evenros que requieran baja capacidad de transmisi6n se puede hacer uso de las redes de telefonfa m6vil celular, las cuales hacen transparente al usuario el medio ffsico de naruraleza inalarnbrica en las aplicaciones. la transmisi6n en campo emple6 la infraestructura de la red de tejefonfa m6vil celular amps, mediante cdpd (cellular digital packet data) en redes de area exrensa, con velocidad de transmisi6n de datos de 19200 bps, full duplex ell modo asincronico, sobre el paquete de protocolos tcpjip. para el monraje realizado se emplearon modems cdpd con potencia de salida 600m w, e interfase aerea de encriptaci6n de datos rsa l28-bits rc4. el proveedor de servicios de cdpd se encarga de asignar un ip fijo valido en la red propia del proveedor y en la red internet, 10 que permite la configuraci6n y adquisici6n de eventos en cualquier punto donde haya acceso a internet. conclusiones se disefio e implemenr6 una estaci6n de campo conformada por un sistema de acondicionamienro y adquisici6n para prop6sitos sismol6gicos, aunque puede ser extendida a otras aplicaciones con sefiaies de baja frecuencia. la estaci6n disefiada posee un range dinarnico de 101db, 9 canales (3 para el sistema triaxial de ge6fonos) y las restanres 6 pueden ser empleadas para aplicaciones con frecuencias de muestreo por debajo de las loomuestras por segundo (temperatura, radiometrfa, inclinomerrfa, baterfa, etc.). dicha estaci6n genera evenros en diferenres modos y los publica en la red de telefonfa m6vil celular e internet mediante paginas activas. ei software de la estacion permite la detecci6n de eventos sfsmicos mediante el algoritmo stajlta, asf como la captura por perdida de range, captura peri6dica y captura continua de los canales. 52 figura /4. prototipo final del sea el sistema de acondicionamiento y adquisici6n de sefiales se disefio como un sistema embebido y orientado por comandos con reloj de tiempo real y memoria de almacenamiento propios. referencias bibliograficas analog devices, 3v/ 5v, cmos, 500 ua. 1998: signal conditioningadc, ad77l4. rev c. www.analog.com, analog devices, inc. basseville, m., benbeniste, a. 1986: detection of abrupt changes in signals and dynamical systems. sv. jarman, david. 1995: a btiefintroduction to sigma delta conversion, intersil application note, www.in/ersil.commayode 1995. kester, walt, bryant,james. 200 i: grounding in high speed systems, hardware design techniques, application note. analog devices. www.analog.com kester, walt, brya1'it,james, buxton,joe. 200l: high resolution signal conditioning adcs, www.analogcom,analogdevices,section3,analog devices application note, pp.15-16 labrosse,jean 1. 2000: embedded systems building blocks, cmp books, second edition. kansas, usa. lee, w. h. k. 1989: toolbox for seismic data acquisition, procesing, and anal ysis, international association of seismology and physics of the earth's interior. lemaire,christophe. 1997: low cost, imems accelerometers sense strong motion seismic activity for hazard mitigation applications. analog devices, inc. www.analog.coll1/publica/iol1s/whilepapers/producls/seisn/{in.hllnl nationallnstitute of standardsandlechnology. nisi. 200 i: internet time service. wwlv.bldrdoc.gov/limejreq/service/ils.hrnl geofisica colombiana, 5, oiciembre de 2001 hernandez & von frese.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 131-146 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data orlando hernández p.1,2 and ralph r. b. von frese 1 (1) dept. of geological sciences, the ohio state university, columbus, oh 43210 usa, fax 614 2927688. hernandez.135@osu.edu , vonfrese@geology.ohio-state.edu, (2) dept. of geosciences, universidad nacional de colombia, bogotá, d.c. colombia ohernandezp@unal.edu.co abstract recently revised models on global tectonics describe the convergence of the north andes, nazca, caribbean and south american plates and their seismicity, volcanism, active faulting and extreme topography. the current plate boundaries of the area are mainly interpreted from volcanic and seismic datasets with variable confidence levels. new insights on the isostatic state and plate boundaries of the northwestern andes mountains can be obtained from the spectral analysis of recently available gravity and topography data. isostatically disturbed terrain produces free-air anomalies that are highly correlated with the gravity effects of the terrain. the terrain gravity effects (tge) and free air gravity anomalies (faga) of the andes mountains spectral correlation data confirms that these mountains are isostatically disturbed. strong negative terrain-correlated faga along western south america and the greater and lesser antilles are consistent with anomalously deepened mantle displaced by subducting oceanic plates. inversion of the compensated terrain gravity effects (ctge) reveals plate subduction systems with alternating shallower and steeper subduction angles. the gravity modeling highlights crustal deformation from plate collision and subduction and other constraints on the tectonism of the plate boundary zones for the region. keywords: north andes, isostasy, gravity, moho. resumen modelos recientes de tectónica de placas describen la convergencia de las placas de los andes del norte, nazca, caribe y sur américa y su sismicidad, volcanismo, fallamiento activo y topografía extrema. los actuales límites tectónicos del área han sido interpretados a partir de bases de datos con varios niveles de confidencialidad. nueva información sobre el estado isostático y bordes tectónicos de los andes del noroeste de sur américa puede ser obtenida del análisis espectral de datos de topografía y gravimetría disponibles recientemente. terrenos isostáticamente distorsionados producen anomalías de aire libre que son altamente correlacionadas con los efectos gravimétricos 131 earth sciences research journal manuscript received april 19 2006. accepted for publication october 29 2006. del terreno. los efectos topográficos del terreno espectralmente correlacionables con las anomalías gravimétricas de aire libre de las montañas de los andes confirman que estas montañas están distorsionadas isostáticamente. fuertes anomalías negativas de aire libre correlacionables con los efectos gravimétricos del terreno a lo largo del margen oeste de sur américa y las antillas mayores y menores son consistentes con un manto anómalamente mas profundo desplazado por la subducción de placas de corteza oceánica. el modelamiento inverso de las anomalías gravimétricas revela sistemas de subducción con inclinaciones sub-horizontales y verticales alternantes. el modelamiento gravimétrico resalta la deformación cortical producto de la colisión de placas y subducción y otras características del tectonismo de zonas de límites de placas de la región. palabras claves: andes del norte, isostasia, gravimetría, moho. 1. introduction the tectonic setting of northwestern south america is poorly understood because of the complex interaction of numerous tectonic plates, and the scarce and incomplete geologic mapping due to the dense vegetation cover and limited accessibility to remote areas. international scientific interest in the northwestern andes mountains is intense because it is the key to improve our understanding of the geological evolution of the backbone of america and the caribbean, and related volcanic and seismic hazards (bird, 2003; cediel et al.,2003). the ability of current physical models to predict neotectonic and other intra-plate lithospheric stresses and strain is limited because the structures and geometry of plate boundaries are largely hidden and poorly understood. however, the deployment of geodetic sensors including the gps-derived ground velocities and high resolution topography have substantially improved the modeling of the lithosphere to analyze surface mass dynamics (kellogg et al., 1985; kellogg et al., 1995). in this paper, spectral correlation theory (e.g., leftwich et al., 2005; von frese and tan, 1999) is applied to terrain and free-air gravity anomalies for new constraints on the boundary zones of the north andes and panama microplates, and the cocos, nazca, caribbean and south american plates. normalization and local favorability indexes are implemented to facilitate the visualization and interpretation of gravity anomalies. this methodology has been applied and validated in other crustal studies such as east asia (tan and von frese, 1997), antarctica (von frese et al., 1992; 1999), greenland (roman, 1999), ohio (kim et al., 2000), and iceland (leftwich et al., 2005). isostatically disturbed terrain produces free-air anomalies that are highly correlated with the gravity effects of the terrain. spectral correlation of the terrain gravity effects (tge) and free-air gravity anomalies (faga) suggested that the andes mountains are isostatically disturbed with an anomalous underlying crust-mantle interface (moho). strong negative terrain-correlated faga along western south america and the greater and lesser antilles are consistent with anomalously deepened mantle displaced by subducting oceanic plates. gravity estimates regionally augment and extend the seismic estimates to highlight crustal deformation from plate collision and subduction in northwestern south america. the gravity anomalies are consistent with a complex evolution for northwestern south america involving the accretion of an oceanic plateau with strong caribbean affinities, obduction of the oceanic crust, over thrusting and strike-slip faulting. these tectonic characteristics are not readily accounted for by the conventional model of “andean type” orogenesis of the central andes (liu et al., 2002). 2. spectrally correlated freeair and terrain gravity the isostasy of northwestern south america was investigated considering the topography and bathymetry data from national imagery and mapping agency (nima) from −8os to 23.5on latitude and from −90ow to −58.5ow longitude. surface and bathymetry elevations from the jgp95e terrain data base (smith and sandwell, 1994, 1997) were processed to produce the isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data 132 digital elevation model (dem) in figure 1 for the water and rock terrain gravity components at 0.5o nodal spacing. free-air gravity anomalies (faga) were estimated from the egm96 spherical harmonic earth gravity model to degree and order 360 (lemoine et al., 1998) at 20 km altitude over the 32o x 32o area at 0.5o nodal spacing in figure 2. the altitude of 20 km was chosen to help minimizing the effects of local density errors in the terrain gravity modeling (e.g., leftwich et al., 2005). the terrain gravity effects were modeled in spherical coordinates at 20 km altitude by gausslegendre quadrature integration in figure 3 (von frese, 1980). the terrain gravity modeling used densities of 2.8 gm/cm3 for the crust and 1.03 gm/cm3 for oceanic water. spectral correlation theory was used to analyze the coregistered faga and tge for their anomaly correlations using matlab (mathworks, 2005). specifically, the fourier transforms t and f of tge and faga, respectively, were used to obtain their correlation spectrum (davis, 1986; von frese et al., 1997a, kim et al., 2000) given by: (1) where cc(k) is the correlation coefficient between the kth wave numbers components f(k) and t(k), and denotes taking the real parts of the wave number components. usually, cc(k) is evaluated from the cosine of the phase difference (δθk) between the two kth wave number components. using the correlation spectrum between faga and tge, spectral correlation filters were designed to extract terrain-correlated hernández and von frese., esrj vol. 10, no. 2. december 2006 figure. 1. topography/bathymetry of northwestern south america with superposed regional tectonic features between −8os to 23.5on latitude and from −90ow to −58.5ow longitude. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm= sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equalarea conic projection. 133 free-air gravity signals. those wave numbers components showing intermediate to high positive (ccp(k)≥0.3) and negative (ccn(k)≤0.3) correlations were identified. the cut off values for the correlation filter were determined to minimize correlative features between the terrain-decorrelated free-air and compensating terrain gravity components. in the other hand, transforming positively and negatively correlated free-air wave number components according to the selected cut off values yielded the terraincorrelated free air gravity anomalies (tcfaga) in figure 4. the residual terrain-decorrelated free-air gravity anomalies (tdfaga) in figure 5 were calculated by subtracting tcfaga from faga, so that faga = tcfaga + tdfaga (2) tcfaga are explained by anomalies associated with the topography while tdfaga include the gravity effects of sources within the crust (e.g., local bodies) and the sub-crust. similarly, spectral correlation filters were used to extract faga-correlated tge (fctge) in figure 6 and faga-decorrelated tge (fdtge) in figure 7. the fctge sharpen up the tge components that may be isostatically disturbed, whereas the fdtge involves relatively marginal amplitudes that appear to reflect mostly noise in the data and other non-geological effects. 3. analysis of anomaly correlations tcfaga and fctge were normalized to facilitate recognizing the anomaly correlations between them (von frese et al., 1997). to enhance the visual perception of the anomaly correlations, the following transformation was applied: (3) figure. 2.free-air gravity anomalies (faga) at 20 km elevation for the study region (lemoine, et al., 1998). map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm= sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection.. 134 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data where μx and x both represent the respective values of the mean and standard deviation of the signal x. the expression in parentheses standardizes the xi coefficients to zero mean and unit standard deviation and is dimensionless. however, the values for mean z and standard deviation of μz of the normalized signal z can be specified by the user to facilitate the visual analysis. the transformations of tcfaga and fctge used the normalization of z = 10 to facilitate plotting the two datasets with common plotting parameters (von frese et al., 1997). local favorability indices (merriam and sneath, 1966) were used to highlight the various anomaly correlations in the normalized tcfaga and fctge. positively correlated features were mapped out by summed local favorability indexes (slfi) obtained by (4): (4) in equation (4), z(x) and z(y) were both respectively the normalized tcfaga and fctge coefficients. the peak-to-peak correlations between the two data sets were mapped out by slfii≥8.7226 in figure 8, whereas troughto-trough correlations were mapped out by the coefficients satisfying slfii≤8.7226 in figure 9. the slfi coefficients brought out the positively or directly correlated features, while suppressing the negatively and null correlated features between tcfaga and fctge. to enhance the perception of inversely correlated features obtained from wave numbers with negative correlation coefficients, the normalized and scaled datasets were subtracted cell by cell for differenced local favorability indexes (dlfi) by (5): (5) figure. 3. terrain gravity effects (tge) at 20 km elevation for the study region by gauss-legendre quadrature integration. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm= sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 135 hernández and von frese., esrj vol. 10, no. 2. december 2006 figure. 4. terrain-correlated faga (tcfaga) at 20 km elevation for the study area. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm = sierra nevada of santa martha, wmid = western-central ranges. this map was produced using the albers equal-area conic projection. cc tge fctge fdtge faga tcfaga tdfaga tge 1 0,97 0,24 0,52 0,59 0 fctge 0,97 1 0 0,56 0,6 0,05 fdtge 0,24 0 1 -0,09 0 -0,02 faga 0,52 0,56 -0,09 1 0,89 0,45 tcfaga 0,59 0,6 0 0,89 1 0 tdfaga 0 0,05 -0,02 0,45 0 1 table. 1. correlation coefficients (cc) between the gravity anomalies at 20 km altitude for the area from −8os to 23.5on latitude and from −90ow to −58.5ow longitude in northwestern south america. 136 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data positive features in tcfaga which were correlative with negative fctge features (peakto-trough) were mapped out by dlfii≥4.8904 in figure 10, whereas negative tcfaga that were correlative with positive fctge were mapped out by dlfii≤4.8904 in figure 11. the dlfi coefficients emphasized the inversely correlated features, while suppressing the positively and null correlated features between tcfaga and fctge. table 1 summarizes the correlations between faga and tge and their correlated filtered components. the increase in the cc between tcfaga and tge (cc=0.59) relative to the cc between the raw faga and tge (cc=0.52) facilitates the isostatic analysis of the tectonic features of the study area. however, the increase in the cc between tcfaga and fctge (cc=0.6) with respect to the cc between tcfaga and tge (cc=0.59) is negligible and the physical meaning of fctge and fdtge is obscured. the cc between tdfaga and faga (cc= 0.45) is significant in showing the strong influence of the data-grid in the anomaly decomposition of faga in tcfaga and tdfaga. therefore, further analyses of tdfaga are required. 4. results isostatically disturbed terrains have gravity effects that directly correlate with faga. the complementary tdfaga reflect the effects of lateral density variations in the crust, mantle and core, as well as errors in the data and data processing. in the subsections below, the gravity effects of the dtm and spectral correlations with free-air gravity anomalies are interpreted for constraints on the tectonic features of the study region. figure. 5. terrain-decorrelated faga (tdfaga) at 20 km elevation for the study area. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm = sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 137 hernández and von frese., esrj vol. 10, no. 2. december 2006 figure. 6. faga-correlated tge (fctge) at 20 km elevation for the study area. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm = sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 4.1. digital terrain model (dtm) the dtm in figure 1 presents major morphological and tectonic features that produce strong terrain gravity effects in the gravity anomalies. the topography data are increasingly being integrated into the techniques to study plate boundary zones (stein & freymueller, 2002). one approach is to use digital terrain models (dtm) showing the forms of mountain ranges, oceanic trenches, volcanic ridges and island arcs, and oceanic basins to constrain models of the processes that produced them. the dtm of northwestern south america suggests a complex tectonic setting with mountain building and uplift as a consequence of the plate convergences that have created the north andes mountains, the sierra nevada of santa marta and the cordillera central mountains. flat low lands are associated with the stable cratonic terrains of the guiana shield. deep pacific ocean trenches along the peru ecuador colombia coastline and the middle america trench are associated with subduction zones. the puerto rico trench in the caribbean is associated with the caribbean − north american subduction zone. the submarine carnegie, cocos and malpelo volcanic ridges are associated with the galapagos hot spot. the volcanic arc islands of the lesser and greater antilles reflect intensive geodynamics in the region, including high seismicity and volcanic activity. 4.2. free-air gravity anomalies (faga) the free air gravity anomalies in figure 2 include the superposed gravity effects of the terrain and subsurface gravity variations. the faga amplitudes are smaller than the tge amplitudes reflecting partial isostatic compensation of the terrain. however, the mean faga of 17 mgals 138 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data indicates that the mean terrain is isostatically disturbed. the largest negative anomalies are over the subduction zones, whereas the strongest positive faga are over the andes mountains and sierra nevada of santa marta. moderate positive and negative faga are distributed along the oceanic volcanic ridges, guiana craton and oceanic plates. the positive correlation between tge and faga (cc = 0.52) reflects the predominant condition of isostatically disturbed terrain in the tectonically complex study region. 4.3. terrain gravity effects the terrain gravity effects (tge) in figure 3 produced by crust air (2.8 gm/cm3) and crust water (1.82 gm/cm3) interfaces are the highest density contrasts that can be defined by the lithosphere. therefore, the minimum and maximum tge values of -452.12 mgals and 285.02 mgals, respectively, are also the extreme values that faga could take in a case of 0% isostatic compensation. negative tge (tgemean = −51.21 mgals) are stronger over the caribbean sea than the pacific ocean. negative tge overlie the puerto rico trench, cayman trough, and colombian and venezuelan basins. the beata ridge, lesser and greater antilles have relatively positive tge. in the pacific, negative tge overlie the panama basin and the peru trench, and positive tge overlie the carnegie, cocos and malpelo ridges. at the continent, positive tge reflect the presence of the mountainous central america, andes mountains, sierra nevada of santa martha and the guiana craton. relatively negative anomalies are associated with the vichada plain and amazon river aulacogens. figure. 7. faga-decorrelated tge (fdtge) at 20 km elevation for the study area. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am) and standard deviation (sd). snsm = sierra nevada of santa martha, wmid = western-central ranges. this map was produced using the albers equal-area conic projection. 139 hernández and von frese., esrj vol. 10, no. 2. december 2006 figure. 8. summed local favorability indices (slfi) for tcfaga and fctge at 20 km elevation for the study region showing tcfaga-peak to fctge-peak correlations for slfi ¸ 8.7226. snsm= sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 4.4. terrain-correlated free-air gravity anomalies tcfaga in figure 4 have a more complex anomaly pattern than tge inferring a wide range of isostatic disturbances of the terrain. tcfaga varies from -190.28 to 131.66 mgals, with a mean value of zero. thus, the tcfaga reflect both negative and positive disturbances of continental and oceanic features. volcanic activity and mountain building are associated with positive tcfaga, whereas negative tcfaga characterize trenches and troughs. arcuate positive and negative tcfaga can be observed along the greater and lesser antilles, along the caribbean north american plate boundary. therefore, the subduction of the north american plate under the caribbean plate is well mapped from the tcfaga. negative tcfaga beneath the trenches are due to the negative density contrast ( 0.37 gm/cm3) of the subducting oceanic slab into the mantle. positive tcfaga over the recently formed volcanic islands are explained by oceanic crustal thickening which is not fully compensated. the tcfaga are smaller than tge, suggesting partial isostatic compensation from the thickening of the oceanic crust into the mantle. negative tcfaga are located along the colombian venezuelan basins and the caribbean − north andes subduction zone (bird, 2003, casa, 1998). the low tcfaga amplitudes suggest that the caribbean north andes subduction zone is incipient with respect to the caribbean north american or nazca north andes subduction zones. in central america, the positive tcfaga from panama to costa rica (60-80 mgals) and from nicaragua to honduras (20-40 mgals) are less prominent than the tcfaga in the andes (80-100 mgals) suggesting that central america may be relatively more isostatically compensated than the andes mountains. the converging plate boundary between the panama and north andes micro-plates proposed by kellogg (1985) is 140 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data not supported by the tcfaga which shows no distinct anomaly between them. at the pacific ocean, negative tcfaga clearly define the middle america trench. relatively positive tcfaga are associated with the flat slab subduction of the cocos ridge under the panama − costa rica micro-plate. negative tcfaga along the western margin of south america vary from -10 to -60 mgals, being comparatively smaller in the colombian trench. these results reflect the lower angle of subduction of the nazca plate in contrast to the steeper angles of subduction and negative amplitudes at the chilean peruvian subduction zone. relative tcfaga highs are associated with the flat slab subduction of the carnegie ridge under the north andes micro-plate. at the continent, positive tcfaga at the andes mountains (90 to 130 mgals) show that these mountains are isostatically disturbed. a remarkable isolated positive tcfaga anomaly is located in northern colombia, in the sierra nevada of santa marta (130 mgals). here, the highest elevations of northwestern south america have also the maximum tcfaga, showing that these mountains, reaching elevations of 5800 m, are even less compensated than the andes mountains suggesting that the sierra nevada of santa marta has been intensively deformed and uplifted after the andean orogenesis. it is not clear how these mountains were formed and why they occupy their present position. these mountains are perhaps allochthonous tectonic blocks that were moved from the east along a left lateral fault (dewey and pindell, 1985). the tcfaga also support this explanation, which implies that lateral stress caused crustal shortening and uplift of the sierra nevada of santa marta without significant isostatic compensation. tcfaga at the guiana craton are close to zero suggesting nearly complete isostatic figure. 9. summed local favorability indices (slfi) for tcfaga and fctge at 20 km elevation for the study region showing the tcfaga-trough to fctge-trough correlations for slfi · −8.7226. snsm= sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 141 hernández and von frese., esrj vol. 10, no. 2. december 2006 figure. 10. differenced local favorability indices (dlfi) between tcfaga and fctge at 20 km elevation for the study region showing the tcfaga-peak to fctge-trough associations for dlfi> 4.8904. snsm= sierra nevada of santa martha, w-mid =westerncentral ranges. this map was produced using the albers equal-area conic projection. compensation. local tcfaga highs and lows may reflect the occurrence of lateral mass variations related to intrusive bodies without topographic expression (tge = 0). here, the tcfaga that are greater than the tge give an example of overcompensated terrain. the negative tcfaga along the eastern andes mountains are associated with continent continent convergence between the north andes microplate and the south american plate with regional thrust faults dipping to the west and intensive compressional and transpressional deformation (cediel et al., 2003). therefore, the negative tcfaga along the eastern cordilleran area appear to reflect the thickening of the continental crust with thick sedimentary sequences that are known to contain oil and gas deposits (stein & sella, 2002). 4.5. terrain-decorrelated freeair gravity anomalies the tdfaga in figure 5 are very poorly correlated with tge (cc = −0.0575) and mostly show gridded patterns due likely to observational and data processing noise. thus, we do not consider the geological implications further for the tdfaga. 4.6. tcfaga and fctge anomaly correlations tectonic analysis of enhanced correlations between the tcfaga and fctge are visualized better using summed local favorability indices with (sfli ≥ 8.7226) in figure 8. high amplitude peak-to-peak zones along the andes mountains overlay mapped mineralized batholithic intrusions (ingeominas, 1987). according to kerr et al. (1997), subduction related batholiths 142 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data and extrusive rocks found around the margin of the caribbean colombian cretaceous igneous provinces are of two distinct events: one suite represents the pre-plateau, collision-related volcanism, whereas the other suite, which is slightly younger than the plateau, may be associated with subduction. therefore, peakto-peak zones along the western cordillera may be associated with subduction, whereas peakto peak zones along the eastern cordillera are associated with the pre-plateau collision suite of igneous rocks. intermediate amplitude peak-to-peak correlations at the guiana craton also indicate isolated topographic highs on relatively denser intrusive rocks of the cratonic basement. these inferred intrusions may be the source rocks of known gold and diamond placer deposits of the guainia and orinoco rivers (romero et al., 1996). the slfi ≤ −8.7226 in figure 9 shows troughto-trough correlations between tcfaga and fctge, respectively. they clearly mark the puerto rico, colombian and peruvian trenches, while the middle american trench and the pacific colombian trenches are relatively poorly defined. the oceanic trenches are the most isostatically disturbed features in the area and also the most seismically active features. of special interest are the intermediate troughto-trough correlations of tcfaga and fctge in the colombian and venezuela basins. these areas are undergoing intensive exploration for oil and gas and have productive fields associated with delta deposits of the magdalena river, the major supplier of sediments to the caribbean sea. therefore, the slfi anomaly correlations may mark strong prospective areas for locating sedimentary basins in the caribbean plate. figure. 11. differenced local favorability indices (dlfi) for tcfaga and fctge at 20 km elevation for the study region showing the tcfaga-trough to fctge-peak associations for dlfi<4.8904. snsm= sierra nevada of santa martha, w-mid = westerncentral ranges. this map was produced using the albers equal-area conic projection. 143 hernández and von frese., esrj vol. 10, no. 2. december 2006 the differenced local favorability indices with dlfi ≥ 4.8904 show tcfaga-peak to fctge-trough correlations in figure 3.10. they mark topographic depressions that are overcompensated by local denser bodies along the lesser antilles, colombian basin, western and central ranges, sierra nevada of santa marta and at the north american plate. the tcfagatrough to fctge-peak correlations with dlfi ≤ 4.8904 in figure 3.11, on the other hand, show the distribution of low density zones along the eastern andean mountains, the upper amazon river and lower orinoco river. the negative dlfi zones mark mapped sedimentary basins including the upper, middle and lower magdalena valley (umv, mmv and lmv, respectively), and the orito, llanos, maracaibo, guajira and orinoco basins with known productive oil and gas fields. thus, the negative dlfi zone outlined at the upper amazon river aulacogen may also infer a prospective sedimentary basin for oil and gas exploration. 5. conclusions the central parts of the nazca, cocos and caribbean plates are mostly in isostatic equilibrium with tcfaga close to zero. this condition changes at the plate boundary zones and volcanic ridges. positive tcfaga are associated with the malpelo, cocos, carnegie and beata ridges. tcfaga may also be associated with dynamic topography by upwelling mantle. thickening the oceanic crust by crustal roots into the mantle partially compensates for the tge. in the case of the malpelo ridge, this volcanic range is partially compensated with a crustal root, in agreement with the seamount model proposed by watts (2001) for isostatic compensation of seamounts and oceanic islands. depth to the moho estimated from gravity data is supported by seismic data (trummer, 2002). the andes mountains are isostatically disturbed. the airy-type isostatic model is consistent with the thickening of the crust at the andes mountains, but lateral density variations between the oceanic and continental crusts must be involved to model tcfaga. the pratt-type isostatic model is not appropriate to model tcfaga of the andean mountains because it can not account for the thick continental roots shown in the seismic data (trummer, 2002). locally positive tcfaga anomalies in the guiana craton are associated with relatively denser intrusive bodies affecting the host cratonic rocks. these intrusive bodies do not have a significant surface expression and may be related to mass variations in the crust and upper mantle. compressional tectonics has played an important role in the mountain building of the north andes micro-plate as a consequence of the relative movements among nazca, south america and caribbean plates and panama and north andes micro-plates. the shortening associated with plate motion, based on gps data only accounts for less than 50% of the mountain building (lamb, 2004). magmatic under-plating and crustal doubling due to over-thrusting may also contribute to the formation of the andes mountains and the sierra nevada of santa marta, as inferred by the positive tcfaga. considering that isostatic equilibrium is the ideal state that any uncompensated mass tends to reach through time, implies that any uncompensated mass of the north andes mountains is under pressure to adjust vertically and horizontally to reach equilibrium. therefore, a direct correlation is expected between disturbed and seismically active terrains, whereas minimal seismic activity is expected for isostatically compensated terrains (artyushkov, 1973). slfi and dlfi enhance tcfaga and fctge anomaly correlation, providing new criteria to locate areas for prospecting mineral and fossil fuel deposits. the results presented here must be used with care. it has been assumed a simplification of the gravity effects at 20 km altitude, where a compensating mass at 35 km depth has only about 40% of the amplitude of the gravity signal of the surface mass that it compensates; therefore, positive and negative faga do not mean, on their own, a lack of compensation. for example, a maximum faga of about 184 mgal is greater than the roughly 110 mgal expected for fully compensated terrain, the difference reflects the 144 isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data true lack of compensation. acknowledgments we thank mohammad asgharzadeh and laramie potts, from the geological sciences department of the ohio state university, for their assistance in the inverse gravity modeling. elements of this research were supported from fulbright laspau, universidad nacional de colombia, colfuturo and sociedad minera kedahda s.a. references artyushkov, e. v. 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(1995). tectonic development of panama, costa rica and the columbian andes: constraints from global position system geodetic studies and gravity, in geol. and tect devel of the caribbean plate boundary in southern central america. geol. soc. am. spec. pap. 295. 75-86. kellogg, j. n., ogujiofor i. j. and kansakar, d. r. (1985). cenozoic tectonics of the panama and north andes blocks, mem. congr. latinoam. geol. 6. 40-59. kerr, a. c., tarney, j., marriner, g. f., nivia, a., and saunders, a. d. (1997). the caribbean colombian cretaceous igneous province: the internal anatomy of an ocean plateau. agu geophysical monograph. 100. 123-144. kim, j. w., von frese, r. r. b., and rae, k. h. (2000). crustal modeling from spectrally correlated free-air and terrain gravity data a case study of ohio. geophysics. 65 (4). 1057-1069. lamb, s. (2004). devil in the mountain, a search for the origin of the andes. princeton university press, new jersey, 335 pp. leftwich, t. e., von frese, r. r. b., potts, l. v., kim, h. r., roman, d. r., taylor, p. t., and barton, m. (2005). crustal modeling of the north atlantic from spectrally correlated free-air and terrain gravity. j. geodynamics. 40. 23-50. 145 hernández and von frese., esrj vol. 10, no. 2. december 2006 lemoine, f. g., chinn, d. s., cox, s. m., factor, j. k. kenyon, s. c., klosko, s. m., luthcke, s. b., olson, t. r., pavlis, e. c., pavlis, n. k., rapp, r. h., smith, w. h. f., and sandwell, d. t. (1994). bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry. j. geoph. res. 99. 21803-21824. liu m., yang y., stein s., and klosko, e. (2002). crustal shortening and extension in the central andes: insights for a viscoelastic model. agu geod. 30. 325-339. mathworks, inc. 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(1997). satellite geopotential models of east asian lithosphere (abstract), trans. agu. spring meeting supplements. 78. 114-121. 146 trummer, i. (2002). s-wave processing and interpretation of wide-angle seismic refraction data, malpelo ridge, eastern panama basin, phd dissertation, der mathematischnaturwissenschaftlichen fakultat, der christian albrechtsuniversitat, zu kiel. watts, a. b. (2001). isostasy and flexure of the lithosphere. first edition, cambridge university press, new york, 458 pp. isostatically disturbed terrain of northwestern andes mountains from spectrally correlated free-air and gravity terrain data maestria en geofisica, una nueva opcion en las geociencias luis alberto briceno guarupe profesor asociado, departamento de geociencias facultad de ciencias universidad nacional de colombia briceno g., l.a. (2000): maestria en geofisica, una nueva opci6n en las geociencias. geofis. colomb. 4:79-80. issn 01212974. santa fe de bogota, d.c. colombia. 1. introduccion ei devenir de las ciencias es mas complejo que lineal. las geociencias en colombia, siguiendo esta hip6tesis, han tenido un desarrollo de ires y venires. la 16gica pareciera ser que se deberian tener primero carreras en las diferentes disciplinas 0 profesiones que las necesidades y planes'de desarrollo de' una comunidad 10 requieran, luego especializaciones, <' maestrias y doctorados. sin embargo, en areas inter 0 multidisciplinarias,. como las geociencias no ha side asi, ni a nivel nacionatni internacional, 10 cual tiene sus pros y contras. ei hecho es que existe la necesidad real de solucionar problemas relacionados con la disciplina de la geofisica, hay la necesidad de profundizar en el tema e investigar sobre una gran variedad de aspectos. la universidad nacional de colombia, en su facultad de ciencias y a traves del departamento de geociencias, con la activa participaci6n del grupo de investigaci6n en geofisica, ha asumido una vez mas el compromiso de impulsar la formaci6n e investigaci6n en el area de las geociencias, reorientando y poniendo en funcionamiento el programa de maestria en geofisica, que habia side aprobado en 1985. en esta ocasion, el acuerdo no. 11, del consejo de sede de bogota, del 20 de mayo de 1998, introdujo modificaciones al plan inicial de estudios, que permitieron reiniciar actividades a partir del primer semestre de 1999, con siete estudiantes, que incluian dos qeoloqos, dos fisicos y tres ingenieros (civil, electrico y catastral y geodesta). para el primer semestre del ano 2000, se recibieron doce estudiantes, que incluian esta vez, siete profesionales en geologia y cinco en ingenierias, fisica y rnaternaticas. como se aprecia, la interdisciplinariedad es tipica del programa, 10 cual permite satisfacer los objetivos de observaci6n, adquisici6n, proceso e interpretaci6n de los procesos naturales que son abarcados por la geofisica. la puesta en marcha de la maestria en geofisica ha contado en esta oportunidad con la activa colaboraci6n directa 0 indirecta de personas e instituciones, ajenas a la universidad, que han entendido 10 positivo del esfuerzo y que estan dispuestos a seguir trabajando para que este posgrado sea cada vez rnejor y sea una opci6n concreta, para quienes quieran formarse a nivel avanzado e investigar en esta area de la geociencias. la maestria en geofisica, junto con el posgrado en meteorologia y el recientemente aprobado posgrado en geologia, fortifican el departamento de geociencias, incentivan a seguir trabajando por otros programas de pre y posgrado y son un paso positive en el desarrollo de las geociencias en colombia. 2. el programa maestria en geofisica objetivos: • obtener un mejor entendimiento de la dinarnica, estructura y composici6n de la tierra y aplicar ese conocimiento en la exploraci6n del subsuelo y en la prevenci6n de catastrofes de origen geol6gico. • formar a un nivel avanzado aquellos profesionales con interes y capacidades de profundizar en la geofisica, mediante la utilizacion de rnetodos cientificos y tecnol6gicos de frontera, a traves de programas ligados a las realidades y necesidades de nuestra sociedad. • proveer al pais y en particular a las instituciones que 10 requieran, de personal id6neo en el area de geofisica, capaces de planear y desarrollar programas investigativos que se integren a las politicas de desarrollo econ6mico, tecnol6gico y social, y que a la vez sean interlocutores en programas cientificos internacionales. • incentivar y fortalecer los grupos de investiqacion en las diferentes lineas de la geofisica, para colaborar con el desarrollo integral de la region. g • colaborar con la capacitaci6n cienlifica de profesionales que puedan contribuir en la formaci6n de futuras generaciones de geofisicos a nivel de pre o posgrado. titulo otorgado: magister en geofisica requisitos de admisi6n: la admisi6n al programa sera anual, comenzando actividades en febrero, con inscripciones en septiembreoctubre del ano anterior. se evaluara: • prueba de conocimientos • hoja de vida • entrevista • comprensi6n ingles 30 puntos 30 puntos 30 puntos 10 puntos puntaje minima de admisi6n 70/100 plan de estudios: • dos asignaturas basicas, que tienen por objeto impartir conocimientos avanzados, fundamentales para cualquier linea de la geofisica, con el objeto de ampliar y profundizar la fundamentaci6n cientifica. • cuatro asignaturas electivas, cuya finalidad es lograr mayor profundizaci6n y diversidad de conocimientos te6ricos y aplicados de temas especializados, referentes a las iineas geofisicas. • tres seminarios de investigaci6n que permiten desarrollar actividades de temas necesarios para el cumplimiento de actividades investigativas. • una tesis de caracter formativo, que permita el planteamiento claro y la soluci6n de un problema de investigaci6n. dependiendo de la formaci6n basica de los participantes, asi como de sus intereses, el trabajo de investigaci6n y tesis se puede orientar en dos iineas: prospecci6n y sismotect6nica. las asignaturas ofrecidas son: basicas: geofisica avanzada; series de tiempo y teoria de inversi6n. electivas: campos potenciales; sismica de reflexi6n; sismologia y ondas sismicas; geologia estructural avanzada; geologia de hidrocarburos avanzada; tect6nica y volcanologia; evaluaci6n geofisica de yacimientos;' geofisica de pozos; sistemas de informaci6n geoqraflca en geociencias. complementarias: geologia especial; maternaticas especiales. duraci6n: cuatro semestres. costos: derechos acadernicos derechos de matricula servicio medico 125 puntos 15 puntos 10 puntos informaci6n complementaria: geoweb@ciencias.ciencias.unal.edu.co geofisic@ciencias.ciencias.unal.edu.co using neuronal networks to determine site response from coda waves: application in armenia, colombia earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004): 34 44 manuscript received june 2004 paper accepted november 2004 34 unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquake-volcanoes interactions. john j. sánchez and stephen r. mcnutt e-mail:jjalaska@giseis.alaska.edu geophysical institute, university of alaska, fairbanks 903 koyukuk dr. fairbanks, ak 99775, usa abstract on november 3, 2002 three segments of the denali fault in interior alaska ruptured during a mw 7.9 earthquake, offering a unique opportunity to study earthquake-volcano interactions. out of the 24 volcanoes that are seismically monitored by the alaska volcano observatory (avo) only mt. wrangell, the closest volcano to the epicenter (247 km), showed a clear response to the shaking in the intermediate-term (weeks to months) time scale. the response was unexpected because it consisted of a decline by at least 50% in the volcano’s seismicity rate (mostly lowfrequency events) that lasted for five months. because most well documented previous instances of short-term (minutes to days) responses of volcanic centers to the passing waves of distant earthquakes, have all been seismicity increases, the decline in seismicity at mt. wrangell poses a controversial puzzle. by using several independent methods to measure the seismicity rate at the volcano from before to after the main shock, and applying rigorous statistical testing, we conclude that the change in seismicity at the volcano was a real effect of the denali earthquake. we suggest that a depressurization of the volcanic plumbing system took place either as a result of sudden decompression (static stress changes) or because of creation of new pathways resulting from the strong shaking (dynamic stresses). at present we cannot distinguish between these two possibilities. key words: alaskan volcanoes, denali fault earthquake, mt. wrangell, seismicity decreases resumen en noviembre 3 de 2002 durante el terremoto de 7.9 mw, se quebraron tres segmentos de la falla denali en alaska, ofreciendo una única oportunidad de estudiar los terremotos por interacción volcánica. aparte de los 24 volcanes monitoreados sismicamente por el observatorio volcanológico de alaska (avo), sólo el mt. wrangell, el volcán más cercano al epicentro (247 km), mostró una respuesta clara ante el movimiento en un término de escala de tiempo intermedio de semanas a meses. la respuesta fue inesperada porque consiste en un declive de por lo menos 50% en la velocidad sismológica del volcán (frecuencia de eventos cada vez más lenta) que duró hasta cinco meses. puesto que muchos casos de corto término (minutos a días), muy bien documentados previamente, responden a olas de distantes terremotos centros volcánicos, todos han tenido incrementos en la sismisidad; por tanto, el declive sísmico en el volcán tuvo un efecto real sobre el terremoto denalie de mt. wrangell que ocasionó unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquakevolcanoes interactions. 35 un controvertido rompecabezas. usando muchos métodos independientes para medir la velocidad sísmica en el volcán desde antes hasta después del principal choque y aplicando rigurosas pruebas estadísticas, concluimos que los cambios sísmicos fueron un efecto real del terremoto denalie. sugerimos también que la despresurización del sistema del volcán tuvo efecto por la descompresión (drásticos cambios estáticos) o por la creación de nuevos caminos fue producto de un movimiento brusco (acentuada dinámica). hasta el momento no podemos diferenciar entre estas dos posibilidades. palabras clave: volcanes de alaska, terremoto de la falla delani, mt. wrangell, decrecimiento de la sismisidad. © 2004 esrj unibiblos. introduction the interactions between earthquakes and volcanoes have been a subject of scientific interest for a long time and, in recent years, a topic of research thanks to the increasing number of reports and the improvement of precise instrumentation. the link between tectonic and volcanic activity is better understood by considering the scales of time and space: at very long times (hundreds to millions of years) and large distances (tens of thousands of kilometers) the connection is explained under the theory of plate tectonics and one only needs to glance at a map of global tectonics to see that the distributions of earthquakes and volcanoes are not the result of mere coincidence. in the very short time scale of minutes to days and distances of kilometers to tens of kilometers, there is documentation of eruptions following the shaking from an earthquake, such interactions are exemplified by the 1975 summit eruption of kilauea volcano an hour and a half after the 7.5 kalapana earthquake (tilling et al. 1976) and the eruption of cordon caulle volcano two days following the magnitude 9.5 1960 chile earthquake (gerlach et al. 1988; gvn, 2004). in the intermediate-term scale of weeks to months or years, new evidence and research points towards a positive link, meaning that a regional earthquake with magnitude 6.5 or larger can trigger unrest at volcanoes located several hundreds of kilometers away (hill et al, 2002). in all instances of earthquake-volcano interactions, all observations indicate that a distant earthquake is able to cause changes in activity at volcanic (and nonvolcanic) areas in terms of increases in seismicity or eruptions. the opposite effect, however, has not been clearly observed and the literature, with only a few exceptions (alvarado, et al. mcnutt and sanchez, 2002; sanchez and mcnutt, submitted), does not include documentation of activity at volcanoes being inhibited by either the transient or permanent changes induced by a distant earthquake. figure 1. map of alaska showing the surface rupture from the dfe (thick solid line) and the epicenters of the october 23, 2002 nenana mountain earthquake (n) and the three sub events of the november 3, 2002 denali earthquake (s1, s2, s3) (surface rupture from eberhart-phillips et. al., 2003, epicenters of n and s1-s3 are from aeic, 2003); mt. wrangell volcano(star); other volcanoes seismically monitored by avo (dots); and main faults and tectonic structures (gray lines). rectangle encloses the area of figure 2. in november 2002, the mw 7.9 denali fault earthquake (dfe, figure 1) triggered short-lived seismicity at volcanic areas located along the rupture direction at distances as far as 3660 km from the epicenter (eberhart-phillips et al. 2003) and an apparent intermediate-term drop in the seismicity at mount wrangell volcano, located in the perpendicular direction with respect to the rupture and roughly 240 km away from the epicenter. this effect may have lasted for five months. in this paper we provide evidence, by means of statistical testing, that the decline in seismicity observed at mount wrangell volcano was real and can john j. sanchez and stephen r. mcnutt 36 be attributable to the dfe. to do so, we analyze the catalog of located earthquakes at mount wrangell, provided by the alaska volcano observatory and test it for the significance of the changes in seismicity following the dfe. data the alaska volcano observatory (avo) monitors the seismic activity at mount wrangell since july 2000, when two seismograph stations were installed. locations of earthquakes were possible beginning august 2001, when two additional stations were deployed (figure “wrangell area map”). since then and until the time of this writing (march, 2003) 5,451 events triggered the automatic detection system, but only 508 could be located (figure 2). the reason for this low detected/located ratio stems from a combination of low signal-to-noise ratio, limited number of seismograph stations and most important of all, the characteristics of the first arrivals of the earthquakes at the different stations. of all located earthquakes, 86.4% have been identified as lowfrequency seismic events (lf). lf events show emergent first arrivals to the seismograph stations, have not distinguishable pand s-arrivals, and have spectra with dominant frequencies between 1 and 5 hz (minakami, 1974). the remaining percentage of seismic events have been identified as high-frequency earthquakes, which may not different in their source mechanism from earthquakes that occur along tectonic areas such as the denali (alaska) , san andreas (california), or north anatolian faults (turkey). because seismic activity in the mount wrangell area is monitored and recorded in continuous and triggermode we can use three largely different datasets: digitally filtered waveform data and unfiltered analog seismograms of station wanc (located 3.2 kilometers west of the vent) and the catalog of located earthquakes. we will refer to each of these datasets as pseudohelicorder, helicorder, and located earthquake counts, respectively. the time period covered by our different data sets is september 1, 2002–december 31, 2002 for the pseudohelicorder and helicorder event counts and august 5, 2001–march 18, 2004 for the catalog of located earthquakes. method we measure the seismicity rates at mount wrangell based on daily event counts out of the three datasets described and the significance of the changes is estimated by applying the z (habermann, 1987) and β (mathews and reasenberg, 1988) statistical tests to the data in both, ordered and randomized time series. figure 2. detail of rectangular area showed in figure 1. red thick line: surface rupture from the dfe; fault plane solutions shown for the dfe sequence earthquakes in lower hemisphere projections with compressional areas in black and dilatational areas in white; green circles: some of the earthquakes located by aeic between october 23, 2002 and october 31, 2003, with depths above 30 km and magnitude 3 and above. (4,653 epicenters are shown); red squares: volcanic earthquakes (lf and hf) at mt. wrangell located by avo between august 5, 2001 and march 18, 2004; white triangles: seismograph stations on mt. wrangell. see other conventions as in figure 1. we define the seismicity rate changes at the volcano in terms of the rates during the periods before and after the dfe. the z test is a parametric test for estimating the difference between two means, in our case the two means being compared are simply the averages of daily earthquake counts for the period preceding and following the dfe. z-statistic formula is: 2 )2( 1 )1( )21( 22 n s n s mmz + − = (1) where m1, m2 mean rates during the two periods (before and after the dfe); s1, s2: standard deviations of the rates; n1, n2: numbers of earthquakes in each time period. the z value computed is interpreted in terms of significance as the number of standard deviations from the mean of a normal distribution (i.e., z = 1.64 represents 90% significance, z = 1.96 1represents 95% significance, and z = 2.57 represents 99% significance) (habermann, 1987). the sign of the z value indicates the polarity of the change (i.e., z < 0 indicates rates increases, z ≈ 0 indicates no change, and z > 0 indicates rate decreases). we also use a moving windows technique to compute the z values. this is done by finding all possible subdivisions of the data in two halves and computing the z value for all pairs of means, allowing us to estimate the significance of a difference in mean seismicity rates at any given point (time) in the curve. unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquakevolcanoes interactions. 37 we investigated the sensitivity of the z values versus time curve to the presence of abrupt changes in seismicity rates by computing the z values of several synthetic data sets with varying amounts of change (decreases) in the number of earthquakes per unit time. our synthetic datasets have the following features: (1) the seismicity rates from before to after the change are different but remain constant within each period; (2) the seismicity rate after the change is some normalized percentage of the seismicity rate before the change; (3) the value of the earthquake count at the middle point of the time series is zero, simulating a useless record the day of the dfe. to somewhat mimic our pseudohelicorder counts data, we created synthetic time series containing one break point and two segments, simulating drops in seismicity of 100% (total drop of seismicity), 95%, 75%, 50%, 25% , and 0% (no change in seismicity). we also investigated the effect of having a seismicity rate that decreases in two steps preceding the main drop. we did this by creating a data set that has two break points and three segments with constant rate each. in this way we simulate a constant rate, and a drop of 25% that precedes a second drop of 50%, after which the seismicity stays at that level. the length of these synthetic data sets is 101 data points. to mimic our data of located lf seismic events, we created a longer synthetic data set in which there are two break points and three segments. in this case the second segment contains a drop in seismicity of 80% and the third segment represents a recovery of the seismicity rates to background values (rate equal to first segment). the length of this synthetic data set was 487 data points. seismicity rates changes, can also be characterized using the -statistic as described mathews and reasenberg (1988): ttn ttnn n nen ab baba a aa / )/( )var( )( − = − =β (2) where: na=number of earthquakes in the region of interest after the perturbing main shock. ta=time period after. e(na)=expected number. e(na) = r.ta, and r = nb/tb is the ambient rate of earthquakes estimated from a background period, tb. nb= number during the background period. note that for a binomial distribution var(na) = npq where p = ta/t, q = 1 – p = tb/t, t = ta + tb and n = na + nb. because the observed value of na may be anomalous, e(na) is used instead, and n = nb(1 + ta/tb). the sign of β in this case indicates the polarity of the change in seismicity rate (i.e., β < o represents decrease in rates, β ≈ 0 means no change, and β > 0 represents rates increases). because there are various methods to compute β that differ in their underlying assumptions and the validity of those assumptions cannot readily be verified (gomberg et al. 2003) we do not ascribe absolute significance level to the rate variations computed with β statistic and only use to spatially assess the nature of the seismicity rate changes in the area of mt. wrangell. results pseudohelicorder and helicorder counts the plots of cumulative daily number of volcanic earthquakes estimated from pseudohelicorder plots and helicorder records are shown in figure 3. the absolute values in numbers of events are different because of the different criteria used to count earthquakes on each type of record and the different noise levels (usually less noise in pseudohelicorder plots) and sensitivities. the change in slope indicating the change in seismicity rate is more obvious in the pseudohelicorder counts curve. the z values at the date of the dfe are z = 6.7358 and z = 2.5531 for pseudohelicorder and helicorder counts, respectively, which correspond to confidence levels for the difference in mean rates before and after de dfe above 99.99% and 99% respectively. from the cumulative number and z values plot for pseudohelicorder counts, we note that the decrease in seismicity begins before the dfe, around the time of the mw 6.7 nenana mountain earthquake (october 23, 2002). the helicorder count data although somewhat noisier also shows the change in rates. we tested the hypothesis that the data showing the suggested decrease in seismicity at mount wrangell is no different from background noise, represented by many random series. to do this we randomly permute our data sets of pseudohelicorder and helicorder counts, plot the cumulative number of earthquakes versus time and compute the z values for the permuted series. this is done 100 times for each data set being analyzed and we always plot the randomly permuted series along with the original data. we observe less spread in the random series for pseudohelicorder counts; also the data from pseudohelicorder records equals the random noise towards both ends of the time series, but is different towards the middle of the series. the helicorder count data is no different from the random noise. john j. sanchez and stephen r. mcnutt 38 figure 3. seismicity rates and z values estimated from earthquake counts on pseudohelicorder and helicorder records from station wanc on mt. wrangell., between september 1, 2002 and december 31, 2002. top: cumulative number of volcanic earthquakes versus time; bottom: plot of z values versus time. squares and circles represent the cumulative sum and z values curves, respectively, for actual data; gray lines represent the result of 100 random permutations of the data in each plot. vertical dashed and solid lines mark the dates of the nenana mountain earthquake (10/23/2002) and the denali fault earthquake (11/3/2002), respectively. percentages indicate the confidence levels at which the two means, before and after the dfe, are different as evaluated by the z-test. analysis of synthetic counts datapseudohelicorder counts. figure 4 shows several cumulative numbers versus time and z values versus time plots for six synthetic data sets all representing a sharp decrease in seismicity, with varying amounts of change. the results show that the spread in the randomly permuted counts increases as the amount of change in seismicity rate increases and so does the separation between observations and random permutations. this means that the stronger the rate change, the more we should be able to see it above random noise. also we see that even in the case of small rate changes the z values versus time curve is sharply peaked near the time of the change. careful examination of the z values curves reveals that exactly one data point before the time where the rate change starts, the peak occurred. this is not the figure 4. cumulative sum and z values plots for six synthetic data sets with one break point in the middle and varying amounts of rate decrease. a vertical dashed line marks the time of the change, in which a value of zero in the counts has been assigned, simulating a useless record. the top plot of z values, for a drop in seismicity of 100%, has been cut off at z≈50 because the peak value is at infinity. other conventions as in figure 3. caused by the number of data points being either even or odd, or by the presence of a zero count at the time of the change, nor by the position of the time of change in the data set. the effect is expected because the z values curve is tracking the evolution of the cumulative sum in time. let n be the total number of data points and ni be the position of the breakpoint. the cumulative curve will be constant until exactly ni1 and then drops to a different rate. that is where the two segments being compared are the most different. an additional interesting result from our testing is the observation that the steepness of the rise in z values depends on the strength of the rate change. unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquakevolcanoes interactions. 39 the z values curve is sharply peaked for large changes in rate and rises more gradually for small rate changes (figure 5). figure 5. the data sets used in figure 4, plotted at the same scale to allow comparison of the z values curve. the inset is an enhanced view of the rectangular at the base of the z values plot. see other conventions as in figure 4. the analysis of a synthetic data set with two break points (figure 6) shows that when a minor decline in rate precedes a larger one, the z values curve is doublepeaked, but surprisingly, the first peak has a larger z value, regardless of the length of the two segments preceding the main break point. as the length of the second segment shortens, the peaks come together into one. similar effects are also present in the pseudohelicorder count data, with the two largest peaks in z values occurring several days before the date of the dfe. the first peak in z values occurs on october 15, 2002, and it is sensing a zero count value (unusable record) on october 16, after which the counts drop for a few days. we tested the influence of this zero value by replacing it with a value similar to its neighbors and the double peak persisted, although with slightly decreased significance. figure 6. cumulative number and z values plots for synthetic data with two break points representing an initial drop in rate by 25% followed after sometime by a larger drop of 50%. vertical arrows mark the time of the drops. the start time of the first (and smaller) drop varies, while the start time of the larger drop is constant. other conventions as in figure 3. the second peak in z values occurs on october 23, 2002 and it is sensing a general decrease in number of daily counts (mostly good days) leading to the strongest drop on november 3. we suggest that the relatively gradual increase in z values leading towards the peak on october 23 reflects the magnitude of the seismicity drop following the dfe, although we cannot reject the possibility that the decrease in seismicity actually started near october 23, 2002. john j. sanchez and stephen r. mcnutt 40 analysis of synthetic counts data-located earthquake counts. when we use a larger data set of 487 data points (equal to the number of located lf seismic events) simulating a time series with a final segment in which the rates recover to the background values, we observed that the z values curve has two peaks, the first one representing the drop in seismicity and a second one, with negative z value, indicating the recovery (increase) to background rates (figure 7). figure 7. z values and cumulative sum versus time for a synthetic data. this simulates a recovery to pre-perturbation rates following a drop in seismicity by 80%. vertical gray and black arrows mark the start times of the drop in rate and recovery, respectively. other conventions as in figure located volcanic earthquakes we examined the catalog of located earthquakes in the mount wrangell area from august 2001 through march 2004. in figure 8 we show a time-depth plot of earthquakes at mount wrangell. notable changes in seismicity rate include the apparent increase associated with the introduction of earthworm acquisition system (johnson and others, 1995; dixon et. al., 2003); the decrease following the dfe, and two other subsequent apparent decreases (july – september 2003 and december 2003) associated with temporary telemetry problems and data disruptions. we used the computer program zmap (wiemer, 2001) which is suited for analysis of located earthquakes and includes routines to estimate changes in seismicity rates. the catalog includes 508 events with magnitude from -0.2 to 2.7. an automatic estimate of the magnitude of completeness indicates the catalog to be complete for magnitudes > 0.9, in agreement for the value reported by dixon et al. (2003). to avoid artifacts introduced by artificial rate changes, we select a subset of earthquakes between early march 2002 and late june 2003 (dotted box in figure 8), also to ensure the completeness of the samples used, we select earthquakes with magnitudes > 0.9 for subsequent analysis. the other decreases in seismicity rates observed in the catalog of are unrelated to large earthquakes along the denali fault because the most recent event of note was the mw 5.7 earthquake on 22 october, 1996 (ratchkovski et al. 2003), but rather these decreases are caused by other factors such as temporary low signal-to-noise ratio or stations outages. we note that the decrease in seismicity at wrangell following the dfe was unrelated to “artificial” causes because the same analyst located all the volcanic earthquakes and the stations were operational during the time of the decrease. figure 8. comparison depth-time plot for located earthquakes between august 5, 2001 and march 18, 2004 at mt. wrangell. red squares and blue circles with gray error bars mark lf and hf earthquakes, respectively. only earthquakes with magnitude > 0.9 are shown. arrows indicated the two largest hf earthquakes and the time of change from iaspei to earthworm acquisition systems. a dotted rectangle encloses the most homogeneous part of the catalog, between march 1, 2002 and june 30, 2003 that is used for all analysis of located earthquakes. other conventions as in figure 3. the catalog of located earthquakes at wrangell from august 5, 2001 to march 18, 2004, includes at least three decreases of noticeable duration. the only unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquakevolcanoes interactions. 41 decrease in seismicity at mt. wrangell following a large earthquake along the denali fault is the one that followed the dfe. the other two decreases are artificial (data disruptions caused by temporary telemetry problems) and are not preceded by large earthquakes in the area or by moderate-sized (with magnitude 5.0 or larger) aftershocks of the dfe (aeic, 2003). thus although the seismicity rates at mt. wrangell fluctuate, the coincidence of the rate decrease onset time with the dfe sequence suggest a causative relation and not the result of chance. to test the hypothesis that the decrease following the dfe, as estimated from located earthquakes, may not be different from random noise, we follow our previous procedure of randomly permuting our data sets of located earthquakes counts, plot the cumulative number of earthquakes versus time and compute the z values for the permuted series. in this case we repeat the permutation 100 times and show the results along with the original data in figure 9. we do the analysis for lf and vt earthquakes separately. for lf events the cumulative number versus time plots shows several changes in slope that represent decreases in seismicity. the decrease associated with the dfe sequence, however, represents the breakpoint with the most significance, because the peak in z value happens near the time of the dfe and nowhere else. we found that for lf events the cumulative number and z values curves are different from their respective permuted series. we note that data of located lf seismic events does not show a negative peak in the z values curve that we observed by creating a synthetic data set in which there was a recovery towards background rates. for hf earthquakes the ordered data is no different from the randomly permuted series. the catalogs of located lf and vt earthquakes show that the decrease in seismicity initiated after the dfe, this is because some earthquakes could be located during the immediate few days following the dfe. spatial mapping of the amount of decrease using βstatistic to examine spatially the decrease in seismicity in the mt. wrangell area we divide the are of interest in a grid with nodes separated 3 km (average of horizontal location errors) and compute the β-statistic using equation (2) for the 30 nearest earthquakes to each node. the two periods being compared to compute the β-statistic are: march 1, 2002 – november 3, 2002 and november 3, 2002 – june 30, 2003. note that the two time periods are of roughly equal duration (eight months each). the map of the β-statistic for the mt. wrangell area is shown in figure 10 and it shows a general decrease in the seismicity rates everywhere on the volcano. a small area located sw of the vent experienced relatively little to no change, but this is a region with sparse seismicity. the catalog used is rather small, but the seismicity is distributed so that figure 9. cumulative sum and z values versus time plot for earthquake counts based on located events between march 1, 2002 and june 30, 2003 (dotted rectangle in figure 8). left: plots for lf seismic events. right: plots for hf earthquakes. conventions as in figure 3. the radius of circles used to enclose the 30 nearest earthquakes to each node varied between < 2 km for areas around and west of the vent to 10 km around the edges of the region mapped. discussion we examined the rate of occurrence of volcanic earthquakes at mt. wrangell from before to after the dfe, at different time scales and using different data sources, to find the amount and significance of an observed decrease in volcanic seismicity. we think we can reliably measure this decrease because mt. john j. sanchez and stephen r. mcnutt 42 wrangell is an area with high rates of background seismicity, and such places are optimal for observation of seismicity declines (toda and stein, 2002). the decrease that we see in the seismicity at mt. wrangell figure 10. map of the mt. wrangell area showing the seismicity rate changes, as evaluated by the β-statistic. the two time periods being compared are: pre-dfe: march 1, 2002 to november 3, 2002; postdfe: november 3, 2002 to june 30, 2003. triangle: vent. crosses: seismograph stations (wanc is the station immediately west of the vent). gray circles and black squares mark the epicentral locations during the pre-dfe and post-dfe periods, respectively. gray lines: topographic contours. following the dfe is not the decay from an aftershock sequence of a large earthquake rupturing along the denali fault. the seismicity along the denali fault is not collocated with the seismicity at mt. wrangell (figure 2) and because we carefully checked all seismograms at station wanc on mt. wrangell, we could easily identify non-local earthquakes and discard them from our counts (see for example figure 3 of sánchez and mcnutt, submitted). we used rigorous statistical testing of the earthquake data and found the rates of local volcanic earthquakes (lf and hf together) to have decreased by 40% when the pseudohelicorder record data is used in the time scale of two months before to two months after the dfe (figure 3). in some areas around the volcano the seismicity may have decreased by as much as 80% when the located earthquake data from eight months before to eight months after the dfe is used (figure 9). although visual inspection of the data suggests that the rates of both lf and hf earthquakes decreased (figures 8 and 9), it is statistically difficult to prove for hf earthquakes because of their low rate of occurrence. from the depth-time plot and the analysis of cumulative number of located earthquakes we found that the decrease lasted for five months after which the seismicity seems to recover to pre-dfe levels. we investigated the question of why the z values curve does not seem to peak exactly on november 3, 2002 (the time of the dfe), but in most cases, before that time. by computing the z values on six synthetic data sets with varying amounts of rate decrease (figures 4 and 5), we found that the z values curve peaks exactly one data point before the drop in seismicity begins, which is expected because the rate before the change is constant exactly until that data point. an obvious conclusion of this test is that if the rates of seismicity are not perfectly constant within each period, we should expect to find variations in the behavior of the z values curve and the presence of multiple peaks of varying significance, which is what we see in our data (figure 3). we also found that the z values curve is highly sensitive to sharp rate changes, regardless of the amount of change. we also tested the effect, in the z values curve, of having a decrease in seismicity in two steps before the main perturbation and the effect of a seismicity rate that fully recovers sometime after the perturbation. the motivation for these two tests is the fact that our pseudohelicorder count data shows an apparent decrease in rates beginning sometime before the dfe (figure 3) and because in the catalog of located earthquakes it appears as if the seismicity recovers to pre-dfe levels (figures 8 and 9). by simulating a two-step decrease in seismicity preceding the main rate change we observed that the z values curve is double-peaked and that the first peak indicates a more significant change (figure 6). this apparently puzzling result comes about because the z values are a combination of the difference in means, the standard deviations of the two samples, and the number of data points in each sample (equation. 1). examination of figure 6 indicates that at the position of the first peak: m1-m2= 0.4221, s1 =0, s2=0.1361, and n2=77; whereas at the second peak m 1 m2=0.3798, s1=0.1262, n1=50, s2=0.07, and n2=51. thus it is apparent that the relative values of the peaks depend mostly on the difference in means and the number of data points on each segment. in the z values curve for our pseudohelicorder counts data we see some of the features identified in the curves for synthetic data. the pseudohelicorder counts data shows a z values curve with “regional” maxima that happens before the dfe but we also observe that this maximum is double-peaked, indicating that there were unexpected response of mt. wrangell volcano, alaska, to the shaking from a large regional earthquake: a puzzle for intermediate-term earthquakevolcanoes interactions. 43 some step-wise changes in earthquake rates at the volcano. from the analysis of the located events at wrangell, on the other hand, we learned that the seismicity may have dropped after a few days following the dfe. we see this in figures 8 and 9. by separating lf events from hf events we infer that both types of events may have decreased in rates, but because the low number of hf events available it makes it statistically difficult to prove. the results of tests with synthetic data suggest that unless the rates of seismicity are fairly uniform before and after the perturbation, it is difficult to use the z values to pinpoint exactly the time of change. this does not mean the plot is useless, because the z values curve does indicate the presence of one or several important changes in rates within the data set under scrutiny, such as in the case of the data presented here. we also simulated a time series in which an 80% drop in rate is followed by a recovery to the original rates (figure 7). in this case we observed that the z values curve has two peaks, one positive and one negative. we made this synthetic data set based on some features of our located earthquakes counts data in which a drop of about 80% in seismicity is followed by an apparent recovery after five months of lowerthan-normal rates (figure 8 and 9). because we do not see the negative peak in the z values curve of our real data, we infer that the seismicity at mt. wrangell did not recover fully to pre-dfe rates, at least until the end time period analyzed (june 30, 2003). although seismicity rates at volcanoes vary in the absence of evident external factors, the coincidence of the decrease with the time of the dfe suggests that the drop in seismicity at wrangell was not the result of chance. the total duration of the catalog of located earthquakes, including those with m < 0.9, is 957 days, or roughly 32 months; we see in this time period (figure 8) three noticeable drops in reported seismicity (one of them following the dfe, the other two are artificial) and the average duration of these three periods of low seismicity is ~ 3 months. thus there are about 11 3-month-long periods during which we may or may not find a decrease. in the context of a binomial distribution, we have n =32/3 = 11 trials and three successes (decreases). the probability p of success in a given binomial trial is p=3/11=0.27. this means that the probability of not finding an anomalously low-seismicity period is q = 1p = 0.73, thus about 73% of the time we find that the seismicity is at background rates. if we consider a more homogeneous catalog, with magnitudes > 0.9, and only during march 1, 2002march 18, 2004 (since data acquisition with earthworm began) the number of available months and 3-month trials get reduced to 25 and 8, respectively, and the probability of finding a decrease rises to p = 0.36, making it a more common fact. but this may be a biased test, because we know that the two latest decreases are man-made. in our view, a more balanced test would only include the period march 1, 2002 – june 30, 2003 (dotted box in figure 8). in this case there are 487 days or roughly 16 months, which means we have only n = 16/5≈3 trials. the probability p of finding a decrease is p =1/3 = 0.33, thus q = 0.69. this means that about 70% of the time we would find the seismicity to be at background rates. because the only large earthquakes along the denali fault during the time period march 1, 2002 – june 30, 2003, were associated with the dfe sequence and because the decrease in seismicity began rather close in time to the dfe, we infer that it was not a chance occurrence but that the cause was a perturbation of the volcanic plumbing system caused by either the dynamic shaking or the sudden drop in pressure around the volcano (sánchez and mcnutt, submitted). a spatial mapping of the β-statistic (figure 10) shows a general decrease in seismicity around mt. wrangell with some patches of larger change located south, north and west of the vent and an area of little to no change southwest of the vent. the mapping should we interpreted with caution because the locations of earthquakes at mt. wrangell are preliminary. conclusions a decrease in volcanic seismicity occurred at mt. wrangell following the denali fault earthquake sequence that began with the mw 6.7 nenana mt. earthquake and ended with the nucleation of the mw 7.9 denali earthquake on november 3, 2002. our data indicates that the volcanic seismicity dropped by as much as 80% during the five months that followed the dfe sequence. our test with synthetic data showed that unless the seismicity rates are fairly constant during the pre and post-perturbation periods, we could not use the z values curve to pinpoint exactly the time of change. the presence of a broad peak, however, indicates a significant change in the rates and that is what we see in the data. because of the time coincidence of the decrease in the seismicity rate with the time of the dfe sequence we think this is not a change occurrence, but an effect attributable to the dfe sequence. a comparison of our data of located earthquakes at mt. wrangell with a synthetic data set reveals that the seismicity at wrangell during the period november 4, 2002 – june 30, 2003 did not recover to the pre-dfe levels. the mapping of the β john j. sanchez and stephen r. mcnutt 44 statistic indicates that the decrease in seismicity occurred around the general mt. wrangell area, with some patches showing a stronger change than others. acknowledgements the authors wish to thank j. grasso, e. brodsky, k. felzer and c. benson for useful comments and discussion. special thanks to s. stihler and j. dixon for their hard work in the location of 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(2003) “aftershock sequence of the mw 7.9 denali fault, alaska, earthquake of 3 november 2002 from regional seismic network data. seismological research letters. 74,6, 743-752. sánchez, j. j. and s. r. mcnutt. (2004) “intermediateterm declines in seismicity at mt. wrangell and mt. veniaminof volcanoes, alaska, following the november 3, 2002, mw 7.9 denali fault earthquake”. submitted to bulletin of the seismological society of america. tilling r. i., r. y. koyanagi, p. w. lipman, j.p. lockwood, j.g. moore and d. a. swanson (1976) “earthquake and related catastrophic events, island of hawaii, november 29, 1975: a preliminary report”. u.s. geol. surv. circular 740: 33 pp. toda, s. and r. stein (2002) “response of the san andreas fault to the 1983 coalinga-nuñez earthquakes: an application of interaction-based probabilities for parkfield”. journal of geophysical research, 107, b6, ese 6, 1-16. wiemer, s. (2001) “a software package to analyze seismicity: zmap”. seismological research letters. 72,2, 374-383. atenuacion de intensidades sismicas en el territorio colombiano elkin salcedo hurtado instituto de investigaciones en geociencias, minerfa y qufmica-ingeominasarea de geoffsica carlos coral gomez profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia salcedo, e. 8< c. coral (1995): atenuaci6n de intensidades sfsmicas en el territorio colombiano. geoffs. colomb. 3:37-44. issn 0121-2974 resumen con base en los mapas de isosistas existentes para los terremotos ocurridos en el territorio colombiano, se han encontrado valores del coeficiente de atenuaci6n para las zonas sismoactivas del pals. ei calculo de los coeficientes de atenuaci6n s y c se hizo para los terremotos cuyo esquema de isosistas contiene no menos de tres isolineas. este catculo se hizo midiendo la longitud media del eje a 10 largo de la ex1ensi6n de las isosistas, utilizando la f6rmula del campo macrosfsmico propuesta por shebalfn (1968) y aplicando el rnetodo de los mfnimos cuadrados. los valores de c obtenidos varfan dentro de un intervalo muy grande de 0.5 a 13.9. la variaci6n del coeficiente s se encuentra en el rango de 3.3 a 7.0. la dispersion de estos valores parece estar relacionada con la complejidad tect6nica del territorio colombiano, 10 cual influye notable mente en la forma como se propagan las ondas sfsmicas. abstract by means of using isoseismal map of the strong colombian earthquakes, attenuation coefficients sand c of the shebalin macroseismic formula (1968) were found. computations were performed measuring the mean radius of isoseismal axes, according to the shebalin macroseismic field method. the obtained c values have a relative broad of variation, from 0.5 to 13.9; and the found s values change from 3.3 to 7.0. dispersion in the observed coefficient values seem to be related to the tectonic complexity and anisotropy of the colombian territory medium which strongly ahects the propagation of seismic energy. 1. introduccion las ondas slsmicas, al igual que otros tipos de energfa, se atsnuan a medida que se propagan y se alejan de su origen. en medios reales, la atenuaci6n de las ondas sfsmicas depende de la dispersi6n del frente de ondas, de la absorcion del medio y de los fen6menos de interferencia que se pueden presentar en su recorrido. en general, la energia liberada por un terremoto se disipa en el espacio debido a atenuaciones de tipo geometrico y rnecanico que caracterizan la transmisi6n de las ondas sfsmicas en el interior de la tierra y que determinan la intensidad del terremoto en la superficie. de 10 anterior se desprende que estudiando la forma y distribuci6n de las curvas de igual intensidad sismica 0 isosistas de los terremotos, es posible deducir las caracteristicas de atenuaci6n de las ondas sismicas de un terremoto. en el presente trabajo se presentan resultados del estudio de isosistas de varios terremotos, realizado con el fin de determinar los. coeficientes de atenuaci6n de intensidades sismicas en el territorio colombiano. ei coeficiente de atenuaci6n de la intensidad es un para metro utilizado para caracterizar las propiedades dinarnicas del medio geoffsico en limites de las zonas epicentrales. su importancia radica en que es un pararnetro fundamental en la estimaci6n del riesgo sfsrnico y es util en estudios de pron6stico de terremotos. 2. determinacion del coeficiente de atenuacion de intensidad sismica generalmente, los efectos causados por los terremotos en la superficie terrestre se pueden relacionar con las caracteristicas del foco de dos maneras: a. en forma de una ecuaci6n denominada ecuaci6n de correlaci6n entre la intensidad en el epicentro, la magnitud y la profundidad del foco (10' m, h) , la cual tiene la forma: 10 = f,(m,h) (1 ) b. en forma de la ecuaci6n liamada f6rmula macrosismica que representa la forma como decae la intensidad al aumentar la distancia desde el epicentro. 10 i, =: f2(8.,h) (2) ambas ecuaciones describen un mismo proceso de atenuaci6n de la intensidad en la superficie terrestre en dependencia de la posici6n y la fuerza de la fuente sisrnoqenica. la expresi6n general que relaciona la magnitud del terremoto y la intensidad i;en cualquier punta de la superficie terrestre a la distancia r; = './(tj.21+ h2 j) del foco, se puede escribir por medio de la f6rmula de sheballn (1968) asi: donde: i; = intensidad de la i-isima isosista; b = constante; m = magnitud; tj.i= distancia media al epicentro (radio medio de la i-isima isosista); tj.i= distancia al epicentro; h = profundidad focal; s y c = coeficientes de atenuaci6n de la intensidad i con la distancia, que definen la dispersi6n efectiva, la que a su vez incluye la dispersi6n propiamente dicha y la absorci6n de las ondas sismicas. en esta f6rmula debe notarse que cuando tj.> 3h y existe un amplio limite de variaci6n de s, la cantidad h2 bajo la raiz puede despreciarse y la f6rmula (3) toma la forma: ei valor del coeficiente b se toma generalmente igual a 1.5 # (sheballn, 1968, 1974), por tanto, 1.5m i; = slgtj.1 c: i ~ 3 donde i = nurnero de isosistas contando desde el epicentro. la ecuaci6n (5) es de la forma (y = sx c), que es la ecuaci6n de una recta. de esta manera, graficando (1.5m ii) contra ig~ y por el metoda de los minimos cuadrados podremos obtener para cada zona sismoactiva del territorio colombiano los valores regionales de los coeficientes s y c. 3. campo macrosismico la complejidad tect6nica del territorio colombiano hace que sea una regi6n sismicamente muy activa. en la cual los terremotos ocurren a causa de diversas fuentes slsmogsnicas. . la ocurrencia de los terremotos en colombia tiene su origen principalmente en las multiples fracturas del terreno que en algunas casos se revelan en la superficie y en la zona de subducci6n donde la placa nazca se introduce bajo el continente suramericano. sin duda alguna, en colombia son muchos los terremotos que ocurren a causa de la dlnamica de las grandes placas litosfericas de nazca, caribe y sur america, las cuales convergen con velocidades diferentes en la esquina noroccidental del continente suramericano. la actividad sismica del territorio colombiano se hace mas evidente en la regi6n andina, parte de la costa del caribe y la regi6n del pacifico. sin embargo, la distribuci6n de los epicentros en esta regi6n no es hornoqenea sino que se notan algunas zonas donde la concentracton de epicentros es mayor y las cuales corresponden a las regiones de santander con el "nido de bucaramanga", choco, caldas y narino (coral-g6mez, 1985). estas zonas epicentrales poseen caracteristicas stsmoloqicas particulares y se diferencian por sus indices de actividad sismica, nivel de magnitud caracteristica, energia liberada por afio, profundidad y mecanismo de los focos, etc., (coral-g6mez, 1987, 1990; salcedo, 1992). (3) en la historia de la sismicidad colombiana han ocurrido terremotos con intensidades epicentrales (10) de hasta x y xi grados en la escala modificada de mercalli. las isosistas de los terremotos colombianos mas importante ocurridos durante los ultirnos 120 anos han sido publicadas por ramirez (1975a,b), ramirez & goberna (1980), meyer et. al. (1986), sarria (1985) y coral-g6mez & salcedo (1992). en el presente trabajo se analizan las isosistas de los terremotos tectonicos superficiales e intermedios mas fuertes ocurridos en el territorio colombiano con intensidad en el epicentro mayor que vi, para los cuales los esquemas de isosistas contienen no menos de tres isolineas. en nuestro estudio hemos utilizado 22 terremotos con las caracteristicas arriba anotadas, los cuales se describen en la tabla 1. (4) tabla 1 lista de terremotos ocurridos en colombia y utilizados en este trabajo (5) fecha coordenadas dfa mes ano 'q>°n "a.°w 18051875 7.9 72.5 31 01 1906 2.6 77.9 08071950 7.6 72.8 21041957 7.0 72.0 16061961 8.8 73.4 20121961 4.6 75.6 30071962 5.2 76.4 09021967 2.9 74.9 21 031967 6.8 73.0 2 3 4 5 6 7 8 9 10 29071967 6.8 73.0 11 0705 1968 6.7 73.0 12 26091970 6.1 77.6 13 0304 1973 4.7 75.6 14 30081973 7.3 72.8 15 19051976 4.5 75.8 16 31 08 1977 7.3 76.3 17 2311 1979 4.8 76.2 18 12 12 1979 1.6 79.4 19 2611 1980 8.0 72.4 20 17 10 1981 8.2 72.5 21 31 031983 2.5 76.7 22 18 10 1992 6.9 76.9 profun. magnitud regi6n h, km mb 20 santander naririo 41 6.7 santander sup. 6.4 santander 120 6.0 santander 176 6.3 caldas 69 6.5 cho~6 60 6.3 huila 161 5.6 nido de b/manga 161 6.2 nido de b/manga 168 5.7 nido de b/manga 8 6.6 choc6 158 6.2 caldas 181 5.7 nido de b/manga 158 5.9 caldas 33 5.7 choc6 108 6.3 choc6 24 6.4 narino 40 5.0 santander 56 5.5 santander 12 5.5 popayan sup. 7.2 choc6 la fig.1 muestra, a manera de ejemplo, esquemas de isosistas de terremotos ocurridos en las principales regiones sismoactivas de colombia y que figuran en la tabla 1. en la region de santander, la cual incluye el "nido de bucaramanga", la forma de las isosistas de los terremotos varia de acuerdo a la profundidad de los focos. para los focos ocurridos por encima del "nido de bucaramanga", es decir, para tocos con profundidad menor de 100 kllomstros, la forma de las isosistas no es elipsoidal y no refleja con claridad una direccion predominante de ruptura que pueda ser tomada como unica causa de ocurrencia de los terremotos de la zona. ei eje mayor de las isosistas, que tienen una forma casi que isometrica, dificilmente puede distinguirse; 10 cual refleja la existencia de una estructura tectonica compleja, que no permite representar una orientacion definida de movimiento interno. esta complejidad tectonica tam bien se refleja en la superficie y se aprecia en el carnbiode orientacion de los esfuerzos nsotectonicos observados en diaclasas recientes de la cordillera oriental (mojica, 1985). un cuadro un poco diferente puede observarse en las isosistas de los terremotos mas profundos ocurridos en la region de santander, es decir, los focos con origen en el "nido de bucaramanga" a profundidades entre los 140 y 180 kilo metros. la forma geometrica de las isosistas para estos eventos tampoco es elipsoidal. ei eje mayor de las isosistas de los terremotos del "nido de bucaramanga" esta orientado principal mente de sw a ne, el cual estaria asociado con una estructura de fallas que puede pensarse serfa la continuacion de la falla de bocono. tarnbien se observa en algunas de las isosistas de estos terremotos una elonqacion orientada en direcci6n nw a se que puede estar asociada con la falla de bucaramanga; se nota, acemas la presencia de otra elonpacion en las isosistas menores que v, orientada en direcci6n un poco mas al este-oeste respecto a la anterior. de esta manera, las isosistas de los terremotos intermedios del macizo santandereano reflejan la existencia de por 10 menos tres discontinuidades orientadas en distintas direcciones que se cruzan en un punta de excitaci6n de las oscilaciones sfsmicas, el cual puede ser el origen del "nido de bucaramanga". isosistas semejantes tarnbien se observan en otras regiones sismoactivas con focos de profundidad intermedia tales como las regiones de brancha y el hindo kush (drumia et. ai., 1990; dobrev & shchukin, 1974). las isosistas de los terremotos ocurridos en la zona del cnoco tarnbien varian en dependencia allugar de ocurrencia. es decir, como la :zona sismica del choco se tocatiza cerca de la zona de subduccion, ocurren en ella terremotos con hipocentro tanto en la parte continental como en el oceano y dependiendo de esto observarnos formas de isosistas diferentes. por ejemplo, para dos (2) terremotos (31.08.77 y 18.10.92) ocurridos entre los ifmites de los departamentos de antioqufa y chaco, la forma de sus isosistas es muy similar, en elias se puede observar una forma elfptica can su eje mayor orientado en direccion nw a se, 10 que supone la presencia de una fractura superficial ubicada en esta direcci6n, la cual ha sido reconocida geologicamente y denominada falla de murindo. de esta manera, podemos decir que la falla de murindo es una falla de actividad considerable, capaz de producir fuertes terremotos, y no de baja actividad como la ha c1asificado page (1986). un poco mas al sur dentro de la misma region sismica del choco, en los limites con el departamento de risaralda, ocurrsn terremotos cuyas isosistas con forma eliptica tienen su eje mayor orientado de ne a sw. de estas isosistas se deduce que las ondas sfsmicas en esta zona atsnuan mas lentamente que en el caso de los terremotos ocurridos en la fuente slsmoqenica de murindo. para hipocentros can origen en el oceano, las envolventes de sus isosistas muestran como las ondas sfsmicas atenuan rapldarnente en el continente, reflejando asi la anisotropla del medio. en la region del antiguo caldas, reconocida tarnbien como una de las regiones sismicamente mas activas del territorio colornbiano, los focos de los terremotos son de origen intermedio. puede observarse que las isosistas tienen su eje mayor orientado de ne a sw, de forma casi igual a las isosistas de los terremotos de la regi6n del choco en su parte limftrofe con el antiguo caldas. los terremotos de estas dos zonas vecinas se diferencian por la profundidad de sus focos, 10 cual revel a la presencia de una fuente sisrnoqenica de gran anchura y profundidad que se extiende hasta el manto superior, que no necesariamente se revela en la superficie. para la regi6n de nariiio donde los focos de los terremotos, al igual que en la regi6n del choco, tienen lugar tanto en el oceano como en el continente, hemos analizado el mapa de isosistas de un (1) terremoto can origen en el oceano; estas isosistas muestran que las ondas sfsrnicas se atenuan raptdarnente en direcci6n al continente, reflejando tarnbien la zona de subducci6n. 4. resul tacos can base en los mapas de isosistas existentes para los terremotos ocurridos en el territorio colombiano descritos anteriormente, se han calculado los valores del coeficiente de atenuaci6n para cad a una de las zonas sismoactivas de colombia. para esto hemos tenido en cuenta que todos los terremotos sean tlpicos para cada zona sismoactiva. cuando existe gran diferencia de sus caracteristicas, por ejemplo la existencia en una zona determinada tanto de terremotos superficiales como profundos, 10 mejor es dividir el material del cual se dispone en dos a tres grupos, como 10 hemos hecho para la zona de santander, de la cual hace parte el "nido de bucaramanga". a continuacion, presentamos los resultados de nuestra observaci6n. fecha, julia 29 de1961 magnitud mb' 6.2 pralundidoddel 1000'161 km. la ritud epicentra' 6.80 n langitud epicentra'73.00w 10 hrs 24 min. 24.6 seg. 50 50 100 zoom. ! ! i i i venezuela brasil "nido· de bucoromongo fecho: abril 3 de 1973 magnltud mb= 6.2 pralundidod del laca= 158km. latitud sp.centro e 4. 70 n langitu epicentra= 75. 60w 13 hr s. 54 min 01.8 segs. so 0 50 100 200km i ! ! i i i venezuela brasil region de cord os peru fecho: septiembre 26 de 1970 mognitud mb e 6.! • valleduparpralundidad del loco' 9 km l latitud epicentra' 6.12 n langitud epicentro'77.594w 12 hrs. 0.2 min .29.3 seg. 50 0 50 100 200m. i , ! ! ! , ecuador region del choco peru figura 1. esquemas de isosistas para sismos en diferentes regiones slsmoactivas de colombia (tomadas de sarria, 1985) fecha: diciembre 12 de 1979 magnitud mb= 6.4 pralundidad de loco: 24 km. latitud epicentro e j584n langitud epicentro= 79.38 w 0.7 hrs. 59mln.0 3.3 seg. so 0 50100 200km , , ! ! , ! venezuela brasil region de norino peru nido de bucaramanga un enjambre de terremotos se l c c a l i z a a una profundidad entre los 140 y 180 kil6metros, al cual se ie ha denominado "nido de bucaramanga" . para nuestro objetivo, fue tacil distinguir el eje mayor de las isosistas de cuatro (4) terremotos fuertes ocurridos en esta zona. se obtuvieron 18 ecuaciones de la forma de la ecuaci6n (5) del campo macrosfsmico de shebalfn, la soluci6n se obtiene mediante la aplicaci6n del metoda de los mfnimos cuadrados; s = 3.8 y c = 5.2 (fig.2). santander como ha sido anotado anteriormente, hemos separado la zona de santander y la del "nido de bucaramanga", con el objetivo de establecer mejor los coeficientes de atenuaci6n regionales. la diferencia de estas dos zonas, como aquf las hemos dividido, consiste en la profundidad de sus focos. en total, para la zona de santander fueron examinados cinco (5) terremotos con profundidad menor que 70 kilometres, y de ellos 23 ecuaciones. obtuvimos que 5 = 2.0 y c = 0.5 (fig. 3). 6d ~----------------:.----------....., 5.!! 5.0 4.5 4.0 j:: 3.5 ~ 3.0 ", 2.5 2.0 1.5 1.0 0.5 od i------.....,..------"'t"""-------,------~ 1.00 10.0 • • • • 150 2.00 100.0 3.00 ig t.i 1000.0 km. 2.50 figura 2. determinacion de los coeficientes s y c para el "nido de bucaramanga" 6.5 6.0 5.5 5.0 4·5 4.0 ~ 3.5 i :i 3.0 i/'l ...l 2.5 2.0 1.5 1.0 0.5 0.0 0.00 0.50 1.0 • • • • • 1.00 10.0 1.50 2.00 100.0 2.50 3.00 lg ai 1000.0 km. figura 3. determinacion de los coeficientes s y c para la region de santander choco debido a las dificultades que existen en la regi6n, por la baja densidad de poblaci6n y la calidad de las construcciones, la intensidad en las zonas epicentrales, no se puede medir con gran precisi6n. en total para esta zona hemos observado cinco (5) terremotos, de los cuales se han sacado 23 ecuaciones. los valores obtenidos para los coeficientes son: s = 5.5 y c = 8.7 (fig.4). caldas tambien es una zona con terremotos de profundidad intermedia. la profundidad hipocentros en casos sobrepasa kil6metros. para nuestro estudio, hemos examinado tres (3) terremotos y 11 ecuaciones, de los cuales se obtiene que s = 3.4 y c = 3.8 (fig.5). de los algunos los 200 7.0 r-------------...o.--------------, 6.5 &0 5.5 5.0 4.5 ~ 4.0 • 3.5 ~ 3.0 2.5 2.0 1.5 1.0 0.5 00 i------"""t------.....,..------r------~ 100 1.50 2.00 2.50 3.00lg ai 10.0 !o().o 1000.0km. • • figura 4. determinacion de los coeficientes s y c para la region del choco , 6.0 5.5 5.0 4.5 4.0 3.5;:; i 3.0~ ~ 2.5 2.0 1.5 1.0 • • 0.5 o.o+-------or-------or-------"t"'""--------f 1.00 1.50 2.00 2.50 3.00li4/ 10.0 1000 1000.0km figura 5. determinacion de los coeficientes s y c para la region de caldal. • narino la mayorfa de los terremotos de esta region ocurren en el oceano, por 10 cual la intensidad en el epicentro (10) casi no se registra. ei terremoto ocurrido el 31 de enero de 1906 para el cual tarnbien existe un mapa de isosistas (rudolph & szirtes, 1911) no ha side tenido en cuenta para los calculos aqui realizados, ya que las isosistas de dicho terremoto fueron trazadas considerando una escala rnacroslsrnica propia de los autores, que no tiene equivalencia con ninguna de las escalas de intensidades que hoy en dia son usadas para estudios del campo macrosismico y en especial con la escala modificada de mercalli con la cual se han elaborado las isosistas de los dernas terremotos analizados en este trabajo. para la region de naririo analizamos solo un (1) terremoto del cual obtuvimos cinco (5) ecuaciones; los valores obtenidos fueron: s = 7.0 y e = 13.9 (fig.6). 6.0 5.5 5.0 4.5 4.0 ;.:; 3.5 i 2 3.0 ~ 2.52.0 1.5 1.0 o.s 0.0 1.00 1.50 2.00 2.50 3.60 l ii d i 10.0 100.0 1000.0 km figura 6. determinacion de los coeficientes 5 y c para la region de nariiio de los resultados arriba descritos, obtenemos una ecuacion de reqresion del campo rnacroslsmico para cada una de las zonas sismoactivas del territorio colombiano. los resultados se resumen en la tabla 2. tabla 2 ecuaciones del campo macrosfsmlco de las zonas slsmoactivas del terrltorlo colomblano zona sismoactiva f6rmula del campo macrosfsmico "nido de bucaramanga" santander choc6 caldas nariiio 1.5 m ,; = 3.blg~, 5.2 1.5 m ,; = 2.0ig~, 0.5 1.5 m ,; = 5.5ig~, b.7 1.5 m ,; = 3.4lg~, 3.b 1.5 m ,; = 7.0ig~, 13.9 en la tabla 2 se puede ver que el coeficiente c varia dentro un intervalo muy grande 0.5 y 13.9. la variacion del coeficiente s para las regiones sismoactivas de colombia se encuentra dentro del range de 2.0 a 7.0. la dispersion de estos valores parece estar relacionada con la complejidad tect6nica del territorio colombiano, 10 cual influye notablemente en la forma como se propagan las ondas slsmlcas en su interior. 5. analisis v discusion analizando los qraficos obtenidos en las figs.2-6 podemos darnos cuenta que para las regiones de santander y el "nido de bucaramanga" se observa una gran dispersion de los puntos de la qraftca respecto a la ecuacion de reqresion obtenida para estas regiones. una de las causas de esta dispersion podna ser la utilizacion de poca informacion sstadlstica en la elaboracion de las isosistas, 10 cual pudo haber ocasionado una interpretacion inexacta de los datos. sin embargo la explicacion mas acertada de dicha dispersion parece ser la complejidad tectonica de la zona, que produce sismos con diferente orientacion de los ejes de esfuerzos en los mecanismos focales, 10cual se manifiesta en la presencia de diferentes patrones de propaqacion de la energia sismica. para las zonas de choco y caldas (figs.4-5) la dispersion de los datos es menor, 10 cual caracteriza una menor complejidad tsctonica del medio. la presencia de diversas fuentes sisrnoqenicas y las condiciones de heterogeneidad y anisotropla del medio influyen para que la forma de las isosistas de los terremotos ocurridos en colombia vane de acuerdo a cada condicion stsmoloqica concreta de la region donde estos ocurran, 10 cual depende de las principales direcciones de las estructuras tectonicas regionales, de la magnitud de los terremotos, del mecanisme y profundidad del foco y de la distancia epicentral. a pesar de que la informaci6n rnacrosfsrnica examinada para los calcutos de los coeficientes de atenuaci6n de la intensidad es poca, podemos comparar los valores de 5 obtenidos para cada regi6n, con la sismicidad que se conoce para todo el territorio colombiano, de 10cual se deduce que los terremotos mas fuertes ocurren en las zonas que tienen mayor valor de s; por ejemplo, recordemos que en al departamento de nariiio han tenido lugar los terremotos mas fuertes ocurridos en colombia y como muestra la tabla 2, a esta regi6n ie corresponde el mayor valor de s. en la regi6n de santander, a pesar de ser una zona con alto grado de sismicidad, no ha sido escenario de fuertes y catastr6ficos terremotos (al menos en 10 que va de este siglo) y como vemos en la tabla 2, a esta regi6n ie corresponden los menores valores de s. los terremotos del "nido de bucaramanga", por ocurrir a profundidades interrriedias, sus efectos producidos en la superficie son leves y muchas veces no son sentidos por la comunidad. ei hecho de que el valor del coeficiente de atenuaci6n s obtenido para el "nido de bucaramanga" sea mayor que el obtenido para la regi6n de caldas se explica por la gran actividad sismica de bucaramanga, que ha dado origen a multiples fracturas en la regi6n; estas zonas de interfase dan origen a los fen6menos ffsicos de reflexi6n y refracci6n de las ondas sfsrnlcas, fen6menos que a su vez causan una mayor atenuaci6n de la intensidad sismica. 6. conclusiones del anal isis cualitativo y cuantitativo de las isosistas existentes para terremotos ocurridos en el territorio colombiano, podemos deducir 10siguiente: 1. las isosistas de los terremotos ocurridos en colombia varian dependiendo de ta regi6n donde estos ocurran, 10cual permite identificar una forma geometrica similar entre elias para cad a regi6n sisrnoactlva, reflejando asi la heterogeneidad y anisotropia del medio y del campo sismico. 2. las isosistas de los terremotos con profundidad intermedia ocurridos en el macizo santandereano revelan la existencia de por 10menos tres fallas orientadas en distintas direcciones que se cruzan, con un punta de excitaci6n de las oscilaciones slsmicas en el lugar de su cruce, el cual puede ser el origen del "nido de bucaramanga". 3. aunque consideramos que la estadlstica del campo macroslsmico en colombia debe aumentarse, la comparaci6n de los valores del coeficiente de atenuaci6n obtenido para cada una de' las regiones sismoactivas del territorio colombiano, permite deducir que el mayor valor de s corresponde a la regi6n donde se han repetido los terremotos mas fuertes y destructores de colombia. 4. la dispersi6n de los valores de s y c obtenidos, refleja la heterogeneidad del medio y esta relacionada con la estructura del medio y la complejidad tect6nica del territorio colombiano, capaz de generar terremotos con diferentes mecanismos focales. agradecimientos los autores agradecen a clara lnes rivas y augusto g6mez l6pera, estudiantes de hsica de la universidad nacional, el haber colaborado en la elaboraci6n de las figuras. referencias bibliograficas coral-gomez, c. (1985): estructura interna y sismicidad del territorio colombiano. mem. vi congr. latinoamericano de geologia, bogota, tomo 2:337-355. coral-gomez, c. (1987): los terremotos en colombia y caracterlsticas de su origen profundo. (mscr), univ. nacional de colombia, bogota, 200 pp. coral-gomez, c. (1990): la convergencia de placas en el noroccidente suramericano y el origen del nido de bucaramanga. rev. acad. colombiana de ciencias exactas, flsicas y naturales. xvii (66):521-529. coral-gomez, c. & e. salcedo. (1992): mapa de lsoslstas del terremoto de murindo de octubre 18 de 1992. ingeominas, informe preliminar. 24p. dobrev, t. b. & y. k. shchukin. (1974): los campos geoffsicos y la sismicidad de la parte oriental de la region carpato-balcanica. m. nauka. 170p [en ruso). drumia, a. v., n. stepanenco & i.a. poiata. (1990): sismicidad y deforrnacion sisrnotectonica en la region del brancha y regiones locales del globo terrestre can profundidad intermedia de los focos. en: ei terremoto de los carpatos del ana 1986. kishiniev. p.21-29. [en ruso). meyer, h., j. duarte & a. paraffan. (1986): caracterlsticas ffsicas del sismo de popayan. en: ei sismo de popayan del 31 de marzo de 1983. p.119-147. m6jica, j. (1985): estilolitos horizontales en sedimentitas del cretacico de la cordillera oriental, y su significado tsctonico. geol. colomb, 14:7-23 page, w. (1986): geologia sismica y sismicidad de noroeste de colombia. woodward-clyde consultants., 156p. ramirez, j. e. (1975a): historia de los terremotos en colombia. bogota. 250p. ramirez, j. e. (1975b): ei terremoto de cucuta cien afios despues. 18 de mayo de 1875-18 de mayo de 1975. bogota. 63p. ramirez, j. e. & j.r. goberna. (1980): terremotos colombianos noviembre 23 y diciembre 12 de 1979. inst. geoffsico, univ. javeriana. bogota. 95p. rudolph, e. & s. szirtes. (1911): "das kolumbianische erdbeden a.m. 31 januar 1906". gerlands beitrdge zur geophysik. vol. xi, no.1. (traducci6n parcial de hans jurgen meyer & alba de cardenas, 1991). salcedo-hurtado, e. (1992), sismicidad y peligro sfsrnlco de colombia. ph. d. tesis. edit. univ. estatal de moscu-busia, 268p. sarria-molina, a. (1985): sismos y riesgo sismico en colombia. en: simposio sabre el peligro y riesgo sismica y volcanico en america del sur, ceresis 14:330-371. shebalin, n. v. (1968): metoda de utilizacion de datos ingeniero-geol6gicos en la zonificaci6n sismica. en: zoniticacion sfsmica de la u.r.s.s. m.: nauka. p.95-111. [en ruso]. shebalin n. v. (1974): focos de terremotos fuertes en el territorio de la u.r.s.s. m.: nauka. 54p. [en ruso). piedrahita and montana.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 35-43 methodology implemented for the 3d-seismic modelling using gocad and norsar 3d software applied to complex areas in the llanos foothills 1 carlos piedrahita. instituto colombiano del petróleo, ecopetrol. piedecuesta colombia. e-mail: carlos.piedrahita@ecopetrol.com.co phone number: (577) 6847104 2 clara l. montaña. instituto colombiano del petróleo, ecopetrol. piedecuesta – colombia. e-mail: clara.montana@ecopetrol.com.co phone number: (577) 6847199 abstract a methodology has been applied in order to use seismic modeling and generate synthetic data which are compared with results from field data. this methodology has been applied to the sirirí area and subsequently it has been extended to other geographic areas. to fully develop this methodology, the gocad and 3d norsar software packages were used to build respectively the geological and 3d seismic models. this methodology involves making structural modeling by edition of the different objects (horizons and faults) in the structural module of gocad, with the help of available seismic and well data, in order to be consistent with reality. from this result a valid seismic model is imported to norsar 3d for making the seismic modeling, using ray tracing to get synthetic data. finally, this information is compared with field data that has been processed. key words: seismic modeling, structural modeling, sirirí, gibraltar. resumen se ha elaborado una metodología con el fin de integrar el modelamiento sísmico y generar datos sintéticos que se comparan con los resultados a partir de datos de campo. esta ha sido aplicada al área de sirirí y posteriormente se ha extendido a otras áreas geográficas. para desarrollarla se utilizó el software gocad y norsar 3d en la construcción de modelos y el modelamiento sísmico 3d, respectivamente. esta metodología consiste en la elaboración del modelamiento estructural mediante la edición de los diferentes objetos (horizontes y fallas) en el módulo estructural de gocad, ayudados de la información existente de sísmica y pozos para que los datos sean coherentes con la realidad. de este resultado se importa un modelo sísmico válido dentro del norsar 3d, en el que procedemos a hacer el modelamiento sísmico utilizando trazamiento de rayos para obtener datos sintéticos. finalmente esta información es comparada con datos reales que se hayan adquirido y procesado anteriormente. palabras claves: modelamiento sísmico, modelamiento estructural, sirirí, gibraltar. 35 earth sciences research journal manuscript received july 10 2006. accepted for publication may 2 2007. introduction the exploratory activity in the country must add new reservoirs to satisfy the national consumption beyond 2008 and to obtain incomes by exploitation of hydrocarbons to help stabilize the national economy. all the efforts are concentrated in increasing the productivity and the lower risk factor that the new prospects must have. new locations have been proposed and already explorated areas have been reinterpreted. in the same manner, new seismic 2d and 3d programs have bee acquired implementing the latest technology regarding design, acquisition, and processing. seismic modeling has turned into the new option for designing acquisition parameters, being able to simulate the whole seismic experiment before acquisition. this is very convenient for testing many alternative designs, in which the lowest exploratory risk and the best seismic quality are expected, especially in such complex areas like the llanos foothills in colombia, characterized by its abrupt topography and geological complexity. the degree of information available in the foothill area is appropriated to perform seismic modeling tests, because deep interpreted structural-crossings and some previously performed seismic 2d lines are available. this methodology was implemented, in this case, on the siriri block located in the area known as the sarare region between arauca, boyacá, and norte de santander departments, specifically in the gibraltar structure (figure. 1). in a regional and geological context, the siriri block is located in the folding and the thrust fault area of the foothills of the colombian eastern mountain range. the current morphology and geology is the result of several phases of changes in the tectonic style varying from extensive to compressive. the siriri block is an area affected by the subduction of the nazca plate in the west and the caribbean plate in the north. the mirador formation’s quartz sands are the main reservoir in the area. rocks in the structural section, ranging from quaternary to cretaceous (campanian), are found in faulting areas, in which parts of the paleocene and cretaceous sections are repeated. the main generator rocks in the region are shale and clay of the la luna formation (cretaceous and turonian). the tertiary section is considered as having some possibilities as a generator rock. methodology figure 2 is showing a flow-chart of the methodology followed to realize the modeling a structural model was realized based on the knowledge of the area and the available seismic data using the time interpretation on the software landmark’s seiswork 2d. figure 3 shows the gibraltar structural model, which was imported into paradigm’s geosec 2d geophysical software, to be converted from time to depth. the exploratory objective was to illuminate the gibraltar structure in the mirador formation. initially, the horizons and interpreted faults were imported and associated to the different thin skins of the mirador formation, in the earth science decisions’ gocad software. gocad is one of the latest state-of-the-art computational integrated applications, in which advanced capacities for 3d modeling, visualization, and geoanalysis are offered. this software has a huge advantage in the integration of information from external sources, and possesses the capacity to export into other applications such as the norsar’s norsar 3d software, where seismic modeling is realized (earth science decisions, 2005). the construction of the 3d model begins by importing the different horizons and faults, editing all the surfaces, assigning the category of fault to each corresponding fault and horizon, and using the algorithms of gocad´s structural software module to obtain the correct relationships between the fault-horizon intersections and the skips of the fault. the final product is a valid three-dimensional model, which will be exported to norsar 3d software (figure 4). norsar 3d is a software for seismic modeling in three dimensions. the method used for modeling is the ray tracing to wave front construction. this algorithm was developed by norsar´s researchers (vinge et al., 1993). methodology implemented for the 3d-seismic modelling using gocad and norsar 3d software applied to complex areas in the llanos foothills 36 piedrahita and montaña, esrj vol. 11, no. 1. june 2007 37 figure 1. location of the area of study containing some seismic lines acquired in the area. figure 2. flow chart that summarizes the methodology followed to realize the seismic modeling 38 figure 3. gibraltar structural model (surfaces m1 to m7 correspond to different thin skin of the mirador formation; and f1-f3, fd1, fd2, fa, ft and fg are the different fault associated to el mirador.). (ecopetrol, 2003). figure 4. 3d model made in gocad and visualized in norsar 3d. the m2 and m4 surfaces correspond to the different scales of the mirador formation. methodology implemented for the 3d-seismic modelling using gocad and norsar 3d software applied to complex areas in the llanos foothills 39 it has the capacity of exchanging data models with other software and importing or exporting in different formats (norsar, 2003a). a description of the ray tracing method is presented by lecompte et al. (2004). the first step to start the modeling is to define the working environment (e.g., choosing the project’s name and defining its extension and coordinate system). the 3d model constructed with gocad is imported to proceed with the geometrical modeling, in which, the blocks are constructed taking into account that each surface or interface has a positive side, specified in this case for the direction of the basement, and a negative side (atmosphere). the system performs this procedure automatically, meaning that false blocks will be generated, making necessary to redraft the model to adapt it to the reality another important step in the validation of the model in norsar 3d is the assignment of the properties of the rocks such as the velocity. these velocities were taken from the vertical seismic profile (vsp) of the gibraltar-1 well and from geological interpretation as well. this allows the model to become suitable for the ray tracing by the wave front method. with a valid seismic model for the ray tracing, it proceeds to generate the seismic design for each one of the lines of the sarare seismic program that passes near the exploratory objective (six 2d lines in total). in the application of the common shot wave front tracer of norsar 3d, all the shots of the design were modeled for each line, obtaining events as a result (databases of all the propagations in the common shot domain), which register the attributes of each ray traced. the common shot wave front tracer is based on a concept developed by norsar called wave front construction. this modeling method is based on ray tracing to interpolate a wave front in intervals of a given time. the purpose of this method is to simulate the seismic wave field and placing a number of shots on the surface, with a certain number of receivers associated to each shot. to execute this tool, three elements were used: (1) a valid seismic model generated in norsar’s model building tool, which should be validated and have assigned properties such as the velocity and the density; (2) a survey, designed from the seismic parameters used, and (3) the ray code, which specifies the trajectory of the rays through the model (norsar, 2003b). the tracer was configured to simulate the wave propagation of the mirador horizon (m4 and m2 mainly) and the deeper (figure 5). this configuration is called signature and usually uses the velocities of the p waves. only waves reflected by this horizon are used. results the final product of the seismic modeling was the generation of illumination maps and synthetic seismograms. the illumination maps evaluate the design of seismic acquisition on the horizon of interest (e.g., the mirador formation). these maps are built in the norsar 3d using the events registered in the seismic modeling. several functions can be visualized on the illumination maps like the hitmat function, equivalent to the seismic fold, which shows the effectiveness of the seismic design according to the parameters required (figure 6). another of the final products of the seismic modeling is the generation of synthetic seismograms. the result of the wave front tracer is stored in files corresponding to events. a trace type and the frequency can be assigned to these events such as a wavelet (ricker of 30 hz). the software generates two types of seg-y files, one internal, for display locally in the project and another external that can exported to landmark´s promax 2d software for processing. a subsoil image is obtained, which can be compared with acquisitions already processed or to propose new seismic prospects (norsar, 2003b). figure 7 shows a comparison among real data (sarare 2004 seismic program) and the synthetic data in the seismic lines on the sirirí block. it can be observed that in the seismic line with real data there is no continuity on the reflectors, being notorious in the right side of the seismic section. this deficiency in the quality of the data is product of the structural complexity presented in the llanos foothills. this is one of the most important uses of this methodology for complex piedrahita and montaña, esrj vol. 11, no. 1. june 2007 40 figure 5a. shot 72 (143 in total) on the horizon m4 of the mirador formation at seismic line sa-95-22. figure 5b. time vs distance graphic (travel time) shot 72. the event indicated with the arrow represents the m4 horizon. methodology implemented for the 3d-seismic modelling using gocad and norsar 3d software applied to complex areas in the llanos foothills 41 areas because some information of the reflectors in the synthetic lines is more notorious, and information of the field data can be enhanced, which is not visible in the field data by itself, because it can be mixed with many types of events. the seismic modeling permits to simulate the effects of the illumination for lines previously acquired and for new lines to be acquired as well. these illumination studies will be utilized for the design of future 3d projects to be executed in the siriri block. conclusions seismic modeling using the methodology described is an excellent tool for the validation of the 2d or 3d seismic designs and it is adequate to use it on complex areas, as the colombian llanos foothills. the complexity of this area makes necessary to use this method. an example of this is observed on the comparison between the images obtained from field data and the ones obtained from synthetic data (figure 7). ray tracing is a tool of great computational efficiency, a very important aspect to be considered, because of the great deal of data that has to be calculated, especially 3d data. the poor knowledge of the seismic velocities on the deep structures is one of the key issues that have to be improved to use the model without limitations. the different variations on the seismic design offer a lot of possibilities of experimentation like testing the illumination of the horizons of interest, especially on geologically complex areas such as the ones located on the colombian foothills (alvarez et al., 2004). figure 6. the illumination map is showing the hitmap function in the mirador horizon in the seismic line sa-95-22. piedrahita and montaña, esrj vol. 11, no. 1. june 2007 42 fi gu re 7 . r ea l d at a (le ft) a nd sy nt he tic d at a (r ig ht ) c or re sp on di ng to th e lin e sa -9 522 o f t he s iri rí ar ea . methodology implemented for the 3d-seismic modelling using gocad and norsar 3d software applied to complex areas in the llanos foothills 43 acknowledgments the authors are deeply grateful with the research division (uin) of the instituto colombiano de petroleo (icp), for the opportunity of having access to the different types of tools and data required on the research. references alvarez, g., pereira, v., and carcione, l. (2004). model-based 3-d seismic survey design as an optimization problem. seg int´l exposition and 74th annual meeting. denver, colorado. earth sciences decision (2005). introducción al módulo base de gocad. houston. lecompte, i., gjoystdal, h., and drotining, a. (2004). efficient and flexible seismic modeling of reservoirs: a hybrid approach. the leading edge. 23 (5). norsar (2003a). norsar 3d ray modeling. user’s guide version 4.0. kjeller, norway. norsar (2003b). norsar 3d ray modeling. interactive ray modeling by norsar-3d, version 3.0.2. course manual. kjeller, norway. piedrahita, c. and salinas, t. (2005). 3d seismic modeling on the sirirí block, colombia: case history. memorias del congreso colombiano del petróleo. acipet. bogotá-colombia. vinje, v; iversen, e., and håvar, g. (1993). traveltime and amplitude estimation using wavefront construction. geophysics. 58 (8). 1157-1166. piedrahita and montaña, esrj vol. 11, no. 1. june 2007 geofisica colombian a pp 57-59 diclembre de 2001 bogota 0 c issn 0121 2974n° 5 migracion mediante apilado medios con por dlfr accton en gradiente constante de velocidad luis antonio castillo lopez profesor a sis t e n t e departamento de geociencias facultad de ciencias universidad nacional de colombia resumen para investigar la reflectividad en el subsuelo de la tierra por rnetodos sfsmicos, se necesita introducir algunas suposiciones acerca del macromodelo de velocidades para la capa suprayacente al reflector. esto se logra al aplicar a los datos de entrada un peso al operador de apilado por difracci6n, cuyo desarrollo te6rico es bas ado en la integral tipo kirchhoff. ai escoger el propio peso para apilado de datos, el resultado del proceso de la migraci6n es una secci6n sismica, donde la amplitud es proporcional al coeficiente de reflexi6n, conocido como migraci6n con amplitudes verdaderas. en el presente articulo se pretende mostrar la cuantificaci6n de un pararnetro s.smico, es decir la amplitud, mediante el uso de un algoritmo de migraci6n, cuyo desarrollo e implementaci6n del mismo permite la evaluaci6n de datos sinteticos, considerando modelos sencillos. este algoritmo se desarro1l6 para trabajar con modelos sinreticos en un medio con gradiente constante de velocidad. para en tender rnejor el comportamiento nurnerico del algoritmo propuesto, se tienen presentes algunas consideraciones, como los aspectos cinernaticos del proceso de migraci6n, por ejemplo, cuando la funci6n peso es la unidad. palabras clave: procesamientq de datos sismicos, migracion en tiempo, modelamiento, problema directo abstract in order to investigate the reflectivity in the earth subsurface by seismic method, we need to introduce some assumption about the macrovelocity model above the reflector. this is achieved by applying to the input seismic data a weighted diffraction stack operator, which theoretical development is based on a kirchhoff type migration integral. by choosing the proper weight for stacking the data, the result of the migration process is a seismic section where the amplitude is proportional to the reflection coefficient, the so-called true-arnpli tude migration. for considering a more realistic situation, we develope a migration algorithm that works very well with constant gradient velocity media and the parameter quantificated for results. in order to better understanding the numerical behavior of the proposed algorithm, we consider in this paper only the kinematic aspects of migration process, i.e. when the weight function is the unit. key words: seismic data processing, time migration, modelling, direct problem introduccion ei operador de apilado por pesos de difracci6n usado para migrar los datos sfsmicos, se basa en la sumatoria de datos de entrada, a traves de una trayectoria definida por superficie de tiempos de transite por difracci6n (superficie de huygens), para un punto dado en el modelo, y diferentes fuentes y receptores sobre la superficie de la tierra. uno de los puntos criticos en esta migraci6n es el incremento de tiempo computacional como consecuencia de la complejidad geometrica del modelo sfsmico. en afios recientes algunos metodos alternatives han sido propuestos, para suplir este problema (vidale, 1988; schneider et al., 1992). en este ensayo se presenta un algoritmo rapido capaz de trabajar en forma cinematica 0 efectuar la migraci6n con amplitudes verdaderas en profundidad, a! considerar una situaci6n mas genera! cuando el modelo de velocidad sfsmica es presentado por una funci6n de las variables espaciales. esta clase de modelo es importante para simular muchas situaciones en la exploraci6n sfsm.ica (lapsen, 1993). operador bidimensional de apllado por difraccion siguiendo el formalisrno usado pol' urban (1999), y bas ado en schleicher et al. (1993), se puede escribir el operador bidimensional de apilado por difracci6n como geofislca colombiana, 5, diciembre de 2001 v(m,t) = ~jd9v @,m)dt'!2u(s,t+td) (i) -v 2n a en la ecuaci6n (i), los pares fuente y el receptor (s,c) estan parametrizados porla variable s ' de tal manera que la difracci6n de la curva de tiempo de transite t d en el plano es definida por todos los puntos de pararnetro s sobre la superficie de la tierra, y un pun to m dentro de un volumen especificado del modelo sfsmico. ei sfmbolo d /! 2 es el operador de la derivada anti-causal de medio tiempo, que corresponde en el dominio de la frecuencia al filtro ,ioj . la funci6n peso w puede ser escogida hasia que el resultado del proceso de migraci6n sea proporcional al coeficiente de reflexi6n. la funci6n u (s , t + t d) representa la componente principal del campo de onda sfsmico prirnario reflejado. ei resultado de la integral (1) es colocado en el punto m dentro del modelo, dando como resultado 10 que aquf se denomina migraci6n en profundidad por apilado de difracci6n. en el caso en que la funci6n peso es la unidad, considerada en este artfculo, se tiene s610 una imagen cinernatica de un reflector sfsmico. curva de t1empos de transito en el procesamiento de apilado por difracci6n, el modelo de macrovelocidad es considerado a priori como conocido. y una curva 57 luis antonio castillo lopez de tiempo de transite de apilado por difracci6n debe ser construida para cada punto dentro de una grill a del modelo. para este caso se considera una situaci6n donde la velocidad en la capa suprayacente esta representada por una funci6n de tipo lineal v(z)=vo +gz donde v0 es la velocidad cerca de la superficie de la tierra, g es el gradiente de la funci6n velocidad en la direcci6n del eje vertical z, en profundidad. esta clase de funci6n de velocidad es la primera aproximaci6n por ser considerada cuando estan siendo estudiadas variaciones regionales de velocidad para muchas rocas sedimentarias (lapsen, 1993). en tales model os con variacione lineares de velocidad, la trayectoria del rayo es circular y los tiempos de transite son dados como soluci6n de la integral (bleistein, 1986) en ja integral (3) zm es la profundidad del punto m en el subsuelo, el f3 es el angulo entre el eje vertical z, y la direcci6n de la trayectoria del rayo en el punto m. la funci6n n( z) = vo / v( z) es el indice de refracci6n. la integral referida tiene una soluci6n analitica, la cual provee una manera de desarrollar un algoritmo rapido y estable para migrar los datos sismicos (figura i). " t,,~······ ,...... t ; ,········:''-·····1 " 1., t,,"······ c c , , : ~ , :>,~ i: ~~'1~"'{vtli figura i. curva de tiempos de transite de reflexi6n (linea interrumpida) y la curva de tiempos de transire par difracci6n (lrnea continua). ei puntoen profundidad coincide con el punta de reflexi6n real ejemplos para medir la inestabilidad del algoritmo de apilado por difracci6n, ha sido aplicado a una serie de datos con una configuraci6n disparo cornun. el modelo sismico (figuras 2 y 4) esta constituido por un reflector curvo debajo de un medio con capas heterogeneas con un medio de gradiente constante de vejocidad, donde la velocidad cerca de la superficie es 2.0 km/s y un gradiente de 0.9375/s. la posici6n del punto de disparo es 1.0 ian a la izquierda, mientras el primer receptor esta a 1500 m, y el mas lejano a 4000 rn. la fuente tiene una frecuencia dominante de 75 hz, y el intervalo de muestreo es de 4.0 ms. en la figura 2 los datos sfsmicos estan sin ruido, en tanto que en la figura 41a raz6n sin es i:0.1. en las figura 3 y 5 se tienen los 58 (2) 1.5 3 43.52 2.51.5 1.8 s 0c. e 1.7"i= 1.6 configuraci6n y trazado de rayos 2 3l-__ ~ ~ __ ~ __ ~ ~ __ --.j 1 3.5 42 2.5 3 (3) figura 2. parte inferior: modelo y trayectorias de rayos. parte superior: sismograma sinterico en un media con gradiente ccnstante de velocidad. (lineas punteadas indican la zona par migrar) imagen de la secci6n sismica migrada en profundldad 25 2 6 .... __ -4+.__ .;.......:. ....... __.. ..........--.:#.;4 ........, 21 2.4 25 26 offset (km) 2.7 figura 3. imagen de la secci6n migrada en profundidad. zona migrada xo= 2.0, x t = 2.8 y zo = 2.0, z 1 = 2.6 datos sismicos migrados en profundidad, despues de aplicar el operador de apilado por difracci6n para una serie respectiva de datos. es importante notar que el algoritmo propuesto provee una buena imagen del reflector, ai igual que en un ambiente con ruido, y puede ser usado para migrar datos con un medio de gradiente constante de velocidad. descripcion del algoritmo ei algoritmo por seguir fue desarrollado e implementado con el fin de efectuar migraciones para medios con un gradiente constante de velocidad, adem as de recuperar los valores de amplitudes verdaderas. este utiliza como para metros de entrada datos referentes a las diferentes caracteristicas del rayo, por ejemplo, angulo de emergencia, de incigeof[sica colombiana, 5, diciembre de 2001 migraci6n mediante apilado por difracci6n en medios con gradiente constante de velocidad figura 4. parte inferior: modelo y trayectoria de rayos. parte superior: sismograma sinretico (can ruido. sin; 1:0.1) en un media con gradiente constante de velocidad. (lfneas punteadas indican zona por migrar) dencia, modelo de velocidades, etc. ei algoritmo es presentado en fortran y utiliza para el modelamiento de trazado de rayos y el sismograma sintetico el algoritmo seis88, desarrollado por cerveny y psensik (1988), iitil en el modelamiento de sismogramas sinteticos para prospecci6n sismica. conclusiones en este ensayo se desarroll6 y prob6 el algoritmo de migraci6n por apilado de difracci6n, siendo aplicado a una serie de datos con configuraci6n disparo cornun, sinteticarnente generado por la teoria de rayo para un medio con gradiente constante de velocidad. ei algoritmo fue probado sin ruido y con ruido sobre los datos, proveyendo una buena imagen de la zona alvo de interes (reflector). este resultado es muy importante en el sentido de que el algoritmo es rapido, estable y se ajusta para ser usado para migraci6n con amplitudes verdaderas. agradecimientos me gustaria agradecer al grupo de sismica del geophysical institute, charles university, prague, czechoslovakia, por permitirme disponer del software para trazado de rayos "the ray tracing software seis88", y al grupo de geoffsica de ja universidad nacional de colombia en la cual encuentro gran soporte para la realizaci6n del trabajo como geofisico. geoff sica colombiana, 5, diciembre de 2001 imlgen de a secci6n sfsrn ca migrada con ruldo ad clonal en profundidad 2.8 272.2 23 24 25 offset (km) figura 5. imagen de la migraci6n preapilado en profundidad (con ruido). zona alvo xo = 2, x i = 2.8 y zo = 2, z i = 2.6 referencias bibliograficas bleistein, n. 1986: two-and -one-half dimensional in-plane wave propagation. geophysical prospecting, 34: 686703. cerveny,y, and psensik, i. 1988: program for computing ray synthetic seismograms in 2d laterally inhomogeneous, czech. lapsen, p. 1993: influence of lithology and neogene uplift on seismic velocities in denmark: implications for depth conversion of maps. the aapg, 77, n. 2,194-211. schleicher,1., tygel, m., & hubral, p. 1993: 3-d true-amplitude finite-offset migration. geophysics, 58(8): 1820-1830. schneider,jr. w. a, ranzlnger, k. a, balch, h. & kuse, c. 1992: a dynamic programming approach to first arrival traveltime computation in media with arbitrarily distributed velocities. geophysics, 57(1): 39-50. urban, j, 1999: two-dimensional true-amplitude migration and introduction to 2.5-d case. master thesis. federal university of para, brazil (en portugues). vidale, j. 1988: finite-difference calculation of the travel times. bssa, 78(6): 2062-2076. 59 archivo03.indd 25 manuscript received june 12, 2006. earth sciences research journal ground surface temperature histories inferred from 15 boreholes temperature profiles: comparison of two approaches l.v. eppelbaum1 , i.m. kutasov1 and g. barak1 1dept. of geophysics and planetary sciences, raymond and beverly sackler faculty of exact sciences, tel aviv university, ramat aviv 69978, tel aviv, israel corresponding author: l.v. eppelbaum, e-mail: levap@post.tau.ac.il abstract understanding the climate change processes requires application of special methodologies for revealing a ground surface temperature history (gsth). it was proved by different authors that the gsth may be determined on the basis of analysis of the temperature field observed in short boreholes. in this paper, the authors analyze four mathematical models describing the gsth: (1) sudden change, (2) linear increase, (3) square root of time increase and (4) exponential increase. fifteen borehole temperature profiles from europe, asia and north america were selected in three groups based on their geographical proximity. after careful analysis of temperature-depth profiles in these boreholes it was found out that two models (linear increase and square root of time increase) provide the best fit with field data. the calculated warming rates in the 20th century were compared with those obtained by a few parameters estimation (fpe) technique. key words: temperature, borehole, climate modeling. resumen para entender los procesos de cambio climático se requiere la aplicación de metodologías especiales que revelen una historia de la temperatura de la superficie del suelo (gsth = por su abreviación en inglés). se ha probado por diferentes autores que gsth puede ser determinado con base en el análisis del de campo temperaturas observado en pozos cortos. en este artículo, los autores analizan cuatro modelos matemáticos describiendo el gsth: (1) cambio súbito, (2) incremento linear, (3) raíz cuadrada del incremento del tiempo e (4) incremento exponencial. quince perfiles de temperatura de pozos de europa, asia y norte américa fueron seleccionados en tres grupos con base en su proximidad geográfica. luego de un cuidadoso análisis de perfiles temperatura-profundidad en estos pozos se encontró que dos modelos (incremento linear y raíz cuadrada del incremento del tiempo) proporcionan los mejores ajustes a los datos de campo. las tasas de calentamiento en el siglo xx fueron comparadas con aquellas obtenidas con la técnica denominada estimación de unos cuantos parámetros (fpe). palabras clave: temperatura, pozo, modelamiento climático. earth sci. res. j. vol. 10, no. 1 (jun. 2006): 25-34 26 l.v. eppelbaum , i.m. kutasov and g. barak introduction at present many efforts are made to determine the trends in ground surface temperature history (gsth) from geothermal surveys (e.g., lachenbruch and marshall, 1986; baker and ruschy, 1993; pollack et al., 2000; majorowicz and safanda, 2005). in this case accurate subsurface temperature measurements are needed to solve this inverse problem − namely the estimation of the unknown time dependent ground surface temperature (gst). the variations of the gst during the long term climate changes resulted in disturbance (anomalies) of the temperature field of formations. thus, the gsth can be evaluated by analyzing the present precise temperature-depth profiles. the effect of surface temperature variations in the past on the temperature field of formations is widely discussed in the literature. three approaches are used in deriving climate information from borehole temperature profiles. in the first case the ground surface temperature history (gsth) is reconstructed as an arbitrary function of time (e.g., cermak, 1971; lachenbruch and marshall, 1986; beltrami et al., 1992; shen and beck, 1992; baker and ruschy, 1993; clauser and mareschal, 1995; harris and chapman, 1995; pollack et al., 2000; jain and pulwarty, 2006). huang et al. (1996) called such an approach an arbitrary function reconstruction (afr). the second approach for inversion of temperature profiles – a few parameter estimation (fpe) technique was suggested by huang et al. (1996). as it was demonstrated by the authors, the fpe technique allows comparison of the inversion results, both spatially and temporally. the third approach is the generalized inverse method named the functional space inversion (fsi) technique (shen and beck, 1991; shen et al., 1995). the fsi method allows for uncertainties in temperature-depth data, thermal properties of formations and heat flow density to be incorporated into the model. in this paper we will compare results of our gsth calculations with those obtained by the fpe technique. for this reason we will consider some of the main features of the fpe method. the main considerations to utilize the fpe technique include (huang et al., 1996): (a) the resolving power of a temperature profile for gsth reconstruction. due to the amplitude attenuation of thermal diffusion and the presence of observational and representational noise in borehole data, vigorous estimations can often be made for only a few parameters such as the trend, duration, and the overall amplitude of the ground surface temperature change in the past. (b) the need to simplify and standardize the procedures for reconstruction of gsth. the problem of inverting borehole temperatures to yield a ground surface temperature (gst) is an ill-posed problem, and some constraints are required for a stable solution. to allow a more consistent comparison of temperature inversion results, a standardization of surface temperature reconstruction is needed. the standardization is a difficult task in an afr because of the high degrees of freedom involved in representing a gsth. (c) convenience in comparing results. an afr techniques attempts to reconstruct a gsth at various time scales and degrees of details. however, in a regional or in a continent-wide study only a comparison of general features is needed instead of the details of gsth’s obtained from different areas. the forward calculation approach (afr) was used in the analysis and interpretation of borehole temperatures in terms of a gsth. fifteen borehole temperature profiles from europe, asia, and north america, were selected (huang and pollack, 1998; www.geo.lsa.umich.edu/~climate). three groups based on geographical proximity were formed (table 1). four mathematical models to describe the gsth (sudden change, linear increase, square root of time, and exponential increase) were used to approximate the temperature-depth profiles of the boreholes. the objective of this study is to calculate the warming rates (r) during the 20th century by the afr method and to compare them with those obtained by the few parameter estimation (fpe) technique. it is also reasonable to assume that for close spaced boreholes the values of r should vary with narrow limits. methodology mathematical models and assumptions let us assume that t x years ago from now the ground surface temperature started to increase (warming) or reduce (cooling). prior to this moment the subsurface temperature was (figure 1): 27 ground surface temperature histories inferred from 15 boreholes temperature profiles: comparison of two approaches table 1. input data for 15 boreholes and results of temperature inversions. (huang and pollack (1998); www.geo.lsa.umich.edu/~climate). w el l # well code longitude latitude h a h b , m h t h c , m t o , oc r, rate after 1900 k/100a north america 1 2 3 4 5 6 ca-9901 ca-9906 ca-9907 ca-9804 ca-9806 ca-9807 -101.50 -101.84 -100.56 -100.76 -101.57 -101.57 54.72 54.77 54.93 55.16 54.79 54.79 49.81-119.58 49.97-149.78 48.00-113.78 48.30-106.25 42.49-84.52 47.06-100.80 189.47-596.47 199.38-599.14 196.03-523.34 154.41-307.76 167.22-498.78 175.81-446.92 2.7 2.1 1.1 2.1 2.8 2.5 2.487 1.219 1.384 0.378 0.803 0.419 europe 7 8 9 10 11 cz-127127 cz-hu-7 cz-hu-9 cz-mj-5 cz-mj-8 14.87 12.81 12.81 14.86 14.58 50.73 50.11 50.11 50.57 50.36 80.80-140.00 50.00-120.00 50.00-100.00 60.00-180.00 50.00-120.00 200.00-440.00 180.00-350.00 200.00-460.00 230.00-290.00 180.00-310.00 6.3 5.6 8.1 5.5 0.855 1.533 3.751 0.212 1.787 asia 12 13 14 15 cn-fj-ql17 cn-gd-c3901 cn-jxck46-25 cn-jxzk59-38 116.94 113.18 116.33 116.33 26.33 25.42 27.97 27.97 60.00-100.00 50.00-120.00 60.00-140.00 50.00-110.0 140.00-430.00 160.00-260.00 200.00-300.00 200.00-380.00 18.5 18.4 19.9 13.7 2.532 1.870 1.181 0.300 figure 1. temperature profiles: a principal scheme. ab – cooling, ac – warming. and γ is the geothermal gradient. is also assumed that the formation is a homogeneous medium with constant thermal properties. now the current (t = t x ) subsurface temperature is (in case of warming): )(),( zftttzt ocxc +== (2) where t oc is the current (at the time (date) of temperature logging) mean ground surface temperature; and f(z) is a function of depth that could be obtained from the field data. in some cases the value of t oc can be obtained by extrapolation of the function t c to z = 0. however, in most cases, the value t oc can be estimated by trial and error method: assuming an interval of values for t oc , calculating for each t oc value of the temperature profiles t c (using a computer program) for various models of change in the ground surface temperature (gst) with time and, finally, finding a best match between calculated and field measured t c profiles. in our study we found that a quadratic regression program performed for the section h a – h b (figure 1) can be utilized to estimate the value of t oc = a o (table 2), zttzt oaa γ+== )0,( (1) where t oa is the mean ground surface temperature at the moment of time t = 0 years; z is the vertical depth 28 l.v. eppelbaum , i.m. kutasov and g. barak table 2. reduced temperatures and results of temperature inversions for the borehole ca-9901. t xc = 52.9 yrs, t xl = 137.5 yrs, t xs = 94.0 yrs, t xe = 282.8 yrs, ∆t rc = 0.092 oc, ∆t rl = 0.043 oc, ∆t rs =0.039 oc, ∆t re = 0.275 oc, a l =0.0245 oc/yr α s =0.3474 oc/yr1/2 a e =0.002971 oc/yr h, m t rc , oc t rl , oc t rs , oc t re , oc t ro , oc 29.89 39.85 49.81 59.77 69.72 79.66 89.62 99.61 109.60 119.58 2.10 1.73 1.39 1.10 0.85 0.64 0.48 0.34 0.24 0.17 2.02 1.68 1.39 1.15 0.94 0.76 0.61 0.49 0.39 0.31 2.04 1.69 1.39 1.13 0.91 0.73 0.58 0.45 0.34 0.26 1.72 1.55 1.39 1.25 1.12 1.01 0.90 0.80 0.71 0.63 2.15 1.71 1.39 1.13 0.92 0.75 0.60 0.48 0.37 0.29 2 21),( zazaattzt oxc ++== (3) where a o , a 1 , and a 2 are the coefficients. the temperature-depth values for all wells were taken from the database (huang and pollack, 1998; www.geo.lsa.umich.edu/~climate). four models of changing gst values with time were considered. the corresponding mathematical solutions are presented in the literature (e.g., carslaw and jaeger, 1959; cermak, 1971; lachenbruch and marshall, 1986; lachenbruch et al., 1988; powell et al., 1988). in the first model (model c) we assumed that t xc years ago the gst value suddenly changed from t o to t oc . the current temperature anomaly (the reduced temperature) is ztzftzt oocr γ−−+= )()( (4) and the solution is xcorrc tt at z xttt =   φ∆== 2 )(* (5) ooco ttt −=∆ (6) where a is the thermal diffusivity of formation and φ*(x) is the complementary error function. in the second model (model l) we assumed that t xl years ago the gst value started gradually to change from t o to t oc . we assumed that gst is a linear function of time and xllooc ttt α+= (7) where a l is some coefficient. the solution is * expt t t at z at z at z at z t t1 2 2 4rl r l xl 2 2 = = + =a r ub d bl n l( 2 (8) in the third model (model s) we also assumed that t xs years ago the gst value started gradually to change from t o to t oc . we assumed that gst is a square root function of time and xssooc ttt α+= (9) where a s is a coefficient. the solution is *expt t t at z at z at z t t 4 2 2 rs r s xs 2 = = =a r ub dl n) 3 (10) and, finally, in the fourth model (model e) we assumed that the gst value exponentially increases with time and )exp( xeeooc ttt α= (11) where a e is a coefficient. the solution is (t re =t r ) * * exp exp exp t t z a at z t z a at z t t t 2 1 2 2 re e e e e e xe = + + = a a a a a u u ] b d b d g l n l n < f (12) 29 ground surface temperature histories inferred from 15 boreholes temperature profiles: comparison of two approaches computer programs were used for processing field data and calculating values of t x , t oc , t o , γ, f(z), a l , a s , and a e . to demonstrate the calculation technique we present below a field example. example of calculations for the interval 49.81-119.58 m of the borehole ca-9901 (table 1 and figure 2) the temperature profile can be approximated by a quadratic equation (standard regression program was applied) 20001927.003626.092.5),( zzttzt xc +−== (13) therefore, the present mean ground surface temperature (gst) is 5.92oc (figure 1). from the observed t-z data it follows that in the 189.47-596.47 m section of the well; the temperature gradient is practically constant. it was assumed that the temperature gradient in this section did not change. for this interval (here a standard regression program was applied) ztztc 01224.056.2)0,( +== (14) therefore, in the past (t x years ago) the mean ground surface temperature t o = 2.56oc and warming occur. figure 2. observed and calculated temperature profiles, borehole ca-9901, north america (see table 1). in our case the observed reduced temperatures (for z < 119.6 m) are: ztzt obsor 01224.056.2)( −−= (15) where t obs values are the measured temperatures (figure 2). for the calculations below we used the depth z = h 1 = 49.81 m, where t r =1.39 oc. the temperature change (anomaly) of the gst is 5.92oc – 2.56oc = 3.36oc. a computer program calculates the reduced temperatures versus depth for the four models of change in gst and compares them with the observed anomalies. for the first model (model c) the reduced temperature is t rc (equation 5). in our case the gst changes from 2.56oc to 5.92oc. in the second model (model l) the reduced temperature is t rl (equation 8) and the ground surface temperature linearly changes from 2.56oc to 5.92oc, a change that can be modeled using the following equation xlltα+= 56.292.5 in the third model (model s) the gst is a square root function of time. the reduced temperature is t rs (equation 10) and xss tα+= 56.292.5 in the fourth model (model e) the gst is an exponential function of time. the reduced temperature is t re (equation 12) and )exp(56.292.5 xeetα= for all models the values of t x and a were estimated by a computer program. the value of thermal diffusivity a = 0.004 m2/hr was used in the temperature inversions. the results of calculations are presented in tables 2 and 3, and in figures 2 and 3. analyzing the data from table 2 and figure 2 we can conclude that the best fit (minimum values of average squared deviations ∆t rl = 0.043 oc and ∆t rs = 0.039 oc) is achieved when the models l or s are applied. 30 l.v. eppelbaum , i.m. kutasov and g. barak table 3. coefficients in equation 1 (b o = t oa , b 2 = γ) and equation 3. t 1,r is the reduced temperature at z = h 1 . w el l # well code h 1 m t 1 , r oc well section h a h b , m well section h t h c , m a o , oc a 1 ·102, oc /m a 2 ·103, oc /m2 b o , oc b 1 ·102, oc /m north america 1 2 3 4 5 6 ca-9901 ca-9906 ca-9907 ca-9804 ca-9806 ca-9807 49.81 49.97 48.00 48.30 42.49 47.06 1.39 1.08 0.76 0.20 0.48 0.52 5.92 4.06 2.51 2.76 3.97 3.70 -3.626 -0.954 -0.133 -0.241 -1.289 -0.210 0.1927 0.0588 0.0317 0.0464 0.1354 0.0515 2.56 2.09 1.19 2.17 2.69 2.65 1.224 1.125 1.211 0.756 1.134 1.092 europe 7 8 9 10 11 cz-127127 cz-hu-7 cz-hu-9 cz-mj-5 cz-mj-8 70.00 50.00 50.00 60.00 50.00 0.91 1.49 0.80 1.31 2.39 8.68 9.73 10.38 9.23 10.46 -0.003 -1.825 -0.781 0.854 -1.042 0.0328 0.2084 0.1606 0.0503 0.1673 6.85 6.24 8.07 6.65 5.86 1.562 3.134 3.056 3.211 4.199 asia 12 13 14 15 cn-fj-ql17 cn-gd-c3901 cn-jxck46-25 cn-jxzk59-38 60.00 50.00 40.00 50.00 0.44 1.08 1.54 2.22 19.77 21.32 24.07 18.64 -0.959 -1.140 -1.327 -1.161 0.1132 0.1023 0.1027 0.1250 18.18 19.02 20.77 14.77 1.723 1.846 2.154 2.813 table 4. results of temperature inversions for two models. ∆t l and ∆t s are the average squared temperature deviations, r l and r s are the warming rates. well codes are presented in tables 1 and 3. w el l # model l model s r l , k/100a r s at t = t xs k/100at xl , yr ∆t l , k al, k/yr t xs yrs ∆t s , k a s, k/yr0.5 north america 1 2 3 4 5 6 137.5 278.6 319.1 92.7 81.9 192.1 0.043 0.055 0.079 0.011 0.027 0.034 0.02450 0.00786 0.00411 0.00636 0.01558 0.00545 94.0 185.4 210.8 64.3 56.5 128.9 0.039 0.032 0.068 0.012 0.033 0.040 0.3474 0.1450 0.0904 0.0734 0.1697 0.0922 2.450 0.786 0.411 0.636 1.558 0.545 1.792 0.532 0.311 0.458 1.129 0.406 europe 7 8 9 10 11 415.2 148.8 98.6 328.1 241.9 0.078 0.092 0.097 0.174 0.082 0.00441 0.02341 0.02343 0.00785 0.01901 279.0 101.4 68.5 219.8 161.7 0.064 0.118 0.079 0.144 0.037 0.10971 0.34591 0.27927 0.17361 0.36149 0.441 2.341 2.343 0.785 1.901 0.328 1.718 1.690 2.482 1.421 asia 12 13 14 15 100.3 183.5 260.7 331.7 0.084 0.082 0.060 0.060 0.01590 0.01251 0.01268 0.01165 70.9 124.0 176.2 219.5 0.078 0.062 0.061 0.041 0.18942 0.20623 0.24911 0.26089 1.590 1.252 1.268 1.165 1.125 0.926 0.939 0.907 31 ground surface temperature histories inferred from 15 boreholes temperature profiles: comparison of two approaches close spaced boreholes (figures 7 and 8). analysis of figure 7 indicates that the duration of the warming period was five centuries and the warming rate is minimal in the last century. however, it is widely accepted that most of the warming occurs in the last century. at the same time the gsth reconstructed from borehole no. 12 temperature-depth data (figure 8) indicates that the cooling period lasted for three centuries and a very high warming rate for the 20th century was calculated by the fpe technique. inversion results in most cases the model l (linear increase) and model s (square root time increase) provide the best fit to the field data (table 4, figure 4). in several cases the model c (sudden change) allows to approximate the measured temperature-depth data with good accuracy (figures 5 and 6). for boreholes no. 16 (north america, table 4, model l) the current warming rates vary in the 0.4112.450 k/100a range. the wide range for the warming rate of 0.328-2.482 k/100a was also determined for five boreholes in europe (table 4, model l). interesting results we obtained for four boreholes in china (table 4). in this case the warming rate (r l ) varies with relatively narrow limits (1.1651.590 k/100a.) it was found that the duration of warming periods (table 4, model l) is: 82-279 years for wells no. 1-6; 99-415 years for wells no. 7-11; and 100-332 years for wells no. 12-15. similar results (r s and t xs , table 4) were obtained for the model s. discussion and conclusions as we mentioned in the introduction section the main objective of our study is to calculate the warming rates (r) during the 20th century by the afr method and to compare with those obtained by the few parameter estimation (fpe) technique (table 1). the warming rate estimated by the fpe technique varied in wide ranges: 0.378-2.487 k/100a (north america); 0.212-3.751 k/100a (europe); and 0.3002.532 k/100a (asia). in our case the fpe method allowed to determine temperature trends (warming or cooling rates) over the past five centuries (huang et al., 2000). in the inversion we employ a priory null hypothesis for the gsth that there has been no climate change. from tables 1 and 4 follows that for the boreholes no. 1-11 (north america and europe) both approaches (afr and fpe) provide practically the same ranges of warming rates. for boreholes in asia (no. 12-15) the afp technique gives a more consistent (narrow) range of warming rates (1.1651.590 k/100a). let us compare the gsth of two figure 3. the possible scenarios of the ground surface temperature history, borehole ca-9901, north america (see table 1). 32 l.v. eppelbaum , i.m. kutasov and g. barak the results of temperature inversion by both techniques show that probably some non-climatic effects (well shut-in periods, vertical and horizontal water flows, sedimentation, uplift, erosion, steep topography, lakes, vertical variation in heat flow, lateral thermal conductivity contrasts, thermal conductivity anisotropy, forest fires, farming, etc.) may have perturbed the borehole temperature profiles (e.g., carslaw and jaeger, 1959; lachenbruch, 1965; kappelmeyer and haenel, 1974; powell et al., 1988, majorowicz and skinner, 1997; guillou-frottier et al., 1998; kutasov and eppelbaum, 2003; gruber et al., 2004; bodri and cermak, 2005; kutasov and eppelbaum, 2005; majorowicz and safanda, 2005; mottaghy et al., 2005). this study shows that extreme caution should be used in the selection of temperature-depth profiles for inferring the ground surface temperature histories. a good example in this regard was demonstrated in the study conducted by guillou-frottier et al. (1998), where only 10 out of 57 temperature profiles were figure 5. the possible scenarios of the ground surface temperature history, borehole cz-127127, europe (see table 1). figure 4. observed and calculated temperature profiles, borehole ca-9807, north america (see table 1). selected for inversion of the past ground surface temperatures. we can conclude that only the calculated warming rates for wells in asia (1.2-1.6 k/100a for model l or 0.9-1.1 k/100a for model s, table 4) can be used for forecasting of short term warming trends. 33 ground surface temperature histories inferred from 15 boreholes temperature profiles: comparison of two approaches references • baker, d. g., and d. l. ruschy (1993). the recent warning in eastern minnesota shown by ground temperatures, geophysical research letters, 20, 371-374. • beltrami, h., a. m. jessop, and j.-c. mareschal (1992). ground temperature histories in eastern and central canada from geothermal measurements: evidence of climate change, palaeogeography, paleoclimatology & palaeoecology (global planetary change section), 98, 167-183. • bodri, l., and v. cermak (2005). borehole temperatures, climate change and the pre-observational surface air temperature mean: allowance for hydraulic conditions, global planetary change, 45, 265-276. • cermak, v. 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one of its first definitions was made by the us geological survey in 1977, stating that geological risk is understood as any geological condition, process or event that involves a potential risk to the health, safety or welfare of a group of citizens or for the functions of a community or economy (brusi-roque, 2003). another definition understood them as a circumstance of distress, loss or damage to social and economical development due to a geological condition(ogura and soares, 2005). man’s interaction with the environment creates potentially hazardous situations since human action itself triggers mechanisms involving natural hazards, or natural geological events may constitute a danger or lead to social harm. some of the most comprehensive geological risks conception posed them as those processes, events or situations occurring in geological environments that can cause damage or harm to communities or infrastructure occupying vulnerable territory. they can also be understood as any geological, natural, induced or mixed situation or event which can produce economic or social harm to a community, where geological criteria may be used for their prediction, prevention or correction (ayala, 1992). management is thus needed to avoid or prevent such geologic risks, a concept bringing together issues such as study, planning, organisation, evaluation, management, analysis, implementation, monitoring and control (koontz and weihrich, 1998). this is an element which when properly inserted in terms of geological risk mitigation is a very useful working tool regarding construction and infrastructure. geological risk management concerns studies regarding phenomena or processes related to geodynamics and earth processes or phenomena induced by human activities affecting projects and/or civil engineering works, or infrastructure, situated or in the future be located on the ground, so that they contribute towards planning, organising, directing, evaluating and controlling organisational, technical and technological measures for such projects or works, aimed at preventing or mitigating the effects of disasters caused by natural or manmade geological events (galban, 2010). the main problem of international risk management for all levels of government is that risk management is a long-term problem and governments have not been particularly good at planning long-term development and have not spent much money on reducing such risks to long-term liber galbán rodríguez, ingrid n. vidaud quintana, tomás jacinto chuy rodríguez, josé maría ruiz ruiz, francisco calderín mestre, eduardo rafael alvarez deulofeu138 (monge granados, 2003). governments have a crucial weakness regarding risk management; specialists also play an important role here but similarly have not demonstrated their expertise in this area. th is analysis indicates that there must be a proper multidisciplinary relationship between specialists, government and the community (figure 1); such interaction infl uences such actors to a greater or lesser extent regarding geological risks concerning man-made structures and therefore whether risks will materialise, meaning that management needs to be contextualised, proposing a mechanism that permit each country working according their real possibilities. cuba, the largest island in the caribbean, is not an isolated case. here, the multidisciplinary management of geological risks developed during construction process deserves an independent analysis according to its singular political and administrative system. data and methods: the cuban experience referred to the multidisciplinary management of geological risk a summary of the bibliographic sources consulted has described multidisciplinarity as being cooperation between diff erent disciplines towards common goals, whether related or not, where there can be coordinated collaboration from the exchange of ideas to the mutual integration of basic concepts and methods aimed at providing solutions and establishing a foundation for such solutions. multidisciplinarity in environmental management and therefore in geological risk management has a close relationship with decision theory which has been studied extensively in the fi eld of economics and engineering science. th e methods used today are the product of research in such areas of knowledge. it is undeniable that every real decision is in fact a compromise between diff erent solutions, each having its own advantages and disadvantages, depending on the position to be adopted (garcia layton, 2004). as in economics, agriculture, industrial engineering and social sciences, decision theory is a necessary tool in geological science applications, particularly in geological risk management regarding construction and infrastructure where specialists from diff erent institutions are involved, including construction, architectural design, projects, scientifi c institutions specialising in implementing geological projects, geotechnical studies, physical planning and government. multicriteria decision-making, or adopting a multidisciplinary approach, involves predicaments in business and public administration; it is diffi cult to make a decision regarding a public issue (such as geological risks) that will satisfy all stakeholders, because the cost often comes into confl ict with other criteria. however, multidisciplinary analysis has to prioritise realism and readability which are important assets in organisations, at a time when the complexity of decision-making is recognised by most actors, even if not all of them show the same sensitivity regarding the different criteria. th ere have been experiences of international multidisciplinary participation and choices being made to reduce geological risks regarding construction and infrastructure; the most commonly seen have been in implementing preparedness, prevention and mitigation so that damage caused by geological events involving construction has been minimal and that rehabilitation and reconstruction eff orts have enabled lower costs, as suggested by wold health organization/pan-american health organization (who / paho) since 2000 regarding management and comprehensive disaster management (figure 2). disaster management is supported by the body of knowledge, measures, actions and procedures, which together with the rational use of human and material resources are geared toward the planning, organizing, directing, implementing and monitoring activities to prevent or reduce the eff ects of disasters on people and infrastructure services and products. th e actions and tasks performed for the management or disaster management are permanent in time and space, forming a cycle comprised of three phases: 1. before (prevention, preparedness and early warning), including measures and actions designed to prevent or reduce the disaster, among these are: • conduct ongoing monitoring and surveillance of natural and human phenomena through information systems and early warning systems interconnected to the a main center. • liaise with all systems generating information on natural hazards and anthropogenic (human induced) that loom over the town or the country in general. • disseminate information in a timely and necessary regarding disaster prevention and care among institutions, authorities and general population. • conduct or processed information sharing between institutions involved in advance in the prevention and relief of national and international levels. figure 1. diagram showing the interrelationship of factors involved in geological risk management. government communityspecialists reflexions on multidisciplinarity and geologic risk management in cuba 139 • investigate, evaluate and analyze natural and human phenomena that can lead to disasters. • develop tools for the analysis of risk factors that facilitate the process of national planning and land use in conjunction with relevant agencies. • apply risk reduction measures in buildings and general infrastructure to minimize possible damage. 2. during (response) formed by the set of activities and measures used during and immediately after a disaster to minimize its effects, the work also includes rescue, search and rescue. 3. after (rehabilitation and reconstruction), consisting of actions to the state’s recovery after the disaster, where in addition (and as part of the development process) again begin to apply the measures provided for in the first phase. as said before, the occurrence of geological phenomena’s could lead to a disaster and there for management is needed. elements related to the management of geological risks regarding construction and infrastructure in cuba become particularly relevant during the initial construction/investment stages. reviewing the pertinent literature on these issues related to cuba confirmed that several actions had been taken aimed at multidisciplinary analysis concerning geological risk management, especially concerning intervention by legal regulations established in cuba regarding aspects such as obtaining an environmental license, compatibility with defence, micro-localisation studies and the authorisation of technology. although cuba has made progress regarding the above, especially when compared to the rest of latin-america and many other third world countries, difficulties do remain. the trouble lies in its effective implementation; there is the need for specialists having different skills to those of the architects, civil and hydraulic engineers and geologists (who mainly work in construction) who become involved in research, ultimately contributing with important elements to key project decision-making. there is the obligatory preparation, study and search for information by specialists working in the construction industry regarding geological risks, a very necessary process today which is facing palpable weaknesses. specialists (most often lacking construction expertise) from other branches of industry which are not related to construction and hydraulics express opinions on issues related to environmental risk management every day. such situation needs to be ordered; the necessary criteria and intentions exist and cuban law regulates such matters in an organised and harmonious way. its achievements in this field demonstrate such statement, only the multidisciplinary integration tools are missing. multidisciplinary action is legally pre-designed regarding construction/investment in cuba. ministry of economics and planning resolution 91/2006 “notes on investment” article 3, section 4, deals with “integrated project management” for promoting multidisciplinary work; however it is not clear from the geological point of view exactly which action can be implemented. the resolution states that an investor must, “hire other subjects to participate in investment to maximise its comprehensiveness”, or, “construction requires guarantees during different investment stages”, but does not consider how or what methods will be used to achieve such integration, or whether to invite or hire different specialists from institutions providing such guarantees (environmental licence, compatibility with civil defence interests, the use of technology, etc.), when it comes to making key decisions regarding the mitigation of geological risk. actions such as integrating risk analysis, multidisciplinary team decision-making and reconciling views with organisations, specialists, society and policy-makers are all elements that help decision-making and therefore better geological risk management. the cuban government is taking action to this end, even though this is still insufficient; the integration methods used are not always the most appropriate ones, resulting in problems regarding geological risk management. results could be even better by applying multicriteria principles (i.e. the multidisciplinary approach proposed here), placing greater emphasis on infrastructure and construction. cuban environmental law is governed by the ministry of science, technology and the environment (citma) (law no. 81/1997). this ministry has worked on environmental issues and risk management in cuba, issues which have been well reflected and identified in its national strategy. the agency has issued some regulations influencing geological risk management involving multidisciplinarity, the key one being resolution 77/1999. this resolution contains the regulations stipulating how to apply for an environmental licence (essential for starting construction work). multidisciplinary participation involves several institutions in obtaining an environmental licence; the national seismic research institute becomes involved concerning geological risk management, carrying out investigation and designing some national standards, such as the cuban earthquake resistance code and geotechnical standards. figure 2. the cycle for managing and integrating disaster management. who / paho 2000. liber galbán rodríguez, ingrid n. vidaud quintana, tomás jacinto chuy rodríguez, josé maría ruiz ruiz, francisco calderín mestre, eduardo rafael alvarez deulofeu140 resolution 77/1999 dealt with geological risk management during a project’s initial stages or planning. if significant, initially unforeseen negative effects arise during the implementation or exploitation of work, the authority could order the cancellation or suspension of the environmental licence regarding the draft work or activity. this implies an immediate halt to the work or project (resolution 77/1999, article 20). civil defence guidelines have been determined in cuba through regulations and state plans, i.e. national defence law 75/1994 and decree 262/1999 “regulations for harmonising the economic and social development of the country’s defence interests.” the latter provides for a judgement to be issued by civil defence national headquarters for all ongoing investments. this may require hazard, vulnerability and geological risk studies which are carried out by entities whose corporate purpose is appropriate and which have been approved by the general staff, a factor also promoting multidisciplinary involvement. several regulations and procedures are involved in physical planning issues which may be involved in risk management: land use plans, microlocation studies and macroand micro-location certificates, as well as plans and strategies designed to meet national or local disasters. it thus follows that environmental protection, disaster prevention and pertinent requirements must be taken into account regarding any construction project undertaken in cuba. environmental management (ipso facto geological risk management) thereby involves various sectors of cuban society in a multidisciplinary manner, without the need for excessive legal provisions being involved, leading to unnecessary legal red tape (galban et al., 2010). the experience of recent years in cuba has indicated that more specialists from fields unrelated to construction and hydraulics express their opinions every day on issues related to environmental risk management, and even propose measures to address certain situations, without even having technical knowledge regarding construction. as stated above, such situation needs to be addressed and harmonically inserted in the construction investment process. nevertheless, some examples highlight multidisciplinary intervention in geological risk management in construction and infrastructure in cuba. results: analysis of the mendive school project a good example of construction projects in recent years which have been executed following the principles of geological risk management involving multidisciplinary participation in cuba would be the rafael maría de mendive senior high school restoration and rehabilitation project in the city of santiago in cuba. this city is located in a known seismic area situated at the contact borders of the north american and caribbean plates (figure 3) which has been affected by more than 1,000 earthquakes since 1528; 20 of them had an intensity equal to or greater than 7, 13 rated 7, 5 rated 8 and 2 rated 9 on the msk scale (table 1), making this area the one having the highest seismic risk in cuba. the mendive project involved action aimed at promoting the structure’s resistance against the impact of geological phenomenon. this project drew on the multidisciplinary involvement of institutions, professors, government and researchers from the santiago de cuba area, including the national seismic research institute, the universidad de oriente (uo) construction faculty’s civil engineering team, the city of santiago de cuba’s conservation project office, ministry of construction provincial group’s enterprise project 15 and the national engineering and architecture company (beltran, 2010). university professors played a decisive role in this work and influenced the implementation of new approaches to management, together with specialists from project company 15 (emproy 15) which was awarded quality management iso 9001 / 2001 in 2005. figure 3. location of the city of santiago de cuba within the boundaries of the southern part of the north american plate. (8º n. lat., 57º w. lon.) – (90º n. lat, 24º w. lon.) reflexions on multidisciplinarity and geologic risk management in cuba 141 a report was prepared containing a seismic hazard, vulnerability and seismic risk study as earthquakes were identified as being the main geological hazard for the building. the report identified high vulnerability for the school as inadequate links with the structural system were observed, being subject to collapse following vigorous shaking. no security measures had been taken in the building and equipment and furniture were not adequate (group of authors, 2007). this report also contained results from tests made by the national applied research company regarding soils and gave conclusions and measures for increasing the building’s security: • calculations were made regarding structural vulnerability to seismic events using different methods, several of which are not included in the cuban earthquake-resistance code; • a strict control plan was designed and implemented based on the above measures. greater security for work was thus achieved, successfully meeting a set of measures, regulations, standards and building codes contributing towards proper geological risk management; • indicators were observed when executing the project due to changes in the original rehabilitation design influencing a reduction in the building’s vulnerability, for example, the behaviour of projected charges in soils was observed by adding concrete and steel structural reinforcement, finding that there was no significant settlement (figure 4); • designing and injecting special concrete foundations and walls for the building (aerated concrete), especially made by the national company for engineering and architecture of havana and the concrete company from the same province, thereby achieving greater compaction and increased resistance to the impact of severe seismic events (figure 4); and • a final report was delivered to the customer (investor) involving adjustments to be made to the building to increase its seismic resistance, also containing information about the conduct of future work so that the measures being taken would be respected. the multidisciplinary group of companies, institutions and specialists who participated in the project were given a prestigious award in 2009 following analysis of the final results and after the national conservation award’s evaluation commission had undertaken a detailed assessment of it. a press note read (beltran, 2010) (figure 5): “[…] quality award was given to restoration companies working on the rafael maría de mendive school in santiago de cuba, on the occasion of cuban architects’ day, in recognition of university student leader josé a. echeverría, who was killed during an assault on the presidential palace and the taking of radio reloj. pedro polo castillo, director of the engineering services company, received the certificate on behalf of all institutions and agencies involved in the restoration from 2001 to 2007. norma diaz ramirez, an architect from the ministry of construction’s development department presented the trophy and diploma. others worthy of honour were the national centre for earthquake research, the faculty of construction’s civil engineering team, the city conservator’s project office, project company 15 and the national engineering and architecture project company. other entities involved were: the ministry of construction’s laboratory (santiago), the construction business group, the escambray comyear month day hour n. lat. w. lon. ms h i (max) 1578 aug (19.90) (76.00) (6.8) (30) 8.0 1580 (19.90) (76.00) (5.8) (30) 7.0 1675 feb 11 (19.90) (76.00) (5.8) (30) 7.0 1678 feb 11 14:59 (19.90) (76.00) (6.8) (30) 8.0 1682 (19.90) (76.00) (5.8) (30) 7.0 1752 oct (19.90) (76.00) (5.8) (30) 7.0 1760 jul 11 (19.90) (76.00) (6.8) (30) 8.0 1766 jun 12 05:14 (19.80) (76.10) (7.6) (35) 9.0 1775 feb 11 (19.90) (76.00) (5.8) (30) 7.0 1826 sep 18 09:29 (19.90) (76.00) (5.8) (30) 7.0 1842 jul 07 (19.90) (76.00) (6.0) (30) 7.0 1852 aug 20 14:05 (19.77) (75.35) (7.3) (30) 9.0 1852 nov 26 08:44 (19.50) (76.25) (7.0) (35) 8.0 1858 jan 28 22:04 (19.90) (76.00) (6.5) (30) 7.0 1903 sep 22 08:09 (19.90) (76.00) (5.7) (30) 7.0 1906 jun 22 07:09 (19.65) (76.25) (6.2) (30) 7.0 1914 dec 25 05:19 (19.45) (76.30) (6.7) (30) 7.0 1930 jan 17 12:00 (19.90) (76.00) (5.8) (25) 7.0 1932 feb 03 06:15 19.80 75.80 6.75 8.0 1947 aug 07 00:40 19.90 75.30 6.75 50 7.0 note: the values in brackets are macroseismic estimations of the parameters. hour (ut), h (km), i (msk). table 1. strong earthquakes reported in santiago de cuba (chuy rodriguez, 1999). liber galbán rodríguez, ingrid n. vidaud quintana, tomás jacinto chuy rodríguez, josé maría ruiz ruiz, francisco calderín mestre, eduardo rafael alvarez deulofeu142 figure 4. structural reinforcement to increase seismic resistance in the mendive project. pictures courtesy of francisco calderín mestre, project member. (a) (c) (e) (f ) (b) (d) reflexions on multidisciplinarity and geologic risk management in cuba 143 figure 5. photos taken before and after the work, courtesy of francisco calderín mestre, project member. (a) (c) (e) (f ) (d) (b) liber galbán rodríguez, ingrid n. vidaud quintana, tomás jacinto chuy rodríguez, josé maría ruiz ruiz, francisco calderín mestre, eduardo rafael alvarez deulofeu144 pany, the endowment entity, the santiago de cuba and las tunas building materials industry, marble yard, panorama equipment, engineering works 11 constructions company, vitral stained glass (country-wide paint seller), marine works, concrete company (havana), copextel, architecture groups 2l, 56, 57 and 58, group 24 engineering, repair and maintenance/conservation group, and prefabricated equipment and service entities. […]” the national bureau of standards evaluated the project before awarding the republic of cuba’s national quality award using renowned specialists’ qualitative and quantitative methodologies based on a total quality management approach and practice (beltran, 2010), whose principles are laid down in iso 9001/2001, further defining management principles. it is worth mentioning that this award directly validated the value of multidisciplinary involvement in geological risk management regarding construction investment from proven experience. it also reflected the need for gradually increasing collaboration between the different actors involved in geological risk management and construction work specialists sharing knowledge about risks. conclusions one issue regarding international geological risk management for all levels of government involves multidisciplinarity amongst specialists, government and the community, aimed at reducing the effect of geological risks (earthquakes, floods, landslides, erosion, sea action on coasts, etc) on man-made structures. a literature review regarding such matters in cuba confirmed that actions aimed at achieving multidisciplinary analysis concerning geological risk management had been taken, especially regarding intervention concerning cuba’s legal regulations. the “rafael maría de mendive” senior high school restoration and rehabilitation project in the city of santiago de cuba validated the value of multidisciplinary involvement in geological risk management regarding construction/investment. bibliography ayala c. (1992). introducción a los riesgos geológicos. geological and mining institute of spain. editorial ríos rosa, 23., 23. 28003. madrid. beltrán, ángel luis., “premio de la calidad a restauración de preuniversitario rafael maría de mendive en santiago de cuba”, sierra maestra digital newspaper, 13th march 2010. issn 16819969. http://edicionesanteriores.sierramaestra.cu/esp/noticia/13calidadmendive0308130. html brusi, david; roqué, carles. (2003). los riesgos geológicos. algunas consideraciones didácticas. third jornades of crecit. chuy rodriguez, tomas j. (1999). macrosísmica de cuba y su aplicación en los estimados de peligrosidad y microzonación sísmica. phd thesis in geophysical sciences. archives of national seismic research institute, 273pp. decree law no. 262/1999 “regulations for the harmonization of economic and social development of the country’s defence interests.” may 14th. executive committee of the council of ministers of cuba. galbán rodríguez, l. (2010). “advances in risk management”, chapter 11: “model for geological risk management in the building and infrastructure processes”. edited by: giancarlo nota, editorial sciyo. croatia. isbn: 978-953-307-138-1. galbán rodríguez, liber.; galbán rodríguez, liuba; vázquez pérez, ársul josé; gago abad, adrián (2010). reflexiones en materia de gestión de riesgos geológicos en procesos constructivos del municipio santiago de cuba: normas y procedimientos jurídicos. revista jurídicas. colombia. vol.25/1, 2010. issn: 1794-2918 galbán rodríguez, liber. (2009). algunas consideraciones teóricas sobre la gestión de riesgos geológicos. revista de geología ufc. volume 22 issue 1 december 2009. brazil. issn-0103-2410 garcía leyton, l.a. 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(2003). la construcción de proyectos de infraestructura multinacionales en centroamérica y sus consecuencias en la generación de riesgos. cd “comarcas vulnerables: riesgos y desastres naturales en centroamérica y el caribe”. compilated by amado fernandez. 1st. digital edition, cries editorial. isbn: 1016 – 9628. buenos aires. argentina. http://www.corteidh.or.cr/tablas/cd0322. pdf no author, “premio nacional de calidad de la república de cuba: bases y cuestionario”, oficina nacional de normalización, ministerio de ciencia, tecnologia y medio ambiente. http://www. nc.cubaindustria.cu/pnc%20bases%20y%20cuestionario.rar (last accessed november 2011). ogura, a.; soares macedo, e. (2005). procesos y riesgos geologicos. division of geology institute for technological research of são pauloipt. tic.uis.edu.co/aula2.1/users/rcorrea/9procesos.pdf (last accessed november 2011). resolution no. 392/98. “essential requirements for construction.” ministry of construction of cuba. resolution no. 77/1999. “regulation of the process of environmental impact assessment. ministry of science, technology and environment of cuba. “ resolution no. 91/2006.”notes on the investment process.” ministry of economy and planning of cuba. undro, “natural disasters and vulnerability analysis”, report of experts group meeting, geneva, july 1979. who / paho (2000). “natural disasters and health protection”. scientific publication. 575; washington dc. usa. geofisica coiombiana n° 5 p p , 32-39 diciembre de 2001 bogota 0 c. issn 0121 2974 estudio de la vartacton solar quieta para el observatorio geomagnetico de fuquene rosa angelica laoino parra posgrado gerencia de recursos naturales facultad del medio ambiente universidad distrital francisco jose de caldas yaoueline figueredo baez emna m,reva valo~s meneses departamento de geociencias maestria en geofisica universidad nacional de colombia resumen el estudio realizado comprende el calculo y anal isis de la variaci6n solar quieta (sq), para el observatorio geornagnetico de fuquene a partir de las tres (3) componentes del campo magnetico terrestre (cmt): declinaci6n (d), componente horizontal (h) y componente vertical (2); de los cinco (5) dfas internacionajmente quietos de cad a mes. el perfodo considerado esta entre ocrubre de 1953 y diciembre de 1994. la metodologfa desarrollada se basa en el analisis individual de cada dfa quieta mediante la aplicacion de una correcci6n por variaci6n no cfclica (cvnc) y su reduccion a un valor medio cero, realizando el respectivo analisis armonico hasta el grado 4. posteriormente se aplic6 la correcci6n por achatarnienro y se reagrup6 la informaci6n en tres niveles de actividad solar: alta, media y baja (en funci6n del mimero de manchas solares); para cada una de elias se promediaron los arm6nicos por mes calendario, perrnitiendo un analisis de la rnodulacion estacional fundamentada en el criterio de promediaci6n de resultados de los meses (solsticio de verano: mayo junio julio agosto; equinoccio: marzo abril septiembre octubre; solsticio de invierno: noviembre diciembre enero febrero). como validacion, se realizaron comparaciones con el modelo de wallace campbell, estableciendose la diferencia de este modelo te6rico con la modelaci6n solar para la misrna selecci6n de dias. a partir de las comparaciones realizadas entre el modelo te6rico y el observado se obtuvo el maximo y minimo valor de sq. los resultados obtenidos son de utilidad para el mejorarniento de los modelos globales de variaciones regulares, variacion solar quieta (sq) y modelo del campo geomagnetico internacional de referencia (lgrf). adicionalrnente facilita la determinaci6n de modelos de propagaci6n ionosferica y propagaci6n de senales satelitales transionosfericas para sistemas de posicionamiento g.p.s. palabras clave' campo magnetico, variacion solar quieta, variacion c!clica, estacion fuquene abstract this study contains the calculation and analysis of solar quiet variation (sq), for the geomagnetic observatory of fiiquene in their three (3) components of terrestrial magnetic field (cmt): declination (d), horizontal component (h) and vertical component (2); of the five (5) internationally quite days of every month. the considered period is between october 1953 and december 1994. the developed methodology is based on the individual analysis of every day by means of the application of a correction for non cyclic variation (cvnc) and its reduction to a value mean zero, carrying out the respective harmonic analysis until the degree 4. later on the correction was applied by flattening and the information was regrouped in three levels of solar activity: hight, medium and low (in function of the number of sunspots); for each one of them the harmonics were averaged per month. the analysis of the seasonal modulation is based in the approach of average of results of the months (solstice of summer: may june julio august; equinox: march april september october; solstice of winter: november december january february). as validation, they were carried out comparisons with wallace campbell's pattern, settling down this theoretical model's difference with the solar modelation for the same selection of days. starting from the comparisons carried out among the theoretical pattern and the one observed it was obtained the maximum and minimum value of sq. this results are useful for the improvement of the global models of regular variations, quiet solar variation (sq) and international geomagnetic reference field (igrf). additionally it facilitates the determination of models of ionosferic and transionosferic propagation for signs satelitales of position systems g.p.s. key words: magnetic field, solar quiet variation, cyclic variation, fuauene's station introduccion tividad minima para los campos solar y terrestre. su determinaci6n e da mediante la observaci6n del mimero de manchas solares y magnetogramas que reflejan cambios principalmente en perfodos de 24, l2, 8 y 6 horas. ei estudio de la variaci6n solarquieta para colombia contribuye al conocimiento general de laactividad solar, permitiendo, principal mente, su uso en el estudio de lapropagaci6n de las ondas transionosfericas ei registro diario de las variaciones geomagneticas sobre cualquier ubicaci6n en el mundo muestra una serie de cambios irregulares en el campo magnetico terrestre (cmt), originados por las corrientes provenientes de la atm6sfera superior, la magnetosfera y la actividad del sol. dentro de estas variaciones se encuentra la solar quieta sq, la cual se obtiene a partir del estudio y selecci6n de dias denominados quietos (q) 0 de ac32 geofisica colombiana, 5, dicii:mbre de 2001 estudio de la variacion solar quieta para el observatorio geomacnetico de fuquene para el modelo electr6nico de la capa ionosferica yen la modulaci6n del cmt con respecto al ciclo solar. el presente articulo esta basado en el proyecto de grado realizado por las estudiantes yaqueline figueredo baez, rosa angelica ladino parra y emna mireya valdes meneses de la universidad distrital francisco jose de caldas, en convenio con ellnstituto geografico agustin codazzi igac, y la universidad nacional de la plata argentina. este trabajo surge ante la necesidad de establecer el comportamiento del campo magnetico terrestre local en el nivel de actividad quieto a partir del procesamiento de datos obtenidos en el unico observatorio geomagnetico permanente del pais (fiiquene). la incorporaci6n de est os resultados a model os intemacionales hacen parte del primer aporte de esta clase realizado en colombia. gen eralidades la tierra se comporta como un iman gigante que, al combinarse con la influencia de la actividad solar, genera una zona de afectaci6n directa conocida como cmt (campo magnetico terrestre). la intensidad del cmt es f (fuerza magnetica) de caracter vectorial y sus componentes sobre los ejes son xyz; la direcci6n de h sefiala el norte magnetico, el angulo que forma esta direcci6n con el norte geografico es d (la declinaci6n magnetica) y el angulo entre f y la horizontal es l iel angulo de inclinaci6n) (udias y mezcua, 1986). (vease figura i). z y' (eslel x ------,>x d ( -\ i ~ f --~'--~h i ir i i i i [norte] z (nadir) figura i. componerues del cmt f=.jx2+y2+z2 h =jx2 +y2 tg-' y d x tg-1 z i h el cmt esta compuesto por un campo rnagnetico intemo y un campo magnetico externo que dependen directamente de las fuentes; un campo conical y un campo magnetico inducido (vease cuadro i). de igual manera, el cmt presenta variaciones de origen extemo e intemo, perrnitiendo comprobar varios fenomenos ffsicos del intelior teltestre. las variaciones externas se emplean para determinar la conductividad geoflslca colombiana. 5, diciembre de 2001 electrica de la corteza y manto terrestre a traves de los campos inducido e inductor, y las variaciones intemas perrniten describir los movirnientos del flujo de conveccion en el micleo, proporcionando informacion acerca del estado termico del manto y su composicion qufrnica (ratcliffe, 1972). las mediciones del cmt y sus componentes se hacen en observatorios permanentes; sus variaciones temporales se registran en un punto fijo de la superficie terrestre, en forma absoluta a traves de las componentes d,h,z; mediante registros continuos de observaciones permanentes (vease figura 2), suministrando informaci6n mas precisa para la determinacion de los valores medios diarios, mensuales 0 anuales y la variacion secular. magnetdgrama observatdrld de fuqu[n[ (5<j28'12" n 7')044'14"\4) 26 .julio 1995 0' 274-70 moo,.... -tooe-t r o ruq. t::!-~olo hof'1'c/r>"to.l 10 ....mh es ccto """("tical h 10 .-.t/(;li',j d lo ,.., 0' e,d'/c:l-" z ~ ,.,1/,;:1\.. r ';> ,.l/<:h .. l7ljo --'--16 ~ 00 ...-----...... --./~ i /"'"" '-hi 27"]70 100 di o zi 17080 33'3do f 332jo 00 09 12 1503 06 18 figura 2. mag~elograma registrado en el observatorio auto matico de fuquene estos observatorios se clasifican en convencionales, caracterizados por realizar un registro fotografico de las variaciones de las componentes, yen automaticos, que registran autornarica y digitalmente las cornponentes (courtillot y le mouel, 1988). en 1948 el instituto geografico agustin codazzi se dio a la tarea de estudiar varios lugares que reunieran todos los requisitos indispensables para el establecirniento de un observatorio de esta indole, escogiendo la isla el santuario (sitio que actualmente ocupa). en la isla y en los alrededores de la laguna se hicieron observaciones de la declinacion con magnet6metros, y se concluyo la ausencia de anomalias magneticas locales (i) cuadro i. fuentes que afectan el cmt (2) fuente medio fenemenns accion -transitorios pulsaciones bahias tormentas hicrornaqnetica terrnlca atmosfera lonizante origen externo sol peri6dicos magnet6sfera variaci6n anual onda de choque cicio undecenal variaci6n solar atm6sfera gravitacional luna variaci6n lunar magnet6sfera hidrotermica tierra nocleo origen interno tierra variaci6n secula dinamo exterior (3) (4) 33 que pudieran perturbar los instrumentos (confirmado en 1995 por ill.unidad de geoffsica del instituto geografico agustin codazzi). localizacion geografica : isla del santuario en ill.laguna de fuquene, cundinamarca, colombia. latitud longitud latitud geomagnetica longitud geornagnctica declinacion teorica: altura : os028 '12.044" n : 73°44'17.133" w : 16°.9 n (1997) : 355°.1 e (1997) : i°.0 (i 997) : 2 543 m.s.n.m. tanto ill. seleccion de ill. isla, de 0,3 km2 de superficie, rodeada por una extension de 30 km2 de agua, como ill. construccion de los edificios, se hizo de tal manera que no se generan perturbaciones magneticas artificiales. actualmente para el desarrollo del trabajo en el observatorio de fiiquene se cuenta con: magnetornetros de tipo convencional. • magnetometros de precesion de protones (ppm) variometros. un observatorio magnetico automatico, magnerometros horizontales de cuarzo (qhm). (notas explicativas a ill.carta geornagnetica para ill.epoca i 958.5, igac, 1958). relaciones terrestres solares la relacion tierra sol esta caracterizada por ill.interaccion de fenornenos ffsicos y qufmicos provenientes del sol y ill.tierra, y se desarrollan en ill. region del espacio cercana a ill.tierra (entre 50 y 70 km sobre ill.superficie terrestre) extendiendose a decenas de radios terrestres. sus lfrnites estan entre ill.materia solar y terrestre (vease figura 3). dichas relaciones incluyen ill.parte superior de ill.atmosfera terrestre, ill.parte exterior del campo geomagnetico y las emisiones solares que las afectan (hargreaves, 1992). geoespacio -. '''-'.......... atmosferil super ior e ionosfero figura 3. relaci6n solar terrestre la actividad solar es ill.principal fuente de variaciones en el cmt y, a su vez, es el indicador de ill.actividad rnagnetica que puede ser analizada a partir de fenomenos que se desarrollan en ill.estructura de ill. superficie 34 ladino et al. solar como son las manchas solares. estas son areas oscuras sobre ill.superficie del sol, de 50 000 km de diametro; aparecen por parejas, tanto en el hemisferio norte como en el hemisferio sur. por ill.cantidad de manchas solares es posible medir el cicio de actividad solar que se determina por el numero de wolf (r) (hargreaves, 1992). r = k u+ 109) (5) f = numero de manchas observadas g = rnimero de regiones perturbadas 0 grupo de manchas k = constante dependiente del observatorio variacion solar quieta sq la determinacion de sq es de importancia en ill. correccion de levantamientos magneticos con fines prospectivos, ya sean globales, continentales, regionales 0 locales mediante el empleo de un modele maternatico en funcion de un analisis de los valores medios horarios, suministrados por los observatorios y el centro mundial de datos. sq es el cambio minima de la actividad magnetica observado en el tiempo y en un lugar determinado, producido por ill. interaccion entre el sol y ill.tierra a traves de la relacion viento solar-cmt. la interaccion se presenta en ill.alta atmosfera a causa del f1uido de la corriente dfnamo en la region e de la ionosfera, provocada por la accion terrnica, ionizante y gravitatoria del sol (wallace y matsushita, 1967). los primeros reportes de variacion diaria quieta fueron hechos por hellibran en 1634 al observar cuidadosamente el movimiento de la punta de una aguja rnagnetica larga. sin embargo, la variacion fue verdaderamente establecida cuando graham (1724) hizo las primeras determinaciones (calibradas) de los cambios minimos de la declinacion sq (d). gauss (1841) desarrollo el metodo mate matico para la separacion de las contribuciones externas e internas del campo superficial por medio de un analisis armonico esferico. existen muchos aspectos de los cambios de sq que aun no son suficientemente conocidos como para permitir una prediccion de los niveles de variacion del campo quieto en todas las posiciones mundiales, como por ejemplo la contribucion al campo sq de las corrientes ionosfericas en las horas nocturnas. la opcion es recopilar, procesar y analizar los datos registrados por los observatorios permanentes. ce t e c t e r i s t i c e s las principales caracteristicas de la variacion solar quieta sq son: amplitudes bajas. dependencia de la latitud, de los cambios estacionales y del nivel de actividad solar. su comportamiento indica actividad magnetica minima. sus valores minimos se presentan en horas de la noche a causa de la disminucion en la produce ion de electrones. variacion de su comportamiento en amplitud y fase a traves del afio, principalmente por el movimiento de rotacion de la tierra con respeeto al plano de la eclfptica solar y por la cantidad de partfculas que transporta el viento solar al interactuar con la magnetosfera. sirnilitud en sus valores registrados por observatorios localizados a una misma latitud; por consiguiente, para un mejor analisis de su comportamiento, la superficie terrestre se ha dividido en cuatro zonas: ecuatorial, latitudes medias, auroral y polar. geofisica colombiana, 5, qiciembae de 2001 estudio de la variacion solar quieta para el observatorio geomagnetico de fuquene su comportamiento sobre la tierra esta influido por el sistema de corrientes propias de la zona; por ejemplo, los observatorios localizados en el area ecuatorial estan afectados poria corriente anillo y el electrochorro ecuatorial. presenta un maximo valor por el calentamiento solar durante horas de mayor iluminaci6n. • la linea base para determinar los cambios de sq toma como referencia los valores registrados en la medianoche, ya que en este intervale de tiempo el proceso de ionizaci6n es minima poria baja densidad de electrones, 10 que hace que la coniente electric a se acerque a cero (wallace y matsushita, 1967). para la determinaci6n de sq extemo en un observatorio, las manchas solares son el elemento que determina la influencia de la actividad solar, principalmente en la capa e de la ionosfera. en la actualidad este valor puede ser estimado por observaciones satelitales (jacobs, 1987). la variabilidad dfa a dfa de sq esta mas influida poria cantidad de vientos generadores de la marea termica en la regi6n e de la ionosfera, que por los cambios que presenta el cmt (jacobs, 1987). identificaci6n de sq para determinar sq se tienen como herramienta primaria los registros (magnetogramas) diarios de las tres componentes del cmt (h, d y z) obtenidas en los observatorios permanentes. en los magneto gram as se puede observar que hay unos dfas mas perturbados que otros; a los menos perturbados se les denomina quietos y la variaci6n para estos dfas es ilamada variaci6n solar quieta (chapman y lindzen, 1970). la selecci6n de dfas quietos intemacionales (iq) se efectiia a partir de tres criterios aplicados a cada dfa de un mes calendario en funci6n de los indices magneticos, indice magnetico es un numero que representa la amplitud de un parametro 0 fen6meno ffsico en estudio, durante un intervale de tiempo fijo acorde a las caracterfsticas constantes del fen6meno. es necesario mencionar que cada valor individual no remplaza el dato original (menvielle y berthelier, 1991) los indices magneticos tienen por objeto suministrar informaci6n del nivel de actividad geornagnetica y analizar los fen6menos presumiblemente unidos a el; por ejemplo, la estructura de la magnetosfera, la influencia del viento solar y el campo magnetico interplanetario (cmi). los criterios para definir el dfa iq son: 1. suma de los 8 valores kp. 8 lkp i~l 2. suma de los cuadrados de los 8 valores kp. ~ lkp2 ;~l 3. determinaci6n del valor maximo kp. {kp;}; = 1,2, , v ei fndice kp es el fndice que permite analizar la actividad rnagnetica para una sola estaci6n 0 para la actividad solar terrestre total, con geofisica colombiana, 5, diciembre de 2001 la contribuci6n de los valores trihorarios de los observatorios; estos son adimensionales. para la determinacion de sus valores a amplitudes medias se asigna a cada fndice un nurnero denominado ak, para una sola estaci6n y ap para un conjunto de observaciones (campbell y matsushita, 1981). estos criterios son asignados con igual peso. en un rnes, los dfas con el quinto mimero medio mas bajo son escogidos como dias quietos internacionales, y aquellos con el quinto valor mas alto como los cinco dias perturbados intemacionales. en la actualidad los dfas son listados de acuerdo con el valor y la escala de quietud 0 perturbacion en un orden crono16gicamente ascendente. teniendo en cuenta los tres criterios mencionados, los dfas de cada mes se ordenan en forma ascendente y los de kp menor corresponden a los dfas quietos. es necesario mencionar que el fndice kp es adimensional. para determinar su valor se hace uso del fndice ap, el cual es dimensional con rango de conversi6n ant; cada unidad de este equivale a 2 nt. para elegir los cinco dfas intemacionales mas quietos se debe cumplir, la siguiente relacion: [± ap ;] = ap i~l 8 (9) si su promedio es menor 0 igual a 6, se tiene con exactitud un dfa quieto. despues de seleccionar los dfas q, se organizan en una !ista conform ada por cinco dfas calmados intemacionales para cada meso estas listas son publicadas por la iaga. su importancia radica en que los usuarios en diferentes observatorios los emplean para analizar los datos y para comparar los resultados en estos dias predeterminados. a partir de estos datos se estima la variaci6n solar quieta en diferentes escalas de tiempo (mensual, semianual, anual y modulaci6n estacional) segun los requerimientos del estudio, para uno 0 varios observatorios, logrando asi concluir su respect iva representaci6n. (6) representaci6n de sq la variaci6n sq se representa mediante expansi6n en arm6nicos esfericos de los 24 valores medios horarios de cada dia quieto registrados en el observatorio. en el analisis es necesario tener un conocimiento tanto de las caracterfsticas generales de sq, como de la minimizaci6n de errores de los coeficientes en la serie de fourier, con el prop6sito de establecer los arm6nicos mas representativos del fen6meno. ei analisis arm6nico esferico se caracteriza por: 1. requerir medidas espaciadas del campo de la tierra sobre la superficie total de la esfera analizada. 2. representar con las series una separaci6n de la funci6n del potencial magnetico terrestre entre la composici6n extema (superficie ionosferica) e intern a (inducida). ei proceso no s610 permite la construcci6n del sistema de conientes de la clinamo atmosferica, sino tambien del campo inducido de informaci6n de la conductibilidad terrestre (chapman y bartels, 1970). el proceso de representar una funci6n en la forma de ondas seno se (7) (8) 35 denomina analisis arm6nico, y los factores an y bn se denominan coeficientes arm6nicos. la representaci6n es unica y para cada funci6n le corresponde un solo conjunto de amplitudes cn y fases (n. (chapman y bartels, l970). los terrninos de amplitudes (en) y fase «n) de los coeficientes, se representan de laforma: c, = ~a,~ + b,; bll ell = arctg an se representan graficamente mediante el dial arm6nico (vease figura 4), donde an es medido ascendentemente y bn a laderecha en un sistema de coordenadas. cad a componente peri6dica es asociada con un punto p y con las coordenadas an, bn. ei vector desde elorigen al punto tiene la longitud (amplitud, (n), con valores para an, bn de: an cii * serub , c; * cos ¢iibn ei analisis de las componentes de fourier proporciona el valor de las variaciones anual y semianual, originados por el calentamiento terrestre; la ionizaci6n durante la trayectoria anual alrededor del sol; la interacci6n de la magnetosfera terrestre con el viento solar, y efectos asociados con el viento solar y cicio solar de ii afios (chapman y bartels, 1970). ....., .£0!f i ~/ ..,i c in/ / "/' -'\-./ in i --f--....t::..------l-....,..---...;;.~ bn bn figura 4. propiedades de un dial arrnonico para la onda seuo, an cos nt + bn sene nt = en seno (nt + (n). metodologia recoleccion de la informacion ei observatorio geornagnetico de fiiquene registra los valores horarios de las componentes del cmt h, d yz, que son recolectados poria unidad de geoffsica del lnstituto geografico agustin codazzi, presentados en archivos ffsicos. elaboracion de la base de da tos se introdujeron los 25 valores medios horarios en tiempo universal (tu) 36 lading et al. de cada dia quieto (q), suministrado por el world data center y el primer valor medio horario del dia siguiente. (10) correccion de los valores medios horarios por el efecto no cicuco (cvnc) debido a que el cmt varia al finalizar cada dia, se hace necesario corregir la sefial por esta variaci6n, mediante el empleo de la siguiente formula: (ii) cvnc(j) = [ ed252~ ed!] * j j = cada una de las 24 horas 1, 2, ...,24 ed25= valor medio horario de la hora 0-1 del dia siguiente edl = valor medio horario de la hora 0-1 del dia q ei valor medio obtenido se reparte a los24 valores medios horarios del dia q. (13) (12) ecj ec j = ed j cvnc i : j = 1,2, ...,2 (14) valor medio horario corregido por el efecto de variacion no cfclica. centrar los valores medios horarios a un nivel cero se centran los valores medios horarios a un nivel cero para que los datos puedan ser comparados con los datos obtenidos por el modelos de wallace campbell. centrar los valores al nivel cero permitira un mejor manejo de estes y su correspondiente anal isis. ~ 24 ec(j) epi-i=\ 24 (15) ~ 0 e j = ec j ep (16) ep = promedio de los 24 valores medios horarios del dia q aoe j = valor medio horario centrado en ceros clasificacion de niveles de actividad magnetica se determinaron tres niveles de actividad (alta, media y baja) basados en el mimero de manchas solares (r) 0 mimero de wolf calculado y suministrado por el world data center asi: • nivel de actividad baja 0 ( r ( 40 • nivel de actividad alta r ( 100 nivel de actividad media 40 ( r ( 100 los archivos de cada componente fueron fragmentados de acuerdo con el mimero de manchas solares. metodo de anal/sis armonico diario se realiz6 el analisis arm6nico clasico del dia q corregido por variaci6n no cfclica, deterrninandose los componentes arm6nicos hasta e) orden n = 4. geoflslca colombiana, 5, diciembaf de 2001 estudio de la variacion solar quieta para el observatorio geomagnetico de fuquene a" 24 * l ec(t) cos(w" * t) 1 12 27m w" 24 b" 1 24 * l ec(t)sen(wn * t) 12 fue necesario emplear un program a de analisis armonico que se aplico a cada dia q en particular elaborado en qbasic denominado armonico.bas, obteniendo para cad a dia cuatro (4) coeficientes armonicos an y cuatro (4) coeficientes bn. se promediaron los coeficientes armonicos en cad a nivel de actividad solar en funcion del mes y quedando en archivo de 12 filas (por cada mes) y 8 columnas que representan los 8 coeficientes. cada coeficiente an y bn se corrigi6 por el efecto achatamiento (vease cuadro 2). cuadro 2. constantes aplicables a la correccron porefecto de achatamiento armonica no. periodo(horas) kn (radianes) 1 24 1.002862 2 12 1011515 3 8 1026172 4 g 1047198 k" [:) sen [ : ) * a" • k "b * • k coeficientes corregidos /i hn n a partir de los coeficientes an* y bn* corregidos por el efecto achatamiento, se determina su amplitud y fase: i * 2 * 2c, = -va" + b" amplitud lpli b* arctg -b-~'-fase ii estos se graficaron en los diales arrnonicos para su respectivo analisis en funci6n del nivel de actividad solar. se utilizaron coeficientes fourier de grado 1,2,3 y 4 porque representan adecuadamente la variacion diaria quieta de folti1areal; estes se ajustan mejor debido a que las principales caracteristicas de sq se observan en el perfodo diurno reflejado en el primer armonico. geofisica colomb1ana, 5, diciembre de 2001 (17) slntesis armonica del dla q a partir de los 8 valores promedio en cada nivel de actividad, se realiz6 el calculo de la variacion solar quieta, sq, empleando el programa sintesis. bas, elaborado en qbasic, suministrado por la universidad nacional de la plata, empleando la formula: (18) 4 27m 27m sqe(t) = l a,'l cos -t + b,: sen --t 11=1 24 24 = 1,2, 24 (21 ) e(t) = para la declinaci6n, componente vertical 0 horizontal, obteniendo asi la sq observada para cada mes y nivel de actividad solar. ( 19) calculo de la variacion solar quieta sq teorica empleando el metodo de wallace campbell para calcular la sq teorica se emple6 un modelo definido por wallace campbell que tuvo como punto de referencia los tres aries de minima actividad magnetic a (1961, 1962 y 1963). este consiste en un modelo idealizado calculado a partir de un analisis armonico de grado 4 en funcion de la latitud, longitud, altura de la estaci6n, y de una correlaci6n con el igrf y las amplitudes de los armonicos. se utiliz6 el programa wdca-sq2.exe elaborado en fortran (sfntesis del campo del modelo global fourier con el modelo de superficie magnetica wdca-campbell para condiciones extremadamente quietas) proporcionado por el world data center y modificado por el departamento de magnetismo terrestre de la universidad nacional de la plata (argentina), cuyos datos de entrada a! programa (vease cuadro 3) son las coordenadas de la estacion y las fechas en que se desea calcular la sq te6rica. este modelo se caracteriza por no tener incorporada la actividad solar en cada uno de los resultados, convirtiendose asf en el mejor parametro de comparacion con el modelo calculado (sq observada) a partir de los datos registrados en el observatorio magnetico de fuquene, cuadro 3. esquema de entrada de datos al progra~·ia nombre componenle lalilud • longilud tiempo universal intervalo 30 minulos hb 52812044 286.558286 2 30 dia mes ann 1 1 1976 2 1 1976 9 1 1976 26 1 1976 28 1 1976 (20) el programa genera el valor de la sq para las componentes (h, d, z, x, y) en gammas; la sq observada en la componente d se obtuvo en minutos, por 10cua! se hizo necesario convertir la sq teorica en la componente d a minutos, mediante la formula: 37 d -i= tg y x la sq te6rica es un modelo de resultados generado por el programa wdca-sq2.exe despues de una selecci6n de datos requeridos y ordenados de tal forma que permitan una facil rnanipulacion. correlacion de resultados se procedio al correspondiente analisis mediante cornparacion de los resultados obtenidos a partir de los datos observados y de los resultados generados por el programa wdca sq2.exe (wallace campbell). esta comparacion se realiz6 sobre las bases de datos y graficas generadas. resultados y conclusiones los resultados obtenidos se generaron teniendo en cuenta: 1. el analisis de la variacion solar quieta sq observada y la variaci6n solar quieta sq teorica en los tres niveles de actividad solar (alta, media, baja) de los perfodos mensuales para las tres componentes del cmt (h, d, z). 2. el analisis de la modulaci6n estacional de sq (equinoccio, solsticio de invierno, solsticio de verano). 3. ei analisis de la variaci6n sq resultante de la diferencia entre la sq te6rica y la sq observada. 4. el dial armonico, las amplitudes y fases de los armonicos generadores de sq, dando lugar a las siguientes conclusiones: la obtencion de la variac ion solar quieta sq para fuquenc asf como para otros observatorios magneticos en el mundo no representa un verdadero nivel quieto, porque ante todo la tierra siempre esta afectada por perturbaciones (internas y extemas), asf como por su constante movimiento, impidiendo que se obtenga un verdadero modelo de quietud. aunque fiiquene se caracteriza por estar ubicado en una zona aislada de anomalfas locales, por el rigor en la toma de datos y por su perrnanencia en el tiempo. la obtencion de la variacion solar quieta sq mediante el metodo de analisis arrnonico esferico para el observatorio geomagnetico de fiiquene permitio llegar a un valor exento de una serie de perturbaciones y efectos planetarios globales, ya que se ernpleo una sola latitud en el analisis (s028 ,44") para reducirlo a un valor afectado por perturbaciones caractensticas de la regi6n como el electrochorro ecuatorial y la corriente anillo. los valores medios horarios observados en las tres componentes del campo rnagnetico terrestre se yen afectados por variaciones reflejadas en las diferencias de valor en los interval os 0-1 y 23-24 de un mismo dfa, indicando el efecto de corrientes de la zona y variacion del cmt; al emplear la cync se redujo a su minima valor el efecto de la corriente anillo en sq, en la componente h. se determine la sq mensual y estacional porque son perfodos representativos de la actividad solar, a fin de obtener un comportarniento general y no particular, debido a que este ultimo limita el analisis a ia obtencion de un valor. en la componente z, la sq obtenida se ve alterada debido a la presencia de corrientes inducidas del campo intemo, siendo proba38 laoino et al. (22) blemente la principal fuente de error en el analisis (manto superior y caracterfsticas geoi6gicas). ei minimo rango de la corriente sq se presento en diciembre y enero para los niveles de actividad alta y baja. el maximo valor de la variaci6n solar quieta se da en horas de la tarde, indicando que en este intervalo la actividad solar tiene su mayor efecto. fuquene presenta su maxima variacion estacional durante el equinoccio, con un valor maximo de 38 gammas y minima de 2 gammas para las cornponentes h y z. excepto la componente z, en elnivel de actividad bajo, tiene su maxima variacion en el solsticio de invierno con una amplitud de 2.7 gammas. al emplear el analisis fourier se logro obtener la variaci6n anual y semianual de sq representada en los coeficientes (al,bl) y (a2,b2), asi como los cambios en amplitud y fase para los meses por separado en el observatorio geornagnetico de fuquene en las tres componentes (h, d, z) del campo rnagnetico terrestre. al final aparece la representaci6n grafica del comportamiento de la variaci6n solar quieta sq, para las componente magnetic as h, d y z en los niveles de actividad alta, media y baja para el observatorio geornagnetico de fuquene (veanse figuras 5, 6 y 7). relacion sq (oeservaod,· teoricaj com ponente h b.l.ja actnidad mes de diciem bre 1353,83 ·1994.99 valores m eoids horarios en r.u. r elacion sq (obse rvaoa . teorica) com ponente h alta ,ll,ctndad mes de diciem bre 1953,83·1934,33 valoresm"dlds horarids ent.u relaciqrl sq (ob$ efiva[i/i.· teoricalcom potlente h m ed\lj. activici'\o mes de dicie mbre 1353,83·1994,99 .,.;valores m eoids hdrarios en t.u ~-sq observada --sqteorica ....... sq observaoa· $qtedrica figura 5. graficos de la variaci6n solar quieta sq para la cornponente rnagnetica h en los niveles de actividad alta, media y baja para el observatorio geornagnetico de fiiquene geofislca colombiana, 5, diciemot=le de 2001 estudio de la variacion solar quieta para el observatorio geomagnetico de fuquene rel,l,ciotl sq (obserlj,i,oa. teoricl.icompoiiente 0 ii eda acrnioao me s de dicieijbrl: 19~3.u .199~.99 ~ \j---o;-~o:-::::'r':-.--;'~~:-;--t:-:-:--~c-:-:~h,...,..,..~ i sc valoresmeoios horp.rios etlt.u relacionsq[obsefn~d,i,·teoricj\)componelheobaj"''''ctivie-.o mesoe oiciembre1953,&3·1994,w >n.ldresilelloshorarioshit u relaclofisq(observa[\l!. teorlcajcomponenteoalta actpjidaomes de ociembre1953.ss·1994,99 v,o,loresmedios horariosent,u. _sqoeser\'ada ~-sqteorica sq oeservada· sqteorca figura 6. graficos de la variaci6n solar quieta sq para la componente magnetica d en los niveles de actividad alta, media y baja para el observatorio geomagnetico de fuquene figura 7. graficos de la variaci6n solar quieta sq para la cornponente magnetica z en los niveles de actividad alta, media y baja para el ob servatorio geomagnetico de fuquene geoff sica colombiana, 5, oiciembre de 2001 referencias bibliograficas chapman, s" and bartels, j, 1940: geomagnetism analysis of the data and physical theories: oxford at the clarendon press, 2, 545-603, 606-637, 639668,669-681,684-697,699-794 chapman, s" and lindzen, r. 1970: atmospheric tides: reidel publishing company, 1, 1-23,24-44,66-104,106-171 courttllot,y., and le mouel, l. 1988: time variations of the earth's magnetic field: reviews earth planet, 16,389-473, hargreaves,1. m, 1992: the solar terrestrial environment an introduction to geoespace, the science of the terrestrial upper atmosphere, ionosphere and magnetosphere: cambridge university press, i, 1-4,6-43,44-96,98-130,132205,208-247,249-310,312-362,364-388,390-413, instituto geografico agustin codazzi. 1958: notas explicativas a la carta geomagnetic a para la epoca 1958,5, jacobs,1. a, 1987: geomagnetism, new york and london: academic press, i, 1527,2, 1-571,3, 1-623, menvielle,m" and erthelier,a. 1991: the k-derivied planetary indices: geophysics, 3, no,3, 415-432, ratcliffe,1. a. 1972: an introduction the ionosphere and magnetosphere: cambridge university press; 9-21,37-99, udl",s,a. y mezcua1. 1986: fundamentos de geofisica. madrid: alambra, 223-255, wallace,c, and matsushita,s, 1967: physics of geomagnetic phenomena: academia press, 1,67-98,181-295,302-622,2,627-900,913-1101. relacion sq [08s efna(\ti, . teorica) com po hente z!l.l ta !l.ctrv'ioad m e5 de diciembre 1953.83 ·1394,99 ._d. . tol_ ..... : .• jt ..... :. _..:_ .' v,il,lores medios horarios en t.u relacion sq {observaqil. te orlca] componente i bo.j,i, actmdad m es de ociem8re 1953.83 -1934j9 0'-"" ',. v,i,lores!\ eoios horarios en t u rel!l.cion sq (085 erv!l. 00. teorlca) com po nente z m edl'l. !l.ct rvi 0.0. 01,\ es de dieie", sre 1953.83· 1994,99 .: -: valores medios hor.o.rios en t.u -sq teortca •. sq observ.aoa· sq te6rica 39 geociencias 15-1 julio 2011.vp scanning electron microscope description of the afowo oil sand deposits in south-western nigeria akinmosin, a1; adeigbe, o.c.2 and oyemakinde, o.o.3 1 geosciences department, university of lagos, nigeria; e-mail: waleakinmosin2001@yahoo.com 2 department of geology, university of ibadan, ibadan, nigeria. e-mail: olukris2009@gmail.com 3 department of earth sciences, olabisi onabanjo university, ago-iwoye, nigeria. contact ; dept of geology, university of ibadan nigeria. e-mail: olukris2009@gmail.com abstract. the afowo oil sand deposits in parts of southwestern nigeria were described by scanning electron microscope for characterizing the reservoir sands, based on their clay content. twelve core oil sand samples were collected, six of which were selected for analysis based on the similarities in their physical and textural characteristics. the prepared oil sand samples were then described by scanning electron microscope studies (vega tescan/lmu scanning electron microscope-sem). the analysis revealed that the oil sands contained minerals which had been precipitated and occurred as pore filling cement; these minerals included sheet kaolinite, vermiform kaolinite, kfeldspar, pyrite crystals, and corrosion quartz and corrosion feldspar. the sem images also showed 2-4 µm micro pores and 2-5µm fractures. the study showed that the clay minerals contained in the afowo formation’s reservoir rocks were mainly kaolinite which was not expected to have any negative effects on reservoir quality, especially during enhanced oil recovery operations. resumen los depósitos de arenas bituminosas de la formación afowo en algunas partes de suroeste de nigeria fueron analizados con microscopio electrónico para caracterizar las arenas del reservorio, basados en el contenido de arcilla. doce corazones de arenas bituminosas fueron recolectados, de estos, se seleccionaron seis para análisis basados en las similitudes de sus propiedades físicas y texturales. las muestras de arena bituminosa preparadas fueron estudiadas con el microscopio electrónico (vega tescan/lmu microscopio electrónico de barrido-sem). el análisis reveló que las arenas bituminosas contuvieron minerales que fueron precipitados y que llenaron espacios de poros con cemento; estos minerales incluyen caolinita laminar, caolinita vermiforme, feldespato potásico, pirita, cuarzo y feldespato corroído. las imágenes sem mostraron micro poros de 2-4 µm a y fracturas de 25ìm. el estudio indicó que los minerales arcillosos contenidos en la rocas del yacimiento de la formación afowo, son esencialmente caolinita la cual no tiene efectos negativos sobre la calidad del yacimiento, especialmente durante las operaciones de recobro mejorado. introduction tar sands, also known as oil sands, consist of bitumen which is a soluble organic matter derived from crude oil degradation, either as seeps coming to the surface or within shallow subsurface reservoirs, and host sediment with associated minerals excluding natural gas. the tar sands deposit found in nigeria outcrops along an eastwest 120km long, 4-6km wide belt extending from the boundary of edo and ondo to ogun states (enu, 1987). these deposits have been known about for quite sometimes now. however, intense investigation started around the 1970’s and earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 19-26 keywords: tar sand, textural characteristics, kaolinite, fracture, pore cement, reservoir and clay. palabras clave: renas bituminosas, características texturales, caolinita, fractura, cementado poro, depósito y arcilla. record manuscript received: 13/01/2011 accepted for publication: 30/05/2011 research groupin geophysics universidad nacional de colombia agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:11 p p p composite 133 lpi at 45 degrees has lasted until now. these deposits’ geology, oil saturation, reserves estimates and, the textural characteristics of the associated sand as well as the physical-chemical characteristics, have been previously described: adegoke et al., (1980), enu,(1987), oshinowo et al., (1982), ekweozor and nwachukwu (1989), nton (2001) and akinmosin et al., (2005 and 2006). a detailed understanding of the geology of bitumen-impregnated sands will provide vital information regarding the sands’ reservoir quality. a good knowledge of tar sand beds reservoir quality vis-à-vis tar recovery is also very important; hence the present study has been aimed at determining the tar sand deposits’ sedimentological characteristics for characterising the reservoirs based on their clay content. dahomey basin’s stratigraphy. the study area was located in the dahomey basin’s tar sands belt extending from latitude 4° 15’ e – 4° 47’e and longitude 6° 36’ n6° 43’n, (fig 1). the stratigraphy of the eastern margin of dahomey basin’s cretaceous to tertiary sedimentary sequence can be divided following in order of age: the abeokuta group, the imo group, the ilaro formation, coastal plain sands and recent alluvium (table 1). the dahomey basin’s sediments range in age from cretaceous to recent. thickness exceeds 2.2km on the coast in western nigeria which thickens markedly into the offshore and then down deep water (whiteman, 1982). the bituminous sands appear to be restricted to the abeokuta group; this unit 20 akinmosin, a; adeigbe, o.c and oyemakinde, o.o table 1: regional stratigraphic setting of the eastern dahomey basin (after idowu et al., 1993) age formation lithology ako et al., 1980 omatsola and adegoke, 1981 tertiary eocene ilaro formation ilaro formation sandstone palaeocene oshosun formation oshosun formation shale ewekoro formation ewekoro formation limestone cretaceous maastrichtian abeokuta group araromi formation shale. turonian afowo formation sandstone/ shale. berremian ise formation sandstone. figure 1: topographical map of the study area, showing sampling locations. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:12 p p p composite 133 lpi at 45 degrees extends throughout the entire basin, unconformably overlying the basement complex .the basin has also been considered to be a positive shallow depression which received a relatively thin sediments supply and was formed during the campanian stage associated with the benue – abakaliki anticlinorium and the subsidence of the anambra platform which gave rise to the anambra basin. the dahomey basin falls within latitude 6° 00| n – 8° 30| n and longitude 0° 15| e – 6° 00| e extending from the volta delta area in ghana to the okitipupa ridge; (figure 2). the miogeosynclinal wedge of the sediments developed through the early cretaceous to tate cretaceous period. the distance from the mouth of the volta to the axis of the okitipupa ridge is about 780 miles and the width of the measured northern onshore margin in benin is 9,840ft (300m); the bathymetric contour lies about 140 miles offshore.the basin thickens towards the republic of benin and dips down towards the atlantic; and floored by crystalline basement complex rocks from a block which became faulted into a series of graben and horst the samples for this study consisted of samples collected from two holes. study materials and methods. twelve core oil sand samples were collected; six of these samples were selected for analysis based on similarities regarding their physical and textural characteristics. the prepared oil sand samples were then subjected to scanning electron microscope (sem) vega tescan/lmu scanning electron microscope (at ibmchina research laboratory). the sem creates images by focusing a high-energy electrons beam onto the surface of a sample and detecting signals from incident electron interaction with a sample’s surface. the type of signals gathered by sem varies and may include secondary electrons, characteristic x-rays, and back-scattered electrons. sem produce high-resolution images of a sample’s surface in its primary use mode, secondary to electron imaging. depending on how such image has been created, sem images have great field depth, yielding characteristic three-dimensional appearance which is useful for understanding a simple surface structure. the prepared oil sand samples were trimmed to an appropriate size into fit in the specimen chamber, they were embedded in resin with further polishing to a mirror-like finish and mounted on a holder for analysis. electrons are thermionically emitted from a tungsten or lanthanum hexaboride (lab6) cathode and are accelerated towards an anode in typical sem; alternatively, electrons can be emitted via field emission (fe).tungsten is used because it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to be heated from a few hundred ev to 100kev, focused by one or two condenser lenses into a beam having a very fine 0.4nm to 5nm focal spot. the beam passes through pairs of scanning coils or pairs of deflector plates in the electron optical column, typically in the objective lens, horizontally and vertically deflecting the beam so that it scans in a raster fashion over a rectangular area of a sample’s surface. results and discussion quantifying clay mineralogy provides sufficient information evaluating the effect of clay minerals on reservoir characteristics. microscopic distribution of small proportions of authigenic clay minerals may have a profound effect on a reservoir regardless of each clay mineral’s specific proportions (sommer, 1978; seeman, 1979; palatt et al., 1984). the oil sand samples were identified having minerals which had become precipitated and occurred as pore filling cement; these included: sheet kaolinite, vermiform kaolinite and k – feldspar, pyrite crystal, corrosion quartz and corrosion feldspar. the sem images were found to have of 2 4µm micro pores and 2 – 5µm fractures. scanning electron microscope description of the afowo oil sand deposits in south-western nigeria 21 figure 2: the dahomey basin’s overall geological framework. (modified from bilman, 1992). agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:12 p p p composite 133 lpi at 45 degrees a. sheet kaolinite kaolinite is a clay mineral consisting of hydrous alluminosilicates having a sheet or layered structure. it is one of the most common authigenic clay minerals in sands and also, as pore filling cements and clay, rims up to 50ìm thick around grains. a authigenic kaolinites characteristically form books of stacked pseudo-hexagonal plates within pores between grains. they are usually formed from feldspar alteration (plate 1). b. vermiform kaolinite vermiform kaolinite also occurs as a result of k-feldspar alteration. however, vermiculite occurrence in small particles as a clay constituent has been recognised and is said to have an expanding lattice; in that expansion can only take place to a limited degree ( macewan, 1948 ). vermiform kaolinite could thus exhibit vermiculites characteristics (plate 2). c. k-feldspar k-feldspar is derived from feldspar hydrolysis because its chemical stability is generally low. feldspar alteration takes place at the weathering site (it is rather chemical than physical weathering) and during diagenesis, either on burial or subsequent uplift (plate 3).k-feldspar corrosion results from chemical erosion (plate4). d. corrosion quartz this is the most common mineral in sands and the most stable of all minerals in sedimentary conditions. corrosion quartz shows the outer parts’ erosion or modification as a result of dissolution (plate 5). e. pyrite crystals pyrite crystals are the most common iron sulphide mineral and are constituents of many iron-rich and other sediments but rarely form major parts. pyrite occurs as localised concretions due to precipitation (plate 6). f. micro pores these are small opening in the interstices of rocks. micro pores are smaller than 2nm in diameter and movement through them is by activated diffusion. the micro pores range in size from 2-4µm, (plate 7). g. fracture this is secondary porosity in rock, which often enhances a rock’s overall porosity. this can be result from the chemical leaching of minerals or the generation of a fracture system. fracture width ranges in size from 2-5µm, (plate 8) 22 akinmosin, a; adeigbe, o.c and oyemakinde, o.o plate 1: scanning electron micrograph of sheet kaolinite. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:13 p p p composite 133 lpi at 45 degrees scanning electron microscope description of the afowo oil sand deposits in south-western nigeria 23 plate 2: scanning electron micrograph of vermiform kaolinite plate 3: scanning electron micrograph of k-feldspar agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:13 p p p composite 133 lpi at 45 degrees 24 akinmosin, a; adeigbe, o.c and oyemakinde, o.o plate 4: scanning electron micrograph of corrosion k-feldspar. plate 5: scanning electron micrograph of corrosion quartz. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:13 p p p composite 133 lpi at 45 degrees scanning electron microscope description of the afowo oil sand deposits in south-western nigeria 25 plate 6: scanning electron micrograph of pyrite crystals. plate 7: scanning electron micrograph of micro pores. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:13 p p p composite 133 lpi at 45 degrees conclusion a scanning electron microscopic description of the afowo oil sand deposits in parts of southwestern nigeria was aimed at characterising reservoir sands based on their clay content. the study revealed that the oil sands contained minerals became precipitated and occurred as pore fillings. these minerals include sheet kaolinite, vermiform kaolinite, kfeldspar, pyrite crystals, corrosion quartz and corrosion feldspar. the sem images also showed 2-4 µm micro pores and 2-5µm fractures. furthermore, the study has shown that the clay minerals contained in afowo formation reservoir rocks are mainly kaolinite which is not expected to have any negative effects on reservoir quality, especially during enhanced oil recovery operations. references adegoke, o. s., ako, b. d. and enu, e. i. (1980): geotechnical investigation of the ondo state bituminous sands. vol.1. geology and reserves estimate. unpub. rept., geological consultancy unit, department of geology, university of ile-ife, 257p. akinmosin a., olabode o. t., and bassey, c. e., (2005): provenance study of bituminous sands in eastern dahomey basin, southwestern nigeria, based on heavy mineral and quartz varieties. ife journal of science, vol 7, no 1, akinmosin, a., oredein, o.s. and odewande, a. a. (2006): sedimentological and reservoir description of afowo oil sands deposits, southwestern nigeria. ife jour. of science, vol. 8. no. 1 pp. 3545. ako, b.d., adegoke, o.s. and petters, a.w. (1980): stratigraphy of the oshosun formation in southwestern nigeria. jour. min. geol. v. 17, p. 9-106. billman, h. g. (1976): offshore stratigraphy and paleontology of dahomey embayment, west africa. proc. 7th ar. micropal. coll. ileife. ekweozor, c. m. and nwachukwu, j. l. (1989): the origin of tar sands in southwestern nigeria. n.a.p.e bull. vol.4. no. 2, pp. 82 -84. enu, e. i. (1987): the paleoenvironment of deposition of late maastrichtian to paleocene black shales in the eastern dahomey basin, geologie en mijinbouww. vol 66 pp. 15-20. idowu, j.o., ajiboye, s.a., ilesanmi, m.a. and tanimola, a. (1993): origin and significance of organic matter of oshosun formation, southwestern nigeria. jour. min. geol. v. 29, p. 9-17. omastola, m. a., and adegoke, o.s. (1981): tectonic evolution and cretaceous stratigraphy of the dahomey basin. jour. min. geol. vol 18, no 1 pp. 130-137. oshinowo, t., ademodi, b. and adediran, a. s. (1982): bituminous tar sands of nigeria: analysis of oils – part 1, jour. nig. soc. chem. engrs. vol. 1, pp. 44-48. seeman, u. (1979): diagenetically formed interstitial clay minerals as a factor in rotliegende sandstone reservoir quality in the dutch sector of the north sea. jour. petrol. geol. vol. 1, pp. 55-62. sommer, f. (1978): diagenesis of jurassic sandstone in the viking graben. jour. geol. soc. london vol.135, pp.63-67. whiteman, a. j. (1982): nigeria: its petroleum geology, resources and potential. vol. 2 grahan and trotman, london 26 akinmosin, a; adeigbe, o.c and oyemakinde, o.o plate 8: scanning electron micrograph of fracture. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:14 p p p composite 133 lpi at 45 degrees geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 17-30 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion josé antonio henao m.1, angélica maría carreño p.2, john alexander quintero d.3, sol ángel candela h.4, carlos alberto ríos r.5, miguel a. ramos g.1 y jesús andelfo pinilla a.1 1 grupo de investigación en química estructural (gique), escuela de química universidad industrial de santander 2 ecopetrol-instituto colombiano del petróleo, piedecuesta, colombia 3 carbones del cerrejón limited, bogotá, colombia 4 ecopetrol s.a., colombia 5escuela de geología, universidad industrial de santander, bucaramanga, colombia corresponding author: c. a. ríos, e-mail: carios@uis.edu.co tel/fax: 57 7 6343457 abstract fine-grained and mainly reddish color, compact and slightly breccious and vesicular pyrometamorphic rocks (natural clinker) are associated to the spontaneous combustion of coal seams of the cerrejón formation exploited by carbones del cerrejón limited in la guajira peninsula (caribbean region of colombia). these rocks constitute remaining inorganic materials derived from claystones, mudstones and sandstones originally associated with the coal and are essentially a complex mixture of various amorphous and crystalline inorganic constituents. in this paper, a petrographic characterization of natural clinker, as well as the application of the x-ray diffraction (rietveld method) by mean of quantitative analysis of its mineral phases were carried out. the riqas program was used for the refinement of x ray powder diffraction profiles, analyzing the importance of using the correct isostructural models for each of the existing phases, which were obtained from the inorganic crystal structure database (icsd). the results obtained in this investigation show that the rietveld method can be used as a powerful tool in the quantitative analysis of phases in polycrystalline samples, which has been a traditional problem in geology. key words: rietveld method, cerrejón, x-ray powder diffraction, natural clinker. resumen rocas pirometamórficas (clinker natural) de grano fino y coloración principalmente rojiza, compactas, y ligeramente brechadas a vesiculares están asociadas a la combustión espontánea de mantos de carbón de la formación cerrejón explotados por carbones del cerrejón limited en la península de la guajira (región caribe de colombia). estas rocas constituyen materiales inorgánicos remanentes derivados de arcillolitas, limolitas y areniscas originalmente asociadas con carbón y son esencialmente una mezcla compleja de diferentes constituyentes inorgánicos amorfos y cristalinos. en el presente trabajo, se llevó a cabo la caracterización petrográfica del clinker natural, así como la aplicación de la difracción de rayos-x de polvo 17 manuscript received: 28/01/2010 accepted for publication: 14/05/2010 (método de rietveld) para el análisis cuantitativo de las fases minerales presentes de las rocas en estudio. el programa riqas fue utilizado para el refinamiento de perfiles de difracción de rayos-x de polvo, analizando la importancia de utilizar los modelos isoestructurales adecuados para cada una de las fases presentes, los cuales se obtuvieron de la base de datos inorganic crystal structure database (icsd). los resultados obtenidos en la presente investigación muestran que el método de rietveld puede usarse como una poderosa herramienta en el análisis cuantitativo de fases en muestras policristalinas, lo cual ha sido un problema tradicional en el campo de las geociencias. palabras clave: método de rietveld, cerrejón, difracción de rayos x de polvo, clinker natural. introduction spontaneous combustion of coal has been reported in different coalfields of the world (usa: heffern et al., 1983, 1993; foit et al., 1987; cosca et al., 1989; heffern & coates, 1997, lyman & volkmer, 2001; india: prakash et al., 1997; romania: radan & radan, 1998; australia: ellyett & fleming, 1974, new zealand: lindqvist et al., 1985, china: zhang, 1998; de boer et al., 2001). fires associated with coal mines, which are preserved in the rock record as reduced volume burnt coal beds and pyrometamorphic rocks (natural clinker), represent global catastrophes that may be initiated for various reasons and are responsible for coronary and respiratory diseases, human death and destruction of ecosystems, contaminating air, water and soil (stracher, 2007). in colombia, candela & quintero (2004) conducted a geological mapping of natural clinker at el cerrejón mining operation, located in the guajira peninsula, northeastern colombia. carbones del cerrejón limited is dedicated to the exploration, production, transportation and shipment of high quality thermal coal), which includes monitoring and structural control at depth. according to quintero et al. (2009), the “in situ” spontaneous combustion of coal, with the consequent development of natural clinker, is a phenomenon that hinders the development of coal mining, not only because it creates areas of geotechnical instability, given the characteristics acquired by the adjacent rock when subjected to high temperatures reached during combustion of coal but also because it represents a reduction in tonnage of workable coal. the term “natural clinker” refers to a series of pyrometamorphic rocks generated as a result of thermal transformation of sedimentary protoliths from the spontaneous combustion of coal beds (figure 1). although many coalfields contain such rocks, there is little information regarding quantitative data about them, and almost nothing about their forming conditions. the mineralogy of natural clinker depends on physical and chemical variables, such as chemical composition of the sedimentary protolith, temperature, degree of fusion and oxidation state, among others, and how one or more of these factors affects the stability of mineral phases may vary on a microscopic scale (cosca et al., 1989). however, some field and petrographic observations combined with analytical data can help to deduce the relative importance of these factors. moreover, the natural clinker should be studied not only in relation to its origin but also to its potential use as an indicator of coal resources. therefore, future studies should consider benefits obtained by using mine tailings as natural clinker based on knowledge of their petrographic, structural, physical and chemical properties. in general, natural clinker is a mineralogical and chemically complex mixture in which the amount of each mineral phase present depends on the original composition of the protolith from which it was formed by pyrometamorphism, the mixture characteristics, the burning conditions, and the thermal history of the material, among others (jones et al., 1984; cosca et al., 1989, coates & heffern, 1999; heffern et al., 1993). although a geological study was not carried out on the protolith from which natural clinker has been generated, we can deduct some mineralogical changes suffered in particular clay strata after burning by spontaneous combustion of coal beds. it can be inferred that the increase in temperature caused by spontaneous combustion evidently produced the dehydration and vitrification of clay minerals, the dissolution of carbonates and the formation of aluminosilicates. therefore, mineralogical and textural changes observed probably occurred on a system with disequilibrium on a small scale, with the development of high-temperature metamorphic reactions (pyrometamorphism). petrographic studies on glass-bearing burned rocks reveal the occurrence solid solution series as magnetite-hercynite (feal2o4), ulvöspinel (fe2tio4), hematiteilmenite (fetio3), y magnesiumferrite along the spinel (mgal2o4)-magnesiumferrite (mgfe2o4) join of sp80mf20 and along the magnesioferrita-hematita join of mf60hm40 (cosca et al., 1989). typical high temperature non magnetic minerals found in pyrometamorphic rocks associated with 18 josé antonio henao m., angélica maría carreño p., john alexander quintero d. coal combustion appear to include several fe and al rich clinopyroxene, melilita, cristobalite, tridymite, mullite, cordierite and fayalite solid solutions, as well as glass (foit et al., 1987; cosca et al., 1989; râdan & râdan, 1998). glass compositions have been used by cosca et al. (1989) to estimate the minimum temperatures of formation of natural clinker, which vary between 1020 and 1400 oc. these authors suggest that different degrees of melting, mixing, crystallization, volatilization and liquid immiscibility in conditions of disequilibrium may be involved in the formation of such pyrometamorphic rocks. however, it is important to establish the type and amount of these magnetic phases in natural clinker, which affect its properties and, therefore, is necessary to identify and quantify them. the determination of mineral phases and chemical composition of natural clinker is vital in resolving problems concerning their genesis, correlation of coal beds, characterization of the protolith and assessment of potential environmental impacts of coal combustion. natural clinker is one of the contributing materials that could affect the environment, especially water chemistry, as important variables like the original composition of the protolith from which this geomaterial comes, burning conditions that generated it, and its particle size and mineralogy, influence the distribution and mobility of trace elements. therefore, in response to environmental problems that occur as a result of the disposal of solid waste from coal mining, within which natural clinker lies, it is important to undertake studies to establish the environmental impacts of using this byproduct of coal in the field of materials science, based on its variability in quality and environmental impacts and human health. the bulk chemical composition similarity of natural clinker with the volcanic materials from which the natural zeolites are originated by post-magmatic hydrothermal activity has motivated attempts of making zeolite from this geomaterial by ríos and co-workers (ríos & williams, 2008; ríos et al., 2008a, 2008b, 2008c; sandoval et al. 2009), justifying the development of future investigations in the field of the synthesis of new materials with potential industrial applications. according to them, zeolitic materials, such as na-phillipsite, hydroxysodalite, hydroxycancrinite, k-chabazite, zeolite k-f, faujasite and zeolite linde type a can be synthesized by alkaline activation of natural clinker. recently, sandoval et al. (2009) synthesized almost pure analcime with a high degree of crystallinity from natural clinker, optimizing the experimental conditions used by the pioneering work of ríos & williams (2008). although its potential application might consume only a small part of 19 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion figure 1. photographs at outcrop scale of natural clinker associated with spontaneous combustion of coal beds, highlighting not only its geometry but also the various shades that present this geologic material. the natural clinker produced by coal combustion, end products could reach a much higher added value than that currently presenting this geomaterial in the coal industry of colombia. the quantitative analysis using the rietveld method has gained popularity in recent years as a rapid and accurate quantitative technique (o’connor & raven, 1988, taylor & matulis, 1991; bish & post, 1993; weidler et al., 1998; raudsepp et al., 1999; gualtieri, 2000; de la torre et al., 2001). quantitative analysis by the rietveld method (rietveld, 1969; young, 1993) has huge advantages over traditional methods using integrated intensities of a small set of reflections in limited angular intervals, since the overlap of the lines of a pattern of powder diffraction, especially in mineralogically complex samples such as the natural clinker, make it virtually impossible to carry out a rigorous analysis by conventional methods. the analytical procedure using the rietveld method is based on the relationship between the refined scale factor for each phase and its weight fraction in the multiphase mixture (hill & howard, 1987; bish & howard, 1988). this method makes efficient use of all information contained in the diffraction pattern and converts it into an effective analysis procedure without the use of standards, because it can be done on the diffraction data collected without the addition of an internal standard and without the need for calibration curves. the only requirement to consider is that there must be a reliable isostructural model of each of phases to be quantified. on the other hand, if the process of quantitative analysis by the rietveld method is successful on high quality diffraction data, the method can be used to quantify crystalline phases whose weight percentage are below 1.0%, and still can be used to determine the amount of amorphous material present in the multiphase mixture. in the last case, it requires adding a known amount of an internal standard to the sample and thus properly quantifies the material (gualtieri, 2000). the purpose of this study was to conduct a petrographic characterization of the natural clinker produced by spontaneous combustion of coal beds cerrejón formation and to apply the rietveld method to the quantitative phase analysis of this geological material. therefore, the implementation of the proposed methodology could be very important to quality control of geological materials for industrial use. materials and methods petrographic analysis of different samples of natural clinker were carried out to establish their textural and structural features as well as their mineralogy, using the following analytical techniques: transmitted light microscopy (nikon triocular microscope, model labophot2-pol) of the school of geology and image analyzer leica of the school of metallurgical engineering and materials science at the universidad industrial de santander, and scanning electron microscope zeiss evo50 of the research center of applied sciences at the university of wolverhampton (analytical conditions: eht = 20.00 kv, beam current 100 ua, signal a = se1, wd = 8.0mm). the preparation and assembly of natural clinker samples for x-ray diffraction (xrd) was carried out at the laboratory of structural chemistry of the school of chemistry at the universidad industrial de santander. samples were homogenized in an agate mortar, and then mounted on an aluminum specimen holder using the technique of lateral filling (mcmurdie, 1986). the process of disintegration and separation of clays (about 2ìm) involved the separation of minerals by particle size using the stokes’ law. chemical and thermal treatments were conducted as follows: saturation with k+ and mg2+ ions, calcinations at 350 and 550 °c and solvation with ethylene glycol. xrd patterns of the natural clinker were recorded with a rigaku d/max iiib diffractometer operating in bragg-brentano geometry with cu-�1 radiation (k = 1.5406 å) at 40 kv and 20 ma and graphite monochromation. the scan parameters were step size 0.02°, dwell time 2 s and 2è range 2-70°. phase identification was performed through the process of comparison search (search/match) using the crystallographic database powder diffraction file (pdf-2) from international centre for diffraction data (icdd). full-pattern rietveld refinement using riqas versión 4.0 (mdi, rietveld quantitative análisis and whole pattern fitting, 2006) was performed to quantify the amounts of phases in the natural clinker using a 20% by weight of aluminum oxide as internal standard. results and discussion outcropping clinker one of the most remarkable features in the areas of cerrejón coal mining complex is the occurrence of natural clinker as an indicator of ancient fires produced by spontaneous combustion of coal seams (candela & quintero, 2004; quintero et al., 2009). natural clinker includes a variety of thermally affected rocks, which are easily recognizable on the field thanks to their mainly red color in a variety of intensities. figure 1 illustrates a typical natural clinker outcrop, showing the reddening intensity due to the rock’s alteration de20 josé antonio henao m., angélica maría carreño p., john alexander quintero d. gree. however, it can also display several tones of black, gray, green and cream or even white, easily contrasting with the grayish hue of coal. it is a fine-grained particle size, compact and highly fractured rock, mainly reddish brown to red in colour due to the presence of hematite. these rocks represent a complex mixture of various amorphous and crystalline inorganic constituents and contain remaining inorganic materials derived from shales, sandy mudstones and grey sandstones originally associated with coal. in general, it is constituted by rocks of mainly clay texture but it could also be sandy. natural clinker also retains much of the original rock characteristics as relict sedimentary structures and even fossil plant remains within layered sandstone and mudstone survive. the occurrence and distribution of natural clinker is described in more detail by quintero et al. (2009). evidence supports the fact that claystones, siltstones and sandstones adjacent to coal beds were metamorphosed by the spontaneous combustion of coal, recording continuous mineralogical and textural changes from relatively unaltered rocks to molten material (vesicular slag) during pyrometamorphism. petrography natural clinker includes a variety of pyrometamorphic rocks, which vary in their degree of thermal alteration, according to their texture and structure, original composition and position on the burning area. these rocks are characterized by fine-grained particle size and usually red color in different shades, although black, gray, green, cream or white varieties also occur. it is relatively hard and compact but disintegrates at the touch, and it can show a sugary and slight brecciate vesicular appearance. general macroscopic characteristics of natural clinker varieties of the cerrejón formation are shown in figure 2. natural clinker usually shows cooked sandy and clay textures (figures 2a and 2b), the last of them similar to a common building brick, which is characterized by its reddish-orange color, shiny fracture rather than opaque similar to jasper and intense oxidation, which has been accompanied by a hardening of the rock through the transformation of clay minerals in hematite. this variety sometimes exhibits very specific characteristics, taking into account that relics of the original sedimentary structure are preserved, which are still recognized by the millimeter scale “tigerstripe" pattern of its color (figure 2c), with orange-red, locally purple, areas and light gray areas, the latter as relics of the original claystone with scalloped and parallel edges to the stratification surface. orange-red areas present red edges, due to the formation of hematite, which tends to permeate the rock, producing relatively hard cement. light gray areas are darker toward the contact with red-orange areas, due to the presence of black magnetite. figure 2d illustrates an example of a natural clinker similar to a vesicular slag with a characteristic spongy texture. the distinguishing characteristics of this variety are shown in figure 3. a glassy material of gray to grayish brown color occurs filling vesicles or along fractures that are arranged perpendicular or diagonal and, in some cases, parallel to the stratification surface (figure 3, top). a microphotography captured in an image analyzer (figure 3, bottom) illustrates the vesicular slag and sponge appearance of this variety. under transmitted light microscopy, it is completely isotropic, except for tiny particles of quartz. rogers (1918) interprets this variety as the result of hot gases (350°c) which escaped from the zone of spontaneous combustion of coal beds and affected the adjacent rock to the molten state, which then hardens into a vesicular glass. nevertheless, faulting locally generates breccia zones that clearly demonstrate that spontaneous combustion existed before the last fault’s activity period since it contains mixed fragments of natural clinker, fresh rock and coal (figure 2e). quintero et al. (2009) reported that calcareous rocks of the cogollo formation in the hanging wall of the cerrejón thrust fault, and immediately over this tectonic discontinuity, are thermally affected and present reddish, cream and pink to purple colours, preserving already all of the original characteristics including fossil fragments (figure 2f). figure 4 illustrates images of scanning electron microscopy (sem) of natural clinker particles, which are characterized by their fragmentation and blocky morphology, with blocky, angular fragments with or without vesicles, occasionally exhibit stepped fractures. x-ray difraction figure 5 shows the xrd patterns of natural clinker in the range of 2è from 2-40°, using cu-�1 radiation, in which sharp reflections are observed corresponding to high intensities of quartz (pdf no. 010-46-1045). similarly, there are broad bands with low relative intensity which correspond to hematite (pdf no. 010-73-2234), anatase (pdf no. 010-72-7058) and minerals of smectite and mica groups. taking into account that the peak intensity corresponding to quartz is larger than those from other mineral phases, it was necessary to carry out the separation of the clay fraction in order to obtain separated xrd patterns (not shown here) for the aluminosilicates. the saturation with mg2+ and subsequent glycosylation of natural clinker samples (tigerstripe type, nc-1, and highly oxidized type, nc-2) produced displacements of the reflection (001) of the band corresponding 21 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion to the smectite group from 15.07 to 17.38 å for sample nc-1 and from 15.17 to 17.52 å for sample nc-2. additionally, strong reflections were observed, indicating the presence of montmorillonite and discarding the presence of vermiculite in both samples with predominance of interchangeable ion mg+2 principally in sample nc-2 in which it looks a greater 22 josé antonio henao m., angélica maría carreño p., john alexander quintero d. figure 2. photographs of hand specimens of natural clinker showing varieties, and textural and structural features. 23 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion slag edge slag inner increase of porosity interface slag-glass porous glass figure 3. above, photographs of a hand specimen of natural clinker, displaying a vesicular slag appearance. below, micrograph of the upper left part of the detail shown above. note the spongy texture and vesicular slag appearance. figure 4. sem images of natural clinker particles. degree of swelling. in the region between 7.14 and 7.20 å, there is a wide band of very low intensity which disappears by thermal treatment at 550 ºc. this behavior is characteristic of kaolinite which collapses at this temperature and is transformed into metakaolin (amorphous phase). kaolinite was not detected in the qualitative and quantitative analysis because it is below the detection level, but it was identified after the thermal treatment of the clay fraction. similarly, the presence of illite (interplanar spacings at 9.93-9.97 å, 4.98-5.00 å and 3.32-3.34 å for sample nc-1 and 9.7-10.04 å, 5.00 å and 3.32-3.33 å for sample nc-2) was confirmed, which remains stable after thermal treatments. xrd profiles of the original (untreated) and treated (saturated with ethylene glycol and heated to 550 °c) clay are shown in figure 6. figure 7 shows the observed and calculated xrd patterns and their differences after refinement by the rietveld method, where it can be observed the good fit of data, indicating that isostructural models chosen for each phase were appropriate. the models used for illite, hematite, quartz and anatase were taken from the inorganic database (icsd, 2007 / i). the montmorillonite model was taken from the literature (viani et al., 2002), assuming a montmorillonite with some degree of disorder and interchangeable ca+2 and na+2 ions. 24 josé antonio henao m., angélica maría carreño p., john alexander quintero d. 3000 2500 2000 1500 1000 500 0 2 6 10 14 18 22 26 30 34 38 2� q ua rt z (1 01) s m ec tit e (0 01 ) m ic a (0 02 ) m ic a (0 04 ) m ic a (1 11) q ua rt z (1 00 ) h em at ite (1 02) a na ta se (1 01 ) h em at ite (1 04 ) m ic a (1 31 ) h em at ite (2 -1 0) q ua rt z (2 -1 0) in te n si ty (c p s) a b 3000 2500 2000 1500 1000 500 0 in te n si ty (c p s) 2 6 10 14 18 22 26 30 34 38 q ua rt z (1 01) s m ec tit e (0 01 ) m ic a (0 02 ) m ic a (0 04 ) m ic a (1 11) q ua rt z (1 00 ) h em at ite (1 02) a na ta se (1 01 ) h em at ite (1 04 ) m ic a (1 31 ) h em at ite (2 -1 0) q ua rt z (2 -1 0) 2� figure 5. xrd patterns of samples nc-1 (a) and nc-2 (b) in the 2è range of 2-40°, using cu-�1 radiation. for illite, hematite, quartz and anatase structure factors were calculated using the atomic scattering factors. the background noise was fitted with a polynomial function and the peak profile was modeled using the lorentzian function of pearson vii. the cell constants, scale factor and coefficients corresponding to sample displacement and asymmetry were also refined. for the refinement of montmorillonite, the same previously described factors were calculated, except atomic positions, which were left fixed. this strategy avoided overestimating the content of this phase in natural clinker samples. similarly, during the refinement of the illite some degree of preferential orientation for the reflection (11-1) was observed. this aspect, which is one of the most serious problems in quantitative analysis, was largely corrected using the march function (dollase, 1986). table 1 shows the residual values defined by riqas, rwp y rbragg, for final refinements by least squares together with the percentages of the phases. rwp values (20.33% and 20.25% for samples nc-1 and nc-2, respectively) indicate that the refinement as a whole has been satisfactory if we consider that natural clinker samples consist of a mixture of minerals which hinders the achievement of lower refining indicators. low values of rbragg (5.05-10.26% and 3.99-12.28% for samples nc-1 and nc-2, respectively) achieved in each phase indicate that the structural model selected was appropriate. however, for the montmorillonite 25 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion m on tm or ill on ite 15 .0 7 ill ite 9. 97 k ao lin ite 7. 14 9. 97 17 .3 8 9. 93 ill ite 4. 99 ill ite 3. 33 5. 00 3. 34 3. 32 4. 98 normal mgci2 ethylene glycol kci calcined 550°c 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 44 .1 4 22 .0 7 14 .7 2 11 .0 4 8. 84 7. 37 6. 32 5. 53 4. 92 4. 44 4. 04 3. 70 3. 42 3. 18 in te n si ty (c p s) d (angstrom) a m on tm or ill on ite 15 .1 7 k ao lin ite 7. 20 9. 99 17 .5 2 9. 97 ill ite 5. 00 ill ite 3. 33 5. 00 3. 32 normal mgci2 ethylene glycol kci calcined 550°c 44 .1 4 22 .0 7 14 .7 2 11 .0 4 8. 84 7. 37 6. 32 5. 53 4. 92 4. 44 4. 04 3. 70 3. 42 3. 18 in te n si ty (c p s) d (angstrom) b 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 ill ite 10 .0 4 3. 33 figure 6. xrd patterns of the clay fraction in samples nc-1 (a) and nc-2 (b). slightly higher values (12.51% and 14.32%) were observed, indicating that some reflections have been completely unadjusted. moreover, we can notice that despite performing the correction for the preferred orientation of the reflection (11-1) of illite, the standard deviation of quantification is the highest (±0.4 and ±0.5 for samples nc-1 and nc-2, respectively), but this value is below the average standard deviations for mineral phases (gualtieri, 2000). in general, very small deviations in the quantitative analysis of the identificated crystalline phases were observed. in accordance, the higher content of amorphous material that could be associated with nc-2 sample was exposed to a higher temperature during spontaneous combustion, which may be related to a possible conversion of quartz to amorphous silica. residual values of rwp and rbragg as a whole indicate that the refinement has been satisfactory and that the structural models used were appropriate. the presence of amorphous material (2.7% for sample nc-1 and 3.3% for sample nc-2) is not an impediment to identify and quantify the mineralogy of the natural clinker. 26 josé antonio henao m., angélica maría carreño p., john alexander quintero d. 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 a in te n si ty (c p s) 2� 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 b in te n si ty (c p s) 2� figure 7. xrd profiles refined by the rietveld method (observed profile in black; calculated profile in red and difference between the observed and calculated profiles in the upper part) from samples nc-1 (a) and nc-2 (b). 27 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion t a b le 1 . c ry st al lo gr ap hi c da ta an d re fin em en td et ai ls by th e r ie tv el d m et ho d fo r th e na tu ra lc lin ke r ph as es h e m a ti te q u a rt z il li te m o n tm o ri ll o n it e a n a ta se c ry st al lo gr ap h ic da ta c he m ic al fo rm ul ae f e 2 o 3 si o 2 k (a l 4s i 2 o 9( o h ) 3 ) (n a, c a) 0, 3( a l,m g) 2s i 4 o 10 (o h ) 2 nh 2 o ti o 2 m (g /m ol ) 15 9, 69 7 60 ,0 84 39 8, 20 4 54 9, 07 79 ,8 8 c ry st al lin e sy st em h ex ag on al h ex ag on al m on oc lin ic tr ic lin ic te tr ag on al sp ac e gr ou p r -3 c [n o. 16 7] p 32 21 [n o. 15 4] c 2/ c [n o. 15 ] p1 [n o. 1] i4 1/ a [n o. 14 1] n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 a (å ) 5. 02 91 (1 ) 5. 02 97 (2 ) 4. 91 16 (1 ) 4. 91 06 (1 ) 5. 18 1 (1 ) 5. 19 28 (7 ) 5. 11 (7 ) 5. 12 (7 ) 3. 78 16 (9 ) 3. 78 56 (4 ) b (å ) 5. 02 91 (1 ) 5. 02 97 (2 ) 4. 91 16 (1 ) 4. 91 06 (1 ) 9. 03 8 (2 ) 9. 01 0 (2 ) 8. 80 (9 ) 8. 42 (9 ) 3. 78 16 (9 ) 3. 78 56 (4 ) c (å ) 13 .7 33 (1 ) 13 .7 39 (1 ) 5. 40 25 (3 ) 5. 40 08 (3 ) 20 .2 00 (3 ) 20 .2 06 (4 ) 15 .2 6 (3 ) 15 .1 6 (4 ) 9. 50 (1 ) 9. 42 (1 ) � (o ) 90 90 90 90 90 90 90 90 90 90 � (o ) 90 90 90 90 95 .6 7 (1 ) 95 .5 7 (1 ) 90 90 90 90 � (o ) 12 0 12 0 12 0 12 0 90 90 90 90 90 90 v (å 3 ) 30 0. 81 (3 ) 30 1. 00 (3 ) 11 2. 87 (1 ) 11 2. 78 (8 ) 94 1. 3( 3) 94 0. 6 (3 ) 68 6. 39 (1 2) 65 3. 88 (1 1) 13 5. 9 (2 ) 13 4. 9 (6 ) � (g /c m 3 ) 5. 28 9 (1 ) 5. 28 6 (1 ) 2. 65 2 (1 ) 2. 65 4 (1 ) 2. 78 9( 1) 2. 79 1 (1 ) 1. 93 (6 ) 2. 02 (6 ) 3. 90 5 (1 ) 3. 93 3 (3 ) z 6 6 3 3 4 4 1 1 4 4 r ef in em en td et ai ls n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 n c -m 1 n c -m 2 n o. re fi ne d va ri ab le s r b (% ) 7, 43 7, 78 5, 05 3, 99 7, 23 6, 83 12 ,5 1 14 ,3 2 10 ,2 6 12 ,2 8 r (% ) 13 ,5 2 13 ,3 8 13 ,5 2 13 ,3 8 13 ,5 2 13 ,3 8 13 ,5 2 13 ,3 8 13 ,5 2 13 ,3 8 r w p (% ) 20 ,3 3 20 ,2 5 20 ,3 3 20 ,2 5 20 ,3 3 20 ,2 5 20 ,3 3 20 ,2 5 20 ,3 3 20 ,2 5 xr d (% ) 9. 4± 0. 1 9. 1± 0. 1 39 .3 ± 0. 3 35 .2 ± 0. 3 42 .4 ± 0. 4 46 .3 ± 0. 5 6. 3± 0. 2 5. 4± 0. 2 0. 4± 0. 1 0. 7± 0. 1 a m or ph ou s (% ) 2, 7 3, 3 2, 7 3, 3 2, 7 3, 3 2, 7 3, 3 2, 7 3, 3 n c ,n at ur al cl in ke r a significant amount of amorphous material appears to reduce the crystalline signals. many of these signals contain a large number of reflections which overlap with those of the other phases. quantitative analysis of amorphous material also looked at all the factors that could affect the results. it is evident that natural clinker contains amorphous material, which cannot be detected directly by xrd, although it is important to understand the thermal history of natural clinker. to quantify the amorphous material by using the program riqas a crystalline standard (free amorphous) was added to the natural clinker in a known percentage. however, the higher content of amorphous material was observed in the natural clinker with glass material. conclusions in the present work, the structural information on natural clinker associated to spontaneous combustion of coal of the cerrejón formation was obtained from high-quality powder diffraction patterns. the obtained crystallographic data were determined by rietveld analysis. rietveld analysis of natural clinker reveals that it contains a large number of crystalline phases, many apparently present in small amounts that could be part of a series of solid solutions as a result of the thermal alteration process. the results obtained in this study indicate that natural clinker is a complex chemical and mineralogical material. nevertheless, it has been shown that quantitative analysis by the rietveld method is appropriate to study the mineralogy and crystallochemistry of phases present in this geomaterial producing reliable results. acknowledgments we thank to carbones del cerrejón limited for supporting s. candela and j. quintero during fieldwork to carry out the collection of geological material covered by this study. we gratefully acknowledge the programme alban, the european union programme of high level scholarships for latin america, scholarship no. e05d060429co, and the universidad industrial de santander for funding the doctoral program of c. ríos. special thanks to these institutions for providing research facilities. the authors also thank the anonymous reviewers for critically reading the article, as well as other persons and entities for the support offered. references bish, d., howard, s., quantitative phase analysis using the rietveld method, j. appl. crystallogr. 21, 1988, pp. 86-91. bish, d., post, j., quantitative mineralogical analysis using the rietveld full-pattern fitting method, am. mineral., 78, 1993, pp. 932-940. candela, s.a., quintero, j.a., cartografía de las zonas de clinker en las áreas de minería de la mina el cerrejón. albania, guajira, tesis de pregrado en geología, universidad 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vol. 11, no. 2 (december 2007): 117-130 geometric model of the nazca plate subduction in southwest colombia patricia pedraza garcia1, carlos alberto vargas1 and hugo monsalve j.2 1 m.sc in geophysics, universidad nacional de colombia – bogotá, colombia e-mail: patriciapedrazag@gmail.com; e-mail: cavargasj@unal.edu.co 2 ceifi, quimbaya group – universidad del quindio, e-mail: hugom@uniquindio.edu.co abstract a geometric model for the subduction of the nazca plate beneath the south american plate in southwestern of colombia is proposed based on the relocation of hypocenters of local and distant earthquakes. by means of the simultaneous inversion of teleseismic p and sh body waves, the depths of the 15 events with mw ≥ 5.8 were constrained, and the hypocenters of the 250 earthquakes recorded between 1990 and 2005 by the international seismological centre (isc) and u.s. geological survey, national earthquake information center (neic) were constrained and relocated. a model is proposed for the hypocentral sections taking into account the trench along of the earth and colombia-ecuador. three different possible shapes of subduction of the nazca plate in the colombia-ecuador trench were obtained: the first configuration, in the cali a segment, the dip angle changes from17º to 45º down to a maximum depth of 100km; the second configuration, in the popayán b and nariño c segments, the dip angle holds approximately constant at 30º down to a maximum depth of 200 km; and the third configuration, in the quito d segment, the dip angle changes of 9º to 50º to a maximum depth 220 km. the maximum depth of seismicity along the colombia-ecuador trench shows two increases, the first between latitudes 4.5ºn-5ºn and the second between the latitudes 1ºs-2ºs, which suggest that the presence of the malpelo and carnegie ridges may generate a differential blockage at the pacific colombia-ecuador basin. key words: seismotectonic, nazca plate, subduction, colombia. resumen se propone un modelo geométrico de la subducción de la placa nazca bajo la placa suramericana en el suroeste de colombia, basado en la relocalización de hipocentros de sismos locales y telesismos. mediante la inversión simultánea de formas de ondas internas p y sh telesísmicas se ajustan las profundidades de 15 eventos con mw ≥ 5.8 y se relocalizan los hipocentros de 250 eventos con lecturas de fases registradas desde 1990 hasta 2005 por las agencias international seismological centre (isc), y u.s. geological survey, national earthquake information center (neic). se propone 117 earth sciences research journal manuscript received march 13 2007. accepted for publication november 20 2007. un modelo de secciones hipocentrales teniendo en cuenta la curvatura de la tierra y la curvatura de la fosa colombia-ecuador. se obtienen tres formas distintas de subducción para la placa nazca en la fosa colombia-ecuador: la primera en el segmento cali a, donde él ángulo de buzamiento cambia de 17º a 45º, a una profundidad máxima de 100km, la segunda en los segmentos popayán b y nariño c donde el ángulo de buzamiento es aproximadamente constante de 30º a una profundidad máxima de 200km y la tercera en el segmento quito d donde el ángulo de buzamiento cambia de 9º a 50º a una profundidad máxima de 220km. la profundidad máxima de sismicidad a lo largo de la fosa colombia-ecuador presenta dos aumentos, el primero entre las latitudes 4.5ºn-5ºn y el segundo entre las latitudes 1ºs-2ºs, sugiriendo la presencia de las dorsales de malpelo y carnegie que generan un bloqueo diferencial respecto a la cuenca del pacífico colombo-ecuatoriano. palabras claves: sismotectónica, placa nazca, subducción, colombia. introduction southwestern colombia is a region of enhanced tectonic activity resulting from the subduction of the nazca plate (np) beneath the south american plate (sa) along the colombia-ecuador trench (cet). several studies have shown that this subduction zone may exhibit lateral variations in the dip angle of the subducted oceanic lithospere as well as changes in the distribution of seismicity along and down dip the trench, specially at intermediate depths (between the latitudes 3ºn-4ºn), where a seismic gap is apparent (pennington, 1981; taboada et al., 1998; monsalve, 1998; gutscher et al., 1999, vargas et al., 2003, mejía and meyer, 2004, chicangana and vargas, 2004). pennington (1981) obtained contours for the wadati-benioff zone (wbz) beneath northwestern south america and determined three segments with different directions and dip angles: the bucaramanga segment from 5.2ºn to 11ºn, which is subducting in direction n109ºe and with dip of 20º25º; the cauca segment in the colombiapanama trench from 7ºn to 1.5ºn, which is subducting in direction n120ºe and dipping approximately 35º; and the ecuador segment in the cet from 1.5ºn to 2ºs, which is subducting in direction n35ºe and a dipping approximatly 35º. the direction of the ecuador segment can be explained by a speculative model related with the activity of the galápagos hot spot. this hot spot is located some 100 km to the south of the presently active cocosnazca plate spreading center and could be subducting beneath the south of ecuador near 1ºs-2ºs. this bathimetric feature could be the cause of the low subduction angle at the peru-ecuador border (barazangi and isacks, 1976). a more recent study made by gutscher et al. (1999) shows a more complex behavior of the plate with the conclusion that the ecuadorian system of subduction seems highly controlled by the subduction of the carnige ridge and by the rupture of the lithospere within the np. mejía and meyer (2004) based on the seismological information compiled by the observatorio sismológico del suroccidenteosso propose a transition zone between active subduction and a possible blocked subduction by the addition of the choco block to the western coast of colombia. recent gps measurements from the casa project (central and south america), reveal a wide plate boundary deformation with escape tectonics occurring along an approximate length of 1400 km of the north andes, locking of the subducting np and strain accumulation in the ecuador-colombia forearc (trenkamp et al., 2004) . associated with the process of subduction of the np there is a volcanic arc, that extends geometric model of the nazca plate subduction in southwest colombia 118 pedraza et al. esrj vol. 11, no. 2. december 2007 119 from 5ºn (cerro bravo volcano, colombia) to 2ºs (sangay volcano, ecuador). the se active volcanism only reappears in perú. the subducted oceanic crust, with ages between 12 and 20 ma, is carrying the carnegie ridge, which is composed of volcanic products from the activity of the galápagos hot spot on the np and that has been subducting for at least 6 ma. (hey, 1977). data and methodology the purpose of this study is to determine the geometry of subducted np based on all the available and reliable locations of earthquakes recorded with local and teleseismic networks, and to explain the decrease in the intermediate-depth seismicity in southwestern colombia. initially, some teleseismic events are constrained from the inversion of long-period body waves (nabelek, 1984). these earthquakes are used as calibration events to relocate local events by means of the joint hypocenter determination-jhd method (dewey, 1971). hypocentral cross-sections are tipically presented as straight cross-sections in different studies at the middle and south american trenches (isacks et al., 1968; isacks and barazangi, 1977; pennington, 1981 burbach et al., 1984, vargas et al., 2003) but more recent works such as those by kawatsu (1986), ekstrom and engdhal (1989), guzmán (1995), and monsalve (1998), consider the arc-trench and earth curvatures, demonstrating a better definition of the wbz. data used in the inversion of body wave earthquakes of events with mw ≥ 5.8 reported by neic-usgs (national earthquake information center) for the 1990-2005 period were selected. the digital registries of the teleseismic p and sh body waves were provided by iris-dmc (incorporated research institutions for seismology) agency. digital records of p and sh waveforms with periods between 1s and 100s were analyzed. the data used corresponded to waveforms recorded by broadband stations with a good azimuthal coverage located at distances from 30º to 90º to avoid the effects of the earth’s core and upper mantle discontinuities on the waveforms. figure 2 shows the location of the stations used in this study. the instrument response was de-convolved from the traces and the waveforms integrated to obtain figure 1. general tectonic setting of the study area. displacement amplitudes. the amplitudes were normalized for a magnification of the instrument equal to one and an epicentral distance of 40 degrees in order to unify the data. the waveforms are band-pass filtered with a three pole butterworth filter with cutoffs at 0.01hz and 1hz to eliminate long and short period noises respectively. synthetic seismograms were generated by a combination of the direct (p or s) and reflected (pp and sp, or ss) phases radiated from a linear source (bouchon, 1976) using the bwidc computer program developed by nabelek (1984). this program is based on the minimization, in the least square sense, of the misfit between observed and synthetic seismograms. the synthetic seismogram at each station is obtained by the convolution of green’s function with the instrumental response and the source time functions. the source time function is parameterized as a single point source or as an event composed of several point sources separated in time and space depending on the complexity of the event. weights were assigned to the stations according to azimuthal density of the data. n 1 for p waves and n2 1 for sh waves were used, where n is the number of stations to avoid the uprising situations from a poor azimuthal cover. table 1a velocity model near the sources (ingeominas, 1993) thickness (km) α (km/s) β (km/s) ρ (gr/cm3) 2.0 4.0 2.3 2.57 3.0 5.5 3.18 2.84 20.0 6.4 3.70 2.95 10.0 7.1 4.10 3.04 ∞ 8.1 4.68 3.30 note: α= pwave velocities; β= s-wave velocities; ρ=densities table 1b.velocity model for the crust in the receiver region thickness (km) α (km/s) β (km/s) ρ (gr/cm3) 60.0 6.0 3.46 2.57 ∞ 6.0 3.46 2.57 note: α= pwave velocities; β= s-wave velocities; ρ=densities table 1a and 1b show the model of velocity structure for the crust near the source proposed by ingeominas (1993) and assumed in this paper. the anelastic attenuation along the propagation path was parameterized using an attenuation value of t* =1s for p waves and a t* =4s for sh waves. data used in the relocated earthquakes earthquakes with magnitude mb ≥3.5 in southwestern of colombia were relocated using the jhd method (dewey, 1971) and the phases readings reported by isc and neic between 1990 and 2004 used. this was done as an effort to reduce the errors in the hypocentral locations determined by isc and neic, since this method permits a better constrained estimate of the focal depth when a considerable number of pp phases are included. the jhd method determines the location of a set of earthquakes, applying time adjustments with a least squares estimation for the p, pp, and s phase readings, using the time interval between pairs of phases. the 2004 pizarro-colombia earthquake (mw=7.2) was used to calibrate the event, along with 14 earthquakes with fixed depths by means of a teleseismic body waveform inversion. results and discussion body waveform inversion in this study, 15 events with a reported mw≥ 5.8 magnitude between 1990 and 2005 were modeled. these earthquakes are the most representative of the seismicity in the colombia-ecuador zone and cover, in a relatively homogenous way, the colombiaecuador subduction zone. ten of the selected earthquakes have shallow hypocentral depths 120 geometric model of the nazca plate subduction in southwest colombia 121 (h≤70km), one earthquake has an intermediate hypocentral depth (70km≤h≤140km), and four earthquakes have deep hypocentral depth (h>140km). the results of the inversion of body waves of this events are summarized in the table 2, and an example of the focal mechanisms, waveform, and source time functions modeled are shown in figure 3 along with a comparison between the observed records (solid lines) and synthetic seismograms (dashed lines). the focal mechanism and the relative location of the teleseismic stations are shown on the lower focal hemisphere. table 2. earthquake hypocentral and fault plane solution data obtained in this study. all events were relocated using jhd method (dewey, 1971). hypocentral sections the colombia-ecuador trench (cet) may be divided into segments of constant curvature, which represent arcs or small circles on a sphere. by means visual inspection, two arcs along the cet were identified (figure 1). for each arc, a center of curvature most closely fitting was obtained by using the gmt mapping programs (wessel and smith, 1991). an arc is concave towards the ocean and it extends approximately from 78ºw to 79ºw in a nw-se direction. in front of the colombia-ecuador boundary, the curvature center is roughly located at 6.387ºn, 82.781ºw, and at a distance of approximately 504km, and other arc is concave towards the continent with the same general nw-se direction, and goes from 79ºw to 81,6ºw to the southeastern end of the study zone, the center of curvature is located at 4.016ºs, 69.984ºw at a distance of approximately of 723km. these segments were divided into two parts each one, according to the distribution of the seismicity (figure 4). perpendicular hypocentral projections and along the arc were done following the propose methodology by guzman, s (1995). in this methodology, for the sections perpendicular to the arc, the hypocenter are project as a function of their latitude with respect to the pole and or depth, holding constant their longitude with respect to the pole and for projection along the arc, the hypocenter are projected onto the arc as a function of their longitude an depth with respect to the pole, that is, through great circles from the pole to the arc then this surface is projecting onto a plane (figure 5). the position of the trench in cross-section view is always located in the upper-left corner of the section. focal mechanims in this study, 15 focal mechanisms are modeled by means of inversion of p and sh body waveform and 14 mechanisms are cmt solutions reported by the harvard university. all events were relocated with the jhd method (figure 7a). these focal mechanisms are also shown on lateral projections on the cross sections (figure 5). in figure 7b, the focal mechanisms reported by harvard university (but not relocated) are shown. in a analogous manner to the seismicity, the stress pattern observed along the subduction zone may be divided in three zones: two zones where the mechanisms correspond to a compressional regime (thrust faulting), the first one from 5ºn to 4ºn, and the second one from 1.5ºn to 2ºs, and a third zone from 4ºn to 1.5ºn where a variety of mechanisms are observed: normal, thrust, and strike-slip, which is something unexpected for shadows subduction. a possible explanation to this behavior is a folding of the subducted plate due to a blockage possibly caused by the subducted malpelo and carnegie ridges within the cali a and quito d segments respectively. pedraza et al. esrj vol. 11, no. 2. december 2007 122 figure 2. location of seismograph stations used in this study. figure 3. plot of p (top) and sh (bottom) waveforms for 4 selected earthquakes representative of the data and their inferred focal parameters. solid lines are observed records and dashed lines are the synthetics records. also shown are their corresponding source time functions. time and amplitude scales for both the p an sh waves are shown in the low part of each figure. geometric model of the nazca plate subduction in southwest colombia 123 the intermediate seismicity is scarce and only two focal mechanisms are obtained (numbered 23 and 24). the first one corresponds to normal faulting and located at 4ºn in front of buenaventura, colombia, and the second one corresponds to inverse faulting and is located at 0.5ºs in front of manta, ecuador. the deep earthquakes are concentrated in two regions, some in the north zone between 4ºn and 5ºn and others located between 1ºs and 2ºs. for the first cluster, the focal mechanisms of two events (numbered 26 and 28) were obtained. both mechanisms correspond to thrust faulting, with some strike-slip component, with depths ranging from 180km to 230km. in this region, tensional movements would be expected; therefore, this rupture patterns could be due to the presence of “asperities” that generate traction on a subducted slab. in the second cluster, the focal mechanisms of two events (numbered 25 and 27) are obtained and both mechanisms correspond to normal faulting. geometry of the subducted plate the wbz shows changes in its dip and maximum depth in all four segments. for the cali segment, a change in the dip angle is considered down to a depth of 100km, to a width of approximately of 180km of the trench. beneath the continent, it increases from 17º to 45º. a maximum depth of 220km is estimated (figure 5). in the popayán b segment, the dip of the wbz decreases to 30º and reaches a maximum depth of 190km. for the nariño c segment, the dip angle is similar to that of the popayán b segment and with a maximum depth of 115km. in the quito d segment, the dip angle increases of 9º to 49,5º with a depth of 100 km to a width of approximately 320km of the trench beneath the continent and to maximum depth of 220 km. a cross section parallel to the trench from (5.5ºn, 78ºw) to (2ºs, 81.6ºw) shows a rapid variations in maximum depth of the hypocenters within southwestern colombia (figure 6). two maximum depth zones are observed that correspond to the cali a and quito d segments. the maximum focal depth is 220 km. the depth extents of various segments of the subduction zone directly correlate with the observed dip of the slab. two transition zones are observed which suggest a steep change in dip angle. the first zone at 4ºn is within the cali a segment and the second zone at 2ºs is within the quito d segment. in the central part of the cet (3,5ºn to 1ºs), a seismic gap for intermediate and deep earthquakes is observed and the dip of the subducted slab becomes shallower (figure 8). in the figure 9 a perspective view of the wbz is shown, which is intended to demonstrate the three dimensional complexity of the nazca plate subduction in the cet. this three-dimensional surface was obtained by means of interpolation of the isodepth contours. several authors have suggested that the principal factors affecting the geometry of a subduction zone are the relative convergence rate, the age of the subducted slab, the absolute motion of the overriding plate, and the subduction of aseismic bathymetric features such as ridges or intraplate seamounts (e.g., jarrard, 1986; gorvatov and kostoglodov, 1997). when comparing the ages calculated using the empirical relations given by gorbatov et al, (1997) with the ages reported by atwater (1989), the age for the popayán b (28±1 m.a). and nariño c (17±3 m.a.) segments are within the margin of 95% of confidence of the gorbatov relation, but the age for the cali a (46±8 m.a.) and for the quito d (58±7 m.a.) segments are almost twice as large. these are precisely the segments that present pedraza et al. esrj vol. 11, no. 2. december 2007 124 figure 4. hypocentral sections of the cali, popayan, nariño and quito segments. relocated hypocenters obtained from the joint hypocenter determination method (dewey, 1971). the location of the quaternary volcanoes (solid triangles). figure 5a. cross-sections perpendicular to the trench for the cali a and popayán b segments. the points mark the hypocenters relocated with jhd method, the solid line shows the tendency at depth, and the dashed line indicates the limits of the wadati-benioff zone. geometric model of the nazca plate subduction in southwest colombia 125 pedraza et al. esrj vol. 11, no. 2. december 2007 figure 5b. cross-sections perpendicular to the trench for the nariño c and quito d segments. the points mark the hypocenters relocated with jhd method, the solid line shows the tendency at depth, and the dashed line indicates the limits of the wadati-benioff zone. figure 6. cross-section along the colombia-ecuador arc, from (5.5n, 78w) to (2s, 81.6w). the hypocenters represented correspond to those in the catalogue of earthquakes relocated with jhd for the period 19762005.. 126 figure 7a. focal mechanisms of the inverted events in this study (solid) and focal mechanisms reported by the harvard university (dashed). all events were relocated with the jhd method. figure 7b. focal mechanisms reported by the harvard university but not relocated with jhd. geometric model of the nazca plate subduction in southwest colombia 127 pedraza et al. esrj vol. 11, no. 2. december 2007 figure 8. isodepth contours of the subducted nazca plate beneath the south america plate in the cet using a spline interpolation of the data relocated with the jhd method and error ellipses ≤ 20km. figure 9. a three-dimensional view of the subducted nazca plate between 5ºn and 2ºs. 128 geometric model of the nazca plate subduction in southwest colombia changes in focal mechanisms. this situation can be explained with two hypothesis: (1) the calculated ages are the result of bias in the velocities or dip angles estimated in the present study and (2) the presence of the malpelo and carnegie ridges generate differentials blockages with respect to the colombo-ecuatorian pacific basin, causing changes in the velocities of convergence in these regions. the second hypothesis agrees with the measured vectors of convergence determined by gps (trenkamp et al., 2002) that show a change in the north component of the convergence velocities from south to north of the cet. other authors such as taboada et al., (2000) propose an overlap zone of the caribbean and nazca plates located between 5,2ºn y 7ºn in colombia. this model would explain the pronounced curve that appears in the isodepth contours in the cali a segment between 3.5ºn-5ºn (figure 7a). gutscher et al., (1999) interpret the seismic gap at intermediate depths to the north of ecuador between 0ºn and 1ºs to the subduction of the carnegie ridge and the changes in the dip angle in the transition zones as tearing of the nazca plate. conclusions the body waveform inversion of 15 telesismic events and the relocation of 250 hypocenters detected during more than 30 years, allowed an approach to the problem or determination of the geometry of the wadati-benioff zone, with a boundary in isodepth contours in the colombia-ecuador trench. the cet was constrained by means of euler poles to two segments with different concavity: the first one is concave towards the pacific ocean located between 5ºn and 3.5ºn and the other one is concave towards the continent between 3.5ºn and 2ºs. two hypocentral sections for each segment were considered. the subduction process of the nazca plate beneath the south america plate is constrained from the seismicity distribution within the cet. three different subduction configurations were observed: the first in the cali a segment, with maximum depth of 290km to 300km up to a distance of 300km from the trench and a dip angle that changes of 17º to 45º to a depth of 100 km. the second in the popayán b and nariño c segments with maximum depth from 130 km to 150 km up to a distance of 280km from the trench and an approximately constant dip of 30º. the third in the quito d segment, with a maximum depth of 200km to 220km up to a distance of 430km from the trench and a dip angle that change from 9º to 50º to a depth of 80km. the maximum depth of the seismicity of the subducted slab in the cet shows two regions of increases: the first one, from 4,5ºn to 5ºn, and the second one, from 2ºs to 1ºs. it is suggested that the presence of malpelo and carnegie ridges generated a differential blockage with respect to the colombiaecuatorian pacific basin. references atwater t. (1989). plate tectonic history northeast pacific and western north america. the geology of north america. v. n, the eastern pacific ocean an hawaii. the geological society of america, chapter 4. barazangi, m and isacks, b.l. (1976). spatial distribution of earthquakes and subduction of the nazca plate beneath south america. geology, 4. p. 686-692. bouchon, m. (1976). teleseismic body wave radiation from a seismic source in a layered medium, geophys. j. r. astr. soc., 47. pag. 515-530. burbach, g.v., frohlich, c., pennington, w.d. and matumoto, t. (1984). seismology and tectonics of the subducted cocos plate. journal geophysical research, 89. p. 61536170. chicangana, g. and vargas, c. (2004). desarrollo y geometría actual de la litosfera en la esquina noroccidental de suramérica: first latin -american congress, mem. (in cd – room). dewey, j.w. (1971). seismicity studies with the method of join hypocenter determination: ph.d thesis, university california, berckeley. ekstrom, g. and engdahl, e. (1989). earthquake source parameters and stress distribution in the adak island region of central aleutian islands, alaska. journal geophysical research, 94. pag.1549915519. futterman, w. (1962). dispersive body waves. j. geophysics res., 67, 5279 -5291. gorbatov a and kostoglodov, v. (1997). maximum depth of seismicity and thermal parameter pf the subducting slab:n general empirical relation and its application. tectonophysics, 277. p. 165-187. gutscher, m.a., malavielle, j., lallemand, s. and collot, j.y. (1999). tectonic segmentation of the north andean margin: impact of the carnegie ridge collision. earth and planetary science letters, 170(1-2). pag. 155-156. guzmán, s. (1995). hypocentral crosssections and arc-trench curvature. geofísica international, v.34, no 1, 131-141. hey, r.n. (1977). tectonic evolution of the cocos-nazca spresding center, geol. soc. am bull, 88, pag. 1404-1420. ingeominas. (1993) boletín trimestral junioagosto. isacks, b., oliver j. and sykes l. r. (1968). seismology and the new global tectonics. j. geophys. res., 73. pag. 5855-5899. isacks, b.l. and m. barazangi. (1977). geometry of benioff zones: lateral segmentation and downward bending of the subducted lithosphere, in: m. talwani and w.c. pitman ii editors), island arcs, deep sea trenches and back-arc basins. american geophysical union. washington, 470pp. jarrard, r. (1986). relations among subduction parameters. review of geophysics. v. 24 pag. 217-284. kawatsu, h. (1986). downdip tensional earthquake beneath the tonga arc: a double seismic zone. journal geophysical research, 91, 6432-6440. mejía, j. a. and meyer, h. (2004). modelo detallado preliminar de la sismicidad en el occidente de colombia. observatorio sismológico del sur occidente. osso. memorias i congreso latinoamericano de sismología. armenia. monsalve, j. h. (1998). geometría de la subducción de la placa de nazca en el noroeste de colombia: implicaciones tectónicas y sísmicas. tesis de maestría. instituto de geofísica unam. 107p. nábelek, j. l. (1984). determination of earthquake source parameter frominversion of body waves. thesis doctoral, massachusets institute of technology. 129 pedraza et al. esrj vol. 11, no. 2. december 2007 pennington, w.d. (1981). subduction of the eastern panama basin and seismotectonics of northwestern south america. journal geophysical research, v.86, 10753-10770. taboada, a. dimaté, c. and fuenzalida, a. (1998). sismotectónica de colombia: deformación continental activa y subducción. física de la tierra. madrid, no. 10. pag. 117-147. taboada, a., rivera, l., fuencalida, a., cisterna, a., philip, h., bijwaard, h., olaya, j. and rivera, c. (2000). geodynaics of the northern andes: subductions and intracontinental deformation (colombia). tectonics. vol. 19. no 5. pag. 787-813. trenkamp,r., kellogg, j., freymuller, j. and h. mora. (2002). wide plate margin deformation, southern central america and northwestern south america. casa gps observations. journal of south american earth sciences, 15,2 p. 157-171. trenkamp r., mora, h., salcedo, e. and kellogg, j. (2004). possible rapid strain accumulation rates near cali, colombia, determined from gps measurements (19962003), earth sci. res. j. 8 (1), 25-33. vargas c.a., pujades ll.g., ugalde a. and canas, j. (2003). tomografía sísmica local del territorio colombiano. revista internacional de métodos numéricos para cálculo y diseño en ingeniería. barcelona, v.19, n.3, p.255 278, 2003. wessel, p. and smith, h. 1998. new improved version of generic mapping tools released, trans. am. geophys. union (eos) 79, 579. 130 geometric model of the nazca plate subduction in southwest colombia geofisica colombiana n° 7 pp. 5-12 issn 0121 2974diciembre de 2003 bogota, d.c. actividad de la falla la salina a partir de lnstrumentaclon slsmoloqica ernesto gomez programa de maestrfa en geofisica. departamento de geociencias, universidad nacional de colombia luis antonio castillo lopez grupo de invesligaci6n en geoffsica. departamento de geociencias, universidad nacional de colombia, e-mail: lacastillol@unal.edu.co resumen la falla la salina se ubica en el departamento de santander, en una region de alta sismicidad y es definida como de tipo inversa. en general, no presenta claras evidencias morfologicas ni de actividad reciente, por 10 cual en el presente trabajo se investiga un area de 294 km2 de la misma. dentro del proyecto hidroelectrico del rio sogamoso, desarrollado por isagen s.a, se establecio una red sismologica de seis estaciones en una area cercana ala falla. durante 124 dias se evaluaron 2.367 sismos, de los cuales se seleccionaron 339 de calidad aceptable para la elaboracion de mapas y perfiles. de estos sismos, la mayor parte estan asociados al "nido de bucaramanga" y 131 eventos estan relacionados a fallas cartografiadas en la region; y de ellos, s610 dos sismos estan asociados a la falia la salina en el area de estudio, que sugieren su actividad actual. palabras clave: sismotectonica, instrumentacion sismologica, neotectonica abstract the "la salina" fault, located in a region of high seismicity was studied in an area of294 km2 in the santander department, it's defined as an inverse fault. in general it doesn't present clear morphological or actual activity evidences. in the area of the hydroelectrical project of the sogamoso river by isagen sa., a six stations seismic network was installed in an area near to the fault. during 124 days, 2.367 seismic events were evaluated; from these, 339 acceptable quality events were chosen for doing maps and profiles. most of the seismicity is related to the "bucaramanga nest", other 131 seismic events ware related to the mapped faults. only two events were related to the "la salina" fault, defining a indicating low activity. keywords: seismotectonic, seismology instrumentation, neotectonic introduccion sismologicamente se ha verificado el grado de actividad de las fallas de bucaramanga y del suarez a partir de su instrumentacion, mediante la implantacion de redes (sismologicas) locales y regionales, en el desarrollo de varios proyectos cientificos que buscan explicar el "nido de bucaramanga" (coral gomez, 1985, a y b; pennington, 1981;rivera, 1984)ytecnicos como el proyecto hidroelectrico de los rios fonce y suarez. estas evidencias de actividad tectonica, junto con el desconocimiento del grado de actividad de otras rupturas importantes, como manuscrito recibido para eva'uacion el15 de abril de 2003. articulo aceptado para publicacion par el comite editorial el 30 de octubre de 2003. geofislca colombiana, 7, diciembre de 2003 es el caso de la falla la salina, y su cercania al area de influencia del proyecto hidroelectrico del rio sogamoso, justifican un estudio local con el fin de evaluar el riesgo sisrnico y su efecto sobre las obras civiles que se construiran dentro del proyecto rio sogamoso. en consecuencia, se implanto una red de seis estaciones sismologicas, como se detail a en la tabla i y figura i, cubriendo un area aproximada de 40 km2 entre san vicente de chucuri al sur, sabana de torres al norte, giron al oriente y lisama al occidente. la red de sismografos opero entre los meses de febrero y junio de 1995. localizaci6n ei proyecto hidroelectrico del rio sogamoso se ubica en el krn 54 de la via que conduce de bucaramanga a barrancabermeja, en el sitio 5 g6mez et al. 6, (stacie·" l,brijo 6,., (l1acian ~iorllto a~ t~i~~r~~~~o~~es) 4:..4. ~'tqeiol'! stc in.. ,so/'! vicente) 65 '::sto:;:;un =ooiclo ai5 e.tociojr: 10 pcl 67 ~~~l~~!nb"os~ct'wq s mvr.icipio univ[rsioad nacional de colombia rio corr.!erc rene ce rum!lc falla il"lv.,.sq rene <i. :'>lormal cl:jdoc r(!) sisio4ologlca ". ...", ttctonico ....ltt ... of: in'lu[~ci'" :xl dro'l'ec rc eel 'uo sogouioso !rnefc gomez :. octubre, 1995 t: 1'000.000 :s figura i. localizacion de la red sismologica en el proyecto hidroelectrico del rio sogamoso. donde el rio sogamoso cruza la cordillera la paz, en jurisdiccion de los municipios de giron, betulia, zapatoca, san vicente de chucuri y los santos, en el departamento de santander (figura 1). 5.10 geologia el estudio geologico regional demuestra que el area de estudio esta situada en la porcion este de la cuenca deposicional del magdalena medio. geologicamente se ubica en el flanco occidental del sinclinal de nuevo mundo, el cual se encuentra limitado por la falla la salina, la cual presenta un trazo sinuoso con direccion preferencial s-n. en el area de estudio afloran rocas sedimentarias: conglomerados, areniscas, arcillolitas y shales del cretaceo superior y terciario, cubiertas en algunas partes por depositos recientes. tectonicamente, se encuentra afectada por el sistema de fallas de la salina, el cual corresponde a un sistema de fallas de tipo inverso con componente de rumbo dextrolateral, que pone en contacto rocas de la formacion umir del cretaceo superior con otras del terciario (shales de la fm. lisama, grupos chorro, chuspas y real), produciendo el hundimiento del valle medio del magdalena, limitandolo en su borde oriental. s.c· :.30 instrumentacion de la falla la salina red sismol6gica para detectar sefiales de actividad sismica y conocer la sismotectonica del area de influencia del proyecto hidroelectrico del rio tabla j. ubicacion y caracteristicas de la red de sisrnografos (e.s.a.p.-meqq800) i i c" i i ! duraclon iinicio deli coordenadas i altura ganancia filtros i deflexioni estaclon im.s.n.m. ubrlmlento unidad litologlca i . x y db [low/high i max. (mm) ~~~ hor~j-:~::--'i i i 1377.'~ 260 i -+~---arenisca formacion 78 i 5/30 j 25 24 i i1-11-95 ila paz 1.073.450 sitio de presa esmeralda (tee) i i --~~~--'----.---~_._---------~~-_.-----------------r-~-l jlisama /1.060.700 1.275.200 i arenisca de grano fino. i 280 falla la salina 84 5/30 25 24 11-24-95 i i i formacion lisama (tpl) i i embalse falla arenisca de grano fino. lei tablazo 1.082.100 1.270.750 370 90 5/30 25 24 i 11-21-95 san vicente formacion tablazo (kit) , ila floresta 1.073.500 1.294.950 200 falla la salina arenisca conglomeratica. 84 5/30 i 25 48 11-11-95 forrnacion real (tmri) i ----1 santa ines 1.078.100 1.255.400 1020 t'ro"" '0"" finoconlutita 84 5/30 i 25 48 i 11-10-95 i falla san vicente gns. formacion tablazo (kit) i i lebrija 1.096.650 1.276.400 i 1100 falla suarez arenisca conglorneratica. 90 5/30 i 25 24 i 11-02-95 , formacion giron (jg) i i • estacion fuera de servicio desde el dia 21 de febrero. 6 geoflslca colombiana. 7, diciembre de 2003 actividad de la falla de la salina a partir de instrument aci6n sismol6gica sogamoso, se establecio una red sismologica en sitios previamente determinados y dentro de un radio de 20 km alrededor del sitio de presa. la red de sismografos consto de seis estaciones sisrnologicas, analogicas portatiles (e.s.a.p) del tipo meq-800 de la fabrica sprengnether, las cuales fueron instaladas a partir del 9 de febrero de 1995. (su fecha, sitio de instalacion, coordenadas, tipo de roca sobre la cual se instal0, cubrimiento dado a cada estructura geologica de interes en el proyecto, asi como la duracion del registro para cada estacion sisrnologica, se presentan en la tabla i). durante los 124 dias de registro se evaluaron 2.367 sismos, de los cuales se pudo hacer localizacion a unos 1.500 eventos sismicos, seleccionandose 339 sismos con calidad "c" (calidad aceptable), para la elaboracion del mapa epicentral y los perfiles hipocentrales. entre estos eventos localizados, 298 sismos tienen una asociacion directa con la sismotectonica de la region; de tales eventos localizados en el mapa epicentral, sobresalen 131 sismos asociados con las fallas cartografiadas en la region como las fallas de surata, rio umbala, bucaramanga, suarez y la salina. asi mismo, los 167 eventos restantes, localizados en el mapa epicentral, son desechados por no estar asociados con las fallas principales que enmarcan el proyecto sogamoso. se resalta que de los 2.367 sismos registrados, solo 30 eventos sismologicos se localizaron dentro del rombo conformado por las estaciones de floresta (sabana de torres), lebrija, santa ines (san vicente de chucuri) y lisama. esto puede significar que para el periodo marzo-junio de 1995, el area de influencia del proyecto presento muy baja actividad sismica. descripcion del procesamiento geofisico de los datos sismologicos procesamiento de datos cada linea horizontal de un sismograma corresponde a lineas de tiempo con diferencias de cinco minutos, que equivalen a un giro del cilindro del equipo registrador; con este criterio y teniendo en cuenta la hora de entrada y salida del sismograma, se colocan en forma manual, sobre el sismograma, las horas correspondientes a las sefiales horarias impresas por la aguja del sismografo, teniendose de esta manera referenciado en el tiempo los sismos que aparecen en el sismograma. conocer el tiempo de arribo de cada tren de ondas sismicas a una estacion georreferenciada es el principio de localizacion de un sismo. ei proceso se inicia con la clasificacion de los sismos; se debe descartar el ruido sismico local y aquellos sismos teoricamente regionales, que tienen un valor de s-p mayor de is segundos. los sismos tectonicos locales, con valores de s-p menores de 15 segundos y de interes para el proyecto, pues se consideran asociados a eventos ocurridos dentro 0 muy cerca de la red sismologica instalada, son marcados en una estacion de referencia, identificados con la hora y minuto de arribo, y comparados con los registros geofislca colombiana, 7, oiciembae oe 2003 sismicos de las otras estaciones, para su corroboracion y posterior registro en el catalogo general de sismos para su estudio. locstizecion de sismos del catalogo general de sismos, se extrae un archivo de lectura (*.inp) para ser utilizado en el programa de localizacion hyp07ipc, de los registros que se reporten minimo en tres estaciones y contengan mayor numero de fases p y s. el porcentaje de confiabilidad que ellector asigne a la observacion depende de la nitidez del sismograma y del tipo de arribo de las ondas; para el programa de computador sera un rango de tolerancia permitido para la diferencia entre el valor leido y el valor calculado por el sistema. asi, entre menor sea esta diferencia de lecturas, mayor confiabilidad tendra el dato de registra. ei programa hyp071pc discrimina la calidad de los eventos en cuatro tipos: a, b, c y d, de los cuales, a presenta un rango maximo de calidad y d un minimo de calidad. asi, un sismo de calidad "c" presenta una regular calidad de los resultados, debido a tres factores fundamentales: error en las lecturas, fases registradas en poco numero de estaciones y sismos fuera 0 en la periferia de la red, que dan lugar a vacios apreciables que bajan la calidad. ei programa de computador se ejecuta con los parametres del modelo de velocidades del subsuelo y los datos de entrada extraidos del catalogo de sismos, genera un archivo de salida (*.pun) que contiene la informacion con las localizaciones realizadas y las respectivas profundidades de iteracion si han sido predeterminadas. marzo 40 35 30 25 20 15 10 11 21 31 dias r--mayo 25 20 is 10 o -'-...j 11 21 31 oias figura 2. nurnero de eventos dia, marzo-junio. abril 35 30 25 20 15 10 11 21 31 oree junio 35 30 25 20 15 10 i 1\ a ./\ ./1/\wvt tj r ./ 11 21 diu 31 7 anausis geofisico de datos numero de eventos sfsmicos por dfa en total se obtuvieron 460 sismogramas, en los cuales se registraron 2.367 sismos, repartidos mensualmente asi: marzo 449, abril 70 i, mayo 374 y junio 843 (figura 2). esta distribucion conformo el catalogo de sismos del proyecto sogamoso, en el cual se anota la fecha, hora, estacion, tiempo de arribo de la onda p, de la onda s y diferencia entre la s y la p. del catalogo de sismos, donde se reportan 2.367 eventos de diferentes magnitudes, ocurridos durante los meses de marzo a junio de 1995, se deduce que para estos cuatro meses se presento un promedio de 20 eventos sismicos por dia, 10 que corrobora la alta sismicidad regional presente, razon por la cual se ubica en una zona de alto riesgo sismico 0 zonas de amenaza sismica. para el mes de marzo de 1995, la mayor cantidad de eventos sismologicos se presento el 30 de marzo con 37 eventos, cuando el promedio de sismos registrados para este mes fue de 15 eventos por dia. para abril de 1995, presenta como mayor cantidad de eventos sisrnicos en un dia, el 25 de abril, con 34 sismos, cuando el promedio para este mes fue de 23 eventos por dia, durante el mes de mayo de 1995, el dia 29 de mayo fueron registrados 22 eventos sismicos, cuando el promedio para el mes de mayo fue de 12 sismos por dia. parajunio de 1995, el dia 16 presento 34 sismos, como el mayor mirnero de eventos registrados en un dia, y el promedio diario para este mes fue de 28 eventos por dia. 50 -r-e-__ m_a~li;~ abril ii ~: ii ~~: 10 -i--'-+----t-ii 1 1 o .l__ -'~ i! ~ o. 200 400i i 0 ii ii~===m=a=y=o===~! 1~======j=u=n=jo======~ :; 1,1' :: ~o ~ rj, ii rj,15 15 ii 10 i i 10 ii 5 i i o . : 0 , i o '100 200 300 i ! 0 200 '-si_sm_os --'i ils_ism_o_s --' ii j i iiuii l~ liil ~ 1'1", '11 ir~"'ii i ii i 40 30 -k---.-+---+-----i~ rj, 20 200 400 sismos ,j 1.1 1i ,ill 11'~i r p ii' 400 figura 3. valores regislrados de s-p en 1995. 8 gomez etal. diferencia s-p en la red sismologica del proyecto sogamoso, la separacion entre estaciones varia de 15 a 30 kilometres, presentandose la maxima separacion entre las estaciones floresta, sabana de torres y santa ines, san vicente de chucuri con unos 35 kilometres de distancia en linea recta. si tomamos valores promedio de vp = 5.5 km/s y vs = 3.10 km/s (con vs '" 0.58 vp), entonces una diferencia de tiempo de arribo de la fase p y de la s, p s = i s. significara que el hipocentro esta a una distancia de 7,5 km del sitio de registro. se calculan las diferencias de s-p para las estaciones de la red, de modo que sea su promedio el valor de interes, de este modo se considerara que diferencias menores de 10 segundos son ocasionadas por eventos locales, dentro de los cuales se consideraran los ocurridos dentro 0 muy cerca de la red sismologica instalada. de este parametro de 10 segundos para s-p, se determine que para el mes de marzo de 1995, solo 3 sismos de los 499, registraron s-p menor 0 igual a 10 segundos; por tanto, para este mes, solo el 0,6% de la sismicidad registrada se considera ubicada dentro de la red 0 muy cerca de ella. para el mes de abril, fueron 10 eventos con s-p menor 0 igual a 10 segundos; par tanto, solo el 14% de la sismicidad registrada se considera dentro de la red 0 cerca de ella. en mayo de 1995, se registraron 8 sismos con s-p menor a 10 segundos, siendo el 2, i % de la sismicidad registrada para ese meso en junio, fueron 7 eventos y representan tan solo el 0,8% de los eventos registrados en este mes, figura 3. en promedio, para los meses de marzo a junio de 1995, solo el 1,2% de la sismicidad registrada se considera localizada dentro de la red 0 muy cerca de ella. estas cifras y porcentajes permiten considerar la zona ubicada dentro de la red y sus sectores aledafios (a menos de 10 km de la red), como de baja a muy baja. magnitudes registradas a causa de las caracteristicas del sismometro (frecuencia natural i hz), del registrador segun la velocidad de tambor y del tipo de sismos registrados (generalmente de alta frecuencia), la amplitud del registro en muchos casos se satura, y por tanto la magnitud de ritcher no se puede medir directamente del sismograma. por esta razon y para ajustarse a la sismicidad propia de una region, se han desarrollado varias formulas ernpiricas que relacionan la duracion del evento (coda) con la magnitud (tsumurak, 1967; crosson, 1972; lee y lahr, 1972). para el caso de la red sismologica del proyecto sogamoso, la ausencia de un sismografo con sismornetro tipo wood anderson no permitio la obtencion de la magnitud richter directamente del sisrnograma, por 10 cua! se utilizo la propuesta por herratz et al. (1991). magnitud = 1.67 log(t) 0.43 (1) donde t es la "coda" 0 duracion en segundos del evento, el resultado se muesta en la figura 4. geofislca colombiana, 7, oiciembre d~ 2003 actividad de la falla de la salina a partir de instrument acion sismologica 2 ~--1,5 0,5 1;' 05, -1 -1,5 -7 i ·2,s i 0 so log 750 nne'00 orden de ev e o to s figura 4. ocurrencia de magnitudes marzo-junio de 1995. correlacion entre epicentros, accidentes neotectonicos y fracturas de los 350 eventos sismicos, con magnitudes (coda) entre -1,5 a 1 en la escala de ritcher, 96 son de calidad "c" (calidad aceptable), los cuales se graficaron en la figura 5 y se proyectaron sobre un mapa tectonico, donde se observa que tan solo 7 eventos estan .:: -: t,i--, ,',\ '"t'\~' ~s:~-:::..~<: _.3)': <., ! !:;.. ~s:~;:: ,~. ~~. "s2 ~'h:::; 0:" scccr-c c e s, ~ \ le-ec e_~::-: ~~r?~:c,,~,~~:~~,r e: ~e:,:s,~~c.,-glc;'i localiz4cio" (""cli"·.lo([y("'06 [~~_"';;o'"om:.-::... ,.,c.::; :'~'.::;:~ '::"ij is::::: 0'.';r ~ -;f~~:;ec~~ .:.i ii.o"''';c:i'c :e ·;: ...cc: figura 5. localizaci6n de eventos tipo c. geoflslca colombiana, 7, diciembre de 2003 v,(j ?l _ ~ ~. , _?_~~..... _. ~ :,)" ~-. ::er!~::j . '/'. "'0~'1 ), ii; -s ilj *~~; i ~", s ."! ,'1., , i -t \:;.~. -' "'--~"~>;;~~'i::"'" h_ i • , / r , /'? ./;,~'/, .if! :.. •..1, r t e -::~l / _ :":s'"<;: j~ ._~:. ":"'2 ~5:gc o~ ~ :.' : s sccc ...c .::~ "c ...::.:;::5:::':.;>~ _ s c r-; !..:. .::',,:c·:~ :;.-:::--':!o,; (sc: c e:e i~~~~~~~:~:=~~~~ .:.:' esrec c~ 3 )-·c-<:· '; ~,ng'::ci.,":;'::: :: "'u-;c'c'o ~d 3:svc':c:c" ~oc"~iz"cio" ("'c("ua~ o( ['((,hqs 'ismolo,tco co .. ''''1.10'''0 ·0· :.locc: figura 6. localizacion de eventos tipo d. localizados dentro del rombo, cuyos extremos son las estaciones de lebrija, floresta, lisama y santa ines (figura 5). para la falla de bucaramanga se presentaron unos 10 epicentros ubicados sobre 0 muy cerca al trazo de la falla, pero a la vez se ubican unos 30 sismos que pueden ser parte de los conocidos como el "nido de bucaramanga" entre piedecuesta los santos y el rio umbala. entre los eventos de calidad "d" (minima), (figura 6), los mas importantes son los asociados a la flexura del chucuri (falla de chucuri 0 falla de san vicente), ramal de la falla la salina. estos representan 22 sismos muy superficiales alineados a 10 largo de la flexion del rio chucuri. de los 1.500 eventos sismicos procesados para su localizacion epicentral e hipocentral, el estudio se concentro en 339 que corresponden a los eventos con calidad "c"; por tanto, solo el 22% de los eventos localizados seran presentados para el analisis de epicentros e hipocentros, de los cuales 41 eventos estan muy distantes del proyecto (menor a 5 0 mayor a 8 de latitud norte). con la utilizacion de tres model os de velocidades (tabla 2), se analizaron cada uno de los resultados, la figura 5 contiene el mapa de epicentral con el modelo de velocidad i, elaborado para .' 9 g6mez et al. i 63 i i 0 i '2.1 0 i i i '.. 74 00 73 lomhtli) 72 figura 7. perfil de hipocentros. este estudio; en el se observa una concentracion de la sismicidad entre las latitudes 6.9 a 7.1 y longitudes 73.00 y 73.30. la figura 6 corresponde al perfil de hipocentros sobre latitud 7 norte. de este perfil de hipocentros se interpreta una asociacion de tres eventos entre i y 35 kilometres de profundidad, a la falla la salina, y a profundidades mayores a 35 kilometres otros dos eventos con inclinacion hacia el oriente. del trazo principal asociado a la falla la salina se desprenden dos sistemas subparalelos con sismicidad entre 10 y 30 kilometres de profundidad; uno de estos sistemas corresponde al trazo asociado a la flexion (falla) de chucuri. en la figura 7 se muestra el perfil hipocentral proyectado sobre el paralelo 7 latitud norte, donde se observa una gran nube de eventos sismologicos a profundidades mayores a 110 kilometres y que pueden liegar a 180 kilornetros, asociados a 10 denominado como "nido de bucaramanga" (pennington, 1981; rivera, 1984 y coral, c. 1985, a y b). modelo de velocldades . para la cordillera oriental area de bucaramanga valores de las profundidades en kilometres y veloctdades de las ondas p en km/seg modelo 1 modelo 2 modelo 3 elaborado rivera (1984) similar a para este estudio munoz et al. (1986) prof. (km) vel. (krn/s) prof. (krn) vel. (krn/s) prof. (km) vel. (krn/s) i o· 10 5.5 i o· 15 5 0-2 4.0 i 10·35 6.5 15·40 7.5 2 5 5.5i 35 70 7.7 l40 6.1 5 25 6.4! >70 8.1 25 35 7.1 i >35 8.1 10 analisis y discusion al evaluar la sismicidad registrada entre marzo y junio de 1995, de 2.367 sismos registrados para 124 dias se obtiene un promedio de 19 sismos por dia (figura 2), de los cuales el 80% corresponden al "nido de bucaramanga", como se puede ver en la figura 3, la gran mayoria de los eventos tienen un valor de diferencia en el tiempo de liegada de las ondas s respecto a la liegada de la onda p, entre 14 y 17 segundos, que para un promedio de 7,5 kilornetros por cada segundo de la diferencia de s-p, sugieren un recorrido de 110 a 120 kilometres entre la fuente y la red sismologica del rio sogamoso; por tanto, ese recorrido coincide con la distancia entre el "nido de bucaramanga" y la red del proyecto. al tratar de localizar sismos regionales por fuera de la red sisrnologica del proyecto sogamoso, se tienen sin embargo, con base en los perfiles hipocentrales (figura 6), que presentan una amplia zona de sismicidad por debajo de 110 kilometres de profundidad. se puede sugerir la siguiente hipotesis, las ultimas investigaciones sismologicas han asociado que este tipo de sismicidad profunda a medianamente profunda no se origina en zonas de subduccion. mas bien se debe a procesos generados en la astenosfera por debajo de la litosfera, la cual presenta unas caracteristicas mas plasticas que la litosfera y perrnite la formacion de un nido de sismos, como los de bucaramanga, caracterizados por tener un area algo irregular e hipocentros de profundidad variable. aunque se asuma que un sismo de magnitud 5,0 0 mayor a 150 kilometres de profundidad, no causa grandes destrozos en superficie, es necesario no descuidar los disefios de grandes obras civiles, porque puede ser destructivo, como ha ocurrido con sismos historicos, como el que afecto a bucarest (rumania) en 1940, y dejo un total de 1.000 victimas. la sismicidad profunda asociada al "nido de bucaramanga", se ha analizado y discutido por ser la que mas se ha registrado en los cuatro meses de muestreo realizado en esa parte del proyecto sogamoso. ei mayor interes y quiza con efectos mas directos sobre el proyecto, es la sismicidad superficial, asociada a las fallas que atraviesan la litosfera (de 0 a a 70 kilometres de profundidad) y obviamente la actividad sismologica asociada con las fallas mas cercanas al sitio de presa y con profundidades de 0 a 30 kilometres. antes de analizar los sismos asociados a las fallas en el area de influencia del proyecto sogamoso, es importante resaltar que los sismos superficiales, con mejor aceptacion y calidad en su ubicae ion epicentral e hipocentral son los que estan dentro de la red sismologica del proyecto (tabla 1). es de anotar que dentro de la misma red, hay diferencias "drastic as", especialmente en la cobertera sedimentaria post-permica entre las estaciones de lebrija y floresta, separadas 27,5 kilometres. mientras la estacion de lebrija puede tener unos 1.800 metros de sedimentos post-permicos, geoflslca colombiana. 7, diciembre dc 2003 acdvidad de la falla de la salina a partir de instrumentaci6n sismol6gica la estacion de floresta (cerca a sabana de torres) tiene unos 17.800 metros. resaltar estas diferencias geologicas no indica que la sismicidad registrada este por debajo de la cobertera sedimentaria post-permica, en algunos casos, como los sismos asociados a la flexura del chucuri, con profundidades de 0,96 a 8 kilornetros, son un ejemplo de sismos que se generan dentro de los depositos sedimentarios post-permicos, a diferencia de los aproximadamente 100 sismos evaluados a nivellocal, con profundidades entre 20 y 80 kilornetros, ubicados hipocentralmente por debajo del contacto basamento cristalino sedimentos post-permicos, existen 298 eventos en el mapa epicentral (figuras 5 y 6), los cuales estan asociados directamente con la sismotectonica regional de los departamentos de santander del sur, parte sur del cesar y norte de boyaca, entre estos 298 eventos, se tienen 131 sismos asociados directamente a las fall as cartografiadas en la region, como fall as de surata, rio urnbala, bucaramanga, suarez y la salina. en la figura 5 se encuentra la representacion grafica de estos 131 eventos sismicos, sobre los cuales se con centra el estudio. se muestra una clara relacion entre los epicentros y el trazo de las principales fallas como se describe a continuacion, en cuanto al sistema de falla salinas, se observa que el trazo principal de la falla en el area de estudio presenta muy baja actividad sismica, el cual muestra solamente dos sismos al norte del rio sogamoso. mientras que en el sector de la flexion del chucuri se presentan 22 eventos sismicos asociados a los principales trazos de la flexion que se ha denominado como falla del chucuri 0 san vicente, debido a sus rasgos geomorfologicos observados en los estudios del area del embalse en el proyecto sogamoso. en la figura 6 se observa que la flexion del chucuri se inicia en el sur, desde la falla la salina. a 10 largo de la flexion se presenta un mayor grado de actividad sismica en cornparacion con el trazo de la salina. esta apreciacion sugiere que la flexion se constituye un ramal activo del sistema de falla salinas. respecto a los eventos sisrnologicos muy superficiales y alineados a 10 largo de la flexura del chucuri entre el sector de la hacienda de mirabel al sur y el puente sobre el rio sucio, al norte, sobre la carretera lebrija sabana de torres, son de mayor interes por confirmar que el trazo de la flexura (falla) del chucuri es sismologicamente activo. adicionalmente a estos eventos superficiales se presentan otros 6 eventos, en su mayoria con hipocentros mayores a 100 kilometres de profundidad, ubicados a unos 10 kilometros al norte del sitio de presa. sin embargo, al no registrar y ubicar epicentros e hipocentros a 5 kilometres alrededor del sitio de presa, se puede presumir que no se genero la actividad sismologic a en esta zona, durante ellapso marzo junio de 1995. geoflslca colombiana, 7, oiciembre oe 2003 conclusiones y recomendaciones durante los 124 dias de registro en la red sismologica del proyecto hidroelectrico del rio sogamoso, se registraron 2.367 sismos, seleccionandose 339 eventos con calidad "c", de los cuales, 131 sismos se relacionan directamente con las fallas cartografiadas en la region, como las fallas de surata, rio umbala, bucaramanga santa marta, flexion del rio chucuri y la salina. ei trazo de la falla la salina en el area de estudio presento muy baja actividad sismica; solo dos eventos se ubican en su linea de trazo, al norte del rio sogamoso. la relacion sismo-falla mas importante es la relacionada con la flexion del chucuri (falla de san vicente), a la cual se asocian 22 eventos de cali dad "d" (minima). esta estructura se desprende de la falla la salina y presenta un mayor grado de actividad sismica con respecto al trazo principal de la salina. a 10 largo de la flexion se presentaron 22 eventos muy superficiales, con profundidades entre loy 30 km, localizados entre la hacienda mirabel al sur y el puente sobre el rio sucio al norte (20 km al este del area de estudio). estos rasgos sisrnologicos confirman que el trazo de la flexion del chucuri tiene actividad sismica. la ocurrencia de un promedio de 20 eventos sismicos diarios corrobora la ubicacion regional dentro de una zona de alto riesgo sismico. se comprueba la importancia del estudio de segmentacion de fallas en el analisis de sismicidad, ya que el trazo principal de la falla la salina presento un grado de actividad mucho mas bajo en el area de estudio, que su ramal de falla, la flexion del chucuri, durante el tiempo de registro. aunque en los estudios de neotectonic a de la falla la salina no se observaron rastros geometricos y geomorfologicos que permitan evidenciar la actividad reciente de esta ruptura geologica, y su expresion morfologica es en general muy pobre en el area de estudio, se cornprobo alguna actividad de la falla la salina a partir de su instrumentacion, 10 que pone de manifiesto la irnportancia de la implantacion de redes sismologicas destinadas para estos fines. se recomienda la instalacion de sismografos a lado y lado de la falla la salina dentro del area de estudio, con el fin de obtener mayor precision en las localizaciones epicentrales asignadas a la salina, porque esta ruptura geologica estaba por fuera de la red sismologica instalada para verificar la sismicidad dentro del area del proyecto hidroelectrico del rio sogamoso. asi mismo, abrir trincheras en la zona de falla, con el proposito de hallar posibles grados de desplazamiento, angulo de buzamiento de la falla, orientacion de su inclinacion, etc. bibliografia coralg6mez, c. 1985 a. condiciones profundas de la sismicidad en la region de santander. mem. vi congreso latinoamericano de geologia, bogota, t. 2, pp. 106-149. 11 g6mez et al. ----, 1985 b. contribucion al estudio de la actividad sismica en santander (colombia). mem. vi congreso latinoamericano de geologia. bogota, t 2; pp. 271-291. crosson, r. 1972. small earthquakes, structure and tectonics of the puget sound region bull seism. soc. am 62(5): 1133-1172 berkeley, california. harrison, j. v. 1930. the magdalena valley, colombia, south america. compt, rend. 15 tho int. geol. congr. south africa, v.2; pp. 399-409, pretoria. herratz, et al. 1991. microearthquake distribution in the granada region. in: seismicity, sismotectonics and seismic risk of the ibero-maghrebian region. ed. j mezcua and a. udias, instituto geografico nacional, spain, pp. 65-78. isagen sa "e.s.p." e ingetec s.a. 1995. estudio de los microsismos obtenidos en la red sismologica del proyecto hidroelectrico del rio sogamoso. lee, w. h. k., and lahr, c. 1972, 1985. hyp071 (revised); a computer program for determining hypocenter magnitude and first motion of local earthquakes. u.s.g.s-ofr 75-311. 12 ___ , and stewart, s. w. 1981. principales and lahr c., 1975 hyp071 (revised); a computer program for determining hypocenter magnitude and first motion of local earthquakes. v.s. g.s. ofr 75-311. ___ , and valdez, c. m. 1985. hypo 71: a personal computer version of the hyp071 pc earthquake location program. united states, department ofinterior, geological survey, california, 47 p. morales, l. c., et al. 1958. general geology and oil ocurrences of middle magdalena valley, colombia. habitat of oil, symposium am. ass. ptr. geol., tulsa, pp. 641-695. pennington, w. /981. subduction of the eastern panama basin and seismotectonics of northern south america. journal geophisics res., 86 10753-10770, washington, d.c. rivera, l. 1984. sismotectonique du nord-est de la colombie. c.n. r.s. no. 195, universite pierre et marie curie. taborda, b. 1963. revision geologica preliminar cuenca putana ecopetrol.lnforme geologico no. 190, ingeominas, bogota. tsumurak l. 1967. determination of earthquake magnitude from total duration of oscillation. bulletin of earthquake research institute. tokyo university, vol. 15, pp. 7-18. geofislca colombiana. 7. diciembre de 2003 enero%2031-geociencias-vol%2012-2%20dic-en%20baja[1].pdf earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 156-168 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet syed mustafizur rahman1, rezaul islam2, mumnunul keramat3 and md. sultan-ul-islam4 1 associate professor, department of applied physics and electronic engineering, university of rajshahi, rajshahi-6205, bangladesh. smrahman@ru.ac.bd 2 associate professor, department of applied physics and electronic engineering, university of rajshahi, bangladesh. r_islam@bttb.net.bd 3 professor, department of applied physics and electronic engineering, university of rajshahi, bangladesh. m_keramat@email.com 4 associate professor, department of geology and mining, university of rajshahi, bangladesh. sultan_ul_islam@yahoo.com abstract seismic attributes analysis is applied to indepth ii seismic profiles to develop a better understanding of the deep crustal structure and geodynamics of the lhasa terrane in the central tibet area. analyses indicate that the frequency anomalies are obtained over the range in between 0-30 hz. some area of the analyzed traces of the seismic profiles have shown dominated frequency anomaly approximately 10 hz with little amount of other frequencies. the variation of frequency anomaly is shown to be the indication of the moho depth of the area. estimated moho depths are found very irregular and varying from 20 km to more than 136 km. the crust is thinner in southern, northern and eastern parts of the area while it is thicker in the western part having asymmetric nature in the central part. this variation might be the result of the complicated tectonic process of the indian and eurasian plates. vertically downward pocket like frequency distributions and similar type frequency distributions at near and far depth are also observed in one of the analyzed sections. it might be indication of uprised asthenospheric matter which exchanged with lithosphere in subduction zones related to the multiple crustal uprising. key words: seismic attributes, central tibet, indepth, moho, subduction. 156 manuscript received manuscript receiver: june 18th, 2008. accepted for publication: october 10th, 2008. resumen el análisis de atributos sísmicos es aplicado a perfiles sísmicos a través del programa indepth ii, que permite un mejor entendimiento de la estructura cortical profunda y la geodinámica de lhasa terrane en el área central del tíbet. los análisis indican que las anomalías de frecuencia se obtuvieron en el rango de 0-30hz. en algunas zonas analizadas de los perfiles sísmicos en las trazas han mostrado que la frecuencia anómala predominante es de10 hz, con poco contenido de otras frecuencias. la variación en la anomalía de frecuencia muestra ser el indicativo de la profundidad del moho en el área. profundidades estimadas del moho son muy irregulares y varían desde 20 kilómetros hasta más de 136 kilómetros. la corteza es más delgada en el sur, norte y este del área, mientras que es más gruesa en el oeste, con una naturaleza simetría en la parte central. esta variación podría ser el resultado de los complicados procesos tectónicos de las placas de la india y eurasia. concentraciones en la parte baja de la distribución de frecuencias y similar tipo de distribución de frecuencia se observan en las partes someras y profundas, en una de las secciones analizadas. esto podría indicar el ascenso de material astenosférico que se intercambió con la litósfera in las zonas de subducción relacionadas al múltiple ascenso de la corteza. palabras clave: atributos sísmicos, tibet central, indepth, moho, subducción. 1. introduction tibet is often called the roof of the world, that comprises table-lands averaging over 4,950 m above mean sea level with peaks at 6,000 to 7,500 m, including themount everest. it is bounded on the north and east by china, on the west by the kashmir region of india and on the south by nepal, bangladesh and bhutan. most of tibet sits atop a geological structure known as the tibetan plateau which includes the himalaya and many of the highest mountain peaks in the world. the plateau was formed by the continuous northward movement of indian plate as well the collision of it with eurasian tectonic plates in the cenozoic period (approximately 55 million years ago). this process is still ongoing. in the process of continental evolution, continent-continent collision substantially affects the earth’s structure. the plateau is considered as an optimal place for studying the basic problems of continental dynamics such as continent-continent collision, convergence and subduction of plates, and formation of the plateau (gao, et al, 2005). project international deep profiling of tibet and the himalaya (indepth) is an interdisciplinary program of geophysical and geological studies designed to develop a better understanding of the deep structure and mechanics of the himalaya-tibet region (brown, et al, 1996). the deep seismic reflection profiles showed fine crustal structure on both sides of the indus-yalu suture (iys), which has exerted a far-reaching influence on theworld. several deep seismic reflection profiles crossing the mountain-basin junction on the northern margin of the plateau were carried out by geophysicists of the chinese academy of geological science, which afforded an effective basis for studying the deep deformation process resulting from collision (gao, et al, 2005). the profiles in this plateau, however, were not intensively studied, due to cost. total length of these was just 1,300 km, 10% of that of the total deep seismic sounding. the moho depth is a significant indicator of crustal structure. the great thickness of the qinghai-tebet plateau is unique in the world. so far, seismicmethods have been used to explore themoho depth, among which deep seismic sounding (dss) is the most effective one. scientists have paid great attention to the changes in the depth of the moho from north to south across the qinghai-tebet plateau. xiong, et al (1985) and hirn, et al (1984) studied 157 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet such changes by using data obtained by the sino-french research project (1981-1982) and later some new probe sections were finished in northern tibet. it is shown that the moho is shallow on the northern and southern margins of the plateau, and deep in the central part (gao, et al, 2005). to date no complete active-source profiles has been done for the purpose of investigating crustal thickness, so the moho depth and its changes beneath the himalayas are still unclear (gao, et al, 2005). it has seen that moho depth varies remarkably in the area. this depths were estimated from various seismic data made by the sino-french project, indepth i, ii, iii projects, siling co-y’angamdo wide-angle seismic profiles, tuotuohe-golmud wide-angle seismic profile, blasting seismic profiles, etc. in various ways along with natural earthquake data. using natural earthquake data scientists are suggested that the thickness of the tibet plate is 120-100 km based on the surface wave dispersion of frequency and body wave inversion (song et al., 1985; zhuang, et al., 1992; chen, et al., 1985; wu, et al., 1998). on the other hand using various seismic profiles, some have concluded that the averagemoho depth of lhasa block is 55 km (hirn, et al., 1984) but others estimated the value at 70 km (xiong, et al, 1985). zhang, et al. (2001) hypothesised a down wrapping form of the moho in nothern tibet, and thought the depth to be at least 80 km. to the south of bangong-nujiang suture (bns), the moho is reportedly as deep as 80-84 km (zeng, et al, 1994). unfortunately, the indepth-ii seismic reflection profile was not able to detect the moho under the lhasa block (gao, et al., 2005). here is our effort to detect themoho in the indepth-ii seismic profiles using the seismic attributes analysis. attributes analyses are the different looks for studying geology of the earth. in the present days, seismic attributes analyses are now relatively standard techniques in the seismic exploration industry. the aim of this paper is to bring attributes analysis from industry to academic phase of deep seismic data for the estimation of moho depth in the central part (figure 1) of the tibet (longitudes 90o23’e to 90o48’e and latitudes 29o45’n to 30o19’n) in the indepth-ii seismic profiles. 2. material and methods 2.1 indepth seismic data the second phase of project indepth is cored by about 200 km of deep seismic reflection profiling in the summer 1994 (nelson, et al., 1996). the reflection profiling and associated wide-angle recording, spans over 300 km from the high himalayas in the south to the central tibetan plateau in the north. broad-band recording of teleseismic events and geologic field mapping have made up the rest of the project activities. seismic crew of the ministry of geology andmineral resources (mgmr) of the people’s republic of china acquired cmp reflection data along nine profile segments with a total length of about 250 km stretching from the crystalline himalaya northward into the tibetan plateau. explosive sources, mostly boreholes containing 50 to 200 kg dynamite, were triggered every 200 m along the profile and recorded by a 6 km-long, 240-channel receiver spread. data have been processed to produce 15-fold stacked sections of the line segments (brown, et al., 1996; alsdorf, et al., 1998 a, b). in this work, the analyses have been made for the unmigrated seismic line segments of tib 6, tib 7, tib 9 and tib 10 of the yangbajain graben and nyinzhong graben of lhasa terrane. the processed 15-fold stacked unmigrated deep seismic reflection data are shown in figures. 2-5. 2.2 seismic attributes analysis seismic attributes like amplitude, phase and frequency attributes analysis can play important roles to study the crustal structure of the earth. computation of complex attributes is basically a transformation that splits apart the amplitude, phase and frequency information into separate displays. frequency attribute is the instantaneous frequency of a time signal or the way of time frequency representation. time frequency representation can be made by sinfit 158 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam 159 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet figure 2. 15-fold stacked unmigrated seismic section of cmp profile tib 6, yangbajain graben, lhasa terrane. figure 1. location map of the study area, blue dark lines indicate the project indepth cmp profiles in the northern yadong-gulu rift (after brown, 1994). (hardy, et al., 2003), aok (steeghs and drijkoningen, 2001), stft (chakraborty and okaya, 1995), zero-crossing and complex trace analysis (taner, et al., 1979) methods. aok and stft methodsan extension of fourier analysis, can estimate the local frequencies but the correct window length must be chosen for better response. fourier analysis (press, et al., 1992) can estimate the multiple frequencies of a signal but it is based on the assumption that frequency does not change over the time. cohen (1995) describes the tradeoff one mustmake between time resolution and frequency resolution for short time fourier transforms i.e. time or frequency resolution depends upon window length. complex trace analysis, not dependent upon window length, in an alternative way is seemed better. trace analysis is 160 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam figure 3. 15-fold stacked unmigrated seismic section of cmp profile tib 7, yangbajain graben, lhasa terrane. figure 4. 15-fold stacked unmigrated seismic section of cmp profile tib 9, nyinzhong graben, lhasa terrane. used in this work for the computation of seismic attributes. trace analysis treats a seismic trace, y(t) as the real part of an analytic signal or complex trace, y(t) (taner, et al., 1979). the complex trace can be written as (hardy, et al., 2003): y t y t jy tr i( ) ( ) ( )= + = + ´ = òy t j s t d a t e j t ( ) ( ) ( ) ( ) p t t t pf2 (1) where the imaginary part, , is given by the hilbert transform y t s t di ( ) ( ) = ´ -ò 1 p t t t (2) the above integration implies taking the caucy principle value of the integral. the instantaneous amplitude and phase are a t y t y tr i( ) ( ) ( )= +2 2 (3) and f(t) y t y t i r = æ è ç ç ö ø ÷ ÷ -tan ( ) ( ) 1 (4) the instantaneous frequency, f(t) is the rate of change of the time-dependent phase: f t d t d t ( ) ( ) ( ) = 1 2p f (5) 2.3 attributes analysis of test signal seismic attributes can be computed from above mathematics. the algorithms along with the programming codes have been developed in this work using the eqns. 1-5. the developed programs or tools are capable to compute amplitude, phase and frequency attributes of signals. effort has been made to show how these tools work with examples. the computed amplitude, phase and frequency attributes of the signals are shown in figures 6-7 respectively. results have shown accurate attributes estimation. frequency attributes are thought to be most useful for the interpretation of subsurface geology. therefore frequency attributes are estimated in the following section for the geologic interpretation of deep seismic data (figures 2-5) 3. frequency attributes analysis of indepth data and interpretation frequency attributes of the indepth deep seismic profiles (figures 2-5) of the southern bns under the lhasa block are estimated and shown in figures 8-11. in each profile, the horizontal axis indicates the distance from southern end and vertical axis indicates the two way travel time in second. in the following, notable features are described from analyzed frequency attributes sections. frequency anomalies are found changes in between 0-30 hz according frequency scale, as shown in figures 8-11. there are some specific regions found dominated by 10 hz (with very little amount of other frequencies) are observed in the frequency attributes sections. the variations of frequency distributions are visible in the all estimated 161 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet figure 5. 15-fold stacked unmigrated seismic section of cmp profile tib 10, nyinzhong graben, lhasa terrane. attributes sections. it has seen a little variation around 5-6 sec all over the sections. it is observed that 15-20 hz frequencies are present at 5-6 second in all most all estimated frequency attributes sections. pocket like frequency distributions shown in rectangle a, b and c are present vertically downward in the tib-7 profile (figure 9). another interesting event, all most similar type frequency distribution is also observed in the attributes section (figure 9). frequency distribution of 25 hz with little amount of other frequencies is observed at higher and shallower depth (in the time scale) at a distance 18 km from south and 9 km from north end along the profile tib 7. these are separated by mainly 10 hz dominated frequency region. from the above observations, some additional crustal properties of the central tibet region are also interpreted as explained. the processed seismic indepth-ii profiles (figures 2-5) made by brown, et al (1996) have producedmany geologic features of the area. makovasky and klemperer (1999) made one of the important contributions from the analysis of the tibetan bright spots that 10% free aqueous fluid present at a depth of 15 km. seismic attributes analysis is well agreed with the tibetan bright spot made at 5-6 second (rahman, et al, 2006). the presence of frequency distributions (15-20 hz) in 5-6 second also indicates the same properties. though, the amount of 162 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam figure 6. a) test time signal, frequency 25 hz, 100 hz, 50 hz and 100 hz respectively in the time span 0.2-0.3 sec., 0.4-0.5 sec., 0.5-0.6 sec. and 0.8-1.0 sec and half amplitude in the time span 0.5-0.6 sec. b) amplitude attribute, c) phase attribute and d) frequency attribute (after rahman et al, 2006). 163 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet figure 7. a) test signal, keeping constant frequency (25 hz) with varied amplitude in the time span 0-0.3 sec, 0.3-0.6 sec and 0.6-1 sec., b) amplitude attribute, c) phase attribute and d) frequency attribute. figure 8. frequency attributes of tib 6, yangbajain graben, lhasa terrane. 164 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam figure 10. frequency attributes of tib 9, nyinzhong graben, lhasa terrane. figure 9. frequency attributes of tib 7, yangbajain graben, lhasa terrane. frequency distributions is not found approximately the same. it is varying and found maximum in the profile tib 10 (figure 11), therefore maximum free water should be present beneath profile tib 10. brown’s (1996) processed section is so far failed to estimate the moho depth. in the present study the moho depths of the region from the same deep seismic profiles are interpreted. the variation of frequency distributions marked by line (figures 8-11) in the attributes sections, is thought to be the interface of the crust and the upper mantle i.e. moho. the sino-french team suggested a depth of 55-53 km for the moho beneath high himalayan organic belt, with gentle rise in its northern part based on the records along a profile with the shot point in tibet and the receiver in nepal (xiong, et al, 1985). the indepth-i seismic reflection profile gave a moho depth of 75 km. with its southern end at pagri, this profile crosses the high peak region of the himalayas, so 75 km may indicate the largest depth of the moho under himalayas (zhao, et al, 1993). however, there high-himalayan crystalline rocks on the southern slope of the himalayas, and therefore, some geophysicists thought that the depth of 75 km is consistent with the value of 75-78 km obtained by the sino-french project along the paiku co-puma yumgco profile (teng, et al, 1983; xiong, et al., 1985). pelkum, et al. (2005) considered that the moho depth is ~45 km from nepal to the southern high himalayas and reaches ~75 km beneath the tethyan himalyas. the moho is offset beneath the iys. hirn, et al. (1984) proposed a larger offset (~20 km), while xiong, et al. (1985) suggested a smaller offset ( ~10 km). at present, natural earthquake are used for the estimation of crustal thickness. song, et al. (1985) assumed that the thickness of the tibet plateau is 120 km, which is same as the findings of zhuang, et al (1992). chen, et al. (1985) reported 100 km by means of the surface wave dispersion of frequency, and wu , et al. (1998) estimated a thickness of 110 km based on body wave inversion. using frequency attributes analysis, the maximum and minimum depths of the moho beneath the central tibet area are estimated at higher than 136 km and 20 km respectively. average rms velocities are estimated to obtained the moho depth from teng,’s et al. (1974) crustal velocity structure. the average 165 deep crustal structure and estimation ofmoho in the lhasa terrane of central tibet figure 11. frequency attributes of tib 10, nyinzhong graben, lhasa terrane. moho depth found varying significantly within the region. along the profile tib 6, moho depth is decreasing towards the north upto 22.5 km from southern end, later towards the end, moho depth is increased. tib 7 profile approximately parallel to the tib 6 shows higher moho depth (>136 km) at the beginning and decreases as well towards the north. tib 9 profile from south to north shows once again higher moho depth (>136 km), which decreases towards the north to a very shallower depth of only 20 km, later increases upto 66 km. while the tib 10 profile, perpendicular to the tib 9 shows the moho depth decreases from 120 km to 96 km. some pockets in the attributes distributions (marked rectangles a, b and c in figure 9) might be the indication of uprised asthenospheric matter which are appeared like vertically connected path from inner core to subsurface. irregular nature of the moho surface might be related to the multiple crustal uprising in the subduction zone and irregular ascending of the asthenospheric matter. tilman, et al., (2003) reported that the front of the indian lithospheric mantle was detached 100 km below the qiantang block, where a mass of asthenospheric matter ascended and was exchanged with the lithosphere. similar type of frequency anomalies are also identified in the attributes section (figure 9) at shallower and higher depths. this distribution property at shallower depth might be the indication of ascended asthenospheric matter. zeng, et al. (2003) observed multiple crustal subductions in the himalayas and southern tibet. the subducting indian crust must have split apart from its upper, and the indian lithospheric mantle subducted continuously in the upper mantle, which can be showed by geophysical evidence (wu, et al, 2004). xu, et al. (2004) reported that indian lithospheric slab the area is being subducted gently 400 km north of the iys, beneath the tanggulamountains. a large high velocity anomalous zone was split into separate high-velocity anomalous bodies, which may be considered as geophysical evidence for abruption brought about by indian lithospheric mantle subduction. the moho depth as appeared larger than 136 kmmight be another indication of subducting process of the indian plateau in the central tibet region. 4. conclusions the attributes analysis of the indepth deep seismic data acquired in 1994 provided a fine crustal structure mainly themoho offset around the southern part (in between bns and iys) of lhasa block. crustal structure indicates that it contains drastic structural variations. moho depths are varying from 20 km to more than 136 km. in general, the crust of qinghai-tibet plateau is thinner in its southern, northern and eastern parts but thicker in the western part. while asymmetrical crustal structure is found in the central part. drastic geologic settings or moho variations might be introduced for the following geologic process such as: initial convergence, deformation of tethyan himalaya, multiple phase emplacement, normal faultingwith strike slip etc. initial convergent is marked suture zone development and this suture zone demarked as the presence of the yzs is characterized by ophiolite i.e. segment of oceanic crust technically transported in suture zone. sediments of the tethyan himalaya is started to deforming and attaining the physiographic position along the series of thrust zones such as: main crystalline thrust (mct), main boundary thrust (mbt) and frontal himalayan thrust (fht). double thickness of the tibetan plateau has attained by multiple phases emplacement of the eurasian plates. tibet block has been inferred as a pericratonic segment of the eurasian plate and double thickness of this plateau is due to the oblique riding over subducted indian plate along thewhole of the trans himalayan belt. tibetan block has been affected by a north-north-west regional push of the indian plate resulted in the development of a right lateral slip motion along tiansan and corrcurum trans-himalayan mega slip zone. this horizontal motion is responsible for multiple phases detachment of tibetan block fromeurasian plate to attain double thickness. the ophiolites or genetically related extensile stress environment have emplaced tectonically during convergence. it is further interpreted that the northern boundary of tibetan block along kunlun and tangla is characterized by normal faulting with strike slip components. very uneven 166 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam moho structure in the central part may indicate further crustal changes in the area. acknowledgement authors acknowledge the project indepth team, especially dr. larry d. brown, institute for the study of the continents, cornell university, ithaca, n.y. for making data available through web site. references alsdorf, d., brown, l. d., nelson, k. d., makovsky, y., klemperer, s. l. and zhao, w. 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(1992). 3-d shear velocity model of crust and uper mantle beneath the tibetan plateau and its adjacent region, acta geophysica sinica. 35, no. 6, 694-709. 168 syed mustafizur rahman, rezaul islam, mumnunul keramat and md. sultan-ul-islam earth sci. res. j. vol. 11, no. 2 (december 2007): 155-162 self-potential signals caused by eruptions of the galeras volcano – colombia greinwald, s.1; ortega, a.2; rugeles, a.3; v. seth, ips, m.1 1 bundesanstalt für geowissenschaften und rohstoffe, stilleweg 2, 30655 hannover germany 2 instituto colombiano de geología y minería (ingeominas) colombia 3 universidad de nariño – colombia abstract the national institute of geology and mining, ingeominas – colombia, and the federal institute for geosciences and natural resources, bgr – germany, perform since 1997 as a joint venture multi-parameter measurements at the galeras volcano, in the southwest of colombia. since the end of 1998 the multi-parameter station at galeras (estación multiparámetro del galeras – emg) includes the continuous monitoring of electromagnetic variations. the electromagnetic (em) station is located at the north-eastern foot walls of the central cone inside the caldera. during almost six years of electromagnetic monitoring, the data did not show significant variations of the electromagnetic field, which could be related to the volcanic activity. in july 2004 a new active period of galeras began with two strong ash emissions. during both emissions strong self potential(sp) signals were recorded lasting for several hours. the present paper will present the data which could give some indications on the movements of liquids during ash emissions. key words: ingeominas; bgr; galeras; electronic monitoring. resumen el instituto nacional de geología y minas, ingeominas – colombia, y el instituto federal para las geociencias y recursos naturales, bgr – alemania, funcionan desde 1997 como una empresa de riesgo compartido tomando medidas multiparamétricas en el volcán de galeras, al sudoeste de colombia. desde finales de 1998, la estación del multiparamétrica del galeras (emg) incluye el monitoreamiento continuo de las variaciones electromagnéticas. la estación electromagnética (em) está situada en las paredes nororientales del cono central dentro de la caldera. durante casi seis años de supervisión electromágnetica, los datos no demostraron variaciones significativas del campo electromagnético, que se podrían relacionar con la actividad volcánica. en julio del 2004 un nuevo período de actividad del galeras comenzó con dos emisiones fuertes de ceniza. durante ambas emisiones, fuertes señales auto-potenciales (sp) fueron registradas por varias horas. este artículo presentará datos que podrían dar algunas indicaciones sobre los movimientos de los líquidos durante las emisiones de ceniza. 155 earth sciences research journal manuscript received october 13 2007. accepted for publication december 20 2007. palabras claves: ingeominas; bgr; galeras; monitoreamiento electrónico. introduction galeras volcano is located in the southwestern part of colombia, in the province of nariño, about 10 km to the west of the city of pasto, the capital of the province of nariño with about 350 000 inhabitants (fig. 1). galeras is a stratovolcano with a caldera of about 4.5 km diameter. a cone shaped crater with about the same height as the caldera rim is located at its center. the caldera is broken and open to the west. galeras is one of the most active volcanoes of colombia. in 1997 a cooperation project between bgr of germany and ingeominas of colombia started. aim of the project was a constant geophysical monitoring of the galeras volcano. the last eruption before occurred in 1993 and caused the death of 9 people which were inside the caldera at the time of the eruption. for that purpose a multi-parameter station was installed at the volcano comprising three broadband seismic stations, an em station inside the caldera, a gas-chemical station, a weather station and regular overflights with a thermalcamera. more information on the concept of the multi-parameter station can be found in seidl et al. (2003). from the start of the emg the volcanoes activity was very low and was further decreasing in the following years. since 2004, however, the volcanic activity is rising. strong external indications began in july 2004 with two ash fall emissions and since that time the activity level has increased. the data, which will be presented in this paper, are connected with the july 2004 emissions. description of the electromagnetic station inside the caldera of the galeras the em station was set up toward the north of the cone of the crater (fig. 2). it was located in a relative flat part at the foot wall of the active cone, at an elevation about 150 m below the rim of the caldera and about 120 m below the peak of the cone. the instruments are powered by batteries and solar panels and therefore independent of regular visits. the electric field of the ground is measured in two directions, north south (ns) and east – west (ew). the ns direction runs more or less perpendicular to the contour lines and in direction of the inclination of the cone walls at the point of the em station while the ew direction runs more or less parallel to the contour lines. the electrodes used were non-polarizable copper-sulphate electrodes. the electrode separation for each component was 100 m. in addition to the electric field, the three components of the magnetic field are recorded, ns, ew and vertical, using a fluxgate magnetometer, with a sensitivity of 0.1 nanot. the data of the two electric channels and the three magnetic channels are digi-tized with a 24 bit guralp digitizer with 50 samples per second for each channel (fig. 3) and are automatically transmitted to the observatorio volcanológico de pasto (ovp) and registered there on hard disc. description of the data figure 4 shows a continuous record of spdata from july 4th to july 25th 2004. only one interruption occurs, lasting from midday july 18th july 19th and was caused by a power failure due to ash fall after the first eruption. after clearing the solar panels the station worked again. the end of the measurements on july 25th was caused by the ash falls on the second eruption. at that time it was too dangerous to visit the station and clean it again. later on the station was completely destroyed by rock fall. the sp data of galeras contain a wide spectrum of time variations. the main sources for these variations are changes of the magnetic field in the ionosphere caused by solar activity. due to the position of the galeras, just about 1° north of the equator, the magnetic field shows constantly and very strong the 24 hour period of the equatorial electro-jet. many other effects induced by the ionosphere are also clearly visible. the average amplitudes of the daily variations are less than 100 µv/m. all short period variations (shorter than 3 minutes), however, are eliminated in the self-potential signals caused by eruptions of the galeras volcano – colombia 156 greinwald et al. esrj vol. 11, no. 2. december 2007 157 figures presented, which are based on data with a reading frequency of one sample per minute. other signals visible all the time are cultural effects caused by technical installations inside the city of pasto in a distance of less than 10 km or by a power line which leads up to the top of the galeras to feed a tv transmitter and some communication systems of the colombian army and police. these signals show constantly irregular variations. during night time the amplitudes are much smaller than during the day due to the reduced human activity. figure 5 represents the changes of the magnetic field for the same time period. here the all dominating effect is the equatorial electro jet. all cultural effects are much smaller than in the electric field. one effect deserves being mentioned: the sudden and strong change of the amplitude in all components on july 16 about one hour before the beginning of the volcanic eruption at that day. it signals the start of a world wide magnetic storm. figure 6 shows a comparison of the magnetic field of this day recorded at the galeras em-station and at the magnetic observatory in kourou (french guyana). this sudden magnetic effect is visible clearly in both stations. while some authors, e. g. yoshino and tomizawa (1989) have reported of electromagnetic precursors to volcanic eruptions, this is an effect clearly without any connection to the eruption of galeras. signals during the ash fall emissions emission of july 16th (fig. 7) on july 16th 2004 the first ash fall emission occurred after a quiet period of more than 11 years. following a rise in the seismic activity, the first ash emissions started on july 16th at 14:01 utc. the seismic activity continued with a constant tremor for several hours. it is difficult to determine the exact end of the tremor, but most probably occurs at about 18:00 utc. in figure 7 the sp-signals are shown together with the signals of the seismic activity, calculated from the vertical component of the seismic station anganoy (location see fig. 2). this information is given to demonstrate the correlation be-tween the seismic and the electric effects. the sp-signals start at the same time as the seismic tremor, in both compo-nents of the electric data. at the ns-component follows a rise in signal amplitude of about 450 μv/m. it continues with a flat top for about 4 hours and finally decays for 8 hours. in the ew-component we see continuous changes of the signal amplitudes up to 100 μv/m. these are followed by a decay of voltage of 120 μv/m. at the same time (17:45 utc) both components of the electric field start to move towards there original level before eruption. the ew-component ends at the same level while the ns-component remains at an offset of about 200 μv. while the short-period features in both components look similar, the figure 1. map of colombia with the location of the galeras volcano in the southwest of colombia. main difference are the much stronger short time movements (periods less than 1.0 hours) in the ew-component. the sp-signals indicate an end of the emission of liquids and perhaps also of the ash eruption at about 17:45 utc. emission of july 21th (fig. 8) the second ash fall on july 21th starts with a sudden rise in the seismic activity at 21:16 utc. the seismic activity stays at a high level until it stops at 3:25 utc on july 22th. this seismic activity consists of a continuous series of tremor pulses. the behaviour of the sp-channels during this ash fall emission appears quite different from the first one. while the start of the seismic activity does not seem to influence visibly the nscomponent, the ew-component shows a strong spike like feature. immediately after the spike the ew-component starts with small short period variations (periods of less than 0.5 hour). at about 22:00 utc additional medium frequency signals are visible (period 1.0 – 1.5 hours). these voltage variations stop about one hour after the end of the high seismic activity level. the total effect in the ew-component appears similar to the one of this component during the first eruption, beginning at a certain level followed by a series of up and down movements and finally ending at more or less the same level as before the event. this time the ns-component shows smaller amplitudes than the ew-component(about half the size compared with the the first eruption). further the component does not mark an increase of the measured voltage during the first part of the eruption but stays at a relatively low level, however, showing medium period voltage changes (period about 1 hour). only at relatively late time during the eruption a sudden rise in voltage occurs. at the end of the high seismic activity level the voltage begins to decay to low values for about five hours down to a level similar to the one at the beginning of the ash fall. results sp-data from a period of almost one month is shown. during this time only two data intervals have been recorded which show in a very obvious way the influence of the volcanic activity. during several years of measurements at the galeras such effects never have been observed on the quiet volcano. these anomalies show very strong amplitudes up to 500 µv/m (compared with about 100 µv/m without such effects). they are strongly correlated with the level of seismic activity. they start at the same time, however end with a certain delay to the seismic signals. the behavior of the sp-signals can possibly be ex-plained by the movements of liquids during the emission. one indication of this is that the shorter sp-variations are associated with the high seismic activity levels. the delay of the spsignals beyond the end of the seismic activity level could be due to the time delay until the movements of liquids has settled after the last transportation of water or vapor to the surface. the differences in the sp-signals between the two ash fall emissions are probably due to the different seismic activity levels which we can seen in the seismic data. at the moment we have very little information on the course of the eruptions and its correlation with the seismic activity. an interesting feature is the different behaviour of both sp-components. this could be caused by the direction of the two components of the sp-measurements in relation to the inclination of the walls of the cone through which the eruption rises and which influences the flow of water during and after the eruption. while the ns-component is more or less in the direction of the inclined cone wall at its lower end, the ew-component runs more or less perpendicular to this geological feature. and therefore the movements of liquids come to a stop after the last emission of liquids earlier. discussion and conclusions it is too early to draw final conclusions from the data we have seen. firstly the evaluation of the seismic and sp-data has not reached its final stage, secondly the correlation between the course of the eruptions and the seismic activity and the spsignals is not too well known. further research in this field is necessary. however, at first sight it seems that the sp-signals could contribute to a better representation of the course of an eruption in the measured data than the seismic activity data alone. aims of our further investigation will be the search for strong correlations between the volcanic activity and the changes of the sp 158 self-potential signals caused by eruptions of the galeras volcano – colombia 159 greinwald et al. esrj vol. 11, no. 2. december 2007 figure 2. map of the galeras caldera with the location of the em station and the seismic station in relationship with the main crater of the volcano. fluxgatemagnetometer+ calibration digitizer+ data transmitter em-station antenna em station antenna ovp data receiver and data storage ovp b ns b vert e ew e ns figure 3. schematic diagram of the em station at the galeras. the distance between the station and the observatorio volcanológico de pasto (ovp) is approximately 10 km. 160 figure 4. sp-signals from the em station inside the caldera of galeras. the upper curve represents the nscomponent, the lower one the ew-component. between july 4th and july 25th of 2004 only two periods show strong signals. both are related to ash emissions from the galeras. figure 5. variations of the horizontal magnetic field at the em station at galeras. the constant strong variations are caused by the equatorial electro-jet. the upper curve represents the ns-component, the lower one the ew-component. the amplitudes are given in nanot. self-potential signals caused by eruptions of the galeras volcano – colombia 161 greinwald et al. esrj vol. 11, no. 2. december 2007 figure 6. magnetic variations from the galeras em-station and the kourou observatory on july 16th , 2004. the strong sudden changes at about 13:00 utc represent the beginning of a worldwide magnetic storm and are not related to volcanic activity. the amplitudes are given in nanot. figure 7. . detailed representation of the sp-effects during the eruption of july 16th. the upper diagram shows the ns-component, the center diagram the ew-component of the sp-measurements. the lower diagram contains the seismic activity level for the time presented. remarkable is the very different behaviour of the two sp-components and the good correlation between the begin of the seismic and electric activity. 162 figure 8. plot of the sp-effects during the eruption of july 21th. the diagram follows the same order as in figure 7 with the ns-component at top, the ew-component at the centre and the seismic activity level in the lowest plot. again the two components show a characteristically different behaviour. self-potential signals caused by eruptions of the galeras volcano – colombia signals as they are demonstrated in the literature e.g. byrdina et al. 2003. references byrdina, s.,friedel, s., wassermann,j., zlotnicki, .j, 2003. self-potential varia-tions associated with ultra-long-period seismic signals at merapi volcano. geo-physical research letters, vol. 30, no.22 zlotnicki, j.,le mouel, j.l., sasai, y., yvetot, p., ardisson, m. h.; 2001. self-potential changes associated with volcanic activity. short-term signals associated with march 9, 1998 eruption on la fournaise volcano (réunion is-land). annali di geofisica, vol. 44, no. 2., april 2001. seidl, d., hellweg, m., calvache, m., gomez, d., ortega, a., torres, r., böker, f., buttkus, b., faber, e., greinwald, s. ; 2003. the multiparameter station at galeras volcano (colombia): concept and realization. journal of volcanology and geothermal research 125, is. 1-2. yoshino, t. and tomizawa, i., 1989: observation of low-frequency electromagnetic emissions as precursors to the volcanic eruption at mt. mihara during november, 1986. physics of the earth and planetary interiors, vol. 57, nos 1-2. claudiabravo.indd 132 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 132-138 (manuscript received august 2005, paper accepted november 2005) wave equation datuming to correct topography effect on foothill seismic data claudia lily bravo niño1 and luis alfredo montes vides2 1 sandox co. bogotá. colombia. e mail: sandox@col.net.co 2 universidad nacional de colombia. departamento de geociencias. e mail: lamontesv@unal.edu.co resumen en el procesamiento convencional se aplican las correcciones estáticas para atenuar el efecto de la topografía, basado en la suposición de una velocidad en la capa superficial mucho menor que la del substrato, lo cual hace que los rayos viajen en una trayectoria cercana a la vertical. sin embargo, cuando el contraste de velocidades entre esas capas no es grande, las trayectorias de los rayos que emergen no son verticales, por lo que las correcciones estáticas posicionan erradamente los reflectores. una mejor alternativa para corregir este efecto es continuar el campo de onda hasta una superficie plana (datum), porque no asume una trayectoria vertical, solucionando la ecuación de onda acústica para extrapolar fuentes y receptores. para conocer el desempeño del algoritmo de kirchhoff se usó para continuar el campo de onda de datos sintéticos. primero se continuaron fuentes y receptores hasta la base de la capa meteorizada y luego hacia arriba hasta un datum plano. comparando los resultados se observa que la solución por continuación del campo de onda provee un realce notable de los reflectores, que exhiben una mejor continuidad y un incremento en el contenido de frecuencia. palabras clave: campo de onda, topografía rugosa, superficie de referencia, trayectoria del rayo, continuación del campo de onda, extrapolación, integral de kirchhoff. abstract the current seismic processing applies static corrections to overcome the effects associated to rough topography, based in the assumption that velocity in near surface is lower than in the substratum, which force going up rays travel near to vertical. however, when the velocity contrast between these layers is not large enough, the trajectory of the up going rays deviate from vertical raveling the reflectors erroneously. a better alternative to correct this is to continue the wave field to a datum, because it does not assume vertical ray trajectory and solves the acoustic wave equation to extrapolate sources and receivers. the kirchhoff approach was tested in synthetic shots continuing their wave field to a datum and finally it was applied instead of static corrections in real data acquired in foothill zones. first shot and receiver gathers were downward continued to the base of weathering layer and later upward continued to a final flat datum. comparing the obtained results we observed that continuation approach provides a noticeable enhancement of reflectors in seismic records, displaying a better continuity of the reflectors and an increment in frequency content. 133 wake equation datuming to correct topography effec on foothill seismic data key words: wave field, rugged topography, datum, ray path, wave equation datuming, extrapolation, kirchhoff integral. © 2005 esrj unibiblos introduction in exploration foothill zones, rugged surface cause damage in seismic data quality. statics corrections have been the conventional technique proposed to overcome this problem. in another approach, berryhill presented the concept of wave field continuation, named wave equation datuming (wed) based in an extrapolation scheme using the kirchhoff integral, applying it to poststack (1979) and prestack data (1984). other solutions emerged based on finite difference (claerbout, 1985) and phase-shift (gazdag, 1978). reshef (1991) gave a datuming principle that permits direct prestack migration of data recorded on a not flat topographic surface. yilmaz and lucas (1986) replaced a layer of arbitrary shape and known velocity with another one with different velocity in order to eliminate bending trajectories in the area with severe velocity contrast. bevc (1997) upward continued the data from the surface to a flat datum above the highest topography using a constant velocity simulating a flooded topography effect without requiring a prior knowledge of the replacement velocity. hindriks and verschuur (2001) presented a redatuming approach based on shot gather amplitude using the cfp (common focus point) technology. benxi and others (2004) presented a novel wave equation datuming method which just uses a single shot gather to downward extrapolate the receivers and vertically time shift the source from the surface to the datum, using the near surface velocity model and depth-velocity curve at the source location respectively. recently effects of topography in foothill data have been studied (malaver et al., 2005; montes et al 2003) and new approaches have been proposed to increase quality in seismic data recorded in these regions (montes et al. 2005). wed approach was applied to a seismic line acquired in region with rough topography and processed using similar seismic steps applied in a flow with static corrections, the obtained results showed an improvement in image quality and a increment in frequency content. static corrections its objective is determine the arrival times of the reflections that could be observed if all measures were done on a flat datum without weathering layer. so, time corrections are applied to data recorded in a, b and c positions of figure 1 to convert them in traces that should be observed if the data were registered in a d , b d and c d points on flat datum. figure 1. static corrections: time correction from the surface to the bottom weathering layer and to the datum figure 2. observe the rays deviating from the vertical trajectory as assumed by static corrections. static corrections estimation needs the replacement velocity, especially if there is a significant change in elevation or in the base of the lvl, a wrong value can produce false structures in the final section. the 134 claudia lily bravo niño and luis alfredo montes vides replacement velocity normally is determined from well data, seismic refraction or tomography. the conventional static corrections apply vertical time shift assuming the waves propagate almost vertically due to the high velocity contrast between the near surface and the substrate as seen in figure 2. wave equation datuming the wed can be done through extrapolation methods as kirchhoff integral which is a solution of the acoustic wave equation, the integral equation corresponds to: , , cos p x z t da r2 1 j 0 2= r i ^ h # , , , ,p x z c r c r t p x z c r i ij i ij0 0 0 02 2 + -x xb bl l; e p j is the scalar wave field propagating with velocity c /r cij =x and is the time used to travel from p i (x 0 , y 0 , z 0 ) on the input datum to the point p j (x, y, z) on the output datum. the objective is to calculate p j for all times at point (x, y, z) on an infinite flat surface a 0 as seen in figure 3. the wave equation datuming is a two-step process: first it is applied to all shot gathers and later to the receiver gathers, extrapolating shot and receiver gathers separately. observed in figure 5a recorded on a flat surface and in figure 5b affected by topography. this dataset was created using a complete processing sequence in seismic unix (the colorado school of mines), because promax does not include wave field continuation through kirchhoff integral. figure 3. using data recorded in an input datum the wave field value in an output datum is estimated. methodology and aplication a synthetic dataset was generated in a two layers model (figure 4), 180 raw records with 25 m shot interval and 8 ms sample rate were obtained. the shots were upward continued to a rough surface through wed, simulating seismic data recorded in foothill zones, one of these synthetic shots is figure 4. two layers model used to generate synthetic data. figure 5. (a) synthetic shot registered on a flat surface, (b) synthetic shot affected by topography. then, the shots were continued to a plane datum. the shot 400 generate on the flat surface is observed in figure 6a, this shot affected by topography is shown in figure 5b and the shot after wed applied to eliminate the effects introduced by the topography is observed in figure 6b. comparing figures 6a and 6b, it is noticeable the kirchhoff integral performance to unravel the reflector. in the second step the wed approach was applied to a 2d seismic line of the colombian foothill area, with 140 m of topographic variations as can be observed in figure 7, 12 km length, 240 channels, 344 shots, 15 m group interval and 15 m shot interval. 135 wake equation datuming to correct topography effec on foothill seismic data (a) (b the stacked section furnished by the use of static corrections is showed in figure 11, there some structures and reflectors are observed, giving an approximation of the geologic model in the zone. at the top-left part of the figure 11 a package of reflectors is observed which conforms the crest of an anticline, being visible until 800 ms. but below the anticline is hard to visualize other geologic features, except between 800 and 1500 ms where there are weak evidence of reflectors. at the central part of the image an anticline structure is seen and in the right part the image is unclear. the stacked section of the figure 12 contains the stacked section obtained replacing wed instead of static corrections in the processing flow, where the reflectors of the left part are enhanced exhibiting a nice continuity also in deeper zone. the right part of the image displays more information content than figure 11, being notorious the anticline structure in the upper section that descends to create a syncline structure. besides, some diffraction points are observed in this central zone indicating the existence of a fault described in previously done surficial geological reports. the presence in figure 11 of unidentified seismic signals crossing the section dipping in both left and right directions are strongly attenuated in figure 12, allowing to depict the geological structure of zone below the foothill zone. conclusions in the conventional processing sequence, vertical static time shift is used to carry out sources and receivers to a flat datum, based in the assumption than the near surface velocity is smaller than the substrate, which is correct in some areas, where its use can correct the nmo curve distorted by the topography. however when the contrast is not great enough the ray path from substrate is not vertical. so, the conventional processing may conduce to wrong location of reflectors. the comparison between the shot gathers and the images obtained by the two considered approaches indicates that the application of wed to attenuate the effect introduced in seismic records by rough topography provides images with higher information content that those images furnished when static corrections are applied in this kind of zones. the nmo curves corrected by wed applications ensures that others methods based in this assumption figure 6. (a) synthetic shot registered in flat surface, (b) shot after corrected by wed. figure 7. topography of the acquired seismic survey the line was processed using conventional sequence in promax including filters, gain, amplitude recovery, static refraction, nmo, residual static to obtain finally a stacked section. the line was processed using a second processing sequence, but instead of use static corrections wed was applied using seismic unix (cwp-su). the difference between the images provided by the two processing sequences is associated to this different step. discussions the raw shot 78 with geometry assigned is showed in figure 8, the same shot after applied static corrections is observed in figure 9 whereas the shot after applied wed is displayed in figure 10. comparing the images in figures 9 and 10 it is noticeable the difference among them. in the figure 9 it is hard to appreciate any defined reflector, whereas in figure 10 several reflectors observed clearly around 1000, 1600, 2000 and 3000 ms, in addition other events with limited continuity are observed. similar results were obtained in others shot gathers, confirming as expected, that the wed enhance the seismic information contained in a record acquired in zones with rough topography. 136 claudia lily bravo niño and luis alfredo montes vides can be correctly applied or others analysis like avo provides information consistent with their own hypothesis. the stacked sections obtained with wed are structurally consistent diminishing the interpretation risk. otherwise although the shot gathers provided by wed should be processed using a different flow sequence to obtain a clearer image, in order to compare the methods performance they were submitted to the same sequence. the wave field continuation method is a solution of the wave equation which allows to sources and receivers at whatever datum, without assumptions about the velocity contrast in shallow layers model. this method has into account the ray path when the velocity contrast between the substrate and the near surface is small, validating the snell law. references • benxi ke, bo zhao, chaoying liu and yunfeng fang. 2004 wave-equation datuming based on a single shot gather. seg expanded abstracts ann. met.23, 2156. • berryhill, j. r., 1979, wave equation datuming. geophysics 44, 1329-1344. • berryhill, j. r., 1984, wave equation datuming before stack. geophysics 49, 2064-2066. • bevc d.,1997, flooding the topography: wave equation datuming of land data with rugged acquisition topography. geophysics 62, 1558-1569. • claerbout, j. f., 1985, imaging the earth’s interior: blackwell scientific publications. • gazdag, j., 1978, wave equation migration with the phase shift method: geophysics 43, pag. 1342-1351. • hindriks c. o. h. and verschuur d. j. 2001, an imaging approach to complex near surface effects eage 2001 p-163 63rd conference & technical exhibition —amsterdam, the netherlands. • malaver l. and montes l. 2005. elastic approach to remove surficial waves from shot gathers. earth sciences researh journal. vol. 9. no. 1, 65-69. • montes l ,vargas c and pérez g. 2005 modeling and removal of back-scattered noise from rough topography in land seismic data, ct&f vol. 3, núm. 1, 131-138. • montes l., quintana r., cespedes s., espíndola n., salinas t. and pérez g. 2003. attenuation of the seismic dispersion associated to foothills topography: application to real data. ct&f vol. 2, núm. 4, 7-12. • reshef, m., 1991, depth migration from irregular surfaces with depth extrapolation methods (short note): geophysics 56, pag. 119-122. • yilmaz, o. and lucas d. 1986. prestack layer replacement. geophysics 51, 1355. figure 8. shot 78 with geometry assigned. 137 wake equation datuming to correct topography effec on foothill seismic data figure 9. shot 78 after applying static corrections. figure 10. shot 78 after applied wed. 138 claudia lily bravo niño and luis alfredo montes vides figure 11. stacked section obtained with static corrections. figure 12. stacked sections provided by the use of wed. analisis de las condiciones que favorecen el desarrollo de la actividad convectiva de las tormentas electricas en la saban a de bogota (*) gerardo de j. montoya gaviria profesor asociado, departamento de geociencias-universiidad nacional de colombia jesus antonio eslava ramirez profesor titular, departamento de geociencias-universidad nacional de colombia montoya g & j. eslava. (2000): analisis de las condiciones que favorecen el desarrollo de la actividad convectiva y de las tormentas electricas en la sabana de bogota. geofis. colomb. 4:23-30. issn 0121-2974. santa fe de bogota, d.c. colombia. resumen la predicci6n de la actividad convectiva en los tr6picos es un problema aun sin resolver. las dificultades para resolverlo estan asociadas con el hecho de que este fen6meno se presenta a menudo como resultado de una compleja interacci6n entre caracteristicas locales y condiciones en el flujo de gran escala. en este trabajo, se analizan registros sobre dias con ocurrencia de tormentas electricas conjuntamente con datos de superficie y de altura de aproximadamente 10 alios, con el objeto de determinar las condiciones tanto de grande escala como de escala local que favorecen el desarrollo de la actividad convectiva en la saana de bogota. como resultado principal del trabajo se concluye que, de acuerdo a los reqistrc... correspondientes a la superficie isobarica de 700 hpa, las tormentas electricas en esta region se presentan con vientos debiles y alto contenido de humedad, situacion que es propia de la zona de convergencia de los alisios y del brazo ascendente ecuatorial de la celda de hadley. el analisis muestra adernas que, la conveccion local 0 terrnica reforzada por el efecto oroqrafico, sirve de mecanismo disparador de las tormentas electricas en la sabana de bogota. estas conclusiones explican el origen y desarrollo de este fen6meno y permiten dar algunas recomendaciones para su diagn6stico y pron6stico. finalmente, se sugiere un modelo de circulaclon del aire durante el desarrollo de la actividad convectiva en la sabana de bogota. abstract convective activity prediction in the tropics are still an unsolved problem. difficulties are mainly related to the fact the phenomenum, very often results as a complex interaction between local and large scale stream conditions. days with thunderstorm activity compared with surface and height data for 10 years are presented in this paper in order to determine both, large and local scale conditions that propitiate the developing of convective activity in the bogota sabana. as a main result it is shown that at the 700 hpa isobaric surface, electrical storms appears whit weale winds and high contents of humidity, which characterize the alisios convergence zone and also the ecuatorial ascendent branch of the hadley cell. besides, the analysis shows that local thermal convection strengthened by orograhapic effect are the trigger mechanism for electrical storm in the "sabana de bogota".these conclusions explain the origin and develop of the phenomenum and allow to give some recomendations for its diagnostic and forecasting. finally an circulation model during the developing of convective activity in the "sabana de bogota". (*) trabajo realizado dentro del marco del grupo de investigaciones en meteorologia-u.n., que cuenta con el apoyo financiero de colciencias y el bid, contrato colciencias-u.n. no.562/98. 1. introduccion la actividad convectiva es el elemento mas importante para predecir en los tr6picos. esto se debe a que primero; la mayor parte de la precipitaci6n registrada durante el ano es originada por nubes convectivas. segundo, muchos fen6menos adversos, tales como tornados huracanes, tormentas electricas, etc., estan asociados con este fen6meno. por ultimo, la convecci6n tropical juega un papel preponderante en la circulaci6n general de la atm6sfera tropical y la formaci6n de las condiciones del tiempo para pron6stico a largo plazo y predicci6n del clima. en particular, en la sabana de bogota, el 21 % de los dias del ario ocurren tormentas electricas causando trastornos, en el transporte, en la red de distribuci6n de energia electrica, etc. por esc, es de gran importancia el mejoramiento de los metodos para pronosticarlas. para mejorar el pron6stico de iluvias convectivas y de tormentas electricas es importante analizar las condiciones atrnosfericas en las diferentes escalas que determinan el origen y desarrollo de estos fen6menos. en la gran escala es conocido por ejemplo, que la circulacion de la atm6sfera en la regi6n tropical y en latitudes medias es bien diferente. mientras que las ultirnas son caracterizadas por un flujo medio cuasihorizontal dirigido de oeste para este, en la regi6n tropical predominan los vientos alisios de ambos hemisferios. estos vientos se encuentran en una franja estrecha liamada zona de convergencia intertropicalzcit, propiciando alii el desarrollo de la actividad convectiva. ei estudio de la zcit ha sido objeto de innumerables contribuciones. no obstante, muchas preguntas aun estan sin resolver. por ejemplo, una primera hojeada de los mapas medios circunglobales rnuestra que la zona de convergencia de los alisios en general coincide con otras bandas de valores extremos tales como; minimo de presi6n, maximo de temperatura de la superficie del mar, y rnaxirnos de convergencia, nubosidad y precipitaci6n. (ver por ejemplo, hastenrath, 1995). lo que no esta muy claro desde el punto de vista fisico, ya que no es facil concebir como el maximo de la temperatura en la superficie del mar obtenida por calentamiento debido a la radiaci6n solar, puede coexistir simultaneamente con maximo en nubosidad; por ejemplo. estudios de mas alta resoluci6n, realizados por hastenrath & lamb (1978), indican que la zona de maxima convergencia, nubosidad y precipitaci6n se encuentra distante del eje de discontinuidad del viento, (definido como minimo en velocidad y persistencia), hasta 350 km en el mar y hasta 1000 km en el continente africano. sin embargo, aun no es claro el mecanismo fisico responsable por esta separaci6n. aunque la zcit es el sistema pluviogenetico mas importante en colombia y en especial en su zona andina, snow (1976), no es el unico. en la costa pacifica colombiana por ejemplo, gran parte de las liuvias acaecidas durante el afio son ocasionadas por complejos convectivos de mesoescala, mesa et al. (1997). otros sistemas productores de liuvia en colombia son: las ondas del este, la brisa marina, las vaguadas asociadas con frentes frios de los hemisferios norte 0 sur y esporadicarnente los huracanes tropicales. las liuvias en colombia, especialmente las de origen convective. a menudo se presenta acornpariadas de tormentas electricas en la mediana y pequena escalas la interacci6n entre los vientos alisios y la cordillera de los andes ha sido poco estudiada. se sabe por ejemplo, que la interacci6n entre los vientos provenientes del pacifico, el calentamiento de la superficie y el factor oroqrafico de la cordillera occidental genera un alto grado de inestabilidad atmosferica provocando fuerte convecci6n a 10 largo de la cordillera occidental colombiana, emmanuel (1994). estos sistemas liuviosos penetran al interior del pais, por la costa pacifica liegando muchas veces mas alia de medellin, velasco & fritsch (1987), nieuwolt (1977). la sabana de bogota, objeto de este estudio, ha sido aun menos estudiada. se sabe por ejernplo, que ella es influenciada en mayor forma por los alisios del atlantico que del pacifico. sin embargo, poco se conoce sobre la interacci6n de estos vientos con factores locales tales como el calentamiento superficial y la orografia en el desarrollo de la actividad convectiva en esta regi6n. ei objetivo de este estudio es analizar la influencia del flujo de los alisios, de la zcit y de factores locales tales como la orografia y el calentamiento superficial sobre el origen y desarrollo de las tormentas electricas en la sabana de bogota. se espera que los resultados de este trabajo sirvan para esclarecer algunas de las caracteristicas de la circulaci6n tropical no conocidas hasta ahora y dar recomendaciones para la predicci6n de liuvias convectivas y tormentas electncas en la sabana de bogota. los resultados de este trabajo tarnbien seran de utilidad para pronosticadores en el tr6pico y estudiosos de la circulaci6n tropical. 2. locallzacion del area estudiada y metodologia utilizada en el estudio la sabana de bogota esta localizada en el altiplano andino a 2547m sobre el nivel del mar y ocupa una extensi6n de 4305 km2 , aproximadamente. esta limitada a la derecha por el costado occidental del ramo oriental del sistema de cordilleras de los andes y a la izquierda por el caluroso y humedo valle del magdalena. los datos utilizados corresponden a la estaci6n aeropuerto el dorado, ubicada sobre la sabana en 4° 43' de latitud norte y 74° 09' de longitud oeste. los datos utilizados en este trabajo fueron: datos sobre direcci6n predominante del viento en superficie y correspondientes a un registro de 10 anos y publicados por barrero (1976), datos sobre altura del geopotencial, deficit de rocio y velocidad del viento correspondiente al nivel de 700 hpa y de los registros correspondientes a dias con ocurrencia de tormentas electricas. estos ultimos datos fueron suministrados por el instituto de hidrologia, meteorologia y estudios ambientales-ideam y, se sometieron a un control de calidad el cual consisti6 en plotear el pararnetro en secuencia temporal. los datos dudosos fueron desechados. 3. la influencia de los vientos alisios la influencia de los vientos alisios sobre la sabana de bogota se puede observar en la tabla 1, la cual contiene la direcci6n predominante horaria del viento en superficie en la estaci6n aeropuerto ei dorado. estos datos fueron publicados por barrero (1976) y corresponden a un registro de 10 aries de duraci6n. como se puede observar en esta tabla, la direcci6n del viento en la sabana de bogota prevalece del ne especialmente durante la noche y varia hacia el e y un poco hacia el se en las horas de la mariana y en los meses de la estaci6n seca. esta particularidad permite afirmar que la sabana de bogota se encuentra bajo el area de influencia de los vientos alisios. los vientos del oeste observados en la tabla 1, especialmente en las horas de la tarde, estan estrechamente relacionados con un fen6meno local el cual se analizara mas adelante. 4. la influencia de la zcit para determinar la influencia de la zeit en la formaci6n de tormentas electricas sobre la sabana de bogota se realize un analisis de los dias de ocurrencia de tormentas electricas, sequn registros del aeropuerto ei dorado y tarnbien correspondientes a un periodo de 10 arios. este analisis muestra que en la sabana de bogota se presentan, en promedio, 76 dias al ario con episodios de tormentas electricas, 0 sea el 21% de los dias del ario. en la fig.1a, se muestra el nurnero medio mensual de dias con tormentas electricas. en la fig.1, se observan 2 maximos durante el ario, uno en marzo-abril y otro en octubre-noviembre. 0 sea que la ocurrencia de tormentas electricas en la sabana de bogota acomparia el paso de la zcit sobre esta regi6n de la misma manera como 10 hace el cicio anual de la precipitaci6n (ver por ejemplo, montealegre, 1986.). en la fig.1b se muestra la distribuci6n horaria de las tormentas electricas en la sabana de bogota. podemos observar en esta figura que la hora de mayor frecuencia de tormentas electricas corresponde a las 13-14 horas 0 sea, coincidiendo con el momento cuando se registra la temperatura maxima. se puede entonces concluir que las tormentas observadas son en su mayoria originadas por la convecci6n local 6 termica. por otro lado, es importante resaltar que las horas con mayor frecuencia de tormentas electricas en la fig.1b coincide con la aparici6n de componentes oeste y resaltadas con negrita en la tabla 1. esta observaci6n puede ser explicada de la siguiente manera: durante las horas y en los meses de mayor actividad convectiva el flujo del este en la sabana de bogota es interrumpido para dar paso a las masas de aire calidas y humedas provenientes del valle del magdalena. este cambio de direcci6n del flujo durante las horas de desarrollo convectivo ha side constatado durante muchos aries por observadores y analistas en el aeropuerto ei dorado y tambien puede interpretarse como la influencia de la orografia en el desarrollo de las tormentas electricas en la sabana de bogota. 5. comportamjento de la presion, la velocidad del viento y la depresion del punto de rocio durante los periodos convectivos y no convectivos. adernas de estas caracterlsticas locales, para que la convecci6n hurneda profunda prospers, como se mencion6 anteriormente, se necesita la existencia de ciertas condiciones, en el f1ujobasico, favorables para su desarrollo. para investigar estas condiciones fueron analizados los campos de geopotencial, velocidad del viento y depresi6n del punta de rocio correspondientes a la superficie isobarica de 700 hpa, (aproximadamente 500-600m sobre la sabana de bogota). en la fig.2 se muestra la altura del geopotencial de la superficie de 700 hpa, correspondiente al sondeo diario ejecutado a las 12 utc y para el periodo comprendido entre 01-ene-1984 y el 31-dic-1985. en esta, y en las figuras siguientes, los 6valos rellenos con negrita serialan los dias con ocurrencia de tormentas electricas. tarnbien se puede observar que los dias con tormenta electrica se situan indistintamente a lade y lado del valor medio. en otras palabras, las tormentas electricas, sin preferencia alguna, suceden tanto para bajas como para altas presiones. esta observaci6n tambien fue caracteristica en otros periodos analizados. a pesar de que te6ricamente se deben esperar bajas presiones asociadas con la actividad convectiva y que los vientos alisios f1uyen desde las altas hacia las bajas presiones, (ver por ejemplo, philander, 1990), estas observaciones no 10 confirman, y por el contrario, corroboran una vez mas 10 afirmado por riehl (1979) en el sentido de que la presi6n atmosferica aporta poca informaci6n uti! para predicci6n del tiempo en los tr6picos. en las fig.3 y 4 se muestra el comportamiento de la velocidad del viento, (en nudos), y de la depresi6n del punta de rocio, respectivamente, dia por dia, y para un periodo de cuatro arios consecutivos. tabla 1. direccion predominante horaria en superficie, en la estacion aeropuerto el-dorado segun barrero (1976). los casos con componente oeste, (resaltados con negrita), corresponden a las horas y los meses con mayor actividad convectiva. mes/ ene feb mar abr may lun .lui ago sep oet nov die bora 00 ne ne ne ne ne ne ne ne ne ne ne n 01 ne ne ne ne ne ne ne ne ne ne ne ne 02 ne e ne ne ne ne ne e ne ne ne ne 03 ne ne ne ne ne ne ne e ne ne ne ne 04 ne ne ne ne ne ne ne ne ne ne ne ne 05 ne ne ne ne ne ne ne ne ne ne ne ne 06 ne ne ne ne ne ne ne ne ne ne ne ne 07 ne ne ne ne ne ne ne ne ne ne ne ne 08 ne ne ne ne ne ne ne ne ne ne ne ne 09 e ne ne ne ne e e e ne e e ne 10 e e e e ne e e e e e e ne ii e ne e e e e e se e e e e 12 e e e e e e e se e e e e 13 e w w e e e se se s w w w 14 se w w w e e se e w w w w 15 w w w w w e se se w w w w 16 w w w w sw e se e w w w w 17 e w w w w e e se w w w w 18 e w w w w e e e w w w w 19 w w w w w e e e e w w w 20 ne w ne e e ne e e n ne w ne 21 ne n ne e ne ne e e n e ne n 22 ne ne ne ne ne ne ne ne ne ne ne n 23 ne ne ne ne ne ne ne ne n ne ne n a) b) 14 ii) 200ii) 12 i1l i1l '0 '<3 10 c c ql 150e ... 8 ...... ::3 ::3 o 100u 6 0 0 ql ql 4 '0 "c 0 50 0 2 z z 0 0 """ t-0 c?> ,," "f? ,,<' 0 "c?> "v"-emeses horas figura 1: a) valor medio mensual de dias con ocurrencia de tormentas electricas y, b) frecuencia horaria de la ocurrencia de tormentas electricas, en esas figuras se observa que las tormentas electricas ocurren preferiblemente para velocidades inferiores a 5 nudes, (",,10 rn/s) y depresiones del punto de rocio por debajo de 2'c, a la altura de 500-600 m sobre la sabana de bogota. estas observaciones tam bien fueron caracteristicas en otros periodos analizados. esto nos permite concluir en general, y al menos cualitativarnente, que las tormentas electricas en la sabana de bogota se presentan con vientos debiles y alto contenido de humedad en relaci6n con los periodos cuando elias no tienen luqar. y tal como se afirm6 anteriormente. con el objeto de tener un estimativo de esta afirmaci6n, se calcul6 la frecuencia relativa fr, de ocurrencia de tormenta electrica por rangos de variaci6n de la depresi6n del punto de recto dr. y de la velocidad del viento w. los resultados se muestran en la tabla 2 y en la fig.5. tanto en la tabla 2 como en la fig.5, la mayor frecuencia de ocurrencia de tormentas electricas en la sabana de bogota. tiende a ubicarse en los primeros rangos. conclusiones ei anal isis de cerca de diez aries y medio de observaciones nos permite concluir que. las tormentas electricas no son un fen6meno apenas local como podria pensarse sino que, en general, se forman debido a una interacci6n compleja entre caracteristicas locales y condiciones en el f1ujo de gran escala. por un lado, elias son determinadas por alto contenido de humedad y vientos debiles, caracteristicas propias de la zona de convergencia de los alisios y de la celda de circulaci6n de hadley. por otro lado, el nurnero de tormentas electricas aurnenta, en la sabana de bogota, por la convecci6n local 0 terrnica, situaci6n reforzada por el efecto oroqrafico. ei efecto oroqrafico en este caso, consiste en el ascenso de las masas de aire candas y humedas del valle del magdalena sobre la sabana de bogota. podemos entonces sugerir el siguiente esquema de circulaci6n del aire durante el desarrollo de tormentas electricas en la sabana de bogota (ver fig.6). de acuerdo a esta representaci6n esquernatica. durante el periodo de desarrollo de las tormentas electricas, el f1ujo de los alisios es interrumpido en la sabana de bogota para dar paso al ascenso de masas calidas y hurnedas provenientes del valle del magdalena. otras conclusiones que se derivan de este trabajo son: para efectos de diagn6stico y pron6stico del tiempo el analisis de la presi6n no es importante perc si 10 son los campos de la humedad y el viento. utilizando la inercia de estos campos. es posible hacer uso del analisls para fines de pronostico a cortos plazos, es decir; a 6, 12 y 24 horas. para efectos de diagn6stico y pron6stico del tiempo es conveniente monitorear los centros de actividad convectiva en los tr6picos a traves por ejemplo de la temperatura de la superficie del mar. la precipitaci6n. la nubosidad, etc. debido a que, como fue mostrado, la actividad convectiva depende de los vientos y humedades superficiales, los modelos hidrodinarnicos para prediccion numerica mas efectivos serian aquellos que tuvieran en cuenta los procesos de la capa limite y la formulaci6n explicita de la convecci6n atmosferica. agradecimientos este trabajo fue financiado por cindec-un (proyecto no 803172). los autores agradecen al instituto de hidrologia, meteorologia y estudios ambientales-ideam por el suministro de los datos usados. referencias bibliograficas barrero, a. (1976): regimen de vientos en superficie de la sabana de bogota. himat, (documento interno). emmanuel, k.a.. (1994): atmospheric convection. oxford univ. press, new york. 580 pp. hastenrath, s. & p.j. lamb (1978): on the dynamics and climatology of surface flow over the equatorial oceans. tellus 30:436-448. hastenrath, s. (1995): climate dynamics of the tropics. kluber acad. publishers. dordrech, the netherlands, 488pp. mesa, s. 0., j.g. poveda, & l.f. carvajal. (1997): introducci6n al clima de colombia. univer. nal de colombia. fac. de minas. medellin. 390 pp. montealegre, j. e., (1986): distribuci6n espacial de la precipitaci6n en colombia. atmosfera 9:37-52. nieuwolt, s. (1977): tropical climatology. john wiley. 207 pp, philander, s. (1990): ei nino. la nina, and the southern oscillation. acad. press, 292 pp. riehl (1979): climate and weather in the tropics. acad press. 611 pp. snow, j. w .. (1976): the climate of northen south america. en: climates of central and south america (w. schwerdtfege. ed.). elsevier, amsterdam. pp.295-403. velasco, i. & m. frisch (1987): "mesoscale convective complexes in the americas. j. geoph. res. 92(d8): 95919613. ]i. m l t u 80 20 • 1984 1985 figura 2. altura del geopotencial en m (para obtener la altura real, sumar 3100m a la ordenada), correspondiente a la superficie isobarica de 770 hpa. los valores estan ploteados dia por dia, durante un periodo de dos an os consecutivos. los ovalos rellenos con negrita indican los dias con registro de tormentas electricas. u v i e 25 n t o 20 n 15 d o 10 5 1984 u v i e 25 n t o 20 n 15 d o 10 5 1986 1985 figura 3. como en la fig.2, para la velocidad del viento en nudos. 1987 d e p 2s r 20 o c 15 i o 5 5 d e p 25 r 20 o c 15 i o 10 figura 4. como en la fig. 2 para la depresion del punto de rocio. tabla 2. frecuencia relativa fr, de dlas con ocurrencia de tormenta electrica en la sabana de bogota, por rangos de variacion de la de presion del punto de rocio (dr) en °c y de la velocidad del viento (w) en nudos respectivamente y para un total de 699 dias registrados con torment a electrica. rango 2 3 4 5 6 7 8 9 10 dr 0.0-1.0 1.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-8.0 8.0-10.0 10.0-15.0 15.0-20.0 no 211 156 110 116 59 20 17 6 3 fr 0.302 0.223 0.157 0.166 0.084 0.029 0.024 0.008 0.004 0.001 w 0-2.5 2.5-5.0 5.0-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 no 111 297 133 110 29 13 3 2 0 fr 0.159 0.425 0.190 0.157 0.041 0.019 0.004 0.003 0.001 0.000 odr .vv 1 2 3 4 5 6 7 8 9 10 rangos figura 5. no. de ocurrencias de tormentas electricas por los rangos en depresion de rocio y velocidad del viento, respectivamente, definidos en la tabla 2. oeste este v. alisios o ~ .. • s ana de 80 'ow valle del agdale llanos orientales figura 6. representacion esquematica de la circulacion del aire en la sabana de bogota, durante el desarrollo de tormentas electricas. nataliapardoet al.indd 148 earth sciences research journal earth sci. res. j. vol. 9, no. 2 (dec. 2005): 148-164 (manuscript received march 2005 paper accepted october 2005) the paipa volcano, eastern cordillera of colombia, south america (part ii): petrography and major elements petrology natalia pardo villaveces1, josé maría jaramillo2, héctor cepeda3 1 universidad nacional de colombia. pobox 101130 bogotá, colombia. e-mail:azufralrouge@yahoo.es 2 latin america enterprise fund managers l.l.c e-mail: jjaramillo@laef.co 3 geological survey of colombia (ingeominasbogota). e-mail: hcepeda@ingeomin.gov.co resumen los productos del volcán de paipa (pardo et al., 2005) son principalmente depósitos de flujos piroclásticos de pómez y ceniza, domos y flujos pirolásticos de bloques y ceniza. se trata de riolitas alcalinas, traquitas alcalinas y riolitas calcoalcalinas altas en potasio. los datos químicos evidencian contenidos de sio2 entre 68 y 72%, y de álcalis (na2o+k2o) entre 7%-10%. los minerales esenciales son fenocristales y glomerocristales de anortoclasa, sanidina hasta de 1,5 cm y anortoclasas con núcleos de plagiocalasa; los minerales accesorios son biotita roja y hastingsita, mientras que los traza son augita, circón, esfena y magnetita. los cristales presentan texturas de desequilibrio, tal como bahías de disolución, bordes de corrosión y reabsorción, zonación normal, inversa, oscilatoria y en parches, así como bordes euhedrales intercalados con bordes fibrosos. en correlación con datos publicados sobre las rocas volcánicas de iza, la geoquímica de elementos mayors de las rocas volcánicas de paipa indican que los magmas alcalinos y ácidos que hicieron erupción en la cordillera oriental de colombia durante el neógeno son distintos a los magmas calcoalcalinos que hacen actualmente erupción en el arco activo de la cordillera central de colombia. aún hacen falta estudios detallados de la geología structural, geoquímica y geofísica para establecer el ambiente geodinámico que gobierna el vulcanismo plio-pleistoceno en la cordillera oriental. palabras clave: composición modal, composición normal, domos, erupción freatomagmática, flujos piroclásticos, ignimbritas, norma cipw, textura de pared de burbuja, pyroclastic flows, shards. abstract paipa volcano products are mainly pyroclastic pumice and ash flow tuffs, lava domes and pyroclastic block and ash flow tuffs. they are classified as alkaline rhyolites and trachytes and high-k calcalkaline rhyolites. chemical data show sio 2 values between 68 and 72%, and alkalis (na 2 o+k 2 o) content of 7%-10%. essential minerals are phenocrysts and glomerocrysts of anorthoclase, sanidine up to 1,5 cm and anorthoclase-mantled 149 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology plagioclase; accessory minerals are red biotite, and hastingsite while trace minerals are augite, zircon, sphene and magnetite. crystals have disequilibrium textures, such as dissolution embayments, corrosion and reabsortion borders, normal, inverse, oscillating and patchy zonation, together with fibrous borders intercalated with euhedral borders. in correlation with published data of iza volcanic rocks, paipa rocks chemical composition confirms that acid and alkaline magmas that have erupted in the eastern cordillera of colombia during the neogene are strongly different from the calc-alkaline magmas that erupt in the westward active arc (central cordillera). detailed structural, geochemical and geophysical research has to be done in future research to establish the geodynamic framework that governs the volcanism of the eastern cordillera. key words: bubble-wall texture, cipw norm, ignimbrites, lava-domes, phreatomagmatic eruptions, pyroclastic flows, modal compositions, normal composition, shards © 2005 esrj unibiblos. introduction the paipa volcano is located at the central part of the eastern cordillera of colombia (ec), in the department of boyaca (figure 1). it has a 3 km diameter caldera possibly related to the interaction of ne high angle faults parallel to the principal structures of the ec, which are mainly inverse with left-lateral component, and nw normal faults that cut the regional structures (pardo-villaveces et al., 2005). pardo-villaveces (2004) defined 14 eruptive units within two eruptive epochs: during the first eruptive epoch (fee) a volcanic caldera was formed and its final collapse occurred at the end. after a quiescence period, caldera resurgence occurred during the second eruptive epoch (see) with the formation of several vents inside it, most of them located at olitas headstream; lava domes, block and ash flow deposits, pyroclastic surges and ignimbrites interbedded with lacustrine siltstones and torrential oligomictic conglomerates remained restricted to the caldera depression, while ash fall tuffs mantled inflow and outflow zones (figure 2)(cepeda & pardo-villaveces 2004; pardo-villaveces et al., 2005). a pliocene and possibly pleistocene age was determined by ar/ar and k/ar radiometric methods: ar/ar analysis gave ages between 2,1 and 2,4 ma for the lava domes that were formed at the beginning of the see, while k/ar methods gave a 1,8 ma date for crystalline flow tuffs accumulated westward from the domes during the see also (cepeda et al., 2004; pardo-villaveces 2004; cepeda & pardo-villaveces 2004; pardo-villaveces et al., 2005). optical petrography and major elements geochemical data are here presented to illustrate the composition of paipa volcano products and to give an initial approach to some of the processes that possibly occurred during magma ascent and eruption. methodology during the field work (pardo-villaveces et al., 2005), we selected 19 samples for petrographic analysis, based on the minimal grade of meteorization and hydrothermal alteration; sites of sample selection are localized on the geologic map shown in figure 2. pyroclastic rocks were classified according to wohletz & heiken (1992) diagram. modal composition was given based on the phenocrist assemblage of each rock, together with their detailed description based on hand samples and thin sections. amphibole, biotites and feldspar compositions were determined by optical properties, such as relief, pleochroic colors, 2v angle amplitude, birefringence, extinction angles, optic figure and sign; for twinned plagioclase michel-levy plagioclase-determination method was used. based on petrography observations we selected five samples for total rock major elements chemical analysis that was done in ingeominas-bogotá laboratory. chemical classification was given by tas diagram (le maitre, 1989) and cipw norm was calculated using norma.xsl. these data were correlated with other 10 data obtained by ingeominas colombia geochemical map project to support our observations (cepeda & pardo150 natalia pardo villaveces, josé maría jaramillo, héctor cepeda villaveces, 2004). the petrologic interpretations and suggestions here presented are based on the 15 major oxides geochemical data and the following results are based on the volcanic stratigraphy determined and discussed in pardo-villaveces et al. (2005). results unit i.1 samples: the oldest ash flow tuff deposit was sampled in ppg-057 and ppg059 stations (figure 2). hand samples are white to red dense and welded pyroclastic rocks, poorly sorted, with up to 40 % of anorthoclase (kfs) and sanidine (sa) phenocrystals in a vitric-clastic ash matrix. feldspar crystals are oriented and aligned, giving a flow texture to the rocks, and are as large as 1,5 cm (figure 3). lithic fragments are accidental blocksized silicic and phosphoric siltstones, chert, green and gray phylites, rarely founded in finer sizes. in thin section (figure 4) these rocks were classified as crystalline-vitric to crystalline pyroclastic flow tuffs (table 1, figure 5). phenocrysts are aligned in a vitrophiric matrix with occasional blocky shards and pumice fragments with chipped edges. essential minerals are anorthoclase and sanidine free crystals and glomerocrysts. most of the anorthoclase is presented as poikilocryst with a plagioclase nucleus that presents reabsortion borders (figure 4). free plagioclase is oligoclase in composition and it is often zoned or twinned. accessory and trace minerals are corroded and oxidized hastingsite (figure 6), euhedral zircon and euhedral sphene. rocks are k-feldspar trachytes determined by modal classification. figure 1. the paipa volcano is located at the eastern cordillera of colombia, eastward of the active arc (pardo-villaveces et al., 2005). 151 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology f ig ur e 2. g eo lo gi c m ap o f pa ip a vo lc an o pr od uc ts ( pa rd ov ill av ec es e t a l., 2 00 5) . 152 natalia pardo villaveces, josé maría jaramillo, héctor cepeda unit i.2 samples: this pumice and ash flow cooling unit was sampled in ppg-006 and ppg-084 gps stations (figure 2). hand samples are poorly sorted pyroclastic flow tuffs, moderately welded, with 30% of block and lapilli-sized juvenile pumice fragments; there are also accessory pumice fragments and accidental red, yellow and gray siltstones, cherts, red, yellow, white and green phylites, black schists and quartzites in a yellow to brown ash and pumice matrix (figure 8). micro-textures (figure 9) are vitric-crystalline (table 1, figure 5), with pumice fragments of different cristallinity in a glassy matrix with abundant bubble-wall shards and pumice. juvenile crystals are the same as in unit i.1 samples, with dominating sanidine crystals, anorthoclase glomerocrysts and poikilocrysts (figure 10). rocks were classified as k-feldspar trachytes on modal observations (table 1), but sample ppg-084i chemical composition suggests high-k calc-alkaline rhyolites (table 2, figure 7). figure 3. a. hand sample of unit i.1 outcropping at gps station ppg-057. juvenile fragments are anorthoclase, plagioclase, sanidine and hastingsite fractured crystals. rocks show a flow texture by crystals alignment. b. plagioclase is often mantled by an anorthoclase rims while k-feldespars are clustered in glomeroporphyritic textures. kfs-mantled pg = anorthoclase-mantled plagioclase; k-gx = k-feldespar glomerocrysts; hs = hastingsite; t-kfs = twinned anorthoclase. figure 4. pyroclastic vitric-crystalline texture of unit i.1 flow tuffs. a. feldespar juvenile crystals, glomerocrystals and poikilocrystals together with subangular volcanic accessory lithics are embedded in a vitroclastic matrix with flow texture. b. k-� k-� k-feldspar glomerocryst; pg = plagioclase (oligoclase). 153 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology table 1. 100% normalized solids percentage (i.e. without glass), textural and modal classifications are shown, based on streckeisen (1978) classification. kfs= anortoclase; sa= sanidine; pl= plagioclase (oligoclase-andesine); hs=hastingsite; oxhnb= oxihornblende; bt= biotite; ti-bt=titanobiotite; aug= augite-aegirine; ttn= sphene; zr= zircon; op= opach minerals. ci= crystallization index; j=juvenile fragments; nj= not-juvenile fragments. sample 057 057j 085 102 102j 006 pv3j 005e 005d 112 113 114 071 104 070 069 69j 119 119j kfs 47 38 38 45 46 53 14 58 41 40 83 70 60 31 62 72 64 sa 19 38 42 39 25 32 68 12 44 88 27 7 81 8 29 8 14 pl 4 13 4 13 9 5 5 4 3 13 13 3 5 4 6 8 10 4 hs 20 16 6 8 3 4 7 7 7 oxhorn 2 4 bt 2 2 5 8 4 6 10 16 1 14 4 5 ti-bt 6 1 20 1 aug 2 ttn 3 4 20 3 2 1 1 2 zr 4 7 4 4 3 9 15 6 6 1 2 1 3 op 7 7 5 3 13 4 4 4 3 1 ci 44 53 57 38 48 24 17 20 23 2 7 17 18 92 69 58 53 48 44 j 99 99 99 97 99 92 98 88 97 88 93 98 90 nj 1 1 1 2 1 8 2 12 3 12 7 2 10 te xt ur al cl as si fic at io n c yi st al -v itr ic c yi st al -v itr ic c yi st al -v itr ic c yi st al -v itr ic c yi st al -v itr ic v itr ic -c ys ta lli ne v itr ic -c ry st al lin e v itr ic -l ith ic v itr ic -c ry st al lin e v itr ic -l ith ic v itr ic v itr ic -c ry st al lin e v itr ic -c ry st al lin e hypocrystalline porphyritic, with 2 kfs generations, glomerocrysts and kfs poikylocrysts hypohialine porphyritic m od al c la ss ifi ca tio n k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te t ra ch yt e t ra ch yt e k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te t ra ch yt e t ra ch yt e k -f el ds pa r tr ac hy te k -f el ds pa r tr ac hy te red biotita bearing k-feldspar trachyte t ra ch yt e k-feldspar trachyte figure 5. textural classification of pyroclastic rocks. 154 natalia pardo villaveces, josé maría jaramillo, héctor cepeda figure 6. a-b. hastingsite phenocrystal with dissolution embayments. c-d. anorthoclase phenocrystal with hollow nucleus and disequilibrium rims. e. hastingsite altered to chlorite and borders are often corroded and oxidized. f. anorthoclase poikilocrysts wi� hs = hastingsite; kfs = anorthoclase; olg = oligoclase; sa = sanidine. figure 7. a. tas chemical composition of paipa volcano rocks. b. peccerillo and taylor chemical classification of paipa volcano rocks. selected samples are alkaline trachytes and rhyolites, and high-k calcalkaline rhyolites. 155 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology figure 8. hand samples from proximal outcrops of unit i.2, which is an ignimbrite. pyroclastic rocks present flow textures with poorly sorted juvenile, accidental and accessory fragments in a vitroclastic brown to yellow matrix. figure 9. unit i.2 pyroclastic flow tuffs. a. juvenile rounded pumice fragments with spherical vesicles and angular sanidine crys� shards are found. b. vesicles are normal to crystal walls giving bubble-wall textures. sa = sanidine; kfs = juvenile anorthoclase crystal; pm = juvenile pumice fragment; pssh = pumice shard with spherical vesicles; pfsh = pumice shard with flatted vesicles; esh = equant shards. unit i.3 samples: the pumice and ash flow tuff cooling unit was sampled in ppg-005 gps station, in el guarrúz puzzolana mine (figure 2). hand samples are poorly sorted white pyroclastic flow tuffs, with abundant juvenile pumice fragments of different crystal content, up to 25-20% of armored lapilli (figure 11) and up to 30 % of accidental metamorphic lithics in an ash and pumice matrix. in thin sections (figure 12) these rocks were classified as vitric-lithic, with a combination of bubble-wall and pumice magmatic shards together with blocky and platy phreatomagmatic shards (cepeda & pardo-figure 10. k-feldespar glomerocrysts. 156 natalia pardo villaveces, josé maría jaramillo, héctor cepeda figure 11. a. hand sample of unit i.3 ignimbrites, composed of juvenile, accessory and accidental poorly sorted fragments in a vitroclastic matrix. b. armored lapilli transversal section shows a metamorphic phylite as nucleus with a reaction border and a pyroclastic ash and pumice rim. c-d: armored lapilli can have more than two(2) layers; in this case 1 = accessory pumice as nucleus; 2 = intermediate layer of green ashes, kfs crystals and pumice fragments. it has a corroded border. 3 = pumice and ash external regular sorted rim. acc = accidental lithic fragments; pm = pumice fragment; ml = accidental metamorphic lithic. villaveces, 2004). pumice vesicles are ellipsoidal and fibrous (figure 12 b). accidental lithics are dominant in field scale, as angular blocks and lapilli phylites, schists, red-yellow and green claystones, chert and quartzites; occasionally they are found in microscopic scale (ash sizes) and are mainly sub-rounded phylites and quartzites. accessory lithics are crystalline vitric and vitric-crystal ash flow pyroclastic rocks, possibly detached from unit i.1 rocks. modal composition is trachytic (table 1) while chemical composition of sample ppg-005 suggests high-k cal-alkaline rhyolites (figure 7). unit i.4 samples: it is a pumice flow unit that was sampled in ppg-112 and ppg 113 gps stations, in los morros puzzolana mine (figure 2). hand samples are white pyroclastic flow tuffs, with abundant highly vesiculated pumice fragments and accidental phylites and quartzites in a glassy matrix (figure 13). in thin section (figure 14) they are classified as vitric and vitric-crystalline flow tuffs (figure 5) with occasional plagioclase, anorthoclase and sanidine free phenocrysts in a vitroclastic matrix. bubbles present different morphologies from spherical to strongly elongated; some fragments are fibrous with tube-like bubbles, shards are mainly bubble-wall type and vesicles tend to surround crystals normal to crystal faces, giving a bubble-wall texture (figure 14). modal classification (table 1) indicates k-feldspar trachytes. 157 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology figure 12. a. pyroclastic textures of unit i.3 ignimbrites, with abundant equant fragments with flattened vesicles, without vesicles and pumice bubble-wall shards. b. vesicles range in morphology from complete spherical to fibrous and show directional flow. deformation causes pull-apart textures. c. accessory lithics are sub-rounded altered pumice fragments while d accidental lithics are mainly metamorphic. pfsh = pumice shard with flattened vesicles; ap = accessory pumice fragment; acl = accidental lithic (here: phylite). figure 13. pyroclastic flow tuffs of unit i.4 are mainly vitric with occasional accidental quartzites, green and yellow phyllites. acl = accidental lithics. unit ii.1 samples: lava-dome rocks, sampled in ppg-069, ppg-070, ppg-104 and ppg-119 gps stations, are hypocrystaline gray rocks, with anorthoclase phenocrysts up to 1,5 cm (figure 15). in thin section (figure 16) some of these phenocrysts are pokilocrysts with an oligoclase nucleus (figure 16 b). feldspar free crystals and glomerocrysts are surrounded by red-microfolded biotite crystals which give a flow texture to the rocks (figure 16 b and d). in the glassy matrix there were identified finer crystals of anorthoclase, hastingsite, oxihornblende and sphene. the finer anorthoclase microcrystals show penetration cracks and hollow nucleus with occasional glass inclusions (figure 16 i). modal (table 1) and chemical classification (figure 7) indicate alkaline affinity trachytes with less than 20% normative quartz. sample m-119i, taken at honda grande creek dome, is alkaline rhyolitic in composition (figure 7); augite-aegirinae was identified as an accessory mineral (figure 16 j-k), and β-quartz as a trace mineral. 158 natalia pardo villaveces, josé maría jaramillo, héctor cepeda figure 14. pyroclastic textures of unit i.4 flow tuffs. bubble-wall textures are common (a and b) around anorthoclase phenocrystals. the main constituents of the rock are juvenile pumice fragments without crystals in the (c). rocks are mainly vitric tuffs with occasional feldespars showing dissolution embayments (d). accidental lithic fragments are angular phyllites and quartzites (e). kfs = anorthoclase. pg = plagioclase. pm = juvenile pumice. ap = accessory pumice fragment. figure 15. a. rock fragment of olitas lava-domes. anorthoclase, oligoclase, albite and red biotite are aligned and oriented suggesting a flow direction. b. rock fragment of honda grande creek dome. mafics are hastingsites, augite-aegirine and red biotite. fe-oxides tend to alter crystals. 159 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology figure 16. a. plagioclase is albite and oligoclase in composition, it is commonly twinned and zoned. b. some plagioclase crystals are mantled by potassic feldespar (anorthoclase) and form glomerocrystals. c. the main mafic mineral is red biotite. in this thin section kfs microcrystals are visible, all of them wit a hollow nucleus. d. commonly, red biotites are folded. e-f. high b� have the hollow nucleus and penetration cracks. j-k. green augite-aegirine occurs as mafic accessory mineral. in summary, figure 7, tables 2 and 3 show chemical data of the five selected samples, which are mainly alkaline trachytes and rhyolites. the oldest rocks are the most silicic rocks (72%) and less aluminous (16%) while the younger are less silicic (68%) and more aluminous (18%). in general, all samples have high contents of alkalis (7-10%), in which na content is greater than k content, which is consistent with anorthoclase, na-amphibole and na-pyroxene crystallization. on the other hand, tio 2 content (< 0,33%) was enough for red-biotite crystallization as an accessory mineral, and sphene as trace mineral. composition is confirmed by other 10 rocks sampled during ingeominas colombia geochemical map project (in cepeda & pardo-villaveces 2004). chemical data were processed to calculate cipw norm and to estimate the physical conditions of magma reported in table 4. 160 natalia pardo villaveces, josé maría jaramillo, héctor cepeda table 2. recalculated whole rock major elements chemical data. sample sio 2 al 2 o 3 na 2 o k 2 o cao fe 2 o 3 feo mgo tio 2 p 2 o 5 119i 70,5 16,44 6,7 3,65 0,41 1,26 0,85 0,05 0,15 0,00 069i 68,2 18,05 5,91 3,43 0,83 2,03 0,67 0,50 0,24 0,10 084i 72,2 17,26 2,69 4,64 0,30 1,68 0,77 0,14 0,33 0,00 005e 72,0 16,77 3,85 4,59 0,24 1,43 0,66 0,11 0,26 0,11 102i 71,6 16,2 6,67 3,71 0,17 0,77 0,71 0,02 0,13 0,00 table 3. álcalis % vs. silica %: chemical classification (for tas diagram in figure 7). sample na 2 o+k 2 o sio 2 119i 10 70,5 ♦ 069i 9 68,2 084i 7 72,2 ο 005e 8 72,0 δ 102i 10 71,6 ´ in order to confirm the obtained composition we compare our five sample analysis with ten other major oxides chemical data (table 5) determined by ingeominas “colombia geochemical map” project to support our observations (cepeda & pardovillaveces, 2004; güiza & muñóz, 2005). rhyolitic and trachytic composition for paipa volcano products are confirmed and shown in figure 17. discussion rocks textures suggest an intratelluric environment in which crystals grew rapidly under various nucleation rates, promoting twinned forms, (pg in figure 3a, hs in figure 3c, pg in figure 16 a, pg and kfs in figure 16 b) and a former viscous melt in which cation diffusion was difficult. while differentiation took place, there was no chemical and no physical equilibrium between plagioclase and hastingsite crystals and the melt, as suggested by dissolution texture, reabsortion and reaction borders (figures 6a, 6b, 6e, 14d). plagioclase and oxi-hornblende rounding (figures 3b, 16e and 16f) could be the result of the convective currents that transport crystals in a possibly thermally stratified magmatic chamber; otherwise it could be the result of system reheating due to hotter magma injection at the base of the chamber (pardo-villaveces, 2004). high growing rates together with low nucleation and low diffusion rates favored feldspar movement along compositional boundaries without reaching the equilibrium, producing compositional and textural zonation: anorthite to albite transformation was faster than growing and crystallization, originating normal-zoned crystals. some crystals show fibrous rings alternated with euhedral rims, which represent adjustment periods to new thermodynamic and compositional conditions (figure 6 c and 6d). on the other hand, anorthoclase poikilocrysts with plagioclase nucleus (figures 3a and 6f) suggest the following hypothesis (pardo-villaveces, 2004): (1) following carmichael et al. (1974), anorthite-poor melts in the water-saturated naalsi 3 o 8 kalsi 3 o 8 -sio 2 subsystem, can crystallize progressively more sodic feldspars that, later, become more potassic. this fact results in progressively more sodic sanidines and anorthoclase overgrowing around corroded plagioclase. (2) fast nucleation plagioclase could get trapped into the slower coeval anorthoclase growth. (3) mixing of trachytic and rhyolitic magmas and/or mixing of magmas of different alkalinity. water introduction could depolimeryzed magma, increasing diffusion and nucleation rates with heat loss and volatile release, allowing the generation of a second generation of alkaline feldspars (figures 3 c, 6b, 16 a and 16 c). 161 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology table 4. cipw norm. results given by norma.xsl. differentiation index, estimated melt temperatures and viscosity (logη) were calculated by the software. the results given by norma.xsl software suggest highly differentiated rocks (di varies between 89 and 97%), low melt density (between 2, 37 a 2, 4 g/c3), high viscosity (given as log η = 8, 9 a 10, 1 for fee rocks and 7, 8 to 8, 4 for the see domes) and melt temperatures between 807 and 879ºc. 162 natalia pardo villaveces, josé maría jaramillo, héctor cepeda once magma encountered one or several conduits, its interaction with external water, as suggested by phreatomagmatic shards, surface pitting and abundance in lithics, reported in cepeda & pardo-villaveces (2004) caused explosive fragmentation and the first eruption known for paipa volcano (pardovillaveces & alfaro, 2005). eruption columns did not reach enough height and pyroclastic flows were originated by their continuous collapse (pardo-villaveces et al., 2005). a former pyroclastic edifice was formed this way, whose eroded walls still evident in the field (figure 18). in this way, the last solidifying phase was the glass and its composition represents the composition of the viscous residual silicic melt, where quartz could not crystallize because of the difficult ion diffusion or because during magma differentiation and feldspar crystallization, eutectic conditions were not reached. after an erosion or no-depositional period inferred by the stratigraphy reported in pardo-villaveces et al. (2005), magma could rise again along the conduit (s) and suffered volatile saturation and exolusion by the lower pressure; system disruption, due to magma injection, volatile introduction or sudden decompression, enhanced the transformation of a melt of liquid, solid crystals and dissolved gasses into a mixture of gasses with liquid drops and solid fragments; melt density reduction (expansion) resulted in a rapid acceleration through and out of the vent, detonating further eruptions (unit i.2). magmatic fragmentation processes allowed vesicle formation and coalescence, particularly around and perpendicular to crystal borders (figure 9 b). bubble inner pressure excess generated bubble wall thinning until the complete rupture, forming glass shards and larger vesicles (figure 9 a). a third eruptive unit was deposited after an explosive interaction between magma and meteoric water (phreatomagmatic eruption. pardo-villaveces et al., 2005); pumice fragments were randomly distributed in a vitroclastic matrix with abundant blocky shards, vesiculated and not vesiculated shards (figure 12 a). rounded and ellipsoidal vesicles grew spherically and then were plastically deformed in the viscous flow along the conduit (figure 12 b). the high input of dense lithic fragments enhanced column collapse and wet conditions allowed the formation of armored lapilli rich pyroclastic flows. cepeda & pardo-villaveces (2004) proposed a combination of table 5. geochemical data obtained by ingeominas “colombia geochemical map” project. by güiza & muñóz, 2005. figure 17. tas diagram showing classification of ten samples based on geochemical data obtained by ingeominas “colombia geochemical map” project (cepeda & pardovillaveces, 2004; güiza & muñóz, 2005). 163 the paipa volcano, eastern cordillera of colombia, south america (pary ii): petrography and major elements petrology magmatic and phreatomagmatic mechanisms. after a quiescence period, bubble nucleation around crystalline faces developed pull-apart textures (figure 12 b) due to the greater effort under rapid exolusion and expansion. foam was formed in the magma until it boiled over the vent, generating a small lobe of vitric flow tuffs over the northeastern edifice flank (pardovillaveces, 2004. pardo-villaveces et al., 2005). based on pardo-villaveces (2004), cepeda & pardovillaveces (2004) and pardo-villaveces et al. (2005) we know that a caldera resurgence that began during the see with conduit opening and alkaline trachytic to rhyolitic magma emission near the caldera margins; initial volcanism was not explosive in nature: magma could reach the surface after complete volatile release through conduit permeable walls, promoting anorthoclase and sanidine nucleation rate increment and amphibole and mica stability reduction, which generated corrosion and opatization borders in mafics. thermodynamic and compositional disequilibrium was permanent leading to the formation of dissolution embayments, reabsortion borders, normal, inverse and oscillatory plagioclase zonation (figure 16) (pardo-villaveces, 2004). figure 18. the paipa volcano has a 3 km-diameter caldera with several vents inside it, most of them located at olitas headstream. outflow facies are mainly ignimbrites while inflow facies are block and ash flows, ignimbrites, surges, domes, sedimentary high energy and low energy deposits (pardo-villaveces et al., 2005). summary and conclusive remarks the paipa volcano produced alkaline rhyolitic and trachytic to high-k calcalkaline rhyolitic magmas eruptions during the pliocene. calculated differentiation index (di) varies between 89 and 97%, while calculated melt density oscillates between 2, 37 a 2, 4 g/c3 (low). viscosity is relatively high (log η = 8, 9 a 10, 1 for fee rocks and 7, 8 to 8, 4 for the see domes) and temperatures were estimated between 807 and 879ºc. under different nucleation, diffusion and growing rates, augiteaegirine, sphene, zircon, oligoclase, anorthoclase and sanidine could crystallize in the intratelluric environment. water content allowed hydrous-phases crystallization, such as hastingsite and biotite, and it was a dominant factor in magma explosivity and in eruptive mechanisms. most of the feldspars and amphiboles show evidences of thermodynamic and compositional disequilibrium with the melt, as indicated by dissolution embayments, different kinds of zonation, reabsortion and corrosion borders. together with the published data of iza volcanic rocks (boyaca) the alkaline and silica-rich magmas that were erupted in the eastern cordillera during the pliocene are strongly distinct from the calcalkaline magmas of the central cordillera, which suggest different geotectonic conditions. while the active arc is clearly related to the subduction of nazca plate under south america, we still don’t know the origin of the eastern cordillera magmas and their volcano-tectonic framework. recommendations since the geotectonic environment and the relation between volcanism and the eastern cordillera regional compressive regime are not well understood, exhaustive geochemical and petrologic studies, together with structural, geophysical, seismic, tectonic and detailed geochronology research still have to be done. it would help to clarify petrogenesis and magma evolution through the continental crust. 164 natalia pardo villaveces, josé maría jaramillo, héctor cepeda references • carmichael i., turner f., verhoogen j. crystallization paths of igneous minerals at low pressures. 1974. igneous petrology. chapter 5. mc graw hill. international series of earth sciences. 739 p • cepeda h. & pardo-villaveces n., 2004. vulcanismo de paipa. informe técnico del ingeominas. ingeominas-bogotá. colombia. 140 p. • cepeda h., pardo-villaveces n., jaramillo j. 2004. the paipa volcano, colombia, south america. abstract in iavcei meeting. november 14-19, pucón, chile. • güiza s., muñoz r. 2005. blancos de mineralización teletermal asociados a vulcanismo en la región de paipa . boyacá. memorias x congreso colombiano de geología. • wohletz k., & heiken g. 1992, volcanology and geothermal energy. university of california press, berkeley, ca, 432 pp. •le maitre, r. w. (1989). a classification of igneous rocks and glossary of terms: recommendations of the international union of geological sciences, subcommission on the systematics of igneous rocks. oxford: blackwell, 193 pp • martínez a., 1989. geologie de la region d’iza, boyacá. cordillere orientale de colombie. diplome de mineralogie et geologie. instituts de mineralogie et geologie, universite de lusanne, nov. 200 p. • pardo-villaveces n., 2004. estratigrafia de las vulcanitas asociadas al volcán de paipa, municipios de paipa y tuta, departamento de boyacá. colombia. tesis de grado universidad nacional de colombia-bogotá. 190 p. • pardo-villaveces n., alfaro, c. 2005. caracterización de cenizas volcánicas por microscopio electrónico para determinación de mecanismos eruptivos. caso volcán de paipa, boyacá. memorias x congreso colombiano de geología, bogotá. • pardo-villaveces n., cepeda h., jaramillo j.m. 2005. the paipa volcano, eastern cordillera of colombia (part i) : volcanic stratigraphy. submitted to the earth sciences research journal for evaluation. • peccerillo a. & taylor t.s. 1976. geochemistry of eocene calc-alkaline volcanic rocks from kastamonu area, northern turkey. contrib. mineral. petrol., 58, 63-81. •streckeisen, a. (1978). classification and nomenclature of volcanic rocks, lamprophyres, carbonatites and melilitic rocks: recommendations and suggestions, iugs subcommission on the systematics of igneous rocks. neues jahrbuch für mineralogie, abhandlungen 134, 1–14. articulo 5.indd earth sciences research journal archaeology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 115 120 quaternary dating by electron spin resonance (esr) applied to human tooth enamel eduar carvajal1, luis montes2 and ovidio a. almanza1 1 departamento de física, universidad nacional de colombia, cra 30 nº 45-03, bogotá 2 departamento de geociencias, universidad nacional de colombia. cra 30 nº 45-03, bogotá. corresponding author: ovidio almanza m, tel. (57 1) 3165000 ext 13048; fax (57 1) 3165635. e-mail oaalmanzam@unal.edu.co abstract this paper presents the results obtained from using electron paramagnetic resonance (epr) to analyse tooth enamel found at the aguazuque archaeological site (cundinamarca, colombia), located on the savannah near bogota at 40 37’ north and 74°17’ west. it was presumed that the tooth enamel came from a collective burial consisting of 23 people, involving men, women and children. the tooth enamel was irradiated with gamma rays and the resulting free radicals were measured using an electron spin resonance (esr) x-band spectrometer to obtain a signal intensity compared to absorbed doses curve. fitting this curve allowed the mean archaeological dose accumulated in the enamel during the period that it was buried to be estimated, giving a 2.10 ± 0.14 gy value. rosy software was used for estimating age, giving a mean 3,256 ± 190y before present (bp) age. these results highlight epr’s potential when using the quaternary ancient ruins dating technique in colombia and its use with other kinds of samples like stalagmites, calcite, mollusc shells and reefs. resumen en este trabajo se presentan los resultados obtenidos al datar por resonancia paramagnética electrónica muestras de esmalte dental provenientes del sitio arqueológico aguazuque, ubicado en la sabana de bogotá en coordenadas 40 37’ de latitud norte y 74°17’ de longitud oeste de greenwich. se presume que el esmalte dental proviene de un entierro colectivo compuesto de 23 individuos entre mujeres, hombres y niños. el esmalte dental fue irradiado con rayos gamma y los radicales libres producidos fueron medidos usando un espectrómetro de resonancia del spin electrónico (esr), banda x, con el fin de realizar una curva de intensidad de la señal versus la dosis absorbida por el esmalte. el ajuste de esta curva permitió obtener la dosis media arqueológica acumulada en el esmalte (da), durante el periodo de enterramiento de la muestra. el valor obtenido fue de 2.10 ± 0.14 gy. para estimar la edad de la muestra se usó el software rosy, obteniendo una edad media de las muestras de esmalte de 3256 ± 190y bp. los resultados obtenidos muestran el potencial uso de la técnica epr como herramienta de datación de muestras cuaternarias en colombia y abre la posibilidad de usarla en otro tipo de muestras como estalagmitas, calcitas, conchas de moluscos y corales. palabras claves: epr, esmalte dental, datación arqueológica. keywords: epr/esr, tooth enamel, colombia, archaeological dating. record manuscript received: 11/05/2011 accepted for publications: 30/11/2011 introduction electron spin resonance (esr) analysis has recently become an alternative c14 and thermo-luminescence dating method which can be applied to a variety of problems in geology, archaeology and paleoanthropology (renfrew c et al., 1992; jonas 1997; walker 2005; grun 1989). esr spectroscopy is the only method for detecting, identifying and quantifying free radicals (halliwell & gutteridge 2007; punchard & kelly 1996). for instance, this method is used for estimating the free radical level and paramagnetic centres produced in some materials by ionising radiation. the some free radicals’ long mean life (above 100 my) make it possible to reveal them in materials as old as 2 my without the risk of overcoming the sample’s saturation limits. esr allows some materials to be dated because free radical concentration is a measurement of the total radiation dose absorbed by the sample during the time it was exposed to radiation and hence their burial time (jonas 1977). esr has been used for dating the formation and more recent re-crystallisation of three types of gypsum samples: massive, bedded and fracture filling gypsum; in this case, the estimated upper miocene–pliocene age agreed with that soil’s stratigraphy (ülkü 2004). molodkov (2001) used esr to date eight terrestrial shell samples from treugolnaya (triangular) cave (northern caucasus), ages ranging from 583,000 to 393,000y old. shigueo et al., (2003) dated a calcite formation found on a rock wall painting at toca da bastiana, rock shelter in serra da capivara national park, piaui, brazil, obtaining an age of 35,000 to 43,000y, indicating that humans lived there before 35,000 years ago (this being the date for the first humans to arrive in brazil). otheduar carvajal, luis montes and ovidio a. almanza116 er papers dating stalagmites, calcite, mollusc shells and reefs are renfrew et al., (1992), jonas (1997), walker (2005) and grun (1989). the present study was aimed at using the esr method for dating tooth enamel from the aguazuque archaeological site in soacha, colombia. theoretical aspect the aguazuque archaeological site (cundinamarca, colombia) is located on the savannah near bogota at 40 37’ north and 74°17’ west (figure 1). the human tooth used in this work was extracted from a skeleton (labelled ag-i) taken from a collective burial site where 23 people were found arranged in a foetal position; men, women and children were identified. the burial site was circular shaped (4.5 m diameter) and as burnt bones and lacerated skeletons were found outside the circle, partially painted red and black, it was deduced that cannibal practices were common by the group responsible for the burial (correal 1990). archaeological research concerning the early and middle holocene period has led to figure 1. location of aguazuque archaeological site (cundinamarca, colombia, 40 37' north and 74°17' west). (taken from groot, a.m., 1992). figure 2. energy levels of an electron placed in a magnetic field. the arrow shows the transitions induced by microwave radiation. reconstructing the way of life for human groups in the region, thereby showing a transition from hunters and gatherers to agricultural societies (groot 1992). a way of life based on hunting several sized mammals has been recorded during a lapse of 11,000 to 5,000 years before today, with rock shelters providing favourites housing places, such as the archaeological sites of el abra, tequendama and nemocón 4 in cundinamarca (correal 1979). sites such as chía i (ardila 1984) and aguazuque (correal 1990) have been investigated and the aforementioned research has led to establishing changes in the form of settlement and adaptation mechanisms (groot, 1992). electron spin resonance (esr) consists of the resonant absorption of electromagnetic energy during electron-spin transitions. a static magnetic field should be applied to resolve different electron-spin levels. unpaired electrons from free radicals have spin equal to ½. there are two magnetic levels (+½ and –½) in a magnetic field, both having different energy (figure 2); the transition between these two levels is possible in the following resonance condition: hν=gβh where ν is resonance frequency, h is plank’s constant, g is the g-factor (which is a constant = 2 for ½ spin), β is the bohr magneton which is an electron magnetic dipole moment and h is the magnetic field. an important conclusion derived from this formula is the linear dependence between the applied magnetic field and resonance frequency. the most frequently used microwave energies drop in what is called the x band. this is because it is a good compromise between sensitivity, sample size and water content effects; ν ≈ 9.8 ghz and h ≈ 350 mt for the x band. esr dating and dosimetry are based on the fact that ionising radiation produces paramagnetic centres having long lifetimes in a number of materials. such centres’ concentration in a given sample is a measure of the total radiation dose to which a particular sample has been exposed. this effect can be used to determine the length of time of such exposure and hence, in many cases, a sample’s burial time (jonas 1997). esr has been used with tooth enamel for determining the age of palaeolithic archaeological sites, mainly because tooth enamel is ubiquitous in these contexts and because it allows dating beyond the 14c dating time range (40–45 ka max.). it is thus also very useful in important sites regarding human evolution studies, sites having palaeontological interest within a quaternary time frame and because many sites lack the volcanic materials needed for k – ar and 40ar/39ar dating (rink 1997). (1) quaternary dating by electron spin resonance (esr) applied to human tooth enamel 117 the aim is to determine the accumulated dose (da) to which a sample has been exposed whilst being buried; the exposure time is then determined by comparing da to the average dose rate over such period: t= da (d) however, no significant difference in the accuracy of dose reconstruction has yet been found using different calibration methods (wieser et al., 2000). each sample’s radiation sensitivity is individually calibrated for dose assessment by the additive dose method; each sample is incrementally irradiated with at least 14 additional doses up to 100 gy or more (avoiding the sample becoming saturated). a linear regression analysis is applied to the esr measurements at each dose increment and the original measurement (laboratory-un-irradiated sample). the original da in the sample is obtained from the negative intercept of the regression line with the dose axis; is estimated as being the total radiation provided by all radioactive elements present in a sample and soil and also cosmic radiation (ikeya 1993; jonas 1997; walker 2005; grun 1989). materials and methods the dentine was initially removed from teeth using a dental drill and water cooling. the enamel was then placed in a 30% naoh solution for one day to disinfect it and separate any remaining dentine (iaea report 2002); 1.4 mm mean enamel thickness was obtained for ag-i. the sample was air-dried at room temperature for three days; a dental drill was used to strip around 50 ± 5 μm from the inside and outside of the enamel surface to ensure that alpha radiation had no effect. the molar enamel powder was divided into nine aliquots. each aliquot was placed inside a plastic capsule for irradiation using 60c standard radiation equipment at room temperature and 8gy/min mean dose rate; the doses ranged from 0 to 740 gy. after irradiation, the esr signal was measured with a bruker esp300 x-band spectrometer. the g  and g  signals’ peak to peak height was used for calculating amplitude. the spectrometer parameters used were: 336 mt central field, 0.02 t scan range, 0.1 m t amplitude modulation, 100 khz modulation frequency, 0.328 s time constant and 20 mw microwave power. the aliquots were then irradiated with additional doses and the same process was repeated until getting 9 data points from esr signal amplitude as a function of γ-ray dose. soil samples were also collected from the aguazuque site and sent for u, th and k content analysis by gamma spectrometry. radiometric analysis is widely used for determining natural radioisotopes in geological samples by means of spectroscopic methods and, especially, for quantifying the uranium, thorium and potassium present in samples. these samples were analysed at ingeominas’ nuclear technology group’s environmental radiometrics’ laboratory which has a low-level gamma spectrometry system with 15 cm lead shielding and high-resolution gehp hyperpure germanium detector (canberra model gc2019), having 1.89% absolute detection efficiency for 661.6 kev. results and discussion figure 3 shows the esr spectra for enamel sample ag-i having an irradiation dose ranging from 54 to 324 gy. a linear model was used for fitting esr signal intensity to dose, as has been done by other authors (baffa et al., 2000; hefne et al., 2002; kinoshita et al., 2001a, 2008b; mascarenhas et al., 1982); very good agreement with linear fit was shown (figure 4). the archaeological da obtained using this method was da = 2.10 ± 0.14 gy. rosy software (brennan et al., 1999) was used for calculating the enamel sample’s age, based on the given da, cosmic radiation dose rate (determined at 251 ± 15 μgy/year), u, th and k content obtained from figure 3. esr spectra of tooth enamel ag-i in a range of 54 to 324 gy. (2) eduar carvajal, luis montes and ovidio a. almanza118 soil sample analysis and enamel (1.4 mm ag-i thickness and 5 mm dentine thickness) in sample. soil water content was 12.1%. the software’s default values were used for the remaining parameters. the cosmic radiation dose rate was obtained using the equation for luminescence and esr dating (prescott & hutton 1994). finding the dose rate to convert da into an age is a crucial step after da has been found. rosy software allows age to be calculated from an enamel sample by taking da due to radioisotopes from adjacent layers into account. the software computes age based on three uranium absorption models (linear, early uptake and a combination of both). table 1 shows the sample concentration (ppm) 238u 232th 40k aguazuque 1.86 ± 0.19 3.72 ± 0.37 1.66 ± 0.09 alpha beta gamma cosmic gamma+cosmic dentine 0.00 0.00 enamel 0.00 0.00 cementum 0.00 0.00 sediment 0.00 46.55 347.31 uranium 0.00 34.54 188.50 thorium 0.00 12.01 158.81 potassium 0.00 0.00 total 0.00 46.55 347.31 251.00 598.31 (alpha+beta) dose rate (µgy/y) = 46.55 total dose rate (µgy/y) = 644.86 table 1. uranium, thorium and potassium concentration as determined by gamma spectrometry. table 2. dose rate obtained from radioisotope concentration, local cosmic radiation and total dose rate. alpha radiation from sediment was not used in the calculation, as about 50 μm was removed from the outer side of the tooth. figure 4. signal versus dose growth showing the linear fitting used. archaeological dose for ag-i was ad = 2.91 ± 0.14 gy; correlation coefficient was 0.9937. soil analysis results. the present work did not determine u, th and k concentration in enamel and dentine as their percentages have been negligible for calculating age in previous work, especially in dates for the holocene period (kinoshita et al., 2008; hefne et al., 2002). typical soil density, enamel and dentine values were used for calculating age (2.00, 3.00 and 2.82 g/cm3, respectively). the layer of enamel stripped off from each side by acid etching was about 50±5 mm; this prevented a possible contribution towards the dose having been made by alpha particles deposited in the sample. using these values and the radioisotope concentrations listed in table 1, rosy software was used for obtaining the alpha, beta and gamma radiation dose rate contribution for each radioisotope in sediment (the results are listed in table 2). these results were the same for any uranium absorption model. considering a 251 ± 15 μgy/year dose rate for cosmic radiation, a 3,256 ± 190y before present (bp) age was thus found for the ag-i sample. it should be noted that rosy software calculates these ages using three uranium accumulation models: linear, early and a combination of both (ikeya, 1993; grun, 1989; fattibene et al., 2010). the way that uranium accumulates in a tooth is established by these models; accumulation increases at the same rate as time in the linear model and accumulation occurs in a short period of time in the early model, relative to a tooth’s age and then remains constant after this short period of time. the tooth’s age was the same when calculated by these three methods; this result was consistent if taken into account that the accumulation of radioactive elements inside the tooth was negligible. conclusions age determined at many countries’ archaeological sites is the result of radiocarbon (14c) analysis of particular items found around the burials and not direct dating of individuals found there. the results regarding either physical or geological phenomena may not be chronologically equal, hence the importance of having techniques like esr which can directly quaternary dating by electron spin resonance (esr) applied to human tooth enamel 119 and absolutely determine the age of individuals of interest. the age of a human burial site found in colombia was determined in this work from a single tooth, a mean age of 3,256 ± 190y bp being found. this result agreed with stratigraphic analysis at sites providing soil ages ranging from 2,800 to 3,500y bp for 50 and 80 cm depths (correal, 1990). esr dating accuracy was obtained relatively straightforwardly, using just a few grams of tooth enamel, thereby highlighting some of the advantages and characteristics of the esr dating method. the results so obtained have contributed towards joining links in the long chain of colombian and american history (where there is still much to do, due to the rich pre-hispanic and colonial past). this paper thus represents a pioneering effort aimed at promoting the dating of our ancestors. acknowledgments we would like to thank the dib, universidad nacional de colombia, for providing financial support (project 8003348), m. peña from ingeominas for soil sediment analysis, the universidad nacional de colombia’s physical anthropology laboratory and its director, j.v. rodriguez, for providing facilities and allowing the use of his samples, as well as am groot (archaeologist) for her support during this research. references a.f. borgonovea, a. kinoshitaa,b, f. chena, p. nicoluccia, o. baffaa, (2007): energy dependence of different materials in esr dosimetry for clinical x-ray 10mv beam. radiation measurements, 42: 1227– 1232. anatoly molodkov; 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(2003): some evidence of a date of first humans to arrive in brazil. journal of archaeological science 30: 351–354. ülkü ulusoy; (2004): esr studies of anatolian gypsum; spectrochimica acta part a 60: 1359–1365. walker m. (2005): quaternary dating methods. john wiley & sons ltd, the atrium, southern gate, chichester. chapters 1, 2, 3 and 4. wieser, a., mehta, k., amira, s., aragno, d., bercea, s., brik, a.,bugai, a., callens f., chumak v., ciesielski b., debuyst r., dubovsky s., duliu o.g., fattibene p., haskell e.h., hayes r.b., ignatiev e.a., ivannikov a., kirillov v., kleschenko e., nakamura n., nather m., nowak j., onori s., pass b., pivovarov s., romanyukha a., scherbina o., shames a.i., sholom, s., skvortsov v., stepanenko v., tikounov d.d., toyoda s. (2000): the second international intercomparison on epr tooth dosimetry. radiat. meas, 32, 549–557. the 1985 mexico earthquake juan manuel moreno murillo profesor asociado departamento de geociencias-facultad de ciencias-universjdad nacional de colombia moreno, j.m. (1995): the 1985 mexico earthquake. geoffs. colomb. 3:5-19. issn 0121-2974 resumen se incluy'e una revision biblioqrafica junto con la descripci6n de varios aspectos relacionados con el terremoto de michoacan, mexico (ms = 8.1), el cual comprendi6 tres eventos. un evento premonitorio ocurri6 el 28 de mayo de 1985 (ms = 5.2). ei evento principal ocurri6 el jueves 19 de septiembre de 1985 a las 7h. 17m. 46.6 s. hora local de la ciudad de mexico. ei foco se determin6 a una profundidad de aproximadamente 18 km. un segundo evento ocurri6 el viernes 21 de septiembre a las 7h. 38 m. p.m., hora local. ei ultimo evento relacionado ocurri6 el 30 de abril de 1986 (ms = 7.0). este evento fue un terrible desastre natural para mexico, por 10 menos con 35.000 victimas y cerca de 30.000 heridos, mas de 100.000 personas quedaron sin vivienda y grandes danos ocurrieron en ciudad de mexico y varios estados del centro del pals. este articulo incluye el ambiente tect6nico global, genesis y localizaci6n del epicentro, una interpretaci6n del mecanismo fuente y un analisis de los resultados a partir de afgunas de las estaciones que registraron este terremoto. ala vez se hace una comparaci6n entre los dos grandes terremotos ocurridos en 1985. sin embargo, se describen unicamente los principales dai'los ocurridos en mexico y una descripci6n de los efectos ocasionados por el tsunami producido por el mismo evento. ei terremoto de mexico '1985, ocurri6 a causa de los deslizamientos ocurridos por los procesos de subducci6n, entre las placas de coco y norteamerica, el cual fue descrito como un fallamiento tipo "thrust" somero entre las placas, ocurrido en la intersecci6n del fracturamiento de orozco con el midlle american trench. abs:rract this paper inciudes a bibliographic review with the description of the various aspects about the (ms = 8.1) michoacan, mexico earthquake, which comprised of three events. the main shock of the september 19, 1985 earthquake occurred on thursday at 7h. 17m. 46.6s. local time in mexico city, and had (ms = 8.1). the focus of the event was a depth of approximately 18 km. a second shock occurred on friday evening 21 september at 7h. 38m. p.m. local time. the last aftershock occurred on 30 april of 1986 (ms = 7.0). a prior event occurred to the september 1985 earthquake, occurred on 28 may, 1985 (mb = 5.2) and is described too. this event, was a terrible natural disaster for that country, at least 9,500 people were killed, about 30,000 were injured, more that 100,000 were left homeless and severe damage occurred in many parts of mexico city and several states of central mexico. according to some sources, it is estimated that the earthquake seriously affected an area of approximately 825,000 square kilometers. this paper describes a summary of the global tectonic setting, genesis and location of the epicenter, an interpretation of the source mechanism and a analyses at these results from some stations that recorded this earthquake and at the same time, a comparison between the two largest earthquake of 1985. moreover, this paper describes the principal damage resulting and a description of effects from tsunami produced from earthquake. the 1985 mexico earthquake occurred as a result of slipping in the subduction process between the cocos and american plates. this was a shallow interplate thrust type event which occurred in the intersection of the orozco fracture with the middle american trench. 1. introduction on september 19-1985, a large subduction earthquake occurred in the michoacan gap, along the mexican trench. figure 1 shows the section of the subduction zone in the vicinity of the, michoacan earthquake. repeat times for large earthquakes along any given section of the central american trench vary from 30 to 70 years. the part of the fault which generated the 1985 event, is one of several sections that has not experienced a major event for a much longer period. the near surface geology of mexico city, locality on the former lake texoco, may be classified into three general zone: the old lake bed, characterized by a deposit of very not hard clay whit high water content; a hill zone, of which is capped by 5-30 meters of lava less than 2,500 years old and the transition zone between the two, consist of river delta and deposits with interbed intervals of clay, figures 20 and 22. the (ms = 8.1) michoacan earthquake, of 19 september 1985 and its aftershocks of 21 september (ms = 7.5) and 30 april, 1986 (ms = 7.0) were well recorded in several stations together with its prior event (mb = 5.2) on 28 may, 1985. this event generated intensities of the order of viii to ix in the epicentral area. the march 3 1985 chilean earthquake (ms = 8.0), ruptured a well studied seismic gap along the chilean subduction zone. this event is another subduction thrust earthquake with a geometry similar to the september event. this paper described the characteristic of the mexico earthquake including the global tectonic setting of mexico, which is strongly influenced by the interaction of several major plates as cocos, pacific and north american. also, is give the focal mechanism for the several events and the analyses of teleseismic long-period seismograms. damage was restricted to 6 of 32 major states of mexico, but caused only moderate damage along the coast. the coast states of jalisco, colima, guerrero and michoacan were strongly affected, figures 5 and 7. though damage was relatively sporadic probably due to their close proximity to the epicenter. in contrast, in mexico city, most of the damage experienced in the earthquake was restricted to buildings. the two major factors behind the severity of the seismic damage of buildings were; the resonance in the lake sediments and the long duration of the shaking. on land, the effects of the double earthquake that affected mexico in september 1985, were devasting, but at sea the effects were relatively minor. according to a report in the december 1985, tsunami newsletter, in honolulu, the tsunamis that rolled across the pacific after the quake were small. along the mexican coast, from manzanillo to acapulco, scientists of international tsunami center in honolulu, estimate that the tsunami ranged from 1 to 3 meters tall, but, ecuador reported some of the tallest waves which were only 60 centimeters high. 2. global tectonic setting-seismic gaps tectonically, mexico is strongly influenced by the interaction of several major plates. the movement of the north american plate against the adjacent pacific and cocos plates has been responsible for the high incidence of volcanic and seismic activity in the west and southwest of the country (fig.1). " ...... hue" pi"i. soo 1000 ..... figure 1. p.... tectonic. of the centrel american region (from 08gg, 1987) the boundary between the cocos plate and north american plate is one the most active plate margins in the world. it is here that the cocos plate, forming part of the pacific floor, is moving north eastward at a rate of between 5 and 11.5 cm for year. in opposition, the north american plate is carrying the mexican land mass in a westerly direction. the zone of interaction between the two plate is marked by the deep oceanic trench (central american trench or middle american trench), formed where the oceanic crust of the cocos plate is thrusting beneath the continental crust of the north american plate. alteration of subduction characteristic such as local decrease in the seismicity, a local change in the dip and depth extent-of the benioff zone, a local change in the stress axes of the earthquake have bean observed in many other areas of topographically anomalous seafloor and are subducting (keller and mccann, 1976 in astiz et ai., 1987). the middle american trench of the coast of mexico, has been the site of numerous large thrust earthquakes and is divided into several regions characterized for their seismic potential. 2.1 benioff zone the hypocentral distribution of locally recorded aftershocks of the great (ms = 8,1 and 7.5), michoacan september 19, and 21, 1985, earthquakes define a narrow wadati-benioff zone structure, roughly 10 km. thick, dipping 14 degrees at n23e. this is in good agreement with the geometry source obtained by waveform modeling of the 1985 mainshock, and the large 1979 petatlan earthquake in the adjoining region. the earthquake epicentral resolution, obtained with this program, is significantly better than that for the conventional approaches (hypo) and looks very promising for use in velocity structures with an important dipping interface like subduction zones. stolte, et ai., (1986), used this program and presented a new velocity model describing a dipping structure derived from inversion as well as precise locations of the aftershock hypocenters based on this model. 2.2 orozco fracture zone lefevre and mcnally (1985, in stolte, 1986), studied stress distribution associated with the subduction of the orozco fracture zone (ofz) (fig. 3), and found only minor local deviations from the overall pattern along the middle american trench of the coast of mexico. coincidentally with the michoacan gap, the orozco fracture zone intersects the middle american trench for about 150 km. the historical seismic record alone was not sufficient to determinate whether there was subduction of the orozco fracture zone. previous to the september 1985 earthquake, the michoacan area with its seismic quiescence and subducting fracture zone, was similar to the southern oaxaca area, where the tehuantepec ridge is subductingand where there are no large earthquakes on historic record. one possibility suggested to explain the seismic quiescence in these areas was that features such as the orozco fracture and the tehuantepec ridge may be locally affecting the subducting process, so that the area is subducting aseismically or more slowly than adjacent regions of the plate boundary (singh, et ai., 1980, in eissler et ai., 1986). figure 2 (a): .how. area with 1985 two maln.hocks a•• tar., the .ix earthquake. used in the inversion are .hown a•• quare •. the three ring. circle the aftershock zone. of recent major earthquake from left to right; colima 1973; playa azul 1981; petatlan, 1979; a·a' cro .. section. (b): .how. cro •• ·.ectlon of the aftershock.; the plate interface inferred from the focal mechanl.m of the maln.hock. i. marked by a .olld line. the stars represent recent major earthquakes, left to right, 1985 mlchoacan second malnshock; 1979, petatlan; and 1985, mlchoacan first malnshocks (from stolte, et al., 1986). the subduction zone of oaxaca (95°-98° w), along with taiwan, was given the highest probability (0.9) of recording accelerations grater than 0.2 g in 10 years (bolt and abrahamson, 1985, in anderson et al., 1986). 100 k~ 1\ a' 104' los 102' fig. 2 -a. 101' figure 3. 8elsmotectonlc setting of the 1985 mlchoacan, mexico earthquake. features of the seafloor shown are rivera fracture zone (rfz), east pacific ri.e (epr), orozco fracture zone (ofz), "ehuantepec ridge (tr), and middle american trench (mat) the trans·mexlcan volcanic belt(tmvb) is also shown. (from, munguia et at 1986). 2.3 guerrero oaxaca region to the south of the petlatan area in the middle of the coast, the area without recent large earthquake activity is the guerrero seismic gap, where four large earthquake occurred along the trench between 1899 and 1911. stall on positions ". ". -'. '. '" i '" -. i . 100 200 distance from trench (km) a' i" i i" i ,ml..lt", i ,cll! ~ trench o '--~'&t-......~-i~.'* :"" . .. . .......~~_.' ~' .............. : .....-.:---;-~ ......... --~i!: 50 0w o fig. 2-b. 20'" 2.4 petlatan area the southeast of michoacan in the northern guerrero, had events in 1943 and 1979 (ms = 7.6), with 36 years interval. it is interesting to note that most of the aftershocks of the 1979 petlatan earthquake, occurred down dip and towards the continent relative to the aftershochs of the 21 september, 1985 earthquake. 2.5 colima seismic area to the northwest, the colima area recently had events in 1941 and 1973 (ms = 7,5) with 32 years interval. the extend of the gap of michoacan was constrained by the rupture areas of this event to the northwest. 2.6 jalisco area the largest earthquake prior to the 1985 event, was located near to jalisco cost, in 1932 (ms = 8.1, fig.4). comparison of seismograms at pasadena with records of other large mexico events shows that the michoacan earthquake is basically the same size as the 1932 jalisco earthquake, and clearly larger than other significant events in mexico since 1932. the intensity in mexico city from the 1932 event, was only v (figueroa, 1959, in eissler et al., 1986). however, it was more complex and of large duration than the september 1985 shock. 106"w 100" 98" 96"104" 102" ("\. \ \ \ \ .,cd guzman '1' '. ....-.j ~ 19321\\' sepl 19 o • \::.1 ..... 'xca1973 .7.}bspp'21 (~ 1981 • '4fji;, • acol'ulco lii04c 1979 ~ () /1982 1,y~ 1957 tvn r-. cut!?!?_ 1968 ~ t!?o~ 1978 oaxaca 2o'n'\ \ \,.. .,me.,co clly 200km figure 4. map of central mexico showing the aftershock areas (ellipses) of subduction events since 1950 m > 7. the september 1985,earthquakes is p-iotted as filled star, and its aftershock as a smaller star. (from, eisler et ai., 1986). 2.7 the ometepec segment this subduction zone, 98.50 -99.50 w has been identified a~ a seismic gap by mccann et ai., (1979, in nishenko and si.ngh, 19~7), based ~>n the amount of the time elapsed in this area since a previous major earthquake. prior large earthquakes in this gap occurred in 1937 (ms = 7,7) and 1950 (ms = 7,3). on 7 june 1982, an earthquake doublet (ms = 6.9; 7.0) occurred in the eastern portion of the ometepec gap. this segment is adjacent to the guerrero segment, which last ruptured as a series of large and great shocks in 1899 and 1911. during this century, the recurrence history observed along the ometepec segment has been what some anomalous when compared to the adjoining segment in oaxaca and has exhibited a variable recurrence history. the majority of ~aveforms can be characterized as complex reflecting irregular rupture propagation and moment release. 2.8 michoacan seismic gap the septe"'!ber 19, 1985 ~ve!'t occurred in this area (figs. 3-4), was designated as seisrmc gap (the michoacan gap) because prior to this event, at least 74 years had elapsed sin.cethe la~t large earthquake (ms = 7,5). the gap has been qurescent sl~ce the 1~11 ev~nt had occurred here. segments of the plate interface immediately adjacent to the michoacan ~ap have experienced recent events at short and regular intervals. . in 1981, the playa azul earthquake (mw = 7,3) occurred in the center of the michoacan gap. the epicenter of the september 1985 event was located in the northern segment of the michoacan gap between the 1973 and 1981 aftershock zones. observations have pointed to the high seismic potential in the michoacan gap: large recent coastal terraces suggesting tectonic uplift were found onshore near orozco fracturing zone (mcnally and minster, 1981, in munguia et ai., 1986). the distribution of the hypocenters and focal mechanisms indicated only minor deviations from the regional patterns of stress distribution along the middle american trench in mexico. this suggested normal seismic subduction (lefevre and mcnally, 1985, in munguia et ai., 1986). the so~rce mechanism of the playa azul earthquake 1981, located in the middle of the gap was similar to other earthquake along the coast of mexico (mcnally and minster 1981, in munguia, et al., 1986). ' 2.9 mexicali-imperial valley eart.hquakes in the imperial valley and its southward extention (called the mexicali valley in mexico) are caused by the strike-slip boundary between the north american and pacific plates. in the historical past, it has been the site of several. moderate sized earthquakes (m = 6 to 7). allen (1968, in anderson and bodin, 1987), suggests that such past.behavior may represent the longer term past and future of this.fault.segme.nt.~~derson and bodin (1987), examined the historical setsmtcity since about 1850. the largest observed earthquake in the area has been of the magnitude (mw = 7.1), during the december 1934 event. 3. seismological activity in mexico ~e~ic~ has long been known to be a country of great seismic risk. it ~sformed as part of the pacific rim, known as the "ring of ~i.re·, because ~f its vigorous earthquake and volcanic actl~l.ty.so, .as me~lco is located in a zone of high tectonic activity, as is see in the section 4,it has been possible to record the seismic activity during the last century (table 1). 3.1 principal earthquakes in mexico the table below lists the principal earthquakes in the mexican subduction zone, obtained from different sources' thus, there are differences between authors for locations' m.agnitudeand depths from, priestley (1986); nishenko and singh (1987); person (1985, 1986); sarria (1986); singh et ai., (1987, 1988); anderson and bodin (1987)' astiz et al. (1987). " 3.2 description of principal historical earthquakes the following historical events occurred in the middle american trench during the last century. on may 3, 1887, a great earthquake took place in the northern mexican state of sonora. the largest town affected was bavispe, which was situated on unconsolidated material and suffered heavy damage. th~re were 1500 inhabitants, of whom 42 were killed. shaking was reportedly felt from prescott arizona to mexico city distances of over 400 miles from the 'probable epicenter (richter, 1958). gutenberg and richter (1954, in umam seismology group, 1986), based on the location of earthquake on june 7, 191.1, .(ms = 7,9~, pointed out a gap of high seismic potential in the region. the literature indicates that the intensity pattern of the 1911 events is similar with the epicenter near coastal michoacan. thus the 1911 events have ~een felt as strongly in mexico city, but were less damaging there because of the smaller population and smaller degree of urban development at that date. the distance from mexico city to the guerrero gap is shorter than to any other region, along the middle american trench (fig.5). the last major events located in the guerrero gap, were in 1899 (m = 8) and 1907 (m = 8). the acapulco earthquake (ms = 7.5) occurred in the southern guerrero in 1957and damaged hundreds of buildings in mexico city. the south of acapulco the plate interface is fairly well filled in with recent large earthquakes. table 1 event date location depth laton longow 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 03251806 05311818 05041820 11221836 03091845 04071845 05051854 06191858 10031864 05111870 03271872 03161874 02111875 09031875 05171879 07191882 05031887 05291887 09061889 12021890 11021894 06051897 01241899 01201900 05161900 01141903 04151907 03261908 03271908 07301909 06071911 12161911 11191912 06021916 12291917 03221928 06171928 09041928 10011928 01151931 01031932 01181932 11301934 07261937 12231937 04151941 02221943 01061948 01061948 12141950 07281957 05111962 05191962 07061964 08231965 02031968 08021968 01301973 08181973 04231975 02011976 06071976 03191978 11291978 01261979 03141979 10241980 10251981 06071982 06071982 07021984 09151985 09191985 09211985 09251985 02031986 04301986 18,9 103.8 19,1 103.6 17.2 99,6 20,0 105.0 16.6 97.0 16.6 99.2 16.3 97.6 19.6 101.6 18.7 97.4 15.8 96.7 15.7 96.6 17.7 99.1 21.0 103.8 19.4 104.6 18.6 98.0 17.7 98.2 31.0 109.2 17.2 99.8 17.0 99.7 16.7 98.6 16.5 98.0 16.3 95.4 17.1 100.5 20.0 105.0 20.0 105.0 15.0 98.0 16.7 99.2 16.7 99.2 17.0 101.0 16.8 99.9 19.7 103.7 16.9 100.7 19.9 99.8 17.5 95.0 15.0 97.0 16.2 95.5 16.3 96.7 16.8 97.6 16.3 97.3 16.1 96.6 19.8 104.0 19.5 103.5 19.0 105.3 18.5 96.4 17.6 101.2 18.9 102.9 17.6 101.2 17.0 98.0 17.0 98.0 17.2 98.1 17.1 99.1 17.3 99.6 17.1 99.6 18.3 100.4 16.3 96.0 16.3 96.0 16.6 97.7 18.4 103.2 18.3 96.5 16.5 98.9 17.0 100.3 17.2 100.88 16.9 99.9 15.8 96.8 17.3 101.0 17.3 101.4 18.2 98.3 17.8 102.6 16.6 98.4 16.4 98.5 16.8 98.5 17.8 97.2 18.5 102.7 17.6 101.8 18.2 102.9 15.0 92.0 18.2 103.1 16 16 16 16 16 16 16 18 16 20 72 20 20 15 47 65 16 28 30 16 16 the mexican earthquake of november 19, 1912. was very widely felt but was damaging only in a rather sharply bounded area measuring about 50 by 20 kilometers centering about 10 km from mexico city. this was a highly unusual event,with its occurrence so far from the principal seismic zone of mexico (richter, 1958). ms the seismic moment and the time since the last large earthquake in michoacan (1911), fit an empirical relation between moment and recurrence time found for the guerrero-oaxaca region of the subduction zone. a large earthquake (m = 7.8) occurred in oaxaca in november 29, 1978, making another large earthquake in that region less likely in the near future (anderson, et al., 1986). the locations of a few aftershocks of the april 15, 1941, event, obtained using s-p times recorded by stations of the mexican network, supports the hypothesis that the rupture area of the 1941 earthquake included part of the faults broken by the 1973 colima earthquake and by the 1985 event. the colima earthquake of 1973 (ms = 7.5), to the nw, and the petatlan earthquake of 1979 (ms = 7.6), to the se (figure 1) constrained the extent of the gap with the areas of rupture in these events. aseismic subduction or large recurrence periods for this gap were suggested by singh et al. (1980, in unam seismology group, 1986). petatlan 1979, event, was a large earthquake that occurred in the adjoining region, to 1985 event. for this event, valdes, et al, (1980 in stolte et. al, 1986), derived a velocity structure from the aftershocks, in an area in the immediate vicinity of the 1985 epicenter region (figures 5 and 6). huajuapan de leon, october 24, 1980, earthquake was located well inland and was deeper than others (65 km.), which was on a normal fault. all mains hocks were shallow, with thrust earthquakes along of the pacific coast. the earthquake of playa azul on october 25,1981 (ms = 7.3) occurred in the central part of the michoacan gap, rupturing an area of over 40 by 20 square kilometers. even after this earthquake, the seismic potential of the gap could not be defined with any precision (unam seismology group, 1986). this event was widely felt in southern mexico, causing damage in the state of michoacan and in mexico city, where 11 people were injured and one person died. the ometepec earthquake of june 7, 1982, (ms = 6.9 and 7.0) occurred in the eastern portion of the ometepec gap, which provided an opportunity to reexamine the rupture zones of earlier events in this region via the joint epicentral determination method (nishenko and singh, 1987). the 1950 (ms = 7.3), 1957 (ms = 7.7), and 1962 (ms = 7.2 and 6,9) earthquakes and their principal aftershock ruptures, compared with that of the great 1907 earthquake (ms = 8.0), suggests that the latter three events may represent a delayed multiple rerupture of the 1907 zone. the time interval between the 1937 and 1950 shocks was short compared to the similar sized events in oaxaca in november 29, 1978. 7.5 7.7 7.6 7.7 7.5 7.9 7.7 7.5 7.3 7.9 7.4 7.3 7.5 7.4 7.0 7.5 7.3 7.2 7.0 7.2 7.4 7.4 7.9 7.9 7.4 8.1 8.0 8.1 7.5 7.4 7.7 7.5 7.0 7.1 7.7 7.5 7.8 7.4 7.6 7.8 8.2 7.8 7.0 7.3 7.5 7.7 7.5 7.0 7.0 7.3 7.5 7.0 7.2 7.4 7.8 5.9 7.4 7.5 7.1 6.2 5.6 6.4 6.4 7.8 6.6 7.6 7.0 7.3 6.9 7.0 6.0 5.9 8.1 7.6 5.2 4.7 7.0 3.3 maps of location of the principal earthquakes the following figures show different maps with the locations of the epicenters and aftershocks zones for the events which occurred in the mexican subduction zone, also including the 1985 event. 4. mexico 1985 earthquake the september 19, 1985, michoacan earthquake (ms = 8,1), was the most serious natural disaster to date in mexico's history. it caused over 10,000 deaths in the mexico city and left an estimated 250, 000 homeless. this earthquake occurred along a segment of the coco-north american subduction zone, that had been identified as the michoacan seismic gap (section 4). this event was followed by another major earthquake on september 21 (ms = 7,5) and the last aftershok occurred on 30 april of 1986 (ms = 7,0). the earthquake occurred at 13h 17m 47.6s gmt; 7h 17m 46,6s local time on thursday 19. the shock had a surface wave magnitude of 8,1 and the aftershock of magnitude 7,5 on friday evening 21 at 7h 38s p.m. local time. 1985 guerrero acce'.rooraph anay • ace.i.'~f.1lh 'tt eplc .... l~ figure 5. map of coastal mexico, epicenters and aftershocks zones of 1985 events and location of strong motion stations in guerrero array on 19 september 1985. short dashed lines show limits of aftershocks of large earthquakes in this region since 1950. peak accelerations are given for the north, east and vertical components in parentheses (from anderson, 1986). 106°w 104° 10zo 100° 98° 96° <, \ \ z~n \ \ ",cd. guzman ",mexico city ~" apr 30, 1986% 193z ---• "sept 19, 1985 zookm 'x ·*~.sept z1, 1985 18° lo/ 1973 ~(9bpetallan (~ 1981 •'414(, -..............aca uleo mlchoocon gap '1c/f04/"' 1979 ~ ri986~ l44~ 7 p~i9s5~' : ~ 195 160 is. ,~@'9si,i985-m2 cue/?, 1968 -, ~ 19850 'r[ro , 1978 iookm '-::'./ ~oaxaca 103· 102· figure 6. map of central mexico showing the aftershocks areas (ellipses) of interplate thrust events since 1950 with m > 7. the september 1985 is plotted as a filled star and its aftershock as smaller star. the epicenter of the m=7.5 aftershock of april 30 1986 is show as an open star (from astiz, et.al., 1987). the modified mercalli intensity was ix at lazaro cardenas, 30 km from the epicenter. astoundingly it was also ix in parts of the capital city 400 km. away. there is no other historic earthquake having an intensity of ix in mexico. analysis of body waves and long-period surface waves from this event, shows that the event was fault plane with a low dip angle (8,9°) striking parallel to the middle american trench; fault plane azimuth of 288° and small component of left lateral motion (=72°) with a point source depth of 18 to 25 km and a seismic moment in excess of 1.0x10 to 1.7x10 dyn-cm. the earthquake was a multiple event with a second source of identical moment, fault geometry and depth occurring approximately 26 seconds after the first. instrumental records seem to indicate that the rupturing process was a complex one with several breaks occurring along the fault line in quick succession. as a result of this, the earthquake that was generated was one of the long duration. ground shaking in coastal regions lasted up to five minutes eefit, (1986 in oegg, 1987), and parts of mexico city were shaken for three minutes. 4.1 epicenter location mexico, has a surface area of approximately two million square kilometers. it is situated between the pacific ocean and the gulf of mexico bordering the united states in the north and guatemala and belize in the south. the epicentral region of the major event was located off the pacific coastline near to the small town of lazaro cardenas in the state of michoacan (figure 5). faulting extend parallel to the coast for a distance of 100 km. north and south of the town. it is estimated that during the earthquake thrust forward more that three meters along the length of this rupture zone. the september 19, event, triggered a digital strong motion array installed along the michoacan-guerrero coast. relocation of first shock, (unam, 1986), useding p and s waves arrival times at stations cal and s waves arrival time at vil and uni (figure 7). constraining the depth to 16 km. suggested by synthetic modeling. the main shock and aftershocks were located using hypo 78 and a crustal model for the adjacent petlatan region (unam, 1986). an approximate location of the second subevent was obtained using the strong motion data. the arrival time of s waves of the second subevent were picked by visually cross correlating the waveforms of the first and second subevents on the near field strong motion records. this analysis suggest the second subevent occurred about 95 km se of the first one probably near uni (unam, 1986), they give this eplcentral iocattons for main shocks: (18,141° n; 102.707°w) and second shock (17,618°n; 101,815°w) respectively. 4.2. acceleration signs aftershock mungia, et.al. (1986) described acceleration signals recorded for nine aftershocks of the september 19, earthquake. they used three a-700 teledyne-geotech digital strong-motion instruments at two sites, la villita dam, and 12 km. to the west of zihuatanejo. peak horizontal accelerations of 0,0005 g. to 0.031 g. were recorded at epicentral distances between 10 and 75 km. for earthquake with magnitude (mb) between 4,5 and 5,3. it was observed that the peak accelerations recorded at a site on the embankment of the dam, (near the crest), are approximately three times those recorded on the abutment bedrock portion of the dam. although these sites were spatially separated by no more than 300 m. differences among their records are also significant. wave forms recorded at the embankment site look more complex than those from the abutment site. this fact, as well as the higher peak accelerations on the embankment provides evidence of a strong influence of the structure of the dam on the ground motion at the embankment site. +-. +1975 _<,j(7~1 i· " i '",\ \ \ .---.-..... ""<,<, ......o ~ 100_ 1...' -'6 ~, aftershocks 20 to 30 sep. 1985 10 s.. lebs sswc<4 • ... ~4 • slronq mollon iiolloni ~ portable itollonl + + + ,.-l-~;__----..;;..-"l.----__;=-----------~~--j,-...• 0" iol" ll)i" figure 7. the location of the first and second main shocks and their aftershocks are shown. solid triangles and dots are epicenters of aftershocks which occurred between the first and second main shocks. from unam (1986). 4.3. factors contributing to the catastrophe there were two major factors contributing to the catastrophe; resonance in the sediments of an ancient lake that once existed in the valley of mexico, and the long duration of shocking compared with other coastal event in the last 50 years. in hall and beck (1986), there are two simplified models which could explain the apparent resonance phenomenon; one involving body waves propagation up through there valley sediments, and the other involving surface waves propagating across the valley. the real situation is sure to be more complex, although a combination of these models may give at least a qijalitative explanation. the importance of the resonance phenomenon in contributing to the structural damage is apparent from the response spectral comparison in figure 7. it is the reason why the damage was confined to areas of mexico city on the lake bed. it also explains why high-rise buildings suffered the most severe seismic attack. 5. interpretation of source mechanisms the focal mechanism of the first main shock from first motion data indicates a thrust-mechanism. the likely fault plane has an azimuth of 2880 and dip of 90 , according to eissler et at (1986). 5.1 teleseismic source mechanisms teleseismic long-periodseismograms showed that the first earthquake consisted of two subevents separated in time by 27 sec subevent occurred about 95 km. se of the first one. teleseismic records of the september shock can be modeled by a single source. 5.1.1. prior event: this has an additional seismic feature. the occurrence of a moderate (mb = 5.2) event downdip from the mainshock approximately 100 km. on may 28,1985 (3.7 months before the mainshock). this normal faulting event, together with the seismic quiescence and aftershock are included two small event (mb = 4,5 and 4,4) on january 20 and june 19 of 1985, which occurred close to the subsequent mainshock epicenter and broke the seismic quiescence figure 9. � 0.6 . z o 0.' 1.0 2.0 3.0 ".0 ~.o natural p[rioo of structure, figure 8, acceleration re.ponse .pectra for the tabacuya and set .trong motion record. (from beck and hall, 1986) n ••••j • 312' 84' -90' ri>= cs = a = • • • •8 figure 9. fault mechanl.m for the may 28,1985 (mb = 5.2) event prior * mat15.8) ffjanp ~shk(6.51 )fhkc "'26,~tss8 m.'8.5 119.2. iog.i126.0" i ~ ioi.o3143153lr aku 82 ,6<88.6" 314" \ / ~. 0.43' '" / i,' 31, 71.2' gua(74) ','31. ~ sep1.19.1985 ~6' wj ·/ rar 4.8 ~;," .•" : lpa95 06.4., .....u·· ~1 291' jmvv~685'" ~.16.1:?i'~f237· ,__ '0 pel 78 67.6" afi6.0 ~8a(5.2) t· 141' ~~~,~ _"!28;<i 75.4't 107.9' 59.~' 229. _il-w 2490 1930 149 _ \j lj v 1.-24s 60s figure 10, p wave. of 19 september earthquake at teleseismic distance., observed and calculated waveforms shown are from long· period wwssn recordings and one geoscope station (ss8), from astlz, et.al. (1987) 5.1.2 the first shock: the main shock was centered about 400 km. from mexico city on the pacific coast, near the town of lazaro cardenas on the border between the states of michoacan and guerrero. the first motion data constrain one steeply dipping nodal plane with dip of 810 and strike equal 1270 and the orientation of the second plane was resolved with wave form modeling. the figure 9 shows observed p·wave seismograms from 12 wwssn stations and one geoscope station (ssb) and synthetic seismograms calculated for the focal mechanism point source depth, and time function that provided,optimal waveform fit for the main shock. the time function is a multiple source consisting of two trapezoids of equal duration (16 sec.) and seismic moment, and the second source beginning 26 sec after the first. the point source depth is 17 km and the focal mechanism shows an overall thrust geometry on a low angle plane (=90; =2880 and =720 ). the horizontal projection of the slip vector orientation is n39°e, which agrees with the local convergence direction of the cocos plate calculated at the epicenter from the rm2 pole of rotation minster and jordan (1978, in astiz et.al., 1987). the seismic moment estimated from the p·waves amplitudes is 7.2 ± l6x10 dyne-em, 5.1.3 the second shock: was on 21 of september, only a few p wave forms of the large event aftershock are available. the waveforms are consistent with the a mechanism identical to the main shock, with a slightly greater source depth of 22 km (figure 11). the aftershock time function is a single source with a duration of 13 sec. the seismic moment recovered from the body waves is 1.2x10 dyn-cm. 5.1.4. the third shock: it was the april 30 aftershok. long period p waves of the aftershock from 15 wwssn stations are shown in figure 12. the synthetic seismograms are calculated for a point source 21 km deep and source time duration of 10 sec. first motion data constrain only one the nodal planes as is common for most large mexican subduction events. the second fault plane was resolved from waveform modeling. the fault parameters determined are = 2800 , = 120 and = 700 • the seismic moment for each station is given next to station code.the seismic moment is 2.0x10 dyn-cm. 5.2, the seismic moment the seismic moment and fault geometry, were resolved from amplitude and phase spectral data of multiple rayleigh and love passages at 256 sec. generally following kanamori and given (1981, in eissler et ai., 1986). the seismic moment is between 1.1-1.7x10 dyn-cm. (mw=7.9-8.1), thus the event is comparable with the largest previous event en the mexico historic record the 1932 jalisco (ms=8.1). the seismic moment range of the aftershock is 2.9-4.7x10 dyn-cm. (mw=7.6-7.7). 5.3, centroit-moment tensor method observations of three dimensional ground motion at a single point on the surface of the earth following an earthquake can be sufficient to determine the zeroth order moment tensor of the event. ekstrom et. al. (1986) adapted the centroid moment tensor method (cmt) and apply to the september 19 event. sept. 21. 1985 i 60s i 1.2 guassb (0.5) 27mo = 1.2x 10 dyne em e =288° 8=90 >..=72° d =22 km 7 10 ..u>: o't 135 figure 11. the second shock waveforms mechanism. ~e,.,"~~,,,t:·" ~e"',"if 00.' roo rh~~v 331" ~l 338" f~ 18' h~~vv-3" ~ 35" -h~r 38. shk (1.51 hon 3.3 ~t::· t:~:0 april 30. 1986 val 2.7 mo 02.0 x 10 26 dyne em ~~ t:j.: 77.4 8=280° d e zl krn t-r;:390 s =120 3 7 >.=70° ds:~.;:,.+.",.:r.59.8' ~~~; 525' ~j~ ',~ 349' ~t:37'"f263· ~ v240· 1\1 149' 1l-142' f84" f 60' ~ figure 12. long-period wwssn recordings of p wave. for the 30 april 1988 (mw=6.9) earthquake are .how by the upper trace •. (from a.tlz, et.al., 1987). the september 19, 1985 michoacan, produced high-quality records at (hrv) harvard, mass. long period surface wave data including the first orbit rayleigh and love waves, were used in a single station source inversion in the mantle wave band. they used seismograms of hrv for three components and the synthetic seismograms corresponding to the obtained mechanism. the variance reduction is 89%. figure 13 shows the geographical location of the epicenter and a lower hemispheric projection of the obtained source mechanism.also shown is the mechanism of the september 21, determined using the same approach and published cmt solutionsfor other earthquake in this region (1977-1985). the solution by a scalar moment given for they, for the 1985 event is in good agreement with the previous mechanisms obtained in this area and with the procedure subduction of the cocos plate. they obtained a scalar moment for the main earthquake of 1.3x10 dyne-cm. 21n 3/9/81 5.9 16tl 105101 figure 13. locetion and beat double couple focel mechaniama for the september 19 and 21 eventa analyzed with cmt. (from ekatrom et al., 1986). the focal mechanism obtained indicates faulting on an shallow plane consistent with the procedure subduction of the cocos plate. a direct deconvolution of the observed p-waves train indicates that the moment release occurred in two subevents each of 20 sec duration with a peak-to peak separation in time of 25 sec (ekstrom et ai., 1986). main shock aflershock yl yl dfl cu p8'tjai0 figure 14. source mechanisms for the michoacan, 19 (left) and 21 (right) earthquakes from spectra mw. t, p and n are the three principal axes. d is double couple and mechanisms vectors (from riedesel et ai., 1986). 6. comparison between 1985 earthquakes 6.1 the 1985 valparaiso, chile earthquake the march 3 1985, central chile earthquake (ms = 7,8) ruptured a well studied seismic gap along the chilean subduction zone. the epicenter of this event is located near the center of an approximately 300 km. long region which ruptured is a great event in 1906 (mw= 8,2). the epicenter of the march 3, 1985 earthquake (33,13° s and 71.87°w) is located at the southern end of the 1971 rupture zone in the same general region as the epicenter of the great 1906 earthquake. the focal mechanism centroid moments tensor solution from the harvard group as listed in the pde report (stri e = 11°; dip = 26°; slip vector = 110°) with a moment of 1,0 x 10 dyne-em. the depth extent of this event is some what difficult to determine. the harvard and usgs moment tensor solutions give depths of 41 and 51 km. respectively, which are relatively deep for an understhrusting event. 6.2. chilean gap the region between 32°s and 35°s along the central chilean trench had been recognized as a mature seismic gap with a strong potential for large or great underthrusting earthquake based on historical seismicity. previous great earthquakes occurred in this trench segment in 1647, 1730, 1822 and 1906 and give average repeat time of 86 ± 10 years (nishenko et al., 1987). in addition to the historic evidence for a seismic gap proposed that the occurrence of a large shallow compressional outer-rise event in this region on october 16,1981 (ms = 7,2) indicated a build-up of compressional stress. 6.3. comparison between earthquakes for comparison, the figure 15 shows peak accelerations recorded during the chile and mexico earthquakes. the chile, is another subduction thrust earthquake with a geometry similar to the september event. peak accelerations for the mexico data show much less scatter than the chile data, and seem to be almost a lower bound. the depths to faulting in chile and mexico may be similar, but site conditions for the two sets of data are different. the mexican stations at distances less than 300 km. are generally on small piers on competent rocks outcrops. the chile site conditions are less uniform, generally in one and two-stored buildings and on a variety of volcanic or sedimentary rocks and alluvial deposits; these conditions result in resonances and amplification relative to basement rock, contributing to the scatter and higher values in peak accelerations (anderson, et al., 1986). 6.4 analysis in the 1985 mexico, the mainshock records, spectrum and time functions contain less high frequency radiation than those of the 1985 valparaiso, chile. apparently, the michoacan ruptured two relatively smooth, strong patches which generated large 30 seconds waves, but small 1 to 10 second waves. such behavior contrasts with the valparaiso event which had a more complex rupture process and generated more 1 to 10 seconds energy. this difference is consistent with the higher nearfield accelerations recorded for the valparaiso event. -~-------...,....----------, o o o ..... tee. ,. sep,.mb., "'5. m•• i.' o chi", 3 yareh 1985. us ~ 7.8 0 0 0 0 0 0 0 00 0 0 0 0 b 0 c 0 10 0 0 0 bi! o • 0o 0 0 "0 0• 0 ga• ii 0• e • a, \.:0 • o "b, 50 .00 .50 200 250 joo oialenee to n•• , •• ' ell.ranoel.. (11m» figure 15. peaks of horizontal components of accelerations plotted against distance outside the boundary of the after-shock zone (from anderson, et al., 1986). houston and kanamori (1986), analyzed teleseismic broadband and short period gdsn (global digital seismic network) records of 19 and 21 of september aftershok. also gdsn records are available for the march 3,1985 valparaiso event, along the chile trench, the second largest earthquake in 1985. figure 16 show the average moment rate spectra for the michoacan and valparaiso events. the spectral values at the low frequency end of the spectra were obtained from the scalar seismic moments determined from long-period waves. theoretical spectra for an w source model with a stress drop of 30 bars are show as a reference. the dashed line shows the average source spectrum for two large interplate subduction earthquakes. relative to the average source spectrum, the spectrum of the michoacan event is enriched at 30 seconds and depleted a 1 to 10 seconds. this behavior contrasts with that of the valparaiso event which had a more complex rupture process and generated more 1 to 5 energy. this observations suggests that the high accelerations in mexico city are not source effect. this spectral character may be typical of earthquake in the mexical subduction zone, figure 16. these differences between the two earthquakes discemed from teleseismic records can also be seen in the near-field strong motions. 7. damage across mexico this earthquake which was felt superficially in an area of approximately 825,000 square kilometers (08gg, 1987). the strength of the quake was felt as far north as houston, texas, 1,500 kilometers from the epicenter, and generated a small tidal wave that was recorded along parts of the mexican and ei salvadorean coast (figure 18). the fact that mexico city is constructed on an old lake bed has led to significant correlations between the geographical distribution of damage from september 1985 and early earthquakes and the type of subsoil underlying the damage area (handon and martin, 1987). ..' fj ", f j ,.' i j 196\ w:choa:an , 1965 ar 1(r5hoc~ . ). 196\ valparaiso ii .....~5 0 •• 110,7. & ~ .•., ", j; f '. i" ..'.. ~ r ..... \! • j . 10' 'j ~~.! .. ..~i'. ..' '~.~ ..' /.' ,.' s,« ~ ,,~;: \ \..' ' i \ 10' ..' ,.i i ~f -, '. \ '\.\ 10' "10-: ,tt-' ... '0-1 u/-1 '0-1 10' figure 16. comparison of the average moment rate spectra for chile and mexico events.the vertical bars shows standard deviations at selected frequencies. the theoretical spectra for an w model are show by thin lines. the dashed line on mexico, show th, averege spectrum of 7 subduction events scaled m • 8.0, from houston and kanamorl, 1986. 1985 v"lparaiso. chile 1985 iolichoacan. mexico asperity distribution~'.~f&@j. 1~~0\-·,·,::···:·---=-'q~"\ \\.. 0 qq.j ,~ ~ 101 \, source time runctions figure 17. schematic comparison of record, time functions, and inferred asperity distributions for (a>the 1985 chile and (b) the 1985 mexico earthquakes. o ~m 200 '--j figure 18. areas affected in mexico (from degg, 1987). 7.1 mexico city geology mexico city lies at an elevation of 2,250 m. above sea level and is situated at the geographical center of mexico, on the western side of the valley of mexico, formed about 30 million years ago by the faulting of an uplifted plate. this is a basin 65 km by 80 km in extent, surrounded by volcanic mountains of up 5,000 meters in height. the volcanoes are mainly of middle to upper tertiary age, but there also some dating from the pleistocene. the city was founded by the spaniards on the site of the great aztec city of tenochtitlan. at the time of the spanish conquest in 1521, tenochtitlan was situated on an island in the middle of the large lake (texcoco), that was one of several covering the floor of the valley (08gg, 1987). since the conquest, more and more of the lake has been reclaimed, so that today only a small remnant survives to the east of the city. figure 20 show that a large part of modern mexico city rests upon the former bed of lake texcoco. this is made up of thick pleistocene lacustrine clays belonging to the tacubaya formation. this clay is montmorillonite and illite and was derived from volcanic ash deposited in the lake (from marsal and mazari, 1959, in oegg, 1987). the thickness of the clay varies across the lake bed but is generally between 7-37 meters followed by the first hard layer of 1-3 meters thick which is mostly sand; a second clay formation to a depth of 50 meters, and the second hard layer again mostly sand; and then a complex conglomerate with soil having all sizes of grains even boulders, down to rock. the western and north-western parts of the city lie outside the ancient lake boundary and are situated upon sands and silts of the tarango formation. these consist of material that was eroded from the volcanic cones surrounding the valley of mexico during the upper pliocene and lower pliocene and subsequently were deposited as alluvial fans. above the highest former lake shoreline, the tarango formation has a thickness of approximately 600 meters. many high-rise buildings on the lake bed have pile foundations which pass through the soft tucubaya clays to the more compacted layers of the tarango beneath. the southern part of the city rests on basalt lava flows, the youngest of which date from about 2400 years ago. the lava flows are referred to in general terms as the pedregal. subsoil divisions: the area and surrounding of mexico city have been divided in three zones on the basis of subsoil properties: athe lake zone: this is lake bed area. the tabacuya clays of this zone have a very high natural water content, of between 200 and 400 percent (oegg, 1987). consisting of a 25 to 80 meters of thickness highly compressible so that small increments of pressure are capable of producing large settlements. this formation is underlain by resistant sands and the subsoil is so soft that many heavy buildings in the central part of the city have experienced some subsidence, which across parts of the lake zone has been great due to intensify due to excessive pumping of water out of the tabacuya clays for industrial and domestic purposes. bthe transition zone: this is between the lake zone and hill zone,it is comparised of a sandy silty layer of alluvial origin with occasional intervals of clay. it comprises areas which previously formed the shores of lake texcoco, so that the thickness of the tabacuya clay is greatly reduced as is its water content 100-200 percent (oegg, 1987). the clay at this zone is much less compressible that the lake zone. cthe hill zone: this zone incorporates the hilly surface layer of volcanic lava flows or volcanic tuff. this part have rest upon the tarango sands and silts which are typically dense and contain much less water that the tabacuya deposits. ex lak e texcoeo b -> ·····7ciiy i boundary ..........~-_ ....~...i ~(~.~.?~, .i~·"\~lja km 5 '--------' a simplified cross-section along line a b a km 5, , a b olacustrine deposits (tacubaya formation) r:.:·(·.::] basaltic lava ~ ~ volcanic rock j:~,'?" ;:) alluvial deposits (tarango formation) 1:::::1 volcanic rocks pliocene _ middle and upper fertiary figure 19. geology of the mexico city area (from 08gg, 1987). 7.2 mexico city damage the earthquake caused considerable damage to modern construction in mexico city and was restricted to the western part of the lake zone and within two to four kilometers of alameda square (figure 20). several hundred buildings collapsed; more would have to be demolished and thousands reinforced. the damage in the adjacent lake bed areas was very sporadic. one factor for this earthquake damage is that the ground-shaking was amplified because mexico city lies on an old lake bed that resonates with the seismic waves that are the most amplified are the low-frequency signals, which can do the most damage to taller building. the earthquake was very selective, and only a relatively small number the buildings suffered severe damage. at to time of the quake, mexico city had one of the world's most stringent building codes, based on experience gained from several quakes in 1957 and 1979. nevertheless, the quakes intensity in particular areas of the city was much larger than what the buildings were designed for. nearly all buildings that collapsed during the earthquake were located on the lake zone (figure 21). ground motion was digitally recorded at four free field sites on the hill and lake zone: unam, viv, cda and sct. other city sites were in buildings and many have been influenced by building response. the characteristics of the shaking at various site are related to the observed damage (anderson et al., 1986). zone of major damage limit of area of investigation former lake shoreline ,.,: , : i • i '. ~ hospital :':-. \ rjj:+~ :-":. . \ ., . .., . •.•• " ~'medical -•••. centre general o km 1 l..---i d park figure 20. the zone of major damage in mexico city (from 08gg, 1987). the combination of intensity, regularity and duration made it selectively devastating. there is no other earthquake with an intensity of ix at such a focal distance anywhere, certainly not an earthquake with a magnitude of 8.1. in fact, there is no record anywhere in the world with a horizontal peak ground acceleration of 0.20 associated with a two second period. the ground motions regularity in the hardest hit area of mexico is manifest in the spectra for the records at the site mentioned. according to the accelerographs installed in the valley, the macroseismic waves arrived with an exceptionally high energy content at around two seconds. the statistics show that the damage was concentrated in buildings of 6 to 15 stories, which were drawn into the most energetic frequency band of the lake bed motions. an interesting characteristic of the damage in mexico city is that a great number of buildings collapsed in their upper stories, leaving the lower portions intact. buildings were destroyed at ciudad guzman, jalisco. damage also occurred in the states of colima, guerrero, mexico, michoacan, morelos, parts of veracruz and in other areas of jalisco (figure 5). the maximum modified mercalli intensity was ix at mexico city, ciudad guzman and the pacific coast, towns of lazaro cardenas, ixtapa and la union. felt reports were received from mazatlan, sinaloa to tuxtla, gutierrez, chiapas and as far away as guatemala city, the earthquake was also felt at the cities of brownsville, mcallen, corpus christy, ingran and ei paso in texas. it was felt very strongly by people on board the ship nedlloyd kyoto located at 17°35,4' north and 102° 36,9'. landslides caused damage at alenquique, jalisco and near jala, colima. rockslides were reported along the highways in the ixtapa area and sand blows and ground cracks were observed at lazaro cardenas. the damage in some parts depended on the population of the states, but also,it depend the hard bedrock that was beneath the coastal region and served to transmit the shock waves without amplification, for example, acapulco, situated on granite bedrock at a distance of 270 km. from the epicenter, was strongly affected by the earthquake. coastal cities such as zihuatanejo and ixtapa, only 90 km. from the epicenter, suffered only moderate damage. in contrast,parts of mexico city were severely shaken and experienced very heavy loss, being located more than 400 km. from the epicenter. it is very vulnerable to earthquake ground motion because a large part of the city is build on the humid and not hard sediment of the former lake bed. the two large hydroelectric plants that the comision federal de electricidad are located in the epicentral region, a subduction zone of thrust faults that dip eastward beneath the mexican coast. the la villita dam, an earth and rockfill structure built on alluvial deposits, experienced a few minor superficial cracks along the downstream edge of its crest. the infiernillo dam, a rockfill structure with a clay core founded on rock, was undamaged figure 5. 7.4 tsunami steel construction is rare in the capital, yet one of the most spectacular examples of damage occurred to the complex of five steel frame buildings at conjunto pino suarez. a 21 story tower overturned at the 3rd floor level and fell to the south onto another tower of 14 stories. many residential buildings in the central part of the city were put cut of use by the earthquake, and thousands of people were made homeless. the residential complex of tlatelolco situated just to the north of the center of the city, was particularly affected. the estate covers 150 hectares and consists of predominantly medium to high rise structures. to the south of the center, the majority of the streets in colonia roma experienced damage, several high rise residential buildings collapsed, and the benito juarez hospital just to the south also collapsed. • coa (81,95,27 n i • accelerograph 6 severely damaged nuildmq • collapsed building ~ zone wun many collapsed oneand two-siorv nouses (brick and aooce: a 1000 m~ figure 21. map of areas of damage in mexico city with accelerographs stations, generalized soli classification and sites cf mort building damage (from anderson, 1986) the number of damaged buildings in mexico city was significant and,therefore, provides an excellent opportunity to extend the knowledge of the behavior of buildings subjected to severe earthquakes. 7.3 damage in near areas this event caused between 3 and 4 billion dollars in damage, was felt by almost 20 million people, four hundred twelve buildings collapsed another 3,124 were seriously damaged in mexico city,and about 60 percent of the the tsunami that was generated by the 1985 earthquake, caused some damage at lazaro cardenas, zihuatanejo, and only rough sea waves observed in manzanillo, with estimated waves heights were 3 meters at zihuatanejo and 2,8 meters at lazaro cardenas located over the mexico coast. tide stations recorded maximum wave heights of 1.4 meters at acapulco, mexico; 60 cm. at la libertad, ecuador; 58 cm. at acojatla, ei salvador; 24 at kahului, hawaii and at pago pago, american samoa; 22 cm. at hilo, hawaii; 21 cm. at baltra island, galapagos; 14 cm. at apia, somoa; 7 em. at rikitea, gambier islands, and 5 em. at paeete, tahiti. there were some reports, still unconfirmed, that some ships off the pacific coast of mexico observed unusually heavy seas up to 30 meters high near the time of the earthquake and that some fishing boats were reported missing. seiches were observed in east galveston bay, texas and in swimming pools in texas, new mexico, colorado and idaho. water well fluctuations were recorded at ingleside, texas; santa fe, new mexico; rolla, misouri; hillsborough county, florida and smithsburg, maryland. other tsunami effects occurred in an island in the balsa river delta when there is a large plant of fertimiex fertilizer. the plant is complete, and there is not any visible damage to buildings, stock, or steel towers, but there was extensive soil failures on all access roads. some sea water was still pooling outside pelleting warehouse; also plenty of ground water was expelled by sand craters. wave heights were variously estimated at 2 to 3 meters, which agrees with observations of runup visible from aircraft on small beaches south of lazaro cardenas, (lomnitz et al., 1986). in on other place, the diammonium phosphate mill, extensive sand cratering and cracking of roads suggests intensity ix. to north of the beach where the tsunami flooding took place (paso de burras), railroad tracks running along open beach just above high sea level, were deformed into s-shaped curves like spaghetti. some wetted sticks showed tsunami over-topped the rails by 30 cm. the lagoon draining into the estuary provided a handy mean sea level reference; estimated elevation of rails was 1.60meters above mean sea level, this suggests tsunami height was 1.90 meters above mean sea level. this was the first positive measurement of a tsunami in mexico and requests resurveying of elevations at this spot. fortunately the same spot was surveyed two weeks before the earthquake, so, the survey should provide evidence of changes in coastal elevation. taken together these observations suggest that: the earthquake of 19 september, 1985 was a tsunamigenetic event, a multiple rupture which probably broke the plate boundary from the trench inland. this would agree with the direction of collapse of filler walls, suggesting one or several lurches of the ground in the ocean-ward direction. the director of international tsunami center in honolulu, hawaii attributes the relatively small size of the tsunami to the shallow angle of this subduction on the small vertical motion of the crust during the earthquake. 8. risk of future earthquakes in mexico the september 19 earthquake of 1985 caused unprecedented damage in mexico. none of the earthquakes along the mexican subduction zone since the 1932 jalisco earthquake (unam, 1986) exceeded rupture lengths of about 100 km. therefore the unusually large rupture length of the 1985 shock, complied with the explosive growth of earthquakes in the last 30-40 years,and may be particularly responsible for the damage. scientists have observed that the seismic gap theory is based upon the stress built up at a relatively constant rate along the zones of plate boundary interaction, so that a section of a given fault might be expected to generate earthquakes at regular intervals.· sections of a fault that have been unusually quiet for a long time are termed "seismic gaps". the identification of these gaps provides the basis for many modern systems of predictions of where earthquakes are most likely to occur next. however, it is much more difficult to predict when the earthquake will actually take place. repeat times of large earthquakes along the central american trench, in the south of the michoacan seismic gaps lies the guerrero seismic gaps, are usually between 30-70 years (fig.22). this shows that the sections of the fault that generated the 1985 earthquake had been quiet for a much longer period than this, and it had not experienced a mayor seismic event for over two centuries and formed part of a quiet zone termed the michoacan seismic gap. the likelihood of an earthquake along this section of the fault therefore remains high, and may have been not seriously increased following the transfer or stress from the rupture in the michoacan gap. still further to the south are other seismic gap that also, have the potential to produce large earthquakes in the near future. most notable of these sections is the fault of acapulco. in other words, the risk of other large earthquakes in mexico in the future, might continue to be considered high. the event of 1985 has not reduced the risk of another large earthquake in mexico, in fact, it may well have increased it. another factor is that the ground vibrations were amplified more or less, five times, because mexico city is situated on an old lake sediments beds, that resonate with the seismic waves. the thickness of the beds is such that they most amplify the low frequency signals, which can do the most damage to taller buildings. this will occur during other earthquakes with the lake beds motions at least as intense as those in september 1985. this must be considered as being a strong possibility in the near future. the september 1985 disaster suggests that conformance to the seismic provisions in the uniform building code is not sufficient to prevent the collapse of buildings in a major earthquake in mexico city. engineers recognize that earthquakes cause structures to vibrate, introducing dynamic loads. with a dynamic analysis, they find that in a major earthquake the structural loads greatly exceed the building code seismic design loads and will cause cracking of the concrete and masonry and yielding of the steel. but for major earthquake loads, reliance is placed on the capacity of structures. at present, ductility and energy absorption in a structure during an earthquake are difficult to quantify. it is also believed that the ground motion will subside before a major earthquake can collapse a building that has been designed according to code provisions. in other words, the safety factors will be revised for a new code, considering the a city's soils and the magnitude of the earthquake. geologic and geophysical properties of the soils tests are being made in mexico. every city in the world has unique geological and soil conditions, and the codes must reflect this. singh et al, (1987) show an empirical prediction of ground motion in mexico city from earthquakes in the mexican subduction zone. they analyzed strong motion data from coastal earthquakes recorded at a single firm site, in the caudad universitaria located in the hill zone of mexico city, because there are more there than at any other in the city. they used a functional form to express peak horizontal ground motion employed the acceleration, velocity and magnitude in the surface waves. the attenatuation relations derived in their paper can provide reliable estimations of peak ground motion from future critical earthquakes along the mexican subduction zone, thus is necessary for more data so that solution will be representative. also, in a study of the mexicali-imperial valley (anderson and bodin, 1987) identified six overlapping fault segments of inferred rupture history and they suggest plausible occurrence times for future events in the six segments. they compared the historical data with predictions of the time-predictable and slip-predictable models, and they suggest the possibility of an earthquake there in the near future. 9. conclusions the 1985 mexico earthquake, occurred as a result of slipping in the subduction process between the cocos and american plates. this event ruptured a seismic gap where unusual oceanic lithosphere is subducting beneath mexico. there are many lessons to be learned from this event in the context of the earthquake engineering, it is important to understand the causes of structural failure. in the seismological context, it is an important event in both a global and regional sense. those had been a global hiatus in the occurrence of large earthquakes from 1980 until 1985 when two large earthquakes occurred; the march 3 chilean (ms=8.0) and september 19 mexican (ms=8.1) earthquakes. -mexico cit v • 19'8 \ gull 0' _ •• ,co '900\ \ ~ 1832 19.32 19"21 .1911 1932 '06" • 19?8 ~9?8 1!65 ~o * epll::silhe 01 19 seolembe e • 1985 -figure 22. epicentres of twentieth century mexican earthquakes (from o8gg, 1987). references anderson, j.g. et al. 1986. strong ground motion from the mlchoacan, mexico earthquake science 233:1043-1049. anderson, j.g. 1986. the september'85 mlchoacan, mexico earthquake, symposium, earthquake notes 57(1), ssa-86. charleston, s.c., january-march. anderaon, j.g., and p. bodin. 1987. earthquake recurrence models and historical seismicity in the mexicali-imperial valley, bull. seismol. soc. am. 77(2):562-578. astlz, l., h. kanamorl and h. eissler. 1987. source characteristics of earthquakes in the mlchoacan seismic gap in mexico, bull. selsmol. soc. am. 77(4):1326-1346. beck, j.l. and j.f. hall. 1986. factors contributing to the catastrophe in mexico city during the earthquake of september 19,1985, geophys. res. lett. 13(6):593-596. degg, m.r. 1987. the 1985 mexican earthquake, modern geology 11:109-131. eissler, h., l. aatlz and h. kanamorl. 1986. tectonic setting and source parameters of the september 19, 1985, michoacan mexico earthquake, geophys. res. lett. 13(6):569-572. ekstrom, g, and a.m. dzlewonskl. 1986. a very broad band analysis of the mlchoacan mexico earthquake of september 19, 1985, geophys. res. lett. 13(6):605-608. ekstrom, g., et aj. 1986. single station cmt, application to the mlchoacan mexico earthquake of september 19, 1985, geophys. res. lett. 13(3):173-176. hall, .j.f. and j.l. beck. 1986. structural damage in mexico city, geophys. res. lett. 13(6):589-592. handon, r.d, m. eerl and h.w. martin. 1987. performance of steel structures in september 19 and 20 1985, mexico earthquake, earthquake spectra 3(2). e.e.r.1. huston, h., and h. kanamorl. 1986. source characteristics of the 1985, michoacan mexicoearthquake at periods of 01 to 30 seconds, geophys. res. lett. 13(6):597-600. lomnltz, c., j.r. gonzalez-rulz and ch. stolte. 1986. selsmogenesis of the 1985 great (ms = 8.1) michoacan mexico earthquake. geophys. res. lett. 13(6):585-588 . munguia, l., .g.w. simlla, k.c. mcnally and h. thompsom. 1986. the september 19, 1985, michoacan earthquake: aftershock acceleration data recorded by a tempory installation of strong motion instruments, geophys. res. lett. 13(6):581-584. nlshenko, s.p., and s.k. sigh. 1987. the acapulco-ometepec, mexico, earthquakes of 1907-1982: evidence for a variable recurrence history, bull. selsmol. soc. am. 77(4):1359-1367. person, w.j. 1985a. seismological notes, july-august 1984, bull. seismol. soc. am. 75(3):893-895. person, w.j. 1985b. seismological notes,september-october 1984. bull. seismol. soc. am. 75(4):1217-1220. person, w.j. 1986a. seismological notes, september-october 1985. bull. seismol. soc. am. 76(4):1161-1165. person, w.j. 1986b. seismological notes, january-february 1986, bull. seismol. soc. am. 76(6):1847-1849. priestley, k.f. 1986. source mechanism of the september 19, 1985, michoacan earthquake and its implications, geophys. res. lett. 13(6):601-604. richter, c.f. 1958. elementary seismology, w.h. freeman and co. san francisco, california, p. 594-595. riedesel, m.a., t.h. jordan, a.f. sheehan and p.g. sliver. 1986. moment-tensor spectra of the 19 sept. 85 and 20 sept. 85, mlchoacan, mexico earthquake, geophys. res. lett. 13(6):609-612. sarrla, m.a. 1986. terremoto de mexico, publicacion de la compaf'\ia central de seguros, bogota, colombia. sigh, s.k., e. mena, r. castro and c. carmona. 1987. empirical prediction of ground motion in mexico city from coastal earthquakes, bull. seismol. soc. am. 77(5):1862-1867. sigh, s.k., e. mena and r. castro. 1988. some aspects of source characteristic of the september 1985 michoacan earthquake and ground motion amplification in and near mexico city from strong motion data. bull. seismol. soc. am. 78(2):451-477. stolte, ch., k.c. mcnally, j. gonzales, g.w. simlla, a. reyes, c. rebollar, l. munguia and i. mendoza. 1986. fine structure of a post failure wadati-benioff lone. geophys. res. lett. 13(6):577-580. unam, seismology group. 1986. the september 1985 michoacan earthquakes. aftershock distribution and history of rupture, geophys. res. lett. 13(6):573-576. untitled earth sciences research journal earth sci. res. j. vol. 13, no.1 (june 2009): 6-15 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding carlos roberto a. candeiro laboratório de geologia, curso de geografia, campus do pontal, universidade federal de uberlândia, avenida josé joão dib, 2545, bairro progresso, zip code 38302-000, ituiutaba, minas gerais state, brazil corresponding author. tel.: ++ (55) 32692389. e-mail address: candeiro@yahoo.com abstract the peirópolis paleontological site (late cretaceous, maastrichtian) in minas gerais state, brazil yielded an important assemblage of fossil vertebrates. the typical occurrence of south american widespread taxa in peirópolis is important for correlation between the brazilian bauru basin and argentinean late cretaceous basins. the fishes, turtles, anuran, crocodilians and dinosaurs known from peirópolis (marília formation) resemble the patagonian latest late cretaceous vertebrate faunas but lacks ornithischian dinosaurs. key words: vertebrates, late cretaceous, marília formation, minas gerais state, brazil. resumen en el sítio paleontológico de peirópolis (cretácico superior, maastrichtiano) ubicado en la província de minas gerais, brasil, hay un contenido muy importante de vertebrados fósiles. la ocurrencia en peirópolis de taxa típicos y de amplia distribución en la américa del sur es importante para la correlación entre la cuenca bauru y cuencas argentinas del cretácico superior. los pesces, tortugas, anuros, cocodrilianos y dinosaurios de peirópolis (formación marília), se equivalen a las faunas de vertebrados del neocretácico de patagonia, sin embargo, sin la presencia de dinosaurios ornitísquios. palabras claves: vertebrados, neocretácico, formación marília, província de minas gerais, brasil. 6 manuscript received: january 10th, 2009. accepted for publication: april 30th, 2009. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:37 p p p composite 133 lpi at 45 degrees introduction the upper cretaceous beds at the peirópolis paleontological site of (uberaba municipality, minas gerais state; fig. 1) consist on an entirely continental sedimentary succession. the upper cretaceous bauru group units at peirópolis are divided into the uberaba formation (coniacian-santonian) and marília formation (late maastrichtian) in ascending order. the uberaba formation occurs as small patches mainly in the peirópolis village. although extensive paleontological studies have been carried out in peirópolis area (e.g., price, 1955; estes and price, 1973; báez and peri, 1989; gayet and brito, 1989; carvalho et al., 2004; frança and langer, 2005; kellner and campos, 2005; kellner et al., 2005; campos et al., 2005; novas et al., 2005; santucci and bertini, 2006; novas et al., 2008; salgado and carvalho, 2008; candeiro et al., 2008), fossil vertebrates are unknown so far from uberaba formation, except for unstudied dinosaur eggs found near peirópolis. on the other hand, abundant vertebrate remains have been reported from the late maastrichtian marília formation (specimens and localities are listed in the tab. 1 and 2). the first discovery of vertebrate fossils was vertebrate fragments from the “ponto 1 do price” from the serra da galga member (marília formation) in the 1940´s (price, 1951; candeiro and bergqvist, 2004). since then, scattered dinosaur bones and other vertebrate fossils including fishes, turtles, and crocodilians have been found in quarries, roads cuts, and stream beds as a result of a centro de pesquisas paleontológicas llewelyn ivor price (peirópolis) team survey. abundant plants (palynomorphs) and mollusks have been located from the peirópolis quarries. they clearly indicate that diversity of vertebrate communities existed during the time of marília formation deposition in the peirópolis area. except for some the crocodilians, however, the vertebrate faunas remain inadequately published. therefore, the purpose of this paper is to review the vertebrate fossils from the marília formation at this locality and report their occurrence to the paleontological community. comments and correlations with other vertebrate-bearing formations of late cretaceous age from argentina are discussed. the potential significance of marília formation vertebrates from peirópolis may be seen especially in the 7 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding n 0° 70°w 10°s 20°s 40°w 50°w 60°w 51”00’ 18”00’ 47”30’ 18”00’ 20”30’ 51”00’ brazil 20”30’ goiás state são paulo state 47”30’ 0 10 20 km legend town localization study area road river paranaiba river araguari river tejuco river peirópolis grande river uberaba prata ituiutaba araguari uberlãndia figure 1. geographic distribution of the upper cretaceous “marília-points” of peirópolis paleontological site. point 1 (5 quarries); point 2 (1 quarry); point 3 (1 quarry); point 4 (2 quarries); point 5 (1 quarry). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:39 p p p composite 133 lpi at 45 degrees context of roughly contemporaneous areas from southern south america (candeiro and bergqvist, 2004). methodology the data on vertebrates from peirópolis paleontological site for the present work are mostly based in literature sources as well as in the direct observation of the specimens deposited at museu dos dinossauros (peirópolis district, uberaba town, minas gerais state, brazil). for the stratigraphic units in the bauru group i follow the arrangement proposed by dias brito et al. (2001). results geological setting the peirópolis paleontological site is northeast of bauru basin (upper cretaceous) (fernandes and coimbra, 1996). the bauru group in triângulo mineiro region is represented by sediments belonging to the adamantina, marília and uberaba formations (fig. 2). these layers overlie basalts belonging to serra geral formation (são bento group, paraná basin), sandstone belonging to botucatu formation (são bento group), metamorphic and proterozoic outcrops belonging to araxá and canastra groups (sanfransiscana basin) and mesozoic intrusion of the 8 carlos roberto a. candeiro figure 2. geological map of bauru group in triângulo mineiro region (modified fernandes and coimbra, 1996). julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:39 p p p composite 133 lpi at 45 degrees soerguimento of alto paranaíba (suguio et al., 1979). outcrops of the marília formation are unevenly distributed throughout the peirópolis paleontological site. according to barcelos and suguio (1987), that unit was deposited by coalescing alluvial fans, and later reworked by a braided system in association with calcretes and lacustrine calcareous sediments. barcelos (1984) subdivided the marília formation into the echaporã, ponte alta, and serra da galga members, although only the latter two members are exposed in the peirópolis area. ponte alta and serra da galga members show the following characteristics (garrido et al., 1992): ponte alta member– a basal “calcareous white member”, characterized by thin and medium calcareous beds with pebbles and calcareous nodules which were deposited in lakes; serra da galga member– “member of white sandstone and conglomerate”, composed of bleached conglomerate and sandstone with feldspar matrix deposited by alluvial fans, braided rivers and lakes. those two members, according to suguio (1973, 1980), were formed in a partially dry climate, which would have favored the fossil preservation found in these layers. dias-brito et al. (2001) assigned a late maastrichtian age to the marília formation. this lithostratigraphic unit yielded invertebrates and plants, and abundant vertebrates (fig. 3). vertebrates from the marília formation are represented by anurans (baurubatrachus pricei), lizards (pristiguana brasiliensis), crocodilians (itasuchus jesuinoi, peirosaurus tormini, uberabasuchus terrificus), chelonians (cambaremys langertoni), dinosaurs (abelisauridae, carcharodontosauridae, maniraptora, and eutitanosaurian baurutitan britoi, trigonosaurus pricei, uberabatitan ribeiroi, aeolosaurus). vertebrate paleontology the vertebrate fossils of the triângulo mineiro region have been known since 1920, however only with the pioneer studies of paleontologist llewellyn ivor price, which started in 1947, the fossils found in the peirópolis paleontological site became not only more known, but at the same time better classified. just a small part of collected material in this paleontological site was described to the level of species due to fragmentary nature of most the specimens which made a more refined description impossible (e.g., fig. 4). abundant fish fossils have recently been found in the peirópolis. to date four localities have yielded isolated scales and teeth. they produced characiformes, perciformes and siluriformes. the species which were described formally are represented by amphibian baurubatrachus pricei báez and peri, 1989; lepidosaurian pristiguana brasiliensis estes and price, 1973 and crocodilians itasuchus jesuinoi, peirosaurus tormini price, 1955 9 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding bauru group peirópolis site serra da galga mb. (marília fm.) ponte alta mb. (marília fm.) uberaba fm. legend plant vertebrate fish frog lizard turtle crocodilian sauropod theropod figure 3. upper cretaceous bauru group generalized geological section in peirópolis showing occurrence of the biota. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:39 p p p composite 133 lpi at 45 degrees and uberabasuchus terrificus carvalho, ribeiro and avilla, 2004. the dinosaurs represent the most abundant remains and they are represented by theropods which were attributed by candeiro (2002) and candeiro et al. (2004) to abelisauridae and carcharodontosauridae, first abelisauridade bone remains by (novas et al., 2008) and maniraptoran claw recently described by novas et al. (2005). the sauropods are represented by the titanosaurians baurutitan britoi campos and kellner, 2005 and trigonosaurus pricei campos, kellner, bertini and santucci, 2005; uberabatitan ribeiroi salgado and carvalho, 2008 and others different elements e.g., teeth, vertebrae, ribs, hemal arches, phalanges and osteoderms. these specimens were studied by campos and kellner (1999), 10 carlos roberto a. candeiro figure 4. reptilian fossils from the peirópolis paleontological site. a, lower jaw itasuchus jesuinoi (modified from price, 1955); b, pelvis of the “titanosauridae” (modified from campos and kellner, 1999); c, abelisauridae teeth; d, carcharodontosauridae tooth; e, theropoda indet. tooth. scale bar in mm. table 1. vertebrate fauna of the marília formation from peirópolis paleontological site. area 1 area 2 area 3 area 4 area 5 ponte alta member x x x serra da galga member x x vertebrata vertebrata indet x x x x x pisces characiform x x perciform x x siluriform x x anura baurubatrachus pricei x reptilia squamata pristiguana brasiliensis chelonia chelonia indet. podcnemidae crocodylomorpha itasuchus jesuinoi peirosaurus tormini uberabasuchus terrificus dinosauria sauropoda titanosauria x x x x x x julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:40 p p p composite 133 lpi at 45 degrees 11 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding area 1 area 2 area 3 area 4 area 5 “titanosauridae” a “titanosauridae” b “titanosauridae” c “titanosaurinae” “titanosaurus” aeolosaurus sp. baurutitan britoi trigonosaurus pricei theropoda theropoda indet. abelisauridae indet. carcharadontosauridae indet. maniraptora x x x x x x x x x x x x x table 2. fossiliferous areas (quarries), in the peirópolis paleontological site, minas gerais state. areas generalities reference selected area 1 (quarry 1) this outcrop has been known as “caieira” campos and kellner (1999) area 1 (quarry 2) this outcrop is inside of “caieira” kellner (1996) area 1 (quarry 3) this quarry has been known as “point 2” price (1955) area 1 (quarry 4) this outcrop has been known as “point 3” campos and kellner (1999), trotta et al. (2002) área 1 (quarry 5) this outcrop is known as “mumbuca” powell (1987, 2003) area 2 the outcrop is situated on the br-262 throughway about 3,5 km east of peirópolis campos and kellner (1999) area 3 the outcrop is situated on the ponte alta district throughway about 18 km east from peirópolis magalhães-ribeiro (2002) area 4 (quarry 1) quarry 1 is known as the “ rio tejuco 1” albuquerque and candeiro (2003) area 4 (quarry 2) quarry 2 is known as the “rio tejuco 2” albuquerque and candeiro (2003) area 5 this area has been known as “partezan quarry” bertini et al. (1993) julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:40 p p p composite 133 lpi at 45 degrees santucci (2002), powell (2003) and santucci and bertino (2006) who attributed part of these remains to titanosauria (see detailed assignments in tab. 1). dinosaur eggs found in sediments of the serra da galga member are related to “megaloolithidae” oofamily, according to magalhães-ribeiro (2002). furthermore there are numerous cranial and post cranial remains belonging to crocodilians and turtles. numerous fragments of disarticulated bones and coprolites are found in marília formation sediments in the peirópolis paleontological site, however they have not been attributed to taxonomic level below vertebrata; they are housed in the centro de pesquisas paleontológicas llewellyn ivor price and museu de ciências da terra (rio de janeiro). fossil sites from marília formation in the peirópolis paleontological site of: the first fossil site was studied by price in the 1940´s. since then, new fossil localities of the marília formation have been found in the uberaba municipality with five mentioned in literature so far (fig. 1; tab. 2). the best known fossil vertebrates of marília formation found in the peirópolis paleontological site of are listed below. discussion the present study and those of the previous works in the marília formation peirópolis paleontological site), it is attempted here to point a comprehensive point of view of this area. the peirópolis paleontological site contains a moderately diverse late cretaceous (maastrichtian) vertebrate assemblage, from which some of the taxa are reported also in argentina (e.g., chubut, neuquén and malargüe groups; candeiro and martinelli, 2003; lamanna et al., 2003; martinelli and forasiepi, 2004; leanza et al., 2004). some of the vertebrate taxa reported at peirópolis occur with other vertebrates known from other areas of argentina, and are important for correlation with argentinean units. supporting this correlation are the mesoeucrocodylian peirosaurus tormini and the titanosaur aeolosaurus, which are found in bajo de la carpa, allen, angostura colorada, los alamitos, and bajo barreal formations in patagonia. abelisaurid and carcharodontosaurid remains are known to occur in the bauru group. in addition to the titanosaurian material from peirópolis area, remains of those sauropods are widely known in upper cretaceous units of argentina. additional support for correlation is given by the presence in peirópolis of the gondwanan “megaloolithidae” eggshells. other support of the correlation between peirópolis area and argentinean areas may be given by other vertebrate groups, but more detailed studies are needed before this can be done. for example, characiform, siluriform and perciform fishes and podocnemid turtles are known in most of the neuquén and malargüe groups, but they cannot be used as correlation tools since their remains are not well known from the marília formation. ornithischian records are remarkable in argentina (late cretaceous), however, not a single ornithischian remain has been identified so far in the marília formation or the bauru group. considering that ornithischians occurred in argentina, this bias may be from lack of extensive or least systematic prospecting in the marília formation rather than from depositional or preservational events. conclusions so far, four fossil sites have been recorded at the peirópolis paleontological site of, the marília formation with a diverse vertebrate fauna of upper maastrichtian age. these fossils are represented mainly by fishes, frogs, turtles, crocodilians, and dinosaurs. the diverse fossiliferous, but poorly known marília formation accumulated in the northwestern bauru basin during the latest cretaceous. deposition occurred on a fluvio-lacustrine environment. the maastrichtian age of the marília formation suggested by previous authors has several important biogeographical and paleontological correlations. first, it indicates that the marília formation assemblage is approximately contemporaneous in age with 12 carlos roberto a. candeiro julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:40 p p p composite 133 lpi at 45 degrees the vertebrate assemblage recovered from the patagonia (e.g., chubut, malargüe, and neuquén groups). this finding is certainly consistent with striking similarities between faunas from these two southern south america areas. second, this maastrichtian marília formation fauna, when considered in the light of the late cretaceous discoveries, was different by the lack of ornithischian records in brazil, but which are found in argentina. acknowledgments this paper forms part of the undergraduate project of the r. candeiro who gratefully acknowledges financial assistance in the form of an undergraduate fellowship (1998-1999) received from the council of scientific from brazil (cnpq/166/98) under dr. ignácio brito advisor (financial supported by fapemig/ufu/801/98 fellow). the manuscript was substantially improved after the revision work of dr. yaneth muñoz-saba (universidad nacional de colombia). i would like to thank prof. luiz carlos borges ribeiro and the centro de pesquisas paleontológicas llewellyn ivor price crew, dr. adriano rodrigues dos santos (universidade federal de uberlândia, ufu), dr. lílian paglarelli bergqvist (universidade federal do rio de janeiro, ufrj) and thiago da silva marinho (ufrj) for reviewing the earlier drafts of the manuscript. the manuscript was substantially improved by of dr. simone magnuco (museo civico di storia naturale di milano, italy), dr. walter hartwig (touro university, united states) and dr. david gillette (museum of northern arizona, united states). the author also would like to thank the friends, lisete, ana and fernando brito, that in some ways helped us to start our paleontological studies, since they are wife, daughter and son of professor ignácio machado brito, who opened the doors of his house and ignited my life into paleontology. references albuquerque, j.a.c. and candeiro, c.r.a., (2003). new potential fossiliferous site uberabauberlândia higway (maastrichtian) – triângulo mineiro, minas gerais state, brazil. paleontologia em destaque. 40. 4-5. báez, a.m. and peri, s., (1989). baurubatrachus pricei, nov. gen. et sp., un anuro del cretácico superior de minas gerais, brazil. anais da academia brasileira de ciências. 61. 447-458. barcelos, j.h., (1984). reconstrução paleogeográfica da sedimentação do grupo bauru baseada na sua redefinição estratigráfica parcial em território paulista e no estudo preliminar fora do estado de são paulo. rio claro. 190 p. dissertation (livre docência). departamento de geociências, universidade estadual paulista. barcelos, j.h. and suguio, k., (1987). correlação e extensão das unidades litoestratigráficas do grupo 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gerais, brasil. rio de janeiro. 136p. msc thesis, programa de pós-graduação em geologia, universidade federal do rio de janeiro. 13 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:40 p p p composite 133 lpi at 45 degrees candeiro, c.r.a. and martinelli, a.g., (2003). comparison of dinosaurs and sedimentary environments between the bauru and neuquénmalargüe groups (upper cretaceous): a preliminary approach. paleontologia em destaque. 17. 48-48. candeiro, c.r.a. and bergqvist, l.p., (2004). paleofauna cretácica do centro de pesquisas paleontológicas llewellyn ivor price, peirópolis, uberaba, minas gerais. revista sociedade & natureza. 16 (31). 27-35. candeiro, c.r.a., bergqvist, l.p., novas, f.e. and currie, p.j., (2004). theropod teeth from the marilia formation (upper maastrichtian), minas gerais state, brazil. journal of vertebrate paleontology. 24 (3, suppl.). 43a. candeiro, c.r.a., santos, a.r., bergqvist, l.p., ribeiro, l.c. and apesteguía, s., (2008). the late cretaceous fauna and flora of the uberaba area (minas gerais state, brazil). journal of south american earth sciences. 25. 203-216. campos, d. a.,; kellner, a.w., bertini, r.j., santucci, r.m., (2005) . description of a titanosaurid caudal series from the bauru group, south-east brazil. arquivos do museu nacional. 63. 565-593. carvalho, i.s., ribeiro, l.c.b. and avilla, l.s., (2004). uberabasuchus terrificus sp. nov., a new crocodylomorpha from the bauru basin (upper cretaceous), brazil. gondwana research. 7 (4). 975-1002. dias-brito, d., musacchio, e.a., castro, j.c., maranhão, m.s.a.s., suárez, j.m. and rodrigues, r., (2001). grupo bauru: uma unidade continental do cretáceo no brasil – concepções baseadas em dados micropaleontológicos, isotópicos e estratigráficos. revue paléobiologie. 20 (1). 245-304. estes, r. and price, l.i., (1973). iguanid lizard from the upper cretaceous of brazil. science, 180. 748-751. fernandes, l.a. and coimbra, a.m., (1996). a bacia bauru (cretáceo superior, brasil). anais da academia brasileira de ciências. 68 (2). 195-205. garrido, a.e., ferreira, a.m. and garcia, a.j.v., (1992). estratigrafia e sedimentologia do grupo bauru em peirópolis município de uberaba, minas gerais. revista da escola de minas. 45. 112-114. gayet, m. and brito, p. m., (1989). ichtyofaune nouvelle du crétacé supérieur du groupe bauru (états de são paulo et minas gerais, brésil). geobios. 22. 841-847. kellner, a.w.a., (1996). remarks on brazilian dinosaurs. memoirs of the queensland museum. 39 (3). 611-626. kellner, a.w.a., campos, d.a. and trotta, m.n., (2005). description of a titanosaurid caudal series from the bauru group, late cretaceous of brazil. arquivos do museu nacional. 63 (3). 529-564. frança, m.a.g. and langer, m.c., (2005). a new freshwater turtle (pleurodira, podocnemididae) from the upper cretaceous (maastrichtian) of minas geris, brazil. geodiversitas, 27 (3). 391-411. lamanna, m.c., luna, m.a., casal, g.a., martinez, r.d., ibiricu, l. and sciutto, j.c., (2003). new crocodyliform and dinosaur discoveries from the upper cretaceous (campanian¿maastrichtian) upper member of the bajo barreal formation, southern chubut province, argentina. journal of vertebrate paleontology. 23 (3, suppl.). 70a. leanza, h.a., apesteguía, s., novas, f. and de la fuente, m., (2004). cretaceous terrestrial beds from the neuquén basin (argentina) and their tetrapod assemblages. cretaceous research. 25. 61-87. magalhâes-ribeiro, c.m.m., (2002). ovo e fragmentos de cascas de ovos de dinossauros, provenientes da região de peirópolis, uberaba, 14 carlos roberto a. candeiro julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:41 p p p composite 133 lpi at 45 degrees minas gerais. arquivos do museu nacional. 60 (3). 223-228. martinelli, a.g. and forasiepi, a.m., (2004). late cretaceous vertebrates from bajo de santa rosa (allen formation), río negro province, argentina, with the description of a new sauropod dinosaur (titanosauridae). revista del museo argentino ciencias naturales “bernardino rivadavia”. 6 (2). 257-305. novas, f.,e., carvalho, i.s., ribeiro, l.c.b. and méndez, a.h., (2008). first abelisaurid bone remains from the maastrichtian marília formation, bauru basin, brazil. cretaceous research. 29. 625-635. powell, j.e., (1987). morfologia del esqueleto axial de los dinosaurios titanosaurios (saurischia, sauropoda) del estado de minas gerais, brasil. atas, 10º congresso brasileiro de paleontologia, sbp/sociedade brasileira de paleontologia, rio de janeiro: 155-171. powell, j.e., (2003). revision of south american titanosaurid dinosaurs: paleobiological, paleobiogepgraphical and phylogenetic aspects. records of the queen victoria museum. 111. 1-173. price, l.i., (1951). ovo de dinossauro na formação bauru, do estado de minas gerais. notas preliminares da divisão de geologia de mineralogia. 53. 1-7. price, l.i., (1955). novos crocodilideos dos arenitos da série bauru, cretáceo do estado de minas gerais. anais da academia brasileira de ciências. 27. 487-498. salgado, l. and carvalho, i.s., (2008). uberabatitan ribeiroi, a new titanosaur from the marília formation (bauru group, upper cretaceous), minas gerais, brazil. palaeontology. 51. 881-901. santucci, r.m., (2002). revisão dos titanosauridae (saurischia, sauropoda) do cretáceo superior continental da bacia bauru. rio claro. 179 p. msc. thesis. instituto de geociências e ciências exatas, universidade estadual paulista. santucci, r.m. and bertini, r.j., (2006). a large sauropod titanosaur from peirópolis, bauru group, brazil. neues jahrbuch für geologie und paläontologie. monatshefte. 6. 344-360. suguio, k., (1973). calcretes of the bauru group (cretaceous), brazil: petrology and geological significance. são paulo. 236 p. dissertation (livre docência). instituto de geociências, universidade de são paulo. suguio, k., (1980). fatores paleoambientais e paleoclimáticos e subdivisão estratigráfica do grupo bauru. resumos expandidos, 1ª mesa redonda: a formação bauru no estado de são paulo e regiões adjacentes, sbg/sociedade brasileira de geologia, são paulo: 15-26. suguio, k., sviero, d.p. and felitti filho, w., (1979). conglomerados polimítricos diamantíferos de idade cretácica de romaria (mg): um exemplo de sedimentação de leques aluviais. atas, 2º simpósio regional de geologia, sbg/sociedade brasileira de geologia, rio claro: 217-229. 15 vertebrates of the marília formation (late maastrichtian) from the peirópolis paleontological site: toward a better understanding julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:10:41 p p p composite 133 lpi at 45 degrees articulofisica 10 augusto a. gómez capera and ilaria leschiutta determining the parameters of earthquake sources in south america from macroseismic intensity data (ceresis database) augusto a. gómez capera 1,2 and ilaria leschiutta 1 1 instituto nazionale di geofisica e vulcanologia, sezione sismologia applicata,viabassini, 15 – 20133 milano, italy e-mail: gomez@ mi.ingv.it 2università degli studi di milano-bicocca, dipartimento di scienze geologiche e geotecnologie, piazza della scienza, 4 – 20126 milano, italy. e-mail leschiutta@mi.ingv.it abstract in the past decades many different procedures have been made to handle the historical data for the determination of the earthquake source parameters. this has been only possible dealing with historical data interpreted and compiled as intensity data points (idp). one of the most interesting tools is the boxer algorithm, a parameterisation method that computes the parameters of the earthquake source in terms of latitude and longitude of the epicentre, magnitude of the event, length, width and azimuth of the box, which represent the surface projection of the sismogenic source. applying the boxer algorithm we have used intensity data points available from the ceresis database (earthquakes with i 0 ≥ 8mm) to obtain a preliminary idea of the possible sources of some historical earthquakes of the south-american region. at a first approach to the south-american historical seismicity we generally can affirm that our results agree fairly well with seismological data and geological background as reported in literature. key words: earthquake source parameters, intensity data points, parameterization techniques, south america. resumen en pasadas décadas han sido desarrolladas diferentes metodologías para determinar parámetros de fuentes de terremotos a partir de datos macrosísmicos. esto ha sido posible usando los datos históricos interpretados y compilados como datos puntuales de intensidad. una de las más interesantes herramientas es el algoritmo boxer, un método que calcula parámetros de la fuente del terremoto en relación con la latitud y longitud del epicentro, magnitud del evento, longitud, ancho y azimut del «rectángulo» que representa la proyección en superficie de la fuente sismogenética. datos de intensidad con i 0 ≥ 8mm, disponibles en la base de datos del ceresis, han sido usados para obtener una idea preliminar de las posibles fuentes de algunos terremotos en suramérica. en esta primera aproximación para la sismisidad histórica suramericana, los resultados obtenidos son discretamente acordes con datos sismológicos y geológicos encontrados en la literatura. palabras clave: parámetros de fuentes de terremotos, puntos de intensidad, técnicas de parametrización, suramérica © 2004 esrj unibiblos. manuscript received may 2004. paper accepted october 2004. 11 determining the parameters of earthquake sources in south america from macroseismic intensity data (ceresis database) introduction macro seismic data are frequently used for parameterisation of the sismogenic sources and evaluation of seismic hazard. consequently, several seismological institutions have formalised their historical data into homogeneous macroseismic databases and have developed online databases to increase the dissemination of data through the web (rubbia, 2004). the increased availability of historical data interpreted in terms of idp has stimulated seismologists to develop standard and repeatable procedures to determine earthquake source parameters, in order to lower the level of subjectivity they are usually determined upon. the boxer program (gasperini et al., 1999) is an example of these new procedures. the algorithm computes the parameters of the earthquake source from idp; it provides the seismological parameters (latitude and longitude of the epicentre, moment magnitude) and a geometric model of the source through a box that represents the surface projection of the modelled sismogenic source. the program represents each source as the surface projection of a fault dipping 45° towards a perpendicular direction to the fault strike (gasperini et al., 1999); the original version is designed only for dip-slip faults and initially calibrated for apennines earthquakes (in most cases shallow events). the projection width constrains the fault at depth. the procedure was largely tested in italy (valensise and pantosti, 2001; mirto et al. 2001) and europe, in the frame of the ec project faust (faults as a seismologist’s tool, http://faust.ingv.it), whose goal was to determine the source parameters (leschiutta and mirto, 2000; stucchi et al., 2000) of some european earthquakes using the european mediterranean intensity database (emid). in this paper we applied the boxer program to some earthquakes of the south-american region, using the data provided by ceresis database available through http://www.ceresis.org. we are perfectly aware that in south america earthquakes are chiefly deep, crustal or due to subduction processes, and the structures are often strike-slip faults. nonetheless, we applied the boxer method to some large earthquakes (above the threshold of io ≥ 8) trying to respect the applicability limit of boxer to dip-slip faults and without considering that the attenuation of earthquake intensity in the algorithm is assessed for the italian territory. as such, our results represent a very preliminary attempt of determining source parameters for some large south american earthquakes. the ceresis database: characteristics and applicability the ceresis database is the main source of intensity data points (idp) for south america, it contains 16,318 idp (fig. 1a) related to 3,183 events (fig. 1b) that occurred from 1471 to 1985 in eight south american countries and one caribbean country (table. 1). figure. 1. a) idp distribution from the ceresis database. b) epicentre distribution from the ceresis database. a b 12 augusto a. gómez capera and ilaria leschiutta table 1. time-windows, number of earthquakes and related idp in the ceresis database table 2. example of ceresis database format. examples of duplicate record, as for different “interpreter” (i.e. callao: dh or utlo) or uncertainties in the intensity assignment (i.e. callao i=8/9), are reported cou code ye mo da ho mi se ilat ilon mlat mlon dph ms io ne hyp pe 120 1586 07 10 00:30:00 -12.300 -77.700 60 10m 12 pe lima -12.10 77.00 100 76 81 8k dh 421 pe callao -12.05 77.15 32 62 74 8k dh 421 pe callao -12.04 -77.09 20 10 41 319 8k utlo 411 pe callao -12.04 -77.09 20 10 41 319 9k utlo 411 pe caraveli -15.77 -73.36 1779 567 568 136 3k utlo 411 pe cuzco -13.52 -71.97 3326 571 573 106 3k utlo 411 pe huanuco -9.92 -76.23 1894 257 261 20 3k utlo 411 pe ica -14.07 -75.72 439 259 262 147 5k utlo 411 pe ica -14.07 -75.72 439 259 262 147 6k utlo 411 pe lima -12.05 -77.05 32 7 41 333 9k utlo 411 pe lima -12.05 -77.05 32 7 41 333 8k utlo 411 pe trujillo -8.10 -79.03 51 495 496 333 3k utlo 411 cou code ye mo da ho mi se ilat ilon mlat mlon dph ms io ne hyp pe19795 1960 11 20 22:01:56 -6.800 -81.000 55 6.8ms 6m 2 pe piura -5.20 -80.62 35 54 81 35 6k utlo 411 pe piura -5.20 -80.62 35 54 81 35 5k utlo 411 the format of the ceresis database includes two types of record as shown in table 2. for each event, the first line provides the earthquake parameters (i.e., epicentral co-ordinates, epicentral or maximum intensity, magnitude, and depth, number of related idp although this is not always reported); in the following lines, the idp related to the earthquake is listed. all intensities are given in either mm (modified mercalli) or msk (medvedev-sponheuer-karnik) macroseismic scales. the investigator who assigned the intensity value is identified as “interpreter” (“dh” or “utlo” in table 2). a more detailed description of the catalogue format is given in vol. 1 of ceresis (1985), including the list of the “interpreters” and the references they used. the database contains some cases of duplicate records for the same events and/or localities. this is either due to different interpreter/source or to uncertainties in the intensity assessment. for instance, if the “interpreter” assess an i=7/8 for a given locality, two records are present in the database, one with i=7 and another with i=8. with the aim of determining source parameters using the macroseismic data provided by ceresis database we focused on large events. as a first input 201 earthquakes above the threshold of io ≥ 8, are considered in this study. figure 2 shows the distribution of earthquakes and number of idp for 50-year time windows, for the 201 selected earthquakes. country time-window nº of earthquakes nº of idp argentina 1692 1985 45 703 bolivia 1650 1981 45 208 brazil 1767 -1981 114 431 chile 1730 1977 61 816 colombia 1566 1981 188 2097 ecuador 15411980 153 1416 perú 1471 – 1981 2023 8354 trinidad & tobago 1825 – 1981 23 78 venezuela 1530 1981 531 2215 total 3183 16318 13 determining the parameters of earthquake sources in south america from macroseismic intensity data (ceresis database) figure 2 a) number of earthquakes per 50-year time-windows. b). number of idp per 50-year time-windows. table 3. number of records in the ceresis database for earthquakes with i 0 ≥ 8 table 4. records from 201 events with i o ≥ 8. ceresis data base ιo ≥ 8 this study ιo ≥ 8 country time-window nº of earthquakes nº of idp nº of idp used argentina 1692 1985 17 483 480 bolivia 1650 1947 6 30 30 brazil 1955 1 10 10 chile 1730 1976 13 382 330 colombia 1644 -1981 43 1287 921 ecuador 15411980 29 912 871 perú 1471 1974 63 1775 990 trinidad & tobago 1825 1 14 14 venezuela 1530 1975 28 622 599 total 201 5515 4245 in some of the studied cases, the ceresis database contains for an individual earthquake duplicated intensity values assessed at the same place. since the boxer program accepts one intensity value only for each place, an “a-priori” choice has been made. both in case of different interpreters and of uncertain intensity, the highest intensity value has been adopted. the number of idp in the ceresis database for the 201 events with io ≥ 8 is 5515, while those used in criteria nº of earthquakes eliminated number of idp <10 89 focal depth 30 >km 45 off shore events 18 subduction events 11 incoherent idp distribution 1 total events eliminated 164 this study are 4245 only (table 3). out of the 201 events above the threshold (io ≥ 8) a more detailed selection was carried out considering the characteristics of the boxer program (table 4). therefore only events with an appropriate number of idp (n° ≥ 10) have been considered; all the earthquakes with a focal depth ≥ 30 km have been disregarded as well as the offshore and subduction events. the identification of the subduction events was supported by the available literature. dorbath et al. (1990) assessed the size of large and great historical earthquakes in zones along the trench in peru. these zones correspond roughly to segmentation defined by the geometry of the subduction zone. other authors, such as lomnitz (1970), kelleher (1972), pennington (1981), kanamori and mcnally (1982), nishenko (1985), comte and pardo (1991), monge (1993), tavera and buforn (1998), tavera (2002) and carpio and tavera (2002) listed the major earthquakes along south a b n˚ of 60 50 40 30 20 10 eqs 0 15 00 -1 55 0 15 51 -1 60 0 16 01 -1 65 0 16 51 -1 70 0 17 01 -1 75 0 17 51 -1 80 0 18 01 -1 85 0 18 51 -1 90 0 19 01 -1 95 0 19 51 -1 98 5 1500 n˚ of idp 1000 500 0 15 00 -1 55 0 15 51 -1 60 0 16 01 -1 65 0 16 51 -1 70 0 17 01 -1 75 0 17 51 -1 80 0 18 01 -1 85 0 18 51 -1 90 0 19 01 -1 95 0 19 51 -1 98 5 14 augusto a. gómez capera and ilaria leschiutta american subduction zone. they are the 1746, 1868, 1912, 1913, 1970 earthquakes in peru and the 1730,1751,1822,1868, 1877, 1928, 1966 earthquakes in chile. these events are not considered by this study. adopting the above described criteria only 37 events have been processed with the method proposed by gasperini et al. (1999). in figure 3, boxes table 5. parameters for the 37 investigated earthquakes in south america. ∆epic. is the distance between epicentre given by ceresis and epicentre computed by boxer program representing the source dimension, size and location obtained from macroseismic data are shown. table 5 gives the ceresis parameters, the parameters computed by boxer and a comparison between them. in general, the moment magnitude calculated by boxer program is smaller than the magnitude ms reported by ceresis. this study ceresis (1985) parameters computed by boxer program comparison of parameters obtained by boxer program with ceresis data id box date country __ sc depth (km) ms nº idp used __ mw fault length (km) fault width (km) fault azimuth ___ mw-ms _epic (km) 1 1698-06-20 ecuador 10 msk 14 10 6.3 20.3 10.5 009±053 00 30.8 2 1766-10-21 venezuela 9 mm 6.0 3 8 5.6 7.7 6.2 027±000 -1.0 -0.4 13.1 3 1785-07-12 colombia 8 mm 6.5 17 8 6.3 20.0 10.5 138±048 0.0 -0.2 37.3 4 1797-02-04 ecuador 11 msk 86 11 7.3 70.3 20.7 067±001 0.0 7.3 5 1812-03-26 venezuela 9 mm 6 6.3 40 9 7.2 62.5 19.4 056±009 0.0 0.9 294.0 6 1834-01-20 colombia 11 mm 7.0 11 10 6.3 20.3 10.5 034±000 -1.0 -0.7 16.2 7 1859-03-22 ecuador 8 msk 20 8 6.2 17.5 9.7 018±029 0.0 14.4 8 1868-08-15 colombia 8 msk 7.0 10 10 6.3 18.4 10.0 068±018 0.0 -0.7 34.8 9 1868-08-16 ecuador 10 msk 52 10 6.7 32.3 13.5 008±016 0.0 7.4 10 1875-05-18 colombia 10 mm 20 7.3 30 10 7.0 45.5 16.3 093±016 0.0 -0.3 9.0 11 1878-04-13 venezuela 8 mm 13 5.9 18 7 5.3 5.2 5.0 096±024 -1.0 -0.6 4.8 12 1894-04-29 venezuela 9 mm 20 7.1 71 9 6.8 38.9 15.0 060±016 0.0 -0.3 12.0 13 1894-10-27 argentina 9 mm 30 8.0 44 9 7.1 53.8 17.9 167±011 0.0 -0.9 215.0 14 1913-11-04 peru 10 mm 20 23 9 6.3 19.7 10.4 106±058 -1.0 4.3 15 1914-12-02 peru 10 mm 15 18 10 6.3 19.3 10.2 073±217 0.0 16.6 16 1928-04-09 peru 9 mm 30 6.9 11 7 6.0 12.7 8.2 101±000 -2.0 -0.9 137.8 17 1928-05-14 peru 10 mm 7.3 16 9 6.9 45.6 16.3 136±143 -1.0 -0.4 91.3 18 1932-03-14 venezuela 9 mm 25 6.8 58 9 6.8 35.8 14.3 114±048 0.0 0.0 32.3 19 1934-06-11 argentina 9 mm 30 6.0 13 8 6.1 14.0 8.6 -1.0 01 28.8 20 1938-08-10 ecuador 9 msk 23 9 5.8 9.5 7.0 090±015 0.0 1.2 21 1942-12-26 colombia 8 mm 6.5 12 8 6.5 24.9 11.8 056±042 0.0 0.0 86.2 22 1944-01-15 argentina 9 mm 30 7.4 39 9 6.9 41.6 15.5 076±165 0.0 -0.5 12.6 23 1946-11-10 peru 11 mm 7.3 34 11 7.3 70.8 20.7 132±016 0.0 0.0 21.1 24 1947-07-14 colombia 9 mm 10 5.5 59 9 5.9 11.3 7.7 159±115 0.0 0.4 36.4 25 1950-08-03 venezuela 8 mm 8 6.4 55 8 6.2 15.7 9.2 043±044 0.0 -0.2 34.6 26 1952-06-11 argentina 8 mm 30 7.0 15 8 6.0 12.2 8.0 177±016 0.0 -1.0 23.9 27 1955-05-11 ecuador 8 msk 6.8 22 7 5.4 5.3 5.1 051±021 -1.0 -1.4 35.0 28 1957-04-21 colombia 8 mm 6.6 18 8 7.0 45.5 16.3 036±037 0.0 0.4 90.3 29 1961-04-08 ecuador 8 msk 24 29 8 5.7 7.9 6.3 155±023 0.0 20.2 30 1966-09-04 colombia 8 mm 8 10 7 5.2 4.1 4.4 121±076 -1.0 19.7 31 1969-10-01 peru 9 mm 20 7 5.3 4.6 4.7 036±009 -2.0 9.7 32 1970-09-26 colombia 8 mm 8 6.6 13 7 6.0 12.0 7.9 000±000 -1.0 -0.6 21.6 33 1974-04-18 colombia 9 mm 24 4.5 10 8 6.3 20.2 10.5 151±158 -1.0 1.8 14.6 34 1974-07-13 colombia 8 mm 12 7.3 17 8 7.0 47.0 16.6 006±042 0.0 -0.3 57.9 35 1976-04-09 ecuador 8 mm 19 6.7 67 7 5.8 5.7 5.3 017±032 -1.0 -0.9 15.4 36 1977-11-23 argentina 9 mm 4 7.4 132 9 6.9 40.2 15.3 108±031 0.0 -0.5 86.4 37 1985-01-26 argentina 8 mm 12 22 8 5.8 9.5 7.0 064±050 0.0 24.4 figure. 3. location of 37 intensity-based sources (box) quoted in table 5. the adjacent number to each box corresponds to id box (table first column). 15 determining the parameters of earthquake sources in south america from macroseismic intensity data (ceresis database) the epicentral intensities are essentially similar. table 5 lists also “∆epic.” that is the distance between the ceresis epicenters and the macroseismic ones computed by the boxer program. this comparison between epicenters shows some cases with distance greater than 150km. the only justifiable difference is the one concerning 9 april 1928, peru earthquake, because in this case ceresis gives the instrumental localization. for the other two events (26 march 1812 venezuela and 27 october 1894 argentina earthquakes) a more detailed analysis would be necessary (but it is out of the scope of this study). examples: the earthquake of 1894, april 29th, in the venezuelan andes the 1894 event is one of the largest historical earthquakes in venezuela. it is known as the great earthquake of the venezuelan andes and it ruined almost completely the towns of santa cruz de mora, zea, mesa de bolívar, tovar, san juan de lagunillas, chiguará and mérida (audemard, 1998). altuve (1998), rengifo and laffaille (1998) and rivera de uzcátegui and torres (1998) investigated and collected historical sources on this event. ceresis (1985), on the basis of the historical work by grases (1979), gives the epicenter as 8.50°n, 71.70°w, at a 20 km depth, magnitude ms = 7.1, and maximum reported intensity ix (mm). also the epicenter given by rengifo and laffaille (1998) is similar: 8.55° ± 0.05°n, 71.69° ± 0.05°w, but with a focal depth estimated in 14 ± 2 km, and a magnitude ranging between 7.1 and 7.4. recently this event has been directly associated to the southern section of the bocono fault through paleoseismological investigations (audemard, 1997; audemard, 1998, audemard et al., 1999) and its magnitude has been estimated between ms 7.1 and 7.3. based on the 71 macroseismic data reported by ceresis (figure 4a), the macroseismic epicenter (8.39°n, 71.70°w) and the moment magnitude (mw = 6.8) were obtained through the boxer program. figure 4. a) the 29 april 1894 earthquake in venezuela: 71 intensity data points reported by ceresis (1985). figure 4. b) the box represents the source dimension, size and location obtained from intensity data following the method proposed by gasperini et al. (1999) table 6. earthquake of 1894, april 29th: comparative table of the earthquake parameters. epicentre author latitude (n) longitude (w) magnitude ceresis (1985) 8.50º 71.70º 7.1 ms this study 8.39º 71.70º 6.8 mw rengifo and lafaille (1998) 8.55º 71.69º 7.1-7.4 ml audemard et al (1997) 7.1-7.3 ms 16 augusto a. gómez capera and ilaria leschiutta the moment magnitude calculated by the boxer (table 6) is smaller than the magnitudes reported by the ceresis catalogue (ms 7.1), by rengifo and lafaille (1998) and by audemard et al. (1997). as shown in figure 4b, the boxer program suggests that the 1894 earthquake ruptured a source with a length of 39 km and an azimuth of n60°e. this result can be considered consistent with the regional tectonic trend, and it is further supported by the conclusions drawn by audemard et al. (2000) (figure 4c). in fact, they mention this earthquake as an event related to historical movement (sense of movement: dextral-normal) of the south of mérida section of bocono fault system, in venezuela. with the quiches fault. doser (1987) re-evaluated firstmotion data and concluded that the epicenter was located at 8°.28.4’s, 77°.51.6’w, the focal depth was 15 to 17 km, and the magnitudes were 6.3 to 6.5 (ms), 6.5 to 6.9 (m b ), and 6.8 (mw). bellier et al. (1991) say the 1946 ancash earthquake produced a surface faulting along the quiches normal fault and given a moment magnitude mw = 7.0 ± 0.1, which is close to the value of mw = 6.8 given by doser (1987). the epicenter coordinates of the event given by ceresis (1985) are 8.5°s, 77.5°w, without information about depth; maximum intensity xi (msk), magnitude ms = 7.3 and the idp are quoted from historical compilation by silgado (1978). this earthquake has a good distribution of idp (34 localities, figure 5a and figure 5b). in such cases the boxer program is rather stable, therefore the results obtained, macroseismic epicenter (8.340°s, 77.603°w), moment magnitude (mw7.3), relative fault length (71 km) and azimuth (n132) are highly valuable to be compared with the ones in literature (table 7 and figure 5c). the moment magnitude computed through the boxer program is similar to the instrumental one (ms7.3) quoted by ceresis. also, in this case, the azimuth of the inferred sources (n132º) is consistent with the strike of quiches fault reported by macharé et al. (2003), located west of the marañon river and north-east of the western cordillera in peru (figure 5c). the author quotes this fault was active during the 1946 ancash earthquake (sense of movement: normal). figure 4. c) box of the 1894 event (blue box), in comparison with the “map of quaternary faults” as published by audemard et al. (2000), modified. the earthquake of 1946, november 10th, in the peruvian andes this earthquake is known as the ancash earthquake, in the northern peru and partially destroyed the towns of pallasca, pomabamba and quiches in the “departamento di ancash”. the “instituto geofisico del peru” (www.igp.gob.pe) locates the event at 8.333°s, 77.833°w, assess maximum intensity ix-x (mm), focal depth 30-40km and magnitude 7.2 (m b ) and indicates that this earthquake mainly affected the towns mayas and quiches. it also gives information about isoseismic map, quotes historical sources and associates this event figure 5. a) the 10 november 1946 earthquake in peru: 34 intensity data points reported by ceresis (1985). 17 determining the parameters of earthquake sources in south america from macroseismic intensity data (ceresis database) figure 5. b) the box represents the source dimension, size and location obtained from intensity data following the method proposed by gasperini et al. (1999). conclusions a recently developed application, which processes macroseismic data (boxer, gasperini et al., 1999) and was originally tested in italy and europe, has been adopted in this study. even though this application should be carefully applied, given the nature of the south-american catalogues, which include both shallow and subduction earthquakes, the results we achieved are to some degree conformable. as a first approach to the south-american historical seismicity we generally can affirm that our results agree fairly well with seismological data and geological background as reported in literature. in particular in the examples we presented in this paper we have observed that the strike of the sismogenic sources inferred from macroseismic data is compatible with the tectonic structures or surface faulting evidence available for large historical earthquakes. indeed the procedure we adopted can be taken as stable for a limited number of large magnitude events due to the source characteristics, that in south america are strongly different from the ones for which the algorithm was initially calibrated. therefore to conclude, we emphasize that more efforts are needed to develop methods that make use of macroseismic intensity data to constrain the essential characteristics of the seismic sources for south america. references altuve, f. (1998): el gran terremoto de los andes venezolanos y las juntas de socorro. revista geográfica venezolana, 39, 1-2, 163-178. audemard, f. (1997): holocene and historical earthquakes on the boconó fault system, southern venezuelan andes: trench confirmation. journal of geodynamics, 24,1-4, 155-167. audemard, f. (1998): contribución de la paleosismología a la sismicidad histórica: los terremotos de 1610 y 1894 en los andes venezolanos meridionales. revista geográfica venezolana, 39, 1-2, 87-105. audemard, f., d. pantosti., m. machette, c. costa, k. okumura, cowan, diederix, h. ferrer, c. and participants of the south american field workshop on paleoseismology (1999): trench investigation along the mérida section of the boconó fault (central venezuelan andes), venezuela. tectonophysics, 308, 1-2, 1-21. audemard, f. a., machette, m. n., cox, j. w., dart r. l and haller k. m. (2000): map and database of figure 5. c) box of the 1946 event (blue box), in comparison with the “map of quaternary faults” as published by macharé et al. (2003), modified. table 7. earthquake of 1946, november 10th: comparative table of the earthquake parameters. epicentre author latitude (s) longitude (w) magnitude ceresis (1985) 8.50º 77.50º 7.3 ms this study 8.34º 77.60º 7.3 mw instituto geofísico del perú 8.33º 77.83º 7.2 mb doser (1987) 8.47º 77.86º 6.8 mw belhier et al. 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(1985) seismic potential for large and great interplate earthquakes along the chilean and southern peruvian margins of south america: a quantitative reappraisal. journal of geophysical research, 90, no. b5, 3589-3615. pennington, w. d. (1981) la subducción de la cuenca oriental de panamá y la sismotectónica del noroeste de sur america. en investigacioness geofísicas sobre las estructuras océano-continentales del occidente colombiano, bogotá: instituto geofísico, universidad javeriana. p. 97-137 rengifo, m. and laffaille, j. (1998) el terremoto del año 1894 en los andes venezolanos. revista geográfica venezolana, 39, 1-2, 141-161. rivera de uzcátegui, a. and torres, r. (1998): estudio de daños originados a las iglesias de la ciudad de mérida por la acción de los terremotos de 1812 y 1894. revista geográfica venezolana, 39, 1-2, 219-256. rubbia, g. (2004): a review of intensity data banks online. annals of geophysics, in printing. silgado, e. (1978) historia de los sismos más notables ocurridos en el perú (1513-1974). instituto geología y minería, boletín no. 3, lima. stucchi, m., leschiutta, i. and mirto, c. (2000) on the influence of the uncertainties of historical data on the determination of earthquake parameters. xxvii general assembly of the european seismological commission (esc), wsa-1. lisboa 10-17 september 2000, abstract, p. 115. tavera, h. and e. buforn (1998) sismicidad y sismotectónica de perú. en física de la tierra, edited by e. buforn and a. udias (servicio de publicaciones universidad de complutense), no. 10, pp. 187-219. tavera, h. (2002) historia de un terremoto anunciado para la región sur del perú. centro nacional de datos geofísicos, instituto geofísico del perú, pp. 11-18. valensise, g. and pantosti d. (eds.) (2001) database of potential sources for earthquakes larger than m 5.5 in italy. ann. di geofisica, suppl, 44 (4): 180, with cdrom. geociencias-vol 12-1 sep.vp earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 7-30 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia carlos alberto ríos reyes1, oscar mauricio castellanos alarcón2 and akira takasu3 1 escuela de geología, universidad industrial de santander, a.a 678, bucaramanga, colombia. 2 programa de geología, universidad de pamplona, colombia. 3 geosciences department, shimane university, japan. corresponding author: carlos alberto ríos reyes, school of applied sciences, university of wolverhampton, wulfruma street, wolverhampton wv1 1sb, uk tel. +44 (0)1902 322679; fax. +44 (0)1902 322714; e-mail: c.a.riosreyes@wlv.ac.uk abstract a barrovian sequence of the silgará formation at the southwestern santander massif, colombian andes, contains zoned garnets in which major and trace element zoning correlates with distribution of mineral inclusions, which may indicate that garnet growth rate varied through time and affected both composition and texture of garnets, although different garnet producing reactions have also played an important role in the chemical zoning of garent. however, a local metasomatism process associated to the action of late magmatic fluids associated to the emplacement of the pescadero pluton (external forcing mechanism) would be also considered. in particular, ca, mn and y zoning patterns in some garnets correspond with inclusion-rich vs. inclusion-free zones, although the distribution of inclusions does not correlate with chemical zoning (i.e., the same inclusions are found in ca-rich and ca-poor zones of the garnet). there is a similar lack of correlation with accessory phases (apatite, monazite, xenotime, ilmenite or rutile). in a garnet from the garnet-staurolite zone, a high mn core contains abundant and randomly oriented apatite, monazite and ilmenite inclusions, while a euhedral low ca mantle zone is inclusion-free and the high ca / low mn rim zone contains apatite, monazite and ilmenite aligned parallel to the margins of the garnet. inclusions in garnet can also represent mineral phases were not completely consumed during garnet growth. association of garnet zoning trends and patterns with inclusion distribution may help differentiate between processes that identically affect major-element zoning but that produced variable textures in the garnet. 7 manuscript received september 7, 2007. accepted for publication january 30, 2008. key words: colombia; garnet; zoning; santander massif; silgará formation. resumen una secuencia barroviana de la formación de silgará en la región suroccidental del macizo de santander, andes colombianos, contiene granates zonados en los cuales la zonación de elementos mayores y trazas se correlaciona con la distribución de las inclusiones minerales, lo cual puede indicar que la tasa de crecimiento del granate varió con el tiempo y afectó la composición y la textura de los granates, aunque diferentes reacciones que producen granate han jugado también un papel importante en la zonación química del granate. sin embargo, un proceso local de metasomatismo generado por la acción tardia de fluidos magmáticos asociados al emplazamiento del plutón de pescadero (mecanismo de fuerza externa) es también aquí considerado. en particular, los patrones de zonación de ca, mn e y en algunos granates corresponden con zonas ricas en inclusiones vs. zonas sin inclusiones, aunque la distribución de inclusiones no se correlaciona con la zonación química (es decir, las mismas inclusiones se encuentran en las zonas ricas y pobres en ca del granate). hay una carencia similar de correlación con las fases accesorias (apatito, monacita, xenotima, ilmenita o rutilo). en un granate de la zona del granate-estaurolita, un núcleo alto en mn contiene abundantes inclusiones aleatoriamente orientadas de apatito, monacita e ilmenita, mientras que una zona euhedral baja en ca carece de inclusiones y la zona de borde rica de alto ca / bajo mn contiene inclusiones de apatito, monacita, e ilmenita orientadas paralelo a los bordes del granate. inclusiones en granate pueden también representar fases minerales que no fueron completamente consumidas durante el crecimiento del granate. la asociación de los patrones de zonación del granate con la distribución de las inclusiones puede ayudar a distinguir entre los procesos que afectan idénticamente la zonación de elementos mayores pero produjo texturas variables en el granate. palabras claves: colombia; granate; zonación; macizo de santander; formación silgará. introduction garnet is one of the most studied minerals in relation with chemical variations in metamorphic rocks since it preserves a record of its growth history expressed by the chemical zoning and inclusions of other minerals. the broad range in mineral composition promotes the growth of garnet in rocks of very different chemical compositions and over a wide spectrum of metamorphic conditions. furthermore, its refractory character allows the preservation of chemical and textural zoning that is important when making interpretations about the metamorphic history of the host rock. of particular interest in recent years have been the trace element zoning in garnet as an important source of information for petrologic processes (hickmott & shimizu, 1990; schwandt et al., 1996; chernoff & carlson, 1999; yang & rivers, 2001; skelton et al., 2002) and for relating the growth of geochronologically important accessory phases (e.g., monazite) to metamorphic reaction histories (pyle & spear, 1999; gibson et al., 2004). major and trace element zoning in garnet have been typically explained by (1) elemental fractionation during mineral growth (e.g., hollister 1966; cygan and lasaga 1982), (2) by intracrystalline diffusion (e.g., anderson and buckley 1973), (3) limitations at the mineral-matrix interface (e.g., carlson 1989), (4) interaction with a metasomatic fluid (e.g., hickmott et al. 1987; young and rumble 1993; chamberlain and conrad 1993; erambert and austrheim 1993; jamtveit et al. 1993; jamtveit and hervig 1994), (5) the breakdown or growth of trace element-rich minerals (hickmott & shimizu, 1990; hickmott and spear 1992) or (6) changes in the garnet-mineral matrix partition coefficients because of changes in pressure and temperature conditions, garnet composition or the number of mineral phases in the assemblage or their proportions 8 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu (yang and rivers, 2002). the majority of these processes occur during crystal growth. diffusional reequilibration is the only primary postgrowth process that leads to zoning in minerals. in an extreme case, compositional zoning can arise in garnet after its crystallization with a homogeneous composition that is in desequilibrium with the matrix. because a chemical gradient exists between the garnet and the surrounding matrix, often biotite-rich, volume diffusion acts to reequilibrate the garnet composition with the matrix (e.g., tracy et al., 1976). diffusion occurs as long as the temperature remains sufficiently high (e.g., lasaga 1983). trace elements are extremely sensitive to changes in accessory mineral assemblage and/or fluid composition and many trace-element diffusivities in garnet must be very much slower than diffusivities for major elements (mg, mn, fe) in garnet, but probably on the same order as the diffusivity of ca in garnet (pyle & spear, 1999). trace element distributions in garnet must, however, be interpreted with caution. previous studies have documented trends in ca, mn and y zoning in garnet (e.g., chernoff & carlson, 1997; pyle & spear, 1999; yang & rivers, 2002) and discussed the possibility that lowor highannuli within garnet are related to local disequilibrium in some elements, but not all. for example, patterns that deviate from normal growth zoning in garnet (e.g., euhedral bands concentric about the garnet core, patches or spiral to curving patterns) might form by garnet overgrowth of phases enriched or depleted in particular major or trace elements (e.g., mn; yang & rivers, 2001; hirsch et al., 2003), by resorption of garnet during garnet-consuming reactions that produce staurolite (menard & spear, 1993) or by growth of garnet after a matrix phase has been depleted (e.g., epidote, chloritoid; whitney & ghent, 1993). the resulting garnet zoning patterns will be a function of the local chemical heterogeneities, the temperature of metamorphism during and after garnet growth and the growth rate of garnet, factors that will affect both the zoning and the distribution of mineral inclusions in garnet. chernoff & carlson (1997, 1999) considered the possibility of independent, local scales of equilibrium for different elements. they demonstrated that many trace elements, as well as ca, reflect disequilibrium at thin-section scale and therefore thermobarometric estimates that involve grossular contents may be in error. garnet zoning may also be affected by fluid flow (e.g., stowell et al. 1996; skelton et al., 2002) and deformation and these processes may be coupled, as deformation will change grain size and adjust grain boundaries, affecting rates and pathways for diffusion. garnets are frequently zoned in the major elements fe, mg, mn and ca. at high temperature, major element growth zoning may be significantly modified by intracrystalline diffusion while trace element zoning may be less susceptible to diffusion (e.g., hickmott & spear, 1992; lanzirotti, 1995; chernoff & carlson, 1999). in this paper, we report zoning patterns for garnets of the metapelitic silgará formation of the southwestern santander massif, colombian andes. we document major and trace element zoning in garnets and evaluate the processes that control and influence zoning during prograde metamorphism. geological setting the santander massif lies within the eastern cordillera of the colombian andes, where it divides into the northeast-trending perijá in colombia and the east-northeast-trending mérida andes in venezuela (figure 1). the metamorphic history of this massif is important for interpretation of the geologic and tectonic evolution of the northwestern continental margin of south america. the santander massif is underlain by deformed and metamorphosed rocks that have been tectonostratigraphically divided into three metamorphic units (bucaramanga gneiss complex, silgará formation and orthogneiss), which are intruded by several igneous bodies, most of them of triassic-jurassic age and some of paleozoic age (e.g., goldsmith et al., 1971; boinet et al., 1985; dörr et al., 1995; ordoñez, 2003). the intrusives form part of the santander plutonic complex and are interpreted as calc-alkaline crustal bodies emplaced after peak-metamorphism. sedimentary rocks ranging in age from devonian to tertiary flank the core rocks. the garnet-bearing samples investigated in the 9 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia present study belong to the metapelitic sequence of the caledonian silgará formation that represents what was originally a very thick pile of volcano-sedimentary rocks. well-exposed sections of this metamorphic unit crop out at the southwestern region of the santander massif (figure 1), which is long established as classic area for the study of rock metamorphism and deformation caused by continental collision during the caledonian orogeny. according to ríos et al. (2003), this region experienced crustal thickenning during the initial stages of collision, followed by a slow uplift and erosion period, resulting in a decompression accompanied by heating. the silgará formation structures and metamorphic features here are complex and early workers had great difficulty in making sense of the geology of the area. the metamorphic rocks of the silgará formation generally strike nw-se and dip to the southwest. the nw-trending bucaramanga fault represents a very major break in the crust in the northeastern part of the 10 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu figure 1. lower left side, map of the colombian andes, showing the location of the santander massif in the eastern cordillera. upper left side, simplified geological map of the santander massif, showing the distribution of the silgará formation metamorphic rocks in grey (modified after goldsmith et al., 1971) and the location of the study area in black. right side, generalized geological map of the study area (modified after ward et al., 1973), showing locations of garnet samples investigated (black stars). metamorphic zones modified after ríos et al. (2003). bf: bucaramanga fault; lsaf. los santos aratoca fault. study area and separates the precambrian bucaramanga gneiss complex from the triassic-jurassic pescadero and mogotes batholiths. this fault is probably strike slip with a protracted history of displacement (goldsmith et al., 1971). in the southwest, near aratoca, the silgará formation is cut by the nw-se-trending los santos aratoca fault and is unconformably overlain by mesozoic sedimentary rocks (figure 1). it is also unconformably overlain by a mesozoic sequence on the northwest and the southwest. the triassic-jurassic pescadero and mogotes batholiths intruded the silgará formation in the northeastern and eastern part of the area after peak-metamorphism (goldsmith et al., 1971). the silgará formation is dominated by metapelitic rocks with minor intercalations of metabasites. the lithology of the metapelitic sequence changes in composition northeastward, from quartz-rich pelitic schists in the southwest to feldspar-rich semipelitic schists in the northeast. a millimeter-scale compositional banding consists of alternating granoblastic quartzplagioclase-rich domains with minor k-feldspar and muscovite-biotite-rich domains. subordinated rock types of the silgará formation are well-foliated amphibole-bearing schists, orthoamphibolites and calc-silicate rocks. microstructural evidence shows that the silgará formation underwent prograde metamorphism during at least three deformation phases, including extensive retrograde metamorphism during the last stage. the dominant schistosity or slaty cleavage in the metapelitic rocks may have developed at the same time as early folds and thrusts. the metapelites display a well-developed schistosity, although well-preserved sedimentary features are locally preserved in low-grade rocks. in general, later structures such as flat-lying crenulations, small chevron folds and kink bands overprint the main foliation. retrograde textures include partial replacement of garnet by chlorite and/or muscovite or chlorite and biotite along cracks; heavily corroded staurolite crystals surrounded by muscovite; biotite and calcic-amphibole replaced by chlorite (although in some cases amphibole is replaced by biotite); and feldspar partly replaced by sericite. a progressive sequence of metamorphic zones has developed in pelitic rocks of the silgará formation. the regional metamorphic grade increases with structural depth from the biotite zone to the staurolite-kyanite zone. garnet-bearing pelites are widespread throughout the region. barrovian regional metamorphism occurred under lowto high-temperature and medium-pressure conditions. the distinction between the staurolite-kyanite and lower sillimanite (fibrolite) zones is not well defined because fibrolitic sillimanite is present in trace amounts in staurolite-kyanite bearing samples. the occurrence of fibrolite in sample pcm-473 lacking other al2sio5 phases near the contact with the pescadero pluton is interpreted as formed in the waning stages of a thermal event. therefore, the fibrolitization process should not be always considered as a polymorphic reaction. ríos et al. (2003) estimated the temperatures and pressures of equilibration of mineral assemblages from metamorphic rocks of the southwestern santander massif. p-t conditions were 500-520°c and 4.4-5.5 kbar in the garnet zone and 590-700°c and 5.5-7.5 kbar in the garnet-staurolite zone. analytical methods x-ray maps were collected and analyses were carried out using the jeol 8800 electron probe microanalyzer at the research center for coastal lagoon environments at the shimane university (japan) and the jeol 8900 electron probe microanalyzer at the department of geology and geophysics at the university of minnesota (usa). accessory mineral phases were identified using back-scattered electron imaging (bei) and energy-dispersive spectroscopy (eds). the analytical conditions were as follows: accelerating voltage of 15 kv and beam current of 20-25 na (for quantitative analyses) and 25-75 na (with dwell times of 40-55 msec/pixel, for major element maps) or higher beam current (100 na) and longer dwell times (up to 90 msec/pixel) for trace element maps. qualitative x-ray maps were obtained first to delineate geochemically significant zones and to guide quantitative spot analysis (in radial traverses and spacing decreasing near the rim). data adquisition and reduction were carried out us11 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia ing the zaf correction procedures. a combination of natural and synthetic minerals were used as standards. chemical zoning and textural relations in garnet a detailed petrographic study of six thin sections was carried out on selected specimens which are the same as those studied by ríos (1999) and corresponding to a series of pelitic rocks of the silgará formation composed of quartz + plagioclase + muscovite + biotite + garnet ± staurolite and accessory fibrolitic sillimanite and rare crystals of andalusite and kyanite (one sample contained the three polymorphs). in the garnet zone, garnets are commonly idioblastic, whereas in the garnet-staurolite and staurolite-kyanite zones they tend to be rounded and embayed, displaying reaction rims. as follows we describe petrographic features of the major and accessory minerals in the analyzed samples as illustrated in figure 2. the mineral assemblages of the analyzed samples are given in table 1. mineral abbreviations are after kretz (1983). all the examined samples contain garnet porphyroblasts that preserve growth zoning to varying degrees. garnet is almandine-rich and exhibits zoning in which mn decreases from core to rim as mg and fe increase. the zoning of fe, mg and mn zoning is strongly correlated with respect to the degree of local equilibrium in contrast with ca zoning that does not mimic fe, mg and mn zoning exactly (e.g., chernoff & carlson, 1997; spear & daniel, 1998). according to hirsch et al. (2003), it reflects a combination of changing availability of these elements to the surface of the growing garnet and changing partitioning of elements among the mineral phases in the rock. garnet shows abrupt variations in ca distribution from core to rim, developing in some cases a complex cyclic zoning with euhedral lowor high-ca annuli. trace element zoning in garnet will depend on the presence or absence of saturing phases, such as xenotime for y, xenotime, apatite or monazite for p and ilmenite, titatine and rutile for ti. analyzed garnets do not show zoning in y, sc, p, cr or ti, except garnet zone sample pcm-441, which exhibits y zoning. yttrium is potentially useful for monitoring reactions involving ca-rich minerals such as epidote and plagioclase because this element substitutes for ca in mineral structures and y discontinuities in garnet zoning may correlate with inflections in ca zoning (hickmott & spear, 1992). in the following sections, we describe the main features of chemical zoning in garnet from the silgará formation pelites. table 2 shows representative chemical compositions of analyzed garnets. analytical points are indicated by white dots and numbers in the x-ray maps, with numbers keyed to the table. pcm-441 garnet. pelitic schist from the garnet-zone, which typically contain a mineral assemblage of muscovite + quartz + plagioclase + garnet ± biotite, with minor k-feldspar, tourmaline, apatite, zircon, epidote, calcite and fe-ti oxides. numerous, very fine-grained (0.03-0.42 mm in diameter), euhedral grains of garnet, with a hexagonal or pentagonal outline, occur in this sample (figure 2a). garnet grew after an early foliation and contain inclusion trails of ilmenite and graphite concordant to the main foliation (crenulation cleavage), although it may have inclusion-free rims. it is replaced along their margins and fractures by chlorite, biotite and fe-oxides. the most striking characteristic of this garnet is the high mn concentration, which from core to rim varies from 54 to 25 mol% spessartine (figure 3a). fe and mg increase from core to rim with a small decrease in fe/fe+mg. garnet has a low-ca core (7 mol%) with an inflection midway (14 mol%) between core and rim, decreasing towards the rim (8 mol%), as shown in figure 4a. trace element maps for garnet in sample pcm-441 show that yttrium is the only one that displays zoning. with the possible exception of yb, which shows a slight enrichment in the garnet core, the other trace elements (sc, p, cr, ti and yb) are in low abundance and homogeneous across the garnet. garnet is characterized by a euhedral outline, high-y core (figure 4b), which is larger than the faint yb-rich region. the y-rich core region corresponds exactly to the low ca core region. relatively y-enriched regions of the garnet also occur discontinuously along the rim and in the ca-poor core of 12 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu 13 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia figure 2. plane-polarized light photomicrographs of garnet-bearing metamorphic rocks of the silgará formation. (a) numerous very fine-grained garnet crystals; sample pcm-441 (garnet zone). (b) garnet with inclusion-rich (quartz and ilmenite) core and inclusion-free rim regions; sample pcm-420 (garnet-staurolite zone). (c) garnet porphyroblast displaying two different inclusion patterns involving ilmenite; sample pcm-361 (garnet-staurolite zone). (d) inclusion trails of quartz and ilmenite preserved in garnet porphyroblasts; sample pcm-516 (garnet-staurolite zone). (e) fibrolitic sillimanite occurring at core of muscovite adjacent to garnet; sample pcm-473 (staurolite-kyanite zone). (f) garnet-bearing calc-silicate rock occurring as numerous fineto medium-grained crystals associated with calcic amphibole; sample pcm-514 (garnet-staurolite zone). 14 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu t ab le 1. m in er al as se m bl ag es of th e an al yz ed sa m pl es of th e s il ga rá f or m at io n pe li ti c ro ck s fr om th e so ut hw es te rn s an ta nd er m as si f m e t. z o n e s a m p le c h l b t m s q z p l k fs g rt s t a n d k y s il c a -a z r a p m g n il m r u t e p m n z x e n t u r c a l g a rn e t p c m -4 4 1 r x x x x m x m m m m m r g a rn e ts t a u ro lit e p c m -3 6 1 r x x ,r x ,i x x x i i i i i i g a rn e ts t a u ro lit e p c m -5 1 6 r x x x ,i x x x i i g a rn e ts t a u ro lit e p c m -5 1 4 r x ,i x ,i x ,i x ,i x x ,i i i i i r g a rn e ts t a u ro lit e p c m -4 2 0 r x x ,i x ,i x x i i s ta u ro lit e k y a n it e p c m -4 7 3 r x ,i x ,r ,i x ,i x ,i x x m ,i i m m q tz ,b t, p l, an d re tr og ra de c hl ar e co m m on in al ls am pl es .x -p re se nt ,b la nk -a bs en t, m -m in or ph as e, iin cl us io ns in ga rn et ,r -r et ro gr ad e m in er al s. m in er al ab br ev ia ti on s af te r k re tz (1 98 3) 15 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia figure 3. zoning profiles rim-core-rim of almandine, pyrope, spessartine and grossular components and fe/(fe+mg) ratio in garnet from samples (a) pcm-441 (garnet zone), (b) pcm-514 (garnet-staurolite zone), (c) pcm-361 (garnet-staurolite zone), (d) pcm-516 (garnet-staurolite zone), (e) pcm-420 (garnet-staurolite zone) and (b) pcm-473 (staurolite-kyanite zone). note the different scales for xsps, xprp and xgrs on the left and for xalm and fe/(fe+mg) on the right. an adjacent small garnet. x-ray maps for ca distribution in garnet and its vicinity indicate that there are no ca-rich accessory phases included in garnet and only one small apatite crystal in the matrix just beyond the field of view in figure 4. pcm-361 garnet. pelitic schist from the garnet-staurolite-zone characterized by the peak metamorphic assemblage of quartz + plagioclase + muscovite + biotite + garnet + staurolite. garnet porphyroblasts (1.5-2.6 mm in diameter) are anhedral to subhedral with an elliptical outline and contain numerous inclusions of ilmenite that define different pattern of distribution (figure 2c). at the core region, the inclusion patterns can be concordant or discordant to the main foliation of the rock or ramdonly oriented, whereas at the rim region, they follow the rim of the garnet, defining an approximately circular pattern. garnet porphyroblasts area wrapped by a penetrative foliation (crenulation cleavege). garnet is strongly zoned in mn (figures 3c, 5b), which decreases from 21 mol% in the core to 1-2 mol% in the rim. fe and mg increase from core to rim, with a small decrease in fe/fe+mg. ca decreases from core (5 mol%) towards the rim, with a slight discontinuity in the zoning midway between core and rim, reaching a minimum (1-2 mol%) represented by an euhedral low-ca annulus (figures 5a, 6a); then increases towards the rim (6 mol%). the low-ca annulus coincides with the euhedral zone of high-mn (6-9 mol%) and is truncated in the bottom part due to partial resorption of garnet. analyzed garnets all exhibit complex ca zoning, but there is no apparent zoning in analyzed trace elements (y, yb, p, ti, cr). garnet is texturally zoned with respect to the distribution and shape orientation of ilmenite inclusions and textural zoning can be related to chemical zoning (figures 2c, 5, 6a). in the mn-rich garnet core region, ilmenite inclusions are randomly oriented. inclusions are absent in the low-ca annulus. in the high-ca / low-mn near-rim zone, ilmenite inclusions are abundant and are aligned parallel to the garnet rim, creating a circular pattern, which is also apparent in matrix ilmenite adjacent to garnet (figure 2c). ilmenite occurs as inclusions in garnet that have been partially pseudomorphed by rutile (figure 7) or as a matrix accessory mineral. other inclusions in garnet are quartz, apatite, monazite, zircon and rutile, which 16 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu figure 4. x-ray compositional maps in garnet from sample pcm-441 (garnet zone). (a) ca map, showing a high-ca band at mantle region and the relationship between the low-ca at core and the high-y annulus. numbered spots are keyed to analyses in table 2. (b) y map, displaying a high-y annulus at core characterized by sharp euhedral and straight outlines. in the x-ray maps, the elemental distributions are indicated by white color for the highest and black color for the lowest concentration. a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia t ab le 2. r ep re se nt at iv e co m po si ti on s of m et am or ph ic ga rn et fr om th e s il ga rá f or m at io n, so ut hw es te rn s an ta nd er m as si f. m a jo r e le m e n ts p c m 4 4 1 p c m -3 1 p c m 5 1 6 p c m 5 1 4 p c m 4 2 0 p c m -4 7 3 ri m co re ri m ri m co re ri m ri m ri m co re ri m co re ri m ri m co re ri m w t% 1 2 3 4 5 1 2 3 4 5 6 7 1 1 2 3 4 5 6 7 8 1 2 3 4 1 2 3 4 s io 2 3 6 ,3 9 3 6 ,7 6 3 6 ,1 7 3 6 ,9 6 3 6 ,8 2 3 7 ,7 3 3 7 ,4 5 3 6 ,7 6 3 6 ,7 9 3 6 ,6 7 3 6 ,6 9 3 7 ,1 0 3 7 ,6 1 3 6 ,8 8 3 7 ,3 1 3 6 ,6 2 3 7 ,4 4 3 6 ,7 6 3 7 ,3 3 3 6 ,8 5 3 7 ,6 6 3 6 ,6 3 3 6 ,5 3 7 ,2 6 3 6 ,7 6 3 7 ,3 8 3 7 ,4 5 3 7 ,3 0 3 7 ,6 7 t io 2 0 ,0 2 0 ,0 6 0 ,0 8 0 ,0 5 0 ,0 5 0 ,0 0 0 ,0 0 0 ,0 9 0 ,1 2 0 ,0 0 0 ,0 1 0 ,0 5 0 ,0 2 0 ,0 4 0 ,0 2 0 ,1 2 0 ,1 6 0 ,4 6 0 ,0 5 0 ,0 3 0 ,0 3 0 ,1 3 0 ,0 2 0 ,0 0 0 ,0 0 0 ,0 8 0 ,0 0 0 ,0 0 0 ,0 4 a l 2 o 3 2 0 ,6 4 2 0 ,3 2 2 0 ,0 3 2 0 ,5 9 2 0 ,6 9 2 0 ,6 1 2 0 ,4 9 2 0 ,5 3 2 0 ,5 5 2 0 ,5 2 2 0 ,6 3 2 0 ,4 1 1 9 ,9 1 2 0 ,2 8 2 0 ,1 9 2 0 ,4 6 1 9 ,8 1 1 9 ,1 7 2 0 ,4 6 2 0 ,3 1 2 0 ,7 0 1 9 ,7 9 2 0 ,2 4 2 0 ,6 3 2 0 ,2 5 2 0 ,1 0 2 0 ,4 6 2 0 ,5 0 2 0 ,2 3 f e o * 2 5 ,4 2 1 7 ,9 5 1 6 ,6 1 2 2 ,1 3 2 6 ,2 6 3 8 ,8 6 3 7 ,0 7 3 3 ,8 4 3 2 ,1 0 3 4 ,6 5 3 4 ,9 4 3 7 ,7 5 2 7 ,9 2 2 3 ,3 9 2 3 ,4 6 2 2 ,6 4 2 3 ,8 0 2 2 ,5 5 2 2 ,9 2 2 1 ,4 5 2 3 ,8 5 2 4 ,6 6 2 5 ,7 2 3 2 ,0 9 3 1 ,2 2 3 4 ,8 3 3 4 ,1 7 3 4 ,9 1 3 4 ,1 5 m n o 1 2 ,2 8 2 0 ,3 4 2 2 ,9 2 1 4 ,5 2 1 1 ,2 0 1 ,9 1 2 ,7 8 6 ,5 1 9 ,2 1 5 ,5 8 4 ,9 9 0 ,7 5 9 ,2 7 8 ,7 2 8 ,8 5 8 ,8 5 9 ,1 4 9 ,2 4 9 ,7 9 1 0 ,1 4 8 ,4 2 1 3 ,8 9 1 1 ,6 3 ,4 3 4 ,6 8 3 ,3 9 2 ,5 7 3 ,7 3 3 ,8 m g o 2 ,2 6 1 ,1 2 0 ,9 8 1 ,7 2 2 ,4 7 2 ,2 9 2 ,7 6 2 ,0 4 1 ,6 8 2 ,2 1 2 ,2 2 2 ,7 7 3 ,5 1 2 ,8 6 2 ,7 6 2 ,4 0 2 ,6 5 2 ,3 6 2 ,5 9 2 ,4 3 2 ,8 1 1 ,5 7 1 ,7 2 2 ,7 7 2 ,4 7 1 ,7 2 2 ,8 5 1 ,7 8 2 ,0 3 c a o 2 ,9 5 4 ,2 2 2 ,5 2 5 ,0 2 2 ,7 6 1 ,2 1 0 ,6 3 1 ,6 0 1 ,6 1 1 ,2 7 0 ,9 7 1 ,9 3 2 ,0 9 6 ,3 9 5 ,9 8 7 ,9 1 6 ,5 6 7 ,5 2 6 ,2 5 7 ,3 1 5 ,9 8 3 ,5 2 3 ,6 0 2 ,9 7 2 ,7 2 1 ,8 8 2 ,1 6 1 ,8 5 2 ,0 0 n a 2 o 0 ,0 0 0 ,0 1 0 ,0 7 0 ,0 3 0 ,0 1 0 ,0 4 0 ,0 6 0 ,0 4 0 ,0 6 0 ,0 5 0 ,0 3 0 ,0 4 0 ,0 4 0 ,0 3 0 ,0 6 0 ,0 3 0 ,0 3 0 ,0 4 0 ,0 3 0 ,0 0 0 ,0 2 0 ,0 1 0 ,0 4 0 ,0 0 0 ,0 1 0 ,0 2 0 ,0 1 0 ,0 5 0 ,0 5 k 2 o 0 ,0 6 0 ,0 4 0 ,0 5 0 ,0 4 0 ,1 7 0 ,0 7 0 ,0 5 0 ,0 5 0 ,0 6 0 ,0 3 0 ,0 5 0 ,0 4 0 ,0 8 0 ,0 8 0 ,0 4 0 ,0 6 0 ,0 5 0 ,0 4 0 ,0 5 0 ,0 3 0 ,0 4 0 ,0 4 0 ,0 5 0 ,0 5 0 ,0 5 0 ,0 4 0 ,0 4 0 ,0 1 0 ,0 8 c r 2 o 3 0 ,0 0 0 ,0 0 0 ,0 0 0 ,0 0 0 ,0 0 0 ,0 4 0 ,0 1 0 ,0 0 0 ,0 3 0 ,0 0 0 ,0 0 0 ,0 7 0 ,0 0 0 ,0 1 0 ,0 0 0 ,0 0 0 ,0 1 0 ,0 2 0 ,0 0 0 ,0 2 0 ,0 0 0 ,0 1 0 ,0 0 0 ,0 1 0 ,0 2 0 ,0 0 0 ,0 1 0 ,0 2 0 t o ta l 1 0 0 ,0 1 1 0 0 ,8 2 9 9 ,4 3 1 0 1 ,0 7 1 0 0 ,4 3 1 0 2 ,2 5 1 0 1 ,2 8 1 0 1 ,4 6 1 0 2 ,2 0 1 0 0 ,9 8 1 0 0 ,8 2 1 0 0 ,9 0 1 0 0 ,4 5 9 8 ,6 8 9 8 ,6 7 1 0 0 ,0 9 9 9 ,6 5 9 8 ,1 6 9 9 ,4 7 9 8 ,5 7 9 9 ,5 1 1 0 0 ,2 5 9 9 ,4 9 9 9 ,2 1 9 8 ,1 8 9 9 ,4 2 9 9 ,7 3 1 0 0 ,1 9 1 0 0 ,0 4 c a ti o n s b a se d o n 1 2 a to m s o f o xy g e n s i 2 ,9 6 1 2 ,9 8 2 2 ,9 9 2 ,9 7 4 2 ,9 7 8 3 ,0 1 2 3 ,0 1 1 2 ,9 7 0 2 ,9 6 3 2 ,9 7 4 2 ,9 9 4 2 ,9 9 2 3 ,0 2 7 2 ,9 9 6 3 ,0 3 3 ,0 0 8 3 ,0 2 0 3 ,0 1 3 ,0 1 3 3 ,0 2 0 2 ,9 9 1 2 ,9 8 9 3 ,0 2 0 3 ,0 2 1 3 ,0 5 2 3 ,0 2 8 3 ,0 2 6 3 ,0 5 2 t i 0 ,0 0 1 0 ,0 0 4 0 ,0 1 0 ,0 0 3 0 ,0 0 3 0 ,0 0 0 0 ,0 0 0 0 ,0 0 5 0 ,0 0 7 0 ,0 0 0 0 ,0 0 1 0 ,0 0 3 0 ,0 0 1 0 ,0 0 2 0 0 ,0 0 7 0 ,0 1 0 0 ,0 3 0 0 0 ,0 0 2 0 ,0 0 8 0 ,0 0 1 0 ,0 0 0 0 ,0 0 0 0 ,0 0 5 0 ,0 0 0 0 ,0 0 0 0 ,0 0 3 a l 1 ,9 7 9 1 ,9 4 2 1 ,9 5 1 ,9 5 2 1 ,9 7 2 1 ,9 3 8 1 ,9 4 1 1 ,9 5 5 1 ,9 5 1 ,9 6 1 1 ,9 6 7 1 ,9 3 9 1 ,8 8 8 1 ,9 4 1 1 ,9 3 0 1 ,9 2 8 1 ,8 8 1 ,8 5 1 ,9 4 1 ,9 5 1 ,9 5 6 1 ,9 0 5 1 ,9 5 3 1 ,9 7 1 1 ,9 6 1 1 ,9 3 4 1 ,9 5 0 1 ,9 6 5 1 ,9 3 2 f e 3 + 0 ,0 9 6 0 ,0 8 7 0 ,0 7 0 ,0 9 3 0 ,0 6 6 0 ,0 3 8 0 ,0 3 8 0 ,0 9 4 0 ,1 0 9 0 ,0 9 1 0 ,0 4 2 0 ,0 7 2 0 ,0 5 6 0 ,0 6 2 0 ,0 1 7 0 ,0 4 1 0 ,0 5 8 0 ,0 6 0 ,0 4 0 ,0 6 0 ,0 0 0 0 ,0 9 7 0 ,0 6 7 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 f e 2 + 1 ,6 3 3 1 ,1 3 1 ,0 8 1 ,3 9 6 1 ,7 1 0 2 ,5 2 2 2 ,4 5 4 2 ,1 9 3 2 ,0 5 3 2 ,2 5 9 2 ,3 2 2 2 ,4 7 4 1 ,8 2 3 1 ,5 2 6 1 ,5 7 1 ,4 7 3 1 ,5 5 1 ,4 8 2 1 ,5 1 0 1 ,4 1 ,5 9 9 1 ,5 8 7 1 ,6 9 4 2 ,1 7 5 2 ,1 4 6 2 ,3 7 8 2 ,3 1 0 2 ,3 6 8 2 ,3 1 4 m n 0 ,8 4 6 1 ,3 9 7 1 ,6 0 ,0 9 0 0 ,7 6 7 0 ,1 2 9 0 ,1 8 9 0 ,4 4 5 0 ,6 2 8 0 ,3 8 3 0 ,3 4 2 0 ,0 5 1 0 ,6 3 2 0 ,6 0 0 0 ,6 0 8 0 ,5 9 9 0 ,6 2 0 ,6 4 0 ,6 7 0 ,7 0 ,5 7 2 0 ,9 6 1 0 ,8 0 4 0 ,2 3 5 0 ,3 2 6 0 ,2 3 4 0 ,1 7 6 0 ,2 5 6 0 ,2 6 1 m g 0 ,2 7 5 0 ,1 3 5 0 ,1 2 0 ,2 0 6 0 ,2 9 8 0 ,2 7 3 0 ,3 3 1 0 ,2 4 6 0 ,2 0 2 0 ,2 6 7 0 ,2 6 8 0 ,3 3 3 0 ,4 2 1 0 ,3 4 6 0 ,3 3 0 ,2 8 6 0 ,3 2 0 ,2 9 0 ,3 1 0 ,2 9 0 ,3 3 6 0 ,1 9 1 0 ,2 1 0 0 ,3 3 5 0 ,3 0 3 0 ,2 0 9 0 ,3 4 4 0 ,2 1 5 0 ,2 4 6 c a 0 ,2 5 7 0 ,3 6 7 0 ,2 2 0 ,4 3 2 0 ,2 3 9 0 ,1 0 3 0 ,0 5 4 0 ,1 3 8 0 ,1 3 9 0 ,1 1 0 0 ,0 8 4 0 ,1 6 7 0 ,1 8 0 0 ,5 5 6 0 ,5 2 0 0 ,6 7 8 0 ,5 7 0 ,6 6 0 0 ,5 4 0 0 ,6 4 0 ,5 1 4 0 ,3 0 8 0 ,3 1 6 0 ,2 5 8 0 ,2 3 9 0 ,1 6 4 0 ,1 8 7 0 ,1 6 1 0 ,1 7 3 n a 0 ,0 0 1 0 ,0 0 3 0 ,0 3 0 ,0 1 1 0 ,0 0 2 0 ,0 1 1 0 ,0 1 8 0 ,0 1 2 0 ,0 1 8 0 ,0 1 6 0 ,0 0 9 0 ,0 1 3 0 ,0 1 3 0 ,0 1 0 0 ,0 2 0 0 ,0 1 0 ,0 1 0 0 ,0 1 3 0 ,0 1 0 0 ,0 0 0 0 ,0 0 6 0 ,0 0 3 0 ,0 1 3 0 ,0 0 0 0 ,0 0 3 0 ,0 0 6 0 ,0 0 2 0 ,0 1 5 0 ,0 1 5 k 0 ,0 1 2 0 ,0 0 9 0 ,0 1 0 0 ,0 0 9 0 ,0 3 6 0 ,0 1 4 0 ,0 1 0 0 ,0 1 1 0 ,0 1 2 0 ,0 0 6 0 ,0 1 1 0 ,0 0 7 0 ,0 1 7 0 ,0 1 7 0 ,0 1 0 ,0 1 3 0 ,0 1 0 ,0 1 0 ,0 1 0 ,0 1 0 ,0 0 9 0 ,0 0 9 0 ,0 1 1 0 ,0 1 1 0 ,0 1 1 0 ,0 0 8 0 ,0 0 9 0 ,0 0 3 0 ,0 1 8 c r 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 0 0 ,0 0 2 0 ,0 0 0 0 ,0 0 0 0 ,0 0 2 0 ,0 0 0 0 ,0 0 0 0 ,0 0 4 0 ,0 0 0 0 ,0 0 1 0 ,0 0 0 0 ,0 0 0 0 0 0 ,0 0 0 0 0 ,0 0 0 0 ,0 0 1 0 ,0 0 0 0 ,0 0 1 0 ,0 0 1 0 ,0 0 0 0 ,0 0 1 0 ,0 0 1 0 ,0 0 0 t o ta l 8 ,0 6 1 8 ,0 5 6 8 ,0 7 8 ,0 6 7 8 ,0 7 1 8 ,0 4 3 8 ,0 4 6 8 ,0 7 0 8 ,0 8 4 8 ,0 6 7 8 ,0 4 1 8 ,0 5 4 8 ,0 5 8 8 ,0 5 8 8 ,0 4 8 ,0 4 3 8 ,0 5 8 ,0 5 3 8 ,0 3 8 8 ,0 4 8 ,0 1 5 8 ,0 6 0 8 ,0 5 8 8 ,0 0 5 8 ,0 1 2 7 ,9 9 1 8 ,0 0 7 8 ,0 1 0 8 ,0 1 2 x m g 0 ,1 4 0 ,1 1 0 ,1 0 0 ,1 3 0 ,1 5 0 ,1 0 0 ,1 2 0 ,1 0 0 ,0 9 0 ,1 1 0 ,1 0 0 ,1 2 0 ,1 9 0 ,1 8 0 ,1 7 0 ,1 6 0 ,1 7 0 ,1 6 0 ,1 7 0 ,1 7 0 ,1 7 0 ,1 1 0 ,1 1 0 ,1 3 0 ,1 2 0 ,0 8 0 ,1 3 0 ,0 8 0 ,1 x a lm 0 ,5 6 0 ,3 9 0 ,3 7 0 ,4 8 0 ,5 8 0 ,8 4 0 ,8 1 0 ,7 3 0 ,6 9 0 ,7 6 0 ,7 7 0 ,8 2 0 ,6 0 0 ,5 1 0 ,5 2 0 ,4 9 0 ,5 2 0 ,4 9 0 ,5 0 ,4 7 0 ,5 3 0 ,5 4 0 ,5 7 0 ,7 2 0 ,7 1 0 ,8 0 0 ,7 7 0 ,7 9 0 ,7 7 x sp s 0 ,2 7 0 ,4 5 0 ,5 2 0 ,3 2 0 ,2 5 0 ,0 4 0 ,0 6 0 ,1 4 0 ,2 0 0 ,1 2 0 ,1 1 0 ,0 2 0 ,2 0 0 ,1 9 0 ,2 0 ,1 9 0 ,2 0 0 ,2 0 0 ,2 2 0 ,2 3 0 ,1 9 0 ,3 1 0 ,2 6 0 ,0 8 0 ,1 1 0 ,0 8 0 ,0 6 0 ,0 9 0 ,0 9 x p rp 0 ,0 9 0 ,0 4 0 ,0 4 0 ,0 7 0 ,1 0 0 ,0 9 0 ,1 1 0 ,0 8 0 ,0 6 0 ,0 9 0 ,0 9 0 ,1 1 0 ,1 4 0 ,1 1 0 ,1 1 0 ,0 9 0 ,1 0 0 ,0 9 0 ,1 0 0 ,1 0 ,1 1 0 ,0 6 0 ,0 7 0 ,1 1 0 ,1 0 0 ,0 7 0 ,1 1 0 ,0 7 0 ,0 8 x g rs 0 ,0 8 0 ,1 2 0 ,0 7 0 ,1 4 0 ,0 8 0 ,0 3 0 ,0 2 0 ,0 4 0 ,0 4 0 ,0 4 0 ,0 3 0 ,0 5 0 ,0 6 0 ,1 8 0 ,1 7 0 ,2 2 0 ,1 8 0 ,2 1 0 ,1 8 0 ,2 1 0 ,1 7 0 ,1 0 0 ,1 0 0 ,0 9 0 ,0 8 0 ,0 5 0 ,0 6 0 ,0 5 0 ,0 6 x m g = m g/ (f e+ m g) * t ot al f e as f eo + f e 2 o 3 also occur as matrix phases. apatite is most abundant in the mn-rich core region (also outlined by the low-ca annulus) and is rare in the outer parts of the garnet except very close to the rim, where a few large grains are located (figure 5, below). monazite increases in abundance from garnet core to rim to matrix, but is entirely lacking from the low-ca zone. xenotime occurs only as a matrix phase. pcm-516 garnet. pelitic schist from the garnetstaurolite-zone, which contains a mineral assemblage of quartz + plagioclase + k-feldspar + muscovite + biotite + garnet. garnet porphyroblasts are subhedral, 0.3-2.5 mm in diameter, pseudopentagonal and poiquiloblastic (figure 2d). they contain inclusion trails of quartz and platy ilmenite that preserve evidence of an earlier fabric occurring at a high angle to the main fabric, which corresponds to a crenulation cleavage. an inclusion-free rim separates the core from the external fabric. garnet is associated with symmetrical and asymmetrical pressure shadows. the fabric outside of garnet porphyroblasts generally displays evidence of recrystallization and grain-size 18 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu figure 5. above, x-ray compositional maps in garnet from sample pcm-361 (garnet-staurolite zone). (a) occurrence of a low-ca annulus characterized by sharp pentagonal and straight outlines. (b) relationship between the euhedral zone of high-mn region and the low-ca annulus. in the x-ray maps, the elemental distributions are indicated by white color for the highest and black color for the lowest concentration. (c) sketch of same garnet showing that the low-ca annulus lacks of inclusions. accessory mineral phases are indicated by black dots (apatite), x marks (xenotime), stars (monazite) and dashes (ilmenite). courtesy of donna whitney. coarsening, respect to the inclusions within garnet. a second stage of garnet growth is indicated by the inclusion-free rim that separates the core from the discordant external fabric. garnet shows decreasing mn from the core (34 mol%) to the rim (16 mol%). fe and mg increase from core to rim, with a small decrease in fe/fe+mg. this garnet is characterized by a small reversal in zoning near the rim. fe, mg and mn distributions are strongly correlated with each other, whereas ca distribution is not (figures 3d, 6b). ca decreases outwards and reaches a minimum at mid-region (4 mole %), then increases towards the rim. inclusions in garnet are ilmenite and quartz in the core region. irregular patches of calcite occur within garnet and in the matrix and may be texturally late, but we note that the distribution of calcite in garnet is confined within the core region, which is bordered by the low-ca ring. calcite does not occur in the grossular-rich outer core/rim of the garnet, although the outer core/rim region is highly fractured. trace elements were not analyzed. pcm-514 garnet. calc-silicate rock from the garnet-staurolite-zone characterized by a mineral assemblage of quartz + plagioclase + k-feldspar + garnet + calcic amphibole. a very interesting texture is observed in this sample, with clusters of numerous fineto medium-grained crystals of garnet, which are 0.1-3.3 mm in diameter, anhedral to subhedral and pseudohexagonal and rounded shape, containing abundant quartz inclusions throughout its core (figure 2f). other inclusions are epidote, plagioclase, biotite and ilmenite. each crystal of garnet has major element zoning (figure 3c), but no detectable trace element zoning in the analyzed elements. the grossular component displays a complex oscillatory zoning, fluctuating between 17 and 24 mol%, opposite to the trend in zoning in almandine (figure 6c). from core to rim, spessartine decreases and fe/(fe+mg) increases. analyzed garnets have at least two high-ca annuli. the variable grossular content may be related to reactions involving other calcic phases in this rock calcic amphibole, plagioclase, epidote). the garnet core, defined as the region within the low-ca ring, contains abundant mineral inclusions. quartz is most common, but epidote, plagioclase, k-feldspar, calcic amphibole, biotite, magnetite and ilmenite also occur. patchy calcite is present in the garnet and appears to be related to fractures. calcite occurs both within and beyond the low-grossular ring in the outer core. figure 8a shows that na is antipathetic with ca, which suggest that the anorthite content in plagioclase varies with the grossular content in garnet and therefore it is necessary a source of ca in addition to the garnet and plagioclase. on the other hand, ti show a positive correlation with ca (figure 8b), which could reflect the consumption of a ti-rich mineral phase such as ilmenite or rutile during garnet growth. similar oscillatory zoning has been reported by other authors (e.g., jamtveit et al., 1995; holten et al., 1997; ivanova et al., 1998; jamtveit, 1999; pollok et al. 2001), probablye due to fluctuations in supersaturation, which might have originated from external (boiling, fluid mixing, temperature, and pressure fluctuations) or internal factors, resulting from self-organization in the interplay of fluid convection and crystal growth in the near vicinity of the growing crystals (jamtveit et al. 1995; holten et al. 1997; pollok et al. 2001). pcm-420 garnet. quartz-feldespatic pelitic rock from the garnet-staurolite-zone that contains a mineral assemblage of quartz + plagioclase + k-feldspar + biotite + garnet. garnet occurs as large (1-2 mm in diameter) subhedral and subrounded porphyroblasts with cores densely packed with very fine inclusions of quartz and minor ilmenite with no preferred orientation that give the host crystal a spongy appearance and inclusion-free rims or as small (0.5-1 mm in diameter) subhedral and subrounded grains with inclusion-free cores (figure 2b). the fabric outside of garnet porphyroblasts shows recrystallization and grain-size coarsening, respect to the inclusions within garnet. small crystals of magnetite occurs adjacent to garnet. garnet exhibits growth zoning: from core to near rim, there is an increase in mg (from 3 to 12 mol%) and a decrease in mn (from 43 to 6 mol%; figure 6d) and in ca (from 18 to 8 mol%), with a small decrease in the fe/(fe+mg) ratio. concentrations of mn, mg and fe in the cores of the small garnets correspond with near-rim compositions in the 19 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia 20 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu larger crystals, but ca composition is not similarly systematic. the concentration of ca in the cores of small garnets does not correspond to near-rim compositions in the larger crystals. no discontinuity in the compositional zoning of mn occurs at the boundary between the inclusion-rich core and the inclusion-free rim. within the high-mn region, which also corresponds to an inclusion-rich region in the garnet, zoning is not concentric about the core (figure 6d). pcm-473 garnet. pelitic schist from the staurolite-kyanite-zone. it is characterized a mineral assemblage of quartz + plagioclase + muscovite + biotite + garnet + staurolite. garnet in this sample ocfigure 6. x-ray compositional maps in pelitic garnets from the garnet-staurolite zone. (a) and (b) illustrate low-ca annuli with sharp pentagonal outlines in garnet from samples pcm-361 and pcm-516, respectively. in (a), at the bottom of the garnet, the low-ca annulus is truncated against biotite; white areas correspond to calcite (garnet core region and matrix) and ca-rich plagioclase (matrix). (c) cyclic ca zoning in garnet from sample pcm-514 with lowand high-ca annuli with sharp hexagonal outlines. (d) mn distribution in garnet from sample pcm-420, showing a small reversal zoning at rim. numbered spots are keyed to analyses in table 2. in the x-ray maps, the elemental distributions are indicated by white color for the highest and black color for the lowest concentration. curs as large porphyroblasts, which are up to 5 mm in diameter, anhedral (generally elongated) and contain a small number of inclusions of quartz, biotite, muscovite, plagioclase, ilmenite and magnetite. it has been highly cracked and partially replaced by quartz, muscovite and sillimanite (figure 2e). fibrolite commonly occurs within muscovite that is associated to garnet or as minute crystals sometimes penetrating quartz grains, which develop embayment in garnet, within which the polymorph of al2sio5 also occurs. the staurolite usually occurs as relicts in muscovite. rutile occurs only as a matrix phase. garnet exhibits reverse zoning, with a decrease in mg (from 11 to 6 mol%), figure 9a, and increase in mn (from 7 to 10 mol%), figure 9b, from core to rim and a small increase in the fe/(fe+mg) ratio. composition is more homogenous in the interior of the crystal and ca content increases slightly within the outer core (5-7 mol%), where it reaches a maximum, then decreases to 5 mol% at rim, developing a low-ca annulus (figure 9c). reaction history a detailed discussion of the reaction history among major mineral phases in metapelitic rocks of the silgará formation at the southwestern santander massif has been presented by ríos (1999) and ríos et al. (2003). therefore, only metamorphic reactions in garnet-bearing pelites will be considered here. the typical barrovian progression of metamorphic reactions in these rocks involve low-grade garnet growth, intermediate-grade garnet consumption during staurolite producing and high-grade resumption of garnet growth during kyanite producing at expenses of staurolite, which is similar to what reported kohn & malloy (2004). the first appearance of garnet was the result of a fe-mg-mn continuous reaction. the characteristic garnet of the garnet zone is almandine-rich and probably grew by two continuous reactions: (1) chl + ms = grt + bt + qtz + h2o (2) chl + ms + qtz = grt + bt + h2o which can explain the formation of almandine and pyrope in garnet, but the high ca amount observed in the garnet core or rim suggests that a ca-rich phase also participated in the garnet-forming reaction. the source of mn is inferred to be mn-rich epidote, probably enclosed in clinozoisite or other matrix minerals. the garnet isograd reaction is 21 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia figure 7. back-scattered electron images of garnet in sample pcm-361 (garnet-staurolite zone), which contains ilmenite inclusions. (a)-(b) unreacted crystal of ilmenite and adjacent pseudomorph of rutile after ilmenite. strongly dependent on bulk rock composition and specially the mno and cao contents. in rocks of high amount of those components, garnet may appear at temperatures below 450°c whereas in rocks of low amount of them, garnet may not appear to well above 500°c (spear, 1993). respect to the cyclic zoning observed in calc-silicate rocks (e.g., sample pcm-514, figure 6c), we consider that the highand low-ca annuli in garnet may have grown, respectively, by the following reactions (3) chl + ms + ep = grt + bt + pl + ca-amp + + bt + h2o, (4) chl + pl + ms + qtz = grt + bt + ca-amp + + h2o however, the behavior of ca-rich fluids should be evaluated further by considering the trace element concentrations in garnet. according to pollok et al. (2001), oscillatory zoning in garnet can be explained through two different sets of processes: local growth dynamics vs external forcing mechanisms. at the staurolite isograd, the first appearance of staurolite can be related to two reactions. textural evidence shows that staurolite may be produced at the expense of garnet by the discontinuous reaction (5) grt + chl + ms = st + bt + qtz + h2o in garnet-free assemblages, staurolite may be produced by the continuous reaction (6) chl + ms = st + bt + qtz + h2o which can be deduced from metamorphic textures, considering that staurolite grew mainly in phyllosilicate-rich layers (graebner and schenk, 1999). when chlorite is completely consumed, as staurolite becomes less abundant it may be preserved as inclusions or armoured relics in muscovite and garnet growth can be explained by the reaction (7) st + bt + qtz = grt + ms + h2o staurolite may dissapears with further growth of garnet at the kyanite isograd as a result of the discontinuous reaction (8) st + ms + qtz = ky + grt + bt + h2o if garnet is not involved, the staurolite breakdown is produced by the continuous reaction (9) st + ms + qtz = al2sio5 + bt + h2o which reflects lower temperatures than the discontinuous reaction (6) and p-t conditions nearly independient of the mno and cao contents of the rock. however, if garnet breaks down as sillimanite 22 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu figure 8. zoning profiles rim-core-rim in garnet from sample pcm-514 (garnet-staurolite zone), showing the correlation between the weight percent of cao and the (a) na2o and (b) tio2 contents. lower axis indicates distance (mm). left axis indicates concentration (wt%). is produced, so the initial growth of sillimanite may be considered to result from two separate continuous reactions operating simultaneously (10) st + ms + qtz = sil + bt + h2o (11) grt + ms = bt + sil + qtz chemical zoning and sillimanite inclusions in muscovite and quartz may indicate decrease in pressure after the formation of sillimanite by reaction (9) and during staurolite-consuming reactions (increasing temperature). there is textural evidence to support the first appearance of kyanite in pelitic rocks of the silgará formation by the polymorphic reaction andalusite � kyanite. on the other hand, a net reaction such as andalusite � sillimanite must not have occurred at some point during metamorphism, because we don’t really know if the andalusite reacted directly to produce the sillimanite or if the andalusite was first dissolved in a fluid phase, with the fluid phase carrying the dissolved components to a new location where the fluid then precipitated the sillimanite. however, the presence of sillimanite, kyanite and andalusite in the area or in some samples seems to suggest a more complex p-t metamorphic evolution. discussion in this study we reveal a number of important observations regarding the major and trace element zoning and its correlation with accesory mineral phases in garnet-bearing pelites and associated lithologies of the silgará formation at the southwestern santander massif. fe, mg and mn zoning is strongly correlated with respect to the degree of local equilibrium in contrast with ca zoning that does not mimic zoning of those elements exactly (e.g., chernoff & carlson, 1997; spear & daniel, 1998). this strong correlation supports the hypothesis that these elements achieved a close approach to local equilibrium during garnet growth. if variations of these elements were produced only by local variations in bulk composition, then mass balance and phase equilibrium considerations would predict antithetic zoning of fe, mg and xfe with respect to mn. the chemical zoning in the analyzed garnets may be originated by growth zoning, as a result of changing external conditions (pressure, temperature or bulk rock chemistry) and by diffusion zoning also due to changing external conditions, but with no growth or consumption of garnet. according to spear (1993), a change in chemical zoning character from growth zoning to diffusion zoning by progressive homogenization is attributed to diffusion with increasing metamorphic grade. however, a local metasomatic 23 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia figure 9. x-ray compositional maps showing (a) mg, (b) mn and (c) ca distribution in garnet from sample pcm-473 (staurolite-kyanite zone). white arrows indicate embayments. numbered spots are keyed to analyses in table 2. in the x-ray maps, the elemental distributions are indicated by white color for the highest and black color for the lowest concentration. event has also played a very important role in controlling metamorphic reactions. major element zoning. the analyzed garnets usually show a normal zoning with mn content decreasing from core to rim, suggesting a prograde metamorphism. a high mn content correlates with a high fe/(fe+mg), which suggests that areas of high mn content formed at lower temperature. therefore, mn zoning can be used as a time line in these rocks (spear and daniel, 1998). crystal size distribution (csd) analysis of garnets is consistent with continuous nucleation and growth of garnet throughout the garnet crystallization episode (ríos et al., 2003), which indicates that large garnets nucleated early and the smallest garnets nucleated last. in contrast to the observations of chernoff & carlson (1997) for garnets in the picuris range, new mexico, high-mn, low-mg+fe cores or low-mn, high-mg+fe cores do not correlate with garnet size in the silgará formation. however, the strong correlation of mn and fe, regardless of garnet crystal size and spatial distribution, suggests a homogenous distribution of these divalent cations in the intergranular medium during garnet growth. ca zoning can exhibit different trends than the fe-mn-mg and is generally unrelated to zoning patterns in fe, mn and mg, although in some cases ca and mn zoning patterns are spatially related (e.g., sample pcm-361, figure 5a, 5b). it is well known that changes in the reactant mineral phases and product assemblages occur at different times during the reaction history of garnet crystals of different size and, therefore, they can not be the result of any event affecting the entire rock, such as a change in pressure, temperature or fluid composition but they reflect kinetic factors that cause ca to fail achieving chemical equilibrium during garnet growth. although garnet outlines are sometimes slightly rounded (e.g., pcm361, figure 6a; pcm-516, figure 6b), it is possible to observe that their rims were basically parallel to the euhedral low-ca annuli and that the majority of garnet consumption occurred at the corners or intersections of growth crystal faces of garnet. a likely explanation for the difference in appearance of the annuli is that a particular thin section plane did not necessarily pass through the centre of the garnet and in some cases a high-annuli at core could be missed. garnet shows abrupt variations in ca distribution from core to rim, probably due to the consumption of ca-enriched mineral phases in garnet-producing reactions, developing zoning patterns with euhedral low-ca annuli (figures 6a, 6b) parallel to the garnet outlines, which is typical of growth zoning and suggest that very little diffusion took place after growth (chernoff & carlson, 1999). cal-silicate rocks contain a variety of ca-rich phases (e.g., epidote, plagioclase, ca amphibole and apatite) in addition to garnet. if the consumption or production of these phases and the changes in how ca and other elements are locally partitioned between garnet and other minerals, accounts for garnet zoning, there should be trends in zoning in other ca-rich phases. matrix plagioclase typically displays normal zoning, with an increase in xan spatially associated with a decrease in xgrs of garnet rims (ríos et al., 2003). low-ca annuli within garnets are unlikely to have been caused by resorption of garnet because the bands are euhedral. a garnet resorption would be expected to produce an irregular, embayed garnet margin and an overgrowth on such irregular boundary should produce a subhedral to anhedral annulus, which has not been observed in this study. a sharp decrease of grossular content from core to the mid-region has been interpreted by menard & spear (1993) as produced by resorption of garnet during productions of staurolite or loss of epidote from the assemblage, which is unlikely because the garnet core is euhedral. a change in mineral assemblage may account for some of the zoning trends observed, but to explain the complex oscillatory zoning observed in calc-silicate rocks (sample pcm-514, figure 6c), this would require appearance and disappearance of ca-rich phases or some other process that control fluctuations in the availability of elements. the ca-rich bands in garnet may correlate with the breakdown of epidote as well as with the presence of a ca-rich fluid. garnet growth continued with decreasing grossular content after each of these events, showing that ca was fractionated into garnet, plagioclase and/or calcic amphibole. ti and ca contents in garnet from sample pcm-514 are positively correlated, which may reflect consumption of ilmenite or rutile to liberate ti during growth of carich garnet 24 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu or metasomatic addition of ti. there is not doubt that a complex oscillatory zoning as described here has a controversial origin and we agree with stowell et al. (1996) in the sense that it is difficult to explain it by a cyclic addition and loss of mineral phase(s) from the chemical system. instead, it could reflect episodic metasomatism by influx of ca-rich fluids during garnet growth near of adjacent to hydrothermal veins of quartz-epidote. therefore, the results obtained by ríos & takasu (1999) for ca concentrations would not be interpreted as event markers or as recording complex barometric histories, when in fact they reflect only local effects that can not have occurred simultaneously throughout the rock. trace element zoning. analysis of the trace element zoning in garnet was done as a useful supplement to major-element zoning to improve the petrological interpretation. according to pyle & spear (1999), trace elements zoning in garnet growing in a close system depends on the presence or absence of “saturing” mineral phases such as xenotime for y, xenotime, apatite or monazite for p and ilmenite, titanite or rutile for ti. however, the accessory phases that likely reflect interactions with major phases could be xenotime and monazite, as a consequence of reaction coupling with garnet and the high compatibility of y in all three phases. in our case, we have documented the presence of these accessory mineral phases, but unfortunately they did not show trace element zoning, except in sample pcm-441 (figure 4b), where y zoning shows a remarkably euhedral core that is not really parallel to crystal faces of garnet, representing a growth zone that probably was not modified by diffusion. high-y annuli in garnet have been reported by pyle & spear (1999) in staurolite-bearing samples, which may form by garnet overgrowth of proximal matrix enrichment in y due to garnet consumption during discontinuous staurolite-forming reactions. therefore, the high-y annulus observed in the garnet studied from the garnet zone can not be explained by a resorption-regrowth process, which also will produce a chemical zoning characterized by an asymmetric annulus with steeper slopes and embayed and irregular shapes. however, the growth history of this garnet can be represented by a period of garnet growth at an approximately constant activity of y (xenotime stable), followed by garnet at much lower y activity (xenotime absent). lanzirotti (1995) describes different mechanisms for annular rings of high-y in garnet, such as open system-fluid infiltration, garnet resorption and renewed growth (unlikely argument for y zoning described above), changes in garnet growth rates and comsumption of y-enriched mineral phases. therefore, the production of an annulus without garnet resorption requieres that the accessory-phase-out reaction be essentially discontinuous or the result would be a gradual, rather than abrupt, increase in garnet major or trace element content (pyle & spear, 1999). hickmott & spear (1992) concluded that concentrations of y, zr and ree in garnet should vary with ca concentrations because these elements susbtitute into epidote-group minerals. therefore, reactions involving consumption of these phases should supply both ca and these trace elements for garnet growth. unfortunately, garnet from sample pcm-514 does not show trace-element zoning in y and therefore is difficult to correlate it with ca in garnet. the addition of calcium would control chemical reactions producing garnet (xgrs increases), plagioclase (xan increases), calcic amphibole and epidote, whereas reactions in absence of metasomatism would return xgrs and xan to previous levels. xenotime is present only as a matrix accessory phase in sample pcm-361 (figure 5c), which can be interpreted as a stable mineral phase throughout the entire growth history of garnet. at this respect, pyle & spear (1999) consider that the scarcity of matrix xenotime at grades above the garnet-zone in garnet-bearing pelites implies that the growth of garnet is accompanied by the consumption of xenotime. monazite is a common accessory mineral in samples from the higher metamorphic zones (garnet-staurolite and staurolite-kyanite), being virtually absent at garnet-grade. monazite inclusions occur in garnet (e.g., pcm-361, figure 5c), suggesting that they grew before or during growing of garnet. garnet growth should be previous to or synchronous with matrix monazite growth. its growth may be related to the prograde consumption of allanite as been reported in different studies, assuming p is derived from apatite (e.g., catlos et al., 2001). although it is probable that a 25 a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombia precursor lree mineral was present, garnet, plagioclase and phyllosilicates host sufficient p and lrees and the reaction between them could stabilize monazite (kohn & malloy, 2004). where analyses are not disrupted by the presence of inclusions, ti concentration decreases from core to rim. the increasing concentration towards the rim in garnet may be a result of changes in the ti-bearing minor phase assemblage from ilmenite-rutile in the garnet core to rutile in the garnet rim. accesory ilmenite, titanite and rutile coexist with the silicates. inclusions of ilmenite (by far the most prominent ti-bearing phase) in garnet have been partly replaced by rutile as observed in figure 7 and show from core to rim an increase in mno and a decrease in feo. apatite is abundant in all rocks throughout the garnet outboard and in the matrix. accessory phases such as zircon, monazite, titanite, epidote and allanite may contain significant y, but these should be of limited modal extent in the samples examined with respect to garnet to explain why there is not apparent trace element zoning in garnet (except for y in sample pcm-441). diffusion and resorption. garnet usually shows a normal zoning with mn content decreasing from core to rim, although a minimum mn content near rim (e.g., sample pcm-420, figure 6d) sometimes is observed, and this type of zoning is characterized as reversal zoning, which in many cases reflects post-peak resorption and reequilibration during cooling by elemental diffusion during retrograde metamorphism. whitney and ghent (1993) consider that no significant increase in mn at garnet rims suggests no major late metamorphic resorption of garnet. however, a resorption process cannot explain a reversal zoning in euhedral garnet. possible explanations involve growth or post-crystallization modification. in the case of the former, asymmetric zoning can be explained by overgrowth of mn-rich phase(s) (top half of crystal in figure 6d) as well as a lower mn phase by garnet at low temperature (so that diffusion was limited and mn was confined to the vicinity of the overgrown phase(s); e.g., hirsch et al., 2003). the core, represented by a low mn concentration, is not situated in the geometrical center of the grain, implying either asymmetrical growth or that a significant amount of resorption has taken place, which is compatible with the corroded appearance of the rim. mn is not concentric about individual parts of the garnet, but rather is zoned in irregular, amoeba-like shapes, a pattern that reflects fast growth along grain boundary surfaces and slower dissolution and replacement of quartz inclusions (spear & daniel, 1999). however, we consider that this garnet where is in contact with progressive shear zones displays a tectonic dissolution, as revealed by the chemical zoning, which is abruptly truncated against the main metamorphic foliation of the rock. chemical zoning in garnet from sample pcm-473 can be affected by a post-growth thermal history. garnet zoning is reversal in this sample and is best explained by diffusion during partial resorption of first stage garnet, with a second stage of growth near the outer rim of garnet, marking the onset of a new prograde garnet producing reaction. resorption is compatible with the corroded appearance of the rim and also with the traditional interpretation of the low-mg (figure 9a) and high-mn (figure 9b) rim. figure 9c illustrates partial preservation of a low-ca annulus at the outermost rim, which is truncated by dissolution and resorption of garnet, developing embayments at rim, the chemical zoning are patchy and the mn or mg (samples pcm-361, pcm-420, pcm-514 or pcm-516) and y (sample pcm-441) distributions are characterized by a small reversal zoning, indicating some garnet resorption and back diffusion after maximum temperature was achieved as a consequence of reaction zones of chlorite or biotite around garnet that could contain inclusions of accessory mineral phases such as xenotime, monazite, epidote, zircon or titanite that can incorporate significant amounts of y as proposed in differet studies (e.g., wopenka et al., 1996; heinrich et al., 1997; finger et al., 1998). conclusions we propose that our zoning and textural observations are most consistent with a continuous garnet growth process, involving various ca-rich phases, and that garnet growth may have varied in terms of different 26 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu garnet producing reactions during prograde metamorphism. the occurrence of inclusion-rich and inclusion-free zones in garnet has been explained by a change in the rate of garnet growth (yang and rivers, 2001), i.e., garnet growth in the inclusion-rich zones must have been sufficiently rapid to allow entrapment of abundant inclusions. therefore, we can interpret inclusion-poor rims and inclusion-rich cores as representing different growth rates, which is supported here by the occurrence of garnet showing low-ca annuli lack of inclusions (e.g., pcm-361 and pcm-516) and high-mn inclusion-rich core (e.g., pcm-420). in sample pcm-361, for example, the high ca / low mn rim zone contains apatite, monazite and ilmenite aligned parallel to the margins of the garnet, whereas the euhedral low-ca annulus within the garnet corresponds to a change in mineral inclusion abundance, but does not correspond to a change in the mineral inclusion assemblage itself (figures 5, 6a). the lowand/or high-ca annuli may be used as time markers during garnet growth history and truncation of annuli is not only an evidence of tectonic dissolution in progressive shear zones as proposed by ríos et al. (2003) but also an excellent indication of subsequent garnet resorption. accessory-phase assemblages may consider an early garnet growth at the expense of chlorite or biotite, with resultant xenotime consumption and production of monazite. most importantly, this study shows that monazite crystallization is not an isolated event, but occurs throughout the metamorphic history of the rock. we have shown that valuable petrogenetic information can be obtained from major and trace-element distribution in pelitic and associated rocks of the silgará formation and that a complete understanding of reaction history may be achieved if accessory phases are considered. however, our interpretation could be tested by running gibbs method simulations, utilising internally consistent thermodynamic datasets and relevant activity-composition models. acknowledgements this research forms part of the m.sc. program carried out by c. ríos at shimane university. we are very grateful to the instituto colombiano para el desarrollo de la ciencia y la tecnología “francisco josé de caldas” (colciencias), as well as the universidad industrial de santander, entities that through grant no. 1102-05-083-95 financially supported fieldwork (research project: “metamorphism and associated metallogeny of the santander massif, eastern cordillera, colombian andes”). we are indebted to the research center for coastal lagoon environments at shimane university for allowing us the use of the electron microprobe analyzer for acquisition and assistance of major element data. we also thank dr. d. whitney from the department of geological and geophysics at university of minnesota for her assistance with the adquisition of major and trace element data, substantial contributions and help in manuscript preparation. references anderson, d.e., and buckley, g.r (1973). zoning in garnets: diffusion models. contributions to mineralogy and petrology, vol. 40, pp. 87-104. boinet, t., bourgois, j., bellon, h., et toussaint, j. 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(1993). prograde reactions and garnet zoning reversals in staurolite schist. british columbia: significance for thermobarometric interpretations. journal of metamorphic geology, vol. 11, pp. 779-788. wopenka, b., jolliff, b.l., zinner, e., and kremser, d.t. (1996). trace element zoning and incipient metamictization in a lunar zircon: application of three microprobe techniques. american mineralogist, vol. 81, pp. 902-912. yang, p., and rivers, t. (2001). chromium and manganese zoning in pelitic garnets and kyanite: spiral, overprint and oscillatory (?) zoning patterns and the role of growth rates. journal of metamorphic geology, vol. 19, pp. 455-474. yang, p., and rivers, t. (2002). the origin of mn and y annuli in garnet and the thermal dependence of p in garnet and y in apatite in calc-pelite and pelite, gagnon terrane, western labrador. geological materials research, vol. 4 (http://gmr.minsocam/papers/v4/v4n1). young, e.d., and rumble, d., iii (1993) the origin of correlated variations in in-situ 180/160 and elemental concentrations in metamorphic garnet from southeastern vermont, usa. geochimica et cosmochimica acta, vol. 57, pp. 2585-2597. 30 carlos alberto ríos reyes, óscar mauricio castellanos alarcón and akira takasu letter from editor a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombiacarlos alberto ríos reyes, oscar mauricio castellanos alarcón and akira takasu 7 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombialorena rayo and carlos a. zuluaga 31 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeriaakintola, o.f. and adekeye, j.i.d. 44 denoising resistivity phosphate “disturbances” using haar mother wavelettransform (sidi chennane, morocco)saad bakkali and mahacine amrani 62 petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”ríos, c.a., castellanos, o.m., gómez s.i. and ávila, g.a. 72 seismic velocity determination in gravel and sands using piezocrystalsarmando luis imhof and juan carlos santamarina 107 modelo hidrogeofisico de la cuenca sur de la habana rosa m. valcarce ortega jacqueline gonzalez espinosa departamento de geofisica-instituto superior politecnico "jose antonio echeverria"-ispjae la habana-cuba valcarce, r.m. & j.gonzalez (1998): modelo hidrogeofisico de la cuenca sur de la habana. geofis. colomb. 4:3-9. issn 01212974. santa fe de bogota, d.c. colombia. resumen como resultado del procesamiento estadistico multivariado aplicado a los registros geofisicos de pozo y a los datos de transmisividad obtenidos de pruebas de bombeo, se presenta un modelo hidrogeofisico de la cuenca sur de la habana que permite sintetizar la informacion hidrodinarnica y geofisica existente. la aplicacion combinada del anabsis euclidiano de aqrupacion, del analisis de correlacion y de las relaciones de uvarov, perrnitio separar la componente estructurada de las variables estudiadas y definir la correlacion entre para metros geofisicos de pozo y la velocidad de filtracion de las aguas subterraneas con coeficientes superiores a 0.8; se establece la relacion existente entre los para metros de dar-zarrouk (calculados a partir del registro electrico) y la transmisividad del acuifero (calculada por pruebas de bombeo), estimandose esta ultima a partir de la conductancia longitudinal con errores menores del 15%. se demuestra que el procesamiento aplicado resulta efectivo incluso para el estudio de acuiferos carsticos donde la heterogeneidad y la anisotropia son la caracteristica principal. abstract a hidrogeophysical model of the south basin of la habana is presented as the result of multivaried statistical processing applied to well logging and transmisivity data from pumping test through combining euclidian group analysis, correlation analysis and uvarov relationships it was possible to separate the structured components of the different variables and: to define the correlation between the geophysical parameters of the well and the filtration velocity of the groundwater, these coefficients are above 0.8; to establish the relation between dar-zarrouk parameters (from electrical well logging) and transmisivity (from pumping tests). the former one was estimated from longitudinal conductance. within less than 15% of error. it is shown that the applied processing is suscefult even in the study of karstic aquifers. where heterogeneity and anisotropy are characteristics. 1. introduccion hidrodinamicos y geofisicos, se hace necesario emplear metodos estadisticos para caracterizar cualitativa y cuantitativamente estas propiedades y mas aun, para realizar evaluaciones cuantitativas de las propiedades hidrodinarnicas del acuifero a partir de los metodos geofisicos. los rnetodos geofisicos son ampliamente utilizados para la caracterizacion cualitativa de las propiedades hidrodinamicas de los acuiferos; sin embargo, sus aplicaciones en estudios cuantitativos son aun limitadas. sobre todo en acuiferos donde la heterogeneidad es la caracteristica principal. tal es el caso de los acuiferos carsticos caracterizados por una marcada heterogeneidad y anisotropla. reconociendo el alto grado de variaci6n de los para metros en este trabajo se explica el procedimiento estadistico multivariado aplicado a los registros geofisicos de pozo y a los datos de transmisividad, obtenidos de pruebas de bornbeo, y se presentan los resultados que permitieron, en el acuifero carstico de la cuenca sur de la habana. esclarecer la relacion entre parametres geofisicos de pozo y la velocidad de filtracion del flujo del agua subterranea, y esclarecer la relaci6n existente entre la conductancia longitudinal (calculada a partir del registro electrico), y la transmisividad(obtenida por pruebas de bombeo). 2. analisis de trabajos anteriores dada la efectividad y eficiencia de los metodos geofisicos en las investigaciones hidrogeol6gicas, a nivel mundial se observa un incremento del volumen de trabajos geofisicos en este campo. la literatura especializada reporta resultados donde los metodos geofisicos han permitido pronosticar el comportamiento de las propiedades hidrodinamicas y en este sentido los de pozo han contribuido de manera importante (jones & bufford, 1951; keys & maccary, 1971; brown, 1988). mazac, kelly & landa (1985), analizan las bases de las relaciones entre las propiedades electricas e hidraulicas, reconocen la existencia de relaciones direetas e inversas entre estas propiedades dependiendo del tipo de aculfero, destacan que las correlaciones a escala de campo dependen de diferentes factores incluyendo el caracter de las correlaciones a nivel de nucleo las que pueden ser establecidas a escala de laboratorio. la relaci6n a nivel de nuc!eo indicara el tipo de correlaci6n de campo que puede ser esperada: positiva 0 negativa. mazac, kelly & landa (1985). tambien senalan que el modelo hidrogeofisico general debe tener en cuenta que: la conductividad hidraulica correlaciona con el tamano de los granos y la porosidad efectiva 0 total para sedimentos libres de arcilla, y correlaciona con el contenido de arcilla y la porosidad efectiva en sedimentos arcillosos. relaciones inversas entre los parametros electricos e hidraulicos pueden ser esperados cuando la porosidad controla las variaciones de la conductividad hidraulica y otros factores como el tamano?e los granos y el contenido de arcilla son relativamente constantes. relaciones directas entre los parametros electricos e hidraulicos aparecen cuando existen correlaciones inversas entre el contenido de arcilla y la conductividad hidraulica. la mayoria de los trabajos que se reportan han sido desarrollados en acuiferos donde existe un predominio de la porosidad intergranular; sin embargo. tambien se reportan trabajos donde ha side posible estudiar el comportamiento de las propiedades hidrodinamicas a partir de los metodos geofisicos en colectores fracturados y cavernosos. gomez rivero (1979) presenta una metodologia para estimar la permeabilidad de las rocas usando solamente registros geofisicos de pozo y una metodologia para relacionar el factor de formaci6n (obtenido de un registro electrico), la porosidad obtenida de un registro de porosidad y la permeabilidad absoluta (obtenida de datos de laboratorio); tarnbien, presenta ecuaciones empiricas obtenidas en carbonatos y arenas. ei metodo desarrollado por este autor puede ser aplicado a rocas con porosidad intergranulary a rocas con porosidad mixta. la metodologia desarrollada por g6mez rivero es aplicada con exlto para el estudio de la permeabilidad de colectores vulcanopeno-sedlmentanos (tobas), donde existe un predominio de la porosidad de fractura (garcia. 1996). katsube & humet (1987) reportan que la relacion entre el factor de formacion obtenido de un registro de corriente enfocada y el obtenido a partir de un registro de densidad, puede ser usada para estimar la conductividad hidraulica de rocas cristalinas fracturadas. estos autores reportan una ecuacion de regresi6n entre la transmisividad obtenida por pruebas hidrogeol6gicas "in situ" y el cociente de los dos factores de formaci6n, para intrusiones graniticas de canada. se reportan tarnbien correlaciones ernpmcas entre la velocidad del flujo del agua subterranea y parametres geofisicos de pozo en calas hidrogeol6gicas del acuifero carstico cuenca sur de la habana (valcarce. 1995). en este trabajo se establece que el comportamiento de la porosidad y la arcillosidad condicionan el comportamiento de los parametros hidrodinamicos del acuifero. tambien reportan correlaciones inversas entre el factor de formacion (obtenido del registro electrico) y la velocidad del agua subterranea (obtenida del metodo de diluci6n de sal) y correlaciones inversas entre la arcillosidad (estimada del registro gamma natural) y la velocidad del flujo; perc para solo tres calas. en el presente trabajo. este analisis se amplia y profundiza para 9 pozos del acuifero, se incorporan otros parametros geofisicos de pozo y los resultados de pruebas de bombeo. 3. metodologia de trabajo y resultados obtenidos la cuenca sur de la habana es un acuifero costero formado casi en su totalidad por rocas del neogeno. principalmente de la formaci6n guines. que presenta en su composici6n una gran variedad de calizas: coralinas. recristalizadas organo-detriticas. organo-relicticas, dolomitizadas, arcillosas; todas con un elevado grade de carstificacion. los parametros hidrogeol6gicos de este acuifero han sido estudiados desde hace ya mas de 50 anos. cuando comenzaron a ejecutarse los primeros proyectos para acueducto. riego y uso industrial. las propiedades de filtracion de las rocas han side calculadas mediante pruebas de caudal y/o aforos en pozos destacandose valores de transmisividad entre 10000 y 50000 m2/dia y aun mayores. esta gran amplitud de los valores esta determinada por el grado de carstificaci6n de las calizas. de acuerdo con la regionalizaci6n hidrogeol6gica de las transmisividades y gastos especificos, es posible observar que existe una gran heterogeneidad del medio y que las propiedades de filtraci6n de las rocas disminuyen con el aumento de la profundidad. a continuaci6n se presenta la metodologia de trabajo y los principales resultados obtenidos, con el objetivo de esclarecer la relaci6n entre los parametres geofisicos de pozo y los parametres hidrodinarnicos. 1. a partir de la interpretaci6n de los registros electricos (rt) y gamma natural (ig) en 9 pozos del area, (t5-106, ts-110, ts-5, ts-9, ts-121, t5-e1e2, cala 2-alquizar, 111-21k, 1-1i-9k);fueron calculados: factor de formaci6n (f), porosidad de archie (~archje), arcillosidad (car), parametro de arcillosidad relativa (l1ar) , resistencia transversal (rt) y conductancia longitudinal (5). de esta forma fueron caracterizados 201 intervalos. 2. a partir de la interpretaci6n del metoda de diluci6n de sal, fue calculada la velocidad de filtraci6n del agua subterranea (vf), el coeficiente de filtraci6n (kf) y la transmisividad (t). 3. para cada uno de estos atributos fue calculada la media (il), desviaci6n estandar (a), valor maximo y minimo, y se analizo tarnbien la ley de distribucion de probabilidades. los resultados se muestran en la tabla 1. los coeficientes de correlaci6n calculados entre las variables con transformaci6n logaritmica fueron siempre superiores a los obtenidos con las variables sin transformar. aunque los coeficientes de correlaci6n no son elevados, se observa correlaci6n positiva entre 5 y t y correlaci6n negativa entre la arcillosidad y t. se destaca la efectividad de la conductancia longitudinal por encima de la resistividad electrica (rt) y la resistencia transversal (rt) para estimar la transmisividad del acuifero. evidentemente, cada uno de los parametres estudiados son variables regionalizadas que adoptan un valor para cada punto x del espacio y en elias pueden ser observados dos aspectos complementarios y aparentemente contradictorios (samper & carrera, 1990): a) un comportamiento aleatorio, asociado con las variaciones erraticas e impredecibles de estas variables regionalizadas, b) un comportamiento general estructurado, que en cierta forma refleja las caracteristicas globales de la variaci6n del fen6meno. por 10 tanto, se hace necesario una formulaci6n del problema que tenga en cuenta ambos aspectos, que haga posible una representaci6n simple de la variabilidad espacial, consistente desde el punta de vista te6rico y operativo como desde el punta de vista practice. por este motive, dada la naturaleza del problema donde los atributos muestran una variabilidad espacial erratica que dificilmente puede ser representada mediante funciones deterministicas, aplicamos tecnicas de c1asificaci6n estadistica sobre el conjunto de datos, con el fin deidentificar grupos de intervalos de composici6n relativamente hornoqenea a partir de las regularidades del conjunto de variables usadas para su descripci6n. en la literatura se describen numerosas tecnicas de clasificaci6n estadistica multivariada (alfonso, 1989). en este caso se aplica una tecnica de clasificaci6n no supervisada. 4. fue aplicado el analisis euclidiano de agrupaci6n (aea) sabre las variables log rt, log rt, log s, log ig, log f, log $arch'€, log car, log llar, log vf; para los 201 intervalos definidos en los 9 pozos bajo estudio. fueron analizados de 2 a 6 modelos de c1asificaci6n, los resultados obtenidos se presentan en las tablas 3 y 4. estos resultados demuestran que a pesar de la marcada heterogeneidad y anisotropia de las propiedades de este acuifero carstico, existe una componente estructural bien definida que permits relacionar la variabilidad de los parametres geofisicos de pozo y de los para metros hidrodinamicos como la velocidad del flujo de agua subterranea y, por 10 tanto, la permeabilidad, la conductividad hidraulica y la transmisividad, aumentan con la disminuci6n del para metro de arcillosidad relativa y con el incremento de la conductancia longitudinal. tambien, en cierta medida un incremento de la porosidad favorece la permeabilidad del acuifero. 5. fue estimada la correlaci6n entre los para metros de dar-zarrouk (resistencia transversal y conductancia longitudinal) calculados a partir del registro electrico y la transmisividad calculada por pruebas de bombeo. ei problema fundamental para realizar este estudio fue la ausencia casi total de coincidencia espacial de las pruebas hidrogeol6gicas y de las mediciones geofisicas de pozo, por 10 que era imposible abordar este analisis de forma convencional. sacasas (1987) referencia un metoda desarrollado por el investigador ruso aauvarov para definir la correlaci6n entre dos atributos x e y que no necesariamente tienen que estar pareados. uvarov establece que teniendo una poblaci6n de x y una poblaci6n de y que caracterizan a un area de estudio, es posible liegar a expresiones de relativa sencillez para evaluar y a partir de x, haciendo uso del metoda estadistico de compatibilizaci6n de muestras. establece que tales comportamientos de las dependencias correlacionables se obtienen siempre que, por un lado la variable x y por otro lade la variable y, tengan distribuci6n tabla 1 estadigrafos y ley de distribucion de las variables bajo estudio (n=201) variable 6 minimo maximo ley de distribucion rt 388.82 552.56 20 4800 normal rt 583.22 1112.04 24 10080 logaritmico-normal s 1.42 3.37 0.0002 0.4 logaritmico-normal f 20.75 24.02 1.5 208.7 logaritmico-normal oarchie 0.18 0.14 0.016 0.73 logaritmico-normal ig 3.65 2.37 0.67 9.72 no normal; no log-normal car 0.33 0.26 0.00 0.99 no normal; no log-normal l;ar 0.58 0.25 0.00 0.96 no normal; no log-normal vf 8.6 4.41 3.58 30 logaritmico-normal kf 2152.48 1103.78 895.0 7500 logaritmico-normal t 361200 39833 3800 30000 logaritmico-norrnal tabla 2 coeficiente de correlacion lineal (r) entre las variables obtenidas de la interpretacion de los registros geofisicos de pozo (n=201 intervalos en 9 pozos) log rt log rt logs loglg log f log eparchie log car log uar logvf -0.27 -0.07 0.38 -0.43 -0.14 0.14 -0.39 -0.42 log kf -0.27 -0.07 0.38 -0.43 -0.14 0.14 -0.39 -0.42 logt -0.19 0.26 0.58 -0.45 -0.01 0.01 -0.43 -0.38 tabla 3 modelo de clasificacion con 6 grupos obtenido oel aea grupo n rt rt s f eparchie ig car uar vf 1 27 730 1092 0.003 38 0.06 2.08 0.07 0.53 8.17 2 33 235 233 0.005 12 0.17 7.37 0.45 0.72 5.38 3 36 66 98 0.04 4.27 0.38 1.96 0.06 0.15 12.5 4 25 1321 1990 0.0012 58.3 0.05 4.6 0.25 0.81 7.26 5 42 123 190 0.016 8.25 0.22 4.85 0.27 0.55 7.8 6 37 268 500 0.008 20.9 0.11 1.10 0.009 0.07 9.86 tabla 4 coeficiente de correlacion lineal (r) entre los centroides del modelo de clasificacion con 6 grupos (n=6) rt rr s f part:hie ig car 1]ar vf -0.35 -0.29 0.83 -0.33 0.61 -0.8 -0.82 -0.85 normal 0 log-normal. ademas es necesario conocer, con anticipaci6n, si la correlaci6m entre ambos parametres es positiva 0 negativa. para profundizar en el conocimiento del tipo de correlaci6n que debe esperarse a nivel de campo entre los parametres de dar-zarrouk y la transmisividad de bornbeo, se insiste en el estudio de la correlaci6n entre los para metros geoelectricos (calculados del registro electrico) y ta transmisividad (calculada por el rnetodo de diluci6n de sal). para los 9 pozos bajo estudio fueron definidos 21 intervalos cuasi-hornoqeneos, caracterizados por los valores promedios de vf, kf, t (obtenidos del rnetodo de diluci6n de sal), rr, resistividad transversal (cr), s, resistividad longitudinal (ot.), resistividad medio cuadratica (am), coeficiente de anisotropia ()j; todos estos para metros calculados a partir del registro electrico. la tabla 5, muestra el coeficiente de correlaci6n lineal entre estos parametres obtenidos de mediciones geofisicas en pozo. nuevamente se destaca la elevada correlaci6n lineal directa entre la conductancia longitudinal y la transmisividad, y que la conductancia longitudinal es el para metro geoelectrico que presenta la mayor eficiencia para el estudio de los para metros hidrodinamicos del acuifero en estas condiciones hidrogeol6gicas. se decide entonces aplicar las relaciones de uvarov entre na s y tbombeopara una profundidad promedio de 25 metros en el acuifero. la tabla 6, refleja los estadigrafos de la variable tbombeoy su ley de distribuci6n. se calcula la conductancia longitudinal hasta 25 metros de profundidad por debajo del nivel freatico para 26 pozos del area. la tabla 7 muestra los estadigrafos y la ley de distribuci6n de este pararnetro. para relacionar s y tbombeo mediante las relaciones de uvarov, se tuvo en cuenta que: la conductividad longitudinal presenta distribuci6n lognormal. aunque la transmisividad presenta distribuci6n normal para estos datos, la literatura reporta que este parametro sigue una ley de distribuci6n log-normal (sam per & carrera, 1990). la correlaci6n entre estos para metros es positiva sequn la informaci6n de los registros geofisicos de pozo. para evaluar la efectividad de esta metodologia en el pron6stico de la transmisividad, se analizaron los resultados en las parejas de pozos (1502 y ts-106) y (688 y ts-110) por coincidir espacialmente. para ello, se presenta la relaci6n de uvarov calculada sin considerar estos pozos y considerando los mismos. tabla 5 valores del coeficiente de correlacion lineal entre los parametros geoelectricos e hidrodinamicos obtenidos de registros geofisicos en pozo (n=21) rt pt s pl pm vf -0.4 -0.45 0.64 -0.47 -0.47 kf -0.31 -0.34 0.39 -0.35 -0.36 t -0.04 -0.31 0.81 -0.34 -0.33 tabla 6 estadigrafos y ley de distribucion de la transmisividad obtenida por pruebas de bombeo (n=21) variable minimo ley de distribuci6n (probabilidad = 95%)(j maximo tsombeo 5697 239 normal4621 19662 tabla 7 estadigrafos y ley de d1stribucion de la conductancia longitudinal obtenida por registros electricos (n=26) variable minima maximo ley de distribuci6n (probabilidad = 95%)a s 0,639 0.46 0,099 1,69 logaritmico-normal a) sin los pozos (1502 y ts-1 06) y (688 y ts-11 0) log (tibombeo) 1,363 (log si + 1.258) + 2,288 evaluando esta expresion para el valor de s en el pozo ts-106 se obtiene que la tbombeoesperada es de 4863 m2/dia, este valor es muy proximo al valor de t obtenido por bombeo en el pozo 1502 que resulto ser de 4953 m2/dia; 0 sea el error del pronostico es de un 2%. repitiendo esta operaci6n para el pozo ts-11 0 se estima una tbombeode 3924 m2/dia que resulto tarnbien una buena estimaci6n teniendo en cuenta que la transmisividad obtenida por el bombeo en el pozo 688 es de 4606 m2/dia. en este caso el error es de 14%, c) considerando los pozos (1502 y ts-106) y (688 y ts110) para calcular la relacion de uvarov, se obtiene: log tibombeo= 1,352 (log si + 1.215) + 2.361 aplicando el mismo procedimiento se comprueba que esta expresion es mas precisa para estimar la transrnisividad. para el pozo 1502 se estima una tbombeode 4902 m2/dia, 10 que representa un 0.8% de error, y para el pozo 688 se estima una bombeode 3994 m2/dia para un 13% de error, conclusiones ei procesamiento estadistico aplicado a la informacion geofisica e hidrogeol6gica permiti6 definir un modelo hidrogeofisico que esclarece la relacion entre los parametres geofisicos de pozo y los para metros hidrodinarnicos. las caracteristicas fundamentales de este modele hidrogeofisico son las siguientes: 1. ei incremento de la actividad gamma natural de las rocas refleja un incremento de la arcillosidad y una disminuci6n de la permeabilidad del acuifero, 2. la porosidad calculada a partir de la ecuaci6n de archie, por si sola no es un pararnetro suficientemente expresivo para explicar las variaciones de los para metros rudrodinarrucos, 3. ei pararnetro de arcillosidad relativa define en gran medida el comportamiento de la velocidad del flujo del agua subterranea. un incremento de este pararnetro hace disminuir la permeabilidad del acuifero. 4. la conductancia longitudinal es el parametro qeoelectrico mas eficiente para explicar las variaciones de los para metros hidrodinarnicos. 5. ei incremento de la conductancia longitudinal esta asociada a un incremento de la transmisividad de los colectores. 6 la conductancia longitudinal calculada a partir del registro electrico correlaciona con la transmisividad obtenida por pruebas de bombeo, sequn un modele de regresi6n logaritmico lineal positive. por primera vez en cuba, se analiza de forma cuantitativa la relaci6n que existe entre parametres geofisicos de pozo y parametres hidrodinamicos. por primera vez se aplican las relaciones de uvarov para abordar un problema de esta naturajeza y se comprueba la eficiencia de esta metodologia de procesamiento. a pesar de las iimitaciones practicas que imponen el insuficiente muestreo y el reducido complejo de rnetodos geofisicos de pozo en la cuenca sur de la habana, el procesamiento estadistico aplicado permiti6 separar la componente estructural de las variables hidrogeol6gicas estudiadas y establecer los nexos mas generales entre las variables geofisicas e hidrodinamicas, sintetizando la informaci6n de los registros geofisicos de pozo en un jenguaje hidrogeoflsico superior. referencias bibliograficas alfonso, j. (1989): estadistica en las ciencias geol6gicas. tomo ii. enpes, pp.239-206. editorial ispjae, la habana-cuba. brown, e. charles. (1988): determination of rock properties by borehole-geophysical and physical testing techniques and ground water quality and movement in the durham triassic basin, north carolina. 29pp. u.s.geological survey professional paper 1432. washington. garcia, c. (1996): tecnicas y metodologia para la interpretacion de registros geofisicos de pozo en yacimientos de origen vulcanoqeno-sedirnentario. 110pp. tesis de doctorado. isctn. la habanacuba. g6mez rivero o. (1979): a semiquantitative technique for predicting producibility limits of reservoirs by well log analysis. 10pp. spwla, 20 annual logging symposium, june 3-6. jones, p.h. & t.b. bufford. (1951): electric logging applied to ground-water exploration. geophysics 16(1):115-139. katsube, t. & p. humet. (1987): permeability determination in crystalline rocks by standard geophysical logs. geophysics 32(3):342-352. keys, w.s. & l. maccary. (1971): application of borehole geophysics to water-resources investigations. in techniques of water-resources investigations of the usgs. 126pp. mazac, 0.; w.e. kelly & i. landa.(1985): a hidrogeophysical model for relations between electrical and hydraulic properties of aquifers. journal of hydrology 79:1-19. sacasas le6n, c. (1987): metodos geofisicos aplicados a estudios inqeniero-qeoloqicos e hidroqeoloqicos. conferencias de post-grado (inedito). dpto flsica ispjae. la habana-cuba.7274pp. samper, f.j. & j. carrera. (1990): geoestadistica. aplicaciones a la hidrologia subterranea. centro internacional de metodos nurnericos en ingenieria. universidad pohtecnica de cataluria. barcelona, espana. 17-64. valcarce, r.m. 1995: metodos geofisicos en pozos para el estudio de acuiferos carsticos. tesis de maestrla. 74. ispjae, la habana-cuba. geotisic a colombian a pp. 53-61 diciembre de 2002 bogota, dc issn 0121 2974 leyes de atenuaclon de la intensidad macrosismica en colombia augusto antonio gomez capera istituto nazionale di geofisica e vulc an o io q l a sezione di milano, via bassini 15-20133, milano-italia elkin de jesus salcedo hurtado departamento de geografia, universidad del valle, a.a. 25360, ciudad universitaria. cali colombia resumen con el fin de establecer los patrones de atenuaci6n de la intensidad macrosismica con la distancia en el territorio colombiano, en el presente articulo se han analizado los mapas de isosistas de 34 terremotos fuertes. usando modelos te6ricos preestablecidos y los valores de profundidad focal macrosismica, se establecen tres rangos diferentes de profundidad (h ~ 20 km, h ~ 60 km y h > 60 km), para los cuales se obtienen tres diferentes relaciones de atenuaci6n: i; =1, 0.019( x-15)0.76in( xi5), con (j' =0.08; para h~20km 1 =( 2.3m6 6.8 )l.lx·o.06 e-o.ooix, con (j' = 0.26; para h~60km 1 = 1 0 1.26e -o.004x , con i' , =0.80; para h>60km para sismos superficiales el coeficiente de atenuaci6n geornetrica adquiere mayor importancia que el coeficiente de atenuaci6n fisica. palabras clave: atenuacion macrosismica, intensidao macrosismica, profundidad focal macrosismica. colombia abstract in order to establish the regional model of attenuation of macro seismic intensity with distance in the colombian territory, the isoseismal maps of thirty four strong earthquakes were analysed in this paper. using some teorical models of attenuation and the macroseismic focal depth we chose three different intervals (h < 20 km, h < 60 km and h > 60 km). from here the following three attenuation relations were obtained: i; =1 0 0.019( x-15)0.76in( x-15), con (j' = 0.08; para h~20km 1=(2.3m 6 _6.8)l.lx·006 e-o . doix , con (j' = 0.26; para h~60km 1 =1 0 1.26e-0.004x, con 1" =0.80; para h>60km for superficial earthquakes, the geometrical spreading coefficient its more important than the coefficient of physical absorption. keywords: macroseismic attenuation, macroseismic intensity, focal macroseismical depth, colombia introduccion geograficarnente, colombia esta ubicada en la esquina noroccidental del continente suramericano, donde convergen tres placas litosfericas: nazca, sudamerica y caribe. el movimiento de estas placas produce grandes zonas de deformaci6n caracterizadas por la presencia de diversos tipos de fallas geol6gicas, que se reactivan debido a la acumulaci6n de grandes esfuerzos que, a su vez, crean zonas sismogenicas generadoras de fuertes y destructores terremotos. debido al crecimiento de las ciudades, la gran densidad de poblaci6n situada en zonas de alta ocurrencia sismica y a las catastrogedfislca cdldmbiana. 6, diciembre de 2002! 53 gomez et al. fes ocurridas en el cursu de los ultirnos an os, en colombia se ha visto la necesidad de evaluar la amenaza y el riesgo sisrnico con mayor precision. uno de los pasos a seguir para la solucion de estos problemas consiste en hacer una modelizacion de las leyes 0 patrones de atenuacion sismica por aceleracion (0 velocidad 0 desplazamiento) de las particulas del terreno. sin embargo, colombia no cuenta con un numero suficiente de acelerogramas 0 registros de movimiento fuerte, ya que solo ahora se esta implementando la red nacional de acelerografos (jngeominas, 1995). por las anteriores consideraciones, y viendo la necesidad de establecer -asi sea de manera preliminar-, una primera relacion de atenuacion sismica, el presente trabajo estudia este problema a partir del analisis e datos macrosismicos, ya que en el pais contamos con informacion de evaluacion de dafios de grandes sismos en una ventana de tiempo relativamente amplia. actualmente es indiscutible el hecho de que en aquellos paises donde no se cuenta con una buena instrumentacion ni gran cobertura de la red para la deteccion y iocalizacion de los sismos, y no se tiene un periodo de registro considerablemente amplio para establecer un patron de comportamiento de la sismicidad, la utilizacion de los datos macrosismicos se convierte en la mejor y unica opcion para obtener relaciones y formulas que contribuyan a solucionar, de manera inmediata, el problema de la amenaza y el riesgo sismico. en este trabajo se recopila la informacion macrosismica disponible, representada en mapas de isosistas, y bajo planteamientos flsicos utilizados en modelos teoricos predeterminados por otros autores, y se hace el estudio macrosismico orientado a la obtencion de formulas 0 patrones de atenuacion de la intensidad macrosismica para el territorio colombiano, las cuales se ex pres an mediante relaciones entre la intensidad macrosismica, la magnitud y la profundidad focal del sismo. de esta manera, esperamos dar un aporte significativo a la solucion de los problemas del riesgo y la amenaza sismica en colombia, puesto que hasta el momenta se ha venido utilizado relaciones de atenuacion sismica tomadas de otras regiones delmundo (lngeominas, 1998), 10cual constituye un primer inconveniente para obtener resultados confiables. algunas definiciones y modelos teoricos de decrecimiento de la intensidad macrosismica intensidad macrosfsmica la intensidad macrosismica se evalua a partir del inventario, en un sitio dado, del conjunto de efectos producidos sobre la poblacion y el entomo por una sacudida sismica. se puede expresar en unidades de escala de mercalli modificada, conocida como unidades en mm o en otras unidades dadas por diferentes escalas; por ejemplo, la msk-64 y la escala macrosismica europea de 1998 (ems-98). 54 estas tres escalas se componen de 12 grados cada una, los cuales se expresan en numeros romanos (i, ii, ii, ... , xii). ei conjunto de estimaciones puntuales de intensidad constituye la base de datos macrosismicos 0 campo macrosismico, campo escalar representado por la funcion intensidad sismica i, que, en los casos mas sencillos, depende de la distancia del epicentro al sitio i-esimo, don de se evaluan los dafios (distancia epicentral), y de la profundidad del hipocentro. los mapas de isosistas indican areas de similar intensidad sismica. estos mapas son una representacion del campo de intensidades. ei contomo de las areas se denomina isosistas. a partir del campo macrosismico es posible evaluar ciertos parametres de un sismo, que podrian ser comparables con los obtenidos instrumental mente pero, en aras de buscar un mejor conocimiento de la sismicidad, es mejor tomarlos como datos complementarios entre los unos y los otros. tales parametres pueden ser la intensidad epicentral (10); el epicentro macrosismico, definido como el centro de masa de las tres isosistas mas altas; la magnitud del terremoto; la profundidad del foco y la distancia epicentral (sheba lin, 1968; bottari et al., 1979). amplitud de onda la energia iiberada en un terremoto es transportada por las ondas sismicas y se disipa haciendo que el medio retome a su estado de equilibrio. la disipacion de la energia se debe a que la onda sismica al atravesar el limite de distintas capas sufre diversos procesos fisicos: division en varios tipos de ondas; expansion de caracter geornetrico, que se denomina divergencia geornetrica; dispersion ocasionada por elmedio turbio y el efecto de transformacion de la energia elastica de las ondas en calor debido a la anaelasticidad del interior de la tierra. la componente espacial de la amplitud para un armonico monocromatico amortiguado esta dado por (landau y lifshitz, 1985; savarensky,1975): (1) donde ao es la amplitud de la onda en la fuente 0 foco; res la distancia de la fuente a un punto sobre la superficie de la tierra; a" es el coeficiente de dispersion; a' es el coeficiente de absorcion por disipacion no elastica, de tal manera que a = a" + a', siendo a el coeficiente de amortiguamiento de la onda armonica rnovil; n = -i para la onda volumetrica y n = y2 para la onda superficial. ei valor de a" puede ser una funcion compuesta de la frecuencia. en gran parte del interior de la tierra se cumple que a' » a". la amplificacion de la amplitud de onda no es explicada por la anterior ecuacion, sino por los fenornenos de resonancia entre elmedio (capas 0 depositos del subsuelo adyacentes a la superficie terrestre) y la onda sismica de entrada. geofislca colombiana, 6, diciembre de 2002 leyes de atenuacion de la intensldad macrosismica en colombia mode/os de decrecimiento de /a intensidad macrosfsmica a medida que una onda sismica se propaga por la corteza terrestre, alejandose de su fuente, se produce dos tipos de fenomenos fisicos relevantes. el primero es la expansion geornetrica, que consiste en que la superficie del frente de onda aumenta ampliamente y la energia, por tanto, se distribuye sobre esta superficie. el segundo es la atenuacion fisica, que consiste en que los materiales que conform an el interior de la tierra absorben parte de la energia de la onda que los atraviesa. suponiendo una onda sinusoidal plana y asumiendo una fuente puntual, y que la propagacion se realiza en un rnedio hornogeneo e isotropico, a partir de la teoria de propagacion de ondas se tiene que la energia en un punto situado a una distancia r de la fuente puede expresarse como (howell and schultz, 1975): e= eo r:«: 41t donde eo es la energia iiberada inicialmente por la fuente, m el coeficiente de expansion geometrica yael coeficiente de atenuacion fisica. teoricarnente el coeficiente de expansion geometrica toma los val ores m = 10m = 2 segun que las ondas que se propagan sean ondas de superficie 0 de volumen, respectivamente. el coeficiente de atenuacion fisica que caracteriza el medio depende de la naturaleza de los materiales que 10 componen. mediante una relacion empirica entre la intensidad y la energia, y a partir del conocimiento del decrecimiento de la energia, se puede deducir el patron de decrecimiento de la intensidad macrosismica. dugue (1989) utilizando criterios clasicos de disipacion de la energia, ha propuesto dos familias de leyes de decrecimiento que relacionan la intensidad sismica respecto a la energia, 1 = i(e), expresadas como: relacion de tipo 1: 1= k, + k2 ine relacion de tipo 2: i =k3 ek, las constantes k], k2, k3 yk4 que figuran en las dos ecuaciones anteriores dependeran de la fuente sismica y de la atenuacion que sufre la onda en el proceso de propagacion por el medio fisico. reemplazando la expresion de energia (2) en las ecuaciones (3) y (4), respectivamente, y teniendo en cuenta que en el epicentro r = h (condicion inicial de frontera, donde i = 10) y utilizando la aproximacion x"23h (shebalin, 1968), se obtiene que ley de tipo i: ii-io =a + blnxi+cxi para la cual se usan los cam bios geofislca colombiana. 6, diciembre de 2d02 (7) (8) ley de tipo 2: in()()=a'+ b'lnxi+c'xi (9) usando los cambios (10) (11 ) (12) (2) donde xes la distancia epicentral; by b' son los coeficientes de atenuaci6n geornetrica, c y c' son los coeficientes de atenuacion fisica. si la atenuaci6n fisica es pequefia comparada con la geornetrica, en la ley de tipo i (5) se obtiene el modelo de blake-shebalin (shebalin, 1968; bottari et ai., 1979), dado por _ ~xi2+h21-1 -y log --i o h2 (13) donde xi es la distancia epicentral medida desde el epicentro hasta un sitio i sobre la superficie terrestre; y es el coeficiente de atenuacion geometrica y h es la profundidad del foco. de este modelo se deriva una expresion tanto para h como para y (shebalin, 1968), dadas como h-~ ~ xi n ~ ~(i0"j"-jx _i) 2 (14) (15)t, 10g(aj ) / ai+'(3) (4) siendo ai el area encerrada por la i-esima isosista. para el caso en que la atenuacion fisica sea mayor que la atenuacion geometrica, de la ley de tipo 2 (ecuacion 9) se deriva un mode10 que decae exponencialmente, representado como (16) donde a"=ea' y c'< 0 (5) igualmente, howell and schultz (1975) han propuesto modelos teoricos del decrecimiento de la intensidad en funcion de la magnitud del sismo (magnitud de onda superficial) y la distancia epicentral, en los cuales la magnitud es una variable instrumental que actua como parametro de control porque reduce la subjetividad del modelo. tales modelos se expresan de la siguiente manera: (6) 55 comez et al. ley de tipo 1 controlada: ley de tipo 2 controlada: in i; in(1.5ms~ 1.5)+ a2 + b2inx;+c2x; los coeficientes b y c representan la atenuacion geornetrica y fisica respectivamente; a y d estan relacionados con las condiciones de frontera de la fuente. £1 termino que acornpafia a c1 actua como componente de fuente lejana en la ecuacion y el termino que acompafia a c2 actua como componente de fuente cercana. notese que la profundidad de los focos no aparece explicita en las ecuaciones, 10 cual bliga a que, para la obtencion de los patrones de atenuacion derivados por estas relaciones, se deban separar los sismos que por su profundidad pertenezcan a diferentes ambientes tectonicos 0 a fuentes sisrnogenicas distintas. solo asi se logran resultados mas confiables. seleccion y evaluacion de datos macrosismico para el estudio estadistico se recopilaron y digitalizaron los mapas de isosistas de 34 terremotos historicos y recientes ocurridos en territorio colombiano y zonas fronterizas con venezuela (figura i). la cifra que aparece en el circulo, (figura i) corresponde a fanumeracion de los eventos en la tabla i. como se puede apreciar, los eventos sismicos fueron seleccionados con el criterio de que la magnitud sea mayor que 5.0 0 que la intensidad maxima observada sea mayor que v. con el fin de respetar la hipotesis de espacio homogeneo e isotropico, mencionada anteriormente, se tome la distancia epicentral igual al radio de la isosista circular, de manera que x = (ails, siendo a el area de la isosista. este parametro fue evaluado directamente de los mapas de isosistas (figura 2), a partir de los cuales tambien se determino el epicentro macrosisrnico de cada evento sismico (tabla i), dado como en baricentro de las tres primeras isosistas, contadas de adentro hacia fuera. tarnbien se calcularon otros parametres macrosismicos: coeficientes de atenuacion de blake y kovesligethy para cada sismo; los ejes mayor y menor de las isosistas y las componentes transversal y paralela de la distancia epicentral de cada isosista, con su correspondiente direccion (salcedo y gomez, 2000). para seleccionar la magnitud de los eventos se tuvieron en cuenta los datos del catalogo sismico de ceresis (1985), el cual es uno de los mas confiables para el territorio colombiano. la escala de intensidad utilizada fue principalmente la de mercalli modificada (mm). la intensidad epicentral (10) se tome con fa isosista de mayor valor, es decir imax. se observo que en las vecindades del epicentro la diferencia entre la intensidad i,y la intensidad epicentral 10 diverge a menos infinita, de manera que para el caso de la ecuacion (5) se tiene que: 56 (17) / ~9 ) l j 1141'\ ."(~29 t ,~~'} :18 '9 (18) \ i. i i i ~j-70 -ee-76'-·76 ·66 figura i. mapa de epicentros de los 34 terremotos utilizados para la obtenci6n de los patrones de atenuaci6n de la intensidad macrosismica en el territorio colombiano. el circulo representa el epicentro y la cifra contenida en su interior corresponde a la numeraci6n de la tabla i. lim~(xj='>-oc! xi--+o mientras que para la ecuacion (9) se tiene: liml2 (x)~ oc! x;--+o l, yl2 se han utilizado para representar ley de tipo i y ley de tipo 2, como se han denominado en las ecuaciones (5) y (9), respectivamente. la divergencia mostrada en los iimites anteriores se debe a los efectos de fuente y refleja los procesos caoticos que se presentan en las vecindades del epicentro, los que a su vez son causados por la gran liberacion de energia en esa region. en el caso instrumental, este efecto se nota por la saturacion que sufren los registros sismicos, mas notoria cuando no se tiene una buena calibracion de las estaciones. de acuerdo con shebalin (1968), para eliminar el ruido nurnerico ocasionado en las vecindades del epicentro, las isosistas de mayor valor no se tuvieron en cuenta para el ajuste de nuestros datos, y en algunos casos solo se consideraron los datos tornados a partir de la tercera 0 cuarta isosista. los valores de profundidad utilizados fueron tornados del trabajo elaborado recientemente por gomez y salcedo (2000), quienes usando los algoritmos de blake-shebalin y koveslighety evaluaron estos parametros para cada uno de los 34 eventos aqui considerados. geofislca colombiana, 6, oiciembre oe 2002 leyes de atenuacion de la lntensidad macrosismica en colombia 2611.1,\rzo 11112 ,., ;, uj iii "iayo 1875 31 agosto 1917 " , ~-' 26di(iefo,1i3ke 1').1.2 s -'~~-' i/4 29 abril 1894 , '5 31 enero 1906 #7 14 marzo 1932 ~r / -"! #21 26 septie,'v1bre1970 #24 19 mayo 1976 --#27 12 dic1e1vll3re 19/9 i 14jl1uoi'j47 21 m1rii. 1957 i "" .~ojuuo 1962 ~ll '" i <)ju1iq 1950 '"12 3 agosto 1950 , r ( j 16jljnlo 1')(,1 itls 20dicie,,·lbre 1%1 / 21,\'iarz01%7 , t, ;' j -#19 29 juljo 1967 #20 7 ,,'!ayo 1968 .~:g n28 26 novlembrb 1980 #f29 6junio 1994 -.. ;~j 180ciub~981 ! #30 31marw 1983 19 enero 1m figura 2. mapas de isosistas de terremotos ocurridos en el territorio colombiano. el numero escrito en la parte inferior de cada mapa coresponde a la numeraci6n de la figura i y la tabla i. i"i") \ e , f 3 abril 1973 1123 30 agosto 1973 r i ' i ) t-__ #25 31 agosto 1977 #26 geofislca colombiana, 6, oiciembre oe 2002 57 comez et al. tabla i. parametros de los terremotos fuertes historicos y recientes ocurridos en colombia, para los cuales existe mapa de isosistas. no. fecha di3 15:05:04 epicentro instrumental epicentro macrosismico profundidad macrosismica magnitud intensidad x ems-92 ado mcs tiempo h m s lat. n 85 -72.9 lat. n 8.5 long. \v h(km) imax escala long. w fuente 1812 03 26 20:07:00 -71.3 -71.5 13 6.2 mcs fiedler. 1961 1827 ii 16 22:4500 1.8 -76.4 1.9 -758 36 62 ix msk espinosa. 1994 1875 05 18 16:15:00 7.9 -72.5 8.0 ~72.5 6.3 xi mm ramirez. 1975 1894 04 29 02:45:00 8.5 -71.7 8.6 -71.6 20 6.2 x mcs fiedler. 1961 1906 01 31 15:36:00 10 -81.5 1.5 -78.5 19 7.4 x msk espinosa, 1992 1917 08 31 ii :36:18 4.0 -74.0 4.3 -74.0 47 6.3 viii msk espinosa. 1994 1932 03 14 22:42:56 8.3 -71.9 8.1 -718 48 61 viii mm fiedler. 1961 1938 02 05 02:23:34 5.1 -75.5 5.3 -75.7 69 6.2 viii r-f ramirez. 1938 1942 12 26 12:31 :40 8.5 -75.5 9.1 -75.8 5.9 viii mm woodward-clyde.1981 10 1947 07 14 06:59:28 1:4 -77.0 1.3 -77.3 18 5.5 ix r-f ramirez, 1948 ii 1950 07 09 02:35:34 7.9 -72.6 7.6 -72.8 18 6.2 x mm ramirez. 1953 1950 08 03 22:50:00 9.7 -69.8 96 -699 27 6.1 x mcs mas vall, 1950 13 1957 04 21 21: 12:26 69 -72.3 7.1 -72.5 106 66 viii mm sarria, 1985 14 1961 06 16 10:31 :56 8.9 -73.4 8.9 -73.3 36 6.2 viii mm sarria, 1985 15 1961 12 20 13:25:34 4.6 -75.6 4.6 -75.6 86 6.9 ix mm sarna, 1985 16 1962 07 30 20 18:52 5.2 -76.4 4.8 -76.1 78 6.9 viii mm sarna, 1985 17 1967 02 09 15:24:45 2.9 -74.8 2.6 -75.0 63 6.3 ix mm sarna, 1985 18 1967 03 01 18:11.43 6.8 -73.0 6.7 -73.1 41 5.4 v mm sarna, 1985 19 1967 07 29 10:24:24 6.8 -73.1 6.4 -73.8 106 6.2 viii mm sarna, 1985 20 1968 05 07 09:00:29 68 -73.0 7.2 -72.6 70 5.7 vii mm sarna, 1985 21 1970 09 26 12:02:30 6:4 -77.5 6.5 -77.8 6.1 vii mm sarria, 1985 22 1973 04 03 13:54:01 4.7 -75.7 4.8 -75.6 63 6.2 vii mm sarria, 1985 23 1973 08 30 18:25:42 7.2 -72.9 7.4 -72.9 55 5.7 viii mm sarna, 1985 24 1976 05 19 04:07: 16 4.5 -75.8 4.8 -75.9 107 5.9 vii mm sarna, 1985 25 1977 08 31 00:42:05 7.4 -76.2 7.6 -76.0 48 5.7 viii mm sarna, 1985 26 1979 11 23 23:40:29 4.8 -76.2 5.0 -76.3 99 6.4 ix mm sarna, 1985 27 1979 12 12 075903 16 -79.4 2.2 -78.5 22 6.4 ix mm sarria, 1985 29 sarria, 1985 28 1980 ii 26 17:35:39 8.0 -72.4 7.9 -72.5 29 5.0 v mm sarria, 1985 1981 10 18 04:31:01 8.2 -75.4 8.0 -72.3 29 5.5 viii mm 30 1983 03 31 13:12:51 2.5 -76.7 2.6 -76.7 10 5.5 viii mm meyer et 01., 1986 31 1990 10 18 15:11:59 7.1 -76.8 7.0 -76.6 21 6.4 x mm coral y salcedo, 1992 salcedo, 1994 32 1993 07 21 04:5709 6.4 -71.1 6.3 -71.1 27 6.1 viii mm romero et 01, 1994 pulido y tapias. 1995 33 1994 06 06 20:47:39 7.9 -76.1 2.7 -76.1 6.4 viii mm 34 1995 01 19 5.0 4.8 -73.2 22 6.4 ix resultados y discusion la confiabilidad de las relaciones de atenuacion depende, en parte, del tratamiento de la y forma como sean considerados los datos disponibles. siguiendo este postulado, para facilitar los calculos y obtener relaciones mas confiables se han separado los 34 eventos considerados de acuerdo con su profundidad macrosismica. de esta manera, el problema de obtener relaciones de la atenuacion de la intensidad sismica para el territorio colombiano se ha dividido en tres. en primer lugar, se busca un patron de comportamiento de la intensidad macrosismica con la distancia para sismos superficiales, con profundidad h ~ 20 kilometres. en segundo lugar, se establece 58 el patron de atenuacion para sismos con profundidades h ~ 60 kilometros; y, par ultimo, se busca una ley de atenuacion para sismos con profundidad mayor de 60 kilometres. es decir, se ha considerado obtener leyes de atenuacion de la intensidad sismica que se ajusten a sismos que ocurren en la parte mas superficial de la corteza, otro rango que abarque estos prim eros y los que ocurren hasta la profundidad de la base de corteza terrestre en el territorio nacional y, por ultimo, para sismos del manto superior. este prolijo procedirn iento, que pennite diferenciar mejor los resultados, esta total mente de acuerdo con 10 establecido por shebalin (1968), quien obro similannente para solucionar el mismo problema en el territorio de la antigua union sovietica. geofislca colombiana, 6. oiciembre de 2002 leyes de atenuacion de la intensldad macrosismica en colombia en cada uno de los intervalos consideradosse aplicaron los diferentes modelos teoricos de atenuacion de la intensidad descritos anteriormente; de los resultados obtenidos se escogi6 como mejor soluci6n aquella que presento mejor parametro estadistico de ajuste, es decir, varianza 0 indice de correlacion, segun las ecuaciones utilizadas. asi, utilizando la ley de tipo 1 (ecuacion 5) y con ayuda del programa minsq (pomentale, 1978), se hizo un ajuste no lineal para sismos superficiales, con profundidad h s; 20 kilometres. el resultado obtenido fue 1i=1 0 -0.019(x-15)-0.76in(x-i5), con (}"2= 0.08 (19) ei numero 15 es un parametro de ajuste que representa el valor medio del paso 10 1= 0, que ayuda a apantallar el ruido numerico que presentan las intensidades en los primeros kilometres ocasionados por los efectos de fuente para sismos superficiales y, al mismo tiempo, armoniza la curva de atenuaci6n de la intensidad sismica en las vecindades del origen (figura 3). i 2 .2 8 6 0'--...:.---' o 50 100 150 200 distancia epieentral (km) figura 3. curva de atenuacion de la intensidad macrosisrnica (f) en funcion de la c!istaneia epicental para terrematas can prafundidad h ,; 20 kilometres; analitieamente esta representada par la ecuacion: mi ~ 0.019(xi 15) + 0.76 in (xi -15), con cr] ~ 0.0841 y distaneias epieentrales, xi > 15 kilometros. para terremotos con profundidades h s; 60 kilometres, el ajuste no lineal se realiz6 con el programa maternatica (wolfram, 1993), especificamente con el algoritmo de levenberg-marquardt (press et ai., 1989), obteniendose que 1-(2 ~ 68)llx-0.06 -o.oou. .jmb .. e , con (}"2 =0.26 (20) la figura 4 muestra la curva correspondiente. para terremotos con profundidades h > 60 kilometres, se obtuvo un modelo de atenuacion dado por la siguiente ecuacion: (21) notese que, en este ultimo modelo de atenuacion, la intensidad decae exponencialmente y s610 aparece el coeficiente de atenuacion fisica. la grafica correspondiente representa una curva de que decae exponencialmente, la cual se muestra en la figura 5. geofisica colombiana. 6, diciembre oe 2002 o """'00 ..0 1-0.2 e <")-0.4 n '-' c: ....l-0.6 i 100 200 300 400 distancia epicentral (km) figura 4. curva de atenuacion de la intensidad maerasismiea (f) en funcion de la distancia epieental para terrernotos can prafundidad h ,; 60 kilometres, control ada por la magnitud (mb); analitieamente esta representada por la ecuacion: in (i !(2.3mb distaneia epicentral (km) modelo que decae exponencialmente 1 ,00 ~--------------0,90 0,80 0,70 s 0,60 0,50 0,40 0,30 0,20 0,10 0,00 +------~-----~----__i 1/10 = 1,2e-o.004• p2 = 0,80 o 100 200 figura 5. curva de atenuacion de la intensidad (f) en funcion de la distaneia epieental para terrematas con profundidad h > 60 kilometres; analitieamente esta representada par la ecuacion: 1 = 101.26e-00i )4" con r' ~ = 0.80. a continuacion, en la tabla 2, se presentan y especifican los diferentes parametres de ajuste obtenidos, los cuales pueden ser facilmente comparados entre si. tabla 2 parametros de ajustes de los patrones de atenuacion de la intensidad sismica obtenidos en el presente estudio rango coeficiente coeficiente de de profundidad de expansion atenuaci6n fisica paramerro (km) geometrica (1/km) de fuente h < 20 0.76 ± 0.22 0.019 ± 0.0055 0.29 ± 0.0841 h < 60 0.06 ± 0.03 0.001 ± 0.0005 1.1 ± 0.55 h > 60 0.004 ± 0.0004 1.26 ± 0.13 es claro que las ecuaciones de atenuaci6n obtenidas no dan evidencias directas sobre cual podria ser la causa de la gran diferencia observada en los patrones de atenuaci6n de los tres niveles de profundidad estimados. pero si se puede establecer que las variaciones 500 • 300 59 gomez et al. de los coeficientes de atenuacion son el resultado de la diferencia de profundidad. en la tabla 2, se puede observar que, para sismos superficiales, el coeficiente de atenuacion geornetrica adquiere mayor importancia que el coeficiente de atenuacion fisica. es factible asumir que la dispersion presentada en el patron de atenuacion para sismos de la corteza, por un lado, se debe al rango de profundidad tomado y, por otro, al fracturamiento 0 las discontinuidades existentes en la corteza (gomez, j 998). conclusiones a partir de datos macrosismicos, se logra un primer resultado de ecuaciones representativas de los patrones de atenuacion de la intensidad sismica con la distancia epicentral en el territorio colombiano. estas ecuaciones han sido establecidas y discriminadas para tres rangos diferentes de profundidad, aplicando modelos teoricos preestablecidos y usando valores de profundidad focal determinada tarnbien por metodos macrosismicos. los rangos escogidos fueron: h :s; 20 ian, h s: 60 ian y h > 60 ian. las ecuaciones y los respectivos parametros estadisticos de ajuste obtenidos son las siguientes: li=i,-0.019(x-15)-0.76in(x-15), con (j2=0.08; para h:s;20km 1 =(2.3m h -6.8)l.lx-006 e-oooix, 1 =1 0 1.26e-o.0o4x, con (j2 = 0.26; para h :s;60km con y2 =0.80; para h>60km los valores de la varianza e) y el indice de correlacion (r') denotan la precision de los model os. la diferencia entre los coeficientes de atenuacion se debe a la diferencia entre los promedios de las profundidades de los focos y a la misma dispersion de los datos de intensidad macrosismica. no obstante el grado de precision y el cuidadoso tratamiento que se ha dado a los datos utilizados, es preciso observar que las ecuaciones de atenuacion obtenidas deben ser consideradas como preliminares, por 10 cual permiten abrir un campo de discus ion e investigacion en los estudios sismologicos de colombia. sin embargo, pese el caracter preliminar indicado, no se resta la importancia que estas ecuaciones tienen para la solucion del problema de la amenaza sismica y el riesgo sismico en colombia, donde tradicionalmente han sido utilizadas ecuaciones de atenuacion tomadas de otras regiones del mundo, como mexico, canada y estados unidos, en las que se evidencian grandes contrastes y diferencias sismologicas y estructurales con respecto a nuestro territorio nacional. por otra parte, es importante considerar los resultados aqui obtenidos puesto que para el futuro representan un primer punto de cornparacion 0 complernentacion con las posibles ecuaciones que se obtengan para todo el territorio 0 para regiones separadas a partir de los datos instrumentales que arrojara la nueva red nacional de acelerografo. para lograr esto, se necesita un considerable periodo de registro. 60 bibliografia bottarj, a.; federjco, b. y lo giudice e. 1979. methodological consideration regarding the determination of some macroseismic field parametres. application to earthquakes in the calabro-peloritan arc. boll. ceo/ teor. appl. xxi (83): 197 225. ceresis. 1985. catalogo de terremotos para america del sur, colombia, datos de hipocentros e intensidades. vol. 4. coral, c. y salcedo, e. 1992. mapa preliminar de isosistas del terremoto del 18 de octubre de 1992. ingeominas. informe interne. dugue, e.1989. modelisation de la decroissance de l'intensite macros ismique. approche regionale du problerne, diplome d'ingenieur geophisicien de l'ecole et observatoire de physique du globe de strasbourg, universite louis pasteur strasbourg, france. espinosa, a. 1992. sismicidad hist6rica y reciente del area de popayan. ingeominas-cee, microzonificaci6n sisrnogeotecnica de popayan. pub/. esp. ingeominas 2: 7-27. espinosa, a. 1994. sismicidad hist6rica de santa fe de bogota y su area, 1500 1994. ingeominas. microzonificaci6n sismica de santa fe de bogota. inedito. fiedler, g. 1961. areas afectadas por terremotos en venezuela. memorias del tercer congreso geologico venezolano. torno iv. gomez, a.a. 1998. atenuaci6n de la intensidad sismica para el territorio colombiano. tesis. departamento de fisica, universidad nacional de colombia & proyecto sismotect6nica del territorio colombiano, subdirecci6n de geofisica-lngeominas. _____ y salcedo e. 2000. determinaci6n de la profundidad focal de algunos sismos ocurridos en colombia a partir de datos macrosismicos. rev. red sismol6gica reg. eje cafetero, no5, p.12-16. gronthal, g. 1998. european macroseismic scale 1998. cahiers du centre europeen de geodynamique et de seisrnologie 15. howell, b. f., jr. and schultz, t. r. 1975. attenuation of modified mercalli intensity with distance from the epicenter. bulletin of seismological society 0/america 62 (3): 651-665. ingeominas. 1995. proyecto red nacional de aceler6grafos de colombia. boletin de movimiento fuerte i (i): 322. _____ . 1998. estudio general de amenaza sismica de colombia. publicaci6n especial. landau, 1.and lifshitz e. 1985. mecanica. volumen i del curso de fisica te6rica. editorial reverte. segunda edici6n. mas vall. 1950. mapa isosismico del terremoto de el tocuyo (3 de agosto de 1950). ministerio de fomento, instituto nacional de mineria y geologia. caracas. referenciado en: grases, j. 1994. terremotos destructores del caribe 1502 1900. unesco. meyer, hj.; duarte, j. y paraffan, a. 1986. caracteristicas fisicas del sismo de popayan. en el sismo de popayan del 31 de marzo de 1983. ingeominas. pomentale t. 1978. cern computer centre. program library. press, w.; flannery, b.; teukolsky, s., and vetterling, w. 1989. numerical recipes. the art of scientific computing (fortran version), cambridge university press. gedfislca cdlombiana, 6, oiciembre de 2002 _____ y gomez, a. a. 2000. atlas macrosismico de colombia. ingeominas, proyecto sisrnotectonica del territorio colombiano. informe interno. lnedito, sarrla, a. 1985. sismos y riesgo sismico en colombia. en: simposio sobre el peligro y riesgo sismico y volcanico en america del sur. ceres is. 14: 330-371. leyesde atenuacionde laintensldad macrosismica en colombia pulido, n. y tapias, m. 1995. aplicacion de la escala macrosismica europea de 1992 en la determinacion de las intensidades del sismo de tauramena (casanare) del 19 de enero de 1995. memorias seminario de sismotectonica del borde llanero colombiano. bogota. ramirez j. e. 1938. algunas notas sobre el terremoto colombiano del4 de febrero de 1938. revista javeriana ix (41): 340.344. ____ . 1948. the pasto, colombia, earthquake of july 14, 1947. bull. seis. soc. am. 38 (4): 247-256. _____ . 1953. el terremoto de arboledas, cucutilla y salazar de las palmas, 8 de julio, 1950. boletin 10. instituto geofisico de los andes. serie a sismologia. _____ .1975. el terremoto de cucuta. cien aiios despues 18 de mayo 1875-18 de mayo 1975.bogota. romero, j.; martinez, j. m.; avila, g. y escallonj. 1994. ei sismo del 21 de julio de 1993. puerto rondon, llanos orientales de colombia. rev. ingeominas (4): 15-34. salcedo e. 1994. mapa de isosistas del terremoto de junio 6 de 1994. inedito. geofislca colombiana, 6. oiciembre de 2002 savarensky,e. f. 1975. seismic waves. mir publishers. shebalin,n. 1968. metodo de utilizacion de datos ingeniero-geologicos en la zonificacion sismica. en: zonificacion sismica de la ur.ss. nauka. wolfram, s. 1993. mathemathica, a system/or doing mathematics by computer. second edition. addison -wesley publishig company. wooward-clydeconsultants.1981. preliminary study of geomorology and quaternary stratigraphy, ituango projct, colombia: report to integral ltda. referenciado en: page, w. 1986. geologia sismica y sismicidad del noroeste de colombia. figura 37. 61 using neuronal networks to determine site response from coda waves: application in armenia, colombia earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004): 19 24 manuscript received april 2004 19 paper accepted october 2004 temporal variation of coda-q at galeras volcano, colombia édgar moncayo1, carlos vargas2 and juan durán3 1universidad nacional de colombia. e-mail cavargasj@unal.edu.co 2universidad nacional de colombia. e-mail cavargasj@unal.edu.co 3instituto de investigaciones geológico minerasingeominas. abstract the temporal variation of coda-q was analyzed for galeras volcano, located in the andes of southwestern colombia. the q -1 value was calculated using the single back scattering (sbs) model working with seismograms filtered in frequency bands centered in 1.5, 3.0, 6.0 and 12.0 hz. the analysis was done in 1990-2002 period, using the urcr station and the frequency dependence q = qo f –n was calculated before and after some eruptions. we used volcano tectonic events with magnitude lower than 1.5 and rms, gap, vertical and horizontal errors lower than 0.1s, 180 and 1km. the temporal change of attenuation occurred in two stages, the first in 1989-1992 period, when the q -1 value increases from 0.015 to 0.070. the second is the 1992-2002 period that is characterized by a decreasing q -1 value. before july 1992 eruption the attenuation increase is associated with the extrusion of an andesitic dome, after this eruption the dome was destroyed and the q -1 value drops. starting from 1994 low attenuation values was associated with low galera’s activity. the analysis shows that q value increases before an eruption and decreases after it. key words: attenuation, coda-q, scattering, colombia, galeras volcano. resumen la variación temporal de coda q fue analizada en el volcán galeras ubicado en el sureste de los andes colombianos. el valor q -1 fue calculado usando el modelo “single back scattering” (sbs) trabajando con sismógrafos filtrados en bandas de frecuencia centradas en 1.5, 3.0, 6.0 y 12.0 hz. el análisis fue hecho durante el periodo 1990-2002, usando la estación urcr y las frecuencias dependientes q = qo f –n fueron calculadas antes y después de algunas erupciones. usamos eventos vulcano tectónicos con magnitud menor que 1.5 y rms, gap, vertical y horizontal con un margen de error menor que 0.1s, 180 y 1km. los cambios temporales en la atenuación se dieron en dos etapas, la primera en el periodo 1989-1992 donde el valor q -1 se incremento desde 0.015 a 0.070. el segundo en el periodo de 1992-2002, caracterizado por un decrecimiento del valor q -1. la atenuación se incrementa antes de la erupción de julio de 1992. esta erupción está asociada a la extrusión de la cúpula de los andes. después de esta erupción la cúpula se destruyó y cayo el valor q -1. iniciando en 1994, los bajos valores de atenuación fueron asociados a la baja actividad del volcán galeras. el análisis muestra que el valor q se incrementó antes de la erupción y decreció después. palabras clave: atenuación, coda-q, dispersión, colombia, volcán galeras. © 2004 esrj unibiblos. edgar moncayo et al. introduction galeras volcano (lat. 1°14’n, lon. 77°22’w) is 4200m-high andesitic volcano in the andes of southwestern colombia and is the most recent active center of galera’s volcanic complex (cepeda, 1986). the active cone has 320 m in diameter and lies within a caldera that is breached to the west. galeras is historically characterized by vulcanian type explosions (calvache, 1990). the most important one recorded, in 1936, generated a pyroclastic flow to the northeast of the volcanic structure. the volcano lies 8 km from pasto city and various figure 1. location of galeras volcanic complex area, and the earthquakes (circles) and stations (s) used for determinations. other towns locate on its flanks. in total, 400.000 people live close to the volcano, inside this hazard zone. since 1988, when galeras was reactivated, after 50 years of repose, it entered in a fumarolic and degassing stage, with seven eruptions between 1989 and 1993. an andesitic dome was extruded in september 1991 and destroyed during july 1992 eruption. since 1994 galeras is in calm stage with lesser explosions and seismic crises in 1995, 1982, 2000 and 2002.many studies have been done since cepeda (1986) and calvache (1990). the latter made a stratigraphic correlation based on c 14 in the 1990-1993 period galeras was declared the volcano of the decade and much research was done. on 1993 january 14 during an international workshop while a fieldwork was in progress in the crater, there was an eruption where several scientists died and others resulted injured. many works were published about galeras in different disciplines: calvache et al., (1997) worked the stratigraphy and evolution of the volcano, a model of degassing was proposed by stix et al., (1993) and geochemistry studies were done by zapata et al., (1997). in the seismology field, the works of gómez and torres (1997) and narvaéz et al., (1997) analyzed the tornillo-type signals whereas the different features seismic sources has been analyzed by ortega et al., (2002). any investigation about temporal variation of the attenuation hasn’t been done, so this is an innovative study that enlarges the knowledge about galeras volcano. geological setting galeras is located in southwestern colombia, a region with a metamorphic rocks basement of precambrian and paleozoic ages. the basement is overlaid by metamorphic rocks of cretacic age with low and medium grade associated with amigdular metabasalts. all of this is covered by volcano sedimentary units of tertiary age that made a plateau, over which the pleistocenic and holocenic volcanoes have emerged. the tectonic plate of this region is very complex, as a result of the collision between the nazca and southamerican plates. this causes the uplift of the andes and the volcanism in the region. the structural trend is n40°e and the principal tectonic feature is the romeral fault zone, which has been interpreted as the limit between continental crust to the east and the oceanic crust to the west (barrero, 1979). this system includes the silvia-pijao and buesaco faults, both of which cross under galeras, and are associated with many old caldera systems (figure 1). method we follow the single back scattering (sbs) model of aki and chouet (1975) for calculating q -1 from the ending portion of the records of local earthquakes, or coda waves. the q factor contains information about seismic energy for two reasons: intrinsic losses (qi) and radiation losses (qr). 20 temporal variation of coda-q at galeras volcano, colombia 21 the method assumes that local earthquake coda is composed by the sum of secondary s waves produced by heterogeneities inside a propagation medium. in this context, coda is the portion of seismogram corresponding to back-scattered swaves, whose travel times are assumed to be twice the s-wave travel time taken from the origin of the earthquake. we assume that the coda portion finishes when the signal-noise ratio is lower than approximately 2 (aki and chouet, 1975). the sbs model considers the back-scattered swaves produced by in homogeneities (scatterers) distributed randomly and uniformly in the lithosphere. the basic and simplified equation is: ln [ t2 aobs (f | r, t)] = c – q-1 2π f t (1) where c is a constant, aobs (f | r, t) is the squared amplitude of the record, f is the frequency, t is the time and q is the quality factor, which contains information about the attenuation of seismic energy. equation (1) has the form y = bx + c where: y = ln [ t2 aobs (f | r, t)] x = 2π f t b = q -1 (2) thus from this model it is possible to calculate q -1 from the slope b, using least-squares techniques. data and processing the seismic data used in this study covered the period between september 1989 and june 2002. the seismograph network installed at galeras is composed of 31 stations equipped with vertical component l-4c seismometers with 1second natural period. the ground motions were telemetric from an observatory located at pasto city and the information taken was signals sampled at a rate of 100s-1. however, seismograph stations have not operated continuously because of changes on network configuration, we used earthquakes with magnitude less than 1.5 to avoid clipping. a total of 435 earthquakes were selected for this study. for hypocenter determinations we used the hypo71 computer program (lee and lahr, 1975). table 1. velocity model used to locate the hypocenters depth at top of the layer (km) p-wave velocity (km/s) 0.0 3.50 2.0 3.70 4.0 4.00 8.0 6.00 26.0 6.80 44.0 8.00 vp / vs 1.78 defined to use regional seismic reflection profiles and based in the work by meissner et al., (1977). we carefully selected seismograms of located earthquakes with root mean square residuals of calculated travel times and observed times (rms) less than 0.1s, azimuthal separation between stations (gap) less than 180°, and formally computed horizontal and vertical errors in locations of less than 1 km. figure 1 shows the location of the seismograph stations and selected earthquakes. the coda-q was calculated for four frequency bands centered at 1.5, 3.0, 6.0 and 12.0 hz, (see figure 2), figure 2. data processing example a. raw seismogram b. bandpass filtered seismogram with centered band at 12 hz c. energy density d. q-1 values. we used the velocity model adopted by the pasto volcano observatory (table 1), using the sbs model with a variation of the in-house routine for matlab developed by vargas and mora (2000) with a 10 seconds window. because variations in q can be introduced by changes in the spatial location of the earthquakes and be misinterpreted as a temporal variation, we show in figure 3. that the hypocenters did not shift through time. edgar moncayo et al. figure 3. space-time and depth-time plots of the selected earthquakes. note that the hypocenters did not shift significantly with time, suggesting that any change in q-1 could be due to temporal changes rather than spatial changes associated with earthquakes’ locations. we used q-values at all stations because we note that during the entire 1989-2002 period the station were not operated, and finally find that these present a similar behavior. additionally we worked with the urcr station, because it has the best record and a wide number of records through the time. analysis and results the coda-q was analyzed for frequency bands centered in 6 and 12 hz. figure 4 shows that q -1 values for the 12 hz band show a clear change in time, maybe contribution of intrinsic absorption to the total attenuation is higher at 12 hz that at 6 hz for galeras volcano. the temporal variations of q -1 value obtained by the sbs model show clear tendencies that follow the changes in volcanic activity. to find the frequency dependence of q -1, we fit the values of q -1 for each frequency band (q = qo f -�). table 2 shows the results obtained from qo and for each eruption (pre and post-eruptive periods). there is a clear difference before and after each eruption. temporal changes can be separated in two stages, the first, before july 1992 eruption, shows a fast rise of q that begins in october 1991 and reaches a maximum value before the eruption. the second stage begins after this eruption, since this date, a long-lasting drop in q is registered, in a short time a little raise happening before june 1993 eruption, and from 22 figure 4. result of q -1 analysis a. 6, b. 12 hz. notice the temporal changes at 12 hz. the q-1 values for 6 hz show scatter. arrows mark the dates of eruptions and periods of unrest at galeras volcano. table 2. frequency dependence parameters before and after of some galeras eruptions period q -1 η before jul.-92 eruption 2.8290 ± 1.32 0.8229 ± 0.12 after jul.-92 eruption 3.2837 ± 1.31 1.4565 ± 0.44 before 16 apr.-93 eruption 5.4490 ± 1.43 0.8541 ± 0.15 after 16 apr.-93 eruption 6.0613 ± 2.60 0.9271 ± 0.15 before jun.-93 eruption 5.2673 ± 1.98 0.9840 ± 0.13 after jun.-93 eruption 6.0890 ± 1.67 1.2225 ± 0.10 1994 to 2002 there were not important changes. the frequency dependence parameters (table 2) show an increase before eruptions and a fall after these, a similar trend shows the q -1 value figure 5. temporal variation of coda-q at galeras volcano, colombia discussion and conclusion evident changes in q-1 occurred in 1990-2002 period. these changes cannot be ascribed to spatial changes in earthquakes hypocenters (figure 2). we concluded that these differences are attributed to changes in the volcano activity and discarded changes related to spatial variations. the values obtained in this work show two stages separated by july 1992 eruption. this date marks an important change at galeras activity, because before this date the attenuation had a clear upward tendency, and after it, the q -1 value decreased, with no similar values being recorded since. finally, the q-1 values show a slow recovery since 1995 to 2002 with a steep increase at the end. our results are according to the study by londoño et al., (1998) about nevado del ruiz volcano, colombia. they observed a systematic decrease in the q-1 values before and after two phreatomagmatic eruptions in 1985 and 1989. at galeras, the temporal variation shows that 12 hz is the best frequency value to discern changes in the activity. these variations in q-1 are clearly associated with changes in the volcanic activity as follows: the fall in the attenuation between 1990 and 1991 can be associated with a degassing process that begins after 1989 eruption. the rise between october 1991 and july 1992 is related to an important change in volcanic activity, such as the extrusion of an andesitic dome with accompanying shallow deformation, increase in the rates and amplitudes of long-period seismic events, and a decrease in the so2 flux. this suggest that the conduit was partially blocked because the viscous magma in the dome acted like a seal that stopped the gas flux, this caused an increase in pressure and attenuation that ended on july 1992 eruption. after this explosive eruption the dome was destroyed and the gas released. for this reasons the pressure drops and attenuation appears. before june 1993 eruption a little rise took place. we suggest this can be caused by small plugs formations that are destroyed in subsequent eruptions. for that reason we didn’t have enough data to get consistent results. attenuation parameters were analyzed on july 1992, april 1993, and june 1993 eruptions. although the error bars are superimposed, (figure 5), the frequency dependence of q shows a similar tendency because we observe three cases in which the attenuation (q-1) figure 5. changes in q -1 values before and after some eruptions. note the decrease of q-1 values after eruption. exhibits a decrease in value before and after each eruption. the parameters also show a long-term decreasing tendency that is according to the fall in the q -1 value during the 1992-1994 periods. in 1994-2002 period the q -1 value were stable. we suggest that it may be associated with repose of the volcano, because it reached an equilibrium state after 1993 eruptions. in this stage little rises in the q -1 value took place. these could be related to minor explosions and ash emissions possibly caused by small gas accumulations (i.e., during june 2002). the changes in the q -1 values show that this parameter can be used as an important tool at galeras volcanic surveillance, because is clearly related to the activity changes, especially before the eruptions. we suggest implementing an attenuation monitoring system. measure by q-1, can be a good complement to the information currently used, for instance, number of seismic events, deformation measurements, and fumaroles gas geochemistry thus helping improve the monitoring. finally we recommend more detailed studies that investigate a possible use of q -1 for forecasting. 23 edgar moncayo et al. 24 acknowledgements we thank ingeominas-corp for technical support in this study. we also thank the comments of our colleagues from volcanological and seismological pasto observatory, ovsp. specially thanks to r. torres for suggestions and comments. references aki, k., chouet, b., (1975). origin of coda waves: source, attenuation and scattering effects. journal of geophysical research, 80: 615-631. barrero, d. (1979). geology of central western cordillera west of buga and roldanillo, colombia. informe ingeominas, bogotá (colombia). calvache, m. (1990). geology and volcanology of the recent evolution of galeras volcano, colombia. m.s. thesis, louisiana state university. calvache, m., cortés, p., williams, s. (1997). stratigraphy and chronology of the galeras volcanic complex, colombia. journal of volcanology and geothermal research, 77: 5-19. cepeda, h. (1986). investigaciones petrológicas en el ámbito de las planchas 429-pasto y 410-la unión, con especial énfasis en el complejo volcánico del galeras. informe ingeominas, medellín (colombia). gomez, d., torrres, r. (1997). unusual lowfrequency volcanic seismic events with slowly decaying coda waves observed at galeras and other volcanoes. journal of volcanology and geothermal research, 77: 173-193. lee, w. h., lahr, j. c. (1975). hypo71: a computer program for determining hypocenter, magnitude, and first motion pattern of local earthquakes. u.s. geological survey. open-file report 75-311. londoño, j. m., sánchez, j. j., toro, l. e., gilcruz, f, and bohórquez, o. p. (1998). coda q before and alter the eruptions of 13 november 1985, and 1 september 1989, at nevado del ruiz volcano, colombia, bulleting of. volcanology, 59: 556-561. meissner, r., fluedh, e., stibane, f., y berg, e., (1977), dinámica del límite de placas activo en el sw de colombia, de acuerdo a recientes mediciones geofísicas. en: proyecto cooperativo internacional nariño "la transición oceáno-continente en el suroeste de colombia": 169-199. narváez, l., torres, r., gómez, d., cortés, p., cepeda, h., stix, j. (1997). “tornillo”-type seismic signals at galeras volcano, colombia, 1992–1993. journal of volcanology and geothermal research, 77: 159-171. ortega, a, torres, r., gómez, d. (2002). caracterización de fuentes sismogénicas de eventos volcano-tectónicos (vt) en el complejo volcánico galeras. memorias. primer simposio colombiano de sismología. stix, j., zapata, j., calvache, m., cortez, g., fischer, t., gómez, d., narváez, l., ordoñes, m., ortega, a., torres, c., williams, s. (1993). a model of degassing at galeras volcano, colombia, 1988-1993. geology, 21: 963-967. vargas, c, mora, p. (2000), conceptos sobre atenuación de ondas coda. red sismológica regional del eje cafetero–viejo caldas y tolima: 37-43. zapata, j., calvache, m., cortés, p., fischer, t., garzón, g., gómez, d., narváez l., ordóñez, m., ortega, a., stix, j., torres, r., williams, s. (1997). so2 fluxes from galeras volcano, colombia, 1989–1995: progressive degassing and conduit obstruction of a decade volcano. journal of volcanology and geothermal research. 77: 195-208. srivastava et al.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 21-33 cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ naveenchandra n. srivastava, brijesh g. rathod, ajay m. solanki, suresh p. machhar, vivek r. patel and a. r. dasgupta bhaskaracharya institute for space applications and geo-informatics nr. ch-0 circle, indulal yagnik marg, nr. info city, gandhinagar – 382 007 (gujarat) india fax: +91-07923213091 corresponding author: naveenchandra n. srivastava, e-mail: naveen0504@yahoo.com abstract cross-section can be described as a two dimensional dataset where the horizontal distances are represented on the x-axis and the depth on the y-axis. a cross-section is a window into the subsurface. this work presents the construction of cross sections with the help of ‘vigat 2005’ – a visual c ++ based software application. its main purpose is to provide cross section views of geoscientific features and to interpret their variation within the area of study. in geological context, profile or cross section is an exposure of the ground showing depositional strata. geological cross sections are very powerful means of conveying structural geometries. they are planar, usually vertical, graphical representations of earth sections showing stratigraphical successions, age, structure, and rock types present in the subsurface. geological cross sections allow a better conceptualization of the 3-d geometry of the structures. by using ‘vigat 2005’, a cross section graphic can be displayed by the user with a simple click of the mouse. it offers much easy to use functionality to facilitate the completion of desired tasks. specific boundary conditions to represent the movement of rock block over the fault can be displayed using the graphical user interface. relief or slope variation of the study area can also be viewed. a topographical map provides an aerial (overhead) view of a landscape. it is possible to create a more pictorial representation of the landscape by making a topographic profile of the region. a topographic profile is a cross section showing elevations and slopes along a given line. a precise method to determine slope variations is to construct a profile or cross section through the topography. the most important advantage of ‘vigat 2005’ is that users do not need to digitize contours. this work focuses on the design and implementation of an optimized interpretive environment that have been built using visual c++ tools. strong programming capabilities of visual c++ have been utilized, at its full extent, for the development of ‘vigat 2005’. key words: vigat, 2005, visual c++, cross section, contour, profile, software, digitization. 21 earth sciences research journal manuscript received october 24 2006. accepted for publication june 20 2007. resumen un perfil puede ser definido como un conjunto de datos bidimensional donde las distancias horizontales son representadas en el eje x, y la profundidad en el eje y. un perfil es una ventana al subsuelo. este trabajo presenta la construcción de perfiles con la ayuda de ‘vigat 2005’ – aplicación basada en visual c++. su propósito principal es proveer perfiles mostrando características geo-científicas que permitan interpretar la variación de las mismas dentro del área de estudio. en un contexto geológico, los perfiles son una exposición del suelo mostrando rocas sedimentarias. estos perfiles son muy útiles para determinar las geometrías de los cuerpos. ellos son representaciones gráficas planares, usualmente verticales, de la tierra mostrando sucesiones estratigráficas, la edad, estructura y tipos de rocas presentes en el subsuelo. los perfiles geológicos permiten una mejor conceptualización de la geometría tridimensional de las estructuras. al usar ‘vigat 2005’, un perfil puede ser mostrado por medio de un simple clic del ratón por parte del usuario. este provee una mejor funcionalidad para completar las tareas deseadas. las condiciones límites específicas para representar el movimiento de un bloque de rocas sobre una falla pueden ser mostradas usando la interfaz gráfica del programa. también puede ser mostrado el relieve o las variaciones de la pendiente el área de estudio. un mapa topográfico permite ver vistas aéreas del terreno. es posible crear una representación más gráfica del terreno al hacer un perfil topográfico de la región. un perfil topográfico muestra elevaciones y pendientes a lo largo de una línea determinada. un método preciso para determinar las variaciones de la pendiente es construir un perfil a través de la topografía. la mayor ventaja de ‘vigat 2005’ es que los usuarios no necesitan digitalizar contornos. este trabajo se enfoca en el diseño e implementación de un ambiente interpretativo optimizado que se construyo con las herramientas de visual c++. las fuertes capacidades de programación de c++ han sido utilizadas completamente para la construcción de ‘vigat 2005’ . palabras claves: vigat, 2005, visual c++, perfil, contorno, software, digitalización. introduction the term ‘vigat ’ means profile or information in gujarati language. profiles or cross-sections are constructed across a contour map to provide a three-dimensional view of the topography. cross-sections construction is a technique used to obtain accurate and consistent information from field, seismic or well data. this is an interactive technique and it depends largely on human interpretation. here, each operation has to be decided by the user (geophysists /geologist). an important method of topographical map analysis is the construction of topographic profiles, which are the graphic representation of graduated elevations along a line segment drawn on a map. the profile view shows the “shape” of the land and demonstrates a line-of sight perspective. the main feature of topographic maps is their representation of topography such as ridges, valleys, mountains, plains and other earth surface features, and natural as well as man-made features. a topographic profile is a cross section of the terrain surface along a given line. some maps also show a cross-section view of the area. a cross-section depicts how features look like below the earth’s surface. a geological crosssection can be termed as a vertical slice through the earth, projecting the dip of the rock units into the subsurface and sometimes above surface. cross-sections are generated from geological maps and can be thought as vertical slices through a map area showing a profile view of subsurface structures. drawing geologic crosssections is not a trivial task and methods for their generation have become very sophisticated because interpretations are made on geometrical features (e.g. bedding and faults) at depth on the basis of surface geometries. some aid in this regard may exist in the form of borehole or geophysical data (university of southern california, 2002). above ground, contour lines represent elevations, or heights. below ground, they represent depths below the surface (the cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ 22 srivastava et al., esrj vol. 11, no. 1. june 2007 23 paleontological research institution, 2003). geological cross-sections are useful for visualizing spatial and temporal relationships of units occurring in an area. steep river canyons, sea cliffs and mountain sides, and roadcuts yield natural cross sections, but most cross sections are made by projecting surface features into the earth. if included, cross-sections are usually printed on the same page of the geological map. they are important accompaniments to geological maps, especially if the map focuses on the geology of the bedrock underneath the soil and loose sediments. topography – the elevation of the earth’s surface – is represented on topographical maps by contour lines. a profile is unexaggerated if the horizontal and vertical scales are the same. to emphasize the differences in relief in a comparatively flat area, the vertical scale may be expanded relatively with the horizontal scale, producing a cross-section with vertical exaggeration. geological maps generally contain the standard topographical information, overlain by the geology of the area. geological maps are often compilations of detailed information gathered from many individuals’ field work over many years. topographical maps depict the terrain of an area as viewed from above. it is possible to convert the aerial view of a topographical map into a view that shows the topography along a given line as viewed from ground level by constructing a topographic profile. the profile is a cross-section with elevation plotted vertically and distance plotted horizontally, and it clearly outlines the relief and slope along a given line on a topographical map. objective cross-sections often accompany geological, geo-morphological, engineering, tectonic, and other maps in scientific and technical bulletins, open file reports, and professional and industrial organizational proceedings. they can be helpful to determine fossil fuels and aquifers, and to locate and describe potential geologic hazards. they can also be utilized to locate subsurface minerals, hydrocarbons, geothermal areas, and construction materials (defense environmental network & information exchange, 2005). the primary way to portray geologic data is through a map. a good geological map shows the types of rocks present in a given area, the nature of the contacts between them (e.g., depositional, intrusive, or fault), and the bedding orientations of the rocks. the problem is that maps present only what is on the surface. often, the main interest lies on not only in the surface geology, but in the subsurface as well. there may be oil and gas or economically valuable deposits or may be in what the subsurface geology can tell about the structural evolution of an area. ideally, it is a picture of what one would see, given a vertical slice through the earth’s crust. in practice, it is not a true picture but an interpretation; an educated guess of what that slice would look like. it is constructed on the basis of the surface geology (the map) and any borehole and/or seismic data that may be available. often, the available data are insufficient to constrain a unique interpretation. if a cross-section is constructed perpendicular to the strike of bedding, the dip of beds in the cross section will be the same as the dip measured in 3-d. one of the basic assumptions in drawing cross-sections is that the rock is neither created nor destroyed; in other words, the volume of rock remains constant. often the best way to visualize a structure on a geological map is with a vertical cross-section. with a cross section view geological principles can be applied superposition, original horizontality, crosscutting relationships, ... -and the sequence of events that produced the materials beneath the surface can be analyzed (butler, 2000). manual technique for crosssection construction these are the suggested steps by maher (2002) to draw a cross-section: choose a cross-section line. sketch out the structure in the cross-section. choose appropriate vertical and horizontal scales. a 1:1 scale provides no distortion. draft the topographic profile. the topography is important for some maps and not as much for others. often, there is no need to plot in every contour, but larger increments can be used. place the surface geological boundaries along the profile and cross section line along • • • • • with dip lines, at the appropriate angle, where available from the map. extrapolate units to depth. reinterpret and revise cross section where inconsistent with the map or other data. extrapolate into the air with dashed lines. place finishing explanatory touches. software functionality the initial screen of ‘vigat 2005’ has been shown in figure 1. the manual technique for drawing cross-sections described by maher (2002) can be performed by ‘vigat 2005’. this task can be completed by implementing the following steps: first of all, the user loads a scanned map or image file of the map (figure 2) as an image in bmp file format (figure 3). now, the user takes a section line on the loaded image file. the user can take a section line in any direction. different colors and thickness can be assigned to a section line. the user enters contour values of parameters along the section line (figure 4). the values of parameters are automatically stored in a database. the user clicks on the display menu to view the cross section. facility for vertical exaggeration is also provided. different profiles can be saved. tools and technology visual c++ has evolved into an incredibly powerful programming environment (young, 2003). it is one of the most comprehensive and sophisticated software development environments available. it has consistently provided a high level programming power and convenience, while offering a diverse set of tools designed to suit almost every programming style. its features include easier application coding, building and debugging, greater support for activex and internet technologies, additional database development options, and new application • • • • • • • • • • • • • architecture and user interface elements (solanki et al., 2005). it makes windows programming far easier by introducing many programming tools such as the menu editor for designing menus and the dialog editor for designing dialog boxes (holzner, 1998). visual c++ consists of the c/c++ compiler, the incremental linker, the resource compiler, and other tools required to generate the 32 bit windows programs. visual c++ manages programs through projects. a program may consist of many different files, and these are all combined into a project (gersting, 1997). the central part of the visual c++ package is developer studio. developer studio is used to integrate the development tools and the visual c++ compiler (williams, 1999). ‘vigat 2005’ requires windows operating system (95/98/2000/xp), microsoft visual c++, and microsoft access for proper functioning. hardware requirement consists of 4 gb hard disk and 64 mb ram. software logic logical inter-relationships of the different functions to be performed by the software are shown in the flowchart. a flowchart is nothing but a pictorial representation of algorithmic steps involved in software development, which helps in systematic analysis and organization of a problem. a flowchart remains as the best choice for communicating the steps needed to solve a problem and the order in which those steps must be executed (rider, 2004). it is an essential and integral part of software development. a flowchart for ‘vigat 2005’ is shown in figure 6. it shows various steps like loading (open and display) image file in bmp file format, drawing section lines, entering contour values, submission of contour values, and display of cross sections. advantages of ‘vigat 2005’ it is completely an interactive software, which provides users with full control over all the operations. it provides a graphical representation of parameters and easy to derive relevant information. no need for digitization of contours. ‘vigat 2005’ does not use dem (digital • • • • 24 cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ 25 srivastava et al., esrj vol. 11, no. 1. june 2007 figure 1. initial screen of ‘vigat 2005’. figure 2. residual gravity anomaly map of auckland volcanic field, contour interval is 1 μn.kg-1, aa’ and bb’ denote section lines (after affleck, 2001). 26 cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ figure 3. loading of image file of contour map in ‘vigat 2005’ in bmp file format. figure 4. dialog box to enter value of contour. 27 elevation model). accurate output is provided. task can be performed repetitively. contrary to ‘vigat 2005’, creating and plotting cross section profiles in geographic information systems (gis) requires the following steps: creating a digital elevation model. creating a section line. creating cross section profiles. plotting cross section profiles. in vigat 2005, instead of contour digitization, users enter contour values from an image file of the map. hence, the number of tasks is reduced considerably, as the user does not need to digitize contours in the map and perform the above mentioned operations. hence, ‘vigat 2005’ is having an edge over available software like ‘quick section’, ‘rockworks2004’, ‘petra’, and ‘terrastation’. limitations of ‘vigat 2005’ ‘vigat 2005’ is only compatible with microsoft windows operating system. multiple operations cannot be performed at once. multiple cross sections cannot be displayed simultaneously. no extrapolation into air. it supports image files with .bmp extension only. developers are trying to modify ‘vigat 2005’ to add more features. microsoft paint may be used at the final stage of the output. utility in constructing the topographic profile, it is easy to exaggerate the vertical dimension by enlarging the vertical scale while keeping the map scale constant. the effect produces profound geometrical changes. this is sometimes useful in geomorphologial or stratigraphical studies to allow greater space for plotting vertical detail. it should never be done for serious structural work because the attitude and form of the structures are also exaggerated; thus, destroying the very • • • 1. 2. 3. 4. • • • • • • • information the structure section seeks to show namely, the true structural relationship at depth. if more space is required to plot a wealth of data, the whole section should be enlarged uniformly. geologists sometimes exaggerate the vertical dimension by enlarging the scale while keeping the horizontal scale the same. application in geo-scientific studies as an illustration of ‘vigat 2005’s capabilities, a gravity profile has been generated from gravity interpretation publication. a published residual gravity map of auckland volcanic field, new zealand is used to generate a cross-section using ‘vigat 2005’. this residual gravity map (figure 2) is characterized by a series of short-wavelength anomalies that correlate only partially with the mapped geology. to the south of mt eden, an elongate gravity high, indicating the presence of dense subsurface rocks, coincides with the location of the former te pouhawaiki scoria cone, which extends and narrows to the south (affleck et al., 2001). small negative anomalies, indicating the presence of low-density deposits, surround this gravity high to the west, south, and east. in the southeast of the region, there is a complex gravity high showing that dense lava flows are probably thick in this region, consistent with the deepening waitemata surface to the south of the ancient divide. figures 1, 2, 3, 4, and 5 illustrate the capabilities of ‘vigat 2005’ by showing the construction of a gravity profile. the west-east cross-section aa’ shows shallow basalt flows in the far west. they get thickened towards east. this gravity profile can be used to make a comparison with observed gravity and borehole data. the surveying techniques, regarding electrical properties, consist of measuring potential at discrete stations along a profile. as in gravity and magnetic surveys, the data are mapped and interpretations of anomalies are based on their geometry. methods used to interpret selfpotential anomalies are often qualitative or based on simple geometric models. visual inspection of mapped anomalies may reveal trends related to elongation of the ore body and crowding of contour lines can indicate its orientation (lowrie, 2002). profiles plotted in known srivastava et al., esrj vol. 11, no. 1. june 2007 cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ 28 figure 5. cross-sections generated by ‘vigat 2005’ along section line aa’ from fig. 2 showing variation in gravity data figure 6. flow chart of ‘vigat 2005’ 29 srivastava et al., esrj vol. 11, no. 1. june 2007 figure 7. hypothetical contour lines of a negative self-potential anomaly over an ore body (after lowrie, 2002) figure 8. the asymmetry of the anomaly along the profile ab generated from fig. 7 suggests that the ore body dips towards a cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ 30 directions across the anomaly can be compared with curves generated from simple models of the source. an example of hypothetical contour lines of a negative self-potential anomaly and a cross section generated by ‘vigat 2005’ has been shown in figures 7 and 8. the asymmetry of the anomaly along the profile suggests that the orebody dips towards a. often in oceanography, there is the need to view the overall relief across the sea floor. it helps to get a sense of its ruggedness. this is best determined by making a bathymetric profile or cross section (reynolds, 1997). depth is always negative below sea level. a bathymetric map is shown in figure 9. figure 10 shows a bathymetric profile generated along section line ab by ‘vigat 2005’. it shows relief across the sea floor and provides a sense of ruggedness. groundwater depth plays a key role in liquefaction evaluation because it results in increased saturation and reduces the effective vertical stress. shearing disrupts the depositional grain packing and results in a more closely packed grain structure, forming a denser material (mansoor, 2004). from the groundwater contour map (figure 11) and the cross sections (figure 12), it can be observed that groundwater movement is mainly from the northeast to the southwest in the northern parts of the city and from east to west in the southern parts of the city. summary cross sections are a valuable method of displaying geological, structural, and other type of information. geological cross-sections show a profile view of subsurface structures. topographical maps present the landscape from a vertical (overhead) point of view. often, it is necessary to know what the landscape looks like from a horizontal perspective in a map (e.g., as though they were standing in the field looking at a vertical section through the landscape). this third dimension is illustrated using a topographical profile. topographical profiles are a special type of cross-section and can easily be constructed. a topographical profile is a two-dimensional diagram that describes the landscape in vertical cross-section. topographical profiles are often created from the contour information found on topographical maps. it represents graphically the skyline viewed from a distance. ‘vigat 2005’ helps to view slope variation along topographic profile. despite of certain limitations, ‘vigat 2005’ offers user friendly interactive graphical user interface (gui) to construct cross-sections and its smooth functioning is a great attraction for the user. as there is no need for digitization of contours, it reduces the number of tasks done by the user considerably. examples of cross section generated from gravity anomaly maps, self-potential anomaly maps, bathymetrical maps, structural maps, and ground water maps specify the vast scope of application of ‘vigat 2005’. it will be of significant use to geophysicist and geologist in various research and application projects. acknowledgements the authors express gratitude to shri t. p. singh, director, bhaskaracharya institute for space applications and geo-informatics, gandhinagar for his encouragement, keen interest, necessary guidance, and permission to publish this work. thanks are also due to reviewers for providing valuable inputs for improvement of quality of manuscript. 31 srivastava et al., esrj vol. 11, no. 1. june 2007 figure 9. a bathymetric map showing 20 m contour interval and section line ab (after reynolds, 1997) figure 10. profile of sea floor generated from fig. 9 along section line ab. figure 11. contours of the depth to the water table in meters in city of aqaba, jordan (after mansoor, 2004). figure 12. cross section generated along section line aq77 – aq76 and aq76 – aq75 from fig. 11 showing the general groundwater elevation within the study area. cross-section generation of various geo-scientific features without contour digitization using a visual c++ based software application ‘vigat 2005’ 32 references affleck, d. k., cassidy, j., and locke, c. a. (2001). te pouhawaiki volcano and pre-volcanic topography in central auckland: volcanological and hydrogeological implications. new zealand journal of geology & geophysics. 44. 313-321. http://www.rsnz.org/publish/nzjgg/2001/24.pdf butler, j. c. (2000). interpreting earth history. http://www.uh.edu/~jbutler/physical/ interpearthhistory.html davis, g. h. and reynolds, s. j. (1996). structural geology. wiley. http://earth.usc.edu/ geol321/labs/prelab2.pdf defense environmental network & information exchange (2005). methods of acquiring and inventorying earth resource data. https:// www.denix.osd.mil/denix/public/es-programs/ conservation/stewardship/chapter4.html. gersting, j. (1997). microsoft visual c++ 4.0. a very basic guide. http://www.cis.temple.edu/ ~koffman/cis67/vcenvirn.doc holzner, s. (1998). visual c++ 6. bpb publication. new delhi, 3. lowrie, w. (2002). fundamentals of geophyics. cambridge university press. 210 pages. mansoor, n. m., niemi, t. m., and misra, a. (2004). a gis-based assessment of liquefaction potential of the city of aqaba, jordan. environmental & engineering geoscience. 10 (4). 297–320. http://geology.newark.rutgers. edu/grad%20students/mansoor/mansoor%20n iemi%20&%20misra%202004.pdf reynolds, b. (1997). gs 108 oceanography, http://web.cocc.edu/breynolds/classes/gs108/ lab%201%20ocean%20floor%20topo-geol.htm rider, j. w. (2004). control flow fundamentals, http://www.jwrider.com/lib/ controlflowfundamentals.htm solanki, a., rathod, b., patel, v. and machhar, s. (2005). tectonic profile viewer, b.e. project report, unpublished, bhaskaracharya institute for space applications and geo-informatics, gandhinagar the paleontological research institution (2003). contour maps explained, http://www.priweb. org/ed/pgws/geology/contour_maps.html university of southern california (2002). g321 structural geology; cross sections and isostasy, http://earth.usc.edu/geol321/labs/prelab2.pdf williams, m. (1999). introducing visual c++ 5, http://cma.zdnet.com/book/visualc/ch01/ch01. htm young, m. j. (2003). mastering visual c++ 6, http://www.mjyonline.com/mmvc.htm 33 srivastava et al., esrj vol. 11, no. 1. june 2007 gómez.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 67-90 seismic hazard map for the italian territory using macroseismic data augusto a. gómez c. istituto nazionale di geofisica e vulcanologia, sezione milano via bassini 15, 20133, milano, italy gomez@mi.ingv.it abstract a seismic hazard map, in terms of macroseismic intensity, is proposed for the italian continental territory and sicily, which has a 10% probability of exceedance in 50 years. the methodology used here was first proposed by cornell (1968), which requires information about the location and seismicity rates within each of the defined seismogenic zones, as well as an attenuation model. in particular, it is proposed an original macroseismic intensity attenuation model derived from the italian macroseismic database dbmi04. the seismic hazard map, obtained in terms of intensity, was subsequently transformed into pga by means of a linear relation between intensity and pga, in order to compare it with the national seismic hazard map mps04. key words: probabilistic seismic hazard, macroseismic data, seismogenic zonation, intensity attenuation, italy resumen se propone un mapa de amenaza sísmica, en términos de intensidad macrosísmica con un 10% de probabilidad de excedencia en 50 años, para italia continental y sicilia. la metodología usada fue propuesta originalmente por cornell (1968), la cual requiere información acerca de la localización y tasas de sismicidad dentro de cada una de las zonas sismogénicas definidas, como también del modelo de atenuación. en particular se propone un modelo nuevo de atenuación de la intensidad macrosísmica desarrollado a partir de la base de datos macrosísmica italiana dbmi04. el mapa de amenaza sísmica obtenido fue transformado en pga usando relaciones lineales entre la intensidad y pga para luego compararlo con el mapa nacional de amenaza sísmica mps04. palabras clave: riesgo sísmico probabilístico, datos macrosísmicos, zonación simogénica, atenuación de intensidad, italia. 67 earth sciences research journal manuscript received july 10 2006. accepted for publication november 9 2006. 1. introduction seismic hazard is generally assessed in terms of peak ground acceleration (pga) for deriving engineering design parameters for new buildings. however, the short time interval covered by the instrumental records can be a problem in regions where the earthquake cycle is rather slow and seismicity not very frequent. in terms of seismic hazard assessment, this can affect the evaluation of seismicity rates in the data sample, because they may not be representative of seismogenic processes. the low density of recording stations determines, in some parts of the world, a limited availability of the strong motion data needed to study the attenuation. it is clear that in these cases the macroseismic data are very important as they may represent the only available data. macroseismic intensity is defined by grünthal (1998) as a classification of the severity of the ground shaking on the basis of observed effects in a limited area. as a consequence of this definition, the macroseismic intensity is a parameter that could be used to evaluate expected ground shaking. in italy, as in other countries, most of the earthquake catalogue data are derived mainly from macroseismic studies (gruppo di lavoro cpti, 2004). the historical research has contributed to the knowledge of the historical seismicity dating back 1000 years (stucchi et al., 1991; albini et al., 2004); the earthquakes occurred before the 20th century and many consecutive events are only qualified with macroseismic intensity data. in the last years in italy, a number of macroseismic databases have been proposed (monachesi and stucchi, 1997; boschi et al., 2000; gruppo di lavoro dbmi, 2005). this wealth of data permits to assess the seismic hazard in terms of macroseismic intensity. a seismic hazard assessment, based on historical earthquakes concerning the local history of seismic effects (site approach), has been proposed by albarello et al. (2002), albarello and mucciarelli (2002), mucciarelli et al. (2000), kijko et al. (2001, 2003); monachesi et al. (1994), gaull and kelsey (1999) and basili et al. (1990). slejko et al. (1998) elaborated a map of seismic hazard of italy, in terms of macroseismic intensity, using the standard probabilistic approach (cornell, 1968), the nt 4.1 earthquake catalogue (camasi and stucchi, 1996), the seismogenic zonation published by scandone (1997), and two intensity attenuation models (grandori et al., 1987; berardi et al., 1993) without introducing the standard deviation. this paper presents a new seismic hazard map of italy mainland and sicily derived by using updated data, a new derived intensity attenuation model, and the cornell methodology as implemented in the seisrisk iii code (bender and perkins, 1987). table 1 describes the input elements used to evaluate the seismic hazard. the first three elements were taken from the national seismic hazard map released in 2004 (gruppo di lavoro mps, 2004); in particular, the cpti04 earthquake catalogue (gruppo di lavoro cpti, 2004), the zs9 seismogenic zonation (figure. 1), and the historical (co-04.2) and statistical (co04.4) completeness time intervals. the other two elements (e.g., seismicity rates in terms of epicentral intensity (i0) and intensity attenuation models) have been computed by gómez (2006). this author presents an original macroseismic intensity attenuation model derived from the most recent italian dbmi04 macroseismic database (gruppo di lavoro dbmi, 2005) that includes different relationships, which are developed in section 2. in section 3, a logic tree approach is used to explore some possible alternatives of epistemic character regarding the catalogue completeness, seismicity rates, and the attenuation models. in the same section, it is described the modification introduced in seisrisk iii (bender and perkins, 1987) to compute hazard in terms of macroseismic intensity. the obtained intensity values have been transformed to pga by using specific empirical relationships developed for italy (margottini et al. 1992; faccioli and cauzzi, 2006). 2. intensity attenuation models 2.1. state-of-the-art macroseismic intensity attenuation is described by musson and cecic (2002) as the rate of decay of shaking with distance from the epicentre. the literature provides a number of empirical relationships that model the intensity decay in varied regions of the world as a function of seismic hazard map for the italian territory using macroseismic data 68 gómez., esrj vol. 10, no. 2. december 2006 epicentral or hypocentral distance. the first model was proposed by kövesligethy (1906) at the beginning of the last century and assumes that the energy of seismic waves declines due to geometrical spreading and absorption of the geophysical media. mathematically, the attenuation of intensity is written as the difference between epicentral and site intensity. where (d) is a function of epicentral distance in km, (h) is the focal depth in km, and (α) is a free parameter. in blake (1941), the kövesligethy relationship is simplified eliminating the linear term (absorption coefficient) but letting the coefficient of the logarithm (geometrical coefficient) as a free parameter (b). following blake (1941), other authors such as howell and schultz (1975), chandra et al. (1979), ambraseys (1985), and dowrick (1992) proposed attenuation intensity models as special cases of the kövesligethy relationship introducing additional simplifications. table 2 illustrates some of the studies proposed in literature; for further references see: neumann (1954), ergin (1969), grandori et al. (1991), peruzza (2000), gómez and salcedo (2002), castro et al. (2002), ecos (2002), fäh et al. (2003), carletti and gasperini (2003), albarello and d’amico (2005), azzaro et al. (2006). the macroseismic intensity attenuation models proposed in the literature are either logarithmic or non-logarithmic (linear, polynomial). the logarithmic models are derived assuming empirically that the macroseismic intensity is proportional to the logarithm or to a power of the seismic energy density (howell and schultz, 1975). the limitation of these models is that the correlation with macroseismic data is not good. for instance, the kövesligethy relationship cannot be used to estimate the coefficients of geometric spreading and absorption, and at the same time estimate focal depth (h) (ambraseys, 1985). in italy, this type of model has been proposed, among others, by albarello and d’amico (2004). their relation describes the intensity decay as a function of the epicentral intensity and hypocentral distance using four free parameters. non-logarithmic models result from a statistical approach that allows finding the best fit of the macroseismic data. two main studies could be mentioned for italy: berardi et al. (1993) and gasperini (2001). the first proposed a simple attenuation model called the cubic root attenuation model (cram) with two free parameters, and the latter proposed a bilinear attenuation model (table 2) with three free parameters. the cram functional model (berardi et al., 1993) has been chosen in the present study to model the attenuation of italian macroseismic intensity data: ∆i=α+βd1/3 (1) it assumes that the intensity decay, expressed by the difference between epicentral and site intensity, is proportional to the cubic root of the epicentral distance (d), without dependence on the earthquakes focal depth. the cram is as fairly simple model as it uses only two free parameters. however, it provides a better fit of the macroseismic data compared to other models such as the logarithmic and square root (berardi et al., 1993). 2.2. macroseismic data set the italian macroseismic database dbmi04 (gruppo di lavoro dbmi, 2005) has been considered in the present study. dbmi04 contains about 60,000 intensity data points (idp), in mcs scale, related to 1042 earthquakes occurred from 217b.c. to 2002. in order to derive an intensity attenuation to be used in probabilistic seismic hazard analysis (psha), a careful selection of the macroseismic data was carried out considering diverse criteria. table 3 describes 13 criteria to filter the intensity records not to be used. first, the macroseismic observations of the etna volcanic zone (zs936 seismogenic zone of zs9) are eliminated because the propagation of the seismic energy in this zone is different than other tectonic zones (del pezzo 69 input element used in this study earthquake catalogue cpti04 * seismogenic zonation zs9 * completeness of the earthquake catalogue historical completeness time intervals (co-04.2) * statistical completeness time intervals (co-04.4) * seismicity rates activity rates (ar) in epicentral intensity classes (io)** gutenberg-richter rates (gr) in epicentral intensity (io)** ground motion attenuation relationship macroseismic attenuation relationship as a function of epicentral distance ** table. 1. elements used to evaluate seismic hazard in terms of macro-seismic intensity. the elements with * are proposed in gruppo di lavoro mps (2004), while those with ** are proposed in gómez (2006). table. 2. examples of attenuation intensity relationships referenced in literature. all relationships are logarithmic, except berardi et. al. (1993) and gasperini (2001). i0= epicentral intensity, r = hypocentral distance (km), d = epicentral distance (km); ml = local magnitude; h = depth of focus (km), m = magnitude; b, d, α, are constants. author attenuation relationship kövesligethy (1906) i0-ii=3 log(di/h)+3 α log(e) (di-h) blake (1941) i0-ij = b log (di/h) howell and schultz (1975) ln (i/i0 ) = 0.364-0.130lnr-0.0019 r gupta and nuttli (1976) i(d) = i0+3.7-2.7logd-0.0011d d ≥ 20 km chandra et al. (1979) i(d)= i0+6.453-4.960log(d+20)-0.00121d sbar and dubois (1984) i =i0+3.2-1.5lnd–0.0015d ambraseys (1985) i0-i=-0.22+0.0024 (r-h)+2.85 log(r/h) greenhalgh et al. (1989) i = i0 e -0.032r/2 / r dugue (1989) i0 i =0.2 ln(d-d)+0.04(d-d), d>10 dowrick (1992) i=2.18+1.41m-1.18lnr-0.0044r berardi et al. (1993) i0-i =-0.729+1.122 zsìros (1996) i0-i =3log( )+3(0.0161)log(e)(d-h) gasperini (2001) albarello and d’amico (2004) i=3.6-0.003r-0.98 lnr+0.705i0 musson (2005) i=3.31+1.28ml-1.22lnr 70 seismic hazard map for the italian territory using macroseismic data # data eliminated reason 1 earthquakes of the etna volcanic zone (zs936). the volcanic zone is excluded because the energy propagation is different from the other zones. 2 earthquakes with epicentral intensity i0<7 the present study is focused on strong earthquakes. 3 earthquakes with number of idp (nidp)≤12. earthquakes with few idp could bias the regression analysis. 4 particular earthquakes, for example the 1117, 1456, 1753 and 1914 events. these earthquakes are not well known, the literature describes them as deep earthquakes. 5 offshore earthquakes. these events could bias the regression analysis because the distribution of idp is inhomogeneous. 6 earthquakes in border regions (as a consequence, the seismogenic zones zs903 and zs904 are rejected). these earthquakes are not well known. 7 earthquakes that does not match the earthquakes catalogue completeness criteria. to be coherent with the earthquakes used to assess the seismicity rates in psha 8 earthquakes with epicenters outside the seismogenic zones of zs9. this study is focused on events inside of seismogenic zones of zs9. 9 special cases (sc) (dbmi04, 2005) with code: te (territory), ss (small settlement), sb (solitary building). the statistical nature of intensity is not met. 10 data for which intensity has not been assessed. these data are not easy to use in statistical study in attenuation. 11 idp with is<3 (is=felt intensities) the present study is focused on strong intensities. 12 data with epicentral distance less than 1 km. earthquakes with few idp could bias the regression analysis. 13 idp rejected according to the following distance criterion. for every i0 is determined the local distance for is=4, called dist_is4, using the relationship of albarello e d’amico (2004); for every i0, idp with d≥dist_ is4 are rejected. i0 class dist_is4 11.0 453.5 10.5 387.4 10.0 326.1 9.5 270.2 9.0 220.0 8.5 176.0 8.0 138.0 7.5 106.0 7.0 80.0 table. 3. criteria for selecting the intensity data points (idp) to be used in a statistical analysis of the intensity attenuation of the italian territory. the total number of selected idp is 20,873, related to 212 earthquakes occurred from 1279 to 2002. 71 gómez., esrj vol. 10, no. 2. december 2006 figure. 1. zs9 seismogenic zonation proposed by gruppo di lavoro mps (http://zonesismiche.mi.ingv.it/ documenti/app2.pdf, 2004). figure. 2. epicentral distribution of the earthquakes selected (macroseismic intensity attenuation; 212 events). 72 seismic hazard map for the italian territory using macroseismic data et al., 1987; ciccotti et al., 2000). other filters are used to remove earthquakes characterised by epicentral intensity i0<7, site intensity is<3, number of macroseismic observation nidp<13 and epicentres outside of zs9. the idp of earthquakes that are not well known have also been disregarded. for instance, the 1456 and 1914 earthquakes are described (meletti et al., 1988; meloni et al., 1988) as deep events, which are not easy to use in a statistical study of the macroseismic intensity attenuation because they are distributed over a very large area. another criterion is based on the distance: for every i0, the local distance for is=4, called dist_is4, is determined using the relationship of albarello and d’amico (2004); for every i0, idp with d≥dist_is4 are rejected. after applying the 13 criteria (table 3), the intensity database is reduced to 20,873 idp related to 212 earthquakes that occurred from 1279 to 2002. figure 2 shows the epicentral distribution of the 212 selected earthquakes. the distribution of the selected idp for each epicentral intensity class and for each site intensity class are shown in tables 4a and 4b respectively. the largest number of idp belongs to the i0 7mcs class and the is 5 mcs class. figure 3 shows the frequency distribution of the intensities as a function of epicentral distance, which indicates that the majority of idp are located within 100km from the epicentre. the idp of the volcanic areas are divided into two datasets: • the first corresponds to the zs921 (etruria), zs922 (colli albani) and zs928 (ischia-vesuvio) seismogenic zones; • the second corresponds to the zs936 (etna) seismogenic zone. from these two datasets, a selection of macroseismic data was used considering the criteria described in table 5, resulting in a subset of 716 idp for ertruria, colli albani and ischiavesuvio and 1328 idp related to 54 earthquakes for the etna zone (zs936). 2.3. development of macroseismic intensity attenuation models according to bommer et al. (2003), the combination of seismic source characterisation, including rupture mechanism and ground-motion prediction equations that explicitly account for style-of-faulting, should produce refined estimates of the seismic hazard. the most recent seismic source zone model of italy, called zs9 (gruppo di lavoro mps, 2004), includes for each zone an average depth of the seismogenic layer, and an indication of the predominant faulting style (figure. 1). this information was used to assess the seismic hazard of italy with regional attenuation relationships, and with the figure. 3. frequency distribution of the site intensities (is) as a function of epicentral distance. 73 gómez., esrj vol. 10, no. 2. december 2006 i0 class (mcs) idp number 11 2385 10/11 899 10 2351 9/10 2259 9 1547 8/9 3606 8 2190 7/8 2021 7 3615 total 20.873 is class (mcs) idp number 11 52 10/11 95 10 223 9/10 198 9 560 8/9 672 8 1545 7/8 1416 7 2560 6/7 1730 6 2412 5/6 1011 5 2886 4/5 1348 4 2475 3/4 608 3 1082 total 20.873 table. 4a. distribution of idp for each i0 (epicentral intensity) class. table. 4b. distribution of idp for each is (site intensity) class. bommer et al. (2003) style-of-faulting scaling factors. in analogy with the pga attenuation relationships, and using the information provided by zs9, gómez (2006) derived a set of macroseismic intensity attenuation relationships, called cub05-reg., from the 20,873 idp described in the previous section. the set includes: 1. a valid relationship for the entire italian territory (cub05-general); 2. a relationship for areas with predominant normal style-of-faulting (pfm-normal); 3. a relationship for areas with predominant strike-slip and reverse-style-of-faulting (pfm-strike-slip+reverse); 4. a relationship for the etna volcanic zone (zs936). table 6 summarizes the values of the parameters of equation 1 obtained for each attenuation relation and their relative standard deviation. the macroseismic data have been fitted by the least squares method using the kaleidagraph software (synergy software, 2005) independent attenuation relationships for areas with reverse and strike-slip style faulting respectively were also derived, but they have been disregarded because the data sample is not statistically significant (gómez, 2006). similarly, attenuation relations for the zs921, zs922 and zs928 volcanic areas have been rejected because they are not significantly different from cub05general. figure 4a shows a plot of the intensity decay (∆i=io-i) as a function of epicentral distance (blue curve), along with the 20,873 idp (small grey points) used to obtain this relationship (cub05-general) with a standard deviation of ±0.94. figure 4b shows the ∆i-residuals (observed-computed) as a function of the epicentral distance grouped in 5 km classes for 74 seismic hazard map for the italian territory using macroseismic data # data eliminated reason 1 special cases (sc) (dbmi04, 2005) with code: te (territory), ss (small settlement), sb (solitary building). the statistical nature of intensity is not met. 2 data for which intensity has not been assessed. these data are not easy to use in statistical study in attenuation. 3 idp with is<3 (is=site intensities). the present study is focused on high intensities. 4 idp with d>40km for zs936 (etna). records outside seismogenic zones 5 idp with d>90km for zs921, zs922 e zs928. records outside seismogenic zones # characteristic of the regression equation nidp α β standard deviation σ 1 cub05-general 20.873 -1,3096 1,1833 0,94 2 predominant focal mechanism normal 13.393 -1,3518 1,2263 0,88 3 predominant focal mechanism reverse + strike-slip 5.020 -0,8904 1,0197 1,00 4 zs936 (etna) 1.328 -0.4860 1,4066 1,15 table. 5. data from dbmi04 (gruppo di lavoro dbmi, 2005) not used to fit the attenuation intensity model for the volcanic areas. the total number of selected idp are 716 for etruria (zs921), colli albani (zs922) and ischia-vesuvio (zs928), and 1328 idp for the etna zone (zs936). table. 6. parameters of the attenuation relationships and their standard deviations. the cub05-general relationship. the intensity attenuation model for predominant normal faulting (pfm-normal) obtained from 11,393 idp is shown in figure 5a, while the distribution of the ∆i-residuals as a function of epicentral distance is shown in figure 5b. the distribution of ∆i residuals for cub05general and pfm-normal models are very similar (figure 4b and figure 5b) and show moderate oscillations from 5 to 150 km, meaning that the computed intensities can be considered as a good estimate of the observed intensities. for epicentral distances greater than 150 km, the attenuation models do not provide a good estimate of the intensity observations. in both attenuation models, the standard error given by the 95 % confidence intervals increases for distances greater than 330 km. figure 6a shows the intensity attenuation relation for both reverse and strike-slip faulting (5,020 idp) while in figure 6b are plotted the ∆i-residuals. in the distance range of 5 to 80 km, the intensity decay calculated with the attenuation relationships overestimates the observed intensities; after 80 km the model tends to underestimate the intensity value decay. the attenuation relationship for the etna seismogenic zone is presented in figure 7a and the distribution of ∆i-residuals in figure 7b, which shows a slight increase of the standard error of 10 km of epicentral distance. figure 8 shows that the frequency distributions of the ∆i-residuals (observed-computed) obtained from the four relationships described so far (cub05-general, pfm-normal, pfmreverse and strike-slip, etna) are gaussian curves (normal distribution). this follows naturally from the fact that intensity equations are written as f(i)=i and not f(i)=ln i (musson, 2005). each gaussian curve in figure 8 has a standard deviation (tab. 6) that can be used to model the uncertainty of the ground shaking in hazard studies. 75 gómez., esrj vol. 10, no. 2. december 2006 figure. 4a. intensity attenuation model cub05-general (solid blue line) obtained from 20873 selected macroseismic data (grey points) (gruppo di lavoro dbmi, 2005). figure. 4b. distribution of the residuals relative to cub05-general as a function of the epicentral distance grouped in intervals of 5 km. the 95 % confidence intervals are shown as error bars. figure 9 compares the four attenuation models obtained in this study (tab. 6). using this, it could be concluded: 1. the etna relationship shows the highest intensity attenuation, consistently with its peculiar geologic setting; 2. within the first 30 km of distance, there is not much difference between cub05general and pfm-normal models. after 30km the pfm-normal model shows that the attenuation is slightly greater than the cub05-general model; 3. the pfm-reverse and strike-slip model is very similar to cub05-general within the first 20km, after this distance the attenuation is slightly lower than the cub05-general. at a given epicentral distance, pfm-normal predicts is lower compared with the cub05general, whereas pfm-strike-slip and reverse predicts higher values. this is similar to empirically observed data from strong ground motion. however, the classification adopted, based on style-of-faulting, carries implicitly a regionalization: the normal faulting style is found, in fact, along the apennines, while the reverse faulting style is found in ne italy and southern italy in the apulian area. recent studies by malagnini et al. (2000; 2002) show that these areas are characterized by different geometric and anaelastic attenuation that leads to a faster decay of the ground motion in central and southern apennines compared with north and eastern italy. the results in figure 9 can be attributed both to the effect of the regionalization and to the style-of-faulting, but it is impossible at this stage to discriminate between them. compared to the models proposed by albarello and d’amico (2004), gasperini (2001) and berardi et al. (1993), the cub05-general model decreases less rapidly within the first 90 km of 76 seismic hazard map for the italian territory using macroseismic data figure. 5a. intensity attenuation model (solid black line) obtained from 13393 selected macroseismic data for areas with predominant normal faulting style (sky-blue points) (gruppo di lavoro dbmi, 2005). figure. 5b. distribution of the residuals relative to pfm-normal as a function of the epicentral distance grouped in intervals of 5 km. the 95% confidence intervals are shown as error bars. epicentral distance (figure 10). for distances greater than 90 km, the attenuation of the cub05-general is greater than albarello and d’amico model, but decreases less rapidly than gasperini (2001) and berardi et al. (1993) model. for epicentral distances equal to zero, the value of ∆i in cub05-general tends asymptotically to zero. 3. macroseismic intensity hazard assessment 3.1. methodology most seismic hazard software (bender and perkins, 1987; ordaz et al., 2003) is designed to work with pga or spectral acceleration such as residuals following a normal logarithmic distribution (sabetta and pugliese, 1987; ambraseys et al., 1996; boore et al., 1997). when computing seismic hazard using macroseismic intensity, it is important to remember that the residuals of the intensity attenuation models follow a normal distribution (see section 2.3; albarello and d’amico, 2004; musson, 2005). in the present study, the program seisrisk iii (bender and perkins, 1987) has been modified in order to evaluate the seismic hazard by implementing a normal distribution scatter and a scale factor equal to 12.0 (gómez and sudati, 2005). the seismic hazard map, in terms of macroseismic intensity, has been evaluated using the same methodology used by gruppo di lavoro mps (2004) to compute the national seismic hazard of italy. some alternatives of epistemic character have been explored using a logic tree approach: a) earthquake catalogue completeness time-intervals determined using either the historical (co-04.2) or statistical (co-04.4) approach (gruppo di lavoro 77 gómez., esrj vol. 10, no. 2. december 2006 figure. 6a. intensity attenuation model (solid purple line) obtained from 5020 selected macroseismic data for areas with predominant style of strike-slip and reverse (orange points) faulting (gruppo di lavoro dbmi, 2005). figure. 6b. distribution of the residuals relative to pfm-strike-slip and reverse as a function of the epicentral distance grouped in 5km intervals. the 95% confidence intervals are shown as error bars. mps, 2004); b) criteria to assess i0max as the maximum epicentral intensity for each seismogenic zone of zs9; c) criteria to compute the seismic rates: activity rates (ar) and gutenbergrichter rates (gr); d) two groups of intensity attenuation relationships. the first group uses the relationships obtained in gómez (2006) (cub05-reg.) with standard deviation. the second group uses the relationship proposed by albarello and d’amico (2004) for all seismogenic zones of zs9, except the etna volcanic zone (zs936), which uses the relationship proposed by azzaro et al. (2006); the standard deviation of the attenuation models are respectively 1.25 and 0.82. figure 11 shows the logic tree and the weighting scheme: 1) the historical completeness co-04.2 and the statistical completeness co04.4 have assigned weights of 60% and 40% respectively; 2) the set of individual seismicity rates (activity rates, ar) (60%) are weighted more than the gutenberg-richter rates (gr-rates) and the set of iomax2 (40%); 3) the weight of the regional attenuation relationship (cub05-reg) is greater (67%) than albarello and d’amico (2004), and azzaro et al. (2006) relationships (33%). 3.2. map of imax (10%/50 years; 475-year return period) the values of imax, with a 10% probability of exceedance within a 50-year exposure time, were computed applying the logic tree methodology (figure 11). results are given as: 78 seismic hazard map for the italian territory using macroseismic data figure. 7a. intensity attenuation model (solid red line) obtained from 1328 selected macroseismic data for the etna volcanic area (zs936) (gruppo di lavoro dbmi, 2005). figure. 7b. distribution of the residuals relative to etna volcanic area (zs936) as a function of the epicentral distance grouped in intervals of 2 km. the 95% confidence intervals are shown as error bars. • distribution of median (50th percentile) of the 8 branches of the logic tree (figure 12); • distribution of 84th percentile (figure 13); • distribution of 16th percentile (figure 14). in figure 12, the maximum intensity value is ix, observed in central and southern italy. the minimum value in the italian peninsula is equal to 6. the values of the distribution of 84th and 16th percentile are not very different (figure 13 and figure 14), as they range between 6 and 9 in the first case, and between 6 and 8/9 in the remaining cases. the difference between the value of the 84th percentile and the median (e.g., the maximum uncertainty on the seismic hazard assessment) can be as high as 0.5. such moderate difference between the percentiles can be due to the small number of branches (only 8) used in the logic tree, or it may be related to the discrete nature of the intensity data. in this case, adding more branches to the logic tree may not introduce enough variability to determine an increase (or decrease) of the intensity level at a given site. 3.3. comparison between the maps in pga (mps 2004) and imax (10%/50years) empirical relationships between macroseismic intensity and pga are useful for comparing maps such as the one of figure 12 with seismic hazard assessment in pga. a few studies deal with the relationships between macroseismic intensity and pga and the majority has been published for the western usa (neuman, 1954; gutenberg and richter, 1956; trifunac and brady, 1975; murphy and o’brien, 1977; wald et al., 1999; boatwright et al., 2001). in italy, relationships between the macroseismic data and pga records have been proposed among others by margottini et al. (1992), panza et al. (1997; 1999; 2001) and by faccioli and cauzzi (2006). some relationships published in literature are shown in 79 gómez., esrj vol. 10, no. 2. december 2006 figure. 8. frequency distribution of the residuals (∆i observed computed) obtained from the intensity attenuation models: a) cub05-general; b) pfm-normal; c) pfm-strike-slip+reverse; d) zs936 (etna). figure 15 as the mean values of the correlation. the seismic hazard map in terms of intensity shown in figure 12, which represents the imax with probability of exceedance 10% within a 50year exposure time, has been converted in pga values using the relationships of margottini et al. (1992) (local intensity) and faccioli and cauzzi (2006). figure 17 shows the converted seismic hazard map representing the weighted mean of the two empirical relationships used according to the scheme proposed in figure 16. faccioli and cauzzi (2006) receives a higher weight (67%) compared with the 33% of margottini et al. (1992), because the dataset used in that study is more recent. the maximum value is equal to 0.34 g for a site located in southern italy. figure 18 shows the seismic hazard map in pga (10%/50 years) proposed by gruppo di lavoro mps (2004), where the maximum value is equal to 0.28 g. figure 19 shows the difference between the maps shown in figures 17 and 18: • the positive values (red shades) indicate areas where the values of the converted map are greater than those of the seismic hazard map mps04 (gruppo di lavoro mps, 2004); • on the contrary, the negative values (blue shades) indicate areas where mps04 values are greater than those 80 seismic hazard map for the italian territory using macroseismic data figure. 9. intensity attenuation models obtained in this study. figure. 10. cub05-general intensity attenuation model compared with those obtained by different studies in italy. 81 gómez., esrj vol. 10, no. 2. december 2006 figure. 11. logic tree and weighted values used for seismic hazard calculation in macroseismic intensity. figure. 12. seismic hazard map in terms of macroseismic intensity (imax with a probability of exceedance 10% within a 50-year exposure time) for the continental italian territory and sicily. the standard deviation in the intensity attenuation model has been accounted for. 82 seismic hazard map for the italian territory using macroseismic data figure. 13. map of the distribution of the 84th percentile (imax with a probability of exceedance 10% within a 50-year exposure time) related to the map in figure 12. figure. 14. map of distribution of the 16th percentile (imax with a probability of exceedance 10% within a 50-year exposure time) related to the map in figure 12. 83 gómez., esrj vol. 10, no. 2. december 2006 figure. 15. comparison of some macroseismic intensity/pga relationships published in literature. figure. 16. weighted values used to convert the imax distribution in pga. of the seismic hazard map converted in pga from imax. the smaller differences between the two maps are located in the interval -0.010g to 0.010g (white zones). the areas located outside the seismogenic zones of zs9 (figure 19) show greater (up to 0.075) pga values in the “converted map” than in mps04. it can be observed that for an epicentral distance greater than 100 km, the macroseismic intensity decays less rapidly than pga, thus providing higher ground motion values. in central and southern italy, the average pga values obtained from imax are 0.025g greater than the values of mps04. however, the intensity is a measure of ground shaking, independent of site conditions, while the seismic hazard map mps04 has been derived specifically for hard ground. negative values (up to 0.075g) in north-east italy, northern apennines and eastern sicily can be related to the use of regional empirical ground-motion attenuation relations in mps04. 4. conclusions macroseismic data has been used in this study to gain a better knowledge of the macroseismic intensity attenuation and seismic hazard of italy. new macroseismic intensity attenuation relationships of regional character for the italian territory are proposed using the most recent italian dbmi04 macroseismic database. these includes a valid relationship for the whole italian territory, a set of relations that account the predominant style-of-faulting (normal, reverse and strike-slip), and a relationship for the etna volcanic zone (these relationships are shown in table 6, and are called cub05-reg). eight years after slejko et al., (1998), a new seismic hazard map, with 10% probability of exceedance in 50 years in terms of macroseismic intensity, is proposed for continental italy and sicily (figure 12). the calculation of the seismic hazard has taken into account uncertainties of epistemic and random character. this is a regular practice in probabilistic seismic hazard assessment in terms of peak ground acceleration. the hazard map has been deduced by applying the cornell methodology, and updated input 84 seismic hazard map for the italian territory using macroseismic data figure. 17. seismic hazard map in terms of intensity converted in pga values using the relationships of margottini et al. (1992) and faccioli and cauzzi (2006) according to the scheme proposed in figure 16. the maximum value is equal to 0.34 g. figure. 18. seismic hazard map in pga (10%/50 years) proposed by gruppo di lavoro mps (http:// zonesismiche.mi.ingv.it, 2004 ). the maximum value is equal to 0.28 g. 85 gómez., esrj vol. 10, no. 2. december 2006 figure. 19. areas located outside the seismogenic zones of zs9 with higher pga values elements such as: • cpti04 earthquake catalogue (gruppo di lavoro cpti, 2004); • zs9 seismogenic zonation (gruppo di lavoro mps, 2004); • historical and statistical completeness time intervals (gruppo di lavoro mps, 2004); • two sets of macroseismic intensity attenuation models: a regional (cub05reg) and a national (albarello and d’amico, 2004; azzaro et al., 2006) were used for the zs936 seismogenic zone (etna zone). • the computer code adopted to calculate the seismic hazard was seisrisk iii (bender and perkins, 1987), which has been modified to be used with macroseismic intensity data (e.g., allowing a normal distribution instead of a normal logarithmic distribution of the residuals). the seismic hazard map proposed concurs with the results obtained by gruppo di lavoro mps (2004). in fact, the differences between the two maps lies within the range of the uncertainties associated to the hazard evaluation. acknowledgements i’m grateful with massimiliano stucchi, carlo meletti and paola albini for their thoughtful suggestion and continuous encouragement, and valentina montaldo for reviewing the manuscript and making helpful comments. references albarello, d. and d’amico, v. 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(1996). macroseismics focal depth and intensity attenuation in the carpathian region. acta geod. geoph. hung. 31 (1-2). 115-125. 90 seismic hazard map for the italian territory using macroseismic data untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 64-73 radiogenic components of the nigerian tar sand deposits akinmosin, a.1, osinowo, o.o.2 and oladunjoye, m. a.2 1department of earth sciences, olabisi onabanjo university, ago-iwoye. nigeria 2department of geology, university of lbadan, lbadan. nigeria corresponding author email: wale.osinowo@mail.ui.edu.ng abstract a combination of factors has prevented the exploitation of the nigerian tar sand deposits to date, among which is the environmental factor which may pose some dangers to both physical and biological components of the area of occurrence. gamma ray spectrometric analysis was carried out on samples of bituminous sand deposits in parts of southwestern nigeria. the aims were to determine the presence and level of radioactivity of selected radionuclides and to assess the possible impact on the environment, and provide geochemical baseline that could be useful in planning appropriate environmental management programs that will reduce potential negative effect of exploiting the resources on the environment. twenty air-dried samples collected for this study were weighed and sealed for 28 days to enable them attain a state of secular equilibrium. they were subsequently analyzed for gamma-emitting radionuclides using gamma-ray spectrometer fitted with a calibrated canberra vertical coaxial high purity germanium detector (hpge) system. the radio nuclides identified with reliable regularity belong to the decay series of naturally occurring radio nuclides headed by 238u, 232th and naturally occurring 40k. result showed that the radiogenic composition of the clay overburden (0.631msvy-1), shale-(0.193msvyr-1), and bituminous sand (0.446msvyr-1), are lower than the normal background value considered harmful to man. key words: spectrometric, gamma ray, radionuclides, nigerian. 64 manuscript receiver: january 07th, 2009. accepted for publication: june 15th, 2009. resumen una combinación de factores ha evitado la explotación de los depósitos de arena bituminosa de nigeria hasta la fecha, en ellos el factor ambiental plantea algunos peligros para los componentes físicos y biológicos de la zona. se realizó un análisis de espectrometría de rayos gamma a las muestras de los depósitos de arena bituminosa en partes del suroeste de nigeria. los objetivos fueron determinar la presencia y el nivel de radiactividad de determinados radio nucleídos para evaluar el posible impacto sobre el medio ambiente, y proporcionar un referente geoquímico que sería útil en la planeación de programas de manejo ambiental que redujeran los posibles efectos negativos de la explotación de los recursos al medio ambiente. para el desarrollo de este estudio se tomaron veinte muestras las cuales fueron pesadas y selladas durante 28 días a fin de que puedan alcanzar un estado de equilibrio secular. un análisis de emisión de rayos gamma por radio nucleídos fue realizado posteriormente con un espectrómetro de rayos gamma equipado con un detector coaxial vertical de germanio de alta pureza (hpge). la radio nucleídos, identificados con una regularidad confiable, pertenecen a la serie de radio nucleídos de desintegración natural, encabezada por 238u, 232th y 40k. los resultados mostraron que la composición radiogénica de la arcilla (0.631msvy-1), de las lutitas-(0.193msvyr-1), y de la arena bituminosa (0.446msvyr-1), son inferiores al valor mínimo, considerado dañino para el hombre. palabras clave: espectrometría, rayos gama, radio nucleídos, nigeria. 1. introduction tar sand is composed of a mixture of bitumen, which makes up about 10-20% and about 80-85% mineral matter including sands, clays and 4-6% water. tar sand has similar composition as the light crude (i.e. h, c and minor amount of s and o). they are believed to have formed from biodegradation and water-washing of light crude due to lack of cap rock. the nigerian tar sand is believed to have formed in a similar process. trace elements occur naturally in rock forming minerals and ore minerals; hence they can reach the environment from natural processes. weathering is a physical and chemical processes that breakdown rocks and then release these trace elements naturally into the environment. the dahomey basin (fig. 1) is a marginal pullapart basin (klemme, 1975) or margin sag basin (kingston et al., 1983), which was initiated during the early cretaceous separation of african and south american lithospheric plates. occurrence of seepage and tar sand deposits over the okitipupa ridge in the dahomey basin provided the initial impetus for oil exploration in nigeria. from the turn of 19th century up till date, no less than over twenty groups comprising public and private ventures have shown degrees of interest. the occurrence of these deposits has been known since early last century, however, intense investigations commenced from mid 70’s till now. the pioneering efforts were initiated by the geological consultancy unit of the university of ife (now obafemi awolowo university). the geology of these deposits, oil saturation and reserve estimates as well as textural characteristics of the associated sands has been described (adegoke et al., 1980, and 1981; enu, 1985). the physiochemical properties of the bitumen in relation to production and processing have been studied (adegoke et al., 1980; oshinowo et al., 1982; ekweozor, 1985; oluwole et al., 1985). the origin of the bitumen has been considered (coker, 1982; ekweozor, 1985). other radiogenic components of the nigerian tar sand deposits 65 relevant studies on the deposit include works done by ako et. al (1983); ekweozor (1986 and 1990); ekweozor and nwachukwu (1989); enu (1987, 1990); enu and adegoke (1984). these works have highlighted relevant aspects of the geochemical and sedimentological characteristic of the deposit. a combination of factors has prevented the exploitation of this resource to date; the most important is the environmental effects that may pose treat to both physical and biological components in the area of occurrence. it is therefore, of utmost importance to know the average background amounts of each metal in the natural uncontaminated geological materials, soils, and waters before assessing the contribution of anthropogenic sources to environmental contamination that may be associated with mining project when it eventually commences. stratigraphy of the dahomey basin the study area lies within latitude 0060381n0060401n and longitude 0040341e0040371e (fig. 2), and falls within the eastern dahomey basin. the reviewed work of omatsola and adegoke (1981) on the cretaceous stratigraphy of the dahomey basin has recognized three formations belonging to the abeokuta group. these are: the ise formation, consisting essentially of continental sands, grits and siltstones, overlying the basement complex uncomformably. neocomian to albian age has been assigned to this formation. overlying the ise formation is the afowo formation, which consists of coarse to medium-grained sandstones with variable interbeds of shales, siltstones and clay. the sediments of this formation were deposited in a transitional to marginal marine environment. turonian to maestritchtian age has been assigned to this formation. the araromi formation consists essentially of sand, overlain by dark-grey shales and interbedded limestone and marls with occasional lignite bands. the formation conformably overlies the afowo formation and maestrichtian to paleocene age has been assigned (omatsola and adegoke, 1981). overlying the abeokuta group conformably is the imo group, which comprises of shale, limestone 66 akinmosin, a., osinowo, o.o. and oladunjoye, m. a. a afowo union volta delta lagos ojo orimedu ise bede gbekebo gilli-gilli benin w. beninflank awaizombe ihuo recent miocene cretaceous miocene a’ oban hills okitipupa highbasement complex dahomey miogeocline eocene paleocene cretaceous aeromgnet t c basemen base ment co mplex 0 10000 20000 30000 feet continental crust transitional c-o crust oceanic crust part of triple junction and abakaliki arm transitional c-o crust continental crust afowo bede basement bas em en t volta delta prolongation of chain and charcot f.z.o. according to emery et al 1978 gilli-gilli oban hills ihuo a’cha rc ot f.z . warri delta nose chain f.z . 0 0 250 150 r . n ig e t port harcourt figure 1: east-west geological section showing the dahomey basin and upper part of the niger delta (after whiteman, 1982) and marls. the two-lithosratigraphic units under this group are: ewekoro formation that consists of thick fossiliferous limestone. adegoke (1977) described the formation as consisting of shaley limestone 12.5m thick which tends to be sandy and divided it into three microfacies. ogbe (1972) further modified this and proposed a fourth unit, which he said is paleocene in age and associated with shallow marine environment due to abundance of coralline algae, gastropods, pelecypods, echinoid fragments and other skeletal debris. akinbo formation lies on the ewekoro formation and it comprises of shale, glauconitic rock bank, and gritty sand to pure grey sand with little clay. lenses of limestone from ewekoro formation grades laterally into the akinbo shale very close to the base. the base is characterized by the presence of a glauconitic rock. the age of the formation is paleocene to eocene. overlying the imo group is the oshoshun formation; it is a sequence of mostly pale greenish-grey laminated phosphatic marls, light grey white-purple clay with interbeds of sandstones. it also consists of claystone underlain by argillaceous limestones with light grey shale at the bottom. there are inclusions of phosphatic and glauconitic materials in the lower part of the formation and the upper part is made up of medium to coarse-grained silty sandstone (adegoke, 1969). the formation is eocene in age (agagu, 1985). the sedimentation of the oshoshun formation was followed by a regression, which deposited the sandstone unit of ilaro formation (kogbe, 1976). the sequence represents mainly coarse sandy estuarine deltaic and continental beds, which show rapid lateral facies change. the coastal plain sands are the youngest sedimentary unit in the eastern dahomey basin, it probably overlay the ilaro formation unconformably, but convincing evidence as to this is lacking (jones and hockey, 1964). it consists of soft, poorly sorted clayey and pebbly sands, of oligocene to recent age. method of study gamma ray spectrometric analysis was carried out on twenty samples. these samples were transferred from the bags into clean air-tight containers and sealed for 28 days in order for the samples to attain secular equilibrium and also to prevent escape of ra-226 gas. moreover, an empty clean container was radiogenic components of the nigerian tar sand deposits 67 a legend river n footpath afowo fm. road settlement 0 1km 4 37e0 6 40n06 40n0 4 37e0 6 38n0 6 38n0 4 34e0 4 34e0 idiobilayo idiopopo tar sand outcrop figure 2: location map of the study area showing tar sands outcrop points. equally weighed and sealed for minimum number of days to serve as the background count. various detectors have been used to measure radiation energy deposited in a medium. the choice of a detector for low counting revolves around many factors, but energy resolution and counting efficiency are the most important. two types of detectors used in this work are: inorganic scintillation counter sodium iodide (nai) activated with thalium (ti), and the other is high purity germanium detector (hpge). after a minimum of 28 days for the sealed samples to attain secular equilibrium, each sample was placed on hpge detector enclosed in a lead shield and counted for 10 hours (36 000 s). an identical but empty container that has been sealed for same 28 days was also counted for 10 hours. a standard soil sample that contains certified radioactivity concentration due to 238u, 232th and 40k by weight was also counted for 10 hours. the high resolution of hpge detector made identification of a wide spectrum of ã-ray energy in the samples possible, and the photo peaks observed with regularity in the samples were identified. the area under each peak from the spectra produced was analyzed using a spectra analysis program, sampo-90, which matched ã-energies at various energy levels to a library of possible radioisotopes. the background peak height area was subtracted from the standard peak area in order to get net-peak area values for both the standard and the sample respectively. results and discussion the measured radioactivity concentration (bqkg-1) of gamma emitting radionuclides in the samples is presented in appendix 1. it was observed that the radionuclides identified and quantified from the gamma ray spectral (apendix2) are decay daughter products of naturally occurring radioactive element 238u and 232th. the radioactive daughter product of 238u, are 214bi, 214pb and 226ra, likewise the daughter products of 232th are 208ti and 228ac. 40k that is also naturally occurring but a non series radioactive isotope was also detected and selected for analysis at 1460.75kev. radionuclides were detected in all samples analyzed for, but in varying proportions. for instance, naturally but non-series 40k was detected at 1460.75kev in all the samples except in tar l2, tar l3, tar sand 13, tar sand 17 and tar sand 18; there activity values range between 7.04 ± 0.45bqkg-1 and 1003.91 ± 48.57bqkg-1 the 238u series are more prominent in the samples analyzed than the 232th series.214bi activity values range between 5.02 ± 0.33bqkg-1 and 193.01 ± 19.94bqkg-1; 226ra activity values range between 6.10 ± 0.84bgkg-1 and 148.07 ± 9.81bgkg-1 ; 228ac activity values range between 2.14 ± 0.60bqkg-1 and 53.66 ± 4.44bqkg-1 ; 208ti values range between 1.09 ± 0.38bqkg-1 and 86.81 ± 2.6bqkg-1. the absorbed dose rates in air calculated from the radioactivity concentration of the various samples are given in tables 1 and 2 for the overburden, shale, and tar sand respectively. the corresponding absorbed dose rates have been calculated using the relationship derived by beck et al (1972) which is given as: d = 0.042ac (k) + 0.429ac (u) +0.666 ac (th) where d is in gyhr-1 and represents the absorbed dose rate in air due to the specific activity concentrations of ac (k). ac (u) and ac (th) in (bqkg-1), respectively. the result may be compared with those units set by the international commission on radiological protection (icrp) as shown in table 3. it could be established that the mean dose equivalent calculated for clay overburden 0.631m svyr-1, shale 0.193m svyr-1 and bituminous sand 0.446m svyr-1 are still lower than the normal background value to all humans on earth considered harmful or hazardous to the environment. conclusion the presence of radioactive elements in an environment has been a source of concern, under certain conditions; they can generate a significant radiation that can be injurious to health. the result of the study using gamma spectrometric method of analysis indi68 akinmosin, a., osinowo, o.o. and oladunjoye, m. a. radiogenic components of the nigerian tar sand deposits 69 table 1: absorbed dose rate in tar sand sample (ngyhr-1). sample no. sample description ac (u) ac226 (bqkg-1) ac (th) ac228 (bqkg-1) absorbed dose rate (ngyhr-1) dose equivalents (msvyr-1) s1 tar sand 35.80 ± 6.92 14.22 ± 0.7 24.69 0.216 s6 tar sand 20.48 ± 1.30 11.13 ±0.56 16.11 0.141 s8 tar sand 88.4. ± 3.27 25.11 ± 0.9 54.64 0.473 s9 slightly impregnated bituminous sand nd 2.23 ±0.28 1.48 0.013 s4 tar sand 148.07±9.81 86.81 ± 2.6 121.33 1.06 s10 tar sand 30.8 ± 1.16 17.76 ±0.52 24.63 0.20 s11 tar sand nd 15.19 ±6.08 65.07 0.57 s12 tar sand 48.80 ± 3.53 20.93 ±7.53 90.01 0.80 s13 tar sand 6.10 ± 0.84 nd 25.86 0.23 s14 tar sand 59.61 ± 4.97 32.40 ±9.54 130.33 1.14 s15 tar sand 66.41 ± 5.00 53.66 ±4.44 92.37 0.81 s16 tar sand 39.19 ± 3.11 26.52 ±5.22 78.36 0.68 s17 tar sand 21.42 ± 1.58 16.85 ±1.56 36.30 0.32 s18 tar sand 8.43 ± 0.76 5.97 ± 1.79 11.89 0.10 s19 tar sand 10.73 ± 0.89 7.92 ± 1.61 14.78 0.13 s20 tar sand 16.95 ± 1.26 12.61 ±1.37 30.01 0.26 mean 37.20 ± 2.78 21.83 ±2.63 51.12 0.446 table 2: absorbed dose rate in clay overburden and shale (ngyhr-1) sample no. sample description ac (u) ra226 ac (th) ac228 absorbed dose rate (ngyhr-1) dose equivalents (msvyr-1) s7 clay overburden 107.20±9.61 39.21±1.26 72.09 0.631 s6 shale 31.49 ± 1.24 12.83±0.53 22.04 0.19 s5 shale 10.50 ± 0.60 2.14 ± 0.60 5.92 0.05 s1 shale 57.70 ± 1.91 21.30 ± 0.7 38.91 0.34 mean 51.72 ± 3.43 18.87±0.77 34.74 0.303 mean absorbed dose rate (ngyhr-1), calculated mean equivalent dose is in svyr-1. cated that the radionuclides identified and quantified belong to the naturally occurring decay series of 238u and 232th. the naturally occurring non-decay isotope 40k was also identified and quantified. although the radioactivity levels of the radionuclides were found to be low. the natural gas 226ra, that is generally associated with petroleum from decay of uranium and thorium is noted in the bituminous sands in relatively low concentration. the specific activity of these radionuclides in the bituminous sands is not expected to cause any health hazards now, but the mode of exploitation and extraction can still raise the level if not cautiously chosen and adopted in future. open cast mining method might concentrate and lead to radiation of some of these elements in the overburden during excavation and thus could have severe health implication. references adegoke, o.s. 1969. eocene stratigraphy of southern nigeria, bulleting bureau de research geologic et miners memoirs. vol. 69 pp 22-243. adegoke, o.s. 1977. stratigraphy and paleontology of the ewekoro formation (paleocene) of sw nig. bull. am. paleontol; vol.71, no 295, 375 pp. adegoke o.s., ako, b.d., enu, e.i. 1980. geotechnical investigations of the ondo state bituminous sands. vol. 1. geology and reserve estimate. rept. geological consulting unit, dept. of geology, university of ife. 257pp. agagu, o.k., 1985. a geological guide to bituminous sediments in southwestern nigeria. unpubl. rept. dept. of geology, university of ibadan. ako, b.d., alabi, a.o., adegoke, o.s., and enu, e.i., 1983. application of resistivity sounding in exploration for nigerian tar sand, energy exploration and exploitation vol. 2, no 2. pp.155-164. beck, h. l, decampo, j. and gogolak, c., 1972. in-situ ge(li) and nai(tl) gamma ray spectrometry. rept. hasl-258, u.s atomic energy comm. coker s.j.l. 1982. some aspects of the geology of the bituminous sands of parts of the benin basin. nig. min. and geosc. soc. 19, pp121. ekweozor, c.m. 1985. nigerian tar sands. bitumen chemical properties and their relation to origin, 70 akinmosin, a., osinowo, o.o. and oladunjoye, m. a. table 3: dose limits and their biological effects radiation dose rate duration of exposure likely effects/implication 10,000msv/yr short-term dose immediate illness and subsequent death within few weeks. 1,000msv/yr short-term dose nausea and decreased white blood cell, but not death. 50msv/yr over 5 year the lowest dose rates where there is evidence of cancer being caused. above this, the probability of cancer occurrence increases with dose. 2msv/yr normal background to all humans on earth 0.3-0.6msv/yr artificial sources of radiation, mostly medical equipment. source: uranium information centre, icrp (2002). production and processing. book of abstracts: 21st ann. conf. nig. min. & geosc. soc., jos nigeria, 11th-15th march 1985. ekweozor, c.m. 1986. characteristics of the non-asphaltene products of mild chemical degradation of asphaltene. org. geochem. 10. pp. 1053-1058. ekweozor, c.m., 1990. geochemistry of oil sands of southwestern nigeria in: b.d. ako, e.i. enu (editors), occurrence, utilization and economic of nigerian tar sands:a workshop held in ogun state university ago-iwoye, nigeria on 29-31 may, 1990.published by the nigerian mining and geosciences society, ibadan chapter, 50-62. ekwezor, c. m., and nwachukwu, j. i., 1989. the origin of tar sands of southwestern nigeria. nape bull. 4, 80-94. enu, e.i., 1985. textural characteristic of the nigerian tar sands. sedimentary geology. v. 44, pp 65-81. enu, e.i., 1987. the paleoenvironment of deposition of late maastrichtian to paleocene black shales in the eastern dahomey basin, nigeria geologie en mijinbouww. 66 pp, 15-20. enu, e.i., 1990. textural and occurrence of tar sands in nigeria in: b.d. ako, e.i. enu (editors), occurrence, utilization and economic of nigerian tar sands: a workshop held in ogun state university ago-iwoye, nigeria on 29-31 may, 1990.published by the nigerian mining and geosciences society, ibadan chapter, 11-16. enu, e.i., and adegoke, o.s., 1984. potential industrial mineral resources associated with the nigeria tar sands. 27th international geological congress mosco. book of abstract. vol. vii, pp 345. international commission on radiological protection (icrp), 2002 jones m.a and hockey r.o., 1964. the geology of part of south western nigeria.nig.geol.surv. bull.no 31, 1-101. klemme, h.d., 1975. geothermal gradients, heatflow and hydrocarbon recovery. in : a.g. fischer and s. judson (eds), petroleum and global tectonics. princeton, new jersey, princeton univ. press, pp. 251-304. kingston, d.r., dishroon, c.p. and williams, p.a. 1983. global basin classification system. american association of petroleum geologists. bulletin 31, 87p. kogbe, c.a., 1976. geology of nigeria. (second revised edition). published by rock view (nig.) 455pp.c.a kogbe (editor). ogbe f.g.a. (1972): stratigraphy of strata exposed in ewekoro quarry, southwestern nigeria. in: africa geology pp305-322. oluwole, f.f., adegoke, o.s., kehinde, l.o., borishade, a.b., ako, b.d., ajayi, t.r., enu, e.i., nwachukwu, j.i., coker, s.j.l., wallace, d., asubojo, o.i and ogunsola, o., 1985. chemical composition of bitumen extracts from nigerian tar sands. in: proceedings of 3rd intern. confer. on heavy crude and tar sands; long beach california; chap. 33, pp 373-379. omatsola m.e and adegoke o.s. (1981): tectonic and cretaceous stratigraphy of dahomey basin jour. of min. geol. vol.54, pp 65-87. oshinowo, t., ademodi, b. and adediran, a.s., 1982. bituminous tar sand of nigeria. analysis of oils-part 1. jour. nig. soc. chem. engrs. 1, 44-48. taylor, s.r., 1964. abundance of chemical elements in the continental crust; a new table. geochim cosmochim acta, 28. 1273-1285. whiteman, a.j., 1982. nigeria: its petroleum geology, resources and potential, vol. 2, graham and trotman, london. radiogenic components of the nigerian tar sand deposits 71 72 akinmosin, a., osinowo, o.o. and oladunjoye, m. a. appendix 1: activity concentration in bgkg-1 (hp ge detector) sample description k40 (1460.75kev) u238 th232 absorbed dose rate (ngyhr-1) equivalent dose rate (msvyr-1) bi214 (609.31 kev) pb214 (351.91 kev) ra226 (911.9 kev) ac228 (911.9 kev) ti208 (583 kev) shale l1 10.32 ± 0.78 51.69 ± 1.9 73.57 ± 7.7 57.7 ± 1.91 21.3 ± 0.7 21.3±0.76 104.71 0.917 tar l2 nd 48.32 ± 2.0 75.52 ± 8.1 35.8 ± 6.92 14.02 ± 0.7 14.00±0.7 87.14 0.76 tar l3 nd 20.48 ± 2.0 82.29 ± 1.3 20.48 ± 1.3 1.13 ± 0.56 11.12±0.59 67.7 0.59 shale s4 70.19 ± 4.04 180.65 ± 5.8 193.01 ± 19.9 148.07±9.81 86.81 ± 2.6 86.81 ± 2.6 242.41 3.009 shale s5 18.16 ± 1.14 10.50 ± 0.6 10.46 10.50 ± 0.95 2.14 ± 0.6 2.137± 1.6 17.09 0.14 shale s6 7.04 ± 0.45 31.49 ± 1.29 35.1± 3.79 31.49 ± 1.24 12.83±0.53 12.82±0.53 59.38 0.52 clay s7 59.903± 0.5 150.36 ± 4.9 165.88 ± 17.2 107.2 ± 9.69 39.21±1.26 19.11± 1.2 236.39 2.07 tar sand 6 92.69 ± 57.5 88.4 ± 3.2 132.46 ± 14.4 88.4 ± 3.27 25.11 ± 0.9 25.1 ± 0.9 170 1.49 tar sand s7 13.13 ± 0.78 nd nd nd 2.23 ± 0.28 2.23 ± 0.27 16.73 0.19 tar sand s8 376.69±18.55 30.79 ± 1.16 42.44 ± 4.47 30.8 ± 1.16 17.16±0.59 17.16±0.58 83.3 0.73 tar sand s9 658.72±29.77 28.66 ± 3.39 15.7 ± 2.4 nd 15.19±6.08 12.39±1.76 65.07 0.57 tar sand10 847.22 ± 38.4 25.79 ± 3.54 12.52 ± 2.72 42.8 ± 3.53 20.93±7.53 8.54 ±1.89 90.01 0.80 tarsand11 518.58±22.75 3.5 ±1.73 nd 6.1 ± 0.84 nd nd 25.86 0.23 tar sand12 1003.91±48.57 39.51 ± 4.68 37.74 ± 3.96 59.61 ± 4.97 32.4 ± 9.54 11.84±2.44 130.33 1.14 tar sand13 nd 16.75 ± 1.98 26.8 ± 0.81 66.41 ± 5.0 53.66±4.44 14.23 ± 1.0 92.37 0.81 tar sand14 458.96 ± 20.58 23.83 ± 2.73 20.36 ± 1.99 39.19 ± 3.11 26.52±5.22 8.48 ± 1.29 78.36 0.68 tar sand15 50.93 ± 3.28 9.68 ± 1.14 15.58 ± 0.4 21.42 ± 1.58 16.85±1.56 4.36 ± 0.36 36.30 0.32 tar sand17 nd 2.20 ± 0.74 5.02 ± 0.33 8.43 ± 0.76 5.97 ± 1.79 1.81 ± 0.55 11.89 0.10 tar sand18 nd 4.11 ± 0.67 5.63 ± 0.28 10.73 ± 0.89 7.92 ± 1.61 1.09 ± 0.38 14.78 0.13 tar sand19 92.45 ± 4.69 8.28 ± 0.97 8.28 ± 0.25 16.95 ± 1.26 12.61±1.37 4.68± 0.36 30.01 0.26 mean 213.944±12.589 34.749±2.21 47.918±2.807 40.104±2.91 21.20±2.393 13.960±1.007 82.992 0.784 appendix 2 radiogenic components of the nigerian tar sand deposits 73 1600 1400 1200 1000 800 600 400 200 0 -200 0 1000 2000 3000 4000 5000 channels spectrum 1 c o u n ts /c h a n n e l 1600 1400 1200 1000 800 600 400 200 0 -200 0 1000 2000 3000 4000 5000 channels spectrum 7 c o u n ts /c h a n n e l geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 111-124 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia john makario londoño b. universidad católica de manizales. facultad de ingenierías y arquitectura email: jmakario@ucm.edu.co abstract an analysis of the seismic activity for volcano-tectonic earthquake (vt) swarms zones at nevado del ruiz volcano (nrv) was carried out for the interval 19852002, which is the most seismic active period at nrv until now (2010). the swarm-like seismicity of nrv was frequently concentrated in very well defined clusters around the volcano. the seismic swarm zone located at the active crater was the most active during the entire time. the seismic swarm zone located to the west of the volcano suggested some relationship with the volcanic crises. it was active before and after the two eruptions occurred in november 1985 and september 1989. it is believed that this seismic activity may be used as a monitoring tool of volcanic activity. for each seismic swarm zone the vp/vs ratio was also calculated by grouping of earthquakes and stations. it was found that each seismic swarm zone had a distinct vp/vs ratio with respect to the others, except for the crater and west swarm zones, which had the same value. the average vp/vs ratios for the seismic swarm zones located at the active crater and to the west of the volcano are about 6-7% lower than that for the north swarm zone, and about 3% lower than that for the south swarm zone. we suggest that the reduction of the vp/vs ratio is due to degassing phenomena inside the central and western earthquake swarm zones, or due to the presence of microcracks inside the volcano. this supposition is in agreement with other studies of geophysics, geochemistry and drilling surveys carried out at nrv. key words: earthquake swarm, volcano-tectonic earthquakes, volcanic activity, vp/vs ratio, volcanic eruption. resumen se llevó a cabo un análisis de a actividad para zonas de enjambres sísmicos tipo volcano-tectónico (vt) en el volcán nevado del ruiz (vnr), para el período 1985-2002, que fue el período sísmicamente más activo hasta el 2010. los enjambres sísmicos se localizaron en fuentes muy bien definidas alrededor del cráter. la fuente sísmica localizada en el cráter, fue la más activa durante este tiempo. la zona ubicada al w del cráter, parece tener una relación con crisis volcánicas posteriores, permaneciendo activa antes de las erupciones de noviembre de 1985 y septiembre de 1989. se cree que la ocurrencia de sismicidad en esta zona sísmica, puede ser usada como premonitorio de actividad volcánica en el vnr. para cada zona sísmica se calculó el vp/vs agrupando sismos y estaciones para cada una de ellas. se encontró que cada zona sísmica tiene un valor vp/vs diferente de las otras. las zonas cráter y oeste (w) presentaron el mismo valor de vp/vs. el valor de vp/vs para la zona cráter y w, fue un 6-7% menor el de la zona norte, y un 3% menor que el de la zona sur. se interpreta que la disminución del vp/vs es debido a desgasificación producida en las zonas sísmicas del cráter y al occidente del volcán, o a la presencia de 111 manuscript received: 10/03/2010 accepted for publication: 25/06/2010 microgrietas en el interior del volcán. esta suposición esta soportada en otros estudios geofísicos, geoquímicos y de datos de pozos geotérmicos, llevados a cabo en el vnr. palabras clave: enjambre de terremotos, terremotos volcán tectónico, actividad volcánica, razón vp/vs, erupción volcánica 1. introduction in many volcanoes around the world, volcano-tectonic (vt) earthquakes are directly or indirectly associated with volcanic activity. in some cases they are associated with magmatic intrusions and rock fracturing (karpin and thurber, 1987). this earthquakes are also known as high-frequency events, and they are thought to be produced by shear failure or slip on faults in the volcano edifice. the only difference with tectonic earthquakes, is their pattern of occurrence. in volcanoes, they occur in swarms, that is a group of many earthquakes of about same size and location with no dominant shock (mcnutt, 2000). the nevado del ruiz volcano (nrv) (fig. 1) began the last eruptive period in december 1984; after this event the most important eruptive processes have been those of november 13, 1985 and september 1, 1989 (bohorquez, 1993). at nrv the vt seismic activity sometimes increased several days before the volcanic crises (eruptions, ash emissions, etc.). vt earthquakes occurred both in a swarm-like pattern and as isolated earthquakes, but mainly in swarm-like pattern (muñoz et al., 1990). several studies of the vt activity at nrv have been carried out with the aim of finding relationships with volcanic activity. all these studies have been done for vt seismicity in nrv but only over short periods of time (e.g., nieto and muñoz 1988; nieto et al., 1990; bohorquez 1993; londono and kobayashi, 1994; londoño 1996, londono and sudo, 2001). nieto et al. (1990) found clusters of vt earthquakes based on the hypocenters obtained for the period july 1985 december 1986. they associated such clusters with a possible ascent of magma. muñoz et al. (1990) identified some geologic features in the area around nrv analyzing hypocenters and composite focal mechanism of vt swarms 112 john makario londoño b. l at it u d e (n ). d eg 5.00° 4.95° 4.90 4.85 ° ° 4.80° 75.41° 75.38° 75.35° 75.31° 75.28° 75.25° longitude (w). deg nevado de santa isabel pico del cisne nevado del ruiz volcano 52 00 olleta crater arenas crater pirana crater. n figure 1. location map of the nevado del ruiz volcano (nrv). the topographic curves of 5200 and 5000 m are shown as reference. from 1986-1987. they conclude that the magma chamber of nrv has a very complex geometry. zollweg (1990) studied vt seismicity from december 1985 to may 1986, examining spatial patterns and information on the orientation of local tectonic stress. he also concludes that these kind of earthquakes are related with a magma-transport process. muñoz (1992) made a study of lateral variations of p wave velocity by using a tomographic technique. the preliminary results obtained with this 3d model suggested that there are several regions of low velocity beneath the active crater and northwest of it. bohorquez (1993) analyzed arrival times of p and s waves, polarity of the p wave, spectra of frequencies and particle motion of vt earthquake swarms occurred at august october 1990. she found spatial concentrations of vt around nrv similar to those found by muñoz et al. (1990). londono and kobayashi studied the vt swarms occurred during 1990 at nrv, by using a portable seismic network of 3-components. they found different seismic sources with different spectral characteristics. londono and sudo (2001) developed a method to determine spectral characteristics of seismic zones at nrv, by using main part of spectra of p and s waves. the calculation of the vp/vs ratio in volcanic regions is another interesting subject to study. although today the 3d tomographic technique for calculation of vp/vs is a powerful tool to find its wide spatial distributions, the modified wadati diagram (chatterjee et al., 1985) is also a useful and simple technique to find differences in vp/vs between small earthquake swarm regions. in that technique, the study area is divided in several sectors, and for each earthquake swarm region a group of earthquakes and stations are choose carefully to calculate the vp/vs ratio, to avoid mixing propagation effects between regions. it is known that changes in vp/vs ratio may be related to changes in the physical and chemical properties of the medium, such as partial saturation of rocks, changes in temperature, density, and steam content (birch, 1961; ito et al., 1979; toksoz et al., 1976). in this study we made an evolutionary study of the whole seismicity of swarm-like vt earthquakes, covering the most active periods of activity of nrv from 1985 to 1999, in order to find some relationships with volcanic activity through time. on the other hand, we calculated the vp/vs ratio for each vt swarm zone in order to determine if there are differences between the swarm zones. this paper does not pretend to map the distribution of vp/vs ratios at nrv area as a whole. since only few specific and small zones around the volcano were investigated, we consider that for the purpose of this study the modified wadati diagram is suitable for vp/vs calculation. 2. data and processing we used vt earthquakes recorded by the telemetered local network at nrv area (fig. 2); the stations feature vertical component seismometers with a natural frequency of 1 hz, model l-4c. the hypocenters of vt earthquakes for each swarm zone were selected from the database of the volcanological observatory of nrv for the period 1985 2002. since that year the nrv has not shown swarm like seismic activity until now. we relocated the earthquakes by using the earthquake location program hypo71 (lee and lahr 1985). we used a p velocity model proposed by zollweg (1990) for nrv (table 1). the arrival times of p phases were accurate to 0.05s and those of s phases to 0.1s. we selected those earthquakes which were recorded at five or more stations, with clear onset of p and s phases. the parameters for the selection of the earthquake locations in this analysis were: residual times less than 0.10s, 113 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia table 1. velocity model for p waves used for earthquake location (zollweg, 1990) velocity (km/s) depth at bottom of the layer (km) 3.6 0.00 4.6 0.95 5.5 1.95 6.1 4.00 6.6 14.00 7.8 30.00 vertical errors less than 0.5km, and horizontal errors less than 0.3km. a total of 600 high-quality earthquakes were used for the analysis. figure 2 shows the hypocenters of these earthquake swarms as well as the seismic stations used in this study. we calculated the seismic energy for the vt selected and correlated the resulting values with volcanic activity. the seismic energy (e) was calculated from the following expressions: log e = 9.9 + 1.9 ml 0.024 ml 2 (richter, 1958) ml = 2.82 log(duration) – 2.59 (nieto et al., 1990), where ml is the local magnitude for vt earthquakes at nrv. a modified wadati diagram was used for the calculation of vp/vs ratios using selected groups of stations for each earthquake swarm zone in order to see differences among them (chatterjee et al., 1985). instead of plotting the s-p as ordinate versus travel time of p (tp) as the abscissa 114 john makario londoño b. 5.00° 4.95° 4.90 4.85 ° ° 4.80° l at it u d e (n ). d eg 75.41° 75.38° 75.35° 75.31° 75.28° 75.25° longitude (w). deg herr told inde rubi piraolle ref3 bis1 fara alf2 cisn reci pico del cisne isa2 piraña crater b arenas crater c a) b) c d d v c) 75.41° 75.38° 75.35° 75.31° 75.28° 75.25° d ep th (k m ) 0 2 4 6 8 10 12 5.00° 4.95° 4.90 4.85 ° ° 4.80° l at it u d e (n ). d eg 0 2 4 6 8 10 12 depth (km) 0 5 km 52 00 figure 2. epicentral and hypocentral location of the earthquake swarms and the stations used in this study. each vt seismic zone is identified with a color and symbol (green circles=crater zone; red squares =north zones; blue rhomboids=west zone; gray circles=south zone). filled triangles represent seismic stations. the topographic curves of 5200 and 5000 masl are plotted as reference. active crater (arenas) and extinct craters (olleta and piraña) of nrv are indicated as well. used in the normal wadati diagram (1933), we plotted the travel time of s (ts) as the ordinate. assuming the poisson’s ratio to be constant along the ray path, the tp versus ts relation is a straight line through the origin with slope equal to the vp/vs ratio. a linear least square fit was made using tp versus ts data. the vp/vs ratio for each earthquake swarm was calculated with a confidence interval of 90% (press et al., 1992). figure 3 shows some examples of the typical seismograms used in this study. 3. analysis sources of error several sources of error are probably involved in the analysis of the evolution of vt seismicity in this study. uncertainty about the earthquake location as well as the origin time are perhaps two of the most important sources of error. the readings of the arrival times of p phase and s phase are also possible sources of error for both vp/vs ratio calculation and analysis of the seismicity. as we are using only the arrival time of the s phase on the vertical component, it is possible to have some error, such as p-to-s conversion, leading to a mispicking of s wave but this possibility is low. although we selected only very clear s phases, we assume an error of 0.5 sec in the arrival time of s phase and it is included in the error of earthquake location and vp/vs calculation. the velocity structure considered for hypocenter location is also a possible source of error, but some tests made previously using controlled blasts showed that the crustal model used for the hypocenter determination at nrv is well constrained. moreover, as zollweg (1990) pointed out, assuming a 1 km/s error in the p velocity model for nrv and an average travel time of 1 sec, the additional error in the location is less than 1km. this systematic error is smaller than a typical 95% confidence ellipsoid major axis lengths that is 1-3km. evolution of the vt seismicity and its relationship with volcanic activity the most recent volcanic activity of nrv started in november 1984. one year later, in november of 1985, a phreato-magmatic eruption occurred, melting about 5% of the ice cap of the volcano. this produced a mudflow, which destroyed armero city located in the eastern part of the volcano, killing about 25000 people. the volume of the eruption was calculated to be about 3.8x107 m3 (naranjo et al., 1986). from 1985 to 1989, several ash emissions occurred. on september 1, 1989 a new phreato-magmatic eruption took place. it was smaller in size than the eruption of november 1985, with a volume of about 1.6x106 m3 (mendez and patiño 1993). more than 1100 earthquakes occurred in only one day. 115 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia figure 3. example of typical seismograms of volcano tectonic earthquake at nrv. the seismograms correspond to an earthquake of crater swarm zone. the line shows the vp/vs ratio for the crater swarm zone. ash continued falling for several days and a small lahar formed. londoño et al., (1998) found a temporal change of coda q before and after these two volcanic eruptions. after the last eruption, the volcanic activity started to decrease gradually. since then, few ash emissions have been observed. table 2 shows a summary of the most relevant volcanic crises of nrv from 1985 to 1999. it seems that a change in the volcanic activity started in 1992. the substantial and continuous decreasing in the vt seismicity is remarkable after that year, compared with previous years. the total energy release for long period (lp) events as well as for all vt earthquakes including those not occurring during swarms is depicted in figure 4. it is clear that as a general tendency, the seismic activity of nrv has decreased gradually since 1992 (except on some particular dates associated with minor volcanic crises), without ash emission or volcanic eruptions. there are two seismic swarm zones that displayed some recurrence in time, but both were low in seismic energy release, and were associated with few volcanic crises. a swarm zone nw far away of the crater exhibited activity a few times, not related with volcanic crises, and was the lowest in seismic energy release. therefore, this swarm zone is not included in this study. there is a seismic swarm zone that has been associated with some of the most important crises at nrv, that is, the eruptions of november 1985 and sep116 john makario londoño b. figure 4. daily total energy (in ergs) releasing (vt and long-period earthquakes) and daily number of vt earthquakes at nrv from 1985 to 2009. asterisks represent volcanic crises. tember 1989. this western swarm-like activity occurred before and after both volcanic eruptions. although this seismic activity recurred only on rare instances, the level of energy was low compared with those recorded before and after both the eruptions mentioned above (figure 5). therefore, it is important to note that not all these vt swarms are related with subsequent volcanic activity. sometimes this was low in energy and in number of events but it was related with a posterior phreatic eruption (fig. 4). in february 1998, another swarm in the western part was recorded. although this swam zone was more energetic than previous mentioned, it did not correlate with any surface volcanic activity (fig. 5). the earthquakes located to the north and to the west were the deepest (4-8 km), while those located beneath the crater and to the south were the shallowest (0-3 km) (figure 2). as is noticed in figure 4, most of the volcanic crises occurred between 1985 and 1992. only two volcanic crises have occurred since 1992. seismic swarm-like activity also occurred concurrently. vt seismicity of this type located in the crater area has been recorded quite continuously after 1992, but it is very low in energy compared to previous years. vp/vs ratios for each vt swarm zone at nrv with the aim to obtain well constrained vp/vs ratios for each vt swarm zone and to avoid mixing propagation effects, we calculated the vp/vs ratio for each swarm zone using different groups of stations for each swarm (figures 2, 6). in order to detect temporal changes of vp/vs for each swarm zone, the vp/vs ratio was calculated for each swarm zone. the figure 7 shows the results. from this figure it is possible to observe that there are some temporal changes in vp/vs with time at nrv for some regions, mainly for crater and west zones. the vp/vs ratios showed low values at the crater and west swarm zones, with vp/vs ratios of 1.67±0.01 and 1.67±0.03 respectively. the vp/vs ratio for the earthquake 117 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia table 2. main volcanic crises at nrv from 1985 to 1999. this list includes phreatic eruptions, phreato-magmatic eruptions as well as strong seismic activity. (lp=longperiod earthquakes; vt=volcano-tectonic earthquakes). no activity date 1 small phreatic eruption september 1985 2 phreato-magmatic eruption 13 november 1985 3 small phreatic eruption january 1986 4 small phreatic eruption may 1986 5,6 small phreatic eruption july 1986 7 small phreatic eruption june 1987 8 small phreatic eruption march 1988 9 intense seismic activity (lp) february 1989 10 intense seismic activity (vt) may 1989 11 intense seismic activity (vt + tremor) june 1989 12 intense seismic activity (vt + lp+tremor ) august 1989 13 phreato-magmatic eruption 1 september 1989 14 small phreatic eruption april 1991 15 small phreatic eruption may 1991 16 intense seismic activity (lp + tremor) april 1994 17 intense seismic activity (vt + lp + hybrids ) july-september 1995 118 john makario londoño b. 0.0e+ 0 2.0 e+ 7 4.0 e+ 7 6.0 e+ 7 8.0 e+ 7 1.0 e+ 8 1.2 e+ 8 1.4 e+ 8 08 1985 08 1989 08 1993 08 1997 08 2001 crater swarm zone 0 .0 e+ 0 1 .0 e+ 8 2.0e+8 3 .0 e+ 8 4 .0 e+ 8 5 .0 e+ 8 6 .0 e+ 8 08 1985 08 1989 08 1993 08 1997 08 2001 west swarm zone 0.0e+0 5.0e+7 1.0e+8 1 .5e+ 8 2 .0e+ 8 2.5e+8 08-1985 08-1989 08-1993 08-1997 08-2001 south swarm zone 0.0e+0 5 .0e+7 1.0 e+8 1.5 e+8 2.0e+8 2 .5 e+8 08-1985 08-1989 08-1993 08-1997 north swarm zone date d ai ly se is m ic en er g y r e l e a s e (e rg s ) 0. 5 figure 5. energy release for each vt swarm (1985-2002). 119 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia 0 2 4 6 8 0 1 2 3 4 crater vp/vs = 1.67 (0.01)stations=ref3, bis1, fara reci,pira 0 2 4 6 8 0 1 2 3 4 west vp/vs = 1.67 (0.03)stations=ref3, bis1, fara told, 0 2 4 6 8 0 1 2 3 4 south vp/vs = 1.73 (0.02)stations=reci,alf2,cisn,isa2 0 2 4 6 8 0 1 2 3 4 north stations=herr, inde, told, rubi, ref3 , olle vp/vs = 1.78 (002) t (s)s figure 6. modified wadati diagrams for each seismic swarm zone. the group of stations used for vp/vs calculation for each swarm zone and the vp/vs ratio and its error are listed. 120 john makario londoño b. v p v s ep -8 5 n ov -8 5 m ar -8 8 s ep -8 9 a pr -9 4 a go -9 5 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 02-jun-85 27-feb-88 23-nov-90 19-ago-93 15-may-96 09-feb-99 crater area 02-jun-85 27-feb-88 23-nov-90 19-ago-93 15-may-96 09-feb-99 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 sep-85 n ov -8 5 m ar -8 8 s ep -8 9 a pr -9 4 a go -9 5 west area 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 02-jun-85 27-feb-88 23-nov-90 19-ago-93 15-may-96 09-feb-99 s ep -8 5 n ov -8 5 m ar -8 8 s ep -8 9 a pr -9 4 a go -9 5 north area 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 02-jun-85 27-feb-88 23-nov-90 19-ago-93 15-may-96 09-feb-99 south area s ep -8 5 n ov -8 5 m ar -8 8 s ep -8 9 a pr -9 4 a go -9 5 date s figure 7. temporal variation of vp/vs for each seismic swarm zone at nrv. asterisks represent volcanic crises. vertical lines represent standard error. swarms located to the south was 1.73±0.02, and for those swarms located to the north was 1.78±0.02. the vp/vs ratios were reduced about 6% for the west and crater swarm zones with respect to the north swarm, and 3% with respect to the south swarm zone. the error bars show that the estimated vp/vs ratios are significantly different among swarm regions, except for the west and crater swarms, whose values of vp/vs are indistinguishable (figure 8). some of the stations used to calculate the vp/vs ratio for the north and the west earthquake swarm zones were the same (figure 6). these two seismic zones were the deepest, as mentioned before. 4. discussion and concluding remarks vt seismic activity is closely related to the volcanic activity at nrv from 1985 to 2002. the swarm-like vt seismic activity was one of the most remarkable phenomena associated with the volcanic crises, occurring before, during or after the crises, although other phenomena such as lp earthquakes and tremor were also associated. the continuous decreasing of seismic activity related to the decrease in volcanic activity for more than 17 years, can suggest that a period of quiescence or calm has occurred since 1992, with some sporadic increment of activity that has not affected the stability of the volcanic system. vt seismic activities located beneath the crater zone and to the west of the volcano were related with the two phreato-magmatic eruptions that had occurred to date at nrv (londoño and sudo, 2001). it is possible that such seismic pattern can be used as a premonitory tool for the major volcanic crises. another volcanic eruption has not occurred yet to confirm this supposition, but we argued, that the level of energy and the conditions of the volcano must be taken into account. it seems that after a certain level of seismic energy release is reached in the western zone, depending on the conditions of the volcano, monitoring of vt becomes an important tool for the surveillance of nrv activity, as is seen on figure 5, where only those dates with high seismic energy release at western zone, are related with phreato-magmatic activity. we suggest that the reduction of 6% of the vp/vs ratio between the north and the west swarm zones is due to the differences in travel path between them. moreover, although it is known that there is a strong relationship between the vp/vs ratios and the depth of the earthquakes, it is interesting to notice that the lowest vp/vs ratios are those for the crater and west zones, which are the shallowest and the deepest seismic zones, respectively. on the other hand, surface geology of nrv around the active crater is fairly homogeneous, consisting mainly of lava flows. based in this, we think that the surface geology did not affect too much the 121 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia 1.90 1.85 1.80 1.75 1.70 1.65 1.60 swarm zone figure 8. comparison of vp/vs for each vt swarm zone at nrv. vertical lines represent standard error. vp/vs ratios. from these results, we suggest that the low vp/vs ratios found in the crater and west zones are the manifestations of different material beneath the volcano. probably these zones have different physical-chemical characteristics from those of the surrounding zones. previous studies carried out in different materials show that factors such as porosity, density, temperature and composition can affect the vp/vs ratio (birch 1961, ito et al., 1979, toksoz et al., 1976). it is possible that such factors affected the vp/vs ratios inside nrv. material such as gas or steam changes the properties of the medium, reducing the vp/vs ratio as was pointed out by chatterjee et al. (1985) who suggested that the low vp/vs ratios in the yellowstone region are due to the presence of steam or gas. we suggest that in the crater and west zones of nrv probably there are more gas available due to continuous degassing of rocks, that affect the vp/vs ratio as well as the volcanic activity. moreover, the temporal changes of vp/vs ratio could be related to temporal changes in gas content inside the volcano. south and north seismic zones did not show any remarkable temporal variation of vp/vs ratio, while west and crater seismic zones did it. on the other hand, the last two seismic zones showed more activity through time (figure 4). the vp/vs ratio found in this study for almost all the vt swarm zones at nrv are rather low. the release of gas or steam from 1985 to 2002, was continuous at nrv, making it possible to observe many fumaroles inside the active crater and huge vapor columns almost daily. on the other hand, a geothermal exploration well, almost 1500m deep was drilled in the nrv area in 1998. the results suggested that: boiling processes are occurring in some places inside the volcano, there is a big hydrothermal system, and the temperatures are higher than 200 °c (monsalve et al., 1998). giggenbach et al. (1990) suggested that there is a huge hydrothermal system in nrv based on petrological and geochemical data. these results support the hypothesis that there is a huge amount of gas or steam inside nrv that affects the vp/vs ratios. this supposition is also in agreement with the idea that there are degassing rocks benath the crater and to the west sectors of the volcano (giggenbach et al., 1990). studies elsewhere suggest that presence of hydrothermal systems at the water-steam transition can explain the reduction of the mean value of the vp/vs ratio (chatterjee et al. 1985; zimmer et al, 2002; maheswar and kalachand, 2007). we think that this is the most likely process occurring at nrv as well. another possible explanation of the low values of vp/vs found in the crater and west swarm zones of nrv compared with the higher values of the north and south zones, is the presence of melting material inside the volcano, or different mechanical properties of the rocks, due probably to the presence of microcracks inside the volcano. a detailed local tomography done at nrv area by londoño and sudo (2002a) seems to support this supposition. they found two low velocity zones to the west and beneath the active crater at nrv. they associated these zones with the presence of degassing magma bodies. no low velocity zones were found at norrh of the volcano. on the other hand, londono and sudo (2002b), modeled the seismic waves attenuation at nrv due to gas content changes through time. they concluded that the gas content is the responsible for phreato-magmatic activity at nrv. therefore, the supposition that the gas content in space and time, is probably one the main causes of spatial and temporal variation of vp/vs ratio at nrv, seems to be reasonable. the relationship between both swarm-like vt seismic zones, the crater and the west zones at nrv, not only with respect to the volcanic activity, but also with respect to the similarity of vp/vs ratios, suggests that these two zones are important for the surveillance and monitoring of the volcano. we think that attention should be raised when vt seismic activity occurs in these places, and when the level of seismic energy release reaches certain levels. of course, it is necessary to correlate such vt seismic activity with other seismic phenomena such as the occurrence of lp earthquakes and tremors, and the conditions of the volcano in order to be more confident about a prediction of volcanic crisis at nrv. acknowledgments the author would like to thank the colleagues at ingeominas, the volcanological and seismological observatory of nevado del ruiz volcano, manizales, colombia, especially álvaro p. acevedo for his help and data processing. this paper was benefited by the discussions with m. yoshikawa, t. mori, and t. hashimoto of kyoto university. special thanks to william hobden for reviewing the english and their comments and suggestions. john jairo sánchez and an anonymous reviewer gave important suggestions and comments that improved the final manuscript. references bohórquez, o.p. (1993). análisis de registros sísmicos digitales en el volcán nevado del ruiz. agosto octubre de 1990. proyecto : “pronóstico de erupciones en los volcanes del p.n.n.”. (in spanish). 122 john makario londoño b. birch, f. (1961). the velocity of compressional waves in rock to 10 kilobars, part 2. elastic properties and equations of state. american geophysical union. 2199-2224. chatterjee, s.n., pitt, a.m., iyer, h.m. (1985). vp/vs ratios in the yellowstone national park region. i. j. volcanol. geotherm. res., 26, 213-230. giggenbach, w.f., garcia, p.n., londoño, c.a., rodriguez, v.l., rojas, g.n., calvache, m.l. (1990). the chemistry of fumarolic vapor and thermal spring discharges from the nevado del ruiz volcanic magmatic hydrothermal system, colombia. j. volcanol. geotherm. res., 42, 13-39. ito, h., de vilbiss, j.,nur, a. (1979). compressional and shear waves in saturated rock during water-steam transition. j. geophys. res. 84, 4731-4735. karpin, t.l., thurber, c.h. 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(1985). hypo71pc: a computer program for determining hypocenter, magnitude and first motion patter of local earthquakes. u.s. geol. surv. open-file rep, 75-311. londoño jm, kobayashi y (1994). spectral analysis of volcanic earthquake swarms in nevado del ruiz volcano (colombia), during august-october 1990. bull. volcanol soc. japan 39, 31-47 londoño, j.m., (1996). temporal change in coda q at nevado del ruiz volcano, colombia. j. volcanol. geotherm. res., 73, 129-139 londoño jm, sanchez jj, toro el, cruz gf, bohorquez op (1998). coda q before and after the eruptions of 13 november 1985, and 1 september 1989, at nevado del ruiz volcano, colombia. bull volcanol 59, 556-561 londoño jm, sudo y (2001). spectral characteristics of volcano-tectonic earthquake swarms in nevado del ruiz volcano, colombia. j volcanol geotherm res 112, 39-54 londoño jm, and sudo y, (2002a). velocity structure and a seismic model for nevado del ruiz volcano (colombia). journal of volcanology and geothermal research. vol. 119, no 1-4. p. 61-87. londoño jm, sudo y, (2002b) a warning model based on temporal changes of coda q for volcanic activity at nevado del ruiz volcano, colombia bulletin of volcanology, vol 64. 303-315. maheswar o. kalachand s (2007). maheswar o. kalachand s (2007). maheswar o. kalachand s (2007). marine and petroleum geology volume 25, no 7, 2008, 637-644. mcnutt s. (2000). volcano seismicity. in: encyclopedia of volcanoes. academic press. 1015-1033. méndez, r,a,, patiño, c.j. (1993). estudio estratigrafico de la zona proximal al volcán nevado del ruiz. internal report. ingeominas. (in spanish). monsalve, m.l., rodríguez, r.a., méndez, r.a., bernal, n.f. (1998). geology of the well nereidas i,nevado de ruiz volcano, colombia. geothermal resources council transactions, 22, 20-23. muñoz, c.f., nieto, e.a., meyer, h.j. (1990). analysis of swarms of high-frequency seismic events at nevado del ruiz volcano, colombia (january 1986 august 1987) : development of a procedure. j. volcanol. geotherm. res., 41, 327 354. muñoz, f. (1992). local earthquake tomography for lateral velocity variations and hypocenters at nevado del ruiz volcano-colombia. master thesis, arizona state university. naranjo, j.l., sigurdson, h., carey, j.n., fritz, w.j. (1986). eruption of nevado del ruiz volcano (colombia), 13 nov 1985: tephra fall and lahars. science, 233, 961-963. nieto, a.h. muñoz, f. (1988). energía sísmica liberada por enjambres de sismos de alta frecuencia en el volcán nevado del ruiz, colombia enero 1986 agosto 1987. bol. geol. ingeominas. vol. 29. 3, 3 – 20 (in spanish). nieto, a.h., brandsdottir, b., muñoz, f. (1990). seismicity associated with the reactivation of nevado del ruiz, colombia, july 1985 december 1986. i. j. volcanol. geotherm. res., 41, 315 326. press, w.h., teukolsky, s.a., vetterling, w.t., flannery, b.p. (1992). numerical recipies. the art of scientific computing, 686-693. raigosa, j. (1993). delimitación de zonas de baja velocidad en el volcán nevado del ruiz a partir de retardos en tiempos de viaje de la ondas p en sismos tectónicos regionales. bachelor thesis. universidad de caldas. (in spanish). richter, c.f., (1958). elementary seismology. freeman, san francisco, ca, 201-264. 123 activity and vp/vs ratio of volcano-tectonic seismic swarm zones at nevado del ruiz volcano, colombia toksoz, m.n., cheng, c.h., timur, a. (1976). velocities of seismic waves in porous rocks. geophys. 41,4, 621-645. wadati, k. (1933). on the travel time of earthquake waves, ii, geophys mag., 7, 101-111 zimmer m, prasad m, mavko g. (2002). pressure and porosity influences on vp-vs ratio in unconsolidated sand. the leading edge; soc. explor. gephys.vol. 21 no. 2. 178-183; doi: 10.1190/1.1452609 zollweg, j.e. (1990). seismicity following the 1985 eruption of nevado del ruiz, colombia. j. volcanol. geotherm. res., 41, 355-367. 124 john makario londoño b. determination of effective elastic thickness of the colombian andes using satellite-derived gravity data remy a. galán, iván f. casallas 7 petrography and application of the rietveld method to the quantitative analysis of phases of natural clinker generated by coal spontaneous combustion josé antonio henao m., angélica maría carreño p., john alexander quintero d., sol ángel candela h., carl enso modulations on streamflow characteristics ali ihsan marti, cahit yerdelen and ercan kahya 31 geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt. magdy a. atya, olga a. khachay, aiman abdel latif, oleg y. khachay, gad m. el-qady and ayman i. taha 44 groundwater investigation in awlad salameh, southern sohag, upper egypt aiman abdel-latif and mohamed el kashouty 63 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, i seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria ekine, a.s. and onuoha, k. m. 88 integrated assessments of possible effects of hydrocarbon and salt water intrusion on the groundwater of iganmu area of lagos metropolis, southwestern nigeria ayolabi, elijah a., folorunso, adetayo f. and obende phillips w. 100 efectos de la erupcion del volcan nevado del ruiz, ocurrida el 13noviembre-1985, sobre la atmosfera en colombia. caso: humedad jesus antonio esla va ramirez profesor titular _ departamento de geociencias-facultad de ciencias-universidad nacional de colombia ealeva, j. (1995): efectos de la erupci6n del volcsn nevado del ruiz, ocurrida ei13-noviernbre-1985, sobre la atm6sfera en colombia. caso: humedad. geof(s. colomb. 3:21-26. issn 0121·2974 resumen este es el tercer arnculo en el que se analiza el electo de la erupci6n del votcan nevado del ruiz del 13-11-85, sobre las diferentes propiedades atrnosfericas. en trabajos anteriores se anahzaron los cambios temporales ocasionados en la insolaci6n (horas de brillo solar) y la temperatura del aire. acs se muestran los efectos que se ocasionaron en la humedad del aire, los cuales comenzaron desde el dla anterior por los gases y cenizas emltidas con anterioridad y que por efeclo de los movimientos locales y generales del aire se hablan acumulado en sectores ubicados al w y nw del volcan. abstract effects on the differents atmospheric properties due to volcano nevado del ruiz activity on nov.13.85, are shows here as a continuation of several publish previous works, were has been analyzed, air temperature, and sunshine changes. this article shows the air humidity effects, starting one day before, due to the action of the previous volcanic gases and ashes emitted, which were located and accumulated to wand nw of the volcano by action of the local and general movements. 1. introduccion en este trabajo se presenta el efecto que la erupci6n del volcan nevado del ruiz del 13-11-85, ocasion6 en los valores de la humedad del aire en colombia. los datos meteorol6gicos de 436 estaciones se recopilaron de los archivos del instituto colombiano de hidrologia, meteorologia y adecuaci6n de tierras-himat (sistema de informaci6n hidrometeorol6gica) y del anuario meteorol6gico de la federaci6n nacional de cafeteros (fnccenicafe) correspondiente al ano 1985. esos datos, verificados y adecuados sequn las necesidades especificas del trabajo, y los resultados de los procesos realizados se consignan en la tabla 1. tal como se ha indicado en anteriores trabajos (eslava, 1993), las cenizas y gases expulsados por el volcan nevado del ruiz formaron capas, en la atm6sfera, de diferentes espesores y densidades que cubrieron rapidaments toda la zona ubicada al noroccidente del tolirna y la mayor parte del pais; ello impidi6 u obstaculiz6 el paso de la energia solar en el resto de esa tarde y dias siguientes. utilizando los registros de los datos meteorol6gicos disponibles se infiere que, 10 anterior, ocasion6 cambios temporales en los valores de los elementos meteorol6gicos que comenzaron, pracficarnente, desde el dia anterior por los gases y cenizas emitidas con anterioridad y que por efecto de los movimientos locales y generales del aire se habran acumulado en sectores ubicados al w y nw del volcan. las caracterrsticas generales de los cambios en los valores de la humedad del aire, se presentan a continuaci6n. 2. efectos generales en la superficie de acuerdo con 10senalado por eslava (1985, 1993), el gran numero de particulas que se introdujeron en la atmosfera por la erupci6n del volcan nevado del ruiz el 1311-85, se difundieron en la atm6sfera formando, principalmente, un penacho de cenizas y una nube gaseosa en forma de arco. sin embargo, las particulas s61idas y gaseosas se hicieron presentes en casi toda colombia y ocasionaron una reducci6n notoria en el nurnero de las horas de sol por dia (brillo solar) que normal mente ocurren en esa epoca del ana; con mayor intensidad, obviamente, en las zonas cercanas al volcan y en las ubicadas por debajo del penacho de cenizas. a su vez, la disminuci6n de las horas de brillo solar, origin6 una correlacionada reducci6n de la radiaci6n solar entrante en el sistema tierra-atm6sfera en cada sitio y una disminuci6n (no exactamente correlacionada, sino modificada por los facto res ffsicos de cada sitio) de las temperaturas medias del aire, con un aumento de las temperaturas rnlnirnas y una disminuci6n de las maxlrnas. esas desiguales variaciones de temperatura, en uni6n con el aporte de vapor de agua y nucleos de condensaci6n (ceniza) afectaron la distribuci6n de la humedad y de los dernas elementos meteorol6gicos, entre ellos el regimen temporal y espacial de las liuvias. las magnitudes de algunas de esas alteraciones fueron pequenas y, muy seguramente, no perceptibles por los metodos de medici6n disponibles. esas innumerables variaciones temporales y espaciales estuvieron sometidas a facto res externos entre los cuales el factor primario fue la radiaci6n solar y sus variaciones; dependieron basicamente de la interrelaci6n que se da entre los elementos meteorol6gicos y entre ellos y el conjunto de procesos zonales de conveccion termica y dinamica, de la influencia extensiva de los diferentes sistemas meteorol6gicos, como la ltc, de la presencia de movimientos del aire determinados 0 condicionados por la geometria tierra-sol, la latitud, la altitud, la configuraci6n del relieve, el suelo, la vegetaci6n, la hora del dia 0 noche en la cuailleg6 a cada sitio la masa principal de partfculas, etc. los irnpactos generales entonces se modificaron por el medio ambiente local y/o regional. todos esos agentes que producen 0 modifican el estado del tiempo, ocasionaron la gran variedad de alteraciones que se indican en la tabla 1 y figuras 1-6. elias muestran anailsls generales y descriptivos de los valores de humedad y sus variaciones, durante los dfas 11 a 16 de noviembre de 1985, antes, en y despuss de la erupci6n. 3. efectos sobre la humedad del aire en la tabla 1 se presentan los valores de humedad relativa media diaria (%) establecida para los dlas 11 a 16 de noviembre de 1985 y la humedad relativa de noviembre-85; al ser comparados esos valores se encuentran diferencias entre ellos que, en muchos sitios y dfas, indican un claro aumento de los valores medios diarios (figs.1-6). antes de resumir los resultados de los analisls respectivos, es conveniente mencionar 10siguiente: a) los valores instantaneos de la humedad relativa durante un dla pueden cambiar dentro de un rango mayor 0 igual a 50%; pero el rango en el que normalmente oscilan los valores medios (horarios "" 30%, 0 de un dla a otro '" 10%, 0 de un mes a otro "" 5%) es mucho menos amplio y cuando se da una variaci6n mayor ella puede ser indicativa de alguna situaci6n especial; b) para que los valores de humedad relativa cambien notoriamente de un dfa a otro 0 de un mes a otro, la situaci6n que gener6 ese cambio debe haber permanecido actuando la mayor parte de ese dfa 0 del meso en el caso de la actividad eruptiva del volcan nevado del ruiz, se presentaron dos situaciones relacionadas con la humedad relativa: la inyecci6n de vapor de agua, produj6 un aumento instantaneo de la humedad relativa del aire en la mayor parte de los sitios ubicados alrededor del volcan: ese aumento de vapor de agua no influy6 en los valores medios de la humedad relativa de ese dfa pero, al unirse con el resto de vapor de agua existente (abundante en esta epoca del a;;o) y formar un numero grande y suficiente de gotas de liuvia, utilizando para ello a las finas partfculas de ceniza como nucleos de condensaci6n, sf logr6 que las liuvias se intensificaran en esas zonas; la disminuci6n del numero de horas de insolaci6n y consecuentemente la disminuci6n de la radiaci6n disponible para calentar la atm6sfera y, por 10mismo, la disminuci6n de la temperatura media diaria, origin6 un aumento de la humedad relativa media diaria ql,le, aun cuando no fue generalizado en todo el pafs, se reflej6 en los dfas 13 y/o 14 de noviembre en algunas grandes regiones. ei 13, se aprecia un <:iaro aumento de la humedad relativa media, con respecto al promedio de nov-1985, enla mayor parte de los departamentos de arauca, boyaca y nari;;o y algunas zonas de antioquia, cauca, huila y meta, entre otros (fig.3). ei 14 el efecto, ademas de notarse mas claramente, cubri6 otras zo.nas del pafs: la mayor parte de antioquia, caldas, cundinamarca, guaviare, huila, quindfo, risaralda, tolima y valle del cauca y, algunas areas de boyaca, caqueta, cauca, meta y putumayo (fig.4). los dras 15 y 16, los efectos practicamente desaparecieron (figs.5-6). 4. conclusiones los analisis efectuados permitieron establecer las siguientes caracterrsticas generales relativas al comportamiento del elemento meteorol6gico humedad, frente a la inyecci6n en la atm6sfera de productos s61idos,ifquidos y gaseosos efectuada por el volcan del ruiz. se ocasion6 una reducci6n notoria en el numero de horas de sol por dfa, del 12 al 16 de noviembre, comparando con las que en promedio ocurrieron en todo el mes de nov-1985. este hecho, a su vez, origin6 una reducci6n de la radiaci6n solar entrante en el sistema tierra-atm6sfera y, a nivel del aire ubicado en la superficie terrestre, una disminuci6n de las temperaturas medias del aire, con un aumento de las temperaturas mfnimas y una disminuci6n de las maxlmas. esas variaciones y el aporte de vapor de agua y nucleos de condensacion (ceniza) afectaron la distribuci6n de la humedad delaire y de los demas elementos, como el regimen temporo-espacial de las liuvias. algunas de esas alteraciones fueron pequenas y, muy seguramente, no perceptibles por los metodos de medici6n disponibles. la comparaci6n entre los valores de humedad relativa media diaria (%) establecida para los dfas 11-16 de noviembre de 1985 y la de noviembre-1985; establece diferencias entre ellos que indican un clare aumento de los valores medios diarios, en muchos sitios y dlas. la inyecci6n de vapor de agua, produjo un aumento mstantaneo de la humedad relativa del aire en la mayor parte de los sitios ubicados alrededor del volcan; ese aumento de vapor de agua no influy6 en los valores medios de la humedad relativa de ese dfa pero sf logr6 que las liuvias se intensificaran. la disminuci6n del numero de horas de insolaci6n y de la radiaci6n disponible para calentar la atm6sfera y, por 10 mismo, la disminuci6n de la temperatura media diaria, origin6 un aumento de la humedad relativa media diaria que, aun cuando no fue generalizado en todo el pafs, se reflej6 en los dfas 13 y/o 14 de noviembre en algunas grandes regiones: el 13, se aprecia un claro aumento con respecto al promedio de noviembre-1985, en la mayor parte de arauca, boyaca y nari;;o y algunas zonas de antioquia, cauca, huila y meta, entre otros; el dfa 14 el efecto, ademas de notarse mas claramente, cubri6 otras zonas del pafs: la mayor parte de antioquia, caldas, cundinamarca, guaviare, huila, quindfo, risaralda, tolima y valle del cauca y algunas areas de boyaca, caqueta, cauca, meta y putumayo. en altitud, se presentaron aumentos notorios en la humedad relativa del aire durante los dfas 14-16 de noviembre, por la abundante recepci6n de vapor de agua (el 13) y el enfriamiento de todo el perfodo (12-16). los dfas 14 y 15 se presentaron aumentos apreciables de 4.000 a 7.000 m de altitud; 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~~~r r< ~~~§>~ " 5 < j ~~ ~§'~= > i< <~ ~ ~~~ ~~~-,jq~ <ig~~ o~<~< ~« .. ~<> '?~~~ ~~ ~ 8",~~~~5 < 8~8 ~ 5<~ ~o < ~«~ ~<§8 ;:<a'i ~ ~~z ~ i~~o~~~~g~~~ j~~~5g~g~~~~ ~~' ~ u !!l < y ~b ~~ < 8 .. o z <u~ ~~~i~~ <8~~~~~j <a~!;;~~~o~'~~~ 1!!i!lllllljll!1 ~<y~ ~ ~~5~~~ ~~~ 8 ;;l5ziil~~~ ~'~~..<~~~b~~a~~ii~~~~~~~~~~ iil§!~llis!~!!i~ll~l '::tal'" ~w ~~1il2~ ~:l ~r~~§<~~~~ ~~~~ ~~~ hiil~i~~~j~<~~ < ~~j>~::1~~g ~u~~~~~~ ~ ~~~~~~~~~ ~ s< ~~:::> 5lli . ~~~~~~~~~~~g~a~ ~~< ",~~g8i<8 ~~~ ~ ~ ~o;<g ij~~ffi~~5§~~~o . ej u <2 gr!;; ::l >< < ~~<g~lil=~..§~~~~o ~ a~~~q~~§s~~~~j§g~~~ ;~~~~~~~ ffi • ~,.~ .. z j~~~j~j wz ::l (/'jq.,lj.l...jc.. j>j~u~j~~~=~ ::t~o o~~,. ~ ,.~=~~~~~~a~~..~~~~uj~~~ ;;~:il~ j.. •. 1 bftasil ~ .. peru / , .1--7-' /la,' /')_1,° f1gui1l1. iloilneu c%) q.... m~ti1in .. dlr.l1iflcl. entl1l i. hum.re"t medle del 11·nov'y" medle dl.ri. de nov'f1gul1l2. ilolimlis c%) quem..... tr.n .. dlr. ...nci. entre •• hum.re••t medl. del f2-nov'y .. medl. dl.ri. de nov'f1gul1l3. 11011_ c%) a.... m~ti1in i.dlr. ...ncl. entre •• hum.rel.t media del 1':j-hov'y ie medle di.ri. de nov'.: ecuador ._--'-"-~ --.-.figul1l 4. isallne.s 1%) quemuestr.n •• dlr. ...nci. entre i.hum.re••t madle del 14-nov'yi. medle dl.ri. de nov',......·~··.....t ,/ j 0=, ;_\~ r-,",.r-j i, , -r->,'-""( " \ brasil ~ \. -~d~.i .. peru flgunl 5. 18011,... r%) aue mumtrlln ie dl"rencl. entrll .. hum.relet ".... del 15-hoy-85 y ie meetle dien de nov-85 ""un! ii. leoll,... r%) que mumtr.n ie dn.rencl. entrlli. hum.r."t meet .. del 1'l1-nov-85 y .. meetl. dl.ri. de nov-lls referencias bibliograficas eslaya, j. 1985: consideraciones sobre los aspectos meteorol6gicos y efectos relacionados con la dispersi6n de fragmentos y gases emitidos por el volcan arenas del nevado del ruiz el 13 de noviembre de 1985. geol.colomb. n1l14.p.165-173. santafe de bogota. 1993a: efectos de la erupci6n del volcan nevado del ruiz,ocurrida ei13-noviembre-1985, sobre la atm6sfera en colombia. caso: insolaci6n (brillo solar). geofis. colomb. 2:37-50. santafe de bogota. 1993b:efectos de la erupci6n del volcan nevado del ruiz,ocurrida ei13-noviembre-1985, sobre la atm6sfera en colombia. caso: temperatura del aire en superficie. zenit 4:51-64. santafe de bogota. geociencias 15-1 julio 2011.vp geological and geophysical evaluation of the ajana area’s groundwater potential, southwestern nigeria ariyo, stephen o.1; folorunso, adetayo f.2; ajibade o.m.3 1 department of earth sciences, olabisi onabanjo university, ago-iwoye, nigeria e-mail: ariyof@yahoo.com 2 department of geosciences, university of lagos, lagos, nigeria e-mail: detayf@yahoo.com; 3 department of earth sciences, olabisi onabanjo university, ago-iwoye, nigeria e-mail: ajibademuyiwa@yaoo.com corresponding e-mail: detayof@yahoo.com abstract a combined geological and geophysical evaluation was made of the groundwater potential of the ajana, remo north area in south-western nigeria; the geology and other structural features of the rocks there strongly influenced and correlated the aquifers’ storability and transmissivity. geological mapping revealed that the area was made up of granite, quartzite and varieties of gneiss, some of which have good secondary porosity and permeability. ten vertical electric soundings (ves) stations were established using a schlumberger electrode array. five geoelectric layers consisting of topsoil, sand, clayey-sandy, fractured / weathered basement and fresh bedrock were delineated. the aquifer layers were the 38.3m thick 283 ?m resistivity sand/sandy clay and 55 – 518 ?m resistivity fractured/weathered basement. other geoelectric parameters used in evaluating the area’s hydrogeological potential included curve type, anisotropy coefficient and reflection coefficient – the qh curve type was predominant in the area. the anisotropy coefficients suggested ves stations having high groundwater potential ranging from 1.4 – 1.56; while the reflection coefficients for the area ranged from 0.21 – 0.99. the overall results showed that ves stations 8, 9 and 10 could be possible groundwater sources having high expected yield. resumen se realizó una evaluación geológica geofísica del potencial hidrogeológico en ajana, remo north en la zona suroeste de nigeria; la geología y las fracturas en las rocas están estrechamente relacionadas con la capacidad de almacenamiento y transmisividad de los acuíferos. el mapeo geológico revela que el área está conformada por granitos, cuarcitas y variedades de gneis, algunos con buena porosidad secundaria y permeabilidad. se realizaron diez sondeos eléctricos verticales (sev) en configuración schlumberger, donde se establecieron cinco capas: suelo, arena, arcilla arenosa, basamento fracturado meteorizado y roca fresca. las capas acuíferas tiene espesores de 38.2m con resistividades de 283 ùm para arena y arcilla arenosa, y resistividades entre 55 – 518 ùm para el basamento meteorizado. otros parámetros geoeléctricos utilizados en la evaluación del potencial hidrogeológico de la zona incluyen el tipo de curva, el coeficiente de anisotropía y el coeficiente de reflexión – las curvas tipo qh fueron predominantes en la zona. los coeficientes de anisotropía sugirieren que los sev tienen gran potencial hidrogeológico con un rango de 1.40 a 1.56; mientras que los coeficientes de reflexión para la zona variaron entre 0,21 y 0,99. los resultados generales mostraron que en los puntos de los sev 8, 9 y 10 podrían ser posibles fuentes de agua subterránea con alto rendimiento. introduction groundwater is a vital natural resource for providing a community’s water supply and plays a fundamental role in human well-being, as well as that of many aquatic ecosystems. it is a preferred water source in many instances because water quality is generally good as it has been filtered by the soil, it is readily available and is often the only source of freshwater available due to its better spatial extension and distribution. there is constantly increasing demand for this resource which has resulted in the tremendous development of groundwater resources during recent years. unfortunately, it is not readily earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 35-40research groupin geophysics universidad nacional de colombia keywords: reflection coefficient, anisotropy coefficient, geoelectric layers, groundwater potential, mapping, ajana palabras clave: coeficiente reflexión, coeficiente anisotropía, capas geoeléctricos, potencial hidrológico, mapeo, ajana record manuscript received: 20/11/2010 accepted for publication: 31/05/2011 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:17 p p p composite 133 lpi at 45 degrees available like surface water and its exploitation involves costs and demands a form of technical-know-how. the early 1900’s brought about the development of the electrical resistivity method in deciphering groundwater potential. since then, improved technologies have led to a more refined approach to the exploration and exploitation of groundwater resources. applying the resistivity method with the availability of computers for processing and analysing data has greatly helped the matter (reynoids, 1997). many researchers such as ako et al, (1986), agwunobi and onuoha (1998), olorunfemi and olorunniwo, (1985), and ariyo, (2003) have successfully located prolific boreholes using this method. physiographic setting the study area lies between latitudes 7o 03’n and 7o 05’n and longitudes 3o43’e and 3o 46’e, covering an area of around 20km². it falls within nigeria’s tropical rain forest belt. the relief is low with elevation above sea level ranging between 16.1 – 117m. the major river draining the area is the river ona which flows souther, having tributaries forming a dendritic drainage pattern. figure 1 the area’s geology the ajana area falls within south-western nigeria’s precambrian basement complex rocks. the basement complex accounts for about 80% of south-western nigeria’s total surface. south-western nigeria’s basement complex rocks have been classified into migmatite–gneiss complexes, meta-sedimentary and metavolcanic rocks (the schist belts), the pan-african granitoids (the older granites) and undeformed acid and basic dykes (rahama, 2006). the study area’s geology consisted migmatised biotite – hornblende gneiss, biotite – hornblende gneiss, quartzite and granite. data acquisition geological mapping rock samples were taken from rock outcrops throughout the area. samples fell into four different rock types: migmatised biotite-hornblende gneiss occupying the study area’s north-eastern part, biotite-hornblende gneiss (a flat-lying outcrop found in one location), quartzite and granite occupying the largest part of the study area. petrographic studies of the rocks revealed a plethora of minerals including biotite, hornblende, quartz, muscovite, microcline and plagioclase feldspar and some accessory minerals, figure 2. the geoelectric survey ten vertical electrical soundings (ves) were made in the area using schlumberger electrode array (figure 3). earth resistance measurements were made with a syscal junior terrameter (a high-powered, fully-automatic resistivity meter for dc electrical surveys); it has been used over the years for groundwater exploration, environmental studies, civil engineering and structural geology investigation. it has the advantage of directly measuring ground material’s apparent resistivity as against earlier instruments which measured earth resistance to obtain the ground’s resistivity. apparent ground resistivity was obtained; ground resistivity was calculated by multiplying by appropriate geoelectric constants. the results were plotted on a log-log graph scale, a sample of which is shown in figure 4. partial curve matching techniques using two-layer model curves and corresponding auxiliary curves were used to arrive at curve types. winrest software was used in this survey to determine appropriate resistivity values. the field curves were compared with the 36 ariyo, stephen o.1; folorunso, adetayo f.2; ajibade o.m. figure 1: ajana location map with nigeria map inset agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:17 p p p composite 133 lpi at 45 degrees computer-generated curves and a good fit was obtained as shown in figure 4. table 1 shows the computer modelling results for the sounding stations and the corresponding depths. the table shows that three to five geoelectric layers were delineated as follows: the first layer was topsoil consisting of 0.9m 1.9m thickness 180 – 5,060 "m resistivity sand/sandy clay and laterite; the second layer consisted of 0.9m thick clay layer having 45 "m resistivity and 0.9 – 31.2m thick and 131 – 803"m resistivity sand/sandy clay layer. the third layer was a 5.3 – 32.2 m thick 14 47"m resistivity clay layer and sandy layer having 755"m resistivity and 0.9 m thickness, weathered layer having 55 "m resistivity and 16.5m thickness and 122 – 518 "m resistivity fractured rock, and a 924"m fresh basement. geological and geophysical evaluation of the ajana area’s groundwater potential, southwestern nigeria 37 figure 2: ajana geological map figure 3: ajana data acquisition map agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:17 p p p composite 133 lpi at 45 degrees the fourth layer was composed of 38.3m thick 283 "m resistivity sand and fresh rock having 909 – 1, 5316"m with 9 – 32.2m depth to basement. the fifth layer is made up of 627"m resistivity fractured rock and fresh basement having 3870"m resistivity and 45.3m depth to basement. the fractured/weathered basement constituted the major aquifer in the area based on their thick overburden and relatively low resistivity values. thus, ves points 3, 4, 6 and 8 10 having 55 – 518 "m resistivity and 7.4 – 45.3m overburden thicknesses respectively could be a good area for groundwater exploration. 38 ariyo, stephen o.1; folorunso, adetayo f.2; ajibade o.m. table 1: summary of ves results ves point � 1 (�m) � 2 (�m) � 3 (�m) � 4 (�m) � 5 (�m) h1 (m) h2 (m) h3 (m) h4 (m) depth to bedrock (m) 1. 510 131 14 909 1.3 9.4 32.2 32.2 2. 398 134 15 1,064 0.9 7.6 15.5 24.0 3. 355 45 518 0.9 6.5 7.4 4. 5,06 0 1,861 755 2809 627 1.9 .9 0.9 10.8 15.4 5. 849 253 47 15,316 1.0 2.7 5.3 9.0 6. 208 650 55 1,889 1.3 2.0 16.8 20.2 7. 229 597 924 1.1 14.6 15.7 8. 181 803 154 1.7 31.2 32.9 9. 180 1,907 122 1.6 6.1 7.7 10. 495 546 1,729 283 3,870 1.1 1.5 4.3 38.3 45.3 � = layer resistivity (ohmm) h = layer thickness (m) figure 4: a typical ves curve obtained in the study area. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:18 p p p composite 133 lpi at 45 degrees ves 3, 6, 8, 9 and 10 had good aquifer quality (layer resistivity and thickness), being favourable for groundwater exploitation based on the geoelectric layers so delineated. ves station 3 consisted of fractured rock but the expected water depth in the borehole was too small to recommend it for borehole drilling (meaning that it was left off the list). curve types curve matching depicted four layer curves (qh, kh) at four ves stations, three layer curves (h, a, and k types) at four ves stations and five layer curves (qhk, akh) at two ves stations (table 2). table 2 shows that the qh curve type predominated in the study area. ariyo (2003) proposed that the qh curve type is generally characterised by high groundwater provided that the aquifer unit is appreciably thick. however, ves points 1, 2 and 5 having qh curve types were clayey aquifers (aquitard) which would have poor groundwater yield. geoelectric parameters the geoelectric layers derived from electrical sounding were described by two basic parameters – layers resistivity and thickness. other geoelectric parameters were derived from the fundamental parameters mentioned above (zohdy et al., 1974), such as anisotropy coefficient (�) and reflection coefficient. they were mathematically derived as: anisotropy coefficient the overburden anisotropy coefficient was given as � � � � c l (keller and frischknecht, 1966) geological and geophysical evaluation of the ajana area’s groundwater potential, southwestern nigeria 39 table 2: various curve types from the ajana area curve type qh h qhk kh a k akh frequency 3 1 1 1 1 2 1 percentage 30% 10% 10% 10% 10% 20% 10% table 3: summary of geoelectric parameters ves no overburden thickness (m) longitudinal resistivity, ��� c (�m) transverse resistivity � l (�m) anisotropy coefficient reflection coefficient curve type 1. 32.1 19.18 63.59 1.82 0.97 qh 2. 24.0 21.56 66.80 1.76 0.97 qh 3. 74.4 50.37 82.68 1.28 0.84 h 4. 15.5 2, 421.88 2, 849.32 1.08 0.63 qhk 5. 9.0 71.94 197.7 1.66 0.99 qh 6. 20.1 63.63 123.94 1.40 0.94 kh 7. 15.7 536.32 571.09 1.03 0.21 a 8. 32.9 681.64 770.48 1.06 0.68 k 9. 7.7 636.15 154.84 1.56 0.88 k 10. 45.2 319.4 436.72 1.17 0.86 akh figure 5: curve type pie chart agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:18 p p p composite 133 lpi at 45 degrees where ñ c = transverse resistivity in"m. � � � �� � � ��# � � c l and ñ l = longitudinal resistivity in"m � � � �� � � ��# hi hi i/� where hi = layer thickness in m ñ i = layer resistivity in m anisotropy coefficients were calculated from the layer resistivity and thickness obtained from qualitative interpretation raging from 1.03 – 1.82. whereas, a range of 1.39 – 1.66 anisotropy coefficient has been judged necessary for boreholes to be considered productive in some parts of south-western nigeria’s basement complex (olorunfemi and olorunniwo, 1985). groundwater yield increases with anisotropy coefficient (olorunfemi et. al., 1991), although ves 5, 6 and 9 had good anisotropy coefficients of 1.4 – 1.66, only ves 6 had appreciable aquifer thickness (16.8m). reflection coefficient the bedrock interface reflection coefficient is an important parameter which shows that a bedrock fracture is filled with water. there must be a direct correlation with the anisotropy coefficient value for this parameter to be considered. however, the present study showed that intermediate reflection coefficient values correlated with sand aquifer were expected to be prolific if developed into a borehole. it was also noticed that reflection and anisotropy coefficients for ves points 8, 9 and 10 had a linear relationship, thus, the parameter could be considered (together with other parameters) as being good factor in selecting a site for developing borehole. this was given as: k n n n n n � � �1 1 1 � � � � where ñn = bedrock resistivity ñn-1 = sub-basement resistivity conclusion this paper has reported a geological and geophysical evaluation of ajana’s groundwater. the area’s geology delineated from geological mapping showed that it consisted of granite, quartzite and varieties of gneiss. rock type has a direct influence on an area groundwater, as easily fractured rock like quartzite, would enhance secondary porosity and permeability while hard and resistant rock like basalt would adversely affect aquifer parameters. ves data was also acquired and interpreted using winrest software to determine the primary geoelectric parameters from which further geoelectric parameters were derived. based on layer thickness and resistivity values, ves 6, 8, 9 and 10 located on a weathered (ves 6) and sandy layer (ves 8 to 10) have been recommended. however, only ves 8 to 10 had low to medium reflection coefficients having a linear relationship with their overburden anisotropy coefficients. ves stations 8 to 10 had good aquifer thicknesses meaning that if developed into water boreholes, the water column would be thick enough to sustain the local people’s water need (i.e. ves stations 8, 9 or 10 could be drilled and developed into producing water boreholes). references agwunobi, o.e., and onuoh, k.m. (1988): geophysical investigation for groundwater in hardrock terrains: experiences from the fobour area of the jos-plateau, nigeria. journal of mining and geol., vol. pp. 45 – 50. ako, b.d., adegoke, o.s., ajayi, t.r., ajayi, j.o., and rahaman, m.a., (1986): groundwater prospecting and exploitation in nigeria; in proceedings of the first annual symposium and exhibition on groundwater resources in nigeria, niwasa, lagos, nigeria, pp. 3–44. ariyo, s.o. (2003): geophysical investigation of the groundwater potential of iseyin. journals of applied sciences, vol. 6, no. 1, pp. 3393–3402. keller, g.v and frischknecht, f.c. (1966). electrical methods in geological prospecting. pergamon press. new york, pp33-37. olorunfemi, m.o. olanrewaju, v.o. and alade, o., (1991): on electrical anisotropy and groundwater yield in a basement complex area south-western nigeria. journal of africa earth sciences, vol. 12, no. 93, pp. 247–327. olorunfemi m.o. and olorunnwo, m.a. (1985): geoelectric parameters and aquifers characteristics of some parts of south-western nigeria. geological application indiogeologic, vol. 20, pp. 99 – 109. rahaman, m.a., (2006): nigeria’s solid minerals endowment and sustainable development in the basement complex of nigeria and its mineral resources, edited by oshin. akin jinad & co., ibadan, nig. pp. 139 – 168. reynoids, j.m. (1997): an introduction to applied and environmental geophysics. john wiley & sons. new york, pp. 417–490. zohdy, a.a.r., eaton, g.p. and mabey, d.r. (1974): application of surface geophysics to groundwater investigations: in tech. of water sources investigations of the u.s. geol. survey, book 2, chap, dl. 40 ariyo, stephen o.1; folorunso, adetayo f.2; ajibade o.m. agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:18 p p p composite 133 lpi at 45 degrees geociencias dic 2010-color.vp contribution of topographic and penetrometric measurements to a site characterization, case of the kekem landslide, national road n°5 (western cameroon) jean jacques nguimbous-kouoh1, 2 and eliezer manguelle-dicoum1, 2 1department of physics, faculty of science, university of yaoundé i, p.o. box 812 yaoundé-cameroon 2 geotech-consulting cameroon, office of geophysical studies and services, p.o. box 6052 yaoundé-cameroon e-mail: nguimbouskouoh@yahoo.fr abstract a topographical survey and a dynamic cone penetrometer testing (dcpt) were conducted over the kekem landslide area. the aim of the work was to unveil new topographic features of the site, and with the penetrometer soundings predict the depth of the potential rupture surface beneath the damaged area. the analysis of penetrometer data has led to the supposition that the fractured surface interface connecting the basement and sediments has a depth of between 2.00 m and 12.20 m. the topographic data was used to determine the average slope percentage over the broken area. the slope percentage gives 33.66% from a point taken over the tearing niche and a point along the road. 2d contours map drawn with the surfer software, over an area of 35 hectares has enabled the observation of a step gradient oriented nne-ssw. considering the steepness of the slope and the thickness of the sediments, the site remains dangerous for the populations especially during the rainy season. resumen se hicieron un levantamiento topográfico y un ensayo con penetrómetro dinámico de cono (dcpt) sobre el área de deslizamiento kekem. el propósito del trabajo es conocer nuevos rasgos topográficas del sitio, y con el penetrómetro predecir la profundidad a la que está la potencial superficie de ruptura debajo del área afectada. el análisis de los datos provistos por el penetrómetro ha permitido predecir que la interfaz de superficie fracturada entre el basamento y los sedimentos está a una profundidad entre 2,00 y 12,20 m. los datos topográficos permitieron determinar la pendiente media de la zona fracturada. medida entre un punto en la zona de desgarre y un punto a lo largo de la vía, la pendiente porcentual es de 33,66%. líneas de contorno 2d dibujadas en el mapa con el programa surfer, sobre una superficie de 35 hectáreas ha permitido observar un empinado gradiente orientado en dirección nne-ssw. tomando en cuenta la empinada pendiente y el espesor de los sedimentos, el sitio sigue siendo peligroso para la población, especialmente durante la estación lluviosa. introduction landslides triggered by the geology, faults or rainfalls are mostly generated in mountainous landscapes. some of these landslides occur suddenly and flow for miles at high speeds. they may pose serious threats to life and property (larsen et al., 2000; agliardi et al., 2001). landslides often take place when a swaying occurs at the fracture interface between superficial sediments and stronger bedrock. this generally results to a complete change of scenery for years or millennia (beven, 1981; iverson, 2000). to study the kekem landslide zone the common subsurface geophysical methods for determining the depth of the basement top were not used because of the unfavorable environment for electrical and electrostatic methods. the slope is very rugged and electrical low tension cable disturbances nearby. we present here, the new topographic features and the results obtained using a dynamic cone penetrometer test which provides a lively alternative to conventional non destructive reconnaissance methods. this process borrowed from geotechnical studies is often used in geophysics to determine the depth of bedrock, when near the surface (iverson and major, 1987; le noble and martinaud, 2003; marmeta et al., 2006). to illustrate how the two techniques can be combined on the field, a case study was conducted over the kekem landslide site. earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 135-144research groupin geophysics universidad nacional de colombia keywords: landslide, national road n°5, kekem, topographic, penetrometer test palabras clave: deslizamiento de tierra, carretera nacional no. 5, kekem, topográficos, prueba de penetrómetro record manuscript received: 20 -052010 accepted for publication: 30-11-2010 geographical and geological setting the study area is located in the western region of cameroon, between latitudes 5°05’ and 5°25’ north and longitudes 9°55’ and 10°26’ east (figure 1). the local climate is tropical with prolonged rainy seasons from april to november. the region is heavily populated because of its economic and agricultural potentials. geologically, the region is located in the central african orogenic belt characterized by gneiss, which is of proterozoic age. the local structural orientation is nne-ssw. the outcrops in the region are in the form of isolated slabs or rocks (kwekam, 1993; talla, 1995; nguiessi et al., 1997). the study area (figure 1) belongs to the mountainous range that borders the mbos plain. most of the hillsides of the main mountain terrain reached their equilibrium profile since the tertiary and quaternary age (belnoun et al., 2008). however, their dismantling by erosion, agriculture and road works remain locally very active. the materials able to slip are varied. they belong either to the bedrock or to the superficial formations which recover it (talla, 1995; nguiessi et al., 1997; nguimbous et al., 2008). the rocks of the bedrock in the sector are granite-gneissic and largely metamorphosed. the shallow formations are inconsistent and are more susceptible to the rupture in the presence of groundwater, notably horizons rich in clayey minerals. the shallow formations were implemented at the rhythm of geological processes (erosion, weathering, and volcanism) of cretaceous and tertiary bedrock and cover the different sides with a thick deposit more or less stable. equipment and methodology to cover the entire study area, we used a laser tachometer (tc 805 l) to measure the various parameters (vertical angles, horizontal and vertical drop, etc.) this equipment is made up of a long range laser (greater than 500 m) to compute inaccessible points with an accuracy of ± 3 mm (variable depending on the support quality). the site was set with a base station and secondary stations materialized by the pillars along the road and within the resident lands. the secondary stations were often used as references. they were uplifted at the beginning and at the end of the trial and led to the lift orientation and precision of successive surveys. the points were not selected according to a regular grid but according to the relief irregularities along the perpendicular and parallel lines of the hillside. a total of 397 study points were obtained including 35 penetrometer sounding points. to measure the thickness and depth of 136 jean jacques nguimbous-kouoh and eliezer manguelle-dicoum figure 1: location map of the study area (from nguimbous et al., 2008) different underground layers in several points on the site, the dynamic cone penetrometer (dl010) was used. the trial test consisted to sink into the soil the stem with a cone containing a free fall mass. the stem that has at its end a cone of 20 mm in diameter sinks by dropping a hammer on its anvil. the drop height (575 mm) and the masses of the hammer (4.6 and 8 kg) are normalized according to astm d6951-03. we also used a gps for the measurement of geographical coordinates, a compass for orientation and positioning, walkie-talkies for communication and a digital camera to save and catalog the event. presentation of the landslide and disaster area in october 20, 2007, after three days of intense rainfall, a landslide occurred at the entrance to the town of kekem along national road no5 (figure 2). the mixture constituting multiple tons of massive materials literally slipped; destroying road, houses, plantations and killing two people. the data obtained by direct measurements on the site were used to estimate the average height of the scratched mountain (1200 m), the average height of the road from the site relative to the sea level (723 m) and average distance between the hill and houses (4 m). the affected area corresponds to a displacement of soft soil and rock along a fracture surface (figure 2). this earth flow occurred on a relatively steep slope, the front mass was found at 272 m from the tearing niche, stripping the soil to a depth of about 4 m. the overall movement is oriented n10 ° e dipping south. the deposits were displaced to less than 5 m from the ngoum river, the main river of the region. the width of the stripped area varies between 18 and 55 m from the foundation of the tearing niche to the base of the material deposits. the volume of material moved is estimated to about 30.000 cubic meters. the soil profile observed at the dislocation surface reveals three different lithological levels namely: an upper horizon made up of litter and humus with thickness of about 5 centimeters, an intermediate horizon, black in colour and about 60 centimeters thick, and a lower brown, silty clay horizon that extends to the underlying bedrock (nguimbous et al., 2008). results and discussion analysis of the topographic maps the description of a topographic map is generally intended to characterize the main aspects of the relief (varnes, 1976; quido and vojtech, 1982). these main aspects can be observed on the contour maps or digital elevation models (tucker and whipple, 2002, glenn et al., 2006; tarolli and tarboton, 2006, eeckhaut et al., 2007). to realize the maps for this study, data were processed with the covadis (2007) software which is an applicative of topography, cartography, and earthwork. the software was used for the topometric calculations and generation of the cartesian coordinate points of the study area. these coordinates were then entered into the surfer (2009) software for interpolation and production of contour maps and the digital elevation model (dem). the longitudinal profile over the slipped area was generated using the covadis (2007) software. • the map in figure 3 presents the site plan of the field measurement points. the points were densified at the collapsed area to better define and simulate the phenomenon. • the 2d contour map (figure 4) shows the distribution of altitudes in meters along the study area. the contour lines reveal a steep gradient oriented nne-ssw, corresponding to a high escarpment of the slope. the shape of the contour lines also implies a convex slope, which is indeed remarkable in the field. the slope is gentle at the beginning but becomes increasingly steep as soon as one goes up. • the longitudinal profile (figure 5) over the damaged area provides an image of the topographic slope and a probable dip of geological formations and structures of the study area when going from top to bottom. from this profile, the quote lands distribution varies approximately between 780.72 and 715.27 m for a cumulative distance of about 200.41 m along the slope. • figure 6 shows the digital elevation model (dem) of the study area reconstructed after topographic data interpolation. it clearly shows contribution of topographic and penetrometric measurements to a site characterization, case of the kekem landslide, national road n°5 (western cameroon) 137 figure 2: photographs of the landslide the notch which represents the kind of landslide. apart from the notch, the morphology of the study area appears to be homogeneous, implying a geology that remained relatively constant over time and space. the data interpretation with the covadis (2007) software was used to estimate the surface area covered by the study at 35 hectares, the perimeter of the area covered by the study at 1200 meters, the slipped surface area at 1.08 hectares and perimeter of 578 meters, the maximum height above the tearing niche to be 804 meters and the slope percentage over the damaged area at 35.18% for a horizontal distance of 201.04 m and a vertical distance of 70.73 m between a point taken over the tearing niche and a point along the road (nguimbous et al., 2008). similarly, the exploitation of the 2d contour map (figure 4) of the study area obtained with the surfer (2009) software agreed with similar characteristics. the slope increases up with a step gradient (figure 4). to analytically calculate the slope, we considered that there are several well-known methods for doing so (monkhouse and wilkinson, 1971; denegre, 1973). the points dcpt32 (x2 y2, z2) and dcpt5 (x1, y1, z1) (table 1) located respectively at the highest point of the tearing niche and along the road were selected. a rapid altimetric and planitmetric calculation gave (denegre, 1973): 138 jean jacques nguimbous-kouoh and eliezer manguelle-dicoum figure 3: the site plan showing locations of the tachometer measurements points figure 4: the 2d contour map of the study area (contour interval is 5 m) (solid black line is national road number 5; dash line is longitudinal profile). p dcpt dcpt z z x x y y ( ; ) ( ) ( ) . %5 32 100 33 66 2 1 2 1 2 2 1 2 � � � � � � � this slope is relatively similar to that obtained by nguimbous et al. (2008). in order to obtain the map (figure 4) and the digital elevation model (figure 6) the kriging method was used. the interpolation procedure gave for 397 introduced points, 395 active data. the semi-variogram components obtained are of linear type and the variogram slope give one. the proportion of anisotropy gives one and null angle. the (x, y) coordinate interpolation grid gave, for values of x ranging between 5594.610 and 6255.829 m and those of y ranging between 6783.676 and 7258.493 m, 133 lines and 96 m columns for an interpolation interval of 5 m. the grids were retained after several tests. the tightening or large interpolation interval introduces distortions on the vicinity of the points. the interpolation radius gave 407 m. the (x, y, z) coordinates were automatically converted into metric distance by utm projection on the clarke 1880 ellipsoid reference. table 2 shows the statistical characteristics that enable the realisation of the digital elevation model (dem) and the 2d contour map. analysis of table and penetrometer sounding maps topographic data are increasingly being combined with geotechnical data used in geophysics to determine the exact depth of basement roof (le contribution of topographic and penetrometric measurements to a site characterization, case of the kekem landslide, national road n°5 (western cameroon) 139 figure 5: the longitudinal profile over the damaged area. figure 6: the 3d digital elevation model (dem) of the study area (solid black line is national road number 5; dash line is longitudinal profile). figure 7: the map of the sediment thicknesses showing locations of the dynamic cone penetrometer testing points. noble and martinaud, 2003; falorni et al., 2005; frankel and dolan, 2007). two types of information were expected from penetrometer tests performed over the kekem landslide area. that is, the height of the bedrock roof and the thickness of the sedimentary layer. the heights of the sounding were leveled with the tachometer and are correct and accurate. thirty-five dynamic cone penetrometer testing (dcpt) were made (nguimbous et al., 2008). the results are presented in the form of maps and a table containing the depths of diverse layers beneath the slipped zone. table 1 highlights the cartesian coordinates of various (dcpt) soundings. the points were identified in altimetry and planimetry. in general, the heights are irregular and ranging between 766.32 and 717.76 meters, which gives a drop of 48.56 meters between the highest point and lowest point where the measurements were made. table 1 equally presents the dcpt points with different depths at which the penetrometer reached. the depths are discontinuous on the entire site. the thickness of sediments was between 2.00 m and 12.20 m. the deepest sounding was obtained in dcpt9 coordinates (5985.99 m; 7015.01 m; 12.20 m). table 1 also shows the basement roof elevations obtained by the difference between the actual heights of the penetrometer soundings and sediment thickness acquired at different points. overall, the elevations are asymmetrical and vary between 761.35 and 708.41 m, which give a drop of 52.94 m between the highest and lowest point of measurement. the map of the sediment thicknesses (figure 7) shows location of the penetrometric testing points. its shows areas where greater sedimentary deposits thicknesses can be observe along the site. the overall sedimentary layer thickness is less important at the center of the slipped zone; it is higher at the foot of the mountain and decreases with altitude. this model-building sedimentary deposit may be correlated to the kenyan shurmai site (kuehn and adovasio, 1999). topographic maps of the slipped area (figure 8a and 8b) have the same morphological features. this implies that the sediments were deposited gradually as the configuration of the basement. these maps also show that topography is irregular through out the site. the maps (figures 7 and 8), were obtained by the same interpolation and with the same variogram model and anisotropic coefficient like figure 4 and 6. for 35 introduced points we obtained 35 active data and 86 m interpolation radius. the (x, y) grid interpolation gave, for values of x ranging between 5959.690 and 6047.003 m and those of y ranging between 6995.620 and 7143.781 m, 59 lines and 100 columns for interpolation interval of 1.50 m. the grids were tested on several occasions. for a smaller or larger interpolation interval the signal witnessed distortions. 140 jean jacques nguimbous-kouoh and eliezer manguelle-dicoum figure 8: topographic maps of the slipped area showing locations of the dynamic cone penetrometer testing points (figure 8a: topographic map of the slipped area; figure 8b: topographic map of the basement roof along slipped area). contribution of topographic and penetrometric measurements to a site characterization, case of the kekem landslide, national road n°5 (western cameroon) 141 table 1: the cartesian coordinates of the various (dcpt) sounding depths and basement roof elevations dcpt x (m) y (m) altitudes (m) depths basement roof elevations (m) dcpt1 6011.62 7006.37 724.84 3.70 721.14 dcpt2 5999.50 7006.44 724.55 10.00 714.55 dcpt3 6000.81 6995.62 722.82 8.50 714.32 dcpt4 5980.90 6995.97 719.31 10.90 708.41 dcpt5 5970.05 6996.07 717.76 8.20 709.56 dcpt6 5959.69 7006.76 718.91 6.20 712.71 dcpt7 5963.63 7013.14 720.55 5.60 714.95 dcpt8 5976.36 7018.08 723.01 10.00 713.01 dcpt9 5985.99 7015.01 724.91 12.20 712.71 dcpt10 5988.42 7007.04 724.04 5.80 718.24 dcpt11 6003.97 7019.03 727.62 2.80 724.82 dcpt12 6009.88 7025.00 729.07 6.60 722.47 dcpt13 5996.60 7044.09 733.38 2.00 731.38 dcpt14 5992.91 7037.68 731.78 4.70 727.08 dcpt15 5979.69 7024.74 723.68 3.20 720.48 dcpt16 5994.64 7023.00 726.57 2.60 723.97 dcpt17 6000.41 7027.02 728.30 2.20 726.10 dcpt18 6020.47 7056.91 731.09 2.00 729.09 dcpt19 6007.41 7036.98 739.37 2.20 737.17 dcpt20 6009.31 7062.99 730.58 3.80 726.78 dcpt21 6015.27 7066.88 741.38 4.00 737.38 dcpt22 6018.21 7079.91 742.36 6.00 736.36 dcpt23 6005.26 7083.63 746.19 3.80 742.39 dcpt24 6013.97 7088.31 747.84 5.80 742.04 dcpt25 6014.29 7094.77 747.70 5.20 742.50 dcpt26 6042.44 7125.82 749.29 3.40 745.89 dcpt27 6047.00 7131.79 761.95 3.20 758.75 dcpt28 6040.41 7131.99 763.42 3.00 760.42 dcpt29 6034.98 7136.66 763.29 4.60 758.69 dcpt30 6026.69 7139.83 765.17 4.80 760.37 dcpt31 6022.71 7143.78 766.15 4.80 761.35 dcpt32 6034.01 7125.43 766.32 6.00 760.32 dcpt33 6019.78 7125.10 760.42 5.50 754.92 dcpt34 6037.48 7106.46 762.09 4.50 757.59 dcpt35 6030.10 7103.54 754.90 4.40 750.50 142 jean jacques nguimbous-kouoh and eliezer manguelle-dicoum table 2: the statistical characteristics of digital elevation model x y z count 395 395 395 1%-tile 5715.624 6907.441 700.798 5%-tile 5761.776 6935.555 712.447 10%-tile 5807.144 6954.467 715.284 25%-tile 5903.493 6981.77 719.265 50%-tile 5994.569 7036.98 722.084 75%-tile 6040.432 7094.77 730.578 90%-tile 6118.551 7160.319 756.09 95%-tile 6177.895 7184.532 765.889 99%-tile 6246.331 7250.973 782.972 minimum 5594.61 6783.676 700.798 maximum 6255.829 7258.493 786.372 mean 5976.29266329 7045.61525063 728.413065823 median 5994.569 7036.98 722.084 geometric mean 5975.15577875 7045.16149688 728.216903234 harmonic mean 5974.01580895 7044.70915771 728.025313695 root mean square 5977.42649565 7046.07041712 728.613895744 trim mean (10 %) 5975.84373315 7042.78160674 726.839859551 interquartile mean 5981.71835859 7035.64368182 722.564277778 midrange 5925.2195 7021.0845 743.585 winsorized mean 5974.95846329 7044.31123544 727.579491139 trimean 5983.26575 7037.625 723.50275 variance 13587.9134253 6430.34244128 293.357287097 standard deviation 116.567205617 80.1894160179 17.1276760565 interquartile range 136.939 113 11.313 range 661.219 474.817 85.574 mean difference 130.194017092 90.0945580672 16.6856108205 median abs. deviation 65.346 55.572 3.46 average abs. deviation 89.6957797468 64.4494481013 10.4998860759 quartile dispersion 0.0114651590662 0.00802754085876 0.00780291383274 relative mean diff 0.0217850805553 0.0127873230175 0.0229067978094 standard error 5.86513263477 4.03476739759 0.861786908809 coef. of variation 0.0195049359501 0.0113814639553 0.0235136859293 skewness -0.160642343012 0.432891861007 1.56173296176 kurtosis 3.1125793314 2.93037169705 4.91813436082 sum 2360635.602 2783018.024 287723.161 sum absolute 2360635.602 2783018.024 287723.161 sum squares 14113202866.8 19610607787.6 209696892.583 mean square 35729627.511 49647108.323 530878.209072 the maximum depth of the basement/rupture surface varied between 2.00 and 12.20 m. these values are compatible with the supposed depths of the sediment-basement surface reached in the other risky sites (figure 1); particularly in marche moume, petit nkam and famke respectively located about 2, 5 and 28 km from the study area (nguimbous et al., 2008). the study of kekem landslide using two techniques led to the actual site characterization. this approach is comparable to conventional approaches in geophysics (brunsden and prior, 1984; crozier, 1986). conclusion this study, combined with the observations carried out on the field conveys evidence of the terrains fragility, the slopes steepness and the great thickness of the sediments. the risk of catastrophe for the road and the populations remains imminent; given their proximity to the site. on the other hand, the combined use of the tachometer and the dynamic cone penetrometer proved a simple and effective employment vis-a-vis of the particular problem of the site. the fluid blending of the two techniques and the data appreciably increases the productivity of the geophysicist in achievement of various tasks of precision measurement of the basement in areas where it is nearer to the surface. this approach should be validated in other sites where the relief is not very broken and the basement nearer to the surface. the method authorizes a total flexibility on the field and a successful data-gathering. the productivity of a great number of geophysics applications is thus largely improved. references astm d6951-03. 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(2006). a new method for determination of most likely landslide initiation points and the evaluation of digital terrain model scale in terrain stability mapping. hydrology and earth system sciences, 10: 663-677. tucker, g. e. and whipple, k.x. (2002). topographic outcomes predicted by stream erosion models: sensitivity analysis and intermodel comparison. j. geophys. res, 107: 2170-2179. varnes, d. j. (1976). slope movement types and processes. national academy of sciences, spec, report 36pp. acknowledgments the authors thank the reviewers for their comments and suggestions which enabled to improve the initial version of the manuscript. contribution of topographic and penetrometric measurements to a site characterization, case of the kekem landslide, national road n°5 (western cameroon) 143 geofisica colombiana n° 6 pp.33-39 diciembre de 2002 bogota, d.c. issn 0121 2974 analisis espacial y temporal de q-coda en el piedemonte llanero (colombia) juan p. duran', carlos a. vargas' y luis a. briceno' resumen la variacion espacial y temporal del pararnetro de atenuacion q-coda en un sector del piedemonte l1anero es estudiada usando las ondas coda. la estirnacion de la atenuacion de la amplitud de la coda se calcula por medio del modelo de dispersion isotropica simple de sato, a partir de sismogramas de sismos que ocurrieron en la region entre septiembre de 1993 a junio 2002. siguiendo la metodologia empleada por aki y chouet (1975), se analizaron los sismogramas en cuatro band as centradas a 1.5, 3, 6 y 12 hz con sus respectivos anchos de banda de 1,2,4 y 8 hz. por regresion linear se calculo el valor de qc a i hz. a partir de la expresion propuesta por puli para modelar las amplitudes de las ondas coda por medio de elipses, se arnpiio esta expresion para calcular la variacion espacial del qc-l, para un medio tridimensional (tomografia anelastica). y por medio de inversion por minimos cuadrados se calculo el valor de atenuacion para cada celda del modelo tridimensional. los resultados se correlacionaron con el marco tectonico regional encontrando que cuando la atenuacion aumenta con la profundidad, corresponde a zonas con tectonica de piel gruesa y por el contrario, cuando la atenuacion disminuye con la profundidad, corresponde a zonas con tectonica de piel delgada. por otro lado, la variac ion temporal de los minimos valores de qc (maximas atenuaciones) se correlaciono cualitativamente con sismos de magnitud mayor de 3.5 y, adernas, se cuantifico la distribucion temporal de qc par medio de la dimension fractal, mostrando que la agrupacion de los valores minimos de qc aumenta antes de la ocurrencia de un evento principal y decrece su agrupacion despues de la ocurrencia del even to principal. palabras clave dndas co'oa, oc, atenuacion, dimension fractal, pieoemonte abstract the space-time variation of the parameter of attenuation q-coda in a sector of the piedemonte l1anero was studied using the coda waves. the estimation ofthe attenuation of the amplitude of coda is calculated by means of the model of simple isotropic dispersion of sa to, from earthquakes seismograms happened in the region from september 1993 to june 2002. following the methodology used by aki and chouet (1975), they analyzed seismograms in four centered bands to 1.5,3,6 and 12 hertz with its corresponding bandwidths of 1, 2, 4 and 8 hertz. and by regression to linear calculated the value of qc to i hertz. from the expression proposed by puli to model the amplitude of the waves coda by means of ellipses, this expression was extended to calculate the space variation ofthe qc-i, (anelastic tomography). and by inverse square minimums, the value of attenuation for each cell of the three-dimensional model was calculated. their results were correlated with the regional tectonic frame showing that when the attenuation increases by depth, it corresponds to zones with tectonics of thick skin and, on the contrary, where the attenuation diminishes by depth, it corresponds to zones with tectonics of thin skin. on the other hand, the temporary variation of minimum values ofqc (maximum attenuations) was correlated to qualitatively earthquakes of magnitude greater to 3,5 and, in addition, the temporary distribution ofqc was quantified by dimension fractal showing that the grouping of the minimum values ofqc increases before ocurring a main event and its grouping decreases after occurring the main event. keywords: coda suaves, oa, attenuation, fractal dimension, foothills estudiante de la maestna de geotlsica, universidad nacional de colombia. profesor del departamento de geociencias, universidad nacional de colombia. ibriceno@ciencias.unal.edu.co introduccion un mecanisme para determinar las condiciones de atenuacion de una zona, son las ondas coda, que son ondas retrodispersadas, resultado de la suma de ondas prim arias dispersadas por la interaccion con dispersores 0 heterogeneidades y por 10 tanto su geofislca cdlombiana, 6, diciembre de 2002 33 duran et al. atenuaci6n en la amplitud se debe principalmente a las condiciones tect6nicas de la regi6n donde se registr6 el sismo, mas que a la misma trayectoria recorrida por la onda des de la fuente hasta la estaci6n sismol6gica. desde el punto de vista temporal, la atenuaci6n de las ondas coda es igualmente interesante, debido a la correlaci6n que existe entre la variaci6n de qc-i y la sismicidad. en general, se ha observado un decrecimiento de valor de qc (aumento en la atenuaci6n del medio) antes del evento principal 0 de la erupci6n y posterior al evento, un aumento en el valor de qc (disminuci6n de la atenuaci6n del medio). la variaci6n de la atenuaci6n se traduce en un cambio del regimen de esfuerzos; por tanto, la variaci6n de qc puede ser usado como un indicador de los cambios temporales de esfuerzos en una area. en este estudio se gener6 un modelo tridimensional de atenuaciones (tomografia anelastica) en un sector del piedemonte llanero entre los departamentos de meta y cundinamarca, entre la latitud 3.50 a 5.30 norte y longitud 74.60 a 72.30 oeste. la variaci6n espacial de la atenuaci6n se correlacion6 con ei marco tect6nico regional. ademas, se analiz6 la correlaci6n entre evoluci6n en el tiempo de qc con la ocurrencia de sismos de magnitud mayor de 3.5 y su distribuci6n temporal por medio de la dimensi6n fractal. marco tectonico y geologico regional a 10 largo de la margen oriental de la cordillera oriental se prolonga una amplia zona de fallas de tipo compresional conocido como sistema del piedemonte llanero 0 sistema frontal de la cordillera oriental, asociado a la reactivaci6n de antiguas fallas originadas durante una fase extensiva de las edades jurasica y cretacea (lngeominas, 1998). algunos autores consideran este sistema como limite entre la placa suramericana y ei bloque norte de los andes (paris, et al., 2000). _ la regi6n se caracteriza por presentar alta actividad sismica, con sismos de todo tipo de magnitud y de profundidades focales hasta unos 50 0 60 km, asociables a la actividad de esta zona de fallamiento. adernas, el analisis de los principales rasgos geomorfol6gicos en abanicos cuatemarios seiialan una alta actividad neotect6nica, 10 cual indica que el proceso orogenico en la cordillera no ha cesado (ingeominas, 1998). ei sector sur de la regi6n se caracteriza por roc as del basamento metam6rfico andino y del paleozoico superior, expuestas en las zonas extern as contra la cuenca de los llanos, principalmente en una serie de altos 0 domos con direcci6n general paralela al rumbo estructural de la cordillera y que definen su culminaci6n estructural, ei sector centro y norte se caracteriza por pliegues que se presentan en trenes mucho menos espaciados que los altos estructurales del sector sur y, por tanto, por un mayor acortamiento de la cobertera cretacica en esta zona (mora y kammer, 1999). los domos del flanco oriental, que definen la culminaci6n estructural de la cordillera, muestran una asimetria variable, que en 34 un caso extremo esta dada por la presencia de una falla inversa en su borde oriental, que levanta un anti forme del bloque occidental. un ejemplo de tal situacion se tiene en el domo de chingaza (mora y kammer, 1999). en cuanto a los pliegues formados en la cobertera cretacica y terciaria del interior de la cordillera, se han propuesto modelos de una tect6nica epidermica, cuyas caracteristicas fundamentales se asumen como validas para toda la cordillera (mora y kammer, 1999). sin embargo, kammer y mojica (1996) sefialan que la dificultad principal de aplicar cualquier modelo de escamaci6n en la cordillera oriental radica en que no existe un contraste ideal entre el bas amento cristalino fragil y poco susceptible a plegarse y una cobertera sedimentaria con propiedades mecanicas muy distintas, separados entre si por un horizonte ductil de deslizamiento. por tanto, el 1110deja que prop one un despegue restringido a un horizonte particular debe ser abandonado en las areas donde exista una deformaci6n entre el sustrato precretacico y su cobertera (mora y kammer, 1999). datos se seleccionaron 505 sismogramas para un periodo comprendido de septiembre de 1993 a junio 2002. su selecci6n, producto de una extensa depuraci6n de registros tuvo criterios como la claridad de arribos de las fases p y s, y que la longitud de la coda garantizara como minimo 50 seg. los eventos seleccionados presentaron rangos de magnitud 1 < m < 5.5 y profundidades focales entre 0 y 30 km. la mayoria de los epicentros se localizaron a 10 largo del sistema de fall as del piedemonte llanero. a partir de los datos seleccionados, se escogi6 una area para hacer la inversi6n de datos, (figura i): '. / / "//r j /. .. ~+" .. s: *, <. • figura i. localizacion de los epicentras de los sismas utilizadas para calcular la tornografla de qc. geofislca colombiana, 6. diciembre de 2002 anausis espacial y temporal de q·coda en el piedemonte llanero (colombia) metodologia analisis espacial de qc para calcular el valor de qc se utiliz6 el mode 10 de dispersi6n isotr6pica simple de sato que asume un medio homogeneo con velocidad y densidad constante, donde las fluctuaciones son debidas a dispersores homogeneos distribuidos aleatoriamente; por ejemplo, fracturas, fall as, contrastes de densidad y an om alias de velocidad. de acuerdo con el modelo de sato, y asumiendo que las ondas coda s610 tienen ondas s dispersadas, la densidad de energia dispersada de la onda coda (es), por la n-esirna heterogeneidad, sera igual a es(r,t if) = w04(f~go k(a );-n/tlq rtr donde r es la distancia hipocentral; t es el lapso de tiempo medido desde el tiempo de origen del sismo; wo es la energia total radiada desde la fuente; go es el coeficiente total de dispersi6n; k(a) = (jla) in [(a + 1)/(a i)], (a> i); a = tits; ts es el tiempo de viaje de la onda directa s. linealizando la ecuaci6n (1), obtenemos inl(as)2 k(ti ts)t in[ 1 jw (t-ts) ac j nocrvt qc entonces el valor de qc se obtiene a partir de una simple regresion linear sobre sismogramas filtrados en unos rangos frecuencia seleccionados. siguiendo la metodologia empleada por aki y chouet (1975), se analizaron los sismogramas en cuatro bandas centradas en 1.5,3,6 y 12 hz con sus respectivos anchos de banda de 1,2,4 y 8 hz. por regresi6n linear se calcul6 el valor de qc a 1 hz. ademas, los valores de qc se calcularon para diferentes tiempos con el fin de rnuestrear la atenuaci6n de las ondas coda a diferentes rangos de distancia. regionalizacion de qc para estimar la variacion espacial de la atenuaci6n de las ondas coda (qc") en un medio tridimensional, se amplio la expresi6n propuesta por puli para modelar las amplitudes de las ondas coda por medio de elipses, a una expresion elipsoidal de la forma don de res la distancia entre la fuente y la estacion, v es la velocidad de la onda s, x, y son las coordenadas en superficie y t es el tiempo total que las ondas coda necesitan para recorrer la regi6n comprendida entre la estacion y el epicentro a velocidad constante con un coeficiente de atenuacion q~i,y es igual a tt=2t + v s 2 geofislca colombiana. 6, diciembre de 2002 tv es la ventana de coda 0 sea el tiempo usado para calcular el valor de qc' si una onda gasta un tiempo t, para recorrer una regi6n con un coeficiente de atenuaci6n q~/, el resultado efectivo de q~ies relacionado al camino de atenuaci6n por la relaci6n (vargas, 2000): (5) para un medio tridimensional, la expresi6n (5) se expresa como un hiperplano de la forma (i) (6) que relaciona el valor de q~ide cada elipsoide con la fracci6n de tiempo (t/t) respecto al tiempo total que las ondas coda necesitan para recorrer la regi6n (en este caso una celda de 3 dimensiones) con un valor de atenuaci6n q;ipara un evento registrado en una estaci6n. la anterior expresi6n se puede expresar en la forma ax = y, siendo a, x y y (2) a= t,iik ["k t" + ...+ 1+ l+ ...+ i1f11p t; 1; 1; (7) 1x= + ...+ + ...+o; a.; e., (8) (9) para determinar el valor de atenuaci6n en cada celda, se utiliz6 la inversion por minimos cuadrados amortiguados, don de el factor de peso es igual a una matriz diagonal de peso (02 ) que consiste en el error estimado en una observaci6n (e.) normalizado por el error medio (e") (3) analisis temporal de qc para estimar la variacion temporal de qc, inicialmente se estudi6 la distribuci6n temporal de los valores qc antes y despues de grandes eventos sismicos (magnitud mayor de 3.5). se espera que los minimos valores de qc (maximas atenuaciones) ocurran antes del evento principal, y que despues del even to principal (minimas atenuaciones) los altos ocurran. si esta relaci6n se cumple, confirmariamos la idea de que para este sector podemos interpretar los cam bios temporales de qc como cambios del estado de esfuerzos en la corteza. adernas de esta aproximaci6n cualitativa al problema de la variacion temporal de la atenuaci6n, se utiliz6 la dimensi6n fractal con el objetivo de cuantificar el agrupamiento temporal de los minimos valores de qc y correlacionar la variaci6n temporal del agrupamiento de qc con la ocurrencia de grandes sismos. (4) 35 duran et al. vinciguerra (200 i) sefiala que un rnetodo sencillo para calcular la dimension fractal (d) es el metoda de la integral de correlacion, si nr<r es el nurnero de pares de eventos separados por una distancia temporal menor que r, la integral de correlacion de una distribucion de n sismos se define como 2nc(r)= r«r n(n -1) (10) (ii) linealizando la ecuaci6n ii, se obtiene 10gc(r) ""d logr (12) por tanto, la dimension fractal (d) puede ser estimada a partir del mejor ajuste lineal de log c(r) vs. log r<r. si los eventos estan concentrados 0 agrupados en un espacio de tiempo muy pequefio 0 = o. £1 caso opuesto ocurre para distribuciones periodicas donde los eventos estan distribuidos igualmente en el tiempo; en este caso no habra agrupacion, y 0 = 1. sin embargo, para intervalos de muestreo temporal muy cortos, 0 sera igual a o y, por el contrario, para intervalos muy largos d tendera a i (latora, rapisarda y vinciguera, 1998). se calculo la dimension fractal para una ventana rnovil de 90 dias, y se asume que el 0 calculado correspond fa al tiempo del ultimo evento en la ventana. resultados y anausis de los resultados varia cion espacial de q;' los valores de atenuacion cal cui ados a partir de los sismogramas fueron filtrados e interpolados por el metodo de minima curvatura para suavizar la respuesta e intentar buscar la tendencia, obviando las anomalias. a partir de la inversion de las atenuaciones calculadas de los sismogramas, se creo un modelo tridimensional de atenuaciones (qc·l) para profundidades menores de 20 km. £1 tarnafio optimo se estimo de los val ores de la matriz de resolucion del modelo y del residual de la inversion. se analizaron tres casos extremos, con diferentes tamafios de grilla. ver tabla 2 y figura 2. tabla 2 parametros para la seleccion del tamano optimo de celda no. de tamaiio de la celda max. valor divisiones (kin) mat. de res. residual 5x6xl0 50.61 x 47.91 x 2.85 0.8 0.0733 8x9x 10 31.6 x 31.0x 2.84 0.6 0.0753 15x 16x 10 16.87 x 18.04 x 2.85 0.28 0.0775 36 a) b) c) figura 2. matriz de resoluci6n para una tomografia de atenuaci6n de a) 5 x 6 x 10 divisiones; b) 8 x 9 x 10 divisiones; c) is xl6 x 10 divisiones. ai analizar las graficas, se aprecia que el residual no varia significativamente entre la inversion en cada una de las grillas; por el contrario, el valor de la matriz de resolucion disminuye al aumentar el nurnero de celdas en la grilla. de acuerdo con menke (1989), cuanto mas cercano sea el valor de la matriz de resolucion a la unidad, el parametro calculado (en nuestro caso, la atenuacion en cad a celda) estara mas cerca del pararnetro real. por 10tanto, entre menor sea el numero de celdas mejor seria la respuesta, pero esto sacrificaria el detalle en el modelo. por tanto se decidio que el tamafio optimo de las celdas es de 31.6 x 31.0 x 2.84 km, donde el maximo valor de la matriz de resolucion es mas 0 menos 0.6, y el numero de eeldas en cada capa permite analizar en detalle la variacion espacial. geofislca colombiana, 6, oiciembre oe 2002 analisis espacial y temporal de q·coda en el piedemonte llanero (colombia) a) perfil no 1 b) ~ .. perfil no 2 perfil no 3 d) figura 3 a) atenuaci6n a 1.5 ian; b) perfil 1-1'; c) perfil 2-2'; d) perfil 3-3'. los resultados de ja variacion espacial qc-i, a partir de los sismogramas (figura 3), muestra una franja de anomalias de atenuacion con sentido sw ne, con una flexion hacia el se en la parte media, marcando cuatro anomalias de atenuacion de sur a norte, y una quinta anomalia en al parte inferior de la figura (sefialadas como a, b, c, dye, respectivamente). las anomalias no s610 varian lateralmente sino tam bien en profundidad. los val ores promedio de la atenuacion de las anomalias a y c, muestran un aumento en su atenuacion a medida que se profundiza de 0.026 a 0.030 y de 0.014 a 0.055, respectivamente. ademas, a medida que se prafundiza, la anomalia a se "desplaza" hacia el norte. por el contrario, las anomalias b y d disminuyen su valor de atenuacion con la profundidad. la anomalia b disminuye su valor de atenuacion promedio de 0.026 a 0.020 y se "desplaza" hacia el oeste. la anomalia d disminuye su atenuacion hasta el punto que a 19 km de profundidad desaparece la anomalia. a diferencia de las cuatro anomalias anteriores, la atenuacion de la anomalia e permanece mas 0 menos constante con un valor de 0.025. de acuerdo con el marco geologico y tectonico regional, la anomalia de atenuacion e se correlaciono con las rocas 'del basamento metamorfico andino y del paleozoico superior. la diferencia en la variacion de la atenuacion de las anomalias a y b plantea un problema que podria ser explicado si asurnimos que geofislca colomb lana. 6. oiciembre oe 2002 donde la atenuacion disminuye con la profundidad, la deforrnacion sola implico la cobertera cretacica (tectonica de piel del gada); por el contrario, donde la atenuacion aumento, con la profundidad, la deformacion implico la cobertera cretacica y el basamento precretacico (tect6nica de piel gruesa), conforme con el planteamiento de mora y kammer (1999). sin embargo, esto plantea otro problema: (,c6mo explicar que la anomalia a y b, que son adyacentes y que en superficie parecen estar controladas por la falla de guiacaramo, tengan dos tendencias tectonicas tan diferentes? de acuerdo con los resultados del trabajo de velasquez (2002), este problema se puede responder si asumimos la existencia de dos basamentos que se comportan mecanicamente diferentes: uno que absorbio la deformacion y se plego junto con la cobertera cretacica y otro mas competente que no se deformo y sirvio como rampa de despegue para la cobertera cretacica. otra solucion al problema la aporta el trabajo rowan y linares (2000), que demuestra, a partir de la interpretacion de datos sismicos, que la falla agua clara 0 guiacaramo se profundiza hacia el norte, sacando rocas mas antiguas al norte, mientras al sur s610 afecta las rocas masjovenes. por tanto, esto explicaria por que la anomalia a aumenta su atenuacion con la profundidad, mientras la anomalia b no la aumenta, ai igual que la anomalia a y b, la anomalia c y d se diferencian por su variacion de la atenuaci6n en prafundidad. esto puede ser explicad por una deformacion homogenea de basamento y cobertera en la anomalia c, mientras que la anomalia d marcaria una zona donde el basamento no se deform a y sirve como despegue de la cobertera. es importante sefialar que la ubicacion geografica de la anomalia c coincide con el area del domo de chingaza, donde mora y kammer (1999) sefialan que hay una concordancia entre las deformaciones y el plegamiento que afecta al basamento precretaceo y la cobertera cretacica. variac/on temporal de qc entre enero 1995 hastajunio del 2002, se correlacionaron los cambios en los valores minimos de qc (rnaximas atenuaciones) con la ocurrencia de eventos de magnitud mayor de 3.5. las variaciones mas drasticas de la tendencia temporal de qc son descritas a continuacion (figura 4). a partir de enero de 1995, los valores qc aumentan hasta un valor maximo, para luego volver a disminuir hasta octubre de 1996 (ver a, en la figura 4). el aumento del valor de qc coincidio con la ocurrencia de tres eventos de magnitud mayor de 4.5; la disminucion de qc, con la ocurrencia de eventos men ores de 3.7. entre marzo de 1997 hasta abril de 1999, la tendencia de los valores de qc fue disminuir; sin embargo, el sismo de magnitud mayor de 4.5, en febrero de 1998, marco un pequefio aumento en el valor de qc (ver b, en la figura 4). ei valor de qc vuelve a aumentar hasta octubre del 2000. sin embargo, esta tendencia puede dividirse en dos: antes y despues de 37 dunan et al. 3:;0 ::: °olm~~~~~~~oo~1~1~lml~l~lboo1~i~w~1160=~ffio~~~woo~iw l~~o(d!;os) figura 4. variacion temporal de qc vs ocurrencia de sismos de magnitud mayor a 3.5. la ocurrencia de dos sismos de magnitud mayor de 4.5 en junio de 1999 (ver c, en la figura 4). desde febrero de 200 i hasta junio de 2002, hay una tendencia a aumentar el valor de qc (ver d, en la figura 4). aunque se aprecia una correlaci6n entre los sismos de grandes magnitudes (mayor de 3.5) y los mfnimos valores qc (menores de 170), todavfa no podemos concluir nada acerca de la evoluci6n de la atenuaci6n en el tiempo ni caracterizarla. debido a esto, se dec idi6 calcular la dimensi6n fractal para la distribuci6n temporal y determinar si la distribuci6n en el tiempo es aleatoria, peri6dica 0 agrupada y, en ultimo caso, si hay agrupaci6n de qc, y determinar si estan relacionados con los eventos de mayor magnitud. el analisis de la dimensi6n fractal d para un rango de qc menores de 170 con ventana m6vil de 90 dias y un r<r: 30,60 y90 dias, mostr6 (ver figuras 5a, 5b y 5c). las variaciones relativas de d, muestran una tendencia de la disminuci6n de la dimensi6n fractal antes de la ocurrencia de un gran evento; por tanto, los valores de qc se agrupan en el tiempo. despues de la ocurrencia del evento, la dimensi6n fractal vuelve a aumentar su valor, 0 sea que la distribuci6n .de qc en el tiempo es aleatoria. de acuerdo con los resultados obtenidos con la correlaci6n cualitativa entre qc y los sismos de gran magnitud,junto con el analisis de la variaci6n de la dimensi6n fractal en tiempo, se interpret6 la disminuci6n de qc (aumento en la atenuaci6n) y su agrupamiento en el tiempo como cam bios en la estructura heterogenea de la corteza (en nuestro caso fracturas y fallas) y por tanto un aumento en el estado de esfuerzos. ei aumento de qc (disminuci6n de la atenuaci6n) y el aumento en la dimensi6n fractal se interpreta como una liberaci6n de esfuerzos. 38 ,+ 'l' "r":' .o z • e 3.5'3./ 3.74 4-.5 ~ 5 co loll) ,''''''''(0''') "" .':,,..: i a ::: i'i'.".::: 'n'" :::w::i~ ~~i~=,"oo o ~ :: 0., :: [t7'fl'• 31·'· ~...., 9 • 4,~ 0 figura 5. analisis de la evolucion temporal de d, con una ventana movil de 90 dias y a) r<r: 30, b) r<r: 60 dias y c) r<r: 90 dias. conclusiones a partir de los resultados del trabajo se lleg6 ados grandes conclusiones: lnicialmente se esperaba que la variaci6n espacial de la atenuaci6n (qc-l) fuera un reflejo del fracturamiento de las rocas; sin embargo, para esta zona, la variaci6n de qc-i marca el contraste entre geofislca colombiana, 6, diciembre de 2002 anallsis espacial y temporal de q·coda en el piedemonte llanero (colombia) dos tipos de deformacion: homogenea e inhomogenea. en el primer caso, existe concordancia entre las deformaciones y el plegamiento entre la cobertera cretacica y un basamento precretacico (tect6nica de escamaci6n gruesa), y que se expresa en un aumento de la atenuaci6n con la profundidad. otra deformaci6n de tipo de escamaci6n delgada, donde la deformaci6n s610 afecta la cobertera cretacica, se presenta como una atenuaci6n que disminuye con la profundidad. por tanto, la atenuaci6n de qc-l permite discriminar las zonas con mayor anisotropia en la zona. la variaci6n temporal de qc representa el cambio temporal del estado de esfuerzos de [a corteza y, por tanto, debe esperarse que la atenuaci6n aumente antes de la ocurrencia de un sismo de gran magnitud, que para esta zona correspondi6 a sismos de magnitud mayor de 3.5. despues del evento principal, al disminuir los esfuerzos, la atenuaci6n disminuye. bibliografia aki, k., and chouet, b. 1975. origin of coda waves: source, atennuation, and scattering effects. journal of geophysical research 80: 3322 3342. ingeominas. 1998. estudio general de amenaza sismica de colombia. segunda edicion. bogota. kammer, a. y mojica, j. 1996. una cornparacion de la tectonica del basamento de las cordilleras central y oriental. geologia colombiana, (20): 93-106. kammer, a. 1999. observaciones acerca de un origen transpresivo de la cordillera oriental. geologia colombiana (24): 29-47. geofislca colombiana. o. diciembre de 2002 latora, v.; rapisarda, a. y vinciguerra, s. 1998. a fractal approach to the temporal distribution of microseismicity at low eastern of mt. etna during 1989-1994. physics on the earth and planetary interiors (109): 115-127. menke, william. 1989. geophysical data analysis: discrete inverse theory. academic press inc. mora, a. y kammer, a. 1999. cornparacion de los estilos estructurales en la seccion entre bogota y los farallones de medina, cordillera oriental de colombia. geologia colombiana (24): 55-82. paris, g.; machette, m. n.; dart, r. and haller, k. m. 2000. map and database of quaternary faults and folds in colombia and its offshore regions. open-file report 00-0284. u.s. department of the interior. u.s. geological survey. rowan, m. g. y lrnares, r. 2000. fold evolution matrices and axial surface analysis offault-bend folds: aplicattion to the medina aticline, eastern cordillera, colombia. aapg, 84(6): 741-763. vargas, c. a. 2000. atenuacion de ondas coda en el territorio colombiano. ingeominas, observatorio vulcanol6gico y sismologico de manizales. velasquez, a. 2002. modelamiento geofisico cortical, por medio de metodos de campos potenciales, con base en un modelo estructural del piedemonte llanero, colombia. trabajo de grado dpto. de geociencias. universidad nacional de colombia. vinciguerra, s. 200 i. deep earthquakes as a precursor of the september-october 1989 flank eruption at mt. etna volcano (italy). benfield greig hazard research centre, department of geological sciences, university college london, gower street, london. 39 geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 76-87 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15 th of may city, helwan, egypt magdy a. atya1, olga a. khachay2, mamdouh m. soliman1, oleg yu khachay3, ahmed b. khalil1, mahmoud gaballah1, fathy f. shaaban1,4 , ibrahim a. el hemali1 1 national research institute of astronomy and geophysics (nriag), 11722 helwan, egypt. 2 institute of geophysics, russian academy of science (ras), ural division, russia. 3 ural state university, russia. 4 king khaled university, faculty of science, physics department, abha, saudia arabia. magdy@igse-eg.com abstract in the present work, we involve the control source electromagnetic (csem) technique to image the dynamic migration center of the near surface fractures, fissures, and cracks in a new dwelling area at 15th of may city close to cairo. this area forms the center of the zonal weakness of the subsoil, which in turn, interact with the weight center of the construction leading to catastrophic collapses. the control source electromagnetic technique has been developed recently to monitor the migration of the weakness center represented as the accumulative fissures and cracks in the near surface. three composite profiles of wide and planshet mesh data have been collected in 2008. this survey has been followed by performing two profiles in 2010; one of these profiles has been repeated to observe the situation changes, and the second profile has been measured between the other two profiles of 2008. the last profile had been performed to study the mutual relation between the measurements of the two cycles. the objectives of the whole process are to validate the capability of the technique to pick the minor changes of the weakness center, consequently, study its relation to the weight center of the adjacent construction to produce a recommended procedure to minimize the destruction resources at the site of investigation. the measured data has been interpreted and represented in graphs showing the distribution of the heterogeneity of the geoelectric parameters in the subsoil, furthermore, in a series of geoelectric cross section representing the applied frequencies used during the survey. the study concludes that (1) the center of the cracked zone is moving upward closer to the surface of the ground and heaver, (2) the water content is moving downward producing soil dryness at the shallow depths, and (3) the site became more stable in 2010 than 2008, however, the destruction resources remain warning with collapse events. furthermore, the situation reflects the relation between the water content and the changes in the weakness center. key words: electromagnetic technique, en helwan, weakness center, control source electromagnetic (csem) 76 manuscript received: 28/03/2010 accepted for publication: 20/05/2010 resumen en el presente trabajo, aplicamos la técnica de fuente electromagnética controlada (csem) para obtenerla imagen del centro dinámico de la migración de las fracturas, fisuras y grietas de la superficie somera en una nueva área de vivienda en la ciudad de 15 de mayo cerca a el cairo. esta zona constituye el centro de la debilidad del subsuelo, que a su vez, interactúacon el centro de peso de la construcción que ha llevado a colapsos catastróficos. la técnica de control de fuente electromagnética se ha desarrollado recientemente para monitorearla migración del centro de la debilidad, representados por la acumulación de fisuras y grietas de la superficie somera. tres perfiles compuestos se han recogido en 2008. este estudio se ha seguido realizando con dos perfiles en 2010. uno de estos perfiles se ha repetido para observar los cambios, y el segundo perfil se ha medido entre los otros dos perfiles de 2008. el último perfil se ha realizado para estudiar la relación mutua entre las mediciones de los dos ciclos. los objetivos de todo el proceso es validar la eficacia de la técnica para recoger los mínimos cambios del centro de la debilidad, y así, estudiar su relación con el centro de peso de construcciones cercanas, para recomendar un procedimiento que reduzca al mínimo la destrucción de los recursosen el sitio de la investigación. los datos medidos se haninterpretado y representado en gráficos que muestran la distribución de la heterogeneidad de los parámetros geo-eléctricos en el subsuelo; además, en una serie de seccionescruzadas geoeléctrica se representa las frecuenciasutilizadasdurante el estudio. el estudio concluye que (1) el centro de la zona agrietada se mueve hacia arriba cerca de la superficie de la tierra, (2) el contenido de agua está bajando, produciendola sequedad del suelo apoca profundidad, y (3) el sitio se volviómás estable en 2010 que en 2008, sin embargo, los producto de la destrucción siguen susceptiblesa colapsos.además, la situación refleja la relación entre el contenido de agua y los cambios en el centro de debilidad. palabras clave: técnica electromagnética, en helwan, centro de debilidad, control de código fuente electromagnética (csem) 1. introduction the idea behind the present paper reflects the capability of the applied and environmental geophysics to be involved in the community problems. accordingly, it is strongly recommended to establish a monitoring system for the detection of possible occurrence of disasters. currently, various kinds of geophysical systems are being operated for environmental monitoring purpose and detecting the subsurface temporal changes. the problem under investigation is related to the unstable subsurface. the subsurface heterogeneity caused by the intensity of the fractures, the fissures, the cracks, and the water content, is a factor of the instability level of the base ground beneath the constructions. such instability of the rock status resulted in several destructions at many places such as the stone block fallen at al deweaka (fig. 1) and at the second level of al mokkatam plateau, furthermore, the constructions deformation like that happened in the 15th of may city, quarter 27 (fig 2), and land subsidence like that occurred at several parts of wadi hof province and along the autostrade form al maadi to the margins of 15th may towards al tebbeen. the research layout is devoted to observe and monitor the near surface structures and to follow the growth of the intensity of the fractures, the fissures and the cracks. the water content and its salinity is also an important factor to be observed and followed. it is planned that, the observation and monitoring phase will run arbitrary at the start of the work to calculate the optimum measurement circulation comparable to the cyclic stress forces or loads affecting the near surface structures. then the situation will be subject of vector analysis to provide a management procedure to reduce the destruction resources and overcome or at least minimize the collapse. geophysics can easily detect the presence of different objects in wide range of applications. however, if those geophysical measurements are repeatedly obtained and compared, additional temporal dimension analysis makes interpretation and characterization of target much easier (hachay, 2004, hachay et al., 2007). the csem approach involves the possibility to survey the subsurface in volume block as wide profiling and planshet pickets along profiles. therefore, the technique is capable to produce an image of the subsurface representing 77 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt the near surface structural setting of the fissures, cracks, fractures, and also the hydrologic situation. getting enough information about the structural image of the surface including the heterogeneity intensity and the concentration depth of the fractures, cracks, and the development of their growth, and also the water content will help to provide an objective procedure assures the best management of the site, therefore, the catastrophic and dramatic accidental results will be minimized. the technique is proposed to be applied on a site with destruction resources that is warning with collapse, the site has been measured in 2008 and repeated in 2010. the comparative study of the subsoil images obtained in the two cycles represents enough information about the subsurface structural changes occurred during the time interval between the two cycles. the study concludes that, the center of weakness zone is moving upwards and became denser as the water saturation became less in 2010 than 2008. that means that the fracture contacts at the subsurface became closer to the surface and became heavier, while the water is moving downwards. the technical impact of this result is the site became more stable but still dangerous and is subject to collapse. 2. objectives the expected objectives and outcome of the present paper could be declared in the following items: 1) full understanding of the site of investigation, proposed here which is the quarter 27 at 15th of may city, and a more instable site in al mokkatam plateau. 2) a documentary archive of the situation including the hydrologic setting, the subsurface structure, the geoelectric heterogeneity move of the center, and upgradable conclusions. 3) vector analysis with locations of the forces applied to the area in reference to the center of heterogeneity. 4) a monitoring protocol and a management procedure for the site of investigation. 5) and finally the main object of the research which is the overcome or at least reduce the catastrophic impact via pre read of the destruction resources and probably prediction of the accident. 78 magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, ibrahim a. el hemali figure 1. accidental fall down of stone block ad al deweaka, 2008. 3. geological background and site description the city of 15th of may is constructed over a limestone plateau on the eastern bank of the river nile (fig. 3). this plateau is covered mainly with wadi garawi and el-qurn formations of middle eocene age. el-qurn formation is of about120 m thick and is composed of chalky and marly limestone intercalated with shale, sandy marls and shale banks (strougo, 1985; said, 1990). in addition, these formations are highly jointed, highly fossiliferous, vuggy and sandy dolomitic limestone. structurally, the area is affected by three major sets of faulting systems striking mainly in nw-se, e-w, and ne-sw directions. all of them are normal faults, with dip ranges from 7u to 40u, where the beds are locally dragged. in addition to faults, there are at least many sets of joints dissect the area (moustafa et al., 1985). these joints trend in e-w and nw-se directions (moustafa et al., 1985; farag, and ismail, 1959). the major fractures in the area around cairo and its vicinity were interpreted based on satellite image of large scale (el-shazly et al., 1980). these fractures were analyzed statistically and each trend of fractures sets was treated separately. it was found that there are three main trends which are ew, wnw and nw. 4. theory and mathematics of csem 4.1 principal the csem provides an enhanced geophysical technique and device, in addition to, the precise analytical interpretation of the geophysical complexes for subjects such as the topic of this research, especially when the primary information about the site is not enough. the device has been established to use the planshet method of electromagnetic induction (hachay, 1997a, hachay and bodin, 1997b, hachay et al., 1999, and 2000, hachay, 2002) in the frequency domain. the method was adapted to map and monitor the high complicated geological mediums, to determine the structural factors and criteria of stability of the rock massif in the mine subsurface. the field observation and the way of interpretation make the new technology differ from other known earlier methods of field raying or tomography (atya et al., 2010, hachay and novgorodova, 1997c, hachay et al., 1999, and 2000, lau and cheng, 1977). the concept to construct the 3d geoelectric medium is based on interpreting the alternating electromagnetic field in the frame of a block-layered isotropic medium with inclusions over three stages (hachay 1997a, and 2002); in the first stage, the geoelectric parameters of the horizontal block-layered medium, which includes the block heterogeneities, are defined, in the second stage, a geometrical model of the different local heterogeneities or groups inside the block-layered medium is constructed based on the data of local geoelectrical heterogeneities, while in the third stage, the surfaces of the searched heterogeneities could be calculated in account of the physical parameters of the anomalous objects. 4.2 data collections the csem s*data collection is a measure of the geoelectric parameters of a medium, through which the electromagnetic waves propagates from the transmitter to the receiver. at each location of the transmitter and re79 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt divergence joints figure 2. constructions showing the joints of divergence at quarter 27, fifteenth of may city. ceiver, the three components of the electromagnetic field are measured. figure (4) explains the layout of the acquisition process. (a) shows an arbitrary position of the transmitter and receiver during a wide profile survey, (b) shows the measurement of the vertical component z, while the receiver coil is placed vertical, (c) shows the measuring of the first horizontal component h1, while the receiver coil is placed horizontal at right angle with the line between the transmitter and the receiver, and (d) shows the measuring of the second horizontal component, while the receiver coil is placed horizontal inline coaxial with the line between the transmitter and the receiver. the transmitter can inject more than a controlled frequency to scan different depths at the survey area. the measuring procedure could be classified into tow configurations, in correspondence to the objects of the survey. planshet data collections; is a procedure to collect the electromagnetic response over a 2d plan underneath the line between the transmitter and the receiver. figure (5) shows the process of gathering the planshet data. in this procedure, the transmitter and receiver are placed on the same line. the transmitter is placed at its first station, as the receiver moves stepwise at pre defined pickets along the profile. then the transmitter is moved to the next station and the receiver repeat the stepwise movement along the profile. this process is repeated until the last transmitter station. wide profile data collection; is a procedure employed to measure the electromagnetic response over a volume, therefore, it may be called volume csem. in this procedure, the transmitter is placed on a line, while the receivers are placed on a parallel line. both of the transmitter and receiver stations on the lines are marked as pickets. the survey volume is margined with the two lines. the data collected on two phases, the first is measures as the transmitter is placed on a line and the receiver is placed on a parallel line (fig. 6a), while in the second phase, the lines are exchanged and the transmitter is placed on the receiver line and vise versa (fig. 6b), the combination of the two phases produces the final image of the volume block. 4.3 interpretation for each array and fixed frequency ù two interpretation parameters are defined: � � � � �eff r r hz hr r h hr( ) ( / ) ( ) ( / ) %� �2 100 where ñeff (r) is the apparent resistivity, r is the distance between the source and the point of observation, ù is the frequency f multiplied by 2ð, �hz� is the module of the vertical magnetic component, �hr� is the horizontal component of magnetic field oriented to the source, ä(r) is the parameter of geoelectrical heterogeneity, and �hö� is the second horizontal component of magnetic field perpendicular to the direction of the source. 80 magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, ibrahim a. el hemali quarter 30 299.5n 299.5n 299n 299n 0 200m 111.0 112.0 111.9 110.0 110.0 105.0100.7108.0 108.0 120.0 59.2 112.0 103.0 100.0 100.0 buildings roads mediterranean sea cairo 15th of may city nile delta quarter 27 34 3e survey site 34 3. 5e sur ve y sit e 34 3. 5e egypt location map s uez g ulf 34 3e figure 3: location map of quarter 27 at 15th of may city. that data are the information base for the further interpretation. on the first stage we define the geoelectrical parameters of the 1-d section for each array and each frequency after the preliminary filtration of the data: ñeff (r): ä(r)<a where, a is a level of data filtration to the area of inverse problem operator for one-dimensional medium (hachay, 1997a). the interpretation is made in a frame of n-layered model for each array and planshet location. after that each point of the planshet is associated with one and only column of layers thicknesses and corresponding fixed column with resistance of the medium in that layers. gathering information of all planshets together we obtain a many-valued function of each point – distribution of thicknesses and resistances of the medium layers. then we calculate the average value for these distributions for each point of the observation set. thus we obtain the unique distribution of thicknesses of horizontal layers and 81 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt (a) (b)(c)(d) transmitter receiver (a) the transmitter receiver layout. (b) measuring the vertical component, the coil is vertical. (c) measuring the first horizontal component, the coil is horizontal and prependicular to the trannsmitter. (d) measuring the second horizontal component, the coil is horizontal and inline to the trannsmitter. second horizontal component first horizontal component vertic al component tr layouth for one reading figure 4. collection of the three component electromagnetic field in csem technique. ground surface subsoil medium interface transmitter receiver tr rays figure 5. the layout of the planshet data collection. resistances, which corresponds to the medium model as a cylinder with vertical axis and with a rectangle at the bottom and with a point of observation located in its center. thus we change over layered model to a block-layered model. then, gathering the values of thicknesses and resistances for all points of observation, located on one and the same profile we obtain the file of an average cross-section along the profile. the next step is combining the neighboring blocks with close-range values of resistance to one block. that operation is made according to the fixed scale of resistance. the second stage of interpretation is used to define the geometrical characteristics of conductive inclusions and their equivalent moments, which are proportional to the ratio of the conductivity difference in the host rock and in the inclusion to the conductivity in the host rock. here the approximation principle is used for alternating electromagnetic fields. the initial model of the inclusion is a current line of fixed length. that approximation construction is used for fitting of the average parameter of geoelectrical heterogeneity, which is calculated as an average value of ä in each point of the profile, located in each point of the profile (hachay, 2002). when the survey has been repeated to estimate the changing of the rock state, during the interpretation, we take all parameters from the previous cycle as the initial values for interpretation data for the next cycle. 5. field measurements in the present paper, two comparative data sets have been collected; the first data set has been obtained during the spring of 2008, and the second has been collected in the winter of 2009/2010. the first data set is composed of three wide profiles (fig. 7) employed to investigate the subsurface among the deformed buildings at quarter 27, fifteenth of may city, the applied frequencies are 80.15, 40.62 and 20.3. the second data set is composed of two wide profiles (fig. 7); the first profile is a repetition profile and the second is a new measured profile laid between two profiles of 2008. 82 magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, ibrahim a. el hemali t1t2t3t4t5t6 transmitters line receivers line r1r2rn r1r2rn receivers line t1t2t3t4t5t6 transmitters line a b figure 6. volume csem data collection along wide profile. the applied frequencies to the system during the two cycles were: 40.62, 20.3, and 10.15 khz. 6. interpretation the two data sets have been analyzed and converted into graphs and cross sections representing the distribution of the geoelectric heterogeneity of the subsoil. in the present paper we give more consideration to the comparative tasks to estimate the discrepancies in the electromagnetic image that might have been occurred with the time interval of the two cycles. therefore, two comparative studies will be discussed; the first will be the repeated profile that has been measured in 2008 and 2010. the second will be between a profile from the data set of 2008 and another from the data set of 2010 measured between the profiles of 2008. firstly; the two profiles represent the same medium while the measurements have been taken over the same pickets in 2010 as in 2008. figure (8) shows the comparative distribution of the average parameter of the geoelectric heterogeneity, it elucidates that the level is higher in range for the cycle of 2010 than the cycle of 2008. this could be interpreted in reference to the soil material that the subsurface beneath the measured profile became less conductive and more resistive, which might be contributed to the migration of the underground water far away either laterally or into deeper depths. the graphs in figure (9) represent the deeper frequency 20.3 khz for the wide profile measured in 2008 and 2010. it could be easily noticed that, the medium became more resistive in 2010 than 2008. the center of the crack concentration at each picket became more regular, parallel, and closer to the ground surface. the start of the profile shows an area (pickets 1-7) where the cracks became denser making the zone more instable, while around the end of the profile; the crack situation produced more stability. the two geoelectric sections of the frequency 40.62 khz (fig. 10), they show that the irregularity is, generally, less, furthermore, it became more regular, but denser and closer to the ground surface in 2010 than 2008.the water content of the general medium became also less producing more resistive soil. secondly, the comparative study between a profile measured in 2008 and another measured in 2010 representing two different mediums. figures (11, 12 and 13) show that, the cross sections of the wide profile (7-8) are more conductive and the disintegration zones are more intensive and are located deeper than of the wide profile (3-4) (fig. 7). the state of the massif 83 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt 1 3 5 7 9 11 13 15 17 19 21 4 10 2 4 6 8 10 12 14 16 18 20 1 7 1 3 5 7 9 11 13 4 1 3 5 7 9 11 13 15 17 4 10 0 10 20 30 40 50 60 70 80 90 100 110 120 0 10 20 30 40 50 60 70 80 90 100 110 120 m wide profile (1-2) wide profile (7-8) wide profile (3-4) 2010 2008-2010 2008 2 0 0 8 w id e pr of ile (5 -6 ) m figure 7. location map of the data sets measured in 2008 and 2010. along the new profile can be estimated as less stable than for the wide profile (1-2) and for the wide profile (3-4). the cross sections give a quantitative informative layout about the structure and state of the massif. having more cycles of observation, a stability table of stability of the rock state will be constructed. 7. conclusion the present work involves the control source electromagnetic approach to use the multi-frequency function for observing the near surface parameters and monitoring the change of geoelectric heterogeneity distribution. the techniques utilizes a series of fixed frequencies to probe the subsurface using the planshet picket profiling to measure the integrated image in depth plan beneath the transmitter to receiver distance, and the wide profiling to investigate the 3d block of the electromagnetic induction between the transmitter line to the receiver line. this survey way provides high resolution informative image about the depth of investigation. the area of investigation is laid at the quarter 27, fifteenth of may city, where the shallow surface of the ground is highly fractured producing a level of danger on the constructions and the block buildings at many sites of the quarter. the move of the fracture center and its combination with the center of weight applied on the ground by the construction loads results in the change of the destruction resources which might lead to collapse, landslide, or divergence over the joints of the blocks. two cycles of measurements have been collected in 2008 and 2010; the first was composed of three wide profiles investigated the area among the block buildings of quarter 27. the selected location of the profiles has been selected to cover areas where the constructions show divergence over the block joints and landslide and fractures. the second cycle of observation has been collected in the winter of 2009/2010 in form of two wide profiles; one of them is a repeat of one of the profiles of 2008 while the second is a new measurement, but lay between the profiles of 2008. 84 magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, ibrahim a. el hemali 5 10 15 20 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 n 5 10 15 20 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 n 2 0 1 0 20.3 khz 40.62 khz 2 0 0 8 20.3 khz 40.62 khz deeper shallower figure 8. the comparison of the distribution of the average parameter of geoelectrical heterogeneity for 2 cycles of monitoring 2008 and 2010, quarter 27. 85 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt -2 -7 -12 -17 -22 -27 -32 -37 -42 0 10 20 30 40 50 m 60 70 80 90 1 3 5 7 9 11 pk 13 15 17 19 1 3 5 7 9 11 pk 13 15 17 19 -2 -7 -12 -17 -22 -27 -32 -37 -42 0 10 20 30 40 50 m 60 70 80 90 mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more frequency 20.62 khz 2008 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (0 m m). frequency 20.62 khz 2008 figure 9. the repetition wide profile at quarter 27, fifteenth of may city with 20.3 khz. 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (0 m m). 1 3 5 7 9 11 pk 13 15 17 19 -2 -7 -12 -17 -22 -27 -32 -37 -42 -2 -7 -12 -17 -22 -27 -32 -37 -42 0 10 20 30 40 50 m 60 70 80 90 frequency 40.62 khz 2008 frequency 40.62 khz 2008 mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more 0 10 20 30 40 50 m 60 70 80 90 1 3 5 7 9 11 pk 13 15 17 19 figure 10. the repetition wide profile at quarter 27, fifteenth of may city with 40.62 khz. 500 450 400 350 300 250 200 150 100 50 0 2 4 6 8 10 12 14 16 18 20 n 81.25 khz 40.62 khz 20.3 khz a figure 11. distribution of the average parameter of the geoelectric heterogeneities using the electromagnetic induction multi-frequency technique, a) wide profile (3-4) of 2008, and b) wide profile (7-8) of 2010. 220 200 180 160 140 120 100 80 60 40 20 0 40.62 khz 20.3 khz 10.15 khz 2 4 6 8 10 12 14 16 n b the data of the two cycles has been analyzed and comparatively interpreted to show the subsurface situations and enhance the relation between the two cycles. a series of 2d sections and analytical graphs have been constructed to show the distribution of the average geoelectric parameters and the fracture and crack distribution considering their weight over the depth of investigation. the interpretation of the two cycles showed that the technique is capable to image the subsurface change recorded in the two data sets. the results showed that the fractures and cracks became denser and their center is moving upwards to the ground surface, while the water content became less and moved deep ward. the rock state became more resistive and stable in 2010 than 2008, but is still suffering from the destruction resources. 8. recommendations it is high recommended to repeat the measurement over arbitrary time interval until reaching the optimum cycling of the force applied on the ground surface and cyclic motion of the center of the cracks and fractures. 86 magdy a. atya, olga a. khachay, mamdouh m. soliman, oleg yu khachay, ahmed b. khalil, mahmoud gaballah, fathy f. shaaban, ibrahim a. el hemali 1 3 5 7 9 11 13 15 17 19 21 -42 -37 -32 -27 -22 -17 -12 -7 -2 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (om"m) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 -42 -37 -32 -27 -22 -17 -12 -7 -2 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 pk m m 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (om"m) a b mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more figure 12. geoelectrical section: a) along the wide profile (3-4) of 2008), and b) wide profile (7-8) of 2010) using the frequency 40.62 khz. 1 3 5 7 9 11 13 15 17 19 21 -42 -37 -32 -27 -22 -17 -12 -7 -2 0 10 20 30 40 50 60 70 80 90 100 pk m m 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (om"m) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 -42 -37 -32 -27 -22 -17 -12 -7 -2 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 pk m m 0 20 50 90 150 200 300 400 500 750 1000 2000 5000 or more (om"m) ba mo~ less .1 .1 .2 .2 .5 .5 .7 .7 1 1 2 2 4 4 6 6 10 10 or more figure 13. geoelectrical section: a) along the wide profile (3-4, 2008), and b) wide profile (7-8, 2010) using the frequency 20.3 khz. 9. acknowledgement the work has been done in cooperation of researchers from the national research institute of astronomy and geophysics (nriag), helwan, egypt and researchers from the institute of geophysics (igf), ural division (ud) of russian academy of sciences (ras), russia. the work has been financed by the local resources of nriag and the instrument belongs to igf. the authors thank the cooperation of the city office of 15th of may city. also, thanks are given for miss mona abu shady and the helping team for collecting the data of the first cycle. 10. references atya m. a., hachay o. a., abdel-lattif a. a., khachay o. yu., el-qady g. m., and taha a. i., 2010: geophysical contribution to evaluate the hydrothermal potentiality in egypt: case study: hammam faraun and abu swiera, sinai, egypt, earth sciences research journal, in press. el-shazly, e.m., el-ghawaby, m.a., salamn, a.b., khawas, s.m., el-amin, h., el-rakaiby, m.m, el-aassy, i.e., abdel-megid, a.a., and mansour, s.i., 1980: geological map of egypt, remote sensing center, academy of scientific research and technology, cairo, egypt. farag, i., and ismail, m., 1959: contribution to the stratigraphy of the wadi hof area (northeast helwan): bull. fac. sci., cairo univ., 34, 147–168. hachay o. a., 1997a: the three stage concept of common interpretation for 3-d electromagnetic and seismic fields and some results of its practical realization, engineering and environmental geophysics for the 21st century, china, chengdu 286-292. hachay o. a., bodin v. v., novgorodova e.n., bodin vd. v., hachay a. yu., and hinkina t.a., 1997b: a new complex near-surface electromagnetic and seismic technique for 3-d research of inhomogeneous and unstationary medium, engineering and environmental geophysics for the 21st century. china, chengdu 181-189. hachay o. a. and novgorodova e. n., 1997c: the experience of area induction research of sharp heterogeneous geoelectrical medium, fizika zemli 1997, 5, 60 -64. hachay o. a., novgorodova e. n, and bodin v. v., 1999: about the problems of near surface geoelectricity and some results of their solution, fizika zemli, 1999, 5, 47–53. hachay o. a., novgorodova e. n.and hachay a. yu., 2000: mapping of 3-d conductive zones with use of area systems of observation in a frame of 3-d frequency-geometrical method, geology and geophysics, 41, 9, 1331–1340. hachay o. a., 2002: new conception of active and passive electromagnetic 3-d monitoring of volcanoes and active faults, iii international workshop on magnetic, electric and electromagnetic methods in seismology and volcanology, moscow, ras 169 –171. hachay o.a., 2004: to the question of structure and state research of geological heterogeneous nonstationary medium in a frame of discrete and hierarchic model, russian geophysical journal, 33, 34, 32-37. hachay o.a., and hachay o. yu., 2007: research estimation and classification of stability of geological medium with use of data of active geophysical monitoring on the base of physical mesomechanics idea, physical mesomechanics, 10, 2, 87-92. hachay o.a., 2007: research of nonstability of the rock massive with use the method of active electromagnetic monitoring, physics of the earth, 4, 65-70. lau k. h. and cheng p., 1977: the effect of dike intrusion on free convection in conduction-dominated geothermal reservoirs, intern .j. heat and mass. trans., 20, 11, 197–209. moustafa, a., yehia, a., and abdel tawab, s., 1985: structural setting in the area east of cairo, maadi and helwan: middle east res. centre, ain shams univ., sci. res. series, 5, 40–64. said, r., 1990: geology of egypt, balkema pub., rotterdam, netherlands. strougo, a., 1985: eocene stratigraphy of the eastern greater cairo (gebel mokattam-helwan) area: middle east res. centre, ain shams univ., sci. res. series, 5, 1–39 87 csem imaging of the near surface dynamics and its impact for foundation stability at quarter 27, 15th of may city, helwan, egypt articulo 6.indd earth sciences research journal seismic processing earth sci. res. sj. vol. 15, no. 2 (december, 2011): 121 -128 thin-layer detection using spectral inversion and a genetic algorithm kelyn paola castaño1, germán ojeda2 and luis montes1 1 universidad nacional de colombia, facultad de ciencias, departamento de geociencias, sede bogotá, colombia. e-mail: kpcastanog@unal.edu.co, lamontesv@unal.edu.co 2 ecopetrol s.a. bogotá, colombia. e-mail: g.ojeda@ecopetrol.com.co abstract spectral inversion using a genetic algorithm (ga) as an optimisation approach was used for increasing the seismic resolution of a particular dataset; by contrast with the conjugate gradient method, a ga does not require a good starting model but rather a search space. the method discriminated layers thinner than λ/8 when tested on synthetic and log data. when applied to a seismic dataset concerning the barco formation in the catatumbo basin, colombia, spectral inversion led to recovering information from seismic data contributing towards the vertical identification of geological features such as thin distributary channels deposited in a deltaic environment having a tidal influence. the results revealed that a ga outperformed traditional minimisation schemes. resumen la inversión espectral, a partir de un algoritmo genético fue usado como estrategia de optimización, e incremento de la resolución de un volumen sísmico. contrario al método del gradiente conjugado, el algoritmo genético no requiere un buen modelo inicial sino un espacio de búsqueda. usado en datos sintéticos y registros de pozo el método discriminó capas tan delgadas como λ/8. al usarse en un volumen de la formación barco en la cuenca del catatumbo, colombia, la inversión espectral recuperó información que contribuyó a la identificación de delgados canales distributarios depositados en un ambiente transicional con influencia de mareas. el resultado mostró que esta estrategia es más eficiente y efectiva que los procedimientos convencionales de minimización. palabras claves: algoritmos genetico, inversion spectral, inversion sismica, formacion barco, cuenca catatumbo, resolucion sísmica. keywords: genetic algorithm, spectral inversion, seismic inversion, barco formation, catatumbo basin, seismic resolution. record manuscript received: 30/04/2011 accepted for publications: 10/12/2011 introduction the reliable estimation of thickness during exploration activities can help to delineate prospects despite a lack of well information; it may help in obtaining more accurate volumetric hydrocarbon production calculations and in planning drilling involving lower risk. vertical seismic resolution is the key to extracting detailed stratigraphy which can distinguish two close seismic events associated with geological events. important oil reservoirs often have layers having a thickness which is below seismic resolution. the transitional palaeocene barco formation in the catatumbo basin consists of varying thickness sand wedges and, as it has several producing oil fields, there is great interest in mapping thin layers within it for drilling future wells. according to the rayleigh criterion, layers thinner than λ/8 cannot be resolved by seismic imaging (kallweit and wood, 1982; widess, 1973); tuning-thickness analysis based on this model has been used as a thickness mapping method for several decades now. amplitude and frequency variations through layers having changing thickness was used as a tool for extracting stratigraphic details (partyka et al., 1999), spectral decomposition using discrete fourier transform (dft) becoming a powerful technique for mapping thickness and geological discontinuity. a set of seismic attributes based on the spectral analysis of seismic reflection was drawn up to map changes in thin reservoirs (marfurt and kirlin, 2001). spectral decomposition has been successfully used for enhancing stratigraphic features. portniaguine and castagna (2004) introduced a method for spectrally decomposing a seismic trace, solving the inverse problem by the conjugate gradient to minimise the objective function; this technique is computationally time-consuming due to the method’s iterative nature. spectral inversion uses interference patterns for identifying and characterising layers (partyka, 2005); other results have shown that inverting seismic data through spectral decomposition provides reflectivity profiles which are then used for estimating layer thickness (puryear and castagna, 2008). seismic data’s vertical resolution has been increased by using the spectral inversion method, examining the thin-bed tuning response in the frequency domain. using a genetic algorithm (ga) meant that spectral inversion gave a global solution by minimising the objective function. the method was tested on synthetic and log data and applied to seismic data for interpreting thin distributary channels deposited in a deltaic environment having a tidal influence. kelyn paola castaño, germán ojeda and luis montes122 (b) geological setting the barco formation consists of interbedded fine-grained sandstone with mudstone; it was deposited in a deltaic environment having a tidal influence (nuñez and saavedra, 2006). thin sequences of bioturbated fine-grained sandstone are cross bedded with fine-grained sandstone and gray mudstone. some coal horizons are present, mainly at the top of this unit. the formation’s thickness varies from 500 feet to 700 feet, running northeast to southeast of the basin. two folds have been interpreted in the area (ojeda et al., 2009); a syncline has been located in the western part by the 3d seismic programme, followed by an anticline structure in the eastern part consisting of two domes. a main fault and secondary fault to the east of the former separate these two domes’ structures (figure 1a). wells p33, p34 and p35 reach the top of the barco formation on the south dome. this formation is divided into four depositional cycles, each representing a maximum flooding surface and hence stratigraphic limits. figure 1b shows x-line 183 where the barco formation and the four depositional cycles link wells p34 and p35. theory an appropriate seismic attribute must be directly sensitive to geological features or reservoir properties, allowing a geologist to interpret a (a) figure 1. (a) top: structural contour map of the barco formation (depth in feet); the prospect is of an anticline formed by two domes. (b) the 183 x-line shows the two domes and wells p34 and p35 reaching the top of the south dome and the four depositional cycles of the barco formation. thin-layer detection using spectral inversion and a genetic algorithm 123 geological structure and its particular environment. coherence integrates information contained in adjacent traces to extract information which may not be easily recognised on time scale maps; high coherence thus indicates lateral lithological continuity whereas abrupt changes may suggest faults and fractures (chopra & marfurt, 2007). fault-to-fault coherence is used to enhance stratigraphic and structural discontinuity, discerning faults and channel geometry. root mean square (rms) amplitude may be applied to recognise seismic anomalies and squaring amplitude values within an analysis window can enhance important amplitudes above noise level. on other hand, combination of attributes has been used to characterize reservoirs (guerrero et al, 2010) and has allowed identify channels in transitional environment with tidal influence. spectral inversion due to the widess model occurring in nature as the exception not the rule, spectral inversion is based on the fact that an impulse pair of reflectors can be decomposed in odd and even parts (castagna, 2004; chopra et al., 2006). seismic resolution is increased because the odd part constructively interferes when thickness becomes thinner. the process differs from conventional methods because inversion is driven by geological knowledge and is based on local frequency spectrum characteristics obtained from spectral decomposition techniques (marfurt and kirlin, 2001). inversion is defined in this method (puryear and castagna, 2008) through an amplitude spectrum’s constant periodicity for a layer of given thickness; it takes advantage of the fact that spacing between spectral peaks and notches is precisely the inverse of layer thickness in the time domain (partyka et al., 1999). when an analysis point lies at the centre of a layer (in the time domain), the impulse pair may be expressed by: here, and are the top and base reflection coefficients and t is layer thickness. the equation’s fourier transform gives the real and imaginary parts: where ƒ is frequency, ro and re are the odd and even component of the coefficient pair. as demonstrated by puryear and castagna (2008) there is a cost function o(t,k) given by: where g(f ) is amplitude regarding frequency and . equation 4 is solvable after evaluating o(t, k) at each frequency and finding the global minimum in a k-t model space. ro is determined by and re from .time sample at t1 top of reflector r1 is given by g(f ) is the complex spectrum for the reflection coefficient pair. extended to multiple layers and assuming seismograms to be the convolution between r(t) and a known wavelet w(t), the spectral decomposition of a seismic trace s(t) within time window length tw can be expressed as: a too short window affects frequency resolution and a too long window deteriorates time resolution. when the wavelet spectrum is known, r(t) and t(t) are estimated by optimising the function: here, fh and fl are high cut-off and low cut-off frequency, αe and αo are weighting functions. the αo/αe ratio is adjusted according an acceptable trade-off between noise and resolution and αo >> αe in the case of the widess model. the best solution is achieved when objective function o (t, re, ro, t), given by equation (6), is minimised over the frequency rank, supplying model parameters re, ro and t. genetic algorithm unlike local optimisation approaches, overall ones attempt to find the misfit function’s global minimum, most of them being stochastic by nature. regarding real data, it can never be known whether an achieved solution is the optimal one; however, even starting with poor initial data, achieved solutions give good results.a genetic algorithm (ga) is widely used in numerical analysis to find accurate solutions for optimising problems because it is one of the best ways to solve a problem about which little is known, thereby creating a relatively quick high-quality solution to a particular problem. a ga tends to thrive in an environment where there are many solutions and in which the search space is uneven and has many hills and valleys. a ga is based on analogies regarding biological evolution (simulated evolution). it uses a population of abstract representations (genome / genotype) of candidate solutions (phenotypes) resolving an optimisation problem. it starts from a randomly-generated population of individuals evolving through successive generations. each individual’s fit in a particular population is evaluated; multiple individuals are stochastically selected (based on their fit) and modified (recombined and randomly mutated) to form a new population. the new population is then used for the algorithm’s next iteration. the algorithm usually ends with the maximum number of iterations (generations) or satisfactory adjustment of the target population has been achieved. a ga review is beyond the scope of this document but one treatise is recommended (glover at al., 2003. chapter 3, pp55) method spectral inversion was first tested on a semi-infinite five-layer model (figure 2, left-hand side) where a 4 ms sample rate synthetic seismogram was produced by convolving the model’s reflectivity series with a 30 hz ricker wavelet (figure 2, right-hand side). the method involved using a 128 ms window centred at 64 ms and applying a 128 m gaussian-tapered fourier transform. it was assumed that αe = αo, i.e. an equal even/odd contribution to the reflection coefficient pair. the selected seismic data covered a 1 km2-area sub-volume from a post-stacked 3d data survey of the catatumbo basin containing 1,271 (1) (2) (3) (4) (5) (6) (8) (7) kelyn paola castaño, germán ojeda and luis montes124 traces. the dataset regarding the top and the bottom of the barco formation was located at the top of the south dome field concerning an oil producing area considered to be stratigraphically complex. good quality borehole data regarding the field was available. data volume was retrodeformed to flatten the top of the barco formation to avoid structural strain effects appearing in the trace due to trace inversion. seismic data quality and vertical seismic resolution were assessed to establish interpretation scope during the initial stage. a 71 m wavelength having 18 m vertical resolution was provided at 40 hz dominant volume frequency together with 2,850 m/s average speed estimated by sonic logs. discussion of results synthetic model figure 3 shows the cost function pattern calculated for a couple of layers where inversion-estimated values were close to optimum values, having lower than 6 ms estimation errors. an additional minimum may be observed at the top left of figure 3a where layer 1 thickness varied from 30 to 60 ms and layer 2 was thinner than 5 ms. figures 3b and 3c contain other minimums where layer 3 thickness vanished; therefore, the layers had to be constrained to non-zero thickness to avoid such border effect. the synthetic seismogram in figure 2 provided an accurate model when it was inverted, considering the 2 ms seismic resolution and the tiny difference between the expected and obtained reflection coefficients. the foregoing was reinforced by comparing the data’s spectra amplitude before and after inversion, depicting a similar pattern and small power decay (figure 4). by contrast with the conjugate gradient optimisation method tested by puryear and castagna (2008), a ga does not require a good starting model and only needs a defined search space containing a particular solution. a conjugated gradient matrix can be analytically formulated, leading to rapid numerical evaluation; the cost of computing its derivatives makes a ga a faster alternative. a ga introduced performance improvement here by doing a fast exhaustive search and finding a good solution quickly. t(ms) rc 40 -0.15 10 0.18 55 -0.12 10 0.18 figure 2. left-hand side, a 5-layer model’s reflectivity values; centre, the reflectivity series; right, the synthetic seismogram provided by convolution with a 30 hz ricker wavelet. figure 3. the optimum point (red star) was close to that estimated by inversion (white star), having lower than 10 ms estimation errors. (a) (b) (c) thin-layer detection using spectral inversion and a genetic algorithm 125 t(ms) rc 36 -0.12 11 0.12 54 -0.07 8 0.19 figure 4. left-hand side, inverted reflectivity series; right-hand side, the spectra before and after becoming inverted. figure 5. in-line 163 shown in bold regarding the area containing the three wells. (a) some stacked traces close to well p33, (b) the lithic column with gr curve used for discriminating shale and sand units, (c) the same traces after inversion. (a) (b) (c) figure 6. in-line 63, above the original stacked data and below the inverted sections (increased resolution). channel features can be observed in the two zoomed-in boxes. (a) (b) (c) (d) kelyn paola castaño, germán ojeda and luis montes126 real data ga was used with spectral inversion with regarding the sub-volume having 1,271 seismic traces and 128 ms window length. the stacked time section was tied to the well and the stacked traces in the volume were inverted. figure 5 shows stacked traces near the well before (figure 5a) and after inversion (figure 5c) and the stratigraphic column (figure 5b) with the tied reflectors associated with sand (yellow) and shale (grey) packages interpreted from well logs. the inverted reflectors discriminated the top and bottom of sand units whereas the reflectors in original data were associated with the thicker layers having a 10 ms uncertainty time position. spectral inversion increased vertical resolution discriminating layers within the predicted resolution limit (6 ms) and a well-defined sand-shale sequence was thus determined. the inversion did not recover the top of the sand package at the bottom of the section, probably due to a border effect regarding the restricted window. stacked in-line 63 is shown in figure 6, before (top) and after inversion (bottom), including time resolution and the number of reflectors having lateral continuity. some discontinuous reflectors in the inverted section’s left zoomed-in box, shown by an arrow around 50 ms, suggested an aggradational stack of layers inside a channel which was not visible before inversion. the reflector’s geometrical pattern inside the right-hand zoomed-in box suggested the base of a channel around 60 ms; two fluvial seismic patters were observed in the 47 ms time-slice shown later on in figure 7. inverted seismic images revealed vertical detail comparable with well information sharply contrasting with the horizontal seismic resolution imposed by the original acquisition; the 20 ms in-line and the 40 ms x-line separations were not modified by the inversion. rms amplitude and seismic volume coherence were calculated before and after inversion. the rms amplitude 47 ms time-slice is shown in figure 7a, a box limiting the zoomed-in area. rms amplitude 17 ms time-slice and inverted data coherence are shown in figures 7b and 7c, respectively, where the rms image shows an n-s sinuous continuity pattern to the east, interpreted as being a small meandering distributary channel which was consistent with the icp palaeoenvironmental interpretation (2006) and supported by core analysis of well p33 drilled in the channel. another sinuous pattern observed to the west was interpreted as a mefigure 7. (a) 47 ms time-slice rms amplitude of complete original seismic volume, (b) rms amplitude of original selected sub-volume, (c) rms amplitude of inverted data showing sinuous patterns, (d) coherency did not reveal structural patterns and (e) interpretation of two sinuous geometry channels having an n-s trend (arrows). (a) (b) (d) (c) (e) thin-layer detection using spectral inversion and a genetic algorithm 127 andering channel; by contrast, these patterns were not observed in rms amplitude before inversion (figure 7b), probably due to the tuning thickness effect. 17 ms time-slice coherence (figure 7d) did not reveal structural seismic patterns as being faults or dipping reflectors, thereby agreeing with the absence of important structures in the sub-volume. increased horizontal resolution would lead to facies identification supporting the above interpretation; however, there were artefacts and noise levels revealing limitations in horizontal resolution which were related to bin size and footprint effect. conclusions thickness constraints in spectral inversion should be used as input for more accurate model-based reflectivity inversion. a ga minimisation algorithm performed well, achieving greater accuracy and faster execution. thickness inversion from real data clearly showed layering below seismic resolution and above 6 ms, i.e. our data’s predicted resolution limit. inversion, however, failed to capture the top of a sand package at the bottom of the section, suggesting that there was a border effect in the analysis window. lower than seismic resolution sand and shale packages can be resolved through spectral inversion and correlating them with stratigraphic information derived from well logs. the inversion method had the necessary power for resolving the vertical top and bottom reflectors associated with thin layers. rms amplitude of inverted data indicated meandering sinuous geometries which were possibly related to small distributary channels on a tidal plain. however, horizontal resolution should be improved to identify facies association thereby leading to a conclusive result. time-slice coherency did not reveal any evidence related to structural features, this being consistent with the absence of important structures in the selected sub-volume. this methodology has provided information making a significant contribution to the vertical identification of geological features such as thin distributary channels in a flat tidal environment. however, it is recommended that future studies should be carried out aimed at solving problems related to seismic horizontal resolution. acknowledgements the authors would like to thank william agudelo, carlos piedrahita and jorge monsegny from the instituto colombiano del petróleo (icp) for their support and contributions and also dr. jonh castagna for discussion regarding the concepts behind spectral inversion. financial support and data permission was provided by ecopetrol s.a. this paper has resulted from an msc thesis in geophysics written by kelyn castaño at the universidad nacional de colombia. references castagna, j. p. 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(1973). how thin is a thin bed? geophysics, 38 (6), 1176-1180. reconstruction of an ancient volcanic edifice near to paipa (boyac, colombia) by means of computer-assisted models for pyroclastic flows earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004): 56 – 62. manuscript received may 2004. paper accepted october 2004. 56 pyroclastic flow modeling to reconstruct a volcanic edifice in paipa (boyacá-colombia). óscar rodríguez 1, milton obando1, luis castillo1 and héctor cepeda2. 1 departamento de geociencias, universidad nacional de colombia, bogotá. e mail: lacastillol@unal.edu.co 2 ingeominas, bogotá. e-mail: hcepeda@ingeomin.gov.co abstract pyroclastic deposits produced by the domes collapse (resurgence of a caldera collapse), at the west of the honda grande creek (paipa, boyacá-colombia) were related by ingeominas. these deposits fill the valleys of olitas, calderitas and a creek at the south of the alto de los volcanes reaching distances near to 3 km from the focus between the alto de los volcanes and el mirador hill. the flows were modeled using 3d software (sheridan and kover, 1996). a volcanic simulation was done obtaining the height and morphology of the volcanic edifice before the collapse during the last eruptive event. keywords: volcanic simulation, pyroclastic flows, computers model resumen depósitos producidos por el colapso de domos (resurgencia del colapso de la caldera), fueron relacionados por ingeominas al oeste de la ensenada honda grande (paipa, boyacácolombia -). estos depósitos llenaron los valles de "olitas" y calderitas, como también la ensenada ubicada al sur del alto de los volcanes, alcanzando distancias de 3 kilómetros desde el foco en un sector entre el alto de los volcanes y el mirador hill. las flujos se modelaron usando software 3d (sheridan y kover, 1996). la simulación del evento volcánico suministró la altura y la morfología que tenía el edificio antes del colapso en el último evento eruptivo palabras clave: flujos piroclásticos, modelos por computador, simulación volcánica. © 2004 esrj unibiblos. pyroclastic flow modeling to reconstruct a volcanic edifice in paipa (boyacá-colombia). introduction in paipa town (boyacá-colombia), a volcanic caldera was discovered and defined by velandia et al. (2004) and also pardo (2004). after the caldera collapsed a volcanism was produced over the cordillera oriental, and pyroclastic flows conformed by pumice and ash ignimbrite were generated. these deposits represents a first volcanic episode. then a new volcanic edifice was formed, representing a volcanic resurgence episode. their products are block and ash pyroclastic flows formed during the dome collapse. pyroclastic flow deposits were mapped and detailed to reconstruct a volcanic edifice generated during the resurgence. the formation conditions were simulated using the flow3d software. the models run many topographic reconstruction until the simulation results were coherent with dispersion of the volcanic products observed in the field, i.e. dome collapse pyroclastic flow. geological setting paipa area is over the cordillera oriental, 120 km north of bogotá d.c. southeastern side of paipa, at boyacá department, between the coordinates x = 1103000e and 1113000e and y = 1116000n and 1127000n, utm bogotá origin. (figure 1) in the area there are cretaceous, paleogene, neogene rocks and sediments corresponding to quaternary deposits. cretaceous older rocks in the interest zone are from the upper cretaceous, which include une formation, churuvita group, conejo, plaenners, arenisca de labor, arenisca tierna and guaduas formation. these units are constituted by a thick section of sedimentary bodies with sandstone layers and shale intercalations. the last formation is characterized by coal seam. cenozoic it consist of volcanic and sedimentary deposits. some of the volcanic deposits are: el volador unit (neogene), paipa volcanic unit and piroclastic rocks (guarrus section, tobas del garrus divided in a1 and a2 deposits, tephras el guarrus deposits: b, c; olitas creek section: volcanic domo, pyroclastic 1,2 flows). the sedimentary sequences are fluvial and alluvial deposits: conglomeratic rocks and sandstone planar and cross structures. two geologic sections: olitas and caldera creek were measured in formed by pyroclastic flows unconformities on the cretaceous formation. figure 1. survey area location 57 oscar rodríguez et al. 58 fundaments to simulate the pyroclastic flows, two models were used: the energy line and the flow3d model. figure 3. energy line model. figure 2. sliding block model. figure 4. energy line for pyroclastic flows. energy line model the energy line principle is the ratio between height of the starting point of the flow (h) and the length of the runout (l), using a type of friction parameter named heim coefficient (µ), after albert heim (figures 2, 3 and 4), who proposed the energy line concept (heim, 1932). energy line inclination is the angle defined by arctan (h/l). the intersection of the energy line defines the distal limits of the flow, being originated at the eruptive source with the ground surface. the vertical distance (h) between the ground surface and the energy line provides a way to estimate the flow velocity in this model. v 2 = 0.5 g h, (1) where v is flow velocity and g is gravitational acceleration. flow3d model. the flow3d is based on generation of a digital elevation model (dem) representing the topographic surface along which the gravity flows move on a triangulated irregular network (tin) of elevations. this kinematics model is constructed from data sets and a geometric configurations (point source, radial distribution, linear or random) used to set the former model. the triangles are contiguous at their boundaries, so that there are no discontinuities such as those that exist with raster data. the tin also serves as the basis for the computations where the gravitational acceleration is assigned to each triangular element of the network. the algorithm considers previous flow models for gravity slides and assumes a constant mass, thickness, and density of the flows. within each specified triangle a single vector represents the driving acceleration due to gravity. the flow3d code (kover, 1995; sheridan and kover, 1996) provides velocity histories of particle streams along flow paths in three dimensions. the calculus of frictional resistance is similar to the mcewen and malin “flow” model (1989), acceleration along the path of steepest descent for each terrain element and the assumption that the center of mass is focused at the ground surface. however, flow3d calculates viscous and turbulent resistances by multiplying the user-defined pyroclastic flow modeling to reconstruct a volcanic edifice in paipa (boyacá-colombia). coefficients of energy dissipation by the flow velocity, which is determined incrementally. multiple flow paths are incremented every 0.1 seconds across triangular elements using as many as three parameters to calculate shear resistance (τr ): basal friction (a0), viscosity (a1) and turbulence (a2) (mcewen and malin, 1989). τr = a0 + a1v+ a2v2. (2) flow3d simulations have been verified by laboratory tests and field studies in the 1991 unzen eruption (kover and sheridan, 1993; kover, 1995) and the 1980 mount st. helens pyroclastic flows (kover, 1995). it has been applied to risk assessment at several other volcanoes including the creation of hazard maps at popocatépetl (macías et al., 1995) and volcán colima (martin del pozzo et al., 1995) and risk probability at volcán colima (sheridan and macías, 1995; saucedo et al., 1997). recently, flow3d was used to simulate the velocity history and runout of block-and-ash flows from “merapi type” dome collapses at soufriere hills montserrat (hooper and mattioli, 2001). in colombia the flow3d has been used in hazard evaluation for pyroclastic flows in the cerro machín volcano (obando and ramos, 2003 and obando et al, 2003). reconstruction procedures the first step in reconstruction procedures is to know the distribution of the pyroclastic flows deposits (figure 1). these deposits are characterized by the presence of fragments of blocks and ash, corresponding to the fourth volcanic event, (pardo, 2004). the pyroclastic flows were formed by dome collapse without the presence of an eruption column, dispersing calderitas and olitas in the valleys and creek, towards the northeast and southeast flanks from the volcanic source (between the alto el mirador and alto los volcanes, map 1). in this work, the term pyroclastic flow 2 is used indicating the pyroclastic flow1 was deposited before. flow3d models were simulated on a topographic model constructed from igac topographic mapsscale 1:25000: 117-iv-c, d, y 191-ii-a, b, which were digitalized using the program autocad. once they were digitalized to generate a tin, taking into account the actual topography expressed by the alto el mirador and alto los volcanes to reconstruct the ancient volcano edifice and saved in dxf format (figures 5 and 6), they were converted to xyz format and loaded to flow3d. 59 x: 1'120.000 y: 1 '1 05 .0 00 alto los volcanes el uvo olitas el molino vereda el tunal q ue br ad a a gu a fr ía q uebrada honda grande alto el mirador q ue br ad a ca ld er ita s alto los volcanes el uvo olitas el molino vereda el tunal q ue br ad a a gu a fr ía q uebrada honda grande alto el mirador q ue br ad a ca ld er ita s x: 1'120.000 y: 1 '1 05 .0 00 figure 5. reconstruction of the volcanic edifice. actual topographic lines (black lines ), and the prolongation of the topographic lines ( gray line ). oscar rodríguez et al. 60 x: 1'118.000 y : 1 '1 05 .0 00 y : 1 '1 10 80 00 x: 1'123.000 alto los volcanes el uvo olitas el molino vereda el tunal alto las peñas q uebrada honda grande q uebrada el ceron q ue br ad a q ui nt al 2800 2800 26 00 28 00 quebrada el tunal alcantarilla el banco el durazno locita blanca 2600 la mesa q ue br ad a cald eri tas alcantarilla alcantarilla 106 a 2871 hato viejo la esperanza 1316 2697 alto el mirador escuela el tunal universidad nacional de colombia facultad de ciencias departamento de geociencias map 1 scenary map made in order to identify parameters escale. 1 : 25.000 0 250 500 750 1000 explanation drainage ways contours pyroclastic flow 2 volcanic dome emplacement area figure 6. final aspect of the volcanic edifice, including a topographic barrier at sw. the distribution of the pyroclastic flows deposits found in the area is important to achieve the best topographic configuration. to found the distribution that getting the best match respect to paipa distribution (figures 7´s and 8), different values for a0, a1, a2, were used for each model so a0 = 0.18, a1 = 0 and a2 = 0, were the best values, for all the cases the value of initial velocity was zero assuming that pyroclastic flows generated by dome collapse were initially static. discussion and conclusions it was found that the altitude of the volcanic build was 3100 m.o.s.l., and the friction coefficient a0 is 0.18, value coherent with observations made in other sites with pyroclastic flow deposits generated by dome collapse (figures 7´s and 8). the lack of evidence of pyroclastic flow thickness occurs because of the few outcrops in the area. then the reconstruction of the paleotopography is difficult, so it would be easier if exist a topography to measure these values. the maximum velocities reached by these flows were about 75 km/h, but it is not enough to override topographic barriers 3150m.a.s.l. to a great altitude localized 12,5km far from the focus of the pyroclastic flow (figure 7a) figure 7a. final model by flow 3d oscar rodríguez et al. vi = 0 m/s figure 7b. final model by flow 3d figure 8. dem corresponding to the reconstruction of the volcanic edifice. 61 oscar rodríguez et al. 62 acknowledgments the authors wish to express their gratitude to the geologists and vulcanologist michael sheridan and the ph.d. student keith dalbey by supplying the flow 3d code as well as the comments and help along this work. special thanks to sloan moreno i.t engineer of laboratorio de geomatica y computacion gráfica of the universidad nacional de colombia. references. heim, a.: (1932). der bergstruz von elm, deutch. geol. gesell. zeitschr. 34, 74–115. hooper, d. m. and mattioli, g. s.: (2001). kinematic modeling of pyroclastic flows produced by gravitational dome collapse at soufriere hills volcano, montserrat, natural hazards 23, 65–86. kover, t. p. (1995). application of a digital terrain model for the modeling of volcanic flows: a tool for volcanic hazard determination, msc thesis, state university of new york at buffalo. kover, t. p. and sheridan, m. f. (1993). numerical models for pyroclastic flows of mt. unzen, japan. geol. soc. amer. abstracts with programs 25, 268. macías, j. l., carrasco-nuñez, g., delgado, h., martin del pozzo, a. l., siebe, c., hoblitt, r. p. martin del pozzo, a. l., sheridan, m. f., barrera, d., hubp, j. l., and selem, l. v. (1995), mapa de peligros volcán de colima, instituto de geofisica, unam, méxico. mcewen, a. s. and malin, m. c., (1989). dynamics of mount st. helens’ 1980 pyroclastic flows, rockslide-avalanche, lahars, and blast, j. volcanol. geotherm. res. 37, 205–231. obando q., m. g. and ramos p., r. a. (2003). evaluación de la amenaza por flujos piroclásticos en la localidad de cajamarca (tolima) ante una probable erupción del volcán cerro machín (trabajo de grado). universidad nacional de colombia, bogotá. obando q., m. g., ramos p., r. a., portilla g, m. e. and cepeda v, h. (2003). modelamiento asistido por computador de flujos piroclásticos en el volcán cerro machín (tolima, colombia). geología colombiana, 28,pp. 2137, bogotá. pardo, n., (2004). estratigrafía de las vulcanitas asociadas al volcán de paipa, municipios de paipa y tuta, departamento de boyacá, colombia. (trabajo de grado), facultad de ciencias, departamento de geociencias, universidad nacional de colombia. bogotá. saucedo, r., komorowski, j-c., macías, j. l., and sheridan, m. f. (1997), modeling of pyroclastic flows generated during the 1913 eruption of colima volcano, méxico. eos, trans. amer. geophys. union 78, f823. sheridan, m. f., and tilling, r. i. (1995), mapa de peligros del volcán popocatépetl, informe final, instituto de geofisica, unam, méxico. 12 pp. & map. sheridan, m. f. and kover, t.: (1996), flow3d: a computer code for simulating rapid, openchannel volcanic flows. in proc. ujst workshop on the technology of disaster prevention against local severe storms. norman ok, pp. 155–163. sheridan, m. f. and macías, j. l. (1995), estimation of risk probability for gravitydriven pyroclastic flows at volcán colima, méxico, j. volcanol. geotherm. res. 66, 251– 256. velandia, f., cepeda, h., alfaro, c., vázquez, l., bernal, n., aguirre, a. (2004). integración geocientífica para la obtención del modelo geotérmico de paipa. primer congreso colombiano de hidrogeología, memorias, medellín. geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 31-43 enso modulations on streamflow characteristics ali ihsan marti1, cahit yerdelen2, and ercan kahya3 1 assistant prof., civil engineering department hydraulic division, selcuk university campus, 42035 konya-turkey. phone: +90 332 223 22 44, fax: +90 332 241 06 35 e-mail: alihsan@selcuk.edu.tr 2 assistant prof., civil engineering department hydraulic division, ege university phone: +90 232 388 60 26 / 131. fax: +90 232 342 56 29 e-mail: cahit.yerdelen@ege.edu.tr 3 prof., civil engineering department istanbul technical university hydraulics division, maslak, 34469 istanbul-turkey work phone: +90 212 285 30 02, fax: +90 212 285 65 87 e-mail: kahyae@itu.edu.tr abstract el niño southern oscillation (enso) has been linked to climate and hydrologic anomalies throughout the world. this paper presents how enso modulates the basic statistical characteristics of streamflow time series that is assumed to be affected by enso. for this we first considered hypothetical series that can be obtained from the original series at each station by assuming non-occurrence of el niño events in the past. instead those data belonging to el niño years were simulated by the radial based artificial neural network (rbann) method. then we compared these data to the original series to see a significant difference with respect to their basic statistical characteristics (i.e., variance, mean and autocorrelation parameters). various statistical hypothesis testing methods were used for four different scenarios. consequently if there exist a significant difference, then it can be inferred that the enso events modulate the major statistical characteristics of streamflow series concerned. the results of this research were in good agreement with those of the previous studies. key words: streamflow, enso modulation, radial based artificial neural network model, turkey resumen la oscilación sureñas de el niño (enso) se ha relacionado con anomalías climáticas e hidrológicas en todo el mundo. este artículo presenta cómo enso modula las características estadísticas básicas de las series de tiempo. para ello, primero se revisaron las series hipotéticas que se pueden obtener de la serie original en cada estación, asumiendo la no-ocurrencia del fenómeno el niño en el pasado. en cambio, los datos que pertenecen a los años con ocurrencia de el niño fueron simulados por el método red neuronal base radial (rnbr). luego comparamos estos datos con la serie original para ver diferencias significativas con respecto a sus características estadísticas básicas (por ejemplo, la varianza, la media y los parámetros de auto-correlación). varios métodos para la prueba de hipótesis estadísticas se utilizaron para cuatro escenarios diferentes. en 31 manuscript received: 12/12/2009 accepted for publication: 16/05/2010 consecuencia, si existe una diferencia significativa, entonces se puede inferir que los eventos enso modulan las principales características estadísticas relacionadas a las series de caudales. los resultados de esta investigación concordaban con los de estudios anteriores. palabras clave: caudales, modulación enso, red neuronal artificial base radial, turquía. 1. introduction the el niño-southern oscillation (enso) occurrence is a well-known natural element of the global climate system. it results from the interactions between large-scale atmospheric and oceanic circulation processes in the equatorial pacific ocean, and related to inter-annual variations in precipitation, temperature, streamflow, evaporation in some regions of the world. el niño refers to describe warm sea surface temperature anomaly conditions in the tropical-subtropical pacific ocean, whereas the southern oscillation refers to the see-saw of pressure differences of atmospheric mass between the australian/indonesian region and the eastern tropical pacific ocean. the warm phase of enso, so-called el niño, is of particular interest in this study. global and regional scale of enso influences on hydrologic and climatologic variables have been extensively documented in the relevant literature. the most comprehensive global-scale studies were carried out by ropelewski and halpert (1987) using data from over 2000 rainfall stations worldwide. for streamflow variable, dettinger et al. (2000) studied multi-scale streamflow variability in relation to enso events using over 700 stations worldwide. similarly chiew and mcmahon (2002) investigated the global enso–runoff teleconnections using data from 581 catchments. it is probable that the enso-streamflow relationship is more noticeable than the enso-rainfall relationship for the reason that precipitation variability is higher than that in streamflow due to the fact that streamflow integrates information spatially. on the other hand, the number of regional-scale (i.e., a selected area like a river basin or national borders) studies regarding the enso-climate variability outnumbers the global-scale studies, including more diverse variables and topics. among those, redmond and koch (1991), kahya and dracup (1993), dracup and kahya (1994), maurer, lettenmaier et al. (2004), twine et al. (2005), and gobena and gan (2006) exemplify streamflow variability in the north america and its relationship to enso occurrences. a recent research tendency is to examine the intended enso relations together with other large-scale climatic oscillations, like pacific decadal oscillation (pdo) and north atlantic oscillation (nao) (e.g., kim et al., 2008; tootle and piechota, 2006). in other domains of the world, among those, nazemosadat and ghasemi (2004) quantified the so-precipitation relation in iran using precipitation composites during warm, cold and neutral phases of the so. shrestha and kostaschuk (2005) examined the impacts of enso on mean-monthly streamflow variability in nepal and found that enso-related below normal streamflow in two core regions. sen et al. (2004) proposed enso templates that can be used for streamflow prediction. for the relationships between enso and droughts; among those, vicente-serrano (2005) and karabörk et al. (2007) documented important evidences for the iberian peninsula and turkey, respectively. in our earlier works, such as kahya and karabörk (2001); karabörk and kahya (2003); kalayci et al., (2004; karabörk et al., (2005), the relations between the both extreme phases of the southern oscillation and surface climate variables (i.e., streamflow, precipitation and temperature) across turkey were well documented using various techniques. the objective of this study is to determine whether enso events modulate the basic statistical characteristics of streamflow data in turkey. furthermore, the results of this study are compared with those of karabörk and kahya (2001), particularly in two large regions in western and eastern turkey, where they determined significant enso signal seasons. for this, we have here developed an empirical approach for analysis which consists of three basic phases: (i) simulation based on an ann method, (ii) defining scenarios, and (iii) hypothesis testing. to our best knowledge, this work presents the first approach and findings in its kind in the germane literature. 2. data and methodology 2.1 data the data network consisting of 78 streamflow gauging stations, approximately uniformly distributed around turkey (figure 1), initially is of primary interest in this study. the streamflow data set spans from 1962 to 2000. owing to the 32 ali ihsan marti, cahit yerdelen, and ercan kahya main idea of this investigation, however we pay more attentions to the stations within the western anatolia (marked by wa) and eastern anatolia (marked by ea) regions (figure 1) where karabörk and kahya (2001) previously determined coherent and consistent enso related streamflow signals. the timing and sign of significant signals are also indicated in figure 1. karabörk and kahya (2001), who used the same data set having a period 1964-1994, set the selection criteria for the stations to be included as: (i) homogeneous distribution; (ii) no missing record; and (iii) no major upstream interference. moreover how this data fulfills the homogeneity condition was discussed in-depth by karabörk and kahya (2001). we here considered the following enso years: 1963, 1965, 1969, 1972, 1976, 1982, 1987, 1991, 1993, and 1997 in the simulation phase of our analysis. 2.2 ann simulation of monthly streamflow records the methodology adapted here requires at the first place the assumption of non-occurrences of enso events in the past records; therefore, original monthly streamflow values corresponding to el niño years should be removed and need to be replaced by another reasonable assumed values using some statistical means. for this reason, we applied the radial based artificial neural network (rbann) model (govindaraju and ramachandra rao, 2000) for the simulation of mean monthly streamflow records. in order to generate synthetic streamflow data by the rbann model, the input variables were taken as the first and second years’ mean monthly records (xi, t-1 ; xi, t-2) (i: year, t: month) and the mean monthly value of that month (xi) not including any records during el niño years. the reason for considering data of the first two years prior to the enso event as input variables is that they have a high correlation with the values of the estimated year. furthermore we checked the possibility of considering similarly preceding third and fourth data values as additional input variables; but we noticed that the length of data becomes small to treat, causing decreased training performance of ann. the mean of each month was calculated without taking in el niño years in the data set. the rbann model was formed by an input unit, a hidden unit and an output unit. the synthetic data generation with the rbann was executed by matlab computer program. as a result, we have two time series at hand at each station: the original (historical) and the synthetic (whose historical data during el niño year were replaced by those generated by the ann model). 2.3 defining scenarios in order to compare the two time series at each station from different perspectives, we first need to define testing scenarios that will be used in the hypothesis testing phase of our analysis. in our earlier study (kahya and marti, 2007) we used two, but here we increased it to four as follows. scenario a: this involves two time series: the original mean annual streamflow records of 39 years and the hypothetical time series whose el niño years were filled with the synthetic data generated by the rbann. scenario b: this involves two time series: the original mean annual streamflow records of 39 years and the hypothetical time series whose values comprises of the mean val33 enso modulations on streamflow characteristics 42° n 41° n 40° n 39° n 38° n 37° n 36° n april (0) october (0) wet anomally wa black sea georgiabulg armenia ea april (0) . november (0) wet anomaly a eg ea n s ea iran mediterranean sea syria iraq 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e gr figure 1. the regions and seasons where significant enso signals occur in turkey. ues of the enso signal seasons specified for the stations existing in the wa and ea regions. scenario c: this involves two time series, the original and the hypothetical, each having 10 values (data points) formed with the mean annual streamflow data of the original el niño years and the synthetic el niño years. scenario d: this involves two time series: the original and the hypothetical, each having 10 values (data points) formed with the mean of the signal season specified for the wa and ea regions in figure 1 identified by karabörk and kahya (2001). 2.4 hypothesis testing having the scenarios known, the following four tests were applied to seek for a statistically significant difference between the two series. in our earlier study (kahya and marti, 2007) we used three tests, but we here added one more test in our analysis as follows. a) testing variances the f-test was used to test the variances of the original and hypothetical series for all testing scenarios. f statistics can be determined using eq. (1) (haan, 1977). f s s s s� �1 2 2 2 1 2 2 2( ) (1) where s: the standard deviation and s2 : the variance. the calculated f value was compared with the critical value from the f-distribution table for the required significance level. the difference between the variances was accepted as significant when the f-value exceeds that critical value. b) testing means the t-test was used to test the mean values of the two data sets (haan, 1977). the length of the data sets was the same in all scenarios (n1 = n2). when no positive correlation between the samples was detected, the following t-model should be used. t x x x x n n n n � � � � � � � �� �� � � � �� �� �� 1 2 1 2 2 2 1 2 1 22 1 1 (2) where n1, n2: the sample lengths and x: the sample mean. in case of positive correlation, the following formula should be used. t x x s n s n s n s n � � � � � � � � � � � � � � � 1 2 1 2 1 2 2 2 1 1 2 2 (3) c) testing populations after testing the samples in terms of variances and means, a non-parametric test, so-called mann-whitney u test, was applied to analyze our original and hypothetical series whether or not they come from the same populations? the u-statistics of the mann-whitney test is defined as (popham, 1967) u u u�min ( , )1 2 (4) u n n n n r1 1 2 1 1 1 1 2 � � � � ( ) (5) u n n n n r2 1 2 1 1 2 1 2 � � � � ( ) (6) where n1, n2: the sample lengths. considering the both series together and arranging them from lowest value to the highest, then assigning a rank value to each element of the series. later r1 and r2 values were calculated by summing the ranks of the first and the second data sets separately. since the calculated u-statistic follows the normal distribution asymptotically, its expected value and standard deviation can be expressed as u n n � 1 2 2 (7) �u n n n n � � �1 2 1 2 1 12 ( ) (8) and finally the standard normal variable can be computed by z u u n n n n n n u u � � � � � � � 1 2 1 2 1 2 2 1 12 ( ) (9) testing this calculated standard normal variable using two-tailed test, a decision can be made on the significance difference between the two samples. dtesting the autocorrelations the original and hypothetical streamflow series were examined to see whether enso events influence the autocorrelation 34 ali ihsan marti, cahit yerdelen, and ercan kahya structure of the observed series. the existence of statistically significant positive lag-1 autocorrelation coefficient (r1) is the important indication of the persistence characteristic of a streamflow time series. therefore, we assume that the r1 is a representative of the autocorrelation structure of the original and hypothetical series and can be expressed as eq. (10) (haan, 1977). r x x x x x x x x k t t k t k t n k t t t k t k t � � � � � � � � � � � � �( )( ) ( ) ( ) 1 1 2 2 11 1 2 n k t n k � � � �� � � � � � � / (10) here, k = 1, xt is the mean of the first n-k terms and xt+k is the mean of the last n-k terms. the confidence intervals at the 95% significance level can be determined by eq. (11). r n k n k k (% )95 1 1 � � � � � � (11) 3. results we have carried out the analysis procedures described in the previous section for all streamflow stations in our data set and presented the results of the hypothesis testing phase in figurative (map) and tabular fashion in this section and make their relevant discussion in the subsequent section. for comparing the variances of the original and hypothetical series, the f-test results are given in figure 2 for the stations in the wa and ea regions. there was no significant variance difference between the two series in the both regions according to scenario a. for comparing the means of the original and hypothetical series, the results of the t-test are illustrated in figure 3. in this case, we found more or less significant results for each of scenarios. for comparing the populations of the original and hypothetical series, the mann-whitney u test was applied to the both series to find out whether they belong to the same population. scenarios a and b did not show any statistically significant differences between the original and hypothetical series. on the other hand we found important distinction of population results for the scenarios c and d and presented the results in figure 4. for comparing the autocorrelation structures based on the lag-1 correlation of the original and hypothetical series, the possible changes in streamflow data due to enso events were analyzed using equations 10 and 11 for calculating correlation coefficients and anderson’s limits for the scenarios a and b. the results of this comparison are presented in table 1. 4. discussion and conclusions a) variances the hypothetical and original time series did not differ statistically for scenario a in the both regions. the both time series exhibited significant differences for scenario b in six stations at the 95% significance level in the wa region (figure 2a); however, none of the stations located in the ea region exhibited any difference for the same scenario. it is worthwhile to note that these identified stations are distinctive in reflecting the enso modulation on streamflow variance among all stations in the entire study domain. the noted number of stations in figure 2a in comparison to the results for scenario a confirms the importance of previously detected regions in association with the enso events. for scenario c, the ea region comprises of 7 stations (corresponding to 41% of the entire ea stations) at the 95% significance level as the wa region had 10 stations at the same significance level (figure 2b). the wa region contains additional 9 stations when considering the 90% significance level, implying that 60% of the entire wa stations demonstrate the enso modulation on streamflow variance. there are stations located other than these regions, mostly in mid-southern turkey, having significant signals for the implied modulation. for scenario d, the number of significant stations dramatically increased in the wa region compared to the results of all other scenarios (figure 2b). among those 19 stations revealed the implied variance modulation at the 95 significance level. this scenario was tailored for the seasonal average considering only the event years and produced an extra confidence on the reliability of the wa region. b) means the computed t-values for the scenarios a and b were determined considering positive correlation between the hypothetical and historical series. the significance of these t-values was then evaluated based on the two-tailed t-distribution. we mapped the results of testing means with respect to the both 90% and 95% significance levels in figure 3. except a few stations, the observed streamflow values in el niño years were greater than those generated by the rbann model (that is to say, wet anomaly responses to el niño events was confirmed in most of the stations). in scenario a (figure 3a), the ea region showed six stations (35% of the total stations in the ea) having a significant difference in the mean and two of them significant at the 95% level. similarly the wa region also showed six stations (19% of the total sta35 enso modulations on streamflow characteristics 36 ali ihsan marti, cahit yerdelen, and ercan kahya black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (a) black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (b) black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (c) figure 2. differences in the variance of the hypothetical and original streamflow series for (a) scenario b, (b) scenario c, and (c) scenario d. solid (open) squares represent the 95% (90%) significance level. 37 enso modulations on streamflow characteristics table 1. the results of lag-1 autocorrelation coefficients for the hypothetical and original streamflow series in the wa and ea regions east anatolia (ea) region station scenario station scenario a b a b ori. hypt. ori. hypt. ori. hypt. ori. hypt. 1413 +� 2147 +� +� +� 1418 +� -� 2151 +� +� 1535 2304 +� +� 1801 +� +� +� 2402 +� 2122 2409 2124 +� +� 2603 +� +� 2131 +� +� 2610 +� +� 2132 +� +� 2612 2145 +� +� west anatolia (wa) region 302 +� 713 +� +� +� +� 311 +� +� +� +� 1203 +� +� +� +� 314 1216 +� +� +� 316 +� 1221 +� +� +� 317 +� +� 1222 +� 321 +� +� +� 1223 +� +� +� +� 324 +� -� 1224 +� +� +� +� 406 +� 1226 +� 407 +� 1233 +� +� 509 +� 1237 +� 510 +� 1242 +� +� +� +� 514 +� +� +� 1243 +� +� +� 518 +� +� +� +� 1302 +� 601 +� +� +� 1307 -� 701 +� +� +� +� 1314 +� 706 +� +� +� +� 1335 38 ali ihsan marti, cahit yerdelen, and ercan kahya black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n (a) 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (b) black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (c) 39 enso modulations on streamflow characteristics black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (d) figure 3. differences in the mean of the hypothetical and original streamflow series for (a) scenario a, (b) scenario b, (c) scenario c, and (d) scenario c. solid (open) squares represent the 95% (90%) significance level. black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (b) figure 4. population differences for the hypothetical and original streamflow series for (a) scenario c and (b) scenario d. solid (open) squares represent the 95% (90%) significance level. black sea georgia armenia bulg gr wa ea a eg ea n s ea mediterranean sea syria iraq iran 42° n 41° n 40° n 39° n 38° n 37° n 36° n 26° e 28° e 30° e 32° e 34° e 36° e 38° e 40° e 42° e 44° e (a) tions in the wa) having statistically significant difference in the mean, but only one station at the 95% level. on the other hand, the results of scenario b exhibit more stations in the wa region having significant t-values as a result of using enso signal season in the original streamflow series (figure 3b). at the same time the ea region exhibited almost same number of significant stations as in scenario a with better results. it is important to note that the number of significant stations in the wa region outnumbers that of the ea region in this case, implying more sensible enso influences streamflow in western turkey. for scenario c, significant differences in the mean were found in about a quarter of the entire stations in the ea region as a slightly high value of percentage was observed in the wa region likewise (figure 3c). as expected, the whole picture was noticeably improved in terms of more and strong signs of the enso modulation on streamflow mean for scenario d (figure 3d). this observation is true for the both regions. it is interesting that the outer stations along the northern border of the ea region indicated highly significant enso modulation on streamflow mean for all scenarios, which was not the case in the enso modulation on streamflow variance. an overall evaluation can be said that the number of significant stations in this analysis reached a maximum for the scenarios b and d that both are based on the enso signal season considered versions of the scenarios a and b. this fact once again approves the importance of the wa and ea regions previously identified by karabörk and kahya (2001). c) populations the hypothetical and original streamflow time series in turkey were subjected to the mann-whitney u test for all the scenarios and the results of this phase are illustrated in figure 4. since significant correlations occur between the two samples for the scenarios a and b, the application of mann-whitney u test with this condition leads the unconformity of “unrelated samples” rules in the theory. the recommended test for the comparison of two such related sample sets is the non-parametric wilcoxon rank sum test. in this case if there are equal samples in the data set, they should be removed from the data set. however, this elimination turns our samples in the scenarios a and b to be the same as those in the scenarios c and d. the other non-parametric tests available in the literature could be cited as the two-sided variance analysis of friedman and the single-sided variance analysis of kruskal wallis, but they are recommended for comparing more than two sample sets (popham, 1967). as a result, we here performed the comparison of the populations using the mann-whitney u test for the scenarios a and b. the results of this phase showed that the wa and ea regions did not reveal any significant differences in the population for the scenarios a and b. however the analysis of scenario d (figure 4b) indicated more significant results than those of scenario c (figure 4a) due to the consideration of the enso signal season. in particular the dominant character of region wa is evident in exhibiting the enso modulation on streamflow population characteristics. meanwhile, the wa region did not show any significant differences for scenario c, while the ea region had three stations having statistically significant results. d) autocorrelations following our earlier results (kahya and marti, 2007) for this analysis phase, we here prefer to present the results of autocorrelation calculations in a tabular fashion (table 1). it is known that the first autocorrelation coefficient (r1) is the most important component of autocorrelation structure of hydrologic time series concerned. therefore we consider only this statistic, to some extent, as a representative of autocorrelation characteristics for the practicability purposes, and calculated for the mean streamflow time series in the da and ba regions for the scenarios a and b. we evaluated the computed r1 values at the 95% significance level and summarized our results according to the following four possible cases concerning the calculated r1 values.. (a) case 1: when the computed r1 values of both the hypothetical and original series fall within the 95% confidence intervals, the both series has no serial dependency property at lag-1. thus this implies that enso events have no influences on the internal dependency of the original streamflow series. we found that 4 stations in the ea region and 2 stations in the wa region confirmed this conclusion for the scenarios a and b. (b) case 2: when the computed r1 value is not significant for the original series, but significant for the hypothetical series. in this case, our perception is that enso events might modify the autocorrelation structure of streamflow series in the form of elimination. in our analysis, this situation was not observed in the ea region, but occurred in the station 1307 in the wa region (table 1). 40 ali ihsan marti, cahit yerdelen, and ercan kahya (c) case 3: the computed r1 value of the original series appears to be significant while that of the hypothetical series does not. in this case, our perception is similar to case 2 in such a way that enso events might modify the autocorrelation structure of streamflow series in the form of generation. this case has occurred in 8 stations in the ea and 14 stations in the wa according to scenario a. moreover 6 stations in each the ea and wa regions showed a significant difference in the first autocorrelation coefficient at the 95% level according to scenario b (table 1). for these stations, it can be speculated that enso events modify some feature of autocorrelation structure of the series, in the manner of increasing the internal dependency. (d) case 4: the computed r1 value of the both series exceeds the 95% confidence intervals. in this case, our perception is that enso events have no influences on the autocorrelation structure of streamflow series unless the opposite sign of the computed significant r1 values emerges. for scenario a, 5 stations in the ea region and 15 stations in the wa region confirmed this case. for scenario b, 10 stations in the wa region and none in the ea region confirmed this case. in general, the results from evaluating the opposite cases b and c lead us to deduce a speculation of enso modulation on streamflow autocorrelation structure. furthermore it can be said that enso events caused modification in the persistency characteristic of turkish streamflow patterns. in order to explain the physical mechanisms behind the observed changes in basic streamflow statistical characteristics in turkey, it is worthwhile to review pertinent discussions presented by kahya and karabörk (2001) who sought some clues based upon the cyclonic activities affecting the overall turkish precipitation regime. in the same context, karabörk and kahya (2003) speculated that the occurrence of enso signals resulted from the interactions between enso events and north atlantic oscillation (nao) as nao has direct impacts on turkish precipitation and streamflow patterns. marshall et al. (2001) indicated that climatic variability in the north atlantic sector is composed of three main phenomena: (i) tropical atlantic variability (tav), (ii) nao, and (iii) atlantic meridional overturning circulation (moc). they argued that the nao, the tav and the moc are intimately connected and should be considered together. the nao may exert extratropical influences on tav and the tav may feedback to the nao by rearranging the hadley circulation. in addition, the pacific enso events influence the tropical atlantic and, in turn, the atlantic influence the eastern equatorial pacific. since the tav is subjected to remote influences of the nao and enso, a feedback effect on the nao by rearrangement of hadley circulation may carry enso signals to mid-latitudes (see marchall et al. (2001: figure 15) for the proposed mechanism). it is important to note that this mechanism needs to be further investigated. terray and cassou (2002) pointed out that when the removed enso global data are considered in the analysis, the resulting forced atmospheric slp responses on the north atlantic sector showed a stronger than usual subtropical high and a deeper than normal icelandic low, namely, a typical positive nao phase. their findings explained how the tropical atlantic acts as a bridge to transfer enso signals to mid-latitudes. this simulated enso-nao relationship indicated by terray and cassou (2002) was also stressed by bojariu and gimeno (2003) in the context of external factors that affect the nao variability. in this perspective, karabörk and kahya (2009) stated that the findings of wang (2002) could be considered noteworthy as the hadley circulation is closely related to the nao, and the nao has significant negative correlations in association with tropical north atlantic ssts. therefore karabork et al. (2005) stated that ‘as both the el nino phase of the so and nao can influence the tropical atlantic climatology, it is reasonable to expect that this sector can play a key role in carrying el nino signals to midlatitudes by means of a feedback mechanism. in conclusion, all results from above statistical tests performed in this study showed that enso events have significant influences on the modulation of streamflow statistical characteristics in turkey. this clear-cut conclusion provides a solid incentive for the water resources planners to consider the large-scale atmospheric oscillation patterns like the southern oscillation and/or north atlantic oscillation for planning and managing purposes. 5. acknowledgements we appreciate dr. karabörk who helped us conduct the hypothesis testing and reviewed the manuscript. this work was supported by the tubitak under research project no: ydabag 102y146. 41 enso modulations on streamflow characteristics 6. references bojariu r. and gimenok, l. 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(2002). atlantic climatic variability and its associated atmospheric circulation cells. journal of climate, 15, 1516–1536. 43 enso modulations on streamflow characteristics enero%2031-geociencias-vol%2012-2%20dic-en%20baja[1].pdf earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 213-234 mineralization potentials of pegmatites in the nasarawa area of central nigeria akintola, o. f.1 and *adekeye, j. i. d.2 1 raw materials research and development council, p. m. b.232, garki, abuja, nigeria fax: 234 9 4136034, e-mail: akintolaolatunde@yahoo.com, phone: 2348055239304 2 geology and mineral sciences department, university of ilorin, p. m. b. 1515, ilorin, nigeria. email: adekeye2001@yahoo.com, phone: 2348033795444. * corresponding author abstract pegmatites in nasarawa area of central nigeria lie within a fracture-controlled east-north-east trending rare metal pegmatite belt closely associated with late panafrican peraluminous granites in nigeria. trace elements of feldspars andmuscovites of pegmatites in nasarawa area of central nigeria were analyzed to determine the raremetals mineralization potentials of the pegmatites. the minerals show fractionation even within units of complex mineralized pegmatites as indicated by the wide ranges of k/rb in na-feldspars (albites), muscovites and k-feldspars and the wide range of k/tl in the latter two minerals. cs, rb and tl and the rare earth elements (rees) ce, la, pr and y are enriched in the k-bearing muscovites and k-feldspars; k-feldspars have the highest average values of tl, ce, la, pr and y while the muscovites have the highest average values of sn, nb, ga, f and zn. average values of ta, sr, ba, w andzr are highest in the albites, which also have the lowest averagenb/ta ratio. this is an indication that these elements are enriched along latena-rich raremetalmineralizing fluids. the rare alkalis rb andcs and tl are positively correlated with the rare lithophile elements (ta, rees, pb, bi and y), in both the pegmatite k-feldspars and muscovites, strongly positively correlated with sn in the muscovites but negatively correlated with nb in the minerals. in the pegmatitic albites, nb has very strong positive correlations with ta (0.868) and zr (0.847), which is indicative of the incorporations/substitution of these elements in the crystal lattice of the ore minerals. there is a general enrichment of the rare elements towards themiddle and inner zones of the complexmineralized pegmatites. fluorine, phosphorus and boron-rich fluids played significant roles in the magmatic complexation/fractionation and concentration of sn-ta-li-cs-be in the albitized zones of the highly evolved pagmatites. k/rb versuscs in thek-feldspars classify the pegmatites into barren, be-, li-be, and li-cs-be-ta types. comparably low nb/ta ratios in the late albites and amblygonites indicate the paragenesis of these minerals with the tantalum ores in the middle/inner zones of the complex mineralized pegmatites. while the be-, and li-be-types are highly prospective for gem tourmaline, beryl and 213 manuscript receiver: october 22th, 2008. accepted for publication: november 4th, 2008. columbite; the most evolved pegmatites in this area are good candidates for tantalite, amblygonite-montebrasite, and probably pollucite prospecting/mining. key words: rare metals, pegmatites, muscovites, feldspars, mineralization, correlations. resumen pegmatitas en el área de nasarawa, nigeria central, se encuentran dentro de un cinturón de pegmatitas conmetales de tierras raras controlado por fracturas, con tendencia este-noreste, y estrechamente asociado con granitos peralumínicos del pan africano tardío en nigeria. elementos trazas en feldespatos ymuscovitas de las pegmatitas del area denasarawa, nigeria central, fueron analizados para determinar el potencial demineralización demetales de tierras raras en las pegmatitas. los minerales muestran fraccionamiento aun dentro de unidades en las pegmatitas complejamente mineralizadas, como lo indica el amplio rago k/rb en feldespatos sódicos (albitas), muscovitas y feldespatos potásicos y el amplio rango de k/ti en estos dos últimos minerales. las muscovitas potásicas y los feldespatos potásicos están enriquecidos en cs, rb y ti y en los elementos de tierras raras (rees) ce, la, pr e y; los feldespatos potásicos poseen el mayor valor promedio de ti, ce, la, pr yy eymientras que las muscovitas tienen losmayores valores promedios de sn, nb, ga, la f y zn. los valores promedios de ta, sr, ba, la w y zr son más altos en las albitas, las cuales poseen también el promedio más bajo de nb/ta. esto indica que estos elementos son enriquecidos por fluidos mineralizantes tardíos ricos en sodio y metales de tierras raras. los elementos alcalinos raros rb andcs y ti se correlacionan positivamente con los elementos litofilos (ta, rees, pb, bi y y), en los feldespatos potásicos y las muscovitas de las pegmatitas, y tiene una fuerte correlacion positiva con sn en la muscovita pero la correlacion es negativa con nb en los minerales. en las albiticas pegmatiticas, nb tiene una correlacion positiva muy fuerte con ta (0.868) y zr (0.847), lo que indica la incorporación o sustitución de estos elementos en la estructura cristalina de los minerales de mena. hay un enriquecimiento general de los elementos de tierras raras hacia la mitad y las partes mas profundas del complejo pegmatitico mineralizado. fluidos ricos en flúor, fósforo y boro juegaron un papel importante en la complejización y fraccionamiento magmatico y en la concentración de snta-li-cs-be en las zonas albitizadas de las pegmatitas altamente evolucionadas. k/rb vs cs en feldespatos potásicos clasifican las pegmatitas en tipos estériles, de be, de li-be, y de li-cs-be-ta. relaciones de nb/ta comparativamente bajas en las albitas y ambligonitas tardias indican la paragenesis de estos minerales con las menas de tantalio en las zonas medias e internas del complejo pegmatitico mineralizado. mientras los tipos be y li-be son altamente prospectivos para tourmalina, berilo y columbita como gemas; las pegmatitas mas evolucionadas en esta area son buenas candidatas para la prospección minera de tantalita, ambligonitamontebrasita, y probablemente polucita. palabras clave:metales raros, pegmatitas, moscovitas, feldospastos, mineralización correlaciones. introduction nigeria possesses a very large pegmatite environment. the pegmatites are widely distributed with a marked concentration ofmineralized pegmatites in a broad belt, which extends from ago-iwoye in the southwest to bauchi in the northeast, an air distance of more than 400kilometers. thousands of pegmatites occur in this belt, most of which have never been mapped or sampled in a systematic, scientific manner. potassium feldspar, albitic plagioclase, quartz, and subordinate, if any muscovite and/or biotite constitute the major minerals while a wide spread spectrum of minerals including lepidolite, tourmaline (green, pink, black and blue), beryl, chrysoberyl, apatite, amblygonite, monazite, lithiophyllitetriphyllite, ghanite, cassiterite, wodginite, nigerite, columbo-tantalite, tapiolite, microlite, bismuthinite, bismuthite, scheelite, cholite, andalusite and 214 akintola, o. f. and adekeye, j. i. d. sillimanite occur as accessory minerals in the pegmatites, bowden and kinnaird (1984). the ages, mineralogy, and composition of these pegmatite units appear to be analogous to those of the pegmatites environment in brazil, canada and australia. the pegmatitic belt and the orientation of the units within it appear to be related to rotational stresses created by the benue trough. from a more global perspective, this trend is probably the northern extension of the brazilian pegmatite belt, which runs from rio grande del sul to rio grande del norte. the pegmatite field of this study area is part of late pan african, (jacobson and webb, 1949; wright, 1970), rare (specialty) metals granitic pegmatites. the primary mineralization of tantalum, niobium, tin, beryllium and lithium is hosted in quartz-feldspar-muscovite pegmatites (kinnaird, 1984). the nasarawa area lies between latitudes 8o 18’ n and 8o 30’ n and longitudes 7o 35’ e and 7o 50’ e, west of the southern boundary of afu complexthe southern most unit of the younger granites complexes (figure 1). mining of tantalite from both pegmatites and the eluvials started in the area some years ago. the mining continues for the major periods of the year except during the very dry months of february to early april when lack of water makes it difficult to mine and concentrate the minerals. columbite and cassiterite are recovered as by-products of the tantalite mining, while other pegmatite minerals like quartz, feldspar, amblygonite-montebrasite, cookeite and mica are still being discarded in waste dumps. area description, methods and materials studied regional geology central nigeria is part of an upper proterozoic mobile belt extending from algiers across the southern sahara into nigeria, benin, and the cameroun. this pan-african belt continues into northeast brazil where analogous rare-metal mineralized pegmatites are also known (schuiling, 1967). bordered to the west by the west african craton (stabilized around 2 ga) the pan-african belt itself is made up of gneiss-migmatites, metasediments, and metavolcanics that have been subjected to polycyclic metamorphism, and emplacement of igneous rocks. these rocks all constitute the precambarian to lower paleozoic basement complex rocks. the gneiss-migmatites bears imprints of the liberian (ca. 2500ma), eburnean (ca 2000ma) and pan african (ca. 600ma) tectonic events (oversby, 1975; turner, 1983). within the sequence are domains of metasediments and metavolcanics intruded by igneous rocks, which constitute the north/south trending schist belts. the schist belt lithologies which consist of fine grained clastics, pelitic schists, phyllites, banded iron-formations, marble and amphibolites are considered to be upper proterozoic assemblages (turner, 1983). they host most of the economic minerals in the basement complex. during the pan-african episode, the proterozoic gneiss-migmatite-schist complex were intruded by various granitoids resulting from oceanic closure, subduction, oblique collision between the west african craton and the hoggar – nigeria shields (black, 1984) and crustal thickening. the pan-african granitoids of nigeria, which is collectively termed older granites, comprise gabbros, charnockites, diorites, granites, and syenites. geochronological data from previous works (rb – sr whole-rock and u-pb zircon) of pan-african granitoids intruding the reactivated archean to lower proterozoic crust of central and south-western nigeria show that intrusive magmatite activity in these areas lasted from at least 630 to 530 ma (van breemen et al., 1977; rahaman et al., 1983; dada et al., 1987; matheis and caen-vachette, 1983; umeji and caen-vachette, 1984; akande and reynolds, 1990). results of the rock ages also show that pegmatities’ emplacement in the southwestern nigeria occurred mainly after the peak of the pan-african orogenic event in this area. the end of the pan-african tectonic event is marked by a conjugate fracture system of the strike-slip faults (ball, 1980). fault directions have 215 mineralization potentials of pegmatites in the nasarawa area of central nigeria consistent trend and sense of displacement; i.e. a ne-sw (nne-ssw) trending system having a dextral sense of movement and a nw-se trending system a sinistral sense (mccurry, 1971; wright, 1976; holt et al., 1978; ball, 1980). both sets crosscut all the main pan-african structures, including older n-s trending shear zones (mylonites) and late orogenic granites (ball, 1980; ajibade and wright, 1989; kuster, 1990; garba, 1992). gold and pegmatities’ rare mineralization are closely associated with the fractures in the pan-african belt (kuster, 1990; ekueme and matheis, 1995; garba, 2002, 2003). about 100km north-east of the area of study at wamba, rare metal pegmatites have also been geochemically linked to peraluminous late pan-african tectonic granitoids, the emplacement of which have largely been controlled by the regional fractures (kuster, 1990). chemical data on granites, and granitic and pegmatitic muscovites show that rb, cs, sn, nb, and ta are enriched during both magmatic and postmagmatic evolution, with the highest contents of these elements occurring in early muscovites of the albitized and mineralized pegmatities (kuster, 1990). albite, k-feldspar, and quartz are the main pegmatitie-forming mineral; white mica is a typical but minor component. from a geochemical point of view, quartz is of no particular interest since it diadochially (substitutionally) hosts trace elements to a negligible extent. feldspars and white micas are the most informative minerals because their element distributions reflect the trace elements contents in either early pegmatite forming fluids or in the later metasomatizing solutions (moller and morteani, 1987). feldspars and white micas of pegmatites in the nasarawa area were sampled and analysed for trace elements to determine the mineralization poten216 akintola, o. f. and adekeye, j. i. d. ilorin ilesha sokoto gusau kano zaria kaduna abuja study area enugulagos maiduguri port-harcourt pan african nigeria belt 10°0° 10° 20° atlantic ocean congo craton0 5001000 50 100 150 kilometres kilometres legend faults barren pegmatites newly metal pegmatites discovered rareknown rarepegmatites metal major cities / towns cretaceoussedimentary cover recent mesozoic ring complexes [younger granites] supracustal (schist belts) basement, gneisses migmatites and pan african (older) granites n atlantic ocean 6° 4° 8° 10° 12°n 6° 10° 16°e12°8°2° west african craton 0° figure 1: geological map of nigeria showing the regional fractures and location of areas of rare-metal and barren pegmatites (after garba, 2003) tials of the pegmatites. data from this area is also compared with published data on well-studied pegmatities for comparison of the potentials. analytical procedures are as stated in akintola and adekeye (2006). the chemical data on the pegmatite mica and feldspars were subjected to bivariate correlations after a lognormal transformation of the data. bivariate correlation coefficient of the elements, r, were interpreted to determine elemental geochemical associations and evolutionary trends in the pegmatites. local geology the geological setting of nasarawa area is shown in figure 2. from field evidence, the oldest rocks in the study area are micaceous pelitic schists. variations in the schist composition for instance, the mica versus the quartz – feldspathic contents are observable in the field. this rock unit has undergone a polyphase metamorphism and ductile deformations evidenced by local variations in the strike and dips of the foliation. like the other schist belt rocks in the country, the schists have a general strike of north-south, and in the area, dips at gentle angle (10o – 30o) to the east. the schists were intruded by older granites, which outcrop as hills especially to the northwest, and west of the area. these older granites have a range of granodiorite to granite compositions. to the east of the area are rocks of the afu complex of the younger granites. the afu complex is made up of high-level anorogenic granites mainly biotic granites and minor quartz porphyry emplaced within precambrian paleozoic basement gneisses and older granites but exposed beneath the cretaceous-recent sedimentary cover of the lower benue valley to the south. the geochemistry of the granites and the mica schists hosting the pegmatites in this area are discussed in akintola and adekeye (2006). two groups of pegmatites are noticeable in the area with minor muscovite in the area: (1) simple, usually barren massive quartz – microcline pegmatites with minor muscovite and accessory tourmalines and (2) complex, albitized muscovite-quartz-microline pegmatites, bearing the rare-metals ta, nb, sn, li and be mineralization. the simple, barren quartz-feldspar (with minor mica) pegmatites occur at the north western and western parts of the area and are spatially closely associated with the main phase older granites. on an outcrop scale, the pegmatites grade into patches of aplite. the complex rare-metal pegmatites are found far away from the granite plutons usually hosted by schists (exterior pegmatites). wall-rock alteration in the simple pegmatites is negligible, but more pronounced in the complex pegmatites especially at contacts with the hanging walls (jacobson and webb, 1946). although tourmalinization is by far the most common type of contact alteration, it is generally accompanied by silicification, albitization, greisenization and sometimes formation of apatites/fluorapatites which give rise to graded contacts. k-feldspars table 1 shows the trace element contents of the microcline, microperthites (k-feldspars) taken from different pegmatites in the study area. for example, one sample of k-feldspar has the highest cs (3489ppm), tl (73ppm), bi (21ppm), rees ce (69ppm), la (166ppm), pr (29ppm) and very high rb (9474ppm). it has the lowest total fe and mn (fe2o3, 0.13%; mno, 0.002%), and k/tl (1558), rb/tl (130), k/cs (33) and k/rb (12) ratios. these characteristic indicate that the feldspar crystallized from a highly fractionated melt and although this particular sample has low concentration of the ore elements, sn-ta-nb, an eluvial concentrate from the mine had earlier yielded high values of ta (1749ppm) with high ta/nb ratio of 5.45, nimamop (1998). the average content of ta in the k-feldspars is very low (2ppm). the k-feldspars have k/rb range of 12-35 and a mean of 19. k/rb has a very high negative correlation (-0.938) with cs in the k-feldspars, figure 3. in figure 3, the cs content is plotted versus the k/rb ratio for k-feldspar together with the boundaries discriminating, according to trueman 217 mineralization potentials of pegmatites in the nasarawa area of central nigeria and cerny (1982), between barren, be-, li-be-, and li-cs-be-ta mineralized pegmatites. from the figure, the following pegmatites in the study area of which the k-feldspars were analyzed belong to the corresponding classes (table 2). it is important to note from this classification that some pegmatites from the same location have different degrees of evolution – some are more evolved belonging to the lithium-beryllium class and some belong to the beryllium class. the pegmatites in the beryllium and lithium-beryllium classes have high prospects for gem tourmaline and beryl mineralization, especially in the albitized zones of the pegmatites (preinfalk et al., 2000). the liberia and onyelow wazobia pegmatites mainly belong to the lithium-cesium-beryllium-tantalum class. it is also important to note here that a sample of the liberia pegmatites plots in the lithium-beryllium class showing that the pegmatites must at least have evolved from the lithium-beryllium class to the lithium-cesium-beryllium-tantalum class. the liberia pegmatite therefore has potentials for bearing ore mineral characteristics of both the lithium-beryllium and lithium-cesium-beryllium-tantalum classes in the different zones of the pegmatite. active mining of sn-nb-ta ores and analysis of mica and other mineral samples such as amblygonite and fluorapatite, confirm the enrichment of the pegmatite in the ores of li-be-b-sn-nb-ta. na – feldspars the framework silicate samples (feldspars and feldspathoids) with low silica (sio2 < 55.95%) and na/k greater than 1 (except cookeite) were grouped together for convenience as na-feldspars (table 2). the true na-feldspars according to deer et al (1966) have the following average contents of ore and lithophile elements: p2o5 (1.17%), f (bdl), ga (27ppm), nb (145ppm), sn (221ppm), ta (195ppm), rb (175ppm) and cs (31ppm). the ore elements are finely disseminated in the albites. the aplitic footwall albite is probably secondary or at least late primary in crystallization.most of the albite in the pegmatites is found in the albite-rich aplitic zone often occurring in the footwalls of pegmatites (jahn and tuttle, 1963; jahn and burnham, 1969). the secondary albite has very low nb/ta ratio of 0.26, which is comparable to the low nb/ta ratio of 218 akintola, o. f. and adekeye, j. i. d. 25 \ akpoku 30 kama tudunwada 30 30 endo nasarawa 7°40’e7°35’e 8°30’e 3000 meters 0 3 6 kilometers3 n older granites granodiorite tonalite gneiss younger granites (afu complex) mica schist tourmalinite basic dyke pegmatite strike and dip of foliation geological boundary 30 legend roads 8°25’n 8°21’n 8°18’n 8°30’e 8°25’n 8°21’n 8°18’n 7°45’e 7°50’e gidankwma figure 2: geological map of nasarawa area. 219 mineralization potentials of pegmatites in the nasarawa area of central nigeria table 1: trace elements of the microline, microperthies (k-feldspars) of the pegmatites. sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a p (ppm) 2400 2461 2662 2579 2130 4774 5398 6642 2854 2138 3024 f 0 0 0 0 0 0 326 0 0 0 0 ba 44 35 34 67 10 30 53 36 69 91 39 bi 20 10 17 15 17 15 10 13 12 10 21 cd 13 7 9 5 6 bdl bdl bdl bdl bdl 16 ce 40 32 32 12 52 21 0 19 35 0 69 co 16 33 18 28 20 28 31 25 23 31 18 cr 10 13 9 14 2 15 35 11 6 1 6 cs 1722 1487 1540 1226 1482 602 111 160 844 692 3489 cu 7 7 14 9 0 13 13 8 4 19 16 ga 18 15 17 15 17 16 44 19 18 14 17 la 95 73 74 58 84 27 2 16 38 31 166 nb 8 5 9 7 9 22 39 8 4 8 8 ni 20 0 31 28 0 0 0 0 0 0 27 pb 70 57 81 79 69 46 0 35 125 140 93 pr 19 15 16 13 18 8 0 6 9 8 29 rb 8546 6536 9534 8303 8420 5537 2593 3069 5440 4089 9474 sn 21 13 187 15 14 28 24 28 9 8 24 sr 38 144 58 44 51 22 30 183 64 70 32 ta bdl 1 2 bdl bdl 9 6 bdl bdl bdl bdl tl 49 38 57 49 49 30 12 16 39 30 73 w 155 241 137 187 176 203 195 154 143 204 160 v 10 0 0 14 18 0 0 0 0 0 20 y 30 18 20 0 19 24 12 11 0 14 24 zn 44 0 0 0 0 0 40 0 0 0 0 k/ba 2521 2906 3492 1694 11058 3628 1696 2841 1571 1165 2916 k/rb 13 16 12 14 13 20 35 33 20 26 12 k/cs 64 68 77 93 75 181 811 639 128 153 33 k/tl 2264 2676 2083 2316 2257 3628 7493 6393 2780 3534 1558 rb/tl 174 172 167 169 172 185 216 192 139 136 130 0.26 average in the amblygonites. obviously, both the secondary albite/mica and the amblygonites are in paragenesis with the tantalum minerals. this observation is in agreement with the findings of moller and morteani (1987). a sample with a very high (8.8%) l. o. i. was also identified (by xrd and icp-oes) to be cookeite – a hydrated lithium aluminum silicate. it has na/k ratio slightly less than 1. cookeite crystallizes from late stage hydrothermal fluids evolved from residual albitic melts on stabilization of tourmaline (and the loss of other fluxing components such as p, b, li, f and h2o) from the pegmatite melts (cerny, 1991b). cookeite is known to occur with other low temperature minerals like adularia and zeolites in miarolitic cavities of the highly evolved complex rare metal pegmatites. white micas (muscovites) in the pegmatites the average chemical composition of the muscovites is typical of rare element pegmatites with high, f, cs, rb and li (deer et al., 1966; gordiyenko, 1971) (table 3). the xrd diffractogram of selected samples of the micas show that the crystal structures of the muscovites are close to the ideal dioctahedral and r2+ free composition typical of muscovites from the rare-element pegmatites class (cerny and burt, 1984). the muscovites have a wider range of rb values (2659-10182ppm) and lower k/rb ratios (8-31) than the k-feldspars’ (2593-9534ppm) and (12-35) respectively (table 4). a late generation mica ð taken from an inner zone of liberia pegmatite has very high cs (2353ppm), rb (9910ppm), sn (647ppm) and ta (103ppm) and correspondingly low k/rb (8), nb/ta (0.66), and k/tl (1600) ratios. these are comparable to average values of cs (2294ppm), rb (8978ppm), sn (665ppm), and ta (464ppm) and the correspondingly low average k/rb (9), nb/ta (0.14), and k/tl (1361) ratios in muscovites from highly mineralized pegmatites in the northern part of the area. nb/ta ratios for the pegmatitic muscovites range from 0.13 to 8.07 with a mean of 2.965. the k/rb ratios for the micas range from 8 and 31 with a mean of 16.20. ta has a very high positive correlation with cs (0.756) but a very high negative correlation with nb (-0.856). linnen (1998) and morteani and gaupp (1989) have observed that the different behaviour of ta and nb during crystallization is due to differences in solubility of nb and ta in li-rich pegmatitic melts. the cs vs. k/rb plots of pegmatite muscovites from the area (figure 4) concentrate in the field typical of rare-element class pegmatites as defined by cerny and burt (1984). only one sample which was collected about 100meters south of the liberia pegmatite plots in the muscovite class. the k/rb ratios of the muscovites range between 7.58 and 30.94. these low ratios of k/rb as well as the high negative correlation of k/rb and cs (-0.830) are typical of muscovites of the rare-metals class of pegmatites (preinfalk et al., 2000) and have been used successfully as exploration tools for the pegmatites. correlation of the trace elements and fractionation trends k/tl, k/rb, k/cs, al/ga, zr/hf and nb/ta ratios give fractionation indices in the granite-pegmatites suites. some of these ratios are compared with the average values of the trace elements in the pegmatitic minerals (table 4). the table also shows that the rare earth elements (rees) ce, la, pr, and y are enriched in the 220 akintola, o. f. and adekeye, j. i. d. barren 10p be li, be li, cs, be, ta cs 100001000100101 1 10 100 1000 k/rb figure 3. classification of the pegmatites using the plots of k/rb versus cs of their k-feldspars. note: the lines discriminating between barren pegmatites and pegmatites with significant be; li-be-, li-beta-mineralization are given according to trueman and cerny (1982). 221 mineralization potentials of pegmatites in the nasarawa area of central nigeria table 2: trace element content of the albitic pegmatite phases phosphates albites cookeite fluorapatite amblygonites sample lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 p2o5 (%) 0.481 0.487 2.169 1.763 0.243 0.017 24.97 49.973 43.25 f(ppm) 1240 0 0 0 0 294 11637 13897 14203 as 0 0 7 8 6 1 0 0 0 ba 19 51 37 26 102 56 31 33 61 bi 14 11 11 16 12 12 38 1 8 ce 14 12 22 14 0 0 443 0 7 cd 7 bdl bdl bdl bdl bdl 27 bdl bdl co 115 47 25 31 37 5 16 8 6 cr 1 0 0 11 0 15 1 2 1 cs 851 23 19 74 9 49 1 28 6 cu 0 0 24 16 4 0 0 16 535 ga 76 24 21 22 39 58 14 21 18 hf 2 3 0 3 3 2 0 3 2 la 48 9 0 3 3 0 204 0 0 mo 0 5 1 1 0 4 0 1 0 nb 88 75 62 326 115 33 190 26 25 nd 3 11 9 8 2 0 144 5 7 ni 14 5 3 4 0 3 9 4 3 pb 25 10 7 0 0 0 74 9 0 pr 10 2 1 2 1 0 65 0 1 rb 4829 69 347 252 31 444 46 290 62 sc 0 0 9 0 4 0 0 0 5 sm 1 3 2 2 1 1 66 1 1 sn 565 659 14 174 35 54 28 231 67 sr 263 52 1037 305 51 8 64 11 188 ta 345 109 67 305 297 37 21 107 86 th 0 0 0 0 0 3 2 1 4 tl 23 bdl bdl bdl bdl 5 bdl bdl bdl u 0 3 0 10 0 1 216 0 7 k-bearing minerals, k-feldspars and muscovites, with the highest concentrations of the element in the k-feldspars. because of their similar geochemical migration patterns, k, rb, cs and tl are concentrated, and have very high positive correlations in the k-feldspars and muscovites (table 6). the muscovites however have higher fractionation indices compared with the k-feldspar as indicated by their low average k/rb, k/tl, and k/cs ratios. the muscovites have high values of sn (383ppm average), nb (155ppm average), ta (96ppm average), ga (196ppm average), f (2128ppm average), and zn (379ppm average) and have higher rb/tl ratio than the k-feldspars. the muscovites have the highest values of ga (with amean value of 161ppm) and lowest al/ga ratio of theminerals. this corroborates the observations of cerny et al (1985) that tourmaline and muscovites are the main concentrators of ga in pegmatites. ba, sr, w, zr and ta are highest in the na-feldspars with the lowest nb/ta ratios, which is an indication that these elements are enriched along with ta in the late mineralizing fluids. the following groups of elements (table 5) have high positive correlation indices in the na-feldspars: rb versus cs (0.922), cs versus zn (0.866), cs versus co (0.802), co versus w (0.974), ga versus co (0.890), ga versus w (0.851), nb versus zr (0.941), nb versus bi (0.850), ta versus bi (0.769), ta versus ga (0.638), ta versus nb (0.629) and ta versus w (0.538).the high positive correlations of these elements reflect their associations in the formation of rare metal ores from the na-rich late fluids. the pegmatitic k-feldspars have very high negative correlations of k/tl and k/rb with the rare lithophile elements such as cs, bi, y, rb, la, pr, pb (table 7). similarly, the muscovites have very high negative correlations of k/tl and k/rb with the rare lithophile elements as well as the ore elements sn and ta. nb is negatively correlated with rb, cs, tl, ta, la, pr and sn but positively correlated with k and ga in both the k-feldspars and muscovites (table 7). nb and ga have positive correlations in both muscovites and k-feldspars (0.704 and 0.472) respectively. obviously, the 222 akintola, o. f. and adekeye, j. i. d. sample lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 v 5 1 4 15 6 5 16 4 8 w 502 346 201 247 298 64 46 89 124 y 6 1 0 0 1 0 1391 1 0 zn 229 63 26 187 12 39 62 26 119 zr 17 12 17 68 18 5 17 54 7 h2o 2.42 0.43 1.1 0.82 0.36 8.8 0.51 5.27 7.54 sum 98.71 98.24 97.67 99.4 99.79 98.01 98.12 102.94 86.53 k 33457 1577 11540 3321 1079 9132 1494 3487 664 k/rb 7 23 33 13 35 21 32 12 11 mg(hx) 13 139 7 23 li(hx) 225 685 16400 20750 li(fusion) 227 2900 13366 17882 na/k 1.17 43.74 5.81 19.80 72.80 <1 21.80 4.23 n.d nb/ta 0.26 0.69 0.93 1.07 0.39 0.89 9.05 0.24 0.29 223 mineralization potentials of pegmatites in the nasarawa area of central nigeria t a b le 3 . t ra ce el em en ts in th e p eg m at it e m ic as s a m p le (p p m ) le 7 le 8 le 1 4 le 1 3 le 1 8 l4 l1 0 b l8 a l9 a lz lu a ls 7 l c1 9 lc 2 0 lc 2 0 a s1 k k a lc 2 3 lc 2 8 w w 2 r lc 3 0 lc 3 1 lc 3 2 lc 3 3 f 2 9 3 3 3 1 9 7 3 2 1 5 3 2 2 0 2 9 7 2 7 4 3 2 3 3 1 3 1 2 8 8 4 4 3 5 2 4 6 5 2 1 0 6 6 1 7 1 0 0 4 3 9 0 3 9 3 3 2 9 6 6 7 4 6 1 2 8 4 1 2 2 5 4 2 6 1 6 2 9 6 5 4 7 5 1 9 4 0 1 0 8 8 8 8 1 3 7 4 b a 1 5 6 0 3 1 2 7 2 4 1 2 3 0 3 5 2 0 3 1 2 3 5 3 7 6 9 3 5 1 9 1 3 2 8 1 5 4 2 2 5 1 0 0 7 4 3 3 1 6 2 3 4 4 1 8 b i 1 7 2 1 2 0 1 6 2 1 2 5 1 6 1 9 1 2 1 9 1 3 1 4 1 2 1 5 2 8 3 4 1 4 2 1 1 7 1 8 2 0 1 1 1 8 1 5 1 3 1 2 1 3 1 7 c e 1 2 2 9 5 2 0 2 6 1 3 0 1 1 0 0 0 0 9 3 2 3 9 3 7 1 8 2 6 1 4 1 2 9 0 2 7 0 2 2 0 2 3 c s 1 7 1 8 7 4 2 9 0 3 0 0 2 6 0 8 7 4 1 0 0 1 0 0 1 2 3 2 8 9 6 2 0 6 3 2 1 1 8 3 4 2 2 1 2 0 2 4 6 7 4 2 4 5 1 5 6 9 4 5 1 6 9 4 2 1 5 1 7 4 9 2 9 1 4 9 9 6 3 1 1 6 3 4 6 g a 1 6 7 1 6 3 1 6 3 1 5 3 1 6 7 1 6 8 1 5 8 1 6 4 1 6 2 1 7 4 1 5 5 2 0 3 2 0 0 1 6 6 1 5 6 1 6 4 1 5 9 1 4 1 1 7 6 1 7 4 1 9 7 1 5 4 1 4 5 1 6 2 9 7 1 4 7 1 6 1 1 3 8 la 0 5 7 1 5 2 0 2 3 5 4 4 4 4 5 1 0 3 6 9 0 0 4 7 9 8 1 0 8 1 7 1 4 3 6 0 4 0 3 2 0 4 9 7 1 0 2 1 5 n b 1 8 1 1 1 6 1 7 8 1 6 5 1 8 2 1 3 1 1 1 9 1 1 3 1 8 5 1 4 4 1 7 8 2 1 8 2 5 6 1 8 7 7 5 5 5 1 4 6 1 5 1 1 6 0 2 1 1 1 4 3 1 9 3 2 0 0 1 2 7 6 4 2 2 3 1 9 0 1 3 5 n d 9 1 7 4 1 4 1 0 6 0 7 2 0 2 0 1 9 1 2 7 7 1 3 2 1 7 0 1 1 0 9 4 5 7 n i 7 2 7 1 7 1 8 2 1 2 3 2 4 2 4 2 1 2 3 1 3 8 9 1 8 2 7 3 1 1 8 2 1 1 6 1 1 2 0 1 0 6 1 7 1 1 1 6 1 8 2 4 p b 1 8 4 7 2 3 2 1 3 0 4 6 4 5 4 5 1 9 4 4 1 7 6 4 2 5 4 7 6 4 1 8 2 6 2 2 7 3 3 1 0 9 3 3 2 7 9 1 8 1 9 p r 3 1 5 6 8 8 1 4 1 2 1 2 6 1 1 6 1 3 1 1 1 9 2 2 6 6 9 2 1 1 7 3 1 1 1 3 2 4 5 r b 4 8 0 3 9 4 1 0 5 0 9 3 5 5 7 8 5 8 3 9 1 3 9 1 5 8 8 0 3 5 2 8 9 8 8 7 4 7 5 1 3 1 3 3 2 6 5 9 5 6 3 8 7 7 7 4 1 0 1 8 4 5 0 4 4 9 4 1 5 5 2 7 3 4 7 1 7 7 4 9 3 2 8 4 2 8 7 0 6 8 0 3 4 5 8 1 3 6 4 5 4 3 2 4 4 2 4 8 s n 2 1 7 4 7 1 2 7 5 3 3 6 3 5 7 5 2 5 5 3 3 5 9 7 2 7 4 6 8 1 2 3 9 6 1 2 6 6 3 9 7 6 8 1 6 4 9 2 9 5 3 6 4 3 9 4 8 7 4 3 7 1 3 9 1 1 8 9 0 2 5 3 9 1 4 7 2 7 1 2 1 9 s r 1 5 5 0 4 1 1 8 1 9 2 4 2 7 2 5 1 7 2 4 1 6 1 1 1 3 1 8 2 5 3 2 1 6 1 8 1 9 1 1 2 3 1 5 1 2 2 1 1 5 1 7 1 7 1 6 t a 4 4 5 8 4 5 5 0 6 3 5 3 5 9 7 1 7 1 7 1 4 1 2 7 4 6 7 2 5 0 2 4 2 5 7 5 8 5 1 1 5 3 1 5 1 6 2 6 4 1 2 0 1 8 3 3 9 3 1 1 4 0 t l 2 2 4 3 2 4 2 7 2 6 4 0 3 9 3 9 2 5 4 0 2 1 1 7 1 5 3 1 5 2 6 4 2 4 2 9 2 8 1 6 3 5 1 8 1 7 3 6 3 3 1 8 2 1 2 5 w 7 1 5 1 6 5 7 1 2 3 7 3 9 3 3 2 8 3 8 7 7 2 5 8 4 6 5 4 0 8 6 8 9 3 4 3 6 4 8 5 6 5 2 4 0 4 9 9 1 5 6 1 1 4 3 2 6 1 z n 4 2 1 1 0 0 0 4 5 2 4 3 5 4 7 2 9 6 1 3 9 1 1 4 1 6 8 0 0 3 7 9 2 3 1 1 3 1 2 1 7 1 1 1 1 2 3 1 4 2 2 4 2 1 4 2 1 8 7 3 4 1 2 4 9 1 6 3 4 5 3 4 7 1 6 1 1 7 7 1 1 2 k /r b 1 7 9 1 6 1 5 1 4 9 9 9 1 5 9 1 7 2 7 3 1 1 4 1 0 8 1 8 1 6 1 5 2 3 1 0 2 6 2 8 1 2 1 7 2 3 1 8 1 9 m g (h x) 6 1 2 0 6 9 6 9 7 6 2 2 2 8 2 2 5 2 2 1 7 4 8 1 1 2 9 7 5 7 0 6 6 2 0 5 0 3 1 8 9 6 6 9 1 1 0 7 8 5 6 6 8 3 3 6 4 4 n b /t a 4 .1 1 2 3 .9 6 3 .3 2 .8 9 2 .4 7 2 .0 2 1 .5 9 2 .6 1 2 .0 3 4 .3 4 8 .0 7 5 .5 7 2 .6 0 .1 5 0 .1 3 1 .9 5 1 .7 8 1 .3 9 6 .8 1 2 .8 3 .1 1 3 .1 3 1 .0 6 0 .3 5 5 .7 2 6 .1 3 0 .9 6 k /r b (h x) 1 0 .9 5 6 .0 1 1 0 .6 6 9 .4 5 9 .6 1 6 .2 4 6 .3 0 6 .4 3 9 .6 0 6 .8 6 1 0 .5 8 1 5 .5 4 1 8 .3 5 9 .2 6 6 .9 5 5 .5 4 1 1 .2 9 1 0 1 0 .0 1 1 3 .4 8 7 .2 5 1 4 .2 4 1 5 .8 9 8 .2 1 9 .9 1 1 4 .0 5 1 1 .6 9 1 1 .1 4 m g /l i( h x) 1 .3 0 .3 4 1 .3 5 1 .1 7 0 .8 4 0 .2 6 0 .1 7 0 .1 9 1 .2 4 0 .2 4 1 .5 7 1 6 .2 3 2 .2 5 2 .2 1 5 7 .3 3 1 0 1 .6 1 1 5 .5 1 1 .1 3 1 1 4 .5 5 4 .4 6 8 .5 2 2 5 .7 2 2 .2 5 1 1 li -a a s (h x) 4 7 5 8 5 1 5 9 9 0 8 6 1 6 8 1 1 5 4 2 8 8 4 7 5 4 3 4 1 4 9 2 3 1 2 8 6 2 0 2 4 1 0 3 1 4 .5 1 6 4 li -a a s (f u s. ) 1 5 4 2 8 3 2 0 1 1 9 8 2 7 2 3 9 5 4 6 8 3 0 3 1 7 4 3 2 4 2 3 1 3 0 4 0 2 0 4 7 3 6 1 1 2 1 5 8 1 0 4 3 3 1 7 6 1 1 1 6 0 2 9 5 4 7 2 3 0 k -a a s (h x) 5 8 0 5 4 1 8 0 5 9 1 8 4 6 8 0 0 0 7 2 2 5 4 2 2 5 4 8 1 5 5 0 5 0 2 8 4 5 4 0 7 5 4 3 1 0 3 5 1 5 5 3 0 5 4 2 5 0 7 4 5 5 5 0 5 5 4 9 3 0 4 1 3 0 5 1 6 5 4 5 5 5 3 8 1 5 5 9 7 0 6 0 8 0 4 2 9 0 r b -a a s (h x) 5 3 0 6 9 5 5 5 5 4 9 5 9 0 0 8 7 5 8 9 5 1 1 2 4 4 4 0 4 4 5 5 3 2 5 1 5 5 4 4 0 6 2 0 6 3 5 4 7 0 4 2 5 7 4 5 3 7 5 6 8 0 2 9 0 3 2 5 5 5 5 3 8 5 4 2 5 5 2 0 3 8 5 -a a s (f u s. ) 2 8 1 9 5 2 9 9 3 7 0 5 3 6 0 4 6 8 6 1 0 6 3 4 3 6 0 3 8 0 8 2 5 1 2 2 3 0 0 3 9 3 4 4 2 3 1 6 8 8 6 3 2 5 3 3 0 8 3 4 1 5 0 2 7 8 3 4 2 9 1 1 4 4 4 2 1 8 3 4 4 3 5 3 0 7 7 2 2 0 7 3 2 5 8 3 5 8 0 224 akintola, o. f. and adekeye, j. i. d. table 4: average contents of trace elements (range given in brackets) in the k-feldspars, na-feldspars and micas of the pegmatites k-fedlspar(n=11) muscovite(n=29) albitesn=4(true albites) ba 46ppm(10-91) 31ppm (3-100) 54ppm (26-102) cs 1214ppm (111-3489) 664ppm (32-2467) 31ppm (9-74) ga 19ppm (14-44) 161ppm (97-203) 27ppm (21-39) la 60ppm (2-166) 33ppm (1-109) bdl nb 12ppm (4-39) 155ppm (55-256) 145ppm (62-326) pb 73ppm (10-140) 27ppm (4-64) bdl pr 13ppm (1-29) 9ppm (1-22) bdl rb 6504ppm (2593-9534) 5930ppm (2659-10182) 175ppm (31-347) sn 34ppm (8-187) 383ppm (61-902) 221ppm (14-659) sr 67ppm (22-183) 22ppm (11-64) 361ppm (51-1037) f bdl 2128ppm (329-4751) bdl ni 9ppm (0-31) 18ppm (6-31) bdl ta 2ppm (0-9) 96ppm (27-502) 195ppm (67-305) ce 28ppm (0-69) 15ppm (0-47) 12ppm (0-22) zn 3.64 379ppm (47-1023) 72ppm (12-187) tl 40ppm (12-73) 30ppm (15-64) bdl w 178ppm (137-241) 62ppm (25-237) 273ppm (201-346) y 16ppm (0-30) 15ppm (0-39) 0.5ppm (0-1) k/tl 3362 (1558-7493) 3096 (1206-5485) rb/tl 168 (130-216) 198 (139-235) k/rb 19 (12-35) 16 (8-31) 26 (13-35) k/cs 211 (33-811) 121 140 k/ba 3226 (1165-11058) 2575 81 al/ga 5359 1197 3680 nb/ta 7.04 2.89 (0.15-8.07) 0.77 nb-dominant minerals crystallize earlier than ta and the positive correlations of nb, k and ga indicate that nb enters the silicate structure more easily than ta. this agrees with the observations of cerny and burt (1989) that nb/ta ratios in silicates are rather high. linnen (1998) has also observed that the different behaviour of ta and nb during crystallization is due to differences in solubility of nb and ta in li-rich pegmatitic melts. such behaviour has also been observed in the bruno and xuxa pegmatites of the aracuai pegmatite district of brazil (preinfalk et al., 2000). in the muscovites (table 7), ta has a very high positive correlation (0.756) with cs but lower correlations with rb in both muscovites and the albites (0.428 and 0.213 respectively). ta and k/rb however have better correlations in the muscovites and albites (-0.441 and -0.592 respectively) when compared with ta and rb. the reason for this relationship between the elements may be explained by the fact that ideal positions for tao 4 3and tao3are lacking in the silicates. therefore, ta partitioning into mica will be small and tao 4 3which grow epitaxially at the mica faces account for much of the ta contents in the micas (moller and morteani, 1987). mg/li has a high negative (-0.856) correlation with zinc; the high positive correlations of zn, li and f (³0.759) in themicas are traceable to the fact that the three elements occupy the octahedral layer in the mica crystal structure. li has negative correlation with ta (-0.244), which shows that li content of the early pegmatite micas are higher than the later ones; obviously increased concentration of li through fractionation in the late fluids crystallized to form discrete li-minerals such as amblygonite-montebrasite rather than enter mica phyllosilicate structure. this is in agreementwith the observation of cerny et al (1985) that the buildup of li concentration in a pegmatite melt that precedes the precipitation of li minerals sensu stricto leads to increased contents of li in early rock-forming mineral phases. sn average value is highest in the muscovites; it also has much higher positive correlations with cs (0.902), rb (0.829), tl (0.868), la (0.799) compared with the very low correlations it has with these elements in the feldspars, which reflects the enrichment of tin with greisenization. using k/rb, k/tl and cs, it is therefore possible to recognize different generations of feldspars and micas within a complex-zoned mineralized pegmatite (table 8). e.g. early and late, which have significant differences in the range, and mean contents of the rare elements. generally, the rare alkali (rb and cs) and tl concentrations in the muscovites and k-feldspars give a good reflection of the concentration of the rare elements in the pegmatites. the liberia (lb) pegmatite deposit in this section, the trace element contents of the k-feldspars and muscovites of pegmatites in the nasarawa area are compared with those of well-studied pegmatites both within and outside nigeria. the liberia pegmatite in the study area is the most exposed of the studied pegmatites, because some blasting and drilling had been carried out on it. twelve (12) muscovite samples of the pegmatite were analyzed. extensive fractionation within the pegmatite is indicated by the wide range of k/rb 225 mineralization potentials of pegmatites in the nasarawa area of central nigeria rare-elements pegmatites’class muscovite class cs 10000 k/rb 10001000 10 100 1000 figure 4: plots of k/rb versus cs for the pegmatites’ muscovites. note: the dash line represents the boundry between the muscovite and rare-metal classes (after cerny and burt, 1984). the solid lines represent the best fit lines for the lct type pegmatites. 226 akintola, o. f. and adekeye, j. i. d. t a b le 5 . c o rr el at io n m at ri x o f so m e tr ac e el em en ts th at h av e si g n if ic an t b iv ar ia te c o rr el at io n s in th e n af el d sp ar s (n = 5 ) b a c s g a n b r b s n t a w z n z r k /r b n b /t a b a 1 .0 0 0 c s -0 .8 6 8 1 .0 0 0 g a -0 .2 3 7 0 .6 6 9 1 .0 0 0 n b -0 .1 8 3 0 .1 2 2 -0 .1 6 8 1 .0 0 0 r b -0 .9 0 0 0 .9 2 2 0 .5 6 5 -0 .0 8 7 1 .0 0 0 s n -0 .4 2 7 0 .6 1 3 0 .3 7 1 0 .1 3 2 0 .3 2 5 1 .0 0 0 t a -0 .1 4 9 0 .4 8 6 0 .6 3 8 0 .6 2 9 0 .2 1 3 0 .5 5 5 1 .0 0 0 w -0 .2 6 5 0 .6 7 7 0 .8 5 1 -0 .1 6 6 0 .4 6 5 0 .4 8 3 0 .5 3 8 1 .0 0 0 z n -0 .8 9 1 0 .8 6 6 0 .2 7 4 0 .4 0 2 0 .7 1 8 0 .7 4 0 0 .3 9 3 0 .4 6 7 1 .0 0 0 z r -0 .3 5 1 0 .1 6 0 -0 .2 7 5 0 .9 4 1 0 .0 6 9 0 .1 5 2 0 .4 5 1 -0 .3 5 5 0 .4 2 7 1 .0 0 0 k /r b 0 .8 3 4 -0 .9 7 3 -0 .6 0 8 -0 .3 0 1 -0 .4 7 6 -0 .8 1 8 -0 .5 9 2 -0 .6 8 5 -0 .9 2 8 -0 .2 9 0 1 .0 0 0 n b /t a -0 .0 1 6 -0 .4 6 1 -0 .9 6 1 -0 .3 1 5 -0 .1 3 1 -0 .3 5 4 -0 .5 4 1 -0 .8 3 6 -0 .0 4 5 -0 .3 9 6 0 .3 9 6 1 .0 0 0 227 mineralization potentials of pegmatites in the nasarawa area of central nigeria t a b le 6 . c o rr el at io n m at ri x o f tr ac e el em en ts th at h av e si g n if ic an t b iv ar ia te co rr el at io n s in th e k -f el d sp ar s (n = 1 1 ) c s g a l a n b p b p r r b s n t l w k /t l r b /t l k /r b k /c s c s 1 .0 0 0 g a -0 .6 6 7 1 .0 0 0 l a 0 .9 4 7 -0 .8 0 1 1 .0 0 0 n b -0 .5 6 6 0 .7 0 4 -0 .6 9 4 1 .0 0 0 p b 0 .7 3 8 -0 .8 5 2 0 .7 9 2 -0 .7 8 3 1 .0 0 0 p r 0 .9 2 3 -0 .8 1 2 0 .9 9 7 -0 .6 8 5 0 .7 7 7 1 .0 0 0 r b 0 .9 4 3 -0 .5 8 5 0 .9 0 3 -0 .4 5 4 0 .6 1 0 0 .8 8 8 1 .0 0 0 s n 0 .2 6 0 .1 4 7 0 .0 3 3 0 .2 9 2 -0 .2 1 6 0 .0 5 7 0 .2 3 6 1 .0 0 0 t l 0 .9 8 0 -0 .6 6 2 0 .9 4 0 -0 .5 6 4 0 .7 6 1 0 .9 2 1 0 .9 6 7 0 .1 1 4 1 .0 0 0 w -0 .1 3 3 -0 .0 3 3 -0 .2 1 9 0 .2 4 5 -0 .2 4 7 -0 .2 3 6 -0 .2 5 4 -0 .4 6 3 -0 .2 7 5 1 0 0 0 k /t l -0 .9 8 9 0 .6 3 6 -0 .9 3 4 0 .5 6 1 -0 .7 4 1 -0 .9 1 1 -0 .9 6 3 -0 .0 8 1 -0 .9 9 7 0 .2 3 8 1 .0 0 0 r b /t l -0 .6 5 0 0 .5 9 3 -0 .6 2 8 0 .6 3 7 -0 .8 6 7 -0 .6 0 2 -0 .4 3 2 0 .3 0 1 -0 .6 4 8 0 .2 1 5 0 .6 5 0 1 .0 0 0 k /r b -0 .9 3 8 0 .5 3 2 -0 .8 8 5 0 .4 2 8 -0 .5 6 1 -0 .8 6 8 -0 .9 9 6 -0 .2 4 3 -0 .9 5 4 0 .2 3 6 0 .9 5 5 0 .3 9 7 1 .0 0 0 k /c s -0 .9 9 9 0 .6 5 1 -0 .9 3 9 0 .5 6 3 -0 .7 2 3 -0 .9 1 3 -0 .9 3 3 -0 .0 0 3 -0 .9 7 1 0 .1 0 4 0 .9 8 3 0 .6 4 6 0 .9 3 1 1 .0 0 0 228 akintola, o. f. and adekeye, j. i. d. t a b le 7 . c o rr el at io n m at ri x o f tr ac e el em en ts th at h av e si g n if ic an t b iv ar ia te co rr el at io n s in th e p eg m at it es ’ m u sc o v it es f c s g a l a n b r b s n t a t l z n l i( x) l i( fu s) r b (x ) r b (f u s ) k /r b n b /t a k /t l r b /t l m g /l i f 1 .0 0 0 c s -0 .1 1 6 1 .0 0 0 g a -0 .1 5 5 -0 .3 2 9 1 .0 0 0 la -0 .0 0 2 0 .9 1 3 -0 .3 2 7 1 .0 0 0 n b 0 .2 1 7 -0 .8 7 5 0 .4 7 2 -0 .7 5 3 1 .0 0 0 r b 0 .1 8 0 0 .8 2 1 0 .0 1 2 0 .7 7 1 -0 .6 9 6 1 .0 0 0 s n -0 .0 3 6 0 .9 0 2 -0 .1 8 5 0 .7 9 9 -0 .7 0 0 0 .8 2 9 1 .0 0 0 t a -0 .5 0 7 0 .7 5 6 -0 .3 9 4 0 .6 7 5 -0 .7 9 6 0 .4 2 8 0 .5 9 5 1 .0 0 0 t l -0 .0 4 8 0 .9 2 2 -0 .1 5 6 0 .8 5 1 -0 .8 5 8 0 .9 3 9 0 .8 6 8 0 .6 8 2 1 .0 0 0 z n 0 .7 5 9 0 .1 6 0 0 .3 4 8 0 .1 9 3 0 .0 7 5 0 .5 5 9 0 .2 8 1 -0 .3 7 5 0 .2 8 7 1 .0 0 0 li (x ) 0 .6 6 6 0 .1 2 7 0 .0 4 4 0 .1 8 3 0 .0 5 6 0 .4 5 2 0 .2 3 2 -0 .2 4 4 0 .2 5 0 0 .8 1 4 1 .0 0 0 li (f u s) 0 .6 3 0 0 .1 6 7 0 .0 0 5 0 .2 1 4 0 .0 0 5 0 .4 5 5 0 .2 7 5 -0 .1 9 4 0 .2 8 4 0 .7 7 3 0 .9 8 0 1 .0 0 0 r b (x ) 0 .2 7 9 0 .5 8 3 0 .0 7 2 0 .5 5 8 -0 .4 3 1 0 .8 5 9 0 .6 1 9 0 .1 8 2 0 .7 0 9 0 .6 0 8 0 .5 1 8 0 .4 4 5 1 .0 0 0 r b (f u s) 0 .1 1 6 0 .6 9 6 0 .1 2 9 0 .6 2 3 -0 .5 6 8 0 .9 1 9 0 .7 6 2 0 .3 3 3 0 .8 3 3 0 .5 1 8 0 .4 0 8 0 .4 3 1 0 .8 3 9 1 .0 0 0 k /r b -0 .1 6 4 -0 .8 3 0 0 .0 0 6 -0 .7 7 2 0 .7 0 8 -0 .9 9 8 -0 .8 4 0 -0 .4 4 1 -0 .9 4 3 -0 .5 3 4 -0 .4 4 1 -0 .4 4 6 -0 .8 5 8 -0 .9 2 1 1 .0 0 0 (8.0-17), k/cs (66-406), k/tl (1600-3764), and nb/ta (1.59-4.11) ratios in the micas (table 9). rb and cs contents and k/rb, k/cs as well as nb/ta ratios in the micas show that there are at least two generations of primary muscovites in the pegmatite, the early and the late. table 8. some trace elements distribution in the different muscovite generations in the liberia pegmatite deposit element early generation late generation rb(%): range 0.48-0.58 0.88-0.99 mean 0.52 0.92 cs(ppm):range 171-300 874-2353 mean 243 1168 tl(ppm):range 21-27 39-47 mean 24.16 41.33 ta(ppm):range 41-71 53-103 mean 52.33 69.16 nb(ppm):range 165-185 68-144 mean 178 115 sn(ppm):range 217-357 471-681 mean 283 575.67 k/rb:range 13.8-17 8.0-9.3 mean 15.58 8.77 k/cs:range 242-406 66-81 mean 301 74.6 k/tl:range 3007-3764 1600-2058 mean 3375 1941 nb/ta:range 2.61-4.11 1.59-2.47 mean 3.54 2.02 number of samples 6 6 the rb, cs and tl contents show enrichment in late muscovites by factors of 1.77, 4.81 and 1.71 respectively with a corresponding enrichment in the ore elements sn and ta by factors of 2.03 and 1.32 respectively but a noticeable depletion of nb by a factor of 1.55 in the late muscovite. thus, the enrichment of ta and sn in the late muscovites is accompanied by decrease in the k/rb, k/cs, k/tl and nb/ta ratios. while there is also enrichment in zn in the late muscovite, they are depleted in mgo and tio2. similar enrichments of the rare elements have been observed in the late primary muscovites of well-studied pegmatites like tanco, noumas, tip-top and harding. such late generation primary muscovites are found in the middle and central zones of the pegmatites where they occur in paragenesis with ores of the rare elements. in table 8, a comparison ismade of the rb, cs, ta and ga contents of muscovites of selected pegmatites in the study area, wamba (100km north-east of the area), and some well studied pegmatites from other parts of the world. the micas have high rb (0.8978%) and cs (2294ppm) which are comparable to those of the higly mineralized tanco pegmatite of manitoba, canada as shown in table 9. pegmatites in the area have a wide range of mineralization potential from low ta – pegmatites through medium ta-pegmatites to high ta-pegmatite. from table 9, it is evident that the rb and cs contents of the micas reflect ta mineralization in pegmatites. extremely low k/rb (8.86 or less), and k/cs (34 or less) characterize such highly mineralized pegmatites like tanco and liberia. the high ta-mineralization potentials of the complex pegmatites in this area is corroborated by the high average ta2o5 content (406ppm) and ta/nb ratio (3) of eighteen concentrate samples taken from different mines in the area (okunlola, 1998). muscovites of the mineralized pegmatites from wamba area have low ta-potential with correspondingly low cs (116ppm), rb (0.3150%) and high k/rb (25) and k/cs (800) nb/ta (4.50) ratios. the wamba pegmatites vary from barren, muscovite class to complex albitized, and sn-mineralized pegmatites (kuster, 1990). on the whole, it appears the pegmatities in nasarawa, the study area, are more fractionated and therefore have higher ta-mineralization potential than those of wamba and the other 229 mineralization potentials of pegmatites in the nasarawa area of central nigeria studied pegmatites in nigeria (matheis and kuster, 2001, personal communication). discussion and conclusion the very low ta contents in the k-feldspars may be attributed to the fact that tao3 cannot enter the fully polymerized (alsi)o4 network of the k-feldspar (moller and morteani, 1987). thus, the rare alkalis rb and cs are better indicators of the rare metalsmineralization potentials in the pegmatites’ k-feldspars than the ore elements ta. the higher contents of the rare earth elements (rees) ce, la, pr, and y in the k-feldspar when compared with those of the other major pegmatite minerals shows that the rees in the fluids from which the pegmatites crystallized were partitioned more into the k-feldspar than the other rock-forming minerals. this corroborates the observations of simmons and heinrich (1980) that ree-bearing minerals occur in k-feldspar-rich parts of pegmatites. the high negative correlation of k/rb versus cs is characteristic of k-feldspars of rare metal pegmatites and has been used along with k/rb versus cs in the micas as reliable prospection aids for rare metal pegmatites (preinfalk et al., 2000). high positive correlations between cs, rb and the rees: ce, la, and pr are notable in the k-feldspars, compared with the muscovites. this may be due to selective complexing of the rees in the residual melt by the framework silicate feldspar. the same process of selective complexing by p and f must have enriched the fluorapatite with high concentrations of the rees. the low average nb/ta ratio for the muscovites (2.965) is still lower than the (4±0.7) upper limit diagnostic of ta-pegmatites (cerny, 1989). higher nb/ta ratios (9.5±1.5) and high nb-concentrations (³200ppm) are diagnostic of the less specialized nb-rich columbite pegmatites. beus (1966) determined that ³20ppm ta concentrations are characteristic of columbotantalite pegmatites and gordiyenko (1971) also determined that 65-75ppm ta concentrations are characteristic of the ta-enriched pegmatite (figure 5). simultaneous enrichment of rb, cs along with ta in the lct granite pegmatite suites makes the alkalis reliable indicators of rare-metals mineralization in pegmatites. thus k/rb versus cs, and ta versus cs plots of primary muscovites have been reliably used to determine the mineralization potentials of pegmatites. 230 akintola, o. f. and adekeye, j. i. d. table 9. geochemical characteristics of some well-studied pegmatites compared with nasarawa pegmatites k(%) rb(%) cs(ppm) ta(ppm) ga(ppm) k/rb k/cs no of samples liberia 8.0831 0.6972 556 57 163 12.6 198 11 loc 20/20a 7.7997 0.8978 2294 464 160 8.86 30 2 k/ka 8.2231 0.5234 605 100 159 15.75 119 2 w/w2 8.3145 0.3077 196 63 150 27.12 374 2 tip top, south dakota (low ta) 8.52 0.355 222 56.0 175 24 384 13 tanco, manitoba (high ta) 8.33 2.450 2420 240.9 433 3.4 34 19 noumas, namaqualand 8.21 0.357 566 74.2 92 23 145 2 harding, new mexico (low ta) 9.47 0.631 1917 64.0 123 15.0 49 21 wamba, central nigeria n.d 0.3150 116 53 n.d 25 800 51 matheis (1979) has earlier shown that mineralized pegmatites in ijero and egbe area of southwestern nigeria are strongly depleted in ba, sr, and zr but are enriched in rb, li, y, be, sn, nb, and ta. kuster (1990) has also observed similar geochemical evolution in late pan african tectonic granites and mineralized pegmatites in wamba area. the relative enrichment of ba, sr, w, zr and ta in the na-feldspars indicate that pegmatites in nasarawa area crystallize from a more fluid and rare metals-enriched melt than those of ijero and egbe, as well as wamba areas. these results agree with observations of cerny et al. (1985) who noted that these elements are enriched in late hydrothermal stages of pegmatite formation. high activity of p during the primary pegmatite crystallization in this area resulted in the formation of the amblygonite subtype (burt and london, 1982; london and burt, 1982b; cerny, 1991b) of the complex pegmatites. the exhaustion of li and f by the phosphates from pegmatitic melt may account for the li-, and f-poor micas with the highest ta contents which probably crystallized after the crystallization of the amblygonites in the area. tl, rb, cs and the rees la, ce, pr fractionation in k-feldspars and white mica demonstrates congruent/similar trends. k/rb, k/tl and k/cs ratios are lower in the white mica than the k-feldspars. ta and cs have the highest positive correlation (0.756) in the micas, therefore low k/rb (16), k/tl (3096) and k/cs (121) as found in the nasarawa area indicate high rare metal ta-nb-sn-li-be mineralization potentials as observed in the nasarawa pegmatites. acknowledgement financial assistance for the fieldwork by the nigerian government through the raw materials research and development council, rmrdc is gratefully acknowledged. the german government through the german academic exchange programme (daad) provided financial assistance for the rocks/minerals chemical analyses in the technical university of berlin (tub) which is also gratefully acknowledged. the authors are grateful to dr. g. matheis of the technical university of berlin (tub) for his assistance on the research. we are also grateful for the assistance of professor peter moller and dr. peter dulski of the geoforshungs zentrum (gfz), potsdam for their assistance in analyzing the rare earth and trace elements of some of the samples. references ajibade, a. c. and wright, j. b. 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(1976) fracture systems in nigeria and initiation of fracture zones in the south atlantic. tectonophys. 34, 43-47. 234 akintola, o. f. and adekeye, j. i. d. geofisica colombiana n° 5 p p 1 2 1 7 diciembre de 2001 bogota d.c. issn 0121 2974 modelacion del problema inverso en geoeh~ctrica 20 mediante elementos finitos luis alberto briceno profesor a s o c ia d o departamento de geociencias facultad de ciencias universidad na c io n a l de colombia pedro mauricio avellaneda l6pez ingeniero civil estudiante de maestria en recursds hidr aulic o s departamento de ingenieria civil facultad de ingenieria universidad nacidnal de colombia resumen se plantea un modelo nurnerico para la solucion del problema inverso en geoelectrica 2d utilizando el metodo de los elementos finitos. la rnetodologia de solucion se fundamenta principal mente en dos procedimientos: a) solucionar nurnericamente la ecuacion fundamental de flujo de corriente para unos valores de resistividad establecidos, para luego calcular su respuesta electrica (resistividades aparentes calculadas), y b) minimizar la diferencia entre las resistividades aparentes medidas y las calculadas utilizando un model a de inversion. el metodo es iterativo. para el primer ciclo se establece un modelo de bloques 0 parametres hornogeneo, que se modificara sucesivamente por el proceso de inversion, procurando que el error minimo cuadratico sea menor que 10%. luego se elabora un programa de computador que sirva como herramienta de calculo y que represente graficarnente los resultados obtenidos para facilitar el proceso interpretativo. finalmente se present an dos casos de secciones tomograficas elaboradas en el parque nacional de la ciudad de bogota d.c. palabras clave' resistividad, tomografia, elementos finitos, modelaci6n numeaica 20. abstract a numerical model for the solution of the inverse problem in geoelectric 2d is proposed using the method of the finite elements. the solution methodology is based mainly on two procedures: a) to solve numerically the fundamental equation of current flow for established values of resistivity to calculate its electrical answer (apparent resistivities calculated), b) to diminish the difference between apparent resistivities measures and the calculated ones using an investment model. the method is iterative. for the first cycle, a model of blocks or parameters settles down homogenous, that successively will be modified by the investment process, trying that the quadratic minimum error is smaller to 10%. a computer program is elaborated that serves like calculation tool and graphically represents the obtained results to facilitate the interpretative process. finally two cases of tomografic sections in the national park in bogota d.c. are presented. key words: resistivity, tomography, finite element, 2d numerical modelling. introduccion los model os numericos como el metodo de los elementos tinitos (mef) se utilizan como herramienta de soluci6n cuando se tiene una ecuaci6n de campo que gobiema el flujo, y ademas se conocen unas condiciones de frontera. para el caso particular, en una tomograffa de resistividades electricas (tre) tinalmente se llega a un mapa de resistividades que resulta de modelar el flujo de una corriente a traves del subsuelo. en el trabajo que aquf se presenta se analiza el modelo bidimensional, que asume que el rumbo de las estructuras geol6gicas es normal al arreglo de electrodos empleado en superficie, es decir, que la geologia perrnanece con stante. ei procedimiento que se utiliza en este trabajo consiste en encontrar la respuesta electrica de una primera hip6tesis que represente las caracteristicas del subsuelo, y compararla con la obtenida en campo, para posteriormente efectuar una correcci6n a esta hip6tesis inicial, y as! sucesivamente encontrar algun modelo que aproxime de mejor manera el comportamiento electrico registrado en campo. el mef se utiliza entonces para encontrar la respuesta electrica del modelo hipotetico. 12 en general, el rnetodo consiste en discretizar el medio continuo mediante un conjunto de puntos que unidos formen una red de elementos, e individualmente se comporten como un medio hornogeneo, es decir, con resistividad constante. de esta manera se modeian medios heterogeneos; basta s610 utilizar un valor de resistividad distinto para cada elemento. las ventajas de este metodo radican en la posibilidad de simular medios heterogeneos con relativa comodidad, ya que s610 se debe ajustar una malla apropiada de elementos finitos; adernas, permite simular fronteras irregulares, tales como la topograffa de la superficie. la principal ventaja radica en que es un metoda que se puede sistematizar, y en el cual el grado de detalle depende de la densidad de elementos de la red. ei trabajo que aqui se presenta consiste en la modelaci6n del subsuelo mediante elementos tinitos utilizando un procedimiento de inversi6n de datos geoelectricos por minimos cuadrados y el rnetodo de quasi newton (loke y barker, 1996). geofislca colombiana, 5, diciembre de 2001 modelacion del problema inverso en geoelectrica 2d mediante elementos finitos marco teorico ecuaciones constitutivas del flujo de corriente eli~ctrica en el subsuelo partiendo de la ley de ohm (dey y morrinson, 1979) se llega a la ecuaci6n 1, que describe matematicamente el flujo de corriente en un espacio heterogeneo tridimensional, en el cuallas fuentes de corriente son puntuales. -v.[ 1 vl/>(x,y,z)l= io(x )o(y)o(z )(1)p(x,y,z) 'j s s s la fuente de corriente se representa maternaticamente por la funci6n delta de dirac 8. ei medio 10representa la resistividad p(x,y,z) en cualquier punto, y el potencial electrico la variable ¢(x,y,z). debido a que el problema se analiza en el plano (x,z) se debe aplicar la transformada de fourier, con el objetivo de representar todo el problema en el plano de analisis, de esta forma, la ecuaci6n (l) se transforma de la siguiente manera: en la ecuaci6n (2) la variable inc6gnita es el potencial transformado ¢t, y ademas se utiliza la variable de transformaci6n kyola selecci6n de la variable de transformaci6n se hace siguiendo la metodologfa propuesta por weller y kample (1996), la cual propone que la variable de transformaci6n 0 mimero de onda es funci6n de la abertura entre electrodos de corriente. k = 0.06 k ymin a, ~ ymax max 6 (3)<; amax=maxima distancia entre la posici6n de un electrodo de corriente y un electrodo de potencial en superficie, para el problema en cuesti6n. amin=mfnima distancia entre la posici6n de un electrodo de corriente y un electrodo de potencial en superficie, para el problema en cuesti6n. kymax= maximo valor de la variable de transformaci6n. kymin= mfnimo valor de la variable de transformaci6n. es necesario recordar que para encontrar los potenciales reales se debe aplicar la transformada inversa de fourier, de acuerdo con la ecuaci6n (4): discretizacion mediante elementos finitos utilizando el metodo de galerkin en general, el metodo de los elementos finitos aplicado a un elemento de la red consiste en encontrar una soluci6n aproximada de la variable inc6gnita, que al ser remplazada en la ecuaci6n de campo (2) produce un error, ya que no es la soluci6n exacta. el metodo de galerkin consiste en minimizar este error, ponderandolo por alguna funci6n, que para el caso particular seran las funciones de forma 0 de interpolaci6n geoflslca colombiana, 5, oiciembre de 2001 que se utilizan para encontrar el valor del potencial dentro del elemento. finalmente, para cada elemento se puede obtener una ecuaci6n matricial de laforma: donde [c] representa la matriz de conductancia del elemento, (i el potencial en los nodos de la red, y s las fuentes de corriente. para el presente caso se utilizan elementos rectangulas; por ella el subfndice cuatro en la ecuaci6n (5), pues representa el numero de nodos de elemento. al sumar todas las contribuciones de los elementos se obtiene el sistema matricial global, que se representa de manera similar que la ecuaci6n (5), s610 que ahora involucrando la totalidad de los nodos de la malla. solucionando el sistema de ecuaciones se encuentran los valores del potencial transformado en los nodos de la red. posteriormente se emplea la ecuaci6n (4) para encontrar el potencial real en los electrodos de corriente. procedimiento de inversion una vez calculada la respuesta electrica del modelo supuesto, se debe realizar un proceso de inversi6n de datos, en el cualla respuesta electrica calculada se relacione con la respuesta electrica obtenida en campo, para de esta forma obtener una matriz correctora del modelo inicial (parametres iniciales). la siguiente expresi6n se utiliza como ecuaci6n de recurrencia en el proceso de inversi6n (sasaki, 1989): ~g :diferencia entre los datos medidos en campo gm en la iteraci6n (i) y las resistividades aparentes calculadas a partir de la modelaci6n mediante elementos finitos. j :matrizjacobiana que contiene las derivadas parciales de la resistividad del bloque respecto a los dernas, ~p : vector correcci6n de los pararnetros del modelo. c: filtro a: factor de amortiguamiento. cada vez que sea utilizada la ecuaci6n (6), se obtiene una matriz columna ~p correctora de los parametros iniciales que servira para la obtenci6n de un nuevo modelo. pi + i = pi +!1p (7) los valores de resistividad del nuevo modelo de bloques se encuentra utilizando la ecuaci6n (7). algoritmo de trabajo los pasos por seguir son los siguientes: • iniciar can un modelo hornogeneo; usual mente se torna el promedio de los logaritmos de las resistividades medidas (lake and barker, 1995). los calculos se efecnian utilizando notaci6n logarftmica, pues de est a forma se evitan resultados negativos, y ademas se est a acorde con resultados experimentales, que rnuestran c6mo usual mente los cambios de resistividad en el subsuelo son 13 briceno £1 al. l\'lalla de elementos finitos rt. --1 .. nodos del elemen/ mediante mef se encuentra: q>1'<pj,<pk,q>m j i t---i--»<: k m <1>, dentro del elemento se encuentra interpolando $. $. $k $ i' j' 'm figura j. malia de elementos finites que une los puntas 0 nodos que discretizan el continuo logarftrnicos. luego se calcula el vector discrepancia entre los datos tornados y 1a respuesta del modelo. como el primer modelo de resistividades es hornogeneo, el computo de la respuesta electric a mediante elementos finitos se puede omitir, pues te6ricamente debe ser igual al modelo supuesto. se calcula la matriz jacobiana, luego se construye la matriz filtro c, y se termina con la soluci6n de la ecuaci6n (6). el factor de amortiguamiento se selecciona segun el nivel de perturbaci6n de los datos tornados. generalmente se toma un maximo valor para la primera iteraci6n (a. = 0.2) y se disminuye en las siguientes, hasta un valor minima de (a. = 0.04). el nuevo juego de parametres resulta de sumar el 'lector lip al modelo inicial. ei procedimiento asi descrito se repite tres 0 cinco iteraciones. para controlar la convergencia del proceso se toma como indice el error minima cuadratico (emc). se aceptan resultados con emc < 10%. programa de computador geomodel es un programa que permite el analisis de datos geoe lectricos tornados en dos dimensiones 0 tomografia de resistividades electricas, cuando se emplean en campo los arreglos wenner-schlumberger, wenner 0 dipolo-dipolo (avellaneda y perez, 2000). el program a lee de un archivo de texto el valor de resistividad aparente tornado en cada lectura horizontal para cada nivel de datos, luego aproxima la profundidad de investigaci6n y efectua una interpo-laci6n de los valores de resistividad. la malla de elementos finitos se genera autornaticarnente; se establecen unas dimensiones a la red y los elementos se construyen de forma tal que puedan ser contenidos por el modelo de bloques. 14 la malla esta conforrnada por una red de elementos cuadrilaterales, y se consideran sus limites hasta donde aproximadamente el valor del potencial sea cero. para efectos de la modelacion, se consideraron cuatro columnas de elementos adicionales a lado y lado de la pseudosecci6n de resistividades, y cuatro filas adicionales para los !imites inferiores. sin embargo, se permite al usuario modificar estos valores para que al final pueda verificar la sensibilidad de los resultados. seguidamente, el programa vincula de red de elementos con la red de parametres 0 "bloques" que se utiliza en el proceso de inversi6n . iterativamente se puede observar la respuesta electrica del modelo, y la pseudosecci6n de resistividades resultante del proceso de inversion. se aprecia la convergencia del modelo cuando la pseudosecci6n de resistividades calculadas se acerca a la tomada en campo, y el error medio cuadratico es menor al 10%. entrada de datos de resistividad, tipo de arreglo, niveles de datos modelo de resistividades hornoqeneo solucionar mediante elementos finitos k'---~ la ecuaci6n de flujo computar la resistividad aparente calculada, con los potenciales nod ales calcular la matriz jacobiana y resolver la ecuaci6n(6)para ~p p (i+1) = p(i) +~p 1 emc < 10% ( fin) figura 2. algoritrno de trabajo geofislca colombiana, 5, diciembre de 2001 modelaci6n del problema inverso en geoelectrica 2d mediante elementos finitos caso de estudio se realizaron dos tomografias en zona aledafia al parque nacional de la ciudad de bogota d.c. ambas se realizaron utilizando un arreglo tipo wenner; schlumberger, para una secci6n de 78 m, con 40 puntos de electrodos de corriente y unidad de espaciamiento entre electrodos de 2 m, 17 niveles de datos para un total de 357 datos y bloques en cada una. en las figuras 3 y 4 se muestra la pseudosecci6n de resistividades aparentes medidas para los ejemplos uno y dos; tambien, se muestra la red de elementos finitos empleada. en total se utilizaron 1892 elementos con 200 i nodos en ambos casos. adicionalmente, la red de elementos fue ampliada en cuatro elementos, a lado y lado de los ifmites laterales del estudio, y cuatro filas de elementos en la profundidad maxima de investigaci6n. la selecci6n de esta condici6n de frontera se obtuvo basicarnente al resolver los problemas con una red mas grande que la mostrada en las figuras 3 y 4, y observar un cambio despreciable en los resultados obtenidos. ,.. 1&2 34.9 .., 53] &),1) ,.. 0.7 i! i ii i ! i i! 33 '" 21.9 ,.. w d ~ " uo ~ n §~ ui lffl ~_______ = ===== "'~m figura 3. pseudosecci6n de resistividades aparentes medidas del ejernplo i. parque nacional, bogota d.c. las distancias en metros .. llj .a.s " '<> "'., ".j 53,7 72.4 i ~ ii ii i 11l"n i + i + i + i i i i i , i i oj 33 ,. .. ". ... ,.. ~ in i~ ~ ~ ~ ro~ ~ i) nw ~_______ = ===== qhm·m geof[sica colombiana, 5, oiciembre de 2001 figura 4. pseudosecci6n de resistividades aparentes rnedidas del ejernplo 2. parque nacional, bogota d.c. las distancias en metros 15 briceno ft al. en las figuras 5 y 6 se muestran los modelos de bloques que fueron utilizados para la soluci6n del problema inverso. en ambos casos se elaboraron tantos bloques como datos del modelo (en total 357 bloques). adernas, la topologfa de la red de elementos finitos fue construida de forma tal que los elementos pudieran ser contenidos por algun bloque del modelo de inversi6n. es necesario anotar que no fue elaborado un adecuado descarte de datos an6malos, y es un factor que se debe estudiar en posteriores investigaciones. valores muy altos respecto al promedio se puntean en las figuras 5 y 6. en las figuras 7 y 8 se muestran las secciones tomograficas finales para los ejemplos respectivos. para el primer ejemplo se realizaron figura 5. modelo rectangular del subsuelo para el ejernplo i parque nacional, bogota d.c. figura 6. modelo rectangular del subsuelo para el ejemplo 2. parque nacional, bogota d.c. cuatro iteraciones y se obtuvieron respectivamente los siguientes errores medios cuadraticos (emc): 17.2,28.1,26.7 y25.4%. para el segundo ejemplo tambien se realizaron cuatro iteraciones y se obtuvieron los siguientes errores medios cuadraticos: 8.6, 10.6,5.0 y 4.7%. en ambos casos se utiliz6 un factor de amortiguamiento (= 0.1). 16 conclusiones se presenta un algoritmo de soluci6n al problema inverso utilizando el rnetodo de los elementos finitos como herramienta para encontrar los potenciales medidos en superficie. como semilla del procedimiento de inversi6n se supone un modelo de resistividades homogeneo, a partir del cual se obtiene la primera matriz de derivadas parciales (matriz jacobiana) que sera corregida iterativamente. geoff sica colombiana, 5, qiciembre de 2001 modelacion del problema inverso en geoelectrica 20 mediante elementos finitos ·z.5 .. ift.2 s3,1 ". n... 0.7 3.3 '.0 e.e 1l.j 14.0 t s 8 20 w& 24) sss 0;9 26.,1 hi24l-~-=~-=~-==-=-=~-==-=-=~i!l!!::lj~'~~=-'===-~-=~-=";===-==='--'====''';===____ o",",m r:'33 f" f"113 ]4.0 ,.. fx ~,e ~.3 ~3,o 17 31 54 'n if,z 291 43'l s<is 14j 1 2s52 ~------=== __ ===== ohrn-m la principal variable que afecta la sensibilidad de los resultados es la selecci6n del mirnero adecuado de elementos laterales y en profundidad. en el trabajo que se presenta, esta condici6n se obtuvo al probar diferentes mall as de elementos y no detectar cambios apreciables en los resultados finales. otro factor que se recomienda investigar es la selecci6n adecuada de los mimeros de onda (ky) minimos y maximos, y c6mo afectan la soluci6n del problema. los valores recomendados por la literatura fueron utilizados en la presente investigaci6n (weller y kample, 1996). modificaciones adicionales que se pueden incluir en posteriores esquemas de soluci6n, consisten en elaborar una adecuada selecci6n y descarte de datos an6malos, permitiendo disminuir el mimero de bloques respecto aj mimero de datos de resistividad medidos en campo. geofisica colombiana, 5, oiciembre de 2001 referencias bibliograficas figura 7. secci6n (01110grafica final para el ejempia i parque nacional, bogota d.c. las distancias en metros. emc: 25.4% figura 8. secci6n rornografica final para el ejempia 2. parque nacional, bogota d.c. las distancias en metros. emc: 4.7% avellaneda, p. m., perez, f. h. 2000: soluci6n al problema inverso en geoelectrica continua mediante elementos finitos. proyecto de grado para optar al titulo de ingeniero civil. facultad de ingenierfa. universidad nacional de colombia. loke m. h. and barker r. d. 1996: rapid least squares inversion of apparent resistivity pseudosections by a quasy newton method, geophysical prospecting 44,131-152. dey a. and morrisonh. f. 1979: resistivity modelling for arbitrarily shaped twodimensional structures. geophysical prospecting 27, 106-136. weller,as. m., and kample,m. a 1996: induced-polarization modelling using comlex electrical conductivities. geophysical joumal lntemational, 127,387-398. sasakl, y. 1989: two-dimensional joint inversion of magnetotelluric and dipoledipole resistivity data. geophysics 54, 254-262. 17 geofisica colombiana pp 27-31 issn 0121 2974n° 5 i u n !o de 2002 bogota d.c utilizacion de redes determinacion de respuestas ondas coda: aptlcaclon para neuronales para de sitio a armenia, la partir de colombia carlos a. vargas~jimenez1, hugo monsalve2, maria l. bermudez3, julio c. cuenca4 resumen se han utilizado datos acelerograficos triaxiales relacionados con replicas del terremoto de armenia, colombia (25-01-99, mw = 6.2), para determinar la funci6n de transferencia formaciones blandas/roca dura en dos sitios de la ciudad de armenia mediante el uso de una red neuronal. dicha red fue entrenada con datos espectrales de roca dura como pararnetro de entrada y de formaciones blandas como pararnetro de salida para las componentes n-s y z. ei comportamiento de la red neuronal respecto a los datos de entrenamiento fue 0.01% yperrnitio simular datos espectrales para la componente e-w de los mismos registros con un error medio cuadratico menor que 8%. se pudo observar que a pesar de los pocos registros utilizados, las redes neuronales facilitan el proceso de deducci6n de funciones de transferencia lineales y no lineales orientadas a la evaluaci6n dinarnica de suelos. palabras clave: redes neuronales, funcion de transferencia, qndas coda, analisis de espectros, parametros de rocas, aespuesta de 81110 abstract triaxial acelerografic data related to replies of armenia earthquake in colombia (25-01-99, mw=6.2) have been used to determine the transfer function of soft formation i hard rock in two sites of armenia city by using a neuronal network. this network was trained with spectral data of hard rock as input parameter and spectral data of soft formation as output parameter for n-s and z components. the error of the neuronal network respect to the training data was of 0,01%. it allowed to simulate spectral data for the e-w component of registries with a quadratic mean error minor to 8%. it was possible to observe that with these few records used, the neuronal networks facilitate the process of deduction of transference functions linear and not-linear oriented to the dynamic ground evaluation. key words: neuronal networks, transfer function, spectral analysis, rock parameters introduccion los metodos de coda normalizada constituyen una herramienta confiable para la determinacion de respuestas de sitio, aspecto relevante dentro de la evaluacion de la amenaza sismica y de la microzonificacion sismica de ciudades. en particular, diversas tecnicas han sido utilizadas en las pasadas tres decadas para establecer parametres que confirrnen la validez de estos metodos. tsujiura (1978) fue el primero en demostrar la confiabilidad de la coda normalizada para encontrar amplificaciones de sitio relativas y compararlas con medidas a partir de ondas directas. tsujiura analizo varios sismogramas filtrados a ciertas bandas de frecuencias para eventos locales registrados en cuatro estaciones en mt. dodaira, japon, con diversa litologfa. establecio relaciones espectrales para la s-coda, medidas en los componentes horizontales en pares de estaciones, y cornparo con relaciones espectrales para las ondas s directas en el rango de frecuencias 0.75 hz a 24 hz. concluyo que las ondas s-coda estaban compuestas por la dispersion de las ondas s, y que la arnplificacion relativa entre dos estaciones podrfa ser determinada par relaciones espectrales de las ondas t universidad nacional de colombia, departamento de geociencias, santare de bogota, e-mail: cvargas@ciencias.unal.edu.co universidad del duindio. facultad de ingenierla. armenia, guindio. ingeominas, santale de bogota. univer stdad nacional aut6noma de mexico, instituto de ingenieria. mexico d.f geofisica colombiana, 5, diciembre de 2001 s-coda. tucker & king (1984) compararon los factores de ampli ficaci6n de sitios a partir del metodo planteado por tsujiura (1978) y concluyen que los resultados utilizando ondas coda presentan menos dispersion que cuando se usan ondas s directas. phillips & aki (1986) presentaron un metodo para invertir las amplitudes relativas en un conjunto de sitios usando una serie de ventanas de tiempo. ellos asumen que la forma de las curvas de decadencia de la coda es la misma para todos los sitios, para todas las fuentes, y que solamente la amplitud de las curvas difieren dependiendo del sitio y la naruraleza de la fuente. despues de aplicar su rnetodo para eventos registrados por 150 estaciones localizadas al none de california encontraron que las amplificaciones de sitio enla mayoria de las estaciones eran independientes de los eventos y ventanas de tiempo utilizadas; unas pocas dependencias parecen estar sujetas a estructuras geologicas locales. mayeda et al. (1991) rnidieron factores de amplificacion de sitio dentro y fuera del area de long valley caldera, california, encontrando que las amplificaciones de sitio en las bandas 1.5 hz a 3.0 hz estaban de acuerdo con los residuos de magnitud de coda para cada sitio. fehler et al. (1992) desarrollaron un mapa de facto res de amplificacion de sitio para la region de tokai, japon, hallando poca dispersion de los resultados para bandas por debajo de los 8 hz, y a diferencia de mayeda, rnuestra que los factores de amplificacion de sitio obtenidos por el metodo de coda normalizada para la banda centrada en 6hz estan de acuerdo con los resi27 duos de magnitud local, obtenidos previamente por noguchi (1990) a partir de medidas de maxima amplificaci6n para las mismas estaciones. kato et al. (1995) compararon factores de amplificaci6n obtenidos mediante el uso de ondas directas s y s-coda para el sur de california a partir de registros digitales de alta calidad para replicas del terremoto de landers en 1992. hallaron que las amplificaciones de sitio usando los dos metodos para seis bandas de frecuencias estaban en concordancia y dentro de un factor de 1.5. aspectos metodologicos en el presente trabajo se ha intentado determinar una funci6n de transferencia no analftica con capacidad de predecir efectos de amplificacion sismica bajo la relaci6n formaciones blandas/roca dura, que pueda en un momenta dado tener en cuenta aspectos no lineales de dicha relaci6n. para el efecto se ha elegido el entrenamiento de una red neuronal basado en observaciones espectrales de los componentes n-s y z para 7 eventos sfsmicos; posteriormente se han realizado simulaciones para el componente e-w en busca de establecer la confiabilidad de la red. de acuerdo con las apreciaciones de vargas (1999) quien realiz6 calculos de qi y qs mediante la utilizaci6n del rnetodo discriminante mltw (hoshiba et al 1993), el principal factor de atenuaci6n de la energfa sismica en esta zona es de caracter dispersivo, por 10 que se puede asumir que las ondas de coda pueden ser modeladas a partir de hip6tesis de distribuci6n hornogenea de dispersores, y puede ser validada esta idea al hallar resultados semejantes de respuesta de sitio mediante el uso de cualquiera de los componentes vectoriales de los registros. de este modo, las simulaciones desde cualquier componente deben mantener tendencias semejantes. redes neuronales las redes neuronales son un paradigma computacional de relativamente nueva aplicaci6n gracias a las bondades tecnol6gicas recientes; se trata de una serie de elementos simples, los cuales operan en paralelo. dichos elementos fueron inspirados en los sistemas nerviosos biol6gicos cuya modelaci6n maternatica se inici6 en la decada de 1940. segun las propuestas iniciales, las neuronas deben responder ante estfrnulos de cierta manera; asf, un modelo de respuesta para una neurona r-esima depende de las entradas e interacciones con otras neuronas segun la expresi6n (crespo, ~996): donde p;j son parametres de la conexi6r. entre la neuron a i y la j; c i son los parametres de la neurona i. la funci6nj, conocida como funci6n de transferencia interna, puede variar segun el disefio entre muchas posibilidades, como por ejemplo la funci6n escal6n, la funci6n lineal, la funci6n sigmoide, etc. de este modo, los disefios topol6gicos plantean que para un trabajo en particular, las conexiones entre las neuronas participantes (pi) deben constantemente ser reajustados (entrenamiento) para que respondan adecuadamente a una solicitud. asi, en el entrenamiento es donde los sistemas neuron ales aprenden. la figura i ilustra una topologia neuronal generica. 28 vargas etal. para el presente trabaj6 se opt6 por disefiar una topologia «backpropagation. (freeman, 1994), la cual establece que para cualquier numero de capas neuronales, los parametres de neurona ci son i, y las funciones de transferencia internas (f) suelen ser de tipo logsig, tansig 0 lineal. en particular fue utilizada una red con dos capas intermedias de 90 neuronas, la primera con funci6n tansig y la segunda con funci6n logsig. una de las ventajas mas importantes de esta topologia radica en su capacidad para adaptarse a problemas lineales y no lineales. dentro de su implementaci6n, la convergencia de error se hace a partir del algoritmo de descenso por gradiente, el cual se basa en la variaci6n de cada pij" input capa neuronal 1 /r----~\ /r----------~ capa neuronal n \ i • •• 0 u• t • p u • t figura i. topologfa generica de una red neuronal. las entradas interacnian con los pesos pij y ilegan surnados a la funci6n de transferencia jpara producir salidas deseadas datos en el presente trabajo fueron utilizados is acelerogramas provenientes de siete replicas asociadas con el terremoto de armenia, colombia (2501-99, mw = 6.2). dichos eventos fueron registrados en una estaci6n referencial donde afloran rocas con metamorfismo regional (calarca) y en dos estaciones sobre suelos organicos con potentes dep6sitos vulcano-clasticos (uniquindio y armeviva). ei sistema de registro consiste en aceler6grafos digitales triaxiales etna-kinemetrics con 24 bits de resolucion y respuesta plana para la banda de frecuencias de interes para este trabajo 2 hz a 25 hz. las localizaciones de los eventos fueron asociadas al sistema de falias activas que circundan la ciudad de armenia. la tabla 1 permite detaliar los eventos y las estaciones utilizadas. tabla 1. localizaci0n de eventos utilizados en el trabajo por las estaciones calarca, uniquindio y armeviva evento tncauzacten prof. mi eslacion estacion estacion calarcil uniquindio armeviva 9904242143 4.49n -75.72w 0200 209 v v 990424.0253 445n -75.70w 10.00 2.23 v v 99050522:05 4.52n -75.69w 01.57 245 v v v 9905072248 4.46n -75.62w 19.26 282 v v 99101313:10 4.32n -75.68w 06.67 410 v v 9910211550 4.47n ·75.62w 0799 345 v v 99110917:02 443n -75.5bw 01.02 3.80 v v geoff sica colombiana, 5, diciembae de 200-\ utilizacion de redes neuronales para la determinacion de respuestas de sitio a partir de ondas coda, aplicacion para armenia, colombia discusion y resultados chen (1999) ha ilustrado como la migraci6n hipocentral puede explicar los cambios temporales en la atenuacion de las ondas coda, y por ende influenciar calculos en el pararnetro b. si se analiza un poco este escenario, queda tacite el cuestionamiento sobre la influencia de este parametro en la respuesta de sitio,y sobre la validez de la hipotesis de phillips & aki (1986) respecto a la independencia de la forma de la coda en una region determinada. siguiendo la hipotesis de dispersores hornogeneamente distribuidos (aki & chouet, 1975), se ha cajculado el factor qc en las band as centradas entre 2hz y 25hz para las tres componentes, en busca de establecer la veracidad en la premisa de phillips & aki (1986), como se puede apreciar en las figuras 2 a 7, existe una independencia de la atenuacion de coda respecto a la distancia, y se observa que la premisa de dispersores homogeneamente distribuidos es valida para las tres componentes. en la figura 8 se puede apreciar una muestra de los registros acelerograficos utilizados, tanto en el dominio del tiempo como de la frecuencia, la amplificacion sismica es evidente en la relacion litologica formaciones blandas/roca dura, asi como su independencia respecto a la ley de atenuacion de ondas coda verificada en las tres cornponentes. bajo esta apreciacion el entrenamiento de la red neuronal fue ilevado a cabo a partir de las componentes n-s y z para todos los eventos, buscando que la red neuronal se comporte como la funcion de transferencia formaciones blandas/roca dura. la figura 9 detalla los espectros simulados a partir de la red neuronal y verdaderos, ambos sobre las formaciones blandas. las dos simujaciones superiores coinciden exactamente con los espectros verdaderos porque fueron tenidos en cuenta durante el entrenamiento de la red neuronal. los siguientes ocho espectros simulados siguen aproximadamente las tendencias de los espectros verdaderos, como se ha mencionado con anterioridad, la premisa de dispersores homogeneamente distribuidos en un espacio tridimensional es <t!en g::,[ ~ ::'1 ~~'" ~ ~ ;';" ;~ j 0 --l 0 c..::> :::: l.lj 10a cl10· 100 10' 10'100 10' 10' --l ~ :::[ g ::,[ ~ '":''' j « ~, ",m~ ~ • j z 0 c3«z 10'· loa clio" 10' la' la' 10' 10' « ( 10' ~ : ':"~; • j ~::f ~:'" : 1 cl -i en ( q 10,l a:: l.lj ~ 10j >-~ ~loa 3la' 10' 10' 100 10' 10' fre.que.rcy (hz) frequency (hz) figura 2. qc-i para las estaciones calarca, uniquindfo y arrneviva en sus ires componentes en el tango de frecuencias 2 a 25hz figura 3. qc·1 para 1a estacion calarca en sus [res ccmponenres en el rango de frecuencias 2 a 25hz g::f ~'t'" ~ j l :::[ >~ j 10~ 10· 100 10' 10' io" 10' 10' ~::'i ~'"~~''' j l::'1 :::~'"":; • j::::::0:;~22 1o~ w· 100 10' 10' 100 10' la' ~ ::,[ ~~." 1 i::f :: ~ • j~~. 10'· , 10~ 100 10' 10' 100 10' 10' frequency (hz) frequency (hz) figura 4. qcj para la estacion uniquindfo en sus ires componentes en el rango de frefigura 5. qc·1 para la estaci6n armeviva en sus rres cornponentes en el range de frecuencuencias 2 a 25hz cias 2 a 25hz geofisica colombiana, 5, diciembre de 2001 29 �:~'f ;;; ~10" 10' 101 o m: f ~;c2;w £sii: 10'~ 10' 10' : :~'f ~>; 10·f gk; 10' 10' frequency (hz) figura 6. qc·1 para las esiaciones calarca, uniquindfo y armeviva en sus tres cornponenres en el fango de frecuencias 2 a 25 hz. en azul calarca, verde uniquindfo y rojc armeviva. x 10~ 1f 990424-2143calarcac1 i_~~---,",;.:-~h~ifllwl'ii>'."'_--,-----------~ 10.3 6 10 12 16 18 2014~+..~~:::~:~::.~~ 1 o 4 10 12 14 s ::1 : ",,"1::1 c. 10 4 (l jl ....oi.i..... _ 10' 10' 10' 0 50 100 frequency(hz) frequency(hzi 16 18 20 figura 8. registros acelerograficos en roca (calarca) y formaciones blandas (armeviva) dande se puede apreciar la amplificaci6n sismica en el dominic del tiempo y la frecuencia, asi como su independencia respecto a la ley de atenuaci6n de ondas coda verificada para la region circundante a la ciudad de armenia en virtud de la similitud de las relaciones espectrales en cualquiera de las componentes utilizados (figura 10); el error medio cuadratico estimado de la red para los datos analizados es menor al 8%, en la figura i l se aprecian los resultados hallados con la red neuronal, donde se puede deducir que dicha red efectivamente puede ser utilizada como funcion de transferencia (pues asimila bastante bien las relaciones espectrales 0 factores de arnplificacion sismica en el dominio de la frecuencia) para deducir comportamientos dinarnicos de las formaciones blandas a partir de espectros de los registros obtenidos en formaciones rocosas como es el caso de la estacion calarca. 30 10' 10' 10' vargas etal. 003,------,----.----,-------.----.---, c1,. ,. u 002 q -0.q1 ,. " "io5~-1l-~;;__---1lj!..,il.-m---'~l.-=-1!-_:!::-'-1l--_:3:-0 -----!l-----:;i35 003r-------,-----,----,-----.----,-----, " " c2 u 002 q 001 q::l' i : ** •',', 0lll.ll-liilji~i_...i__,ill' , -u...l-. ~_ ii---,----,il-: _ 5 10 15 20 25 30 35 distance (.km) figura 7. qc·1 respecto a la distancia hipocentral para las estaciones calarca, uniquindio y armeviva en sus ires componentes. los diferentes colores estrin relacionados con las frecuencias centrales entre 2 hz y 25 hz, donde se infiere independencia de la coda respecto a 1a distancia en las bandas de frecuencia analizadas <i &! 05 ci 0 ~ -0 5 '-__ ~ __ ~ __ ___.j o 05 o -05o 991021-1550uniquindio c3 10 20 30 frequency (hz) figura 9. espectros simulados (azul) y verdaderos (negro) basados en los espectros de la estaci6n sabre roea (calarca). las dos simulaciones superiores coinciden exaciamente con los espectros verdaderos porque fueron tenidos en el entrenaruiento de la red neuronal. los siguientes ocho espectros simulados siguen aproximadamente las tendencias de los espectros verdaderos conclusiones han sido utijizados registros acelerograficos triaxiales para determinar la funcion de transferencia formaciones blandas/roca dura en las estaciones armeviva y calarca de la ciudad de armenia, colombia, mediante el uso de una red neuronal. asurniendo un medio con heterogeneidades hornogeneamente distribuidas y q frecuencio-dependiente entre la zona epicentral y las estaciones de registro, se han podido establecer condiciones semejantes de arnplificacion sismica en las componentes n-s, z y ew; esta situacion fue aprovechada para entrenar la red neuronal con datos espectrales de roca dura como parametro de entrada y de formaciones blandas como pararnetro de salida para las cornponentes n-s y z, la red geofislca colombiana, 5, dicii:.i'abae de 200-1 utiliz.acion de redes neuronales para la determinacion de respuestas de sitio a partir de ondas coda, aplicacion paraarmenia, colombia 10.----_----, ~ ~o~~ 5 990424-2143arm/cal .100 10 20 1o.----------~ 'il ~ __ ~ o' ._ ..~~ 5 990505.2205armical 10 20 w ~o~~ 5 990505-2205uno/cal 10 20 io~ 5 991021-1550unoical .100l---=-=--=-=1'=-0--"-=::.:.=::20:::..::::-=-----:' frequency 1hz) 10,-----_--_--, j 0 5 990427-02:53 armical ·100~---,,:"::0---2:co-----,j30 10.--------_--\l;' ~ o~~·~=~ 5 990507-22'47ar~ncal .100:----1c':0----20c------:c30 10,.------_--;; ~ o~-:.-5 991013-13:10 uno/cal 30 30 10 20 30 1 ~-c :;( :s 991109·17:02u~jq/cal 30 10 20 30 froquancv (hz) figura/d. relaciones espectrales para los tres componentes (clazul; c2 negro; c3purpura) para los registros verdaderos 10.-------------, !o~ ~ 99042~·2·1'43armical .100 10 20 0.-----------, =:," ~ o?"~~ :s 990505·12'05 armical ·10o 10 20 10.----------~ 'i ~ or=--~ :s 990505·12'05 uno/cal .100 10 20 10.-------_-----, 'ffi ~o~ :s 99'1021·1550uno/cal .100 10 20 30 frequency (hz) 10,-----_--_--, 30 figura ii. relaciones espectrales verdaderas (azul) y simuladas (negro). los resultados simulados fueron a partir de registros no tenidos en cuenta durante el entrenamiento de la red neuronal 30 30 99'1'i0s-1702unqlcal 10 20 frequency 1hz) neuronal final tuvo un error respecto a los datos de entrenamiento del 0.0 i%, y permitio simular datos espectrales para la componente ew con un error medio cuadratico menor al 8%. de este modo, se puede concluir que, a pesar de los pocos registros utilizados, una topologja neuronal bien seleccionada puede facilitar el proceso de deduccion de funciones de transferencia lineales y no lineales orientadas a la evaluaci6n dinarnica de suelos, geofisica colombiana, 5, oiciembre de 2001 30 chen, x. q. 1999: on the characteristics of coda scatterers for shallow earthquakes, ph.d. thesis, sch. of geophys. sci., ga. inst. of technol., atlanta. crespo, j.l. 1996: redes neuronales artificiales para ingenieros. universidad de cantabria . fehler,m., hoshtba, m., sato,h. & obara, k. 1992: separation o. scattering and intrinsic attenuation for the kantotokai region, japan, using measurements of s-wave energy versus hypocentral distance, geophys. 1. int. 108, 782-800. freeman, 1. 1993: redes neuronales. addison wesley, iberoamericana. hoshtba, m. 1993: separation of scattering attenuation and intrinsic absorption in japan using the multiple lapse time window analysis of full seismogram envelope, j. geophys. res 98,15809-15824. kato, k., aki, k. & takemura, m. 1995: site amplification from coda waves: validation and application to s-wave site response, bull. seismol. soc. am. 85,467-477. mayeda. k., koyanagt,s. & aki, k. 1991: site amplification from s wave coda in long valley caldera region, california, bull. seismol. soc. am. 81,21942213 noguchi, s. 1990: regional difference in maximum velocity amplitude decay with distance and earthquake magnitude. res. notes nat. res. ctr, disast. prevo 86, 1-40. philllps, w.s. & aki, k. 1986: site amplification of coda waves from local earthquakes in central california, bull. seismol. soc. am. 76, 627-648. tsujuira, m. 1978: spectral analisis of the coda waves from local earthquakes. bull. earthq. inst. univ. tokyo 53. 1-48. tucker, b. & king, 1. 1984: dependence of sediment-filled valley response on input amplitude and valley propierties, bull. seismol. soc. am. 74, 153-165. vargas,ca. 1999: atenuaci6n de ondas coda en la regi6n central de los andes de colombia (eje cafetero). tesis de master, ingenieria sismica y dinarnica estructural, universidad politecnica de cataluna, barcelona. referencias bibliograficas 30 30 31 earth sci. res. j. vol. 11, no. 2 (december 2007): 108-116 accurate gravity anomaly interpolation: a case-study in cameroon, central africa j. kamguia1, c. t. tabod2, j. m. tadjou1, e. manguelle-dicoum2, r. nouayou2 and l. h. kande1 1 national institute of cartography (nic) – cameroon, po box 157, yaounde – cameroon 2 department of physics, faculty of science, university of yaounde – cameroon corresponding author: joseph kamguia: kjerryfr@yahoo.fr; j.kamguia@geomaticsystem.com; 0023799440426 abstract many treatments in geodesy and geophysics require regularly gridded gravity anomalies. the gridding of gravity data needs interpolation. for the predicted data to be accurate, the smoothest type of gravity anomaly should be used along with the most indicated prediction method. this paper presents the comparison of various prediction methods applied on different types of gravity anomalies and considering the relative geological complexity of the study area. many algorithms are tested and the suitability of each type of anomaly and each prediction method discussed in a case-study in cameroon (central africa), using a set of 43,000 gravity data points to determine the must accurate prediction technique. key words: prediction, gravity anomaly, residual free-air anomalies, global geopotential model, complex geology, geophysics. resumen muchos tratamientos en geodesia y geofísica requieren que las anomalías gravimétricas se encuentren en grillas regulares. esta disposición de los datos de gravedad requiere interpolación. para que los datos predichos sean exactos, la anomalía gravimétrica más fina debe ser utilizada junto con el método más indicado para predicción. este artículo presenta la comparación de varios métodos de predicción aplicados a diversos tipos de anomalías gravimétricas y considerando la relativa complejidad geológica del área del estudio. se probaron una gran cantidad de algoritmos y la conveniencia de cada tipo de de anomalía y método de predicción fueron discutidos en una aplicación en camerún (áfrica central), usando un conjunto de 43.000 datos gravimétricos para determinar la técnica de predicción más adecuada.. 108 earth sciences research journal manuscript received july 15 2007. accepted for publication december 5 2007. palabras claves: predicción, anomalía gravimétrivca, anomalías residuales libres de aire, modelo geopotencial global, geología compleja, geofísica. introduction interpolation is a process widely used in earth science. it estimates the value of a parameter at a point from neighbourings. the process may be extrapolation in some areas (heiskanen and moritz, 1967). in geodesy, gravity anomalies are interpolated when computing geoid undulations or the quasi-geoid height anomalies. in geophysics, prediction is used to compute regular grids of gravity anomalies required for presentation purposes or further treatments. the accuracy of the predicted gravity anomalies depends mainly on their smoothness and on the method used. it also depends the data density, their geographical distribution and the grid intervals. in general, the smoother the gravity anomalies, the more accurate the predicted values. many interpolation methods are now proposed (el abbas et al., 1990), with some algorithms available in open sources such as the generic mapping tools or gmt (wessel and smith, 1995). comparisons have already been made between surface fitting algorithms, in order to check their reliability in predicting bouguer gravity anomalies (el abbas et al., 1990). however, many types of gravity anomalies are routinely used in geodesy and in geophysics. it may sometimes be difficult to determine which type is a priori suited for prediction. a constant crustal density value generally used in different computations might be too far from the reality below the earth topographic surface in complex geological areas. residual anomalies that are deduced from free-air (faa), simple bouguer (ba), complete bouguer (cba) and isostatic (ia) anomalies might also be smoother than the original anomalies in some areas. these anomalies are now easily computed using the spherical harmonic coefficients of a global geopotential model (ggm) (kamguia et al., 2007). this paper presents the results of comparison of various types of gravity anomaly predicted with some methods commonly and in different areas of cameroon. these anomalies were computed from a homogeneous database containing 43,000 data points. the aim is to give some hints as to the most appropriate gravity anomaly prediction strategy according to the density of the gravity net, their geographical distribution and the relative complexity of the geology of the study area. to achieve this, four areas with different geological characteristics are chosen. in these areas of different extension and gravity net density, six prediction algorithms are tested: the minimum curvature splines in tension (smith and wessel, 1990); the least square polynomial fitting (two variants); krigging (krige, 1978) and the inverse distance to a power method (two variants). 2. gravity anomaly prediction 2.1. characteristics of gravity anomalies different types of gravity anomaly are used in earth sciences. gravity data recorded on land must be adjusted for elevation above or below sea level to yield the free-air anomalies (faa). they are nearly equivalent to what would be observed if all the topographic masses were condensed onto this reference surface (blakely, 1996). simple and complete bouguer gravity anomalies (ba and cba) are obtained by completely removing the masses that exist between the level of observation and sea level. they are in theory smoother and then more accurate gravity anomaly interpolation: a case-study in cameroon, central africa 109 kamguia et al. esrj vol. 11, no. 2. december 2007 110 suitable for prediction than the faa (heiskanen and moritz, 1967). however, in complex geological contexts, the gravity field may have special characteristics. therefore, ba and cba may be rougher than faa. the isostatic reduction regularizes the earth’s crust according to a model of isostasy. depending on the complexity of the geology of the study area, these anomalies may be less or more suited in prediction processes than faa, ba and cba. from spherical harmonic coefficients of global geopotential models (ggm), the long wavelength components of faa, ba, cba and ia are easily computed. after subtraction from respective total field gravity anomalies, the residual anomalies obtained may be smoother in some areas. the long wavelength anomalies ( )ggmg∆ are computed from the harmonic coefficients of the ggm using equation (1) and subtracted from the total field gravity anomalies )( g∆ (heiskanen and moritz, 1967): (1) where gm is the geocentric gravitational constant; ),,( qlr the spherical coordinates of the computation point; g the normal gravity on the reference ellipsoid; a the equatorial radius of the earth; (sin )n mp q the fully normalized associated legendre functions for degree n and order m ; mnc∆ and mns∆ the normalized egm-ggm harmonic coefficients, reduced for the even zonal harmonic for the ellipsoid and complete to degree and order 360max =n . 2.2. prediction of gravity anomalies and statistical analysis the gravity is a regionalized field, since it varies from place to place in a continuous manner but with no possibility of associating a specified mathematical function. the most accurate gravity anomaly prediction technique is the one that maintains the broad and fine features of the original gravity data processed, without introducing undue distortions. therefore, the predicted and the measured values must at the same point should be mathematically and/or physically related. to achieve this, the criterion of crain and bhattacharyya (1967) is important. data point grid node with a reference point of gridded data (i) input data in gridding algorithms (ii) gridded cells waiting for gridded data at nodes   (i) (ii) figure 1. choice of reference points of gridded data   max 2 2 0 ( 1) cos sin (sin ) nn n ggm n m n m n m n m gm ag n c m s m p r r                to apply the criterion, some data points must be selected to serve as reference points (fig. 1). a reference point is a data point with coordinates nearly coinciding with those of a grid node. the predicted and known values are therefore comparable in a simple statistical analysis of their differences. 3. a case-study in cameroon (central africa) 3.1. characteristics of the gravity field in cameroon free-air gravity anomalies are smoother than ba and cba in cameroon, as is indicated in table 1. in this table, the standard deviation (std) of the ba and cba are nearly 10 mgal greater than the faa’s. this means that the gravity field in cameroon possesses special features. one can therefore conclude that: ● a constant density )67.2( =r used in the bouguer reduction does not effectively suppress the effect of the topography above the reference surface in cameroon; ● the disturbing geological masses responsible of the observed gravity anomalies cannot be easily determined here; ● the geology of cameroon is more complex than it looks like. in this context, interpolation of gravity anomalies may be subject to many errors if the more indicated method and especially the suitable type of gravity anomaly are not used. table 1: statistical comparison of bouguer and free-air anomalies in cameroon (unit: mgal) four test areas ),,,( 4321 zzzz (fig. 2) were selected in order to precise the special features of the gravity field and its behaviour in prediction processes. their selection is based on the differences in geological characteristics, the density and distribution of the data points, and the relative roughness of the topography relief of the area. the principal characteristics of these areas are summarized in table 2. table 2: characteristics of the four test areas the first test area 1z is located in the northern sedimentary region of cameroon. the area is characterized by a long wavelength relatively positive bouguer anomaly attributed by adighije (1981), and fairhead and okereke (1987) to an uplift of the moho. the topographic relief has a mean orthometric height of about 1000 m. gravity data are more denser in this test area. the second test area 2z is situated on the north eastern extension of the cameroon volcanic line (cvl). it is characterised by large negative bouguer anomalies oriented e-w from western cameroon to western c. a. r. these are attributed to a light mass anomaly structure in the upper mantle beneath the adamawa uplift (fairhead and okereke, 1987; poudjom et al., 1992). the mean elevation is greater than 1000 m. the gravity data density is the smallest in this test area. 3z is located on the continental part of the cvl. this line is associated with a series of nesw trending gravity lows not well defined by the reconnaissance survey of collignon (1968). the mean height of the area is nearly 2000 m, with a maximum of 4010 m for mount-cameroon. the gravity data density is intermediate to those of 1z and 2z . the area 4z is situated in southern cameroon. the mean height is lower than in the 111 anomaly max. min. mean std variability free-air 315,5 –85,4 –0,03 25,0 smoother simple bouguer 253,4 –125,5 –26,5 34,4 rougher complete bouguer 254,2 –121,4 –25,5 33,6 rougher number oftest area points used control points data mean density 1z  ee  1613  ;  nn  118  2179 36 1 point per 25 km 2z  ee  1513  ;  nn  85  672 16 1 point per 2100 km 3z  ee  119  ;  nn  74  1465 9 1 point per 250 km 4z  ee  1412  ;  nn  52  987 16 1 point per 2100 km accurate gravity anomaly interpolation: a case-study in cameroon, central africa 112 previous test areas and the gravity data density is comparable to what is observed in 3z . the distributions of data in each test area are shown in figures 3 to 6. the reference points are well distributed in 1z . the numbers of these points in the test areas are enough for an informative analysis of predicted gravity anomalies. table 3 shows the statistics of the anomalies to be interpolated in the test areas. from the analysis based on the std, one can see that the total field bouguer and isostatic anomalies are not always the smoothest. the gravity field has special characteristics in cameroon and its surroundings. the high frequency components of this field, created by the topography relief, are not entirely removed after the bouguer and isostatic reductions. by smoothing faa, ba, cba and ia with subtraction of the longer wavelength components computed using equation (1), four residual types of anomaly specified by (res) in table 3 are computed. table 3: statistics based on std of different types of anomaly in the test areas (unit: mgal) the differences between the measured and interpolated anomaly at the reference points are computed in each area and for each type of anomaly. a statistical comparison is made. the analysis is made on the standard deviations (std) of the differences between anomalies. where the std are small, the interpolation technique is said to be most accurate. the ggm used in residual anomaly computations is egmggm, the most representative of gravity data in the central african subregion (kamguia et al., 2007). the residual free-air anomalies faa(res) are smoother than all the other types of anomaly, except in area 2z where the total gravity bouguer anomalies (ba and cba) are the smoothest (std = 13 mgal). the difference between the std of the residual anomalies is nearly 1 mgal. the std of residual bouguer and isostatic anomalies are greater than the std of total gravity bouguer and isostatic anomalies and than the std of residual free-air anomalies in some areas. in conclusion, cba and ba are the smoothest anomalies in 2z while faa(res) is the smoothest in 1z , 3z and 4z . faa(res) is therefore smoothest type of anomaly in cameroon. it is the only residual anomaly used in the analysis made below. 3.2. comparison of prediction results the faa(res) were treated with the total gravity free-air, bouguer (simple and complete) and isostatic anomalies with the above interpolation methods (and their variants). table 4 shows the results of statically analysis based on std of the differences between predicted and known data at the reference points. table 4: summary of the std of interpolation results in test areas (unit = mgal) from table 4, one can conclude that the accuracy of predicted anomalies depends both maximum minimum mean standard deviation anomaly z1 z2 z3 z4 z1 z2 z3 z4 z1 z2 z3 z4 z1 z2 z3 z4 faa 71 89 155 67 -30 -60 -40 -30 2,4 26 25 9,9 13 22 32 20 ba 3,7 -46 105 -12 -73 -126 -119 -105 -41 -90 -37 -62 13 13 42 22 cba 4,3 -43 106 -9 -72 -121 -95 -102 -39 -83 -30 -59 12 13 41 20 ia 68 85 153 65 -30 -63 -44 -32 0,9 23 22 7,7 13 22 39 20 faa (res) 56 57 87 27 -34 -84 -84 -37 -1,6 5,8 -6 -1,2 11 19 29 11 ba (res) 3 -55 34 -40 -94 -163 -187 -109 -44 -110 -68 -73 15 20 58 13 cba (res) 4 -52 34 -37 -86 -153 -172 -106 -43 -102 -60 -70 14 18 54 12 ia (res) 53 53 85 26 -36 -87 -87 -39 -3,1 2,4 -8,4 -3,5 12 20 30 12 kamguia et al. esrj vol. 11, no. 2. december 2007 faa ba cba ia faa (res) methods z1 z2 z3 z4 z1 z2 z3 z4 z1 z2 z3 z4 z1 z2 z3 z4 z1 z2 z3 z4 minimum curvature 4 6 7 2 3 4 3 3 4 4 3 3 4 6 7 2 3 6 3 1 polynomial quadratic 17 27 29 16 12 12 13 19 12 13 16 19 17 26 29 16 10 26 11 13 polynomial cubic 16 27 27 15 11 12 13 18 11 13 15 19 17 27 27 15 10 26 11 12 krigging 3 3 6 2 3 3 2 4 3 3 2 4 3 3 6 2 2 3 2 1 inverse distance (1/r2) 5 5 7 2 4 4 4 4 4 4 4 4 5 5 7 3 3 5 2 1 inverse distance (1/r3) 4 4 7 2 3 4 3 4 3 3 3 4 4 4 7 2 3 4 2 1 113 longitude la tit ud e z1 niger cameroon nigeria chad c. a. r. gabon p. r. congo e. guinea atlantic ocean z2 z4 z3 study area lac chad figure 2. location of the four test areas 13 14 15 16 8 9 10 11 figure 3. data and control point distributions in + = data point ; ● = reference point accurate gravity anomaly interpolation: a case-study in cameroon, central africa 114 kamguia et al. esrj vol. 11, no. 2. december 2007 13 14 15 5 6 7 8 figure 4. data and control point distributions in + = data point ; ● = reference point 9 10 11 4 5 6 7 figure 5. data and control point distributions in + = data point ; ● = reference point 115 accurate gravity anomaly interpolation: a case-study in cameroon, central africa on the interpolation method, the variability of the anomaly used and the test area considered. however, the choice of the type of anomaly to interpolate is more pertinent than the method and the area. the smoother the anomaly interpolated, the less are the interpolation errors, represented here by the std. for the same type of anomaly interpolated, the krigging and the inverse distance to power 3 methods gave the most accurate results, with approximately the same precision. the best precisions are 2 mgal, 3 mgal, 2 mgal and 1 mgal in test areas ),,,( 4321 zzzz respectively. hence, when the best gridding method is used in conjunction with the smoothest anomalies, a precision of about 2 mgal is obtained after interpolation. 4discussions and conclusion accurate interpolated gravity anomalies are needed in geodetical and geophysical analyses. these anomalies are obtained if the interpolation method and the type of anomaly used are carefully selected. theoretically, bouguer gravity anomalies are smoother and therefore suited for prediction processes than free-air anomalies. however, this is only true in an area with simple geology where a constant crustal density of 67.2 is nearest to the real value. for specific geological, geophysical and geodetical studies in such areas, if bouguer anomalies are interpolated, the information deduced from the gravity interpretations (such as subsurface modelling, basin exploration, geoid computation …) will be biased. residual free-air anomalies can be smoother than total gravity free-air, bouguer and isostatic anomalies in many areas. they are easily computed using the spherical harmonic coefficients of a global geopotential model. after reconstruction of the total gravity free-air anomalies from interpolated residual anomalies, any type of gravity anomaly may be deduced. these residual free-air anomalies are associated 11 12 13 14 2 3 4 5 figure 6. data and control point distributions in + = data point ; ● = reference point 116 kamguia et al. esrj vol. 11, no. 2. december 2007 to smaller errors in gravity anomaly prediction in cameroon. the krigging method and the inverse distance to power 3 performed approximately equally as best prediction methods. the geology of cameroon seems more complicated and only a digital density model may introduce adjusted crustal density values into bouguer gravity reductions. the number of interpolation methods tested is very limited, just for clarity reasons. if the data were very dense in cameroon, many grid intervals would have been tested. acknowledgments the authors would like to thank the following individuals and organizations: mr. sylvain bonvalot of ird (institut de recherche pour le developpement), for giving the ird gravity data during the computation of the first geoid model of cameroon, and mr. bernard langellier of bgi (bureau de gravimetrie international) for making these data accessible. references adighije, c. i., 1981. a gravity interpretation of the benue trough, nigeria. tectonophysics, 79, pp. 109-128. blakely, j. r., 1996. potential theory in gravity and magnetic applications. cambridge university press. 441 pages. collignon, f., 1968. gravimetrie de reconnaissance, cameroun, rapport orstom, 35 p. crain, i. k. and bhattacharyya, b. k., 1967. treatment of non-equispaced two dimensional data with a digital computer. geoexploration, vol 5, pp. 173-194. el abbas, t., jallouli, c., albouy, y. and diament, m., 1990. a comparison of surface fitting algorithms for geophysical data. terra-nova, vol. 2, n° 5, pp. 467-475. fairhead, j. d. and okereke, c. s., 1987. a regional gravity study of the west african rift system in nigeria and cameroon and its tectonic interpretation. tectonophysics, 143, pp. 141-159. heiskanen, w. a. and moritz, h., 1967. physical geodesy. freeman, sanfrancisco, 364 pages. kamguia, j., manguelle-dicoum, e., tadjou, j. m., tabod, c. t. and kande, h. l., 2007. the first geoid model of cameroon (central africa). accepted for publication in the nordic journal of surveying and real estate research (njsr). krige, d. g., 1978. geostatistics for ore evaluation. south african institute of mining and metallurgy, johannesburg, south africa. 50 pp. poudjom-djomani, y. h., diament, m. and albouy, y., 1992. mechanical behaviour of the lithosphere beneath the adamawa uplift (cameroon, west africa) based on gravity data. journal of african earth sciences, 15 (1), pp. 81-90. smith, w. h. f. and wessel, p., 1990. gridding with continuous curvature splines in tension. geophysics, vol. 55, n°3, pp. 293-305. wessel, p. and smith w. h. f., 1995. new version of gmt released: transactions of the american geophysical union 72 (441), pp. 445-446. geociencias-vol 12-2 dic.vp earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 181-193 hydrologic homogeneous regions using monthly streamflow in turkey ercan kahya1, mehmet c. demirel2 and osman a. bég3 1 prof., civil engineering department. hydraulics division, istanbul technical university. maslak, 34469 istanbul-turkey. phone: (90) (212) 285 3002 – fax: (90) (212) 285 65 87 e-mail: kahyae@itu.edu.tr 2 department of water engineering and management. university of twente, po box 217, 7500 ae enschede, the netherlands. work tel: (31) 53 489 3911 – fax: (31) 53 489 35377 e-mail: m.c.demirel@utwente.nl 3 assoc. prof., mechanical engineering department sheffield hallam university sheaf building, room 4112 sheffield, s1 1wb, england, uk e-mail: o.beg@shu.ac.uk abstract cluster analysis of gauged streamflow records into homogeneous and robust regions is an important tool for the characterization of hydrologic systems. in this paper we applied the hierarchical cluster analysis to the task of objectively classifying streamflow data into regions encompassing similar streamflow patterns over turkey. the performance of three standardization techniques was also tested, and standardizing by range was found better than standardizing with zero mean and unit variance. clustering was carried out using ward’s minimum variance method which became prominent in managing water resources with squared euclidean dissimilarity measures on 80 streamflow stations. the stations have natural flow regimes where no intensive river regulation had occurred. a general conclusion drawn is that the zones having similar streamflow pattern were not be overlapped well with the conventional climate zones of turkey; however, they are coherent with the climate zones of turkey recently redefined by the cluster analysis to total precipitation data as well as homogenous streamflow zones of turkey determined by the rotated principal component analysis. the regional streamflow information in this study can significantly improve the accuracy of flow predictions in ungauged watersheds. key words: cluster analysis, ward’s method, streamflow, homogeneous region, regionalization, turkey 181 manuscript receiver: august 18th, 2008. accepted for publication: october 10th, 2008. febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:39 p p p composite 133 lpi at 45 degrees resumen el análisis de nidos de registros de flujos de corrientes calibrados en regiones homogéneas y robustas es un instrumento importante para la caracterización de sistemas hidrológicos. en este artículo hemos aplicado este análisis para clasificar objetivamente datos de flujos de corrientes en una región que comprende patrones similares en turquía. el desempeño de las tres técnicas de estandarización probado y estandarizado por rangos, fue mejor que la estandarización con media cero y varianza 1. el anidamiento se llevó a cabo utilizando el método de mínima varianza de ward el cual se torna prominente en el manejo de recursos acuíferos con medidas de dis-similaridad cuadráticas euclidianas sobre 80 estaciones de flujos de corriente. las estaciones poseen regímenes de flujos donde no ha ocurrido regulación intensiva sobre los ríos. una conclusión general es que las zonas que tienen patrones similares de flujos de corriente, no fueron bien cubiertas con las zonas climáticas convencionales de turquía. palabras clave: análisis racimo, método de ward, flujo de corriente, región homogénea, regionalización, turquía. 1. introduction streamflow characteristics provide information needed in design of structures built in or along stream channels, for avoiding flood hazards, for defining the available water supply and in the large scale provides a useful tool for extrapolation of hydrological variables and for the identification of natural flow regimes where intensive river regulation has occurred. because climate factors, such as precipitation, temperature, sunshine, humidity, and wind, all affect streamflow but topography, soil characteristics, precipitation and temperature account for major differences among the river catchments (haines 1988; riggs 1985). for instance high temperature variability generally leads to more potential evaporation so that the water cycle turns in a warmer environment. hence the higher content of water vapour in a warmer atmosphere will increase precipitation. but in summer, the streamflow will be decreased by higher temperatures and higher evapotranspiration (stahl 2001). it is important to document climatic and hydrologic regionalization in planning water resources systems. this requires similar pattern and clustering characteristics. in this context, fovell (1993) was among the first pioneering studies which attempted to develop a regionalization for climatic variables over the us using monthly temperature means and precipitation accumulations from 344 climate divisions. gaffen and ross (1999) applied a modified version of eight-cluster solution to analyze trends in us temperature and humidity. stahl (2001) correlated the monthly averages of the regional streamflow deficiency index (rdi) series of the 19 european clusters with the nao index and noted weak relations. however seasonal correlations were much higher except for the summer season in northern europe. in europe, most rivers show a strong seasonal regime; therefore, seasonal variability is important to assess the impact of climate changes on the complex hydrological system (stahl 2001). the seasonality of streamflow varies widely from stream to stream and is influenced mostly by the local distribution of precipitation, local seasonal cycle of evaporation demand, timing of snowmelt, travel times of water from runoff source areas through surface and subsurface reservoirs and channels to stream gauge, and human management (chiang 1996). dettinger and diaz (2000) worked with the global dataset of monthly streamflow series and pointed out that the timing and amplitude of streamflow seasonality depends on the local month of maximum precipitation 182 ercan kahya, mehmet c. demirel and osman anwar bég febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:39 p p p composite 133 lpi at 45 degrees and the extent to which precipitation is trapped in snow and ice at most gauges. acreman (1986) classified 168 basins in scotland using normix multivariate clustering algorithm. they used logarithmically transformed basin characteristics; area, stream length, channel slope, stream density, rainfall, soil moisture deficit, soil type, and lake storage. in cluster analysis, the choice of variables, clustering technique and dissimilarity measure significantly influence the results (fovell 1993; stooksbury and micheals 1991). the final groups may or may not be geographically contiguous. if robust clustering is done, strong relationship in streamflow properties (e.g., mean, standard deviation, and correlation of monthly streamflow) and river basin characteristics can be determined. these links can be utilized to develop useful streamflow information at ungauged watersheds featuring similar patterns (chiang 1996). using temperature and precipitation data, some climate classifications to delineate regions with similar climate conditions in turkey were previously presented by türke� (1996) and ünal et al., (2003). the former applied a common approach (thornthwaite classification method) as a priori definition of a set of climate types or rules that were then used to classify climate of turkey. the latter applied cluster analysis for the same purpose. since streamflow is an integrated variable of atmospheric and land processes, it would be wise to explore clustering schemes from the hydrological standpoint using nation wide streamflow network in turkey. in this study we carry out the cluster procedures for delineating the geographical zones having similar monthly streamflow variations. 2. data and methodology 2.1 streamflow data our study domain includes 26 river basins across turkey (figure 1). because of unreliable records we, however, had to eliminate the basins 2, 10, 11, and 25 from the analysis. table 1 presents gauging stations and their basins used in this study. most of the drainage basins are medium to large size (>1000 km) and are located in an elevated area (>500m). the maximum flow per unit area can be observed in antalya basin as the eastern black sea basin has the highest precipitation measurements. turkey is located in semiarid zone where precipitation is mainly characterized by high spatial and temporal variability. readers are referred to ünal et al., (2003) and karaca 183 hydrologic homogeneous regions using monthly streamflow in turkey figure 1. locations of streamflow gauging stations used in this study. the boundaries of river basins are shown along with station id numbers. febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:39 p p p composite 133 lpi at 45 degrees et al. (2000) for a recent review of the general climate features in turkey. monthly streamflow recorded at 80 stations used in this study are compiled by general directorate of electrical power resources survey and development administration (abbreviated as eie). each streamflow station contains a 31-year period spanning from 1964 to 1994. karabörk and kahya (1999) and kahya and karabörk (2001) showed that the data set used in this study fulfils the homogeneity condition at a desirable confidence. following suggestion of arabie et al. (1996), original streamflow data first were standardized by the following equation prior to the cluster analysis. 184 ercan kahya, mehmet c. demirel and osman anwar bég table 1. gauging stations used in this study and their locations basin no name of river basin number of the gauging stations’ 1 maritza (meriç) 101 2 marmara 3 susurluk 302, 311, 314, 316, 317, 321, 324 4 northern aegean 406, 407 5 gediz 509, 510, 514, 518 6 small menderes 601 7 big menderes 701, 706, 713 8 western mediterranean 808, 809, 812 9 antalya 902, 912 10 burdur lake 11 akarçay 12 sakarya 1203, 1216, 1221, 1222, 1223, 1224, 1226, 1233, 1237, 1242, 1243 13 western black sea 1302, 1307, 1314, 1335 14 yeþilýrmak 1401, 1402, 1413, 1414, 1418 15 kýzýlýrmak 1501, 1517, 1524, 1528, 1532, 1535 16 konya closed. 1611, 1612 17 eastern mediterranean 1708, 1712, 1714 18 seyhan 1801, 1805, 1818 19 orontes (asi) 1905, 1906 20 ceyhan 2006, 2015 21 euphrates (fýrat) 2122, 2124, 2131, 2132, 2145, 2147, 2151 22 eastern black sea 2213, 2218, 2232, 2233 23 chorokhi (çoruh) 2304, 2305, 2323 24 arax (aras) 2402, 2409 25 van lake 26 tigris(dicle) 2603, 2610, 2612 febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:40 p p p composite 133 lpi at 45 degrees z s s s s � � � min ( ) max( ) ( ) eq. (1) where z is normalized streamflow series and s is raw monthly streamflow series at a station. 2.2 cluster analysis cluster analysis is an unsupervised learning procedure that group names and number of groups are not known in priori. classification differs from clustering since it is a supervised learning procedure in which group names and numbers of groups are known. since the purpose of cluster analysis is to organize observed data into meaningful structures, it combines data objects into groups (clusters) such that objects belonging to the same cluster are similar as those belonging to different clusters are dissimilar (anderberg 1973; everitt 1993; karaca et al. 2000). to measure the distance between two stations x and y, the euclidean distance function, d is frequently used (chiang 1996; gong and richman 1995) and expressed as d x y x yi i i n � � � � � � 2 1 eq. (2) where x and y is the station pair and n is the number of months. although there are many other distance metrics, the euclidean distance is the most commonly used dissimilarity measure in the clustering algorithms. a literature review provided by gong and richman shows that the majority of investigators (i.e., 85%) applied this metric in their study. the ward’s algorithm and squared euclidean metric were selected in this study because this linkage method aims to join entities or cases into clusters such that the variance within a cluster is minimized (everitt, 1993). to be more precise; each case begins as its own cluster then two clusters are merged if this merger results in a minimum increase in the error sum of squares. readers are referred to everitt (1993) and romesburg (1984) for further details concerning cluster analysis. determination of the appropriate number of clusters to retain is considered as one of the major unresolved issues in the cluster analysis. in this study, we applied an informal method which includes an examination of the differences between a conjunction level in the dendrogram and cutting the dendrogram when large changes are observed (everitt, 1993). it is then possible to define different cluster numbers by moving the dashed horizontal line up and down in the graph of dendrogram until achieving a desirable result. moreover, we applied two other statistics to decide an appropriate number of clusters i.e. root-mean-square standard deviations (rmsstd), pseudo f statistic (sarle 1983). 4. results and discussion a group of 80 stations was analyzed using the hierarchical clustering method described in the preceding section. an agglomerative clustering method show which stations or clusters are being clustered together at each step of the analysis procedure. this requires a total of 79 (80-1) steps to converge to one single cluster. the ward’s minimum variance method was applied to the distance matrix constructed from the standardized monthly mean variables. for each variable, the analysis process was stopped at the 60th (calculated as 80-20) step to detect variation in the cluster memberships and to get more consistent clusters. at the beginning of the analysis, we carried out the cluster procedures up to 20 steps using both standardized and original variables to see which type of variables seems to be proper for the analysis. the 20 steps was the possible reasonable largest number of cluster (hereafter abbreviated as ncl). if it was a larger number, it would not be practical to handle the analysis outcomes. the results for the monthly streamflow variables were presented in a mapping fashion for the cluster level of 6 which was selected among possible 20 different cluster levels. this cluster level seemed to account more for compact and reasonable solutions in a manageable manner and is consistent that of ünal et al. (2003). different colours for each cluster will be 185 hydrologic homogeneous regions using monthly streamflow in turkey nov 10pagina 185.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp martes, 10 de noviembre de 2009 20:30:39 p p p composite 133 lpi at 45 degrees used to demonstrate the analysis results on the maps afterward. in addition, we calculated the rmsstds and pseudo f statistics to decide an appropriate number of clusters and presented their results together with those of dendrogram in table 2 for each month. as a result, all three different techniques suggested us more or less same number. we considered single digit for the number of clusters, namely 6, for each month. figure 2 illustrates 6 distinctive clusters, each showing a hydrologically homogeneous region across turkey. for january, four clusters appeared to be prevailing, mainly having a stripe-like shape extending from north to south. for the coastal areas of mediterranean sea, two clusters come out not only in this month but also in the february and march, dividing the entire coastal area into the west and east parts. for february, six clusters emerged almost equally in size, and each extending more or less from west to east. in this month, the patterns of streamflow variation in the marmara and aegean areas differs each other as opposed to the case in january in which the both areas were confined in region a (figure 2a). the entire black sea coast lines were represented by a single cluster, namely region b. southern part of kizilirmak basin and konya closed basin together were identified by region c in february, representing the boundary of characteristic mid turkey hydroclimatology. for march, the overall pattern seems somewhere in-between those of january and february, resembling to the latter’s map for the west side and to the map of the former for the east side. region c of march cluster solution remains unchanged but its designation was changed to region e in february solution. region f includes konya plateau in all monthly analysis except january and february months where annual rainfall frequently is less than 300 mm. in this region, may is generally the wettest month and july and august are the driest season. similar detailed evaluations can be made for the remaining months (figures 3, 4 and 5); however, we will introduce common and striking features of the map patterns after this point. there is immense similarity between cluster solutions of january and april, both belonging to different season. in the same context, the map pattern of may, in general, is said to be a replication of that of may and april, implying that spring months demonstrate nearly common cluster pattern. region f in the cluster solution of april is often appears in most months, composing of the basins 16, 17, 18 and 20, and the stations 2124, 2145 and 2131. this cluster region was also noted by (kahya and kalayc1 2002; kahya et al., 2008). they used an alternative approach that is the 186 ercan kahya, mehmet c. demirel and osman anwar bég table 2. examination of the number of clusters in monthly domain january february march april may june suggested selected suggested selected suggested selected suggested selected suggested selected suggested selected rmsstd 6 8 7 8 7 8 4 8 6 8 8 8pseudo f 5 8 8 6 7 5 dendrogram 6 6 6 6 6 6 july august september october november december suggested selected suggested selected suggested selected suggested selected suggested selected suggested selected rmsstd 6 8 4 8 5 8 6 8 5 8 4 8pseudo f 4 5 5 7 5 6 dendrogram 6 6 6 6 6 6 febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:40 p p p composite 133 lpi at 45 degrees 187 hydrologic homogeneous regions using monthly streamflow in turkey figure 2. homogeneous streamflow regions for the months: (a) january, (b) february, and (c) march febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:41 p p p composite 133 lpi at 45 degrees 188 ercan kahya, mehmet c. demirel and osman anwar bég figure 3: homogeneous streamflow regions for the months: (a) april, (b) may, and (c) june febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:42 p p p composite 133 lpi at 45 degrees 189 hydrologic homogeneous regions using monthly streamflow in turkey figure 4. homogeneous streamflow regions for the months: (a) july, (b) august, and (c) september febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:43 p p p composite 133 lpi at 45 degrees 190 ercan kahya, mehmet c. demirel and osman anwar bég figure 5: homogeneous streamflow regions for the months: (a) october, (b) november, and (c) december febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:45 p p p composite 133 lpi at 45 degrees rotated principal component analysis. region e (figure 3b) is another spatially widespread cluster covering eastern turkey. the comparison of all twelve monthly cluster patterns together led us to draw the following features for hydrologic regionalization of turkish streamflow patterns. (i) region c in the cluster solution of october may be the most disaggregated scheme which includes the basins 17, 19 and 22. (ii) figure 4 designates that there is no significant change in the map patterns of july, august, and even september. (iii) the eastern black sea basin where current national energy politics mainly rely on this basin was divided into two sub-regions in the analysis of the following months: january, april, may, june and july. (iv) in two monthly patterns (i.e., june and december), there exists a single cluster (i.e., region f in figure 3 and region c in figure 4, respectively) occupying almost half of the entire country. (v) in particular during the months january and april, streamflow stations in the marmara and aegean area (marked as region a in the both cases) reveal common variation modes. for the remaining months, the both regions were included by separate clusters. (vi) the area described by region c cluster during the months march, april, may, june, july, and august sequentially appears to delineate the same geographical extent (referring kizilirmak and yesilirmak basins), indicating a noticeable temporal persistency as well. (vii) the cluster defined by region f constantly emerges with almost same geographical extent (referring antalya, konya closed, eastern mediterranean, ceyhan, and some parts of firat basins) during the months march, april, may, july, august, and september. it should be noted that this cluster also comes out during the months january, june, and november with a large size. it might be concluded that it is very stable throughout the year. (viii) the geographical extent defined by the region e cluster in figure 2c shows a temporal consistency during the months march, partially in june, july, august, and with much larger area coverage in december. (ix) the cluster region d defined in figure 3b, implying similar streamflow variation mode in the marmara and western black sea areas, appears during the months may, august, september, and with little larger area in october. this cluster includes the basins meric, western black sea, and some parts of kizilirmak basins. isik and singh (2008) also applied cluster analysis to the same domain to demonstrate hydrologic regionalization. they included 1410 stations from 26 river basins having at least 5 year data. they used flow duration curves in the analysis to estimate streamflow values at desired ungagued sites after the homogeneous regions were defined by cluster analysis methods. the number of clusters was chosen by hierarchical methods and homogenous regions were delineated by k-means method. they used standard euclidean distance instead of squared euclidean that we used in our application as a measure of dissimilarity. our results for the number of cluster is consistent with those of isik and singh (2008). readers are referred to our recent discussions regarding methodological aspects of the cluster analysis (demirel et al., 2008a; demirel et al., 2008b). explanations concerning climatological reasoning are out of the scope of this investigation. however, it is very important to document the patterns of monthly precipitation and temperature variables in similar manner, and to relate to the results presented here. 5. conclusions within a river basin, hydrologic processes are integrated into streamflow characteristics; thus, 191 hydrologic homogeneous regions using monthly streamflow in turkey febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:45 p p p composite 133 lpi at 45 degrees streamflow data provide a natural filter for precipitation data (piechota et al., 1997) and was preferred unique study-variable in this study. we then set the goals of this investigation as to determine streamflow zones of turkey, as objectively as possible, and evaluate the stability of solutions based on the feedbacks obtained from variable pre-processing, choosing the best algorithm, and ncl. we specifically considered two measures: the euclidean and squared euclidean. ward’s minimum variance method was decided to yield acceptable results in our solutions. ünal et al., (2003) redefined 7 climatic zones for turkey as quite different boundaries than conventional classification. hence in this paper, the number of homogenous cluster is chosen as 6 for month domain according to visual inspection of the dendrogram and two statistics of clusters. the initial data consisted of monthly average streamflow resulting in a data matrix of 80-station x 31-variable. (a) the ward’s method with squared euclidean was found more effective in producing homogenous cluster schemes when comparing to the other hca methods. (b) using monthly patterns seem to be favourable in regard to defining streamflow regions. (c) standardization by range is superior to the other techniques. the outcomes of this study were in a good agreement and relation with earlier studies conducted for, in general, turkish hydrologic and climatological surface variables. for example, precipitation and streamflow variables showed some significant downward trends in western turkey (partal and kahya 2006; kahya and kalayci 2002; kahya and kalayci 2004) where we usually assigned region a in this study. ünal et al., (2003) who developed a regionalization of climate in turkey using cluster analysis, pointed out eight regions of similar climate pattern. some of their regions are quite similar to those found here. however the zones having similar streamflow pattern appeared not overlapped well with the conventional climate zones of turkey. although river drainage basins were not a major impetus for the climate divisions in turkey, which are based primarily on political boundaries with limited attention given to topography and other factors, streamflow variable is strongly recommended for inclusion in the dataset used for climate clustering studies. 6. references acreman, m. c., and sinclair, c. d. (1986). classification of drainage basins according to their physical characteristics; an application for flood frequency analysis in scotland. journal of hydrology, 84(3/4), 365-380. anderberg, m. r. (1973). cluster analysis for applications, academic press, new york ny and london. arabie, d., hubert, l. j., and de soete, g. (1996). clustering and classification, world scientific publ., river edge, nj. chiang, s. m. (1996). hydrologic regionalization for the estimation of streamflows at ungauged sites based on time series analysis and multivariate statistical analysis., syracuse university, new york. demirel, m. c., kahya, e., and rivera, d. (2008a). discussion of “hydrologic regionalization of watersheds in turkey” by sabahattin isik; and vijay p. singh in asce journal of hydrologic engineering, sep 2008, vol. 13, no. 9, pp.824-834. doi: 10.1061/(asce) 1084-0699(2008) 13:9(824) (accepted for publication). demirel, m.c., kahya, e. and rivera, d. (2008b). discussion of “clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano” by mauricio orozco-alzate, and césar germán castellanos-domínguez, earth sciences research journal (in press). dettinger, m. d., and diaz, h. f. (2000). global characteristics of streamflow and variability. journal of hydrometeorology, 1, 289-310. 192 ercan kahya, mehmet c. demirel and osman anwar bég febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:45 p p p composite 133 lpi at 45 degrees everitt, b. (1993). cluster analysis. 3rd edn. , halsted press, division of wiley, new york. fovell, r. g., and fovell m.y.c. (1993). climate zones of the conterminous united states defined using cluster analysis. j.climate, 6, 2103-2135. gaffen, d. j., and ross, r. j. (1999). climatology and trends of u.s. surface humidity and temperature. journal of climate, 12(3), 811-828. gong, x., and richman, m. b. (1995). on the application of cluster analysis to growing season precipitation data in north america east of the rockies. . j. climate, 8, 897-931. haines, a. t. a. f., b.l. and mcmahon, t.a. (1988). a global classification of river regimes. applied geography, 8(4), 255-272. isik, s., and singh, v. p. (2008). hydrologic regionalization of watersheds in turkey. journal of hydrologic engineering, 13(9), 824-834. kahya, e., and kalaycý, s. (2002). streamflow variability in turkey. agroenviron 2002, cairo, egypt. kahya, e., kalaycý, s., and piechota, t.c. (2008). streamflow regionalization: case study of turkey. journal of hydrologic engineering, 13(4), 205-214. karabörk, ç., and kahya, e. (1999). multivariate stochastic modeling of streamflows in the sakarya basin (in turkish). turkish journal of engineering and environmental sciences, 23(2), 133-147. kahya, e., and karabörk, m.ç. (2001). the analysis of el nino and la nina signals in streamflows of turkey. international journal of climatology, 21(10), 1231-1250. kahya, e. and kalaycý, s. (2004). trend analysis of streamflow in turkey. journal of hydrology, 289, 128-144. karaca, m., deniz, a., and tayanc, m. (2000). cyclone track variability over turkey in association with regional climate. international journal of climatology, 20, 122-136. partal, t. and kahya, e. (2006). trend analysis of turkish precipitation patterns. hydrological processes, 20 (9), 2011-2026. piechota, t. c., dracup, j. a., and fovell, r. g. (1997). western us streamflow and atmospheric circulation patterns during el nino-southern oscillation. journal of hydrology, 201, 249-271. riggs, h. c. (1985). streamflow characteristics, elsevier, amsterdam. romesburg, h. c. (1984). cluster analysis for researchers, lifetime learning publications, belmont, california. sarle, w. s. (1983). cubic clustering criterion. sas institute, cary, nc. stahl, k. (2001). hydrological drought: a study across europe, albert-ludwigs-universität, freiburg. stooksbury, d. e., and micheals, p. j. (1991). cluster analysis of southern u.s. climate stations. theoretical and applied climatology, 44, 143-150. türkeþ, m. (1996). spatial and temporal analysis of annual rainfall variations in turkey. international journal of climatology, 16, 1057-1076. ünal, y., kindap, t., and karaca, m. (2003). redefining the climate zones of turkey using cluster analysis. international journal of climatology, 23(9), 1045-1055. 193 hydrologic homogeneous regions using monthly streamflow in turkey febrero 20-geociencias-vol 12-2 ultima version.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp viernes, 20 de febrero de 2009 13:08:45 p p p composite 133 lpi at 45 degrees hernandez et al.indd earth sci. res. j. vol. 11, no. 1 (june 2007): 81-94 crustal thickness variations and seismicity of northwestern south america orlando hernandez-pardo1,2, ralph r. b. von frese 1, jeong woo kim3 (1)school of earth sciences, the ohio state university, columbus, oh 43210 usa, fax 614 2927688, hernandez.135@osu.edu, vonfrese@geology.ohio-state.edu. (2)dept. of geosciences, universidad nacional de colombia, bogotá, d.c. colombia ohernandezp@unal.edu.co (3)dept. of geoinformation engineering, sejong university, seoul, korea, jwkim@sejong.ac.kr abstract any uncompensated mass of the northern andes mountains is presumably under pressure to adjust within the earth to its ideal state of isostatic equilibrium. isostasy is the ideal state that any uncompensated mass seeks to achieve in time. these pressures interact with the relative motions between adjacent plates that give rise to earthquakes along the plate boundaries. by combining the gravity moho estimates and crustal discontinuities with historical and instrumental seismological catalogs the correlation between isostatically disturbed terrains and seismicity has been established. the thinner and thicker crustal regions were mapped from the zero horizontal curvature of the crustal thickness estimates. these boundaries or edges of crustal thickness variations were compared to crustal discontinuities inferred from gravity and magnetic anomalies and the patterns of seismicity that have been catalogued for the last 363 years. the seismicity is very intense along the nazca-north andes, caribbean-north american and north andes-south american collision zones and associated with regional tectonic compressional stresses that have locally increased and/or diminished by compressional and tensional stress, respectively, due to crustal thickness variations. high seismicity is also associated with the nazca-cocos diverging plate boundary whereas low seismicity is associated with the panama-nazca transform fault and the south american plate. keywords: crustal thickness, gravity, seismicity, northwestern south america. resumen cualquier masa sin compensar al norte de las montañas de los andes se encuentra presumiblemente bajo presión para ajustarse en la tierra a su estado ideal de equilibrio isostático. la isostasia es el estado ideal que cualquier masa sin compensar busca a través del tiempo. estas presiones interactúan con los movimientos relativos de las placas adyacentes para producir terremotos a lo largo de los límites entre las placas. al combinar los estimados de la gravedad de moho y discontinuidades de la corteza con catálogos sismológicos históricos e instrumentales, la correlación entre terrenos isostáticamente anómalos y la sismicidad ha sido establecida. las regiones delgadas y gruesas de la corteza han sido cartografiadas desde la curvatura original cero de los espesores estimados de la corteza. estos 81 earth sciences research journal manuscript received december 21 2006. accepted for publication june 19 2007. límites o bordes de las variaciones de espesor de la corteza fueron comparadas con discontinuidades de la corteza inferidas de anomalías magnéticas y gravimétricas, y los patrones de sismicidad que han sido catalogados en los últimos 363 años. la sismicidad es muy intensa a lo largo de las zonas de colisión de nazca-norte de los andes, caribe-américa del norte y el norte de los andes-sur américa y se encuentra asociado con esfuerzos tectónicos regionales compresionales que localmente han aumentado y/o disminuido por esfuerzos compresionales y tensionales respectivamente, debido a las variaciones de espesor de la corteza. la alta sismicidad se encuentra asociada con el límite de placas divergente de nazca-cocos, mientras que la baja sismicidad se encuentra asociada con la falla de transformación de panama-nazca y la placa suramericana. palabras claves: espesor de la corteza, gravedad, sismicidad, noroeste de sur america.. introduction the increased number of seismological networks established during the last century led to the discovery that earthquakes are not randomly distributed, but tend to occur along well defined earthquake belts (shearer, 1999). these belts are largely concentrated along the margins of tectonic plates that shift slowly over geologic time. the relative motions between adjacent plates give rise to earthquakes along the plate boundaries that include spreading oceanic ridges, converging subduction zones, collisional continental plate boundaries, and transform faults along which they shear past each other (bird, 2003; turcotte and schubert, 2002; cediel et al.,2003). for northwestern south america, the improvement in instrumentation and expansion of seismological networks has led to the production of relatively complete and accurate catalogs of earthquake locations and ground motions. the response of the ground during an earthquake is commonly attributed to the elastic rebound theory whereby the low accumulation of shear stress at a point along a fault builds up until the elastic strength of the rock is exceeded and it fractures releasing energy to produce the earthquake (reid, 1906 in shearer, 1999). crustal thickness variations also can contribute with the compressional stress where the crust is thinner than normal and with the tensional stress where it is thicker than normal (artyushkov, 1973). therefore, the crustal thickness estimates derived from gravity anomalies can be included in the analysis of earthquake behavior. spectrally correlated terrain and free air gravity anomalies the isostasy of northwestern south america was investigated considering the topography/ bathymetry data from national imagery and mapping agency (nima) from −8os to 23.5on latitude and from −90ow to −58.5ow longitude. surface and bathymetry elevations from the jgp95e terrain data base (smith and sandwell, 1994; 1997) were processed to produce the digital elevation model for the water and rock terrain gravity components at 0.5o nodal spacing. free-air gravity anomalies (faga) were estimated from the egm96 spherical harmonic earth gravity model to degree and order 360 (lemoine et al., 1998) at 20 km altitude over the 32o x 32o area at 0.5o nodal spacing. the altitude of 20 km was chosen to help minimize the effects of local density errors in the terrain gravity modeling (leftwich et al., 2005). the terrain gravity effects were modeled in spherical coordinates at 20 km altitude by gauss-legendre quadrature integration (von frese, 1980). the terrain gravity modeling used densities of 2.8 gm/cm3 for the crust and 1.03 gm/cm3 for oceanic water. spectral correlation theory was used to analyze the co-registered faga and tge for their anomaly correlations using matlab (mathworks, 2005). specifically, the fourier transforms t and f of tge and faga, respectively, were used to obtain their correlation spectrum (davis, 1986; von frese et al., 1997a; kim et al., 2000) given by: cc(k) = cos(δθk) = re f(k) t(k) (1) t(k)||f(k)|| crustal thickness variations and seismicity of northwestern south america 82 where cc(k) is the correlation coefficient between the kth wavenumber components f(k) and t(k), and denotes taking the real parts of the wavenumber components. usually, cc(k) is evaluated from the cosine of the phase difference (δθk) between the two kth wavenumber components. using the correlation spectrum between faga and tge, spectral correlation filters were designed to extract terrain-correlated free-air gravity signals. those wavenumber components showing intermediate to high positive (ccp(k) ≥ 0.3) and negative (ccn(k) ≤ 0.3) correlations were identified. the cut off values for the correlation filter were determined to minimize correlative features between the terrain-decorrelated free air and compensating terrain gravity components. inversely transforming positively and negatively correlated free-air wavenumber components according to the selected cut off values yielded the terraincorrelated free air gravity anomalies (tcfaga). the residual terrain-decorrelated free-air gravity anomalies (tdfaga) were calculated by subtracting tcfaga from faga, so that faga = tcfaga + tdfaga (2) tcfaga are explained by anomalies associated with the topography while tdfaga include the gravity effects of sources within the crust (e.g., local bodies) and the subcrust. crustal modelling a new crustal thickness model for northwestern south america was developed using the compensated terrain gravity effects (ctge) that resulted when the tge were subtracted from the tcfaga. the ctge represent isostatically adjusted complete bouguer anomalies that correspond to the gravity effects of the terrain in isostatic equilibrium. this approach is feasible because 90% of the earth is in equilibrium with the mean global free-air gravity anomaly being zero (heiskanen and moritz, 1967). moho and related crustal thickness variations were modeled from the ctge by inversion, assuming the constant nominal density contrast of 0.4 gm/cm3 of the mantle relative to the crust. this methodology has been successfully applied in studies of the mantle-crust interface for east asia (tan and von frese, 1997), antarctica (von frese et al., 1999), greenland (roman, 1999), ohio (kim et al., 2000), and iceland and the north atlantic (leftwich et al., 2005; leftwich, 2006). negative ctge values are located along the eastern andes mountains suggesting some degree of partial compensation and thickening of the crust. the cc between the tcfaga and the ctge is -0.3377 showing that most of the topography/bathymetry is isostatically disturbed. areas where the tcfaga values are excessively negative or positive are more prone to seismic activity than areas where the tcfaga values are closer to zero (song and simons, 2003). this paper analyzes and compares seismic data from the advanced national seismic system (anss) and the red sismológica nacional de colombia (rsnc) catalogs with crustal thickness estimates from gravity anomalies by understanding improved large-scale dynamic models of earthquakes and tectonics. gravity anomalies along trenches, continental converging margins, crustal discontinuities, mountain ranges, and cratonic areas are compared with their seismic signatures from earthquake data collected over the last 363 years. zero curvature of crustal thickness variation the terrain-correlated free-air anomalies (tcfaga) in figure 1 highlight regions with isostatically disturbed crustal features. the zero anomalies mark areas of crustal equilibrium so that the positive and negative anomalies reflect the compression and tension of the crust, respectively. positive and negative tcfaga values can mark crust that is isostatically too thin (under-compensated) or thick (overcompensated), respectively, and hence under pressure to equilibrate by the compressive inflow or expansive out-flow of crustal material (artyushkov, 1973). thus, these anomalies can reflect lithosphere in subsidence or uplift (kim et al., 2000; von frese et al., 1999a), or alternatively dynamic surface topography that is too high or too low, respectively, to be in isostatic equilibrium. hernández et al., esrj vol. 11, no. 1. june 2007 83 figure 2 gives the crustal thickness variations for northwestern south america obtained by adding the gravity moho estimates (hernandez, 2006). the zero curvature contour that estimates the edges of thickness variations was obtained by calculating the second vertical derivative of the crustal thickness (figure 2). the use of vertical derivatives has been always a standard method of enhancing high frequency features in potential field data. second order vertical derivatives were computed using convolution filters and laplace’s equation (sheriff, 1994). the intervening yellow and blue regions of figure 2 reflect the thicker and thinner crustal components, respectively. seismic data compilation for comparison with the crustal thickness estimates, the regional seismic data of northwestern south america were compiled from the advanced national seismic system catalog (anss; usgs, 2006). a more local study was also considered using the historic and instrumental seismic catalogs of the red sismológica nacional of colombia (rsnc; ingeominas, 2006). anss catalog the anss catalog is a world-wide earthquake catalog created by merging the master earthquake catalogs from the contributing anss institutions and then removing duplicate solutions (usgs, 2006). the anss catalog currently consists of earthquake hypocenters, origin times, and magnitudes. the anss database was searched for data from 1966 to 2006 with ritcher magnitudes m of 3-10. the earthquake epicenters were converted from geographic to cartesian coordinates in figure 2 with the proportional symbols for ritcher magnitudes given in the legend. only those events with hypocenters from 0 to 60 km depths were considered in this study. the crustal discontinuities interpreted from the tcfaga in figure 1 are also superposed in figure 2. the seismic events in figure 2 are concentrated along the plate boundary zones of the nazca, caribbean, north american and south american plates, the north andes, panama and costa rica microplates, the cocos-nazca spreading system, and the intraplate discontinuities. they are predominantly located in the thicker figure 1. terrain-correlated faga (tcfaga) at 20 km elevation for the study area. map annotations include the amplitude range (ar) of (min; max) values, the amplitude mean (am), and standard deviation (sd). snsm = sierra nevada of santa martha, w-mid = western-central ranges. this map was produced using the albers equal-area conic projection. 84 crustal thickness variations and seismicity of northwestern south america 85 crustal sections and along the inferred edges of the thickness variations. however, the thicker oceanic sections of the nazca, malpelo and cocos ridges in the pacific and the beata ridge in the caribbean are aseismic. comparatively few earthquakes are concentrated in the thinner crustal regions with relatively negative tcfaga. the guiana craton and the interior of nazca and caribbean oceanic plates also lack significant seismic events and are seismically quiescent. to account for the intraplate earthquakes, gravity and magnetic anomalies that infer crustal discontinuities are useful to analyze deformation in the plate interior. rsnc historical and instrumental seismic catalog the rsnc catalog was created by merging the master earthquake catalogs from the observatorio sismológico del sur occidente (osso), centro regional de sismología para america del sur (ceresis), instituto geofísico de los andes, red sismológica nacional, and regional institutions of northwestern south america and the caribbean (ingeominas, 2006). this catalog currently consists of earthquake epicenters, origin times, and magnitudes. the rsnc database compiled data from 1643 to 1991 for ritcher magnitudes from m = 3.0 to m = 10. the rsnc catalog contains an enormous number of events. thus, the events were sorted by magnitude for the minor earthquakes (m = 3 to 4), moderate earthquakes (m= 4.1 to 5.0), and strong earthquakes (m > 5.0) as shown in figures 3, 4, and 5 respectively. minor and moderate seismic events in figures 3 and 4, respectively, are concentrated along the pacific subduction zone and intracrustal discontinuities of the north andes microplate. the concentration of events in the northern part of the eastern cordillera is known as the “bucaramanga nest.” few events are recorded for the sierra nevada of santa martha, guiana craton, and the northwestern flat lands of colombia at the north of the andes mountains. at the continent, areas with thicker crust are more seismically active, while areas of thinner crust are less seismically active. an exception is the sierra nevada of santa martha with a thick crust and low seismic activity. major earthquakes in figure 5 are located along the converging nazca-north andes and south america-north andes continental boundaries. the major earthquakes are located further landward from the pacific subduction zone relative to the minor and intermediate earthquakes that are concentrated closer to the coastline. the few seismic events of the guiana craton are mostly localized along local crustal discontinuities and are not associated with crustal thickness variations. recent seismicity in the rsnc catalog the rsnc started operating since 1992 and has accumulated accurate instrumental information that consists of earthquake epicenters, hypocenters, origin times, and magnitudes. the epicenters were sorted by magnitude for minor events (m= 3 to 4) in figure 6, moderate events (m= 4.1 to 5) in figure 7, and major events (m > 5.0) in figure 8. the minor and moderate earthquakes in figures 6 and 7, respectively, define seismic corridors oriented along the major crustal discontinuities of the north andes microplate. the sierra nevada of santa marta also shows more intensive seismic activity than previously recorded by the older data. major earthquakes in figure 8 are located along the major crustal discontinuities and plate boundary zones of the north andes microplate. in general, seismic events are associated within thicker crust, and therefore can be associated with tensional stress (artyushkov, 1973). hypocenter depths from the modern rsnc were sorted into shallow earthquakes (from 0 to 10 km) in figure 9, intermediate earthquakes (10 km to 30 km) in figure 10, deep earthquakes (30 km to 70 km) in figure 11, and very deep earthquakes (70 km to 300 km) in figure 12. the tendency of the deepening hypocenters to migrate landward of the pacific subduction zone is indicative of the oceanic plate subducting under the andes at an hernández et al., esrj vol. 11, no. 1. june 2007 86 figure 2. anss seismic catalog for 1966-2006 for three categories of earthquake magnitudes: m=3-4 (minor earthquakes), m= 4.1-5.5 (intermediate earthquakes), and m ¸ 5.5 to 10 (major earthquakes). crustal discontinuities interpreted from gravity anomalies and zones of crustal thickness variations are also displayed. yellow and blue zones correspond to thicker and thinner crust, respectively. this and the succeeding figures were created using the capabilities of oasis montaj (geosoft, 2006). figure 3. rsnc historical seismic catalog for 1643-1991 of earthquakes with magnitudes from m=3.0 to 4.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. crustal thickness variations and seismicity of northwestern south america 87 figure 4. rsnc historical and instrumental seismic catalog for 1643-1991 of earthquakes with magnitudes from m=4.1 to 5.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. figure 5. rsnc historical seismic catalog for 1643-1991 of earthquakes with magnitudes m > 5.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. hernández et al., esrj vol. 11, no. 1. june 2007 88 figure 6. rsnc modern seismic catalog for 1992-2006 of earthquakes with magnitudes from m=3.0 to 4.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. figure 7. rsnc modern seismic catalog for 1992-2006 of earthquakes with magnitudes from m=4.1 to 5.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. crustal thickness variations and seismicity of northwestern south america 89 figure 8. rsnc modern seismic catalog for 1992-2006 of earthquakes with magnitudes m > 5.0. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. figure 9. rsnc modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 0 km to 10 km. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. hernández et al., esrj vol. 11, no. 1. june 2007 90 figure 10. rsnc modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 10 km to 30 km. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. figure 11. rsnc modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths from 30 km to 70 km. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. crustal thickness variations and seismicity of northwestern south america 91 angle that is less than vertical. discussion gravity and topography are related to seismic activity by the frictional behavior of plate boundary zones and crustal discontinuities. when two plates push against each other, friction between the plates builds up causing the plates to lock. the continued build up of tectonic stress also causes the plates to deform, creating variations in topography and gravity. in addition to deforming the plates, the rocks of the sticking point can break when the stress build up exceeds the breaking strength of the rocks. the resulting strain released at the locking point produces the ground shaking of an earthquake. for northwestern south america, plate tectonic stress plays out notably in the ocean-continent, and ocean-ocean and continent-continent plate collision zones, and also along oceanic spreading ridges and transform faults. in the next sections, these plate boundaries and associated earthquakes zones areas are further analyzed. ocean-continent collision zones in figures 2 to 12 the subduction of the nazca plate under the north andes microplate is clearly parallel to the andes mountains. the collision caused the creation of the andes mountains by stacking thrust slices of the crust. the lateral movement of these thrust slices in the welded continent has continued after the collision generating minor, moderate and major earthquakes with shallow, intermediate and deep hypocenter depths defining seismic corridors that are oriented along the subduction zone and the andes mountains. the general pattern of the seismicity and focal mechanism solutions of earthquakes are in agreement with the easterly subduction of the nazca plate beneath the andes. the intensive seismicity along the middle american trench (mat) has a broad pattern reflecting the low angle of inclination of the subducting nazca and cocos plates under the central american continent. the seismicity along the caribbean-north andes plate boundary is relatively low, with few minor (figures 3, 6, and 9), moderate (figures 4, 7, and 8), and major (figures 10 and 11) historical seismic events. this seismicity reflects the predominantly dextral lateral movement of the caribbean plate with respect to the north andes microplate and south american plate that may include minor subduction that is incipient when compared with the subduction of the nazca plate underneath the andes mountains. ocean-ocean collision zones figure 2 shows a broad arcuate seismic zone along the lesser antilles. a large number of events occur along the caribbean-north american plate boundary, which dips to the west away from the underthrusting oceanic plate. shallow, intermediate, and deep focus earthquakes occur at progressively greater distances from the site of underthrusting. the topographic expression of the trench is largely obscured by thick sediment fill derived from the orinoco river of venezuela, but its presence is indicated by the belt of negative tcfaga in figure 1. the seismic events are located in thicker oceanic crust associated with the doubling of the collisional oceanic crust. focal mechanism solutions of the lesser antilles reflect regional compressional stress. however, the relatively thicker oceanic crust reflects local tensional stress. continent-continent collision zones the continent-continent collision boundary zone between the north andes microplate and the south american plate in figures 2 to 12 is characterized by intensive seismicity. minor, moderate, and major historical and recent seismic events in figures 3 to 8 have occurred along the llanos front fault. shallow events in figure 9 can be associated with tensional stress of the sub-andean thrust belt or fault zone, which is located on a relatively thicker continental crust that implies the local presence of additional tensional stress. hernández et al., esrj vol. 11, no. 1. june 2007 92 oceanic spreading ridges minor and moderate earthquakes occur along the galapagos spreading ridge and the panama fracture zone in figure 2. focal mechanism solutions indicate regional tensional events associated with plate accretion and strike-slip events where the ridges are offset by transform faults. however, the panama fracture zone is located on relatively thinner oceanic crust that may be locally associated with compressional stress. transform faults the southern panama fault zone constitutes a conservative plate boundary between the nazca plate and panama microplate, where the plates are in tangential contact and are affected by limited or no subduction or accretion. therefore, events along this fracture are rare as shown in figure 2. cratonic zone the intraplate areas of the south american plate are relatively aseismic. however, a few minor, moderate, major historical and recent seismic events in figures 3 to 8 have occurred at shallow, intermediate, deep, and very deep hypocenter depths in figures 9 to 12. these events are located along some of the crustal discontinuities interpreted from the surface faga in figure 1. their occurrences suggest the presence of continental sutures in the south american plate, with infrequently seismic activity. although, these earthquakes are rare, they are important because they indicate the nature of directions of stress within the south american plate. the instrumental seismic events in the above plate boundary zones are analyzed in the next section for their focal mechanisms. focal mechanism solutions figure 13 gives the compressional stress regime for the study region from stress inversions of the focal mechanisms of the instrumental earthquakes of northwestern south america (vargas and duran, 2005). these solutions infer the type of faulting or focal mechanisms from the radiated seismic energy. the costa rica-panama-north andes converging margins show compressional stress oriented west-east, generating the relative movement of local tectonic blocks of the north andes microplate. shallow earthquakes associated with the pacific subduction zone show compressional stress oriented nw-se. vectors of displacement and compressional stress in southwestern colombia are oriented west-east. the deep seismicity in the bucaramanga nest reflects compressional stress oriented nnw – sse associated with the subducting caribbean plate under the north ande microplate. complementary examples of global focal mechanisms for the study area can also be obtained from the harvard centroid moment tensor catalog (harvard, 2006). these results reflect the inversions of long-period body and surface waves for the source moment tensors and best-fitting double-couple solutions. conclusions seismicity in northwestern south america is mainly restricted to the plate boundary zones of the caribbean, north america, and south america plates and the panama, costa rica and north andes microplates. seismic corridors are oriented predominantly sw-nw, showing their association with compressional stress generated by the convergence of the nazca and south american plates, and the shortening of the continental crust and mountain building of the andes mountains. the seismicity is extremely high along the northern margin of the pacific subduction zone. the hypocenter depths show the tendency of the earthquakes to migrate from the coastline eastwards as the hypocenter depths increase, confirming the eastward motion of the nazca plate under the north andes microplate. the spatial distribution of earthquakes is consistent with crustal discontinuities interpreted from the gravity anomalies. seismic events are preferentially concentrated in zones of relatively thicker continental crust. this crust is inherently under tension by virtue of its enhanced thickness, but it is also being subjected to overwhelming compression from plate subduction forces. the analysis of the isostatic state of compensation crustal thickness variations and seismicity of northwestern south america 93 figure 12. rsnc modern seismic catalog for 1992-2006 of earthquakes with hypocenter depths to the hypocenter from 70 km to 300 km. crustal discontinuities interpreted from gravity anomalies and the zero curvature contours of crustal thickness variations are also displayed. the yellow and blue zones correspond to thicker and thinner crust, respectively. figure 13. main stress axes determined by the reches method (reches, 1983; reches, 1987) from 94 focal mechanism solutions of the central moment tensor catalog (cmt) reported by harvard university (harvard, 2006) for the period 1976-2000 and event magnitudes m = 5.0 (vargas and duran, 2005). hernández et al., esrj vol. 11, no. 1. june 2007 94 from terrain and gravity anomalies of northwestern south america help to differentiate probable earthquake zones. integrating the local stress of the crustal thickness variations with regional tectonic stresses may help to predict local ground motions. the focal mechanisms reflect regional stress and more local stress from the thickness variations of the crust. references artyushkov, e.v. (1973). stresses in the lithosphere caused by crustal thickness inhomogeneities. journal of geophysical research. 78. 7675-7708. bird, p. (2003). an updated digital model of plate boundaries. geochemistry, geophysics geosystems g3. 4 (3). 1-52. cediel, f., shaw, r. p., and caceres, c. (2003). tectonic assembly of the northern andes block. the circum-gulf of mexico and caribbean hydrocarbon habitats, basin formation and plate tectonics: aapg memoir. 79. 815-848. geosoft (2006). geosoft oasis montaj: software for earth sciences mapping and processing. http://www.geosoft.com. harvard, (2006). centroid moment tensor catalog (ctm). http://www.seismology.harvard. edu/cmtsearch.html. hernandez, o. (2006). tectonic analysis of northwestern south america from integrated satellite, airborne and surface potential field anomalies. phd dissertation, department of gelogical sciences. the ohio state university, columbus, oh. usa. 177 pages. ingeominas, (2006). red sismológica nacional de colombia (rsnc) catalogo de sismos. kim, j.w., von frese, r.r.b., and rae, k.h. (2000). crustal modeling from spectrally correlated free-air and terrain gravity data a case study of ohio. geophysics. 65 (4). 1057-1069. shearer, p. m. (1999). introduction to seismology. cambridge university press, new york. 259 pages. song t. r. a. and simons, m. (2003). large trench parallel gravity variations predict seismogenic behavior in subduction zones. science, reports. 301 (5633). 630-633. turcotte d.l. and shubert, g. (2002). geodynamics, second edition. cambridge university press, new york. 456 pages. usgs, (2006). advance national seismic system (anss). http://earthquake.usgs.gov/ research/monitoring/anss/ vargas c. a. and duran, j. p. (2005). state of stress of northwestern south america. earth sciences res.j. 9 (1). 41-49. von frese, r. r. b. and tan, l. (1999). antarctic crustal modeling from spectral correlation of free-air gravity anomalies with the terrain. journal of geophysical research. 104, b11, 25275-25,296. crustal thickness variations and seismicity of northwestern south america geofisica colombiana n°.7 pp. 19-23 diciembre de 2003 bogota, d.c. issn 0121 2974 modelo integrado de informacion gravimetrica, geologica y de terrenos para los perfiles: turbo san jose del guaviare y quibdo puerto berrio ramiro leon dial campos leonardo dial yaoueline figueredo baez grupo de investigaci6n en geofisica, departamento de geociencias, universidad nacional de colombia, e-mail: rldiaz@unal.edu.co resumen ei modelo estructural de la zona de estudio se genera a partir de la integraci6n de la informaci6n gravimetrica, la geologia regional y terrenos tectono-estratigraficos, se analizan dos perfiles aa' (turbo san jose del guaviare) y bb' (quibd6 puerto berrio), que permiten confirmar la existencia de dos zonas, una con corteza oceanica (oeste de romeral) y otra con corteza continental (al este). entre los dos perfiles se encuentra un punto de inflexi6n en el rumbo de la falla de romeral a partir del cual se presentan cambios importantes en las componentes de acrecion y de csfuerzos, dando origen a variaciones considerables en el modelo de subducci6n. al norte del punto de inflexi6n aparecen dos zonas de subducci6n: una actual, asociada a la falla del simi, y otra f6sil asociada a la falla de romera!. ademas, localmente se encuentran evidencias tanto de geologia como en gravimetria de una zona de obducci6n. al sur del mismo punto se determinaron tres zonas de subducci6n: la actual, asociada a la fosa del pacifico, y otras dos fosilizadas asociadas a las suturas 0 fallas de romeral y fallas del rio atrato. en el perfil aa', en la zona de corteza continental se observan bien definidos sectores de las cordilleras central y oriental, y de los llanos orientales, en el perfil bb' se aprecia hasta el valle medio del magdalena. la diferencia entre estas zonas radica en el tipo de roca del basamento y en el estilo de deformaci6n. palabras clave: gravimetria, problema inverso. modeld abstract the structural model of the studied zone, was generated from the integration of the gravimetric, regional geology and area teetono-stratigraphics. two profiles aa' (turbo san jose del guaviare) and bb' (quibdo puerto berrio), analized corroborate the existence of two zones, one with oceanic crustal (west of romeral) and other with continental crustal (to the east); among both profiles there is a point of inflexion in the direction of the attitude of romeral fault, from which they present important changes in the accretion components and efforts, giving origin to important variations in the model of subduction. to the north of the point of inflexion two zones of subduction appear, the current one associated with the sinu fault and a fossil were area associated to the with romeral's fault. besides, locally they find evidences so much in surface geology as gravimetry ofa obduction zone. to the south of this point theree subduction zones were determined, the current one associated to the pacific grave and two fossilized associated to the sutures or romeral's faults and of the atrato river fault's. in the profile aa', in the zone of continental crust are observed the sectors of cordillera central, cordillera oriental and llanos orientales. in the profile bb ' is appreciated until the valle medio del magdalena. the differences among these areas they reside in the type of rock of the basement and the deformation style. keyworos: gravimetry. inverse problem, mooel antecedentes en trabajos anteriores, barrera (\ 979), dengo (1993) y g6mez (2001), se presentan diferencias en el modelo de subducci6n y en la direcci6n de los perfiles, la cual es perpendicular a las estructuras geomorfol6gicas 0 a los trazos de los planos de falla; en este trabajo manuscrilo recibido para evaluacion el 23 de junio de 2003. articulo aceptaco para publicacion por el comtta editorial el 30 de octubre de 2003. geoflslca colombiana. 7, diciembre de 2003 19 se plantea paralela a la componente de acrecion resultante de los sentidos de desplazamiento de las placas. para la elaboracion de los modelos estructurales se utilizaron: el mapa geologico de ingeominas (1998), el mapa de terrenos (etayo et al., 1996) y datos gravimetricos (igac, 1998). localizacion se analizan dos pefiles gravimetricos, el perfil aa' se localiza desde el golfo de uraba hasta los llanos orientales y el perfil b-b' cabo corrientes serrania de san lucas (figura i). a-a' tiene una direccion n45w y se eligio par ser perpendicular a la tendencia del rumba and ina y par tener en cuenta la componente de rumba dextral generada par la acrecion de los diferentes terrenos teetono-estratigraficos, el perfil b-b' tiene una direccion n45e perpendicular a a-a', es paralelo a la componente de acrecion de terrenos y a los esfuerzos compresivos; adicionalmente, estos perfiles presentan un buen cubrimiento de datos gravimetricos. integracion de la informacion ei perfil a-a' turbo bogota san jose del guaviare (figura 2) pasa par las fallas de simi, romeral, honda y guaicaramo, las cuales a su vez son limites de los terrenos simi, canas gordas, cajamarca, cordillera oriental y llanos. el perfil b-b' quibdo medellin puerto berrie (figura 3) pasa par las fallas de baudo, rio atrato, cauca, romeral y pales tina, que a su vez son limites de los terrenos baudo, chaco, canas gordas, cauca romeral, cajamarca, payande y parte w de la cordillera oriental, cuyas caracteristicas primardiales como tipo de roca predominante y ambiente tectonico en el cual se desarrollaron, se presentan en las tab las i y 2: le6net al. figura j. localizaci6n perfiles. tabla 1. tipo de roca predominante de cada uno de los terrenos : sinu i a corteza oceanica basaltos tonalitas i 2 7 subduccion actual i i---"c'-"a"-'n-"'as"----"g'-"o'-"rd"'a""s'--_--c!ra'-'--'b~+_--"'c"'o-'-'rt""e""za~oc'""e....a"'n'-"ic....a -t--ob"'a""s""a"lt""os"--+_-=2'-1-7 -jls"'-u""b"'d".u"-'c""c-'-'io<'-n'-cfi'-"o""s-'-'il1 ii bcauca romeral corteza oceanica basaltos 2 7 subduccion fosil j baudo chaco ! b corteza oceanica 2 7 subduccion actu~1 i cajamarca i a b corteza continental metarnorficos alto grado 2,6 subduccion fosill.ocalmente se l +_1 !--------____________ _ ~_ _ -+--'-a'l'nj~re.....c~i~a~o'_"b~d....u~c:".cl'-"·o~n'.£.'ne~r-,"fi~1.!..a~_ i payande i b i corteza continental igneos y metamorficos 2,6 bloque levantado y limitado i ~ i i nor fall as i cordillera oriental i a b ! carteza continental i igneos ~ menor proporcion i 2,6 estructura invertida sobre sus i i _~ _+!---+-i---------+-i .:..:m-'-'e"'ta".m=or,-ofi-'-'lc'-"o"'s-+ i dos flancos ------1 illanos i a ii corteza continental i paleozoico sedimentario i 2,6 antepais i : _ i y metamorfico de bajo zrado i i 20 geofislca colombiana. 7. oicie~.bre oe 2003 modelo integrado de informacion gravimetrica, geologica y de terrenos para los perfiles tabla 2. cobertera del basamento cristalino y ambiente tect6nico de cada uno de los terrenos. -~ terreno corte cobertera del basamento densidad promedio observaciones icristalino (gm/em3) i simi a terciario sedimentario 2.3 alta deforrnaci6n de escama gruesa i baud6 choc6 b terciario sedimentario 2.3 alta deformaci6n de escama g~ canas gordas ab cretacico'terciario 242.3 alta deformaci6n de escama zruesa cauca romeral b cretacico'terciario 2,42.3 alta deformacion de escama grue§~ cajamarca ab no es importante la cobertera alta deforrnaci6n de escama gruesa sedimentaria ~ payande b cobertera sedimentaria muy deformaci6n de escama gruesa delzada a inexistente cordillera oriental ab cretacico'terciario sedimentarios 2.42,3 alta deforrnaci6n de escama gruesa en espesores de miles de metros v delzada i llanos a cretacico terciario sedimentarios; 2,42,3 baja deforrnaci6n de escama gruesa l. 2redomina el terciario en esnesores _-.j figura 2. perfil gravirnetrico aa' con interpretacion geologica. se observa como varian los cambios de anomalia a partir de las zonas de falla principales. km 50 ~ ffi ~ 0.0 ++-+_1-+00 20t-0__ f--i300 wc 'i,;01ii;0'iii/il.50~~100 km 5 e'. i ~z..l c? ~o ~'z oceano serrania< § -50 0 ~ 8 ~ sw pacifico del baudo ~ ct 8 g (3 omsnm,~~~~~~~~~-15000m~ ~ ,," "" ~ _50000 m z.subd~· .subdu~6n ~~~ ~ cordillera ~s central ~ vmm :::;=;;:;::=5~bo / .: -15doc ~."'"""'mi..c....c.j esism<ls figura 3. perfil gravimetrico bb' con interpretacion geologica. geofislca colombiana. 7. oiciembre oe 2003 modelo estructural la variaci6n de la anomalia bouguer total registra una correlaci6n con la geologia regional y con los terrenos tectono-estratigraficos cuyos limites son fallas de caracter regional. con base en la interpretaci6n gravimetrica y el modelamiento geol6gico se observaron los dos grandes domini os de basamento uno, corteza oceanica al oeste de la falla de romeral, y el otro, la corteza continental al este (figuras 4 y 5). para el perfil aa' en la zona donde predomina la corteza oceanica se aprecian variaciones marcadas por las fallas de sinu y romeral; estas variaciones responden a la ubicaci6n respecto a los frentes de falla. como podemos observar en el "front dip" del sinu, la anomalia marca el prisma acrecentivo del sinu, 10 que puede explicarse por los apilamientos estructurales de "fore arc". la zona de subducci6n actual asociada a la falla del sinu y la zona de subducci6n f6sil asociada a la falla de romeral presentan diferencias debido a que la acreci6n no fue perpendicular en todos los sectores y gener6 una serie de esfuerzos transpresivos; el no desarrollo de un arco volcanico en la parte norte por la convergencia oblicua y la geometria generada por la variaci6n del rumbo de la falla de romeral marca el limite entre las placas de sur america y nazca en la parte nw. con base en la situaci6n anterior se produjo un evento de obducci6n, que se evidencia y manifiesta geol6gicamente con la presencia de escamas de corteza oceanica en el bloque colgante de la fall a de romeral, y se manifiesta en la anomalia de bouger total (figura 4). en la anomalia de bouguer total se notan apreciables cam bios entre la zona de corteza continental, cordillera central y cordillera oriental, debido posiblemente al cambio de roca dominante. las 21 figura 4. modelamiento geofisic del perfil gravirnetrico aa·. fallas en el area de la cordillera central tienen trazos muy rectos con alto grade de buzamiento y escarnacion gruesa y de rumbo fuerte, exhumando rocas del paleozoico y precambrico, en la cordillera oriental las fallas son de bajo angulo involucrando cobertera sedimentaria del terciario y cretacico, por tal motivo el basamento se encuentra mucho mas abajo, dando como resultado una raiz mas profunda que se refleja muy bien en el perfil gravimetrico interpretado. geornetricamente se observa un cambio muy marcado en la cordillera oriental que representa una estructura tipo "pop up", con una raiz profunda. hacia el piedemonte llanero se encuentra la falla de guaycaramo, como limite este del basamento tipo cordillera (raiz profunda); en el bloque yacente de esta falla el basamento es el escudo de guyana (raiz superficial). ei perfil bb' con orientacion s45w n45e desde el oceano pacifico pasando por el cabo corrientes hasta el valle medio del magdalena a la altura de la serrania de san lucas, en el area de corteza oceanica (al w de romeral) presenta una deformacion de fallas de cabalgamiento a inversas con una vergencia principalmente al w y de tipo de escama delgada; localmente se aprecia decapitada por fallas con vergencia este. ei desarrollo del sector estuvo dominado por acreciones consecutivas de diferentes terrenos tectono-estratigraficos a 10 largo de las suturas de romeral, rio atrato, que demarcan dos zonas de subduccion fosilizadas; las series de cabalgamientos enmarcan un ambito tectonico de "fore arc" para cada una de las subducciones. en las figuras 2 y 3 se muestra un modelo integrado de los perfiles aa' y bb' que contienen la informacion de geologia regional, los terrenos tectono-estratigraficos y las anomalias gravimetric as. estos modelos integrados estan soportados por modelos geofisicos en 20 (figuras 4 y 5), ajustados utilizando las anomalias de bouger total y referenciados a los hipocentros de algunos sismos que evidencian los buzamientos de las zonas de subduccion, se presenta una muy buena correlacion entre las anomalias calculadas y las anomalias medidas, 10 cual permite plantear con una alta 22 leon et at. figura 5. modelamiento geofisico del perfil gravirnetrico bb·. probabilidad las estructuras regionales. se utilize el programa computacional gmsys para realizar los calculos de ajuste para las anomalias. discusion de resultados ei modele con base en estos dos perfiles proporciona una vision muy aproximada de la estructura regional. entre los perfiles aa' y bb' casi perpendiculares entre si, se observa un punto de inflexion (figura 6) 0 cambio de direccion de las suturas como se presenta en la falla de romeral. ei cambio de direccion de estas suturas se manifiesta en la variac ion del senti do de acrecion de los terrenos. la componente de acrecion para el corte aa' es oblicua a romeral con un angulo aproximado de 40°; para el corte bb', las componentes de esfuerzos son perpendiculares a romeral; esto figura 6. esquema de fallas y punto de inflexion de romeral geofislca colombiana. 7, diciembre de 2003 modelo integraoo de informacion gra vimetrica, geol6gica y de terrenos para los perfiles marca muy fuertemente la diferencia de deformacion y subduccion para las margenes pacifica y caribe, como se aprecia en los cortes. conclusion los cambios de direcci6n de las suturas generan variaciones morfologicas muy importantes en los mecanismos de subduccion, las cuales se reflejan en el modelo estructural que se plantea en este trabajo. este modelo esta soportado por la correlacion que existe entre la informacion de geologia regional, la division de terrenos tectono-estratigraficos, el modelamiento geofisico de la anomalia bouguer total en 2d y la informacion sismol6gica de la zona. geofislca colombiana, 7, digiembre de 2003 bibliografia barrera, d. 1979. geology of western cordillera, west of bug a and roldanillo colombia. pub. geol. esp. ingeominas 4. dengo c. and covey m. 1993. structure of the eastern cordillera of colombia: implications for trap styles and regional tectonics. aapg bulletin, vol. 77, pp. 1315-1337. etayo et al. 1996. mapa de terrenos geologicos de colombia. ingeominas. gomez e. (200 i). tectonic control underline cretaceous to cenozoic sedimentary field of the middle magdalena valley basin. eastern cordillera and llanos basin colombia. ph. d. degree tesis, cornell university. igac. 1998. gravimetria. ingeominas. 1998. mapa geologico de colombia. 23 using neuronal networks to determine site response from coda waves: application in armenia, colombia earth sciences research journal earth sci. res. j. vol 8, nº 1 (dec. 2004): 45 51 manuscript received april 2004 45 paper accepted august 2004 fine thermohaline structure of the colombian pacific ocean nancy villegas department of earth sciences, metheorology, universidad nacional de colombia bogotá, colombia e-mail: nlvillegasb@unal.edu.co abstract the present document shows strata classification of the colombian pacific ocean – columbian pacific ocean, done by first time according its fine thermohaline structure, based on temperature and salinity fields analysis. layers, where different mechanisms of fine structure predominate, were determined and everywhere in the area a stable stratification was observed, although conditions for not stability as a result of the double diffusion were present. key words: fine thermohaline structure, small-scale thermohaline structure, stability of the ocean, stratified ocean, salt finger stratification, differential-diffusion convection, and step structure resumen el presente documento muestra la clasificación de las capas de la cuenca del pacífico colombiano (cpc), hecha por primera vez de acuerdo con su estructura termohalina fina, basada en el análisis de los campos de temperatura y salinidad. las capas fueron determinadas con predominancia de diferentes mecanismos de estructura fina y se observó que en toda la región existe una estratificación estable, aunque se encontraron condiciones de inestabilidad como resultado de una doble difusión. palabras clave: estructura termohalina fina, estructura termohalina de baja escala, estabilidad en el océano, estratificación oceánica, estratificación de salinidad, convención de difusión diferencial, paso de estructura. © 2004 esrj unibiblos. nancy villegas 46 introduction on a fine structure scale, the profiles of hydrophysical fields show alternated sections with small and high gradient properties, the first one called quasi-homogeneous layers and the second one by layers. inside temperature (t) and salinity (s) profiles, sections with inverted properties distributions are frequently encountered. the fine structure is understood as a model of physical fields represented by a set of layers instead of homogeneous properties in strata ranging from 0.1 to 1 m, divided by even more thin layers with sharp gradients of temperature and salinity. the vertical gradients are 10 to 100 times higher and more, exceeding corresponding average gradients values (karlin, 1988). the fine stratification of water layer is testified by numerous measurements, done in different regions of the ocean with low-inertia sounding equipment, in particular ctdprobes. contrasting profiles obtained by standard hydrological instruments, these profiles contain many structural details, reproduced by repeated soundings and, therefore, which live long. the frequently curves of vertical distribution t and s take the form of correct steps or variables on the sign of deviations from the average profiles. there are fine-structure heterogeneities in the seasonal and main thermoclines on big depths and even in the upper quasi-homogeneous layer of the ocean (karlin et al., 1988). fedorov showed (fedorov, 1976) that below the upper quasi-homogeneous layer, a fine structure is formed in the majority of cases against the hydrostatic stability background on the vertical distribution density. on these conditions, two mechanism of fine structure formation are possible. the first one requires kinetic energy from an external source, used to increase the potential energy in the liquid layer. the mechanisms present are shift instability of currents, internal waves, etc. the second mechanisms do not require external energy sources. there, the structurization occurs by liberation of accessible potential energy, dissipated later as heat. the mechanism presents instability due to differential-diffusion convection or dual diffusion (zhubas and ozmidov, 1984, ozmidov, 1983). in shift instability of currents and internal waves, turbulence and elements of fine structure of hydrophysical fields arise under action of strong hydrodynamical instability in zones with high vertical gradients of speed current, like located areas observed as separate turbulent spots (benilov, 1985, maderich and nikishev, 1986). turbulent mixing regions are outlined on the vertical profiles of hydrophysical fields as the fine structure elements. the mixture process in the turbulent volume decreases temperature and salinity gradients inside it, being critical on its boundaries. spots formation due developed turbulence may also occur because instability or breaking of internal waves (fedorov, 1976, fedorov, 1991, konyaev and sabinin, 1992). this mechanism is observed when the richardson´s number achieve a value below some critical value, being the probability of formation of spots of turbulence at instability higher than at breaking of internal waves (ohotnikov and panteleis, 1985). the sizes of vertical instable areas of internal waves vary from decimeters to meters. the differential-diffusion instability is due to diffusion of two or more components of hydrodynamic system at different speeds. such instability forms a fine structure of temperature and salinity fields in water with a wide circulation at the ocean. because differences in the molecular thermal conductivity coefficients and salt diffusion in the system, the convective instability appears, producing intensive mixing and forming quasi-homogeneous layer divided by layers with sharp gradients properties (turner, 1985). a special attention is paid to the mechanism of differential-diffusion instability, it allows accelerated heat and salts transfers without energy from external sources (karlin et. al, 1988, fedorov et. al, 1986, karlin, 1988). this mechanism is observed, when temperature and salinity gradients produce opposite contributions to the vertical density gradient. here, the potential energy associated to vertical stratification is used, contributing to destabilize the density gradient. liberation of instability energy is due to differences in coefficients of molecular exchange of heat and salts. differential-diffusion instability is manifested in the regime of salt fingers and layered or diffuse convection. methodology initial natural data the calculation of stability of layers in waters of colombian pacific ocean (villegas, 2001) was made according to the fields of temperature and salinity, obtained in the scientific expedition during may 2000 (otero y pineda, 2000). the vertical distribution of water stability and the fine thermohaline structure of the colombian pacific ocean 47 vaisala-brunt’s frequency (villegas, 2002b, karlin and villegas, 2003) were made based on the example of 3 hydrometeorological stations obtained in this expedition: 14, 49 and 111 (figure 1). 84 8 3 82 81 80 79 78 77 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 n or th la tit ud e western longitude panama c olombia ecuador figure 1. hydrometeorological stations on the colombian pacific ocean. these stations, covering the 3 more interesting zones: coastal zone-station 14 (at 4°n/78°w), open sea-station 111 (at 3°n/84°w) and zone of mixing waters-station 49 (at 2°n/80°w), were selected on basis to the quasi-homogeneous zones on this study region (villegas, 2002a). diagnostic of possible fine structure activity in the ocean the temperature (et) and salinity (es) components of stability (e) are widely applied in the solution of different problems, including the typification of thermohaline conditions for stratification, diagnosing possible forms of mixing and processes of structurization, including the scales of fine structure (karlin et al, 1988; malinin, 1998). diagnostics of possible fine structure activity in the ocean is carried out usually along background ts-profiles with the aid of some parameters and criteria. one of them relates temperature and salinity components of stability, and also density relationship rp (karlin et al, 1988; filimonov, 1990, fedorov and pereskokov, 1991). based on these parameters and criteria, the diagnostics of possible fine-structure activity in colombian pacific ocean is represented below. according to data obtained on colombian pacific ocean during may 2000 (otero and pineda, 2000), and the help of the hesselberg sverdrup’s criterion (1) ([malinin, 1998; valerianova and zhukov, 1974; kamenkovich and monin, 1978), stability is calculated and contributions of temperature and salinity are located into general stability colombian pacific ocean (villegas, 2001). dz dt tdz dt tdz ds s e a ∂ ∂ + ∂ ∂ + ∂ ∂ = ρρρ (1) with: s∂ ∂ρ as density variation with a change in the salinity, kg/m3‰; dz ds as vertical gradient of salinity, ‰/m; t∂ ∂ρ as density gradient with change in temperature, kg/m3°cm; dz dt is the vertical gradient of temperature, °c/m; dz dta as vertical gradient of adiabatic temperature, °c/m; the temperature and salinity stability components, and the density relationship rp (2) (karlin et al., 1988, malinin, 1998, fedorov, 1991) have been used for carrying out of diagnostics of possible fine structure activity at ocean on background t-s profiles. s t e e dz ds dz d r −≈ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = β θα ρ (2) with dz dθ as vertical gradient of potential temperature, °c/m; dz ds as vertical gradient of salinity, ‰/m; te as temperature component of stability, kg/m4; se as salinity component of stability, kg/m4. on the same data is calculated vaisala-brunt’s frequency (villegas, 2002b) according to the formula (malinin, 1998, konyaev and sabinin, 1992, kamenkovich and monin, 1978; valerianova and zhukov, 1974): nancy villegas 48 dz dgn ρ ρ = (3) where dz dρ is the vertical gradient of density. results the stability of layers in waters colombian pacific ocean n, according to temperature and salinity data of may 2000 expedition (villegas, 2001), showed a vertical distribution of stability and the vaisala-brunt’s frequency with a jump in the range 10 75 m. see station 14 (figures 2 and 3); station 49 (figures 4 and 5) and station 111 (figures 6 and 7) (villegas, 2002b; karlin and villegas, 2003). de pt h, m stability, кg/m4 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 -400 -350 -300 -250 -200 -150 -100 -50 0 e et es figure 2. vertical distribution of stability on colombian pacific ocean. may 2000, station 14 de pt h, m frequency, s-1 0.005 0.01 0.015 0.02 0.025 0.03 0.035 -400 -350 -300 -250 -200 -150 -100 -50 0 figure 3. vertical distribution of vaisala-brunt’s frequency on colombian pacific ocean. may 2000, station 14 besides that, colombian pacific ocean has a positive stability on all vertical and prevalence of temperature stability above salinity stability. the hydrological stations near the coast showed that, in a superficial layer, the stability in salinity prevailed over temperature stability. the contribution of salinity to stability decreases with removal from the coast. d ep th , m s tab ili ty, кg /m 4 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 e et es figure 4. vertical distribution of stability on colombian pacific ocean. may 2000, station 49. fine thermohaline structure of the colombian pacific ocean 49 d ep th , m frequency, s-1 0.005 0.01 0.015 0.02 0.025 0.03 0.035 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 figure 5. vertical distribution of vaisala-brunt’s frequency on colombian pacific ocean. may 2000, station 49. vertical distribution vaisala-brunt’s frequency has similar behavior with vertical distribution of stability. vaisala-brunt’s frequency has values from 0.0025 to 0.036 s-1 which decrease with removal from coast. the greatest values took place in the center section of the coast, and minimum in the southeastern region colombian pacific ocean. de pt h, m stability, кg/m4 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 -400 -350 -300 -250 -200 -150 -100 -50 0 e et es figure 6. vertical distribution of stability on colombian pacific ocean. may 2000, station 111 d ep th , m frequency, s-1 0.005 0.01 0.015 0.02 0.025 0.03 0.035 -400 -350 -300 -250 -200 -150 -100 -50 0 figure 7. vertical distribution of vaisala-brunt’s frequency on colombian pacific ocean. may 2000, station 111 in 100 150 and 75 100 m layers, the greatest values of stability were located on the north along the coast and in the region from 78 to 80° w and from 5 to 7° n. in the 50 75 m layer an absolute stability on the north and southwest colombian pacific ocean is observed, while minimum values were observed around the gorgona island and southeast colombian pacific ocean. the layer 25-50 m shows absolute stability. the minimal values were located on the northeast and southwest colombian pacific ocean. in the 10-25 m layer negative values of stability are observed colombian pacific ocean . in general, in colombian pacific ocean area stable stratification is observed, but there are conditions for not stability as result of double diffusion (villegas, 2002b; villegas, 2003). table 1 shows the results of conducting diagnostics of the possible fine structure activity in colombian pacific ocean by the example of stations located on all researched area, with the help of vertical distribution of temperature and salinity, combinations of the contributions and to the general stability, and also criterion . in the given table four types of stratification are submitted as follows: te se ρr • full, or absolute stability (as): 0<∆t , 0>∆s , , , ; 0>te 0>se 0<ρr • salt fingers type (sf): , 0<∆t 0<∆s , , 0>te 0<se , ; 0>ρr nancy villegas 50 • type of layered convection (lc): 0>∆t , , , , ; 0>∆s 0<te 0>se 0>ρr • absolute instability (ai): , 0>∆t 0<∆s , , , . 0<te 0<se 0<ρr it is evident from table 1 that in colombian pacific ocean predominates the complete stability upper 150 m layer. below 200 m the salt the salt finger stratification is observed. in the layer 0-10 m only at separate stations 17, 42 and 114 absolute instability takes place. the type of salt finger stratification is at stations 49, 81 and 107, and the type of layered convection at the station 1. in the open colombian pacific ocean, in all layers, the alternation by stratification of the type pu and sp is evident. mixing here is irregular, i.e., in the form of spots, which forms steps in the ts-profiles. table 1. types of stability stratification of layers on columbian pacific ocean. may 2000 numbers of station and type of stability layers, m 1 5 14 17 29 33 42 43 47 49 79 81 107 111 114 0-10 lc as as ai as lc ai as as sf as sf sf as ai 10-25 as lc as as as as as as as as as as as as as 25-50 as as as as as as as as as as sf sf as as sf 50-75 as as as sf as as as as as as as as as as as 75-100 as as as as as sf sf as sf sf sf sf as sf as 100-150 as as as as as as as as sf as as sf sf as sf 150-200 as sf as sf sf sf as sf sf as sf as sf sf as 200-300 sf sf sf as sf sf sf sf sf sf sf sf sf sf sf 300-400 – sf sf sf sf sf sf sf sf sf – sf sf sf sf 400-500 – sf sf – – sf as sf – sf – as sf sf sf 500-600 – – – – – – – – – sf – – sf – sf conclusions layers with predominance of different mechanisms of fine structure were determined. in columbian pacific ocean, over the 150 meter layer, the total stability stratification predominates. below 200 m the salt fingers type stratification and the diffusion instability were observed on the southwest of columbian pacific ocean. in the vertical line on the open columbian pacific ocean the alternation of the total stability stratification type and the salt fingers stratification type were observed. the other remaining regions have stable stratification. in general, in columbian pacific ocean everywhere stable stratification is observed, but there are conditions for not stability as a result of the double diffusion. references benilov a.y. (1985) about the turbulent mechanism of formation to fine structure in the ocean //. izv as ussr. serie. fao. -. t.21, no. 9. p. 973-981. fedorov k.n. fine thermohaline structure of waters. l.: gidrometeoizdat. 1976. 184 p. fedorov k.n., and. pereskokov, a.i. (1991) typification of the thermohaline conditions of stratification in the world ocean: physical oceanology. l.: gidrometeoizdat. p. 127-135. fedorov k.n., pack v.t. et al. (1986) analysis of convective steps in the sea temperature inversion// izv. as ussr. serie. fao. t.22, no. 9 p. 969-978. fedorov, i.n. (1991) chosen transactions on physical oceanologyl.: gidrometeoizdat, p 309. filimonov, ag. (1990) typification of fine thermohaline structure in two hydrological sections in the subtropical part of the northeastern atlantic. hydrology of the southern ocean and north atlantic// sb.nauch.tr. l.: publ. lgmi, p. 145-149. kamenkovich, v.m., and monin, (1978) a.s. physics of ocean. vol. 1 hydrophysics of the ocean/ussr. m.: science, p 456. (ser. oceanology). fine thermohaline structure of the colombian pacific ocean 51 karlin l.n., kluykov y.y. and kutko, (1988) v.p. small-scale structure of hydrophysical fields on the upper layer of ocean. m.: gidrometeoizdat,. 164 p. karlin, l.n. and villegas, n.l. (2003) isolation of layers in the columbian pacific ocean (colombian pacific ocean) with different fine structure mechanisms of formation// materials of academic council session of on 27 28 january part.ii. sections of oceanology, ecology and physics. st. petersburg: rshu, c 26-28. karlin, l.n. (1988) processes of formation and interaction of the different-scale vertical thermohaline structure on the upper layer ocean and their numerical simulation. thesis to the competition of the scientific degree of dr. of phys. and math. l.: lgmi p. 289. konyaev, k.v. and sabinin, k.d. (1992) waves inside the ocean. st. petersburg: gidrometeoizdat, p 272. maderich, v.s. and nikishev, v.i. (1986) the diffusion-viscous stage of spreading variable spots in the stratified liquid// izv. as ussr. serie. fao. t. 22, no. 6. p 656-658. malinin, v.n. (1998) general oceanology, part of 1 physical processes. st. petersburg: publ. rshu, p 342. ohotnikov, i.n. and panteleis, n.a. (1985) the shearing instability of internal waves and vertical exchange in the ocean// sea hydrophysical periodical. no. 3. p 13-20. otero l., y pineda r. (2000) cruceros oceanográficos pacífico xxxii erfen xxx y pacífico xxxiii erfen xxxi 2000: reporte final.– tumaco.: cccp,.–200 p. ozmidov r.v. (1983) small-scale turbulence and the fine structure of hydrophysical fields on in the ocean// oceanology. 23, ed. 4. p. 533-537. turner, j.s. (1985) convection in multicomponent systems//natur wissenschaften. 72, no. 2. – p. 70–75. valerianova, n.a. and zhukov, l.a. (1974) practical works of general oceanology. l.: publ. lgmi. villegas, n. (2001) estructura vertical de las aguas en el océano pacífico colombiano: informe final.– tumaco.: centr. contr. cont. pac. villegas, n.l. (2002a) definition of uniform zones in the surface water of the columbian pacific ocean// materials of academic council session of on 30 31 january. st. petersburg: rshu,. – pp. 126-128. (a). villegas n. (2002 b)estabilidad de las aguas en el océano pacífico colombiano en mayo del 2000. boletín científico cccp.– tumaco.: centr. contr. cont. pp. 1-11 villegas n.l. laws governing the formation of the thermodynamic structure of the columbian pacific ocean. thesis to the competition of the scientific degree of dr. of phys. and math..: rshu. 2003. 189 p. zhubas, v.m. and ozmidov, r.v. (1984) stepped structures form of oceanological thermocline and the mechanisms of their generation// oceanology. 2a. ed., pp. 197-204. nota editorial hei comenzar las casas es tenerlas media acabadas" miguel de cervantes saavedra en el trasegar de las actividades cotidianas, con frecuencia olvidamos los esfuerzos que hemos llevado a cabo en la realizacion de las mismas, solo cuando volvemos la vista al camino andado y confrontamos los desafios que se ten ian y las metas conseguidas, percibimos la incertidumbre ante la empresa y las satisfacciones de los logros. henos un afio despues, nuevamente aqui, con la publicacion del septimo numero de la revista geofisica colombiana, en la cual, con gran teson y mediante el esfuerzo encomiable de investigadores latinoamericanos de las ciencias de la tierra, presentamos a la comunidad cientifica los resultados de investigaciones en los campos de la geofisica. coincide esta ocasion con los cinco afios de reapertura del programa de maestria en geofisica en la universidad nacional de colombia, en su sede bogota, labor que se ha visto fructificada con una nueva promocion de egresados de este programa de estudios avanzados, que con certeza va a redundar en el devenir de un mayor desarrollo de esta rama del saber en beneficio de la nacion colombiana. otras dos efernerides nos circundan; luctuosa la primera, el proximo 25 de enero se cumplen cinco afios del terremoto de armenia, even to que puso a cuestionar a la ciencia sobre el conocimiento de la corteza bajo los andes colombianos; de regocijo la otra, los diez afios de creacion de la red sisrnologica nacional, que ha suministrado un gran volumen de informacion valiosa en el entendimiento de la geodinamica colombiana. recibimos con beneplacito la propuesta de creacion de una carrera de estudios de pregrado en geofisica por parte de! departamento de geociencias de la universidad nacional de colombia, la cual sera bien recibida por parte de la industria y de los circulos intelectuales del pais, por su naturaleza innovadora ante los retos que asume frente a la sociedad y la comunidad cientifica. invitamos a nuevos iniciados en esta ciencia, para que se acerquen a la luz del saber y el conocimiento cienrifico, proyectando su interes al tenor de la geofisica, ciencia que, por su naturaleza aplicada, reconforta al entre gar sus frutos, y por su fundamentacion teorica henche de curiosidad la mente de inquietos jovenes, finalmente, extendemos la invitacion a toda la comunidad cientifica a participar en el primer congreso latinoamericano de sismologia que va a realizarse en la ciudad de armenia, tanto con la presentacion de sus alcances en investigaciones como con su grata presencia en esta acogedora ciudad; para ella incluimos en este numero la informacion pertinente de este acontecimiento. por el avance de la ciencia y en beneficio de todos, los esperamos. luis alfredo montes vides editor application of artificial neural network for predicting shaft and tip resistances of concrete piles ehsan momeni¹, ramli nazir¹, danial jahed armaghani², harnedi maizir³ ¹department of geotechnics and transportation, faculty of civil engineering, universiti teknologi malaysia. ²young researchers and elite club, qaemshahr branch, islamic azad university, qaemshahr, iran. ³jurusan teknik sipil, sekolah tinggi teknologi pekanbaru, indonesia. correspondent author: mehsan23@live.utm.my abstract resumen key words: axial bearing capacity, artificial neural network, high strain dynamic testing, pile shaft resistance, pile tip resistance. palabras clave: capacidad axial de soporte; red neuronal artificial; dinámicas de alto esfuerzo de pilotes; resistencia de punta. record manuscript received: 11/07/2013 accepted for publication: 26/10/2014 geotechnics axial bearing capacity (abc) of piles is usually determined by static load test (slt). however, conducting slt is costly and time-consuming. high strain dynamic pile testing (hsdpt) which is provided by pile driving analyzer (pda) is a more recent approach for predicting the abc of piles. in comparison to slt, pda test is quick and economical. implementing feed forward back-propagation artificial neural network (ann) for solving geotechnical problems has recently gained attention mainly due to its ability in finding complex nonlinear relationships among different parameters. in this study, an ann-based predictive model for estimating abc of piles and its distribution is proposed. for network construction purpose, 36 pda tests were performed on various concrete piles in different project sites. the pda results, pile geometrical characteristics as well as soil investigation data were used for training the ann models. findings indicate the feasibility of ann in predicting ultimate, shaft and tip bearing resistances of piles. the coefficients of determination, r², equal to 0.941, 0.936, and 0.951 for testing data reveal that the shaft, tip and ultimate bearing capacities of piles predicted by ann-based model are in close agreement with those of hsdpt. by using sensitivity analysis, it was found that the length and area of the piles are dominant factors in the proposed predictive model. la capacidad axial de soporte (abc, en inglés) de un pilote de construcción se determina usualmente a través de una prueba de carga estática (slt, inglés). sin embargo, estas pruebas son costosas y demandan tiempo. la evaluación de las dinámicas de alto esfuerzo de pilotes (hsdpt, inglés), que la provee el programa de análisis de excavación (pda, inglés), es una forma de aproximación más reciente para preveer la capacidad axial de soporte. en comparación con la prueba de cargas estática, la evaluación pda es rápida y económica. la implementación de redes neuronales arficiales (ann, en inglés) que permita resolver problemas geotécnicos ha ganado atención recientemente debido a su posibilidad de hallar relaciones no lineales entre los diferentes parámetros. en este estudio se propone un modelo predictivo ann para estimar la capacidad axial de soporte de pilotes y su distribución. para fines de una red de construcción se realizaron 36 pruebas pda en pilotes de diferentes proyectos. los resultados de los análisis de excavación, las características geométricas de los pilotes, al igual que los datos de investigación del suelo se utilizaron para probar los modelos ann. los resultados indican la viabilidad del modelo ann en predecir la resistencia de los pilotes. los coeficientes de correlación, r², que alcanzaron 0.941, 09.36 y 0.951 para la evaluación de los datos, revelan que la capacidad del pilotaje en el último rodamiento, en el cojinete del eje y en la punta que se predijeron con el modelo ann concuerda con las establecidas a través del hsdpt. a través del análisis de respuesta se determinó que la longitud y el área de los pilotes son factores dominantes en el modelo predictivo propuesto. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 85 93 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.38712 86 ehsan momeni, ramli nazir, danial jahed armaghani, harnedi maizir 1. introduction pile foundations are used extensively to transfer structural loads deep enough into the ground. proper estimation of axial bearing capacity is of prime importance in designing geotechnical structures. the ultimate amount of the load, which can be carried by the pile shaft, determines the type of pile as piles are classified according to their load-transfer mechanism (friction piles and end-bearing piles). hence, in addition to determining ultimate bearing capacity of piles, obtaining the pile shaft capacity is also of advantage (nazir et al, 2013). there are numerous methods for assessing pile bearing capacity and its distribution. although many attempts have been made to develop analytical or empirical methods for pile bearing capacity estimation (e.g. meyerhof, 1976; vesic, 1977; coyle and castello, 1981), most of these methods rely on empiricism and they are site specific (randolph, 2003). the most direct way for determining the axial bearing capacity of piles is static load test (slt). the test is standardized by american standards test methods (astm d114307). however, conducting slt is time consuming, expensive and difficult (likins and rausche, 2004). high strain dynamic testing (hsdt) of piles is a current approach for predicting the abc of piles and its distribution. hsdt is based on one dimensional wave propagation theory and is performed by using a pile driving analyzer (pda). in essence, in hsdt, a pile is hit by a hammer while pda monitors and records the necessary data for implementing wave equation analysis. consequently, through an iterative procedure using capwap software, the pile bearing capacity and its distribution can be predicted. the hsdt procedure is standardized by astm (astm d4945-08). utilization of artificial neural network (ann) in civil engineering has recently drawn considerable attention. it is generally attributed to the ann power in finding complex relationship between different parameters when the contact nature between them is unknown (garret, 1994). although many researchers have attempted to show the superiority of ann in predicting the bearing capacity problems, most of them focused on the prediction of ultimate bearing capacity of piles rather than its separate shaft and tip resistances (goh, 1995; goh, 1996). the main objective of this study is to propose an ann-based predictive model of bearing capacity using real pda and site investigation data. the predictive model is built for predicting shaft, tip and ultimate resistances (qs, qp and qu) of piles. nevertheless, it is worth mentioning that this study uses the capwap predicted pile bearing capacity rather than the determined bearing capacity of piles through slt. there are several published works concerning utilization of artificial intelligence techniques for predicting abc of piles (chan et al. 1995; chow et al. 1995; abu-kiefa, 1998; shahin, 2001; lok and che, 2004; das and basudhar, 2006; ardalan et al. 2009; shahin, 2008; shahin, 2010; adarsh et al. 2012; alkroosh and nikraz, 2012). among researchers who have addressed anns for predicting bearing capacity of pile foundations, goh (1995; 1996) developed an ann-based predictive model to estimate the ultimate load capacity of driven piles in sandy soils. his findings suggest that compared to conventional methods of pile bearing capacity estimation, ann-based predictive model works better. in another study, lee and lee (1995) employed ann for estimation of pile bearing capacity. their study focused on small scale laboratory tests where the horizontal and vertical chamber pressure, the number of hammer blows, pile penetration depth ratio, and mean normal stress of the soil were set as inputs of the network model while the ultimate bearing capacity was selected as the model output. according to their conclusion, ann can provide good prediction performance in bearing capacity problems. teh et al. (1997) also addressed the workability of neural network for predicting the pile bearing capacity. abu-kiefa (1998) implemented ann to predict the abc of driven piles in cohesionless soils. for network construction purpose, he compiled the data of 59 recorded cases of good-quality pile load tests. in his study, friction angles of the soil, the effective overburden pressure around the tip of the pile, the length of the pile and its equivalent cross-sectional area were considered as input layers of the ann model. his conclusion showed the feasibility of ann for predicting shaft and tip resistances of piles. however, among more recent studies, pal and deswal (2008) studied the ann application in predicting the total capacity of concrete spun pipe piles. they used stress-wave data for building their annbased predictive model. based on their conclusion, in comparison to support vector machines, the prediction performance provided by ann was more reliable. shahin and juksa (2009) proposed an ann-based predictive model of bearing capacity for drilled shafts. their model dataset comprised cone penetration test results and drilled shaft load tests on 94 recorded cases. jianbin et al. (2010) developed an ann model for predicting the ultimate abc of pipe piles. the influential parameters that they have considered for network construction included the effective length and diameter of pile, unit weight, cohesion and internal friction angle of soil as well as the standard penetration test (spt) results. benali and nechnech (2011) suggested an ann-based predictive model of pile bearing capacity in cohesionless soils. for training the ann, they had collected the mechanical properties of purely coherent soil and geometrical characteristics of 80 axially loaded piles. the correlation coefficient, r, equals to 0.92 reveals the reliability of their ann-based predictive model. 2. methods 2.1 high strain dynamic testing of piles it was mentioned earlier that high strain dynamic testing (hsdt) is an innovative method for estimating the abc of piles. in dynamic testing of pile (pda test), it is hypothesized that a pile with uniform cross section is a slender element surrounded by material with much lower stiffness. hence, any mechanical impact through a hammer leads to downward propagation of wave (timoshenko and goodier, 1951; salgado, 2008). therefore, the principle of one dimension wave propagation theory can be implemented for piles. details of the solution to the partial differential equations of wave propagation theory for predicting axial bearing capacity of piles can be found elsewhere (salgado, 2008). nevertheless, the finite difference-based model introduced by smith (1960) was considered as the bench mark for dynamic testing of piles. to estimate the axial bearing capacity, smith developed a discrete solution for wave propagation in piles. in smith’s model, the pile was simulated using a number of masses attached to each other by elastic springs, while the soil was modelled by a number of springs, and linear viscous dampers to represent its behavior. however, there were some deficiencies in smith`s model due to lack of knowledge on hammer energy and cushion characteristics. smith`s model later on was enhanced by a group of researchers at case western reserve university (goble et al.1970; rausche et al. 1972; rausche et al. 1985). in their developed method also known as case method, it was not necessary to model the hammer and driving systems. instead, they reported the use of force and acceleration records in a simplified model for predicting the static bearing capacity. as stated by fellenius, (1999) the full power of the wave equation analysis was first realized when it was coupled with dynamic monitoring of piles. the latter can be determined by installing a pair of accelerometer and strain transducer on top of the pile (see figure 1). subsequently, the data recorded by aforementioned instruments are transmitted by means of a cable to pda. the pda will then transform the recorded data into force and velocity. in the next step, the bearing capacity of the pile is predicted using capwap program. capwap combines the measured force and velocity with the wave equation analysis to determine soil resistance and its distribution along the pile. the capwap approach is based on an iterative curve-fitting technique in which pile response, estimated through wave equation analysis of a model pile, is matched to the measured response of the actual pile for a single hammer blow (fhwa, 2006). 87application of artificial neural network for predicting shaft and tip resistances of concrete piles figure 1. example of installed accelerometer (right) and strain transducer (left) on the pile. it should be mentioned that in performing pda tests, to obtain more reliable abc, some criteria such as hammer weight and impact condition must be considered. for full mobilization of soil strength along pile shaft, susilo (2006) suggested minimum hammer weight equals to 1% of the required ultimate pile capacity whereas for piles with larger expected end bearing contributions, the recommended percentage is increased to at least 2% of the ultimate pile capacity. 2.2 artificial neural network artificial neural network is a flexible non-linear function approximation tool that estimates a relationship between given input and output parameters. simpson (1990) reported that a specific ann can be defined using three important components: transfer function, network architecture and learning law. more details on ann structure are addressed elsewhere (e.g. hechtnielsen, 1990; maren, et al. 1990; zurada, 1992; fausett, 1994; ripley 1996). however, study by haykin (1999) recommends that the most wellknown type of feed-forward anns is multi-layer perceptron (mlp). in feedforward anns, the neurons are usually grouped into layers. using neuron connections, signals move from input layer through the hidden layer(s) to output layer. in essence, anns are composed of a set of parallel layers and several interconnected nodes or neurons. there is also a transfer or activation function along each node which transmits signals to either other nodes or output of the network. the activation function in each node is applied to the net input of that node. the net input of the node is obtained by summation of connection weights as well as a threshold value known as bias. among different algorithms for training anns, back-propagation (bp) algorithm is recognized as the most common training algorithm (dreyfus, 2005). basically, bp algorithm consists of two passes; a forward pass and a backward pass. in the former, using transfer function, the outputs are calculated and the errors at the actual output unit are determined (demuth et al. 2007). if the obtained error (mean squared difference between the actual and predicted outputs) is more than adequate, then the error is propagated back through the network and updates the individual weights. this procedure is called backward pass. forward and backward passes are repeated several times until the error is converged to a level specified by a cost function such as mean square error (mse) or root mean square error (simpson 1990, kosko 1994, singh et al. 2004). 3. dataset using the procedure suggested by astm (d4945-08), 36 pda tests were conducted at various project sites in indonesia. the tested piles were reinforced and pre-stressed concrete piles with different diameters and lengths. most of the tests were conducted in cohesionless soils. an example of pda test performed in one of the project sites is shown in figure 2. to represent soil characteristics, the results of spt were collected. the average spt (n) values along the pile shaft and tip were calculated. it is worth mentioning that for obtaining the average spt (n) value around the pile tip, the meyerhof’s recommendation (1976) was considered. the average spt (n) value for 10d above and 4d below the pile tip was obtained where d represents pile diameter. table 1 lists the pda test results including ultimate bearing capacity of piles, pile set, pile shaft and tip resistances. in addition to pda results, pile geometrical characteristic and the average spt (n) values around the pile shaft and tip are also tabulated in table 1. figure 2. pda test 4. model development the prediction performance of anns is closely related to the architecture of the selected network. therefore, defining the optimum network architecture is crucial in designing ann models. hornik et al. (1989) mentioned that a network with one hidden layer can approximate any continuous function in ann. lawrence (1994) reported that in designing ann architectures, increasing the number of the hidden layers should be the last options; instead focus should be on adding the number of hidden nodes. nevertheless, for network construction, the optimum number of hidden nodes should be determined. several researchers suggested that numbers of hidden nodes are related to the number of input and output parameters. in table 2 some of the formulas suggested by a number of researchers for obtaining the optimum number of hidden nodes are presented. however, the network performance is often evaluated based on the root mean squared error (rmse) as well as regression values. that is to say, using the trial-and-error method, several network architectures are trained with same input and output data and the network that performs best is selected as the optimum network. 88 ehsan momeni, ramli nazir, danial jahed armaghani, harnedi maizir table 1. dataset used for ann-based predictive model. no. pile name pile length (m) pile set (mm) pile area (cm2) nshaft ntip qu (kn) qs (kn) qp (kn) 1 kn 189 25 16 1159.25 9 30 2988 1665 1323 2 kn 204 24.7 23 1159.25 9 30 3940 1445 2495 3 kn 215 19 19 1159.25 3 10 3200 1966 1234 4 p 105 bp6 22.7 3 11309.73 7 37 6753 3649 3103 5 fe p3 bp5 23 4 11309.73 4 21 8501 6035 2466 6 fe p6 bp6 23 3 11309.73 5 21 5322 3529 1794 7 p 112 bp 12 21 3 11309.73 5 15 6089 4293 1796 8 p 108 bp 11 23 5 11309.73 3 12 8500 5903 2597 9 p 109 bp 16 23 4 11309.73 3 12 10930 6412 4518 10 p 105 b2 23 7 11309.73 4 13 11601 7211 4390 11 p 105 b6 22.7 4 11309.73 4 13 8755 5535 3220 12 p 4 a 30 7 1159.25 20 27 1347 1197 150 13 p 18 f 30 7 1159.25 20 27 950 800 150 14 p 33 c 30 6 1159.25 20 27 1176 980 196 15 p 01 34.9 8 876.5 6 10 829 789 40 16 p 02 _ 1 34.9 8 876.5 6 10 605 591 14 17 p 04 34.8 13 552.92 6 10 772 744 28 18 p 05 34.9 11 552.92 6 10 781 774 7 19 p 1 as6bc.187 12.3 16 625 13 11 864 764 100 20 p 02 as21ed.349 10.6 8 625 15 13 689 637 52 21 p 03 as19ed.355 12.8 13 625 14 12 1520 1386 134 22 p01 10.3 9 735.13 18 26 500 485 15 23 p02 8.8 11 735.13 19 27 452 438 14 24 p03 10 6 735.13 18 25 584 556 29 25 p04 101 10 7 735.13 18 25 603 576 27 26 p06-112 10.3 6 735.13 18 26 811 768 44 27 p07-122 10 9 735.13 18 25 770 733 36 28 p08-46 10 6 735.13 18 25 748 700 48 29 p1-p330_3 sp 350 5.3 18 678.58 13 15 788 696 92 30 p2-p169 sp 350 5.3 15 678.58 13 15 794 700 94 31 p3-p144 sp 350 5.2 15 678.58 13 15 824 721 103 32 p1-e7 squar 25 19.6 15 625 11 22 1604 1437 167 33 p2-h 5_4 squar 25 19.7 12 625 11 22 1419 1269 150 34 p3-h 12 squar 25 19.1 12 625 9 19 1920 1720 200 35 b2 _1 5 16 625 8 11 764 670 94 36 d4_3 7.4 17 625 13 19 650 509 141 89application of artificial neural network for predicting shaft and tip resistances of concrete piles to determine the optimal network architecture of the ann model which is designed for predicting qs, qp and qu of piles respectively, using a matlab code created stochastically between -1 and 1, nine networks made of different hidden nodes in the range of 2 to 10 (based on the recommendations presented in table 2) were trained and tested. it should be mentioned that each model was iterated 5 times. for training the networks, levenberg-marquardt (lm) algorithm was used. several studies reported that lm converges while other gradient descendent training algorithms diverge (e.g. hagan and menhaj, 1995). details of this algorithm reported elsewhere (martin et al. 1995). it is also worth mentioning that in developing ann models, the sigmoid function was used as transfer function. assessments of the networks performance were made based on the obtained coefficient of determination, r² as well as rmse. the former indicates the reliability and strength of the correlation between actual and predicted outputs. table 3 lists the obtained r² and rmse for training and testing datasets. as shown in this table, the fourth model which comprises 5 hidden nodes in one hidden layer performs best. the obtained r2 and rmse values for the selected model are 0.998, 0.942, 0.012 and 0.091 for training and testing datasets respectively. the architecture of the selected model is shown in figure 3. it is worth noting that in designing the ann models, 80 percent ( 29 out of 36) of the datasets were assigned randomly for training purpose, and the last 20 percent was used for testing the performance of the model. table 3. ann model performances. model no. nodes in hidden layers network result iteration 1 iteration 2 iteration 3 iteration 4 iteration 5 train test train test train test train test train test r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse r2 rmse 1 2 0.974 0.078 0.761 0.136 0.978 0.064 0.910 0.515 0.902 0.096 0.674 0.392 0.988 0.089 0.879 0.153 0.961 0.081 0.853 0.250 2 3 0.980 0.065 0.674 0.288 0.978 0.166 0.783 0.226 0.990 0.065 0.832 0.152 0.982 0.104 0.842 0.149 0.976 0.133 0.891 0.155 3 4 0.994 0.071 0.783 0.178 0.994 0.043 0.906 0.135 0.990 0.058 0.784 0.149 0.992 0.037 0.637 0.217 0.968 0.036 0.861 0.194 4 5 0.998 0.016 0.896 0.155 0.998 0.040 0.904 0.107 0.998 0.012 0.942 0.091 0.998 0.019 0.774 0.177 0.998 0.095 0.758 0.157 5 6 0.998 0.017 0.763 0.156 0.998 0.032 0.776 0.252 0.998 0.021 0.794 0.237 0.998 0.024 0.605 0.277 0.998 0.033 0.777 0.117 6 7 0.998 0.022 0.721 0.167 0.998 0.028 0.740 0.187 0.998 0.043 0.629 0.209 0.998 0.031 0.921 0.125 0.998 0.067 0.790 0.168 7 8 0.998 0.049 0.850 0.150 0.998 0.116 0.885 0.139 0.998 0.044 0.589 0.231 0.998 0.033 0.784 0.189 0.998 0.020 0.785 0.211 8 9 0.998 0.051 0.851 0.185 0.998 0.042 0.571 0.262 0.998 0.039 0.752 0.201 0.998 0.098 0.870 0.262 0.998 0.072 0.767 0.291 9 10 0.998 0.038 0.706 0.321 0.998 0.055 0.794 0.202 0.998 0.088 0.887 0.282 0.998 0.075 0.715 0.371 0.998 0.061 0.727 0.299 figure 3. architecture of the selected ann-based predictive model. 5. result and discussion the reliability of the ann-based predictive model of bearing capacity can be seen in figures 4 to 6. these figures show the predicted qs, qp and qu of piles versus their measured values. figure 4 shows a comparison between predicted and measured qs for training and testing data. the obtained r² values equal to 0.999 and 0.941 suggest the reliability of the model in predicting qs. similarly figure 5 suggests that the predicted qp is in good agreement with the measured qp. as displayed in figure 5-b, the coefficient of determination equals to 0.936 for testing data recommends the feasibility of the ann-based predictive model of bearing capacity. table 2. the proposed number of neurons for hidden layer ni: number of input neuron, n0: number of output neuron. 90 ehsan momeni, ramli nazir, danial jahed armaghani, harnedi maizir figure 4. shaft bearing capacity (qs) of piles predicted by ann model versus their measured values. figure 5. end-bearing capacity (qp) of piles predicted by ann model versus their measured values. in figure 6, a comparison is made between the measured and predicted qu of piles for both training and testing data. coefficient of determination equals to 0.951 for testing data suggests that the ann-based predictive model is good enough in capturing the ultimate bearing capacity of piles. figure 6. ultimate bearing capacity (qu) of piles predicted by ann model versus their measured values 91application of artificial neural network for predicting shaft and tip resistances of concrete piles to have a better understanding of the prediction performance of ann, in figures 7 to 9 the predicted qs, qp, and qu are checked against their measured values for testing dataset. in fact, the idea behind using testing dataset is to verify the generalization capability of proposed neural network model. close agreement between measured and predicted values suggests that by using pile geometrical characteristics, pile set and the results of spt insitu test, the ann-based predictive model can be implemented for estimating qs, qp, and qu of piles. figure 7. ann-model performance in predicting pile shaft resistance (testing data). figure 8. ann-model performance in predicting pile tip resistance (testing data). figure 9. ann-model performance in predicting ultimate bearing capacity of piles (testing data). the overall prediction performance (for all dataset) of the annbased model is summarized in table 4. in this table, apart from r², rmse and value account factor (vaf) were also used to control the capacity performance of the model. for determining rmse and vaf, equations 1 and 2 were used respectively. rmse = vaf = (1) (2) in the aforementioned equations, y and y′ denote the obtained and estimated values, respectively and n is the total number of data. it is worth mentioning that the model is excellent if the rmse is zero and vaf is 100. table 4. performance indices of the predictive model output r2 rmse vaf (%) qp 0.982 0.081 98.199 qs 0.984 0.075 98.361 qu 0.988 0.064 98.770 overall, the general trend of the results shows that ann as a method which does not require prior assumptions and can provide a relatively reliable solution for assessing the pile bearing capacity and its distribution. although direct determination of pile bearing capacity through slt is still recommended due to the amount of uncertainties in other semi empirical methods (momeni et al, 2013), the use of proposed ann-based predictive model is of advantage as it can reduce the required number of pda tests in each project. 6. sensitivity analysis sensitivity analysis was performed to recognize the importance of each input variable on the axial bearing capacity of piles. for this reason, the strength of the relations between the output parameters and the input parameters was evaluated using cosine amplitude method (cam). this use of this sensitivity analysis is reported in several studies (yang and zhang, 1997; jong and lee, 2004). to utilize cam, all data pairs were expressed in common u-space. the data pairs used to construct a data array u is defined as: u= the elements ui in the array u is a vector of lengths m that is: (3) (4) therefore, each data pair can be considered as a point in m-dimensional space, where each point requires m-coordinates for a full description. the strength of relation between data pairs, ui and uj, is represented by the following equation: (5) the strength of the relation (rij value) indicates the influence of different input parameters on one of the output parameters. the larger the value of rij becomes, the higher is the effect on the output. for example, if the output has no relation with the input, then the rij value is zero, while the value of rij closer to 1 expresses the further influence of the input parameter. nevertheless, the obtained strength of relations of the problem in hand is shown in figure 10. this figure suggests that the most influential parameters on qs, qp and qu are pile area and pile length. 92 ehsan momeni, ramli nazir, danial jahed armaghani, harnedi maizir figure 10. strengths of relation (rij) between qs and qp and input parameter 8. summary and conclusion to develop an ann-based predictive model for estimating the axial bearing capacity of piles, 36 pda tests were performed on different concrete piles with various diameters and lengths. the tests were mostly performed in cohesionless soils. for network construction purpose, the pda results, pile length and cross sectional area, and the average spt (n) values along the pile shaft and tip were used as inputs while the pile bearing capacity and its distribution were set as outputs. through a trial-and-error procedure, it was found that a network with five hidden nodes in one hidden layer yields the best performance. the coefficients of determination equal to 0.941, 0.936, and 0.951 for testing data revealed the reliability of the proposed ann model in predicting the shaft, tip and ultimate bearing capacities of piles, 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(1992). introduction to artificial neural systems, west publishing company, st. paul. articulo 3.indd earth sciences research journal hydrogeology earth sci. res. sj. vol. 15, no. 2 (december, 2011): 101 108 hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india k. brindha and l. elango department of geology, anna university, chennai600 025, india. corresponding author. e-mail: elango34@hotmail.com, elango@annauniv.edu abstract hydrochemical study was carried out in madhuranthakam located near chennai in tamil nadu, india with an objective of understanding the suitability of local groundwater quality for domestic and irrigation purposes. twenty groundwater samples were collected in february 2002 and analysed for physical and chemical parameters. groundwater in this area was found to be within the desirable bureau of indian standards and world health organisation limits for drinking water. ca-hco3 was the dominant groundwater type. groundwater in this area was assessed for irrigation purposes on the basis of sodium percentage (na%), magnesium hazard (mh), residual sodium carbonate (rsc), sodium absorption ratio (sar), permeability index (pi) and united states department of agriculture (usda) classification. most of the groundwater samples were suitable for irrigation, except in a few locations (15%) based on mh. overall the groundwater quality was suitable for drinking and domestic purposes and permissible for irrigation activities. resumen un estudio hidroquímico fue llevado en la localidad madhuranthakam cerca chennai en tamil nadu, india, con el propósito de evaluar la calidad de las aguas subterráneas locales para usos domésticos y fines de riego. veinte muestras de agua subterránea fueron recogidas y analizadas en términos de parámetros físicos y químicos en febrero 2002. las aguas subterráneas en esta área fueron encontradas aptas como potables según los limites sugeridos por el consejo de estándares de india y los límites permisibles por la organización mundial de la salud para el agua potable. ca-hco3 es dominante en las aguas subterráneas. las aguas subterráneas en esta área fueron asignadas para propósitos de riego sobre la base del porcentaje de sodio (na%), magnesio peligroso (mh), carbonato sodico residual (rsc), proporción de absorción de sodio (sar), índice permeabilidad (pi) y la clasificación del departamento de agricultura de los estados unidos (usda). la mayoría de las muestras de las aguas subterráneas son aptas para el riego, excepto en unas pocas ubicaciones (15%), basadas sobre mh. en general, la calidad del agua subterránea fue apta para el consumo, uso doméstico y permisible para las actividades de riego. palabras claves: agua subterraneas, madhuranthakam, calidad de agua potable, calidad de agua de riego, tamil nadu, india. keywords: groundwater, madhuranthakam, drinking water quality, irrigation water quality, tamil nadu, india. record manuscript received: 10/03/2011 accepted for publications: 24/11/2011 introduction groundwater is a precious source of fresh water, being the most distributed form on the earth, excluding the polar icecaps and glaciers. groundwater studies are gaining more importance in the present day as it is used for almost all purposes such as domestic, industrial and agricultural activities in most parts of the world. improper management of this replenishable resource may lead to groundwater contamination and scarcity. when some ions and minerals are present beyond the permissible limit, they become unsuitable for drinking and irrigation purposes, which may be due to natural and also anthropogenic causes. groundwater quality has been given lot of importance and studied worldwide (lahermo and backman, 1999; omo-irabor et al., 2008; baalousha, 2010). several regions in india have encountered degradation in groundwater quality too, due to rapid urbanisation and an exponential increase in population (ramesh and elango, 2005; brindha and elango, 2010; brindha et al., 2011). the present study was carried out with the aim of understanding groundwater quality and its suitability for drinking and irrigation purposes in madhuranthakam, located near chennai in tamil nadu, india. groundwater is the major source for domestic and irrigation practices in this area. there has been an increase in the demand for groundwater due to the growth of the local population. groundwater is usually put to direct use in rural areas without proper monitoring and treatment. groundwater may also become contaminated by the agrochemical products used for irrigation. the groundwater quality in the nearby regions, namely chennai, kancheepuram and chengalpet, k. brindha and l. elango102 has been studied earlier (elango and manickam, 1987; ramesh, 1999; rajmohan et al., 2000; elango et al., 2003a; elango et al., 2003b; kumaresan and riyazuddin, 2006). however, no studies have been carried out in the madhuranthakam region pertaining to groundwater quality. the suitability of groundwater for domestic and irrigation purposes thus had to be determined based on the presence of major ions in the groundwater of this region. the present study, which was carried out in 2002, will serve as baseline data for comparing future groundwater quality. study area the study area is situated 76 km south of chennai city and forms a part of madhuranthakam taluk, kancheepuram district, tamil nadu, india (figure 1). most of the annual rainfall of 1,206 mm is received from october to december and the rest during the southwest monsoon season from june to september. climatic condition in the study area varies from 39oc to 40oc during summer (april to june) and 20oc to 24oc during winter (october to december). the study area is surrounded by a number of tanks which are used for drinking and agricultural purposes. this area is intensively cultivated by pumping groundwater from dug wells as well as surface water resources. geologically this area is characterised by charnockites of precambrian era. outcrops of these rocks are found in many parts of the study area. sandy clay and clayey sand overlay the charnockites with thickness ranging from 1.5 to 5m. groundwater occurs under unconfined condition mainly in the weathered and fractured part of this charnockites in this area. figure 1. location of study area and sampling wells. parameter unit minimum maximum mean ph 7.2 8.2 7.69 ec µs/cm 200 1900 807.6 calcium mg/l 11.84 93.9 52.21 magnesium mg/l 3.65 132.46 24.37 sodium mg/l 27.59 82.76 50.35 potassium mg/l 0 2.74 1.17 carbonate mg/l 6 54.01 28.5 bicarbonate mg/l 30.51 256.27 136.9 chloride mg/l 20.35 32.41 21.91 sulphate mg/l 30.48 44.89 35.64 table 1. statistical summary of physicochemical parameters. hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 103 sampling and analysis a well inventory survey was carried out in february 2002 to obtain background information about the well population, lithology, well use etc. forty-seven wells were investigated and 20 wells (figure 1) were chosen as representative wells for groundwater sampling based on electrical conductivity (ec). the groundwater samples were collected in clean 500 ml polyethylene bottles. the sampling bottles were soaked in 1:1 diluted hcl solution for 24 hours and washed twice with distilled water before sampling. they were washed again in the field with groundwater sample filtrates. the groundwater samples were collected from bore wells after pumping the water for about 10 minutes. field parameters such as ec and ph were measured in the field using portable digital meters. samples were analysed for major ions in the laboratory using the standard recommended methods (apha, 1998). sulphate concentration in the groundwater samples was analysed using a uv visible spectrophotometer. sodium and potassium content was determined by using a flame photometer and calcium, magnesium, chloride, carbonate and bicarbonate by titration technique. total ions measurement precision was checked by calculating the ion balance error (ibe). the ibe was within ±10%. total dissolved solids (tds) were calculated by using the formula: tds (mg/l) = ec (µs/cm) x 0.64; total hardness (th) was calculated by using: th = 2.497ca + 4.115mg in mg/l. results and discussion table 1 shows the maximum, minimum and mean concentrations of various parameters. the order of cation dominance was ca2+>na+>mg2+>k+ and hco3 ->so4 2->co3 2->clfor anions. the general chemical nature of groundwater can be understood by plotting major cation and anion concentrations on a piper trilinear diagram (piper, 1944). ca-hco3 was the major water type dominant in this area (figure 2). the next dominant water type was mixed ca-na-hco3. drinking water quality groundwater used for domestic purposes, such as drinking and cooking, should be free from toxic chemicals and pathogens. domestic water quality indicates that a particular parameter at a given concentration may be suitable for the human body beyond which it is unsuitable. the concentration of various ions in the groundwater samples was compared with bureau of indian standards (bis, 2003) and world health organisation (who, 1993) standards, which are given in table 2 wherein all the groundwater samples were found to be within the suitable limits. the groundwater in this area was thus seen to be fit for domestic consumption, based on the major ion analysis carried out in this study. the groundwater samples were classified regarding tds and th (tables 3, 4 and 5). most of the groundwater was fresh and suitable for drinking purposes based on tds (freeze and cherry, 1979; davis and deweist, 1966). groundwater in this study area varies from soft to very hard (sawyer and mccarty, 1967). irrigation water quality good quality irrigation water is essential for achieving maximum crop productivity. groundwater suitability for irrigation purpose in this study area was assessed using sodium percentage (na%), magnesium hazard (mh), residual sodium carbonate (rsc), sodium absorption ratio (sar), permeability index (pi) and united states department of agriculture (usda) classification. groundwater in most of the study area was found to be suitable for irrigation. irrigation water having high ec content will affect root area and water flow. groundwater in this area was grouped according to the guidelines figure 2. groundwater type. k. brindha and l. elango104 parameter bis (2003) who(1993) percentage of samples above maximum allowable limit highest desirable limit maximum permissible limit highest desirable limit maximum permissible limit ph 6.5– 8.5 6.5 – 9.2 6.5 – 8.5 6.5 – 9.2 nil th (mg/l) 300 600 100 w500 nil calcium (mg/l) 75 200 75 200 nil magnesium (mg/l) 30 100 50 150 nil chloride (mg/l) 250 1000 200 600 nil sulphate (mg/l) 150 400 200 400 nil sodium (mg/l) 200 nil table 2. comparison of groundwater samples with bis and who standards. table 3. freeze and cherry (1979) classification of groundwater based on tds (mg/l). table 4. david and deweist (1966) classification of groundwater based on tds (mg/l). table 5. sawyer and mccarty (1967) classification of groundwater based on th (mg/l). table 6. suitability for irrigation based on usda classification. tds (mg/l) water type number of samples percentage <1,000 fresh 19 95 1,000 10,000 brackish 1 5 10,000 1,00,000 saline nil nil >1,00,000 brine nil nil tds (mg/l) classification number of samples percentage <500 desirable for drinking 11 55 500 1,000 permissible for drinking 8 40 1,000 3,000 useful for irrigation 1 5 >3,000 unfit for drinking and irrigation nil nil total hardness (mg/l) type of water number of samples percentage <75 soft 2 10 75 150 moderately high 7 35 150 300 hard 7 35 >300 very hard 4 20 ec (µs/cm) salinity class percentage of samples remark on quality <250 c1 5 excellent or low 250-750 c2 45 good or medium 750-2250 c3 50 permissible or high 2250-5000 c4 nil unsuitable or very high hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 105 established by the united states salinity laboratory (freeze and cherry, 1979) based on ec (table 6). this showed that groundwater in this area was permissible for irrigation. sodium is an important parameter for irrigation water and is denoted as na% which was calculated from the formula given in equation 1 (wilcox, 1955) and all concentrations were expressed in meq/l. the na% in this area is given in table 7. groundwater was suitable for irrigation in 60% of the samples while 40% were permissible to doubtful. ec and na% are plotted in figure 3 which showed that most of the groundwater samples were good for agriculture. the concentration of bicarbonate and carbonate higher than calcium and magnesium will influence the suitability of water for irrigation purposes. the rsc value was computed using the following formula (equation 2) where ions were expressed in meq/l. rsc = (co3 2+ hco3 -) – (ca2+ + mg2+) however, regarding rsc, all samples fall within the safe category for irrigation. sar is another important parameter for determining the desirability of irrigation water. the sar values were calculated by using equation 3 (richards, 1954), where all the concentrations were given in meq/l. all the groundwater samples collected from this area were excellent on the basis of sar. water used for irrigation can be classified into four typesc1, c2, c3 and c4 based on salinity hazard and s1, s2, s3 and s4 based on sodium hazard. figure 4 shows the plot of groundwater samples grouped on the above basis. most groundwater samples fall under c1s1 and c2s1 which are suitable for irrigation. comparatively few samples were grouped under c3s1 (which is permissible for irrigation). thus the groundwater in this area was seen to be suitable for irrigation based on salinity hazard and sodium hazard. mh for irrigating water was calculated using the formula mh = mg/ (ca + mg) x 100, where concentrations were given in meq/l (szabolcs and darab, 1964). mh above 50 is considered to be unsuitable for irrigation. groundwater had an mh above 50 in 15% of the samples, thus not being fit for irrigation. table 7. suitability for irrigation based on sodium percent. % na suitability for irrigation number of samples percentage <20 excellent 5 25 20-40 good 7 35 40-60 permissible 6 30 60-80 doubtful 2 10 >80 unsuitable nil nil (1) (2) (3) )( 100)(% 22 ++++ ++ +++ + = knamgca xknana 2 22 ++ + + = mgca nasar figure 3. suitability of irrigation water, based on ec and sodium percent. k. brindha and l. elango106 the suitability of groundwater for irrigation based on pi was calculated using equation 4, where concentrations were in meq/l. class i and class ii waters are considered to be good and suitable for irrigation while class iii water is unsuitable for irrigation (doneen, 1964). figure 5 shows that one groundwater sample was not suitable for irrigation based on pi whereas the rest of the samples were good. in general, the groundwater fall within the permissible category for irrigational use, except for a few locations where it was unsuitable based on mh. conclusion groundwater quality of an area must be studied to understand its suitability for domestic and irrigation purposes. all the groundwater samples collected from the madhuranthakam area, tamil nadu, india, showed that the major ions fall within the permissible range. the dominant groundwater type was ca-hco3. based on tds, 95% of the groundwater was fresh and permissible for drinking. the groundwater varied from soft to very hard on the basis of th. the suitability of groundwater for irrigation was assessed from na%, ec, rsc, sar, mh, pi and usda classification. the groundwater in this area was seen to be good and suitable for drinking and domestic purposes. however, the groundwater was unsuitfigure 4. irrigation water salinity and alkalinity hazard. figure 5. classification of irrigation water based on permeability index. )( 1003 +++ + ++ + = namgca xhcona pi (4) hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india 107 able for irrigation in a few places, based on mh. overall, groundwater in the madhuranthakam area remains usable. the present groundwater quality status must be maintained by taking precautionary measures such as rainwater harvesting, less use of chemical fertilisers and the ongoing monitoring of groundwater quality in this region. acknowledgements the authors would like to thank the department of science and technology-funds for improvement in science and technology (dst-fist) (grant no. sr/fst/esi-106/2010) and the university grants commission-special assistance programme (ugc-sap) (grant no. ugc drs ii f.550/10/drs/2007(sap-1)) for providing financial support which helped in creating facilities to 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(2005). groundwater quality assessment in tondiar basin, indian journal of environmental protection, 26, no. 6, 497 –504. ramesh, r. (1999). groundwater quality management: pollution perspectives impacts of urban growth on surface water and groundwater quality, proceedings of iugg 99 symposium hs5, birmingham, july, iahs publ. no. 259, 47-55. richards, l. a. (1954). diagnosis and improvement of saline and alkali soils, us department of agriculture, hand book 60. sawyer, g. n. and mccarty, d. l. (1967). chemistry of sanitary engineers, 2nd edn, mcgraw hill, new york, 518 pp. szabolcs, i. and darab, c. (1964). the influence of irrigation water of high sodium carbonate content of soils, proceedings of 8th isss, trans vol. ii , 802-812. who (1993). guidelines for drinking water quality, vol. 1, 2nd edn, recommendations, who, geneva, 130 pp. wilcox, l. v. (1955). classification and use of irrigation waters, usda, circular 969, washington dc, usa. orozco et al.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 57-65 dissimilarity-based classification of seismic signals at nevado del ruiz volcano mauricio orozco1,3, marcelo e. garcía 2, robert p.w. duin 3, and césar g. castellanos1 1 universidad nacional de colombia sede manizales, grupo de control y procesamiento digital de señales, campus la nubia, km 7 vía al magdalena, manizales, colombia. 2ingeominas observatorio vulcanológico y sismológico de manizales, avenida 12 de octubre no. 15-47, manizales, colombia. 3information and communication theory group, mekelweg 4, 2628 cd delft, delft university of technology, the netherlands. corresponding author: mauricio orozco, e-mail: morozcoa@unal.edu.co abstract automatic classification of seismic signals has been typically carried out on feature-based representations. recent research works have shown that constructing classifiers on dissimilarity representations is a more practical and, sometimes, a more accurate solution for some pattern recognition problems. in this paper, we consider bayesian classifiers constructed on dissimilarity representations. we show that such classifiers are a feasible and reliable alternative for automatic classification of seismic signals. our experiments were conducted on a dataset containing seismic signals recorded by two selected stations of the monitoring network at nevado del ruiz volcano. dissimilarity representations were constructed by calculating pairwise euclidean distances and a non-euclidean measure on the normalized spectra, which is based on the difference in area between spectral curves. results show that even though euclidean dissimilarities have advantageous properties, non-euclidean measures can be beneficial for matching spectra of seismic signals. key words: classification, dissimilarity, nevado del ruiz volcano, seismic signals. resumen la clasificación automática de señales sísmicas se ha llevado a cabo típicamente sobre representaciones de características. trabajos de investigación recientes han mostrado que construir clasificadores sobre representaciones de disimilitud es una solución más práctica y, algunas veces, más precisa para ciertos problemas de reconocimiento de patrones. en este artículo consideramos clasificadores bayesianos construidos sobre representaciones de disimilitud. mostramos que tales clasificadores son una alternativa viable y confiable para la clasificación automática de señales sísmicas. nuestros experimentos fueron llevados a cabo sobre 57 earth sciences research journal manuscript received july 5 2006. accepted for publication october 24 2006. una base de datos que contiene señales sísmicas detectadas por dos estaciones seleccionadas de la red de monitoreo del volcán nevado del ruiz. las representaciones de disimilitud fueron construidas mediante el cálculo de distancias euclidianas y de una medida no euclidiana sobre los espectros normalizados, ésta última está basada en la diferencia de área entre curvas espectrales. los resultados muestran que aunque las disimilitudes euclidianas tienen propiedades ventajosas, las medidas no euclidianas pueden resultar benéficas para comparar espectros de señales sísmicas. palabras claves: clasificación, disimilitud, volcán nevado del ruiz, señales sísmicas. introduction nevado del ruiz volcano (nrv) is capped by a large volume of snow and ice, forming a glacier which has a volume of about 1200~1500 million cubic meters. nrv has three craters: arenas —the currently active vent—, and two parasite craters: olleta and piraña. since the start of digital recording at the volcanological and seismological observatory in manizales (vsom), a large and increasing amount of data has been recorded by the monitoring networks. classification of seismic signals is a crucial issue in order to discover the interaction between volcanic earthquakes and volcanic processes. the database available from vsom is suitable for applying automatic classification/learning techniques. in this study, we consider three classes of seismic signals originating from nrv: volcanotectonic (vt) earthquakes, long-period (lp) earthquakes and icequakes (ic); of course, for every seismically monitored volcano, seismologists use their own classification with more detailed descriptions of every subtype of earthquakes (zobin, 2003). vsom staff currently classifies volcanic earthquakes by visual inspection; such a method imposes a great deal of workload for the seismic analysts. in consequence, an automatic classification tool dramatically reduces this arduous task and also makes the classification reliable and objective, removing errors associated to tedious evaluations and changing of personnel. among the applications of pattern recognition techniques to volcanic-seismic signals, two recent works are highlighted: automatic classification of seismic signals at mt. vesuvius volcano, italy (scarpetta et al., 2005) and automatic classification of seismic events at soufrière hills volcano, montserrat (langer et al., 2006). both of them propose the application of artificial neural networks (ann) to classify seismic events. the former work uses a multilayer perceptron (mlp) to distinguish between vt events and transient signals due to other sources such as underwater explosions, quarry blasts, and thunders; spectral features and amplitude parameters are used for characterization. in the latter work, an ann is used to classify five fundamental types of signals: vt events, regional (re) events, lp events, hybrid (hb) events and rockfalls (roc); autocorrelation functions, high order statistical moments and amplitude ratios are introduced as features to the input nodes; a mismatch rate of 30% is reported, which was reduced up to 20% after a manual revision of the original a-priori classification. typically, in the context of volcanic seismology, neural networks have been preferred rather than other classical statistical pattern recognition methods; they are still being used for discrimination of seismic signals, including modifications in the feature-based representation (e.g. the modified approach used in benbrahim et al., 2005). the popularity of neural networks models to solve pattern recognition problems has been primarily due to their seemingly low dependence on domain-specific knowledge and also because of the availability of efficient learning algorithms (jain et al., 2000). recently, a number of studies showed advantages of learning from dissimilarity representations as opposed to learning from feature-based representations (duin et al., 1998; pękalska et al., 2001; pękalska and duin, 2002; paclík and duin, 2003; pękalska and duin, 2005). a dissimilarity representation of objects, seismic events in our particular case, is based on pairwise comparisons and can be expressed as a dissimilarity matrix , where each entry corresponds to dissimilarity between pairs of objects. dissimilarity representations are more general dissimilarity-based classification of seismic signals at nevado del ruiz volcano 58 than feature-based representations; in fact, the notion of dissimilarity is more fundamental than that of a feature (pękalska and duin, 2005). for dissimilarities the geometry is contained in the definition, giving the possibility to include physical background knowledge; in contrast to feature-based representations which usually suppose a euclidean geometry. this paper is devoted to explore dissimilarity representations to classify volcanic-seismic signals. in dealing with this particular problem, we advocate the dissimilarity-based classification of seismic signals as an advantageous and feasible alternative to the feature-based classification. dataset the signals were selected from data collected by the monitoring network deployed by vsom staff on nrv. the stations of the nrv monitoring network are located strategically; for instance near to glaciers and craters. signals from two stations (olleta crater station and glacier station) have been selected for the experiments because, according to the experiences reported by vsom staff, these two stations are a reference for the volcanic and ice-related events. the stations are located at distances of 4.08 km and 1.8 km from the active crater, respectively. signals were digitized at 100.16 hz sampling frequency by using 12 bits analogue-to-digital converter. a description of the nrv data set is provided in table 1. typical waveforms are shown in figure orozco et al., esrj vol. 10, no. 2. december 2006 figure 1. typical waveforms of the three classes considered: vt, lp and ic events. 59 1. table 1. composition of the data set (number of events per class). vt and lp events were detected by the olleta crater station. ic events were detected by the glacier station. vt lp ic 483 580 782 vt and lp earthquakes are commonly detected at nrv. in our experiments, the vt and lp events are analyzed as a separate problem (the ruiz-lp,vt two-class problem), and we also analize all classes together (the multi-class ruiz-all problem). dissimilarity representations and classifiers dissimilarity representations can be derived in many ways, e.g. from raw (sensor) measurements such as images, histograms or spectra or, from an initial representation by features, strings or graphs (pękalska et al., 2006); nonetheless, the particular way in which dissimilarities are computed is crucial, and relies on the additional knowledge that experts —volcanic seismologists— have about the problem. the spectra of seismic signals are commonly used for classification and monitoring of volcanic activity. since differences in spectral content allow a visual discrimination of different types of volcanic earthquakes (zobin, 2003), we have calculated the spectrum for each signal by using two different approaches: (i) n-point fast fourier transform (fft) and, (ii) parametric estimation of the power spectral density (psd). in such a way, we explore the difference between deriving dissimilarities from a data-based spectral estimation and from a model-based spectral estimation such as the yule-walker ar method. dc bias was removed before computing the spectra; in addition, when spectra are directly compared, they are required to be normalized and to have the same length. in consequence, considering the length of the shortest event and a length-resolution trade-off, we calculated 128point spectra. two different dissimilarities measures have been computed between spectra: (i) pointwise euclidean distance and (ii) area difference: the area of non-overlapping parts ( -norm) as shown in figure 2. dissimilarity-based classification of seismic signals at nevado del ruiz volcano figure 2. dissimilarity measured as the difference between normalized spectra. 60 the k nearest neighbor classifier (k-nn) k-nn is considered a direct approach for dissimilarity-based classification. this rule classifies a new object by assigning it the class label most frequently represented among the k nearest prototypes (e.g., by finding the k neighbors with the minimum distances between the new object and all the prototypes). for , the rule is called 1-nn. even though k-nn is asymptotically optimal in the bayesian sense, it is sensitive to noise and erroneously labelled prototypes. linear and quadratic normal density based classifiers previous studies (pękalska et al, 2001; pękalska and duin, 2002; paclík and duin, 2003) have shown that bayesian (normal density-based) classifiers, particularly the linear (ldc) and quadratic (qdc) normal based classifiers, perform well in dissimilarity spaces. for a 2-class problem, the ldc based on the representation set is given by (1) and the qdc is derived as (2) where is the sample covariance matrix, and are the estimated class covariance matrices, and and are the mean vectors, computed in the dissimilarity space , and are the class´ a-priori probabilities. if is singular, a regularized version must be used. in this study, the following regularization is used: . (3) we have fixed λ to be 0.01. nonetheless, this regularization parameter should be optimized in order to obtain the best possible results for the normal density-based classifiers. experimental results experiments were conducted to compare the results of the k-nn rule and the ldc and qdc classifiers built on the dissimilarity representations described above. experiments were performed 25 times for randomly chosen training and test sets. since we are particularly interested in accuracy recognition rather than computational complexity and storage requirements, the entire training set has been used as the representation set . nonetheless, may be properly reduced by prototype selection procedures (pękalska et al., 2006). training and testing sets were generated by selecting equal partitions for the classes. the results of our experiments are shown in figures 3 and 4. they present the generalization errors as a function of the number of randomlychosen training objects. figure 3 presents the results for four dissimilarity representations of the ruiz-vt,lp two-class problem; similarly, the results for the ruiz-all problem are shown in figure 4. standard deviations for averaged test errors decrease rapidly, varying around 0.02 after at least 10 training objects per class become available; for clarity reasons, standard deviations are not presented in figures 3 and 4. final errors and their standard deviations are summarized in table 2. orozco et al., esrj vol. 10, no. 2. december 2006 61 figure 3. ruiz-vt, lp data. average classification error of the bayesian classifiers and the 1-nn classifier in different dissimilarity spaces ( fft+euclidean, fft+areas, psd+euclidean and psd+areas) as a function of the number of prototypes per class. dissimilarity-based classification of seismic signals at nevado del ruiz volcano 62 figure 4. ruiz-all data. average classification errors of the bayesian classifiers and the 1-nn classifier in different dissimilarity spaces (fft+euclidean, fft+areas, psd+euclidean, and psd+areas) as a function of the number of prototypes per class. orozco et al., esrj vol. 10, no. 2. december 2006 63 table 2. averaged final classification error (in %) with its standard deviation for the four representations and the three classifiers considered. ruiz-vt,lp two-class problem representation k-nn ldc qdc fft + euclidean 19.85 (1.84) 14.79 (1.55) 16.36 (2.15) fft + areas 19.90 (2.04) 14.94 (1.00) 19.35 (1.98) psd + euclidean 17.99 (2.03) 13.35 (1.24) 15.45 (1.38) psd + areas 17.48 (2.09) 13.40 (1.06) 16.23 (2.05) ruiz-all problem fft + euclidean 24.53 (1.66) 18.57 (1.13) 19.31 (1.33) fft + areas 24.28 (1.52) 18.94 (1.02) 26.38 (2.73) psd + euclidean 27.76 (2.08) 21.19 (0.80) 24.10 (1.47) psd + areas 26.70 (1.67) 20.11 (1.25) 22.99 (1.85) discussion and conclusions we have explored and tested a dissimilaritybased strategy for classifying three different types of volcanic-seismic signals recorded by the monitoring network at nrv. two classification problems were conducted: a two-class problem including vt earthquakes and lp seismic events, and a multi-class problem including ice-related seismic events. four dissimilarity representations were derived by combining two different approaches for spectral estimation: n-point fft and parametric psd estimation, as well as two dissimilarity measures: euclidean distance and area difference between spectral curves. these dissimilarity representations allowed the usage of traditional statistical decision rules, particularly normal density based classifiers. the 1-nn rule was employed as a reference for performance comparison. the two-class ruiz-vt, lp problem seems the easiest because it contains signals detected and identified by the same station (olleta crater station); thus it is expected that sensor and noise conditions are the same influencing the subsequent steps for representation and classification. in addition, it is well know that, in general, multi-class problems are more difficult to deal with. for the two-class problem, experiments based on parametric psd estimation outperform those based on the fft. this makes sense because event lengths are, in general, short and, consequently, a parametric spectral estimation yields a higher resolution; in addition, the autoregressive methods (ar) tend to adequately describe spectra of peaky data, which is precisely the spectral nature of many volcanic-seismic signals. in contrast, for the multi-class problem, the fft yields better results but, in these particular cases, differences are not significant. our experiments confirm that bayesian classifiers outperform the 1-nn classifier, when a sufficient number of prototypes are provided. the ldc constructed on the different dissimilarity representations, for both ruiz-vt,lp and ruizall problems, always outperforms the 1-nn rule. ldc accuracies for the ruiz-vt,lp problem vary between 85% and 87% when the average classification error curve reaches a steady state. similarly, classification accuracies for the ruizall problem vary between 81% and 84%. qdc shows accuracy loss when certain number of prototypes is provided. therefore, a further study on a proper regularization for the qdc should be conducted in order to obtain an improvement of this classifier. ldc accuracies could be an intrinsic limit of our classification problem; however, a further study on other dissimilaritybased classifiers is needed as well as a re-analysis of the original a-priori classification, in order to find more suitable classifiers and to confirm the labels assigned by the experts. dissimilarity-based classification of seismic signals at nevado del ruiz volcano 64 acknowledgements this work was partially supported by the research project: “técnicas de computación de alto rendimiento en la interpretación automatizada de imagines médicas y bioseñales”. we thank the vsom staff for providing the raw data set, prof. ricardo henao for his valuable suggestions, and two anonymous reviewers for their useful comments. references benbrahim, m., daoudi, a., benjelloun, k., and ibenbrahim, a. (2005). discrimination of seismic signals using artificial neural networks, 2nd world enformatika conference, istanbul, turkey, february, 4-7. duin, r. p. w., de ridder, d., and tax, d. m. j. (1998). featureless pattern classification, kybernetika. 34 (4). 399-404. jain, a. k., duin, r. p. w., and mao, j. (2000). statistical pattern recognition: a review, ieee trans. pattern anal. machine intell. 22 (1). 437. langer, h., falsaperla, s., powell, t., and thompson, g. (2006). automatic classification and a-posteriori analysis of seismic event identification at soufrière hills volcano, montserrat, journal of volcanology and geothermal research. 153. 1-10. paclík, p. and duin, r. p. w. (2003). dissimilaritybased classification of spectra: computational issues, real time imaging. 9. 237-244. pękalska, e., paclík, p., and duin, r. p. w. (2001). a generalized kernel approach to dissimilarity based classification, j. mach. learn. res. 5. 175211. pękalska, e. and duin, r.p.w. (2002). dissimilarity representations allow for building good classifiers, pattern recognition lett. 23. 943-956. pękalska, e. and duin, r. p. w. (2005). the dissimilarity representation for pattern recognition: foundations and applications. world scientific, singapore. pękalska, e., duin, r. p. w., and paclík, p. (2006). prototype selection for dissimilaritybased classifiers. pattern recognition. 39. 189208. scarpetta, s., giudicepietro, f., ezin, e. c., petrosino, s., del pezzo, e., martini, m., and marinaro, m. (2005). automatic classification of seismic signals at mt. vesuvius volcano, italy, using neural networks. bulletin of the seismological society of america. 95 (1). 185196. zobin, v. (2003). introduction to volcanic seismology. elsevier, the netherlands. orozco et al., esrj vol. 10, no. 2. december 2006 65 v16n1c.indb earth sciences research journal gravity exploration methods earth sci res sj vol 16, no 1 (june, 2012): 5 9 introduction the study area was located on the northern margin of the congo craton (cc) (2 5°-4 5°n, 11°-13°e) the study area was affected by a series of tectonic activity due to pan-african fold belt interaction with the cc forming the south cameroon area’s structural features (castaing et al , 1994; abdelsalam et al , 2002; toteu et al , 2006) the collision between the cc and pan-african belt (pab) led to the pan-african units overthrusting onto the craton by about 50 to 150 km (manguelle-dicoum, 1998; tadjou et al , 2004; shandini et al , 2010) the basement configuration and faults cannot be mapped directly using conventional field methods due to the blanket of pan-african units whose thickness can be considerable in places even though geological and geophysical studies in cameroon began more than 50 years ago, little is known about the south interpreting gravity anomalies in south cameroon, central africa yves shandini1 and jean marie tadjou2* 1 physics department, science faculty, university of yaoundé i, cameroon 2 national institute of cartographynic, yaoundé, cameroon * corresponding author e-mail: shandiniyves@yahoo fr abstract the area involved in this study is the northern part of the congo craton, located in south cameroon, (2 5°n 4 5°n, 11°e 13°e) the study involved analysing gravity data to delineate major structures and faults in south cameroon the region’s bouguer gravity is characterised by elongated sw-ne negative gravity anomaly corresponding to a collapsed structure associated with a granitic intrusion beneath the region, limited by fault systems; this was clearly evident on an isostatic residual gravity map high gravity anomaly within the northern part of the area was interpreted as a result of dense bodies put in place at the root of the crust positive anomalies in the northern part of the area were separated from southern negative anomalies by a prominent e-w lineament; this was interpreted on the gravity maps as a suture zone between the south congo craton and the pan-african formations gravity anomalies’ total horizontal derivatives generally reflect faults or compositional changes which can describe structural trends the local maxima of the bouguer gravity data’s horizontal gradient grid and its upward continuation at various altitudes were used to highlight the deepest lineament faults and their dip and direction these features led to producing a structural map of the study area resumen el área involucrada en este estudio es parte del norte del cratón del congo, localizada en el sur de camerún, (2 5°n 4 5°n, 11°e 13°e) se realizo un análisis de los datos de gravedad para delinear las principales estructuras y fallas en el sur de camerún la gravedad de bouguer de la región se caracteriza por una alargada anomalía de gravedad negativa, con tendencia sw-ne, que corresponde a una estructura colapsada asociada con una intrusión granítica debajo de la región, limitada por un sistema de fallas; esto fue claramente evidenciado sobre un mapa de gravedad residual isostático las altas anomalías de gravedad dentro de la parte norte de la zona fueron interpretadas como resultado de cuerpos densos colocadas en la raíz de la corteza las anomalías positivas en la parte norte de la zona fueron separadas de las anomalías negativas del sur por un lineamiento prominente con tendencia e-w; esto fue interpretado en los mapas de gravedad como una zona de sutura entre el sur del cratón del congo y las formaciones pan-africano las anomalías de gravedad de los derivados horizontales totales generalmente reflejan fallas o cambios en la composición, las cuales pueden describir las tendencias estructurales la máxima local derivada de los datos de la malla de gradiente horizontal de la gravedad de bouguer y su continuidad hacia arriba en diferentes altitudes fueron usadas para resaltar los lineamientos más profundos de las fallas y su inclinación y dirección estas características permitieron producir un mapa estructural del área de estudio palabras claves: falla, datos gravitacionales, gravedad residual isostática, sur camerún, derivada horizontal total keywords: fault, gravity data, isostatic residual gravity, south cameroon, total horizontal derivative record manuscript received: 14/10/2011 accepted for publications: 06/05/2012 yves shandini and jean marie tadjou6 cameroon area’s subsurface structure this study’s main purpose was to compile available geological and gravity data regarding southern cameroon to gain better understanding of its structural framework, basement configuration and construct a tectonic map of the study area geological setting the area being studied lies in south cameroon, located in the transition zone between the cc and the west african pab; its very complicated tectonic history has unfolded as it has undergone its geological evolution a simplified geological sketch map of the study area illustrating the main geological units is shown in figure 1 the ntem complex represents the north-western part of the cc in central africa and is well exposed in south cameroon (maurizot at al , 1985) the complex predominantly consists of younger intrusive complexes and banded series composed of gneisses intrusive complexes primarily consist of ttg suite rocks (nédélec et al , 1990) the ttg unit is made up of three rocks types: the tonalitic suite (known as so’o granite), the charnockitic suite and the granodioritic suite massifs (shang et al , 2004) the tonalitic suite is exposed to the north and is strongly mylonitised and retrogressed along the fault boundary with formations from the yaoundé group the yaoundé group is a huge allochtonous nappe thrust southward onto the cc it comprises lowto high-grade garnet-bearing schist, gneisses and orthogneisses transformed in mediumto high-pressure granulite facies metamorphism (toteu et al , 2004) the yaoundé group in the study area consists of the mbalmayo-bengbis series and the yaoundé series known as “intermediate series ” the mbalmayo-bengbis series are composed of schist and re-crystallised quartzite in greenschist facies (vicat, 1998) the yaoundé series consist of strongly deformed meta-sedimentary rocks and migmatites (nzenti et al , 1988) the structural data has shown definite surface deformation characterised by flat structures gently sloping to the north and generalised tilting towards the south or south-west, indicating a significant intermediate formation overlap on the ntem complex basement such deformation may be seen by the presence of northward sloping folds the region has a complex and uneven tectonic structure the region’s tectonic evolution seems to have given rise to the basement’s vertical movement, with subsidence to the north and uplift to the south (manguelle-dicoum, 1988) this basement movement must have provoked irregularities in deep formations, giving rise to faults, horsts and grabens which are characteristic of the boundary between the cc and the pan-african fold belt data and methods the gravity data used in this work was obtained from the institut français de la recherche scientifique pour le développement en coopération’s (orstom) database the data were collected in the area, using worden gravimeters (n° 69, 135, 313, and 600), 0 2 mgals/division scale precision, and lacoste & romberg gravimeters (model g n° 471 and 823) the altimeter readings were looped along with gravimeter reading to obtain fairly reliable height differences between base stations, thereby guaranteeing reasonably accurate elevation values for intermediates stations maximum error in determining the height of any station by means of altimeters did not exceed 10 m consequently, bouguer anomaly value figure 1. geological sketch map of the study area (modified from maurizot et al , 1985) 1 tonalitic suite (cc); 2 charnockitic suite (cc); 3 bonded series (cc); 4 micashists (pab); 5 epischists (pab); 6 gneiss (pab); 7 granulites (pab); 8 fault; 9 major thrust interpreting gravity anomalies in south cameroon, central africa 7 maximum error for any station due to the above height determination error was not expected to exceed 0 15 mgal the bouguer anomaly values resulted from free-air reduction referring to the ellipsoid, infinite plate reduction with 2 670 g/cm3 constant reduction density and topographic reduction (crenn, 1967; collignon, 1968) gravity survey accuracy was estimated to be about ± 0 5 mgal a bouguer anomaly map (constant 5 mgal contour interval) was thus prepared to interpret the study area’s crustal structure (figure 2) the map revealed a large-scale negative bouguer anomaly tending sw-ne to e-w over the central part of the area closer geological and structural observation (figures 1 & 2) of this anomaly’s axes suggested that its general trend followed an inferred granite intrusion area known as so’o granite the quite different nature of the bouguer gravity map on the northern side was marked by gravity highs, bounded by relatively steep gradients occurring over or near higher metamorphic formations (granulite, migmatite and micaschist) and other granitic plutons, suggesting the existence of a suture zone between two of the crust’s blocks (kennedy, 1964) positive bouguer gravity in the southern area tending sw-ne marked the intrusion of dense rocks (charnockites) in this area interpreting gravity and magnetic data can be aided by applying advanced processing: total horizontal derivative and isostatic residual/regional processing for the bouguer gravity map such techniques were applied here to aid understanding regional tectonics and structures total horizontal derivative for gravity data this operation measured the rate of change of field in x and y directions and created a resultant grid (cordell and grauch, 1985): the horizontal gradient method was used to locate density boundaries from gravity data (cordell and grauch, 1985) this highlighted high gradient areas such as those which might occur at faulted boundaries local peaks (or ridges) in gravity horizontal gradient magnitude gave the steepest gradients’ locations, this being intuitively similar to taking the first derivative of a curve a 3x3 window was passed over the horizonfigure 2. bouguer anomaly map of the study area showing the gravity stations (dots) thd= + 22 ∂m ∂x ∂m ∂y( ) )( (1) tal gradient magnitude (hgm) grid in the automated method (blakely, 1986) to fit parabolic peaks to the four 3-point scans passing through the centre of the window if a sufficient number of peaks or maxima were found (usually 2 to 4), the location of the largest peak was taken as the contact location this technique has been applied to upward-continued data at five depth levels: 4, 6, 8, 10 and 12 kilometres above measurement surface the upward continuation processing of the bouguer gravity map at various altitudes, followed by determining horizontal gradient maxima for each level yielded these maxima’s progressive migration while increasing upward continuation altitude indicating the dip direction faults were presumed to produce a single alignment in map view, or one prominent hgm maxima “track” isostatic residual gravity anomaly an isostatic reduction was applied to the bouguer anomalies to quantify the isostatic compensation effects allowing isostatic equilibrium to be restored the regional correction was calculated from topography using airy’s model of crustal compensation and subtracted from the bouguer gravity anomaly, which then produced an isostatic residual gravity map of the study area a crust having constant density c but changing thickness was hydrostatically supported by a denser substratum whose density was ρm if t were the thickness of the sea-level standard crust (moho initial depth), h the height above sea-level of any crustal column having r as a root relative to the standard crust base, then the value of r was determined using an isostatic equilibrium equation (schoeffler, 1975): (h + t + r) c = tc + rm or hc = r (m c) the isostatic reduction would then result in determining the r value for each h and computing its gravity effect from the relationship given below: r = hc δ where δ = m c mantle-crust density contrast isostatic anomalies depict deviations from compensation, resulting from crust restructuring (density changes corresponding to short wavelength effects) and the lithospheric plate or upper mantle’s elastic behaviour (deepest sources corresponding to long wavelength effects) the isostatic residual anomaly was calculated by removing the long wavelength gravity effects caused by isostatic compensation, i e variations in the crust/ upper mantle boundary theoretically, the isostatic residual should have only retained the gravity effects from the upper crust to the surface results and discussion analysing the results of the horizontal gradient map (4, 6, 8, 10, 12 km altitude; figure 3) aided in defining the location of linear features related to the trend of the deep faults in this area faults could be traced easily along these salient features the resulting set of lineaments (figure 4) showed that most faults in the study area had sub-vertical dips interpreting the so’o granite complex’s structural elements, in the central part of the area, revealed that it was bounded to the north and southwest of the sangmelima region by main fractures interpreted as being normal faults ene-wsw sets of faults appeared in the southwest part of the study area, extending from south of ebolowa to mbalmayo the nnwssw tending fault in the mbalmayo area was described as representing an intra-granitic fault (manguelle-dicoum et al , 1992) the gravity data for the northern part of the area was characterised by gravity highs, bounded by relatively steep gradients which could be interpreted as the result of mantle upwelling associated with high temperature; deep-seated basement (2) (3) yves shandini and jean marie tadjou8 structure associated-melting had thus occurred in the lower crust overall horizontal derivative analysis results for gravity anomalies produced many features regarding trend and dip direction marking the faulted cc and the pab boundary and revealed a complex subsurface geological structure regional correction was calculated from topography using airy’s crustal compensation model using geosoft software (2009) the following parameters were selected for computation: 30 km initial compensating depth and 0 60 g/cm3 mantle-crust density contrast the isostatic residual anomaly was calculated by removing the long wavelength gravity effects caused by isostatic compensation, i e variations in crust/upper mantle boundary the main isostatic residual map (figure5) feature was similar to that of the bouguer map shown in figure 2 prominent low gravity seen in the isostatic anomaly might have indicated a deepening of the basement, assuming that the isostatic correction had appropriately accounted for deep crustal gravity effects low regional gravity reflected low density granitic rocks (so’o granites) making up the upper 8 km of a batholith in this region (tadjou et al , 2004; shandini et al , 2010); however, its strongest correlation with surface geology was not evident the consistent relationship between broad gravity lows and granitic rock distribution was perhaps most clearly seen locally along the major thrust marking the contact between the cc and the pan-african orogenic belt, near latitude 3 5°n although major low gravity sources appeared to be shallow, they were most likely concealed throughout much of the area some gradients defining the borders of the low occurred over exposed pan-african formations it could have been inferred that the faults dissecting and crosscutting the central low gravity area might have been fracturing the crust associated with massive granitic intrusion in the upper, brittle part of the crust local gravity highs were the dominant feature in the northern part of the area the positive anomalies occurring over or near higher metamorphic formation outcrops appeared to be associated with mafic igneous bodies shandini et al , (2010) used spectral analysis and 3d gravity modelling to provide evidence that the bouguer anomaly on the northern edge of the cc could not be modelled without high-density intrusive-like deep bodies and attributed positive anomalies in this region to deep and dense intrusive bodies a positive nw-se trending anomaly was the dominant feature in the southern part of the area which could be interpreted in terms of ttg group igneous charnockite intrusion 11 5 12 12 5 13 4 3 5 3 2 5 figure 3. bouguer anomaly horizontal gradient maxima and its upward continuation to different heights in km (black 4km; blue 6 km; green 8km; red 10km; yellow 12km) 4 3 5 3 2 5 11 11 5 12 12 5 13 the transition from low to high background gravity values occurred over relatively short distances, suggesting that the density distributions producing these anomalies were shallow the northern part of the area was separated from the southern part by a prominent e-w lineament expressed on bouguer and isostatic residual gravity maps such extensive gravity gradients mark regional crustalscale subsurface structures considered suture zones between two of the crust’s block the general abundance of faults observed in the northern part of the area revealed complex subsurface geological structure crustal faulting in this area could have been related to the inferred tectonic boundary separating the cc and the pab and was associated with deep-seated basement structures conclusion gravity anomaly field maps present positive/negative anomalies separated by strong gradient zones multi-scale analysis of horizontal gravifigure 4. map showing the distribution of interpreted gravity faults in the area being studied arrows indicate dip direction figure 5. isostatic residual bouguer gravity map of the study area interpreting gravity anomalies in south cameroon, central africa 9 metric gradient led to identifying the main faults responsible for structuring the studied area and so obtaining indications regarding their dip the study area was characterised by one gravity anomaly in the south characterised by having a long wavelength and high anomaly expression in the north low gravity anomaly expression was associated with granite intrusions while high gravity anomaly expression was associated with basement outcrop these two areas were separated by a pronounced e-w lineament recognised on both bouguer and isostatic residual gravity maps and was interpreted as being a basement fault references abdelsalam, m g , liégeois, j p and r j stern (2002): the saharan metacraton journal of african earth sciences, 34, 119–136 archibald, n and f 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no. 1 (june, 2013): 33 40 drastic-based methodology for assessing groundwater vulnerability in the gümüşhacıköy and merzifon basin (amasya, turkey) arzu firat ersoy* and fatma gültekin* *karadeniz technical university, engineering faculty, geological engineering department, 61080, trabzon, turkey corresponding author: arzu firat ersoy e-mail:arzufirat@gmail.com phone: + 90 462 377 20 63 fax: + 90 462 325 74 05 hydrogeology preparing aquifer vulnerability maps has become crucial during recent years for preventing adding new ones to aquifers which have been contaminated due to environmental effects and been out of use. gis techniques and drastic method were used when preparing vulnerability maps for the basin in which the gümüşhacıköy and merzifon aquifers are located. groundwater flow is approximately directed west-east and many villages are located across the aquifer in the basin which contains two sub-provinces and is characterised by intensive agricultural activity. drastic layers were created when preparing vulnerability map, using parameters such as groundwater level, recharge, aquifer environment, topography and hydraulic conductivity. the aquifer vulnerability map was prepared by overlapping the layers by means of gis. , three different vulnerability zones were determined in the gümüşhacıköy basin according to drastic scores low (<100), medium (100-140) and high (>140). based on the vulnerability map, it was found that the gümüşhacıköy basin had a low contamination potential. it was established that 16% of the basin had high vulnerability and 47% low vulnerability. areas having high vulnerability generally overlapped areas where the slope was gentle soil above the aquifer was permeable. la preparación de mapas de vulnerabilidad acuífera se ha convertido en una actividad crucial en los últimos años para prevenir la contaminación por efectos ambientales de un afluente y su posterior inutilización. técnicas gis y el método drastic fueron utilizados en la preparación de mapas de vulnerabilidad en la cuenca donde están localizados los acuíferos gümüşhacıköy y merzifon, en turquía. el flujo de las aguas subterráneas corre aproximadamente oeste-este y varias poblaciones están ubicadas al paso del acuífero por dos subprovincias que se caracterizan por la actividad agrícola. se crearon capas en el método drastic cuando se preparó el mapa de vulnerabilidad con parámetros como nivel, recarga, ambiente del acuífero, topografía y conductividad hidráulica. la representación de vulnerabilidad se logró al sobreponer estas capas a través de técnicas gis, lo que permitió determinar tres zonas diferentes de vulnerabilidad en la cuenca de gümüşhacıköy basado en los puntajes del método drastic: baja (<100), medio (100-140) y alta (>140). con base en este mapa, se concluye que la cuenca de gümüşhacıköy tiene un bajo potencial de contaminación. se estableció que el 16 % de la cuenca es altamente vulnerable y el 47 % de baja vulnerabilidad. en aquellos lugares identificados con alto potencial de contaminación se suelen sobreponer áreas donde la inclinación de tierra sobre el acuífero es permeable. key words: vulnerability mapping, drastic, geographic information system (gis), turkey palabras claves: mapa de vulnerabilidad, drastic, sistema de información geográfica (gis), turquía. record manuscript received: 23/10/2012 accepted for publication: 04/04/2013 abstract resumen introduction groundwater has been considered as an important source of water supply due to its relatively low susceptibility to pollution compare to surface water. groundwater quality is usually subject to contamination especially in agriculture-dominated areas having intensive activity involving the use of fertilisers and pesticides. vulnerability assessment has been recognised for its ability to delineate areas which are more easily contaminated than others as a result of anthropogenic activity on/or near the earth’s surface. vulnerability studies can thus provide valuable information for stakeholders working on preventing further deterioration of the environment (mendoza and barmen 2006). figure 1. location map of the gümüşhacıköy-merzifon basin (gmb) arzu firat ersoy and fatma gültekin the concept of groundwater vulnerability to contamination was introduced in the 1960s in france by margat (1968). several approaches for developing aquifer vulnerability assessment maps were adopted such as drastic (aller et al. 1987), god (foster 1987), avi (van stempvoort et al 1993), and sintacs (civita 1994). conventional methods (i.e. drastic, avi, god, sintacs) can distinguish degrees of vulnerability on a regional scale involving different lithology (vias et al 2005). drastic is a familiar method developed for the us environmental protection agency (epa) by aller et al (1987) it has been used in several regions (merchant 1994; melloul and collin 1998; cameron and peloso 2001; al-adamat et al 2003; baalousha 2006; jamrah et al 2007; sener et al 2009; massone et al 2010). the area studied in this research is located in amasya (mid black sea region), namely the gümüşhacıköy-merzifon basin (gmb) (figure 1). groundwater is the major source of irrigation in the amasya district in turkey. surface water has been the main source of water supply for irrigation during the last few decades (fırat ersoy and gültekin 2008). however, water demand has increased and groundwater is now used as a secondary source. annual groundwater exploitation yield was only 3.5x106m3 during the 1970s, and rose to 5.5x106 m3 in 2005 (fırat ersoy, 2007). due to the excessive exploitation of groundwater, water levels have significantly decreased. groundwater quality has also been affected by over exploitation. the town of, gümüşhacıköy is located in this basin. some well water’s nitrate concentration has reached 15.6 mg/l in the gümüşhacıköy basin. nitrite and ammonium concentration have reached 0.03 and 0.3 mg/l respectively, around the town (fırat ersoy et al, 2006). this paper was aimed at assessing groundwater vulnerability to pollution in a shallow aquifer using the drastic model and geographical information system (gis) techniques combined with hydro-geological data layers, i.e. depth of water, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity. a vulnerability map, showing high, medium and low vulnerability areas was produced for the mentioned basin. study area the gmb covers a 1,060 km2 area elevation ranging from 5501,873 m (figure 1). average annual rainfall is 458 mm average annual temperature is 13.6°c (url-1) and the average annual potential evaporation is 680 mm (fırat ersoy, 2007). the most important body of surface water flowing through the basin is the gümüşsuyu river, which discharges 8.5x106m3/ year (fırat ersoy, 2007). groundwater in the basin draws from both alluvium aquifers, one being confined, (the gümüşhacıköy aquifer) and the other unconfined (merzifon). agriculture is widespread in the basin, and fertiliser and pesticide application have caused groundwater contamination through leaching. 193 wells had been drilled in the gmb up to 2006, 173 wells were aimed at irrigation, and 20 well for domestic purposes. geology the paleozoic metamorphic rocks in the study area, represent the oldest formation. these rocks consist of clayey schist, chlorite schist and green schist, marble and re-crystallised limestone. upper jurassic-lower cretaceous limestone, in the area has fossils which are generally compact, thick-bedded, very hard and fissured lower cretaceous limestone is pink, very hard, thick bedded and micritic, overlying jurassic limestone. cretaceous limestone outcrops on the plain. the flysch series having mixed volcanic material composed of conglomerate, green and black sandstone, shale, marl, limestone, andesite, tuff and tuffite are deposited in the upper cretaceous limestone. cenozoic beds started with the middle eocene age flysch series in the study area. flysch consists of sandstone, shale, sandy limestone, marl, local conglomerate, tuff and agglomerate. the miocene series consist of thick blue claystone and marl and the pliocene beds of micro conglomerate, sandstone, sand, clay, gravel and a mixture of these drastic-based methodology for assessing groundwater vulnerability in the gümüşhacıköy and merzifon basin (amasya, turkey) layer thickness ranges from 10 to 50 cm in these series. the very loose layers are not continuous and change their lithology over short distances. this unit gradually become harder as one goes deeper and turns out to be conglomerate. some blocks of the gravels are 50 cm thick and about 5-10 cm-in diameter 95% of such gravel and blocks are usually rounded and are composed of volcanic material. the quaternary is characterised by alluvium and an alluvium cone consisting of detrical material that comes from north and south with the flood waters. alluvium and cone (10-60 m thick) take the form of gravel, sand, clay and a mixture of these, along the gümüşsuyu, köseler and salhan rivers. hydrogeology the gmb’s hydrogeological setting has been outlined by fırat ersoy (2007). the most impotant geological units for groundwater transport in the basin are quaternary alluvium and pliocene clay, sand, gravel and a mixture of these. unconsolidated quaternary and pliocene sediments are around 350 m thick. the other units underlying the alluvium do not bear significant amounts of groundwater. the gmb can be divided into discrete hydrogeological units, including permeable (alluvium), semi-permeable (weak cemented pebble and sandstone, silty clay and volcanic rocks) and impermeable (massive marble and limestone, silty clay and schist). alluvial materials and the pliocene units consisting loosely cemented pebbles, sand and clayey-silt materials outcrop in most parts of the basin. known as the gümüşhacıköy aquifer, this part is crucial to groundwater storage and transfer since it is characterised by high conductivity and storage capacity. deposits have a heterogeneous structure, being formed as alluvial cones at the end of tributary rivers. the alluvial cone formed by the alluvial unit of the paşa and yakacık river is called the merzifon aquifer. well logs show that the cone’s middle sections consist of clayey levels between pebble and sand layers and that level becomes thin along the eastern edge, dominated by clay and silt. since the section between the east of the gümüşhacıköy aquifer and the south of the merzifon aquifer consists of miocene clay and marl, it is not important in terms of groundwater. the late eocene volcanic rocks outcropping across the north, northwest and northeast of the basin form a catchment area with their fractured and fissured structure. natural discharge in the basin is provided by a stream flowing through it from west to east. the gümüşhacıköy aquifer naturally discharges into the gümüşsuyu river, located in the east of basin. the basin contains numerous springs discharging from geological units, faults and fractures. some are exploited as potable for drinking water and others are used for irrigation, consequently recharging the groundwater. the springs’ total flow rate is 720 l/sec. three ponds in the basin are used for irrigation. infiltration into the groundwater from these ponds has been estimated as being 41.5 m3/year on average (fırat ersoy, 2007). the basin contains 193 wells, 167 in the gümüşhacıköy aquifer and 26 in the merzifon aquifer (dsi̇, 2006). well depth varies from 39 to 290 m and pump flow-rates from 5 to 60 l/sec (dsi̇, 2006). as most of these wells are used for irrigation, pumps operate from may to october. the increased number of wells drilled in the aquifer during recent years and accordingly, the increased amount of water pumped from the aquifer has resulted in a decrease in groundwater level by 15 to 20 m (dsi, 2006). several pumping tests have been performed at existing wells in the gümüşhacıköy and merzifon aquifers. data interpretation has indicated 89.7 1727 m2/day transmissivity, 0.7619.17 m/day hydraulic conductivity and 1.5x10-57.9x10-3 storage coefficient for the basin (fırat ersoy, 2007). materials and methods drastic, proposed by the us environmental protection agency (aller et al., 1987) and its modification termed sintacs (civita, 1994), parameter range rating description relative weighting depth to water (d) (feet) 0-5 5-15 15-30 30-50 50-75 75-100 >100 10 9 7 5 3 2 1 refers to the depth to the water surface in an unconfined aquifer. deeper water table levels imply lesser chance for contamination to occur. depth to water is used to delineate the depth to the top of a confined aquifer. 5 net recharge (r) (in) 0-2 2-4 4-7 7-10 >10 1 3 6 8 9 indicates the amount of water per unit area of land which penetrates the ground surface and reaches the water table. recharge water is available to transport a contaminant vertically to the water table, horizontal with in an aquifer. 4 aquifer media (a) massive shale metamorphic/igneous weathered met./igneous bedded sandstone, limestone, shale sequences massive sandstone massive limestone sand and gravel basalt karst limestone 2 3 4 6 6 6 8 9 10 refers to the consolidated or unconsolidated medium which serves as an aquifer. the larger the grain size and more fractures or openings with in an aquifer, leads to higher permeability and lower attenuation capacity, hence greater the pollution potential. 3 soil media (s) soil thin or absent gravel sand peat shrinking and/or aggregated clay sandy loam loamsilty loam clay loam muck non-shrinking and non-aggregated clay 10 10 9 8 7 4 5 4 3 2 1 refers to the uppermost weathered portion of the vadose zone characterised by significant biological activity. soil has a significant impact on the amount of recharge which can infiltrate into the ground. 2 topography (t) (slope%) 0-2 2-6 6-12 12-18 >18 10 9 5 3 1 refers to the slope of the land surface. it helps a pollutant to runoff or remain on the surface in an area long enough to infiltrate it. 1 impact of vadose zone (i) silt/clay shale limestone sandstone bedded limestone, sandstone, shale sand and gravel with significant silt and clay metamorphic/igneous sand and gravel basalt karst limestone 1 3 6 6 6 6 4 8 9 10 is defined as unsaturated zone material. the significantly restrictive zone above an aquifer forming the confining layers is used in a confined aquifer, as the type of media having the most significant impact. 5 hydraulic conductivity (c) (gpd/ft2) 1-100 100-300 300-700 700-1,000 1000-2,000 >2,000 1 2 4 6 8 10 refers to the ability of an aquifer to transmit water, controlling the rate at which groundwater will flow under a given hydraulic gradient. material within the groundwater system 3 table 1 drastic weighting factors (aller et al. 1987) arzu firat ersoy and fatma gültekin drastic-based methodology for assessing groundwater vulnerability in the gümüşhacıköy and merzifon basin (amasya, turkey) are two methods for evaluating vertical vulnerability based on the following seven parameters: depth to water (d), net recharge (r), aquifer media (a), soil media (s), topography (t), vadose zone impact (i), and hydraulic conductivity (c) figure 2). each mapped factor is classified into ranges (continuous variables) or significant media types (thematic data) having an impact on pollution potential. weighting multipliers are then used for each factor to balance and enhance their importance, the typical rating ranging from 1 to 10 (table 1). the final vulnerability index is a weighted sum of the seven factors. the drastic index (di) can be computed using expression (1): di = dr dw + rrrw + araw + srsw + trtw + iriw + crcw (1) di drastic index for a mapping unit w weighting factor for each parameter r rating for each parameter d, r, a, s, t, i, and c the seven parameters figure 2. drastic method flowchart results depth to water (d) depth to water is defined as the distance (in meters) from the ground surface to the water table. groundwater table depth in the gmb has been measured since 1976. this present study, has used the 2005 values for groundwater table depth. the 167 wells’ location was digitised from the accompanying digital elevation model (dem).groundwater table depth changed between 9-40 m in the gmb the merzifon aquifer has the lowest groundwater depth in the gmb. the deepest groundwater occurred at the end of the impermeable layer over the aquifer media in the mostly confined gümüşhacıköy aquifer. the depth to water table map was then classified into ranges defined by the drastic model and assigned rates ranging from 1 (minimum impact on vulnerability) to 10 (maximum impact on vulnerability) (figure 3). figure 3. depth to water table map of the study area net recharge (r) net recharge is the total quantity of water infiltrating from ground surface to an aquifer on an annual basis. local recharge in the study area comes from inflow by the gümüşsuyu river and its branches, irrigation return flow and direct recharge. the main groundwater recharge source are the gümüşsuyu river, springs, located high in the basin, and irrigation leakage. the average direct annual volume of recharge into the aquifer from the surface of the basin and from the springs is about 11334316 m3 (fırat ersoy, 2007). irrigation pond canals contribute 41.5 m3 recharge in the area (fırat ersoy, 2007) the recharge map was then classified into ranges and assigned ratings from 1 to 8 (figure 4). figure 4. net recharge map of the study area aquifer media (a) aquifer media refers to consolidated or unconsolidated rock which serves as an aquifer. the main aquifer being exploited and that most vulnerable to contamination is partially confined, here called the gümüşhacıköy aquifer the central and northern parts of the gümüşhacıköy aquifer are confined. the clayey layer over high permeability uncemented sediments is 210 m thick from the drilling data. clayey layer thickness of gradually changes from the centre to the north of the basin. the merzifon aquifer is unconfined. the aquifer media was obtained using a subsurface geology map, geological sections, and drilling profiles of the gümüşhacıköy and merzifon aquifers. the main aquifer includes quaternary alluvium and pliocene gravel, sand and clayey levels. the merzifon aquifer’s alluvial fan is relatively thin, consisting of coarse-grained gravel and sand with silt and clay interbeds. the aquifer media were mapped as shown in figure 5. figure 5. aquifer media map of the study area soil media (s) soil media refers to the uppermost portion of the vadose zone characterised by significant biological activity. soil plays a significant role in the amount of recharge which can infiltrate into the ground and hence on a contaminant’s ability to move vertically into the vadose zone. a soil’s pollution potential is largely affected by the type of clay present, such clay’s shrink/swell potential, and soil grain size. soil media in the gmb was determined using drilling profiles. the gümüşhacıköy and merzifon aquifers are covered by clayey gravel and sand and alluvial plains. fractured volcanic rocks are located west and south-west of the gmb and limestone outcrops south of the basin. thickness over the volcanic rock and limestone gradually changes form bottom to top, the hills especially, having little or no soil. the soil media map was then classified into ranges and assigned ratings from 3 to 10 (figure 6). topography (t) topography refers to land surface slope variability. slope degree will determine the extent of pollutant runoff and settling long enough to infiltrate. a digital elevation model (dem) was used to extract the slope of the study area, while 27% of the gmb has a gentle slope, most of the basin has a steep slope. the areas in the extreme east and south consist of ridges which may reach 1050 m.. the topography has been mapped as shown in figure 7. figure 6. soil media map of the study area figure 7. topography map of the study area vadose zone impact (i) the vadose zone is defined as the zone above the water table which is unsaturated. unconsolidated clayey gravel and sand represents the vadose zone in the plain, volcanic rocks and limestone is the vadose zone in the mountain areas. the map of vadose zone impact is shown in figure 8. the aquifer’s hydraulic conductivity (c) hydraulic conductivity is important because it controls the rate of groundwater movement in the saturated zone, thereby controlling the degree and fate of contaminants. hydraulic conductivity values used in this study were derived from pumping test data. hydraulic conductivity varied from 8.79 × 10−6 to 2.21 × 10−4 m/s in alluvium (fırat ersoy, 2007). the hydraulic conductivity of the other rock in the basin was available from the pertinent literature. hydraulic conductivity values for various rock types have been proposed by domenico and schwartz, (1990). the hydraulic conductivity of the limestone and fractured volcanic rocks, located in the west and south part of the basin, were 10-3 m/s and 3x10-4 m/s, respectively. clayey unit permeability is 10−9 to 10−10 m/s. hydraulic conductivity rating distribution is shown in figure 9. arzu firat ersoy and fatma gültekin drastic-based methodology for assessing groundwater vulnerability in the gümüşhacıköy and merzifon basin (amasya, turkey) figure 8. vadose zone impact map of the study area figure 9. hydraulic conductivity map of the study area the aquifer vulnerability map the vulnerability map was obtained using the seven hydro-geological data layers in the arcview gis software environment. drastic scores ranged from 58 to 177, taking into consideration the determined ratings and weightings. these values were reclassified into three classes using the natural breaks (jenks) classification method. the study area’s vulnerability was classed as low (<100), medium (100-140) and high (>140) according to data obtained from hydrogeological investigations (figure 10). the gmb’s high groundwater vulnerability risk zones were mainly located in the centre of the basin where some villages are located and also in the northern and southern parts of the basin. these vulnerable zones covered around 16% of the studied area. four springs in the southern area had as high vulnerability risk. the gmb’s middle groundwater vulnerability risk zones were mainly located in the groundwater recharge area (figure 10), these vulnerable zones covered around 37% of the studied area. the gmb’s low groundwater vulnerability risk zones were mainly located in the west and south-east of the study area (figure 10), these vulnerable zones covered 47 % of the studied area. figure 10. vulnerability index map of the study area the resulting vulnerability map indicated that the highest potential areas for contamination were the central part of the basin where the slope is gentle. in the southern area where karstic limestone outcropped, the high drastic index probably represented the effects of aquifer media and hydraulic conductivity. impermeable volcanic rocks and clay, silty-clay units located in the west and east respectively had low drastic index. conclusions the merzifon-gümüşhacıköy (amasya-turkey) basin is an important agricultural centre for the central black sea section groundwater is a major water source for such activity. groundwater quality has deteriorated due to excessive abstraction of groundwater. this study involved using a gis model and the drastic method for determining the vulnerability of the groundwater in the basin. the aquifer vulnerability map was prepared using depth to water, net recharge, aquifer media, soil media, topography, vadose zone impact, and hydraulic conductivity. the study area was divided into three zones according to groundwater vulnerability assessment results: low (risk index <100); middle (risk index 100–140) and high groundwater vulnerability risk (risk index >140). the drastic method results should be useful in designing aquifer protection and management strategies. the drastic index map indicated that overall potential for groundwater becoming polluted was low for the gmb. low sensitivity areas lay outside the agricultural areas in the basin. the alluvium and most pliocene sediments were used for agriculture in the gmb. the town of gümüşhacıköy is located on an aquifer recharge area. areas determined by the drastic method should thus be given priority in research in terms of contamination. high nitrate concentrations were mainly near urban areas according to the the study area’s analysis (fırat ersoy et al, 2006). high nitrate concentration was likely to be related to wastewater leakage from industrial activities, urbanisation and agricultural practices. two towns and many villages were situated in the study area involving agricultural activities many wells were used for springs. the prevention of groundwater pollution caused by waste and wastewater in gümüşhacıköy’s recharge area was significant owing to groundwater flow being west to east in the basin. regarding urban planning and organisation of agricultural activities in the merzifon and gümüşhacıköy districts, the vulnerability risk map prepared in the study could be most important when considering protection off groundwater quality arzu firat ersoy and fatma gültekin acknowledgements this work was supported by the karadeniz technical university’s scientific research fund project number: 2008.112.005.14. the authors are grateful to geomatics engineer dr. r. nişancı and y. s. erbaş for training in gis mapping. references al-adamat. ran., foster. idl., baban. smj. 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(1987). fundamental concepts in aquifer vulnerability, pollution risk and protection strategy. in: van duijvenbooden w, van waegeningh hg (eds) vulnerability of soil and groundwater to pollutants. committee on hydrological research, the hague, pp 69–86. jamrah a., futaisi aa., rajmohan n., al-yaroubi s. (2007). assessment of groundwater vulnerability in the coastal region of oman using drastic index method in gis environment. environ monit assess. doi:10.1007/s10661-007-0104-6. margat j. (1968). vulnerabilite des nappes d’eau souterraine a la pollution [groundwater vulnerability to contamination]. bases de al cartographie, (doc.) 68 sgc 198 hyd, brgm, orleans, france. massone h, mauricio mq, martínez d. (2010.) enhanced groundwater vulnerability assessment in geological homogeneous areas: a case study from the argentine pampas. hydrogeology journal, 18: 371–379. melloul aj., collin m. (1998). a proposed index for aquifer waterquality assessment: the case of israel’s sharon region. j environ manage 54(2):131–142. mendoza ja. barmen g. (2006). assessment of groundwater vulnerability in the río artiguas basin, nicaragua. env geol 50:569–580. merchant jw. (1994). gis-based groundwater pollution hazard assessment: a critical review of the drastic model. photogramm eng remote sens 60(9):1117–1127. sener e., sener s., davraz a. (2009). assessment of aquifer vulnerability based on gis and drastic methods: a case study of the senirkent-uluborlu basin (isparta, turkey). hydrogeology journal, 17: 2023–2035. url-1.,http://www.dmi.gov.tr/verideğerlendirme/yıllık-toplam-yagisverileri-amasya,15 march 2011. van stempvoort d., ewert l., wassenaar l. (1993). aquifer vulnerability index (avi): a gis compatible method for groundwater vulnerability mapping. can water resour j 18:25–37. vias jm., andreo b., perles mj., carrasco f. (2005). a comparative study of four schemes for groundwater vulnerability mapping in a diffuse flow carbonate aquifer under mediterranean climatic conditions. env geol 47:586–595. geociencias dic 2010-color.vp reservoir characterization by multiattribute analysis: the orito field case jairo g. guerrero1,2, carlos a. vargas2 and luis a. montes2 1 ecopetrol ltda. bogotá, colombia. e-mail:guerreguimo@yahoo.com 2 universidad nacional de colombia. dpto. de geociencias. e-mail: lamontesv@unal.edu.co, cavargasj@unal.edu.com abstract in order to characterize the caballos formation reservoir in the orito field in the putumayo basin colombia, a multiattribute analysis was applied to a 50 km2 seismic volume along with 16 boreholes. some properties of the reservoir were reliably estimated and very accurate when compared with well data. the porosity, permeability and volume of shale were calculated in the seismic volume by at least second order multivariate polynomial. a good correlation between porosity and acoustic impedance was observed by means of crossplot analysis performed on properties measured and estimated in cores or borehole logs as well as on properties calculated in the seismic volume. the estimated property values were well behaved according to the rocks physics analysis. with the property maps generated and the geological environments of the reservoir a new interpretation of the caballos formation was established. high correlation coefficients and low estimated errors point out competence to calculate these three reservoir properties in places far from the influence of the wells. the multiple equation system was established through weighted hierarchical grouping of attributes and their coefficients calculated applying the inverse generalized matrix method. resumen el análisis de múltiples atributos sísmicos fue usado para caracterizar el yacimiento a nivel de la formación caballos en el campo orito ubicado en la cuenca del putumayo colombia, para ello se usaron 50 km2 de sísmica y 16 pozos. las propiedades del yacimiento fueron confiablemente estimadas y la validación con registros de pozo y datos de corazones indicó una apreciable exactitud. la porosidad, permeabilidad y volumen de lutitas fueron estimadas en los datos sísmicos a través de la aplicación de ecuaciones polinómicas multivariadas de segundo orden. una alta correlación entre la porosidad e impedancia acústica fue estimada tanto en propiedades calculadas por múltiples atributos como en las calculadas mediante registros de pozo y corazones. los resultados fueron consistentes con los establecidos mediante el análisis de física de rocas. la relación entre los mapas de propiedades y los estudios geológicos disponibles del yacimiento hicieron posible interpretar y caracterizar el yacimiento. los resultados obtenidos muestran una importante capacidad para calcular propiedades del yacimiento en áreas fuera de la influencia de los pozos. las ecuaciones polinómicas usadas para los cálculos fueron establecidas a través del agrupamiento jerárquico ponderado de atributos y los coeficientes fueron estimados usando la matriz inversa generalizada. introduction the orito field in the putumayo basin, one of the more developed and well known reservoirs in colombia, is located at the southwest of colombia near the ecuadorian border (figure 1). the orito field is located on an asymmetrical anticline (north dome) limited by orito fault at east, and at the north with a system of inverse faults oriented in ne-sw. the caballos formation is part of the sandstone belt deposited during the aptian –albian over a triassic-jurassic eroded surface in an extent littoral system. in 2001 the colombian state petroleum co. ecopetrol, acquired a 3d volume of seismic data over an area of 50 km2 to provide high quality structural images. the field has 26 production wells located in this zone but only 16 drilled the caballos formation. due to the absence of outcrops of the cretaceous unit in the basin and the access restrictions to the zone, the geological information is mainly provided by ecopetrol´s internal reports. the more recent researches have established a geological model and also identified the depositional environment due to the absence of outcrops of the cretaceous unit in the basin and the access restrictions to the zone, the geological information is mainly provided by earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 173-180research groupin geophysics universidad nacional de colombia keywords: reservoir characterization, seismic attributes, seismic inversion, caballos formation, orito field. palabras clave: caracterización de reservorios, atributos sísmicos, inversión sísmica, formación caballos, campo orito. record manuscript received: 20/04/2010 accepted for publication: 15/11/2010 ecopetrol´s internal reports. the more recent researches have established a geological model and also identified the depositional environment (amaya, 1996; amaya and centanaro, 1997). according to them, the caballos formation represents the oldest cretaceous unit deposited immediately above the triassic-jurassic surface and is a retro gradational sequence deposited in an estuarine environment dominated by tides (figure 2) and it is constituted predominant by medium to fine sandstone with local coarse grained and grey mudstone interbedded with organic matter, glauconite and pyrite. by years, the main goal of seismic information has been to provide time depth structural interpretation, discarding its capacity to predict rock properties although is widely accepted that 3d seismic data provides more information about the reservoir than borehole data, but with less vertical 174 jairo g. guerrero, carlos a. vargas and luis a. montes figure 1. the orito field is at sw of colombia in the putumayo basin, near de ecuadorian border. at left the 3d survey with the 16 hole positions. figure 2. at left the generalized stratigraphic column of the putumayo basin. at right the facies of the caballos formation. resolution. the basic assumption for extracting seismic attributes from seismic information is a good quality data (tanner et al., 1979); however in colombia just few studies have been published using the approach of seismic attributes involving multiattribute analysis (gomez et al., 2005). the attributes are extracted from basic seismic parameters (time, amplitude and frequency, etc) with widely available tools to quantify and analyze geologically this information (brown, 1996). every attribute has a particular usefulness to give information about reservoir and to predict rock´s properties. this paper shows the advantage of using seismic information to predict reservoir properties by means of multiattribute analysis instead of the traditional use of wells and single seismic attribute analysis. seismic attributes discriminate wave characteristics related to rock and fluid properties, as well as give them more validity according the area of influence of the wells. however it is necessary to corroborate the behavior of the calculated parameters according to its geological and physical properties to support the interpretation. geophysical data the seismic volume was acquired on a rectangular area gridded in 25x25m2 bin size with 10 km inline and 5 km xline geometry survey, 2 ms sample rate and 4 seconds record length. after data processing, a post stack migrated volume was provided which depicts strong and well defined reflectors with frequency content ranging from 20 to 60 hz. sixteen wells are crossing the caballos formation whose thickness varies from 200 to 300 feet along the field. each well possess a set of well logs including gamma ray, sp, density, neutron, sonic, resistivity and caliper used to calculate petrophysical and physical properties like density, porosity, permeability, shale volume, p-wave velocity, acoustic impedance and to define lithology (figure 3). the presence of washouts in wells was taken into account to evaluate the confidence of the calculated values. attribute analysis there are several attribute classification, according to their use, dynamic/kinematics features and reservoir properties (chen and sydney, 1997), wave geometry, basic seismic characteristics (browm, 1996), and others criteria. the dynamic/kinematic traces parameters and organized attributes are related to reservoir characteristics, identifying relationships between seismic attributes and petro physical properties. the single attribute analysis (sa) uses a linear or no-linear relationship between a seismic attribute and a property using a single trace parameter. the multiattribute analysis (ma) term includes all geo statistical methods that use more than one attribute to estimate reservoir properties. there are three types of ma analysis methods: the co kriging technique that uses several attribute to predict a property, the neural networks which combine attributes by means of learning and training methods, and finally the covariance matrix that makes predictions using a weighted sum of incoming attributes (rusell, 1997). these statistical approximations use polynomial relationships that match seismic attributes calculated and measured rocks properties in wells and may identify the set of seismic attributes which forecast the properties. the simplest case is a linear relationship involving only one attribute: p w w ao j j� � (1) wj is a weighted coefficient, defines the lowest value of the attribute, aj is an attribute and p is the estimated property. assuming a linear relationship in a ma case, any physical or petro physical property is estimated as a linear weighted sum: p w w ao j jj n � � �� 1 (2). reservoir characterization by multiattribute analysis: the orito field case 175 figure 3. log of the orito 5 well, with gamma ray (grcab), sonic (dtcab), calculated p wave velocity (vp), calculated porosity (por_dt), calculated permeability (kh_log), calculated shale volume gr (vsh_gr) y caliper (calidt). where wj are weighted coefficients, aj the related attributes and p the estimated property. although complex relationships may be represented by different order equations, order higher that 2 are rare and without physical meaning, as: p w w aoo kj j k j n k n � � �� �� 11 (3) k is the order of the polynomial, wkj the weighted factor, a j k the seismic attribute and p the property. to establish wjk the generalized inverse matrix was used: � �w a a a pjk t t� �1 (4). the a matrix contains the attributes values inside the seismic volume, p represents the property values measured in wells, at and a-1 are respectively the transpose and inverse matrix of a. the term [ata] is the covariance matrix, which is a powerful statistical tool for multivariate data analysis, used in wiener-levinson deconvolution and others applications (rusell et al., 1997). the selection of attributes to be used as key predictors was done through hierarchical sorting of attributes and using correlation coefficient r and estimated error � to measure similarity. procedure the migrated volume was tied to the wells and dominant frequency and vertical resolution were calculated to interpret the seismic volume. porosity, permeability, shale volume, p-wave velocities and acoustic impedance were estimated first in the well using the borehole logs gamma ray, sp, sonic, neutron-density, resistivity and caliper. to be more confident, porosity and permeability in available core samples were measured and used to calibrate these properties in the well. in the research five properties were calculated in the well and forty two attributes were considered and generated in the seismic volume to know the more robust to predict properties in the caballos formation. physical and petrophysical information in the well and attributes in the seismic volume were used to estimate petrophysical and physical properties according to rock physics. the ma analysis applied hierarchical grouping using highest correlation coefficients as criterion of similarity. the attributes were clustered according to correlation coefficients provided by the sa analysis between the attributes and the rock properties. the first selected attribute exhibits the highest single correlation coefficient, the second the higher of the remaining, then the next high value and so on, increasing the correlation coefficient and establishing the best relationship. to check the predictive confidence of the technique, two wells’s information was not fed to define the polynomial relationships and instead their values were predicted by the method (o-15 and o-24 wells), as observed in table 1. to estimate porosity by the ma, in the clustering procedure the following attributes were included: first average frequency zero crossing followed by average trough amplitude, then peak spectral frequency and finally correlation length. the second order polynomial relationship established between the porosity and the four mentioned attributes a correlation coefficient r = 0.995 and an error � = 0.00079 were achieved. the acoustic impedance was related by a second order polynomial with the attributes average zero crossing, slope of reflection strength, dominant frequency rating (series), average signal to noise ratio and average instantaneous phase, provided an r = 0.998 and an � � � �� � with an r = 0.990 and � = 0.00137, the permeability was set by a third order polynomial related with dominant frequency – f2, bandwidth rating, variance amplitude, average trough amplitude and number of troughs. finally, a second order polynomial relates shale volume with correlation window time shift to next cdp attribute, average zero crossing, slope of instantaneous frequency and maximum trough amplitude, with r = 0.989 and � = 0.00737. for a wide and precise descriptions and application of attributes the references brown (1996), pal (2001) and chen et al. (1997) are suggested. 176 jairo g. guerrero, carlos a. vargas and luis a. montes table 1. accuracy achieved by ma and sa compared with the value measured in the well. well measured sa ma 0-5 0,097 0,101 0,097 0-6 0,107 0,112 0,107 0-10 0,069 0,083 0,069 0-14 0,089 0,104 0,089 0-15 0,087 0,966 0,087 0-21 0,105 0,108 0,105 0-24 0,084 0,083 0,084 0-34 0,075 0,079 0,075 0-35 0,081 0,080 0,081 0-36 0,103 0,109 0,103 0-37 0,065 0,090 0,064 0-38 0,086 0,091 0,085 0-39 0,089 0,094 0,089 0-40 0,095 0,080 0,094 0-43 0,102 0,083 0,098 table 2. correlation coefficients and errors in the calculus of the attribute. no attributes attributes c. correlation error a1 average zero crossing 0,79834 0,00537551 a2 avarage trough amplitude 0,93521 0,003047 a3 peak spectral frequency 0,97097 0,00203953 a4 correlation length 0,99565 0,000789 reservoir characterization by multiattribute analysis: the orito field case 177 figure 4. the same trend is present in core and well logs analysis (left) and in predicted seismic attributes (right), where the samples were discriminated according to environment previously identified in wells. figure 5. the same trend is present attributes established by cores and well logs analysis (left) and in seismic attributes predicted in the volume(right). figure 6. net to gross map obtained from well information with an approximately ns body depicted at left, inside the marked square (modified from amaya, 1996). to evaluate the validity of results provided by the sa and the ma methods, crossplot analysis were done. the first one relates porosity with acoustic impedance (figure 4) and the second one relates porosity with shale content (figure 5). the first analysis was done in attribute dataset calculated inside the volume: (figure 4a) and also in properties calculated with borehole data (figure 4b), with a similar trends observed in both. the second analysis threw similar trend in attributes measured in wells and in attributes estimated in the volume (figures 5a and 5b). finally the properties predicted by ma method inside the caballos formation were mapped for a new geological interpretation in accord with the identified geological environments. using the information provided by the wells in the area, a net sand map for the caballos formation was generated and showed in figure 6. the net sand distribution confirmed the presence of a body that was deposited in a paleo valley with an approximately ns main direction and the presence of isolated bodies that run almost parallel to the main body, as was interpreted in the facies map before amaya (1996). he identified four events in the caballos formation deposition, the lower unit consisting of fluvial deposits with lower tidal influence (see at right of figure 6), which grades toward estuarine deposits formed in tidal channels and tidal flats overlain by tidal channel deposits, which are eventually were eroded and overlying by deposits of mouth bar. results and discussion although many properties were obtained using sa and ma, just porosity, impedance and shale volume were considered in this paper, because they show the most outstanding results obtained in the project. the property calculations started with the attribute that owns the smallest error and the highest correlation coefficient and then more attributes were added until the highest correlation coefficient as r=0.995 and lowest error were achieved, see table 1. the properties predicted in the caballos formation volume supplied by ma and sa were compared against values established in the well, see table 2. the ma method always provided values closest to true property values (porosity, acoustic impedance, etc), included those in o-24 and o-15 wells which were not involved as input data, whereas the values achieved with sa drastically depart from the real values. the ma was applied to the caballos formation volume to estimate porosity, acoustic impedance and shale content were. to verify that the predicted value trend are agree with that of data observed in wells, the estimated porosity was crossploted against estimated acoustic impedance. in figures 4a and 4b a similar trend in predicted and measured in well data is observed assuring a reliable output, although the predicted porosity, acoustic impedance and shale volume values were calculated by polynomials independent of borehole data. according to petrophysical and physical considerations the observed results were interpreted so, highest porosities and smallest impedances are related to material with high sand content as found deposited in channel environment and lowest porosities and highest impedances are associated to materials with high content of shale found in a tidal flat marsh. two maps of porosity distribution in the caballos formation were built, a first one generated by the sa method in figure 8a and a second one by the ma method in figure 8b. comparing these two maps a higher contrast in porosity is clearly visible in the figure 8b, providing a better discrimination of porosity distribution in the formation. a map of acoustic impedance generated by the ma is seen in figure 9. a shale content map (figure 8) was generated with multiattribute analysis, consistent with the net sand map (figure 6) where most of the low values of shale volume (yellow and red) coincide with the main body trend. however, there are outliers to the east side of the area which are not easy to explain except due to the possible presence of another channel along the orito fault. finally through figures 6,7,8b and 9, a geological interpretation depicted in figure 8a was done. it is possible there to identify the environments in the caballos formation, including fluvial channels in the lower part (unit u1), tidal channels (units u2 and u3), tidal flat marsh and crevasse splays, with the possible presence of channel bars. the interpretation map of figure 8a is geologically consistent with a previous interpretation map done by amaya (1997) in figure 8b, but a better discrimination of the channel and the other facies are visible in figure 8a. 178 jairo g. guerrero, carlos a. vargas and luis a. montes figure 7. porosity maps for the caballos formation, on the left was calculated using the sa and on the right through the ma. the patterns and trends are clearer in the ma map. it allowed to discern different environments in the caballos formation according with facies described in figure 2, from fluvial channels in the lower part of the formation (unit u1), tidal channels in the intermediate units (u2 and u3), tidal flat marsh and crevasse splay with the possible presence of channel bars. the production wells were plotted on the interpretation map as seen in the figure 9, the bubble sizes indicates the accumulated production in each well. it is noticeable that the wells with higher production are associated to main fluvial channel deposits (u1), in a second place the wells in tidal channels of u2 and u3 units whereas the lowest productions are from wells in deposits of tidal flat marsh. note the similarity in the bodie trend and the arrangement of the identified facies. conclusions single and multiattribute analyses were applied to the caballos formation using 3d seismic information of the orito field, putumayo basin colombia. comparative analysis indicated that ma estimates property values in the reservoir more confident than sa approach. the inverse relationships porosity impedance and porosity – shale content observed in predicted in volume dataset were consistent with similar trend noted in estimated in wells dataset ensuring reliable predictive behavior and geological validness. the sa analysis discriminates samples where the highest porosities with smallest impedances characterizes high sand content such as channel and where the lowest porosities with highest impedances characterizes high content of shale such as tidal flat marsh. moreover, maps of properties estimated by the ma method depicted higher contrast than those provided by the sa, indicating a higher content of information. the equations relating the attributes depends on data and geological characteristic, so in areas with different facies the deduced equation certainly are not applicable, being necessary to establish a new polynomial, besides that the success of the anterior approach depends on quality of data and appropriate attribute selection. the porosity and acoustic impedance maps permitted to create an interpretation map, identifying in the caballos formation, fluvial channels in the lower part of the formation (unit u1), tidal channels in the intermediate units (u2 and u3), tidal flat marsh and crevasse splay with the possible presence of channel bars. these environments were previously identified in well logs and available cores. reservoir characterization by multiattribute analysis: the orito field case 179 figure 8. shale content map generated by mutiattribute analysis. figure 9 .the acoustic impedance map shows a pattern of contrasts, better than the map obtained by simple attribute. finally, plotting production wells on the interpretation map enhanced the fact that higher production are associated to main fluvial channel deposits, medium production wells to tidal channels whereas the lowest productions wells are in related to tidal flat marsh. acknowledgments the authors would like to thank colombian company of petroleum ecopetrol, and also to the national university of colombia for the support provided to carry out this research. references amaya c., 1996. archetecture of estuarine reservoir of cretaceous caballos formation, orito field, putumayo basin, colombia. thesis master of arts. the university of texas-austin. taner, m.t., koehler f. and sheriff r. 1979. complex seismic trace analysis. geophysics: 44, 1041-1063. brown, a.r. 1996. seismic attributes and their classification. the leading edge: 10, 1090. chen, q. and sydney s. 1997. seismic attributes technology for reservoir forecasting and monitoring. the leading edge: 16, 445-456. pal, 2001. poststack family reference manual, landmark graphics, september, p. 200 –332 russell b., hampson d., schuelke j. and quirein j. 1997. multiattribute seismic analysis. the leading edge: 10, 1439-1443. gomez f., castagna j. 2005. lithology and fluid determination for acae area, puerto colom oil field colombia. ct&f. yilmaz o. 1992. seismic data processing, seg. 180 jairo g. guerrero, carlos a. vargas and luis a. montes figure 10. facies’s distribution interpreted from the ma’s map (on the left author, 2004) and its comparison with the facies’s distribution generated from wells information (on the right, modified from amaya, 1997). figure 11. the higher production is associated to main fluvial channel, medium to tidal channels and lowest to tidal flat marsh. geociencias 15-1 julio 2011.vp analysis and comparison model for measuring tropospheric scintillation intensity for ku-band frequency in malaysia js mandeep, rm zali department of electrical, electronic & systems engineering ,faculty of engineering & built environment, universiti kebangsaan malaysia, 43600, ukm bangi,selangor darul ehsan,malaysia email: mandeep@eng.ukm.my, reeza79@yahoo.com abstract this study has been based on understanding local propagation signal data distribution characteristics and identifying and predicting the overall impact of significant attenuating factors regarding the propagation path such as impaired propagation for a signal being transmitted. predicting propagation impairment is important for accurate link budgeting, thereby leading to better communication network system designation. this study has thus used sample data for one year concerning beacon satellite operation in malaysia from april 2008 to april 2009. data concerning 12ghz frequency (ku-band) and 40° elevation angle was collected and analysed, obtaining average signal amplitude value, ÷ and also standard deviation ó which is normally measured in db to obtain long-term scintillation intensity distribution. this analysis showed that scintillation intensity distribution followed gaussian distribution for long-term data distribution. a prediction model was then selected based on the above; karasawa, itu-r, van de kamp and otung models were compared to obtain the best prediction model performance for selected data regarding specific meteorological conditions. this study showed that the karasawa model had the best performance for predicting scintillation intensity for the selected data. resumen este estudio se basa en la comprensión de las características y distribución de los datos de la señal de propagación local, identificar y predecir el impacto general de los factores atenuantes más significativos relacionados con la trayectoria de propagación, tal como el deterioro de una señal propagada durante su transmisión. la predicción del deterioro en la propagación es importante en la exactitud del enlace presupuesto, permitiendo mejorar la red de comunicación del sistema diseñado. este estudio utilizo una muestra de datos de un año del funcionamiento del satélite beacon en malasia desde abril 2008 a abril 2009. los datos se refieren a una frecuencia de 12 ghz (band ku) y un ángulo de elevación de 40°, recogidos y analizados, y entonces obteniendo un valor promedio de amplitud de señal, ÷ y una desviación estándar que normalmente se mide en db para obtener a largo plazo una distribución de la intensidad de centelleo. este análisis mostró que la distribución de la intensidad de centelleo corresponde a una distribución gaussiana para datos de distribución a largo plazo. con base a lo anterior se selecciono un modelo de predicción; los modelos de karasawa, itu-r, van de kamp and otung fueron comparados para obtener el mejor modelo de predicción para los datos seleccionados para condiciones meteorológicas específicas. este estudio mostró que el modelo karasawa tuvo el mejor desempeño para predecir la intensidad de centelleo para los datos seleccionados. introduction radio-wave propagation through the earth’s atmosphere has a major impact on system design; several propagation effects increase in importance when comparing lower frequency bands, having a high degree of accuracy and comprehensiveness concerning their prediction (agunlejika, et al., 2007). propagation impairment regarding satellite communication links, especially in the ku band and signal level fluctuation caused by attenuation due to rain and tropospheric scintillation, must be is carefully considered to ensure accurate link budgeting. tropospheric scintillation concerns rapid signal amplitude and phase fluctuation throughout a satellite link. it is caused by irregularities and earth sciences research journal earth sci. res. s j. vol. 15, no. 1 (july, 2011): 13-17research groupin geophysics universidad nacional de colombia keywords: tropospheric scintillation, ku-band, satellite communication, atmospheric attenuation. palabras clave: centelleo troposférico, band ku, comunicación satelital, atenuación atmosférica. record manuscript received: 25/01/2011 accepted for publication: 28/05/2011 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees turbulence in the first few kilometres above the ground, thereby affecting atmospheric refractive index measurement (mandeep et al., 2006). a link for propagation through the troposphere consists of combining random absorption and scattering from a continuum of signals along a path causing random amplitude and random scintillation in the waveform being received. scintillation effect varies as time elapses and is dependent upon frequency, elevation angle and weather conditions, especially dense cloud. the greatest effect caused by tropospheric scintillation is signal fading, thereby acting as a limiting factor on system performance (akhondi and ghorbani, 2005). this is why accurate prediction is important when evaluating a link budget, especially in highly tropospheric scintillation conditions. scintillation occurs continuously, regardless of whether the sky is clear or rainy. when it is raining, signal level fluctuation (known as scintillation) can change together with rain attenuation affecting signal level. signal log-amplitude level will rise dramatically and such extreme level data should be carefully eliminated (mandeep et al, 2006). data analysis the measurement of data collected from a beacon satellite having 12 ghz frequency, 2.4m antenna diameter and 40° elevation angle were obtained by monitoring and collecting data from april 2008 to april 2009. disanayake et al., (2002) have mentioned that most available beacon data has been analysed regarding clear sky conditions and this essentially removes the bulk of low-attenuation-producing phenomena. table 1 gives measurement site specifications. signal attenuation due to rain is the most remarkable signal propagation effect in ku-band frequency and this kind of loss due to the above can be greater than 15 db over a short period of time (otung, 1996). all data which has become changed due to attenuation caused by rain is eliminated. table 1. satellite specifications ground station location 5.170n, 100.40e beacon frequency 12.255 ghz elevation angle 40.10 polarisation horizontal antenna configuration offset parabolic antenna diameter 2.4m satellite position 1440e antenna height 57m above sea level considering a clear sky (with or without rain), all data having a spike regarding extreme amplitude values due to rain attenuation has been removed by comparing it to rain gauge data values. visual inspection was needed and performed for all data sequences to eliminate spurious and invalid data (garcia, 2008). full attention must be paid during inspection to ensure obtaining accurate result from studies. scintillation variance values can be best described for scintillation intensity in the present study and have been calculated as the standard deviation of signal amplitude given in decibels (db). comparison prediction model four prediction models were selected for this study: karasawa (karasawa et al., 2002), itu-r (2009), van de kamp (van de kamp et al., 1999) otung (otung, 1996). the model so selected depended on its correlation with wet refractivity index value, and meteorological conditions, i.e. relative humidity (rh) and temperature, t (°c), these being suitable with scintillation data for a satellite beacon (van de kamp, 1998). prediction model comparison was based on signal fading and enhancement. the chosen model was also able to predict long-term distribution propagation signals. the karasawa model karasawa has presented a prediction model for signal standard deviation regarding scintillation intensity as follows; pre n af g d for � � � � � 0 45 1 3 5 . . ( ) sin (1) where n is normalised intensity, f is frequency in ghz, is elevation angle and g(da) is antenna aperture averaging factor as given by: g d d l for d l d l fora a a a( ) . . . . . .� � � � 1 0 1 4 2 0 2 0 5 0 5 0 4 2 0 � � � 5 2 1 0 0 1 1 0 2 � � � � � � � � � � � � d l for d l a a � � . . . (2) where � is wavelength in m, is effective antenna diameter and l is the distance of the turbulent part of the path and can be determined as follows: l h h a e � � � 2 22sin sin (3) concerning equation (1), karasawa obtained the following expression for scintillation enhancement: y p p ppred � � 0 06 0 08 1 25 2 6 10 3 10 2 10 . (log ) . (log ) . log . 7 0 01 50 � � �� � � �� � �for p. (4) signal fading can be expressed as: y p p ppred � � � 0 061 0 072 1 71 3 10 3 10 2 10 . (log ) . (log ) . log .0 � � �� � � �� (5) the itu-r model the long-term tropospheric scintillation prediction model proposed by the international telecommunication union-radiocommunication sector (itu-r) was used for calculating the standard deviation of signal fluctuation 14 js mandeep, rm zali agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:10 p p p composite 133 lpi at 45 degrees due to scintillation. this model uses the wet term of earth refractivity wet í, regarding relative humidity and temperature, averaged at least once a month as input (agunlejika et al., 2007). this model is applicable for frequencies ranging from 7ghz to 20 ghz and 4° to 32° elevation angles. the following equation can be used for the itu-r prediction model; � ref f g x db7 12 1 2/ .[ ( ) (sin ) ] (6) where, = standard deviation (db) ref = reference standard deviation (db) g(x) = antenna averaging factor and, ref wetn db� � � � 3 6 10 103 4. ( ) (7) n e t wet � �3 732 10 5 2 . (8) referring to equation 6, scintillation fading can be calculated from the following equation for 0.01 � p � 50. no prediction model has been recommended by the itu-r for scintillation enhancement. y p p p � � � � � 0 061 0 072 1 71 3 0 10 3 10 2 10 . (log ) . (log ) . log . �� � � �� (9) the van de kamp model the van de kamp prediction model represents a slight modification from the itu-r model. scintillation standard deviation for long-term distribution can be estimated from the equation given below; x n f g x � 0 45 1 3 . . ( ) (sin ) (10) the percentage of time for scintillation intensity can be identified from the above equation, as in equations 11 and 12. a p p p p1 3 20 0515 0 206 181 2 81( ) . (log ) . (log ) . log .� � � (11) a p p p2 20 172 0 454 0 274( ) . (log ) . log .� � (12) signal fading and enhancement can be determined as follows: a a p a pp x x� �1 2 2( ) ( ) (13) e a p a pp x x� �1 2 2( ) ( ) (14) the otung model this model is similar to the itu-r model, except for elevation angle dependent value which is sin 11 12 and this is shown as equation 15; x xref f g d � � � 7 12 11 12 ( ) sin ( ) (15) hence, fading and enhancement for signal level can be determined by using this equation: a p p p px( ) . exp . [ . . ] ln ( )� � � � � � � � � � �3 6 0 00095 0 4 0 002 (16) � �e p p p px( ) . exp . [ . . ] ln ( )� �3 17 0 00095 0 272 0 004 (17) the analysis and comparison model figure 1 shows monthly cumulative distribution for scintillation variance considering average standard deviation of scintillation intensity over a one-month time period. such variance was determined by considering clear sky conditions without rain. percentage time value was lower than scintillation variance value for april 2008 and that for april 2009 was slightly higher than for the other month. figure 2 shows that average monthly scintillation distribution followed gamma distribution for long-term distribution data collection. analysis and comparison model for measuring tropospheric scintillation intensity for ku-band frequency in malaysia 15 figure 1. monthly cumulative distribution for scintillation variance figure 2. average scintillation distribution agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:11 p p p composite 133 lpi at 45 degrees long-term distribution data should be analysed more than once a month while only a few minutes are needed for short-term data analysis. figure 2 gives values regarding negative state for signal level enhancement while correct or positive state is for signal level fading. it obviously shows that variation in variance scintillation value for fading and enhancement was not equally likely. signal fading had a long tail compared to enhancement and the shape was not symmetrical, as has been mentioned by van de kamp (1998). fading and enhancement represent two types of scintillation signal level. both have their own use and functionalities which can have a large effect on the propagation of a signal being transmitted through the atmosphere. when propagation signals are affected by rain, especially during the raining season, fading value will suffer a drastic change due to changes in signal amplitude. however, the enhancement value is not affected by rain or can become negligible. figure 3 represents cumulative distribution for signal level fading and enhancement. variance distribution for fading was slightly higher when comparing enhancement value for the lower percentage of time. such cumulative distribution was for a local data study with specific meteorological conditions due to geographical conditions. 26°c temperature and 76% humidity were used in the present study. prediction model selection was based on their relationship to meteorological conditions. comparing these four models showed that the karasawa model was the best model for predicting scintillation data intensity, as shown in figure 4 for scintillation signal fading (~26°c temperature (t) and 76% relative humidity, (rh)). figure 4 shows that the karasawa model gave good prediction, having 0.007db minimum signal variance, 1.8% of this referring to the measured data. the karasawa was thus a suitable model for predicting local data regarding scintillation intensity for signal fading compared to the other models while the otung model did not perform well in predicting scintillation data (0.12db and 35% from measured data as reference). however, only three models performed well regarding signal enhancement, as shown in figure 5. this was because no prediction model has been proposed by the itu-r for signal enhancement (itu-r, 2007); only the karasawa, van de kamp and otung models will thus be compared. figure 5 shows signal enhancement, at ~26°c and 76% humidity value. this comparison obviously showed that the karasawa model also performed well for predicting signal level enhancement regarding scintillation data intensity. a small difference regarding variance value with 0.0052db and 2.6% as reference compared to the other models. the otung model was the worst model (0.0414db and 20.96% referencevalues). conclusions tropospheric scintillation predication models have been reviewed and evaluated, including models for predicting signal log-amplitude cumulative distribution and models for predicting scintillation intensity. this tropospheric scintillation intensity study responded to the requirement for better understanding of propagation impairment in satellite communication systems. better understanding can produce better system design. this study thus concluded that the karasawa prediction model can be best used for predicting overall propagation impairment regarding scintillation on the malaysian propagation path. 16 js mandeep, rm zali figure 3. cumulative distribution of scintillation signal for fading and enhancement figure 4. comparison model for cumulative signal fading figure 5. comparison model for cumulative signal enhancement agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:11 p p p composite 133 lpi at 45 degrees acknowledgement the author would like to acknowledge the universiti kebangsaan malaysia, universiti sains malaysia, mosti grant science fund (01-01-92-sf0670), ukm-ggpm-ict-108-2010, the association of radio industry business (arib) of japan for providing the instruments used for collecting the data and the research university postgraduate research grant scheme (usm-ru-pgrs). references agunlejika, o., t.i. raji and o.a. adeleke (2007). “tropospheric scintillation prediction for some selected cities in nigeria’s tropical climate”, international journal of basic and applied science, 9, 10, 12-19. akhondi, m. and a. ghorban (2005). “long-term cumulative distribution modeling of tropospheric scintillation for the earth-satellite links in the 40/50 ghz band”, 4th wseas international conference on electrical, hardware., wirel. and optical commmunication., 1, wisconsin, usa, 56-59 . dissanayake a., j. allnutt and f. haidara (2002). “a prediction model that combines rain attenuation and other propagation impairments along earth-satellite paths”, ieee transaction on antenna and propagation, 45, 10, 1546-1558. itu-r recommendation p.618-9 (2009). “propagation data and prediction methods required for the design of earth-space telecommunication systems”, karasawa,y., m. yamada and j. allnutt (2002) “a new prediction method for tropospheric scintillation on earth-space paths” ieee transaction on antenna and propagation, 36, 11, 1608 – 1614. mandeep, s. j. s., i.s.h syed,, i. kiyoshi, t. kenji and i. mitsuyoshi (2006). “analysis of tropospheric scintillation intensity on earth to space in malaysia”, american journal of applied science, 3, 9, 2029-2032. otung, i. e. (1996). “prediction of tropospheric amplitude scintillation on a satellite link”. ieee transaction on antenna and propagation 44, 9, pp 1600–1608. van de kamp, m.m.j.l, j.k tervonen, e.t. salonen, and j.p.v.p baptista,. ‘improved models for long-term prediction of tropospheric scintillation on slant paths’. ieee transaction on antenna and propagation , 47, 12, 249–260. van de kamp, m.m.j.l. (1998) “asymmetric signal level distribution due to tropospheric scintillation”, electronic. letters, 34, 17, 1145 – 1146. analysis and comparison model for measuring tropospheric scintillation intensity for ku-band frequency in malaysia 17 agosto 25-portada geociencias-15-1 2011.prn d:\geociencias julio 2011\geociencias 15-1 julio 2011.vp jueves, 25 de agosto de 2011 11:28:11 p p p composite 133 lpi at 45 degrees earth sci. res. j. vol. 11, no. 2 (december 2007): 131-138 clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano mauricio orozco-alzate, and césar germán castellanos-domínguez universidad nacional de colombia sede manizales, grupo de control y procesamiento digital de señales, campus la nubia, km 7 vía al magdalena, manizales, colombia. corresponding author: mauricio orozco-alzate, email: morozcoa@unal.edu.co abstract classification of seismic signals at colombian volcanoes has been carried out manually by visual inspection. in order to reduce the workload for the seismic analysts and to turn classification reliable and objective, the use of supervised learning algorithms has been explored; particularly classifiers built in dissimilarity spaces. nonetheless, the performance of such learning methods is subject to the availability of a representative and a priori well classified training sets. to detect mislabeled events, the use of clustering techniques on the dissimilarity representations is proposed. our experiments, performed on re-analyzed seismic signals, show a significant improvement respect to recognition accuracies for the original data sets. key words: clustering, dissimilarity, mislabeling, seismic signals. resumen la clasificación de las señales sísmicas en los volcanes de colombia ha sido llevada a cabo manualmente mediante inspección visual. con el fin de reducir la carga de trabajo de los analistas y para tornar la clasificación confiable y objetiva, se ha explorado el uso de algoritmos de aprendizaje supervisado; particularmente, clasificadores construidos en espacios de disimilitud. no obstante, el desempeño de dichos métodos de aprendizaje está sujeto a la disponibilidad de un conjunto de entrenamiento representativo y, a priori, bien clasificado. para detectar eventos mal clasificados, se propone el uso de técnicas de agrupamiento sobre las representaciones de disimilitud. los experimentos, realizados sobre las señales sísmicas verificadas, muestran una mejora significativa respecto a las tasas de reconocimiento para los datos originales. palabras claves: agrupamiento, disimilitud, etiquetado incorrecto, señales sísmicas. 131 earth sciences research journal manuscript received september 9 2007. accepted for publication november 30 2007. introduction in many applications of pattern recognition, it is extremely difficult or expensive, or even impossible, to reliably label a training sample with its true category (jain et al., 2000). particularly, in automatic classification of seismic-volcanic signals, night and rotating shift work schedules, tedious evaluations, and changes of personnel turn the task of recognition by visual inspection susceptible to human errors. besides, analysts often engage in differences of opinion about interpretations of dubitable signals. in order to reduce the workload for the seismic analyst and the risks associated to subjective judgments, a number of supervised classification methods have been used (scarpetta et al., 2005; langer et al., 2006; orozco-alzate et al., 2006a). it is supposed for those supervised classification techniques that a well-labeled data set is available. however, due to the same reasons cited above, it is highly likely that training sets include mislabeled events. in langer et al. (2006), an automatic classification of seismic events at soufrière hill volcano was carried out. in addition, a careful manual revision of the original a-priori classification was achieved by an expert not involved in the previous labeling of the data set. it was found that a considerable number of the events were erroneously attributed to other classes. as a result, a remarkable improvement in classification accuracy was obtained when the revised data set was used. the nevado del ruiz volcano is monitored by the volcanological and seismological observatory at manizales (vsom). because of the considerable amount of data, the labelling task of the recorded seismic signals is distributed among several analysts (e.g. one trainee per volcanic station). a second or third opinion is requested just in case of serious doubt. as a result, classifications performed by different experts are not available and an analysis of concordance for such a-priori labels was not conducted. in this study, a revision of the original labelled nevado del ruiz volcano (ruiz) data set is conducted. in contrast to the approach followed by langer et al. (2006), the revision by using clustering techniques was automated. several clustering algorithms on a given data set were used due to the lack of a single appropriate clustering algorithm (jain et al., 2000). therefore, experiments were conducted by using the most popular clustering approaches, which belong to two basic strategies: hierarchical and partitioning methods. in addition, the ruiz data set was arranged to consider two separated problems: the ruiz-vt,lp (two classes) and the ruizall (three classes) data sets. revised data sets were used according to our previous dissimilarity-based classification approach (orozco et al., 2006a, orozco et al., 2006b) and compared against the performances obtained with the original data sets. dissimilarity representation and classifier differences in spectral content allowed a visual discrimination of different types of volcanic earthquakes. therefore, spectra of seismic records are commonly used for classification and monitoring of seismic activity (zobin, 2003). in addition, recent studies have claimed that the dissimilaritybased classification approach is a feasible and sometimes advantageous alternative to the feature-based method (duin et al., 1998, pękalska et al., 2001, pękalska and duin, 2002, paclík and duin, 2003b, pękalska and duin, 2005). according to those facts, a dissimilarity representation for the ruiz data set can be derived as follows: (i) the power spectral density (psd) for each clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano 132 orozco and castellanos. esrj vol. 11, no. 2. december 2007 133 record is estimated via the yule-walker autoregressive method: dc bias must be removed before computing the spectra, (ii) a dissimilarity measure between normalized spectra is calculated as the area difference of the non-overlapping parts (l1-norm) between spectra, see fig. 1. figure 1. dissimilarity measure as the difference between normalized spectra. a dissimilarity matrix d(t,t) was constructed by having those pairwise measures. each entry dij of d corresponds to the dissimilarity between a pair of seismic records from the training set t. then, a proper classifier can be defined on such a dissimilarity representation, either by using the entire training set t or a representation set r⊆t. linear normal density based classifier a number of studies have showed that normal density based classifiers perform well in dissimilarity spaces (pękalska et al., 2001, pękalska and duin, 2002, paclík and duin, 2003b, paclík and duin, 2003a, pękalska et al., 2004, orozco et al., 2006a). particularly, in our previous study with the nevado del ruiz volcano data set (orozco et al., 2006b), the linear normal density based classifier (bayesnl) outperformed the nearest neighbor rule 1-nn and the quadratic normal density based classifier (bayesnq). for a two-class problem, the bayesnl classifier is given by              2 1 21 1 21 log 2 1),()),(( p p crxdrxdf t       mmmm (1) where c is the sample covariance matrix; m(1), m(2) are the mean vectors and p(1), p(2) are the class prior probabilities. if c is singular, a regularized version must be used. the following regularization is typically used with λ equals 0.01 or less (pękalska et al., 2006):    cdiagccreg   1 . (2) clustering techniques unsupervised classification refers to figure 1. dissimilarity measure as the difference between normalized spectra. situations where the objective is to construct decision boundaries based on unlabeled training data (jain et al., 2000). hierarchical and partitioning methods are the two basic strategies to find clusters. in this study, the following clustering techniques are used: single linkage (sl), average linkage (al), complete linkage (cl), k-means and k-centres. sl, al, and cl are hierarchical, whereas the latter are partitioning methods. a brief description of these approaches is given below. hierarchical clustering the most popular hierarchical techniques for clustering are the agglomerative methods. at the beginning, each object is considered as a single cluster; then, the closest two clusters are merged iteratively until a specified number of clusters is reached (pękalska and duin, 2005). let ck and cl be two clusters of the cardinalities nk and nl respectively, and let ρkl be a dissimilarity measure between them. three basic criteria for the agglomerative methods are summarized in table 1. partitioning clustering partitioning methods group the objects into k clusters, usually by using representatives or by assuming a specific geometrical structure. objects are assigned to the clusters, new representatives are estimated and the process is repeated until a stable solution is reached. two typical partitioning methods are k-means and k-centres; see table 2 for a brief description, a detailed one can be found in pękalska and duin (2005). table 2. clustering methods. experimental results volcano-tectonic (vt) earthquakes, longperiod (lp) earthquakes and icequakes (ic) are the seismic signals classes considered in this study. they are contained in the ruiz-all data set. the ruiz-vt,lp data set includes only the first two classes. signals were digitized at 100.16 hz sampling frequency by using a 12 bits analog to digital converter. 134 method � kl emphasis/comment sl  jicpcp ppd ljki ,minmin  connectedness. resulting clusters are elongated and chain-like. cl  jicpcp ppd ljki ,maxmax  compactness. it performs well when the objects form naturally distinct clouds. al     ki ljcp cp ji lk ppd nn ,1 connectedness and compactness. it performs well for naturally distinct clouds and elongated clusters. table 1. hierarchical clustering methods. clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano 135 recording stations are located near to the olleta crater and the glacier at nevado del ruiz volcanic complex. in order to explore the level of agreement/ disagreement between the labels given by the experts and the ones produced by the clustering algorithms, the number of mismatches for the entire data sets is considered. the averaged number of mismatches over 10 runs is reported in table 3. hierarchical methods report the same number of mismatches over the runs, therefore their standard deviations are zero. sl hierarchical criterion for both the ruizvt,lp and the ruiz-all problems presents a rate of disagreement considerable high; similarly, mismatches of al results for the ruiz-all problem reach 45%. in fact, even though the number of cluster is fixed, sl and al find second and third clusters of a few objects only. as a result, valid data subsets, i.e. randomly generated and including enough objects per class, are not always warranted. in consequence, al for the ruiz-all problem and sl in both cases are not considered in the subsequent classification experiments. orozco et al. (2006b) observed an asymptotic behaviour for training set sizes greater than 60 examples per class. in addition, the bayesnl provided the best overall performance, outperforming the 1-nn and the bayesnq. according to that, the experiments were conducted with the bayesnl using training sets of a fixed size of 60 objects per class. clustering was performed on the entire data sets. then, training and test sets are randomly extracted for each run. the results are shown in table 4. for comparison, the results using the original data are also presented. it is clear that performances for re-labelled data sets are much better than those for the original data. discussion and conclusion a revision of the original labelled seismic events recorded by the vsom staff provides a significant improvement in the performance of supervised dissimilarity-based classifiers such as the observed for the bayesnl classifier. the use of events labelled by clustering confirmed that labelling errors are frequent and recurrent. clustering uses a notion of proximity, judged in a numerical way. in contrast, labels assigned by experts obey to the method description k-means representatives are estimated by cluster mean vectors. the dissimilarity is the euclidean distance of an object to the cluster means. k-centres centre objects are chosen such that the maximum of the distances over all objects to the nearest centre is minimized. results depend on random initialization. table 2. clustering methods. orozco and castellanos. esrj vol. 11, no. 2. december 2007 136 clustering method ruiz-vt,lp ruiz-all sl 482 1108 cl 367 495 al 164 861 k-centres 158.2 (25.2049) 507.5 (52.2797) k-means 135.6 (0.5164) 506.6 (0.5164) total 1063 1891 table 3. averaged number of mismatches between the class labels assigned by the vsom staff and labels assigned by the clustering method over the entire data sets. clustering method ruiz-vt,lp ruiz-all sl — — cl 2.3494 (0.4045) — al 4.9646 (1.1176) 3.77 (0.76) k-means 2.7524 (0.7405) 5.6722 (0.6109) k-centres 2.8810 (0.7441) 4.6792 (0.8413) total 13.0075 (1.0354) 20.02 (0.81) table 4. classification error (in % and averaged over 25 runs) with its standard deviation (in %) for the rnlc applied to the revised data sets. clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano 137 visual resemblance between the event and a canonical waveform which analysts have learnt by reference or experience. obviously, such a method is highly subjective and supposes that differences are easily detected by visual inspection but in many cases this is not true. since the final rule used (stand-alone) was calculated from all the data, clustering methods were used on the entire data sets instead of applying them to the training sets only. al and cl offer the smallest errors for the ruiz-vt,lp and ruiz-all problems respectively. even tough, the best clustering is hierarchical in both cases; differences are not enough to claim that hierarchical methods should be preferred over the partitioning ones. nonetheless, a general conclusion can be drawn from our study: the use of a clustering method to confirm labels assigned by experts is highly beneficial for constructing reliable and accurate supervised classifiers of seismic events. acknowledgements we thank the vsom staff for providing the raw data set. references duin, r. p. w., de ridder, d., and tax, d. m. j. (1998). featureless pattern classification. kybernetika, 34, no. 4, 399–404. jain, a. k., duin, r. p. w., and mao, j. (2000). statistical pattern recognition: a review. ieee trans. pattern anal. machine intell., 22, no 1, 4–37. langer, h., falsaperla, s., powell, t., and thompson, g. (2006). automatic classification and a-posteriori analysis of seismic event identification at soufrière hills volcano, montserrat. journal of volcanology and geothermal research, 153, 1–10. orozco, m., garcía, m. e., duin, r. p. w., and castellanos, c. g. (2006a). dissimilaritybased classification of seismic volcanic signals at nevado del ruiz volcano. 2nd latin-american congress of seismology, bogotá, colombia, august, cd-rom. orozco, m., garcía, m. e., duin, r. p. w., and castellanos, c. g. (2006b). dissimilaritybased classification of seismic volcanic signals at nevado del ruiz volcano. earth sciences research journal, 10, no. 2, 57– 65. paclík, p. and duin, r. p. w. (2003a). classifying spectral data using relational representation. proceedings of the spectral imaging workshop, graz, austria, april, 3134. paclík, p. and duin, r. p. w. (2003b). dissimilarity-based classification of spectra: computational issues. real time imaging, 9, no. 4, 237–244. pękalska, e. and duin, r. p. w. (2002). dissimilarity representations allow for building good classifiers. pattern recognition lett., 23, no. 8, 943–956. pękalska, e. and duin, r. p. w. (2005). the dissimilarity representation for pattern recognition: foundations and applications. world scientific, singapore, 636pp. pękalska, e., duin, r. p. w., günter, s., and bunke, h. (2004). on not making dissimilarities euclidean. proceedings of structural and statistical pattern recognition, lisbon, portugal, august, 1143–1151. pękalska, e., duin, r. p. w., and paclík, p. (2006). prototype selection for dissimilaritybased classifiers. pattern recognition, 39, no. 2, 189–208. orozco and castellanos. esrj vol. 11, no. 2. december 2007 138 pękalska, e., paclík, p., and duin, r. p. w. (2001). a generalized kernel approach to dissimilarity based classification. j. mach. learn. res., 2, no. 2, 175–211. scarpetta, s., giudicepietro, f., ezin, e. c., petrosino, s., pezzo, e. d., martini, m., and marinaro, m. (2005). automatic classification of seismic signals at mt. vesuvius volcano, italy, using neural networks. bulletin of the seismological society of america, 95, no. 1, 185–196. zobin, v. (2003). introduction to volcanic seismology. elsevier, amsterdam, the netherlands, 302pp. clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano geociencias-vol 12-2 dic.vp earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 265-268 discussion of “clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano” by mauricio orozco-alzate, and césar germán castellanos-domínguez mehmet c. demirel1, ercan kahya2 and diego rivera3 1 ph.d. student, department of water engineering and management, university of twente, po box 217, 7500 ae enschede, the netherlands. e-mail: m.c.demirel@utwente.nl 2 civil engineering department, american university of sharjah, po box 26666, sharjah, united arab emirates (corresponding author). e-mail: ekahya@aus.edu 3 professor, department of water resources, university of concepcion, vicente méndez 595, chillán, chile. e-mail: dirivera@udec.cl the authors are to be congratulated for a systematic investigation of the accurate and non subjective classifying approach in seismic research. the authors have conducted several clustering algorithms to the seismic event records from volcanological and seismological observatory at manizales. their objective was to improve the grouping of seismic data (i.e., volcano-tectonic earthquakes, long-period earthquakes and icequakes) digitized at 100.16 hz sampling frequency. their study seems adding new approach to their previous work of langer et al. (2006) who applied different classification techniques to seismic data. the discussers have the following suggestions to improve the author’s investigation on ruiz volcano data and to be a guide for similar future studies. 1. there are five empirical steps that should be followed in the application of cluster analysis which are (i) selection of variables, (ii) selection of standardization technique (if necessary), (iii) dissimilarity metric, (iv) selection of an appropriate method, and (v) test of stability or validation (demirel 2004; everitt 1993; green et al. 1990). these steps are difficult to distinguish in the present study by orozco-alzate and castellanos-domínguez (2007). the users` experience and preferences in these steps may radically affect the resultant cluster structures. for instance, the selected distance metric was not clearly mentioned in the text. did the notation �kl refer to the correlation coefficient between entities k and l? how many stations were selected near the olleta crater and the glacier at nevado del ruiz volcanic complex? were there any scale issues in the dataset which may perturb the dissimilarity matrices? henceforth 265 nov 10-discussion.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp martes, 10 de noviembre de 2009 20:50:03 p p p composite 133 lpi at 45 degrees the span of the data and some statistical information on the data structure should be explained for easy follow of readers and to avoid the aforementioned questions. standardization priori to analysis phase is necessary when the scale differences emerged in a dataset (demirel et al. 2008; everitt 1993; gnanadesikan et al. 1995; milligan and cooper 1988). 2. the authors applied several algorithms on their data and reasoned the following statement: “the lack of a single appropriate clustering algorithm”. however most of the algorithms were already tested in the literature and the relevant shortcomings are given in many text books (bacher 2002; everitt 1993). single linkage produces chain type cluster which is not be desirable for many applications, and complete linkage may create small and compact clusters (demirel 2004; everitt 1993). on the other hand the wards method emerged to make more successive clusters with small inner variance. hence it is herein suggested to use the wards method with the squared euclidean metric to get more distinct clusters in future investigations. 3. in the context of text indications in notation wise, at page 133: the notation “dc” was not explained in the text. at page 133: d(t,t) designates to distance/dissimilarity measure; however, the notation “d” was used for the same purpose in table 1. it is important to maintain consistent use of notations for the dissimilarity measure throughout the text. 4. the mismatches in labeling were counted for the performance comparison and number of runs was given as 10. the author also mentioned that “hierarchical methods report the same number of mismatches over the runs”. it should be noted that cluster structure in the hierarchical methods do not differ in any run as the steps in dissimilarity calculations and cluster delineation has concrete algorithm; thus, it is herein encouraged that issues similar to these unclear points should be justified in the manuscript. 5. the clustering results were not given in the text. the labels of clusters and statistics (i.e., variance, mean) of each cluster should be summarized in the result section. only the averaged numbers of mismatches between class labels were presented but this was not adequate for the readers to have appropriate insights regarding the main objective of the study. since the article is about signal clustering, it would have been very illustrative to put 2 figures: one graph including 3 representative signals (e.g. langer et al., 200) and one figure representing the topological structure of the clusters, e.g. dendrogram. both figures allow analyzing in an intuitive way dissimilarities among signals. 6. at page 135: the authors mentioned that “even though the number of cluster is fixed, single linkage and average linkage find second and third clusters of a few objects only”. the single linkage and average linkage methods are in the group of unsupervised clustering techniques which has no priori knowledge on number of clusters as partitioning methods; therefore, a justification should have been indicated for that matter (demirel and kahya 2007; kahya et al. 2007). 7. as was noted by morlet et al. (1982), seismic signal does vary in amplitude, shape, frequency and phase, versus propagation time. therefore, for clustering it is necessary to analyze signal’s frequency content, as well as to localize in time changes in both, frequency and amplitude. for this task, wavelet transform is a joint time-frequency signal representation that can give the frequency content of the signal at a particular instant of time by filtering (sheikholeslami et al., 1998). it is well suited for signal whose frequencies change with time, but also for signal containing noise and transients (rouyer et al., 2008). also, its multi-resolution property can help detecting the clusters at different levels of accuracy (sheikholeslami et al., 1998). we 266 mehmet c. demirel, ercan kahya and diego rivera nov 10-discussion.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp martes, 10 de noviembre de 2009 20:50:04 p p p composite 133 lpi at 45 degrees propose for further research to apply this technique to ruiz volcano data. a good reference are kumar and foufoula-georgiou (1997) and torrence and compo (1998) for methods. indeed, arciniega-ceballos et al. (2008) applied bandpass filters before clustering in seismic data and rouyer et al., (2008) applied a wavelet-based clustering technique. references arciniega-ceballos a, chouet b, dawson ph. and g asch (2008) broadband seismic measurements of degassing activity asociated with lava effusion at popocatépetl volcano, mexico. journal of volcanology and geothermal research, 170: 12-23. bacher, j. (2002). “cluster analysis.” lecture notes, nuremberg. demirel, m. c. (2004). “cluster analysis of streamflow data over turkey,” istanbul technical university, istanbul. demirel, m. c., and kahya, e. “hydrological determination of hierarchical clustering scheme by using small experimental matrix.” 27th agu hydrology days, fort collins, colorado, 161-168. demirel, m. c., kahya, e., and rivera, d. (2008). discussion of “hydrologic regionalization of watersheds in turkey” by sabahattin isik; and vijay p. singh in asce journal of hydrologic engineering, sep 2008, vol. 13, no. 9, pp. 824-834. doi: 10.1061/(asce) 1084-0699 (2008) 13:9(824) (accepted for publication). everitt, b. (1993). cluster analysis. 3rd edn. , halsted press, division of wiley, new york. gnanadesikan, r., kettenring, j. r., and tsao, s. l. (1995). “weighting and selection of variables for cluster analysis.” journal of classification, 12(1), 113-136. green, p. e., kim, j., and carmone, f. j. (1990). “a preliminary study of optimal variable weighting in k-means clustering”. journal of classification, 7(2), 271-285. kahya, e., demirel, m. c., and piechota, t. c. “spatial grouping of annual streamflow patterns in turkey ” 27th agu hydrology days, fort collins, colorado, 169-176. kumar, p. and foufoula-georgiou, e. (1997). wavelet analysis for geophysical applications. reviews of geophysics, 35:385–412. langer, h., falsaperla, s., powell, t., and thompson, g. (2006). “automatic classification and a-posteriori analysis of seismic event identification at soufri?re hills volcano, montserrat.” journal of volcanology and geothermal research, 153(1-2), 1-10. milligan, g. w., and cooper, m. c. (1988). “a study of standardization of variables in cluster analysis.” journal of classification, 5(2), 181-204. morlet, j., arens, g., fourgeau, e. and glard, d. (1982) wave propagation and sampling theory-part i: complex signal and scattering in multilayered media. geophysics 47, 203-221. orozco-alzate, m., and castellanos-domínguez, c. g. (2007). “clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano.” earth sci. res. j., 11( 2), 131-138 rouyer t, fromentin j-m, stenseth n and b cazelles (2008) analysing multiple tiem series and extending significance testing in wavelet analysis. marine ecology progress series, 359:11-23. sheikholeslami, gh., chatterjee, s. and zhang, a. (1998) wavecluster: a multi-resolution clustering approach for very large spatial databases. in proceedings of the 24th vldb conference. torrence, c. and compo, g. (1998). a practical guide to wavelet analysis. the bulletin of the american meteorological society, 79:61–78. 267 discussion of “clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano” by mauricio orozco-alzate, and césar germán castellanos-domínguez nov 10-discussion.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp martes, 10 de noviembre de 2009 20:50:05 p p p composite 133 lpi at 45 degrees nov 10-discussion.prn d:\geociencias v-12-2-dic 2008\geociencias-vol 12-2 dic.vp martes, 10 de noviembre de 2009 20:50:05 p p p composite 133 lpi at 45 degrees a new approach on the tectonometamorphic mechanisms associated with p–t paths of the barrovian-type silgará formation at the central santander massif, colombian andes castellanos a. óscar m., ríos r. carlos a. and takasu akira 125 deep crustal structure and estimation of moho in the lhasa terrane of central tibet syed mustafizur rahman, m. rezaul islam, mumnunul keramat and sultan-ul-islam 156 geophysical prospecting of the transition zone between the congo craton and the panafrican belt in cameroon tadjou j. m., njingti-nfor, kamguia j. and manguelle-dicoum e. 169 hydrologic homogeneous regions using monthly streamflow in turkey ercan kahya, mehmet c. demirel and osman a. bég 181 intraseasonal variability of rainfall over northern south america and caribbean region j.d. pabón and j. dorado 194 mineralization potentials of pegmatites in the nasarawa area of central nigeria akintola, o. f. and adekeye, j. i. d. 213 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa carlos a. zuluaga c. and harold h. stowell 235 discussion of “clustering on dissimilarity representations for detecting mislabelled seismic signals at nevado del ruiz volcano” by mauricio orozco-alzate and césar germán castellanos-domínguez mehmet c. demirel, ercan kahya and diego rivera 265 geofisica colombiana p p 5 -1 1 0121 2974n° 5 d l c i e mb r e de 2001 bogota 0 c. iss n interval velocity determination by downward continuation of the traveltime function: paraxial ray approximation luis montes v,des profesor asociado departamento de geociencias facultad de ciencias universidad nacional de colombia. resumen present amos un rnetodo para estirnar las velocidades de intervalo, la profundidad y la geometrfa de los reflectores en modelos 3d que constan de un apilado de capas hornogeneas e isotr6picas con velocidades y densidades arbitrarias, separadas por interfaces suaves. el tiempo de transite de cualquier rayo reflejado en una interfaz particular y registrado en la vecindad de un rayo zero-offset se expresa mediante una funci6n referida a un sistema de coordenadas centrado en el rayo; tal funci6n se estima en la superficie superior del modelo. la funci6n tiempo de transite de reflexi6n asociada a cada superficie reflectora se deterrnina en la superficie superior en la vecindad del rayo central. la geometria de la superficie limitante superior de una capa particular y el tiernpo de transite estirnado sobre la misma permite calcular la velocidad de intervalo de la capa en cuesti6n y la geometria de la interfaz limitante inferior. con la velocidad de intervale y la geometrfa de las interfaces limitantes, se estima la funci6n de tiempo de transite del siguiente reflector sobre la interfaz limitante inferior. en este paso se simula el posicionamiento de las fuentes y los detectores sobre la superficie anterior de la pr6xima capa subyacente. el proceso se repite recursivamente en las capas mas profundas hasta obtener la soluci6n completa sin conocimiento previo, excepto el obtenido en las capas superiores y la funci6n tiempos de transite de cada superficie reflectora. se desarrollaron programas de computador que expresan el algoritmo del metodo y, posteriorrnente, se probaron con datos sinteticos, suministrando velocidades de intervalo y profundidades de los reflectores con errores considerados aceptables. palabras clave: trazamiento de rayos, teoria del ravo, problema inverso, continuacion hacia abajo, velocidad de intervalo abstract we present a method to estimate interval velocities, reflector depths and geometries in 3d models consisting of a pile of isotropic and homogeneous layers of any velocities and densities separated by smooth interfaces. the travel time of a ray reflecting on a particular interface and registered in the vicinity of a zero-offset ray is expressed by a function referred to a ray-centered coordinated system, fnnction which is estimated at the uppermost surface of the model. the reflection travel time function associated to each reflecting surface is determined at the superior surface in the neighborhood of the reference ray. the geometry of the upper limiting surface of a particular layer and the travel time function estimated on this interface allow to calculate the interval velocity of the layer and the geometry of the bottom limiting interface. with the interval velocity and geometry of the two limiting interfaces of the layer, the travel time function of the following reflector is estimated at the bottom interface. this step simulates positioning the sources and detectors on the anterior surface of the next subjacent layer. the procedure is repeated recursively at deeper layers getting the complete solution without a priori knowledge but the upper determined layers and the estimated travel time functions of each reflecting surface. computer's programs expressing the algorithm of the method were developed and tested with synthetic data, providing the interval velocities and reflector's depths with errors considered acceptable. key words: ray tracing, ray theory, inverse problem, downward continuation, interval velocity introduction the interval velocity is a basic parameter in seismic data processing, specially in depth migration and interpretation of seismic sections. in the high-frequency range the propagation of seismic wave is well described by paraxial ray theory. in order to estimate the interval velocity using a travel time inversion method we recourse to the mathematical formalism developed by bortfeld (1989). the bortfeldss originally ideas were presented for earth models consisting of homogeneous layers. using dynamic ray tracing, hubral et al. (1992) extended posteriorly the validity of the formalism to layered media with lateral inhomogeneity. the parameter describing a paraxial ray are its distance to geofislca colombiana, 5, diciembre de 2001 the central ray and the deviation of its slowness vector from that of the central ray. at any point of a paraxial ray, the parameters are linearly dependent on those at its initial point. the dependency is described by the propagator matrix, which is written as a product of many ray-segment propagator matrices, each one associated to each layer. after kahn (1987) the ray-segment propagator matrix through one layer is decomposed in a product of matrix operators containing the parameters of the layer, and to determine the parameters of that layer is necessary to know the travel time function on its anterior surface. at the uppermost surface the travel time function of each reflecting interface can be estimated measuring the travel times of several rays in the neighborhood luis montes vides of the central ray. based on the anterior ideas a method to estimate the interval velocities and reflector's depth was developed. test on synthetic data showed that the algorithm representing the method provides results with errors considered acceptables. the solutions obtained are valid only in the second order approximation of the travel times function and its range of applicability corresponds to the hyperbolic dynamic correction. traveltime function according the formalism developed by bortfeld (1989), the earth model consists of a pile of homogeneous layers of any velocities and densities. the layers are separated by arbitrarily curved but smooth interfaces. the uppermost interface, called anterior surface, is assumed to be plane and contains sources and detectors. the reflecting interface is called the posterior surface. one normal ray to the reflecting interface, called central ray, intersects the anterior surface at the origin of the xyz coordinate system and the posterior surface at the origin of the x'y'z' coordinate system. in the vicinity of the central ray, any transmitted ray is described by its initial position x and slowness 13 vectors, and by its final position x' and slowness p' vectors. representation is by twocomponent vectors. the two-component position and slowness vectors are obtained by projecting the three-component vectors into the tangent plane to the anterior and posterior surface at the intersection point with the central ray. the vectors x and 13 of any transmitted ray determine the vectors x' and p'. the first-order approximation of x and 13 sets a linear relationship: a, fl, ~ and d are 2x2 jacobian matrices and po is the slowness vector of the central ray. similar equations apply for reflected events. according bortfeld, in the second-order approximation, the travel time of a transmitted ray from any source position at the anterior surface x to any receiver position x' at the reflecting interface is given by: t(x,x') = to po ·x++x'· dfl-iy' +1..i· b-1 ax -i· b-1i' 2 two transmitted paraxial rays (one starting at x and x" other at) combine into a reflected one, with the travel time function t(x, x") given by the sum of the travel times-of the two transmitted rays: t(x, x") = to 2po + (x + x") + + (x + x") . d -i c 1.. (x + x") + 1.. (x" x) . b-1 a 1.. (x" x) -2 2 -2 where to is the two way travel time of the central ray. expression (3) is called the parabolic approximation of the travel time along the paraxial ray, and is not the best approximation. it is known a long time ago that in simple layered media, seismic nearvertical reflections are better approximated by hyperbolic rather than parabolic travel times curves. ifwe square (3) and retain only its terms up to the second-order in x and x', the hyperbolic approximation of the travel time for the paraxial ray is obtained (ursin, 1982). ( ")2 _ i" 2t x,x -(to-2po'z(x+x)) + 2to(1(x + x")· o' c t (x + x") + t (x" x)· b-1 a t (x" x)/ (4) equations (3) and (4) contains the same nine unknown parameters: one in to' two in po' three in ll-ia and three in d -i ~ (because the matrices are symmetric), in consequence (3) and (4) can be determined registering at least nine travel times for rays in the vicinity of a central ray at uppermost surface. for each reflector interface in the model, a travel time function like equation (4) will be estimated at the uppermost surface. interval velocity considering the travel time function determined at the anterior surface of a layer and known the geometry of that interface ( k n-i)' we will show how the interval velocity of the layer can be determined, uniquely. the total transfer matrix, which describes the ray propagation in the n" layer, can be decomposed after kahn (1987) in a product of matrices: (i) [an bn] [ i a~vnsni] en dn v;;ikn 1 [~ o ][ l ~] (5) -i 1 -il -cos({3n)v;; kn-i [cos({3n) ~]with l = 'l({3n) = 0 (6) (2) the capital letters in the matrix relation (6) are 2x2 matrices: e.g., 1 is the unity matrix, q is the zero matrix and k n is the matrix of curvature of the n" interface. the n" interval velocity is vn ' and sn the path length of the central ray in the layer. the angle between the normal vector of the (n i )th interface at the intersection point with the central ray and the slowness vector of the central ray is f3 n . the matrices on the right side of the transfer matrix are responsible for the following transfers of positions and slowness vectors, from right to left: (i) from the (n i )th interface into the tangent plane, (2) from the tangent plane into the plane perpendicular to the central ray, (3) from the plane perpendicular to the central ray at the (n i )th interface into a perpendicular plane to the central ray at the n'" interface, and finally from the tangent plane into the n" interface, i.e. into the reflector. !1t n is the one way travel time of the central ray through the n" layer. in this step, besides k n-i ' we assume to know the interval velocity of the superior layer vn-i and the angle of incidence of the paraxial ray an _i at the anterior surface. multiplying the matrices in expression (5), we obtain the following equations: (3) geofislca colombiana, 5, diciembre de 2001 interval velocity determination by downward continuation of the traveltime function, i'araxial ray approximation an ='l({3n)-cos({3n)sn'l-1({3n)kn-1 en = vnsn'l-'({3n) c, =v;/kn'l({3n)-v;;'cos({3n) [snkn + l]'l-' ({3 n )kn-' dn =[snk n +!ji-i({3n) determining the inverse of !l n from (8) and multiplying by (7) we obtain !1~'an' that we recalled u: equation (12) is equivalent to the following three equations: 1 1 u22 = --cos({3n )kn_, 22 vnsn vn ' at the anterior surface of the layer the snell's law is used: cos\{3n)= 1-sin2(an_1){ vn }2 vn_1 where an_i is the angle of incidence and {3n is the angle of transmission of the central ray, see figure i. by definition the interval velocity is: v 2sn n (17) /)"t n solving simultaneously the equations (13), (14), (15), (16) and (17), see appendix a, the interval velocity of the layer vn is determined, except in case of a spherical reflector as was pointed by krey (1989).this step permits to know also the three others unknowns sn' /)"t n' fj n ' and in con sequence to determine a nand !l n . geometry of posterior surface to determine the geometry of the posterior surface is necessary to know sn' /)"t n' fj n and vn' i.e., which were solved the anterior step. to calculate the inverse of d n equation (10) is used: d~' =i({3n)[snkn -rr' the product of (18) by (9) gives the matrix geofisica colombiana, 5, diciembre de 2001 (7) (8) t(sn, gn) ~n.j (9) anterior surface l-----i--~ (10) (12) posterior surface (13) (14) figure j. left: a seismic system and the travel lime function of rays reflected on the posterior surface, sources and detectors on the anterior surface. right: the central ray cross the anterior surface with angles of incidence and transmission. (15) ( 16) (19) reorganizing terms in (19) vn d-;" cn = 'l({3 n )[s n kn + ir' {kn 'l({3 n) cos({3n)[s n kn + i]'l-' ({3 n)kn-d (20) then 'l({3n )[s nkn + ir' kn 'l({3n) = vndn'cn +cos({3n)kn-1 (21 ) to find: [snkn +ir'kn = 'l-1{({3n)vndn1c.n + cos({3n)kn-i }'l-' ({3n) (22) term simplified by recalling ( i 8) q = i.-' ({3n) {v n d'n' c, + cos({3 n)kn-' }i.-1 ({3n) to get: (23) luis montes vides ~---i------7 t3 ~ __ -+-__ -;? t2 ~----t-----? t1 r 3 v 3 figure 2. the model is analyzed like three seismic systems, each one with its own central ray and each reflector like its posterior surface. there are travel time functions associated to each system and estimated on the anterior surface (flat). finally the matrix of curvature of the posterior surface in the vicinity of the central ray k n is obtained: k = q{i s q}-l_n _ _ n_ now, we can know numerically the transfer matrix which describes the ray propagation through the n" layer, substituting the terms vn' sn' /).t n' f3n' k n in equation (5). traveltime function in depth it will be necessary to introduce different central rays in the model, a central ray for each "piece" of seismic system. the first niece of seismic system is defined between the uppermost surface (zero interface) and the first reflecting surface, the second between the first one and the second one, and so on. each piece is considered a seismic system with a central ray perpendicular to the corresponding reflecting surface at the midpoint position (see figure 2). . the determination of the travel time function in depth is interpreted like if sources and detectors were moved from the uppermost surface, following the raypaths, to the anterior surface of the deepest layer of the seismic system (see figure 3). we will show how to determine the travel time function at the anterior surface of the deepest layer from the knowledge of the travel time functions at the upper interfaces of the seismic system. the propagator matrix describes the ray propagation of the paraxial rays through the system, and can be decomposed in a product of manyray segment propagator matrices. in a system with n layers the propagator matrix is written as the product of the n individual propagator matrices of each "piece": b ]k~n-l[a b]_n ii -] -] d . c d_n ]~l -] -] (25) if we have solved the system composed by the upper n ilayers, i.e., the transfer matrix for each superimposed layer is known, then (25) is rewritten: (26) where the matrix in the middle of (26) propagates the ray in the nth layer (assumed unknown), and the left most matrix in (26) corresponds to the product of the n ipropagator matrices of the upper n i solved layers, i.e., it has been assumed that the transfer matrix for each superimposed layer is numerically known. the transfer matrix posses the property of simplecticity (borfeld & kempert, 1990), this property means that the inverse of the propagator matrix of the system composed of n iupper layers is: (27) the super index means the matrix transposed, and by the same argument the inverse of the propagator matrix of the n" layer is: [~ !l]-i [d' b ]__ n _n !2 -~n a~ (28) (24) using (27) and (28) we transform (26) in: [ [2~ -c'_n [ d~ -c'_n !in.][ [2' !l'] d -c' a'-n· _ _ (29) su~dipoksurfa \~ ~0 layers : traveltime an . ~functionin tenor de th surfa~cs*\-/g* p unknown layer poste~ v ~ <»: surfa~~-' ~ figure 3. if the sources and detectors were moved from the superior surface to the anterior surface following the ray paths, then the travel time function is determined at the top of the deeper system. geoff sica colombiana, 5, diciembre de 2001 interval velocity determination by downward continuation of the traveltime function, paraxial ray approximation factorizing d' at left of (31) d~cn =[dnkc -cnkd'][(ank -bnkc d-1)d'r1 (32) the product of symetric matrices is conmutative, then d~cn =[dnkc -cnkdl]dl[(ank -bnkc d-1)j1r1 (c33) dt)cn =[dm(c' d-')-cnd[ank -bw(c d-t)r1 (34) by symmetry d-1 ~ = c' d-' then d~' ~n = [dnk (d-i~) ~nk harlk ~nk (d-1 ~)]-i (35) following a similar process we demonstrate that ~~ian = u2nk [~-i a] ~nk hank ~nk [~-i an (36) this step makes possible to know the matrices u; an and !2~1~n in depth. the two-way time ton and the slowness vector jj on complete the knowledge of the travel time function in depth, these last two parameter are calculated by ray tracing through the model, as it is shown in the next procedure. ray tracing in this step, the interfaces limiting the n upper layers and the their interval velocities are known. the paths of rays traveling across those layers can be calculated (see figure 4). in the vicinity of the central ray, the nth interface is approximated by the second order polynom z = + (x, y) . k n (x, y) , where k n = (k11 ' k12, kn) is the matrix ~f curvature. central 1'1 ray r3 paraxial rays v. figure 4. ray tracing of the c;,nl~~l and the paraxial rays through the last solved layer. the new coordinate system (i. ,.i ) has rotated an angle e geofislca colombiana, 5, diciembre de 2001 according figure 5, the normal ray to the n + i interface is refracted at the n surface in the point: (x, y, z) = -rl + £...r + r2 (37) where si' sz are the respective path lengths of the paraxial and central ray in the nth layer, ~ = (0,0, s i) and £...r = (£...rl ' /sr, ,£...r3) are known. therefore (37) is equivalent to equation (38). dr\ r2 law n normal ~~ector (x, y, z) ii ii figure 5. the ray '2 is normal to the next interface in depth, been refracted in (x, y, z). a, f3: incidence and transmission angles. substituing x, y in the third expression of (38) we obtain: s;[sinz f3(~ kll cos' 8 +~ k22 sin' 8 + k'2 sin8cos8)] + s2[sinf3{k11 cos8fuj + k2z sin8&z + 'sz(sin8fuj + cos8&2)} cosf3] + [s, -11r" + ~(kl,fujz + k22&z2 + 's2fuj&2)] = a (39) which has the form as; + bs z + c = o. with s2 known, we calculated: the position (x, y, z), the normal vector in this point v· by n = 1'11:::1 and the incidence angle in the nth interface by cos an = f,"1 . using the snell's law, the refracted angle f3n+i of the paraxial ray is determined. now is possible to estimate the time used by the paraxial ray to travel through the upper known layers and the remaining time to reach the posterior interface of the next unknown layer, i.e, we estimate the travel time function at the anterior surface of the unknown layer, to be solved. application to synthetic data synthetic data were generated in several models using the ray tracing program anis ray_3d, developed by costa et al, at the ufpa in brazil (1993), but only two of them are shown here. two programs were developed, the first one to estimate the travel time function at the uppermost interface of the seismic system, and the second one to determine the interval velocities by the estimation of the travel time in depth as was explained in before. luis montes vioes o -05 -i ~-15 r:r ·2 -25 -3 -5 xikml vjm/s ] vlm/s ] erro[m/s] erro[m/s] 0 0 0 0 200 200 200 200 400 400 400 400 goo goo goo goo 800 800 l800 lj 800 k 1000 000 e 000 k 000o£ o£ n n n n '1200 200 200 200 1400 400 ) 400 400 11 goo goo 600 600 1800 800 800 800 2000 000 000 000 0 5000 0 5000 0 50 0 50 figure 6. on top the model used to test the algorithm. on bottom interval velocities and erros obtained in two points at the superior surface. besides, the second program also calculates the matrices of curvature of the interfaces an the points where the rays intercept them. figure 6 shows on top the first model used to generate the travel time and in bottom the interval velocity model obtained by the program, in two different positions fo the model. the two leftmost graphics in figure 6 shows the errors in estimating the interval velocities, being fewer than 50 mls. a second model was used to show the program estimating the points where the rays reach the interfaces; the model is shown in top, figure 7, and in bottom the final position reached by the rays calculated by the program. in the third interface the program srnoothes it, satisfying the restriction impossed on the surfaces of the models. the others points show a well fitted with the interfaces. conclusions the second order approximation of the travel time and the paraxial ray theory make possible to develop an inversion method to estimate the interval velocities in models with no structural complexity or strong dipping interfaces. the method is a tool to estimate interval velocity and reflector depths, based on post stacked seismic data. due to the paraxial ray theory difficulties could occur when the positions of sources and detectors exceed the validity of the approximation, being valid only for small values of distance to the zero-offset ray position. its range of applicability correspond to the hyperbolic dynamic correction. because 10 :~-------------0----5 5 ·5 figure 7. on top the second model and on bottom the interfaces and the points where the algorithm estimates final positions for normal rays. the input data are travel time measurements on seismic traces, a good quality on seismic data is impossed. nevertheless the method provides the inverse solution without a priori knowledge of the model, just using travel time measurements on the uppermost surface. bibliography bortfeld,r. 1989: geometrical ray theory: rays and travel times in seismic systems. geophysics, 54, 342-349. bortfelo,r., and kemper, m. 1991: geometrical ray theory: line foci and point foci in the anterior surface of seismic systems (second order approximation of the travel times). geophysics, 56, 806-811. costa, 1., schoenber, m. and urban, 1. 1993: 3d ray tracing through no-flat homogeneous anisotropic layers. 3rd international congress of the brazilian geophysical society. expanded abstracts, vi, 47-52. hubral, p, schleicher,j. and tygel, m. three-dimensional paraxial ray properties i basic relations: 1. seis. expl., i, 265-279. kahn, s. u. 1987: die aufdeckung der mehrdeutigkeiten in der reflexionsseimik: ph.d. tesis, tech. univ. clausthal. krey, t. 1989: the non uniqueness of the determination of interval velocities from moveout velocities. geophysics, 54, 1209-1211. ursrn, b. 1982: quadratic wavefront and travel time approximations in inhomogeneous layered media with curved interfaces. geophysics, 47, 1012-1021. acknowledgements this paper is the result obtained in the montes's ph. d. thesis at the federal university of para, brazil. the project was supported by a fellowship from coordenacao de aperfeicoamento de pessoal de nivel superior (capes). appendix determining the interval velocity to determine the interval velocity in the n" layer is necessary to solve the set of simultaneous equations, represented by the matrix equation b-1 a: geoflslca colombiana, 5, qiciembre de 2001 interval velocity determination by downward continuation of the traveltime function, paraxial ray approximation (a i) (a2) 1 ui2 = ---cos ([3n)kn-i12 vnsn ' the snell's law at the anterior surface of the nth layer (a3) cos2([3n)=i-sin2(an_1){ vn }2 vn_1 with angles an-l of incidence and f3 n oftransmission, and the interval velocity (m) 2sn vn =& (as) n in order to solve (ai), (a2), (a3), (a4) and (as) we will consider five different cases: first: when k n-i,ll = k n-i,12 = k n-1,22 = 0 , i.e, the anterior surface is a flat one, or when kn-1,12 = kn_1,22 = 0 and kn_1,11 '" 0, substituting (as) in the reduced equation (a3) (m) second: when k n-1.i2 * 0 we use equations (a2) and (a4) to get [ k ]2[ 22_ n-1,l2 ,2 vn vn u i sin (0. n i ) v 2 12 ~i and finally (a 7) vn=[kui2 ]2+(sin~(~n-i)j (a8) n-1.12 n-i third: when k n-l,12 = k n-i,ll = 0 and k n-i,22 '" 0 we substitute (as) in (ai) _ 1 [ 1 ' 2 ) v; ]ull -1sin (an-1 -0vnsn vn'_i to get (a9) v i1tnu11 + sin 2 (an_1) n0 (aid) 2 vn'_i fourth: when kn-1,22 * kn_1,1i * 0 and kn-1,12 = 0 we rewrite equation (ai) cos 2 ([3n) vnsnkn_1,11 (a it) kn_i,ii and equation (a3) geofislca colombiana, 5, diciembae de 2001 ~ 1 cos(f3n) kn-1,22 vnsnkn-1,22 vn (ai2) subtracting (a 12) from (a 1i) and factorizing the factor vn sn we obtain v s ( kn_1,ll j+ ~n-l,11 = n null ul2 kn-1,12 kn-1,12 1 ' 2( ) vn-sin an-1 -vn_1 (ai3) including (as) in (al3) and reorganizing tenns,the interval velocity is v -n(a 14) r: k-i n-i n-i,n fifth: when kn-1,22 = kn_1,11 * 0 and kn-1,12 = o,i,e"theanterior surface is a spherical one, from (a3) vnu22 kn-1,22 (ais) and using (a4) in (a is) and squaring the resulting expression we have 1 '2( )[vnj2-( 1 vnunj2-sin a ----n-l vn_1 kn-1,22s n kn-1,22 and substituting (as) in (a 16) (a 16) (a 17) the expression (a 17) is reorganized in terms of vn (a l8) the expression (ai8) has the form a(v[~) +b(v; )+c = 0 and its solution is given by v~ = (2a)-1 (-b ±.j b2 4ac), in consequence the square of the interval velocity can be known but there is an ambiguity to determine the interval, i.e., the unique solution does not exist for this particular situation. the result shown here was first reported by kahn (1987) and later by krey (1989). 11 .' una propuesta tematica para un curso de mecanica newtoniana dora deyanira bernal nieto profesora asociada departamento de ffsica-facultad de ciencias-universidad nacional de colombia bernal, d. (1995): una respuesta tematica para un curso de mecanica newtoniana. geolfs. colomb. 3:67-71. issn 0121-2974 resumen en este trabajo se busca explicitar, hacienoolos presentes en el debate hist6rico, algunos de los presupuestos epistemol6gicos involucrados en la ifsica anstoteuca, la ifsica newtoniana y en las bases de la relatividad de einstein. se propone trabajar, con los estudiantes del primer curso de ffsica para las carreras de ciencias, score el cambio de pensamiento entre la ffsica aristotelica y la newtoniana, y esta y la relativista, actarando las "reglas de juego" involucradas en cada una de estas teorfas para que su conocimiento mejore los resultados de su trabajo en ffsica. abstract according with a analysis from a historical debate, this work tray to clear some epistemologic concepts, involved within the aristotelic and newtonian physics and an the einstein relativity theory bases. its proposed to works with students of physics from each sciences academic programs about of the thought change between the anstotelic and newtonian physics ans this last one with the relativistic theory, in order to clear the "play rules" involved in each theory so that its knowledge to improve the results from the physics works. 1. introduccion los estudiantes que normalmente recibimos para iniciar su formaci6n universitaria en ffsica consideran que esta disciplina tiene poco que ver con su cotidianidad. no es cornun encontrar en ellos la capacidad para dar explicaciones de aconteceres diarios de acuerdo con formulaciones vigentes de la tfstca, y reconocer que las explicaciones que se aprenden en la academia hacen parte de un modo de pensamiento aceptado a partir de un momento hist6rico es aun menos cornun, . la comprensi6n cabal de los conceptos de la ffsica exige la explicitaci6n de ciertos presupuestos epistemol6gicos involucrados en esos conceptos (el marco te6rico en el cual se definen, la amplitud del campo de fen6menos a los cuales se aplican, el lenguaje en el que deben expresarse y las caracterfsticas de ese lenguaje, la metaffsica implfcita 0 explfcita a la que corresponden). estos presupuestos se hacen presentes en el debate entre las distintas teorlas (los presupuestos de la ffsica aristotetica se hacen presentes en su comparaci6n con la ffsica newtoniana y los de esta en el surgimiento de la ffsica einsteniana). ei trabajo en ffsica tiene ciertas reg las de juego y los presupuestos a los que se hace referencia son partes de las reg las de juego que es necesario conocer. muchos de los errores que cometen los estudiantes corresponden precisamente al desconocimiento de esas reg las fundamentales. se propone iniciar el curso de ffsica general, la parte de mecanlca, haciendo que el estudiante reflexione sobre el cambio de pensamiento, cambio de modelo de explicaci6n, de la ffslca aristotelica a la ffsica newtoniana, logrando, cuando sea el caso, que reconozca que su propio pensamiento debe modificarse para acceder a la comprensi6n rnaternatica de los fen6menos del movimiento. se propone tambien terminar el curso logrando que comprendan otro cambio hist6rico importante: de la concepci6n de espacio y tiempo absoluto de newton a la de espacio y tiempo relative de einstein. se cuenta con la evidente curiosidad de los estudiantes por saber algo de la liamada ffsica moderna y de la tan nombrada teorfa de la relatividad. otra idea comun en nuestros estudiantes es que en los cursos de ffsica aprendsran "la verdad" sobre c6mo funciona la naturaleza. seguramente, si se muestran ejemplos hist6ricos concretos, se loqrara que comprendan que "ia ciencia" es hecha por hombres que plantean "modelos de explicaci6n", teorfas que fundamentalmente no contradigan los hechos empfricos, pero que es posible que existan modelos contradictorios que den explicaciones igualmente validas de situaciones comunes. 2. c,donde caera? retomando algunas de las preguntas planteadas en la epoca galileana y respondidas por galileo para la ffsica de la tierra en movimiento, es posible descubrir los modos de explicaci6n de nuestros estudiantes. para ellos, como para todo el mundo, es obvio que la tierra se mueve y facilrnente liegan a los valores de la velocidad de rotaci6n y de traslaci6n: la rapidez de un punta sobre el ecuador es de 1.600 km/h 0 450 m/s para el movimiento de la tierra sobre su propio eje y de 103.000 km/h 0 31.000 m/s para su movimiento alrededor del sol (valores aproximados asumiendo los valores de las vslocldades constantes). comparando estos valores de velocidades con valores cotidianos conocidos, rapidez de un atleta en una carrera de 100 m (aproximadamente 10 m/s) , rapidez de un autom6vil en carretera (120 km/h) y otros, es facil reconocer los enormes valores de la rapidez de los movimientos de la tierra. si se formulan las siguientes preguntas: ~d6nde volvera a caer un proyectil disparado en llnea recta hacia arriba? ~d6nde caera cualquier objeto que se suelta desde la ventana del sal6n de c1ase? (se pueden dar longitudes aproximadas de la cafda y pedirles que calculen el tiempo de calda y las distancias recorridas por un punta sobre la tierra en ese tiempo). la respuesta es la esperada para un sistema de referencia sobre la superficie de la tierra. pero, aunque inicialmente no se tenga suficiente claridad sobre el problema implicado en la no definici6n del sistema de referencia lrnphcito en la pregunta, se generan importantes inquietudes. la discusi6n rica e interesante se da al solicitar la explicaci6n de las respuestas dad as y de las que aparecen al formular las siguientes preguntas: ~c6mo es posible que la tierra se mueva a estas tremendas velocidades y sin embargo no oigamos silbar el aire que deja tras de sf? ~porque los pajaros se atreven a soltarse de los arboles, volar hasta el suelo y comer gusanos situados tanto al este como al oeste del arbor? estas preguntas fueron planteadas a copernico y a galileo, perc mantienen vigencia y permiten retomar modos de enserianza siempre validos. la experiencia es diversa en cada uno de los grupos de estudiantes, perc 10 mas probable es que encontremos intentos de explicaci6n que correspondan al modo de pensamiento aristotelico, es posible que se tenga que regresar sobre muchas de las respuestas dadas, en puntos pertinentes del desarrollo del curso, y que sea necesario dar una explicaci6n aun en el caso de respuestas acordes con el modo de pensamiento newtoniano. la dificultad para dar explicaciones a interrogantes sobre el movimiento en relaci6n con una tierra que se mueve, permite mostrar el gran cambio de pensamiento entre la ffsica arlstotelica y la ffsica newtoniana. 3. fisica aristotelica ~c6mo se pensaba antes de newton? busquemos caracterizar, en pocos enunciados, la ffsica antigua, ffsica aristotelica por ser arist6teles su mas grande representante, ffsica adaptada a los conceptos de una "tierra inm6vil". ei cosmos es finito y esterlco, con un centro, donde la tierra tiene su lugar "natural". la tierra y "el cielo" son entidades diferenciadas. hay una ffsica diferente para cada una de estas entidades: los objetos de la tierra se componen de cuatro elementos: aire, tierra, fuego y agua. son livianos 0 pesados dependiendo de la proporci6n en que intervienen los cuatro elementos. la tierra es "naturalmente" pesada, el fuego es "naturalmente" iiviano. agua y aire estan entre estos dos extremos. ei movimiento "natural" de un cuerpo pesado es hacia abajo. ei movimiento "natural" de un cuerpo iiviano es hacia arriba. en consecuencia, el movimiento "natural" (sin obstaculos) de un objeto terrestre es en linea recta hacia arriba 0 hacia abajo. obviamente arist6teles sabra que hay movimientos en direcciones distintas a las indicadas. tales movimientos son "violentos", contrarios a la naturaleza del cuerpo y s610se presentan cuando alguna fuerza actua para iniciarlos y mantenerlos en su movimiento "antinatural". en los cielos nada cambia, todo permanece igual, los cielos son "incorruptibles". los planetas, las estrellas y el sol se consideran perfectos. la luna es el punta divisorio entre la regi6n terrestre de los cambios y la regi6n celeste incorruptible. los cuerpos celestes estan formados por "eter". ei movimiento "natural" de un cuerpo de "eter" es circular. ei movimiento de los astros es cerrado, peri6dico, no hay principio ni fin en ellos. la tierra es diferente a los cuerpos celestes por su composici6n y "propiedades esenciales", el movimiento circular serla contrario a su naturaleza. ei movimiento natural de todos los pedazos de materia terrestre es hacia el centro del universo, centro que coincide con el de nuestro planeta. si la tierra se moviera, al arrojarse un cuerp,o hacia arriba y luego caer, el cuerpo no caeria exactamente sobre el mismo punta desde el cual se 10arroj6. 4. ciencia moderna los acontecimientos hist6ricos sobresalientes que permitieron el cambio de pensamiento de la ffsica aristotelica a la ffsica newtoniana son en una rapida enumeraci6n los siguientes: en el ano 1543 se publica el libro de nicolas copernico "sobre las revoluciones de las esferas celestes", que presenta un nuevo sistema de astronomla opuesto a la noci6n de inamovilidad de la tierra. en la antigua grecia, aristarco de samos habra sugerido que la tierra pudiese tener un movimiento de rotaci6n sobre su propio eje y describir una 6rbita alrededor del sol. la obra de copemico, publicada cas; 2.000 afios mas tarde, basa su descripci6n en esos dos movimientos terrsstres. a pesar de que esa obra no encontr6 aceptaci6n general, fue el inicio de un roceso que culmin6 con la formulaci6n de isaac newton. • copsrnlco plantea que todos los planetas, incluida la tierra, giran alrededor del sol y el sol es el centro del cosmos. ei hombre de ciencia a quien se debe fundamentalmente la introducci6n del telescopio como instrumento cientffico, y que estableci6 los fundamentos de la nueva ffsica, fue galileo galilel galileo hizo su primera contribuci6n a la astronomia antes de usar el telescopio. en 1604 apareci6 en la constelaci6n serpentario una "nova" 0 nueva estrella. galileo demostr6 que se trataba de una verdadera estrella ubicada en los espacios celestes y no dentro de la esfera de la luna. con ello mostr6 que podia haber cambios en los cielos, a pesar de arist6teles para quien los cielos eran inmodificables. ai dirigir el telescopio al cielo donde quiera que miraba encontraba pruebas para defender el sistema copernicano, 0 por 10menos, para debilitar la autoridad de las teorias antiguas. ei sistema kepleriano, publicado en 1609, presenta un universo de estrellas fijas en el espacio, un sol fijo y una sola elipse para la 6rbita de cada planeta, con una adicional para la luna; y tres leyes (basadas en el analisis de las observaciones de tycho brahe) sobre la cinernatica de los planetas. kepler y galileo exigian la existencia de una sola ffsica, aplicable a los cuerpos celestes y a los cuerpos terrestres comunes. la pubncacion de los "principia" de isaac newton, en 1687, constituye uno de los acontecimientos mas importantes en la historia de la fisica. si bien la fisica de los "principia" ha sido modificada, mejorada, objetada, aun hoy enfrentamos la soluci6n de la mayoria de los problemas de la mecanica celeste y de la fisica de los cuerpos macrosc6picos como 10hizo newton hace 300 afios. en los "principia" newton desarrolla los principios de la dlnarnlca de los cuerpos, enuncia sus tres leyes; aplica estos principios al mecanismo del universo, enuncia la ley de la gravitaci6n universal; estudia la rnecanica de los f1uidos y el movimiento de los cuerpos en medios resistentes y otros aspectos de la fisica general. para esto utiliza los trabajos de galileo y los de kepler. aplica la misma fisica a los cuerpos celestes y a los cuerpos terrestres. hace desaparecer la dicotomfa entre mundo sublunar y mundo supralunar. 5. fisica newtoniana ei primer curso de tlsica esta centrado en la comprensi6n de las leyes de newton, la aplicaci6n de elias a la resoluci6n de problemas de mecanica ctasica y a la comprensi6n y aplicaci6n de los principios de conssrvacion derivados de elias. este empeno de comprension y manejo no resulta una tarea facil con estos grupos de estudiantes, las dificultades las 'conocemos todos muy bien aqui se busca hacer una corta reflexi6n sobre los problemas de metodo en la obra de isaac newton y aclarar el significado de los conceptos de espacio y tiempo, conceptos previos a sus leyes y necesarios para entenderlas, para luego centrar las crfticas sobre la mecanica de newton en estes conceptos con el animo de situarnos hist6ricamente en el momento de su modificaci6n por parte de einstein. para newton los principios te6ricos deben encontrarse por "inducci6n" a partir de observaciones y experimentos 0 bien deben provenir de "otras verdades ciertas" (granes, 1978). procesos de observaci6n y de analisis conducen a los principios basicos de la teoria. pero al mismo tiempo, la teorfa permite pensar cada fen6meno en terrnmos de conceptos, principios y leyes. estos dos procesos: mirar la realidad y sacar conclusiones sobre ella, se alimentan y se determinan mutuamente. la teorfa es previa a la reconstrucci6n racional de la realidad perceptible, los aspectos que aparecen como determinantes estan definidos por la perspectiva te6rica adoptada. definidos los fundamentos te6ricos la realidad ffsica puede pensarse a traves de los conceptos y de los principios generales de la teorfa. la observaci6n newtoniana no es observaci6n pura, no contaminada por preconcepciones (10que, por otra parte, no seria posible); el analisis esta orientado desde la teoria, hay un "modelo" que se debe seguir en cualquier anatisis: argumentar a partir de los fen6menos, deducir las causas a partir de los electos. ei modele newtoniano es un esquema racional entre causa y efecto. para newton la explicaci6n rnaternatica del fen6meno es insuficiente para entender el mundo real que se oculla dstras del fen6meno, perc puede mostrar el camino a verdades fundamentales. la ciencia en newton, no es totalmente ajena a perspectivas metaffsicas y teol6gicas. existen ideas previas a sus leyes, ideas que el considera necesarias para entenderlas, los conceptos de "espacio absoluto" y "tiempo absoluto" que trascienden 10 empfricamente observable. newton hace distinci6n entre ideas absolutas y relativas, verdaderas y aparentes, rnaternaticas y vulgares, ("en las disquisiciones filos6ficas deberfamos abstraer de nuestros sentidos y considerar las cosas en sl mismas, distintas de 10 que no son sino medidas sensibles de elias"; newton, 1987). ei espacio y el tiempo, tal como son concebidos por el cornun de la gente son relatives, aparentes y vulgares. para newton el espacio y el tiempo son absolutos, verdaderos, matsrnaticos. maternaticos porque en ellos no se hace referencia alguna a la constituci6n material de los entes. ei tiempo absoluto, verdadero y rnaternatico iluye siempre de manera uniforme, es una realidad objetiva, independiente de los hombres, sus propiedades no dependen de nada 5610 de si mismo. ei tiempo relative, aparente y cornun es una medida sensible, externa de la duraci6n de un movimiento (dfa, ario, nora, ...). en la practica debemos trabajar con tiempos relativos. ei espacio absoluto por su propia naturaleza y sin relaci6n alguna con nada externo permanece siempre similar a si mismo e inm6vil. ei espacio es el continente de todo. ellugar donde se pongan las cosas debe estar antes de las cosas, sus propiedades son previas a las cosas. ei espacio es isotr6pico: infinito, autosuficiente. ei espacio relativo, ligado al cuerpo, se mueve con el cuerpo a traves del espacio absoluto. movimiento absoluto es un movimiento con relacion al espacio absoluto y el relativo la traslacion de un lugar relativo a otro. "asf en un barco que navega ellugar relativo de un cuerpo es esa parte del barco que posee el cuerpo y que se mueve junto con el barco y el repose relativo es la continuaci6n del cuerpo en la misma parte del barco. si pensamos a la tierra en reposo, el cuerpo que reposa en rslacion al barco se rnovera real y absolutamente con la misma velocidad que tiene el barco sobre la tierra. mas si la tierra se mueve el movimiento verdadero y absoluto del cuerpo se derivara en parte del movimiento verdadero de la tierra en el espacio inrnovil y en parte del movimiento relativo del barco sobre la tierra. y si el cuerpo se mueve tamoten relativamente al barco, su movimiento verdadero se debara en parte al movimiento verdadero de la tierra en el espacio inmovil y en parte a los movimientos relativos tanto del barco sobre la tierra como del cuerpo en el barco" (newton, 1987). las causas de los movimientos verdaderos son las fuerzas. si aparece un efecto no causado por una fuerza, sera causado por una "fuerza ffcticia 0 inercial" y sera este un movimiento relativo. si observamos un movimiento que no sea en ifnea recta con rapidez constante y no ie podamos atribuir la causa a la accion de un cuerpo en su medio ambiente, podemos concluir que se trata de un movimiento relativo. con estes conceptos previos se snunclaran las leyes de newton y se aplicaran a la soluclon de problemas de la mecanica, teniendo en cuenta que para el estas leyes son las mismas en todos los sistemas inerciales de referencia. ai trabajar con movimientos relativos, la transformaci6n de las variables clnernaticas de un sistema a otro se hace de acuerdo con las transformaciones de galileo. los presupuestos datras de las transformaciones de galileo son: ' ei intervalo de tiempo entre eventos medido desde cualquier sistema de referencia en movimiento relativo es el mismo. la longitud de un metro es la misma medida desde cualquier sistema de referencia en movimiento relative. 6. critica a los principios de la mecanica newtoniana ei libro mas importante de ernst mach, "the science of mechanics· (1883), se conoce principalmente por la discusion de los "principia", de newton, en particular por la crftica de 10 que mach llama "el concepto patologico de espacio y tiempo absoluto", concepto patologico porque es "una pura construcci6n mental que no se puede encontrar en la experiencia" (mach, 1960). para mach es imposible determinar un tiempo absoluto, 10 que podemos es comparar la duraci6n de dos movimientos, (ia duraci6n del movimiento de un pandulo con la duraci6n de otro movimiento). rechaza el tiempo como realidad objetiva, el tiempo es una construcci6n humana. un movimiento nunca es uniforme en sf mismo, es uniforme con relaci6n a otro. en la realidad comparamos movimientos, se necesita mas de un objeto en el universo para hablar de quietud 0 movimiento. ei comportamiento de un objeto depende de la presencia de los otros cuerpos, hay una dependencia entre las condiciones que determinan los movimientos. todas las cosas estan interrelacionadas entre sf. para mach no hay necesidad de suponer una realidad desconocida que se esconda detras de las sensaciones. ei conocimiento cientffico del mundo consiste simplemente en la descripcton mas sene ilia posible de las conexiones entre los elementos, sensaciones, y tiene como urnco objetivo el dominio intelectual de esos hechos con el menor esfuerzo posible de pensamiento, ("no existe nada real excepto las percepciones, y toda la ciencia natural es en ultima instancia una forma econ6mica de adaptar nuestras ideas a nuestras percepciones"; mach, 1960). ei programa de la ffsica del siglo xix consisti6, principalmente, en la reconciliaci6n de las nociones de ater, materia y electricidad por medio de imagenes e hip6tesis mecanicistas. esto condujo a deformaciones como la propuesta de larmor que consideraba al electron como una torsion 0 estado de tension en el ater, permanente perc m6vil, que forma part feulas discontinuas de electricidad y tal vez de toda la materia ponderada. habfa una fe doqmatica en la rnecarnca como base ultima de todo pensamiento ffsico. "the science of mechanics", de mach, hizo tambalear esta fe en el joven einstein. para einstein: "ia grandeza de mach radica en su escepticismo e independencia a toda prueba" (holton, 1992). reconoce que la posici6n epistemol6gica de mach influy6 en el en sus afios j6venes. esto coincide con el momento en que el estilo de pensamiento inspirado en mach apuntaba hacla observables. 7. bases del pensamiento einsteniano einstein identifica realidad con 10 que nos es dado a travss de las sensaciones, los "acontecimientos", en lugar de colocar a la realidad en un plano que esta mas alia 0 detras de la experiencia sensorial: "tenemos que tener en cuenta que todos nuestros juicios en los que el tiempo entra a formar parte son siempre juicios sobre acontecimientos slrnultaneos, si digo por ejemplo "ese tren lieg6 aquf a las siete en punto" 10 que quiero decir es: "ia liegada del tren y el que la manecilla pequefia del reloj apunte a las siete en punta son simumneos. ei tiempo en que se produce un acontecimiento es el que esta dado simumneamente al acontecimiento por un reloj estacionario colocado en ellugar del acontecimiento" (holton, 1992). la definicion de simultaneidad en la teorfa especial de la relatividad se basa en el requisito de mach de que todo enunciado de la ffsica tiene que establecer relaciones entre cantidades observables. de la misma forma que el tiempo en que se produce un acontecimiento cobra significado cuando se conecta con la experiencia sensorial, (cuando se mide p'lr medio de un reloj situado en el mismo lugar), ellugar 0 coordenada espacial de un acontecimiento tiene significado sola mente en el caso en que entre dentro de nuestra experiencia sensorial por medio de una medici6n en su aspecto fundamental, (por medio de una regia presente en el mismo instante). einstein modifica los conceptos de espacio y tiempo absolutos de newton. de abstracciones de nuestros sentidos, independientes del hombre, estos conceptos pasan a corresponder a mediciones sensoriales, siguiendo concepciones de mach. pero en el desarrollo posterior de su teorfa se apartara fundamentalmente de mach. otro hecho hist6rico que genera conflicto a finales del siglo xix es la idea del ater, idea que se hace eficaz a partir de las teorfas ondulatorias de la luz. newton habfa rechazado la hip6tesis ondulatoria de la luz y de esta manera habfa favorecido la concepci6n de la luz como un flujo de partfculas. esta idea se habfa mantenido durante los siglos xvii y xviii. en el siglo xix se hace muy diffcil explicar con esta teorfa las franjas de interferencia y difracci6n, (experiencias de fresnel y de young), que s610 pueden ser explicadas si se acepta la idea de la luz como una onda. y una onda es una perturbaci6n que se propaga en un medio que debe tener ciertas caracterfsticas de elasticidad. es entonces cuando la idea de ater comienza a tener el papel protag6nico que mantiene durante toda la historia de la ffsica del siglo xix. la luz se concibe, por analogfa con las ondas senoras, como una perturbaci6n rnecanica del ater, su velocidad es independiente del movimiento de la fuente, como en el caso sonoro, y solo depende de las propiedades del medio. ei ater es un medio material que permea todo el espacio y penetra incluso en los poros de las sustancias s6lidas; permanece en reposo coincidiendo con el espacio absoluto de newton y siendo practicamente una raplica material de aste. los ffsicos se dedican a comprobar su existencia real y a medir sus caracterfsticas ffsicas por medio de experimentos 6pticos, cuyo resultado siempre negative obligara a reformular una y otra vez sus caracterfsticas y llevara finalmente a descartar esa idea. es en este medio historico don de surge la teorfa de la relatividad como una respuesta a la situaci6n de crisis de la ffsica del siglo xix creada por las crfticas a la mecanica newtoniana, los experimentos con electrones a altas velocidades que no concordaban con la teorfa conocida hasta el momento y las dificultades con la teorfa del eter y la teorfa electrodinamica de maxwell. conclusiones se espera que concluido este curso los estudiantes den explicaciones acordes con la teorfa newtoniana de los movimientos en una tierra que se mueve. no se pretende que los estudiantes entiendan la teorfa de la relatividad especial, solamente que les quede claro el cambio de concepto de espacio y tiempo absoluto de newton al concepto de espacio y tiempo relativo de einstein. en la corta experiencia que se tiene, (tres semestres consecutivos con el curso de rnecanlca para estudiantes de segundo semestre de la carrera de qufmica), solamente ha side posible utilizar las ultirnas dos semanas de clase para la discusion sobre el surgimiento de la teorfa de la relatividad, contando con el estudio previo, por parte de los estudiantes, del capftulo sobre cinernatica relativista que viene comunrnente en cualquiera de los textos de ffsica general. se espera que los estudiantes comprendan los postulados basicos de la relatividad especial; entiendan el experimento ("crucial?) de michelson-morley; entiendan que, como consecuencia de los postulados basicos, el intervalo de tiempo entre eventos es relative al sistema de referencia desda el cual se mide y que la longitud de un metro medida desde sistemas de referencia en movimiento relativo con velocidades comparables a la de la luz no es igual; que entiendan, por ultimo, que la transformaci6n de variables cinsmaticas de un evento en un sistema de referencia al otro debe hacerse por una transformaci6n, diferente a la de galileo, la de lorentz. pero este entendimiento se facilita si se tiene informacion sobre las condiciones de la ciencia en el momento hist6rico en que surge una nueva teorfa. referencias bibliograficas granas, j. 1978: "newton: problemas de matodo y fines de la ciencia". serie pensamiento universitario. apun 5:12 holton, g. 1992: "ensayos sobre el pensamiento cientffico en la epoca de einsten". pp.170. alianza universidad. alianza editorial, madrid 1992. mach, e. 1960: "the science of mechanics: a critical an historical account of its development". pp/273. the open court publishing company. illinois. newton, i. 1987: "principios matematicos de la filosoffa natural". pp.129. alianza universidad. alianza editorial. madrid. earth sciences research journal.indd earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 13 16 software engineering’s contribution to earthquake engineering zamora-hernández abigail1, gonzález-lópez gloria inés2, gonzález-navarro félix fernando3 and ordaz-schroeder mario4 1 uabc. email: azamorah@yahoo.com 2 universidad autónoma de baja california (uabc). email: giglzlzy@yahoo.com.mx 3 uabc, department of computing and informatics. email: fernando.gonzalez@uabc.edu.mx 4 universidad autónoma de méxico (unam), seismic engineering area. email: mordazs@iingen.unam.mx earthquakes are natural disasters regularly affecting mexico. it is very important that all information about them is quickly obtained to speed up attention being provided for the affected population and, if possible, applying preventative action. computer and specialised software are currently used for obtaining essential knowledge concerning seismic events. 22 software applications for seismic analysis have been examined in this study to define predominant characteristics, strengths and limitations, to determine the current status of this particular field and suggest lines for the advance for new developments or prototypes. this country’s existing technological dependency is clearly shown (hardware and software). academic cooperation between different branches of engineering should thus be encouraged for creating new domestic products. méxico es un país generalmente afectado por terremotos. es muy importante que toda la información acerca de este tipo de desastres naturales sea obtenida rápidamente para acelerar la atención y si es posible implementar acciones preventivas. a través de programas computacionales especializados se reúnen datos esenciales sobre eventos sísmicos. en total, en este estudio se examinaron 22 aplicaciones de software para análisis sísmicos que permiten definir las características predominantes, sus fortalezas y limitaciones, al tiempo que se determina el estado actual del campo y se sugieren líneas de avance para nuevos desarrollos o prototipos. además, se expone la dependencia del país en materia tecnológica (software y hardware). la cooperación académica entre diferentes ramas de la ingeniería podría entonces motivar la creación de nuevos productos domésticos. key words: seismic analysis software, seismic engineering, software engineering. palabras clave: software de análisis sísimico, ingeniería sísimica, ingeniería de software record manuscript received: 08/10/2012 accepted for publication: 04/06/2013 abstract resumen introduction earthquakes are natural disasters which have scourged mankind throughout history, leading to recurrent destruction, desolation and death. this kind of event is referred to several times in the bible, for example in psalms 114:4; “the mountains skipped like rams, the hills, like lambs,” and in apocalypse 11:13 can be read, “and at that hour there was made a great earthquake, and the tenth part of the city fell: and there were slain in the earthquake names of men seven thousand: and the rest were cast into a fear, and gave glory to the god of heaven.” entire cities have been razed and reduced to rubble because of the violent shaking of the ground. considerable damage has been caused and many have been made victims because of seaquakes, tsunamis, fires and conflagration resulting from an earthquake; these produce a severe impact on a country or region’s economic activity and social life [4]. scientific techniques and modern tools must be used in dealing with these problems. 2. earthquakes in mexico mexico is located on top of five tectonic layers whose relative movement produces some of the highest seismic and volcanic hazards in the world. industrial cities are especially vulnerable (i.e. tijuana, tecate, mexicali and some others in baja california) because they share similar hazards with los angeles and san francisco cities in the usa [2]. figure 1 shows that the main hazard is represented by earthquakes occurring along pacific coasts. this area is where earthquakes are frequently produced, in addition to those having most intensity, so they represent a serious danger for coastal locations but also affect the valley of mexico (even although mexico city is 200 km away from here), due to the conditions of the soil on which the city was built. table 1 presents a historical extract of seismic data recorded, giving details about seismic events which have occurred in mexico. this table concentrates information related to all earthquakes having a magnitude higher than 6 and where many have caused fatalities [6]. seismology figure1: seismic regions and volcanic areas in mexico [5] zamora-hernández abigail, gonzález-lópez gloria inés, gonzález-navarro félix fernando and ordaz-schroeder mario 3. studying earthquakes in mexico using instruments for measuring earthquakes in mexico began late in the nineteenth century when the central meteorological observatory was set up. later, on september 5th 1910, the national seismological service (nss), an agency of national geological institute, was created and installed by presidential decree. telegrapher operators provided data about earthquakes so that this could be published in monthly bulletins once a telegraphic network had been set up [8]. the nss was created for providing early information for authorities, means of communication and the general public concerning earthquakes occurring in mexico for determining the main parameters, such as magnitude and epicentre. such data is published in monthly catalogues enabling researchers to evaluate the country’s seismic hazards and the creation of an extensive database [7]. the seismic observation network covers the set of seismic recording instruments (seismographs and accelerographs) available in a particular area for analysing local or regional seismicity. the nss currently has a national network of modern seismographs; however, there are other local seismic networks operated by several institutions, such as the valley of mexico’s geophysics institute’s network, the autonomous university of mexico’s (unam) engineering institute’s telemetric information system, the seismic network of the colima university (resco) and the centre for scientific research and higher education’s (resnor) north-western mexico network, baja california (cicese), the autonomous university of puebla’s (buap) puebla city network, the centre for seismic instrumentation and registration (cires) and the national centre for disaster prevention (cenapred) [9]. damage to property and economic loss also increase in the event of an earthquake due to an increasing population, urbanisation and industrialisation being closer to areas of high seismic risk and the increasing cost of real estate. this is why the origin of earthquakes and their patterns should be studied to provide better understanding of their effects and, whenever possible, reduce consequent damage and economic loss. earthquakes are studied by seismic engineering to mitigate the damage they produce by designing, constructing and managing earthquake-resistant structures. this is essential for knowing where and why earthquakes originate and the type of soil where they occur because this influences the amplification or reduction of ground motion intensity when determining a region’s seismic hazard. 4. software for earthquake study according to statistics, 11,515 earthquakes occurred in mexico between 1st january 2009 and may 3rd 2012; 2,182 of them occurred in 2009, 3,402 in 2010, 4,162 in 2011 and 1,769 such events occurred in 2012. it seems that values have increased but in fact what has increased is the ability to detect them [11]. this has resulted from more equipment gradually and software engineering’s contribution to earthquake engineering constantly becoming integrated into seismic monitoring different networks. such increase in information has enabled understanding different soil types, their behaviour during earthquakes and the effects occurring in them, as well as how they affect the society living on top of them. not surprisingly, as in many other areas of human performance, computer equipment has been used as support for processing large volumes of data thereby enabling seismic event to be processed quickly and effectively for producing useful information. so that specific software can be used for earthquake analysis, functions must be able to do repetitive tasks quickly and resolve common problems for people managing such data. the main computer applications for seismic data processing were examined, i.e. bap, degtra, earthworm, fuente, geogiga microtremor, hypo71, icorrect, insite, loca 1.0, pase, pitsa, preproc, promax, sac, seisan, seismosignal, seispace, seisplus, seisware, sigview, stk, zmap (figure 2). the following characteristics were considered when analysing them: release date, country of origin, platform architecture and operating system, programming language used for developing them and, last but not least, their cost. 4.1 by release date figure 2 shows that the development of this kind of software began in 1971, advancing steadily throughout the 1990s as a consequence of the growth of processing power together with a reduction in the cost in computer equipment. after 20 years, production slowed due to the increasing complexity of user requirements, because complete systems were being demanded which had to incorporate new components such as sophisticated graphical interfaces, compatibility between different file systems and the inclusion of data in several media with their corresponding interfaces. software production 2012 2000 2010 ye a r 1990 1980 1970 hyp o7 1 pa se pre pro c bap 1 . eart hw orm sa c pit sa pro max se isp ace zmap se isw are se isa n degt ra fu en te loc a 1 .0 se ism osi gn al st k se isp lus sig vie w geo gig a m . ico rre ct ins ite figure 2: years when applications for processing earthquake data were launched 4.2 by country of origin reviewing the countries producing the aforementioned software, they could be classified by continent or origin (i.e. america produced 14 and europe 8), distributed as follows: canada 3, usa 9 and mexico and peru one each. germany produced 1, spain 2 and france, italy, norway, the uk and switzerland one each. a graphical representation of software production is presented in figure 3. spain 25% usa 64% canada 22% switzerland 13% uk 13% norway 13% germany 12% france 12% italy 12%mexico 7% peru 7% america europe figure 3: countries in america and europe producing seismic analysis software 4.3. by type of access some applications are free software, being extensively used in the academic institutions which created them; those which were created for commercial purposes obviously require a licence to be able to use such software legally. information was requested from the companies which created them in the case of licensed software; some provided technical data and product prices, whereas others only provided information if there were real interest in acquiring a product (i.e. data could not be obtained). information obtained about licensed software is given below; it includes country of origin, name of product and cost in 2011: canada geogiga microtremor $ 490 usd usa sigview $139 usd a single computer $1,290 usd a suite italy seismosignal 60 € switzerland zmap matlab 2,000 € 4.3. by type of code special mention should be made about open source applications which provide access to source codes so that users know details about their operation and can modify them according to their requirements. earthworm and stk do not require any monetary contribution whereas other open source ones do such as promax, seispace and seisplus. summarising, many european countries develop software for seismic analysis but the us is the leader in this area in america because it has largest number of software applications. latin-america’s low participation is also clear; only the degtra application, developed by unam in 2002, is widely used in mexico and some other countries [3]. 4.3 by their platform table 2 presents the type of architecture that can be used by the software applications described above. it is clear that all the aforementioned applications considered personal computers because technological advances have increased their processing power, storage, portability and communications (obviously, encouraging more new users). table 2: architecture for applications types of equipment total vax workstation 1 sgi workstation 2 sun workstation 8 hp workstation 1 pc 22 mac 2 when considering an operating system it can be observed that as most applications use personal computers, then pcs predominate regarding different types of operating system (table 3). the redhat version for linux is included. different versions of windows have been included (i.e. win32, 98, 2000, millennium, xp, vista and version 7). table 3: operating system name total dos 5 hp-ux 2 irix 2 linux 7 osx 2 solaris 7 sunos 1 unix 2 vms 1 windows 15 4.4. by implementation language another evaluation parameter considered was that related to programming language used for creating analysis applications. applications created around the 1970s were developed in several versions of fortran, like 77, 90 and later even vfortran. regarding c language, it was found that some applications were complemented with functions or libraries created in some other language or tool, like fortran (for reusing previous codes) and oracle. however, the kind of application where it was almost impossible to get information about programming language used in development is considered in the last category. table 4 contains the most commonly used programming languages for developing seismic analysis software; half of them were created with free access software like c language. four products involved using licensed software but some did not specify the programming language they used in development. high complexity interfaces and sophisticated functions were observed for these applications, meaning that they were probably created with licensed software. table 4: programming language name total c 2 c with other 3 fortran 6 gtk 1 matlab 3 not specified 7 this study thus found that interest in seismic engineering has motivated all people involved in earthquake studies (such as geophysicists, geologists, engineers) regarding intensive and extensive use of computer equipment and software tools. this has promoted their participation in implementing their own algorithms in convenient and available programming languages, such as fortran and the emerging c language. however, not only computer architectures have evolved but also user requirements. simple interfaces became replaced by those having high definition and graphical displays, new components being included such as mouse, camera and speakers to offer multimedia information. this is why this type of application almost stopped being produced as it is not so easy to produce a programme as it was 40 years ago. several things have evolved such as equipment, requirements, programming languages and also how to use them, as have the people who zamora-hernández abigail, gonzález-lópez gloria inés, gonzález-navarro félix fernando and ordaz-schroeder mario create engineering software. this implies that we must encourage and support national scientific development resulting in new technologies for producing effective solutions to a country’s problems. academic institutions can take a first step towards low-cost application development for resolving different problems in all engineering areas. conclusions earthquake engineering and related sciences’ contribution towards understanding and evaluating earthquakes means that their effects must become minimised and the losses affecting society reduced. it should also be considered that the effects of natural disasters depend on economic, social and political factors [1]. it is the duty of science to transform the natural misfortune involved in an earthquake occurring into an opportunity for innovation and technological development in diverse areas such as earthquake engineering and of course, software engineering. producing earthquake analysis software is definitely an essential tool for achieving such goals more efficiently, quickly and accurately. software developers’ participation in other disciplines must thus be expanded to contribute towards the understanding and management of national seismic hazards and ensure technological independence. references [1] cisternas, m. (2011). responsabilidad social de los geocientistas en el manejo de riesgos: enseñanzas derivadas de la experiencia chilena con el megaterremoto del 27 de febrero 2010”, geo-risk management – a german latin american approach field workshop organized by the geo-network of latin american german alumni (goal). [2] gutiérrez, c., quaas, r., ordaz, m., guevara, e., muriá, d., krishna, s. (2008). fascículo sismos. cenapred. 1ª reimpresión de la quinta edición [3] ordaz, m., montoya, d., (2002). programa degtra, versión 3.2.4, instituto de ingeniería, universidad nacional autónoma de méxico, méxico, 2002 [4] sauter f., f. (1989). introducción a la sismología.fundamentos de ingeniería sísmica, editorial tecnológica de costa rica [5] dirección general de materiales educativos (dgme). (2010). atlas de méxico. subsecretaría de educación básica, secretaría de educación pública del gobierno de méxico. [6] u.s. geological survey. (2012). internet site for: “terremotos históricos: méxico”. http://earthquake.usgs.gov/earthquakes/world/ historical_country.php last access (may/2012) [7] national seismological service (ssn). internet site for: “el ssn y la sismicidad de méxico” http://secre.ssn.unam.mx/ssn/doc/mx_sismo/mhist.htm last access (may/2012) [8] national seismological service (ssn). internet site for: “terremotos y ondas símicas: una breve introducción”. http://secre.ssn.unam.mx/ssn/doc/cuaderno1/ch5.html last access (may/2012) [9] national system for civil protection in mexico. internet site for: “sismos: serie fascículos” http://portal.proteccioncivil.gob.mx/upload/publicaciones/tecnicas/ fsismo.pdf last access (may/2012) [10] boletín unam-dgcs-302 ciudad universitaria. internet site for: “en méxico han ocurrido 11 mil 515 sismos entre 2009 y 2012” http://www.dgcs.unam.mx/boletin/bdboletin/2012_302.html last access (may/2012) geoflslca colombiana n!! 1, octubre de 1992 37 perfil altitudinal de la temperatura media del aire en colombia jesus esla va profesor titular depto de geociencias-facultad de ciencias-universidad nacional de colombia eslava, j.: perfil altitudinal de la temperatura media del aire en colombia. geofis. colomb. 1:37-52, 1992. issn 0121-2974 resumen se establecen ecuaciones que relacionan la temperatura media anual del aire en colombia con la altitud. por medio de elias se posibilita estimar los valores de esa temperatura en cualquier lugar, con el requisito de conocer unicarnente el dato de su altitud y ubicacion regional; tarnbien se determinan los gradientes altitudinales que, a su vez, posibilitan conocer cualquier otro dato de temperatura del aire, conociendo los datos de otro sitio 0 estacion base. las ecuaciones muestran que los valores a nivel del mar y la variacion altitudinal de la temperatura del aire en colombia no son unico s, sino que cambian sequn las condiciones fisicas de cada region 0 subregion y se diferencias sustancialmente de los valores propuestos con anterioridad por otros auto res. abstract equations that establish relations between annual average temperature and altitude are defined for colombia. these equations allow to estimate temperature values at any place, just by knowing altitude and regional location data. it is also possible to determine altitudinal gradients, which in turn allows to know the temperature of a given place, from the data for a base. the defined equations show that temperature is not a unique function of altitudinal changes, but it also depends on specific physical conditions for each region or subregion. 1.introduccion con base en los valores medios anuales determinados para 338 estaciones rneteoroloqicas de superficie que han funcionado durante el periodo 1923-1990 y utilizando rnetodos estadisticos, se determinan ecuaciones que relacionan esa temperatura y la altitud en colombia. las ecuaciones obtenidas permiten estimar los valores de temperatura media anual en cualquier lugar de colombia con el requisito de conocer unicarnente el dato de su altitud y su ubicacion geogratica (region 0 subregion); ademas, con base en los gradientes y los datos de una estacion base, se pueden definir otros datos de temperatura del aire. ai igual que como se ha mencionado en otras oportunidades (p.ej., eslava, 1988), el creciente interes en la definicion de los mecanismos del clima, sus variaciones y f1uctuaciones y su prediccion, ha incrementado los estudios que tienden a definir la estructura de la atmosfera y la variabilidad espacial y temporal de los componentes atrnosfericos del sistema climatico ; el conocimiento de esa estructura y variabilidad es requisito indispensable para el optirno desarrollo de modelos atrnosfericos (fisicos, dinarnico s 0 estadisticos) 0 del sistema terrestre. ese conocimiento se puede lograr mediante el analisis de la informacion basica contenida en las series temporales de los elementos clirnafico s el que, a su vez, posibilita probar la sirnulacion de los procesos atrno sterico s a partir de los resultados obtenidos usanda modelos climatico s, en colombia se han elaborado varios estudios tendientes a describir las caracteristicas basicas 38 eslava: perfil altitudinal de la temperatura del aire de elementos climatico s como la temperatura, prectpitacicn, evaporacion e insolacion: sin embargo, los trabajos han liegado a solo una 0 des relaciones de qeneralizacion para toda colombia y de tipo gratico, no rnatematico. este trabajo constituye el primer aporte al conocimiento del regimen de la temperatura del aire en colombia en el cual se intenta ofrecer una base rnaternatica para conocer la variacion altitudinal de sus valores medias anuales (perfil altitudinal) con el fin de posibilitar la estirnacion de eso s y cualquier otro valor de la temperatura donde no se tenga informacion instrumental. se parte de generalizaciones ya conocidas, como es que la temperatura del aire varia inversamente y de manera lineal en relacion con la altitud y se liega a establecer ecuaciones de reqresicn que muestran esa relacion. a fin de alcanzar ese objetivo y teniendo en cuenta que en la practica no existen datos elimaticos perfectos, se pro cedio a una veriticacion preliminar de lodos y cada uno de las datos disponibles de temperatura media mensual y anual, can el propo sito de descubrir errores (sistematicos a no), para luego pro ceder a su correccio n a eliminacion. una vez depuradas las series de datos extractadas de diferentes fuentes, se formularon modelos rnaternatico s para establecer relaciones entre los datos de las estaciones meteorolo qicas y el factor clirnatico de la altitud, para todo colombia y para sus diferentes regiones y subregiones. 2. informacion utilizada para efectuar el analisis de los datos de temperatura del aire, primero se establecio una relacion de los diferentes trabajos en los cuales se publico informacion sobre ese elemento clirnatico , posteriormente se recopilo la informacion referente a temperaturas medias mensuales y anuales publicada en las siguientes referencias: cortes (1989) 245 estaciones; stanescu & diaz (1971) 209 estaciones; instituto colombiano de hidrologia, meteorologia y adecuacion de tierras-himat (1981, 1984, 1985a) 74, 72 y 29 estaciones; servicio colombiano de meteorologia e hidrologia-scmh (1973) 54 estaciones; eslava (1992) 42 estaciones. los datos de altitud de las diferentes estaciones utilizadas y su ubicacion geografica fueron definidos can base en la informacion aportada par cada uno de los autores mencionados, esos datos fueron contrastados y corregidos can los que aparecen en las siguientes publicaciones: instituto geogrmico "agustin codazzi"·igac (1960), bernal (1978), federacion nacional de cafeteros-fnc (1941-1989) e himat (1985b). los procesos de verificacion, depuracion, homogeneizacion, etc., de la informacion original utilizada para definir los valores medias, fueron realizados por cada uno de los autores mencionados de los cuales se recopilo la informacion. en este trabajo se efectuo una revision y verificacion de todos y cada uno de los datos medias mensuales y anuales disponibles, can el proposito de descubrir errores de tipo numerlco , de lectura, de transcripcion, etc.; luego se procedio a su cornprobacion, oorreccion 0 eliminacicn, sequn fuera necesario. la cornparacion. verificacion y revision de la informacion aportada por los diferentes autores (725 series de datos) perrnitio seleccionar finalmente los datos de temperatura media anual sobre 339 sitio s diferentes entre si (tabla 1), despues del analisis de la calidad de la informacion y de asegurar que dos 0 mas de elias no perfenecen a un mismo sitio y que no tienen involucrados errores siatematico s, datos extremos, combinaciones excepcionales 0 causantes de heterogeneidades. las series de datos muy cortas provenientes de estaciones meteorclo qicas ubicadas en areas geograticas criticas, se utilizaron para poder estimar la situacion en esas areas; de todas formas, eso s datos se consideran satisfactorios a nivel media anual ya que los resultados de las estimaciones se ajustaron al anaiisis espacial general. 3. resultados obtenidos 3.1 control de caudad de los datos can base en los valores de temperatura media anual del aire, de las 339 estaciones rneteoroloqicas seleccionadas, se comprobo la hipotesis de que de los posibles modelos de reqr esion entre la temperatura media anual (y en "c) y la altitud (x en m), el mas adecuado corresponde a uno de reqresion lineal simple y=a+bx [1] ya que los resultados de los analisis de los otros modelos (iogaritmico, potencial, exponencial) demostraron que los modelos de reqresion lineal presentan los mas altos coeficientes de corre-acion y determinacion, el mas bajo error tipico de estimacion, el mas bajo coeficiente de variabilidad y los menores residuos entre la temperatura determinada can mediciones y la calculada a partir de los modelos. un control de calidad aplicado con el fin de encontrar errores en los valores de temperatura a altitudes incorrectas (similar ala propuesto por eslava, 1988), consistio en los siguientes pasos: a) encontrar la ecuacion (recta de regresion de minimos cuadrados) que mejor represente la relacion entre la temperatura media anual del aire (yen 0c) y la altitud (x en m), b) calcular para cada estacion el valor de la temperatura (tc), con base en esa ecuacicin de regresion, c) encontrar los residuos (t-tc) entre el valor de la temperatura medido (t) y el calculado (tc), d) encontrar los residuos que no cumplan can la siguiente condicion: i t-tel < 2 sy, [2] sy" = error tipico de estimacion de y en x e) rectificar, si ella es posible, el valor de la temperatura y la altitud de la estacion cuyo geoflslca colomblana n9 1, octubre de 1992 39 line chart of resldulil: tlmp,meoanualresiduo no cumpla con 10 p1anteado en d) y sea el mas alto, f) aplicar nuevamente todos los pasos hasta obtener que todos los residuos cumplan 10 siguiente: i t-tc 1< 3 s,.• [3] g) suprimir la estaclorues) que presente(n) residuo(s) fuera del limite dado por la ecuacion [3]. ei paso e), roctificacion de las temperaturas y altitudes se realize contrastando nuevamente a las fuentes originales y, ademas, en el caso de las altitudes se etectuo con base en la informacion que aparece en las referencias ya citadas de igac (1960), bernal (1978), fnc (1941-1989) e himat (1985b). las altitudes solo se modificaron (en 15 casos) cuando se cornprobo que el nuevo dato, adernas de tener un soporte bibliogratico adecuado, mejoraba las condiciones del mode/o, en general, y el residuo para el sitio especifico. 3.2 determinacion del modelo de correlaclon para toda colombia despues de efectuar veinte veces ei cicio de control de calidad, se ileqo , con base en los datos de 338 estaciones (solo se suprirnio una, codiqo 5201502). a la siguiente ecuacion 0 recta de reqresion de minimos cuadrados: y = 28,1 0,00553x [4] y = temperatura media anual del aire en °c x = altitud en m esta ecuacion esquematizada en la fig.1 (los residuos en las figs.2 a 4), es la que mejor representa, para toda colombia, la relacion entre las dos variables (temperatura-altitud) puesto que es la que majores valores presenta en todo sentido: los mas altos coeficiente de determinacion y de correlacio n (0,95766 y 0,9786), los menores errores tipicos de estirnacion (1.0926) y los menores residuos (-2.9 a 2.6) (tablas 2 y 3). v : -.00553)( + 28.07603 r-squared: .95766 30 27,s -.:'..~.. 25 ~\'. 225 20 17.5 ...' 15 : 12.5 ' .. 10 '.'1. 75 0 500 1000 15(.j 2000 2500 3000 3500 al rltuo (m) figura 1. modelo de regresi6n entre la temperatura media anual (y en °c) y la altitud ex en m). colombia. h+lhi-i----++e ·'l4--4--ftllh--'h---'-i--+-'-f-hir -e -2 -3 coservaucns figura 2. variaci6n de los residuos; 8=desviaci6n tlpica y bx=error lipico. colombia z score of residual: temp.med.anual 140 120 100 c o u n t 80 60 40 20 o--~",.. z scale figura 3. distribuci6n de las variables normalizadas de los residuos. colombia ei modelo de reqresion seleccionado (ecuacion [4], fig.1) muestra el perfil altitudinal de la temperatura media anual en superficie para colombia, permite determinar con una relativa buena apcoximacion los valores de temperatura para cualquier sitio ubicado en colombia a condicio n de conocer solo su altitud. adicionalmente, permite establecer que el gradiente vertical de la temperatura media (5,53°c/km) e s diferente del que tradicionalmente ha sido aceptado {6 a 6,5°clkm; p.aj.: ver cortes (1989), stanescu & diaz (1971)}, pero coincidente con el insinuado por trojer (1958). 3.3 regionalizacion y modelos definidos no obstante 10 senalado en los parrafos anteriores, se considero que el modelo mencionado podria no ser 10 suficientemente 40 eslava: perfil altitudinal de la temperatura del alre adecuado para la mayor parte de las utilidades practices ya que, a pesar de presenlar valores estadisticos optimos, algunos de los residuos son un poco relativamente altos (> 2 sr.,) y al aplicar ei control de calidad a nivel mensual, se deben suprimir varias estaciones y no se logra mejorar los resultados generales. 70 oer chert or residue.: tettp.tted."nu"l 1~====~ode edlanean 60 50 30 '20 10 4 5 6 7 b 9 10 bar· figura 4. dlslrlbuci6n de frecuencla de los reslduos (tabla 3), colombia. , en consecuencia se efectuo un anal isis mas detallado de cada uno de los resultados y residuos obtenidos en las 20 veces que se aplico el control de calidad y los resultadc.i de ese analisie permitieron concluir que, si bien es cierto que la temperatura del aire depende en gran medida de la altitud y que esa dependencia predomina sobre los demas factores climaticos, existen otros facto res que, aun cuando en menor forma, influyen y as necesario tenerios en cuenla para asi lograr resultados mas optimos. un anal isis delallado de la variaoion espacial mensual y anual de la temperatura y de los residuos ya mencionados, perrnitio delimilar el territorio colombiano en diferentes regiones (tipificadas por la latitud, longitud, confiquracion del relieve, posicion de las cordilleras, valles interandinos, serranias, lianuras, circulacion general de ia atmosfera, distribuci6n de otros elementos clirnaticos y otras caracteristicas del conjunto fisiografico natural). diferentes regionalizaciones dieron lugar a modelos de reqresion, coeficientes de determinacion y residuos d1ferentes. ei afinamiento paulatino posibilito alcanzar ia regionalizacion optima que se muestra en la fig.5 y que coincide con los conceptos de region natural y cuenca hidrogratica. quedo asi, colombia, caracterizada (para estos efectos) por siete regiones y eubreqlcnes: region 1: region del caribe. region 2: region anona. 2.1 : region andina-gatatumbo. 2.2: region andina-guenca del rio magdalena. 2.3: region andina-guenca del rio cauca. region 3: region de la orinoquia. region 4: region de la amazonia. region 5: region del pacifico (cuenca hidrogratica y region natural del pacifico). una vez definido 10 anterior, se aplic6 ell olclo de control de calidad, especificado en la seccion 3.1, con los datos de las estaciones que conforman cada region 0 subregion, se establecieron los modelos de reqresion lineal (tabla 2) y se comprob6 la bondad de la reqicnahzacio n: los residuos dieron inferiores a 3~..l! y en terminos generales son inferiores a £::iy" con algunas pocas excepciones (tabla 2). . los residuos pasaron de oscilar entre -soc y +3°c para ei modelo que cubre toda colombia, a oscilar entre -2°c y +2°c para los modelos regionales 0 subregionales y, en algunas de elias, entre -1,5° y l,o°c (tabla 3). ei analisis de las caracteristicas de los modelos y sus residuos se presenta graticamente en las figuras que se senalan a continuacion: region region 1: figuras 6a9 2: figuras 10 a 13 2.1 : figuras 14 a 17 2.2: figuras 18 a 21 2.3: figuras 22 a 25 3 y 4: figuras 26 a 29 3: figuras 30 a 33 4: figuras 34 a 37 5: figuras 38 a 41 region region region region geoffslca colomblana ~ 1, octubre de 1992 41 tabla 1. estaclonea metllorol6glcas con datos de tempersturs medls del sire en colombls n° codigo nombre estacion subcuenca dpto municipio laton longow alti temp fe'ox) 1. region del caribe 1 1701501 ~~~m~ul~5~tenario ~~~~~~8fbur ~t~~~~~btanores 12"35' 81'43' 1 27,4 19582 1508504 11'42' 71°49' 50 28,6 19713 1508503 esc.agrop,carraipia carraipta guaj maicao 11°13' 7'?22' 118 26,8 19684 1508502 nazareth a sharimajana guaj uribia 12'11' 71'17' 85 26,6 19715 1508501 apto.maicao av,i<asutoru guaj maicao 11'24' 72'15' 53 28,0 19716 1507504 uribia av,kutamahana guaj uribia 11°43' 72'16' 18 29,2 1961-78 7 1507503 manaure mar caribe guaj manaure 11'46' 72'27' 1 28,7 19408 1507501 bahia honda mar caribe guaj uribia 12'19' 71°48' 10 28,1 19719 1506504 la paulina rancheria guaj fonseca 10'55' 72'49' 170 27,5 196610 1506501 apto.almirante padilla av.zongo guaj riohacha 11'32' 72'56' 4 28,2 197211 1504501 matitas taplas guaj riohacha 11°15' 73°03' 20. 27,4 196412 1501506 san lorenzo piedra magd santa marta 11'07' 74'03' 2.250 13,6 196913 1501505 apto.simon bolivar mar caribe lolago santa marta 11'08' 74°14' 4 28,1 195214 1501503 punta betin mar caribe mago santa marta 11°15' 74°13' 15 27,6 1965-79 15 1501502 la ye cordoba magd cienaga 11'00' 74°13' 20 28,4 196716 2906503 prado sevilla sevilla magd cienaga 10'46' 74°10' 18 27,4 197017 2906502 patuca tucurinca magd cienaga 10'42' 74°12' 20 27,7 196718 2906501 zacapa fundacion magd aracataca 10'35' 74°15' 30 28,0 196719 2904511 juan de acosta magdalena atla in 0 acosta 10'50' 75'03' 20 27,7 196420 2904502 apto.e. cortissoz magdalena atla soledad 10'53' 74°47' 14 27,4 194021 2903512 el limon cga.guajaro atla manati 10'25' 75'04' 7 28,6 1965 22 2903511 san pablo ay.hondo boll lola la baja 10'03' 75'15' 20 27,3 196323 2903508 normal manati cga.guajaro atla manati 10'27' 74°!i8' 10 26,9 196324 2903506 veracruz cga.guajaro atla repelon 10'34' 75'05' 12 28,0 1963-77 25 2903505 bella fuente ay.molinero atla sabanalarga 10'39' 75°01' 145 27,3 1963-71 26 2903504 nueva florida ay.el viento boll lola la baja 09°57' 75'21' 13 27,8 196327 2903503 presa ay grande presa ay.gran boll lola la baja 09°51' 75°19' 60 27,7 196928 2901502 carmen de bolivar ay.alferez boll c de bolivar 09°43' 75'07' 152 27,6 193129 1401503 escuela naval mar caribe boll cartagena 10'23' 75'32' 1 27,7 194730 1401502 apto.crespo cga. de tesca boll cartagena 10'27' 75'31' 2 27,9 194131 1401501 galerazamba mar caribe boll sta catalina 10'47' 75'16' 20 26,8 194532 2804502 algarrobo ariguani lolago fundacion 10"09' 74'04' 60 27,5 196433 2804501 pueblo bello ariguani cesa publo bello 10'22' 73'38' 1.000 20,8 195434 2803503 apto.alfonso lopez guatapuri cesa valledupar 10'26' 73°15' 138 28,7 196835 2803502 el callao azucarabuena cesa valledupar 10'23' 73'14' 110 28,2 196836 2803501 villa rosa diluvio cesa valledupar 10°12' 73'33' 70 28,7 196837 2802510 la laguna jobo cesa robles.j..ap 10'16' 73'04' 1.500 20,8 1962-75 38' 2802507 motilonia-codazzi pernambuco cesa codazzi 10'00" 73°15' 129 28,7 197239 2802506 la europa sicarare cesa codazzi 09'56' 73'21' 100 29,0 1969-76 40 2802504 san jose de oriente chiriaimo cesa rbles-la pa 10'21' 73'03' 850 24,1 197541 2802503 san diego chiriaimo cesa rbls.j..apaz 10'21' 73°1 l' 165 28,6 1964-78 42 2802502 el rincon jobo cesa san diego 10'16' 73°10' 170 28,1 196443 2801507 urumita nacho guaj urumita 10'34' 73'01' 255 27,1 197544 2801504 guatapuri guatapuri cesa valledupar 10'33' 73'22' 150 26,7 1964~7 45 2801502 la dorada marquezote guaj villanueva 10'30' 73'07' 255 27,5 1964-75 46 2502515 ayapel cga.ayapel cord ayapel 08°19' 75'08' 22 27,4 196747 2502514 nechi nechi anti caucasia 08'06' 74°46' 200 27,6 197048 2502510 apto.baracoa bzo.de loba boll magangue 09'16' 74'49' 18 28,3 195449 2502509 apto.las florez magdalena magd wl banco 09'04' 73°59' 34 29,1 195250 2502508 apto.rafael barvo corozal sucr corozal 09'20' 75°17' 166 27,5 194051 2502504 apto.la florida ay.late grande sucr san marcos 08'42' 75'11' 30 28,5 193252 2502501 el banco mac;dalena magd el banco 09'00' 73°59' 33 29,1 1932-46 53 2502004 la esmeralda catjo guataca boll sn fernando 09°11' 74'21' 25 28,6 192554 1308504 apto,los garzones catjo bugre cord monteria 08°49' 75'51' 20 28,2 197455 1308503 s bernardo del viento cano cicara cord s bdo viento 09'22' 75°58' 22 27,2 196456 1308502 lorica sinu cord lorica 09°16' 75'49' 30 27,2 196457 1308501 la doctrina siny cord lorica 09'18' 75°54' 20 27,3 196858 1307507 cerete can m..gre cord cerete 08°54' 75'47' 13 27,7 1960-70 59 1307506 monteria sinlj cord monteria 08°45' 75°53' 20 28,1 1962-71 60 1307504 apto.berastegui catjo bugre cord cga de oro 08°53' 75°40' 40 28,4 1944-74 61 1307503 turipana cano bugre cord cerete 08°51' 75'49' 20 27,1 196062 1307502 el salado ay.agus prieta cord cga de oro 08'55' 75'35' 40 27,7 196463 1306501 la torpeza a.cuchuco cord monteria 08'26' 75°58' 42 28,0 1965-79 64 1305501 la voragine san diego cord monteria 08'36' 76'02' 80 28,0 1965-79 85 1304501 jaraguay a.jaraguay cord valencia 08°18' 76'05' 70 26,8 196666 1303502 el manso manso cord tierralta 07'51' 76'18' 160 26,0 1~7 67 1303501 puerto nuevo sinu cord tierralta 07°57' 76'17' 145 26,6 196568 1202501 apto.gonzalo mejia caribe anti turbo 8'08' 76°44' , 27,3 194869 1201503 idema-montecristo chigorodo anti chigorodo 07'39' 76°41' 40 26,4 1972-82 70 1201502 uniban zungo anti apartado 07'47' 76'38' 23 26,4 197771 1201501 villarteaga villarteaga anti mutata 07'21' 76"29' 132 25,8 19722.1 region andina·catatumbo 72 1605504 centro adm abrego algodonal n.sa abrego 08'05' 73'14' 1.430 20,3 196973 1603501 tlbu tlbu n.sa tibu 08'39' 72'42' 66 27,3 194074 1602506 francisco romero salazar n,sa salazar 07'46' 72'48' 1.000 21,7 195275 1602505 l1moncito zulia n,sa cucuta 08'06' 72'34' 280 26,7 1968-81 76 1602504 cinera-villaolga a.la floresta n.sa cucuta 08'09' 72'29' 100 27,2 196577 1602503 salazar salazar n.sa salazar 07'47' 72'49' 1.000 21,6 197378 1602502 risaralda cano picha n.sa zulia 08"09' 72'34' 140 27,0 195279 1602501 carmen de tonchala a.tonchala n.sa cucuta 07'51' 72'34' 285 27,1 196880 1601508 el dove pamplonita n.sa cucuta 08'19' 72'25' 290 27,8 1968-71 81 1601506 blonay tachira n,sa chinacota 07'35' 72"36' 1.235 19,9 195182 1601505 san faustino a.faustinera n,sa cucuta 08'04' 72'25' 420 26,3 1968-83 83 1601504 santa isabel pamplonita n.sa cucuta 08°14' 72'26' 90 27,0 196884 1601502 iser pamplona pamplonita n.sa pamplona 07'22' 72"39' 2.340 14,7 197285 1601501 apto, camilo daza pamplonita n.sa cucuta 07°56' 72"31' 250 27,5 194186 1601027 cucuta pa~plonita n.sa cucuta 07'57' 72:'3iy 216 28,4 1955~7 87 1601005 alto vlento cano negro n.sa cucuta 08'10' 72'29' 280 27,3 19342.2 region andina·magdalena 88 2405505 aguas blancas sogamoso sant san vicente 06°57' 73"26' 1.000 22,2 1967continua 42 eslava: perfil altitudinal de la temperatura del alre tabla1 (contlnuaclon). estaclon8ll meteorologlcas con datos de temperatura media del alre en colombia n° codigo nombre estacion subcuenca dpto municipio laton longow alti temp ~ 89 2403525 chita q.pena blanca boya chita 06°13' 72"28' 2.888 12,1 197()' 90 2403518 san rafael chicamocha boya tibasosa 05°47' 72"59' 2.548 15,3 196991 2403517 tunguavita salitre boya paipa 05°45' 73"08' 2.470 14,1 196892 2403515 belencito chicamocha boya nobsa 05°47' 72"53' 2.530 15,1 196793 2.403513 u.p.t.c. chula boya tunja 05"34' 73"22: 2.690 12,9 196294 2403512 surbata-bonza surba boya duitama 05°49' 73"04' 2.485 14,3 194495 2402504 esc.agrop.mogotes mogoticos sant mogotes 06"29' 72"58' 1.667 18,6 197396 2402503 la sierra la rucia boya duitama 06"00' 73°10' 2.700 12,2 196797 2402502 el cucharo fonce sant pinchote 06"31' 73°13' 975 24,8 195398 2401522 villa carmen gachaneca boya samaca 05"32' 73"30' 2.600 13,7 196899 2401517 pasadena moniquira boya villa de leyva 05°41' 73"36' 2,133 18,1 1962100 2401512 isla del santuario lag.fuquene cund fuquene 05"28' 73°44' .2.580 14,3 1942101 2401503 ubate ubate cund ubate 05°19' 73°49' 2.600 14,1 1929-58 102 2401502 granja bertha suarez boya moniquira 05°5z 73"24' 1.700 18,4 1929103 2321503 aguas blancas buturama cesa aguachica 08°15' 73"37' 208 27,6 1973104 2321501 gamarra magdalena cesa gamarra 08"20' 73°45' 50 28,1 1931-36 105 2319520 cachiri cachiri sant surata 07"29' 73"00' 1.850 17,3 1971· 106 2319518 esc.agrop.cachira cachira n.sa cachira or44' 73°03' 1.882 16,6 1972107 2319513 apto. palonegro de oro sant lebrija 07"06' 73°1z 1.189 21,1 1974108 2319509 vivero surata surata sant surata 07"21' 73°00' 1.725 18,6 1968109 2319506 la mariana surata sant tona 07"07' 73°00' 2.250 16,9 1968-75 110 2319505 vivera chimita surata sant bucaramanga 07"07' 73°09' 830 25,5 1968iii 2319504 univ.ind. de satlitander tona sant bucaramanga 07"08' 73°06' 1.018 22,9 1961112 2319503 apto.gomez nino de oro sant bucaramanga 07"08' 73"08' 931 24,0 194()'74 113 2318501 villa leiva q.stos gutierr sant sabana de tor 07"26' 73"30' 328 27,2 1966· 114 2317502 apto.otu otu anti remedios or-or 74°43' 630 24,8 194()' 115 2315504 el centro magdalena sant barranimeja 06°52' 73°46' 162 27,5 1937116 2315503 apto. yariguiez magdalena sant barranimeja 07°01' 73°48' 126 28,4 1931117 2312504 campo capote q.capote sant velez 06"35' 73°55' 178 27,0 1968118 2311501 puerto boyaca magdalena boya pta boyaca 05°57' 74"36' 350 27,9 1974119 2310002 yolombo san lorenzo anti yolombo 06"36' 75°00' 1.680 19,8 1947-50 120 2308519 puerto bela guatape anti san carlos 06°13' 74°51' 790 23,1 1973-83 121 2308515 apto. rionegro negro anti rionegro 06°07' 75"26' 2.140 15,5 1973122 2308513 san c....rlos san carlos anti san c....rlos 06°12' 74°59' 1.060 22,7 1973-ao 123 2308511 el penal rionegro anti el penol 06°13' 75°14' 2.075 17,5 1973124 2308508 gja.exp.el nus nus anti san roque 06"29' 74°49' 835 22,9 1972125 2308504 la selva negro anti rionegro 06°11' 75"23' 2.090 16,2 1958126 2303502 apto. palanquero magdalena cund puerto salgar05"29' 74"39' 172 27,8 1957127 2302502 la esperanza magdalena toll honda 05°15' 74°45' 122 27,9 1970128 2302501 llanadas guarino cald manzanares 05°13' 75°09' 1.470 19,7 1951129 2207502 varsovia cucuana toll san luis 04"01' 75°10' 400 27,0 1964-al 130 2205503 santa helena saldaf::ja toll purificacion 03°58' 74°55' 300 27,8 19631:>1 2205502 mesa de pole saldana toll ataco 03"28' 75"33' 500 25,5 1968132 2204501 gja.demos.chaparral amaya toll chaparral 03°43' 75"32' 1.040 24,2 1964133 2202501 apto. planadas ata toll planadas 03°14' 75°45' 1.355 21,7 1964134 2125508 el salta magdalena toll ambalema 04°48' 74°48' 450 28,2 1968135 2125505 ingenio pajonales magdalena toll ambalema 04°46' 74°47' 250 28,3 194().50 136 2125502 l1bano recio toll l1bano 04°56' 75"03' 1.495 19,1 1934-48 137 2125501 granja armero maracaibo toll armera 04°58' 74°55' 400 28,2 1930-57 138 2124504 apto. perales apia toll ibague 04"26' 75"09' 928 23,7 197()' 139 2124501 perales hato apia apia toll ibague 04"27' 75°04' 750 24,7 1965140 2121512 san jorge coello toll ibague 04"26' 75°15' 1.250 22,2 1923-61 141 2121510 cajamarca bermellon toll cajamarca 04"26' 75"26' 1.920 18,2 1964142 2121509 maranones coello toll coello 04°13' 74°56' 370 27,1 1959-71 143 2121508 chicoral coello toll espinal 04°15' 75°00' 475 27,1 1963144 2121507 chapeton-mno melendr coello toll ibague 04"27' 75°16' 1.300 20,4 1954145 2121505 el paso tochecito toll cajamarca 04"30' 75"34' 3.264 8,3 1945-ao 146 2120612 anolalma bogota cund anolaima 04°46' 74"28' 1.916 18,8 1962-65 147 2120609 zipaquira bogota cund zipaquira 05°01' 74"01' 2.620 12,9 1931-54 148 2120579 pista 1-2 eldorado bogota cund bogota d.c. 04°43' 74"09' 2.547 13,3 1972149 2120567 la florida bajamon cund anolaima 04°46' 74"26' 1.915 16,8 1970150 2120566 las mercedes bogota cund anapoima 04"34' 74"33' 810 24,8 197()' 151 2120562 guatavita tomine cund guatavita 04°53' 73"22: 2.625 13,0 1967152 2120558 vivero venado de oro bogota cund bogota d.c. 04"36' 74"04' 2.725 12,3 1965153 2120542 tibaitata balsillas cund mosquera 04°42' 74°1z 2.543 13,2 1954154 2120539 represa sisga sisga cund choconta 05"05' 73°44' 2,675 12,7 1952155 2120537 pantano redondo bogota cund zipaquira 05°0z 74°02' 3.000 11,0 1971156 2120532 gja.usme-buenavista bogota cund bogota d.c 04°26' 74°08' 3.175 10,2 1947-59 157 2120524 vitelma san cristobal cund bogota d.c, 04"34' 74"04' 2.800 12,1 1941158 2120523 obs.meteorol. nal. bogota cund bogota d.c 04"38' 74"06' 2.556 14,2 1941159 2120521 techo-aerodromo fucha cund bogota d.c 04"38' 74°09' 2.550 13,6 1942-59 160 2120516 la ramada bogota cund funza 04°4z 74°11' 2.545 13,8 1937161 2120515 el cucharo bogota cund girardot 04°18' 74°48' 326 28,5 1936-61 162 2120510 psto monta-<:hoconta bogota cund choconta 05°09' 73°41' 2.780 13,2 1951-54 163 2120509 regadera tunjuelo cuno bogota d.c 04"24' 74"09' 3.050 10,2 1941164 2120504 la picota tunjuelo cund bogota d.c. 04"33' 74"08' 2.800 13,3 1929-53 165 2119507 pasca cuja cund pasca 04°19' 74°18' 2.256 15,1 1969166 2119506 pandi sumapaz cund pandi 04°1z 74"29' 950 23,5 1969167 2119503 alberto williamson sumapaz cund tibacuy 04"21' 74"25' 1.550 19,2 1951168 2118506 espinal coello toll espinal 04"09' 74°53' 438 27,5 193().59 169 2118504 apto.santiago vila magdalena toll flandes 04°17' 74°48' 286 28,3 1951170 2118503 guamo luisa toll guamo 03°59' 74°59' 360 27,6 1953171 2118502 nataima magdalena toll espinal 04°1z 74°56' 431 27,9 1971172 2115502 la montana prado toll dolores 03"33' 74°54' 1.260 20,9 1954173 2114504 san alfonso cabrera hull villavieja 03"23' 75°06' 440 28,5 1963174 2114501 el banquito cabrera hull colombia 03"23' 74°51' 800 25,5 1963-83 175 2113504 media luna chenche toll coyaima 03°48' 75°01' 410 28,0 1964176 2113503 anchique anchique toll nata~ima 03"35' 75"07' 415 27,8 1963177 2113502 jabalcon chenche toll saldana 03°51' 75"01' 425 28,1 1963178 2111514 los laureles villavieja hull baraya 03°06' 74°55' 2.100 14,9 1963179 2111510 palacio-vegalarga fortalecillas hull neiva 02"56' 75°02' 1.100 22,3 1971continua geoflslca colomblana ~ 1, octubre de 1992 43 tabla 1 (contlnuaci6n), eataclones meteorol6glcaa con datoa de temperatura media del aire en colombia n° codigo nombre estacion subcuenca dpto municipio lapn long'w alti temp ftflox) 180 2111508 villavieja ffcc magdalena hull villavieja 03°14' 75°13' 430 28,0 1964181 2111507 el portal villavieja hull tello 03°02' 75°03' 1.300 22,0 1963182 2111505 santa helena villavieja hull baraya 03°08' 75°04' 635 26,4 1963-73 163 2111504 granja neiva ceibas hull neiva 02°51' 75°18' 438 27,6 1961-75 184 2111502 apto,la manguita las ceibas hull neiva 02"58' 75°18' 439 27,5 1930185 2110505 los rosales neiva hull campoalegre o~t 75~5' 553 26,5 1973186 2110502 lavuelta neiva hull campoalegre 0~9' 75"22' 500 25,8 1968-73 187 2109501 el juncal magdalena hull palermo 02"50' 75"20' 460 27,0 1965188 2106504 zuluaga loro hull garzon 0~16' 75oj2' 1.623 19,6 1971189 2106503 jorge villamil-gigante q,guandinosa hull gigante o~' 75oj3' 1.500 19,5 1954190 2105504 toez paez cauc paez 02"49' 76"05' 1,997 16,8 1971191 2105502 esc,agrop,la plata la plata hull la plata on5' 75°55' 1.070 '21,9 1969192 2101503 parque arqueologico naranjo hull san agustin 01°51' 76°18' 1.800 17,5 19712.3 region andina-cauca 193 2703503 santa margarita nechi anti el bagre or44' 74°46' 179 27,6 1970-80 194 2702503 anori anori anti anori 07°04' 75°09' 1.610 18,7 1975195 2702502 charcon liberia anori anti anori or23' 75°00' 165 26,4 1970196 2701519 guayabito rosario anti sto domingo 060j3' 75°08' 1.700 20,6 1970197 2701511 aragon grande anti s rosa 0 os os 06°41' 75°34' 2.600 12,6 1970198 2701509 tullo ospina medellin anti bello 06°19' 75oj2' 1.438 21,3 1950199 2701507 apto.olaya herrera medellin anti medellin 06°13' 75oj6' 1.490 21,8 1940200 2701505 fac de agronomia medellin anti medellin 06°16' 75oj5' 1.475 21,5 1937-56 201 2625504 cacaoteras del dique cauca anti caucasia or59' 75°0' , 55 27,2 1968202 2624503 taraza cauca anti taraza 07oj5' 75~4' 120 27,0 1975203 2623501 san jeronimo aurra anti san jeronimo 06°26' 75°44' 764 25,0 1945-62 204 2622504 hda piunti q,tesorero anti buritica 06°44' 75°55' 1.540 20,1 1970205 2620502 el rosario q,mina anti venecia 05°58' 75°43' 1.600 19,9 1967206 2620501 esteban jaramillo cauca anti venecia 05°55' 75°43' 1.450 20,3 1950-65 207 2619502 ita-andes san juan anti andes 05°42' 75°53' 1.250 22,0 1970208 2619501 miguel valencia san juan anti jardin o5oj2' 75°51' 1.570 18,9 1952209 2618504 san felix san felix cald salamina o502j' 7502j' 2.821 11,1 1970210 2618502 mesopotamia buey anti la union 05°53' 75°18' 2.370 14,5 1970211 2618501 sonson arma anti sonson 05°42' 75°18' 2.530 15,3 1955-56 212 2617502 inst.los fundadores sucio cald riosucio 05~6' 75°43' 1.820 18,6 1970-79 213 2615514 gja.luker-santagueda chinchina cald palestina 05°05' 75°41' 1,020 22,5 1964214 2615511 apto.la nubia chinchina cald manizales 05°02' 75~8' 2,080 16,2 1968215 2615509 santagueda chinchina cald palestina 05°05' 75°40' 1.010 22,6 1965216 2615507 las palomas chinchina cald manizales 05°08' 75~8' 2.700 12,8 1959-72 217 2615505 facultad agronomia chinchina cald manizales 05°03' 75"29' 2.150 16,5 1955218 2615502 cenicafe chinchina cald chinchina 05°00' 75°38' 1.310 20,9 1941219 2614503 la camelia mapa risa santuario 05°05' 75°58' 1.670 19,0 1963220 2614502 bella vista guatica cald anserma 05°11' 75°49' 2.000 16,7 1963221 2614501 la cecilia risaralda cald viterbo 05°04' 75°53' 1.028 24,2 1963-73 222 2613510 la bohemia cauca risa pereira 04°52' 75°54' 1.020 23,5 1963223 2613508 universidad de pereira otun risa pereira 04°48' 75°42' 1.354 21,1 1963-71 224 2613507 el cedral otun risa pereira 04°47' 75oj2' 2.120 14,9 1960225 2613505 naranjal chinchina cald chinchina 04°58' 75°42' 1.400 20,6 1950226 2613504 apto.matecana otun risa pereira 04°49' 75°44' 1.342 21,9 1947227 2613503 dos quebradas san eugenio risa sta rosa de c 04°51' 75°42' 1.470 19,7 1946-70 228 2612513 cumbarco barragan vall sevilla 04°10' 75°41' 1.740 18,4 1973229 2612510 arturo gomez-alcala cauca vall alcala 04°40' 75°48' 1.320 21,0 1960230 2612508 paraguaycito sto domingo quin buenavista 04'23' 75°44' 1.250 21,6 1951231 2612507 la bella sto domingo quin calarca 04oj1' 75°40' 1.450 19,5 1950232 2612506 aptoel eden quindio quin armenia 04'27' 75'46' 1.204 22,1 1949233 2611505 las gramas cauca vall rodanillo 04~8' 76'01' 940 25,1 1967-83 234 2611504 centro adm la union cauca vall la union 04oj2' 76°03' 920 24,2 1967235 2611503 tierra blanca cauca vall roldanillo 04°25' 76°06' 930 24,2 1967236 2610516 apto.farfan tulua, vall tulua 04'06' 76°13' 955 24,1 1948237 2610509 heraclio uribe saldai,a vall sevilla 04'16' 75°55' 1.540 19,3 1936238 2609511 ica-balboa cauca vall buga 03°58' 76°18' 960 23,7 1961239 2607505 ing manuelita-porvenir nima vall palmira 03'34' 76°11' 1.020 23,6 1900240 2607504 apto.palmaseca guachal vall palmira 030j3' 76~3' 961 23,7 1971241 2607502 cali puerto parraga vall candelaria 03~6' 76~4' 930 24,7 1940-71 242 2607501 palmira ica palmira vall palmira 03oj1' 76°19' 975 23,8 1930243 2606502 miranda desbaratado cauc miranda 03°15' 76°14' 1.200 23,0 1965244 2606501 ingenio cauca desbaratado cauc miranda 03°11' 76°19' 1.000 23,1 1965245 2605507 universidad del valle melendez vall cali 03~4' 76oj2' 970 23,8 1965246 2604501 ingenio bengala palo cauc pto tejada 03°15' 76~5' 1.000 23,4 1965247 2603506 t ambo-manuel mejia cauca cauc el tambo on4' 76°48' 1.700 18,3 1953248 2603505 la florida-j.m.obando cauca cauc popayan ont 76oj5' 1.850 17,8 1949249 2603504 la salvajina cauca cauc buenos aires 0~58' 76°42' 1.100 23,0 1972250 2603503 apto.machangara cauca cauc popayan 02°26' 76oj5' 1.730 18,8 1941251 2603502 munchique sucio cauc tambo oz-ar 76°58' 3.012 10,7 1971252 2603501 universidad del cauca cauca cauc popayan 0~26' 76°36' 1.730 18,4 1925-66 253 2602509 venta de cajibio cajibio cauc cajibio 0~1' 76oj5' 1.800 18,5 1972254 2602507 gabriel lopez palace cauc-totoro o~' 76°.11' 3.000 10,5 1971255 2602504 santander-hd s julian cauca cauc santander 03'06' 76oj1' 960 24,0 1966258 2602503 japio cauca cauc caloto 03001' 76~8' 1.015 23,2 1965257 2601502 paletara cauca cauc coconuco o~' 76~5' 2.900 10,6 19713. orinoquia 258 3801503 puerto carreno orinoco vich pto carrer!lo 06°11' 67~9' 50 28,1 1972259 3801502 el merey orinoco vich pto carreno 06°10' 67oj2' 55 28,0 1968-72 260 3705501 apto.arauca ai3auca arau arauca 07°04' 70°44' 128 27,1 1950261 3704501 saravena cno.la pava arau saravena 06°51' 71°53' 148 ~:54 1971262 3701502 berlin jordan sant tona 07°11' 7~52' 3.214 1968263 3510502 barranca de upia upia meta cumaral 04'34' 7~59' 250 26,2 1976264 3509503 el tunel lago de tota boya cuitiva 05°34' 72°51' 3.000 11,1 1971265 3509502 don antonio upia casa sabanalarga 04°41' 73°02' 300 25,3 1968266 3508502 rondon mueche boya rondon 05~1' 73°13' 2,120 16,2 1970267 3507501 nuevo colon turmeque boya nuevo colon 05°21' 73°21' 2.438 14,0 1965268 3506501 gacheta gacheta cund gacheta 04°49' 730jt 1.850 18,2 1972continua--44 eslava: perfil altitudinal de la temperatura del aire tabla 1 (contlnuaci6n). estaciones meteorol6gicas con datos de temperatura media del aire en colombia n" codigo nombre estacion subcuenca dpto municipio lapn longow alti temp f'ef"xx) 269 3505501 el japon humea cund medina 04"23' 73°18' 280 26,0 1968-<15 270 3503503 salinas de upin guatiquia meta restrepo 04°18' 73°34' 850 23,3 1960271 3503502 apto, vanguardia guatiquia meta villavicencio 04°10' 73°31' 423 25,7 1924272 3502503 pachaquiaro negro meta puerto lopez 04°04' 73°11' 200 26,2 1968273 3502502 la l1bertad negro meta villavicencio 04°03' 73°29' 336 25,6 1968274 3501502 granja iraca ca~o camoa meta san martin 03°41' 73°42' 400 25,1 1960275 3501501 la nutria ca~o guaroa meta castilla 03°48' 73~6' 230 27,2 1968-75 276 3401501 las ga viot as cano urimica vich sn j de oc\jne 04°33' 70°55' 171 26,0 1967277 3306501 jamury. vichada vich pto carreno 04°22' 69°10' 154 25,6 1976278 3303501 carimagua muco meta pto gaitan 040j4' 71ow' 200 26,1 1972279 3207503 vistahermosa guejar meta vistahermosa 03°02' 73°44' 325 25,8 1969280 3207502 puerto limon ariari meta fuente de oro 03~' 7303q' 255 25,6 1968281 3207501 los naranjos ariari meta granada 03~8' 73°43' 220 25,7 1968-76 282 3204501 bocas del ariari guayabero guav s j de guavia 02"35' 7~4t 260 25,6 1967283 3203501 la macarena guayabero meta la macarena 0~03' 73°56' 350 25,4 1967284 3109501 puerto inirida inirida guai pto inirida 03°50' 67°55' 100 26,2 19724. amazonia 285 4206501 apto, mitu vaupes vaup mitu 01°16' 70°14' 207 25,8 1948286 4401501 villagarzon mocoa putm v!lla amazoni 01°00' 76036' 440 24,6 1964287 4401503 valencia caqueta cauc san sebastian 01°51' 76°31' 2,900 10,6 1971288 4403501 centro adm, florencia hacha caqu florencia 01031' 75°36' 280 25,1 1969-<14 289 4403502 apto, capitolio hacha caqu florencia 01036' 75°32' 244 25,0 1969290 4404501 san jose de fragua fragua caqu belen de los a 01°18' 76°00' 320 24,7 1972291 4404502 valparaiso pescado caqu florencia 01°15' 75°36' 200 25,3 1967292 4404503 la mono pescado caqu belen de los a 01~o' 75°48' 220 24,7 1967293 4405501 tres esquinas orteguaza caqu florencia 00°45' 75°13' 219 25,1 1971294 4601501 san vicente 0 caguan caguan caqu s vicente 0 c 0~09' 74°48' 300 24,6 1963295 4603501 maguare guayas caqu doncello 01031' 75'08' 270 24,9 1967296 4603502 puerto rico guayas caqu puerto rico 01°58' 75°09' 400 25,5 1972297 4701501 la menta putumayo putm mocoa 01°11' 76'56' 2,067 15,6 1968-<12 298 4701503 sibundoy putumayo putm s!bundoy 01°11' 76°55' 2.100 16,1 1957299 4701502 puerto asis putumayo putm puerto asis 00°30' 76'30' 254 25,4 1965300 4801501 apto,vasquez cobo amazonas amaz leticia _04°09' 69°51' 84 25,9 19585. region del pacifico 301 1112501 la teresita tr\jando choc r!o.sucio roo' 7703q' 50 26,0 1972302 1111502 canasgordas canas gordas anti canasgorda 6°44' 76°02' 1.200 21,6 1966-73 303 1110501 murindo murindo anti murindo 6°58' 76°45' 23 26,6 1972-<11 304 1108501 la loma bojaya choc bojaya 6032' 76°59' 20 26,2 1980305 1104501 apto,el carano-{)uibdo atrato choc quibdo 5°43' 76°31' 53 26,2 1947306 1103501 lloro atrato choc lloro 5°30' 76°34' 90 25,8 1983307 1102501 la mansa atrato choc c de atrato 5°53' 76°01' 2,100 16,6 1973308 5601501 panamer!cana pacifico choc bahia solano 6°14' 77°24' 4 25,6 1963309 5408501 noanama san juan choc istmina 4°40' 76°56' 40 25,8 19733:0 5407504 la mision san juan vall bnentura 4°11' 77°11' 5 25,6 1968311 5407502 bajo calima calima vall bnentura 4°00' 76°51' 50 25,6 1931312 5403502 argella-el recreo las vueltas vall argelia 4°42' 76°11' 1.600 19,6 1972313 5403501 sipi taparo choc sipi 4°40' 76°39' 110 26,0 1973-<14 314 5402504 andagoya san juan choc istmina 5°08' 76°43' 35 26,8 1962-75 315 5402502 apto,condoto condoto choc condoto 5°06' 76°41' 66 25,6 1971316 5402501 san jose de palmar ingara choc s,jose palma 4°51' 76°11' 1.000 20,6 1973317 5311504 la cumbre cauca vall la cumbre 3°38' 76035' 1.580 18,5 1966-<58 318 5311503 julio fern-restrepo grande vall restrepo 3°49' 76"31' 1.360 19,7 1962319 5311502 colpuertos pacifico vall bnentura 3°53' 77°04' 10 25,9 1969320 5311501 apto.buenaventura pacifico vall bnentura 3°51' 76°58' 14 25,8 1946321 5304503 apto,guapi pacifico cauc guapi 2035' 77°53' 10 25,9 1978322 5304501 bonanza guapi cauc guapi 2031' 77°49' 10 25,8 1966323 5206502 barbacoas telembi nari barbacoas 1°40' 78°08' 32 25,8 1972324 5205507 ospina perez guaitara nari consaca 1°16' 77~8' 1.700 18,9 1951325 5205503 bombona guaitara nari consaca 1°11' 77~8' 1.493 19,4 1968326 5205502 el paraiso guaitara nari tuquerres 1°05' 77031' 3,120 10,8 1968327 5205501 apto,san luis guaitara nari ipiales 0°49' 77038' 2962 11,2 1941328 5204502 apto,antonio narii~o pasto nari pasto 1'25' 71'16' 1.796 18,9 1957329 5204501 obonuco pasto nari pasto 1°14' 77°16' 2,710 13,3 1953330 5204013 l1ceo la merced pasto nari pasto 1°13' 77°11' 2,380 15,3 1941-<56 331 5202505 la sierra guachicono cauc la sierra 2°10' 76°46' 1.870 17,7 1971332 5202503 mercaderes 2 sambingo cauc mercaderes 1°46' 77°10' 1.174 22,3 1971333 5202502 los milagros sambingo cauc bolivar 1°44' 76°54' 2.300 15,8 1972334 5202501 bolivar sambingo cauc bolivar 1"53' 76°58' 1,510 21,0 1971335 5201503 el tambo sucio cauc el tambo 2°30' 76°49' 1.720 18,1 1963336 5201502 la fonda patia cauc elbordo 2°09' 77°03' 580 26,6 1974337 5103501 apto,la florida caunapi nari tumaco 1°49' 78°45' 3 26,2 1948338 5102502 ricaurte guiza nari ricaurte 1°12' 77°59' 1,381 19,3 1968339 5102501 granja el mira caunapi nari tumaco 1°34' 78°41' 16 25,4 1968tabla 2, caracterlstlcae de 108model08 de regresl6n (recta de rll9resi6n de mlnimos cuadrados) entre la temperatura media anual(y en ·c) y la attltud (x en 01) region ecuacion de coeficiente coeficiente error tlpico gradiente temp residuos regresion determinacion variabilidad estimacion °clkm ao m min max colombia y_28,hl.00553x 0,95766 4,80291 1,09260 5,53 28,1 -2,86 2,59 1. caribe y -28,l.o,d0589x g:m~~ 3,01927 0,82362 5,89 28,1 -1,54 1,55 2. andina y _29,3.o,00613x 3,94834 0,83064 6,13 29,3 -1,88 1,90 2.1 catatumbo y -28,4.o,oo606x 0,96344 3,14692 0,78240 6,06 28,4 -1,28 1,05 2,2 magdalena y -29,6.o,00624x 0,98125 3,91322 0,81451 6,24 29,6 -1,81 1,60 2.3cauca y -29,4.o,00617x 0,96773 3,67909 0,75263 6,17 29,4 -1,62 1,96 3 y 4. orin-amaz y_27,l.o,00544x 0,98122 2,92275 0,68522 5,44 27,1 -1,36 1,19 3, orinoouia y-27,4.o,00551x 0,98535 2,71371 0,64245 5,51 27,4 -1,06 1,20 4, amazonia y -26,5.0,00529 0,99008 2,05499 0,47380 5,29 26,5 -1,09 0,67 5, pacifico y -26,2.0,00462 0,98093 3,00984 0,65836 4,62 26,2 -1,77 1,32 geoffslca colomblana ~ 1, octubre de 1992 45 tabla 3. distribuci6n de frecuencia de los reslduos region -3a-2,5 -2,5 a-2 -2.-1,5 -1,5.-1 -1 s-o.s -0,5. a a a 0,5 0,5.1 1 a 1,5 1,5 a 2 # % # % # % # % # % # % # % # % # % # % barra 1 barra 2 barra 3 barra 4 barra 5 barra 6 barra 7 barra 8 barra 9 ew1ra 10 colombia 2 6,0 10 3,0 18 5,3 40 11,8 40 11,8 50 14,8 71 21,0 46 13,6 27 8,0 21 6,2 barra 1 barra 2 barra 3 barra 4 barra 5 barra 6 barra 7 barra 8 1, caribe 2 2,8 9 12,7 10 14,1 14 19,7 12 16,9 16 22,5 6 8,5 2 2,6 2. andina 6 3,2 19 10,2 27 14,5 35 18,8 49 26,3 32 17,2 9 4,8 9 4,8 2.1 catatumbo 2 12,5 3 18,8 1 6,2 6 37,5 1 6,2 2.2 magdalena 3 2,9 10 9,5 19 18,1 11l 17,1 24 22,9 19 18,1 10 9,5 2 1,9 2.3 cauca 3 4,6 4 6,2 7 10,8 19 29,2 16 24,6 12 18,5 1 1,5 3 4,6 3 y 4. orinoouia y amazonia 9,3 5 11,6 11 25,6 10 23,3 2 4,7 3. orinoquia 3,7 5 18,5 6 22,2 11 40,7 2 7,4 2 7,4 4. amazonia 6,25 1 6,25 4 25,0 8 50,0 2 12,5 5. pacifico 2 5,3 6 15,8 8 21,1 15 39,5 6 15,8 2,6 figura 5_reglonallzacl6n utlllzada para ei anallsis de,a temper4tura media anual del alre en colombia v » -.00589)( + 28.08832 r-squared: .85912 t e m p 30 28 ~ ';;'r, 8 26 .• v 24 22 20 18 16 14 12,1--~~~~~~~~~~~~~~o 250 500 750 1000 1250 1500 1750 2000 2250 altttud tm) figura 6. modelo de r~resi6n entre la temperatura media anual (y c) y la altitud (x m) regioncarise 1.5 hl-----i\------f\--""*"if-------=----1h'-i+.~ -.5 f-l---i+------+rr-------,4-=~f_----~ -1 -1,5 -2 figura 7. variaci6n de los residuosi.&:desviaci6n tlplca y ox=error tldlco. region caribe 46 eslava: perfil altitudinal de la temperatura del alre 25 22.5 20 17.5 c 15 0 u 12.5 n 10t 7.5 5, 2.5 0 z scale figura 8. distribuci6n de las variables normatizadae de los residuos. region caribe c o u n l o r:==f======eode edlan ean bar .. figura 9. distribuci6n de frecuencia de 108 residuos (tabla 3). region caribe v » -,00613x" 2931416 r-sauared: .9758 30 t e 27.5 m p 25 225 20 17.5 15 : 12.5 10 7.5 0 500 1000 1500 2000 2500 3000 3500 altitud fml figura 10. modelo de rep,resl6n entre la temperatura media anual (yc) y la altitud (x-m) region andi~a 2 1 ~. i !ij! ,ii ~i ja ii. 0 ~ 11 )v~~ ii ~i i~ 1 ii 1.5 '.5 -).5 ·2 figura 11. vamicion de 105 residuos; o=desviaci6n upica y ox=error upico. region andina 70 60 50 c o u n t 40 30 20 100--_ -1 z scale figura 12. distribuci6n de las variables normalizadas de los residuos region andina 50 45 ~ode ~t:•..=. ======--11= ~~~an c o u n t bar .. figura 13. distribuci6n de frecuencia de los residuo8 (tabla 3) region andini( geofislcli! colomblana tf11, octubre de 1992 47 t e m p y. -.00606)( + 28.43175 r~quared: .96344 30 26 0 'lll 26 24 22 20 c 0 16 16 14':--,"-:::::-:=-,-::c:-~--o;-~~~~-.:::::5:"o 250 500 750 1000 1250 1500 1750 2000 2250 2500 altituo (m) figura 14. modelo de regresi6n entre la temp media anual iv en ·c) y la altitud ix en m) regioiol andina·catatum~o 125 751----.=:=-=---7"c-----+-\--,1,----.~ 5 ~ .~o't--f------¥----+-+-i+---\,---h _,~ -b-5 -.75;1-----------\--j--------\--!_ -~ -, -125 -1.5 7 6 5 4 3 2 010..... figura 15. variaci6n de los residuos; lboesvlaci6n trpica y l)x=error tlgico region andina-catatumb z scale figura 16. oistribuci6n de las variables normalizadas de los reslduos region andina-catatumbo 6 ~-----~ode r-;::=+======~edi6neen c o u n i 5 4 3 2 3 4 5 6 8 9 10 bar· figura 17. distribucl6n de !recuencia de los residuos (tabla 3). region andina·catatumbo y. -,00624)( + 29.60002 r~quared: .98125 20 175 15 125 1075j-__ ~ ~'::o; o 500 1000 1500 2000 2500 3000 3500 altiruo tm; figura 18. modelo de regresi6n entre la temp media anual iv en ·c) y la altitud ix en m) regioiol andina-magdalena * +++...t + + : 15 .5 -.5 _i htlhfff---tt----i.--lif-f------i---+i-*ih-"----~ -1.5 -2 figura 19. variaci6n de los residuos; l)=1)esvlaci6n trpica y l)x::error trplco region anoina-magdalena 48 eslava: perfil altitudinal de la temperatura del aire 35 30 25 20 15 10 5 0"'-_"'; z scale figura 20. distribuci6n de las variables normalizadas de los residuos region andina·magdalena ~======~~odeedlan . ean25 225 20 17.5 c 15 0 u 12.5 n l 10 7.5 5 2.5 0 10 bar • figura 21. distribuci6n de rrecuencia de los residuos, (tabla 3). region andina·magdalena y -0061711' 29,37792 c-ecoerec 96773 30 28 .~ 26 m 24 ~e 22 >.,. / 0 :-~20 18 ~ 16 14 :~1 0 500 \000 1500 2000 2500 3000 3500 alfltud (m) figura 22. modelo de regresi6n entre temp media anual iv en ·cl y altitud ix en m) r~gion andina·ca(jca i--+j--+i-----\i---+-+f~~--~ '8 -i -1.5 -2 figura 23. variaci6n de los residuos; o=uesviaci6n trpica y ox=error trpico region andlfila·cauca 30 c o u n t 25 20 15 10 5 o~--1 1 z scale figura24:15istrfbuci6'n'de las variables normalizadas de los residuos region andina·cauca mode ----l1edlan 20 1ean 18 16 i~ c 12 10 8 6 2 6 bar' figura 25. distribuci6n de frecuencia de los residuos (tabla 3). regio andina·cauca geofislca colomblana ~ 1,octubre de 1992 49 v • 00544)< + 27 08676 q-~clu~red' 98\22 i;~.·";'l~ ::j .~ \-11 l2i loj 1;1 r---. 500 1000 \500 2000 2500 3000 3500 altltud (m) figura 26. modelo de regresi6n entre tamp media anual (v en ·c) y altitud ix en m) region orinoquia v ""'azonia 15 \--++-------r-----/o.-i¥-+--~ 'b f-+---4a-i--i+-------~-b -i -15 figura 27. var;aci6n de los residuos; o=oesviacl6n t1pica y ox=error t1pico region ornoquia v amazonia 16 c o u n l 14 12 10 8 6 4 2 01""-.. figura 28. distribuci6n de las variables normalizadas de los residuos region orinoquia v amazonia edian 12 ean 10 c 0 u n l 4 o 9 105 6 bar' figura 29. distribuci6n d .• frecuencia de los residuos it abla 3) region orinoqllla v a"'azonia y 0055 h ~ 27 41259 r-'qulired 98535 ~ e o 30 280 26 "22 20 16 16 14 12 10bil-~ -~__=o" o 500 1000 1500 2000 2500 3000 3500 al titud tm l figura 30. modelo de regresi6n entre la temp media anual iv en ·c) y la alii iud (x en m) region ol'llnoquia 125 75 [---+-+--------+-\-----1·f .5 25 25 -.5j + .-+i-">.l_-++_ -75 -1 -12sj figura 31. variaci6n de los residuos; o=l>esviaci6n tfpica y ox=error t1pico region orinoquia eslava: perfil altitudinal de la temperatura del aire50 10 9 8 7 c 6 0 u 5 n 4l 3 2 0~--~-3~-2-10 1 2~3--z scale figura 32. distribuci6n de las variables normalizadas de los residuos region ornoquia 12 f:::f~~~~~~~~ode edian 1ean 10 9 10 8 2 o 8456 bar" figura 33. distribuci6n de frecuencia de los residuos (tabla 3). region orinoquia 2 y -.00529k + 26.5312 r~quared: ,99008 28 26 2' 22 20 18 16 " 12 ioil---~~~-~~~-_--~~=-o soo 1000 1500 2000 2500 3000 altitud (m) figura 34. modelo de regresi6n entre la temp media anual iv en ·e) y la altilud (x en m) rl:gion amazonia c o 'u n l .8 f----a-----+-\---~-----.b .2 .3· +---i-~_v-------_\_-r__-_i'~~_ ., --'f--=~----------+--+---\::7---b -.6 -8 -i -1.2 figura 35. variaci6n de los residuos; o=l>esviaci6n tfpica y ox=error tfpico region amazonia 7 6 5 4 3 2 0""-... figura 36. distribuci6n de las vanaores normalizadas de los residuos region amazonia ode median 8 ean 7 6 c 5 0 u 4 n t 3 2 o 5 6 bar" figura 37. distribuci6n de frecuencia de los residuos (tabla 3). region amazonia geofislca colomblana ~ 1, octubre de 1992 51 y -.00-46211 + 26,20071 r..,quarocl: ,96093 26 t 26, m 2. p 22 20 16 16 ,4 12 10 0 oo o 00 o 0 500 1000 1500 2000 2500 3000 3500 altitud (m) figura 38. modelo de regresion entre la temp media anual iv en ·c) y la altitud ix en m) r~gion pacifico 1.5 r, s i o u os i----.....--+-\-------h---~f_· 'b w-1----\l-..lj-----'---1i-hf-t---b -1.5 -2 figura 39. variaci6n de 108 residuos; o=oeaviaci6n tlpica y ox=error tlptco region pacifico 18 c o u n t 16 14 12 10 8 6 4 2 o..... ....;;~_3!ii"-2 -1 0 1 z scale figura 40_ distribuci6n de las variables normalizadas de los reslduoe region pacifico lode edian 16 mean 1~ 12 c 10 0 u n l b 4 5 6 8 9, 10 8ar' figura 41. distribucl6n de trecuencia de 108 residuce (tabla 3) region pacifico 52 eslava: perfil altitudinal de la temperatura del aire referenciasconclusiones ei analisis de los valores medios anuales de la temperatura del aire en colombia condujo al establecimiento de modelos de reqresion salislactoriamente exactos, ellos permiten cuanlilicar ese elemento y sus variaciones espaciales, para las dilerentes regiones colombianas, en funcion de la altitud. las roctas de reqresion de minimos cuadrados que expresan las leyes naturales de variacion a nivel anual de este elemento del tiempo y el clima, constituyen instrumentos concretos y de tacit empleo en la determinacion de los valores de temperatura media anual, para cualquier lugar ubicado en colombia, con solo conocer su ubicacion geogratica (region 0 subregion) y su altitud,se posibilita asi su utiiizacion en cualquier actividad cientilica 0 tecnica. los resultados obtenidos muestran que cada region 0 subregion presenta valores de temperatura media anual a nivel del mar y un regimen altitudinal distinto, por 10que no parece conveniente establecer un unico modele para toda colombia. ademas, esos valores a nivel del mar y los gradientes altitudinales son diferentes a los que tradicionalmente se han aceptado y que habian side propuestos por otros autores. estos primeros resultados maternatico s obtenidos, sugieren que el trabajo continue su desarrollo y se amplie, que se establezcan las relaciones temperatura-altitud a nivel mensual y anual para todos los parametres indicativos de este elemento y se electuen nuevamente los procesos de recopilacio n, verificacio n, horno qeneizacion y unificacion del periodo de datos, partiendo de los datos medios mensuales originales. asi mismo, .es recomendable una revision "in situ" de las altitudes y coordenadas geograticas de los sitios donde estan ubicadas las estaciones meteorologicas, puesto que estos datos pueden ser una luente importante de errores. bernal, e. (1978): red meteorol6gica de colombia. public.aperi6dica n"39 himat, 115 pp. himat, santate de bogota. cortes, e. (1989): estudio del regimen de temperaturas en colombia. (mscr). 103 pp. hima t, santate de bogota. (parcialmente, tam bien en: memorias del lv congreso interamericano de meteorologia y ii congreso colombiano de meteorologla. pp.191-198. sociedad colombiana de meleorologia-socolmet, santafe de bogota). eslava, j. (1988): perfil altitudinal de la presi6n atrnosferica en colombia. memorias del iii congreso interamericano de meteoroloqla 'i iii congreso mexicano de meteorologia. pp.279-283. mexico. (en prensa): regimen termico del pacifico colombiano. rev.acad.colomb.cienc. santafe de bogota. federaci6n nacional de cafeteros-fnc. (1989): anuario meteorol6gico 1989. cenicafe, ch inchina-caldas, instituto colombiano de hidrologla, meteorologiay adecuaci6n de tierras-himat. (1981): calendario meteorol6gico 1981. himat, santafe de bogota. ---(1984): calendario meteorol6gico himat, santate de bogota. (1985a): calendario meteorol6gico himat, santale de bogota. -(1985b): catalcqo de estaciones hidrol6gicas y meteorol6glcas (lista de computador). 141 pp. himat, sanlafe de bogota. 1984. 1985. instituto geografico "agustin codazzi"-igac. (1960): nlvelaci6n geodeslca, resultados definitivos, puntas y cotas. 286 pp. igac, santafe de bogota. servicio colombiano de meteorologia e hidrologla-scmh. (1973): promedios multlanuales de precipitaci6n, temperatura, humedad relativa y brillo solar. 44pp. scmh (actualmente himat), santate de bogota. stanescu, s. & j.r. diaz. (1971 ): estudio preliminar de la temperatura del aire en colombia. public.aperi6dica no.26, 9opp. scmh, santafe de bogota. trojer, h. (1958): meteorologia y climatologia de la vertiente del pacifico colombiano. rev.acad.colomb.cienc. 10 (40):199-219. santafe de bogota. geofisica colombiana pp 53-56n° 5 d i c i e m b r e de 2001 bogota d.c issn 0121 2974 la teoria del rayo paraxial en la sismica de pozo: aproxlmaclon de segundo orden en los tiempos de transite luis alfredo montes v,oes profesor asociado departamento de geociencias; universidad nacional de colombia facultad de ciencias resumen utilizando la aproximacion del rayo paraxial de la llamada 6ptica geornetrica se encontr6 un conjunto de expresiones que caracterizan los eventos de reflexi6n que han sido registrados en detectores situados dentro de un pozo. una de ell as es la funci6n caracterfstica que estima los tiempos de transite de eventos vsp de fuentes situadas en la cercanfa de la boca del pozo, la cual puede ser conocida nurnericarnente a partir de la medici6n de al men os seis tiempos de transite apropiadamente seleccionados. se deducen expresiones que permiten transformar la funci6n caracterfstica de tiempos de transite de los eventos de reflexi6n de la sfsmica de pozo en la equivalente funci6n caracterfstica de tiempos de transite obtenida en configuraci6n pun to medio cormin, correspondiente a eventos registrados en superficie. este resultado hara posible la soluci6n del problema inverso en profundidad en la vecindad del pozo, mediante la aplicaci6n de tecnicas de continuaci6n hacia abajo de la funci6n de tiempos de transite, los resultados son validos en la aproximaci6n de segundo orden de los tiempos de transite en model os constituidos par capas homogeneas e isotr6picas con cualesquiera velocidades y densidades, separadas por interfases suaves arbitrariamente curvadas. abstract palabras clave: sismica de pozo, sismica geometrica, teoria del ravo, contlnuacion hacia abajo, problema inverso, tiempqs de transito. using the paraxial ray approximation of the named geometric optics a set of numerical expressions were found, these expressions characterize reflecting events recorded in detectors placed in a borehole. one of them is the characteristic travel time function which estimates the travel times of reflecting events in a vertical seismic profile configuration with sources placed in surface in the vicinity of the boreholes's top. this characteristic vsp function can be numerically estimated using at least six measurements of appropriated selected travel times. others expressions were deduced to allow the transformation of the vsp travel time function in the equivalent cop travel time function corresponding to reflecting events recorded in surface in a common midpoint array. the use of this result will make possible to solve the inverse problem in the neighborhood of the borehole by the application of downward continuation techniques of the travel time function. the results are valid only in the approximation of the second order of the travel times in models which consist of a pile of homogeneous and isotropic layers of any velocities and densities, separated by smooth interfaces arbitrarily curved. key words: vertical seismic profile, geometric seismic, ray theory, downward continuation, inverse problem, travel times. introduccion uno de los pasos import antes en el procesamiento de datos para la obtenci6n de imageries del subsuelo es la transformaci6n de datos desde el dominio de datos obtenidos en la configuraci6n de sismica de pozo vsp (vertical seismic profile) ala configuraci6n de adquisici6n de datos en superficie cmp (common midpoint). asf se obtienen tiempos de transito de rayos recibidos en superficie a partir de datos obtenidos en un polo para su proceso e interpretaci6n. usando la teorfa del rayo paraxial en la aproximaci6n de segundo orden de los tiempos de transite, se presenta un desarrollo te6rico que permite llevar a cabo la transformacion entre los dominios mencionados. como resultado se obtiene un conjunto de ecuaciones que permite transformar los tiempos de trans ito vsp a tiempos de transite de eventos de reflexi6n registrados en superficie. la soluci6n del problema inverso se obtiene a partir de la soluci6n de este conjunto de ecuaciones, conociendo as! las velocidades de intervalo y la geometria de las interfaces en la vecindad del rayo central (montes, 1988 y montes, 1999). geof(sica colombiana, 5, diciembae de 2001 boca de polo (ova po(oxictlroyo po=.~ figura 1. sistema sismico con el pozo, interceptando las superficies anterior y reflectora. por sirnplicidad no se muestran otras interfaces. el rayo central queda definido entre estes dos punros de inrerseccion. el rayc paraxial parte de la fuente en la superficie anterior y alcanza al ge6fono situado en el pozo luego de reflejarse en la superfide posterior 53 luis alfredo montes vides en consistencia con los planteamientos de la teona del rayo paraxial (bortfeld, 1989), la tierra se model a como una superposici6n de capas homogeneas, sin restricciones sobre sus valores de velocidad y densidad. estas capas estan separadas por interfaces suaves arbitrariamente curvadas y aproximadas por superficies de segundo orden. el modelo sismico, como se muestra en la figura i, esta limitado en su parte superior por la superficie de la tierra hamada superficie anterior, y en su parte inferior por la superficie donde tienen lugar las reflexiones denominadas superficie posterior. un pozo arbitrariamente curvado atraviesa el sistema sismico.el rayo que inicia en la boca del pozo sobre la superficie anterior e incide sobre la superficie posterior se selecciona como rayo central, aclarando que el rayo central no es en general un rayo normal, y se define iinicamente por dos puntos en los extremos superior e inferior del sistema sismico. ei punto inicial del rayo central define el sistema de coordenadas (i, i. k) y el punto final, a su vez, el sistema cartesiano (i i, i', k ') .los pianos (i i, j ') y (i, j) son tangentes a las superficies anterior y posterior en los puntos de intersecci6n con el rayo central. tiempos de transito asociados a rayos paraxiales cualquier rayo en la vecindad del rayo central se describe por cuatro vectores bidimensionales, sobre la superficie anterior por los vectores posici6n x (i, j) y p (i, j) lentitud, y en la superficie posterior por los correspondientes vectores x' (i i, j ') y p'(i, j). estos vectores bidimensionales se obtienen proyectando los respectivos vectores tridimensionales sobre los pianos tangentes en los puntos de intersecci6n del rayo central con las superficies anterior y posterior, eliminando asi un componente en ellos, en la aproximacion de segundo orden de los tiempos de transite, los vectores posici6n y lentitud sobre la superficie posterior y anterior estan relacionados a traves del operador matriz de transferencia, dados por la relaci6n: -i x -i po donde a 0 ' b 0 ' co' do son matrices jacobianas 2x2. se observa que aparece un terrnino p' adicional en la ecuaci6n (1), que es igual a cero por ser rayo normal en la equivalente expresion de bortfeld (1989). la ecuaci6n (1) pennite expresar p y j/ como funciones de x y x i exclusivamente i i c d b i a + do bo-i x ip = po + ox 0 ; ox todo evento de reflexi6n se compone de dos rayos transmitidos, uno que inicia en la posicion x con lentitud inicial p , y el otm que parte de x" con lentitud inicial p", arribando ambos al mismo punto final (x") i = x i situado sobre la superficie posterior, donde se cumple la ley de snell (p") i = p'. 54 aplicando la ecuaci6n (i) al segundo rayo y usando las pareja de condiciones (x")' = x' y (p")' = _p' seobtieneunaexpresi6n para el vector lentitud inicial del rayo reflejado v = i\ tr'c; 1(x8 + x) b,~'ao 1(x8 x) d,~'p;' (4) asicomo para el vector lentitud inicial del rayo transmitido p = i\ d;'co 1(x8 + x) + b;'ao + (x" x) d,~' p;' (5) se observa que la diferencia entre estos vectores, lentitud inicial del rayo transmitido y del reflejado, es independiente del vector punto medio comun ~ (x" + x), mientras la suma de tales vectores es inde1 (-" -)pendiente del vector offset medi0"2 x x . en la situaci6n zero-offset, cuando x" = x, sobre la superficie anterior los dos rayos poseen iguallentitud p" = p, siendo esta en general diferente de po' tiempo de transito del rayo transmitido para determinar el tiempo t (x, x ') que toma al rayo ir desde la superficie anterior a la posterior, se usa la ecuaci6n principal de hamilton (bortfeld, 1989): dt ( x, x ') = p' . dx i p . dx , (6) la cual representa el diferencial de tiempo de trans ito desde cualquier pun to en la superficie anterior a cualquier punto en la superficie posterior. de (6) se obtiene p a partir de la derivada parcial de t (x, x ') respecto a x, mientras p' se obtiene mediante la derivaci6n parcial respecto a x i. usando las cuatro ecuaciones diferenciales correspondientes, el tiempo de viaje de un rayo paraxial desde la superficie anterior hasta i posterior sera: ( -i) + -i -i i -i d b-1 -i +t x, x = to po x po' x + '2 x· 0 0 x -2 1 x . b~' a x x . b-1 x' 000 (i) (7) to: tiempo que tom a el rayo central para atravesar el sistema sisrnico, en el caso de la sismica de pozo 0 ysp, un rayo paraxial viajara desde cualquier punto en la superficie de la tierra, en la cercania del rayo central, hasta un detector situado dentro del pozo que coincida -i 0-con una superficie reflectora en particular, en cuyo caso x = yel tiempo de viaje sera segun la ecuaci6n (7). (2) ( -i -0) i b-1 a -txx = =t -p ·x+-x· x, 0 0 2 0 0 (8) (3) expresi6n que puede estimarse numericamente a partir de por 10 menos seis tiempos para seis fuentes distintas, ya que contiene seis inc6gnitas. tiempo de transito del rayo reflejado sea la ecuaci6n principal de hamilton para el rayo reflejado dada por: dt (x, x") = p" . dx" p . dx. (9) geofisica colombiana, 5, diciembre de 2001 la teo ria del rayo paraxial en la sismica de pozo, aproximaci6n de segundo orden en los tiempos de transito la expresi6n (9) representa el diferencial del tiempo de trans ito de un rayo que parte desde cualquier fuente, en x hasta cualquier detector en x" . la derivada parcial de t t; x, x") respecto a x provee el vector lentitud 13 en la posici6n de la fuente, mientras que la derivaci6n respecto a x" da el vector lentitud 13" en la posici6n del receptor. al expresar el par ( 13, 13") por el correspondiente par de ecuaciones (4) y (5), usando adem as las cuatro ecuaciones diferenciales correspondientes, se obtiene la expresi6n para el tiempo de transite del rayo reflejado: t(x, x") = to 2po . + (x" + x) + 2d,~1p: + + (x" + x) . d,~ico + (x" + x) + (i" x) . s;l ao + (x" x) al considerar el caso del detector situado en el pozo en el pun to x" = 6, los tiempos para un iinico receptor cormin vendran dados por: ((x, x" + b-'a ) l o 0 2 x (ii) expresi6n estimable numericamente a traves de al menos seis medidas en tiempos de transite en configuraci6n receptor corruin vsp. transformacion del espacio vsp a espacio cdp se busca transformar los tiempos de trans ito t vsr (x:, x), correspondientes a rayos reflejados en las interfaces y registrados en los detectores situados dentro del pozo, a sus correspondientes a tiempos ( ( x, x") de reflexi6n registrados en superficie en levantamientos figura 2. el sistema sfsmico muestra let superficie virtual plana perpendicular -"ala trayectoria del rayo central, coincidiendo con la posici6n x* del ge6fono en el pozo geoflslca colombiana, 5, diciembre de 2001 (10) convencionales en configuraci6n punto medio cornun. esta operaci6n corresponde a una transformaci6n de los datos desde un espacio vsp a un espacio cdp. en la figura 2 se ilustra un levantamiento vsp con un sistema sismico donde no se muestran las interfaces intermedias para claridad del rnismo. entre las superficies limitadoras superior e inferior del modele se incluye un plano virtual, virtual en el sentido de no separar medios con propiedades litol6gicas distintas, la cual es perpendicular al rayo central y contiene el detector del pozo situado en la posici6n x: en el nuevo sistema de coordenadas definidas para dicho plano. el vector posici6n x: es un vector bidimensional definido en el sistema cartesiano 1. , } * que tiene su origen en el punto de intersecci6n del rayocentral con el plano. cuando el rayo reflejado pasa por el pozoen la posici6n, el tiempo de trans ito medido en el receptor del pozo puede expresarse como la diferencia: (-if) (-if) (-" _if)(vsp x, x * = l x, x t x ,x * ( 12) donde (( x, x") representa el tiempo de reflexi6n de un rayo que parte de x en la superficie y es detectado en un receptor situ ado en x" sobre la superficie, mientras (( x ", x:) corresponde al tiempo de transito desde el detector del pozo posicionado en x: hasta x" en la superficie. la transformaci6n vsp-cdp puede formularse ahora de ja siguiente manera: primero, hallar para una deterrninada fuente situada en superficie la posici6n correspondiente del receptor rayo, un rayo entre estas posiciones que pase por la posici6n en el pozo donde se halle un detector; segundo, estimar el tiempo de transite tt; x", x:) del rayo transmitido, y tercero, adicionar el tiempo de transite (( x ", x:) al tiempo de transite medido en el pozo (vsp (x, x:) . la relaci6n entre las posiciones x y x: esta establecida a traves de la relaci6n: x: = aokx" + s ; (p" po) (13) esta ultima relaci6n determina la transformaci6n de la pareja de vectores posici6n y lentitud desde la superficie de la tierra hasta la posici6n x: en el pozo. las matrices aok y b ok estan conformadas por parametres que dependen exclusivamente de las caracteristicas del medio situado por encima del receptor. ai expresar el vector lentitud 13" de la relaci6n (13) por el vector lentitud de la ecuaci6n (4) del rayo reflejado, se obtiene: -. a -. b l d -i c 1 (_. -) b -i a 1 (-. -) d -i 'jx * = ok x + ok 0 0 2" x + x 0 0 2" x x (1 p 11 (14) don de se obtiene el rayo transmitido como parte del rayo reflejado que pasa por el punto x: situado en el pozo. el tiempo de trans ito (( i",x:) necesario para la transforrnaci6n vsp-cdp se obtiene a partir de la ecuaci6n (7); basado en el conocimiento previa de las matrices que transmiten el rayo a traves de la capas situadas encima del detector en el pozo, es: 55 luis alfredo montes vides donde representa el tiempo de transite del rayo central para llegar hasta el detector. la expresi6n (15) representa el tiempo necesario para completar el tiempo vsp en tiempo de transite cdp de acuerdo con la ecuaci6n ( 12). conclusiones se dedujeron funciones estimables numericamente con tiempos de transite registrados en detectores colocados en un pozo, asi como aquellas que permiten su transformaci6n en funciones equivalentes de eventos registrados en superficie. las matrices ao k y b 0 k ' como parametres de la funci6n caracteristica, estan constituidas por elementos que dependen exclusivamente de las propiedades elastodinamicas del medio que esta situado encima del reflector. segun kahn (1988) las velocidades pueden determinarse utilizando la matriz de curvatura de los tiempos de transite b ~~ a 0 n de una configuraci6n punto medio cormin situada sobre la ultima superficie antes de la interfaz reflectora. la continuaci6n hacia debajo de la funci6n tiempo de transite (montes, 1988) permite conocer la funci6n tiempos de transite sobre la interfaz anterior de la ultima capa del sistema sismico y, en consecuencia, la soluci6n del problema inverso, que se manifiesta en el conocimiento de las velocidades de intervalo y la geometria de las interfaces en profundidad. las expresiones (13), (14) y (15) permiten relacionar la informaci6n sismica en la vecindad del pozo con la informaci6n sismica obtenida en configuraciones convencionales, estableciendo correlaciones numericas entre la informaci6n de pozo y su entorno. los resultados logrados son validos en la aproximaci6n de segundo orden de los tiempos de transite en model os constituidos por capas hornogeneas e isotr6picas con cualesquiera velocidades y densidades, separadas por interfaces suaves arbitrariarnente curvadas. 56 referencias bibliograficas (15) bortfeld, r. 1989: geometric ray theory: rays and travel times in seismic systems. geophysics, y54, 342-349. kahn, s. u. 1988: die aufdeckung der mehrdeutigkeiten in der reflexionsseismik: dissertation. tech. univ. clausthal. germany. montes, l. a. 1988: determinacao das velocidades interval ares usando a teoria do raio paraxial: aproximacao de segunda ordem dos tempos de transite. ph. d. thesis. universidade federal do para. ufpa, brazil. montes, l. a. 1999: estimaci6n de las velocidades de intervalo a traves de la funci6n de tiempos de difracci6n de la sismica geometrica, geologia colombiana, 24, 177-183. geofisica colombiana, 5, diciembfle de 2001 seismic event characterization is often accomplished using algorithms based only on information received at seismological stations located closest to the particular event, while ignoring historical data received at those stations. these historical data are stored and unseen at this stage. this characterization process can delay the emergency response, costing valuable time in the mitigation of the adverse effects on the affected population. seismological stations have recorded data during many events that have been characterized by classical methods, and these data can be used as previous “knowledge” to train such stations to recognize patterns. this knowledge can be used to make faster characterizations using only one three-component broadband station by applying bio-inspired algorithms or recently developed stochastic methods, such as kernel methods. we trained a support vector machine (svm) algorithm with seismograph data recorded by ingeominas’s national seismological network at a threecomponent station located near bogota, colombia. as input model descriptors, we used the following: (1) the integral of the fourier transform/power spectrum for each component, divided into 7 windows of 2 seconds and beginning at the p onset time, and (2) the ratio between the calculated logarithm of magnitude (mb) and epicentral distance. we used 986 events with magnitudes greater than 3 recorded from late 2003 to 2008. the algorithm classifies events with magnitude-distance ratios (a measure of the severity of possible damage caused by an earthquake) greater than a background value. this value can be used to estimate the magnitude based on a known epicentral distance, which is calculated from the difference between p and s onset times. this rapid (< 20 seconds) magnitude estimate can be used for rapid response strategies. the results obtained in this work confirm that many hypocentral parameters and a rapid location of a seismic event can be obtained using a few seconds of signal registered at a single station. a cascade scheme of svms or other appropriate algorithms can be used to completely classify an event in a very short time with acceptable accuracy using data from only one station. los algoritmos de determinación de parámetros hipocentrales empleados en la actualidad, se basan específicamente en la información recibida en las estaciones de monitoreo mas cercanas al epicentro y no tienen en cuenta la valiosa información histórica registrada a lo largo del tiempo en dichas estaciones. es por esto que los procesos de caracterización toman varios minutos, tiempo precioso que podría ser de gran utilidad en la generación de alertas tempranas que permitan una oportuna reacción ante el evento. el registro de información, a lo largo el tiempo, de sismos ocurridos en los alrededores de la estación, puede ser empleada para dotarla de algún grado de experiencia que le permita, mediante detección y clasificación de patrones, realizar una caracterización previa mucho mas rápida, mediante el empleo de técnicas modernas las cuales pueden ser algoritmos bio-inspirados o métodos estocásticos mas recientes conocidos como métodos kernel. en el presente trabajo se emplea un método conocido como maquinas de soporte vectorial (msv), entrenando dicho algoritmo con información de la relación del área bajo la curva de la potencia de la transformada de fourier de las componentes n-s, e-w y vertical, calculada para 5 ventanas de 2 segundos, desde la onda p, de 123 sismos de magnitud superior a 3, desde 2004 hasta 2008, alrededor de la estación el rosal, de la red sismológica nacional de ingeominas. el algoritmo clasifica sismos que superen un umbral predeterminado de la relación entre el logaritmo de la magnitud y la distancia, que refleja, de alguna manera, la intensidad del sismo. con la obtención de este parámetro será posible conocer la magnitud del evento, debido a que la distancia puede ser calculada, con base en picado de la onda s, y de esta manera establecer una aproximación rápida de la magnitud en un tiempo aproximado de 20 segundos después del evento. los resultados obtenidos permiten confirmar que con poco tiempo se señal en una sola estación sismológica es posible obtener información confiable para ser empleada en alertas tempranas. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 115 122 abstract resumen key words: machine learning, seismology, single station, magnitude, distance. palabras clave: aprendizaje de máquina, sismología, una estación, magnitud, distancia. record manuscript received: 06/12/2013 accepted for publication: 05/09/2014 severity classification of a seismic event based on the magnitude-distance ratio using only one seismological station luis h. ochoa¹, luis f. niño¹, carlos a.vargas¹ ¹ universidad nacional de colombia seismology issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.41083 118 luis h. ochoa, luis f. niño, carlos a.vargas 1 introduction seismic activity is a natural phenomenon with enormous and uncontrollable power, and damage in populated areas can impede appropriate responses when earthquakes occur. recent developments have allowed efficient and alternative solutions inspired by natural and biological processes that have been converted to stochastic models and have been used in seismology with acceptable results. when an earthquake is detected, seismologists determine its main features, such as magnitude, hypocenter coordinates, fault mechanisms, etc., to help authorities respond appropriately and allow scientists to study the earth internal structures. seismometers are distributed across an area of interest, many with three-component broadband sensors that detect, at the same point, movement in three mutual, orthogonal directions (north-south, east-west and up-down). these stations transmit information in near real time to seismological centers. the traditional characterization of earthquakes takes a few minutes, depending on the degree of automatization, network density and the number of stations involved in the process. earthquakes usually must be detected by at least five stations, and features such as compressive (p) and shear (s) wave arrival times are determined by expert analysis or an automatic algorithm. subsequently, the hypocenter coordinates can be calculated. in colombia, an institute called ingeominas can obtain seismic event localization in approximately 8 minutes due largely to the low-density network. in some seismological centers with many stations, this time can be reduced by half because earthquake energy requires additional time to arrive at stations farther away from the hypocenter. however, we are mostly interested in earthquakes occurring near population centers, where seismic energy can arrive in tenths of a second, making warning impossible. long processing times are associated with conventional methods that are based on information related only to the particular event and that ignore valuable previously collected historical information. therefore, new alternatives must be explored to reduce the processing time and allow rapid generation of warnings. the main purpose of this study is to use historical information from only one station to detect patterns correlated with known event features available from very accurate, traditional processing. 2 problem definition colombia has a central seismological network, managed by ingeominas, and local networks located in certain cities, such as armenia (quindio) and cali (valle). it is important to develop a system based on these valuable resources to discover patterns that will allow more rapid seismic early warnings. recent developments in computational intelligence (ci) techniques allow this goal to be achieved by solving certain seismological problems in near real time. the automatic detection and discrimination of different events recorded with a seismometer, including seismic and non-seismic signals, using ci have been achieved using artificial neural networks (anns) in many cases around the world. anns use a probe that is an excellent pattern recognition tool. anns were created to reproduce the mechanisms of human reasoning and allow discrimination of signals to simulate the brain processing of the operator. it is an efficient processing strategy for solving seismic signal discrimination problems, in which multiple hypotheses must be controlled in parallel, the number of input parameters is large and a well-defined solution is not always well known. the onset time determination is usually the first step to locating the hypocenter of an event. the first arrival of a p wave can be compressive or tensile, which is key to determining the focal mechanism. interactions between body waves and changes in the mechanical properties of rocks at strata interfaces generate diffraction and reflection phenomena, generating new body waves. these new onsets can provide clues to the distribution of strata in the subsurface, and these clues can then be used as seismic tomography data to determine the earth’s internal structure. the automatic detection of p waves can be accomplished using comparison algorithms, such as the one reported by allen 1978, who developed an application to determine p wave onset by comparing discrete signal parameters. the p onset is present where the short-term average (sta) of the signal increases abruptly with respect to the long-term average (lta). later, allen 1982 established that technological development will lead to more efficient s wave detection through the use of a signal-in-frequency domain. baer and kradolfer, 1987, developed an improved methodology based on signal processing. the complexity of the s wave onset requires alternative methods to achieve better solutions. cichowicz, 1993, developed an algorithm that identifies the s wave onset using three-component seismometers in local networks. recently, a neural network approach has been proposed to solve this problem because many simultaneous parameters can be included and the approach is easy to implement and flexible. it is also not necessary to be proficient with the interactions of the analyzed variables. wang and teng, 1997, applied a back propagation neural network (bpnn). dai and mcbeth, 1997, implemented a bpnn to identify and detect p and s waves from threecomponent recordings of local earthquake data. two years later, zhao and takano, 1999, used bpnns for p wave detection by combining three bpnn detectors (long, mid and short term). aldersons, 2004, developed a system (mannekenpix) that automatically detects the phase of a wave. this program is based on a weighting mechanism calibrated with reference detections and weights provided by the user; in other words, it is a learning process that uses examples and statistical techniques to separate groups by criteria similitude. this approach is a linear combination of predictors to maximize differences among groups, while minimizing differences within groups. di stefano et al. 2005 applied this algorithm to achieve precise phase detection using pg, pn and their polarities. these authors demonstrate that such data are suitable for high-precision earthquake location, focal mechanism determination and highresolution seismic tomography. once signal spectral characterization is complete and arrival times for p and s signals are determined, it is possible to generate the subsurface structure. this issue has been resolved using computational intelligence techniques, such as genetic algorithms (gas) and simulated annealing (sa). a good subsurface structural model is an important step for calculating the focal parameters of seismic events. the high model combination rates obtained with ga can perform a very wide exploration of the solution’s space, and this computation alternative is preferred when solving this kind of problem. bhattacharyya et al. 1999, analyzed regional seismograms using a ga to determine crustal structure in a four-layered crustal model of the eastern great basin and western colorado plateau, in which the optimizing variables were thickness and p-wave velocity. the resulting model agreed well with results reported in several geological publications. chang et al. 2004, implemented a ga and modeled crustal structure in southern korea using teleseismic p-wave receiver function data and rayleigh– wave phase velocity measurements. the genotype describes the distribution of wave velocities and is determined using 40 1-km-thick layers. lawrence and wiens, 2004, combined the inversion of body wave receiver functions and rayleigh wave phase velocities using a niching genetic algorithm (nga). using non-unique inversion models, it is possible to explore many solutions to problems associated with the determination of focal parameters. pezeshk and zarrabi, 2005, developed an inversion procedure for spectral analysis of surface waves (sasw) using a genetic algorithm. kim et al. 2006, proposed a method for determining hypocentral parameters for local earthquakes in 1-d using genetic algorithms and twopoint ray tracing, which calculates seismic ray trajectories that cross a known velocity structure. in this case, the velocity structure is not known, but the arrival times at different stations are available. with ga, it is possible to explore many subsurface velocity distributions, and a comparison between the calculated and observed arrival times can be a useful fitness selection function. assimaki and steidl, 2007, used a hybrid algorithm that combined a genetic algorithm with a local least-squares fit operator for inversion of weak and strong motion among an array of data obtained during the mw 7.0 sanriku-minami earthquake. this combination accelerates the convergence of the ga by improving the parental population with the local hill-climbing operator. many of the processes mentioned above can be described by computational intelligence tools, which must be designed, or trained, for 119severity classification of a seismic event based on the magnitude-distance ratio using only one seismological station particular geological, environmental and structural settings. additionally, these techniques can be applied to function in near real time by processing information at the seismological center or directly in the sensor using electronic devices with embedded models. these devices are faster and can transmit processed information via parallel link, requiring fewer resources for transmission. however, these processing tasks only use information transmitted from current seismic activity registered by the nearest stations. the processing takes approximately 5 to 10 minutes, which does not allow for early warnings and alerts. more stations can be used to determine seismic source magnitude and location, but more time is needed for processing; thus, less time is be available for reaction. the generation of early warnings depends on extracting the most accurate and reliable magnitude and location values using the fastest signal time. new data-driven pattern analysis models (e.g., kernel methods) with solid statistical foundations have been applied to classification and regression issues. these pattern analysis models apply historical data available in seismological databases to determine signal features (descriptors) and relationships between signal features and previously processed results. the descriptors and their relationships with previously processed results can be used to “train” certain simple algorithms, which can be used to process new data more rapidly. here, we present the first results obtained from a single three-component seismic station with our rapid and accurate characterization of a seismic event using only a few seconds of signal following the p wave onset. this station is near the source, where energy does not saturate the data, and uses support vector machine models to obtain a magnitude-distance ratio from 14 seconds of signal. 3 data set description the data set used in this research was obtained from ingeominas’s national seismological network. we have waveform files and calculated data from approximately 45,000 registered seismic events from 1993 to mid2008. we selected 130 of the events registered at the “el rosal” station that had magnitudes (mb) greater than 2.1 and epicentral distances less than 120 kilometers. this station has digital, three-component broadband seismometers and is located near bogota, which is vulnerable to mediumor high-magnitude events. this research is proposed as a first step to developing a reliable early warning system for the city. only events registered after late 2003 were used because the three-component station data were not available prior to this time. the location of selected events is presented in fig. 1, in which symbol size is proportional to event magnitude. 4 methodology one of the most important steps in kernel method processing is to determine the best “descriptors” to be used as input variables for the model. these descriptors are the key to obtaining good correlations with desired output, and the selection of descriptors depends on the type of information available for processing. in this case, we have time signals, which have certain characteristic features that can be described by signal transformations, such as fourier transform and others. • fourier transform was applied to each of the three components of the signal, starting at the p wave onset. • this transform was divided into 2-second windows (80 samples). • the integral of the squared frequency spectrum was calculated for each window, which generated one value for a given two-second partition. • finally, the ratios between the values of each component calculated in the step above are the input descriptors of the model and are calculated as follows: (1) (2) (3) where x, y and z are the values of the squared fourier transform integral at each x, y, and z component registered at the seismological station. these descriptors are used in our classification model as input variables. an output value is needed to separate the two classes that represent, in some way, the severity of a given earthquake’s effects at the station site. the effects of an earthquake at any place depend directly on magnitude and inversely on epicentral distance. a ratio between the logarithm of magnitude and epicentral distance is proposed here as a classifier based upon the following equation: , where m is body wave magnitude and d is epicentral distance, based on reports processed by ingeominas for each seismic event. the physical context of this classifier variable is related to the energy that an earthquake has when it reaches the seismic station; very near events with low magnitudes are perceived to be similar to distant events with high magnitudes. higher classifier values are obtained for high-magnitude earthquakes occurring close to the station. to achieve a balance between the number of events over and under a background limit, a value of 0.5 was determined to be the classification limit. with calculated descriptors and the information available for the classification variable, an svm was trained to find the optimal separator hyperplane that discriminates the events corresponding to classes a and b. appendix i includes the events used in the model and the calculated severity values from known magnitudes and distances, as well as the predicted svm model. 5 classifier evaluation a machine learning classifier performance depends entirely on the data used for training, validation and testing. therefore, the method’s evaluation must consider answers to certain questions about the data, such as ‘do we have enough training data to represent the model?’; ‘do we have balanced training data for each considered class?’; ‘what is the effect of re-using data in the training sets?’; etc. to answer these questions, the algorithms and data used in this research were evaluated using 10-fold cross validation techniques, and the results are summarized below. [complexity c=0.1] predicted real a b a 9 32 b 0 98 [complexity c=1] predicted real a b a 20 12 b 4 94 [complexity c=10] predicted real a b a 22 10 b 3 95 table 1: confusion matrix. 120 luis h. ochoa, luis f. niño, carlos a.vargas % correctly classified instances 87.69% % incorrectly classified instances 12.31% mean absolute error 0.12 mean square root error 0.35 relative mean absolute error 32.95 relative mean square root error 81.39 table 2: svm classifier with quadratic kernel results. a b true positives 62.5% 95.9% false negatives 4.0% 37.5% precision 0.830 0.887 recall 0.625 0.959 mean-f 0.714 0.922 roc curve area 0.792 0.792 table 3: estimated error of the svm quadratic kernel classifier. table 1 shows the results from the determination algorithm of the meta parameters, which determine the curve roc that was used to evaluate the value of c for which the classification model has the best results. figure 2 shows the curve roc for multiple values of c, achieving the best value as a compromise between hits and false alarms. similar curves were generated to locate other parameters, such as the degree of polynomial function used for mapping vectors in the feature space. using this approach, the quadratic kernel was selected as the best mapping function for this problem and dataset. figure 2 shows the relationship between the numbers of hits and false alarms, which are determined using the following equations: figure 2: roc curve for c estimation. wrongly classified events that exceeded the background limit of c, known as “false negatives”, are cases where no alarm was generated but a moderate event was imminent. this unique situation indicates that there is a local condition that must be analyzed carefully to determine the area’s unique pattern (figure 3). to increase accuracy and obtain better results, it is necessary to explore other descriptors. figure 4 plots the values of the confusion matrix. there is a very good correlation for a severity threshold of 0.5. this result can be used as an initial early warning parameter. figure 3: wrongly classified events (red points). figure 4: plotted confusion matrix. 6 results results obtained in this study using the classification methods described above are presented in tables 2 and 3. class b events were classified with high accuracy, but class a events were wrongly classified in 31% of the cases. future work will focus on the identification of new descriptors to better characterize the input signal using a priori knowledge. it is possible to implement new strategies using bagging, which combines many algorithms. the described algorithm proposes a new classification method for early warning purposes only. 7 conclusions • the structure of the input data is important for classifier behavior and determining relationships within the model. one of main challenges in this work was selecting the best signal descriptors and determining the relationships between them. • with a small signal interval processing from only one seismological station, it will be possible to estimate parameters such as magnitude and distance, which will allow the rapid classification of a seismic event based only on the effects at this station. • previous determinations of hypocentral parameters ignore historical data from previous seismic events and instead use only information recorded during current earthquakes. the proposed method uses historical data to calculate a relationship between magnitude and distance and allows rapid warning generation times. • the results suggest that it is possible to use the proposed method to calculate other hypocentral parameters, such as hypocenter location, azimuth, distance and others. 121severity classification of a seismic event based on the magnitude-distance ratio using only one seismological station • improvements to the technique can be accomplished by including a priori information in descriptors calculated from signals and spectral characteristics. these techniques have been used previously as input in some artificial intelligence approaches and resulted in better accuracy and faster processing times. • the adjustment results obtained using this approach cannot be compared with conventional strategies because they do not use the same information. this approach is based on a single station, whereas others are based on multiple stations (no fewer than 5). • early detection systems must consider combined strategies to optimize the identification of descriptors that can be used more efficiently in these types of approaches. references allen, r. v. 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(1999). application of quadratic neural networks to seismic signal classification, physics of the earth and planetary interiors. 113, 103-110. zhao, y. and takano, k. (1999). an artificial neural network approach for broadband seismic phase picking. bulletin of the seismological society of america. 89. 670-680. zhou, r., tajima, f., and stoffa, p. l. (1995). application of genetic algorithms to constrain near-source velocity structure for the 1989 sichuan earthquakes. bulletin of the seismological society of america. 85. 590-605. 122 luis h. ochoa, luis f. niño, carlos a.vargas figure 1: selected earthquakes. appendix i: list of used events and parameters. date time lat lon dist mag depth hypo dist log mag dist hipo classification real (0.5) predicted 07/11/2002 22:22 4.008 -74.15 96 2.2 0 96.0 0.36 b b 13/11/2002 23:23 5.775 -73.906 112 2.5 115 160.5 0.25 b b 08/01/2003 09:09 4.492 -74.898 75 2.2 6 75.2 0.46 b b 14/04/2003 22:22 4.658 -74.535 32 2.5 3.1 32.1 1.24 a a 29/04/2003 01:01 4.05 -74.11 92 2.9 2.6 92.0 0.50 a b 08/06/2003 10:10 5.258 -73.649 88 2.4 156.2 179.3 0.21 b b 12/06/2003 11:11 4.636 -74.647 43 2.8 22.9 48.7 0.92 a a 20/06/2003 01:01 4.684 -74.785 54 2.7 21.9 58.3 0.74 a a 28/06/2003 23:23 5.59 -74.352 81 2.2 100.3 128.9 0.27 b b 01/07/2003 21:21 4.159 -75.146 119 2.8 0 119.0 0.38 b b 20/08/2003 23:23 5.519 -73.858 90 2.4 136 163.1 0.23 b b 01/09/2003 11:11 5.543 -74.389 76 2.6 104.3 129.1 0.32 b b 19/10/2003 02:02 5.618 -73.915 96 2.8 117.3 151.6 0.30 b b 20/10/2003 11:11 5.778 -74.675 109 2.2 6.8 109.2 0.31 b b 31/10/2003 00:00 5.213 -73.714 79 2.8 155.5 174.4 0.26 b b 03/11/2003 20:20 5.704 -74.413 94 2.2 0 94.0 0.36 b b 09/11/2003 14:14 5.375 -74.927 88 2.2 8.2 88.4 0.39 b b 20/12/2003 19:19 5.219 -74.546 47 3.8 2.2 47.1 1.23 a a 26/12/2003 12:12 5.779 -73.873 114 2.9 129.4 172.5 0.27 b a 02/01/2004 09:09 5.74 -74.311 98 2.5 1 98.0 0.41 b b 12/01/2004 17:17 5.589 -73.874 96 2.3 134 164.8 0.22 b b 24/01/2004 07:07 5.805 -73.878 116 2.8 122 168.3 0.27 b a 01/02/2004 10:10 5.431 -73.741 91 2.7 142.3 168.9 0.26 b b 10/03/2004 07:07 4.54 -75.06 88 2.3 26 91.8 0.39 b b 24/03/2004 01:01 5.868 -74.003 118 2.4 138.8 182.2 0.21 b b 123severity classification of a seismic event based on the magnitude-distance ratio using only one seismological station 01/04/2004 15:15 5.443 -73.781 89 2.2 140.6 166.4 0.21 b b 04/04/2004 16:16 4.347 -73.853 77 3.2 4 77.1 0.66 a b 17/04/2004 20:20 5.577 -73.884 94 2.6 135.2 164.7 0.25 b b 19/04/2004 00:00 4.579 -74.776 58 2.5 0 58.0 0.69 a a 24/04/2004 13:13 5.315 -73.774 80 2.6 140 161.2 0.26 b b 16/05/2004 01:01 5.629 -73.893 98 2.4 135.7 167.4 0.23 b b 25/05/2004 17:17 4.678 -74.679 43 2.6 14 45.2 0.92 a a 24/06/2004 23:23 4.686 -74.807 56 3.2 21.7 60.1 0.84 a a 28/06/2004 12:12 5.435 -73.769 89 2.7 140.7 166.5 0.26 b b 24/07/2004 22:22 5.215 -74.552 47 2.4 3.2 47.1 0.81 a a 22/08/2004 02:02 5.067 -74.761 53 2.5 13.6 54.7 0.73 a a 22/08/2004 17:17 5.596 -73.856 97 2.5 124 157.4 0.25 b b 26/08/2004 04:04 5.628 -73.986 93 2.3 111.9 145.5 0.25 b b 27/09/2004 14:14 5.785 -73.924 112 2.2 102 151.5 0.23 b b 07/10/2004 23:23 5.605 -74.381 83 2.2 96.4 127.2 0.27 b b 21/11/2004 14:14 5.361 -73.725 87 2.9 147.8 171.5 0.27 b b 22/11/2004 10:10 4.583 -75.138 95 3.1 0 95.0 0.52 a a 19/12/2004 14:14 4.053 -74.039 95 2.3 0 95.0 0.38 b b 01/01/2005 15:15 5.013 -74.844 60 2.4 18.3 62.7 0.61 a a 21/01/2005 07:07 5.419 -73.742 90 2.3 148 173.2 0.21 b b 23/01/2005 07:07 5.241 -73.686 83 2.3 153.1 174.2 0.21 b b 08/02/2005 09:09 5.367 -74.91 86 2.4 7.8 86.4 0.44 b b 11/02/2005 22:22 5.278 -73.777 77 2.4 155.4 173.4 0.22 b b 06/04/2005 19:19 5.791 -74.028 109 2.3 1.8 109.0 0.33 b b 16/04/2005 05:05 5.453 -73.817 87 2.2 134.4 160.1 0.21 b b 04/05/2005 12:12 5.291 -73.736 82 2.7 146 167.5 0.26 b b 26/05/2005 09:09 4.681 -74.863 62 2.9 22.2 65.9 0.70 a a 30/06/2005 03:03 5.326 -73.756 82 2.4 143.5 165.3 0.23 b b 10/07/2005 03:03 4.415 -75.05 94 2.3 8.7 94.4 0.38 b b 16/07/2005 12:12 5.594 -73.891 95 2.2 122 154.6 0.22 b b 16/07/2005 02:02 5.92 -74.499 119 2.2 28.7 122.4 0.28 b b 29/07/2005 22:22 5.632 -74.61 91 3 4 91.1 0.52 a a 05/08/2005 01:01 5.716 -74.38 95 2.2 58.7 111.7 0.31 b b 15/08/2005 18:18 5.608 -73.867 98 2.3 131.7 164.2 0.22 b b 19/08/2005 23:23 5.304 -73.751 81 2.4 150.5 170.9 0.22 b b 09/10/2005 04:04 5.325 -73.721 85 2.3 146.8 169.6 0.21 b b 15/10/2005 01:01 5.749 -73.973 106 2.2 101.5 146.8 0.23 b b 23/10/2005 13:13 5.744 -73.906 109 2.8 100.9 148.5 0.30 b b 12/11/2005 09:09 5.289 -73.751 80 2.2 144 164.7 0.21 b b 17/11/2005 03:03 4.728 -74.708 44 2.4 8.7 44.9 0.85 a a 21/11/2005 00:00 5.325 -73.761 82 2.2 150.1 171.0 0.20 b b 02/01/2006 00:00 5.45 -74.159 68 3.5 48 83.2 0.65 a b 07/02/2006 03:03 5.321 -73.76 81 2.7 143.6 164.9 0.26 b b 27/03/2006 02:02 5.286 -73.813 74 2.2 151.6 168.7 0.20 b b 04/06/2006 06:06 5.6 -73.869 97 2.2 131.5 163.4 0.21 b b 17/06/2006 05:05 5.287 -73.724 82 2.4 148.3 169.5 0.22 b b 20/06/2006 18:18 4.177 -74.54 79 2.6 0 79.0 0.53 a b 21/06/2006 03:03 5.435 -73.802 87 2.3 138 163.1 0.22 b b 10/07/2006 07:07 5.284 -73.738 81 3.4 155.7 175.5 0.30 b b 06/08/2006 04:04 5.615 -74.045 90 2.3 107.3 140.0 0.26 b b 16/08/2006 04:04 5.301 -73.754 81 2.3 155.2 175.1 0.21 b b 06/09/2006 00:00 5.641 -73.88 100 2.4 132 165.6 0.23 b b 06/09/2006 09:09 5.299 -73.735 82 2.4 149.5 170.5 0.22 b b 124 luis h. ochoa, luis f. niño, carlos a.vargas 18/09/2006 17:17 5.312 -73.745 82 2.5 154.9 175.3 0.23 b b 06/10/2006 03:03 5.281 -73.713 83 2.5 151.4 172.7 0.23 b b 23/10/2006 22:22 5.289 -73.709 84 3.4 148.9 171.0 0.31 b b 01/11/2006 12:12 5.206 -74.013 52 3.9 126.5 136.8 0.43 b b 07/11/2006 07:07 5.274 -73.708 83 3.5 151.8 173.0 0.31 b b 12/12/2006 07:07 5.09 -74.781 56 2.8 22 60.2 0.74 a a 18/01/2007 19:19 5.591 -74.086 86 2.5 103.3 134.4 0.30 b b 27/01/2007 05:05 5.515 -74.078 78 2.7 112.2 136.6 0.32 b b 31/03/2007 01:01 5.69 -73.865 106 2.4 131.7 169.1 0.22 b b 12/04/2007 16:16 5.485 -73.82 90 2.4 144.5 170.2 0.22 b b 16/04/2007 07:07 4.582 -74.754 56 3 28.3 62.7 0.76 a a 03/05/2007 07:07 5.737 -73.902 108 2.2 98.5 146.2 0.23 b b 19/05/2007 07:07 4.526 -75.309 115 2.6 0 115.0 0.36 b b 29/06/2007 17:17 5.303 -73.705 85 2.2 148.3 170.9 0.20 b b 10/07/2007 17:17 5.272 -73.664 87 2.4 153.6 176.5 0.22 b b 14/07/2007 10:10 5.238 -73.701 82 2.3 143.2 165.0 0.22 b b 16/10/2007 10:10 5.293 -73.669 88 2.5 142.7 167.7 0.24 b b 12/11/2007 09:09 5.459 -73.803 89 2.7 133.4 160.4 0.27 b b 17/11/2007 21:21 5.771 -74.26 102 2.9 62 119.4 0.39 b b 02/12/2007 11:11 4.696 -73.647 77.8 2.6 15.7 79.4 0.52 a b 17/12/2007 09:09 5.263 -73.673 85.7 3.1 150 172.8 0.28 b b 29/12/2007 06:06 5.265 -73.717 81.6 3.3 152 172.5 0.30 b b 11/01/2008 02:02 5.305 -73.739 82 2.3 146.3 167.7 0.22 b b 13/01/2008 04:04 5.244 -73.649 87 3 151.4 174.6 0.27 b b 14/03/2008 16:16 4.88 -74.821 55 3.8 4 55.1 1.05 a a 07/04/2008 04:04 5.345 -73.785 81 2.2 136.3 158.6 0.22 b b 13/04/2008 02:02 5.622 -73.941 95 2.2 110.2 145.5 0.24 b b 24/05/2008 19:19 4.422 -73.83 73 4.5 6.3 73.3 0.89 a a 24/05/2008 21:21 4.409 -73.762 80 3.4 2.9 80.1 0.66 a a 24/05/2008 19:19 4.395 -73.82 76 3.6 4 76.1 0.73 a b 24/05/2008 21:21 4.452 -73.766 77 2.9 0 77.0 0.60 a b 24/05/2008 17:17 4.453 -73.83 71 2.9 0 71.0 0.65 a b 24/05/2008 21:21 4.512 -73.657 84 2.5 22.8 87.0 0.46 b a 24/05/2008 23:23 4.58 -73.726 74 2.4 17.6 76.1 0.50 b b 24/05/2008 19:19 4.431 -73.797 76 2.3 4 76.1 0.48 b b 25/05/2008 11:11 4.452 -73.799 74 2.8 12 75.0 0.60 a a 25/05/2008 17:17 4.475 -73.729 79 2.6 15.9 80.6 0.51 a a 25/05/2008 00:00 4.483 -73.825 70 2.9 0 70.0 0.66 a b 25/05/2008 02:02 4.406 -73.698 86 2.5 0 86.0 0.46 b b 25/05/2008 06:06 4.439 -73.76 78 2.2 10.8 78.7 0.43 b b 26/05/2008 01:01 4.461 -73.775 75 2.3 3.4 75.1 0.48 b b 30/05/2008 19:19 4.439 -73.767 78 2.3 0 78.0 0.46 b b 02/06/2008 12:12 5.634 -74.382 86 2.9 89.6 124.2 0.37 b b 03/06/2008 02:02 4.446 -73.762 78 3.4 12.4 79.0 0.67 a a 07/06/2008 20:20 4.432 -73.83 73 2.6 0 73.0 0.57 a b 11/06/2008 18:18 5.399 -73.7 92 2.5 141.8 169.0 0.24 b b 02/07/2008 14:14 5.315 -73.747 82 2.2 149.4 170.4 0.20 b b 07/07/2008 05:05 4.429 -73.824 73 2.2 0 73.0 0.47 b b 09/08/2008 12:12 5.297 -73.741 82 2.7 142.5 164.4 0.26 b b 15/08/2008 01:01 5.311 -73.765 80 2.2 143.7 164.5 0.21 b b 02/09/2008 18:18 5.42 -73.757 89 2.3 139 165.1 0.22 b b 03/09/2008 02:02 5.517 -74.052 79 3.8 120.8 144.3 0.40 b b untitled earth sciences research journal earth sci. res. j. vol. 13, no. 1 (june 2009): 82-89 comparative evaluation of the influencing effects of geomagnetic solar storms on earthquakes in anatolian peninsula sadik cengiz yesugey dokuz eylul university, faculty of architecture, campus of tinaztepe, dogus str. no.209 kurucesme, buca, ýzmir turkey. phone: +902324535716 fax: +902324532986 corresponding author email: cengiz.yesugey@deu.edu.tr abstract earthquakes are tectonic events that take place within the fractures of the earth’s crust, namely faults. above certain scale, earthquakes can result in widespread fatalities and substantial financial loss. in addition to the movement of tectonic plates relative to each other, it is widely discussed that there are other external influences originate outside earth that can trigger earthquakes. these influences are called “triggering effects”. the purpose of this article is to present a statistical view to elaborate if the solar geomagnetic storms trigger earthquakes. as a model, the research focuses on the anatolian peninsula, presenting 41 years of historical data on magnetic storms and earthquakes collated from national and international resources. as a result of the comparative assessment of the data, it is concluded that the geomagnetic storms do not trigger earthquakes. keywords: earthquake, earthquake triggering, geomagnetic storms. resumen los terremotos son eventos tectónicos que tienen lugar dentro de las fracturas de la corteza terrestre, nominalmente las fallas. por encima de cierta escala los terremotos pueden resultar considerables pérdidas humanas y financieras. adicionalmente de los movimientos de las placas tectónicas, es ampliamente discutido que hay otras influencias originadas en el exterior de la tierra que pueden desencadenar terremotos. estas influencias son llamadas “efectos disparados”. el propósito de este artículo es presentar una observación estadística sobre si las tormentas solares geomagnéticas desencadenan terremotos. como un modelo, la investigación se centra sobre la península anatolia, presentando 41 años de datos históricos sobre tormentas 82 manuscript receiver: december 12th, 2008. accepted for publication: july 28th, 2009. julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:18 p p p composite 133 lpi at 45 degrees magnéticas y terremotos cotejados con datos nacionales e internacionales. como resultado de la evaluación comparativa de los datos, esto esta concluyendo que las tormentas geomagnéticas no desencadenan terremotos. palabras claves: terremoto, desencadenar terremoto, tormentas geomagnéticas 1. introduction earthquakes are tectonic events that take place within the fractures of the earth’s crust, namely faults. in addition to the movement of tectonic plates relative to each other, it is widely discussed that there are other external influences originate outside earth that can trigger earthquakes. these influences are called “triggering effects”. examples of such external events are the lunar and solar eclipses, planetary alignment within the solar system and the influences of geomagnetic storms taking place in the sun. the most influential of these events is considered to be the extraordinary solar activities influencing the earth’s geomagnetic field. anatolian peninsula is a historically active earthquake region within the borders of turkish republic. the peninsula, surrounded by black sea in the north, aegean sea in the west, mediterranean sea in the south, lies at the intersection of asia, europe and africa continents. hundreds of earthquakes in various scales take place every year in this active region. the most recent major earthquake, marmara earthquake, took place on 17th august 1999 and measured 7.3 on richter scale, causing extensive damage and widespread fatalities. the purpose of this article is to present a statistical view to elaborate if the solar geomagnetic storms trigger earthquakes. as a model, the research focuses on the anatolian peninsula, presenting 41 years of historical data on magnetic storms and earthquakes collated from national and international resources. 2. the causes of earthquakes and geomagnetic storm and measurement methods this section outlines the principles behind the occurrence of earthquakes and geomagnetic storms and the methods used to measure them. 2.1 the causes of earthquakes and measurements methods in 1911, professor reid established the elastic-rebound theory, supported with [reid,h.f, 1911]. a mathematical scale was required in order to obtain the data about the effects of these vibrations on the structures and charles f. richter and beno gutenberg applied “magnitude” concept on this field in 1930 [gutenberg, b.,et al., 1954]. among these; mb is calculated by taking as basis the magnitude of p and s waves (body-wave magnitude), md is calculated by using the durations of very small and close earthquakes, ms is calculated by taking as basis the magnitude of surface waves (surface-wave magnitude) and mw takes as basis the seismic moment of the released energy (moment magnitude). these are the most used magnitude measures. different magnitude methods can give different values around 0.2m. this difference can reach up to 0.5m after saturation. the magnitude of an earthquake does not change wherever on earth it is measured. giving different magnitude values of earthquakes must be associated with the method used and the fact that the geology of the place in which the recorders are located is not adequately taken into account [gürbüz, c., 2003]. 2.2 the reasons for the occurrence of geomagnetic storms and measurement systems the solar material, which accelerates as a result of the energy discharges occurring in the sun, “solar flares”, “prominences” and “coronal mass ejections”, reaches the earth from the gravitational field of the sun as different radiation forms and particles, and it interacts with the magnetic field of the earth. in cases where these effects are dense, sudden changes occur in the magnetic field and 83 comparative evaluation of the influencing effects of geomagnetic solar storms on earthquakes in anatolian peninsula julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:19 p p p composite 133 lpi at 45 degrees they create the phenomenon which is called “geomagnetic storm.” although different systems are used, one of the most important scales used in measuring this is the dst “disturbance storm time” indexes which have been recorded since 1957 [sugiura, m, 1964]. dst index’s showing high negative values means that there is a big magnetic storm [sugiura, m, 1991]. for the magnetic fields, changes in the dst index are evaluated in five major groups. weak storms -30 nt�dst >-50 nt, moderate storms -50 nt>dst >-100 nt, strongstorms -100 nt>dst>-200 nt, intense storms -200 nt>dst >-350 nt, heavy storms dst >-350 nt [loewe, c.a., at al., 1997], [natural environment research council british antarctic survey, 2001 ]. dst values’ falling to these values in their changes within 24 hours which are measured hourly is regarded as a geomagnetic storm. within the internet, data centers which provide service to the entire world on this subject are also concentrated in the usa. one of these institutions is the national environmental satellite, data, and information service which operates under the usa national oceanic and atmospheric administration, [national oceanic and atmospheric administration (noaa) space weather prediction center (swpc) 2003]. another institution is the national geophysics data center where all observations on all solar activity conducted in the entire earth, the effects of the activity on the near perimeter of the earth and its results on the atmosphere are collected, and this center provides international data coordination, [nooa national geophysics data center, 2005]. world data center system which is conducted by the usa national science foundation provides the communication between scientific research institutions all over the world. another one of these institutions is the world data center for geomagnetism, kyoto, data analysis center for geomagnetism and space magnetism graduate school of science, kyoto university, [world data center for geomagnetism, kyoto, 2005]. 3. the method used for the detection of the triggering effects of the earthquakes in accordance with the earthquake magnitudes and the changes in the dst values the occurrence of the earthquakes is dependent on very different factors. especially, the structures of the rocks are the most important features. due to the fact that some rocks show more resistant features, they may not be broken or may be broken later than usual even though they have the same energy accumulations and they are in close regions. however, it is doubtless that they all have limit values. this can be resembled to the discharge of a condenser, which is loaded with current and which reached the limit value for the discharge, with a very low current; or overflow of a glass, which is filled with water up to its utmost capacity, with a drop of water. in this study, the breakage probabilities of the rocks, which have energy accumulation that reached up to the limit value, with geomagnetic effects apart from the other factors have been examined. the below-mentioned methods have been used in order to obtain these statistics. 3.1 sources for the earthquake data and shortcomings in calculating the earthquake triggering effects, the study area is limited to the anatolian peninsula and surrounding seas, between coordinates 35°, 00’n-43°, 00’n and 25°,00’e-45°,00’e. the earthquakes which fall outside these areas are excluded from this study. the dataset used in this study is based on 41 years of data collected between 1965 and 2005. this data has been collected from the archive of the following institutions, local to the study area; republic of turkey, ministry of public works and settlement, general directorate of disaster affairs, earthquake research department, [general directorate of disaster affairs, earthquake research department, 2005], bogazici university kandilli observatory and earthquake research institute, national earthquake monitoring center –nemc, 84 sadik cengiz yesugey julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:19 p p p composite 133 lpi at 45 degrees [bogazici university kandilli observatory, 2005]. the scientific and technological research council of turkey, marmara research center, earth and marine sciences institute, [the scientific and technological research council of turkey, marmara research center, 2005]. in order to ensure that the dataset is complete, the data from the above listed sources have been validated by the archives of; u.s. geological survey, national earthquake information center usgs-neic, earthquake center, [usgs-neic, 2005], u.s. geological survey, earthquake hazard program, advanced national seismic system-anss, [anss, 2005], incorporated research institutions for seismology iris, [seismology iris, 2005]. the data from various sources was sorted by year, month, day and coordinates, and compared with each other to identify discrepancies. in the event of discrepancies, the data from the local sources was used. during this assessment, it was identified that slight differences in coordinates could result in the same earthquake event being recorded more than once. if there are a number of earthquakes reported by various sources for the same time frame, the coordinates were compared and if the coordinates measure within ± 30’ (30 minutes) of each other, it was assumed that all sources record the same event, therefore, a single earthquake was included in the study. as it has been stated in section 2.1, the measurements conducted by different institutions in different dates are archived as magnitude from time to time in accordance with mb, ms, md and mw scales, and there is no possibility to reduce them to a common unit. however, as it has been stated before, these scales take values which are very close to each other. in this condition, the values and scales provided by the turkish institutions at the same time and date have been primarily accepted. no scale difference was inquired in the different and incomplete data that were obtained by scanning various national and international institutions, and those with the greatest magnitude values were added to the list as a data. the dataset contains all the earthquakes within the coordinate limits set for the anatolian peninsula for the last 41 years. this data is sorted by year, month, day and magnitude. the total number of earthquake measurements within the study period is 122838. the earthquakes with magnitudes below 3.0m was excluded in order to prevent misleading results. the limiting upper bound was set to 7.9m, the highest earthquake recorded between 1965 and 2005. as widely known, there are many relatively minor preshocks leading up to the earthquake and aftershocks following the earthquake, which sometimes continue to occur for months. however, it is not practical to differentiate between the major event and its pre and after shocks. it can also be argued that geomagnetic storms could amplify the aftershocks. for this reasons, the study includes all pre shocks and aftershocks, as long as they coincide with a geomagnetic storm. 3.2 sources for the geomagnetic storm data and shortcomings in this study, data from world data center for geomagnetism, kyoto, data analysis center for geomagnetism and space magnetism graduate school of science, kyoto university which are constantly and regularly broadcasted on the internet and globally acknowledged have been taken as basis [world data center for geomagnetism, kyoto, 2005]. the dst -30 nt value specified in section 2.2 for the geomagnetic storm dst values has been accepted as the minimum storm value. with a computer program that has been prepared, 41 year old data on a 24 hour basis have been scanned, the days on which the dst values were inside the storm limits have been listed, and the values apart from those have been marked as “0” value. thus, “yearly and daily value” lists spanning over 41 years have been obtained. 4. methods used in the assessment of data the data assessment has been conducted with three different methods. 85 comparative evaluation of the influencing effects of geomagnetic solar storms on earthquakes in anatolian peninsula julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:19 p p p composite 133 lpi at 45 degrees in the first method; all the earthquakes occurring in a day within the range of 3.0m-7.9m remaining within the given coordinates between the years 1965-2005 and in different regions have been collectively and on a daily basis compared with the dst values within the same range in terms of their date, magnitude and dst values and presented in the form of tables (for instance; each of the values with such magnitudes as 3.1m, 3.2m, 4.5m, 5.2m that occurred on the same day and within the range of 3.0-7.9 has been taken as data). again within the same range; the earthquakes with magnitudes of 3.0m–3.9m, 4.0m–4.9m, 5.0m–5.9m, 6.0m–6.9m and 7.0m–7.9m have been compared in the form of tables and on a daily basis with the dst values corresponding to their date and magnitude values within their own range. in this comparison, all the earthquake magnitude values within the mentioned range and occurring on the same day have been included in the comparison. (for instance; each of the values with such magnitudes as 3.1m, 3.2m, 3.5m occurring within 3.0-3.9 range and on the same day has been taken as data). in the second method; the greatest rates of earthquakes detected within a day within the range of 3.0m-7.9m and with the magnitudes of 3.0m–3.9m, 4.0m–4.9m, 5.0m– 5.9m, 6.0m–6.9m and 7.0m–7.9m remaining within the given coordinates between the years 1965-2005 and in different regions have been within their own range collectively and daily compared with the dst values according to their date and magnitude values and in the form of tables. however, in this comparison, among the earthquakes occurring within mentioned range and on the same day, the earthquake with the greatest magnitude detected within the day has been included in the comparison. (for instance; as far as such values as 3.1m, 4.2m, 5.5m occurring on the same day were concerned, 5.5m being the greatest value has been taken as data. similarly, when such values as 3.1m, 3.2m, 3.5m within the range of 3.0m-3.9m occurring on the same day are concerned, only 3.5m being the greatest value has been taken as data). in the study conducted; it has been considered that in the past years, especially the records between the 1960s and 1970s might not have been as sound as the records of our present time and as a third method, the comparisons have been repeated for the next 20 years (1986-2005). in these comparisons made with a prepared computer program, the data and statistical values below have been obtained. the evaluation of the data was carried out using a bespoke computer model and the below listed data and statistical values have been obtained. input; number of earthquakes corresponding to geomagnetic storm within the magnitude interval (m), number of earthquakes not corresponding to geomagnetic storm within the magnitude interval (n), total number of earthquakes within the magnitude interval (w), number of days without geomagnetic storms (-d-days), number of days with geomagnetic storms (+d-days), total number of days between 1965-2005 (days), total number of days between 1986-2005 (days1). output; ratio of number of earthquakes corresponding to geomagnetic storms to total number of earthquakes within the magnitude interval (m/w %), ratio of number of earthquakes not corresponding to geomagnetic storms to total number of earthquakes within the magnitude interval (n/w %). 5. evaluation tables based on the approach outlined in section 4, the following evaluation tables were produced. 6. results the ratio of all the earthquakes within the range of 3.0m-6.9m magnitude between the years 1965-2005 which corresponded to the days with dst=-30nt and occurred during the day, to the number of earthquakes (w) which are within the same range differs between 19.56% and 27.50%. this ratio is 45.46% in the 7.0m-7.9m magnitude range. (table 1). the ratio of the greatest earthquake which were detected during the day among the earthquakes be86 sadik cengiz yesugey julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:19 p p p composite 133 lpi at 45 degrees tween the years 1965 and 2005, and which corresponded to the days with dst -30nt, to the number of earthquakes (w) occurring within the same range differs between 19.8% and 20.8%. this ratio is 28.57% in the 7.0m-7.9m magnitude range (table 2). the ratio of the earthquakes within the range of 3.0m-5.9m magnitude between the years 1986-2005 corresponding to the days with dst -30nt and occurring during the day, to the number of earthquakes (w) which occurred within the same range differs between 18.90% and 22.96%. this ratio is 31.15% in the 6.0m-6.9m magnitude range and 50% in the 7.0m-7.9m magnitude range (table 3). the ratio of the greatest earthquake which was detected during the day among the earthquakes within the 3.0m-5.9m range and corresponding to the days with dst -30nt between the years 1986 and 2005, to the number of earthquakes (w) that occurred within the same range differs between 20.00% and 22.67%. this ratio is 23.08% in the 6.0m-6.9m magnitude range and 25.00% in the 7.0m-7.9m magnitude range (table 4). 87 comparative evaluation of the influencing effects of geomagnetic solar storms on earthquakes in anatolian peninsula table 1. evaluation using all earthquakes within the same calendar day between 1965 and 2005. magnitude interval m (num.) n (num.) w (num.) m/w (%) n/w (%) m /(+d-day) (num./num.) n /(-d-day) (num./num.) 3.0-7.9(total) 11060 40294 51354 21.50 78.46 3.743 3.352 3.0-3.9 8966 32979 41945 21.38 78.62 3.034 2.744 4.0-4.9 1897 6552 8449 22.45 77.55 0.642 0.545 5.0-5.9 170 699 869 19.56 71.44 0.0575 0.0582 6.0-6.9 22 58 80 27.50 72.50 0.0075 0.0048 7.0-7.9 5 6 11 45.46 54.55 0.0017 0.0005 +d-day = 2955(num.) -d-day = 12020(num.) day= 14975(num.) table 2. evaluation using the earthquakes with the highest magnitudes within the same calendar day between 1965 and 2005. magnitude interval m (num.) n (num.) w (num.) m/w (%) n/w (%) m /(+d-day) (num./num.) n /(-d-day) (num./num.) 3.0-7.9(total) 2411 9174 11585 20.81 79.19 0.816 0.763 3.0-3.9 1415 5304 6719 21.06 78.94 0.479 0.441 4.0-4.9 882 3413 4295 20.54 79.46 0.298 0.284 5.0-5.9 104 421 525 19.81 80.19 0.0352 0.0350 6.0-6.9 8 31 39 20.51 79.49 0.0027 0.0026 7.0-7.9 2 5 7 28.57 71.43 0.00068 0.00042 +d-day= 2955(num.) -d-day= 12020(num.) day= 14975(num.) julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:19 p p p composite 133 lpi at 45 degrees in addition, there is not a significant quantitative increase of the empirical values m/(+d-day) ratio (table 1-4). 7. conclusion when the ratios above are examined and the highness of the ratio of dst�-30nt values to total days are taken into account, both between the years 1965-2005 and 1986-2005, an extraordinary quantitative increase can not be seen in the earthquakes which have magnitudes of 3.0m-5.9m and which correspond to days with dst�-30nt. when it is considered that the aftershocks are also included in the number of earthquakes which have been taken into the comparison, in the area within the given coordinates, there is no finding that shows that the earthquakes occur as a result of a triggering under the effect of geomagnetic storm. in order to mention such a finding, more number of earthquakes should occur with percentages that correspond to geomagnetic storms. this is not verified by the results. 88 sadik cengiz yesugey table 3. evaluation using all earthquakes within the same calendar day between 1986 and 2005. magnitude interval m (num.) n (num.) w (num.) m/w (%) n/w (%) m /(+d-day) (num./num.) n /(-d-day) (num./num.) 3.0-7.9(total) 8318 30248 38566 21.57 78.43 5.020 5.356 3.0-3.9 7096 26089 33185 21.38 78.62 4.282 4.619 4.0-4.9 1103 3701 4804 22.96 77.04 0.666 0.655 5.0-5.9 96 412 508 18.90 81.10 0.0579 0.0729 6.0-6.9 19 42 61 31.15 68.85 0.0115 0.0074 7.0-7.9 4 4 8 50.00 50.00 0.00241 0.0007 +dst-day = 1657(num.) -dst-day = 5648(num.) day1 = 7305(num.) table 4. evaluation using the earthquakes with the highest magnitudes within the same calendar day between 1986 and 2005. magnitude interval m (num.) n (num.) w (num.) m/w (%) n/w (%) m /(+d-day) (num./num.) n /(-d-day) (num./num.) 3.0-7.9(total) 1527 5238 6765 22.57 77.43 0.922 0.927 3.0-3.9 972 3317 4289 22.66 77.34 0.587 0.587 4.0-4.9 500 1706 2206 22.67 77.33 0.302 0.302 5.0-5.9 48 192 240 20.00 80.00 0.0290 0.0339 6.0-6.9 6 20 26 23.08 76.92 0.0036 0.0035 7.0-7.9 1 3 4 25.00 75.00 0.00060 0.00053 +dst-day = 1657(num.) -dst-day = 5648(num.) day1 = 7305(num.) julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:20 p p p composite 133 lpi at 45 degrees earthquakes with magnitudes of 6.9m-7.9m are very few in number. the fact that these partially have greater percentages gives the impression that they are coincidental values rather than findings that form correlation with geomagnetic storms. as a result of all these data, a hypothesis can not be put forward which suggests that geomagnetic storms trigger earthquakes in the anatolian peninsula. however, these results should not hinder the conduction of further research. a global study on this subject can potentially provide new approaches. references gutenberg b. and richter c. f. (1954), seismicity of the earth and associated phenomena, 2nd edition, princeton univ. pres, new jersey. loewe c. a., prölss, g. w. (1997), classification and mean behaviour of magnetic storms, journal of geophysical research, a 102, 14209-14213. reid, h. f., (1911), the elastic-rebound theory of earthquakes, university of california department of geology bulletin, 6(19), 413–444, berkeley. sugiura, m., (1964), hourly values of equatorial dst, annals of the international geophysical year, vol. 35, pp. 945-948, pergamon press, oxford. sugiura, m.,(1991), on dst index, http:// swdcwww.kugi.kyoto-u.ac.jp/dstdir/dst2/ondsti ndex.html, paper presented at iaga bulletin no. 40. bogazici university kandilli observatory and earthquake research institute, (2005), internet site for national earthquake monitoring center –nemc, http://www.koeri.boun.edu.tr/ (last accessed 2008). gürbüz,c. (2003). internet site for earthquake magnitude, bogazici university kandilli observatory and earthquake research institute, national earthquake monitoring center –nemc, http:// www.koeri.boun.edu.tr/jeofizik/html/info_magnitude.htm (last accessed october 2008). incorporated research institutions for seismology, (2005), internet site for iris, http:// www. iris.edu/quakes/quakes.htm (last accessed 2008). natural environment research council british antarctic survey (2005), internet site for satellite risk index, http://www.antarctica.ac.uk/ satellite risks/dstdetails.html (last accessed 2008). national oceanic and atmospheric administration (noaa) space weather prediction center (swpc) (2005), internet site for ace real time solar wind news and announcement, http://www.swpc.noaa.gov/ace/ (last accessed march 31, 2003). noaa, national geophysics data center (2005), internet site for space weather and solar events, http://www.ngdc.noaa.gov/ (last accessed july 30, 2008). world data center for geomagnetism (2005), internet site for data analysis center for geomagnetism and space magnetism graduate school of science, kyoto university, http:// swdcwww.kugi.kyoto-u.ac.jp/ (last accessed 2008). republic of turkey, ministry of public works and settlement, general directorate of disaster affairs, (2005), internet site for earthquake research department, http://www.deprem.gov.tr/ indexen.html (last accessed 2008). the scientific and technological research council of turkey, (2005), internet site for marmara research center, earth and marine sciences institute http://www.mam.gov.tr/eng/institutes/ ydbe/(last accessed october 07, 2008). u.s. geological survey, (2005), internet site for national earthquake information center usgsneic, http://earthquake.usgs.gov/ eqcenter/(last accessed july 16, 2008). u.s. geological survey, earthquake hazard program, advanced national seismic system, (2005), internet site for anss http://earthquake.usgs.gov/research/monitoring/anss/ (last accessed september 10, 2008). 89 comparative evaluation of the influencing effects of geomagnetic solar storms on earthquakes in anatolian peninsula julio 6-geociencias-vol 13-1 2009.prn d:\geociencias 13-1 jun 2009\geociencias-vol 13-1 2009.vp martes, 07 de julio de 2009 18:11:20 p p p composite 133 lpi at 45 degrees geofis. colomb. santafe de bogota d.c issn ·0121·2974 efectos de la erupcion del volcan nevado del ruiz, ocurrida el 13-noviembre-1985, sobre la atmosfera en colombia. caso: insolacion (brillo solar). jesus a. eslava r. profesor titular departamento de geociencias-facultad de ciencias-universidad nacional de colombia eslava, j: efectos de la erupci6n del volcan nevado del ruiz, ocurrida ei13-noviembre-1985, sobre la atm6sfera en colombia. caso: insolaci6n (brillo solar). geofis. colomb. 2:37-50,1993. issn 0121-6120 resumen este es el primero de una serie de artlculos que muestran el efecto que la psquena erupci6n del volcan nevado del ruiz del 13-11-85, ocasion6 en los valores de algunos elementos meteorol6gicos, en la capa de aire superficial y en las de mayor altitud del aire en colombia. aca se muestra un anatisis de las variaciones de la insolaci6n (brillo solar) del 11 al 16 de noviembre-1985 (antes, en y despues de la erupci6n). se destaca que la notable disminuci6n de la insolaci6n ocurri6 desde 11dfa 12, por efectos directos e indirectos d3e los gases y cenizas emitidas con anterioridad abstract this article is the first one from a serie that shows the effects on the values of some basic meteorological elements on differents levels of the atmosphere, ocasiones by the eruptive event from volcano nevado del ruiz on november 13 of 1985. variations on the sunshine are analysed during november 11 to 16 period after and before of the eruptive event, where is notary the value disminution of the sunshine from november 12 by direct and indirect effects of the gases and ashes erupted before. 1. introduccion en general, los autores de trabajos relacionados con los efectos de las erupciones volcanicas, adernas de los comentarios sobre la muerte de seres humanos, exponen sus puntos de vista respecto a la destrucci6n de bienes materiales, muebles e inmuebles; en ocasiones los hacen extensivos a los problemas digestivos 0 respiratorios, por inhalaci6n de gases 0 cenizas digeridas; en algunos casos a la reducci6n de la visibilidad, la importancia del movimiento del aire en el trans porte de las cenizas y de la liuvia en los posibles f1ujos de escombros. no es facil encontrar referencias cuantitativas (a veces ni siquiera cualitativas) que muestren la modificaci6n de los valores de los elementos meteorol6gicos ocasionada por la introducci6n en la atm6sfera de s61idos, ifquidos y/o gases durante un perfodo de actividad volcanica, este es el primero de una serie de arttculos que intentaran mostrar, cuantitativa y/o cualitativamente sequn las posibilidades, el efecto que la relativamente pequeria erupci6n del volcan nevado del ruiz del 13-11-85, ocasion6 en los valores de algunos de los elementos meteorol6gicos basicos, tanto en la capa de aire superficial como en las capas de mayor altitud. aca, se analiza la reducci6n de la radiaci6n solar entrante al sistema tierraatm6sfera, a traves del estudio de la cantidad de horas diarias de brillo solar (insolaci6n) que ocurrieron durante el perfodo 11-16-nov-1985 (antes, en y despues de la erupci6n). los datos recopilados en las estaciones meteorol6gicas se extractaron de los archivos de el himaty la federaci6n nacional de cafeteros. los datos originales, verificados y adecuados sequn las necesidades especfficas del trabajo y los resultados de los procesos realizados, se encuentran parcialmente consignados en la tabla y figuras involucradas en el trabajo. 38 eslava: efectos erupcl6n del vnr sobre la atm6sfera. caso: insolaci6n 2. generalidades las cenizas y gases expulsados por el volcan nevado del ruiz formaron capas, -en la atmosfera, de diferentes espesores y densidades que cubrieron rapidamente toda la zona ubicada al noroccidente del tolima, desde los comienzos de la actividad y posteriormente a la mayor parte del pais; ello impidi6 u obstaculiz6 el paso de la energfa solar en el resto de esa tarde y dlas siguientes. utilizando los datos meteorol6gicos disponibles se infiere que se ocasionaron cambios temporales en los valores de los elementos meteorol6gicos que comenzaron desde el dla anterior por los gases y cenizas emitidas con anterioridad y que por efecto de los movimientos locales y generales del aire se habfan acumulado en sectores ubicados al w y nw del volcan, ei gran nurnsro de partfculas que se introdujeron en la atmosfera por la erupci6n del volcan nevado del ruiz el 13-11-85, se difundio en la atm6sfera formando, principalmente, el penacho de cenizas y la nube gaseosa en forma de arco, descritas por eslava (1985) y krueger (1985, citado en smithsonian institution, 1985), respectivamente. sin embargo, las partlculas s61idas y gaseosas se hicieron presentes en casi toda colombia y ocasionaron una reducci6n notoria en la cantidad de horas de sol por dfa que normalmente ocurren en esa epoca del ana; con mayor intensidad, obviamente, en las zonas cercanas al volcan y en las ubicadas por debajo del penacho de cenizas. a su vez, la disminuci6n de las horas de brillo solar, origino una correlacionada reduccion de la radiacion solar entrante en el sistema tierraatm6sfera en cada sitio y una disminucion (no exactamente correlacionada, sino modificada por los factores ffsicos de cada sitio) de las temperaturas medias del aire, con un aumento de las mfnimas y una disminuci6n de las maximas. esas desiguales variaciones de temperatura, en union con el aporte de vapor de agua y nucleos de condensaci6n (ceniza), afectaron la distribuci6n de la humedad y los de mas elementos meteorol6gicos, entre ellos el regimen temporal y espacial de las liuvias. las magnitudes de algunas de esas alteraciones fueron pequenas y, muy seguramente, no perceptibles por los metodos de medici6n disponibles. esas innumerables variaciones temporales y espaciales estuvieron sometidas a factores extern os entre los cuales el factor prima rio fue la radiaci6n solar y sus variaciones; dependieron basicamente de la interrelaci6n que se da entre los elementos meteorologicos y entre ellos y el conjunto de procesos zonales de convecci6n termica y dinamica, de la influencia extensiva de los diferentes sistemas meteorol6gicos, como la ltc, de la presencia de movimientos del aire determinados 0 condicionados por la geometrfa tierra-sol, la latitud, la altitud, la configuraci6n del relieve, el suelo, la vegetaci6n, la hora del dfa 0 noche en la cuailleg6 a cada sitio la masa principal de partfculas, etc. los impactos generales entonces se modificaron por el medio ambiente local y/o regional. todos esos formadores, condicionantes 0 agentes que producen 0 modifican el estado del tiempo, ocasionaron una gran variedad de alteraciones. las caracterfsticas generales de los cambios en uno de los mas importantes elementos meteorol6gicos, se presenta aca. 3. efectos sobre la insolacion (brillo solar) como ya se mencion6, el efecto prima rio del gran nurnero de partlculas introducidas en la atm6sfera por la erupci6n del volcan nevado del ruiz el 13 de noviembre de 1985 fue el de obstaculizar la entrada de radiaci6n solar en la troposfera y superficie terrestre, disminuyendo el nurnero de horas de sol (brillo solar) y la cantidad de energfa que normal mente se recibe en cada sltio por dfa. la intensidad de la disminuci6n fue variable, en espacio y tiempo, dependiendo en cada lugar del espesor de las capas de ceniza y gases, del dia y la hora en que esas partlculas liegaban a cada sitio y de los muy variados factores que influyeron (iatitud, altitud, configuraci6n del relieve, suelo, vegetaci6n, distancia al volcan, movimiento general y local del aire, etc.). los datos (tabla 1), muestran los valores de insolaci6n para el periodo comprendido entre el 11 y el 16 de noviembre de 1985 (antes, en y despues de la erupci6n) y el promedio para todo el mes de noviembre. las diferencias entre esos valores indican los cambios que normalmente ocurren, en unos dias y sitios, por efecto de los sistemas meteorol6gicos normal mente actuantes y en otros dfas y sitios por la uni6n de los efectos meteorol6gicos normalmente actuantes y por los gases y cenizas inyectados en la atm6sfera por el volcan nevado del ruiz (antes, en y despues de la erupci6n). a manera descriptiva y con el prop6sito de poder efeetuar las eomparaciones neeesarias, se muestra en la fig.1 un anal isis de la distribuci6n espacial promedia de la insolaci6n durante el mes de noviembre de 1985. esa distribuei6n es aproximadamente la que en forma tfpiea se darfa si s610 se presentaran los efeetos de los sistemas meteorol6gieos normal mente actuantes. se eoncluye, en forma general, que se pueden eonsiderar (para este mes y elemento) dos grandes zonas: a) una zona que eubre la regi6n del pacifico, la regi6n andina y la regi6n del catatumbo; en esta zona los sistemas meteorol6gieos aetuantes (por ej., la itc) obstaeulizan el paso de la radiaei6n solar y log ran que s610 se den menos de 5 horas de brillo solar por dfa y; b) una zona eomprendida por la mayor parte de la orinoquia, amazonia y la regi6n del caribe; en esta los sistemas meteorol6gieos actuantes no obstaeulizan mayormente la entrada de radiaci6n solar y oeurren mas de 5 horas de insolaei6n por dfa/promedio. la fig.2 i1ustra la variaei6n de la insolaci6n, del 11 al 16-nov-1985, en 12 silios localizados en areas eubiertas per el penaeho de eenizas; en ella se sintetiza el e/deto de la eeniza y gases, que origin6 una clara disminuei6n de la insolaci6n durante el perfodo crftieo del 13 a116. geofisica colombiana n° 2, abril de 1993 39 to· to· ... i ._.........1-...-../.1 c./ \. .-:'> ......._ .....'t _,j' ',__ -,-, i . ',) ,) k_.~ \..~' . '\ (.j'.a..! '<; !»< !r. \ r'''/ \~.'v'. ". brasil ..r--...rj (') \s'·'\..\..h 5 ,..---...... 1 peru :~. ~ 5 .. o· '0 .. .. .. o· to· ",. to· .. . f"gura 1.•sone"as (horas-<leclmos) que indican la insolacion (brillo solar) media dlarla de noylembre de 1985 40 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n tabla 1. insolael6n (brilio solar en horas y deelm0t) re~lstrada en estaelones meteorol6",leas ubleadas en colombia; antes, en y despues de la erupei6n del ole n nevado del rulz del13 de novlernbre de 1985 estacion valores diarios med diferencias nombre-municipio 11 12 . 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m amazonas araracuara-araracuara 4.8 4.6 0.2 1.1 0.4 0.2 5.4 -0.6 -0.8 -5.2 -4.3 -5.0 -5.2 apto.vasquez cobo-leticia 5.6 0.0 2.4 0.0 1.5 1.2 2.9 2.7 -2.9 -0.5 -2.9 -1.4 ·1.7 antioquia urrao-urrao 5.6 0.0 2.4 0.0 1.5 1.2 2.9 2.7 -2.9 -0.5 -2.9 ·1.4 -1.7 canas gordas-canas gordas 6.9 0.0 3.9 0.0 2.9 0.1 4.3 2.6 -4.3 ·0.4 -4.3 -1.4 -4.2 musinga-frontino 6.6 0.0 2.1 0.0 2.1 0.1 3.6 3.0 -3.6 -1.5 -3.6 -1.5 -3.5 villarteaga-muta ta 1.7 0.0 3.1 0.0 4.6 1.3 3.3 -1.6 -3.3 -0.2 -3.3 1.3 -2.0 uniban-apartado 3.7 0.0 5.7 0.3 4.0 2.6 4.4 ·0.7 -4.4 1.3 -4.1 ·0.4 -1.8 riogrande-apartado 1.2 0.2 4.5 2.8 4.0 -2.8 -3.8 0.5 -1.2 tulenapa-chigorodo 3.5 0.0 4.1 0.1 4.3 4.1 ·0.6 -4.1 0.0 -4.0 0.2 apto.los cedros-apartado 3.8 0.0 4.5 0.4 4.8 4.0 4.8 1.0 -4.8 -0.3 -4.4 0.0 -0.8 el mellito-neclocli 6.5 0.0 5.0 1.4 7.9 3.9 4.8 1.7 -4.8 0.2 -3.4 3.1 -0.9 la selva-rionegro 8.0 0.0 1.5 0.0 1.7 0.7 4.0 4.0 -4.0 -25 -4.0 -2.3 -3.3 gja.exp.el nus-san roque 6.9 1.1 3.6 0.2 3.7 2.4 4.8 2.1 -3.7 -1.2 -4.6 -1.1 -2.4 corrientes-san vicente 1.3 9.2 10.0 4.7 4.9 4.3 -3.0 4.9 5.7 -0.2 0.6 las violetas-san rafael 7.6 1.2 1.6 0.0 4.5 1.4 4.1 3.5 -29 -2.5 -4.1 0.4 -2.7 san carlos-san carlos 7.0 0.3 1.8 0.0 2.8 0.5 3.7 3.3 -3.4 -1.9 -3.7 -0.9 -3.2 apto.pto.berrio-pto.berrio 7.7 2.0 6.0 0.1 5.9 5.6 6.2 1.5 -4.2 ·0.2 ·6.1 -0.3 -0.6 apto.otu-remedios 1.5 5.4 0.7 1.6 5.6 6.1 5.1 -36 0.3 -4.4 -3.5 0.5 1.0 gja.la nacional-tamesis 7.3 0.0 2.8 0.0 3.1 1.2 5.0 2.3 -5.0 -2.2 -5.0 -1.9 -3.8 hda.montenegro-valparaiso 8.2 0.1 3.0 0.0 4.1 2.0 5.4 2.8 -5.3 -2.4 -5.4 -1.3 -3.4 mesopotamia-la union 7.1 0.0 0.3 0.0 0.2 2.0 3.9 3.2 -3.9 -3.6 -3.9 -3.7 -1.9 ita andes-andes 8.1 0.0 3.6 0.0 3.9 3.1 4.7 3.4 -4.7 -1.1 -4.7 -0.8 -1.6 el rosario-venecia 8.1 0.0 2.2 0.0 2.4 2.2 4.7 3.4 -4.7 -2.5 -4.7 -2.3 -2.5 la plata-venecia 7.0 0.0 4.6 0.0 4.3 2.0 5.4 1.6 -5.4 -0.8 -5.4 -1.1 -3.4 concordia-concordia 4.6 0.0 0.5 0.0 1.0 0.4 2.1 2.5 -2.1 -1.6 -2.1 -1.1 -1.7 hda.cotove-s.f de antioquia 8.0 0.1 1.5 0.0 2.2 0.6 4.8 3.2 -4.7 -3.3 -4.8 ·2.6 -4.2 hda.piunti-buritica 3.5 0.0 19 0.7 0.0 0.0 3.2 0.3 -3.2 ·1.3 -25 -3.2 -3.2 sta.isabel-sta.isabel 5.8 1.5 4.0 0.8 1.7 3.1 4.1 1.7 -2.6 -0.1 -3.3 -2.4 -1.0 apto.olaya herrera-medellin 6.0 0.1 1.5 0.0 3.2 1.2 4.2 1.8 -4.1 ·2.7 -4.2 -1.0 -3.0 tullo ospin"'-bello 6.4 0.0 1.2 2.0 0.7 4.5 1.9 -4.5 -3.3 -2.5 -3.8 aragon-santa rosa de os os 6.5 0.0 0.1 0.0 2.5 0.1 3.9 2.6 -3.9 -3.8 -3.9 -1.4 -3.8 guayabito-santo domingo 7.0 1.0 2.8 0.0 3.8 0.8 4.7 2.3 -3.7 -1.9 ·4.7 -0.9 -3.9 el vivero-guarne 4.2 0.0 0.0 0.0 2.0 0.3 2.3 1.9 -2.3 -2.3 -2.3 -0.3 -2.0 la salada-caldas 6.0 0.0 0.7 0.0 5.6 0.9 3.8 2.2 -3.8 -31 -3.8 18 -2.9 anori-anori 3.6 0.4 4.9 0.5 0.8 2.3 3.6 0.0 -32 1.3 3.1 -28 -1.3 arauca sara vena-sara vena 4.2 2.3 3.5 3.1 3.8 0.0 4.7 -0.5 -2.4 -1.2 -1.6 -0.9 -4.7 apto.arauca-arauca 9.2 9.8 10.7 4.1 7.8 0.0 6.8 2.4 3.0 3.9 -2.7 1.0 -6.8 atlantico repelon-repelon 9.6 8.8 2.2 5.6 4.5 7.0 6.8 2.8 2.0 -4.6 -1.2 -2.3 0.2 normal mana ti-mana ti 9.4 8.3 2.3 4.0 3.6 5.0 6.6 2.8 1.7 -4.3 -2.6 ·3.0 -1.6 el limon-mana ti 9.2 7.8 20 4.0 5.1 6.6 6.6 2.6 1.2 -46 -2.6 -1.5 0.0 gja.sta.lucia-santa lucia 8.2 7.0 2.6 4.8 3.5 8.0 6.1 2.1 0.9 -3.5 -1.3 -2.6 1.9 apto.e.cortissoz-soledad 10.6 9.3 5.5 5.8 70 9.2 7.2 3.4 2.1 ·1.7 -1.4 -0.2 2.0 base naval-barranquilla 9.0 2.9 4.4 6.2 8.2 6.6 2.4 -3.7 -2.2 -0.4 1.6 las florez-barranquilla 9.5 9.2 4.1 5.4 7.1 7.8 6.9 2.6 2.3 -2.8 -1.5 0.2 0.9 bolivar galerazamba-santa catalina 10.3 7.4 0.8 3.3 7.6 9.8 7.4 2.9 0.0·6.6 -4.1 0.2 2.4 apto.rafael nunez-cartagena 10.6 9.8 1.7 4.3 8.0 10.4 7.7 2.9 2.1 -6.0 -3.4 0.3 2.7 escuela naval-cartagena 10.1 8.7 2.4 6.7 10.3 7.5 2.6 1.2 -5.1 -0.8 2.8 islas del rosario-cartagena 10.2 6.1 1.5 4.0 3.7 9.5 7.2 3.0 -1.1 -5.7 -3.2 ·3.5 2.3 sta.rosa de simiti-simiti 6.7 4.8 3.0 2.1 1.2 5.0 5.4 1.3 -06 -2.4 -3.3 -4.2 -0.4 canelos-simiti 6.1 3.3 2.7 0.4 15 4.2 19 -0.9 -1.5 -38 -2.7 carmen de bolivar-c. de b. 8.2 6.3 2.7 4.6 6.7 9.1 6.3 1.9 0.0 -3.6 ·1.7 0.4 2.8 presa arroyo grande-m.la baja 8.6 3.7 1.9 4.8 7.5 89 6.1 2.5 -2.4 -4.2 -1.3 1.4 2.8 nueva florida-maria la baja 9.3 5.4 2.3 6.8 7.8 6.7 6.5 2.8 -1.1 -4.2 0.3 1.3 0.2 san pablo-maria la baja 9.7 5.9 2.1 6.7 4.4 5.9 6.3 3.4 -0.4 -4.2 0.4 ·1.9 -0.4 boyaca esclusa tolon-chiquinquira 8.5 6.7 1.8 0.0 4.5 1.2 3.3 5.2 3.4 -1.5 ·3.3 1.2 ·2.1 villa carmen-samaca 7.5 1.8 0.2 3.5 6.9 5.3 2.2 -3.5 -5.1 -1.8 1.6 gachaneca-samaca 9.8 4.8 1.7 1.1 3.2 3.8 4.4 5.4 0.4 -2.7 -3.3 -1.2 -0.6 villa de leyva-villa de leyva 6.6 6.2 2.2 0.0 3.0 2.5 4.1 2.5 2.1 -1.9 -4.1 -1.1 -1.6 la sierra-duitama 4.4 1.8 0.0 1.0 5.4 3.8 0.6 -2.0 -3.8 -2.8 1.6 surbata bonza-duitama 5.9 7.6 3.3 0.0 0.0 0.0 4.8 1.1 2.8 -1.5 -4.8 -4.8 -4.8 univ.pedag.tec de col-tunja 8.3 6.8 2.0 1.1 3.9 3.6 4.8 3.5 2.0 -2.8 -3.7 -0.9 -1.2 belencito-nobsa 8.0 8.1 0.0 2.8 2.0 4.2 3.8 3.9 -4.2 -1.4 -2.2 tunguavita·paipa 5.8 6.0 2.6 0.6 3.5 0.8 4.6 1.2 1.4 -2.0 -4.0 -1.1 -3.8 san rafael-tibasosa .5.9 6.8 2.2 0.0 3.3 2.5 4.1 1.8 2.7 -1.9 -4.1 -08 -1.6 sierra nevada cocuy-guican 0.6 1.1 32 -2.6 -2.1 chiscas-chiscas 6.3 6.8 3.9 0.2 3.1 3.8 4.9 1.4 1.9 -1.0 -4.7 -1.8 -1.1 boavita-boavita 6.6 6.2 4.5 0.2 5.7 2.3 5.1 1.5 1.1 -0.6 -4.9 0.6 -2.8 aptoalleras.c.-sogamoso 10.2 8.6 2.5 0.0 0.0 1.6 5.0 5.2 3.6 -2.5 -5.0 -5.0 -3.4 nuevo colon-nuevo colon 9.3 5.7 3.2 0.4 2.7 4.0 5.1 4.2 0.6 -1.9 -4.7 -2.4 -1.1 sutatenza-sutatenza 6.3 3.7 5.9 0.0 2.3 0.9 4.7 1.6 -1.0 1.2 -4.7 -2.4 -3.8 rondon-rondon 6.2 4.0 4.4 0.0 2.2 0.4 4.2 2.0 ·0.2 0.2 -4.2 -2.0 -3.8 el tunel-cuitiva 9.5 7.9 2.5 3.9 4.6 0.9 5.4 4.1 2.5 -2.9 ·1.5 -0.8 -4.5 caldas samana-samana 2.5 0.1 0.8 0.1 0.3 0.3 2.2 0.3 -2.1 -1.4 -2.1 -1.9 ·1.9 santa helena-marquetalia 6.0 2.4 0.1 0.8 1.7 2.1 3.7 2.3 -1.3 -3.6 -3.1 -2.0 -1.6 naranjal-chinchina 7.2 4.0 5.2 0.0 4.0 3.0 5.0 2.2 ·1.0 0.2 -5.0 -1.0 ·2.0 cenicafe-chinchina 7.5 4.5 4.9 5.3 2.2 -0.8 -0.4 fac.agronomia-manizales 6.1 3.9 3.0 0.0 3.5 23 43 1.8 -0.4 -1.3 -4.3 -0.8 -2.0 santagueda f.-palestina 6.9 4.8 5.5 0.0 4.8 5.2 5.8 1.1 -1.0 -0.3 -5.8 -1.0 ·0.6 apto.la nubia-manizales 4.8 4.4 1.4 0.0 2.3 1.0 4.7 0.1 -0.3 -3.3 -4.7 -2.4 -3.7 granja luker-palestina 6.7 4.4 5.3 0.0 4.0 4.6 5.0 1.7 -0.6 0.3 ·5.0 -1.0 -0.4 -------------------------------------------continua geofisica colombiana n° 2, abril de 1993 41 tabla 1 {conllnuacl6nl' insolacl6n (brlllo solar en horas y declmo~ reglslrada en eslaclones meleorol6glcas ublcadas en colombia; an es, en y despues de la erupcl6n del volc'n evado del rulz del 13 de novlembre lie 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14·m 15-m 16-m las brisas-villamaria 1,5 0.7 2.2 8.2 4,2 2,7 ·3,5 ·2.0 4.0 rafael escobar-supia 7.4 1.0 4.0 0,0 4.2 2.4 5.0 2.4 -4.0 ·1.0 ·5.0 -a.8 ·2.6 san fellx·salamina 4,6 3.0 0.6 0.0 0.3 0.4 3.6 1.0 -a.6 ·3.0 -3.6 -3.3 ·3.2 caqueta apto.g.artunduaga·florencia 6.4 7.6 3.3 0.0 4.4 2.1 5.7 0.7 1.9 -2.4 -5.7 ·1.3 ·3.6 macagual-florencia 5,8 7.5 3.5 0.0 5.5 0,8 5.9 -0.1 1.6 -2.4 -5.9 ·0.4 -5.1 valparaiso-florencia 8,6 2.5 0,0 3.6 0.1 6.5 2.1 -4.0 -6.5 ·2.9 -6.4 la mono-belen de los andaquies 6.4 8.3 4.1 4.5 0.1 5.8 0.6 2.5 -1.7 ·1.3 -5.7 san vicente del caguan= 4,9 10.6 3.2 0,0 1.9 3,9 5.9 -1.0 4.7 -2.7 ·5.9 -4.0 -2.0 maguare-doncello 6.3 4,0 0.0 1.9 07 5.4 0,9 .1.4 -5.4 -3.5 -4.7 puerto rico-puerto rico 5.8 7,0 1,5 0.0 5.9 0.0 4.9 0.9 2.1 -3.4 -4.9 1.0 -4.9 casanare aguazul-aguazul 9.2 5,6 3,8 0,7 3.9 2.7 5,6 3.6 0.0 ·1.8 -4.9 ·1.7 -2.9 apto.yopal-yopal 10.2 5.2 5,7 1.4 3.5 1.5 5.2 5.0 0.0 0.5 ·3.8 ·1.7 -3.7 modulos-orocue 8,1 4.9 9,5 0,3 4.0 0.0 6.4 1.7 -1.5 3.1 ·6.4 ·2.4 -6.4 cauca santa leticia-purace 7.9 6.4 0.4 0.9 0.0 3.9 4.0 2.5 -3.5 -3.0 ·3.9 paletara-coconuco 8,3 2.9 1.5 0.0 3.3 4,1 4.0 4,3 ·1.1 -2.5 -4.0 -0.7 0.1 gabriel lopez-totoro 5,0 2,5 0.0 0,0 2.0 3,1 3.6 1.4 ·1.1 ·3.6 -3.6 -1.6 ·0.5 venta de cajibio-cajibio 29 1,9 0.0 0.1 2.1 3,1 3.3 ·0.4 -1.4 -3,3 -3.2 -1.2 ·0.2 apto.machangara-popayan 4.2 2.1 0.8 0.0 4.2 1.1 4.5 -03 -2.4 -3.7 -4.5 -0.3 -3.4 la florida-popayan 5.3 3,4 0.9 0.0 2.1 3.5 4.8 0,5 ·1.4 -3.9 -4.8 -2.7 -1.3 manuel mejia·el tambo 8,1 5.4 3.4 0.9 5,6 5.5 5.6 25 -0.2 ·2.2 -4.7 0.0 ·0.1 ing.cauca-miranda 7,0 3.6 0.8 0.0 3.8 3.9 5.0 2,0 ·1.4 -4.2 -5.0 -1.2 -1.1 miranda-miranda 4,7 2.4 0.3 0.0 2.7 1.9 4.4 0,3 -2.0 -4.1 -4.4 -1.7 -2.5 valencia-san sebastian 46 2,1 0.2 0.0 0.8 2,2 3.6 1.0 -1.5 -3.4 ·3.6 -2.8 -1.4 la fonda-el bordo 6.8 6,8 1.2 0.2 5.3 5.6 6.3 0.5 0.5 -5,1 ·6.1 ·1.0 -0.7 bolivar-bolivar 5.6 0,9 1.7 0.1 1.7 2.3 5.0 0.6 -4.1 -3.3 -4.9 -3.3 ·2.7 los milagros-bolivar 5,2 4.2 0.1 0,0 2.7 6.6 5,1 0,1 -0.9 -5.0 ·5.1 ·2.4 1.5 mercaderes-mercaderes 7.3 4.5 2.1 0.0 6.1 4.4 5.9 1.4 -1.4 -3.8 ·5.9 0.2 -1.5 la sierra-la sierra 3,0 0.8 0.0 1.5 1.1 4.3 -1.3 ·3.5 -4.3 -2.8 -3.2 bonanza·guapi 2,9 1,1 2.3 0.0 2,3 2,9 0.0 -1.8 ·0.6 -2.9 ·0.6 cesar la mata-la gloria 8,0 3,9 6.6 1.8 1.5 1,8 4.1 3.9 -02 2.5 -2.3 -2.6 -2.3 chiriguana-chiriguana 8,5 9.0 3.5 9,2 7.8 0.7 1.2 -4,3 1.4 la gloria-la gloria 93 5.6 8.7 5.0 2.8 7,0 6.6 2.7 ·1.0 2.1 -1.6 -3.8 0.4 s,j. de oriente-robles-la paz 7,3 8.0 5.4 6.7 6.3 6.1 5.9 1.4 2.1 ·0.5 0.8 0.4 0.2 motilonia codazzi-codazzi 9,3 8.3 6.7 5.7 5,3 9.6 6,7 2.6 1.6 0.0 ·1.0 -1.4 2.9 socomba-becerril 8.7 8,2 5.0 7.1 6.0 2.7 2.2 -1.0 1.1 villarosa·valledupar 7.8 7,9 5.6 5.4 5.9 8.3 6.5 1.3 1.4 -0.9 -1,1 ·0.6 1.8 el callao-valledupar 9.5 8,6 7.4 6.0 6,2 8.8 7.5 2.0 1.1 -0.1 ·1.5 ·1.3 1.3 guaymaral·valledupar 100 6.3 5.8 3.6 9.2 7.5 2.5 -1.2 ·1.7 ·3.9 1.7 pueblo bello-pueblo bello 8.1 7.3 4.1 3.7 6,2 6.1 6.1 2.0 1.2 -2.0 ·2.4 0.1 0.0 choco la mansa-carmen de atrato 6,5 0,0 0.6 0.0 1.6 1.1 3.1 3.4 -3,1 ·2.5 -3.1 -1.5 -2.0 apto.el carano-quibdo 1.4 0,0 0.8 0.0 1.1 1.2 3.2 -1.8 -3.2 ·2.4 -3.2 -2.1 ·2.0 la teresita·riosucio 3,0 0.0 0.0 2.4 0.0 1.4 3.3 ·0.3 -3,3 -3.3 -0.9 -3.3 -1.9 san j. del palmar-s,j,del pal 2,2 0.0 0.0 0.0 0.3 0.3 1.6 0.6 -1,6 ·1,6 ·1.6 -1.3 -1.3 apto.condoto·condoto 0,0 0.0 0.4 0.0 2.1 1.9 2,9 -2.9 -2.9 -2.5 ·2.9 -0.8 -1.0 noanama·istmina 0.4 0.0 0.0 0.0 0.0 1.2 2,1 -1.7 ·2.1 -2.1 -2.1 ·2.1 -0.9 panamericana-bahia solano 0.1 0.0 2.8 0.7 0.0 2,3 ·2.2 -2.3 0.5 -1.6 -2.3 cordoba puerto nuevo-tierralta 6,5 0.3 6.2 4.2 3.2 4.9 1.6 -4,6 1.3 ·0.7 -1.7 turipana-cerete 8.1 3.0 5.2 1.8 7.2 8,5 6.0 2.1 -3.0 -0.8 -4,2 1.2 2,5 univ.de cordoba-monteria 8.3 3.2 7.1 2.9 7.3 6.5 6.7 1.6 -3.5 0.4 -3.8 0.6 ·0.2 la doctrina·lorica 9,5 3.0 4.3 3.0 9.4 7.7 6.2 3.3 ·3.2 -1.9 ·3.2 3.2 1.5 ayapel-ayapel 8.9 2.0 4.7 1.9 5.4 4,7 5.4 3.5 -3.4 ·0,7 -3.5 0.0 -0.7 planeta rica-planeta rica 8.7 4.2 2.0 4.2 7.2 5.5 3.2 -1.3 -3.5 -1.3 1.7 cundinamarca alberto williamsontibacuy 7.7 3,8 2.7 1.8 1.7 2.4 4.1 3.6 -0.3 ·1.4 -2.3 ·2.4 -1.7 pandi-pandi 8.6 2.8 4,2 0.0 1.6 2,5 5.2 3.4 ·2.4 -1,0 ·5.2 -3.6 ·2.7 pasca·pasca 3.6 6.1 1,8 3.7 0.5 1.2 3.4 0.2 2.7 ·1,6 0.3 ·2.9 -2.2 tibaitata·mosquera 5.7 3.8 4.5 0.0 2.4 1.1 4.6 1.1 ·0.8 -0.1 -4.6 -2.2 ·3.5 las mercedes·anapoima 5.8 4.1 3.5 0.0 4.6 2.7 4,9 0.9 -0.8 -1.4 -4.9 ·0.3 -2.2 granja san jorge-soacha 5.8 1.6 2.2 0.0 0,5 1.2 3.5 2.3 -1.9 ·1.3 -3.5 ·3,0 -2.3 ~ilos-choconta 8.3 52 3.0 0.0 3.8 5.2 5.0 3.3 0.2 -2.0 ·5,0 -1.2 0.2 base aerea madrid-madrid 6.3 2,2 4.8 0.0 2,7 1.3 4.1 2.2 .1.9 0.7 -4.1 -1.4 ·2,8 misiones-mesitas del colegio 7.7 1.3 2.1 0.0 0.9 0.3 3.4 3.3 ·2.1 -2.3 -3.4 ·2.5 -3.1 mesitas de sta.ines·cachipay 6.0 1,7 1.7 0.1 3,8 1.5 4.2 1.8 ·2.5 -2.5 -4.1 ·0.4 -2.7 apto.palanquero-puerto salgar4.7 2.6 3,5 0.0 2.1 4.1 5.1 ·0.4 ·2.5 -1.6 ·5.1 -3.0 -1.0 yacopiyacopi 5.4 4.6 4.3 0.0 1.2 0.5 4.8 0.6 ·0.2 -0,5 -4.8 -3.6 -4.3 montelibano-yacopi 6.3 3.4 4.6 0.0 1.2 0.2 4.4 1.9 -1.0 0.2 -4.4 -3.2 -4.2 san cayetano-san cayetano 6,8 4.3 1.4 0.4 0.4 0.1 3.3 3.5 1.0 ·1.9 -2,9 ·2.9 -3.2 isla del santuario-fuquene 7.1 6.8 4.1 0.3 7.0 3.4 5.5 1.6 1.3 -1.4 -5.2 1.5 -2,1 el japon-medina 3.7 1.8 5.7 0.2 2.4 0.9 3.8 ·0,1 ·2.0 1.9 -3.6 ·1.4 ·2.9 gacheta-gacheta 4.8 2.1 4.4 0.2 3.6 3.3 4.5 0,3 -2.4 ·0.1 -4,3 -0.9 -1.2 guajira 6.8 0.8 0.7 -4.4 0.2ma titas-riohacha 7.6 7.5 2.4 7.0 6.4 -0.4 la paulina-fonseca 9.2 9.5 8.9 8.4 8,8 9,5 8.0 1.2 1.5 0.9 0.4 0.8 1.5 manaljre-manaure 10.2 8.6 7,2 4.6 8,2 8.4 7.6 2.6 1.0 -0,4 -3.0 0.6 0.8 apto.maicao-maicao 9.8 9,2 6.0 6.7 4,7 8,2 6.8 3.0 2.4 -0.8 -0.1 -2.1 1.4 nazareth-uribia 8.0 6,9 6.5 5,9 3,6 2,8 5.7 2.3 1.2 0.8 0.2 -2.1 -2.9 esc.agr.carraipia·maicao 7.0 7.8 4.8 5.6 5.4 8.2 6.0 1.0 1.8 ·1.2 ·0.4 -0.6 2.2 urumita·urumita 7,3 6.4 5.7 6.0 5.5 6.6 0.7 -0.2 -0.9 ·0.6 .1.1 guaviare 5.0 -5.0 0.8 -0.6 -4.7 -4,6 san j. de guaviare-sn j.de g, 0.0 5.8 4.4 0.3 0.4 0.5 -4.5 continua 42 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n tabla 1 (contlnuac'6nl' inaolacl6n (brilio solar en horas r declmo~ reglatrada en eataclonea meteorol6glcaa ublcadas en colombia; an aa, an y deapuea da la arupcl6n dal volc'n avado del rulz del 13 da novlambra da 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m huila sevilla-pitalito 6.7 7.6 3.5 0.2 4.3 0.6 4.4 2.3 3.2 ·0.9 -4.2 ·0.1 ·3.8 parque arq.·san agustin 7.7 7.2 3.1 0.4 5.1 0.1 5.1 2.6 2.1 ·2.0 -4.7 0.0 ·5.0 altamira-altamira 8.8 6.9 4.8 0.0 5.3 1.6 5.5 3.3 1.4 -0.7 ·5.5 ·0.2 ·3.9 resina-guadalupe 3.2 5.7 0.8 0.2 7.0 1.0 3.6 ·0.4 2.1 ·2.8 ·3.4 3.4 ·2.6 cueva los guacharos-acevedo 3.9 7.1 3.8 0.0 2.8 0.5 3.3 0.6 3.8 0.5 ·3.3 ·0.5 ·2.8 esc.agr.la plata-la plata 8.2 8.2 3.5 0.0 4.2 2.4 4.8 3.4 3.4 ·1.3 -4.8 ·0.6 ·2.4 jorge villamil-gigante 4.3 5.9 2.4 0.0 3.1 0.7 4.0 0.3 1.9 ·1.6 -4.0 ·0.9 ·3.3 iquira-iquira 6.4 7.3 2.1 0.1 4.4 2.2 4.7 1.7 2.6 ·2.6 -4.6 ·0.3 ·2.5 el juncal·palermo 7.0 10.0 4.5 0.0 6.4 3.0 6.3 0.7 3.7 ·1.8 ·6.3 0.1 ·3.3 algeciras·algeciras 7.5 7.3 2.7 0.1 3.3 2.8 4.6 2.9 2.7 ·1.9 -4.5 ·1.3 ·1.8 apto.benito salas-neiva 8.3 10.2 3.4 0.0 6.2 1.9 5.5 2.8 4.7 ·2.1 • ·5.5 0.7 ·3.6 palacio vegalarga·neiva 7.1 9.2 0.0 0.0 2.1 1.7 4.1 3.0 5.1 -4.1 -4.1 ·2.0 ·2.4 villaranza-neiva 7.8 9.3 4.3 0.0 4.8 3.3 5.4 2.4 3.9 ·1.1 ·5.4 ·0.6 ·2.1 santa maria-santa maria 7.3 6.1 1.2 0.0 1.7 1.1 3.9 3.4 2.2 ·2.7 ·3.9 ·2.2 ·2.8 san alfonso-villavieja 8.7 6.0 0.5 0.0 6.9 0.4 4.6 4.1 1.4 -4.1 -4.6 2.3 -4.2 la legiosa-colombia 7.3 8.0 4.8 0.0 6.2 1.4 4.7 2.6 3.3 0.1 -4.7 1.5 ·3.3 magdalena la ye-cienaga 10.5 9.3 3.4 5.5 6.2 9.5 7.2 3.3 2.1 ·3.8 ·1.7 ·1.0 2.3 apto.simon bolivar-sta marta 10.5 10.1 4.9 6.0 7.8 9.6 7.9 2.6 2.2 ·3.0 ·1.9 ·0.1 1.7 san lorenzo-santa marta 4.9 2.1 0.6 0.3 0.6 1.0 2.0 2.9 0.1 ·1.4 ·1.7 ·1.0 ·1.0 parque tayrona·santa marta 8.0 6.9 5.3 5.5 6.9 6.8 6.3 1.7 0.6 ·1.0 ·0.8 0.6 0.5 apto.las flores-el banco 9.7 7.6 7.0 6.2 1.4 7.1 6.2 3.5 1.4 0.8 0.0 -4.8 0.9 los alamos-san sebastian 10.0 6.9 5.7 4.3 5.5 7.5 6.4 3.6 0.5 ·0.7 ·1.9 ·0.9 1.1 algarrobo-fundacion 7.8 8.2 3.1 3.9 6.2 7.5 6.0 1.8 2.2 ·29 ·2.1 0.2 1.5 zacapa-araca taca 9.3 8.6 3.3 5.6 3.7 9.3 6.2 3.1 2.4 ·2.9 ·0.6 ·2.5 3.1 patuca-cienaga 9.6 8.7 2.1 5.7 5.2 7.8 6.4 3.2 2.3 -4.3 ·0.7 ·1.2 1.4 prado sevilla-cienaga 9.7 9.1 3.7 6.4 5.2 6.7 6.6 3.1 2.5 ·2.9 ·0.2 ·1.4 0.1 meta la macarena·la macarena 0.8 6.9 0.9 0.0 1.8 1.1 4.6 ·3.8 2.3 ·3.7 ·4.6 ·2.8 ·3.5 vista hermosa-vista hermosa 4.8 5.3 0.8 0.0 2.6 2.7 4.4 0.4 0.9 ·3.6 -4.4 ·1.8 ·1.7 la holanda-granada 4.7 7.5 6.9 0.1 4.7 2.0 4.8 ·0.1 2.7 2.1 -4.7 ·0.1 ·2.8 mesetas·mesetas 3.9 8.2 3.5 0.0 5.8 2.0 3.9 0.0 4.3 ·0.4 ·3.9 1.9 ·1.9 carimagua-puerto gaitan 8.9 2.2 8.9 6.7 2.2 -4.5 2.2 san carlos de guaroa·s.c.de g. 5.7 7.0 8.1 1.0 1.8 1.5 5.3 0.4 1.7 2.8 -4.3 ·3.5 ·3.8 la l1bertad-villavicencio 6.2 6.4 6.2 0.8 4.0 0.9 4.5 1.7 1.9 1.7 ·3.7 ·0.5 ·3.6 pachaquiaro·puerto lopez 7.7 7.4 0.9 2.8 5.8 5.3 2.4 2.1 -4.4 ·2.5 0.5 univ.del llano-villavicencio 4.6 6.1 0.2 4.0 3.8 4.5 0.1 16 -4.3 ·0.5 ·0.7 hda.la cabana-cumaral 4.7 4.9 5.3 0.6 2.9 1.1 4.2 0.5 0.7 ·1.1 ·3.6 ·1.3 ·3.1 barranca de upia-cumaral 4.2 2.9 4.7 0.0 2.0 0.4 4.1 0.1 ·1.2 0.6 -4.1 ·2.1 ·3.7 narind monopamba-puerres 2.1 2.8 2.0 0.7 3.6 2.1 4.3 ·2.2 ·1.5 ·2.3 ·3.6 ·0.7 ·2.2 el encano-pasto 3.9 1.1 0.6 0.0 1.4 2.5 4.3 ·0.4 ·3.2 ·3.6 -4.3 ·3.9 ·1.8 gja.el mira·tumaco 0.2 0.4 0.4 3.5 0.1 0.5 2.4 ·2.2 ·2.0 ·2.0 1.1 ·2.3 ·1.9 ricaurte·ricaurte 0.6 1.1 0.0 0.0 3.2 1.1 3.3 ·2.7 ·2.2 ·3.3 ·3.3 ·0.1 ·2.2 apto.la florida-tumaco 1.3 0.0 0.1 4.9 4.9 2.2 4.1 ·2.8 -4.1 -4.0 0.8 0.8 ·1.9 la union·la union 3.6 2.4 1.3 0.0 1.0 5.3 5.6 ·2.0 ·3.2 -4.3 ·5.6 ·4.6 ·0.3 obonuco-pasto 4.5 3.4 0.6 0.0 1.3 2.6 4.3 0.2 ·0.9 ·3.7 -4.3 ·3.0 ·1.7 apto.antonio narino-pasto 5.5 3.3 0.9 0.0 1.8 1.3 5.6 ·0.1 ·2.3 -4.7 ·5.6 ·3.8 -4.3 apto.san luis·ipiales 5.7 2.1 0.2 0.6 2.5 5.8 5.2 0.5 ·3.1 ·5.0 ·4.6 ·2.7 0.6 el paraiso·tuquerres 6.5 1.3 0.4 0.0 1.7 6.2 5.0 1.5 ·3.7 -4.6 ·5.0 ·3.3 1.2 bombona·consaca 5.7 3.0 0.4 0.0 4.4 5.9 ·0.2 ·2.9 ·5.5 ·5.9 ·1.5 botana·pasto 6.6 3.9 0.0 0.0 1.1 6.6 5.3 1.3 ·1.4 ·5.3 ·5.3 -4.2 1.3 ospina perez-consaca 7.1 4.0 1.6 0.0 1.6 3.6 5.8 1.3 ·1.8 -4.2 ·5.8 -4.2 ·2.2 barbacoas-barbacoas 0.6 0.1 0.3 1.0 0.5 1.9 3.1 ·2.5 ·3.0 ·2.8 ·2.1 ·2.6 ·1.2 norte de santander apto.camilo daza-cucuta 5.0 2.2 5.3 3.6 4.2 5.1 4.7 0.3 ·2.5 0.6 ·1.1 ·0.5 0.4 iser-pamplona 5.5 6.3 5.0 1.6 2.9 1.7 3.3 2.2 3.0 1.7 ·1.7 ·0.4 -1.6 santa isabel-cucuta 2.3 4.7 0.8 2.7 1.2 3.1 3.8 ·1.5 0.9 ·30 ·1.1 ·2.6 ·0.7 blonay-chinacota 3.4 2.6 4.0 1.5 2.7 1.0 2.4 1.0 0.2 1.6 ·0.9 0.3 ·1.4 carmen de tonchala·cucuta 1.1 1.8 4.1 1.7 1.7 2.9 3.5 ·2.4 ·1.7 0.6 ·1.8 ·1.8 ·0.6 risaralda-zulia 0.9 0.2 0.7 0.8 0.0 0.8 0.1 -o.e ·0.1 0.0 ·0.8 cinera villa olga-cucuta 10.1 4.0 8.4 5.4 0.6 0.0 3.6 6.5 0.4 4.8 1.8 ·3.0 ·3.6 francisco romero-salazar 2.1 1.0 0.7 1.2 1.6 4.6 2.8 ·0.7 ·1.8 ·2.1 ·1.6 ·1.2 1.8 tlbu-tibu 3.7 2.8 1.8 3.1 0.6 ·0.3 ·1.3 apto.aguas claras-ocana 2.7 3.4 5.1 2.8 1.9 3.4 4.5 ·1.8 ·1.1 0.6 ·1.7 ·2.6 .1.1 centro adm.abrego·abrego 7.3 5.0 2.2 3.8 2.0 3.0 4.6 2.7 0.4 ·2.4 ·0.8 ·2.6 ·1.6 la playa-la playa 0.0 3.6 1.9 0.0 0.0 1.6 3.6 ·3.6 0.0 ·1.7 ·3.6 ·3.6 ·2.0 esc.agrcachira·cachira 6.8 4.9 3.5 0.5 1.9 4.5 4.0 2.8 0.9 ·0.5 ·3.5 ·2.1 0.5 silos-silos 7,.6 5.3 3.3 0.3 2.5 1.5 3.0 4.6 2.3 0.3 ·2.7 ·0.5 ·1.5 putumayd villa garzon·villa amazonica 5.3 5.5 4.2 0.0 3.3 0.5 5.1 0.2 0.4 ·0.9 ·5.1 ·1.8 -4.6 acueducto mocoa-mccoa 6.4 5.7 3.7 2.5 0.0 5.0 1.4 0.7 ·1.3 ·2.5 ·5.0 sibundoy-sibundoy 7.2 2.5 2.6 0.0 2.7 0.2 4.6 2.6 ·2.1 ·2.0 ·4.6 ·1.9 -4.4 michoacan-sibundoy 3.8 2.3 0.3 0.0 1.3 0.0 3.7 0.1 ·1.4 ·3.4 ·3.7 ·2.4 ·3.7 pto leguizamq.pto leguizamo 1.1 5.9 0.4 0.0 5.8 -4.7 0.1 ·5.4 -5.8 quindid maracay-quimbaya 5.6 3.8 2.2 0.0 3.8 3.0 4.8 0.8 ·1.0 ·2.6 ·4.8 ·1.0 ·1.8 la bella·calarca 4.8 2.0 1.6 0.0 2.1 2.9 4.0 0.8 ·2.0 ·2.4 ·4.0 ·1.9 ·1.1 paraguaycito-buenavista 6.9 3.3 3.8 0.0 3.8 3.7 5.0 1.9 ·1.7 ·1.2 ·5.0 ·1.2 ·1.3 el sena-armenia 2.5 1.4 1.7 0.0 1.2 0.5 3.4 ·0.9 ·2.0 ·1.7 ·3.4 ·2.2 ·2.9 risaralda la joya-pereira 5.2 4.7 4.0 0.0 5.0 6.0 5.2 0.0 ·0.5 ·1.2 ·5.2 ·0.2 0.8 apto.ma tecana-pereira 5.2 3.5 5.5 0.0 5.5 3.6 5.0 0.2 ·1.5 0.5 ·5.0 0.5 ·1.4 el jazmin-santa rosa 4.9 2.9 1.9 0.0 1.7 3.3 3.7 1.2 ·0.8 ·1.8 ·3.7 ·2.0 ·0.4 el cedral-pereira 2.4 2.0 2.5 0.0 1.2 1.0 2.5 ·0.1 ·0.5 0.0 ·2.5 ·1.3 ·1.5 la bohemia-pereira 6.4 5.0 3.3 0.0 4.7 5.2 5.6 0.8 ·0.6 ·2.3 ·5.6 ·0.9 ·0.4 continua geofislca colomblana n° 2, abril de 1993 43 tabla 1 (continuacl6nt. insolacl6n (brlllo solar en horal y diclmo~ reglltrada en eltaclonel meteorol6glcal ublcadal en colombia; an el, en y delpuil de la erupcl6n del volcen evado del rulz del 13 de novlembre de 1985 estacion valores diarios med diferencias nombre-municipio 11 12 13 14 15 16 mes 11-m 12-m 13-m 14-m 15-m 16-m veracruz-sta.rosa de cabal 3.4 3.4 3.7 0.0 1.3 2.8 3.7 -0.3 -0.3 0.0 -3.7 -2.4 ·0.9 la camella-santuario 4.4 0.9 0.4 1.7 7.3 2.9 4.0 0.4 ·3.1 ·3.6 -2.3 3.3 -1.1 pueblo rico-pueblo rico 3.8 0.0 0.6 0.0 0.4 1.0 3.2 0.6 ·3.2 ·2.6 -3.2 -2.8 ·2.2 apto.sesquicentenar-s.andres san andres y prdvidencia 9.3 8.2 3.7 7.7 9.6 10.1 5.6 3.7 2.6 -1.9 2.1 4.0 4.5 apto.providencia-providencia 9.8 5.7 6.7 7.6 8.8 9.5 6.0 3.8 -0.3 0.7 1.6 3.5 3.5 santafe de bogota d.c eldorado didactica 5.7 3.5 4.3 0.0 2.9 0.7 4.3 1.4 ·0.8 0.0 -4.3 -1.4 ·3.6 vivero venado de oro 4.5 1.9 0.0 0.0 5.3 2.8 1.7 -0.9 -2.8 ·2.8 2.5 apto.eldorado pista 1-2 5.5 4.1 4.6 0.0 3.9 0.4 4.8 0.7 -0.7 -0.2 -4.8 -0.9 -4.4 santander campo capote-velez 5.7 2.4 4.0 0.1 6.2 3.0 4.4 1.3 -2.0 ·0.4 -4.3 1.8 ·1.4 albania-albania 5.4 1.2 0.3 0.0 2.2 0.1 3.4 2.0 ·2.2 -3.1 ·3.4 -1.2 ·3.3 cimitarra-cimitarra 7.9 2.1 5.3 0.4 7.2 2.5 5.8 2.1 ·3.7 -0.5 -5.4 1.4 ·3.3 apto.yariguies-barrancabermej 6.5 3.1 3.6 0.3 8.6 6.8 5.9 0.6 -2.8 -2.3 -5.6 3.0 0.9 villa leiva-sabana de torres 7.3 5.6 4.2 0.0 7.0 3.2 4.8 2.5 0.8 -0.6 -4.8 2.2 -1.6 univ.ind.sant.-bucaramanga 2.7 2.4 1.0 0.0 2.7 6.0 2.7 0.0 -0.3 -1.7 -2.7 0.0 3.3 apto.palonegro-lebrija 4.4 3.2 2.3 0.2 2.5 4.2 3.3 1.1 -0.1 -1.0 ·2.1 -0.8 0.9 provincia-sabana de torres 7.3 6.3 2.1 0.0 2.9 8.7 5.4 1.9 0.9 -3.3 ·5.4 -2.5 3.3 cachiri-surata 4.6 4.9 3.6 0.7 1.7 3.2 3.5 1.1 1.4 0.1 ·2.8 .1.8 -0.3 granja velez-velez 5.8 2.6 3.7 0.0 4.0 4.5 4.7 1.1 ·2.1 ·1.0 -4.7 -0.7 ·0.2 el cucharo-pinchote 0.7 3.7 7.9 8.4 6.4 ·5.7 ·2.7 1.5 2.0 esc.agr.mogotes-mogotes 5.2 4.0 3.7 0.2 4.1 5.9 4.9 0.3 -0.9 ·1.2 -4.7 -0.8 1.0 charala-charala 7.0 6.0 5.2 1.3 4.8 5.4 5.3 1.7 0.7 -0.1 -4.0 ·0.5 0.1 capitanejo-capitanejo 7.5 7.3 6.0 1.0 7.0 7.9 6.2 1.3 1.1 -0.2 -5.2 0.8 1.7 granja t1naga-cerrito 7.1 7.0 4.5 1.2 0.6 3.8 4.1 3.0 2.9 0.4 ·2.9 -3.5 -0.3 zapatoca-zapatoca 5.9 2.5 2.3 1.1 1.7 0.0 4.0 1.9 -1.5 ·1.7 ·2.9 -2.3 -4.0 aguas blancas-san vicente 0.6 2.2 1.9 0.0 1.3 2.7 2.5 -1.9 -0.3 -0.6 -2.5 -1.2 0.2 berlin-tona 6.8 1.0 5.4 1.8 4.4 4.6 2.2 -3.6 0.8 ·2.8 ·0.2 sucre majagual-majagual 7.1 4.8 6.0 1.4 4.7 8.6 5.0 2.1 -0.2 1.0 ·3.6 -0.3 3.6 tolima anchique-natagaima 9.1 6.2 4.6 0.0 5.8 3.3 5.7 3.4 0.5 -1.1 ·5.7 0.1 -2.4 la montana-dolores 5.5 6.8 3.7 4.1 7.0 5.8 5.0 0.5 1.8 -1.3 ·0.9 2.0 0.8 hda.madron-dolores 6.3 6.8 3.3 0.0 5.3 3.4 4.7 1.6 2.1 -1.4 -4.7 0.6 -1.3 luis bustamante-vlllarica 2.7 2.9 1.1 0.0 0.0 0.0 2.0 0.7 0.9 -0.9 -2.0 -2.0 -2.0 lozania-purificacion 8.7 6.6 6.2 0.0 4.2 3.0 5.3 3.4 1.3 0.9 -5.3 ·1.1 -2.3 nataima-espinal 10.7 5.4 3.0 0.0 6.9 0.7 5.3 5.4 0.1 -2.3 ·5.3 1.6 -4.6 guamo-guamo 10.0 6.8 3.4 0.0 6.7 2.3 5.5 4.5 1.3 -2.2 -5.5 1.2 -3.2 apto.santiago vila-flandes 9.8 5.5 5.4 0.0 5.6 4.2 5.8 4.0 -0.3 -0.4 ·5.8 -0.2 -1.6 chapeton-ibague 3.1 3.4 0.2 1.0 0.1 2.3 4.2 ·1.1 -0.8 -4.0 ·3.2 -4.1 -1.9 cajamarca·cajamarca 8.7 5.1 2.1 5.1 0.2 0.0 4.2 4.5 0.9 -2.1 -0.9 -4.0 -4.2 hato de opia perales-ibague 9.1 6.4 1.2 0.0 4.8 1.6 4.9 4.2 1.5 -3.7 -4.9 ·0.1 -3.3 apto.perales-ibague 8.5 5.9 3.6 1.6 4.0 0.1 4.5 4.0 1.4 -0.9 -2.9 ·0.5 -4.4 el sal to-ambalema 8.5 4.2 6.0 0.0 33 3.3 5.4 3.1 ·1.2 0.6 -5.4 -2.1 ·2.1 villa hermosa-villa hermosa 6.2 2.1 2.5 0.0 2.6 0.6 3.3 2.9 -1.2 -0.8 ·3.3 -0.7 ·2.7 la trinidad-l1bano 7.8 2.8 0.6 0.0 1.1 1.4 3.1 4.7 -0.3 -2.5 ·3.1 -2.0 -1.7 la quinta-lerida 8.9 5.3 8.5 0.0 3.1 4.3 5.9 3.0 -0.6 2.6 -5.9 ·2.8 ·1.6 apto.planadas-planadas 8.5 4.5 2.8 0.0 1.8 0.9 3.8 4.7 0.7 ·1.0 -3.8 -2.0 ·2.9 gja.demostracion-chaparral 9.0 4.9 3.5 0.0 4.6 0.3 4.7 4.3 0.2 -1.2 -4.7 -0.1 -4.4 mesa de pole-ataco 6.2 4.1 4.3 0.0 2.2 0.1 3.7 2.5 0.4 0.6 ·3.7 -1.5 -3.6 quinta san antonio-s.antonio 8.2 2.7 1.2 0.0 3.8 0.6 4.5 3.7 -1.8 -3.3 -4.5 ·0.7 ·3.9 el limon-chaparral 6.9 5.3 1.6 0.0 3.1 0.8 3.6 3.3 1.7 -2.0 -3.6 ·0.5 -2.8 la esperanza-honda 6.9 4.2 3.5 0.0 1.4 3.9 5.3 1.6 -1.1 ·1.8 -5.3 -3.9 ·1.4 valle del cauca potrerito-jamundi 7.8 4.5 0.8 0.0 6.7 3.8 5.2 2.6 -0.7 -4.4 -5.2 1.5 ·1.4 univ.del valle-cali 8.2 2.7 1.2 0.0 4.9 4.2 5.0 3.2 -2.3 ·3.8 -5.0 ·0.1 -0.8 palmira ica-palmira 8.8 3.4 2.4 0.0 3.8 4.9 5.1 3.7 -1.7 ·2.7 -5.1 ·1.3 -0.2 aptoabonllla a.-palmira 9.4 4.9 1.9 0.0 5.5 5.5 5.2 4.2 -0.3 ·3.3 -5.5 0.0 0.3 ~~~~a~'f~l'b~~g~laria 9.1 3.3 2.2 0.0 4.8 5.5 5.6 3.5 ·2.3 -3.4 ·5.6 -0.8 -0.1 7.0 3.5 1.4 0.0 3.6 4.7 5.1 1.9 ·1.6 -3.7 ·5.1 -1.5 -0.4 manuel mallarino-trujlllo 5.1 0.3 0.4 0.0 2.3 4.8 4.0 1.1 -3.7 ·3.6 -4.0 ·1.7 0.8 tenerife-cerrito 5.1 2.4 0.0 0.0 20 4.1 4.2 0.9 .1.8 -4.2 -4.2 ·2.2 ·0.1 el vinculo-buga 7.3 1.5 1.5 0.0 4.8 5.3 5.4 1.9 -3.9 ·3.9 -5.4 -0.6 -0.1 heraclio uribe-sevilla 2.9 3.3 1.2 0.0 0.3 2.1 3.1 -0.2 0.2 -1.9 ·3.1 ·2.8 ·1.0 tierrablanca-roldanlllo 7.2 2.3 0.1 0.0 4.7 6.7 4.9 2.3 -2.6 -4.8 -4.9 ·0.2 1.8 cent.adm.la union-la union 6.5 2.8 0.4 0.0 2.4 5.9 4.7 1.8 -1.9 -4.3 -4.7 ·2.3 1.2 arturo gomez-alcala 5.6 2.8 3.9 0.0 5.6 3.2 5.0 0.6 ·2.2 -1.1 ·5.0 0.6 ·1.8 cum barco-sevilla 6.1 1.9 0.0 0.0 1.3 2.6 3.5 2.6 ·1.6 -3.5 ·3.5 -2.2 -0.9 colpuertos-buenaventura 3.1 0.2 0.0 0.0 3.4 1.6 3.4 -0.3 ·3.2 ·3.4 ·3.4 0.0 -1.8 julio fernandez-restrepo 5.6 3.2 0.0 0.2 2.1 0.0 3.3 2.3 -0.1 ·3.3 ·3.1 ·1.2 ·3.3 argelia el recreo-argelia 3.9 1.5 0.5 0.0 1.5 1.9 3.3 0.6 ·1.8 -2.8 ·3.3 ·1.8 -1.4 subestacion alban-el cairo 3.6 0.2 0.8 0.0 1.4 2.4 3.5 0.1 -3.3 ·2.7 -3.5 ·2.1 ·1.1 la mision-buenaventura 0.0 0.0 0.0 0.0 0.3 1.6 1.7 -1.7 ·1.7 -1.7 ·1.7 -1.4 -0.1 vaupes apto.mitu-mitu 5.8 4.5 2.0 0.2 4.1 1.2 5.1 0.7 -0.6 ·3.1 -4.9 ·1.0 -3.9 vichada las gaviotas-san jose de ocune 6.3 1.6 7.7 0.9 4.0 0.2 5.5 0.8 -3.9 2.2 -4.6 -1.5 -5.3 apto.pto carreno-pto carreno 9.2 7.8 9.3 6.1 1.4 1.5 6.0 3.2 1.8 3.3 0.1 -4.6 -4.5 med mes = valor medio diario en noviembre de 1985 diferencias: ej: 11-m = valor del dfa 11 menos valor medio diario en noviembre de 1985 (med mes) 44 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n morall/ella 10.0 d.d • dia11 ~dla.l6 _ dla12 o dlale lit] via 1s • dia 1+ a ¥idia nov figura 2. insolaci6n (hora-deetme) registrada del 11-16 de nov-1985 y promedia en nov-1985, en 12 poblaciones ubicadas en areas cubiertas por el penacho de cenizas: chinchina-caldas (chin), palestina-caldas (pale), espinal-tolima (espi), ambalema-tolima (amba), pandi-cundinamarca (pan d), puerto salgar-cundinamarca (salg), nuevo col6n-boyaca (nuev), sutatenza-boyaca (suta), yopal-casanare (yopa), salazar-n.de santander (sala), sabana de torres-santander (saba) y saravena-arauca (sara). dla a dla, la situaci6n durante el perfodo 11 a 16 de noviembre-1985. puede sintetizarse asi: 11-nov-85. la dlstnbuci6n del numero de horas de brillo solar, en este dla, si bien no es igual al promedio para el mes de noviembre (fig.1), se considera muy cercana a 10que normalmente puede ocurrir en un dla cualquiera: los valores en algunas areas son superiores y en otras inferiores a los promedios. en la regi6n andina, llanos orientales y regi6n del caribe, la tendencia fue la de superar los promedios hasta en 2 horas (en algunas pequeiias areas hasta en 3 horas) y en la amazonia y algunas areas de los departamentos de choc6 y nariiio la tendencia muestra valores inferiores al promedio en 1 y. en algunos casos, hasta 2 horas. esas diferencias. analizadas porcentualmente (fig.3), muestran que en la mitad dei pals se presentaron valores que representan entre el 80% y el 120% respecto al promedio. 10 cual puede considerarse normal dadas las diferentes caracterlsticas fisiograficas. aumentos superiores al 20% ocurrieron en la regi6n del caribe, algunos sectores de los departamentos de arauca, casanare y una parte de la regi6n andina (en mayor % al sur de antioquia, boyaca, tolima y norte del huila); descensos superiores a120%se presentaron en la mayor parte de la amazonia, sw del departamento del choc6 y w de nariiio. 12-nov-85. en este dfa, la distribuci6n de la insolaci6n aparentemente coincide con las caracterrsticas generales que muestran los promedios (fig.1): una zona con valores relativamente bajos que cubre la regi6n del catatumbo, la regi6:l andina, la regi6n del pacifico y otra zona con valores altos que abarca la regi6n del caribe y la mayor parte de la orinoquia y amazonia. sin embargo. los valores de la primera zona son mas bajos y los de la segunda zona son mas altos que los promedios. se encuentran dos importantes situaciones, cada una de elias con areas crrticas especlficas (fig.4): a) un notorio aumento de 2 hasta mas de 3.5 horas en la mayor parte de huila, tolima, suroccidente del meta, la mayor parte de caqueta y el centro de boyaca; b) una extraordinaria. mas que notoria, disminucion de la insolaci6n diaria en toda la regi6n andina, norte de la regi6n del pacifico. suroccidente de la regi6n del caribe y algunas areaspequerias como leticia y ifmites entre meta y vichada. esta disminuci6n fue de tal magnitud que en casi toda el area cubierta por los departamentos de antioquia y choc6 la insolaci6n fue 0.0 horas (cero.cero), 0 muy cerca a cero, durante este dla. lo anterior, aparentemente, fue ocasionado por una reactivaci6n 0 intensificaci6n de la itc y la acci6n de gases y fina ceniza (altamente concentradas en esa zona) que transportadas por los vientos generales y locales (principalmente estos ultirnos) y de altitudes inferiores a 7.000 m, procedlan de las emisiones. pequeiias perc constantes, que se estaban dando en el volcan nevado del ruiz desde los dlas previos. la ceniza y gases, como nucleos de condensaci6ncongelaci6n, originaron 'lubes de gran espesor e impulsaron la intensificaci6n de la itc. 13-nov-85. la dlstnbuci6n de la insolaci6n en este dfa presenta variaciones sustanciales con respecto al promedio. en todo el pals se present6 una notable disrninucion del brillo solar, con sectores crlticos ubicados en caldas. norte del tolima, santander, boyaca, amazonas, nariiio (fig.5). la forma y ubicacion de esta zona con disrninucion en ef brillo solar es muy similar a la nube de gases detectada el 14 de noviernbre y que fuera reseiiada por krueger (en smithsonian institution, 1985). esto sugiere que la capa de gases, formada e intensificandose desde los dras anteriores, fue en gran parte responsable de la disminuci6n de la insolaci6n ocurrida en los dras 12 y 13 de noviembre. unicamente en guajira. cesar, norte de bolivar, arauca, vichada, guainla y oriente del meta se conservaron las caracterlsticas generales de presentar mas de 5 horas de insolaci6n per dra, aun cuando en la orinoquia se presentaron aumentos hasta de 2.5 horas (40%) y, al contrario, en la parte correspondiente a la regi6n del caribe ocurrieron disminuciones de hasta 2 horas (20%). 14-nov-85. en todo el territorio colombiano, a excepci6n de la guajira y el norte del cesar, se registraron valores de insolaci6n inferiores a 5 horas/dla y una notable disminuci6n con respecto al promedio de noviembre1985. esa disminuci6n fue superior a 3 horas, excepto en la regi6n del caribe, regi6n del catatumbo, norte de arauca y nororiente del vichada, donde la disminuci6n oscil6 entre 1 y 3 horas, en general. porcentualmente la insolaci6n de este dfa represent6, en la mayor parte del pals, menos del 20% de 10 normal; en algunas pequeiias areas, relativamente, represent6 entre el 20% y el 100% (por ej., la regi6n del caribe. la regi6n del catatumbo, norte de arauca, nororiente del vichada. litoral pacffico en nariiio y norte del choc6 (fig.6). 15-nov-85. la insolacl6n continu6 siendo inferior al promedio en la mayor parte del pars, aun cuando en un grado menos fuerte que los dras anteriores. valores de brillo solar superiores a 5 horas/dfa s610 se presentaron en la regi6n del caribe y algunas otras pequeiias areas (por ej., arauca y occidente de santander). excepto en algunas areas de c6rdoba, santander y huila, en el resto del pals hubo disminuci6n en el numero de horas de sol cc'nrespecto al promedio. esa disminuci6n tuvo valores de 1.5 a 2 horas. en los casos mas fuertes la insolaci6n represent6 s610el 30-40% de 10normal (fig.?). geofislca colomblana n° 2, abril de 1993 45 to' to' escala .' ..' '0 ., .' -----..:.-130 \ o' ../.r--""""......\ 'i ) ",/ g.\. o· peru ·0 o· ,.' to' ./nulo.g ....... figura 3. isohelias que seiialan, en %, la diferencla entre la insolacl6n del 11-nov-1985 y la media dlarla de nov-1985 46 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n ?o' ..',,' s·-.......r\ ..) l s c s c a o so '00 "o~ •. .. venezuela .' cgo brasil peru .. iluld.6.l.*. ,.' ,,' ..' 10 .' .' .' cfigura 4. isohelias que seilalan, en "10, la diferencia entre la insolaci6n del 12-nov-1985 y la media diaria de nov-1985 geoffsica colombiana n° 2, abril de 1993 47 to· to· e $ca l a o· \ 120 \ .././"".-"'\ .......\, \ ,/.-c, o· ecuador o,.., 40 peru ... '0 o· .. .. t • j""-1).g.ljiiii. to· ,.. figura 5. isohelias que seilalan, en %, la diferencia entre la insolaci6n del 13-nov-1985 y la media diaria de nov-1985 48 eslava: efeetos erupci6n del vnr sobre la atm6sfera. caso: insolaei6n to" 0" ." 0" l.==---...... __ to· peru t." escal" o so '00 venezuela .." '0 0" .. <,"\' 30 ',\ r-. " ,"l''\..,..... \.. \. _.} \ . \ \ brasil 20 h" 12" to",.. figura6. isohelias que seilalan, en %, la diferenela entre la insolaei6n del 14-nov-1985 y la media diaria de nov-1985 geofisica colombiana n° 2, abril de 1993 49 ... 7'· 70· escai.a o '0 '00 rso r». ," venezuela o· cbrasil ··l.,/1iulo.:i.l,in. peru 72· 70·71· 71· ii· '0 i· .. o· figura 7. isohelias que sei'lalan, en %, la diferencia entre la insolaci6n del 15-noy·1985 y la media diaria de -r noy·1985 50 eslava: efectos erupci6n del vnr sobre la atm6sfera. caso: insolaci6n 16·nov·85. en este dla el efecto de disminuci6n de la insolaci6n fue menos intense que los dlas anteriores y la situaci6n empez6 a normalizarse. los valores estuvieron cerca de 10 normal (100-120%) en la regi6n del caribe y sector central de santander, areas en las cuales tam bien los valores fueron superiores a 5 horas/dia. en el resto del pals el brillo solar fue menos intense que 10 normal, con un mayor efecto de disminuci6n (70-80%) en choc6, antioquia y algunos sectores de huila, tolima, caldas, cundinamarca, boyaca y las regiones de orinoquia y amazonia. conclusiones los analisis efectuados permitieron establecer las siguientes caracterlsticas generales relativas al comportamiento de los elementos meteorol6gicos frente a la inyecci6n en la atm6sfera de productos s61idos, ifquidos y gaseosos efectuada por el volcan nevado del ruiz. las partlculas s6lidas y gaseosas se hicieron presentes en casi toda colombia y forma ron capas en la atm6sfera, de diferentes espesores y densidades; ello impidi6 u obstaculiz6 el paso de la energfa solar y ocasion6 cambios temporales en los valores de los elementos meteorol6gicos, en el aire superfcial y a diferentes altitudes. esos cambios comenzaron desde el dla anterior, por efecto de los gases y cenizas emitidas con anterioridad y que por los movimientos locales y generales del aire se habfan acumulado en sectores ubicados al w y nw del volcan. en general, los efectos, perduraron hasta el dla 16. se ocasiono una reducci6n notoria en el numero de horas de sol por dla, durante el perfodo del12 al16 de noviernbre, comparando con las que en promedio ocurrieron en todo el mes de noviembre de 1985. este hecho, a su vez, origin6 una reducci6n de la radiaci6n solar entrante en el sistema tierra-atm6sfera y, a nivel del aire ubicado en la superfcie terrestre, una disminuci6n de las temperaturas medias del aire, con un aumento de las temperaturas minimas y una disminuci6n de las rnaxirnas. esas variaciones, en uni6n con el aporte de vapor de agua y nucleos de condensaci6n (cenizal afectaron la distribuci6n de la humedad del aire' y de los de mas elementos meteorol6gicos, eiltre eum: el regimen temporal y espacial de las ii'jvias. las magnitudes de algunas de esas alteraciones fueron p&queiias y, muy seguramente, no perceptibles por ios metod os de medici6n dis;jooibles. la distribuci6n de la inso'aci(~n en el perfodo 12 a 16 de noviembre presenta variaciones· sustanciales con respecto al promedlo. en todo el pals se present6 una notable disminuclon del orillo solar; la forma y ubicaci6n de las areas con dism;nuci6n en el brillo solar es muy similar a la nube de gases, esto sugiere que la capa de gases fue en gran parte responsable de la disminuci6n de la insolaci6n ocurrida en los dlas 12 y 13 de noviembre. ei dia 12-nov-1985 ocurri6 una extraordinaria, mas que notoria, disminuci6n de la insolaci6n diaria en toda la regi6n andina, norte de la regi6n del pacifico, suroccidente de la regi6n del caribe y algunas areas pequeiias como leticia y iimites entre meta y vichada; esta disminuci6n fue de tal magnitud que en casi toda el area cubierta por los departamentos de antioquia y choc6 la insolaci6n fue de 0.0 horas (cero.cero), 0 muy cerca a cero. ei dla 14, por ej., en todo el territorio colombiano, a excepci6n de la guajira y el norte del cesar, se registraron valores de inso/aci6n inferiores a 5 horas/dla y una notable disminuci6n con respecto al promedio de noviembre-1985; esa disminuci6n fue superior a 3 horas, con algunas excepciones donde la disminuci6n oscil6 entre 1 y 3 horas. porcentualmente la insolaci6n de este dia represent6, en la mayor parte del pals, menos del 20% de 10 normal. uno de los efectos colaterales, entre los muchos que se originaron, fue el ocurrido en la regi6n del cocuy, departamento de boyaca: las cenizas convirtieron el sector occidental de la sierra nevada del cocuy en una superfcie oscura en la que la nieve presentaba un color grisaceo; este cambio intempestivo en la composici6n y color de la cubierta de sus cumbres modific6, aumentando, sustancialmente la capacidad de absorber la energla solar y, por 10 tanto, de la aptitud para efectuar el proceso de iicuefacci6n 0 deshielo, hecho que paulatinamente ocurri6. las condiciones generales resumidas en las conclusiones anteriores no fueron uniformes para cada dia ni para cada regi6n 0 zona, ya que se detectaron variaciones locales que diferian de la situaci6n general establecida. esas innumerables variaciones temporales y espaciales estuvieron sometidas a factores externos entre los cuales el factor prima rio fue la radiaci6n solar y sus variaciones; ademas, la complejidad de los procesos atrnosfericos y su continua y temporal modificaci6n dependieron basicarnente de la interrelaci6n que se da entre los elementos meteorol6gicos y entre ellos y el conjunto de procesos zonales de convecci6n terrnica y dinarnica, de la infiuencia extensiva de los diferentes sistemas meteorol6gicos (por ej., la ltc), de la presencia de movimientos del aire determinados 0 condicionados par la geometrfa tierra-sol, la latitud, la altitud, la configuraci6n del relieve, el suelo, la vegetaci6n, la hera del dia 0 noche en la cual lieg6 a cada sitio la masa principal de partlculas, etc. todo 10 anterior justifica el hecho de que las caracteristicas atmosfericas no presentaran una uniformidad en los cambios mencionados. los impactos generales entonces se modificaron por el medio ambiente local y/o regional. bibliografia eslava, j. 1985. consideraciones sobre los aspectos meteorol6gicos y efectos relacionados con la dispersi6n de fragmentos y gases emitidos por el volcan arenas del nevado del ruiz el 13 de noviembre de 1985. geol.colomb. 14:165-173. santafe de bogota. federacion nacional de cafeteros. 1986. anuario meteorol6gico 1985. 360pp. cenicafe. chinchina, caldas. instituto colombiano de hidrologla, meteorologia y adecuacion de tierras (himat). 1991. informaci6n meteorol6gica de 1985 existente en los archivos de himat (sistema de informaci6n hidrometeoroi6gica). santafe de bogota. smithsonian institution. 1985. ruiz volcano, colombia. sean bt. letin 10 (10):2-4, 25-35. washington. geociencias-vol 12-1 sep.vp earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 31-43 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia lorena rayo and carlos a. zuluaga department of geological sciences, universidad nacional de colombia, edif. manuel ancizar, ofic. 301, ciudad universitaria, bogotá, colombia abstract the contact between the unit porphyry andesite and the cajamarca group is observed in the “túnel de la linea” section. the integration of petrographic, geochemical and textural (crystal size distribution, csd) analysis allows description of physical and chemical processes that took place in the contact zone in order to propose a model for the intrusion. material assimilation produced quartz enrichment towards pluton’s boundaries associated to a simple process of melt injection. the difference between host rock and hot melt rheologies caused shear stress that produced crystal breaking, folding and foliation rotation. keywords: cajamarca complex, cordillera central, csd, igneous and metamorphic petrology, andesite-dacite porphyry, la línea tunnel. resumen el contacto entre la unidad porfido andesitico y el complejo cajamarca es observado en la sección del “tunel de la linea”. la integración de análisis petrograficos, geoquímicos y texturales (distribución de tamaño de cristales, csd) permiten la caracterización de los procesos físicos y químicos que se dan en la zona de contacto y que sirven como base para proponer un modelo de intrusión. la asimilación de material produjo enriquecimiento de cuarzo hacia los limites del pluton y esta asociada a un proceso simple de inyección de fundido. la diferencia de reología entre la roca encajante y el fundido caliente ocasionó cizallamiento que resultó en rompimiento de cristales, plegamiento y rotación de la foliación. palabras clave: complejo cajamarca, cordillera central, csd, petrología metamorfica e ignea, porfido andesitico. 31 manuscript received may 02, 2008. accepted for publication june 10, 2008. introduction the colombian andes is a result of the interaction of several tectonic plates that have interacted since the paleozoic; because of this, the orogen is an important record of all tectonic processes that have taken place in south america northwest corner from the paleozoic to the present. the central cordillera is one of the most prominent geomorphologic features in the colombian andes and its central section consists of a set of metapelitic and metavolcanic rocks of early paleozoic age (restrepo-pace 1992), association that was intruded by mesozoic and cenozoic plutons probably related to subduction of oceanic lithosphere below the colombian andes (aspden & mccourt 1986). in the axial zone of the central cordillera, between calarca and cajamarca towns, the digging of “la linea” tunnel (by “instituto nacional de vias invias”), in both sides of the cordillera (fig. 1), provided an excellent opportunity to have access to fresh rocks of the lithologic units present in the area. this paper presents the study of the emplacement of an igneous body that involves juxtaposition of a hot and viscous liquid in movement against a cold and stationary solid of a different composition in the section cut by “la linea” tunnel. the conjugation of contrasting material properties and relative movement produced characteristic structures and textures related to chemical and mechanical interactions in the contact zone as reported in similar diapiric intrusions (e.g., deflection of regional markers and evidence of stoping; see for example miller and patterson, 1999; tikoff et al., 1999). the study presented here aims to a better understanding of the emplacement process of a small interpreted diapiric forceful intrusion at shallow crustal levels. with this purpose in mind, the use of traditional geochemical and petrographic techniques is complemented with a textural analysis of the porphyritic body to relate nucleation and growing rates with the emplacement process. geological setting the eastern flank of the central cordillera, located inside the central andean terrane (restrepo & toussaint 1988, restrepo-pace 1992), chibcha terrane (toussaint 1993), or cajamarca terrane (etayo-serna et al., 1983), consists of polymetamorphic, low to medium pressure, metapelitic and metavolcanic rocks of continental and marine origin. the terrane is limited at the east by the otú-pericos fault and at the west by the romeral fault system. the area was first described by botero (1946), but the work of nelson (1962) was the first to identify that the area is characterized by igneous bodies mostly in tectonic contact wit metamorphic rocks, both lithologies covered by recent volcanic (fig. 1). metamorphic rocks are grouped into a unit known as cajamarca complex (núñez 2001), and are characterized by a sequence of amphibole and graphite schist metamorphosed under the greenschist to amphibolite-epidote facies (restrepo-pace 1992). mayor and trace element geochemistry indicates that the protolith were rocks related to an intraoceanic island arc and a continental margin (restrepo-pace 1992). radiometric dating gives a wide spectrum of ages that range from paleozoic to paleogene (315± 15 ma to 63±2.3 ma), where the oldest ages could reflect the age of the protolith (restrepo-pace 1992), and the youngest may reflect isotopic resetting caused by overprinting of dynamothermal events. the protolith could be even older than the oldest age obtained by radiometric dating according to silva et al. (2005) who argues a neoproterozoic – early cambrian age based on c and o stable isotope analysis. the metamorphic association was intruded by mesozoic-cenozoic plutons (e.g., ibagué batholith, payandé and dolores stocks and minor associated intrusions); these rocks are predominantly of quartz-diorite composition (nelson 1962, alvarez 1979). mojica & kammer (1995) associated the smaller intrusions to discrete mesozone and epizone plutons intruded during early and middle jurassic and associated with contact metamorphic aureoles, skarn zones, and copper and gold mineralizations. small, porphyritic bodies are thought to be related to nearby intrusives of batholithic dimensions because most of the small bodies intrude the batholiths (sillitoe et al., 1982). 32 lorena rayo and carlos a. zuluaga cause of magmatism is also a contentious issue, one view relates magmatism to evolution of a convergent margin where oceanic lithosphere is subducted below the andes (alvarez 1979, aspden & mccourt 1986, núñez 1986, 2001, bayona et al. 1994); an alternative explanation is that magmatism is associated with distensive tectonics (rifting) caused by gradual continental separation across a paleorift (mojica & kammer 1995). structural styles in the region have a typical character of high angle inverse faults; seismic data indicates that these structures feed into a 20 km deep west-dipping décollement (butler & schamel 1988). two of the most prominent faults of the region, the chapetón-pericos fault and the palestina fault, separate different deformational styles. between the two mentioned faults, the style is marked by isoclinals folds in all scales, while west of the palestina fault and east of a third fault, the aranzazu-manizales fault (la soledad zone fault), a superimposed s-c fabric characterized the deformation style (restrepopace 1992). the romeral fault system is the main structure near la linea tunnel and it is also the main source of earthquakes; however, the 1999 armenia’s 33 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia venezuela brasil perú ecuador 0 200 km c e n tr al c o rd ill e ra colombia 10 km0 cajamarca complex: paleozoic? quartzite and schist cenozoic alluvial, pyroclastic and glacial deposits quebrada grande complex: cretaceous metavolcanic and metasedimentary paleozoic “intrusivo neisico de la línea”cenozoic dacites andesites cretaceous igneous complexes t p t8 tieradentro: precambrian neiss and amphibolite t p t12 t p t13 t p t14 t p t15 t p t17 t p t19 t p t16 t p t20 t p t21 t p t22 t p t23 t p t24 t p t11 t p t10 t p t -9 nw se km 8+000 km 8+200 figure 1. regional geologic map from the area around “la línea” tunnel (central cordillera, colombia). the tunnel is located towards the centre of the figure and cross mainly paleozoic metamorphics (cajamarca complex) and cretaceous metasedimentary and metavolcanic rocks (quebrada grande complex). the map was redrawn from generalized geologic maps of quindio and tolima (rodiguez & nuñez 2001, mosquera 2000). the schematic section below the map show sample localities in the tunnel between abscissa k8+000 and k8+200 (samples tpt-8 to tpt-24, represent by numbers 8, 10, 22, etc. without the prefix tpt). earthquake, showed the presence of nnw faults with recent activity (monsalve & vargas 2002). additionally, there are several e-w systems that generate differential horizontal displacements and segment nnw faults (vargas et al. 2008). methodology sample rocks were collected by a systematic way along the tunnel depending on the tunnel’s walls covering. the contact zone between the metamorphic and the porphyritic body (abscissa k8+081 to k8+108, fig. 1) was sampled with 2 m separation between samples to allow a detailed characterization of the zone. beyond the contact zone, samples were taken with a separation of 10 m, 50 m and 200 m approximately. sampling was accompanied by detailed structural characterization including data collection from joints, foliation, folds, cleavage and veins. a second auxiliary section was sampled in the surface in order to collect more information from the contact zone between the cajamarca schist and the andesite porphyry. nearly 200 structural data and 27 rock samples were collected (tpt-1 to tpt-24, qc-1 and qc-2). thin sections of each one of the samples were obtained and three selected sections were polished for microprobe analysis. the petrographic characterization of the samples consisted of mineral identification, microstructure description and modal analysis (counting of 300 to 400 points). rock microstructure descriptions include: grade of crystallinity, grain size, grain shape and crystal spatial relations. a fei quanta 2000 scanning electron microscope, hosted at universidad nacional de colombia – bogotá, was used to obtained point analysis and backscattered electron images (bei) of polished thin sections coated with a mix of au-pd (1:1 anode). bulk rock chemical analysis were obtained from glass discs with the universidad nacional de colombia – bogotá magixpro pw-2440 philips x ray fluorescence spectrometer, fitted with a rh tube, maximum power of 4 kw, and calibrated with international standards (mbh and nist). crystal size distribution analysis was done for three pluton samples (tpt-16, tpt-20 and qc-2) at different scales, covering an average area of 2x3.5 cm2. photomicrographs, taken in 6 to 10 fields in each sample, and scanned thin sections (1000 dpi resolution) were processed with a drawing program to trace the maximum length of each crystal. the minimum and maximum number of crystals measured in all sections was 317 and 1678, for a total of 4793 analyzed crystals. the data was then analyzed with the software csd correction 1.37 (higgins 2002). porphyry andesite the body is exposed in an area of approximately 5 km2, has an elongated n-ne shape geometry and its age is uncertain. it was probably originated by neogene plutonism (see for example aspden et al., 1987), but could also belong to the porphyry mineralized bodies associated to a jurassic calc-alkaline suite described by mccourt et al. (1984). the main constituents are plagioclase, amphibole and quartz, with minor biotite, apatite, pyrite, chalcopyrite, sphene and ilmenite and chlorite, sericite, epidote and carbonates as alteration minerals. in the qapf modal classification of volcanic rocks (le maitre et al., 2003) samples from this body felt in the basalt – andesite field (fig. 2) and the rock is classified as a porphyry hornblende andesite. the quartz content increases towards the pluton´s boundary (most quartz-rich samples are located towards the pluton boundary) suggesting assimilation of material from the country rock. the rock has prophyritic microstructure, is holocrystalline and contains phenocrysts of plagioclase up to 6 mm in diameter and hornblende from 0.5 mm to 2.5 mm in diameter. micro-phenocrysts of plagioclase, hornblende and quartz with diameters of less than 0.4 mm are also present. the matrix (24 to 36%) is criptocrystaline; however, microlites of quartz and plagioclase were suspected under the petrographic microscope and confirmed by electron microscopy analysis. in general, crystals are subhedral to euhedral, but in some cases are highly fragmented due to deformation related to the intrusive process (e.g., tpt-21b; fig. 3). some samples 34 lorena rayo and carlos a. zuluaga show also microfaulting originated by post-emplacement processes as the microfractures also cut the country rock (fig. 3). euhedral tabular plagioclase is the primary constituent of the rock (40 to 60%). plagioclase composition ranges from andesine to oligoclase, phenocrysts range in composition from an26 to an43 and microphenocrysts range from an28 to an39 while matrix plagioclase has a composition of an31. phenocrysts have inclusions of pyrite and amphibole and are partially or totally replaced by sericite (12 to 43%). epidote and calcite are also present as secondary minerals within plagioclase probably originated by action of hydrothermal fluids. the presence of euhedral fe-rich epidote associated with plagioclase is restricted to the contact zone (samples tpt-21b and qc-1) suggesting schist partial melting and assimilation of the country rock material in the melt. olive green brown to green yellow amphibole (13 to 55%) in euhedral, prismatic, rarely twinned crystals is the second most abundant constituent of the rock. it contains inclusions of plagioclase, ilmenite and pyrite and is incipiently zoned. this amphibole is ca-rich with intermediate to low si content and relatively high al, whose compositional classification ranges between pargasitic hornblende to ferrous pargasitic hornblende and to edenitic hornblende and silicic edenite. most hornblende crystals are altered to chlorite and biotite along cleavage planes and occasionally they are completely replaced. anhedral quartz is present in less than 10% modal proportion. it usually has rounded edges with reaction and corrosion bays, reaction textures that suggest disequilibrium of this mineral with the melt. opaque minerals (up to 18% modal proportion) include pyrite, chalcopyrite, rutile, and intergrown titanite – ilmenite. accessories phases (<3%) include euhedral lath-shaped and locally kinked biotite (0.7 mm), euhedral apatite (0.1-0.2 mm), and euhedral zircon. cajamarca complex (pzc) quartz-biotite-graphite schist: they consist mostly of quartz (34 54%), graphite (1 23%), biotite (2 -34%), and muscovite (1 14%), with minor plagioclase, calcite, actinolite and chlorite (all <10%). accessory minerals include apatite, monazite, pyrite, chalcopyrite, ilmenite, rutile and titanite. these rocks have schistose microstructure with folded microlithons of plagioclase and biotite-quartz-graphite-muscovite 35 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia figure 2. diagram showing compositional variations of samples from the porphyritic intrusive body in the “la linea” tunnel section. note that most samples are within the basalt – andesite field (only sample qc-1 with a higher quartz content felt within the dacite field). the observed trend of increasing quartz content in the triangle corresponds with an increasing trend of quartz towards the contact between the intrusive and the metamorphics. qapf modal classification of volcanic rocks (le maitre et al., 2003). (fig. 3). the presence of biotite is indicative of the beginning of the biotite zone in the greenschist facies. these rocks probably were originated from an impure psamitic to pelitic protolith consisting of thin interbeded sandstone and quartz claystone, very rich in organic matter, with some proportion of carbonates. the presence of multiple foliations indicates at least two deformative events. mica-quartz schist: they are composed mainly of quartz (45-70%), muscovite (10-25%), biotite (10%), minor plagioclase (7-11%), calcite (6-17%), chlorite (4-13 %), and graphite (3%), and accessory apatite, titanite, pyrite and chalcopyrite. the protolith was a psamitic sequence with quartz sandstone and small proportions of claystones and limestones. the parageneses of quartz-chlorite-muscovite suggests a regional metamorphism in the greenschist facies. amphibole-epidote schist: it consists of hornblende (40-60%), epidote (9-67%), plagioclase (2-18%), and minor calcite, chlorite, titanite, zircon 36 lorena rayo and carlos a. zuluaga and opaques (<10%). they have a schistose microstructure marked by preferred orientation of hornblende and epidote. the parageneses talc-epidote-calcite indicates that these rocks were metamorphosed under regional metamorphism in the greenschist facies. quantitative analysis of crystal size distribution (csd) quantitative csd analysis complements results from petrography and from chemical analysis to reveal the magmatic processes that affect the evolution of the body. this technique is based on textural analysis of rocks and considers the crystal content as a function of size, shape and orientation (marsh 1998; higgins 2002). crystallization is mainly controlled by the rate of heat removal from the system, which results from interactions between kinetics, time and temperature; for example, high temperatures and large diffusion rates favored a few large crystals (vernón 2004). the size of crystals is primarily the result of heterogeneous nucleation, where material is rapidly and continuous added to a crystal boundary (growing rate) during all stages of crystallization (marsh 1998). the subsequent states of nucleation not only depend on 37 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia figure 3. thin section photomicrographs of samples from the tunnel showing evidence of fragile and ductile deformation, left: ppl, right: xpl. a. sample tpt-21, microfaults in apatite and biotite crystals, b. sample tpt-21b, plagioclase broken crystals, c. sample tpt-22, fractures in plagioclase filled with quartz, d. sample tpt-21b, bended biotite crystals, e. sample tpt-19, microshear zones. mineral abbreviations after kretz (1983). the cooling rate but also in the process of growth of the nuclei initially formed. therefore, changes in the rates of nucleation (n) and growth (g) are the result of the interplay of various factors such as temperature diffusion and time and are reflected in the crystal size population. the most important part of the csd curves is their shape and not the absolute values in the graphs (marsh 1998). a linear logarithmic csd is basically originated from an exponential change of nucleation rate over time; thus, changes in the slope reflect changes in the relative rate of nucleation. under stable conditions, the maximum size of crystals should increase systematically with the increase of crystallinity, so that a curved csd clearly reflects the addition of natural crystals. for example, if a csd suggests multiple states, then nucleation can be interpreted as induced by different thermal regimes. csd is a statistical method and the frequency depends on the size of the crystals; thus, the analyzed samples must be large enough to get a statistically valid analysis, a minimum of 200 crystals must be measured to get a reasonably valid csd (mock and jerram 2005). that is why samples selected for this study have a population of at least 317 measurements obtained at two different scales. results csd graphs for two crystalline phases (plagioclasehornblende) show a variable slope and concave shape (fig. 4). the abnormal changes of the slope are interpreted as measurement errors and fall within the error bars that indicates wide dispersion of the data. the csd curvature can be explained in two different ways. first, the shape could reflect two nucleation events (�n) with a super exponential increase in its final part that explains the higher frequency of small crystals. this increase in nucleation rate could be interpreted as a product of addition of country rock material, in agreement with variations in the slope of the csd away from the edges of the intrusive, which becomes more linear, and with the interpretation of quartz and mica assimilation supported by rock compositional variations. second, the shape of the curves could be caused by a growth rate dependent or proportional to size (�g) (eberl et al. 2002), this is in discrepancy with mineral analysis that shows an overlap in the compositional range of phenocrysts, microphenocrysts and matrix crystals indicating that these may have nucleated simultaneously. however, an alternative explanation is that some nuclei may have begun to grow more rapidly than others, the larger crystals have lower surface energy and growth more at expenses of the smaller ones (ostwald ripening) favoring the emergence of phenocrysts and the greater number of small crystals is favored by selective and concentrated growth of the larger crystals. discussion and conclusions since relationship between country rock and type of intruding magma is the governing factor for the type of generated contact, the characterization of the host rock is important to determine the effects of the approximating heat source in the pressure and temperature regime. for example, a type of relationship in forceful intrusions develops when magmatic fluids move into fractures opened in the country rock. these fracturing is created or enhanced by a momentum generated by the intrusive itself. in the case here, petrographic characterization and field evidence (fig. 5) suggest that the contact between the porphyry andesite and cajamarca group is intrusive, this contact was later affected by a deformation event causing a faulted contact in some parts of the intrusive (n to nnw predominant direction). the suggested forceful intrusion is also characterized by assimilation of quartz in the igneous body and hydrothermal fluid exchange between country rock and the igneous body. the presence of fe-rich epidote restricted to the contact zone support the interpretation of partial melting of the schist country rock and assimilation of that material into the melt and/or hydrothermal fluid exchange. csd curves show an increase in nucleation rate that is interpreted here as a product of addition of country rock material, it is possible that the shape of the csd was influenced by nucleation and growth, however geochemical evidence of country rock assimilation is in agreement 38 lorena rayo and carlos a. zuluaga with the interpretation of csd affected by rock assimilation. structural data also support the interpretation of a forceful intrusion. the country rock near the contact has a different foliation orientation than the regional trend suggesting rotation of foliation that could be originated by the emplacement of the pluton (fig. 6). the two lithologies have contrasting mechanical behavior, while the cajamarca group schist 39 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia 40 lorena rayo and carlos a. zuluaga figure 4. csd diagrams, samples tpt-16 (top), qc-2 (middle), and tpt-24 (bottom). the diagrams illustrate the crystal size distribution for plagioclase (left) and hornblende (right). size of the crystals, given in mm, plotted as a function of the natural log of population density. figure 5. field intrusive evidence of the porphyry (top) and subsequent faulting affecting the intrusive contact (bottom). and quartzite have ductile behavior that is expressed in tight folds developed in two orientations (0/72 and 162/60) the intrusive body is affected by faults and microfaults (fragile deformation, figs. 3 and 5) that also cut the country metamorphic rock suggesting a post-emplacement fragile deformation event. additionally, crenulation cleavage that affects the foliation in several directions (fig. 3e) and quartz, carbonate and sulfurs veins that cut in different directions the foliation suggests that other processes affect the rock after peak metamorphism. we suggest that the magma formed a gradational contact zone in a simple injection process, where fragments of the country rock were trapped into the magma and some of the fragments were not completely melted and formed xenoliths (see fig. 5). however, a faulted contact between the intrusive and the country rock is observed in the area (fig. 5), this faulted contact has a trend of n to nne consistent with the regional trend of the structures such as the otú pericos fault and romeral fault system. there is not field evidence in the form of dikes or sills that suggest pervasive invasion of melt, this is because of the characteristics of the country rock (quartz-rich schist) that acted as an impermeable unit. acknowledgments this work had financial support from the universidad nacional de colombia (project hermes number 6214). the authors thank the instituto nacional de vías (invias) for their logistical support during field work within the ”la linea” tunnel. this project was initiated by the student group geoymco. the authors thank carlos vargas and an anonymous reviewer for his comments on an earlier version of the manuscript. references alvarez, a. j. (1979). geología de la cordillera central y el occidente colombiano y petroquímica 41 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia figure 6. stereograms of structural data between km. 5+640 and km. 8+400. the section runs from the nw to the se (pq = “portal quindio”, pt = “portal tolima”). blue lines represent foliation and red lines represent joints. note the change in foliation orientation near the contact between the intrusive and the metamorphics (sector 2). de los intrusivos granitoides mezo-cenozoicos. ph.d. thesis. universidad de chile. 359 p. aspden, j.a. and mccourt, w.j. (1986). mesozoic oceanic terrane in the central andes of colombia, geology. 14, 415-418. aspden, j.a., mccourt w.j., and brook m. 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(1993). evolución geológica de colombia: precámbrico, paleozoico, universidad nacional de colombia, medellín, colombia. vernon, r. (2004). a practical guide to rock microstructure. cambridge unversity press, 594 pp. vargas, c., nieto, m., monsalve, h., montes, l., valdes, m. (2008) the abanico del quindio alluvial fan, armenia, colombia: active tectonics and earthquake hazard. journal of south american earth sciences. 25, 64-73. 43 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombia earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 44-61 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria akintola, o.f.1 and adekeye, j.i.d.2 1 raw materials research and development council, p.m.b. 232, garki, abuja, nigeria. fax: 234 9 4136034 e-mail: akintolaolatunde@yahoo.com 2 geology and mineral sciences department, university of ilorin, p.m.b. 1515, ilorin email: adekeye 2001@yahoo.com abstract the pegmatites of nasarawa area occur in the central part of nigeria. they are mainly hosted by phyllonites in a nne-ssw trending shear zone lying east of some foliated pan-african and west of jurassic afu complex younger granites. a geological mapping of the area was followed by petrographic and mineralogical studies of selected rock and mineral samples. a total of 72 samples consisting of 25 rocks, 22 feldspars and 25 white micas were analyzed for various elements. the pegmatites are peraluminous and are genetically linked to the late pan-african leucogranite with the shear zone. the pan-african granites have very low ree abundances and non-chondritic ratios of zr/hf and y/ho and low nb/ta ratios indicating crystallization from a liquid-rich melt. barren pegmatites are closely associated with the primitive hornblende biotite pan-african synorogenic granites while snnb – ta mineralized granites are correspondingly enriched in pegmatites spatially associated with pan-african synorogenic granites with enhanced values of rare lithophile elements such as rb, cs, mn, sn and nb-ta. the primary control of rare metal mineralization in the pegmatites is the composition of the source rock since the ta-nb-sn-li-be-w mineralized pegmatites crystallized from fluid (h2o-b-p-f) rich melts. it is hereby proposed that the late pan-african tectonic granite which is parental to the highly mineralized pegmatites in this area originated from anatexis of undepleted mica-rich metasediments at depth, followed by a magmatic fractionation of the fluid rich melt as it ascended through reactivated ancient fractures. the heat for the partial melting might have been supplied mainly by the reactivation of ancient fractures, which controlled the emplacement of the fertile granites and the related pegmatites. keywords: pegmatites, nasarawa, shear zones, mineralization, anatexis, magmatic fractionation, nigeria. 44 manuscript received may 11, 2008. accepted for publication june 19, 2008. resumen las pegmatitas del área de nasarawa se dan en la parte central de nigeria. ellas están principalmente emplazadas en filonitas de una zona de cizalla con una tendencia nne, ssw reposando al e de algunos complejos graníticos como el pan africano joven y al w el complejo jurasico afu. un mapa geológico del área fue seguido mediante estudios petrográficos y mineralógicos de rocas seleccionadas y muestras minerales. se realizó un análisis de varios elementos sobre un total de 72 muestras compuestas por 25 rocas, 22 feldespatos y 25 micas blancas. las pegmatitas son peraluminosas y están relacionadas genéticamente con el leucogranito pan africano tardío y con la zona de cizalla. los granitos pan africanos tienen muy bajos contenidos ree y proporciones no condríticas de zr/hf y y/ho y las bajas relaciones de nb/ta indican cristalización a partir de un fundido rico en líquido. las pegmatitas barren están muy relacionadas con la biotitas y orblendas primitivas de los granitos sinorogénicos pan africanos, mientras que los granitos mineralizados con sn-nb-ta son correspondientes con las pegmatitas enriquecidas espacialmente relacionadas con los granitos sinorogénicos pan africanos con valores altos de elementos litófilos raros tales como_: rb, cs, mn, sn y nb-ta. el control primario de la mineralización de metales raros en las pegmatitas es la composición de la roca fuente a partir de las pegmatitas mineralizadas en ta,-nb-sn-li-be-w cristalizadas a partir de un fundido rico en fluidos (h2o-b-p-f). aquí se propone que el granito tectónico pan africano tardío, el cual es padre de las pegmatitas altamente mineralizadas en esta área se originó a partir de la anatexia de metasedimentos no empobrecidos en micas, seguido por un fraccionamiento magmático del fundido rico en fluidos que ascendió a través de fracturas antiguas reactivadas, las cuales controlaron el emplazamiento de granitos fértiles y las pegmatitas relacionadas. palabras clave: pegmatitas, nasarawa, zonas de cizalla, mineralización, anatexia, fraccionamiento magmático, nigeria. introduction the pegmatite field belongs to the pegmatites related to syn to late pan-african tectonic granites occurring in the pan-african mobile belt east of the west african craton. the field occurs in an area bounded by 70351e – 70 051e, 80081n – 80301n covering an area of 531 km2 (fig. 1). nb-ta-sn-be-liw primary mineralization is hosted in quartz-feldspar-muscovite pegmatites. intrusion of the older granites into the reactivated archean to lower proterozoic crust of central and southwestern nigeria have been shown by rb-sr whole rock and u-pb zircon age determinations) to have lasted at least 630 to 530ma. pegmatites in the same area have been dated 562-534 ma (matheis and caen-vachette, (1983) indicating that the pegmatite emplacement occurred at the end of pan-african magmatic activity. the nasarawa pegmatite field is also in close spatial relationship with the granites of afu complex, which is the southernmost occurrence of the 1250 km-long belt of ring complexes extending across niger and nigeria. rb/sr age decreases from ordovician in northern niger to late jurassic (141 ma) of the afu complex in nigeria. the younger granites as this later suite of rocks are called, are notably mineralized in sn and nb. the two geochemically distinct and economically important types of primary sn-nb-ta mineralization were already recognized by raeburn (1924). in wamba area, kuster (1990) has shown that the emplacement of late pan african granites with similar geochemical characteristics with the mineral45 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria ized pegmatites was fracture-controlled and mylonitized along a conjugate set of ne-sw and nw-se to nnw-ssestriking faults. in more recent times, new rare-metal pegmatite fields have been discovered both within a ne-sw belt recognized by the earlier workers, jacobson and webb (1946) and wright (1970) as well as other areas already known for gold mineralization northwest of the pegmatite province especially the kushaka schist belt, the magami and maradun areas of northwestern nigeria, garba (2003). all the rare metal and gold mineralizations are associated with prominent regional faults in the basement complex of nigeria. this paper discusses the geology and geochemistry of the nasarawa area in relation to the source and controls of mineralization of rare metal pegmatites in the nasarawa area. regional geology nigeria lies within the zone of pan-african reactivation (ca.550 ma) to the east of the west african craton, which has been stable since approximately 1600ma. this mobile belt extends from algeria across the southern sahara into nigeria, benin and cameroon. rocks of the nigerian basement complex which is part of the pan african mobile belt are intruded by mesozoic ring complexes of jos area and overlain unconformably by cretaceous to quaternary sediments forming the sedimentary basins. three broad lithological groups have been distinguished in the nigeria basement complex: a polymetamorphic migmatite-gneiss complex with ages ranging from liberian (ca. 2800 ma) to pan-african (ca. 600ma). ages >3000ma have lately been obtained from some of the rocks (dada, 2006). metamorphism is generally in the amphibolite to granulite facies grade. younger members of this group are n-s to nne-ssw trending belts of low grade (greenschist to amphibolite facies) metasedimentary and minor metavolcanic supracrustals of late proterozoic age. the schist belts which are concentrated in the western half of nigeria are seldomly found east of 80 e longitude, (ajibade and wright, 1989). the schist 46 akintola, o.f. and adekeye, j.i.d. lokoja 534 egbe 632 igbeti osu ijero ibadan makurdi wamba study area kaduna 580 toro bauchi2.27 609 panyam 605 555 nassarawa-eggon 535 545 550 562 631 617 586 idanre 11° 00'e jos ilorin aromoko akure/ijere kainji lake cretaceous to recent jurassic "younger granittes" pan african "older granittes" meta-volcano sedimentary belts polycyclic basement complex 9°6° 6° 9° 175km 0 175km river benue r iver n ig er figure 1. geological sketch map of central and south-west nigeria showing the location of the nasarawa pegmatite field (study area) and the distribution and ages of pan-african older granites and pegmatites (underlined). geochronogical data sources are van breemen et. al. (1977), rahaman et. al. (1983), matheis and caen-vachette (1984), tubosun et. al. (1984), dada et. al. (1987). belt rocks host the gold and rare metal mineralized pegmatites and veins, which are associated with prominent regional fractures. the older granites, which are pan-african orogeny-related, range from synthrough late to postorogenic granitoids of upper proterozoic to lower paleozoic age (ca. 873-500 ma). they intrude both the schist belts and migmatite-gneiss complex rocks and comprise diorites, tonalites, granodiorites, granites, syenites, gabbros and charnockites. the end of the pan-african tectonic event is marked by a conjugate fracture system of strike-slip faults (ball, 1980). fault directions have a consistent trend and sense of displacement; i.e. a nne-ssw trending system having a dextral sense of movement and a nw-se trending system with a sinistral sense (mccurry, 1971; ball, 1980). both sets crosscut all the main pan-african structures, including older n-s trending shear zones (mylonites) (ball, 1980; ajibade and wright 1989, kuster 1990, garba 1996). other parallel pan-african fracture systems with structural trends (n300e and n600e) appear to have been precursors to the development of the cretaceous benue trough and its associated volcanics. the pattern of these fracture systems was probably established during the pan-african orogeny (mccurry, 1971), and the main transcurrent movement probably occurred then – but may well represent lineament of much greater age. wright (1970) was of the opinion that the regional faults had some influence on the direction of migration of hot spots within the mantle that culminated in the formation of the mesozoic ring complexes. late pan-african granites parental to rare metal pegmatites and gold-bearing veins are closely associated with the fractures in the pan african mobile belts (kuster, 1990; ekwueme and matheis, 1995; garba, 2002; okunlola, 2005). the pegmatites, both rare metal mineralized and non-mineralized, are associated with the older granites. the pegmatites were initially thought to be concentrated in a ne-sw zone extending from ago-iwoye in the southwest through wamba-jema’a to bauchi area in the northeast. however, other pegmatite fields have more recently 47 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria g9 g8 18pg18 7p lc19 lc20 lc2 3p 11p 10p ch2 25 ch1 lc21 25 g4 g5 lc 26 kj 04b 31 33 4a 4a kw 30 g25 or lc7 g27 23 28w 30 lb5 tg 32 12p sj 30 a b 8° 18'n 7° 50' 7° 35' 8° 30'n 8° 18'n 7° 50' 8° 30'n 7° 35' ap 6kilometers303000meters 8° 25' 8° 21' 8° 25' 8° 21' 10002000 7° 40' 7° 45' 7° 45'7° 40' scale 1:150,000 ba l e g e n d symbol inferred rock boundary description granite mica schists phyllonites with minor amphibolites undifferentiated strike and dip of foliation pegmatite mine shear zone pegmatites in cross section basic dyke pegmatitic granite g25 sodic aplite g27 main phase pan african older granite granodiorite tonalite gneiss younger granite complex (afu) pegmatite sample point schistlb5 foliation planes in cross section (of the schists) tourmalinite figure 2. geological map of nasarawa tantalite field (sheet 229 northwest udegi). been known around zuru-gusau in the northwest (garba, 2002 and okunlola, 2005), and obudu area in the southeast of nigeria (ekwueme and matheis, 1995). “younger granites”a 1250km-long belt of ring complexes extending across niger and nigeria, with the rb/sr age decreasing from ordovician in northern niger to late jurassic in central nigeria. these are high level anorogenic volcanic-plutonic ring complexes (jacobson, snelling et al, 1964) intruded into the older precambrian to paleozoic basement complex rocks. granites overwhelmingly predominate in the province, but in some complexes their emplacement was preceded by basic and intermediate intrusions, ranging from olivine-gabbro to quartzmonzonite and syenite. the basic, intermediate and porphyritic members of the younger granites carry pb-zn-cu-fe sulphide mineralization. the granitic members are mainly peralkaline arfvedsonite granites and the metaluminous to peraluminous biotite granites: these are the commonest and carry most of the sn-nb mineralization. chemical analysis geological maping of the area was followed by petrographic and mineralogical studies of the rocks. whole rocks chemical analysis of selected representative samples of the granites and the simple graphic quartz-feldspar pegmatites was done using phillips 1404 automatic x-ray fluorescence (xrp) spectrometer on their powder pellets and glass discs in the geochimistry laboratories of the technical university of berlin. icp-ms measurements of ta, nb and rees were performed for 14 samples in geoforschungs zentrum potsdam, using an elan 5000a quadrupole icp mass spectrometer (perkinelmer/sciez), canada. details of laboratory procedures used in analyzing some of the samples by icp-ms are as published by dulski (2001) in geostandars newsletter. few samples with peculiar assays were analyzed by the xrd method to determine their mineralogy. the framed powder samples using phillips pw 1820 diffractometer in the technical university of berlin x-rays were generated at 50kv, 30ma. analitycal condition for each sample were: 0.02° 2ø/step, 2.5 seconds per step with analysis completed from 3-80°2 ø. results geology and petrology of the area the nasarawa area comprises metasedimentary rocks (mainly mica schists and sericitized/chloritized phyllonites) intruded by a pan-african granodiorite/ granite batholith (ca. 600ma), fracture controlled elongate late pan-african granite and pegmatites (fig.2). abutting the mica schist southeast of the area is the afu complex of late jurassic (ca. 141ma) composed mainly of biotite granites with minor quartz porphyry. there is a shear zone that trends north-north east in the area within which the rocks are mylonitized. below is the detailed description of the geology of the area and figure 2 is the geological map of the area. the schists occur as relics and xenoliths in the older granites and pegmatites with thick successions in a north-south trending low-lying area that lies between the older granite and granodiorite/tonalite gneisses rock suite in the west and the jurassic younger granite (afu) complex in the east (see figure 2). the schists generally have measurable north-south trending foliations that dip to the east at low angles (250-300). the north-south foliations are interpreted as pan-african structures superimposed on earlier tectonite fabrics, which sometimes give contorted appearances to the schists. compositionally, the rocks range between metamorphosed pelitic to semi-pelitic and psammitic rocks with biotite, quartz and minor muscovite, as the major minerals. compositional changes related to pegmatites’ intrusion are noticeable at the contacts of the pegmatites and the schist. towards the south of the area, the schist becomes gneissic with appearance of feldspars and pale amphiboles. accessory minerals found in the schist include opaques (ilmenite and magnetite), sphene and garnet. tourmalines and apatites are common accessory minerals in the schist at contact zones with pegmatites and in some cases may constitute more than 20% of the rock the tour48 akintola, o.f. and adekeye, j.i.d. malines in the pegmatites’ exocontact zones in the schist are usually zoned which shows that they crystallized from highly fluid-rich melts (london and manning, 1995). radiogenic haloes are formed around inclusions of radioactive minerals (monazite and zircon) in biotites. the schists as well as other rocks within the shear zone are mylonitized. the mylonitized mica schist-phyllonite is composed of porphyroclasts of biotite and chlorite in a matrix of fine-grained groundmass of muscovite and quartz. within the schist at the center of the area, and in proximity to the schist at the northern part of the area are tourmalinites, which are essentially composed of tourmalines and quartz with accessory to minor apatites. within the schist are fragments of foliated amphibolites that are too small in dimensions to be represented as discrete bodies on the map. the mineral assemblages of biotite, muscovite, and garnet in the schists as well as amphiboles and feldspar porphyroblasts indicate that the rocks must have reached amphibolite grade of regional (barroviantype) metamorphism, during the pan african orogenic cycle. the greenschist facies minerals such as chlorite and green biotite, recrystallized fine-grained muscovite and quartz within the shear zone are products of retrograde metamorphism of the rocks by the post-tectonic processes of shearing/mylonitization that probably accompanied the emplacement of the pegmatites. coincidentally, the boundary of the shear zone marks the boundary of the zone of occurrences of the mineralized pegmatites within the schists. the mylonitic micro-textures of these rocks within the shear zone provide evidence of fracturing/shearing of the rocks. this phenomenon is observable in the leucogranitic samples and schistose samples in which porphyroclasts of biotite, chlorite and quartz are set in a groundmass of recrystallized fine grained muscovite and quartz. the shear zone is occupied by schistose metapelites/metapsammites in a north-northeast belt. within this belt are also the leucocratic pegmatitic granite. ocan and okunlola (2001) have also observed zones of mylonitization in the rocks (granite and schist) associated with the mineralized pegmatites at angwan doka, north-east of this area. similarly at wamba, about 100kilometers northeast of this area (kuster, 1990), there are elongated granitic plutons that are partly affected by deformation (mylonitization) along a conjugate set of strike-slip (transcurrent) faults. the emplacement of these granites appears to be fault-controlled and the directions of relative movements are dextral along ne-sw striking faults and sinistral along nw-se to nnw – sse strikingfaults. it thus appears that there is a regional northeast trending shear/fracture-zone characterized by mylonitization of the rocks coinciding with the zone of mineralized pegmatites, and movement along the faults must have been active before and after the emplacement of the granites and the related pegmatites. older granites of batholitic dimensions intrude these schists, which range in composition from hornblende-biotite granodiorite/tonalite gneiss to biotite granites at higher elevations. this suite of rocks appears to represent the first major episode of granite plutonism in the area. while the granodiorite/tonalite gneiss occupies the western part of the area, the biotite granites, which outcrop as inselbergs, occur in the northwestern part of the area. the granodiorites are strongly foliated with the quartzo-feldspathic phenocrysts developed into porphyroblasts or augen structures. in thin sections, the granodiorite/tonalite gneiss consists of quartz, plagioclase, bluish and brownish amphiboles, biotites with accessory titanite (sphene) and apatite. feldspar and quartz sometimes form wart-like intergrowths –myrmekites. in the biotite granite, quartz, biotite, microcline and plagioclase feldspar are the essential minerals. there may be minor or no hornblende. at the northernmost part of the area, the biotite granite is fluid-rich. some plagioclase shows some sericitisation and pegmatites close to this granite have enhancement of the rare lithophile elements compared with pegmatites close to a less fluid-rich granite. granites at the south-central part of the area are smaller bodies than the main phase granites and have some distinct characteristics in their mode of occurrence in the field. the pegmatitic granite in thin section consists of phenocrysts of quartz in a groundmass of felsic quartz, alkali feldspar and white mica, with very little biotite. the 49 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria quartz phenocrysts are strongly deformed showing wavy/undulose extinction in cross-polarized light; and in some cases, they are recrystallized due to shear movement. the rock is however not foliated and the elongated mode of emplacement is obviously controlled by a northeast-southwest trending fault. some mineralized pegmatites are close to this pegmatitic granite. simple pegmatites with mineral assemblage of microcline-quartz and minor plagioclase (albiteoligoclase) with accessory garnet, tourmaline (schorl), biotite and magnetite intruded the biotite granites-granodiorite suites. within the schists, the pegmatites become richer in muscovites and the rare metal minerals. there is a tendency towards the arrangement of the pegmatites in sub-parallel groups akin to an en-echelon emplacement, and in some cases there are two or more intersecting sets of dykes. a rose diagram plot of the pegmatites indicate two major directions, viz:east-west and north to north north-east. many of the richly mineralized pegmatites occur as sill-like bodies. the swellings are generally loci of intense albitization and mineralization. while majority of the pegmatites in the study area strike north-east/south-west, some have north-westsouth-east and east-west strike directions. strike and dip may change even in one dyke, following planes of weakness (joints, fractures and foliation planes) in the country rock. majority of the pegmatites generally cut across the foliation of the host schists and gneisses. many of the complex pegmatites display a textural and mineralogical zonation parallel with the walls of the intrusion. a zone of tourmalinization (black tourmalines) within the host rock at the contact with the pegmatites is followed by a prominent zone of quartz-mica margins of the dykes. in the complex pegmatites the marginal facies may be up to two feet or more in thickness and as observed in the liberia pegmatite with a paragenesis of cloudy (and in some rare cases smoky) quartz, mica, microcline, albite and accessory large crystals of alkali enriched (blue-green) tourmalines and fluorapatite. the mica is coarse-grained and oriented at right angles to the contacts. within the quartz-mica marginal zone is the quartz-microcline-albite-muscovite-beryl zone. this is followed by a quartz-muscovite-albitetourmaline-amblygonite-montebrasite zone. at the inner zone, there is albite-fine grained muscovite-quartz (clear and colourless). in liberia pegmatite, an albite-rich footwall zone with finely disseminated nb-ta mineralization was observed. from the outer to the inner zones there is enrichment in ta, li and cs and their ores, and the toumalines become albite with increasing contents of na and attractive colours. most of these zones are observable in the complex pegmatites with some minor variations due to variations in their bulk chemistry; at kilimanjaro, hydrated lithium-aluminosilicates (cookeite) were crystallized (no lithium aluminophosphates was sampled from this pegmatite) with albite, mica and quartz in the inner zone. pegmatite-country rock relationships (sharp contacts, unfractured wall rocks, variations in strike, dip and thickness of the dykes) suggest an emplacement level transitional between ductile to brittle host-rock behaviour (kuster, 1990). xenoliths of the foliated host rock, quartz-biotite schist, are present in some pegmatites, suggesting that the pegmatites are younger than the schists. the barren simple quartz-feldspar pegmatites found in proximity to the biotite older granites at the western part of the area are composed essentially of quartz, microcline-microperthite and minor plagioclase (albite-oligoclase). the minor plagioclase appears to be replacing the perthite with sericite by-product. garnet, magnetite and tourmaline are accessory minerals observed in the simple pegmatites. the more complex and mineralized pegmatite deposits occurring in the area show a more pervasive albitization. some pegmatites show subparallel micro cracks with a large perthite crystal, which are filled with albite and sericite. such cracks provide the channel ways by which the soda-rich late stage mineralizing fluids deposit the ores of nb-ta-snli-be. in a favourable environment especially in the middle to inner zones (close to the quartz cores of the mineralized, complex and zoned pegmatites), replacement of microcline by albite is complete, giving rise to the formation of secondary feathery albites and fine-grained muscovite“gilbertile”. east of the 50 akintola, o.f. and adekeye, j.i.d. 51 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria table 1: trace elements of the microcline, microperthites (k-feldspars) of the pegmatites trace elements of pegmatitic k-feldspars sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a p (ppm) 2400 2461 2662 2579 2130 4774 5398 6642 2854 2138 3024 f 0 0 0 0 0 0 326 0 0 0 0 ba 44 35 34 67 10 30 53 36 69 91 39 bi 20 10 17 15 17 15 10 13 12 10 21 cd 13 7 9 5 6 bdl bdl bdl bdl bdl 16 ce 40 32 32 12 52 21 0 19 35 0 69 co 16 33 18 28 20 28 31 25 23 31 18 cr 10 13 9 14 2 15 35 11 6 1 6 cs 1722 1487 1540 1226 1482 602 111 160 844 692 3489 cu 7 7 14 9 0 13 13 8 4 19 16 ga 18 15 17 15 17 16 44 19 18 14 17 la 95 73 74 58 84 27 2 16 38 31 166 nb 8 5 9 7 9 22 39 8 4 8 8 ni 20 0 31 28 0 0 0 0 0 0 27 pb 70 57 81 79 69 46 0 35 125 140 93 pr 19 15 16 13 18 8 0 6 9 8 29 rb 8546 6536 9534 8303 8420 5537 2593 3069 5440 4089 9474 sn 21 13 187 15 14 28 24 28 9 8 24 sr 38 144 58 44 51 22 30 183 64 70 32 ta b.d.l. 1 2 b.d.l. b.d.l. 9 6 b.d.l. b.d.l. b.d.l. b.d.l. tl 49 38 57 49 49 30 12 16 39 30 73 w 155 241 137 187 176 203 195 154 143 204 160 v 10 0 0 14 18 0 0 0 0 0 20 y 30 18 20 0 19 24 12 11 0 14 24 zn 44 0 0 0 0 0 40 0 0 0 0 k/ba 2521 2906 3492 1694 11058 3628 1696 2841 1571 1165 2916 k/rb 13 16 12 14 13 20 35 33 20 26 12 area is the western flank of the afu younger granite complex. the complex is the southernmost occurrences of the nigerian anorogenic ring complexes, which extend through jos and northwards to the arid region of niger republic. the complex was dated 144+ 2 ma (bowden et al., 1976). it is elliptical in outline and about 50km in maximum diameter. it shows broad similarities (geochemical, mineralogical, etc.) to the other younger granite complexes emplaced during the early to late jurassic (jacobson et al., 1958 and macleod et al., 1971). the afu complex is composed mainly of biotite granites with minor quartz porphyry (imeokparia, 1982). the biotite granites show a somewhat fractionation trend with an enrichment of nb, li, f, sn, in the more evolved albitized granites with low biotite contents. mineralogically, the biotite granites are composed of quartz, k-feldspar, albite, and biotite, with fluorite, zircon, cryolite, magnetite, hematite and less commonly cassiterite, columbite, thorite, apatite and monazite as accessory minerals. geochemistry in the older granites g8, g9 and g18b on the one hand have similar geochemical characteristics, which differentiate them from g25 and g27 (see table 1). g8, g9 and g18b are calc-alkali granites with higher contents of ca and mg. their k/rb ratios range between 155 and 261. the lowest value 155 in the range is that of g18b. field evidence shows that these three granites belong to the main-phase pan african older granites. sample g18b with the lowest k/rb ratio as well as ce among these three samples has relatively enhanced values of rb-254ppm, w 241ppm, ta 2ppm, mn 700ppm, sn 32ppm, cs 11ppm, and nb 36ppm. the spatially associated pegmatites 18p and 18ap have enhaced concentrations of rb, 917ppm and 1718ppm; cs, 62ppm and 779ppm, low k/rb ratios 73 and 40 and correspondingly enriched in the ore elements sn, 13ppm and 17ppm; nb, 65ppm and 80ppm; and ta, 15ppm and 21ppm, respectively. on the other hand, the pegmatite 10p which is spatially associated with the less geochemically evolved older granite in the area has low concentrations of the rare elements rb, 450ppm; cs, 2ppm; sn, 9ppm ; and ta 3ppm and a high k/rb of 155. major elements composition of sio2 73.16, 71.1; fe2o3 1.54, 2.02; cao 0.88, 1.75 and mgo 0.42, 0.5 respectively show that g25 and g27 are more leucocratic than the main phase pan african granites. however, trace elements’ compositions of the two granites show a lot of differences and indicate different levels of fractionation and possibly origins for the two granites. g27 has very high ba (1377ppm), sr (677ppm), ba/rb (17.21), and very low k/ba (22) and rb/sr (0.12), which may indicate a metamorphic origin of the rock. the g25 has enhanced mn (600ppm), rb (295ppm), ta (13ppm), nb (24ppm) low k/rb, al/ga, zr/hf and nb/ta ratios of 134, 2486, 19.96 and 1.85, respectively. such high values of lithophile rare elements and low k/rb, al/ga, zr/hf and nb/ta ratios are characteristic of 52 akintola, o.f. and adekeye, j.i.d. trace elements of pegmatitic k-feldspars sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a na/k 0.10 0.17 0.06 0.08 0.09 0.12 0.20 0.16 0.12 0.12 0.06 k/cs 64 68 77 93 75 181 811 639 128 153 33 k/tl 2264 2676 2083 2316 2257 3628 7493 6393 2780 3534 1558 rb/tl 174 172 167 169 172 185 216 192 139 136 130 continuación tabla 1 highly evolved granites parental to rare metal pegmatites (raimbault et al., 1995). thus the major and trace element distribution in the sampled older granites show that samples g25 and g18 are more highly evolved with lct geochemical affinities (cerny, 1991c) than g8, g9 and g27. both granites also have negative ce anomaly which may be an indication of oxidizing conditions during rare-metal mineralization (piper, 1974). the sampled afu complex younger granites g4 and g5 are depleted in ca, mg and sr; have high fe/mg, and are enriched in nb, y, f and zr, thereby showing the characteristics of the nyf suites (cerny, 1991c) compared with the older granites. they have lower asi (aluminum saturation indices) and more alkaline. in both the older and the younger granites, mg, ti, ba and zr are depleted in the granites with enhanced values of rb and therefore amply depict the degree of magmatic fractionation within the suites (figures 3 and 4). the granites with enhanced values of rb are also enriched in cs and the ore elements of sn-nb-ta. in the younger granites, g5 is more leucocratic and coarser grained with less biotite than g4 it also has more enhanced values of the lithophile rare elements like ta, nb, f, p, rb, mn, y,u, cs, th, mo, w, and low k/rb (104) and al/ga (2280) ratios. it however has a higher nb/ta ratio (4.65) when compared with that of 1.85 of g25. the younger granites have on the average, higher f content that the older granites while negative ce anomaly (very low ce content) is observed only in the mineralized older granites g25 and g18. kinnaird (1984) and barchelor (1987) have observed similarly distinct geochemical characteristics in the older and younger granites of nigeria. the ree concentration (fig. 5) of pegmatites 10p and 11p show a decrease of an order of magnitude from those of the granite g25 while the ree in the pegmatitic white mica are the least (see table 2). the ree generally have sub-horizontal to heavy rare earth element-depleted patterns; minerals with the lowest ree abundances have nearly horizontal patterns. ree contents of the granite, g25, which is highest in the samples analyzed, is very low (< 2x chondritic). such 53 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria barren 10p be li, be li, cs, be, ta cs 100001000100101 1 10 100 1000 k/rb figure 3. classification of the pegmatites using the plots of k/rb versus cs of their k-feldspars according to trueman and cerny (1982). rare-elements pegmatites’ class muscovite class cs 1000010000 k/rb 1000100010001000 10 100100 10001000 figure 4. plot of k/rb versus cs for the pegmatites’muscovites (after cerny and burt, 1984). gordiyenko (1971) beus (1968) 10000100001000100010010010 11 1010 100 1000 cs ta figure 5. plot of ta versus cs for the muscovites of nasarawa pegmatites. (after beus (1968), gordiyenko (1971)). 54 akintola, o.f. and adekeye, j.i.d. table 2: trace element contents of the albitic pegmatite phases sample albites cookeite phosphates fluorapatite amblygonites lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 p2o5 (%) 0.481 0.487 2.169 1.763 0.243 0.017 24.97 49.973 43.25 f (ppm) 1240 0 0 0 0 294 11637 13897 14203 as 0 0 7 8 6 1 0 0 0 ba 19 51 37 26 102 56 31 33 61 bi 14 11 11 16 12 12 38 1 8 ce 14 12 22 14 0 0 443 0 7 cd 7 bdl bdl bdl bdl bdl 27 bdl bdl co 115 47 25 31 37 5 16 8 6 cr 1 0 0 11 0 15 1 2 1 cs 851 23 19 74 9 49 1 28 6 cu 0 0 24 16 4 0 0 16 535 ga 76 24 21 22 39 58 14 21 18 hf 2 3 0 3 3 2 0 3 2 la 48 9 0 3 3 0 204 0 0 mo 0 5 1 1 0 4 0 1 0 nb 88 75 62 326 115 33 190 26 25 nd 3 11 9 8 2 0 144 5 7 ni 14 5 3 4 0 3 9 4 3 pb 25 10 7 0 0 0 74 9 0 pr 10 2 1 2 1 0 65 0 1 rb 4829 69 347 252 31 444 46 290 62 sc 0 0 9 0 4 0 0 0 5 sm 1 3 2 2 1 1 66 1 1 sn 565 659 14 174 35 54 28 231 67 sr 263 52 1037 305 51 8 64 11 188 ta 345 109 67 305 297 37 21 107 86 th 0 0 0 0 0 3 2 1 4 tl 23 bdl bdl bdl bdl 5 bdl bdl bdl u 0 3 0 10 0 1 216 0 7 v 5 1 4 15 6 5 16 4 8 low ree abundances (mostly between 20x and 1x chondritic) with sub-horizontal to heavy rare earth element-depleted patterns are typical of rare metal granites and associated pegmatites, (cerny, 1991c; raimbault et al, 1995; and morteani et al, 1995; preinfalk et al., 2000). despite the low ree abundances in the rocks/minerals, evidence of magmatic fractionation is given by the negative eu anomalies in the pegmatites and muscovites. g25 shows a rather horizontal ree pattern and slight inflections with minima corresponding to nd, gd, and ho, which indicate a fractionation reflecting the lanthanide tetrad effect (bau, 1996). the tetrad effect is more noticeable in white micas and the pegmatitic samples with some showing the v-shaped pattern with strong negative eu anomaly. according to zhao and cooper (1993), v-shaped patterns indicate an extensive crystal fractionation involving feldspar, biotite and accessory ree minerals such as monazite and zircon. the extremely negative euanomalies also correlate positively with the rare-element accumulation (matheis, 1991). the ree-depleted and rb-enriched nature of the g25 is also characteristic of peraluminous lct (enriched in rb, be, ga, sn, mn, li, cs, nb, and ta) granite intrusions (cerny, 1991c). petrogenesis of the rare metal pegmatites the peraluminous pegmatite granites parental to the rare-metal pegmatites were formed by partial melting of mica-rich metasediments along the regional fracture zones as enunciated by wright (1970), matheis (1991) and garba (2002). although, the anatexis of the metasediments occurred at deeper levels below the currently exposed surface, evidence of shearing of the rocks at the earth’s surface is provided by 55 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria sample albites cookeite phosphates fluorapatite amblygonites lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 w 502 346 201 247 298 64 46 89 124 y 6 1 0 0 1 0 1391 1 0 zn 229 63 26 187 12 39 62 26 119 zr 17 12 17 68 18 5 17 54 7 h2o 2.42 0.43 1.1 0.82 0.36 8.8 0.51 5.27 7.54 sum 98.71 98.24 97.67 99.4 99.79 98.01 98.12 102.94 86.53 k 33457 1577 11540 3321 1079 9132 1494 3487 664 k/rb 7 23 33 13 35 21 32 12 11 mg(hx) 13 139 7 23 li(hx) 225 685 16400 20750 li(fusion) 227 2900 13366 17882 na/k 1.17 43.74 5.81 19.80 72.80 <1 21.80 4.23 n.d. nb/ta 0.26 0.69 0.93 1.07 0.39 0.89 9.05 0.24 0.29 continuación tabla 2 56 akintola, o.f. and adekeye, j.i.d. t ab le 3: t ra ce el em en ts in th e pe gm at it e m ic as s a m p le u n it le 7 le 8 le 1 4 le 1 3 le 1 8 l4 l1 0 b l8 a l9 a lz lu a ls 7 l c1 9 lc 2 0 lc 2 0 a s1 k k a lc 2 3 lc 2 8 w w 2 r lc 3 0 lc 3 1 lc 3 2 lc 3 3 f p p m 2 9 3 3 3 1 9 7 3 2 1 5 3 2 2 0 2 9 7 0 2 7 4 5 3 2 3 3 3 1 3 1 2 8 8 4 4 3 5 1 2 4 6 5 2 1 0 6 6 1 7 1 0 0 4 3 9 0 3 9 3 3 2 9 6 6 7 4 6 1 2 8 4 1 2 2 5 4 2 6 1 6 2 9 6 5 4 7 5 1 9 4 0 1 0 8 8 8 8 1 3 7 4 b a p p m 1 5 6 0 3 1 2 7 2 4 1 2 3 0 3 5 2 0 3 1 2 3 5 3 7 6 9 3 5 1 9 1 3 2 8 1 5 4 2 2 5 1 0 0 7 4 3 3 1 6 2 3 4 4 1 8 b i p p m 1 7 2 1 2 0 1 6 2 1 2 5 1 6 1 9 1 2 1 9 1 3 1 4 1 2 1 5 2 8 3 4 1 4 2 1 1 7 1 8 2 0 1 1 1 8 1 5 1 3 1 2 1 3 1 7 c e p p m 1 2 2 9 5 2 0 2 6 1 3 0 1 1 0 0 0 0 9 3 2 3 9 3 7 1 8 2 6 1 4 1 2 9 0 2 7 0 2 2 0 2 3 c s p p m 1 7 1 8 7 4 2 9 0 3 0 0 2 6 0 8 7 4 1 0 0 8 1 0 0 1 2 3 2 8 9 6 2 0 6 3 2 1 1 8 3 4 2 2 1 2 0 2 4 6 7 4 2 4 5 1 5 6 9 4 5 1 6 9 4 2 1 5 1 7 4 9 2 9 1 4 9 9 6 3 1 1 6 3 4 6 g a p p m 1 6 7 1 6 3 1 6 3 1 5 3 1 6 7 1 6 8 1 5 8 1 6 4 1 6 2 1 7 4 1 5 5 2 0 3 2 0 0 1 6 6 1 5 6 1 6 4 1 5 9 1 4 1 1 7 6 1 7 4 1 9 7 1 5 4 1 4 5 1 6 2 9 7 1 4 7 1 6 1 1 3 8 la p p m 0 5 7 1 5 2 0 2 3 5 4 4 4 4 5 1 0 3 6 9 0 0 4 7 9 8 1 0 8 1 7 1 4 3 6 0 4 0 3 2 0 4 9 7 1 0 2 1 5 n b p p m 1 8 1 1 1 6 1 7 8 1 6 5 1 8 2 1 3 1 1 1 9 1 1 3 1 8 5 1 4 4 1 7 8 2 1 8 2 5 6 1 8 7 7 5 5 5 1 4 6 1 5 1 1 6 0 2 1 1 1 4 3 1 9 3 2 0 0 1 2 7 6 4 2 2 3 1 9 0 1 3 5 n d p p m 9 1 7 4 1 4 1 0 6 0 7 2 0 2 0 1 9 1 2 7 7 1 3 2 1 7 0 1 1 0 9 4 5 7 n i p p m 7 2 7 1 7 1 8 2 1 2 3 2 4 2 4 2 1 2 3 1 3 8 9 1 8 2 7 3 1 1 8 2 1 1 6 1 1 2 0 1 0 6 1 7 1 1 1 6 1 8 2 4 p b p p m 1 8 4 7 2 3 2 1 3 0 4 6 4 5 4 5 1 9 4 4 1 7 6 4 2 5 4 7 6 4 1 8 2 6 2 2 7 3 3 1 0 9 3 3 2 7 9 1 8 1 9 p r p p m 3 1 5 6 8 8 1 4 1 2 1 2 6 1 1 6 1 3 1 1 1 9 2 2 6 6 9 2 1 1 7 3 1 1 1 3 2 4 5 r b p p m 4 8 0 3 9 4 1 0 5 0 9 3 5 5 7 8 5 8 3 9 9 1 3 9 9 1 5 0 8 8 0 3 5 2 8 9 8 8 7 3 4 7 5 1 3 1 3 3 2 6 5 9 5 6 3 8 7 7 7 4 1 0 1 8 2 4 5 0 4 4 9 4 1 5 5 2 7 3 4 7 1 7 7 4 9 3 2 8 4 2 8 7 0 6 8 0 3 4 5 8 1 3 6 4 5 4 3 2 4 4 2 4 8 s n p p m 2 1 7 4 7 1 2 7 5 3 3 6 3 5 7 5 2 5 5 3 3 5 9 7 2 7 4 6 8 1 2 3 9 6 1 2 6 6 3 9 7 6 8 1 6 4 9 2 9 5 3 6 4 3 9 4 8 7 4 3 7 1 3 9 1 1 8 9 0 2 5 3 9 1 4 7 2 7 1 2 1 9 s r p p m 1 5 5 0 4 1 1 8 1 9 2 4 2 7 2 5 1 7 2 4 1 6 1 1 1 3 1 8 2 5 3 2 1 6 1 8 1 9 1 1 2 3 1 5 1 2 2 1 1 5 1 7 1 7 1 6 t a p p m 4 4 5 8 4 5 5 0 6 3 5 3 5 9 7 1 7 1 7 1 4 1 2 7 4 6 7 2 5 0 2 4 2 5 7 5 8 5 1 1 5 3 1 5 1 6 2 6 4 1 2 0 1 8 3 3 9 3 1 1 4 0 t l p p m 2 2 4 3 2 4 2 7 2 6 4 0 3 9 3 9 2 5 4 0 2 1 1 7 1 5 3 1 5 2 6 4 2 4 2 9 2 8 1 6 3 5 1 8 1 7 3 6 3 3 1 8 2 1 2 5 w p p m 7 1 5 1 6 5 7 1 2 3 7 3 9 3 3 2 8 3 8 7 7 2 5 8 4 6 5 4 0 8 6 8 9 3 4 3 6 4 8 5 6 5 2 4 0 4 9 9 1 5 6 1 1 4 3 2 6 1 z n p p m 4 2 1 1 0 0 0 4 5 2 4 3 5 4 7 2 9 6 1 1 0 2 3 9 1 1 4 1 6 8 0 0 3 7 9 2 3 1 1 3 1 2 1 7 1 1 1 1 2 3 1 4 2 2 4 2 1 4 2 1 8 7 3 4 1 2 4 9 1 6 3 4 5 3 4 7 1 6 1 1 7 7 1 1 2 mylonitization of the rocks within the fracture zone. high heat flow and shear movement along the regional fractures might have contributed significantly to the heat for the partial melting of the metasediments. an evolution of the magma as it ascends through the fractures would be toward an increase of the depolymerizing elements f, p and al (raimbault et al., 1995). these elements would depress the liquidus of the magma, thereby reducing the viscosity of the melt while aiding both its flow along the fractures and the extreme fractionation of the elements. roofward enrichment of rare elements (such as b, li, rb, cs, ti, be, mn, sc, y, h, rees, sn, th, mo, ta>nb, and w) can be expected ( cerny, 1991c) in the lct granite-pegmatite suite. the magmatic evolution of the melts is towards an increase in the al, rb and cs as documented in granites and pegmatites in the area while evolution from silicate-dominated melt to water-dominated b, f, li and p-rich fluids is marked: petrologically by common accessory apatites and zoned tourmalines in the pegmatites’ exocontact host schists, as well as deuteric alteration, sericitisation and albitization of feldspars in the granitic rocks parental to the mineralized pegmatites. the most complex of the pegmatites in this area belong to the amblygonite subtype of the classification of cerny (1991b) enriched in p, f, li, rb, cs, be, ta>nb. geochemically by low mg, ti, ba, zr and ce, as well as low ba/rb, low k/rb, nb/ta and k/cs ratios in the pegmatites and the related granites as well as non-chondritic y/ho and zr/hf ratios, high negative eu anomaly and lanthanide tetrad effect in the ree distribution patterns as documented in the pegmatitic granite g25. as already noted by bau (1996) the rare elements are transported as complexes in such fluid-rich melts. it is also clear from this area that there are two distinct rare metal generating events associated with the pan african orogeny viz: enrichment of the rare metals in the fluid-rich and deuterically altered main phase pan african granite g18 and the spatially/genetically related pagmatites. this granite is the northernmost extension of older granites in this area and obviously represents marginal part of the batholith. it is characterized by a widespread strong alteration of plagioclase (a replacement of the plagioclase by perthite, and finally albitisation/sericitization). this deuteric alteration resulted from the late metasomatic fluids that mobilized the ore elements, sn and nb, and concentrated them in the granite. such deuteric alteration is marked in the border zones and in the uppermost parts of granitic bodies (pedrosa and siga, 1987). enhancement of the rare metals in the geochemically distinct g25 is fracture controlled and postdates the emplacement of the main phase pan african granites. the fact that the pegmatitic granite g25 was emplaced into fractures and again mylonitized after emplacement (see also kuster, 1990), shows that these fractures were active before and after the emplacement and were probably reactivated during the emplacement of the younger granites during the jurassic. matheis and caen vachettee (1983) have documented biotite ages of 185/183ma from southwestern nigeria. basalt intrusions of 165ma north of zaria in northwestern nigeria, which as inferred by matheis and caen vachette (0p cit) indicate a regionally more extensive thermal event in association with the central nigeria younger granites emplacement. the low p and f contents of the pegmatitic granite g25 (p2o5 0.068%, f35ppm table 1) may be explained in that the elements which would have been concentrated at the roof of the granite, would at the current level of exposure of the granite have been lost to erosion. it appears, based on the low elevation of the g25 and the outcrop of ne-sw and nw-se trending tourmalinites, which are most probably related to the pegmatitic granite intrusion in the area that some of the granites parental to the rare metal pegmatites are not yet exposed at the current erosional level (that is they are still lying buried). similar views were very recently expressed by garba (2002) who inferred from the studies of gold and rare-metal pegmatite occurrences in the kushaka schist belt of north-western nigeria that the mineralizations are controlled by, postdates the pan-african tectonism and related to ne-sw and nne-ssw trending regional fractures. it is worth noting that pan-african sn-w bearing quartz-veins occur in the reactivated crust of the central hoggar and 57 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria are probably related to a 521ma old peraluminous differentiated granitic plutons. helba et al. (1996) also reported higher ta/nb ratios in the more differentiated albitized eastern part of nuweibi albite granite, in the eastern desert region of egypt. 207pb/206pb ratios in zircon from the granite yielded 450 – 600ma-a post-kinematic pan-african age. discussion cerny (1991c) and douce (1999) have observed that anatexis of mica-rich supracrustal sequences as well as ortho and para-lithologies of their basement in both the classical orogenic cycles and non-orogenic magmatic events give rise to peraluminous granites. these observations are corroborated by high intial 87sr / 86sr ratios (0.710 to 723) of the pan african older granites as well as the post-kinematic intrusives i.e. the pan african older granites (matheis and caen-vachette, 1983; bertrand et al., 1987), which strongly suggest crustal influence in the generation of the late pan african peraluminous granites. thus the older granites share some geochemical affinities with the lct suites while the younger granites share geochemical affinities with the ayf suites as recognized by cerny (1991c.) kuster (1990) observed that the late pan african tectonic granites at wamba (about 100km northeast of nasarawa) are all subalkaline, peraluminous, and highly siliceous rocks with their peraluminosity more pronounced with increasing differentiation. the major elements si, al, k, and na show only slight variations; only na is enhanced toward the end of granite evolution. in the course of evolution from the biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is a pronounced enrichment of rb, li, cs, sn, nb, mn, and p whereas b is only slightly enhanced. strong depletion is evident for ba, sr, zr, y, la, and ce together with ti, mg, ca, and fe. these results support the observation that the rare-metals are related to highly differentiated granitic magmas and represent strongly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals, (cerny, 1991b and london, 1990). conclusions it is conclude that anatexis of mica-rich supracrustal sequences as well as ortho and paralithogies of their basement give rise to peraluminous granites. these observation are corroborated by high initial 83 sr /86sr, ratios (0.710 to 723) of the pan african older granitesas well as the younger granites which strongly suggest crustal influence in the generation of the late pan african peraluminous granites. thus the older granites share some geochemical affinities with the lct suite while the younger granites share geochemical affinities with the ayf suites. in the course of evolution from the biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is pronounced enrichment of rb, li, sc, san, nb, mn and p whereas b is only slightly enhanced. strong depletion is evident for ba, sr, zr, y, la, and ce together with ti, mg, ca, and fe. these results support the observation that the rare metals are related to highly differentiated granitic magmas and represent strongly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals. the ne-sw and nne-ssw regional fractures controlling the mineralization are deep seated. reactivation of the fractures in the mesozoic probably influence the emplacement of the anorogenic younger granites and initiated the formation of benue trough in the mesozoic period. the benue trough, which is parallel to the ne-sw trending belt of mineralized pegmatites hosts pb-zn-cu-fe sulphides, fluorites and barites. the afu complex younger granites are more alkaline than the other granites as reflected in their lower a/n+k ratios; they have high fe/mg ratios and low tio2 contents which tend to agree with lameyre and bowden’s (1982) documentation of the younger granites of nigeria as continental epeirogenic uplift granitoids (ceug). peraluminous granites are known, according to cerny (1991c) to be parental to the granite-pegmatite suites. trace element studies of the 58 akintola, o.f. and adekeye, j.i.d. suites also show that extreme igneous fractionation aided by the fluids rich in b, p and f leads to the concentration of the rare metals in the residual melts that form the pegmatites. acknowledgements financial assistance for the fieldwork by the nigerian government through the raw materials research and development council, rmrdc is gratefully acknowledged. the german government through the german academic exchange programme (daad) provided financial assistance for the rocks/minerals chemical analyses in the technical university of berlin (tub). we are grateful to dr. g. matheis of the technical university of berlin (tub) for his assistance on the research and review of this paper. we are also grateful for the assistance of professor peter moller and dr. peter dulski of the geoforshungs zentrum (gfz), potsdam for their assistance in analyzing the rare earth and trace elements of some of the samples. references ajibade, a.c. and wright, j.b., (1989). the togo-benin-nigeria shield:evidence of crustal aggregation in the pan african belt. tectonophysics 165, 125-129 ball e., (1980). an example of very consistent brittle deformation over a wide intra-continental area: the late pan-african fracture system of the tuareg and nigerian shield. tectonophysics 61, 363-379. barchelor r.a. 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(1982): exploration for rare-element granitic pegmatites. in cerny, p. ed., granitic pegmatite science and industry. mineralogical association of canada, short course handbook, 8: 463 494 wright, j.b., (1970). controls of mineralization in the older and younger tin fields of nigeria. econ. geol. 65, 945-951 zhao, j. x. and cooper, j.a., (1993). fractionation of monazite in the development of v-shaped ree pattern in leucogranite systems; evidence from a muscovite leucogranite body in central australia. lithos 30, pp. 23-32. 61 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria letter from editor a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombiacarlos alberto ríos reyes, oscar mauricio castellanos alarcón and akira takasu 7 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombialorena rayo and carlos a. zuluaga 31 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeriaakintola, o.f. and adekeye, j.i.d. 44 denoising resistivity phosphate “disturbances” using haar mother wavelettransform (sidi chennane, morocco)saad bakkali and mahacine amrani 62 petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”ríos, c.a., castellanos, o.m., gómez s.i. and ávila, g.a. 72 seismic velocity determination in gravel and sands using piezocrystalsarmando luis imhof and juan carlos santamarina 107 geofisica colombiana n° 6 pp. 17-23 diciembre de 2002 bogota, d.c. issn 0121 2974 cambios temporales del valor de b: ,posibles correlaciones con la actividad hidrotermal del volcan tungurahua? c. indira molina p., mario c. ruiz instituto geofisico escuela po l it e c n ic a nacional quito-ecuador. mail: cimp@tera.com.co resumen ei volcan tungurahua, localizado en la cordillera real del ecuador, se encuentra desde septiembre de 1999 en estado eruptivo. mediante el rnetodo de maxima verosimilitud con ventanas de 100 eventos sobrelapadas el 50%, se calcul6 el valor de b para eventos volcano-tectonicos, y se encontraron cinco picos maxim os: de septiembre a octubre de 1995: b = 1.295 (c = 0.081); de junio a diciembre de 1996: b = 1.399 (o = 0.084); de enero a febrero de 1999: b = 1.370 (c = 0.083); de marzo a mayo de 1999: b = 1.427 (o = 0.084), y de junio ajulio de 1999: b = 2.120 (c = 0.104). parece que los picos de b se encuentran antecedidos por sallos en la curva de energia trem6rica, 10 cual sugiere una posible correlaci6n. durante el perfodo estudiado, la evoluci6n espectral del tremor volcanico muestra que existen tres grupos predominantes de frecuencias: (i) alrededor de 0.5 hz, (2) alrededor de 2 hz y (3) alrededor de 5 hz. ei grupo (2) es mas comun en el tiempo y presento una buena correlacion con las precipitaciones. en cambio, los grupos (i) y (3) aparecen antes del primer pico de b en 1999. por tanto, el incremento del valor de b en aquella epoca podria ser indicativo de un mayor tlujo de calor, 10 cual fue evidenciado en una actividad fumarolica mayor y en el inicio de la actividad eruptiva del volcan, palabras clave: volcanes, valor de b, tremor, frecuencia, ef'lergia, y sistema hiorotermal abstract tungurahua volcano, located in the cordillera real of ecuador, has been in mild eruptive activity from september 1999 to the present. using the maximum verosimilitude method with windows of i 00 events overlapped by 50%, the b-value was calculated for the volcano-tectonic events. five peaks of maximum b values were recognized: from september to october/1995 b = 1.295 (c = 0.081), from june to decemberl1996 b = 1.399 (c = 0.084), from january to february/1999 b = 1.370 (c = 0.083), .... from march to mayl1999 b = 1.427 (c = 0.084), and from june to julyl1999 b = 2.120 (o = 0.1 04). it appears that peaks ofb at tungurahua are preceded by jumps in the tremoric energy curve: suggesting a possible correlation. during the same study period, the spectral evolution in the volcanic tremor shows that three predominate groups of frequencies exist: (i) around 0.5 hz, (2) around 2 hz y (3) around 5 hz. group (2) was most common during the time period and presented a good correlation with the precipitation regime. in contrast, groups (i) and (3) appear before the occurrence ofthe first peak of bin 1999. therefore, the increase in the b value recorded in that epoch could be indicative ofa larger heat flow, a situation which was evidenced through greater fumarolic activity and the beginning of eruptive activity in the volcano. key words: volcanoes, b-value, tremor, freouency, energy and hydrothermal system log., n (m)=abm (i) tante que depende del periodo tomado y b es la pendiente de la relaci6n denominada valor de b. mogi (1962) y vinogradov (1959,1962) estudiaron experimentalmente la relaci6n frecuencia-magnitud de los eventos de microfractura en roea. un resultado sorprendente fue que esta relaci6n es igual a la dada por la relaci6n de gutenberg y richter en la ecuaci6n (i). en esta relaci6n, el valor de b es la pendiente de la recta de mejor ajuste entre ellogaritmo del numero de eventos observados a una magnitud dada, y la magnitud. este valor es inversamente prointroduccion ishimoto elida (1939)? gutenberg y richter (1944, 1(49) propusieron la relaci6n empirica entre la frecuencia de ocurrencia y la magnitud de los sismos en una ecuaci6n logaritmica de la forma: donde m es magnitud, n(m) es el nurnero de sismos de magnitud > m que ocurren en una regi6n durante cierto periodo; a es una consgeofislca cdlombiana, 6. diciembre de 20021 17 molina p. et al. porcional a la magnitud media, por 10 que diferencias en el valor de b retlejan diferentes tamafios de grietas promedio que generan sismos (power et al., 1998). algunos valores de b reportados en el mundo los valores de b generalmente estill cercanos a i en muchos lugares de la corteza terrestre (fronlich y davis, 1994). sin embargo, en varias regiones volcanicas, el valor de b suele ser mucho mas alto, algunas veces muy cercano a 2. particularmente para sismos volcano-tectonicos tipo a, el valor de b es alrededor de 1.5. mediante el estudio experimental en muestras de roca sometidas a cargas mecanicas, se ha1l6 que 10 val ores de b son normalmente cercanos a i (mogi, 1962, scholz, 1968). por otra parte, en pruebas de laboratorio consistentes en aplicar amplios gradientes termicos sobre muestras de roca no confinadas, se encontr6 que los valores de b variaron entre i y 3 (warren y latham, 1970). para enjambres sismicos, el valor de b suele ser bien distinto de 1 y pueden alcanzar valores altos, cerca de 2.5 (mogi, 1962). scholz (1968) encuentra que, para enjambres sismicos, el valor de b suele ser bien distinto de i y puede alcanzar valores altos (cercanos a 2.5). esto se debe a que, en este tipo de fen6meno, no existe un sismo principal identificable al cual se encuentre asociada la secuencia de replicas y, por el contrario, el conjunto esta formado por una serie de eventos de tamaiio semejante. en el volcan off-ito se encontr6 que las regiones que presentaban los valores mas altos de b retlejaban zonas muy fracturadas alrededor de las carnaras magmaticas, mientras que los val ores de b "norm ales" (es decir menores que i) se encontraron en areas circundantes (wyss, et al., 1997). en la caldera de long valley, los valores de b variaron entre 0.6 y 2. los valores mas altos se registraron cerca del domo resurgente (wiemer, et al., 1998). en el volcan soufriere hills, el valor de b vari6 entre 0.5 y 1.5; dichos valores tan altos fueron explicados como el resultado de un incremento en la heterogeneidad, la temperatura y las condiciones de esfuerzo en el medio, mientras que los val ores mas bajos de b estan dados probablemente por un medio menos fracturado y con menos alteraci6n termica. igualmente, las areas que presentan rocas mas competentes podrian soportar sismos mas grandes, 10 cualse retleja en valores de b mas bajos (power et al., 1998). factores que influyen en el valor de b entre los factores que pueden alterar el valor de b se encuentra el cambio en la heterogeneidad del material, es decir, que el valor de b aumenta proporcionalmente con la heterogeneidad y densidad de grietas en la roca. ademas, un esfuerzo no uniforme tam bien da lugar a val ores mas altos de b (mogi, 1962). un incremento en el esfuerzo de cizalla aplicado (scholz, 1968; urbancic et al, 1992) 0 un incremento en el esfuerzo efectivo disminuye el valor de b. warren y latham (1970) sugirieron que el fracturamiento producido por es18 fuerzo termico da como resultado valores de b muy altos y una gran proporci6n de eventos pequeiios. datos como bien se sabe, un catalogo contiene informaci6n de periodos irregulares de observaci6n; en consecuencia se presentan desviaciones importantes en la zona lineal de la regresi6n gutenberg-richter (weichert, 1980; bender, 1983). para minimizar el valor de b se calcula mediante datos obtenidos de la base de datos a partir del sistema anal6gico, ya que los registros digitales son incompletos porque los eventos muy pequeiios (probablemente de magnitudes menores de 1.7) no se graban por las condiciones de detecci6n del sistema de adquisici6n. asi, el valor de b fue calculado con 2153 eventos clasificados como volcano-tect6nicos. las magnitudes fueron determinadas por medidas de codas de los eventos en la estaci6n de mson (3.2 km de distancia desde el crater), usando la expresi6n: (2) donde a, ~y 8 son constantes determinadas por consideraci6n de varios sismos referencia de magnitudes conocidas (lee et al., 1972, encontraron a = -0.87, ~ = 2 y 8 = 0.0035); t es la duraci6n del evento en una estaci6n y l1es la distancia focal del sismo a la estaci6n. metodo y resultados existen dos metodos para calcular las constantes a y b de la ecuaci6n (i): minimos cuadrados y maxima verosimilitud (mogi, 1962; aki, 1965; utzu, 1974; weichert, 1980). mediante el metodo de minimos cuadrados se calcul6 la recta de mejor ajuste a partir de la dispersi6n de datos que resulta de graficar log(n) vs. m, cuya pendiente b seria el valor de b y la intersecci6n con el eje m, la constante a 0 magnitud maxima esperada (rnmaj. asimismo, el valor de b se calcul6 mediante el metodo de maxima verosimilitud que, a su vez, utiliza el rnetodo iterativo de newton-raphson (press, 1986) que tiene en euenta la magnitud media regional esperada e(rn) deserita por bender (1983) y el valor de b ealeulado por aki (1965). ai ealeular b para todo el catalogo, se tomaron dos valores supuestos de magnitud de minima homogeneidad (rna); los resultados muestran una difereneia muy pequefia para el mismo metodo (figura 1). si!1 embargo, los valores de b ealculados muestran, de un metodo a otro, una difereneia del 48% para rna = 1.65 y del 62% para rna = 1.75. en la figura i, se puede observar que,. sobre una magnitud de 3.4, la relaei6n de gutenberg-richter no es lineal, 10 eual puede deberse a un catalogo ineompleto 0 a que esta ley no es totalmente aplicable en ambientes volcanicos, ei metodo de maxima verosimilitud es una tecnica de mayor solidez que la de minimos euadrados y se aeerea mas al valor real de b cuando el numero de sismos grandes es variable (sha y bolt, 1982). ei metodo de minimos euadrados ajusta los val ores a una recta en geofislca colombiana, 6, diciembre de 2002 cambios temporales del valor de b: iposibles correlaciones con la actividad hidrotermal del volcan tungurahua! 40 -,-------------------------, 2.5 • 3.5 jmots i •••• +,+ ••• -i•• ... ..-~-_. •30 • • • • • 1.5 • • 1.0 metoda de minimos cuadrados mcf165 b=1.59+/0078 mo=1.75 b=1.63+/o,mu 0.5 metodo de maxima verosimilitud mo=165 b-ll l +/0016 mo=j.75 b=i.ol +10,015 0.0 -f==~~~~~~~~---+--_+--_+_-......---4 o 0.5 15 2 25 magnitud (md 3.5 figura 1. distribucion de gutenberg-richter para eventos volcano-tectonicos. observese que se tomaron dos valores de mn, con sus correspondientes valores de b obtenidos mediante los dos metodos. funcion de la densidad de datos que existan en determinada zona (press et al, 1987), mientras que el metodo de maxima verosimilitud ajusta la recta al valor medio de los datos sobre la magnitud minima de homogeneidad, incluida la maxima magnitud observada (weichert, 1980; press et al, 1987) 10que a su vez normaliza el aporte que hacen los sismos de diferente magnitudes. estas diferencias perm iten explicar que ei valor de b que reflejaria mejor el estado de esfuerzos en el volcan tungurahua es ei hallado mediante el metodo de maxima verosimilitud con una rna de 1.65. se escoge esta rna porque la diferencia en los resultados de b, en ambos metodos, es mucho mas pequefia que la obtenida con una rna de 1.75. variacion temporal de b para calcular el valor de b en el tiempo, se tomaron ventanas de 100 eventos sobrelapadas ei 50% de manera consecutiva. ei promedio de las desviaciones estandar del calculo de b en las diferentes epocas por el metodo de minimos cuadrados es 0.121, mientras que por maxima verosimilitud es 0.075, por 10cual se toma este metodo como el que mas se ajusta a los datos. asi, para calcular las ventanas temporales de b se tuvo en cuenta una rna de 1.65 y una rnmin: de 3.8. de esta manera se obtuvieron 5 picos y 3 valles importantes en la "curva" temporal de b (figura 2). los cinco picos maxirnos importantes ocurrieron de septiembre a octubre de 1995: b = 1.295 (a= 0.081); dejunio a diciembre de 1996: b = 1.399 (a = 0.084); de enero a febrero de 1999; b = 1.370 (a = 0.083); de marzo a mayo de 1999 b = 1.427 (a = 0.084), ydejunio a julio de 1999: b = 2.120 (a = 0.104). los 3 valles fueron observados de enero a diciembre de 1994: b = 0.571 (a = 0.04), de diciembre de 1995 a junio de 1996: b = 0.833 (a = 0.061) y de noviembre de 1997 a agosto de 1998: b = 0.686 (a = 0.05) .• geofislca colombiana. 6. oiciembre de 2002 2.25 400 1.75 s-2 j 4 1 7\ ,ztn.... 1'\ /~ :t1¥ \~ / / ~ c j ..... """-~~_...a "~~ fr '0 350 300 i., 250 1.25 200 iso 0.75 100 0.' 0.25 1994 1995 ]9% 1997 1998 1999 fecha figura 2. variac ion temporal de b. con nurneros de i a 5 se sefialan los picos maximos mas importantes: septiernbre a octubre de 1995: b = 1.295 (= 0.081); de junio a diciembre de 1996: b ~ 1.399 ( ~ 0.084); de enero a febrero de 1999: b = 1.370 ( = 0.083); de marzo a mayo de 1999: b ~ 1.427 (~0.084); y de junio a julio de 1999: b = 2. j 20 (= 0.104). con letras de a a c se sefialan picos minimos mas irnportantes: de enero a diciembre de 1994: b = 0.571 (= 0.04); de diciembre de 1995 ajunio de 1996: b= 0.833 (= 0.061) y de noviembre de 1997 a agosto de 1998: b = 0.686 (= 0.05)(observado en la estacion de mson), entre las variables que podrianinf1uir en el calculo de b, se encuentran directamente relacionadas el calculo de la magnitud para cada evento y el numero de eventos de determinada magnitud. al indagar que variables influian en dicho calculo, se observo que la diferencia entre los tiempos de arribo de las ondas s y p ("s-p") jugaba un papel bien importante, de manera que un fuerte aumento en dicho parametro, en septiembre de 1996, podria haber afectado el resultado incremental de b. respecto a la duracion del evento, esta -parece ser parte influenciable en el calculo de b durante enero, abril y junio de 1999, de modo que un leve incremento de la duracion corresponde directamente con un aumento en el valor de b. en octubre de 1995 y julio de 1999, el incremento en el valor de b podria estar relacionado directamente con el incremento en el numero de sismos. correlacion entre cambios de b y la actividad tremorica debido a que no es claro definir cual de los factores causan un valor anomalo de b, es necesario buscar otros parametres que ayuden a desarrollar evidencias para conocer las causas de la anomalia. con el objeto de determinar suficiente numero de posibles correlaciones entre los distintos tipos de actividad sismica que se registran en ei volcan, se ploteo la curva de energia tremorica acumulada con la "curva" del valor de b (figura 3). los picos maximos de b se encuentran inmediatamente antecedidos por saltos importantes en la curva de energia del tremor volcanico, 10cual sugiere que existe 19 molina p. et al. 2.5e+1s,----------------------,2.5 2e+18f--------------------!1y 1,5e+18f---------~--------,.-'4ff1 ie+18+-------f--""""'.......,.'--..~=r---_i_h~="l-'--1 5e+17t=======~'-----b---_.j::::=-~----~ o l--~ --' 1994 1997 1998 19991995 1996 figura 3. variaci6n temporal del valor de b y de la energia trem6rica acumulada. las flechas sefialan los saltos en la curva de energia acumulada del tremor (observado en la estaci6n de mson). correlacion entre la actividad sismica y la tremorica. unicarnente dos saltos en la curva de energia acumulada del tremor en julio de 1997 (flecha d') y mayo de 1998 (flecha e ') no anteceden de manera inmediata un pico maximo de b. probablemente estos saltos son consecuencia del segundo pica de b (ocurrido entre julio y diciembre de 1996) 0 tal vez, sigan precediendo el tercer pico de b (ocurrido entre enero y febrero de 1999). lgualmente, hay una excepcion con el pico 4, el cual coincide con el mayor saito en la curva de energia tremorica acumulada, comprendida entre los meses de abril y mayo de 1999 (flecha g '), por otro lado, las frecuencias dominantes del tremor volcanico consideradas como altas (alrededor de 5 hz) 0 bajas (alrededor de 0.5 hz) se acentuan de manera importante cuando el valor de b-disminuye entre 1997 y 1998 (valle c) precediendo con gran anticipacion los picos de b (3, 4 y 5) en 1999 (figura 4). estos hechos podrian indicar, en la epoca del valle de b, una acumulacion importante de esfuerzos que justamente estaria precediendo la fracturacion de un medio en el que el calor y la densidad de grietas sedan la principal causa de los altos valores de b observados en 1999. probablemente, debido a dichos hechos, desde mayo de 1999, nota un mayor flujo de calor 0 esfuerzo termal evidenciado por alta actividad fumar6lica, incremento en el olor y concentracion de azufre en julio de 1999, y el comienzo de la actividad eruptiva del volcan desde septiembre de 1999 con importantes caidas de ceniza en los alrededores. correlacion con cambios geoquimicos en el tungurahua hay cuatro fuentes termales monitoreadas desde 1994. dichas fuentes se encuentran localizadas en los flancos ne y so del volcan, entre 3.3 y 9.3 km de la cumbre. en dichas fuentes se 20 12 2.5 10 ··2 0 e 8~ ~ 15 0 6 ~ ! ~ i , ~ 0.5 15 u b; 0.5 1996 1<;)8 199919971994 1995 pecha figura 4. variaci6n temporal del valor de b y la frecuencia dominante diaria del tremor (observado en la estaci6n de mson). han medido distintos parametres geoquimicos y fisicos que pueden ser interpretados como inform adores potencialmente utiles de la actividad magmatica, si las condiciones 10 permiten. es decir, que mientras haya ascenso de magma, los cam bios en las condiciones subterraneas podrian ser reflejados en las composiciones de los gases y compuestos quimicos disueltos y emitidos por las fuentes termales y furnarolicas del volcano mediante el metodo analitico, espectroscopia de absorcion atomica, cromatografia de gases y potenoiometria, entre otros, se estudian las especies quimicas de sodio (na), potasio (k), calcio (ca), magnesio (mg), silice (si02), bicarbonatos (hc03), c1oro (ci) y sulfatos (s04)' adicionalmente se toman datos del ph, temperatura y conductividad de las fuentes termales del volcan tungurahua. en todos estos parametros geoquimicos se notaron po cos pero importantes cam bios respecto a la media de datos, que podrian estar rei acionados con aportes magmaticos en las distintas epocas del muestreo. en la tabla i se muestran los cam bios geoquimicos observados de los muestreos colectados entre 1994 y 1999, que sobrepas an 0 igualan el resultado de sumar la media y el 25% de dicha media de muestreo por cada especie quimica en por 10 menos dos estaciones simultaneamente. tambien se muestran los cam bios que sobrepasan 0 igualan el resultado de sumar la media y el 2% de dicha media en el caso de la temperatura, 0 el 10% en el caso de la conductividad 0 el ph. las concentraciones de silice, sodio, potasio, magnesio y bicarbonatos en las fuentes termales crecieron y decrecieron simultaneamente con los cambios temporales del valor de b (figura 5). solo en dos casos excepcionales, el ph y la conductividad parecen haber seguido incrementandose pese a los descensos del valor de b. la correspondencia entre el incremento de las concentraciones geoquimicas y el valor b estarta de acuerdo con la hipotesis de que geofislca colombiana. 6. diciembre de 2002 cambios temporales del valor de b, iposibles correlaciones con la actlvidad hidrotermal del volcan tungurahua? tabla l. fechas y fuentes termales que registraron los cambios mas importantes oe los diferentes patrones geoquimicos. en negrilla se senalan las fechas comunes en que se presentaron los cam bios para las d1stintas especies quimicas y todas las' entes termales que registraron los cam bios geoquimicos simultaneamente fuentes termales especie fecha en i.que se registro que registraron un cambio quimica el cambio positivo (incremento) sodio 30-0ic.-97 1,3,4 22-0ic.-98 1,2,3,4 potasio 10-ago.-98 1,2,3,4 calcio 24-feb.-94 1,2,3 06-jun.-95 1,2,3 23-sep.-97 1,3,4 magnesio 30-dic.-97 3,4 27-may.-98 1,2,4 silice 19-0ct.-95 1,2,3 24-sep.-99 3,4 bicarbonatos sulfatos 10-nov.-94 otros parametres (ftsicos) temperatura 17-abr.-96 1,2,3 o i-jul.-97 y 22-abr.-97 1,2 23-sep.-97 y 30-0ic.-97 1,2,4 17··mar.-98 y 28-may.-98 1,2,3,4 10-ago.-98 1,2,4 18-ago.-99 1,2 conductividad 01-jul.-97,22-abr.-97 1,3 30-dic.-97, 3 i -ago.-99 1,3 15-sep.-99,27-0ci.-99 1,3 ph(*) 26-may.-94,27-mar.-95 1,2,3 06-jun.-96, 17-abr.-96 1,2,3 o i-jul.-97, 22-abr.-97 1,2,3 " 22-dic.-98, 18-ago.-99 1,2,3,4 15-sep.-99 1,2,3,4 1 el salado; 2, palitagua; 3 la virgen; 4 santana. * como se registraron muchos cam bios en dos estaciones, unicarnente se muestran en mas de dos estaciones. cuando se determinan valores de b mas altos,implicitamente estaria reflejando un medio mas fracturado y mas caliente, 10cual facilitaria que losfluidos magmaticos puedan viajar mas facilmente a traves del volcan y ser portadores de manera significativa de los incrementos en las concentraciones de los aniones y cationes medidos en las fuentes termales. en contraste, la baja correspondencia entre las vageofislca colombiana, 6, oiciembre de 2002 riaciones del ph y la conductividad con el valor de b podria ser el resultado de la baja confiabilidad de las mediciones 0 ser indicativo que el efecto de la excitacion magmatica subsiste despues de un corto periodo de calma 0 intensa actividad. sin embargo, seria importante tener en cuenta los incrementos del ph en sus fases "iniciales" ya que tales incrementos podrian deberse a que cuando aumenta la temperatura cerca de los cuerpos magmaticos, el nivel de las aguas de las fuentes termales podria disminuir, de modo que el dioxide de carbono, el sulfuro de hidrogeno y otros gases se pierden durante el boiling. en este caso, el ph deberia incrementarse debido a la perdida de los gases acidos. discusion en defmitiva, desde marzo a abril de 1995 el valor de b viene aumentando hasta llegar a un pico maximo entre septiembre y octubre de 1995,10 cual coincide a su vez con un aumento en la frecuencia y energia del tremor, ademas del incremento en el numero de sismos volcano-tectonicos. sin embargo, vale la pena mencionar que la energia tremorica alcanza su maximo en abril de 1995. el descenso de b se hace paulatino desde octubre de 1995 hasta fines de marzo de 1996, coincidiendo con un descenso en la energia trernorica. luego, desde fines de marzo de 1996, el valor de b vuelve a aumentar hasta un pico maximo de junio a diciembre de 1996. antes de este pico maximo fue posible observar dos picos rnaximos en la energia trernorica uno en mayo y otro en julio de 1996. desde septiembre de 1996 el valor de b disminuye paulatinamente hasta agosto de 1998; sin embargo, se registran dos picos importantes en la energia tremorica en febrero y julio de 1997, y en enero de 1998, que podrian 0 no coincidir con la disminucion de b 0 incluso ser "causa" del siguiente ascenso en b desde marzo de 1998 hasta re8e+16r;:=======;--------------~2.5 silice sodic magnesio poteso bicarboneros conductividad ph 7e+16 ~ ~ 6e+16 .~ 5e+16 '5 e ~~ 4e+16 e~ ~§]e+16 . ~ .~ 2e+16 -·~---l ...... .....,-4;al tli l5 -c "-e 0 ;;; 05 ie+16 ~ fecha figura 5. correspondencia entre los cambios del valor de b y los cam bios geoquimicos registrados. las fiechas ascendentes indican incrementos; las fiechas descendentes indican disminucion del patron geoqufmico (estos cam bios no concuerdan con una escala de concentraciones). 21 molina p. et al. gistrar un pica maximo entre enero y febrero de 1999. despues de 10 anterior ocurre un pico maximo de energia tremorica a inicios de marzo de 1999, el cual coincide con un pico maximo de b registrado entre marzo y mayo de 1999. inmediatamente despues de este ultimo pica de b ocurren varios picas de energia tremorica antes de liegar al maximo pico de b determinado en este periodo de estudio. despues de este pico, aunque el valor de b disminuye un poco, la energia trernorica liega a su maximo valor registrado desde enero de 1994. es importante notar dos aspectos que se relacionan. a pesar de que el valor de b disminuye drasticamente hasta agosto de 1998 y despues de julio de 1999, la energia trernorica continua aumentando sin obedecer el patron que hasta el momento se registraba. ello puede ser indicio de un efecto de rea 'cion debido a los picos maximos de b registrados de junio a diciembre de 1996 y de junio a julio de 1999. en estas epocas aun no hay claridad acerca de estos dos fenomenos y cualquiera de ellos pod ria ser la causa 0 el efecto del otro. conclusion es los val ores de b (entre i y 2.5) obtenidos en el presente estudio son tan altos como los hallados experimentalmente por warren y latham (1970). como es de esperarse, estos valores de b son consistentes para la actividad sismica relacionada con volcanismo, en que podrian esperarse amplios gradientes termales. esto permite inferir que los esfuerzos internos en el volcan tungurahua podrian corresponder principalmente a efectos termales. dichos valores fueron importantes entre septiembre y octubre de 1995, junio a diciembre de 1996, enero a febrero de 1999, marzo a mayo de 1999 y j unio a julio de 1999. igualmente, un esfuerzo no uniforme y un medio mas fracturado en dichos period os podrian haber propiciado val ores de b tan altos, mientras que los valores de b mas bajos (entre 0.5 y i) ocurridos entre enero y diciembre de 1994, diciembre de 1995 y junio de 1996, noviembre de 1997 y agosto de 1998, podrian estar relacionados con un aumento del esfuerzo aplicado que, justamente, antecedio la formacion de una(s) fractura(s) rnacroscopicats) que tuvieron lugar en los, periodos en que se produjeron los valores mas altos de b durante 1999. debido a que carnbios en el estado de esfuerzos del volcan podrian facilitar variaciones en el comportamiento de los fluidos internos, se intento hacer correlaciones entre el valor de bylas actividades tremorica y geoquimica. se observo que las variaciones en la concentracion de bicarbonatos, sodio, magnesio y silice, medidas en las fuentes termales, coincidieron con cam bios en el valor de b. dicha correspondencia probablemente sefiale que, al existir mayor circulacion de tluidos magmaticos en el interior del edificio volcanico, podria ocurrir un incremento en la concentracion de los aniones y cationes mencionados anteriormente. se asume que dicha circulacion de tluidos seria importante cuando aumente la densidad de grietas en el medio y los esfuerzos dominantes sean de tipo termal principalmente. es decir, se 22 veria retlejado cuando se determinen valores de b muy altos (probablemente mayores que 1.5). en cuanto a la actividad tremorica, se observo que las frecuencias alrededor de 0.5 y 5 hz aparecen varios meses entre 1997 y i 998,justo antes del primer pico de b en 1999. esto, mas los altos valores de b registrados en 1999, podria reflejar el inicio de una actividad termica anormallo cual fue evidenciado por una alta actividad furnarolica en mayo de 1999, ei incremento del s02 en julio de 1999 y el inicio de la actividad erupt iva subsiguiente a traves de explosiones freato-rnagmaticas en octubre de 1999, y magmaticas desde noviembre de 1999. bibliografia aki, k. 1965. maximum likelihood estimate ofb in the formula log n= a-bm and its confidence limits. bull. earth. res. inst. 43: 237-239. tokyo university. bender,b. 1983. maximum likelihood estimation ofb values for magnitude grouped data bull. seism. soc. am. 73: 831-851. fronllch,c., and davis,s. 1994. teleseismic b values: or much ado about 1.0. j geophys. res. 98: 631-634. gutenberg,b., and richter,c.f. 1944. frequency of earthquakes in california. bull. seism. soc. am. 34: 185-188. gutenberg,b., and richter,c.f. 1949. seimicity of the earth. princeton university press. ishimoto,m., elida, k. 1939. observations of earthquakes registered with the microseismograph constructed recently. bull. earthq. res. int. 17: 443-478. lee, w.k.h.; bennett,r.e., and meagher,k.l. 1972. a method of stimating magnitude oflocal earthquakes from signal duration. geol. surv. open-jile rep. 28. mogl,k. 1962. study of elastic shocks caused by the fracture ofheterogeneous materials and its relations to earthquake phenomena. bull. earthquake res. inst. 40: 125. tokyo university. power,l.a., wyss, m., and latchman,l. 1998. spatial variation in the frequency-magnitude distribution of earthquakes at soufriere hills volcano, monserrat, west indies. j res. l. 25( 19): 3653-3656. press,w. h.; flannery,b. p.; teukolsky,s. a., and vetterling, w. t. 1986. numerical recipes. sha, y., and bolt, b.a. 1980. the standard error of the magnitud-frequency b value. bull. seism. soc. am. 72: 1677-1687. scholz,c.h. 1968. the frequency-magnitude relation of microfracturing in rock and its relation to earthquakes. bull. seism. soc. am. 58(1): 399-415. urbancic,t. i.; trlfu,c. i.; long,l. m., and toung,r.p. 1992. space-time correlations of b-values with stress releases. pageoph, (139): pp.449-462. utzu,t. 1974. a three-parameter formula for magnitude distribution of earthquakes. j phys. earth. 22: 71-85. geofislca colombiana, 6. oiciembre de 2002 cam bios temporales del valor de b ipos1bles correlaciones con la actividad hidrotermal del volcan tungurahun vinogradov, s.d. 1959. on the distribution of the number of fractures in dependence on the energy liberated by the destruction of rocks. bull (izv) acad. sci. ussr geophys. ser. 12, pp. 1850-1852. _____ .1962. experimental study of the distribution of the number of fractures in respect to the energy liberated in the destruction of rocks. bull. (izv) acad. sci. ussr geophys. pp. 171-180. warren, n., and latham, g. 1970. an experimental study of thermally induced microfracturing and its relation to volcanic seismicity. j geophys. res. 75(23): 4455-4464. weichert, d. 1980. estimation of earthquake recurrence parameters for unequal observations for different magnitudes. bull. seism. soc. am. 70: 1337-1346. wiemer, s.; mcnutt, s., and wyss, m. 1998. temporal and three-dimensional spatial analyses of the frequency-magnitude distribution near long-valley caldera, california. geophys. j int. 134: 409-421. wyss, m. 1973. towards a physical understanding of the earthquake frequency distribution. j r. astr. soc. 31: 341-359. wyss, m., shlmasakj, k., and wiemer, s. 1997. mapping active magma chambers by b-values beneath off ito volcano, japan. j geophys. res., in press. geofislca colombiana, 6. oiciembre de 2002 23 geoflsica colombiana pp. 35·41 bogota, d.c.n°.7 diciembre de 2003 issn 0121 2974 estudio y slmulaclon de las deformaciones corticales asociadas a la varlaclon del campo gravitacional, usando redes neuronales artificiales yaaueline figuereoo baez ramiro dial grupo de investigaci6n en geof1sica, universidad nacional de colombia, e-mail: ramdiaz@unal.edu,co resumen para este trabajo se utilizaron dos estrategias de procesamiento: la computaci6n neuronal y la computaci6n convencional. en la computaci6n convencional, la base rnatematica para la simulaci6n es la teoria mecanica del medio continuo, mediante la cual se obtiene un algoritmo evolutivo en funci6n de la estimaci6n de un tiempo evolutivo, que permite la simulaci6n de modelos que muestran las deformaciones de la corteza en terminos de variaciones geornetricas, de tal manera que estas deformaciones se pueden utilizar para el calculo de las anomalias gravitacional y magnetica a traves del rnetodo de talwani. la computaci6n neuronal asimila las neuronas biol6gicas como elementos de proceso, que en el problema de inversi6n se asumen como informaci6n gravimetrica e informaci6n magnetica, siendo procesadas en una red neuronal artificial con arquitectura hopfield y en una red neuronal artificial con estructura cluster. palabras clave: campo gravitacional, red neuronal, defdrmaci6n, cdrteza abstract this work is development with two strategies of processing: the neural computation and the conventional computation. in the conventional computation the mathematical base for the simulation is the theory mechanic of the continuum medium, by means of which an evolutionary algorithm is obtained in function of the evolutionary time estimated; that allows the simulation of models that showing the deformations ofthe crust in terms of geometric variations. these deformations can be used for the calculation of the gravity anomaly and magnetic anomaly through the method oftalwani. the neural computation assimilates the biological neurons as process elements that are assumed as the gravity information and the magnetic information in the inverse problem, being processed in a neural network artificial with architecture hopfield and in a neural network artificial with structure cluster. keywords: gravity field, neural network, stress, crustal introduccion ei desarrollo matematico en la computacion convencional esta basado en la teoria del medio continuo aplicada a un medio espacial homogeneo e isotropico estratificado, con comportamiento viscoso newtoniano, que genera un algoritmo de evolucion (para un sistema no comprimido y para un sistema comprimido horizontalmente), permitiendo la simulacion de las deformaciones de la corteza, asociadas tanto a la variacion del campo gravitacional como a la variacion del campo magnetico, utilizando para ella el metodo de talwani. manuscritorecibidaparaevaluactonel 30 demayode 2003. articulo aceptado parapubncaclon por el comttl!editorialel 30 de oclubrede 2003. gedflslca cdldmbiana, 7, diciembre de 2003 no obstante, el desarrollo directo del algoritmo evolutivo no se puede asimilar enel problema de inversion, limitandose la evolucien a instantes de tiempo, en los que se mide la anomalia gravitacional 0 magnetica; por tanto, para acelerar la convergencia y evitar la inestabilidad en el problema de inversion de los datos de gravedad y magnetismo se utiliza el problema inverso probabiiistico en la teoria de redes neuronales artificiales (rna) con arquitectura hopfield y con estructura cluster. 35 figueredo et al. figura 1. fucrzas tcctonicas y de flotabilidad en una region de tipo dinamico, deformaciones corticales considerando la gravedad la rnecanica de la deforrnacion en una region de tipo dinamico esta controlada por fuerzas tectonicas y de flotabilidad (porth, 1998); vease figura i: i. los esfuerzos compresivos ob: actuan en la frontera de la region y se transmiten al interior, controlados par propiedades reologicas, propiedades rnecanicas de la roca y por el empuje del fluido sobre el que flota (principio de arquimedes). 2. la fuerza de gravedad fg: actua sobre las masas (rocosas 0 magrnaticas) con el fin de controlar la estabilidad del sistema (0 region), debido a que la suma de los esfuerzos de frontera estaticos y dinamicos igualan la fuerza de graved ad (hans, 1981). 3. esfuerzos intemos normales cjy y de cizalla 'xy, debidos al arrastre viscoso y la deformaci6n elastica dentro del sistema. 4. fuerza de flotabilidad fb: resulta de las diferencias de densidad. para el estudio de un modelo estratificado (figura 2), las ecuaciones de velocidad en el caso del movimiento de perturbaciones sinusoidales en el plano x, y. se plantean a partir del potencial de flujo \f. considerando que cada capa tiene un espesar hi y la longitud de onda de la perturbacion a se mide en la direccion x (hans, 1981). las velocidades que dan solucion al bilaplaciano del potencial de flujo iji son: [ a exp(wy)+ bexp(-wy)] u(x,y) +cyexp(wy) +dyexp(-wy) [ a exp(wy)bexp(-wy)] v(x,y)= ( ij+c y+~ exp(-wy) los coeficientes a, b, c yd se expresan en terminos de las velocidades de amplitud en las fronteras superior e inferior de la 36 h,=ld superficie iibre capai .-..... < ............ ..._--------_ ...--_ ...interfase i+ 2 capai+3 l..----------------fronterarigida -y figura 2. sistema de tipo dinamico estratificado donde se observa la relacion de velocidades para un flujo bidimensional. capa (hans, 1981) y la frecuencia angular w, en donde w=2/'a. y 'a. es la longitud de onda. las ecuaciones de movimiento de un fluido viscoso que reflejan el efecto de las fuerzas de presion y de gravedad son (hans, 1981): (3) en donde eyes la rata de cambio de la deforrnacion longitudinal, i-l es el coeficiente de viscosidad y pes la presion litostatica. 'xy=i-lyxy (4) y es la rata de cambio de la deformacion de cizalla. bv e =y by (5) las ecuaciones de difusion relacionan la presion con las componentes de la velocidad (u, v), i-l, densidad (p) y la gravedad (g). (6) (1) (7) (2) en el caso de un sistema comprimido horizontalmente, las ecuaciones de esfuerzo normal y de cizalla se complementan con el esfuerzo compresivo paralelo a la capa cjb (figura i). para que el esfuerzo cjb supere la rigidez del medio, se requiere que este alcance un cierto limite que depende de las condiciones elastomecanicas del medio (hans, 1981). geofisica colombiana. 7. diciembre de 2003 estudio y simulaci6n de las deformaciones corticales asociadas a la variaci6n del campo gra vitacional, usando redes... e es el modulo de young y i la relaci6n de poisson. las condiciones de continuidad para un modelo estratificado con "n" interfases flexibles y "(n+ i)" capas, son: '11='21 interfase i crt i= (j21 (9) '22 ='32 interfase 2 cr22=(j32 'tnn =1(,,+1)n interfase n an" =o(n+l)n al solucionar el sistema de ecuaciones (9) en funci6n de la amplitud de deformaci6n y se obtiene: para un sistema no comprimido horizontalmente: <1" exp(k,q/) ~" exp(k2q/ (10) y" para un sistema comprimido horizontalmente: (ii) en" p,-p(i+i) donde q,=---h(i+'jg 2~(i+') (12) k representa los valores propios, cij son los vectores propios y r indica el valor de referencia. se debe destacar que el sistema de ecuaciones (l0) y (11) permite relacionar la deformaci6n con el tiempo de evoluci6n de la misma; por tanto la variable t se considera como un estimador de evoluci6n, que permitira reconstruir paso a paso el proceso de deformaci6n. ei acortamiento de la longitud de onda para un sistema comprimido horizontalmente es funci6n de k, c, fl, t, yo, y se expresa mediante la siguiente relaci6n (hans, 1981): geoflslca colombiana, 7, oiciembre de 2003 (8) (13) (j inlo donde-t=-~ k-~ (14) simulecion numerics del movimiento de las interfases una deformaci6n se considera como el cambio de la forma del cuerpo producido por el desplazamiento en sus interfases. la funci6n de potencial de flujo '¥, para un sistema no comprimido horizontalmente, esta en funci6n del potencial de flujo inicial , el valor propio k (relacionado a la longitud de onda dominante), la relaci6n de densidades qi y el tiempo de evoluci6n de la deformaci6n t. \jj= !(a, cos w,x+ bsenwxve?" i~o (15) mientras que la funci6n potencial de flujo \jj, para un sistema comprimido horizontalmente, esta en funci6n del potencial de flujo inicial 'f, la relaci6n esfuerzo compresivo-viscosidad (cr,l~r) de la cap a que se selecciona como capa de referencia a partir de la cual se calculan las maximas amplitudes de deformaci6n (relaci6n vectores propios) en las otras capas et) -k~1 \jj=l(a,cosw,x+b,senw,x)e f', j=o (16) donde a", b", son los coeficientes de la serie discreta de fourier. entrenamiento de una red hopfield, y de una red neuronal cluster basadas en la solucion del problema inverso probabilistico en el primer caso, se implementa una red neuronal artificial con arquitectura hopfield, estructurada sobre un sistema de entrenamiento supervisado (figueredo y diaz, 2002) basado en la formulaci6n probabilistica para la soluci6n del problema inverso no lineal; esto implica un procesamiento iterativo y altemado (red con arquitectura hopfield) de los dos problemas (para el campo gravitacional y para el campo magnetico), en el segundo caso, se utiliza una arquitectura tipo cluster, basada tambien en el sistema de entrenamiento supervisado con un procesamiento iterativo pero sirnultaneo (procesamiento en paralelo) de los dos problemas. 37 figueredo et al. formulecion probabilfstica del problema inverso no lineal solucionar el problema de inversi6n en geofisica, significa determinar los parametres m de un modelo que describe un sistema fisico s, a partir de relaciones te6ricas de los valores de los parametros del modelo con los valores reales observables d (datos) sobre s. una buena soluci6n depende de la combinaci6n del estado de la informaci6n de los datos, los para metros y las leyes fisicas. por tanto, en este desarrollo estocastico se deben involucrar las incertidumbres de estos tres estados de la informaci6n, asumiendo una distribuci6n de probabilidad gaussiana (tarantola, 1987). los procesos itera ivos siguen una distribuci6n gaussiana, donde cada iteraci6n es un cambio de densidad de probabilidad (cortijo, 2000). de acuerdo con las consideraciones anteriores, la ecuacion de actualizaci6n es (mosegaard y tarantola, 2000): (17) g es la matriz jacobiana cd=e 2 1 cdes la matriz covarianza de los datos. (18) la matriz covarianza de los parametres cm esta definida como: (19) (menke, 1989). optimizecion basada en la arquitectura de la red hopfield la arquitectura hopfield corresponde a una red neuronal artificial monocapa (figura 3) en la que cada neutona de la red se encuentra conectada a todas las demas (conexiones laterales) pero no consigo misma. utiliza el entrenamiento no supervisado para desarrollar tare as de optimizaci6n, que representen la evoluci6n de un sistema fisico (kari, 2002). optimizecion basada en la estructura de cluster ei uso de la estructura cluster (figura 4), que se caracteriza por su procesamiento paralelo, permite acelerar la convergencia mediante la ponderaci6n de los coeficientes de correlaci6n (entrenamiento supervisado) entre los datos observados y los calculados para cad a una de las anomalias. 38 figura 3. arquitectura de la red hopfield, w representa los pesos (parametres) en el entrenamiento. x aes :aigqri1lllode entrenarnjeato supetvis"do figura 4. estructura cluster para dos fuentes de informacion (informacion gravimetrica e informacion rnagnetica). modelos numericos ejemplo de un sistema no comprimido horizontalmente con este ejemplo se muestran los resultados obtenidos para un modelo no comprimido horizontalmente, estratificado de tres capas con dos interfases (figura 5). los resultados obtenidos fueron: • longitud de onda dominante: a, = 15840 m • valor propio: k = 2.1048x 10-2 • amplitud maxima, interfase i: yi = 1512 m • amplitud maxima, interfase 2: y2 = 1584 m • tiempo estimado para alcanzar la estabilidad: t = 47 millones de afios. • velocidad normal promedio: v = 0.003 em/afro. geofislca colombiana, 7. diciembre de 2003 estudio y simulaci6n de las deformaciones corticales asociadas a la variaci6n del campo gra vit acional. usando redes... capa 1 superfeje libre hi =00 pi=o pi =0___------------~l~ h,=3km capa 2 p, =2.6 grjcm' corteza ,il, = ixl o" poises ~--capa3 corteza h, :8km p$=2.4~jcrnl j.1, '" lxlo2l poises figura 5. modelo 1, muestra la distribucion de los parametres fisicos en cada una de las capas. 1*9",";'",·11,',. ad£1an1£ i ae .... a .... i i...... figura 6. simulacion para el modele 1, desarrollada en visual basic. en este modelo las variaciones promedio en la anomalia gravitacional y magnetica son de 0.00 12 ugals/afio y 0.608 teslas/afio. modelo comprimido horizontalmente en este modelo se involueran esfuerzos eompresivos que superan la rigidez del medio, y se ealculan teniendo en cuenta el modulo de young y la relacion de poisson (figura 7), que generan un cambio geometrico en las interfases. los resultados obtenidos fueron: • longitud de onda dominante: a. = 17360 m • acortamiento longitud de onda 7,99% • valor propio: k = 3.803xl 0-2 • amplitud maxima, interfase i: yl = 1516 m • amplitud maxima, interfase 2: y2 = 1736 m geoflslca colomb lana. 7. oiciembre de 2003 capa 1 superficie libre hi =a;t pl=o ].11 =0____...----------------~l_____ h,3km capa 2 p,.2.6vlc»j' corteza p, -i xi0" poi8e8 youlj&. • 5xl011 didtej cm~~_---~_m.0.2 capa3 corteza h,-ikm ~ ",.'aer/""" jl;, -i xl cf' poise:" y..,o« -sxio"~q»' ~siqq.·o.'} figura 7. modelo 2; muestra la distribucion de los parametres fisicos en cada una las capas. -.. ·..:.:::iii-........ ft__ -arlo.olalia m~iti~ ·m:itl.'hlt i ~ ~ variacjom~:j :'::;;:'m _v~likio()t':i hn'''.,zo,ntoloer: figura 8. simulacion para el modelo 2, desarrollada en visual basic. • tiempo estimado para alcanzar la estabilidad: t = 556000 afios. • veloeidad normal promedio: v = 0.3 em/ana en este modelo se observa una mayor deformacion, y por tanto una mayor variacion en las anomalias gravitacional y magneticas del orden de 0.4706 igals/afio y 0.0019354 gauss/afio, desarrollo de un modelo para el problema de inversion en este caso se utilize un modelo de dificil eonvergeneia, de tal manera que el problema inverso probabilistieo ayudado por la red de arquitectura hopfield y la estruetura cluster ayuda a la convergenera. 39 figueredo lit al• • i '_ g observede "'.\ 9 calculada "'" t observada ~.91832~ ~.o]31191~ ~~:9~"w;;\',\. ~~ calculada ~iili'-l ij""'! 40 .. tal\ilaniinverso ~!!,'31 mg'.! ~~ ~~ / ,/ figura 9, resultados obtenidos por la inversi6n con la red de arquitectura hopfield en la iteraci6n i y 10, geoflslca colombiana, 7, diciembre de 2003 estudio y simulaci6n de las deformaciones corticales asociadas a la variaci6n del campo gra vit acional, usando redes... conclusiones las conc1usiones que se presentan a continuacion se hacen con respecto a tres itemes: el primero se relaciona con la determinacion del algoritmo de evolucion y con respecto al problema de inversion; el -segundo ite~e refiere a la comparacion de estos dos ternas desarrollados, para por ultimo conc1uir con los modelos numericos. determinacion del algoritmo de evotucion y problema de inversion • la utilizacion de un modelo mecanico en el medio continuo permite el desarrollo de un algoritmo evolutivo que esta en funcion de los valores y los vectores propios. no obstante, este algoritmo evolutivo al ser asimilado en el modele de talwani permite evidenciar las variaciones en los campos gravitacional y magnetico, de tal manera que el campo gravitacional y el campo rnagnetico sirven para estudiar la estructura deformada, dado que toda deformacion involucra un cambio en estos campos. pero tambien a partir de estos campos se puede determinar la frontera deformada; de ahi que se desarrollara el problema inverso probabilistico. • ei problema inverso probabilistico es un programa rnatematicamente accesible a la informacion a priori en geofisica, de tal manera que el contar con datos, como por ejemplo de pozo, ayuda a garantizar una buena convergencia. algoritmo de evolucion y problema de inversion en redes neuronales artificiales • infortunadamente, la metodologia algoritmo evolutivo (problema directo) y el problema de inversion (usando redes neuronales artificiales) no se pudieron trabajar como una metodologia unica en la simulacion, ya que no hay un acoplamiento en la forma de procesamiento y los objetivos por cumplir. ei algoritmo evolutivo es un proceso secuencial, mientras que las redes neuronales geofislca colombiana, 7, diciembre de 2003 no asimilan este tipo de procesamiento, siendo mas versatiles en procesos aleatorios que buscan optimizar, c1asificar y reconocer patrones. modelos numeticos • en el modele i, la variacion del campo de gravedad y el campo magnetico es menor con respecto at modele 2. • por presentarse las capas de densidad similar, la deformacion en cada una de las interfases es aproximadamente igual. • la optimizacion utilizando la estructura cluster y de arquitectura hopfield son dos metodologias que permiten una buena convergencia, inc1uso para estructuras complicadas. bibliograffa cortijo, f. (2000), lafuncion de densidad de probabilidad normal, universidad de granada; departamento de ciencias de la computaci6n e inteligencia artificial. http://www-etsi2.ugr.es/depar/ccia/rf. figueredo, y. y diaz, r. (2002). "optimizacion en la soluci6n del problema inverso en geofisica usando el algoritmo de entrenamiento supervisado de levenberg-marquardt", geojisica colombiana, no.6, pp.66-69. hans, r. (1981). gravity, deformation and the earth's crust, academic press inc. (london) ltda. karl, t. (2002). eee511: artificial neural computation systems, http://www.eas.asu.edureee51i11handouts.html. menke, w. (1989). geophysical data analysis: discrete inverse theory, academic press, san diego new york, boston. porth r. (1998). the mechanics of andean plateau evolution a numerical modelling study. university berlin. http://sfb267.geoinf.fu-berlin.de/web/oldlabstracts/ tarantola, a. (1987). inverse problem theory: methods for data fitting and model parameter estimation, elsevier science. 41 geofls. colomb. santaf' de sogota d.c ls5n • 0121-2974 los potenciales espontaneos: aspectos basicos y sus aplicaciones luis a. briceno g. profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia robinson leon departamento de geociencias carrera de geologia facultad de ciencias universidad nacional de colombia briceno, l.a & le6n, r.: los potenciales espontaneos: aspectos baslcos y sus aplicaciones. geofls. colomb. 2:21-24, 1993. issn 0121-2974 resumen los potenciales generados espontaneamente en un medio, sp, se han observado en suelos infrayacidos por arcillas y por areniscas. en cada uno de los casos se miden su dependencia temporal y se establecen rangos de valores: 10 mv para arcillas y 0.6 mv para areniscas, en el caso de que los electrodos de medici6n se coloquen sobre medios homoqsneos. las rapidas y econ6micas medidas ofrecen una gran posibilidad de investigaci6n del subsuelo. abstract the spontaneous potentials, sp, has been observed in two different mediums. in one of them, soil is underlayed by clays and in the other by sandstones. in both cases, time dependency has been observed, and range of values has been established: 10 mv for clays and 0.6 mv for sadstones, if there exists lateral homogenity. because the sp method is really fast and low cost, it provides a wide range of possibilities for doing research. 1. definici6n y causas los potenciales espontaneos "sp",son diferencias de potencial naturales que suelen existir entre dos puntos cualesquiera del terreno. su origen se puede atribuir a diversas causas como: la presencia de capas i6nicas; diferencias de ph, temperatura, salinidad; electrofiltraci6n 6 electro-osmosis. estas causas pueden originar acciones electroqufmicas entre los diferentes minerales y las soluciones con las cuales estas estan en contacto. generalmente. estas soluciones son de composici6n variada 10 que origina presiones de disoluci6n diferenciales en las respectivas superficies de contacto. estas diferencias de disoluci6n generan diferencias de potencial las que a su vez originan un f1ujo de corriente en el subsuelo. 2. utilidad las corrientes naturales de la tierra general mente se miden de manera potenciomstrlca con dos electrodos no polarizables, 0 hechos en plomo 0 grafito y un milivoltlmetro de buena sensibilidad. se puede decir que la profundidad alcanzada en la medida es aproximadamente igual a la separaci6n de los electrodos. esta sencillez operativa caracterizada por un bajo costo del equipo, su simplicidad de rnane]o, la facilidad de transporte y el poco nurnero de operarios requeridos, hacen de este un metodo de alto rendimiento, y sus resultados saran utiles en tanto que la medida de datos sea sistematica y cuidadosa. \22 briceno & le6n: potenciales espontaneos: aspectos basicos y aplicaciones $p (mv) 12 .--.-:..-.....:-------------------------, 10 ;:::j:i;:a::e.e-81b-1;r1:*hh:h3.a:a ..a::...e.:!.-~-~....~....~-.~....~.....~...~...~~.....~...~.....~..~...:...-:..=-....=....~-.:..~....~..-:...-~--.-.............................. _._................................ . -_ __ ._._-._ _._ -_ -._ -_ _ __ _._ __ __ _ _--_ _-_ __ _--8 6 ....................... _ _ .-...-..__ __ _ _ _ _-_ -._ -. 4 ._ -_ _._ _-_ _--.--_ _-_ _.__ _-----_.-_ __ ._ _---_.--_ .._ ..__ .._ __ •.._---_ -_ __ .. 2 ...................................................... _ ••• ,••• _ .. w, , ,_._._._.,_, .. , •••, •••••• _,_ •••• __ ., ••••••••• _._._ -.••,.-.----, •• ,-,,-, ••• -•••••• ,•••• _-••••• _ --.qi9..l.. ...l....i-..... o 4010 20 30 t (min) -t:tserial! d figura 1. sp sustrato arcllloso. ciudad universitaria 07-11-92 ei "sp" depende en gran medida de la composici6n intrfnseca de la roca, por eso sus valores y comporlamiento son reflejo de su iitologia. una aplicaci6n sencilla de la observaci6n superficial de sp, puede ser la de caracterizar rocas sedimentarias con arreglos estructurales sencillos, que sstsn bajo coberleras de poco espesor (hasta 1 m), ya que se espera que exista un cierto rango de sp para una litologia determinada. otra aplicaci6n imporlante es su ulilizaci6n en zonas de actividad votcanica, donde el potencial geoelectrico sufre grandes variaciones inducidas por esta actividad. los cambios en la amplitud, direcci6n y fase de la corriente de la tierra se pueden deber, entre otras, a cambios de resistividad, temperatura, presi6n y condiciones quimicas, que normalmente preceden a los' eventos de actividad rnaqmatica. la debilidad de las corrientes generadas es la principal limitante del metodo. a la que se suman otros factores como la dependencia de las concentraciones quimicas no propias de la roca (aguas subterransas, diagenesis, etc.); la "rigidez" quimica de las rocas cristalinas (igneas y metam6rficas) y su gran irregularidad de distribuci6n geometrica, dificultan la aplicaci6n del rnstodo "sp". por 10 dsrnas es de gran ayuda en la detecci6n de la variaci6n facial de las rocas sedimentarias. 3. objetivos ei presente trabajo se realiza con el objeto de establecer unacaracterizaci6n electrica de "tipos faciales" de rocas mediante observaciones de sp. se han tomado dos de las litologias mas representativas: arcillas y arenas; ambas claramente reconocibles, diferenciables, de una amplia presencia en la columna estratiqrafica y ademas de facil caracterizaci6n como patrones por otros rnetodos. se plantea la hip6tesis de que para una iitologia dada existe un rango de potencial generado sspontaneamente en el subsuelo (naturaleza) y observable en la superficie. ai realizar las medidas se observa su comporlamiento con el tiempo y con las condiciones de su medida (calidad y tipo de equipos, condiciones del subsuelo y metodologia de adquisici6n de datos). de modo general, se puede abrir un canal de inquietud sobre el fen6meno inherente a la electricidad sspontanea de la naturaleza y que puede afectar en algun modo la concepci6n sobre la misma, en cuanto a su caracter permanentemente activo. geofisica colombiana n° 2, abril de 1993 sp (mv) 23 0,6 .. _ • ..co>. •• •..... , .. ... .. ..__ ..... ..__ .._. .._--.--.__ .----_ ...if 1.5 ~-.................-................--.---.-----...-.-..---.....-...........-1, 0 ..~-----_--_._ _ ..--.-...._ _ ....-._ -.-.-.-..--,-.,..--5 0.0 l..-. l-' .....l ---ll....j o 2010 t (min) -.serie. c figura 2. sp sustrato arenoso. la punta e1 06-02-93. 4. metodologia en la adquisici6n de datos para nuestro experimento se tuvieron en cuenta cuatro aspectos relacionados con: . a) equipo: utilizaci6n de un milivoltimetro anal6gico marca phoenix y electrodos no polarizables de cobre en soluci6n saturada de sulfato de cobre. b) dispositivo: los electrodos se enterraron aproximadamente 15 cm y la distancia entre ellos se mantuvo constante e igual a 2 m. en cada localidad y ocasi6n se hicieron por 10 menos 4 lecturas en sitios diferentes. c) ei medio: las lecturas se realizaron generalmente sobre suelos 0 'zonas de meteorizaci6n que varian entre 0.2 y 1.0 m de espesor, infrayacidas en unos casos por formaciones arcillosas recientes y en otro por areniscas. d) control temporal: para asegurar la estabilidad y calidad de las medidas, se hicieron en cada ocasi6n medidas multiples en el tiempo, que generaron series de largos perfodos, para observaciones de mas de 30 minutos. 5. resultados la fig.1 corresponde a variaciones de sp, contra el tiempo, sobre un suelo orqanico, de aproximadamente 0.6 m de espesor, que 15 suprayace una unidad arcillosa cuaternaria de varios metros de espesor. las medidas se realizaron en predios de la universidad nacional de colombia en santate de bogota. la curva obtenida es de tipo exponencial creciente, con una amplitud del orden de 10 mv. las figs.2 y 3 corresponden a observaciones realizadas en la vereda la punta (sabana de bogota), donde afloran unas areniscas. elias se tomaron sobre unas lodolitas arenosas, con espesores entre 0.2 y 1.0 m, que suprayacen los bancos de areniscas. en el caso de la fig.2, los electrodos de medici6n se ubicaron sobre las lodolitas, mientras que en la fig.3, uno de los electrodos se ubic6 sobre las areniscas. se observa que en el caso de colocar ambos electrodos sobre las lodolitas, los valores de sp son bastante estables en el tiempo (excepci6n del primer punto), tendiendo hacia un valor de 0,6 mv. cuando se colocan los electrodos en diferentes medios, hay f1uctuaciones en los valores del sp auncuando, en este caso, las amplitudes observadas fueron menores que en los casos anteriores. 6. analisis los rnetodos geoelectricos de prospecci6n que incluyen medidas galvanicas de potencial, deberan siempre tener en cuenta la presencia de potenciales espontaneos. sp, los cuales son 24 briceno & le6n: potenciales espontaneos: aspectos basicos y aplicaciones sp (mv) 0.4 ;=-..:..:::.....:------------------------, 0.3 ~----------_._.0.2 0.1 0.0 l-l l......l.. ---' o 206 10 16 t (min) ~ seriell e figura 3. sp sustrato arenoso. la punta e2 06-02-93. funci6n, tanto de litologias como del tiempo. se puede afirmar que si el medio es aparentemente hornoqeneo, conviene esperar unos pocos minutos, para acercarse a un valor mas estable. fluctuaciones del sp, pueden estar asociadas a inhomogeneidades laterales del medio en el cual se realizan las observaciones. la unidad infrayacente a profundidades del orden de separaci6n de los electrodos, define la amplitud del sp observado. se observ6, en el presente caso, que los sp sobre formaciones arcillosas, son por 10 menos un orden de magnitud mayor que sobre areniscas. este tipo de resultados coincide con estudios realizados en pozos (ver p.ej. shelton, 1973), en donde se muestra la clara diferenciaci6n entre iitologfas, a partir del simple valor del sp. 7. conclusiones los experimentos realizados permiten concluir, con medici ones seneillas pero cuidadosas del sp en superficie: a) si los valores de sp son relativamente estables, existe homogeneidad lateral del medio; b) la amplitud del sp, depende de la litologia a profundidades del orden de separaci6n de los electrodos. para arcillas, el valor medido fue del orden de 10 mv, sobre suelos orqanlcos: mientras que para areniscas sobre lodolitas arenosas, su valor fue del orden de 0.6 mv. se recornienda, observar con detalle los sp, sobre litologias especificas para establecer un rango mas confiable de valores, que permitan su facil uso como identificador facial. observar los sp, en otro tipo de condiciones hidrodinamicas, eventos volcanicos p.ej., pues parece abrirse un gran campo de acci6n y aplicaci6n. bibliografia dobrin. m.b. (1961): introducci6n a la prospecci6n geofisica. omega, barcelona. parasnis. 0.5. (1970): principios de geofisica aplicada. barcelona. metodos electricos cap. 4. davidson. m.j. (1987): unconventional methods in exploration for petroleum an natural gas. geophysics 9. shelton, j. w. (1973): models of sand and sandstone deposits: a methodology for determining sand genesis and trend. oklahoma geological survey, bulletin 118. unesco. (1971): the servillance and prediction of volcanic activity. paris. meto os geofisicos. geofisica colombian a n° 5 pp. 18-26 diciembre de 2001 bogota 0 c issn 0121 2974 implicaciones sismotectonicas de las soluciones mecanismo focal de algunos terremotos de la central de colombia del region elkin de jesos salcedo hurtado*1.2 clara ines rivera perez1,3 augusto antonio gomez capera1.3 resumen se han analizado las s luciones del mecanismo focal de veinte terremotos ocurridos a diferentes profundidades y arnbientes tectonicos para establecer el marco sismotect6nico de la regi6n central del territorio colombiano. los sismos intermedios ocurridos en la parte occidental de la regi6n, en su mayorfa responden a fuerza de traccion y corresponden a la subduccion del material litosferico de la placa oceanica bajo la placa continental, mientras que los eventos superficiales que tienen lugar en el continente en su mayorfa son de tipo compresivo y estan relacionados con la actividad de las diferentes fallas que atraviesan la regi6n. esta actividad es derivada de la colisi6n de la placa nazca con el bloque andino. en ei sistema de fallas del borde lianero se nota la continuaci6n de los esfuerzos tect6nicos desde la base de la corteza hasta la superficie terrestre. en general, el regimen geodinamico de la esquina noroccidental del continente suramericano est a gobernado por la interaccion de tres grandes placas, que a su vez causan la deformaci6n y el desplazamiento del bloque andino en direcci6n nne. palabras clave: mecanismo focal, sismotectonica,tectonica de placas abstract focal mechanism solutions of twenty recent earthquakes ocurred at different depthes and tectonics environment have been analysed to establish the seismotectonic framework of the central region of colombian territory. the intermediate earthquakes in the western part of the region, in the majority respond to traction force and correspond to the subduction phenomenon of the oceanic lithospheric material beneath continental plate. while the superficial events ocurred at the continent crustal in the majority being of compressional type are related with the activity of different faults that cross the region. this activity is derived from the collision of the nazca plate with the andean block. on the east andean frontal fault system is noted a continuation of the tectonic stresses at the interior of the crustal to the earth's surface. in general, the geodymanic regimen of the nothwestern corner of the south american continent is governed by the collision of three major tectonic plates, that cause the deformation and displacement of the andean block in nne direction. key words: focal mechanism, seismotectonics, plate tectonics introduccion tradicionalmente las soluciones de los mecanismos focales de los terremotos han sido una de las principales herramientas para el analisis sismotect6nico de regiones continentales y zonas oceanicas, como 10 muestran trabajos realizados en diferentes regiones del mundo (sykes, 1967; isaacks et al., 1968; molnar and sykes, 1969; nowroozi, 1972; udias et al., 1976; y muchos otras) y para el estudio del tipo de desplazamiento y movimiento relativo entre los bordes de los lfmites de diferentes placas y fall as tectonicas (tandon and srivastava, 1975; philip y cisternas, 1985; udias and bufom, 1985; aztis and kanamori, 1988; adamek et al., 1988; salcedo et al., 1995; bufom and udias, 1991, y otros). ingeominas, subdirecci6n de geofisica, a. a. 4865, santare de bogola, d.c. colombia. actualmente depto. de geogralia, universidad del valle, cali (valle) colombia depto. de fisica, universidad nacional de colombia, santale de bogota, d.c. colombia. corresponding author. tel.: +57 (2) 321-2189; fax: +57 (2) 333-4909. e-mail: elsalced@mafalda.univalle.edu.co 18 la tecnica de obtenci6n del mecanismo focal de los terrernotos ha evolucionado considerablemente. la mas cormin a nivel mundial ha sido la utilizaci6n de la polaridad del primer impulso de las ondas p. cuando el numero de lecturas de polaridades de las ondas p no son suficientes para obtener una soluci6n confiable, general mente para algunos sismos moderados, se usa la tecnica de inversi6n de ondas. actualmente, con ayuda del desarrollo de las tecnicas computacionales, el estudio del mecanismo de los terremotos y la aplicaci6n de los resultados de sus investigaciones son mejor utijizadas tanto en tect6nica regional como para el conocimiento de las estructuras locales. ei uso de estos rnetodos, los cuales definen los parametres del mecanismo focal de los terrernotos, muestran una enorrne concordancia de sus resultados con los obtenidos por observaciones geol6gicas y geoffsicas. para el tenitorio colombiano en particular, algunos autores han realizado importantes trabajos sobre interpretacion del mecanismo focal de terrernotos, que ayudan a la comprensi6n del marco sismotect6nico de una de las regiones tectonicas mas complejas de la tierra, la esquina noroccidental del continente suramericano. entre los principales trabajos aplicados al territorio colombiano se pueden enumerar los realizados par mendiguren, 1973; bune et al. 1975; jordan, 1975; herd et al., 1981; pennington, 1981; pennington geof!sica colombiana, 5, oiciembre de 2001 implicaciones sismotectonicas de las soluciones del mecanismo focal de algunos terremotos de la region central de colombia et ai., 1981;kellogg and bonini, 1982;mendoza and dewey, 1984;lomnitz and hashizmine, 1985; page, 1986; adamek et al., 1988; rivera, 1989; salcedo, 1992; salcedo et al., 1995. ei presente trabajo hace parte de los resultados del proyecto "sismotect6nica del territorio colombiano" llevado a cabo en ingeominas (instituto de investigaciones en geociencias, minerfa y qufmica) durante el afio 1995. aqui se usan los datos de las soluciones del mecanismo focal de una poblaci6n de terremotos para el analisis del marco sismotect6nico de la regi6n central de colombia. los datos utilizados en este estudio fueron, en buena parte, obtenidos durante el desarrollo del proyecto, y tarnbien tornados de otras fuentes como se muestra mas adelante. aqui no se analizaran las particularidades de la sismicidad de la regi6n, puesto que esto se hizo en un articulo previa realizado tambien por los autores del presente trabajo (salcedo et al., 1997). ubicacion geografica de la region de estudio la regi6n de estudio escogida esta enmarcada entre las coordenadas 2,y 6,50 latitud norte y 72,00 76,00 longitud oeste en el territorio colombiano, encerrando los departamentos de cundinamarca, boyaca, tolima, caldas, quindfo, risaralda y meta; adernas, cubre el sur de los departamentos de antioquia y santander, la parte occidental del casanare y el norte de huila (figura i). esta regi6n, que sin duda corresponde a la zona mas poblada y donde mas frecuentemente ocurren los sismos en el pais, se escogi6 con el objeto de evaluar el ambiente sismotect6nico de la parte central del territorio colombiano, yencontrar la relaci6n entre la sismicidad y las principales estructuras tect6nicas allf presentes. la figura i tambien muestra los principales sismos ocurridos en la regi6n. o to r-, to to m " to n r-, ", to to r-, s!siolo\.ocicas >l~=•• ... a •• m r-, convenciones » o capjlolde icfll!pjbl;c,o, cop;loldco..l>cw'\<l""""lo, figura i. localizaci6n de la regi6n de estudio: adernas se ruuestran los principales sisrnos ocurridos. geofisica colombiana, 5, diciembre de 2001 19 salcedo et al. datos utiuzados y procesamiento para las soluciones del mecanismo focal de los terremotos se ha utilizado la polaridad de la primera liegada de las ondas p, como se publica en los boletines del centro sismo16gico internacional (isc). los puntos convencionales tomados represent an las estaciones, que son definidas en cada caso por el angulo entre la vertical y la tangente al rayo sismico en el foco y por el azimut de la estaci6n con respecto al epicentro; los angulos de incidencia en el foco fueron leidos de las curvas tiempodistancia de jeffreys y bullen. estos fueron gratificados sobre una proyecci6n de igual area del hemisferio focal inferior en la red de wulff. a partir de la distribuci6n espacial de los signos del primer impulso en cada estaci6n, fueron determinados los dos pianos nodales, que representan los posibles pianos de fallas. convencionalmente estos pianos son descritos por tres angulos: !poazimut, medido desde el norte; 8° buzamiento, medido desde la horizontal y ie ° angulo de deslizamiento, medido entre la horizontal sobre el plano de falla y el vector deslizamiento. la arnbiguedad que presenta la soluci6n por el rnetodo de las primeras liegadas de las ondas p fue removida en algunos casos por media de datos geol6gicos, y en otros por la distribuci6n de las replicas en los casos en que estas se conocieron. adernas, fueron definidos los ejes de los esfuerzos principales de compresi6n y tensi6n. las soluciones obtenidas en este trabajo se lograron utilizando metodos manuales 0 mediante el programa sphere (kohler, 1991). ei criterio de sejecci6n de los datos utilizados fue basicamente escoger aquellos sismos con magnitud mb;::: 4.8 dentro del periodo 1960-1992, con por 10 menos is lecturas de polaridad de llegada del primer arribo de las ondas p. asi, se obtuvieron soluciones del mecanismo de trece terremotos; tarnbien se utilizaron para el estudio otras siete soluciones tomadas de otras fuentes (pennington, 1981; salcedo, 1992 y bulletin isc), que se ajustaban a nuestros criterios de selecci6n. en total se utilizaron los resultados de veinte terremotos (tabla i). la figura 2 muestra la localizaci6n de los terremotos y las soluciones del mecanismo focal analizadas; la base tect6nica (fallas geol6gicas) fue tomada de paris y romero (1993). los resultados numericos estan dados en la tabla 2. los datos de las primeras liegadas para las soluciones encontradas por este estudio son dibujadas en la figura 3, donde aparecen representadas las compresiones, dilataciones y los ejes de los esfuerzos principales, compresivo p y extensivo t. anausis y discusion los mecanismos focales de los veinte terremotos analizados son mostrados en la figura 2, junto con las principales fallas geol6gicas de la regi6n. ei rnimero allado de cada even to corresponde a la numeraci6n usada en las tablas i y 2, donde aparecen por orden cronol6gico. de aquf se puede ver que en la parte occidental, entre las coordenadas 4.00n y 6.00n, se agrupa una serie de terremotos (mimeros 1,5,6,8,9, 11,14,15, 17y 18)deprofundidadinterrnediaentre60y 170ki16metros aproximadamente. entre ellos encontramos eventos de tipo extensivo, en ellos se nota un plano de falla orientado de nw a se y buzando con un alto angulo, bien sea al noreste 0 hacia el suroeste. ei desplazamiento sobre este plano en cada evento es dextrolateral, excepto en el terremoto del 29 de marzo de 1986 (no. 14), en el cual el desplazamiento en el plano de falla es lateral izquierdo. estos sismos ocurren en la placa oceanica de nazca, que se subduce bajo el continente suramericano. la direcci6n del desplazamiento en los pianos de falla concuerda con el sentido del vector que muestra la direcci6n en la cual se introduce la placa de nazca. ei desplazamiento dextrolateral en el plano de falla de estos eventos es generado por esfuerzos de orientaci6n ns, variando su direcci6n desde nwn hasta nen. fecha tiempo coordenadas del foco dia mes ado hora min. seg. <p°n x-w prof. (km.) mb 03 01 1966 18 16 5.1 4.65 76.00 98 5.1 04 09 1966 22 14 50.0 4.57 74.12 9 5.0 18 10 1966 18 43 37.6 3.60 74.50 42 5.0 09 02 1967 15 24 47.9 2.92 74.94 60 6.3 03 04 1973 13 54 1.0 4.70 75.60 158 6.1 24 04 1973 18 42 320 5.20 75.80 117 5.3 18 10 1973 9 18 34.8 2.94 74.88 61 5.1 13 04 1975 1 53 30.3 4.85 75.71 139 5.1 19 05 1976 16 54 33.0 4.50 75.80 166 5.8 21 01 1978 8 19 29.3 6.43 72.31 27 5.1 25 06 1980 12 4 56.9 4.40 75.80 162 5.7 17 01 1982 14 52 14.0 5.76 74.21 42 5.3 18 12 1985 22 36 29.3 5.11 74.65 44 5.0 29 03 1986 9 48 55.6 4.59 75.63 163 5.0 31 12 1988 20 58 31.3 5.10 75.78 118 4.9 11 10 1989 14 42 97 2.95 73.64 35 4.8 10 01 1990 2 57 507 5.65 75.79 62 5.0 23 1l 1990 22 35 34.0 4.75 75.55 136 5.6 25 06 1991 23 6 580 5.22 72.91 15 5.0 02 01 1992 19 41 46.4 5.67 73.84 146 5.7 20 tabla 1. terremotos usa dos para el estudio de mecanismos focales en la regi6n central del territorio colombiano, con magnltud mb~ 4.8 para el periodo 1960 y 1992 geofislca colombiana, 5, digiembre de 2001 implicaciones sismotect6nicas de las soluciones del mecanismo focal de algunos terremotos de la regi6n central de colombia 6.50 i / / / .> j. eo .-e 16 convenciones estructurales geograflcast copital de lc republica, a capitol de departamento. o pcbroctcn . ________ trurno de folio con ;fldlc'os 0 eviceoclcs ptmlvoles de oct; ...\ooo -eetectentcc. rupturos p:rindpolm.lltt' pleislol;cn;ccs. -------tramo de folio inoc:livtl • "t' • • • • • •• folio de cobcrqormentc. los puntas ncc:c et bloque revoetodo. • • • • • • • • •• folio in....ersc. los rec.l6ngulos noctc !!i brcqoe tevcrncdo. sismdl6gicas falla coi'i indicoci6n del seouoc de 10 ccropeeente tlo(;lootol del movimtentc r0110 definido. folio cproximcdc a ini.erida. unecmtento ictoqectoqtco. soluci6n del mecanisme focal. lo porte sombreodo deooto ccmoreston y 10 porte en bronco diiotocion. • ecicentre anticlinal ceooaolco. figura 2. principales sistemas de fallas en el area de estudio (adapt ada de paris y romero, 1993) juiho can los diagramas del mecanisme focal de los ierremotos. los cfrculos representan los epicentros. los diagramas representan el hemisferio inferior de esfera focal can cuadrantes sombreados para las cornpresiones y blancos para las dilatauones. geofislca colombiana, 5, diciembre de 2001 21 salcedo et al. tabla 2 soluc10nes del i\·lecanis/ v 1q focal de los terremotos de la. region central de coloi'.,tbia. fecha ejes p, t planos de falla t1po de falla referencias no. dia mes aiio ci>p el,. <p 8 t.. 03 01 1966 p: 215 25 a: 351 78 -157 normal t: 122 7 b: 256 67 -13 2 04 09 1966 p: 117 6 a: 252 70 169 inversa t: 210 21 b: 345 80 20 3 18 10 1966 p: 352 7 a: 43 57 27 lnversa t: 256 41 b: 297 68 144 4 09 02 1967 p: 142 7 a: 232 39 90 inversa 2 t: 142 83 b: 52 51 90 5 03 04 1973 p: 339 35 a: 21 12 -32 normal 2 t: 63 7 b: 122 60 -160 6 24 04 1973 p: 335 8 a: 23 82 21 casi puramente de 3 t: 68 20 b: 110 68 in deslizamientocon componente inversa 7 18 10 1973 p: 109 15 a: 243 53 163 rumbo deslizantecon t: 210 37 b: 344 76 39 componente inversa 13 04 1995 p: 195 29 a: 232 54 -6 normal t: 93 21 b: 325 85 -143 9 19 05 1976 p: 352 50 a: 40 74 -130 normal 3 t: 110 17 b: 156 44 -30 10 21 01 1978 p: 246 27 a: 23 73 -159 normal t: 155 2 b: 287 70 -18 11 25 06 1980 p: 301 20 a: 200 80 40 inversa 2 t: 69 34 b: 102 50 167 12 17 01 1982 p: 243 47 a: 41 44 -153 normal t: 352 17 b: 291 12 -49 13 18 12 1985 p: 2 14 a: 48 53 18 inversa t: 260 37 b: 306 76 142 14 29 03 1986 p: 280 6 a: 230 74 154 lnversa t: 186 29 b: 327 65 18 15 31 12 1988 p: 357 14 a: 46 48 27 lnversa t: 253 45 b: 298 71 135 16 ii 10 1989 p: 162 28 a: 67 73 83 1nversa t: 327 62 b: 268 18 110 17 10 01 1990 p: 145 3 a: 15 12 24 inversa t: 23'( 30 b: 277 67 160 18 23 11 1990 p: 341 34 a: 16 24 17 lnversa 4 t: 204 47 b: 270 83 113 19 25 06 1991 p: 96 7 a: 228 62 159 inversa t: 191 34 b: 328 12 30 20 02 01 1992 p: 29 24 a: 170 55 180 rumbo deslizantecon 4 t: 131 24 b: 260 90 35 componente inversa referencias: 1soluciones obtenidas en este estudio 2-salcedo(1992) 3pennington(1981) 4bolerines delisc 22 geofisica colombiana, 5, diciembre de 2001 [mpllcac[ones s[smotecton[cas de las soluc[ones del mecan[smo focal de algunos terremotos de la reg[on central de colomb[a i. 1966-[-03 18:16:5.1 7. 1973-x-18 9: 18:348 13. 1985·x11-18 22:3629:3 16. [989-x-[1 14:42:9.7 8. 197hv-13 1:53:30.3 ~~/< ! / ,,: .a/ ""1 // \~ t',~.v \l~7!\...)•.-.~ -. -.i-"j> ----------_. -14. 1986-ih-29 9:48:55.6 17. 1990-1-10 2:57:50.7 es sabido que en la zona de transici6n del continente al oceano del territorio colombiano ocurre la interacci6n de gran des elementos litosfericos, yen correspondencia con la teorfa de la tect6nica de placas, es evidente que ocurre la deformaci6n y la penetraci6n de la placa oceanica frfa en el manto de la tierra. aqui el hipocentro de los terremotos refleja la posici6n de la zona de wadatti-benioff. la frta y fragil placa de nazca, como resultado del acercamiento con el continente suramericano, continua sumergiendose bajo el material mucho mas caliente del manto, formando con la masa rocosa continental un fuerte contacto entre los bloques que constituyen la litosfera de las dos placas, oceanica y continental, y a la vez entre los bloques intern os de cada una de elias mismas (figura 4). la fuerza de contacto entre los bloques de cada una de las placas en cuesti6n aparece por la presencia de la fuerte presi6n lateral de geofislca colombiana, 5, diciembre de 2001 3. 1966-x-18 18:43:37.6 10. 1978-1-21 8: 19:29.3 /---. i "l~-~) /'------' i ' ~-----..1~~-i---~ \ [ i /~) \ : ~ ~, \,,--~! /',,-., /.~ 15.1988·xii-3120:58:31.3 19. 1991-vi-25 23:06:58.0 figura 3. soluciones del mecanismo focal de los terremotos encontradas par este estudio, representadas par compresiones + y dilataciones 0, sabre una proyecci6n de area igual del hemisferio inferior de 13esfera focal. p y t denoian los ejes de presion y tension, respecrivarnente, y la lfnea punteada que los contiene represerua el plano ecuatorial. el nurnero que se ante pone a la fecha de cada evento corresponde a la numeraci6n adoprada en las rablas i y 2. calentamiento que se genera con la penetracion de la placa oceanica. la existencia de estas grandes fuerzas genera a su vez fuertes rupturas dentro de la placa, las cuales son las causantes de los complejos y simultaneos movimientos que segun lobkovskyi (1988) ocurren en los focos superficiales de terremotos que suceden en los bordes sismoactivos de zonas oceanicas, como se observa en la figura 2 y la tabla i, los terremotos del 9 de marzo de 1967 (no.4) y del 18 de octubre de 1973 (no.7) con profundidad de 60 y 61 kilometres respectivamente, ocurrieron en condiciones de esfuerzos compresivos en direcci6n nw-se. ambos eventos representan falla de tipo inverso y, aunque notandose una alta componente de rumbo en el 7, tienen el plano preferencial de ruprura orientado transversalmente en direcci6n suroeste-noreste, representando un desplazamiento lateral derecho, el cual concuerda estrechamente 23 salcedo et al e o 0 0 0 -76.'" -75.5 -75.'" -74.50r-.--------;..~-----'_t_-=---------=----,:...:, ·50 i-l°o "0 o :? -o § 150 "ci: -250 '-----.j convenciones vectc-es de roe-zoe de 01;;,66,.. y reoccioo de 10 il'lieroccion de 10 meso de 10 crocc subcoc.do y 10 meso roc oso y del monto de 10 ciccc cootleento! vector de dlrec ctsn general de cesptcrcmeotc en et prone de rupture de los iocca eeroccs en 10 cicco subducido nr-ecctoo en 10 que se introduce 10 prcco oceanica bcjc el ccntjoente [squemot'loci6n de jo ploc:osl,lbducidcl figura 4. esquema de los vecrores gecdinamicos en la zona de subduccion de la placa de nazca bajo el cont inenre suramericano en e l territorio colombiano entre las lati tudes (4.0( . 6.00n. con la direcci6n del sistema de fallas del borde lianero. por su profundidad, estos eventos pueden considerarse como corticales, puesto que la corteza bajo la cordillera oriental tiene aproximadamente un espesor de 66 kilometres (gobema, 1981; coral-g6mez, 1987). ei sismo del 18 de octubre de 1966 (no.3), ocurrido al nororiente respecto a los dos anteriores, tiene su plano preferencial orientado de sw a ne con desplazarniento lateral izquierdo a 10 largo de el; el esfuerzo de compresi6n esta orientado horizontalmente. ei terremoto del 25 de junio de 1991 (no. 19) ocurre superficialmente, bajo condiciones de esfuerzos compresivos vertic ales, que generan un desplazamiento lateral derecho en los pianos de falla; la orientaci6n del plano de ruptura coincide con la orientaci6n de la falla santa maria mas pr6xima a el, mas al nororiente, despues que la cordillera oriental y el sistema de fallas del borde llanero cambian de rumbo, se localiza el terremoto del 21 de enero de 1978 (no. 10), el cual ocurri6 superficialmente y bajo condiciones de esfuerzos compresivos orientados en direccion sww-nee. ei plano preferido es el que esta en la direcci6n sws-nen. en estos eventos (3, 4, 7, 10 y 19) la direcci6n del plano preferido de ruptura es aquel paralelo al sistema de fallas del borde ijanero. e1terremoto del 18 de diciembre de 1985 (no. 13), ocurre bajo esfuerzos compresivos que tienen una orientacion ns, entre las faijas alto del trigo y de com bras; el plano nodal del mecanismo obtenido esta orientado en la misma direcci6n que tienen estas fall as, es decir, sw-ne, representando una falja con desplazamiento lateral izquierdo y componente inverso. 24 por ultimo,el terremoto del 17de enero de 1982 (no. 12)es de tipo distensivo. el esfuerzo principal es subhorizontal orientado de wsw-ene, mientras que el plano nodal preferido esta orientado ssw-nne. este plano y el desplazamiento que en el se indica coinciden estrechamente con el rumba y desplazamiento de la falla salinas, que recorre la zona de estudio desde el centro de esta, dirigiendose hacia el norte casi hasta la latitud de 5s norte, donde cambia de rumba en la direccion nne. ei terremoto no. 20, ocunido el2 de enero de 1992, tuvo lugar en una profundidad interrnedia de 144 kilometres, en este caso el mecanismo tiene un plano casi horizontal e-w. ei eje de compresi6n buza hacia el ne. implicaciones sismotectonicas en la region ei patron 0 esquema sismotect6nico, que resume la sismicidad y las observaciones de mecanismos focales de terremotos para esta regi6n, es mostrado en la figura 5. como puede observarse, la mayor parte de la sismicidad esta localizada en el sector occidental, en un area que abarca los departamentos de caldas, quindio, risaralda, parte de tolima y valle del cauca. en esta zona los sismos son caracterfsticos a una profundidad interrnedia, y estan relacionados a la zona de subducci6n. ei regimen de esfuerzos predominante es extensivo y es gobemado par la tracci6n de la placa subductada, como es habitual para sismos de profundidad intermedia en este tipo de zonas. este esfuerzo y el movimiento de la placa estan bien documentados por datos sismol6gicos (jordan, 1975; pennington, 1981; adamek et al., 1988 y otros) y geodesicos por g.p.s. (freymueller et al., 1993; kellogg and vega, 1995; mora and kellogg, 1995; mora, 1995). la direcci6n del movimiento en los pianos de fallas deterrninados, en la mayorfa de los terremotos de profundidad interrnedia en este sector, coincide con la direcci6n del vector de penetraci6n de la placa oceanica bajo el continente. las flechas grandes rellenadas en la figura 5 muestran la direcci6n de los ejes de esfuerzos de presi6n y tension inferidos de las direcciones de estes en cada mecanismo focal, los cuales varian mas corminrnente de nw a w. en este lugar, la subducci6n alcanza una profundidad aproximada de 180 kil6metros, inferida de la actividad sismica interrnedia. se observa un cordon sismico que se extiende desde el norte del departamento del huila, por el piedemonte de la cordillera oriental, sobre la zona que corresponde al sistema de fallas del borde llanero, que a su vez representa la zona de contacto del bjoqueandino y la placa suramericana en el territorio colombiano (pennington, 1981). la soluci6n obtenida para cuatro eventos (4,7, loy 19) ocurridos a 10 largo de la zona de separaci6n de estos dos grandes bloques es bastante consistente con la direcci6n del plano de falla en cada uno de los tramos del sistema de falja, el cual experimenta cambios de direccion a diferentes latitudes. en los dos sectores identificados en el sistema de falla del borde llanero, de acuerdo con el cambio de direcci6n que experimenta, se nota la predominancia del desplazamiento lateral derecho, siendo de mayor magnitud en el sector sur (salcedo et al., 1995). las soluciones de los mecanismos focales de los terremotos de esta zona de falla muestran una reorientaci6n de los esfuerzos compresivos en la corteza a 10 largo del borde oriental de la cordillera oriental (figura 5). una reorientaci6n similar de esfuerzos en la superficie terrestre a 10largo de esta zona, tambien ha sido notada por mojica (1985), mediante observaciones geol6gicas. en la figura 5 las flechas rellenas mas pequefias muestran la direccion del eje de esfuerzo de presi6n a 10 largo de la zona. geofislca colombiana, 5, oiciembre de 2001 implicaciones sismotectonicas de las soluciones del mecanismo focal de algunos terremotos de la region central de colombia 6.0" , ~\6 2.5" '" ll> "" 'd -o ll> ill ,; t-, t'. r-, ~ '" ld s: er m l") r-, " ~ convencion(s _____ it'jmo <);> ,,,ho f"'~ li>dit;h <> ... ~,,~'''s w~h .. ,i.p~ "" <><:u"""'1 ~!«i""'co, "'.,pi......:.. "''''''''~''\'' ~'t"<e'\<x-:m ... a • • • • • • • • •• 1"0;10 !lot <:aool9'ilm~nlo. lo" "",,'!.n. i'>a<:.;g 1'1 ~ 'l'»<l-nllxlo_ -....................... ra1k, .......... <>. ul' 'ed<:n.;:>l",. r.o,::;o "i h~" ........~i.m". .._ .••.•... ~-~._-r"l<i ~"" it<.j~<>(;i6<l 6f.l '~"(..k>~ '" <:~"....t .. "'yi!ltn'oi ""'''-'''''-<:'''0 ----f<l~p 0:»1;1'\.00, ...••••.•.......•.•.• 'cji,) qp,.,.<!'nmq 0 in!~"i"<l, l,,,,,,oiy\,tnlo fcto<;'l:oi.bq;cq --+--a,n.;tlillai <:tt>'noocd • otros eventos, como 12 y 13, estan representando la gran compresi6n regional producida por la deformaci6n de grandes bloques litosfericos. esta compresi6n ha sido la causa de la generaci6n de los sistemas de falla generalmente de tipo inverso en el territorio colombiano, mientras que la ocurrencia del evento 20 a profundidad intermedia y su condici6n de esfuerzos puede interpretarse como un estado causado por el material litosferico que es empujado por debajo de la corteza desde el sw este mecanismo focal esta soportado por el desplazamiento del bloque andino que es movido en direcci6n ne como resultado de las colisiones recientes o acreciones tect6nicas entre la fosa de carnegie y el continente suramericano. la flecha grande no rellena en la figura 5 muestra la direcci6n del movimiento del bloqueandino respecto a la placa suramericana, conclusiones del estudio de las caracterfsticas de la sismicidad en concordancia con la estructura tect6nica que se refleja en el territorio colombiano entre las geofislca colombiana, 5, diciembre de 2001 coordenadas (2.5° 6s)n y (72° 76°)w, se pueden deducir las siguientes conclusiones: en el sector occidental se observa una concentraci6n de terremotos de profundidad intermedia, entre 60 y 170 kil6metros dentro de la placa oceanica subductada; muchos de ellos responden a la fuerza de tracci6n tipica de estas zonas, mostrando que la direcci6n del movimiento en el plano de falla coincide con la direcci6n de penetraci6n de la placa oceanica bajo el continente. adernas, los esfuerzos derivados de estos focos dan evidencias de las fuerzas de accion y reaccion que ocurren entre la placa subducida y la masa rocosa del bloque continental. el patr6n general de esfuerzos en la region, dentro del continente, deducido del mecanismo focal de los terremotos evaluados, es de compresiones horizon tales, los que a su vez se derivan de la interaccion de las placas de nazca, caribe y suramericana. como producto de la gran com presion regional se generan grandes deformaciones asociadas a zonas de fallas y el desplazamiento del bloque andino en direccion nne. a pesar de que los datos de los mecanismos focales de los terrernotos a 10 largo del sistema de fallas del borde llanero no son muy abundantes y dejan grandes sectores al descubierto, es interesante notar, en esta etapa, que en su recorrido dentro de la corteza se presenta una reorientacion de esfuerzos cornpresivos. en este sentido es importante aumentar la estadfstica de los datos para poder tener una mejor representaci6n de este fenomeno. sin embargo, esto esta de acuerdo con otros datos geologicos que son representativos para la parte mas superficial de la corteza. slsmolo(;icas o;'~(<6'l o"! 10 c<:l'1'"lp<l""tll. !'>on,r.on!ol d. i~ ejf!:j ee p'~ll'~"'. de·'i/(lo(l(l.$ oer mec""';smo lll!l l~("emelo oorfl;c'oo oel it)l! d.e c"'n.6il at cvotro tetftm()tol! q 10 tof~ del ere oe loll<n g'~l 9or<h: tjcoerc a"'" p'e ce ~"ie o~ iii l:crool\e1"lj or'er>loi d:"ii-(.d6f1 (1" 1011t'!li".,,-:cl de ~~silin 'i d;~\",ns,~ (,if"· s de los me'!j",ernol rccetee ~!d aceo de~i"'~cio;:leiq5;it>q'acci6n~io~1i61cq agradecimientos focos. las fechas mas grandes representan la direccion promedio del eje de presion y tension en la lana. figura 5. marco sismotect6nico de la region central del territorio colombiano. las rechas pequerias indican la direcci6n de los ejes de presion en el foco, derivados del mecanismo de los terremotos. los cfrculos muestran los este trabajo se elaboro dentro del proyecto "sismotectonica del territorio colombiano", identificado con el codigo p92fo i, en ellnstituto de investigaciones en geociencias, minerfa y qufmica de colombia (ingeominas), a cuya directiva expresamos nuestro mas sincero agradecimiento por el apoyo institucional brindado al hacer posible est a investigacion y su respectiva publicacion. 25 salcedo et al. referencias bibliograficas adamek, s, frohlich,c. & pennington,w. d. 1988: seismicity of the caribbean-nazca boundary: constraints on microplate tectonics of the panama region, j, geophys, res" vol. 93 (b3): 2053-2075. aztis, l., lay, t. & kanamori, h. 1988: large intermediate depth earthquakes and the subduction process. phys. earth planet. inter. 53: 80-166, buforn, e, & udias, a 1991: focal mechanism of earthquakes in the gulf of cadiz, south spain and alboran sea, publicacion i.g.n, serie monografia no, 8: 29-40, bulletin of the international seismological center (isc), edinburgh scotland, 1960-1990 bune, v i., balakina,l. m. & pavlova,g, i. 1975: sismicidad de sur america, en: tectonica, sismicidad y geodinamica del sector suroriental del oceano pacifico (investigaciones geodinamicas). no.1: 82-99. (en ruso). coral gomez, c. 1987: los terremotos en colombia y caracteristicas de su origen profundo. universidad nacional de colombia, bogota, 199 pp. freymueller,1. t, kellogg, 1. n, & vega, v 1993: plate motion in the north andean region. 1. geophys. res" 98: 21853-21863. goberna,j. r, (ed.). 1981: investigaciones geofisicas sobre las estructuras oceanocontinentales del occidente colombiano. en: proyectos cooperativos internacionales, narifio ii (1976), llj (1978). bogota, 137 pp. herd, d. g., yound, l., meyer, h., arango, j. l., person, w. j., & mendoza, c. 1981: the great tumaco, colombia earthquake of 12 december 1979. science., vol211 (4481): 441-445. isaacks, b., oliver, j. & sykes, l. r, 1968: seismology and new global tectonics, 1. geophys. res., 73: 5855-5899. jordan, t h. 1975: the present day motion of the caribbean plate. j, geophys. res" vol 80 (32): 4433-4439. kellogg,]. n. & bonini, w. e. j 982: subduction of the caribbean plate and basement uplifts in the overriding south american plate. tectonics, i: 251-276. kellogg, j, n. & vega, v 1995: tectonic development of panama, costa rica, and colombian andes: constraints from global positioning system geodetic studies and gravity. in: geologic and tectonic development of the caribbean plate boundary in southern central america: boulder, colorado, geological society of america special paper. mann p., ed., 295: 75-90. kohler, w. 1991: sphere, program for numerical search for optimal fault plane solution of a given set of event. postdam, ajemania. lobkovskyi l., y. 1988: geodinarnica de la zona de spreading, subduccion y la tectonica de placas. nauka, 252 pp. (en ruso). lomnttz, c. & hashtmine, m. 1985: the po ayan, colombia, earthquake of 31 march 1983. bull. seism. soc. am., vol 75 (5): 1315-1326. mendiguren, j. 1973: identification of free oscillation spectral peaks for 1970, july 31, colombian deep shock using the excitation criterion. geophys. j. royal asrron. soc., 33: 281-321 mendoza, c. & dewey, 1. w. 1984: seismicitv associated with the great colombia-ecuador earthquakes of 1942, 1958 and 1979: implications for barriere models of earthquake rupture. bull, seism. soc. arn., vol 74 (2): 577-593. mojica, j. 1985: estilolitos horizontales en sedimentos del cretacico de la cordillera oriental y su significado tectonico. rev. geol. colombiana, no. 14: 7-25. molnar, p. & sykes, l. r. 1969: tectonics of the caribbean and middle america regions from focal mechanism and seismicity. geol. soc. am, bull., 80: 1639-1684 mora, h. 1995: central and south america gps geodesy: relative plate motions deterrnined from 1991 and 1994 measurements in colombia, costa rica, ecuador, panama and venezuela. m.sc. thesis, university of south carolina. 26 mora, h. & kellogg, j. n. 1995: tectonics of northwestern south america, casa gps measurements. american geophysical union. abstract form. nowroozi, a. a, 1972: focal mechanism of earthquakes in persia, turkey, west pakistan and afganistan and plate tectonics of the middle east. bull seism. soc. am. 62: 823-850. page, w. d. 1986: geologia sismica y sismicidad del noroeste de colombia. isa, integral, woodward-clyde consultantes, 281 pp. paris, g. & romero, 1. 1993: mapa de fallas activas de colombia. bol. geol., vol. 34 (2 3), ingeominas, 5-25. pennigton, w. d. 1981: subduction of the eastern panama basin and the seismotectonics of northewestern south america. 1. geophys. res., vol. 86 (b ii): 10753-10770. pennington,w. d., mooney, w. d., hissenhoven,r., meyer, h., ramirez, j. e., & meyer, r. 1981: resultados de un estudio de reconocimiento de microsismos en bucaramanga, colombia. en: investigaciones geoffsicas sobre estructuras oceano-continentales del occidente colombiano.instituto geoffsico, universidad javeriana, bogota, 49-62. philip, h. & cisternas,a. 1985: elterremoto de ei asnam del 10 de octubre de 1980. en: mecanismo de los terremotos y tectonica. a. udias, d. munoz, e. buforn, editores, 176-196. rivera, a 1989: inversion du tenseur des constraintes et des mecanismes au foyer it partir des donnees de polarite pour une population de seismes, application it l'etude du foyer de seismicite intermediaire de bucaramanga (colombie). these de doctorat. universite louis-pasteur de strasburg. 266 pp. salcedo, e. 1992: sismicidad y peligro sismico de colombia. tesis de ph. d. universidad estatal de moscu "m. v lomonosov", moscu-rusia, 268 pp. salcedo, e., rivera, c. i. & gomez, a. 1997: analisis de la frecuencia de terremotos en la region central de colombia mediante parametres del regimen sismico. rev. academia colomb. ciencias. vol. xxi (79): 57-72. salcedo, e., gomez capera, a. & rivera perez, c. 1995: deforrnacion y desplazamiento de bloques corticales en la zona de falla del borde llanero colombiano (mediante datos sobre mecanismos focales de terrernotos). mem. serninario de sismotectonica del borde llanero colornbiano. noviembre 9 y 10 de 1995, bogota, 66-81. sykes, l. r, 1967: mechanism of earthquakes and nature of faulting on mid-oceanic ridge. j. geophys. res., vol. 72: 2131-2153. tandon, a. n. & srivastava,h. n. 1975: focal mechanisms of some recent himalayan earthquakes and regional tectonics. bull. seism. soc. am., vol. 65 (4): 963-969. udias, a. & buforn, e. 1985: mecanismo focal de los terremotos y sismotectonica del area ibero-mogrebf. en: mecanismode los terremotos y tectonica. a. udias, d. munoz, e. buforn, editores: 217-224, udias, a, lopez, arroyo & mezcua, j, 1976: seismotectonics of the azoresalboran region, tectonophysics 31: 259-289, geof[sica colombiana, 5, diciembae de 2001 perez & marfurt.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 147-155 improving vertical and lateral resolution by stretch-free, horizon-oriented imaging gabriel pérez1 and kurt marfurt2, agl, university of houston (1) gabriel pérez: geosciences department. university of houston. houston, texas 77204-5007. geophysics ph.d research assistant. gabriel.perez@mail.uh.edu (2) kurt marfurt: geosciences department. professor, director of agl. university of houston. houston, texas 77204-5007. kmarfurt@uh.edu abstract the pre-stack kirchhoff migration is implemented for delivering wavelet stretch-free imaged data, if the migration is (ideally) limited to the wavelet corresponding to a target horizon. avoiding wavelet stretch provides long-offset imaged data, far beyond what is reached in conventional migration and results in images from the target with improved vertical and lateral resolution and angular illumination. increasing the range of imaged offsets also increases the sensitivity to event-crossing, velocity errors and anisotropy. these issues must be addressed to fully achieve the greatest potential of this technique. these ideas are further illustrated with a land survey seismic data application in texas, u.s. keywords: pre-stack kirchoff migration, seimic data, wavelet stretch-free data, texas. 147 earth sciences research journal manuscript received november 2 2006. accepted for publication december 20 2006. introduction surface seismic data are a major source of information about the subsurface. roughly, this comes in two different ways: through seismic images that illuminate structure and stratigraphy, and from the study of data attributes such as amplitude, arrival time or frequency content that are sensitive to lithology and fluid content. exploration challenges steadily place increasing demands on all factors that impact the usefulness and quality of this information such as frequency content and availability of long-offset data. increasing frequency content is desired to achieve the level of lateral and vertical resolution required for even smaller and/or more elusive targets. increasing the usable range of offsets in the data is desired to support avo studies, anisotropy and velocity analysis, or simply to increase the fold of stacking. unfortunately, these two factors are sometimes counteracting in common seismic exploration practice. specifically, when it comes to imaging, either in conventional nmo or in pre-stack migration, loss of frequency content and wavelet distortion due to stretch is a major problem for far-offset data. typically, imaged data beyond a certain offset, roughly between once to twice the reflection depth, is severely distorted due to stretch that it needs to be discarded by harsh muting. while several methods have been proposed to improving vertical and lateral resolution by stretch-free, horizon-oriented imaging alleviate nmo stretch (dunkin and levin, 1973; rupert and chun, 1975; barnes, 1992), little attention has been devoted to stretch due to migration. in addition, recent approaches attack the problem during the stacking process, thereby resulting in an improved stacked image but not delivering stretch-free pre-stack traces (trickett, 2003). in that case, an increased fold of stack but none of the other desired benefits mentioned above is achieved. recently, wavelet stretch has been recognized as a major adverse factor in avo (swan, 1997; dong, 1999) and increasing attention has been given to develop methods to quantify and correct for its effects. though most approaches focus on the improved estimation of attributes such as avo intercept and gradient or 3-term avo/ava inversion, some directly or indirectly attempt to correct for stretch on the data itself (shatilo and aminzadeh, 2000; castoro et al, 2001; brouwer, 2002; lazaratos and finn, 2004). hilterman and vanschuyver (2003), working in long offset avo, introduced a horizon-oriented pre-stack migration implementation that generates stretch-free pre-stack imaged data. besides from the avo focus, the impact of such horizonoriented stretch-free prestack imaging and the availability of long-offset stretch-free imaged data on image quality and lateral and vertical seismic resolution will be explored, through data from a land survey application in texas, u.s. the impact of such imaging on a land seismic dataset is assessed through the use of multitrace geometric seismic attributes. the attributes computed from pre-stack seismic data may reveal subtle geologic features that are lost in conventional stacked images. in the interest of maximizing the science with a reasonable computational effort, pre-stack time migration has been used as the preferred imaging method. horizon-oriented stretch-free prestack imaging as discussed by levin (1998), wavelet stretch occurs in all pre-stack imaging methods. within the context of kirchhoff migration, this stretch is associated to the variation in the curvature of isochron surfaces for different times on an input seismic trace (figure 1). using the analogy of kirchhoff migration as a moving operation, if “migration paths were somehow parallel”, stretch would be avoided. as suggested in figure 2, it would be possible to “move” a block of input samples along a single path. in this way, relative separation within the samples in the block is not altered so no stretch is introduced. this idea can be actually implemented in practice by computing a traveltime operator that is exact for a single output position and then, migrating a window of the data centered on the computed arrival time, instead of migrating just the single sample matching that time. the central sample is migrated to the “exact” position in the output and, for time migration; surrounding samples are migrated to positions in which the difference in time with respect to the central migrated sample are preserved. in depth migration, a similar outcome can be achieved by scaling the time differences by a factor given by the local value of the migration velocity. this simply amounts to conversion from uniform sampling in time to uniform sampling in depth with scaling given by that factor (see yu et al., 2004 for an application of this idea to wavelet-based depth migration). notice that samples other than the central matching sample are not migrated properly, that is, to the position they would be migrated in conventional migration. in that regard, this approach is only accurate locally, at most in a relatively narrow window around the central matching sample, where the error introduced is small. in the other hand, the migration operation just described makes perfect sense if the matching sample corresponds to a reflector, and instead of just a block of samples operating over the wavelet corresponding to the reflector is considered. in that case, the wavelet is simply kept (with proper scaling to depth, in the case of depth migration) attached to the image of the reflector that carries it. the whole operation amounts to the migration of a broad-band impulse representing the reflector, followed by convolution with the wavelet. using a block of samples is simply a practical way of avoiding the added complication of determination of the wavelet; the size of the block has to be large enough to include the wavelet. regardless of which of the two points of view just discussed is preferred, it is clear that the imaging approach described is limited, be 148 figure. 1. kirchhoff migration schematics and its action on a band-limited wavelet. the data sample whose timing matches the total traveltime from source location to subsurface position and back to receiver location, with proper scaling, is the contribution from the input trace to the image at the given position. in time migration, if the timing within the samples in the wavelet is changed in the output, relative to the input, the wavelet will be distorted accordingly. in depth migration, variations in the separation between output samples, assuming uniform sampling in time in the input data, will also result on differential stretch or squeeze on the wavelet. figure. 2. instead of “moving” individual samples independently, a block of samples is moved along “parallel” paths so that relative separation between samples is not altered, hence no stretch is introduced. ideally the block of samples should include the wavelet associated to the event of interest. this is a modification of the operation shown in figure 1. pérez and marfurt., esrj vol. 10, no. 2. december 2006 149 figure. 3. comparison of imaged gathers pertaining to the same image location, for the: (a) conventional and (b) target-oriented migrations. notice that, by contrast with the conventional migration, the target-oriented migrated data is stretch-free. the event used as a target for the target-oriented migration is shown in blue in both panels. as depicted by the dotted blue line, the desired definition of this event in the longest offsets in the target-oriented migration is obscured by interference from crossing events such as the one marked in yellow. the inferred location of this event on the conventional migrated data is shown with a broken line. cause of either accuracy or scope, to a relatively narrow window around a previously chosen target horizon or target depth. because of this, the migration is limited to a narrow window of input trace samples (usually within 50ms to 200 ms thick) surrounding the originally selected time. similar methods have been developed elsewhere. wyatt et al. (1997) describe an application with emphasis on the efficiency in computation and human intervention provided by imaging limited to a narrow zone around a horizon. our interest arises from the work of hilterman (2004) on improving the frequency content of long-offset data for avo studies, which evolved into a related but different implementation (hilterman and vanschuyver, 2003). the next sections will describe the application of the horizon-oriented migration to the imaging of a field dataset from the fort worth basin, texas, usa. as previously stated, this paper focuses on the impact of the improved frequency content from far-offset imaged data, on image quality, and on lateral and vertical resolution. data processing data was processed twice, initially with a conventional pre-stack time migration and then, with the horizon-oriented stretch-free implementation. the conventional migration work-flow included a migration velocity estimation step. the same velocity field was used for the conventional migration and the horizon-oriented migration. as appropriate for time migration, velocities were computed using a “deregowski-loop” approach where an initial migration was performed with the same velocities used for nmo correction in earlier time processing. using this velocity field, nmo was restored on selected gathers and an additional step of conventional hyperbolic moveout analysis yielded updated velocities, used for the final migration. as a target for the horizon-oriented migration, a horizon interpreted on the stack of the conventional pre-stack migrated data was used as corresponding to the top of one 150 improving vertical and lateral resolution by stretch-free, horizon-oriented imaging figure. 4. comparison between images for a selected inline from the 3d volume for the: (a) conventional and (b) target oriented migrations. images stacked over the full range of offsets available to each migration. the blue arrows in each image point to the location of the event used as a target in the target-oriented migrations. the yellow arrow points to the location of a fault that is better resolved in (b). figure. 5. comparison between images from the conventional and the target oriented migration. images stacked in the far-offset range. once again the fault pointed-to by the yellow arrow is better resolved in the target-oriented image. 151 pérez and marfurt., esrj vol. 10, no. 2. december 2006 figure. 6. comparison of time slices on the full offset range for the: (a) conventional and (b) target oriented migrations. slice is taken at 1.2 seconds, approximately the time of the target horizon. faults trending to the ne can be identified between the yellow arrows in each image. figure. 7. comparison of time slices on the far-offset range for the: (a) conventional and (b) target oriented migrations. time of the slice is 1.2 seconds. 152 improving vertical and lateral resolution by stretch-free, horizon-oriented imaging of the major formations of interest in the area. as seen in figures 4 and 5 this horizon runs roughly between 1.1 and 1.2 seconds; the size of the data block window was 200 ms, centered on the time in the input trace corresponding to the horizon picked time. figure 3 compares the results of the two imaging approaches. in the conventional migration, stretch shows up at the longer offsets, and it is notably stronger at the shallower times (i.e., where reflection angle is relatively large). in contrast, the target-oriented migrated data is stretch-free with nicely aligned reflections for the full range of offsets. at the longer offsets, however, crossing events from earlier and later times interfere with proper imaging of the target events. vertical resolution for the analysis the data was divided in three offset ranges: 0-4000 ft, 4000 – 8000 ft and 8000 – 12500 ft, corresponding to near, middle and far offset ranges. the absolute maximum offset present in the data was around 25000 ft but on average the maximum offset was closer to 15000 ft. the depth of the target horizon lies in the 7000 to 8000 ft range. figures 4 and 5 compare stacked images from the full and far offset ranges for the two migrated datasets. it can be observed that for the fullrange stack the target-oriented migration image is better resolved than the conventional migration. in the far-offset range images in figure 5, the target horizon and the fault zone are more sharply defined by the target-oriented migration. this should be attributed to the combined effect of improved frequency content on the far offsets and increased fold of stack available to the target-oriented images. notice that because of the lower fold, the offset-limited images are of infefigure. 8. comparison between principal component coherence images in the far-offset range stack for the: (a) conventional and (b) target oriented migrations. time of the slice is 1.2 seconds. 153 pérez and marfurt., esrj vol. 10, no. 2. december 2006 154 rior quality compared to those with the full range of offsets. besides, the target-oriented migration images are slightly noisier and more impacted by migration artifacts than the conventional migration images. this is explained because for this dataset, the signal-to-noise ratio is poorer (figure 3) and fold distribution is less uniform in the extended offset range included in the faroffset target-oriented migration. in the very-long offsets (beyond 12500 ft) in the horizon-oriented migration, overcorrected events from the later times in the imaged data window interfere with properly aligned data from earlier times (figure 3). failure to image the data at the very-long offsets is a limitation of the hyperbolic moveout assumption implicit in conventional prestack time migration. inclusion of the overcorrected, not properly imaged events in the stack, results in a distorted and defocused stacked image. avoiding major distortion in the images is the main reason to limit the offsets from the horizon-oriented migrated data to a maximum of 12500 ft, as described above. reduction of the offset range is a drastic and suboptimal solution. a better approach should consider non-hyperbolic moveout in the computation of the operator for the horizon-oriented migration. for the conventional migration, imaging of long-offset data is simply not attempted and, as observed in figure 3, it is severely distorted by the migration. as usual, this data was muted prior to stacking to obtain stacked images from the conventionally migrated data. lateral resolution an improved lateral resolution is fully achieved from the target-oriented migration (figures 6 and 7). just as for the vertical images, the pickcorrected migration is comparatively much less resolved and is not shown. in those figures, the ne-trending faults, indicated by the arrows, are more sharply defined in the target-oriented images for both the far-offset and full offset ranges. though subtle, definition of features like these is potentially very important for the exploration goals in the survey area. a final comparison is made in figure 8 between the conventional and target-oriented migrations at the far-offset range, this time using principalcomponent coherence. principal-component coherence (gersztenkorn and marfurt, 1999) is a multitrace attribute that provides an image of lateral changes in waveform by computing the similarity between neighboring traces in a 3d data volume. multitrace attributes such as coherence may reveal subtle geologic features that are lost in conventional images. these attributes are used as a way to assess the lateral resolution in the migrated images generated in this paper. the ne trending faults and other features pointed to by the arrows, are once again better defined by the target-oriented migration (figure 8). conclusions a block-moveout, horizon-oriented implementation of kirchhoff pre-stack migration effectively delivers stretch-free pre-stack imaged data. imaged data is augmented by long offsets that are discarded by conventional imaging algorithms. this contributes to an improved image quality and larger vertical and lateral resolution. fully achieving the potential of this technique requires that non-hyperbolic moveout is properly accounted for in the migration. the possibility to apply it in a target-oriented fashion has been repeatedly mentioned as one of the strengths of kirchhoff migration. as shown by others before, and reinforced here, if the target is a horizon, kirchhoff migration can be implemented so that it is also stretch-free. this imaging approach provides an increased fold of imaged far-offset data with improved frequency content. this impacts image quality and resolution. in the application of this approach to a 3d dataset from the fort worth basin, good results have been achieved in imaging subtle fault features that are nevertheless important for the exploration targets in the area. finally, moving further in the long-offset range of what is conventionally available, opens up a whole can of worms of new issues that have been swept under the rug or discarded by harsh muting in the conventional approach. improving vertical and lateral resolution by stretch-free, horizon-oriented imaging acknowledgments this work was supported by the sponsors of agl and the state of texas. field data was provided by devon energy. references barnes, a. e. (1992). another look at nmo stretch. geophysics. 57. 749-751. brouwer, j. h. (2002). improved nmo correction with a specific application to shallow-seismic data. geophysical prospecting. 50. 225– 237. castoro, a., white, r. e., and thomas, r. (2001). thin-bed avo: compensating for the effects of nmo on reflectivity sequences. geophysics. 66. 1714-1720. dong, w. (1999). avo detectability against tuning and stretching artifacts. geophysics. 64. 494-503. dunkin, j. w. and levin, f. k. (1973). effect of normal moveout on a seismic pulse: geophysics. 28. 635–642. gersztenkorn, a. and marfurt, k. j. (1999). eigenstructure based coherence computations as an aid to 3d structural and stratigraphic mapping. geophysics. 64. 1468-1479. hilterman, f. (2004). personal communication. hilterman, f. and van schuyver, c. (2003). seismic wide-angle processing to avoid nmo stretch. 73rd annual international meeting: society of exploration geophysicists, expanded abstracts. 215-218. lazaratos, s. and finn, c. (2004). deterministic spectral balancing for high-fidelity avo. 74th annual international meeting, society of exploration geophysicists, expanded abstracts. 219–223. levin, s. a. (1998). resolution in seismic imaging: is it all a matter of perspective? geophysics. 63. 743-749. rupert, g. b. and chun, j. h. (1975). the block move sum normal moveout correction. geophysics. 40. 17–24. shatilo, a. and aminzadeh, f. (2000). constant normal-moveout (cnmo) correction. a technique and test results. geophysical prospecting. 48. 473-488. swan, h. w. (1997). removal of offset-dependent tuning in avo analysis. 67th annual international meeting, society of exploration geophysicists, expanded abstracts. 175–178. trickett, s. (2003). stretch-free stacking. 73rd annual international meeting, society of exploration geophysicists, expanded abstracts. 20082011 wyatt, k. d., valasek, p. a., wyatt, s. b., and heaton, r. m. (1997). velocity and illumination studies from horizon−based psdm, 67th annual international meeting: society of exploration geophysicists, expanded abstracts, 1801−1804. yu, z., mcmechan, g. a., anno, p. d., and ferguson, j. f. (2004). wavelet transform-based prestack multiscale kirchhoff migration. geophysics. 69. 1505-1512. 155 pérez and marfurt., esrj vol. 10, no. 2. december 2006 geociencias-vol 12-1 sep.vp earth sciences research journal earth sci. res. j. vol. 12, no. 1 (june 2008): 44-61 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria akintola, o.f.1 and adekeye, j.i.d.2 1 raw materials research and development council, p.m.b. 232, garki, abuja, nigeria. fax: 234 9 4136034 e-mail: akintolaolatunde@yahoo.com 2 geology and mineral sciences department, university of ilorin, p.m.b. 1515, ilorin email: adekeye 2001@yahoo.com abstract the pegmatites of nasarawa area occur in the central part of nigeria. they are mainly hosted by phyllonites in a nne-ssw trending shear zone lying east of some foliated pan-african and west of jurassic afu complex younger granites. a geological mapping of the area was followed by petrographic and mineralogical studies of selected rock and mineral samples. a total of 72 samples consisting of 25 rocks, 22 feldspars and 25 white micas were analyzed for various elements. the pegmatites are peraluminous and are genetically linked to the late pan-african leucogranite with the shear zone. the pan-african granites have very low ree abundances and non-chondritic ratios of zr/hf and y/ho and low nb/ta ratios indicating crystallization from a liquid-rich melt. barren pegmatites are closely associated with the primitive hornblende biotite pan-african synorogenic granites while snnb – ta mineralized granites are correspondingly enriched in pegmatites spatially associated with pan-african synorogenic granites with enhanced values of rare lithophile elements such as rb, cs, mn, sn and nb-ta. the primary control of rare metal mineralization in the pegmatites is the composition of the source rock since the ta-nb-sn-li-be-w mineralized pegmatites crystallized from fluid (h2o-b-p-f) rich melts. it is hereby proposed that the late pan-african tectonic granite which is parental to the highly mineralized pegmatites in this area originated from anatexis of undepleted mica-rich metasediments at depth, followed by a magmatic fractionation of the fluid rich melt as it ascended through reactivated ancient fractures. the heat for the partial melting might have been supplied mainly by the reactivation of ancient fractures, which controlled the emplacement of the fertile granites and the related pegmatites. keywords: pegmatites, nasarawa, shear zones, mineralization, anatexis, magmatic fractionation, nigeria. 44 manuscript received may 11, 2008. accepted for publication june 19, 2008. resumen las pegmatitas del área de nasarawa se dan en la parte central de nigeria. ellas están principalmente emplazadas en filonitas de una zona de cizalla con una tendencia nne, ssw reposando al e de algunos complejos graníticos como el pan africano joven y al w el complejo jurasico afu. un mapa geológico del área fue seguido mediante estudios petrográficos y mineralógicos de rocas seleccionadas y muestras minerales. se realizó un análisis de varios elementos sobre un total de 72 muestras compuestas por 25 rocas, 22 feldespatos y 25 micas blancas. las pegmatitas son peraluminosas y están relacionadas genéticamente con el leucogranito pan africano tardío y con la zona de cizalla. los granitos pan africanos tienen muy bajos contenidos ree y proporciones no condríticas de zr/hf y y/ho y las bajas relaciones de nb/ta indican cristalización a partir de un fundido rico en líquido. las pegmatitas barren están muy relacionadas con la biotitas y orblendas primitivas de los granitos sinorogénicos pan africanos, mientras que los granitos mineralizados con sn-nb-ta son correspondientes con las pegmatitas enriquecidas espacialmente relacionadas con los granitos sinorogénicos pan africanos con valores altos de elementos litófilos raros tales como_: rb, cs, mn, sn y nb-ta. el control primario de la mineralización de metales raros en las pegmatitas es la composición de la roca fuente a partir de las pegmatitas mineralizadas en ta,-nb-sn-li-be-w cristalizadas a partir de un fundido rico en fluidos (h2o-b-p-f). aquí se propone que el granito tectónico pan africano tardío, el cual es padre de las pegmatitas altamente mineralizadas en esta área se originó a partir de la anatexia de metasedimentos no empobrecidos en micas, seguido por un fraccionamiento magmático del fundido rico en fluidos que ascendió a través de fracturas antiguas reactivadas, las cuales controlaron el emplazamiento de granitos fértiles y las pegmatitas relacionadas. palabras clave: pegmatitas, nasarawa, zonas de cizalla, mineralización, anatexia, fraccionamiento magmático, nigeria. introduction the pegmatite field belongs to the pegmatites related to syn to late pan-african tectonic granites occurring in the pan-african mobile belt east of the west african craton. the field occurs in an area bounded by 70351e – 70 051e, 80081n – 80301n covering an area of 531 km2 (fig. 1). nb-ta-sn-be-liw primary mineralization is hosted in quartz-feldspar-muscovite pegmatites. intrusion of the older granites into the reactivated archean to lower proterozoic crust of central and southwestern nigeria have been shown by rb-sr whole rock and u-pb zircon age determinations) to have lasted at least 630 to 530ma. pegmatites in the same area have been dated 562-534 ma (matheis and caen-vachette, (1983) indicating that the pegmatite emplacement occurred at the end of pan-african magmatic activity. the nasarawa pegmatite field is also in close spatial relationship with the granites of afu complex, which is the southernmost occurrence of the 1250 km-long belt of ring complexes extending across niger and nigeria. rb/sr age decreases from ordovician in northern niger to late jurassic (141 ma) of the afu complex in nigeria. the younger granites as this later suite of rocks are called, are notably mineralized in sn and nb. the two geochemically distinct and economically important types of primary sn-nb-ta mineralization were already recognized by raeburn (1924). in wamba area, kuster (1990) has shown that the emplacement of late pan african granites with similar geochemical characteristics with the mineral45 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria ized pegmatites was fracture-controlled and mylonitized along a conjugate set of ne-sw and nw-se to nnw-ssestriking faults. in more recent times, new rare-metal pegmatite fields have been discovered both within a ne-sw belt recognized by the earlier workers, jacobson and webb (1946) and wright (1970) as well as other areas already known for gold mineralization northwest of the pegmatite province especially the kushaka schist belt, the magami and maradun areas of northwestern nigeria, garba (2003). all the rare metal and gold mineralizations are associated with prominent regional faults in the basement complex of nigeria. this paper discusses the geology and geochemistry of the nasarawa area in relation to the source and controls of mineralization of rare metal pegmatites in the nasarawa area. regional geology nigeria lies within the zone of pan-african reactivation (ca.550 ma) to the east of the west african craton, which has been stable since approximately 1600ma. this mobile belt extends from algeria across the southern sahara into nigeria, benin and cameroon. rocks of the nigerian basement complex which is part of the pan african mobile belt are intruded by mesozoic ring complexes of jos area and overlain unconformably by cretaceous to quaternary sediments forming the sedimentary basins. three broad lithological groups have been distinguished in the nigeria basement complex: a polymetamorphic migmatite-gneiss complex with ages ranging from liberian (ca. 2800 ma) to pan-african (ca. 600ma). ages >3000ma have lately been obtained from some of the rocks (dada, 2006). metamorphism is generally in the amphibolite to granulite facies grade. younger members of this group are n-s to nne-ssw trending belts of low grade (greenschist to amphibolite facies) metasedimentary and minor metavolcanic supracrustals of late proterozoic age. the schist belts which are concentrated in the western half of nigeria are seldomly found east of 80 e longitude, (ajibade and wright, 1989). the schist 46 akintola, o.f. and adekeye, j.i.d. lokoja 534 egbe 632 igbeti osu ijero ibadan makurdi wamba study area kaduna 580 toro bauchi2.27 609 panyam 605 555 nassarawa-eggon 535 545 550 562 631 617 586 idanre 11° 00'e jos ilorin aromoko akure/ijere kainji lake cretaceous to recent jurassic "younger granittes" pan african "older granittes" meta-volcano sedimentary belts polycyclic basement complex 9°6° 6° 9° 175km 0 175km river benue r iver n ig er figure 1. geological sketch map of central and south-west nigeria showing the location of the nasarawa pegmatite field (study area) and the distribution and ages of pan-african older granites and pegmatites (underlined). geochronogical data sources are van breemen et. al. (1977), rahaman et. al. (1983), matheis and caen-vachette (1984), tubosun et. al. (1984), dada et. al. (1987). belt rocks host the gold and rare metal mineralized pegmatites and veins, which are associated with prominent regional fractures. the older granites, which are pan-african orogeny-related, range from synthrough late to postorogenic granitoids of upper proterozoic to lower paleozoic age (ca. 873-500 ma). they intrude both the schist belts and migmatite-gneiss complex rocks and comprise diorites, tonalites, granodiorites, granites, syenites, gabbros and charnockites. the end of the pan-african tectonic event is marked by a conjugate fracture system of strike-slip faults (ball, 1980). fault directions have a consistent trend and sense of displacement; i.e. a nne-ssw trending system having a dextral sense of movement and a nw-se trending system with a sinistral sense (mccurry, 1971; ball, 1980). both sets crosscut all the main pan-african structures, including older n-s trending shear zones (mylonites) (ball, 1980; ajibade and wright 1989, kuster 1990, garba 1996). other parallel pan-african fracture systems with structural trends (n300e and n600e) appear to have been precursors to the development of the cretaceous benue trough and its associated volcanics. the pattern of these fracture systems was probably established during the pan-african orogeny (mccurry, 1971), and the main transcurrent movement probably occurred then – but may well represent lineament of much greater age. wright (1970) was of the opinion that the regional faults had some influence on the direction of migration of hot spots within the mantle that culminated in the formation of the mesozoic ring complexes. late pan-african granites parental to rare metal pegmatites and gold-bearing veins are closely associated with the fractures in the pan african mobile belts (kuster, 1990; ekwueme and matheis, 1995; garba, 2002; okunlola, 2005). the pegmatites, both rare metal mineralized and non-mineralized, are associated with the older granites. the pegmatites were initially thought to be concentrated in a ne-sw zone extending from ago-iwoye in the southwest through wamba-jema’a to bauchi area in the northeast. however, other pegmatite fields have more recently 47 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria g9 g8 18pg18 7p lc19 lc20 lc2 3p 11p 10p ch2 25 ch1 lc21 25 g4 g5 lc 26 kj 04b 31 33 4a 4a kw 30 g25 or lc7 g27 23 28w 30 lb5 tg 32 12p sj 30 a b 8° 18'n 7° 50' 7° 35' 8° 30'n 8° 18'n 7° 50' 8° 30'n 7° 35' ap 6kilometers303000meters 8° 25' 8° 21' 8° 25' 8° 21' 10002000 7° 40' 7° 45' 7° 45'7° 40' scale 1:150,000 ba l e g e n d symbol inferred rock boundary description granite mica schists phyllonites with minor amphibolites undifferentiated strike and dip of foliation pegmatite mine shear zone pegmatites in cross section basic dyke pegmatitic granite g25 sodic aplite g27 main phase pan african older granite granodiorite tonalite gneiss younger granite complex (afu) pegmatite sample point schistlb5 foliation planes in cross section (of the schists) tourmalinite figure 2. geological map of nasarawa tantalite field (sheet 229 northwest udegi). been known around zuru-gusau in the northwest (garba, 2002 and okunlola, 2005), and obudu area in the southeast of nigeria (ekwueme and matheis, 1995). “younger granites”a 1250km-long belt of ring complexes extending across niger and nigeria, with the rb/sr age decreasing from ordovician in northern niger to late jurassic in central nigeria. these are high level anorogenic volcanic-plutonic ring complexes (jacobson, snelling et al, 1964) intruded into the older precambrian to paleozoic basement complex rocks. granites overwhelmingly predominate in the province, but in some complexes their emplacement was preceded by basic and intermediate intrusions, ranging from olivine-gabbro to quartzmonzonite and syenite. the basic, intermediate and porphyritic members of the younger granites carry pb-zn-cu-fe sulphide mineralization. the granitic members are mainly peralkaline arfvedsonite granites and the metaluminous to peraluminous biotite granites: these are the commonest and carry most of the sn-nb mineralization. chemical analysis geological maping of the area was followed by petrographic and mineralogical studies of the rocks. whole rocks chemical analysis of selected representative samples of the granites and the simple graphic quartz-feldspar pegmatites was done using phillips 1404 automatic x-ray fluorescence (xrp) spectrometer on their powder pellets and glass discs in the geochimistry laboratories of the technical university of berlin. icp-ms measurements of ta, nb and rees were performed for 14 samples in geoforschungs zentrum potsdam, using an elan 5000a quadrupole icp mass spectrometer (perkinelmer/sciez), canada. details of laboratory procedures used in analyzing some of the samples by icp-ms are as published by dulski (2001) in geostandars newsletter. few samples with peculiar assays were analyzed by the xrd method to determine their mineralogy. the framed powder samples using phillips pw 1820 diffractometer in the technical university of berlin x-rays were generated at 50kv, 30ma. analitycal condition for each sample were: 0.02° 2ø/step, 2.5 seconds per step with analysis completed from 3-80°2 ø. results geology and petrology of the area the nasarawa area comprises metasedimentary rocks (mainly mica schists and sericitized/chloritized phyllonites) intruded by a pan-african granodiorite/ granite batholith (ca. 600ma), fracture controlled elongate late pan-african granite and pegmatites (fig.2). abutting the mica schist southeast of the area is the afu complex of late jurassic (ca. 141ma) composed mainly of biotite granites with minor quartz porphyry. there is a shear zone that trends north-north east in the area within which the rocks are mylonitized. below is the detailed description of the geology of the area and figure 2 is the geological map of the area. the schists occur as relics and xenoliths in the older granites and pegmatites with thick successions in a north-south trending low-lying area that lies between the older granite and granodiorite/tonalite gneisses rock suite in the west and the jurassic younger granite (afu) complex in the east (see figure 2). the schists generally have measurable north-south trending foliations that dip to the east at low angles (250-300). the north-south foliations are interpreted as pan-african structures superimposed on earlier tectonite fabrics, which sometimes give contorted appearances to the schists. compositionally, the rocks range between metamorphosed pelitic to semi-pelitic and psammitic rocks with biotite, quartz and minor muscovite, as the major minerals. compositional changes related to pegmatites’ intrusion are noticeable at the contacts of the pegmatites and the schist. towards the south of the area, the schist becomes gneissic with appearance of feldspars and pale amphiboles. accessory minerals found in the schist include opaques (ilmenite and magnetite), sphene and garnet. tourmalines and apatites are common accessory minerals in the schist at contact zones with pegmatites and in some cases may constitute more than 20% of the rock the tour48 akintola, o.f. and adekeye, j.i.d. malines in the pegmatites’ exocontact zones in the schist are usually zoned which shows that they crystallized from highly fluid-rich melts (london and manning, 1995). radiogenic haloes are formed around inclusions of radioactive minerals (monazite and zircon) in biotites. the schists as well as other rocks within the shear zone are mylonitized. the mylonitized mica schist-phyllonite is composed of porphyroclasts of biotite and chlorite in a matrix of fine-grained groundmass of muscovite and quartz. within the schist at the center of the area, and in proximity to the schist at the northern part of the area are tourmalinites, which are essentially composed of tourmalines and quartz with accessory to minor apatites. within the schist are fragments of foliated amphibolites that are too small in dimensions to be represented as discrete bodies on the map. the mineral assemblages of biotite, muscovite, and garnet in the schists as well as amphiboles and feldspar porphyroblasts indicate that the rocks must have reached amphibolite grade of regional (barroviantype) metamorphism, during the pan african orogenic cycle. the greenschist facies minerals such as chlorite and green biotite, recrystallized fine-grained muscovite and quartz within the shear zone are products of retrograde metamorphism of the rocks by the post-tectonic processes of shearing/mylonitization that probably accompanied the emplacement of the pegmatites. coincidentally, the boundary of the shear zone marks the boundary of the zone of occurrences of the mineralized pegmatites within the schists. the mylonitic micro-textures of these rocks within the shear zone provide evidence of fracturing/shearing of the rocks. this phenomenon is observable in the leucogranitic samples and schistose samples in which porphyroclasts of biotite, chlorite and quartz are set in a groundmass of recrystallized fine grained muscovite and quartz. the shear zone is occupied by schistose metapelites/metapsammites in a north-northeast belt. within this belt are also the leucocratic pegmatitic granite. ocan and okunlola (2001) have also observed zones of mylonitization in the rocks (granite and schist) associated with the mineralized pegmatites at angwan doka, north-east of this area. similarly at wamba, about 100kilometers northeast of this area (kuster, 1990), there are elongated granitic plutons that are partly affected by deformation (mylonitization) along a conjugate set of strike-slip (transcurrent) faults. the emplacement of these granites appears to be fault-controlled and the directions of relative movements are dextral along ne-sw striking faults and sinistral along nw-se to nnw – sse strikingfaults. it thus appears that there is a regional northeast trending shear/fracture-zone characterized by mylonitization of the rocks coinciding with the zone of mineralized pegmatites, and movement along the faults must have been active before and after the emplacement of the granites and the related pegmatites. older granites of batholitic dimensions intrude these schists, which range in composition from hornblende-biotite granodiorite/tonalite gneiss to biotite granites at higher elevations. this suite of rocks appears to represent the first major episode of granite plutonism in the area. while the granodiorite/tonalite gneiss occupies the western part of the area, the biotite granites, which outcrop as inselbergs, occur in the northwestern part of the area. the granodiorites are strongly foliated with the quartzo-feldspathic phenocrysts developed into porphyroblasts or augen structures. in thin sections, the granodiorite/tonalite gneiss consists of quartz, plagioclase, bluish and brownish amphiboles, biotites with accessory titanite (sphene) and apatite. feldspar and quartz sometimes form wart-like intergrowths –myrmekites. in the biotite granite, quartz, biotite, microcline and plagioclase feldspar are the essential minerals. there may be minor or no hornblende. at the northernmost part of the area, the biotite granite is fluid-rich. some plagioclase shows some sericitisation and pegmatites close to this granite have enhancement of the rare lithophile elements compared with pegmatites close to a less fluid-rich granite. granites at the south-central part of the area are smaller bodies than the main phase granites and have some distinct characteristics in their mode of occurrence in the field. the pegmatitic granite in thin section consists of phenocrysts of quartz in a groundmass of felsic quartz, alkali feldspar and white mica, with very little biotite. the 49 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria quartz phenocrysts are strongly deformed showing wavy/undulose extinction in cross-polarized light; and in some cases, they are recrystallized due to shear movement. the rock is however not foliated and the elongated mode of emplacement is obviously controlled by a northeast-southwest trending fault. some mineralized pegmatites are close to this pegmatitic granite. simple pegmatites with mineral assemblage of microcline-quartz and minor plagioclase (albiteoligoclase) with accessory garnet, tourmaline (schorl), biotite and magnetite intruded the biotite granites-granodiorite suites. within the schists, the pegmatites become richer in muscovites and the rare metal minerals. there is a tendency towards the arrangement of the pegmatites in sub-parallel groups akin to an en-echelon emplacement, and in some cases there are two or more intersecting sets of dykes. a rose diagram plot of the pegmatites indicate two major directions, viz:east-west and north to north north-east. many of the richly mineralized pegmatites occur as sill-like bodies. the swellings are generally loci of intense albitization and mineralization. while majority of the pegmatites in the study area strike north-east/south-west, some have north-westsouth-east and east-west strike directions. strike and dip may change even in one dyke, following planes of weakness (joints, fractures and foliation planes) in the country rock. majority of the pegmatites generally cut across the foliation of the host schists and gneisses. many of the complex pegmatites display a textural and mineralogical zonation parallel with the walls of the intrusion. a zone of tourmalinization (black tourmalines) within the host rock at the contact with the pegmatites is followed by a prominent zone of quartz-mica margins of the dykes. in the complex pegmatites the marginal facies may be up to two feet or more in thickness and as observed in the liberia pegmatite with a paragenesis of cloudy (and in some rare cases smoky) quartz, mica, microcline, albite and accessory large crystals of alkali enriched (blue-green) tourmalines and fluorapatite. the mica is coarse-grained and oriented at right angles to the contacts. within the quartz-mica marginal zone is the quartz-microcline-albite-muscovite-beryl zone. this is followed by a quartz-muscovite-albitetourmaline-amblygonite-montebrasite zone. at the inner zone, there is albite-fine grained muscovite-quartz (clear and colourless). in liberia pegmatite, an albite-rich footwall zone with finely disseminated nb-ta mineralization was observed. from the outer to the inner zones there is enrichment in ta, li and cs and their ores, and the toumalines become albite with increasing contents of na and attractive colours. most of these zones are observable in the complex pegmatites with some minor variations due to variations in their bulk chemistry; at kilimanjaro, hydrated lithium-aluminosilicates (cookeite) were crystallized (no lithium aluminophosphates was sampled from this pegmatite) with albite, mica and quartz in the inner zone. pegmatite-country rock relationships (sharp contacts, unfractured wall rocks, variations in strike, dip and thickness of the dykes) suggest an emplacement level transitional between ductile to brittle host-rock behaviour (kuster, 1990). xenoliths of the foliated host rock, quartz-biotite schist, are present in some pegmatites, suggesting that the pegmatites are younger than the schists. the barren simple quartz-feldspar pegmatites found in proximity to the biotite older granites at the western part of the area are composed essentially of quartz, microcline-microperthite and minor plagioclase (albite-oligoclase). the minor plagioclase appears to be replacing the perthite with sericite by-product. garnet, magnetite and tourmaline are accessory minerals observed in the simple pegmatites. the more complex and mineralized pegmatite deposits occurring in the area show a more pervasive albitization. some pegmatites show subparallel micro cracks with a large perthite crystal, which are filled with albite and sericite. such cracks provide the channel ways by which the soda-rich late stage mineralizing fluids deposit the ores of nb-ta-snli-be. in a favourable environment especially in the middle to inner zones (close to the quartz cores of the mineralized, complex and zoned pegmatites), replacement of microcline by albite is complete, giving rise to the formation of secondary feathery albites and fine-grained muscovite“gilbertile”. east of the 50 akintola, o.f. and adekeye, j.i.d. 51 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria table 1: trace elements of the microcline, microperthites (k-feldspars) of the pegmatites trace elements of pegmatitic k-feldspars sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a p (ppm) 2400 2461 2662 2579 2130 4774 5398 6642 2854 2138 3024 f 0 0 0 0 0 0 326 0 0 0 0 ba 44 35 34 67 10 30 53 36 69 91 39 bi 20 10 17 15 17 15 10 13 12 10 21 cd 13 7 9 5 6 bdl bdl bdl bdl bdl 16 ce 40 32 32 12 52 21 0 19 35 0 69 co 16 33 18 28 20 28 31 25 23 31 18 cr 10 13 9 14 2 15 35 11 6 1 6 cs 1722 1487 1540 1226 1482 602 111 160 844 692 3489 cu 7 7 14 9 0 13 13 8 4 19 16 ga 18 15 17 15 17 16 44 19 18 14 17 la 95 73 74 58 84 27 2 16 38 31 166 nb 8 5 9 7 9 22 39 8 4 8 8 ni 20 0 31 28 0 0 0 0 0 0 27 pb 70 57 81 79 69 46 0 35 125 140 93 pr 19 15 16 13 18 8 0 6 9 8 29 rb 8546 6536 9534 8303 8420 5537 2593 3069 5440 4089 9474 sn 21 13 187 15 14 28 24 28 9 8 24 sr 38 144 58 44 51 22 30 183 64 70 32 ta b.d.l. 1 2 b.d.l. b.d.l. 9 6 b.d.l. b.d.l. b.d.l. b.d.l. tl 49 38 57 49 49 30 12 16 39 30 73 w 155 241 137 187 176 203 195 154 143 204 160 v 10 0 0 14 18 0 0 0 0 0 20 y 30 18 20 0 19 24 12 11 0 14 24 zn 44 0 0 0 0 0 40 0 0 0 0 k/ba 2521 2906 3492 1694 11058 3628 1696 2841 1571 1165 2916 k/rb 13 16 12 14 13 20 35 33 20 26 12 area is the western flank of the afu younger granite complex. the complex is the southernmost occurrences of the nigerian anorogenic ring complexes, which extend through jos and northwards to the arid region of niger republic. the complex was dated 144+ 2 ma (bowden et al., 1976). it is elliptical in outline and about 50km in maximum diameter. it shows broad similarities (geochemical, mineralogical, etc.) to the other younger granite complexes emplaced during the early to late jurassic (jacobson et al., 1958 and macleod et al., 1971). the afu complex is composed mainly of biotite granites with minor quartz porphyry (imeokparia, 1982). the biotite granites show a somewhat fractionation trend with an enrichment of nb, li, f, sn, in the more evolved albitized granites with low biotite contents. mineralogically, the biotite granites are composed of quartz, k-feldspar, albite, and biotite, with fluorite, zircon, cryolite, magnetite, hematite and less commonly cassiterite, columbite, thorite, apatite and monazite as accessory minerals. geochemistry in the older granites g8, g9 and g18b on the one hand have similar geochemical characteristics, which differentiate them from g25 and g27 (see table 1). g8, g9 and g18b are calc-alkali granites with higher contents of ca and mg. their k/rb ratios range between 155 and 261. the lowest value 155 in the range is that of g18b. field evidence shows that these three granites belong to the main-phase pan african older granites. sample g18b with the lowest k/rb ratio as well as ce among these three samples has relatively enhanced values of rb-254ppm, w 241ppm, ta 2ppm, mn 700ppm, sn 32ppm, cs 11ppm, and nb 36ppm. the spatially associated pegmatites 18p and 18ap have enhaced concentrations of rb, 917ppm and 1718ppm; cs, 62ppm and 779ppm, low k/rb ratios 73 and 40 and correspondingly enriched in the ore elements sn, 13ppm and 17ppm; nb, 65ppm and 80ppm; and ta, 15ppm and 21ppm, respectively. on the other hand, the pegmatite 10p which is spatially associated with the less geochemically evolved older granite in the area has low concentrations of the rare elements rb, 450ppm; cs, 2ppm; sn, 9ppm ; and ta 3ppm and a high k/rb of 155. major elements composition of sio2 73.16, 71.1; fe2o3 1.54, 2.02; cao 0.88, 1.75 and mgo 0.42, 0.5 respectively show that g25 and g27 are more leucocratic than the main phase pan african granites. however, trace elements’ compositions of the two granites show a lot of differences and indicate different levels of fractionation and possibly origins for the two granites. g27 has very high ba (1377ppm), sr (677ppm), ba/rb (17.21), and very low k/ba (22) and rb/sr (0.12), which may indicate a metamorphic origin of the rock. the g25 has enhanced mn (600ppm), rb (295ppm), ta (13ppm), nb (24ppm) low k/rb, al/ga, zr/hf and nb/ta ratios of 134, 2486, 19.96 and 1.85, respectively. such high values of lithophile rare elements and low k/rb, al/ga, zr/hf and nb/ta ratios are characteristic of 52 akintola, o.f. and adekeye, j.i.d. trace elements of pegmatitic k-feldspars sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a na/k 0.10 0.17 0.06 0.08 0.09 0.12 0.20 0.16 0.12 0.12 0.06 k/cs 64 68 77 93 75 181 811 639 128 153 33 k/tl 2264 2676 2083 2316 2257 3628 7493 6393 2780 3534 1558 rb/tl 174 172 167 169 172 185 216 192 139 136 130 continuación tabla 1 highly evolved granites parental to rare metal pegmatites (raimbault et al., 1995). thus the major and trace element distribution in the sampled older granites show that samples g25 and g18 are more highly evolved with lct geochemical affinities (cerny, 1991c) than g8, g9 and g27. both granites also have negative ce anomaly which may be an indication of oxidizing conditions during rare-metal mineralization (piper, 1974). the sampled afu complex younger granites g4 and g5 are depleted in ca, mg and sr; have high fe/mg, and are enriched in nb, y, f and zr, thereby showing the characteristics of the nyf suites (cerny, 1991c) compared with the older granites. they have lower asi (aluminum saturation indices) and more alkaline. in both the older and the younger granites, mg, ti, ba and zr are depleted in the granites with enhanced values of rb and therefore amply depict the degree of magmatic fractionation within the suites (figures 3 and 4). the granites with enhanced values of rb are also enriched in cs and the ore elements of sn-nb-ta. in the younger granites, g5 is more leucocratic and coarser grained with less biotite than g4 it also has more enhanced values of the lithophile rare elements like ta, nb, f, p, rb, mn, y,u, cs, th, mo, w, and low k/rb (104) and al/ga (2280) ratios. it however has a higher nb/ta ratio (4.65) when compared with that of 1.85 of g25. the younger granites have on the average, higher f content that the older granites while negative ce anomaly (very low ce content) is observed only in the mineralized older granites g25 and g18. kinnaird (1984) and barchelor (1987) have observed similarly distinct geochemical characteristics in the older and younger granites of nigeria. the ree concentration (fig. 5) of pegmatites 10p and 11p show a decrease of an order of magnitude from those of the granite g25 while the ree in the pegmatitic white mica are the least (see table 2). the ree generally have sub-horizontal to heavy rare earth element-depleted patterns; minerals with the lowest ree abundances have nearly horizontal patterns. ree contents of the granite, g25, which is highest in the samples analyzed, is very low (< 2x chondritic). such 53 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria barren 10p be li, be li, cs, be, ta cs 100001000100101 1 10 100 1000 k/rb figure 3. classification of the pegmatites using the plots of k/rb versus cs of their k-feldspars according to trueman and cerny (1982). rare-elements pegmatites’ class muscovite class cs 1000010000 k/rb 1000100010001000 10 100100 10001000 figure 4. plot of k/rb versus cs for the pegmatites’muscovites (after cerny and burt, 1984). gordiyenko (1971) beus (1968) 10000100001000100010010010 11 1010 100 1000 cs ta figure 5. plot of ta versus cs for the muscovites of nasarawa pegmatites. (after beus (1968), gordiyenko (1971)). 54 akintola, o.f. and adekeye, j.i.d. table 2: trace element contents of the albitic pegmatite phases sample albites cookeite phosphates fluorapatite amblygonites lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 p2o5 (%) 0.481 0.487 2.169 1.763 0.243 0.017 24.97 49.973 43.25 f (ppm) 1240 0 0 0 0 294 11637 13897 14203 as 0 0 7 8 6 1 0 0 0 ba 19 51 37 26 102 56 31 33 61 bi 14 11 11 16 12 12 38 1 8 ce 14 12 22 14 0 0 443 0 7 cd 7 bdl bdl bdl bdl bdl 27 bdl bdl co 115 47 25 31 37 5 16 8 6 cr 1 0 0 11 0 15 1 2 1 cs 851 23 19 74 9 49 1 28 6 cu 0 0 24 16 4 0 0 16 535 ga 76 24 21 22 39 58 14 21 18 hf 2 3 0 3 3 2 0 3 2 la 48 9 0 3 3 0 204 0 0 mo 0 5 1 1 0 4 0 1 0 nb 88 75 62 326 115 33 190 26 25 nd 3 11 9 8 2 0 144 5 7 ni 14 5 3 4 0 3 9 4 3 pb 25 10 7 0 0 0 74 9 0 pr 10 2 1 2 1 0 65 0 1 rb 4829 69 347 252 31 444 46 290 62 sc 0 0 9 0 4 0 0 0 5 sm 1 3 2 2 1 1 66 1 1 sn 565 659 14 174 35 54 28 231 67 sr 263 52 1037 305 51 8 64 11 188 ta 345 109 67 305 297 37 21 107 86 th 0 0 0 0 0 3 2 1 4 tl 23 bdl bdl bdl bdl 5 bdl bdl bdl u 0 3 0 10 0 1 216 0 7 v 5 1 4 15 6 5 16 4 8 low ree abundances (mostly between 20x and 1x chondritic) with sub-horizontal to heavy rare earth element-depleted patterns are typical of rare metal granites and associated pegmatites, (cerny, 1991c; raimbault et al, 1995; and morteani et al, 1995; preinfalk et al., 2000). despite the low ree abundances in the rocks/minerals, evidence of magmatic fractionation is given by the negative eu anomalies in the pegmatites and muscovites. g25 shows a rather horizontal ree pattern and slight inflections with minima corresponding to nd, gd, and ho, which indicate a fractionation reflecting the lanthanide tetrad effect (bau, 1996). the tetrad effect is more noticeable in white micas and the pegmatitic samples with some showing the v-shaped pattern with strong negative eu anomaly. according to zhao and cooper (1993), v-shaped patterns indicate an extensive crystal fractionation involving feldspar, biotite and accessory ree minerals such as monazite and zircon. the extremely negative euanomalies also correlate positively with the rare-element accumulation (matheis, 1991). the ree-depleted and rb-enriched nature of the g25 is also characteristic of peraluminous lct (enriched in rb, be, ga, sn, mn, li, cs, nb, and ta) granite intrusions (cerny, 1991c). petrogenesis of the rare metal pegmatites the peraluminous pegmatite granites parental to the rare-metal pegmatites were formed by partial melting of mica-rich metasediments along the regional fracture zones as enunciated by wright (1970), matheis (1991) and garba (2002). although, the anatexis of the metasediments occurred at deeper levels below the currently exposed surface, evidence of shearing of the rocks at the earth’s surface is provided by 55 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria sample albites cookeite phosphates fluorapatite amblygonites lb6a lb8 sj3 rna lc19 kj2fsp lb9 lb10 ch1 w 502 346 201 247 298 64 46 89 124 y 6 1 0 0 1 0 1391 1 0 zn 229 63 26 187 12 39 62 26 119 zr 17 12 17 68 18 5 17 54 7 h2o 2.42 0.43 1.1 0.82 0.36 8.8 0.51 5.27 7.54 sum 98.71 98.24 97.67 99.4 99.79 98.01 98.12 102.94 86.53 k 33457 1577 11540 3321 1079 9132 1494 3487 664 k/rb 7 23 33 13 35 21 32 12 11 mg(hx) 13 139 7 23 li(hx) 225 685 16400 20750 li(fusion) 227 2900 13366 17882 na/k 1.17 43.74 5.81 19.80 72.80 <1 21.80 4.23 n.d. nb/ta 0.26 0.69 0.93 1.07 0.39 0.89 9.05 0.24 0.29 continuación tabla 2 56 akintola, o.f. and adekeye, j.i.d. t ab le 3: t ra ce el em en ts in th e pe gm at it e m ic as s a m p le u n it le 7 le 8 le 1 4 le 1 3 le 1 8 l4 l1 0 b l8 a l9 a lz lu a ls 7 l c1 9 lc 2 0 lc 2 0 a s1 k k a lc 2 3 lc 2 8 w w 2 r lc 3 0 lc 3 1 lc 3 2 lc 3 3 f p p m 2 9 3 3 3 1 9 7 3 2 1 5 3 2 2 0 2 9 7 0 2 7 4 5 3 2 3 3 3 1 3 1 2 8 8 4 4 3 5 1 2 4 6 5 2 1 0 6 6 1 7 1 0 0 4 3 9 0 3 9 3 3 2 9 6 6 7 4 6 1 2 8 4 1 2 2 5 4 2 6 1 6 2 9 6 5 4 7 5 1 9 4 0 1 0 8 8 8 8 1 3 7 4 b a p p m 1 5 6 0 3 1 2 7 2 4 1 2 3 0 3 5 2 0 3 1 2 3 5 3 7 6 9 3 5 1 9 1 3 2 8 1 5 4 2 2 5 1 0 0 7 4 3 3 1 6 2 3 4 4 1 8 b i p p m 1 7 2 1 2 0 1 6 2 1 2 5 1 6 1 9 1 2 1 9 1 3 1 4 1 2 1 5 2 8 3 4 1 4 2 1 1 7 1 8 2 0 1 1 1 8 1 5 1 3 1 2 1 3 1 7 c e p p m 1 2 2 9 5 2 0 2 6 1 3 0 1 1 0 0 0 0 9 3 2 3 9 3 7 1 8 2 6 1 4 1 2 9 0 2 7 0 2 2 0 2 3 c s p p m 1 7 1 8 7 4 2 9 0 3 0 0 2 6 0 8 7 4 1 0 0 8 1 0 0 1 2 3 2 8 9 6 2 0 6 3 2 1 1 8 3 4 2 2 1 2 0 2 4 6 7 4 2 4 5 1 5 6 9 4 5 1 6 9 4 2 1 5 1 7 4 9 2 9 1 4 9 9 6 3 1 1 6 3 4 6 g a p p m 1 6 7 1 6 3 1 6 3 1 5 3 1 6 7 1 6 8 1 5 8 1 6 4 1 6 2 1 7 4 1 5 5 2 0 3 2 0 0 1 6 6 1 5 6 1 6 4 1 5 9 1 4 1 1 7 6 1 7 4 1 9 7 1 5 4 1 4 5 1 6 2 9 7 1 4 7 1 6 1 1 3 8 la p p m 0 5 7 1 5 2 0 2 3 5 4 4 4 4 5 1 0 3 6 9 0 0 4 7 9 8 1 0 8 1 7 1 4 3 6 0 4 0 3 2 0 4 9 7 1 0 2 1 5 n b p p m 1 8 1 1 1 6 1 7 8 1 6 5 1 8 2 1 3 1 1 1 9 1 1 3 1 8 5 1 4 4 1 7 8 2 1 8 2 5 6 1 8 7 7 5 5 5 1 4 6 1 5 1 1 6 0 2 1 1 1 4 3 1 9 3 2 0 0 1 2 7 6 4 2 2 3 1 9 0 1 3 5 n d p p m 9 1 7 4 1 4 1 0 6 0 7 2 0 2 0 1 9 1 2 7 7 1 3 2 1 7 0 1 1 0 9 4 5 7 n i p p m 7 2 7 1 7 1 8 2 1 2 3 2 4 2 4 2 1 2 3 1 3 8 9 1 8 2 7 3 1 1 8 2 1 1 6 1 1 2 0 1 0 6 1 7 1 1 1 6 1 8 2 4 p b p p m 1 8 4 7 2 3 2 1 3 0 4 6 4 5 4 5 1 9 4 4 1 7 6 4 2 5 4 7 6 4 1 8 2 6 2 2 7 3 3 1 0 9 3 3 2 7 9 1 8 1 9 p r p p m 3 1 5 6 8 8 1 4 1 2 1 2 6 1 1 6 1 3 1 1 1 9 2 2 6 6 9 2 1 1 7 3 1 1 1 3 2 4 5 r b p p m 4 8 0 3 9 4 1 0 5 0 9 3 5 5 7 8 5 8 3 9 9 1 3 9 9 1 5 0 8 8 0 3 5 2 8 9 8 8 7 3 4 7 5 1 3 1 3 3 2 6 5 9 5 6 3 8 7 7 7 4 1 0 1 8 2 4 5 0 4 4 9 4 1 5 5 2 7 3 4 7 1 7 7 4 9 3 2 8 4 2 8 7 0 6 8 0 3 4 5 8 1 3 6 4 5 4 3 2 4 4 2 4 8 s n p p m 2 1 7 4 7 1 2 7 5 3 3 6 3 5 7 5 2 5 5 3 3 5 9 7 2 7 4 6 8 1 2 3 9 6 1 2 6 6 3 9 7 6 8 1 6 4 9 2 9 5 3 6 4 3 9 4 8 7 4 3 7 1 3 9 1 1 8 9 0 2 5 3 9 1 4 7 2 7 1 2 1 9 s r p p m 1 5 5 0 4 1 1 8 1 9 2 4 2 7 2 5 1 7 2 4 1 6 1 1 1 3 1 8 2 5 3 2 1 6 1 8 1 9 1 1 2 3 1 5 1 2 2 1 1 5 1 7 1 7 1 6 t a p p m 4 4 5 8 4 5 5 0 6 3 5 3 5 9 7 1 7 1 7 1 4 1 2 7 4 6 7 2 5 0 2 4 2 5 7 5 8 5 1 1 5 3 1 5 1 6 2 6 4 1 2 0 1 8 3 3 9 3 1 1 4 0 t l p p m 2 2 4 3 2 4 2 7 2 6 4 0 3 9 3 9 2 5 4 0 2 1 1 7 1 5 3 1 5 2 6 4 2 4 2 9 2 8 1 6 3 5 1 8 1 7 3 6 3 3 1 8 2 1 2 5 w p p m 7 1 5 1 6 5 7 1 2 3 7 3 9 3 3 2 8 3 8 7 7 2 5 8 4 6 5 4 0 8 6 8 9 3 4 3 6 4 8 5 6 5 2 4 0 4 9 9 1 5 6 1 1 4 3 2 6 1 z n p p m 4 2 1 1 0 0 0 4 5 2 4 3 5 4 7 2 9 6 1 1 0 2 3 9 1 1 4 1 6 8 0 0 3 7 9 2 3 1 1 3 1 2 1 7 1 1 1 1 2 3 1 4 2 2 4 2 1 4 2 1 8 7 3 4 1 2 4 9 1 6 3 4 5 3 4 7 1 6 1 1 7 7 1 1 2 mylonitization of the rocks within the fracture zone. high heat flow and shear movement along the regional fractures might have contributed significantly to the heat for the partial melting of the metasediments. an evolution of the magma as it ascends through the fractures would be toward an increase of the depolymerizing elements f, p and al (raimbault et al., 1995). these elements would depress the liquidus of the magma, thereby reducing the viscosity of the melt while aiding both its flow along the fractures and the extreme fractionation of the elements. roofward enrichment of rare elements (such as b, li, rb, cs, ti, be, mn, sc, y, h, rees, sn, th, mo, ta>nb, and w) can be expected ( cerny, 1991c) in the lct granite-pegmatite suite. the magmatic evolution of the melts is towards an increase in the al, rb and cs as documented in granites and pegmatites in the area while evolution from silicate-dominated melt to water-dominated b, f, li and p-rich fluids is marked: petrologically by common accessory apatites and zoned tourmalines in the pegmatites’ exocontact host schists, as well as deuteric alteration, sericitisation and albitization of feldspars in the granitic rocks parental to the mineralized pegmatites. the most complex of the pegmatites in this area belong to the amblygonite subtype of the classification of cerny (1991b) enriched in p, f, li, rb, cs, be, ta>nb. geochemically by low mg, ti, ba, zr and ce, as well as low ba/rb, low k/rb, nb/ta and k/cs ratios in the pegmatites and the related granites as well as non-chondritic y/ho and zr/hf ratios, high negative eu anomaly and lanthanide tetrad effect in the ree distribution patterns as documented in the pegmatitic granite g25. as already noted by bau (1996) the rare elements are transported as complexes in such fluid-rich melts. it is also clear from this area that there are two distinct rare metal generating events associated with the pan african orogeny viz: enrichment of the rare metals in the fluid-rich and deuterically altered main phase pan african granite g18 and the spatially/genetically related pagmatites. this granite is the northernmost extension of older granites in this area and obviously represents marginal part of the batholith. it is characterized by a widespread strong alteration of plagioclase (a replacement of the plagioclase by perthite, and finally albitisation/sericitization). this deuteric alteration resulted from the late metasomatic fluids that mobilized the ore elements, sn and nb, and concentrated them in the granite. such deuteric alteration is marked in the border zones and in the uppermost parts of granitic bodies (pedrosa and siga, 1987). enhancement of the rare metals in the geochemically distinct g25 is fracture controlled and postdates the emplacement of the main phase pan african granites. the fact that the pegmatitic granite g25 was emplaced into fractures and again mylonitized after emplacement (see also kuster, 1990), shows that these fractures were active before and after the emplacement and were probably reactivated during the emplacement of the younger granites during the jurassic. matheis and caen vachettee (1983) have documented biotite ages of 185/183ma from southwestern nigeria. basalt intrusions of 165ma north of zaria in northwestern nigeria, which as inferred by matheis and caen vachette (0p cit) indicate a regionally more extensive thermal event in association with the central nigeria younger granites emplacement. the low p and f contents of the pegmatitic granite g25 (p2o5 0.068%, f35ppm table 1) may be explained in that the elements which would have been concentrated at the roof of the granite, would at the current level of exposure of the granite have been lost to erosion. it appears, based on the low elevation of the g25 and the outcrop of ne-sw and nw-se trending tourmalinites, which are most probably related to the pegmatitic granite intrusion in the area that some of the granites parental to the rare metal pegmatites are not yet exposed at the current erosional level (that is they are still lying buried). similar views were very recently expressed by garba (2002) who inferred from the studies of gold and rare-metal pegmatite occurrences in the kushaka schist belt of north-western nigeria that the mineralizations are controlled by, postdates the pan-african tectonism and related to ne-sw and nne-ssw trending regional fractures. it is worth noting that pan-african sn-w bearing quartz-veins occur in the reactivated crust of the central hoggar and 57 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria are probably related to a 521ma old peraluminous differentiated granitic plutons. helba et al. (1996) also reported higher ta/nb ratios in the more differentiated albitized eastern part of nuweibi albite granite, in the eastern desert region of egypt. 207pb/206pb ratios in zircon from the granite yielded 450 – 600ma-a post-kinematic pan-african age. discussion cerny (1991c) and douce (1999) have observed that anatexis of mica-rich supracrustal sequences as well as ortho and para-lithologies of their basement in both the classical orogenic cycles and non-orogenic magmatic events give rise to peraluminous granites. these observations are corroborated by high intial 87sr / 86sr ratios (0.710 to 723) of the pan african older granites as well as the post-kinematic intrusives i.e. the pan african older granites (matheis and caen-vachette, 1983; bertrand et al., 1987), which strongly suggest crustal influence in the generation of the late pan african peraluminous granites. thus the older granites share some geochemical affinities with the lct suites while the younger granites share geochemical affinities with the ayf suites as recognized by cerny (1991c.) kuster (1990) observed that the late pan african tectonic granites at wamba (about 100km northeast of nasarawa) are all subalkaline, peraluminous, and highly siliceous rocks with their peraluminosity more pronounced with increasing differentiation. the major elements si, al, k, and na show only slight variations; only na is enhanced toward the end of granite evolution. in the course of evolution from the biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is a pronounced enrichment of rb, li, cs, sn, nb, mn, and p whereas b is only slightly enhanced. strong depletion is evident for ba, sr, zr, y, la, and ce together with ti, mg, ca, and fe. these results support the observation that the rare-metals are related to highly differentiated granitic magmas and represent strongly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals, (cerny, 1991b and london, 1990). conclusions it is conclude that anatexis of mica-rich supracrustal sequences as well as ortho and paralithogies of their basement give rise to peraluminous granites. these observation are corroborated by high initial 83 sr /86sr, ratios (0.710 to 723) of the pan african older granitesas well as the younger granites which strongly suggest crustal influence in the generation of the late pan african peraluminous granites. thus the older granites share some geochemical affinities with the lct suite while the younger granites share geochemical affinities with the ayf suites. in the course of evolution from the biotite granites through biotite-muscovite granites, muscovite granites to the apogranites, there is pronounced enrichment of rb, li, sc, san, nb, mn and p whereas b is only slightly enhanced. strong depletion is evident for ba, sr, zr, y, la, and ce together with ti, mg, ca, and fe. these results support the observation that the rare metals are related to highly differentiated granitic magmas and represent strongly fractionated residual melts rich in silica, alumina, alkali elements, water and other volatiles, lithophile elements, and rare metals. the ne-sw and nne-ssw regional fractures controlling the mineralization are deep seated. reactivation of the fractures in the mesozoic probably influence the emplacement of the anorogenic younger granites and initiated the formation of benue trough in the mesozoic period. the benue trough, which is parallel to the ne-sw trending belt of mineralized pegmatites hosts pb-zn-cu-fe sulphides, fluorites and barites. the afu complex younger granites are more alkaline than the other granites as reflected in their lower a/n+k ratios; they have high fe/mg ratios and low tio2 contents which tend to agree with lameyre and bowden’s (1982) documentation of the younger granites of nigeria as continental epeirogenic uplift granitoids (ceug). peraluminous granites are known, according to cerny (1991c) to be parental to the granite-pegmatite suites. trace element studies of the 58 akintola, o.f. and adekeye, j.i.d. suites also show that extreme igneous fractionation aided by the fluids rich in b, p and f leads to the concentration of the rare metals in the residual melts that form the pegmatites. acknowledgements financial assistance for the fieldwork by the nigerian government through the raw materials research and development council, rmrdc is gratefully acknowledged. the german government through the german academic exchange programme (daad) provided financial assistance for the rocks/minerals chemical analyses in the technical university of berlin (tub). we are grateful to dr. g. matheis of the technical university of berlin (tub) for his assistance on the research and review of this paper. we are also grateful for the assistance of professor peter moller and dr. peter dulski of the geoforshungs zentrum (gfz), potsdam for their assistance in analyzing the rare earth and trace elements of some of the samples. references ajibade, a.c. and wright, j.b., (1989). the togo-benin-nigeria shield:evidence of crustal aggregation in the pan african belt. tectonophysics 165, 125-129 ball e., (1980). an example of very consistent brittle deformation over a wide intra-continental area: the late pan-african fracture system of the tuareg and nigerian shield. tectonophysics 61, 363-379. barchelor r.a. 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(1982): exploration for rare-element granitic pegmatites. in cerny, p. ed., granitic pegmatite science and industry. mineralogical association of canada, short course handbook, 8: 463 494 wright, j.b., (1970). controls of mineralization in the older and younger tin fields of nigeria. econ. geol. 65, 945-951 zhao, j. x. and cooper, j.a., (1993). fractionation of monazite in the development of v-shaped ree pattern in leucogranite systems; evidence from a muscovite leucogranite body in central australia. lithos 30, pp. 23-32. 61 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria letter from editor a new interpretation for the garnet zoning in metapelitic rocks of the silgará formation, southwestern santander massif, colombiacarlos alberto ríos reyes, oscar mauricio castellanos alarcón and akira takasu 7 material assimilation in a shallow diapiric forceful intrusion: evidence from microstructures and csd analysis in a porphyritic intrusive body, “la línea” tunnel, central cordillera, colombialorena rayo and carlos a. zuluaga 31 mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeriaakintola, o.f. and adekeye, j.i.d. 44 denoising resistivity phosphate “disturbances” using haar mother wavelettransform (sidi chennane, morocco)saad bakkali and mahacine amrani 62 petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”ríos, c.a., castellanos, o.m., gómez s.i. and ávila, g.a. 72 seismic velocity determination in gravel and sands using piezocrystalsarmando luis imhof and juan carlos santamarina 107 geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 63-75 groundwater investigation in awlad salameh, southern sohag, upper egypt aiman abdel latif1 and mohamed el kashouty2 1 national research institute of astronomy and geophysics, helwan 2 cairo university, faculty of science, geology department aiman_aziz21@yahoo.com abstract the groundwater was the only water resources in the reclaiming area in awlad salameh village in the western side of nile valley in sohag governorate, egypt. the soil was salinized and decrease the income per capita due to the increase in water salinity. fifty groundwater samples was analyzed for major ions, besides the hydrogeological data. the groundwater salinity increased in the northwestern, northern, and northeastern part, attributed to geological, hydrogeological, and anthropogenic sources. the structural pattern enhances the downward, lateral and upward intrusion of saline eocene limestone and nubian sandstone aquifers. the salinity decreased in areas of wadis deposits, which characterized by high infiltration of rainfall through gravelly and cobbly sediments. the k concentration is mainly caused by aquitard diffusion rather than anthropogenic. the saturation index approach and statistical analyses were determined and discussed with respect to the geomedia and anthropogenic source. the groundwater is unsuitable for drinking and irrigation purposes due to increase in total dissolved solids. key words: awlad salameh, groundwater, soil salinization, hydrogeochemical processes resumen el agua subterránea es el único recurso de este tipo en el área de recuperación en el pueblo awlad salameh ubicado en el lado occidental del río nilo en la provincia de sohag, en egipto. el suelo esta salinizado y disminuyó el ingreso per cápita debido al aumento de la salinidad del agua. cincuenta muestras de aguas subterráneas fueron analizadas en busca de iones importantes, además de otros datos hidrogeológicos. la salinidad del agua subterránea aumentó en el noroeste, norte y noreste, debido a factores geológicos, hidrogeológicos y de fuentes antropogénicas. el patrón estructural realiza la intrusión hacia abajo, lateral y hacia arriba de la caliza salina del eoceno y del acuífero de la arenisca de nubia. la salinidad disminuyó en las zonas del depósito, que se caracterizan por la filtración de las precipitaciones a través de la grava y los sedimentos gruesos (adoquín). la concentración k es causada principalmente por difusión lenta en lugar de antropogénica. el índice de saturación y el análisis estadístico fueron determinados y discutidos con respecto a la media y a la fuente antropogénica. el agua subterránea no es apta para beber ni para riego, debido al aumento del total de sólidos disueltos. palabras clave: awlad salameh, aguas subterráneas, salinización del suelo, procesos hidrogeoquímicos 63 manuscript received: 22/12/2009 accepted for publication: 16/05/2010 1. introduction sohag governorate constitutes an important hydrographic part in the nile valley in upper egypt. sohag governorate lies in the middle part of the nile valley which situated south of cairo by about 460 km and has a total area of 11022 km2. groundwater constitutes an important source of water in the strip of the desert fringes on both sides of the nile valley which has been partially reclaimed. because of the development, the government and the people have been focused on the new areas in the western part of the cultivated land boundary (investigated area). the studied area occupies a region including both the floodplain and the desert fringes between longitudes 31o 30` and 31 o 45`e and latitudes 26 o 15` and 26 o 30`n (fig. 1a). the nile valley, in sohag area, is bounded from east and west by the eocene limestone plateau. these plateaus are dissected by a number of drainage basins that drains towards the valley from the east and the west. the surface areas of these basins are around 740 km2. the strip of the desert fringes on both sides of the nile valley has been partially reclaimed and irrigated completely with groundwater. in the last ten years, rapid increase in the use of groundwater in the desert areas has occurred and that can affect the sustainability of the supply both quantitatively and qualitatively. recent activities of the land reclamation are carried out in the area covering about 295 km2 where 182 km2 could be reclaimed. the remaining area of about 263 km2 is not reclaimed yet. sohag was represented by 5 % of egypt (gasg 2004), the developments were relamation projects, new urban communities, industraial areas, and wastewater dis[osal sites in the desert zones of the nile valley. untreated wastewaters dumped by industrial facuility impact the environmental helth (el gohary et al 2002). many problems in the production of crops and death of plants in the reclaiming area was found in awlad salameh, resulted from irrigation by saline groundwater. groundwater contamination is one of the principal environmental and public health problems in sohag governorate. it is attributed to intensive use of agro-chemical fertilizers, septic tanks, and pesticides and wastewater disposal (ayman 2009). then, the main objective is to study the hydrogeochemical and hydrogeological characteristics of the shallow quaternary aquifer. fifty water samples were collected from the shallow (10-50 m water depth) quaternary aquifer. 2. geology and hydrogeology the geology of the study area is very important especially in the hydrochemistry of groundwater, it is lightly influenced by the type of rocks, surficial soils, and structures in the area. the study area was in the unstable shelf of egypt (meshref 1990). the structural features developed in the nile valley are pre-paleozoic or older basement trends and rejuvenated later on during tertiary times (sigaev, 1959; said, 1961 and 1962; brioussov, 1968; yossef, 1968; youssef et al. 1978). the eocene plateau in nile valley is effected by normal faults and drag folds (yallouze and knetsch, 1954; abdel kareem, 1972; omara et al. 1970 and 1973; mustafa, 1979; mahranand el haddad, 1992). they stated that the structural features are originally induced from tensional rather than compressional forces. structurally, the river nile is drained in central part and bounded by two limestone plateaus on both sides. the river nile is occupied on both sides by low lying areas (floodplain), represented by graben that created by upthrow of two plateaus. the latter gently dips about 3o due the central low part (zaki 2001) and about 54 recorded surface fault planes. the fault and interconnected of them influence greatly on the rate of precipitation-infiltration into the aquifer system and the enhance the upward leakage from the underlying aquifers (saline) toward the pleistocene aquifer. generally, the geology of sohag area can be outlined as follows: 1) thebes formation (eocene); it was first introduced by said (1960) for the lower eocene limestone of the nile valley. he described the thebes formation as massive to laminated limestone with flint bands or nodules and marl rich with nummulites and planktonic foraminifera. amer et al. (1970) and said (1971), subdivided the eocene rocks exposed between luxor and assiut into two formations, namely the thebes formation at the base and the manfalut formation at the top. the thebes formation includes the massive to laminated limestone with flint bands and concretions. the exposed part of the formation decreases gradually toward the north due to the regional gentle sloping northward. 2) muneiha formation (pliocene); issawi et al. (1978) mentioned that muneiha formation includes the estuarine fine clastic sediments accumulated due to the invasion of the mediterranean sea which create a long gulf from cairo to aswan in pliocene time, these sediments are equivalent to the madmoud formation (said 1981). omer 1996 divided this formation into two main divisions (lower and upper members) according to the facies and the depositional environment. 64 aiman abdel latif and mohamed el kashouty groundwater investigation in awlad salameh, southern sohag, upper egypt 65 43 well loction legend: scale: 26° 22’ 15.57” 31° 40’ 55.9” 26° 25’ 0.8” 31° 45’ 27.19” 31° 43’ 13.7” egypt a a` a) 100 m. 60 20 -20 -30 v er tic al s ca le : gravel fine sand medium sand coarse sand clay northwest a 21 24 26 49 southest a legend horizontal scale 2.5 0 2.5 km b) figure 1. location map of the awlad salameh (a), geological cross section nw-es (b), and general hydrogeologic section of the sohag area (rigw 1990) (c). 3) issawia formation (pliocene/pleistocene); said (1981 and 1990) mentioned that, by the end of the pliocene and beginning of the pleistocene, arid climatic conditions prevailed over egypt causing the cessation of the riverine activities. this left local depressions, which received water only during seasonal rainy storms; lacustrine sediments were accordingly accumulated within these depressions. these lacustrine sediments were treated as the issawia formation (issawi, et al. 1978 and omer, 1996). sediments of the issawia formation include two interfingered facies: the carbonate facies and clastic facies. 4) the pleistocene sands; they consist of a thick succession of the pleistocene sand-gravel association, which is widely distributed along the nile valley in both the surface and subsurface. these sediments have been studied by many authors (e.g. wendorf and said, 1967, said, 1975, 1981; paulissen and vermeersch, 1987; issawi and mccauley, 1992; omer, 1996; omer and issawi, 1998). 5) dandara formation; during the late middle pleistocene, the ethiopian water coupled with the dry phase in egypt resulted into an exotic suspended-load river depending mostly on the seasonal rainfall in ethiopia (said 1975, omer, 1996, omer and issawi, 1998). sediments accumulated during this stage (dandara formation) were thus entirely derived from the upper reaches of the nile in ethiopia and southern sudan. these sediments are formed mainly of fluviatile fine sand-silt intercalation and accumulated at low-energy environment. 6) recent wadi deposits; generally, the intermittent activities of the transverse channels led to the accumulation of flashflood deposits covering the surface of the older sediments throughout the desert areas outside the cultivated land. these deposits vary greatly in both the thickness and texture depending upon the land morphology and the intensity and regime of the flashflood rainfall. they range in thickness from few centimeters to more than 30 meters. they are formed of the disintegrated product of the nearby eocene carbonates, in addition to the reworked material from the pre-existing sediments. the aquifer is 66 aiman abdel latif and mohamed el kashouty 240 220 160 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 e le va ti o n (m as l) c) r iv er n ile south west north east 400 380 360 340 320 300 280 260 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 gravel, sand and clay (young and old nile and wadi deposits) semi-confined silty-clay layer (holocene) shale (paleocene) sand and clay intercalations with gravel at fringes (plio-pleistocene) nubian sandstone (cretaccous and pre-cretaccous) graded sand and gravel intercalated with clayey lenses (pleistocene) shale and carbonates (late cretaccous) clay (pliocene) limestone (eocene) 0 1 2 3 4 5 10 km continued figure 1. semi confined around the river nile, shifted to unconfined away from the floodplain (fig. 1b). the investigated aquifer is the pleistocene that overlies the fissured eocene limestone, the holocene silt and clay act as capping bed (fig. 1c). the pleistocene aquifer thickness is 260 m around the river nile and vanished due the two plateaus. the aquifer mainly was recharged from the irrigation and draining canals and the rainfall. groundwater flow generally at sohag governorate is towards the river nile. the highest contour lines are at the valley fringes 65 m (masl) and lowest contour are close to the river nile 53 m (masl). the groundwater is recharging the river nile which acting as a natural drain (ahmed 2007a). the horizontal and vertical hydraulic conductivity of the capping sediments are 0.06 and 0.0086 m/d, the specific storage coefficient is 7.6 *10-6 (abdel moneim 1999). the pleistocene aquifer horizontal hydraulic conductivity ranged from 40-100 m/d (zaki 2001). 3. materials and methods a total of 50 boreholes was collected from the pleistocene aquifer and one sample from river nile in awlad salameh village in sohag governorate. they were analyzed in the centre of reclaiming new land in ministry of agriculture at january 2009. the location site is determined by gps instrument. ph, ec, and temperature are measured in situ using portable field kite. cl, hco3, ca, and mg were measured by titration, while so4 is estimated by turbidity method, and na and k were analyzed by flame photometer 4. results and discussions 4.1 water chemistry the tds concentration increased due the northwestern, northern, and northeastern part (fig. 2a), attributed to high numbers of fault planes interconnected together to build a groundwater investigation in awlad salameh, southern sohag, upper egypt 67 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.4 eca, ppm c.i. 25 ppm fcl, ppm c.i. 150 ppm 100 250 400 550 700 850 1000 1150 1300 1450 1600 1750 1900 26.37 26.38 26.39 26.41 26.37 26.38 26.39 26.4 26.41 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 atds, ppm c.i. 250 ppm bna, ppm c.i. 100 ppm 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 26.38 26.39 26.4 26.41 26.37 c ultivated land aw la d s al am ah c ultivated land aw la d s al am ah aw la d s al am ah c ultivated land c ultivated land aw la d s al am ah 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 good conduits. these conduits facilitate the upward and downward leakage from the lower brackish (tds 1000-5000 ppm) to saline ( tds > 5000 ppm) aquifers and agricultural and sanitary wastewaters, respectively. the saline aquifers are eocene limestone and nubian sandstone, they non renewable and influenced by lithogenic. the pleistocene aquifer is influenced by agricultural activities in old cultivated area and reclaimed lands and development projects in the desert zone (el kola wastewater disposal in the eastern part and el dair wastewater disposal site in the western part, ayman 2009). the irrigation water is applied in flooding, which was at frequebcy of two and three times a month (abdel moneim 1992; shamrukh et al. 2001). the pesticides detection in the cow’s milk confirm the agricultural impact (dawood et al 2004). the aquifer thickness increased due the northwestern part of the study area (zaki 2001) to be increased the capacity of the received water from different sources through these fault planes. the tds concentration decline in the southeastern, southwestern, and central-western part, caused by the presence of wadis deposits, which composed mainly of very gravelly and even cobbly (fig. 1b) (zaki 2001). these wadis deposits drain in the pleistocene aquifer and is characterized by high precipitation and infiltration rate (zaki 2001). the wadi el rashaydah drainage basin is the main wadi passing in the areas of low salinity. the basin length, streams orders numbers and lengths are 526, 2163, and 1132.94 km, respectively (zaki 2001). these data indicate elongation of the basin, which facilitate the passage of runoff and infiltrate downward to dilute the groundwater system. the drainage frequency is high and reflect a good collection of runoff (zaki 2001). the thickness of 68 aiman abdel latif and mohamed el kashouty 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 borehole location gso4, ppm c.i. 150 ppm 100 300 500 700 900 1100 1300 1500 1700 1900 2100 2300 26.41 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 ck, ppm c.i. 2 ppm 1 3 5 7 9 11 13 15 17 19 21 2 km0 c ultivated land aw la d s al am ah c ultivated land aw la d s al am ah 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.4 hhco3, ppm c.i. 15 ppm 120 145 170 195 220 245 270 295 320 345 26.37 26.38 26.39 26.41 aw la d s al am ah c ultivated land 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 dmg, ppm c.i. 25 ppm 10 35 60 85 110 135 160 185 210 235 260c ultivated land aw la d s al am ah figure 2. the distribution of the major ions in groundwater in awlad salameh, sohag. the aquifer increased in the low saline groundwater areas mentioned previously. it can dilute the groundwater through the rainfall infiltration into the low water saline areas. na concentration resemble the tds trend (fig. 2b). the k concentration anomaly shifted from the tds trend to be in the central-western part (fig. 2c), because of aquitard diffusion intercalated and tapped the aquifer. the mg, ca, cl, and so4 concentrations are similar to the tds trend (fig. 2d, e, f, and g). the ca and so4 concentrations are mainly attributed to the agricultural wastewater especially fertilizers. the hco3 concentration increased in the low saline groundwater areas (fig. 2h), confirm the dilution from the surface meteoric water (rainfall) into the aquifer system. the ph values is very limited (8.1 -8.5) by the continuous leakage from the rainfall. 4.2 saturation index approach the calcite and dolomite are super saturated in groundwater (fig. 3a & b), they increase in saturation in the northeast and northwest directions. the southwestern, southeastern, and central map parts are the lowest saturation indices with respect to calcite and dolomite. only the southwestern part match with low tds concentration, while the southeastern and central map parts coincide with high tds concentration. it reflect the ca and mg concentrations partially contributed by anthropogenic and lithogenic sources. the groundwater is under saturated with respect to gypsum and anhydrite (fig. 3c & d), they decrease in saturation due the southwestern part, by dilution from the rainfall infiltrated through the wadis deposits. 4.3 statistical analyses tds is strongly correlated with na, mg, ca, cl, and so4, reflect the dissolution of limestone, evaporation, and anthropogenic source (agricultural wastewater). the k concentration is not included in the groundwater salinity correlation, indicate that it is contributed by the aquitard diffusion groundwater investigation in awlad salameh, southern sohag, upper egypt 69 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 borehole location acalcite bdolomite cgypsum danhydrite 2 km0 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 -2.6 -2.4 -2.2 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -2.8 -2.6 -2.4 -2.2 -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 c ultivated land awlad salamah c ultivated land awlad salamah c ultivated land awlad salamah c ultivated land awlad salamah figure 3. saturation indices of selected minerals of the groundwater system in awlad salameh, sohag 70 aiman abdel latif and mohamed el kashouty dendrogram using average linkage (between groups) rescaled distance cluster combine case 0 5 10 15 20 25 label num +– – – –– – – – –+– – – –– – – – –+– – – –– – – – –+– – – –– – – – –+– – – –– – – – –+ case 21 21 -+ case 47 47 -+ case 16 16 -+ case 31 31 -+ case 8 8 -+-+ case 37 37 -+ i case 15 15 -+ i case 18 18 -+ i case 30 30 -+ i case 2 2 -+ i case 34 34 -+ i case 38 38 -+ i a case 19 19 -+ i case 41 41 -+-+ case 10 10 -+ i case 4 4 -+ i case 5 5 -+ +-+ case 29 29 -+ i i case 39 39 -+ i i case 45 45 -+ i i case 49 49 -+ i i case 3 3 -+ i i case 7 7 -+ i i case 6 6 -+ i +-+ case 22 22 -+-+ i i case 48 48 -+ i i case 26 26 -+ i i case 24 24 -+ i i cluster i case 23 23 -+ i +----+ case 12 12 -+ i i i iccase 33 33 ------+ i i case 1 1 -+---+ i i case 42 42 -+ i i i case 28 28 -+ +-+ i b case 40 40 -+-+ i i cluster ii case 27 27 -+ +-+ +-------------+ case 11 11 ---+ i i case 44 44 -+ i i case 46 46 -+-+ i i case 43 43 -+ i i i case 9 9 -+ +-----+ i i independent cases case 50 50 -+-+ i i +----------------------------------+ case 35 35 -+ i i i i i case 13 13 -+ i +----+ i i case 14 14 -+ i i i i case 32 32 -+-+ i i i case 20 20 -+ i i i iccase 36 36 ----------+ i i iccase 25 25 -------------------------+ i iccase 17 17 -------------------------------------------------------------+ figure 4. dendrogram analysis of the groundwater system in awlad salameh, sohag table 1. average concentrations of the clusters identified by hca cluster sub cluster number of samples ph e.c. µs/cm tds ppm na ppm k ppm i a 31 8.27 3891.5 2487.3 671.58 2.5806 b 6 8.27 1058.3 677.33 172.73 3.519 ii 10 8.24 6607 4228.9 1107.3 3.9491 i.c.37 1 8.1 9140 5849.6 1502.1 3.128 i.c.26 1 8.3 4420 2828.8 634.98 23.46 i.c.18 1 8.1 11730 7507.2 1535.9 5.083 mg ca cl so4 hco3 ppm ppm ppm ppm ppm 61.5 91.7 642 776.10 278.4 18.7 28.7 124.44 124.71 272.5 112.2 171.6 1147.7 1475.6 156.2 163.7 250.5 1630.8 2035.9 183 78.9 120.6 765.43 960.56 158.62 317. 6 485.6 2127.2 2636.3 146.4 ic independent case from the pliocene clay and the clay interbedded with sand and gravel of pleistocene aquifer (moukana and koike 2008). ca is strongly correlated with mg reflect a common mineralogic sources, may be weathering of amounts of calcite and dolomite (stauffer and wittchen 1991). the principle component analysis (pca) shine et al., 1995; ruiz, 2001; lui et al., 2003 and zhou et al., 2004) have been using spss program (statistical program of social science) under windows version 12.0 (statistical program) in order to study the interrelationship among the chemical elements in the groundwater. two factors were identified, factor 1 positively loaded with tds, na, mg, ca, cl, so4, and more or less hco3. it reflect the groundwater system was contributed by evaporation, recharge, anthropogenic, and dissolution of geomedoia. the 2nd factor loaded with ph and k, clarify the aquitard diffusion that depend upon the ph condition. the dendrogram investigation revealed two clusters and three independent cases (fig. 4), based on hydrogeochemistry of the groundwater. cluster i was subdivided into two sub cluster a and b. sub cluster a include 31 samples and has average concentrations as shown in table 1. it is distributed allover the study area, caused by lithogenic and anthropogenic sources and also recharge from freshwater. it contained the different groundwater salinity (moderate and high). sub cluster b was represented by 6 samples, it is distributed due the central northwestern part. it is characterized by the lowest tds concentration, because of dilution from the infiltrated rainfall through wadis. cluster ii contain 10 samples, it is distributed in the northwestern and southeastern part. it is distinguished by higher salinity than the previous cluster, attributed to closer to the upward and lateral leakage of saline water from the eocene limestone aquifer, therefore the groundwater salinity increased. the independent cases no. 18, 26, and 37 are located separately and have highest tds , k, and mg concentrations, respectively. 4.4 netpath and aquachem application in netpath program, mixing of two or more initial waters is modeled to show hydrogeochemical processes taking place due to mixing and migration of waters from two or more sources of recharging groundwater aquifer. the prevailing minerals in the formations, through which the migrating water passes and interacts, were used in the model. the initial waters are different, water from river nile, canals, runoff, and limestone aquifer. ground-and surface-water chemistry cannot be dealt with separately where surface and subsurface flow systems interact. the movement of water between two solutions provides major pathways for chemical transfer between terrestrial and aquatic systems. the limestone aquifer recharge rate increased in the northwestern, eastern, and northeastern part (fig. 5a). the contribution by fresh water (river nile, canals, and runoff) was increased in the southeastern and central-western part (fig. 5b). 4.5 water purposes. the quality of groundwater is a function of physical, chemical, and biological parameters, so the standards should follow (who 1984, 1993, and 1997). besides the anthropogenic sources, the geology also imposes its chemistry on the groundwater system. drinking water standards are generally based upon two main criterion 1) objectionable taste, odor, and color; 2) presence of substances with adverse physiological (health) effects. the suitability of groundwater for groundwater investigation in awlad salameh, southern sohag, upper egypt 71 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 2 km0 31.695 31.7 31.705 31.71 31.715 31.72 31.725 31.73 31.735 31.74 31.745 31.75 26.37 26.38 26.39 26.4 26.41 b-mixing proportion of fresh water (river nile, canal, and runoff)a-mixing proportion of limestone aquifer borehole site 5 15 25 35 45 55 65 75 85 5 15 25 35 45 55 65 75 85 95 % %c ultivated land awlad salamah c ultivated land awlad salamah figure 5. percentage of irrigation canals and drainage wastewater in the groundwater samples drinking and irrigation has been determined using world health organization (who) guidelines and united states environmental protection agency (usepa 1976) and a modified wilcox class model. irrigation water quality is dependent on its specific conductance, relative proportion of na to cations, plant types, soil, and climate. the tds of groundwater samples exceeds the permissible upper limits (tds = 1000 ppm) for drinking in the study area. the groundwater is high to very high salinity hazards. the water class was permissible to doubtful, which reduce the permeability and therefore decline the income per capita. the increase of total salinity in the irrigation water caused an excessive increase of its content in the soil. consequently, an accumulation of salts in plant cells will cause the growth damage of such plants. according to the u.s. salinity laboratory staff (1954), the analyzed water samples are related to the following categories (figure 6a); 1) medium salinity and low s.a.r. (c2-s1), it is used for irrigation for all crops; 2) high salinity and low s.a.r. (c3-s1), it apply in soil of medium texture; 3) high salinity and medium s.a.r. (c3-s2), it is suitable for medium texture and high permeability; 4) high salinity and high s.a.r. (c3-s3), it is hazardous to most soils and requires good drainage and chemical amendments; 5) very high salinity and medium s.a.r. (c4-s2), it is not suitable due to its harmful water management for salinity control and 72 aiman abdel latif and mohamed el kashouty a) b) figure 6. u.s. salinity laboratory staff (1954) classification (a) and piper graph (b) of quaternary aquifer in awlad salma village in sohag governorate leaching is necessary to remove the excess salts and is hazardous to most soils requires good drainage and chemical amendments; 6) very high salinity and high s.a.r. (c4-s3), it is not suitable due to its harmful water management for salinity control and leaching is necessary to remove the excess salts; 7) very high salinity and very high s.a.r. (c4-s4), it is not good for irrigation because its required chemical amendments and good drainage and most remove the excess salts. the samples numbers 13, 17, 20, 21, 24, 27, 38, 40, 41, 48, 49 and 50 are out of scale due to excessive salinity water, not advice to use for irrigation. piper graphical representation indicate that almost all the samples fall in one zone (fig. 6b) indicate a similar chemical signature. the cationic composition of the waters ranges from na+k dominated through a mix of mg+na+k dominated. the anionic composition are chloride dominated. it is alkaline type with prevailing sulphate and chloride types. conclusion this paper presents some of preliminary results of groundwater chemistry in awlad salameh village in sohag governorate, upper egypt. the problem of soil salinization was confirmed by this research. the groundwater salinity was very high in most of the samples. caused by lithogenic and anthropogenic sources. the salinity decline where the infiltration enhanced in wadi deposits. the groundwater generally unsuitable for irrigation purpose and can cause soil salinization and water logging. the boreholes locations recommended in the areas of gravelly wadi deposits and after modeling of the aquifer system. references abdel kareem, h.h. 1972. geology of the area west of the nile valley between sohag and girga. m.sc. thesisi, fac of sci., geol dept., assiut univ., 129p. abdel moneim, a.a. 1992. numerical simulation and groundwater management of the sohag aquifer, the nile valley, upper egypt. ph.d. thesis, university of strathclyde, glasgow, scotland, uk. abdel moneim, a.a. 1999. geoelectric and hydrogeological investigation of the groundwater resources on the area to the west of the cultivated land at sohag, nile valley, upper egypt. geol soc of egypt. v 43/2, pp. 253-268. ahmed aa (2007a) using lithologic modeling techniques for aquifer characterization and groundwater flow modeling of sohag area, egypt. second international conference on geo-resources in the middle east and north africa, 24–28 feb, 2007, cairo university, egyp amer, a.f., krintsov, m.l. and hanna, f.l. 1970: the egyptian carbonate rocks and the possibilities of their utilization. in: studies on some mineral deposits of egypt. geol. surv. egypt, p.195-208. ayman, a. 2009. using generic and pesticides drastic gis-based models for vulnerability assessment of the quaternary aquifer at sohag, egypt. hydrogeology journal, v17: 1203-1217. brioussov, b.a. 1968. tectonic divisions and definition of basement relief in northern uar according to gravity and magnetic data. the geol soc of egypt. 6th ann meet pp. 12-14. dawood, a., abdel el maaboud, r., helal, a., mohamed, s., and ali, w. 2004. detection of organochlorine pesticides residues in samples of cow’s milk collected for sohag and qena governorates. assiut university, bull environ res 7: (2). el gohary, f., wahaab, r., nasr, f., and ali, h. 2002. three egyptian industrial wastewater management programs. the environ 22: 59-65. gasg (general authority of sohag governorate), 2004. periodical reports of information center. july, 2001, gasg sohag. issawi, b. hassan, m.w., and osman, r. 1978: geological studies in the area of kom ombo, eastern desert, egypt. ann. geol. survey, egypt. v viii, p.187-235. issawi, b., and mccauley, j. f. 1992: the cenozoic rivers of egypt: the nile problem. in: freidman, r. and adams, b., (eds.), the followers of horus, egypt: studies assoc. public., no. 2, oxbow monog. 20, park end place, oxford, p.121-138. liu wx, li. xd, shen zg, wang dc, wai ow, and y.s. li. 2003. multivariate statistical study of heavy metals enrichment in sediments of the pearl river estuary. environ pollut v121: 377-388. meshref, w.a. 1990. tectonic framework in said, r. 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recommendations. wendorf, f., and said, r. 1967. paleolithic remains in upper egypt. nature, v.215, p.244-247. yallouze, m., and knetsch, g. 1954. linear structures around the niule basins (comprehensive analysis of tectonic evolution in north africa). bull soc geograph , egypt. v22, pp. 154-207. 74 aiman abdel latif and mohamed el kashouty youssef, m.i. 1968). structural pattern of egypt and its interpretation. amer assoc petrol geol bull, v52, pp. 601-614. youssef, m.m., riad, s., and mansour, h.h. 1978. surface and subsurface structural study of the area arounfd assiut, egypt. bull fac sci assiut univ., v6, no 2, pp. 293-306. zaki, s., 2001. hydrogeological studies and application of gis for evaluation of water resources and land use projects in the southern pasrt of sohag govornorate. m.sc. thesisi, fac of sci., geol dept., menoufia univ., 179p. zhou, h.y., peng x. t., and j.m. pan. 2004. distribution, source, and enrichment on some chemical elements in sediments of the pearl river estuary. china continental shelf res, 24: 857-1875. groundwater investigation in awlad salameh, southern sohag, upper egypt 75 rainfall intensity-duration-frequency (idf) curves are of particular importance in water resources management, for example, in urban hydrology, for the design of hydraulic structures and the estimation of the flash flood risk in small catchments. idf curves describe rainfall intensity as a function of duration and return period, and they are significant for water resources planning, as well as for the design of hydraulic constructions and structures. in this study, scaling properties of extreme rainfall are examined to establish the scaling behavior of statistical non-central moment over different durations. idf curves and equations are set up for all stations by using the parameter obtained from scaling behavior, the location and scale parameters μ24 and σ24 of the gumbel distribution (evi) sample of annual maximum 1440 min rainfall data. in another hand, we have established the idf curves for ten selected rain gauge stations in the northern (oueme valley) parts of benin republic, west africa by using the simple scaling approach. analysis of rainfall intensities (5 min and 1440 min rainfall data) from the ten rainfall stations shows that rainfall in north-benin displays scales invariance property from 5 min to 1440 min. for time scaling, the statistical properties of rainfall follow the hypothesis of simple scaling. therefore, the simple scaling model applies to the rainfall in (oueme valley). hence, the simple scaling model is thought to be a viable approach to estimate idf curves of hourly and sub-hourly rainfall form rainfall projections. the obtained scaling exponents are less than 1 and range from 0.23 to 0.59. the empirical model shows that the scaling procedure is a good estimator as it is more efficient and gives more accurate estimates compared with the observed rainfall than the traditional method which only consists the gumbel model in all stations for lower return periods (t<5 years) but not for higher return periods. las curvas de precipitación intensidad-duración-frecuencia (idf) son de particular importancia en el manejo de los recursos hídricos, como es el caso de la hidrología urbana o para el diseño de estructuras hidráulicas y la estimación del riesgo de crecidas en pequeñas captaciones. las curvas idf describen la intensidad de las precipitaciones como una función con períodos de duración y recurrencia, lo que las hace significativas en la planeación de recursos hídricos así como en el diseño de construcciones y estructuras hidráulicas. este estudio examina las propiedades de escala en precipitaciones extremas para establecer un comportamiento en momentos estadísticos marginales en diferentes períodos de duración. se establecieron las curvas idf y las ecuaciones para todas las estaciones a partir del parámetro obtenido del comportamiento de escala, la ubicación y los parámetros de escala μ24 and σ24 de la muestra de información de precipitación máxima anual de 1440 minutos de la distribución de gumbel (evi). por otro lado, se establecieron las curvas idf para 10 estaciones pluviométricas seleccionadas en el valle oueme, al norte de la república de benín (áfrica occidental), con el uso de aproximación simple de escala. el análisis de las intensidades de precipitación en las diez estaciones pluviométricas muestra que la precipitación en el norte de benín expone propiedades de poca variación en la escala 5 min y 1440. en el tiempo de escala, las propiedades estadísticas de precipitación confirman la hipótesis de escala simple; además, este modelo so corresponde a la precipitación del valle oueme. por lo tanto, el modelo de escala simple se considera una aproximación viable para estimar las curvas idf en las proyecciones de precipitación de cada hora y sub-hora. los exponentes de escala obtenidos son menores a 1 y oscilan de 0,23 a 0,59. el modelo empírico muestra que el procedimiento de escala es un buen estimativo, más eficiente y con cálculos más exactos que el método tradicional, el cual consiste solamente en el modelo gumbel aplicado en todas las estaciones pluviométricas en períodos de menor recurrencia (t<5 años) pero no en lapsos de mayor recurrencia. abstract resumen keywords: simple scaling, idf curves, annual maximum rainfall, frequency analysis, oueme valley. palabras clave: escala simple, curvas de intensidadduración-frecuencia, precipitación máxima anual, análisis de frecuencia, valle oueme. estimation of idf curves of extreme rainfall by simple scaling in northern oueme valley, benin republic (west africa) record manuscript received: 02/03/2015 accepted for publication: 08/01/2016 how to cite item agbazo, m., koton'gobi, g., kounouhewa, e., alamou, e. & afouda, a. (2016). estimation of idf curves of extreme rainfall by simple scaling in northern oueme valley, benin republic (west africa). earth sciences research journal, 20(1), d1-d7. doi: http://dx.doi.org/10.15446/esrj.v20n1.49405 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.49405 estimación de las curvas idf de extrema precipitación por escala simple en el valle oueme, al norte de la república de benín (africa occidental) m e t e o r o l o g y earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): d1 d7. m.agbazo1, g. koton’gobi2, b. kounouhewa2, e. alamou3, a. afouda3 1 international chair in physics mathematics and applications (cipma-chair unesco), university of abomey-calavi, benin bp: 526 uac benin 2 radiation physics laboratory (lpr), university of abomey-calavi, bp: 526 uac benin 3 applied hydrology laboratory (lha), university of abomey-calavi, bp: 526 uac corresponding author: m.agbazo; e-mail: agbazomedard@yahoo.fr d2 m.agbazo, g. koton’gobi, b. kounouhewa, e. alamou and a. afouda 1. introduction west african countries have been struck these last years by floods and inundations of unprecedented magnitude (danida, 2008; zannou, 2011; g. panthou, 2014). for example we can cite the inundations that occurred from 2007-2009 in benin, niger and burkina-faso (world bank, 2009) which caused drastic consequences in more field of development agriculture (destroyed cultivation), aquatic ecosystems, economy, urban drainage systems, energy distribution and social security by displacing people (danida, 2008, descroix et al., 2009). likewise, the emblematic niger river floods of 2010 and 2012 are the two most important floods ever recorded at the niamey station since the beginning of the observation in 1929, causing heavy casualties to people living close to the river (panthou, 2014). projection in the northwest of benin, one of the high-risk regions, predicts trends including lower average annual rainfall (15% up to 2025); more variation in precipitation; more frequent extreme climatic elements such as heavier rains and more powerful storms; shorter, delayed monsoons with irregular precipitation; and higher long-term average temperatures (danida, 2008). this illustrates that extreme events can appear in dry or moderately wet years anywhere in west africa where benin republic is located and in which wet years can occur without any extreme rainy event being recorded. in urban drainage systems, short duration rainfall events are one of the most critical elements for hydrological investigation (bougadis and adamoski, 2006), useful for the control of storm run-off such as flood detention, reservoirs, sewer systems etc. in particular, rainfall extremes with high temporal resolution (one hour or shorter) are necessary for the design of drainage systems, usually characterized by fast response. the temporal resolution of rainfall data usually available for practical applications are often lower than the data requested for the design procedures or mathematical models application that greatly affect their reliability (nhat et al, 2008) because they depend on many assumptions such as the distribution selection for durations. rainfall data generally require a large number of parameters and are time dependent. such information is often expressed in intensityduration-frequency (idf). the establishment of such relationships goes back to the 1930s (bernard, 1932). since then, different forms of idfs have been constructed for several regions of the world. in africa we can cite the studies which utilized statistical method to construct idf curves: in congo (mohymont et al., 2004; mohymont & demarée, 2006; van de vyver & demarée, 2010), in ghana (endreny & imbeah, 2009), in cote d’ivoire (soro et al., 2008; soro et al., 2010) and in nigéria (oyegoke & oyebande, 2008; okonkwo & mbajiorgu, 2010). traditionally the idf curves are constructed by performing statistical analysis on annual maxima series (ams) or partial duration series (pds) by fitting probability distribution for several pre-selected rainfall durations (bougadis and adamowski, 2006). such a procedure has several disadvantages including fitting distribution and parameters estimations for each duration, and requirements to extrapolate result to different time-scales. the usefulness and accuracy of such procedures are limited because of fitting uncertainties and lack of ability to adequately describe rainfall properties at different time scales (van nguyen and wang, 1996). in addition, this traditional method has certain limitations. for example, a high number of parameters are involved, which makes it none-parsimonious from the statistical point of view (baghirathan and shaw, 1978; bell, 1969; sherman, 1931; bernard, 1932; chen, 1983; garcia-bartual and schneider, 2001; takara, 2005; nhat et al., 2008). traditional method cannot take into consideration characteristics of rainfall for different durations; and it is based on the annual extreme data available at local site only. it is therefore advantageous to develop a model, which could adequately describe rainfall characteristics through a continuum of times scales including inferences at time resolutions that may not have been observed and to reduce the number of parameters to be estimated in order to increase their reliability. consequently, alternative approaches to construct idf relationships are used. for example the fractal properties of rainfall which implies scaling invariance were developed (e.g., menabde, et al., 1999) where the scaling properties of the statistical moments of rainfall in south africa over a range of durations from 30 min to 1440 min were examined. yu, et al., (2004) used regional idf formulas for estimating scaling exponents for 46 non-recording rain gauge stations in taiwan. molnar and burlando (2005) investigated the variability of the scaling properties from 62 rain gauge stations in switzerland and özger, et al. (2010) focused on the scaling properties of rainfall at 43 rain gauge stations in texas. recently, scaling formulas are proposed to extend the idf relationships from daily time scale to shorter time intervals based on scaling properties. gupta and waymire (1990) studied the concepts of simple and multiple scaling to characterize the probabilistic structure of the precipitation process, koutsoyiannis and foulfoula-georgiu (1993) used a scaling model to predict storm hyetographs. menabde et al. (1999) showed that based on the empirically observed scaling properties of rainfall and some general assumptions about the cumulative distribution function for the annual maxima of mean rainfall intensity, it is possible to derive simple idf relationships. bendjoudi, et al. (1997) used a multifractal point of view on rainfall idf curves. rosso and burlando (1990) and later burlando and rosso (1996) used this concept to study traditional forms of depth-durationfrequency relationships. de michele et al. (2002) developed idf curves design for storms in milan (italia); yu, et al. (2004) developed regional idf formulas for non-recording sites in taiwan, molnar and burlando (2005) examined the variability of scaling exponents in alpine mountainous region of switzerland; nhat, et al. (2007) developed regional relationship for ungauged locations based on the scaling theory in japan. molnar and burlando (2008) investigated the variability of the scaling properties from 62 rain gauge stations in switzerland, acar, et al. (2008) used a multilayer perception artificial neural network model to assess idf relationships for short duration rainfalls. recently, özger, et al. (2010) focused on the scaling properties of rainfall at 43 rain gauge stations in texas; ceresseti (2011) used a multifractal point of view on rainfall idf curves in france to characterize idf curves; ghanmi (2014) developed regional relationship for ungauged locations based on the scaling theory in tunisia. scaling properties of extreme rainfall has never been investigated in benin republic. the objectives of the paper in this study area are: (i) to examine the scaling properties of extreme rainfall in the oueme valley (benin), (ii) to establish scaling behavior of statistical non central moment over different durations of rainfall intensity and (iii) to derive the idf relationships for short-duration rainfall from daily data using the scaling methodology and to compare with empirical method. 2. methodology the scaling or scale-invariant models enable transforming hydrologic information from one temporal or spatial to another one, and thus, help overcome the difficulty of inadequate hydrologic data. a natural process fulfills the simple scaling property if the underlying probability distribution of some physical measurements at one scale is identical to the distribution at another scale. the basic theoretical development of scaling has been investigated by many authors and considerable amount of studies were devoted to extreme rainfall and its scaling properties, including waymire and gupta (1981); waymire, et al.(1984); rodriguez-iturbe, et al. (1984); marien and vandewiele (1986); sivapalan and wood (1987); gupta and waymire(1990); rosso and burlando (1990); smith (1992); koutsoyiannis and foulfoula-georgiu (1993); burlando and rosso (1996); veneziano et al. (1996), bendjoudi et al. (1997); willems (2000); hubert et al. (2002) ; de michele et al. (2002); molnar and burlando (2005); ceresetti (2011); panthou (2013); ghanmi (2014). in this work the simple scaling hypothesis is adopted to test the scaling behavior of rainfall in northern of benin, following the methods described in menabde et al. (1999); yu et al. (2004); kuzuha et al. (2002); nhat et al. (2007); bara et al. (2009) and ghanmi (2014) and are briefly outlined in the ensuing sections. 2.1 scaling characteristics of extreme rainfall let id and id denote the annual maximum rainfall intensity series for time durations d and d, respectively. in the model proposed by menabde et al. (1999); yu et al. (2004); veneziano and furcolo (2002); bara et al. d3estimation of idf curves of extreme rainfall by simple scaling in northern oueme valley, benin republic (west africa) (2009) rainfall intensity id of duration d is said to exhibit simple scale invariance behavior if the following equation holds true. (1) hold true (2) the equality refers to identical propability distributions in both sides of the equations; denotes a scale factor and k is a scaling exponent. it follows that by raising both sides of equation (1) to the power of q and taking the ensemble average that is, taking the qth moment of both distributions, we obtain where k (q) represents the scaling exponent of order q. in order to obtain the value of k (q), we simply take logarithm of both sides of equation (2), which transforms it into: (3) (8) (10) (9) (4) (5) (7a) (7b) (6) when the moments are plotted on a logarithmic chart versus the scale λ for various moments order q, each slope will represent k (q) for the respective q. the various slope k(q) are then plotted against q: if the graph is linear, it implies that k is a constant and i(d) exhibits characteristics for simple scaling, while in other cases, the multi-scaling approach has to be considered. 2.2 estimation of idf rainfall extremes the idf curves are often fitted to the extreme value type i, evi developed by gumbel and it is still the most often used distribution by many national meteorological services in the world to describe rainfall extremes. it’s also used in this study along with the method of moments. the annual maximum rainfall intensity i(d) has a cumulative probability distribution (cdf) (gumbel, 1958). according to the scaling theory, the idf formula can be derived (menabde et al, 1999; nhat et al., 2007 and ghanmi, 2014) as: where and are respectively the location and the scale parameter for gumbel model at d hour data series; and represent the rainfall intensity for a duration d and d for a return period t. λ denotes a scale factor and k is a scaling exponent. it was also shown in menabde et al. (1999); nhat et al. (2007) and ghanmi (2014) that: where the location parameter μd and scale parameter ϭd of gumbel model are calculated from daily data series by using the equations of landwehr et al. (1979) given as: and are probability weight moment (pwm) for order 0 and 1, and are estimated by using the daily annual maximum rainfall intensity. the idf relationships can be derived from longer duration data series based on three parameters: scale exponent, the location and scale parameter of evi. the location and scale parameters (μ24 and σ24) of the evi (gumbel model) sample of annual maximum 1440 min rainfall data are calculated using equation (7a) and (7b). the results are recorded in table 2. 2.3 model performance as an indication of goodness of fit between the observed and predicted values, the coefficient of determination (r2), the root mean square error (rmse), the mean absolute percent error (mape) are calculated. the rmse and mape are defined as: -the root mean square error (rmse): and -the mean absolute percent error (mape): xobs,i is the ith observed data point, xest,i is the ith estimated value, n is the number of data point. idf curves are plotted together with the actual observed data, and shown in fig.4. to evaluate the goodness-of-fit graphically, idf curves derived by fitting gumbel extreme value distribution to actual observed data (of various durations d) are also plotted on the same graphs (fig.4). the statistical root mean square error (rmse) and mean absolute percent error (mape) are also used to evaluate the performance of the simple scaling model. the results are presented in table 3 are for 5 min, 10 min, 15 min, 20 min, 30 min and 60 min idf curves simple scaling. 2.4 study sites and data analysis the hydro meteorological observatory of upper oueme valley (ohhvo) has an area of about 14,000 km2 and is located in northern of benin republic, an area of humid tropical climate of sudanese type and delimited by latitude 9°n-10°n and longitude 1.5°e-3°e. this area was instrumented during the catch (coupling of tropical atmosphere and hydrological cycle) and amma (multidisciplinary analyses of african monsoon) projects. ohhvo is an area characterized by a single rainy season from mid-march to late october, the heart of the rainy season is between july and august with an interannual average rainfall of about 1200 mm (lawin, 2007). the study area has a subdued topography with an elevation of about 200 m (le barbé et al., 1993). the location of ohhvo in benin is shown in fig. 1. for a good representativeness of the samples, ten rain stations of ohhvo which operating rate (the ratio of the total number of days of operation to the total number of days in the year) during the year is greater than 90% have been chosen. the locations of these stations are shown in fig. 1. the data used are time series of rainfall intensity 5 min and 1440 min, measured from 1999 to 2012. matlab (v10.2) programming code is used to accomplish the tasks, allowing the aggregation of data over periods of 5 min and more and longer (i.e. 10 min, 20 min, 30 min, 60 min, 90 min, 120 min, 180 min, 240 min, 1440 min), using disjoint windows. missing in these raw data in the sets are provided by interpolating the neighboring values using the proposed formula of hoang, (2008), given as: p is the intensity of the calculated length, n is the total number of time steps of 5 min from the calculated time, n’ is the number of time steps of 5 min measured from the calculated time and p’i is the ith measured intensity pitch 5 min i. d4 m.agbazo, g. koton’gobi, b. kounouhewa, e. alamou and a. afouda table1: name of stations fig. 1. study site: (a) benin republic location in west africa, (b) ohhvo location in benin republic and (c) rain gauge location in ohhvo (1 = affon, 2 = corpargo, 3 = djougou, 4=wewe, 5= koko, 6= pélébina, 7= sarmanga, 8= sonoumon, 9= tobre and 10= wewe). 3. results and discussions 3.1 scale invariance properties of rainfall the property of simple scaling of rainfall intensities in the wide sense is demonstrated in the ten selected sample stations. the scaling exponents are derived by including all the analyzed durations of rainfall (5 min, 10 min, 15 min, 20 min, 30 min, 60 min, 90 min, 120 min, 180 min and 1440 min) to the analysis. fig. 2 displays the relationships between the log-transformed values of moments of various orders q (q values of 0.5 to 5 at increments) against various rainfall durations at copargo station. it can be observed that, for all the considered moment orders, log (moment) and log (duration) have a linear relationship for durations between 5 min and 1440 min. it’s not broken between the short and long duration data. linear regression performed for the regime confirmed the linear relationship, the correlation is consistently strong with coefficient of determination r2, greater than 0.94. the behavior (linearity) found from copargo station is similar to the other examined stations in the study area. the linearity of the moment from this study is also observed in others region of world. for example ghanmi (2014) in tunisia (northern of africa) find that data from (tunis) present linearity, ceresseti (2011) in france; bara, et al. (2009) in slovakia; nhat (2006) in japan, bougadis and admowski (2006) in canada have found linearity in such study. fig. 2. relationships between the log-transformed values of moments of various orders q and various rainfall durations at the station copargo. the orders q1 – q10 have values from 0.5 – 5 in steps of 0.5 respectively. the scaling exponents of the moments of various orders are estimated as the slopes of the linear regression between these moments and the rainfall duration. fig. 3 shows the relationships between the scaling exponents of the moments and the orders of the moments at copargo station. it’s obvious that the scaling exponents decrease with the order of moments and a linear relationship exists between scaling exponents and the moment orders. this implies that the property of wide sense simple scaling of rainfall intensity exists in this station. the scaling exponents and order of moments were linearly related with r2 value of 0.9932 for copargo station. data from other gauges of the 10 stations showed similar scaling relationships, indicating that scaling may be applicable in all the stations. fig.3 relationship between k (q) and order of moment q (q= [0.5:0.5:5]) for range, 5 min to 1440 min at copargo station. table 1 presents the characteristics of the examined stations, showing the stations and results of the scaling exponent factor k of the 10 stations in ohhvo. catchments are shown with high coefficients of determination ranging from 0.98 to 1. table 2: characteristics of examined stations: the values scale exponent k, and gumbel’s parameters of statistical analysis for 1440 min (σ24, μ24) the result indicates that for all the stations, the scaling exponents are less than 1 and in ranging from 0.23 to 0.59 in all the stations, showing considerable variability. the highest (the lowest) scaling exponent was found at sonoumon (pelebina) station. it is likely that the variability of the scaling exponents is influenced by the geographical location of the stations and/or the different mechanisms of rainfall generation. however, the results of olsson et al. (1992) which identified [0.6 0.8] as an adequate interval of scaling exponents at 45 min are not coherent with ours. these differences in the number of regime of scaling can be attributed to the differences of rainfall threshold detection of different used devices. comparing the value of scaling exponents k to those found by bara et al. (2009) in slovakia and chang (2013) in china, our estimation for all the 10 stations are different to other authors results, because these workers found the scaling exponents k higher than 6.2; but for few stations (sarmanga, wewe and sonoumon) where the value of k is approximately equal to those obtained by nhat (2006) d5estimation of idf curves of extreme rainfall by simple scaling in northern oueme valley, benin republic (west africa) in japan, bougadis et al. (2006) in canada and ghanmi (2014) in tunisia. the results indicate a strong validity of the simple scaling properties of the extremes rainfall in time series for the stations. 3.2 estimation of idf curves and evaluation of model performance the figures (fig.4) show the idf curves estimated by simple scaling and by fitting evi to the observations. it can be observed that the estimated idf curves of the short-duration rainfall by simple scaling fit the observed data point well at lower return periods (t<5 years) but not as well at higher return periods. this result may be due to the short observation period (only 13 years) of qualitative available rainfall data. from table 2, the location parameter μ24 and scale parameter σ24 calculated using equation (7) for the 10 stations is shown. subsequently, the mean μd and standard deviation σd for various sub-daily durations d are calculated from μ24 and σ24 using equations (5) and (6). fig. 4 idf curves estimated by simple scaling and by fitting evi to the observations for: (a) 5 min, (b) 10 min, (c) 15 min, (d) 20 min), (e) 30 min and (f) 1440 min rainfall. table 3. rmse and mape for 5 min, 10 min, 15 min, 20 min, 30 min, and 1440 min idf curves estimate using (a) simple scaling assuming evi and (b) fitting evi to observed data. the table 3 shows that mape values (scaling assuming evi) are in the range of 3.7-10%, which is slightly higher than the mape values when fitting evi to the observed data, ranging from 3.4-6.3%. the rmse values are in the range of 1-2.5 which is approximately higher than the rmse values when fitting evi to the observed data, ranging from 0.4-2.4. these results validate the performance of the simple scaling model in estimating the idf curves of short-duration rainfall in ohhvo valley (northern benin). 4. conclusion properties of the time scale invariance of selected rainfall data at 10 rainfall station in ohhvo valley (northern benin republic) are studied. a simple analytical formulation for rainfall idf relationship, which utilizes the scaling behavior, is presented. the benefit of using the principals of scaling d6 m.agbazo, g. koton’gobi, b. kounouhewa, e. alamou and a. afouda reduces the amount of parameters required to compute the quantiles. the main result of the study can be summarized as follows: (i) rainfall displays scale invariance property in one scaling regime from 5 min to 1440 min, (ii) the properties of rainfall follow the hypothesis of simple scaling, following the result of menabde et al. (1999), (iii) it is possible to derive the rainfall idf curves with only three parameters (k, μ24 and σ24) for duration shorter than a day. from only the location and scale parameters μ24 and σ24 of the evi sample of annual maximum 1440 min rainfall data and the scaling exponents, the idf curves of short-duration rainfall for low return periods (t<5years) could be estimated but may not be a reliable estimate for higher return period. in future studies, we should consider to exclude higher return periods, (iv) for the all stations the scaling exponents are less than 1 and range from 0.23 to 0.59, showing a considerable variability in values for the all stations. since it was not possible to find any clear dependence of the variability of the scaling exponents, it is believed that this variability may be accounted for by either for the geographical location of the stations or it the result of it is the sampling variability of the data (different mechanisms of rainfall generation), (v) idf relationships show acceptable result in comparison with the idf curves obtained from at-site short duration rainfall data. the key advantage is that idf curves of shorter rainfall duration for low return periods (t<5years) that are not measured or unavailable can be derived solely based on the statistical properties of the annual maximum 1440 min rainfall. this is relevant for benin republic daily data, available for long duration period, but data for short duration are often not available for the required site. the finding from this study may have significant bearing beyond the study area. acknowledgements the authors would like to thank the general directorate of amma for providing the data. we also thank the anonymous reviewers for their suggestions to improve the manuscript. we also thank the international foundation for science (ifs) for its financial support. 5. references acar, r., celik, s., senocak, s. 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(2011). analysis and modeling of the continental hydrological cycle for integrated water resources management in benin: beterou basin at oueme. university of abomey-calavi, cipma. phd thesis. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 5 6 letter from editor dear readers and contributors of the earth sciences research journal, thanks again to be part of this interesting professional journal. with pretty proud i want to present this new issue (17:1, june-2013) with interesting research products coming from antarctic continent, mexico, botswana, india, turkey, nigeria and iran. currently, esrj is covered in the top scientific index and bibliographic database as academic search complete, doaj, fuente académica, georef, scopus, isi web of knowledge, latindex, scielo, ulrich, publindex, e-revistas, and academic onefile. and we expect in the future turn into one of the most referred journal in this field of knowledge. the first article is related to antarctic’s crustal structure. “reference crust-mantle density contrast beneath antarctica based on the vening meinesz-moritz isostatic problem and crust2.0 seismic model” looks for a moho density accurate contrast in spite of the data lackness in this part of the world. then, the authors used a combination of methods to reach their main objective and give us major features of the antarctic’s continental and surronding oceanic tectonic plate configuration. our second paper presents a mexican inventory in seismic specialised software. “contribution of software engineering in earthquake engineering”, which is a suggestion for new development lines, prototypes in seismic software aimed to apply preventive actions related to seismic events in this country. third manuscript comes from botswana and presenting an electrical resistivity study carried out in the thata island, one of the numerous islands in the okavango delta. based on seven data profiles, study indicates the centre of the island had low resistivity values, while those values increase laterally outside the island. results of this work showed how high salinity levels may have formed in near surface aquifers in natural inland delta environments in semi-arid areas. concerned about drinking water in maharashtra province, in india, our authors have deployed an important study about anjani and jhiri rivers catchment area, which provide the jalgaon district. trace elements were analysed in ten groundwater samples and the results were compared to bis drinking water standards. concentration of lead, cadmium and nickel exceed the maximum permissible limit marked by bis. coming from turkey, other study shows the preparation of vulnerability maps to prevent contamination in aquifers. using gis techniques and the drastic method, a vulnerability map is prepared for the basin in which the gümüşhacıköy and merzifon aquifers are located, across many villages characterised by intensive agricultural activity. this analysis ends with the conclusion the basin had a low contamination potential. located in a crude oil prospecting area of ondo state in nigeria, we present a speciation metals study aimed to assess the bioavailability, fate and mobility of as, cd, cr, fe, mn, ni, pb, and v. this paper divides the studied metal in five operationally-defined categories and follows their presence in different places of the area for being concluded that sediment from the crude oil prospecting zone may serve as non-point sources for metals contaminating the coastal system, they have higher metal bioavailability and higher toxicity risk potential than other sites. in nigeria too, an active dumpsite was monitored to test natural attenuation efficiency in reducing contaminants by determining soil and groundwater biogeochemical and engineering properties within and around the site. after measuring unsaturated zone thickness and permeability, and grading the soil as clay or silt according to casagrande plasticity chart, study found that the concentration of contaminants in the soil are very high and inversely proportional to depth. our two last works in this edition were prepared in iran. one of them is focused on the stratigraphy of the permian-triassic boundary in the haftad gholleh area. this boundary of erosional unconformity contains dolomitic limestone and shale and sandstone disconformably located below the triassic sequence and represents a hiatus of about 10 million years. finally, we close this edition with the work “late permian brachiopoda fauna in north-western iran”. this paper found in the area twenty-seven species from 13 brachiopoda genera. the recognised fossil community was compared to brachiopod communities in some regions of iran, and they belong to the orders athyris, rhynconellida, productida and strophomenida. these interesting papers will constrain much better new stratigraphic correlations in this region of the world. carlos a. vargas j. chief editor departamento de geociencias universidad nacional de colombia keywords: crustal structure; gujarat region; surface wave; dispersion; genetic algorithm. palabras clave: estructura de la corteza; region gujarat; onda superficial; dispersión; algoritmo genético how to cite item joshi, v. (2019). crustal velocity models retrieved from surface wave dispersion data for gujarat region, western peninsular india. earth sciences research journal, 23(2), 147-155. doi: https://doi.org/10.15446/esrj.v23n2.58570 the physiographic features of gujarat state of western india are unique, as they behaved dynamically with several alterations and modifications throughout the geological timescale. it displays a remarkable example of a terrain bestowed with geological, physiographical and climatic diversities. the massive 2001 bhuj earthquake (m 7.7) over the kachchh region caused severe damage and devastation to the state of gujarat and attracted the scientific community of the world to comprehend on its structure and tectonics for future hazard reduction. in the present study, three clusters of wave paths a, b1, and b2 have considered. in each cluster, dispersion data were measured station by station which collectively formed a dispersion data file for a nonlinear inversion through genetic algorithm. in this way, three crustal velocity models were generated for entire gujarat. these models are 1) across cambay basin (path a), 2) along saurashtra kathiawar horst (path b1) and 3) along narmada basin (path b2), which were formed at different times during the mesozoic. the average thickness of the crust estimated in the present study for paths a, b1 and b2 are 38.2 km, 36.2 km, and 41.6 km respectively and the estimated s-wave velocity in the lower crust is ~ 3.9 km/s for all the paths. the present study will improve our knowledge about the structure of the seismogenic layer of this active intraplate region abstract crustal velocity models retrieved from surface wave dispersion data for gujarat region, western peninsular india modelos de velocidad cortical obtenidos de la información de ondas de dispersión superficiales en la región de gujarat, en el occidente peninsular de la india issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.58570 las características fisiográficas del estado occidental de la india son únicas, ya que ellas actúan dinámicamente con varias alteraciones y modificaciones a través de la escala de tiempo geológica. estas características son el ejemplo de un terreno con diversidades climáticas, fisiográficas y geológicas. el terremoto de bhuj en 2001 (de magnitud 7.7.) en el distrito de kutch causó varios daños y devastación al estado de gujarat, al tiempo que atrajo a la comunidad científica del mundo para estudiar su estructura y tectónica con el fin de reducir futuros peligros. en el estudio presente se consideraron tres grupos de trayectorias de ondas, a, b1 y b2. en cada grupo la información de dispersión se midió estación por estación y el conjunto de datos de dispersión recolectados permitió una inversión no lineal a través de un algoritmo genético. de esta forma, tres modelos de velocidad cortical se generaron para todo gujarat. estos modelos son: 1. a lo largo de la cuenca cambay (trayecto a). 2. a lo largo de saurashtra kathiawar horst (trayecto b1). 3. a lo largo de la cuenca narmada (trayecto 2), los cuales se formaron en momentos diferentes durante el mesozoico. el promedio del grosor de la capa estimado en este estudio para las trayectorias a, b1, y b2 es de 38,2 km, 36,2 km y 46,1 km respectivamente, y la velocidad de la onda s en la parte baja de la capa es de ~ 3.9 km/s para todas las trayectorias. este trabajo debe contribuir el conocimiento acerca de la estructura de la capa sismogénica de esta activa región que yace entre placas resumen record manuscript received: 23/06/2017 accepted for publication: 29/03/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 147-155 vishwa joshi institute of seismological research (isr), gujaratuniversity, navrangpura, ahmedabad gujarat, india * corresponding author: vishwajoshi@isr.res.in g eo ph ys ic s 148 vishwa joshi introduction the western continental margin (including state of gujarat) of india evolved mainly during the drift stage of the indian plate in late cretaceoustertiary times, following the late triassic rifting of the eastern gondwanaland. kachchh, saurashtra, cambay and narmada are four pericratonic basins which evolved during the upper gondwana time. these basins are located in the northern part of the western continental shelf (biswas, 1987, 2005). the state of gujarat comprises three distinct zones: mainland gujarat, saurashtra and kachchh. the kachchh zone is one of the most seismically active zones in india. during the last 200 years, since 1819, this zone has produced two largest devastating intraplate earthquakes, the june 1819 earthquake mw 7.8 and the january 2001 earthquake mw 7.7 (fig. 1) (kayal et al., 2002a). in addition, this zone was hit by a strong earthquake mw 6.0 in 1956 (fig. 1) (chung and gao, 1995), which also caused severe damages and loss of lives. some parts in the saurashtra zone are also found to be producing minor to moderate magnitude local earthquakes. after the january 2001 large earthquake, the institute of seismological research (isr) has established a local close spaced gujarat seismic network (gsnet) since 2006. this network comprises of some 60 broadband seismographs and 50 strong motion accelerographs. the network provides data of large aftershock sequence of the 2001 event till date, and many local, regional and teleseismic events. a few studies have been done to find out crustal structure in the epicenter area of the 2001 earthquake using temporary network and the gsnet data by seismic tomography (kayal et al., 2002b; mandal, 2006; singh et al., 2011). from early 1980s to 1990s, a few deep seismic sounding (dss) profiles were undertaken to investigate crustal structure in different parts of the gujarat region (fig. 1) (kaila et al., 1981, 1985, 1989, 1990). in the recent years, some limited studies have been done to find out crustal structure using surface wave data of the local networks (mandal, 2006; prajapati et al., 2011; joshi and rastogi, 2015). in this study, data of the gsnet was used and an attempt was made for comprehensive study on the crustal velocity structure beneath the whole gujarat region through inversion of both love and rayleigh wave group velocities. the results of this study are highlighted in this paper. the present study will provide detailed knowledge about the structure of seismogenic layer of this active intraplate region. geological setup of gujarat region the state of gujarat with unique physiographic features comprises the three distinct zones, i.e. 1) mainland gujarat, 2) saurashtra and 3) kachchh. its physiography has remained very dynamic with alterations and modifications taking place throughout the geological history of the region. it illustrates an interesting example of a terrain endowed with geologic, physiographic and climatic diversities. the geological evolution of the northern and eastern parts of mainland gujarat has been controlled by the precambrian orogenies aravalli and delhi cycles, and the older crystalline rocks ideally show folds, faults and magmatism related to the two orogenies. a major part of mainland falls within the cambay and the narmada grabens and the eastern and northeastern precambrian rocks mark a tectonic boundary (merh, 1995). the central and southern parts are dominantly covered with thick pile of quaternary sediments. the peninsula of saurashtra has been described as a horst, foundered between the fractures related to the three intersecting rift trends, viz, delhi (ne-sw), narmada (ene-wsw) and dharwar (nnw-sse) (biswas, 1987). the triangular shape of the saurashtra horst is a reflection of the various bounding faults on all sides (merh, 1995). in case of kachchh the main structural features that have played a vital role in its geological evolution include a group of e-w trending uplift (highlands and islands) surrounded by a residual depression, plains of the great and the little ranns. six major uplifts namely pachcham, khadir and bela islands, chorar hills, wagad highlands and the kachchh mainland occur along three sub-parallel trending e-w lines (merh, 1995). the kachchh-saurashtra, cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton. these basins were formed by rifting along precambrian tectonic trends. interplay of three major precambrian tectonic trends of the western india, dharwar (nnw-sse), aravalli-delhi (ne-sw) and satpura (ene-wsw), controls the tectonic style of the basins. the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults. the kachchh basin opened up first in the early jurassic (rifting was initiated in late triassic) along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend (biswas, 1982, 1987). the evolution of the basins took place in four stages. these stages are synchronous with the important events in the evolution of the indian sub-continent-its breakup from gondwanaland in the late triassic-early jurassic, its northward drifting during the jurassiccretaceous and collision with the asian continent in the early tertiary. the most important tectonic events occurred in late cretaceous time. the present style of the continental margins of india evolved during early tertiary time (biswas, 1982, 1987, 2005). the saurashtra arch, the extension of the aravalli range across the western continental shelf, subsided along the eastern margin fault of the cambay basin during the early cretaceous. it formed an extensive depositional platform continuous with the kachchh shelf, for the accumulation of thick deltaic sediments. a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous (biswas, 1987). the continental flood basalts (deccan volcanics) cover large parts of northwest india. these consist of several flows of mainly tholeiitic lava that extruded from the reunion plume during the passage of the indian plate over the plume in cretaceousearly tertiary time (morgan, 1981; rao et al., 2005). it has been proposed that the volcanic eruption from the reunion plume at ~65 ma was mainly responsible for the widespread volcanic activity in western india (white and mc kenzie, 1989; rao et al., 2005). the center of the plume at that time was close to the west coast of india. the saurashtra peninsula was close to the trace of this plume on the earth’s surface, during the passage of india over it, in late cretaceous. the trace of the plume passed through the cambay basin to the immediate east of the saurashtra peninsula (campbell and griffiths, 1990; kaila et al., 1990; kennett and widiyantoro, 1999; rao et al., 2005; rao et. al., 2013). thus, whole gujarat region is tectonically complex, geologically in much diversity and seismically most active compared to any other part of the intraplate region in peninsular india. data surface wave data used in this study was recorded by the gsnet. study area with selected earthquakes and recording stations are shown in figure 1. eight stations data of ssnnl (sardar sarovar narmada nigam limited) network for the path a, four stations data of ssnnl for path b2 and seven stations data of kachchh region for path b1 have been used. observatories at kachchh are equipped with broadband triaxial seismometer cmg3t (guralp) and those at ssnnl are equipped with trillium 120 (nanometrics) connected to 24-bit recorder with gps time synchronization. the hypocentral parameters of earthquakes used for the surface-wave dispersion study are listed in table 1. epicentral distance range for path a, b1 and b2 are 361-505 km, 241-306 km and 277-461 km respectively. group velocity measurement the broadband data of earthquakes listed in table 1 were used to obtain the group velocity measurements. the digital waveform records were decimated to 5 samples per second, corrected for the seismograph response, and converted to vertical, radial, and transverse components with known back azimuth. then, the group velocities were measured using a technique called the frequency time analysis (ftan) following bhattacharya (1981, 1983). vertical components are used to obtain the rayleigh wave group velocities, and transverse components are used to obtain the love wave group velocities for each cluster. the measured love wave group velocities sample periods from 4–62 s and 4-50 s for path a and b1 respectively and rayleigh wave group velocities sample periods from 4–66 s, 4-46 s and 4-72 s for paths a, b1 and b2 respectively. fig. 2 shows observed dispersion data of both love and rayleigh wave at different stations for path b1, similarly we get dispersion curve data for path a and b2 also. for path b2 only rayleigh wave dispersion curves are used because the quality of dispersion curves of love wave was not good. the mean and standard deviations of the observed data is shown in figures 3, 4 and 5. these data are used to retrieve the average shear-wave velocities along the wave paths. 149crustal velocity models retrieved from surface wave dispersion data for gujarat region, western peninsular india figure 1. map shows three clusters of wave paths: a, b1 and b2. the dss profile, geological section, major fault lines, pericratonic rift basins and past damaging earthquakes (1819, 1956 and 2001) are also shown. table 1. list of earthquakes used for evaluation of group velocity path date (dd/mm/yyyy) origin time (hr:mm:ss) longitude (°e) latitude (°n) magnitude used for* path a 9/3/2008 23:36:00 70.347 23.389 4.9 lq, lr 17/5/2011 16:00:00 70.569 23.559 4.2 lq, lr 19/6/2012 13:44:00 70.279 23.639 5.1 lq, lr path b1 6/11/2007 0:27:29 70.520 21.134 4.8 lq, lr 6/11/2007 9:38:04 70.544 21.133 5.0 lq, lr 20/10/2011 17:18:32 70.536 21.129 5.1 lq, lr path b2 6/11/2007 0:27:29 70.520 21.134 4.8 lr 6/11/2007 9:38:04 70.544 21.133 5.0 lr 20/10/2011 17:18:32 70.536 21.129 5.1 lr *lq =love wave; lr = rayleigh wave 150 vishwa joshi 2,4 4 3,6 3,2 2,8 4,4 0 5040302010 60 2,4 4 3,6 3,2 2,8 4,4 0 402010 5030 bdr bel bhu des gdd suv vam average bdr bel bhu des gdd suv vam average rayleigh wavelove wave figure 2. observed dispersion curves of love and rayleigh wave for path b1 2,6 3,6 3,4 3,2 4 0 4020 60 3,2 4 3,8 3,6 3,4 4,2 80 3 2,8 3,8 0 4020 60 80 3 2,8 rayleigh wave love wave period (s) period (s) g ru p ve lo ci ty (k m /s ) g ru p ve lo ci ty (k m /s ) figure 3. observed group velocities across path a are shown by small triangles; vertical lines show corresponding standard deviations. the theoretical group velocity curve for the accepted model is shown by the continuous line. misfit φ of accepted model is 0.80 and estimated thickness of the crust is 38.2 ± 1.2 km. 151crustal velocity models retrieved from surface wave dispersion data for gujarat region, western peninsular india 2,6 3,6 3,4 3,2 4 0 4020 60 3,2 4 3,8 3,6 3,4 4,2 80 3 2,8 3,8 0 3020 40 50 3 2,8 rayleigh wave love wave period (sec) period (sec) g ru p ve lo ci ty (k m /s ec ) g ru p ve lo ci ty (k m /s ec ) 2,6 10 4,2 figure 4. observed group velocities across path b1 are shown by small triangles; vertical lines show corresponding standard deviations. the theoretical group velocity curve for the accepted model is shown by the continuous line. misfit φ of accepted model is 0.84 and estimated thickness of the crust is 36.2± 1.0 km. rayleigh wave period (s) g ru p ve lo ci ty (k m /s ) 3,6 3,4 3,2 4 3 2,8 3,8 4,2 3020 40 5010 60 70 80 figure 5. observed group velocities across path b2 are shown by small triangles; vertical lines show corresponding standard deviation. the theoretical group velocity curve for the accepted model is shown by the continuous line. misfit φ of accepted model is 0.79 and estimated thickness of the crust is 41.6 ± 0.7 km. inversion to retrieve the crustal shear-wave velocity models, a nonlinear inversion with the genetic algorithm (ga) following suresh et al. (2008), bhattacharya (2009), prajapati et al. (2011) has used as, the surface wave dispersion curve is a nonlinear function of medium parameters. the ga inverts the dispersion and gives a fully nonlinear solution in a large model space (lomax and sneider, 1995). the ga does not improve a solution, but it works on a population of possible solutions. however, the ga is an iterative directed search operating on a population of trial solutions within a user defined search space to find new solutions with lower misfit in each generation; the misfit is obtained from the difference between observed data and theoretical values based on a solution (suresh et al., 2008). for inversion through ga, adequate knowledge of starting model is required. in present study, the model evaluated by prajapati et al. (2011) has been used as starting model. since the travel paths used to evaluate crustal model by prajapati et al. (2011) crosses the present study area, it is presumed that the structure along the present study region is similar to the model evaluated by them. this model is taken as a starting model, which consist of 4 layers of crust and a layer of subcrust, whose base is fixed at 100 km depth. the ranges of model parameter were set around the corresponding parameters of this model. theoretical group velocities were obtained for an isotropic elastic layered medium following bhattacharya (1987). the 14 variables in our ga search are (1–4) thickness of the top four solid layers of the crust, (5–9) vs of the top five solid layers, and (10–14) vp/vs of the top five solid layers (table 2). here, vp is p-wave velocity, and vs is s-wave velocity. 152 vishwa joshi table 2. five layer structure with search space of the solutions path layer number thickness (km) depth of bottom (km) density (gm/cm3) vs (km/s) vp/vs path a 1 2.0-5.0 2.20 2.50-3.20 1.65-1.78 2 3.0-6.0 2.30 2.80-3.30 1.65-1.78 3 7.0-12.0 2.60 3.00-3.50 1.65-1.80 4 34.0-44.0 3.01 3.80-3.95 1.65-1.80 5 100 3.30 4.55-4.69 1.70-1.80 path b1 1 0.8-3.0 2.20 2.50-3.00 1.70-1.85 2 1.0-3.0 2.30 2.50-3.20 1.70-1.85 3 7.0-12.0 2.60 3.40-3.75 1.70-1.85 4 32.0-44.0 3.01 3.80-3.95 1.60-1.75 5 100 3.30 4.65-4.75 1.60-1.75 path b2 1 0.8-3.0 2.20 2.50-3.00 1.68-1.78 2 1.0-3.0 2.30 2.50-3.20 1.68-1.78 3 7.0-12.0 2.60 3.30-3.75 1.68-1.78 4 34.0-44.0 3.01 3.80-3.95 1.68-1.78 5 100 3.30 4.65-4.80 1.68-1.78 the density in each layer has been kept constant because it has the least effect on the dispersion curve (bloch et al., 1969; suresh et al., 2008). the density in this region is based on prajapati et al. (2011) and radhakrishna et al. (2002). the misfit function is considered as similar to prajapati et al. (2011) as ϕ=max ,i r i r j l j l δ σ δ σ         where i r is the absolute difference between the observed and theoretical group velocities of a given structure, and i r is the standard deviation (s. d.) of the observed group velocity at period ti (i = 1, 2,…, nr) for rayleigh waves.  j l and  j l are the corresponding values at period tj (j = 1, 2,…, nl) for love waves. nl and nr are the numbers of observations for love and rayleigh waves. the misfit φ gives a measure of closeness of the theoretical curve to the observed data with respect to standard deviation. if φ < 1, the theoretical curve is within the standard deviation at each observation (prajapati et al., 2011). however, study done by suresh et al., (2008) and prajapati et al., (2011) considered a misfit function as ϕ0 1 1 1 1=max nr , nl i nr i r i r j nl j l l = = ∑ ∑           δ σ δ σ , j according to bloch et al., (1969), the partial derivatives of group velocities with respect to the model parameters show that the group velocities at different periods are affected by different depth regions of the structure. thus, the group velocity at each period has its own importance in evaluating model parameters of different depth regions of the structure. the misfit φ gives equal importance to δ/σ for each period of rayleigh and love waves so, is better than φ0. the misfit φ0 considers the averages, so the importance of individual data is ignored; further, if φ0 < 1, the theoretical curve is not necessarily within the standard deviation at each observation. (prajapati et al., 2011). the ga searches with a random initial population of k models within the search limits. 100 generations, with the population size k =300 at each generation is considered for all the path, and the best model of the last generation is accepted if the misfit value <1. this procedure is repeated 50 times for each path. the mean and s.d. of each variable of all the 50 accepted solutions for each path are obtained and accepted as final solutions. figures 3, 4 and 5 show the observed group velocity curve and the theoretical group velocity curve of the model obtained through ga for path a, b1 and b2 respectively. results and discussion in this study, crustal velocity models have been obtained for three different paths (a, b1and b2) that crosses through three different tectonic blocks of the region as mentioned above. the average thickness of the crust estimated for paths a, b1 and b2 are 38.2 km, 36.2 km and 41.6 km respectively and the estimated s-wave velocity in the lower crust is ~ 3.9 km/s for all the paths. the path a that crosses through cambay basin; opened up in the early cretaceous along the dharwar trend (biswas, 1982, 1987), and it has developed in three structural stages. the lower stage is represented by mesozoic rocks, middle stage by thick basaltic lava flow during late cretaceous which is better known as deccan trap and finally the upper stage is represented by cenozoic sequences. the total thickness of the crust estimated for path a is 38.2 ± 1.0 km with s-wave velocity of 3.9 km/s. the top most sedimentary layer is 3.2 km thick followed by 4.6 km thick deccan trap. similar results were found by biswas (1987), he identify ~4 km thick deccan trap in cambay graben (geological section bb’ in fig. 1). through deep seismic sounding (dss) study, kaila et al. (1981, 1990) found moho at ~38 km depth in the cambay basin, where our path a crosses their dss profile between gandhinagar and broach (narmada basin) (fig. 1). these results are much in agreement with study done by biswas (1987) and kaila et al. (1981, 1990). 153crustal velocity models retrieved from surface wave dispersion data for gujarat region, western peninsular india second path b1 is along the saurashtra; which was uplifted as a horst during the major tectonic phase in the late cretaceous (biswas, 1987). this event was followed by eruption of the deccan volcanics due to interaction between the reunion hotpot and the overlying lithospheric plate (mckenzie and sclater, 1971; raval and veeraswamy, 2000; rao and tewari, 2005). almost entire saurashtra is covered by this volcanics (rao and tewari, 2005). the estimated average thickness of the crust for path b1 is 36.2 ± 1.0 km with s-wave velocity of 3.9 km/s. the top most layer of deccan trap is 2.0 km thick, which is followed by 1.6 km thick volcanic upper crust. these observations are comparable with those of biswas (1987) (geological section aa’ in fig. 1). further the crustal thickness estimated through dss study along navibandar and amreli profile shows ~36 km thick crust beneath junagadh (rao et al., 2005). which is also in a good agreement with present estimate, as present path crossing the profile near junagadh (fig. 1). moreover through receiver function study mandal (2006) found moho at the depth of 35-42 km. path b2 is along the narmada basin that, opened up during late cretaceous along the satpura trend. it separates the region of proterozoic vindhyan deposition to the north from gondwana deposits to the south. according to biswas and deshpande (1983, 1987) widespread volcanic eruption towards the end of the cretaceous covered the mesozoic sedimentary rocks of all the (kachchh-saurashtra-narmada) basins. the estimated average thickness of the crust for path b2 is 41.6 ± 0.7 km with s-wave velocity of 3.9 km/s. according to tewari et al. (2000), the deccan traps thickness in this basin varies from 1000 to 2700 m. moreover, through dss study along dharimanna-billimora profile kaila et al., (1981, 1985, 1990) and tewari et al. (1995) revealed ~40 km thick crust near broach (narmada basin) where present path crosses the profile (fig. 1). another dss study along thuadara-sendhwasindad profile done by kaila (1989) across narmada basin found 38-43 km moho depth variation. which indicates the results of present study are fairly comparable with the results of kaila et al., (1981, 1985, 1989, 1990) and tewari et al. (1995). structural parameters of all the paths are shown in table 3. in present study, standard deviations for the estimated thicknesses vary from 0.8 to 1.3, 0.2 to 1.0 and 0.2 to 0.7 km for the paths a, b1 and b2 respectively. standard deviations for the s-wave velocity vary from 0.1 to 0.2 km/s for path a, b1 and b2. for each solution vp is obtained from vs and vp/vs. standard deviations for the p-wave velocity vary from 0.1 to 0.4 km/s for path a, b1 and b2. misfit φ of the accepted model is 0.80, 0.84 and 0.79 for path a, b1 and b2 respectively. these show that present observations are fairly well constrained, and within acceptable errors. variations of s-wave velocity with depth for the three paths (a, b1 and b2) are illustrated in step plot (fig. 6). the maximum moho depth (~ 42 km) is found along the path b2 beneath the narmada basin. the moho depth is minimum at ~36 km along the path b1, below the saurashtra horst, and an average moho depth ~38 km along the path a, below the cambay basin, mainland gujarat. the plot further illustrates the marked variations in s-wave velocity in the upper crust at depths between 5 km and < 20 km. it is much low beneath the cambay basin (path a) compared to that beneath the saurashtra horst (path b1) and narmada basin (b2). presence of mesozoic sediments below the deccan traps might be the possible reason for the low s-wave velocity beneath the cambay basin kaila et al., (1981, 1990). on the other hand, the s-wave velocity in the lower crust at depth below 20 km, is much consistent along all the paths a, b1 and b2 beneath all the tectonic zones studied here. observations have summarized in figure 7 with inferred geological sections for each path. table 3. crustal and subcrustal structure of paths a, b1 and b2 path layer thickness (km) vs (km/s) path b1 sediments 3.2±0.8 3.0±0.1 deccan trap 4.6±0.8 3.1±0.1 upper crust 9.4±1.2 3.2±0.1 lower crust 21.1±1.3 3.9±0.1 subcrust 61.8±1.0 4.6±0.1 path b1 deccan trap 2.0±0.2 2.7±0.1 volcanic upper crust 1.6±0.4 3.0±0.1 upper crust 8.4±0.6 3.6±0.1 lower crust 24.2±0.5 3.9±0.1 subcrust 63.8±1.0 4.7±0.1 path b2 deccan trap 1.0±0.2 2.9±0.2 shallow crust 1.5±0.4 3.0±0.2 upper crust 7.9±0.7 3.4±0.1 lower crust 31.1±0.5 3.9±0.1 subcrust 58.4±0.7 4.8±0.1 path a 20 30 40 60 10 0 32,5 3,5 42 4,5 5 5,5 50 d ep th (k m ) path b1 path b2 s-wave velocity (km/s) figure 6. s-wave velocities in the crustal and subcrustal region along wave paths 20 30 40 60 10 0 50 d ep th (k m ) deccan trap volcanic upper crust 70 80 100 90 sediment shallow crust upper crust lower crust sub crust path b1 path a path b2 20 30 40 60 10 0 50 70 80 100 90 figure 7. geological section of the crust along paths b1, a and b2 respectively. 154 vishwa joshi according to merh (1995), the geology of gujarat comprises a precambrian basement over which younger rocks of jurassic through cretaceous, tertiary and quaternary are deposited. further, the jurassic rocks are found only in kachchh basin, so the major geological events of gujarat are confined to mesozoic and cenozoic. uplift of saurashtra as horst and formation of cambay graben is shown in first two columns of figure 7. from the stratigraphy succession given by biswas (1987) and merh (1995) it is concluded that the top two layers along all three paths are formed during cretaceous to quaternary, and the layers below top two layers were formed in proterozoic because paleozoic era are totally absent. if present results are supplemented with other geophysical and seismological investigation, it can provide better understanding about the seismogenic depth and related hazard for this active intraplate region. conclusions crustal structure of gujarat region has been investigated by inverting group velocity measurement of fundamental mode rayleigh and love waves using a genetic algorithm. important outcomes of this study are as follows: i) the maximum estimated crustal thickness is 41.6 ± 0.7 km, beneath the narmada basin, followed by 38.2 ± 1.0 km beneath the cambay basin and 36.2 ± 1.0 km beneath the saurashtra horst. ii) the deccan trap thickness is maximum in the cambay basin (path a) as compared to saurashtra horst (path a) and narmada basin (path b2). iii) the cambay basin exhibits maximum sediment layer thickness of about 3km while it is absent for in the saurashtra horst. it may be mentioned that the cambay basin is an oil producing basin. iv) the s-wave velocity at the lower crust remains constant at 3.9 km/s, while it varies from 3.2 to 3.6 km/s in the upper crust. the younger formations above the upper crust (say surface to 5 km depth) have different lithology as these are formed after the evolution of the basins and horst in the cretaceous. the velocity models retrieved through the present study would be much useful in seismic hazard assessment and to understand the crustal configuration and to understand evolution and tectonic history of the western part of india. acknowledgments the author is thankful to 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(2014). comparison of stochastic optimization methods on two analytic objective functions and on a 1d elastic fwi. 76th eage conference and exhibition 2014. doi:10.3997/2214-4609.20140857 sridhar, a. r., & tewari, h. c. (2000). existence of a sedimentary graben in the western part of narmada zone: of seismic evidence. journal geodynamics, 31(2001), 19-31. suresh, g., jain, s. & bhattacharya, s. n. (2008). lithosphere of indus block in the northwest indian subcontinent through genetic algorithm inversion of surface-wave dispersion. bulletin of the seismological society of america, 98(4), 1750-1755. tewari, h. c., dixit, m. m. & sarkar, d. (1995). relationship of the cambay rift basin to the deccan volcanism. journal of geodynamics, 20(1), 85-95. white, r. s. & mckenzie, d. p. (1989). magmatism at rift zones; the generation of volcanic continental margins and flood basalts. journal of geophysical research, 94, 7685-7729. palabras clave: basurero municipal; índice de compresion; propiedades físicas; modelo anfis; modelo ann; estándar estadístico. key words: municipal solid waste, compression ratio, physical properties, anfis model, ann model, statistical criteria the compression ratio of municipal solid waste (msw) is an essential parameter for evaluation of waste settlement. since it is relatively time-consuming to determine compression ratio from oedometer tests and there exist difficulties associated with working on waste materials, it will be useful to develop models based on waste physical properties. therefore, present research attempts to develop proper prediction models using anfis and ann models. the compression ratio was modeled as a function of the physical properties of waste including dry unit weight, water content, and biodegradable organic content. a reliable experimental database of oedometer tests, taken from the literature, was employed to train and test the ann and anfis models. the performance of the developed models was investigated according to different statistical criteria (i.e. correlation coefficient, root mean squared error, and mean absolute error) recommended by researchers. the final models have demonstrated the correlation coefficients higher than 90% and low error values; so, they have capability for acceptable prediction of municipal solid waste compression ratio. furthermore, the values of performance measures obtained for ann and anfis models indicate that the anfis model performs better than ann model. el índice de compresión de residuos sólidos es un parámetro esencial para la evaluación del asentamiento de un basurero municipal. debido al desgaste de tiempo para determinar el índice de compresión a partir de pruebas edométricas y debido a las dificultades asociadas al trabajo con materiales desechados es necesario desarrollar modelos basados en las propiedades físicas de los desechos solidos. además, la presente investigación pretende desarrollar modelos de predicción apropiados a partir de los esquemas anfis y ann. el índice de comprensión se modeló como una función de propiedades físicas de desechos que incluyen el peso seco de una unidad, el contenido de agua y el contenido orgánico biodegradable. de la literatura se tomó una base de datos confiable de pruebas edométricas experimentales que fue empleada para preparar y evaluar los modelos anfis y ann. el desempeño de los modelos desarrollados fue investigado de acuerdo con diferentes criterios estadísticos (por ejemplo, el coeficiente de correlación, el error cuadrático medio y el error medio absoluto) recomendados por investigadores. los modelos finales han demostrado coeficientes de correlación mayores al 90 por ciento y valores bajos de error. esto significa que estos modelos tienen una capacidad de predicción aceptable para el índice de comprensión del basurero municipal. además, los valores de las medidas de desempeño obtenidos para los modelos anfis y ann indican que el modelo anfis tiene una mayor asertividad que el modelo ann. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 165 171 abstract resumen record manuscript received: 10/02/2014 accepted for publication: 29/07/2014 geotechnics compression ratio of municipal solid waste simulation using artificial neural network and adaptive neurofuzzy system maryam mokhtari¹*, ali akbar heshmati r¹, nader shariatmadari¹ ¹ school of civil engineering, iran university of science and technology (iust), narmak, tehran 16846-13114, iran, tel: +98 21 77240565 ¹ school of civil engineering, iran university of science and technology (iust), narmak, tehran 16846-13114, iran, tel: +98 21 77240565, corresponding author: mokhtari_ma@iust.ac.ir ¹ school of civil engineering, iran university of science and technology (iust), narmak, tehran 16846-13114, iran, tel: +98 21 77240565 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.41988 http://dx.doi.org/10.15446/esrj.v18n2.41988 166 ali akbar heshmati r, maryam mokhtari, nader shariatmadari introduction settling municipal solid waste (msw) is a significant issue considered in geotechnical design of a landfill. waste settlement influences the stability, deformation, and structural performance of landfills (chen et al., 2010). the settlement of msw is composed of three mechanisms: primary compression, mechanical creep, and bio-compression (sowers, 1973; edgers et al., 1992; wall and zeiss, 1995; el-fadel and khoury, 2000; and bareither et al., 2011). primary compression occurs quickly in response to self-weight and an external load applied to the waste. secondary compression (i.e. mechanical creep) is due to long-term slippages, reorientation of particles, and delayed compression of some waste constituents. bio-compression is related biochemical processes associated with the decomposition of biological solids and gas generation. a commonly used parameter for predicting the primary compression strain in msw is the compression ratio (c’c): (1) where εi is the primary compression in msw, σ vo ' is the existing vertical effective stress and δσ v ' is the change in the vertical effective stress. the compression ratio represents the slope of the curve of the strain versus the logarithm of the effective pressure. it is conventionally determined through an oedometer test and is related to the compression index (cc), which is commonly applied for soils by the equation c 'c = cc 1+ e0 , where e0 = initial void ratio. considering some difficulties in calculating the initial void ratio of the waste materials, it is usually preferred to use the compression ratio instead of the compression index. there have been several studies investigating the effect of various parameters on waste compressibility. chen et al. (2010) and landva et al. (2000) examined waste samples with different initial densities. their results demonstrate that an increase in the waste density reduces the c’c. experimental works conducted by sowers (1973), swati and joseph (2008), and chen et al. (2009) on waste settlement revealed an increase in organic compounds and compressible materials could bring about an increase in c’c. karimpour-fard and machado (2009) and chen et al. (2009) tried to examine the compressibility of waste samples that were at different ages. the results demonstrated that an increase in the age; thus, further waste decomposition decreased the waste compressibility and compression ratio. durmusoglu et al. (2006) tested waste materials with a natural moisture content and field capacity moisture. the results indicated that wastes with field capacity moisture had a higher c’c than those with a natural moisture content. other researchers, such as vilar and carvalho (2004) and reddy et al. (2009b), examined waste samples with a similar composition and a dry unit weight. they reported higher c’c values for wastes with higher water contents. earlier experimental works introduced such parameter as dry unit weight, water content, and contribution percentage of biodegradable organic materials (paper, food wastes, and yard wastes) as the most significant factors which affect the compressibility of municipal solid waste. earlier studies have addressed the effect of some parameters on waste compression ratio. it is to be noted that determining compression ratio through an oedometer test is time-consuming. another problem with doing this test on waste materials is the scale effect. owing to the fact that the particles of waste materials are much larger than those of fine-grained soil, it is not possible to precisely determine the waste compressibility by commonly used oedometers such as casagerande oedometer. in many cases, therefore, any precise investigation of the compressibility behavior of waste entails designing and manufacturing oedometer instruments that suit the size of waste materials. also, owing to the specific nature of waste materials, it would be hard to work on them. therefore, it seems necessary to develop models that are based on effective parameters measured through more simple experimental methods. in recent years, soft computing methods such as neural networks and neuro-fuzzy have been used successfully to solve a wide variety of problems in geosciences εi =cc ' σ v0 ' +δσ v ' σ v0 ' and geotechnical engineering. these include the estimation of the prediction of compression ratio of soils (park and lee, 2011; and ozer et al., 2008), strength parameter modeling of different soils (sezer, 2013; khanlari et al., 2012; and heshmati et al., 2009), simulation of waste generation (noori et al., 2009), estimation of probability of the liquefaction (hanna et al., 2007; and venkatesh et al., 2013). in the present study, two soft computing methods, i.e. adaptive neuro fuzzy inference system (anfis) and neural network (ann), have been utilized to propose efficient models for predicting the compression ratio of msw. artificial neural network (ann) a neural network consists of parallel layers of simple computational elements called ‘artificial neurons’. all of the layers are connected to each other by interconnection weights. figure 1(b) shows an artificial neuron model. the artificial neurons basically include three parts: weight, bias and transfer function. in each neuron, coming signals from the previous layer, ix , are multiplied by the related adjustable connection weight, ijw , and then summed in addition to an bias value, θ j , . then the combined input, ji , is passed through a nonlinear transfer function, )( jif , to yield the output of the neuron (yi) (jaksa et al., 2008). output of each neuron creates the input of the neuron in the next layer. this process is summarized in equations (2) and (3), and illustrated in fig 1(b): (2) (3) fig. 1 (a) the feed forward neural network structure used for modeling compression ratio of msw, (b) the artificial neuron model one of the most practical artificial neural networks for solving the problems of engineering and geotechnics is multi-layer perceptron (mlp) network (khanlari et al., 2012). the mlp networks contain an input layer, one or more hidden layers, and one output layer. this network is a feedforward network, the information of which only flows in one direction, from the input to the output. figure 1 depicts a three-layered feed-forward network. mlp utilizes a supervised learning technique called ‘back propagation’ for training the network. back propagation algorithm is one of the most common supervised learning algorithms for training feed forward multilayer networks. this algorithm involves two main phases, namely forward pass and backward pass. in the forward pass, an input training pattern is shown to the network, and its effect propagates in the network layer by layer until the output of the network is obtained. in the forward pass, the error between the network prediction and the target value is calculated and distributed; then, by means of a technique called gradient descent, it is propagated from the output layer to the input layer in order to reduce the error value. this process is iterated, and the weights are updated until the summed square of the errors comes below an acceptable value. to enhance speed and performance of the back propagation algorithm, levenberg-marquardt algorithm can be alternatively used instead of the conventional gradient descent method. i j = wji∑ xi +θ j yi = f (i j ) 167compression ratio of municipal solid waste simulation using artificial neural network and adaptive neurofuzzy system adaptive neuro-fuzzy inference system (anfis) neuro-fuzzy systems are the result of the integration of fuzzy logics and artificial neural networks. so they benefit from the advantages of both fuzzy systems and neural networks. in this hybrid method, the fuzzy system provides the relation between input and output variables, while parameters of membership functions of the fuzzy system are optimized by the neural network. one of the well-known neuro-fuzzy models is adaptive neuro-fuzzy inference system (anfis) which is capable of modeling any nonlinear function with a desirable accuracy (jang et al., 1997). anfis uses a feed forward network to optimize the parameters of a fuzzy inference system and perform well on a given task. the most common fuzzy inference system with this compatibility located in an adaptive network is takagi-sugeno fuzzy system. the first order sugeno model was adopted in this study as it is also extensively used in many engineering problems. an example of the first order sugeno fuzzy inference system with two inputs, one output and two membership functions for each of the inputs is illustrated in figure 2(a). two typical if-then rules can be stated for such a model (sugeno and kang, 1998): (4) if x = a2and y = b2 then f2(x,y) = p2x + q2y+k2 (5) where a1, a2, b1, and b2 are the membership functions for inputs x and y, respectively, while k1, p1, q1 and k2, p2, q2 represent the parameters of output functions for the two defined rules. the discussed architecture of the anfis is also depicted in figure 2(b). it can be seen that calculations of the anfis model are implemented in five layers. the detail and formulation on anfis are addressed by jang et al. (1997). anfis utilizes a back propagation learning algorithm or a hybrid algorithm which is an integration of back propagation and least square techniques in order to generate fuzzy rules and tune member function. fig. 2 (a) the first order sugeno fuzzy model, (b) corresponding anfis architecture (padmini et al., 2008) experimental database a valid database was used for modeling. it included the results of 64 oedometer tests on msw as well as information related to the tested waste properties such as dry unit weight (γd ), dry weight water content (ωd ), and percentage of biodegradable organic materials (om ) obtained in previous f x = a1and y = b1 then f1(x,y) = p1x + q1y+k1 studies. a considerable part of these data and references cited by bareither et al. (2012) were combined with the data provided by hyun ii et al. (2011), reddy et al. (2011), and karimpour-fard and machado (2012). the employed data are indicated in table 1. the physical properties of wastes (i.e ωd,γd, and om) were used as the input parameters and the cc ' as the target in the anfis and the ann models. table 1. the sources of the database reference no. of oedometer test beaven and powrie (1995) 5 chen and lee (1995) 1 gabr and valero (1995) 6 landva et al. (2000) 4 olivier and gourc (2003) 1 vilar and carvaleho (2004) 4 olivier and gourc (2007) 2 stoltz and gourc (2007) 1 chen et al. (2009) 5 reddy et al. (2009a) 3 reddy et al. (2009b) 4 reddy et al. (2009c) 3 reddy et al. (2011) 5 chen et al. (2010) 2 stoltz et al. (2010) 9 breither et al. (2011) 6 hyun ii et al. (2011) 1 karimpourfard and machado (2012) 2 in order to simulate the compression ratio, the database was randomly divided into two subsets, training and testing subsets. the training subset was used to calibrate the models, and the testing subset was used for validation of the developed models based on the training subset. the selection was such that the maximum, minimum, and mean value as well as the standard deviation of the parameters got to be close to each other in the training and the testing of the subsets. eighty percent of the data (51 cases) were assigned for the training subset and twenty percent (13 cases) for the testing subset. to control network overtraining and over-fitting, the training set was divided into two subsets: training and cross validation. the use of about 10% of the training data as a cross validation set has been suggested by researchers (kucuk 2008). as a result, in this study, 10% of the training subset was randomly taken and used for cross validation. the statistical properties of the divided subsets are summarized in table 2. 168 ali akbar heshmati r, maryam mokhtari, nader shariatmadari table 2. statistical properties of the training and testing sets parameter statistics training data testing data d (%)ω max 151.00 135.00 mean 71.02 73.79 min 15.60 31.30 std dev 33.72 34.59 γd (kn/ m3) max 12.50 9.14 mean 5.48 5.85 min 2.45 1.86 std dev 1.86 2.18 om (%) max 60.00 60.00 mean 32.91 34.12 min 0.49 2.00 std dev 19.79 20.03 c’c max 0.39 0.34 mean 0.24 0.21 min 0.09 0.14 std dev 0.06 0.07 performance measures various statistical parameters including correlation coefficient (r), root mean squared error (rmse), and mean absolute error (mae) were investigated in order to evaluate the efficiency and accuracy of the proposed models. these parameters are defined as: (6) (7) (8) in equations (6), (7) and (8), n is the number of the data, mi and pi display the measured and predicted values, respectively. also, m and pare the mean values corresponding to the measured and predicted values, respectively. development of ann model as the topology of a network directly influences its computational complexity and generalization capability, it is important to devise appropriate network architecture in neural network modeling. this needs determination of the number of hidden layers and the number of neurons in each layer. theoretical works have revealed that a multilayer feed forward network with one hidden layer is able to approximate any continuous and complex nonlinear function with the desired degree of accuracy (hornik et al., 1989). consequently, one hidden layer was used in this study. the number of neurons in the input and output layers is dependent on the number of model inputs and outputs. the input layer of the ann model, developed in this study, has tree neurons, including one for each input. the output layer has only one neuron representing the measured value of compression ratio. the ann performance is influenced by the number of neurons in hidden layers. however, there are no specific guidelines that can r = (mi −mi=1 n ∑ )(pi −p) (mi −m ) 2 ( (pi −p) 2 i=1 n ∑i=1 n ∑ rmse = (mi −pi ) 2 i=1 n ∑ n mae = 1 n mi −pii=1 n ∑ be used to select the optimum number of neurons in a hidden layer for a given problem. this parameter is often selected through a trial and error method. to obtain the optimum network architecture, different numbers of hidden-layer neurons, from 3 to 8, were employed to train different networks. finally, an appropriate ann network with best performance was selected by considering r and rmse values of the testing data set and the training data set. a back-propagation algorithm was used to train the mlp neural network. specifically, the levenberg–marquardt algorithm was used to improve the speed and efficiency of back propagation algorithm. also, a tan-sigmoid function was used as the activation function in the hidden and output layers. after the training phase, the generalization capability of the trained model was evaluated using the test data set (13 data). the output value of the optimal network, compression ratio, can be calculated by equation (18): (9) where w1 and w2 are the weight matrices, and b1 and b2 are the biases vectors. the values of these parameters are presented in table 3. development of anfis model in the anfis system, each input parameter must be clustered into several class values in order to form fuzzy rules, and each fuzzy rule is made up of two or more membership functions .various methods have been proposed for the clustering of the input parameter and development of the rules, the most common of which are: “subtractive clustering” method (chie, 1994) and “grid partitioning” method (jang and sun, 1995). the former was adopted in this study because it required less computational effort and calculation time. the subtractive clustering algorithm assumes that every data point is a candidate that can become a cluster center. then, a measure of density for each data point is used to determine the potential of a point to become a cluster center (chiu, 1994). the data point with the highest potential is selected as the first cluster center, and the data points near the first cluster center are removed, as determined by the influential radius. the algorithm searches for a new data point having the highest number of neighbors as the next cluster center. then the data points near the new cluster center with fewer neighbors are removed. this is repeated until all the data points are examined and sufficient numbers of cluster centers can be generated. the influential radius is necessary for determining the number of clusters. a smaller radius leads to many smaller clusters in the data space, thereby resulting in more rules, and vice versa. the value of the optimal radius in a trial and error process was determined by changing the radius from 0.1 to 1 with an increment of 0.05. at last, the radius value was chosen to be 0.9 that resulted in approximately two clusters. therefore, each input and output was characterized by two membership functions and, hence, two rules were created. the gaussiantype membership function was assigned to each input variable. after constructing the initial fis structure, the training stage was done. the hybrid learning algorithm was employed as an optimizing method in the training stage. in this step, the number of iterations of the hybrid algorithm for the correction of model parameters and goal error were considered 100 and 0, respectively. the constructed anfis model has 20 parameters (8 linear and 12 nonlinear). the membership function (mf) plots of the dry weight water content, dry unit weight, and biodegradable organic compounds used in the training stage are shown in fig. 3. cc ' = f2 w2[ ] f1 w1[ ] ωd γd om ⎡ ⎣ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ + b1[ ] ⎡ ⎣ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ + b2[ ] ⎡ ⎣ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ 169compression ratio of municipal solid waste simulation using artificial neural network and adaptive neurofuzzy system fig. 3 membership function plots of the compression ratio, (a) dry weight water content, (b) dry unit weight, and (c) biodegradable organic compound results and discussion ann model fig. 1(a) schematically depicts the ann architecture used in this study. a study on different networks with different hidden neurons showed that a network comprising five neurons in a hidden layer approaches the lowest prediction errors and maximum correlation coefficient. a comparison of the prediction made by the ann with the experimental value for each of the training and testing data set is shown in fig 4. as it can be seen, most of the cases in the training and testing subsets are around the middle, indicating a good agreement between the ann model prediction and the experimental values. fig.4. measured versus predicted compression ratio of msw using ann model, (a) training data set, (b) testing data test table 3 shows the weights of the input-hidden and hidden-output layer connections and biases of the proposed ann model after the end of the training procedure. the relative importance of the input parameters can be assessed by partitioning the hidden-output connection weights into components connected with each input variable. table 4 lists the relative importance of the input variables in the ann model. as seen in this table, the dry density and the dry weight water content have the most significant effect on the predicted compression ratio with average relative importance values of 41.88 and 43.47%, respectively. table 3. connection weights and biases bias connection weights hidden neurons b2b1outputom -3.400870.85305-0.16996-0.54242.3920481 -1.01113-0.305351.3268221.993881-0.997252 0.108780.4829270.9026261.1143041.0685023 -0.18422-0.4320.1127751.8204131.3946824 2.493959-0.49832-0.415312.31615 0.445335 0.876667output neuron wd γd table 4. the relative importance of the ann model input variables. om dgdwinput parameter 14.6541.8843.47relative importance (%) according to smith (1986), if a proposed model provides r> 0.8 and the error values (e.g. mae and rmse) are at the minimum, there is a strong correlation between the measured and predicted values. the training and testing results of the ann model are presented in table 5. the results show that the ann model performs well, with r, rmse, and mae at the values of 0.93, 0.0187, and 0.0219 in the training stage and 0.928, 0.0378, and 0.0404 in the testing stage, respectively. anfis model fig. 5 represents the structure of the proposed anfis model in this study. the fuzzy radius of this model is 0.9, and it is formed of two rules. fig. 5 the schematic of anfis architecture based on sugeno fuzzy model developed in this study fig. 6 compares the anfis predictions with the experimental values of the compression ratio of msw for the training and testing data sets. the proposed model shows a very good correlation to the training and testing data. the predictive performance of the anfis model is summarized in table 5. the results demonstrate that the anfis model with a high r and low rmse and mae values is trained well and able to estimate the target values with an acceptable degree of accuracy. fig.6 measured versus predicted compression ratio of msw using anfis model, (a) training data set, (b) testing data test table 5. performance of the proposed models testingtrainingmodel rmsemaerrmsemaer 0.05150.0480.90.06020.02350.904ann 0.03960.03470.9230.02220.01870.923 anfis 170 ali akbar heshmati r, maryam mokhtari, nader shariatmadari prediction results obtained from the ann and anfis models are compared in table 5. the high correlation coefficient and the lower error in both models demonstrate the capability of those models in acceptably predicting the compression ratio. as suggested by the results, the error rates of the anfis model are lower than ann model; so, anfis has performed more precisely. table 5 also shows that the results obtained for each of the models during testing are generally consistent with those obtained during training. this suggests that the proposed models are able to generalize within the range of data used for training. it is noteworthy that, although the predictive capability of the proposed models is limited to the range of the data used for their calibration, if a new data set becomes available, these models can be retrained and updated easily without repeating the development procedures from the beginning. conclusions in this research, anfis and ann models were used to predict the compression ratio of municipal solid waste. the models were developed based on a reliable database obtained from the literature. the following emerge as the conclusions of the study: 1 the proposed models developed through ann and anfis modeling techniques can precisely predict the compression ratio of msw. 2 the relative importance study indicates that dry unit weight and dry weight water 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(1995). municipal landfill biodegradation and settlement, environmental engineering journal. 121, 214-224. based on actual land use in yancheng city, this paper establishes the ecological security of the land index system from the perspectives of economy, society and ecological environment. using the catastrophe theory and pressure-stateresponse (psr) model, the purpose of the research was to judge the degree of land ecological security in yancheng city from 2002 to 2011 and determine the requirements to ensure the sustainable development of this region. the results showed that over the ten years, the pressure, state, response and synthetic values of the land ecological security were evaluated as safe in 2003 and 2007 and at middle level in 2002 and for 2004 to 2006 and then dropped to insecure for 2008 to 2009 and very insecure for 2011 to 2012. the ecological security level and circumstance of land use in yancheng city has generally declined. during this period, the land use pattern of yancheng city was under tremendous pressure from the conflict between rapid urbanization and economic development, as well as conservation and rehabilitation of the eco-environment. this research shows that conditions for the land ecosystems of yancheng city are not optimistic and, as such, should draw the attention of responsible government departments. future policy options should aim to mitigate these problems through the control of population growth and the improvement to quality of life, protection of wetland and forest land, application of scientific concepts of development, coordination of economic development and land utility, and strengthening the control functions of land-use planning. de acuerdo con el uso actual del suelo en la ciudad de yancheng, en el este de china, este artículo establece la seguridad ecológica del sistema de tierras desde las perspectivas económica, social y ecológica. a partir de la teoría de las catástrofes y el modelo ambiental de presión-estado-respuesta (psr), el propósito de esta investigación fue determinar el grado de seguridad ecológica del suelo en la localidad de yancheng entre 2002 y 2011 y determinar las condiciones para asegurar el desarrollo sustentable de la región. los resultados muestran que sobre estos diez años los valores de presión, estado, respuesta y sintéticos fueron evaluados como seguros en 2003 y 2007, como nivel medio en 2002, entre 2004 y 2006, cayeron a inseguros en 2008 y 2009, y muy inseguros para 2011 y 2012. el nivel de seguridad y las condiciones de suelo en yancheng se han disminuido generalmente. durante este período, el patrón de uso de la tierra en la ciudad de yancheng estuvo bajo una gran presión por el conflicto entre la rápida urbanización y el desarrollo económico y la conservación y la rehabilitación ecoambiental. esta investigación muestra que las condiciones para los ecosistemas terrestres no son óptimas y que por lo tanto es necesario llamar la atención de los departamentos de gobierno responsables. las posibles políticas futuras deben enfocarse en mitigar estos problemas a través del control al crecimiento urbano y el mejoramiento de la calidad de vida, la protección de los humedales y los bosques, la aplicación de conceptos científicos al desarrollo, la coordinación del avance económico y el suelo, y el fortalecimiento de las funciones de control en la planeación del uso de la tierra. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 181 187 abstract resumen key words: land ecological security evaluation, catastrophe theory, yancheng city. palabras clave: evaluación de la seguridad ecológica del suelo; teoría de las catástrofes; yancheng. record manuscript received: accepted for publication: land ecological security assessment for yancheng city based on catastrophe theory wang liang¹,2. bian zhengfu¹, chen hongquan2 ¹school of environment and spatial informatics, china university of mining and technology, xuzhou , china.e-mail:wangsantai123@163.com 2school of urban and resource environment, yancheng teachers university, yancheng , china ecology issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.44642 http://dx.doi.org/10.15446/esrj.v18n2.44642 182 wang liang, bian zhengfu, chen hongquan introduction land is the foundation of global development and ecological survival. land ecosystems play a very important role in social-economic progress at the regional, national and international scales, and land ecological security is related to regional sustainable development. currently, land ecosystems are under great stress with the intensification of agriculture, population growth, urbanization and industrialization, all of which impede human sustainable development. there is significant evidence that the land ecosystems of many regions have become highly stressed and dysfunctional due to continuous and excessive utilization and exploitation of land resources (mark 2006; honson and marvin 2009; wessels et al. 2004; garcı´a et al. 2008; su et al. 2011; salvati and bajocco 2011). aggravation of the ecological environment has caused the ecological security situation to become one of the most pressing problems that society has to face and attempt to solve. in this regard, there is a necessity to develop methods and indicators to accurately diagnose the state of land ecosystems in order to prompt ecological restoration, management and regulation. land ecological security (eco-security) assessment based on an analogy of the threat to survival is such a procedure. most of the currently used methods to evaluate the security of land ecosystems (fuzzy ahp, entropy method, matter element model) have disadvantages in subjectivity and complexity, especially associated with the weight determination procedure (shi et al. 2003; su et al. 2011). as a mathematical model characterized by dialectic and simple construction, catastrophe theory (ct) generally applies to systems that may respond to continuous changes in control variables through a discontinuous change from one equilibrium state to another. ct is naturally chosen to effectively model and analyze the discrete transition mechanism resulting from changes in the internal and external circumstances (xu et al. 2014). by combining catastrophe theory with fuzzy mathematics and only considering the relative importance of the indices, the catastrophe method avoids the subjectivity in weight decisions. it has therefore been used in many fields, including evaluation of rural information levels (zhang et al. 2013), business cycles (khademian 1988), agricultural insect pest-control (piyaratne et al. 2013) and environmental pollution assessment (wang et al. 2011). although the pressure-state-response (psr) model has been developed further to the dpsir, which includes ‘driving force’ and ‘impact’, the decision was made to use the psr model framework in this research, as explained later in the paper in the section on driving force and response. in recent years, yancheng city has faced great change as a result of the jiangsu coastal development to prompt urbanization and industrialization. with a huge population (the population of yancheng city in 2011 was 8.207×106 with an annual growth rate of 5.311%), this region’s land eco-security level has been in continuous decline. the change in the land eco-security state can be considered as a particular catastrophic behavior, a small and gradual change in the steady equilibrium state of a sub-system can rapidly cause the whole system to reach a crash state (su et al.2011). following this idea, this paper proposes a catastrophe model combined with the psr model framework for land ecosecurity assessment, using yancheng city, china, as a case study. the objective is to demonstrate the method and evaluate the degree of land ecological security in yancheng city from 2002 to 2011 as a tool to determine the requirements to ensure sustainable development of this region. study area and data study area yancheng city (32°34’~34°28’ n, 119°27’~120°54’ e), which has the largest land area (16, 972 km2) and the longest coast line in the jiangsu province, is located on the western side of the yellow sea in eastern china. the urbanization rate and gross domestic product (gdp) rate of increase for this region in 2011 was 55.9% and 12.8%, respectively. investment in tertiary industry accounted for 37.82% of gdp, while investment in environmental protection was only 2.09%. the land ecosystems of yancheng city have become highly stressed and dysfunctional due to this continuous, excessive utilization and exploitation of land resources. the trend for both gdp and the total ecological footprint in yancheng city from 2002 to 2011 has steadily increased, putting still greater pressure upon the ecological environment of the region (figure. 1). consequently, there is considerable need for methods and indicators to accurately diagnose the state of the land ecosystem and to prompt ecological restoration, management and regulation. 0 500 1000 1500 2000 2500 3000 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 year 0 500 1000 1500 2000 2500 per gdp($) total ecological footprint(100km2) figure1. changes in per gdp and ecological footprint in yancheng city from 2002 to 2011 data source the data used in this research are from the “statistical yearbook of yancheng city 2003-2012”, “statistical yearbook of jiangsu 2003-2012” and data supplied by the yancheng city prefecture land and resources bureau. some additional data were obtained from field surveys and questionnaires. all the index values are calculated based on the data collected from these sources. materials and methods catastrophe theory catastrophe theory was proposed in an attempt to rationally account for the phenomenon of discontinuous change in behaviors (outputs) resulting from continuous change in parameters (inputs) in a given system. in this section, a brief discussion of the basic assumptions and results of catastrophe theory is presented in a form useful for such applications (woodstock and poston 1974). let f: tk×tn t: a smooth (infinitely differentiable) function representing a dynamical system m. tk: the space of input variables (controls, parameters). tn: the space of output variables (responses, behaviors). the fundamental assumption is that m attempts to locally minimize f. given any such function f, if the point c∝tk is fixed, then a local potential function fc: t n→t is obtained. therefore, fc can be expressed as follows: v=v(x,u) where v: the potential function, x: response variables, and u: control variables. the critical points of the potential function fc form an equilibrium surface (zeeman 1976). the equation of that surface is obtained by calculating the first derivative of fc, f′c（x）= 0, and the singularities are obtained by calculating the second derivative of fc, f′′c（x）= 0. a bifurcation set of the catastrophe system is obtained by eliminating x between f′c（x）= 0 and f′′c（x）= 0. a normalization formula is derived by decomposing the bifurcation set. the values of x and all control variables in the normalization formula range between 0 and 1. these variables are called ‘‘catastrophe progression’’. the catastrophe progression of each control variable can be obtained from the initial membership function using a recursive algorithm subject to the normalization formula. supposing that the response variable is one dimensional, catastrophe models can be classified into four categories according to the dimension of control variables. summary descriptions of these models are given in table 1. 183land ecological security assessment for yancheng city based on catastrophe theory table 1. description of catastrophe models model types control variables function bifurcation set normalization formula fold 1 v(x)=x3+u1x u1=-3x2 xu1= u cusp 2 v(x)=x4+u1x 2+u2x u1=-6x2 , u2=8x3 xu1= 1u ,xu2= 3 u swallowtail 3 v(x)= 5 1 x5+ 3 1 u1x 3+ 2 1 u2x 2+u3x u1=-6x2, u2=8x3, u3=-3x4 xu1= 1u ,xu2= 3 u xu3= 4 u butterfly 4 v(x)= 6 1 x6+ 4 1 u1x 4+ 3 1 u2x 3+ 2 1 u3x 2+u4x u1=-10x2,u2=20x3,u3=-15x4,u4=4x5 xu1= 1u ,xu2= 3 2u xu3= 4 u ,xu4= 5 u the psr model the psr model is based on the concept of causality: human activities exert pressures on the environment and change the quality and quantity of natural resources, which then lead to responses in human behavior. three categories of indicators are distinguished. first, eco-environmental pressure indicators describe pressures on the environment by human activities and climate change. second, eco-environmental state indicators describe the status quo of the natural environment and ecosystem function. third, societal response indicators show the degree to which society responds to eco-environmental changes and concerns. this could be the number and type of measures taken, the efforts of implementing measures or the effectiveness of those measures. indices selection and standardization based on previous studies and the framework, as well as the principles explained above, this paper generates a set of assessment indices. initially, a set of 55 indices was developed. subsequently, a three-round delphi process (linstone and turoff 1975) was established, from which 28 indices were selected that favored the consensus needed to validate the analysis. experts with skills in the appropriate fields of study evaluated the set of 28 indices for relevance to the assessment. after performing principal component analysis to reduce data dimensionality, a total of 18 indices were generated (table 2). table 2. land ecological security indices framework of yancheng city target layer project layer first code factor second code index layer properties land ecological security pressure a1 social economy b1 c1 population growth rate（‰） c2 population density（person/km2) c3 urbanization（%） c4 per capita gdp（yuan/person） + environ ment b2 c5 the usage of fertilizer（kg/hm2） c6 water resource（m3/person） + c7 dwelling areas（m2/ person ） state a2 social economy b3 c8 gdp growth（%） + c9 cultivation index（%） + c10 rural per capita net income（yuan/person） + c11 city per capita net income （yuan/person） + environ ment b4 c12 per farmland（hm2/ person ） + c13 per public greenland（m2/ person ） + c14 proportion of irrigated farmland（%） + response a3 method b5 c15 proportion of tertiary industry（%） + c16industrial solid wastes utilized proportion （%） + c17industrial wastewater up to the discharge standards proportion (%) （%） + c18 proportion of environment invest （%） + 184 wang liang, bian zhengfu, chen hongquan note: development of a tertiary industry can be an effective method to alleviate deterioration of land ecosystems, so it is feasible to put c15 in the project layer of response. as different indices have no comparability because they differ completely in dimensions, it is necessary to convert original data to a dimensionless form. the evaluating indicators can be divided into positive term targets and negative term targets. the model takes the following two forms. 1) standardization of positive effect indices: for some indices, the higher their value, the greater the positive effect they will bring to land ecological security, which means this type of index presents much less risk to ecological security. their standardization method is: 2) standardization of negative effect indices: for other indices, the higher their values, the greater the negative effect they will bring to land ecological security, which means this type of index presents much more risk to ecological security. their standardization method is: in the formula, zij is the i standardized value of the j index included in the assessment; xij is the i th actual value; max is the biggest value of this index; and min is the minimum value of this index. application of catastrophe theory the catastrophe assessment index system can be divided into hierarchical sub-systems. if the index at a higher level (response variable) contains two lower level indices (control variable), it can be assumed as a cusp system. the relative importance of these two control variables should be determined (u1, important; u2, less important), and the control variable can then be obtained from the membership function using a recursive algorithm subject to the normalization formula. similarly, when the index at a higher level contains one, three or four lower level indices, it can be, respectively calculated based on fold, swallowtail and butterfly membership functions. the catastrophe assessment model for yancheng city was therefore developed using such an approach (su et al. 2011). the catastrophe model for land eco-security assessment in yancheng city is shown in figure. 2. figure 2 . catastrophe model for land eco-security assessment of yancheng city. score transformation the synthetic values of catastrophe assessment are generally high, and the differences are not obvious (poston and ian 1978). this can be attributed to the fact that catastrophe progression is calculated based on the normalization formula (su et al. 2003). therefore, it is difficult to determine the actual secure level directly using the results obtained by the catastrophe assessment. usually, the synthetic values of multi-attribute assessment are divided into five grades using an equality distribution function (xiong et al. 2007). accordingly, the land eco-security level can be divided into five grades: 0.2 (very insecure), 0.4 (insecure), 0.6 (middle), 0.8 (secure) and 1.0 (very secure). the problem is how to find a way to transform the results obtained by the catastrophe assessment into the ordinary-used synthetic values. the method for score transformation used in this paper is described as follows: suppose the relative membership degree for all indices equals n, then the relative membership degree for higher level indices should also equal n. consequently, the synthetic membership degree can be obtained by applying a suitable catastrophe model. by virtue of this method, the catastrophe progression value for each secure grade was calculated (table 3). table 3. corresponding values between the results of the catastrophe model and ordinary-used values at different security levels security level land eco-security level obtained by catastrophe model ordinary values pressure state response synthetic very secure >0.973 >0.975 >0.985 >0.991 >0.8 secure 0.973-0.948 0.975-0.934 0.985-0.926 0.991-0.977 0.6-0.8 middle secure 0.948-0.897 0.934-0.880 0.926-0.872 0.977-0.957 0.4-0.6 insecure 0.897-0.814 0.880-0.831 0.872-0.839 0.957-0.933 0.2-0.4 very insecure <0.814 <0.831 <0.839 <0.933 <0.2 method demonstration this section is provided to demonstrate this method in terms of a simple example using the 2002 data from the study area (table 4). 185land ecological security assessment for yancheng city based on catastrophe theory table 4. land eco-security value of yancheng city in 2002 index layer primary data standard application of catastrophe theory c1 population growth rate（‰） 1.82 0.343 343.0 0.791 b 1 791.0 0.922 a 1 0.967 c2 population density（person/km2) 531.01 0.902 3 902.0 c3 urbanization（%） 35 0.602 4 602.0 c4 per capita gdp（yuan/person） 8464 0.212 5 212.0 c5 the usage of fertilizer（kg/hm2） 399.69 0.538 538.0 0.872 b 2 3 872.0 c6 water resource（m3/person） 18.69 1 3 1 c7 dwelling areas（m2/ person） 29.3 0.606 4 606.0 c8 gdp growth（%） 11.4 0.973 973.0 0.817 b 3 817.0 0.872 a 2 c9 cultivation index（%） 162.77 0.454 3 454.0 c10 rural per capita net income（yuan/person） 3867 0.266 4 266.0 c11 city per capita net income （yuan/person） 9015 0.317 5 317.0 c12 per farmland（hm2/ person） 0.097 0.065 065.0 0.591 b 4 3 591.0 c13 per public greenland（m2/ person） 6.5 0.893 3 893.0 c14 proportion of irrigated farmland（%） 46.45 0.095 4 095.0 c15 proportion of tertiary industry（%） 32.4 0.215 215.0 0.784 b 5 784.0 0.885 c16industrial solid wastes utilized proportion （%） 92.7 0.445 3 445.0 c17industrial wastewater up to the discharge standards proportion (%) （%） 72.30 0.870 4 870.0 c18 proportion of environment invest （%） 2.13 0.751 5 715.0 results by using the model described above with the statistical data and other data from 2003 to 2012, the land eco-security of yancheng city was calculated (figure. 3). 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 pressure state response synthetic0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 figure3. the land eco-security level of yancheng city between 2002 and 2011. it can be seen that the land eco-security level for ‘pressure’ was evaluated as secure in 2003 and 2007, a middle level in 2002, 2004, 2005 and 2006, insecure in 2008 and 2009, and very insecure in 2010 and 2011. the maximum value of the land eco-security level for ‘pressure’ was 0.951 in 2003, and the minimum value was 0.192 in 2011. in the past decade, rapid urbanization, increasing from 35% in 2002 to 55.9% in 2011, accompanied by population growth (increasing by two hundred thousand between 2002 and 2011), has been a fundamental cause for the current land ecological problems in yancheng city. with a high population density and increasing wealth, demands for public infrastructure (roads, water facilities, and utilities), housing, industrial and commercial growth and additional construction projects start to appear as ‘‘rural sprawl’’ (mann 2009). statistical data provide evidence that urbanization has greatly stimulated the up-scaling and expansion of the road systems in yancheng city. the great opportunities available in the city have also attracted a significant influx of people into the city, which exerts further pressure on the land ecosystems. in addition, overuse of chemical fertilizers, pesticides, and plastic film in croplands has led to land pollution and contributed to the degradation of land ecosystems in yancheng city. similar to pressure, the land security level with regard to the ‘state’ index also generally decreased over the period between 2002 and 2011. the ‘state’ index of land eco-security was considered as very insecure in 2010 and 2011. experience in other parts of the world suggests that intensive urbanization necessitates the transfer of significant areas of land to secondary and tertiary sectors (chan and shimou 1999). in many cases, farmland has to be given up for development, and large areas of high-quality farmland are lost to urban build-up (long et al. 2009). this is exactly the case in yancheng city. during the studied 10 years, per capita farmland and effectively irrigated land significantly declined, posing considerable threat to regional food security. the increased concentration of factories with low treatment efficiency for waste gas, water and other wastes has also contributed to the insecure state. according to the statistics, the load 186 wang liang, bian zhengfu, chen hongquan of industrial wastewater increased from 28,800 ton/km2 in 2002 to 35,400 ton/km2 in 2011. the case of industrial solid wastes was even more serious, with the quantity doubling during the study period. over the decade, there is a great difference of quality change between various land types. from 2002 to 2011, the area of farmland decreased the most in the region by 3,679.77 km2, while forest lands declined by 61.21 km2. all these phenomena indicate that the ecological service function of the land ecosystem in yancheng city has been deteriorating rapidly. if appropriate and timely measures are not adopted, the land ecosystem will continue to degrade. as for ‘response’, the security level was evaluated as secure in the initial 2 years. however, insecure and very insecure levels were identified for the following years. while it is true that yancheng city has invested in a number of environmental protection projects and the proportion of scientific researchers has been increasing, the industrial pollution issues have not been effectively tackled. performance evaluation by local officials is mainly associated with economic growth, and environmental protection is not listed as one of the criteria, unless serious environmental disasters are made public and raise concerns. industry is the major sector for the local labor market and the main financial source for the local government. most officials ignore such industrial activities, and although pollution levy systems have been introduced in the city, they are largely ineffective, as the levies are too low to give polluters an incentive to reduce emissions. similarly, water pollution fees are small relative to the additional costs of pollution control (sinkule and ortolano 1995). these responses have led to the continuing decline of the land eco-security level. over the studied 10 years, the synthetic land eco-security was evaluated as very secure in the initial 2 years, middle and insecure from 2004 to 2009, and very insecure in the last 2 years. these results show that the land ecosystem conditions are not optimistic for yancheng city and should be brought to the attention of policy makers and the public. at present, serious problems, such as rapidly decreasing farmland with a significant reduction in amount of farmland per person, the high demand for construction land, prominent contradiction between land resources and population growth, land utilization increasing yearly, over-exploitation of land resources and declining ecological land, especially forest and natural reserve land, exert huge pressure on the regional environment. land ecosystem conditions are equally pessimistic for shanghai (su et al.2011), liaoning province (han 2009) and hubei province (xu 2012). discussion and conclusions the catastrophe model integrated multiple assessment indices of land ecological security according to the inherent contradictions and relative importance of indices without calculating weights (thom 1975; vadrevu et al 2008; weidlich and huebner 2008; zeeman 1976). the aim of this study on the land eco-security assessment was to develop a means of quantifying the state of land ecosystems over time. the land ecosystems of yancheng city generally remained at or under the middle grade of eco-security across the 10 years between 2002 and 2011. during the 10 year period, the land ecosystems sustained intensified pressure, presented a worsened security state and received reduced positive responses from society. these have led to a significant decline in the synthetic land eco-security. the results presented here, showing the changes over time and the factors that have led to the present state, demonstrate a method that can be used to predict and manage land ecosystems with considerable utility. it is noted that this assessment relied mainly on the available statistical data set, and the evaluated eco-security levels, whether secure or insecure, are thus a relative concept. however, although this approach lacks absolute accuracy, the study clearly demonstrates the changes in land ecosystem conditions over time and should assist officials and citizens in resolving issues regarding regional land eco-security improvements. the land eco-security index described here should therefore be a useful tool for policy makers and land managers to develop measures that strengthen land ecosystem protection and promote ecological reconstruction. further policy options that should be considered to mitigate the problems more thoroughly include control of population growth and in-migration; protection of wetland and forest land; application of scientific concepts of development; coordination of economic development and land utility; and strengthening the control functions of land use planning. acknowledgments the study was supported by the national natural science foundation of china (41071083), and the priority academic program development of jiangsu higher education institutions. references chan rck, shimou y.1999, urbanization and 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hydrometeorological university, san petersburg, russia, e-mail: rector@rshu.ru 2 departamento de geociencias, universidad nacional de colombia, sede bogotá, e-mail:nlvillegasb@unal.edu.co resumen este artículo muestra los resultados obtenidos en el estudio de la estructura fina termohalina de la capa sub-superficial realizado en la cuenca del pacífico colombiano. se formularon modelos matemáticos para diferentes regímenes de capas sub-superficiales. basado en tres estaciones se realizó un análisis horario de información meteorológica, variación de espesor, temperatura y salinidad de la capa fina. las leyes de formación de la estructura fina termohalina para la capa sub-superficial en la cuenca del pacífico colombiano están relacionadas con convección nocturna, mezcla viento-ondas, absorción de energía radiante y precipitación. palabras clave: capa sub-superficial, estructura termohalina fina, termoclina, haloclina, mezcla vientoondas, mezcla convectiva. abstract this paper presents the results provided by the fine-scale thermohaline structure of near-surface layer (nsl) study done in the colombian pacific ocean (cpo). mathematical models for different regimes of the nearsurface layer are formulated. based on three stations, an hourly analysis of meteorological data, thickness change, temperature and salinity of this layer was done. the principles of formation of the thermohaline structure for nsl over cpo are related to night convection, wind-wave mixing, volume absorption of radiant energy and precipitation. key words: near-surface layer, fine-scale thermohaline structure, thermocline, halocline, wind-wave mixing, convective mixing. © 2005 esrj unibiblos manuscript received august 2004, paper accepted june 2005 introduction fedorov and ginsburg (1988) found for thermohaline structure formation in the ocean-nsl, the following regimes: intensive wind-wave mixing (wind speed > 8 m/s), intensive convection (night, autumn-winter, during the cooling or rise of salinity in ocean), langmuir circulation (wind speed between 3 and 10 m/s) and desalt due to precipitation. 111 simulation of near-surface layer of the colombian pacific ocean first, karlin et al. (1988) and later rodhe (1991) found another regime related to horizontal advection. finally, the regime connected to special features of the offshore thermohaline structure in polar and circumpolar regions was determined (timokhov, 1989; timokhov et.al, 1993). we described four mathematical models used for nsl regimes over cpo provided by data analysis in 3 stations. hourly weather data and vertical thermohaline profiles at these stations were examined in detail, obtaining nsl thickness values, top and bottom boundaries and the corresponding temperature and salinity (villegas, 2003). in conclusion, the predominant nsl regimes of cpo are night convection, wind-wave mixing, the volume absorption of radiant energy and regime with precipitation. methodology the former data was obtained at hydrometeorological stations 14 (4° n , 78° w), 49 (2° n, 80° w) and 111(3° n, 84° w), in august and september, 2001 (otero and pineda, 2001), recording data during 24 hours, trying to verify the models that describe the different nsl regimes, see figure 1. these stations covered the coastal (station 14), the offshore (station 111) and the mixing water zone (station 49), being selected according to the quasi-homogeneous classification zones (villegas, 2002). wind-wave mixing regime the quasi-homogeneity in temperature and salinity vertical distribution of the mixed layer in diurnal time, simplify the differential simulation without loss of accuracy, using integral methods. it is assumed the presence of mixed layer during the calculation period. the simplified expression of nsl thickness is: h g q mu2 * p t 0 3 = a (1) where, m is an empirical constant of proportionality; h is the upper mixed layer thickness, m; u c u * a p v = t t is the dynamic speed, m/s; cp is the resistance coefficient; uv is the wind speed, m/s; pa is the thermal expansion coefficient, 1/°c; qt0 is the heat flux through water-air boundary, °cm/s. the equation to calculate temperature evolution with constant heat flux is: t h q t t 0 0 =d d (2) where, t0d is the change of upper mixed layer temperature for the calculated step on the time td , °c. equation (1) determines the upper mixed layer thickness in wind-wave mixing regime, in the case of its decrease. this happens until maximum heat flux is observed, when diurnal thermocline is formed. thickness remain constant until the heat flux in airwater boundary decreases, then thickness increases and expansion of diurnal mixing layer is observed. in this case, the temperature and thickness evolution formulas of mixed layer must consider the formed diurnal thermocline thickness and heat flux qht on mixed layer bottom boundary. in accordance with the similarity theory of profile t in the thermocline (kitaygorodskiy, 1970; miropolskiy, 1970 and garnich and kitaygorodskiy, 1978), we have: t h g h t t mu t t q2 * p h t h t t 0 3 0 0 2 2 = -a a a^ ^h h (3) ; h q gh mu t t h h2 2 1* t p h t t0 2 3 0 a a a d ^ ] ] n h g g t t h q gh mu2 2 * t p 0 0 2 3 2 2 = a (4) figure 1. cpo hydrometeorological station locations. 112 lev karlin and nancy villegas where, t0 is the water temperature in upper mixed layer, °c; th is the water temperature in bottom boundary of mixed layer, °c; ta is the dimensionless coefficient of self-similarity determined according to observed data and h is the thermocline depth, m. formulas (1), (2), (3) and (4) calculate the thickness and temperature evolution of mixed layer in windwave mixing regime. convective mixing regime this type of convection, in absence of dynamic factors on water-air boundary, is also called free convection. in this work, convective-wind mixing took place at night, when heat flux on water-air boundary is negative, the mixed layer descends and heat flux through mixed layer bottom boundary becomes defined by the involvement hypothesis (karlin, 1988). the equations of thickness h ad temperature t evlution of nsl in convective mixing regime are: ; t h t t k q g h t t mu2 * h q t p h0 0 0 3 2 2 =+ -a ^ h 5) . . t t h q m g h u 1 2 2 * t p 0 2 3 2 2 = a 6) where, k q is dimensionless coefficient for free convection (woods and barkmann, 1986, varfolomeyev and sutyrin, 1981). convective mixing regime is characterized by layer mixed deepening even for weak wind, due to negative heat flux in water-air boundary. these two factors together (intensive refreshing and strong wind) contribute to the deepening of layer, causing the diurnal thermocline vanish. sunny and weak wind weather regime if in the ocean surface exists a convective mixed layer and the mixture of this layer is so strong that the temperature vertical gradient is zeroed, then we use heat equation in vertical direction, integrating from surface to inferior boundary of convective layer h: t t h q q q qt so h t h 0 02 2 = + -r 7) here, q so is the radiation flux through ocean-atmosphere boundary, °cm/s; q h is the radiation flux on bottom boundary of convective layer, °cm/s; qt0r is the heat flux through ocean surface caused by evaporation, turbulent heat exchange and long-wave radiation, °cm/s. equation (7) requires the expression of turbulent energy balance integrated in limits of convective layer. the production of energy (generated due to forces of archimedes) is made until z depth with the bouguer’s law (radiation flux determined exponentially) and considering dissipation according to works of zilitinkevich and dearfor (1974); dearfor et al. (1980); dearfor and willis (1985); kofi (1985) and zilitinkevich (1987), it is possible to write the energy balance turbulence equation integrated in the limits of convective layer as follows: q q h q h e r e 2 2 1 1 1 t h t s rh rh0 + + + --r ] ]g g: d 8) . q h0 4 t 0= r where, q s is the short wave emission flux to ocean surface, °cm/s; r is the effective coefficient of weakening solar energy in sea water, m-1. in expression (8), qht is undetermined doing necessary to analyze two stages of nsl evolution, in the first one, layer thickness increases when solar energy flux decreases or decreases when heat loss increases; causing heat exchange (involvement), the previous processes make necessary to estimate qht . in the second stage, when solar energy flux increases or heat loss decreases, the mixed layer thickness decreases, i.e., a new convective mixing layer takes place with a thickness smaller than previous one. when h decreases q 0h t = then: . rh e q q e 1 0 2 2 1 rh s t rh 0 = + + r c ] m g 9) because rh is dimensionless, it is necessary to use the expression /h r1* = in order to calculate it. defining /hr h h h*= = u as the thickness of the mixed convective layer, so /h h r= u . from (9), hu is depending on / ;q q qt s s0 has important role because its value is maximum at noon and practically zero at night. in consequence h* decreases in sunrise to noon interval, producing a minimum convective layer thickness. after noon, mixed convective layer thickness increases and q 0h t ! , which is defined as: 113 simulation of near-surface layer of the colombian pacific ocean q t h t th t z02 2 = -^ h 10) where, t tz0 -^ h– is the difference between water temperature of convective layer and level ,z h 0= + ˚c. after noon /q qt s0 icreases and, it leads to an increase of h. this occurs until values qt0r and qs become equal, that indicates the warming up ends and usual convection begins due to surface cooling. precipitation regime nsl thickness according to experimental simulation (fedorov and ginsburg, 1988), is determined as: lnh gi s u t t* c 0 3 0 = b m 11) where, m – s the involvement coefficient, which is function of falling drops diameter; t – s the time counted from the beginning of freshened layer formation, s; t 0 – s the time interval from the beginning of rain to the beginning of the freshened layer formation, s; i – is the precipitation intensity, m/s; cb – s the salt compression coefficient, 1/ ‰; s 0 – is the top boundary salinity of mixed freshened layer, ‰. freshening value in layer generated during precipitation can be calculated by: ln s u is g t t t * c 3 0 2 0 =d m b^ h 12) these values depend on precipitation intensity, rain duration and wind speed. in katsaros and buettner (1969) work, it is shown that u * during precipitation will be equal to the sum of traditional dynamic speed u * with some u *1 : u iu 2* k 1 3 2 = 13) where, u*13 – is the dynamic speed in water due to turbulent influence of falling drops, m/s; uk – is the vertical speed of falling drops, m/s. the presence of thin freshened layer separated from great bulk of water over the mixed layer clearly can be observed due to a discontinuity layer that plays an important role in the evolution of this freshened layer after rain. when rain stops, salt flux through surface is equal to zero and thickness of freshened layer increases. then, its characteristic can be defined as follows (karlin et. al, 1988): h h g s h u s t5 * t 0 0 0 3 0= + d 14) where, h t is the thickness of the freshened layer at time t, easured since the end of the precipitation, m; h 0 is the thickness of the freshened layer at moment of end precipitation, m; s0d is the jump in salinity in the bottom of the freshened layer at the end of precipitation, ‰. this jump at time t ater end of precipitation is determined by the equation: s s h h t t 0 0=d d (15) equations (14) calculate the freshened layer thickness and (15) the evolution of the mentioned jump on its bottom boundary. results determination of regimes according to weather data and calculated fluxes on ocean-atmosphere boundary depending on change of wind, air temperature, water temperature, cloudiness and precipitation, a mathematical model for calculations is determined. inaccuracy in meteorological data can influence the behavior of used model in certain hydrometeorological station. in this section, obtained hour weather data and results of heat and salt calculated fluxes in each station, are analyzed. station 14 was observed from 5 am (8 september, 2001) to 4 am (9 september, 2001), with precipitation registered from 5 to 10 am and at 3 pm. in those hours, values of air (ta) and water temperature (tw) should be smaller than other hours, an upper thin freshening layer should appear, rain be accompanied by hard wind and cloudiness must be of 6 – 8 okt. (ginsburg et al., 1980). the figure 2 shows hourly weather characteristics and figure 3 shows results of heat and salt flux. the cloudiness data match with atmospheric aspects described below (figure 2a). during 6-7 am period, 114 lev karlin and nancy villegas wind speed was very low, and an increase in tw was observed (figure 2b). at night, convective process and a decrease in tw must occur, instead at 10 p.m., a tw increase was registered. similar behavior occurred from 1 am to 2 am (tw increased 0,2 °c), figure 2. weather data at station 14 on cpo figure 3. calculated heat and salt fluxes on water-air boundary at station 14 on cpo also at 4 am. calculated salt flux shows high values during rain (figure 3a). minimum calculated heat flux was observed at night and maximum from 10 am to 1 pm (figure 3b). a) cloudiness, okt ___ and wind speed, m/s --b) tw, °c ___ and ta, °c --a) calculated heat flux, °c cm/s b) calculated salt flux, ‰cm/s time, hours time, hours 115 simulation of near-surface layer of the colombian pacific ocean according to weather characteristics in station 14, nsl thickness and thermohaline structure will be properly described with precipitation and night convection regime models. after rain, nsl water must have less salt than before rain. at the beginning of rain nsl thickness will be small, and at the end, it will increase. due to wind, the nsl formed by precipitation will disappear immediately after rain. wind will create a new greater thickness nsl, more salinity and warmer water than previous one. night convection plays an important role in nsl thickness increase. hourly observations from 1 am (28 august, 2001) to 00 am (29 august, 2001) do not register precipitation at station 49 (figures 4 – 5) exhibiting a constant wind with relatively high speed. the nsl thickness will change depending on wind intensity: minimum thickness and warm water with low wind speed and maximum thickness and cold water with high wind speed. figure 4. weather data at station 49 on cpo figure 5. calculated heat and salt fluxes on water-air boundary at station 49 on cpo a) cloudiness, okt ___ and wind speed, m/s --b) tw, °c ___ and ta, °c --a) calculated heat flux, °ccm/s b) calculated salt flux, ‰cm/s time, hours time, hours time, hours time, hours 116 lev karlin and nancy villegas figure 4a shows very low cloudiness only from 10 am to 11 am, and the cloudiness at 8 am was 7 okt, figure 4b shows high ta and tw. ta and tw had highest values between 1 pm and 2 pm, although wind was constant and strong enough (5 m/s). since 6 pm to midnight there was no positive heat flux into the ocean, but an increase in tw at 7 pm, 10 pm and 00 am was registered. the anterior erroneous observations make hard to calculate nsl thermohaline characteristics. figure 5a shows calculated heat flux increasing due to erroneous data but at night this flux must be constant. figure 5b shows minimum salt flux associated to no rain. nsl thermohaline characteristics will be described by night convection and wind wave mixing regime models. figures 6 and 7 represent weather data and calculated heat and salt fluxes for station 111. this station observed from 00 (30 august, 2001) to 11 pm (31 august, 2001) registered three precipitations: from 6 to 7 am, from 9 am to 1 pm and from 6 pm to 9 pm. cloudiness and wind speed show values associated to rain, whereas ta and tw increased from 1 am to 5 am and from 8 pm to 11 pm, but usually ta and tw usually must decrease at this time. figure 6. weather data at station 111 on cpo according to early hourly weather observation nsl thickness must expand at 5 am and tw decrease. from 6 am, due to rain, a new thin layer should be generated and increase. after one hour, rain again is observed contributing to create a new thin layer with tw change, increasing its thickness at 1 pm due to wind. the described process will be repeated during night, appearing night convection with nsl thickness expansion and tw decreasing. taking into account this process, nsl thermohaline characteristics of station 111 can be well described by precipitation and night convection regime model. nsl thermohaline structure of cpo according to simulation results calculations of tw evolution were performed with all regimes, because temperature factor reacts immediately and more adequately to meteorological influences. calculations of salinity evolution were performed only with precipitation regime. station 14 results are represented in figures 8, 9 and 10. used regimes do not provided observed nsl thickness behavior, but shows properly tw and salinity difference evolutions. wind-wave mixing regime describes better tw evolution than a) cloudiness, okt ___ and wind speed, m/s --b) tw, °c ___ and ta, °c --time, hours time, hours 117 simulation of near-surface layer of the colombian pacific ocean figure 8. nsl characteristics evolution of cpo station 14: night convection and wind-wave mixing regimes figure 7. calculated heat and salt fluxes on water-air boundary at station 111 on cpo other ones. salinity difference evolution is well described by precipitation regime model from 5 am to 7 pm. until now it is impossible to say with confidence, that used models are not reliable to describe nsl characteristics in station 14 and verify data validness. a) calculated heat flux, °ccm/s b) calculated salt flux, ‰cm/s a) depth, cm b) tw, °c time, hours time, hours time, hours time, hours 118 lev karlin and nancy villegas figure 9. nsl characteristics evolution of cpo station 14: night convection and sunny and weak wind weather regimes figure 10. nsl characteristics evolution of cpo station 14: night convection and precipitation regimes calculated nsl characteristics at hydrometeorological station 49 are shown in figures 11-12. here, night convection regime describes well nsl depth behavior from 1 am to 5 am and from 7 pm to 12 pm. also there are good results with wind-wave mixing regime for the other hours. theoretically, when there is a constant wind, nsl depth should increase (in this case so it was calculated from 9 am to 2 pm), but according to observed data it decreased, allowing to conclude that oceanographic data or wind data are erroneous (wind should decrease). for this period, sunny and weak wind weather regime, gave a very sharp decrease of nsl depth, although this change is similar to natural data. here we also can assume not correctly registered wind data. a) depth, cm b) tw, °c a) depth, cm b) salinity, ‰ time, hours time, hours time, hours time, hours 119 simulation of near-surface layer of the colombian pacific ocean precipitation regime at this station did not adapt, since rain was not observed. nsl tw at station 49 is described well with examined regimes. sunny and weak wind weather regime shows abrupt changes in tw in day time, but preserves similarity with natural data. in general, calculations with different regimes at this station show better results, than at station 14. figure 11. nsl characteristics evolution of cpo station 49: night convection and wind-wave mixing regimes figure 12. nsl characteristics evolution of cpo station 49: night convection and sunny and weak wind weather regime figures 13, 14 and 15 show results of calculations at hydrometeorological station 111. here, evolution of nsl depth, tw and salinity difference are well described with all regimes. a) depth, cm b) tw, °c a) depth, cm b) tw, °c time, hours time, hours time, hours time, hours 120 lev karlin and nancy villegas figure 13. nsl characteristics evolution of cpo station 111: night convection and wind-wave mixing regimes figure 14. nsl characteristics evolution of cpo station 111: night convection and sunny and weak wind weather regimes simulation results of sunny and weak wind weather regimes, have a high differences from others regimes and preserve natural data behavior. calculated nsl depth with night convection regime from 0 to 5 am is more similar to natural data than those obtained from 7 pm to 11pm. the precipitation regime provided the best performance from 6 pm. in general, the sunny and weak wind weather regime was more suitable due to presence of precipitation with wind. salinity difference evolution was well enough described by precipitation regime. these models can be used for researches of nsl thermohaline characteristics of cpo, but it is necessary quality improvement of natural data. a) depth, cm b) tw, °c a) depth, cm b) tw, °c time, hours time, hours time, hours time, hours 121 simulation of near-surface layer of the colombian pacific ocean figure 15. nsl characteristics evolution of cpo station 111: night convection and precipitation regimes conclusions nsl thermohaline structure of cpo according to simulation results with different regimes was described. on basis of data analysis and simulation results, over cpo occur four nsl regimes: night convection, wind-wave mixing, volume absorption of radiant energy and precipitation. models 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turbulent convection// by izv. as ussr. serie. fao. 1987. t.23, no.6. p. 593–610. • zilitinkevich, s.s. and j.w. dearfor. similarity theory for the planetary boundary layer of time-dependent height// j. atmos. sci.– 1974.– v.31, no.5.– p.1449–1452. :marzo, ~2000 tomografia geoelectrica: dos ejemplos en la sabana de bogota camilo ernesto sanin ge610go departamento de geociencias facultad de ciencias universidad nacional de colombia luis alberto briceno guarupe profesor asociado, departamento de geociencias-facultad de ciencias-universidad nacional de colombia sanin c.e. & l.a. briceno (2000): tomografia qeoelectrica: dos ejemplos en la sabana de bogota. geofis. colomb. 4:7378. issn 0121-2974. santa fe de bogota, d.c. colombia. resumen con base a las nuevas tecnologias en exploraci6n geoelectrica en cuanto a algoritmos para resolver el problema inverso y software de interpretaci6n, se hicieron dos secciones tornoqraficas 0 "imaging". los dos ejemplos se realizaron, en ubicaciones diferentes y sobre sedimentos no consolidados de edad cuaternaria. ei primero, ubicado en cercanias de tenjo, se realiz6 para analizar el comportamiento del modelo geoelectrico sobre materiales de baja resistividad y contraste electrico (dep6sitos aluviales de la formaci6n sabana); mientras que el segundo, ubicado sobre los dep6sitos originados por procesos de tipo fluvio-glaciar, como 10son los del rio tunjuelito, se implement6 para ver efectos sobre materiales altamente resistivos y con grandes variaciones laterales de resistividad. utilizando columnas litol6gicas, en ambos casos se evaluaron los resultados obtenidos en las secciones, presentandose en general, resultados coherentes y de buena resoluci6n. las profundidades de investigaci6n alcanzadas fueron de 30%, para el primer ejemplo y 40%, para el segundo, ubicando electrodos con arreglos geometricos de wenner y de wenner-sciumberguer, respectivamente. sequn los resultados de estos ejemplos, ciaramente se puede inferir que la profundidad de investigaci6n y la resoluci6n depende claramente del medio, de la geometria y de la longitud de la secci6n. se recomienda utilizar esta herramienta geofisica para aplicaciones de poca profundidad (hasta 100 m) y en donde se necesiten datos de alta resoluci6n y confiabilidad. abstract with the new technologies about math algorithms and software interpretation to solve the inverse problem, two examples of electric tomography were done in two different places and over quaternary deposits that correspond to sabana de bogota. the first, placed, near the location of tenjo, was done to see the behaviour of the geoelectrical model with low resistivity and electric contrast materials (alluvial deposits of formaci6n sabana). while the second, over deposits of fluvial-glacier origen, like rio tunjuelito's deposits, shows effects over high resistivity materials with great lateral variations of resistivity. the evaluation of the quality of data was made with lithologic columns in both cases, in general, the results were coherent and showed high resolution. the reached depths of investigation were 30% for the first example and 40%, for the second, situating the electrodes with wenner amd wenner-sciumberguer arrays. using the results of these two examples, clearly we can see that the depth of investigation depends on the media, the geometric array and the lenght of the section. this geophysic tool is very useful in shallow applications (100 m depth), and where high resolution and quality are needed. iibliouca ce t al sala u i'4eil&idao .. "e,o 11 1. introduccion la exploraci6n geofisica ha avanzado durante los ultirnos 20 alios, gracias a los proqresos en software. investigadores como loke & barker, (1995) y sasaki, (1992), han enfocado la soluci6n del problema inverso hacia los modelos 20 y 3d, desplazando a los antiguos modelos unidimensionales en que se basaba la exploraci6n electrica de sondeos y perfiles. este nuevo modele geoelectrico incluye variaciones laterales y verticales que se acercan mas a la realidad. ei presente articulo esta basado en el trabajo de grade realizado por sanin, (1999). en donde se utiliza el software de interpretaci6n qeolectrica res2dinv de la compaliia sueca abem, desarrollado por loke & barker, (1995), adaptandolo a la instrumentaci6n convencional de sondeaje electrico, en dos ejemplos escogidos en la sabana de bogota. ei objetivo es implementar esta tecnologia a las posibilidades reales de logistica para exploraci6n geofisica con que se cuenta actualmente en colombia y evaluar los resultados para visualizar las posibles aplicaciones practicas en ingenieria y geologia. se escogieron dos sitios, con base en la facilidad de la adquisici6n de la informaci6n, la logistica y las condiciones electricas del medio. dentro de estas condiciones se busc6 una zona en donde el medio tuviera bajos valores de resistividad y poco contraste electrico. para este efecto se escogi6 un area cercana al municipio de tenjo, en donde se tenia informaci6n litol6gica de dos pozos y los dep6sitos aluviales de la formaci6n sabana. ei segundo ejemplo se realiz6 en una cantera de extracci6n de materiales para construcci6n. ei medio, constituido por gravas y arenas conqlorneraticas correspondientes a los dep6sitos de origen f1uvio-glaciar del cono del rio tunjuelito, permi!ia que se dieran las condiciones de altos valores de resistividad y variaciones laterales del espesor de los dep6sitos de grava, que se intercalan con materiales finos. 1.1. concepto de tomografia la tomografia tiene sus raices en "torno" que viene del latin: tomus, que significa secci6n, y "grafia", que viene del griego y significa relativo a escritura un concepto general de tomografia en ciencia seria: representaci6n de una propiedad, magnitud 0 campo fisico en un corte 0 secci6n. la tomografia geofisica se plantea como un problema inverso que tiene el mismo principio de la tomografia acustica generalmente se plantea la tomografia como herramienta para obtener informaci6n de la distribuci6n horizontal y vertical de parametros en el subsuelo. en geofisica se conocen la tomografia sismica y la tomografia electrica, dentro de esta ultima se conocen la "tomografia de impedancia" (eit) y la "tomografia de resistividad" (ert), dentro de la que se desarroll6 este trabajo. 1.2. problema inverso como en todo problema inverso se tiene un modele, unos parametros que definen dicho modele y los datos de campo. los parametres del modelo estan definidos por la resistividad aparente en cada uno de los bloques de que se compone el modelo. para un ensayo fisico dado, donde se tomen n mediciones, para un modele fisico en el que se tengan m parametros, se tendran los vectores respectivos: en donde t significa transpuesta. los valores d son los datos que obtenemos del experimento y los valores de m son los parametros del modele que vamos a encontrar. dependiendo del tipo de problema y de la forma que se plantee la resoluci6n de este, habran mas 0 menos numeros de parametres y de mediciones. generalmente se plantean ecuaciones matriciales que se resuelven mediante operaciones de transpuestas e inversas para obtener la matriz de parametres: g m = d 1.3. modelo planteado para resolver el problema inverso de la tomografia loke & barker (1995) explican las ecuaciones que rigen el problema directo para el modele rectangular ya antes utilizado por otros autores (de groot-hedlin & constable, 1990). ei modele rectangular (fig.1) consiste en dividir el subsuelo en varios rectanqulos de determinadas dimensiones, cada uno de los rectanqulos contiene un pararnetro (resistividad aparente y real) que es el que se utilizara posteriormente en la inversi6n para la obtenci6n de la tomografia final. para el manejo de ecuaciones en donde se obtienen los parametres mediante minimos cuadrados y matrices jacobianas (derivadas parciales) se necesita conocer con c1aridad las derivadas parciales del potencial en cada uno de los bloques imaginarios r que contienen los parametros del modelo. para cada bloque rectangular se supone un medio hornoqeneo en el que se puede aplicar facilrnente la ecuaci6n de poisson: v'¢ • 11 ii " 2u ii!ii vo n 12 1) ,.. 15 "..."" 'i ,~ """-, .. ! ! i -0----<--+-1 --+----. ~-"'~-.-'p18i . ~ ••••••. __ ~,,,,,,1. .-r" .....__.~._..~ ..._.._--....-.•. -p:u p\>o .-------._ -----_ .._-. __ ..._----_=-=.. ..---_...... . __.. __.figura 1. modelo recl angular del subsuelo en donde ~ es el potencial producido por una fuente puntual de corriente en un medio homoqeneo (que en este caso es el bloque rectangular del modelo) que transmite una corriente continua is, localizada a una distancia xs. sequn park & van (1991). la ecuaci6n principal para el calculo de matrices jacobianas que utilizan loke & barker (1995), y en la que se basa el software res2dinv es conocida como: "smoothness-constrained, least-squares inversion", (de groot-hedlin & constable, 1990). en donde j es la matriz jacobiana, con las derivadas parciales correspondientes a los bloques rectangulares del modelo, a es un factor denominado multiplicador de lagrange (factor de amortiguamiento 0 damping factor), 9 es el vector de discrepancia que contiene las diferencias logaritmicas entre las resistividades aparentes medidas y calculadas, y p es el vector de correcci6n de los parametros del modele. la matriz ~ tiene un efecto de filtrado 2d que suaviza los parametros del modelo hacia un valor constante "smoothness constrain"(sasaki,1992). 1.4. proceso de inversion la inversi6n utilizada en el trabajo de loke & barker (1995) se basa en los mismos conceptos de sasaki (1992), perc modifica los procesos de inversi6n con el fin de ahorrar pasos en el computador. con esta inversi6n loke disena el programa para windows que se utiliza en este trabajo (res2dinv). la inversi6n utiliza como base a la ecuaci6n de' minimos cuadrados y consiste en tres pasos principales para la practica: 1. ei calculo de los valores de resistividades aparentes iniciales (qo) para el calculo de las inversiones posteriores. esta resistividad aparente inicial se calcula mediante el promedio logaritmico de los valores que se miden en superficie utilizando la siguiente ecuaci6n: 2 la matriz jacobiana se calcula para la configuraci6n (arreglo qeometnco), junto con los valores de derivadas parciales, en un archivo que tiene el programa de interpretaci6n. 3. con los valores de la matriz jacobiana se resuelve la ecuaci6n de minimos cuadrados y se obtiene el vector de correcci6n de parametres "p", de donde se obtienen un q1, mediante: en donde este q1 corresponde a los nuevos pararnetros calculados para el modelo, mientras que los qo son los parametros iniciales. 2. metodologia 2.1. adquisicion de datos en la tomografia qeoelectrica se pueden implementar arreglos geometricos como el schlumberguer, wenner, wenner-schlumberguer, polo-dipolo y dipolo-dipolo. la adquisici6n se hace sobre una linea en la que hay una serie de estaciones de datos, como se muestra en la fig.2. para cada arreglo se debe diseriar una grilla (fig.3) para visualizar la secuencia de toma de datos, ya sea que se cuente con una unidad de qeoelectrica continua 0 una convencional de toma de dato por dato. station 3. stllt'on 2r··-···---··------~----____, l6ptop reslstlully computer c1 pi pz cz i 2.. i 2" i 2... i sllluufi 1 ,-------'-------, n = 1 n = ? n = ) n = " n = 5 n 6 i • 2' 3 • 4' 5" ii figura2. diagrama de grilla de toma de datos y niveles de informacion a) . wenner .... + ... .... .... .... + + ....+ + ... ... ... ... + ... + + ... ....+ + ... ... ................+ + + + + + .... .... + + + + + + ... + ........+ .... .... + .... .... + ... + + ... ............+ + + + ....+ .... ... + +++++++.++ .... .... .... .... .... .... +... .... .... .... b) . schlumberger .. .. -, .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... .. .. .. + + .. . .. + .. .. .. .. + .. .. .. .. .. + .. .. .. .. + . .. .. .. .. .. .. .. .. .. .. +.. .. + + + + .. + .. .. .. .. + .. .. .... + + .. + .. .. .. .. .. .. + .. +.. .. + + .. + .. .. .. + .. .. + .. + + .. .. .. .. .. + + + + + .. .. .. + + + + .. .. .. + .. .. .. figura 3. arreglos de wenner y schlumberguer 2.2. procesamiento de datos mediante el software de interpretaci6n res2dinv se leyeron los datos de los dos ejemplos, utilizando opciones qraficas de optimizaci6n de los datos que trae, como opciones de lectura, el programa. se pueden editar tarnbien opciones de los pasos inversi6n, tales como el factor de amortiguamiento, filtro de suavizado, espesor de los niveles de bloques del modele y la optimizaci6n del factor de amortiguamiento. ei programa realiza las inversiones con los pasos previamente mencionados y genera una qrafica en colores despues de realizar tres iteraciones se iimpia el ruido con el descarte de datos an6malos y cambiando los valores del factor de amortiguamiento. ei proceso se realiza hasta obtener la secci6n mas adecuada para la interpretaci6n. 3. resultados 3.1. ejemplo 1: zona de tenjo esta tomografia se realiz6 dentro de los predios de una propiedad privada, de la vereda guangata, municipio de tenjo, sobre dep6sitos aluviales de edad cuaternaria correspondientes a la formaci6n sabana. ei arreglo utilizado fue de wenner, para una secci6n de 126 m, con 42 puntos de electrodos de corriente, unidad de espaciamiento entre electrodos de 3 m, con 15 niveles de datos para el modelo, para un total de 112 datos y 93 bloques. ei modele litol6gico final obtenido. junto con la columna de pozo,se muestran en la figa f\j1()dei.cj tomogi.;'(/',fic() flf~i\l. 24.0 48.000 "8 0.8 ,-' '"0 6.5 ~ ~ 13.0~ 0 17.6..... . 0-,-----r o _ 9.62 13.7 19.6 28.0 40.0 57.2 817 117 resistividad (ohm-m) 00 ~ 0.8 e '-' '"0 51;ro .j '"0 "s ~ no 0 17.66:: 48.0 96.024.0 .. flill'":':2tj 720 y 120m. unidad i. suelo, materia organica, capas de turba yarcillas unidad 2. arena tina de composici6n cuarzosa. unidad 3. arcillas y arenas parcialmente saturadas. acuiferos en zona saturada .. localizaci6n pozo l y sondeo 1 localizacion pozo2 y sondeo 2 y y 1.._-----,,------,----figura 4. seccion tomografica y modelo l1tologico del ejemplo de tenjo se interpretaron tres unidades que se pueden distinguir mediante la qeoelectrica: una unidad superficial de alta resistividad (80-120 ohm-m) que esta conformada por suelo, materia orqanica, capas de turba y arcilla, una unidad de resistividad mas baja que la anterior consistente en una arena de grana fino de composici6n cuarzosa, y una capa mas profunda, que liega hasta los 18 m aproximadamente, y que define la base de la secci6n tornoqrafica, de baja resistividad y constituida por arcillas. la profundidad de investigaci6n (",18 m), alcanzada en esta secci6n, se debe al arreglo utilizado y a que se presentan unidades de baja resistividad que impiden la profundizaci6n de la corriente. en este caso la profundidad de investigaci6n fue, aproximadamente, un 30% de la distancia ab/2. 2.2. ejemplo 2: zona de tunjuelito se ubic6 al sur de bogota en una cantera de explotaci6n de gravas, sobre dep6sitos no consolidados de edad cuaternaria y origen fluvio-glaciar correspondientes al cono del rio tunjuelito. ei arreglo utilizado fue wenner-schlumberguer, con una longitud total de 160 m, espaciamiento entre electrodos de 5 m, 32 puntos de electrodos, 10 niveles de datos, un total de 120 datos y 93 bloques para el modelo. en la fig.5, se aprecia el modelo obtenido y la columna litol6gica utilizada para evaluar la secci6n. 81bliotfca cent "'~ , sa~~ ultlvuliulag malllo [v10de:lotomogr/\fico final 40000 uj--'-----'---j..-'--"''''"----'----'----'-~-'---'-----'---'-"'--...l..--'---j..-'---'----'---'---'--'--""""---'-----'----'o'-"'---'-----'----'-7(1 121j800 15.7 217 29.3 resistivid?d (ohm-m)____ .. bl-") aw) ~ _ [4e'iib [=:j _ ~ ... _ 200 500100 140 180 220 250 280 modelo l1tologico final bmj40000 131---'-----'----'~..&....-.........----"-"--~------''--..&....---'-----''-''---'----'----''--'---'----.-''---'---'--'--"'-.........---.-''-.........--'---'70 120 2u ~57. 29.3 figura 5. seccion tomografica final y modelo l1tologico. zona del cono del rio tunjuelito se interpretaron dos unidades: una mas superficial de bajas resistividades, que se compone principalmente de suelo, materia orqanica e intercalaciones de arcillas y limos (20-100 ohm-m), y otra capa mas profunda, cuyo tope se encuentra en este modelo hacia los 16 m aproximadamente, con valores de resistividad altos (200300 ohm-m), que corresponden a los conglomerados del cono del rio tunjuelito. conclusiones se obtuvieron secciones tornoqraficas. que comparadas con los sondeos electricos verticales, mostraron una mejor resolucion y valores similares de resistividades electricas con una metodologia que podria ser competitiva, tanto logistica como econornicarnente. con los otros metodos que se hacen en el pais con el estado actual de mstrumentacion geoelectrica. por medio de los resultados obtenidos en ambas tomografias se ve que la profundidad de investiqacion de la tomografia qeoelectrica depende de la composicion de los materiales sobre los que se va a trabajar y del arreglo qeornetrico utilizado. se vio que los datos obtenidos coinciden en gran parte con 10 observado en la informacion disponible, sin embargo, se encontro la limitacion que tiene la exploracion qeoelectrica. que es la perdida de resolucion, aunque menor que en el sev, con la profundidad. referencias bibliograficas degroot-hedlin, c. &, s. constable. 1990: occam's inversion to generate smooth, two-dimensional models from magnetotelluric data. geophysics, vol 55:16131624. loke, m. & r.d. barker. 1995: least-squares deconvolution of apparent resistivity pseudosections geophysics, vol 60:1682-1690. parke, s.k. & g.p. van. 1991: inversion of pole-pole data for 3d resistivity structure be neath array of electrodes. geophysics 50:951-960. sanin, c.e. 1999: tomografia geoelectrica: adaptacion y aplicacion en dos ejemplos de la sabana de bogota. (trabajo de. grado) 90pp. universidad nacional de colombia. sasaki, y. 1992: resolution of resistivity tomography inferred from numerical simulation, geophysical prospecting. 40:453-463. earth sciences research journal seismology earth sci. res. sj. vol. 16, no. 2 (december, 2012): 31 38 adaptive seismic ground roll attenuation using the double density dual tree discrete wavelet transform (dwt) method ali reza goudarzi1 and mohammad ali riahi2 1 institute of geophysics, tehran university, islamic republic of iran. e-mail: aligoudarzi@ut.ac.ir 2 institute of geophysics, tehran university, islamic republic of iran, phd in exploration geophysics (associate professor of geophysics). phone: +98 (21)61118219. e-mail: mariahi@ut.ac.ir abstract seismologists working on the reflection of seismic data have been dealing with noise and trying to attenuate noise as much as possible without damaging a particular signal. many methods have been used to remove noise types, each being partially effective. none of the transforms have been ideal and, even at the point of transform, noise becomes added to data. the double density dual tree (dddt) discrete wavelet transform (dwt) method was used in this paper to attenuate noise in pre-stack and post-stack seismic data. wavelet domain ground roll analysis (wdga) was used for finding ground roll energy in the wavelet domain; it is a substitute method for thresholding in seismic data processing. the dddtdwt method has four wavelets and two scaling functions. using this method, shot gather data decomposes to sub-scales and each scale contains coefficients related to a particular signal. the procedure involved applying distinct wavelets and scaling functions to data and then eliminating the coefficient of noise from inverse dddtdwt using the wdga technique. the inverse transform provided a section without noise; double density dwt and dual tree dwt methods were combined (i.e. each method having its own characteristics and advantages). resumen los sismólogos que trabajan en la reflexión de los datos sísmicos se han ocupado del ruido y en tratar de atenuarlo tanto como sea posible sin dañar una señal particular. muchos métodos se han utilizado para eliminar ciertos tipos de ruido, cada uno siendo parcialmente efectivo. ninguna de las transformadas han sido ideal e incluso en el punto de transformación, el ruido es añadido a los datos. el árbol de doble densidad dual (dddt) y el método de transformada de ondicula discreta (dwt) fueron los métodos utilizados en este trabajo para atenuar el ruido en los datos sísmicos la sección preapilda y postapilada. el análisis de ground roll en el dominio de la ondícula (wdga) se utilizó para encontrar su energía. este es un método sustituto de umbralización en procesamiento de datos sísmicos. el método dddtdwt tiene cuatro ondiculas y dos funciones escalares. usando este método, datos shot gather fueron descompuestos a subescalas, donde cada escala contiene coeficientes relacionados a una señal particular. el procedimiento consistió en aplicar distintas ondiculas y funciones escalares a los datos y posteriormente eliminar el coeficiente de ruido a partir de la inversión de dddtdwt utilizando la técnica wdga. la transformada inversa proporcionó una sección sin ruido; los métodos de doble densidad dwt y doble árbol dwt se combinaron (es decir, cada método tiene sus propias características y ventajas). palabras claves: sismica, ruido, transformada ondicula, disparo-receptores , sección apilada, wdga. keywords: seismic, noise, wavelet transform, shot gather, stack section, wdga. record manuscript received: 14/10/2011 accepted for publications: 06/05/2012 introduction this study presents a wavelet-transform-based method and wavelet domain ground roll analysis (wdga) technique for seismic ground roll reduction. the double density dual tree (dddt) discrete wavelet transform (dwt) was used with the wdga technique for seismic denoising. exploration seismology has usually dealt with a variety of noise having determined and undetermined sources. this paper was aimed at attenuating ground roll using the dddtdwt method on post-stack (residual noise) and pre-stack data. more than two-thirds of total generated seismic energy is imparted ali reza goudarzi and mohammad ali riahi32 into rayleigh waves in most surface seismic surveys when a compressional wave source is used (richart et al., 1970; choon b, park et al., 1999). receiver arrays are used to attenuate undesired coherent noise in certain wave numbers. shot hole depth and charge size are the important parameters in surface wave generation in land seismic acquisition. since the surface’s physical conditions can vary significantly along a seismic line, measurements taken in the field to eliminate surface waves may be insufficient (saatcilar r and canitez n, 1988). in recent years, halliday d (2011) has used an adaptive interferometry method for ground-roll suppression and henley d (2003) has tried to attenuate coherent noise in the radial trace domain. neelmani et al., (2008) used the curvelet domain to attenuate coherent and random noise in seismic section. more recently, bonar and sacchi (2012) have applied a nonlocal means algorithm to seismic noise attenuation. signal and noise frequency and time domain together have considerable overlap. hence, the fourier transform for noise removal is not always quite efficient because its basic premise is that stationary signal and seismic signals have non-stationary characteristics. the dwt technique can be a viable alternative for the removal of coherent and random noise in seismic data processing. wavelet transform is a simple and reliable tool for signal and image processing and has overcome the limitations of the fourier transform. the conventional dwt uses a wavelet and scaling function; this method was used by deighan et al., (1997) to remove ground roll noise. defects in this type of dwt are aliasing, its shift invariant property and a lack of directivity in higher dimensions (because coefficients related to trace ground rolls traced at the same scale are different and hamper making all coefficients zero, ground rolls therefore not being completely removed). surface waves provide common sources of noise in seismic land data. creating surface waves is closely related to regional geology and the location of sources and receivers (al-husseini et al, 1981; dobrin 1951). deeper sections’ seismic data is similar in bandwidth to that of ground rolls, but ground rolls have higher amplitude. a regular filtering method is 1d frequency filtering; however, the intersection between seismic signal and noise frequency time components means that 1d frequency filtering is not a useful tool for such purpose. another technique that is often used is 2d fourier transform or f-k transform; nevertheless, it disagrees with seismic data’s natural behaviour because of stationary signal assumptions. also, there is no distinct border between signal and noise in 2d fourier transform, consequently it distorts a seismic signal because of windowing and changes its frequency component. the dwt applied by deighan et al., (1997) had various restrictions; different techniques have been proposed to overcome such restrictions, including aliasing which may happen because of dwt’s low redundancy factor. it is also a shift-invariant transform making denoising difficult in practice and it cannot be expanded to m-dimensional applications (i.e. difficult directivity). it has been suggested that a proper wavelet could be applied to avoid aliasing and reduce the frequency band of scales intersection in the wavelet domain. some newer wavelet transform structures have been introduced instead of critically sampled dwt. a common type of dwt is udwt which was introduced by guo et al., (1995); redundancy grows with level and is perfectly shift-invariant in this method. udwt’s redundancy characteristics generate a huge amount of information which cannot be simply applied to seismic data processing because it is time-consuming. udwt can thus be replaced by practical methods. after methods were introduced by kingsbury (2001) and selesnick (2001), a combination of such methods (selesnick et al., 2005) was applied to denoising and image processing. the proposed techniques were based on multi-wavelet frames to avoid aliasing characteristics and achieve better time-frequency representation between scales. kingsbury applied the dwt in separate parallel trees; he used two wavelets that were approximately hilbert transform pairs (a dual tree cwt uses two real dwts: the first dwt gives the real part of the transform while the second dwt gives the imaginary part (selesnick et al., 2005)). selesnick (2001) used figure 1. iterated filter bank for the double-density dual-tree dwt (selesnick, 2004). a similar strategy but one of the wavelets had one sample delay and the wavelets were real. the double density discrete wavelet transform (dddwt) was based on oversampled fir filter banks. this method has much smoother wavelets than orthonormal wavelets (like daubechies’ orthonormal wavelets) from the same support. as ground roll occurs at higher scales, it can be picked up and suppressed clearly in the wavelet domain. dddwt and dtcwt are nearly shift invariant and have redundancy factors of 2, so they might be considered as perfect substitute methods. dddtdwt uses their advantages in signal processing and has a redundancy factor of 4. the wavelet domain ground roll analysis technique (wdga) has been used here to address ground roll coefficients in the wavelet domain, each signal being analysed to scales and each scale has its time-frequency resolution. note that frequency was related to scales (abry, 1997): fa = fc a. δ where a was a scale, δ was the sampling period, fc was the centre frequency of a wavelet in hz and fa was pseudo-frequency corresponding to scale a, in hz. methodology h, i(t) and g, i(t) (for i = 1, 2) were wavelets in the dddtdwt method. wavelets were applied with a sample apart and were nearly the same, so that h, 1(t) ≈ h, 2(t – 0.5) . other pairs had an approximate hilbert pair of wavelets so that g, 1(t) ≈ h { h, 2(t)}. the results gave wavelets where the zero moments had to be determined, having a high degree of smoothness and the shortest possible length. the dddtdwt method is nearly shift-invariant and has a redundancy factor of 4. double density dwt is nearly invariant with shift, it uses a scaling function and two wavelet functions, it can remove noise, its redundancy factor is 2 and wavelets are smooth and their lengths are minimised. the adaptive seismic ground roll attenuation using the double density dual tree discrete wavelet transform (dwt) method 33 dual tree dwt method is almost invariant with shift, it is a fairly good approximation for continuous wavelet transform, its redundancy factor is 2, time-frequency width is enhanced and wavelet length is minimised. the two methods do have differences, such as both wavelets become linked together in quite different manners, the degree of freedom in dual tree dwt design is less than that of double density dwt and filter bank structure is distinct from each other. dddtdwt has advantages over both these methods. the relationship between wavelets is: g, 1 (t) ≈ h { h, 1 (t)} (1) g, 2 (t) ≈ h { h, 2 (t)} (2) h, 1(t) ≈ h, 2(t – 0.5) (3) g, 1(t) ≈ g, 2(t – 0.5) (4) figure 1 shows the dddt dwt filter bank. dual tree dwt uses critically-sampled dwt and the structure is such that the filter banks are applied iteratively and in parallel. reconstruction filters in the filter bank are time-reversed analysis filters. the filters applied in this study had the following features which had to satisfy certain conditions: they had to satisfy perfect reconstruction, wavelets had to be approximately hilbert pairs, they had to satisfy zero moment property and they had to have a minimum length. figure 2 shows the wavelets considered for seismic data processing (selesnick et al., 2004). selesnick et al., (2005) encountered four problems and solved them by using the dddtdwt technique used here. oscillations mean that since wavelets are band pass functions then wavelet coefficients tend to oscillate positively and negatively around singularities. shift variance means that a small shift in the signal greatly perturbs wavelet coefficient oscillation pattern around singularities. substantial aliasing arises from wavelet figure 2. the wavelets and scaling functions (selesnick, 2004). the wavelets and scaling functions of double density dual tree dwt 5 0,5 0 -0,5 0 2 4 6 8 10 12 14 1 0,5 0 -0,5 -1 0 2 4 6 8 10 12 14 1 0,5 0 -0,5 -1 0 2 4 6 8 10 12 14 1 0,5 0 -0,5 -1 0 2 4 6 8 10 12 14 1 0,5 0 -0,5 -1 0 2 4 6 8 10 12 14 5 0,5 0 -0,5 0 2 4 6 8 10 12 14 h(t) h, 1(t) h, 2(t) g, 2(t) g, 1(t) g(t) ali reza goudarzi and mohammad ali riahi34 coefficient samples’ wide spacing or, equivalently, the fact that wavelet coefficients are computed via iterated discrete-time down-sampling operations interspersed with non-ideal low-pass and high-pass filters. a lack of directionality would concern multi-dimension wavelet transforms (but was not the case in this study). unlike dwt based on real-valued oscillating wavelets, the fourier transform is based on complex-valued oscillating sinusoids. the oscillating cosine and sine components (real and imaginary parts, respectively) form a hilbert transform pair (selesnick et al., 2005). the fourier transform does not suffer from these problems. fourier transform magnitude does not oscillate positively and negatively but rather provides a smooth positive envelope in the fourier domain. fourier transform magnitude is perfectly shift-invariant, having a simple linear phase offset encoding the shift. fourier coefficients are not aliased and do not rely on a complicated aliasing cancellation property to reconstruct a signal. the m-d fourier basis sinusoids are highly directional plane waves (selesnick et al., 2005). the idea behind dddtdwt is to use characteristics from fourierbased methods but in short support wavelets and using a filter bank parallel strategy. thresholding the following nonlinear transform was applied to empirical wavelet coefficients to suppress noise: f (x) = x i (|x| > t) where t was a certain threshold. the choice of threshold was a very delicate and important statistical problem. a large threshold leads to atdata preparation shotgather f-k transform of shotgather f-k analusis of the groundroll inverse fourier transform forward dddtdwt of ground roll forward dddtdwt of shotgather inverse dddtdwt output figure 3. wdga technique flowchart. comparing performs of coefficients of ground rol al shot gather in the wavelet domain and set them of zero tenuating the seismic signal inside the ground roll whereas a small threshold provides inappropriate denoising. theoretical considerations yielded the following threshold value: (5)t = √ 2 2 log(n) n where n was input vector length and 2 variance of noise. the only difference between hard and soft thresholding procedures lie in the choice of nonlinear transform on empirical wavelet coefficients. the following nonlinear transform was used for soft thresholding: s(x) = sign(x)(|x| – t)i(|x| > t) (6) where t was a thresholding parameter. soft thresholding, like hard thresholding, has encountered similar problems in seismic denoising, especially for ground roll elimination. thresholding is well behaved for seismic random noise attenuation but causes trouble for data-processors for coherent noise attenuation. there is a trade-off between noise attenuation and signal damage; a practical approach was thus needed to overcome such restriction. the wavelet domain ground roll analysis technique (wdga) was thus used to address ground roll coefficients in the wavelet domain, each signal analysed to scales and each scale having its time-frequency resolution, frequency being related to scales (abry, 1997). it was obvious that ground roll low-frequency signal would be seen in low frequency scales, based on applying a wavelet transform strategy. filter banks of these lowfrequency scales may be in lower scales or upper scales. setting thresholding value depends on user experience; if a user chooses an incorrect thresholding value then thresholding algorithms go through all the scales in a particular wavelet domain. thresholding value means limiting to keep or kill a coefficient and will not ask if it is a part of a particular signal or not. where ground roll is based on the level of decomposition present in a few low-frequency scales, then thresholding procedures are inappropriate for attenuating them. the wdga strategy used in this paper consisted of: 1) transforming seismic data to the f-k domain and analysing it; 2) eliminating the energy relating to a signal because of ground roll linear characteristics in the fourier domain; 3) the inverse transform of the analysed data in the fourier domain providing the seismic section containing ground roll energy; 4) transforming the section containing signal and ground roll energy to the wavelet domain; 5) transforming the analysed section containing just ground roll energy to the wavelet domain by the same level of decomposition as the previous step; 6) comparing two domains from steps 4 and 5 and the coefficients present in step 5 from step 4 set to zero; and 7) step 6 inverse dwt regarding time domain providing a section with no ground roll energy. figure 3 shows a flowchart of the wdga technique. dddtdwt resolution level was controlled by signal length. maximum value was used for better time frequency representation. two types of transform were thus applied to a chain, where each transform’s abilities helped to diagnose noise having a linear pattern. linear analysis was used for ground roll elimination and it was believed that the wdga technique could attenuate each linear complex noise in the wavelet domain, e.g. air waves, multiples and tube waves. the main point concerned the choice of transform (in the chain) to be applied before discrete wavelet transform. no transform in seismic signal processing was ideal and perfect, but a combination of them led to reliable data-processing results. adaptive seismic ground roll attenuation using the double density dual tree discrete wavelet transform (dwt) method 35 application seismic data usually involves the following equation: y(t) = x(t) * w(t) + n(t) (7) where x(t) was the earth’s impulse response. the source wavelet w(t) was convolved with x(t), then noise n(t) was added to the equation (n(t) containing all types of noise). this noise could not be eliminated completely because its time and frequency overlapped the seismic signal. the wavelet used in generating a synthetic seismogram was a ricker wavelet having 30 hz central frequency; the ground rolls were in the 5-15 hz frequency range. figure 4a shows the synthetic section; this shot gather contained four reflectors where ground roll masked them, especially the first low velocity reflector. ground rolls have low frequency and high amplitude, features which can be very useful for attenuating ground roll. the dddtdwt method was thus used here; the data involved 2050 samples and figure 4b gives the sampling interval as 4 ms. selected traces’ frequency spectra can be seen before and after dddtdwt filtering. note in figure 5 that noise amplitude in the 5-15 hz frequency range has been attenuated. the first low velocity reflector appeared successfully and other reflectors’ continuity was enhanced. the dddtdwt method (due to time frequency representation) eliminated all ground roll energy while reflector continuity was enhanced without damaging the signal. note that the dddtdwt method preserved each frequency’s amplitude in the pass band. good shift invariance (and wavelet smoothness) required that cascaded multirate filters’ frequency response side lobes be small (kingsbury (2001). figure 4. a) synthetic seismic section, b) synthetic seismic section after dddtdwt filtering figure 5. a) the 20th signal’s frequency spectrum before (green) and after (red) dddtdwt filtering. sa m bl e n o. 200 400 600 800 1000 1200 1400 1600 1800 2000 100 200 300 400 500 trace no. sa m bl e n o. 200 400 600 800 1000 1200 1400 1600 1800 2000 100 200 300 400 500 trace no. input shotgather filtered shotgather a m pl it ud e 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 20 40 60 80 100 120 frequency (hz) amplitude spectrum of input data amplitude spectrum of output data (a) (b) ali reza goudarzi and mohammad ali riahi36 after applying the method to synthetic data, the method’s performance was investigated on real shot gathers. the real data had 1.92 second record length with 2 ms sampling interval, and the trace interval was 20m. ground roll frequency band in this example was estimated to be 3-12 hz by f-k mapping. dddtdwt was applied to attenuate ground roll (figure 6); this method had an acceptable result and noise became significantly reduced. events in a deeper part of the section had more continuity and signal distortion did not happen with this filtering technique. the signal was still visible and most ground-roll energy was attenuated. figure 7 shows the 20th spectrum’s data trace amplitude and its filtered section. the second type of data used here was a stacked section having 2 ms sampling interval, distance between adjacent traces being 25 m. record length was 1.92 seconds or 960 samples. the wavelet transform method’s efficiency was then tested on shot gather for remnant noise reduction of post stack data. all shot gather noise was attenuated and true velocity analysed to give the best results; the data was then stacked without ground roll attenuation using these velocity values (figure 8a). residual noise in post stack data occurred in some field data examples with complicated behaviour of ground roll or complex geological areas being generated; it might thus be of interest for geophysicists to investigate filtering methods’ behaviour in this respect. figure 8 shows noisy and denoised sections. comparing the sections shown in figure 8, it can be seen that the reflections were quite recovered without distortion, and horizontal continuity was extended and improved by using the dddtdwt method. it is thus worth emphasising that dddtdwt can be used to attenuate ground roll, having an application to both synthetic and real data. the algorithm produced satisfactory results regarding the preservation of wavefigure 6. a) real seismic section, b) real seismic section after dddtdwt filtering figure 7. the 20th signal’s frequency spectrum before (green) and after (red) dddtdwt filtering of real data. input shotgather filtered shotgather sa m bl e n o. 100 200 300 400 500 600 700 800 900 10 20 30 40 50 60 trace no. sa m bl e n o. 100 200 300 400 500 600 700 800 900 10 20 30 40 50 60 trace no. (a) (b) a m pl it ud e 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 50 100 150 200 120 frequency (hz) amplitude spectrum of input data amplitude spectrum of output data adaptive seismic ground roll attenuation using the double density dual tree discrete wavelet transform (dwt) method 37 form, frequency and bandwidth and improved resolution of seismic data. filtered output had better s/n ratios and reflected signals’ trace-to-trace continuity. conclusions the dddtdwt method was used for seismic ground roll attenuation in this study. dddtdwt is an efficient way to filter seismic data; this method does not suffer from conventional dwt’s limitations. it can be quickly applied to a large volume of data and coherent noise in seismic data was successfully attenuated with this type of discrete wavelet transform. the wdga is an efficient and applicable method for seismic denoising and could be a substitute method for thresholding. wdga gains from two transforms: a regular transform (e.g. f-k) and discrete wavelet transform; the latter overcame fourier restrictions. this denoising method preserved signal information and generated little processing-related noise; filtering partly reduced noise. the advantage of dddtdwt, in conjunction with wdga, was that ground roll was suppressed by insignificant distortion of a particular signal. a reflection seismogram’s vertical resolution was not affected by applying dddtdwt, as happens when using frequency filters. acknowledgments we would like to thank the university of tehran’s institute of geophysics and the earth physics department which supported us in this study. we appreciate the kind advice given by professor kingsbury who helped us in this study. we would like to offer our regards and blessings to all who supported us in any respect during the completion of the study. references abry, p. 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gcalexandere@unal edu co abstract disseminated deposits are low-grade, large tonnage deposits which are mainly mined for gold, copper, molybdenum and tin these deposits are normally associated with intermediate to acid plutonic intrusive rocks and all are characterized by intense, extensive hydrothermal alteration of host rocks disseminated deposit exploration in the tropics suffers from difficult geological mapping due to dense vegetation, lack of rock outcrops and extensive but variable saprolytic weathering geophysical exploration of these deposits normally includes magnetic, induced polarization and gamma ray spectrometry surveys the largest rtp total magnetic field anomaly highs are usually associated with diorite porphyry intrusive bodies diorites are usually less magnetic than basaltic rocks; thus when magnetic anomaly highs are related to intrusions they can be explained by basic metals’ intensive mineralization (including magnetite) associated with porphyry intrusions magnetic anomaly gradients regularly help delineate local discontinuities which can be interpreted as local faults and joints that can be mineralized gamma ray spectrometry is useful for determining anomalous potassium, uranium and thorium concentration as it can map alteration halos and linear discontinuities statistical analysis of the data also can determine the intermediate intrusive / extrusive nature of porphyry intrusions gamma ray spectrometry can help to determine elemental potassium concentration, regardless of the associated potassium mineral species, thereby enabling alteration mapping in a geological setting related to volcanic-associated massive sulphide base metal and gold deposits an induced polarization survey is useful in outlining disseminated sulphide distribution in porphyry deposits related to chargeability anomaly highs the results of the quinchía project’s geophysical exploration program are discussed resumen los depósitos de sulfuros diseminados son depósitos de bajo tenor y alto tonelaje, los cuales son importantes fuentes en la explotación de oro, cobre, molibdeno y zinc estos depósitos están asociados a rocas plutónicas intermedias a acidas y se caracterizan por una intensa y extensa alteración de la roca huésped o encajante la exploración de estos depósitos en las zonas tropicales presenta una gran dificultad debido a la densa vegetación, escasez de afloramientos y alteración saprolítica variable la exploración geofísica de los depósitos de sulfuros diseminados, por lo general incluye la aplicación de los métodos de magnetometría, polarización inducida y espectrografía de rayos gamma las anomalías positivas magnéticas, reducidas al polo, más intensas usualmente están asociadas a la presencia de cuerpos intrusivos porfiríticos de dioritas estos cuerpos dioríticos son por lo general menos magnéticos que las rocas basálticas que intruyen, por lo tanto, cuando las anomalías magnéticas positivas están asociadas a las intrusiones se debe a la intensa mineralización de metales básicos (incluyendo magnetita) asociada con las intrusiones los gradientes de las anomalías magnéticas permiten delinear discontinuidades locales las cuales son interpretadas como fallas locales y diaclasas que pueden estar mineralizadas la espectrografía de rayos gamma es útil para determinar anomalías en la concentración de potasio, uranio y torio permitiendo la delimitación y mapeo de halos de alteración y detección de discontinuidades lineales el análisis estadístico de los datos de rayos gamma también permite determinar la naturaleza intrusiva o extrusiva de las intrusiones la espectrografía de rayos gamma permite determinar la concentración de potasio sin implicar la presencia de minerales de potasio, permitiendo el mapeo de las zonas de alteración en un palabras claves: depósitos diseminados, estovercas, exploración magnética, espectrografía de rayos gamma, polarización inducida, cordillera occidental, colombia keywords: disseminated deposit, stockwork, magnetic exploration, gamma ray spectrometry, induced polarization, western andes, colombia record manuscript received: 16/02/2012 accepted for publications: 03/05/2012 orlando hernández pardo, gretta c alexander e and iraida m pintor b 12 introduction disseminated deposits are low-grade, large tonnage deposits which are mainly mined for gold, copper, molybdenum and tin (evans, 1993; ariosa, 1990) the ore minerals in these deposits are scattered throughout host rock as what is called disseminated mineralization or they are largely or wholly restricted to quartz veinlets forming a ramifying complex called stockwork; both forms of mineralization occur in many deposits many alkaline and calc-alkaline porphyry au-cu deposits have extensive potassic hydrothermal alteration halos, varying both laterally and vertically, with changes in pressure, temperature, eh and ph during magmatic, hypogene and subsequent supergene processes (wilson, 1986) a well established alteration-mineralization potassic zoning sequence may be evident within a single deposit, ranging from a central, orthoclase and/or biotite core, sericite as fracture-controlled and pervasive replacements, outwards through successive phyllic, sericitic, argillic and propylitic areas although phyllic areas may contain less bulk potassium gain than potassic cores, their peripheral distribution usually offers much larger targets for gamma ray spectrometry detection a number of potassium anomalies are directly associated with intermediate intrusive bodies and their associated alteration systems within porphyry copper belts potassium anomalies’ proximity to mineralization provides evidence of significant mineralizing event-associated fluid/rock interaction being able to map and quantify this potassium alteration has important implications for porphyry gold and copper exploration and mapping (shives et al , 1997) the study area is located at the southern end of the mid-cauca gold belt along the romeral fault system in colombia, north-western south america (figure 1) tertiary gold and cooper mineralised intrusive bodies have been mapped in this area various miocene intrusive centres have been identified (i e la cumbre, manzanillo and dos quebradas) holding gold and copper mineralization additional targets have been identified from comprehensive ground geophysics, geochemical and field mapping programs (www baterogold com, 2012) this paper describes the application of an integrated geophysical exploration program based on contrasting magnetic susceptibility, chargeability, electrical resistivity and th, u and k concentration between the porphyry intrusions and host rocks the results were useful for modelling figure 1. location map of the study area on the eastern flank of the western cordillera of the andes mountains, colombia, north-western south america the quinchia project is situated at the southern end of the mid-cauca gold belt, within marmato and la colosa porphyry gold projects the local geology consists of cretaceous basaltic host rock from the barroso formation intruded by tertiary mineralized porphyry bodies such lithology is partially covered by pyroclastic rocks from the combia formation these rock units have been affected by intense fracturing mid-cauca gold belt quinchia project ambiente geológico relacionado con sulfuros de metales bases y depósitos de oro asociados a sulfuros masivos volcánicos el estudio de polarización inducida es útil para delimitar la distribución de sulfuros diseminados en los depósitos porfiríticos asociados a valores anómalos positivos de cargabilidad los resultados del programa de exploración geofísica integrada del proyecto quinchia son discutidos geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 13 mineralized bodies’ 3d geometry and recommending a drilling program to estimate the mining project’s gold and associated mineral reserves ground geophysical survey the quinchía project is located about 190 kilometres wnw of the colombian capital, bogotá, at the southern end of the mid-cauca gold belt (figure 1), within 100 kilometres of two world-class gold deposits: marmato (20 km n) having 12 4 million ounces of gold at a 0 3g/t cut-off and la colosa (100 km s) having 16 27 million ounces of gold at 0 3g/t cut-off (pintor, 2011; www.baterogold.com, 2012) this belt also hosts other gold and copper porphyry deposits, including titiribi, la mina and quebradona compared to the projects noted above, quinchía has been the least explored in the mid-cauca gold belt the quinchía concessions cover 1,407 hectares three miocene intrusive centres (la cumbre, el centro and dos quebradas) have been identified as having a north-south trend and strike figure 2. ground geophysics data acquisition array with 24 ns profiles (50m line spacing), magnetic and gamma ray spectrometry station (10m spacing) and pole–pole ip electrode spacing (50m to 350m) extension of around three kilometres at 1,600 and 1,950 meters above sealevel these centres all host gold and copper mineralization all mineralised areas identified to date form part of a large, regional mineralizing system covering more than 300 hectares la cumbre porphyry is a high-priority area, which continues to be open at depth to the north-northwest and south-southeast significant elevated gold mineralization from near surface and at depth is present in la cumbre porphyry a geophysical exploration program was carried out along 24 north–south profiles having different lengths (2 4 km predominant length), 50m line spacing and 10m station spacing for gamma ray spectrometry and magnetic and pole–pole array for an induced polarization survey (angoran and madden, 1977; bleil, 1953: ward, 1990: ward et al , 1995) with 50m dipole spacing for a total of 57 6 km the survey covered a 2 4 km long by 1 2 km wide area (2 88 km2) (figure 2) gamma ray spectrometry 5,417 stations were acquired using a pico envirotec pgis21 spectrometer the gamma ray spectrometric results were represented by four maps: total count (figure 3a), potassium (figure 3b), equivalent uranium (figure 3c) and equivalent thorium (figure 3d) to present the four primary measured variables four additional maps were produced to represent derived products eth/k (figure 4a), eu/eth (figure 4b), eu/k (figure 4c) ratios and ternary k-eu-eth ratio (figure 4d; keary & brooks 1992; lowrie, 2007; telford et al , 1994) oasis montaj mapping software was used for data processing (geosoft, 2010) magnetic survey magnetic readings were taken every 10 meters with two (base station and rover) scintrex envi proton precession magnetometers the current international geomagnetic reference field model (igrf2011) parameters for the study area were f=31822 5 nt total magnetic field, i=+30 47° inclination an d=-5 28° declination magnetic rover and base data was processed in a number of steps (keary & brooks 1992; lowrie, 2007; telford et al , 1994; merrill, 2010) diurnal variation was removed from the raw magnetic data thereby obtaining a total magnetic field intensity map the regional total magnetic field intensity map was obtained by data interpolation of the igrf11 values total magnetic field anomaly map (tfma) values were obtained by subtracting regional igrf tfmi from tfmi (figure 5a) a reduction-to-the-pole (rtp) filter was applied thereby obtaining the rtp total magnetic field anomaly map (cooper and cowan, 2005; figure 5b) to simplify magnetic anomaly dipolar asymmetry regarding survey area magnetic inclination and declination other frequency domain filters were applied to the total magnetic field intensity map including analytic signal (figure 5c) only the longer wavelength components were taken for modelling magnetic sources from the surface to hundreds of meters depth euler 3d depth solutions were obtained by calculating the first derivative in x, y and z and their fast fourier transforms (fft) to obtain point data solutions for structural indexes for sphere and barrel causative bodies the database was represented as proportional symbols for depth solutions (figure 5d) induced polarization the ip survey was carried out by taking readings every 50 meters, using pole-pole (2-array) electrode configuration, with a plotting point at mid-distance between moving electrodes c1 and p1; successive “a” 50m, 100m, 150m, 200m, 250m, 300m and 350m spacing was used to measure chargeability and electrical resistivity the transmitter was an iris vip4000, a 220v-60hz motor generator energising field with direct current (time-domain) with 2” pulse duration, a maximum available 3,000 volts tension and applied current up to 3a true intensity the receiver was orlando hernández pardo, gretta c alexander e and iraida m pintor b 14 an iris elrec pro with 1mv resolution pole–pole time domain-induced polarization chargeability and resistivity data was interpreted regarding the ore deposit model to indicate possible alteration areas associated with porphyry style copper and gold mineralization chargeability and resistivity data were 3d modelled and displayed in a series of chargeability and resistivity maps at 310m, 260m, 210m, 170m, 100m, 50m and 20m depth discussion the total count map (figure 3a) showed three areas a, b and c, where areas a and c were interpreted as geophysical units associated with the recognition of alteration related to intrusive bodies located in topographic highs which were partially outcropping and had been differentially weathfigure 3. (a) total count map (cps) showing areas a, b and c and local discontinuities, (b) potassium content (%) showing areas a, b and c and local discontinuities, (c) uranium content (ppm) showing interpreted local discontinuities associated with local faults, (d) thorium content (ppm) showing local discontinuities (a) (c) (d) (b) geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 15 ered while area b was interpreted as the intrusive bodies’ host rock located in topographic lows some linear patterns were recognized, inferring the presence of geological discontinuities or local faults the k map (figure 3b) shows the same three areas a, b and c observed on the total count map indicating the dominance of k in the radioactive elements the spatial distribution of k was interpreted as k-alteration related to intermediate intrusive rocks close to the surface the areas of intense k-feldspar enrichment were less directly associated with porphyry gold and copper mineralization than sericitic and chloritic alteration; potassium anomalies’ proximity to mineralization provided evidence of significant fluid/rock interaction associated with mineralization areas a and c were interpreted as potassium alteration providing powerful exploration guidance for locating mineralised areas figure 4. (a) th/k x 10-4 ratio showing low areas (blue) associated with evident alteration in porphyry gold and copper belts, (b) eu/eth ratio showing local discontinuities (yellow) affecting the area’s lithological units, (c) eu/k x 10-4 ratio showing low ratio areas (blue) associated with potassium alteration, (d) ternary k-eueth ratio showing the predominance of potassium in the southern area while eu and eth are dominant in the northern area (a) (c) (d) (b) orlando hernández pardo, gretta c alexander e and iraida m pintor b 16 the uranium map (eu; ppm; figure 3c) shows several high frequency local anomalies peripheral to the potassium anomalies and related to numerous small discontinuities which could be associated with mineralized veins this pattern suggested that the uranium moved laterally away from the hydrothermal centre of the system some linear patterns were recognized, inferring the presence of geological discontinuities or local faults orientated ne-sw, nw-se and n-s the thorium map (eth; ppm; figure 3d) shows several high frequency local anomalies similar to those recognized on the uranium map, inferring the presence of geological discontinuities or local faults orientated ne-sw, nw-se and n-s as thorium enrichment does not usually accompany potassium during hydrothermal alteration, the eth/k ratio map provided an excellent distinction between potassium associated with alterafigure 5. (a) total magnetic field anomaly map showing tfma highs and lows, (b) reduced to pole total magnetic field anomalies showing prospective rtp-tfma highs and lows and magnetic gradients interpreted as local discontinuities, (c) analytic signal of the total magnetic field intensity values, (d) proportional euler depth solution symbols for pole total magnetic field anomalies and discontinuities interpreted from the rtp–tfma map depth solutions are concentrated in the first 200 m (a) (c) (d) (b) geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 17 figure 6. (a) chargeability model at 310m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing with six chargeability anomaly highs (a1 to a6) related to sulphur mineralization and local discontinuities, (b) resistivity model at 310 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing four resistivity highs (h1 to h4) and four resistivity lows (l1 to l4) the lows are related to sulphur mineralization and altered mineralised fracture areas figure 7. (a) chargeability model at 260m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing showing nine chargeability anomaly highs (b1 to b9) related to sulphur mineralization and local discontinuities, (b) resistivity model at 260 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing five resistivity highs (h1 to h5) and four resistivity lows (l1 to l4) the resistivity lows are related to sulphur mineralization of diorite porphyry intrusion and altered mineralised fracture areas (a) (a) (b) (b) orlando hernández pardo, gretta c alexander e and iraida m pintor b 18 figure 8. (a) chargeability model at 210m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing showing nine chargeability anomaly highs (c1 to c9) related to sulphur mineralization and local discontinuities, (b) resistivity model at 210 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing eight resistivity highs (h1 to h8) and four resistivity lows (l1 to l4) the resistivity lows are related to sulphur mineralization of diorite porphyry intrusion and altered mineralised fracture areas figure 9. (a) chargeability model at 170m depth in mv/v obtained from pole-pole array with 50m to 350m electrode spacing showing twelve chargeability anomaly highs (d1 to d12) related to sulphur mineralization and local discontinuities, (b) resistivity model at 170m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing eleven resistivity highs (h1 to h11) and seven resistivity lows (l1 to l7) the resistivity lows are related to sulphur mineralization of the diorite porphyry intrusion and altered mineralized fracture areas (a) (a) (b) (b) geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 19 figure 10. chargeability model at 100m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing showing 25 chargeability anomaly highs (e1 to e25) related to sulphur mineralization and local discontinuities, (b) resistivity model at 100 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing 37 resistivity highs (h1 to h37) and 24 resistivity lows (l1 to l24) the resistivity lows are related to sulphur mineralization of the diorite porphyry intrusion and altered mineralized fracture areas (a) (b) tion and anomalies related to normal lithological variations (figure 4a) this important correlation of low eth/k ratio with alteration was evident in porphyry gold and copper belts the eth/k map ratio showed the characteristic lows interpreted as alteration associated with mineralization produced by potassium, this being clearly differentiated from normal lithology the potassium alteration was more intensive in shear hosted gold deposits the strong potassium anomaly was characterized by less than 2 5 x 10-4 eth/k ratio values unaltered lithologies typically reflect the normal k and th crustal abundance ratio (5 x 10-4) thorium does not usually accompany potassium during potassium alteration; however, the resulting low eth/k ratio thus enabled distinguishing potassium anomalies having exploration significance from those solely related to lithological variations uranium enrichment, evident on the eu/eth ratio map, was peripheral to the potassium anomaly and related to numerous small pitchblende veins (figure 4b) such mineralization may have shown that uranium moved laterally away from the system’s hydrothermal centre local discontinuities infer an intensive fracture system that has affected intrusive intermediate porphyry intrusions uranium enrichment, evident on the eu/kx10-4 ratio map, was peripheral to the potassium anomaly and related to mineralized veins (figure 4c) this mineralization may have shown that uranium moved laterally away from the system’s hydrothermal centre local discontinuities infer an intensive fracture system that has affected the intrusive intermediate porphyry intrusions the ternary k-eu-eth radioelement map (figure 4d) was a k-eu-eth ternary colour presentation which is useful in outlining the areal extent of the potassium alteration but ambiguous for interpreting local discontinuities a ground magnetic survey is useful for modelling the surface and hidden geology associated with eocene gold and cooper mineralised intrusive bodies and related mineralization and structural features affecting them magnetic data has been useful for detecting magnetic susceptibility contrast among diorites, basalts, pyroclastic rocks, breccias, mineralised areas and halos of alteration thereby helping to define drill targets together with the results of radiometry and induced polarization surveys the extreme maximum and minimum values were related to cultural anomalies tfma highs (~1000 nt) were concentrated in the southern region interpreting magnetic field data at low magnetic latitudes is difficult because the vector nature of the magnetic field increases the complexity of anomalies from magnetic rocks magnetic data is reduced to the magnetic pole to simplify the observed anomalies the rtp total magnetic field anomaly map (rtp-tfma) (figure 5b) showed that the largest magnetic anomaly highs were located in the southern region of the study area, in la cumbre and matecaña sectors having values ranging from 1600 nt to 2000 nt these anomalies were attributed to diorite porphyry intrusive bodies which are intensively mineralised intermediate anomaly highs, having values greater than 2000nt, were observed in the central region in mandeval, la lenguita and el cedral these anomalies were attributed to diorite porphyry intrusive bodies that seemed to be smaller in size than those from la cumbre and matecaña anomaly areas but could be more mineralised, as inferred from their higher amplitudes the smallest magnetic anomaly highs were located in the northern region in dos quebradas and san luis, having anomalies similar in size and amplitude regarding those from mandeval, la lenguita and el cedral but more isolated, separated by anomaly lows rtp-tfma magnetic anomalies gradients were interpreted as two local discontinuity systems orientated ne-sw and nw-se these discontinuities could be associated with local faults and joints affecting the intrusive bodies the tfmi map’s vertical integration analytic signal (macleod et al., 2004; figure 5c) showed that 3-d analytic signal amplitude produced maxima over magnetic contacts regardless of the direction of magnetisation the absence of magnetization direction in the shape of analytic signal anomalies was a particularly attractive characteristic for interpreting magnetic field data near the magnetic equator although analytic signal amplitude was dependent on magnetization strength and the direction of geological strike regarding the magnetization vector, this dependency was orlando hernández pardo, gretta c alexander e and iraida m pintor b 20 figure 11. (a) chargeability model at 50m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing showing 55 chargeability anomaly highs (f1 to f55) related to sulphur mineralization and local discontinuities, (b) resistivity model at 50 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing 59 resistivity highs (h1 to h59) and 36 resistivity lows (l1 to l36) the resistivity lows are related to sulphur mineralization of the diorite porphyry intrusion and altered mineralized fracture areas figure 12. (a) chargeability model at 20m depth in mv/v obtained from a pole-pole array with 50m to 350m electrode spacing showing 82 chargeability anomaly highs (g1 to g88) related to sulphur mineralization and local discontinuities, (b) resistivity model at 20 m depth in ohm-m obtained from a pole-pole array with 50m to 350m electrode spacing showing 104 resistivity highs (h1 to h104) and almost 100 resistivity lows related to sulphur mineralization of the diorite porphyry intrusion and altered mineralised fracture areas (a) (b) geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 21 easier to deal with when interpreting analytic signal amplitude than original total field data or pole reduced magnetic field euler 3d deconvolution processing was applied to automatically locate and determine depth for gridded rtp total magnetic field anomalies the euler depth solutions were obtained by calculating the first derivative of the rtp tfma in x, y and z directions and applying the fast fourier transform to estimate the depth location of the bodies causing local rtp tfma anomalies the point data solutions were mainly from 0m to 170m in depth these depth solutions were represented by proportional symbols (figure 5d) showing that the main mineralised areas were located from the surface to the first 170 m depth (32m to 50m depth) shallow depth solutions from 0m to 30m-50m could be associated with the weathered–fresh rock interface the chargeability maps at 310m (m310; figure 6a), 260m (m260; figure 7a), 210m (m210; figure 8a), 170m (m170; figure 9a), 100m (m100; figure 10a), 50m (m50; figure 11a) and 20m depth (m20; figure 12a) showed several chargeability anomaly highs located within the cumbre, matecaña, mandeval-la lenguita, el cedral, san luis and dos quebradas target areas, respectively these chargeability anomaly highs were related to sulphur mineralization of diorite porphyry intrusions the resistivity maps at 310m (r310; figure 6b), 260m (r260; figure 7b), 210m (r210; figure figure 13. (a) chargeability discontinuities interpreted at depths from 310m to 20 m compiled from figures 2 to 8 showing the intensive fracturing system characterized by fewer deep fractures split up into branches (ramified) at shallower depths, (b) resistivity discontinuities interpreted at depths from 310m to 20 m compiled from figures 9 to 15 showing the intensive fracturing system characterized by few deeper fractures split up into branches (ramified) at shallower depths figure 14. (a) 3d view from the sw of the quinchía mining project showing chargeability data distribution red and pink areas are related to sulphur mineralization having economic interest, (b) 3d view from the se of the quinchía mining project showing chargeability data distribution red and pink areas are related to sulphur mineralization having economic interest orlando hernández pardo, gretta c alexander e and iraida m pintor b 22 8b), 170m (r170; figure 9b), 100m (r100; figure 10b), 50m (r50; figure 11b) and 20m depth (r20; figure 12b) showed several resistivity highs associated with diorite rocks without significant mineralization while resistivity lows were related to sulphur mineralization of diorite porphyry intrusions and mineralised fracture areas with intensive alteration the induced polarization survey was useful in outlining sulphide distribution in porphyry deposits related to chargeability anomaly highs the survey identified few major chargeability highs and few discontinuities at depths from 170m to 310m and several small chargeability highs and several discontinuities at depths from 20m to 100m depth these chargeability highs and discontinuities suggested that diorite porphyry intrusion had been intensively fractured and sulphur mineralization had been emplaced throughout these fracture areas (figure 13a, b) interpreting inferred structural features was complex most linear features did not coincide with 20m to 310m or the interpreted applied radiometry, magnetic and ip methods however, the interpreted linear features showed that intrusive bodies and host rocks had been intensively fractured, allowing the flow of mineralized solutions these linear features’ vertical and horizontal offset also suggested that most of them were dipping at high angles only a few of these linear features seemed to be vertical the 3d target distribution inferred a 3d model of intensively fractured porphyry rocks which were also intensively altered at shallow depths from the surface to 20m and 50m depth (figure 14) chargeability anomalies were locally coincident with mapped diorite porphyry rocks at la cumbre, matecaña, la lenguita, el cedral, mandeval, san luis and dos quebradas and were also coincident with gold soil geochemical anomalies at la cumbre and were proximal to electrical resistivity lows interpreting the geophysical data was based on discussions regarding quinchia geology; mineralization models including both sulphides and magnetite (chargeable) could have been associated with cu and au mineralization and the presence of cu and au mineralization between 170m and 300m inferred a possible characteristic geophysical response for alteration and mineralization consisting of a moderate resistivity / resistivity low, coincident with moderate chargeability high / edge of a chargeability high (brett, 2007; figure 15) conclusions gamma ray spectrometry has been useful in determining the anomalous potassium, uranium and thorium concentration interpreted as alteration halos and linear discontinuities, associated with intermediate intrusive / extrusive rocks (dickson and scott, 1997) the total counts, k, th/ kx10-4, u/kx10-4 and ternary k-u-th maps clearly showed areas a and c, located in the south and northwest, respectively, as being the most prospective areas the u, th, and u/th maps revealed linear gradients which were interpreted as local discontinuities associated with local fault and/or joint systems orientated ne – sw and n-s, which were also prospective for basic and precious metals depth estimates of the bodies causing magnetic anomalies showed that the main mineralized areas were located from the surface to the first 170 m depth (32m to 50m depth average mean value) targets were identified on all induced polarization maps, with priority target areas identified for immediate drilling to test and develop the exploration model these targets included deeper and shallower chargeable areas, having apparent line-to-line continuity the highest priority targets were interpreted to conform to an idealized model for sulphide or magnetite associated alteration areas which might be associated with gold mineralization and are recommended for follow-up via drilling further geophysical, geological and geochemical data integration work and model development is recommended to expand the quinchía mining project’s potential the induced polarization survey has also been useful in outlining sulphide distribution in porphyry deposits related to resistivity anomaly lows the induced polarization survey identified few major resistivity lows and few discontinuities at depths from 170m to 310m and several figure 15. idealised porphyry model with key mineralization types postulated intensive fracturing is indicated which would be associated with cu, mo, au, ag in alteration areas and au, ag in veins (brett, 2007, adapted from sinclair, geological survey of canada) this model can be adapted to explain the chargeability, resistivity, magnetic and radiometry anomalies modelled in the quinchía mining project geophysical exploration of disseminated and stockwork deposits associated with plutonic intrusive rock: a case study on the eastern flank of colombia’s western cordillera 23 small resistivity lows and several discontinuities at depths from 20m to 100m depth these resistivity lows and discontinuities suggested that the diorite porphyry intrusion has been intensively mineralized and altered throughout these fracture areas the chargeability anomaly highs and resistivity anomaly lows that were outlined at each depth solution map were spatially superimposed and inversely correlated the chargeability highs had values ranging from 35 mv/v and exceptionally up to 170mv/v to 75mv/v against a background of 20 mv/v related to intermediate to high grade sulphur mineralization the highest mean chargeability values and most associated mineralized areas were located at 170m depth the resistivity lows had values from 7 ohm-m to 300 ohm-m related to sulphur mineralization and altered igneous rocks it was concluded that about 40% of the studied area covered by the induced polarization survey had the potential to host porphyry style alteration areas and sulphide areas which might be mineralized with base and precious metals in the porphyry intrusion of la cumbre, matecaña, la lenguita, el cedral, mandeval, san luis and dos quebradas the 3d target distribution inferred a 3d model of intensively fractured porphyry rocks which were also intensively altered at shallow depths from the surface to 20m and 50m depth it should be noted that the chargeability anomalies had moderate to high intensity (up to 170mv/v against a background of 30mv/v) and were interpreted here to be consistent with the intermediate sulphide concentrations typical of low-grade porphyry targets acknowledgements elements of this research were produced with support from minera quinchía sas, tep ltda and the universidad nacional de colombia’s geosciences department’s geophysics group we thank the anonymous reviewers for their comments, corrections and contributions references angoran, y , and madden, t r , 1977, induced polarization: a preliminary study of its chemical basis: geophysics, 42, 788-803 ariosa, j 1990 curso de yacimientos minerales metálicos, editorial pueblo y educación la habana, cuba, 306 p bleil, d f , 1953, induced polarization: a method of geophysical prospecting: geophysics, 18, 636-661 brett, j s 2007 report on the interpretation of induced polarization data, kenville gold project, prepared for anglo swiss resources, mhp geophysical interpretation report, p 1-25 cooper, g r j cowan r d, 2005 differential reduction to the pole computer & geosciences, elsevier 31 (2005) 989–999 dickson b l and scott k m 1997 interpretation of aerial gamma-ray surveys – adding the geochemical factors agso journal of australian geology and geophysics 17:187–200 evans, a m 1993 ore geology and industrial minerals: an introduction geosciences texts, blackwell scientific publications, 389 p geosoft, 2010, geosoft oasis montaj: software for earth sciences mapping and processing (www geosoft com/) keary, p and brooks, m 1992, an introduction to exploration geophysics, second edition, blackwell series, london, 254 pp lowrie, w 2007 fundamentals of geophysics, second edition, cambridge university press, new york, usa macleod, i n , jones k, ting fan dai, 2004 3-d analytic signal in the interpretation of total magnetic field data at low magnetic latitudes geosoft technical notes merrill, r t 2010 our magnetic earth the science of geomagnetism the university of chicago press 261 p pintor, b i m , 2011 exploración geofísica integrada de los pórfidos auríferos del sector de la cumbre, municipio de quinchía, risaralda, colombia tesis msc en ciencias geología, universidad nacional de colombia, shives, r b k , b w charbonneau, ken l ford, 1997 the detection of potassic alteration by gamma ray spectrometry recognition of alteration related to mineralization, radiation geophysics section, mineral resources division geological survey of canada telford, w m , geldarty, l p , sheriff, r e , 1994, applied geophysics second edition, cambridge university press, new york, 770 pp ward, s h , 1990, resistivity and induced polarization methods, in ward, s h , , geotechnical and environmental geophysics: volume i, review and tutorial, soc explor geophys , 147-190 ward s h , sternberg b k , labrecque d j , and poulton m m , 1995, recommendations for ip research: the leading edge, april 1995, 243-247 wilson, m 1989, igneous petrology, winchester, unwyn hyman, 466 pp (www baterogold com, 2012) articulo 1.indd earth sciences research journal surface geophysics earth sci. res. sj. vol. 15, no. 2 (december, 2011): 81 88 application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina armando luis imhof1, manuel sánchez2, carlos calvo3 and adriana martín4 1 instituto geofísico sismológico volponi / facultad de ciencias exactas físicas y naturales. unsj. ignacio de la roza y meglioli. rivadavia. c.p. 5400, san juan, república argentina. fax: (54) 264 4234980. tel.: 54 264 4945015. e-mail: aimhof@unsj.edu.ar 2 instituto de mecánica aplicada. facultad de ingeniería. unsj. e-mail: msanchez@unsj.edu.ar 3 departamento de matemática aplicada. facultad de ingeniería. unsj. e-mail: ccalvo@unsj.edu.ar 4 departamento de informática. facultad de cs. exactas, fcas y naturales. unsj abstract a geophysical survey involving seismic refraction tomography (srt) for mapping ‘p’ waves was carried out in sierra santa clara, san juan province, argentina in july 2009. the purpose of the geophysical survey was to determine the degree of fracturing and the rigidity of the rock mass through which it is planned to build a 290 m long road tunnel traversing the mountain almost perpendicular to the axis thereof, at around 100 m depth from the summit. several difficulties arose from the operational point of view which made it almost impossible to conduct fieldwork in normal circumstances. firstly, the topography had almost 45º slopes and 100 m research depths which would have involved having had to use explosives to generate seismic waves reaching sensors which had sufficient signal-to-noise ratio for distinguishing them. legal restrictions regarding the use of explosives on the one hand and insufficient power when using hammer blows on the other made it necessary to design and build a gas-powered gun to achieve the minimum energy (2 kj) required for detecting seismic signals. secondly, using conventional interpretation methods involving layered models was inoperable in such geological structures; seismic tomography methods were thus used which make use of the velocity gradient concept (both lateral and in-depth). this allowed mapping subsurface velocity variations in the form of velocity contour lines. the methodology used with the new seismic waves’ source generator, as well as srt application in this type of geological structure, demonstrated that satisfactory results could be obtained for this kind of geophysical study for geotechnical purposes. resumen se llevó a cabo un estudio geofísico de tomografía sísmica por refracción (srt) con ondas compresionales en la sierra de santa clara, provincia de san juan, república argentina en julio de 2009. el propósito del estudio geofísico fue determinar las variaciones de velocidad de ondas p en el interior del macizo rocoso con el fin de inferir el grado de fracturación y rigidez del mismo, y proyectar la construcción de un túnel vial de 290 m de longitud, que atravesará la montaña de forma casi perpendicular al eje de la misma, a 100 m de profundidad desde la cumbre. varias dificultades se presentaron desde el punto de vista operativo, que tornaron altamente compleja la realización del trabajo de campo en circunstancias normales. en primer lugar las condiciones topográficas, con pendientes de 45º y profundidades de investigación de 100 m, que implicaba el uso de explosivos para generar ondas sísmicas que alcanzaran los sensores con suficiente relación señal-ruido para distinguirlas. restricciones legales respecto al uso de explosivos, por una parte, y la potencia insuficiente en caso de uso de golpe de maza por otra, hicieron necesario el diseño y construcción de un cañón impulsado por gas en busca de generar la energía mínima (2 kj) necesaria para la detección de las señales sísmicas. en segundo lugar, no fue posible la utilización de los métodos convencionales de interpretación que emplean modelos de capas, inoperable en este tipo de estructuras geológicas; por ello resultó imprescinpalabras claves: tomografía sísmica – sísmica refractiva – ondas de compresión – procesamiento de señales. keywords: seismic tomography, p wave, seismic refraction, compressional waves, signal processing. record manuscript received: 15/12/2010 accepted for publications: 30/11/2011 armando luis imhof, manuel sánchez, carlos calvo and adriana martín82 dible utilizar el método de tomografía sísmica que hace uso del concepto de gradientes de velocidades, tanto laterales como en profundidad. ello posibilitó mapear las variaciones de velocidad en subsuelo en forma de curvas de nivel. la metodología empleada con el nuevo sistema de generación de ondas sísmicas, así como la aplicación en este tipo de estructuras geológicas de la srt, demostraron la posibilidad de obtener resultados satisfactorios para este tipo de estudios geofísicos con fines geotécnicos. introduction conventional refraction inversion methods use a layered model approach. the subsurface is divided into a number of continuous constant velocity layers having velocities and thicknesses which are modified through interactive forward modeling in an effort to match traveltimes determined from the field data. several interpretation methods have been based on this premise (see among others jakosky, 1940; heiland, 1940; palmer, 1986; dobrin, 1975). these conventional methods require sections of traveltime curves to be mapped to refractors, a task that can be difficult in several geological situations. for example, the presence of extremely hard topographic conditions in mountains surely implies that there can be large and sudden changes in the shape of the bedrock. there may also be localised features such as voids that contradict the assumption of continuous constant velocity layers. the progress of computer technology has simplified calculus procedures, thereby leading to a great development in geophysical data processing methodologies, particularly for application to soil studies regarding civil engineering infrastructure. the amazing increase in microcomputer processing velocity during the last few years, added to their huge storage capability, has enabled the development of algorithms from different theoretical backgrounds, giving birth to refraction tomography (schuster et al, 1993; sethian, 2007; zhang et al., 1998). refraction tomography unlike conventional refraction methods, seismic refraction tomography (srt) does not require that the model be broken into continuous layers having constant velocity. instead, the model is made up of a large number of small constant velocity grid cells or nodes. inversion is performed by an automated procedure involving raytracing through an initial model and comparing the modelled traveltimes to the field data, and adjusting the model grid-by grid to match the modeled traveltimes to these. this process is iteratively repeated until a preset number of iterations has been reached. because there is no assumption of continuous constant velocity layers, srt can model localised velocity anomalies. figure 1. study zone. application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 83 another difficulty related to traditional seismic refraction methods for data interpretation is that they are only applicable to relatively gentle topography, as mentioned in previous paragraphs. several difficulties appear regarding geophysical studies in mountain areas, namely: • the terrain is extremely rugged; and • such material has great lateral variation, making layered model theory inapplicable. survey area location and general geology the study area was located in calingasta, sarmiento, in the southern sector of the province of san juan, argentina, and the district of las heras in the north of the province of mendoza and about 150 km southwards of the capital city of san juan, on the eastern slope of the front range and western edge of the pre-cordillera at an average height of 2,500 m above sea level (masl) (figure 1). the region’s climate can be described as semi-arid to arid, being of the mesothermal type. the present study was conducted in a mountainous area having an ancient marine environment, with the presence of high-angle folding sedimentary rocks, general lithology consisting of sandstones interbedded with siltstones. the area is characterised by the presence of two major mountain bodies, the west one belonging to the front range and the eastern one belonging to the pre-cordillera with a central mountain body, corresponding to the central and western pre-cordillera subprovinces (figure 2) the pedernal-los pozos hills form an outstanding feature of the eastern mountain range, extending 25 km nne-ssw and 8 km w-e. they are mainly made up of paleozoic age calcareous rocks, being more than 2,000 masl high. figure 2 shows the prevalent geological formations in the study area. the main formations are as follows: • the marquesado group (bordonaro, 1980), which includes la laja, zonda and la flecha formations and is composed of limestone, dolomitic limestone, marl, shale and chert; • the mariño formation, made up of sandstones and tuffs presenting strongly structured (high tilt) intercalated materials; and • the uspallata triassic formation, which is composed of medium to coarse-grained olive green metasandstone alternating with olive green shale (cortés et al., 1997). there is abundant folding and fracturing (jointing). the formations dip at a high angle towards the nw, where there are greater shale levels. basic theory refraction interpretation methods leads to seismic velocity determination (typically calculated from the first break pick of the p-wave) as part of the interpretation results. travel time inversion is nonlinear because a perturbation in the velocity field results in changes in the ray paths through it. linearised inversions involve local linearisation of such nonlinear problem. this makes them initial-model dependent. a poor choice of starting model could cause an inversion to converge to an incorrect solution at a local minimum, due to most algorithms looking for a solution close to the starting model. obviously, this hampers interpretation. global optimisation several investigators have developed algorithms that search for global minimum, instead of local ones. the algorithm used in this work for seismic tomography data interpretation used this type of scheme; it is known figure 2. geologic map. sector of exploration (from gonzález díaz y fauqué, 1993). armando luis imhof, manuel sánchez, carlos calvo and adriana martín84 as simulated annealing optimisation and it was developed by kirkpatrick et al., (1983) and bohachevsky et al., (1986). although the original theory was developed for statistical mechanics related to temperature, it soon became adapted to geophysical problems (e.g. chunduru et al., 1996). it follows the basic procedure given below: 1. travel times were computed through an initial model. there are several ways of doing this, basically grouped into two methods: straight or curved ray theory (cerveny, 2001) or numerically resolving the wave equation with some constraints, such us the eikonal equation (vidale, 1988). the latter option has two main advantages: it accounts for curved rays from all types of primary arrivals (i.e. direct ones, refractions or diffractions) and a ray tracing algorithm does not need to be used. there are several schemes for the numerical solution of the wave equation, most being grouped into two methods: finite differences and finite elements methods. here, a fast finite difference algorithm was used. the domain was divided into rectangular blocks or cells having a lateral extension generally half the distance between geophones and depth was defined by the interpreter. 2. the least-square error, e0, was calculated among the measured times (tmeas) and the first theoretical predicted ones (tpred). the leastsquare error was defined for any iteration it, which is named the objective function, as: where n is the number of observations and j denotes each observation. 2. the velocity model was then altered by repeating step 1 and the new least-squares error e1 was computed. 3. the new model was unconditionally accepted if its error was small (e1 <= e0 ), and conditionally accepted with probability: when e1>e0 . in the above equation, δe = e0 e1, t was temperature, q was an empirical parameter and emin global minimum objective function value (ideally, equal to zero). conditional acceptance of models having larger least square errors enabled the algorithm to escape from local minima in its search for the global minimum. (emin-e1)q made the probability of accepting a detrimental step towards local minima tend to zero as the inversion approaches the global minimum. 4. the procedure was repeated until the optimisation converged. the software performed thousands of iterations to obtain the best matches between: i) actual arrival times and distances between sources and receivers; and ii) modelled, calculated times based on ray paths through the finitedifference mesh. each finite-difference array element forming part of ray paths was characterised with a velocity value throughout the optimised interpretation. the extension of the area being researched was the part of the finite-difference mesh having optimised velocities. the interpretation results were typically presented in colour bar form, different colours representing different velocities. interpretation results could also be converted into spreadsheet files for further analysis. methodology figure 3 a-c shows a top view and cross-section, as well as an overview of the study area where source points t1 to t3 were located at the coordinates shown in table 1. figure 3. a) aerial view (schematic) of study zone with indication of seismic profiles, shotpoints (ti) and projected tunnel axis location. (b) lateral view (slice) showing geophone and shots’ spread. (c) photograph of the same sector. (a) (b) (c) (1) (2) application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 85 figure 3-b shows that the trace of the projected tunnel is about 100 m deep from the apex of the mountain, which made logistical tasks become very difficult. a 24-digital-channel seismic device was configured from 12 physical-channel recording equipment having a three pointed shot: a central one (t2) and two at both ends of the profile (t1 and t3). figure 3.b represents the seismic survey location along the hillside. due to the seismograph having 12 channels, shooting had to be repeated at each emission point (in addition to particular shotpoint’s corresponding stacks), moving the line of geophones each time to cover all 24 channels. the total layout length was 280 m with 5 m to 15 m spacing between stations (figure 4a). figure 4b shows the distribution of in-depth information content (coverage) regarding the rays with this three-shot device according to the topography in question. source wave system a new system for generating body waves (p and s), called tamper (imhof, 2009), was designed and implemented with the aim of generating waves at the corresponding shooting points (t1-t3, figure 3 a-b and 4). normally, a 5 kg to 10 kg hammer is used as p wave source, making it possible to generate power up to 0.2 kj (fernandez, 2000). this power scarcely allows reaching 20 m research depths, which was insufficient for this study. another option was to use explosives, which was then discarded as the work also required s wave generation for other kinds of measurements (not described here, see imhof, op.cit.). ordinary seismic wave generation systems use various energy sources, such as elastomeric bands (damper), gunpowder or compressed air buffalo-gun type (pullan et al., 1987), to name but a few. the excellent articles written by miller et al., (1986) and herbst et al., (1998) have compared several of them (see also figure 5). these systems are not very portable, and basically they only allow wave trains to be generated (since they emit vertically). the tamper works like a cannon loaded with nitrogen gas at high pressure (150 bars) as a source of energy that fires a 3 kg bullet at high velocity. the release of energy is variable and depends on the bullet’s trajectory length and gas volume in the chamber, being able to reach up to 20 kj. it can be placed horizontally to generate sh waves, as well. figure 5.b shows an image of this device. shotpoint geographical coordinates gauss-kruger latitude longitude x y t1 s32º08’07,1” 69º00’19,3’’ 6445378,60 2499494,14 t2 s32º08’05,0’’ 69º00’23,1’’ 6445443,28 2499394,53 t3 s32º08’03,0’’ 69º00’26,6’’ 6445504,88 2499302,79 figure 5. amount of energy necessary related to depth of investigation for seismic sources (from herbst et al, 1998). (a) table 1. seismic refraction shotpoint coordinates. (b) figure 4. (a) 24 channels refraction seismic survey array and distances among stations. (b) ray coverage for refraction tomography. armando luis imhof, manuel sánchez, carlos calvo and adriana martín86 figure 6. tamper. vertical-horizontal seismic wave generation system. vertical implementation. figure 7. (a) seismic record ensambled (from t1). (b) time-dist representation (t1 to t3). 2.5 kj were enough for this work, which allowed reaching a 100 m depth research (herbst et al., 1998) (figure 5.a) with a seismic device like that shown in figure 6. results time-distance graphs (dromochrones) six 500 msec. recording time records were obtained for p-waves (see figure 7.a; a typical record for t1). contiguous records for the same shot were assembled in pairs to build one of the 24 channels which allowed observing consistency among the first arrivals, thus facilitating choosing them. the tasks of extracting information from records and transferring it to the time distance graphs were very complicated due to the poor quality of the seismic signals, resulting from prevailing weather conditions (wind) in the study area combined with the terrain’s extreme unevenness which hampered geophone deployment. pre-processing records in seg-1 format were transformed into ascii with seg1to-ascii software prepared by the working group; each signal was then processed separately with bandpass filters. the whole record was reconstructed to enable choosing them more accurately and achieve a more adjusted dromochrone. filtering algorithms (fourier transforms) were prepared in mathcad and matlab platforms. after pre-processing and choosing all records, data were represented in the form of time-distance graphs (dromochrones; figure 7.b). (a) (b) application of seismic refraction tomography for tunnel design in santa clara mountain, san juan, argentina 87 input information required by the seismic modelling software consisted of the device’s geometry (figure 4.a) and the first arrivals (i.e. dromochrones). the grid size for finite difference modelling depended on the tomogram’s desired resolution; the highest was used here, so minimum grid size was considered, applying the expressions (pullammanappallil et al, 1994): hx = 0.375 . receiver spacing hz = 0.1875 . receiver spacing figure 8 shows contour lines that represented boundaries between compressional wave velocity gradients. discussion the area being investigated consists of coarse-grained sandstones intermixed with dark shale (the latter most noticeable on the nw side), having bz = 65º nw and az = 100º. the rock is extremely fractured (altered). regarding shale, it should be noted that it has a wide range of seismic wave velocity variation, according to the degree of fracturing. for example, velocity can reach up to 1,500 m/s when weathered, increasing significantly up to 5,000 m/s if not having undergone external alteration. the seismically surveyed area could be divided into two sections: the first one from t1 position up to about 120 m nw where the se mountain side is made up of coarse-grained sandstones which, although somewhat fractured, are nevertheless less altered from around 15 m deep downwards. the situation changed towards the nw side where the presence of altered black shale caused lower vp values. nevertheless, at projected tunnel depths, velocities and hence material strength tended to increase (about 1,600 m/s in the extreme nw; figure 8, under t3). the interblended shales were not very thick. conclusions geophysical refraction tomography was an important tool for evaluating rock rigidity, adding extra information regarding lithological characteristics. p velocity variation was determined at depths greater than 100 m in the survey area. this is the first time it has been done in argentina, bearing the depth of investigation plus hard topographical conditions and refraction tomography in mind. the tamper system would appear to be a valuable tool for countless applications in the future, since it allows p and s waves to be generated at greater depths than the traditional hammer (its maximum range not having been studied yet) without the risk of using explosives. moreover, its portability allows it to be carried into high mountain terrain, allowing the complete characterisation of rock masses, something which could not be performed by means of surface geophysical methods only, having to employ cross-hole studies which require expensive drilling and also provide data having much more bounded distances. acknowledgements we would like to thank juan marcet, director of “escuela de ingeniería de caminos de montaña” at the universidad nacional de san juan and his team for their tremendous cooperation in logistical tasks and positive attitude towards developing new geophysical methods, as well as their valuable contributions so that this daunting task could be carried out. figure 8. seismic refraction tomography profile. (3) armando luis imhof, manuel sánchez, carlos calvo and adriana martín88 we would also like to thank manuel sánchez phd, who was in charge of designing the tamper emission system, for his continued willingness to cooperate with the project. references bohachevsky, i. o.; johnson, m. e. and stein, m. l. (1986). generalized simulated annealing for function optimization, technometrics, 28, 209-217 bordonaro, o.l. (1980). el cámbrico en la quebrada de zonda, provincia de san juan, revista de la asociación geológica argentina. 35, 26-40 cerveny, v. (2001) seismic ray theory. cambridge university press, 700pp. chunduru, r. k.; sen m. k. and stoffa, p. l. (1996) 2-d resistivity inversion using spline parameterization and simulatd annealing. geophysics, 61, 151-161.  cortés, j; gonzález bonorino, g; kourkharsky, m; brodtkorb, a y pereyra, f. (1997). memoria y mapa geológico a escala 1:100.000 de la hoja 3369-03 yalguaraz, provincia de mendoza, dir. nacional de servicio geológico. informe inédito. buenos aires. dobrin, m. (1975). introducción a la prospección geofísica. ed. omega. españa. fernandez, a.l. (2000). tomographic imaging the state of stress. phd thesis, georgia institute of technology. atlanta. usa. gonzález díaz, e & fauqué, l (1993) geología y recursos naturales de mendoza. v.a. ramos (editor). relatorio xii congreso geológico argentino. ii congreso exploración de hidrocarburos. buenos aires i-17:217-234 heiland, c.a. (1940). geophysical exploration, prentice hall. ny. 1013 pp. herbst, r; kapp, i; krummel, h & luck, e. (1998). seismic sources for shallow investigations: a field comparison from northern germany, journal of applied geophysics, 38, 301-317. imhof, a. l. (2009) estudio geofísico ruta nacional nº153. san juanargentina. informe técnico. escuela de ingeniería de caminos de montaña (eicam). universidad nacional de san juan. kirkpatrick, s., gelatt jr., c.d., vecchi, m.p. (1983). optimization by simulated annealing. science 220, 671–680. miller, r.; pullan, s. e.; waldner, j. s. & haeni, f. p. (1986). field comparison of shallow seismic sources, geophysics 51, 2067-2092. palmer, d. (1986). refraction seismics: the lateral resolution of structure and seismic velocity. ed. geophysical press ltd. london. isbn: 0-946631-13-1. 269 pp. pullammanappallil s.k. & louie, j.n. (1994) a generalized simulatedannealing optimization for inversion of first-arrival times, bulletin of the seismological society of america, 84-5, 1397-1409 pullan, s.e.; macaulay, h.a. (1987). an in-hole shotgun source for engineering seismic surveys, geophysics, 52, 985-996. schuster, g. t. & quintus-bosz, a. (1993). wavepath eikonal traveltime inversion: theory. geophysics, 58, 1314-1323. sethian, j. a. (2007). level set methods and fast marching methods: evolving interfaces in computational geometry, fluid mechanics, computer vision, and materials science. cambridge university press, 367 pp. isbn 0521645573 vidale, j. e. (1988). finite difference calculation of travel times, bulletin of the seismological society of america, 78, 2062-2076. zhang, j. & toksoz, m.n. (1998). nonlinear refraction traveltime tomography, geophysics, 63, no.5, 1726-1737. the study area is located in northwestern iran in the central iran zone, specifically the western alborz sub-zone south of the tarom-hashtjin metallogenic zone. the exposed rock units in this area generally include eocene volcanic rocks (lava flows and pyroclasts belonging to the karaj formation) and oligocene granitoid intrusive bodies. the intrusive bodies in the area have a petrographic composition of granite, syenite and monzonite and are mostly metaluminous. the dual characteristics of these intrusives (for example, the behavior of elements such as rb, p, ga/al, y/nb, k/ na, and feo/fe2o3, the rb/nb ratios, the a/cnk molar ratios and the acf and a/cnk-fe2o3+feo diagrams), some of which are consistent with the i nature and others with the s and a natures, show that the rocks are among hybrid granitoids and, in terms of the tectonic setting, lie within the wpg range. according to the rb/sr, zr/hf, k/rb ratios, the granite melts that form the aforementioned bodies are not extremely evolved and have not undergone post magmatic activity, which would lead to mineralization. the sm/eu and rb/ba ratios and the behavior of rb, ba and sr within the aforementioned granitoids show that the rocks are similar to average granitoids unrelated to li, be, sn, w and ta deposits; they fall within the range of barren granitoids but are partially fertile in cu. el área de este estudio está localizada en el noroeste de la zona central de irán, específicamente en el oeste de la subzona de alborz y al sur de la zona metalogénica de tarom-hashtjin. las unidades de roca expuesta en esta área se clasifican generalmente como rocas volcánicas del eoceno (flujos de lava y piroclastos pertenecientes a la formación karaj) y como cuerpos granitoides intrusivos del oligoceno. los cuerpos intrusivos en el área tienen una composición petrográfica de granito, sienita y monzonita mayormente metaluminosa. las características duales de estas intrusiones (por ejemplo, el comportamiento de de elementos como rb, p, ga/ al, y/nb, k/na, y feo/fe2o3, los índice de rb/nb, la proporción molar de los a/cnk y los diagramas acf y a/cnk-fe2o3+feo), algunas de las cuales son consistentes con la índole i y otras con las índoles s y a, muestran que las rocas son granitoides híbridos y, en términos de orden tectónico, subyacen en la cadena wpg. de acuerdo con los índices rb/sr, zr/hf, k/rb, los granitos fundieron la forma de los cuerpos sin desarrollarse completamente y sin registrar actividad magmática posterior, lo que llevó a la mineralización. los índices sm/ eu y rb/ba y el comportamiento del rb, ba y sr al interior de los granitoides mencionados muestran que las rocas son similares al promedio de los granitoides no relacionados con los depósitos de li, be, sn, w y ta; estos se incluyen en el rango de granitoides estériles, pero son parcialemente fértiles en cu. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 123 129 abstract resumen key words: intrusive, valis, tarom, hybrid, barren. palabras clave: intrusión, valis, tarom, híbrido, estéril. record manuscript received: 06/08/2014 accepted for publication: 09/09/2014 study of the mineralization potential of the intrusives around valis (tarom-iran) mojtaba bahajroy1, saeed takiֿ ² email: taki_saeed 2002@yahoo.com 1&2: department of geology, lahijan branch, college of basic sciences, islamic azad university, lahijan, iran. ore deposits issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.44799 http://dx.doi.org/10.15446/esrj.v18n2.44799 124 mojtaba bahajroy, saeed taki introduction the study area is located in northwestern iran and is part of the alborz mountain range. alborz itself is affected by the alp-himalayan orogeny and generally trends northwest-southeast. this area is geographically located between the eastern longitudes of 48º 30ʹ to 48º 40ʹ and northern latitudes of 37º to 37º 5ʹ. in terms of iran’s structural division, this area is located in the western alborz subzone in central iran (alavi, 1996) (figure 1), and from metallogenic point of view, it belongs to the multimetallic belt of tarom-hashtjin (ghorbani, 2013). the tarom-hashtjin belt covers an area between qazvin (west of taleghan) and north-northwest of mianeh; it is limited by the manjil depression and talesh mountains to the north and by the alborz-zanjan-mianeh axis to the south. in fact, this belt is structurally confined to the north by the sefidrood fault, to the south by the continuation of the tabriz–soltanieh and soltanieh–takestan faults, and to the west by the astara–marivan fault (ghorbani, 2013) (figure 1). almost the entire study area is composed of cenozoic volcanic, intrusive, and sedimentary rocks. the rocks along the tarom-hashtjin axis, which are considered equivalent to the karaj formation, are different from those in central alborz in terms of the lithology and chemical composition because the lava flows do not consist only of volcaniclastic rocks along this axis and their compositions are more basic. in fact, the rocks in the karaj formation in central alborz are predominantly acidic tuffs, while their equivalents along the tarom-hashtjin axis are mostly andesites and basaltic andesites. the ancient rocks in the tarom-hashtjin belt are not exposed within the central parts (ghorbani, 2013), and the basement is mostly composed of tertiary magmatic rocks. the volcanic rocks in the tarom area vary from rhyodacite and dacite to basalt. these rocks are observed in the form of lava flows, tuffs and sometimes tuffites. according to the conducted studies, the following volcanic rocks have been observed in this belt: basalt, basaltic andesite, andesite, trachyte, latite, trachyandesite, dacite and rhyodacite ignimbrite and acidic to intermediate tuffs. among these rocks, andesites are the most voluminous. moein-vaziri (1985) believed that most volcanic rocks from the tarom-hashtjin are potassic-alkaline, while some are sodic-alkaline or calc-alkaline. most of the andesites are shoshonitic. the emplacement of oligocene age acidic to intermediate intrusive bodies within the eocene volcanic rocks and the existence of the mining indices of zehabad, barikabad, khalifehlou, aliabad and golouje in the tarom region along with vast alteration zones have caused this area to be geologically considered as a prime geological district for the detection and identification of metallic and non-metallic deposits (ghorbani, 2013). the studies conducted by different researchers (peyrovan h., 1992; torkamani e., 1997; moayyed m., 2001) show that the granitoid bodies in this zone are of the i type. haj alilou (2000) believed that the intrusive bodies have been emplaced at a depth between 1400 to 3000 m and their formation temperature ranged from 700 to 880 degrees centigrade. because of the high water content and sulfur fugacity, the bodies managed to create very extensive hydrothermal alterations together with vein, veinlet and scattered mineralizations in tuffites and eocene volcanic rocks. the magmatic series of these intrusive bodies are shoshonite and high k calc-alkaline and are of the i type (ghorbani, 2013). shallower bodies are observed in the form of prophyrite with the general composition of monzonite porphyry and occurred along with major bodies with important roles in mineralization and hydrothermal alteration. the structural geology features similar anticline and syncline axes, and faults trending east-west and northwest-southeast had important roles in the emplacement of intrusive bodies and the development of hydrothermal alteration areas. sericitic, argillic (advanced, intermediate and weak), silicic, chloritic, propyltic, zeolitic and alunite alteration zones have been recognized along this axis. the igneous bodies, which have economic concentrations of either genetic or paragenetic chemical elements, generally show specific geochemical patterns (beus, 1968), the identification of which potentially distinguishes metalliferous geological units from barren types. when determining the mineralization potential of felsic intrusive bodies, it is important to recognize their nature, which generally accompanies special types of ore deposits. to achieve this purpose and determine the mineralization ability of the granitoid bodies around valis, petrography, petrology, and geochemical ratios and the distribution of major and rare elements have been studied; in addition, this granitoid body was compared with the world’s most well-known fertile and barren granitoid bodies. figure 1. location of the study area (red square) on iran’s structural divisions map (alavi, 1996). this area is located in alborz in the taromhashtjin metallogenic zone according to the structural zoning implemented by ghorbani (2013). geological setting this area is separated from the talesh mountains by manjil basin and from the soltanieh mountains by the zanjan-abhar plain. the rock units in the area mainly include eocene volcanic rocks (lava flows and pyroclasts belonging to the karaj formation) and oligocene granitoid intrusive bodies (figure 2). the volcaniclastic succession lithology in tarom, as in other areas in alborz, consists of green tuffs and shaly and sometimes calcareous intercalations. the eocene volcanic rocks in the area include volcaniclastic and extrusive rocks. the extrusive rocks in this area include andesite, basaltic andesite, quartz trachyandesite and basalt, though the bulk of these rocks are basaltic andesites and quartz trachyandesite. many intrusive bodies have been injected into eocene volcaniclastic assemblages, so these bodies are post-eocene (most likely oligocene) in age. one characteristic of the oligocene intrusive bodies is the creation of alteration areoles in eocene volcaniclastics, and their hydrothermal phases have been generally accompanied by the formation of elements such as epithermal gold, copper, lead, zinc and kaolin. the main body is exposed at the surface in the form of a prolate batholith trending northwestsoutheast. subvolcanic bodies are mostly observed in dyke form. most faults in the area generally trend northwest-southeast, but some faults trend northeastsouthwest. the aforementioned bodies were injected along the structures and longitudinal faults in the highs in tarom. most structures in this area follow the fault system so that a set of alteration zones occurs along the faults. figure 2. simplified geological map of the study area (davis et al, 1972). 125study of the mineralization potential of the intrusives around valis (tarom-iran) methodology the rock samples were collected from the intrusive bodies in the study area. forty-five thin sections were prepared for petrological study. to study the chemical characteristics, 11 of the most representative rock samples taken from the intrusives were selected for whole rock major oxide, trace element, and ree analysis and sent to acme company in canada and the atomic energy organization of iran. at the acme company, the samples were first dried and then crushed and pulverized to pass a 200 mesh sieve. a lithium borate fusion and dilute nitric acid digestion of a 0.2 g sample pulp followed by icp emission spectrometry was used to carry out the whole rock analysis for 11 major oxides and some minor elements. the loss on ignition (loi) was obtained by sintering at 1000°c. two separate icp-ms analyses were done to determine the trace elements. the rare earth and refractory elements were collected from a lithium borate decomposition (same as that used for the major elements) to determine the total abundances. the precious and base metals and their associated pathfinder elements were generated from an aqua regia digestion (table 1). at the atomic energy organization of iran, the pulverized samples were mixed with boric acid and then pressed and analyzed using the xrf method (table 2). the geochemical data were processed using the minpet 2.02 software. because iron has been reported in its unseparated form, irvine and baragar’s (1971) method was used in this study to calculate the bivalent and trivalent iron. table 1. representative whole rock major oxides (wt %), trace elements (ppm) and rare earth elements (ppm) for samples from the intrusives around valis (icp-aes and icp-ms) (implemented by the acme laboratory). b8b7b3b2b1 sample no. analyte 67.3656.8366.0068.9961.08sio2 14.6416.3314.5014.3415.30al2o3 3.596.293.823.115.58fe2o3 0.463.391.080.651.38mgo 1.527.452.011.631.78cao 3.223.293.583.012.64na2o 7.333.996.726.407.49k2o 0.580.940.560.460.75tio2 0.100.350.120.090.18p2o5 0.080.230.140.140.61mno 0.0190.0110.0060.0190.007cr2o3 20<20<20<20<20<ni 5187511sc 0.80.61.20.92.8loi 99.7199.7099.7699.7799.58sum 4816726762861293ba 61<842be 6.614.87.74.511.2co 3.84.27.812.24.4cs 17.116.615.415.813.3ga 19.011.37.910.45.5hf 53.328.248.653.317.7nb 224.2131.3347.8338.0256.1rb 1<1443sn 307.9492.1197.1210.3277.1sr 3.52.03.93.81.3ta 60.430.154.050.312.2th 14.46.414.213.93.3u 31178532785v 1.42.14.45.92.0w 843.8474.1267.2392.4221.8zr 26.232.135.336.420.8y 26.234.640.855.737.5la 46.569.076.7105.866.3ce 5.408.428.1911.787.36pr 20.632.028.742.227.5nd 4.226.495.406.794.70sm 0.681.060.930.631.14eu 4.316.545.586.504.39gd 0.650.940.880.950.63tb 3.945.865.605.923.97dy 0.851.201.251.220.80ho 2.913.263.893.712.24er 0.440.500.620.630.35tm 2.733.294.104.312.25yb 0.480.520.620.710.33lu 2.21.51.32.31.2mo 23.713.674.119.36.8cu 15.022.242.632.9361.5pb 375084113870zn 5.212.84.15.34.6ni 5.44.19.75.42.9as 0.10.1<0.20.22.0cd 1.60.62.82.00.6sb 0.1<0.1<0.1<0.1<0.1bi 0.1<0.1<0.10.1<0.1ag 0.5<0.5<1.11.01.8au 0.01<0.01<0.01<0.01<0.01hg 0.1<0.1<0.1<0.1<0.1<tl 0.5<0.5<0.5<0.5<0.5<se table 2. representative whole rock major oxides (wt %) and trace elements (ppm) for samples from the intrusives around valis (xrf) (implemented by the atomic energy organization of iran). va-11va-08va-04hs-03hs-02hs-01 sample no. analyte 69.33069.80169.22666.86563.73863.904sio2 14.57814.43013.75115.23115.84416.003al2o3 2.2142.3572.3123.0373.5203.361fe2o3 1.8551.4451.7782.193.3763.194cao 1.9213.0222.8263.0243.0573.023na2o 0.7270.4280.7860.7711.1651.025mgo 6.1476.0025.6766.0385.7695.775k2o 0.3330.3240.3290.4570.5270.511tio2 0.0350.0670.0420.0580.0710.066mno 0.0870.0880.0890.1270.1700.166p2o5 316334236368530531ba 4574116775566ce 3377108co nnn44ncr 31339664860cu 423646433634nb 13171721136ni 204123482234pb 302333330281237238rb 249210199233361354sr 555534mo 383638496157v 394042373434y 280271267403410378zr 418444625475zn 16151814610u 465055472229th discussion and results geochemistry and tectonomagnetic setting of the intrusives around valis the petrographical studies and chemical classification diagrams show that the intrusive bodies in the study area have granite, syenite and monzonite (granitoid) compositions and are mostly metaluminous (figure 3). contradictory features in the intrusives (such as the behavior of p, rb, ga/al, y/nb, k/na, and feo/fe2o3, the rb/nb ratios, the a/cnk molar ratios and the a/cnkfe2o3+feo and acf diagrams), some of which are consistent with the i nature and others with the s and a natures, show that the rocks are classified as hybrid granitoids (figures 4-7). 126 mojtaba bahajroy, saeed taki figure 3. plot of chemical data from the intrusives around valis on a) chemical rock classification (cox et al, 1971) and b) aluminum saturation index (maniar and piccoli, 1989) diagrams. figure 4. plot of chemical data from the intrusives around valis on a) acf (chappell and white, 1992) and, b) na2o versus k2o diagrams (chappell and white, 1974). the data from the study area are consistent with both i and s granites. figure 5. the chemical data from the intrusives around valis plot in the s field on the acnk-fe2o3+feo diagram (pearce et al, 1984). figure 6) plot of chemical data from the intrusives around valis on a) normative corundum versus rb and b) p2o5 versus rb diagrams for i (solid squares) and s type (hollow circles) granites related to the lachlan fold belt in australia (pearce et al, 1984). the data from the study area are consistent with both i and s granites. figure 7. plot of chemical data from the intrusives around valis on a) rb/nb versus y/nb (eby, 1992) and b) ga/al *10000 versus nb diagrams (whalen et al, 1987). the data are plotted in the a type granite field. plotting the data on the granitoid tectonomagmatic discrimination diagrams (figure 8) shows that they usually fall within the wpg fields. figure 8. plot of chemical data from the study area’s intrusives on a) rb-(y+yb) and b) nb-y diagrams (pearce et al, 1984). these data usually plot in the wpg field. using geochemical features to study the mineralization potential of the intrusives around valis determining the mineralization potential of felsic intrusive bodies is important because every specific type of granite is usually accompanied with a specific type of ore deposit. for example, palingenetic granites have mineralization potential for cu, au, nb and minor amounts of sn and w (beus, 1968). high amounts of li, rb compared to k (lower k:rb ratio) and sr compared to rb (high rb:sr ratio) are geochemical characteristics of magmatic production that can be used to study the separation of volatile substances and magmatic rare metals from magma, increasing the mineralization potential (beus, 1968). special indicator minerals can indicate the economic concentrations of some metallic rare elements; for example, the existence of tourmaline and topaz in granitoid bodies can imply tin mineralization as cassiterite. in addition, pink, robellite type tourmalines are abundant in lithium bearing pegmatites (rozendaal et al, 1995); therefore, the economic value and mineralization type of the pegmatites can be partially recognized according to the color of the existing tourmalines (beus, 1968). aplite can be seen in some granite bodies in the study area, especially in marginal zones or in the veins that form across the bodies. these aplites have tourmalines that are black in hand specimen. on the other hand, there is no sign of topaz; therefore, the mineralization of tin is unlikely. tungsten has different distributions in barren and fertile intrusives. in a few samples from barren igneous bodies, the w content is higher than 5 ppm, and no sample is observed with w content higher than 10 ppm. however, at least 10% of the samples from fertile bodies contain more than 10 ppm of tungsten (jonasson and boyle 1972). the intrusive bodies, which have been affected by post magmatic hydrothermal processes (jonasson and 127study of the mineralization potential of the intrusives around valis (tarom-iran) boyle, 1972). the amount of this element in the study area’s intrusives ranges from 1.4 to 5.9; therefore, they are barren in terms of w. the degree and type of differentiation and oxidation state of the magmas that formed granites are important to determine the potential and type of associated mineralization (blevin, 2003). the k/rb ratio is used to determine the transition state of granite melts; if the ratio is under 100, the granite is highly evolved (rossi et al, 2011) (figure 9). this observation occurs because rb tends to be differentiated in the melt during the segregation stage of aqueous liquid phases from the remaining silicate melts (clarke, 1992). this ratio ranges from 142.8 to 271.4 in the study area’s intrusives. the average value of k/rb in the intrusives around valis (table 3) is higher than all the values in table 4; according to this table, the greater the mineralization in the granitoid bodies the lower the ratio is, as rb tends to remain in the melts and be differentiated among the silicate melts and aqueous liquids. therefore, these granitoids are not strongly differentiated; in other words, the granite melts that created the intrusives in the study area are insufficiently evolved (table 4). thus, the aforementioned bodies have not undergone sufficient post magmatic activity to cause mineralization. the value of k/rb in the study area’s intrusives is similar to the average for granitoids unrelated to li, be, sn, w and ta ore deposits, so the intrusives are classified as barren granites (table 4). the evolved nature of the granites is also recognizable using the sr-rb-ba triangular diagram by el bouseily & el sokhary (1975) (figure 10). most samples plot in the normal granite field, which shows that the intrusives in the study area are not completely evolved. the ratio of compatible to incompatible elements (such as sr/rb) is also a useful tool to recognize the differentiation type of granite magmas (blevin 2003, ishihara and tani 2004, blevin and chapell, 1992) (figure 11); in particular, the granite magmas in the study area have been slightly differentiated. bea et al (2006) suggested another index to recognize the evolution of magmas; if zr/hf<20, strong magmatic hydrothermal alteration has occurred, while if zr/hf>20, magmatic hydrothermal alteration has not occurred. the zr/hf ratio in the study area’s intrusives is between 33.8 and 44.4; therefore, magmatic hydrothermal alteration has not occurred. the sm/eu and rb/ba ratios and amounts of rb, ba and sr in the intrusives around valis are higher than those of the s type granite source rocks of porphyry tin deposits around the world (rongfupei and dawei hog, 1995; lehmann et al, 1989) (table 5). diagrams using the aforementioned parameters can show the fertility of the granitoids with respect to sn (karimpour, 1999); accordingly, the intrusives in the study area lack tin mineralization (figure 12a and b). to discriminate granitoids and recognize their economic potential for tin, molybdenum or porphyry copper, the rb/sr and ce/yb ratios and color index (obtained from [ci=(sio2+k2o+na2o)/ (mgo+cao+feo)]) can be used (karimpour et al, 1983) (figure 13a and b). tin, high grade and low-grade molybdenum and porphyry copper deposits have been completely discriminated in these diagrams. the chemical data imply that the intrusives in the study area are barren in sn and mo but somewhat fertile in cu (especially sample b7). the geochemical behaviors of the copper and zinc in magnetic crystallization products are different. unlike copper, which is found in the chalcopyrite phase, zinc replaces iron in iron and magnesium silicates. in intermediate rocks (with 52 to 60 wt% silica), the amount of zinc is fixed at approximately 85 ppm. in rocks with over 60% silica, the zinc amount decreases linearly with increasing sio2. in rhyolite with 75% sio2, the amount reaches 35 ppm (wolf, 1975). wolf believed that practically all rocks with total iron higher than 10% can be used in regional explorations of zinc. considering prior research and using the sio2 versus fe and zn diagrams by wolf (1975), we recognize the barren nature of the study area’s intrusive bodies with respect to zinc (figure 14). table 3. average of the rare and major elements in the intrusives around valis. zr/ hfrb/srba/ rbk/rbeu (ppm) hf (ppm) zr (ppm) ba (ppm) sr (ppm) rb (ppm) k (%)element 34.10.9621.99189.430.8910.82369517.752692594.9average table 4. k/rb ratios of barren and fertile granites (beus, 1968) compared to those in the intrusives around valis. type of granitoid k/rb average of the granitoids average of the granitoids unrelated to li, be, sn, w and ta deposits. average of the granitoids related to li, be, sn, w and ta deposits. average of the biotite granites related to li, be, cs and ta pegmatitic deposits. average of the biotite granites related to ta pegmatitic and apogranitic deposits. 170 170 130 160 126 average of the granitoids in the study area. 189/43 table 5. amount and ratios of some rare elements in s type granites around the world that contain tin compared to the intrusives in the study area (not analyzed = na). rb/basm/eurb(ppm)ba(ppm)sr(ppm)mine’s name 25-16251100-80065-3523-18china, yanbei granites na18nananasouth of china, yanyan granites 2165-204300-36003535southeast of china, gianlishan granites na75nananasoutheast of china, yoaganixian granites na45nananasoutheast of china, xihuashan granites 1535-20543-15003531-9australia, blue tier batholith 100-20120-40700-50035-519-5australia, emuford herberton region 115-3395-155100-6703220brazil (northeast), maderia granites 1770-503502520southwest of japan, sanyo 55-30na2570-110085-389-1thailand, thai-barmese granites 75-30 na1390-68085-2044-9south of thailand, phuket island, kato body 1.4-0.210.784.12347-1311293236 492199the intrusives around valis figure 9. plot of k/rb versus sio2 (blevin, 2003). the study area’s intrusives are semi-evolved. figure 10. plot of the study area’s samples on sr, rb and ba triangular diagram (el bouseily and el sokhary, 1975). 128 mojtaba bahajroy, saeed taki figure 11. diagram of rb/sr versus sio2 (blevin, 2003) and a plot of the study area’s samples. figure 12. plot of the study area’s intrusives on a) sm/eu versus rb and b) rb-ba-sr diagrams, which discriminate tin-bearing granites from barren ones (karimpour et al, 1983). figure 13. the discrimination source rock diagrams of cu, sn and mo porphyry deposits using a color index and ratios of a) rb/sr and b) ce/yb (karimpour, 1999), and a plot of the study area’s samples. figure 14. the relationship between changes in a) fe and b) zn versus sio2 in barren and fertile granites (wolf, 1975), and a plot of the data related to the study area. results based on the information obtained from field studies and the petrography and analysis results of the intrusives around valis, the following results are determined: 1 these intrusives have granite, syenite and monzonite petrographic compositions (granitoid) and are mostly metaluminous. 2 contradictory features in the intrusives (such as the behavior of p, rb, ga/al, y/nb, k/na, and feo/fe2o3, the rb/nb ratios, the a/cnk molar ratios and the a/cnk-fe2o3+feo and acf diagrams), some of which are consistent with the i nature and others with the s and a natures, show that the rocks are classified as hybrid granitoids. 3 in terms of the tectonomagmatic setting, the rocks are classified as wpg granitoids. 4 based on the k/rb ratio, these granitoids are not strongly differentiated and have not undergone post magmatic activity, which would lead to mineralization. the amount of k/rb in the study area’s intrusives is similar to the average of granitoids unrelated to li, be, sn, w and ta deposits, classifying the intrusives as barren granites. 6 the sm/eu and rb/ba ratios and the behavior of rb, ba and sr in the intrusives around valis are completely different compared to the s type granite source rocks of porphyry tin deposits around the world and are barren in terms of tin. 7 the chemical data (rb/sr, ce/yb ratios and color index) imply that the intrusives in the study area are barren in terms of sn and mo but are somewhat fertile in cu. 8 the changes in zn and fe versus sio2 show the barren nature of these intrusives. references alavi, m. (1996) tectonostratigraphy synthesis and structural style of the alborz mountain system in northern iran. geodynamics journal, 21, 1–33. bea, f., montero, p. and ortega, m. (2006) a la-icp-ms evaluation of zr reservoirs in common crustal rocks: implications for zr and hf geochemistry, and zircon forming processes. the canadian mineralogist, 44, 693–714. beus, a. a. (1968) geochemical criteria in theoretical principles of exploration for mineral deposits, moscow, pp. 127-145. blevin, p. (2003) metallogeny of granitic rocks, the ishihara symposium, granites and associated metallogenesis, 14, 5–8. blevin, p. l. and chappell, b. w., (1992) the role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern australia, transaction of the royal society of edinburgh: earth sciences, 83, 305–316. chappell, b. w. and white, a. j. r., (1974) two contrasting granite types. pacific geology, 9: 173-174. chappell b. w., white a. g. r. (1992) i and s type granites in the lachlan fold belt, transactions of the royal society of edinburgh sciences, 83, 1-26. clarke, d. b. (1992) granitoid rocks: chapman and hall, pp. 238. cox k. g., bell j.d., pankhurst r.j. (1979) the interpretation of igneous rocks, allen and unwin, london, 405 p. davis, r. g., jones, c. r., hamzepour b. and clark, g. c. (1972) the geology of the musuleh sheet (northwest iran): geol. survey of iran, report no. 24, 110 pp. eby, g. n., (1992) chemical subdivision of the a-type granitoids, petrogenetic and tectonic implications, geology 20, 641-644. el bouseily, a. m. and el sokkary, a. a. (1975) the relation between rb, ba and sr in granitic rocks, chemical geology, 16, 207–219. haj ailou, b. (2000) metallogeny of tertiary in western alborz-azerbaijan (miyaneh-siyahroud) with specific view on hashtjin area, phd thesis, shahid beheshti university. ghorbani, m. 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(eds) granite-related ore deposits, geological society, london, special publications, 350, 175–186. rozendaal, a., and bruwer, l. (1995) tourmaline nodules: mineralization in the cape granite suite, south africa. journal of african earth sciences, 21, 141–155. torkamani, e., (1997) petrologic study of intrusive rocks north of abharkhoramdarreh. msc thesis, shahid beheshti university whalen, j. d., currie, k. l., and chappell, e. w. (1987) a-type granites: geochemical charactristics, discrimination and petrogenesis, cont. min. pet., 95, 407-419. wolf, m. b., (1975) estimation of parameters affecting rapid fluid transfers in the whole body, isotopic infusions, ann. biomed eng., 3: 209-224. geociencias dic 2010-color.vp laboratory testing of the monotonic behavior of partially saturated sandy soil noureddine della1 1 laboratory of sciences of materials and environment, department of civil engineering university of chlef, road of sendjas pb 151 chlef 02000 algeria e-mail: nour_della@yahoo.fr abstract this paper presents a laboratory study on the influence of the saturation evaluated in term of skempton’s pore pressure coefficient b on the behavior of chlef sand. the study is based on drained and unnonno drained compression tests which were carried out for skempton’s pore pressure coefficient varying between 13 and 90%.the tests were conducted on medium dense sand samples having an initial relative density id = 0.50 at an effective stress of 100 kpa. the paper is composed of two parts. the first one presents the characteristics of the sand used in this study. the second provides an analysis of the experimental results and discusses the influence of skempton’s pore pressure coefficient (b) on the mechanical characteristics of the sand. the tests show that the increase in the skempton’ s pore pressure coefficient (b) reduces the soil dilatancy and amplifies the phase of contractancy and reduces the frictional and characteristic angle of the sand. the residual strength decreases with the increase ininin the skempton’s pore pressure coefficient b. resumen este artículo presenta un estudio de laboratorio sobre la influencia de la saturación evaluada en términos coeficiente de presión de poro de skempton b en el comportamiento de la arena chlef. el estudio se basa en pruebas de compresión drenados y no drenados, llevados a cabo con coeficiente skempton variando entre 13 y 90%. las pruebas se llevaron a cabo en muestras de arena con una densidad inicial relativa = 0,50 unidades y un esfuerzo efectivo de 100 kpa . este trabajo se compone de dos partes. el primero presenta las características de la arena utilizada en este estudio. el segundo ofrece un análisis de los resultados experimentales y se evalúa la influencia del coeficiente de presión de poro de skempton b en las características mecánicas de la arena. las pruebas muestran que el aumento del coeficiente de presión de poro de skempton b reduce la dilatancia del suelo, amplifica la fase de la contractancia y reduce el ángulo de fricción y característico de la arena. la resistencia residual disminuye con el aumento de la presión del poro del coeficiente b de skempton introduction in the neighbourhoods of the town of chlef (algeria), unsaturated zones exist on the top of the phreatic ground which underwent a significant folding back following the dryness which touches the area since the eighties. during the past decades, advanced researches helped us better understand the liquefaction of the grounds based on experiments carried out in laboratory, the physical modeling and the numerical analysis. the majority of the investigations on the liquefaction of the granular soils were based on completely saturated material. the study of the influence of the saturation degree on the liquefaction of the soils is of practical interest, because we often find structures built on the top of the phreatic ground; what implies the presence of partially saturated grounds. the incidence of a partial saturation on cyclic resistance was approached in a theoretical way by martin et al. (1978). mulilis et al. (1978) examined the effect of the saturation degree on the liquefaction of monterrey sand. they noted that the variation of the of skempton’s coefficient b between 0.91 and 0.97 does not significantly affect the liquefaction of this sand. this influence depends on the type of soil, the density and the initial confining pressure. however, the recent results of the in-situ tests include the measurements of the velocity of the compression waves (vp), and indicate that the condition of partial saturation can exist above the level of the phreatic ground for a few meters due to the presence of bubbles of air (ishihara et al., 2001 and nakazawa et al., 2004) or the presence of gas bubbles in the marine sediments and sands containing oils as noted by mathiroban and grozic (2004).the effects of a condition of partial saturation on liquefaction were approached by some researchers (as by atigh and byrne, 2004; mathiroban and grozic, 2004; pietruszczak et al., 2003; yang and sato, 2001).the condition of saturation of soil samples in laboratory can be evaluated by measuring the value of the coefficient of skempton b or the speed of compression waves vp as suggested by ishihara et al. (2001). in situ, the saturation can be evaluated by the earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 181-186research groupin geophysics universidad nacional de colombia keywords: sand, compression, drained, undrained, saturation, frictional angle. palabras clave: arena, compresión, drenaje, sin drenaje, saturación, ángulo de fricción. record manuscript received: 03/06/2010 accepted for publication: 18/11/2010 measurement of the velocity of the of compression wave (vp).the results of the laboratory tests showed that the resistance to the liquefaction of sands increases when the saturation degree decreases (martin et al., 1978; yoshimi et al., 1989; bouferra, 2000; ishihara et al., 2001 and 2004; yang, 2002; yang et al., 2004; bouferra et al., 2007). mullilis et al., (1978) and tatsuoka et al..(1986) showed that in the case of loose sands, a good saturation requires high values of the coefficient b .on the other hand, for stiffer materials, the problem seems less critical. sheriff et al. (1977) show that a fine or clayey sand can be considered saturated if the value of b exceeds 0.8, chaney (1978) stressed that the coefficient b must exceed 0.96 so that the soil is well saturated. on the other hand, giroud and cordary (1976) noted that for values of b superiors to 0.85, the degree of saturation is very close to 1.tests of liquefaction were carried out by yoshimi et al. (1989) on the sand of toyoura of medium density (id = 0.60) with samples having various degrees of saturation. the results show that the degree of saturation significantly affects the resistance to liquefaction: with a coefficient b higher than 0.8, it is enough to apply three cycles to have a liquefaction of the sample; whereas we need eight cycles to obtain liquefaction of the specimens having a coefficient b close to 0.5. in this article, we present the results of a laboratory investigation on the influence of the saturation degree evaluated in term of coefficient of skempton (b) varying between 13 and 90% on the behavior of granular sand. these tests allow us to better understand the influence of saturation on the mechanical behavior of granular sand. the article is composed of two parts. in the first part, we present the material used, the second part gives an analysis of the experimental results of the tests carried out and discusses the influence of the degree of saturation evaluated in term of coefficient of skempton (b) on the resistance to liquefaction. material used the tests were carried out on the sand of chlef (algeria) containing 0.5% of silt of the chlef river which crosses the town of chlef (formerly el-asnam). the tests have been carried out on specimen collected from the region where the phenomenon of liquefaction was observed during the last earthquake (october 10th, 1980) (see fig. 1) with an index of density id = 0.50 and at initial confining pressure of óc’= 100 kpa. the granular grading curve of sand used is given in fig. 2. the sand of chlef is a medium sand, rounded with an average diameter d50 = 0.45mm.the silt contained is not very plastic with an index of plasticity of 6%. table 1 gives the physical characteristics of the sand used. experimental procedures the used experimental device is presented in fig. 3.it contains: • an autonomous triaxial cell type bishop and wesley (bishop and wesley, 1975), • two controllers of pressure / volume type gds (200cc), • a void pump joined to a reservoir in order to deaire the demineralized water, • a microcomputer equipped with software permitting the piloting of the test and the data acquisition. sample preparation the sample preparation method used is dry pluviation where the dry soil is deposited in the mould using a funnel with a rigorous control of the drop height 182 noureddine della table 1: properties of soil testd material emin emax �dmin (g/cm3) �dmax (g/cm3) �s (g/cm3) cu (d60/d10) d50 (mm) d10 (mm) particle shape o/chlef 0.54 0.99 1.34 1.73 2.67 3.2 0.45 0.15 rounded where emin and emax indicate the minimum and the maximum void ratio, respectively, ãdmin=minimum dry unit weight; ãdmax=maximum dry unit weight; ãs=solid grain unit weight; cu=uniformity coefficient; d50=average diameter; d10= effective diameter. figure 1. sand boils due to the liquefaction phenomenon at chlef region of the sand which must be quasi-null for the loose samples. with the aim of having medium dense homogeneous samples, we have used the method recommended by ladd (1978); this method consists in dividing the sample in several layers. the relative density of each layer varies to1% from bottom upwards. the average layer has the same value of the relative density as the sample. the used samples are cylindrical in shape of 70mm of diameter and 70mm height (l/d=1). to make sure of a good homogeneity of the stresses and deformations within the samples, several researchers (lee, 1978; robinet et al., 1983; tatsuoka et al., 1984; colliat 1986, and al mahmoud 1997) insisted on the need for reducing frictions between the sample and the superior and inferior base plates; this can be obtained by base plates of smooth or lubricated surface (antifrettag system).the antifrettage system used is represented on fig. 4 (al mahmoud (1997)). the sand mass to be set up is evaluated according to the desired density (the initial volume of the sample is known), the state of density of the sample being defined by the relative density: id = (emax e) / (emax – emin) (1) saturation of the sample saturation is a significant stage in the experimental procedure because of its quality depends the answer of the sample under drained and undrained loading.to obtain a good degree of saturation, the technique of carbon dioxide worked out by lade and duncan (1973) was used. the sample is firstly swept by carbon dioxide during twenty minutes, then we let circulate the deaerated and demineralized water until collecting a volume of water superior to one and a half the volume of the sample. to be able to obtain samples with various degrees of saturation, we have varied the time of passage of carbon dioxide and the duration of duct drainage deaerated water through the sample. consolidation of the sample the phase of consolidation consists in applying simultaneously a rise of pressure in the cell by means of the volume/pressure controller (gds1) and inside the sample (gds2).the application of back pressure to the sample using the gds2 improves the quality of the saturation by compressing the microbubbles of the interstitial gas which remains imprisoned between the grains of the soil. we maintain the pressures in the cell and the sample until stabilization of volumes (cell and sample).the quality of saturation is evaluated by measuring the coefficient of skempton (b).this coefficient (b) is equal to äu /äó. we give an increment äó of 100 kpa to the gds1 connected to the cell and we measure the variation of the pore water pressure äu by the gds2 connected to the sample. the back pressure used is of 400 kpa. the degree of saturation is controlled during a triaxial compression test by the coefficient of skempton which can be related to the degree of saturation by the following relation (lade and hernandez 1977): b nk s k s w s r w r a � � � �� � � � � 1 1 1( (2) where ks and kw indicate the bulk modulus of the soil skeleton and the water, respectively; n=the soil porosity, ua= water pore pressure. results of the tests drained compression tests figure 5 shows the results of the drained compression tests carried out for coefficients of skempton b ranging between 13 and 90%. it is clear from this laboratory testing of the monotonic behavior of partially saturated sandy soil 183 100 90 80 70 60 50 40 30 20 10 0 10.00 1.00 0.10 0.01 grain size (mm) p er ce n t fi n er (% ) sand of chlef figure 2. grain size distribution of tested material. figure 3. the experimental device used pastille latex pastille latex grease teflón base plate figure 4. antifrettage system used figure that the coefficient of skempton (b) significantly affects the variation of the deviatoric stress (fig. 5a) and the volumetric deformation (fig. 5b). the increase in the coefficient of skempton (b) from 13 to 90% induces a reduction in the initial stiffness and resistance of the soil (maximal deviator). with regard to the volumetric deformation, we note that the increase in the coefficient of skempton (b) delays the appearance of dilatancy; the sample with a degree of saturation b=13% dilatancy appears after 3% of axial deformation, while the sample with a degree of saturation b=90% we observe an amplification of the phase of contractance and dilatancy is delayed and appears after 12% of axial deformation. also shown in fig. 6 are the curves of variation of the volumetric deformations at the phase change (contractance-dilatancy) and at the steady state versus the coefficient of skempton (b). we notice that the difference between these curves decreases with the increase in the coefficient of skempton (b), showing the progressive disappearance of the phase of dilatancy and the appearance of only the phase of contractance for the sample with a coefficient of skempton (b) equal to 90%. undrained compression tests figure 7 shows the results of the undrained triaxial compression tests performed in this study for various values of coefficient of skempton (b) between 32 and 90% with an initial confining pressure of 100 kpa. as can be seen, increase in the degree of saturation characterized by the coefficient of skempton (b) lead to a reduction in the resistance of the deviatoric stress (fig. 7a) and an increase in the water pressure (fig. 7b). this increase in the water pressure results from the role of the degree of saturation in the increase in the phase of contractance observed during the drained tests. the increase in the pore water pressure leads to a reduction of the effective confining pressure and consequently with a reduction of resistance as fig. 7a illustrates. the stress path curve in the plan (p’, q) shows well the role of the degree of saturation in the reduction of the effective mean stress and the maximal deviatoric stress (fig. 7c). variation of the residual strength when sands are subjected to an undrained shearing; after the peak of deviatoric stress, the resistance to the shearing falls with an almost constant value on a broad deformation. conventionally, this resistance to the shearing is called residual strength or the shearing force at the quasi steady state (qss). the residual strength is defined by ishihara (1993) like: sus = (qs/2) cos�s (3) where qs and �s indicate the deviatoric stress and the mobilized angle of interparticle friction at the quasi steady state. fig. 8 shows the evolution of the residual strength with the coefficient of skempton (b).we note that the residual strength decreases in a significant and linear way with the increase in the coefficient of skempton b resulting from the role of saturation as for the amplification of the contractance of the studied soil. conclusion this experimental study investigated the effect of the saturation characterized by skempton’s pore pressure coefficient (b) on the behavior of a granular soil. both drained and undrained triaxial compression tests were performed on specimens with an initial medium relative density id = 0.50, at an initial effective confining pressure of 100 kpa. the study of the influence of saturation on the potential of liquefaction was carried out on the chlef sand of varying the coefficient of skempton (b). the results of the tests showed that the saturation has a detectable effect on the drained and undrained behavior. the tests show that the increase in the coefficient of skempton induces a reduction of the initial stiffness and the resistance of the soil (maximal deviatoric stress); and increases the phase of 184 noureddine della b=13% b=21% b=54% b=88% b=90% chlef 250 200 150 100 50 0 d ev ia to ri c st re ss (k p a 0 5 10 15 20 25 30 axial strain (%) 0 5 10 15 20 25 30 -12 -8 -4 0 4 8 12 16 b=13% b=21% b=54% b=88% b=90% chlef v o lu m et ri c st ra in (% ) axial strain (%) figure 5. influence of the coefficient of skempton (b) on drained 0 20 40 60 80 100 -15 -10 -5 0 5 10 15 stable state phase change coeficient skempton (%) v o lu m et ri c st ra in (% ) figure 6. influence of the coefficient of skempton (b) on the volumetric deformations at the phase change and steady state contractance. this, results in a significant effect on the volumetric response inducing an amplification of the phase of contractance when the coefficient of skempton b increases. the residual effort (sus) decreases in a significant and linear way with the increase in the coefficient of skempton b resulting from the role of saturation as for the amplification of the contractance of the studied sand. the reduction of the coefficient of skempton, induces a reduction of the increasing rate in the pore water pressure, an improvement of the residual strength of the soil and consequently an increase in the resistance to the liquefaction. a partial saturation can eliminate any risk of liquefaction, this can have practical applications in the high-risk zones of liquefaction. references al mahmoud, m. (1997). etude en laboratoire du comportement des sables sous faibles contraintes, thèse de doctorat en génie civil à l’ustl, lille. atigh, e. and byrne, p. m. (2004). “liquefaction flow of submarine slopes under partially undrained conditions: an effective stress approach”, can. geotechnical j., vol. 41, n° 1, pp.154-165. bouferra, r. (2000) etude en laboratoire de la liquéfaction des sols, thèse de doctorat, ecole universitaire des ingénieurs de lille ustl lille. bouferra, r., benseddiq, n. and shahrour, i. (2007). “saturation and preloading effects on the cyclic behavior of sand.” international journal of geomechanics, asce, vol. 7, n° 5, pp. 396-401. laboratory testing of the monotonic behavior of partially saturated sandy soil 185 0 5 10 15 20 25 30 0 100 200 300 400 500 axial strain (%) d ev ia to ri c st re ss (k p a b=32% b=54% b=90% chlef (rd = 50%) 0 5 10 15 20 25 30 500 520 540 560 580 600 b= 32% b= 54% b= 90% p o re p re ss u re (k p a) axial strain (%) chlef (rd = 50%) 0 100 200 300 400 500 0 100 200 300 400 500 d ev ia to ri c st re ss (k p a) effective mean stress (kpa) b = 32% b = 54% b = 90% chlef (rd = 50%) (a) (b) © figure 7. influence of the coefficient of skempton (b) on undrained behavior of chlef sand 0 2 0 4 0 6 0 8 0 1 0 0 r es id u al st re n g th , s u s (k p a) skempton’s coefficient b (%) 0 20 40 60 80 100 figure 8. influence of the coefficient of skempton (b) on the residual strength at the quasi steady state bishop, a. w. and wesley, l.d. (1975) “a hydraulic triaxial apparatus for controlled stress path testing ” geotechnique, vol.25, n° 4, pp. 657-670. chaney, r.c. (1978). “saturation effects on the cyclic strength of sand” proc., earthquake engineering and soil dynamics, asce, vol. 1, new york, pp. 342-359. colliat, j.l. (1986). comportement des matériaux granulaires sous forte contraintes, influence de la nature minéralogique du matériau étudié, thèse de doctorat de spécialité, img, grenoble. giroud, j.p. et cordary, d. (1976). “prevision du tassement initial et du tassement différé des fondations superficielles.” annales de l’institut technique du bâtiment et des travaux publics, 135, sp 127, janvier, pp. 105-135. ishihara, k. (1993) “liquefaction and flow failure during earthquakes” geotechnique, vol. 43, n°3, pp. 351-415. ishihara, k., tsuchiya, h., huang, y. and kamada, k. 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(1978). “end restraint effct on undrained static triaxial strength of sand.” j. geotech. eng. div., proc. of asce, vol.104, n° 6, pp.687-704. martin g.r., finn w.d.l. & seed h.d. (1978) “effects of system compliance on liquefaction tests”, journal of geotechnical engineering division, asce, vol. 104, n° 4, pp. 463-479. mathiroban, s., and grozic, j. (2004). “a model to predict the undrained behaviour of loose gassy sand.” proceeding 57th geotech. conf., session 6g, pp.16-22. mullilis, j.p., townsend, f.c. and horz, r.c. (1978). “triaxial testing techniques and sand liquefaction.” astm stp 654, dynamic geotechnical testing, pp. 265-279. nakazawa, h., ishihara, k., tsukamoto, y., and kamata, t. (2004). “case studies on evaluation of liquefaction resistance of imperfectly saturated soil deposits.” proc., conf., cyclic behavior of soils and liquefaction phenomena, germany. pietruszczak, s., pande, g. and oulapour, m. (2003). “a hypothesis for mitigation of risk of liquefaction.” geotechnique, vol. 53, n° 9, pp. 833-838. robinet, j.c., mohkam, m., doanh, d. and torrenti, j.m. (1983). “amélioration des appareils triaxiaux pour les sols et les bétons.” 18ème colloque annuel du groupe français de rhéologie, méthodes et appareils de mesures rhéologiques, paris. sherif a.m., tsuchiya, c. and isibashi, i. (1977) “saturation effects on initial soil liquefaction.” journal of geotechnical engineering division, vol. 103, n° 8, pp. 914-917. tatsuoka, f., maeda, s., ochi, k. and fujii, s. (1986). “prediction of cyclic undrained strength of sand subjected to irregular loading.” soils and foundations, vol. 26, n° 2, pp.73-89. tatsuoka, f., molenkamp, f., torii t., hino, t. (1984). “behavior of lubrication layers of platens in element tests.” soils and foundations, vol. 24, n°1, march, pp.113-128. yang, j. (2002). “liquefaction resistance of sand in relation to p-wave velocity.” geotechnique, vol. 52, n°4, pp.295-298. yang, j., and sato, t. (2001). “analytical study of saturation effects on seismic vertical amplification of a soil layer.” geotechnique, vol.51, n° 2, pp. 161-165. yang, j., savidis, s. and roemer, m. (2004). “evaluating liquefaction strength of partially saturated sand.” journal of geotech. and geoenviron. eng., asce, vol. 130, n° 9, pp. 975-979. yoshimi, y., tanaka, k., tokimatsu, k. (1989). “liquefaction resistance of partially saturated sand.” soils and foundations, vol.29, n°3, pp.157-162. 186 noureddine della modelo gravimetrico del espesor de la corteza terrestre en colombia oscar eduardo hurtado briceno erick leuro parra universidad distrital "francisco jose de caldas", facultad de ingenieria catastral y geodesia hurtado 0., & e. leuro. (2000): modelo gravirnetrico del espesor de la corteza terrestre en colombia. geofls, colomb, 4:11-22, issn 0121-2974. santa fe de bogota, d.c, colombia, resumen la determinacion del espesor de la cortez a terrestre en colombia, a partir de datos gravimetricos, se hizo utilizando datos de anomalia total de bouguer y altura topoqrafica suministrados por el instituto geoqrafico agustin codazzi. estos fueron representados mediante series bidimensionales de fourier, que al ser combinadas con expresiones matematicas del potencial gravitacional terrestre, permitieron plantear el grosor cortical en terminos observables (gravedad y altura) sobre la superficie terrestre. los resultados han sido dispuestos en una malla de puntos que se interpolan a una superficie utilizando el algoritmo de minima curvatura. para la estimaci6n de la confiabilidad de los resultados se ha realizado una confrontacion con las deducciones geologicas obtenidas de algunas interpretaciones sismicas realizadas en el pais. abstract the gravimetric model of the crustal thickness of the earth in colombia was made from total bouguer anomaly and topographic height given by the instituto geoqrafico agustin codazzi. they were transformed to bidimensional series of fourier, that combined with mathematics expressions of the gravitational potential, both, they made possible to find the crustal thickness starting from observable values across the earth's surface (gravity and topography). the results of several points were converted to surface through minime curvature algorithm. in order to obtain reliability of the results, seismic control was applied from another works. 1. introduccion ei conocimiento de la estructura interna del planeta tierra es el objeto fundamental de la geofisica, parte de ello corresponde al estudio de la corteza terrestre, la cual no s610 es la capa mas externa, sino que adem as, en ella se presentan la mayor cantidad de fen6menos que afectan al hombre. la determinaci6n de la profundidad de la cortez a terrestre, involucra conocer, en un espacio y tiempo dados, la dinamica de las caracteristicas de la estructura terrestre tales como densidades, tectonica de placas, valores de anomalia, etc., variables que se relacionan mediante el desarrollo de metodologias que pretenden ajustarse a las particularidades geol6gicas y fisicas de cada regi6n, con el objeto de acercarse a los valores mas probables del grosor cortical terrestre. no obstante el gran numero de estudios de la corteza terrestre, el dimensionamiento de su espesor ha revestido dificultad en raz6n de la imposibilidad de poder formular metodologias deductivas que permitan unificar el tratamiento de las variables densidad, estructuras presentes, movilidad y dimensi6n entre otras. en el presente estudio se utiliza la metodologia desarrollada por el geofisico japones chuji tsuboi (1938a, 1938b, 1981), quien deduce la profundidad de la corteza a partir de la soluci6n del potencial gravitacional apoyado en la hip6tesis isostatica de airy. adicionalmente, este procedimiento se compara con otras metodologias gravimetricas aplicables a escala mundial, con el prop6silo de establecer similitudes y grado de confiabilidad de los resultados obtenidos. 2. metodologia la metodologia desarrollada en el presente trabajo consiste en calcular el valor del espesor de la corteza terrestre a partir de la soluci6n de la ecuaci6n del potencial qravitacionat expandido en series de fourier. este metodo tiene la caracteristica de ser deductivo, porque, parte de una relacion flsico maternatica, que es el potencial de gravedad, en don de una de sus soluciones esta en funci6n de la profundidad del plano subterraneo de rnasas, responsable del campo gravitatorio observado en superficie (anomalia total de bouguer). de acuerdo al modele de airy existe una distribuci6n de masas -p veces el valor de la altura topografica a 10 largo de la superficie de la corteza terrestre con un promedio de espesor d. la anornalla total de bouguer y la altura topoqrafica se expanden mediante el uso de las series de fourier, que son de acuerdo con la teoria matematica, la funci6n que rnejor minimiza el error (tsuboi, 1981), es decir, hace que el error minimo cuadratico tienda a cero 0, en otras palabras la serie de fourier es la funci6n que nos produce el mejor ajuste de minimos cuadrados. otras metodologias parten de valores observados de gravedad, los que se ajustan por minimos cuadrados a la funci6n maternatica mas representativa, esta interpolaci6n distorsiona las mediciones disminuyendo la precision. 2.1 solucion de la ecuacion de laplace para el potencial gravitacional la soluci6n de la ecuaci6n detaplace en coordenadas cartesianas puede ser utilizada para encontrar la distribuci6n de las mas as subterraneas responsables de las distorsiones gravitatorias observadas en superficie (tsuboi,1981). en el caso bidimensional la ecuaci6n de laplace es: (1) donde z se toma positivo hacia el interior de la tierra. si se considera que el potencial es una cantidad escalar, cada miembro de la ecuaci6n debe corresponder a un ente;o d,e igual magnitud y signa algebraico diferente, m2 y -m asi: 82u 1 d2x 2--=---=m 8x2 x dx' 82u 1 d2z 2--=---=-m 8z2 z dz' (2) las soluciones para las ecuaciones (2) son: cosx(x) = mx sen (3) el producto de las expresiones (3) soluciona el potencial u(xz), que multiplicado por una con stante am y expandido en series se presenta como: cos u(xz) = i a mx e':" m m sen (4) se observa un doble signo algebraico en el terrnino exponencial de mz, la elecci6n del signo depende de la direcci6n en que el eje z se asuma positivo con respecto a su origen; para fines opera cion ales la direcci6n del eje z se toma positivo descendentemente y el origen z = 0 sobre el nivel medio del mar (tsuboi, 1981). 2.2 deducci6n de la formula para el espesor de la corteza terrestre estudios a nivel mundial indican que el campo gravitacional y la superficie del relieve estan conectados muy cercanamente con el espesor de la corteza terrestre (mironov, 1977). algunos autores como bowie, heiskanen, hayford, wollard y otros intentaron determinar el mejor ajuste para el espesor de la corteza terrestre buscando que valores satisfacian la condicion de error minima cuadratico i(observado calculadot "un metoda mucho mas simple y directo es buscando el espesor de la corteza terrestre a partir de las series de fourier" (tsuboi, 1981). ei desarrollo del modele se basara en la hip6tesis isostatica de airy ya que aparece generalmente, consistente con la evidencia sismol6gica (dobrin, 1984); ademas, desde el punta de vista geol6gico es mas verosimil (mironov, 1977). es necesario aclarar que la hip6tesis de pratt parece ser mas acertada para ciertas regiones del globo en las que el espesor cortical es regular en lugares como plan ides costeras y algunos tipos de montanas (dobrin, 1984). si por alguna raz6n se prefiere el modelo de pratt al de consideraci6n un factor de 2 (tsubol, 1981). sup6ngase una superficie topoqrafica expresada en series de fourier: cos h(x) =" n; mxl...m sen (5) de acuerdo con la hip6tesis de airy, existe una distribuci6n de masas -p veces h(x) a 10 largo de la interface entre corteza y manto que coincide con el espesor de la corteza terrestre a un valor de profundidad d. la distribuci6n de anomalia total de bouguer sobre la superficie que es producida por esta distribuci6n de masas es: (6) de otra parte, la anomalia total de bouguer observada puede ser: cos i1go (x) =" bm mxl...m sen (7) de acuerdo con airy las dos expresiones anteriores de li9o(x) son iguales; despejando d y extendiendo ados dimensiones se obtiene: (8) la ecuaci6n (8) corresponde a ia deducci6n de la f6rmula del espesor de la corteza terrestre en forma bidimensional de acuerdo con la hip6tesis isostatica de airy y que en realidad "actua como un filtro gravimetrico de tipo regional" (diaz ramiro. comunicaci6n personal. departamento de geociencias, universidad nacional) el cual, elimina todo efecto gravitatorio diferente al del espesor de la corteza terrestre (anomalias de frecuencia alta y media). si, los valores encontrados son el resultado de suponer que el espesor de la corteza terrestre se puede asumir como el de una ondulaci6n h(x) entre dos capas rocosas de densidad diferente p y p'. que corresponden a la corteza y el manto respectivamente, en dicha ondulaci6n (te6ricamente) se condensa la masa correspondiente a la capa de menor densidad (corteza). en la ecuaci6n (8) se observa un signo menos en la parte derecha: para obtener valores positives de d es necesario que: iog[bmll lj e 2rrgph mn (9) resulte negativo y esto requiere que: sea mayor que: tamblen hay un signa menos en: esta expresi6n tiene que resultar positiva, con el fin de aplicar logaritmo, por 10 tanto 8mn y su correspondiente hmn deben ser opuestos en signa algebraico. en consecuencia, existen dos condiciones necesarias que son: 1) 8mn y hmn deben ser opuestos en signa algebraico. 2) 12ngphmn i > i 8mn i a partir de esto 8mn y hmn se denominan pares racionales de la funci6n profundidad del plano de masas 0 profundidad de la corteza terrestre (tsuboi, 1981). 2.3 solucion de los coeficientes 8mn y hmn de acuerdo a la expresi6n (8), el espesor de la corleza terrestre esta en funci6n de la expansi6n bidimensional de fourier para la anomalia bouguer total y la altura topoqrafica, ello se logra haciendo: cos cos bmn nix nysen sen (10) cos cos /-fmn nix ny ~'mk-dn sen sen hattar los terminos bml! y hmn siqnifica encontrar los coeficientes discretos de fourier. si las observaciones estan recolectadas en forma de malia, la longitud de onda fundamental en la direccion x1 va a ser igual a la longitud del grupo de datos en aquella 0 111; la longitud de onda fundamental en la direccion x2 puede ser definida similarrnente como 112. asi los coeficientes para cualquier armonico pueden ser encontrados a traves de (davis, 1984): a i. 21rk1x1 21rk2xj'j cos_ ..~-.-.--'.coo;-~~--~;.. 111 i 21rk1x1,. 21rk2x2y,j cos -----~ sen ------.....!.. 111 112 ·1 i 2lrklx11 2lrk2x2/},/sen-----cos---111 112 (11 ) donde: k ::: 2 si k1 ::: a 6 k2::: 0 pera no ambos k 4 si k1 > a y k2> a k1 ::: numero del armonico en la direccion x1 k2::: numero del armonico en la direccion x2 n1 ::: numero de puntos en la malla en la direcci6n x1 112::: numero de puntos en la malla en la direccion x2 3. espesor cortical en colombia de acuerdo con tsuboi ei metodo de tsuboi se aplico en una region cuadrada de 2376 km de longitud, extendido entre i, :::86°w f, ::: 64'w y <p ::: is <p ::: 14'n. cuyos datos de anomalia de bouguer total y altura topografica se encuentran dispuestos en mallas espaciadas a 0,25 unidades de grado. estos datos se expandieron a coeficientes discretos de fourier aplicando la ecuacion (11) que, al remplazarla en la ecuacion (8) se obtienen los valores de espesor de corteza terrestre mostrados en la fig.1. en esta figura se observa que el pais presenta una tendencia general de 40 a 45 km de espesor, siendo el mayor valor de profundidad cortical de 52 km y el menor de 16 km en la region oriental del pais la tendencia es suave y oscila entre 35 y 45 km, presentando una pendiente maxima de 7,5% (<p ::: 5'n y f, :::70' w) y una pendiente minima de 2,12% (rp ::: 0' y i. :::70' w). en la region de los andes su tendencia es abrupta y se presenta el mayor valor del pais de 52 km en (<p ::: 2 n y /, ::: 76'w) , sus espesores oscilan entre 45 y 52 km, presentando una pendiente minima de 0,9% (entre <p ::: 6'n. /. ::: n'w y <p ::: 3'n, i. ::: 74' w) y una pendiente maxima de 6,04% (<p ::: 2'n y i. :::74'w). en la region caribe su tendencia es homoqenea, presenta espesores entre 40 y 45 km, con una pendiente minima de 4,92% (<p ::: 9'n y i. :::73'w) y una pendiente maxima de 6,02% (p::: 10'n y i. :::74'w). en el oceano pacifico su tendencia es suave presentando valores de 19 y 40 km con tendencia a disminuir rapidamente mar adentro, el valor de pendiente minima corresponde a 0,12 % (entre <p ::: 5'n y <p ::: 8°n, sobre /, ::: 81 w) y un valor de pendiente maxima de 6,02% (<p ::: in y /. ::: 83'w) , en el oceano atlantico su tendencia es suave y presenta el menor valor del pais de 16 km, (<p ::: 14'n y i. :::84°w) , sus valores oscilan entre 16 y 43 km, con tendencia a disminuir rapidamente mar adentro, el valor de pendiente minima corresponde a 1% (entre <p ::: 12"n, i. ::: 74°w y <p ::: 14°n, i. ::: now) y un valor de pendiente maxima de 6,02% (<p ::: 13'n y /, ::: 83'w). en general la tendencia del pais es suave y homogenea, aunque se observa que existecontraste entre las regiones geograficas con respecto a valores maximos y minimos de espesor cortical, distribucion de pendientes maximas y minimas y diferenciacion entre la corteza oceanica y la continental, a pesar que la transicion entre las dos es bastante suave. 4. modelos gravimetricos del espesor de la corteza terrestre de acuerdo a otros auto res los modelos expuestos a continuacion corresponden a metodologias desarrolladas para ser aplicadas en cualquier lugar de la tierra, estos sirven como patr6n de comparacion con la metodologia del presente trabajo. 4.1 modelo gravimetrico del espesor de la corteza terrestre en colombia sequn r.m. demenitskaya la formula empirica para calcular el valor mas probable del espesor de la corteza terrestre en cualquier parte del globo sequn demenistkaya es (mironov, 1977): h = 35(1 tanh 0,0037 m) donde: (12) h: es el espesor de la corteza terrestre en km /';g: es la anomalia total de bouguer. este modelo coincide satisfactoriamente con el desarrollado independientemente por wollard en 1958 (demenitskaya, 1969). ei error relativo de determinacion del espesor de la corteza oscila entre el 15 y el 40% siendo el promedio alrededor de un 20%. para distintas regiones el error es diferente: en los oceanos es de 1,3-4 km, en los continentes de 5-8 km. en los valores de espesor de corteza terrestre rnostrados en la fig.2, se observa que el pais presenta una tendencia general de 40 km de espesor. ei mayor valor de profundidad cortical corresponde a 55 km y el menor de 20 km. en la region oriental del pais la tendencia es suave y predomina un valor de 35 km; en la region de los andes se presenta el mayor valor del pais, 55 km en <p = 25' y i, = 76·w, sus espesores oscilan entre 35 y 50 km; en la region caribe presenta espesores entre 40 y 45 km; en los oceanos pacifico y atlantico presenta valores de 35 y 20 km con tendencia a disminuir rapidarnente mar adentro. 4.2 modelo gravimetrico del espesor de la corteza terrestre en colombia sequn wollard ei siguiente es el modelo desarrollado por wollard en 1958 (deminitskaya, 1969), donde: h = 32 o,08~g (13) h: profundidad de la discontinuidad de mohorovicic en km. /';g: anomalia total de bouguer. en los valores de espesor de corteza terrestre mcstrados en la fig.3, se observa que el pais presenta una tendencia general entre 30 y 40 km de espesor. ei mayor valor de profundidad cortical corresponds a 40 km y el menor de 20 km. en la region oriental del la tendencia es suave y predomina un valor de 25 km; en la region de los andes se presenta el mayor valor del pais, 45 km en tp = 2"n y i, = 75'w, sus espesores oscilan entre 35 y 45 km; en la region caribe presenta espesores entre 30 y 35 km. en los ocean os pacifico y atlantica presenta valores de 30 y 20 km can tendencia a disrninuir rapidamente mar adentro. 4.3 modelo gravimetrico del de la corteza terrestre en colombia sequn wollard & strange ei siguiente es el modele desarrollado por wollard y strange en 1962 (deminitskaya, 1969), donde h ( "').5 h ~g + 75'j.l, tan 275 (14)40,5 h: profundidad de la discontinuidad de mohorovicic en km. i\g: anomalia total de bouguer, en los valores de espesor de corteza terrestre mostrados en la figa, al igual que 10 mostrado en la seccion anterior, se observa que el pais presenta una tendencia general entre 30 y 40 km de espesor. ei mayor valor de profundiad cortical corresponde a 40 km y el menor de 20 km. en la region oriental del pais la tendencia suave y predomina un valor de 25 km; en la region de los andes se presenta el mayor valor del pais, 45 km en (0 = y = 75'w, sus espesores oscilan entre 35 y 45km; en la region caribe presenta espesores entre 30 y 35 km. en los oceanos pacifico y atlantico presenta valores de 30 y 20 km con tendencia a disminuir rapidamente mar adentro. considerando que ta forma de la anomalia de bouguer total muestra la tendencia de la discontinuidad de mohorovicic. en las figs.1-4 se muestra la modelos entre sl, principalmente en los exr)onen,8iales. de los comparando los modelos gravimetricos entre si se observa que el sugerido por tsuboi mantiene la tendencia de los otros modelos, pero de una forma mas suavizada. es de anotar que los modelos exr)orlenciales se parecen entre si ya que su estructura matematica es muy semejante. j,. ol ~ -43 .aq ~ 'tp longitud figura 1. espesor de la corteza terrestre segun tsuboi 1 8~ 6 0 ::::> i4i::s 1.0 c') 2 c\c")\ r: \ 0 i\,j i & -2 eo <0 i longitud figura 2. espesor de la corteza terrestre segun demenitskaya <0 <0 i 1 1 8 longitud figura 3. espesor de la corteza terrestre segun wollard -4 c ::::> t4 5 tabla 1. control sismico de los modelos gravimetricos region tsuboi demeniwollard wollard proyecto rangos de aspectos modelo tskaya modelo strange narino anomalia geologicos por series modelo lineal en modelo modelo bouguer de fourier hiperb61ico km hiperb61ico sismico en total en en km en km en km km mgal oriental 40 35 25 25 -41a +20 tipo de corteza: i de continente ! i medio i ii tect6nicamente estable andes 50 55 45 45 tipo de (jl == 5,5'),== 76'w 45 38 34 34 30 corteza: orogenesis (jl == 5,5'i, == 75,5"w 43 46 38 41 45 -30a -150 reeiente (jl == 2' i. == 76'w 47 53 43 48 45 fallamiento normal reciente volcanismo e intrusion: gran promedio en i altura caribe 40 40 30 30 -93a +140 corteza de planicies j6venes oceano 35 20 20 20 tipo de pacifico 40 22 19 18 30 +30 a +250 eorteza: grandes areos (jl == 3,6'a == 78,5'w 36 5 15 15 20 de isla (jl == 5,5'),== 78'w 43 30 alta intensidad(jl == 4'y a == 81'w volcanica, fallamiento y plegamiento oceano 35 20 20 20 +150 a +250 corteza atlantico oceanica, ausencia de capa de sedimentos y capa basaltica en el rnetodo de tsuboi sobre el oceano pacifico se presenta una divergencia del orden de 15 a 25 km con respecto a los otros modelos y adernas la rata de decrecimiento en los oceanos es muy lenta en direcci6n mar adentro. para el oceano atlantico el comportamiento es analoqo. en la corteza continental la similitud entre tsuboi y todos los modelos en general as notable, con una divergencia minima del orden de ±5 km sobre la regi6n de los andes; presenta una divergencia del orden de ±10 km sobre la regi6n oriental y, sobre la regi6n caribe, el comportamiento es muy similar en cuanto a valores corticales y distribuci6n de isolineas. 5. control sismico de los resultados obtenidos a partir del metodo gravimetricos los datos gravimetricos del espesor de la corteza terrestre estan en funci6n de la hip6tesis isostatica elegida, por tanto, es imposible decidir cual de los modelos lsostaticos esta mas cerca de la realidad, partiendo solo de datos de gravedad (tsuboy, 1981). se necesita apoyo de otras fuentes geofisicas como la refraccion y reflexion sismicas y la sismologia. los valores de espesor obtenidos a partir de datos slsmicos se calculan considerando que "uno de los grandes contrastes de impedancia dentro de la iitosfera es el limite de la corteza y el manto 0 moho, el que se obtiene a partir de la generacion de una prominente conversion de ondas pas y ps, despues del arribo de la onda directa p, la medicion del tiempo entre el arribo de la onda directa p a la ps es una funcion tanto del promedio de las velocidades de la corteza y el espesor de esta" (beck et al., 1991). los promedios para las velocidades de las ondas sismicas dependen de las caracteristicas del medio y los factores de deformacion, esfuerzos propios de este, las velocidades promedio en la corteza tienen un valor de 6,5 km/s y en el manto un promedio de 8,5 km/s (dobrin, 1978). ai contrastar valores promedios de espesores de la corteza terrestre mediante los metod os gravimetricos y slsrnicos usualmente difieren en algunos grados; estas diferencias pueden atribuirse a (demenitskaya, 1969): 1) posibles errores en las determinaciones slsmicas, un exceso de ±2 km. 2) cam bios significativos en la composicion de la corteza asociada a estructuras mayores en el orden geol6gico. 3) inhomogeneidad en el manto. los trabajos efectuados sobre la determinacion del valor esperado de la corteza terrestre en colombia a partir de registros sisrnicos son: proyecto nariiio (lnstituto geofisico de los andes, 1977, 1981), bogota, 17 km (tovar, 1980) y mar caribe, norte de cartagena, 16 km (ewing, 1960; en tovar 1980), en el control sismico mostrado en la tabla 1 se descartan los dos ultirnos valores ya que estos no coinciden con ningun modele qravimetrico. conclusiones en este trabajo se construyo el primer modelo gravimetrico integral de variaciones del espesor de la corteza terrestre en colombia. ei comportamiento de las isolineas de la discontinuidad de mohorovicic se ajusta al modele isostatico de airy, ya que se observa que la tendencia de la altura topoqrafica es directamente proporcional al espesor de la corteza terrestre. adicionalmente se observa que el relieve de la discontinuidad de mohorovicic sigue la tendencia de la anomalia total de bouguer. de acuerdo con las caracteristicas geologicas del pais se puede concluir que los valores de corteza obtenidos corresponden a los supuestos iniciales dados por el control sismico como se observa en la tabla 1. de los resultados obtenidos de acuerdo al modele cortical se concluye que el comportamiento del espesor de la corteza terrestre por zonas corresponde a la siguiente descripcion: • oceano pacifico: alii se observa que los valores de espesor cortical son mayores que los esperados para cuencas oceanicas, sin embargo este resultado coincide con los valores calculados por rnetodos slsrnicos del proyecto nariiio, en el que efectivamente los val ores de profundidad son mayores de 10 esperado para cuencas oceanicas, esto se debe a tres factores: a) en primer lugar la presencia de una zona de subduccion a causa del choque de las placas tectonicas de surarnerica y de nazca, siendo esta ultima la que subduce. b) en segundo lugar, los datos sismicos ponen de manifiesto una zona bland a del manto oceanico, 10 cual induce a un flujo termico superior a 10 normal haciendo que la corteza penetre mas en el manto. c) por ultimo, la presencia sobre la cuenca oceanica de las cordilleras oceanicas de malpelo, carnegie y cocos 10 cual obliga a un reacomodamiento isostatico superior al normal para una cuenca oceanica (lnstituto geofisico de los andes, 1977). • oceano atlantico: tarnbien alii se observa al borde de la planicie costera valores altos para una cuenca oceanica, esto se debe a un probable buzamiento lento de la discontinuidad de mohorovicic de la sierra nevada de santa marta que penetra sobre el oceano. • regi6n de los andes: observando el comportarnrento de las isolineas de la discontinuidad de mohorovicic se ve que las tres cordilleras estan compensadas por una (mica antiraiz, sin distinguir un ajuste isostatico individual para las diferencias geologicas existentes entre cada cordillera, tales como: edades, composici6n de rocas y factores oroqenlcos. la gran profundidad de este sistema montafioso corresponde a regiones tect6nicamente activas con su respectiva actividad sismol6gica y volcaruca que elevan el espesor cortical. • region oriental: a pesar de ser una gran planicie presenta altas variaciones del espesor cortical debido a la presencia del macizo guayanes que aflora en la regi6n del orinoco colombiano, debido a su gran edad geol6gica ie corresponde el valor promedio global de corteza de 35 a 45 km. • region caribe: esta influenciada en su mayor parte por la sierra nevada de santa marta donde se observa la mayor descornpensacion isostatica del pais ya que la antiraiz de 45 km obtenida no es suficiente para compensar una elevaci6n montafiosa de 5700 m. de 10 anterior se desprende que los modelos desarrollados concuerdan satisfactoriamente con los datos de control sismico reahzados en el proyecto narino dentro de la corteza continental, presentandose diferencias maximas de ±5 km, poniendose de manifiesto la validez de la informacion gravimetrica y las metodologias desarrolladas. de acuerdo con consideraciones fisicornaternaticas se concluye que el modelo mas confiable es el desarrollado por series de fourier, a los correspondientes a analisis de reqresion lineal y exponencial porque sequn la teoria maternatica es la funcion que mejor minimiza el error medic cuadratico. referencias bibliograficas beck,s., i. drake, s. meyers, p. silver, t. wallace. (1996): crustal thickness variations in the central andes. geology. 24: (5), pp. 407. davis, j.e. (1984): data analysis in geology. pp. 284. willey. usa. deminitskaya, r (1969): the relation between the earth's crust, surface and gravity field in the urss en the earth's crust and upper mantle, pembroke and hart, washington d.c. dobrin. (1978): introduction to geophysical prospecting. second edition. pp 31. mac graw hill. newyork. dobrin. (1984). introduction to geophysical prospecting. third edition pp. 498. mac graw hill. newyork. hurtado, o. & e. leuro. (1997): modelo gravimetrico del espesor de la corteza terrestre en colombia. universidad distrital francisco jose de caldas. tesis de pregrado, facultad de ingenieria, departamento de ingenieria catastral y geodesia. instituto geofisico de los andes. (1977): la translcion oceano continente en el sur oeste de colombia (proyecto narifio). guadalupe, santa fe de bogota. instituto geofisico de los andes. (1981): investigaciones geoflsicas sobre las estructuras oceano-continentales del occidente colombiano. (proyecto narifio) guadalupe. santa fe de bogota. mironov, v. (1977): curso de prospeccion gravirnetrica. revette. barcelona. tovar, j. (1980). estlmacion del espesor de la corteza terrestre en bogota mediante registros sismicos. tesis de pregrado. universidad de los andes, facultad de ingenieria, departamento de ingenieria civil. santa fe de bogota. tsuboi, c. (1938a). relation between gravity anomalies and the corresponding subterranean mass distribution (ii). no 15 pp. 272-283. bulletin of earthquake research institute, tokio imperial university. tsuboi, c. (1938b). relation between gravity anomalies and the corresponding subterranean mass distribution (iv). no 17 pp. 384-410 isostasy in the united states of america.. bulletin of earthquake research institute, tokio imperial university. tsuboi, c. (1981). gravity. pp 204. allen and unwin. londres. geofisica colombiana pp. 61-64 bogota, d.c.n°.7 dlciembre de 2003 !ssn 0121 2974 modelo de velocidades de intervale en profundidad a partir de funclon de dlfracclen: apllcaclon a datos reales carlos augusto ortega galvis luis alfredo montes vioes grupo de invesligaci6n en geofisica, universidad nacional de colombia, e-mail: lamontesv@unal.edu.co resumen se implanto un algoritmo de solucion al problema inverso para estimar las velocidades de intervalo y construir modelos en profundidad. ei metodo basado en la teoria del rayo paraxial de la sismica optica se aplic6 a registros del program a majayura72, suministrando un modelo inicial para la migracion pre-apilada en profundidad. las posiciones fuente, receptor y tiempos de viaje de los eventos reflectores necesarios para estimar la funcion tiempos de transite son tornados en configuraciones cmp y zero offset. usando la funcion tiempos de transite de reflexion se calcula la funci6n tiempos de difraccion, y a partir de ella se trazan rayos difractados en profundidad que al enfocar permiten conocer la velocidad de intervalo. palaras clave: migracion, inversion sismica, velocioad de intervalo abstract an algorithm to solve the inverse problem was implemented, estimating the interval velocity and building seismic models in depth. based in the paraxial ray theory of seismic optics, the method was applied to seismic data of majayura project, providing a former model for pre stack depth migration. traveltimes of reflectors, source and receiver position, used to estimate tha travetiem function, were obtained from cmp gathers and zero offset sections. the diffraction function is estimated from the traveltime function allowing to calculate the interval velocity through focusing of traced rays. keywords: migration, seismic inversion, interval velocity introduccion durante el procesamiento se utilizan diferentes tipos de velocidades con el propos ito de obtener una imagen del subsuelo; en cada de sus etapas se requiere asignar un campo de velocidad que funcione, independiente de las caracteristicas del medio. caso muy distinto son los procesos de migracion en profundidad, en donde se necesita el valor de velocidad con que la serial viaja a traves de las sucesiones rocosas, es decir la velocidad de intervalo. al interprete ie interesa la velocidad de intervalo en profundidad a fin de realizar la rnigracion en profundidad. si el objetivo final es colocar los eventos buzantes en su verdadera posicion y colapsar las difracciones, es obvio que en areas de gran complejidad estructural debe existir diferencias entre un apilado final y una migracion en profundidad. luego de la aparicion del concepto de migracion, y de sus efectos en el mejoramiento de la imagen de subsuelo, es aun mas manuscrrto recibido para evauacicn ei 15 de junio de 2003. articulo aceptadc para publicacion por el cornte editorial el 30 de octubre de 2003, geofislca cdlombiana, 7, diciembre de 2003 clara la necesidad de desarrollar e implementar metodos y tecnicas que ofrezcan mayor seguridad y confiabilidad en modelamiento final. la velocidad de la sefial sismica no es otra cosa que "una propiedad fisica del medio" (hubral and krey, 1980). calcular la velocidad de intervalo a partir de los tiempos de transito es resolver el problema inverso a partir de las caracteristicas cinematicas del medio. la optica geornetrica brinda una soluc io n a la ecu ac ion de onda al propagarse en un medio hornogeneo-isotropico, cuando el pulso que se propaga tiene longitud de onda pequefia comparada con las dimensiones estructurales del modelo y cuando los cambios espaciales de los parametros de lame y la densidad son pequefios sobre la distancia de una longitud de onda. la estimacion de un modelo de velocidad de intervalo en profundidad requiere informacion obtenida fundamentalmente de registros sonicos de pozo, la cual no siempre se encuentra disponible para los procesos de obtencion de imagenes del subsuelo en profundidad. dicho modelo puede estimarse a partir de un procedimiento cuidadoso de calculo de la funcion 61 tiempos de transite y la posterior inversion de rayos en profundidad (montes, 1999). fundamento teorico teniendo en cuenta las ideas de hamilton sobre rayos de luz, bortfeld (1989), asumiendo un modelo de capas homogeneas e isotropicas, separadas por interfases suaves, con velocidades y densidades arbitrarias, determino la funcion de los tiempos de transite en la vecindad del rayo central: donde i., =(xs' ys) posicion de la fuente i,=(x, ,y, ) posicion del receptor to = tiempo de transite de reflexion del rayo central (rayo normal) s; ao= matriz de curvatura de la onda cmp d~l co = matriz de curvatura de la onda normal la ecuacion (1) por simetria "tiene nueve incognitas: una en ta, dos en po' tres en b~l ao y tres en d~lco' las cuales pueden estimarse registrando al menos nueve tiempos de transite, especialmente seleccionados; para ella se toman los datos al menos en dos configuraciones de la sismica, en este caso common mid point y zero offset (montes, 2000). la curva de tiempos de difraccion depende de la velocidad de intervalo, y la suma de las amplitudes a 10 largo de ella permite el proceso de colapso de las difracciones a su apice durante la migracion, razon de su importancia en los procesos de obtencion de imagenes del subsuelo. en la vecindad del rayo central esta determinada a partir de la funcion de tiempos de trans ito (montes, 2000), por la ecuacion (1). donde i es la posicion del detector. la funcion tiempos de difraccion (2), permite conocer el tiempo que tarda la onda desde un punto difractor en subsuelo hasta un punto en la superficie. con el valor de velocidad de intervalo calculado para el primer medio y de la geometria del primer reflector, asi como de los angulos de buzamientos de los rayos difractados, se calcuia el valor de velocidad del medio siguiente que los hacen converger en un punto. mediante un procedimiento iterativo se realiza la 62 ortega it al. (1) figura 1. registro emp gather. en este registro correspondiente at cdp 319 se ilustra el reflector correspondiente a la discordancia al tope del cretacico superior. en cada una de las trazas se toman los datos de posicion y tiempos de viaje en el maximo valor de amplitud. figura 2. regisrro cero offset. en este registro se muestra el picado de tiempos tfpico elosel cdps 328-312. (2) continuacion hacia debajo de la funcion tiempo de difraccion para cada horizonte de interes, obteniendo un modelo de velocidad en profundidad. aplicacion a datos reales la toma de datos y calculo de velocidades se hizo sobre familias de trazas em? y la seccion zero offset. estos registros deben llevarse al mismo datum para facilitar la identificacion del reflector de interes. se definio un nuevo sistema de coordenadas con origen en el comienzo de la linea y se hizo coincidir esta con el eje, aplicando los siguientes pasos. geofislca colombiana. 7. diciembre de 2003 modelo de velocidades de interv alo en profundidad a partir de funcion de difraccion: aplicacion a datos reales t d'~.', figura 3. analisis grafico veloeidades l(xm;g y2m;,). curva que muestra el ajuste del valor de velocidad de intervalo dado al program a central, en el cdp 319. ei valor de velocidad es de 2220 m/s yla apertura de rnigracion de 60 metros. identiticecton del reflector en el cop alrededor del rayo central en una secci6n apilada se registraron posiciones de fuente-receptor y quince tiempos de trans ito (figura 1). igual procedimiento se realize en una familia cmp a 10 largo del reflector (figura 2). luego el programa con el algoritmo implantado permite ingresar el valor de la velocidad de migraci6n y muestra una grafica de forma hiperbolica, con va!ores xmig contra zmig muy cercanos cuando la velocidad es correcta (figura 3). con los valores de velocidad obtenidos en cada punta de la grilla se elaboro el modele de velocidades (figura 4). una vez calculado y validado minuciosamente el modelo de velocidades para la linea sismica m-72-0 1 con los datos de pozo, se cargaron los datos al software promax y se implementa un flujo de procesamiento para efectuar la migracion pre-apilado en profundidad lograndose resultados satisfactorios, obviamente susceptibles de ser mejorados mediante la utilizacion de la tomografia en un proceso interactivo y repetitivo. figura 4. modelo final de veloeidades de intervalo para la linea m-72-0j. este modelo es cl resultado de cargar al sistema las velocidades de intervalo obtenidas mediante la aplicacion de la teoria paraxial. ei modelo fue validado con la informacion de pozo y mejorado en un proceso interactivo. geoflslca colombi.~na, 7, diciembre de 2003 63 ortega it al. figura 5. migraci6n pre-apilado en profundidad linea m-72-01. esta secci6n muestra las caracteristicas del subsuelo. la resoluci6n del programa y la longitud del tendido limitan de alguna manera la calidad de la secci6n. finalmente se hace la prestack depth migration en profundidad usando promax y se obtiene la secci6n migrada en profundidad (figura 5). la secuencia de pasos implementada se apoyo en el modulo de analisis de yelocidades de migracion (migration velocity analisis mva) del software mencionado. aunque en el my a se presentan varias opciones para construir y depurar el modelo de velocidad de intervalo -la metodologia implementada fue muy escueta-, simplemente el modelo de velocidades cargado fue revisado y ajustado hasta obtener una seccion migrada admisible. conclusiones el calculo minucioso de los tiempos de viaje y la posici6n fuente-receptor de las secciones sismicas, y su posterior utilizaci6n mediante la aproximacion hiperbolica, son la 'base para la estimaci6n de la funci6n caracteristica de punto de hamilton, que esta asociada a cad a superficie reflectora en la vecindad del rayo central. la estirnacion mediante la teoria paraxial de la funcion de tiempos de transito de difraccion a partir de los tiempos de transite de reflex ion permite calcular el valor de la velocidad de intervalo en profundidad, independiente de la complejidad geologica, la geometria del subsuelo y las variaciones de velocidad, y en consecuencia es de gran utilidad en los procesos de migracion en profundidad de las secciones sismicas. la aplicaci6n de !a teoria del rayo paraxial en el calculo de velocidades de intervalo en profundidad se constituye en un metodo eficiente, robusto y econ6mico, que favorece procesos de imagen 64 como migraciones pre-apilado y pos-apilado en profundidad. el metcdo presenta gran estabilidad y garantiza desde el punto de vista geofisico un acercamiento al valor de velocidad real, con rnargenes de error entre el 3% y el 10%. esto es muy importante ya que procesos como la migraci6n son demasiado sensibles a las velocidades. en la actualidad los retos de la industria -yen particular de teenicas como la migraci6n pre-apilado en profundidaddemand an trabajo en equipo permanente entre procesador e interprete, dado que se trata de deterrninar la geologia del subsuelo, mejorar la capacidad prospectiva y disminuir los factores riesgo y costa en la operaci6n. bibliografia bortfeld, r. "geometrical ray theory: rays and traveltimes in seismic systems (second order approximations of the traveltimes)". geophysics, vol 54, number 3, march 1989. hubral, peter, & !(rey, theodor. 1980. interval velocities from seismic reflection time measurements, society of exploration geophysicists, tulsa, oklahoma. montes, l. 1999. estimaci6n de las velocidades de intervalo a traves de la funci6n de tiempos de difracci6n de la sismica geometrica. geologia colombiana, no. 24. universidad nacionai de colombia, santafe de bogota. montes, l. 2000. analisis de sensibilidad en la estimaci6n de la funci6n caracteristica de punto de hamilton de la sismica geometrica. geofisica colombiana, no.4. universidad nacional de colombia, santafe de bogota. geofislca colombiana, 7. oiciembre de 2003 the shock response and dynamic fracture of concrete gravity dams under impact load are the key problems to evaluate the antiknock safety of the dam. this study aims at understanding the effects of impact shock on the elastic response and dynamic fracture of concrete gravity dams. firstly, this paper uses acceleration records of a concrete gravity dam under impact to establish the correct way to determine the concrete gravity dam of the fundamental frequency and present cut sheets multi-degree-of-freedom dynamic modeling. under strong impact loading, the constitutive relation of concrete gravity dam and the highest frequency of the impact are uncertain. so, the main advantage of this method is avoiding the use of elastic modulus in the calculation. the result indicates that the calculation method is a reliable computational method for concrete gravity dams subjected to impact. subsequently, the failure process of dam models was numerically simulated based on abaqus commercial codes. finally, this paper puts forward suggestions for future research based on the results of the analysis. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): m1 -m6 abstract keywords: reservoir, gravity dam, underwater explosion, failure mode, dynamic analysis. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54133 h id r o l o g y lu lu1,2,3*, xin li2, jing zhou2, genda chen4, dong yun1 1 faculty of management engineering, huaiyin institute of technology, huai’an 223001, china 2the state key laboratory of coastal and offshore engineering, dalian university of technology, dalian, 110623, china 3the state key laboratory of structural analysis for industrial equipment, dalian university of technology, dalian 116023 china 4 the center for infrastructure engineering studies, missouri university of science and technology, rolla, missouri 65401, usa *corresponding author. e-mail: llzhxy@aliyun.com numerical simulation of shock response and dynamic fracture of a concrete dam subjected to impact load record manuscript received: 12/11/2015 accepted for publication: 05/02/2016 how to cite item lu, l., li, x., zhou, j., & chen, g. (2016). numerical simulation of shock response and dynamic fracture of a concrete dam subjected to impact load. earth sciences research journal, 20(1). m1-m6. doi: http://dx.doi.org/10.15446/esrj.v20n1.54133 introduction recently, the novosibirsk hydroelectric plant after a report about a planted explosive, russia has declared a state of emergency. so, this event should cause the attention of researchers and administrative departments. hydraulic dams are critical infrastructure in geotechnical engineering. they are often designed and built to store water for drinking and irrigation in adjacent areas, to add water recreation spaces, to create a water way for the short-distance transport of people and goods across deep canyons in mountainous regions, and to regulate the river during a flood event. a water-filled dam can boost the local economy through various personal and business activities, leading to the establishment of a new community center such as a village or a town, and representing a major capital and long-term investment. on the other hand, the breaching and an accidental damage of a dam can lead to a catastrophic flood event and its chain effects such as engulfing downstream residential areas and washing away agriculture lands. therefore, design and maintenance of dams are not only a serviceability issue but also a life-threatening matter to millions of people. when extreme events such as earthquakes, tsunamis, hurricanes, and tornadoes took place, concrete dams can be subject to extensive shaking and wave impact. the march-11, 2011, japan earthquake event testified the destructive power of the earthquake-induced tsunami. many scholars have studied the high dam subjected to earthquake action. among these are zhou et al. (2000), mir et al. (1995), kong et al. (2012). equally, if not more important, dams are also vulnerable targets for man-made explosion events, particularly with the advent of advanced long-range and precision missile technologies. since the september 11 attacks by terrorists, there has been increasing public concern about the threat of bomb attacks on dam structures (federal emergency management agency, 2003). therefore, protection of dam structures against impact loads is a critical component of homeland security (lu et al., 2013). indeed, as respectively studied by lu et al. (2012, 2014a, 2014b), and zhang et al. (2014), the risk of a tall concrete dam being subjected to underwater explosion shock wave cannot be neglected. m2 lu lu, xin li, jing zhou, genda chen and dong yun 0.075 m 0.1 m 0.56 m 0.075 m 0.1 m 0.56 m 0.75m hammers initial position water bag pressure sensor vertical rope 1#hammer 2#hammer 3#hammer 4#hammer 5#hammer model dam moving hammers fixed point base anchorage hammers initial position water bag pressure sensor vertical rope 1#hammer 2#hammer 3#hammer 4#hammer 5#hammer model dam moving hammers fixed point base anchorage currently, with the development of computational techniques and numerical simulation methods, as well as commercialization of nonlinear dynamic software (e.g. abaqus, ly-dyna), major developments in understanding the structural responses and failure modes of concrete structures under blast load have taken place. many researchers have conducted comprehensive experimental and numerical investigations related to the effects of explosions on building structures(tian et al., 2008; jayasooriya et al., 2011), marine structures (jin, et al., 2011; zhang et al., 2011), underground structures (ma et al., 2011; li et al., (2013), and bridge structures (hao et al, 2010; son et al., 2011). in the modeling of transient loading, it is very critical to describe the propagation velocity of the stress waves correctly. in fact, the value of this velocity depends on the material elastic modulus that is given by the material constitutive relation. from the material point of view, concrete shows an increase in elastic modulus with the strain rate increases, a phenomenon called strain rate effect (georgin, 2003). the relationship between concrete strength and strain rate was extensively investigated by bischoff and perry (1991), georgin and reynouard (2003), grassl(2006) and tai, y.s. (2009). because many problems of the concrete material constitutive parameters and constitutive model have not been a clear understanding, the numerical results obtained by different calculation models are very different. moreover, the modeling of the strain rate effect on concrete is not very easy to tackle. these problems, to a great extent, comprise the uncertainty existing in the macroscopic numerical simulation. one of the efficient methods to study the failure modes and mechanisms of structures is to carry out a large number of model experiments and obtain data from them. considering that the experimental study has its limitations, as well as great difficulties and expensive costs for the underwater explosion test, only a small amount of data can be obtained. for example, lu et al. (2014b) obtained only a small quantity of damage to the relationship between the state and the maximum pressure of the shock wave through model tests. overall, to the best of our knowledge, experimental investigations of concrete gravity dams under underwater shock wave effects have not yet been conducted to date. concrete dams are thoroughly studied in this paper both experimentally and numerically to understand their behavior and failure modes. specifically, this paper is to use acceleration records of the concrete structure under strong impact to establish the correct way to determine the concrete structure of the fundamental frequency and present cut sheets multi-degree-of-freedom dynamic modeling. the dams were numerically modeled to understand further their shorttime failure process in the order of msec based on abaqus. examples of model tests the same test setup and test results as presented by lu (2012), lu (2014a), lu (2014b)were used in this paper which will be briefly depicted as follows. the dimension of the model dam and test layout are shown in figure 1. figure 1. dimension of the model dam and test layout (lu, 2012; lu, 2014a; lu, 2014b). -0.01 0 0.01 0.02 0.03 0.04 0.05 0.2 0.3 0.4 0.5 0.6 rt pr es su re (m pa ) time(sec)-0.01 0 0.01 0.02 0.03 0.04 0.05 0.2 0.3 0.4 0.5 0.6 rt pr es su re (m pa ) time(sec) the mechanical properties of the individually tested samples and their average values are given in table 1. therefore, the actual material density, young’s modulus, compressive strength, and tensile strength of the small-scale model are ρm = 2,900 kg/m3, em = 355 mpa, fcm = 205 kpa, and ftm = 13.2 kpa, respectively. table 1. mechanical properties of concrete material for scale model dams (lu, 2012; lu, 2014a; lu, 2014b). -100 -50 0 50 100 150 200 0 0.2 0.4 0.6 0.8 1 st ra in () time(sec) 610 � -100 -50 0 50 100 150 200 0 0.2 0.4 0.6 0.8 1 st ra in () time(sec) 610 � the main test results of model test are shown in table 2. the pressure, strain recorded and acceleration recorded are shown in figures 2, 3 and 4. table 2. main results of the model tests (lu, 2012; lu, 2014a; lu, 2014b). -12 -7 -2 3 8 13 18 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) time(sec) 22.521 =ow1#acceleration sensor -12 -7 -2 3 8 13 18 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) time(sec) -12 -7 -2 3 8 13 18 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) time(sec) 22.521 =ow1#acceleration sensor figure 2. time history of the impact pressure on the dam surface (lu, 2012; lu, 2014a; lu, 2014b). figure 3. recorded strain (lu, 2012; lu, 2014a; lu, 2014b). m3numerical simulation of shock response and dynamic fracture of a concrete dam subjected to impact load -6 -4 -2 0 2 4 6 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 a cc el er at io n( g) time(sec) 2#acceleration sensor -6 -4 -2 0 2 4 6 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 a cc el er at io n( g) time(sec) 2#acceleration sensor -3 -2 -1 0 1 2 3 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 3#acceleration sensor time(sec) -3 -2 -1 0 1 2 3 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 3#acceleration sensor time(sec) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 4#acceleration sensor time(sec) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 4#acceleration sensor time(sec) -1.5 -1 -0.5 0 0.5 1 1.5 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 5#acceleration sensor time(sec) -1.5 -1 -0.5 0 0.5 1 1.5 0.2 0.3 0.4 0.5 0.6 a cc el er at io n( g) 5#acceleration sensor time(sec) figure 4. recorded acceleration time histories from the model test (lu, 2012; lu, 2014a; lu, 2014b). as shown in the measured pressure-time curve, the impact load can be simplified to a triangular distribution. the failure mode and damage area are shown in figure 5. figure 5. failure mode and damage area[6-8] note that the accelerometers and pressure sensor were not synchronized during the test. as a result, the peak accelerations seem to occur before the application of the peak pressure. the maximum time delay among the five accelerometers is 23.44 msec. the time delay was likely caused by different periods when five hammers applied impact forces. based on the arrival time of peak accelerations, hammers 4 and 5 were in contact with the model dam later than hammers 1-3. 3. cut sheets dynamic modeling for that, the concrete dam is divided into i unites, the governing equation for the system with i freedom is as follows: [m] {a}+[k]{x}={p} (1) where [m],[k] denote the mass matrix and stiffness matrix; {x}, {a} and {p} are the displacement, acceleration and force vectors, respectively. displacement and acceleration using the vibration mode expand can be got (2) (3) where is {φi} i-th mode vector of the system, ξi is general coordinate corresponding to {φi}. equation 1 after substituting equations 2 and 3 becomes (4) this equation left multiplied by {φj}t becomes (5) use of the orthogonality of the vibration mode on the and , that is (6) by the equations 5 and 6 can be got (7) the governing equation for ξi is (8) (9) (10) (11) (12) let m4 lu lu, xin li, jing zhou, genda chen and dong yun rt p t maxp rt2 p t maxp rt2 p t maxp rt (a) (b) (c) rt p t maxp rt2 p t maxp rt2 p t maxp rt (a) (b) (c) (13) (14) (15) where superscript t denotes transformation of matrix or vector. pi* is the generalized force the corresponding to i-th vibration mode. mi* is the generalized masscorresponding to i-th vibration mode. mii is known quantity, so mi* can be determined. to give expression of , must be assumed for pi*(t) .therefore, three forms of the impact force-time curve were examined, as shown in figure6 (a-c), with the same pmax and impulse pmtr. figure 6. generalized impact force-time curve solutions of for figure 6, a, b, and c, are represented by equations13, 14 and 15, respectively. because |ẍ|t=0 = |ẍ|max for equations13 and 14 that are inconsistent with experimental results, figure 6 (a) and (b) are impossible. to determine the value of must also be determined ω0i and θc from the measured acceleration record. the time of acceleration extremal point(including the maximum and minimum values) tm1, tm2, tm3... tmi and the time of acceleration zero point t01, t02, t03... t0i can be obtained by the measured acceleration records. if tr infinite,by the first type of the equation 15, | |max occurs at the following times: , respectively. by the second type of the equation 15, the following equations can be obtained (16) (17) (18) the specific calculation process is as follows: let tm1 <tr , then the equation 17 substituting ω01 of the solution to equation 18 and t01 to determine tr. if tr > tm1, then the ω01 and the corresponding tr is solution. if tm1 <tr, then by tm1 and t01 from the measured acceleration records, and equations 16 and 17 , the simultaneous solution to get ω01 and tr . the maximum acceleration (shown in table 2) is gradually decreased from top to bottom, which reflects the first mode characteristics of the dam. experiment obtained vibration mode and natural frequency, so do not need too much freedom in the calculation. for that, the model dam is divided into five unites(as shown in fig.7.). y x (x, y) (0, 0) (0, 0.75) (0.56, 0) 1#acceleration sensor(0.06, 0.75) unit: m 2#a(0.126, 0.681) 3#a(0.252, 0.413) 4#a(0.378, 0.244) 5#a(0.500, 0.080) 11m 22m 33m 44m 55m (0.075, 0.65) y x (x, y) (0, 0) (0, 0.75) (0.56, 0) 1#acceleration sensor(0.06, 0.75) unit: m 2#a(0.126, 0.681) 3#a(0.252, 0.413) 4#a(0.378, 0.244) 5#a(0.500, 0.080) 11m 22m 33m 44m 55m (0.075, 0.65) figure 7. discrete model dam and the locations of acceleration sensors mii (shown in fig.7.) in kg, it is reasonable to assume that the measured acceleration distribution is proportional to the first mode, so for the model test, {φ1} t=(1,0.323,0.161,0.111,0.074).from above m* 1=5.1928kg, (p* 1)max = 1510.016n. for given pressure-time curve shown in figure 6(c) the equation 15 becomes (19) (20) for given case the maximum acceleration of 1# appears in t <tr (where tr is rise time of the pressure pulse, as shown in fig.6.), so is obtained 1 from and the value of ω01 equal to 52.22 by equation 18, we can get calculation results and experimental results are in good agreement. it indicates that the cut sheets dynamic calculation model is a reliable computational model for the concrete dam subjected to impact load. the main advantage of this method is avoiding the use of elastic modulus in the calculation. under impact loading, the constitutive relation of concrete structures and the highest m5numerical simulation of shock response and dynamic fracture of a concrete dam subjected to impact load frequency of the impact are uncertain. the computational work is much smaller than step by step integration. failure process by simulation during impact tests, it is difficult, if not impossible, to observe the failure process of concrete dams over the duration of pressure pulse (~60 msec) except at the end of the tests. on the other hand, the cut sheets dynamic model can calculate the elastic response of the dam under impact, but that cannot simulate the dynamic fracture process, and must assume that the functional form of load. therefore, numerical simulations with a finite element model (fem) were supplemented for further understanding of the concrete dam behavior over time. a fem of the model dam was established with 4-node linear tetrahedron three-dimensional (3d) elements in abaqus commercial codes. due to symmetry in both boundary conditions and loads, only half of the dam was numerically modeled as shown in figure 8. the model dam was fixed at its base. the locations of five accelerometers are also shown in figure 8. the mechanical properties of concrete used for the model are included in table 1. besides, the poisson ratio was taken to be 0.18 in various simulations. figure 8. 3d meshes and locations of five accelerometers of half of the model for convenience, the pressure-time curve in fig. 2 was cleaned up and only the main pulse was included in simulation as shown in red line in figure9. after numerical treading, the peak pressure slightly increases from 48.6 kpa to 49.7 kpa due to the initial negative value of the recorded pressure time history. the horizontal stress at the location of strain gauge was calculated from the fem under the registeredpressure. figure8 compares the calculated stress (black line) with the recorded pressure (red line) and the so-called “predicted stress” (blue line) by multiplying the recorded strain by the modulus of elasticity. it is clearly seen that the three-time histories are in good agreement because the horizontal stress expects to be in equilibrium with the applied pressure at the upstream of the model dam. therefore, the fem is partially validated. figure 9. recorded pressure, calculated stress and measured stress from the measured strain figure 10 shows the distribution of the vertical bending stress at different time instances and the progression of structural failure. since the maximum bending stress at 13.3 msec. exceeds the tensile strength of concrete, 18 kpa, concrete was considered to start cracking at 13.5 msec. in numerical simulations. crack initiated at 0.57 m above the base, which is close to the fracture location observed at 0.5 m high during the testing of the model in table 3. at 20.0 msec, the fracture was extended to the downstream face and, by the end of pressure loading at 50.0 msec, another fracture section was fully developed across the middle of the model dam as well as at the bottom of the dam. the fracture damage at the base is similar to the pattern observed from the photo of the model in table3. the fracture in the middle of model dam seems close to the damage scenario observed during the testing of model underlarger impact loading. figure 10. distribution of vertical stress (bending effect) at various time instances in practical applications, there is no anchorage of dam foundation. it is 150 m tall (h) and 112.5 m wide (l) with a vertical upstream slope and a downstream slope of 1:0.75. let p1 is the tensile strength of concrete material. there p1 = 0.3 mpa p2 isconcrete gravity against sliding stress. mpa when g is the dam gravity. f=1 is the friction coefficient. p3 is concrete gravity against overturning. (a) before damage (b) crack initiated at 13.5 msec. (d) model collapse at 50.0 msec. (c) fracture through the top section at 20.0 msec. m6 lu lu, xin li, jing zhou, genda chen and dong yun to simplify the calculation of the dam as a triangle, centroid dam in 1 / 3h and 2/3 l department, the resultant moment m (t) can be obtained (21) (22) where f is the uniform external force on upstream. hydrostatic pressure pw is expressed as: where ρw is the density of water. if m (t) ≥ 0, when the dam starts to rotate, so the concrete dams may be first toppled off, then fracture at their top of the damand, finally, fracture at their middle height. the performance of concrete under impact load and static load strength and deformation have a certain difference, with the increase of loading rate, the mechanical properties of concrete exhibit different behavior, the so-called rate dependent. at present, the research work is not enough, the research data is relatively small, and the conclusions of the study are not unified. this kind of research needs mesoscopic and macroscopic numerical analysis methods(multis cale model). conclusions the following conclusions can be drawn: under strong impact loading, the constitutive relation of concrete structures and the highest frequency of the impact are uncertain. this paper uses acceleration records of concrete structure under strong impact to establish the correct way to determine the concrete structure of the fundamental frequency and present cut sheets multi-degree-of-freedom dynamic modeling. the main advantage of this method is avoiding the use of elastic modulus in the calculation. calculation results and experimental results are in good agreement. the computational work is much smaller than step by step integration of the finite element. however, the cut sheets dynamic model can not simulate the dynamic fracture process, and must assume that the functional form of load. therefore, numerical simulations with a finite element model (fem) were supplemented for further understanding of the concrete dam behavior over time. the results of the analysis indicate that concrete damage mainly occurred at the dam head and bottom using the material property from table 1.concrete dams may be first toppled off, then fracture at their top of the damand, finally, fracture at their middle height. the performance of concrete under impact load and static load strength and deformation have a certain difference, with the increase of loading rate, the mechanical properties of concrete exhibit different behavior, the so-called rate dependent. at present, the research work is not enough, the research data is relatively small, and the conclusions of the study are not unified. acknowledgements this work was supported by the [the natural science foundation of china] under grant [no. 51379028,71301060], [the research funds of the state key laboratory of structural analysis for industrial equipment of dalian university of technology] under grant [no. gz1409], and the [huai’an applied research and scientific and technological research funds] under grant [no. has2014021-4]. references: bischoff, p.h. and perry, s.h. 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(2000). experimental investigation into seismic failure of high arch dams.journal of structural engineering. 126(8), 926-935. earth sciences research journal earth sci. res. j. vol. 20, no. 3 (september, 2016) letter from the editor the earth sciences research journal volume 20, number 3, with eight high-quality scientific articles, is now available online for all our readers, all the related scientific community. the publishing committee has decided to increase to four the numbers issued per year starting 2016. that will be just possible with your support. this time we want to call the attention to the necessary balance between good judgment and environmentalism. just for a different way of thinking about environment and management resources, some self-proclaimed environmentalists take a right for pointing out, for blocking those aimed at development or exploiting resources. their origin is located in germany during the 60's with the vorsorgeprinzip, or the precautionary principle. green activists depict themselves as victims of economic empires of government's neglected policies to turn into persecutors over development, competitivity, and innovation plans. as it was recently stated by the social issues research centre, this principle poses of a harmless policy. is not our right to be protected when the unscrupulous science is applied, especially when the environment and our lives are at risk? but the use of the principle goes beyond the protection of known or foreseen risks. this principle has been recalled to block plans with just little risks which will be balanced out thanks to the reached advantages. the precautionary doctrine is claimed whenever the little risk perception appears, no matter if the actions to take are necessary to allow future benefits. thousands of cases around the world would be checked. not only in mining or petroleum exploitation but genetically modified food, technology gadgets, fracking. this principle is a serious obstacle for the rational analysis. if the risk can not be demonstrated, the vorsorgeprinzip is baseless. the science works assuming the hypothesis can not be probed, but refuted. the evidence and probes are provided to the scientific community, and they will proceed under logic and rational methods; usually, the precautionary principle is not an important matter at this stage of the process. but is there when that policy can turn into dangerous. it origins a fanatism, close to a religious belief. the senseless debate shows a society with a limited future, without options. all the progress is neglected for standing up at the "safe place." our present has been just possible thanks to the bold decisions by some people with the enough vision to calculate the risks by the advantages. the trip to the moon and the sun system study, development of communications and treatment to sicknesses have been no possible under the precautionary principle. our scientific community should pick the good judgment up to allow all the small steps for humanity. carlos alberto vargas jiménez. editor in chief, earth sciences research journal le tt er f r o m t h e ed it o r the eastern anatolia provides one of the best examples of an area of rapid deformation and intense contraction that is the consequence of an active continental collision between the arabian and eurasian plates leading to large and devastating earthquakes. the latest evidence of the active tectonism in the region is revealed by two remarkable seismic events; vantabanli (mw 7.2, october 23, 2011) and van-edremit (mw 5.6, november 9, 2011) earthquakes. the study of the earthquake cycle and observation of geodetic and seismic deformation in this region is very important to hazard assessments. in this study, the inter-seismic, co-seismic, and post-seismic movements caused by the above-mentioned earthquakes were investigated using the time series of 2300 days of global navigation satellite systems (gnss) observations of the local stations selected from the network of the continuously operating reference stations, turkey (cors-tr). for the interseismic period, approximately 1100 daily data were obtained from 21 cors-tr stations (prior to the earthquakes between october 1, 2008 and october 23, 2011) and evaluated using the gamit/globk software. the behaviour of these stations was investigated by processing 1 hz data from the gnss stations during the earthquakes on the gamit/track software. in addition to october 23 and november 9, the gnss data on one day before and after the earthquakes was assessed to determine co-seismic deformations. during the october 23 earthquake, hanging-wall deformation of about 60 mm was detected in the sw direction at the mura station. however, at the vaan station, deformation of 200 mm (value predicted by time series) was observed in the footwall block in the nw direction. there were not any significant changes at the stations during the november 9 earthquake. for the post-seismic period, the gnss data from 2012 to 2015 was evaluated. according to the observations, post-seismic deformation continued at the stations close to the epicenter of the earthquake. abstract keywords: van earthquake, gnss, cors-tr, time series analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series análisis de los terremotos de octubre 23 (7.2 mw) y noviembre 9 (5.6 mw) de 2011 en van a través de series temporales gnss a largo plazo issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.62812 i. tiryakioglu1, h. yavasoglu2, m. a. ugur1, c. ozkaymak3, m. yilmaz1,*, h. kocaoglu1, b. turgut1 1 geomatics engineering, faculty of engineering, afyon kocatepe university, tr-3200, afyonkarahisar, turkey. 2 department of geomatics engineering, istanbul technical university, tr-34467, istanbul, turkey. 3 geology engineering, faculty of engineering, afyon kocatepe university, tr-3200, afyonkarahisar, turkey. * correspondence: mustafayilmaz@aku.edu.tr (m.y.). anatolia oriental provee uno de los mejores ejemplos de un área de rápida deformación e intensa contracción, consecuencia de una colisión continental activa entre la placa arábiga y la euroasiática, lo que causa grandes y devastadores terremotos. dos notables terremotos son evidencia de la actividad tectónica de la región: los terremotos van-tabanli (7.2 mw. 23 de octubre de 2011) y van-edremit (5.6 mw, 9 de noviembre de 2011). el estudio del ciclo sísmico y la observación de las deformaciones geodésicas y sísmicas en esta región es fundamental para la evaluación de riesgo. en este estudio, se investiga las fases inter-, coy post-sísmicas asociados a estos terremotos a través de 2300 días de observaciones del sistema global de navegación por satélite (gnss, por sus siglas en inglés) de estaciones locales seleccionadas de la red de estaciones de referencia de funcionamiento continuo en turquía (cors-tr). para el periodo intersísmico se obtuvieron aproximadamente 1100 datos diarios de 21 estaciones cors-tr (anteriores a los terremotos entre el 1 de octubre de 2008 y el 23 de octubre de 2011) y evaluados con el software gamit/gblok. el comportamiento co-sísmico fue investigado procesando datos de 1 hz de las estaciones gnss durante los terremotos en el software gamit/track. adicionalmente se evaluaron datos del gnss un día antes y un día después de los terremotos para determinar las deformaciones cosísmicas. durante el terremoto del 23 de octubre, en el bloque colgante se detectó una deformación de 60 mm en dirección sw (estación mura). en la estación vaan en el bloque yacente se observó una deformación de 200 mm en dirección nw (valor predicho por series temporales). durtante el terremoto del 9 de noviembre no se observó cambios significativos. para la fase post-sísmica se evaluaron los datos de la gnss del 2012 al 2015. según las observaciones, la deformación postsísmica continuó en las estaciones cercanas al epicentro del terremoto. resumen palabras clave: terremotos de van; sistema global de navegación por satélite; estaciones de referencia de funcionamiento continuo en turquía; frecuencias. record manuscript received: 21/02/2017 accepted for publication: 28/07/2017 how to cite item tiryakioglu, i., yavasoglu, h., ugur, m.a., ozkaymak, c., yilmaz, m., kocaoglu, h., & turgut, b. (2017). analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series. earth sciences research journal, 21(3), 147 156. se is m o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 147-156 148 i. tiryakioglu, h. yavasoglu, m. a. ugur, c. ozkaymak, m. yilmaz, h. kocaoglu, b. turgut introduction the active tectonics of turkey are dominated by the westward tectonic escape of the anatolian plate, relative to eurasia, from a collision zone between the converging african and eurasian plates (~5 ma) (bozkurt 2001). the east anatolian contractional province is under the control of a n-s directed compressional-contractional tectonic regime (bozkurt 2001; kocyigit et al. 2001; özkaymak 2003; özkaymak et al. 2004a, 2004b, 2011; sengor and kidd 1979; sengor and yilmaz 1981; şaroğlu and yılmaz 1986; yilmaz et al. 1987). the lake van basin located on the north of the bitlis-zagros suture zone formed by the collision of the eurasian and arabian plates suffered from historical and instrumental earthquakes of a magnitude greater than 6.0 (ambraseys and finkel 1995; ergin et al. 1967; soysal et al. 1981). the most recent of these were the mw=7.2 tabanli-van earthquake of october 23, 2011 and the mw=5.6 edremit earthquake of november 9, 2011 (figure 1) (usgs 2011). according to the field-based geologic and seismic geomorphology studies performed soon after the earthquake (akyuz et al. 2011; emre et al. 2011; özkaymak et al. 2011), the mw 7.2 earthquake occurred on an ene-striking and north-dipping 70-km-long main thrust fault which was named the van fault zone (vfz) by emre et al. (2011). the northern block (hanging-wall) of the thrust fault moved towards the southern block. the reported depth of the hypocenter is about 16 km and the coseismic thrust offset was approximately 2 m around the hypocenter (akyuz et al. 2011; koeri 2011; usgs 2011). according to studies using synthetic aperture radar interferometry after the earthquake, the hanging-wall uplift was about 80 cm (akyuz et al. 2011). the focal mechanism solution of the november 9, 2011 earthquake and the results of geological studies in the region indicate that the mw 5.6 earthquake occurred on the ne-sw-trending edremit fault (ef) with a sinistral strike-slip character. figure 1. (a) tectonic outline of the eastern mediterranean area. abbreviations: eacp, east anatolian contractional province; dsfz, dead sea fault zone; eafz, east anatolian fault zone; nafz, north anatolian fault zone; (b) lake van basin and simplified geological map showing the main tectonic structures of the lake van basin and the surrounding area. abbreviations: gfz: gurpinar fault zone, af: alabayir fault, vfz: van fault zone, efz: edremit fault zone, ef: ercis fault (compiled from emre et al. (2012); özkaymak et al. (2004a), (2011); şenel (2002)). the results of other social and geodetic studies in relation to the tectonics of the region can be found in the relevant references (mcclusky et al. 2000; ozener et al. 2010; reilinger et al. 2006; tatar et al. 2012; yavasoglu et al. 2011; zare and nazmazar 2013). studies carried out after the october 23, 2011 earthquake investigated the amount of dislocation caused by the earthquake using the data from the cors-tr stations in the region. however, these studies only focused on co-seismic deformation (altiner et al. 2013; yildirim et al. 2014). dogan et al. (2014) determined the post-seismic deformation using the data obtained from the gnss network after the earthquake. however, research in this area is still limited, thus there is a need for further studies to investigate post-seismic deformation to provide a better and complete understanding of the earthquake cycle of the region. in this study, the inter-seismic, co-seismic, and post-seismic movements of the two earthquakes were investigated simultaneously with the gnss time series of the local reference stations selected from the cors-tr network. for the inter-seismic period, approximately 1100 daily data were obtained from 21 cors-tr stations (during the pre-earthquake period from october 1, 2008 to october 23, 2011) and evaluated using the gamit/globk software. the behaviour of these stations was investigated by processing 1 hz data from the gnss stations during the earthquakes on the gamit/track software. in addition to october 23 and november 9, the gnss data in one day before and after the earthquakes was assessed to determine co-seismic deformations. for the post-seismic period, the gnss data from 2012 to 2015 was evaluated and time series were obtained. findings from the time series analysis showed that there were local movements in some of the cors-tr stations (malz, muus, sirn and hakk). these movements indicate that the post-seismic deformation continued around the stations located close to the earthquake epicenter. the tectonic structure of the region the lake van basin is located just north of the bitlis-zagros thrust zone that was formed by the collision of the arabian and eurasian plates (figure 1). it is also situated to the east of the karliova triple conjunctions at the intersection of the north anatolian fault and east anatolian fault. since the collision to date, the region has been uplifted approximately 2 km (kocyigit et al. 2001). the active tectonic structures observed in the region include nw-se-trending right lateral (e.g., ercis, caldiran, and alabayir faults), ne-sw-trending left-lateral strike-slip faults (eg. edremit fault zone), approximately e-w-trending reverse/ thrust faults (e.g., van fault zone and gurpinar fault zone), and nstrending extensional faults associated with volcanic activity (e.g., nemrut opening and suphan opening). these structures indicate approximately n-s-directed compressional-contractional tectonic regime in the region (figure 1) (emre et al. 2012; özkaymak et al. 2004a; 2011; şenel, 2002). the van fault zone (vfz) consists of a large number of e-w-trending north-dipping parallel/semi-parallel thrust fault splays. the first of these was identified by özkaymak (2003) in the holocene lacustrine/fluvial sediments striking 70º n and dipping 45º nw with a dominant thrust and minor left-lateral character. according to the surface deformations of the october 23, 2011 earthquake and geomorphologic observations, vfz is mapped approximately 30 km along the zone between lake van to the west and south of the lake ercek to the east (emre et al. 2011) (figure 1). another e-w-trending and north-dipping major neotectonic structure in the lake van basin is the gurpinar fault zone (gfz) located in the southeast of lake van (figure 1). özkaymak et al. (2004a) suggested that this thrust fault could also be the source of the 2003 earthquake (mw 4.7) in edremit. according to the focal mechanism solution of the 2003 earthquake and the geological mapping studies in the region, as a result of this event, the quaternary travertine deposits were thrust over the younger fluvial deposits towards the south. the fault generating the november 9, 2011 earthquake (mw 5.6) was mapped by özkaymak (2003) and called the edremit fault. the ne-sw-trending left-lateral strike-slip edremit fault cuts and deforms the quaternary edremit travertines and can be traced to a distance of approximately 10 km. the alabayir fault located in the east of van is a nw-se-trending right149analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series lateral strike-slip fault of approximately 32 km (özkaymak 2003). datasets of historical and instrumental earthquakes indicate that many destructive earthquakes and surface ruptures occurred along these active structures reflecting the geodynamic setting of the lake van basin and the surrounding area. several events with the intensity of 9 and higher have been reported for the historical period, including 1110, 1441, 1647, 1696, and 1881 (ambraseys and finkel 1995; ergin et al. 1967; soysal et al. 1981). however, the source faults of these events are still not clear. during the instrumental period, the focal mechanism solutions of some moderate-scale earthquakes that occurred between 1988 and 2002, the most recent earthquake on october 23, 2011 (mw 7.2), and a large number of aftershocks demonstrate that e-w-trending thrust faults are active. furthermore, ne-swand nw-se-trending strikeslip faults are also observed in the region. gnss network and data analysis cors-tr is a nationwide gnss network established in turkey through a project implemented by istanbul kultur university (iku) in collaboration with the general command of mapping and general directorate of land registry and cadastre between 2006 and 2009 and sponsored by the scientific and technological research council of turkey (tubitak) (eren et al. 2009). one of the objectives of this project was to establish a spatial infrastructure to monitor the tectonic plate movements using continuous gnss data, ultimately to contribute to the prediction of earthquakes and early warning studies within the network area (uzel et al. 2013). the cors-tr network has 21 reference stations located near the epicentres of the van earthquakes, covering a total area of ~ 100000 km2 (figure 2). the closest station to the epicentres of both van earthquakes is vaan at a distance of 21 km and the furthest is ardh at a distance of approximately 275 km. figure 2. the cors-tr stations evaluated in the study in this study, 2300 days of gnss observations of the 21 corstr stations were recorded at 30-second intervals in the rinex data format (01.10.2008-31.12.2014, covering inter-seismic, co-seismic and post-seismic periods). the data from the gnss day 2008.75 to 2015.00 was processed on a daily basis on the gamit/globk software (herring et al. 2010). data from each daily measurement were processed using two international terrestrial reference frames (itrf); itrf2008 and itrf08_ eura. as in many studies, the following processing parameters are used in this research. the antenna phase center was derived according to the height-dependent model. the precise orbit information (igs-f) in the sp3 format required by the gamit process was downloaded from the scripps orbit and permanent array center (sopac) database (herring et al. 2010). the earth rotation parameters (erps) were obtained from united states naval observatory bulletin b (usno bull. b). lc (l3) the ionosphereindependent linear combination of the l1 and l2 carrier waves was used during the analysis. the fes2004 ocean tide loading (otl) grid was used to interpolate the components of the displacement caused by ocean tides (gulal et al. 2013; herring et al. 2010; lyard et al. 2006; ozener et al. 2010; tiryakioglu et al. 2013; tiryakioglu 2015). analysis of inter-seismic deformation with long-term time series and inter-seismic strain field for the inter-seismic period, time series of station coordinates (itrf08 and itrf08_eura) were obtained from the gamit software starting with the first measurement taken at the time of the earthquake (2008.75-2011.85). with the time series analysis, it is possible to determine the linear (trend), periodic and irregular (stochastic) movements of the reference stations. for the north, east, and up coordinates of the reference stations, the times series of (ti) depending on the time of ti (i= 1,2,3,...,n) can be expressed as follows (gulal et al. 2013, 2015): where, ak : trend component parameters, bc; sc : periodic component parameters, fs : frequency, αj : auto-regressive (ar (p)) model parameters, βjj : average movement (ma (q)) model parameters, v(ti) : random errors with average zero and variance σ2 m,k; r,s; p,j; q,jj: degree and order of the time series. in order to perform the time series analysis of x (ti) , the data must be continuous and systematic. in the analysis of the time series, first, the general state of time series should be visually interpreted based on the time-dependent graphics of the series. in the time series of the 21 reference stations located in the study area, some measurements were observed to have sudden and non-recurring peaks. these measurements called ‘outliers’ provide values that are either significantly smaller or larger than the average values of the time series. in this study, we discuss the ‘additive outliers’ (ao), first defined by fox (1972) as the first type outliers that occur due to incorrect data entry or measurements, and affect only one measurement. after the removal of the measurements that are significantly larger or smaller than the expected values, the time series returns to its normal course (gulal et al. 2013). in cases where some of the measurements of the (ti) series are affected by ao at time ti, the new series that is obtained can be expressed by y (ti) as in the following eq. (2). outliers in time series are usually determined using a function that fits the series. the function used to eliminate outliers in the y(ti) series can be expressed as the sum of linear and trigonometric functions (periodic motion) (erdogan 2012; gulal et al. 2013). the analysis of the trend component in the coordinate time series derived from the gnss data is crucial for earth sciences since the linear movements of time series, ‘ak • t’ in the trend component are used to determine the plate movements (or velocity) based on the time unit and the direction (gulal et al. 2013; gulal et al. 2015). it is also assumed that annual periodic (1) (2) 150 i. tiryakioglu, h. yavasoglu, m. a. ugur, c. ozkaymak, m. yilmaz, h. kocaoglu, b. turgut movements are produced by the effect of atmospheric and hydrologic loading (blewitt and lavallee 2002; wdowinski et al. 1997). furthermore, van dam et al. (2001) and romagnoli et al. (2003) suggested that due to the climatic effects such as the accumulation of ice on antennas and antenna shields, there are periodic movements that occur quarter-yearly, half-yearly and yearly. the outliers in the time series of y(ti) were detected using the method described by gulal et al. (2015) and then these outliers were removed from the series. to avoid complexity, the stochastic component was not added to the function. the same function can be used to estimate new values for missing data due to the removal of outliers. furthermore, as shown in figures 3a and 4a, this function can estimate values to fill the gaps caused by gnss data loss. to determine the temporal movements of the gnss stations, the coefficients of the linear function that fits the time series were estimated using the software developed in matlab (figure 3b-4b). the inter-seismic velocities of the stations were determined by the estimated coefficients (figure 5) using the core stations of the international gnss service (igs). figure 3. time series (column a) and linear function (column b) for the components in the north, east, and up coordinates of the erzr station in itrf08 figure 4. time series (column a) and linear function (column b) for the components in the north, east, and up coordinates of the ozal station in itrf08 figure 5. velocities obtained from the time series analyses; the red arrows indicate eurasia-fixed velocities and the green arrows shows itrf08 velocities. based on figure 5, it was observed that the malz, muus, sirn, and hakk stations move in different directions compared to other stations due to local movements (ozdemir 2016). furthermore, some sudden fluctuations were identified at the malz and muus stations 151analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series (figure 6). the normalized root mean square (nrms) values of these stations are ±4.1 mm, ±5.8 mm, ±3.1 mm; ±6.0 mm, ±4.6 mm, ±1.3 mm, respectively. furthermore, for these stations, the annual, semi-annual, and seasonal periodic movements explained earlier were found to be greater. this can be attributed to the changes in water level according to seasons and varied amount of precipitation. although no fluctuations were observed in the times series of the hakk and sirn stations, they acted in a way contrary to the region’s tectonic structure. this might be due to local deformations that occurred at these stations. figure 6. time series of the components in the north, east, and up coordinates of the malz and muus stations in itrf08. calculation of the inter-seismic strain rate the strain analysis was performed using the inter-seismic velocities to obtain the strain fields of the region. for reasons explained in detail earlier in the paper, the malz, muus, hakk, and sirn stations were excluded from the analysis. after determining the velocity vectors, the stress and strain parameters of the region were calculated. in the literature, various methods have been suggested for the determination of stress and strain parameters (brunner 1979; deniz 1990; dogan 2002; kakkuri and chen 1992; lambeck 1988). in this study, the gnss sites were used to create geodetic triangles, and from these triangles, the strain rates were calculated. the general approach associated with this method is to obtain a strain field of the east and north velocity components determined through the analysis of time series. figure 7 presents the calculated strain rates of the generated triangles. the midy station was excluded from figure 7 because of the geometric distortion. figure 7. geodetic triangles and the strain rates in figure 7, the dominant extension areas, pure compression areas and the strike-slip faults are indicated by red, blue and green arrows, respectively. figure 7 shows that compressions consistent with the tectonic regime are widely seen. the largest compressions in the region were observed in the nnw-sse direction, located in the triangles of mura-ozal-vaan (-128 e-09/yr), hini-tvan-sirt (-88 e-09/yr) and hini-agrd-tvan (-84 e-09/yr). the october 21 earthquake occurred in the mura-ozalvaan triangle. this demonstrates, once again, that the focal mechanism solution of the earthquake complies with the strain. the united states geological survey (usgs) observed a rotation of 66 degrees in the nne direction from the earthquake solutions and 50-70 degrees from the surface measurements (usgs 2011). in the strain analysis, the rotation of the axis direction was found to be 70 degrees in the nne direction. the largest extension was observed in the ene-wsw direction and located in the vaan-ozal-bask (46 e-09/yr) triangle. furthermore, strike-slip faults were observed in the south of lake van in the tvan-vaan-catk and tavn-catk-sirt triangles. the efz fault is another example of these strike-slip faults. the focal mechanism solutions of the november 9 earthquake in efz in the tvan-vaan-catk triangle comply with the strain. in addition, in the agrd-igir-mura triangle, strike-slips faults are active in the north of lake van. the caldiran fault located in the same region is another strike-slip fault that is in compliance with the 1976 earthquake (ms:7.3) solutions (tan et al. 2008). similarly, the solutions of the november 15, 2000 earthquake (mw: 5.6) in the same triangle indicate the presence of strike-slip faults in the region (pinar et al. 2007). determination of co-seismic motion and deformation in order to determine the co-seismic motion, 1 hz gnss data obtained from the cors-tr stations in the region were analyzed (10:00 to 11:00 utc on oct. 23, 2011, doy296). since the 1 hz gnss data of the november 9 earthquake was not available, this earthquake was excluded from this study. furthermore, some of the cors-tr stations in the region were not operational on october 23, 2011; therefore, the co-seismic motions of these stations were not analyzed due to the lack of data. as a result, the co-seismic motions of 11 stations were analyzed (malz, igir, hakk mura, ozal, tvan, semd, sirt, muus, hini, and hors). 152 i. tiryakioglu, h. yavasoglu, m. a. ugur, c. ozkaymak, m. yilmaz, h. kocaoglu, b. turgut the kinematic positions of the 11 stations were determined using the kinematic data processing program, track developed at massachusetts institute of technology. track version 1.21, released with the gamit/ globk version 10.4 (herring et al. 2010) and the igs final orbits were used to determine the kinematic baseline lengths. the displacement components of the 11 stations were plotted as differences in one-second-epoch coordinates relative to the reference coordinates of the stations. figure 8 presents the co-seismic motions of the malz, hakk, and mura stations between 10:40:00 and 10:45:00 utc. similar results were observed in the tvan, semd, ozal, igir stations. there were no significant changes in the sirt, muus, hini, and hors stations. the coordinates of the stations obtained in the days before and after the earthquakes were evaluated to determine the co-seismic deformations of both earthquakes. as mentioned earlier, outliers were detected and removed from the series. new values were estimated using a linear function that fit the series. from the gnss data one day before and after the earthquake was not available for all stations; the missing coordinates in the series were estimated using the linear functions. the post-seismic earthquake data for the vaan station was derived using a logarithmic function as explained in section 3.4. figure 8. velocity waveforms at the malz, hakk, and mura stations during the earthquake along the three coordinates the inter-seismic 1 hz kinematic solution of mura (the closest station to the earthquake epicenter, van-ercis) shows the maximum movements as 11.5 ± 1.5 cm in the direction of n-s, 14.8 ± 1.3 cm in the direction of e-w, and 6.9 ± 3.2 cm in the vertical direction. similarly, the maximum movements of the ozal station were 14.2 ± 1.5 cm in the direction of n-s, 7.6 ± 1.6 cm in the direction of e-w, and 7.0 ± 3.6 cm in the vertical direction. the highest displacement was obtained from the malz station with the movement being 17.5 ± 1.5 cm in the direction of n-s, 20.5 ± 1.6 cm in the direction of e-w and 12.1 ± 3.2 cm in the vertical direction. contrary to altiner et al. (2013), in the current study, the malz station was observed to have shaken more, despite being further from the earthquake epicenter compared to the mura and ozal stations. the analysis of co-seismic deformation showed that the november 9, 2011 earthquake did not cause any deformation in the trends of the gnss stations, whereas the october 23, 2011 earthquake caused deformation to the gnss stations, as mentioned in konca 2015. (figure 9) 153analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series figure 9. co-seismic deformations. the red arrows indicate horizontal movements (a) and the blue arrows show vertical movements (b). figure 9 shows that the largest co-seismic deformation occurred at the vaan station with a movement of 226 ± 3 mm in the direction of nw. this supports the data reported by other studies (altiner et al. 2013; dogan et al. 2014). in addition, at this point, a positive movement (169 ± 5 mm) was found in the vertical direction. this shows that the vaan station is on the footwall block (figure 9b). in the stations to the north of the earthquake epicenter (on the hanging block), deformations were in the s-sw direction, whereas in the stations to the south of the earthquake epicenter (on the footwall block) deformations were in the direction of n-nw. this indicates that deformations are compatible with the characteristics of the respective earthquake. despite the high number of interseismic movements in the malz station (figure 8), there was almost no coseismic deformation. furthermore, no co-seismic deformation was observed in the btmn, midy, sirt, and muus stations. post-seismic deformation in order to determine the post-seismic deformation, 1100-days of data from post-earthquake years (between 2012.0 and 2015.0) was processed using the gamit software. the coordinates of the stations relative to the eurasian plate were analyzed using time series as described in section 3.1. according to the time series analysis, the post-seismic deformation of the vaan station did not show any linear trends. the linear function is not suitable for the analysis of time series data obtained after a megathrust earthquake. the time series of the vaan station can be characterized with a logarithmic relaxation function (figure 10). satirapod et al. (2008) found that a logarithmic decay function presented by marone et al. (1991) better fits the post-seismic time series of the 2011 earthquakes. figure 10. time series of the vaan station. the thin curved line in red fits the changes in coordinates as a logarithmic function of a + b log(t) (where t is time after the earthquake in years, and a and b are constants) (savage et al. 2005). 154 i. tiryakioglu, h. yavasoglu, m. a. ugur, c. ozkaymak, m. yilmaz, h. kocaoglu, b. turgut the functions of time series are used to derive velocity combinations with respect to different years to determine whether the post-seismic movements continue (figure 11). the accuracy of all velocity combinations was found to be lower than ±1mm/year. the black arrows in figure 11 indicate the pre-seismic velocities presented in figure 5. at the vaan station, which is the nearest station to the epicenter of the earthquake, the post-seismic velocity between 2012 and 2013 is 131 ± 1 mm in the direction of ew which is significantly different from the pre-seismic velocity. furthermore, the mura and agrd stations which are also close to the earthquake epicenter move in the se direction contrary to the pre-seismic velocity. however, in the bask, catk, tvan, ozal, and hini stations the pre-seismic and post-seismic directions are similar with the only difference being an increase in the post-seismic velocities. the post-seismic and inter-seismic velocities are almost the same in other stations further from the epicenter of the earthquakes; such as the sirn, midy, btmn, and sirt stations. after the earthquake, the vaan station has been re-constructed 10 km away from its initial location and was renamed as van1. this new location is on the same hanging block as the vaan station. the postseismic velocity of van1 for the years between 2012 and 2014 is 26 mm greater than the pre-seismic velocity of the vaan station. however, both velocities are in the nw direction. the mura and agrd stations changed their direction of motion from se to e. contrary to the previous year, their post-seismic velocities decreased getting closer to their inter-seismic velocities. despite the 6mm decrease in the post-seismic velocity of the van1 station from 2012 to 2015, it was still 20 mm greater than the pre-seismic velocity. the tvan and mura stations were renamed as tva1 and mur1 following their movement after the earthquake. for all stations except the van1, ozal, bask, catk, mura, and agrd stations, the post-seismic and pre-seismic velocities were almost the same. however, the stations closer to the earthquake epicenter such as van1, bask, and catk still had post-seismic movements. the analysis of the post-seismic velocities of the gnss stations between 2013 and 2015 demonstrated that the post-seismic motion was transformed into an inter-seismic motion. figure 11. post-seismic motions by periods. the red, green, purple and turquoise arrows indicate horizontal movements. conclusions in this paper, a total of 2300 daily data was obtained from the gnss observations of 21 cors-tr stations (between 2008.75 and 2015.00) using the gamit software. the coordinates of the stations were determined on a daily basis according to itrf08 and itrf08_eura. the outliers were eliminated from the time series. the trends of these time series were used to derive the velocities of the gnss stations. the period from october 1, 2008 to october 23, 2011 was evaluated in terms of the inter-seismic movements. based on the analysis of the inter-seismic time series at the malz and muus stations, the annual periodic differences were considered to have been caused by changes in ground water level. it was also observed that there were some local deformations at the hakk and sirn stations. therefore, these four stations were not included in the strain analysis. the largest compressions in the region were observed in the vaan-mura-ozal triangle, which indicates that the thrust faults are active. the focal mechanism solutions of the october 23, 2011 earthquake showed that the earthquake occurred on a thrust fault. the directions of the compressions were found to be in agreement with the solutions of the earthquake. furthermore, the focal mechanism solutions of the 2011 earthquakes and the previous earthquakes that had occurred in the region were consistent with the results of the strain fields. this also indicates the possibility that the thrust faults are active particularly in the south of lake 155analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series van. strain analysis is a powerful method to predict the possible strong ground motions in a region. as in the case of hini-agrd-sirt region (figure 7), strain values indicate the approximately nnw-sse directional compressional tectonic regime. it means that the thrust faults located at the west of vfz can produce destructive earthquakes in the future. the co-seismic displacements were determined only for 11 stations due to the lack of 1 hz gnss data for the co-seismic period for the remaining stations. despite not being the closest station to the earthquake epicenter, the malz station was affected most (figure 8). earthquakes may cause different movements at the roof level depending on the dynamic character of the buildings. for a building with a lower horizontal rigidity, the motion of the roof is greater than the ground motion. the ground facility type of the malz station is a pillar. therefore, it is considered that malz had been affected most since its gnss receivers on the building measured not only the ground motion, but also the motion of the building relative to the ground motion. the vaan station had a logarithmic trend after the earthquake, which continued for approximately a year. after re-construction to new location (van1), its trend became linear. according to the analysis of inter-seismic deformation calculated from the stations on the footwall block, the direction is nnw while the stations on the hanging block, the direction is ssw. the vaan station was located close to the deformation zone, 400 meters away from the surface rupture caused by the earthquake. therefore, it was observed that the vaan station was uplifted about 17 cm due to the thrust fault movement. in terms of the post-seismic deformation, after slip was normally the main physical mechanism for the ground motion after the main shock. we processed the continuous gnss data to create deformation maps and calculate the variations in time-series. furthermore, the results indicate the potential presence of a low angle fault splay as suggested by dogan et al. (2014). we also processed the post-seismic data in four periods, 2012-2013, 2012-2014, 2012-2015 and 2013-2015 (figure 11). the post-seismic movement was composed of thrust slip with a significant left-lateral component in the region affected by the main shock. both left-lateral and thrust components of the post-seismic slip are in agreement with the results of earlier studies (dogan et al. 2014; feng et al. 2014). the post-seismic deformation was found to have different rotational components when the velocities were determined from the time for different periods (figure 11). the igir, mura and agrd stations, in particular, underwent unequal rotational deformation throughout the four periods. after the earthquake, the stress may have been shifted to the northern and western part of the region. acknowledgment this study was supported by afyon kocatepe university’s project numbered 13.fen.bil.038. we thank the university staff for their invaluable support and recommendations throughout the study. references akyüz, s., zabci c. & sançar, t. 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(2013) van, turkey earthquake of 23 october 2011, mw 7.2; an overview on disaster management. iranian journal of public health, 42(2), 134-144. beryl crystals are found within stream sediments transecting schists in the northeast of eskişehir, western anatolia. this paper studied the eskişehir beryl crystals with optical microscopy, scanning electron microscopy (sem-edx), infrared spectroscopy (ir) and geochemical analyses. beryl is accompanied by garnet, glaucophane, quartz, epidote, muscovite and chlorite in the stream sediments. the crystals are euhedral emerald (green gem beryl) and light bluishgreen aquamarine, with ideal sharp ir bands. wet chemical analysis of eskişehir beryl yielded 61.28% sio2, 15.13% al2o3, 12.34% beo, 0.18% cr2o3, 1.49% mgo, 1.69% na2o, 0.98% fe2o3, and 0.008% v2o3, resulting in the formula (al1.75cr0.01mg0.22fe0.08)(be2.90si6.00)(na0.32)o18. large ion lithophile elements (lile) (barium, strontium), some transition metals (cobalt, except nickel) and high field strength elements (hfse) (niobium, zirconium, and yttrium) in stream sediments that are associated with beryl exhibited low content about metamorphic rocks. beryl formation appears to be controlled by upthrust faults and fractures that juxtaposed them with cr-bearing ophiolitic units and a regime of metasomatic reactions. such beryl crystals have also been found in detrital sediments that are derived from the schists. cristales de berilio fueron encontrados en sedimentos de corrientes que atraviesan en esquistos en el noreste de eskisehir, al oeste de anatolia. en este artículo se presentan resultados sobre el estudio de los cristales de berilio de eskisehir con microscopio electrónico de barrido (sem-edx, del inglés scanning electron microscopy), espectroscopia infrarroja y análisis geoquímicos. el berilio estaba acompañado de granate, glaucofana, cuarzo, epidota, moscovita, y clorito en las corrientes sedmientarias. los cristales son esmeraldas de formas definidas (gema verde de berilio) y aguamarinas color verde celeste, con bandas de espectroscopia infrarroja de buena nitidez. el análisis químico húmedo del berilio de eskisehir mostró 61.28 % de sio2, 15.13 % de al2o3, 12.34 % de beo, 0.18 % de cr2o3, 1.49 % de mgo, 1.69 % de na2o, 0.98 % de fe2o3, y 0.008% de v2o3, lo que resulta en la formula (al1.75cr0.01mg0.22fe0.08)(be2.90si6.00)(na0.32)o18. los elementos litófilos de ion grande (bario, estroncio), algunos metales de transición (cobalto, excepto níquel) y los elementos de gran campo de fuerza (niobio, circonio e itrio) en las corrientes de sedimentos que están con berilio mostraron un bajo contenido sobre las rocas metamórficas. las formaciones de berilio aparecen controladas por fallas de cabalgamiento y fracturas que se yuxtaponen con las unidades ofiolíticas relacionadas con cromo y un régimen de reacciones metasomáticas. estos cristales de berilio se han encontrado también en sedimentos detríticos que se derivan de los esquistos. earth sciences research journal earth sci. res. j. vol. 20, no. 3 (september, 2016 ) : a1 a10 abstract resumen keywords: beryl, kaymaz, schist, sem-edx, ir. palabras clave: berilio, kaymaz, esquistos, microscopio electrónico de barrido, espectroscopia infrarroja. occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey record manuscript received: 04/12/2014 accepted for publication: 26/08/2016 how to cite item erkoyun, h., & kadir, s. (2016). occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey. earth sciences research journal, 20(3), a1-a10 doi:http://dx.doi.org/10.15446/esrj.v20n3.47677 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n3.47677 hülya erkoyun and selahattin kadir * eskişehir osmangazi university, deparment of geological engineering, tr−26480 eskişehir, turkey herkoyun@ogu.edu.tr skadir.euroclay@gmail.com *corresponding author o r e d e po si t s presencia de berilios relacionados con cromo en corrientes sedimentarias de eskisehir, noroeste de turquía hülya erkoyun and selahattin kadir a2 introduction the study area is in the village of halilbağı, approximately 72 km northeast of the eskişehir, western anatolia (fig.1). the area was previously explored to understand its structural geology and stratigraphy (erentöz, 1975; bingöl, 1976, kulaksız, 1981; gözler et al., 1996). groat et al. (2008) and arif et al. (2010) has classified the genetic types of beryl deposits. worldwide, beryl deposits occur 1) in association with granitic pegmatite and hydrothermal veins in mafic-ultramafic rocks, with or without schist in contact zones (kazmi and snee, 1989; sinkankas, 1994; dereppe et al., 2000; schwarz and giuliani, 2001; sabot, 2002). emerald deposits in poona (australia) occur in quartz and pegmatite veins related to metasomatic reactions between quartz-muscovite or quartz-topaz-greisens and ultramafic units (groat et al., 2008); 2) in schist-type deposits that are controlled by thrust faults and shear zones, with or without pegmatites (sinkankas, 1994; schwarz and giuliani, 2001; zwaan, 2006). the emeralds from santa terezinha (brazil) formed in metasomatized mica schist related to hydrothermal fluids along thrust fault associated with pegmatite veins (giuliani et al., 1997a,b); 3) related to black shale with veins and breccias controlled by thrusts and faults (beus, 1979; dereppe et al., 2000; schwarz and giuliani, 2001). the colombian (cordillera basin) emerald is hosted in the lower cretaceous sediments (sandstone, limestones, black shales and evaporates) associated with stratiform breccias and mobilized black shales. firstly, calcite, pyrite and muscovite filling veins related to hydrothermal fluids developed along faults. secondly, extensional faulting and folding resulted in the formation of emeralds in voids accompanied by fluorite, dolomite, and quartz as breccia filling of defects (giuliani et al., 2000; pignatelli et al., 2015). 4) about oceanic suture zones (dereppe et al., 2000). the emerald occurrences in panjshir valley (afghanistan) associated with continental suture zone within intrusion of tourmaline-bearing leucogranite into serpentinite units that result of phlogopite bands containing jewels (sabot et al., 2000); 5) in carbonate rocks (sabot, 2002). the swat valley (pakistan) emeralds occur as veins and stockworks within fractured quartz-bearing magnesite rocks and chromium-rich tourmaline in limonitized shear zones (schwarz and giuliani, 2002); 6) in granitic cupolas (dereppe et al., 2000). the emeralds from nigeria formed in pegmatite having contact with alkaline granite bedrock where the chromium is originated from basement schists or younger volcanic units (schwarz et al., 1996); and 7) rarely, in alluvial-fluvial placer deposits (dill, 2010). the gem deposits (beryl, chrysoberyl, amethyst, corundum, etc.) of the ratnapura (sri lanka) developed by transportation from gem-bearing source rocks and deposition in old stream terraces and flood plains such as well-rounded gravel or lenses of sand (dissanayake et al., 2000). there has been no prior research on the chromium-rich beryl [be3al2(sio3)6] in the schists from the halilbağı area (which are in tectonic contact with ophiolites), although prime ministry archives (1993) reported that emeralds (a green gem variety of beryl) were discovered and mined in the sivrihisar area during the ottoman empire. therefore, the beryl crystals in proximal metamorphic rocks prompted the present study, with emphases on geochemistry, the genetic relationship between the beryl and metamorphic rock, and the mineralogy and geochemistry of beryl crystals that occur in sediments derived from the metamorphic rocks. figure 1. geological map of the study area (modified from gültekin et al., 2003; sarıfakıoğlu et al., 2009). occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey a3 figure 2. photomicrographs showing views of a) garnet-glaucophane schist (plane-polarised light) (h1); b) garnet-lawsonite schist (plane-polarised light) (h6); c) epidote-chlorite schist (plane-polarised light) (h2); d) epidotite (plane-polarised light) (h7). geological setting the basement of the study area consists of triassic phyllites, phyllitic schists and partially metamorphosed clastic rocks (metaarkose, metagraywacke) (kahya and kuşcu, 2014)(fig. 1). triassic metasediments are overlain unconformably by jurassic conglomerates and sandstones and cretaceous sivrihisar massif blueschists (garnet-glaucophane schist, lawsoniteglaucophane schist, epidote-chlorite schist, epidotite, and chlorite-lawsonite schist) (davis and whitney, 2006). the blueschist facies comprise of sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz characterizing a broad range of pressure-temperature conditions (12-16 kbar, 380500 °c) (okay, 2002). these units are tectonically overlain by upper cretaceous ophiolitic rocks (harzburgites, dunites, wehrlites, pyroxenites, gabbros, and serpentinites). these units are overlain by the oligo-miocene rocks comprising silicified and altered trachytic tuffs as well as fresh intrusive alkaline trachytes and phonolites. 40ar/39ar geochronological analyses of sanidine crystals from the phonolites have yielded an age of 25 ma (late oligocene-early miocene) for the alkaline volcanic rocks (sarıfakıoğlu et al., 2009). temel (2001) concluded that the alkali basaltic and trachytic rocks of the sivrihisar (eskişehir) area are products of post-collisional miocene alkaline volcanism in an extensional tectonic regime. pliocene and quaternary sediments overlie these units. structural features indicate that the study area is affected by major and minor e-wand n-s-trending faults that have been under the tectonic influence of the north anatolian fault zone (şengör, 1979; şengör and yılmaz, 1981; gültekin et al., 2003). materials and methods twenty-nine sediment samples and twenty-nine rock samples were collected along creek valleys that transect the metamorphites, phonolites, trachytes, and pegmatites (outside the study area) in compliance with the principles of geochemical prospecting. transparent and opaque minerals were separated from sediment samples using a stereomicroscope (nikon smz-1b), and these minerals were grouped based on color and crystal form. transparent, colored minerals considered to be beryl were separated, and the refractive indices (r.i.) of these minerals were determined under polarized light using 0.002 r.i.-spaced immersion oils; other optical properties were also measured. the samples were analyzed for their mineralogical characteristics by polarized light microscopy (leitz laborlux 11 pol), x-ray powder diffractometry (xrd) (rigaku-geigerflex), and scanning electron microscopy (sem-edx, jeol jsm 84a-edx). xrd analyses were performed at 40 kv and 30 ma with cukα radiation; slits with divergence 1°, scatter 0.15°, receiving 1° and monochrometer 0.30 mm; and a scanning speed of 1° 2θ/min. un-oriented mounts of four powdered sediment samples were scanned to determine the mineralogy of bulk samples. representative beryl crystal samples were prepared for sem-edx analyses by adhering each beryl sample onto an aluminum sample holder that had been covered with double-sided tape and coated with a thin film (~ 350 å) of gold using a giko ion coater at eskișehir osmangazi university. infrared spectroscopy was performed using pressed pellets of powdered beryl crystal samples (2 mg of < 2 µm) mixed with 200 mg kbr and a perkinelmer 100 ft-ir spectrometer; scans were made at 4 cm-1 resolution. data were collected over the range 400-4000 cm-1. major element analyses and trace element analyses of four metamorphic sediment samples and one metamorphic rock sample were performed using the inductively coupled plasma atomic emission spectroscopy (icp-aes) method at acme laboratory ltd. (canada) to ascertain their beryllium contents. the detection limits for these analyses were between 0.01 and 0.1 weight percent for major elements, 0.1 and 0.5 ppm for trace elements, and 0.01 and 0.1 ppm for rare earth elements. the detection limit for beryllium was 1 ppm. hülya erkoyun and selahattin kadir a4 figure 3. various views of study-area beryl collected from stream sediments: a) stereomicroscopic view of crystals of aquamarine (a) on the left and emerald (z) on the right; b) emerald crystal viewed using a stereomicroscope; c) view of a beryl grain in an immersion fluid with an r.i. of 1.60; d) emerald crystal (plane-polarized light); e) basal cleavage (0001) in aquamarine (plane-polarized light). occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey a5 one emerald crystal was analyzed for all major and trace elements using both inductively coupled plasma – optical emission spectroscopy (icp-oes) and inductively coupled plasma – mass spectroscopy (icp-ms) methods in the analytical services laboratory of the university of greenwich (uk). the sample was analyzed by icpoes and icp-ms after dissolution using a lithium metaborate fusion (jarvis and jarvis, 1992). these methods are accredited to iso 17025, by ukas. in essence, 0.25 g of each sample was mixed with 1.25 g of lithium metaborate flux and fused at 900 °c. the molten bead was poured figure 4. sem images of a) beryl crystals collected from stream sediments; b) a close-up view of a hexagonal prismatic beryl crystal. figure 5. infrared spectroscopy spectra for an eskişehir beryl crystal. figure 6. aluminum versus the sum of mg+cr+fe-site cations of octahedral beryl in atoms per formula unit; from beryl analyses from the literature (leitmeier, 1937; abdalla and mohamed, 1999; groat et al., 2002; seifert et al., 2004; gavrilenko et al., 2006). into weak nitric acid and stirred until dissolved. the resultant solution was made to volume. the analysis of major elements and some trace elements was undertaken using a thermo icap 6500 icp-oes with calibration via matrix-matched synthetic standards. certified reference materials and internal qc powders were included within the analytical batch. analysis of the remaining trace elements and rare earth elements was completed using a thermo x series 2 icp-ms, again with calibration via matrix-matched synthetic standards and the use of crm s and internal qc powders. data quality was assessed post analysis via the use of shewhart charts. hülya erkoyun and selahattin kadir a6 table 2. chemical compositions of ophiolitic units, metamorphic sediments, and rocks that are associated with beryl in the study area. table 1. mineralogical variation in stream sediments of the study area. acc: accessory; +: relative abundance of mineral. occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey a7 table 3. the chemical composition of an eskişehir octahedral emerald in schist compared to another emerald in metamorphic rocks of similar nature. hülya erkoyun and selahattin kadir a8 results petrography and mineralogy the metamorphic units comprise garnet-glaucophane schist (lepidoporphyroblastic texture), epidote-chlorite schist (lepidoblastic texture), garnet-lawsonite-glaucophane schist and epidotite (nematoblastic texture) (fig. 2ad). these units consist of glaucophane, garnet, lawsonite, muscovite, epidote, chlorite, augite, quartz, sphene and zircon. locally, garnet crystal exhibit chloritization and carbonatization developed in microfracture. the results of the stereomicroscopic and xrd analyses of stream sediments showed lawsonite, quartz, garnet, augite, glaucophane, muscovite, epidote and sphene. beryl is associated with garnet, glaucophane, quartz, epidote, muscovite and chlorite in the stream sediment. sediment samples from the kaymaz stream, which transects the metamorphic rocks, were defined as emerald and aquamarine by transparency and color, with crystal sizes between 200 µm and 1 mm (e.g., h1 sample) (fig. 3). the examination of green crystals using a stereomicroscope revealed that the hexagonal morphology (fig. 3a-b); the refractive indices of these grains were found to be no= 1.568 and ne= 1.584 (fig. 3c). based on optical studies of the emeralds (fig. 3d) and light bluish green aquamarine (fig. 3e) using polarized light and immersion oils, the analyzed grains were determined to be uniaxial (-), euhedral hexagonal-prismatic crystals. stereomicroscopically determined beryl crystals were also analyzed by sem (fig. 4a-b); these crystals exhibit euhedral hexagonal prismatic forms, with locally corroded crystal edges. infrared spectra bands at 3594, 1634 and 1602 cm-1 correspond to the characteristic stretching vibrations of oh and h2o molecules in the channel coordinated to be2+ cations in tetrahedral coordination and al3+ cations in octahedral coordination along the c-axis direction of the beryl crystal (łodziński et al., 2005; makreski and jovanovski, 2009) (fig. 5). 1184, 1144, 952, 749, 519 and 441 cm-1 bands are characteristic of the si-o stretching vibration (farmer, 1974; łodziński et al., 2005). bands at 808, 749, 680 and 650 cm-1 correspond to be-o stretching, and bands at 519 and 492 cm-1 correspond to al-o vibrations (hofmeister et al., 1987; viana et al., 2002; łodziński et al., 2005; makreski and jovanovski, 2009). geochemistry major element and trace element analyses of selected schist, schist sediment, peridotite and pyroxenite samples are given in table 2. bulk stream sediment samples from the karakız stream, which cuts the metamorphic rocks, are composed of high cao (8.58-18.75%), fe2o3 (4.18-7.11%), moderate mgo (3.14-5.56%), chromium (356-554 ppm), nickel (229-306 ppm), strontium (133-140 ppm), vanadium (103 ppm) and zirconium (7-91 ppm). the metamorphic rocks (garnet-glaucophane schist, lawsonite schist and lawsoniteepidote schist) contain appreciable amounts of fe2o3 (13.12%), mgo (8.279.20%), moderate al2o3 (11.91-18.46%), nickel (97-291 ppm), chromium (196619 ppm), zirconium (117-285 ppm), cobalt (50-111 ppm), zinc (33-134 ppm), vanadium(18-228 ppm) and low beryllium (0.5-21 ppm). large ion lithophile elements (lile) (potassium, rubidium, and lead) of the metamorphic rocks are depleted, whereas the metamorphic rocks are enriched with barium (119190 ppm) and strontium (280-490 ppm). in general, lile (barium, strontium except for lead, potassium), some transition metals (cobalt, except nickel) and hfse (niobium, zirconium, yttrium except thorium) elements are relatively low or depleted compared to metamorphic rocks. however, chromium and nickel are enriched in stream sediments about the metamorphic rocks. ophiolitic rocks (peridotite, pyroxenite) of the area contain 20.6-37.84 wt% mgo, 6.276.56 wt% fe2o3, 5515-7116 ppm chromium and 1369-2103 ppm nickel. beryl chemistry a chemical analysis of an eskişehir beryl crystal is given in table 3. this study reveals that the eskişehir beryl has crystal chemistry of emerald compared to the composition of similar occurrences of octahedral beryl in metamorphic units of similar nature (table 3). the calculated structural formula for this eskişehir beryl is ( a l 1 . 7 5 c r 0 . 0 1 m g 0 . 2 2 f e 0 . 0 8 ) ( b e 2 . 9 0 s i 6 . 0 0 ) ( n a 0 . 3 2 ) o 1 8 . the high chromium value compared to vanadium and the ratio of fe2+ to fe3+ indicates that the green coloration of the eskişehir emeralds is derived from chromium. the atoms per formula unit (apfu) for the emerald crystal were calculated based on 18 oxygen. the small aluminum value (1.746 apfu) in the eskişehir emerald versus a high total value for the octahedral cations (mg2+, cr3+, fe2+), compared to the compositions of other schist-hosted emeralds, shows that the eskişehir emerald is poor in aluminum (al<1.75 apfu), similar to canadayukon emeralds, as reported by groat et al. (2002). magnesium (0.217 apfu) and iron (0.08 apfu) cations are substituted for aluminum at the octahedral site of the emeralds structure, thus allowing this to be characterized as an octahedral beryl (aurisicchio et al., 1988) (fig. 6). sodium was assigned to the channel site. discussion beryl rarely occurs in stream sediments of the study area as a result of resistance to alteration processes via short stream transportation distances and accumulates as euhedral to subhedral crystals in alluvial-fluvial placer deposits (dill, 2010). similarly, developed occurrences of beryl and other gem crystals in stream sediment by chemical and physical alteration of pegmatite, metamorphic and greywacke type units, were described in the state of minas gerais, brazil (proctor, 1984); the ratnapura gem deposit, sri lanka (munasinghe and dissanayake, 1981; dissanayake et al., 2000); and ilakaka river gravels in madagascar (pezzotta and simmons, 2001). beryl-bearing sediments of the study area that are derived from metamorphic units consist of garnet-glaucophane schist, lawsoniteglaucophane schist, epidote-chlorite schist, epidotite, and chlorite-lawsonite schist (kulaksız, 1981) reflecting blueschist facies (bucher and frey, 1994; davis and whitney, 2006). green and light bluish-green, hexagonal prismatic beryl crystals occur in stream sediment that transects the metamorphic units, suggesting that the chromium-rich beryl developed within the metamorphic rocks consisting of garnet, glaucophane, epidote, chlorite, feldspar, quartz, lawsonite, muscovite, augite and sphene. its formation in the metamorphic units was controlled by synto post-tectonic deformation due to the upthrust of chromium-rich ophiolitic units, which resulted in metasomatic reactions. thus, the concentration of magnesium-rich and beryllium-rich fluid favored the crystallization of beryl (turner et al., 2005; grundmann and morteani, 2008; groat et al., 2008). the host rock of this type of beryl occurrence is similar to beryl (emerald) deposits that are formed via synto post-tectonic reactions under low-grade regional metamorphism and within biotite schist tectonically overlain by ophiolite, such as in southern egypt (abdalla and mohamed, 1999) and habachtal, austria, (grundmann and morteani, 1989) where the source of chromium is believed to be metasomatized ophiolitic rocks (groat et al., 2008). the green coloration of the eskişehir emerald is due to the presence of chromium and, potentially, the ratio of fe2+ to fe3+, similar to the emeralds of zambia, canada, egypt, austria and brazil (table 3). aurisicchio et al. (1988) documented substitutions involving fe2+, mg2+, mn2+, fe3+, cr3+, v5+, and sc3+, as well as v5+ and ti4+, for octahedrally coordinated aluminum, as well as lithium for tetrahedrally coordinated beryllium. additionally, eskişehir beryl can be classified as chromium-containing beryl according to the gemological institute of america (sinkankas, 1994). the presence of chromium and nickel in the eskişehir emerald crystals and the small values of barium, strontium, and rubidium suggest that crystallization of this beryl (emerald) crystals was due to a metasomatic-fluid reaction regime controlled by the upthrust of ophiolitic units onto the schists. eskişehir beryl contains high na2o (1.69%) compared to k2o (<0.29%) and indicates alkali-rich, sodic beryl occurrence of cr-bearing beryl in stream sediment from eskişehir, nw turkey a9 (łodziński et al., 2005). the presence of type-ii h2o molecules narrow 3594 cm-1 and strong 1634 cm-1 bands corresponds to the presence of alkalis in the structural channels of beryl, similar to that described for beryls of the sudety mountains and macedonian regions (łodziński et al., 2005; makreski and jovanovski, 2009). these stretching bands decrease, and type-ii h2o disappears when the alkali content of the beryl falls (manier-glavinaz et al., 1989). magnesium is the main constituent of emerald that is derived from metamorphic rocks. the high magnesium contents of the metamorphic units may indicate a genetic relationship to beryl (schwarz and giuliani, 2001). the chromium (619 ppm) and nickel (681 ppm) contents in the garnet-glaucophane schist can be explained by the close association with ophiolitic rocks, which contribute chromium and nickel via leaching (grundmann and morteani, 2008). furthermore, lile (barium, strontium), transition metals (cobalt, except nickel) and hfse (niobium, zirconium, and yttrium) element values that were low in stream sediments relative to metamorphic rocks suggest that these elements were transported during weathering of primary minerals, similar to what has been reported for lile values in sri lanka (chandrajith et al., 2000). the low beryllium content of stream sediment (1 ppm) and rocks (0.46-21.09 ppm) in the study area is similar to that of other beryl-bearing metamorphic rocks and sediments in nature (grundmann and morteani, 1989; grew, 2002). all major oxide (except sio2 and cao) values decrease in stream sediment, possibly during transportation from altered metamorphic units. beryllium-bearing fluid associated with sio2 and al2o3 may indicate thermodynamic conditions (p, t) that are related to the alkalinity of melts and fluids through reactions (barton and young, 2002). thus, beryl is stable under high silica activities and temperature (> 600°c) conditions. by contrast, a decrease of the silica activity of beryl may result in replacement with other beryl crystals, such as chrysoberyl and phenakite, which are absent in the study area. the occurrence of alluvial gemstones was identified for the first time in the eskişehir region (turkey) during this study. this research may be significant for future use in investigations of beryl crystals that are associated with metamorphics and related sediments. additionally, this study will also help identify the effect of tectonism and metasomatism on the formation of beryl crystals near ophiolite and metamorphic units and the local occurrence of beryls in sediment under short distance transportation from metamorphics. conclusions the eskişehir green color beryl crystals formed in schists via the upthrust in the upper cretaceous of chromium and magnesiumrich metasomatized ophiolites onto metamorphic units. locally, the contact with ophiolites metamorphic unit is transected by streams, the sediments of which contain micrometric accessory beryl crystals. the euhedral, hexagonal beryl crystals have low refractive index value and between 200 µm and 1 mm sizes. the eskişehir beryl’s chemical composition is characterized by high chromium, iron and magnesium, reduced sodium, cesium and low potassium, vanadium and rubidium. acknowledgements this research comprises further work on the first author’s m.sc. study, supervised by the second author. the authors are indebted to doctors güneş kürkçüoğlu (eskişehir osmangazi university) and david wray (university of greenwich) for conducting the infrared spectra and chemical analyses of the emerald crystals, respectively. we also thank professor rifat bozkurt (eskişehir osmangazi university) for his help during the m.sc. study. the authors are much indebted to professor fahri esenli (i̇stanbul technical university) and an anonymous reviewer for their extremely careful and constructive reviews that improved the quality of the paper significantly. we are also incredibly grateful to editor in chief, professor andrés felipe torres-villegas for his insightful editorial comments and suggestions. references abdalla, h. m, & mohamed, f. h. 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(2006). gemmology, geology and origin of the sandawana emerald deposits, zimbabwe. scripta geologica, 131, 1-211. geociencias-vol 13-2 2009.vp earth sciences research journal earth sci. res. j. vol. 13, no. 2 (december 2009): 108-118 characterization of vermiculite by xrd and spectroscopic techniques a. campos, s. moreno, r. molina1 1 estado sólido y catálisis ambiental, departamento de química, facultad de ciencias, universidad nacional de colombia. ak 30 no. 45-03, bogotá, colombia; fax: 57-1-3165220 e-mail autor:ramolinag@unal.edu.co abstract a natural mineral from santa marta-colombia used as the starting material in the synthesis of pillared clays has been characterized by several techniques, including x-ray diffraction, x-ray fluorescence, electronic paramagnetic resonance, aluminum nuclear magnetic resonance and scanning electron microscopy. the information revealed that the mineral corresponds to trioctahedral vermiculite. the identification of this mineral is valuable in the control of reduction charge and pillaring processes on these materials to obtain more complex solids like the ones required to specific catalytic applications. key words: vermiculite, structural formula, layer charge. resumen un mineral natural de la región de santa marta en colombia, el cual usado como el material de partida en la síntesis de arcillas pilarizadas, ha sido caracterizado por diversas técnicas tales como difracción de rayos-x, fluoresencia de rayos-x, resonancia electrónica paramagnética, resonancia magnética nuclear de aluminio y microscopía electrónica de barrido. la información en conjunto indica que el mineral corresponde a vermiculita trioctaédrica. la identificación de este mineral es muy importante en la comprensión y el control de los procesos de reducción de carga y de pilarización de estos materiales para la obtención de sólidos más complejos, como los requeridos en aplicaciones catalíticas específicas. palabras clave: vermiculita, fórmula estructura, capa de carga. 1. introduction vermiculite is a clay mineral usually of secondary origin due to the alteration of mica, pyroxene, chlorite or similar minerals (brown 1961). the substitutions of si by al in tetrahedral sheet predominate in vermiculite. in this way, the negative charge generated on the tetrahedral sheets limits the expansion properties of the clay and this factor determines the high layer staking of its structure (mac ewan and wilson 1980; tunega et al. 2003). 108 manuscript received: 24/03/2009 accepted for publication: 26/05/2009 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:17 p p p composite 133 lpi at 45 degrees from the catalytic point of view, the vermiculite is a very attractive material due to the thermal resistance (suvorov and skurikhin 2003) and the number of tetrahedral substitutions, which ensure the presence of a larger number of brønsted-type acid sites (del rey-pérez-caballero and poncelet et al. 2000); these substitutions are less numerous in other hydrous 2:1 minerals such as smectites. on the whole, these properties offer considerable interest in the production of pillared clays, which are distinguishable from an intercalated layered solid due to their micro and/or mesoporosity and high thermal stability with preservation of the layer stacking (moreno et al. 1997, schoonheydt et al. 1999; stefanis et al. 2006). the development of pillared clays from vermiculite during the past few years has offered particular interest especially as a heterogeneous catalyst in acid-catalyzed reactions (campos et al. 2005; campos et al. 2007; campos and gagea et al. 2008; campos and moreno et al. 2008; cristiano et al. 2005; del rey-pérez-caballero and poncelet et al. 2000; del rey-pérez-caballero and sánchez et al. 2000, hernández et al. 2007). however, as direct intercalation of vermiculite is impossible due to a very high potential of stabilization of interlayer cations (suvorov and skurikhin 2003) it has been necessary to develop alternative methodologies to achieve pillared vermiculite (cristiano et al. 2005; f. del rey-pérezcaballero 2000). one of these methodologies has been the hydrothermal treatment prior to a conventional process of intercalation, in order to reduce negative charge and make possible the subsequent cationic exchange (cristiano et al. 2005). the catalytic profile of al and al-zr modified vermiculites by means of this methodology has confirmed the potential expected for this acidic acid solid, since they are highly active with respect to other clays in alkane hydroisomerization (campos 2005; campos et al. 2007; campos and gagea et al. 2008; campos and moreno et al. 2008; cristiano et al. 2005; del rey-pérez-caballero and poncelet 2000; del rey-pérez-caballero and sánchez et al. 2000; hernández et al. 2007). in order to produce practical information to control the modification processes described previously, to achieve a clay mineral with potential catalytic and adsorptive properties, this paper focuses on the characterization of natural vermiculite issued from a colombian deposit. x-ray diffraction (xrd) and several spectroscopic techniques such as electronic paramagnetic resonance (epr) and nuclear magnetic resonance (nmr) were used. 2. experimental 2.1 materials the vermiculite selected for this work was labeled v, and it corresponds to a commercial mineral, which comes from a natural deposit in the santa marta region in colombia. its characterization was carried out with the fraction of a particle size smaller than 150 ìm, separated from the commercial form by sieve, without purification or fractioning treatment. 2.2 characterization methods the cationic exchange capacity was determined on the clay previously interchanged with an ammonium acetate solution, by means of the micro-kjeldahl method (chapman 1965). the x-ray diffraction study obtained from flakes, which had been gently pressed onto the plates by using a philips pw 1820 diffractometer (ká radiation of cu , ë=1.54056 å, 40 ma, 40 kv) in 2è geometry and bragg–brentano configuration, a step size of 0.05 and a step time of 2 s. diffraction patterns were taken at room temperature, 20 °c and 65% of humidity on the average. the identification of the mineral was assessed according to the position of basal reflection 001 in three patterns: i) natural sample, ii) after solvation in the presence of ethylene glycol for 24h and iii) after heating at 500 ºc for 2 h. in addition, mg2+ and k+ saturations were performed in the mineral to achieve a better characterization (thorez 1976). adsorption-desorption isotherms of nitrogen at the temperature of liquid nitrogen were analyzed using a micromeritics tristar 3000 instrument on samples previously degassed at 200 °c for 6 h. scanning electron microscopy images were recorded in a philips scanning electron microscope xl30 feg. the gravimetric and differential thermal-analysis were performed at a heating speed of 10 ºc min-1 in air atmosphere using a thermal analyzer tg and dsc rheometric instruments. the chemical analysis was performed by x-ray fluorescence in an xrf 2400 instrument. in addition, the fe2+ analysis was carried out through the analytic method described in wilson m. (wilson 1995). electron paramagnetic resonance spectra in the x band (9.8 ghz) was carried out in a brucker esp 3220 spectrometer adopting a modulation frequency of 100 khz whose 109 characterization of vermiculite by xrd and spectroscopic techniques enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:17 p p p composite 133 lpi at 45 degrees width was fixed at 0.4 mt for wide lines and reduced to 0.05 mt for acute bands, at a temperature of 100 k. the nuclear magnetic resonance of 27al was performed in a brucker advance 400 spectrometer; the sample was placed in 2.5mm rotors, while 1200 scannings were accumulated with a time of 100ms of recycle. rotor speed was 5000 hz. xrf, epr and nmr measurements were done additionally in the mineral washed with nitric acid 0.01 m (10 ml g-1 clay) by 1h at 30 °c. 3. results and discussion 3.1 x-ray fluorescence (xrf) and cation exchange capacity (cec) table 1 records the chemical composition of the mineral obtained by x-ray fluorescence. the structural formula of the colombian vermiculite estimated from its chemical composition and following the methodology described in wilson m. (wilson 1995) is: [(si3.04al0.92ti0.04)(al0.11fe3+ 0.35 fe2+ 0.07mg2.41mn0.003)o10 (oh)2] ca0.21k0.05na0.10 on the other hand, the cationic exchange carried out with ammonium-vermiculite is 1.10 meq nh g4 1� . 3.2 x-ray diffraction analysis table 2 shows the comparison between the reflections in xrd for mg-vermiculite (brown 1961), those corresponding to a diffraction pattern calculated using difk software (wiewióra et al. 2003) and the reflections observed in the pattern for natural vermiculite v. this parallel makes it possible to observe an analogue pattern between the clays; thus the clay mineral under study corresponds to vermiculite (wiewióra et al. 2003). likewise, a high layer stacking is highlighted, which is verified by the presence of reflections in series 02l and 11l (wiewióra et al. 2003). reflection 060 located in 0.154 nm indicates that the vermiculite structure is of the trioctahedral type (dixon and weed 1989). in general, vermiculite can be dioctahedral or trioctahedral, but trioctahedral type is common in soils with a similar morphology to that of mica (moore and reynolds 1997), as observed in vermiculite (figure 3). on the other hand, 6th, 8th, and 10th order reflections located at 0.480, 0.360 and 0.288 nm respectively show a growing intensities serie typical for vermiculites (wiewióra et al. 2003). interstratified material is absent; otherwise the sequence should be abnormal (wiewióra et al. 2003). trioctahedral vermiculites usually formed by the weathering of biotite (gordeeva et al. 2002) may be found in the company of this mineral and also talc is possible. in the case of clay v we can suggest that talc is found as a non significant impurity, indicated by its characteristic diffractions in 0.93 nm, 0.48 nm and 0.31 nm (fagel et al. 2001) (figure 1). in parallel, starting from the positions of reflections 001 and 004 it can be estimated that the vermiculite is found in a proportion larger than 90 % (thorez 1976). in order to verify the purity of the mineral, table 3 records the follow up on reflection 00l of raw natural clay powder, when the sample is solvated in the presence of ethylene glycol and, when the latter is calcined at 500 ºc. this table shows that the v material exhibits the sequence typical of vermiculites (thorez 1976; dixon and weed 1989). however different authors (thorez 1976, dixon and weed 1989, malla and douglas 1987) suggest complementary analysis xrd in mg2+ and k+-vermiculite to distinguish between soil vermiculites and oh-interlayer vermiculites. as well they suggest a relation between the d001 signal and the layer charge. after saturation with magnesium followed by glycerol the basal reflection should be located in 1.4 nm compared with 1.8 nm if the clay 2:1 were a smectite (thorez 1976; fagel et al. 2001). in this sense, the basal reflection of v located at 1.4 nm was not modified. the impossibility for expansion in mg-v is then a consequence of the strong retention of magnesium between layers with a high negative 110 a. campos, s. moreno, r. molina table 1. chemical composition of colombian vermiculite. oxide al2o3 sio2 fe2o3 feo mgo mno cao k2o na2o tio2 % 13.06 45.40 6.91 1.24 24.10 0.11 2.88 0.60 0.26 0.73 enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:18 p p p composite 133 lpi at 45 degrees density like vermiculites (slade et al. 1976, tunega et al. 2003) (table 4). malla and douglas (malla and douglas 1987) established that the saturation with this potassium originates interlayer space reduction in the region between 1.12 -1.20 nm if the charge is lower than 0.57 and, between 1.00 and 1.06 nm when it is at least 0.6. in this sense, when clay v was saturated in the presence of k+ the basal spacing of 1.4 nm was reduced to 1.0 nm after dried at room temperature and calcinations of the sample at 500 °c (table 4). the reduction of interlayer space after potassium saturation, qualitatively indicate that the layer charge must be higher than 0.57 (malla and douglas 1987). in addition, the previous characteristic is a tool for the clear distinction between vermiculite and chlorite, due to the fact that the latter keeps the 001 reflection at 1.4 nm when k-chlorite, while the basal spacing of k-vermiculite is reduced to 1.0 nm (dixon and weed 1989). for this reason, the absence of chlorite is verified as a possible impurity in the mineral. 3.3 thermal analysis the thermal-gravimetric (tg) and differential (dta) analysis of vermiculite is presented in the figure 2 where a total weight loss of 21.1 % is distributed in several regions. in the first one, the dehydration of the clay takes place in two stages: i) between 20 and 110 ºc which corresponds to the loss of water physically absorbed on the surface and ii) from 110 ºc to 500 ºc, with a weight loss of 14.3 % attributed to the exit of water molecules, which are in contact with the cations in the interlayer region (pérez et al. 2003). 111 characterization of vermiculite by xrd and spectroscopic techniques table 2. xrd patterns of mg-vermiculites, for simulated pattern and the pattern of colombian vermiculite. hkl v reference (nm) (nm) 1a 2b 002 1.430 1.44 1.43 004 0.719 0.718 006 0.480 0.479 0.477 020 0.468 0.460 0.460 008 0.360 0.360 0.358 115 0.339 0.341 0010 0.288 0.287 0.287 200 0.265 0.265 0.264 132 0.259 0.260 0.258 202 0.254 0.255 0.254 204, 0012 0.240 0.238 0.239 136 0.221 0.227 138, 206 0.206 0.209 208 0.201 20î2, 1310 0.187 2010 0.185 0.184 0016, 20î4 0.179 0.174 2012 0.168 0.167 1314, 2016 1.580 0.158 060 0.154 0.154 0.154 062, 330 0.153 2018, 0020 0.144 0.144 338 0.136 0.136 a mg-vermiculite macroscopic of west chester (brown 1961). b simulated pattern (wieióra et al. 2003). table 3. identification of vermiculite from the basal reflection 001 (nm). clay mineral n eg 500 vermiculite 1.4 1.4 0.96-1.0 smectite 1.2-1.5 1.7 1.0 chlorite 1.4 1.4 1.4 v 1.4 1.4 1.0 natural clay (n), after solvation treatment with ethylene glycol (eg) and the latter heated at 500ºc (500). enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:18 p p p composite 133 lpi at 45 degrees 112 a. campos, s. moreno, r. molina a b 0. 93 3 1. 43 0 (0 02 ) 0. 71 9 (0 04 ) 0. 48 0 (0 06 ) 0. 46 8 0. 36 0 (0 08 ) 0. 33 9 (1 15 ) 0. 31 2 0. 28 8 (0 01 0) 12000 10800 9600 8400 7200 6000 4800 3600 2400 1200 o in te n si ty (a rb it ra ry u n it s) 5 10 15 20 25 30 35 2theta (degrees) 0. 24 04 0. 20 6 (1 38 ,2 06 ) 1500 1350 1200 1050 900 750 600 450 300 150 o in te n si ty (a rb it ra ry u n it s) 35 40 45 50 55 60 65 70 0. 18 7 (1 31 0. 20 2)1 0. 18 5 (2 01 0) 1. 79 9 (0 01 6. 20 4)1 1. 68 04 (2 01 2) 0. 15 8 (1 31 4. 20 6)1 0. 15 4 (0 60 , 3 32 ) 0. 14 4 (2 01 8) 0. 13 6 (3 38 ) 2theta (degrees) figure 1. xrd patterns of colombian vermiculite, in the regions of 5-35 º2 (a) and 35-70 2 (b). table 4. comparison of the first order reflection (nm) for high and low charge vermiculites and the colombian mineral (v). vermiculite composition n eg 500 mgn mgeg kn k500 low charge 1.4 1.6 1.0 1.4 1.6 1.1 1.0 high charge 1.4 1.4 1.0 1.4 1.4 1.0 1.0 v 1.4 1.4 1.0 1.4 1.4 1.0 9.3 natural vermiculite after mg2+ saturation (mgn). mgn after solvation with ethylene glycol (mgeg). natural vermiculite after k+ saturation (kn). kn heated at 500 ºc (k500). enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:18 p p p composite 133 lpi at 45 degrees the region between 500 and 850 ºc records a loss of 6.8 % in weight with a linear tendency with respect to temperature, which is attributed to dehydroxilation (pérezrodríguez et al. 2004). in fact, the unusual endothermic peak at 800 °c is associated with the thermal stability on vermiculite which is characteristic of this mineral as well this signal could be assigned to the formation of a new enstatite crystalline phase, which has been reported previously (pérez et al. 2003). 3.4 texture and morphology the selected fraction for vermiculite v corresponds to the mineral observed in figure 3, where large vermiculite layer crystals are detected, which display soft surfaces, with small protuberances. polygonal sheets with flaked borders are observed in vermiculite formed by the alteration of parental materials (kishk and barshad 1969). the specific surface area (7.41 m2 g-1) corresponding to external surface area without any microporosity. the low surface area is related to the particle but as well with the surface charge, since micas with similar particle size and charge higher than vermiculites have less surface area (del rey-pérez-caballero and sánchez et al. 2000). the strong hydrogen bonds between the tetrahedral sheet and the water of interlayer cations in vermiculite can keep the interlayer zone obstructed (tunega d. et al. 2003), which could reduce superficial nitrogen adsorption considerably. by other hand, the relation observed between the interchange cations and the penetration in the interlayer space has been indicated by different authors (pérez et al. 2003). in fact, when monovalent cations in the interlayer region are replaced by those of divalent type, a significant reduction in the superficial area for this mineral clay is observed (pérez et al. 2003). 3.5 electronic paramagnetic resonance (epr) and nuclear magnetic resonance of aluminum ( 27 al-nmr) natural vermiculite can be accompanied by iron oxides and hydroxides on its surface (iron that is not substituting aluminum in the octahedral sheet), which will cause an overestimation of iron in vermiculite if it is evaluated by chemical analysis. in this way, epr could be used to determine the presence of structural fe3+ in vermiculite. additionally, to evaluate the structural environment of paramagnetic ions found in the clay. as can be observed in figure 4, the epr of v is dominated by signals associated with fe3+ g = 4.3 and 2.1. in addition, a sextet of lines in g = 2 can be appreciated, which is typical of mn2+ species with distorted octahedral symmetry (tus¡ar et al. 2005). the signal at g = 4.3 is related with fe3+ located at an orthorhombic environment within the clay structure (bensimon et al. 2000). in general, smectites exhibit signals for fe3+ between g = 4.3 and g = 2.0, which can be attributed to the combination of two different octahedral and two tetrahedral environments. their formation depends on the possible organizations of the oh groups in the feo4(oh)2 unit (wilson 1995). mc bride et al. (mc bride et al. 1975) suggest that the difference between the two octahedral sites occupied by 113 characterization of vermiculite by xrd and spectroscopic techniques temperature ºc 30.0 20.0 10.0 0.0 -10.0 -20.0 -30.0 15.0 14.5 14.0 13.5 13.0 12.5 12.0 11.5 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 1000.0 w eight( ) [m g] h ea tf lo w ( ) [m w ] figure 2. differential and gravimetric thermal curves of colombian vermiculite. figure 3. sem image of vermiculite v. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:19 p p p composite 133 lpi at 45 degrees fe3+ ions depends on the nature of the adjacent octahedral cation (divalent or trivalent). according to timofeeva et al. (timofeeva et al. 2005), the iron found in the form of aggregates in the surface is related with signals in the region comprised between g = 2.3-2.6. alternatively, different contributions to the wide signal located around g = 2.1 are related with fexoy or feooh species (chung et al. 2004). in the case of the smectite minerals the signal at g = 4.3 is assigned to the presence of isolated fe3+ in tetrahedral or octahedral coordination, which corresponds to the iron located in the interior of the clay sheets (iron substituting aluminum in the octahedral layers). on the other hand, the signal at g = 2.0 is associated to the presence of clusters of iron (iron extra-red species) (carriazo et al. 2005). 114 a. campos, s. moreno, r. molina a b a figure 4. epr spectra of colombian vermiculite at 100 k. (a) natural mineral, (b) natural mineral after mild acid treatment. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:20 p p p composite 133 lpi at 45 degrees as observed in figure 4a, several signals point to the presence of iron extra-red species on the surface of the clay v. for this reason, the clay was submitted to a soft acid wash to remove impurities from its surface. in figure 4b, the absence of signals between g = 2.3-2.6 indicates that these species had been removed. likewise, signals of mn2+ ions located in the region at g = 2, and g = 1.9 (figure 4a), and the multiple signal recorded is interpreted in terms of the interaction between structural ions mn2+ and fe3+ and other environments (kessissoglou 1999). after acid wash mn2+ signals remain invariable which suggest structural positions for this cation. respect to aluminum, in order to evaluate the structural environment as well as the al(iv)/al(vi) ratio to compare with the ratio estimated in the structural formula, 27al mas-nmr spectra was made. the spectrum is shown in figure 5, in which an intense signal at 60 ppm is observed (with two sidebands at -60 and 190 ppm). this signal is associated with al(iv) substituting the silica in the tetrahedral sheets (klinowski 1999). a second signal corresponding to al(iv) at 0 ppm (klinowski 1999) was observed. the intensity ratio al (iv)/ al (vi) was 10.4. 3.6 correlation of results weaver (weaver 1958) suggested that vermiculite derived from mica weathering has a high laminar charge and its 001 reflection decrease to 1.0 nm after potassium saturation, as happens in clay v (table 4). in contrast, vermiculites generated by the weathering of materials like amphiboles or volcanic material exhibit a limited layer contraction with the same treatment. in fact, the laminar charge is a fundamental element in identification of 2:1 silicates. in fact, the aipea (international association for the study of clays) established that vermiculite is a phylosilicate 2:1 with a charge of 0.6 to 0.9 per unit cell (guggenheim et al. 2006). in this sense, to determine the layer charge of v, it was performed with the structural formula, which was determined through xrf elemental analysis. in it formula can be deduced that the tetrahedral sheet originate an excess negative charge of -0.9 e/ o10(oh)2, which is compensated by ca2+, na+ and k+ cations. as regards to the octahedral sheet, formed specially by fe3+ and mg2+ it carries a positive charge of 0.35, resulting in a total negative charge of 0.6 e/ o10(oh)2. in order to confirm the approach to the elemental organization and to evaluate the environment of the elements found in the structure, epr and nmr provide very valuable information. thus, by means of epr spectroscopy, iron species on the natural clay surface were detected, which can be easily removed through a soft acid wash (figure 4b). in consequence, the chemical analysis by xrf to evaluate the structural formula was done in the mineral washed with soft acid because unwashed sample could overestimate the iron content. regarding the nmr the al (iv)/al (vi) ratio (10.4) compared with the ratio estimated with the structural formula (8.9) is a satisfactory approach of vermiculite structure take into account that paramagnetic species such as iron can modify the tetrahedral/octahedral ratio (gates et al. 1996). the total negative charge (malla and douglas 1987; guggenheim et al. 2006) observed in the structural formula and the slight difference with the cec as well as the xrd signals and the follow up of the basal reflection 001 after submitting the sample to different treatments (thorez 1976; 115 characterization of vermiculite by xrd and spectroscopic techniques 2060140 al iv * -140-100-60-20100180220 (ppm) al vi * figure 5. 27al mas-nmr spectra of the colombian vermiculite. (*) spinning sidebands. enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:20 p p p composite 133 lpi at 45 degrees wilson 1995; moore and reynolds 1997) indicate that mineral v is trioctahedral vermiculite. the knowledge derived from the present work is very useful in further understanding, controlling and predicting the modifications that can be performed on this attractive natural mineral in order to obtain materials with technological impact such as catalyst and adsorbents (campos et al. 2005; cristiano et al. 2005; campos et al. 2007; campos and gagea et al. 2008; campos and moreno et al. 2008, hernández et al. 2007). 5. conclusion mineral clay from a deposit in the region of santa marta, colombia has been characterized through different techniques. the correlation of results indicates that the mineral corresponds to trioctaheral vermiculite as result of the mica-biotite weathering process. the layer stacking, its high thermal stability, low superficial area and the magnitude of its negative charge are characteristic of vermiculite. the details about the structure of this mineral are highly useful in the latter pillaring process of these mineral to enhance their catalytic applications. acknowledgements this research has been supported by projects code hermes 9887 vri-dib-universidad nacional de colombia. the authors wish to thank professor pierre jacobs, director of the centrum voor oppervlaktechemie en katalyse at katholieke university of leuven for carrying out the nmr and epr analysis. references bensimon y., deroide b., dijoux f., martineau m. (2000). nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance. journal of physics and chemistry of solids 61, 1623-1632. brown b. (1961) the x-ray identification and crystal structures of clay minerals. mineralogical society; london. campos a., moreno s., molina r. (2005). acidez e hidroisomerización de heptano en una vermiculita colombiana modificada con aluminio. revista colombiana de química. 34, 79-92. campos a., gagea b., moreno s., jacobs p., molina r. (2007). hydroisomerization of decane on pt/al,ce-pillared vermiculites. studies in surface science and catalysis. 170, 1405-1409. campos a., gagea b., moreno s., jacobs p., molina r. (2008). decane hydroconversion with al-zr, al-hf, al-ce-pillared vermiculites. applied catalysis a. 345, 112-118. campos a., moreno s., molina r. (2008). relationship between hydrothermal treatment parameters as a strategy to reduce layer charge in vermiculite, and its catalytic behavior. catalysis today. 133-135, 351-356. carriazo j., guélou e., barrault j., tatibouët j., molina r., moreno s., 2005. synthesis of pillared clays containing al, al-fe or al-ce-fe from a bentonite: characterization and catalytic activity. catalysis today 107–108, 126–132. chapman h. (1965). cation exchange capacity. in: black c, evans d, white j, ensminger l, clark e, editors. methods of soil analysis (agronomy 9), american society of agronomy. chung h., sang-won c., yong-sik o., jinho j. (2004). epr characterization of the catalytic activity of clays for pce removal by gamma-radiation induced by acid and thermal treatments. chemosphere 57, 1383-1387. cristiano d., campos a., molina r. (2005). charge reduction in a vermiculite by acid and hydrothermal methods: a comparative study. journal of physical chemistry b 109, 19026-19033. del rey-pérez-caballero f., poncelet g. (2000). microporous 18 å al-pillared vermiculites: preparation and characterization. microporous and mesoporous materials. 37, 313-327. del rey-pérez-caballero f., sánchez m., poncelet g. (2000). hydroisomerization of octane on pt/al-pillared vermiculite, and comparison with zeolites. studies in surface science and catalysis. 130, 2417-2422. dixon j., weed s. (1989). minerals in soil environments. second edition. united states. fagel n., robert c., preda m., thorez j. (2001). smectite composition as a tracer of deep circulation: the case of the northern north atlantic. marine geology. 172, 309-330. gates w., stucki j., kirkpatrick r. (1996). structural properties of reduced upton montmorillonite. physics and chemistry of minerals 2, 535-541. 116 a. campos, s. moreno, r. molina enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:20 p p p composite 133 lpi at 45 degrees gordeeva l., moroz e., rudina n., aristov y. (2002). formation of porous vermiculite structure in the course of swelling. russian journal of applied chemistry. 75, 357-361. guggenheim s., adams j., bain b., bergaya f., brigatti m., drits v., formoso m., galán e., kogure t., stanjek h. (2006). summary of recommendations of nomenclature committees relevant to clay mineralogy: report of the association international pour l’etude des argiles (aipea) nomenclature committee for 2006. clays and clay minerals. 54, 761-772. hernández w., centeno m., odriozola j., moreno s., molina r. (2007). acidity characterization of a titanium and sulfate modified vermiculite. materials reserach bulletin. 43, 1630-1640. kessissoglou d. (1999). homoand mixed-valence epr -active trinuclear manganese complexes. coordination chemistry review. 185, 837-858. kishk f., barshad i. (1969). the morphology of vermiculite clay particles as affected by their genesis. american mineralogist. 54, 849-857. klinowski j. (1999). solid-state nmr studies of molecular sieve catalysts. chemical reviews. 91, 1459-1479. laird d. (1999). layer charge influences on the hydration of expandable 2:1 phyllosilicates. clays and clay minerals. 47, 630-636. malla p., douglas l. (1987). layer charge properties of smectites and vermiculites: tetrahedral vs. octahedral. soil science society of america journal. 51, 1362-1366. macewan, d. and wilson, m. (1980). interlayer and intercalation complexes of clay minerals. pp. 197 248 in: crystal structures of clay minerals and their x-ray identification (g.w. brindley and g. brown, editors). mineralogical society, london. mc bride m., pinnavaia t., mortland m. (1975). exchange ion positions in smectite: effects on electron spin resonance of structural iron. clays and clay minerals. 23,162-164. moreno s., gutiérrez e., alvarez a., papayannakos n., poncelet g. (1997). al-pillared clays: from lab syntheses to pilot scale production characterization and catalytic properties. applied catalysis a 165, 103-114. moore d., reynolds j. (1997) . x-ray diffraction and the identification and analysis of clay minerals. second edition. oxford university press. pérez l., balek v., potayo j., pérez j., subrt j., bountsewa i., beckman i., málek z. (2003). study of natural and ion exchanged vermiculite by emanation thermal analysis, tg, dta and xrd. journal of thermal analysis and calorimetry. 71, 715-726. pérez-rodriguez j., potayo j., jiménez de haro m., pérez-maqueda l., lerf a. (2004). thermal decomposition of nh4 � -vermiculite from santa olalla (huelva, spain) and its relation to the metal ion distribution in the octahedral sheet physics and chemistry of minerals 31, 415-420. schoonheydt r., pinnavaia t., lagaly g., gangas n. (1999). pillared clays and pillared layered solids. pure applied chemistry. 71: 2367-2371. slade p. (1976). telleria m., radoslovic w. the estructures of ornithine-vermiculite and 6-aminohexanoic acid-vermiculite. clays and clay minerals 24, 134-141. suvorov d., skurikhin v. (2003). vermiculite: a promising material for high-temperature heat insulators. refractories and industrial ceramics. 44, 186-193. stefanis a., tomlinson a. (2006). towards designing pillared clays for catalysis. catalysis today 114, 126-141. thorez j. (1976). practical identification of clay minerals. g. lelotte (ed). dison, belgique. timofeeva m., khankhasaeva s., badmaeva s., chuvilin a., burgina e., ayupov a., panchenko v., kulikova a. (2005). synthesis, characterization and catalytic application for wet oxidation of phenol of iron-containing clays. applied catalysis b. 59: 243-248. tunega d., lischka h. (2003). effect of the si/al ordering on structural parameters ad the energetic stabilization of vermiculitesa theoretical study. physics and chemistry of minerals 30, 517-522. tus¡ar n., zabukovec n.,vlaic g., arc¡on d., daneu n., kauc¡ic¡ v. (2005). local environment of manganese incorporated in mesoporous mcm-41. microporous and mesoporous materials. 82, 129-136. weaver c. (1958). effects of geologic significance of potassium “fixation” by expandable clay minerals derived from muscovite, biotite, chlorite and volcanic material. american mineralogist. 43, 839-861. 117 characterization of vermiculite by xrd and spectroscopic techniques enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:20 p p p composite 133 lpi at 45 degrees wiewióra a., perez-rodriguez l., perez-maqueda l., drapala j. (2003). particle size distribution in sonicated highad low charge vermiculites. applied clay science. 24, 51-58. wilson m. (1969). mineralogy: spectroscopy and chemical determinative methods. champan & hall ed; london. 118 a. campos, s. moreno, r. molina enero 30-geociencias-vol 13-2 2009.prn d:\geociencias 13-2 dic 2009\geociencias-vol 13-2 2009.vp sæbado, 30 de enero de 2010 18:16:21 p p p composite 133 lpi at 45 degrees earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 15 30 this study reports the occurrence and petrologic significance of ti-clinohumite for the first time in the ciénaga skarn, sierra nevada de santa marta massif. ti-clinohumite occurs as thin bands hosted in the ciénaga marbles of lower cretaceous age, which consist of quartz-wollastonite-diopside-garnet-clinohumite marbles. we focus the attention on the occurrence of ti-clinohumite-bearing domains, highlighting some interesting textures as well as the mineralogy and chemistry of the ti-clinohumite. on the basis of its geologic situation, petrography, mineral composition and metamorphic history, we discuss the origin of ti-clinohumite. el estudio reporta la presencia y el significado petrológico de ti-clinohumita por primera vez en mármoles del depósito de skarn de ciénaga, macizo sierra nevada de santa marta, de edad cretácico inferior. la ti-clinohumita ocurre como bandas delgadas en mármoles de cuarzo-wollastonita-diópsido-granateclinohumita. centramos la atención en la ocurrencia de los dominios con ti-clinohumita, destacando algunas de sus texturas interesantes, así como la mineralogía y la química de la ti-clinohumita. con base al contexto geológico, a las características petrográficas, a la composición mineral y a la historia metamórfica, se discute el origen de la ti-clinohumita. abstract resumen key words: ti-clinohumite; marbles; ciénaga skarn; mineralogy; sierra nevada de santa marta massif. palabras clave: ti-clinohumita; mármoles; skarn de ciénaga; mineralogía; macizo sierra nevada de santa marta. ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance carlos alberto ríos r.¹*, oscar mauricio castellanos a.², carlos alberto chacón avila³ ¹*grupo de investigación en geología básica y aplicada (gigba), escuela de geología, universidad industrial de santander, bucaramanga, colombia, e-mail: carios@uis.edu.co ²grupo de investigación en geofísica y geología (pangea), programa de geología, universidad de pamplona, colombia ³grupo de investigación en óptica y procesamiento de señales (gots), escuela de física, universidad industrial de santander, bucaramanga, colombia petrology record manuscript received: 11/07/2013 acepted for publication:26/10/2014 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.42020 16 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila introduction the humite-group minerals have a limited paragenesis and occurs in marbles and skarns generated by contact metamorphism. however, they may also form in rocks affected by metasomatic processes linked to the regional metamorphism or hydrothermal alteration. clinohumite is an uncommon member of the humite group, a magnesium silicate with the chemical formula (mg,fe)9(sio4)4(f,oh)2, which is essentially a hydrated olivine. it is a mineral found as small grains in marbles of contact metamorphic environments. it is sporadically mined gem-quality in the pamir mountains of tajikistan, and the taymyr region of northern siberia. other (non-gem quality) occurrences of clinohumite include several localities around the world, such as buell park and san juan county (usa), ambasamudram (southern india), jacupiranga complex (brazil), isua supracrustal belt (west greenland), malenco (italy), almirez massif (spain), northeastern jiangsu, su-lu, dabie mountains (china), western liguria (italy), kokchetav massif (kazakhstan). ti-poor and f-rich clinohumite have been reported in metamorphic and metasomatic carbonate-rocks adjacent to acid plutonic intrusions (fujino and takeuchi, 1978; satish-kumar and niim, 1998). contrary to this, ti-rich and f-poor clinohumite has been reported most frequently in ultramafic rocks. it was described from ultramafic xenoliths in carbonatites (gaspar, 1992) and kimberlites (mcgetehin et al., 1970; aoki et al., 1976), found to be associated with antigorite in serpentinites (trommsdorff and evans, 1980; sánchezvizcaíno et al., 2005) and also occurs in metadunite (dymek et al., 1988), in several garnet peridotites from ultrahigh-pressure (uhp) metamorphic terranes (yang et al., 1993; okay, 1994; zhang et al., 1995; yang, 2003), in metagabbros (scambelluri and rampone, 1999) and in a garnet-rich rock from the kokchetav uhp metamorphic terrane of northern kazakhstan (muko et al., 2001). in this study, we report and discuss data concerning this unusual humite-group mineral at the ciénaga skarn, sierra nevada de santa marta (snsm) massif, with the aim of characterize its occurrence and determine its petrologic significance. geological setting the snsm massif constitutes an isolated triangular-shaped range in the northern caribbean region of colombia and represents an uplifted area located along the southern caribbean plate boundary (figure 1), a margin owing its character to the oblique convergence and right-lateral shearing between the caribbean plate and northwestern south america (moreno-sánchez and pardotrujillo, 2003). its tectonic limits are the right-lateral oca fault, the left-lateral santa marta-bucaramanga fault, the cerrejón thrust, and the romeral suture. this massif is mainly composed of crystalline rocks and can be divided into three different belts (geotectonic provinces) with a well-defined outboard youngling pattern from east to west: sierra nevada, sevilla and santa marta. the southeastern and oldest belt (sierra nevada province) includes ca. 1.01.2 ga high grade metamorphic rocks represented by granulites, gneisses and amphibolites related to the grenvillian orogenic event (restrepo-pace et al., 1997; ordoñez et al., 2002, cordani et al. 2005). jurassic plutons and volcanites intrude and cover these metamorphic rocks and minor carboniferous and late mesozoic sedimentary sequences rest in unconformity towards the southeast flank (rabe, 1977). the intermediate belt (seville province) represents a polymetamorphic complex that includes gneisses and schists of paleozoic age with permian mylonitic granitoids (tschanz et al., 1969; cardona et al., 2006; cardona et al., 2010). the northwestern and youngest belt (santa marta province) comprises two metamorphic belts; an inner belt composed of imbricated metamorphic rocks (greenschists and amphibolites) of cretaceous age and an outer belt formed of mesozoic amphibolites, greenschists and phyllites separated from the lower to middle cenozoic santa marta batholith (doolan, 1970; macdonald et al., 1971; tschanz et al., 1974). the cesarranchería basin is exposed on the southeastern flank of the santa marta massif and represents a sedimentary record that evolved from cretaceous passive margin to maastrichian-paleogene orogenic deposits that are linked to an accretionary and subduction related event of the caribbean figure 1. geologic sketch map of the ciénaga marbles (modified after hernández and maldonado, 1999), showing its distribution in the study area. the open star indicates the location of the ti-clinohumite marble. smgp, santa marta geotectonic province; sgp, sevilla geotectonic province; sngp, sierra nevada geotectonic province. 17ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. plate (bayona et al., 2007). the geological unit of interest in this study corresponds to the ciénaga marbles, initially reported by tschanz et al. (1969) to describe marbles and limestones outcropping in two small bodies located eastward of the ciénaga town, lying in apparent discordance with the lower unit of the gaira schists and cut by the santa marta batholith. interbedded marble layers in schists have also been reported by tschanz et al. (1970). hernández (2003) assigned a geological age of 50.7 ± 2.1 ma to the ciénaga marbles on the basis of their stratigraphic position within the gaira schists. the ciénaga marbles include carbonates and siliceous carbonates affected by regional metamorphism. in the ciénaga marbles, the typical skarn mineral association has been recognized by castellanos et al. (2013) and they will be the goal of the present contribution. field sampling and analytical methods ti-clinohumite-bearing marble was collected from a marble quarry of the ciénaga skarn. the preparation of polished sections and thin sections for microscopic analysis performed according to the standard method in the sample preparation laboratory of the geology program at the universidad de pamplona (colombia), and the mineralogical and petrographic analysis of samples performed at the laboratory of microscopy of the research group in basic and applied geology of the school of geology at the universidad industrial de santander (colombia), using a trinocular nikon (labophot2pol) transmitted light microscope to establish the modal percentage of mineral constituents and mineral assemblages, with emphasis on textural relationships between mineral phases. additional analysis was developed at the laboratory of microscopy of the, using a trinocular olympus (bx51) transmitted light microscope in order to take microphotographs using a nis-elements br microscope imaging software of the geology program at the universidad de pamplona (colombia). mineral abbreviations are after kretz (1983), siivola and schmid (2007) and whitney and evans (2010). polished thin sections were coated by carbon evaporation technique using a carbon cord quorum q150r es system before sem analysis. sembse/eds imaging and analysis were carried out by field emission gun environmental scanning electron microscopy (fei quanta 650 feg sem) of the laboratory of microscopy at the universidad industrial de santander, guatiguará technological park (colombia) to examine the mineral phases, textures and cross-cutting relationships in the ti-clinohumite marble, under the following analytical conditions: magnification = 150-600x, wd = 9.4-10.6 mm, hv = 20 kv, signal = z cont, detector = bsed. 17 seconds counting for points analyzed with 12.000 counts. semiquantitative eds analysis (standardless quantitative analysis) which does not require calibration standards was performed. the typical detection limit for eds is 0.1wt%. quantification was performed with type zaf matrix correction. field occurrence the field occurrence of marbles in the ciénaga skarn reveals that they show a variable morphology (with sharp contacts) and thickness with a transition into carbonate-silicate rocks, which consecutively pass into calc-silicate and carbonate-bearing silicate rocks. when carbonate tends to disappear, they pass into granodiorites. the marble horizon in which ti-clinohumite occurs (figure 2) forms part of the ciénaga marbles geological unit. marbles are white and sometimes pale green, gray or yellow in color, and mediumto coarse-grained, and their thickness ranges from 7.5 to 10 cm up to 1.5 m in thickness (figures 2a-c). they show banding foliation and they are characterized by the alternation of carbonate-rich zones (from 1 to 3.5 mm up to 1 cm in thickness) with different mineral assemblages, which display sharp contacts and discontinuous bands of lenticular geometry within the metamorphic sequence. marbles are frequently arranged in bands or patches with different colors being some shade of red, pink, yellow or green, which are caused by the presence of impurities, such as clay minerals or iron oxides, which were originally present as grains or layers in their protoliths, and add to their beauty when they are cut and polished. these marbles contain different proportion of calcite and dolomite and were intensely replaced by skarn mineral association. ti-clinohumite-bearing marble occurs as reddish-brown tiny bands interbedded in white marble parallel to the regional schistosity (figures 2d-f). it is a white to reddishbrown coarse-grained rock composed of calcite, dolomite, ti-clinohumite, forsterite, diopside, accessory clinochlore, muscovite, fluorapatite and shining flakes of graphite. serpentine is the main alteration product. ticlinohumite-bearing folded bands were also observed (figures 2d and 2e). these marbles have been locally replaced by abundant skarn mineral association near the contact with orthoamphibolites, which are crosscut by pegmatitic veins. contacts between marbles and orthoamphibolites are subparallel to the main mesoscopic foliation, although the development of the skarn mineral association is not always clearly related to these contacts. figure 2. field (a-c) and hand-specimen (d-f) photographs of ti-clinohumite marble in the ciénaga skarn, sierra nevada de santa marta massif. petrography the ciénaga marbles are mainly calcite-rich marbles with bands parallel to the regional trend (n35-45one and 20-45ose) showing different mineral assemblages. these are composed of calcite bands in a proportion of more than half of the mode and the variations in the mineral assemblages are defined by the mg-rich phases. castellanos and co-workers (personal communication) described in detail the characteristic mineral assemblages from reaction zones recognized in marbles, which include calcite + graphite ± quartz; calcite + diopside ± tremolite ± epidote-group minerals ± muscovite; calcite + wollastonite + quartz + garnet + diopside; calcite + dolomite + ti-clinohumite + diopside + forsterite + clinochlore ± graphite; calcite + forsterite + tremolite; dolomite + calcite + clinochlore. accessory minerals are fluorapatite, muscovite, rutile and titanite. the rock of interest in this study corresponds to a ti-clinohumite marble, which display a granoblastic texture. it is composed of several reaction zones (figure 3), which are described as follows: (1) grey color reaction zone (2-8.5 mm in thickness) consisting of calcite + dolomite + graphite; (2) light grey color reaction zone (1-3.5 mm in thickness) consisting of diopside + calcite ± tremolite; (3) brown reddish color reaction zone (1.520 mm in thickness) consisting of ti-clinohumite + calcite + clinochlore ± muscovite ± fluorapatite ± graphite; (4) orange color reaction zone (26.5 mm in thickness) consisting of ti-clinohumite + calcite + dolomite + clinochlore + rutile ± muscovite; (5) light yellow color reaction zone (2-5.5 mm in thickness) consisting of dolomite + calcite + forsterite (antigorite). 18 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila figure 3. left, a photograph of the polished section of the ti-clinohumite marble, showing different reaction zones. right, photomicrographs in plane polarized light (ppl) and cross polarized light (xpl) of the reaction zones. dol, dolomite; cal, calcite; di, diopside; clc, clinochlore; ti-chu, ti-clinohumite; tr, tremolite; atg, antigorite; ms, muscovite; rut, rutile; gr, graphite. ti-clinohumite associated mineral phases calcite (80-95%) occurs in two generations. the first one occurs as medium-grained individuals with heteroblastic to subidioblastic shape (figure 4), showing characteristic rhombohedral cleavage and polysynthetic twinning. it occurs through most of the sample as an intergrowth with dolomite (15-75%) rhomb sections (figures 4a-b, 8d-f). domains with ti-clinohumite (figures 5, 6, 7, 8a-b) and fluorapatite (figures 7, 8a, g) are cross-cut by veinlets of very fineto medium-grained xenoblasts of calcite. diopside (5-20%) occurs in the sample as a very fine-grained granoblastic intergrowth with calcite (figures 4c-d). it shows a heteroblastic to subidioblastic contours, high relief and first and second order interference colors (from red to violet and blue). clinochlore (5-10%) is colorless to pale green and can be recognized by its flaky to fibrous character, excellent one direction cleavage and very low birefringence (figures 4e-f, 7, 8a-b). tremolite (1-5%) occurs as colorless crystals of moderate to high relief and first to second order interference colors, which are in contact with calcite (figures 4c-d). graphite (1-5%) occurs as scarce tiny flakes (figure 7). forsterite (<1%) is characterized by cracked round grains, which show up in bright interference colors (figure 7). it can be replaced by antigorite (figure 8h). fluorapatite (<1%) occurs as small grains (up to 250 µm) are usually idioblastic and closely associated with ti-clinohumite grains and clinochlore (figures 7, 8a). rutile (<1%) displays its typical brown reddish color (figures 4e-f, 5c-d). titanite (1%) use to develop a reaction rim around rutile (figures 5c-d). muscovite (<1%) is colorless and can be recognized by its flaky character and excellent one direction cleavage (figure 6). antigorite (0-10%) appears as a fibrous product of alteration along irregular fractures of forsterite (figure 7) and pseudomorphic replacement after diopside grains (figures 5e-f) grains. 19ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. figure 4. photomicrographs showing the representative textural relationship between minerals in the ti-clinohumite-bearing marbles in (a), (c), (e) ppl and (b), (d), (f) xpl. dol, dolomite; cal, calcite; di, diopside; clc, clinochlore; ti-chu, ti-clinohumite; tr, tremolite; rut, rutile; ttn, titanite. 20 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila figure 5. photomicrographs showing the representative textural relationship between minerals in the ti-clinohumite-bearing marbles in (a), (c), (e) ppl and (b), (d), (f) xpl. cal, calcite; ti-chu, ti-clinohumite; clc, clinochlore; rut, rutile; ttn, titanite; atg, antigorite. ti-clinohumite alternating calcite-rich with dolomite and dolomite-rich with calcite bands are observed in the analyzed sample. however, domains rich in ticlinohumite are mainly related to the calcite-rich with dolomite bands as shown in figure 7. the ti-clinohumite-bearing domains show a granoblastic polygonal texture (figures 5a-b, 8c-d, f), with the mineral assemblage: calcite + dolomite + ti-clinohumite + forsterite + diopside + clinochlore + fluorapatite ± rutile ± muscovite ± graphite, where the small grains of ti-clinohumite can be preserved within large calcite grains (figures 7, 8i). ti-clinohumite (10-55%) occurs predominantly as xenoblasts (up to 2 mm in diameter) in granoblastic calcite-rich bands. these xenoblasts usually show patchy pleochroism varying within individual grains due to compositional variations and irregular zoning patterns and also can develop twinned grains (figure 6). ti-clinohumite grains can also occur spatially related with clinochlore and muscovite (figure 6). very fine-grained and rounded pseudomorphs of antigorite after diopside disseminated around calcite are illustrated in figures 5e-f. rough intergrowths of ti-clinohumite with forsterite are very common (figures 8f, i). ti-clinohumite occasionally contains scarce inclusions of clinochlore and rutile. titanite rims around rutile are commonly observed near ti-clinohumite. 21ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. figure 6. photomicrographs display the occurrence of the ti-clinohumite and associated mineral phases. (a), (c), (e) ppl and (b), (d), (f) xpl. cal, calcite; di, diopside; chu, ti-clinohumite; ms, muscovite; clc, clinochlore. an overview high resolution image mosaic of the ti-clinohumite bearing marble is presented in figure 7. an irregular crack cross cut at high angle the metamorphic foliation of the rock. note the highdensity fluorapatite (to the right) from the backscatter electron image contrast. figure 8 shows several examples of the textural relationships between ti-clinohumite and associated mineral phases. 22 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila figure 7. high resolution backscattered electron image mosaic of the ti-clinohumite bearing domain in calcite-rich marble. an irregular crack cross cut at high angle the metamorphic foliation of the rock. note the high-density fluorapatite (top-right corner of the image). fo, forsterite;¸cal, calcite; dol, dolomite; ti-chu, ti-clinohumite; clc, clinochlore; f-ap, fluorapatite; gr, graphite; brt, barite. 23ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. figure 8. backscattered electron images of the textural relationships between ti-clinohumite and associated mineral phases. cal, calcite; dol, dolomite; ti-chu, ti-clinohumite; fo, forsterite; atg, antigorite; clc, clinochlore; f-ap, fluorapatite; gr, graphite. (a) relationship between ti-clinohumite and clinochlore, calcite and fluorapatite; observe secondary veins of calcite cross cutting minerals and non-optical continuity of gray color in clinochlore. (b) numerous secondary veins of calcite cross cutting ti-clinohumite, which is associated with calcite and clinochlore. (c) association dolomite + calcite, which are in contact with ti-clinohumite; observe secondary veins of calcite cross cutting minerals and non-optical continuity of gray color in clinochlore. (d) the occurrence of ti-clinohumite along with dolomite + calcite. (e) dolomite predominance with scarce calcite, which occurs as a matrix phases or as secondary veins cross cutting dolomite. (f) association calcite + dolomite + ti-clinohumite + forsterite (replaced by antigorite). (g) occurrence of fluorapatite along with calcite and clinochlore; numerous veins of calcite cross cut fluorapatite; with graphite filling microfractures. (h) association clinochlore, calcite and highly corroded ti-clinohumite. (i) replacement of forsterite by antigorite; notice also the presence of ti-clinohumite, dolomite, calcite and clinochlore. 24 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila the sem image in figure 9 shows the textural relationships between ti-clinohumite and its associated mineral phases. the semiquantitative energy dispersive spectroscopy (eds) analyses at different points marked with stars allowed identifying the particular elements and their relative proportions in the mineral phases analyzed in the ti-clinohumite-bearing domains of the calciterich marbles. the eds analysis revealed that mineral phases in the analyzed sample correspond to: calcite (1) has 79.95 wt% cao and 15.65 wt% co2, with minor mgo (4.40 wt%); dolomite (2) with 50.10 wt% cao, 34.59 wt% mgo and 15.31 wt% co2. forsterite (3) is characterized by high contents of mgo (50.15 wt%) and sio2 (47.47 wt%), with minor feo (2.38 wt%). antigorite (4) reveals the presence of 62.91 wt% sio2, 24.72 wt% mgo, and minor fe2o3 (2.59 wt%) and cao (0.93 wt%). diopside (5) indicates the presence of cao (19.48 wt%), mgo (18.91 wt%) and sio2 (61.61 wt%). clinochlore (6) reveals 31.18 wt% mgo, 13.71 wt% al2o3, 1.07 wt% feo, 53.69 wt % sio2. fluorapatite (7) shows the presence of cao (53.87 wt%), p2o5 (42.79 wt%) and f (2.87 wt%). ti-clinohumite (8) reveals that it consists of mgo (50.87 wt%) and sio2 (44.01 wt%), with minor feo (2.12 wt%). it can be considered as a ti-clinohumite taking into account the incorporation of up to 2.57 wt% of ti in its mineral structure. eds spectra are in agreement with literature data (http://www.sfu.ca/~marshall/sem/mineral.htm). figure 9. sem image and eds spectra of the analyzed mineral phases (marked with stars on the image) of ti-clinohumite-bearing domains in magnesium calcite-rich marbles. the appearance of c element is attributed to the carbon coating on the sample before sem analysis. 25ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. figure 10 shows the elemental mapping of ti-clinohumite and associated mineral phases, which however will only give a qualitative illustration of the distribution of elements. ti and fe show a similar pattern of distribution with low concentration of these elements in the ti-clinohumite, except by the occurrence of rutile that displays a high content of ti. the content of mn is regularly distributed in the sample. note also a similar pattern of distribution for mg and si with a high concentration of these elements in the ti-clinohumite. however, the highest contents of mg reveal the occurrence of mg-rich chlorite (clinochlore), which occurs both as inclusions in ti-clinohumite and as a matrix phase. the distribution of al also reveals not only its low content in ti-clinohumite but also the high content in clinochlore. 500 µm ti 500 µm mg 500 µm fe 500 µm mn 500 µm al 500 µm si figure 10. compositional maps of ti-clinohumite and associated mineral phases for ti, mg, fe and mn (above), and al and si (below). light colors show areas of high concentration while dark colors represent areas of low concentration (black is very low concentration). figure 11 illustrates a binary diagram of mgo/(mgo+feo+mno) vs. tio2 for the compositions of ti-clinohumite from several geological environments. according to gaspar (1992), this diagram takes into account the two most effective substitutions in ti-clinohumite: mg by fe (and mn) and mg (foh)2 by tio2. even the analyzed ti-clinohumite from the ciénaga marbles does not match with any of the compositional fields defined by gaspar (1992), it represents not only a ti-clinohumite almost pure in mg but also may be some of the most mg-rich and tio2-rich varieties around the world for skarn environments. figure 11. mgo/(mgo+feo+mno) vs. tio2 diagram, showing the compositions of the ti-clinohumite from several geological environments, including the analyzed sample in this study indicated in black star (adapted and modified after gaspar, 1992). 26 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila discussion on the origin of ti-clinohumite and its petrologic significance the ciénaga marbles were affected by emplacement of granodioritic rocks belonging to the santa martha batholith, developing a skarntype mineralogy as suggested by castellanos and co-workers (personal communication), which gives rise to thin reaction zones. however, we suggest that the skarn-type mineralogy hosted in this metamorphic unit should be related both magmatic and postmagmatic stages of the santa martha batholith, and is the result of a very complex skarn-type mineralogy zonation (castellanos and co-workers, personal communication). calcareous rocks (marbles, metacarbonates and calc-silicate rocks) represent only a relatively small fraction of the crust but their metamorphism is important because it can reveal significant information about the composition of the metamorphic fluid phase as well as p-t conditions (spear, 1993). the regional fluid flow transporting volatiles including h2o and co2 is an integral part of prograde regional metamorphism (ague, 2003). according to connolly and trommsdorff (1991), isothermal or isobaric phase diagram sections as a function of the fluid composition are widely used for interpreting the genetic history of these rocks, although this approach has the following disadvantages: (1) the influence of key metamorphic variables such as p and t are obscured, and (2) the diagrams are inappropriate for systems that are not fluid-saturated. these problems are avoided by constructing phase-diagram projections in which the volatile composition of the system is projected onto a pt coordinate frame, i.e., a petrogenetic grid (connolly and trommsdorff, 1991). mineral reactions in calcareous rocks can be modeled in the system cao-mgo-sio2-h2oco2 (cms-hc) however, other components (e.g., feo, al2o3, na2o, k2o and tio2) are also present, playing a very important role in the stabilization of additional mineral phases such as micas, epidote-group minerals, garnet, feldspar, titanite, rutile. a petrogenetic grid for these rocks is most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled. therefore, pt projection for this chemical system suggests that many assemblages in a mixed-volatile environment have stability fields that make them useful as p-t indicators (connolly and trommsdorff, 1991). one of the implicit assumptions to construct a t-xco2 reaction grid in this system is that there is a h2o and co2 fluid present in all mineral assemblages, which are considered in “excess”, consequently, only the remaining three “inert” components (cao-mgo-sio2) are considered (spear, 1993). figure 12 illustrates a composition triangle for the system cms-hc showing the plotting position of mineral phases considered in the figure 13. sio2 cao mgo cal wo di tr tlc fo qtz dol en figure 12. composition triangle for the chemical system cms-hc projected from h2o and co2 showing the plotting position of mineral phases (modified after spear, 1993). according to spear (1993), t-xco2 diagrams can be used to predict the sequence of metamorphic mineral assemblages and there are three mechanisms by which the calcareous rocks may evolve: (1) metamorphism at constant fluid composition, (2) metamorphism driven by infiltration, and (3) metamorphism in a closed system. the reactions encountered in progressive metamorphism generate h2o and co2 that will tend to change the fluid composition, representing a problem for the first mechanism to evolve. this is why metamorphism at constant fluid composition, although easy to understand, is probably not representative of calcareous rocks of the ciénaga marbles. instead the fluid infiltration process serves very well to explain driving devolatilization reactions in the ciénaga marbles. the sequence of mineral assemblages in the first two process is similar with the exception that diopside is not encountered up to ~ 570 oc (spear, 1993), which can explain the occurrence of diopside associated with wollastonite in the ciénaga marbles. we suggest from field and petrologic evidence (occurrence of ti-clinohumite and wollastonite, and the mineral assemblages calcite + quartz + diopside, calcite + diopside + wollastonite and quartz + diopside + wollastonite) that infiltration of h2o-rich fluids within these marbles was probably driven by channeling along fractures and grain boundaries. additional evidence includes the fact that ti-clinohumite is not stable in pure h2o-co2 fluids as suggested by bucher and frey (1994). on the other hand, calcareous rocks of the ciénaga marbles are surrounded by the pelitic gaira schists, which can drive decarbonization reactions as suggested by spear (1993). this can be explained by a chemical reaction such as caco3 (calcite) + sio2 (quartz) = casio3 (wollastonite) + co2 (e.g., bucher and frey, 1994; ferry, 2000; ferry et al., 2001), which can explain the formation of wollastonite in the ciénaga marbles by the interaction of marble with an h2o-rich fluid as suggested by bucher and frey (1994). the h2o released from the dehydration of pelitic schists probably infiltrated the calcareous rocks promoting decarbonization, which can explain the transformation of forsterite to ti-clinohumite. we consider that infiltration may have been the dominant mechanism during contact metamorphism, with h2o-rich fluids from the crystallization of a granodiorite intrusion penetrating calcareous rocks in the contact aureole giving rise to a skarn-type mineralogy and the formation of ti-clinohumite, discussed in detail below. in the third mechanism, the fluid generated by devolatilization reactions remains in the rock and is mixed with the fluid that is already present, producing a fluid of a new composition (spear, 1993). in this case, the evolution of the fluid is controlled by the mineral assemblage and most of the reaction progress will occur at isobaric invariant points. the process in which the fluid composition evolved in response to mineral reactions is called buffering. t-xco2 diagrams are used to predict the sequence of metamorphic mineral assemblages that would be observed at constant fluid composition as shown in figure 13. according to winter (2001), there is a fluid-internal vs. external control. in the internal control (buffering assemblage), the mineral assemblage controls the composition of the h2o-co2 fluid, representing a closed system; the buffer capacity is present until one of the phases completely consumed. in the external control, the extensive influx of fluids overrides buffer capacity mineral assemblages, representing an open system. however, taking into account that ti-clinohumite is not stable in co2-rich fluids, it is better to consider that mineral assemblages with the presence of ti-clinohumite reveal the influence of h2o-rich fluids. 27ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. a textural intergrowth between calcite and dolomite can be interpreted as the result of exsolution of dolomite from calcite. quartz and calcite, which are commonly found in contact in other samples from the study area, reveal sign of reaction to produce wollastonite. therefore, since quartz and dolomite are considered to have been in disequilibrium everywhere and wollastonite was found in some of the samples of the ciénaga marbles, the area of interest in figure 13 lies between the line defined by reactions 1-3 and the wollastonite curve (reaction 9) at high temperatures. however, this area is divided into two sub-areas, separated by the forsterite-forming reactions 5 and 6. tremolite formed from talc + calcite (reaction 1) at xco2 £ 0.5 or from quartz and dolomite (reaction 2) at xco2 ³ 0.5. however, taking into account that tremolite contains al, na, and k in its structure, these elements may have been derived from detrital plagioclase and muscovite as suggested by kretz (1980). there is evidence of the occurrence of forsterite in marbles. forsterite can be the result of the presence of silica in the protolith. however, as quartz is incompatible with forsterite, it could not have been part of the precursor assemblage. hence it is more likely that forsterite, in the presence of calcite, dolomite and diopside, grew by a reaction, such as: diopside + dolomite → forsterite + calcite + co2. diopside has probably formed from tremolite via reactions 4, 7 or 8, with al, na, k, and fe derived from tremolite and, alternatively, it can be formed directly from dolomite by reaction 3, which appears to be responsible for the occurrence of pyroxene-rich assemblages. ti-clinohumite can be preserved within calcite, which isolated ticlinohumite from ulterior retrogradation and allowed its preservation. on the other hand, ti-clinohumite, clinochlore and muscovite are very often spatially associated, which indicates the supply of al and k to the system. several different metamorphic events on the ciénaga marbles have resulted in a great variety of mineral assemblages, including calcite + dolomite + ti-clinohumite + forsterite + diopside + clinochlore + fluorapatite ± rutile ± muscovite ± graphite, which corresponds to the middle amphibolite facies. locally, retrograde antigorite after forsterite is common. however, what is the origin of ti-clinohumite, metasomatic or metamorphic? calcite-rich marbles are usually impermeable to fluids except for fracture-controlled fluid infiltration (holness and graham, 1991). ti-clinohumite-bearing assemblages at the ciénaga marbles are in calcite-rich domains and hence support the fact that they were affected by fracture-controlled fluid infiltration. ti-clinohumite that occurs in the ciénaga marbles is associated with calcite, dolomite, forsterite, diopside, clinochlore, muscovite, rutile and fluorapatite. antigorite forms pseudomorphs after forsterite and diopside. alkali amphibole, which is a common phase in marbles, has not been found so far in association with ti-clinohumite in the study area. eventually, ti-clinohumite may be a reaction product of preexisting forsterite, and, therefore, based on textural evidence and mineral assemblages, we suggest that ti-clinohumite can grow by a reaction such as forsterite + dolomite + tio2 + h2o → ti-clinohumite + calcite + co2. it is obvious from the compositions of these minerals that the replacement of forsterite by ti-clinohumite implies the incorporation of ti, f, and oh in the forsterite. ti-clinohumite is the only phase in the forsterite-rich layers that can accommodate significant amounts of ti and f (chattopadhyay et al., 2009), consequently, the presence of f has a strong effect in increasing the stability of ti-clinohumite (rice, 1980). because ti-clinohumite preferentially accommodates f rather than oh, the presence of f stabilizes ti-clinohumite (stalder and ulmer, 2001). the origin of ti-clinohumite with presence of f can be attributed to a metasomatic process resulting from the external influx of f-bearing fluids or to internal buffering during metamorphism (satish-kumar and niim, 1998). according to moore and kerriek (1976), metasomatism is normally the process operating in contact aureoles, where granodioritic intrusions inject fluids into the impure marbles causing the formation of ti-clinohumite with presence of f. however, rice (1980) stated that it is possible that all marbles that contain this mineral have not resulted from metasomatic introduction of f and its concentration can result from high partitioning of f into hydrous minerals. if the marbles were infiltrated by aqueous f-bearing fluids from unconnected granitoids, then the oxygen isotopes should preserve the isotopic signatures (satish-kumar and niim, 1998). the origin of f in ti-clinohumite can be also attributed to the isochemical reactions involving (oh-f) silicates such as amphiboles (tremolite), which are abundant in the ciénaga marbles. however, it can be also associated with the occurrence of fluorapatite. rice (1980) attributed to the partitioning of f into hydrous silicates relative to h2o-co2 fluids, although this is not probably due to the presence of h2o-rich fluids that have strongly controlled the formation of ti-clinohumite. however, we consider that evidence of products of retrograde hydration metamorphism can be found in all these rock types, which is revealed by the occurrence of antigorite from forsterite and occasionally from diopside and ticlinohumite from forsterite. however, the chemical reactions described above can take place in response to an increase in the activity of h2o relative to that of co2 and a decrease in temperature. figure 13. t-xco2 diagram for the system cms-hc, showing boundary curves for carbonate-silicate reactions, according to bucher and frey (1994). 28 carlos alberto ríos r., oscar mauricio castellanos a., carlos alberto chacón avila what is the role that extra components (e.g., f, ti al, fe) play in the mineralogy of marbles in the system cao-feo-mgo-a12o3-sio2tio2-h2o-co2 (cfmast-hc)? according to bucher and frey (1994), metamorphic fluids often have a small partial pressure of hydrofluoric acid (hf), and the replacement of hydroxyl groups by fluorine (f(oh)-1 exchange) rapidly decrease with increasing xfe. however, additional exchange components operating in ti-clinohumite, such as h2mgti-1 and femg-l, are thus potentially important in determining its stability (engi and lindsley, 1980). ti-clinohumite (low phf), a mineral of the humite group, formed in forsterite marbles and may be stabilized by the presence of ti (probably as a result of the formation of titanite reaction rims around rutile. the presence of al in marbles promoted the formation of several minerals, such as clinochlore, epidote-group minerals, garnet, muscovite, actinolite and tremolite distributed in different reaction zones of marbles. however, a detailed study of these reactions zones is currently under study by castellanos and co-workers (personal communication). the formation of ti-clinohumite suggests that solutions were reduced in f. this mineral occurs in calc-silicate marbles with no ultrabasic rocks occurring either as enclaves or as discrete bodies in the vicinity. however, comparisons between ti-clinohumite from the ciénaga marbles and that found in jac carbonatites, xenoliths in kimberlites, rgp alpine peridotites and archean ultramafics may carry out as shown in figure 11, highlighting that the ti-clinohumite analyzed in this study represents not only an almost pure in mg but also may be considered as the most mg-rich and tio2-rich varieties around the world for skarn environments. numerous mineral reactions should take place in the calcareous rocks of the ciénaga marbles during metamorphism. however, there are obstacles that normally encounter in carrying out an interpretation of mineral assemblages in these petrochemical type of rocks, such as: (1) complex identification of reaction textures; (2) primary or secondary nature of dolomite and quartz sometimes is unclear; (3) a gaseous phase may not be present and molecules of h2o and co2 and other relatively volatile constituents may be concentrated in grainboundaries; (4) a mineral is not normally a “pure” substance and therefore one or more compositional variables must be considered in addition to variation in the concentration or activity of h2o and co2. nonetheless, figure 14 shows a preliminary model of the metasomatic process that probably affected the ciénaga marbles, which include the following steps: (1) h2o-rich fluid by the dehydration of surrounding pelitic schists infiltrated by fractures and discontinuities into carbonate layers; (2) infiltrated h2o-rich fluid decreased decarbonization temperatures, promoting decarbonization reactions in carbonate layers; (3) h2o-rich fluid carried several chemical elements. conclusions calcite-dolomite marbles from the ciénaga marbles, sierra nevada de santa marta massif (colombia) contain ti-clinohumite, which has been recognized and reported in this study. the mineral assemblage including the presence of ti-clinohumite within marble layers may be attributed to prograde reactions from impure dolomitic limestones. the skarn-type mineralogy of these marbles can be modeled in the system cmast-hc. the formation of wollastonite by the interaction of marble is a very important key to consider the influence of h2o-rich fluids. the h2o released from the dehydration of pelitic schists probably infiltrated the calcareous rocks promoting decarbonization, which can explain the transformation of forsterite to ticlinohumite. infiltration may have been the dominant mechanism during contact metamorphism, with h2o-rich fluids derived from the crystallization of a granodiorite intrusion penetrating calcareous rocks in the contact aureole giving rise to a skarn-type mineralogy and the formation of ti-clinohumite. the extra components (e.g., f, ti, al, fe) play a role in the mineralogy of marbles in the system cfmast-hc. several exchange components (f(oh)1, h2mgti-1 and femg-l) operate in ti-clinohumite. it may be stabilized by the presence of ti (probably as a result of the formation of titanite reaction rims around rutile. the replacement of forsterite by ti-clinohumite implies the incorporation of ti, f, and oh in the forsterite. because ti-clinohumite preferentially accommodates f rather than oh, the presence of f stabilizes ticlinohumite but it is necessary a very few amount of this element in the fluid or in the chemical system. the origin of ti-clinohumite with the presence of f can be attributed to a metasomatic process resulting from the external influx of f-bearing fluids or to internal buffering during metamorphism. the origin of f in ti-clinohumite can be also attributed to the isochemical reactions involving (oh-f) silicates such as amphiboles (tremolite). field and petrologic evidence (occurrence of ti-clinohumite and wollastonite, and mineral assemblages calcite + quartz + diopside, calcite + diopside + wollastonite and quartz + diopside + wollastonite) reveals that ti-clinohumite can be developed due to infiltration of h2o-rich fluids within forsterite-rich layers in marbles probably driven by channeling along fractures and grain boundaries. acknowledgments we are grateful to l. mantilla for guiding a fieldwork in the study area where the ti-clinohumite-bearing marble was collected. discussions with him aided our understanding of the ciénaga skarn geology. authors also thank geologist h. cotes for providing us support to visit marble quarries. we thank laboratory facilities and their technical personnel of the school of geology (universidad industrial de santander) and geology program (universidad de pamplona). thanks to the laboratory of microscopy of the guatiguará technological park and its professional staff for assistance with sem data acquisition. the manuscript was greatly improved based on the critical and helpful reviews and comments by anonymous reviewers. we are most grateful to the people and institutions mentioned earlier for support. figure 14. schematic illustration showing the ciénaga marbles metasomatism (adapted and modified after ogasawara, 2014). 29ti-clinohumite in the ciénaga skarn-type mineralogy, sierra nevada de santa marta massif (colombia): occurrence and petrologic significance. references ague j., 2003. fluid infiltration and transport of major, minor, and trace elements during regional metamorphism of carbonate rocks, wepawaug schist, connecticut, usa. american journal of science 303, 753-816. aoki k., fujino k., and akaogi m., 1976. titanochondrodite and titanoclinohumite derived from the 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clinohumite–garnet–pyroxene rock from the su-lu uhp metamorphic terrane, china: chemical evolution and tectonic implications. lithos 70, 359-379. zhang r.y., liou j.g., and cong b.l., 1995. talc-, magnesiteand ticlinohumite-bearing ultrahigh-pressure meta-mafic and ultramafic complex in the dabie mountains, china. journal of petrology 36, 1011-1037. http://www.sfu.ca/~marshall/sem/mineral.htm mineral energy dispersive spectra (eds) consulted on 15 december , 2013. using neuronal networks to determine site response from coda waves: application in armenia, colombia earth sciences research journal earth sci. res. j. vol. 8, no. 1 (dec. 2004): 25-33 manuscript received july 2004 25 paper accepted october 2004 possible rapid strain accumulation rates near cali, colombia, determined from gps measurements (1996-2003) robert trenkamp1, héctor mora p.2, elkin salcedo h. 3 and james n. kellogg 1 1 university of south carolina, department of geological sciences 2 ingeominas, observatorio vulcanológico y sismológico, manizales, av. 12 de octubre 15 47 3 universidad del valle, facultad de humanidades abstract global positioning system (gps) data from southern central america and northwestern south america collected between 1991 and 1998 reveal wide plate margin deformation along a 1400 km length of the north andes. also associated with the oblique subduction of the nazca plate at the colombia-ecuador trench is the ‘escape’ of the north andes block (nab). the nab is delineated by the bocono-east andean fault systems and the dolores guayaquil megasheare to the east, the south caribbean deformed belt on the north and the colombia-ecuador trench and panama on the west. within the nab many damaging crustal earthquakes have occurred which is most recently exemplified on january 25, 1999 (mw = 6.1) armenia earthquake. preliminary analysis of recent occupations (2003 geored gps) of several previously observed (1996-2001) gps sites suggest shear strain accumulation rates in the cauca valley near cali of approximately 2.1 x 10-7 yr1 and 1.6 x 10-7 yr-1. these strain rates are measured within 2 delaunay triangles with common vertices at cali and restrepo, which encompass areas, located north and west of cali. seismicity has been monitored in the cauca valley for the last 17 years by the “observatorio sismológico del suroccidente” (osso) since 1987 and by the red sismológica nacional del ingeominas since 1993. their catalogs list numerous shallow earthquakes near cali but nothing larger than magnitude 5. historically, however, several large earthquakes are associated with the “falla cauca almaguer” in locations both to the south and north of cali in the cauca valley. preliminary calculations using the strain rates determined for these delaunay triangles and a simplified kostrov formula suggest possible decadal (30 – 90 years) recurrence intervals for mw = 6.0 – 6.3 earthquakes, centenary (90 – 900 years) recurrence intervals for mw = 6.4 – 6.9 earthquakes and millennial (900+ years) recurrence intervals for mw ≥ 7 earthquakes. key words: gps, kostrov formula, recurrence intervals, seismotectonics, strain rates resumen los datos del sistema de posición global registrado desde la zona meridional de centro américa y el noroccidente de sur américa, tomados en el periodo que comprende 1991 y 1998 revelan un amplio margen de deformación a lo largo de los 1400 kilómetros de longitud al norte de los andes. igualmente robert trenkamp et al. 26 asociado a la subducción oblicua de la placa de nazca en el corte de colombia y ecuador es el escape del bloque norandino (nab). el nab está delineado por el sistema andino de fallas bocono y el gran corte de dolores, guayaquil al este; el cinturón del caribe sur; el corte de colombia ecuador, al norte, y panamá, al oeste. dentro de esta zona han ocurrido muchos terremotos; como ejemplo está el más reciente, ocurrido el 25 de enero de 1999 en armenia (mw = 6.1). los análisis preliminares de recientes ocupaciones (2003 geored gps) de muchos gps previamente observados (1996-2001) sugieren que la tensión en la acumulación de los cortes en el valle del cauca, cerca de a cali, son de aproximadamente 2.1 x 10-7 yr-1 y 1.6 x 10-7 yr-1. estas velocidades de tensión son medidas dentro de los dos triángulos delaunay con vértices comunes en cali y restrepo, que comprometen áreas localizadas al norte y occidente de cali. desde 1987 la sismología del valle del cauca ha estado monitoreada por el observatorio sismológico del sur occidente (osso) y desde 1993 por la red sismológica nacional de ingeominas. se cataloga una lista de leves terremotos cerca a cali, pero ninguno mayor a una magnitud de 5. sin embargo históricamente muchos terremotos están asociados a la “falla cauca almaguer” en localidades cerca al sur y al norte de cali en el valle del cauca. cálculos preliminares realizados con velocidades de tensión determinadas por estos triángulos delaunay y la fórmula simplificada de kostroy que sugiere posibles intervalos recurrentes de algunas décadas (30 a 90 años) para terremotos mw = 6.0 – 6.3; intervalos centenarios (90 – 900 años) de terremotos mw. = 6.4 – 6.9 e intervalos milenarios (900 años o más) para terremotos de mw ≥ 7 palabras clave: formula kostrov, gps, intervalos de recurrencia, parámetros de tensión, sismotectónica, tasas de esfuerzo. © 2004 esrj -unibiblos. introduction the central and south america (casa) gps project was initiated in 1988 to study plate motions and crustal deformation in a tectonically active area of complex interaction among the nazca, cocos, caribbean and south american plates. data from the casa project collected between 1991 and 1998 reveal wide plate margin deformation along a 1400 km length of the north andes (trenkamp et al. 2002). associated with the oblique subduction of the nazca plate, the colombia-ecuador trench is the ‘escape’ of the north andes block (nab). the nab is delineated by the bocono-east andean fault systems and the dolores-guayaquil megasheare to the east, the south caribbean deformed belt to the north and the colombia-ecuador trench and panama on the west (figure 1). within the nab many damaging earthquakes have occurred. with this continuing threat of damaging earthquakes to major metropolitan centers, a large combined geological and geophysical study, microzonificacion sismica de la ciudad de santiago de cali, was proposed and performed by ingeominas (alvarado et al. 2003). in general, the project was a broad-based geological and geophysical approach toward understanding the stresses and deformation responsible for the neotectonics of the area. as a part of this study to assess the earthquake hazard potential near cali and areas adjacent to the cauca valley, a gps project, geodesia: red de estudios de deformacion 2003 (geored03) (mora and trenkamp, 2003) was coordinated and executed between july and september 2003 by the volcanological and seismological observatory of ingeominas in manizales. the objective of the field project was a reoccupation of a subset of casa stations and other previously occupied gps stations in the cauca region focusing on the city of cali. plate tectonic models for southwest colombia, based on regional mapping of the tertiary and quaternary deposits, show a series of graben shaped sub-basins located between the cauca patía and romeral fault systems. palinspastic reconstruction shows restraining bend, releasing bend and pull apart basins containing several tectonic blocks (valle and eduardo, 1999). possible rapid strain accumulation rates near cali, colombia determined from gps measurements (1996-2003) 27 figure 1. generalized tectonic map of northwestern south america and southern central america including the fault slip planes for the 1906, 1942, 1958 and 1979 earthquakes. slip planes are from kanamori and mcnally (1982). a gravity survey completed in 1964 shows two anomalies of interest on the west side of the upper cauca basin, the vijes and cali anomalies. these two anomalies are bounded on the west by the cauca fault and on the east by the candelaria fault and separated by transverse faults. seismic lines/sections confirm the gravity interpretations and show that the anomalies are not intrusions but are uplifted basement blocks (noel, 1996). also obvious from the seismic lines is an upper crust broken by numerous faults. seismic sources that affect cali and the surrounding cauca region are diverse. during the 20th century a four (4) large sequence and great subduction related earthquakes occurred. the first and largest event occurred in 1906 (mw=8.8) and ruptured a 500 km length of the subduction interface on the colombia-ecuador trench between manta, ecuador and buenaventura, colombia (kelleher, 1972, kanamori and mcnally, 1982). kanamori and mcnally (1982) report that three smaller events in 1942 (mw= 7.9), 1958 (mw=7.8) and 1979 (mw=8.2) re-ruptured most of the thrust fault plate boundary segment that ruptured during the 1906 event (figure 1). these earthquakes are shallow (< 50 km) on the subduction interface. other deeper earthquakes also occur that seem to be associated with deeper segments of the subduction interface. a third source of earthquakes is intraplate and occurs on faults in the shallow crust. these earthquakes have been most recently exemplified by the january 25, 1999 (mw=6.1), armenia earthquake and pose the greatest threat of damage to the populated areas of the cauca valley and environments. the seismicity in the cauca valley has been monitored for the past 17 years by the observatorio seismológico del suroccidente (osso) since 1987 and also by red sismológica nacional de ingeominas since 1993. their catalogs list numerous shallow earthquakes near cali but nothing larger than magnitude 5. page (1986) reports faulted alluvium in the vicinity of palmira, a city near cali, which suggests shallow seismicity during the quaternary and historically several large earthquakes are associated with the “almaguer fault” in locations to the north and south of cali. data and data analysis casa data in colombia consists of hundreds of sites, which have at least one epoch measurement. of these sites and sites previously occupied by ingeominas, a subset of 36 sites was chosen for the larger geological and geophysical project looking at zones of interest within and surrounding the cauca valley and the city of cali. data was collected using 3 trimble 4000 ssi receivers and dorne margolin choke ring antennas. site occupations consisted of a minimum of three 8-hour observation days and were continuous over the 3 days when security conditions permitted (table 1). all of the gps data presented in this analysis and the larger geored03 study was processed using the jpl/nasa developed gipsy/oasis-ii (gps inferred positioning system/orbit analysis and simulation software) software (release 5) (zumberge et al. 1997). loosely constrained solutions were obtained using jpl's fiducially free trenkamp robert, mora p. héctor salcedo h. elkin, kellogg james n 28 orbit (a ''non-fiducially'' orbit is one that is estimated without significant a-priori site position constraints), which were then transformed into the international terrestrial reference frame 2000 (itrf2000). in order to transform loosely constrained solutions into the itrf2000 reference frame, daily 7 parameter transformations were determined using all reference sites (igs tracking stations) used in our daily solutions and which are contained in the itrf2000 position and velocity model. these are then applied to the daily solution, which transforms them to the desired reference frame (itrf2000). horizontal and vertical velocities were determined using a combination of all 352 daily solutions and 174 stations from the 1994, 1996, 1998, 1999, 2001 and 2003 surveys. a least squares inversion was used to estimate site velocities and position at an arbitrary epoch from the daily itrf2000 coordinates weighted by the full covariance matrix of the coordinates. a scaling factor of 7.2 was applied to the input covariances so the reduced chi-square statistic equals 1.0. reduced chi-square statistics that equal one indicate that the formal errors agree with the scatter of the measurements. the scaling by 7.2 is equivalent to scaling the sigmas by 2.68. variance scaling assumes a gaussian (normal) error distribution, removal of all human errors (blunders) from the dataset and systematic underestimation of the true errors by gps software, which is generally accepted. table 1. the east, north and vertical velocities with one sigma errors relative to itrf2000 of the 30 stations with multiple year observations made during the project geored03. six sites were established and observed for the first time. these sites were aqu2, café, fres, letr, uval and vers and will be reoccupied in the near future in order to add their vectors to the growing sw colombia gps database. on. lat. e n u σe σn σu site deg deg mm mm mm mm mm mm 287.12 5.55 0.0 12.6 4.8 3.2 1.5 6.4 aqui 282.61 6.20 7.3 12.3 1.1 0.7 0.4 1.3 bhsl 285.92 4.64 1.3 12.1 -35.3 0.5 0.4 0.6 bogt 283.01 3.82 3.9 14.2 1.5 0.9 0.5 1.8 buen 284.04 4.75 9.0 14.0 -7.8 1.3 0.7 2.7 cago 283.64 3.50 4.0 13.9 -3.4 0.7 0.4 1.2 cali 284.33 5.92 6.2 13.3 -3.1 2.0 0.7 3.1 frda 284.42 6.26 2.2 14.3 3.0 1.4 0.7 2.7 mede 283.40 0.98 2.1 9.2 4.7 8.7 2.3 8.2 moco 285.11 5.20 1.0 15.5 1.9 1.7 0.8 3.5 mqta 284.53 5.03 5.0 14.2 -4.2 1.1 0.6 1.9 mzal 284.57 6.18 7.4 14.1 -0.8 1.8 0.7 2.7 negr 284.70 2.94 2.1 13.2 -0.3 1.4 0.6 2.5 neiv 284.87 4.47 1.4 14.3 0.2 1.5 0.6 2.7 ombl 284.54 5.53 5.7 15.2 -2.1 1.3 0.6 2.6 paco 282.74 1.22 1.5 12.2 -0.5 0.7 0.5 1.6 past 284.27 4.82 3.7 14.7 15.1 1.5 0.7 2.7 pere 286.63 3.27 -2.9 10.0 -0.3 1.6 0.6 2.7 plle 283.42 2.48 1.7 13.5 13.2 0.8 0.5 1.8 ppyn 283.58 3.22 1.2 12.8 2.7 1.4 0.7 2.7 ptej 283.46 3.81 1.3 14.0 29.8 1.3 0.7 3.0 rest 284.57 6.18 6.4 14.1 -0.5 0.7 0.5 1.3 rion 283.85 4.40 2.4 12.8 -1.3 1.8 0.8 3.5 roln 284.39 4.87 3.8 16.6 -11.9 1.6 0.7 2.7 srdc 283.78 4.09 5.6 13.1 1.2 1.5 0.7 3.1 tul2 281.25 1.81 13.3 9.8 6.4 0.8 0.5 1.7 tuma 286.62 4.07 -3.7 9.5 -1.0 0.8 0.5 1.7 vill 284.40 4.98 1.4 15.2 -9.1 2.8 1.0 4.0 chin 284.70 2.94 2.1 13.2 -0.3 1.4 0.6 2.5 neiv 284.34 4.56 2.5 13.9 -15.6 5.0 1.5 6.1 uniq trenkamp robert, et al. velocities, in this study, are determined relative to the itrf2000 reference frame (figures 2 & 3). due to an assumption that each daily solution is uncorrelated (white noise error model) uncertainties in the velocity estimates may be underestimated even though individual position uncertainties have been scaled to match the observed scatter. recent analysis (zhang et al. 1997; mao et al. 1999; dong et al. 2002) have shown that a better description of noise in a continuous gps time series can be obtained by incorporating both white noise and either ''flicker'' noise or ''random walk'' noise (both time related noise processes) into the error model. however, since every continuous gps site will have its own ''unique noise spectrum'' it is not certain how best to apply these results to campaign style occupations which are too infrequent for noise characteristic determination. it is also not clear what noise model and weighting between available noise model should be applied to relative position time series over baselines a few hundred kilometers in length (freymueller et al. 2000) much less baselines which are less than 100 kilometers. figure 2. vector field for sites observed during geored03. a few sites near pere are not shown to maintain clarity of the figure. one site chip was added. it was re-observed at the end of the project because it was near the ingeominas office and had been previously observed in 1999 after the armenia earthquake. the chip/mzal vectors and the rion/negr are interesting examples of the consistency of gps measurements. (see table 2 for the rion/negr vectors magnitudes). overlain on the vector field is the 31 block grid used for the project microzonificacion sismica de la ciudad de santiago de cali and a small sampling of the faults (light gray lines) in and around the cauca valley. results for this study, only four stations are analyzed which are the four nearest to the city of cali and which are also in a position favorable for the creation of delaunay triangles. these stations are tul2, rest, cali and ptej (figure 3). for these sites casa data collected prior to 1996 was removed from the strain analysis to minimize the effect of viscoelastic overprinting of the geodetic signal due to suggested ongoing viscoelastic effects from the 1979 (mw=8.2) earthquake as reported in white et al. (2003). in order to solve for the velocity gradient tensor components ),,,( 21 ωθεε &&& and to estimate the shear strain (γ& ) (table 2), the loosely constrained itrf 2000 daily solutions for all 174 stations were input as quasi observations into the quasi observations combination analysis (qoca) software developed at jpl (dong et al. 1998). the qoca software permits forward and backward filtering of the daily solutions and tightening and loosening of geodetic constraints on selected stations. this is done to achieve 29 trenkamp robert, mora p. héctor salcedo h. elkin, kellogg james n 30 consistency in results after all constraints are applied to the dataset. figure 3. map showing the topography of the region; the stations used in the strain analysis and the delaunay triangles for which the strain was calculated. for our purposes the data were processed forward and backward with tight and loose constraints on a subset of the global sites until the solutions were consistent. at all times the stations used in the strain analysis were completely unconstrained so that the results forthcoming were completely determined by the data and not artificially determined by tight constraints. at this step, the data prior to 1996 was removed from the input data to qoca for the four stations used in the strain analysis and the strain rates were determined. two separate data runs using both forward and backward filtering were processed and the average strain rates were calculated from a combination of these separate runs (table 2) the simplified kostrov formula was used to determine the time for release of accumulated strain for different moment energy release events these calculations were iterated using seismic moments for various synthetic moment magnitude events and the calculated shear strains until natural breaks (decadal, centenary and millennial) were discovered in the release time, see figure 4. table 2. separate processing results of the runs using the qoca software. listed is the eigenvalue parameterization ),,,( 21 ωθεε &&& along with theγ& evaluation and the average strain rates used in the kostrov recurrence rate calculations. delaunay vértices 1ε& 2ε& ω& θ γ& 10-8strain yr1 10-8 strain yr1 10-8strain yr1 degrees 10-8 rad yr-1 cali-resttul2 15.7 ± 5.60 -0.98 ± 1.20 -0.43 ± 1.80 11.13 ± 6.88 16.74 ± 6.88 cali-resttul2 14.8 ± 5.62 -0.99 ± 1.19 -0.49 ± 1.80 10.46 ± 7.22 15.84 ± 6.88 average used 16.29 ± 6.88 cali-restptej 24.5 ± 8.71 2.80 ± 1.41 -0.66 ± 2.69 -6.45 ± 6.59 21.65 ± 10.24 cali-restptej 23.2 ± 8.71 2.64 ± 1.41 -0.89 ± 2.69 -7.45 ± 6.94 20.59 ± 10.23 average used 21.12 ± 10.24 these strain rates were then input into a simplified kostrov formula γµ &v m o 2 ≅τ where mo is the moment energy release in dynecm, µ is the shear modulus taken to be 3x1011 dyne/cm2, t is time, v is the volume being strained in cm3 and γ& is the shear strain rate. possible rapid strain accumulation rates near cali, colombia determined from gps measurements (1996-2003) 31 figure 4. results of the recurrence rate calculations using the simplified kostrov formula. decadal recurrence intervals (30-90 years) include earthquakes of magnitude (mw = 6.0 – 6.3), centenary (90 -900years (mw = 6.4 -6.9) and millennial (900+ years) (mw ≥ 7.0+) discussion instantaneous velocity gradients within the continental lithosphere in wide plate margin deformation zones such as the nab are surely driven by plate tectonic forces. with the inception of space based geodesy, especially the global positioning system, the possibility of determining increasingly dense, instantaneous, three dimensional surface velocity fields has become a reality. the density and precision of these measurements has made possible the determination of strain rates which can be calculated to a certain degree of accuracy and the denser the measurements, the more accurately the straining areas can be mapped. pollitz (2003) points out that there is a distinction between instantaneous velocity and steady state velocity in that instantaneous velocity is typically represented by geodetic measurements obtained over short time periods while steady state velocity is best described by fault slip rates over geologic time. and so, if the instantaneous surface velocity measurements represent the interseismic velocity field and not the steady state velocity field, this implies that the procedure of relating geodetically determined velocity fields directly to rates of seismic moment release and, subsequently, moment recurrence rates is not altogether correct. in fact, if, as determined in pollitz (2003), the instantaneous velocities are potentially underdetermined steady state velocities the recurrence rates determined must be considered as maximal. the strain rates determined in this study for these two delaunay triangles near cali (table 2), while not high compared to subduction zone rates (10-5 – 10-6) (bilham et al. 1989), are consistent with the seismicity that has been observed in this region of the nab. the energy release rates were determined using conservative volumes, which increase the recurrence intervals but are believed to be valid for 1an area with a thin sedimentary cover similar to this section of the cauca valley. these data are, however, very preliminary and all of the vectors except the vector at cali are based on 2 epoch measurements. although two measurements are enough for obtaining a vector, are certainly not very robust vectors and all of the sites have measurements, which are within the effective viscoelastic window reported in white et al. (2003) and will also have an affect on the results. the historical record only goes back approximately 450 years and if the release is to be greater than mw=6.6 then it may be an event to be trenkamp robert, mora p. héctor salcedo h. elkin, kellogg james n 32 expected in the near future. this would seem unusual or unlikely in the area since the largest recent events near this area have been nearer the mw = 6.1 range. however recent paleoseismic work near the pereira-armenia region produced evidence of faulted offsets that may have generated an mw=6.6 for a ne-sw trending fault and ‘at least’ mw=6.9 for an e-w trending fault (lalinde et al. 2003). maximum seismic magnitudes had been believed to be between 6.2 and 6.5, but this paleoseismic evidence brings a better understanding of the seismicity of the area over the last 30,000 years. in other words, this work and the work of many other scientific investigators indicate a need to study further the geodynamics of the area and apply the findings to the seismic risk in a more informed manner. this area is an area in need of site densification and multiple epoch observations in order to improve the strain analysis and focus the strain search to a localized set of faults. this information would then aid other researchers in the search for paleoseismic study locations, multiple front geologic and geophysical projects and possible permanent gps site installation and observation. conclusions the cauca valley of colombia is a seismically active region of high potential risk to large population centers. a myriad of faults are being tectonically stressed which have repeatedly, in the past, slipped with devastating results. unfortunately, the number of observable points throughout the valley and surrounding areas and resources for expanding the observational database are, at present, small but the need for a minimum 100 station gps network throughout the valley surrounding the population centers of cali, armenia, pereira and medellín along with the two bounding cordilleras, occidental and central, is obvious. our results while not conclusive and rife with difficulties are important in context with several other studies and historical documentation of seismic activity, which has affected cali and other large population, centers in the region. acknowledgments the casa project was supported by nsf grants ear-8617485, ear-8904657 and nasa. the gps measurements were made with the assistance from ingeominas and the instituto geografico agustin codazzi in colombia. the project microzonificacion sismica de ciudad santiago de cali was supported by funding from dagma and performed by ingeominas. special consideration and gratitude to the facultad de ingeniería of the universidad de manizales for use of office space and sun workstation for data processing and to the andean geophysical laboratory at the university of south carolina for the use of two trimble ssi receivers and choke ring antennas. figures 1-3 were created using the generic mapping tool (gmt) (wessel and smith, 1995). references alvarado, c., et al. (2003) projecto microzonificación sismica de santiago de cali, reporte preliminar, ingeominas, dagma. 2003. bilham, r., yeats, r.s. and zerbini, s. (1989) space geodesy and global forecast of earthquakes, eos, transactions, american geophysical union, 70, 65, 73. dong, d., herring, t.a. and king, r.w. (1998) estimating regional deformation from a combination of space and terrestrial geodetic data, journal of geodesy, 72 (4), 200-214. dong, d., fang,p. bock,y. cheng, m.k. and miyazaki. s. (2002) anatomy of apparent seasonal variations from gps-derived site position time series. journal of geophysical research, b, solid earth and planets, 107 (4), 13. freymueller, j.t., cohen, s.c. and fletcher, h.j. (2000) spatial variations in present-day deformation, kenai peninsula, alaska, and their implications, journal of geophysical research, b, solid earth and planets, 105 (4), 8079-8101, 2000. kanamori, h., and mcnally, k.c.(1982) variable rupture mode of the subduction zone along the ecuador-colombia coast. bulletin of the seismological society of america, 72 (4), 12411253. kelleher, j.a. (1972) rupture zones of large south american earthquakes and some predictions, journal of geophysical research, 77 (11), 20872103. kostrov, v.v., (1974) seismic moment and energy of earthquakes, and seismic flow of rocks, lzv. acad. sci. ussr phys. solid earth, 1, eng trans l., 23-44. lalinde pulido, c.p., toro v., g.e. lópez, m.c. velásquez, a. and audemard, f.a. (2003) paleoseismic evidence at taller san miguel, pereira-armenia region, colombia south america, gsa conference, reno, nevada. mao, a., harrison, g.a. and dixon, t.h. (1999) noise in gps coordinate time series. journal of geophysical research, b, solid earth and planets, 104 (2), 2797-2816,.mora, h. y trenkamp, r., investigaciones geodesicas satelitales, projecto microzonificación sísmica de santiago de cali, reporte preliminar, ingeominas, 2003. noel, j.a.,(1996) landsat analysis of the upper cauca basin, colombia, annual meeting abstracts – american association of petroleum geologists and possible rapid strain accumulation rates near cali, colombia determined from gps measurements (1996-2003) 33 society of economic paleontologists and mineralogists, 5, p.106. page, w. geología sísmica y sismicidad del nw colombiano. interconexión eléctrica s.a. medellín colombia. pollitz, f.f., (2003) the relationship between the instantaneous velocity field and the rate of moment release in the lithosphere, geophysical journal international, 153, 595-608. trenkamp, r., kellogg, j.n. freymueller, j.t. and h. mora p., (2002) wide plate margin deformation, southern central america and northwestern south america, casa gps observations, journal of south american earth sciences, 15, 157-171. valle, p., and eduardo, l. (1999) evolución de subcuencas cenozoicas en el valle-cauca-patia; evolution of cenozoic basins at valle-cauca-patia, colombia, boletín de geología (bucaramanga), 21, 27-36. wessel, p., smith, w.h.t., (1995) new versión of the generic mapping tools released. eos trans. am. geophys. u. 76, 329. white, s.m., trenkamp, r. and kellogg, j.n. (2003) recent crustal deformation and the earthquake cycle along the ecuador-colombia subduction zone, earth and planetary science letters, 216, 231-242. zhang, j., bock, y. johnson, h. fang, p. williams, s. genrich, j. wdowinski, s. and behr, j. (1997) southern california permanent gps geodetic array; error analysis of daily position estimates and site velocities, journal of geophysical research, b, solid earth and planets, 102 (8), 18,035-18,055. zumberge, j.f., heflin, m.b. jefferson,d.c. watkins, m.m. and webb, f.h. (1997) precise point positioning for the efficient and robust analysis of gps data from large networks, journal of geophysical research, b, solid earth and planets, 102 (3), 5005-5017, it is vital for analyzing groundwater chemical characteristics to develop, manage and protect groundwater resources. hydrogeochemistry simulation, graphic method, and other methods are applied to analyze 22 pieces of groundwater chemical data in the urban area of zhumadian city. the results show that shallow groundwater is mainly of hco3-ca type. besides, from northeast to southwest, salinity and hardness increase gradually, the hydrodynamic condition becomes worse, and groundwater is significantly affected by human activities. this study is of important significance to scientifically using and managing groundwater resources in the urban area of zhumadian city. el análisis de las características químicas de las aguas freáticas o subterráneas es determinante para desarrollar, administrar y proteger los recursos hídricos. este trabajo se basó en la simulación hidrogeoquímica, el método gráfico y otros procedimientos para analizar la información química de 22 muestras de aguas freáticas en el área urbana de zhumadian, en el este de china. los resultados muestran que las aguas freáticas poco profundas son principalmente del tipo hco3-ca. además, del noreste al suroeste, la salinidad y la rigidez aumentan gradualmente, las condiciones hidrodinámicas empeoran y las aguas freáticas son afectadas por las actividades humanas. este estudio es de una importancia significativa para el manejo científico y la administración de los recursos de aguas subterráneas en el área urbana de zhumadian. earth sciences research journal earth sci. res. j. vol. 20, no. 3 (september, 2016) : g1 g5 abstract resumen keywords: urban area of zhumadian city; groundwater; hydrochemical characteristics palabras clave: área urbana de zhumadian, aguas freáticas, características hidroquímicas. chemical characteristics of groundwater and its spatial distribution in urban area of zhumadian city record manuscript received: 16/12/2015 accepted for publication: 21/11/2016 how to cite item zhongpei, l., jianhua, p., & yuping, h. (2016). chemical characteristics of groundwater and its spatial distribution in urban area of zhumadian city. earth sciences research journal, 20(3), g1-g5. doi:http://dx.doi.org/10.15446/esrj.v20n2.54710 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.54710 liu zhongpei1,2, ping jianhua3, han yuping1* 1 department of water resources, north china university of water resources and electric power, zhengzhou-450045, china 2 china institute of water resources and hydropower research, beijing-100038, china 3 school of environment and water conservancy engineering, zhengzhou university, zhengzhou 450001, china *for correspondence: hanyp@ncwu.edu.cn g e o c h e m is t r y características químicas y distribución espacial de las aguas freáticas en el área urbana de zhumadian (china) liu zhongpei, ping jianhua, han yupingg2 analysis of groundwater chemical characteristics is an essential content of hydrogeology subject. it will be of important significance to both use and management of region groundwater resources and construction and protection of ecological environment (zhang, deng, & he, 2006). chemical characteristics of the groundwater are determined by sedimentary environment and sedimentary association characteristics of provenance. it is also subject to the interaction of natural and human factors such as flood, saline intrusion, ecologic water conveyance and agricultural irrigation (an, zhao, ye, liu & ding, 2012). through study on groundwater chemical characteristics, the scientific basis will be provided for interaction mechanism of groundwater and environment, description of groundwater recharge, runoff and discharge conditions, as well as groundwater resources management. in recent twenty years, for rapid economic and social development and climate changes in zhumadian city, the conflict between water resources supply and demand in the urban area of zhumadian city increase prominently. the urban area of zhumadian city is located at the slight area of the mountain front, where surface water resources are deficient and limited surface water resources are not enough to meet the increasing water resources demand, and groundwater plays a vital role in development and use of water resources (liu, zhang, & ren, 2002). study on chemical characteristics of shallow groundwater in the urban area of zhumadian city is conductive to understand further of typical features of current shallow groundwater in the area. moreover, it can provide scientific bases and management concept for groundwater use in this area, and it is good to safeguard sustainable development of water resources. also, it is of great theoretical guidance significance and practical value of reasonable use and protection of groundwater resources in the urban area of chinese cities. 1.2 geology and hydrogeology shallow groundwater in the urban area of zhumadian city mainly lies in silty, clayey soil porosity of quaternary pleistocene series. aquifer group structure is single-layer clayed soil structure. water storage capacity of shallow groundwater is different in different parts of the study area. quaternary pleistocene series lithology in this area is thick-layer brownish yellow, auburn yellow, yellowish-brown and ginger silty clay with muddy medium-coarse sand, muddy medium-fine sand, fine muddy silt and muddy gravel layer. silty clay porosity and fracture from earth surface to the layer 30m deep are well developed and connected and constitute a storage space of shallow groundwater. a supply source of shallow groundwater in the urban area of zhumadian city is mainly from atmospheric precipitation, following lateral groundwater flow and well irrigation area infiltration. shallow groundwater flows from west to east, without obvious depression cone. the drainage mainly includes industrial and agricultural exploitation, evaporation and lateral flow. 2 data and methods 2.1 data twenty-two sampling wells were arranged in the study area, as shown in figure 1, with a radius of 0.25-0.50m and depth of 15-60m to reveal the chemical characteristics and spatial distribution characteristics of shallow groundwater in the urban area of zhumadian city. the analysis items of groundwater quality mainly include bicarbonate ion, carbonate ion, free carbon dioxide, calcium ion, magnesium ion, potassium-sodium ion, chloride, sulfate, ammonia nitrogen, total hardness, total dissolved solids, ph value, microelement, metal ion, conductivity, total acidity and total alkalinity. 2.2 methods this paper studies water chemistry of groundwater and its spatial distribution characteristics in the urban area of zhumadian city using graphical method (zhang et al., 2006; an et al., 2012; lu, zhou, wang, yang, & lu, 2010), hydrogeochemistry simulation (tang, chen & qian, 2012; jiang, li, & hu, 2009; li, hou, zhou, & liu, 2009) and other methods. 1.1 physical geography the study area is located zhumadian city, henan province, with geographical coordinates of east longitude 113°56′02″～114°08′52″ and northern latitude 32°54′29″～33°04′31″. the total area is 360 km2, and the number of population is 570400. the study area is a relatively gentle terrain and ground slope gradient of 1～10 ‰. bounded by guanwang temple, shun river, and li tower, its western part is alluvial-proluvial plain, and its eastern part is alluvial-lacustrine plain, as shown in figure 1. the earth surface is composed of hard and dense silty clay of the middle pleistocene and slight yellow muddy clay limnetic facies of upper pleistocene. it is in the transition zone from northern subtropics to warm temperate zone and belongs to a mainland monsoon type climate, with primary weather features of prominent monsoon climate, four distinctive seasons, moderate climate, the same season of rain and hot, multi-year mean evaporation capacity of 935.5mm and multi-year mean precipitation of 966.0mm. the water system is not well developed in the study area, in addition to a few of streams, only a relatively large lianjiang river passes the area from the south boundary. lianjiang river is a branch of ru river and flows into suya lake reservoir from west to east through the urban area. figure 1. sampling point location chemical characteristics of groundwater and its spatial distribution in urban area of zhumadian city g3 3 results and discussion 3.1 spatial distribution characteristics of water chemistry composition 3.1.1 spatial distribution characteristics of tds and hardness under the usual state of many water chemical properties, tds can reflect the overall condition of chemical composition field, and total hardness is an important indicator of water quality and significantly affects industrial water (zhang & shen, 2000). based on the results of chemical analysis of water samples in the study area, contour map of tds of shallow groundwater in the urban area of zhumadian city (fig. 2) and contour map of total hardness (by caco3) (fig.3) are drawn as below. it can be learned from the analysis as below. (1) according to tds level, groundwater can be classified into fresh water (tds<1g /l), brackish water (1g /l≤tds≤3g /l) and salt water (tds>3g /l). shallow groundwater in the urban area of zhumadian city is mainly fresh water, with brackish water in southwestern part and no salt water. tds becomes lower from southwest to northeast, and it changes rapidly in the southwest part, but changes slowly in other regions. based on water quality monitoring data, tds of well 18 and 19 is relatively high, contents of cland so4 2ions at these two points are far superior to that of other monitoring points and may be affected by pollution. (2) according to hardness classification standard (by caco3), groundwater can also be classified into soft water (<150mg/l), little hard water (50-300mg/l), hard water (300-450mg/l) and very hard water (>450mg/l). hard water and very hard water mainly distribute around the urban area, and groundwater is of relatively small hardness in the center area. based on features of the city, groundwater around urban area of zhumadian city is relatively high and may be polluted by urban domestic water, industrial water, and other wastewater. (3) based on figure 2 and figure 3, tds and salinity of the southwest part of the urban area of zhumadian city are both relatively high, which less likely result from natural causes. by access to information, it is found that there is a quarrying transport factory at xiangshan town at the southwest part of the urban area of zhumadian city, and there are power plants and coal mines in gucheng town, so it is speculated that such human activities result in pollution of local groundwater. 3.1.2 spatial distribution characteristics of hydrochemical type piper trilinear chart of shallow groundwater (figure 4) is drawn, and graphical method is applied to analyze and study hydrochemical characteristics and its spatial distribution rule of shallow groundwater in the study area. figure 4. piper trilinear chart of shallow groundwater. from the figure, it can be drawn as below. (1) water sampling points in the study area are mainly distributed at the left bottom of the delta-shaped region, indicating groundwater in the study area is primarily of hco3-ca type, or hco3-ca·mg, so4-ca·mg and so4-ca type. (2) hydrochemical types of the study area from southwest to northeast are transiting from hco3-ca to hco3-ca·mg, so4-ca·mg and so4-ca· and spatial variation degree of the anion is higher than that of positive ion. (3) carbonate hardness of water sample point is greater than 50%, indicating that chemical composition of water is mainly of carbonate lixiviation. besides, groundwater is dominated by hco3 with little mineralization degree and also reflecting that shallow groundwater in the urban area of zhumadian city is mainly from atmospheric precipitation. 3.2 ion proportionality coefficient characteristics in chemical composition of groundwater, content proportionality coefficient among all compositions is often used to study some figure 2. contour map of tds figure 3. contour map of total hardness (by caco3) liu zhongpei, ping jianhua, han yupingg4 hydrogeochemical problems and judge the groundwater causes, groundwater composition source and groundwater classification li, sun, & liu, 2000). coefficient ƴna/ƴcl and ƴcl/ƴca are respectively used to reflect the causes and hydrodynamics of shallow groundwater in the study area jiang et al., 2009. figure 5. [ na+ ] vs [ cl] figure 6. [cl/ca2+] vs tds (1) ƴna/ƴcl coefficient is called as groundwater cause coefficient and is a hydrogeochemical parameter of sodium ion concentration degree of natural groundwater. the mean value of ƴna/ƴcl coefficient of standard saline water is 0.85; low salinity water has relatively high ƴna/ƴcl coefficient (ƴna/ƴcl>0.85) and high salinity water has relatively small ƴna/ƴcl coefficient (ƴna/ƴcl<0.85). from figure 5, it can be drawn that analysis point of shallow groundwater sample in the urban area of zhumudian city is at or close to line 1:1. it indicates that groundwater will continuously dissolve rock minerals by the hydrolysis and acidification action in the runoff process and release na+ from feldspar so as to make na+ concentration larger than clconcentration. it also indicates that groundwater has a rock-salt lixiviation action in the runoff process. sampling well 18 is abnormally large due to clmilliequivalent and ƴna/ƴcl coefficient is relatively small, which reflects southwestern part of the study area may be polluted. (2) ƴcl/ƴca coefficient can be used as a parameter describing hydrodynamics. from figure 6, it can be drawn that when tds<1000mg/l, ƴcl/ƴca coefficient does not change significantly with tds. however, when tds>1000mg/l, ƴcl/ƴca coefficient increases sharply. it shows that northeast part of the study area is a relatively good hydrodynamic condition and southeast part is a relatively weak hydrodynamic condition. at the same time, there are two significant deviations sampling points, well 18 and 19, which also reflects that they are most likely polluted. 3.3 saturation index analysis saturation index refers to the parameter which is used to determine the status of water and minerals through getting water-rock interaction information, represented by “si.” when si>0, minerals in water are saturated and will subside; when si<0, minerals in water are unsaturated and will be dissolved; when si=0, minerals in water are in an equilibrium state of dissolution and sediment, that is, the minerals will not subside and also not be dissolved (li et al, 2009). phreeqc software is used to simulate and calculate saturation index of mineral facies of shallow groundwater in the study area to quantitatively describe evolution state of the chemical field of shallow groundwater. because shallow groundwater is mainly of hco3-ca type, calcite saturation index is selected to draw a contour map of mineral saturation index to analyze hydrochemical characteristics of groundwater in the study area. figure 7. calcite si contour map from calcite si contour map (fig.7), it can be drawn as below. (1) calcite mineral si in the study area mostly is less than 0, and the fields with an si larger than 0 are mainly at southwest, southeast and northeast part of the study area, reflecting the study area is dominated by calcite dissolution. however, at the southwest, southeast and northeast part of the study area, calcite sediment is separated out. (2) mineral saturation index may be affected by groundwater circulation and exchange degree. calcite saturation indexes at the southwest, southeast and northeast part of the study area are larger than that of other regions, and that of the southwest part is the largest. it reflects runoff condition at such sections is relatively worse than that of other areas, water circulation and exchange are relatively slow, and mineral saturation index is relatively high, especially water circulation and exchange in southwestern part are the weakest. (3) parts with relatively high mineral saturation index are around the urban area, similar to hardness distribution rule. based on the study area including large urban, living and industrial activity strength and shallow groundwater, groundwater chemistry is more easily affected by human endeavor, reflecting that shallow groundwater in the urban area of zhumadian city is significantly affected by human activity. chemical characteristics of groundwater and its spatial distribution in urban area of zhumadian city g5 4. conclusion based on spatial distribution characteristics, ion proportionality coefficient and saturation index analysis results of chemical composition of water in the study area, conclusion can be drawn as below: (1) shallow groundwater in the urban area of zhumadian city is mainly of hco3-ca type, salinity, and hardness decrease from southwest to northeast and hydrodynamic conditions become worse from northeast to southwest. (2) intensive human activities occur in the study area, which may significantly affect groundwater, and the groundwater at the southwest part of the study area is likely polluted. acknowledegment this study was supported by the national natural science foundation of china (grant no. 51209090 and 71271086), the “948” program of the ministry of water resources in china (grant no. 201328). references an, l. s., zhao, q. s., ye, s. y., liu, g. q., ding, x. g. (2012). hydrochemical characteristics and formation mechanism of shallow groundwater in the yellow river delta. chinese journal of environmental science, 33(2), 370-378. jiang, l., li, p., & hu, a. (2009). the groundwater chemical characteristics in the yaoba oasis of alxa area, inner mongolia. journal of arid land resources and environment, 23(11), 105-110. li, x., sun, z., & liu, j. (2000). hydrogeochemical. atomic energy press, 3rd. edition. li, x. q., hou, x. w., zhou, z. c., & liu, l. x. (2009). hydrogeochemical characteristics and formation evolutional mechanism of the groundwater systems in the taiyuan basin. geoscience-journal of graduate school, china university of geosciences 23(1), 1-8. liu, g., zhang, j., & ren, h. (2002). assessment of shallow groundwater environmental in zhumadian urban. henan water conservancy, 4, 75-80. lu, x., zhou, a., wang, m., yang, l., lu, h. (2010). characteristic analysis of phreatic water equality evolution by piper diagram in huaihe river drainage area, jiangsu province. geotechnical investigation & surveying, 2, 42-47. tang, h., chen, j., & qian, h. (2012). application of saturation index to research of water-rock interaction and its sensitivity analysis. journal of water resources and water engineering , 6, 180-183. zhang, g. x., deng, w., & he, y. (2006). hydrochemical characteristics and evolution laws of groundwater in songnen plain, northeast china (in chinese). advances in water science, 17(1), 20-28. zhang, z., & shen, z. (2000). evolution of groundwater environment in north china plain. geological publishing house. geottsic a colombiana n° 7 pp. 24-29 diciembre de 2003 bogota, d.c. issn • 0121 2974 caracterlzacion preliminar de la susceptibilidad sismica en el entorno de la localidad de sampacho, cordoba, argentina daga romina virginia grosso guillermo sagripanti monica villegas diego villalba departamento de geologia. universidad nacional de rio cuarto. agencia postal 3. cp x5b04zab rio cuarto. c6rdoba. argentina email: rdaga@exa.unrc.edu.ar resumen la localidad de sampacho, situada en el departamento rio cuarto, provincia de cordoba, centro de la republica argentina, se encuentra emplazada en una region de intraplaca con probada actividad sismica historica y actual, donde el nivel de sismicidad, que es importante, pone en una constante geoamenaza sismica a toda la region. es de destacar que esta fue epicentro de dos terremotos historicos (afio 1934), considerados dos de los mas destructores ocurridos dentro de la zona de sismicidad reducida. en el presente trabajo se caracterizo de manera preliminar la susceptibilidad sismica en el entomo de la localidad de sampacho, como base para futuros estudios de microzonificacion sismica y evaluacion del riesgo sisrnico. la metodologia seguida hizo enfasis en el estudio de los factores litologicos, geornorfologicos e hidrologicos locales, los cuales pueden tener una importante incidencia en el desplazamiento del frente de ondas elastic as. se definieron cinco clases de susceptibilidad a partir de la superposicion de niveles de informacion litologia y profundidad del nivel freatico, de las cuales predomina la moderadamente baja. la distribucion areal de las mismas se presenta en una carta de susceptibilidad sismica preliminar en soporte digital, siendo este el primer documento sobre esta tematica generado para la region. palabras clave: terremotos; riesgo sismico; microzonificacion; licuefaccion abstract the city ofsampacho, situated in rio iv departmen, center of argentina, it is located in an intraplate region with proved historical and current seismic activity, where the level of seismicity, which is important, makes a low constant seismic geoamenace to the all region. it is worth mentioning that it was epicenter of two historical earthquakes (year 1934), considerated two of the most destructive ones within of the zone of reduced seismicity. in this paper characterizes preliminarily the seismic susceptibility in the sourrounding areas ofsampacho city, as a base for future studios of seismic rnicrozoning and evaluation of seismic-risk. the following metodology emphatized the local iithologic, geomorphologic and hydrologic factors, which may have an important incidence in the displacement of elastic wave front. it also defines five classes of susceptibility from superimpose of information levels (lithology and depth oflevel freatic), from wich the moderately low prevails. the areal distribution of them is presented in a preliminary map of seismic susceptibility in digital support, being this one the first document developed on this theme for the region. keywords: earthquake. sismic hazard, microzonification, liquefaction ubicacion ei area de estudio esta situada al sw del departamento rio cuarto, cordoba, centro de la republica argentina (figura i), manuscrrto recibido para evaiuacton ei 24 de julio de 2002. articulo aceptado para publicacien por el comite edrtorialel 30 de octubre de 2002. 24 geofislca colombiana. 7, diciembr~ de 2003 caracterizacion preliminar de la susceptibilidad sismica en el entorno de la localidad de sampacho, cordoba, argentina republica argentina provincia de cordoba 35 ~ area de [studio figura j. ubicaci6n area de estudio. borde occidental de la provincia geologica llanura chacopampeana, que comprende una extensa llanura compuesta predominantemente por materiales loessicos y fluviales subordinados, de edad cenozoica y con bajo a nulo grado de diagenesis. emplazada en el area sismotectonica sierras de cordoba y san luis, en una region sismicamente activa, donde el nivel de sismicidad es importante, por 10 que la region esta sometida a una constante amenaza sismica (sagripanti et al., 2001). introduccion en regiones donde la actividad sismica es manifiesta y somete al hombre, sus bienes y el normal desarrollo de sus actividades a una constante amenaza sismica, como ocurre en la zona de estudio, es imprescindible que los habitantes adquieran conciencia para revertir .el estado de negacion sismica existente. ei riesgo sismico se asume como la probabilidad de perdidas de vidas, materiales y economic as frente a la ocurrencia de un terremoto de una magnitud dada, en un tiempo preestablecido, con consecuencias especificas sobre el medio ambiente (paniza, 1990). para su estimacion es necesario lograr previamente la caracterizacion de la amenaza 0 peligro sismico, que depende de la situacion sismotectonica de la region, de la vulnerabilidad que es el grado de perdidas esperadas con relacion a las perdidas maximas posibles y de la susceptibilidad sismica. uno de los aspectos que condiciona la magnitud de los dafios producidos por los terremotos es la susceptibilidad sismica, que comprende las caracteristicas propias del medio fisico, como la profundidad del agua subterranea, litologia y geomorfologia, que tienen incidencia directa en el potencial de licuefaccion de los sedimentos sueltos, siendo esta una de las principales causas de destruccion de edificios e infraestructurasde servicios. si bien las construcciones en zonas sismicas suelen estar preparadas para resistir estos fenomenos, en cuanto a pautas de sismorresistencias, el hecho de que geofislca colombiana, 7, diciembre de 2003 esten asentadas en suelos granulares poco consolidados las transforma en construcciones vulnerables de sufrir las consecuencias de la licuefaccion: destrucci6n por asentamiento diferencial del terreno, agrietamientos en paredes y techos, desmoronamientos, depresiones y hundimientos en la superficie. este fen6meno se pone de manifiesto con la disminucion parcial 0 total de la resistencia de cizalla, como consecuencia de una agitaci6n externa inducida 0 natural, como una sacudida sismica, transformando un material del estado solido al fluido. si bien el origen de la licuefacci6n no siempre es sismico, es asociada a terremotos cuando se encuentran en zonas de riesgo sismico. los materiales mas susceptibles a la licuefacci6n son arenas de grana fino a medio, bien seleccionadas. en una region sismicamente activa, el desconocimiento de la posicion y comportamiento del nivel freatico y de la respuesta de los materiales que forman el subsuelo frente a una agitacion sismica puede generar graves dafios en las estructuras edilicias emplazadas en la zona, como es el caso de la poblacion en estudio. por ello el objetivo de este trabajo es lograr la caracterizaci6n preliminar de la susceptibilidad sismica para la localidad de sampacho y sus alrededores, como base para una futura estimacion del riesgo sismico, focalizando el analisis en los factores iitolcgicos, geomorfol6gicos e hidrol6gicos locales, los cuales pueden amplificar 0 atenuar el frente de ondas elasticas. propuesta metodologica para la obtencion de cartografia de susceptibilidad sismica de acuerdo con 10 planteado por paniza (1990), para la valoraci6n de la susceptibilidad sismica se deben considerar factores relacionados con el medio fisico, como la litologia, geomorfologia y profundidad del nivel freatico. desde este punto de vista, y para la obtencion de las unidades cartograficas de susceptibilidad, se elabor6 el mapa geologico-geornorfologico donde las unidades se definieron teniendo en cuenta rasgos estructurales, litologicos y morfol6gicos, todos ellos relacionados geneticamente. por esta raz6n, y considerando la escala de trabajo, los limites de unidades fueron utilizados para generalizar la distribucion areal de la litologia. para la tipificaci6n de la litologia se utiliz6 el sistema unificado de clasificacion de suelos que se aplica en los estudios geotecnicos de mecanica de suelos, y que se considera el mas apropiado para la ponderacion preliminar del potencial de licuefaccion. los otros parametres geornecanicos que intervienen en la definici6n final, como presi6n de confinamiento inicial y densidad relativa, se consideran homogeneos a esta escala de trabajo y deben ser definidos al hacer estudios de mayor detalle (microzonificacion sismica). 25 para completar esta propuesta, se establecio la profundidad del nivel freatico mediante metodos hidrogeologicos convencionales. para la elaboracion de la carta preliminar de susceptibilidad sismica se aplic6 la metodologia de superposicion de niveles de informacion (overlay). para ello se establecieron cuatro clases litol6gicas definidas sobre la base de las litologias presentes y tres clases derivadas de la profundidad del nivel freatico, siguiendo 10 propuesto por ramirez rayo (1997), considerando la mayor probabilidad de licuefaccion entre la superficie topografica y los seis y siete metros de profundidad como maximo y por encima de los tres metros de profundidad del nivel freatico, en ambos, casos a cada clase se ie asigno una valoraci6n cualitativa y, a partir del cruce de las mismas, se lograron cinco clases de susceptibilidad, las cuales representan la cantidad maxima de combinaciones posibles entre dichas variables, esto ultimo para permitir una mejor cornparacion al aplicar el metodo en otras zonas. marco geologico-geomorfologico ei area de estudio se encuentra dentro la provincia geomorfol6gica llanura chacopampeana, region eolica-loessica subhumeda (cantu et al., 1984), dentro de la cual pueden -a su vezdistinguirse zonas con caracteristicas geologicas-geomorfologicas diferentes (figura 2). se realize el foto-analisis de la zona, con el proposito de elaborar una carta geologica-geornorfologica de base para el area de estudio. todas las unidades responden a un fuerte control estructural que definen bloques tectonicos de basamento ascendidos diferencialmente, los que han condicionando el comportamiento del agua superficial y subterranea, generando asi depositos palustres y wpa ge<l.oolco-~oolco """""""+f,truah.€asa =lj):= ~"'i'wxjiw. ~~~~~~ _... kjoerw£s u1 ~ <) ......... ___ 0i'w. ~ gwflo q ne«d<s i"_ figura 2. mapa geo!6gico-geomorfoi6gico. 26 romina et al. lagunares que interdigitan y se suceden verticalmente con otros eolicos 0 fluviales. unidad 1 (ul) bloque hundido de fa (alia las lagunas esta unidad deprimida se caracteriza por la presencia de una serie de lagunas permanentes, entre elias las lagunas suco y goyo, como tambien de otras temporarias y zonas de baiiados originados por el aporte de aguas subterraneas, y en menor grado por escurrimientos superficiales. son de origen tectonico, controladas por la falla las lagunas (az: 0400n). presentan una morfologia elongada en el mismo senti do de su alineaci6n; son sub-redondeadas, con bordes netos y se extienden a 10 largo de la expresi6n de la escarpa. en su mayoria son asirnetricas, con la mayor profundidad hacia el bloque levantado. se reconocen depositos lagunares, intercalados con loess, y materiales mas arenosos posiblemente de origen fluvial. unidad 2 (u2) bfoque hundido de fa (alia las rosas esta unidad esta emplazada en una depresi6n de origen tectonico, controlada por la falla las rosas, de rumbo 0400n y bloque hundido al se. genera una zona de bajos y humedales alineados y elongados en sentido noroeste-sudeste, alimentados en su mayoria por el nivel freatico. esta depresi6n es cabecera de las nacientes del arroyo sampacho, area de relieve plano, con escaso microrrelieve donde aflora la freatica, salada, originando eflorescencias de sales al final del periodo de sequia (rasgos hidroalom6rficos). los materiales reconocidos son sedimentos finos (lagunares), arenosos finos y loessicos, la falla las rosas, si bien no tiene expresi6n en superficie, puede establecerse por las morfologias asociadas en el area. unidad 3 (u3) pfanicie ondufada con medanos sobreimpuestos esta unidad presenta un relieve ondulado constituido por medanos activos y otros estabilizados, de orientacion nw -se, que se sobreimponen a paleorasgos fluviales, correspondientes probablemente a una antigua traza del arroyo del gato. pueden reconocerse materiales loessicos y arenosos finos. unidad 4 (u4) pfanicie ondufada con afloramiento de areniscas al sw de sampacho se encuentra un afloramiento de areniscas permicas (cantera sampacho) simi lares a las aflorantes en el cerro suco. estos son considerados un relieve residual producto de erosion diferencial y de la actividad neotectonica de estructuras. unidad 5 (u5) bfoque efevado y bascufado de fa (alia las lagunas la falla las lagunas se caracteriza por presentar una escarpa de falla continua con frente bien abrupto inclinando al oeste y con su espalda mas tend ida al este; tiene una altura promedio de 2.5 m, pendientes que alcanzan 25°, y sobre ella se evidencia una alta tasa geoflslca colombiana. 7, diciembae de 2003 caracterizaci6n prellminar de la susceptibilidad sismica en el entorno de la localidad de samp acho. c6rooba. argentina de erosion hidrica, con la instalacion de sistemas de carcavas y la consecuente modificacion del perfil de la escarpa; tambien puede mencionarse la presencia de sistemas de medanos coronando la parte superior de la misma. los sitios de mayor elevacion dentro de esta unidad corresponden a los cerros sampacho y suco. ei primero, cuya litologia es un granitoide de grana medio, es atravesado por algunos filones de pegmatitas. dicho material ha intruido un gneis biotitico muy esquistoso que predomina en la parte norte del mismo (mingorance, 1991). junto al cerro suco, conformado por areniscas permicas, forman relieves residuales productos de la erosion diferencial y de la actividad neotectonica de las estructuras presentes en el area. las areniscas permicas tambien se reconocen aflorando en las proximidades de la localidad de sampacho. ei cerro sampacho es atravesado por una falla (az: 1200n) que nace aproximadamente en las iiltimas manifestaciones de la sierra de comechingones (faja de cizalla las lajas), la cual atraviesa toda la unidad, con direccion a la localidad de sampacho, y hacia el sudeste controla la posicion del arroyo que lieva el mismo nombre. hacia el oeste, y recorriendo toda la unidad, como tambien la unidad 6, se ubica un canal de origen antropico que nace en la laguna suco, aetna como desaguador de la misma y luego desemboca en el arroyo sampacho. unidad 6 (u6) planicie ondulada toessice pueden observarse en esta unidad rasgos paleofluviales con orientacion nw-se, a los que se ie sobreimpone una importante secuencia de material eolico, toda la unidad tiene eventos eolicos sobreimpuestos, y el material que se encuentra predominantemente son sedimentos arenosos finos y muy finos de origen eolico (holoceno). unidad 7 (u7) alto estructural de sampacho esta unidad se considera una morfoestructura elevada, controlada estructuralmente hacia el noreste y sudoeste, siendo estas estructuras limites de la unidad. los materiales identificados pueden clasificarse como arenas, limos y arcillas. unidad b (ub) oepresion longitudinal arroyo sampacho dicho arroyo presenta un marcado control estructural que se refleja en el patron rectilineo que presenta el mismo. su faja fluvial actual se limita al propio cauce y sus barrancas. presenta a su alrededor zonas con problemas hidrohalomorficos, que se manifiestan tanto en el suelo como en la vegetacion. estos bajos escurren por sistemas de drenaje encauzados con carcavas y surcos que ponen en evidencia el poder energetico desarrollado en epocas de liuvia (nicolau, 1989). los materiales presentes son arenas gruesas a finas. geofislca colom81ana, 7, diciem8re de 2003 unidad 9 (u9) faja fluvial y paleoderrames del arroyo del gato el sistema de drenaje mas importante presente en toda el area de estudio es el arroyo del gato, con una direccion de drenaje nw-se, con tramos rectos, evidenciando un marcado control estructural, con barrancas bastante pronunciadas y poco desarrollo de terrazas en todo su recorrido. se reconocen con una importante distribucion areal, rasgos paleofluviales con orientacion nw-se, a los que se ie sobreimpone una secuencia de material eolico que alcanza en algunos sitios los 5 m de potencia, predominando los sedimentos arenosos finos y muy finos de edad holocena. factor hidrogeologico segun custodio (1983), se denomina acuifero a aquel estrato 0 formacion geologica que permite la circulacion del agua por sus poros 0 grietas. en el area de estudio este esta constituido por depositos de materiales no consolidados, tales como arcillas, limos, arenas 0 gravas y mezclas de las mismas, de origen principalmente fluvial, lagunar y eolico. de acuerdo con la presion hidrostatica del agua encerrada en los poros, pueden reconocerse distintos tipos de acuiferos, libres, no confinados 0 freaticos y acuiferos cautivos, confinados 0 a presion. en este caso, los estudios se realizaron en los del primer tipo. con el objetivo de determinar la profundidad del nivel freatico se llevo a cabo el relevamiento de pozos (molinos, perforaciones), con una densidad de un punto de muestreo cada 16 krrr, considerada apropiada para el area estudiada, a1canzando un total de 48 puntos relevados. la profundidad del acuifero se determine mediante el uso de un piezometro con sefialluminica y sonora. a partir de los datos obtenidos en los puntos de muestreo (pozos), incorporados a una base topografica en soporte digital, se obtuvo un mapa de isoprofundidad del acuifero libre. las curvas de isoprofundidad son lineas que unen puntos de igual espesor de la eapa de sedimentos que constituyen la zona de aireacion, ubieada por encima del nivel freatico y se trazan con el propos ito de definir la topografia del techo del acuifero libre 0 superficie hipsometrica. en el presente trabajo las curvas se obtuvieron mediante la representacion de los datos de profundidad del teeho del aeuifero libre medidos directamente en el campo y se presentan en un mapa de isoprofundidad (figura 3). factor litologico la caracterizacion litologica del subsuelo para determinar las variaeiones de los materiales en profundidad se realize por medio de sondeos con pala vizcachera, e igualmente se extrajeron muestras disturbadas para su posterior determinacion. la litologia presente en el area de estudio esta compuesta por roeas y sedimentos. entre los primeros, se reconocen rocas graniticas y metamorficas, restringidas al sector que ocupa el cerro 27 ....'( ... ii"'~~~~'"d=~_~~ i -~ csa ~~"""~ figura 3. mapa de isoprofundidad. sampacho (chafiaritos) y afloran tambien areniscas permicas en cerros aislados del entorno de la localidad. en general, los sedimentos presentes en el area ocupan la mayor superficie de la misma y estan constituidos por materiales arenosos finos y muy finos de origen fluvial y edad pleistocena-holocena, limos y arenas limosas de origen eolico (loess y depositos medanosos) de edad holocena, que supera en algunos sitios los 5 m de potencia, asi como tambien se identifican depositos limo-arcillosos y arcillas de media a baja compresibilidad y genesis lagunar a palustre. los materiales reconocidos en el area y tipificados de acuerdo con el sistema unificado de clasificacion de suelos, son sm, ml, cl-mly cl. de acuerdo con ramirez rayo (1997), los'materiales mas susceptibles a la licuefaccion por la ocurrencia de fenornenos sismicos son las arenas de grana fino a medio bien seleccionadas, que en el caso en estudio corresponderian al tipo sm. determinacion de la susceptibilidad sismica para la elaboracion del mapa preliminar de susceptibilidad sismica de la zona de estudio (figura 4), se analizaron las variables: litologia y profundidad del nivel freatico, de la superposicion de ambas surgieron las clases de susceptibilidad que pueden presentarse en el area segun las diferentes combinaciones. la valoracion se realize de manera cualitativa. del analisis de los materiales presentes en el area pudieron determinarse cuatro grupos litologicos, y a cada uno un valor cualitativo de susceptibilidad: 28 romina it al. t ltolngia psc£.ptibili~ arenas finas a limosas (sm) alta i i limos (ml) media i arcillas / limos-arcillosos (cl-ml) baja i roca nula i i para la otra variable considerada, profundidad del nivel freatico, se establecieron rangos 0 !imites de profundidades del nivel freatico segun la mayor 0 menor influencia que tengan sobre el proceso de licuefaccion. los rangos de valoracion considerados fueron los siguientes: profundidad (metros) susceptibilidad i 0-3 alta j 3-6 media ~ i>6 baja i j por ultimo se realize el cruce de variables y se obtuvo como resultado cinco clases de susceptibilidad, cuyas respectivas caracterizaciones de litologias y profundidad del nivel freatico se resumen en el siguiente cuadro: clase de suscepcaracterizaci6n tibilidad materiales arenosos finos a limosos con ~ alta profundidades menores a 3 m del nivel freatico. imateriales arenosos finos a limosos con moderadamente alta i presencia del nivel freatico entre 3 y 6 m. i materiales loessicos con nivel freatico enmedia itre 3 y 6 m, 0 limo-arcillosos con profundidad del nivel freatico menor a 3 metros. i i iarenas finas a limosas con nivel freatico moderadamente baja mayor a 6 metros de profundidad, 0 limos arcillosos con el nivel entre 3 y 6 metros. materiales limo-arcillosos con el nivel [baja freatico a mas de 6 metros de profundii dad.rocas sedimentarias y cristalinas. i geofisica colombiana, 7, dici[mbre de 2003 caracterizaci6n preliminar de la susceptibilidad sismica en el entorno de la localidad de samp acho, c6rdoba. argentina mapa preuminar de susceptibilidad tag.:goyo i =cvosiioesusa ...... """" .... ,. d~·"'u 0"'''''' ~ rooxiefiacwoil!n'u ............ figura4. mapa preliminar de susceptibilidad sismica discusion y conclusiones se determine que en la localidad de sampacho y sus alrededores, las profundidades minimas del nivel freatico se encuentran entre 9 y 17 metros. se puede concluir que, si bien los materiales presentes en el area pueden llegar a ser susceptibles de licuefaccion, el nivel freatico se encuentra a demasiada profundidad como para ejercer influencia sobre los sedimentos suprayacentes, en caso de ocurrencia de un evento sismico importante. debe considerarse que, ante un eventual ascenso del nivel freatico por causas naturales 0 inducidas, la localidad puede considerarse como propensa a fenornenos de licuefaccion debido a las granulometrias presentes en el area. por ella se recomienda llevar un control de las oscilaciones del nivel freatico, ya que la zona se encuentra bajo amenaza sismica. sobre la base de los resultados obtenidos de la superposicion de niveles de informacion, expresados en el mapa preliminar de susceptibilidad sismica, se concluye que en la zona de estudio predominan las clases moderadamente baja y baja (en la localidad de sampacho). los sectores que presentan una susceptibilidad mas alta coinciden can areas en las que se encuentran materiales arenosos y presencia del nivel freatico cercano a la superficie. agradecimientos al programa "geoindicadores para la evaluacion y el monitoreo ambiental", proyecto "calidad ambiental. evaluacion y monitoreo geofislca colombiana. 7. oiciembre oe 2003 de riesgos naturales", financiado por secyt-unrc, y al proyecto "pautas para la planificacion y gestion de los recursos hidricos del departamento rio cuarto a partir del analisis geoambiental de su estado actual y su tendencia evolutiva", financiado por agencia cordoba ciencia, por su apoyo para el desarrollo del presente. bibliografia consultada cantu, m. (1992). el holoceno en laprovode cordoba. holoceno de la rep. arg. torno 1. cadinqua. parana. argentina, m. iriondo ed., pp. 1-16. ___ y degioy anni s. b. (1984). geomorfologia de la region centro sur de la provincia de cordoba. noveno congreso geologico argentino, s. c. de bariloche. aetas iv, pp. 76-92. custodio, e. y llamas, m. r. (1983). hidrologia subterranea. 2a. ed., barcelona: ediciones omega, s. a. moll, l. l., rocca, r. j. yredolfi, e. r. (1987). curso de mecanica de suelos. facultad de ciencias exactas, fisicas y naturales, universidad nacional de cordoba. mingorance, f. (1991). "analisis y evaluacion del tectonismo reciente asociado a la falla sampacho, provincia de cordoba, argentina central". revista geojisica, n° 35, instituto panamericano de geografia e historia, pp.13-39. nicolau, l. (1989). evaluacion del potencial de licuacion de suelos en la localidad de sampacho. provo de cordoba. unrc, inedito, olsacher,1. (1935). "el terremoto de sampacho, provo de cordoba". revista del museo de ciencias naturales, afio i, n° i, cordoba. panizza, m. (1990). geomorphology and seismic risk. earth science reviews, pp. 11-20. ramirez ra yo, m. y de vicente munos, g. (1997). paleosismicidad en emplazamientos nucleares. consejo de seguridad nuclear, madrid, espana, pp. 250. sagripanti, g., origlla, d. ycampanella, o. (1997). estimacion del riesgo sismico en el ejido urbano de la ciudad de rio cuarto. cordoba. aetas asagai, vol. xi, pp. 188-202. ___ , (1998). sismicidad de una region de intraplaca, departamento de rio cuarto, provincia de cordoba. argentina. aetas confereneia intemaeional "sistemas modemos de preparacion y respuesta ante riesgos sisrnicos, volcanicos y tsunamis". santiago, chile, pp. 281-293. 29 earth sciences research journal geomorphology earth sci. res. sj. vol. 16, no. 2 (december, 2012): 87 94 the geomorphic history of the ainoura plain, kyushu, japan, based on excavation of the monzen ruins hirotsugu mori1, shoichi shimoyama1 and kazuaki soejima2 1 earth and planetary sciences department, kyushu university, fukuoka, fukuoka, japan. e-mail: hmori0622@gmail.com 2 sasebo property research office, education agency, nagasaki prefecture, sasebo, nagasaki, japan abstract the complex coastline of north-western kyushu, japan, consisting of cliffs, steep-sided inlets and archipelagos, has resulted from subsidence in mountainous terrain; the region’s populated river valleys, having narrow alluvial plains, often suffer rock avalanches. the ainoura plain is one such alluvial plain. excavation of the monzen ruins on the ainoura plain by the nagasaki prefectural board of education has revealed the plain’s geomorphic history regarding how the gravels and associated muds and sands were deposited during the last several thousand years. regarding alluvial succession, mud and sand first filled in the trough, possibly as part of the transgression caused by the holocene climatic optimum, and gravel later began to replace parts of the sand and mud layers by successive erosion and sedimentation, thereby raising riverbed level. it was confirmed that such conglomerate from the ainoura plain was derived from the hokusho landslide area in the hinterland because there is no conglomerate layer in the hino plain (a small plain southeast of the ainoura plain). the ainoura river’s thalweg shows that the monzen ruins are located at a sudden reduction in its gradient, thereby accounting for the thickness of coarse clastic material in that area. the reconstructed ainoura plain’s paleoshoreline during the holocene climatic optimum is also reported. the paleoshoreline did not move much until people began to reclaim land; it would thus be expected that the similar deposition would happened on the ainoura plain inside this paleoshoreline. resumen la compleja línea de costa en la región noroccidental de kyushu, japón,  está compuesta por acantilados, ensenadas de laderas inclinadas y archipiélagos, y es producto de la subsidencia de un cinturón montañoso. los valles aluviales y las estrechas llanuras de inundación de esta región se encuentran densamente pobladas y están sujetas a continuos movimientos en masa.  ainoura es un ejemplo de esta llanura de inundación. la excavación de monzen ruin realizada por  “nagasaki prefectural board of education” en la llanura de inundación ainoura revela la historia geomorfológica; la depositacion de gravas, arenas y lodos en los últimos milenios. en la sucesión aluvial, los primeros en llenar el canal fueron los depósitos de lodos y arenas, probablemente como resultado de la transgresión del óptimo climático que ocurrió durante el holoceno.  consecutivamente,  los depósitos de grava empezaron a reemplazar las capas de arena y lodo  mediante la erosión y depositación  sucesiva, elevando el lecho del rio. nosotros corroboramos que el conglomerado de la llanura de inundación ainoura proviene del deslizamiento de tierras de la zona interior de hokusho, ya que la llanura de inundación hino, ubicada al sureste de ainoura, no contiene capas de conglomerados. el lecho del rio ainoura muestra que el monzen ruin se localiza en la abrupta reducción del gradiente, lo cual explica el material clástico grueso en esta área. adicionalmente, nosotros reportamos la paleo-línea de costa de la llanura de inundación de ainoura durante el optimo climático que ocurrió durante el holoceno. la paleolínea de costa no se movió significativamente hasta que la población empezó a recuperar tierra. por consiguiente, nosotros predecimos que el mismo proceso de depositacion podría ocurrir en la llanura de inundación de ainoura al interior de la línea de costa. palabras claves: basalto de kitamatsuura, corrimiento de tierra de hokusyo, llanura de inundación, kyushu, transgreso keywords: alluvial plain, hokusyo landslide, kitamatsuura basalt, kyushu, transgression. record manuscript received: 03/10/2011 accepted for publications: 06/05/2012 hirotsugu mori, shoichi shimoyama and kazuaki soejima88 20 m 100 m 100 m 100 m 300 m 300 m 300 m 500 m 20 m kitamatsuura basalts ainoura river monzen ruin ainoura plain 129°38’e 129°42’e 129°46’e 33°10’n 33°12’n legend reclaimed land sand stone and conglomerate sand and conglomerate sand, mud, and tu� stone basalts fluvial sediment landslide sediment kitamatsuura basalt quaternary rough counter line remnants of landslides 1 km figure 1. topomap of the ainoura plain, sasebo, and its location in northern kyushu, japan. a detailed map around the monzen ruin is shown in figure 2. note that the higher portion of the mountain range is covered by basalts, and landslides have occurred around the boundary between the basalt layer and underlying sand, mud, and tuff layer. small islands other than kyushu island are not depicted. modified from hatano et al. (1974) and matusi et al. (1989). introduction the japanese archipelago is geologically active and therefore suffers from geological disasters; millions of people still live on the islands, coping with disasters. rock avalanches forming part of the process of sedimentation represent an example of such disasters. the hokusho landslide area is located in north-western kyushu (noda, 1957) and the region’s flood plains have suffered many rock avalanches. even so, the area is populated by nearly 400,000 people. understanding the region’s geomorphic history is thus important for predicting future disasters. the ainoura plain is one of the region’s small plains; several ruins have been excavated on this plain, indicating that people have been living on the ainoura plain for at least 6 ka. the monzen ruins represent one such site. the nagasaki prefectural board of education excavated the ruins the geomorphic history of the ainoura plain, kyushu, japan, based on excavation of the monzen ruins 89 figure 2. terrain classification in the ainoura plain showing the major rivers. the monzen ruins occupy a large portion of the narrow part of the floodplain surrounded by mountains and river terraces. mollusk shells were collected from the location indicated by the stars. and published a report (2006); this paper is partly based on their excavation work. their results were used to reveal the plain’s geomorphic history, including being struck by many rock avalanches. geological data the complicated shoreline of north-western kyushu, japan, has resulted from active subduction (shimoyama et al., 1999) combined with erosion. this area lacks flatlands, but there are occasional wide river valleys; the ainoura plain in western sasebo, in which monzen ruins are located, is one of these. figure 1 gives a geological map of the ainoura plain and vicinity. the most widespread stratigraphic units in the region are the sasebo group and the ainoura group consisting of paleogene sand and mud (matui et al., 1989). these groups lie beneath the 6-9 ma “kitamatsuura basalt” (which includes basalt and andesite; kaneoka and aramaki, 1971; matsui and uto, 1998). basalt and andesite cover mountain tops and the structure is unstable. erosion of fragile sand and mud layers causes landslides; accordingly, this type of landslide has been named a hokusho-type landslide by noda (1957) and a detailed landslide map was prepared by hatano et al. (1974a,b). the ainoura plain is an alluvial plain on the ainoura river; the hino river is a tributary of the ainoura river and these two rivers converge near ainoura bay. several ruins are present on the ainoura plain (figure 2). aso (1972) reported discoveries of houses, shell and animal bone middens and tombs from the early jomon era (c.a. 4,000 bc). 400 meters upstream of the shimomotoyama cave ruin, the shitanda ruins were built during the final jomon to middle yayoi era (c.a. 3,000 to 1,000 bc). sasebo municipal council (smc) excavated houses, paddy fields and stone tombs in this ruin from 1990 to 1995 (kumura, 1994). takebe c ruins were built on a hill and were excavated by the nagasaki prefectural board of education (npbe) (nagasaki prefectural board of education, 2008). this included the monzen ruins, located in the middle of the ainoura plain (n 33˚12’06’’, e129˚40’09’’, wgs84), where the ainoura river curves to the south (figure 2). npbe excavated the ruins and published a report in 2006 and 2008; they discovered archaeological remains from the initial jomon era (c.a. 5,000 bc) to the modern era, such as earthen vessels, shell middens, houses and tombs. the wooden tombs from the yayoi era (c.a. 300 bc) have special archeologically importance as they are some of the oldest wooden tombs found in japan. their excavation also unearthed clay pots, ceramic wares made in china and in the kinki region (near kyoto), as well as agricultural tools, such as a wooden spade. they also found at least five ancient channels from inside the monzen ruins and unearthed several archaeological items from these channels’ conglomerate deposits. most items from the ruins showed little evidence of abrasion. npbe and smc have conducted research on plant residues and phytoliths from the monzen and shitanda ruins. research on the monzen ruins indicates that the site was a moist environment surrounded by evergreen broadleaf forest. however, the amount of rice phytolith was not enough to affirm rice cultivation (npbe, unpublished). on the contrary, research on the shitanda ruins revealed rice phytoliths, indicating that rice might have been cultivated there (kumura, 1994). the non-abraded items and existence of the rice phytolith suggested that people were living on the plain. however, these ruins are characterised by having been built on an alluvial plain which has experienced several destructive avalanches. the chronology of japanese history, especially concerning when the yayoi era began, is an active area of research, currently lacking consensus (ozuka, 2000; onishi, 2005; yamamoto, 2007: nakahashi and iizuka, 2008). this research has been based on the conservative view that sets the start of yayoi era around 2,300 years ago (ozuka, 2000). methods a geological cross-section of the ainoura plain was first prepared along the ainoura river and the hino river from drill core data provided by smc, northern nagasaki prefecture development and promotion bureau and the fujinaga geological and construction corporation. a topographic cross-section was created based on a 50 m-mesh digital elevation model published by the geological information authority of japan (2002), using kashmir 3d software. hirotsugu mori, shoichi shimoyama and kazuaki soejima90 figure 3. geological and topographic sections along the ainoura (a) and hino rivers (b). drilling logs were used to draw the closed circles and triangles to draw a geological cross-section of figure 4; the former is along the transect ab and the latter is along the transect cd of the small topographic map. flood deposits in the ainoura plain could not be found in the hino river flood plain. the monzen ruins are located near the sudden reduction in the gradient. (a) the paleoshoreline’s approximate position was determined by using drill data: holocene deposits including marine shells and others which were continental. the location of the paleoshoreline was estimated from the intersection of the lowest marine deposits with the current terrain. the holocene marine layer’s maximum extension represents the paleoshoreline during the holocene climatic optimum (4000bc), when the sea level was at its highest. shell fossils were collected from the bank of the hino river (figure 2). the plain’s depositional history was investigated by plotting sediment layers’ elevation changes. the ages of layers determined by npbe (2008) was used mainly by means of seriation of potteries. when cultural objects were not found, 14c from fallen trees and tephra derived from the sakurajia volcano was also used. referring tephra helped to solve the problem of secondary deposition in some players. the elevations were adjusted by applying the modern gradient of the ainoura river near the ruins, (b) the geomorphic history of the ainoura plain, kyushu, japan, based on excavation of the monzen ruins 91 measured as 3.89/1,000 using an auto level. accordingly, 38.9 cm would be added to the elevation of a sediment layer 100 m downwards from the centre of the ruins along the paleoriver channel, for example. results a cross-section (figure 3) of the ainoura plain showed that the monzen ruins were located near a sudden reduction in the gradient and that the flood deposit layer was relatively thick here. figure 3a also represents the cross-section of the ainoura plain. from bottom to top, the plain consists of a paleocene unit, a non-marine pleistocene unit, a non-marine holocene unit, flood deposits after the holocene climatic optimum and human-made deposits. many basaltic conglomerates were included in this plain in the non-marine holocene unit. a geological cross-section of the hino river is shown in figure 3b. although layers similar to those of the ainoura river were found in this region, no flood deposits were observed. the following mollusk shells were collected from the bank of the hino river: granosa, macoma tokyoensis makiyama, anadotia stearnsina, pephia undulata, pistris capsoides, batillaria zonalis, cerithideopsilla cingulata, turbo cornatus coreensis, cryptonatica tigrina, cerithideopsilla djadjariensis, figure 4. the distribution of the drilling data and the estimated paleoshoreline of the ainoura plain. the paleoshoreline was estimated based on marine deposit distribution, shown as the intersection of the lowest marine deposits with the current terrain. the marine deposit was reported from locations marked with closed and open circles, but not from locations marked with closed and open triangles. the number in italics is the marine layer elevation at each location. hirotsugu mori, shoichi shimoyama and kazuaki soejima92 nassarius livescens and hemifusus ternatanus. most of these mollusks lived in the intertidal zone. the 4,000 bc paleoshoreline was estimated (figure 4). beside the drill data, a wave-cut notch found on a hillside was considered as evidence of the paleoshoreline. figure 4 shows that the monzen ruins were about 1 km away from the shoreline; the shoreline at the hino river was about 1 km away from the current shoreline. figure 5 shows how mud, sand and conglomerate layers developed on the plain. figure 5a show that sand and mud layer levels from the initial jomon era were 1 to 2 m lower than those of later periods. the levels of sand and mud layers rose suddenly about 4,000 bc, then stopped rising. figure 5b shows that levels of conglomerate layers have risen since the last 5,000 bc and four to five debris flows presumably brought the gravels striking the monzen ruins. discussion the ainoura plain’s paleogene unit consists of sand and mud layers having nearly horizontal stratification, covered by kitamatsuura basalts. landslides easily occur in such an area when underground water pressure rises. noda (1957) reported that the area has experienced many such landslides. comparing the cross-sections of the hino and ainoura rivers (figure 3) revealed differences in their depositional histories. considering the shell fossils from the hino river, it was inferred that the hino plain was originally an intertidal flat dominated by marginal marine sedimentation that subsequently became damp, subaerially exposed ground. by contrast, basaltic gravels from the ainoura plain must have either been derived from landslides near the ruins, or carried from farther upstream. since there is no evidence of landslides near the ruins (figure1), it was assumed that the basaltic gravels of the monzen ruins were carried by the ainoura river; fluvial influence is thus more pronounced than marine influence on the ainoura plain. a geological cross-section of the ainoura plain (figure 3) revealed that the monzen ruins were located near the sudden reduction in the gradient, and where the flood layer is thick. this suggested that such location was where the river’s flow rate suddenly drops and sedimentation occurs. these gravels did not begin to be deposited until 1,000 bc (figure 5b) at the location of the monzen ruins. gravel became successively deposited over the conglomerate layers after that, leading to the rising of the riverbed. such trend was dissimilar to that of mud and sand layers (figure 5a). mud and sand layers most likely responded to changes in river water level which was influenced by changes in sea level. yokoyama et al., (1999) reported sea level change in tsumizu, western kyushu, 50 km south-east of the ainoura plain. the sea level change there was characterised by a sudden rise in sea level around 5,000 to 3,000 bc and a subsequent gradual fall, similar to the sand-mud deposition pattern on the ainoura plain. since the geomorphic history of western kyushu is complicated (yokoyama et al., 1996; shomoyama et al., 1999), one cannot be certain whether the ainoura plain experienced the same change in sea level as at tsumizu, but it is possible that changes in ainoura plain elevation record/reflect changes in sea level in western kyushu. figure 4 shows that the paleoshoreline was located about 1-2 km seaward from the monzen ruins. after the holocene climatic optimum, the shoreline has been gradually prograding due to sedimentation; however, the cross-section (figure 3) showing locations between the paleoshoreline and the end of flood deposition is quite close. it was thus concluded that progradation did not alter the shoreline much, at least until the 17th century, when the japanese began to reclaim the land (nagata 1994). it can thus be expected that the deposition which happened at the location of the monzen ruins described above probably also happened in the region between the monzen ruins and the estimated paleoshoreline, accounting for almost half of the ainoura plain. conclusions this research was focused on the geomorphic history of the ainoura plain based on the monzen ruins. most of the strata of the ainoura plain consists of basaltic conglomerate (flood deposit) and sand. such basaltic gravels were derived from the kitamatsuura basalts in the hinterland; such gravel represents landslide debris carried by floods to the plain to be deposited. the geographical cross-section revealed that the monzen ruins were located at the sudden reduction in the river gradient, more flood sediments thus being deposited there. it was also found that mud and sand first filled in the trough, probably as part of the jomon transgression caused by the holocene climatic optimum. later, conglomerate began to replace sand and mud layers by successive erosion and sedimentation, thus raising the riverbed. around the final jomon era (c.a. 500 bc), depositional elevation levels of gravel surpassed those of sand and mud. considering the position of the paleoshoreline, such scenario is probably applicable to about half of the ainoura plain. acknowledgments data regarding the monzen ruins was kindly provided the nagasaki prefectural government’s excavation office, saseabo city municipal office and the northern nagasaki prefecture’s development and promotion bureau. the fujinaga geological and construction corporation provided the drilling core data. professor yoshiyuki tanaka from the social and cultural studies graduate school, kyushu university, suggested how to interpret wooden tombs. dr. ramues gallois from the british geological survey, dr. patrick druckenmiller and dr. rainer newberry proofread the paper. ms. susana salazar jaramillo prepared the spanish abstract. this research was financially assisted by emeritus professor tatsuro matsumoto takachiho scholarship, kyushu university. references aso, s. (1972): “shimomotoyama iwakage” otsuka kogei sya, tokyo, japan. hatano, s., okabe, f., and watanabe, y. (1974a): “compilation of 1:50,000 geomorphological classification map of landslide landforms in hokusho district, kyushu, japan”. notes of cooperative research for disaster prevention 32, 3-6. hatano, s., okabe, f., watanabe, y., and furukawa, t. (1974b): “a catalogue of large-scale landslide landforms formed in the past in hokusho (kitamatsuura) district, north-western kyushu, japan”. notes of cooperative research for disaster prevention 32, 7-23. kaneoka, i., and arakami, s. (1971): “k-ar age of plagioclase from a kitamatsuura basalt and the reliability of plagioclase k-ar age”. journal of the geological society of japan, 77. 6. 389-392. kumura, s. (1994): “excavation of shitanda ruin report” sasebo city, sasebo, japan. matsui, k., and utsu, k. (1998): “1:200.000 geological map “karatsu””. chishitsu news 521, 23-24. matsui, k., suda, t. and hiroshima, t. (1989): “1:200.000 geological map, nagasaki prefecture, ni-52-17”. geological survey of japan, tokyo, japan. nagasaki prefectural board of education, (2006): “excavation report of buried cultural properties involved in the construction of sasa-sasebo national route 497 vi: monzen ruin”. research report cultural property research office of sasebo, nagasaki prefecture, 190. nagasaki prefectural board of education, (2008): “excavation report of buried cultural properties involved in the construction of sasasasebo national route 497 v: takebe c ruin and takebe d ruin”. research report cultural property research office of sasebo, nagasaki prefecture, 3. the geomorphic history of the ainoura plain, kyushu, japan, based on excavation of the monzen ruins 93 figure 5. depositional history of the sand, mud (a) and gravel layers (b) on the ainoura plain. horizontal bars represent rough possible age ranges and vertical bars represent altitudes for layers connected along the river gradient. cross-section diagrams prepared during excavations were referred to for the elevations of the layers. the layers’ ages were determined by seriation, 14c dating and tephra. bars marked with an * are layers whose ages were determined by 14c and # by tephra. sand and mud layers’ depositional elevations rose around 5,000 to 4,000 bc, thereby agreeing with the change in sea level. the gravel layers began to be deposited around 1,000 bc, then already becoming replaced by deposited sand and mud layers. (a) (b) initial jomon early jomon middle jomon middle jomon final jomon kofunyayoi kodai mid-ageedo m od er n sea level legends mud sand conglomerate el ev at io n (m ) 4.5 4.0 4.0 5.0 5.5 6.0 6.5 7.0 7.5 8.0 el ev at io n (m ) age 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 4 0 2 -2 -6 -8 -4 se a le ve l c ha ng e of w es te rn k yu sh u (m ) 5000 b.c. 4000 b.c. 3000 b.c. 2000 b.c. 2000 a.d.1000 b.c. 0 b.c. 1000 a.d. depositional trend of sands and muds depositional trend of gravels elevation range possible age range # # # * * hirotsugu mori, shoichi shimoyama and kazuaki soejima94 nagasaki prefectural board of education, (2008): “excavation report of buried cultural properties involved in the construction of sasasasebo national route 497 vi: monzen ruin ii”. research report cultural property research office of sasebo, nagasaki prefecture, 4. nagata, k. (1994): “evolution of land improvement districts in japan”. short report series on locally managed irrigation, 6. noda, m. (1957): “karatsu sasebo ryo tanden nai no jisuberi ni tuite (report on landslides in the coal field of karatsu and sasebo)”. kyushu kozan gakkai shi (journal of mining engineering of kyushu), 25, 11, 443-452. nakahashi, t.and iizuka, m. (2008): “anthropological study of the transition from the jomon to the yayoi periods in the northern kyushu using morphological and paleodemographical features (2)” anthropological science (japanese series), 116, 2, 131-143. onishi, s. (2005): “a review of the recent debate about the date of yayoi period”. anthropological science (japanese series), 112, 2, 95-107. ozuka, s. (2000): “desk-sized chronological table of japanese history augmented edition” iwanami books, tokyo, japan. shimoyama, s., kinoshita, h., miyahara, m., tanaka, y., ichihara, t. and takemura, k. (1999): “mode of vertical crustal movements during the late quaternary in kyushu, japan, deduced from heights of ancient shorelines”. tectophysics, 302, 1-2, 9-22. yamamoto, n. (2007): “archaeology integrates the humanities and science” takashi shoiun, tokyo, japan. yokoyama, y., nakada, m., maeda, y., nagaoka, s., okuno, j., matsumoto, e., sato, h. and matsushima, y. (1996): “holocene sea-level change and hydro-isostasy along the west coast of kyushu, japan”. palaeogeography, palaeoclimatology, palaeoecology, 123, 29-47. the study investigated the feasibility of applying the depth-invariant index algorithm to delineate the shallow underwater geomorphological features using quickbird imagery. although the technical procedures of the depthinvariant index are already demonstrated in the professional literature, the algorithm needs to be applied and tested in several coastal regions with different seawater conditions and properties. some of the geomorphological features in the coastal zone of the gaza strip are seasonal features such as underwater sand bars. however, since these features are non-permanent and dynamic, it would be useful to utilize satellite data to detect changes in both locations and depth, and also to monitor this feature in shallow coastal water. the importance of identifying the underwater sand bars in the coastal zone can be significant in the case of rip currents, and swimming conditions. additionally, using satellite imagery for detecting dynamic underwater coastal morphologies may enable local authorities to adopt such an algorithm with a combination of satellite imagery for fast, efficient and extensive coverage mapping. while information about seafloor features can be obtained using active satellite data such as radar or lidar imagery, active satellite data are not available and require advanced processing and logistical resources, which are difficult to obtain in the case of the gaza strip. the study instead used passive quickbird satellite imagery to map and delineate the seabed morphologies through the calculating the depth-invariant index for the spectral bands. by using the depth-invariant index, it was possible to distinguish and delineate the major underwater sand bars within the entire coastal zone of the gaza strip. also, the spectral and spatial resolution of quickbird imagery exhibited a substantial capability in mapping and delineating the underwater seabed morphologies. el estudio investigó la factibilidad de aplicar el algoritmo índice de profundidad invariante para delinear las características geomorfológicas del suelo bajo el agua a través de las imágenes quickbird. a pesar de que los procedimientos técnicos del índice de profundidad invariante se han demostrado ampliamente en la literatura profesional, el algoritmo debe ser aplicado y verificado en varias regiones costeras con diferentes condiciones y propiedades del agua marina. algunas de las condiciones geomorfológicas en la zona costera de la franja de gaza dependen de las estaciones, como las barreras de arena bajo el agua. sin embargo, como estas condiciones son dinámicas, sería útil usar la información satelital para detectar cambios en las locaciones y en la profundidad, y para vigilar estas condiciones en el agua costera poco profunda. la importancia de identificar las barreras de arena en la zona costera puede ser determinante en el caso de corrientes violentas o para las condiciones de inmersión. adicionalmente, el uso de imágenes satelitales para determinar las dinámicas morfológicas costeras bajo el agua podría permitir a las autoridades locales adoptar un algoritmo que combinado con dichas imágenes presente un mapeo más rápido, más eficiente y de mayor cobertura. si bien la información sobre las características del suelo oceánico pueden obtenerse con información satelital activa como las imágenes de radar o lidar, la información no está disponible, además de requerir un procesamiento avanzado y recursos logísticos, que son difíciles de obtener en el caso de la franja de gaza. este estudio, por su parte, utiliza imágenes satelitales pasivas de quickbird para mapear y delinear las morfologías del lecho del mar a través de calcular el índice de profundidad invariante para las bandas espectrales. al utilizar el índice de profundidad invariante fue posible determinar y delinear las mayores barreras de arena en toda la costa de la franja de gaza. la resolución espacial y espectral de las imágenes de quickbird mostraron una capacidad sustancial en el mapeo y delineamiento de las morfologías del lecho marino abstract resumen keywords: remote sensing, seabed morphology, depth-invariant index, gaza strip, palestine. palabras clave: teledetección, morfología del suelo marino, índice de profundidad no variable, franja de gaza, palestina. mapping of underwater seabed morphology of the gaza strip coastal zone using remote sensing technique record manuscript received: 19/04/2015 accepted for publication: 26/04/2016 how to cite item abualhin, k. (2016). mapping of underwater seabed morphology of the gaza strip coastal zone using remote sensing technique. earth sciences research journal, 20(2), c1-c7. doi:http://dx.doi.org/10.15446/esrj.v20n2.50256 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.50256 khaldoun abualhin al-azhar university-gaza, gaza, gaza strip, p.o. box 1277, palestine, khaldon@alazhar.edu.ps earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : c1 c7, mapeo de la morfología del suelo oceánico en la zona costera de la franja de gaza a través de la técnica de detección remota h y d r o g e o l o g y c2 khaldoun abualhin introduction several techniques and algorithms have been developed to use satellite imagery for environmental monitoring and coastal habitats mapping. the mapping of underwater seabed morphologies includes several features such as massive valleys, canyons, rocky outcrops, geomorphological features, and sand bars [goudie, 2004; pavlopoulos et al., 2009]. seabed morphologies are an important element of coastal protection and planning elements and also in mapping marine habitats and their management. accordingly, it is necessary for the researcher to get detailed information about seabed morphology and habitats in the nearshore coastal zone. recently, modern methods of marine surveying such as echo sounder and active satellite sensors like lidar data are used for scanning the seabed. the advantages of these modern techniques of mapping are on the accuracy and also a full range of depth coverage. on the other hand, these methods of seabed mapping are expensive, time-consuming, and difficult to process. in contrast, passive remotely sensed data have been shown to offer a reasonable alternative in providing reliable and low-cost information about coastal habitats and morphologies [mumby and edwards, 2002; mumby et al., 2004]. different passive satellite imagery has been used in studying coral reef and classification, benthic habitats classification, sand bottoms delineation and in other studies [ahmad and neil, 1994; andréfou t et al., 2003; edwards et al., 1999; green et al., 2000; hochberg and atkinson, 2003; michalek et al., 1993; mishra et al., 2006; mumby et al., 1998; mumby and edwards, 2002; mumby and harborne, 1999; mumby et al., 2004; ong et al., 1998; pasqualini et al., 2005; stumpf et al., 2003; vanderstraete et al., 2003]. mishra et al. (2006) utilized quickbird multispectral data for identifying and classifying tropical-marine benthic habitats. in 1998, ong and hick combined spectral data and digital bathymetry to auto-classify larger benthic habitats. pasqualini et al., (2005) used spot5 satellite imagery to map seagrasses. mumby et al., (1997a,b, 1998) compared the descriptive resolutions of various remote sensing methods for mapping coral reef habitats of the turks and caicos islands. other researchers simulated ikonos data for different seabeds and coral reef habitat mapping [andréfouet et al., 2003; hochberg and atkinson, 2003]. mumby et al., (2004) provided a general review of what can and cannot be mapped using passive remote sensing data and summarized the capacity of various methods to predict and derive bathymetry from different types of optical imagery [jupp, 1988; stumpf et al., 2003]. in terms of satellite spectral and spatial resolutions, multispectral imagery with coarse and medium spatial resolution, landsat tm, landsat etm+, aster, and ikonos satellite data are considered useful tools for monitoring the spatial dynamics of habitats [mumby and edwards, 2002; mumby and harborne, 1999]. however, taking into consideration the similarity in spectral resolution and the width of the spectral band between quickbird and ikonos sensors, the ability of quickbird data to discriminate seabed morphology in the shallow coastal region is comparable to ikonos data. in this study, quickbird satellite imagery was used to delineate the seabed morphology of sand bars mainly in the nearshore of the gaza strip coastal zone. in addition to oceanographic factors, the coastal region of the gaza strip suffers from intensive anthropogenic pollution and other manmade interventions along the entire coastal zone. moreover, as a result of israel’s blockade of the gaza strip and the subsequent limitation on resources, it is not always possible to use the most recent technologies and tools for mapping and monitoring the coastal zone, including the mapping of underwater morphologies. as it is important to get up-todate information about seabed morphologies for monitoring the changing conditions of the coastal zone, a quickbird satellite image was used to extrapolate the underwater morphology of sand bars along the coastal zone of the gaza strip. underwater bars can defined as an aggregation of ridges of sediments whose formation, morphology and behavior were determined by interactions between waves, currents, tides, local slope and grain size. bowman and goldsmith (1983) delineated a wide range of nearshore sand bars and shoal patterns along the southeastern mediterranean coastline. their study identified three major types of sand bars: non-rhythmic parallel-meandering bar; inner single-crescentic bars; and, double-crescentic bars [bowman and goldsmith, 1983]. study area the gaza strip is a part of historical palestine and is located on the south eastern mediterranean coast and bordered by sinai from the south and the negev desert from the east (figure 1). the current area of the gaza strip is 360 km² with an estimated human population density of 4167 per km². the geographic coordinates of the lower left and upper right corners of the gaza strip are 31.31 n, 34.2 e; and 31.53 n, 34.56 e. the shoreline of the gaza strip is a smooth line that extends 42 km. the coastal zone includes sand dunes in the south and north, coastal cliffs, and the wadi gaza mouth area. wadi gaza is the main seasonal river in the gaza strip and influence the biodiversity in the gaza strip than it does today. figure 1. map of the gaza strip, palestine methodology seabed feature and underwater patterns can be mapped from passive remote sensing imagery if the influence of the water column was removed. the implementation of the physical relation between spectral reflectance and light attenuation is the base of extracting information from multispectral satellite imagery over water bodies. the incident radiation on the water column is absorbed and scattered or reflected unequally. the attenuation influence can be estimated over the entire multispectral image by getting water depth at several pixels in the image [lyzenga, 1981]. however, to neutralize the influence of seawater on the reflected signal, an attenuation coefficient must be calculated at each pixel in the image [green et al., 2000]. lyzenga (1978, 1981) proposed a simple image-based technique to combine the information in various spectral wavelengths to produce the depth-invariant index. in this method, the ratio of water attenuation coefficients are required and optical properties considered uniform. depth-invariant index the concept of extracting bottom-type information in shallow water depends on the fact that the reflected seabed-radiance is a linear function of bottom reflectance and an exponential function of water depth [lyzenga, 1978]. the technique to calculate depth-invariant index involves three main steps: (1) generate a linear relationship between depth and radiance; (2) calculate the ratio of attenuation coefficients of band pairs; and, (3) calculate the depth-invariant index of bottom type (figure 2). c3mapping of underwater seabed morphology of the gaza strip coastal zone using remote sensing technique figure 2. flow chart for generating depth-invariant index since the relationship between the depth and reflected radiance over water bodies is an exponential relation, this relationship can be transformed using a natural logarithm (equation 1) to become a linear relationship. for a given bottom type, the transformed radiance in bands (i) and (j) is linearly (figure 3). the transformed radiance will attenuate linearly with increasing water depth. xi = ln(li) (1) where has transformed reflectance in the band (i). li has atmospherically corrected radiance in the band (i). ln is the natural logarithm. accordingly, the transformed radiance in the band (i) is plotted versus the transformed radiance in the band (j) at variable depth and constant bottom type (figure 3. the result falls along a straight line. consequently, different bottom types should provide different parallel lines [lyzenga, 1981]. the slope of the lines represents the ratio of attenuation coefficients for each spectral band (figure 4 and figure 5). figure 3. linear exponential relationship between reflected radiance and depth using natural logarithms. the gradient of the line represents the ratio of attenuation coefficient. (band i has a shorter wavelength, and, therefore, attenuates less rapidly, than band j) figure 4. scatter plot of the natural logarithm (log) of quickbird radiance. quickbird band 4 was plotted versus band 3, 2 and 1. (the dn values were obtained from 257 pixels over sandy bottom-type at varying depths in the coastal zone of the gaza strip.) figure 5. scatter plot of the natural logarithm of quickbird radiance in the band (3) versus radiance in the band (4) (the quickbird image for the gaza coastal zone was acquired on 21 january 2009) the ratios of the attenuation coefficients of the spectral bands were derived from the scattering bi-plot. the slope of the scatter plot trend line represents the ratio of attenuation coefficients (figure 5). the slope of the trend line will depend on the variable chosen to be the dependant variable. accordingly, using equation 2, the ratio of attenuation coefficients (ki/kj) will not depend on the order the variable. where, (2) (3) where σii and σjj are the variances for i and j measurements respectively, and σij is the covariance between i and j bands. once the ratios of attenuation coefficients are calculated, the depthinvariant indexes can be calculated using equation 3 for the entire image. each pair of spectral bands produces a single depth-invariant index for the bottom type. implementation of depth-invariant index algorithm on quickbird image a quickbird satellite image acquired on 21 january 2009 at 08:35 gmt was used to calculate the depth-invariant indices and then to map the seabed morphology along the coastal zone of the gaza strip. the quickbird image was radiometrically and geometrically corrected. the image was registered to the topographic map with rms less than 0.4 pixels (table 1). the atmospheric correction was then performed using the envi flaash module. the homogeneous regions in the satellite image with a uniform bottom reflectance were selected to be used for attenuation coefficient calculation. the ratio of attenuation coefficients was calculated based on the equation 2. table 2 shows the calculated attenuation coefficients ratios (ki/kj) for each pair of spectral bands. table 1. satellite data characteristics c4 khaldoun abualhin table 2. the calculated variance (σii, σjj), covariance (σij) and the ratio of attenuation coefficients (ki/kj) for each pair of logarithmic transformed quickbird bands results and discussion several depth-invariant indices were calculated from quickbird imagery for the coastal zone of the gaza strip, portraying different abilities to recognize seabed morphology. for example the calculated index from band 1 and band 3 demonstrated the ability to map features in shallow depth up to 10 m. the index derived from band 2 and band 3 showed the feasibility of mapping features at shallow water depth. the index of band 2 and band 4 as well as an index of the band 3 and band 4 obtained reasonable results in mapping the very shallow coastal bars at depth varying from 0.0 -1.5 m. these bars welded to the coastline. accordingly, different types of seabed bars were clearly observed and delineated at different locations in the entire coastal zone and nearshore zone of the gaza strip. types of bars are often distinguished based on their shape and orientation about the shoreline. they may be linear, sinuous or crescentic, or they may consist of alternating transverse bars, which are welded to the shoreline and are separated by channels occupied by rip currents, as seen in figure 6 [goudie, 2004]. figure 6. idealized cross-shore barred profile (a) and some examples of bar types [goudie, 2004]. the mapped sand bars in the coastal zone of the gaza strip were similar to those observed by [almagor and hall, 1984; bowman and goldsmith, 1983; goldsmith et al., 1982], in the south-eastern mediterranean coast. some of these sand bars are seasonal occurrences while others occur throughout the year. the delineated underwater sand bars in the gaza strip coastal zone were as follows: single-crescentic bars; anvil-shaped transverse bars, oblique transverse bars, initial crescentic bars, double crescentic bars and triple crescentic bars. figure 7 shows the different types of underwater sand bars delineated in the nearshore of the gaza strip. as mentioned above, depth-invariant indices demonstrated disparities in their sensitivity to mapping seabed features in the shallow coastal region. for instance, double and triple crescentic bar systems can be delineated using the ratio index of band 1 and band 2. the ratio of band 2 and band 3 achieved a respectable result in mapping different sand bar systems at different locations. the indices of band 2 and band 4 and of band 3 and band 4 showed less capability in mapping sand bar features. in addition, the calculated depth-invariant indices provided a dimensionless bottom relief in the nearshore zone of the gaza strip. several bottom relief profiles were generated perpendicular to the coastline (figure 7 and figure 8). figure 8 shows a variation in the seabed relief on both sides of the fishing port. this variation may be the result of sediments being transported from the nile delta to the southeastern mediterranean nearshore shelf by northward longshore currents [rohrlicht and goldsmith, 1984; zviely et al., 2007]. since the sediments are trapped and accumulated at the southern side of the fishing port, the sand supply to the northern side of the fishing port has decreased. two sandbars appeared at the southern side of the fishing port and located at a distance of 100 m up to 400 m from the coastline. figure 9 illustrates the bottom relief of the general morphology and distribution of the underwater sand bars. a. transverse bars (oblique) c. single crescentic bar d. double crescentic bars b. double crescentic bars system figure 7. example of sand bars observed along the coastal zone of the gaza strip. (the bars were extracted from quickbird imagery using depth-invariant index technique [lyzenga, 1981]) c5mapping of underwater seabed morphology of the gaza strip coastal zone using remote sensing technique d. double crescentic bars b. double crescentic bars system figure 8. the bottom relief on the southern side of the seaport and up to 400m seaward. (the figure reveals the bars crest and trough about the distance of 500 m from the shoreline. the profile was obtained from the calculated depth-invariant index for quickbird imagery.) figure 9. bottom relief profiles. (the profiles were extracted from the calculated depth-invariant index for quickbird band 2 and band3. the horizontal component of the patterns was relatively accurate where the vertical component is pseudo-depth.) c6 khaldoun abualhin utilizing the envi topographic modelling tool, the slope, the aspect and the shaded relief image were derived from the seabed indices (figure 10). index from band 1 and band b3 demonstrated a slope gradient of the gaza strip coastal zone ranging from 0.57° to 0.41° at a distance of 50 m to 100 m from the coastline. figure 10. shaded relief surface generated from the depth-invariant index of quickbird band 2 and band 3. (the sand bars appear in three sets: the first one attached to the coastline, the second located within a very shallow water from 2-4 m, and the third located at 200-400 m distance from the coast at a depth of 7-10 m.) conclusion gathering information about seabed morphology and bottom profiles requires advanced instruments and sometimes it is a time-consuming process. data collection may also be complicated by challenging field and logistical circumstances beyond the control of researchers, as has been the case in the gaza strip. nevertheless, remotely sensed imagery has provided an alternative method for generating bathymetric maps with reasonable accuracy. in particular, the utilization of the depth-invariant index to map and delineate underwater seabed morphology has proven effective. the study confirmed the possibility of deriving underwater seabed morphology in shallow water. the calculated indices provided interesting results on mapping seabed morphology such as sand bars and bottom profile. the method enables mapping and delineates the major underwater sand bars along the entire coastal zone of the gaza strip. importantly, various types of seabed morphologies were clearly observed and defined at different regions and depths in the gaza strip coastal zone such as crescentic bars, single-crescentic bars, double-crescentic bars, triple crescentic bars and transverse bars. some bar systems were located onshore and attached to the shoreline at a very shallow depth of up to 2 meters, as was the case for transverse bars, which then moved offshore to form crescent bars. consequently double crescent bars are formed at a deeper depth and distance from 150 up to 400 meters offshore. finally, as the nearshore region of the gaza strip is homogeneous and consists mainly of sandy sediments, the spectral and spatial resolution of quickbird imagery were useful in distinguishing natural variations and contrasts due to optical physical properties of coastal habitats. quickbird imagery exhibited a substantial capacity to delineate underwater seabed morphology by using depth-invariant index technique. the achieved findings were quite satisfying and consistent with published information on the region. acknowledgment special gratitude is extended to the daad (german exchange service) for funding support and the purchase the quickbird image. references ahmad, w., & neil, d. 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(2009). mapping geomorphological environments. berlin: springer verlag. rohrlicht, v., & goldsmith, v. (1984). sediment transport along the southeast mediterranean: a geological perspective. geo-marine letters, 4, 99-103. stumpf, r., holderied, k., & sinclair, m. (2003). determination of water depth with high-resolution satellite imagery over variable bottom types. limnology and oceanography, 48, 547-556. vanderstraete, t., goossens, r., & ghabour, t. (2003). can aster-data be used for bathymetric mapping of coral reefs in the red sea using digital photogrammetry? in, proceedings of the second workshop of the earsel special interest group on remote sensing for developing countries (pp. 212-218). bonn, germany. zviely, d., kit, e., & klein, m. (2007). longshore sand transport estimates along the mediterranean coast of israel in the holocene. marine geology, 238, 61-73. enero%2031-geociencias-vol%2012-2%20dic-en%20baja[1].pdf earth sciences research journal earth sci. res. j. vol. 12, no. 2 (december 2008): 235-264 multidisciplinary approach to studymigmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa carlos a. zuluaga c.1 and harold h. stowell2 1 departament of geosciences, universidad nacional de colombia. 2 department of geological sciences, university of alabama, tuscaloosa, al 35487. abstract the nason ridge migmatitic gneiss of the cascades core is a migmatitic unit comprising concordant pelitic schist and gneiss, amphibolite, and tonalite gneiss, and cross cutting tonalite, quartz-rich granitoid, and pegmatite. there are several generations of ‘igneous’ lithologies (leucosomes = tonalite, quartz-rich granitoid, and pegmatite) some of which are concordant; others clearly crosscut the strongly deformed host rocks. the host rocks are interpreted to be chiwaukum schist with metasedimentary (pelitic schist and some gneiss) and metavolcanic (amphibolites) origins. metamorphic fabric in the nason ridge migmatitic gneiss is characterized by preferred orientation of platy minerals (continuous schistosity), compositional layering, mineral lineations (elongate grains and grain aggregates), and non-coaxial deformational features (asymmetric augen, grain offsets, rotated porphyroblasts, etc.). compositional layering is characterized by quartz-plagioclase lenses and patches (mm to cm scale) and by large variations in biotite content. this composite fabric is faulted and folded by mesoscopic structures. the most strongly foliated leucosomes (gneissic tonalites) are generally concordant with the regional trend of foliation, while weakly foliated leucosomes (tonalites) and pegmatite veins crosscut host rock and tonalite gneisses. thin melanosome layers (biotiteand amphibole schist) are developed locally around quartz – plagioclase lenses and patches. metamorphism in the nason ridge migmatitic gneiss and the nearby chiwaukum schist likely peaked after intrusion of the mt. stuart batholith ca. 91-94 ma. peak temperatures and pressures for the nason ridgemigmatiticgneiss in thewenatchee ridge and pacific crest areas were 650 720 °c and 6 9 kbar with a pressure increase of £ 2.0 kbar during metamorphism. thermodynamic modeling indicates that hydrous partial melting would begin at ca. 660 °c and is relatively pressure independent. field and petrographic observations, mineral chemistry and thermobarometry, and bulk rock chemistry and thermodynamic modeling of phase equilibria (pseudosections) applied to the nason ridge 235 manuscript received: june 10th, 2008. accepted for publication: november 11th, 2008. 236 carlos a. zuluaga and migmatitic gneiss indicate that at least some of the leucosome bodies were derived by local partial melting. the clearly intrusive character and the sharp contacts between some tonalite leucosome bodies and host rock support an externally derived origin for these tonalite melts. however, some of these bodies may have originated from partial melting of the host chiwaukum schist and traveled a short distance before crystallization, or have been modified by deformation so as to obscure textural evidence for local derivation. results are compatible with derivation of leucosome rocks in the nason ridgemigmatitic gneiss from two non-exclusive processes: partial melting of the host rock and intrusion of externally derived tonalite melts. resumen el canto del nason migmatitico gneiss de las cascadas core es una unidad migmatìtica que concuerda con el pelitico schist y gneiss, amphibolite, y tonalite gneiss, y atraviesa cortando el tonalite, rico en cuarzo granitoide, y pegmatita. hay varias generaciones de lithologias ígneas (leucosomes = tonalite, cuarzo rico en granitoide y pegmatite) algunos de los cuales son concordantes; otros claramente cortan transversalmente las rocas deformándoles. las rocas son interpretadas para ser chiwaukum schist con metasedimentary y metavolcanic rigins. la telametamórfica en el canto nasonmigmatiticgneiss es caracterizada por la orientación preferida de minerales platy, la acodadura compositiva, el mineral lineations, y rasgos de deformational no coaxiales. la acodadura compositiva es caracterizada por lentillas de-cuarzo-plagioclase y parches y por variaciones grandes en el contenido de biotite. esta tela compuesta es tachada y doblada por estructuras demesoscopic. el más fuerte foliated leucosomes son generalmente concordante con la tendencia regional de foliación, mientras débilmente foliated leucosomes y el corte transversal de venas pegmatite reciben la roca y tonalite gneisses. melanosome delgado(fino) se encama son desarrollados en la zona alrededor de lentillas de-cuarzo-plagioclase y parches. metamorphism en el canto nason migmatitic gneiss y chiwaukum cercano schist probablemente alcanzó su punto máximo después de la intrusión del mt. california de estuardo batholith 91-94 mamá. temperaturas máximas y presiones para el canto nason migmatitic gneiss en el canto wenatchee y la cresta pacífica áreas eran 650-720 °c y 6-9 kbar con una disminución de presión de £ 2.0 kbar durante metamorfismo. el modelado termodinámico indica que la fusión hydrous parcial beging en la california 660 ºc y es relativamente la presión independiente. el campo y observaciones petrographic, la química mineral y thermobarometry, y la química de roca de bulto y el modelado termodinámico de fase equilibra aplicado al canto nason migmatitic gneiss indican que al menos algunos cuerpos leucosome fueron sacados por la fusión local parcial. el carácter claramente intruso y los contactos agudos entre algún tonalite leucosome cuerpos y la roca de anfitrión apoyan un origen por fuera sacado para estos tonalite se derrite. sin embargo, algunos de estos cuerpos pueden haber provenido de la fusión parcial del anfitrión chiwaukum schist y viajar una distancia corta antes de la cristalización, o han sido modificados por la deformación para obscurecer pruebas de textural para la derivación local. los resultados son compatibles con la derivación de rocas de leucosome en el canto nason migmatitic gneiss de dos procesos no exclusivos: la fusión parcial de la roca de anfitrión y la intrusión de tonalite por fuera sacado se derrite. palabras clave: introduction this paper presents a multidisciplinary methodology to fully characterize a migmatitic unit: the nason ridge migmatitic gneiss (nrmg). the nrmg is one of three metamorphic culminations in the cascades magmatic arc of the cascades crystalline core (cascades core). the origin and metamorphic history of the nrmg constrains the deep crustal evolution of the magmatic arc; however, its origin is enigmatic and few data are available to constrain interpretations. the unit has been interpreted as one of themost deeply exhumed parts of the nason terrane (brown andwalker, 1993;miller and paterson, 2001). multiple techniques are used to elucidate the origin of the nrmg migmatites exposed in the wenatchee ridge area (figure 1). techniques include: petrographic analysis, thermobarometric calculations and p-t pseudosections. pseudosections are used to construct quantitative p-t paths for metamorphism and to predict conditions for partial melting. thermobarometry and p-t pseudosections indicate that garnet grew over temperatures from 550 to 700 °c with a negligible to moderate pressure increase of £ 2.0 kbar. p-t estimates from thermobarometry and pseudosection modeling support petrographic interpretations that partial melting produced leucosome quartz – plagioclase lenses in the nrmg. methods textural analysis changes that rocks experience during metamorphism may be recorded in the mineralogy and texture. partial melting of a rock suite generally produces identifiable petrographic characteristics that yield information about metamorphism and tectonic events. macroscopic textures are the first and the simplest criteria that can be used to identify if a suite of rocks had been formed by partial melting. the presence of melanosome layers or patches (e.g., biotite selvages) provides the best evidence of local melt formation, and the presence of leucosome (rich in non-ferro237 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 1. generalized geologic map of the wenatchee block in the cascades core, wa. note the distribution of the main geologic units in the nason terrane: chiwaukum schist, nason ridge migmatitic gneiss, and mt. stuart batholith. magnesian minerals – generally quartz and feldspar), where the melt collected (sawyer, 1999). thin section analysis of textures and mineral assemblages was used to identify mineral assemblages that may have undergone melting and the potential melt forming reactions (e.g., sawyer, 1999) and/or microscopic textures generally linked with partial melting processes (sawyer, 1999; mehnert et al., 1973; ashworth and mclellan, 1985). these microscopic textures include: 1) thin films of plagioclase, quartz, and k-feldspar along grain boundaries (crystallized melt), and 2) melt-solid reaction textures. macroscopic features, assigned to partial melting, are readily identified in some mesosome rocks from the nason ridge migmatitic gneiss. on the other hand, microscopic features related to partial melting cannot be identified in nason ridge migmatitic gneiss rocks; likely because of extensive deformation. however, mineral parageneses identified in thin sections are important for constraining thermodynamic models. bulk rock chemistry whole-rock compositions were determined by x-ray fluorescence from fused glass discs (samples were analyzed by activation laboratories, ltd. and at the university of alabama analytical facilities). one sample (00nc9d) was analyzed in both laboratories for interlaboratory comparison. bulk rock samples were ground on a diamond embedded lap to remove surfaces that were obviously weathered or cut by the rock saw. approximately 30 g of resulting ‘fresh’ sample was washed, and rinsed in acetone and 2m hcl before jaw crushing and grinding to a powder in a steel ring-and-puck mill. samples prepared and analyzed at the university of alabama were dried in two steps (120 ºc followed by ca. 1000 ºc), mixed with flux (lithium tetraborate 67% lithium metaborate 33%) in a 1:9 proportion (sample/flux), and combined with a drop of lithium bromide non-wetting agent. this mix was fused in a platinum-gold crucible using a gas burner, and cast into a 32 mm diameter disc using a platinum-gold mold. glass discs were analyzed with the university of alabama phillips pw2400 x-ray fluorescence spectrometer equipped with a rh x-ray tube. calibration was based on 15 to 20 certified rock standards per element. mineral chemistry quantitative mineral analyses and x-ray maps were collected with the jeol 8600 electron probe microanalyzer at the university of alabama using wavelength dispersion spectrometry. major element analyses were collected with a 1 to 20 mm diameter beam at a current of 20 na under a 15 kv accelerating potential. raw counts from characteristic x-ray peaks were converted to weight percent oxides by comparison to natural mineral and synthetic standards, using the citzaf correction technique of armstrong (1984). count times ranged from 30 to 45 seconds. operating conditions for collection of x-ray mapswere 15 kv accelerating potential, 75 to 300 na beam current, and a 1 mm beam. count times ranged from 50 to 100 ms pixel. thermodynamic modeling and p-t paths for metamorphism several methods have been used for constructing p-t paths for rocks (spear and selverstone, 1983; spear, 1988; st-onge, 1987; stowell et al, 2001, tinkham, 2002). the p-t paths constructed here follow the methods of vance and mahar (1998), stowell et al. (2001), and stowell and tinkham (2003). garnet rim thermobarometry was used to estimate p-t at peak metamorphic conditions with the average p-t routine of thermocalc (v. 3.21; powell and holland, 1988; powell et al., 1998) using externally calculated activities. activities were calculated using pressures and temperatures close to the estimated p-t conditions, then input in thermocalc for linearization of reactions or for average p-t calculation (powell and holland, 1994). activities and p-t estimates were refined by iteration until calculation and estimated temperatures and pressures differ in less than 5 ºc and 0.1 kbar, respectively. estimates for peak pressures and temperatures were further refined with pseudosection fields following the technique presented in zuluaga et al. (2005). garnet core composi238 carlos a. zuluaga and tions were plotted as the compositional variables spessartine, grossular, and iron number (fe# = fe/fe+mg) in p-t pseudosections (isopleths). ideally, the three isopleths intersect at a single point, but frequently this is not the case because of the uncertainties in analytical data and in model calculations. however, the area bounded by isopleth intersections provides an estimate for initial garnet growth p-t conditions. initial garnet growth p-t estimates were integrated with garnet rim thermobarometry to provide a simplified finite p-t path for garnet growth. pseudosections were constructed using the computer program thermocalc and the thermodynamic database of holland and powell (1998) with the silicate melts model extension (holland and powell, 2001; white et al., 2001; th pdata files created february 13, 2002). all thermodynamic models used the ninecomponent oxide system: mno, na2o, cao, k2o, feo, mgo, al2o3, sio2, and h2o (mnnckfmash) because this is the minimum system needed to realistically predict mineral stability for garnet-bearing pelites (tinkham et al., 2001). except for the melt phase, activity models used here are the same as those used and discussed in tinkham et al., (2001). melt activity models are the same as those presented in holland and powell (2001) andwhite et al (2001). regional geology the cascades core of the north cascades and the coast plutonic complex to the north represent the roots of a mesozoic to early tertiary magmatic arc. mesozoic metamorphic rocks and cretaceous to tertiary plutons crop out in the cascades core in a mosaic of amalgamated terranes. the overall tectonic history of the cascades core has been discussed in several publications (e.g., brown et al., 1994; evans and davidson; 1999; miller et al., 1994; tabor et al., 1993). the post metamorphic high angle entiat fault divides the cascades core into two tectonic blocks with different thermal histories, the wenatchee and chelan blocks (miller et al., 1994; miller and paterson, 2001; haugerud et al., 1991). this paper focuses on the wenatchee block and does not discuss the chelan block. the most prominent metamorphic rock units in the wenatchee block are part of the nason terrane. the nason terrane consists of dominantly metasedimentary chiwaukum schist and the migmatitic nason ridge migmatitic gneiss. the earliest metamorphic event in the chiwaukum schist was a poorly understood pre-mount stuart amphibolite facies regional metamorphic event (m r 1 )(e.g., evans and davidson, 1999). m r 1 mineral assemblages were overprinted by minerals that grew during buchan style dynamic contact metamorphism associated with the mount stuart batholith (m c 2 )(evans and berti, 1986) and other late cretaceous plutons. late cretaceous contact metamorphism was followed by barrovian style regional metamorphism (m r 3 )(evans and berti, 1986; evans and davidson, 1999; tinkham, 2002). rocks adjacent to late cretaceous plutons typically containm c 2 andm r 3 mineral assemblages: for example, andalusite + cordierite ± garnet are typical of m c 2 , and staurolite + kyanite + garnet are typical of m r 3 (evans and berti, 1986; tinkham, 2002). chiwaukum schist dominantly comprises metasedimentary rocks (aluminous biotite-rich schists) and lesser amounts of metavolcanic rocks (amphibolite) with penetrative foliation, predominantly continuous schistosity, and lineation defined by mineral alignment. the nason ridge migmatitic gneiss contains biotite-rich and/or muscovite-rich schist, amphibolite, quartzite, and minor calc-silicate layers, and layers, lenses, patches, and veins of granitoid rocks yielding a migmatitic texture (van diver, 1967). thermobarometry in the nason terrane yields temperatures of 500-700 °c and systematic trends in pressure increasing from ca. 3 kbar, in the south to ca. 9 kbar in the northeast (brown and walker, 1993; tinkham, 2002). several tectonic models have been developed to explain the metamorphic and structural features of the cascades core. these models can be grouped into two types: (1) orogen normal contraction, produced by bulk-shortening in a pure shear setting (whitney and mcgroder, 1989; mcgroder, 1991; whitney, 1992a; whitney et al., 1999; paterson et al., 2004; stowell et al., 2007) and (2) orogen 239 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa parallel strike slip in a simple shear setting (brown and talbot, 1989; brown and walker, 1993; brown et al., 1994; walker and brown, 1991). thermal relaxation signature characterized by a fairly rapid pressure increase followed by temperature increase during garnet growth supports the orogen normal contraction model with loading by a tapered thrust sheet (stowell et al., 2007). late cretaceous thrusting is preserved at the southern margin of the wenatchee block, where the ingalls ophiolite complex was thrust over the chiwaukum schist (windy pass thrust). other evidence for thrusting is observed on the western border of the cascades core where an assemblage of oceanic sedimentary and volcanic rocks, were thrust onto the magmatic arc along a complex array of faults known as the northwest cascades system. metamorphic stretching lineations throughout the cascades core show a horizontal nw-se preferred orientation and shear sense features indicate non-coaxial right-lateral motion (brown and talbot, 1989). evidence for non-coaxial deformation includes asymmetric augen and porphyroclasts, rotated (snowball) porphyroclasts, s-c fabrics, and grain offsets. strain partitioned folding might have been the cause for the lack of structural evidence for thrusting and steepening of paleobarimetric gradients (stowell et al., 2007). textural and compositional description of the nason ridge migmatitic gneiss rosenberg (1961) and van diver (1967) reported the first detailed studies of the nason ridge migmatitic gneiss. rosenberg (1961) subdivided the chiwaukum schist into the “whittier peak unit” and “poe mountain unit”, the last being the equivalent of the nason ridge migmatitic gneiss. van diver (1967) produced a detailed petrographic study of these rocks in the wenatchee ridge area. he interpreted that this unit formed by granitization of chiwaukum schist following a migmatization front that focused around the wenatchee ridge orthogneiss. magloughlin (1986, 1989, and 1993) determined metamorphic conditions from thermobarometric calculations and described pseudotachylites and other cataclastic rocks in the chiwaukum schist and nason ridge migmatitic gneiss on wenatchee ridge. taylor (1994) and miller and paterson (2001) presented results from structural studies on the chiwaukum schist, which displays a strong composite schistosity resulting from transposed cycles of folding. tinkham (2002) and stowell and tinkham (2003), reported garnet sm-nd geochronology and p-t-t paths for rocks at the western end of the nason ridge migmatitic gneiss near heather lake (figure 1). these studies indicate that garnet grew at ca. 86 – 88 ma (after mt. stuart emplacement ca. 93.5 ma) during the latter stages of crustal loading recording 0 to £ 2 kbar of pressure increase along the heating path. the nason ridge migmatitic gneiss is an elongate northwest to southeast oriented body within the chiwaukum schist (figure 1). gradational contacts with the adjacent chiwaukum schist have been used to infer that the nason ridgemigmatitic gneiss originated from a chiwaukum schist protholith. in the wenatchee ridge area, nason ridge migmatitic gneiss is composed mainly of schist and gneiss with lesser volumes of tonalites, pegmatites and amphibolites. the rocks are classified into leucosomes, mesosomes, and melanosomes; following that scheme, textures and mineralogy for each lithology are discussed below. figures 2 and 3 portray the lithological and structural features of the nason ridge migmatitic gneiss and sample localities discussed here along wenatchee ridge and the pacific crest. table 1 summarizes the most important petrographic features of samples described in the text. leucosomes leucosomes include a variety of igneous-like lithologies, which have variable composition, textures, and field relations with other units (figure 3; tables 1 and 2). these units are generally tabular to sub-tabular in geometry and have thicknesses that range from cm to m scale (figure 3). three compositional groups are observed: 1) tonalites, 2) granodiorites, and 3) quartz-rich granitoids. varia240 carlos a. zuluaga and 241 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 2. geologic map of the wenatchee ridge area, cascades core, wa. location provided on fig. 1. data to the northeast and to the southwest from van diver (1967). 242 carlos a. zuluaga and f ig u re 3 . o u tc ro p m ap o f n as o n r id g e m ig m at it ic g n ei ss ea st o f p o e m o u n ta in , w en at ch ee r id g e, w a . t h is m ap il lu st ra te s co n ta ct re la ti o n s an d d is tr ib u ti o n o f m es o so m e an d le u co so m e li th o lo g ie s. n o te al so st ru ct u re s at ce n ti m et er to m et er sc al e. 243 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa t a b le 1 . p et ro g ra p h y o f n as o n r id g e m ig m at it ic g n ei ss li th o lo g ie s, c as ca d es c o re , w a . s a m p le s q tz p l k fs m s b t h b l c a m g rt k y s il t u r e p a cc e ss o ri e s n a m e ; te xt u re s l e u co so m e s 0 1 n c 2 a 3 5 3 0 1 0 2 0 5 g rt , c z o , z rn , r t g ra n o d io ri te ; f g -s f , iz -m i, d 0 1 n c 2 b 4 5 4 0 1 5 r t. p e g m a ti te to n a lit e ; w f , d 0 1 n c 2 c 7 0 3 0 t r t r t r q tz -p l le n s; g ra n o b la st ic 0 1 n c 3 b 3 0 3 5 3 5 e p , a p , z rn , r t, g rt t o n a lit e ; m g -w f , m i, d 0 1 n c 8 a 4 0 3 5 1 5 5 a p , z rn , e p t o n a lit e ; s f , m i, c 0 1 n c 8 c 2 3 7 5 1 m s, s p n , a p , z rn , c h l p e g m a ti te to n a lit e ; c g -w f , iz , c 0 1 n c 9 a 3 5 3 7 1 5 e p , a p , z rn . g ra n o d io ri te ; f g -m g -s f , m i, p i, d 1 0 1 n c 9 b 4 0 3 5 1 5 1 0 b t, a p , z rn , e p , c z o , o p g ra n o d io ri te ; m g -s f , m i, d 0 1 n c 1 5 c 4 5 3 5 1 5 5 e p , a p , c h l t o n a lit e ; f g -w f , iz , d 0 1 n c 1 5 a 6 0 2 0 2 0 g rt , a p , z rn . p e g m a ti te q tz -g ra n it o id ; w f , m i, d m e so so m e s 0 1 n c 2 c 5 5 1 5 2 2 0 5 2 r t, z rn , a p g rt -k y -t u r g n e is s; le p -g ra n 0 1 n c 3 a 5 5 2 0 t r 2 5 t r a p , z rn , r t b tg rt sc h is t; le p id o b la st ic 0 1 n c 6 5 0 2 0 3 1 5 5 5 t r t r a p , z rn , r t( 2 % ), c z o g rt -k y sc h is t; le p id o b la st ic 0 1 n c 8 d 7 8 t r 8 0 2 s p n (3 % ), a p , r t, c h l g rt a m p h ib o lit e ; le p id o b la st ic 0 1 n c 9 c 4 0 3 3 1 2 5 1 t r a p , r t, z rn g rt sc h is t; le p id o b la st ic 0 1 n c 9 d 4 0 3 0 2 1 7 5 3 t r c h l (3 % ), r t, z rn , a p g rt -k y sc h is t; le p -g ra n 244 carlos a. zuluaga and s a m p le s q tz p l k fs m s b t h b l c a m g rt k y s il t u r e p a cc e ss o ri e s n a m e ; te xt u re s 0 1 n c 1 0 7 7 5 5 3 0 c z o , r t, s p n a m p h ib o lit e ; le p id o b la st ic 0 1 n c 1 5 b 4 5 3 0 1 5 5 1 1 z rn , r t( 3 % ), a p , c h l g rt -k y sc h is t; le p id o b la st ic 0 1 n c 5 2 a 4 0 3 5 2 0 2 1 m n z , a p , r t, il m , z rn , g r g rt -s il g n e is s; le p id o b la st ic 0 1 n c 5 4 3 0 4 0 5 1 5 7 t r z rn , il m (3 % ), a p , c h l g rt -h b l sc h is t; le p id o b la st ic m e la n o so m e s 0 1 n c 2 c 8 4 1 0 7 0 5 3 b t sc h is t; le p id o b la st ic 0 1 n c 8 b 7 t r t r 3 9 0 z rn , a p , r t, e p a m p h ib o lit e ; le p id o b la st ic 0 1 n c 4 0 3 t r 2 9 5 z rn , a p b t sc h is t; le p id o b la st ic w f = w ea k fo li at io n s f = st ro n g fo li at io n iz = p l id io b la st ic zo n in g m i = m ir m ek it ic in te rg ro w th p i = p er th it ic in te rg ro w th f g = fi n e g ra in ed m g = m ed iu m g ra in ed c g = co ar se g ra in ed p eg . = p eg m at it ic c = c o n co rd an t d = d is co rd an t m in er al ab re v ia ti o n af te r k re tz (1 9 8 3 ). a ll n u m b er s ar e m o d es o b ta in ed m o st ly b y v is u al co m p ar is o n w it h m o d e es ti m at io n ch ar ts . 245 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa t a b le 2 . w h o le ro c k a n a ly se s o f n a so n r id g e m ig m a ti ti c g n e is s lit h o lo g ie s, c a sc a d e s c o re , w a . s io 2 t io 2 a l 2 o 3 f e o f e 2 o 3 m n o m g o c a o n a 2 o k 2 o p 2 o 5 l o i t o ta l l e u co so m e 0 1 n c 2 a 7 0 .3 0 .1 9 1 6 .1 n .d . 1 .5 0 .0 2 0 .4 1 .6 3 .7 2 .9 0 .0 6 1 .2 5 9 8 .2 1 n c 2 b 7 5 .8 0 .0 4 1 4 .8 n .d . 0 .9 0 .0 0 0 .2 1 .2 4 .3 1 .6 0 .0 3 1 .2 3 1 0 0 .1 0 1 n c 2 c 7 8 .8 0 .0 5 1 2 .3 n .d . 0 .8 0 .0 0 0 .2 2 .6 3 .9 0 .2 0 .1 1 0 .5 6 9 9 .5 0 1 n c 3 b 7 5 .3 0 .0 5 1 7 .2 n .d . 0 .8 0 .0 0 0 .2 1 .5 3 .4 2 .4 0 .0 4 1 .1 7 1 0 2 .0 0 1 n c 8 a 7 1 .2 8 0 .2 8 5 1 6 .2 1 1 .8 4 0 0 .0 3 0 .9 4 2 .5 5 3 .9 6 2 .0 5 0 .1 0 .9 7 1 0 0 .2 0 1 n c 8 c 7 4 .2 4 0 .0 6 4 1 5 .1 6 0 .8 6 0 0 .0 0 8 0 .2 3 3 .0 5 5 .2 8 0 .7 2 0 .0 2 0 .7 8 1 0 0 .4 0 1 n c 9 a d k 6 9 .9 0 .3 8 1 7 .1 n .d . 2 .4 0 .0 0 0 .6 2 .4 4 .2 2 .8 0 .1 3 0 .6 8 1 0 0 .7 0 1 n c 9 a lt 6 8 .9 0 .2 0 1 6 .4 n .d . 1 .6 0 .0 0 0 .4 2 .1 3 .8 3 .1 0 .0 8 0 .8 6 9 7 .4 0 1 n c 9 b 7 2 .7 0 .0 8 1 6 .0 n .d . 0 .9 0 .0 0 0 .2 1 .3 3 .3 4 .3 0 .0 7 0 .8 2 9 9 .7 0 1 n c 4 0 7 2 .1 0 .1 4 1 6 .6 n .d . 0 .9 0 .0 2 0 .4 2 .5 5 .3 1 .0 0 .0 4 0 .6 1 9 9 .7 0 3 n c h s 3 b 7 0 .0 0 .2 6 1 6 .6 n .d . 2 .4 0 .0 0 1 .0 2 .6 4 .3 0 .8 0 .0 6 0 .3 7 9 8 .4 l e u co so m e -l e n se s 0 1 n c 1 5 b 8 5 .8 0 .1 0 7 .3 n .d . 1 .9 0 .0 0 0 .3 1 .5 1 .7 0 .3 0 .0 7 0 .3 9 9 9 .3 0 1 n c 5 2 a 8 1 .1 0 .0 7 1 0 .1 n .d . 1 .5 0 .0 1 0 .3 2 .6 2 .6 0 .2 0 .2 4 0 .3 0 9 9 .0 0 2 n c 7 7 9 .8 0 .0 3 1 1 .8 n .d . 0 .9 0 .0 0 0 .1 2 .3 3 .2 0 .1 0 .0 4 0 .1 6 9 8 .5 0 2 n c o u t1 9 7 2 .4 0 .1 1 1 4 .8 n .d . 2 .1 0 .0 1 0 .5 3 .2 4 .5 0 .5 0 .0 7 0 .4 4 9 8 .6 0 3 (9 9 )n c 5 7 p 7 3 .2 0 .0 5 1 4 .8 n .d . 1 .2 0 .0 0 0 .4 4 .4 4 .5 0 .3 0 .7 2 0 .2 3 9 9 .9 0 3 n c h s 3 b 7 6 .0 0 .0 6 1 5 .2 n .d . 1 .1 0 .0 0 0 .3 2 .8 4 .0 0 .2 0 .1 1 0 .2 0 1 0 0 .0 246 carlos a. zuluaga and s io 2 t io 2 a l 2 o 3 f e o f e 2 o 3 m n o m g o c a o n a 2 o k 2 o p 2 o 5 l o i t o ta l m e so so m e 9 6 n c 6 7 (0 3 ) 6 2 .5 0 .8 1 1 7 .3 n .d . 8 .4 0 .1 5 3 .0 1 .7 1 .9 2 .0 0 .0 9 2 .0 7 9 9 .9 9 9 n c 3 7 6 6 .5 0 .7 8 1 5 .8 5 .3 2 0 .7 0 .1 1 2 .9 1 .7 2 .0 2 .3 0 .1 7 1 .1 0 9 9 .4 0 1 n c 2 -c 5 9 .6 0 .8 1 1 7 .7 n .d . 7 .7 0 .1 3 2 .8 2 .4 3 .4 3 .0 0 .2 6 2 .0 5 9 9 .8 0 1 n c 3 -a 6 3 .1 0 .8 2 1 6 .1 n .d . 7 .4 0 .1 0 3 .0 2 .2 3 .2 2 .9 0 .1 7 0 .9 8 1 0 0 .1 0 1 n c 6 5 8 .4 0 .8 8 1 8 .8 n .d . 8 .1 0 .1 1 3 .1 2 .6 3 .3 2 .6 0 .2 0 2 .7 6 1 0 0 .9 0 1 n c 8 d 4 8 .3 2 .2 8 1 3 .3 1 0 .0 9 2 .6 0 .2 3 6 .3 3 1 2 .0 1 .4 0 .7 0 .2 2 0 .8 7 9 8 .3 0 1 n c 9 c 6 5 .6 0 .7 2 1 5 .8 n .d . 6 .6 0 .1 0 2 .8 2 .2 3 .0 2 .6 0 .1 5 0 .8 4 1 0 0 .4 0 1 n c 9 d 6 4 .9 0 .8 3 1 5 .7 5 .5 8 0 .8 0 .1 2 3 .1 2 .0 2 .5 2 .6 0 .1 6 1 .8 8 1 0 0 .2 0 1 n c 1 5 b 6 7 .3 0 .7 3 1 3 .2 n .d . 6 .5 0 .0 8 2 .6 2 .4 2 .4 1 .7 0 .1 7 1 .3 9 9 8 .4 0 1 n c 5 2 a 6 0 .3 0 .8 9 1 8 .3 n .d . 8 .0 0 .1 0 3 .1 2 .8 2 .8 1 .8 0 .2 2 2 .4 3 1 0 0 .7 0 1 n c 5 4 4 9 .6 1 .6 4 2 1 .2 9 .7 1 .9 0 .2 5 2 .5 7 .6 3 .4 0 .9 0 .1 6 0 .2 7 9 9 .2 0 2 n c 3 b 6 4 .1 0 .7 8 1 6 .0 n .d . 7 .3 0 .1 0 2 .7 2 .4 3 .0 2 .1 0 .1 3 1 .3 2 9 9 .9 m e la n o so m e 0 1 n c 2 c 4 5 .8 1 .8 2 2 .6 4 n .d . 1 6 .1 0 .3 5 .6 1 .0 1 .1 2 6 .3 1 0 .3 n .d . 1 0 1 .1 (d ry ) 0 1 n c 8 b 5 4 .4 0 .2 1 8 .4 6 .4 1 1 .1 0 .2 4 1 3 .9 1 0 .6 0 .8 0 .7 0 .1 7 1 .5 8 9 8 .6 0 1 n c 4 0 4 1 .5 2 .6 7 1 8 .1 n .d . 1 7 .7 0 .1 4 7 .8 0 .5 0 .4 8 .4 0 .3 3 2 .2 0 9 9 .8 (1 ) w h en n o f eo is re p o rt ed al l ir o n is as su m ed as f e3 + a ll v al u es re p o rt ed as w ei g h t p er ce n t. b u lk -r o ck co m p o si ti o n s w er e d et er m in ed b y x -r ay fl u o re sc en ce an al y si s o f fu se d g la ss d is cs w it h t h e u n iv er si ty o f a la b am a p h il li p s p w 2 4 0 0 x -r ay fl u o re sc en ce sp ec tr o m et er eq u ip p ed w it h a r h x -r ay tu b e. c al ib ra ti o n w as b as ed o n 1 5 to 2 0 ce rt if ie d ro ck st an d ar d s p er el em en t. 247 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 4. outcrop photograph and sketch showing general relations between lithologies in the nason ridge migmatitic gneiss, wenatchee ridge, wa. locality 01nc9 (see fig. 2). the sledgehammer in the center of the picture is 0.4 m long. a thick amphibolite layer (~25 m) is observed above and left of a tonalite gneiss, these lithologies are cross-cut by pegmatites. within the tonalite gneiss and just below teh amphibolite contact is observed a dark lens of amphibole schist. tion in the type of mica present (muscovite, biotite, or both) and variation in the content of mica, garnet, and tourmaline are the most definitive expression of compositional differences between groups. texturally, these rocks can be differentiated on the basis of grain size and metamorphic foliation. grain size is variable from fine grained to pegmatitic; pegmatites are tonalites to quartz-rich granitoids, but fineto medium-grained leucosomes have a larger compositional range from granodiorite to tonalite. the de248 carlos a. zuluaga and figure 5. examples of cross cutting relations between lithologies in the nason ridgemigmatitic gneiss, wenatchee ridge, wa. a. a strongly-foliated tonalite crosscuting gneissose mesosome and weakly foliated tonalite. note the two thin concordant non-foliated tonalites. b. two foliated leucosomes interfingering with schistose mesosome. c. weaklyto non-foliated leucosomes crosscutting schistose mesosome. d. weakly-foliated pegmatite leucosome cross cutting schistose mesosome and weakly-foliated fine-grained tonalite concordant with schistose mesosome. 249 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa table 3. mineral chemistry, nason ridge migmatitic gneiss, cascades core (wa) 01nc15b (grt-ky schist) garnet core 5 anal. 01nc52a (grt-sil gneiss) garnet core 29 anal. rep. average cations rep. average cations oxides oxides s.d. oxides oxides s.d. sio2 36.72 36.71 0.09 2.96 37.01 37.13 0.16 2.96 tio2 0.11 0.22 0.23 0.01 0.02 0.05 0.10 0.00 al2o3 21.32 21.20 0.20 2.02 21.63 21.74 0.15 2.05 cr2o3 feo 26.13 26.45 0.29 1.78 28.81 28.71 0.20 1.93 mgo 1.33 1.33 0.02 0.16 2.18 2.30 0.06 0.28 mno 5.81 5.59 0.16 0.38 5.32 5.30 0.12 0.35 cao 8.05 8.06 0.24 0.70 5.18 5.33 0.13 0.45 na2o k2o total 99.47 99.56 100.15 100.56 02nc3b (grt-ky schist) garnet core-20 anal. 01nc8b (amp. schist) amphibole 20 anal. rep. average cations rep. average cations oxides oxides s.d. oxides oxides s.d. sio2 36.78 36.55 0.15 2.95 47.81 47.90 0.43 6.93 tio2 0.15 0.14 0.02 0.01 0.28 0.30 0.03 0.03 al2o3 21.13 21.31 0.13 2.03 11.23 10.94 0.38 1.86 cr2o3 0.22 0.16 0.04 feo 27.38 27.44 0.23 1.85 8.55 8.43 0.22 1.02 mgo 1.21 1.25 0.04 0.15 14.68 14.70 0.20 3.17 mno 5.16 5.16 0.08 0.35 0.28 0.27 0.03 0.03 cao 7.98 7.91 0.15 0.68 11.81 11.91 0.15 1.85 na2o 1.02 1.04 0.07 0.29 k2o 0.75 0.69 0.10 0.13 total 99.80 99.76 96.63 96.33 (1) the average of all analyses is reported together with the standard deviation and a representative analysis (2) oxides = oxide weight percent (3) cation are calculated based on 11 oxygens for biotite, 12 oxygens for garnet, 8 oxygens for plagioclase, and 23 oxygens for amphibole gree of foliation development varies from almost non-foliated to gneissic. foliation is defined by alignment of micas, compositional layering, and elongation of quartz and plagioclase. compositional layering is more common within the thickest concordant gneissic leucosomes, where it is defined by variations in biotite content. foliation in leucosomes is dominantly parallel to the regional trend of foliation observed in mesosomes (figure 3). pegmatite leucosomes are non-foliated to weakly-foliated and generally crosscut mesosomes, but fineto medium-grained leucosomes show both concordant and discordant relations with mesosomes (figure 4 and figure 5). fine-grained leucosome bodies are weaklyto strongly foliated and show complex outcrop interrelationships, some weakly-foliated bodies crosscut strongly-foliated bodies (figure 4) and some gneissic bodies crosscut weakly-foliated bodies (figure 5a). contacts with adjacent rocks are generally sharp and there is no evidence for contact metamorphism. small-scale relict igneous textures are common in pegmatite leucosomes, but are less common in finer grained rocks. these textures include idioblastic compositional zoning in plagioclase (most commonly simple zoning, and rarely oscillatory) and myrmekitic intergrowths of quartz and plagioclase. leucosome rock types are grouped, according to the degree of foliation development, into strongly foliated and weakly to non-foliated. this scheme emphasizes the variable deformation style and is readily appli250 carlos a. zuluaga and figure 6. characteristics of strongly foliated leucosomes, nason ridge migmatitic gneiss. locality 01nc8. a. compositional layering at cm scale observed in outcrop. b. thin section photomicrograph of sample 01nc8a showing foliation defined by muscovite and biotite alignment, and compositional layering expressed as variable biotite content. cable as objective criteria for field classification. quartz-plagioclase lenses are grouped with weakly to non-foliated rocks because they do not show internal foliation features. leucosome bodies with the exception of quartz – plagioclase lenses, constitute 16% to 43% of the outcrop area, along wenatchee ridge (figure 2). the proprotion of these leucosomes drops to ca. 8% near the contact with the adjacent chiwaukum schist in the north. strongly-foliated rocks. strongly-foliated leucosomes are both concordant (figure 4) and discordant (figure 5a) with mesosome fabrics. mica alignment and alternating ferromagnesian-rich and quartz feldspathic-rich layers (figure 6a) define foliation. variation in biotite content at cm scale is the most notable expression of compositional layering (figure 6b). variation in grain size between layers is from fineto medium-grained. this group of rocks ranges in composition from tonalite to granodiorite, contains one (muscovite) or two (muscovite + biotite) micas, and commonly contains: quartz + plagioclase + muscovite ± biotite ± garnet. bulk rock chemical analyses (table 2) reveals that these rocks are composed mainly of silica and aluminum (sio2+al2o3 ~ 90%) and that they have low iron and magnesium contents (fe2o3+mgo < 2%). in general, these rocks are gneissic with granoblastic texture. within the light colored biotite-poor layers, quartz-feldspar microlithons (< 0.2 cm) alternate with thin discontinuous muscovite microlithons (< 0.02 cm). quartz is xenoblastic with arrested grain boundaries, undulatory extinction, and needle-like rutile inclusions aligned parallel to gneissosity. plagioclase is xenoblastic or less commonly subidioblastic, regularly has idioblastic compositional zoning (figure 7), and albite twinning. pericline and carlsbad twinning are rare. plagioclase commonly contains relatively large muscovite grains parallel to cleavage (figure 7a), epidote, and occasionally quartz inclusions. plagioclase can rarely be seen in vermicular intergrowths with quartz (figure 7b). potassium feldspar is xenoblastic and is pervasively altered to white mica (kaolinite, muscovite, and/or paragonite). muscovite and biotite are subidioblastic and platy; both define gneissosity. zircon inclusions are common in biotite. garnet is an 251 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 7. photomicrographs illustrating relict igneous textures in leucosomes lithologies, nason ridge migmatitic gneiss, wa. a. subidioblastic simple zoning in plagioclase. b. quartzplagioclase mirmekitic intergrowth. c. idioblastic oscillatory zoning in plagioclase. accessory phase, idioblastic to subidioblastic and nearly inclusions-free. weaklyto nonfoliated rocks. these leucosome rocks are discordant and concordant bodies with compositions that range from tonalite to quartz-rich granitoid. grain size varies from fine grained to pegmatitic (figure 8a) and mineralogy is quartz + plagioclase + muscovite with biotite and garnet as minor to accessory phases. quartzplagioclase discontinuous lenses that occur in varying proportions within the gneisses and schists are grouped in this category. chemically, these lithologies are very similar to strongly foliated bodies (table 2), except the quartz – plagioclase lenses, which have lower al2o3 content (~ 12%), slightly higher sio2 content, and slightly lower fe2o3+mgo. weakly gneissose bodies have a granoblastic texture (figure 8b). quartz is xenoblastic with undulatory extinction. plagioclase is xenoblastic, but some small grains are subidioblastic with lath shape. twinning is uncommon, but when present is pericline or rarely albite twinning. quartz inclusions are common in plagioclase, and as in gneissic bodies, plagioclase is altered to muscovite along cleavage planes and in some cases to sericite, 252 carlos a. zuluaga and figure 8. characteristics of weak to non-foliated leucosomes, nason ridge migmatitic gneiss,wa. a. outcrop aspect of a non-foliated (two-mica tonalite) and a weak foliated leucosome (muscovite pegmatite tonalite). b. photomicrographs of the two-mica tonalite, no foliation is observed and mica grains are randomly orientated. 253 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 9. characteristics of mesosome garnet-kyanite gneiss, sample 01nc2c, nason ridge migmatitic gneiss, wa. a. layering at cm scale with abundant leucosome lenses. b. thin melanosome layers are observed on each side of the leucosome lenses. c. photomicrograph of the gneiss, mm scale leucosome, melanosome, and mesosome layers can be recognized. d. photomicrograph under plane polarized light of the mesosome portion of the gneiss, observe the abundance of garnet. e. photomicrograph of the melanosome portion, biotite, garnet, and quartz are the main components, tourmaline and muscovite are also present. f. photomicrograph of the mesosome portion showing the most common mineral paragenesis: garnet + kyanite + quartz + plagioclase + biotite + muscovite. 254 carlos a. zuluaga and figure 10. characteristics of mesosome schist and amphibolite, nason ridge migmatitic gneiss, wa. a. fineto medium-grained with < 10% leucosome lenses. b. photomicrograph in plane polarized light of a garnet-kyanite schist, note the large garnet porphyroblasts and the quartz-plagioclase lens (no associated selvage) in the lower portion of the photograph. c. amphibolite layer with alternating dark hornblende-rich bands and thin light colored bands with quartz, plagioclase, and epidote. d. photomicrograph of a garnet amphibolite composed mainly of hornblende, quartz, epidote, and small garnet porphyroblasts, sphene is accessory and observed evenly distributed. 255 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa figure 11. mesoscopic structures observed on wenatchee ridge, wa. a. quartz veins with tight isoclinal folding. b. boudinage in pegmatite. c. similar isoclinal folding in biotite-garnet-kyanite gneiss. d. similar folding in biotite-garnet schist. epidote, and clinozoisite. muscovite is idioblastic to subidioblastic, with platy form, and its alignment defines the gneissosity. large grains of plagioclase and quartz have cracks filled with finer grained aggregates of quartz, muscovite, and plagioclase. the quartz – plagioclase lens-shaped bodies have long dimensions of 0.05 to >1 m and are typically parallel or at low angle to the dominant foliation. they are composed mainly of quartz and plagioclase with minor amounts of biotite (figures 9b and 9c). quartz is xenoblastic and plagioclase is xenoblastic to subidioblastic. the plagioclase is commonly altered to muscovite and epidote. quartz-plagioclase lenses are locally associated with thin discontinuous selvages of biotite-rich schist that are described below as melanosomes. melanosomes biotite schist. these thin (generally £ 2 cm thick) biotite-rich layers only occur directly adjacent to quartz-plagioclase leucosome lenses in the gneisses (figure 9b). the mineral assemblage for these lenses is biotite + garnet + quartz + plagioclase ± muscovite ± tourmaline (figure 9e). they are schistose with a strong lepidoblastic texture as a result of the high biotite content. biotite is subidioblastic and tourmaline is idioblastic, other minerals present are generally xenoblastic. garnet, when present, is generally strongly elliptical and elongate in the foliation orientation. the chemical composition of these rocks is controlled by the high biotite mode. the most important constituents are sio2 + al2o3 + fe2o3 + mgo + k2o + tio2 (table 2). sio2 content is notably lower than other nason ridge migmatitic gneiss lithologies and they have a high tio2 content (~2.7%). amphibole schist. amphibole schist is coarseto medium-grained and composed mainly of a calcic amphibole (table 3). other minerals present are quartz and plagioclase, and locally muscovite. the texture is strongly schistose and lepidoblastic (linear fabric defined by amphibole orientation). the amphibole is idioblastic with lamellar and simple twinning. it has abundant inclusions of quartz-plagioclasemuscovite-biotite, and is locally poikiloblastic. quartz is xenoblastic with undulatory extinction, and fills voids between amphibole grains. plagioclase is subidioblastic to xenoblastic. muscovite is present mainly as inclusions in amphibole. these melanosome have also lower sio2 content that mesosomes or leucosomes (table 2). their chemical composition is similar to that of biotite schist except that cao is a major component and k2o is a minor component. mesosomes mesosomes comprise schist, gneiss, and amphibolite. gneiss described here as mesosome is compositionally distinct from leucosome gneiss, has contacts that are concordant with adjacent mesosome schists, and differs from mesosome schist only by containing lesser amounts of biotite. mesosome gneiss differs mineralogically from leucosome gneiss in that it contains abundant garnet and kyanite. schist and gneiss. fineto medium-grained schist and medium grained gneiss with porphyroblasts of kyanite (1 – 5 mm), garnet (1 – 30 mm), and tourmaline (< 2 mm). the most common prograde mineral assemblage is biotite + quartz + plagioclase with variable amounts of muscovite, garnet, staurolite, and kyanite/sillimanite. accessory phases include tourmaline, apatite, rutile, ilmenite, and zircon. chemical composition of schist and gneiss is characterized by sio2 between 60 and 70%, al2o3 between 13 and 18%, fe2o3 between 6 and 8%, and mgo+cao+na2o+k2o between 2 and 4% (table 2). mesosome gneiss is chemically different from leucosome gneiss in that fe2o3+mgo is greater than 9% and sio2 is lower than 70% in mesosomes while fe2o3+mgo is lower than 3% and sio2 is greater than 70% in leucosomes. gneiss shows compositional layering at mm scale with alternating biotite-rich and biotite-poor layers, and at cm scale with < 2 cm thick leucosome lenses bounded by < 0.5 cm thick melanosome layers (figure 9). compositional layering is also expressed by subtle variations in garnet and kyanite content, and porphyroblast size. schist contains quartz-plagio256 carlos a. zuluaga and clase leucosome lenses of < 10% with lengths from 2 cm to 10 cm and thickness < 5 mm (figure 10a). mineral alignment is commonly observed in hand sample and includes parallel orientation of mica (defining foliation) and sub-parallel orientation of kyanite and tourmaline porphyroblasts (defining lineation). foliation along the crest of wenatchee ridge generally strikes west-northwest and dips north-northeast (figure 2 and figure 3). lineation is subhorizontal west trending (figure 3). other fabrics observed at thin section and outcrop scale include asymmetric augen and microfolding to mesofolding with axial surfaces parallel to subparallel with foliation (figure 11). at the microscopic scale penetrative foliation is characterized by parallel orientation of biotite and muscovite grains. locally, garnet porphyroblasts have helicitic snowball structures compatible with rotation and large biotite grains have mica fish morphology also compatible with porphyroblast rotation. gneiss has lepidoblastic texture in melanosomes, granoblastic texture in leucosomes, and granoblastic predominating over lepidoblastic textures in mesosomes (figure 9). schist varies from strongly schistose to weakly gneissose, with subtle compositional layering formed by quartz-rich lenses and varying amounts of biotite. grain contacts show generally arrested morphology, but some minerals are subidioblastic to idioblastic with well-developed faces (especially porphyroblasts). muscovite and chlorite occur as retrograde minerals that commonly crosscut foliation and/or form epitaxial intergrowths with biotite.muscovite is also present in some layers apparently as a prograde mineral. quartz is fine-grained to locally coarse, xenoblastic, and in some cases elongated in the foliation direction, with undulatory extinction and abundant fractures probably reflecting late brittle features. plagioclase is fine-grained, xenoblastic to subidioblastic with irregular shape, but a few laths are observed. pericline and albite twinning are common, and carlsbad twinning is rare. simple idioblastic compositional zoning is observed in some grains, with an inclusion rich albitic core. larger grains contain rounded inclusions of quartz. locally, plagioclase is altered to muscovite and epidote. biotite is subidioblastic, deep brown in color, contains abundant zircon inclusions, and a lesser number of apatite and opaque mineral grains. biotite preferred orientation generally defines the dominant foliation. in some rocks, biotite is replaced by chlorite in epitaxial intergrowth. muscovite is subidioblastic and most commonly occurs as fine aggregates of randomly oriented grains replacing kyanite porphyroblasts. garnet is xenoblastic to subidioblastic in gneisses (figure 13, figure 14, and figure 15) and idioblastic to subidioblastic in schists. locally garnet is poikiloblastic, and elongated parallel to the foliation. poikiloblastic crystals contain inclusions of quartz, plagioclase, and biotite, and are surrounded by coronas or pressure shadows of quartzplagioclase aggregates. x-ray maps and quantitative mineral analyses across grains indicate that xenoblastic to subidioblastic garnet grains generally have weak compositional zoning while other subidioblastic to idioblastic garnet grains have strong compositional zoning.weakly-zoned garnet displays smooth zoning profiles with no significant central zoning and with relatively wide (> 0.2 mm) rimswith increased spessartine mole fraction and fe/(fe +mg) (figure 12b) which is refer to as reverse zoning to indicate that this pattern is the reverse to that which would be predicted for growth during increasing temperatures (hollister, 1966). the lack of strong central zoning and wide ‘reverse’ zoned rims in these subidioblastic to xenoblastic grains is interpreted to result from post-growth diffusion and partial resorption. strongly zoned garnet grains have smooth bell-shaped zoning profiles and wide to thin reverse zoned rims. almandine and pyrope mole fractions show enrichment from core to rim (xalm ~0.60 to ~0.73, xprp ~0.05 to ~0.18), and spessartine and grossular mole fractions are correspondingly depleted (xspss ~0.15 to ~0.02, xgrs ~0.23 to ~0.09). a reversal in zoning is also present near the rims, but this is typically a zone less than 0.1 mm wide. the strong zoning in these subidioblastic to idioblastic grains is interpreted to result from growth during prograde metamorphism. grain size distribution for garnet is bimodal with a median for larger grains between 1-3 mmand amedian for smaller grains at ~0.1 257 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa mm. larger grains generally have abundant inclusions of quartz, plagioclase, biotite, muscovite, ilmenite, and graphite. smaller garnet grains generally lack inclusions. kyanite is idioblastic to subidioblastic, bladed with an orientation parallel to foliation, and varies from pristine in some samples to completely replaced by muscovite in others. sillimanite is present as prismatic, isolated, less than 5 mm long grains and as aggregates of fibrolite. chlorite only occurs as a retrograde mineral replacing biotite. tourmaline is acicular idioblastic, aligned with foliation, and color zoned (green core – brown rim). amphibolite. amphibolite varies from fine to medium grained layers that are up to 25 m thick (figure 2 and figure 4). the typical mineral assemblage is hornblende + quartz + plagioclase ± garnet ± sphene ± epidote ± zoisite (fig. 10d). bulk rock chemical analysis for one amphibolite sample reveals that their sio2 content is lower than 50% and that they have higher fe2o3+mgo (> 25%) and cao (> 12%) than other mesosome lithologies (table 2). the texture is schistose with compositional layering characterized by alternating light green quartz-plagioclase-epidote and dark green hornblende-rich layers (figure 10c and figure 10d). dark green layers have lepidoblastic texture defined by hornblende alignment. quartz-rich lenses with granoblastic texture are commonly present. hornblende is idioblastic to subidioblastic in elongated prisms defining schistosity. it contains abundant inclusions of quartz and epidote that are poorly aligned with schistosity. epidote is equant xenoblastic to subidioblastic. plagioclase is xenoblastic to subidioblastic (in quartz-rich lenses), strongly altered to muscovite-epidote-clinozoisite, but albite twinning and a subidioblastic zoning are still recognizable. quartz is present as a minor phase mainly filling spaces between amphiboles. sphene is diamond shaped, idioblastic, and regularly distributed. garnet is equant, poikiloblastic and xenoblastic, concentrated in hornblende-rich layers, and has abundant inclusions of plagioclase, quartz, and epidote. interpretation of textural features the nason ridge migmatitic gneiss leucosomes described above (non-foliated to weakly-foliated pegmatites and tonalites, strongly-foliated tonalites, and quartz-plagioclase lenses) are interpreted as: undeformed (some pegmatites), weakly deformed (some pegmatites and tonalites), and strongly deformed (tonalite gneiss). these differing amounts of deformation likely reflect emplacement of magmatic bodies over a protracted period of time during variable states of stress. unfortunately, no geochronological data are available to quantify the timing and duration of emplacement. the non-foliated quartzplagioclase lenses define foliation in mesosomes and thus were probably affected by or related to the deformation event that produced foliation in the strongly deformed tonalite gneiss. they are also locally rimmed with selvages composed mainly of biotite and garnet. these textural features support the local development of partial melts (quartzplagioclase lenses) from mesosome lithologies. segregation of melt into the lenses would leave behind an un-melted restite. there are no other unambiguous textures supporting partial melting; however, low generation of partial melts hindered melt segregation and extensive deformation and metamorphic re-equilibration subsequent to melting could have erased other partial melting textures. nason ridge migmatitic gneiss rocks show variable evidence for retrogression. the most commonly observed retrograde features are poikiloblastic garnet with wide rims of reverse or retrograde zoning, kyanite partially or completely replaced by muscovite, and chlorite replacing fe-mg minerals. wide (up to 0.2 mm) rims of manganese enrichment and increased fe/(fe+mg) are interpreted to represent resorption of garnet. manganese that was preferentially incorporated into garnet during growth is inferred to have been re-incorporated into remaining garnet near the rim during consumption of the grain. other components in the garnet are distributed into matrix phases, and there likely was fe-mg exchange with other matrix minerals and equilibration associ258 carlos a. zuluaga and ated with diffusion of these elements into the garnet at high temperature. melanosome origins have been interpreted in several ways (brown, 2002; kriegsman, 2001). the most common interpretation for melanosomes is that they are sites of melt extraction, and represent the non-reactive un-melted fraction of the rock or restite (mehnert, 1968; brown et al., 1995). a second interpretation, which has received recent attention, is that melanosomes result from retrograde back reactions between crystallizingmelt (leucosome) and host rock (kriegsman and hensen, 1998; kriegsman, 2001). a third interpretation is that melanosomes are formed by mafic minerals crystallizing from a melt (kriegsman, 2001). the interpretation that selvages are an un-melted fraction is disregarded here based on thermodynamic modeling (see chapter 3). although, as predicted by modeling, the probable melting reactions involve very little biotite and the resulting melts would have low iron and magnesium, back-reactions during partial melt crystallization and retrograde metamorphism would produce biotite modal increase in restite rimming leucosomes. an argument against formation of selvages by back-reactions is that they are not always observed around leucosome lenses, even within the same mesosome lithologies. it seems reasonable, if retrogression was an extensive selvage forming mechanism, that the melanosome would be present around all leucosome lenses in the nason ridge migmatitic gneiss. however, the biotite selvage forming mechanism envisioned here requires that the volume of equilibration between leucosome and restite must be close to a 1:1 proportion or higher (see figure 11, chapter 3) and that this back-reactions focused on a thin layer surrounding the segregated melt. if back-reactions are taking place between equal proportions of leucosomes and restite in a larger scale (not focusing in mm to cm rimming restite) modal proportion of biotite will be uniform across the re-equilibrated portion of the rock and no biotite-rich rimming selvage will be observed. sharp contacts between leucosome gneiss and adjacent rocks and the strongly deformed character of the gneiss suggest that these bodies were emplaced as magmas before the last deformation event. these rocks most probably formed by injection of foreign magma because the volume of magma (up to 43%) is too great to have formed locally. there is a general absence of evidence for contact metamorphism around leucosomes; however this is readily explained by overprinting of later metamorphic events (m r 3 ) and/or by small temperature differences between the igneous bodies and the country rock. the origin of discordant weakly foliated leucosome is uncertain, because they could have originated by partial melting at lower crustal levels within the same unit, then traveled short distances to their emplacement position. p-t paths for metamorphism in the nason ridge migmatitic gneiss three p-t paths from nrmg (see stowell et al., 2007) based on metamorphic peak p-t estimates from thermobarometry and pseudosections and initial garnet growth p-t estimations from garnet chemistry compositions plotted in p-t pseudosections show that garnet initial growth was in all samples in a range of 550 ºc ± , and 6.5 kbar ± ; estimated peak p-t conditions are, however, variable and they are evidence of the degree of exhumation of a particular area. the finite p-t paths reflect a pressure increase during garnet growth of less than 2 kbar (figure 12) in agreement with other estimates in the nrmg and the nearby chiwaukum schist. pseudosection models of partial melts in metapelitic rocks of the nason ridge migmatitic gneiss pseudosection models of two samples from the nrmg (zuluaga, 2004) support wet partial melting as the origin for leucosome lenses and associated biotite selvages. water likely saturated the system during metamorphism and partial melting as indicated by t-x(h2o) pseudosections. temperature predictions for wet partial melting, using p-t pseudosections, are in the range of 655 ºc (10 kbar) – 259 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa 260 carlos a. zuluaga and figure 12.metamorphic p-t paths for nason ridge migmatitic gneiss, wa. garnet rim and core p-t estimates plotted on the mnnckfmash p-t pseudosection define the garnet growth segment of the path. final ‘peak’ p-t for garnet growth is estimated from garnet near-rim compositions and matrix mineral chemistry using avergae p-t with thermocalc. the intersection of the uncertainty ellipse for rim thermobarometry with the fields for the peak mineral assemblage provides the best estimation of peak metamorphic conditions. a. sample 01nc15b (wenatchee ridge). b. sample 01nc52a. (pacific crest) c. sample 02nc3b (nason ridge). 703 ºc (3 kbar); melts were produce likely by reactions that involve comsumption of quartz and plagioclase. p-t pseudosection predictions also include production of leucocratic melts, peritectic garnet, and peritectic kyanite, and consumption of biotite and muscovite. predictions are also consistent with the presence of biotite selvages as product of retrograde back-reactions (zuluaga, 2004; see also kriegsman, 2001). discussion four general models have been proposed to explain the origin of migmatitic rocks: metamorphic differentiation, metasomatism, injection of foreign magma, and partial melting. metasomatism was inferred for formation of the nason ridge migmatitic gneiss by van diver (1967). later, other authors have inferred that this unit originated mainly from magmatic injection (miller and patterson, 2001). results from thermodynamic modeling and petrographic observations suggest that partial melting was responsible for some of the leucosomes observed within the nason ridge migmatitic gneiss. water content and temperature are the two most important variables controlling the formation of partial melts. in the model presented in chapter 3, water is assumed to be the product of dehydration reactions and that remained in the system (closed system) or it was sequentially expulsed from the system during prograde metamorphism (open system). lack of water availability would cause low volumes of partial melt and unlikely preservation of partial melting textures because of metamorphic re-equilibration. temperature estimates for nason ridge migmatitic gneiss (625 ºc – 806 ºc) are close to or above the estimated wet solidus (655 ºc at 10 kbar – 703 ºc at 3 kbar). estimated peak metamorphic conditions and rock textures for sample 01nc52a support a partial melt origin for leucosome quartz – plagioclase lenses. in outcrops close to sample 01nc15b locality textures are also compatible with partial melt origin for quartz – plagioclase lenses. however, estimated peak metamorphic conditions for this sample are at temperatures lower than those predicted for initiation of melting. this discrepancy may be explained by back-reactions that are modeled thermodynamically in chapter 3, where the absence of textures indicative of partial melting are explained by low melt generation at some levels that hindered melt segregation and allowed retrograde re-equilibration. the p-t paths calculated for samples 01nc15b and 02nc3b are similar top-t paths determined in the nearby chiwaukum schist (tinkham, 2002), where p-t paths show zero to £ 2 kbar pressure increase during garnet growth. the p-t path for sample 01nc52a is not well constrained; however, the possible p-t paths in this sample are consistent with the interpretation of paths with less than 2 kbar pressure increase. results show considerably smaller pressure increases than those proposed by previous workers (e.g., brown and walker, 1993; whitney et al., 1999). the working hypothesis proposed for the nason ridge migmatitic gneiss origin include three events: preto syn-tectonic intrusives (gneissic tonalites), melting with formation of leucosome lenses, veins and patches, and a late intrusive event, that might or might not be related to partial melting of the same unit at lower crustal levels. the concordant character and the strong foliation interpreted to have resulted from the main deformation event are the arguments supporting a preto syn-tectonic intrusion origin for the gneissic tonalites. the presence of selvages and thermodynamic model predictions suggest partial melting for the origin of discontinuous leucosome lenses present in nason ridge migmatitic gneiss lithologies. these lenses did not form an interconnected net of melt and thus partial melts generated at this crustal level did not migrate far from the melting site. the interconnected array of pegmatites and weakly foliated tonalites are probably of postor syn-tectonic origin. acknowledgements bob miller and scott patterson are thanked for their generous help during field work. nsf ear–9628232 (green and others) and nsf ear–0207777 (stowell) provided partial analytical and field support. the university of alabama 261 multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa hooks fund, the university of alabama mobil fund, the university of alabama graduate student association research fund, the geological society of america, and the southeastern section of the geological society of america provided direct support to carlos zuluaga for this research. references armstrong, j.t. 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(1999) p-t-t evidence for mechanisms of vertical tectonic motion in a contractional orogen; north-western us and canadian cordillera. journal of metamorphic geology, 17, 75-90. zuluaga, c.a., 2004. thermodynamic modeling applied to metamorphic processes in migmatites: an example from the nason ridge migmatitic gneiss (cascades core, wa). 177 p. ph.d. thesis, university of alabama, tuscaloosa. zuluaga, c.a., stowell, h.h., tinkham, d.k., 2005. the effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic p-t paths. american mineralogist 90, 1619-1628. 264 carlos a. zuluaga and in recent years, due to the increase in providers of orbit and clock corrections of satellites for data evaluation in real-time and post-processing the method of precise point positioning (ppp) using measurements of global navigation satellite system (gnss) and web-based online positioning services have become widespread. owing to some advantages, such as work-duration and cost-effectiveness, many of users have implemented ppp method instead of the traditional relative positioning method for several applications. on gnss applications the quality of satellite ephemerides products used for data evaluation is a significant factor that affects the results in post-processing solutions either applying relative or ppp methods on analyses. these products, classified as ultra-rapid, rapid and final orbits, are regularly provided by several national and international organizations to the users. in this paper, the accuracy of ppp method has been studied comparing the outcomes from various online web services using different software and satellite ephemerides products. for this purpose, three test points were established on a place with completely free satellite visibility (ac01) and on the other two places with partially (yc01) and vastly (kc01) prevention of satellite signals near and within a forest area at campus of davutpaşa of the yildiz technical university of istanbul. at these stations, static observations have been conducted with a time span of 6 hours on 4th may 2015. the dataset collected using topcon hiperpro receiver, a receiver for gps and glonass data, was evaluated manually by means of the bernese v5.2 (bsw) and gipsy-oasis v6.3 (gipsy) scientific software. moreover, the gnss data were also proceeded using six different web-based online services (auspos, opus, csrs-ppp, apps, gaps, trimble-rtx) with ultra-rapid, rapid and final satellite ephemerides products. for the station with free satellite visibility (ac01), the analyses of outcomes indicate a coordinate accuracy of 1 cm for web-based and manual data processing. that suggests relative good quality of orbit and clock corrections of satellites used by online data processing services for ppp. furthermore, for station with partially (yc01) prevention of satellite signals, manual solutions have approximately 1 cm coordinate accuracies for n, e and u components. moreover, when we compare the results among web-based services, auspos in relative solution and trimblertx in ppp solution provides the best results. here, one must emphasize that trimble-rtx is a web-based processing service for gnss data (gps, glonass, beidou, and qzss) and operates an own global network of approximately 100 stations around the world for this purpose. for station with vastly (kc01) prevention of satellite signals, the best solutions were provided by apps and csrs-ppp using ppp approach among the web-based services. here, trimble-rtx could not produce any solution. these results could demonstrate the benefit of multi constellation of gnss in the areas with limited satellite visibility because of increasing of the number of measurements. en los años recientes, debido al incremento de operadores satelitales de corrección de órbita y tiempo para la evaluación de información en tiempo real y de postproceso, se ha generalizado el método de posicionamiento preciso (ppp, del inglés precise point positioning) que utiliza medidas del sistema global de navegación por satélite (gnss, del inglés global navigation satellite system) y servicios web de posicionamiento en línea. debido a ventajas como el tiempo de ejecución y la relación costo-efectividad, muchos de los usuarios han implementado el método ppp en varias aplicaciones antes que abstract resumen keywords: global navigation satellite system (gnss), precise point positioning (ppp), relative positioning, forest environment, satellite ephemerides, web based positioning services palabras clave: sistema global de navegación por satélite, método de posicionamiento preciso, posicionamiento relativo, ambiente boscoso, efemérides satelitales, servicios web de posicionamiento accuracy investigation of ppp method versus relative positioning using different satellite ephemerides products near/under forest environment issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.59496 comparación de precisión del método de posicionamiento preciso frente al posicionamiento relativo a través de diferentes productos de efemérides satelitales cerca y bajo un ambiente boscoso earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): d1 d9. taylan ocalan, bahattin erdogan, nursu tunalioglu, utkan mustafa durdag yildiz technical university, department of geomatic engineering, davutpaşa campus, 34220esenler, istanbul, turkey author for correspondence: nursu tunalioglu (e-mail: ntunali@yildiz.edu.tr). g e o d e sy d2 taylan ocalan, bahattin erdogan, nursu tunalioglu, utkan mustafa durdag 1. introduction global navigation satellite system (gnss) has been routinely used in several scientific and engineering applications including positioning, navigation and timing systems for many years. traditional relative positioning method has been handled for gnss-based positioning applications when high precision and accuracy are required. however, precise point positioning (ppp) method, a state space solution to the processing of gnss data acquired from a single receiver, has become an alternative to the relative positioning method by providing high absolute accuracy (gao and shen, 2001; rizos et al., 2012). traditionally, in all of the relative positioning methods depend on gnss technique, simultaneous observations should be made at one or more reference points. in other words, minimum two or more gnss receivers should be occupied at; one reference station, whose coordinates are known, and the other/s should be established to the point/s, whose coordinates will be estimated. however; in ppp technique to make observations on one point, whose coordinates will be estimated is enough for the estimation with one gnss receiver using absolute positioning approach. with regard to this, ppp technique allows a significant advantage to the users in terms of time and budget saving (ocalan, 2015). many factors such as preferred positioning method (relative or absolute), survey mode (static or kinematic), observation duration, equipment, signals and codes, data processing algorithms, reference receiver/receivers utility, satellitereceiver geometry, post-processing analyses or real-time applications provide different levels of performance for gnss positioning (rizos et al., 2012). several error sources affect positioning accuracy in gnss surveys. today relative techniques provide better solutions than ppp technique in terms of point positioning accuracy. the basic reason of this is that the effects of satellite orbit errors in relative techniques are lower than ppp technique. moreover, clock errors can be eliminated in relative techniques using double differencing of phase measurements. however, these effects are quite significant and dominant error sources in ppp technique. although relative positioning method removes some of these error sources either by applying phase differences techniques or by modeling of them, no modelled errors occurred in the forested areas, where especially satellite vision is limited, and orbital errors have significant effects on solutions (hoffmanwellenhof et al., 2001). this is a significant problem for ppp method. since precise satellite orbit and clock information are used, several error models are inevitable for ppp method. satellite visibility problem is the critical issue that should be taken into consideration for selection of the point location. it is required to ensure that there should not be any blockage or limitation of signals transmitted from satellite to receiver while establishing the points. this may cause problems at some of the engineering applications. at the applications where especially are performed around the forested areas, satellite vision has significant effects on positioning accuracy (naesset, 1999; pirti, 2005; tachiki et al., 2005; hasegawa and yoshimura, 2007; pirti, 2008). the other significant problem is orbital errors. several organizations such as igs, jpl, code and nrcan present several data and products freely to the users to be used for decreasing or eliminating the orbital errors. these products are classified in terms of their accuracies as broadcast, ultra-rapid, rapid and final. igs also presents some other standardized products such as satellite and station clocks, ionospheric and tropospheric parameters, besides satellite ephemerides. the principle of gnss method is based on resection problem using satellites, so satellite ephemerides are crucial on analyses. these orbit parameters have different levels of accuracy as seen in table 1. therefore, accuracies of the results have been altered according to the used product. el método tradicional de posicionamiento relativo. en las aplicaciones gnss, la calidad de los productos de efemérides satelitales utilizados para la evaluación de información es un factor significante que afecta los resultados en las soluciones postproceso sea con la aplicación de métodos relativos o ppp en los análisis. estos productos, clasificados como ultrarrápidos, rápidos y de órbita final, los proveen a los usuarios diferentes organizaciones nacionales e internacionales. en este artículo se estudia la exactitud del método ppp en comparación con los resultados de varios servicios web que utilizan diferentes productos de software y de efemérides satelitales. con este objetivo se establecieron tres puntos de evaluación: el primero en un lugar con completa visibilidad satelital (ac01) y los otros dos con parcial (yc01) y reducida (kc01) señal satelital, en los alrededores y al interior de una zona boscosa del campus de davutpaşa , en la universidad técnica yildiz de estambul. en las estaciones se llevaron a cabo observaciones estáticas con intervalos de seis horas el 4 de mayo de 2015. la información se recolectó con el programa topcon hiperpro, un receptor de gps y glonass (sistema gnss desarrollado por la unión soviética), y se evaluó manualmente con el software científico bernese v5.2 (bsw) y gipsy-oasis v6.3 (gipsy). además, la información también se procesón con seis servicios web en línea (auspos, opus, csrs-ppp, apps, gaps, trimble-rtx) con productos de efemérides ultrarrápidas, rápidas y finales. para la estación con completa visibilidad satelital (ac01), el análisis de los resultados indican una precisión coordinada de 1 cm en los procesamientos web y manual. esto sugiere una buena calidad de las correcciones satelitales de órbita y tiempo usadas por los servicios de procesamiento de información en línea con el método ppp. además, para la estación con señal parcial (yc01), las soluciones manuales tienen aproximadamente 1cm de coordinación en la precisión para los componentes norte y este. luego, cuando se comparan los resultados entre los servicios web, los mejores resultados fueron de auspos en soluciones relativas y trimble rtx en el método ppp. es importante recalcar que trimble rtx es un servicio de procesamiento web para información gnss (gps, glonass, beidou y qzss) y opera una red global propia de unas 100 estaciones en el mundo para este fin. para la estación con una reducida señal satelital (kc01), las mejores soluciones fueron ofrecidas por apps y csrs-ppp con el método ppp entre los servicios web. en este caso, trimble-rtx no puede producir ninguna solución. estos resultados demuestran los beneficios de la multiconstelación gnss en las áreas con visibilidad satelital limitada gracias al incremento del número de medidas. table 1. gps/glonass satellite ephemerides (url-1, url-4 and url-6) how to cite item ocalan, t., erdogan, b., tunalioglu, n., & durdag, u. m. (2016). accuracy investigation of ppp method versus relative positioning using different satellite ephemerides products near/ under forest environment. earth sciences research journal, 20(4), d1-d9 doi:http://dx.doi.org/10.15446/esrj.v20n4.59496 record manuscript received: 09/08/2016 accepted for publication: 09/12/2016 d3accuracy investigation of ppp method versus relative positioning using different satellite ephemerides products near/under forest environment in this study, it is aimed to investigate the accuracy of ppp method versus relative positioning especially around or under the forested areas (limited satellite visibility) considering the developments of ppp method and webbased online services. for this purpose, gnss data collected at three points with long observational durations have been examined for static positioning and analyzed by different software and web-based online services by selecting different satellite orbit products (ultra-rapid, rapid and final). these dataset was first processed manually using gipsy-oasis v6.3 software in ppp mode with jpl products, and then analyzed by the various web-based online ppp services, namely csrs-ppp, apps, gaps and trimble-rtx. moreover, the related dataset was processed manually by bernese v5.2 software using code and igs products (dach et al., 2015), and then evaluated by web-based online services namely auspos and opus. hereby, we investigate the accuracy of ppp method versus relative positioning according to the outcomes of analysis, especially for the points established around the forested areas. methods mentioned in this study use different satellite ephemerides provided from various services. auspos, opus, gaps and csrs-ppp services use igs products; apps uses jpl products and trimble rtx uses its own orbit products. 2. ppp technique versus relative positioning gnss users prefer relative positioning method in surveying applications if high accuracy is needed. all gnss methods depending on relative positioning principle require simultaneous observations occupied at least one reference station whose coordinates are well known. therefore, minimum two receivers should be used on surveys: one is busy at the reference station and the other is occupied at the point whose coordinates will be determined. the primary factors for point positioning accuracy are the baseline length between two receivers and the observation duration. in this context, establishing continuously operating reference stations (cors) networks has a significant contribution to the relative positioning (abd-elazeem et al., 2011; rizos et al., 2012). relative positioning determines the point coordinates by using double differences on phase measurements as given below with eq.1 (hoffmanwellenhof et al., 2001): ∇δφ t =∇δr(t, t-τ) + ∇δds(t-τ) ∇δdiono + ∇δdtropo+∇δλν + ∇δε(φ) (1) where ∇δ is the double difference operator at the time of receiving data, φ is the phase measurement, (t) is the time of receiving data, (t-τ) is the satellite time, τ is the travel time from the satellite to the receiver, r(t, t-τ) is the true geometric range, ds is the orbital prediction error, diono and dtropo are the ionospheric and tropospheric errors, respectively, λ is the wavelength, ν is the integer phase ambiguity and ε is the noise components. scientific software is used for long baselines in post processing solutions of relative positioning, and commercial software is used for short baselines. the list of the most commonly known web-based online services can be seen in table 2. table 2. web based online services using relative solution approach (url-2, url-3) ppp method is the particular case of traditional absolute point positioning approach that became widespread after the establishment of global gnss networks with permanent stations. for a worldwide positioning in sub-meter level, an accurate determination of corrections of satellite orbits and clocks is possible using data of a global gnss network. data collection with a dual frequency receiver at the point, whose coordinates will be computed accurately, is enough in ppp method for determining high accurate position. so, by using code and carrier phase observations with a doublefrequency receiver that are utilized by the un-differenced and ionosphere-free combinations, decimeter to centimeter level point positioning accuracies may be achieved due to observation durations (zumberge, 1997; gao and shen, 2001; kouba and héroux, 2001; gao and shen, 2002; rizos et al., 2012). the accuracies of orbit and clock correction products and error models to be used in determination have significant importance on the point positioning accuracies to be achieved in ppp method. therefore, considering the errors and biases separately shown in table 3 for ppp depending on the quality of the application is the key factor to increase the position accuracy. table 3. biases and errors that need to be applied for in typical ppp and differential gnss positioning techniques (rizos et al. 2012) in typical ppp method, to reduce the effect of ionospheric errors, double frequency gnss observation models (code observations and ionosphere-free carrier phase observations) have been adopted. the first effective use of the functional model in ppp has been discussed and represented in the study of zumberge et al. (1997). kouba and heroux (2001), as given in equations 2 and 3, has presented mathematical observation models of typical ppp ℓp = ρ + c(dtdt) + tr + εp (2) ℓф = ρ + c(dt-dt) + tr + nλ + εф (3) here ℓp (p3) is the ionosphere-free combinations of p1 and p2 code observations, (p3)=(2.546p1-1.546p2), ℓф (l3) is the ionosphere-free combinations of l1 and l2 carrier phase observations, (l3)=(2.546 λ1 ф11.546 λ2 ф2), ρ is the geometric distance between satellite and receiver antennas in meters, c is the speed of light in meters per second, dt is the receiver clock offset according to gps time in seconds, dt is the satellite clock offset according to gps time in seconds, tr is the signal delay in atmosphere (prior tropospheric delay) in meters, n carrier-phase ambiguities on l1 and l2 frequencies in cycles, respectively. λ1, λ2, λ are the carrierphase wavelengths on l1, l2 frequencies, and combined l3 (10.7 cm) carrier phases, respectively, and εp, εф are the components of related survey noises including multipath and un-modeled errors. d4 taylan ocalan, bahattin erdogan, nursu tunalioglu, utkan mustafa durdag 3.2 data post-processing all data obtained from observations were converted to rinex format and quality check of data was done by bkg ntrip client (bnc) v2.11.1 software. later, gnss data were uploaded and processed on the web-based online services using ppp and relative approaches. ac01 has the best satellite visibility among three points, thus the positional dilution of precision (pdop) values are better than other points’ values (fig. 2 and 3). the pdop values and cycle slips for point yc01 are worse than the pdop values for ac01 (fig. 2 and 4). the worst pdop values and satellite visibility are belonged to the point kc01 (fig. 2 and 5). here, improvement of the satellite visibility spatial geometry brings the pdop values are decreased. in figure 4, the cycle slips in satellite visibility is seen in south-west direction for yc01 where the forest area (with approximately 10 m-15 m tree heights) are placed. moreover, as seen in figure 5, for kc01 that has the limited satellite visibility, the satellite visibility is only traced in north-west direction limitedly. despite the fact that traditional ppp method has had several advantages aforementioned, the most significant disadvantage of the method is to require long convergence time to solve the carrier phase integer ambiguities. to resolve an ambiguity float solution a convergence time up to 20 minutes is expected for centimeter-level positioning accuracy depending of accuracy of apriori coordinates of station and the number of received satellites, (rizos et al., 2012). therefore, another significant factor to increase the position accuracy is the growth of the observation duration. since the usage percentage of ppp method is growing due to supports and gnss products provided by several organizations such as igs, jpl, nrcan, bkg and code to the users, many software algorithms for ppp in real-time and post-process have been developed (i.e. bernese, gipsy-oasis, bnc, rtk-lib, panda, wappp). in addition to this, web-based ppp services, csrs-ppp, gaps, apps, magic-gnss, trimble centerpoint rtx, etcetera, listed in table 4, offer to the users the possibility of processing of uploaded data (el-mowafy, 2011; ocalan and alkan, 2013; ocalan et al., 2013; alkan and ocalan, 2013; alkan et al., 2013). similarly, these services constantly improve to provide a userfriendly service without any fees for usage. table 4. web-based services using ppp solution approach (url-4, url-5, url6, url-7 and url-8) 3. case study 3.1 test area and control points characteristics to investigate the accuracy of the relative and ppp methods by using different satellite ephemerides and software with different satellite visions around the forested areas, three test points were established at campus of davutpaşa, yildiz technical university, istanbul. points namely; ac01, yc01 and kc01 were installed in the open area, close to the forest and within the forest area, as shown in figure 1, respectively. the average height of the trees is approximately within 10 m-15 m in the forest area. topcon hiperpro gnss (gps+glonass) geodetic receiver was used for collecting the data with a time span of 6 hours from 08:30 am to 02:30 pm (utc) on the 4th may 2015 (day of year: 124) at static mode. during the view, the antenna slope-heights were measured carefully minimum from two sides, and then vertical antenna heights from antenna reference point (arp) were computed by using antenna parameters. figure 1. test area and the points figure 2. pdop plots for points ac01,yc01, kc01 figure 3. analysis sky plots of point ac01 d5accuracy investigation of ppp method versus relative positioning using different satellite ephemerides products near/under forest environment figure 4. analysis sky plots of point yc01 figure 5. analysis sky plots of point kc01 scientific software and web-based online services were used in data processing and analyzing stages. for manual solutions, bernese v5.2 and gipsy-oasis v6.3 scientific software were used by applying relative positioning approach and ppp method, respectively. for webbased data processing, auspos, opus (relative technique), csrsppp, apps, gaps and trimble-rtx (ppp technique) were preferred. in table 5, a brief description has been given according to used gnss satellite ephemerides for all computations of post processing web-based online services and manual solutions. table 5 types of gnss satellite ephemerides used in post processing the data processing strategy of the gaps v5.5.0 was applied to solve the gps observations by using ultra-rapid and rapid orbit datasets. during the analyzing process, the updated version of gaps v5.9.0 has been presented to the users, thus data were processed according to the final orbit datasets with updated version. datasets were uploaded to the web-based online services within 17 hours for ultra-rapid solutions, within 3 days for rapid solutions and approximately 14 days later for final solutions. to compare the solutions, the outcomes of the relative solution is considered as reference coordinates of stations (target coordinates) computed by bernese v5.2 software using final orbit products. stations bucu, crao, ankr, sofi and nico, involved in igs global network, were settled as datum stations for bernese v5.2 solutions. here, all computed coordinates were based on the igs realization of itrf 2008 reference frame at observation epoch. in the evaluation process, the offsets and correction values for the antenna phase variations of the antenna types available from the us national geodetic survey were used. in the first processing step, single point positioning has been performed by using ionosphere-free (l3) observable, the receiver clock errors have been eliminated, and then baselines in the network have been pre-determined. cycle slips and outliers have been checked by using triple differences. after this process, the outliers have been removed. later, ambiguity parameters have been solved. in order to solve the ambiguity parameters, quasi ionosphere free (qif) strategy has been applied. consequently, precise cartesian coordinates (x,y,z) of the points in itrf 2008 reference frame at related observation epoch have been computed. we processed our gps data using gipsy-oasis v6.3. it is an automated, fast, ultra-precise high precision gps data processing software package for ppp rather than relative positioning (zumberge et al., 1997). all of the cartesian coordinates (x,y,z) provided by the systems and determined manually in itrf2008 datum were then converted to the local topocentric coordinates (north, east and up) and differences for all components were calculated as reads by equation 4. difference (δ) = solution – reference bernese v5.2 solution (4) 3.3 results and analysis the point ac01 was established on the area with free satellite visibility. coordinate differences of ac01 estimated by using different orbit products for north (n), east (e), up (u) directions and their standard deviations are listed in tables 6 and 7 and illustrated in figures 6, 7 and 8, respectively. the horizontal-axis shows the solution methods and verticalaxis corresponds the coordinate differences and standard deviations derived by mentioned orbit ephemerides. the solutions estimated manually by scientific software indicate that the differences computed with bernese v5.2 using ultra-rapid and rapid orbit products are less than 1 mm in all coordinate directions. this proves the good quality of both orbit products. in contrary to this, the results obtained by gipsy-oasis v6.3 software are in the range of 1 and 4 mm for n and e directions and the difference amounts to maximum 8.6 mm for u direction of coordinates. these differences to the outcomes of the bernese v.5.2 mainly accumulate due to the different modelling and default values for the signal delay in the troposphere. when the results mentioned above compare to their standard deviations, only the coordinate differences of u component for rapid ephemerides are greater than the standard deviations. moreover, an increase has been occurred on coordinate differences when comparing the solutions of ultra-rapid orbit product and rapid orbit product. then, a decrease has been determined when comparing the results between rapid orbit solution and final orbit solution. these differences have been in limits of their standard deviation values. when the results of web-based service for the ac01 point are analyzed, it is found that relative methods give better solution than ppp method. the maximum difference between n and e directions for auspos is determined as 2.0 mm. on the u direction, the difference is increased to 12.7 mm. in auspos system, a decrease has been occurred on coordinate differences, when comparing with ultra-rapid orbit solution, rapid orbit solution and final orbit solution, respectively. when the opus system has been analyzed, maximum 17.1 mm difference has been determined on n and e directions. all values are lower than standard deviation values. in spite of this, differences on u direction are greater and all of these values are bigger than their standard deviations. according to the results, it is determined that coordinate differences obtained from ultra-rapid orbit products in opus system are closer to the reference coordinates. d6 taylan ocalan, bahattin erdogan, nursu tunalioglu, utkan mustafa durdag table 6. coordinate differences for point ac01 table 7. standard deviations of the coordinate differences for point ac0 figure 6. coordinate difference and standard deviation of point ac01 for n direction figure 7. coordinate difference and standard deviation of point ac01 for e direction figure 8. coordinate difference and standard deviation of point ac01 for u direction when web-based ppp service solutions have been examined for ac01 point, it is seen that maximum difference on n and e directions is as 13.2 mm for csrs-ppp, apps and trimble-rtx services. among them, especially apps service has the lowest differences. for u direction, all of the service solutions except trimble-rtx give the highest difference. trimble-rtx service gives approximately similar solutions for all type orbit products. however, although gaps service provides worse results for ultra-rapid and rapid orbit products, it produces better solutions especially for u direction than the other services while using final orbit products. the point, namely yc01 was selected on the relatively forest area. the observation duration was 6 hours. coordinate differences of n, e, u directions determined by using different orbit products and standard deviations are presented in tables 8 and 9, respectively. moreover, the graphical representation of coordinate differences and standard deviations are given in figures 9, 10 and 11 for point yc01. table 8. coordinate differences for point yc01 when the statistical results of yc01 point have been examined, it is seen that the standard deviations have been increased due to the decrease of satellite vision. when the results of web-based services have been examined, it is seen that there is a deviation from reference coordinates on u direction, especially when comparing with ultra-rapid and rapid orbit products. at online services, there is progress on results when comparing with ultra-rapid orbit products and final orbit products. when considering the coordinate differences of n and e directions and standard deviations of them, the best results have been provided by apps service. moreover, if n, e and u directions have been considered together, trimble-rtx service provides the best results. table 9. standard deviations of the coordinate differences for point yc01 figure 9. coordinate difference and standard deviation of point yc01 for n direction d7accuracy investigation of ppp method versus relative positioning using different satellite ephemerides products near/under forest environment figure 10. coordinate difference and standard deviation of point yc01 for e direction figure 11. coordinate difference and standard deviation of point yc01 for u direction as seen in figure 10, gaps service solutions for ultra-rapid ephemerides provide the worst solutions. moreover, csrs-ppp and gaps services solutions for u direction provide great coordinate differences. the point, namely kc01 was selected under the forested area. the observation duration is 6 hours. coordinate differences of n, e, u directions determined by using different orbit and standard deviations are given in tables 10 and 11, respectively. moreover, the graphical representation of coordinate differences and standard deviations are given in figures 12, 13 and 14 for point kc01. table 10. coordinate differences for point kc01 table 11. standard deviations of the coordinate differences for point kc01 the kc01 point has the worst satellite vision among the points. problems on solving the carrier phase integer ambiguity were occurred during the analyses. trimble-rtx could not produce any solution on point kc01. the most appropriate results for ultra-rapid and rapid orbit solutions were achieved by bernese v5.2 scientific software. when the differences and standard deviations of them determined from web-based services have been examined, it is found that the deviations are greater than the results of services, which use relative model. results obtained from apps and csrs-ppp services among the other web-based services give efficient solutions. figure 12. coordinate difference and standard deviation of point kc01 for n direction figure 13. coordinate difference and standard deviation of point kc01 for e direction figure 14. coordinate difference and standard deviation of point kc01 for u direction as seen in figures 12, 13 and 14, the gaps service for ultra-rapid ephemerides provides the worst solutions among the web-based online ppp services. in addition, opus service solutions for all directions provide great coordinate differences among the web-based online relative services. d8 taylan ocalan, bahattin erdogan, nursu tunalioglu, utkan mustafa durdag 4. conclusions and recommendations satellite vision in gnss survey has a significant importance for the accuracy of the estimated coordinates. this constraint has been encountered especially in surveys performed near the forested area. moreover, accuracy of the products used in studies affects the expected results. after the survey stage, due to the determination process, one of the products, ultra-rapid, rapid or final orbit products can be used. today, besides commercial and scientific software, web based positioning services have been used for analyzing gps/gnss data, which are free. due the development and common use of these services, ppp method for next generation positioning has come to the forefront. in this study, the accuracy of ppp method versus relative method has been investigated according to the different satellite orbit products where satellite vision is limited. for this purpose, analyses have been performed with six different webbased services, then performances of them have been compared. for manual solutions, the estimation was achieved by bernese v5.2 and gipsy-oasis v6.3 scientific software. post processing solutions for csrs-ppp and trimble-rtx web-based online services use gps+glonass observations. however, the other web-based online services and manual solutions in post processing use gps only observations. in survey areas, where the satellite visibility is limited such as forest regions as illustrated in this study, the use of gps+glonass observations in post processing could be an advantage due to the increase of number of observations. but, this is not the unique parameter to improve the point positioning accuracy in post processing solutions. the analyses of outcomes show that web-based services (except gaps and trimble-rtx) and manual solutions used in this study estimate accuracy of coordinate components of north and east as approximately 1 cm level (in all cases when ultra-rapid, rapid and final satellite orbit products were used) for the station with free satellite visibility (ac01). for ac01, considering the ephemerides used in post processing solutions, the significant improvement on coordinate components are provided by gaps service in respect to use of ultra-rapid, rapid and final products. for the point yc01 located near the forest area where the satellite vision decreases or partially disturbed, the positioning accuracy decreases. here, manual solutions processed by scientific software (bernese and gipsy-oasis) give the best results. among the web-based online services applying relative model approach, auspos offers the best solution for partially prevention of satellite signals. furthermore, the ppp-solution of the trimble-rtx delivers a better positioning accuracy than the outcomes of the other web-based services using ppp approach for yc01. this outcome for yc01 estimated by trimble-rtx, using ppp solution and its own satellite ephemerides products could be related to use of multi constellation of gnss in the areas with limited satellite visibility because of increasing of the number of measurements. in contrary to this, trimble-rtx could not produce any solution for point kc01 (vastly prevention of satellite signals). here, the best solution was provided by apps service using ppp approach. among the web-based online services applying relative model approach, auspos offers the best solution for kc01. consequently, using scientific software manually on the analyses provides advantages to the users to achieve high accuracy on results in all conditions. the fundamental reason of it is that the user has an option to interfere the estimation and analyzes stages during the process. in spite of this, the professional use of scientific software requires a long-term experience. however, web-based services represent basic internet based interface, which users of all level may easily use. they produce fast and easy solutions for post-processing stage at positioning applications. however, several factors affect the positioning accuracy under the different satellite visibilities. these factors can be listed as; used satellite observations (gps only, gps+glonass etc.), the solution algorithms (relative or ppp), satellite ephemerides product types (ultra-rapid, rapid and final) and the other correction models, that the users should take into consideration of them acknowledgements we are grateful to organizations, which present web-based online services (opus, auspos, csrs-ppp, gaps, apps, trimble-rtx) and thank to nasa’s jet propulsion laboratory for providing gipsy oasis ii software. we are grateful to anonymous referees for their helpful comments to the manuscript. references abd-elazeem, m., farah, a. & farrag, f. a. 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(1997). precise point positioning for the efficient and robust analysis of gps data from large networks. journal of geophysical research. 102(b3): 5005-5017. url url-1: international gnss service (igs) web page, http://www.igs.org (last accessed february 2016) url-2: australian online gps processing service (auspos) web page, http://www.ga.gov.au/bin/gps.pl (last accessed february 2016) url-3: online positioning user service (opus) web page, http://www.ngs. noaa. gov/opus/ (last accessed february 2016) url-4: canadian spatial reference system-precise point positioning (csrs-ppp) web page, http://webapp.geod.nrcan.gc.ca/geod/toolsoutils/ppp.php (last accessed february 2016) url-5: gps analysis and positioning software (gaps) web page, http:// gaps. gge.unb.ca/ (last accessed february 2016) url-6: automatic precise positioning service (apps) web page, http:// apps.gdgps.net/ (last accessed february 2016) url-7: magicppp-precise point positioning solution (magicgnss) web page, http://magicgnss.gmv.com/ppp/ (last accessed february 2016) url-8: trimble centerpoint rtx post-processing service web page, http:// www.trimblertx.com/ (last accessed february 2016) geofisica colombiana pp. 42-48 issn· 0121·2974n°.7 dtciembre de 2003 bogota, d.c. caracterizacion de depositos de suelo utilizando geo-radar francisco javler moreno v. departamento de geociencias, maestria en geofisica, universidad nacional de colombia luis alfreoo montes vioes departamento de geociencias, facultad de ciencias, grupo de investigaci6n en geofisica, universidad nacional de colombia, lamontesv@unal.edu.co resumen la tecnica del geo-radar, 0 g.p.r. por su sigla en ingles (ground penetrating radar), es una forma de prospeccion del subsuelo utilizando ondas electromagneticas con frecuencias entre 10 y 1200 mhz. una imagen de geo-radar obtenida por reflexion contiene contrastes generados por la variacion espacial de las propiedades electromagneticas, la velocidad de las ondas electromagnetic as en un suelo depende fundamentalmente de la constante dielectrica equivalente, la cual se puede correlacionar con la fabrica, la estructura y el contenido de agua. por medio de pruebas en laboratorio se establecieron correlaciones para arena, recebo y arcilla. palabras clave: gpr, radar, prospeccion electromagn~tica abstract the radar technique prospects the subsoil with electromagnetic waves with frequencies between 10 and 1200 mhz. a reflection radar image shows contrasts of the subsoil generated by its inhomogeneous electromagnetic properties. the velocity of the dielectric constant, correlated with mineral fabric, structure and water contained in rocks. meanwhile lab assays, correlations between this properties for sand, cloy and a mixture of the, were stablished. keywords: gpr, radar, electromagnetic prospecting introduccion ei conocimiento de las caracteristicas y disposicion espacial del subsuelo es la plataforma de partida a la planeaci6n de cualquier actividad que este relacionada con la ingenieria civil ambiental y con las disciplinas que involucren el recurso suelo. ei empleo de ondas electromagnetic as para la caracterizaci6n del subsuelo es una tecnica en desarrollo en estados unidos, canada y europa, esta promete convertirse en una herramienta fundamental en la prospecci6n debido a su versatilidad, rapidez y bajo costo, comparada con los tradicionales metodos invasivos. durante la primera mitad del siglo pasado se inici6 la investigaci6n de las ondas electromagneticas en el campo de las comunicaciones; en la segunda mitad se emplearon las ondas electromagneticas para estudiar el subsuelo. el said (1956) intento manuscrito recibido para evaluacion el 30 de mayo de 2003. articulo aceptado para publicaci6n par el comite edijorial el 30 de octubre de 2003. 42 medir la profundidad de la tabla de agua. en los afios 1960 a 1970 varios grupos de investigadores se dedicaron al estudio en zonas polares y glaciares; bentley (1964) y walford (1964). holser (1972), cook (1973), unterberger (1978) y thierbach (1973) iniciaron la evaluaci6n del subsuelo para la exploracion de minas de carbon y sal. la era espacial permitio un gran avance de estas tecnicas; un radar similar al empleado en el estudio de las zonas polares fue utilizado para hacer medidas sobre la superficie lunar desde su orbita. durante el periodo de 1980 a 1995 se inici6 una gran explosion en el avance y comercializacion del geo-radar: geophysical survey system inc. en estados unidos, mala geosciences en suecia, era technologies en el reino unido, sensors and sofware inc. en canada e ids en italia, entre otros. las aplicaciones del geo-radar se pueden dividir en dos segun las frecuencias de las antenas. para aplicaciones en geologia geofislca colombiana, 7, oiciembre de 2003 caracterizacl6n de dep6srros de suelo ufillzando geo.radar donde la profundidad de penetracion tiende a ser mas importante que la resolucion, se utilizan antenas con frecuencias men ores a 500 mhz. en ingenieria 0 en ensayos no destructivos se emplean antenas con frecuencias mayo res a 500 mhz. algunas de las areas en las que se puede implementar son: • ei mapeo, la delimitacion y el monitoreo de trayectorias y contaminantes en el subsuelo. • facilidades y utilidades enterradas que no tienen registro de construccion y se requiere conocer su ubicacion espacial (acueductos, alcantarillados). • la caracterizacion del subsuelo para obras longitudinales como vias, oleoductos, poliductos. • determinacion de superficies de falla. • programas de control de caiidad y exploracion en los que con la menor inversion se obtenga la mejor informacion (pavimentos). control de humedad del suelo para cultivos. • en general, procedimientos y circunstancias que no perrnitan el ejercicio de tecnicas invasivas. el geo-radar un georadar es un dispositivo electronico, constituido por un generador de seflal, una antena transrnisora, una receptora, una unidad de control de altisima precision (mide tiempos de viaje de orden de nanosegundos) con funciones de archivo de datos y un monitor. ei transmisor genera un pulso de ondas a una frecuencia y ancho de banda segun las caracteristicas de la antena; esta transmite el tren de ondas al subsuelo conforme a un patron de radiacion segun su diseflo. la antena transmisora es un transductor que convierte la corriente electrica en trenes de onda electromagnetica y las transmite al subsuelo; la antena receptora capta las ondas electromagneticas reflejadas, refractadas, difractadas 0 directas y las convierte en corriente. la unidad de control genera y administra los disparos de los pulsos en el transmisor y mide los tiempos de viaje de la onda en el subsuelo. cada pulso generado y recibido es representado por una traza, de manera que al ir moviendo las antenas se generan consecutivas trazas que representan un corte del subsuelo a manera de un radargrama. el sistema de radar permite tres modos de adquisicion de datos o exploracion: reflex ion, sondeo variando el offset y tomografia. marco teorico las ecuaciones de maxwell, las cuales describen matematicamente la fisica electromagnetica y las relaciones constitutivas que cuantifican las propiedades geoelectricas del rnedio, son el fundamento para describir cuantitativamente las seflales del geo-radar. ecuaciones de maxwell las ecuaciones de maxwell predicen la variacion de los campos electrico y magnetico, los cuales son normales entre si y perpendiculares a la direccion de propagacion. geoflslca colombiana, 7, oiciembre oe 2003 ul 0.00 (l) h .lj (l) 1.25 e 1 .~ 2.50 i ..c::b. 3.75 (l) o 5.00 a 2 1064 b position in metres figura 1. metoda de reflexi6n antena. antena ~ ~j a. : onoadirectaajre g. : onda direct a terreno r 1 : onoa reflejaoa -0.25 de 0.00 pt 0.25h-in 0.50 -m 0.75 et re 1. 00 s 1.25 1. 50 1. 75 0.00 0.25 0.50 0.75 1.00 1.75 2.00 figura 2. metoda c m p 43 figura 3. metodo tornografico ee vxe=-(i) at ab vxh=j+(2) at v·d=q (3) v·b=o (4) donde e = campo electrico b = densidad flujo magnetico (rnagnetizacion) d = desplazamiento electrico (polarizacion) h = intensidad de campo magnetico q = densidad de carga j = densidad corriente electrica las ecuaciones constitutivas son el medio de cuantificar las propiedades fisicas, electricas y magnetic as ide los materiales; prove en una descripcion de como los electrones, los atornos, y las moleculas responden en masa a la aplicacion de un campo. conductividad electrica (a) dada por la expresion j=e describe como las cargas libres fluyen para formar una corriente cuando un campo electrico esta presente. la permitividad dielectrica (8) expresa en la ecuacion (6) describe como las cargas son forzadas a desplazarse en respuesta a un campo electrico y la permeabilldad magnetica (j:i) a su vez b=il·h 44 moreno it al. muestra como los momentos rnagneticos atornicos responden a un campo magnetico, las ecuaciones de maxwell describen el acople entre los campos electrico y magnetico cuando estos varian en el tiempo. cambios en los campos electricos generan campos magneticos, los cuales inducen campos electricos, esta continua sucesi6n de inducciones y variaciones resulta en campos que se desplazan a traves del medio. dependiendo de las magnitudes relativas de las perdidas, los campos pueden difundirse 0 propagarse como ondas. matematicamente, la ecuaci6n de onda se obtiene partiendo de las ecuaciones de maxwell y colocando el campo magnetico en funcion del campo electrico. --a-vxvxe=--(vxllf!) at (8) a (adj ae &"evxvxe=-flj+=-fla--flf;at at at at2 (9) ae &"e vxvxe+flcr·-+ fle-2=0 at at . (10) propiedades de las ondas los campos electromagneticos cuando se propagan como ondas pueden ser caraeterizados por los frentes de onda 0 por trayectorias de rayo. ei frente de onda es la superfieie en la eual todas la sefiales estan en fase; para un tren de ondas emitido por una fuente puntual, el frente de onda se define como la superfieie a iguales tiempos de viaje. la trayectoria de rayo es perpendicular al frente de onda; los campos electromagneticos cuando se modelan como rayos viajan segun la trayeetoria de estes. los campos electrico y magnetico son perpendiculares a la trayectoria del rayo. las propiedades de las ondas electromagneticas son: velocidad de fase, v, atenuacion, , y la impedancia electromagnetica, ·z. v=c/ {( erflr/ 2)((i+py +)y (5) c = velocidad de la luz en el espacio e, = constante dielectrica relativa. flr = permeabilidad magnetic a relativa (i para los materiales no magneticos). p factor de perdida p = ale»: c conductividad ro 2nf donde f es la frecuencia e permitividad = ergo eo = permitividad del espacio libre eo = permitividad del espacio libre (8.854* 1o-12f/m) en materiales de baja perdida p "" 0 v=o.3/,je; (6) (7) geoflslca colombiana, 7, oiciembre oe 2003 caracterizacion de depositos de suelo utilizando geo·radar a=z ._g_ o 2.je: z=)~=jt zo = es la impedancia del espacio libre experimentacion la metodologia de investigaci6n fundarnentalmente consisti6 en medir la variaci6n de la velocidad de las ondas electrornagneticas como funci6n de la naturaleza del suelo, y de parametres como la humedad, la densidad y la resistividad. para establecer un patr6n de cornportarniento y deducir relaciones maternaticas entre los parametres fue necesario controlar las diferentes variables involucradas para cad a tipo de suelo. por ella se construy6 una celda de prueba que consisti6 en un paralelepipedo en estructura de madera, reforzado can perfiles de acero. en una de sus caras se coloco una lamina de vidrio para tener control sobre el cornportamiento interno de la masa del suelo. la celda tiene una capacidad aproximada de 3 rrr'. ei experirnento se inici6 con la serie de ensayos en arena, luego en recebo y final mente en arcilla, una serie de ensayos cornprendio una medicion de velocidad, treinta y dos pruebas de humedad, dieciseis pruebas de densidad y una medicion de resistividad. en la arena se realizaron once series, en el recebo seis y en la arcilla veinte. la variable que se cambiaba era la humedad 0 la compactaci6n, y se media el cambio en velocidad y resistividad. ai terminar una serie de ensayos, el suelo se sacaba total mente de la celda, se iniciaba un nuevo llenado con el rnismo suelo al cual previamente se dosificaba sisternaticamente agua si se trataba de una variaci6n de hurnedad 0 se compactaba con otra energia, de rnanera que cada serie de ensayos requeria llenar y desocupar la celda para modificar la humedad 0 la compactaci6n segun fuera el caso. para la obtenci6n de imagenes de radar se utiliz6 un equipo sensors and software ekko 1000 de fabricaci6n canadiense y un conjunto de antenas con frecuencia de 450 mhz, las antenas se colocaron inicialmente con una distancia de 0.25 m y para cada pulso se separaban entre si 0.05m en forma equidistante a un punto central; 1a distancia maxima de separaci6n entre antenas que permitia la celda fue de 2.50 ill. la informacion obtenida se traslad6 a un cornputador para asi imprimir la matriz de datos tendiente al calculo de la velocidad. resultados en la tabla adjunta se resumen los resultados de laboratorio obtenidos. dentro del rango de humedades experimentado (5% 24%) se encontr6 una relaci6n lineal entre la hurnedad y la velocidad con geofislca colombiana. 7. diciembre de 2003 w('/~ ys cte dieleclrlca 25 20 <{ u ;;: 15 • arena.... . o w • recebo...j w • • arcillais 10 -*---w •.... u 5 ... ~'l!; 0 0 5 10 15 20 25 w('/~ figura 4 w('/~ ys cte dieleclrica .. ~ et: ~ 15r~-------~ ------.~-----"'--::7""'----j w ...j ~ 10(------------..,....~~_+_-----_:i w.... o oj.-. ------+----_-------+__--=='\ o 10 15 w/o figura 5 cte dielectrica ys vel(m'ns) 0.16 0.14 ""'"'.\~ ''i>"". ........ >w . . . ,'j" 0.12 0,1~ .!".s 0.08 ...j w > ojj5 0.04 0,02 o o 10 15 20 2s cte dielectrica figura 6 45 vi('/~vs coniluctmdillj 25 f--~---++---++-~/~-~---'1 2dl--~_-+/_'_!!f~--4 15+------7"/'--,-" --------1 10 1. ~-='/'t----~-~4jj r-r,(' ,arenii .~cebo .arciua vi('/~ figura 7 un coeficiente de correlaci6n de 0,94; la velocidad para la arena fluctuo entre 0,137 m/ns y 0,091 m/ns; para e! recebo entre 0,100 m/ns y 0,078 m/ns, y para la arcilla entre 0,12 mlns y 0,067 mlns. veanse figuras 4 y 5. conociendo la velocidad se puede calcular la constante dielectrica (tp(0,3n/) y se obtiene la figura 6. en la figura 7 se representa la relacion que existe ente la conductividad y la humedad para cada material en particular. conociendo la con stante dielectrica relativa para un suelo, la cua! involucra la parte salida y el fluido paral que puede ser agua y aire segun el grado de saturaci6n, y utilizando la siguiente f6rmula empirica propuesta para sue los se tiene: tr 0 vt = t5 ovs + ef .v; vi' volumen total vo!umen de vacios volumen de agua constante dielectrica de la parte salida con stante dielectrica del fluido poral constante dielectrica de! agua con stante dielectrics del aire vw tr• v r = t5 0 v, + tlv 0 viv + ta • va s=~ 5 = saturaci6n v, e. 'vr =£svs +(vv '5)+ £a (v" -v",) £1" vr =e,v, +(v" '5)+ £a (v" -vv '5) £1" vr =£,' v, + £",-( v,.'5+ £a' vv ,(1-5)) £1'=(_vr_-_v"]£s + s., -( v, '5) + -,£ae.:-·-,v"---,-(,---l-_s-l.) vr vr vr n = porosidad 46 moreno et al. 10 15 aj 25 gte llb.ectrca ,8=10'/0 s=2j'/0 • s;dl/o x s=>v'/o , s=6j'/0 .8'6j'/o + 8=70% .s8j'/0 s=ro'/o 8=1lx1'/o figura 8 cte dielectrica vs porosidad arenas 50 45 '" 35 ;f'. 0 3l -c 9 25 vj 0 "" 2d 0 ll 15 10 10 15 2d 25 30 cte dtelectrica figura 9 £1'=£5 -n'£5 + £", '5'/1+ £a'/1 -£a '5'/1 simplificando se obtiene la siguiente ecuaci6n: /1 e,-£",'s-£a(l-s) si:£5=4.0 £",=81 £a=l 4.0-£, n=--3-80'5 ,5=10% .5=20% .5=30% ,5=40% ,5=50% .5=60% + 5=70% .5=80% -5=90% 5=100% tabulando las expresiones anteriares, se obtienen las figuras 8 y 9 que permiten conocer la porosidad partiendo de e., estimando un parcentaje de saturaci6n. geofislca colombiana, 7, diciembre de 2d03 caracterizacl6n de dep6sltos de suelo utllizando geo.radar humedad i densidad resistividad velocidad conductivldad contante ensayo descrlpcion w% (kg'm3) (ohm-rn) (m/ns) (ms-m) dlelectrlca r i wn 5.25 1649.56 163.40 0.137 6.12 4.80 2 ei 5.42 1756.69 127.11 0.134 7.87 5.01 3 e2 5.36 1777.50 138.73 0.129 7.21 5.41 a 4 e3 5.48 1826.69 130.19 0.126 7.68 5.67 r 5 wi 5.55 1382.50 151.24 0.131 6.61 5.24 e 6 w2 6.35 1319.75 170.4 0.127 5.87 5.76 n 7 w3 8.24 1618.81 105.43 0.124 9.48 5.85 a 8 w4 11.91 1772.56 59.75 0.105 16.74 8.16 9 e3 9.90 1873.10 54.98 0.103 18.19 8.48 10 w5 13.39 1765.94 45.24 0.094 22.10 10.19--ii w6 15.32 1830.75 37.01 0.091 27.02 10.87 r i wn 13.06 1834.75 8570.27 0.100 0.12 9.00 e 2 el 12.96 1795.38 12528.67 0.105 0.08 8.16 c 3 e2 12.921 1923.88 8972.39 0.103 0.11 8.48 e 4 e3 12.77 1956.69 8884.42 0.096 0.11 9.77 b 5 wi 15.38 2118.81 4926.02 0.080 0.20 14.06 0 6 w2 15.24 2206.19 3056.77 0.078 0.33 14.79 1 wn 8.34 1553.50 3257.80 0.120 0.31 6.25 2 ei 9.31 1640.19 1985.50 0.118 0.50 6.46 3 e2 9.12 1666.56 1749.87 0.116 0.57 6.69 4 e3 8.91 1707.81 1658.76 0.118 0.60 6.46 5 wi 11.26 1555.75 1297.48 0.109 0.77 7.58 a 6 el 10.34 1646.44 810.85 0.115 1.23 6.81 r 7 e2 10.23 1662.63 865.19 0.113 1.16 7.05 c 8 e3 10.34 1663.63 850.43 0.113 1.18 7.05....-~. i 9 w2 11.90 1416.31 612.61 0.103 1.63 8.48 l 10 el 11.85 1526.56 696.49 0.106 1.44 8.01 l ii e2 11.90 1564.94 699.63 0.109 1.43 7.58 a 12 e3 11.82 1605.50 639.63 0.108 1.56 7.72 13 w3 14.19 1493.19 624.86 0.089 1.60 11.36 14 w4 16.20 1410.17 633.03 0.093 1.58 10.41 15 w5 19.66 1484.08 453.65 0.090 2.20 11.11 16 ei 17.80 1557.75 429.46 0.090 2.33 lui 17 e2 17.40 1642.19 407.46 0.095 2.45 9.97 18 e3 17.41 1653.75 407.15 0.088 2.46 11.62 19 w6 21.14 1646.13 243.47 0.072 4.11 17.36 20 w7 23.73 1747.60 225.56 0.067 4.43 20.05 geoflslca colomb lana. 7, oiciembre oe 2003 47 conclusiones y recomendaciones las correlaciones encontradas permiten evaluar la humedad, la porosidad y parametres geoelectricos como la conductividad y la constante dielectrica, una vez sea medida la velocidad de las ondas electromagneticas en el suelo prospectado. existe una relaci6n muy definida entre la humedad y la velocidad de las ondas electromagneticas para los suelos estudiados, de manera que el geo-radar, se constituye en una tecnica de evaluaci6n practica y rapida de este parametro. los datos presentan un buen grado de correlaci6n; sin embargo, se deben ajustar con ensayos en terreno y de esta forma llegar a relaciones confiables que se puedan utilizar en el ejercicio profesional. la prospecci6n con geo-radar permite conocer la con stante dielectrica del subsuelo, la cual se puede correlacionar con la litologia facilitando los procesos de caracterizaci6n para las capas someras, de igual forma puede ser asociada a la naturaleza del fluido intersticial convirtiendose en un medio de evaluaci6n de la contaminaci6n. para los niveles de compactaci6n utilizados no se encontr6 variaci6n significativa en la velocidad; sin embargo, el grado de compactaci6n se puede asociar con la porosidad, y por tanto, se debe continuar investigando sobre este tema. se deben desarrollar proyectos que permitan modelar y estudiar la interacci6n de la permitividad dielectrica y permeabilidad magnetica ante la influencia de ondas electromagneticas en suelos. el empleo de las ondas electromagneticas en la caracterizaci6n y monitoreo de procesos relacionados con sedimentaci6n, 48 morenoit al. consolidaci6n y cementaci6n en suelos, es un campo en pleno desarrollo que ofrece oportunidades de investigaci6n. referencias annan, p. a. ground penetrating radar workshop notes, july, 2002. bussell!, g., barber, c., davis, g. b. and salama, r. b. (1990). "detection of groundwater contamination near waste disposal sites with transient electromagnetic and electrical methods". in: ward, s. h. (ed), geotechnical and environmental geophysics. vol. 2: environmental and groundwater. tulsa: society of exploration geophysicists, 27-39 daniels, d. j., gunton, d. j. and scott, h, f. (1998). introduction to subsurface radar. iee proceeding, 135 (f,4): 278-320. das, b. m. (1983). advanced soil mechanics, mcgraw-hill international editions. da vis, j. l. and annan, a. p. (1989). "ground-penetrating radar for high resolution mapping of soil and rock stratigraphy". in: geophysical prospecting, 37(5): 531-551. dobrin, m. b. and savit, c. h. (1988). introduction to gen!"l,ysicalprospecting, 4th edn. new york: mcgraw-hill. johnson, r. w., glaccum, r. and wojtasinski, r. (1979). "application of ground penetrating radar to soil survey. soil survey". soil and crop science society of florida proceedings, 39,2-4 october: 68-72. reynolds, john m. (1998). an introduction to applied and environmental geophysics. new york: john wiley and song. santamarina, carlos. soils and waves, primera edici6n 2001. john wiley and sons ltd. sarria, alberto. (1991). introduccion a los metodos geofisicos en la ingenieria civil, 2a. edici6n, facultad de ingenieria uniandes. sheriff, r. e. (1991). encyclopedic dictionary of exploration geophysics, 3rd ed. tulsa: society of exploration geophysicists. geofislca colombiana, 7, oiciembre oe 2003 an artificial neural network was used for forecasting of long-term wind speed data (24 and 48 hours ahead) in la serena city (chile). in order to obtain a more effective correlation and prediction, a particle swarm algorithm was implemented to update the weights of the network. 43800 data points of wind speed were used (years 20032007), and the past values of wind speed, relative humidity, and air temperature were used as input parameters, considering that these meteorogical parameters are more readily available around the globe. several neural network architectures were studied, and the optimum architecture was determined by adding neurons in systematic form and evaluating the root mean square error (rmse) during the learning process. the results show that the meteorological variables used as input parameters, have influential effects on the good training and predicting capabilities of the chosen network, and that the hybrid neural network can forecast the hourly wind speed with acceptable accuracy, such as: rmse=0.81 [m·s−1], mse=0.65 [m·s−1]2 and r2=0.97 for 24-hours-ahead wind speed prediction, and rmse=0.78, mse=0.634 [m·s−1]2 and r2=0.97 for 48-hours-ahead wind speed prediction. abstract keywords: wind speed; time series forecasting; artificial neural network; particle swarm optimization; meteorological data. long-term prediction of wind speed in la serena city (chile) using hybrid neural network-particle swarm algorithm predicción a largo plazo de la velocidad de viento en la ciudad de la serena (chile) utilizando un algoritmo híbrido de red neuronal-enjambre de partículas issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.50337 juan a. lazzús* and ignacio salfate departamento de física y astronomía, universidad de la serena, casilla 554, la serena, chile *jlazzus@dfuls.cl una red neuronal artificial fue utilizada para la predicción de datos de la velocidad de viento a largo plazo (24 y 48 horas en adelanto) en la ciudad de la serena (chile). para obtener una efectiva correlación y predición, se implementó una optimización de enjambre de particulas para actualizar los pesos de la red. se emplearon 43800 datos de velocidad de viento (años 2003-2007), y los valores pasados de velocidad del viento, humedad relativa y temperatura del aire fueron utilizados como parámetros de entrada, considerando que estos parámetros meteorológicos se encuentran fácilmente disponibles en todo el mundo. se estudiaron varias arquitecturas de redes neuronales y la arquitectura optima se determine añadiendo neuronas de forma sistemática y evaluando la raíz del error cuadrático medio (rmse) durante el proceso de aprendizaje. los resultados muestran que las variables meteorológicas utilizadas como parámetros de entrada, tienen un efecto positivo sobre el correcto entrenamiento y capacidades predictivas de la red, y que la red neural híbrida puede pronosticar la velocidad del viento horaria con una precisión aceptable, como un rmse=0.81 [m·s−1], mse=0.65 [m·s−1]2 y r2=0.97 para la predicción de la velocidad del viento de 24 horas en adelanto, y un rmse=0.78, mse=0.634 [m·s−1]2 and r2=0.97 para la predicción de la velocidad del viento de 48 horas en adelanto. resumen palabras clave: velocidad del viento; predicción de series de tiempo; redes neuronales artificiales; optimización de enjambre de particulas; datos meteorológicos. record manuscript received: 30/04/2015 accepted for publication: 17/02/2017 how to cite item lazzus, j. a. & salfate, i. (2017). long-term prediction of wind speed in la serena city (chile) using hybrid neural network-particle swarm algorithm. earth sciences research journal, 21(1). 29-35. doi:http://dx.doi.org/10.15446/esrj.v21n1.50337 m e t e o r o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 29 35 30 juan a. lazzús and ignacio salfate 1. introduction energy derived from wind has played a vital role in the history of mankind and is again receiving considerable attention because of its free and non-polluting character. with the development of wind energy technologies and the decrease of wind power production cost, wind power has rapidly developed around the world in recent years (akdağ and guler, 2011). as a clean energy source wind is considered an alternative to fossil fuels, which actually accelerate global warming. the first scientific research to utilize wind for generating electricity, was initiated by the danish in the 1960s. the 1973 energetic crisis, forced many governments to realize the value of wind, as a renewable and independent energy source (hanağasioğlu, 1999). electricity generation using wind energy has been well recognized as environmentally friendly, socially beneficial, and economically competitive for many applications (monfared et al., 2009). prediction of wind speed (ws) at the surface or near the surface, is essential in many areas of science and technology, e.g., wind energy generation, aviation, space vehicle launching, weather forecasting, and agro-meteorology (kulkarni et al., 2008). wind field prediction at the level of wind farm, is still a challenging problem. different methods have been developed (kallos et al., 2007); and several studies have been performed to estimate the wind potential in different parts of the world (çam and yildiz, 2006). there are various strategies for wind speed prediction that can be classified into two categories: (1) statistical methods that can be subdivided into numerical weather prediction (nwp) and persistence and (2) artificial intelligence techniques that have subdivisions such as artificial neural networks (ann) and fuzzy logic (monfared et al., 2009). with the developments made in chaos theory, researchers have looked for the determinism in various seemingly chaotic-looking fluctuations from different disciplines such as physics, chemistry, hydrology, atmospheric sciences, etc (karunasinghe and liong, 2006). the time series prediction is one of the most important aspects in chaos theory. time series contain much information about dynamic systems (han and wang, 2009). these systems are usually modeled by delay-differential equations. some of them, for example, the mackey–glass equation (mackey and glass, 1977), the ikeda equation (ikeda, 1979), and equation for an electronic oscillator with delayed feedback (chua et al., 1992), are standard examples of time-delay systems (bezruchko et al., 2001). the main problem of the time series study consist of predicting the next value of a series known up to a specific time, using the known past values of the series. in time series prediction, this is usually first embedded in a state space using delay coordinates: where x(t) is the value of the time series at time t, τ a suitable timedelay and d the order of the embedding. this embedded vector is then used to predict the next value of the series x(t + τ). therefore, the non-linear dependence of the level of a series on previous data points is of interest, partly because of the possibility of producing a chaotic time series. note that short-term prediction for chaotic time series have been widely investigated by several techniques (karunasinghe and liong, 2006), however, the long-term prediction has not been widely studied in the literature. in this work, chaotic time series data taken from the mackey–glass differential equation were used to develop a neural network. in order to still obtain a more effective correlation and prediction, particle swarm algorithm has been introduced to update the weights of all layers of the network. next, this hybrid algorithm was used in the long-term prediction of the next 24 and 48 hours of the wind speed time series. to the best of the authors’ knowledge, there is no application for the prediction of the wind speed that includes the long-term prediction, such as the one presented here. 2. computational method a feed-forward neural network was used to represent non-linear relationships among variables. this ann was implemented by replacing standard back-propagation algorithm with particle swarm optimization (pso). pso is a population-based optimization tool, where the system is initialized with a population of random particles and the algorithm searches for optima by updating generations (eberhart and kennedy, 1995). in each iteration, the velocity of each particle j is calculated according to the following formula (lazzús, 2011): where s and v denote a particle position and its corresponding velocity in a search space, respectively. k is the current step number, ω is the inertia weight, c1 and c2 are the acceleration constants, and r1, r2 are elements from two random sequences in the range (0,1). k js is the current position of the particle, ψ k jy is the best one of the solutions that this particle has reached, and ψg is the best solutions that all the particles have reached. in general, the value of each component in v can be clamped to the range [–vmax ,vmax] control excessive roaming of particles outside the search space (kennedy et al., 2001). after calculating the velocity, the new position of each particle is: the total steps to calculate the output values, using the input values of the network were as follows (lazzús et al., 2014): where xi is the input variables i, min ix and max ix are the smallest and largest value of the data, thus the input data are normalized using this equation. next, the net inputs (n) are calculated for the hidden neurons coming from the inputs neurons. for a hidden neuron: where pi is the vector of the inputs of the training, , h i jw is the weight of the connection among the input neurons with the hidden layer h, and the term , h i jb corresponds to the bias of the neuron of the hidden layer h, reached in its activation (freeman and skapura, 1991). the pso algorithm is very different from any of the traditional methods of training (lazzús, 2011). each neuron contains a position and velocity. the position corresponds to the weight of a neuron while the velocity is used to update the weight . starting from these inputs, the outputs (yi) of the hidden neurons are calculated, using a transfer function f h associated with the neurons of this layer (freeman and skapura, 1991). to minimize the error, the transfer function f should be differentiable. in the ann, the hyperbolic tangent function (tansig) was used as all the neurons of the ann have an associated activation value for a given input pattern; the algorithm continues finding the error that is presented for each neuron, except those of the input layer. after finding the output values, 1 1k+ k k+ j j js = s +v 31long-term prediction of wind speed in la serena city (chile) using hybrid neural network-particle swarm algorithm the weights of all layers of the network are actualized by pso, using equations 2 and 3 (lazzús et al., 2014). the velocity is used to control how much the position is updated. on each step, pso compares each weight using the data set. the network with the highest fit ness is considered the global best. the other weights are updated based on the global best network rather than on their personal error or fitness (pérez ponce et al., 2012; lazzús et al., 2014). in this article, we used the mean square error (mse) to determine network fitness for the entire training set: where yi is the output value obtained from the normalized output (yi) of the network. this process was repeated for the total number of patterns to training. for a successful process the objective of the algorithm is to modernize all the weights by minimizing the total root mean squared error (rmse): figure 1 presents a block diagram of the ann+pso algorithm developed in this study. in pso, the inertial weight ω, the constant c1 and c2, the number of particles npart and the maximum speed of particle summarizes the parameters to synchronize for their application in a given problem. an exhaustive trial-anderror procedure was applied for tuning the pso parameters. table 1 shows the selected parameters for this hybrid algorithm. 3. simulations 3.1. mackey-glass time series to evaluate the capability of the proposed hybrid algorithm in the long-term prediction, the mackey–glass time series was used. thus, a set of data points were generated from the mackey–glass time-delay differential equation (mackey and glass, 1977; farmer, 1982) which is defined by: (11) where t is a variable, x is a function of t, and τ is the time delay. the initial values of the time series are α = 0.2, β = 0.1, and x(0)=1.2. if τ ≥ 17, the time series show the chaotic behaviour (farmer, 1982; mirzaee, 2009). the goal of the task is to use known values of the time series up to the point x=t to predict the value at some point in the future x=t+t. the standard method for this type of prediction is to create a mapping from d points of the time series spaced apart, that is , to a predicted future value x(t+t). in order to solve the mackey–glass equation, the fourth-order runge–kutta method was applied to find the numerical solution. the time series was obtained evaluating the solution of eq. (11) at each integer points. step size of 0.1 was used to generate a time series, and x(t) is thus derived for with x(t)=0 for t <0 in the integration. four non consecutive points in the time series are given to generate each input vector xi (where i=1, 2, …, n) of the input matrix x, as: (12) a similar criterion was used to create the output matrix, as: (13) figure 1. flow diagram for training of the ann using pso algorithm table 1. parameters used in the hybrid ann+pso algorithm. 32 juan a. lazzús and ignacio salfate then, the ann+pso method was used in the long-term prediction. this hybrid algorithm was trained to predict the future value x(t+84) from the current value x(t) and the past values, using the standard form applied in the literature, for d=4 and ∆=t=6 (chng et al., 1996; mirzaee, 2009). (14) one thousand data points of the above format were collected. the first 500 were used for training while the others were used for testing the ann+pso method. then, the following case was simulated with α = 0.2, β = 0.1, x(0) = 1.2 and τ = 17. from this case, several network architectures were tested. the most basic architecture normally used for the analysis of chaotic time series involves a neural network consisting of three or four layers (lazzús et al., 2014). the input layer contains one neuron for each input parameter: x(t), x(t–6), x(t–12), and x(t–16). the output layer has one node generating the scaled estimated value of the chaotic time series x(t+84). the number of hidden neurons needs to be sufficient to ensure that the information contained in the data utilized for training the network is adequately represented (lazzús et al., 2014). there is no specific approach to determine the number of neurons of the hidden layer, many alternative combinations are possible. the optimum number of neurons was determined by adding neurons in systematic form and evaluating the mse and rmse of the sets during the learning process (pérez ponce et al., 2012; lazzús et al., 2014). for our case the optimum architecture was 4-12-1. the results obtained with the ann+pso method present a mse=0.000063 and rmse=0.0079 for training set, and mse=0.000065 and rmse=0.0080 for prediction set. these results show that the ann+pso model can be accurately trained and that the chosen architectures can predict the long-term x(t+84) with acceptable accuracy. table 2 shows a comparison between some computational methods found in the literature (martinetz et al., 1993; whitehead and choate, 1996; bersini et al., 1997; awad et al., 2009) and the result obtained with the ann+pso method. this comparison was made using the normalized root mean squared error (nrmse), defined as: (15) table 2. comparison between computational methods found in the literature for the long-term prediction. 3.2. wind speed time series once the capability of the hybrid algorithm was proved, it was used to forecast the long-term of wind speed time series. this study is based in data collected from a meteorological stations located on the semi-arid norte chico of chile (29º54’ s; 71º15’ w; 10 m), located at south of the hyper-arid atacama desert. the region is characterized by complex topography with altitudes varying from sea level until 5000 m at the high andes cordillera. the climatic characteristics are influenced by the south-eastern pacific subtropical anticyclone and the cold humbolt current which results in low precipitation rates (kalthoff et al., 2002). the zone is one of the most sensitive areas in south america (kalthoff et al., 2006), and recent studies of oceanic and atmospheric variability have confirmed the implications of the dynamics of el niño-southern oscillation (enso) cycle on the local climate of this zone (meinen and mcphaden, 2009). 43800 data points (years 2003-2007) were used. the future values of wind speed was predicted using the past values of the time series of wind speed ws(m·s−1), relative humidity rh(%), and air temperature t(k). figure 2 shows the time series of the selected meteorological data used. the data ranges and the properties of interest are listed in table 3. as seen in this table, hourly ws cover wide ranges, going from ≈0 to 10 (m·s−1). other wide range for the input parameters are: t from 270 to 305 (k), and rh from 2 to ≈100 (%). the influence of the selected meteorological data in this study (t, rh, and ws) over the climate of the semi-arid zone of the atacama desert has been revised and evaluated in other communications (kalthoff et al., 2002; kalthoff et al., 2006). figure 3 shows ws as a function of the selected meteorological data: t and rh. fig. 3a shows ws as a function of t with a coefficient of linear correlation (r2) of 0.5979. fig. 3b shows ws as a function of rh with r2 of 0.2757. note that the coefficient of linear correlation in this figure shows a non-linear relationship between table 3 summary of data used in this study. figure 2. time series of the meteorological data used in this study. (a) wind speed ws/ms-1, (b) air temperature t/k, and (c) relative humidity rh/%. 33long-term prediction of wind speed in la serena city (chile) using hybrid neural network-particle swarm algorithm ws and the input parameters for this climate zone. then, the relationship between ws and these meteorological data is highly non-linear, and consequently an ann is the best alternative to model the hourly ws. two new cases were studied with this methodology, the long-term prediction of the next 24 hours ws(t+24) and the long-term prediction of the next 48 hour ws(t+48). to select the best input parameters for solving the problem, the hourly data from the current value to 23 past hours (t-23, t-22, t-21,… , t), were considered. then, the sum of absolute values of weights (savw) was used (lazzús, 2013). figure 4 shows the input with more contribution for the prediction of future values of ws. this figure shows the great significance of the past values (t-18), (t-12), (t-6), and the current value (t) on the three meteorological data (ws, t, and rh). figure 3. experimental data of ws from 2003 to 2007 as a function of the selected meteorological parameters. a) ws vs t →r2=0.5979; (b) ws vs rh →r2=0.2757. thus, the optimum input vector was: (16) where τ is the future value to predict: 24 and 48 for the long-term prediction. figure 4. influence of several points of time series on prediction of future values of wind speed. bars represent the sumsof absolute values of weights (savw) of the ann+pso. the leave-20%-out cross-validation method was used to estimate the predictive capabilities of the model. 34996 data points were used in the training set, and 8760 data points (not used in the training step) were used in the prediction set. 4. results and discussion several network architectures were tested for the long-term wind speed prediction (t(t+24) and t(t+48), separately). the optimum architecture was checked using the objective function (eq. 10). figure 5 shows mse found in correlating the ws as function of the number of neurons in the hidden layer (nn). fig. 5a shows the best topology found for the prediction of the ws for the next 24 hours t(t+24) with a network architecture 12-28-1. fig. 5b shows the prediction of the ws for the next 48 hours t(t+48) with an optimum network architecture of 12-42-1. once the best architectures were determined, the optimum weights and biases required to carry out the estimate of future values of ws were obtained. figure 6 shows a comparison between real data (black line) and calculated values (points) of long-term prediction of ws(t+24). fig. 6a shows the forecasting of ws(t+24) with a correlation coefficient (r2) of 0.974 for the training set (years 2003-2006) with a slope of the curve (m) of 0.965 (expected to be 1.0), and with rmse=0.787 [m·s−1] (mse=0.620 [m·s−1]2 and msemax=10.028 [m·s-1]2). fig. 6b shows the forecasting of ws(t+24) with r2=0.973 during the prediction step (year 2007) with m=0.963 (also expected to be 1.0), and with rmse=0.807 [m·s−1] (mse=0.651 [m·s−1]2 and msemax=7.110 [m·s−1]2). figure 5. deviations found in the correlation of wind speed as a function of the number of neurons in the hidden layer for: (a) long-term prediction ws(t+24), and (b) long-term prediction ws(t+48). in both graphics, training step (■) and prediction step (○). 34 juan a. lazzús and ignacio salfate figure 6. comparison between real and calculated values of ws(t+24) using the proposed ann+pso model. (a) training set, period 2003-2006; and (b) prediction set, period 2007. figure 7 shows a comparison between real data (black line) and calculated values (points) of long-term prediction of ws(t+48). fig. 6a shows the forecasting of ws(t+48) with r2=0.970 for the training set with m=0.963, and with rmse=0.788 [m·s−1] (mse=0.622 [m·s−1]2 and msemax=9.386 [m·s−1]2). fig. 6b shows the forecasting of ws(t+48) with r2=0.969 for the prediction set with m=0.962, and with rmse=0.796 (mse=0.634 [m·s−1]2 and msemax=8.210 [m·s−1]2). a comparison was made with a neural network with standard back-propagation (bpnn) algorithm (hagan and menhaj, 1994), and similar architecture and database. this bpnn show results of mse higher than 1 [m·s−1]2 and r2 lower 0.8 for the forecasting of ws(t+24) and ws(t+48). and other comparison was made with a multiple linear regression (mlr) method, and similar database. the mlr method shows mse higher than 4 [m·s−1]2 and r2 lower 0.7 for both cases. note that, the predictions with the proposed ann+ pso method shows mse a little higher than 0.6 [m·s−1]2 and r2 higher than 0.97. table 4 summarizes the deviations obtained in the long-term prediction using the proposed method versus bpnn and mlr methods. these results show that the ann+pso can be accurately trained and that the chosen topologies can estimate the future values of ws with acceptable accuracy. these results represent a tremendous increase in accuracy for forecasting this important meteorological property and show that not only the optimum architecture obtained was crucial, also the appropriate selection of the independent parameters (t and rh). this is important because air temperature and relative humidity are commonly available parameters. note that the coefficients of linear correlation of these parameters show a non-linear relationship with the ws for the climate of several geographic zones. then, the relationship between ws and these meteorological data is highly non-linear, and consequently the ann+pso is a good tool for modeling ws for several applications. figure 6. comparison between real and calculated values of ws(t+48) using the proposed ann+pso model. (a) training set, period 2003-2006; and (b) prediction set, period 2007. the results obtained by other models and other sites can be usefully compared with our results. zhang et al. (2012) shows the performance analysis of four modified approaches for wind speed forecasting for four observation sites in gansu (china), with mse higher than 2 [m·s−1]2. liu et al. (2013) shows the forecasting models for wind speed using wavelet, wavelet packet, time series and artificial neural networks with mse higher than 1 [m·s−1]2. for spain, the wind speed estimation was made using a multilayer perceptron with mse higher than 1 [m·s−1]2 and r2 below 0.75 (velo et al., 2014). recently, wang et al. (2014) shows the mean hourly wind speed prediction in the hexi corridor of china based on the seasonal adjustment method (sam), exponential smoothing method (esm), and radial basis function neural network (rbfn), with rsme higher than 0.7 [m·s−1]. it must be mentioned that these results were obtained from different sites and based on different weather conditions, and the results cannot be compared directly with one another. however, results from the different methods show that the accuracy of ann+pso model employed in this study is good. 5. conclusions in this work, a neural network was used for the forecasting of longterm wind speed time series. in order to obtain a more effective correlation and prediction, particle swarm algorithm has been introduced to update the weights of all layers of the network. 43800 data points (years 2003table 4. summary of the deviations obtained with the ann+pso algorithm for the long-term prediction of wind speed.. 35long-term prediction of wind speed in la serena city (chile) using hybrid neural network-particle swarm algorithm 2007) of wind speed were used. to distinguish between the different values of hourly data considered in this study, so that the network can discriminate and learn in optimum form, the following past values of the time series of meteorological data were used as input parameters: wind speed ws(m·s-1), relative humidity rh(%), and air temperature t(k). based on the results and discussion presented in this study, the following main conclusions are obtained: i) the results show that the proposed ann+pso can be properly trained for predicting the hourly wind speed, with acceptable accuracy; ii) the meteorological variables used (ws, t, and rh), have influential effects, on the good training and predicting capabilities, of the chosen network; iii) the low deviations found with the proposed ann+pso method indicate that it can predict the future values of ws with better accuracy than other methods; and iv) the values obtained with the proposed method are believed to be sufficiently accurate for engineering calculations, among other uses. acknowledgements the authors thank the direction of research and development of the university of la serena (diduls) through the research project peq16141, and the department of physics of the university of la serena (dfuls) for the special support that made possible the preparation of this paper. special acknowledgements to dr. pedro vega (uls) for the support that made possible the implementation of the meteorological station and to mg. julio marín (uls) for providing the meteorological data set. references akdağ, s.a. and güler ö. 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(2012). performance analysis of four modified approaches for wind speed forecasting. applied energy, 99, 324–333. new models using scale analysis and perturbation methods were derivated starting from the shallow water equations based on barotropic fluids. in the paper, to discuss the irregular topography with different magnitudes, especially considering the condition of the vast terrain, some modified quasi-geostrophic barotropic models were obtained. the unsteady terrain is more suitable to describe the motion of the fluid state of the earth because of the change of global climate and environment, so the modified models are more rational potential vorticity equations. if we do not consider the influence of topography and other factors, the models degenerate to the general quasi-geostrophic barotropic equations in the previous studies. este trabajo deduce nuevos modelos con el uso de los métodos de análisis a escala y de perturbación a partir de las ecuaciones de aguas poco profundas con base en fluidos barotrópicos. en este artículo se obtuvieron algunos modelos semigeostróficos barotrópicos para aplicar en zonas de topografía inestable con diferentes magnitudes y considerar especialmente la condición del extenso terreno. la topografía inestable es más propicia para describir el movimiento del estado fluido de la tierra debido al cambio del clima y ambiente, por lo tanto los modelos modificados son ecuaciones de vorticidad potenciales más razonables. si no se considera la influencia de la topografía y otros factores, los modelos se reducirían a las ecuaciones generales semigeostróficas barotrópicas de estudios anteriores. abstract resumen keywords: quasi-geostrophic; potential vorticity; scale analysis; topography. palabras clave: semigeostrofía; vorticidad potencial; análisis a escala; topografía. the modified quasi-geostrophic barotropic models based on unsteady topography record manuscript received: 01/03/2017 accepted for publication: 31/03 /2017 how to cite item zhao, b., sun, w., & zhan, t. (2017). the modified quasi-geostrophic barotropic models based on unsteady topography. earth sciences research journal, 21(1), 23-28. doi:http://dx.doi.org/10.15446/esrj.v21n1.63007 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.63007 modelos semigeostróficos barotrópicos modificados con base en topografía inestable g e o m o r ph o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 23 28 baojun zhao 1, wenjin sun 1,*, tianming zhan 2 1 college of oceanography, hohai university, nanjing 210098, china 2 school of technology, nanjing audit university, nanjing 211815, china *email of corresponding author: sunwenjin950925@126.com 24 baojun zhao, wenjin sun and tianming zhan 1. introduction in recent decades, many scholars have conducted extensive research on large-scale atmospheric and oceanic dynamics. among them, the topographic effect has great influences on the dynamic mechanism of potential vorticity equations (pedlosky, 1974; collings, 1980; pedlosky, 1980; treguier, 1989). in the atmosphere, the topographic height plays an importance role in atmospheric cyclone and anticyclone changes. luo (1990) pointed out the topographic effect was also an important factor of forming atmospheric blocking, and the global atmospheric circulation would even get affected by the topographic effect. lu (1987) discussed the effects of topographic height and shaped on rossby wave activation and the influences of topographic south-north and east-west slopes on waveform and energy propagation. chen (1998) and jiang (2000) derived the quasi-geostrophic potential vorticity equation with large-scale topography, friction, and heating under the barotropic model, and the large-scale effects of qinghai-tibet plateau on atmosphere were discussed. in addition, the oceanic topography is very complex, such as the north west shelf of australia (holloway, 1997), portugal shelf sea area (sherwin, 2002), etc. the relationship between topography and ocean circulation was pointed out in the literature (roslee et al., 2017b; kamsani, 2017; la, 1990; marshall, 1995; alvarez, 1994; sou, 1996). cessi (1986) discussed the important role of topography in ocean circulation. holloway (1992) introduced the interaction of eddies with seafloor topography and argued that ocean circulations would be significant interaction between turbulent vortices and topography rather than gravity wave drag. then, general expressions for the eddy-topographic force, eddy viscosity, and stochastic backscatter, as well as a residual jacobian term, are derived for barotropic flow over mean topography by frederiksen (1999). all the above researches, actually the topography height also changes with time in the earth fluid. changes in topography can lead to tsunamis, floods and natural disasters(abdullah, 2017; elfithri, 2017; rahim, 2017). yang (2011,2012) and song (2012, 2013) discussed topography changes over time discussed topography changes over time t, the influence of the nonlinear long wave amplitude and waveform. da (2013) discussed the shallow water equation forms when underlying surface slowly changes with time and obtained the vorticity equation with an underlying surface. this model considered the actual circumstances that the topography changes with time-space (erfen et al., 2017; roslee, 2017a) . in this paper, we discuss different magnitude topography under spatial-temporal variable and obtain some modified models, which have important effects on the future discussion about the waveform changes of nonlinear long waves. this paper is organized as follows: in section 2, starting from the rotating shallow water equation set, we give the bottom topography which is not smooth boundary and simplify the equation set with unsteady topography. section 3 is given scale analysis and perturbation methods; then we obtain new equation set by topographic conditions with different magnitudes. later we derive the equations with different orders and get some modified models in section 4. finally, make the relevant conclusions in section 5. 2. basic equations assuming the static equilibrium condition, the fluid can be regarded as barotropic, incompressible, frictionless state. the upper boundary height is ( , , )h x y t , and , free surface pressure intensity is constant. the basic equation set can be written as (pedlosky, 1987) (1a) (1b) (1c) (1d) especially, the bottom topography is about spatial-temporal variable ( , , )b bh h x y t= , the coriolis parameter is (2) first of all, we integrate eq. (1c) (3) and get (4) so (5) (6) eqs. (5) and (6) indicate that the horizontal pressure gradient under the barotropic model can be expressed by the gradient of the free surface gravitational potential (taharin & roslee, 2017). we assume that the preliminary horizontal velocity is independent of the z , eqs. (1a) and (1b) can be rewritten as (7a) (7b) then integrate continuity eq. (1d) from ( , , )bz h x y t= to the free surface ( , , )z h x y t= , we can get (7c) assuming that the free surface height h is constant when the fluid is static (pedlosky, 1987). (8) the barotropic equation set with unsteady topography can be written as (9a) 25the modified quasi-geostrophic barotropic models based on unsteady topography (9b) (9c) if (10) the equation set can be written as (11a) (11b) (11c) 3. scale analysis, perturbation methods making dimensionless analysis on the eqs. (11a), (11b) and (11c), for the large-scale motions (12) where l is an undetermined parameter. substituting eq. (12) into eqs. (11a), (11b) and (11c), then (13a) (13b) (13c) eqs. (13a) and (13b) divided by 0f u , (13c) divided by 2 0c u ghu l l = , we get (14a) (14b) (14c) where (15) let (16) meantime so (17) then we get (18a) (18b) (18c) where (19) 4. derive the barotropic models making classified discussion on the magnitude of λ. 4.1. λ ~ 1 magnitude the scale of the ϕb is consistent with the small amplitude function ϕ, most of the topography parameters following with this situation in the real world. 0ro order approximation (20) or a dimensional form (21) (0) (1) 1 1 1u u rou= + + (0) (1) 1 1 1v v rov= + + (0) (1) 1 1 1 ˆ̂̂ rof f f= + + 26 baojun zhao, wenjin sun and tianming zhan 1ro order approximation (22a) (22b) (22c) after calculation, the dimensionless vorticity equation is derived (23a) (23b) eliminating from the eqs. (23a), (23b), we get (24) namely (25) where (26) λr is the rossby radius of deformation, eq. (25) is a modified model. 4.2. λ ~ 10 magnitude large-scale atmospheric motion of large topography is suitable for such conditions (l ~ 106 m, u ~ 10m/s, f 0 ~ 10-4 s -1 ). for example, a case study of tibetan plateau topography, the height is 3 4 ×103m approximately which is suit for the situation. 0ro order approximation (27) or a dimensional form (28) in order to get the results, we should consider the magnitude of , according to the eq. (18c), 2 0m can be considered as a parameter. 1ro order approximation (29a) (29b) (29c) or dimensional form (30a) (30b) (30c) eqs. (30a), (30b) and (30c) can be transformed into the vorticity equations (31a) (31b) eqs. (31a), (31b) are quasi-geostrophic barotropic models, so (32a) (32b) where (33) 27the modified quasi-geostrophic barotropic models based on unsteady topography assuming , we eliminate from eqs. (32a) and (32b), then (34) finally (35) eq. (35) is a modified model. 4.3. λ ≤ 10 -1 magnitude according to (18c), it is observed that the magnitude of topography expression form λ is too small, which approximately ignores the influences of topographic effect. now we can observe that the eqs. (25), (35) are two quasigeostrophic barotropic models under variable topographic conditions with time t . if the topographic effect is independent of time, equations degrade into the following forms. under the stable topography, eq. (35) degenerates into (36) where (37) the model (25) under the stable topography can be written as (pedlosky,1987) (38) where (39) otherwise, if we don’t consider the topographic effect and the divergence of the fluid, eqs. (36) and (38) degenerate into (liu, 1991) (40) eq. (40) is a classics dynamics model used by the large-scale atmospheric and oceanic motions. 5. conclusion (a). under the unsteady topography, some new modified models (25), (35) are derived. these models meet the general rule that the topography changes with time in reality. when the topography has nothing to do with the time, eq. (35) degenerates into a dynamics model (36), eq. (25) degenerates into eq. (38) which is a quasi-geostrophic barotropic model under the spatial topography. (b). the modified models under the topographic effect with different magnitudes are presented, we can see the pattern under the condition of large terrain, which is the improvement of the model. after the above-modified models are given, we will also derive the mathematical model for rossby wave in the further study, and the further exploration of the large-scale factual influences of topography on atmosphere and ocean are required. acknowledgements this work was supported by the national nature science foundation of china under grant (no. 61502206), key projects of water conservancy science and technology of jiangsu province, china (no. 2010500312), and the research innovation program for college graduates of jiangsu province (no .cxzz12_0224). references abdullah, m. p. (2017). stakeholders’ response and perspectives on flood disaster of pahang river basin. malaysian journal of geosciences, 1(1) 43-49. alvarez, a. (1994). effect of topographic stress on circulation in the western mediterranean. journal of geophysical research atmospheres, 99(c8), 16053-16064. cessi, p., & pedlosky, j. (1986). on the role of topography in the ocean circulation. journal of marine research, 44 (3), 445-471. chen, j., & liu, s. k. (1998). the solitary waves of the barotropic quasi-geostrophic model with large-scale orography. advances in atmospheric sciences, 15 (03), 404-41. collings, i. l., & grimshaw, r. (1980). the effect of topography on the stability of a barotropic coastal current. dynamics of atmospheres and oceans, 5(2), 83-106. da, c. j. (2013). dynamic modification of the shallow water equation on the slowly changing underlying surface condition. acta physica sinica, 62(3), 039202. elfithri, r. (2017). pahang flood disaster: the potential flood drivers. malaysian journal of geosciences, 1(1) 34-37. erfen, h. f. w. s. (2017.) geochemical characterization of sediments around nukakatan valley, tambunan, sabah. geological behavior, 1(1)13-15. frederiksen, j. s. (1999). subgrid-scale parameterizations of eddytopographic force, eddy viscosity, and stochastic backscatter for flow over topography. journal of the atmospheric sciences, 56(11), 1481-1494. holloway, g. (1992). representing topographic stress for large-scale ocean models. journal of physical oceanography, 22(9), 1033-1046. holloway, p. e., et al., 1997. a nonlinear model of internal tide transformation on the australian north west shelf. journal of physical oceanography, 27(6), 871-896. jiang, x. (2000). a barotropic quasi-geostrophic model with largescale topography, friction and heating. journal of tropical meteorology, 6(01), 65-74. la, v., & paul, e. (1990). the western mediterranean circulation experiment (wmce): introduction. journal of geophysical research: oceans, 95(c2), 1511-1514. liu, s. k., & liu, s. d. (1991). atmospheric dynamics. beijing, bj: peking university press. lu, k. l. (1987). the effects of orography on the solitary rossby waves in a barotropic atmosphere. acta meteorologica sinica, 45, 267-273. luo, d. h. (1990). topographically forced rossby wave instability and the development of blocking in the atmosphere. advances in atmospheric sciences, 7(4), 433-440. marshall, d. (1995). influence of topography on the large-scale ocean circulation. journal of physical oceanography, 25(7), 1622-1635. 28 baojun zhao, wenjin sun and tianming zhan pedlosky, j. (1974). longshore currents, upwelling and bottom topography. journal of physical oceanography, 4(2), 214-226. pedlosky, j. (1980). the destabilization of shear flow by topography. journal of physical oceanography, 10(11), 1877-1880. pedlosky, j. (1987). geophysical fluid dynamics second edition. springer, new york, 57-246. rahim, i. a. (2017). kampung mesilou landslide: the controlling factors. geological behavior, 1(1) 19-21. roslee, r. (2017). gis application for comprehensive spatial soil erosion analysis with musle model in sandakan town area, sabah, malaysia. geological behavior, 1(1) 01-05. roslee, r. (2017). flood potential analysis (fpan) using geo-spatial data in penampang area, sabah. malaysian journal of geosciences, 1(1) 01-06. sherwin t. j., vlasenko v. i., & stashchuk, n. (2002). along-slope generation as an explanation for some unusually large internal tides. deep sea research part i: oceanographic research papers, 49(10), 1787-1799. song, j., et al., 2012. beta effect and slowly changing topography rossby waves in a shear flow. acta physica sinica, 61(21), 210510, http:// dx.doi.org/10.7498/aps.61.210510. song, j. (2014). nonlinear rossby waves excited slowly changing underlying surface and dissipation. acta physica sinica, 63(6), 060401, http://dx.doi.org/10.7498/aps.63.060401. sou, t. (1996). effects of topographic stress on caribbean sea circulation. journal of geophysical research: oceans, 101(c7), 16449-16453. taharin, m. r., & roslee, r. (2017). comparison of cohesion (c’), and angle of internal friction (ф’) distribution in highland area of kundasang by using ordinary kriging and simple kriging. geological behavior, 1(1) 16-18. treguier, a. m. (1989). topographically generated steady currents in barotropic turbulence. geophysical and astrophysical fluid dynamics, 47, 43-68. yang, h. w. (2011). forced solitary rossby waves under the influence of slowly varying topography with time. chinese physics b, 20(11), 120203. yang, h. w. (2012). generation of solitary rossby waves by unstable topography. communications in theoretical physics, 57(3), 473-476. geociencias dic 2010-color.vp petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 1akinmosin, a., and 2osinowo, o.o. 1 department of geosciences,university of lagos, lagos 2 department of geology, university of lbadan, lbadan e-mail: waleakinmosin2001@yahoo.com abstract limestone deposits can be found in every system of the geologic column since the precambrian. in classifying this rock, various attempts have been made and notable among them is the classification scheme based on three components: allochems, cements and matrix. another classification divided limestone on the basis of texture into grainstone, packstone, mudstone, wackestone and boundstone. texturally, limestone ranges from fine to coarse grained with aragonite and calcite as the most common minerals. this work was aimed at identifying and describing both the allochemical and orthochemical components of the limestone deposits in order to classify and deduce their depositional environment. eight core samples of limestone deposits representing the ewekoro formation of the dahomey basin were studied petrographically. the result of the analysis carried out on the samples showed that the ibese limestone is composed of the following allochems: sponges, echinoids, bivalves, coralline algae, and pelecypod which represent the skeletal content and intraclasts, pellets and ooids which represent the non skeletal grains. the orthochemical constituents include the carbonate mud matrix and the sparry calcite cement. these constituents observed from the slides indicate that the ibese limestone belongs to the class of wackestone and could have been deposited in a quiet and low energy environment. resumen los depósitos de caliza se pueden encontrar en todo sistema de la columna geológica a partir del precámbrico. para su clasificación se han hecho varios intentos y entre ellos es notable el sistema de clasificación basado en alo-químicos, cemento y matriz. otra forma de clasificación es según la textura. texturalmente, la caliza va del grano fino a grueso con aragonito y calcita como los minerales más comunes. este trabajo permitió identificar y describir los componentes alo-químicos y ortho-químicos de los depósitos de caliza con el fin de clasificar y deducir su ambiente de deposición. se estudiaron petrográficamente ocho muestras de núcleos de depósitos de caliza representativos de la formación ewekoro en la cuenca dahomey. los resultados de los análisis mostraron que la caliza ibese está conformada por los siguientes alo-químicos: esponjas, equinodermos, moluscos, algas coralinas, y pelecípodos e intraclastos, pellets y ooides . los componentes ortho-químicos incluyen carbonato en la matriz y cemento de calcita espática. estos componentes observados en las secciones delgadas indican que la caliza ibese es de la clase wackosa y podría haber sido depositado en un ambiente tranquilo y de baja energía. keywords: carbonate, depositional, environment, petrographic, allochems, micrite and fossiliferous. palabras clave: carbonato, deposicionales, medio ambiente, petrográfico, aloquímicos, micrita y fosilíferas record manuscript received: 10-05-2010 accepted for publication: 05-12-2010 earth sciences research journal earth sci. res. s j. vol. 14, no. 2 (december, 2010): 187-196research groupin geophysics universidad nacional de colombia introduction approximately 20% of the sedimentary rocks is made up of carbonate rocks especially limestone and dolomite (chilingar et al. 1972). the dahomey basin (fig.1) is a marginal pull-apart basin (klemme, 1975) or margin sag basin (kingston et al., 1983), which was initiated during the early cretaceous separation of african and south american lithospheric plates. the eastern flank of this basin is delineated by the okitipupa high, while the western flank extends as far as into togo and ghana. the dahomey basin contains piles of sediments that have been found to be about 3000m and thickens offshorewards. studies (geology, stratigraphy, textural and geochemical) on the eastern dahomey basin have been undergone by previous authors some of which include jones and hockey, 1964;reyment, 1965; omatsola and adegoke, 1981 coker and ejedawe, 1983; billman, 1992; elueze and nton, 2004 and akinmosin et al, 2005. in classifying limestone, various attempts have been made, notable among them are the classification scheme of folk (1959,1962) which was based on three components: allochems, matrix and cements. the classification of dunham, 1962 divides limestone on the basis of texture into grainstone, packstone, mudstone, wackestone and boundstone. this work is aimed at identifying and describing both the allochemical and orthochemical components of ibese limestone deposits based on petrographic studies and to deduce the depositional environment. stratigraphy the area of study covers ibese, ogun state, southwestern nigeria. it is within the eastern dahomey basin of nigeria. (fig. 2). ibese lies at the southern edge of the ewekoro depression and is overlooked by the escarpment capped by the ilaro formation which forms a prominent feature in the area. ibese quarry is located along ilaro-idiroko road and it lies between latitudes 6 0 9 1 n and 7 0 n and longitudes 3 0 e and 3 0 3 1 e. ibese quarry is operated by the dangote cement company. the reviewed work of omatsola and adegoke (1981) on the cretaceous stratigraphy of the dahomey basin has recognized three formations belonging to the abeokuta group. these are: the ise formation, consisting essentially of continental sands, grits and siltstones, overlying the basement complex. neocomian to albian age has been assigned to this formation. overlying the ise formation is the afowo formation, which consists of course to mediumgrained sandstones with variable interbeds of shales, siltstones and clay. the sediments of this formation were deposited in a transitional to marginal marine environment turonian to maastritchtian age has been assigned to this formation. the araromi formation consists essentially of sand, overlain by dark-grey shales and interbedded limestone and marls occasional lignite bands. the formation conformably overlies the afowo formation and maastrichtian to paleocene age has been assigned (omatsola and adegoke, 1981). overlying the abeokuta group conformably is the imo group, which comprises of shale,limestone and marls. the two-lithosratigraphic units under this group are: ewekoro formation and akinbo formation. adegoke (1977) described the formation as consisting of shaly limestone about12.5m thick which tends to be sandy and divided it into three microfacies. ogbe (1972) further modified this and proposed a fourth unit. it is paleocene in age and associated with shallow marine environment due to abundance of coralline algae, gastropods, pelecypods, echinoid fragments and other skeletal debris. akinbo formation lies on the ewekoro 188 akinmosin, a., and osinowo, o.o. figure 1: east-west geological section showing the dahomey basin and upper part of the niger delta (after whiteman, 1982). formation and it comprises of shale, glauconitic rock bank, and gritty sand to pure grey and with little clay. lenses of limestone from ewekoro formation grades literally into the akinbo shale very close to the base. the base is characterized by the presence of a glauconitic rock. the age of the formation is paleocene to eocene. overlying the imo group is the oshoshun formation. it is a sequence of mostly pale greenish-grey laminated phosphatic marls, light grey white-purple clay with interbeds of sandstones. it also consists of claystone underlain by argillaceous limestone of phosphatic and glauconitic materials in the lower part of the formation. eocene age has been assigned to this formation (agagu, 1985). the sedimentation of the oshoshun formation was followed by a regression, which deposited the sandstone unit of ilaro formation (kogbe, 1976). the sequence represents mainly coarse sandy estuarine deltaic and continental beds, which show rapid lateral facies change. the coastal plain sands are the youngest sedimentary unit in the eastern dahomey basin. it probably overlay the ilaro formation unconformably, but convincing evidence as to this is lacking (jones and hockey, 1964). it consists of soft, poorly sorted clayey sand and pebbly sands. the age is from oligocene to recent. methodology samples used for this work are core samples retrieved from a well in ibese. eight samples were selected at an average vertical interval of 1.5m. part of the field work included thorough optical examination of the samples. afterwards, the selected samples were clearly labeled and kept in sample bags for laboratory analysis. prepared slides from the collected samples were closely examined under plane polarized light.the stage of the microscope was rotated continuously to attain different views of the slides. photomicrographs of each slide were taken under crossed nicols and comparison was made with carbonate photomicrograph catalogue to ascertain the compositional features. results and discussion lithology the limestone samples from ibese quarry are medium grained with colour ranging from light brown to cream. dark brown stains were also observed on petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 189 contour line dispensary river egba river oke 0 1 2km footpath 3 3.01 3.02 3.03 3 3.01 3.02 3.03 6.95 6.96 6.97 6.98 6.99 7 6.95 6.96 6.97 6.98 6.99 7 ile owo joga abule egba araromi disp. sch. sch school settlement major road ibese study area figure 2: location map showing the study area some of the exposed samples which is an indication of oxidation of the iron content. the samples were observed to be highly fossiliferous. petrographic description allochemical constituents, carbonate mud matrix (micrite) and sparry calcite cement are the major components of carbonate rocks, folk (1959, 1962). the petrographic description of ibese limestone was based on these three parameters. the term “allochem” is applied to carbonate grains greater than 0.062mm rich, 1964. discussion allochems the non skeletal grains identified from the petrological study are intraclast, pellets and ooids while the skeletal grains represent the fossil content. skeletal grains (fossil content) for the identification of the fossil content of the ibese limestone, the following points were noted: shape, internal micro-structure and presence of spines or pores. the following were identified with varying depth: sponges, echinoderms, pelecypods, bivalves, coralline algae and foraminifera globorotalia cerroazulensis. sponges the sponge spicules are composed of silica or calcite. sponges were identified in plates 1 and 6 at depths 23m and 40m respectively. they serve as source of silica for the formation of chert nodule and the silicification of the limestone. echinoderms it was observed that sparite cement crystal had grown syntaxially around the echinoderm fragment identified in the lbese limestone deposit. (see plate 3a). bivalves bivalve shells identified in the deposit consisted of several layers of specific internal microstructure composed of micron-sized crystallites. (see plate 3b). non skeletal grains intraclasts these are fragments that have been reworked within the area of deposition thereby forming new sediment. sizes of identified intraclasts ranged from sand size to pebble or boulder size. sub-angular or subrounded types were rare and some of them were identified to possess irregular protuberances. some have complex internal structure and contain fossils, pellets, quartz silt as seen in plates 1, 2, 5, 7 and 8. pellets these are rounded, spherical to elliptical or ovoid aggregates of microcrystalline calcite ooze which are devoid of any internal structure. they show a uniformity of shape and size (see plate 6). ooids these are particles that show either radical or concentric structure (plates 2 and 5). ooid typically form in agitated waters. 190 akinmosin, a., and osinowo, o.o. table 1: summary table of components of ibese limestone plate depth (m) fossil intraclast ooid pellet support sparry calcite 1 23.0 sponge spicules v x v mud/grain v 2 24.5 sponge spicules v v v mud v 3 34.0 echinoid sponge spicules bivalves x x x mud x 4 37.5 pelecypod x x x grain v 5 38.5 pelecypod v v v mud/grain v 6 40.0 sponge spicules x x v mud x 7 47.5 v x v grain v 8 48.5 globorotalia cerroazulensis v x v mud v carbonate mud matrix (micrite) it is considered as forming very largely by rather rapid chemical or biochemical precipitation in sea water settling to the bottom and at times undergoing some later drifting by weak currents. micrite was identified in most of the samples studied (plates 2,3,4,7 and 8) sparry calcite cement this type of calcite generally forms grains or crystals 10 microns or more in diameter and is distinguished from microcrystalline calcite by it’s clarity as well as coarser crystal size. large coarse grains of sparry calcite which were white and grey in colour were observed to be dominant in plates 1 and 7. environmental interpretation this is based on the fossil content and the dominant support of the ibese limestone (table 1). the fossil contents include sponges, echinoids, bivalves, coralline algae, pelecypod which are typical of an open shelf environment. the presence of echinoid indicates deeper waters. the dominant support is mud and muddiness is generally a property of rocks deposited in quiet water and a low energy environment. the absence of shell fragments is indicative of this. conclusion petrographic studies of ibese limestone deposit have shown that the rock is highly fossiliferous with the identified fossils indicating deposition in an open shelf environment. moreover, the limestone deposit was equally observed to be principally mud supported which is indicative of rocks deposited in quiet water and a low energy environment. from the aforementioned textural characteristics, ibese limestone deposits can be classified as wackestone according to dunham, 1962. references adegoke o. s. (1977): stratigraphy and paleontology of the ewekoro formation (paleocene) of south western nig. bulletin american paleontologist, vol. 71, no 295, 375pp. agagu o.a (1985): a geological guide bituminous sediments in south western nigeria. unpublished report, department of geology university of ibadan. akinmosin a., odewande a.a., and akintola a.i. (2005): geochemical composition and textural features of some carbonate rocks in parts of southwestern nigeria. ife journal of science, vol. 7, num. 1. billman h.g. (1992): offshore stratigraphy and paleontology of dahomey embayment, west africa. nigerian association of petroleum explorationist bulletin, vol.7, no. 2 pp. 121-130. chilingar, g.v., mannon r.w. and h.r. riecke (1972): oil and gas production from carbonate rocks ed by g.larson and g.v. chilingar pp 408. elsevier, new york. coker s.l. and ejedawe j.e.(1983): hydrocarbon source potential of cretaceous rock of okitipupa uplift, nigeria journal of mining and geology; vol. 20 pp 168-169. dunham r.j. (1962): classification of carbonate rocks according to depositional texture, in: classification of carbonate rocks (ed. by w.e. ham), pp 108-201. elueze, a.a., and nton, m.e. (2004). organic geochemical appraisal of limestones and shales in part of eastern dahomey basin, south western nigeria. journal of minning and geology, vol. 40 (1), pp. 29-40. folk r.l (1959): practical petrographic classification of limestone. bulletin of american association of petroleum geologist, vol. 43, pp1-38. folk r.l (1962): spectral subdivision of limestone types in: classification of carborate rocks (ed. by w.e ham), 1, pp 62-82 jones m.a. & hockey r.o. (1964): the geology of part of south western nigeria. nigeria geological survey bulletin, no 31 pp 1-101 kogbe c. a. (1976): the cretaceous and paleocene sediments of south eastern nigeria in: kogbe, c. a. (ed) geol. of nigeria. elizabethan publishing co. lagos. ogbe, f.g.a., (1972): stratigraphy of strata exposed in the ewekoro quarry, saith western nigeria. in: african geology, pp. 305-322. omatsola m.e. and adegoke o.s. (1981): tectonic and cretaceous stratigraphy of the dahomey basin. journal of mining and geology, vol. 54 pp 65-87. reyment r.a. (1965): aspect of the geology of nigeria. ibadan university press, 145pp. rich, m. (1964): petrographic classification and method of description of carbonate rocks of the bird spring group in southern nevada: journal of sedimentary petrology, vol.34, pp 367-378. whiteman, a.j. (1982). nigeria: its petroleum geology, resources and potentials, vol.2, graham and trotman, london. pp. 268. petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 191 appendix 1 192 akinmosin, a., and osinowo, o.o. plate 1: photomicrograph of ibese limestone (depth 23m). plate 2: photomicrograph of ibese limestone (depth 24.5m). petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 193 plate 3: photomicrographs of ibese limestone (depth 34.0m) plate 4: photomicrograph of ibese limestone (depth 37.5m) 194 akinmosin, a., and osinowo, o.o. plate 5: photomicrograph of ibese limestone (depth 38.5m). plate 6: photomicrograph of ibese limestone (depth 40.0m) petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 195 plate 7: photomicrograph of ibese limestone (depth 47.5m). plate 8: photomicrograph of ibese limestone (depth 48.5m) contenido using the micro – resistivity method to detect hispanic ancient floors at nombre de dios, panamá alexis mojica1, louis pastor2, richard vanhoeserlande3, maría salamanca heyman4 127 contribution of topographic and penetrometric measurements to a site characterization,case of the kekem landslide, national road n°5 (western cameroon) jean jacques nguimbous-kouoh1, 2* and eliezer manguelle-dicoum1, 2 135 thermal modeling of pluton emplacement and associated contact metamorphism:parashi stock emplacement in the serranía de jarara (alta guajira, colombia) lina fernanda martínez and carlos a. zuluaga c.1 145 potential littoral sediment transport along the coast of south eastern coast of india s. saravanan and n. chandrasekar 153 x-ray color maps of the zoned garnets from silgará formation metamorphic rocks,santander massif, eastern cordillera (colombia) carlos alberto ríos r.1, oscar mauricio castellanos a.2, akira takasu3 161 reservoir characterization by multiattribute analysis: the orito field case jairo g. guerrero1,2, carlos a. vargas2 and luis a. montes2 173 laboratory testing of the monotonic behavior of partially saturated sandy soil noureddine della1 181 petrographical study of ewekoro carbonate rocks, in ibese, south western nigeria 1akinmosin, a., and 2osinowo, o.o. 187 geociencias-vol 14-1 2010.vp earth sciences research journal earth sci. res. j. vol. 14, no. 1 (june 2010): 88-99 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria ekine, a.s.1 and onuoha, k. m.2 1 department of pure and applied physics, university of port harcourt, portharcourt 2 department of geology, university of nigeria, nsukka, nigeria asekine2001@yahoo.co.uk abstract the methods of seismic geohistory analysis and differential interformational velocity analysis have been applied using data from some interpreted seismic sections within the anambra basin. these two techniques for basin analysis have been applied in the basin in an attempt to aid the identification of anomalous velocity zones. results from the seismic geohistory analysis indicate that some faults arising probably from compressional stresses, due to the upliftment of the abakaliki anticlinorium, were observed to be significant mostly in the northern and southeastern regions of the basin. these faults which originated in the santonian are probably related to the first folding episode in the evolution of the benue trough. the maestrichtian / campanian faults observed towards the north of anambra river-3 well may also have been influenced by these stresses or are directly related to the post-maestrichtian folding episode in the benue trough. the fault system kp identified on the seismic section is a pre-eocene event which cuts across from the south of okpo-1 and nzam-1 to the north of iji-1 well. sediments to the north of nzam-1 well site have experienced more faulting during the lower-maestrichtian times. fault systems that occurred after the paleocene times are more prominent in the southern parts of nzam-1 towards iji-1 well sites. generally most of the major faults occurred in the paleocene. the major ‘diva’ anomaly observed southeast of anambra river-1 well correlates with the zone of overpressures within which liquid hydrocarbon, water and gas have been discovered in the basin. the maestrichtian to paleocene sediments in the southern and mainly in the southwestern sector of the anambra basin should be the major sedimentary strata with liquid hydrocarbon potentials, whereas the lower cretaceous and particularly the santonian sediments exhibit the highest potentials for gaseous hydrocarbons. key words: seismic geohistory; seismic section; differential velocity; anambra basin. resumen los métodos de análisis sísmicos histórico y análisis de velocidad interformacional diferencial se han aplicado usando datos de algunas secciones sísmicas interpretadas de la cuenca anambra. estas técnicas se han aplicado en un intento de ayudar a la identificación de zonas de velocidad anómalas. los resultados del análisis sísmico histórico indican que algunas fallas probablemente surgen de esfuerzos de compresión, debido a la elevación del anticlinorio abakaliki, importantes sobre todo en las regiones norte y el sureste de la cuenca. estas fallas que tuvieron su origen en el santoniano, están probablemente relacionados con el primer episodio de plegamiento en la evolución de la depresión de benue. las fallas maestrichtiense / 88 manuscript received: 17/12/2009 accepted for publication: 15/05/2010 campaniano observadas hacia el norte del pozo anambra river -3, también pueden haber sido influenciadas por estos esfuerzos o están directamente relacionados con el episodio de plegamiento post-maestrichtiano de la depresión de benue. el sistema de fallas kp identificado en la sección sísmica es un evento pre-eoceno que atraviesa desde el sur de okpo-1 y nzam 1-al norte del pozo iji-1. los sedimentos al norte de la ubicación del pozo nzam-1 han experimentado mayor fallamiento en el maestrichtiano temprano. el sistema de fallas producido después del paleoceno es más prominente en las partes meridionales desde nzam-1 hacia el pozo iji-1. en general la mayoría de las principales fallas se produjeron en el paleoceno. la gran anomalía ‘diva’ observada al sudeste del pozo anambra river -1 se correlaciona bien con la zona de sobrepresiones en la que hidrocarburos líquidos, agua y gas han sido descubiertos. los sedimentos que van del maestrichtiano al paleoceno en el sur y principalmente en el sector suroccidental de la cuenca anambra deberían ser los principales estratos sedimentarios con potencial de hidrocarburos líquidos, mientras que los del cretácico inferior y en particular los sedimentos santonianos exhiben el más alto potencial de hidrocarburos gaseosos. palabras clave: geohistoria sísmica, sección sísmica; velocidad diferencial; cuenca anambra. introduction the anambra basin has been identified as one of the major in-land sedimentary basins in nigeria. it is bounded on the east by the abakaliki anticlinorium and on a south-westerly direction by the benin hinge-line, while the southern extreme is marked by the upper limits of the eocene growth faults of the niger delta (merki, 1972). the basin is about 300 km long in a northeast – southwest direction, extending between the onitsha environs in anambra state to the loko area in benue state of nigeria. its southwestern tip is about 160 km wide, while the northeastern extreme is about 48 km wide (whiteman, 1982) (fig. 1). the geologic history of the basin has been that of nearly continuous subsidence and sedimentation, and had therefore remained largely unaffected by major tectonism. the stratigraphic successions in the anambra basin and environs 89 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria cratonic stable or block foulted basinal orogenic hinge line site of shoal carbonate oil, tar seapage ameki fm. mamu, ajali nsukka transgressive phose regregressive phose imo shale nkporo shale lower campanian lower eocene figure 1. tectonic map of southern nigeria (adapted from murat 1970). have been discussed by several authors which include (reyment, 1965; murat, 1972; adeleye, 1975; peters, 1978; whiteman, 1982; hoque and nwajide, 1984; agagu et al, 1985). the reconstruction and the analysis of the geologic history of a sedimentary basin are very significant for the analysis of the subsidence and thermal histories of such a basin. this method of reconstruction or geohistory analysis has been applied by many authors using stratigraphic and other geophysical data (angevine and turcotte, 1981; middleton, 1982; onuoha, 1985; onuoha and ekine, 1999; ekine and onuoha, 2008). however, in recent times, geohistory analysis has been successfully applied to seismic data (middleton and falvey, 1983; middleton, 1984). seismic geohistory analysis is a technique whereby the geologic history of a sedimentary basin is reconstructed in terms of two-way travel times from observed seismic reflection data. this method provides a means of rapidly reconstructing the geologic history from an interpreted seismic section. again in recent times, the great advances in exploration tools, techniques and experiences have compelled and motivated many explorationists to shift their exploration emphasis from the quest for simple structural to stratigraphic traps. the difference between structural and stratigraphic traps can either be quite clear or so subtle for differentiation where both traps play significant roles as trapping mechanisms. the use of seismic techniques for the identification and definition of stratigraphically trapped hydrocarbons has been made possible by the introduction of sophisticated packages and approaches in seismic data interpretation. the differential interformational velocity analysis, originated by neidel and beard (1984), attempts to delineate anomalous low velocity zones by tracking laterally the interval velocity (stacking velocity) along a seismic profile by using the travel times for the reflectors that have been identified. these two techniques for basin analysis have been applied in the anambra basin using some interpreted seismic sections (see fig. 2). this attempt is to aid in the identification of anomalous ve90 ekine, a.s. and onuoha, k. m. 7°00’6°32’e 6°40’ 7°20’e 7°00’ ald-1 opl 447 atu-1 129/128-776/77-18 anr-3 anr-1 anr-2 odr-1 nzam-1 447-84-216 okp-1 alo-1 44-84-259 iji-1 ajr-1 447-84-248 447-84-217 alo-1 o p l 44 7 aku-1 ama-1 6°30’ 0 20 km 6°00’n figure 2. location of seismic sections. locity zones which in normal circumstances could be attributed to the occurrence of reefs, sand bars, geomorphic features and other features commonly associated with stratigraphic traps. also the study will demonstrate the applicability of these non conventional techniques in the search for the subtle traps. methodology p-waves travelling through sedimentary layers are found to obey a power law equation given by acheson (1981) as t az bn� � (1) where t = the one-way travel time; z = depth to reflector; a, b and n = constants. consequently, the two-way travel time t can be expressed as t = 2(a zn + b) (2) according to falvey and middleton (1981), and middleton, (1984), the behaviour of porosity �z with depth z can be sufficiently approximated by the expression 1 1 0� �( )z kz� � (3) where �0 = the depositional porosity and k = a constant. the above expression has been used in this study as the main thrust of the study is not the evaluation of reservoirs as in ehrenberg et al., (2009). to decompact and reconstruct the geologic history of a sedimentary basin from interpreted seismic section, a procedure similar to burial history reconstruction is applied. in this case depths are replaced with arrival times. according to sclater and christie (1980), the height of sediment grain hsg for a unit cross-sectional area between the intervals z1 and z2 is given by the expression � �h dzsg z z z � �� 1 1 2 � ( ) (4) and z h z k n k z k n k zstg2 1 0 1 0 2 1 1 1 1 1 1’ ’ ’( ) ( )� � � � � �� � (5a) or z k n k z h z k n k zstg2 0 2 1 0 1 1 1 1 1 1 1’ ’ ’ ’( ) ( )� � � � � �� � (5b) where z1 ’ and z2 ’ are the depths to the top sedimentary units when z z1 1� ’ is at the surface. substituting equation (2) into equation (3) gives 1 1 2 20 1 � � ( ) � � � � � � �k t b a n (6) and equation 5(b) now becomes t b a k k t fn n2 1 0 22 2 1 1 1 2 ’ ’ ( � � � � � � � � � � � � � � � � � � �n 2b a � t t t1 1 2, , )’ (7) with fn t t t t b a t tn n ( , , )’ ’ ’ 1 1 2 1 1 2 1 12 2 2 � � � � � � � � � � � � � 2b a � � � � � � � � � � � � � � 2b a n 2b a 2b a 2 1 1 1 2 1 2 1 0 2 0 2 n k k t k t � � ’ ’ � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1 1 1 2 0 1 1 k k t n n 2b a � ’ � � � � (8) t1 ’ and t2 ’ are the reflector times when t t1 1� ’ is at the surface. (t t z2 1 2 ’ ’, and have been defined in the equations as required) equation (8) is the basis for the seismic geohistory analysis. this equation is solved numerically for t2 ‘, with t1 ‘ = 0 at the surface for the first instance. fig. 3 is a schematic representation of the process of seismic history analysis. for subsequent reflectors and cycles a similar procedure as employed for the burial history reconstruction was used in this study (ekine and onuoha, 2008). using the two-way travel time versus depth plots obtained at anambra river-1, oda river-1 and nzam-1 well sites (figs. 4a, b, and c), a time-depth relation was obtained for the basin. this relation can be expressed as t = 0.46 z0.83 + 18.99 (9) where t is in milliseconds and z is in feet. seismic geohistory analysis was carried out along three interpreted seismic sections which were calibrated at iji-1, nzam-1 and okpo-1 wells. the generalized porosity-depth relations were obtained from each well along individual seismic line and not a generalized relation for the whole basin. this approach significantly reduced the possible errors associated with simple assumed basinal generalized functions and thus enhances the representativeness of the porosity-depth and the time-depth functions. the reconstructed 91 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria seismic sections as obtained from the solutions of equation (7) are shown on figs. 5(i), (ii) and (iii). finally, the method of differential interformational velocity analysis (diva) was applied to some interpreted seismic section within the basin. these velocity sections include those calibrated at anambra river-1, anambra river-3, iji-1 and nzam-1 wells. the graphics of the analysis are displayed on figs. 6a, b, c and d. discussions the results of seismic geohistory analysis carried out along three interpreted seismic sections indicate that most of the major faults in the basin originated in the post-maestrichtian times and mainly in the paleocene. the observed major faults are found to commence mainly within the nkporo shales at okpo-1 well site (fig. 5(i) (d, e)); within the lower coal measure at iji-1 (fig. 5(ii)(d, e)) and nzam-1 (5(iii)(c)) well sites. these formations were deposited in the post-maestrichtian times. this observation correlates with the renewed high rate of subsidence and sedimentation revealed by the reconstructed burial analysis (ekine and onuoha, 2008). a very good correlation also exist between sediments depths obtained from the decompaction method and that obtained from seismic geohistory analysis by converting travel-times to depths using the time-depth relation obtained for the basin. similarly, some faults arising probably from compressional stresses, due to the upliftment of the abakaliki anticlinorium (ekine, 1989) were observed to be significant mostly in the northern and southeastern regions of the basin. these faults which originated in the santonian are probably related to the first folding episode in the evolution of the benue trough. the maestrichtian / campanian faults observed towards the north of anambra river-3 well may also have been influenced by these stresses or are directly related to the post-maestrichtian folding episode in the benue trough. the fault system kp identified on the interpreted seismic section [see figs. 5(i)(b) and 5(iii)(b)], is a pre-eocene event which cuts across from the south of okpo-1 and nzam-1 to the north of iji-1 well. it is observed that sediments to the north of nzam-1 well site have experienced more faulting during the lower-maestrichtian times. however, fault systems that occurred after the paleocene times are more prominent in the southern parts of nzam-1 towards iji-1 well sites. generally, we observed that most of the major faults occurred in the paleocene. some other faultings which occurred in the santonian were mainly significant in the northern and the southeastern parts, whereas 92 ekine, a.s. and onuoha, k. m. 0 1.0 2.0 3.0 4.0 5.0 t3 reflector a t = 01 2 reflector b reflector c reflector c reflector b reflector a t1 t2 0 1.0 2.0 3.0 4.0 5.0 t3 t2 reconstructed seismic section (reflector a time) interpreted seismic section (present day) figure 3. schematic diagram of the seismic geohistory analysis method. 93 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria 0.2 0.4 0.6 0.8 1.0 1.2 1.4 one-way time (sec) 0 1.0 2.0 3.0 4.0 true t-z curve computed (c) nzam i well d ep th (k m ) 1 = 0.46 z + 18.990.83 figure 4. time vs depth. 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0 1.0 2.0 3.0 t = 0.46 z + 18.990.83 (a) anambra river i well true t-z curve computed 0 1.0 2.0 3.0 true t-z curve t = 0.46 z + 68.980.83 t = 0.46 z + 18.990.83 one-way travel time (sec) (b) oda river i well d ep th (k m ) 94 ekine, a.s. and onuoha, k. m. 80 120 160 200 240 280 320 360 90 120 160 200 240 280 320 360 iji 0.0 1.0 2.0 3.0 4.0 base of eocene horizon within imo hf ftb gl top icm hk ft vb top lcm kr 0.0 1.0 2.0 (d) top ucm at surface (a) present day (b) top imo shale at surface 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 (c) lower imo shale at surface top ucm top lcm (e) top lcm at surface ucm = upper coat measure lcm = lower coat measure ftb = major faults as indentified on seismic sections figure 5(ii). reconstructed line 447-84-259 at iji-1. 610 650 690 730 770 810 610 650 690 710 770 810 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 3.0 0.0 1.0 2.0 0.0 1.0 2.0 okpo base of eocene kp kn top ucm top nkp (a) present day (b) top imo shale at surface (c) top ucm of surface (d) top nkporo shale at surface (e) end if santonian ucm = upper coal measure rp = major faults as identified on seismic sections figure 5(i). reconstructed line 447-84-216 at okpo-1. 95 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria 140 200 260 320 380 440 500 560 620 680 740 800 860 920 980 1040 1100 1160 nzam 0.0 1.0 2.0 3.0 4.0 base of eocene gl hk too ucm top lcm vb ha gl ga kp kn mp kh le la lb (a) present time 0.0 1.0 2.0 3.0 (b) top imo shale at surface (c) top ucm at surface ucm = upper coat measure lcm = lower coat measure ga = major faults as indentified on seismic sections kr figure 5(iii). reconstructed line 447-84-217 at nzam-1. 1.5 3.5 1.5 3.5 1.5 3.5 2.5 4.5 1.5 3.5 2.5 4.5 v el o ci ty (k m s ) -1 cdp 1331 1033 735 437 138 h2 h1 h1 h3 h3 h1 h4 h4 h2 h2 h4 h3 h2 = top mamu fm.h1 = top nsukka fm. h4 = top awgu shale.h3 = top nkporo shale. figure 6(a). “diva” display for line 129/128-76/77-78. the post-eocene faults were predominantly in the southwestern portions of the basin. the differential interformational velocity analysis, ‘diva’, carried out along four seismic sections indicated mainly minor ‘diva’ anomalies. however, the ‘diva’ display for line 129/128 – 76/77 – 78, calibrated at anambra rivers-1 and -3 wells, show major anomaly beyond cdp 437 and to the end of the seismic line southeast of anambra river-1 [see fig. 6(a)]. this anomaly is within the tops of mamu formation and the agwu shales. a similar anomaly, not clearly defined, is indicated northwest of anambra river-3 well. this anomaly is within the same levels with beds dipping southwest, or shows the trend of migrating fluids and hydrocarbons. the top of agwu shales contains the column of water and liquid hydrocarbons observed at the anambra river-1 well. the major ‘diva’ anomaly observed southeast of anambra river-1 well correlates with the zone of overpressures within which liquid hydrocarbon, water and gas have been discovered in the basin. the absence of any other well defined anomaly along this section implies that the overpressured condition and the hydrocarbon deposit do not extend in that direction or that the section analyzed is not long enough to clearly show this trend. 96 ekine, a.s. and onuoha, k. m. h2 h1 h1 h3 h3 h1 h4 h4 h2 h4 h3 2.3 4.3 2.3 3.3 2.3 3.3 2.3 3.3 2.3 4.3 2.3 4.3 v el o ci ty (k m s ) -1 0 50 100 200 350 iji-isp h2 = top nsukka fm. h4 = top mamu fm.h3 = horizon in imo shale. h1 = top imo shale. figure 6(b). “diva” display for line 447-84-259. conclusions the method of seismic geohistory analysis has shown that the timing of faulting in a basin is very important for the understanding of the problem of hydrocarbon migration and accumulation. the lack of adequate stratigraphic or structural trappings may lead to vertical or lateral migration of liquid hydrocarbon and their possible dissemination. when the time of hydrocarbon maturation (ekine, 1989), is related to the timing of the observed fault systems, the absence of accumulated liquid hydrocarbons in the north-central parts of the anambra basin can be explained. results of the differential interformational velocity analysis have further confirmed the existence of velocity anomalies resulting from changes in rock type and or fluid content, which substantially enhances the magnitude of the anomaly when the presence of gas is indicated. the diva results indicate that the maestrichtian to paleocene sediments in the southern and mainly in the southwestern sector of the anambra basin should be the major sedimentary strata with liquid hydrocarbon potentials, whereas the lower cretaceous and particularly the santonian sediments exhibit the highest potentials for gaseous hydrocarbons. the above geophysical tools employed in this study for basin analysis, 97 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria 2.5 3.5 0 100 200 350 400 2.5 3.5 2.5 3.5 2.5 4.5 2.5 45 2.5 4.5 sp iji-1 h2 h1 h1 h3 h3 h1 h4 h4 h2 h4 h3 h2 h2 = top nsukka fm. h4 = top mamu fm.h3 = horizon in imo. h1 = top imo shale. figure 6(c). “diva” display for line 447-84-248. have demonstrated their effectiveness for obtaining information about the tectonism, hydrodynamics, hydrocarbon and geothermal resource potentials of basin. consequently, we suggest that the methods of seismic geohistory analysis and diva be further pursued using high fidelity seismic sections with greater coverage. references acheson, c. h., 1981. time-depth and velocity-depth relations in sedimentary basins: a study based on current investigations in the arctic islands and an interpretation of experience elsewhere. geophysics, 46: 707 – 716. adeleye, d. r., 1975. nigerian late cretaceous stratigraphy and paleogeography. a. a. p. g. bull., 59: 2302 – 2313. agagu, o. k., e. a. fayose and s. w. peters, 1985. stratigraphy and sedimentation in the senonian anambra basin of eastern nigeria. nig. journ. min. geol., 22: 25 – 36. angevine, c. l. and d. l. turcotte, 1981. thermal subsidence and compaction in sedimentary basins: application to baltimore canyon trough. a. a. p. g. bull., 65: 219 – 225. ehrenberg, s. n., p. h. nadeau and ø. steen, 2009. petroleum reservoir porosity versus depth: influence of geological age. a. a. p. g. bull., 93(# 10): 1281 – 1296. ekine, a. s., 1989. empirical heat flow studies, geohistory analysis and hydrocarbon maturation modelling in the anambra basin, nigeria. unpublished ph. d. thesis, university of nigeria, nsukka. ekine, a. s. and k. m. onuoha, 2008. burial history analysis and subsidence in the anambra basin, nigeria. nig. journ. phys., 20 (#1): 145 – 154. falvey, d. a. and m. f. middleton, 1981. passive continental margins: evidence for break-up deep crustal metamorphic subsidence mechanism. oceanologica acta, suppl., 103 – 110. hoque, m. and c. s. nwajide, 1984. tectono-sedimentological evolution of an elongate intracratonic basin (aulacogen): the case of the benue trough of nigeria. nig. journ. min. geol., 21: 19 – 26. merki, p., 1972. structural geology of the cenozoic niger delta. in african geology, ed. t. j. f. dessauvigie and a. j. whiteman. university of ibadan press, pp. 251 – 268. 98 ekine, a.s. and onuoha, k. m. 2.0 4.0 2.0 4.0 2.0 4.0 v el o ci ty (k m s ) -1 0 138 338 538 738 938 1138 sp nzam-1 h1 h2 h1 h3 h2 h3 h2 = top nsukka fm. h3 = top mamu fm. h1 = top imo shale. figure 6(d). “diva” display for line 447-84-2178. middleton, m. f., 1982. the subsidence and thermal history of the bass basin, southeastern australia. tectonophysics, 87: 383 – 397. middleton, m. f. and falvey, d. a. 1983. maturation modelling in otway basin, australia. a. a. p. g. bull., 67(#2): 271 – 274. middleton, m. f., 1984. seismic geohistory analysis – a case history from the canning basin, western australia. geophysics, 49: 333 – 343. murat, r. c, 1972. stratigraphy and paleogeography of the cretaceous and lower tertiary in southern nigeria. in african geology. ed. t. j. f. dessauvigie and a. j. whiteman, university of ibadan press, pp. 635 – 648. neidel, n. s., and j. h. beard, 1984. progress in stratigraphic seismic exploration and the definition of reservoirs. distinguished author series, journ. pet. tech. pp. 709 – 726. onuoha, k. m., 1985. basin subsidence, sediment decompaction and burial history modelling techniques: applicability to the anambra basin. n. a. p. e. proc., 2: 6 – 17. onuoha, k. m. and a. s. ekine, 1999. subsurface temperature variations and heat flow in the anambra basin, nigeria. journ. african earth sc., 28(#3): 641 – 652. peters, s. w., 1978. stratigraphic evolution of the benue trough and its implication for the upper cretaceous paleogeography of west africa. journ. geol., 86; 311 – 322. reyment, r. a., 1965. aspects of the geology of nigeria: the stratigraphy of the cretaceous and cenozoic deposits. university of ibadan press. sclater, j. g. and p. a. f. christie, 1980. continental stretching: an explanation of the post-mid-cretaceous subsidence of the central north sea basin. journ. geophysical research, 85: 3711 – 3739. whiteman, a. j., 1982. nigeria: its petroleum geology, resources and potential. vols. i and ii, graham and trotham, london. 99 seismic geohistory and differential interformational velocity analysis in the anambra basin, nigeria earth sciences research journal geotechnical earth sci. res. sj. vol. 16, no. 2 (december, 2012): 13 19 introduction due to their low permeability, clays are the main material used as a liner in solid waste disposal landfills. they become exposed there to various chemical, biological and physical events and the clay is affected by the resulting leachate (yılmaz et al., 2008); researchers are thus interested in surfactants and polymers for modifying clays’ engineering properties. surfactants are surface-active agents which alter fluid interface properties. surfactant modified clay (smc) or surfactant–clay complexes have been considered appropriate landfill liners (lo, 2001; ashmawy et al., 2002; gates et al., 2004; matott et al., 2006) and also potential sorbents for wastewater and contaminated soils (zhu and zhang, 1997; mulligan et al., 1999a, b; mulligan et al., 2001; al-asheh et al., 2003; li et al., 2003; wibulswas, 2004; ghiaci et al., 2004; yang et al., 2005). consistency limits (atterberg limits) have been repeatedly shown to be useful indicators of clay behaviour (jefferson and rogers, 1998; dolinar et al., 2007). reconstituted clay sample liquid and plastic limits can be correlated with various engineering properties, such as specific surface area, cation exchange capacity, permeability, shrinking and swelling behaviour, shear strength and soil compressibility (sharma and lewis, 1994; abdullah et al., 1999; yukselen and kaya, 2006; dolinar et al., 2007). consequently, soil consistency limits are determined, then some other geotechnical properties whose determination may take a long time can be easily estimated with acceptable accuracy. evaluating consistency surfactant modified clays’ consistency limits and contact angles suat akbulut, z. nese kurt and seracettin arasan* ataturk university, civil engineering department, 25240 erzurum, turkey. * corresponding author. phone: + 90 4422314618; fax: + 90 4422360957. e-mail: arasan@atauni.edu.tr (s. arasan). abstract this study was aimed at preparing a surfactant modified clay (smc) and researching the effect of surfactants on clays’ contact angles and consistency limits; clay was thus modified by surfactants for modifying their engineering properties. seven surfactants (trimethylglycine, hydroxyethylcellulose, octyl phenol ethoxylate, linear alkylbenzene sulfonic acid, sodium lauryl ether sulfate, cetyl trimethyl ammonium chloride and quaternised ethoxylated fatty amine) were used as surfactants in this study. the experimental results indicated that smc consistency limits (liquid and plastic limits) changed significantly compared to those of natural clay. plasticity index and liquid limit (pi-ll) values representing soil class approached the a-line when zwitterion, nonionic, and anionic surfactant percentage increased. however, cationic smc became transformed from ch (high plasticity clay) to mh (high plasticity silt) class soils, according to the unified soil classification system (uscs). clay modified with cationic and anionic surfactants gave higher and lower contact angles than natural clay, respectively. resumen este estudio tiene como objetivo la preparación de un surfactante de arcilla modificada (smc) y la investigación de sus efectos en los ángulos de contacto y los límites de consistencia de las arcillas es decir, sus propiedades de ingeniería. siete surfactantes (trimetilglicina, hidroxietilcelulosa, octil fenol etoxilato, ácido lineal alquilbenceno sulfónico, sulfato sódicol auril éter, cloruro de cetil trimetil amonio cuaternario y amina grasa etoxilada) fueron utilizados como en este estudio. los resultados experimentales indicaron que los límites de consistencia de smc (límites líquido y plástico) cambiaron significativamente en comparación con los de arcilla natural. el índice de plasticidad y límite líquido (pi-ll) que representan el tipo de suelo, se acercaron a la línea a cuando aumentó el porcentaje de zwitterion, nonionic y el surfactante aniónico. sin embargo, catiónicos smc se transformaron a partir de ch (arcilla de alta plasticidad) a los suelos de la clase mh (limo alta plasticidad), según el sistema de clasificación de suelos unificado (uscs). la arcilla modificada con surfactantes catiónicos y aniónicos dio mayores y menores ángulos de contacto que la arcilla natural, respectivamente. palabras claves: limite de cosistencia, angulo de contacto, surfactante, arcilla. keywords: consistency limit, contact angle, surfactant, clay. record manuscript received: 14/10/2011 accepted for publications: 06/05/2012 suat akbulut, z. nese kurt and seracettin arasan14 chemical composition clay sio2 % 53.28 al2o3 % 20.67 fe2o3 % 6.13 cao % 1.71 mgo % 2.82 k2o % 0.82 na2o % 0.02 ti2o % 0.63 loi % 13.9 table 1. the chemical composition of clay. table 2. clay’s index properties table 3. some properties of the surfactants used in the tests index properties clay clay content <0.002 mm (%) 74 finer content <0.075 mm (%) 99 specific gravity gs 2.72 liquid limit wl (%) 88.4 plastic limit wp (%) 38.0 plasticity index ip (%) 50.4 cation exchange capacity (meq./100 g dry soil) 38.59 contact angle o 35 2005; dharaiya and jana, 2005). rogers et al., (2005) examined the contact angles of some compatibilisers for polymer-silicate layer nanocomposites. dharaiya and jana (2005) and cipriano et al., (2005) have also investigated smc contact angles. cipriano et al., (2005) observed that imidazolium bf4 surfactant becomes adsorbed onto a mica surface, resulting in a hydrophobic surface. similarly, jouany and chassin (1987) indicated that a cationic surfactant will become adsorbed with its positively-charged head group next to the negatively-charged clay surface, thereby forcing the hydrophobic surfactant tail to adsorb and become exposed to the solution. thus, while native montmorillonite surface is hydrophilic, adsorption of a small amount of surfactant on the surface can render it hydrophobic. many studies can be found dealing with surfactant (anionic, cationic and nonionic) modified clays (fu and qutubuddin, 2000; gungor et al., 2001; he et al., 2005; isci et al., 2008; guegan et al., 2009; gurses et al., 2009). most have been focused on investigating smcs’ electrokinetic properties, such as zeta potential, cation exchange capacity, electrical conductivity, etc; however, no comprehensive study has been found comparing smc contact angles and consistency limits. this paper was thus aimed at investigating the effect of seven surfactants (zwitterion, nonionic, anionic, and cationic surfactants) on smectite clay’s consistency limits and contact angles. the surfactants used in suspensions were 5% (250 ppm), 10% (500 ppm), and 15% (750 ppm) dry natural clay by weight. materials and methods clays the clayey soil sample used came from a clay-pit in the oltu-narman deposits in erzurum, turkey, classed as high plasticity clay (ch) according to the unified soil classification system (uscs). these deposits are concentrated in two different stratigraphic horizons, namely late oligocene and early miocene sequences. clay-rich fine-grained sedimentary units were deposited in shallow marine and lagoon-type mixed environments. their clay minerals originated from the alteration of eocene calc-alkaline islandarc volcanics, mainly from pyroclastics (trachite and andesite flow) which form the basement for the oltu depression (kalkan and bayraktutan, 2008). x-ray diffraction (xrd) and x-ray fluorescence (xrf) methods (table 1) have been used to identify the major minerals and chemical compounds present in these clays. the soil’s chemical compositions and xrd patterns are given in table 1 and fig. 1, respectively. table 2 gives some of the clay’s index properties. smectite (56%) appeared to be the predominant clay mineral according to x-ray diffraction (figure 1), whereas kaolinite (34%) and illite (3%) appeared in lower proportions. silt and sand-size particles were composed of quartz (4%) and feldspar (3%). surfactant abbreviation formula surfactant type trimethylglycine tmg (ch3)3n +ch2co2zwitterion hydroxyethylcellulose hec (c6h10o5)n nonionic octyl phenol ethoxylate triton x-100 c14h22o(c2h4o)n nonionic linear alkylbenzene sulfonate acid labsa ch3(ch2)11c6h4so3h anionic sodium lauryl ether sulfate sles ch3(ch2)10ch2(och2ch2)noso3na anionic cetyl trimethyl ammonium chloride ctac (c16h33)n(ch3)3cl cationic quaternised ethoxylated fatty amine qefa (fatty amine) r (och2ch2)n nh cationic limits provides some very basic mechanical data about a particular soil and a first insight into a particular clay’s chemical reactivity. the liquid limit and plasticity index are mainly influenced by clay minerals’ ability to interact with liquids (schmitz et al., 2004) and hydraulic conductivity tends to decrease when the liquid limit and plasticity index become increased (alawaji, 1999; met et al., 2005; arasan and yetimoglu, 2008). contact angle measurements are often used to indicate clay wettability and interfacial tension (rogers et al., 2005) and provide an insight into surfactant behaviour. the pertinent literature contains only a limited number of studies on smc contact angles (jouany and chassin, 1987; janczuk et al., 1989; lopez-duran et al., 2003; cipriano et al., surfactant modified clays’ consistency limits and contact angles 15 surfactants and modified clay preparation seven surfactants were used in this research; table 3 gives the surfactants’ abbreviation, formula and type. smcs were prepared following the procedure described by xi et al., (2007) and liu et al., (2008). briefly, 40 g of clay was first dispersed in 8 l of deionised water, then stirred with a magnetic stirrer at about 1,000 rpm for about 2 h. previously-prepared surfactant solution was slowly added to the clay suspension at 30oc. all modified products were dried at room temperature. the surfactants were used at 5% (250 ppm), 10% (500 ppm), and 15% (750 ppm) of clay by weight. consistency limits consistency limit tests (i.e. liquid limit and plastic limit) were carried out following the procedure outlined in bs 1377, part 2, 1990. the cone penetrometer (fall cone) method was used to determine liquid limit. specimens were prepared for the liquid limit tests by mixing an air-dried smc mass (passing through a 425-μm sieve). plastic limit tests were performed on material prepared for the liquid limit test; both liquid and plastic limit tests were conducted at room temperature. contact angle measurements a contact angle is defined as being the angle made by the liquid/solid interface intersection with the liquid/air interface; alternately, it can be described as being the angle between a solid sample’s surface and the tangent of a droplet’s ovate shape at the edge of the droplet. a high contact angle indicates low solid surface energy or chemical affinity; this is also referred to as a low degree of wetting. a low contact angle indicates high solid surface energy or chemical affinity and a high, or sometimes complete, degree of wetting (anonymous, 2009). figure 2 illustrates a drop of the reference liquid (water for fig. 2a and air for fig. 2b) resting on a solid surface in the presence of another fluid (air for fig. 2a and water for 2b). the interface between the two fluids meets the solid surface at contact angle  (mitchell and soga, 2005). the simplest way of measuring a contact angle is with a goniometer which allows the user to measure the contact angle visually. the droplet is deposited by a syringe pointing down vertically onto the sample surface and a high-resolution camera captures the image which can then be analysed either by eye (with a protractor) or using image analysis software. the contact angles were measured with a goniometer (cam 101, ksv instruments, finland) in this study, using clay and modified clay pellets. figure 1. a typical xrd analysis result for clay figure 2. wetting two fluids (water and air) on a solid surface (taken from mitchell and soga, 2005) figure 3. wtmg effect on clay’s consistency limits (taken from akbulut et al., 2010). figure 4. hec effect on clay’s consistency limits. water (reference fluid) water (reference fluid)   solid surface solid surface air air (a) (b) c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (tmg/clay), % c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (hec/clay), % plastic limit liquid limit plasticity index plastic limit liquid limit plasticity index suat akbulut, z. nese kurt and seracettin arasan16 results and discussion consistency limits figures 3, 4, 5, 6, 7, 8 and 9 give the variation of modified clays’ consistency limits by tmg, hec, triton x-100 labsa, sles, ctac, and qefa, respectively. for clays modified with zwitterion, nonionic and anionic surfactants (tmg, hec, triton x-100, labsa and sles), it was seen that the liquid limit and plasticity index increased when surfactant percentage was increased. the plastic limit also decreased when increasing the percentage of surfactant. on the other hand, the liquid limit and plasticity index for clays modified with ctac and qefa decreased drastically when surfactant percentage was increased. however, plastic limit values’ variation was insignificant for ctac and qefa (figures 8-9). the consistency limit test results were marked on the cassagrande plasticity chart to determine the new soil classification according to the uscs. figure 10 shows the changes after modification with anionic surfactants and figure 11 shows the changes after modification with cationic surfactants. it can be clearly seen that the points representing soil classification approached the a-line when anionic surfactant percentage was increased. however, cationic surfactants changed the clay class when the figure 5. triton x-100 effect on clay’s consistency limits. figure 7. sles effect on clay’s consistency limits (akbulut et al., 2010). figure 9. qefa effect on clay’s consistency limits. figure 8. ctac effect on clay’s consistency limits. figure 6. labsa effect on clay’s consistency limits (taken from akbulut et al., 2010) c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (triton x-100/clay), % c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (sles/clay), % c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (qefa/clay), % c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (ctac/clay), % c on si st en cy l im it s, % 140 120 100 80 60 40 20 0 0 5 10 15 20 surfactant precent (labsa/clay), % plastic limit liquid limit plasticity index plastic limit liquid limit plasticity index plastic limit liquid limit plasticity index plastic limit liquid limit plasticity index plastic limit liquid limit plasticity index surfactant modified clays’ consistency limits and contact angles 17 percentage was increased, resulting in cationic smcs becoming mh (high plasticity silt) (figure 11). consistency limit test results would suggest that clay water affinity became significantly increased by labsa and hec; however, tmg and sles did not significantly change water affinity. ctac and qefa also decreased water affinity. it should be pointed out that there has been no general consensus regarding the effect of surfactants and chemicals on clays’ consistency limits. most researchers have reported that chemicals have decreased the liquid limit of clays (gleason et al., 1997; shackelford et al., 2000; schmitz et al., 2004; lee et al., 2005; jo et al., 2005). however, little research has indicated that chemicals have increased cl or kaolinite clay liquid limit (rao and mathew, 1995; sivapullaiah and manju, 2005; park et al., 2006; arasan and yetimoglu, 2008). different patterns have most likely arisen from a difference in clay mineralogy and surfactant/chemical type (i.e. zwitterion, anionic, nonionic and/or cationic). in line with previous studies, this research has shown that anionic smcs were the most hydrophilic and cationic smcs were the most hydrophobic (figures 12, 13). nevertheless, it could be said that the net electrical forces between clay mineral layers were affected by surfactant percentage and type; anionic and cationic surfactants would result in an increase and a decrease in net repulsive forces, respectively (increased and decreased repulsive forces cause dispersion and flocculation of clay particles, respectively). figure 12. zwitterion, nonionic and anionic surfactants modified clays’ contact angle results. figure 13. cationic surfactants modified clays’ contact angle results. figure 14. the contact angles for some modified clays. a) contact angle image of natural clay (35o); b) contact angle image of tmg (10%) modified clay (14o); c) contact angle image of qefa (15%) modified clay (54o) figure 10. zwitterion, nonionic, and anionic surfactants modified clay results on the plasticity chart. figure 11. the cationic surfactants modified clays results on the plasticity chart. c on ta nc t a ng le , ° 50 40 30 20 10 0 0 5 10 15 20 surfactant precent (surfactant/clay), % c on ta nc t a ng le , ° 90 70 50 30 10 0 5 10 15 20 surfactant precent (surfactant/clay), % tmg labsa sles hec triton x-100 qefa ctac pl as ti ci ty in de x, % 80 60 40 20 0 0 20 40 60 80 100 120 liquid limit, % pl as ti ci ty in de x, % 80 60 40 20 0 0 20 40 60 80 100 120 liquid limit, % cl cl ml ml mh mh ch ch a-line a-line tmg labsa sles hec triton x-100 ctac qefa (a) (b) (c) suat akbulut, z. nese kurt and seracettin arasan18 contact angle measurements figure 12 gives the effect of zwitterion, nonionic and anionic surfactants on modified clays’ contact angles and figure 13 shows the cationic surfactants effect. figure 14 presents some images of the smallest contact angle obtained from natural and smcs. figure 12 shows that zwitterion, nonionic and anionic surfactants significantly decreased modified clays’ contact angles; however, cationic surfactants increased the contact angles (figure 13). tmg and qefa were the most effective surfactants when contact angle results were taken into consideration; similar to consistency limit results, contact angle measurements indicated that clay water affinity was increased by zwitterion, nonionic and anionic surfactants and also became decreased by cationic surfactants. cipriano et al., (2005) indicated that cationic surfactants increased modified clays’ contact angles and produced a hydrophobic surface. conclusions the following conclusions were thus drawn: • consistency limits were significantly changed compared to those for natural clay. the points representing soil class came further towards the a-line when zwitterion, nonionic and anionic surfactant percentage increased. cationic surfactants changed the clay classification from ch to high plasticity silt (mh) when the percentage of surfactant added to the clay was increased; • clays modified with zwitterion, nonionic and anionic surfactants gave the lowest contact angles compared to those for natural clay; however, the clays modified with cationic surfactants gave the highest contact angles; and • it could also be said that clay water affinity was increased by zwitterion (tmg), nonionic (hec, triton x-100) and anionic surfactants (labsa, sles), also that cationic surfactants (ctac and qefa) decreased the water affinity used in this research. hence, zwitterion, nonionic and anionic smcs may be used as hydrophilic materials in waste water remediation and the cationic smcs may also be used as hydrophobic materials (liner) in waste disposal landfills and dams. it should be pointed out that further studies on smcs’ engineering properties (e.g. xrd, xrf, dta and tg for mineralogy and cation exchange capacity, zeta potential for electro-kinetic properties) are needed to make more reasonable judgments. such studies 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surfactant-enhanced electrokinetic removal of phenanthrene from kaolinite. engineering geology, 77: 243–251. yılmaz, g., yetimoglu, t., arasan, s., 2008. hydraulic conductivity of compacted clay liners permeated with inorganic salt solutions. waste management & research, 26: 464-473. yukselen, y., kaya, a., 2006. prediction of cation exchange capacity from soil index properties. clay minerals, 41: 827-837 zhu, l., li, y., zhang, j., 1997. sorption of organo-bentonites to some organic pollutants in water. environ. sci. technol. 31: 1407–1410. cambios de las anomalias magneticas de campo total debidos a las variaciones de la inclinacion y de la declinacion ramiro leon diaz campos profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia dlaz, r. (1995): cambios de las anornatlas magneticas de campo total debidos a las variaciones de la inclinaci6n y de la declinaci6n. geoffs. colomb. 3:51·57. issn 0121-2974 resumen en este trabajo se plantea un rnetodo para rotar datos magneticos, con el ffn de reducir a las mismas condiciones, datos muestreados en diferentes epocas. ei proceso se realiza en el dominio de la frecuencia, teniendo como base los cambios producidos en los parametres declinaci6n e inclinaci6n maqnetica en el area de muestreo. abstract a method for rotation of magnetic data is presented. it allows to reduce, to the same temporary conditions, data sampled at different times. the process is done in the frequency domain, and it has as a base, the observed changes in magnetic declination and inclination in the sampled area. 1. introduccion en cualquier punto sobre la superficie de la tierra, el campo que genera una roca magnetizada por efectos de inducci6n del campo geomagnetico, cambia en forma y amplitud en la medida que cambia la posici6n de los polos rnaqneticos de la tierra. la direcci6n y sentido del campo geomagnetico es funci6n de los parametres dec,linaci6ne inclinaci6n rnaqnetica: as! que cualquier cambio en estes parametros, refleja un cambio en el vector campo maqnetico total. ei objeto de este trabajo es plantear un rnetodo para rotar datos rnaqneticos, teniendo como guia los cambios producidos en los parametres declinaci6n e inclinaci6n maqnstica en el sitio de muestreo. 2. consideraciones teoricas ei campo rnaqnetico y el campo gravitacional de la tierra, son campos vectoriales conservativos, asociados a una funci6n de potencial; estos campos se comportan en forma semejante, por 10tanto, se puede establecer una relaci6n entre el campo maqnetico y el campo gravitacional producidos por una misma fuente, si se conocen algunos parametres rnaqneticos y gravitacionales, como la susceptibilidad maqnetica, el momento dipolar rnaqnetico, iii densidad etc. eicampo rnaqnetico es muy sensible al cambio en la ubicaci6n de los polos geomagneticos, mientras que el campo gravitacional presenta una relativa estabilidad temporal. la ecuaci6n de laplace, la definicion de la funci6n de potencial asociada y la relacion entre el campo y la funci6n de potencial, determinan el comportamiento de los campos rnaqnetico y gravimetrico (telford, 1980). para el campo rnaqnetico: v'!a(/') = 0 a(/') = jm(fo)'v[ ~ldvo va t:.. t(r) = va(/') [1] para el campo gravitacional: v oir, = 0 v(r) = jgp(foh ~ldvo va t:..g(f) = vv(/') [2] los cambios en la magnitud del campo rnaqnetico total debido a rocas magnetizadas, se pueden expresar por: [3] considerando que m(r) = mt y las ecuaciones [1] y [2], se obtienen las siguientes relaciones: ei campo geomagnetico se puede expresar en terminos de la siguiente serie arm6nica: 1 n-l m-l b.t(f') '" l l (ct(kje.zlk] eik.r nm n.o m.o cuya transformada discreta de fourier esta dada por: n-1 m-l ct( k) '" l l b. t( r) e-ik.r /.0 k.o como la serie es finita, el primer coeficiente de fourier se debe calcular usando los datos de muestreo mediante la siguiente relaci6n en el dominic del espacio (geckinli & davra 1983): n-l m-1 ct( 0) = l l b. t( t) /., k.1 o la siguiente relaci6n en el dominic de la frecuencia (laurenco, 1970): n-l m-l ct(o) = l l ctis: n·o m·o es claro que la transformada de fourier permite pasar del dominic del espacio al dominio de la frecuencia y viceversa; en forma esquernatica se representa de la siguiente manera: a t( r) <-------> ct( k) en donde. k: vector de onde e '"k~ + k" 1kf2 = k~n k~ + k~ o : intervalo de muestr80 r; vector de posicion r = xex + yey = jdex + kdey o :; j,m :; m-1; 0:; k,n :; n-1 es posible plantear en el dominio de la frecuencia, operaciones equivalentes a las derivadas direccionales en el dominio del espacio: [4] ex' v t:. rir, <-----> t«; ct(k) t§y' \l t:. rir, < > tx; ct(k) t§z·\lt:.t(f) < > ikmn ct(k) i. \l t:. t(i') < > qmn ct(k) [9] con: f", a ex + ~ 9y .. yez a '" cos(d) cos(/); ~ = sen(d) cos(/); y '" sen(1) qmn = [ykmn + i(akm + ~kn)l [5] la serie arm6nica que representa a la ecuaci6n [5] sera: [6] l6gicamente: debido a que ninqun otro termino en los coeficientes se anula: [7] mt:. t(t) <:----> [qmnsmnl cu(k) gp [10] es conveniente usar subindices, ya que el metoda se aplicara a series discretas. [8] se consideran dos situaci6nes rnaqneticas diferentes en un proceso de variaci6n, que se pueden relacionar con una tercera situaci6n, la gravimetrica, cuyo comportamiento es relativamente estable. condiciones para la situaci6n inicial: anomalfa inicial de campo rnaqnetlco total, coeficientes de fourier para el potencial gravitacional, oeclinaci6n e inclinaci6n inlciales. oeclinaci6n e inclinaci6n del momento dipolar maqnettco. se asume que toda la magnetizaci6n es inducida si '0 = 1'0 y do = 0'0' m: momento dipolar magnetico por unidad de volumen, p: oensidad, qmn= [sen(lo)] kmn+ i{[cos(oo) cos(lo)] km+ [sen(oo) cos(lo)] ~} smn= [sen(i'o)]kmn+ i{[cos(o'o) cos(i'o)]km + [sen(o'o) cos(l'o)]kn} l'1tjk<------> ct mn ct mn= [m/gp][qm smn]cumn[11] 0-:;:j,k-:;:m-1; 0-:;: k,m-:;:n-1 condiciones para la sltuacion final: dt"jk: anomalfa final de campo maqnetico total, cumn: coeficientes de fourier para el potencial gravitacional, i" dr: decllnacion e inchnacion finales, 1',.d',: declinacion e inclinaclon del momento dipolar magnetico. q'mn= [sen(i,)] k",n+ i{[cos(d,) cos(l,)] km+ [sen(d,) cos(i,)] kn} s'mn= [sen(i',)]k",n + i{[cos(d',) cos(i',)]km + [sen(d',) cos(i',)]kn} osj,ksm-1; osk,msn-1 de las relaciones [11] v [12], se obtiene una nueva relaclon entre los coeficientes de fourier para la serie anomalfa de campo rnaqnstico total inicial v los coeficientes de fourier para la serie final correspondiente. como son series finitas, se deben tener en cuenta las relaciones [7] v [8] en el calculo del coeficiente ct 00' va que este depende de los datos de muestreo en el dominio del espacio v de los cam bios en los coeficientes de fourier en el dominio de la frecuencia. ctoo" n-l m-l n-' m-l 8 l l d t(" + b [ l l c t( k)) /.1 k.l n·o moo (8 + b) a v b son constantes reales. concretamente: d 1jk <------:> cr.; i d 1j~ <-----> ct;n [13] os m,j s m-1; 0 s n,k s n-1 queda asf planteado el algoritmo que permite efectuar la rotadon de datos magneticos en funcion de los cambios en los parametros, inclinacion v declinacion magneticas. 3. validacion del metodo con el fin de validar el metodo propuesto, se usa el sigljiente modelo teo rico que simula un dique magnetizado v permite calcular el campo total producido como efectos de induccion del campo geomagnetico (reford & summer, 1964): r2t1 t/ " 2pb2 sem(d)[os8n(2/-d) + ln[-) cos(2/oj] r1 [14] en donde: d: angulo de busamiento del dique, p: polarizacion rnaqnetica, b: b2 = 1 cos'(l) sen2(a) a: angulo entre el eje positivo de las "x" v el meridiano rnaqnetico. i: lnclinacion del campo maqnstico de la tierra, 0, r" r2: parametres que determinan la posicion del punta de rnuestreo, con respecto al dique, i: tan (i) = tan(i)/cos(a) se considera un campo inductor de 35.000 y, un intervalo de muestreo d = 3.8 m, un nurnero de intervalos de muestreo n = z" = 28 = 256 v una longitud del registro l=nd. ei modelo [14], permite calcular las series que representan los perfiles de campo rnaqnetico total inicial v final. estas series tienen por objeto: la primera, servir como condiciones iniciales para el metoda propuesto v la segunda como punto de cornparacion para los datos obtenidos mediante el desarrollo del algoritmo [13]. en la fig.1 se muestran los datos del modelo, en linea continua, v los del rnetodo, en linea punteada. en la validacion del rnetodo v en sus aplicaciones, se usa el algoritmo de la transformada rapida de fourier (fft), planteado por cooley & tukey (1965) v cooley, lewis & welch (1969) para calcular los coeficientes de fourier. usando como condiciones iniciales los promedios de inclinacion v declinacion para el mes de junio de 1993 en el observatorio de fuquene (d = 4.0° 28'; i = 32.0° 7'), se consideran siete rotaciones diferentes con el fin de hacer comparaciones estadfsticas entre los resultados obtenidos con el modele [14] v el rnetodo [13]. para cad a caso se calcula: a) ei coeficiente de correlacion cruzada normalizado [cdo)], entre la tuncion campo rnaqnetico total dtjc calculado con la ecuacion [14] v la tuncion campo rnaqnetico total dtj" calculado con el metodo propuesto en (13). este coeficiente involucra comparaciones en frecuencia de las seiiales. c'2(0)c (0) ---'-=-'----'--12 [c'1(o) c22(o)]1/2 c11 (t) b) la il1lscion tmtre el e tlpico de e5timacion de t1tf y la d8sviacion tipica de /s funci6n t1tf con respecto a . aksu meats (-). a $e considers que t17j* represents s a t1tf, si ~ se eproxlme a cero. a se realiz6 el proceso, primero sin tener en cuenta ninguna simetrfa de la matrfz de datos (tabla 1), luego se construy6 una matrfz palfndroma vertical (fandiiio & cepeda, 1983), con simetrfa par (dlaz, 1983) y se realiz6 el mismo proceso: los resultados se muestran en la tabla 2. tabla 1 resultados en la comparacion de las series t1rjk y ~tolk'cuando nc?existe simetria par en la matriz de datos 10=32° 1,=32° 1,=45° 1,=15° 1,=75° 1,=25° 1,=90° 00=-4,5° 0,=20° 0,=5° 0,=-2° ' 0,=-5° 0,=0° 0,=0° c,,(o) 1.0000 0.9999 0.9983 0.9974 0.9913 0.9995 0.9932 a/a 0.0000 0.0562 0.1099 0.1356 0.2922 0.0589 0.2299 i) • 0.0000 1.2226 3.3055 3.9477 8.5429 1.7314 8.7934 6t -5.2645 -3.9495 3.0733 -14.1100 18.5626 -9.4028 20.6922 6t" -5.2645 -4.5621 0.0777 -10.6070 10.5781 -7.8632 12.2702 tabla 2 resultados en la comparacion de las series ~rlk y ~to'k'cuando existe simetria par en la matrfz de datos 10=32° 1,=32° 1,=45° 1,=150 1,=75° 1,=250 1,=900 00=-4.5° 0,=20° 0,=5°, 0,=_2° 0,=-5° 0,=0° 0,=0° c12(o) 1.0000 0.9999 0.9997 0.9993 0.9981 0.9998 09983 a/a 0.0000 0.0521 0.0265 0.0426 0.0620 0.0196 0.0770 o , 0.0000 1.2057 0.7246 1.1559 1.5872 0.5266 1.7418 6t -5.2645 -4.9228 -0.2279 -9.8870 11.0328 -7.5594 13.8868 6t" -5.2645 -4.5621 0.0777 -10.6070 10.5781 -7.8632 12.2702 en los resultados de las tablas 1 y 2, se observa que cuando se construye a partir de los datos de muestreo una matrfz palfndroma con simetrfa par, los coeficientes c,2(0) son mayores, las relaciones entre las desviaciones tfpicas (a/a) son menores (bath, 1974), las desviaciones medias son menores y los promedios ~to,se acercan mas a los promedios~r,;estas observacionesmuestranque se puede mejorar sustancialmente la convergencia del rnetodo, si el proceso se realiza organizando los datos en una matrfz palfndroma vertical con simetrfa par. los sectores del registro que presentan mayores desviaciones, son los extremos, debido a los efectos de truncaci6n de la serie, en el resto del registro las desviaciones no presentan una tendencia definida, por 10 que se puede considerar que tienen caracter aleatorio (fig.1). ai comparar los espectros de energfa correspondientes a ~to,y a ~r" se ve claramente que presentan el mismo contenido de frecuencias e igual amplitud espectral (fig.2). ei metoda es muy raroido,emplea aproximadamente 30 segundos en procesar 2 0 puntos muestra. como se ve en las tablas, se puede tomar para una situaci6n final, los valores if = 90° y of = 0°, que son las condiciones de reducci6n al polo, siendo entonces este metoda general y la reducci6n al polo un caso particular. 4. aplicaciones como una aplicaci6n directa se toma un perfil de campo maqnetico total, muestreado el 26 de marzo de 1:990 en el area de planeta rica (c6rdoba) (montana & pimiento, 1991), cuando el campo rnaqnetico en la zona tenfa una declinaci6n 0 = -2.49°, 1= 36.29°, la variaci6n de la anomalfa es relativamente pequena, teniendo en cuenta que la inclinaci6n ha tenido un cambio de aproximadamente 5° en los ultimos 10 afios, pero podemos proyectar la anomalla y observar las variaciones cuando el campo alcance un angulo i = 45° 0 un angulo de 75°, estos resultados se muestran en las figs.2 y 4, ~osdatos representados con lineas continuas corresponden a los datos originales observados y filtrados, las graficas representadas por lineas punteadas corresponden a los datos observados rotados y sus correspondientes filtrados. 5. conclusiones ei algoritmo planteado es muy eficiente; presenta una buena y rapida convergencia, que se manifiesta en los coeficientes de correlacion cruzada normalizados c12(0), en los valores para las relaciones a/a, en las desviaciones de los promedios, en los espectros de energfa y en el tiempo de caiculo. la proyecci6n de datos maqneticos, usando el algoritmo propuesto y la fft, se convierte en un proceso sencillo que se realiza en el dominic de la frecuencia, mediante operaciones elementales con nurneros complejos. colocar los datos rnaqneticos bajo las mismas condiciones, permite obtener ideas claras sobre las fuentes rnaqneticas en labores de interpretaci6n. en colombia se puede realizar el proceso de proyecci6n de levantamientos maqnetlcos en forma muy aproximada, ya que en el observatorio de fuquene, se tiene suficient informaci6n sobre el cambio real de los parametros inclinaci6n y declinaci6n magneticas. leo 120 ---(/) e eoe e o s 40 oo:;:; ii) 0 ,-,:::..~::....-.... ......c: 01o .' e .2 -40 0 e 0 -soc-c -120 _ ..-_._ ..,,--_._._-----------_. __ ._----:;;;;~.~~;~ ,-.... 100 (/) ~ 90 g 60 ~ 70 "0 <5 eo fo0 50o :oj '" 40c c\ 0 30:ii 0 200e c 10 u 0 -10 -20 0 -180+----,----r---~---_.---r_--___r---,...---_r_--___,----, figura 2. anomalia de campo magnetico total. planeta rica (cordova). registro original y rotado hasta 1=45°, 0=5° -500 -200 -100 0 100 distoncio (m.) -400 -300 figura 1. rotaci6n de datos l~agnetico8 120 110 __ anomaua wagnetlca oe calapo total. •••• _ anoimua ioiagnetlca oe campo total. rotaoa. 10 20 30 40 50 espocio (krn) 120 110 ..........100 '"0 90e e 800 ~ 70 "0 <5 80 f0 50c:;:; '" 40c 0\ 0 j()~ 0 200e 0 10 u 0 -10 -20 0 ~ """ anolaalja llagnet1ca de cai<lpo total. ----anomaua ..... gnet1ca de c-o total rotaoa. 10 20 7030 40 50 esoocio (krn) 60 60 90 figura 3. anomalia de campo magnetlco total. planeta rica (cordova). reglstro original y rotado hasta 1=750 , 0=50 eooo 5000 , 4000 i ~ 3000· t 2000 eepectro d. 10 •• rl. cah;uloda para i 45 u8anda ,i modelo 12 e.~ctro de 10 eerie rotollo deed' i 25 holjta i ~5 uaonllo 'i metoda 11 1000 0-+---· -0:05 0.00 0.05 0.10 0.15 frecuencio (1/1lm) 0.20 0.25 figura 4. eapeclroa de energra referencias bibliograficas bath, m. (1974): spectral analysis in geophysics. elsevier scientific publishing company. amsterdam -oxtordnew york. cooley, j.w., p.a.w. lewis & p.o.welch. (1969):the fast fourier transform algorithm. ibm corporation,thomas j. watson research center. new york. cooley, j.w. & j.w. tukey. (1965): an algorithm for the machine calculation of complex fourier series. math. compo19 (90):297-301. ofaz, r.l. (1983): tratamiento cuantitativo de datos aeromagneticos. tesis magister en geoffsica. universidad de chile. santiago. fandino, r., & f. cepeda. (1983): la transposlciony otras simetrias de matrices cuadradas. revista colombiana de estadfstica no 7. universidad nacional de colombia, departamento de matematicas y estadfstica. pp.55-63. bogota. geckinli, n.c., & y. oavras. (1983): discrete fourier transformation and its aplications to power spectra estimation. elsevier scientific publishing company. amsterdam -oxtordnew york. montana, j.c., & e. pimiento. (1990): estudio magnetometrico en las areas de planeta rica y montelibano (c6rdoba). trabajo de grado, carrera de geologia. universidad nacional de colombia. bogota. reford, m.s., & j.s. summer. (1964): aeromagnetics. geophysics 29 (4):482-516. telford, w.m., l.p. gerald, r.e. sherifff & o.a. keys. (1980): applied geophysics. cambridge university press. mine water is a menace to coal mining. mine water quantity is detected by drilling in the coal mine roadway, and the disadvantage is the high workload and low efficiency. therefore, transient electromagnetic method (tem) was proposed, and tem data was processed to detect the water yield property of a targeted layer in the coal mine. through a mine-oriented 3d transient electromagnetic method observing system, the signal of induced voltage is obtainable. transient electromagnetic method 3d data volume can be calculated through the calculation of all-time resistivity and time-depth conversion. after an appropriate apparent resistivity value is set, the spatial distribution range of an aquifer can be determined. then, with the water-filling coefficient of the aquifer, its water quantity can be estimated. the water yield property detection results in the no.4 coal seam goaf of the no.80101 workface in jude mine of shanxi, china, demonstrates that the apparent resistivity of this goaf is less than 3 ω.m, and the projection area of low resistivity anomaly zone is 22,383 m2. by using the formula q=kms, we can estimate that the water volume is 33,574 m3. three boreholes have been constructed for the next dredging and drainage project, which results in a total water yield of 33,089 m3. the error percentage of the predicted water quantity is less than 1.5%. it can thus be concluded that it is feasible to predict aquifer water content with tem data. abstract keywords: transient electromagnetic method, apparent resistivity calculation, time-depth conversion, water quantity quantitative calculation of aquifer water quantity using tem data cálculo cuantitativo del volumen de aguas subterráneas a través del método electromagnético transitorio (tem) issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.63002 earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 51 56 lanying huang1, shengdong liu1, bo wang1*, fubao zhou2 1 state key laboratory of deep geomechanics & underground engineering and school of resource and earth science, china university of mining and technology, xuzhou 221116, china 2 school of safety engineering, china university of mining and technology, xuzhou 221116, china email of corresponding author: wbsyes@126.com el agua al interior de las minas es una amenaza para la minería de carbón. la cantidad del agua en las minas se detecta al perforar en las vías del socavón, con la desventaja de la carga de trabajo y la baja eficacia que significan. por esto, el presente trabajo propone el método electromagnético transitorio (tem, del inglés transient electromagnetic method) y utiliza la información generada para detectar las propiedades de producción de agua en una capa específica de una mina de carbón. a través de un sistema de observación tridimensional tem orientado a minas se puede obtener la señal del voltaje inducido. la capacidad de la información del sistema tem tridimensional se puede calcular al obtener la resistividad y la conversión tiempo-profundidad. tan pronto se establece el valor apropiado de la resistividad aparente se puede determinar el rango de distribución de un acuífero. luego, con el coeficiente de saturación del acuífero se puede calcular la cantidad de agua. los resultados de la detección de las propiedades de producción de agua en la mina abandonada número cuatro de frente de trabajo número 80101 de la mina jude, en shanxi, china, demostró que la resistividad aparente de esta excavación es de 3 ω.m, y el área de proyección de la zona anómala de baja resistividad es 22.383 m2. al usar la fórmula q=kms, se estimó que el volumen de agua es de 33,574 m3. tres pozos se han construido para el próximo proyecto de dragado y drenaje, cuyos resultados de producción de agua es de 33,089 m3. el porcentaje de error de la predicción de la cantidad de agua es menor al 1.5 %. se concluye, por ende, que es factible producir el contenido de agua de un acuífero con la información del tem. resumen palabras clave: método electromagnético transitorio, cálculo de la resistividad aparente, conversión tiempo-profundidad, cantidad de agua. record manuscript received: 01/03/2017 accepted for publication: 31/03/2017 how to cite item huang, l., liu, s., wang, b.,* zhou, f. (2017). quantitative calculation of aquifer water quantity using tem data. earth sciences research journal, 21(1). 51-56. doi:http://dx.doi.org/10.15446/esrj.v21n1.63002 h y d r o g e o l o g y 52 lanying huang, shengdong liu, bo wang and fubao zhou 1. introduction china is the largest producer of coal in the world because coal plays an essential role in the economic development of china (figure 1). coal mining accidents have become a frequent occurrence in recent years (zhang et al., 2014; wang et al., 2016). although the death rate resulting from these accidents in china has declined, it is still 70 times higher than that in the united states and 17 times higher than that in south africa. against this backdrop, it is urgent to study how to find effective ways to detect water quantity in the aquifer and reduce coal mining accidents (zhang et al., 2015). among some geophysical methods currently available, tem technology is one that has received much attention and has been widely used to reduce coal mining accidents. figure 1. global coal production structure and energy consumption ratio in china. in a tem application, an ungrounded loop-line first transmits downward a pulse-type primary field, which induces an eddy current. the spatial and temporal distribution of a secondary field caused by the eddy current can then be observed through the coil (yu, 2007; wang et al., 2011; tanaka and kunisada 2011). by measuring the variation rule of the secondary magnetic field over time during the timeoff period, the geoelectricity features at different depths can be obtained (guillemoteau et al., 2011; tuncer et al., 2014). from figure 2 it can be seen that tem receives induced signals during the time-off period, which will be attenuated over time. by using the tem method, we can solve some geological problems (cheng et al., 2013; hu et al., 2013). due to its features such as light equipment, little lateral influence, and high resolution, tem can be widely applied in water disaster prevention and control (zhang et al., 2010; mollidor et al., 2013; xue et al., 2013; tao et al., 2013). however, because of the focus on theory development and processing method restraints, past analyses have usually been qualitative, which cannot meet the standard of quantification, and, to certain extent, hinders the application of tem on a wider basis(xu et al., 2012; wang et al., 2014). hence, in this study, we will develop a theory and propose a method to calculate tem responses. to do so, we will use equivalent substitution, interpret the diffusion law of tem field from the perspective of physics, calculate physical parameters, and conduct 3d apparent resistivity imaging and quantitative analyses of detection results. figure 2 schematic diagram of tem survey. 2. tem data processing 2.1. apparent resistivity calculation the transmitting loop center induction electromotive force of horizontal round loop in uniformity whole space medium is: (1) (2) (3) where is the vacuum permeability, n is the turn number of transmitting loop, i is the respective emission current, s is the receiving loop similar area, 0r is the transmitting loop radius, t is the observation time and is the true resistivity of the medium. from equation (3), we can obtain an equation as below: (4) if the loop forms a square about b meters on a side, and , then, with equations (1) ~ (4), the all-time apparent resistivity can be calculated with the binary search algorithm. the calculation process is illustrated in figure 3. for any observation window t, u is set as [0,10], and the searching region of u can be seen as an absolute value, and among them, us is the initial value of the searching region, ue is final value, ue is a median value which is u value. u is calculated by a binary search algorithm. is actual measured magnetic value, and h(ti ) is: 2.2. depth conversion the detection depth d of the tem method is related to transmitting magnetic torque, all-time apparent resistivity, and the minimum resolutive voltage. the time-depth conversion equation for mine-oriented tem method is: (5) (7) (8) (9) (10) (6) 53quantitative calculation of aquifer water quantity using tem data figure 3. the solving procedure of apparent resistivity. where is the conversion coefficient, which is about the heterogeneous conductor in the vicinity of the roadway and usually values between 0.6 and 1.8; is the factor of proportionality and , where is the corresponding resistivity value of moment it . (11) 2.3. interpolation operation due to the unstable workload, the measured and computational data of experiment and field are always not accurate enough, so interpolation is introduced as a supplement. ( , , )k k kk x y z is an arbitrary point in the three-dimensional space. there is given point which is the vicinity of k.the inverse distance weighting method can be used to attribute the value xk with the equation: where id is the distance between the interpolating point and the given point in the vicinity. yk and zk can be calculated in the same way as xk . 2.3. 3d slice extraction 3d visualization is a technique to reveal and describe spatial data, receive the subsurface geological structure and features, which provides information for accurate description of the 3d geological structure, and facilitate the exploration and development of coal mine. according to the data received from different detection directions, it can be seen that 3d visualization can clearly display the spatial distribution of anomalies with different resistivity values that correspond to different threshold values, the scattered points, and the range of apparent resistivity. however, a cross-section map can be clearer in showing the anomaly areas at a particular depth on a profile. cross-sections can be extracted from different depths of plane xy, yz, and zx. apparent resistivity data can be extracted from any depth or vertical profile. 2.4. aquifer water quantity estimation through the cross-section map, the aquifer water volume can be calculated with the following formula: (12) in this equation, q is the aquifer water quantity, k is the water containing coefficient, m is the aquifer thickness, and s is the water-rich area in the aquifer. 3. field application 3.1. geological setting the no.80101 workface of jude mine in shanxi province is under l1 limestone in the middle and lower part of taiyuan formation, 60.6m beneath the 4# seam. the seam occurrence is stable with a thickness range of 1.8 to 3.2m, and a 2.8m thickness on average. part of the area contains a 0.05 to 0.1m dirt band of mudstone or carbonolite (carbonaceous mudstone). at the bottom of the seam is a pyrite seam in forms of lamella and nodule. the lithological column is shown in figure 4. 3.2. field detection to detect the spatial distribution of the goaf and estimate the water quantity, mtem was adopted with the ycs360 electromagnetic system (as shown in figure 5). the detection of the no.80101 workface starts at the intersection of the return airway and the concentrated rail roadway, then goes on from the return airway to the openoff cut, and ends at the intersection of intake airway and concentrated rail roadway. the total detection workload is 3470 m, including 1660 m along the return airway, 150 m in the openoff cut, and 1660 m along the intake airway. monitoring points are set every 10 meters, and the detection is conducted in 6 directions, including 60°, 45°, and 30° to the external wall roof, standard to the ceiling, and 30° and 45° to the interior wall roof. the tem detection direction diagram is shown figure 4. lithological column. 54 lanying huang, shengdong liu, bo wang and fubao zhou figure 4. lithological column. figure 5. ycs360 electromagnetic system. figure 7. arrangement diagram of mtem. table 1. drilling log table of anomaly yc7 in no.80101 workface figure 6. tem detection direction diagram. in figure 6, and the arrangement diagram of mtem is shown in figure 7. 3.3. data interpretation and result analysis the data is processed based on the standard flow chart (figure 8), and data interpretation is carried out from the perspective of geophysical characteristics. the features are shown in 3d images. water quantity is higher in areas where the fractures are concentrated, indicated by higher electrical conductivity and lower resistivity (i.e., high potentiallow resistivity) anomalies. this characteristic identifies aquifer areas as those where roof resistivity is less than 3ω.m. figure 9 is a 3d spatial distribution graph of anomalies at different depths above the no.80101 workface. 40 meters above the roof is a water-rich stratum, and key preventions should be focused on region yc1, yc7, and yc10. anomaly area yc7 is located between 1085m and 1850 m along the return airway (between monitoring point kf13 to kf15), 0-120m along the inclination slope (120m within return airway and workface), and 40m to 60m above the roof (more details in fig 9), with an anomaly-impacted area of 22,383 m2. the goaf water-filling coefficient is 0.25 to 0.35, and the detection result of the actual ratio is 0.3. it is indicated by the effect sketch that m is 5m, based on the equation q=kms. thus we can deduce that q=33,574m³. the verification of the calculated values based on field measurements is shown in table 1. h192, h19-3 and h19-s1 boreholes have been constructed for the later next and drainage project, which results in a total water quantity of 33,089m3, and the error percentage of the predicted water volume is less than 1.5%. 55quantitative calculation of aquifer water quantity using tem data figure 8. processing flow chart figure 9. the 3d spatial distribution of anomalies at different depths above the no.80101 workface. 56 lanying huang, shengdong liu, bo wang and fubao zhou 4. conclusions tem resistivity imaging boasts rapid imaging, high quantification level, and high resolution, especially when it comes to detecting low resistivity body with a high-resistivity overlayer. with the projection area s of low-resistivity anomaly zone, the water containing coefficient k and the aquifer thickness m, the aquifer water quantity can be estimated with an error percentage of the predicted water quantity below 2%. the tem 3d presentation and analysis has offered a new method to determine the water amount in an aquifer. acknowledgments this research has been performed under national natural science foundation project (grant no. 41604082, u1261202), and the fundamental research funds for the central universities (2014xt02), and a project funded by the priority academic program development of jiangsu higher education institutions. references cheng, j. l., qiu, h., ye, y. t., yan, g. c., zhou, j., cheng, f. b., & zhang, s. s. (2013). research on wave-field transformation and data processing of the mine transient electromagnetic method. journal of the china coal society, 38(9), 1646-1650. guillemoteau, j., sailhac, p. & béhaegel, m. (2011). regularization strategy for the layered inversion of airborne transient electromagnetic data: application to in-loop data acquired over the basin of franceville (gabon). geophysical prospecting. journal of immunoassay, 59(6), 1132-1143. hu, x. w., zhang, p. s., cheng, h., wu, r. x., & guo, l. q. (2013). quantitative assessment of interference induced by roof bolt during advanced detection with transient electromagnetic method in mine. journal of rock mechanics and engineering, 32, 3275-3282. lin, z., ge, s., li, d., & peng, w. (2015). structure characteristics of acidic pretreated fiber and self-bind bio-boards for public health. journal of pure and applied microbiology 9, 221-226. liu, z. l. (2014). the groundwater pollution and environmental protection in china. biotechnology: an indian journal, 10, 1883-14886. liu, z. x., liu, x. c., & liu, y.g. (2009). research on transient electromagnetic field of mine water-bearing structure by physical model experiment. journal of rock mechanics and engineering, 28(2), 259-266. mollidor, l., tezkan, b., bergers, r., & löhken, j. (2013). float-transient electromagnetic method: in-loop transient electromagnetic measurements on lake holzmaar, germany. geophysical prospecting, 61(5), 1056-1064. tao, f., li, w. g., wang, p., & an, s.p. (2013). research on fine interpretation for water containment of coal mine rock strata by mt imitated tem depth inversion method. journal of the china coal society, 38(z1), 129-135. tanaka, y., & kunisada, e. (2011). study on meshless method using rpim for transient electromagnetic field. ieee transactions on magnetics, 47(5), 1178-1181. tuncer, o., shanker, b., & kempel, l. c. (2014). a hybrid vector generalized finite-element method for transient electromagnetic simulations. electromagnetics, 34 (3-4), 286-297. wang, b., liu, s. d., liu, j., huang, l. y., & zhao, l. g. (2011). advanced prediction for multiple disaster sources of laneway under complicated geological conditions. international journal of mining science and technology, 21, 749-754. wang, b., liu, s. d., lu, t., & sun, h. l. (2014). coal seam thickness detection in mine roadway by using advanced prediction method. electronic journal of geotechnical engineering, 19, 4753-4762. wang, b., liu, s. d., zhou, f. b., lu, t., huang, l. y., & gao, y. j. (2016). polarization migration of three-component reflected waves under small migration aperture condition. acta geodynamica et geomaterialia, 13(1), 1-12. xue, g. q., cheng, j. l., zhou, n. n., chen, w. y., & li, h. (2013). detection and monitoring of water-filled voids using transient electromagnetic method: a case study in shanxi, china. environmental earth sciences, 70(5), 2263-2270. xu, j. p., liu, s. d., & wang, b. (2012). electrical monitoring criterion for water flow in faults activated by mining. mine water and the environment, 31(31), 172-179. yu, j. c. (2007). mine transient electromagnetism prospecting. china university of mining and technology press, xuzhou. zhang, s. f., meng, l. s., & du, x. j. (2010). transient electromagnetic method to investigating potential safety hazard of mine-out area in tailings pond of a gold mine. journal of jilin university (earth science edition), 40(5), 1177-1182. zhang, w., zhang, d. s., wang, h. z., & cheng, j. x. (2015). comprehensive technical support for high-quality anthracite production: a case study in the xinqiao coal mine, yongxia mining area, china. minerals, 5(4), 919-935. zhang, w., zhang, d. s., wu l. x., & wang h. z. (2014). on-site radon detection of mining-induced fractures from overlying strata to the surface: a case study of the baoshan coal mine in china. energies, 7(12), 8483-8507. geotisica colombiana pp.71-73 bogota, d.c.n° 7 dieiembre de 2003 issn 0121 2974 true amplitude migration in constant gradient velocity media luis antonio castillo lopez protesor asistente universidad nacional de colombia, curso posgrado de geofisica tacultad de ciencias. departamento de geociencias, grupo de investigaci6n en geofisica e-mail: lacastillol@unal.edu.co resumen ei algoritmo de kirchhoff con trazado de rayo se basa en una funcion de velocidad con stante. esta solucion incorpora informacion de trayectoria de rayos dentro del algoritmo, el cual puede extenderse a un medio de velocidad variable. el modelo de veloci dad incluye la localizacion de reflectores e interval os de velocidades entre dos reflectores adyacentes, generados a traves del analisis de velocidad, pero importante en el procesamiento sismico tradicional. este proceso puede alcanzarse al aplicar un operador de funcion peso de apilado por difraccion, cuyo desarrollo teorico se bas a en la integral de migraci6n tipo kirchhoff. al escoger el propio peso para el peso de apilado de datos, el resultado del proceso de migraci6n es una seccion sismica donde se recupera la amplitud y su valor es proporcional al coeficiente de reflexion, liamado rnigracion con amplitudes verdaderas (recuperacion de amplitudes verdaderas). palabras clave: recuperacion de amplitudes verdaderes, psdm, medid con gradiente de velocidad. abstract the kirchhoff conventional algorithm using ray tracing is based on a constant velocity function. this solution, where is incorporated ray path information into conventional algorithm, can be extended to a variable velocity medium. velocity models include the locations of reflectors and interval of velocities between any two adjacents reflectors that are generally generated through the velocity analyses, process important in traditional seismic processing. this process could be achieved by applying a weighted diffraction stack operator, which theoretical development is based on a kirchhoff type migration integral. by choosing the proper weight for stacking the data, the result of the migration process is a seismic section where the amplitude are recovered and its value are proportional to the reflection coefficient, the so-called true-amplitude migration (true ampiltude recovering). migration process was applied into different models. the objetive is to reproduce an accurate picture of the subsurface model from the data registered in areas where the velocity changes. the migration process was obtained after trying several testes made with different weigths. key words: true amplitude recovery, psdm. gradient velocity media introduction by incorporating ray path information into conventional migration algoritm, the solution can be extended to variable velocity medium. the rays are traced from each depth point in the survey of each source-geophone location in order to determine the difraction traveltime surface difraction (huygens surface). then, the weighted diffraction stack operator could be used to migrate the seismic data. in this paper we present a fast algorithm that is able to recover true amplitude in depth migration process, by considering manuscrrto recibido para evaluacion el15 de junio de 2003. articulo aceptado para publicae ion por el comne editorial el 36 de octubre de 2003. gfofislca cdlombiana. 7, diciembre de 2003 a more particular situation when the seismic velocity model is represented by a linear distribution variable with the depth. this kind of model is important for simulating many situations in the seismic exploration. for instance, variations of the velocity with depth, inversion velocity (negative gradient) and increasing of velocity in a short layer (positive gradient). modelling asymptotic ray theory provide useful extrapolating techniques for modelling, migration and inversion, using ray theory of coordinates tracing equations which determine the second derivatives of the traveltime curve. this is desired in order to determine geometrical spreading, and then true amplitude recovery. in order to know 71 luis antonio castillo l6pez ray irajelory with positive gradient velocity 0.5 e ~1.5 ! 2.51------3l-'::-----'--,j-'---' o 0.5 1.5 2.5 offset (km) 0.5 ec -e 1.5 ! 2.51------........... 3l-----''--' ---'--'----l o 0.5 1.5 2 offset (kml figure i. models for differents gradients in a configuration common shot. gradient positive (left) and gradient negative (rigth), the stability of the diffraction stack migration algorithm, we have applied it to a set of seismic data in a common-shot configuration. diffraction stack operator after data input, two-dimensional diffraction stack operator can be written following the expression given by urban (1999), based on martins et al. (1998) 72 seismogram distance o 0 5 (km) i 1.5 2 i -i------=+=~~~_rt'-i-r-,....,...,~.,....,..-,--l----___+ e 0.5 -i------+-++-i-++-;.+:~~f.l=m"!-'l-+-----____+ f.== o-l------j-lllllllllj...ljlulu--'--'--'--,------t ray trajectories in constant gradient velocity media with commom shot configuration o r-~-----''''-'-"t't'rr-rt'crroct>--rt..--.rrr~~~---' vo=2kml 0.2 i ;s 0.4 c. ojo 0.6 0.8l--~-~~~~-~-~-~-~-~----j o 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 offset (km) figure 2. ray trajectory model(lower) and synthetic seismogram(upper). v(m ,t)= rbfd~w(~,m)~ u (~,t+td) v21t a (1) with the source and receiver pairs (s,g) described by the vector parameter and the traveltime curve defined by all the points of on the earth surface, and a point m in depth. ar~u (~, t +,d ) is the anti-causal half-time derivative (operator that corresponds in the frequency domain to the filter.j -iw) of the seismic trace u (~,t + 'd) registered on the geophone position g( ~), and it represents the principal component of the seismic primary reflected wavefield. the result of the integral (i) is put at the point m into the model. providing that we call depth difraction stack migration, with all lines summed along the stacking line and multiplied by the true amplitude (weight function) w(~,t-'d) and summed, then the result of migration process is proportional to the reflection coefficient recovered. 2.5 diffraction traveltime stack curve being known the macrovelocity model with a distribution of velocity varying linearly with depth the diffraction traveltime stack curve can be built, for the diffraction stack processing v(z)=vo +gz (2) where vo is the velocity near the earth surface, g is the gradient of the velocity function in the direction of the vertical axis z, in depth. this kind of velocity function is the first approximation to be considered when investigating the regional variations of velocity for most sedimentary rocks (lapsen, 1993). geofisiga golombiana, 7, oigiembre de 2003 true amplitude migration in constant gradient velocity media 1.9,----,------,-----,,---,---r----,--,.--...., 1.85r········· ,........... t···········, l ;..............,............."... ~ ~ 1.8 r············ .j ,........... , , ,...........w, ,..,?<. , ~ ~ 1.75 r············,··········· "',....... ; ; ; ····:<'a::···············l ~ ;:: 1.7 r···········+···············f."-:~~==~4:···w! .]. ~ 1.65 r···········t··········· t··········· ; ,........... .i. ; ;.... ~ 0.5 1.5 2.5 distance (km) 3.5 4.5 figure 3. traveltime difraction and traveltime reflection curve for a depth point. inside such model with linear variation of velocity, the ray trajectory is circular and the traveltime is given as solution of the integral (bleistein, 1986) i tan ( a~) i t=-in -in(b,bg) g tan(a~) g (3) being b, = 1 + (g2p + 2g(ji) / 2cco. all the values of this result are known; ii and e, are the circular trajectory and parameter ray, respectively. computational examples the seismic model consists of by a curved reflector bellow an inhomogeneous domo with constant gradient velocity medium, where the near surface velocity is 2.0 kmls (varying with the depth), and its gradient is 1.25/s. the shot position is 0.4 km on the left, with 0.5 km and 1.45 km for the near and far offset, respectively. the source dominant frequency is about 70 hz, and the sample interval is of 2.0 ms. in the figure 2 are represented the tracing rays(lower) and the synthetic seismogram(upper) for a asymmetric common shot configuration. the seismic data is noise free. in the figures 3 and 5 we geofislca colomb lana. 7, diciembre de 2003 have the image and depth migrated seismic data, after appliying of the diffraction stack operator to the respective set of input seismic data. it is important to note that the proposed algorithm provides a good image of the target reflector, and it is able to be used for migrating seismic data in constant gradient velocity media. conclusions in this paper, was develop and tested the diffraction stack migration algorithm, applying it to a set of common-shot seismic data, syntheticaly generated by the ray theory for a constant gradient velocity medium. the algorithm was tested providing us a good image of the target reflector. this result is very important in sense that the proposed algorithm is fast, stable and suitable to be used for true amplitude depth migration. references bleistein, n., 1986, two-and -one-half dimensional in-plane wave propagation. geophysical prospecting, 34, 686-703. japsen, p., 1993, influence of lithology and neogene uplift on seismic velocities in denmark: implications for depth conversion of maps. the aapgb, 77, n. 2, 194-211. schleicher, 1.; tygel, m.; and hubral, p., 1993, 3-d true-amplitude finite-offset migration. geophysics, 58(8): 1820-1830. schneider, jr. w. a.; ranzinger, k. a.; balch, h.; and kuse, c, 1992, a dynamic programming approach to first arrival traveltime computation in media with arbitrarily distributed velocities. geophysics, 57, n. 1,39-50. urban, 1.,1999, two-dimensional true-amplitude migration and introduction to 2.5-d case. master thesis. federal university of para, brazil. (in portuguese). acknowledgments we would like to thank the seismic group of the geophysical institute, charles university, prague, czechoslovakia, for making available the ray tracing software seis88, and the universidad nac10nal de colombia for supporting the author. 73 as hydrological models become more prevalent in water resources planning and management, increasing levels of detail and precision are needed. currently, reliable models that simulate the hydrological behavior of a basin are indispensable; however, it is also necessary to know the limits of the predictability and reliability of the model outputs. the present study evaluates the influence of uncertainty in the main input variable of the model, rainfall, on the output uncertainty of a hydrological model. using concepts of identifiability and sensitivity, the uncertainty in the model structure and parameters was estimated. then, the output uncertainty caused by uncertainties in i) the rainfall amounts and ii) the periods of the rainfall was determined. the main conclusion is that uncertainty in rainfall estimation during rainy periods produces greater output uncertainty. however, in non-rainy periods, the output uncertainty is not very sensitive to the uncertainty in rainfall. finally, uncertainties in rainfall during the basin filling and emptying periods (apr. – jun. and sep. – nov., respectively) alter the uncertainty in subsequent periods. therefore, uncertainties in these periods could result in limited ranges of model predictability. los modelos hidrológicos se han vuelto cada vez más necesarios en la planificación y gestión de recursos hídricos, donde un aumento en los niveles de detalle y precisión es necesario. actualmente disponer de modelos para simular el comportamiento hidrológico de una cuenca resulta indispensable, sin embargo, también es necesario conocer los límites de predictibilidad y de confiabilidad de las salidas de un modelo. en este estudio se evalúan la influencia de la incertidumbre en la principal entrada de un modelo, la precipitación, sobre la incertidumbre de las salidas de un modelo hidrológico. utilizando conceptos de identificabilidad y sensibilidad se estima la incertidumbre de los parámetros y estructura de un modelo. luego, la incertidumbre en las salidas causadas por incertidumbre en i) los montos de precipitación, ii) los períodos de precipitación fue calculada. como conclusiones se obtuvo que la incertidumbre en la estimación de la precipitación en períodos de lluvia produce mayor incertidumbre sobre las salidas. en períodos no lluviosos, la incertidumbre de las salidas es poco sensible a incertidumbre sobre las precipitaciones. finalmente, incertidumbres en periodos de llenado y vaciado (abril-junio y septiembre-noviembre respectivamente) afectan la incertidumbre en las salidas en los períodos subsecuentes. por lo tanto incertidumbres en aquellos períodos pueden resultar en rangos limitados de predictibilidad de un modelo. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 69 75 abstract resumen key words: uncertainty, hydrological predictability, surface hydrology, water resources. palabras clave: incertidumbre, predictibilidad hidrológica, hidrología superficial, recursos hídricos. uncertainty in rainfall input data in a conceptual water balance model: effects on outputs and implications for predictability enrique muñoz, pedro tume, gabriel ortíz1 1. alonso de ribera 2850, concepción. departamento de ingeniería civil, universidad católica de la santísima concepción, concepción, chile mail: emunozo@ucsc.cl, ptume@ucsc.cl, gaortiz@ing.ucsc.cl meteorology record manuscript received: 16/07/2013 accepted for publication: 29/04/2014 70 enrique muñoz, pedro tume, gabriel ortíz introduction considering that i) water is a limited resource, ii) its demand increases with development and population growth, iii) its availability changes year to year due to local, global, natural and anthropogenic phenomena, iv) the impact of climate change on the water supply is of fundamental importance and v) a large part of the world’s population is already experiencing water stress (vörösmarty et al., 2000), it is necessary to develop tools for efficient water resources management and to support the prediction of future conditions under different scenarios caused by local (orographic), regional (el niño southern oscillation) or global effects (climate change). one alternative for efficient water resources management and planning is the use of hydrological models. a model attempts to reproduce a physical phenomenon that occurs in an object or area. therefore, in hydrology, a model seeks to represent an area defined by a watershed, the phenomena of rainfall-runoff processes and the associated water movement. the objective of reproducing these processes is to simulate and predict future conditions for management, administration and optimization of water uses. consequently, it is essential to have reliable models that adequately represent the hydrological behavior of a basin. for example, when using predictive models or alternative sources of information (e.g., global datasets) (mahe et al., 2008, muñoz, 2010), it is necessary to know the predictability limits of the model outputs and their uncertainty. normally, a conceptual hydrological model requires at least two input variables (potential evapotranspiration and rainfall) to quantify the inputs and water losses in the water balance. rainfall is the main input variable for a hydrological model (olsson and lindström, 2008). therefore, the potential predictability and the range of the predictability of the basin flows depend on the uncertainty associated with the input variables and their quality. uncertainty is defined as the degree of the lack of knowledge of or confidence in a certain process or result (caddy and mahorn, 1995). therefore, in a hydrological model, the sources of uncertainty are associated with the input variables (lack of knowledge of the quality of the measurements and predictions) and with the model structure and calibration parameters (lack of knowledge and simplification of the simulated hydrological processes in a basin). the present study aims to quantify how uncertainty in the outputs of a conceptual hydrological model is affected by uncertainty in the main input, rainfall, while keeping the uncertainty associated with the model structure and calibration parameters fixed. methods case study the polcura river basin (figure 1) is located in the temperate zone of south-central chile, between 37°20’s 71º31’w and 36°54’s 71°06’w. it is bounded by the andes to the east, the nevados de chillán volcanic complex to the north, and lake laja to the south. it comprises an area of 914 km2 that is between 700 and 3,090 masl; the area is characterized by steep slopes (≈ 26° on average) that are mainly composed of partially eroded volcanosedimentary sequences (om2c, ppl3 and m3i) (sernageomin, 2006). figure 1: location, boundaries and characteristics of the polcura river basin. in addition, the area is located in an extratropical zone that is greatly affected by el niño southern oscillation (enso) (grimm et al., 2000), which introduces seasonality and interannual variability to the local hydrometeorological patterns (grimm et al., 2000; montecinos and aceituno, 2003). the average annual precipitation in the basin is 2300 mm, with a pluvial period during winter and an ice-melt and snowmelt period in spring and early summer. the average monthly temperature is 9° c, ranging from 2.5° c in winter to 16.5 ° c in summer. because of the location of the basin, its mountainous nature and its geomorphology (figure 1), it exhibits high temporal variability with respect to hydro-meteorological characteristics, where the orographic effect on the eastern slope of the andes produces an increase in rainfall (garreaud, 2009; vicuña et al., 2011). additionally, the area is affected by enso. enso is a coupled ocean-atmosphere phenomenon that is characterized by irregular periodicity (2 to 7 years), where the alternation between el niño and la niña episodes is associated with above and below average rainfall and warmer and colder than normal air temperatures, respectively (garreaud, 2009). therefore, the characteristics of the basin, such as high rainfall variability and enso effects, provide an ideal case study in which uncertainty in precipitation could be the main source of uncertainty in predictions. description of the water balance model the model used in this paper is the snow-rain conceptual water balance model presented by muñoz (2010) and muñoz et al. (2014). this model separately simulates pluvial and snow-melting processes and includes external alterations, such as irrigation or transfer canals, by adding or subtracting flows. this model has been successfully implemented in andean basins in southcentral chile (e.g., zúñiga et al., 2012, arumí et al., 2012). therefore, it is an adequate option for analyzing the study area. the pluvial component is modeled using a lumped monthly rainfall-runoff model that treats the watershed as a double storage system: subsurface (ss) and underground storage (us). ss represents the water stored in the unsaturated soil layer as soil moisture. us is the water that covers the saturated soil layer. the model requires two inputs: rainfall (pm) and potential evapotranspiration (pet). the model output is the total runoff (etot) at the watershed outlet, and it includes both subterranean (es) and direct runoff (ei). these amounts are calculated by six calibration parameters and two input modifiers (useful in case of non-representative pm and pet data). the snowmelt model calculates snowfall (psnow) based on precipitation above the 0 (°c) isotherm. psnow is stored in the snow storage system (sn), for which the melting calculations are performed using the degree-day method (rango and martinec, 1995). thus, potential melting (psp) is estimated, and depending on the snow stored, real melting (ps) is calculated. then, ps is distributed in the pluvial model through calibration parameter f. every calibration parameter has a conceptual meaning for integrating spatial and temporal variability. table 1 presents a brief description of the model parameters and their influence on the model. the external alterations module permits the incorporation of alterations, such as irrigation or transfer canals, and simulates inflows and/or outflows to/ from the basin by adding or subtracting flows as follows: (1) where the basin outflow (qout) at time t is the basin runoff (etot) plus the contributions (qcontributions) and less the extractions (qextractions) during the same period. for a further explanation of the model, refer to muñoz (2010) and muñoz et al. (2014). q out (t) = etot (t) + qcontributions (t) qextractions(t) 71uncertainty in rainfall input data in a conceptual water balance model: effects on outputs and implications for predictability model inputs to run the described model, it is necessary to obtain precipitation, temperature and potential evapotranspiration series, as well as the geomorphological characterization of the basin to compute the monthly 0°c isotherm elevation. for the geomorphological characterization of the basin, a digital elevation model was constructed using data from the shuttle radar topography mission (srtm) of 3 arc-seconds (90 m). for the inputs, rainfall series from the rain gauge stations of las trancas, san lorenzo and trupán were obtained (administered by the dirección general de aguas, dga). temperature series from the center for climatic research of the university of delaware (ud) (willmot and matsuura, 2008) were collected. climatological model inputs were constructed using both sources of information. additionally, the potential evapotranspiration series were calculated using the thornthwaite method and the ud temperature data series. the spatial distribution of these variables in the basin was determined using thiessen polygons. based on prior experience, both the thornthwaite and thiessen polygons methods are adequate for estimating potential evapotranspiration and for determining the meteorological spatial distribution of monthly data in basins of south-central chile (e.g., muñoz, 2010, zúñiga et al., 2012). therefore, both methods were used in this study. because of the availability and quality of the input data, the analysis was performed on a monthly scale for a period of 13 years (1990 – 2002). the stream gauge station for controlling the basin outflows is the polcura antes de descarga central el toro station (figure 1). uncertainty analysis to quantify the uncertainty, the monte carlo analysis toolbox (mcat) (wagener and kollat, 2007) was used. mcat is a tool that operates under the generalized likelihood uncertainty estimation methodology (beven and binley, 1992), and it contains a group of analysis tools to explore the identifiability of a model and its parameters. mcat was chosen because it is an easy-to-use tool and is an adequate and simple option for evaluating model behavior and uncertainty. to evaluate identifiability and quantify the uncertainty in a model, mcat operates by performing repetitive simulations using a set of randomly selected parameters within a range defined by the user. the program stores the outputs and values of the objective function(s) for subsequent analyses. in this case, due to the lack of knowledge of the parameter distribution, a prior uniform distribution of the model parameters was used. parameters of hydrological models cannot be identified as unique sets of values. this is mainly because changes in one parameter can be compensated by changes in one or more other parameters due to their interdependence (bárdossy, 2007) and because the processes simulated in a hydrological model are commonly interrelated. in the case of the muñoz (2010) model, for example, direct runoff depends on cmax; therefore, the processes that occur in the subsurface storage layer depend on the amount of rainfall that is not transformed into runoff (i.e., 1-cmax); consequently, the processes that occur in this storage layer depend on the identifiability of cmax. this generates an interconnection among the identifiability of the calibration parameters of a model. therefore, it is necessary to perform various iterations as a means of restricting the range of the validity of the parameters that first show identifiability and to observe identifiability in the remaining (dependent) parameters as a means of reducing the identifiability range of these parameters. because the monte carlo method is based on random trials, it normally requires a large number of simulations that cover a wide spectrum of possible simulations. in this case, the number of monte carlo simulations was estimated via trial and error. the “stop” criterion was met when the correlation (according to the pearson correlation coefficient) between uncertainty bands (calculated as a linear correlation between the time series of the upper and lower limits of the uncertainty bands) of two trials with the same number of simulations was equal to or greater than 0.999. under this criterion, it was determined that the adequate number of simulations for this study was 25,000. the procedure to estimate the rainfall uncertainty and to differentiate it from the model structure and calibration parameters was as follows: through an identifiability analysis of the factors for the modification of the inputs (a comparison of these factors with the value of the objective function), these factors were limited to precise values. it is necessary to set these factors to unique values, as they ensure the long-term water balance in which the precipitation input volumes are equal to the evapotranspiration and flow output volumes during the entire period of the simulation. with the values of these fixed factors, new simulations were performed, in which the ranges of variation assigned to each parameter were reduced in table 1: description of the model parameters, adjustment factors, and conceptual physical range for the pluvial and snow-melting model. parameter description influence on range pl uv ia l m od ul e pa ra m et er s cmax maximum runoff coefficient when the sub-surface layer is saturated. ei 0.00 – 0.60 plim (mm) limit of rainfall for which ppd exists. ppd 0.00 500 d percentage of rainfall for which plim transforms into ppd. ppd 0.00 – 0.60 hmax (mm) maximum capacity of the soil layer to retain water. cmax and er 180 500 porc fraction of hmax that defines the soil water content restriction of the evaporation processes. hcrit and er 0 100 ck subterraneous runoff coefficient. es 0.05 – 0.60 a adjustment factor of the precipitation data. pm 0.80 – 2.50 b adjustment factor of the potential evapotranspiration data. pet and er 0.80 – 2.50 sn ow m od ul e pa ra m et er s m (mm °c-1) fraction of snowmelt over a base temperature (tb) at which melting starts. psp, ps 1 – 12 dm minimum rate of melting when tm < tb. psp, ps 0.00 – 0.50 f fraction of the real snowmelt that contributes to ei. ei 0.0 – 1.00 fgt adjustment factor of the thermic gradient data. psp, ps 1.00 – 5.00 72 enrique muñoz, pedro tume, gabriel ortíz accordance with the observed identifiability. the established stop criterion was defined as when identifiability was not observed in the new defined range (the reduced range) of each parameter. after the identifiability analysis, the resulting uncertainty in the model outputs was quantified. this uncertainty was defined as the uncertainty associated with the model structure and calibration parameters. the confidence level was 90% (a range between 5 and 95% of the outputs for each time step of the series). the established rejection criterion was set according to the kling-gupta efficiency objective function (kge) (gupta et al., 2009). the kge function (eq. 2) is an improvement of the nash-suctliffe efficiency index (nse) (nash and sutcliffe, 1970) in which correlation, deviation and variability are equally weighted to resolve systematic underestimation of the maximum values and low variability identified in the nse function (gupta et al., 2009). the kge varies between and 1, where the best model is closest to 1. because the glue methodology is based on likelihood, the kge was transformed into 1-kge. with this change, 1-kge represents a measure of likelihood in which the best model is closest to 0 and the worst model is + . where r is the linear correlation between the simulated and observed flows, α is the ratio between the standard deviation of the simulated and observed flows and represents the variability in the values, and β is the ratio between mean simulated and mean observed flows (i.e., the bias). then, with the uncertainty associated with the model structure and parameters quantified, the rainfall series were modified for different magnitudes and periods, as shown in table 2. it is important to note that this variation corresponds to a punctual variation of the rainfall series of different magnitudes and periods, and uncertainty is not added as a rainfall range in each time step. this analysis aims to identify how output uncertainty would be affected by an error in the precipitation prediction. table 2: precipitation percentage change in different time periods variation period variation (%) mean rainfall season (mm) entire year (jan. – dec.) ±5 ±5 ±15 ±20 ±25 2253 winter (jun. – aug.) ±5 ±10 ±15 ±20 ±25 805.4 summer (dec. – feb.) ±5 ±10 ±15 ±20 ±25 126.6 basin-filling period (apr. – jun.) ±5 ±10 ±15 ±20 ±25 808.4 basin-emptying period (sep. – nov.) ±5 ±10 ±15 ±20 ±25 368.7 results and discussion using the previously described methods, uncertainty associated with the model structure and parameters was found; then, the uncertainty related to rainfall data were computed and analyzed. the major findings and a discussion are presented as follows. uncertainty associated with model structure and calibration parameters figure 2 shows the identifiability and sensitivity analysis performed for the model parameters. the graphs show the probability distribution function (pdf) and the cumulative distribution function (cdf) of each parameter for the best 10% of the simulations according to a determined objective function (kge in this case). as observed, five parameters presented identifiability (according to the pdf): a, b, cmax, hmax and ck. according to the cdf, a, cmax and ck were the most sensitive (i.e., greater slope within a determined range) to the model outputs. a 1 1.5 2 0.5 1 d 0.2 0.3 0.4 0.5 0.5 1 plim 200 400 600 800 0.5 1 porc 20 40 60 80 0.5 1 dm 0.05 0.1 0.15 0.5 1 f 0.2 0.4 0.6 0.8 0.5 1 m 2 4 6 8 10 0.5 1 fgt 3.5 4 4.5 0.5 1 b 1.3 1.4 1.5 1.6 1.7 1.8 1.9 0.5 1 cmax 0.3 0.35 0.4 0.45 0.5 0.5 1 hmax pd f kg e; c df kg e 250 300 350 400 450 0.5 1 ck 0.1 0.2 0.3 0.4 0.5 0.5 1 [0.5 2.5] [0.5 2.5] [200 500] [0.0 0.6] [50 1000] [0 100] [0.05 0.60] [0.05 0.80] [0.0 0.2] [0 1] [1 12] [3 5] figure 2: analysis of the identifiability and sensitivity of the model parameters. the black line and the histogram in each graph represent the cdf and pdf of the best 10% of the simulations, respectively. the range indicated in the upper left corner is the initial range of the analysis for each parameter. because of the identifiability analysis, the ranges of variations in these five identifiable model parameters were reduced. a complementary analysis was performed in which the value of each parameter versus the value of the objective function was plotted. based on this analysis, it was determined that the factor that modifies the inputs (a) has an optimal value of 1.58. therefore, the differences between the simulated and observed flows, according to the kge, are minimal when a is 1.58. this analysis allows the value of the parameter to be fixed and the influence of the model structure and parameters on the output uncertainty to be separated from later analyses. the resulting value of parameter a suggests that the distributed rainfall over the basin was underestimated. the underestimation is caused by the orographic effect and the related enhancement of rainfall in mountainous areas (garreaud, 2009); these effects are not well recorded by meteorological stations, mainly because they are located in the lower part of the basin. finally, the aforementioned parameters were limited to the following ranges: i) b [1.2 – 1.7], ii) cmax [0.3 – 0.52], iii) hmax [220 – 500], and iv) ck [0.25 – 0.52]. the remaining parameters, which exhibited neither greater sensitivity nor identifiability, retained their initial ranges in the simulations. then, considering the aforementioned parameter ranges and using mcat, the output uncertainty associated with the model structure and calibration parameters was estimated. next, using this data, the influence of the input variables on the uncertainty in the output was estimated. uncertainty associated with the input rainfall data by incorporating the variations in the rainfall in accordance with table 2, new ranges of uncertainty were estimated. to identify these new ranges and observe the differences generated by the modification of the input variables, figures 3 to 7 show the uncertainty bands associated with the model structure and calibration parameters (white area between the black lines), the uncertainty associated with the positive variations in the rainfall (blue area), and the uncertainty associated with the negative variations in the rainfall (red area). additionally, to quantify the propagation of rainfall uncertainty effects, table 3 presents a summary of the average width of the uncertainty bands (in m3/s) for the different variations in the rainfall data. additionally, table 4 presents the percentage of variations in these ranges. figure 3 shows that the model outputs are sensitive to changes in the kge = 1 − (r −1)2+(α −1)2+(β −1)2 (2) 73uncertainty in rainfall input data in a conceptual water balance model: effects on outputs and implications for predictability rainfall amounts, where the range of uncertainty increases by an average of 15% for positive variations and 14% for negative variations in the rainfall (table 4). upon comparing these results with figure 3, it can be observed that overestimations of the rainfall amounts result in an increase in the uncertainty toward greater flows, while underestimations of the rainfall amounts result in an increase in the uncertainty of the outputs toward greater or reduced flows. figure 3: uncertainty bands associated with the model structure and calibration parameters (white area between the black lines) and uncertainty bands for the positive (blue) and negative rainfall variations (red) of 5, 10, 15, 20 and 25% (plots a, b, c, d and e, respectively) during the entire simulated period. figure 4 shows that errors in the prediction of the rainfall amounts during the winter period generate an effect on the flows of the recession curve by increasing the range of the uncertainty of the outputs of the model in the spring-summer (oct.-feb.) and early fall (mar.-apr.) (see the periods prior to the peak flows in figure 4) and by increasing the discharge amounts estimated by the model. (b) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (c) q (m 3 /s ) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (d) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (e) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (a) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 tiempo (meses) figure 4: uncertainty bands associated with the model structure and calibration parameters (white area between the black lines) and uncertainty bands for the positive (blue) and negative rainfall variations (red) of 5, 10, 15, 20 and 25% (plots a, b, c, d and e, respectively) during the winter months (jun. – aug.) figure 5 shows that errors in the estimation of the rainfall in summer do not significantly affect uncertainty in the model outputs, most likely because the earlier and later flows of the period mainly depend on the base flow and water storage in the basin during winter, rather than the runoff that can be produced in summer. (a) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (b) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (c) q (m 3 /s ) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (d) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (e) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 tiempo (meses) figure 5: uncertainty bands associated with the model structure and calibration parameters (white area between the black lines) and uncertainty bands for the positive (blue) and negative rainfall variations (red) of 5, 10, 15, 20 and 25% (plots a, b, c, d and e, respectively) during the summer months (dec. – feb.) figure 6 shows that overestimations in the prediction of the rainfall during the basin-filling period produce a displacement of the output uncertainty bands toward greater flows but maintain a relatively constant uncertainty band width (tables 3 and 4). this result suggests that the model is highly sensitive to overestimations of the rainfall data in such a period, most likely because the model is not capable of redistributing the water in the basin processes; therefore, higher flows and similarly wide uncertainty bands are obtained. in contrast, underestimations of the rainfall in this period do not result in greater differences in the uncertainty band widths, suggesting that the uncertainty is related to different processes and/or parameters, such as the underground storage. (a) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (b) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (c) q (m 3 /s ) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (d) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (e) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 tiempo (meses) figure 6: uncertainty bands associated with the model structure and calibration parameters (white area between the black lines) and uncertainty bands for the positive (blue) and negative rainfall variations (red) of 5, 10, 15, 20 and 25% (plots a, b, c, d and e, respectively) during the basin-fill months (apr. – jun.). figure 7 shows that overestimations of the rainfall in the recession period of a basin produce an increase in the output uncertainty bands, increasing the bands toward greater flows while keeping the lower limit constant. (a) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (b) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (c) q (m 3 /s ) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (d) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (e) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 tiempo (meses) 74 enrique muñoz, pedro tume, gabriel ortíz (b) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (c) q (m 3 /s ) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (d) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (e) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 (a) 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 0 100 200 tiempo (meses) figure 7: uncertainty bands for the positive (blue) and negative rainfall variations (red) of 5, 10, 15, 20 and 25% (plots a, b, c, d and e, respectively) during the basinrecession months (sep. – nov.). figures 3 to 7 show that errors in the estimation of the rainfall in a given period would affect the uncertainty in the model in subsequent periods. additionally, errors in the estimation of the rainfall during rainy periods, especially during the filling period of the basin, generate greater uncertainty in the model outputs. this is reasonable because the greatest water intakes occur in winter, when the basin produces water storage that influences the flows of the recession curve, in addition to surface runoff. these results are also consistent with the pluvial-snow regime of the basin, in which the generated flows are directly dependent on the rainfall inputs. figure 7, which analyzes the influence of the rainfall in the recession period of the basin, presents similar results for the uncertainty in the estimation of the rainfall in the summer months (figure 5); this result supports the concept of the important dependence of the model and its outputs on the inputs of water to the basin in rainy periods. tables 3 and 4 show that the model structure and parameters generate a 13 (m3/s) range of output uncertainty on average. after estimating how the model uncertainty is affected by uncertainty in the rainfall amounts during different periods, it is observed that the outputs are more sensitive to rainy periods. specifically, greater ranges of uncertainty and greater percentage changes in the uncertainty bands are observed. a smaller influence is observed for low water periods (recession and summer) when the model outputs are less sensitive to errors or uncertainties in the estimation of the rainfall amounts. however, in the filling period of the basin, it is observed that the width of the uncertainty bands does not significantly increase (8% on average), but it is displaced toward greater flows when rainfall is overestimated. table 3: average range (m3/s) of the uncertainty associated with the different variations (positive/negative) in precipitation. the range indicated for a variation of 0% corresponds to the range of uncertainty associated with the model structure and parameters. period 0% 5% 10% 15% 20% 25% entire year (jan. – dec.) 13 14/14 15/14 15/15 16/14 15/16 winter (jun. – aug.) 13 13/14 14/14 14/15 14/15 15/16 summer (dec. – feb.) 13 14/14 15/14 14/14 14/14 14/14 basin-filling period (apr. – jun.) 13 13/13 14/14 14/13 14/13 15/13 basin-emptying period (sep. – nov.) 13 14/14 14/14 15/14 15/18 15/13 table 4: flow percentage change in the range of the uncertainty produced by positive/negative variations in precipitation period 0% 5% 10% 15% 20% 25% entire year (jan. – dec.) 0 8/8 15/8 15/15 23/8 15/23 winter (jun. – aug.) 0 0/8 8/8 8/15 8/15 15/23 summer (dec. – feb.) 0 8/8 15/8 8/8 8/8 8/8 basin-filling period (apr. – jun.) 0 0/0 8/8 8/0 8/0 15/0 basin-emptying period (sep. – nov.) 0 8/8 8/8 15/8 15/18 15/0 conclusions the model is more sensitive to rainy periods; therefore, greater uncertainty in the estimation of rainfall during these periods directly influences uncertainty 75uncertainty in rainfall input data in a conceptual water balance model: effects on outputs and implications for predictability in the model outputs and increases the range of the related uncertainty bands. the output uncertainty is highly sensitive to rainfall. therefore, in dry periods, when the rainfall amounts are relatively small, the model is less sensitive to uncertainties in the rainfall than in the wet periods. currently, it is common to estimate flows and their uncertainty bands using a hydrological model. in the present study, it was observed that the uncertainty associated with the main water input (rainfall) in an andean basin can generate a significant increase in the uncertainty bands of the outputs. therefore, when using alternative sources of information to feed a hydrological model (e.g., global gridded data or data interpolated at a global scale), it is necessary to reduce the uncertainty associated with the predictions by ascertaining and reducing the deviation of the rainfall data in rainy periods (with respect to the observed or ground values) and/or by ascertaining the quality of the presented predictions. additionally, for predictive hydrological models, high uncertainty in periods of low rainfall does not greatly influence uncertainty in the output. however, when predicting flows in rainy periods or in recession periods, the uncertainty associated with pluviometric predictability should be evaluated. acknowledgements this research has been supported by the eu erasmus mundus external cooperation window, the conicyt programa nacional de formación de capital humano avanzado scholarship, the bmbf/conicyt project 2008099 impacto de la variabilidad climática en la disponibilidad de recursos hídricos y requerimientos de riego en la zona central de chile, and the fondecyt 11121287 project hydrological process dynamics in andean basins. identifying the driving forces, and implications in model predictability and climate change impact studies. the authors thank the dirección general de aguas for providing the rain gauge and stream gauge data. references arumí, j.l., d. rivera, e. muñoz, m. billib. 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(2012). estudio de los procesos hidrológicos de la cuenca del río diguillín. análisis mediante un modelo hidrológico conceptual, obras y proyectos, 11, 69-78. road construction has increased significantly worldwide in the last decades to meet the demands of the increasing human population and this has led to serious soil erosion problems, the bulk of which is unaccounted for, especially in the developing world. for comprehensive land management decisions and monitoring strategies, a review of work that has been done to assess soil erosion due to roads is critical. this article, therefore, reviews the causes of road‒related soil erosion, assessment methods and available control measures. specifically, this work provides an overview of (i) the linkages between roads and soil erosion; (ii) measurement and prediction of road‒related erosion; and (iii) erosion control and rehabilitation techniques. literature shows that road construction results in hill-slope profile modification; removal of vegetation cover; as well as the formation of steep slopes that are prone to severe erosion. furthermore, there is a variety of erosion control measures for controlling road‒related erosion although no study has demonstrated the method that is cost efficient and operational across different landscapes. we are of the view that this study provides guidance in future research on road‒related soil erosion across the developing world where sophisticated monitoring techniques are limited due to resource scarcity for assessing large areas. la construcción de carreteras se ha incrementado ampliamente en todo el mundo durante las últimas décadas para cumplir con las demandas de la creciente población humana, lo que ha llevado a serios problemas de erosión de suelos, muchos de los cuales no se previeron, especialmente, en los países en desarrollo. sobre las decisiones y supervisión de estrategias de un manejo completo del terreno se realizó una revisión al crítico trabajo que se ha hecho para medir la erosión en suelos causados por las carreteras. por esta razón, este artículo revisa las causas de la erosión relacionada con la construcción de rutas y evalúa los métodos y medidas de control disponibles. específicamente, este trabajo ofrece una revisión de (a) las relaciones entre las carreteras y la erosión de los suelos; (b) la medida y la predicción de la erosión vinculada a las carreteras, y (c) las técnicas de control de erosión y rehabilitación. la literatura muestra que la construcción de carreteras produce modificaciones en el perfil inclinación, remueve la vegetación superficial y aumenta la inclinación en pendientes propensas a erosión severa. además, existen varias medidas para controlar la erosión causada por la construcción de carreteras, a pesar de que ningún estudio ha demostrado el método que sea más eficiente y operacional para diferentes paisajes. este estudio guía futuras investigaciones en la erosión causada por la construcción de caminos en los países en desarrollo donde las técnicas de supervisión sofísticas para la evaluación de grandes áreas son limitadas debido a la escasez de recursos. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 73 80 abstract resumen key words: roadcut and fill embankments, road drainage structures, runoff, soil loss, measurement and prediction, revegetation. palabras clave: diseño de carreteras, llenado de diques, estructuras de desagüe de carreteras, escorrentía, pérdida de suelos, medida y predicción, revegetación. a review of road‒related soil erosion: an assessment of causes, evaluation techniques and available control measures khoboso elizabeth seutloali, heinrich reinhard beckedahl record manuscript received: 04/06/2014 accepted for publication: 30/05/2015 geotechnics issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.43841 74 khoboso elizabeth seutloali, heinrich reinhard beckedahl introduction road construction has increased significantly worldwide in the last decades for the provision of effective human mobility and transportation of commodities (bochet et al., 2010). this development has resulted in permanent alteration of the geomorphic and hydrological settings of the landscape leading to increased soil erosion (ramos-scharron and macdonald, 2007). for instance, road construction can result in the modification of natural hill-slope profiles, the construction of roadcut and fill embankments and impervious roadbeds that concentrate runoff (jordan and martinezzavala, 2008). roads concentrate runoff, critical for enhancing increased hill-slope soil loss and sediment yield which later impairs the quality of surrounding open waterbodies (forsyth et al., 2006; lane and sheridan, 2002; ramos-scharron and macdonald, 2007; sheridan and noske, 2007). for instance, lane and sheridan (2002) in their study observed a water quality deterioration as shown by increased turbidity and total dissolved solids downstream of a road stream crossing. the major sediment source at the road stream crossing was the result of erosion on the road verge and the road fill slopes. environmental challenges caused by the accelerated soil erosion due to roads have economic ramifications related to soil rehabilitation and water treatment. it is therefore, a necessity to provide an overview of literature on road-related soil erosion for a better understanding of the causes and methods of assessment that have been considered so as to (1) guide future development; and (2) provide the necessary guidance and informed recommendations on possible efficient and cheap monitoring approaches and erosion control efforts especially in resource-scarce environments. this review therefore seeks to provide an overview of: (i) the effects of armoured roads on soil erosion by water, (ii) related structural designs that facilitate soil erosion processes, and (iii) available approaches for assessing roadrelated soil erosion and the available erosion control techniques. so far, to the best of our knowledge, no studies have been done to assess soil erosion related to paved roads. for instance, previous studies on road‒ related erosion have been dominated by the work on forest roads (i.e. unpaved roads) which include those by burroughs and king (1989) who addressed the potential for reduction of onsite sediment production by different treatments on various components of the forest road prism. croke and hairsine (2006) reviewed the interaction of forest road and track network with both sediment and runoff delivery in managed forests. the study by macdonald and coe (2008) discussed the underlying processes of forest roads sediment production from surface erosion and land sliding. although baird et al. (2012) also reviewed forest road erosion, their focus was on the processes of erosion and sediment delivery from these roads, whereas the other studies either considered land sliding or the process of runoff from the forest road network only. the limitation of the reviews mentioned above is that none addressed the post-construction case of armoured roads except focusing on erosion from unpaved forest roads. furthermore, none of the studies conceptualized assessment of road-related erosion, as well as its control. road-related soil erosion road construction creates numerous roadcut and fill embankments, as well as ditch relief or culvert sites (figure 1) that contribute to runoff and high sediment production that cause extreme land degradation (ramos-scharron and macdonald, 2007). roadcut and fill embankments have bare and steep gradients that cause the generation of runoff and sediment yield (bochet and garcía fayos, 2004). lack of vegetation cover also intensifies soil detachment by raindrops and proliferates susceptibility to erosion as a result of reduced cohesion and shear strength of the soil (jankauskas et al., 2008). similarly, steep gradients increase erosion on these slopes due to reduced water infiltration and increased runoff accumulation (arnaez et al., 2004; cerdà, 2007). numerous studies have documented soil erosion on roadcut and fill embankments (arnaez et al., 2004; jordan and martinez-zavala, 2008; megahan et al., 2001; xu et al., 2009). for example, a study by arnaez et al. (2004) recorded a significant generation of runoff and sediment from roadcut embankments and fill slopes in the iberian range, spain. roadcut embankment soil loss rates exceeded those from the fill slopes by 16 times, and this was attributed to the steep gradients, presence of embedded gravels and low vegetation cover. similarly, jordan and martinez-zavala (2008) recorded a total soil loss of 106 g m-2 and 17 g m-2 from roadcut and side-cast fills respectively in southern spain. the highest erosion rate was observed on the roadcuts due to steep slopes, low vegetation cover and the presence of loose colluvium. moreover, megahan et al. (2001) evaluated the effects of slope gradient, slope length, slope aspect, rainfall erosivity and ground cover density on erosion on the roadcuts in idaho, usa. the multiple regression analysis showed that slope gradient was the most significant of all site variables in affecting roadcut erosion. xu et al. (2009) on the other hand investigated the effects of rainfall and slope length on runoff and soil loss on the qinghai-tibet highway side-slopes in china and found that rainfall intensity correlated with sediment concentration and soil loss, while soil loss decreased with increasing slope length. in summary these studies highlight that slope properties (viz. slope gradient and length, vegetation cover and soil properties, particularly soil texture) of the roadside embankments are critical in determining the degree of soil erosion along these areas. roads initiate soil erosion through drainage structures diverting water from their impervious surfaces as well as from roadcut embankments. road surfaces (including unpaved roads) increase runoff generation (ziegler and giambelluca, 1997). furthermore, the road surfaces transect the hillslope hydrology, creating the need for draining the roadcut embankment and road surface through culverts at regular intervals (as indicated by point 1, in figure 1), with the consequential change from diffuse surface flow downslope to concentrated flow. extensive surface erosion may occur where this concentrated flow is discharged down-slope at discharge points (point 2 and 3 in figure 1). geomorphic impacts of concentrated runoff from road drainage have been documented by numerous studies (jungerius et al., 2002; kakembo, 2000; montgomery, 1994; beckedahl and de villiers, 2000). montgomery (1994) conducted a field survey of road drainage concentration in the western united states and observed that the discharge of road surface concentrated runoff and of intercepted subsurface flow result in initiation and enlargement of a gully and slope instability below the drainage outfall. gully initiation was related to ground slope and contributing area thresholds. kakembo (2000) reported a case of ephemeral stream incision triggered by runoff concentration through a series of railway culverts on a steep hillslope at kwezana, eastern cape, south africa and concluded that concentrated runoff coupled with the steep slope of the drainage discharge area and the rainstorms of high magnitude influenced gully initiation. although not a case study of roads, the scenario is similar in this instance too, the slope hydrology is disrupted and concentration of runoff initiated gullies and triggered hillslope instability. jungerius et al. (2002) reported gully formations where concentrated surface water was diverted to the verges alongside the road in west pokot, kenya. the study found that gully formation is influenced by the steep slopes, lack of vegetation cover, torrential rainfall and the fine grained soils of the alluvial fans. beckedahl and de villiers (2000) investigated the causal relationship between road drainage and pipe erosion in the eastern cape province, south africa. their findings showed that soil pipes and gullies developed where road drainage resulted in a high concentration of surface water on sensitive or dispersive soils. these studies have shown that erosion initiation at road drainage discharge sites is influenced by the contributing area, slope steepness, rainfall intensity and soil properties. the studies by kakembo (2000) and montgomery (1994) however, did not include the estimation of the quantity of soil loss in their agenda. investigations of the impact of concentrated road runoff on soil erosion, to be complete and comprehensive, should also consider an estimation of the amount of soil loss rather than simply focussing only on the contributing factors. these estimations are necessary as they could provide a clear and detailed evidence of the effects of concentrated road runoff discharge on the actual soil loss. after analysis of the possible impacts of road construction on soil erosion, it is important to highlight the methods that can be used to investigate road‒ related erosion. this knowledge will help for accurate assessment of erosion levels and soil loss along the road networks. 75a review of road‒related soil erosion: an assessment of causes, evaluation techniques and available control measures figure 1. a typical cut and fill road cross section and features. the numbers one (1) to four (4) refer to potential impacts, and these are discussed in the text. adapted from (fu et al., 2010). methods of assessing road‒related soil erosion road‒related soil erosion field measurement techniques available field methods of measuring road-related erosion have been principally based on rainfall simulation and volumetric surveys of erosion features. the choice of a particular technique primarily depends on the part of the road component that will be monitored (table 1). for instance, rainfall simulation method has been widely used to explore runoff and soil loss processes related to roadcut and fill slopes, as well as unpaved road surfaces in many parts of the world (table 1). rainfall simulators create controlled rainfall events (jordan and martinez-zavala, 2008), and their design depends on the type of experiments to be carried out (clarke and walsh, 2007). control of rainfall allows determination of the relationship between soil loss and rainfall parameters (lascelles et al., 2000) as well as generation of runoff and soil loss under repeatable conditions (hamed et al., 2002). moreover, in semi-arid regions, with high rainfall variability and recurrent droughts, rainfall simulation could be useful (cerdà, 2007). however, rainfall simulation is uncertain for extrapolating results to larger scale (arnaez et al., 2004) and also underestimates soil loss as compared to natural rainfall as it supplies a constant rainfall intensity (boix-fayos et al., 2006) and short duration rainfall (jin et al., 2008). nevertheless, simulation results remain useful for comparative purposes (foster et al., 2000; jordan and martinez-zavala, 2008) and for planning, despite challenges of underestimating soil loss and limitation to small scale applications. on the other hand, the volumetric survey of erosion features for assessing road-related soil erosion involves the use of measured dimensions (viz. lengths, widths and depths) of the erosion features either directly in the field or from the use of photographic images to estimate soil loss (okoba and sterk, 2006). these dimensions are then utilized to calculate the volume of the erosion features excavated, which is equivalent to the volume of soil lost (hagmann, 1996). although actual soil loss is underestimated since inter-rill erosion is excluded when measuring pipe, gully, and rill erosion, the approach produces the best approximation of erosion (bewket and sterk, 2003). a number of studies have been carried out using the volumetric survey of erosion features to estimate soil erosion on roadcut and fill embankments and most of these have focused on measurement of erosion related to concentrated runoff from road culverts (table 1). other studies used an erosion index for rill and gully erosion to determine its severity on motorway slopes, such as that of bochet and garcía‐fayos (2004), in valence, spain. the erosion index is based on the percentage cover of erosion on the sampling area. however, unlike other studies based on quantitative estimation of erosion, this semi-quantitative approach did not reveal the effect of aspect on erosion intensity and this was attributed to the fact that this method might not be precise enough to detect such differences. although field methods provide the necessary understanding of erosion processes, the obtained results are, however, difficult to generalize due to the complex interaction of erosion processes and field conditions (ande et al., 2009). prediction of road-related erosion could, therefore, help consider the complex interactions that affect erosion rate. modeling of road-related soil erosion soil erosion models vary from simplified procedures, such as the universal soil loss equation (usle) to more complex methods requiring a series of input parameters, such as water erosion prediction project (wepp) (oliveira et al., 2012). usle, and its modifications, the revised universal soil loss equation (rusle) computes the average annual soil loss caused by rill and inter-rill erosion by multiplying the natural factors (rainfall erosivity-r, erodibility-k, slope length and steepness-ls) and anthropogenic factors (cover and management-c, and conservation practices-p) (angima et al., 2003; oliveira et al., 2012). literature has shown that usle/rusle approaches give better estimates of erosion on an overall basis. for instance, oliveira et al. (2012) stated that the usle/rusle provides a right approach for soil loss prediction since it is applicable in terms of required input data, and the obtained soil loss estimates are reliable. however, the use of this model is based upon erosion rates from landscapes larger than road plots hence application for roads is at a smaller scale than for which it was intended (riedel, 2003). in contrast to the usle/rusle, the wepp model was developed to provide a spatial and temporal distribution of soil loss (baird et al., 2012; clinton and vose, 2003). this model utilizes climate, infiltration, water balance, soil chemistry, plant growth and residue decomposition, tillage and consolidation to predict soil erosion deposition and sediment delivery (baird et al., 2012; clinton and vose, 2003). wepp model is applied to roads by including multiple road features such as road surface, cut-slope, ditch, fill slope and lower hillslope (fu et al., 2010; cheng et al., 2013; elliot et al., 1995; forsyth et al., 2006). the road features are modeled separately by defining them as different overland flow elements with unique soil and vegetation parameters assigned (fu et al., 2010). although some models exist for predicting road-related erosion, these are primarily used to predict erosion from the road surfaces (forsyth et al., 2006; sheridan et al., 2006) and few studies have focused on modelling erosion on roadside slopes and erosion due to road drainage ditches/culverts (elliot and tysdal, 1999; megahan et al., 2001) (see table 2). erosion models, however, suffer from a range of problems (barrett et al., 1998). firstly, the model development was often based on data derived from the united states or european conditions and the application of these models to different climatic and management conditions in other regions has not yet been fully established. secondly, the models were created for field plot scale, and application for large scales is still questionable. thirdly, the model predictions are not entirely accurate as a result of incomplete knowledge of the entire set of aspects and interaction processes arising from a limited set of variables. for instance, the disturbance associated with construction frequently exposes the subsoil (or new soil may be brought in from elsewhere) hence the erodibility values along the road will differ to those of the region (barrett et al., 1998). therefore, for road applications, these models still require further testing, and modifications to include additional factors specially designed for road erosion (fu et al., 2010). measurement of soil erosion using the volumetric survey of erosion features, therefore, could provide a reasonable estimation of erosion (sidle et al., 2004) and does not involve expensive instrumentation, long lead times and/or sophisticated modeling (bewket and sterk, 2003). 76 khoboso elizabeth seutloali, heinrich reinhard beckedahl table 1: overview of the techniques of field measurement of road-related erosion used to date table 2: mathematical models used for predicting road-related erosion 77a review of road‒related soil erosion: an assessment of causes, evaluation techniques and available control measures methods used to control road-related soil erosion soil erosion control measures, i.e. non-engineering and bioengineering (e.g., vegetation, soil erosion control blankets, silt fences and geotextiles) and engineering techniques (e.g., diversion drains and lattice) are formulated to reduce accelerated soil erosion rates on roadside slopes (rickson, 2006; xu et al., 2006). this is because roadside slopes have been demonstrated as major contributors towards road-related soil erosion, accounting for 70 to 90% of the total soil loss from the disturbed roadway area (grace iii, 2000). most of the erosion control measures are specifically designed to minimise the contact of rainfall with the soil as well as reduce runoff velocity (de oña et al., 2009). while these soil erosion control methods are effective in minimising road-related soil erosion, however, some of these methods are failing to meet their intended objectives while others are even expensive to use especially in resourcescarce environments. amongst all these control methods, vegetation cover is probably the most widely used measure for controlling erosion on roadside slopes (xu et al., 2006). this is because vegetation cover intercepts rainfall and increases water infiltration (claridge and mirza, 1981; faucette et al., 2006), stabilizes the soil with roots that hold soil particles together (bochet and garcíafayos, 2004; collison and anderson, 1996), and moderates and dissipates the energy exerted by water (lal, 2001; ande et al., 2009). grace iii (2000) and xu et al. (2006) emphasised the importance of vegetation cover in reducing soil erosion and their findings are also supported by the inserts (figure 2) that indicate the importance of vegetation cover on roadside slopes. grace iii (2000) observed a reduction in sediment yield by over 30% on vegetated roadcut and fill slopes compared to the bare roadside slopes and concluded that vegetation has the greatest potential to mitigate soil erosion through stabilizing the roadside slopes. similarly, xu et al. (2006) found that vegetation provided a longterm soil erosion control on roadside slopes and concluded that soil erosion is significantly reduced when vegetation cover is well established. the effectiveness of vegetation cover to control erosion, however, starts when the vegetation is established (rickson, 2006) and mature (vishnudas et al., 2006). for instance, vetiver grass (vetiveria zizanioides l. nash) application significantly controls soil erosion and stabilizes the slopes, although it may take at least one year to become fully effective (sanguankaeo et al., guangzhou, china). this implies that a site may be susceptible to erosion during the period when there is no vegetation or immature stage, also making the establishment of vegetation difficult, since there is no immediate and adequate protection (vishnudas et al., 2006). additionally, the absence of initial binding material in the slope soils may result in poor vegetation growth (bhattacharyya et al., 2008). for these reasons, soil erosion control blankets and geotextiles are short-term vegetation cover replacement that have been used to offer immediate soil protection (smets et al., 2009). erosion control blankets reduce runoff and soil erosion by improving soil quality (bhattarai et al., 2011) and enhancing vegetation (faucette et al., 2006) that would offer a permanent erosion control. likewise, geotextiles control rain splash and runoff (bhattacharyya et al., 2010) and promote a micro-climate for subsequent vegetation growth (sutherland and ziegler, 2006). geotextiles are applied on bare slopes after spreading seed mixture for long-term erosion protection (sutherland and ziegler, 2007). erosion control geotextiles are made from natural or synthetic material (smets et al., 2009) with synthetic geotextiles dominating the commercial market (jankauskas et al., 2008). synthetic geotextiles such as silt fences are used for highway and other construction projects to provide a temporary sediment control (barrett et al., 1998). silt fences reduce runoff velocity and filters sediments thereby enhancing sedimentation (barrett et al., 1998). silt fences are preferred because they are cheap and easy to install (robichaud et al., 2001; wachal et al., 2009). the limitations of synthetic geotextiles, however, are that they are non-degradable and may cause soil pollution, and their production may cause air and water pollution (bhattacharyya et al., 2010). according to jankauskas et al. (2008), however, natural geotextiles constructed from organic materials are more efficient in controlling soil erosion since they adhere to the surface’s microtopography and can follow slope contours and stay in close contact with the soil (bhattacharyya et al., 2010). additionally, natural geotextiles are easily available in many parts of the world, less costly to produce, apply and are environmentally friendly as they are made of biodegradable material (bhattacharyya et al., 2008). some previous studies have evaluated the effectiveness of erosion control blankets and geotextiles in reducing erosion on roadside slopes and found that they reduce soil loss as a result of improvement in vegetation growth (bakr et al., 2012; de oña and osorio, 2006; jankauskas et al., 2008; pengcheng et al., 2008). bakr et al. (2012) examined the influence of compost/mulch on stormwater runoff rates on highway embankments in louisiana. they found out that compost/mulch was effective for soil erosion control since it increased crop cover and reduced soil loss. others such as pengcheng et al. (2008) evaluated the application of sewage sludge compost on highway embankments in china and observed an improvement of soil quality parameters, increased growth of ryegrass and a reduction in volume of runoff and soil loss. similarly, osorio and de ona (2006) observed that compost application on road embankments in southern spain increases vegetation cover and reduces soil loss. additionally, it was found that soil loss decreased with the addition of greater quantities of compost. jankauskas et al. (2008) investigated the use of palm-leaf geotextiles to control erosion on roadside slopes in lithuania. they found that soil erosion from bare fallow soil was reduced by 91.15 – 94.8%, and this was attributed to the multiple benefits such as soil conservation and improved soil moisture that encouraged better plant growth. on the other hand, engineering soil erosion control techniques (e.g. diversion drains and lattice structures) like non-engineering methods, also reduce erosion on roadside slopes by diverting runoff away from the surface of the roadside slope (claridge and mirza, 1981) and intercepting runoff (xu et al., 2006), respectively. these techniques, however, do not provide a protective layer on the surface of the roadside slope; hence soil detachment from direct rainfall impact could still occur. the combination of engineering and vegetation measures could, therefore, provide an effective method for reducing runoff and direct rainfall impact thereby reducing soil loss on roadside slopes (xu et al., 2006). on the basis of the above discussion, the most efficient and economic soil erosion control strategy is re-vegetation. this is because vegetation cover provides a cheap long-term erosion control (benik et al., 2003), requires less maintenance than complex engineering structures (montoro et al., 2000) and improves the landscape aesthetic value (albaladejo montoro et al., 2000). hence, soil erosion control through the establishment of a dense vegetation cover is a priority for restoration of roadside slopes (garcía-palacios et al., 2010). for instance, figure 2a illustrates roadside slopes that have successfully stabilized due to the use of vegetation cover as a control mechanism. on the other hand, it can be observed in figure 2b that areas without vegetation cover are prone to erosion. while the use of soil erosion control techniques has been widely recognised and investigated, these investigations have, in most cases, focused on the non-engineering and bio-engineering techniques, and less attention has been given to engineering measures although they could provide an efficient erosion control on roadside slopes (xu et al., 2006). therefore, there is a need to test the effectiveness of 78 khoboso elizabeth seutloali, heinrich reinhard beckedahl engineering measures for erosion control on roadside slopes. figure 2. (a) successful application of vegetation cover to control erosion on a roadside slope and (b) signs of erosion on a roadside slope due to the absence of vegetation cover. conclusion roads and road construction result in soil erosion due to the impacts of rainfall affecting geomorphic and hydrologic processes. research has shown that the creation of roadcut and fill embankments with steep slopes and little vegetation cover, as well as the concentration of runoff from the road surface and intercepted subsurface flows influence the hydrologic and geomorphic processes. roadcut embankments, however, are the major sources of erosion than other parts of the road with slope gradient being the most important factor influencing soil erosion. a variety of techniques are used to investigate roadrelated erosion, ranging from field measurements to soil erosion prediction models. these methods could assist in understanding the nature and severity of road-related erosion and can help guide future development and erosion control efforts. however, besides the strengths of erosion measurement methods, soil erosion prediction models, although appropriate for predicting soil loss for the field plot scale, have challenges when applied to small land parcels. therefore, there is a need for further testing and modification of soil erosion prediction models for road application. it has been shown in the literature that soil erosion control techniques have the potential to reduce runoff and soil loss. numerous studies that have investigated the effectiveness of soil erosion control techniques utilised on roadside embankments showed that the most effective methods are those that promote revegetation and reduce both velocity and quantity of runoff. since the extent of road networks is ever-increasing, lessons learned from this research may be applied in the future construction of road systems. as such, research still needs to be done (i) to fully understand the underlying determinants of soil erosion related to road design and construction to limit the effect of embankments; (ii) to quantify road-related soil loss; (iii) to evaluate the effectiveness of erosion control methods on both roadcut and fill embankments; and (iv) to identify new approaches such as remote sensing technologies, to try to improve soil erosion mapping along roads for future monitoring and management strategies. this review therefore provides the necessary insight and inspiration to geomorphologists, road engineers and environmentalists to move towards identifying the most suitable, cheap and readily available techniques for assessing and controlling soil erosion, necessary for reliable and informed approaches for monitoring and managing road-related soil erosion across the world, especially in under resourced-countries. acknowledgements authors would like to thank the university of kwazulu-natal for funding this research. references albaladejo montoro j., alvarez rogel j., querejeta j., diaz e., castillo v. 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(2018). a modified approach for elastic impedance inversion due to the variation in the value of k. earth sciences research journal, 22(3), 205-213. doi: https://doi.org/10.15446/esrj.v22n3.61135 elastic impedance inversion is the latest development in the field of hydrocarbon exploration and production. the present research focuses on the improvement of the use of elastic impedance inversion, easing exploration of hydrocarbons. the seismic velocities change with variation in geological constraints. constant k, which is s-wave to p-wave ratio of the nth layer and n+1 layer across the interface, it must be changed accordingly. this research focuses on testing the effects of k as a constant in the elastic impedance equation. as using the same value of k for all types of formations can give rise to severe errors in the interpretation of data. the importance of the value of k for particular amplitude variation with offset avo type (i-iv) is studied using different elastic impedance equations. the reflection coefficient (rc) curves for each avo class are generated using zoeppritz approximation and elastic impedance equations. the comparison of rc curves shows significant variations at far offsets in each avo type using the constant value of k. when k calculated is used, avo type i and type ii shows a good match at near, mid and far offsets. type iii does not change due to the changing value of k. type iv gives good agreement at near and intermediate offsets. this variation in curves, with the change in the value of k, indicates that it is a significant factor of interpretation using elastic impedance. the application of findings on well logs has given a satisfactory confirmation of the present results. this research can be helpful to resolve severe errors in the interpretation due to the constant value of k. abstract keywords: zoeppritz; amplitude variation with offset (avo) classification; inversion; fatti ei and connolly ei, well-log; k value. a modified approach for elastic impedance inversion due to the variation in the value of k enfoque modificado de la inversión de impedancia elástica con variación del valor k issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.61135 la inversión de impedancia elástica es el último desarollo en los campos de la producción y exploración de hidrocarburos. la presente investigación se enfoca en el mejoramiento del uso de la inversión de impedancia elástica para facilitar la exploración de hidrocarburos. las velocidades sísmicas cambian con la variación de las condiciones geológicas. la constante k, que es el resultado de la velocidad de la onda s y la onda p en una capa n y en la capa n+1 a lo largo del punto de contacto, debe ser cambiada de acuerdo con estos condicionantes. este trabajo busca medir los efectos de k como una constante en la ecuación de impedancia elástica. si se usa el mismo valor de k para todos los tipos de formaciones se puede incurrir en errores severos al interpretar la información. se analizó el valor de la constante k en la variación de la amplitud con el desplazamiento (avo, del inglés amplitude variation with offset) tipos i al iv a través de diferentes ecuaciones de impedancia elástica. las curvass del coeficiente de reflexión para cada tipo avo se generaron a traves la ecuaciones de zoeppritz y impedancia elástica. la comparación de las curvas de reflexión muestran variaciones determinantes en los desplazamientos más lejanos en cada tipo avo si se usa un valor constante de k. cuando se utiliza el valor modificado de k, los tipos i y ii de avo muestran coincidencia en los desplamientos cercanos, medianos y lejanos. la avo tipo iii no cambia cuando se utiliza el valor k constante o modificado. la avo tipo iv concuerda en los desplazamientos cercanos e intermedios. esta variación en curvas, con los cambios en el valor de k, indican que esta constante es un factor significante en la interpretación a traves de la impedancia elástica. la aplicación de los hallazgos de este trabajo en registro de pozos han confirmado satisfactoriamente los resultados. esta investigación puede ser de ayuda para resolver errores en la interpretación de información que tenga un valor de k constante. resumen record manuscript received: 21/11/2016 accepted for publication: 30/05/2017 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 205-213 saiq shakeel abbasi1,*, jiangping liu1, nayima hameed2, muhsan ehsan3 1institute of geophysics and geomatics china university of geosciences, wuhan. 2halliburton co, houston, tx, usa 3department of earth and environmental sciences, bahria university islamabad. *corresponding author: saiqshakeel@gmail.com g e o ph y si c s 206 saiq shakeel abbasi, jiangping liu, nayima hameed & muhsan ehsan introduction seismic velocities play a vital role in the determination of lithology, porosity and fluid content. seismic velocities vary horizontally and vertically depending upon the geological constraints, the depositional environment, overburden pressure, lithological variations, and others. elastic impedance inversion is based on the assumption of using s-wave to p-wave ratio of the nth layer, and n+1 layer across the interface which is known as k and its value is 0.25. in this research, the effects of using k as constant in elastic impedance inversion have been studied. as the seismic velocities change with the variation in both the elastic properties and other geological constraints, k also changes accordingly. conventional acoustic impedance (ai) inversion, which is the product of p-wave velocity and density, assumes that the p-wave strikes the subsurface interfaces at normal incidence (lindseth, 1979). this assumption remains valid when the offset range is small, and the reservoir is deep. however, in the presence of avo at larger offsets the reflection coefficients and polarities may change from those at normal incidence (ma, 2004). the elastic impedance approach provides better inversion results than the standard intercept-gradient approach which can be influenced by noise (cambois, 2000). elastic impedance inversion is the generalization of acoustic impedance for the variable angle of incidence. elastic impedance inversion (ei) is the latest development in the field of geophysics. the idea of elastic impedance was presented by connolly (1999). he describes the importance of elastic parameters inversion to identify the fluid and lithology. aki and richards (2002) approximation of zoeppritz (1919) equations and connolly (1999) defined equation 3 (appendix i) for elastic impedance. it is the product of p-wave, s-wave velocities, and density. elastic impedance enables geophysicists with an additional tool for the reservoir prediction. acoustic impedance alone cannot distinguish and identify the fluid type and lithology. the acoustic impedance log ignores avo effects, which enhances the chances of information loss (verwest et al., 2000). as the p-wave velocity is a function of the mineral constituents, it can only give information regarding the geometry of pores and volume of fluid (cosban et al., 2002). elastic impedance uses the necessary elastic parameters of the earth such as p-wave, and s-wave velocities and density. poisson ratio is lithologically sensitive and helps in distinguishing sandstone from mudstone, in reservoir characterization, and fluid identification. verwest et al. (2000) presented another form of elastic impedance inversion by introducing a planar boundary between two layers of acoustic media, which depends on ray parameters. whitcombe (2002) gave a normalized form of elastic impedance equation 7 (appendix i). other researchers presented different approximations of the zoeppritz equation to provide several kinds of elastic impedance equation. whitcombe et al. (2002) introduced the term extended elastic impedance (eei) to identify both fluid and lithology using the elastic impedance. duffaut et al. (2000) gave the shear-elastic impedance to estimate the ratio of p-wave velocity and s-wave velocity using joint p-p and p-s elastic impedance inversion. to deal with the dimensionality problem, savic et al. (2000) defined the elastic impedance as the function of p-wave, s-wave velocities, and ray parameters. martins (2002)set out the elastic impedance in weak anisotropic media. based on fatti approximation of the zoeppritz equation, wang et al. (2008) proposed an elastic impedance equation by using fatti approximation equation 9 (appendix i). other elastic impedance relations are presented using different media or parameterization. in this research, the importance of the value of k for each avo type has been used. as seismic velocities increase with depth, then their effect at large offsets in depth also varies. the principal purpose of this study is to check the impact of variation in the value of k when used for specific data sets. usually, k= 0.25 is used in elastic impedance inversion. however, due to lateral and horizontal variation in the velocity, k should also change accordingly, instead of using a constant value. avo classes have been evaluated by applying the elastic impedance equations for a continuous and calculated k. this study will be helpful for geoscientists as well as exploration and production companies to identify the lithological reservoirs more accurately without erroneous results. constant k connolly’s elastic impedance equation assumes using s-p wave velocity ratio k as a constant for the whole data set, which is an unrealistic approach. it is the average v vs p 2 2 of the nth layer and n+1 layer across the interface which gives different values of k at the top and bottom of a layer. hence giving different elastic impedance values. k v v v v sn pn sn pn= + + 2 2 1 2 1 2 2  (1) however, for the multilayer strata, its value is taken as an average of for each layer. mostly the rocks have v vs p 2 2 = 0.5, i.e., k = 0.25 (connolly, 1999). it is not reasonable value as it assumes that the velocity ratio of p and s waves of each medium remains the same which is not possible because seismic velocity generally increases with depth (ma, 2004). lu and a. mcmechan (2004) used a variable value of k based on empirical approximations. elastic impedance inversion with different k values gives different values of poisson’s ratio estimation, which shows that it is sensitive to the choice of k (mallick, 2001). ma (2004), and santos and tygel (2004)introduced the term reflection impedance (ri), which is derived by integrating the reflectivity along the ray path to overcome the limitations of constant v vs p ratio and normalization of the elastic impedance. in their study, the density is assumed to be related to the s-wave velocity(potter et al., 1998). most of the avo/ei equations assume the angle of incident and value of k as a constant which generates errors with the increase in angle, prominently seen from the avo/ei equations. the value of k should be used as a constraint rather taking as constant to minimize the errors that arise in the inversion (jin et al., 2015). the interpretation of the results of the normalized form of the elastic impedance depends on k and the normalization parameters (zhang et al., 2012). classification of avo anomalies the intercept/gradient method is not only a good indicator of reservoirs, but this approach can also be used to classify avo anomalies. rutherford and williams (1989) categorized various avo trends as they diversified with increasing incident angle for the top of gas-saturated reservoirs. based on field studies, they developed an avo anomaly classification scheme for gas-saturated reservoirs. the rutherford and williams (1989) classification is as follows: class i. gas-sand reservoir has larger impedance than the impedance of the encasing shale. typically, normal-incident values (ni) are higher than +0.03. class ii. gas-sand reservoir has nearly the same impedance as the encasing shale. ni typically ranges between ±0.03. class iii. gas-sand reservoir has a lower impedance than the encasing shale. ni is usually less than -0.03. class iv. gas-sands for which the reflection coefficients decreases with increasing offset. rutherford and williams (1989) also indicated that the slope of the reflection coefficient curve is usually negative for all classes and magnitude of the gradient decreases from class i to class iii anomalies. j.p castagna et al. (1998) added class iv to the rutherford and williams classification of avo. they concluded that certain class iii gas-saturated anomalies have slowly decreasing amplitudes with offset and may even reverse polarity 207a modified approach for elastic impedance inversion due to the variation in the value of k within the common depth point cdp gather. it is a reservoir with a negative avo intercept and positive avo gradient. their classification demonstrates that reflection coefficients do not always have negative slopes for gas sands. however, they cautioned that the avo gradient for class iv brine-saturated sand might be almost identical to the avo gradient from class iv gassaturated sand. hence the gas may be difficult to detect by partial offset stacks. methodology in this paper, two-layer models based on four avo classes which included avo type i, type ii, type iii, and type iv were used. avo type is a continuously distributed property of the seismic data (young & lopiccolo, 2003). the input parameters of all four models are given in table 1. from each derived elastic impedance equation, different elastic parameters by inversion can be obtained. connolly’s elastic impedance equation and fatti based elastic impedance equations were used for checking the behavior of value of k and determining which value of k should be utilized. reflection coefficient (rc) curves based on the zoeppritz equation and elastic impedance equations are used for comparison of the results. matlab software tool is utilized to generate the plots between the reflection coefficient and offset angle. the reflection coefficient is plotted on y-axis while offset angle of 0°-40° is plotted on the x-axis. sayers and rickett (1997) provided the parameters for the avo’s type i to type iii, while john p. castagna and swan (1997) provided the type iv. the value of k is calculated for each avo class using equation 1. this value varies with the change in the p-wave and s-wave velocities as shown in table 1. these estimated values of k are used during generation of reflection coefficient curves for elastic impedance. two steps are involved in seismic inversion. first is the estimation of the subsurface reflectivity as a function of the angle of incidence for each point. second is the inversion of reflectivity to estimate the corresponding earth parameters (grossman, 2003). the elastic impedance allowed to calibrate far offset angle stack like acoustic impedance logs are used to calibrate zero-offset data. there are various approaches for quantitative estimation of reservoir properties from seismic inversion. in this research, the generalized matrix inversion was used. well-x is used in the application of the findings from which the synthetic angle traces at 10º, 15º, 25º were obtained using fatti’s three-term elastic impedance formula. after generation of elastic impedance values at different angles, inversion procedure was applied; p-impedance, s-impedance, and density of the following equation were inverted. (equation 2 in appendix 1 will give rise to the equation.). a a a a b b b b c c c c t n t n t θ θ θ θ θ θ θ θ θ θ θ 1 2 1 2 1 2 ( ) ( ) ( ) ( ) ( ) ( ) • • ( ) ( ) ( ) ( ) ( ) θθn p t p s t s in i i in i i ( )                                  ( ) ( ) 0 0                                    = ( ) ( ) in in ie t i tρ ρ ,θ 0 1 pp p t p n in ie t i in ie t i in ie t 0 0 0 2       ( )      • • ( )      ( ,θ ,θ ,θ ))                                           i p0 (2) the 5% of random noise with gauss distribution is added to the synthetic angle traces to test the elastic impedance inversion method. calculated log of k is generated for each value. results and discussion type 1 avo here, it is observed the elastic impedance equations of connolly and fatti with zoeppritz approximation reflection coefficient curves with constant k = 0.25 and calculated k = 0.333. the behavior of both curves is shown in figure 1 along with near and far angles gather along type 1 avo. for avo type i when k = 0.25, the plot between reflection coefficients derived from zoeppritz approximation and both elastic impedance equations from connolly and fatti start with a similar trend. the curves start to deviate from each other from 10° and continue to deviate as they go towards far angles. by the time the curve reaches 30° both the curves have clear space among them. it is evident from the figures that at far offsets the separation of curves is prominent. the reflection coefficient is generated using calculated k =0.3393 using equation 6, and the plot between both the reflection coefficient starts with a similar trend and matches correctly. both the curves begin to deviate from each other from 30°, and the deviation is slight. avo type ii avo type ii shows in figure 2 the reflection curves using connolly elastic impedance equation for the constant and calculated value of k. it starts from near to zero reflection coefficient and the trend is more negative table 1. parameters used for each avo type for elastic impedance inversion, modified from sayers and rickett (1997)and john p. castagna and swan (1997) avo type lithology p-wave velocity km/s s-wave velocity km/s density gm/cc k v v v v sn pn sn pn= + + 2 2 1 2 1 2 2  i shale 3.30 1.70 2.35 0.33932 sand 4.20 2.70 2.49 ii shale 2.96 1.38 2.43 0.32395 sand 3.49 2.29 2.14 iii shale 2.73 1.24 2.35 0.28921 sand 2.02 1.23 2.13 iv shale 3.24 1.62 2.34 0.3432 sand 1.65 1.09 2.07 208 saiq shakeel abbasi, jiangping liu, nayima hameed & muhsan ehsan with the increase in the angle. both curves begin to deviate from each other from 10° and continue to more negative as they go towards the far angles when the constant value of k is used. connolly reflection coefficient curves match mostly but show slight separation as illustrated in figure 2. when calculated k = 0.32395 is used, it was noticed that the elastic impedance curves become more negative than the zoeppritz curve. both elastic impedance curves closely match the zoeppritz reflection coefficient curve. avo type iii the elastic impedance reflection coefficient curve for avo type iii gives a peculiar behavior for both constant and calculated value of k. the deviation of reflection coefficient curves of zoeppritz and the elastic impedance is similar, and it increases with the angle. as the estimated value of k is 0.28921, very close to k constant 0.25, connolly and fatti elastic impedance equations are only valid up to 20° for both constant k and calculated k. figure 1. reflection coefficient curves avo type i using connolly and fatti elastic impedance equations with constant and calculated value of k. figure 2. reflection coefficient curves avo type ii using connolly and fatti elastic impedance equations with constant and calculated value of k. 209a modified approach for elastic impedance inversion due to the variation in the value of k avo type iv type iv is gas sands for which reflection coefficients decrease with increasing offset figure 4. they are noticeable when the s-wave velocity in the gas sand is lower than in the overlying shale(john p. castagna & swan, 1997). the reflection coefficient curves for connolly and fatti based elastic impedance equations tends to deviate from the zoeppritz curve from 10 while using k = 0.25. while when the calculated k 0.3432 is used, the deviation starts from ° 27 for both equations. from the results, it can be seen that the value of k is a significant factor in elastic impedance inversion and its variation also affects the results. avo type i and type ii give excellent agreement with the zoeppritz when calculated k is used. avo type iii, on the other hand, does not change with the change in the value of k. with computed k type iv agrees with the zoeppritz equation until the angle of 27°; this happens when the value of k becomes significant upon the use on a considerable scale. so, for a constant value of k, the results are good for the near-offset data, but an figure 3. reflection coefficient curves avo type iii using connolly and fatti elastic impedance equations with constant and calculated value of k. figure 4. reflection coefficient curves avo type iv using connolly and fatti elastic impedance equations with constant and calculated value of k. 210 saiq shakeel abbasi, jiangping liu, nayima hameed & muhsan ehsan accurate shear wave and density information from using near-offset cannot be gotten. thus, it can be deduced that if the calculated value of k is used, a good agreement at far offset will give good control over a shear wave and density information. there exists a contradiction which can be resolved using calculated value of k. avo type i and type ii will give better results with calculated k even at far offsets. while avo iii is more accurate at near angles for constant and calculated k. type iv has a range from close offset for constant and near to mid offset for an estimated value of k and these are more accurate at near angles table 2. application of the well data the present study has been tested using 70-meter part of well log x in the study area. figure 5a shows the comparison of the well-logs with p-impedance, and s-impedance, p-wave velocity, s-wave velocity, and density. the inverted p-impedance, s-impedance, p-wave velocity, s-wave velocity, and density is shown as ip1, is1, vp1, vs1, and p1. results are generated using fatti’s three-term elastic impedance inversion for the angle gathers with 5% noise. the error in the real and inverted value is shown in figure 5b. it is evident that when k constant is used, the inverted logs deviate more from the normal well-logs. these errors arise because the constant k, as k = 0.25, or average k of well-log data has been taken; it means each layer has the same value of vs2/vp2. when some segments deviate from this assumption, significant errors are introduced into inversion. the inverted and real curves using the calculated value of k are shown in figure 6a. when calculated k is used, it is noticed that the error in s-impedance and s-wave velocity is reduced considerably, while in p-impedance and p-wave velocity error is also reduced to some extent as seen in figure 6b. density in both cases is very unstable while inverting. it gives large errors even for the slight presence of noise. these results agree with the findings that k is a significant factor in the interpretation of data in elastic impedance inversion, and it should not be considered as a constant value. the calculation of elastic impedance log is a significant step towards the elastic impedance inversion of seismic data. if the elastic impedance logs are not generated with proper care, it can deprive us of some valuable information in the interpretation of data. conclusions elastic impedance equations coupled with the zoeppritz equation can successfully be utilized to identify lithological reservoirs as well as avo table 2. maximum angles for agreement of zoeppritz and elastic impedance reflection coefficient curves avo type maximum matching curve constant k maximum matching curve calculated k i 10° 40° ii 20° 40° iii 20° 20° iv 10° 27° figure 5. (a) real and inverted logs p-impedance s-impedance p-wave, s-wave and density logs using a constant value of k. (b) error in the real an inverted logs. 211a modified approach for elastic impedance inversion due to the variation in the value of k cosban, t., helgesen, j., & cook, d. (2002). beyond avo: examples of elastic impedance inversion. paper presented at the offshore technology conference, houston, texas. duffaut, k., alsos, t., rognø, h., al-najjar, n. f., & landrø, m. (2000). shearwave elastic impedance. the leading edge, 19(11), 1222-1229. jin, z., huaishan, l., siyou, t., xing, l., xiangpeng, c., & chaoguang, s. (2015). estimation of elastic parameters using two-term fatti elastic impedance inversion. journal of earth sciences, 26(4), 556-566. lindseth, r. o. (1979). synthetic sonic logs-a process for stratigraphic interpretation. geophysics, 3-26. lu, s., & a. mcmechan, g. (2004). elastic impedance inversion of multichannel seismic data from unconsolidated sediments containing gas hydrate and free gas. geophysics, 69, 164-179. ma, j. (2004). the extending of elastic impedance and generalized elastic impedance. paper presented at the expanded abstracts, cps/seg geophys. conf, beijing, china. mallick, s. (2001). avo and elastic impedance. the leading edge, 20, 10941104. martins, j. l. (2002). an approach for elastic impedance in weakly anisotropic media seg technical program expanded abstracts 2002 (pp. 185188): society of exploration geophysicists. potter, c. c., dey, a. k., & stewart, r. r. (1998). density prediction using p-and s-wave sonic velocities: geotriad. paper presented at the cspg, cseg, cwls joint convention: http://www. crewes. org/foroursponsors/ conferenceabstracts/1998/cseg-1998/den_pred. pdf. rutherford, s. r., & williams, r. h. (1989). amplitude-versus-offset variations. geophysics, 54, 680-688. santos, l. t., & tygel, m. (2004). impedance-type approximation of the p-p elastic reflection coefficient: modeling and avo inversion. geophysics, 69, 592-598. classes. the value of elastic impedance equations constant k and its impact on avo classes and computed reflection coefficient curves were discussed and analytical studied. sometimes, a fixed value of constant k generates an error in lithological reservoirs characterizations. the constraint k value was successfully applied to study avo classes. results indicate that it improves the matching with zoeppritz curve to near, mid and far angle for avo type i and ii, and near and mid-offset for avo type iv. avo type iii does not show significant change with the change in the value of k. the decrease in the percentage of error in the inversion of the well's logging segment using calculated value of k also agrees with the findings. thus, it can be induced the findings that keeping k constants would give errors in the results, but it is necessary to have proper control over the value of k to make the results accurate acknowledgments authors would like to thanks the china university of geosciences for giving adequate facilities for this research. they would also like to extend the gratitude to mr. ahsan shafi of the china university of geosciences for his moral support and help in improvement of the manuscript. references aki, k., & richards, p. g. (2002). quantitative seismology (2nd ed.): university science books. cambois, g. (2000). avo inversion and elastic impedance. seg technical program expanded abstracts 2000, 142-145. castagna, j. p., & swan, h. w. (1997). principles of avo crossplotting. the leading edge, 16(4), 337-344. doi:10.1190/1.1437626 castagna, j. p., swan, h. w., & foster, d. j. (1998). framework for avo gradient and intercept interpretation. geophysics, 63, 948-956. connolly, p. (1999). elastic impedance. the leading edge, 18, 438-452. figure 6. (a) real and inverted logs p-impedance, s-impedance, p-wave, s-wave and density logs using a calculated value of k. (b) error in the real and inverted logs. 212 saiq shakeel abbasi, jiangping liu, nayima hameed & muhsan ehsan savic, m., verwest, b., masters, r., sena, a., & gingrich, d. (2000). elastic impedance inversion in practice seg technical program expanded abstracts 2000 (pp. 689-692): society of exploration geophysicists. sayers , c. m., & rickett, j. e. (1997). azimuthal variation in avo response for fractured gas sands. geophysical prospecting, 45, 165-182. verwest, b., masters, r., & sena, a. (2000). elastic impedance inversion. paper presented at the 70th annual seg 2000. wang, b.-l., yin, x.-y., zhang, f.-c., & li, a.-s. (2008). elastic impedance equation based on fatti approximation and inversion. progress in geophysics, 23(1), 192-197. whitcombe, d. n. (2002). elastic impedance normalization. geophysics, 67(1), 60-62. whitcombe, d. n., connolly, p. a., reagan, r. l., & redshaw, t. c. (2002). extended elastic impedance for fluid and lithology prediction. geophysics, 67(1), 63-67. young, r. a., & lopiccolo, r. d. (2003). a comprehensive avo classification. the leading edge, 22(10), 1030-1037. zhang, f., wang , y., & li, x. (2012). viabilities of seismic ray impedance and elastic impedance for hydrocarbon-sand discrimination. geophysics, 58, 1350-1360. zoeppritz, k. (1919). erdbebenwellen viiib, on the reflection and propagation of seismic waves. göttinger nachrichten, i, 66-84. 213a modified approach for elastic impedance inversion due to the variation in the value of k appendix i connolly elastic impedance equation elastic impedance is a generalization of acoustic impedance for the variable angle of incidence. connolly obtained this expression given in equation 1 by the integration and exponentiation of three-terms of aki and richards (2002) approximation of zoeppritz equation, and it is more sensitive than acoustic impedance for narrow distributions. with p-wave velocity (vp), s-wave velocity (vs), and density (ρ), connolly gave the following formula. ei v vp tan s ksin ksinθ ρθ − θ − θ( ) = +1 1 42 2 2 (3) where a tan= +1 2 b ksin=− θ8 2 c ksin= 1 2−4 θ k in the equation is constant; it is defined as the average v vs p 2 2 . for a two-layered model, authors get the following equations. ei v vp tan s ksin ksin 1 1 1 1 4 1 2 1 2 2 θ ρθ − θ − θ( ) = + (4) ei v vp tan s ksin ksin 2 1 2 1 4 2 2 2 2 2 θ ρθ − θ − θ( ) = + (5) the elastic impedance for a given angle of incidence θ, the reflection coefficient r as a function of  is given as, θ − + ( ) =rc ei ei ei ei 2 1 2 1 (6) ei and ei1 2 are the elastic impedances of the first and second layer, respectively. the normalized form of elastic impedance elastic impedance and acoustic impedance cannot be displayed together because the elastic impedance values change dramatically with the change in incident angle. to overcome this problem, whitcombe (2002) introduced constants v vp s0 0 , and 0 which allow elastic impedance values to remain constant for a layer and does not change with the angle  and modified the elastic impedance function as following. ei v v v v p p a s p b c θ ρ ρ ( ) =                   0 0 0 (7) furthermore, whitcombe introduced a scale factor vp0 0  by which authors could predict the correct value of acoustic impedance ai  from ei () at =0. ei v v v v vp p p a s p b c θ ρ ρ ρ ( ) =                  0 0 0 0 0 (8) equation 8 has enabled more control over the elastic impedance values as they will not vary rapidly with the change in the angle . it also sets the elastic impedance function to normalized due to the addition of the constants for a layer. fatti based elastic impedance equation based on fatti approximation of zoeppritz, wang et al. (2008) proposed the elastic impedance equation as shown below ei i i i i ip p p a s p b c θ ρ ρ ( ) =                             0 0 0 0 (9) where a tan= +1 2 b ksin=− θ8 2 c ksin tan= 4 2 2θ− θ i ip s0 0 0 , ,  are constants for normalization and can be taken as the p-wave impedance, s-wave impedance, and density of the upper layer, respectively. letter from the editor great challenges come for the world. climatic change, clean energy, petroleum, politic points of view using science for their own benefit. during the last months, the catastrophes have been in the spotlight, not only for those who work on the sciences side to relieve the pain caused or avoid the consequences, but also those states' managers trying to reach political objectives as the earth stills moving, the water continues running through streets and housings; when the forest fires are not completely extinguished, some people is looking for economic or ideology tributes. it's part of our history. from the character rhett butler in gone with the wind, the inspiring american novel, who made a vast fortune with the war, toward the contractors around the world drying public budgets as they explain the "risk management" concept. but science and politic are not going hand-in-hand. recently, the andean region in south america has been the scenery of record-breaking floods and landslides; peru was hit by heavy rains causing damages in the capital city lima and the northern part of the country. in colombia, the same heavy rains caused by el niño climate cycle destroyed two neighborhoods in mocoa, southern city capital of a department, state equivalent. three hundreds of people died. at the same time, bolivia is struggling with the drought consequences of climatic change. country by country in this subcontinent the landscape is similar. a political earthquake was the recent marches celebrated around the globe claiming for respect and support for the science. "we are marching today to remind people everywhere, our lawmakers especially, of the significance of science for our health and prosperity," said in washington bill nye, a science educator with the fame of a rock star. people knows the solution for a better future is in the hands of our politicians; their decisions will allow the development or the constraint of the science. "science, not silence," "oceans are rising, and so are we," "fund science = fun future," "policy without science is a biohazard," "science is not a partisan issue, it is the foundation of our society," "science is not an opinion," were some of the messages exhibited by demonstrators in their marches. humbly, from the earth sciences research journal, we want to adhere the same message: “it’s not magic, it’s science!” we present to you this new volume, is not so so much, but we hope every little bit helps. carlos alberto vargas jiménez. editor in chief, earth sciences research journal le tt er f r o m t h e ed it o r earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017) the study discuss about the soil moisture estimation using dual polarimetric risat-1. the semiempirical approach of modified dubois model (mdm) derived by (srinivasarao 2013) is worked out using (σ˚hh) and (σ˚hv) for soil moisture estimation using dual polarimetric backscattering image. irs liss iv data have been used to analyze the site suitability of different land use/cover types. the retrieval of backscattering coefficient values (σ˚) from sar is the common principle factor for soil moisture estimation. the surface roughness were measured in the selected sample location, for which the same backscattering values derived from the sar is linearly correlated showing r2 = 0.93. the estimated surface roughness is used for retrieval of dielectric constant using mdm. the dielectric constant derived from mdm in combination with the topps model proposed by (topp 1980), is used to derive satellite based soil moisture estimation. linear regression analysis was performed and the soil moisture derived from sar are well correlated with the volumetric soil moisture showing value of r2 = 0.63. abstract keywords: soil moisture, surface roughness, modified dubois model, topp’s model, risat 1-sar. derivation of soil moisture using modified dubois model with field assisted surface roughness on risat-1 data issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.59972 p. thanabalan*, and r. vidhya institute of remote sensing, anna university, chennai – 600025, tamilnadu, india. *thanabalanp@gmail.com earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 13-18 este estudio discute la estimación de la húmedad del suelo medida con el radar de imágenes satelitales (risat-1) doble polarimétrico. la aproximación semiempírica del modelo modificado de dubois (mdm), derivada por srinivasa rao (2013), se resuelve a través de (σ˚hh) y (σ˚hv) para la estimación de la húmedad del suelo usando la imagen de retrodispersión doble polarimétrica. la información de las imágenes liss iv se utilizó para analizar la pertinencia de diferentes tipos de uso del suelo y coberturas. la recuperación de los valores de coeficientes retrodispersos (σ˚) del radar de apertura sintética (sar, del inglés synthetic aperture radar) es el factor común principal para la estimación de la húmedad del suelo. la desigualdad de la superficie se midió en la ubicación del ejemplo seleccionado, por lo que los mismos valores retrodispersos derivados del sar están correlacionados linealmente con r2 = 0.93. la rugosidad de la superficie estimada se utilizó para la recuperación de la constante dieléctrica a través del modelo modificado de dubois. la constante dielétrica derivada del mdm, combinada con el modelo de topps, se utilizó para obtener la estimación de la húmedad del suelo partir de datos satelitales. se realizó el análisis de regresión lineal y la húmedad del suelo derivada del sar está bien correlacionada con el índice volumétrico de húmedad del suelo con un valor r2 = 0.63 resumen palabras clave: : húmedad del suelo; desigualdad de la superficie; modelo modificado de dubois; modelo de topp; radar de apertura sintética; record manuscript received: 08/09/2016 accepted for publication: 17/11/2017 how to cite item thanabalan, p., & vidhya, r. (2018). derivation of soil moisture using modified dubois model with field assisted surface roughness on risat-1 data. earth sciences research journal, 22(1). 13-18. doi: http://dx.doi.org/10.15446/esrj.v22n1.59972 medición de la húmedad del suelo a través del modelo modificado de dubois con superficie desigual, basado en información del radar imaging satellite-1 r em o te s en si n g 14 p. thanabalan and r. vidhya introduction in hydrological studies, soil moisture is a critical component that controls the infiltration and runoff rates. the wetness of the soil controls the evapotranspiration rate. although only a small percentage (~ 1%) of the total fresh water budget is contained in the soil layers of the earth surface, it acts as a key parameter in the exchange of mass and energy at the soil-atmospheric layer and also in hydrological processes. the soil surface conditions, soil moisture content, and roughness are important parameters in agriculture and vegetation growth monitoring. technological advancements in satellite remote sensing provide various techniques for measuring the soil moisture across a wider area (anguela 2010). remote sensing is an advanced technology that provides accurate and repetitive spatial data; microwave remote sensing synthetic aperture radar (sar) is particularly useful as they have a tendency to measure the soil parameters under any weather condition (mirmazloumi 2016; radar 2016). for bare agricultural soils, the backscattered radar signals depend strongly on the geometric characteristics (roughness) and dielectric properties (moisture content and soil composition) of the soil (singh 2007; tiwari 2014; gharechelou 2015). the dynamic relationship between the radar signal and surface roughness is stronger in the l, c, and x bands. moreover, sar data is more sensitive to soil roughness at high incidence angles (baghdadi 2006). since the microwave radiation can penetrate through clouds, it aids the acquisition of images in all weather conditions, and also penetrates into the soil for at least 1-5 cm and often upto 15 cm of the root zone level of soil moisture for volumetric analysis. the large variation in the dielectric constant, which depends on the radar backscattering coefficient, has a linear relation with the soil moisture if the roughness condition is absent (terrain 2007). previously, many attempts have been made for the development of semi-empirical algorithms (radar 2016; khabazan 2013; oh 2004; dubois 1995). semi-empirical models have the advantages of both empirical and theoretical models. according to the research carried out by many researchers in barely and sparsely vegetatedareas (capodici et al., 2013), soil moisture predicted by the dubois model is better than the integrated equation model (iem). till date, the dubois model is one of the most successful soil moisture prediction models, with some limitations because it requires quad pole data (hh, hv, vh, and vv). in this study, the risat crs image (spatial resolution of 36 m) acquired at hh & hv polarization have been used for the retrieval of soil moisture in and around the theni district of southern tamil nadu. risat-1 measures the volumetric soil moisture content in all weather conditions. the soil moisture content was derived using modified dubois model for risat-1 data. thus, the overall aim of the study explains the risat sar retrieval of soil moisture both spatially and temporally to improve our understanding of the hydrological behavior of soil. materials and methods a. study area the study area (theni district of tamil nadu) is geographically located from latitudes 9°39’n to 10°30’n and from longitudes 77°00’e to 78°30’e. it is a part of the western ghats, mostly consisting of hilly area with intermittent plains. in theni, agriculture is the primary occupation and 67% of the population lives in rural areas. the part of theni district that has been taken into consideration for this study lies within 10°1’38.22”n and 77°20’18.70”e 9°49’37.12”n and 77°38’43.79”e (fig.1) parts of villages, namely, andipatti, virapandi, odapatti, and chinamanur lie within the geographical extension being studied. it is a semi-arid region with an annual maximum temperature of 38.5°c and minimum temperature of 26.3°c. the average normal rainfall is about 829.8 mm (tamil nadu agriculture university). the soil, however, is sandy and clayey according to is 2720: part 4 (1985). the district falls within the southwest monsoon region, accounting for 21% of the total annual rainfall; northeast monsoon accounts for 46%, winter 6%, and summer 27% of the total annual rainfall. the district depends on the northeast monsoon rains, which are brought by the troughs of low pressure established in the southern part of the bay of bengal between october and december. the maximum amount of water is supplied by periyar river. figure 1. location of the study area (theni, tamil nadu) b. sar data risat-1 is india’s first microwave remote sensing satellite carrying a sar payload operating in the c-band (5.35 ghz). risat-1 was successfully launched by pslv-c19 on april 26, 2012. it started imaging on may 1, 2012. in risat-1, quad pole data is available only for high-resolution spot light (hrs) and fine-resolution stripmap (frs). however, hrs and frs imaging modes have very small swaths, 10 m and 25 m, respectively, which cover a very small area. the c-band frequency of operation and risat-1 sar capability of imaging in hh, vv, hv, vh, and circular polarization has ensured its wide applicability. in this study, risat -1 datasets were acquired with hh, hv polarization in a crs mode with a standard geo-referenced product. the product was purchased from nrsc data centre. the project system used for this image is wgs_1984_utm_zone_43n and datum: wgs_84. the acquisition data of pass was obtained on march 1st, 2016, with an incidence angle of 27.394. majority of the area covers terrain and agricultural lands. data analysis and processing were carried out using envi 4.8 software. land use/cover classes have been prepared by digitizing the image with resourcesat 2 liss iv satellite sensor with a resolution of 5.8 m to identify the different land use and land cover classes for field sample collections. figure 2. field sampling locations 15derivation of soil moisture using modified dubois model with field assisted surface roughness on risat-1 data. c. field measurements a field visit was carried out for this study from march 1 to 4, 2016. a total of 15 field sites were selected for sampling using land use and land cover classes (fig.2). soil samples were collected at 0-15 cm depths using the core cutter sampling method as per civil engineering standards (is 2720 part 29, 1975). the ground control points (gcp) for each of the 15 locations were collected using handheld gps. during the time of field data collection, the humidity of that area was very low. no rainfall was recorded in theni district from march 1 to 5, 2016. d. surface roughness derivation the surface roughness model helps to describe the surface height variations with respect to the ground surface level. surface roughness and soil moisture are two important factors that affect sar backscattering from a bare agricultural field (dobson 1981; ulaby 1979; casb 2000). in order to quantatively compare the profiles and measure the spatial characteristics the spatial wave number (k) and spatial scale (2π) is used for estimating the surface roughness from soil moisture data using backscatter sar (evans 1992; wang 2011; fresnel 2005). the radar return pulse at different times shows that the effect of surface roughness is considerable enough to study the soil moisture retrieval (baghdadi 2002; terrain 2007). thus, surface roughness is one of the most crucial and sensitive parameters in the analysis of hydrological parameters (y.oh 2002). meanwhile, the measurement of surface roughness was carried out in all 15 sampling locations using transecthull method. the roughness profiles taken across the direction of tillage and opposite to the look direction of the satellite pass for each sample training site. the statistical approach of measuring the surface roughness is derived by using the root mean square height (r.m.s -vertical variations). where, – estimated pixel; total pixels; – number of counts e. soil moisture measurement & descriptions gravimetric soil moisture (mg) value was obtained from laboratory analysis using civil engineering standards (is 2720 part 2, 1973), which was converted to volumetric soil moisture (mv) by multiplying the dry density(ɣd). soil is an important factor for vegetation and agriculture as well as for drought assessment. soil texture has been grouped into five types on the basis of analysis of collected samples from 15 sampling sites. the most abundant type of soil texture is sandy clay (69%) and clay (31%) as per is 1498-1970. f. modified dubois model the dubois model (1995) is a semi-empirical model, which is studied for soil moisture estimation in indian context (srinivasa rao 2013). the model was used to extract the dielectric constant from the backscattering coefficient of risat sar data. the parameters such as incidence angle, wavelength and frequency of sar data, and field surface roughness is used. the model consists of two equations (hh, hv) having independent parameter as surface roughness (say root mean square height). the model was applied for hh polarization to derive dielectric constant (ɛ) (1) (2) (3) f. conversion of dielectric constant to sar soil moisture the dielectric constant was derived using eq. (3). soil moisture can be derived from the dielectric constant using topp’s model (khedikar 2014). this model has been effective for soil moisture estimation for a very prolonged period (oh 1992; song 2009). this model does not require any prior knowledge about the soil texture, but needs the dielectric constant. the relative soil moisture (θv) related with the dielectric constant (ɛ) is shown in the following equation (4). the methods are clearly shown in (fig.3). figure 3. methodology chart results and discussion a. landuse/landcover map land use and land cover maps were prepared by digitizing the object using resourcesat 2 liss iv. simple first level of classification was used for the land use and cover maps. the classification is based on the nrsc lulc level 1 classification system. the classification results show plantation (10.40%), cropland (25.47%), agricultural land (43.23%), forest (0.75%), built-up area (2.20%), water bodies (1.94%), and wasteland (15.98%). the agricultural land with respect to the total geographical area is about 28%. figure 4 shows the land use/cover classification. b. surface roughness derivations a linear regression is performed by comparing the field measured roughness of 15 samples having roughness for smooth to very roughness (shaohua zhao, 2016). the ratio hv/hh derived polarization showing the trend analysis with positive effect in the study site. the results thus obtained are well correlated and the coefficient of determination (r2) = 0.93 is shown in (fig. 4 &5). the roughness measurement is relatively depending upon wavelength k = 2π/λ and frequency. in real field the propagation of the electromagnetic wave, the surface appears rougher than in the far field, which can be compared with reflection of the sunset (reflectivity 2005) of was done for the last image as it was assumed that there was no variation in the roughness. 16 p. thanabalan and r. vidhya figure 4. land use statistics of the study area figure 5. surface roughness (satellite-derived vs. observed) c. retrieval of soil moisture the soil moisture was derived from mdm in combination with topp’s model. the study site comprises land use and land cover classifications such as (cropland, plantation, wasteland, forest and buildup). the derived soil moisture extracted from risat-1 sar shows the cropland ranges from (0.4 to 0.67), plantation ranges from (0.69 to 1.87), wasteland ranges from (0.45 to 0.59). the cropland and wasteland shows similar range of db values extracted from risat-1 sar data. the date of pass is acquired on 1st march 2016. this is post-harvested period, therefore some field would have been harvested. this is mixing up with open lands. so, the agricultural area is masked to identify the changes in landuse classes. other than the ancillary parameters such as seasonal rainfall helps to understand the water availability during the crop sown period. however, this also can be taken as indicator pertaining to dryness of the agricultural area, measuring the indicator of the soil moisture condition within the agricultural area. when superimposed with the land use map having agri/open land, decision regarding agricultural management may be derived. this can be achieved by varying with hh, hv cross polarization combination, that can be an indirect method for irrigation management practice ii) crop condition assessment iii) also a direct indicator of drought. the incorrect estimation of backscattering amount and distribution will affect the retrieval of soil moisture. this effect is more severe in the cropland, as the date of pass comprises during rabi-zaid seasons. according to the indian cropping season are classified into two main seasons i) kharif for pre-monsoon period (june – september) and ii) rabi for post-monsoon period (october – february), when rabi sowing area is also over some of the irrigation practice follows during (march – june) are called as zaid seasons. the plantation is the annual crops, which is having very good soil moisture profile. the observed soil moisture derived from risat-1 sar is found to be well correlated with ground measured soil moisture are described in (fig: 6&7). this validation is mainly to compare the soil moisture derived from risat-1 sar data. the performance of the algorithms were evaluated on the basis of a standard evaluation criterion suggested mainly for rmse calculation (nagamani 2008; sikdar 2004) which states that coefficient of determination r2 = 0.63 was obtained from the mdm – topp’s model. the overestimation of predicted soil moisture values is due to the mixed landuse classes particularly the settlement/built-up area gives high back scattering values, which is reflect in comparison with barren field. otherwise the retrieval of soil moisture over agricultural field is highly correlated with other field measured values which can be clearly seen in (fig7) the previous research finding explains that the dielectric constant of the soil-water ratio is related to the microwave brightness, temperature, and emissivity or reflectivity of the soil (jackson 1981; mekonnen 2009; su 2015). the horizontal (hh) and vertical (hv) distributions of soil moisture control the partitioning of the incident radiant energy into latent and sensible heat fluxes via evapotranspiration. the near-surface soil moisture distribution helps to determine the status and trend in agricultural drought monitoring. moreover, it helps to understand the behavior of the top 5 cm of soil characterized by near-surface level soil moisture (engman 2000). the classification accuracy was better identified in the c band dual polarization and incidence angles (ponnurangam 2011). hence risat-1 sar data gives better results on soil moisture studies in the study site. the comparison and validation of soil moisture with ground measured data is clearly positive. figure 6. soil moisture estimation (mdm &topp’s model) 17derivation of soil moisture using modified dubois model with field assisted surface roughness on risat-1 data. figure 7. soil moisture estimation (observed vs predicted)conclusions the irrespective of the field measurements, a number of inversion models have been developed to retrieve the soil moisture data using active and passive microwave remote sensing. sar has proved its high accuracy in soil moisture estimation at a regional scale. even though the backscattering coefficient was determined with respect to the surface features, the semi-empirical model, i.e., mdm and topp’s model were subsequently approached for more simple and realistic efforts to estimate the soil moisture. similarly, the spectral reflectance of different land use and cover types are classified to derive the optical remote sensing parameters, such as land surface temperature (lst) and normalized difference vegetation index (ndvi). the result shows changes in the vegetative biomass at a spatial and temporal scale. the study found that the cross polarization (hv/hh) is sensitive to the surface roughness. thus, the roughness influence can be removed inform the soil moisture estimation (2133(4). the complex permissivity of wet soil and dry soil ratio with different soil textures depends on the variations in dielectric constant and dielectric loss of various types of soil in effective measurements of sensitive hydrological parameters. the derived model (mdm and topp’s model) is efficient in soil moisture estimation with the present availability of risat data in hh, hv with medium-resolution scansar (mrs) and coarse-resolution scansar (crs). hence, the synergistic approach of using both optical and microwave remote sensing helps to strengthen and to assess the validation of the sigma naught (σ°) against the vegetative biomass with low and high incidence angles. the surface roughness and thermal variations are normalized for agricultural drought assessment. furthermore, to assess the spatial and temporal patterns of soil moisture, its physical and hydrological properties need to be studied using suitable techniques of very high-resolution microwave remote sensing. references anguela, t., zribi, m. 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(2016). assessment of different backscattering models for bare soil surface parameters estimation from sar data in band c , l and p. 261–278. nagamani, p. v., chauhan, p.., dwivedi, r. m., (2008). development of chlorophylla algorithm for ocean colour monitor onboard oceansat-2 satellite, 527–531. oh, y., (2004). quantitative retrieval of soil moisture content and surface roughness from multipolarized radar observations of bare soil surfaces. ieee trans. geosci. remote sens. 42(3), 596–601. oh, y., sarabandi, k.., ulaby, f. t., (1992). an empirical model and an inversion technique for radar scattering from bare soil surfaces. ponnurangam, g.g., rao, y.s., (2011). soil moisture mapping using alos palsar and envisat asar data over india, 606–609. radar, s. a. (2016). 2 . 2 analysis methods for sar data. 2–5. fresnel, j., augustin, p., fresnel, t. et al. 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(2007). an efficient modeling with ga approach to acknowledgement the authors gratefully acknowledge prof. s. s. ramakrishnan, director, institute of remote sensing, anna university, chennai, india. the authors want to thank national data center (ndc) and national remote sensing centre (nrsc), hyderabad, for providing the risat satellite data used during the analysis. sincere thanks to dr. v. kumar, professor and head, department of agricultural engineering, agricultural college & research institute, madurai campus, and to all those who helped during the ground truth survey and analysis of satellite data. 18 p. thanabalan and r. vidhya retrieve soil texture, moisture and roughness from ers-2 sar data. prog. electromagn. res. 77, 121–136. song, k., zhou, x.., fan, y., (2009). empirically adopted iem for retrieval of soil moisture from radar backscattering coefficients. ieee trans. geosci. remote sens. 47(6), 1662–1672. srinivasa rao, s., dinesh kumar, s., das, s. n., nagaraju, m. s. s., venugopal, m. v., rajankar, p., et al. (2013). modified dubois model for estimating soil moisture with dual polarized sar data. j. indian soc. remote sens. 41(4), 865–872. capodici, f., maltese, a., ciraola, g., loggia, l., urso, g. d., et al., (2013). coupling two radar backscattering model to assess surface roughness and surface water content at farm scale. j. hydrological sciences. 58(8), 1677–1689. su, z. b., (2015). microwave remote sensing of soil moisture, pp (6-18). terrain, d., house, m., (2007). simulation of radar backscattering coefficients using jem a tool for surface soil moisture retrieval, 383–388. tiwari, r., singh, r. k., chauhan, d. s., singh, o. p., prakash, r.., singh, d. (2014) microwave scattering for soil texture at x-band and its retrieval using genetic algorithm, 120–127. topp, g. c., davis, j. l.., annan, a. p. (1980). electromagnetic determination of soil water content: measurements in coaxial transmission lines. water resour. res. 16(3), 574–582. ulaby, f. t. g. a. b. and mc. d. (1979). microwave backscatter dependence on surface roughness, soil moisture and soil texture: part ii-bare soil. ieee trans. geosci. rem. sens. 16(4), 286–295. wang, s. g., li, x., han, x. j.., jin, r., (2011). estimation of surface soil moisture and roughness from multi-angular asar imagery in the watershed allied telemetry experimental research (water). hydrol. earth syst. sci. 15(5), 1415–1426. y. oh., k. sarabandi., f. t. ulaby., (2002), semi-empirical modelof the ensemble-averaged differential mueller matrix for microwave backscattering from bare soil surfaces. ieee trans. geosci. remote sensing, 40, 1348 –1355. statistical embedded markov chain processes were used to analyze facies transitions and to determine the stacking pattern of the lithofacies of the guaduas formation. twelve lithofacies were found and characterized based on lithology and sedimentary structures in four stratigraphic sections. the findings were compared with a previous assemblage of lithofacies, interpretations of sedimentary environments, and depositional systems. as a result, four depositional systems were established. through the statistical analyses of facies transitions it was found that tidal facies are prevalent in the socota section, more specifically in the upper part. whereas in the sogamoso, umbita and peñas de sutatausa sections, fluvial facies are prevalent in the upper part of the sections, and follow a regressive sequence with more continental deposits around the upper part of the sections. for each of these sections the markov chain transition matrices illustrate a strong interaction between tidal facies and fluvial facies, especially in the peñas de sutatausa matrix, where facies 6, made up of tidal deposits, appears several times. from the facies model and markov chain analyses, it is evident that the guaduas formation is a cyclic sequence in which the markov facies repetitions are consistent with the lithofacies analyses conducted in previous stratigraphic studies. the results reveal that the markov chain statistical process can be used to predict stratigraphy in order to correlate contiguous geologically unexplored areas in the guaduas formation, where much work relating the correlation and the continuity of coal beds has yet to be done. se utilizaron los procesos estadísticos de las cadenas de markov para analizar las transiciones de facies y para determinar el patrón de apilamiento de las litofacies de la formación guaduas. se encontraron y caracterizaron doce litofacies en cuatro secciones estratigráficas según litología y estructuras sedimentarias. los resultados fueron comparados con un ensamblaje anterior de litofacies, interpretación de ambientes sedimentarios y sistemas deposicionales. como resultado, se establecieron cuatro sistemas deposicionales. a través de los análisis estadísticos de las transiciones de la facies se encontró que la facies de marea son frecuentes en la sección de socotá, más concretamente en la parte superior. mientras que en las secciones de sogamoso, umbita y peñas de sutatausa, las facies fluviales son frecuentes en la parte superior de las secciones y siguen una secuencia regresiva con depósitos continentales hacia la parte superior de las secciones. para cada una de estas secciones, las matrices de las cadenas de markov presentan una fuerte interacción entre facies mareales y facies fluviales, especialmente en la matriz de penas de sutatausa, donde la facie 6, compuesta de depósitos mareales, aparece varias veces. a partir de los análisis de facies y de las cadenas de markov, es evidente que la formación guaduas es una secuencia cíclica en la que las repeticiones de facies de markov son consistentes con el análisis de litofacies en estudios estratigráficos anteriores. los resultados revelan que el proceso estadístico de las cadenas de markov se puede utilizarse para predecir la estratigrafía y para correlacionar áreas contiguas y geológicamente inexploradas en la formación guaduas, donde aún hay mucho trabajo que hacer en la correlación y la continuidad de los yacimientos de carbón. earth sciences research journal earth sci. res. j. vol. 20, no. 3 ( september, 2016 ) : b1 b9 abstract resumen keywords: litofacies, lithofacies, reservoir environment, facies maps, guaduas, lithofacies maps. palabras clave: : litofacies, ambientes de depósito, mapas de facies, formación guaduas, mapas de litofacies. lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains determinacion de la ciclicidad de las facies en la formacion guaduas (colombia) usando las cadenas de markov record manuscript received: 15/07/2014 accepted for publication: 20/09/2016 how to cite item mariño, j., & amaya, e. (2016). lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains. earth sciences research journal, 20(3), b1-b9. doi:http://dx.doi.org/10.15446/esrj.v20n3.44429 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n3.44429 mariño m. jorge e.1, amaya erika2 1 universidad pedagógica y tecnológica de colombiaescuela de ingeniería geológica; jorge.marino@uptc.edu.co 2 universidad pedagógica y tecnológica de colombiaescuela de ingeniería geológica; amayaer@gmail.com st r a t ig r a ph y mariño m. jorge e. and amaya erikab2 introduction the guaduas formation is the main source of thermal and coking coal for central colombia and is located on the eastern cordillera of the andes mountain range. according to certain authors, the cyclic patterns found in this formation allow the use of statistical methods. sarmiento (1994) identified the repetition of transitional environments during the transition between the cretaceous sea invasion and the early tertiary with its transitional deposits, and in 2010 amaya et al. (2010) found a prevalent muddy composition with some key sandstones (guias) and coal beds, in which such coal beds are interbedded with mudstone and sandstone in a pattern beginning with tidal deposits followed by coastal deposits (such as lagoons) and lastly fluvial deposits. markov chains statistical analyses have been used by several authors to differentiate geological processes and construct facies models based on transition frequency (harper, 1984) and can also be used to determine which lithofacies are generally found above or below other lithofacies. each lithofacies has an assigned number according to its facies description (miall, 1992). using markov chain analyses, mariño and morris (1996) found 37 transitions between eolian and marginal facies, which helped in the development of the depositional model. and additionally found that most of these transitions follow the depositional model, although some random facies could indicate sudden events such as storms. the purpose of this research is to use markov chains to confirm the patterns of repetition found in the guaduas formation by other authors using standard lithofacies analyses (sarmiento, 1994; amaya, 2009; amaya et al., 2010). amaya et al. (2010) described and reviewed 10 stratigraphic sections in outcrops and cores, and found 12 lithofacies that helped to establish four depositional systems (figure 1). four of such stratigraphic sections were chosen with the intent of discovering a pattern by using the statistical method known as the markov chain, which orders successions of mutually exclusive states (i.e. facies) wherever a change of state occurs to find the significance of the succession. a statistical pattern is suspected in this formation as a result of the cretaceous cyclicity that has been well documented in colombia (etayo-serna et al, 1976). these patterns could help to understand the dynamics that allow the sediment accumulation in this part of the eastern cordillera by the end of cretaceous and beginning of paleocene, and can also be used to improve the correlation and learn more about the continuity of the coal beds in the guaduas formation. figure1. location of the stratigraphic sections in central colombia which cover parts of the boyacá and cundinamarca provinces. cyclicity in sedimentary rocks and markov chains long ago people began to evidence that the sea once covered areas that are currently dry land. episodic sea invasions were found to be periodical and important through the study of sedimentary successions and as a result, cyclic patterns were found. in any stratigraphic succession one can see changes in lithologies, called facies, and several statistical methods have been used to differentiate geologic processes and construct facies models on the basis of transition frequency (harper, 1984). in this study, the term facies is called lithofacies because it represents a rocky unit defined by its lithologic characteristics, which include composition, grain size, and sedimentary structure (miall, 1990). each lithofacies represents an individual depositional event, and is grouped according to characteristics of a depositional environment based on facies association (miall, 1990; amaya, 2009). the cyclic patterns, represented by cyclical lithofacies, go beyond relative and periodical changes in sea level, and include eustatic or global changes in sea level, non eustatic changes in sea level related to local and regional events, subsidence, compaction, climate, tectonics amongst other factors (miall, 1990). the statistical method best suited for the analysis of facies transitions data in stratigraphic sections is the markov chain transition matrix. markov chain analyses can be used to determine which lithofacies are usually found above and below other lithofacies, and thus is one of the methods used to differentiate geologic processes and construct facies models based on transition frequencies, some of which can be used as patterns to predict events or facies related to minerals resources and so on (harper, 1984; walker and james, 1990). a markov chain transition matrix groups lithofacies into a two dimensional array which tabulates the number of times that all possible vertical lithofacies transitions occur in a given stratigraphic succession. the analysis becomes more meaningful with increasing numbers of transitions. each lithofacies has an assigned number or code according to the facies description (miall, 1990). there are two ways to make a transition count matrix: transitions may be recorded at each new bed when lithofacies change in character, regardless of bed thickness, or transitions may be constructed by sampling the section at fixed vertical intervals. in this study, the first method, or “embebed markov chain analysis,” was chosen because it emphasizes changes and focuses on depositional processes. repeating lithofacies are measured as one, hence the row of zeros that spans from the top-left to the bottom-right corner of the transition count matrix (table 1). table 1. example of a 6x6 embedded transition matrix in which the transition numbers in the diagonal are all zero. geologic setting of the guaduas formation the guaduas formation is part of the eastern cordillera basin in east central colombia and has a muddy composition with some sandy units and coal beds; such coal beds are of economic importance in colombia due to their high rank, with some of them being coking coals (luna et al., 2004). the formation presents a systematic thinning to the north and to the east. in the southwest, its thickness can reach 1100 m (checua-lenguazaque lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains b3 syncline), to the north 250-400 m, and to the east 80 m (sarmiento, 1994; cooper et al., 1995; amaya et al., 2010: duarte y mariño, 2000). the guadalupe group, chipaque formation, and une formation underlie the guaduas formation, and the cacho or socha sandstone overlies the guaduas. the northern part of the eastern cordillera basin was bisected by the santander high (the santander and floresta massives) until the emergence of the cretaceous, which allowed for the development of two separated sub-basins: the cocuy and the magdalena-tablazo. this barrier controlled deposition during the lower cretaceous, with the accumulation of marine shallow facies mainly (fabre, 1983), to the south, these sub-basins merged as the cundinamarca sub-basin, where the cretaceous sequence reaches maximum thickness (sarmiento-rojas et al., 2006). the sub-basins tablazo and cocuy combined as one basin during hauterivian due to the flooding of the santander-floresta high (fabre, 1983). during the aptian, a major transgression, followed by a relative sea-level rise, caused the flooding of the current area of the eastern cordillera, including the cundinamarca sub-basin. by that time, the santander high was not a barrier that would prevent the sediments movement (cooper et a.l, 1995). in the albiancenomanian, a relative fall in the base level favored the progradation of deltaic and littoral sands une formation (fabre, 1983; sarmiento-rojas et al., 2006). during the cenomanian-turonian the eustatic level reached its maximum and allowed for the deposition of the chipaque formation (sarmiento rojas et al., 2006). by the santonian, campanian, maastrichtian and paleocene, a regression and general progradation lead to the accumulation of littoral and transitional facies during the deposition of the guadalupe group; it was followed by coastal and alluvial flats and coal facies during the deposition of the guaduas formation (sarmiento, 1994). finally, the entire sequence was covered by the fluvial deposits of the cacho-socha sandstone. the guaduas formation was deposited throughout the upper cretaceous and lower tertiary during the regression of the cretaceous seas. superimposed upon this big-tendency scale there are several smaller transgressive-regressive cycles that allow the repetition of the lithofacies and made the formation suitable for statistical analyses. the subsidence of the basin was fast, and the predominance of shallow muddy marine facies also suggests a quick deposition. (cooper et al., 1995). the 12 lithofacies found in the guaduas formation can be grouped in 4 lithofacies associations (see tables 2 and 3). lithofacies association a consists of interbeddings and interdigitation of sandstones, siltstones, claystones, and at times thin coal beds, which are interpreted as lagoon deposits based on the association, lateral continuity, and organic content of the lithofacies. lithofacies association b is characterized by the interbedding of sandstones, and claystones with plant debris and coal, and is interpreted as being formed by tidal flats based upon heterogeneous stratification, and bidirectional cross-stratification. lithofacies association c consists of claystones, coal, and certain sandstones. this association represents a great percentage of the stratigraphic record and is classified as an alluvial flooding flat. lithofacies association d consists mainly of sandstones and is related to meandering channels based on sedimentary structures, especially big scale cross-stratification (see table 3) (amaya et al., 2010). data and methods the purpose of this study is to use markov chain analysis to verify the facies stacking pattern discovered in the guaduas formation using conventional lithostratigraphic facies analysis (amaya et al., 2010; sarmiento 1994). figure 1 depicts the places where markov chain analysis was performed in the four stratigraphic sections of socota, sogamoso, umbita, and peñas de sutatausa. these sections are well distributed and represent guaduas formation stratigraphy in the provinces of boyacá and cundinamarca. facies determination and distribution was conducted in a five step process: (1) stratigraphic data recollection, (2) facies definition and description (3) depositional environment determination, (4) markovian analysis of lithofacies, and (5) stratigraphic correlation and analysis. each facies description was based on grain size and rock type (folk, 1974), sedimentary structures, bed thickness, vertical and lateral change relationships based upon panoramic sections (miall, 1990), as well as color and texture. after describing the stratigraphic sections of the guaduas formation in the studied area, 12 lithofacies were found: 1) sandstone with bidirectional cross-stratification, 2) sandstone with irregular wavy bedding, 3) rhythmic successions of sandstones and claystones, 4) sandstones with heterolithic bedding, 5) claystones with sandy lenses, 6) claystones with thin lamination, and plant debris, and coal, 7) claystones and siliceous limestones with paleosols, 8) sandstones with through-cross stratification, 9) sandstones with planar cross-stratification, 10) sandstone with planar stratification, 11) sandstone with climbing cross-stratification, and 12) massive sandstones (table 2). table 2. lithofacies number and lithofacies according to the facies analysis, the repetition of different attributes was identified throughout the sections and along the basin. this association of lithologic attributes is presented in table 3, which shows a prograding sequence that characterizes the transition between transitional and continental deposits. the lithofacies association allowed the determination of the depositional environments and the depositional systems. the facies model, by which the depositional environments were interpreted, indicates that tidal flat deposits were followed by coastal lagoons and topped by a fluvial system with alluvial flats interbedding with meandering channels. description and interpretation of depositional environments was supported by research done by reineck y singh (1980), walker y james (1990), boggs (1987), kvale y archer (1990), miall (1990), walker and james (1990), fichter (1993), choi and dalrymple (2004), and mariño and morrison (1996). for the construction of the markov chains, each section was labeled with facies number defined during the facies study (table 2); this procedure eased the transition counting between one lithofacies and an adjacent one (above or below). transitions were recorded based on the changes in character of lithofacies regardless of bed thickness. through this process the transition between the same types of facies was not taken into account, this is why the elements of the main diagonal in each matrix are all zero, which follows the method known as “embedded markov chain analysis” (ethier, 1975). these transitions were tabulated in a two-dimensional arrangement where the columns represent the overlying lithofacies while the rows represent underlying ones (table 1). markov chains are based on two considerations, the first is the supposition that lithology at any point n depends upon the lithology of the proceeding point n-1. this supposition is possible due to the fact that the guaduas formation apparently behaves as a cyclic sequence, in which the same lithology is observed at different points in a stratigraphic section. the second is to consider a transition as a change between two different lithologies where the transition in the same lithology is not taken into account, thus the elements of the main diagonal in the matrix are all zero (table 1). mariño m. jorge e. and amaya erikab4 results the facies analysis using markov chains in the four stratigraphic sections was based upon that carried out by amaya (2009) and amaya et al. (2010), which determined 12 repeating lithofacies throughout the sections (table 2). these facies were associated in order to establish a facies model with depositional environments and depositional systems for the guaduas formation. four depositional systems were established, from coastal lagoons, passing through tidal flats, to fluvial systems with meandering channels interbedded with alluvial flooding flats (table 3). the change between transitional and continental environments seems to be oscillatory, which reflects sea level variations for that time. the markov chain analyses were used to test the stacking pattern found during the previous facies study. the stratigraphic sections and the resulting matrices of socota, sogamoso, umbita, and peñas de sutatausa are as follows: socota stratigraphic section. figure 2 presents the stratigraphic section and the markov chain matrix for the socota area. in this matrix, the presence of lithofacies 6 (claystones with thin lamination, plant debris, and coal) stands out, as it appeared 8 times in the section. facies 6 is followed 4 times by lithofacies 1 (sandstones with bidirectional crosssection), and 3 times by lithofacies 5 (claystones with sandy lenses). sogamoso stratigraphic section. from the matrix completed for the sogamoso stratigraphic section (figure 3), lithofacies 6 (claystones with thin lamination, plant debris, and coal) is the most abundant, appearing 6 times, 4 of which lithofacies 8 is underlain by lithofacies 6, and 3 of which by lithofacies 12 (massive sandstones). lithofacies 5 was found below lithofacies 12 in three instances. lithofacies 12 was found 4 times below lithofacies 6. umbita stratigraphic section. from figure 4 in the umbita’s section, it is clear that lithofacies 6 (claystones with thin lamination, plant debris, and coal) is the most abundant and usually relates with facies 1 (sandstone with bidirectional cross-stratification) and facies 8 (sandstones with through-cross stratification). peñas de sutatausa stratigraphic section. figure 5 presents the stratigraphic section and markov chain matrix for the peñas de sutatausa area. from the matrix, it is concluded that the most related facies are 6 (claystones with thin lamination and plant debris and coal), 1 (sandstone with bidirectional cross-stratification), 5 (claystones with sandy lenses), and 8 (sandstones with through-cross stratification). lithofacies 6 and 8 are closely related and follow each other 13 times (below and above). table 3. prograding lithofacies model for the guaduas formation indicating continentalization of the basin. the lithofacies association result in a deposition environment that is related to a depositional system (amaya et al. (2010). lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains b5 figure 2. stratigraphic section of the guaduas formation in socota with lithofacies numbers and markov chain transition matrix. mariño m. jorge e. and amaya erikab6 figure 3. stratigraphic section of the guaduas formation in sogamoso with lithofacies numbers, and markov chain transition matrix. lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains b7 figure 4. stratigraphic section of the guaduas formation in umbita with lithofacies numbers and markov chain transition matrix. discussion and conclusions from the highlighted areas or more frequent transitions in each of the matrices, certain conclusions can be made: 1) for the socota area, the presence of lithofacies 6 stands out due to its repetition (claystones with thin lamination, plant debris, and coal), followed by lithofacies 1 and 5. the transitions of lithofacies 6 to other lithofacies are tenfold in this section. from this matrix, the presence of muddy areas with coal and the strong relationship between facies 1 (sandstone with bidirectional cross-stratification) and 6 (claystones with thin lamination, plant debris, and coal) is evident, and indicates the prevalence of transitional facies to the top, and a lateral relationship between fluvial and tidal flats, supporting the conclusions of the previous model in which the coal in that area was originated in tidal swamps (amaya et al, 2010; sarmiento, 1994). the socota area is the only section where the transitional facies seem to be to the top of a section with tidal deposits at the base and top of the section, and alluvial facies in the middle. 2) from the sogamoso matrix, it is evident that lithofacies 6 (claystones with thin lamination and plant debris and coal) is the most abundant and is most related to lithofacies 8 (sandstones with through-cross stratification) and 12 (massive sandstones). this indicates the interbedding of clays and coal from swamps (which are part of an alluvial flat), with sandy units that represent the fluvial meandering channels. the prevailing lithofacies analysis in the sogamoso section indicates continental dominance towards the top of the section, unlike that of the north-east section (socota section) where tidal and fluvial environments prevailed. this could be explained by the progressive thinning of the formation to the north and to the east (amaya et al, 2010). 3) for the umbita area, the strongest associations are between the tidal facies (facies 6, 5 and 1), which could indicate that the coal beds in this area are more related to tidal flats and have some interaction with fluvial meandering channels, lateral bars, and fluvial channels (8 and 9 lithofacies). this follows the conclusions of previous studies that have shown a balance between tidal and fluvial environments, with the fluvial deposits prevailing to the top of sections (amaya et al, 2010). mariño m. jorge e. and amaya erikab8 figure 5. stratigraphic section of the guaduas formation in sogamoso with lithofacies numbers, and markov chain transition matrix. 4) in the peñas de sutatausa area two associations are important: the first one relates tidal lithofacies (1-6), and the second associates tidal and fluvial facies (6 and 8), supporting the facts that in this area, fluvial, tidal, and fluvial with tidal facies influence were the prevailing lithofacies during the coal deposition. the fluvial influence is stronger to the top of the peñas section (sarmiento, 1994; amaya, 2009). in peñas, the fluvial influence is stronger than in the umbita area. from the facies model and markov chain analyses, it is evident that the guaduas formation is a cyclic sequence in which the markov facies repetitions are consistent with the lithofacies analyses done in the stratigraphic sections. the statistical facies analyses proved that the tidal facies prevails in the socota section, especially to the upper part; the markov chain transition matrix found little interaction of the tidal facies with the fluvial facies, indicating fewer fluvial facies in that section (figure 2). note that in figure 2 the facies repetitions are between tidal lithofacies 1 and 6, supporting the results of previous facies analyses (amaya et al., 2010). on the contrary, the stratigraphic correlations of the sogamoso, umbita and peñas de sutatausa sections revealed fluvial facies prevailing to the upper part of the sections, following a regression sequence with more continental deposits toward the upper part of the sections (table 2). in these latter sections, the markov chain transition matrix illustrates a strong repetition of fluvial facies following tidal facies, indicating strong interaction between them, as is evident in the peñas de sutatausa´s matrix where facies 6 related to tidal deposits repeats 13 times with facies 8 of fluvial origin (figure 5). lithofacies cyclicity determination in the guaduas formation (colombia) using markov chains b9 the finding of transitional tidal facies located most often to the bottom, followed by fluvial flats and fluvial channel deposits shows a shallowing upward sequence with continental deposits more frequent near the top that was developing at the upper cretaceous and early paleocene epoch as a result of the regression forced by the andean orogeny (cooper et al., 1995). however, in each transition, smaller cycles are present, allowing the cyclic deposition of different lithologies like key sandstones (guias) and 12 to 14 coal beds. this cyclicity allowed the deposition of a great deal of coal, especially in the south, making of this area one of the best coal sources in colombia (mainly coking coals) (luna et al., 2004). not only the vertical changes are evident in this area, but also the horizontal changes, due to the thinning of the stratigraphic succession to the north and east of the study area. the northward thinning of the guaduas formation may be explained from a smaller subsidence to the north (fabre, 1983, amaya et al, 2010). the thinning may also be explained by erosion (sarmiento, 1994). these hypotheses would explain why the guaduas formation is thicker in the cundinamarca province and has two coal bed members. on the contrary, to the north (boyacá province), the formation is thinner and has just one coal member, as the upper one possibly eroded. this along with erosional unconformity would better explain the thinning of the guaduas formation to the north (sarmiento, 1994; duarte y mariño, 2000). the discovery of more continental facies found to the top by use of markov chains is in agreement with facies maps, which indicate an upward shallowing of the environments with an increased presence of fluvial facies to the top, especially in the south (amaya et al., 2010; amaya, 2009). initially the sea regression-transgression had a sw-ne tendency, which later became more e-w. these changes could be related to the reactivation and stratigraphic control of big trust faults like soapaga that have caused differential subsidence (krammer, 1999; duarte y mariño, 2000). from the facies association in the guaduas formation, a progressive shallowing in the environments due to fluvial facies dominance is evident near the top of the section, with three main west to east transgressive episodes with tidal and fluvial environments prevailing to the top and ne of the basin, which could be explained by the progressive thinning of the formation to the north and east as a result of the sea receding to the west. this follows previous findings that used conventional facies analyses (amaya et al, 2010). these results demonstrate that the markov chain statistical process can be used as a quantitative tool to distill lithofacies transitions and define lithofacies trends with precision, as well as to predict stratigraphy in order to correlate contiguous unexplored geologic areas in the guaduas formation where much work on correlation and coal bed continuity is still to be done. acknowledgments the authors are grateful for, the financial support from the smithsonian tropical research institute and to the research office (din) of the universidad pedagógica y tecnológica de colombia (sgi 978), the technical support from: kyunk choi, cedric carpentier, angela leguizamón, and josh reed; the support and information during data collection and access to mines and wells from the companies, carbones de los andes, carbones del caribe, coprocarbon, minergéticos. thanks to sandra pedraza for her help in drawing figures, and matthew mcgee and cynthia marino for reviewing the manuscript. references amaya, e. 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(1990). facies models: response to sea level change. canada: geological association of canada. 530p. identifying remaining oil distribution is an essential study at the third district in north saertu of the west block in daqing oilfield. this field is known as a flooding fine potential tapping demonstration zone, characterized by a long-developing history and complex well history. based on tectonic features and sedimentary characteristics of the study area, the methods of facies controlled reservoir 3d geological modeling and numerical simulation are used in the process of establishing the geological 3d static model. in this paper, we summarized the causes and distribution law of remaining oil in the study area by using the method of fine reservoir numerical simulation to provide a reliable basis for the development and adjustment of the oil field. in combination with fine exploration such as water drive fracturing, water plugging, reperforating and injection-production segment, the recoverable reserves recovery rate could be effectively increased. la identificación de la distribución del remanente de petróleo es un estudio esencial en el tercer distrito del norte de sartu, que corresponde al bloque occidental del campo petrolífero de daqing. este campo es conocido como una zona ejemplar para aprovechar el potencial de explotación por inundación y que se caracteriza por una historia compleja y de largo desarrollo de sus pozos. con base a las características tectónicas y sedimentarias del área de estudio se utlilizaron los métodos de modelado geológico 3d en depósitos con facies controladas y la simulación numérica en el proceso de establecer el modelo geológico 3d estático. en este artículo se establecen las causas y la ley distributiva del remanente de petróleo en el área de estudio a través del método de simulación numérica de depósitos de alta resolución que provea una base fiable para el desarrollo y ajuste del campo petrolero. con la combinación de métodos de exploración como la fractura dirigida con agua, taponamiento acuático, reperforación y segmentos de inyección-producción, el índice de recuperación de reservas podría incrementar efectivamente. abstract resumen keywords: remaining oil distribution, 3d geological modeling, numerical reservoir simulation, sedimentary microfacies, north saertu. palabras clave: distribución del remanente de petróleo, modelado geológico 3d, simulación numérica de depósitos, microfacies sedimentarias, norte de sartu. study of distribution of remaining oil in west block of the third district in north saertu record manuscript received: 10/04/2016 accepted for publication: 29/07/2016 how to cite item wei, h., zhu, h., shi, s., yang, x., zhang, j., wang, j., & ashraf, m. a. (2016). study of distribution of remaining oil in west block of the third district in north saertu. earth sciences research journal, 20(2), a1-a4. doi:http://dx.doi.org/10.15446/esrj.v20n2.55167 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.55167 huabin wei1a, huanlai zhu1, shangming shi1, xinliang yang2, jingjun zhang1, jian wang3, muhammad aqeel ashraf4 1geoscience college of northeast petroleum university; 2school of petroleum engineering of northeast petroleum university; 3daqing china national united oil corporation eighth oil production plant; 4faculty of science and natural resources, university malaysia sabah 88400 kota kinabalu, sabah, malaysia awhbinemail@163.com earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : a1 a4 estudio de la distribución del remanente de petróleo en el bloque oriental del tercer distrito, en el norte de sartu, china h y d r o c a r b o n s a2 huabin wei, huanlai zhu, shangming shi, xinliang yang, jingjun zhang, jian wang and muhammad aqeel ashraf 1. introduction the west block of the third district in north saertu water flooding fine potential tapping study area is located in the west of the pure oil of saertu oilfield of daqing placanticline and belongs to the delta facies lithologic-structural reservoir (yang, 2004). saertu, putaohua, and gaotaizi reservoirs developed in the study area (tong et al., 2014). since 1964, the district has experienced some major adjustments: the implementation of the fracturing, fill holes, water shutoff, subdivision injection production and other essential measures. because of the long development time and complex well history conditions, it is not very clear for the remaining oil distribution law to find the appropriate area of remaining oil (sun et al., 2011), the distribution of the remaining oil in the study area and the cause of formation. those factors were quantitatively studied in this work. 2. study area survey west block of the third district in north saertu study zone is located at the west end of the anticlinal structures and having area of 18.5km2 and it is in the northern part of saertu oil field. the structure is gentle, with a 2~3 degrees formation dip. the average altitude of the ground is about 150m. there are 625 water-drive oil & water wells in the study area. well density is 33.8 well/km2, the injection production ratio is 1.47, the average thickness of sandstone is 21.6m. so far, three types of subfacies developed in the study are have been seen: si group, sii group, siii group. pii group formation belongs to delta distributary plain subfacies and delta front subfacies sedimentary facies, and below gi9 oil layer in gaotaizi, all belong to the sand body deposition in the outer edge of the delta front except gii1+2b, gii12+13 (wang et al. 2007). 3. 3d facies controlled geological modeling due to the heterogeneity of saertu, putaohua, and gaotaizi reservoirs at the west block of the third district in north saertu (han, 2010), it was established the use of the static three-dimensional geological model of the software petrel 2009 to control the morphology of the geological body, ensuring the accuracy of modeling, the thickness of stratum and well pattern density (guo et al., 2015). the plane grid spacing was set to be 20m×20m to characterize the reservoir plane distribution characteristics (zhang et al., 2010). the vertical average is a grid for each 0.5m, and the minimum is 0.25m, which is used to meet the requirement of resolving the thinner layer (fig. 1). the sedimentary facies modeling is based on construction model (li et al., 2013), under the guidance of sedimentary model, using the theory of assign to closest point to plane interpolation for phase data, and take the result of interpolation as constraints to establish sedimentary microfacies model (chong et al., 2011). si4+5 small layer pii1 small layer figure 1. microfacies model of reservoir sedimentary facies in the west block of the third district in north saertu study zone 4. numerical reservoir simulation there is no visible crack in the study area, and the in-place oil is a conventional heavy oil. that is, the reservoir does not have an anticoagulation phenomenon (guo et al., 2013). it combines the actual situation in the study area to follow the first overall, after the local; the first principle of the block and single well numerical simulation to carry out the numerical simulation (yang et al., 2011). among 379 production wells in the region, the number of wells which have statistical production of single well fluid volume fitting relative error less than 20% is more than 65% of the total number of wells (hu, 2012), the oil production fitting rate could reach to 100%, and the fitting accuracy of the study area is relatively high to meet the requirement of single well-fitting index (fig. 2). figure 2. the fitting curve of generating oil and fluid in the west block of the third district in north saertu study zone 5 .residual oil analysis 5.1 remaining oil distribution law. the numerical simulation results were carried out, the remaining oil saturation were subtracted from the residual oil saturation (fu et al., 2010), and the residual oil reserves were calculated using the formula of reserves calculation (fig. 3). figure 3. movable remaining oil abundance distribution in west block of the third district in north saertu study zone a3study of zdistribution of remaining oil in west block of the third district in north saertu the distribution of remaining oil in the reservoir of saertu oil layer is relatively uniform, and the remaining oil reserves are moderate, in the 20~50×104t/km2, which are distributed in the north and south area. putaohua reservoir movable remaining oil reserves abundance high-value area is not relatively wealthy, with the moving remaining oil reserves abundance between 10~50×104t/km2. the high-value areas are in south and northeast. gaotaizi reservoir remaining oil reserves plenty are between 5~50×104t/km2; the high-value area concentrated in the southern half of area. the potential of the remaining oil is mainly accumulated in the nonprincipal parts of <1.0m, by using the existing well nets to do the injection-production relationship, adjust layer, water blocking and other measures, the estimated total remaining recoverable reserves are expected to reach 113.1×104 tons, the ultimate recovery rate could be 49.8%. 5.2 analysis of the causes of the remaining oil. the formation of the remaining oil is influenced by the reservoir heterogeneity and the mining condition (li et al., 2011; wei et al., 2008). in this paper, the structure of the remaining oil in the block are affected by the following factors: the pattern can not control it. this kind of remaining oil is in the original well network, while drilling but not perforated, or original well pattern not being drilled and new infill wells drilled encountered reservoir is formed. alternatively, is due to the relatively narrow oil-bearing sand body, passes through the middle of the two wells, due to well away from the ohara pattern did not hold and the formation of remaining oil (fig. 4). siii9b gii9 figure 4. remaining oil formation (well pattern can not control) the type whose injection production is not perfect. such residual oil is only well drilling encountered or only wells drilled, or oil wells drilling met for interlayer, the cementing quality and the same layer and water control, water and other aspects of the reasons and no perforation of remaining oil (fig. 5). injection production system adjustment can improve this potential, well network encryption and hole filling and so on, which are used to raise the level of use. si2 gii1+2a figure 5. remaining oil formation (the type whose injection production is not perfect) the kind of detention area. the remaining oil is formed by the pressure balance between adjacent wells, which is a certain proportion in the thick layer and thin layer, but its distribution area is relatively small. the encryption can be used to improve the injection production relationship (fig. 6). gi4+5 siii10 figure 6. remaining oil formation (the type of detention area) the kind of poor reservoir. the of this kind of residual oil is mainly due to the thin reservoir, poor physical properties, the original well pattern and well spacing conditions not established an effective drive system or the plane and inter layer interference. one is a thin layer with large distribution area, and the other is the area with poor physical properties in the side of the river. the remaining oil must be adjusted by the well network encryption and fracturing and other measures to improve the use of the situation (fig. 7). figure 7. gi11+12 comparison of permeability and residual oil saturation of the type of poor reservoir the relationship between residual oil and sedimentary microfacies. with good physical properties of the channel sand reservoir, in the case of injection production, the water channeling occurred along the high permeability channel in the injection-production well, the water flooded in the first place, the formation of high permeability channel, and its adjacent non-main channel sand, front sand and other physical properties of the reservoir with low oil saturation, low level of reserves, the formation of the remaining oil enrichment (fig. 8). figure 8. sii12 the contrast between sedimentary microfacies and remaining oil saturation a4 huabin wei, huanlai zhu, shangming shi, xinliang yang, jingjun zhang, jian wang and muhammad aqeel ashraf no water flooded type in the layer. this kind of remaining oil mainly exists in the thick oil layer. because of the good heterogeneity of the layers, good physical properties, high permeability of the site development effect is good, while the physical properties of poor, low permeability of the part because of the high permeability of the impact of the use of poor or non-use of the remaining oil, the use of the well network encryption and injection production system adjustment can be improved. the remaining oil is mainly the third times such polymer flooding oil potential object. 6. conclusion (1) based on the geological modeling and numerical simulation, it was stablished that the remaining oil potential in west block of the third district in north saertu study zone is large, and saertu oil reservoir remaining oil is mainly distributed in the area to the north and south; relatively few in the center. residual oil of putaohua reservoir is in the south and northeast area. gaotaizi reservoir remaining oil is concentrated in the southern half of area. (2) the remaining oil potential in west block of the third district in north saertu study zone is concentrated in the non-main parts below 1.0m, the residual oil potential distribution is not balanced, and the local area has the encryption adjustment condition. it needs to use the existing well pattern and make the injection production relationship, adjustment layer, water shutoff and so on. the final recovery rate can reach 49.8%. combined with water flooding fine exploration the recoverable reserves recovery rate could increase 1.1%. acknowledgments the authors would like to thank shi shangming for supporting the publication of this work and zhu huanlai for supporting the shale-gas reservoir modeling studies. the authors would also like to acknowledge yang xinliang, zhang jingjun and wang jian for his many contributions to shale-gas reservoir modeling that were instrumental in this work. this paper is supported by the state key laboratory breeding base co-constructed by unconventional oil and gas accumulation & development province department, and it is funded by the national natural science foundation of china (fund number: 41302210) and the research fund for young teachers of northeast petroleum university (fund number: 2013nq136). references chong, r.j., & yu, x.h. 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(2010). application of three-dimensional geological modeling and numerical simulation in exploitation of fractured water-gas reservoir. natural gas geoscience, 21(5), pp. 863-867. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 17 23 human health risk due to trace element contamination in groundwater from the anjani and jhiri river catchment area in northern maharashtra, india golekar r. b.1, patil s. n.2 and baride m. v.3 1department of applied geology, school of environmental and earth sciences, north maharashtra university jalgaon, 425001 (m.s.), india e-mail: rbgolekar@gmail.com 2department of applied geology, school of environmental and earth sciences, north maharashtra university jalgaon, 425001 (m.s.), india e-mail: sanp2@rediifmail.com 3department of geology, z.b. patil college, dhule 424002 (m.s.), india e-mail: baridemv@gmail.com corresponding author: m. v. baride (e-mail): baridemv@gmail.com hydrogeology the present study was aimed at assessing ground water chemistry, with reference to drinking water quality; the anjani and jhiri river catchment area in the jalgaon district, northern maharashtra, india, was selected. ten ground water samples were collected from different locations in two urban areas (daharangaon and erandol) and some samples from rural water supply wells during the pre-monsoon 2011 and post-monsoon 2011 seasons. trace elements were analysed from collected groundwater samples using standard techniques. the ground water pollution observed at some study area sites was mainly due to waste disposal and agricultural activities and vehicles burning fossil fuel. the present investigation was based on the occurrence of trace elements such as fe, zn, cu, ni, mn, pb and cd in groundwater samples, concentration ranging from 0 to 0.096 ppm, 0.066 to 0.427 ppm, 0 to 0.012, 0.026 to 0.361 ppm, 0 to 0.160 ppm, 0.968 to 1.516 ppm and 0 to 1.064 ppm, respectively. the analysis of ground water chemistry and results were compared to bis drinking water standards. the concentration of lead, cadmium and nickel exceeded the maximum permissible limit, as per bis drinking water standards. el presente estudio está enfocado en una evaluación química con referencia a los estándares de calidad del agua potable. para su ejecución se seleccionó la zona de captación de los ríos anjani y jhiri, en el distrito jalgaon, en maharashtra, al norte de la india. se recolectaron diez muestras de agua en diferentes locaciones de dos áreas urbanas (daharangaon y erandol) y algunas muestras en pozos de zonas rurales antes y después de la temporada monsónica de 2011. los elementos traza fueron analizados en las muestras con técnicas estandarizadas. la contaminación de las aguas subterráneas en algunas zonas de estudio se debe, principalmente, a la disposición de aguas residuales, a la actividad agrícola y al residuo generado por motores que trabajan con combustibles fósiles. la presente investigación se basó en la presencia de elementos traza como fe, zn, cu, ni, mn, pb y cd en los ejemplos hechos y que se concentran en rangos de 0 a 0.096 ppm; de 0.066 a 0.427 ppm; de 0 a 0.012 ppm; de 0.026 a 0.361 ppm; de 0 a 0.160 ppm; de 0.968 a 1.516 ppm, y de 0 a 1.064 ppm. el análisis químico de las aguas subterráneas y los resultados fueron comparados a los estándares de agua potable de bis. las concentraciones de cadmio y níquel, sobre todo, exceden el límite permitido por bis. key words: trace element, ground water, health hazard, anjani jhiri river catchment area, northern maharashtra, india. palabras clave: elementos traza, riesgos de salud, captación, ríos anjani y jhiri, maharashtra, india. record manuscript received: 24/10/2012 accepted for publication: 20/03/2013 abstract resumen 1. introduction ground water pollution is a major concern in urban areas in many countries around the world. the study area, the anjani and jhiri river catchment areas (tapm006), covered part of northern maharashtra, india. location map of study has shown in figure 1. this area’s drinking water supply depends upon the anjani dam but rainfall has been very scanty during the last few years in the study area, groundwater being the only remaining source of water; ground water level has declined in the study area due to overexploitation. some wells or bore holes’ ground water level figure 1: location map of the study area golekar r. b., patil s. n. and baride m. v. is deeper than 30 to 60 meters below the surface. another major current issue in the study area concerns ground water quality which has been badly affected by urban waste water and the excessive use of pesticides/fertiliser in agricultural areas. another source of ground water pollution may be heavy transport vehicles burning fossil fuel. the study area is geologically covered by deccan trap rocks from the cretaceous to the lower eocene age, consisting of amygdaloidal and vesicular basalts. some areas are covered by thick alluvium from the quaternary age along river channels. geological map of study area has shown in figure 2. hydro-geologically, the study area has two types of aquifer (shallower under unconfined and deeper under confined types). the study area’s ground water has been polluted due to the influence of waste water in groundwater and agricultural activities and transport burning fossil fuel. the results from of analysing ground water chemistry were compared to recommended guidelines as per bureau of indian standards (bis-2003). figure 2: geological map of the study area 2. materials and method the 10 ground water samples were collected during may 2011 and november 2011 for pre-monsoon and post-monsoon periods, respectively. water samples were collected from wells in clean one-litre polyethylene human health risk due to trace element contamination in groundwater from the anjani and jhiri river catchment area in northern maharashtra, india sample bottles. sampling locations are shown in table 1.water samples were analysed by adopting standard procedure (apha, 1998). trace elements viz. ni, mn, zn, cd, pb, cu and fe were analysed from ground water samples by atomic absorption spectrometer (thermo scientific) using acetylene gas and hollow cathode lamps. the data obtained from hydro-chemical analysis was compared to bis 2003 and who guidelines for drinking water standards. a global positioning system (gps, garmin make) was used for determining precise sampling point locations in the field and giving the exact latitude, longitude and altitude. iso-concentration/contour maps were prepared with the help of surfer 7.0 software to delineate spatial variation regarding trace elements from the study area’s groundwater samples. 3. result and discussion 3.1 trace elements trace elements primarily come from source rocks due to weathering and, in moderate contamination levels, are usually harmless to organisms; however, when their concentration increases they become considerably harmful to human and other living organisms. trace elements have increased in ground water during the last few years as it has become contaminated by human activities, like agriculture, fossil fuel burning and industrial effluent, etc. it is clear that trace elements are entering aquatic systems from either point or non-point sources (elder 1988). trace element concentration in ground water was thus determined for the anjani jhiri river catchment area, mainly consisting of urban and rural water supply wells. the results for trace elements are shown in tables 2 and 3; the recommended guidelines as per bureau of indian standards (bis-2003) are given in table 4. figures 3, 4, 5, 6, 7, 8 and 9 indicate spatial variation for fe, zn, cu, ni, mn, pb and cd, respectively, regarding the present study area. iron fe concentrations were found during the pre-monsoon period in 2011, ranging from bdl to 0.096 ppm (average 0.047) and the postmonsoon period 2011 from bdl to 0.068 ppm (average 0.026 ppm). a maximum permissible fe concentration of 1 mg/l for drinking purpose (bis 2003) suggested that the all the samples from the study area were excellent. zinc zinc concentration during the pre-monsoon period 2011 ranged 0.096 to 0.427 ppm (average 0.177 ppm) and ranged from 0.066 to 0.248 ppm post-monsoon period 2011 (average 0.158 ppm). zinc concentration in all samples of the study area within the permissible limit of (15 ppm bis-2003). the main source of zinc was fertilisers, pesticides and herbicides used for agricultural purposes. zinc is an essential element for human health and agriculture; it plays an important role in protein synthesis and carbohydrate metabolism (tylor and demayo, 1980). zinc is relatively non-toxic up to 25 ppm in drinking water (mc neely et al., 1979). copper cu ranged from 0.002 to 0.010 ppm (average 0.007 ppm) during the pre-monsoon period in 2011 and bdl to 0.012 during the post-monsoon period 2011 (average 0.005). cu concentration in all water samples collected from the study area came within the maximum permissible limit (1.5 ppm, bis 2003). cu is an essential element for human health. it has a toxic effect on human health if the concentration is below the desirable limit. anaemia, diarrhoea etc., can be caused due to cu deficiency (md. abdugolekar r. b., patil s. n. and baride m. v. llah khan and sarfaraz ahmad, 2010). overdoses of copper may also lead to neurological disorder, hypertension, liver and kidney dysfunction (larocque and rasmussen 1998; rao et al. 2001; krishna and govil, 2004). nickel ni concentration during pre-monsoon season 2011 ranged from 0.070 to 0.361 ppm (average 0.235 ppm) and ranged from 0.026 to 0.350 during the post-monsoon season 2011 (average 0.225 ppm). nickel concentration in most groundwater samples from the study area exceeded the permissible limit (0.07 ppm, bis 2003). a higher nickel concentration is harmful to human health as this may lead to lung cancer (mc neely et al., 1979). manganese mn concentration during the pre-monsoon 2011 season ranged from 0.003 ppm to 0.026 (average 0.010 ppm) and ranged from bdl to 0160 table 2.trace elements in ground water samples from the anjani and jhiri river catchment area, northern maharashtra, india sample id fe fe average zn zn average cu cu average season pre 2011 post 2011 pre 2011 post 2011 pre 2011 post 2011 1 0.0469 0.0362 0.0416 0.4269 0.1963 0.3116 0.0019 0.0012 0.0016 2 0.0634 0.0644 0.0639 0.1196 0.2034 0.1615 0.0096 0.0062 0.0079 3 0.0963 bdl 0.0963 0.1369 0.1481 0.1425 0.0062 0.0122 0.0092 4 0.0063 bdl 0.0063 0.0962 0.0882 0.0922 bdl 0.0012 0.0012 5 0.0696 0.0326 0.0511 0.1369 0.1228 0.1299 bdl bdl bdl 6 0.0596 0.0436 0.0516 0.1789 0.2202 0.1996 bdl bdl bdl 7 0.0096 0.0097 0.0097 0.1963 0.1723 0.1843 0.0063 bdl 0.0063 8 bdl bdl bdl 0.1196 0.1183 0.1190 0.0085 bdl 0.0085 9 bdl bdl bdl 0.0963 0.0658 0.0811 0.0065 0.0039 0.0052 10 0.0236 0.0682 0.0459 0.2636 0.2481 0.2559 0.0091 bdl 0.0091 maximum 0.0963 0.0682 0.0823 0.4269 0.2481 0.3375 0.0096 0.0122 0.0109 minimum bdl bdl bdl 0.0962 0.0658 0.0810 0.0019 bdl 0.0019 average 0.0469 0.0262 0.0366 0.1771 0.1584 0.1678 0.0069 0.0049 0.0059 all trace elements are expressed in ppm, where bdl = below the detected level ppm post-monsoon season 2011 (average 0.054 ppm). mn concentration in the study area was below the maximum permissible limit (0.5 ppm, bis 2003). manganese is an essential nutrient for humans; mn deficiency can disrupt the central nervous system and reproductive functions (mc neely et al., 1979). lead pb level during the pre-monsoon season 2011 varied from 0.006 to 0.968 ppm (average 0.227 ppm) and 0.018 to 1.516 during the postmonsoon season 2011 (average 0.719 ppm). 60% of water samples from the study area had a pb concentration above the maximum permissible limit (0.05 ppm, bis 2003); this suggested that the water was not suitable for drinking purposes in the study area. lead is naturally present in trace amounts in all biological materials, i.e. in soil, water, plants and animals. the main source of lead contamination was due to transport burning fossil fuel (smirjakova s et al., 2005). table 1: sampling locations from study area sample id latitude in decimals longitude in decimals masl (in meters) water level premonsoon 2011 water level postmonsoon 2011 1 20.98 75.28 232.6 14.6 10.6 2 20.86 75.36 216.9 6.5 4 3 20.91 75.32 212.6 9.5 7 4 20.9 75.33 211.8 * * 5 20.94 75.32 222 12.6 8.9 6 20.83 75.38 218.9 9.5 7.5 7 21.01 75.27 232.4 * * 8 21.01 75.26 220.6 10.5 6.5 9 21.01 75.28 224.6 12.9 7.9 10 20.99 75.29 238.2 11.6 8.5 (masl indicates metres above sea-level, * bore/tube well water) human health risk due to trace element contamination in groundwater from the anjani and jhiri river catchment area in northern maharashtra, india table 3.trace elements in ground water sample from anjani and jhiri river catchment area, northern maharashtra, india sample id ni ni average mn mn average pb pb average cd cd average season pre 2011 post 2011 pre 2011 post 2011 pre 2011 post 2011 pre 2011 post 2011 1 0.2966 0.0263 0.1615 0.026 0.16 0.093 0.846 0.637 0.7415 0.096 0.057 0.0765 2 0.1963 0.2605 0.2284 0.005 0.002 0.0035 0.09 0.367 0.2285 0.316 0.095 0.2055 3 0.1789 0.1787 0.1788 0.003 0.002 0.0025 0.036 0.018 0.027 0.096 0.042 0.069 4 0.3216 0.2916 0.3066 0.006 bdl 0.006 0.006 1.516 0.761 0.896 bdl 0.896 5 0.3606 0.3245 0.3426 0.006 bdl 0.006 0.089 0.958 0.5235 1.026 0.126 0.576 6 0.1789 0.1978 0.1884 0.004 bdl 0.004 0.079 0.56 0.3195 0.79 0.095 0.4425 7 0.0695 0.0618 0.0657 0.01 bdl 0.01 0.046 0.186 0.116 0.169 0.047 0.108 8 0.3165 0.3498 0.3332 0.016 bdl 0.016 0.01 0.964 0.487 0.963 bdl 0.963 9 0.1963 0.3370 0.2667 0.006 bdl 0.006 0.968 1.181 1.0745 1.064 0.1 0.582 10 0.2369 0.2174 0.2272 0.013 bdl 0.013 0.098 0.805 0.4515 0.959 0.107 0.533 maximum 0.3606 0.3498 0.3552 0.026 0.16 0.093 0.968 1.516 1.242 1.064 0.126 0.595 minimum 0.0695 0.0263 0.0479 0.003 bdl 0.003 0.006 0.018 0.012 0.096 bdl 0.096 average 0.2352 0.2245 0.2299 0.01 0.054 0.032 0.227 0.719 0.473 0.638 0.084 0.361 table 4.bis limit regarding drinking water standards (all values are expressed in ppm) trace element fe zn cu ni mn pb cd bis desirable limit 0.3 5 0.05 0.07 0.1 0.05 0.01 maximum permissible limit 1.0 15 1.5 no relaxation 0.3 no relaxation no relaxation cadmium cd in the pre-monsoon season 2011 varied from 0.096 to 1.064 ppm (average 0.638 ppm) and varied from bdl to 0.16 post-monsoon 2011 (average 0.084 ppm) in ground water samples from the study area. cd concentration in 10 (55%) water samples was above the maximum permissible limit (0.01 ppm, bis 2003); this suggested that the water was not suitable for drinking purposes in the study area. cd is found in very low concentrations in most rocks; other sources of cadmium in groundwater come from burning fossil fuels and applying fertiliser, etc. cd is found in basaltic rock (0.15 ppm) (krasukopf k and bird d, 1994), but is not essential for plants, animals and humans. long-term exposure (over years or decades) to cadmium in drinking water may be a cause of kidney damage (bis 2003) and can also lead to anaemia, cardiovascular disease and hypertension (mielke h. w et al., 1991; robards and worsfold, 1991). cadmium’s effect on the cardiovascular system can be explained by relating nutritional cadmium to hypertension (schroeder, 1965). seasonal variation of trace elements in ground water the percentage of average trace element concentration in ground water samples during the pre-monsoon and post-monsoon periods is given in tables 2 and 3. a change in trace element concentration was observed with a change in season. monsoon rainfall affected the concentration of most elements in different ways; the above table shows that average fe, mn, cd, ni, zn and cu concentrations in ground water samples during the post-monsoon season were lower by varying degrees due to the dilution effect. on the other hand, pb had a higher concentration in samples during the post-monsoon season. surface run-off from the agricultural land where chemical fertilisers, pesticides and herbicides, etc, were present also contributed to enriching trace element concentration. fe, mn, cd, ni, zn and cu show had lower concentrations in post-monsoon ground water samples. the dilution effect shown by these elements was due to the influx of a greater amount of rainwater due to monsoon rainfall. 4. conclusion ground water in the study area was polluted due to the influence of waste water from urban areas, excessive use of fertilisers and pesticide from agricultural areas and from transport burning fossil fuels. most ground water samples had pb, ni and cd concentrations exceeding the permissible limits recommended by bis (2003) for drinking purposes. trace element concentrations in ground water were found to decrease in the following sequence: cd >pb> zn > ni > fe > cu >mn. this study has thus shown that ground water in the study area was polluted by toxic and trace elements from samples collected during the pre-monsoon (may 2011) and post-monsoon seasons (december-2011). the results showed that the ground water was significantly contaminated by cd, pb and ni which might have led to various health problems. however, it has been assumed that the anomalous concentrations of nickel in the vicinity of highways and industry resulted from anthropogenic input. cd concentration in the study area was mainly due to its presence in basaltic rocks and pb concentration was due to transport burning fossil fuels and farmers applying fertilisers. acknowledgements financial assistance, in the form of a major research project (sr/s4/ es-328/2008), awarded to one of the authors (mvb) by the department of science and technology (dst) govt. of india, new delhi, is gratefully acknowledged. the co-operation of north maharashtra university officials is also acknowledged. the author would like to thanks unknown reviewers for their valuable comments and suggestions, which improved the quality of this manuscript. 5. references apha (1998) standard methods for the examination of water and waste water, 19th edition. apha, washington dc, usa bis (2003) drinking water standards (is: 10500 91 revised 2003) edmonds, w. m. (1971) hydro geochemistry of ground waters in the derbyshire with special reference to trace constituents reports 71/7, inst geol. sci. great britain elder (1988) metal biogeochemistry in surface water system a revise of principles and concepts united states geological survey circular 1013 pp 43 golekar r. b., patil s. n. and baride m. v. figure 7.mn spatial variation figure 9.cd spatial variation figure 8.pbspatial variation figure 3.fe spatial variation figure 5.cu spatial variatio figure 6. ni spatial variation figure 4. zn spatial variation krasukopf konrad and bird dennis k. (1994) textbook of introduction to geochemistry published by mcgraw hill inc 591 pp krishna, a. k., & govil, p. k. (2004). heavy metal contamination of soil around pali industrial area, rajasthan, india. environmental geology, 47, 38–44 larocque, a. c., & rasmussen, p. e. (1998). an overview of trace metals in the environment: mobilization to remediation. environmental geology, 33, 85–91 mcneely r n neimania v p and dwyer l (1979) water quality source book a guide to water quality parameter inland water directorate water quality branch ottawa canada pp 1-65 md. abdullah khan and sarfaraz ahmad (2010) trace metal concentration in the waters of rivers and ponds in valley field intermontane basin in the eastern himalayas journal of applied geochemistry volume. 12 no 2 pp 242-252 mielke, h. w., adams, j. l., chaney, r. l., mielke, p. w., and ravikumar, v. c. (1991). the pattern of cadmium in the environment of five minnesota cities. environmental geochemistry and health, 13, 29–34 rao, m. s., gopalkrishnan, r., and venkatesh, b. r. (2001) medical geology-an emerging field in environmental science. in national symposium on role of earth sciences (pp. 213–222). integrated and related social issues gsi special. pub. no. 65(ii). robards, k., and worsfold, p. (1991). cadmium: toxicology and analysis, a review. analyst, 116, 549–568 schroeder, h. a. (1965). cadmium as a factor in hypertension journal of chronic diseases, 18, 647–656 smirjakova and ondrasovicova o., kaskova a., lakticova k (2005) folia veternaria 49, 3 supplementum the effect of cadmium and lead pollution on human health pp 3132 human health risk due to trace element contamination in groundwater from the anjani and jhiri river catchment area in northern maharashtra, india alvarez et al.indd earth sci. res. j. vol. 10, no. 2 (december 2006): 117-129 attenuation of diffracted multiples with an apex-shifted tangentsquared radon transform in image space gabriel alvarez1, biondo biondi2, and antoine guitton3 (1) gabriel alvarez: stanford university, stanford, ca. usa. geophysics ph.d research assistant, stanford exploration project. gabriel@sep.stanford.edu (2) biondo biondi: stanford university, stanford, ca. usa. associate professor, stanford exploration project. biondo@sep.stanford.edu (3) antoine guitton: stanford university, stanford, ca. usa. geophysics ph.d research assistant, stanford exploration project. antoine@sep.stanford.edu abstract in this paper, we propose a method to attenuate diffracted multiples with an apex-shifted tangentsquared radon transform in angle domain common image gathers (adcig) . usually, where diffracted multiples are a problem, the wave field propagation is complex and the moveout of primaries and multiples in data space is irregular. the method handles the complexity of the wave field propagation by wave-equation migration provided that migration velocities are reasonably accurate. as a result, the moveout of the multiples is well behaved in the adcigs. for 2d data, the apex-shifted tangentsquared radon transform maps the 2d space image into a 3d space-cube model whose dimensions are depth, curvature and apex-shift distance. well-corrected primaries map to or near the zero curvature plane and specularly-reflected multiples map to or near the zero apex-shift plane. diffracted multiples map elsewhere in the cube according to their curvature and apex-shift distance. thus, specularly reflected as well as diffracted multiples can be attenuated simultaneously. this approach is illustrated with a segment of a 2d seismic line over a large salt body in the gulf of mexico. it is shown that ignoring the apex shift compromises the attenuation of the diffracted multiples, whereas the approach proposed attenuates both the specularlyreflected and the diffracted multiples without compromising the primaries. key words: diffracted multiples, adcig, gulf of mexico 117 earth sciences research journal manuscript received november 10 2006. accepted for publication december 20 2006. introduction surface-related multiple elimination (srme) uses the recorded seismic data to predict and iteratively subtract the multiple series (verschuur et al., 1992). 2d srme can deal with all kinds of 2d multiples, provided enough data are recorded given the offset limitations of the survey line. diffracted multiples from scatterers with a cross-line component cannot be predicted by 2d srme but in principle can be predicted by 3d srme as long as the acquisition is dense enough in both in-line and cross-line directions. with standard marine streamer acquisition, the sampling in the cross-line direction is too coarse and diffracted multiples need to be removed by other methods (hargreaves et al., 2003) or the data need to be interpolated and extrapolated to a dense, large aperture grid (van dedem and verschuur, 1998; nekut, 1998; biersteker, 2001). in general, multiples may not have their moveout apex at zero offset on a cmp gather. peg-leg multiples “split” into independent events when reflectors dip. these events look similar to diffracted multiples and may similarly hamper standard radon demultiple and velocity analysis. hargreaves et al. (2003) proposed a shifted hyperbola approach to attenuate split or diffracted multiples in cmp gathers. this approach, however, relies on the moveout of the multiples to be well approximated by hyperbolas in data space, which is problematic in complex media. a similar apex-shifted radon transform was proposed by trad (2002) for data interpolation. in most situations in which diffracted multiples are a serious problem, the wave propagation is rather complex (e.g., multiples diffracted off the edge of salt bodies). thus, the moveout of primaries and multiples tend to be very complex, making the application of data-space moveoutbased methods for the removal of multiples difficult. in adcigs, however, since the complexity of the wave field has already been taken into account by pre-stack migration (to the extent that the presence of the multiples allows a pretty accurate estimation of the migration velocity field), the residual moveout of multiples is generally smoother and better behaved (sava and guitton, 2003). this paper focuses on attenuating diffracted multiples in adcigs by redefining the tangentsquared radon transform of biondi and symes (2003) to add an extra dimension to account for the shift in the apexes of the moveout curves of the diffracted multiples. it is shown with a 2d seismic line from the gulf of mexico that the approach is effective in attenuating both, the specularly-reflected and the diffracted multiples, and ignoring the apex shift results in poor attenuation of the diffracted multiples. the real impact of the method for attenuating diffracted multiples is likely to be in 3d rather than in 2d, though the results that are shown in this paper are limited to 2d. biondi and tisserant (2003) have presented a method for computing 3d adcigs from full 3d pre-stack migration. these 3d adcigs are functions of both the aperture angle and the reflection azimuth. simple ray tracing modeling shows that outof-plane multiples map into events with shifted apexes such as 2d diffracted multiples, and different reflection azimuth than the primaries. attenuation of these multiples from 3d adcigs can be fully accomplished with the methodology presented in this paper. diffracted multiples on adcigs figure 1 shows a stack over aperture-angle of a wave-equation migrated 2d line from the gulf of mexico over a large salt body. the presence of the salt creates a host of multiples that obscure any genuine sub-salt reflection. most multiples are surface-related peg-legs with a leg related to the water bottom, shallow reflectors or the top of salt. below the edges of the salt are found diffracted multiples (e.g., cmp position 6000 m below 4000 m depth in figure 1). figure 2 shows four adcigs obtained with wave-equation migration as described by sava and fomel (2003). the top two adcigs correspond to lateral positions directly below the left edge of the salt body (cmp positions 6744 m and 22056 m in figure 1). notice how the apexes of the diffracted multiples are shifted away from the zero aperture angle (e.g., the “seagull”-looking event at about 4600 m in panel (a)). for comparison, attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space 118 the bottom panels in figure 2 show two adcigs that do not have diffracted multiples. figure 2c corresponds to a adcig below the sedimentary section (cmp 3040 m in figure 1) and figure 2d to a adcig below the salt body (cmp position 12000 m in figure 1). in these adcigs all the multiples are specularly-reflected and thus have their apexes at zero-aperture angle. although the data are marine, the adcigs show positive and negative aperture angles. reciprocity was used to simulate negative offsets and interpolation to compute the two shortest-offset traces not present in the original data. the offset gathers were then converted to angle gathers. the purpose of having both positive and negative aperture angles is to see more clearly the position of the apexes of the diffracted multiples. apex-shifted radon transform in order to account for the apex-shift of the diffracted multiples (h), it is defined the forward and ad-joint radon transforms as a modified version of the “tangent-squared” radon transform introduced by biondi and symes (2003). the transformation is defined from data space (adcigs) to model space (radontransformed domain) as: and from model space to data space as where z is depth in the data space, is the aperture angle, z´ is the depth in the model space, q is the moveout curvature and h is the lateral apex shift. in this way, the two dimensional data space of adcigs, , is transformed into a three-dimensional model space, m(z´,q,h). in an ideal case, primaries would be perfectly horizontal in the adcigs and would thus map in the model space to the zero-curvature (q = 0) plane (e.g., a plane of dimensions depth and apex-shift distance (h,z´)). specularly-reflected multiples would map to the zero apex-shift distance (h = 0) plane, e.g., a plane of dimensions depth and curvature (q,z´). diffracted multiples would map elsewhere in the cube depending on their curvature and apex-shift distance. alvarez et al., esrj vol. 10, no. 2. december 2006 figure. 1. angle stack of migrated adcigs of 2d seismic line in the gulf of mexico. notice that multiples below the salt obscure any primary reflections. 119 figure. 2. angle domain common image gathers. (a) under the left edge of the salt, cmp at 6744 m; (b) under the right edge of the salt, cmp at 22056 m; (c) below the sedimentary section, cmp at 3040; (d) below the salt body, cmp at 12000 m. 120 attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space sparsity constraint as a linear transformation, the apex-shifted radon transform can be represented simply as (1) where d is the image in the angle domain, m is the image in the radon domain and l is the forward apex-shifted radon transform operator. to find the model m that best fits the data in a least-squares sense, the objective function is minimized as: (2) the second term in the equation is a cauchy regularization that enforces sparseness in the model space. here n is the size of the model space, and and b are two constants chosen a-priori: which controls the amount of sparseness in the model space and b related to the minimum value below which everything in the radon domain should be zeroed (sava and guitton, 2003). the least-squares inverse of m is (3) in equation (3) diag defines a diagonal operator. because the model space can be large, m was estimated iteratively. notice that the objective function in equation (2) is non-linear because the model appears in the definition of the regularization term. therefore, a limited-memory quasi-newton method (guitton and symes, 2003) is used to find the minimum of f (m). a look at the 3d radon domain in this section the mapping of the different events between the image space and the 3d radon space (z´,q,h) will be illustrated using the adcig at cmp 6744 m (figure 2a). this adcig has no discernible primaries below the salt, but nicely shows the apex-shifted moveout of the diffracted multiples. figure 3a shows the h = 0 plane from the 3d volume. this plane corresponds to the zero apexshift and is therefore similar to the standard 2d radon transform. primaries are mapped near the figure. 3. radon transforms of the adcig in figure 2b. (a): 2d transform. (b): h d 0 plane of the apex-shifted 3d transform. 121 alvarez et al., esrj vol. 10, no. 2. december 2006 q = 0 line, and specularly-reflected multiples are mapped to other q values. diffracted multiples, since their moveout apex is not zero, are not mapped to this plane and do not obscure the mapping of the specularly-reflected multiples. for comparison, figure 3b shows the 2d radon transform, plotted at the same clip value as figure 3a. notice how the diffracted-multiple energy is mapped as background noise, especially at the largest positive and negative q values. notice also that the primary energy is higher than in figure 3a since with the 3d transform the primary energy is mapped not only to the h = 0 plane but to others h planes as well. this is further illustrated on figure 4, which shows the zerocurvature (q = 0) plane for the same adcig. the nearly flat primaries have zero curvature for all values of the apex-shift distance h, so in this plane they appear as horizontal lines. neither the specularly-reflected nor the diffracted multiples map to this plane. attenuation of diffracted and specularly-reflected multiples with ideal data, attenuating both specularlyreflected and diffracted multiples could be, in principle, accomplished simply by zeroing out (with a suitable taper) all the q-planes except the one corresponding to q = 0 in the cube model m(z´,q,h), and taking the inverse apex-shifted radon transform. in practice, however, the primaries may not be well-corrected and primary energy may map to a few other q-planes. energy from the multiples may also map to those planes and so we have the usual trade-off of primary preservation versus multiple attenuation. the advantage is that the diffracted multiples are well focused to their corresponding h-planes and can be easily attenuated. rather than suppressing the multiples in the domain model, we chose to suppress the primaries and inverse transform the multiples to the data space. the primaries were then recovered by subtracting the multiples from the data. figure 5 shows a close-up comparison of the primaries extracted with the standard 2d transform (sava and guitton, 2003) and with the apex-shifted radon transform for the two adcigs at the top in figure 5. the standard transform (figures 5a and 5c) was effective in attenuating the specularly-reflected multiples, but failed at attenuating the diffracted multiples (below 4000 m), which are left as residual multiple energy in the primary data. again, this is a consequence of the apex shift of these multiples. there appears not to be any sub-salt primary in figures 5a and 5b, and only one clearly visible sub-salt primary in figures 5c and 5d (just above 4400 m). this primary was well preserved with both transformations. figure 6 shows a similar comparison for the extracted multiples. notice how the diffracted multiples were correctly identified and extracted by the 3d radon transform (figure figure. 4. zero curvature plane from 3d radon transform of adcig in figure 2b. flat primaries are mapped to the zero curvature line for all h values. 122 attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space figure. 5. comparison of primaries extracted with the 2d radon transform (a) and (c), and with the apexshifted radon transform (b) and (d). notice that some of the diffracted multiples remain in the result with the 2d transform. 123 alvarez et al., esrj vol. 10, no. 2. december 2006 figure. 6. comparison of multiples extracted with the 2d radon transform (a) and (c) and with the apex-shifted radon transform (b) and (d). 124 attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space 6b). in contrast, the standard 2d transform misrepresent the diffracted multiples as though they are specularly-reflected multiples as seen in figure 6a. the 3d model representation could be successfully used to separate the diffracted multiples from the specularly-reflected ones (figure 7). the diffracted multiples are clearly seen in figure 6c. in order to assess the effect of better attenuation of the diffracted multiples on the angle stack of the adcigs, a total of 310 adcigs corresponding to horizontal positions 3000 m to 11000 m were processed (figure 8). figure 9 shows a closeup view of the stack of the primaries extracted with the 2d radon transform, the stack of the primaries extracted with the 3d radon transform and their difference. notice that the diffracted multiple energy below the edge of the salt (5000 m to 7000 m) appearing as highly dipping events with the 2d transform has been attenuated with the 3d transform. this is shown in detail in figure 9c. it is very difficult to identify any primary reflections below the edge of the salt, so it is hard to assess if the primaries have been equally preserved with both methods. it is known, however, that for this dataset, there are no multiples above a depth of about 3600 m, between 3000 m to 5000 m cmp positions. the fact that the difference panel appears nearly white in that zone shows that the attenuation of the diffracted multiples did not affect the primaries. of course, this is only true for those primaries that were correctly imaged, so that their moveout in the adcigs was nearly flat. weak sub-salt primaries may not have been well-imaged due to inaccuracies in the migration velocity field and may therefore have been attenuated with both the 2d and the 3d radon transform. figure 9 shows the extracted multiples with the 2d and the 3d radon transform and their difference. again, the main difference is largely in the diffracted multiples. discussion the results shown in the previous section demonstrate that with the 3d radon transform it is possible to attenuate, although not completely remove, the diffracted multiples. it should be noted, however, that in this seismic section it is very difficult to find a legitimate primary reflection below the salt and in particular below figure. 7. comparison of (a) diffracted and (b) specularly-reflected multiples. notice the lateral shifts in the apexes of the diffracted multiples. 125 alvarez et al., esrj vol. 10, no. 2. december 2006 figure. 8. comparison of angle stacks for primaries. 126 attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space figure. 9. comparison of angle stacks for multiples. 127 alvarez et al., esrj vol. 10, no. 2. december 2006 the edge of the salt, where the contamination by the diffracted multiples is stronger. it is somewhat disappointing that the attenuation of the diffracted multiples did not help uncovering any meaningful primary reflections in this case. it is expect that this situation will be different with other datasets. as stated earlier, working with space images (adcigs in this case) is convenient because the migration takes care of the complexity of the wave field propagation, but attenuating the multiples after migration does not come out without a price. the estimation of the migration velocities may be more difficult and less accurate because of the presence of the multiples. there is an inherent trade-off when choosing to work in space-image. good migration velocities for weak sub-salt primaries may be particularly difficult to estimate in the presence of the multiples. on the other hand, the parabolic or hyperbolic assumption for the moveout of the multiples in data space may not be appropriate at all in complex media. an alternative could be doing a standard radon demultiple before prestack migration to facilitate the choice of the migration velocities, and 3d radon demultiple on the adcigs to attenuate residual multiples, in particular diffracted multiples. it is also emphasized that adding the extra dimension to deal with the diffracted multiples does not resolve by itself the usual problem that non-flat primaries may map to the multiple region, and primary preservation is trade-off for multiple attenuation. this limitation forced to let some residual multiple energy leak into the extracted primaries. obviously, the flatter the primaries in the adcigs, the better are the chances to reduce the residual multiple energy. conclusion the combination of choosing the space image in the form of adcigs and the apex-shifted tangent-squared transformation from to (z´,q,h) has proven to be effective in attenuating specularly-reflected and diffracted multiples in 2d marine data. the residual moveout of both multiples in adcigs is well-behaved and the extra dimension provided by the apex-shift allows the attenuation of the multiples without compromising the integrity of the primaries. acknowledgments we thank the sponsors of the stanford exploration project for their support to carry out this study and westerngeco for providing the dataset. references biersteker, j. (2001). magic: shell’s surface multiple attenuation technique. in: ‘71st ann. internat. mtg soc. of expl’. geophys. 1301– 1304. biondi, b. and symes, w. (2003). angle-domain common-image gathers for migration velocity analysis by wavefield-continuation imaging. sep–113. 177–210. guitton, a. and symes, w. (2003). robust inversion of seismic data using the huber norm. geophysics. 68 (4).1310–1319. hargreaves, n., ver west, b., wombell, r., and trad, d. (2003). multiple attenuation using an apex-shifted radon transform. 73rd ann. internat. mtg., soc. of exp. geophys., expanded abstracts. 1929–1932. nekut, a., (1998). 3d surface-related multiple elimination. 68th ann. internat. mtg., soc. of exp. geophys., expanded abstracts. 1511– 1514. sava, p. and fomel, s. (2003). angle-domain common-image gathers by wavefield continuation methods. geophysics. 68 (3). 1065–1074. sava, p. and guitton, a., (2003). multiple attenuation in the image space. sep–113. 31– 44. tisserant, t. and biondi, b. (2003). kinematics of 3-d angle-domain common-image gathers for migration velocity analysis. sep–114. 71–82. 128 attenuation of diffracted multiples with an apex-shifted tangent-squared radon transform in image space trad, d. (2002). interpolation with migration operators. 72nd ann. internat. mtg., soc. of exp. geophys., expanded abstracts, 2174–2177. van dedem, e. and verschuur, d., 1998, 3d surface-related multiple elimination and interpolation: 68th ann. internat. mtg., soc. of exp. geophys., expanded abstracts, 1321– 1324. verschuur, d., berkhout, a., and wapenaar, c., (1992). adaptive surface-related multiple elimination. geophysics. 57 (9). 1166–1167. 129 alvarez et al., esrj vol. 10, no. 2. december 2006 archivo04.indd 35 manuscript received april 6, 2006. oil decontamination of bottom sediments experimental work results sergey v. lushnikov1, yulia a. frank1 and danil s. vorobyov2 1 research and technical association “priborservice” ltd., komsomolsky av., 70, tomsk, russia, 634041 2 tomsk state university, lenina av., 36, tomsk, russia, 634050 corresponding author: sergey v. lushnikov, e-mail: pribor@mail.tomsknet.ru abstract this article presents the results of experimental work during 2004-2005 on oil decontamination of bottom sediments of lake schuchye, situated in the komi republic (northern russia). the cause of the contamination were huge oil spills occurred after a series of accidental ruptures on the harjaga-usinsk and vozej-usinsk oil-pipe lines in 1994. flotation technology was used for the cleaning of bottom sediments. 157 tons of crude oil were removed during the course of 2-year experimental work from an area of 4,1 ha. the content of aliphatic and alicyclic oil hydrocarbons was reduced from 53,3 g/kg to 2,2 g/kg, on average. hydrobiological investigations revealed that bottom sediments started to be inhabited by benthos organisms, dominantly oligochaeta. besides oligochaeta, chironomidae maggots and bivalvia were detected. the appearance of macrozoobenthos organisms can serve as a bioindicator of water quality. key words: bioindication, bottom sediments, macrozoobenthos, oil contamination and decontamination of water ecosystems. resumen este artículo presenta los resultados del trabajo experimental realizado entre 2004-2005 en la descontaminación de petróleo de los sedimentos de fondo del lago schuchye en la república de komi (norte de rusia). la causa de la contaminación del lago fueron enormes derrames de petróleo ocurridos después de una serie de ruptures accidentales en los oleoductos de harjaga-usinsk y vosey-usink en 1994. la tecnología de flotación fue usada para limpiar los sedimentos de fondo. en el transcurso de los dos años de trabajo experimental, fueron removidas 157 toneladas de petróleo crudo en un area de 4,1 ha. el contenido de hidrocarburos alifáticos y alicíclicos se redujo en promedio de 53,3 g/kg a 2,2 g/kg. las investigaciones hidrobiológicas mostraron que los sedimentos de fondo empezaron a ser habitados por organismos bentónicos, predominantemente por oligohetas (oligohaeta). también fueron detectadas larvas de chironomides (chironomidae), y bivalvos (bivalvia). la aparición de organismos macrozoobenthos puede servir como un bioindicator de la calidad del agua. palabras claves: sedimentos de fondo, contaminación y descontaminación petrolífera de ecosistemas acuáticos, bioindicación, macrozoobenthos. earth sciences research journal earth sci. res. j. vol. 10, no. 1 (jun. 2006): 35-40 36 sergey v. lushnikov, yulia a. frank and danil s. vorobyov introduction professional ecologists are fully aware of the harmful impact the development of oil fields has on the environment. the monitoring research of these technogenic territories almost always reveals a negative effect on environment (laffon, et al., 2006; paasvirta, et al., 1982). serious accidents lead to irreparable damage to natural ecosystems, and mitigation of effects of pollution may last for decades (laffon, et al., 2006). in regions with lakes and rivers the aquatic ecosystems are heavily impacted by oil spills. oil and oil products are most intensively washed down into the river network during the spring tide when runoff is formed at all geomorphologic levels and in summer at the heaviest rain period (kondratyeva, 2000; zhadin & gerd, 1961). liquidation of oil spills on a water surface is carried out by booming, by means of different types of skimmers, dispersants and sorbents. additionally bacterial isolates are also used for such purposes (gentili, et al., 2006; mukherji, et al., 2004; reed, et al., 2004). if measures for localization and gathering the pollutant are not taken at the proper time, it may lead to oil and oil products sorption by suspended organic and mineral particles and aquatic flora, and in the long run to their sinking to the bottom of the reservoir, which produces a negative effect on bottom organisms associations causing their total degradation (ke, et al., 2005; reed, et al., 2004). oxygen deficiency, low photo-oxidation indices, low water temperature prevents the destruction of the pollutant. under anaerobic conditions, organic substances undergo fermentation but not oxidation, and sulfate reduction and methane formation take place (kondratyeva, 2000). at the same time toxic substances accumulate in water (petrov, 1978; vinogradov, 2002). at choosing technological schemes for cleaning of bottom sediments in lake schuchye several variants were taken into consideration. the first one is the use of pumping units for removing contaminated soil. pumping-out of bottom sediments contaminated with oil requires adding some technological stages for cleaning the contaminated soil which considerably complicates the decontamination work and increases its cost. in lake schuchye, the oil accumulated on the surface of bottom sediments in the form of layers with aggregates of different sizes. on the off thicknesses upper layer of bottom sediments where the “soil-oil” contact was contaminated. there is no doubt that if oil was removed from the upper contaminated layer, a large amount of pollutionless sediments would be pumped out, acting as a sorbent for oil while moving along the pipeline. removing the sediments layer with a thickness of 10 cm (of lake schuchye, with an area of 6,26 ha.) would result in more than 6000 m3 of contaminated soil, which should undergo further decontamination. therefore, application of this technological scheme was considered inexpedient. the second variant is more acceptable: hydropneumatic decontamination of bottom sediments, based on the ability of molecular adhesion of oil to the phase at the interface between air and water (flotation). the given scheme allows removing the oil from the surface of bottom sediments. determination of the necessary decontamination level of bottom sediments of the reservoir, after which mechanisms of natural self-purification start actively working, is very important for a wide application of flotation technology to oil decontamination of water bodies. methodology lake schuchye no.1 (figure 1), where experimental work on oil decontamination of bottom sediments was carried out, is a thermokarstic slowflow lake with an area of 6,26 ha. and a maximum depth of 7 m. it is a part of a system of 4 lakes (figure 1) and situated in the komi republic (northern russia). monthly, samples of water and bottom sediments were taken at monitoring stations and samples of bottom invertebrates were taken every ten days. surface water sampling was carried out according to the general technical conditions, (2002) (gost 17.1.5.04-81) from the depth of 0,4 m up. bottom sediments sampling was carried out by means of a petersen bottom sampler with a grip area of 1/80 m2. after lifting of the bottom sampler containing bottom sediments, water was poured out; bottom sediments were put into a polyethylene bag and labeled. for collection of zoobenthos samples, the petersen bottom sampler was also used. to eliminate probable errors resulting from hydrobiontes distribution in homogeneity each zoobenthos sample consisted of a set of three or four bottom sediments samples taken separately. 37 oil decontamination of bottom sediments experimental work results figure 1. map of russia, (top) showing the location of the komi republic and a map of the schuchye lake system (lakes no. 1-4). the lake 4, (1 ha.) by means of a flotation technology. in 2005 the equipment was improved, which made possible to reduce the oil content in bottom sediments and during the summer period of 2005, 92 tons of crude oil were extracted. during 2 seasons of cleaning work in 2004-2005, only the hydropneumatic cleaning scheme was used. the content of aliphatic and alicyclic oil hydrocarbons was reduced from 53,3±7,46 g/kg to 2,2±0,39 g/kg on average (figure 2a). reduction of oil content in the lake water mass during cleaning operations was not so significant (figure 2b). hydrobiological research carried out in the lake in 2005 showed that after two years of integrated decontamination work in the lake zones where the content of oil in soil did not exceed 3,3 g/kg, bottom sediments contained benthonic organisms. three groups of macrozoobenthos were found: there types of oligochaeta, chironomidae maggots, and bivalvia. the number of benthos came up to 2320 specimens per m2, and measured biomass was as high as 3,940 g/m2. oligochaeta were the basic benthos because of their quantity (90-100%) and biomass (80-100%) in lake schuchye. the samples were washed in a rinse bag made of fine gauze no. 28 to remove fine fractions of silt. residual contents were examined with the help of a magnifying glass, and zoobenthos organisms were taken out with tweezers, placed into glass containers with 70% ethyl alcohol and labeled (morduhaiboltovsky, 1975). oil contents in the water mass and bottom sediments were determined by means of an infrared spectrophotometrical method. the method allows the determination of aliphatic and alicyclic oil carbohydrates and is based on the extraction of oil by ccl 4, chromatographic fractionation with aluminum oxide and on quantitative determination in the infrared region using the «specord m 80». oil decontamination of bottom sediments and water was carried out according to the technology developed and patented by the authors of this paper (lushnikov & vorobyov, 2005). results in 2004, as much as 65 tons of oil were extracted from bottom sediments of the experimental zone of figure 2. oil content in bottom sediments (a) and in water mass (b) before remediation (june 2004) and after remediation (june 2005 and september 2005). ( ) ( ) 38 sergey v. lushnikov, yulia a. frank and danil s. vorobyov discussion in water reservoirs and waterways oil spreads mostly under the influence of water currents and wind. the currents of water are the basic factor of oil spread in waterways. relative proximity of the waterside and a winding river-bed allows oil to reach waterways quickly. the best conditions for oil accumulation exist in backwaters, creeks, and sections of rivers where there is no current. in these sections the content of oil and oil products in bottom sediments is high in comparison with that of riverbeds. sedimentation of contaminated suspended particles and their accumulation in soil exert a negative influence on bottom associations of organisms (rusanova & vorobyov, 2001). in lakes oil spreads on the water surface under the influence of wind. irregularity of the waterside leads to formation of zones with conditions favorable for oil accumulation. removing such oil patches presents no difficulties if special oil-gathering equipment is used, although the solution to this problem is versatile. at first, it is necessary to locate the oil patch. furthermore, the decontamination technology depends on availability of access roads to the reservoir and the need for transporting localized oil on water surface; in the case of large reservoirs the use of specialized oil-spill boats is optimal. gathering oil from the water surface and cleaning the waterside of a reservoir is complicated by the presence of higher aquatic vegetation. to a greater extent this refers to water-resistant plants which can be found in the littoral zone, on floating bogs, on swampy and highly moist shores, and to aquatic plants with aerial generative organs. after water plants die, oil kept by the vegetation on the surface of water accumulates in the bottom sediments of the waterside where its destruction slows down. therefore, contaminated vegetation is removed from the reservoir along with oil. figure 3 shows a part of the shore of lake schuchye shore with oil contaminated aquatic vegetation before decontamination and with recovered phytocenosis after removing oil and parts of the plants rising to the water surface. to avoid secondary pollution of decontaminated areas near the waterside it is necessary to use barriers to prevent further spread of oil. as a rule, secondary pollution occurs, in reservoirs where bottom sediments are highly contaminated with oil. in summer, when the temperature in the bottom layers of water rises, in shallow zones there occurs spontaneous raise of oval and globular-shaped oil aggregates to the surface. this phenomenon took place in the zones of the lake with a depth up to 2 m. in 2004 in the experimental zone of lake schuchye more than 12 tons of oil rose to the water surface. figure 3. the waterside of schuchye lake no.1 before (top) and after decontamination (bottom) (photo by d.s.vorobyov). fertilizer application, the use of aerators in the water area of the reservoir, and optimal temperature conditions in july 2004, the temperature in the bottom layers of water at a depth of 5 m reached 19-20 oc, were favorable to the intensive raise of oil. to reduce the content of oil absorbed by sediments during the second step of cleaning operations we activated microbiological destruction processes. the content of nutrients and oxygen in water is the key factor in the processes of microbiological decomposition, thus, when carrying out work in the lake we used aerators, set up active booms on the surface of water, and applied dosified amounts of fertilizers. the flotation technology applied allowed us to remove oil (in the form of a continuous layer and aggregates) from the surface of bottom sediments of the lake. during the cleaning up work period in 39 oil decontamination of bottom sediments experimental work results lake schuchye in 2004-2005 as much as 157 tons of oil were extracted from the bottom sediments. however, a large amount of oil is kept in the sediments due to the high sorption capacity of fine soils. it is well known that there is a connection between the granulometric composition of bottom sediments and the amount of oil absorbed by them – fine sediments possess higher sorption surface, hence they can retain more oil; 1 gram of bottom sediments under optimal conditions absorbs up to 12-25 mg of oil products (zyulkina, 1975), (i.e. the concentration of this pollutant can run up to 25 g/kg). hydrobiological research carried out in the lake in 2005 showed that after two years of integrated decontamination work in the lake zones where the content of oil in soil did not exceed 3,3 g/kg, bottom sediments contained benthonic organisms (in 20032004 benthonic organisms were not found there). the number of benthos measure up to 2320 specimens per m2, and the biomass was as much as 3,940 g/m2. many characteristics of zoobenthos (species diversity, quantity, biomass etc.) depend on the physical properties of the soil and sediments quality, and the amount of easily assimilated organic substance present in it transformed by bacteria. (vinogradov, et al., 2002) noted that the quality criteria of bottom sediments should involve parameters such as the level of quantitative development of zoobenthos. the three groups of macrozoobenthos in the lake schuchye correspond to a benthos group which can endure oil pollution best of all. oligochaeta feed from organic substances in sediments and play an important role in their formation. during a 24-hour period, a worm digests an amount of detritus which weighs 4 times as much as the worm itself (zhadin & gerd, 1961).). experiments carried out on another group of hydrobiontes, polychaeta, showed that aromatic structures of organic substances of excrements acquired a different character from that in initial forms. these hydrobiontes transformed oil products, namely, tailing (georga-kopulus & alemov, 1990).). oligochaeta also take part in the process of transformation of oil-contaminated soil. and although they do not play the main role in the self-purification processes (as compared with microbial associations), the whole activity of the complex biosystem of a reservoir provides a rapid recovery. conclusions the flotation technology allowed effective oil decontamination of the bottom sediments of schuchye lake situated in komi republic (northern part of russia). during the period of experimental cleaning work (2004-2005) a total of 157 tons of crude oil were removed. a significant amount of oil was kept by sediments because of the high sorptive capacity of fine soils. it was shown that the aliphatic and alicyclic oil hydrocarbons content in bottom sediments reduced in the course of the experiment from 53, 3±7, 46 g/kg to 2, 2±0, 39 g/kg. after a 2-year integrated decontamination work in the lake zones where the content of oil in soil did not exceed 3,3 g/kg bottom, sediments started to be inhabited by benthonic organisms. three groups of macrozoobenthos have been found oligochaeta, chironomidae maggots and bivalvia. oligochaeta were the basic benthos according to their quantity (90-100%) and biomass (80-100%). the total number of benthos came up to 2320 specimen per m2, and the biomass totaled 3,940 g/m2. the fact that macrobenthos organisms appear in the bottom sediments of lake schuchye is a strong bioindicative proof of an improving in the habitat quality of bottom organisms associations. currently we initiated the task of working out integral criteria for a quantitative assessment of recovery in water bodies using hydrochemical and hydrobiological tools. to solve this problem, monitoring research of the lake and research of other water bodies undergoing decontamination in different climatic zones will be continued. acknowledgements the project was put into practice by the research and technical association “priborservice” ltd. supported by “lukoil-komi” ltd., federal state department “komirybvod”, the department of water industry of komi republic and usinsk regional environmental protection committee. we also thank coworkers from department of ichthyology and hydrobiology of tomsk state university. references • gentili a.r., m.a., cubitto, m. ferrero and m.s. rodríguez (2006). bioremediation of crude oil polluted seawater by a hydrocarbon-degrading bacterial strain immobilized on chitin and chitosan flakes, international 40 sergey v. lushnikov, yulia a. frank and danil s. vorobyov biodeterioration and biodegradation. 57, no. 4, 222228. • georga-kopulus l.a., and s.v. alemov (1990). the role of nereis sp. in transforming oil tars in marine bottom sediments, hydrobiological journal. 26, no. 2, 60-64. 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(1975). the technique of studying ecosystems of inland water reservoirs, “nauka” publishing house, moscow, 240 pp. (in russian). • vinogradov g.a., n.a. berezina, n.a. lapteva, and g.p. zharikov (2002). the use of structural characteristics of bacterioand zoobenthos for quality rating of bottom sediments (by the example of reservoirs of the upper volga basin), water source. 29, no. 3, 329-336 (in russian). • zhadin v.i., and c.v. gerd (1961). rivers, lakes and storage pools of the ussr: their fauna and flora, publishing house of the ministry of education of rsfsr, moscow, 600 pp. (in russian). • zyulkina v.y. (1975). the importance of suspended sediments and bottom sediments in self-purification of the river selenga from oil products, hydrobiological journal. 3, 63-65 (in russian). v16n1c.indb earth sciences research journal magnetic exploration methods earth sci res sj vol 16, no 1 (june, 2012): 25 33 introduction the magnetic method is one of the best geophysical techniques used for determining depth to magnetic source bodies (and possibly sediment thickness) and delineating subsurface structures large-scale aeromagnetic surveys have been used to locate faults, shear zones and fractures such zones may serve as potential hosts for a variety of minerals and may be used as guidance for exploration of the epigenetic, stress-related mineralisation in the surrounding rocks (paterson and reeves, 1985) sediment thickness required for hydrocarbons (oil and/gas) to form or be generated analysis of aeromagnetic data over wamba and its adjoining areas in north-central nigeria g k anudu1, 5*, l n onuba2, 5, a g onwuemesi3 and a e ikpokonte4 1 department of geology and mining, nasarawa state university, pmb 1022 keffi, nasarawa state nigeria 2 department of geology, anambra state university, pmb 01uli, anambra state nigeria 3 department of geological sciences, nnamdi azikiwe university, awka anambra state nigeria 4 department of geology, ahmadu bello university, zaria, kaduna state nigeria 5 department of geology and petroleum geology, university of aberdeen ab24 3ue, united kingdom * corresponding author e-mail: anudugoodluck@yahoo co uk; goodluck anudu@abdn ac uk abstract an aeromagnetic intensity contour map of wamba was acquired, digitised and analysed the study was aimed at mapping metalliferous mineral deposits in basement rocks, delineating structural lineaments and their trends as well as estimating the depth to magnetic source bodies regionalresidual separation was applied to a total aeromagnetic intensity contour map to produce a residual aeromagnetic intensity contour map the rose diagram of the orientations or azimuth direction of the structural lineaments delineated on the maps depicted major trends being ne-sw, nw-se and nnessw (pan-african trends) with minor nnw-sse and e-w trends (pre-pan-african trends) depths to the magnetic source bodies were estimated using peter’s (half-slope) method, indicating two depth source models the deeper sources ranged from 0 88km to 3 15km and had -32nt to -225nt magnetic intensity, whereas the shallower sources ranged from 0 23km to 0 76km and had -10nt to 20nt magnetic intensity the deeper sources were most likely due to intra-basement basic intrusive bodies underlying the sediment-covered marginal areas of the benue trough the sedimentary cover in the area was generally low and therefore not likely to favour hydrocarbon generation resumen un mapa del contorno de la intensidad aeromagnética de wamba fue adquirido, digitalizado y analizado el estudio tuvo como objetivo mapear los depósitos minerales metalíferos  en  rocas basamentales, delinear las configuraciones  estructurales y sus tendencias, así como estimar la profundidad a los cuerpos de la fuente magnética la separación  residual  regional  fue aplicada a un mapa del contorno de intensidad aeromagnética total para producir un mapa del contorno intensidad aeromagnética residual el diagrama rosa de las orientaciones o la dirección del azimut de los lineamientos estructurales delineados sobre los mapas represento las principales tendencias come ne-sw, nw-se y nne-ssw (tendencias panafricanas), con menor tendencias nnw-sse y ew (tendencias pre-panafricanas) las profundidades a los cuerpos de la fuente magnética se calcularon a partir del método de peter (método half-slope), lo que indica dos modelos sobre la profundidad de la fuente las fuentes más profundas oscilaron de 0,88 km hasta 3 15 km y tuvieron una intensidad magnética de 32nt hasta 225nt, mientras que las fuentes menos profundos oscilaron de 0,23 km hasta 0 76 km y tuvieron una intensidad magnética de -10nt hasta 20nt muy probablemente las fuentes más profundas fueron debido a los cuerpos intrusivos básicos intra-basamentales que subyacen a las zonas marginales cubiertos de sedimentos de la depresión de benue en general, la cubierta sedimentaria en la zona fue baja y, por lo tanto, no es probable que favorecen la generación de los hidrocarburos palabras claves: wamba, aeromagnético, lineamientos, fuente magnética, método de peter keywords: wamba, aeromagnetic, lineaments, magnetic source, peter’s method record manuscript received: 11/07/2011 accepted for publications: 29/05/2012 g k anudu, l n onuba, a g onwuemesi and a e ikpokonte26 varies from place to place the minimum sediment thickness required for producing oil usually varies from 2km to 4km, compared to 3km to 7km for gas production/formation (dow, 1978; cornford, 1990; gluyas and swarbrick, 2005) aeromagnetic maps usually reflect variations in the earth’s magnetic field resulting from the underlying rocks’ magnetic properties (e g magnetic susceptibilities) sedimentary rocks have the lowest magnetic susceptibility, whereas metamorphic and acidic igneous rocks intermediate and basic igneous rocks have the highest magnetic susceptibility (kearey et al , 2002) the largest proportion of a magnetic signal or anomaly is thus generated at crystalline (igneous or metamorphic) basement level (getech, 2007) magnetic anomalies are caused by magnetic minerals contained in rocks; such anomalies are usually caused by underlying basement (igneous and/or metamorphic) rocks or by igneous features such as intrusive plugs, dykes, sills, lava flows and volcanic centres when magnetic anomalies are observed over sedimentary terrain (gunn, 1997) nevertheless, high sensitivity measurements could also be associated with cultural iron contamination and authigenic alterations in sedimentary rocks, possibly caused by hydrocarbon migration (costanzo-alvarez et al , 2000; aldana et al , 2003) several articles have been published on the nigerian basement complex’s structural and tectonic framework, based on analysing aeromagnetic data (ajakaiye et al , 1986; olasehinde et al , 1990) ajakaiye et al , (1986) studied the benue trough’s tectonic framework and that of parts of the adjoining nigerian basement complex using aeromagnetic maps, delineating ne-sw and ene-wsw directions as being the dominant aeromagnetic lineament trends they stated that these aeromagnetic lineaments depicted a possible continental continuation of the four atlantic fracture zones (st paul’s, romanche, chain and charcot) abutting the west african coast into the nigerian basement complex according to olasehinde et al , (1990), analysing aeromagnetic data over central nigeria’s basement complex has shown that the nigerian basement complex’s structural and tectonic framework comprises ne-sw and nw-se lineaments superimposed over a dominant n-s trend overall ne-sw aeromagnetic lineaments of the benue trough and atlantic fracture zones apparently signified an ancient zone of weakness in the nigerian basement complex according to nur et al , (1994), depths to magnetic basement in the middle benue trough varied from 0 066km to 4 938km, whereas onyedim et al , (2006) have stated depth values ranging from 0 11km to 5 5km the present study was focused on analysing and interpreting aeromagnetic data over wamba and its adjoining areas its purpose was to map metalliferous mineral deposits in the basement rocks, delineate structural lineaments and their trends as well as to determine the depth to magnetic source bodies (and possibly sediment thickness) in the sedimentary part of the area such structural lineaments (i e fractures, faults, shear zones and veins) usually serve as potential hosts or migration paths for groundwater, hydrocarbons and minerals sediment thickness is very important in any sedimentary basin regarding to hydrocarbon formation the study showed the usefulness of aeromagnetic data in mapping metalliferous mineral deposits (especially iron ore having a high magnetite to haematite ratio), delineating structural lineaments and estimating sedimentary cover (sediment thickness) in wamba and its adjoining areas this study is novel as there has been no record regarding this aspect of research in the area figure 1. geological map of wamba and its adjoining areas (modified from, macleod et al , 1971) analysis of aeromagnetic data over wamba and its adjoining areas in north-central nigeria 27 the study area’s location and geology the study area is located in nasarawa state, north-central nigeria, covering wamba, nasarawa eggon, garko, shabu, adogi, assakio, tunga, utani, panda, nakere, ninkade, angwa madaki, lange, arikya, kumuru, zamatak and monkwar it is located between latitudes 8o 30’n and 9o 00’n, and longitudes 8o 30’e and 9o 00’e (figure 1) it is accessible through three major roads, namely abuja-keffi-akwanga-lafia road, makurdi-lafia road and jos-namu-lafia road the study area’s climate is characterised by rainy season (april to october) and dry season (november to march) with annual rainfall and temperature varying between 1300mm-1500mm and 28oc-36oc, respectively (adefolalu, 2002) the study area is underlain by the basement complex rocks, younger granites and cretaceous middle benue trough’s sedimentary rocks (figure 1) (macleod et al , 1971) the basement complex rocks in the area are grouped into two, namely migmatite-gneiss complex and the older granites (or pan-african granitoids) the migmatite-gneiss complex is neoproterozoic to meso-archean (542ma-3200ma) in age and composed of migmatites and gneisses it is the most extensive rock type in the area the older granites (or pan-african granitoids) are pan-african (600 ± 200ma) in age and consist mainly of granites, diorites and dolerites both the migmatites and gneisses were deformed and intruded by the older granites (or pan-african granitoids) during the 600 ± 200 ma pan-african episodes the pegmatite veins associated with these basement complex rocks usually show n-s, nne-ssw, ne-sw and nnw-sse trends (figure 1) the younger granites in the study area are jurassic (145-210ma) in age and are high-level, anorogenic granites; they mainly consist of microgranites and biotite granites the southern part of the area forms part of the middle benue trough and is underlain by cretaceous sedimentary rocks, namely the awgu shale and lafia formation the awgu shale is composed mainly of bluish-gray to black shales, whereas the lafia formation consists mainly of sandstones and claystones according to obaje et al , (1994), paleogene basalt flows and dolerite sills have been encountered within the lafia formation materials and methods an aeromagnetic contour map of wamba (sheet 210) was used in this study; it was compiled by the geological survey of nigeria, published on scale of 1:100,000 the aeromagnetic data was acquired by using a proton precession magnetometer towed in a housing known as a ‘bird’ along a series of nw-se flight lines, spaced 2km apart, with 20km tie-line spacing and 500ft (152m) nominal flight altitude the geomagnetic gradient was removed using the january 1974 international geomagnetic reference field (igrf) the map was digitised at 1km intervals to avoid aliasing error this interval imposed a 1/2km-1 (i e 0 5km-1) nyquist wavenumber and therefore, the thinnest magnetic source body that would have been delineated from the digitised data would have had around 2km width the digitised data was gridded, applying the minimum curvature method and the resulting grid was contoured at 20nt intervals using surfer 8 software to produce the total aeromagnetic intensity contour map (figure 2) a plane surface regional anomaly was fitted to the digitised data by least squares multi-regression analysis to obtain the residual anomaly data the expression obtained for the regional fields t(r) was: figure 2. total aeromagnetic intensity contour map of wamba and its adjoining areas (20nt contour interval) g k anudu, l n onuba, a g onwuemesi and a e ikpokonte28 t(r) = 7820 + 0 0961x + 1 7767y where, x and y were units spaced along the horizontal and vertical axes the regional field values obtained were subtracted from the observed (total aeromagnetic intensity) data to obtain the residual anomaly data furthermore, the residual anomaly data obtained was subsequently gridded using the minimum curvature method and the resulting grid was contoured at 20nt intervals to produce the residual aeromagnetic intensity contour map (figure 3) results and discussion qualitative interpretation the qualitative interpretation of aeromagnetic data directly illustrates geological information by looking at an aeromagnetic map without any calculations (grant and west, 1965) a visual study of magnetic maps can be fruitful for preliminary interpretation (telford et al , 1998) visual studies of total aeromagnetic intensity contour map (figure 2) and the residual aeromagnetic intensity contour map (figure 3) of wamba and its adjoining areas revealed variations in magnetic field intensity throughout the area magnetic field intensity usually increases from the south towards the northern part of the area; such variation is a function of the rock types the southern part of the area is part of the middle benue trough and is underlain by cretaceous sedimentary rocks (awgu shale and lafia formation) and thus exhibited low (highly negative) magnetic intensity, whereas the pre-cambrian basement rocks (migmatites, gneisses and older granites) and jurassic younger granites occupied the northern portion of the area and hence had high (positive) magnetic intensity the magnetic anomaly contours in the southern portion were smooth and characterised by highly negative, long wavelength, magnetic anomalies having e-w and ne-sw direction trends (benue trough trend), while the magnetic anomaly contours in the northern portion had complex, positive, short wavelength, magnetic anomaly variations trending nesw and nw-se furthermore clear, localised, well-defined, high magnetic anomalies could be observed around the western and west-central portions of the aeromagnetic maps of the study area (figures 2 and 3) total magnetic intensity values in the aforementioned areas varied from 7960nt to 8020nt, whereas residual magnetic intensity values ranged from 160nt to 220nt iron ore is being mined by unskilled miners about 4km north of the town of nasarawa eggon; it could thus be inferred that the localised and welldefined high magnetic anomalies may have been associated with the presence of iron ore (having high magnetite to haematite ratio) or other metalliferous mineral deposits within the basement rocks in the area quantitative interpretation structural lineament trends residual aeromagnetic intensity anomaly data was used to produce the residual aeromagnetic intensity shaded relief maps of the area to delineate structural lineament trends in the area (figure 4) the residual aeromagnetic intensity shaded relief maps were modelled using the cenfigure 3. residual aeromagnetic intensity contour map of wamba and its adjoining areas (20nt contour interval) (1) analysis of aeromagnetic data over wamba and its adjoining areas in north-central nigeria 29 nb : hpla= horizontal position light angle and vpla= vertical position light angle figure 4. residual aeromagnetic intensity shaded relief map depicting structural lineament trends in wamba and its adjoining areas figure 5. the structural lineament map of wamba and its adjoining areas g k anudu, l n onuba, a g onwuemesi and a e ikpokonte30 tral difference gradient relief method, the lommel-seeliger law shading method, 0 02262781 z-scale factor and different horizontal and vertical position light angles the horizontal and vertical position light angles used in the modelling were hpla= 0, vpla= 0; hpla= 45, vpla= 0; hpla= 90, vpla= 0; and hpla= 135, vpla= 0 the aforementioned position light angles were used in modelling the residual aeromagnetic intensity shaded relief maps of the study area, because they are very useful in enhancing, illuminating and delineating structural lineament trends on shaded relief maps hassanein and soliman (2009) have stated that changing the horizontal position light angle (hpla) helps define subsurface structures’ predominant trends in an area the trends generated and shown on the residual aeromagnetic intensity shaded relief maps of the area depicted the structural lineament trends on the magnetic basements beneath the area (figure 4) the major structural lineament trends were ne-sw, nnessw and nw-se with minor ene-wsw, e-w trends most of these trends agreed with previous work carried out in the benue trough and parts of the adjoining basement complex by ajakaiye et al , (1986), olasehinde et al , (1990) and anudu et al , (2012) furthermore according to dobrin and savit (1988), lineation in the magnetic contours usually follows regional geology (e g , intrusive bodies or large faults’ strikes) and is thus useful in mapping structural trends they also stated that well-defined boundaries between zones having appreciably different degrees of magnetic relief often indicate the presence of major basement faults and fractures the structural lineaments in the area were thus mainly faults and fractures, as well as veins and foliations a structural lineament map of the area (figure 5) was produced using dobrin and savit’s (1988) aforementioned assertions together with figure 4 figure 6. rose diagram of structural lineament trends regarding wamba and its adjoining areas the comparative study of the geological map (figure 1) and generated structural lineament map (figure 5) of wamba and its adjoining area showed that structural lineaments were more concentrated in the pre-cambrian basement rocks (migmatites, gneisses and older granites) and jurassic younger granites than on the cretaceous sedimentary rocks (awgu shale and lafia formation) this was due to the effect of several tectonic deformations which occurred throughout the nigerian basements figure 7. depth estimation from magnetic profile using peter’s half-slope method (telford et. al , 1998) half m ax im um slo pe lin e half m axi mum slo pe lin e p analysis of aeromagnetic data over wamba and its adjoining areas in north-central nigeria 31 structural lineament orientation or azimuth direction on the structural lineament map (figure 5) was measured and plotted on a rose diagram using grapher 5 software the rose (azimuth-frequency) diagram depicted the structural lineaments’ major trends being ne-sw, nw-se and nnessw, with minor nnw-sse and e-w trends (figure 6) structural lineament (fractures, faults, shear zones, veins and foliations) orientation or trends’ measurement can be used to date the events that produced the rocks from which they were obtained according to obiora (2009), nnessw / n-s trends are pan-african, while e-w / nnw-sse trends are pre-pan-african hence the ne-sw, nw-se and nne-ssw trends in the study area may probably have been pan-african (younger than 800 ma), whereas the nnw-sse and e-w trends may have been pre-panafrican (over 800 ma) figure 8. location of profiles a-a1, b-b1, c-c1, d-d1 and e-e1 on the residual aeromagnetic contour map of wamba and its adjoining areas (20nt contour interval) profiles depth (km) a-a1 0.95, 0.76, 1.01, 0.88 and 0.25 b-b1 1.25, 1.13, 3.15, 1.64, 1.26, 0.76 and 0.32 c-c1 0.76, 1.89, 2.01 and 0.34 d-d1 1.26, 1.07, 1.13, 2.52, 0.47 and 0.23 e-e1 1.98, 1.76, 1.04, 1.61 and 0.26 table 1. depth estimates for the magnetic source bodies (and possible sediment thicknes) estimating depth peter’s half-slope method was used to estimate the depth to magnetic source body it is much simpler and faster and provides more depth estimates than model curve fitting analysis it uses magnetic profiles’ half maximum sloping flanks (figure 7) to estimate depth especially in aeromagnetic interpretation (peters, 1949; nettleton, 1971; rao and babu, 1984; telford et al , 1998; milsom, 2003) the depth to the source body beneath these portions of the curve was given by: h = 0 63p, where p was the horizontal distance between points where the slope was half the maximum slope (figure 7) five profiles were taken across the sedimentary part of the residual aeromagnetic intensity contour map of the area to estimate the depth to magnetic bodies (and possibly sediment thickness), namely a-a1, b-b1, c-c1, d-d1 and e-e1 (figure 8) they were taken perpendicular to the most prominent anomalies’ strike direction to obtain the best estimate of body parameters from the profiles so selected the afore-mentioned five profiles were plotted, as shown in figure 9, and then interpreted using peter’s half-slope method table 1 above shows the magnetic source bodies’ depth estimates (and possibly sediment thickness) throughout the study area’s sedimentary part two depth source models were established using depth values determined by peter’s half-slope method; the deeper sources ranged from 0 88km to 3 15km, anomaly values varying from -32nt to -225nt, characterised by longer wavelength anomaly, while the shallower sources ranged from 0 23km to 0 76km, anomaly values g k anudu, l n onuba, a g onwuemesi and a e ikpokonte32 acterised by high negative anomaly values having longer wavelength and depicted basic intrusive rock at depth since the southern part of the area lies on the sediment-basement boundary (part of the cretaceous benue trough’s northern margin), the causative sources were considered to be deeper-seated intra-basement basic intrusive rock bodies the anomaly occurred in the ne-sw and e-w trend following the benue trough’s general trends the results were in good agreement with interpreted positive gravity anomalies characterising the trough’s northern margin (ikpokonte, 2009; fairhead and okereke, 1987) the depth values obtained for the study area also agreed with those computed from aeromagnetic data over some parts of the middle benue trough by nur et al , (1994) and onyedim et al , (2006) using spectral analysis and hdep software, respectively, hence depicting the sedimentary cover (sediment thickness) across the sedimentary part of the study area being generally low and therefore not likely to favour hydrocarbon (oil and/gas) generation or formation conclusions a total aeromagnetic intensity contour map, residual aeromagnetic intensity contour map and structural lineament map of the area being studied were obtained in this work visual studies of total aeromagnetic intensity contour map and residual aeromagnetic intensity contour map depicted that the southern part of the area underlain by cretaceous sedimentary rocks (awgu shale and lafia formation) had low and smooth magnetic intensity, whereas the northern part underlain by the pre-cambrian basement rocks (migmatites, gneisses, older granites) and jurassic younger granites had high and complex magnetic intensity structural lineament orientation suggested that they were products of pan-african orogeny (ne-sw, nw-se and nne-ssw trends) and pre-pan-african orogeny (nnw-sse and e-w trend) magnetic source bodies’ (and possibly sediment thickness) depth estimated across the sedimentary part of the area using peter’s half-slope method, indicating two depth source models deeper sources varied from 0 88km to 3 15km, while shallower sources varied from 0 23km to 0 76km this showed that the sedimentary cover was generally low and hence would not likely to favour hydrocarbon (oil and/gas) generation acknowledgement the authors are grateful to the two anonymous reviewers for their useful suggestions and criticism which have greatly improved the revised manuscript we would also like to thank jason garry for comprehensive editing of this paper which added significantly to its quality and content figure 9. plots of residual magnetic anomaly profiles a-a1, b-b1, c-c1, d-d1 and e-e1 across the sedimentary part of the study area ranging from -10nt to 20nt, characterised by short wavelength anomaly the shallower sources probably depicted depths to pre-cambrian basement or near surface basic igneous intrusive rocks (such as basalt flows and dolerites) and/or low-lying river valleys the deeper sources were charprofile a-a1 profile c-c1 profile d-d1 profile e-e1 profile b-b1 r es id ua l a no m al y (n t ) r es id ua l a no m al y (n t ) r es id ua l a no m al y (n t ) r es id ua l a no m al y (n t ) r es id ua l a no m al y (n t ) a-a1 c-c1 d-d1 e-e1 b-b1 distance (km) distance (km) distance (km) distance (km) distance (km) 5 5 5 5 5 0 0 0 -20 50 10 10 20 5 0 -50 -40 -50 -10 -5 0 10 -100 -20 -10 -60 -150 -30 -15 -10 -200 -40 -20 -20 -50 -25 -30 -30 -80 -250 -60 -35 -40 -100 100 20 15 30 20 10 10 10 10 10 15 15 15 15 15 20 20 20 20 20 25 25 25 30 30 30 30 30 25 25 analysis of aeromagnetic data over wamba and its adjoining areas in north-central nigeria 33 references adefolalu, a d 2002 climate in: africa atlases; atlas of nigeria les editions j a aux e’dition du jaguar 57 bis, rue d’auteuil-75016 paris-france aldana, m , costanzo-alvarez, v and diaz, m 2003 magnetic and mineralogical studies to characterise oil reservoirs in venezuela the leading edge, vol 22, no 6, pp 526 anudu, g k , obrike, s e , iyakwari, s and ikpokonte, a e 2012 preliminary structural study of landsat imagery over wamba and environs, nasarawa state north-central nigeria research journal of applied sciences, vol 7, no 1, pp 1-9 ajakaiye, d e , hall, d h , miller, t w, verherjen, p j t , awad, m b and ojo, s b 1986 aeromagnetic anomalies and tectonic trends in and around the benue trough nigeria nature, 319, pp 582-584 cornford, c 1990 source rocks and hydrocarbons of the north sea in glennie, k w (ed ) petroleum geology of the north sea basic concepts and recent advances blackwell science, oxford, pp 376-462 costanzo-alvarez, v , aldana, m , aristeguieta, o , marcano, m c and aconcha, e 2000 study of magnetic contrasts in the guafita oil field (south-western venezuela) physics and chemistry of the earth, part a: solid earth and geodesy, vol 25, no 5, pp 437445 dobrin, m b and savit, c h 1988 introduction to geophysical prospecting, 4thedition, mcgraw-hill, new york 867pp dow, w g 1978 petroleum source beds on continental slopes and rises (2) american association of petroleum geologists (aapg) bulletin vol 62 fairhead, j d and okereke, c s 1987 a regional gravity study of the west african rift system in nigeria and cameroun and its tectonic interpretation tectonophysics, vol 143, pp 141-159 getech 2007 advanced processing and interpretation of gravity and magnetic data getech plc , kitson house elmete hall leeds, uk 27pp gluyas, j and swarbrick, r 2005 petroleum geoscience blackwell science, oxford, 358pp grant, f s and west, g f 1965 interpretation theory in applied geophysics new york: mcgraw-hill gunn, p j 1997 application of aeromagnetic surveys to sedimentary basin studies australian geological society organisation (agso) journal of australian geology and geophysics, vol 17, no 2, pp 133-144 hassanein, h i e and soliman k s 2009 aeromagnetic data interpretation of wadi hawashiya area for identifying surface and subsurface structures, northeastern desert, egypt journal of kau: earth sci , vol 20, no 1, pp 117-139 ikpokonte, a e 2009 a continental structural junction: evidence of shallow crustal discontinuities deduced from the gravity data over the niger-benue troughs (confluence) area, nigeria nigerian journal of physics, vol 21, no 1 kearey, p , brook, s m and hill, i 2002 introduction to geophysical exploration 3rd edition blackwell scientific publication, oxford 262pp macleod, w n , turner, d c and wright, e p 1971 the geology of jos plateau geological survey of nigeria, bull no 32, vol 2, pp milsom, j 2003 field geophysics 3rd edition john wiley and sons ltd, london 232pp nettleton, l l , 1971 elementary gravity and magnetics for geologists and seismologists: society of exploration geophysicists monograph series no 1, pp 83-87 nur, m a , onuoha, k m and ofoegbu, c o 1994 spectral analysis of aeromagnetic data over the middle benue trough, nigeria journal of mining and geology, vol 30, no 2, pp 211-217 obaje, n g , ligous, b and abaa, s i 1994 petrographic composition and depositional environment of cretaceous coals and coal measures in the middle benue trough of nigeria international journal of coal geology, 26, pp 233-260 obiora, s c 2009 field measurements in descriptions of igneous and metamorphic rocks in lambert-aikhionbare, d o and olayinka, a i (eds ) proceedings of field mapping standardisation workshop, ibadan university press, pp 105-125 olasehinde, p i , pal, p c and annor, a e 1990 aeromagnetic anomalies and structural lineament in the nigerian basement complex journal of african earth sciences, vol 11, no 3/4, pp 351-355 onyedim, g c , awoyemi, m o, ariyi, e a and arubayi, j b 2006 aeromagnetic imaging of the basement morphology in part of the middle benue trough, nigeria journal of mining and geology, vol 42, no 2, pp 157-163 paterson, n r and reeves, c v 1985 applications of gravity and magnetic surveys: the state-of-the-art in 1985 geophysics 50, pp 2558-94 peters, l j 1949 the direct approach to magnetic interpretation and its practical application geophysics 14, pp 290-320 rao, d a , and babu, h v r 1984 on the half-slope and straight-slope methods of basement depths determination geophysics 49, pp 1365 8 telford, w m , geldart, l p and sheriff, r e 1998 applied geophysics, 2nd edition, cambridge university press, usa 770pp la buena interpretación de datos estadísticos de refracción sísmica depende de la identificación acertada y confiable de los tiempos de llegada. los primeros tiempos de llegada se pueden identificar en un gráfico o imagen por picado convencional, pero este proceso depende de factores externos como la escala y la calidad de información de la imagen, el índice de amplitud, la sensibilidad del cursor de recolección y la experiencia del usuario. bajo estas consideraciones, la identificación de los tiempos de llegada bajo información ruidosa se vuelve más compleja e inestable. en este estudio, la técnica de correlación cruzada (cct, en inglés), que es ampliamente trabajada en el proceso de análisis de datos de reflexión, se utilizó para seleccionar los primeros tiempos de llegada en información sísmica ruidosa o no ruidosa con un proceso semiautomático. la cct redujo la dependencia en el usuario y bajó el nivel de selección incorrecta causada por el ruido ambiental al desplegar características y factores de escala. la cct se ha probado en modelos sintéticos con diferentes contenidos de ruidos y diversa información de campo. el error de la norma chi-cuadrado se utilizó para evaluar el desempeño de las selecciones. en adición, los efectos de las pequeñas diferencias de tiempo entre el proceso convencional de selección y la cct se han demostrado en una tomografía reflexiva de velocidad. además, se estima que el método propuesto es una contribución útil a los métodos existentes de la recolección de los primeros tiempos de llegada. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 107 113 abstract resumen key words: first arrival picking, first break, cross-correlation, seismic refraction, semi-automatic picking. palabras clave: primeros tiempos de llegada, primer arribo, correlación cruzada, refracción sísmica, selección semiautomática. record manuscript received: 10/12/2012 accepted for publication: 26/09/2014 seismology the high-quality interpretation of seismic refraction data depends on the accurate and reliable identification of the first arrival times. first arrivals can be identified on a graphic or image by conventional picking, but this process depends on external factors, such as the scale and quality of the imaging data, amplitude ratio, sensitivity of the picking cursor and user experience. under these considerations, identifying first arrivals in noisy data becomes more complex and unstable. in this study, the cross-correlation technique (cct), which is widely used in the process of analyzing reflection data, has been used to pick the first arrival times in noisy or noiseless seismic refraction data by a semi-automatic process. the cct has reduced the dependence on user and decreased incorrect picking caused by environmental noise, displaying characteristics and scaling factors. the cct has been tested with synthetic models with different noise contents and various field data. the chi-square error criterion was used to assess the performance of the pickings. in addition, effects of small-time differences between the conventional picking process and the cct have been demonstrated on a refraction tomography velocity section. therefore, we believe that our proposed method is a useful contribution to the existing methods of first arrival picking. a semi-automatic approach to identify first arrival time: the cross-correlation technique (cct) mustafa senkaya, hakan karslı department of geophysics engineering, karadeniz technical university, trabzon, turkey e-mail: msenkaya@ktu.edu.tr and hkarsli@ktu.edu.tr issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.35887 http://dx.doi.org/10.15446/esrj.v18n2.35887 108 mustafa senkaya, hakan karslı 1. introduction one of the principal methods for near-surface exploration is the analysis of seismic refraction data. this method conventionally requires the picking of first arrivals of direct and head waves on many shot records. determining the arrival times of seismic events is the first step in the conversion of seismic observations to geological models. to obtain a high degree of consistency between travel time data and the seismic model, it is important to use appropriate data for f the inversion process (leung, 2003). generally, the identification of first arrivals in seismic refraction data depends on shallow geological structure, source type, and the signal to noise ratio (snr). if the data are collected from complex geological structures or have low snr, the automatic picking of the first travel times becomes a difficult process (yilmaz, 2001). first arrival picking techniques can be divided into manual and automatic (cox, 1999). manual picking depends on visual inspection of the amplitudes, operator’s visual estimation capacity and experience, the scaling factor, and the quality of data imaging. these issues in data processing are time consuminging and can lead to inaccurate picks due to picking-operator subjectivity. moreover, if seismic data contain various types of noise, the picking of first arrivals may not be simple and reliable. as a result, when the data volume is large and the data quality is poor, the picking procedure can take up to 20–30% of the total processing time (sabbione and velis, 2010). automatic picking techniques, on the other hand, involve machine picking according to certain criteria. they are more efficient than manual picking but may also lead to false picking (mausa et al., 2011). depending on the increase in the volume of data and the significant development of computer technologies in recent decades, many automatic or semi-automatic techniques have been developed to pick first arrivals. first studies for this purpose were based on the correlation of sequential traces and delay times between sequential arrival times (peraldi and clement, 1972). hatherly (1982) used the least squares method with the first kick concept to define first arrival times. gelchinsky and shtivelman (1983), ervin et al. (1983) suggested using the correlation properties of signals and statistical criteria. mcevilly and majer (1982), coppen (1985), baer and kradolfer (1987), gu et al. (1992), and earle and sharer (1994) obtained first arrivals by using the energy ratios of signals. murat and rudman (1992), kusuma and fish (1993), and mccomack et al. (1993) applied neural network algorithms on the seismic data to detect first breaks. boschetti et al. (1996) and xu et al. (1999) conducted fractal dimensional analysis to determine first arrivals on seismic traces. yung and ikelle (1997) used a bispectrum process instead of the traditional correlation method. bois (1980), liu and fu (1982), and wu and nyland (1987) incorporated pattern recognition into the picking process. keho and zu (2009) presented an alternative approach using peak spikes and comparison of adjacent traces. blais (2011) suggested the use of an optimization technique with an objective function and similarity of wave features. liao et al. (2011) picked first breaks by spectral decomposition using minimum uncertainty wavelets. mausa et al. (2011) presented a new technique for first arrival picking of refracted seismic data based on digital image segmentation. all techniques described above have advantages and disadvantages. in this study, we propose to apply the cct, which is an efficient approach in vibro-seismic reflection data processing, to extract reflectors, inon refraction data and detect first arrival times more accurately by semi-automatically process and. the cross-correlation (cc) process often produces side-lobes that arrive before the first break, thus complicating the picking process. to overcome this problem, we have developed a semi-automatic picking procedure. the entire process involves three steps. the first step of the method consists inof obtaining a source wavelet according to the noise content of the dat aa and using the source wavelet as an input to the cc process. the second stage is the crosscorrelation between the source and traces in seismic refraction data. in the last stage, the first arrival times are automatically determined in the area marked by the user with linear lines on the cross-correlated output. the effectiveness of the cct is tested by means of noisy, noiseless synthetic and real data. tests have shown that the method provides a faster and more reliable picking process, and it is an efficient approach to detect first breaks in all data types. 1.1 factors affecting the first arrival pickings it is difficult to detect the first breaks in noisy seismic refraction data. even ifwhen it is possible, general opinion is that the first arrivals are in the form of a nearly straight line, and since because the human eye works in this direction, the arrival times for each receiver loses sensitivity. filtering is the first option to make noisy data more understandable. however, the amplitudes of the first arrivasl have low energies and variable waveforms compared to other traces in the data. in that case, filtering is a troublesome process and sometimes it may cause erroneous results. moreover, different filter band ranges and slopes have some characteristic effects on signals. filtering low frequencies increases the side lobe of the signal. in contrast, the signal waveform the signalis extended without affecting the any side lobes when using high-cut filters. the filter slope is another parameter impacted by the filtering process. the energy of the wavelet tends to shift with an increasing slope of the filter (geldart and sheriff, 2004). these effects are shown on a zerophase ricker wavelet in fig. 1 by butterworth filtering, with different frequency bands and slope values. red ticks on the filter outputs indicate the real first break times. however, when the slope and high cut frequency of the filter increase, the first break points apparently move forward from the real point. in contrast, when the slope and low cut frequency of the filter increase, the first break points remain constant, but side lobes exist, leading to false picking. figure 1. effects of low and high cut filters, and slopes of filters on zero phase ricker wavelets with a center frequency of 50 hz (upper row). the parameter “n” shows the filter degree of the butterworth filter. red ticks show the real first arrival time on the original ricker signal. the difficulties in the conventional process of first arrival picking, especially for noisy data, are related to numerous effects, such as human operator experience, sensitivity and capacity of the operator’s eyes, scale of the imaging and data amplitude, sensitivity of the picking cursor axes, and snr. therefore, the process is time consuming. the imaging scale and amplitude of the data directly affect the accuracy of the picked time (douglas et al. 1997). the first arrivals picked at low-level amplitudes have delays compared to those picked at high levels because some earlier low-amplitude events will be masked by high levels, and normalization is not sufficient in these cases. time scale is important to the sensitivity of the picking cursor. if the cursortime axis is too narrow, the sequenced picked times may appear very similarsimultaneously, which will cause an inaccurate picking. usesthe alsthe values all the above-mentioned factors appear in the conventional picking process of the first breaks, which is performed on “an image” by clicking or marking. to overcome the difficulties inherent toin the traditional process, 109a semi-automatic approach to identify first arrival time: the cross-correlation technique (cct) picking must be done directly with the numerical values of amplitudes and be performed independently for each channel by mathematical approaches and attributes. in this paper, an attempt is made to devise a simple and quick procedure, the cct, to obtain accurate and sensitive first travel times in refraction data. 2. method our method is based on the simple and practical cc technique. it is well known that the cc process is not affected by random noise in signals. this is an unique and useful feature of the cc process under noisy conditions. it can permit us to obtain travel times independently of noise, and it is possible to increase the quality of first breaks. r sw (τ ) = s t ⋅w t+τ t=−∞ +∞ ∑ (1) from eq. (1), cc requires two inputs in the time domain: data st and the source wavelet wt. if the seismic data are modeled as a linear system, the source wavelet becomes an input, the linear system is a homogeneous layered earth, and the output is the refraction data. according to this linear system, each seismic refraction datum is influenced by its source. therefore, the correlation function, which is generated by the cc of the refraction signals and source, has a maximum correlation value where the data and source wavelet appear to be the same waveform or shape. consequently, the first break time is defined as the time of the maximum correlation value on the time axis. the theoretical accuracy of the proposed method has been shown in fig. 2(a) on a simple and noiseless synthetic signal generated by the previously mentioned linear system. the correlation function, the cc of the source and data, has a maximum correlation value where itthe function entirely matches with the first break at 17.25 ms. to demonstrate the success of the cc process under the random noise condition, this basic application was repeated with 5% and 15% random noise (fig. 2(b), 2(c), respectively). in both cases, the maximum correlation value indicates itself on the same theoretical first break time as in the noiseless one. figure 2. examination of noise effects on a simple theoretical waveform for the cct for a (a) nnoiseless signal, with (b) 5% and (c) 15% random noise. the vertical green dashed line shows the maximum value of the cc and indicates the first break point. note that although the noise increases, the location of the first break point remains equalunchanged. 3. synthetic applications the cct was tested by means of synthetic models under different noise conditions. in the synthetic tests, data were generated by the convolution of a modified sine wave as a source with an impulse series that had a single impulse for each channel on the calculated theoretical wave travel time sample (fig. 3). our earth model was based on the superposition of a horizontally layered, homogenous model, and we confine ourselves to a single interface with parameters as given in table 1. figure 3(d) indicates the results of the cct on 24 channels of noiseless synthetic seismic refraction data. the semi-automatic picked times havehad high accuracy compared to theoretical wave arrivals. this consistency was demonstrated by a chi-squared error value with a 95% confidence interval. figure 3(c) shows flagged lines, which were used to restrict the automatic search area of first breaks. this restriction must be applied to shorten the process time and reduce failed picks under real field conditions that may have several maximum correlation values at late times caused by side-lobes. table 1. synthetic model parameters synthetic model parameters synthetic data parameters velocities (m/s) v1: 350 time sampling (ms) 5 v2:1200 number of samples 401 interface depth (m) 10 number of receiver 24 receiver interval (m) 2 offset (m) 5 figure 3. application of the cct to synthetic data: (a) source wavelet, (b) impulse series, (c) correlation section and first break searching area (red lines), (d) comparison of output of cct and theoretical first breaks. the synthetic tests were extended to data with three different noise contents: i) 30% random noisy data as high-frequency noises (fig. 4), ii) 0.15% system noise as low frequency, coherent noise (fig. 5), and iii) mixed noise as both 30% random and 0.15% system noise (fig. 6). while first breaks were determined with 9.30*10-2 ms and 6.92*10-1 ms chi-squared error for the data for random and system noises data, respectively, the error value was 4.96*10-1 ms for the mixed noisy data. these results demonstrate that the cct is an useful process even in the case of different levels of noise because of the nature of the cc process. figure 4. cct performance in the case of 30% random noise: (a) data with random noise, (b) first arrival times from theoretical and the cct. figure 5. cct performance on data with 0.15% system noise: (a) data with system noise, (b) first arrival times from theoretical and cct. 110 mustafa senkaya, hakan karslı figure 6. cct performance on data with mixed noises (30% random, 0.15% system): (a) data with mixed noise, (b) first arrival times from theoretical and cct. 4. real data examples data were gathered from different fields containing various frequencies and types of noise to demonstrate the flexibility of this method. data sets were provided by the seismic data process group at the geophysical engineering department of karadeniz technical university, in trabzon, turkey. all field data were collected at 12 receivers with varying spreads on a es3000 seismograph by 8 kg hammer impacts, using a time sampling of 0.25 ms. in the semi-automatic process, we hadencountered two major issues: i) polarity changes between channels, ii) source wavelet estimation for the data that is acquired from uncontrolled sources. polarity changes of waves are caused by complex or inordinate near-surface conditions. in the last stage of the cct, first breaks are searched in the marked area as a maximum correlation function value. in the case of a polarity change, the correlation function assumes a negative rather than positive maximum value. therefore, the automatic searching for first breaks must be applied to absolute values of the correlation function to avoid affecting polarity changes. the selection of an optimum source type in field studies relates to the desired source wavelet, portability, cost, repeatability, and environmental damage and safety factors. considering these factors, the uncontrolled sources, such as hammer and weight-drop apparatus, are the most suitable ways to acquire seismic refraction data sets. however, in addition tothe source shapes and the frequency band ranges of uncontrolled sources generally change between shots, so they are not completely explained by fixed mathematical functions. this situation is a considerable issue for cct applications in uncontrolled source data. eq. (1) shows that a source wavelet function is necessary for cc and is a single and dominant input to generate a reliable correlation function and travel time. therefore, a reliable estimation of the seismic source can be a pivotal point for a successful picking process. we offer three ways to determine the source wavelet function from uncontrolled source data: i) by choosing a wavelet on the trace with high snr, ii) by inverse fourier transformation according to center frequency of a smooth average power spectrum of the data, iii) by generating suitable mathematical operator functions. according to noise content, the user must decide which option is most appropriate. our experiences have shown that seismic refraction data can be divided into four categories according to noise content and ease of picking the first arrival times: a) nearlynearly noiseless data with clear first arrivals b) data containing high frequency environmental noises (i.e. industrial and traffic) c) data with an altered waveform for most receivers and unknown noise sources in the subsurface or on the ground d) data with very low snr, in which it may be impossible to pick first breaks our experimental analyses show that the source wavelet can be estimated directly on a clear channel for a-type data, whilea the smooth spectrum approach is very useful on b-types. to generate a source function in the cases of c and d-types, a mathematical operator will be suitable. moreover, a mathematical function can be adapted to data with numerous minimum phase function approaches. we shall concentrate on a source estimation step because of its direct importance to the accuracy of the time picking. if possible, it has been suggested that the source wavelet to be used should initially be chosen on a clear channel. cct applications have been illustrated several times for each type of real data to verify effectiveness and usability. first arrivals picked from clean data with high accuracy can be observed in fig. 7(a) and 7(b). applications of the proposed method on high and low frequency noises are shown in between fig. 7(c) and 7(f), and fig. and respectively. fig. 7(g) and 7(h) illustrate instances of the picking with a mathematical operator and smooth spectrum application on hard data compared with others. we used the earlier part of a modified sine function as a mathematical operator. furthermore, the estimated source wavelets have been shown in fig. 8, following the same sequences in the data imaging. effectiveness and accuracy of the cct directly depend on the estimated source wavelet. we suggest using the half the length of the estimated source wavelet in the cct. however, our tests indicate that when the data includes more noise, the length may be longer. refraction tomography is becoming a common method to estimate the accuracy of near-surface velocity models. this process also depends on the picking of first arrivals of refracted data and tomographic inversions are very sensitive to each individual travel time. one of the main difficulties with the refraction tomography is the low snr characterizing the first break waveform, especially for far-offset receivers. moreover, our experiences have shown that other seismic refraction data interpretation methods, such as the delay time method, can also be affected. therefore, we want to demonstrate that small time differences between first arrivals may have a substantial effect on seismic velocity and thickness sections. 111a semi-automatic approach to identify first arrival time: the cross-correlation technique (cct) figure 7. first break picking (red dots) on real data by the cct (horizontal and vertical axis, respectively, offset (m) and time (s)): (a-b) nearly clean data, (c-d) data with random noises, (e-f) altered waveform and low frequency noises, (g-h) data with more noise in each channel. figure 8. the estimated source wavelets from data in fig. 7 that have been used in cct processes. we used two different seismic refraction data sets (cases 1 and 2) that were recorded with 12 channels to search sliding surfaces in a land-fill area interface. some shots for cases 1 and 2 are illustrated in fig. 9. is the data does not include more noise than the data of types a and b. we chose this data set specifically to demonstrate the importance of small time differences between traditional picking and the cct in tomographic solutions. however, because we lack well-log information about the working area, we avoid comparing both tomographic results in terms of structural interpretation. therefore, we prefer to work with reliable and fairly clear data sets. both data sets were picked using the traditional method and cct (fig. 10). the obtained maximum, minimum and mean absolute time differences of first breaks for each shot are shown in table 2 for each case. we used to the same tomographic inversion parameters for both data sets in each case to makeprovide a better comparison. tomographic results for cases 1 and 2 are shown in figs. 11 and 12, respectively. generally, the depth of layer interfaces are nearly the same, but their resolution and stability are comparable. this is exactly our main intention. in table 2, the mean absolute time difference for case 1 is 1.212 ms.. although this value seems small, it can cause unexpected effects on the solution. at the first glancesight of fig. 11, layer interfaces are flatter in the traditional result, so the lateral resolution is lower than that obtained by the cct. this result occurs because the human operator inherently desires to pick first breaks on an approximately straight line and based on a trend of first breaks on neighboring channels. therefore, each channel cannot be evaluated independently in the traditional picking procedure; thus, the dependency inherently causes loss of resolution. in contrast, in the cct, first breaks are picked independently on each channel. in this way, the lateral resolution of velocity sections can increase, and more reliable results can be obtained. in fig. 11, the area between 23-26 m on the first layer interface (marked by i) is unexpectedly truncated instead of continuing toward the end of the line. in contrast, the continuity of the first layer interface in this area can be clearly observed from the cct section. moreover, the traditional velocity section has a scattered and nebulous area for the second layer interface between 20-23 m (marked by ii), but this area is more stable in the cct section. in case 2, the increases in lateral resolution for the first and second interface are clearly seen in the whole cct section, even though the mean difference is only 1.370 ms between data sets. 2 4 6 8 10 1214 16 1820 22 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (a) 2 4 6 8 10 1214 16 1820 22 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (a)(b) 2 4 6 8 1012 1416 18 2022 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (c) 112 mustafa senkaya, hakan karslı 2 4 6 8 10 1214 16 1820 22 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (d) 2 4 6 8 10 1214 16 1820 22 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (e) 2 4 6 8 10 1214 16 1820 22 24 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 (f) figure 9. (a,b,c) seismic refraction records from different shot points for case 1 (a,b,c) and case 2. (d,e,f). examples for case 2. 0 2 4 6 8 10 12 14 16 18 20 22 24 26 distance (m) 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 ti m e (m s) 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 tim e (m s) manual cct (a) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 distance (m) 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 ti m e (m s) 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 tim e (m s) manual cct (b) figure 10. picking of first arrival times by the cct and traditional method for (a) case 1 and (b) case 2. the green stars show the shot locations. table 2. comparison of time differences between cct and traditional method. case 1 case 2 shot number max(abs(dif)) (ms) min(abs(dif)) (ms) mean(abs(dif)) (ms) max(abs(dif)) (ms) min(abs(dif)) (ms) mean(abs(dif)) (ms) 1 1.506 0.033 0.654 2.682 0.082 0.911 2 3.292 0.187 1.091 3.450 0.130 1.423 3 3.825 0.099 1.427 4.225 0.095 1.547 4 3.063 0.283 1.848 3.380 0.250 1.798 5 2.862 0.025 1.039 2.350 0.125 1.170 mean 1.212 1.370 max(abs(dif)): maximum absolute differences min(abs(dif)): minimum absolute differences mean(abs(dif)): mean of absolute differences for a shot mean: mean of absolute differences for all shots figure 11. comparison of the tomographic inversion results for case 1 using (a) the traditional method and (b) the cct. the color scale shows p-wave velocity. figure 12. comparison of tomographic inversion results for case 2 using (a) the traditional method and (b) the cct. the color scale shows p-wave velocity. 113a semi-automatic approach to identify first arrival time: the cross-correlation technique (cct) 5. conclusions in this paper, we introduce a new procedure based on a semi-automatic first arrival pick, called the cct. the procedure overcomes traditional picking difficulties and provides an accurate, reliable method of picking first arrival times with a basic mathematical approach. the obvious benefits of the cct are that the algorithm allows detection of travel times on each channel independently and reduces misleading human operator effects. moreover, the picking of first arrivals can be performed without random noise and complex near surface effects due to the nature of cc. synthetic and real data examples show that the estimation of a seismic source is the pivotal decision for success because the source function is the basic information being searched for during the cc process. hence, if the input of cc has ambiguities, the correlation section may produce deceptive correlation values. to avoid possible erroneous picking, the user must maintain an active position during the determination of the possible source wavelet and mark borders of an automatic searching area on the correlation section. the resulting tomograms show that small time differences in first break pickings are important and effective on all results that can be obtained from refraction data. acknowledgements this work was funded by karadeniz technical university-scientific research foundation (project code no: 2009.112.007.2). additionally, we would like to thank the anonymous reviewers and editors for their useful comments, which allowed this paper to be improved. references baer m and kradolfer u. 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hydrocarbon deposits; neo-tectonic activity; horizontal compressive strain; vertical forces. formation of oil and gas reservoirs in the great depths of the south caspian depression issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n4.64008 earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017): 169-174 gahraman nariman ogly gahramanov state oil company of azerbaijan republic gahraman@inbox.ru el objetivo de este artículo es la identificación y caracterización de los complejos factores que actúan en la formación de depósitos de gran tamaño en lo profundo de la cuenca al sur del mar caspio, al igual que el desarrollo de un modelo de formación de depósitos de hidrocarburos basado en la combinación de procesos naturales sincrónicos. este artículo analiza las causas de la presión de las capas altas en los campos de hidrocarburos ubicados al sur de la depresión cáspica y sus factores de impacto. además, se propone el componente tectónico y el modelo de densidad barométrico como los criterios más importantes para la selección de proyectos prioritarios de exploración y la escogencia de lugares para perforación de pozos. los resultados obtenidos permiten realizar estudios de exploración y desarrollo en los campos de petróleo y gas condensado en el sur de la depresión cáspica. resumen palabras clave: formación de plegamientos, depósito de hidrocarburos; actividad neotectónica; esfuerzo horizontal de compresión; fuerzas verticales. record manuscript received: 13/04/2017 accepted for publication: 25/08/2017 how to cite item gahraman gahramanov, o.n.(2017). formation of oil and gas reservoirs in deep water areas of the south caspian depression. earth sciences research journal, 21(4). 169-174 doi:http://dx.doi.org/10.15446/esrj.v21n4.64008 formación de depósitos de petróleo y gas en las profundidades al sur de la depresión cáspica g eo lo g y o f o il a n d g a s 170 gahraman nariman ogly gahramanov introduction the scd is the most deep-sunk sedimentary basin in the world and occupies the entire southern deep-water part of the caspian sea and its adjoining kur and western-turkmen depressions in the west and east, respectively. the scd is the most deep-sunk sedimentary basin in the world and occupies the entire southern deep-water part of the caspian sea and its adjoining kur and western-turkmen depressions in the west and east, respectively. it is a unique geological structure in the earth’s crust in terms of sedimentation conditions, thickness and rate of deposition, history of geotectonic development, oil and gas formation, oil and gas accumulation and depth of their deposition, reserves, volumes, and geological structure of reservoirs. the scd is a relic of the large caucasian south caspian back-arc marginal sea, which emerged in the middle jurassic period and differs from deep-water basins in inland and marginal seas of the world: the scd has the thinnest (6-8 km) consolidated oceanic crust and the thickest sedimentary cover (25-30 km). the thickness of pliocene-quaternary sediments is 10-12 km. at that, the thickness of the main oil and gas -pay red rocks horizons of the pliocene – is 7-8 km. the scd was formed in the arabian-eurasian collision zone. it is surrounded by the mountain ranges of the greater caucasus, kopet dag, talysh, and elbors (mamedov, 2008; aliyev, 2005; eybatov, 2010). effective exploitation of hydrocarbon (hc) resources in the scd requires a development of a new scientific bases and methodological solutions of relevant tasks. the creation of theoretical models of these scd deposit formations is an unprecedented and complicated task. at present, hydrodynamic modeling methods for hydrocarbon deposits, computer and programming technologies are being developed rapidly, which allows designing detailed numeric hydrodynamic models with account for the most factors that affect the development of deposits. in particular, this approach is described in detail in by j. hunt (2013). this allows using in the output data for mathematical modeling the entire source material that is accumulated at oil fields by the documentation of the state of development by wells and the deposit in general. however, one should not forget that a numerical geological model is the basis for the detailed hydrodynamic model (mstislavskaya et al., 2005; zhang, 2012; carvalho, 2013) shaymardanov et al. (2003) and abasov et al. (2007) established that the most productive structures (deposits) at great depths are formed in favorable conditions of earliergenerated and preserved hydrocarbon gases, i.e. where gas did not escape through cap rocks or they were not destroyed by mud volcanoes or other geological processes. preservation of hc at great depths is also facilitated by “sealed” deposits and lithological traps (bochever et al., 1972). under these conditions, hc preservation is determined by a weakened draining-out of formed gas during oil destruction due to significant deceleration of its diffusion. theory and practice showed that the probability of finding “sealed” deposits and high-capacity lithological traps is low (kartsev et al, 1992; gurbanov et al., 2014). the creation of comprehensive geological three-dimensional filtration models, based on the combination of their preconditioning natural simultaneous processes (norman, 2001), is a promising area of theoretical and applied geology that allows taking a big step towards the solution of this problem. intensive hc generation due to the existence of a powerful deep source is a geochemical prerequisite of hc deposit formation (abasov, 2008). another question of interest: can folded structures of the sedimentary basin, which appeared in the scd, or poor folds be further developed by means of internal force impact in the hc deposit itself? at that, the creation of a theoretical model of formation of such deposits is a complicated task. the difficulties are determined by a wide range of geological conditions and the lack of relevant information (popkov et al, 2014). limited data on these underground processes in hc deposits, located in the scd (kazmin et al., 2011), allow designing only a conceptual model of hc reservoir formation (baak, 2010), based on the natural peculiarities of oil and gas deposits at great depths. the conditions of the tectonic development of the scd and its subsidence primarily in the neogene-quaternary period determined the accumulation of a thick flysch formation and the accumulation of large oil and gas deposits thereon (gurbanov et al., 2014). interdisciplinary modelling shows that subsidence and sedimentation in the scd resulted from thermal subsidence following jurassic rifting and further enhanced subsidence due to sedimentation during in the neogene-quaternary (alizadeh et al., 2017). based on advanced analytical methods, hydrocarbons of the lower pliocene productive series (the main reservoir) are of epigenetic origin (feyzullaev et al., 2015). they have been formed due to subvertical impulsive migration from the oligocene-miocene sediments. two main phases of oil migration produced commercial accumulation in the productive series. the following questions arise: 1. which factors preconditioned the accumulation of such thick hydrocarbon-bearing sedimentary layers and which forces played the main role in the formation of large oil and gas deposits? 2. why are deposits with satisfactory reservoir properties (rp) encountered at such great depths in the scd area? this should not happen at such depths. the purpose of this study is to determine and characterize the complex of simultaneously acting factors of the formation of large reservoirs at great depths of the scd, and the development of a model, based on the combination of their preconditioning natural simultaneous processes. method kuliyev et al. (1998) shows that there are several variants of the mechanism of formation of folds by means of “internal” factors of the sedimentary basin. in the present paper, these results were taken as the theoretical aspects of the proposed concept. under certain conditions, abnormally high reservoir pressures (ahrp) are formed and, consequently, sediments are compacted. this process mostly occurs in the central, the most submerged parts of the sedimentary basin, mostly comprised of clay rocks. rock compaction primarily depends on the effective pressure (pef) (kuliyev et al., 1998): pef = prc cppor (1) where (prc) is the pressure of the overlying sedimentary rocks, (ppor) is the pressure of fluids that saturate rocks (pore pressures); c is the load coefficient that varies from 0 to 1. the studies conducted in this area, showed that rock porosity can be determined by the following formula (2) where , is the rock porosity under pef pressure and without compacting pressure, respectively, β is the irreversible compaction coefficient. taking into account the fact that porosity is related to density by formula (3) where is the matrix density, is the proportionality coefficient, formula (3), taking into account (1), can be written as (4) 171formation of oil and gas reservoirs in the great depths of the south caspian depression figure 1. left: location of the petroleum and anticlinal structures of the south caspian megadepression. 1 anticlinal structures; fields: 2 oil, 3 gas, 4 oil and gas, 5 oil and condensate. from formulas (1) and (4) it follows that under other various conditions, marginal areas of basins will be characterized by the greatest level of effective pressure and rock density (kuliev et al., 1998). in the central parts, where pore pressures are close to geostatic ones, effective pressure will be significantly lower and can even theoretically be close to zero (mamedov, 2008; kuliyev et al., 1998). consequently, such discrepancies in the levels of compacting load directly affect rock density, in particular: rock density in the horizontal section reduces from the periphery to the center of the basin. results the location of oil and gas and anticlinal structures of the south caspian megadepression, which gives an idea of the location of major hydrocarbon deposits in the study region is presented in the fig. 1. due to specific geological, geochemical, pressure, and temperature conditions of hc accumulation in traps and its preservation in deposits at great depths, excessive reservoir pressure is a crucial factor. it is determined by the size of the reservoir, the presence of ahrp beyond its boundaries, and the difference in fluid density. the formation of larger and thicker structures is investigated as a process of accumulation, selforganization, and self-preservation of natural oil and gas reservoirs at great depths. this allows for a new approach to the problem of folding and formation of large fields on a local scale in the scd itself. the formation of anticlinal traps is known to be accompanied by an almost simultaneous development of the sedimentary basin itself. the theoretical substantiation and adequate modeling of values, the direction and nature of the effect of pef, prc, and phyd (hydrostatical pressure) factors, and the possibility of gas-dynamic and geodynamic pressure effect are the determinants of the scale and form that implement structural changes. in the anticlinal structures of a gas condensate field, especially with low hc density, in this phase, pressure, which corresponds to the far-wing parts of the structure can be transferred to its crest without significant changes. this zone is characterized by excessive reservoir pressure. the considered mechanism of formation and reformation of gas condensate fields up to large sizes due to excessive pressure in the elevated structure parts of the scd provides a more reasonable explanation of their vertical and horizontal zonation. in the authors’ opinion, the preservation of large hc accumulations in the scd deposits can be governed, primarily, by two reasons. the first one is the type and size of traps, with anticlinal structure and deposit thickness being of great importance. the second one is the intensive generation of gas due to catagenetic destruction of organic matter (om) under the elevated pressure and temperature conditions and influx of gaseous fluids into traps. it is worth noting that the scale of oil catagenetic transformation is affected by both its genetic peculiarities, i.e. the initial hc content of oil, and the pressure and temperature conditions, required for the change of the hc phase structure. therefore, liquid hcs, which have the greatest resistance to thermal action, are located at great depths. at present, the initial level of reservoir pressure decreased significantly in the gas condensate fields of many regions. due to the development of gas condensate fields, the reduction of reservoir pressure could affect the changes of hc phase structure in these fields to the liquid phase, since under favorable temperature conditions, the level of reservoir pressures is important during this process. the decisive factor of great depths is the capacity and reservoir properties (rp) of rocks. the temperature conditions in the scd are favorable for the preservation of hc. the temperature reaches 120°c at 6 km depth (guliyev et al., 2011). therefore, the main factor that deteriorates the rp of rocks with depth is pressure, i.e. increased rock pressure (yusifzade, 1985, 2013). in the authors’ opinion, the preservation of large hc accumulations in the south caspian depression deposits can be determined by two reasons. hc-containing reservoirs, in the thick elevated anticlinal structures, have a relatively increased reservoir pressure due to the growth of excessive pressure. the growth of excessive pressure in the elevated part of an anticlinal structure is caused by the difference in the density of deposit water and gas. it increases with temperature growth. the reservoir pressure in the contour, where the relative depth increases, in thick structures in gas fields, is almost transferred to the crest of the deposit. the deposition depth in the crest of the anticlinal structure decreases significantly compared to the wings. reducing the effective pressure level facilitates the deceleration of rock compaction, sometimes even decompaction. this helps preserve or even slightly reduce the primary capacity and reservoir properties of rocks. in reducing the negative impact of rock pressure on the horizon and preserving rp, high reservoir pressure in the crest of a depressed structure during simultaneous hc accumulation facilitates the loss of stability of the considered environment, the growth of the anticlinal structure itself and, subsequently, the continuation of hc accumulation in this trap, which ultimately forms large reservoirs. the most important criterion for the estimation of oil and gas basins’ prospect is the volume of sediment accumulation and the frequency of discovery of large and giant oil and gas fields (aliyev, 2007). the scd volume of sediment accumulation is estimated to be ca. 3.25 million km3. the thickness of the sediment complex is 25 km. the complex consists of a thick stratum of sediments of primarily terrigenous and carbonate content from the jurassic to the quaternary period. most oil and gas (gas condensate) deposits, discovered in the pay horizon of azerbaijan are limited to the “absheron facies”, formed by deltaic deposits of the paleo-volga (aliyev, 2007). the sediment accumulation of compacted sand and clay, alongside a mass of diatoms, high temperatures in the depression zone (absheronbalkhan depression) and the counter count of high-temperature fluids, created ideal conditions for oil and gas accumulation (abasov et al., 2007). since the 1970s, a number of promising oil and gas condensate fields have been discovered in the absheron archipelago of the azerbaijan sector of the caspian sea. these include such large fields as azeri-chyrag-guneshlil, shah deniz, large structures of kyapaz, umid, safar-mashal, nakhchivan, and others. the main deposits of hydrocarbons (over 70%) in abovementioned fields are concentrated in the “pereriv” suite and the x layer of the pay horizon (yusifzade, 1985). the geological structure of the area is formed mostly by pliocene-quaternary sediments with a thickness of 3700-4000 m. on the regional scale a certain growth in reservoir pressures is observed, where the anomaly coefficient reaches 1.05-1.07 on average. on the local scale, i.e. within hc deposits, the level of reservoir pressures can exceed the hydrostatic pressure significantly. at that, the anomaly coefficient can go up to 1.7 and more. 172 gahraman nariman ogly gahramanov of pef, prc, and ppor strains (figure 2). in the anticlinal structures of gas condensate fields, especially in the conditions of low hc density, the pressure that corresponds to the far wings of the structure also affects its crest without significant changes. thus, such zones are characterized by excessive reservoir pressure. due to excessive pressure in the elevated parts of the structure in the scd, distribution of vertical forces in the deposit area provides for a more substantiated explanation of the mechanism of formation and reformation of gas condensate fields up to large sizes (kuliyev, 1998). estimations of excessive pressures for various thicknesses and depths of gas condensate deposits were carried out in order to determine the regularities of the changes in excessive pressure and the gradients of reservoir pressure with depth (table 1). figure 2. schematic cartoon that explains the distribution mechanism of pressures (prc, ppor, pef) in the mentioned fields that created excessive pressure in the crest of the anticlinal structure, which causes structure growth (a1, a2, a3) with increase of deposition depth. a) density-baric model; b) discovered and assumed structure; c) diagram of the hydrostatic, geostatic, and reservoir pressures. 1 – current position of the structure; 2 – structure at the early stage of development; 3 – mud volcano; 4 – assumed structure; 5 gas-dynamic system; 6 sea bottom; 7 smooth bank; 8 sea level; 9 water and gravity system; 10 decompaction zone; 11, 14 pef – 38 mpa; 12, 13 ppor – 70 mpa; 15 compaction zone; 16 0,1 mpa/m; 17 prv (excessive pressure) in well no. 1; 18 0,24 mpa/m; 19 well no. 1; 20 well no. 2. table 1. information on the distribution of primary geological reserves of gas with depth at the western edge. the following question arises can folded structures of the sedimentary basin in the scd, or poor folds be further developed by means of internal force impact in the hc deposit itself notwithstanding the external force impacts, in particular, tectonic forces? the reason for the emergence of intrabasin linear structures (there are seven such lines in the absheron archipelago and four in the baku archipelago) is the horizontal compressive strain, which causes folding in the sedimentary cover in the scd and subducting in the consolidated crust. another question arises: can folded structures of a sedimentary complex be formed by internal force impact of the basin itself? during sedimentation, horizontal gradients of effective pressure and density are formed in the basin, which are directed from the center to the periphery. i.e. the basin, essentially, generates internal compressive horizontal forces, independent on the external forces effect. the magnitude of such strain can be sufficient for the development of folds. the probability of folding in this region is high. the period of origination and youth of the sediment basin are inherent in all basins. therefore, the described mechanism is typical of all basins, irrespective of their geological age. both lithological and geotectonic reconstruction should consider the possibility of folding due to the internal forces of the sediment cross-section (kuliyev et al., 1998; babayev et al., 2014). a similar process occurred in the pay horizon (ph) of the scd, since in the deep-lying horizons, there is a high probability of favorable conditions for the formation of hc in cases of deposition at depths of less than 2.54.5 km. the constructed profile for the determination of the influence of effective pressure shows changes in the sizes of the structure with the increase of the deposition depth of the pay horizon and the distribution figure 3 presents charts of changes in the anomaly of reservoir pressures with depth, depending on deposit thickness, which shows how important the thickness of gas condensate deposits for the formation of large fields and the conditions of preservation of deposits (reservoirs) at great depths. in the crest and far wings of the structure, pef values differ significantly and increase with the growth of the structure and, consequently, the preservation of reservoir properties occurs primarily in the elevated parts of the structure. the local maximum pef in the elevated part of the structure creates the difference in anomalies both horizontally and vertically. the ∆pef gradient, which affects the long period even at small values, affects the formation (growth) of the anticlinal structure. thus, other thing being equal, the greatest levels of rock pressure and density will be characteristic of the marginal and aquifer zones of the deposit. in the central parts, the effective pressure will be significantly lower. in the crest of the structure, due to a relatively small deposition depth, rock pressure will be the lowest, while internal pressure will be the highest. it is also expedient to examine the mechanism of emergence of horizontal compressive strains in a quick subsidence regime (figure 4). in a submerging basin, other mechanisms of formation of horizontal compressive strains are possible. assume that within the abovementioned geological situation, i.e. due to substance differentiation, the values of forming horizontal compressive strains are insignificant. 173formation of oil and gas reservoirs in the great depths of the south caspian depression figure 3. gas condensate deposits. changes in excessive pressure (prv/pex) with depth, depending on the deposit thickness of 1, 2, 3, 4 – 500, 1000, 1500, 2000 m, respectively. figure 4. the schematic cartoon of structural elevation under the effect of forces. it is considered geostatic, hydrostatic, effective, and excessive pressure, with account for sedimentary basin pressures. sedimentary basin pressure is only explained by influence of sedimentary cover. more preferential mechanism is the combination of loading the edges of the basin and large horizontal compressive forces due to the syn compressional bending associated with subduction of the scd (abdulaev et al., 2015). after a certain geological time, the magnitude of such horizontal strains can be sufficient for the process of elastic instability of the competent bed (kuliyev et al., 1998). next, it is expedient to investigate the mechanism of formation of scd structures during the emergence of vertical forces. the studied structures differ from other folds in the studied region in relatively large sizes and mostly isometric dislocation. these structural features confirm the dominance of normal vertical forces in their formation. a very important peculiarity of the folding process is the possibility of the growth of folds together with sedimentation. this process should result in the natural change of thicknesses and facies of individual stratums. their thickness should mostly be reduced in the crests of anticlines and increased in the most depressed parts. the facies in the crests of the anticlinal syngenetic horizon should include several coarse sediment fractions, while in the depressions they should be thinner. the sediments will not “die out” with depth, while the dying out of deformations can occur in the upper intervals. in certain cases, epigenetic deposits can occur at great depths. for example, due to intensive subsidence of ph deposits in the scd, the structures of the absheron peninsula were subjected to strong effect of folding. consequently, the crests of the lower and upper structures of the ph often do not coincide, sometimes shifting over large distances. discussion in general, the formation of large reservoirs at great depths of the scd is determined by the effect of a set of simultaneous factors, the most important of which, in the authors’ opinion, are: 1. the presence of excessive pressure determined by the hc deposit thickness, which height changes depending on the deposit thickness and depth, and hc density. for example, at a depth of 7 km and a deposit thickness of 2000 m, excessive pressure is 14-15 mpa. 2. the presence of anomalously high reservoir pressures in ph, which increase significantly with hypsometric depth. for example, in the study region of the scd, at a depth of 7 km, ahrp level can reach relatively high levels, since in marine fields at a depth of 6-6.5 km, the prv/phyd value varies from 1.3 to 1.7. 3. under intensive subsidence of the scd due to the effect of avalanche sedimentation and flow, the thickness of the ph and its depth created appropriate pressure and temperature conditions at great depths of the scd, where hc migrated from the area with high reservoir pressures to the area with low reservoir pressures. in the conditions of intensive submersion of the scd because of the effect of abovementioned forces, depositional structures are formed and grew, which created large reservoirs. hc migrated from the area of higher initial gradients of reservoir pressure (depressed zones) to areas of low gradients (the most elevated zone). traps, filled to the hinges ended up an enclosed on all sides by high potential zones, where the process of hc migration and accumulation continued, which required a growth of the gas-containing capacity of traps at great depths under high reservoir pressures. at that, high potential zones grew under the effect of abovementioned forces, creating a range of high strains across the entire ph section above the structure. consequently, the mechanism of formation of large reservoirs in the ph at great depths of the scd is determined by a simultaneous effect of a set of factors and the presence of conditions, related to the peculiarities of the geological structure of the depths. the peculiarities are lithofacial change in the sediments of the ph and its intermittent structure along the section, which are characterized by the presence of reservoirs or interconnected reservoirs that form a single hydrodynamic system, on the one hand, and a powerful cap of clay strata with continuous migration of hc under pressure gradients from submerged zones and their accumulation and self-preservation in traps, on the other hand. from this, it follows that at such great depths, small deposits cannot exist. favorable conditions for the natural preservation of reservoir properties are created only in large and thick deposits. the suggested density-structure-fluid theory of formation of large fields in the scd can be used for both explored fields and for explorations in this region, with a view to modelling above mentioned fields. the suggested theory can increase the probability of discovering oil and gas fields during explorations of regions, located in similar geological conditions. at that, the main factor that resists the negative effect of rock pressure on the rp of rocks at great depths is the reservoir pressure. thus, the main factor that resists the negative impact of rock pressure on rp of rocks at great depths is the reservoir pressure, which is directly dependent on its size. under these conditions, hc-containing reservoirs in the thick elevated anticlinal structures have a relatively increased reservoir pressure due to the growth of excessive pressure. the growth of excessive pressure in the elevated part of an anticlinal structure is caused by the difference in the density of deposit water and gas. the excessive pressure increases with temperature growth and the reservoir pressure in the contour, where the relative depth increases, in thick structures in gas fields, is almost transferred to the crest of the deposit. in addition, the deposition depth in the crest of the anticlinal 174 gahraman nariman ogly gahramanov structure decreases significantly compared to the wings. (other sources exist, which facilitate the reduction of rock pressure towards the crest). reducing the effective pressure level facilitates the deceleration of rock compaction, sometimes even decompaction. this, in turn, helps to preserve or even slightly reduce the primary capacity and reservoir properties of rocks (perhaps, form fractures). thus, continuous accumulation of hc in traps, growth of anticlinal structure sizes, especially the increase in the thickness of the structure, deceleration of rock compaction, especially in the elevated part of the structure are interconnected. these facilitate the preservation of large hc clusters at great depths of the scd, which is an important scientific substantiation of hydrocarbon extraction processes and ensures enrichment of scientific knowledge with the possibility of their direct use for the solution of practical problems. conclusions the result of the study is the compilation of a list and characteristics of the set of simultaneously acting factors invoked in forming reservoirs at great depths in the scd. they are the presence of excessive pressure; the presence of abnormally high reservoir pressure in pay horizons, the thickness and occurrence depth of which created respective pressure and temperature conditions at great depths of the scd, as well as developing a model based on the combination of their preconditioning natural simultaneous processes. this paper suggests the possibility of development of autonomous folding due to the formation of internal pressures in the hc field (deposit) itself which the author called the structure-fluid theory of large-amplitude structure formation in the scd. the density-baric model reflects one of the most important physical, geological, and industrial characteristics of reservoirs. consequently, the tectonic factor and its density-baric model can be the main criterion of the primary objects for exploration, the optimal choice of primary programs for drilling, and for the placement of exploratory and industrial wells. this model allows distinguishing potentially dangerous areas for drilling wells at the beginning of the field exploitation. to conclude, it should be noted that both studied types of folding with differences in their formation mechanism, could have correlations that are more complex. in the authors’ opinion, folds of the singular large structures can be relatively simple. noted regularities in the distribution of facies and thicknesses of deposits that form the studied large folds, are convincing proof of the leading role of vertical forces in their formation. references abasov, l. g., kondryushkin, m.t., aliyarov, yu.m., krutykh & r.yu. 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(2012). north ustyurt basin structure evolution and basic petroleum geological characteristics. inner mongolia petrochemical, 3(1), 133-134. there are enormous resources of unconventional gas in coal measures in ordos basin. in order to study the geological characteristics of unconventional gas in coal measures in ordos basin, we analyzed and summarized the results of previous studies. analysis results are found that, the unconventional gas in coal measures is mainly developed in upper paleozoic in eastern ordos basin, which including coalbed methane, shale gas and tight sandstone gas. the oil and gas show active in coal, shale and tight sandstone of upper paleozoic in ordos basin. coalbed methane reservoir and shale gas reservoir in coal measures belong to “self-generation and selfpreservation”, whereas the coal measures tight sandstone gas reservoir belongs to “allogenic and self-preservation”. the forming factors of the three different kinds of gasses reservoir are closely related and uniform. we have the concluded that it will be more scientific and reasonable that the geological reservoir-forming processes of three different kinds of unconventional gas of coal measures are studied as a whole in ordos basin, and at a later stage, the research on joint exploration and co-mining for the three types of gasses ought to be carried out. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): q1 q5 abstract keywords: ordos basin, unconventional gas in coal measure, coalbed methane, shale gas, tight sandstone gas, upper paleozoic. geological characteristics of unconventional gas in coal measure of upper paleozoic coal measures in ordos basin, china record manuscript received: 12/11/2015 accepted for publication: 19/02/2016 how to cite item he, j., zhan, x., ma, l., wu, h., & ashraf, m.a. (2016). geological characteristics of unconventional gas in coal measure of upper paleozoic coal measures in ordos basin, china. earth sciences research journal, 20(1), q1-q5. doi: http://dx.doi.org/10.15446/esrj.v20n1.54140 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54140 pe t r o l e u m g e o l o g y jinxian he,1,2* xiaoli zhang,1,2 li ma,1,2 hongchen wu,1,2 muhammad aqeel ashraf3 1school of resources and geosciences, china university of mining and technology, xuzhou, jiangsu, 221116, china; 2 key laboratory of coalbed methane resource and reservoir formation process, ministry of education(china university of mining and technology), xuzhou 221008 china; 3 faculty of science and natural resources, university malaysia sabah 88400 kota kinabalu sabah, malaysia * corresponding author: jinxian he, email: jxhe@cumt.edu.cn introduction unconventional gas in coal measures is all natural gasses which are generated through evolution when the parent materials of source rocks underwent coalification of biochemistry and physiochemistry. it includes gasses such as coalbed methane, shale gas, tight sandstone gas, natural gas hydrate (cao et al., 2014; wang et al., 2014). unconventional gas resources of coal measures have a huge amount (table 1). according to the principle of “giving priority to coal, comprehensive exploration, comprehensive evaluation” of the geological survey of coal resources in china (deng et al., 2006), improving the research of the gas resources associated with coal in coal measures is of great significance. table 1. distributional situation of gas resources of coal measures in ordos basin and china (zhao et al., 1998; feng et al., 2002; liu et al., 2009; jie, 2010; zhang et al., 2012a; yang et al., 2015) in chinese coal measure strata, because the coal seam, dark mud shale, and tight sandstone interbedded deposit, the geological phenomenon which several kinds of coal measures gasses jointly preserve is relatively prevalent. scholars have agreed to take unconventional gas in coal measures as a system of research and development. they also agreed to carry out joint exploration and exploitation for multi-gasses (qin et al., 2014; cao et al., 2014; wang et al., 2014; gao et al., 2014). the comprehensive research of unconventional gas in coal measures is the trend in the future(qin et al.,2016; liang et al.,2016a).it may improve the production of the single well, reduce production cost and increase economic benefit that multigasses are jointly exploited from a well(qin et al.,2016; liang et al.,2016a). additionally, in coal mining, because the surrounding rock gas of coal seam is a primary source of the coal mine gas, the coal measures’ gas (not only coalbed methane) drainage helps reduce the hazard of coal mine gas and efficiently reduce greenhouse gas emissions. 1 regional geological characteristics as the second largest sedimentary basin in china, ordos basin is also an important coal-bearing and oil-bearing one. the whole basin may be divided into six first-order tectonic units. in the basin, the gas resources of q2 jinxian he, xiaoli zhang, li ma, hongchen wu and muhammad aqeel ashraf upper paleozoic coal measures mainly distribute in the east and principally includes the east of yanan area in the yishan slope and the deflection fold belt in western shanxi (fig. 1). in the eastern ordos basin, greater total thickness, multi-beds, and stable distribution of coal seams developed in the upper paleozoic coal measure strata; at the same time, some dark mud shale and tight sandstone interceded with these coal seams. it provides the abundant material basis for preservation and development of coalbed methane, shale gas, and tight sandstone gas. previous researches indicated that coalbed methane, shale gas, tight sandstone gas jointly developed in the upper paleozoic strata in the eastern ordos basin (including linxing area) (guo et al., 2012; guo and zhao, 2014), therefore providing a favorable geological condition for carrying out joint multi-gases exploration and exploitation. figure 1. sketch of regional tectonic division of ordos basin. the developed lithologies in upper paleozoic of ordos basin are mainly coal seam, sandstone, mud shale, and few carbonatite. benxi formation, taiyuan formation, shanxi formation, shihezi formation, and shiqianfeng formation, from bottom to top, developed in sequence. of them, taiyuan formation and shanxi formation are leading strata containing coal, followed by the development degree of benxi formation (table 2). therefore, taiyuan formation and shanxi formation are also the most important target beds for exploration and exploitation of coal measures gases. 2 coalbed methane coalbed methane resource in ordos basin reached 10.72 × 1012 m3 and accounted for about 1/3 of the country’s total volume. therefore, it played a significant role in the distribution of chinese coalbed methane resource. in the basin, about 1/2 of the coalbed methane resources distribute in its eastern margin (table 1), making the east margin of ordos basin one of the most principal explored and exploited areas for coalbed methane in china. at the same time, it is also the area which is most explored in ordos basin (feng et al., 2002). at present, commercial exploitations for coalbed methane have been launched. in eastern ordos basin, it was well developed that upper paleozoic carboniferous-permian coal measure strata, the layers of coal seams are more, and the distribution is relatively stable. coal seams concentratedly drawn up in taiyuan formation and shanxi formation. of which taiyuan formation contains 3~7 layers of coal seams, and 2~4 layers can be exploited; total thickness is 4.3~7.1 m. shanxi formation contains 4~8 layers of coal seams, and 2~3 layers can be exploited; total thickness is 3.1~21.8 m. burial depths of coal seams are less than 1500 m. in the eastern margin of ordos basin, main coal seams overall take on a tendency of “thick north and thin south” (zhang et al., 1996). the distributional ranges of coal are from long flame coal to meager coal, concentrating on medium rank coal. moreover, the vitrinite reflectance ro is between 0.5 and 2.2 % (jie, 2010). table 2. stratigraphic division and lithological developing characteristics of upper paleozoic in ordos basin (yan et al., 2013) table 3. geological characteristics data of main coal seam of upper paleozoic in ordos basin (jie, 2010) in this area, the fractures of cleats of main coal seam were comparatively developed; density of endogenetic fracture is 10~30 pieces/ 5 cm. overall, the physical characteristics of coal reservoir are relatively favorable, and permeability of main coal seam is 0.22×10-3 μm2 ~ 8×10-3 μm2. reservoir pressure gradient of the main coal seam is 0.5~0.9 mpa/100 m; coal seam desorbing pressure is 1.7 ~ 4.0 mpa; and gas saturation of coal seam is 55 – 86 % (table 3) (jie, 2010). the gas-bearing feature of the coal seam is relatively favorable, and gas content of the main coal seam is 4 ~ 20.87 m3/t. in particular, coalbed gas content of weibei block is 6 ~ 14 m3/t; the coalbed gas content of linfen block is 9.03 ~ 20.87 m3/t; the coalbed gas content of lvliang block is 4 ~ 14 m3/t (jie, 2010). the above data suggests an excellent potential of coalbed methane resources. some scholars also studied the origin of coalbed methane in the eastern margin of ordos basin. the studying results show that the methane carbon isotope distributes in -70.5 ‰ ~ -36.19 ‰ and evidently from north to south it gradually becomes weighting. mixed gas, including resulting biogas and thermogenic gas, occurs in the peripheral and shallow part, and thermogenic gas is within the middle and profound part (li and zhang, 2013). through the study of the origin of coalbed methane in hancheng region of southeastern ordos basin, ma et al. (2011) discovered that the methane carbon isotope of coal seam was in -42.978 ‰ ~ -32.200 ‰ and ethane carbon isotope belonged to -21.619 ‰ ~ -9.751 ‰. therefore, they considered that the origin type of coalbed methane in this region was predominantly thermal degradation gas. 3 shale gas in coal measures in ordos basin, upper paleozoic carboniferous – permian source rocks are predominantly dark mud shale; organic matter is abundant; thickness is stable; the distributional area is large; the maturity is high. the rocks of origin mainly distribute in benxi formation, taiyuan formation, and shanxi formation. q3geological characteristics of unconventional gas in coal measure of upper paleozoic coal measures in ordos basin, china the total thickness of mud shale is relatively high, with that of dark mud shale in benxi formation averaging at about 29.3 m, that of taiyuan formation at 39.6 m and that of shanxi formation at 63.2 m (table 4). the buried depth of upper paleozoic mud shale is between 2000 and 3500 m (wang et al., 2011). therefore, it provides an appropiate material basis for the formation of shale gas reservoir of coal measures, and its resource total extent is also tremendous. table 4. geological characteristics data of mud shale of upper paleozoic in ordos basin (yan et al., 2013) shale gas is an unconventional kind that is self-generation and selfpreservation. mud shale is not only a hydrocarbon-generating bed but also reservoir. geological characteristics data of the shale of upper paleozoic in ordos basin can be seen in table 5. in the basin, the average organic carbon content of upper paleozoic mud shale reaches 1.72 %; kerogen type is mainly iii; higher maturity of organic matter; ro value distribute in 1.6 ~ 2.82 %, thus it belongs to the mature-postmature stage and begins to generate a large quantity of gas at present. the porosity is averagely 1.12 %; permeability is 0.037×10-3μm2. brittle mineral content in shale is generally 46.51 % ~ 54.04 %; clay content, 43.61 % ~ 47.81 %. on the whole, upper paleozoic mud shale possesses the condition for shale gas reservoir to form (wang et al., 2011; yan et al., 2014). gas-bearing coal rock mud shale and gas-bearing tight sandstone mud shale are the optimal source-reservoir-cap combination type (yan et al., 2014). table 5. geological characteristics of mud shale of upper paleozoic in ordos basin (zhang et al., 2012b; guo et al., 2012; yan et al., 2013; yan et al., 2014) the gas content of the mud shale is an important parameter. several scholars have studied the gas content of upper paleozoic mud shale in ordos basin, and the data were collated in table 6. usually, 1.0 m3/t is taken as the lowest limit of industrial standard for the gas content of mud shale. from table 6, it can be known that gas content of mud shale in shanxi formation distribute in 0.96 ~ 2.71 m3/t and the average value is 1.94 m3/t, therefore, exceeding the minimum threshold of industrial standard. however, gas content of the mud shale of benxi formation of mi 35 well is slightly low, and this preliminarily reflects that the reservoir forming conditions of mud shale of shanxi formation are more favorable than benxi formation. in xiasiwan – yunyan region of eastern basin, gas measure and groove face of the dark mud shale interval of shanxi formation from more than 30 wells show active, and display good gas indicator and produced low-yield industrial gas flow (fig. 2) (gong et al., 2013). table 6. gas content characteristics of partly upper paleozoic mud shale in ordos basin (wang et al., 2011; fu et al., 2013; gong et al., 2013) figure 2. gas logging of shanxi formation of x24 well in eastern ordos basin (gong et al., 2013) several suits of mud shale developed in ordos basin. middle ordovician pingliang formation is difficult to form a large scale of shale gas reservoir; triassic yanchang formation mud shale possesses the reservoir forming condition for shale gas and tight oil. however, exploration potential of the tight oil is broader; carboniferous-permian sea-continental transitional facies shale is relatively favorable for shale gas reservoir. it is also the most certain strata series for exploration of shale gas (wang et al., 2011). 4 tight sandstone gas tight sandstone gas resource of upper paleozoic coal measures in ordos basin is very enormous, and the resource is preliminarily estimated to be more than 5x10-12 m3 (table 1). upper paleozoic in the basin extensively developed fine sandstone, siltite, argillaceous siltite, and other tighter sandstone. porosity and permeability of these sandstones average at 6.58 % and 0.77×10-3 μm2, respectively (deng, 2003). for these reasons, they belong to low porosity and low permeability tight sandstone. in ordos basin, it was developed that the fluvial-delta facies tight sandstone reservoir of shanxi formation and shihezi formation (fig. 3), thus this paper mainly discussed the geological characteristics of tight sandstone in these two horizons. the thickness of the sandstone is large, linxing area may be taken as an example. in this area, the thickness of taiyuan formation reaches 4.0 ~ 40.89 m, additionally the thickness of shanxi formation is larger than of taiyuan formation and the distributional scope is also wider (guo et al., 2012). the tight sandstone of coal measures staggeringly developed with mud shale and coal seam and adjoins gas source rock; therefore, it possesses the potential of tight sandstone gas. currently, exploration in ordos basin has made a significant breakthrough and entered the scale development stage. the lithologic character of shanxi formation is mainly feldspar debris quartz sandstone, followed by a few of feldspathic lithic sandstone and lithic sandstone. cement is predominantly silicic, and calcite cementation; grain-supported structure; type of cementation includes mainly pore and mosaic cementation (fig. 3). box 8 segment is chiefly feldspar debris quartz sandstone; apart from that, it also consists of a few of lithic sandstone and lithic quartz sandstone (zhou et al., 2013). q4 jinxian he, xiaoli zhang, li ma, hongchen wu and muhammad aqeel ashraf figure 3. developed characters of tight sandstone of coal measures in ordos basin (zhou et al., 2013) when the organic matter of coal measures transformed to hydrocarbons, organic acid was constantly produced, therefore coal measures strata are rich in organic acid. organic acid has a greater impact on the diagenesis of tight sandstone reservoir because organic acid may corrode feldspar to form a secondary pore. organic acid plays an active and constructive role in improving the physical properties of tight sandstone reservoir. in the basin, average porosity of the sandstone in taiyuan formation is 8.2 % and the permeability, porosity and permeability of shanxi formation average at 0.58×10-3μm2, 7.2% and 7.68×10-3μm2, respectively. also, the median porosity of sandstone and permeability in shihezi formation are 9.2 % and 10.42×10-3μm2 (table 7). therefore, physical properties of the sandstone of shihezi formation are better than those of shanxi formation and those of taiyuan formation are comparatively poor. meanwhile, coal measures tight sandstone staggeringly deposited and developed with coal seam and dark mud shale riching organic matter (fig. 2) to make a favorable condition for source rocks of tight sandstone. the gas show active in tight sandstone of upper paleozoic in ordos basin and especially in linxiang area(xie et al., 2016;liang et al.,2016b). in the southern basin, tight sandstone of segment 6 of lower shihezi formation underwent fracturing testing to attain daily capacity 8457 m3 gas flow, thus, it exhibits excellent exploration potential of tight sandstone gas. sandstone of shan 2 segment of yan 6 well is 12 m thick (fig. 3). logging showed gas anomaly, log interpretation revealed that gas reservoir was 8 m, the daily capacity including the underlain no. 2 coal seam was 6 000 m3 gas flow (zhou et al., 2013). table 7. physical properties characters of coal measures tight sandstone of upper paleozoic in ordos basin (fu et al., 2013) 5 reservoir formation model unconventional gas in coal measures belongs to “continuous oil and gas reservoir”, which belongs to different types. reservoirs of coalbed methane, shale gas, and tight sandstone gas are coal rocks, mud shale, and sandstone respectively, and all of these different types of reservoirs coexisted in the same coal-bearing series. their source types, such as sedimentary environments, structural, thermal evolutions, were in remarkable consistency. therefore, unconventional gas in coal measures possesses paragenesis characteristics in the same basin (cao et al., 2014). in coal measures, coal seam and dark mud shale are rich in organic matter, which constitutes source rocks of unconventional gas in coal measures. when these organic matters reach the threshold of hydrocarbon formation to generate gaseous hydrocarbon, and then it may supply hydrocarbon for coal seam, mud shale, and tight sandstone to altogether form gas reservoirs of a coal seam, shale, and tight sandstone respectively (fig. 4.). however, gas reservoirs of the coal seam and shale belong to self-generation and self-preservation type that display no straightforward migration process. therefore, they are distinctly different from the formation of the conventional oil-gas reservoir. the hydrocarbon gasses generated from coal seam and mud shale migrate to tight sandstone via a short-distance to gather into reservoir. therefore, the formation of tight sandstone gas reservoir of coal measures is similar to that of the conventional oil-gas reservoir. the slight difference that exists is that reservoir bed of tight sandstone gas reservoir of coal measures is close to hydrocarbon source rock bed, and the migration distance of natural gas is shorter than that of oil and gas of conventional oil-gas reservoir. figure 4. gas reservoir model of upper paleozoic coal measures in ordos basin(wang et al.,2014;cao et al.,2014) 6 conclusions (1) in ordos basin, unconventional gas in coal measures, e.g. coalbed methane, shale gas and tight sandstone gas, developed in upper paleozoic, and the resources amounts are enormous. the geological condition of ordos basin is favorable for three kinds of gasses occur together. therefore, at a later stage, the research on joint exploration and exploitation of the three types of gasses ought to be carried out. (2) unconventional gas of upper paleozoic coal measures mainly distributes in the eastern ordos basin. currently, the geological studying level to the coalbed methane in the eastern ordos basin is relatively high, and several areas have entered the commercial development stage. whereas studying level to the shale gas and tight sandstone gas of coal measures in this region are relatively small. available data indicate that in this region the oil gas shows of upper paleozoic mud shale and tight sandstone are favorable, and resource potential is conspicuous, about which further work is needed (3) unconventional gas of coal measures is accumulated through slightly different processes, of which coalbed methane reservoir and shale gas reservoir of coal measures belong to “self-generation and selfpreservation”, and the gas migration process almost did not happen. whereas the coal measures tight sandstone gas reservoir belongs to “allogenic, and self-preservation” and the natural gas experienced a short distance migration. although reservoir types of coal measures unconventional gas belonging to the same suit of strata are different, the forming factors of the gasses reservoir are closely related and uniform. therefore, it will be more scientific and reasonable that the geological reservoir-forming processes of three different kinds of unconventional gas of coal measures are studied as a whole. acknowledgement this study was supported by the fundamental research funds for the central universities (2015xkzd07) and a project funded by the priority academic program development of jiangsu higher education institutions. references cao, d.y., yao, z. and li, j. 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(2013). analysis of natural gas exploration potential of upper paleozoic tight sand of south yanchuan area in ordos basin.progress in exploration geophysicsreservoir evaluation and development, 3(5), 17-23. 115 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 115 118 an approximate method for estimating nutrient loads in drainage water from a coastal irrigated area giakoumakis s., alexakis d. and gotsis d. laboratory of reclamation works and water resources management, school of rural and surveying engineering, national technical university of athens, athens, greece dial@survey.ntua.gr, dalexak@gmail.com hydrogeology in this work, an estimation of nutrient loads, expressed as dissolved inorganic nitrogen (din) and inorganic phosphorous (ip), in drainage water from a coastal irrigated area was undertaken. the procedure is based on the calculation of drainage discharge with the use of data from the drainage pumping station and the assessment of the concentrations of nutrients in drainage water. the case study examined is the cultivated land of agoulinitsa (peloponnese greece). the results of this study could be useful for taking the necessary measures for environmental protection. en este trabajo, se emprendió una estimación de las cargas de nutrientes (expresados como nitrógeno inorgánico disuelto y fósforo inorganico) en las aguas bombeadas desde una área de irrigación costera. el procedimiento está basado en el cálculo de descarga de la cuenca con el uso de datos desde la estación de bombeo y con la medida de concentración de nutrientes en el agua bombeada. el caso de estudio propuesto es la región cultivada de agoulinitsa (peloponeso grecia). los resultados de este estudio podrán ser útiles para tomar las medidas necesarias para la protección del medio ambiente. palabras clave: bombeo de agua; carga de nutrientes; tierra cultivada. abstract resumen key words: drainage water, nutrient loads, cultivated land record manuscript received: 26/01/2013 accepted for publication: 10/09/2013 introduction nutrients and heavy metal inputs from drainage water and surface water to the sea environment has been a major environmental concern in many regions of the world; both agricultural practices as well as land use practices are among the main factors that control the nutrient concentration and quality of water (alexakis 2011; brindha and elango 2011; gamvroula et al. 2013). in recent years, environmental studies have revealed elevated contents of nutrients in drainage water, surface water, groundwater and soil (alexakis et al. 2014; brindha and elango 2011; golekar et al. 2013). in this work, an effort was made in order to estimate both drainage water quantities and transported nutrients in an agricultural area of greece adjacent to the sea, which suffers seriously from pollution problems of this type. the findings of this work could be useful for planning the necessary protection measures by the local authorities. study area the study area is the agoulinitsa irrigation district, the downstream part of the alfeios river basin in the north-west peloponnese peninsula in greece. the irrigation network was constructed between 1960 and 1970, after the artificial draining of the agoulinitsa lagoon. the study area lies between latitudes 37o32´-37o37´n and longitudes 21o26´-21o36´e and is located 7 km southwest of pyrgos city. it is surrounded by the ionian sea in the west, with elevations ranging from 0.5 to -1.5 m below sea level, and covers approximately 35 km2 (fig. 1). the district is irrigated using the water of the alfeios river. water from the main irrigation channel is distributed to three pumping stations (a1,a2,a3), which supply sprinkler irrigation. the drainage network consists of 1st, 2nd and 3rd class drainage ditches that drain excess water from the cultivated land to the sea through a drainage pumping station (e). the net irrigated area of the agoulinitsa network is 24 km2. according to the local management authorities, the cultivated land is approximately 80 % of the net area. the irrigation season begins in april or early may and generally ends in september or october. the most common agricultural crops are corn, cotton, alfalfa, cereals, vegetables and citrus. there is no detailed information about the amount of commercial fertilizers applied by farmers in the agoulinitsa district. the most common type of fertilizer applied in the study area is npk (14 % of n, 28 % of p and 14 % of k). soils are derived from quaternary alluvial deposits. the quaternary alluvial deposits cover the study area, which includes lagoonal deposits, recent delta deposits, red clays, gravel and sandy materials. soil infiltrability varies between 0.04 and 118 mm h-1 (gotsis, 2007). materials and methods data collection data were collected from literature sources and relevant databases (alexakis et al. 2012; public power corporation of greece). monthly pre116 cipitation and temperature data obtained from the meteorological station of n.m.s. (national meteorological service), which is located in pyrgos city, were used in this study (fig. 1). this study uses all the available systematically collected data concerning water quality of the study area over the of the year 2010. both irrigation and drainage water samples were collected during pre-irrigation and post-irrigation seasons. a location map of the water samples and a detailed description about the sampling campaign are given by alexakis et al. (2012). inflow and outflow volumes electrical power for a pumping station is calculated from the following equation: (1) where: p, is the electrical power (kw) q, is the water discharge (m3/h) h, is the manometric head (m) n, is the total efficiency of the pumping station transforming eq. 1, the electrical energy consumption for a pumping station can be calculated from: (2) where: w, is the electrical energy consumption (kwh) v, is the volume of discharged water (m3) monthly records of electrical energy consumption for the pumping stations (a1, a2, a3 and e) were used (public power corporation of greece/pyrgos city). by using eq. 2, the volume of water discharged each month was calculated (i.e., irrigation water using data from pumping stations a1, a2, a3 and drainage water using data from pumping station e). the monthly discharged volume of water was then expressed in mm/month according to the data reported in table 1. figure 1. the irrigation district of augoulinitsa giakoumakis s., alexakis d. and gotsis d. 117 table 1 basic characteristics of the pumping stations. pumping station irrigation drainage a1 a2 a3 e manometric h (m) 76 71 79 6 motor power (hp) 2040 2380 2040 860 total efficiency n 0.6 0.6 0.6 0.6 covered area (km2) 6.2 6.2 7.8 35.0 the estimated values of the mean monthly inflows for the period of the years 2000-2010 (precipitation, irrigation water) and outflows (potential evapotranspiration, drainage water) in the study area are presented in fig. 2. the potential evapotranspiration losses, also shown in fig. 2, were estimated with the use of the modified blaney-criddle equation (gotsis and giakoumakis, 2006): (3) where: pet, is the potential evapotranspiration for the reference crop (clipped grass) in mm/day t, is the mean monthly temperature (oc) p, is the mean daily percentage of total annual daylight hours (taken from tables) ab , bb: are the adjustment factors based on minimum relative humidity, sunshine duration and wind speed (taken from tables) according to the crop and the month, the appropriate crop factor kc was then selected. nutrient loads the mean annual concentrations of din (dissolved inorganic nitrogen), which is the sum of no3 -n, nh4-n and no2-n and ip (inorganic phosphorous), that were used in this study were calculated based on the no3-n, nh4-n, no2-n, and po4 3concentrations, respectively (table 2). the conversions between no3 and no3-n is the no3 result multiplied by 0.226, while the conversions between nh4 + and nh4-n is the nh4 + result multiplied by 0.777. the recorded no2 water contents were below the detection limits. a conversion factor of 0.326 was applied for the conversion between po4 3-and p. table 2 mean annual concentrations of nutrients in drainage water of the agoulinitsa irrigation district. mean din mean ip (mg l-1) (mg l-1) irrigation water 0.45 0.05 drainage water 1.32 0.16 because po4 3is the most readily available form of phosphorous for uptake during photosynthesis, the study of this type of phosphorous is generally of high importance with regards to algal blooms and eutrophication (yanagi et al. 1992). annual loads of nutrients were then calculated by multiplying the mean annual concentrations mentioned above by the corresponding volumes of annual inflowing (irrigation) water and drainage water, respectively. the total annual loads (in tn yr-1) of din and ip discharged from the agoulinitsa drainage network into the ionian sea were finally estimated by the following equations: an approximate method for estimating nutrient loads in drainage water from a coastal irrigated area fig. 2 mean monthly inflows and outflows in the study area. 118 ldin-total = ldin-inflowing water + ldin-drainage water (4) lip-total = lip-inflowing water + lip-drainage water (5) results and discussion concentrations of din and ip in drainage water are higher than the concentrations of din and ip in alfeios surface water. this is attributed to the extensive use of fertilizers by farmers in the agoulinitsa district. the results are shown in table 3. table 3 annual loads of nutrients for irrigation and drainage water. volume din ip (106 m3 yr-1) (tn yr-1) (tn yr-1) irrigation water 18.7 8.4 0.9 drainage water 21.4 28.2 3.5 total volume   40.1   36.6   4.4 from this table, the following remarks can be outlined: (i) the annual volume of drainage water from the study area is greater than that of irrigation water, which is percolated into the soil by sprinkler irrigation, because drainage water is also composed of runoff from precipitation. (ii) annual loads of din and ip are far greater for drainage water than for irrigation water. namely, the contribution of inflowing water (alfeios river water feeding the irrigation network) in the annual din and ip loads are 23% and 21%, respectively, whereas the contribution of drainage water in the annual din and ip loads are 77 % and 79 %, respectively (fig. 3). (iii) total annual loads of din and ip discharged from the agoulinitsa drainage network into the ionian sea are 36.6 tons of din and 4.4 tons of ip. to contribute to the environmental protection of the sea coast and to control the nutrient loads that are discharged into the seawater, a method of reusing drainage water and mixing with freshwater for irrigation use was proposed (gotsis et al., 2011). conclusions it was found that the drainage water of the study area contains nutrients expressed as dissolved inorganic nitrogen (din) and inorganic phosphorous (ip), due mainly to the extensive use of fertilizers in the cultivated land. the annual transported loads of nutrients din and ip discharged into the sea were calculated to be 36.6 and 4.4 tons, respectively. these findings could contribute to the establishment of the appropriate policy measures for environmental protection (i.e., drainage water reuse after mixing with fresh water). references alexakis, d. (2011). assessment of water quality in the messolonghietoliko and neochorio region (west greece) using hydrochemical and statistical analysis methods, environmental monitoring and assessment, 182, 397-413. alexakis, d., gotsis, d. and giakoumakis, s. (2012). assessment of drainage water quality in preand post-irrigation seasons for supplemental irrigation use, environmental monitoring and assessment, 184, 5051-5063. alexakis, d., gotsis, d. and giakoumakis, s. (2014) evaluation of soil salinization in a mediterranean site (agoulinitsa district—west greece), arabian journal of geosciences, doi 10.1007/s12517-014-1279-0 brindha, k. and elango, l. (2011). hydrochemical characteristics of groundwater for domestic and irrigation purposes in madhuranthakam, tamil nadu, india, earth sciences research journal. 15, no. 2, 101-108. gamvroula, d., alexakis, d., stamatis, g. (2013). diagnosis of groundwater quality and assessment of contamination sources in the megara basin (attica, greece), arabian journal of geosciences. 6, no.7, 2367-2381. golekar, r. b., patil, s. n. and baride, m. v. (2013) human health risk due to trace element contamination in groundwater from the anjani and jhiri river catchment area in northern maharashtra, india. earth sciences research journal, 17, no. 1, 17 – 23. gotsis, d. and giakoumakis, s. (2006). update of planning variables for the irrigation network of alfeios in ilia. 10th hellenic hydrotechnical association’s conference, xanthi, greece, 761-768 (in greek). gotsis, d. (2007). estimation of irrigation and drainage planning variables for the network of agoulinitsa with the use of experimental data, msc dissertation, interdepartmental program of postgraduate studies in water resources science and technology, school of rural and surveying engineering, national technical university of athens, greece (in greek). gotsis, d., spiliotis, m . and giakoumakis, s. (2011). reuse of drainage water in irrigation with the aid of 0-1 linear programming, irrigation and drainage systems, 25, no.4,385-394. yanagi, k., yasuda, m. and fukui, f. (1992) reexamination of the fractionation of total dissolved phopshorous in seawater using a modified uv-irradiation procedure, and its application to samples from suruga bay and antarctic ocean. journal of oceanography, 48, 267-281. giakoumakis s., alexakis d. and gotsis d. din ip fig.3 pie-charts showing the contribution (%) of each type of water to the annual loads of nutrients discharged from the agoulinitsa drainage network into the ionian sea. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014) letter from editor during the outgoing year, successful events took place, as latin american and caribbean seismological commission assembly, on july 23, 24, 25, in bogotá, colombia. in that academic event, the relevant results associated with the seismotectonic behavior of the latin america and caribbean region were addressed, as well as the implications of active exploration of the tectonic conditions, the socio-economic impacts of exploration and extraction of economic resources, and the implementation of nonconventional techniques for exploring and mining that can contribute to sustainable development. another event was the shale colombia 2014, unconventional oil and gas development congress, aimed to determine the next steps towards the commercialization of unconventional oil and gas development for the country. the event hosted case studies from some of the pioneers in colombian development, using the latest reservoir data fiscal updates and global scale technology case studies, to evaluate real potential colombia holds for unconventional development. in the outlook of 2015, one of the main events is the 26th international union of geodesy and geophysics general assembly, to be realized between june 22 and july 2, in prague, czech republic. you are on time to participate in this event yet. in colombia will take place the xv colombian congress of geology on 31 august to 5 september. this event is a bi-annual meeting gathering scientists, academicians, and researchers from the region, with an extensive participation of international guests. the congress wants to provide a scenery to discuss the perspectives of mineral resources, new exploration techniques related to energy supply, proposals for optimized management of mining and mineral resources, between other topics. finally, we want to introduce you the manuscripts selected for this edition. there is a broad range of information about seismology with four papers coming from different countries. the first one is an analysis of the elastic structure beneath australia shown by means of s-velocity maps and determined by the regionalization and inversion of rayleigh wave dispersion. the obtained s-velocity models suggest the existence of lateral and vertical heterogeneity in the structure. from turkey, the second manuscript presents a site response analysis and estimation of s-wave velocity depending on acceleration data. the other two articles in seismology for this edition are based on improving accuracy and reliability of issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.49641 http://dx.doi.org/10.15446/esrj.v18n2.49641 the first arrival times through cross correlation technique, and in the characterization of seismic events using only one seismological station. in other knowledge field, it is presented a mineralization potential study of the intrusive bodies in valis, northwest of iran. a very graphical manuscript coming from the gaza strip shows a study in sand dunes, from morphology, textural characteristics, and associated environmental impacts. from morocco, a study aimed to evaluate geostatistical methods for delimitation of anomalies detected in the phosphatic series of sidi chenane found that the exponential model is the best one to represent the spatial variability of their geoelectric data; models of the geology were successfully obtained from geostatistical method, which help mapping the phosphate deposit inclusions and the estimations of phosphate reserves were improved and better constrained. in geotechnics, three important works are displayed. one is about the experimental triaxial tests developed to estimate undrained critical shear strength under monolithic loadings; the second one reads about classification of caving depth by feature extraction in cutting images, and the other is a simulation using artificial neural network and adaptative neuro-fuzzy system of compression ratio in a municipal solid waste. last two works talk about gis and kriging techniques applied to map a spatial distribution of sediments pollution in the khajeh kory river and an assessment of the land ecological security for yancheng city, in china. we hope you may find interesting information in our esrj. carlos alberto vargas earth sciences research journal editor in chief earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 5 letter from editor more than two decades ago we have been working steadily to provide readers and scientific community with articles on a broad range of geosciences field around the world. this achievement would not be possible without the support of the universidad nacional de colombia, mainly, and partners who help bring these high-quality papers to light. one of those partners is colciencias, a colombian government-owned agency that support research in our country. currently, colciencias is carrying out a national strategic plan for science, technology and innovation in geosciences aimed to promote and develop megaprojects in the area, provide a management tool, and create a fund with resources coming from contracts of exploration and exploitation of petroleum. this 2015-2018 plan should result in the creation of new research groups, improvement of investigation centers, recognition of researchers and the double of resources in funding for projects. similar strategies are carried out by the centro de geociencias, at the national autonomous university of mexico. their primary objective is to gather the experience of their academicians to produce fundamental and applied investigation under the high-quality requirements. in venezuela, the non-government organization fundación geoparques de venezuela, under the motto "geosciences for human development", promotes research and divulgation of earth sciences with communities as the head axis. just these examples but in several countries of latin america government or non-government entities work for the development of geosciences. our call is for regional collaboration optimizing resources, promotion of open journal publications, no isolated academic communities, etcetera. on the other hand, find available our newest volume with a selection of ten works of the best quality in different subjects of the earth sciences. the first of them is a complete map of the seismotectonic and geologic hazards in iran evaluated using seismicity records, structural trends, tectonic settings, fault ruptures and neotectonics activities in the country. from colombia, the second manuscript is an approaching to the skarn and formation of the ti-clinohumite from its occurrence and petrologic significances. from china, but also in petrology subject, a paper presents geochemistry and petrography of the amphiboles and plagioclase from fenghuangshan granodiorite. combining with the barometry and thermometry analysis, the author estimates the conditions of the fenghuangshan magmatic rocks and explains the origin of the magma. moreover, evaluate the deeper exploration of copper mineralization prospecting potential. in applied geophysics, a paper coming from jordan provides information about integration of data obtained from geophysical methods, gis and the previous works and literature review of geology in southwestern jordan. about geomorphology, it is presented an altimetric and hypsometric analysis, with a longitudinal profile and geomorphological studies, for soil and water conservation based in the basins of anjani and jhiri rivers. as part of the strategies to warrant water resources, the method presented in next work is accomplished to be more efficient and cheaper to estimate runoff in a case based in tamilnadu district, in india. related to climatic change, one of the authors investigated the correlation of some climatic indicators with precipitation and discharge of madarsoo (iran) watershed. it is a meaningful study in the context of global climate change. the last block is dedicated to three works in the field of geotechnics, in areas of roads, construction of concrete piles and evaluation of polypropylene fibers for mining and civil engineering applications. we hope this articles will be useful for everyone of you. carlos vargas editor in chief, earth sciences research journal quantitative x-ray diffraction analysis of rock, x-ray diffraction analysis of clay minerals types and components, field emission scanning electron microscope (fesem) and sensitivity flow experiments methods were used to research the effects of clay minerals on the porosity and permeability of ordos basin’s tight chang 7 reservoir (zhenbei area). these methods were also used to analyze the type, degree, and factors affecting reservoir sensitivity. the research showed that the reservoir possessed poor water, salt, and alkali sensitivity, poor to strong acid sensitivity and none to poor velocity sensitivity. acid sensitivity among them had comparatively large differences, mainly because acid sensitivity was not only affected by chlorite but also by components of carbonate minerals. stress sensitivity experiment results showed that the maximum degree of permeability damage in the rocks of this reservoir was median to very strong; irreversible damage level was weak to strong. consequently, the experimental studies are favorable to not only determine the factor dominating the petrophysical properties of the tight formation, but also to optimize the follow-up development strategies, e.g. injection schedule and hydraulic fracturing implement. este trabajo utilizó análisis cuantitativos de rocas por difracción de rayos x, análisis de los tipos y componentes de arcillas minerales por difracción de rayos x, análisis con el microscopio electrónico de efecto de campo (fesem, del inglés field emission scanning electron microscope) y ensayos de respuesta de flujo para investigar los efectos de las arcillas minerales en la porosidad y permeabilidad del depósito chang 7, en la cuenca del ordos (región zhenbei). estos métodos también se utilizaron para analizar el tipo, el grado y los factores que afectan la respuesta del depósito. la investigación demuestra que el depósito posee poca agua, sal y respuesta alcalina, baja a fuerte respuesta de acidez, y ninguna a baja respuesta de velocidad. entre estas características, la respuesta de acidez presentó grandes diferencias comparativas debido a que está afectada tanto por el clorito como por los componentes de minerales carbonatos. los resultados de los ensayos de respuesta de tensión muestran que el máximo grado de daño por permeabilidad en las rocas del depósito es de mediano a muy fuerte; el nivel de daño irreversible va de débil hasta fuerte. por lo tanto lo estudios experimentales son favorables no solo para determinar el factor dominante en las propiedades petrofísicas de la formación compacta sino también para optimizar las futuras estrategias de desarrollo, como una programación de las tareas de inyección y la implementación de la fractura hidráulica. earth sciences research journal earth sci. res. j. vol. 20, no. 2 (june, 2016) : b1-b6 abstract resumen keywords: analysis of clay minerals, porosity, permeability, sensitivity experiments, formation damage. palabras clave: análisis de arcilla mineral, porosidad, permeabilidad, ensayos de respuesta, daños de la formación. experimental study of petrophysical properties of a tight formation by considering the clay minerals and flow sensitivities estudio experimental de las propiedades petrofísicas de una formación compacta al considerar las arcillas minerales y las respuestas de flujo record manuscript received: 02/12/2015 accepted for publication: 27/05/2016 how to cite item liu, g., li, s., gu, d., lu, y., & asamoah, e. (2016). experimental study of petrophysical properties of a tight formation by considering the clay minerals and flow sensitivities. earth sciences research journal, 20 (2), b1-b6. doi:http://dx.doi.org/10.15446/esrj.v20n2.54508 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.54508 guangfeng liu1, shoujun li1*, daihong gu1, yang lu2 and emmanuel asamoah1 1cmoe key laboratory of petroleum engineering, china university of petroleum, 18 fuxue road, changping, beijing, 102249, china 2 china university of geosciences, beijing, 29 xueyuan road, haidian, beijing, 100083, china h y d r o c a r b o n s b2 guangfeng liu, shoujun li, daihong gu, yang lu and emmanuel asamoah 1. introduction clay minerals in tight reservoirs have a significant impact on its physical properties. reservoir sensitivity analyses have theoretical and practical significance for the prevention and treatment of tight sandstone reservoirs. since the 1960s, experts had noticed that formation damage is caused by clay mineral (hewitt, 1963). hower (1974) studied the distribution of clay minerals in the united states gulf coast and pointed out that the formation damage degree depended on the contents, types of clay mineral and their position in the rock. krueger (1986), amaerule, kersey, norman, and shannon. (1988), porter (1989) and reed (1989) comprehensively reviewed the research of formation damage, which reflected the achievements of the 80s. aase, bjorkum and nadeau (1996) and pittman, larese, and heald (1992) studied the influence of different types and occurrences of clay on the formation and protection of secondary porosity. baker, uwins, and mackinnon (1993a, 1993b, 1994) used environment scanning electron microscope (esem) to research the sensitivity of clay minerals. ‘clay mineral cements in sandstones,’ by worden and morad (2003), described the relationship between clay minerals and sandstone reservoir properties and looked to the future research direction of clay minerals. ordos basin is located at the western margin of north china, with an area of 25×104km2. it is a large cratonic basin of a multiconfiguration system and multicycle evolution (zhao, liu, yu, and wang, 2008; feng, huang s., huang p., zou and wu, 2009; wang et al., 2010). zhenbei area is a typical sandstone reservoir, situated at the north-western part of the basin. it possesses closed sediment supply and fast deposition rate. chang 7 reservoir of this region is located in the southwest lake basin, mainly affected by the northwest provenance. chang 7 reservoir is a deep-water turbidite fan sedimentary system with developed turbidite sand body; it is the central horizon of the zhenbei area (lu et al., 2011; cui, feng, qin, and peng, 2013). in this article, x-ray diffraction analysis, fesem and sensitivity flow experiments were used to research and evaluate the clay mineral features of the zhenbei area’s chang 7 reservoir. 2. analysis of reservoir minerals the x-ray of whole rock analysis is usually adopted in laboratories for the quantitative analysis of rocks to get the clay mineral contents and x-ray diffraction analysis of clay minerals. thus obtaining the corresponding contents of each type of clay mineral in the reservoir. the porosity and permeability of rocks can be gotten through gas logging porosity and permeability experiment. through those experimentations carried out on ten wells of zhenbei area, the results were as shown in table 1 and table 2. table 1. analysis of reservoir rock minerals table 2. analysis of clay minerals *note: k-kaolinite, c-chlorite, i-illite, s-smectite, i/s-illite smectite mixed layer, %s-the content of smectite in mixed-layer minerals of illite and smectite. the data in table 1 showed that the content of clay minerals in zhenbei area’s chang 7 reservoir was high, the reservoir had a small porosity and ultralow permeability. from table 2 it could see that the main contents of clay minerals in the reservoir were the illite-smectite mixed layer, followed by illite and kaolinite; smectite was not independently developed but occupied a slight proportion in the illite-smectite mixed layer. fesem photographs showed that kaolinite in chang 7 reservoir often filled in the porosity with booklet shapes (as shown in figure 1a); chlorite was usually distributed on the surfaces of particles with leaf shapes (figure 1b) and the form of the illite and the illitesmectite mixed layer were flocculent structure (figure 1c and figure 1d). figure 1. fesem photographs of zhenbei area chang 7 reservoir. 3. influence of clay minerals on the reservoir properties the total content of clay minerals and types of clay minerals affected the properties of the reservoir. as shown in figure 2, rock mineral components formed an inversely proportional relationship with both porosity and permeability. as fewer mineral content in the rock, the higher porosity and permeable the reservoir would be. b3experimental study of petrophysical properties of a tight formation by considering the clay minerals and flow sensitivities b3 figure 2. influence of clay mineral content on porosity and permeability. 3.1 influence of authigenic kaolinite on the reservoir properties authigenic kaolinite was the primary dissolution product of the feldspar (huang et al., 2009), so the content change of authigenic kaolinite was closely related to the feldspar content in the sandstone reservoir. as shown in figure 3, authigenic kaolinite and detrital feldspar had a significant negative correlation; it could be seen that authigenic kaolinite content increase with the decrease of feldspar. the content of authigenic kaolinite in chang 7 reservoir was small and usually filled in the pores and throats. the kaolinite dissolution from feldspar grew much mineral intracrystal porosity, which had a positive effect on the reservoir property. as shown in figure 4, as higher the content of kaolinite, higher the porosity and permeability. figure 3. relationship between authigenic kaolinite content and feldspar content. figure 4. influence of authigenic kaolinite content on porosity and permeability. 3.2 influence of authigenic chlorite on the reservoir properties the formation of chlorite membrane was suitable for the reservoir (ehrenberg, 1993; hurst & nadeau, 1995; he, 2010): (1) chlorite membrane could increase the mechanical strength of rock and resist the compaction to some extent, thus making the primary intergranular pore unable diminish further by compaction. (2) feldspar could even be dissolved and formed a moldic pore, but the chlorite membrane would remain. if the chlorite rims had sufficient strength, they could support themselves and protect the secondary porosity formed by dissolution. (3) quartz secondary enlargement caused porosity loss, but the chlorite film could prevent authigenic quartz cement from nucleating on the surface of detrital quartz by dividing pore water from quartz particle surface (4) authigenic chlorite transformed part of the intergranular pores it occupied to intercrystalline pores, some of which might be effective for reservoir spaces. therefore, the development of authigenic chlorite was favorable for reservoir physical property. as shown in figure 5, the porosity and permeability were positively associated with the content of authigenic chlorite. figure 5. influence of authigenic chlorite content on porosity and permeability. 3.3 influence of authigenic illite and illite-smectite mixed layer on the reservoir properties common forms of authigenic illite are hair-like, fibrous and bridging. they often cut lots of pores and pore throats into micro bound pore and reduce the effective radius (liu, qu, sun, yue & zhu, 1998, lander & bonnell, 2010). zhenbei area chang 7 reservoir has a high content of illite-smectite mixed layer whose main constituent is illite and fesem showed that the attitude of the illite-smectite mixed layer was similar to illite, so the influence of it on the reservoir was also similar to that of illite. as shown in figure 6, as higher the contents of illite and illite-smectite mixed layer, the lower the porosity, and permeability. figure 6. influence of authigenic illite and i-s content on porosity and permeability. a. illite b. i/s b4 guangfeng liu, shoujun li, daihong gu, yang lu and emmanuel asamoah 4. sensitivity evaluation reservoir sensitivity is referred to the degree of change in reservoir permeability caused by the physicochemical interactions between the reservoir and fluid. tight sandstone reservoir is different from the conventional low permeability reservoir, so its experimental methods require more precise and complex (zhang, chen, & yan, 2006; ju, fan & wan, 2007; wu, wang, cui, zhang, 2013). this experiment used cores in six wells to make systematic sensitivity evaluation for chang 7 reservoir. the device schematic diagram is as shown in figure 7. *note: 1-pump. 2-intermediate container. 3-pressure gage. 4-core holder. 5-measuring cylinder. 6-confining pressure pump. figure 7. experiment device schematic diagram. 4.1 water and salt sensitivity evaluation among reservoir clay minerals, montmorillonite has the strongest effect on water sensitivity; even a tiny amount of montmorillonite caused very strong sensitivity. secondly, the illite-smectite mixed layer could also cause water sensitivity. the expansion of illite and chlorite upon contacted with water was feeble, and kaolinite was not easily hydrated (cui, d. l. liu, tao, li, & y. b. liu, 2004) the water sensitivity experiment results of zhenbei area chang 7 reservoir were as shown in table 3; results of salt sensitivity were as illustrated in figure 8. table 3. results of water sensitivity experiment on zhenbei area’s chang 7 reservoir. figure 8. salt sensitivity experiment results. it could be seen from table 3 that the region had weak water sensitivity. the salt sensitivity analysis results of figure 8 shows that when formation salinity declines, the change in permeability was not obvious and had no critical salinity. the content of i-s in zhenbei area chang 7 reservoir was high, but the montmorillonite only took a slight percentage (less than 25%). it was close to the end-member composition of illite. the conversion content of montmorillonite was less than 10%, so the water and salt sensitivity was weak in this reservoir. 4.2 acid sensitivity evaluation in the process of water injection, tight oil reservoirs were prone to be blocked up. therefore, acid fluids were to be infused to broken down reservoir to improve its productivity. acid fluids upon entering the reservoir could react with sensitive acid minerals to produce deposits and released microparticles and blocked the reservoir, causing a reduction in permeability. acid-sensitive rocks in clay were mainly chlorite, which produced ferric hydroxide upon contact with acid causing blockage in the reservoir. the acid sensitivity experiment results of zhenbei area’s chang 7 reservoir were as shown in table 4. table 4. acid sensitivity test results of zhenbei area’s chang 7 reservoir. table 4 showed that the acid sensitivity of each well at the zhenbei area’s chang 7 reservoir had great differences. from table 2, it could be known that difference in the chlorite content of six wells was not big, but the difference in the degree of acid sensitivity was huge. through analysis, it could be seen that the calcite content of each well at the zhenbei area had a particular difference. calcite was carbonate mineral which compared to chlorite readily reacted with hydrochloric acid (tian, q. guo, y. li, y. guo, & y. z. li, 2009). through x-ray analysis, it could be seen that the contents of calcite in z-1, z-4 were large. z-5, z-6 only had small quantities of calcite and the contents of calcite in z-2, z-3 were between them (as illustrated in table 1). therefore, the acid sensitivities were different. b5experimental study of petrophysical properties of a tight formation by considering the clay minerals and flow sensitivities b5 4.3 alkaline sensitivity evaluation during comparatively low temperatures, ca2+, mg2+ on the surface of clay minerals exchanged with other alkaline minerals to produce hydroxide precipitation and the calcium ions coverage was reduced. then the surface of clay minerals was easier to expand or to be released from the sand surface (li, gao, yang & hua, 2012). also, under high temperature and ph>9 conditions, it reacted kaolinite to form montmorillonite and caused further damage of the reservoir. the alkaline sensitivity experiment results of zhenbei area’s chang 7 reservoir was as shown in figure 9. figure 9. alkaline sensitivity experiment results. the results in figure 9 revealed that chang 7 reservoir had weak alkaline sensitivity. data demonstrated that critical ph of z-1 well is 7, critical ph of z-2 well is 10 and other wells did not possess critical data. 4.4 speed sensitive evaluation kaolinite was the clay mineral that most likely caused speed-sensitivity, and the second was hair-like illite. results of a speed-sensitive experiment on zhenbei area’s chang 7 reservoir were as showed in figure 10. figure 10. speed sensitive test results. figure 10 showed that the speed-sensitivity of chang 7 reservoir was none to poor and permeability of z-1 was even improved. this phenomenon was due to different particle sizes were scoured by the fluid when the flow velocity got increased. when the particle size was greater than the pore throat radius, particles washed down by fluid would accumulate in the throats and form a ‘bridge’, which made the reservoir permeability decline (yang & wei, 2004). however, if particles size were less than the pore throat radius, particles would be carried out with fluid and the pore radius would increase, thereby the permeability of the reservoir got improved. also, indoor experiment samples were too little, particles might be washed out of the experimental model, but these particles in the reservoir would still stay in it and block the throats. so the results gotten from the laboratory were less than that in the actual reservoir. 4.5 stress sensitive evaluation during reservoir development, with the output of the internal fluid, the pore pressure became decreasing, and the original force balance of the rocks got changed. that led to elastic-plastic deformation of the rocks and the changing of reservoir pore structure and permeability (zhao, yue, & lv, 2009; feng, huang, chen, wang, che, 2010; ru, liu, fan, li, yu, 2011; sun, yang, li, 2011). the experiment used a variable confining pressure to test the stress-sensitivity of three samples with different permeability. the trial included both ascending and descending net confining pressure processes. the results were as shown in figure 11. figure 11. stress sensitive experiment results. as shown in the figure above, the solid lines represented ascending net confining pressure processes, and dotted lines indicated the descending net confining pressure processes. it could be seen the stress-sensitivity features of the reservoir were as below. (1) during the ascending net confining pressure process, with the increasing of confining pressure, the core permeability reduced dramatically at first and then it slowed down to a modest pace. the smaller the core permeability was, the faster it declined and the greater the degree of permeability lost. (2) part of the reservoir permeability loss would be restored in the process of descending net confining pressure, but it could not be returned to its initial state. so the permeability loss caused by stress-sensitivity was irreversible. the core with smaller permeability would have a greater irreversible permeability loss. (3) the content of clay mineral in z-1, z-2 and z-3 are 15%, 22%, and 26% respectively (as shown in table 1). the results showed that the higher content of clay mineral meant a higher stress sensitivity. in general, the maximum degree of permeability damage in the rocks of this reservoir was median to very strong and irreversible damage level was weak to strong. 5. results and conclusions (1) chang 7 reservoir in the zhenbei area had high contents of clay minerals; the main compositions of these were illite-smectite mixed layer and chlorite, illite and kaolinite only took up a small proportion of them. (2) the reservoir porosity and permeability were negatively correlated to the total content of clay minerals. kaolinite and chlorite were advantageous to reservoir porosity and permeability, illite and i-s were not good for them. (3) the water, salt, alkali, speed sensitive were weak in chang 7 reservoir in the zhenbei area, and acid sensitive was weak to strong; the maximum degree of permeability damage in the rocks of this reservoir was median to very strong and irreversible damage degree was weak to strong. (4) since the reservoir heterogeneity, sensitivities of each well might be different in chang 7 reservoir of the zhenbei area. so it should be treated separately for each well when implementing acidification or other technological measures. taking all sensitives into consideration, water with low salinity or high alkaline did not have severe impacts on permeability but acid fluid might b6 guangfeng liu, shoujun li, daihong gu, yang lu and emmanuel asamoah change it significantly, so weak alkaline substance could be added to protect the reservoir when fluids are injected for some technological measures. in the course of water flooding, considering the ability of wellhead pressure bearing and the formation fracture pressure, it could properly use a high injection rate. during production, it should select a consistent production pressure and control the bottom-hole pressure so that the damage degree of the stress-sensitivity to the reservoir would be reduced and the oil production would be kept. acknowledgement the work and results reported here were part of project 51404282, supported by national natural science foundation of china, project 2014d-5006-0215, funded by petrochina innovation foundation, and project 2462015yq0217, backed by science foundation of china university of petroleum, beijing. references: aase, n. e., bjorkum, p. a., & nadeau, p. h. 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(2008). the transfer of depocenters and accumulation centers of ordos basin in mesozoic and its meaning. acta geologica sinica, 82(4), 540-549. the present paper contains longitudinal profile, hypsometric analysis of river basin and geomorphological studies for future planning of soil and water conservation in anjani and jhiri river basin, jalgaon district, maharashtra, india (20°45’ to 21°08’n latitude and 75°09’ to 75°27’e longitude). the main aim of the present study is to recommend a suitable site for soil and water conservation in anjani and jhiri river basin, northern maharashtra, india. the study area is geologically covered by deccan trap basalts of the cretaceous to the lower eocene age, and some part is covered by thick alluvium of the quaternary age. drainage map of the study area reveals two drainage patterns including dendritic and sub dendritic patterns, indicating that the uniform lithology of the basaltic landforms has uniform resistance to erosion. this condition has caused the anjani and jhiri river get little recharge from the rainwater and the surface drainage flows during only monsoon season and latter becomes dry. therefore, water conservation is a prime need for minimize water scarcity problems in the study area. in the present study, landscape information, such as longitudinal profile and hypsometric integral, has been analysed. the longitudinal profile reveals the presence of nick points at three locations in anjani river and two locations in jhiri river. the nick points indicate that the rapids are formed due to different erosion, relief and presence of lineament (joint). the hypsometric integral values for the anjani and jhiri river are 0.52 and 0.51, respectively. the present research illustrates that both basins fall under equilibrium stage and are in a mature phase of development. el presente artículo contiene el perfil longitudinal, el análisis altimétrico de las cuencas ribereñas y los estudios geomorfológicos para la planeación a futuro de la conservación de suelos y aguas de los ríos anjani y jhiri, en el distrito jalgaon, maharashtra, india (20° 45' a 21° 08' n de latitud y 75° 09' a 75° 27' e de longitud). el objetivo principal de este estudio es la identificación de un lugar apropiado para la conservación de agua y de suelos en las cuencas de los anjani y jhiri, al norte de maharashtra. el área de estudio está geológicamente cubierta por basalto de las traps del decán, en la era del cretáceo al bajo eoceno, y por aluvión espeso de la era cuaternaria. el mapa hidrográfico de la zona de estudio muestra dos modelos de drenaje, el dendrítico y el subdendítrico, lo que indica que la litología uniforme de los accidentes geográficos basálticos tiene una resistencia uniforme a la erosión. esta condición ha causado que los ríos anjani y jhiri tengan una pequeña recarga proveniente de las aguas lluvias y de los drenajes superficiales durante la única estación monsónica del año y que luego se secan. es por esto que la conservación del agua es una necesidad primaria para minimizar los problemas de escasez en el área de estudio. en el presente trabajo se analizó la información del paisaje, como el perfil longitudinal y la altimetría integral. el perfil longitudinal revela la presencia de cambios de gradiente en tres puntos del río anjani y en dos puntos del jhiri. los cambios de gradiente indican que los rápidos se formaron debido a la erosión diferente, al relieve y presencia de lineamiento (convergencias). los valores de altimetría integral para los ríos anjani y jhiri son de 0.52 y 0.51, respectivamente. la presente investigación muestra que ambas cuencas se encuentran en un escenario de equilibrio y en una fase madura de desarrollo. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 51 58 abstract resumen key words: longitudinal profile, hypsometric analysis, soil and water conservation structures, anjani-jhiri river basin, deccan trap, india palabras clave: perfil longitudinal, análisis de altimetría, estructuras de conservación de agua y suelos, cuencas anjani y jhiri, traps del decán, india. altimetric and hypsometric analysis for soil and water conservation: a case study of anjani and jhiri river basin, northern maharashtra, india golekar r. b.¹*, baride m. v.², patil s. n.³, adil s. h.4 ¹department of applied geology, school of environmental and earth sciences, north maharashtra university, jalgaon 425001 (m.s.), (india) ²department of geology, z. b. patil college, dhule 424002 (m.s.), (india) ³department of applied geology, school of environmental and earth sciences, north maharashtra university, jalgaon 425001 (m.s.), (india) 4department of applied geology, dr. h. s. gour university, sagar – 470003 (m.p.), india *corresponding author: r. b. golekar (e-mail): rbgolekar@gmail.com *present address: department of civil engineering, rajarambapu institute of technology, rajaramnagar, islampur district sangli 415414 (m.s.), (india) record manuscript received: 23/08/2014 accepted for publication: 20/04/2015 geomorphology issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.45175 52 golekar r. b., baride m. v., patil s. n., adil s. h. introduction the maharashtra state, with its peculiar geohydrological, geomorphological and climatic conditions, faces multifold problems in exploration, exploitation and development of its groundwater resource in a scientific and planned manner. the operation of groundwater on a large scale for meeting the demands of rapid industrialization, agricultural sector, and domestic purpose has further aggravated the problem. currently, maharashtra, especially in marathwada and northern maharashtra regions, faces drinking water problem and supply for drinking water in this area depends on water tankers. the anjani and jhiri river basin was chosen for catchment hydrogeomorphological studies, both of which are tributaries of girna river. girna is one of the major tributaries of river tapi. anjani and jhiri river basin covers an area of about 1243.65 km². the present study area is located in jalgaon district (fig. 1). it is confined to latitude 20° 45’ n to 21° 08’ n and longitude 75° 09’ e to 75° 27’ e, covering topographic maps no. 46 p/1, 46 p/5 and 46 o/8 of the survey of india. the data on anjani jhiri river basin for the present study can serve as a pilot study based on hydrology and hydrogeomorphology. the anjani jhiri river basin is characterized by an arid to semi-arid climate, and its available water depends on the amount of rainfall in this basin. the water in the basin is used for agricultural and domestic purposes. most of the domestic water supply is through common well in rural areas funded under rural water supply schemes of the state government. people use water from unprotected open pits (dug) well, and/or depend on supply by tankers. the scarcity of water is also one of the main reasons that compel the farmers to cultivate their land under rain fed condition annually. the anjani jhiri river basin enjoys a hot and dry summer, cool winter and fair monsoon with an average annual rainfall of about 750 mm. the anjani river has an elevation difference of 135 m, with the highest elevation (305m) at titvi village located at the southern edge of the catchment, and drops to its lowest elevation (170m) at the northern edge where anjani river joins girna river. the jhiri river has an elevation difference of 125 m, with highest elevation of 300 m amsl near padamalay reserve forest, and the lowest being 175 m. amsl near its confluence with the river girna. the contour map of this area (fig. 2) shows elevation ranging from 300m to about 180m from southern part of the basin to the confluence of girna river. the hydrogeomorphological study helps to explore, plan and manage the water and the natural processes which produce landforms in association with their hydrological characteristics (babar, 2005). figure 1. location map of the study area 53altimetric and hypsometric analysis for soil and water conservation: a case study of anjani and jhiri river basin, northern maharashtra, india figure 2. contour map and river flowing in the study area figure 3. dark grey massive basaltic flows. (location: 20°58’ n 75°27’ e) geologically, the major part of study area is covered by deccan basalt of the cretaceous to the lower eocene age and the rest by alluvium of the recent period (gsi, 2000). the deccan trap lava sequence is grouped under sahyadri group, which was formed during the upper cretaceous to lower eocene. the lava assemblage of sahyadri group consists of alternating sequence of pahoehoe and ‘aa’ flows (gsi, 1976). dark grey massive basaltic exposed in the study area has been depicted in figure 3. those parts that are covered by deccan basalts consist of amygdaloidal and vesicular basalts. alluvial deposits can be observed along the lower reaches of river valleys and watercourses (fig. 4). the thickness of the alluvium ranges from less than a meter to several tens of meters (10 – 100 m). alluvium consists of unconsolidated to consolidated sand, brownish yellow silt and calcrete (locally known as kankar) along the girna, anjani and jhiri rivers. in the study area, however, the alluvium is characterized by unconsolidated clays and silts that are associated with a very limited thickness (0.5-2.0m). figure 4. geological map of study area (after golekar et al., 2013) the soil cover in the study area is derived from the basaltic lava flows and is classified as a) deep black soils b) medium black soils c) sandy soils and d) black alluvial clay (cgwb, 2009). a deep black soil covers the central part of the catchment area. medium black soils occur within the valley and southern hills, and the alluvial basin soil (black alluvial clay) covers the northern part of the study area. sandy soils are observed along the foothills of satmala stretching near padamalay hillocks. the gross and net water availabilities in the anjani jhiri river basin were calculated by using equation 1 (pilgrim and cordery, 1993), available water (in hectare) = area (in ha) × rainfall (in mm) (1) the gross and net water availabilities in the anjani jhiri river basin have been presented in table 1. 54 golekar r. b., baride m. v., patil s. n., adil s. h. table 1 gross and net water availability in the anjani and jhiri river basin methodology the field work was carried out in the summer of 2012 for observing geomorphological units in the study area. after completion of the field work, we carried out the hypsometric analysis of anjani and jhiri river basin by strahler methods (strahler, 1964). the topographic maps numbered 46 p/1, 46 p/5 and 46 o/8 on 1:50000 scale from the survey of india are considered. the soi toposheet is geometrically rectified and georeferenced to world space coordinate system (wgs 1998) using digital image processing software (autocad 2004 and arc gis version 9.0) and digitization work was carried out for entire basin. percentage hypsometric curve (strahler, 1952) that involves a ratio of relative height expressed in percentage (cumulative (hx100)/h) was plotted on ordinate, and the relative area expressed in percentage (cumulative (ax100)/a) was plotted on abscissa. where ‘a’ and ‘h’ denote area and height between successive contours, and ‘a’ and ‘h’ denote total area and height of the basin, respectively. the areas between successive contours and their respective heights are the basic data required for the study of area-height relationship. the areas between successive contours were measured by digital planimeter, and contour heights were obtained from topographic maps used for this work. after plotting the respective value on simple arithmetic graph paper and joining all the points, a smooth line percentage hypsometric curve can be obtained. the hypsometric integral (hi) were calculated mathematically from the graph. result and discussion hydrogeomorphological parameters the delineation of a hydrogeomorphological parameter is aimed at demarcating the areas of groundwater recharge/discharge and potential zone for the development of groundwater (raghu and reddy, 2011). alluvial plain represents the runoff zone while valley belongs to discharge zone, and the denudational hills constitute the infiltration area. it is classified into three zones on the basis of their groundwater potential zone as a) very favorable zones b) good to moderate zones and c) poor zone. the groundwater potential zones are identified with the help of geomorphological units. the morphometric data are enclosed in details and are described below (table 2 and 3). dissected plateau (dpt) an extensive flat top and steep slopes formed over horizontally layered deccan basalts that may be crossed by fractures, joints and lineaments are called as plateaus (babar, 2005). these units can be expressed regarding the slope of the area, runoff characteristics, drainage density, stream frequency and relief ratio of the area (babar, 2005). in the present study area, two plateaus were observed, i.e. moderately dissected plateau and un-dissected plateau. moderately dissected plateau the soils covering in this plateau are moderately thick and well drained. high moisture capacity suggests that the irrigation requirement is moderate in the moderately dissected plateau area. the groundwater potential in these units is moderate to high. un-dissected plateau the land of this unit is dissected by the streams of giving rise to un-dissected terrain consisting of flat-topped hills and steep scarps. the groundwater potential in these units is very poor (babar, 2005). the runoff from these areas can be arrested through the construction of check dams and other strategies. hydrogeomorphological characters of anjani river basin moderately dissected plateau moderately dissected plateau is presented in upper part of the anjani river catchment (fig. 4). this unit is observed between denudational hills and un-dissected plateau. un-dissected plateau un-dissected plateau is distributed in lower part of the anjani river catchment (fig. 4). this unit is observed between moderately dissected plateau and eroded lad by stream. valley fills (vf) valley fills are linear depression presented in between the younger alluvium composed of cobbles, pebbles, gravels and sands. valley fills are also observed in between the denudational hills near galapur, khadke bk, mangrul and palskhede bk villages of upper part of the anjani river catchment. denudational hills a group of hills with basalt as resistant rock are formed due to different erosional and weathering processes, and they occupy the low area. denudational hills are observed in a few patches of the upper part of the anjani river catchment (fig. 4). alluvial plains (younger and older alluvium) alluvial plains are composed of sand slit and clay layer. fine to coarse sand layers form the potential aquifer zone. it is the most prospective hydrogeomorphological unit for groundwater exploration. younger alluvial plain can provide sufficient amount of water than older alluvial plains. older alluvium is distributed along the northern part of the anjani river catchment area. 55altimetric and hypsometric analysis for soil and water conservation: a case study of anjani and jhiri river basin, northern maharashtra, india hydrogeomorphological characters of jhiri river basin moderately dissected plateau moderately dissected plateau is mainly distributed in the upper part of the jhiri river catchment (fig. 4). this unit is observed between denudational hills and un-dissected plateau. un-dissected plateau un-dissected plateau is mainly located in the lower part of the jhiri river catchment (fig. 4). this unit is observed between moderately dissected plateau and eroded lad by stream. valley fills (vf) valley fills are found between the younger alluvium composed of cobbles, pebbles, gravels and sands. valley fills are observed at padmalaya and umarde villages. denudational hills a group of hills with hard rock is formed due to different erosional and weathering processes and they occupy the region where the river jhiri originates (fig. 4). alluvial plains (younger and older alluvium) alluvial plains with fine to coarse sand layers form potential aquifer zone. younger alluvial plain can provide sufficient amount of water than older alluvial plains. older alluvium occurs in the northern part of the jhiri river catchment, especially at vaktuki, anjan vihire and jhurkheda (fig. 5). table 2 linear aspects of the anjani and jhiri river basin (after golekar et al., 2013) where, (nu) = number of streams; (lu) = total length of streams in km; (l) = mean stream length in km; (rb) = bifurcation ratio table 3 aerial and relief aspects of the anjani and jhiri river basin (after golekar et al., 2013) 56 golekar r. b., baride m. v., patil s. n., adil s. h. figure 5. geomorphological map of study area (mdp: moderately dissected plateau, udp: un-dissected plateau) profile analysis profiles drawn from the contour map is an image observation of the real environment of the landscape. the longitudinal section of the valley is called as longitudinal profile. the entire distance from source to the mouth of a particular river is considered. the graph drawn reveals the relief impact of the river course (singh, 1997). the longitudinal profile provides breaks in the longitudinal course of the river flow. these breaks may indicate nick points, and helps in examining the nature and control of landform development (singh, 1997). longitudinal profile gives geomorphologists an insight of relief and topographical impact on river flow (babar, 2005). longitudinal profile of the anjani river basin the anjani river originates from the titvi village at 305 m amsl, and flows almost straight towards ne. the slope is gentle with a drop in elevation from 292m to 240m, spreading over 25 km. at the village pashtane budruk, it turns toward ne. after that, the channel gradient decreases steadily till its confluence with the river girna at village babhulgaon (fig. 6). a study of longitudinal profile reveals the character of anjani river. it suggests the presence of nick points at locations chorvad (292m amsl), pharkande (240m amsl) and waghlud budruk (197m amsl). the difference in elevation suggests that the river has developed “rapids”. it is formed due to different erosion, relief and presence of lineament (joints). lineament map of jalgaon district has shown in figure 8. longitudinal profile of the jhiri river basin jhiri river originates from village padmalaya at 300m amsl, and it runs almost parallel with anjani river and at anjan vihire village at 176m amsl it takes a nw turn and meets girna river. the longitudinal profile curve for jhiri river basin indicates the presence of nick points at vikhran (212m amsl) and vanjari khapat (186m amsl). the jhiri river has a steep gradient in its earlier course with a drop from 284m to 231m up to village umarde (8 kms away from the origin of jhiri river), and thereafter the slope become gentle up to village vikhran and vanjari khapat. there is a drop in elevation at vanjari khapat and the river becomes gently sloping after village anjan vihire. longitudinal profile of the jhiri river channel has shown in figure 7. the nick points indicate that the rapids are formed due to different erosion, relief and presence of lineament (joint). figure 6. longitudinal profile of anjani river figure 7. longitudinal profile of jhiri river figure 8. lineament map of jalgaon district source: central ground water board (cgwb, 2009) hypsometric (area-altitude) analysis the hypsometric curve is a normalized cumulative frequency distribution of elevation (strahler, 1964). this curve is constructed by summing the number of data points higher than incremental thresholds 57altimetric and hypsometric analysis for soil and water conservation: a case study of anjani and jhiri river basin, northern maharashtra, india of elevation, starting at the maximum elevation. classically, hypsometric analysis has been used to differentiate erosional landforms at different stages of their evolution (strahler, 1952; schumm, 1956). the hypsometric analysis is necessary to determine the runoff, recharge, and storage conditions of the ungauged basins and is helpful for hydrogeological investigations. hypsometric curves show the relationship between relative heights (h/h) and relative areas (a/a). the relative area is a ratio of the area above a particular contour to the total area of the basin. according to sarangi et al. (2001), the relative elevation is calculated as the ratio of the height of a given contour (h) to the maximum basin elevation (h). hypsometry reveals the relationship between altitude and basin area to understand the degree of dissection and the stage of the erosional cycle (strahler, 1952; schumm, 1956). area height curve, hypsometric curve, and percentage hypsometric curve are used to obtain the relationship between altitudes and area of the basin. convex-up curves with high integrals indicate typical youth, undissected (disequilibrium stage) landscapes, while smooth, s-shaped curves crossing the center of the diagram represent the characteristics of mature (equilibrium stage) landscapes and concave curves with low integrals exhibit old and deeply dissected landscapes (strahler, 1952). hypsometric (area-altitude) analysis of the anjani river basin the hypsometric curve of anjani river (fig. 9) suggests that the larger part of the area is moderate to gentle sloping. the curve can be characterized as mature/equilibrium stage of landscape development. the curve shows 1) elevation in 15 % of the area ranges between 300m -280m 2) 50 % between 280m -240m and 3) 35 % between 240m-180m. these results indicate that about 85% of the study area is moderate to gently sloping and about 15 % area is steep sloping (fig. 6). figure 9. hypsometric curve (anjani river) hypsometric (area-altitude) analysis of the of the jhiri river basin hypsometric curve of jhiri river (fig. 10) shows 5 % are of steep slopes (300m -260m)), 80 % are of moderate slope (280m220m) and 15 % are on gentle slope (220m-180m), suggesting that jhiri river flows over moderate slope to gentle slopes. this indicates a mature (equilibrium) stage of river development (fig. 7). figure 10. hypsometric curve (jhiri river) hypsometric integral (hi) the hypsometric integral (hi) is the area beneath the hypsometric curve which relates the percentage of total relief to its cumulative percentage. this provides a measure of the distribution of landmass volume remaining beneath or above a basal reference plane. integration of the hypsometric curve gives the hypsometric integral. pike and wilson (1971) has mathematically defined the elevation-relief ratio e as where, e is the elevation-relief ratio equivalent to the hypsometric integral hi; elev mean is the weighted mean elevation of the basin estimated from the identifiable contours of the delineated basin; elev min and elev max are the minimum and maximum elevations within the basin, the hypsometric integral is expressed in percentage units. the hypsometric curves and hi values give valuable information about the geological stage of the development of a basin. according to strahler (1952) and kusre (2013), it can be classified as a) hi ≥ 0.6 = the basin is in-equilibrium (youthful) stage. b) 0.35 ≤ hi < 0.6 = the basin is at equilibrium stage. c) hi< 0.35 = the basin is at monadnock stage. in in-equilibrium (youthful) stage, the basin is still under developed. the equilibrium stage is the mature phase of basin development, and the monadnock phase occurs, particularly when isolated bodies of resistant rock from major hills (monadnock) are found above the subdued surface. it is a rejuvenation stage controlled by structural features (kusre, 2013). narayanpethkar et al. (1991) and sable et al. (2009) used the hypsometric integrals for understanding groundwater recharge in different basins of basaltic terrains. the characteristics of hypsometric integrals of the watershed were also reported by vivoni et al. (2008). they stated that the watershed with higher hi values indicated higher soil moisture, whereas a watershed with lower hi values is characterized by soil moisture being concentrated at the shallow depth. this means that watershed with lower hi values has less total runoff contributed from surface runoff. whereas, watershed with higher hi values shows subsurface runoff is major process contributing the total runoff. 58 golekar r. b., baride m. v., patil s. n., adil s. h. hypsometric integral (hi) for anjani river basin the hypsometric integral value of the anjani river is 0.52. according to strahler classification (strahler, 1952), the value falls under the equilibrium stage and that the basin is in a mature stage of development. hypsometric integral (hi) for jhiri river basin the hypsometric integral values of the jhiri river is 0.51. according to strahler classification (strahler, 1952), the value also falls under the equilibrium stage and is in a mature stage of development. the values for both rivers are similar. the river has attained mature stage since it has been controlled by climate, lithology, and relief. the nick points observed for both of the rivers (figure 5 and 6) do not show any indication of structural control or rejuvenation of the river. conclusion and recommendation drainage morphometry data of the area suggests that the maximum number of streams is of the first order (table 2). it is also observed that there is a decrease in stream frequency as the stream order increases. the total and mean lengths of stream segments are maximal in first order streams and decrease as the stream order increases. an exception to this is anjani river basin that shows the 6th order stream has more length then the 5th order stream (table 2). this is due to the moderate to steep slope. topography and also shows nick points at pharkande where the 5th order stream begins. the average bifurcation ratios of anjani and jhiri river are 3.63 and 4.13, respectively, suggesting low runoff, high recharge and somewhat elongated basin (table 2). drainage density values of anjani river (1.25/ sq.km) and jhiri river (1.49/ sq.km) suggest that the jhiri river is flowing through more weathered basalts than anjani river (table 3). elongation ratios of the both basins are categorized as more elongated suggesting the areas has a high infiltration capacity and low runoff (table 3). both rivers flow through basalts with moderate slope and low relief. the ruggedness number for both river basins show lower value due to the presence of hard resistant basalts (table 3). longitudinal profile results reveal the presence of nick points at chorvad (292m amsl), pharkande (240m amsl) and waghlud budruk (197m amsl) in anjani river and at vikhran (212m amsl) and vanjari khapat (186m amsl) in jhiri river (fig. 7). the hypsometric curve of anjani river suggests that a larger part of the area is moderate to gently sloping. the curve can be characterized as mature/ equilibrium stage of landscape development whereas the hypsometric curve of jhiri river shows 5 % steep slopes (300m -260m)), 80 % moderate slope (280m220m) and 15 % gentle slope (220m-180m) suggesting that jhiri river flows over moderate to gentle slopes. this fact indicates a mature (equilibrium) stage of river development. the hypsometric integral values of both the anjani and jhiri rivers fall into the equilibrium stage and that the basins are in a mature phase of development. the area, therefore, suggests relief, topography and climate controlled development. following are preventive measures recommended for the protection of the groundwater resources in the study area. i. cement concrete bandhara (bunds) is most feasible for water conservation structures in the moderately dissected plateaus and construct it across the 3rd and fourth order streams of both the anjani and jhiri river basin. ii. nala bunding, gabion structures and vegetative bunds feasible structures for soil and water conservation and construct it across the 2nd or third order streams of both the anjani and jhiri river basin. iii. in the alluvial tract of the study area, recharge wells or shafts are suggested, because the groundwater can be tapped through deep tube wells. this type of structure can be constructed in the lower part of the anjani and jhiri river catchment, mainly at ahire bk, pashtane bk and tarde bk villages in anjani river basin and at vaktuki, anjan vihire and jhurkheda village in jhiri river basin. the most feasible artificial recharge structure in such areas are recharge wells/shafts on the river bed of the tributaries. iv. controlling urban expansion in the study area is suggested. the study area is a recharge area for the shallow and deep aquifers, and the urban expansion will reduce the area of recharge. acknowledgements i would like thanks to department of science and technology, government of india for valuable research grant sanctioned to dr. m. v. baride (research guide) and the project fellowship awarded to one of the author rbg under this scheme. the co-operation of north maharashtra university officials is also acknowledged. authors also would like thanks to survey of india for provide a topographic map. references babar md. (2005). a text book hydrogeomorphology fundamentals application and techniques new india publishing agency, new delhi 137 p. cgwb (2009). manual on hydrogeology of jalgaon district, maharashtra cgwb, nagpur ground water information jalgaon district maharashtra 1606/dbr/2009 19 p. geological survey of india (2000). district resource map jalgaon district, maharashtra geological survey of india (1976). geology of the jalgaon district, maharashtra geological survey of india 125th annual celebration pp. 6 golekar r. b., baride m.v., patil s. n. (2013). human health risk due to trace elements contamination in groundwater from anjani and jhiri river catchment of northern maharashtra, india earth science research journal vol 17, no. 1, pp 17-23 golekar r. b., baride m.v., patil s. n. (2013). morphometric analysis and hydrogeological implication: anjani and jhiri river basin maharashtra, india archives of applied science research v. 5 (2) pp 33 41 kusre b. c. (2013). hypsometric analysis and watershed management of diyung watershed in north eastern india journal geological society of india vol.82, pp. 262-270 narayanpethkar a.b., zambre m. k., mallick, k. (1991). geophysical studies for groundwater in adila basin around solapur, maharashtra proc. of seminar association of exploration of geophysist pp 255 to 261 pike r.j., wilson s.e. (1971). elevation-relief ratio, hypsometric integral and geomorphic area-altitude analysis geol. soc. london vol 157, pp.303-316 pilgrim d. h., cordery i. (1993). chapter 9: flood runoff. in: maidment dr (ed.) handbook of hydrology. mcgraw-hill, new york, usa. raghu v., mruthyunjaya reddy k. (2011). hydrogeomorphological mapping at village level using high resolution satellite data and impact analysis of check dams in part of akuledu vanka watershed, anantapur district, andhra pradesh. j. ind. geophys. union, v. 15(1), pp.1-8 sable s. m, ghodake v. r., narayanpethkar a. b. (2009). influence of weathering and fracting on aquifer parameters in the basaltic terrain: a case study in dhubdubi basin, solapur district maharashtra, india gondwana geological magazine vol. no. 24 (2) pp – 131 – 136 sarangi a, bhattacharya a k., singh a., singh, a.k. (2001). use of geographic information system (gis) in assessing the erosion status of watersheds indian jour. soil conservation v. 29, pp.190-195 schumm s. a. (1956). evolution of drainage systems and slopes in badlands at perth ambos, new jersey. geol. soc. amer., v. 67 pp 597-646. singh savindra (1997). geomorphology, prayag pustak bhawan, allahabad, pp 18107 strahler a. n. (1952). hypsometric (area-altitude) analysis of erosional topography geo. soc. am. bull v. 63, pp. 1117-1142 strahler, a.n. (1964). quantitative geomorphology of drainage basins and channel networks, in hand book of applied hydrology (edited by v. t. chow), pp 439-476. vivoni e.r., moreno h.a., mascaro g., rodriguez j.c., watts c.j., garatuza‐payan, j., scott, r.l. (2008). observed relation between evapotranspiration and soil moisture in the north american monsoon region geophysical research letters 35: doi: 10.1029/2008gl036001 pronostico de valores horarios de la temperatura del aire en el observatorio meteorologico nacional-santafe de bogota ramiro leon diaz campos profesor asociado departamento de geociencias-facultad de ciencias-universidad nacional de colombia jesus antonio eslava ramirez profesor titular departamento de geociencias-facultad de ciencias-universidad nacional de colombia diaz, r. & j. eslava (1995): pron6stico de valores horarios de la temperatura del aire en el observatorio meteorol6gico naclonal-santate de bogota. geotrs. colomb. 3:45·50. issn 0121-2974 resumen aplicando una metodologfa que incluye un anal isis de fourier, se obtiene un modelo rnaternatico que representa la variaci6n diu rna de la temperatura del aire en santate de boqota y permite calcular sus valores medios para cualquier momento del dfa de cada mes y afio, con base en el valor promedio diario 0 en los valores que determinan ese promedio. ei modelo tambien indica que existe una gran posibilidad de pronosticar los valores horarios de ese elemento para perfodos de 6 0 12 horas con base en el promedio de las 24 horas anteriores y en alguna otra variable dependiente de la amplitud diaria de la temperatura y/o ta existencia 0 no de nubosidad. abstract a time model for temperature, has been established for the data of the national meteorological observatory station located at the campus of the national university in santate de bogota. to get this model, fourier analysis has been applied to the hourly mean values of the before mentioned parameters. the results provide not only diurnal variations, but allow the forecasting of the parameters for periods of six and twelve hours, from the average of the last 24 hours. 1. introduccion en este traba]o, segundo de una serie que cubre tres elementos meteorol6gicos, se define un modelo matematico que permite determinar y cuantificar la evoluci6n de la temperatura media horaria del aire en santate de bogota. ei modelo se caracteriza mediante el analisis de fourier, en tsrmlnos de los parametres de frecuencia y longitud de onda de las sefiales de temperatura media horaria definidas para la estaci6n observatorio meteorol6gico nacional. en el modelo se utiliza la temperatura media que se determina para cualquier perfodo continuo de 24 horas del cual se conozcan las temperaturas de las 07, 13 y 19 horas (0 cualesquiera otras con las cuales se pueda definir el promedio para ese perfodo), corregida para adecuarla al valor mas exacto que se determinarfa si se utilizaran los 24 valores horarios. ai igual que 10 que se realiz6 con la presi6n atmosterica (vease dfaz & eslava, 1993), despues de una descripci6n de la metodologfa utilizada y el analisis de los datos, se liega al modelo que representa la variaci6n horaria y, con base en el, se pronostica la temperatura media horaria y se incluye un ejemplo y un analisls de los resultados. 2. generalidades sobre modelos mate maticos y meteorologia tal y como 10mencionan dfaz & eslava (1993) es posible construir modelos maternaticos, que simulen desde cada una de las caracterfsticas de cad a uno de los elementos involucrados con la atmosfera, hasta supermodelos que permitan pronosticar no s610 el pr6ximo estado del tiempo, sino el clima mismo. cada una de las propiedades 0 caracterfsticas del sistema atmosterico, interactua con una 0 todas las propiedades 0 caracterfsticas restantes y, ademas, 10hace en un completo rango de escalas temporales y espaciales, que abarcan desde los fen6menos de muy pequeria escala, baja amplitud y corto perfodo (rnllimetricos y de muy corta duraci6n), hasta aquellos que cubren toda la tierra e inclusive el universo mismo y duran muchos anos (alta amplitud y largo perfodo). puesto que las propiedades ffsicas que definen el estado y evoluci6n del tiempo y el clima son muy complejas y variadas es imposible, en el estado actual de la ciencia, concebir un modelo que defina a la atm6sfera en forma completa. por 10 anterior, es necesario concebir modelos unitarios y/o parciales que consideren toda 0 una parte de una 0 varias de esas propiedades. los modelos tarnbien pueden usarse con cierta confianza para investigar la reacci6n del clima ante las imaginables perturbaciones naturales y las provocadas por el hombre y saber si esos cambios pueden distinguirse de las fluctuaciones cumancas naturales. obviamente, para lograr establecer esa distinci6n, es necesario determinar muy bien 10 relacionado con las fluctuaciones naturales, las cuales se intenta conocer y cuantificar con base en modelos que usan los datos observados, tal y como es el caso del presente trabajo. 3. metodologia utilizada la metodologia aplicada en este trabajo es la misma que se utiliz6 en un trabajo anterior sobre la presi6n atmosterica (dfaz & eslava, 1993) y que se considera conveniente reproducir aqui. tarnbien se considera necesario recalcar que 10 que se acostumbra en general, y en particular en meteorologia, es tratar, por medio de 'regresiones, de acomodar un modele rnaternatlco preestablecido (lineal, logaritmico, exponencial, etc.) a un conjunto de datos reales de un muestreo, sin tener en cuenta los contenidos de frecuencias y longitudes de onda de las sefiales. mediante el anal isis de fourier (cooley et al., 1970), se posibilita la determinaci6n de un modele maternatico, no preestablecido y en tsrmlnos de los parametres de frecuencia y longitud de onda ya mencionados y que son los verdaderamente determinantes en los ana/isis de variabilidad espacio-temporal de los elementos meteorol6gicos. las funciones de temperatura, presion, humedad del aire, etc., son funciones temporales y espaciales continuas. un muestreo en el tiempo de cualquiera de elias, por ejemplo la temperatura (9, se puede considerar como la convoluci6n de t(x) con la funci6n delta de dirac, cuyo resultado es una funci6n discreta y peri6dica. ~ = t(x), i: o(x-na) = n=-oo = i:t(x-na) = i:t(jo-na) n=-oo n=--oo , en el intervalo 0 5: j 5: n-1, los parametres n, 0 y no representan el numerode intervalos de muestreo, el intervalo de muestreo y la longitud del registro (iongitud de onda fundamental). ei procesamiento cuantitativo de datos en meteorologfa, se inicia representando la informaci6n en terminos de una serie arm6nica, la cual permite un tratamiento matematico adecuado. para muestreos en una dimensi6n la longitud del registro es: l = a = no ei vector de onda esta dado por: k = k",ax k = (27tnll),ax 05: n 5: n-1 y el vector posici6n: r = x.ax r+= jo.ex 05: j 5: n-1 cada observaci6n se puede representar en terminos de la serie arm6nica: n·' tj = k cn·e12mv'< n=o n·' tj = k cn·e lk ., n=o [2] i = -j ("l la suma de las desviaciones cuadraticas con respecto a la serie inicial n·' n-' i (tj tl = k rl j=o j=o es mfnima, siempre y cuando los coeficientes cn, correspondan a los coeficientes discretos de fourier n-'c, = (1/n), k tj'ei2.nj/n 1=0 n-' cn= (1/n). k tj.e ik • j=o [3] las transformadas de fourier se denotan abreviadamente asf: [4] esto significa que se puede pasar del dominic del tiempo, al dominic de la frecuencia y viceversa, usando la transformada directa y la transformada inversa de fourier. oebidoa la longitud finita de la sefial en el dominic del tiempo, se hace necesario establecer ifmites en el dominic de la frecuencia, los cuales dependen de su longitud y de su intervalo de muestreo. la longitud del registro define el ifmite inferior: u 1 = 1/l y el intervalo de muestreo define el ifmite superior liamado de frecuencia de nyquist un=y20 de las dos condiciones anteriores se establece que el intervalo de frecuencia en el espectro es: 1/l 5: u 5: y2 d [1] y el intervalo entre dos frecuencias consecutivas esta dado por: au = 1/l liamado tamblen co-intervalo de nyquist. en la practica, para lograr una 6ptima resoluci6n, es necesario tomar la longitud del registro tan grande como sea posible. los datos de muestreo tj son reales y los resultados que se desea obtener tambien deben ser reales; por 10 tanto, al expresarlos en terminos de una serie de fourier, se debe tener en cuenta la siguiente propiedad: tj es real y par «==" cn es real y par [5] en general tj no cumple con esta condici6n pero, usando los datos de muestreo, es posible construir una nueva funci6n ti, en el intervalo 05: i 5: 2n-1, que tenga simetrfa par con respecto al ultimo punta de la serie haciendo: (ver fig.1) • t, i i d •. i i i l_l__l-.l l__l-.l l-.l l__l-.l__, i -2 i n i 2n·' iiio , 2 figura 1. -funci6n tl construida a partir de los datos de muestreo, con simetria par 2n-l _ c, = (l1ln).1:. t1·e1kr j=o se puede construir tarnbien la funci6n par, con respecto al origen de la misma, tomando: t, =t., en esta nueva funci6n 2n, d y2nd representan respectivamente: el numero de intervalos, el intervalo de muestreo y la longitud de la nueva funci6n par. construida la funci6n par y haciendo: las ecuaciones [6] y [7] se transforman en las ecuaciones [8] y [9] que son la base te6rica para la determinaci6n 0 construcci6n de modelos que representen las senates en el tiempo y la frecuencia de los diferentes elementos meteorol6gicos. ei espectro de potencia permite identificar las componentes en frecuencia de una senal y separarlas posteriormente en el dominic del tlempo. es posible tarnblen realizar procesos cuantitativosqueen el dominic del tiempo son dispendiosos, perc que en el dominic de la frecuencia se reducen a operaciones algebraicas. n-1 t,=(aj2)+ 1:. an·cos(k.r)+ (aj2).cos(1ti) [8] n=1 n-1 an=(2/n)[to+21:.tr.cos(k.r) + tn"cos(1tn)] [9] 1=1 ei coeficiente aode fourier, se define para cada mes, como: 24 a, = 2.(1/24). ~ t, [10] i=l y corresponde a los valores que aparecen en la tabla 2, como nurnero de onda cero. en todos los casos: el subfndice n significa el valor del punta 24 (hora 24 0 cero), n es un punta cualquiera dentro de los ifmites senalados, to valor del elemento a la hora cero (24 horas), r, valor horas 1 a 23, t; valor horas 1 a 24, tn valor a la hora 24 (0 cero). 4. analisis de los datos los datos utilizados corresponden a los valores medios de temperatura del aire para cada hera, de cad a mes y afio (tabla 1) determinados por eslava (1990, 1991) para la estaci6n observatorio meteorol6gico nacional, que estuvo ubicada en el extreme ne de la ciudad universitaria de santats de bogota, dentro del area ocupada por el instituto geoqrafico agustin codazzi (igac). para definir esos valores se utilizaron los valores horarios medidos en ese sitio, durante el perfodo 1941-1960, previa una verificaci6n, homogeneizaci6n y procesamiento sequn los rnetodos meteorol6gicos y estadfsticos mas adecuados y com unmente usados en estos casos. 4.1 reproducci6n de la senal mediante la serie de fourier ai igual que 10 senalado para ta presi6n atrnosterica (diaz & eslava, 1993), la serial tj (ecuaci6n [1]) se puede reproducir mediante t, (ecuaci6n [7]) con desviaciones mfnimas y se pueden lograr correlaciones que tienen un coeficiente de 0.9999. como se serialo antes el coeficiente ao corresponde al doble del promedio diario determinado para cad a mes con base en 24 observaciones horarias. por su parte, a" a2, [6] a3, .... ,a24' definen la presencia de frecuencias en orden ascendente en el espectro y, por 10 tanto, determinan la forma de la serial, mientras que a, solamente ubica un desplazamiento de la sefiat a partir del origen. tabla 1 temperatura media horaria del aire, en santafe de bogota, (dc) hora enero febrer marzo abril mayo junio julio agosto septie octubr noviem diciem ano 01 9.5 9.6 10.6 10.7 10.9 10.5 9.9 9.9 9.5 10.2 10.2 10.2 10.1 02 9.0 9.2 10.2 10.4 10.5 10.1 9.6 9.5 9.0 9.8 9.8 9.7 9.7 03 8.6 8.7 9.8 10.0 10.2 9.8 9.3 9.2 8.7 9.5 9.5 9.3 9.4 04 8.2 8.4 9.6 9.8 10.0 9.5 9.0 8.9 8.5 9.2 9.3 9.1 9.1 05 7.8 8.1 9.3 9.7 9.8 9.3 8.8 8.7 8.3 8.9 9.0 8.7 8.9 06 7.6 7.7 9.1 9.6 9.8 9.3 8.8 8.5 8.1 8.9 8.9 8.6 8.7 07 8.5 8.5 10.2 11.1 11.5 10.8 10.5 10.3 10.1 10.6 10.7 10.2 10.2 08 11.8 11.8 12.9 13.6 13.8 13.3 12.7 12.1 12.8 13.3 13.4 12.7 12.9 09 14.3 14.3 15.2 15.5 15.4 14.7 14.4 14.4 14.9 15.1 15.4 14.9 14.9 10 16.5 16.4 16.8 16.5 16.4 15.8 15.4 15.6 16.2 16.5 16.9 16.8 16.3 11 17.9 18.2 17.9 17.7 17.1 16.6 16.1 16.5 16.9 17.3 17.8 17.9 17.3 12 18.7 19.1 18.6 18.0 17.6 17.1 16.6 16.9 17.4 17.6 18.3 18.6 17.9 13 19.1 19.4 19.2 18.5 18.1 17.6 17.1 17.5 17.8 17.6 18.4 18.7 18.2 14 18.9 18.9 19.0 18.2 18.0 17.6 17.2 17.4 17.9 17.2 17.8 18.5 18.0 15 18.6 18.5 18.6 17.8 17.7 17.3 17.1 17.3 17.8 16.9 17.2 17.8 17.7 16 17.6 17.7 17.9 17.1 17.3 16.9 16.7 16.8 17.2 16.3 16.5 16.9 17.1 17 16.5 16.6 16.9 16.3 16.4 16.0 16.0 16.0 16.4 15.6 15.4 15.8 16.2 18 15.1 15.4 15.5 15.1 15.3 15.0 14.9 14.9 15.0 14.4 14.4 14.7 15.0 19 13.6 14.1 14.2 13.8 14.2 13.9 13.7 13.7 13.8 13.4 13.4 13.7 13.8 20 13.0 13.3 13.5 13.2 13.5 13.2 12.9 12.9 13.0 12.8 12.9 12.9 13.1 21 12.1 12.3 12.8 12.5 12.6 12.3 12.0 11.9 12.0 12.0 12.0 12.1 12.2 22 11.4 11.5 12.3 12.0 12.1 11.8 11.3 11.4 11.2 11.4 11.6 11.5 11.6 23 10.7 10.8 11.8 11.5 11.8 11.3 10.8 10.8 10.6 11.0 11.2 11.1 11.1 24 10.1 10.2 11.1 11.2 11.4 10.8 10.3 10.4 9.8 10.6 10.7 10.7 10.6 m 13.1 13.3 13.9 13.7 13.8 13.4 13.0 13.0 13.0 13.2 13.4 13.4 13.3 los valores que aparecen en la tabla 2, son los coeficientes de fourier obtenidos al aplicar la transformada de fourier (ecuaci6n [9]) a los datos de la tabla 1. [7] tabla 2 coeficientes de fourier, obtenidos al aplicar la transformada de fourier (ecuaci6n 9) a los promedios horarios de la temperatura del aire en santafe de bogota (tabla 1) #onda ener febrer marzo abril mayo junio julio agosto septie octubr noviem diciem ano o 2 3 26.26 25.94 -1.81 -1.31 -4.95 -3.87 0.71 0.57 1.44 0.66 0.81 0.48 0.27 0.44 0.02 0.16 -0.32 -0.15 -0.14 -0.14 -0.04 -0.12 0.02 -0.03 0.14 0.08 0.09 0.06 0.06 0.04 0.02 0.05 -0.05 0.00 -0.03 -0.01 0.00 -0.01 -0.02 -0.04 0.12 0.02 0.05 -0.02 0.01 0.01 -0.01 0.02 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 26.58 26.03 -1.84 -1.38 -4.94 -4.02 0.76 0.53 1.43 0.80 0.89 0.53 0.19 0.43 -0.10 0.13 -0.25 -0.15 -0.11 -0.15 -0.05 -0.12 0.01 -0.03 0.15 0.07 0.16 0.09 0.08 0.07 0.00 0.06 -0.05 0.03 -0.06 -0.04 -0.02 -0.03 0.00 -0.05 0.10 0.05 0.02 0.00 0.00 -0.01 -0.02 0.05 27.77 26.09 -1.50 -1.48 -4.38 -4.53 0.56 0.54 1.20 0.77 0.61 0.66 0.27 0.47 0.08 0.19 -0.28 -0.25 -0.11 -0.18 -0.10 -0.11 0.04 -0.01 0.15 0.10 0.09 0.11 0.03 0.06 0.03 0.08 0.00 0.01 -0.03 -0.02 0.00 0.01 -0.02 -0.06 0.06 0·02 0.00 0.00 0.00 0.03 -0.01 0.01 27.50 26.38 -1.11 -1.09 -4.04 -3.99 0.27 0.12 0.96 0.89 0.54 0.73 0.35 0.44 0.11 0.07 -0.16 -0.20 -0.11 -0.11 -0.11 -0.11 -0.03 -0.05 0.13 0.06 0.08 0.11 0.08 0.04 0.05 0.08 . -0.03 -0.01 -0.05 -0.02 -0.01 -0.01 -0.01 -0.04 0.05 0.05 -0.03 0.02 -0.04 0.02 0.03 -0.02 27.64 26.75 -1.16 -1.07 -3.81 -4.22 0.34 0.02 0.77 1.06 0.46 0.78 0.42 0.33 0.15 0.07 -0.12 -0.17 -0.14 -0.15 -0.13 -0.12 -0.03 -0.04 0.08 0.07 0.10 0.13 0.03 0.04 0.07 0.06 0.02 0.03 -0.02 -0.02 -0.01 -0.02 -0.02 -0.04 0.04 0.05 -0.02 -0.01 0.02 0.02 0.02 0.03 26.72 26.78 -1.22 -1.31 -3.79 -4.45 0.46. 0.29 0.75 1.25 0.50 0.75 0.38 0.27 0.17 0.02 -0.12 -0.15 -0.13 -0.14 -0.13 -0.06 -0.03 0.01 0.07 0.05 0.10 0.08 0.06 0.07 0.06 0.06 0.01 0.03 -0.06 0.00 -0.02 0.00 -0.01 -0.02 0.06 0.01 0.00 0.01 0.03 0.04 0.02 0.00 26.67 -1.36 -4.25 0.44 0.99 0.65 0.37 0.07 -0.20 -0.14 -0.10 -0.02 0.10 0.11 0.05 0.04 -0.01 -0.03 -0.q1 -0.03 0.05 0.01 0.02 0.02 4.2 pron6stico de valores horarios usando el promedio de 24 horas anteriores y aplicando la serie de fourier: mediante la ecuaci6n [9] se calculan los coeficientes, para el mes que corresponda, (tabla 2) y se define la funci6n en cualquier punta (hora) en la serie mediante la ecuaci6n [8]. asf se pueden determinar con buena precisi6n, en primera instancia, los valores horarios de temperatura para perfodos de 6 0 12 horas, segun los datos de entrada. en caso de no disponer del valor promedio determinado con base en las 24 horas ante rio res, se puede usar el promedio determinado por cualquiera de las f6rmulas alternativas que existen, previa aplicaci6n de una correcci6n para convertirlo en promedio de 24 horas. puesto que en colombia se utiliza generalmente la f6rmula: [11 ] entonces, el primer coeficiente de fourier sera: [12] ei t:.t que se utiliza en este trabajo es el determinado por eslava (1990, 1991) y que se relaciona para cada mes en la tabla 3. td = temperatura media diaria, . t07 = temperatura del aire a las 07 horas, este dato puede ser el cronol6gicamente anterior 0 posterior al de las 13 0 19 horas que se utilice en la f6rmula, t'3 = temperatura a las 13 horas, este dato puede ser el cronol6gicamente anterior 0 posterior al de las 07 0 19 horas que se utilice en la f6rmula, t19 = temperatura a las 19 horas, este dato puede ser el cronol6gicamente anterior 0 posterior al de las 07 0 13 horas que se utilice en la f6rmula, t:.t = diferencia media mensual entre la temperatura media diaria basada en las observaciones de las 07, 13 y 19 horas, y la temperatura media diaria determinada con base en 24 horas. un ejemplo de la aplicaci6n del modele al pron6stico se presenta en las tablas 4 a 6, en las cuales con los datos de temperatura del aire medidos en el observatorio meteorol6gico nacional el 6 y 7 de octubre de 1960 (tomados de igac, 1961), se pronostican los valores horarios para perfodos de 12, 6 y 6 horas, respectivamente. tabla 3 tabla 5 valores medios horarios de temperatura del aire en santafe de bogot~, medidos (m) y pronosticados (p) (6 y 7 de octubre de 1960) aplicando el mceete, °c. diferencia media mensual (at) entre la temperatura \oia 6 media diaria determinada con base en 24 hora m observaciones [1] y la temperatura media diaria determinada con base en observaciones de las 07,13 y 07 19 horas [2], °c. 08 09 10 11 12 13 21.0 19 12.4 mes temperatura media at [1] [2] [1)-[2] enero 13.1 13.7 -0.6 febrero 13.3 14.0 -0.7 marzo 13.9 14.4 -0.5 abril 13.7 14.3 -0.6 mayo 13.8 14.5 -0.7 junio 13.4 14.0 -0.6 julio 13.0 13.8 -0.8 agosto 13.0 13.8 -0.8 septiem 13.0 13.9 -0.9 octubre 13.2 13.8 -0.6 noviemb 13.4 14.0 -0.6 oiciemb 13.4 14.1 -0.7 ano 13.3 14.0 -0.7 tabla 4 valores medios horarios de temperatura del aire en santafe de bogot~, medidos y pronosticados (6 y 7 de octubre de 1960) aplicando el modelo hora 6-1q-60 06·10-60 medida pronos medida 07-10-60 pronos medida.. 01 02 03 04 05 06 07 13 19 20 21 22 23 24 9.4 7.7 1.7 9.0 7.4 1.6 8.7 6.4 2.3 8.4 6.2 2.2 8.1 5.3 2.8 8.1 5.8 2.3 9.8 7.2 2.66.2 21.0 12.4 12.0 12.2 -0.2 11.2 12.6 -1.4 10.611.1 -0.5 10.2 10.3 -0.1 9.8 9.4 0.4 7 7-10-60 m p m a 7.2 12.8 13.6 -0.8 14.6 14.6 0.0 16.0 16.8 -0.8 16.8 18.4 -1.6 17.1 18.9 -1.8 17.1 18.7 -1.6 tabla 6 valores medios horarios de temperatura del aire en santa" de bogot~, medidos (m) y pronosticados (p) (6 y 7 de octubre de 1960) aplicando el modelo, °c. \ora 6 7 7-10-60 hora m m p m 07 7.2 13 18.7 14 16.1 18.0 -1.9 15 15.8 17.7 -1.9 16 15.2 17.0 -1.8 17 14.5 18.4 -3.9 18 13.3 15.6 -2.3 19 12.4 12.3 14.6 -2.3 .. las desviaciones (a) entre los valores obtenidos aplicando el modele y los medidos son relativamente altas (en algunos casos cercanas a 3°g), dependen de condiciones meteorol6gicas propias de cada dia, en particular de la amplitud de la temperatura y/o de la presencia 0 no de nubosidad (fig.2). los ensayos indican que es posible lievar estas desviaciones a valores minimos mediante la aplicaci6n de un filtro que involucre las variables que las ocasionan. esos ensayos tamblen muestran que las desviaciones grandes pueden estar indicando errores en la serie original de datos o la presencia de una situaci6n meteorol6gica particular 0 no usual; en este sentido el modelo puede ser usado como un control de calidad de la informacion. x , . , . i i i i i i i e , 20 r--. 15 '".~ '"q) u '" 100 'd 0 co <:> '-' 0 co 5:j '0 co q) o, e q)~ 0 -5 202122232412345678910111213141516171819 20 horos de16-10 a los 19 del 7-10-60 d medida ~ pronosticada _ diferencia figura 2. valores de temperatura (dc), medidos (6 y 7 de octubre de 1960) y pronosticados con el modele (tablas 4-6) 5. conclusiones eimodelo definido presents una excelente correlaci6n con los datos de entrada, perc al aplicarlo como pronosticador no se encontr6 una rnuy buena concordancia entre los valores generados y los observados, debido a la influencia de variables no consideradas en el modelo. aun asi los resultados obtenidos son muypromisorios pues se trat6 de una aplicaci6n preliminar de la metodologia, que puede mejorarse mediante el manejo del procedimiento estadistico que determma la temperatura mediay la utilizaci6n de filtros que contemplen las variables que originan esas diferencias y las disminuyan a un grado aceptable. referencias bibliograficas cooley, j.w., p.a. lewis & p.o. welch. (1970): the fast fourier transform algorithm: programming considerations in the calculation of sine. cosine and laplace transforms. journal sound vib. 12 (3):315-337. u.s.a. olaz, r. & j. eslava. (1993): pron6stico de valores horarios de ta presi6n atmostsrica en el observatorio meteorol6gico nacional. zenit 4:5-12. acig, santafe de bogota. eslava, j. (1990): caracterislicas de la variaci6n diurna de la temperatura del aire en bogota. memorias del xi congreso colombiano de geogratra. pp.222-23t. acoge, monterfa. --(1991): variaci6n temporal de la temperatura del aire en bogota. rev. acad. colomb. cienc. 18 (68):65-74. santate de bogota. instituto geogralfico "agustin codazzi" (igac). (1961): anales del observatono meteorologico nacional-ciudad universitaria 1960. 215pp. igac, santafe de bogofa. _ sand dunes are wide spread in the gaza strip and are present in its northern, central and southern regions. thirty sand samples were collected at seven locations along the middle region of the strip. the coordinates for each sampling site were positioned using gps and processed with arcgis software. mechanical and chemical properties were examined to determine the textural characteristics and carbonate contents of the dune sands. the mean grain size is fine to medium, and the sands range from moderateto well-sorting. the skewness is fine to very fine, and the kurtosis ranges from mesokurtic to very leptokurtic. additionally, the results show that the carbonate content ranges from 1.5% to 5%. the high permeability, good porosity, and low carbonate content of the sand dunes in the gaza strip have led to more groundwater pollution via leachates percolating from the solid waste landfills and basins from wastewater treatments constructed above the sand dunes. sand quarries have also changed the natural landscape of the gaza strip. these quarries were excavated randomly, used for a short period and then disused. this random and unregulated excavation has led to the destruction of natural habitats for flora and fauna. las dunas de arena se extienden ampliamente en la franja de gaza y hacen presencia en las regiones del norte, centro y sur. para este trabajo se recolectaron treinta muestras de arena en siete puntos a lo largo de la región media de la franja. las coordenadas de cada muestra se marcaron con el gps y se procesaron con el programa arcgis. se examinaron las propiedades químicas y mecánicas para determinar las características de textura y los contenidos de carbonato de las dunas de arena. el tamaño del grano está entre fino y medio y las arenas oscilan entre moderadas y bien definidas. la asimetría estadística va desde fina hasta muy fina y la variedad de la curtosis se ubica entre mesocúrtica y muy leptocúrtica. adicionalmente, los resultados muestran que los rangos de contenido de carbonato están entre 1,5 % y 5 %. la alta permeabilidad, la buena porosidad y el bajo contenido de carbonato en las dunas de arena de la franja de gaza han permitido una mayor contaminación a través de lixiviados que se filtran desde vertederos de desechos sólidos y desde estaciones de tratamiento de aguas residuales construidas sobre las dunas de arena. las canteras de arena, además, han cambiado el paisaje natural de la franja de gaza. estas canteras fueron excavadas de manera aleatoria, utilizadas por un tiempo corto y luego cayeron en desuso. estas excavaciones aleatorias y sin regulación han llevado a la destrucción de hábitats naturales de flora y fauna. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 131 142 abstract resumen key words: beach dunes, desert dunes, grain size distribution, gaza strip, palestine. palabras clave: dunas de playa, dunas de desierto, distribución del tamaño de grano, franja de gaza, palestina. record manuscript received: 23/02/2013 accepted for publication: 16/07/2014 sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts khalid f. ubeid 1 alhasan s. albatta 2 1 department of geology, faculty of science, al azhar university – gaza, p.o. box 1277, gaza, palestine. email: k.ubeid@alazhar.edu.ps 2 department of coastal and marine environment, general directorate of environmental resources, palestinian environment quality authority “eqa” – gaza. email: geologist-hasan@hotmail.com sedimentology issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.37238 http://dx.doi.org/10.15446/esrj.v18n2.37238 132 khalid f. ubeid, alhasan s. albatta introduction sand dunes cover vast land areas and are primarily located in arid regions. sand dunes are concentrated in africa, australia and asia, where approximately half of the sand dunes have stable, fixed configurations. most of the currently fixed dunes were active during the last glacial maximum (e.g., hesse et al., 2003; fitzsimmons et al., 2007; stone & thomas, 2008; ashkenazy et al., 2012) when the weather was colder and drier, and air turbulence was strong (e.g., harrison et al., 2001; lunt & valdes, 2001; 2002; duller & augustinus 2006). in palestine, sand dunes cover an area of approximately 1,600 km2; approximately 81% of the area (1,300 km2) is located in northwestern negev, and the remaining area is primarily located in the southern coastal plain. the northwestern negev dunes are older than those on the coastal plain, where they invaded the area more than 15 thousands years ago. the southern coastal plain dunes are younger, approximately 100 1,500 years old (goring-morris & goldberg, 1990; magaritz & enzel, 1990; rendell et al., 1993; tsaor et al., 2004; singhvi & porat, 2008; tsaor, 2009). in the gaza strip, sand dunes overlay the gaza formation, which consists of alternations of calcareous sandstones and reddish-brown, fine-grained deposits along the middle region of the gaza strip between the coastal ridge and al montar ridge. in addition, the sand dunes cover the beach along the western margin of the strip. the western and middle gaza strip sand dunes stabilized at the end of the previous century when land use drastically changed. stabilization of the gaza strip sand dunes contains considerable vegetation coverage, and architectural activities occur in the middle part of the strip. these dunes stabilized rapidly due to the low wind power in palestine (tsaor, 2009). the aim of this study was to examine the sand dunes in the gaza strip, specifically the morphology, textural parameters, and anthropogenic environmental impacts associated with these dunes. geography and geology of the gaza strip figure 1. map of the study area showing the topography and the sand dune locations in the gaza strip. bv=bedouin village; zt=al zahraa town; nst=north asdaa town; sst=south asdaa town; khp=khan younis; rr=rafah road; and rb=rafah border. the gaza strip is located in southwestern palestine at the southeastern coast of the mediterranean sea, between 34o 2` east and 31o 45` north (fig. 1). the strip covers an area of approximately 365 km2, has a length of 45 km along the coastal zone, and its width ranges from 5 to 8 km in the central and northern regions to a maximum of 12 km in the south. the gaza strip is located in the transitional zone between a temperate mediterranean climate in the west and north and the arid climate of the sinai peninsula on the east and south. the temperature gradually changes throughout the year, reaches its maximum during the summer in august and its minimum in january during the winter. the average monthly maximum temperature ranges between 17.6 co for january and 29.4 co for august. the average monthly minimum temperature for january is approximately 9.6 co and 22.7 co for august. the mean annual rainfall is 335 mm per year, and the average annual evaporation is 1300 mm. the topography of the gaza strip is defined by three ridges (locally termed kurkar ridges), depressions, dry streambeds and shifting sand dunes (fig. 1). the ridges and depressions generally extend in the ne-sw direction parallel to the coastline. the surface elevation ranges from mean sea level to approximately 110 m above mean sea level. the depressions, which contain alluvial deposits, are approximately 20 40 m above mean sea level. in terms of stratigraphy, these ridges belong to the kurkar group (bartov et al., 1981). this group is of a pliocene-pleistocene age, and within this group, three formations can be distinguished: ahuzam fm, pleshet fm, and gaza fm (bartov & arkin, 1980; frechen et al., 2004; galili et al., 2007; ubeid, 2010; 2011b). the kurkar ridges, which belong to the gaza fm, consist of calcareous sandstones (locally termed kurkar) alternated with brown-reddish, fine-grained deposits (locally termed hamra) (horowitz, 1975; abed & al weshahy, 1999; frechen et al., 2004; ubeid, 2010; 2011b). in the southern part of the gaza strip, these features tend to be covered by sand dunes. the soil in the gaza strip is primarily composed of three types: sandy soil, clayey soil, and loess soil (ubeid, 2011b; 2013). the sandy soil is found along the coastline and middle parts of the gaza strip in the form of sand dunes. the clayey soil is found along the northeastern part of the gaza strip. the loess soil is found around the wadis area. materials and methods the dune field was first studied using aerial photographs and landsat images. the sand dunes were mapped from orthophotos at a scale of 1:5000 in a gis program and validated with field surveys. field observations were noted and documented using field photographs. seven localities, characterized by the presence of a dune field were selected for study, which from north to south, are the following: bedouin village (bv), al zahraa town (zt), north asdaa town (nst), south asdaa town (sst), khan younis (khp), rafah road (rr), and rafah border (rb) (fig. 2). at each location, the length, width and height of the dunes were measured using meter tape, and the slope of the dunes was measured in both the lee and stoss directions. the coordinates for each location were recorded using portable gps (table 1). the sand flow direction was determined from ripple marks and dunes using a brunton compass. figure 2. sampling sites within the study area. (a) bedouin village; (b) al zahraa town; (c) north asdaa town; (d) south asdaa town; (e) khan younis; (f) rafah road; (g) rafah border. 133sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts table 1. gps coordinates of the sampling sites in the study area. thirty samples were collected from the crest, lee and stoss sides of the dunes for sedimentological analysis. samples were taken during the spring season (may-june 2012). approximately 100 g of each sample were used to determine the grain size. the grain size distribution was obtained using the sieve analysis method. the data were processed using gradistat software following blott and pye (2001) to determine the grain size parameters. in this software, linear interpolation was used to calculate the statistical parameters as per folk and ward (1957). the software also provides a physical description of the textural group that the sample belongs to and the sediment type, which are named according to folk (1954). the total calcium carbonate content was measured using volumetric calcimetry. the coordinates of each sampling location were integrated onto a digital map using arcgis software. results dune morphology and field relations during the fieldwork, it was observed that the dunes were of the barchans type (fig. 3). the dunes had a low-density distribution with tens of meters between dunes. table 2 presents a summary of the dimensions of the dunes, and figure 4 shows the morphology of the dunes in the studied area. dunes differ in their sizes. smaller dunes (fig. 4a) had heights ranging from 0.4 m to 1.6 m and widths ranging from 2.4 m to 12 m, whereas relatively larger dunes (fig. 4b) ranged in height from 1.8 m to 20 m and in width from 25 m to 78 m. there is a direct relationship between the length and width of the dunes and between their length and height (fig. 5). the relationship between dune length and width is stronger because the fit line of the curve passes through the original point, which suggests that the wind effects that transport the sand were uniform throughout the dunes. the curves also show that the smaller dunes are closer to the line of fit; increased size may lead to some deformation in shape. figure 3. field photograph of a barchans dune, at the southern part of the gaza strip. 134 khalid f. ubeid, alhasan s. albatta table 2. the measured field dimensions of the sand dunes. figure 4. the morphology of sand dunes in the study area. (a) dunes with a ripple index ranging from 6 to 25. (b) dunes with a ripple index ranging from 6 to 9 (except sst9 at south asdaa town). the size distributions of the dunes are random and not dependent on their location. it was observed that architectural activities, in addition to sand supplements and wind directions, affected the size and distribution of the dunes. the larger dunes were observed in open areas with low relief and were far from buildings or foundations. additionally, the larger dunes were found at localities where the coastal ridge height was at the minimum, such as in the middle and southern parts of the gaza strip; these locations experienced maximum wind speeds. additionally, sand excavation at the sand quarries affected the size of the dunes: the creation of large, flat areas in the quarries allowed sand to move easily through these areas. figure 5. the relationships between the dimensions of the dunes in the study area. 135sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts figure 6. current flows of dune sands at the study sites in the gaza strip. the table below the figure shows the mean vector of the directions at each site. the wind direction was sse in most locations, except in the northernmost location (bedouin village) and in the southern part (north asdaa town), where the wind direction was ees (due to open areas with a low relief at these locations, fig. 6). the rose diagrams in figure 6 show the vector mean and dominant direction data separately at all sites in the study area. it was observed that the vector means are not coincident with the maxima of each rose diagram. this could be due to the wind regime, which is affected by the topography of the different sites (zu et al., 2008). it was observed that the differences are smaller in the southern sites than in the northern sites. furthermore, the wind energy increases in the south due to the low relief of the coastal ridge in the west (fig. 1). however, the density of the vegetation on the sand dunes was low to very low in the northern and middle parts of the gaza strip, whereas it was low to moderate in the southern part. the vegetation preserves the stability of the dunes, and the sands accumulate around the plants and form a nabkha, particularly in the south (fig. 7). figure 7. field photographs of vegetation cover. (a) the vegetation cover on the sand dunes in the south, at the khan younis locality. (b) the nabkha in the southernmost part of the gaza strip, at the rafah locality. textural parameters the grain size parameters of the sands from the seven sampling locations are shown in tables 3-5. twenty-two of the histograms that represent the mechanical composition exhibit unimodal distributions in which the modal class falls into the medium grade, whereas eight display bimodal distributions that are of fine and medium grades. the statistical grain-size parameters show that in all the studied locations, the sand dunes are primarily formed by medium sands, except in the north asdaa town location in the southern part of the gaza strip; there, the sand dunes are mostly formed by fine sands. the dune sands in all the studied locations are primarily moderately well sorted, except those at al zahraa town and rafah road, where the dune sands are moderately sorted to well sorted. the skewness of the dune sands of the northern and middle locations is primarily very finely skewed, whereas in the southern part of the gaza strip (i.e., in south asdaa town, khan younis, and rafah) the dune sands are finely skewed. the dune sands in the northern and middle parts of the gaza strip are primarily very leptokurtic, whereas in the middle and southern parts, they are leptokurtic to mesokurtic. the fine grain size at the north asdaa town is due to selectiveness of the wind, which is concentrated at the dune. the sorting values suggest that the al zharaa and rafah road dunes have been subjected to high transport by winds that control the grain size distributions within the dune system; this factor was strongly related to the extremely low height of the coastal ridge at these localities, where the effect of wind is larger. 136 khalid f. ubeid, alhasan s. albatta table 4. average values of grain size parameters for each location in the study area. table 3. grain size parameters of sands from the dunes. 137sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts table 5. grain size distribution of the different sand samples. the gaza strip sand dunes are rich in quartz with smaller amounts of plagioclase. bioclasts of land snail shells were found in the middle and southern parts of the gaza strip (fig. 8). the carbonate contents of the sand dunes range from 1.5% -to 5% (table 6). figure 8. the bioclasts of land snail shells (white color), at south asdaa town in the southern part of the gaza strip. table 6. calcium carbonate contents for the different sand samples. discussion classification and inferred sources of sand dunes based on geographic location, two main categories of sand dunes can be recognized: (1) beach sand dunes and (2) desert sand dunes. beach sand dunes these dunes are located along the beach at the western margin of the gaza strip. the sand is composed of a high percentage of a light yellow to transparent sugary quartz and approximately 1% dark-colored accessory minerals. the sand grains are characterized by medium-grained sand that is moderately well sorted (ubeid, 2011a). the sand of this type comes from marine sediments 138 khalid f. ubeid, alhasan s. albatta that were transported along the eastern shore of the mediterranean sea from the nile delta by longshore currents (nir, 1989; goldsmith & golik, 1980; goldsmith et al., 1990; ben-dor et al., 2006; zviely et al., 2006; 2007; ubeid, 2010; 2011a). desert sand dunes the desert sand dunes are located along the middle part of the gaza strip (fig. 1). the characteristics of these dunes are outlined above (see results) and are presented in tables 35. the characteristics of the sand reflect the aeolian sediments that were supplied from the sinai desert (neev et al., 1987; crouvi et al., 2008; ubeied, 2011b) (fig. 9). sediment supply and strong winds are both crucial contributors to the dune encroachment and sand transport from the sinai desert to northwestern negev and the gaza strip (amit et al., 2006; enzel et al., 2008; 2010; roskin et al., 2011a & 2011b, 2013). figure 9. regional map showing the distribution of sand dunes in the northern sinai desert, which extends from the nile delta eastwards into northwestern negev and gaza strip (after dan & raz, 1970; bartov, 1990).the large ne arrow marks the westerly winds prevalent during sand incursion (e.g. amit et al., 2006; enzel et al., 2010). associated environmental impacts the anthropogenic environmental impacts associated with sand dunes in the gaza strip are the following: figure 10. field photographs of the sand quarries in the study area. (a & b) the quarries at bedouin village in northern part of the gaza strip. (c & d) the quarries at north asdaa town in the southern part of the gaza strip. (e) the trucks used during the excavation sands from a sand quarry in the southern part of the gaza strip. (f) an abandoned sand quarry at south asdaa town in the southern part of the gaza strip. 139sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts 1 sand quarries. randomly established sand quarries were distributed throughout most of the studied sites (fig. 10). sand was excavated from each quarry for a short period of time; then, workers moved the sand to another place, which left relatively large low-relief areas behind. the mineral composition of the sand dunes (i.e., primarily quartz) encourages people to excavate the sands from these quarries for commercial and industrial purposes. for example, approximately 664,759 m3 of sand was used for construction. figure 11 shows the amount of sand excavated from the quarries in the studied sites during 2011. the excavation was performed without any plan for the future, which led to the destruction of the natural landscape and the habitat for flora and fauna in the gaza strip. figure 11. the amount of sand excavated from the sand quarries in the studied sites during 2011. 2 wastewater treatment basins. most of these basins were established on the sand dunes (fig. 12). the geochemical analysis results of the groundwater for the wells around the wastewater treatment basins in the rafah location at the southernmost tip of the gaza strip showed that the groundwater of these wells are polluted, where most of them are not fit for drinking according to the world health organization (who) (table 7a and 7b). therefore, high seepage from these basins could possibly occur. the sand dunes of the strip are characterized by high permeability and good porosity due to moderateto well-sorted grain sand and low calcium carbonate contents. these characteristics lead to wastewater leakage into the aquifer and subsequent pollution of the groundwater. in addition, a large amount of rainfall that would otherwise be used to recharge the aquifer is lost; these basins occupy an area of at least 872 km2 above the sand dunes (table 8). figure 12. field photographs of wastewater treatment basins above the sand dunes in the study area. (a) at beit lahya locality near bedouin village, in the northernmost part of the gaza strip (basins are shown in a rectangle). (b & c) at khan younis locality in the southern part of the gaza strip. note in (c) the sewage from the basin thrown above the sands. (d) at rafah border, in the southernmost part of the gaza strip. 140 khalid f. ubeid, alhasan s. albatta table 7. geochemical analysis of the groundwater for wells around the waste water treatment basins in rafah locality at southern most part of gaza strip. (b) who’s drinking water standards. note the high concentration of the results in the wells especially chloride and nitrate concentration comparing with who’s standards. table 8. depicts the wastewater treatment basins above the sand dunes in the studied locations. 3 landfills. several solid waste landfills were established in the sand dunes at the abandoned sand quarries above the low-relief area after the sand excavations were terminated (fig. 13 and table 9). solid waste leachate from these landfills causes groundwater pollution in areas where the sand dunes are characterized by high permeability. the landfills also led to the destruction of natural habitats. figure 13. photographs of solid waste landfills above the sand dunes in the study area. (a) at khan younis locality, in the southern part of the gaza strip. (b) near rafah border in the southernmost of the part gaza strip. note that both locations were established at abandoned sand quarries. table 9. depicts some waste solid landfills above the sand dune in the study area. conclusions in palestine, sand dunes cover an area of approximately 1,600 km2. in the gaza strip, the sand dunes cover the beach area along the western margin and the middle of the strip between the coastal ridge and the al montar ridge. according to the sand dune sources, the sand dunes in the gaza strip can be classified as either beach sand dunes or desert sand dunes. the beach sand dunes are located along the western part of the strip between the beach and the coastal ridge. the origin of these dunes is the nile delta, from which the sands are transported by longshore currents. however, the desert sand dunes are found along the middle part between the coastal and al montar ridges. they cover the northern part of the bedouin village, the middle region in al zahraa town, and the southern areas from khan younis up to rahah. the sands from these dunes originated from the sinai desert. the sand dunes in the gaza strip have a low-density distribution and differ in size, ranging from 0.4 20 m in height and 2.4 78 m in width. architectural activities, in addition to sand supplements and wind directions, affect their sizes and their distribution. in addition, vegetation contributes to the stability of the dunes. 141sand dunes of the gaza strip (southwestern palestine): morphology, textural characteristics and associated environmental impacts the sands from the middle part of the gaza strip are primarily mediumgrained, except in north asdaa town, where they are fine-grained. the sands are primarily moderately well sorted, except in al zahraa town and in the southernmost part of the strip, where the sands are moderateto wellsorted. the skewness is very fine to fine, and the sands are very leptokurtic to mesokurtic. the carbonate content in the dune sands ranges from 1.5% -to 5%. there are several wastewater treatment basins and solid waste landfills above the sand dunes in the gaza strip. these basins and landfills pollute the groundwater due to the high permeability and good porosity of the sand dunes, which are composed of fineto medium-grained sands and are moderate to well-sorting. the random distribution of the sand quarries, wastewater treatment basins, and solid waste landfills have destroyed the natural landscape of the gaza strip. references abed, a. and al weshahy, s. 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(2006). the holocene evolution of haifa bay area, israel, and its influence on the ancient human settlements. the holocene, 16, 849-861. coastal plains with a unique geographic setting and renewable natural resources are promising for sustainable development; however, these areas may be subjected to some environmental hazards due to their geological setting. one of those hazards is the seasonal flash flood that can threaten existing and future development projects in such critical areas. southwestern sinai, egypt, is a coastal plain that is characterized by complex geological setting an arid climate with seasonal rainfall which can result in a high runoff. the aim of this work is to model spatially the runoff amount and density related to flash flood development and to create a flash flood hazard map of the plain as an example of coastal plain in a desert environment with large and complex hydrologic setting. in this research, aster images are used to develop a digital elevation model (dem) and land use/land cover (lulc) data sets of the study area. geographic information system (gis) was used to perform runoff and flash potential flood analyses of the created databases and to show distributed runoff and flooding potential in spatial maps. a module was created in a gis environment to develop a flash flood potential index map. it was clear that the main two factors controlling runoff amounts and flash flood potential in such kinds of areas are the slope and soil types. the final dataset map procedure by this work can be very helpful in land use planning by highlighting the areas subjected to flash floods. las llanuras costeras que poseen recursos naturales renovables y una configuración geográfica única son promisorias para el desarrollo sostenible. estas áreas, sin embargo, son objeto de algunas amenazas ambientales debido a su escenario geológico. una de estas amenazas es la temporada de inundaciones rápidas que pueden poner en riesgo los proyectos de desarrollo existentes y los futuros en estas zonas críticas. el suroccidente de sinaí, egipto, es una llanura costera que se caracteriza por su compleja configuración geológica de clima árido, con temporadas de lluvia que pueden resultar en una gran escorrentía. el objetivo de este trabajo fue modelar espacialmente la cantidad y densidad de escorrentía relacionada al desarrollo de inundaciones rápidas y elaborar un mapa de amenazas de inundación rápida en este valle, como un ejemplo de llanura costera en un ambiente desértico con un escenario hidrológico grande y complejo. en este trabajo se utilizaron imágenes aster para desarrollar un modelo de elevación digital (dem, en inglés) y establecer la información de uso del suelo/cobertura del suelo (lulc, en inglés) en el área de estudio. a partir del sistema de información geográfica (gis) se analizaron la escorrentía y el potencial de inundación de las bases de datos creadas, y se mostró la escorrentía y el potencial de inundación en mapas espaciales. se creó un módulo en un ambiente del gis para desarrollar un mapa del índice inundación rápida potencial. se estableció que los dos factores que controlan la cantidad de escorrentía y el potencial de inundaciones rápidas en estas áreas son la inclinación y los tipos de suelo. el mapa final de procemiento con el conjunto de datos de este trabajo es de gran ayuda en la planeación del uso de suelos, ya que evidencia las áreas con posibilidad de inundaciones rápidas. abstract resumen keywords: flash flood, remote sensing, gis, hydrology, geo-environment, dem. palabras clave: inundación rápida, teledetección, sistema de información geográfica (gis), hidrología, geoambiente, modelo de elevación digital (dem). geospatial analysis for the determination of hydro-morphological characteristics and assessment of flash flood potentiality in arid coastal plains: a case in southwestern sinai, egypt record manuscript received: 13/03/2015 accepted for publication: 18/12/2015 how to cite item wahid, a., madden, m., khalaf, f., & fathy, i. (2016). geospatial analysis for the determination of hydro-morphological characteristics and assessment of flash flood potentiality in arid coastal plains: a case in southwestern sinai, egypt. earth sciences research journal, 20(1), e1-e9. http://dx.doi.org/10.15446/esrj.v20n1.49624 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.49624 análisis geoespacial para determinar las características hidromorfológicas y evaluar las inundaciones potenciales en llanuras costeras áridas: caso de estudio en el suroccidente de sinaí, egipto g e o m o r ph o l o g y earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): e1 e9 ahmed wahid 1, marguerite madden 2, fikry khalaf 3, and ibtehal fathy 4 1 geology department, faculty of science, port said university, port said, egypt, postdoctoral researcher, center for geospatial research; department of geography, university of georgia. email: awahid@sci.psu.edu.eg; 2 center for geospatial research; department of geography, university of georgia, athens, georgia; email: mmadden@uga.edu 3 geology department, faculty of science, port said university, port said, egypt; email: fikry_khalaf@hotmail.com 4 geology department, faculty of science, suez canal university, ismaelia, egypt; email: ibtehal_fathy@yahoo.com e2 ahmed wahid, marguerite madden, fikry khalaf and ibtehal fathy 1. introduction some of the main applications of hydrologic models are in water resources assessment, irrigation design, prediction of erosion and sedimentation, aquatic ecology, evaluation of climate change impacts and subsurface water contamination (refsgaard and abbott, 1996; ogden et al., 2000). parameters derived from hydrologic models usually focus on the physical parameters of the catchment area, which as noted by ogden et al. (2000), is an advantage of such models. one of the significant applications of hydrologic modeling is predicting and monitoring flash floods and their hazard, especially for urban areas and areas planed for development activities. flash flood is one of the supreme enemies to mankind in an environment. flooding is one of the worst weather-related hazard, causing loss of life and excessive damage to property (carpenter et al., 1999). however, human often place themselves in the heart of risk and human activity is expanding into more hazardous areas, particularly in or next to mountainous regions (montz and gruntfest, 2002). recently, and because of global climate changes, concern about the flash flood hazard has increased (habets et al., 1999, becker and grunewald, 2003; wmo, 2003, hattermann, et al and knebl et al., 2005). flash flood events in arid regions have a particular behavior, they happen locally, suddenly and in short duration . commonly, desert storms are highly localized, with only part of a catchment receiving rain and contributing directly to surface runoff along the wadis (valleys) (foody et al., 2004). in such arid regions, it is important to be concerned about flash flood potential. there are different ways to predict flash floods. the simplest way has been used by the u.s. national weather service since it was developed in the 1970s as described by konstaintine and georgakakos (2006), that depends on the idea of comparing in real time the observed (for now-casting) and predicted (for forecasting) rainfall volume of a given duration and over a given catchment area. if the nowcast or forecast rainfall volume is greater than the characteristic rainfall volume then flooding in the catchment is likely. in the absence of detailed field measurements, the most appropriate method for predicting the sites subjected to flash flooding is to use a spatially distributed hydrological model (foody et al., 2004). as, the climate data for the study area is discontinuous since 1967, so that spatial hydrologic modeling is the most favorable way to be used for the study area. the study area; southwestern sinai coastal plain is located between 27 o 45/ –28 o 45/ n and 33o13/ –34 o 10/ e (figure 1). it is considered one of the most promising areas for sustainable development in sinai, as it has the main constituents for establishing development projects. figure 1. location of the qaa plain and meteorologic stations depicted in a display of aster satellite mosaic at 15-m resolution (wahid et al, 2009). el-qaa plain is an elongated coastal plain occupies an area of about 3500 km2 with low-lying land. it contains important natural resources such as a large quaternary groundwater aquifer, sources of constructing and ornamenting materials and rich soils. however, some environmental and anthropogenic circumstances surrounding the plain need to be under considered during decision making for land use planning. there are a considerable number of valleys running over the plain known as wadis. these separate drainage systems are of significant importance to the plain. the plain is associated geologically and environmentally with the south sinai mountainous ranges and the northern granitic and sedimentary hills. the drainage vallies deliver sediments from the source rocks of the granitic rocks to this coastal gravelly plain. they also represent the main recharging source for coastal groundwater aquifer , which are fed by seasonal floods as surface runoff, or as ground water (wahid et al, 2009). although, land use covers less than 2% of the study area at present, the area is one of the most promising parts of the sinai for development. the egyptian government is currently planning to address various development activities and increase the population density of the area. thus it is important to delineate the flash flood potential index for the area to help decision makers for rational and sustainable land use planning. southwestern sinai has been extensively studied and still remains interesting for different kinds of investigations of geology and stratigraphy (abdallah,1982; bunter, 1982; adawy, 1986), tectonic pattern (gilboa, 1980; ibrahim and ghoneimi, 1992; meshref and el-kattan, 1989; mcclay et al., 1998) geomorphology and hydrology (el-refai, 1984; eweida et al., 1985a and 1985b), hydrogeology (abdel rahman et al., 1994; shendi et al., 1999; gorski and ghodeif, 2000; ghodeif, 2001; sayed et al., 2004), and soil and mineralogy (el-assy and sadek, 1992; hammad et al., 1998; el nahry and saleh, 2005). recent works have investigated how to use the promising natural resources in this interesting area while being mindful of the potential surrounding hazards. ghodeif (2001) studied the underground water resources and how to manage and protect them for sustainable development. he used topographic maps and aerial photos to delineate the watershed of the basins recharging the ground water aquifer. el nahry and saleh (2005) studied the influence of seasonal flash floods on soil erosion of the plain using landsat enhanced thematic mapper plus (etm+) images and gis. wahid (2008) used aster images, gis modeling and programming to model the wind transported sand along the coastline of the qaa plain, southwestern sinai. among the huge number of works that studied the study area, none tried to estimate the environmental impact of runoff or assess the flash flood hazard on the existing and expecting areas of development activities. the overall objective of this study is to estimate the level of hazard that can threat such promising coastal plains. the final product of this work is essential for interested agencies of planning and decision makers. this aim is accomplished by defining the hydrologic setting and developing a framework for flash flood potential modeling using the advanced gis analysis techniques and remotely sensed data. 2. materials and methods the aster images (level 1b data with 15-m resolution) were used for the present study. eight scenes were required to cover the study area dated between april 2003 and february 2004. data was obtained from the u.s. geological survey (usgs). aster is an instrument on board the terra (eo-1) satellite platform launched in december of 1999. the visible near infrared radiometer (vnir) sensor is a high performance, moderate resolution optical instrument of 15-m spatial resolution. the aster raw data were processed using erdas imagine, leica geosystem, inc. and subjected to geometric and radiometric corrections. egyptian military survey topographic maps (scale 1:50,000) were scanned and geo-referenced to be used along with the aster images for extracting the land use /land cover (lulc) of the study area. geologic map scale 1: 250,000 from the egyptian authority for mineral resources were used to delineate the geologic setting of the study area. precipitation records of the study area during a 23-year period (from 1980 to 2003) were obtained from the egyptian meteorological authority. data were derived from the eltor and st. kathrin meteorological stations that are located close to the watershed of the study area(figure 1). e3geospatial analysis for the determination of hydro-morphological characteristics and assessment of flash flood potentiality in arid coastal plains: a case in southwestern sinai, egypt a soil map in the form of a gis shape file of the study area was obtained from the egyptian national authority for remote sensing and space sciences ( narss ) created by el nahry and saleh, 2005. for the needs of the present study, some modifications were carried out to this soil map such as spatially extending of some soil types to the parts of the study area that were not in the original map. the eastern granitic mountains were not represented in the original soil map of el nahry and saleh (2005). using the geologic map of the study area, soil types were compared to the geologic rock units in the same site to define the equivalent soil type of the granitic rocks. this lets the missed parts to be filled and presented in the soil map. elevation variation of the examined area were produced and presented according to (wahid et al., 2009) and summarized here after. dems extraction process matches the two scenes of aster images, nadir-looking vnir (band 3n) and the backward looking-vnir (band 3b), on a pixel by pixel basis, notes the parallax and then calculates the relative or absolute height for each image separately and then mosaiced using erdas imagine. the resulting dem grid has 30-m post spacing resolution. the vertical accuracy assessment were done from collected check points extracted from topographic maps. dems of the study area yielded rmsez of 3± to 15±. this result is considered reasonable and dems were not applied to correction. the primary lulc classification scheme used for this work was derived from the anderson classification system for level i and ii (anderson et al., 1976). in addition to field observation, supervised and unsupervised classification were used for identifying the classes in the study area. also manual editing were used for small features defined from field work. with the classification for the area complete an accuracy assessment of the final data product was performed using randomly selected field reference points. to overcome reference accessibility issues the random reference sites were generated within a specified distance of existing transportation features. the accuracy is evaluated using computed producer accuracy, user accuracy, overall accuracy and kappa index (wahid et al., 2009). almost all errors occurred in the boundary area between different land use /land covers. the overall accuracy is 82.4% and overall kappa is 0 .79 (table 1). 2.1. hydrologic and watershed analysis the hydrologic parameters were identified using the archydro tool that can be added to esri arcgis™. the parameters defined for the needs of the present study are; flow directions, flow accumulation, catchment areas, drainage lines and drainage points. 2.2. curve number (cn) and runoff the curve number (cn) method is an empirical approach to estimate the direct runoff from the relationships between rainfall, land use and hydrologic soil groups (lim et al., 2006). the high cn, the low soil infiltration and the high runoff and vise versa. the arccn-runoff tool developed by zhan and huang (2004) is used to spatially model the watershed cn and runoff. this table 1. lulc accuracy assessment results (wahid et al, 2009). tool functions with vector data of lulc and soil of the watershed and is based on the u.s. department of agriculture (usda) soil conservation service (scs) method for cn calculation (zhan and huang, 2004). precipitation quantities fall over the study area are also required for the calculation. figure 2 . steps of cn-runoff tool operations for computing cn and runoff (after zhan and huang , 2004) overall accuracy =82.36% kappa = 0.79 the procedure for calculating cn and runoff using the arccn-runoff tool involves defining the infiltration of each soil type from its components and physical characteristics according to usda-natural resources conservation service (usda-nrcs, 2007). there are four hydrologic soil groups (a, b, c and d) where soils of the a hydrologic group have high infiltration rates and d soils have the slowest infiltration rates. the clastic constituent of each soil type were defined according to usda-nrcs, 1999 and then their infiltration rates were delineated. for example, soils that mainly consist of sand and gravel have high rate of infiltration, soils with high percentage of clay and mud have lower infiltration rate, where the hard rock’s have no infiltration. area of study found to include the four types of the soil hydro-groups. the lulc layer was converted to a polygon layer using arcgis. the soil and lulc layers were then clipped to the study area and intersected to generate a new composite layer that has the polygon associated with both soil hydro-group and lulc name. the arccn-runoff tool was then applied to calculate the cn and runoff for the study area watershed. calculation of cn and runoff requires three sets of data: precipitation records, landuse/ land cover (lulc) and soils. the steps of calculating cn and runoff using the cn-runoff tool is illustrated in figure 2. e4 ahmed wahid, marguerite madden, fikry khalaf and ibtehal fathy 2.3. creating flash flood model four sets of gridded data were prepared for this analysis. these data sets are, slope layer derived from 30-m resolution dem, soil layer, landuse layer and basins shape factor layer (table 2). dem is used to prepare the slope grid. the modified soil after el nahry and saleh, 2005 is used for soil layer that represents the different types of soils and its filtration capacity. landuse layer is extracted from classified image and topographic map of the study area. shape factor is calculated for each basin resulted from archydro model. shape factor is calculated as the square of maximum straight-line length of basin (from mouth to water divide ) divided by total area (jena and tiwari, 2006). these data sets were then re-sampled to 30-m resolution using a bilinear method. a relative flash flood potential index (rank) ranging from 1 to 10 was assigned to each data layer based on the layer attributes associated with the hydrologic response of each category. using model builder in arcgis, a module integrating the four data sets was created (figure3). the four raster data sets were reclassified according the given rank. for example the slope grid values were reclassified (re-valued) into 10 values as follows: slope 0-5 = 1, 5-10 = 2, 10-16 = 3 and so on (table 2 ). after giving each category of each layer the appropriate rank, weighting overlay were subjected to the four reclassified layers according to its importance in flashflood occurrence (table 2 ).the result of the model is a new grid was thus created that represents the overall flash flood index of the study area. a buffer zone is then created around the highest risk zone as a precaution. table 2. weights of layers contributing in ffpi model and ranks of each layer category. figure 3. module of calculating flash flood potential index (ffpi) for the study area. 3. result and discussion 3.1. geomorphology there is a close relationship between the geomorphlogical setting and the hydrologic pattern of the study area. the hydrology of the study area is mainly controlled by structural elements defined in shape, size and slope by long term fluvial process during the pleistocene period. the hydromorphic structure combined with current precipitation controlling factors influence runoff and flash flood in the study area. el-qaa plain was and still, is subjected to the process of the fluvial geomorphic cycle for long time intervals (el-refai, 1984).the effect of extensive amounts of past rainfall precipitation on the area is evident by the aerial distribution of drainage systems, large alluvial fans, abundant terraces and dispersed playa deposits. according to wahid et al, 2014, dem and aster stacked vnir_ swir image were displayed in arcgis arcscene to show the major geomorphological units of the study area in a 3d terrain view. several perspective scenes and ‘fly-by’ simulations were developed showing the study area from different view angles and in different scales (figure. 4). these views highlight the major geomorphic features described below. a) the gravelly coastal plain (el qaa plain) that extends along the gulf of suez and trends in a nw-se direction parallel to the suez rift with elevations ranging from mean sea level to 200 meters above sea level (figure. 4a). it generally slopes to the west towards the gulf of suez. b) precambrian mountains are represented by the eastern mountainous basement that bounds the eastern part of the study area and is composed of highly fractured igneous and metamorphic basement rocks that are dissected by many wadies and drainage lines (figure. 4b). the drainage pattern of this unit represents about 80% of the watershed of the study area that drains to the gravelly coastal plain. c) there are some precambrian basement hills distributed along the northwestern part of the qaa plain and other isolated small hills that are e5geospatial analysis for the determination of hydro-morphological characteristics and assessment of flash flood potentiality in arid coastal plains: a case in southwestern sinai, egypt scattered in the southern part of the study area (figure. 4c). d) sedimentomorphic units include the sedimentary hilly ranges distributed mostly in the northwestern part of the study area with low elevation and relief and wide spread alluvial beds deposited by wadis dissecting the eastern mountainous unit (figure. 4d). these wadis drain large amounts of water during floods accumulating rock debris at the mouths of wadis and, accordingly, forming the alluvial deposits called alluvial fans. 3.2. hydro-morphology and watershed the study area is a clear example about the relationship between the geologic history of a region and its hydrologic pattern. hydrology of southwestern sinai coastal plain is mainly controlled by two main factors. the first is the structural elements defined in shape, size ,slope and fracture density . the second factor is long term fluvial process during the pleistocene period. the hydrographic basins of the study area are controlled by endogenous and exogenous processes (hammad, 1980; el nahry and saleh, 2005). the endogenous factors are represented by structural lines (faults, domes, synclines, and anticlines along with the variation in lithologic formations). the study area is dominated by two large nw-se trending fault systemsthe rift border fault system and the eastern marginal fault system that defined the locations and the direction of wadis in the eastern granite massive (gilboa, 1980; khalil, 1998). the exogenous factor is the environmental conditions that started in the past (pleistocene fluvial period) and present time conditions as the area was and still, is subjected to the process of the fluvial geomorphic cycle for long time intervals (el-refai, 1984).the effect of extensive amounts of past rainfall precipitation is evident by the aerial distribution of drainage systems, large alluvial fans, abundant terraces and dispersed playa deposits. by defining the flow directions and flow accumulation, the catchment boundaries (hydrobasins), drainage streams and drainage points were established (figure 5). figure 5. hydrologic patterns of the study area. the area of study is characterized by a considerable number of drainage basins. it has 186 drainage basins that vary in area from 0.3 km2 to 264.6 km2 and mostly run in the granitic mountainous range, and some in the sedimentary hills. drainage pattern refers to the particular plan or design which the individual stream courses collectively form (thornbury 1969). the drainage patterns of southwestern sinai are mostly of dendritic and rectangular type, especially in the faulted and jointed mountainous basement rocks. 3.3. surface runoff several works studied the watershed and rainfall analysis of sinai peninsula (issar 1982, ghodeif 2001, cools et al 2012,afandi et al. 2013 and elsayad et al., 2013). figure 6 modified after elsayad et al., 2013 shows that the rainfall heights for 50 years return period is 20 to 60 mm. figure 6. isohyetal map of rainfall heights for 50-yr. return period in south sinai (after elsayad et al., 2013) runoff is the excess amount of rainfall that can accumulate over a drainage basin in a period of time and floods at the outlet of the draining stream of the basin. estimating the runoff of watersheds is an essential component of flash flood warning systems that is highly dependent upon surface characteristics of the lulc. table 3 shows the percentage of the area occupied by each of the 13 varieties of lulc (figure 7). the area of study is mainly covered by gravelly plain (45.2%), igneous rocks (33.6%), sedimentary rocks (14.0%) and sandy plain (5.0%). the remaining land use classes represent less than 2% of the study area. figure 4. 3d views of the study area showing major geomorphologic units, (after wahid et al, 2104) e6 ahmed wahid, marguerite madden, fikry khalaf and ibtehal fathy table 3. landuse/land cover classes in southwestern sinai coastal plain. figure 7. land use/ land cover classification of the study area.. the physical properties of soils also govern the hydrological processes, such as transmission and storage of received water. figure 8 shows the hydrological soil groups according to the infiltration capacity, where soil a has the highest infiltration rate and d has the lowest rate. the highest infiltration rate occurs within the gravelly plain, while the eastern basement mountainous ranges have the lowest infiltration ability. figure 8. soil map showing hydrologic groups (converted from el nahry and saleh, 2005). the third factor affecting surface runoff is the climatic conditions. the sinai peninsula is generally characterized by a mediterranean climate, with mild winters and hot summers, relatively low precipitation and high evaporation. study area receives rare, but sometimes noticeable and irregular, amounts of rainfall mainly in winter and autumn where summer is a dry season (jica, 1999; ghodeif, 2001). the analysis of the annual rainfall data of el-tor and st. katherine stations shows that, the mean annual precipitation varies from about 9 mm at el-tor to 33 mm at st. katherine. the maximum quantity of rainfall that fell in one day since 1980 until 2003 was 37.1 mm on may 27, 1995. this value (37.1mm) was used in the runoff calculation to determine the highest volume of runoff in case of the maximum rainfall recorded amount.cn values range from 30 to 98 with average 78, while runoff values vary between 0 and 479,224,14.0 m3 with average 609,468.8 m3 (figure 9 and table 4). the highest cn and runoff values are noticed in the eastern mountainous basement heights in contrast to coastal areas and gravelly plains. also, there is a dramatic decrease in runoff production when moving from the eastern mountainous areas to the coastal sandy plain in the west .this refers to the absence of connection between runoff production zones, and a partial down-slope re-infiltration of the runoff (bergkamp, 1998; braud et al, 2001). it emphasizes the fact that areas close to the heights are more subjected to receiving more water from the drainage streams to store or to flush away, whereas the coastal zone absorbs all or most of the rainfall and does not receive much water from the high runoff areas. e7geospatial analysis for the determination of hydro-morphological characteristics and assessment of flash flood potentiality in arid coastal plains: a case in southwestern sinai, egypt figure 9. runoff amounts for different units in the study area. table 4. summary of runoff analysis results on southwestern sinai coastal plain. 3.4. flash flood potential index map a flash flood is generated when precipitation saturates the drainage capacity of the basin slope causing impoundment of the drainage network and resulting in exceptionally high discharge amounts at the basin’s outlets. (carpenter et al., 1999; al-rawas et al., 2001; montz and gruntfest, 2002; rosso and rulli, 2002). the study area is located in an arid area and receives small amounts of precipitation. however, seasonally intensive rainfall, along with its geological and geomorphological settings, creates potential flash flood hazards in the area. in 2004, an extreme rainfall happened over the study area and caused some damage of properties. the flash flood potential index is an indicator of the hydrologic response of an area to the intense rainfall in its watershed. this index helps in predicting the flash flood prone areas and planning development accordingly. the higher rank values of index point to the high hydrologic response for flash flood potential and threat. factors contributing to flash flooding in an area depend on the physiographic characteristics of its watershed. slope is an important factor in the momentum of the flood velocity . also, a basins geometry reflects the way that runoff will move toward the outlet. a comprehensive study conducted in three selected sites in the egyptian desert suggests that circular basins create flash floods with sharp discharge peaks (quick flow), whereas elongated basins are generally characterized by slower flood-water flow (al-rawas et al., 2001). soil structure and texture are important in determining water holding and infiltration characteristics. land use practices can play a significant role in water infiltration, concentration and runoff behavior. the concept of creating this flash flood index is to analyze these characteristics together to derive information about the hydrologic response and flash flood potential inherent to a specific area. the produced flash flood index map (figure 10) shows that the areas of highest risk are those close to the eastern mountainous ranges, followed by those close to the northern sedimentary heights. the safest area is the part close to the sea. figure 10. flash flood hazard map of the study area. it is important to point out that slope is the most effective factor in defining the flood potential index. the slope of the study area gradually changes eastward. this makes the runoff move out strongly in the eastern side where the slope is very steep. this could be very dangerous for any development activities built close to this high flood potential index. in addition, the high infiltration soils of the low land infiltrate the incoming water faster and reduce runoff possibility, whereas, the impervious rocks of high mountainous areas in the east and north increase the possibility of runoff. shape factor results shows that most of the basins of the watershed are elongated and basins of high shape factor values are those of very small areas. the least effective factor is the landuse factor, as it covers a very small area of the study area. the flash flood hazard map (figure 10) contains the flash flood potential index, buffer zones around the high index areas, boundary of each basin, main drainage streams and points. the hazard map can help national and international agencies interested in landuse planning and sustainable development to make the right decisions in establishing development projects. e8 ahmed wahid, marguerite madden, fikry khalaf and ibtehal fathy 4. conclusions coastal plains could are promising area for sustainable development activities as long as the existing resources are managed wisely and development proceeds in a planned and organized manner. southwestern sinai coastal plain is an example of these promising areas. however, its geological and morphological characteristics along with the climatic conditions are responsible for seasonal flash floods that may threat the landuse planning in this area. it is very important for landuse planning to illustrate in detail the hydro-morphological setting of such kinds of area and define the spots prone to flash floods (montz and gruntfest, 2002). for more accurate results, aster images were used to create a dem with 30-m resolution and lulc cover maps with a resolution of 15 m, along with field work. although the study area is arid, intensive seasonal rainfall can cause catastrophic flash floods. the geomorphological parameters of this area defines its drainage pattern, with the highest amount of originating runoff flows from the eastern basement rocks followed by the northern and northeastern sedimentary hills. slope is the most effective factor in defining the flash flood index along the study area. the most area subjected to flash floods is that closest to the eastern side where the slope is very steep, the soils are impervious and flooded water can flush the sediment and boulders through the drainage streams. coastal zone is mainly composed of sandy and gravelly sediments. this makes this zone the safest from flash hazards because of the high infiltration capacity of the gravelly and sandy sediments forming this gentle slope plain. the results of this work are very important for sustainable development of the coastal plains close to rocky heights, because spatially explicit flash flood hazard is very critical for landuse planning of infrastructure, water management and establishment of new residential and tourism areas. using this map, it is easy to find the suitable sites for running roads and pipes (oil or water) far from the floodhazards . also it can help in groundwater management, as it can be used to find the best locations for building dams to preserve surface water for ground water recharging. future work to improve the accuracy and detailed of flash flood potential may require additional spatial factors such as distance from urban and economic areas and proximity to main streamlines. gis modeling of risk from natural disasters before development plans are initiated to management planners and decision makers hopes to prevent loss of life, damage to property and ecological impacts. acknowledgements authors would like to express appreciation to professor e. lynn usery, u.s. geological survey (usgs) for providing the aster images. we acknowledge with sincere appreciation dr. ned gardiner, american museum of natural history for technical support in extracting the dems of the study area. we also appreciate the support of the staff of the center for geospatial research; department of geography, university of georgia references and notes abdallah, m.a., 1982. straigraphy of the miocene deposits of the elqaa plain, west southern sinai, egypt, m.sc thesis, faculty of science, ain shams university. cairo, egypt. abdel rahman, s. i., zagloul, e.a., younes, h.a., 1994. evaluation of land and water resources of el-qaa plain, south sinai, for sustainable agriculture, agro. kemia e’s talajtan, budapest. 43 (3-4), 408-414. adawy, h.f., 1986; geophysical interpretation of qaa plain area, gulf of suez, ms.c, faculty of science, cairo university, egypt. al-rawas,g., koch,m., and el-baz, f., 2001. using gis for flash flood 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[online] available: http://soils.usda.gov/technical/handbook/ wahid, a. m (2008) gis-based modeling of wind-transported sand on the qaa plain beach, southwestern sinai, egypt. journal of coastal research: vol. 24, no. 4 pp. 936–943. wahid, a., madden, m., khalaf, f., and fathy, i., 2009. land suitability scenarios for arid coastal plains using gis modeling: southwestern sinai coastal plain, egypt, journal of urban and environmental engineering (juee), v.3, n.2, p.73-83 wahid, a., madden, m., khalaf, f., and fathy, i.,2014, geospatial analysis for determination of the geomorphologic forms in southwestern sinai coastal plain, egypt. international journal of geosciences and geomatics, vol.2, issue 1, 2014 wmo (world meteorological organization), 2003. our future climate (publication wo-952). zhan, x., and huang, m., 2004. arccn-runoff: an arcgis tool for generating curve number and runoff maps, environmental modelling and software. 19, 875-879. the tatuí formation, one of the brazilian paraná basin geological units, is developed solely in the state of são paulo, southeast brazil. it is studied herein, through outcrops at the eastern border of the basin, which are siltstones and fine sandstones, interbedded with some coarse and conglomeratic sandstones. published papers on its fossils are sparse, dealing with some groups but not involving the entire association. this paper deals with ichnofossils, arthropods (conchostracans, isopods, and indeterminate crustaceans), plants (pteridophytes and spermatophytes) and fish remains (scales and teeth of actinopterygii and sarcopterygii). foraminifers and sponge spicules were reported in short communications without illustrations and from uncertain stratigraphic positions. the tatuí formation paleoenvironmental deposition evolved through three phases. the first contains only the euryhaline fossils in siltstone beds. the second phase is regressive, with continental fossils (conchostracans and plants) in massive or laminated siltstone beds and conglomerates. the third stage is transgressive with hummocky cross – stratification in sandstone and increasing salinity. la formación tatuí es una unidad litoestratigráfica de la cuenca del paraná que aflora exclusivamente en el estado de são paulo, brasil. en su borde nororiental está formada por limolitas y areniscas finas que poseen intercalaciones de areniscas gruesas y conglomeráticas. este trabajo caracterizó en esta formación diversas evidencias de fósiles: huellas, artrópodos (conchostracos, crustáceos isópodos e indeterminados), vegetales (helechos y fanerógamas) y restos de vertebrados (escamas y dientes de actinopterygii y sarcopterygii). foraminíferos y espículas de esponjas fueron reportados en comunicaciones breves, entretanto, sin estar acompañado de ilustraciones y de sus posiciones estratigráficas. tres fases de deposición permiten reconstruir el paleoambiente de la formación tatuí. en la primera fase hay presencia solamente de fósiles eurihalinas en limolitas. en la segunda fase, fósiles continentales (conchostracos y vegetales) que están presentes en camadas limolíticas masivas o laminadas y en conglomerados vinculados a una etapa regresiva. finalmente, la tercera fase se caracteriza por una etapa transgresiva con la presencia de estratificación cruzada “hummocky” y aumento de la salinidad. . abstract resumen keywords: rock mass classification, rock slope, q, rmr, smr, sqr. palabras clave: permiano, paleontologia, estratigrafía, cuenca del paraná record manuscript received:17/04/2015 accepted for publication: 16/05/2015 how to cite item chahud a., and setembrino p. (2015). the tatuí formation (early permian, paraná basin), brazil: paleontology and paleoenvironmental considerations. earth sciences research journal, 19(2) 153 158. http://dx.doi.org/10.15446/esrj.v19n2.50245 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.50245 la formación tatuí (pérmico temprano, cuenca del paraná), brasil: consideraciones paleontológicas y paleoambientales earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 153 158 pa l e o n to l o g y the tatuí formation (early permian, paraná basin), brazil: paleontology and paleoenvironmental considerations artur chahud 1, setembrino petri2 1instituto de geociências da universidade de são paulo, departamento de geologia sedimentar e ambiental. rua do lago, 562. cidade universitária 05508900 são paulo, sp – brasil e-mail: arturchahud@yahoo.com 2universidade de são paulo, instituto de geociências, departamento de geologia sedimentar e ambiental. rua do lago, 562. cidade universitária 05508900 são paulo, sp – brasil. tel. profissional. (11) 3091-4662. e-mail: spetri@usp.br 154 artur chahud and setembrino petri introduction pennsylvanian – permian lithostratigraphic units of the brazilian paraná basin are, from the oldest to the youngest: itararé, guatá, and the passa dois groups. they are joined in the gondwana i supersequence by milani and ramos (1998). the early permian tatuí formation, studied in this contribution, is chrono-correlated with two formations of the guatá group, rio bonito, and palermo, from southern brazil, and from south africa, prince albert formation (milani et al., 2007; holz et al., 2010; chahud, 2011). fine sandstones, sometimes bearing hummocky cross stratification, and siltstones are the main lithologies of the tatuí formation. coarse and conglomeratic sandstones, locally rather thick, are minor lithologies (washburne, 1930; soares, 1972; massoli, 1980; fúlfaro et al., 1984; stevaux et al., 1986; assine et al., 2003; holz et al., 2010; chahud, 2011; chahud et al., 2012). most of these contributions dealt with lithostratigraphy and paleoenvironments, but the fossils are recorded in poor communications without descriptions and illustrations. thick conglomeratic beds were informally known as the ibicatu facies (soares, 1972). the type section of this facies, now destroyed, was covered by deposits of the irati. so, this facies was sometimes wrongly allocated to the irati basal beds (taquaral member). the ibicatu facies is indeed present at different stratigraphic positions within the tatuí formation (fúlfaro et. al., 1984; stevaux et al., 1986; assine et al., 2003; holz et al., 2010; chahud, 2011). all known fossiliferous outcrops of the tatuí formation are located in the uppermost 10m of the unit (mezzalira 2000; chahud, 2011; chahud et al., 2010a; chahud et al., 2012). the present contribution stands out from previous investigations by different approaches as to details of fossil taxa, granulometry, depositional evolution, biostratigraphy and paleoenvironments. figure 1. outcrop belt of the tatuí formation in the state of são paulo. tatuí lithostratigraphy older researchers considered this formation as essentially made up of two lithologic bodies, recognized by different colors, with a diastem between (soares, 1972, fulfaro et al., 1984). the lower body, a thick brown – purplish finely laminated siltstone (whitish locally), with interbedded chert-limestone, a few millimeters to 30 cm thick. this lower body is seldom exposed. there are occasional references to foraminifers, without description and illustration (fúlfaro et. al., 1984) and never confirmed by later researchers (mezzalira and martins neto, 1992; assine et al., 2003; holz et al., 2010; chahud, 2011; chahud et al., 2012). the second body is gray-greenish to yellow-greenish siltstones and fine sandstones and conglomerates exhibiting rather round clasts with crossbedding and ripple marks. from angular to rounded chert pebbles are reported. three lithologies are reported, correlated with the classical lithologic bodies. the herein described lithologies are the only surely fossiliferous tatuí beds. they are the following: fine to very fine sandstones with hummocky these sandstones are well sorted, yellow whitish or greenish, sometimes bearing dots and blots. they mostly lie right under the irati formation beds. the outcrops are centimeters to two meters thick, the hummocky cross – beds may reach 0.5m high. chahud (2007) reported small parallel ripple marks apparently symmetric, striking n20e, less than 0.5cm high, on upper beds of this facies. very fine, thinly laminated or massive whitish sandstones, outcrop right under an irati formation bed. sometimes these sandstones acquired a light yellow color, possibly due to weathered pyrite nodules, from the basal irati bed. this facies is interbedded with other tatuí lithologies, discussed below. it is scarcely fossiliferous, small osteichthyes scales (ragonha, 1978; chahud and petri, 2008a, 2008b, 2009b, 2010b, 2012a, 2012b, 2014) are rarely present. ichnofossils, right under the irati bed, were reported (chahud et al., 2010a) very fine sandstones and siltstones, massive or finely laminated defined by very well sorted brownish white, very fine sandstone to siltstone, mostly massive, sometimes parallel or cross laminated. some lateral variation in granulometry occurs but never over fine sandstones. scarcely fossiliferous, these strata contain rare arthropods (conchostracans and indeterminate crustaceans), fish scales and loose remains. the fossils are recognized only in unweathered samples. occasionally they occur right in sharp contact with the taquaral member of the irati formation, which are either coarse sandstones or siltstones (assine et al., 2003; chahud et al., 2010a; chahud and petri 2013a; 2013b). conglomerates conglomerate, informally known as the ibicatu facies since soares (1972), contains siliceous pebbles one to three centimeters in size, “floating” in a light brown coarse sandstone matrix. the type locality was a road cut later destroyed by road widening. it was six meters thick, according to fúlfaro et al. (1984) and stevaux et al. (1986). right above, in sharp contact, came deposits of the taquaral member of the irati, reported by these researchers. the type section had a significant amount of large plant fossils, specimens that reach nearly one meter (figs. 2 and 3). conglomerates, like the ibicatu, were reported in other outcrop localities from the state of são paulo but stratigraphically not in contact with the irati (stevaux et al., 1986; assine et al., 2003; chahud, 2011; chahud et al., 2012), with sigmoidal structure, and ichnofossils are present in the upper beds. however, these structures were not present in the type locality (stevaux et al., 1986). figure 2 . fern stem. ibicatu type locality. 155the tatuí formation (early permian, paraná basin), brazil: paleontology and paleoenvironmental considerations the upper tatuí formation, here discussed, is sketched in figure 3. it must be placed in the late paleozoic third order sequence of the tatuí, lpts-4, of holz et al. (2010). these authors believe this sequence to be middle artinskian in age. however, it is possible that the base of tatuí formation, not discussed in that paper, might be placed in an older sequence, lpts3. the columnar section (fig. 3) was based on center-east outcrops in the state of são paulo (stevaux et al., 1986; assine et al., 2003; chahud et al., 2010b; chahud and petri, 2010b, 2015). most of the tatuí outcrops occur in this area. figure 3. sketched columnar section of the upper tatuí formation as occurring in the state of são paulo center-east. 1) fine to very fine sandstones with hummocky; 2) very fine sandstones and siltstones, massive or finely laminated: 3) conglomerates: 4-5) taquaral member of the irati formation: 4) basal sandy beds; 5) silty-shale facies paleontology ichnofossils bioturbated deposits were recorded with two morphotypes in two tatuí formation lithologies. the first was described by chahud et al. (2012) in lithology 1, below deposits interpreted by the authors as representing a freshwater paleoenvironment (lithology 2). the structures are simple irregular conical excavations, irregular oval or elongate openings lay down in seen both from above and perpendicular to the beds (fig 4a-b). several ichnogenera might be applied to this kind of excavation. it is not a good paleoenvironmental index, present under fresh or salty shallow waters or even subaerial. figura 4. ichnofossils. a-b) excavations in fine sandstones a) plan view, b) profile view. bar-scale 2mm c) trails in coarse sandstone. bar-scale 40mm. the second morphotype (fig. 4c) is seen in poorly sorted sandstone with sigmoidal structure interbedded with silty beds (lithology 3 fig. 3), observed in outcrop as a lateral extension of the remainder of the destroyed ibicatu type-locality. it is a set of horizontal trails of 5mm width with simple ramifications, without preferential direction, many tracks cut perpendicular or askew other trails. there are no spines, paws, claws or any ornamentation, just smooth trails and structureless fill with lithology similar to host rock. the structures are preserved in negative and positive relief, this being the most common. this ichnofossil might be ascribed to following four ichnogenera, known in the guatá group: gordia, helminthopsis, palaeophycus and planolites (fernandes et al., 2002; netto et al., 2012) gordia, helminthopsis, and planolites are widespread and common in three ichnofacies: cruziana, mermia, and nereites. palaeophycus is restricted to the cruziana, is the genus attributed to this ichnofossil. it also appears in the palermo formation (fernandes et al., 2002; netto, 2000; netto et al., 2012), partially chrono-correlated to the tatuí formation. several biogenic perforations occur at the interface tatuí-taquaral (lithologies 1 and 4, fig. 3). animals that lived at the beginning of taquaral deposition, right after the final of tatuí deposition, were responsible for these structures. plant fossils only sparse, isolated, fragmented remains of plant fossils in the tatuí beds have been reported in the literature. the first occurrences of larger and better-preserved specimens came from the ibicatu conglomerate (fúlfaro et al., 1984; assine et al., 2003; chahud and petri, 2009a). these fossils, the largest of the present study, are pteridophyta and spermatophyta trunks. the great pteridophyta fragments of elongate stems (fig. 5a and b), several centimeters long, were investigated by chahud (2011), who placed them in the genus tietea (solms-laubach, 1913), by the shape of the meristeles, similar to those described by derby (1915) and tavares (2007), at the corumbataí formation, units younger than tatuí, and also by the absence of the organized meristeles in the central part of the stele, features which exclude the genus psaronius, with well defined and organized meristeles. herbst (1987) described similar plants from the permian of paraguay, proposing the genus tuvichapteris, also with unorganized meristeles and the 156 artur chahud and setembrino petri stem center like tietea. the differences between these two genera would be the number of fronds sprouted out from the trunk. the ibicatu specimens did not preserve them. tavares et al. (2011), however, argued against the value of the frond differences, which would be caused by the difference of ontogenetic stages, so the priority goes to the name of the genus as tietea. another kind of ibicatu stem, but smaller and apparently less frequent, was attributed to spermatophyta. it is a gymnospermic cylindrical type (fig. 5c-d), compact and deformed and irregularly elongate. the plant fossils were dispersed among the conglomerate with clasts 5mm to 3cm in size. none of the fossils are in “life position”, each laid down horizontally by the current, which direction was registered as e-w for the two kinds of the stems. previously stevaux et al. (1986) reckoned a sole n-s direction for the fossils, before the destruction of the outcrop. diagenesis acted before final fossilization, mainly through compression by sediment inflow, increasing the fossil widths and internal deformations. figure 5 . ibicatu stems. a-b) pterydophyta. a) tietea sp. bar-scale 10mm. b) meristeles details of tietea sp. bar-scale 5mm.c-d) spermathophyta. c) stem. bar-scale 50mm d) cross-section. bar-scale 20mm. arthropods conchostracans they are preserved as impressions with few external morphological features. sometimes the umbo is easily seen. the entire outline of growth lines is scarcely observed. no thicknesses neither concave – convex shapes are detected. the best-preserved specimens allowed distinction of the valves as oval rounded in outline (fig. 6a and b) similar to both recent and paleozoic forms (brito et al., 2000; ferreira de oliveira, 2007). however, it is not possible to classify them at the genus level, based only on the features noted above. a lot of packed conchostracans were reported in a silty laminated bed (fig. 3) with characteristics of free – settling and absence of traction action (chahud et al., 2012). these fossils are packed as a “shell bed”, interbedded with others with less numerous fossils. the assemblage concentration changes upward and sideward. other crustaceans mezzalira and martins neto (1992) are the only researchers to record crustaceans (no conchostracan) in the tatuí formation. the two genera, pseudopalaega, and protourda are rather poorly preserved isopods, associated with fish remains and conchostracans. chahud et al. (2012) reported unidentified crustaceans in massive finely laminated sandstones and siltstones (fig. 6c) paleovertebrates some vertebrates are also associated with the conchostracans and others scarcely present in the fine sandstones with hummocky (ragonha,1978; chahud and petri, 2009c). a unique identification of a semi-articulated fish, actinopterygii, a kind of platysomoidea, was reported by silva santos (1991). the mezzalira and martins neto (1992)’s crustaceans came from the same outcrop. the fossils, as a rule, were preserved as scales and loose teeth of the palaeonisciformes and coelacanthimorpha, dispersed and fragmented among the matrix (fig. 6e-f). figure 6. tatuí formation fossils a-b) conchostracans from the center-east state of são paulo. bar-scale 1mm c) crustacean fragment. bar-scale 1mm d) palaeonisciformes scale associated with conchostracans . bar-scale 5mm e) fragment of palaeonisciformes tooth. bar-scale 2mm. f) coelacanthimorpha scale. bar-scale 2mm. e-f) found out near the contact with taquaral member. other fossils rare fossils, assigned to the tatuí beds, were mentioned in papers published before this contribution. most of them were placed in beds wrongly thought to belong to the tatuí formation. barbosa and almeida (1949) and almeida and barbosa (1953) reported sponge spicules in beds believed to be tatuí. they neither described nor illustrated the spicules, furthermore the stratigraphic position of the beds is uncertain. three foraminifera genera, erlandia, ammodiscus, and hyperamina, were reported by fúlfaro et al. (1984) in silty sandstones. these fossils came from the basal beds of the tatuí, and would be probably a continuation of the conditions of deposition of the upper itararé group. lima et al. (1976) recognized ammodiscus and hyperamina in the itararé beds. 157the tatuí formation (early permian, paraná basin), brazil: paleontology and paleoenvironmental considerations cabral jr. et al. (1988) reported agglutinated foraminifera, ostracods, and acritarchs from core wells from the base and top of a section attributed to the tatuí. however, the lower cores might belong to the itararé, and the upper might belong to the base of the taquaral member. cabral jr. et al. (1988) didn’t describe and illustrate them, and only acritarchs were confirmed by marasco et al. (1993) in the base of the taquaral member. conclusions and paleoenvironmental considerations the record of the entire tatuí formation fossils are ichnofossils, plant fossils, bones of fishes and arthropods. only ichnofossils and fishes are reported in more than one lithology (coarse and fine sandstones). the tatuí formation paleoenvironment of deposition evolved through three phases. the first apparently contains only the euryhaline fossils, agglutinated foraminifera mentioned by fúlfaro et al. (1984). these fossils are common in brackish waters that may be transitional from coastal estuary complex to coastal marine. another two phases, from the upper tatuí, reflect the interplay of two paleoenvironments. the deposits of the second phase are regressive and continental, with conchostracans dropped downward without further transportation, in a lacustrine deposit, within the siltstones massive or laminated beds (lithology 2, fig. 3), and large plant stems in the conglomerates (lithology 3, fig. 3), fluvial deposits. the third phase is transgressive with hummocky cross – stratification and, probably, increasing salinity, similar to the first stage (lithology one contact with lithology four, fig. 3). acknowledgments the authors thank the national council for scientific and technological development (cnpq) (grant: 500755/2013-2) for financial support, prof. dr. thomas rich fairchild and the dr. ‎annabel pérez aguilar for support and jointing field researchers. the authors also thank the departamento de geologia sedimentar e ambiental of the institute of geoscience of the university of são paulo which allowed the preparation of fossils in its laboratories. references almeida. f.f.m. and barbosa, o. 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(1930). petroleum geology of the state of são paulo – brazil. são paulo, boletim da comissão geográfica e geológica do estado de são paulo, 22, 282p. this study uses experimental triaxial tests with monotonic loading to develop empirical relationships to estimate undrained critical shear strength. the effect of the fines content on undrained shear strength is analyzed for different density states. the parametric analysis indicates that, based on the soil void ratio and fine content properties, the undrained critical shear strength first increases and then decreases as the proportion of fines increases, which demonstrates the influence of fine content on a soil’s vulnerability to liquefaction. a series of monotonic undrained triaxial tests were performed on reconstituted saturated sand-silt mixtures. beyond 30% fines content, a fraction of the silt participates in the soil’s skeleton chain force. in this context, the concept of the equivalent intergranular void ratio may be an appropriate parameter to express the critical shear strength of the studied soil. this parameter is able to control the undrained shear strength of non-plastic silt and sand mixtures with different densities. este estudio utiliza evaluaciones experimentales triaxiales con cargas repetitivas para desarrollar relaciones empíricas y estimar la tensión crítica de corte bajo condiciones no drenadas. el efecto de contenido de finos en la tensión de corte sin drenar se analizó en diferentes estados de densidad. el análisis paramétrico indica que, basado en la porosidad del suelo y las propiedades del material de finos, la tensión de corte sin drenar primero se incrementa y luego decrece mientras la proporción de finos aumenta, lo que demuestra la influencia de contenido de finos en la vulnerabilidad del suelo a la licuación. una serie de las evaluaciones se realizó en mezclas rehidratadas y saturadas de arena y cieno. más allá del 30 % de los contenidos finos, una fracción del cieno hace parte principal de la cadena de fuerza del suelo. en este contexto, el concepto de porosidad equivalente intergranular puede ser un parámetro apropiado para expresar la tensión crítica de corte del suelo estudiado. este parámetro nos permite controlar la tensión de corte sin drenar de cieno no plástico y mezclas de arena de densidades diferentes. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 149 156 abstract resumen key words: sand, silt, static liquefaction, undrained, strength, density. palabras clave: arena, cieno, licuación estática, no drenaje, tensión, densidad. record manuscript received: 10/03/2014 accepted for publication: 03/09/2014 critical undrained shear strength of sand-silt mixtures under monotonic loading mohamed bensoula ¹˴², hanifi missoum ², karim bendani ² ¹ department of civil and architecture, university of mostaganem, bat 72 n°03 cité 05 juillet, mostaganem 27000 algeria, bensoulamoh@hotmail.com ² laboratory of construction, transports and environment protection (lctpe), bp 227, route de belhacel, university of mostaganem, cp 27000, mostaganem, algeria geotechnics issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.42492 http://dx.doi.org/10.15446/esrj.v18n2.42492 150 khadija baba, lahcen bahi, latifa ouadif list of symbols cu: coefficient of uniformity cc: coefficient of curvature dr: initial relative density : equivalent relative density e: global void ratio emax: maximum void ratio emin: minimum void ratio es: intergranular void ratio e*: equivalent void ratio fc: fines content gs: specific gravity of soil grain gd: solid dry density p’: effective mean stress p’cr: effective mean stress at critical state : effective mean stress at steady state q: deviatoric stress qcr: deviatoric stress at critical state qs: deviatoric stress at steady state sucr: undrained critical shear strength ea: axial strain di: grain diameter corresponding to i% finer sc: confining pressure ip: plasticity index m: slope of critical state line fs: mobilized angle of inter-particle friction at the steady state introduction northern algeria is located on a tectonic plate boundary (ruptured fault), and consequently, many earthquakes occur in this region. recent seismic activity over the last twenty years in algeria includes the mascara earthquake in 1994 (m: 5.6), the algiers earthquake in1996 (m: 5.7), the ain-temouchent earthquake in 1999 (m: 5.8), the beni-ouartilane earthquake in 2000 (m: 5.4) and the boumerdès earthquake in 2003 (m: 6.9).these earthquakes caused major structural damage to constructions and dramatic losses of human life. these events also included numerous cases of ejection of great masses of sandy soils onto the ground surface level and large displacements of equipment foundations. the region of mostaganem is in the northern part of the country, is situated on thick geological deposits of silty sand, and has a relatively shallow phreatic water table. these conditions enhance the liquefaction phenomena. due to the soil failure phenomenon, there is a need to characterize this granular media as an engineering material. under undrained conditions, the shear resistance of sandy soils may decrease by either monotonic (e.g., erosion at toe of a slope, reservoir filling, or rapid sediment accumulation) or dynamic (e.g., earthquake, vibration, or blast) loading, leading to liquefaction. the pore water in a saturated sandy soil will increase due to volume contraction if the drainage of the water is impeded. this results in a lower effective stress and, consequently, a reduction in soil strength. several liquefaction flow failures can be triggered by static and cyclic mechanisms. liquefaction may lead to catastrophic failure when the shear resistance of a soil vulnerable to liquefaction drops below the existing initial static load (e.g., driving shear stress on a slope). many cases of static liquefaction flow failures have been reported by olson and stark (2003). to evaluate the occurrence of flow deformation, the undrained shear strength of sandy soils during flow failures and liquefaction is an essential parameter in undrained stability analysis. the correct estimation of the undrained liquefied strength is of paramount importance for the design of soil structures, such as earthen dams, bridge supports and building foundations, with the aim of protecting them from liquefaction failure. the understanding of soil liquefaction is continuously being revised due to new field and laboratory observations. obvious evidence has significantly influenced the research trend in investigations of liquefaction in natural sandy soil deposits. the effects of fines on the mechanical behavior of sandy matrix soil are not yet fully understood. therefore, the present challenge is to improve fundamental research on the mechanical behavior of soils susceptible to liquefaction to unify the treatment of such soils. the presence of fines may either increase or decrease liquefaction vulnerability. many contributions have tried to characterize the influence of fines on liquefaction behavior in sandy matrix soils. the chinese criteria (wang, 1979) are the well-known liquefaction susceptibility identification. these criteria are essentially based on field observations. these criteria have undergone many revisions, but they still constitute the basic principles and guidelines for many methodologies. nevertheless, recent findings suggest some inadequacy and deviation associated with the presence of fines, especially plastic fines. due to discrepancies in the research results in the laboratory and post-liquefaction events, many researchers agreed that such procedures should be discontinued and should be reviewed in a general context (prakash and puri, 2010). silty sands are the most common type of soil involved in liquefaction flow events. static and earthquake-induced liquefaction failure events have been described by yamamuro and lade (1998). several studies have been conducted on the influence of fine particles on the liquefaction behavior of sandy soils. seed et al. (1983) observed that certain types of soil with high contents of fines may be highly susceptible to liquefaction vulnerability. after performing several tests, yamamuro and lade (1997) demonstrated that an increase in silt in a sand-silt mixture leads to a decrease in undrained shear resistance of the silty sand at a constant total void ratio. the behavior of clean sand differs from that of sand-silt mixtures. there are contradictory conclusions regarding this difference: some researchers have concluded that silt in the sand matrix reduces the undrained shear resistance of sandysoil mixtures, e.g., chang et al. (1982), whereas other researchers have reached the opposite conclusion, e.g.,troncoso and verdugo (1985), and vaid (1994). therefore, based upon the contradictory results presented in the literature, the fines content of a sandy soil does not lead to uniform behavior, and although recent advances have been achieved in this area of research, our understanding of this behavior is still non-unified and controversial. the stability of structures built on liquefied soil depends on the postliquefaction effects of the soil shear strength. the strength of soils at the phase transition is critical for engineering design (ishihara, 1993). consequently, it is important to clarify the main parameters that exert substantial influence on the shear resistance and to delineate the necessary principles for the design process. yamamuro and lade (1997) showed that the intergranular void ratio (es) likely 151critical undrained shear strength of sand-silt mixtures under monotonic loading controls the undrained resistance of sand-silt mixtures. this parameter, (es), represents the space within sand grains and can be computed as follows: es = (e + fc) / (1 fc) (1) where e is the global void ratio and fc is the fines content (the ratio of the silt weight to the total sample weight). thevanayagam et al. (2002) stated that, beyond a certain value of fc (the threshold value), a fraction of the silt participates in the soil skeleton chain force, and these authors introduced a new parameter defined as the equivalent intergranular void ratio. this parameter is able to influence the undrained shear resistance of non-plastic silt and sand mixtures and is described by the following equation: e* = (e + α.fc)/(1 α.fc) (2) where e is the equivalent void ratio and a represents the silt fraction that participates in the soil skeleton strength. when a = 1, the equivalent granular void ratio is reduced to the intergranular void ratio. this paper presents a parametric study focusing on estimating the undrained liquefied shear strength of silty sand using state parameters and the geotechnical properties of the mixtures. this work investigates the effects of non-plastic silty fines content on the undrained liquefied shear strength of silt-sand mixtures. our main aim in this study is to introduce relationships that express the critical undrained shear resistance in terms of the various soil parameters of mostaganem sandy soil at different relative densities and fines contents (up to 40%). to achieve this aim, a series of monotonic undrained triaxial tests were performed on reconstituted saturated sand-silt mixtures. materials tested soil samples from the coastal region of mostaganem were collected at various depths. in these samples, the silty fines content did not exceed 30%. in our experiments, the sand and silt were separated and used to generate the test specimens. the specific gravities of the sand and silt were 2.67 and 2.69, respectively. the tests were conducted on mixtures of the collected sand and silt. the plastic limit and liquid limit of the silt were 20% and 25%, respectively. because most static liquefaction and earthquake-induced liquefaction occur in silty sand and sandy silt, silt contents ranging from 0% to 40% were tested. additionally, liquefaction vulnerability is highly affected by the relative density of the soil (yamamuro and kelly, 2001; maheshwari and patel, 2010). in this work, a variety of soil density states were also studied. the geotechnical properties of the soil used in this work are presented in table 1. 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.11 sand silt grain size (mm) pe rc en t p as si ng by w ei gh t figure 1. grain size distribution curves of the tested materials. the grain size distribution curves for all sand silt mixtures under investigation are illustrated in figure 1. di corresponds to the soil diameter at which i% of the soil weight is finer. the plasticity index of the silt is ip = 5%, and cu and cc are the coefficients of uniformity and curvature, respectively. according to the astm d2487-11(2011) classification, the sand under study is poorly sorted (sp), and the silt is inorganic (ml). table 1. geotechnical properties of sand-silt mixtures. index properties clean sand (0%fc) silty sand (10%fc) silty sand (20%fc) silty sand (30%fc) silty sand (40%fc) silt (100%fc) fc (%) 0 10 20 30 40 100 gs g/cm3) 2.670 2.672 2.674 2.676 2.679 2.690 emin 0.519 0.461 0.418 0.401 0.457 0.707 emax 0.844 0.791 0.753 0.727 0.744 1.429 d10 0.350 0.130 0.050 0.020 0.010 0.0012 d30 0.550 0.290 0.210 0.140 0.090 0.017 d50 0.770 0.480 0.400 0.300 0.230 0.035 d60 0.820 0.600 0.520 0.450 0.330 0.044 cu 2.34 4.62 10.40 22.50 33.00 36.67 cc 1.05 1.08 1.70 2.18 2.45 5.47 the parameter emax is the maximum void ratio (lowest density state), and emin is the minimum void ratio (densest density state). these parameters, emax and emin, are obtained by fixing the height drop of the funnel (figure 2). the variation of emax and emin in term of the fines content fc is shown in figure 2. the variation in both indices clearly follows the same trend. the two indices decrease with as the fines content increases to 30% and then increase beyond this value. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 20 40 60 80 100 dr = 15% dr = 55% emax emin fines content (%) v oi d r at io s ( e m ax , e m in ,e ) emin emax figure 2. extreme and global void ratios of samples for different relative densities versus fines content. laboratory experiment procedures preparation method: sample preparation methods considerably affect the liquefaction behavior of soils (ladd, 1974; mulilis et al., 1977). the sample must be prepared in such a way as to replicate the soil conditions in the field. therefore, the choice of proper sample preparation techniques is important in determining the liquefaction potential of sandy soils. numerous sample reconstitution 152 khadija baba, lahcen bahi, latifa ouadif methods have been discussed for use in the laboratory, including moist tamping, dry funnel pluviation and water sedimentation. the dry pluviation deposition method tends to reproduce field performance well, as shown by vaid et al. (1999). in this study, dry funnel pluviation was used as the sample preparation method to reproduce the soil conditions in the field. the experimental device fabricated for the dry funnel depositional method is shown in figure 3. raising system sand-silt mixture funnel mold deposited sand-silt mixture figure 3. dry funnel deposition system. soil sample preparation: dry sand and silt were mixed according to the different specified weight ratios. the samples were prepared by means of a mold consisting of two semicylindrical shells (figure 4). the two parts of the mold can be easily joined by a hose clamp. figure 4. experimental device. the samples are cylindrical in shape and are 70 mm in diameter (d) and 140 mm in height (h) (h/d=2.0). the mass of sand-silt mixture to be put inside the mold is calculated according to the required relative density (the initial volume is known). the relative density of sample is defined as follows: dᵣ = (emax emin) / (emaxemin) (3) where e is the global void ratio. the tests were carried out on reconstituted saturated samples. degree of saturation during the test: first, the samples were purged with carbon dioxide gas for more than 30mn and subsequently saturated with de-aired water. skempton’s pore water parameter b controls the saturation state. according to astm d 4767-02 (2004), samples can be considered fully saturated if b is equal to or greater than 0.95. in all the undrained tests, this condition was fulfilled. in this investigation, a backpressure of 100 kpa was applied during the experiments to achieve a completely saturation state. consolidation and loading: samples were isotropically consolidated at a mean effective stress of 100 kpa and then subjected to undrained monotonic triaxial loading. a constant strain rate of 5% per hour was applied during all the tests to stabilize the pore water pressure build up throughout all the samples. all the tests reached 25% axial strain. estimation of undrained critical shear strength: the behavior of saturated sand specimens in undrained triaxial tests is presented in figure 5 (vaid and chern, 1983). points a, b, and c represent isotropically compressed specimens. in undrained (constant volume) conditions, the effective mean stress decreases as excess pore water pressure develops. during this process, the soil sample reaches peak undrained shear strength su (the yield strength) at positions a1, b1 and c1. liquefaction is triggered when shear stress applied to the soil specimen exceeds the peak shear strength as a result of static or dynamic loading. beyond that point, an unstable regime (prunier et al., 2009) develops in the specimen until the critical state is reached at point d on the critical state line (csl). this position is called steady state. if the effective stress path reaches the flow liquefaction surface (fls), the soil element becomes unstable and its effective stress state rapidly moves to the steady state, where the soil element undergoes sudden failure with large deformation. (a) figure 5. graphical explanation of the flow liquefaction surface (vaid and chern, 1983) a) stress paths (q-p’) in an undrained test. b) flow liquefaction susceptibility. in undrained situations, the shear strength at steady state is conventionally the critical shear strength. in the framework of critical state soil mechanics theory, the following relationship can be written: qs= m.p's (4) where m is the slope of the critical state line. according to schofield and wroth (1968) and triaxial tests, sin øs = (3.m) / (6 + m) (5) where qs, and fs indicate the deviatoric stress (), the effective mean principal stress , and the mobilized angle of inter-particle friction at steady state, respectively. the critical shear strength sucr can be written as follows: sucr = (qs / 2). cosøs (6) (b) 153critical undrained shear strength of sand-silt mixtures under monotonic loading results and discussion the results of the undrained monotonic compression triaxial tests performed for different fines contents (ranging from 0 to 40%) at an average confining pressure of 100 kpa within two separate density ranges (dr = 15 and 55%) are shown in figures 6 to 10. the stress paths (p’, q) recorded during the tests, as well as the axial strain versus the deviatoric stress q, were recorded and represented graphically. for fc values ranging from 0% to 30%, we observe that the critical deviatoric stress (qcr) is reached for axial strain values ranging from 0.8% to 1.6% (figure 6). when the fine fraction is fc = 0%, the critical deviatoric stress (qcr) values are 34.12 kpa and 38.69 kpa corresponding to effective mean stress (p’cr) values of 57.86 kpa and 63.11 kpa for the initial relative densities dr = 15% and 55%, respectively (m = 0.84, fs = 21.62°). when the fine fraction is 10% (figure 7), the critical deviatoric stress (qcr) values are 32.08 kpa and 35.84 kpa (m = 0.83, fs = 21.38°). for fc = 20% (figure 8), the critical deviatoric stress (qcr) values are 28.54kpa and 31.08 kpa (m = 0.82, fs = 21.14°), and for fc = 30% (figure 9), the critical deviatoric stress (qcr) values are 24.09 kpa and 27.32 kpa (m = 0.80, fs = 20.67°). when fc = 40% (figure 10), the mixture exhibits a dilatant behavior and does not develop a contractile phase. the deviatoric stress (qcr) increases again and has values of 27.52 kpa and 30.85 kpa for dr = 15% and 55%, respectively (m = 0.78, fs = 20.19°). we observed that an increase in the amount of fines of 0% to 30% leads to a decrease in the deviatoric stress. this decrease occurs because of the role of the fines in reducing the soil dilatancy and amplifying the phase of contraction in the sand-silt mixtures, which leads to a reduction in the confining effective pressure and, consequently, a decrease in the peak strength of the mixtures. the stress path in the (p’, q) plane clearly shows the role of the fines in the decrease in the average effective pressure and the maximum deviatoric stress (figures 6.b-10.b). in this case, the effect of fines on the undrained behavior of the mixtures is observed for the fines contents lower than 30%. these results are in good agreement with the observations of shen et al. (1977), troncoso and verdugo (1985), koester (1994) and kokusho et al. (2014). figure 6. a) test results of deviatoric stress versus axial strain (q, ea), fc = 0%. b) stress paths (q, p’) for undrained loading tests, fc= 0%. figure 7. a) test results of deviatoric stress versus axial strain (q, ea), fc= 10%. b) stress paths (q, p’) for undrained loading tests, fc= 10%. figure 8. a) test results of deviatoric stress versus axial strain (q, ea), fc= 20%. b) stress paths (q, p’) for undrained loading tests, fc= 20%. figure 9. a) test results of deviatoric stress versus axial strain (q, ea), fc= 30%. b) stress paths (q, p’) for undrained loading tests, fc= 30%. figure 10. a) test results of deviatoric stress versus axial strain (q, ea), fc= 40%. b) stress paths (q, p’) for undrained loading tests, fc= 40%. when the proportion of fines is 40%, there is an increase in the deviatoric stress, which is due to the role of the fines in increasing the dilatancy in the soil and the absence of a contractile phase. this result shows that % in mixtures with greater than 30% fines, the fines participate in the strength of the mixture and reverse the behavior trend. table 2 presents the summary of the undrained monotonic compression triaxial tests. table 2. undrained monotonic test results for different initial relative densities of sand-silt mixtures. fc (%) e e* e* min e* max dr(%) d* r (%) m sucr/σc clean sand (0%fc ) 0% 0.795 0.795 0.519 0.844 15% 15% 0.84 0.1615 0.665 0.665 55% 55% 0.1811 silty sand (10%fc ) 10% 0.742 0.897 0.591 0.951 15% -16.26% 0.83 0.1506 0.610 0.753 55% 27.98% 0.1682 silty sand (20%fc ) 20% 0.703 0.978 0.648 1.037 15% -41.34% 0.82 0.1347 0.569 0.823 55% 6.56% 0.1466 silty sand (30%fc ) 30% 0.678 1.051 0.712 1.111 15% -63.66% 0.80 0.1149 0.548 0.892 55% -14.62% 0.1303 silty sand (40%fc ) 40% 0.701 1.167 0.856 1.222 15% -99.38% 0.78 0.1288 0.586 1.021 55% -54.37% 0.1444 these results suggest that the soil under investigation exceeds the threshold set by the chinese criteria (wang, 1979) and clearly show that the soil under study is susceptible to liquefaction at a proportion of 30% fines. these results coincide with the actual observations in three recent earthquakes, the northridge (1994), kocaeli (1999), and chi-chi(1999) earthquakes, which all showed signs of liquefaction in soils with greater than 15% fines content. this means the soils 0 20 40 60 80 100 120 140 0 50 100 150 200 250 dr = 15% dr = 55% effective mean pressure, p' (kpa) de via to r s tre ss, q (k pa ) sand-silt mixture (0%fc) (b) 0 20 40 60 80 100 120 140 160 0 50 100 150 200 dr = 15% dr = 55% effective mean pressure, p' (kpa) d ev iat or s tre ss , q (k pa ) sand-silt mixture (10 %fc) (b) 0 20 40 60 80 100 120 140 160 0 5 10 15 20 25 30 dr = 15% dr = 55% axial strain (%) d ev iat or s tre ss , q (k pa ) sand-silt mixture (10 %fc) (a) 0 20 40 60 80 100 120 140 0 10 20 30 dr = 15% dr = 55% axial strain (%) d ev ia to r s tre ss , q (k pa ) sand-silt mixture (0%fc) (a) 0 20 40 60 80 100 120 140 160 0 5 10 15 20 25 30 dr = 15% dr = 55% d ev ia to r s tre ss , q (k pa ) axial strain (%) sand-silt mixture (20 %fc) (a) 0 20 40 60 80 100 120 140 160 0 50 100 150 200 dr = 15% dr = 55% d ev ia to r s tre ss , q (k pa ) effective mean pressure, p' (kpa) sand-silt mixture (20 %fc) (b) 0 20 40 60 80 100 0 5 10 15 20 25 30 dr = 15% dr = 55% d ev ia to r s tre ss , q (k pa ) axial strain (%) sand-silt mixture (30 %fc) (a) 0 20 40 60 80 100 120 0 50 100 150 dr = 15% dr = 55% effective mean pressure p '(kpa) d ev ia to r s tre ss , q (k pa ) sand-silt mixture (30 %fc) (b) 0 20 40 60 0 10 20 30 dr =15% dr = 55% sand-silt mixture (40 %fc) axial strain (%) d ev ia to r st re ss , q (k pa ) (a) 0 20 40 60 80 100 120 0 50 100 150 dr =15% dr = 55% sand-silt mixture (40 %fc) d ev ia to r s tre ss , q (k pa ) effective mean pressure, p '(kpa) (b) 154 khadija baba, lahcen bahi, latifa ouadif did not meet the chinese criteria and yet still liquefied. the same observations are evident in the catastrophic static liquefaction failure events reported by kramer and seed (1988) and fourie and tshabalala (2005). in these events, the post-liquefied soil was found to have more than 20% silt content. in this context, holzer et al. (1999) noted that soils that would have been declared safe from liquefaction by the chinese criteria did in fact liquefy and lead to permanent ground deformation, and many recent studies, such as bray and sancio (2006) and boulanger and idriss (2006), have recommended that workers should not rely on the chinese criteria. correlation of critical undrained shear resistance (sucr) and equivalent void ratio (e*): in equation (1), the fine particles are simply assumed to occupy voids in the sand skeleton. the concept of the intergranular void ratio suggests that the fines fill the voids formed between the sand grains; thus, the behavior of sand with a moderate amount of fines should be governed by the intergranular void ratio instead of the global void ratio. however, when the intergranular void ratio exceeds the maximum void ratio of the clean sand, there are sufficient fines to prevent grain-to-grain contact of the sand particles. in this case, the fines constitute the dominant structure and carry the shear forces, while the coarse grains may act as reinforcing elements (thevanayagam and mohan 2000). zlatovic and ishihara (1995) and pitman et al. (1994) reported that fines particles started to interfere with sand particle contacts at 5% fines content and that sand particle contacts vanished completely at approximately 25% silt content. to satisfy these experimental observations (thevanayagam et al. 2002), expression (1) was modified to equation (2). generally, e* is obtained from the correlation between soil sorting properties and back-analyzed values (ni et al., 2004; yang et al., 2006). rahman et al. (2008) analyzed the experimental data of mcgeary (1961) on binary packing studies and concluded that this parameter is dependent on both the particle size ratio and fines content, fc. the size ratio r is defined by where d50(fine) is the median size of the fines and d10(sand) is the particle size at 10% fraction of the host sand. the correlation was suggested in the following form: α = 1 [{1exp([-0.3(fc / fthre ) / k) ]}] (rfthre / fc) r (7) where k = 1 –r0.25 and fthre is threshold fines content or transitional fines content, which is characterized by the predominance of finescontrolled behavior. thus, the value of fthre is defined by the point at which the behavior trend reverses with further increases in fines content. based on our tests, this transition point is located at 30% fines content for all two initial relative densities as shown in figures 6.b to 10.b. for the studied sand-silt mixtures, r and k were calculated to be 0.100 and 0.438, respectively. the variations in the equivalent void ratios versus fines content for the initial relative densities (dr = 15% and 55%) are shown in figure 11. for all studied initial relative densities, a decrease in the global void ratio generates an increase in the equivalent or intergranular void ratio. the absolute rate of variation in both void ratios tends to decrease as long as the fines content increases. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 10 20 30 40 50 dr = 15% dr = 55% fines content (%) eq ui va len tv oi d ra tio s ( e* ) figure 11. variation in the equivalent void ratio (e*) versus fine content fc. figure 12. variation in the undrained critical shear strength versus equivalent void ratio (e*) (σc =100 kpa). figure 12 shows the variation in the undrained critical shear strength versus the equivalent void ratio for the initial relative densities (dr = 15% and 55%). it is obvious from this figure that the undrained critical shear strength decreases linearly as the equivalent void ratio decreases and fines content increases for all densities (dr = 15% and 55% during reconstitution) up to 30% fines content. thus, when the global void ratio decreases and the fines content increases, the undrained critical shear strength also decreases. we conclude that the global void ratio does not represent the real behavior of silty sandy soil with 0-30% fines content. moreover, the equivalent void ratio is a key parameter for characterizing the mechanical state of clean sand and sand-silt mixtures. the following correlation of normalized undrained shear strengths and equivalent intergranular void ratio is obtained: sucr / σc = -0.165 e* + 0.290 (8) correlation of critical undrained shear resistance (sucr) and equivalent relative density (): because the equivalent intergranular void ratio (e*) is presented as an essential representative parameter, the equivalent relative density should consequently be defined as follows (thevanayagam et al., 2002, 2003; shenthan, 2005): d*r = [(emax,cs e* ) / (emax,cs emin,cs )]x100 (9) where emax,cs is the maximum void ratio of clean sand and emin,cs is the minimum void ratio of clean sand. this relative index compares an equivalent granular state of sandy soils with two extreme density states of clean sand. equivalent relative density. dr* (%) figure 13. undrained critical shear strength versus equivalent relative density (sc =100 kpa). 155critical undrained shear strength of sand-silt mixtures under monotonic loading figure 13 shows the variation of the normalized undrained critical shear strength in term of the equivalent relative density. a linear relationship is obtained as follows: sucr / σc = 0.0537dr* + 0.1506 (10) where d* r is the relative density expressed as a decimal ( d* r =d* r /100) the critical undrained shear strength increases with increasing equivalent relative density for fc ≤ 30%. in the range 0-30% fine content, we may deduce that, for both obtained relationships, the equivalent intergranular void ratio and the equivalent relative density are key parameters in characterizing such soil’s susceptibility to instability. conclusions a series of monotonic undrained triaxial tests were performed on reconstituted saturated sand-silt mixtures. two levels of initial relative density (15% and 55%) and an initial effective confining stress of 100 kpa were used in the experimental program. the soil sample’s proportions of fines ranged from 0% to 40%. in the framework of the steady state concept and liquefaction susceptibility of sands, the tested sand-silt mixtures experienced limited liquefaction behavior and dilation in the present undrained triaxial tests under monotonic loading. the obtained results show that the critical undrained shear strength is considerably affected by the fines content. the participation of silty fines in the skeleton forces in the soil depends on their content. consequently, the equivalent intergranular void ratio was introduced to describe the fine participation. in our tests, this parameter accurately represents the soils until a fines content of 30%. the advantage of this parameter is that it may be able to define the inter-grain friction properties of sand-silt mixtures. the critical undrained shear strength decreases in a linear manner with increasing equivalent intergranular void ratios, whereas it increases with increasing equivalent relative densities. however, this behavior is only valid for fc ≤ 30%. the factor α should be related to the mechanical parameters of sand silt mixtures via the critical state soil mechanics framework. in the range 0-30% fine content, we may deduce that, for both obtained relationships, the equivalent intergranular void ratio and the equivalent relative density are key parameters in characterizing such soil’s susceptibility to instability. furthermore, these results mean that the studied soil exceeds the threshold set by the chinese criteria (wang, 1979) and clearly show that this soil is susceptible to liquefaction at fines content of 30%. these results agree with recent observations reported by researchers studying static and cyclic liquefaction failures that have caused catastrophic events. references astm d2487-11(2011) “standard practice for classification of soils for engineering purposes (unified soil classification system)” astm international, west conshohocken, pa. doi: 10.1520/d2487-11. astm d4767-02 (2004). “standard test method for consolidated undrained triaxial compression test for cohesive soils.”american society for testing and materials, vol. 04.08, pp. 1-13. boulanger, r. w., & idriss, i. m. 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(1995). “on the influence of non-plastic fines on residual strength.” proceedings of the first international conference on earthquake geotechnical engineering, tokyo, pp. 14-16. continuously operating reference station (cors) networks consisting of multiple gnss systems have been set up and operating in many developed countries. in turkey, cors-tr has been operating. the aim of cors-tr is fast, correct, and reliable collection of all kinds of geographic data, thus, speeding up the activities of cadastre, assuring organized urbanization, constituting the spatial infrastructure for relevant works of e-government, and monitoring plate tectonics. therefore, in this paper, we investigated 788 days of linear and periodic of akhr, beys, ciha, kamn, yuna, aksi and knya cors-tr stations by linear trend analysis and wavelet transform in located in the central anatolia region of turkey. as a result of trend analyses of time series it was determined that with the exception of a single station (kamn), other stations were moving in the southwest direction (17.78 mm/year). this finding is in consistency with the region’s tectonic plate movements. in the periodical analyses, stations’ daily, monthly, seasonal, annual and even biennial movements resulting from atmospheric and hydrologic loading, climatic and other effects was revealed. moreover, detail (d) component, which protect the largest percentage of energy, was determined by calculating the energy spectrum. energies of detail components are generally similar in horizontal and especially d7-d8 have the highest energy. however, the components obtained the maximum energy was varied in up coordinate time series. these results indicate that stations of cors-tr network located in central anatolia were rather stable and able to be providing reliable, accurate and continuous data for national and international studies to be conducted in many fields abstract keywords: cors-tr; gnss; wavelet analysis; tectonic plate; power spectrum. research of behaviours of continuous gnss stations by signal analysis methods issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.62552 o. oktar1, h. erdoğan1* 1 geomatics engineering, faculty of engineering, aksaray university, tr-68000, aksaray, turkey. * correspondence: hediye.erdogan@gmail.com. earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 19-27 las redes de estaciones de referencia de operación continua (cors) conformadas por el sistema satelital de navegación global (gnss) se han establecido y están operando en muchos países desarrollados. en turquía este proyecto se conoce como cors-tr y tiene el objetivo de hacer una recopilación rápida, correcta y confiable de todo tipo de información geográfica, y por lo tanto de agilizar la actividades de catastro, garantizar la urbanización organizada, constituir la infraestructura espacial de los trabajos relevantes de gobierno electrónico y monitorizar las placas tectónicas. además, en este artículo se analizó la información lineal y periódica durante 788 días en las estaciones akhr, beys, ciha, kamn, yuna, aksi y knya a través del análisis de tendencia lineal y el método de transformación por ondículas para la región central de anatolia en turquía. a través del análisis de tendencias en series de tiempo se determinó que, excepto kamn, todas las estaciones se movieron en la dirección suroccidente (17.78 mm/año). este hallazgo es consistente con los movimientos de las placas tectónicas de la región. en los análisis periódicos se revelaron entre otros los efectos climáticos a partir de los movimientos diarios, mensuales, estacionales, anuales e incluso bianuales de las estaciones por la carga atmosférica e hidrológica. además, el componente detalle (d), que consiste en el mayor porcentaje de energía, se determinó al calcular el espectro de energía. las energías de los componentes detallados son similares en su forma horizontal y especialmente el d7 y el d8 tienen el mayor nivel de energía. sin embargo, los componentes que obtuvieron la máxima energía son diferentes en las coordenadas de las series de tiempo. estos resultados indican que las estaciones de la red cors-tr en anatolia central son estables y pueden proveer información confiable, exacta y continua para estudios nacionales e internacionales en las áreas de conocimiento relacionadas. resumen palabras clave: estaciones de referencia de operación continua; sistema satelital de navegación global; transformación por ondiculas; placas tectónicas; espectro de energía. record manuscript received: 09/02/2017 accepted for publication: 19/01/2018 how to cite item oktar, o., & erdogan, h. (2018). research of behaviors of continuous gnss by signal analysis methods. earth sciences research journal, 22(1), 19-27. doi: http://dx.doi.org/10.15446/esrj.v22n1.62552 investigación de comportamientos en una estación con el sistema satelital de navegación global continua a través de métodos de análisis de señales g e o m at ic s 20 o. oktar, h. erdoğan introduction a satellite navigation or sat nav system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. it allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites. receivers calculate the precise time as well as position, which can be used as a reference for scientific experiments. a satellite navigation system with global coverage may be termed a global navigation satellite system (gnss) (http://en.wikipedia.org/wiki/satellite_navigation, 2016). the original motivation for satellite navigation was for soldiers and military vehicles, planes, and ships in accurately determining their locations worldwide. today, the uses of gnss have extended to include both the commercial and scientific worlds. commercially, gnss is used as a navigation and positioning tool in airplanes, boats, cars, and for almost all outdoor recreational activities such as hiking, fishing, and kayaking. in the scientific community, gnss plays an important role in the earth sciences. meteorologists use it for weather forecasting and global climate studies (ahmet, 2010), and geologists can use it as a highly accurate method of surveying and in earthquake studies to measure tectonic motions during and in between earthquakes. also, gnss is used in determination of geodetic datum (geoid determination), creating a reference system, geophysicalgeodynamic studies, sea-level determination, engineering, cadastral and mining surveying, precision agriculture studies and machine control systems. such practices caused national cartography institutions in countries, local institutions and the private sector to set up gnss networks created by stable gnss stations which provide continuous and real-time observations. particularly, for tectonic studies, gnss based geodynamic projects were developed at a regional level. we can refer the works from (larsen and reilinger, 1992) in the california region, (straub and kahle, 1995; reilinger et al., 1997) in the mediterranean area, (miyazaki et al., 1996) in japan and (erdoğan et al., 2009; eren et al., 2009; gülal et al., 2013) in turkey. in parallel, projects with a broader geographical (intercontinental) scope also started to be developed. examples are the casa uno (kellogg and dixon, 1990) in central and south america and the tango (bastos et al., 1991 in the azores–gibraltar region, which were among the first gps networks established to support geodynamic studies. a nationwide gnss infrastructure network known as cors-tr was established in turkey by istanbul kültür university (iku), general command of mapping and general directorate of land registry and cadastre during 2006–2009. as is known, cors stations are continuous signal receiving points. after obtaining a time series of mean daily positions derived from cors stations observations, one can estimate the station movements as periodical movements from these data. mostly a linear movement is thus assumed to be equal to the plate movement, also called a linear trend, on which the station is located. furthermore, it has been observed that most gnss position time series exhibit annual, semi-annual, seasonal and daily periodical movements with amplitude of a few millimeters (bos et al., 2010; scherneck et al., 2002). blewitt and lavallée (2002) demonstrated that an annual movement within the data deteriorates the accuracy of the estimated linear trend in time series with an observation time span of a few years, even when this annual movement is taken into account during the estimation process. although the causes of periodical movement are not yet completely understood, the most likely explanations are a combination of atmospheric loading (van dam et al., 1997), hydrological loading (van dam et al., 2001), and thermal expansion of the gps stations (romagnoli et al., 2003). in this study, 788 days of gnss observations using 7 cors-tr stations (1.10.2008–26.11.2010) located in the central anatolia region of turkey were processed with gamit/globk program package. then, trend analysis, continuous wavelet transform (cwt) and discrete wavelet transform (dwt) are applied to coordinate time series and correlations between the original time series and details obtained with dwt are investigated. method fast fourier transform (fft) is widely used and well known method for time series analyzers. it is a mathematical technique that transform signal from time to frequency. fft of signal is defined with where n is the number of the signal y(t), t is time and i is complex unit. for 0≤k≤n-1, the calculation of x(k) in eq. (1) requires n complex multiplications and n-1 complex sums. computing all n of the x(k) values demands n2 complex multiplications and n2-n complex additions. the fft coefficients x(k) are in the complex plane but this representation does not aid interpretation. therefore, the power of the fft coefficient is calculated by power spectrum pxx(k)=|x(k)|2 and existing frequencies in the series are obtained by determining the density frequencies of the signal (allen and mills, 2004). while transforming time information of the signal is lost. in addition, fft supposes signals to be stationary. however, noisy gnss signals are nonstationary. thus, fft can’t detect specific event in time series. for the reasons stated above, wavelet transform (wt) was developed about three decades ago. wt was first applied to analyze seismic waves in geophysics studies (morlet et al., 1982). then, wavelet analysis has gained huge popularity among scientists and engineers who are interested in analyzing the time-frequency characteristics of signals. dividing the signal into different frequencies and researching each component in terms of scale is the main idea of wt. the wt breaks up the signal into its “wavelets”, which are scaled and shifted versions of the “mother wavelet”. the two basic criteria for a function (t) to be considered a wavelet function are (2) that the average value of the wavelet must be zero and (3) that it must have unit energy. the criteria can also is written mathematically as follows: (1) (2) (3) (4) a number of wavelet functions, which have different features, have been identified such as haar, daubechies, symlets, morlet, and mexican hat etc. different wavelets may be used depending on the application. a couple of mother wavelet as shown in fig. 1 also they were used in this study. wt use a time-scale region as shown in fig. 1, so we can detect when specific events occur. wt can be classified in two types: the continuous wavelet transform (cwt) and the discrete wavelet transform (dwt). cwt is a result of multiplication between analyzed time series and scaled and shifted versions of wavelet function at all time (misiti et al., 2002). for the original signals y(t) and wavelet function ψ(t) cwt is defined as (misiti et al., 2002): cwt (a,b) is the coefficients as a result of transition, a is the scale (dilation) parameter of the wavelet, b is the translation parameter of the wavelet, and t is time. scalogram is a method of using wavelet analysis to obtain spectral information. wavelet function has an importance of detecting frequency location. it can be detect that which frequency components is in the time series and when they join the time series with scalogram. figure 1. a) wavelet transform, b) morl wavelet, and c) db1 wavelet. cwt requires lots of time and memory because of calculating wavelet coefficients at every possible scale. therefore, discrete values, which are depended on powers of two, of scale and positions are chosen in the dwt. qiao (2005) developed an effective method, which utilizes filters, to 21research of behaviours of continuous gnss stations by signal (5) (8) (6) (7) apply this template. for the discrete signal y(k) and the wavelet function ψ(n), the dwt is defined as: the dwt filtering process effectively separates the low and high frequency components. from the point of resolution, the low frequency components will have a large scale and the high frequency components will have smaller scale. the low frequency components called as approximations (a) and the high frequency components called as details (d). approximations contain main features of the original signal. noise is generally in detail components. energy of the a and d are able to be calculated with parseval’s theorem. through this, it is possible to compare components each other to detect which d component has a high correlation with original signal. energy of the d components is defined as: energy of a components is defined as: where edi and eai represent energy of the i decomposition level. dij and aij are discrete wavelet transform coefficient of component at ith decomposition level and j time step. n represents the number of the coefficients (qiao, 2005). study area and data the time series analysis was carried out using the cors positions from 7 gnss stations including akhr, beys, ciha, kamn, yuna, aksi, and knya of the cors-tr network (national gnss network), located in the central anatolia region of turkey (fig. 2). figure 2. the cors-tr network in the central anatolia region of turkey. for the time series analysis, 788 days (1.10.2008–26.11.2010) of gnss observations of the cors-tr stations were first processed with gamit software and produced daily time series of cors-tr stations. gamit is a comprehensive suite of programs for analyzing gnss observations and developed by mit, the scripps institution of oceanography, and harvard university with support from the national science foundation. the initial step in the processing of gnss observations is the determination of the exact absolute position of the receiver. exact positioning can be done by two methods, single point positioning and precise positioning (pany, 2007). these two methods are pre-assessments to determine the station coordinates and clock errors. mathematical models of gnss pseudo range code signals for single point positioning system is given in eq.(8). here, c code is the value, c is the speed of light, k index is the satellite and i index is the receiver while δtk is the clock error of the satellite at the times tr and te ,δion ,δtrop ,δtide, and δrel denote the ionospheric, tropospheric, tidal and relativistic effects respectively. the remaining error is denote as ɛc. in eq. (8), satellite clock errors are corrected by using atomic clocks according to igs precise orbit information and broadcast ephemeris information. after the errors are corrected, there are only four unknowns left in eq. (8), receiver clock errors and receiver coordinates. for an acceptable solution, initial value for receiver clock error is considered as null and the eq. (8) is made linear. this linear equation is given below. (9) (10) (11) (12) (13) equation (9) can be written in a more general form as below. in the above equation, akj describes the coefficients in eq.(9), describes the differences between the coordinates, kv the residual errors and receiver clock errors. if we apply the observations made for all satellites to eq. (10), eq. (10) becomes a general equation and single point positioning equation system with p being the weight matrix is as follows ,l ax v p= + here, l is the observation vector, x is unknowns vector and a is coefficient matrix. in equation (11), unknowns vector x is calculated according to least squares method (lsm) with eq. (12). here, coefficient matrix is created by using initial coordinate values. however, most of the time, the initial coordinates are not exactly known. therefore, for single point positioning problems, iterative solutions are mostly used. this means, final coordinates calculated from initially used station coordinates are again processed as initial coordinates. this process is continued until the difference between initial and final coordinates are reduced to minimum. as mentioned above, there are four unknowns in single point positioning equation. therefore, the solution of the problem requires at least four equations. for the formulation and solution of these equations, observation values from at least four satellites are obtained for single point positioning. when receiver station coordinates are calculated, in eq. (10) becomes null and eq. (10) is written as eq. (13), 22 o. oktar, h. erdoğan here, k indicates the total number of satellites observed in one epoch. receiver clock errors can similarly be calculated by phase measurements (pany,2007). after the observations (coordinates or signals) processing, 27 time series were obtained from 7 stations. because of data loss in aksi and knya time series, they were divided into two separate time series including aksi1, aksi2, knya1, and knya2. thus, 27 time series for 7 stations were obtained. these time series show the changes in the signals in the north, east, and up directions of stations according to coordinate system of the gnss. the time series of the ciha, aksi1 and knya1 stations are shown in fig. 3. in fig. 3, the signal change s(ti) of the time series was acquired assuming that s(t1) =0 for the time initiation of ti (i = 1). in this case, the signal change at any time point during the time series is in relation to the initiation time of the signal. a filtering procedure, third-order floating weighted average (fwa), was applied to the series to partially eliminate the noise effect, also big outlier data in the raw time series (gülal et al., 2013). fwa is )]/4s(t)s(t2)[s(ty 2i1ii ++ ++= . this procedure also has a type of lowpass filtering. since the number of time series is too much, just the raw and filtered time series of the ciha and aksi1 stations are shown in fig. 3. (14) and receiver clock error is calculated with eq. (14). figure 3. time series of ciha and aksi1 stations for raw and filtered signals. results and discussion in this study, in the time series linear changes were determined by linear functions and also periodical movements were investigated by cwt and fft applied for details and approximations obtained with dwt. in addition, approximations and details are obtained by applying dwt to the time series and correlations between approximations details and original signals were investigated. linear trend analysis it was determined that if time series for the signal changes of gnss stations north, east an up component contained any linear trend component by calculating a and b parameters of y=a+bt linear functions and standard deviations of these parameters by lsm. whether the calculated parameters are significant or not is decided by t-distribution test. test value t̂ is calculated from parameter and standard deviation of parameter for each parameter, t̂ =parameter/standar deviation of parameter, and this value is compared with pre-determined 1-α confidence level and tf,1-α/2 confidence limit of t-distribution dependent on the f degree of freedom. if, | t̂ |<tf,1-α/2 parameters are insignificant and if | t̂ |>tf,1-α/2 parameters are significantly different from zero at a 1-α confidence level. for this study, t-distribution table values according to the degree of freedom f at 95% confidence level; (f>500,t500,1-0.05/2=1.96) 1.96 for akhr, beys, ciha, kamn, yuna, and aksi1, (f>200,t200,1-0.05/2=1.97) 1.97 for knya1 and knya2, and (f>60,t60,1-0.05/2=2) 2 for aksi2. for all stations, these values from the t-distribution table are compared with the calculated test values and the results showed that test values are greater than table values at 95% confidence level. 23research of behaviours of continuous gnss stations by signal figure 4. north and east time series and linear functions for akhr thus, it is concluded that the time series of north, east and up coordinates of all stations have a linear trend component (there is a linear movement in the stations) and the results are given in table 1. in addition, the north and east coordinates time series and linear trend models of the akhr station are given in fig.4 respectively. statistically significant linear trend component functions (linear model) of the stations and annual linear changes calculated from this model in other words linear movements of stations are shown in table 1. the b parameter of the trend function in table 1 shows the linear variation of the time series of stations and “t” parameters is day. a change in (+) direction indicates increase of movement, a change in (-) direction indicates decrease of movement. using these parameters it is possible to determine the linear changes or movements in stations together with the direction of movement. on the other hand, the change in the (+) direction shows the movements of the stations in the north and east directions, and the change in the (-) direction shows the movements in the south and west directions horizontally. vertically, (+) change indicates that the height is higher, and (-) change indicates that the height is lower. as shown in table 1, while the cors-tr stations have linear movements in west and south directions, kamn have linear movement in west and north directions (+3.65 mm / year). linear movements of stations in horizontal position: the highest linear movement in the south direction is at the akhr station with 5.84 mm/ year, the lowest linear movement in the south direction was observed at knya1 station with 0.66 mm/year. table 1. linear models of gnss stations, y=a+bt, (trend components) and annual velocities. the highest linear movement in the west direction was detected at the akhr station with 20.81 mm/year, and the lowest linear movement was detected at aksi1 station with 6.57 mm/year. except for kamn station, the average resultant movement obtained from the linear movements in the west and south direction of the stations is 17.78 mm/year and the direction is southwest. yet, the linear movement of the kamn station is 13.46 mm/year and the direction is northwest. the results of the linear movements in the up coordinates of the stations show that at akhr, ciha, yuna and knya2 stations have linear movements in (+) direction, that is, vertical values increased. the highest linear movement was calculated at yuna station with 2.63 mm/year value and the lowest linear movement was calculated at ciha station 0.99 mm/year value. linear movements in up coordinates of beys, kamn, aksi1, aksi2 and knya1 stations are in (-) direction. the highest linear movement was detected at kamn station with -21.39 mm/year value and the lowest linear movement at beys station -0.29 mm/year. in the time series analysis of gnss stations, linear changes or movements obtained from the trend component analysis are extremely important in terms of plate movements. the reason is that bt in the trend component is used to determine the time unit and direction dependent plate movements (or velocities). fig. 5 shows that the majority of turkey is located on the anatolian plate where the arabian, eurasian and the african plates conjunct. the anatolian plate is being squeezed by the movement of the eurasian and arabian plates. thus, anatolia is one of the most seismically active regions of the mediterranean basin. historical records of seismicity, as well as instrumental recordings of earthquakes that have occurred in the past 20 years, reveals the active tectonic features of this area. the north anatolian, east anatolian, aegean (western of southwest anatolia), and burdur-fethiye fault zones, indicate the tectonic diversity and activity of anatolia (gilbert et al., 1994; kahle et al., 1996). between 1988 and 1992, measurements were concentrated during alternate years in western and eastern turkey, with sufficient overlap to allow the integration of the networks, but since the 1994 survey, almost all the stations were reoccupied during each survey (reilinger 1997). in addition to these broad-scale surveys in turkey a small and relatively dense network of stations spanning the various strands of the north anatolian fault in the marmara region was observed in 1990, 1992, 1994 and 1996 (straub et al., 1997). to aid in orbit determination and to strengthen the reference frame, (mcclusky et al., 2000) analyzed data from 50 to 200 globally distributed stations from the international gps service (igs) acquired between june 1992 and april 1999. altogether, their solution included data spanning 9 years from 450 stations, included 189 in the mediterranean region. also, for the southern aegean, the gps velocity field is dominated by a southwest motion relative to eurasia of about 30 mm/year. to better show the possible deformation in the southern aegean (<2 mm/year), and the motion of the southern aegean relative to the surrounding areas, they calculated an euler vector for the anatolia and determined the residual velocities that stations indicated increasing velocities toward the trench and counterclockwise rotation in and around the region. also, gps strain fields and velocity fields for the period 20032006 in the southwest of turkey were indicated by (erdoğan et al., 2009). the measurements, which were performed as periods, were evaluated using daily measures. it was found that the region is affected by a southwestern movement of 15-30 mm/year according to a eurasia-fixed stabilization. strain 24 o. oktar, h. erdoğan vectors show a clockwise rotation in the study area. the velocity field and strain fields are characterized by a system that indicates nondeforming and deforming regions separated by fault zones. in addition, the velocity field is in agreement with location of seismicity, earthquake focal mechanisms, and mapped fault locations. according to the gps velocity vectors there is a shows slower movement in that south compared with the north. this movement of area is agreement with the results obtained in this study (table 1). for the whole of turkey, the last study was made by eren et al., (2009) taking short term observations of the gnss stations and the maximum estimated displacement from the 146 cors-tr stations with respect to the eurasian plate are 19.91 mm/year (southwest direction) and 23.16mm/year (northwest direction) at fini and uder stations. also, tectonic activity inferred from velocity field of gnss measurements for the period 2003-2010 in southwest of turkey were studied by gülal et al., (2013), ıt was found that the anatolian plate is moving to the southwest at a rate of 12–40 mm/yr with respect to the eurasian reference frame. in a different study, 20 cors-tr stations located in the southwest of turkey for period 2008-2010 were studied by gülal et al., (2013). the average movement of the stations with the exception of kamn is 21.7 mm/year in southwest direction. point velocities (vertical) of tusaga-active obtained by eren et al., (2009) support the results obtained in this study. figure 5. tectonic setting of turkey (mcclusky et al., 2000) as reported in this paper, the results of 788 days of linear trend component analyses acquired from the gnss stations are consistent with the results of previous short term, long term and campaign type observations based on tectonic studies on the whole of the anatolian plate and around this region (fig. 5 and table 1). continuous wavelet analysis scalograms were obtained by applying cwt at different scales to gnss coordinate time series of north, east, and up coordinate components. morlet wavelet was used to cwt. the morlet wavelet is both complex and contains more oscillations, and hence the wavelet power combines both positive and negative peaks into a single broad peak. scalogram provides to detect periodicities in time series and this information depends on time. in scalogram, while vertical axis provides scale information, horizontal axis gives time information. percentage of energy for wavelet coefficient increases from blue to red. therefore, while detecting any scale, it should be maxima of energy. it cannot be explained clearly that how many times the periodicity is repeated, nevertheless we ensure the existence in this analysis. also the number of excess periodicity should be regarded as a more important criterion than the high-energy scale. since we have a lot of scalograms, determined periodic movements in them are given in table 2. the scalogram of the up direction of station yuna (a) and north direction of station knya1 (b) coordinate time series using cwt are plotted in fig. 6. figure 6. (a)scalogram of yuna for up signals, (b)scalogram of knya1 for north signals fig. 6 shows that there are 60, 90, and 312 days of periodic movements in up direction of station yuna coordinate time series and 15, 42,76, and 129 days of periodic movements in north direction of station knya1 coordinate time series. in this way, it can be reached that when the period is active when we look at the horizontal axis of the graph. regarding the analysis performed for all gnss stations used in this study, up coordinate time series have the most periodicity whereas east coordinate time series have the least periodicity. table 2. periodic movement times (days) of gnss stations obtained from scalograms. table 2 shows that the up signals of the gnss stations in the central anatolia region of turkey contain more movements in comparison with their southern and northern signals. daily (from 2 to 28 days), monthly (28, 30, 66 days etc.), seasonal (87, 95, 112), semi-annual (150, 170, 190 days etc.), and annual (360, 380, 390 days etc.) periodic movements based on various factors related to the signals have been determined (detail information were given in section periodical analysis). discrete wavelet transform when we analyze gnss daily coordinate time series by cwt, it is not possible to examine in detail the behaviors of the gnss stations. therefore, studying on approximation and detail components of these series obtained from dwt make possible to get more information. also with the help of the dwt, we are able to discuss time series in different scales separately. this process gives us positive opportunities on the analysis of nonstationary gnss coordinate time series. for example, we are able to examine any scale-level component that we want depending on time. dwt was carried out to gnss coordinate time series of north, east, and up coordinates components of stations with multiresolutional analysis. for 25research of behaviours of continuous gnss stations by signal translation, db1 wavelet from daubechies wavelet family was chosen because it is practicable to dwt. considering the number of data, decomposition levels were set 9 for akhr, beys, ciha, kamn, yuna, and aksi1, 6 for aksi2, 7 for knya1, and 8 for knya2. in this way, a and d were obtained with dwt applied for all series. approximations (a) and details (b) of station yuna for up signals are shown in fig. 7. figure 7. approximations (a) and details (b) componenets of station yuna for up signals fig. 7 (a) shows that a1 has a high correlation with the original signal while a9 has a low correlation. in this case correlation between the original signal and approximations is going to be weakened from a1 to a9. besides, a8 and a9 have very low correlation with the original signals. fig. 7 (b) shows that d1 and d2 (especially d1) have vibrations caused from noise at 0. also, these components have the highest frequency changes of the original signal. d1 has the highest frequencies and it decreases from d1 to d9. furthermore, it is clearly seen that d9 has low correlation with the original signal. periodical analysis after the elimination of the linear trend component in the time series for the movements of the cors-tr in the north, east and up directions, the periodical components in the approximation and detail series, which are obtained with dwt, were determined. the periodical components are the long-term periodical changes (low frequencies) which occur in the cors-tr as a result of climatic events, atmospheric and hydrological loading, satellite and reference system errors effecting the stations over a period of time (hours, half a day, semi-annual and annual etc.). in order to obtain the frequencies in the signals of the stations, the series was transformed to frequency domain by fft (eq.1) and the power spectrums pxx(k) were calculated. the frequencies of the periodical movements were calculated for a and d of the north, east and up signals of stations from the power spectrum figures (peak-peaking method). for example, the power spectrum figures of d1-d9 of stations beys are given for the north signals in fig. 8. the calculated frequencies of stations in the north, east and up directions indicated which cors-tr station had many periodical changes or movements. it must be defined that whether a large number of periodic movements, which are shown in fig. 8, is statistically significant. for this purpose, periodical component analysis can be made by modeling the sp(ti) signals with the use of the trigonometric function defined as (erdogan and gülal, 2009). in this equation, the frequency fs of signal is estimated by eq. (1) and (2). these periodical movements were tested for statistical significance. figure 8. power spectrum of d1-d9 of station beys for north signals. during the calculations, the frequencies were used in eq. (15), the as and bs parameters of the periodical changes and covariance matrices of these parameters were estimated by lsm. whether the calculated values are significant is decided by t-distribution/t-test. according to “0” expected value and degrees of freedom f at %95 confidence level, t-distribution values are; 1.96 (f>500,t500,1-0.05/2=1.96) for akhr, beys, ciha, kamn, yuna ve aksi1, 1.97 (f>200,t200,1-0.05/2=1.97) for knya1 and knya2, and 2 (f>60,t60,1-0.05/2=2) for aksi2. the parameters with larger test values than the confidence limit were considered as significant. the insignificant frequencies were excluded from eq. (15) and this procedure was repeated until the significant frequencies were determined. the time of the periodical movements was calculated from significant frequencies and time of significant periodical movements as given in table 3. daily (from 2 to 28 days), monthly (28, 30, 66 days etc.), seasonal (87, 95, 112 days etc.), semiannual (150, 170, 190 days etc.), annual (360, 380, 390 days etc.) and even biennial termed periodic movements based on various factors related to the signals have been determined. it is stated that the gps coordinate time series includes periodic changes (signals) ranging from half-day to years, due to various factors (blewitt et al., 2002; mao et al., 1999; poutanen et al., 2001). (15) 26 o. oktar, h. erdoğan table 3. significant periodical movements for cors-tr stations. seasonal movements are generally related to gravitational excitation, of a thermal origin coupled with hydrodynamics, sources that are indirectly due to geophysical processes or resulting from instrument, or modeling deficiency (dong et al., 2002). gravitational excitation consists of rotational displacements due to seasonal polar motion (ut1), and loading induced displacement caused by solid earth, ocean and atmospheric tides, which are modeled in the raw data processing step. however, the residual ocean tide effects which are still present, and are mainly due to the use of global ocean tide models instead of higher quality local tidal models. the pole tide loading also belongs to gravitational excitation, with the spectrum of mostly annual and chandler wobble periods (409 and 512 days or possibly ~682 days). this long time signal with a large amplitude and nearly 450 days period is described in calais (1999). due to this signal not being seasonal and being larger in east and west components, it is concluded that this is probably caused by errors in the reference systems or satellite orbit or ground guidance parameters according to calais (1999). the deformation caused by pressure field variation, non-tidal sea surface fluctuation, ground water changes in both liquid and solid form, bedrock expansion beneath the gps benchmark, and wind shear are all in the category of being of thermal origin coupled with hydrodynamics. also, other error sources, which generate apparent seasonal variations, such as orbit modeling errors caused by an imperfect reference frame, which is defined through a set of stations that are subject to seasonal variations. there are other important phenomena that can affect the position time series through seasonal signals; these involve tectonicinduced deformation, due to possible seasonal movements in the regional fault slip or regional stress, e.g., slow earthquakes; (miller et al., (2002). the acquired periodical movements and their impact on gnss stations in this region are consistent with other gnss stations throughout the world. this indicates that the recently set up cors-tr network for the whole country is capable of providing stable, reliable, accurate and continuous data for national and international studies that can be conducted in many fields. the results obtained in this section support periodicities acquired from scalogram in section cw analysis. energy spectrums of details and correlations as approximations and details obtained with dwt, there are one or several components to show the best fit to original time series. in this study, detail component, which protect the largest percentage of energy, was determined by calculating the energy spectrum. because the latest level represents the series with the minimum coefficient, high energy at these levels was not considered. the energies of details of north, east, and up coordinate time series of each stations are shown in the same graph. thus, details affecting the coordinate components may be compared to each other. because we have a lot of graphs, only results of yuna and aksi1 are given in fig.9. fig.9 shows that d3 has the highest energy in east and up coordinate time series and d8 has the highest energy in north coordinate time series for yuna stations. considering all energy spectrum graphs, detail component energies of north and east time series are usually similar and the highest energies are obtained in d7 and d8 components. however, the components in which the highest energy obtained have varied in up time series. d3 has the periodicity between 8 and 16 days and d8 has the periodicity between 256 and 512 days. in this way, activities having these periods of specified components effect characteristic of north, east, and up coordinate time series of yuna stations. in conclusion, energies of detail components are generally similar in vertical and especially d7-d8 have the highest energy. however, the components obtained the maximum energy was varied in up coordinate time series. d7 has the periodicity between 128 and 256 days and d8 has the periodicity between 256 and 512 days. figure 9. percentage of energy spectrum for yuna and aksi1 north, east and up signals. 1 2 3 4 5 6 7 8 9 0 10 20 30 40 50 level (d1-d9) e n e rg y yuna north, east, and up energies yuna-n yuna-e yuna-u 1 2 3 4 5 6 7 8 9 0 10 20 30 40 50 level (d1-d9) e n e rg y aksi1 north, east, and up energies aksi1-n aksi1-e aksi1-u conclusions in this paper, 788 daily gnss time series were obtained using gamit software to process the gnss observations from seven stations of cors-tr network in turkey. firstly, linear models of north, east and up coordinate components of the stations are formed and the annual velocities are calculated. it was determined that the calculated linear movements were in the north-west direction (13.46 mm/year) for kamn station and in the south-west direction (17.78 mm/year) for the other stations. linear movements in up coordinate time series of the stations are: in (+) direction for akhr, ciha, yuna, and knya2 stations and in (-) direction for beys, kamn, aksi1, aksi2, and knya1 stations. the linear movements obtained for these stations were found to be in agreement with the plate velocity values of the zone where these stations are located. then periodical movements in the time series are investigated by cwt and fft. although we can obtain periodic movements by both cwt and fft, fft provides more clear results. however, the results of both methods support each other. furthermore, correlations between the original time series and details were investigated by using parseval’s theorem. in dwt, different 27research of behaviours of continuous gnss stations by signal decomposition levels are applied to the time series. this provides that we can study and concentrate on which component we want. also, in the analyses of the periodic movements, it was determined that signals of stations contained daily (from 2 days to 28 days), monthly (28, 30, 66 days etc.), seasonal (87, 95, 112 days etc.), semi-annual (150, 170, 190 days etc.), annual (~ 360 days) and even biennial termed periodic movements changing from daily to annual terms. these periodic movements acquired for the stations in this region are consistent with those found for other continuous gnss stations in other parts of the world. in the analyses of correlations, energies of detail components are generally similar in vertical and especially d7-d8 have the highest energy. however, the components obtained the maximum energy was varied in up coordinate time series. the acquired important results from cors-tr for this region are consistent with other gnss stations throughout the world. this indicates that the recently set up cors-tr network for the whole country is capable of providing stable, reliable, accurate and continuous data for national and 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(2001). crustal displacements due to continental water loading. geophysical research letters, 28, 651–654. this work analyses soil loss over the last 150 years due to urban growth in one of the boroughs of highest environmental value around madrid (spain), i.e. colmenar viejo. the extent and patterns followed by urban extension are examined retrospectively using a present cartographic base layer. this evolutionary approach considering space and time has required a thorough compilation of cartographic sources that were created using classic topography, photogrammetry or remote sensing, some of which have been rarely used so far. these documents have been georeferenced, edited and handled using geographic information systems (gis) and remote sensing programs. urban growth in colmenar viejo has been steady, has registered exponential increases (soil sealing affected 0.14% of the total municipal territory in 1861 and covers 2.64% nowadays) and has followed similar patterns to other spanish towns influenced by metropolitan processes. abstract keywords: soil sealing; cartography; topography; photogrammetry; remote sensing; geographic information systems (gis). soil sealing in madrid (spain), study case of colmenar viejo issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.51450 andrés aristegui cortijo1*, maría eugenia pérez gonzález2 1*instituto nacional geográfico de españa. 2 universidad complutense de madrid earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 111-116 este trabajo analiza la pérdida de suelo en los últimos 150 años debido al crecimiento urbano en uno de los municipios de mayor valor ambiental de madrid (españa), colmenar viejo. la extensión y los patrones seguidos por la expansión urbana se examinan retrospectivamente usando una capa base cartográfica actual. este enfoque evolutivo que considera el espacio y el tiempo, ha requerido una recopilación completa de fuentes cartográficas, que fueron creadas usando la topografía clásica, la fotogrametría o la teledetección, algunas de las cuales han sido raramente utilizadas hasta ahora. estos documentos han sido georeferenciados, editados y manipulados utilizando sistemas de información geográfica (gis) y programas de teledetección. el crecimiento urbano en colmenar viejo ha sido estable, ha registrado aumentos exponenciales (el sellado del suelo afectó al 0,14% del territorio municipal total en 1861 y cubre el 2,64% en la actualidad), y ha seguido patrones similares a otros pueblos españoles influenciados por procesos metropolitanos. resumen palabras clave: sellado del suelo; cartografía; topografía; fotogrametría; teledetección; sistemas de información geográfica. record manuscript received: 23/06/2015 accepted for publication: 30/06/2017 how to cite item aristegui, c. a., & perez, g. m. e. (2017). soil sealing in madrid (spain), study case of colmenar viejo. earth sciences research journal, 21(3), 111-116. doi: http://dx.doi.org/10.15446/esrj.v21n3.51450 g e o l o g ic a l e n g in e e r in g sellado de suelo en madrid (españa). el caso de estudio de colmenar viejo 112 andrés aristegui cortijo, maría eugenia pérez gonzález introduction major demographic changes have taken place in europe since the 19th century. this social evolution has resulted on a relevant urban growth and on significant transformations on land use and land cover (meirich, 2008). these variations can be approached from several disciplines since they are fostered by complex processes. however, in this specific study these changes have been analysed from a cartographic and geographic perspective. soil sealing is one of the main consequences of urban growth. this is a matter of grave concern as it entails ground waterproofing and leads to almost irreversible damages on soil ecosystems (nizeyimana, et al, 2001; montanarella, 2007; scalenghe & ajmone-marsan, 2009; eea, 2012; siedentop & fina, 2012). spatio-temporal dynamics of soil sealing have been regularly studied with the use of 20th century maps, aerial photographs, orthophotographs and satellite images (ridd, 1995; zhang, 2001; wu, 2004; moeller, 2005; kampouraki et al., 2006, etc.). however, the use of information predating the mid-20th century is rather unusual since only a few organisations have digitised ancient documents on a large scale and made them accessible to the general public. instead, this study has included 19th century maps provided by the national geographic institute of spain (ign). they have been used to analyse patterns of urban growth and the extent to which land has been lost to sealing over the last 150 years in a borough located in the outskirts of madrid. this is a rather innovative approach that enables this retrospective analysis to be extended in time an additional century to what is usually considered. the effects of soil sealing resulting from urban growth are intensely felt in spain on coastal regions (ojeda & villar, 2006; valera et al., 2011a) and on major metropolitan areas, like madrid (garcía & pérez, 2007, 2011 and 2016; garcía et al., 2014). this study has focused on a specific town called colmenar viejo, which is located in the northern outskirts of madrid (40º 39’ 31’’ n, 3º 45’ 58’’ w). this borough has been chosen because it has got both common characteristics to other spanish towns affected by metropolitan processes, but also distinctive attributes which are only to be observed in this territory, the most outstanding being: an environmentally significant and well preserved extension (18,256 ha). proximity to the city of madrid (30 km) and to several royal estates, i.e. el pardo y el soto de viñuelas, la granja de san ildefonso, san lorenzo de el escorial and aranjuez (fig. 1). colmenar viejo is well connected to the main city and to the surrounding towns by both road and rail. an urban character right from the beginning of the period under study as colmenar viejo has been a medium-size town since the 15th century. nevertheless, it is still nowadays physically apart from other built-up areas within the metropolitan area of madrid. this eases its separate study. various attempts to accomplish a scientific map of spain based on a geodetic network were carried out in the surroundings of colmenar viejo since the 1860´s. therefore, many maps of this area from the mid and late 19th century can be found at the national geographic institute of spain (ign), which is the only organisation that stores this type of scientific ancient cartography. this large amount of available ancient maps is most uncommon in spain, where some territories had not been scientifically surveyed even a century later. one of the main disadvantages of analysing soil sealing is that it is rarely confined to one administrative unit (borough, province, region or country) (seifert, 2010). as a result, analysis is more about how it has evolved in a specific territory than about comparing its evolution with other areas. instead, this study has aimed to show the spatio-temporal results of sealing at a municipal level so that it can be compared with other boroughs or territorial units. all information has been integrated in a gis so it can be measured and updated. figure 1. situation of colmenar viejo within the province of madrid and within spain sources a large variety of sources of information has been used to measure and analyse soil sealing. this variability results from the different techniques used for mapping over the last 150 years, i.e. classic topography (18601960 approx), photogrammetry (1960-2005 approx) and remote sensing (2005-2017 approx). this enables the use and integration of various cartographic techniques and its use in combination with gis programs, what makes this study rather innovative. in the first place, a careful selection of 365 ancient maps of colmenar viejo has been provided by the topographic archive at the national geographic institute of spain (ign). from 1860, when the spanish geodetic network started being calculated, to 1960, when photogrammetry started being widely used, several mapping projects using classic topographic methods were carried out in colmenar viejo. the material gathered for this lengthy period ranges from highly detailed land registry records at a scale of 1:500, to sheets from the national topographic map at a scale of 1:50,000. the characteristics of these old maps are summarised in table 1 and some examples of this ancient cartography are shown in fig. 2. should anyone be interested in going in depth in these sources, see muro et al., 1996; arístegui, 2014; urteaga & nadal, 2001; moreno, 2008. table 1. ancient maps of colmenar viejo created using classic topography [period 1860-1960] (source: national geographic institute of spain (ign)) 113soil sealing in madrid (spain), study case of colmenar viejo figure 2. examples of ancient maps of colmenar viejo created using classic topography [period 1860-1960] (source: national geographic institute of spain (ign)) in the second place, the period from 1960 to 2005 has been analysed by means of visually interpreting some aerial photographs. these images have been chosen from various photogrammetric flights, one per decade. they were captured at different scales, varying from 1:18,000 to 1:40,000, on analogue or digital formats, and on a colour or on a black & white scale, as shown on table 2. table 2. aerial photographs [period 1960-2005] (source: national geographic institute of spain (ign)) figure 3. base layer: each polygon represents an urban land plot within the builtup area as it stood in the year 2013 (source: spanish cadastral service (catastro)) finally, the different population censuses carried out in spain since the mid-19th century have been acquired at the national institute for statistics of spain (ine) (table 3). table 3. population registered in colmenar viejo on various censuses (source: national institute for statistics of spain (ine)) in the third place, the most recent period has been examined by means of remote sensing images from the year 2011. a panchromatic image (with a spatial resolution of 2.5 m) and a multispectral image (with a spatial resolution of 10 m) from spot 5 satellite (path 69, row 033) were analysed. these images have been supplied by the national geographic institute of spain (ign) over the national remote sensing plan (pnt). in the fourth place, some auxiliary sources have also been used. on the hand, an orthophoto from the year 2013 has been selected. it has been provided by the national geographic institute of spain (ign) over the aerial orthophoto national plan (pnoa). this layer has been used for the first step of the method, which has consisted of georeferencing all maps. on the other hand, an up-todate cadastral map of the built-up area has been chosen. this map has been provided by the spanish cadastral service. it is in a polygonal vector format (.shp) and represents urban land plots in the year 2013 (fig. 3). this map has been used as a base layer for the second stage of the method, which has consisted of editing this layer to help calculating soil loss by means of sealing. 114 andrés aristegui cortijo, maría eugenia pérez gonzález method the first step of the method has included changing all maps to a raster format and georeferencing them in datum etrs89, projection utm, zone 30 t, which is the official reference system in central spain at present. this process has been carried out with global mapper 14 gis program, using the 2013 orthophoto as a base layer. between 30 and 50 control points have been assigned to each map. these control points have been located in places that seem to have remained invariable over the last decades. afterwards a rubber-sheeting conversion with triangulation has been used and rotations, transformations and changes in local and directional scales have been applied. mean rms at control points has been 3.4 m. all control points with a rms above 6 m have not been taken into account. finally, georeferenced maps have been saved in .ecw format. the second stage of the method has included part of the method proposed by valera et al. (2011 b). first, the urban area has been analysed using maps dating from 1861, 1918 and 1952, photos from 1968, 1979, 1983, 1991 and 2002, and satellite images from 2011. second, the 2013 cadastral base layer, downloaded on global mapper 14 gis application, has been progressively edited in inverse chronological order by means of visually interpreting maps, photos and satellite images for each date. as the process has gone back in time, plot erasing has always been needed (it has never been necessary to add them) since the built-up area has always shown an increase with time, never a decrease. due to the differing nature of mapping techniques, certain decisions had to be taken. on the one hand, spectral analysis using erdas imagine 2011 remote sensing program has been used to edit the 2013 cadastral base layer with respect to the 2011 satellite image. on the other hand, to estimate the urban area in 1952, for which only rural maps were available, it has been necessary to erase those plots that appeared on the 1968 edited base layer, but not on the 1952 map. however, the territory comprising the urban area could not be modified because no information from 1952 was available. therefore they have been left the same as they were in 1968. also the variability in the different urban units represented on ancient maps had to be overcome. for instance, when it came to adjusting the base layer to the urban area in 1918, maps referred to city blocks and not to land plots. in other words, the available information was less detailed on the 1918 map than on the base layer, since each block includes several plots. less spatial information meant that blocks had to be edited as a whole as it was not possible to discriminate information related to each plot. the third level of the method has included analysing patterns and determining urban growth by means of using arcgis10.1 gis program. in this regard, pinho & oliveira, 2009 and tombolini et al., 2015 have been consulted. two aspects have been assessed: two-dimensional growth considering only the x and y axis. these calculations help examining the extent of soil sealing in quantitative terms. the sealed soil area (in hectares), the sealing increase between consecutive dates (in percentages), the mean annual sealing growth (in hectares per year) and the sealed soil among the total municipal territory (in percentages) have been calculated for each date. three-dimensional growth considering also the z axis. these figures help analysing the patterns followed by urban extension from a qualitative perspective. since building heights were not represented on maps and there was therefore a lack of information, this analysis has been based on auxiliary information related directly to vertical growth, i.e. population and population density. population of the town for each date from 1861 to 2011 has been obtained from a linear function between the two population censuses closest in time. population density for each date has been determined by dividing the estimated population by the urban area. three-dimensional urban growth has been calculated by means of the following parameters: population (in inhabitants), population increase between consecutive dates (in percentages), mean annual population growth (in inhabitants per year), population density (in inhabitants per sq km) and population density mean annual increase (in percentages). the fourth step of the method has consisted of examining the territory in situ and verifying the obtained figures and calculations. results the main calculations related to soil sealing and the characteristics of urban growth have been summarised in table 4 and fig. 4. table 4. two-dimensional and three-dimensional growth with respect to the two-dimensional growth of the urban area, which refers to the extent of the process from a quantitative perspective, the following should be noted: the old town was small in relation to the present-day built-up area. in 1861 it comprised 25.3 hectares, i.e. only 0.14% of the municipal territory. from 1861 to 1918 urban growth was subdued, with a total increase in the built-up area of 40%, which implies a mean annual growth of less than 0.20 hectares. this means that the loss of soil due to sealing in colmenar viejo affected hardly 0.19% of the municipal territory by 1918. from 1952 to 1991 a considerable urban extension took place and thus the built-up area of the town easily tripled. mean annual growth showed ever greater increases, going from 3 hectares/year at the initial time to nearly 6 hectares/year at the final stage. at the beginning of the 1990´s, 1.31% of the municipal territory was covered by built-up area. the greatest urban growth has taken place over the last two decades. from 1991 to 2011 the built-up area has doubled, with a mean annual growth of between 9 and 16 hectares. at the end of the period, the percentage of sealed soil has already reached 2.64%. therefore, the two-dimensional growth of the built-up area throughout the period under study has been both progressive, i.e. all periods have recorded positive increases, and exponential, i.e. the mean annual growth has increased over an ever larger area. the urban area has doubled in size during the period of fastest growth which comprises only twenty years, i.e. from 1991 to 2011. as a result, 2.64% of the municipal territory is now sealed as a consequence of urban processes. this percentage is lower than in other european cities, where soil sealed from 1990 to 2006 within a range of 6% to 9% (eea, 2012), and it is also lower than in other boroughs within the region of madrid, where these figures vary from 8.5% to 13.5% (garcía et al., 2014). these discrete figures are probably related to the proximity of colmenar viejo to some royal estates and the influence of their environmental protection. it could therefore be concluded that the data recorded in colmenar viejo should not give rise to much concern. however, it should also be borne in mind that since the borough under study covers the second largest 115soil sealing in madrid (spain), study case of colmenar viejo municipal territory within the region of madrid, small quantities in relative terms are highly significant in absolute terms. •1952-1968: detached houses for holiday home use •1979-1991: detached and semi-detached houses with ample gardens •2002-2011: vast urban extension and exorbitant soil sealing; extensive residential fabric with a predominance of houses over blocks conclusions this study has analysed the spatio-temporal dynamics and has enabled the measuring of the soil sealing process due to urban growth over the last 150 years in a borough located in the northern outskirts of madrid, i.e. colmenar viejo. this town is one of the boroughs of highest environmental value around the capital city of spain and has got both common characteristics to other spanish towns influenced by metropolitan processes and specific attributes that are only to be observed in this territory. the sources and the method used in this study are rather innovative. on the one hand, most soil sealing analysis only consider maps since the mid-20th century. instead, this study has enabled this retrospective study to go back in time an additional century with the use of 19th century maps obtained from the national geographic institute of spain (ign). on the other hand, integrating in a gis program a large volume of maps made by different cartographic techniques, i.e. classic topography, photogrammetry and remote sensing, has guaranteed the capture of a detailed and reliable urban cartography since the mid-19th century and has enabled the use and integration of various mapping techniques. furthermore, this study has aimed to show the spatio-temporal results of sealing at a municipal level so that it can be measured, updated and compared with other boroughs or territorial units. calculating the extension of urban growth from a quantitative perspective has been fulfilled by georeferencing ancient maps, digital maps, aerial photographs and satellite images, integrating them into a gis program and analysing some parameters regards the twodimensional urban extension. observing the patterns followed by urban growth from a qualitative point of view has been achieved by using population censuses accomplished since the mid-19th century and using an indicator system to analyse the three-dimensional urban growth. part of valera´s et al. (2011b) method for editing and adjusting maps to the urban area according to different dates has been used. on the one hand, always using the same layer as the base map avoids slivers. on the other hand, using a vector layer for a plot detail provides accurate and precise information for quantifying and analysing urban growth. results show that, in terms of quantity, urban growth has been both progressive, i.e. all periods have recorded positive increases, and exponential, i.e. the mean annual growth has increased over an ever larger area. along the whole period, urban extension has increased from 25.3 hectares to 482.5 hectares, i.e. from 0.14% to 2.64% of the municipal territory, what means multiplying 19-fold its size. the trend followed by urban growth has been therefore similar to other spanish towns influenced by metropolitan processes. however, the intensity of this process has remained slightly under the european and madrilian average, which is probably related to its proximity to some royal estates, what has conferred this territory a stronger environmental protection. in terms of quality, the patterns observed on urban extension comply with the general urban growth model recorded during the 20th and 21st centuries in the outskirts of major spanish cities. acknowledgments this paper has been supported by the science and innovation office of the spanish government under the research program cso-2012-34785, called “sellado de suelo en la comunidad de madrid y áreas limítrofes. estudio mediante teledetección y sistemas de información geográfica (soil sealing in madrid and surrounding areas. analysis using remote sensing programs and geographic information systems)”. figure 4. sealed soil in the borough of colmenar viejo from 1861 to 2011 with respect to the three-dimensional growth, which shows the patterns followed by urban processes from a qualitative perspective, the main features are: population growth has been steady throughout the whole period under study. the greatest mean annual growths were recorded in the 1970´s, with around 700 inhabitants per year, and during the final stage, with over 1,000 inhabitants per year. population density has varied constantly. it registered a plus sign over some periods, which shows a rise on population density, and a minus sign during some others, which denotes a drop on population density. variations in population density have remained modest, with mean annual changes within a range of ±1.5%. if only population statistics were to be taken into account, it could therefore be presumed that while population has risen steadily in numerical terms, it has not followed the same patterns in terms of population density. this infers that there have been some periods in which the built-up area has spread loosely and population density has dropped, and others in which population density has risen as a result of high building construction. however, if, in addition, these data are broken down into periods corresponding to available maps, and statistics are complemented with the ground truth, it can be concluded that the dominant processes are consistent with the general model of urban growth followed by metropolitan outskirts of major spanish cities. these can be summarised as follows: initial period from 1861 to 1918: modest yet steady extension, following the same patterns observed historically up to that time, i.e. narrow streets and one-storey buildings periods of vertical growth: • 1968-1979: tower blocks with very few gardens and leisure areas •1991-2002: residential estates of 3-4 storey buildings in landscaped surroundings periods of horizontal growth: •1918-1952: similar houses to those in the old town but including bigger courtyards and vegetable patches 116 andrés aristegui cortijo, maría eugenia pérez gonzález references arístegui, a. 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(2001). detection of urban housing development by fusing multisensor satellite data and performing spatial feature post classification. international journal of remote sensing, 22/17, 3339 3355. lake water temperature is one of the key parameters in determining the ecological conditions within a lake, as it influences both chemical and biological processes. therefore, accurate prediction of water temperature is crucially important for lake management. in this paper, the performance of soft computing techniques including gene expression programming (gep), which is a variant of genetic programming (gp), adaptive neuro fuzzy inference system (anfis) and artificial neural networks (anns) to predict hourly water temperature at a buoy station in the yuan-yang lake (yyl) in north-central taiwan at various measured depths was evaluated. to evaluate the performance of the soft computing techniques, three different statistical indicators were used, including the root mean squared error (rmse), the mean absolute error (mae), and the coefficient of correlation (r). results showed that the gep had the best performances among other studied methods in the prediction of hourly water temperature at 0, 2 and 3 meter depths below water surface, but there was a different trend in the 1 meter depth below water surface. in this depth, the ann had better accuracy than the gep and anfis. despite the error (rmse value) is smaller in ann than gep, there is an upper bound in scatter plot of ann that imposes a constant value, which is not suitable for predictive purposes. as a conclusion, results from the current study demonstrated that gep provided moderately reasonable trends for the prediction of hourly water temperature in different depths. la temperatura del agua es uno de los parámetros básicos para determinar las condiciones ecológicas de un lago, ya que está influenciada por procesos químicos y biológicos. además, la exactitud en la predicción de la temperatura del agua es esencial para el manejo del lago. en este artículo se evalúa el desempeño de técnicas de soft computing como la programación de expresiones de genes (peg), que es una variante de la programación genética (pg), el sistema neuro-fuzzy de inferencia adaptativa (anfis, en inglés) y las redes neuronales artificiales (rna) para predecir la temperatura del agua en diferentes niveles de una estación flotante del lago yuan-yang (yyl), en el centro-norte de taiwán. se utilizaron tres indicadores estadísticos, el error cuadrático medio (ecm), el error absoluto medio (mae, en inglés) y el coeficiente de correlación (r) para evaluar el desempeño de las técnicas de computación. los resultados muestran que la peg es más exacta en la predicción de la temperatura del agua entre 0,2 y 3 metros de profundidad. sin embargo, se evidencia una tendencia diferente a partir del metro de profundidad. a esta distancia de la superficie, las rna son más exactas que la peg y el anfis. los resultados de este estudio probaron claramente la usabilidad del peg y las rna en la predicción de la temperatura del agua a diferentes profundidades. abstract resumen keywords: soft computing techniques, statistical indicators, subalpine lake, water temperature. palabras clave: técnicas soft computing, indicadores estadísticos, lago subalpino, temperatura del agua. water temperature prediction in a subtropical subalpine lake using soft computing techniques record manuscript received: 29/04/2014 accepted for publication: 26/02/2016 how to cite item earth sciences research journal, 20(2), b-b. doi:http://dx.doi.org/10.15446/esrj.v20n2.43199 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.43199 saeed samadianfard1,*, honeyeh kazemi1, ozgur kisi2, wen-cheng liu3 1department of water engineering, university of tabriz, tabriz, iran (*corresponding author, e-mail: s.samadian@tabrizu.ac.ir, tel: +989141101845, fax: +984113356007) 2department of civil engineering, canik basari university, samsun, turkey 3department of civil and disaster prevention engineering, national united university, miao-li, taiwan earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : d1-d11. h y d r o l o g y d2 saeed samadianfard, honeyeh kazemi, ozgur kisi and wen-cheng liu 1. introduction water temperature is a fundamental physical property with a direct impact on all organisms inhabiting the aquatic environment (webb et al., 2008). because predicting the water temperature is important for maintaining water quality and for ecosystem management, several authors have investigated methods for simulating water temperature of lakes (lawrence et al., 2002; lee et al., 2009; schwab et al., 2009). hondzo and stefan (1996) simulated daily water temperature and dissolved oxygen profiles in minnesota lakes by deterministic process-based water quality models with daily meteorological conditions as input. derived empirical formulas for lake water quality and stratification indicators from the simulation results gave good predictions of temperature and dissolved oxygen characteristics estimated from measurements in seven minnesota lakes. fang and stefan (1996) substantially modified a water temperature and ice cover model for freshwater lakes and combined it with a summer model to simulate water temperature structures and ice thickness in two small lakes in the north central us. the best value of volume averaged water temperature for the two studied small lakes was higher than was previously found for a larger lake. benyahya et al. (2007) provided an overview of the existing statistical water temperature models. they categorized them in two major groups: deterministic and statistical/stochastic models. they stated that the deterministic models require numerous input data and they are appropriate for analyzing different impact scenarios, but the main advantage of the statistical models is their relative simplicity and relative minimal data requirement. sharma et al. (2008) developed models to predict annual maximum near-surface lake water temperatures for lakes across canada using four statistical approaches: multiple regression, regression tree, artificial neural networks and bayesian multiple regression. although artificial neural networks were marginally better for three of the four data sets, multiple regression was considered to provide the best solution based on the combination of model performance and computational complexity. trumpickas et al. (2009) tried to construct empirical relationships between surface water temperatures and local air temperatures that could be used to estimate future water temperatures using future air temperatures generated by global climate models. zhao et al. (2011) calculated the surface water temperature, deep water temperature and mean annual epilimnetic temperature and compared the obtained values with empirical data in lake taihu by using the dynamic water temperature model. the simulated values were consistent with empirical data. results showed that this model could be used for the temperature simulation in the studied lake. thiery at al. (2014) evaluated a set of one dimensional lake models for lake kivu, east africa. in this study, meteorological observations from two automatic weather stations were used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, was used to assess the model’s performance. simulations were performed over the freshwater layer only (60 m) and over the average lake depth (240 m). the good agreement between the deep simulations and the observed meromictic stratification also showed that a subset of models was able to account for the salinityand geothermal induced effects upon deep-water stratification. also, some attempts have been made to relate lake water temperature to meteorological parameters such as air temperature. in this context, piccolroaz et al. (2013) developed air2water, a simple physically based model to relate the temperature of the lake superior (usa–canada), considering a 27 years record of measurements, to air temperature only. the results proved that their model was suitable to be applied over long timescales (from monthly to interannual), and could be easily used to predict the response of a lake to climate change, since projected air temperatures were usually available by large-scale global circulation models. toffolon et al. (2014) reconstructed temperature of the surface layer of temperate lakes by means of a simplified model on the basis of air temperature alone. the comparison between calculated and observed data showed a remarkable agreement for all 14 lakes investigated (mara, sparkling, superior, michigan, huron, erie, ontario, biel, zurich, constance, garda, neusiedl, balaton, and baikal, in west-to-east order), which presented a wide range of morphological and hydrological characteristics. given the significant impact of lakes on surface atmosphere interactions, the need for an accurate simulations of lake temperatures at different depths arises. for this purpose, soft computing techniques such as gene expression programming (gep), adaptive neuro fuzzy inference system (anfis) and artificial neural networks (ann) can play undeniable roles in mentioned simulations. gene expression programming (gep) has been applied to a wide range of problems in artificial intelligence, artificial life, engineering and science. gep can be successively applied to areas where (i) the interrelationships among the relevant variables are poorly understood (or where it is suspected that the current understanding may well be wrong), (ii) finding the size and shape of the ultimate solution is difficult, (iii) conventional mathematical analysis does not, or cannot, provide analytical solutions, (iv) an approximate solution is acceptable (or is the only result that is ever likely to be obtained), (v) small improvements in performance are routinely measured (or easily measurable) and highly prized, (vi) there is a large amount of data in computer readable form, that requires examination, classification, and integration, e.g., molecular biology for protein and dna sequences, astronomical data, satellite observation data, financial data, marketing transaction data, or data on the world wide web (banzhaf et al. 1998). during the last decade, genetic programming has been used as a viable alternative approach to physical models. aytek and kisi (2008) applied gp to suspended sediment transport, and found it to perform better than conventional rating curve and multi-linear regression techniques. shiri and kisi (2011) compared gep and anfis methods for predicting groundwater table depth fluctuations and found gep to be better than anfis in this regard. samadianfard (2012) examined the potential of the gep technique in estimating flow friction factor in comparison with the most currently available explicit alternatives to the colebrook–white equation. results revealed that by using gep, the friction factor could be identified precisely. samadianfard et al. (2012) studied the capabilities of the gp in simulating the wetting patterns of drip irrigation. results showed that the gp method had good agreement with the results of hydrus 2d software considering the full set of operators in estimation of radius and depth of wetting patterns. also results obtained from field experimental in a sandy loam soil showed reasonable agreement with the gp results. results of the study demonstrated the usefulness of the gp method for estimating wetting patterns of drip irrigation. anfis is a neuro-fuzzy system, which uses a feed-forward network to search for fuzzy decision rules that perform well on a given task. using a given input/output data set, anfis creates a fuzzy inference system whose membership function parameters are adjusted using a back-propagation algorithm alone or a combination of a back-propagation algorithm with a least mean square method. this allows the fuzzy systems to learn from the data being modeled. kisi (2006) investigated the ability of anfis technique to improve the accuracy of daily evaporation estimation. based on his results, the anfis computing technique could be used successfully in modeling evaporation process from the available climatic data. shiri et al. (2011) compared anfis to ann to estimate daily pan evaporation values from climatic data and found anfis to be better than ann. the artificial neural network (ann) approach provides a viable solution to the environmental problems because it is based on training not on analytical models or statistical assumptions. ann models can be trained to predict results from examples and once trained; they can perform predictions at very high speed (mellit et al., 2006). ann is an intelligent data-driven modeling tool that is able to capture and represent complex and non-linear input/output relationships. anns are massively parallel, distributed processing systems that can continuously improve their performance via dynamic learning. moghaddamnia et al. (2009) explored evaporation estimation methods based on ann and anfis techniques. they found that ann and anfis techniques had much better performances than the empirical formulas. zaier et al. (2010) developed ann ensemble models to improve the results of single artificial neural network (single ann) for the estimation of the ice thickness in a number of selected canadian lakes during the early winter ice growth period. ann ensemble models for the estimation of ice thickness proved to be more accurate than single ann models. kisi et al. (2012) applied three artificial intelligence approaches, d3water temperature prediction in a subtropical subalpine lake using soft computing techniques namely anns, anfis and gep to forecast daily lake-level variations of lake iznik in turkey. the results obtained by the gep approach indicated that it performed better than anfis and anns in predicting lake-level variations. liu and chen (2012) compared the performance of the ann technique with a physically based three-dimensional circulation model for prediction of water temperature at a buoy station in the yuan-yang lake in north-central taiwan at various measured depths. the simulated results revealed that the accuracy of the three-dimensional circulation model was better than the ann model. fallah-mehdipour et al. (2013) investigated the capability of anfis and gp as two artificial intelligence tools to predict and simulate groundwater levels in three observation wells in the karaj plain, iran. results indicated that gp yielded more appropriate results than anfis when different combinations of input data were employed in both prediction and simulation processes. the main objective of this paper is to investigate the accuracy of soft computing techniques such as gene expression programming, adaptive neuro fuzzy inference system and artificial neural network methods for the prediction of hourly water temperature in a lake at different measured depths. some statistical parameters for error estimation are used herein as comparing criteria for the evaluation of the performance of the studied models. 2. material and methods 2.1. study site and data collection yuan-yang lake (yyl, 24°34’60.00’n, 121°24’0.00’e, area= 3.7×104 m2; maximum depth= 4.5 m) in north-central taiwan is a subtropical, humid lake in the clilan national forest preserve (figure 1). figure 2 shows the bathymetry of the yyl (contours in m). also, yyl is a subalpine lake situated in a natural reserve area that has been virtually undisturbed by human activity for a long time (chen and wu, 1999). it is surrounded by a cloud belt forest of taiwan yellow cypress, an important timber tree and a relic species. the lake is located 1670 m above the mean sea level which is at the subalpine region. the geography of the drainage basin allows large quantities of terrestrial runoff from the surrounding mountains to enter the yyl (wu et al., 2001). the mean annual air temperature is approximately 13°c (monthly averages range from -5 to 15°c), and annual precipitation can exceed 4000 mm. wind speed over the lake, which was measured 1 m above the lake by an anemometer, (between 0 to 4.220 m.s-1) is relatively weak. the dominant wind directions are from the east and the south west because of the v-shaped valley facing east to west. concerns about the water quality in yyl, have been rapidly increasing recently due to the natural and anthropogenic pollution. in order to understand the underlying physical and chemical processes as well as their associated spatial distribution in yyl, liu et al. (2011) analyzed fourteen physico-chemical water quality parameters recorded at the eight sampling stations by using multivariate statistical techniques and a geostatistical method. their results showed that four principal components i.e., nitrogen nutrients, meteorological factor, turbidity and nitrate factors, account for 65.52% of the total variance among the water quality parameters. the spatial distribution of principal components further confirmed that nitrogen sources constitute an important pollutant contribution in the yyl. since april 2004, the deepest point of yyl (approximately 4.5 m) was instrumented with a buoy that measures environmental parameters every 10 min (see figure 2) ; these data are accessible at the global ecological lakes observatory network (gleon) website. all data from the instrumented buoy and associated meteorological variables were downloaded from the gleon publicly accessible database. the yyl buoy measured surface dissolved oxygen, wind speed, wind direction, water temperature profiles, and air temperature. a weather station approximately 1 km from the lake also measured rainfall, humidity, and soil temperature. high-resolution water temperature profiles collected from the buoy were used for the training and validation of the studied models. also meteorological variables given in table 1 were used to develop the soft computing techniques. figure 1. location of yuan-yang lake in taiwan figure 2. the bathymetry of the yyl (contours in m) table 1. meteorological variables used to develop the models table 2 represents the hourly statistical parameters of the applied variables. in this table, the terms xmean, xmin, xmax, sx, cv and csx denote the mean, minimum, maximum, standard deviation, coefficient of variation and skewness coefficient, respectively. as can be seen clearly, rainfall has the maximum skewness. solar radiation and relative humidity also indicate a skewed distribution. air pressure, soil temperature and air temperature show normal distributions because they have significantly low skewness values. table 3 shows the correlations between the meteorological and hourly water temperature parameters. as can be seen from the table, the soil temperature has the highest correlations with hourly water temperatures in all depths. air temperature also has higher correlations than the other variables. time variation graphs of the meteorological variables used as inputs to the gep, anfis and ann models are illustrated in figure 3. d4 saeed samadianfard, honeyeh kazemi, ozgur kisi and wen-cheng liu table 2. hourly statistical parameters of the observed data. note: the terms xmean, xmin, xmax, sx, cv and csx denote the mean, minimum, maximum, standard deviation, coefficient of variation and skewness, respectively. table 3. correlations between meteorological and hourly water.temperature parameters. 0 200 400 600 800 1000 1200 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date so la r r ad ia tio n (w m -2 ) 610 615 620 625 630 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date a ir pr es su re (h pa ) 70 80 90 100 110 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date r el at iv e hu m id ity (% ) 24 25 26 27 28 29 30 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date so il te m pe ra tu re (c ) 10 15 20 25 30 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date a ir te m pe ra tu re (c ) 0 1 2 3 4 5 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date w in ds pe ed (m .s -1 ) 0 10 20 30 40 50 5/1 5/6 5/11 5/16 5/21 5/26 5/31 6/5 6/10 6/15 6/20 6/25 6/30 date r ai nf al l ( m m ) figure 3. time variation graphs of the meteorological conditions. d5water temperature prediction in a subtropical subalpine lake using soft computing techniques 2.2 general overview of genetic programming in this section, a brief overview of the gp and gep is given. detailed explanations of gp and gep are provided by koza (1992) and ferreira (2006), respectively. gp was first proposed by koza (1992). it is a generalization of genetic algorithms (gas) (goldberg, 1989). the fundamental difference between ga, gp, and gep is due to the nature of the individuals. in the ga, the individuals are linear strings of fixed length (chromosomes). in the gp, the individuals are nonlinear entities of different sizes and shapes (parse trees), and in gep the individuals are encoded as linear strings of fixed length (the genome or chromosomes), which are afterwards expressed as nonlinear entities of different sizes and shapes (ferreira, 2001 a,b). gp is a search technique that allows the solution of problems by automatically generating algorithms and expressions. these expressions are coded or represented as a tree structure with its terminals (leaves) and nodes (functions). gp applies gas to a “population” of programs, typically encoded as tree-structures. trial programs are evaluated against a “fitness function” then the best solutions are selected for modification and re-evaluation. this modificationevaluation cycle is repeated until a “correct” program is produced. there are five major preliminary steps for solving a problem by using gep. these are the determination of (i) the set of terminals, (ii) the set of functions, (iii) the fitness measure, (iv) the values of the numerical parameters and qualitative variables for controlling the run, and (v) the criterion for designating a result and terminating a run (koza, 1992). a gep flowchart is presented in figure 4. figure 4. gep flowchart there are five major steps in preparing to use gep of which the first is to choose the fitness function. the fitness of an individual program i for fitness case j is evaluated by ferreira (2006) using: where p is the precision and e(i,j) is the error of an individual program i for fitness case j. for the absolute error, this is expressed by: (2) where p(i,j) is the value predicted by the individual program i for fitness case j (out of n fitness cases) and tj is the target value for fitness case j. again for the absolute error, the fitness fi of an individual program i is expressed by: (3) where r is the selection range. the second major step consists of choosing the set of terminals t and the set of functions f to create the chromosomes. for this study, the function set consists of 7 functions including four basic arithmetic operators, i.e., (+, -, ×, /) and some basic mathematical functions, i.e., (√, ln(x), exp) selected among all the available functions in gep. the function selection was based on simplicity and its relevance to the nature of the problem thus ensuring a simple and efficient final gep model. the third major step is to choose the chromosomal architecture, i.e., the length of the head and the number of genes. values of the length of the head, h = 10, and six genes per chromosome were employed based on the discussion in ferreira (2001b). the fourth major step is to choose the linking function. in this study, the subprograms were linked by addition on the basis of recommendations made by ferreira (2001a) and findings of other studies (e.g. guven and aytek, 2009). finally, the fifth major step is to choose the set of genetic operators that cause variation along with their rates. a combination of all genetic operators, i.e., mutation, transposition and recombination, was used for this purpose. the parameters of the training of the gep are given in table 4. table 4. parameters of the gep model 2.3. general overview of adaptive neuro-fuzzy inference system (anfis) anfis (jang, 1993), using a given input/output data set, constructs a fuzzy inference system (fis) whose membership function parameters are tuned (adjusted) using either a back propagation algorithm alone or in combination with a least squares type of method. this adjustment allows fuzzy systems to learn from the data set. figure 5 shows the anfis structure for a fuzzy inference system with two inputs. d6 saeed samadianfard, honeyeh kazemi, ozgur kisi and wen-cheng liu figure 5. anfis structure firstly, the fis type should be selected. in this regard, the sugeno method was selected for the present study (ozger, 2009; ullah and choudhury, 2013). to train a fis, a training data set that contains the desired input/output data of the system to be modeled must be loaded. before training, an initial fis model structure must be specified: (1) hybrid algorithm was selected as the optimization method. the optimization methods train the membership function parameters to emulate the training data. (2) the number of training epochs and the training error tolerance were selected. these values are entered to set the stopping criteria for training. the training process stops whenever the maximum epoch number is reached or the training error goal is achieved. to validate the trained fis, after loading the test data, the number of membership functions (mfs) and the type of input and output membership functions were selected. there are only two choices for the output membership function: constant and linear. this limitation of output membership function choices is because anfis only operates on sugeno-type systems (jang, 1993). the builtin membership function composed of difference between two sigmoidal membership functions (dsigmf) and the constant membership function were selected as the input and output membership functions, respectively. the number of mfs assigned to each input was three. it should be noted that there is no basic rule for selecting the optimization method and determining the optimal number and type of mfs and they are usually considered by trial and error. nevertheless, it should be taken into consideration that large numbers of mfs will increase the calculation time and efforts (keskin et al., 2004). the values assigned to each parameter for the anfis model are given in table 5. table 5. parameters for the anfis model 2.4. general overview of artificial neural networks (anns) the qnet neural network development system which is a complete solution for back propagation neural network modeling (qnet, 2000) was used for the present study. back propagation type neural networks process information in interconnecting processing elements termed nodes. these nodes are organized into groups termed layers. there are three distinct types of layers in a back propagation neural network: the input layer, the hidden layer(s) and the output layer. a network consists of one input layer, one or more hidden layers and one output layer. connections exist between the nodes of adjacent layers to relay the output signals from one layer to the next. information enters a network through the nodes of the input layer. the input layer nodes are unique in that their sole purpose is to distribute the input information to the next processing layer (i.e., the first hidden layer). figure 6 shows the structure of the anns with one hidden layer including three nodes. figure 6. the structure of the artificial neural networks to design the neural network, the modeler must specify the layer and node quantities, transfer functions and network connections. including the input layer, one hidden layer and the output layer, the specified number of the layers for the network was three. the number of nodes in the input layer will be equal to the number of input data values in the model. according to table 1, it was specified seven for the prediction of hourly water temperature for the present study. the number of output nodes for the network must correspond to the number of outputs in the network. it was one because we only had one output. choosing the number of hidden layers and the number of hidden nodes in each layer is not so trivial. the construction of the hidden processing structure of the network is arbitrary. generally, it is best to start with simple network designs that use relatively few hidden layers and processing nodes. however, in practice, it is usually better to employ multiple hidden layers for solving complex problems. a single hidden layer including three single nodes was specified herein to avoid an unnecessary large and complex model. the sigmoid function is qnet’s default transfer function and it is the most widely used function for back propagation neural networks. another network design consideration concerns how to control the network’s connections. while the connection editor gives the modeler almost unlimited flexibility in designing a network, the fact is that the vast majority of designs work best fully connected. qnet’s connection editor is best suited for highly advanced models that require groups of input data to be processed through separate network pathways. the default fully connected configuration was used for the present study. the values assigned to each parameter for the ann model are given in table 6. as it is explained above, qnet has a high and intelligent ability to simulate complex networks easily while preparing a code for these networks could be excruciating. for practical problems, using an easy method, which is usable for different cases, is more acceptable than sophisticated methods. in summary, qnet is professional user friendly software and it has been used for simulating different complex problems (kuo et al. 2004, yang et al. 2009) and that is why it was used in this research. table 6. parameters for the ann model the performance of three soft computing techniques, namely gep, anfis and ann to predict the hourly water temperature at yyl at various measured depths was compared. hourly water temperature data from may 1 to june 11, 2008 were taken as the training data set, while the measured d7water temperature prediction in a subtropical subalpine lake using soft computing techniques data from june 12 to 30, 2008 served as the validation data set. furthermore, time-series meteorological conditions served as inputs for the studied models for data from may 1 to june 30, 2008 (figure 3). 2.5. evaluation parameters several parameters can be considered for the evaluation of the model-predicted values of hourly water temperature in the yyl lake. in this study, root mean squared error (rmse), mean absolute error (mae) and correlation coefficient (r) were used as the evaluation criteria and they can be computed as follows (liu and chen, 2012): where tp(i) and to(i) represent the model-predicted and the observed water temperature, respectively, and n is the number of observations. 3. results and discussion a comprehensive comparison was made to compare the accuracy of gep, anfis and ann methods for the prediction of hourly water temperature in different depths in the yyl. rmse, mae and r values of each model for the prediction of hourly water temperature at different depths are shown in table 7. it is clear from the table that in the case of surface water temperature, the gep model has the lowest rmse (1.49 oc), mae (1.21 oc) and the highest r (0.73) values. the ann model seems to be the second best from the rmse and mae viewpoints. also, in the case of 1 meter depth, the ann model shows better accuracy than the gep and anfis models. anfis model is ranked as the second best. despite the error (rmse value) is smaller in ann than gep, there is an upper bound in scatter plot of ann (see figure 7.e) that imposes a constant value, which is not suitable for predictive purposes. furthermore, at the 2 meter depth below water surface, the gep model has the lowest rmse (0.35 oc) and mae (0.24 oc) values. also, the ann model has a better accuracy than the anfis. finally, at the 3 meter depth below water surface, the superiority of the gep model over the other models is clearly seen from table 7. the ann model seems to be the second best from the rmse, mae and r viewpoints. figure 7 shows the observed (x-axis) and predicted (y-axis) hourly water temperature values in different depths and in validation period in the form of scatter plots. gep model seems to be better than the other soft computing techniques. in the case of surface water temperature, the estimates of gep model seem to be closer to the exact line than those of the ann and anfis models. the anfis model’s estimates are also less scattered than the ann. in the case of 1 meter depth, although the ann model has the least scattered estimates, but there is an upper bound in scatter plot of ann that imposes a constant value, which will make additional problems in future predictions. so, gep model having less scattered estimations seems to be the best. table 7. performance assessment of different models for predicting hourly water temperature at different depths at the 2 meter depth below water surface, the estimates of the gep model closer to the exact line than those of the other models. the ann and anfis models underestimate all the high values (>13.5 oc). the ann model has a better accuracy than the anfis because anfis model significantly underestimates high values (>13 oc). it is clear from the figure, at the 3 meter depth below water surface, the estimates of the gep model are closer to the ideal line than those of the other models. the ann model’s estimates seem to be less scattered than the anfis model. it should be noted that actual physical processes of the lakes such as vertical mixing, which controls the vertical distribution of temperature, have undeniable roles in changing water temperatures at different depths. vertical mixing, in small, seasonally stratified lakes such as yyl, might result in partial or complete mixing of the water column, depending on the thermal conditions of the lake and the strength of meteorological forces driving the mixing process. due to the fact that the purpose of the current research is statistical investigation of the effects of metrological parameters in predicting hourly water temperatures of yyl, the roles of physical phenomena including vertical mixing and phase lag between the sub-daily variations of the variables, affecting the heat flux, have been ignored. so, ignoring the physical factors might be one of the sources, which increased the error parameters of the predictions. figure 8 shows the observed and predicted values as time series plot in the validation period. from the figure, it can be said that the gep model is generally more successful than the ann and anfis model especially for the high water temperature values. one of the advantages of gep in comparison to the other theories is its ability in producing analytical formula for determination of output parameters. table 8 summarizes the gep mathematical equations for the prediction of hourly water temperature in different depths. from the table, the ap seems to be not effective variable on the water temperature in the case of 2and 3-meter depth below water surface. it can be said that the ap has a more effect on wt0 than the wt1. decreasing ap effect by increasing depth is clearly seen from the table. d8 saeed samadianfard, honeyeh kazemi, ozgur kisi and wen-cheng liu 10 15 20 25 10 15 20 25 measured wt0 (°c) g ep p re di ct ed w t0 (° c ) r=0.733 mae=1.206 °c rmse=1.492 °c a 10 15 20 25 10 15 20 25 measured wt0 (°c) a n n p re di ct ed w t0 (° c ) r=0.636 mae=1.315 °c rmse=1.637 °c b 10 15 20 25 10 15 20 25 measured wt0 (°c) a n fi s pr ed ic te d w t0 (° c ) r=0.630 mae=1.341 °c rmse=1.770 °c c 12 14 16 18 12 14 16 18 measured wt1 (°c) g ep p re di ct ed w t1 (° c ) r=0.644 mae=0.568 °c rmse=0.675 °c d 12 14 16 18 12 14 16 18 measured wt1 (°c) a n n p re di ct ed w t1 (° c ) r=0.796 mae=0.351 °c rmse=0.441 °c e 12 14 16 18 12 14 16 18 measured wt1 (°c) a n fi s pr ed ic te d w t1 (° c ) r=0.414 mae=0.508 °c rmse=0.639 °c f 12 14 16 12 14 16 measured wt2 (°c) g ep p re di ct ed w t2 (° c ) r=0.821 mae=0.236 °c rmse=0.345 °c g 12 14 16 12 14 16 measured wt2 (°c) a n n p re di ct ed w t2 (° c ) r=0.842 mae=0.307 °c rmse=0.395 °c h 12 14 16 12 14 16 measured wt2 (°c) a n fi s pr ed ic te d w t2 (° c ) r=0.586 mae=0.665 °c rmse=0.807 °c i 10 12 14 10 12 14 measured wt3 (°c) g ep p re di ct ed w t3 (° c ) r=0.782 mae=0.245 °c rmse=0.319 °c j 10 12 14 10 12 14 measured wt3 (°c) a n n p re di ct ed w t3 (° c ) r=0.746 mae=0.442 °c rmse=0.552 °c k 10 12 14 10 12 14 measured wt3 (°c) a n fi s pr ed ic te d w t3 (° c ) r=0.521 mae=0.576 °c rmse=0.684 °c l figure 7. scatter plots of observed (x-axis) and predicted values (y-axis) of hourly water temperature in different depths. d9water temperature prediction in a subtropical subalpine lake using soft computing techniques 14 16 18 20 22 24 26 0 100 200 300 400 500 time (hr) w t0 (° c ) observed gep ann anfisa 12 13 14 15 16 17 18 0 100 200 300 400 500 time (hr) w t1 (° c ) observed gep ann anfisb 12 13 14 15 0 100 200 300 400 500 time (hr) w t2 (° c ) observed gep ann anfisc 11 12 13 0 100 200 300 400 500 time (hr) w t3 (° c ) observed gep ann anfisd figure 8. time series plots of observed and predicted values of hourly water temperature in different depths. table 8. mathematical expressions of gep model. d10 saeed samadianfard, honeyeh kazemi, ozgur kisi and wen-cheng liu as a conclusion, output results showed that gep provided reasonable and moderately accurate trends for the prediction of hourly water temperature in different depths. 4. conclusion in the present study, the performance of some soft computing techniques, namely gene expression programming, adaptive neuro fuzzy inference system and artificial neural network to predict hourly water temperatures at different layers of the yuan-yang lake in north-central taiwan has been compared. a time-series set of data for the hourly water temperature at different measured depths from may 1 to june 11, 2008 was taken as training dataset, while the data measured from june 12 to june 30, 2008 were served as a validation dataset for the studied models. the relative performances of these models were comprehensively evaluated using various statistical indices including rmse, mae and r coefficients. results showed that the gep had the best performances in predicting hourly water temperatures at the surface and both 2 and 3 meter depths below water surface, whereas, a different trend was seen for the 1 meter depth below water surface. in this depth, in spite of smaller error in ann than gep, there is an upper bound in scatter plot of ann that imposes a constant value, which is not appropriate for predictive purposes. conclusively, results obtained from this study showed that gep can provide reasonable trends for the 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temperature of lake taihu. american geophysical union, fall meeting. the ordovician carbonatites of the maigaiti slope have formed the conditions for the development of large oil and gas fields with karst reservoirs. this study systematically analyzed the isotopic characteristics of carbon, oxygen, and strontium and enrichment trace elements regularity to examine the geochemical features of the paleo-karstification products, various periods of paleo-karstification, and paleoenvironmental conditions of the ordovician. affected by terrigenous clasts, the lianglitage formation is composed of limestone with a relatively high 87sr/86sr value and a frequently fluctuating sea level, which exhibits an overall increasing-to-decreasing trend variation. of all samples affected by the atmospheric freshwater leaching effect, δ18o values were negative, and δ13c values varied greatly toward both ends. the fissure/cave infills were rich in fe and mn but poor in sr, and the sr/ba value was considerably less than 1, which confirms the existence of ancient weathered crust and the development of an atmospheric freshwater karst environment. four different paleo-karstification periods were identified according to the carbon and oxygen isotopic characteristics of calcite. combined with the trace element characteristics of the infills, the paleo-karstification in the yingshan formation was subdivided based on three hydrological environment conditions. erosion modification of buried acid compaction-released water from late-period corrosion pores, caves, and fissures formed by syndiagenetic paleo-karstification and weathered crust bare paleo-karstification due to atmospheric freshwater leaching can significantly improve the reserving and permeability characteristics of the karst reservoir to develop a large paleokarst reservoir. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): l1l10 abstract keywords: ordovician paleokarst, paleoenvironment, carbon–oxygen–strontium isotope, trace elements, maigaiti slope, tarim basin issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54142 g e o c h e m is t r y qingyu zhang1,2,3, liang bin2,3, qin fengrui 2,3*, cao jianwen1,2,3, dan yong2,3, hao yanzhen2,3, li jingrui2,3, muhammad aqeel ashraf4 1school of environmental studies, china university of geosciences, wuhan 430074, china; 2institute of karst geology, chinese academy of geological sciences∕karst dynamics laboratory, mlr & gzar, guilin 541004; 3international research center on karst under the auspices of unesco, guilin, guangxi 541004 4 faculty of science and natural resources, university malaysia sabah 88400 kota kinabalu, sabah, malaysia * corresponding author: qin fengrui, email: qinfengrui@karst.ac.cn geochemical characteristics and paleoenvironmental significance of the ordovician paleokarst reservoir in the maigaiti slope of tarim basin record manuscript received: 12/11/2015 accepted for publication: 05/02/2016 how to cite item qingyu, z., bin, l., fengrui, q., jianwen, c., yong, d., yanzhen, h., jingrui, l., & ashraf, m.a. (2016). geochemical characteristics and paleoenvironmental significance of the ordovician paleokarst reservoir in the maigaiti slope of tarim basin. earth sciences research journal, 20(1), l1-l10. doi: http://dx.doi.org/10.15446/esrj.v20n1.54142 1 introduction paleo-karstification is a transformative process of soluble rocks by the action of ancient surface water and groundwater and the sum of resulting surface and underground geologic phenomena (wang and alaasm, 2002; wen et al., 2014). in domestic and global carbonatite rock strata, reservoirs formed by the result of paleo-karstification are very common and often develop large to very large oil and gas fields (yang et al., 2011) that contain rich oil and gas resources (kerans, 1988; guo, 1993; loucks, 1999; chen et al., 2004). paleokarst carbonatite reservoirs are widely distributed in china. with the continuously expanding application of new technology and new methods for petroleum exploration, a large number of paleokarst reservoirs have been found in the tarim (zhang et al., 2004; wu et al., 2012), qaidam (feng et al., 2013), ordos (xia et al., 2007), and sichuan basins (zhang et al., 2011). these findings have led to the development of various mediumand large-scale oil and gas fields such as the tahe, lunnan, changqing, and weiyuan. this outlook indicates that the paleokarst carbonatite reservoirs in china are highly promising with significant exploration potential (jin et al., 2006). therefore, expanded research into paleokarst reservoirs in marine oil and gas exploration is crucial (jia and cai, 2004; liu, 2007; su et al., 2010). in recent years, researchers have applied geochemical characteristics in paleokarst reservoir research and have greatly promoted global research on paleokarst and diagenesis fluids (shields et al., 2003; veizer et al., 1999; liu et al., 2012; yan et al., 2005; huang et al., 2008). the maigaiti slope is located in the northern slope zone of the southwestern depression of the tarim basin, where oil and gas exploration remains at a relatively low level. only the bashituopu oilfield was found in the western maigaiti slope (liu et al., 2004). nevertheless, a series of ordovician marine carbonatite oil and gas fields has been discovered in the northern and central paleohigh areas of the tarim basin (kang, 2007; zhou, 2010). therefore, the discovery of replacement areas with a similar paleohigh background is of great importance in the exploration and development of oil and gas fields (shi et al., 2014). previous research has indicated that the maigaiti slope and its marginal area developed paleohighs and formed multiple sets of reservoircap combinations, leading to migration and accumulation of hydrocarbons during various periods (li, 2009; zhao, 2012; tian, 2013). however, this exploration has not achieved a breakthrough in the past 20 years. also, early research and exploration of the maigaiti slope were focused on the carboniferous system as a major target layer (zheng, 1995; he et al, 1999; zhou et al., 2006); yang et al., 2007). no studies on the formation l2 qingyu zhang, liang bin, qin fengrui, cao jianwen, dan yong, hao yanzhen, li jingrui and muhammad aqeel ashraf figure 2. distribution map of the maigaiti slope and the periphery of the ordovician weathered crust the northwest-banded distribution of weathered crust in the upper ordovician lianglitage formation is developed south of the upper ordovician sangtamu formation mudstone pinch-out line. the strata are mainly based on the platform facies of mud and limestone, and the thickness of the biological debris is more than 400 m. the maigaiti slope is based on the weathered crust of the middle and lower ordovician yingshan formations. the northeast locality exhibits the weathered crust of the lianglitage formation. the western yingshan formation is based on the open platform of the low-energy micrite development phase and has a lithological density. the eastern yingshan formation’s top surface mainly presents high-energy particle and dust-grain limestone beach facies, mainly based on a thick layer of sparry calcarenite and micrite interbeds. 1 introduction paleo-karstification is a transformative process of soluble rocks by the action of ancient surface water and groundwater and the sum of resulting surface and underground geologic phenomena (wang and al-aasm, 2002; wen et al., 2014). in domestic and global carbonatite rock strata, reservoirs formed by the result of paleo-karstification are very common and often develop large to very large oil and gas fields (yang et al., 2011) that contain rich oil and gas resources (kerans, 1988; guo, 1993; loucks, 1999; chen et al., 2004). paleokarst carbonatite reservoirs are widely distributed in china. with the continuously expanding application of new technology and new methods for petroleum exploration, a large number of paleokarst reservoirs have been found in the tarim (zhang et al., 2004; wu et al., 2012), qaidam (feng et al., 2013), ordos (xia et al., 2007), and sichuan basins (zhang et al., 2011). these findings have led to the development of various mediumand large-scale oil and gas fields such as the tahe, lunnan, changqing, and weiyuan. this outlook indicates that the paleokarst carbonatite reservoirs in china are highly promising with significant exploration potential (jin et al., 2006). therefore, expanded research into paleokarst reservoirs in marine oil and gas exploration is crucial (jia and cai, 2004; liu, 2007; su et al., 2010). in recent years, researchers have applied geochemical characteristics in paleokarst reservoir research and have greatly promoted global research on paleokarst and diagenesis fluids (shields et al., 2003; veizer et al., 1999; liu et al., 2012; yan et al., 2005; huang et al., 2008). the maigaiti slope is located in the northern slope zone of the southwestern depression of the tarim basin, where oil and gas exploration remains at a relatively low level. only the bashituopu oilfield was found in the western maigaiti slope (liu et al., 2004). nevertheless, a series of ordovician marine carbonatite oil and gas fields has been discovered in the northern and central paleohigh areas of the tarim basin (kang, 2007; zhou, 2010). therefore, the discovery of replacement areas with a similar paleohigh background is of great importance in the exploration and development of oil and gas fields (shi et al., 2014). previous research has indicated that the maigaiti slope and its marginal area developed paleohighs and formed multiple sets of reservoir-cap combinations, leading to migration and accumulation of hydrocarbons during various periods (li, 2009; zhao, 2012; tian, 2013). however, this exploration has not achieved a breakthrough in the past 20 years. also, early research and exploration of the maigaiti slope were focused on the carboniferous system as a major target layer (zheng, 1995; he et al, 1999; zhou et al., 2006); yang et al., 2007). no studies on the formation and evolutionary mechanisms of the deep paleokarst reservoirs in the ordovician weathered crust have been reported, which has resulted in a lack of knowledge of this area’s changing patterns. by using ordovician carbonatite rocks and paleokarst fissure/cave-filling calcite as study subjects in addition to carbon–oxygen and strontium isotopes and trace element techniques, this study attempts to examine the development of paleokarst reservoirs and the larger paleoenvironment to provide geochemical evidence for the exploration and development of karst reservoirs. 2 study area and methods 2.1 study area the maigaiti slope is located in the southern bachu uplift in the tarim basin (figure 1) and extends in a northwest–southeast direction with an area of approximately 5 × 104 km2. the maigaiti slope is a foreland slope that was developed after multistage tectonic movement in a foreland basin southwest of the tarim basin with a regional southward-dipping shape. the current maigaiti slope has an overall structure of large-scale broad and gentle slopes inclined to the south. oil and gas resources in the foreland basin southwest of the tarim basin are abundant (jin et al., 2006), although progress has been limited due to the difficulty in exploration. thus far, the bashituopu oil/gas field with the carboniferous system as the production layer in the west of the maigaiti slope and the hetianhe gas field with the ordovician carbonatite rock-containing buried hill and carboniferous system as the production layer in the mazhatage tectonic belt on the northern border of the eastern maigaiti slope have been identified. figure 1. location map of the maigaiti slope in the tarim basin well-drilling exploration has revealed the absence of the middle ordovician yijianfang and upper ordovician tumuxiuke formations in the central tarim uplift between the mid–lower ordovician yingshan and the upper ordovician lianglitage formations of the bachu uplift (yang et al., 2011). besides, the development of a large area of yingshan formation weathered crust and m2 near the maigaiti slope was drilled within the set of stratigraphic unconformity. the south bachu area west of the sh2 well and wells to the south of the bachu county-gd3 well all lacked upper ordovician sangtamu formation mudstone, and the lianglitage formation was missing up to the south of the j1 well-k2 well-s1 well of the maigaiti slope. the yingshan formation emerged on the top of the ordovician development in the northwest to the southwest of the paleo uplift, and an extensive distribution of the weathered crust of ordovician carbonatite rocks is noted. the development zone is about 430 km long and about 210 km wide, and the area is about 9.1 × 104 km2 (figure 2). l3geochemical characteristics and paleoenvironmental significance of the ordovician paleokarst reservoir in the maigaiti slope of tarim basin this layer becomes thinner to the south of the paleo uplift and includes two pieces of the yingshan formation lacking in the previous zone. the penglaiba formation dolomite and calcite-dolomite are exposed (figure 3). (a) m401 well, o3l 1, 2242.0 m, sparry bioclastic limestone, transmission of single polarization, 10 × 2.5; (b) k1 well, o3l 3, 3283.56 m, mud micrite, transmission orthogonal polarization, 10 × 10; (c) ln1 well, o3l 5, 4728 m, microcrystalline sand–dust bioclastic limestone, transmission of single polarization, 10 × 10; (d) m2 well, o1-2y2, 2377.5 m, powder fine-grained dolostone, transmission orthogonal polarization, 10 × 10; (e) m4 well, o1-2y1, 2021.6 m, microcrystalline limestone, transmission orthogonal polarization, 10 × 10; (f) k2 well, 3821.95 m, o1-2y3, fine-grain dolostone transmission orthogonal polarization, 10 × 2.5. figure 3. petrological characteristics of ordovician carbonatite rocks of maigaiti slope 2.2. samples and experiment the trace element sampling layer is the lower middle ordovician yingshan formation (o1–2y), from which a total of 37 rock-seam and karst cave infill pieces were collected including 12 pieces of seam-hole bedrock (limestone), 12 pieces of mechanical sand–shale infills, 8 pieces of chemical calcite infills, and 5 pieces of karst cave in karst breccia (limestone breccia). testing and analysis were completed in the karst geological resources and environmental supervision and inspection center of the ministry of land and resources by using axios x-ray fluorescence spectrometry to conduct measurements of trace elements in the sample. the measurement instrument is 2000 dv with a detection limit of 0.001%, an error of 0.002%, and measurement according to the y/t05-1996 inductively coupled plasma (icp) spectrum method. in the analysis of carbon and oxygen isotopes, nine samples of the ordovician bedrock drilling core and 45 samples of the paleokarst cave-filling calcite were used. testing and analysis were completed in the karst geological resources and environmental supervision and inspection center of the ministry of land and resources. the accuracy of analysis adopted the chinese national carbonate standards (gbw04405, gbw04406, gbw04416, and gbw04417) for correction and the level of carbon isotopes in carbonatite (nbs18) for monitoring. the standard deviation of δ13c and δ18o was 0.005‰ and 0.07‰, respectively. the test results are given in the pee dee belemnite (pdb) standard; the test temperature and humidity were 25 °c and 60%, respectively. in the sr isotope analysis, 21 samples were obtained from the m2 well of the lianglitage formation of the liang second section to the liang fifth section (o3l2–5) bedrock and 2 were obtained from the lower yingshan formation (o1–2y) bedrock; both are composed of limestone. the tests were performed in the wuhan geological survey center’s isotope superclean laboratory of the china geological survey. the instrument used is a mat261 thermal ionization mass spectrometer with a testing standard obtained from a standard sample of the national bureau of standards of the united states. the measured value error (2σ) of 87sr/86sr was less than 0.006%. the chemical process adopted the general international standard, and separation and purification were performed by using a cation-exchange column following that performed in the sample solution. the entire process blank was <5 × 10–10g, the laboratory temperature was 22 °c, and the humidity was 50%. sample collection of ordovician limestone and the paleokarst seamhole filling calcite was performed at a buried depth range of 2023.32 – 4698.30 m in the yingshan (o1–2y) and lianglitage formation (o3l) strata, and the sampling wells were distributed in the entire zone. a limited number of drilling cores was available because the exploration zone is new. samples were collected on three separate occasions. 3 results and analysis 3.1 geochemical characteristics of sr isotope because of the unique nature of the sr isotope, it is widely applied in such fields as marine stratigraphic dating (dingle et al., 1997; denison et al., 1998), global geological event comparisons (crame et al., 1999; jiang et al., 2001), diagenesis fluid research (huang et al., 2004; roger et al., 2004), and paleoclimatic and ancient marine environmental analyses (huang et al., 2002; shi et al., 2005; liu and ji 2008). however, the application of the sr isotope in the study of paleokarst reservoirs is a new concept. few reports on this topic are available (wen 2014; zheng et al. 2008); only diagenesis effects appear in published reports that applied the trace elements of sr and 87sr/86sr values (montanez, 1994; banner, 1995; heydari, 2003). the sr isotope analysis results (table 1) in 23 samples of the m2 well in the lianglitage formation marine carbonatite rocks showed that the 87sr/86sr sample value was within the range of 0.70851–0.70913; the average value was 0.70877, and the average error was around 4.71 × 10-5. because of old-tonew stratum transitioning, the sr isotope ratio displayed an overall decreasing trend (table 1, figure 4), and its value was within the range of the ordovician 87sr/86sr (0.7078–0.7092) from shields et al. (2003) of brachiopod shell animal fossils and the ratio range obtained by burke et al. (1982) and veizer et al. (1999). the 87sr/86sr of this sample was higher than global normal of the marine carbonatite rocks in the late ordovician, which was 0.7078–0.7082 (denison et al., 1998; mcarthur et al., 2001). however, the 87sr/86sr value reported by liu et al. (2012) in six wells in the central tarim area lianglitage formation was 0.708136–0.719936, and the average value was 0.7099. the 87sr/86sr value of the lianglitage formation of bachu area reported by zhao et al. (2013) was 0.708032–0.708759, and the average value was 0.7099. the late ordovician 87sr/86sr average value of the central tarim gz4 well obtained by huang et al. (2006) was 0.709173. the late ordovician sr isotopic ratio of most areas of the tarim basin is larger than the global maximum range of normal sr isotopes of marine carbonatite rocks, at 0.7082. this result is relevant to the higher 87sr/86sr ratio of seawater caused by the lianglitage formation of terrigenous material entering the basin during the deposition period (zheng et al., 2008). table 1. analysis results of the sr isotope of carbonatite rocks in 23 samples obtained from the study area. l4 qingyu zhang, liang bin, qin fengrui, cao jianwen, dan yong, hao yanzhen, li jingrui and muhammad aqeel ashraf average value was −8.74‰. these values are similar to those of the carbon and oxygen isotope samples in tahe oilfield ordovician karst cave-filling, in which the average values of δ13c and δ18o from 21 data points were −0.807‰ and −9.14‰, respectively (liu et al., 2008). these results exhibited no significant differences from those of the central tarim area tz12 wells, in which the average values of δ13c and δ18o from four data points were −1.08‰ and −7.05‰, respectively (jiang, 2002). moreover, they are similar to the average values of δ13c and δ18o in the eastern lungu area, which from 25 data points were −1.40‰ and −10.74‰, respectively (zhang, 2015). the phenomenon of individual points shifting to the two ends was noted. the δ18o values in carbonatite bedrock were between −7.57‰ and −5.94‰, and the δ13c values were from −0.86‰ to 0.56‰. 3.3 geochemical characteristics of trace elements the test results (table 3) show that the average range of sr content in all types of bedrock and karst cave-filling calcite and karst breccia was between 132.564 × 10-6 and 183.683 × 10-6, and the average content of sr in karst cavefilling sand–shale was 77.219 × 10-6. overall, the sr content values were less than the lower limit of the sr content value (200 × 10-6) in the accurate representation of homogenized seawater samples proposed by derry et al. (1989). the average sr content value that best represents seawater micrite was only 183.683 × 10-6, which indicates leaching and dissolving of carbonatite rock by meteoric water. the mean value of fe content (1232 × 10-6 – 5651.8 × 10-6) in the karst cavefilling materials such as karst breccia, sand–mud rocks, and filling calcite was larger than that in bedrock (385 × 10-6 – 669.9 × 10-6). in addition, the mn element content in the karst cave-filling calcite and karst breccia was higher than that in the bedrock, indicating that they were formed through complete open meteoric water karstification conditions (huang et al., 2006; huang et al., 2008) because the respective contents of sr and mn are lower and higher in meteoric water than those in seawater (walter et al., 2000; huang et al., 2008). also, under oxidizing conditions, fe and mn in high states would be leached and filled by meteoric water in the matrix between the karst breccia fragments, and their contents would be very high (wen, 2014). because sr and ba are alkaline earth metals with similar chemical properties, they were separated owing to their geochemical behaviors in various environments. usually, terrigenous sediments are relatively rich in ba and poor in sr, whereas marine sediments are relatively rich in sr and poor in ba. thus, researchers have analyzed the changes in carbonatite rock salinity during the formation process by using the sr/ba ratio (he et al., 2004; huang, 1990; tao et al., 2009). further, a sr/ba ratio greater than 1 indicates a marine sedimentary environment, whereas that less than 1 indicates a freshwater sedimentary environment. the sr/ba ratios for the three types of infills in the ordovician karst cave in the markit slope were considerably less than 1. this confirms that their formations are related to fresh water and proves the existence of ancient weathered crust karstification (li et al., 2009). also, multivariate statistical methods used to handle large amounts of data can summarize the trace element data according to the relationships of quantity, behavior, and characteristics and can provide a series of trends to draw a reasonable inference (fan et al., 1999). correlation analysis was conducted on cao and sio2, which reflected the sand–shale rock composition and trace elements (table 4). the correlation coefficient of cao and sio2 was −0.991, illustrating that both were highly irrelevant; those of sio2, p, cr, ba, co, and ni were greater than 0.8, indicating that similar to sio2, the trace elements of p, cr, ba, co, and ni were enriched in sandstone and mudstone. the correlation coefficients of zn, pb, and cd as trace elements were larger than 0.8, although the negative correlation was noted with p, cr, ba, co, and ni. these results imply that the enrichment mechanism of zn, cd, and pb differed from that of sio2. the mechanical filling in sand–shale was compared with that of bedrock. the former had a high combination of trace elements including p, cr, ba, co, and ni, indicating that its source was mainly exposed or shallow burial conditions in which sand–shale rich in p, cr, ba, co, and ni trace elements was weathered when flowing water entered the karst cave space. however, the cu content in sand–shale reached 16.291 possibly owing to the burial conditions. the contents of heavy metal trace elements that easily migrated within compaction-released water or hot water in the karst cave was significantly higher than that in bedrock, even forming compounds with s. the correlation of calcite and bedrock was 3.2 geochemical characteristics of carbon and oxygen isotopes the geochemical characteristics of carbon and oxygen isotopes in the paleokarst have great significance in analysis of the origin and forming conditions of carbonatite sedimentary environment, diagenesis, and cementation filling (zhang et al. 2011). different types of karst encountered different fluidic environments during the formation process; thus, the use of a geochemical method could better identify and distinguish various types of corrosion. the carbon and oxygen isotopic characteristics of carbonatite rocks and karst cavefilling calcite in markit slope ordovician are shown in table 2. table 2. carbon and oxygen isotopic characteristics of karst cave-filling calcite and bedrock. numerous test results using pdb as the standard showed that most modern marine inorganic carbonatite δ13c values are close to 0‰ (liu et al., 2004); the δ18o values are also close to 0‰ (gu, 2000). according to the sample test results (table 2, figure 3), the δ13c and δ18o of carbonatite karst cave filling in the markit slope ordovician deviated significantly from these values. the alteration span was large; the δ13c (pdb) values were from 3.06‰ to −5.02‰, and the average value was −0.3‰; the δ18o values were between −4.56‰ and −15.53‰, and the l5geochemical characteristics and paleoenvironmental significance of the ordovician paleokarst reservoir in the maigaiti slope of tarim basin se is m o l o g y relatively high, although the calcite was rich in mn and poor in sr. this indicates that the calcite formed in a meteoric water environment. the comparison between collapse breccia and bedrock revealed that although higher contents of p, cr, ba, co, and ni occurred in the collapse breccia, they were lower than those in the sand–shale filling. the average value of sr was lower than that in the bedrock and higher than that in the sand– shale filling, which reflects the cave development mode of cementation of collapsed bedrock roof by calcium shale. table 3. trace element contents of various rock types and filling materials. table 4. analysis of correlation between different elements 4. discussion 4.1. environmental and geological significance of sr isotope the sr isotope ratio is a sensitive indicator of sea level change. a geological period including a rise and fall of the sea level would cause changes in the sedimentary environment that are recorded in sedimentary carbonatite rocks in the same period; thus, changes in the sr isotopic ratio in a rock can reflect the geological period of sea level settlement and ancient marine environment characteristics (li and guan, 2001). with an increase in sea level, the weathering rate is reduced, surface runoff decreases, and surface rock weathering and denudation decreases. moreover, a relatively small amount of sr isotopes enter the oceans, resulting in a decrease in the marine sr isotope ratio. however, a decrease in sea level leads to an increase in the marine sr isotope ratio. therefore, sea-level fluctuation and the 87sr/86sr value change is negatively correlated (yan et al., 2005). l6 qingyu zhang, liang bin, qin fengrui, cao jianwen, dan yong, hao yanzhen, li jingrui and muhammad aqeel ashraf figure 4. comparison of sea level changes among the markit and bachu, keping, laurentia, and baltica continents. in their global sr isotope study, mcarthur et al. (2001) used locally weighted scatterplot smoothing (lowess) to establish a 0–509 ma sr isotope fitting curve; their results created a complete sr isotope evolution curves at that time (huang et al., 2004) (figure 5a). the sr isotopic evolution curve in the late ordovician lianglitage formation sedimentary period was established by combining the sr isotope data of bachu reported by zhao, 2013 and that of central tarim reported by liu et al. (2012) (figure 5b). the sr isotope ratios determined from the global curve fitting in the late ordovician (455 × 106a – 445 × 106a) seawater displayed a monotonically decreasing trend with forwarding time whereas after 445 × 106a, the 87sr/86sr ratio generally presented an upward curve trend, which had good consistency with the deposition from the sea level change of the lianglitage formation. this result reflects that the sea level change of the markit slope had a relatively good correlation with bachu (zhao, 2013), central tarim (liu et al., 2012), laurentia (ross and ross, 1995), and the baltoscandian paleocontinent in the late ordovician (figure 5). the isotope test results of m2 well carbonatite rock sr (figure 4) indicated many high 87sr/86sr values in the fifth liang section at 1935– 1960 m. the highest value, 0.70913, occurred in the middle of this section and may correspond to the closure of the southern tianshan ocean in this period and the entry of large amounts of terrigenous detrital materials (zheng et al., 2008). between the end of the fifth liang section (o3l5) and the fourth liang section (o3l4), sea levels underwent a slight decline and then increased at a relatively rapid rate, marked by the minimum 87sr/86sr ratio of 0.70851. this process may have been influenced by the keping uplift, which offsets the sea level rise to a certain extent (zhao, 2013). between the fourth liang section (o3l4) and the third liang section (o3l3), the sedimentary environment changed to foreslope facies. the lithology was varied from nuclear-shaped limestone to marl and micritic limestone, and the stratigraphic thickness changed from thin to thick layers. moreover, the 87sr/86sr values reached a low value of 0.70856, and the sea level rose significantly. during the period of the second liang depositions (o3l 2), sea level began to decline gradually, and the sedimentary environment changed to platform sediment with an open platform. figure 5. evolution trend of sr isotopes in the ordovician marine strata and evolution curves of sr isotopes in the late ordovician lianglitage formation. for the carbon isotopes, carbon is stored in two large carbon pools including organic carbon (carbon reduction) and inorganic carbon (carbon dioxide). the average value difference of δ13c between these two is about 25%. therefore, of the factors affecting carbon isotopes in marine carbonatite rocks, oxidation and relative burial volume of organic carbon was the most important at that time (huang, 1997; wang et al., 2009). when the relative burial volume of organic carbon was higher than the amount of oxidation, more 12c entered the organic carbon, which made the δ13c value of the marine carbonatite rocks in the same period move toward the positive direction. also, the increasing salinity of the deposition in the water medium would have caused increases in carbonatite δ13c (clayton and degens, 1959; keith and weber, 1964); atmospheric precipitation and terrestrial freshwater injection would have caused δ13c value decreases. oxygen isotopes are greatly influenced by the temperature and the concentration of the medium. moreover, the δ18o value will significantly increase with an increase in water media salinity in the sedimentary environment and a strengthening in evaporation (clayton and degens, 1959; keith and weber, 1964). the δ18o values of calcite and bedrock fill in the markit slope karst cave had negative characteristics such that the span of δ13c values was significant. this result is related to the ordovician surface uplift, the overall open environment, meteoric water eluviation (land, 1980; rosen et al., 1989; gasparrini et al., 2006) of widespread δ12c, and all δ16o of atmospheric co3 2-sources (veizer et al., 1999; azmy et al., 2009). the relationship diagram of δ13c–δ18o (figure 6) indicates that the filling materials in the markit slope ordovician carbonatite fracture cave can be divided into four classes indicating differences in formation’s environment (table 5). l7geochemical characteristics and paleoenvironmental significance of the ordovician paleokarst reservoir in the maigaiti slope of tarim basin figure 6. the cross-diagram of carbon and oxygen isotopes for filling materials in the markit slope ordovician karst cave system. table 5. response characteristics of karst development stage and filling environment class i is the marine environment during the syndiagenetic stage, which reflects the isotope characteristics of early paleo-karstification environment filling. the δ18o of the karst cave calcite was −9.28 ‰ to −5.76‰, and the δ13c was −1.60‰ to −1.07‰. the distribution ranges were similar to those of the limestone background value area. individual points drifted toward both ends, indicating that the oxygen and carbon isotope compositions of the fractures and cave calcite were controlled mainly by δ13c and δ18o in the original rock, followed by subsequent freshwater leaching and transformation (land et al., 1980; rosen et al., 1989). the values for some points of the δ13c displayed negative bias. class ii is weathered crust exposed in a karst freshwater filling environment. the δ18o values of the filling calcite were −11.24‰ to −6.21‰, and δ13c was between −5.02‰ and −2.06‰. owing to the influence of fresh atmospheric water (gasparrini et al., 2006), the δ13c values were apparently smaller than the bedrock carbon isotope values, which were formed in the open environment. class iii is a shallow buried karst environment. the δ18o values of the karst cave filling calcite were between −9.95‰ and −4.56‰, and the δ13c values were from 1.58‰ to 3.06‰. the δ13c showed obvious positive bias likely caused by decomposition and methanation (ch4) of organic matter caused by methane bacteria. ch4 formed from biochemical processes was rich in d12c, whereas the co2 had abundant d13c. because co2 released during the reaction of methane bacteria fermentation participated in the process of karstification and sedimentary filling, the sedimentary filling was rich in d13c and presented higher positive values. class iv is a deep-burial high-temperature environment. the δ18o in calcite was between −15.53‰ and −9.97‰, and the δ18o values were obviously negatively biased, reflecting the clear relationship between the formation of calcite filling and hydrothermal activity. owing to the influence of heat loss in the transformation of deep δ18o in the burial stage (tritlla et al,, 2001; lavoie and chi, 2006), the δ18o values presented negative bias. also, the diagenesis stage was a period of organic matter thermal maturation. a large amount of organic acid-based compaction-released water caused dissolution and alteration in the carbonatite rocks and early-stage fracture calcite. moreover, it led to organic carbon injection, which resulted in δ13c loss and negative bias (boni et al., 2000; azmy et al., 2009). 4.3 geoenvironmental significance of trace elements the element content in a mineral is attributed mainly to the concentration and kinetic effect of that element in a precipitation solution (huang, 1990). because seawater is an environment of low mn and fe, the contents of these elements are far less than those in fresh water. according to the characteristics of trace elements in all types of bedrock and infill examined in this study, paleokarst fractured reservoirs in the markit slope ordovician yingshan formation can be divided into the following three paleoenvironment hydrological conditions: (1) active oxidizability freshwater environment in the seepage-active hyporheic zone. during the karst process, surface clay or bauxite enters the underground karst fissure caves with meteoric water, which results in enrichment of fe, k, ti, p, cr, ba, co, and ni trace elements in the sandstone and mudstone in the cave. also, cave collapsing breccia retains the characteristics of trace elements in the bedrock, although shale calcium cementation retains traces of the meteoric effect. (2) subcurrent stagnant freshwater environment. the calcite precipitates easily owing to the low flowability of the fresh water. because the main components originate from the bedrock, they are affected by meteoric water, resulting in loss of sr, and enrichment of fe and mn. (3) burial environment. through changes in temperature and pressure, compaction-released water is formed and flows. this fact leads to migration and depletion of elements, particularly in the enrichment of heavy metals such as zn, cd, pb, and cu due to the involvement of s in the chemical reactions. 5 conclusions (1) markit slope is located in the hetian paleo uplift, which belongs to three large paleo uplifts in the tarim basin. exposed ordovician carbonatite weathered crust is widely distributed and is particularly developed in penglaiba formation (o1p), in which a cave-type karst carbonatite reservoir layer is developed. this area is ideal for the exploration and development of oil and gas in the tarim basin. (2) the limestone in upper ordovician lianglitage formation has high 87sr/86sr values, which shows good consistency with the 87sr/86sr values of the keping and central tarim areas in the tarim basin of the same period. the caledonian movement, in which a lot of clastic terrigenous was generated, might be the main reason for the increase in 87sr/86sr values. during the sedimentary period of the lianglitage formation, sea level fluctuation was frequent. however, an overall trend of increase followed by decrease was identified, which has good correlation with global sea level change. (3) the δ18o values of filling calcite and bedrock in the ordovician karst cave indicate negative bias. the δ13c values vary significantly at both ends, which presents an obvious deviation from the δ13c and δ18o values of normal marine carbonatite rock. however, these values are similar to those in the tarim river, central tarim, and the ancient regions. four different formation environments are indicated: class i, a marine environment during the syndiagenesis stage; class ii, exposed weathered crust karst freshwater filling environment; class iii, shallow buried karst environment; and class iv, deep-burial high-temperature environment. (4) the analysis results of trace element for weathered crust bedrock and filling materials in the ordovician yingshan formation (o1-2y) showed that fissure/ cave infills are enriched in fe, and mn and are depleted in sr, which means that that filling materials were formed in a freshwater environment with low salinity. moreover, the sr/ba ratio was markedly less than 1, confirming the existence of ancient weathered crust and the development of freshwater karst. according to the correlation analysis of trace elements in the bedrock and karst fissure/cave infills, the paleokarst system in the yingshan formation can be subdivided into three hydrological conditions of paleoenvironment: an active oxidizability freshwater environment in a seepage-active hyporheic zone; a sub-current stagnant freshwater environment; and a burial in a hydrothermal water environment. l8 qingyu zhang, liang bin, qin fengrui, cao jianwen, dan yong, hao yanzhen, li jingrui and muhammad aqeel ashraf acknowledgments this research was funded by the national natural science foundation project (41302122), national key basic research development (973 program) project (2011cb201001), and public welfare industry scientific special research (201211082) in ministry of land and resources. the authors express sincere thanks to professor wenqing pan of the petrochina tarim oilfield exploration and development research institute for helpful discussions about paleokarst fracture hole filling in a paleoenvironment and to the reviewers and editors who offered valuable advice for this paper. references: azmy, k., knight, i., lavoie, d. and chi, g. 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(2010). advances and problems in hydrocarbon exploration in the tazhong area, tarim basin (in chinese with english abstract). petroleum science, 7(2), 164-178. in order of abundance, the meta-sedimentary rocks along pos selim highway in perak state malaysia comprise quartz mica schist, graphitic schist and quartzite layers. field investigations revealed that these meta-sedimentary rocks have gradational weathering profile based on differences particularly in textures, hardness, lateral changes in colour, and consistency of material extension. the results from uniaxial compressive strength tests confirmed field observations whereby failure occurred mostly on outcrops having joints almost perpendicular to foliation. from the kinematic analyses, the investigated cut slopes are unstable with possibilities of wedge and planar failures. application of rock mass classification schemes including rock quality designation (rqd) and rock mass rating (rmr) yielded almost similar poor to good quality ranges for each investigated rock mass. while slope mass rating (smr) classified the cut slopes from stable to unstable slopes, this study categorized them into one actively unstable, four marginally stable and five stable slopes. en orden de abundancia, las rocas metasedimentarias a lo largo de la carretera pos selim, en el estado perak de malasia, se componen de esquistos de cuarzo mica, esquistos de grafito y capas de cuarzo. las investigaciones de campo revelan que estas rocas metasedimentarias tienen perfiles de meteorización progresiva basados en diferencias particulares como textura, dureza, cambios laterales de color y consistencia del material de extensión. los resultados de los ensayos uniaxiales de esfuerzo de compresión confirmaron las observaciones de campo por las cuales se estableció que las fallas ocurrieron mayormente en los afloramientos con coyunturas perpendiculares hacia la foliación. de los análisis cinemáticos se desprende que los taludes de corte investigados son inestables con posibilidades de fallas planas y de cuña. la utilización de esquemas de clasificación rocosa como el índice rqd (del inglés rock quality designation) y la clasificación geomecánica de bienawski o rmr (del inglés rock mass rating) evidencia rangos similares de baja y buena calidad para cada masa rocosa estudiada. mientras que el índice de taludes smr (del inglés slope mass rating) clasificó los taludes de corte de estables a inestables, este estudio los categorizó de uno activamente inestable, cuatro marginalmente estables y cinco estables. abstract resumen keywords: weathering, uniaxial compressive strength, slope stability, rock quality designation, rock mass rating, slope mass rating. palabras clave: meteorización, esfuerzo de compresión uniaxial, estabilidad de talud, designación cualitativa de roca, clasificación geomecánica de bienawski, índice de smr. classification of cut slopes in weathered meta-sedimentary bedrocks record manuscript received: 30/08/2015 accepted for publication: 04/03/2016 how to cite item nkpadobi, j. i., raj, j. k., & ng, t. t. (2016). classification of cut slopes in weathered meta-sedimentary bedrocks. earth sciences research journal, 20(2), j1-j9. doi:http://dx.doi.org/10.15446/esrj.v20n2.52781 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.52781 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ng earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : j1 j9. clasificación de taludes de corte en sustratos rocosos metasedimentarios meteorizados g e o t h e c n ic s j2 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ng 1. introduction information gathered from field investigations by geologists and engineers is insufficient to predict geotechnical behaviour of rocks and rock masses, which calls for laboratory investigations to ascertain the response of rocks under a wide variety of disturbances. the uniform definition of geotechnical engineering by murthy (2002), venkatramaiah (2006), and das and sobhan (2013) is that it deals with the application of the principles of soil and rock mechanics to the design of foundations, retaining structures, and earth structures. in this investigated area, previous studies by mohd asbi & associates (2005) and andrew malone ltd (2007) classified the stability of the cut slopes using kinematic analyses. to ensure further understanding of the characterization of the slopes, this study carried out field observations, laboratory and computational analyses of the geotechnical properties of the rocks and then utilized the recorded and derived data for rock mass and slope mass classifications. 2. geological setting of investigated area the study area is along pos selim highway in perak state malaysia. it traverses through the titiwangsa main range mountainous terrain reaching highest altitude of 1587m above sea level at gunung pass and terminating at 1420m above sea level at perak/pahang states boarder. this study begins at latitude 4° 33’ 44” and longitude 101° 18’ 86” extending to 17.7km and terminating at latitude 4° 35’ 95” and longitude 101° 20’ 80”. granite and schist are the two major formations covering 60% and 40% of the area respectively (fig. 1). this investigation focused on the schist, which according to tajul (2003) might probably be of upper palaeozoic. in order of abundance the schist consists of quartz mica schist, graphitic schist and quartzite layers. petrographic analyses by mohd asbi & associates (2005), andrew malone ltd (2007), nkpadobi (2014) and nkpadobi et al. (2015) classified the schist as quartz-mica schist, graphitic schist, phyllite and quartzite, and photomicrographs confirm that most of the quartz mica schist are mylonitized. the major identified failure types in the area include planar sliding, wedge failure, and complex failures characterized by slump and earth flow which occurred along the gunung pass axis. the geometrical properties of failed slopes are shown in table 1. however, the plausible triggering factors for these different types of failures varied. figs. 2(a) (c) represent these planar, wedge and complex failures in the area respectively. figure 1. geological map of the study area. table 1. geometrical properties of failed slopes. figure 2. (a) planar failure. (b) wedge failure. (c) complex failure. 2.1 description of weathering profile over quartz mica schist since the failures in quartz mica schist occurred in the weathered zones, the characteristics of these weathered earth materials were studied in order to interpret the features of the morphological zones. the photograph and schematic diagram of weathering profile of location 010 shown in fig. 3 is typical of quartz mica schist units. employing bsi (1981) code of practice for site investigations, the weathering profile was graded. by identifying the lateral changes in colour, texture, hardness, and consistency of material extension, the weathering profiles over the quartz mica schist are broadly differentiated into three zones; pedological soil zone, intermediate zone and bedrock zone corresponding to zones i, ii and iii respectively. based on differences particularly in textures and structures of original bedrock as well as degree of preservation of the constituent minerals, these three broad zones are further differentiated into thinner characteristic horizons. j3classification of cut slopes in weathered meta-sedimentary bedrocks figure 3. field photograph and schematic diagram of weathering profile over quartz mica schist at location 010. zone i with vertical thickness of less than 8m which corresponds to grade 6 is further sub-divided into ia, ib, and ic . horizons ia and ib are brownish to reddish brown pedological soil profile, whereby ia which is less than 1m is friable sandy clay, whereas ib which is less than 2m is firm sandy clay. horizon ic is completely weathered, stiff yellowish brown sandy clay devoid of any distinct discontinuity plane. greater attention is focused on zone ii because of its extensive thickness, complexity, and easy accessibility. with vertical thickness measuring up to 72m, this zone ii comprises about 24% vertical thickness of higher horizon iia (corresponding to grade 5) of highly weathered brownish unit devoid of distinct discontinuity plane, but with intercalations of relicts of moderately weathered units. the lower horizon iib is about 55% vertical thickness of the entire zone ii, comprising moderately weathered grayish coloured quartz mica schist. it corresponds to grade 4, exhibits conspicuous quartzite veins, and distinct joint and foliation planes, but indistinct fault plane. horizon iic is the lowest horizon of zone ii, and it corresponds to grade 3. with vertical thickness of about 21% of the entire zone ii, this slightly weathered dark gray unit comprises distinct relict discontinuity planes and unweathered core-boulders which are very prominent. zone iii which is schematically represented only by morphological zone iiia is unweathered bedrock which experiences the effect of weathering only along and between structural discontinuity planes. 3. materials and methodology 3.1 measurement of rock mass jointing to ensure adequate representation of the joint parameters, the joint orientations were recorded using scanline and random survey techniques. joint spacing and persistence were measured with ruler and tape while the aperture was measured with caliper. according to palmström (2005), joint frequency is defined as the number of joints per meter length, and this was calculated as the inverse of joint spacing. the variations of joint spacing and frequency of joint sets are shown in table 2. the average volumetric joint count jv was calculated as the sum of the average frequency of the joint sets while field recording of the conditions of the joints include their hydraulic conditions, material infill and roughness. the general characterization of the rock mass jointing for the investigated slopes is presented in table 3 and these data shall be applied in rock mass and slope mass classifications. table 2. summary of spacing and frequency of individual joint sets. j4 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ngj4 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ng table 3. general characteristics of the rock mass jointing. 3.2 kinematic stability assessment kinematic stability assessment method was applied in order to identify critical planes of weakness in the rock slopes and outline potential danger and likely modes of failure. measurements of discontinuities were carried out with the use of both scanline and random survey techniques. the slope face orientation was also measured. these analyses were based on markland’s test as described in hoek and bray (1981). pole intensity greater than 4% was regarded as a major discontinuity and assumed friction angle along the discontinuities in the rocks is averaged at 25°. table 4 shows the summary of the orientations of the major discontinuity planes and slope face orientations. table 4. summary of the orientations of slope locations and major discontinuities. 3.3 uniaxial compressive strength test very large rock blocks were collected in the field with use of sledge hammer and chisel for this very test, and the laboratory determination of the strength of the rock samples was carried out using isrm (1981a) standard. as shown in fig. 4(a), the rock samples were prepared into right circular cylinders ranging from 12.5 to 15cm in height and 5cm in diameter using diamond embedded rock core drilling machine while fig. 4(b) shows the set up of this test. the uniaxial compressive strength of the samples was then determined under dry condition which is their ultimate strength. figure 4. (a) some of the rock core samples. (b) set up of uniaxial compressive strength test. 4. results and discussions 4.1 uniaxial compressive strength (ucs) it was observed that the tested samples failed axially following joint trends and fissures, almost perpendicular to foliation. this confirmed field observations whereby failure occurred mostly on outcrops having joints almost perpendicular to foliation. although some metamorphic rocks tested by horino and ellickson (1970) and broch (1974) showed that uniaxial compressive strength of the rocks are almost the same at different orientations of foliation, but in these tested quartz mica schist samples, there are significant differences in the strength in different orientations of the well developed foliations. where the orientation of the foliation to the horizontal is less than 45˚ in some quartz mica schist samples, failure occurred along the foliation plane. several tests showed consistency of failure modes in graphitic schist, whereby majority of the failures occurred along orientations of foliations at 45˚ with minimal oblique joint trends. the determined uniaxial compressive strength of quartz mica schist samples shown in table 5 ranges from 56mpa to 117mpa, while graphitic schist has value of 87mpa. according to classification of the uniaxial compressive strength of rocks by deere and miller (1966), bieniawski (1978), isrm (1978 and 1981b), the rocks of the study area fall within medium to high strength. this gives insight into the observed failure mechanisms whereby most of the failures were controlled by discontinuities. j5classification of cut slopes in weathered meta-sedimentary bedrocks j5classification of cut slopes in weathered meta-sedimentary bedrocks j5 table 5. uniaxial compressive strength (ucs) and rock quality designation (rqd). 4.2 rock mass classification rock mass classification has been developing over the years and various classification schemes have considered a lot of factors such as water content, discontinuities and rock strength. in this study, the considered rock mass classification schemes include rock quality designation (rqd) and rock mass rating (rmr). 4.2.1 rock quality designation (rqd) rock quality designation (rqd) index was developed by deere (1963). it is defined as the percentage of intact rock mass length that are 10cm or longer from borehole drill cores. in absence of drill core logs but where discontinuity traces are visible in rock surface exposures, palmström (1982) suggested that rock quality designation (rqd) might be estimated from the number of discontinuities per unit volume using below equation: rqd = 115-3.3jv (1) where jv known as the volumetric joint count is the sum of the number of joins per unit length for all joint sets. using already determined values of jv and applying equation 1, the rqd values for the rock masses are presented in table 5. according to deere (1968) relationship between rqd and the engineering quality of rock mass, the rqd values determined in this work range from 34.5% to 90% covering a broad range of poor, fair and good quality rocks. in view of non-consideration of joint orientation, joint condition, type of joint, infilling and stress condition by rqd index, singh and goel (1999) advised that its sole consideration for classification is insufficient to provide adequate description of a rock mass. 4.2.2 rock mass rating (rmr) rock mass rating (rmr) or geomechanics classification was initially developed by bieniawski (1976). the system has evolved due to a better understanding of the importance of the different parameters and increased experience leading to changes to the ratings of parameters. as a result of these advancements, the bieniawski (1989) version was employed in this study. this scheme uses six parameters: uniaxial compressive strength of rock material, rock quality designation (rqd), spacing of discontinuities, condition of discontinuities, groundwater conditions and orientation of discontinuities. as explained in bieniawski (1989), estimation of rmr is the sum of the total ratings of each of the above listed six parameters. considering the dip angles of the joints in these high cut slopes, -5 rating adjustment for discontinuity orientations was used. as presented in table 6, the discontinuity condition is the sum of joint persistence, aperture, roughness, infilling and weathering ratings. this rmr scheme has five rock mass classes determined from total ratings ranging from very poor rock to very good rock. in order to determine the rmr for the investigated extended cut slope, the already determined values of the six parameters were substituted with their individual ratings and the rock masses range from fair to good rocks. the estimation of the rmr of the investigated rock masses are presented in table 7. unlike in rqd index where quartz mica schist at location 004 yielded highest value of 90%, in this rmr only quartz mica schist at locations 010 and 014 and graphitic schist at location 015 yielded highest value of 62 and designated good rocks. although the remaining cut slopes are in quartz mica schist and designated fair rocks, the quartz mica schist at locations 008 and 009 maintained the lowest rmr values of 43 and 44 respectively. the advantage of rock mass classification using rmr is that it incorporates geological, geometric and engineering parameters in arriving at a quantitative value of the rock mass quality. table 6. determination of condition of discontinuities. j6 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ng table 7. rock mass rating (rmr) of investigated rock masses. 4.3 slope stability analysis of the rock cuts the kinematic analyses of the discontinuity sets revealed the possibility of both wedge and planar failures. at location 004, analysis shows intersection of j2 and j3 along 267˚/27˚ line orientation. four intersections within the critical zone were recorded at location 005; j2 and j3 intersected along 280˚/34˚, j3 and j4 intersected along 265˚/30˚, j2 and j4 intersected along 266˚/46˚, and j3 and j5 intersected along 136˚/54˚ line orientations. location 006 recorded three intersections; j1 and j4 intersected along 49˚/29˚, j1 and j3 intersected along 128˚/37˚, while j1 and j2 intersected along 155˚/27˚. at location 007, only one intersection between j1 and j3 along 53˚/31˚ was recorded. the same single intersection was also recorded at location 008 between j1 and j3 along 58˚/32˚. j1 and j3 intersected along 242˚/54˚ at location 009. there was also only one intersection between j2 and j4 at location 010 along 48˚/65˚. only j3 and j4 intersected along 262˚/56˚ at location 011, while j1 and j3 intersected along 222˚/61˚ at location 014. analysis on the graphitic schist at location 015 yielded only one intersection between j3 and j4 having orientation of 326˚/28˚ line of intersection. as shown in fig. 5(a), there are possibilities of only wedge failures at locations 004, 010, 011, 014 and 015, while fig. 5(b) shows possibilities of wedge and planar failures at locations 005, 006, 007, 008 and 009. based on the kinematic analysis of these discontinuity sets, it is anticipated that all cut slopes in this investigated area are unstable. figure 5a. stereographic projections showing possibilities of wedge failures. j7classification of cut slopes in weathered meta-sedimentary bedrocks figure 5b. stereographic projections showing possibilities of wedge and planar failures. 4.4 classification of the rock cut slopes it may be useful to visualize slopes as existing in one of the following three stages: i. stable the margin of stability is sufficiently high to withstand all destabilizing forces. ii. marginally stable likely to fail at some time in response to destabilizing forces reaching a certain level of activity. ii. actively unstable slopes where destabilizing forces produce continuous or intermittent movements. slope mass rating (smr) proposed by romana (1985) is a method to access the stability of both natural and cut slopes. according to romana (1993) and romana et al. (2003), smr is obtained from rmr of bieniawski (1989) as shown in equation 2 by adding a factorial adjustment factor which depends on the relative orientation of joint and slope and by adding another factor depending on the method of slope excavation. rating adjustment for discontinuity orientations in rmr is not considered and this basic rmr is designated rmrb. smr = rmrb + (f1.f2.f3) + f4 (2) whereby f1 ranges from 1.0 to 1.5 and depends on the parallelism between discontinuity and it fits into the relationship: f1 = (1-sin a)² (3) and a = the angle between the dip directions of the slope and joint. f2 depends on the joint dip angle (βj). for toppling failure, this parameter maintains 1.0 value and f2 thereby fits into the relationship: f2 = (tan βj)² (4) f3 depends on the relationship between dips of slope (βs) and joint (βj). f4 is a correction factor that depends on the excavation method used and it is fixed empirically. in table 8, p represents plane failure, αs represents slope dip direction, while αj represents joint dip direction and t represents toppling failure. as proposed by romana (1993), the adjustment ratings of these factors are presented in table 8 while the classes of smr are presented in table 9. the smr was carried out using the independent joint sets from each investigated cut slope and finally applying equation 2. by analyzing the joint sets as shown in table 10, smr classified the stability of slopes as completely stable, stable, partially stable, unstable and completely stable. using these five categories of smr stability by romana (1993), this study categorized the slopes as existing in three stages: stable, marginally stable and actively unstable as presented in table 10. table 8. adjustment rating of f1, f2, f3 for joints and method of slope excavation (romana, 1993). j8 johnbosco ikenna nkpadobi, john kuna raj and tham fatt ng table 9. classification of slope according to smr (romana, 1993). table 10. slope mass rating of the investigated cut slopes. j9classification of cut slopes in weathered meta-sedimentary bedrocks 5. conclusion the field and laboratory tests, computation of results and schematic representations were aimed at classifying the stability of the investigated cut slopes. from the uniaxial compressive strength tests, it was observed that the combination of joints and foliations induced structurally controlled failures in the tested samples. rqd and rmr schemes utilized for rock mass classification yielded almost similar poor to good quality ranges for each investigated rock mass. while the rqd values range from 34.5% to 90% covering a broad range of poor, fair and good quality rocks, rmr yielded values from 43 to 62 which cover only fair and good quality rocks. rmr applied in this study is very useful as a tool for the preliminary assessment of slope stability whereby it classified seven of the slopes cut into fair rocks and only three slopes cut into good rocks. although based on the kinematic analyses carried out in this study, it is anticipated that the entire cut slopes are unstable with possibilities of wedge and planar failures, smr gave comparative stability classes among these cut slopes, categorizing them actively unstable, marginally stable and stable slopes. considering the determined unstable slopes, there is need for the use of expensive and wider in-situ and laboratory testing equipment in order to reasonably predict the stability of these cut slopes before any further infrastructural development in the area is carried out. acknowledgement special thanks to university of malaya for sponsoring this research through postgraduate research fund, grant number: ps362/2010b. references andrew malone ltd. 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(2013.) principles of geotechnical engineering. 8th edition. cengage learning, stamford, ct 06902, usa, 726 pp. deere, d. u. (1963). technical description of rock cores for engineering purposes. rock mechanics and rock engineering, 1, 16-22. deere, d. u. (1968). geological considerations. in: rock mechanics in: engineering practice. r.g. stage and d.c. zienkiewicz eds. wiley. new york, 1-20. deere, d. u. & miller r.p. (1966). engineering classification and index properties of rock. technical report no. afnl-tr-65-116. albuquerque, nm: air force weapons laboratory. hoek, e. & bray, j. w. (1981). rock slope engineering. the institute of mining and metallurgy, london, england, 358 pp. horino, f. g. & ellickson, m. l. 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(1985). new adjustment ratings for application of bieniawski classification to slopes. proceedings of international symposium on role of rock mechanics in excavations for mining and civil works. isrm, mexico city, 59-68. romana, m. r. (1993). a geomechanical classification for slopes: slope mass rating. in: comprehensive rock engineering, v. 3, ch. 23 (hudson, j. d., ed.) pergamon press, oxford, 575-599. romana, m., serón, j. b. & montalar, e. (2003). smr geomechanics classification: application, experience and validation isrm– technology roadmap for rock mechanics. south african institute of mining and metallurgy, 1-4. singh, b. & goel, r. k. (1999). rock mass classification: a practical approach in civil engineering. elsevier science. 282 pp. tajul, a. j. (2003). engineering geological assessment and slope failures along the pos selim cameron highland highway. seminar penyelidikan jangka pendek (vot f), 11th-12th march 2003, university of malaya. venkatramaiah, c. (2006). geotechnical engineering. revised edition. new age international, 926 pp. 151 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 151 158 titaniferous magnetite and barite from the san gregorio de polanco dike swarm, paraná magmatic province, uruguay rossana muzio1, fernando scaglia1 and henri masquelin1 1instituto de ciencias geológicas, facultad de ciencias, iguá 4225, cp 11400, montevideo, uruguay fax number: (00598) 2525 8617 ore deposits the san gregorio de polanco dike swarm (tacuarembó department, uruguay) is the southernmost set of dikes in the paraná magmatic province of uruguay. five major dikes have been identified with two main structural trends: n140º–n170º and n50º–n80º. the dikes have tholeiitic affinities and are composed of plagioclase (an55), augite and augite-pigeonite, relicts of olivine and opaque minerals. these rocks have high contents of fe–ti oxides (titanomagnetites), the mineralogical and textural characteristics of which have been studied using scanning electron microscopy and energy dispersive spectrometry techniques (sem – eds). these features, along with other mineralogical and textural relationships, have been used to propose the following crystallization sequence for the dikes: (i) crystallization of olivine, plagioclase and ca-rich pyroxene phenocrysts; (ii) precipitation of the first population of ti-magnetite; (iii) crystallization of plagioclase and pyroxene in the groundmass; (iv) partial dissolution of ti-magnetite by reaction with magmatic fluids; (v) crystallization of the second population of ti-magnetite and finally, (vi) crystallization of interstitial barite. el haz de diques de san gregorio de polanco (departamento de tacuarembó, uruguay) es la ocurrencia más meridional de diques pertenecientes a la provincia magmática paraná en uruguay. fueron identificados cinco 5 diques principales con dos direcciones estructurales principales: n140º n170º y n50º n80º, respectivamente. son diques de afinidad tholeítica compuestos por plagioclasa (an55), augita y augita-pigeonita, relictos de olivina y minerales opacos. estos diques se caracterizan por el alto contenido de óxidos de fe y ti (titanomagnetitas), cuyas características mineralógicas y texturales fueron estudiadas con microscopio electrónico de barrido y espectrometría de energía dispersiva (sem-eds), incluyendo mapeos composicionales. estas características junto con otras relaciones mineralógico-texturales presentes en estas rocas permitieron proponer la siguiente secuencia de cristalización: (i) cristalización de fenocristales de olivina, plagioclasa y piroxenos cálcicos; (ii) precipitación de una primera población de titanomagnetita, (iii) cristalización de plagioclasa y piroxenos conformando la matriz; (iv) disolución parcial de la primera población de titanomagnetitas por reacción con fluidos magmáticos; (v) cristalización de la segunda población de titanomagnetitas y finalmente, (vi) cristalización de barita intersticial. palabras clave: titanomagnetita, barita, diques máficos, mesozoico, uruguay record manuscript received: 22/10/2013 accepted for publication: 01/11/2013 abstract resumen key words: titanomagnetite, barite, mafic dikes, mesozoic, uruguay introduction the paraná magmatic province (pmp) is located in central south america, and it is one of the largest igneous provinces in the world (figure 1a). it is mainly composed of flood basalts, dike swarms and sills, and other intrusive bodies related to the breakup of gondwana and the opening of the south atlantic ocean (peate 1997). the age of this magmatism has been constrained to 138–120 ma, with most ages between 132–129 ma (renne et al., 1992; 1996; turner et al., 1994; stewart et al., 1996; deckart et al., 1998). concomitant dike swarms (132–131 ma; féraud et al., 1999) distributed across the province are interpreted to be potential feeders of the sills and lava flows (bellieni et al., 1984; peate et al., 1992; turner et al., 1994), providing a view of the upper levels of the plumbing system of the pmp. in uruguay, the pmp covers an area of approximately 40,000 km2 and consists of lava flows of basaltic composition and related intrusions (figure 1b). the intrusive rocks resulting from mafic dike swarms and sills are known as the cuaró formation (preciozzi et al., 1985), and they can be grouped according to their three main areas of occurrence: melo, tacuarembó and san gregorio de polanco. the first studies of this sub-volcanic unit only involved the 152 post-permian doleritic dikes that outcrop near the city of melo (walther, 1927; 1938). féraud et al. (1999) were the first to relate these sills and dikes to the pmp, providing them with 40ar/39ar ages of approximately 132 ma. additional background information on the petrographic aspects and regional distribution of the cuaró formation can be found in bossi and shipilov (1997; 2007), bossi (2006), masquelin et al. (2009) and muzio et al. (2012). the main petrographic features of these intrusions are the presence of glomeroporphyritic textures, plagioclase phenocrysts (an55), a groundmass with plagioclase and clinopyroxene (augite – augite pigeonite) and partially iddingsitized relicts of olivine and titaniferous magnetite (muzio et al., 2012). among all of the dike swarms in the cuaró formation, the san gregorio de polanco dike swarm (sgpds) is the least-known, and it contains the southernmost mafic dikes related to the pmp in uruguay (figure 1c). detailed petrologic studies concerning the sgpds have been published recently by masquelin et al. (2009) and scaglia (2010). a remarkable mineralogical feature observed in these studies is the abundant presence (close to 10% of the modal volume) of two generations of iron-titanium oxides (scaglia, 2010), as well as evidence of hydrothermal activity (masquelin et al., 2009; scaglia, 2010). the available lithochemical data outlined by muzio et al. (2010) allow these dikes to be classified as basalts and andesitic basalts that correspond to the low-tio2 suite within the sub-alkaline tholeiitic series. based on trace-element analyses, the parental composition of the magma indicates that these dolerites are related, with at least some contribution of a subcontinental lithospheric mantle (bossi, 2006; muzio et al., 2010; 2012). this is consistent with other gondwana-related flood basalts and dikes in the province (marques and ernesto, 2004). this work is focused mainly on characterizing the mineral content in the sgpds by means of scanning electron microscopy (sem) and presenting the main mineralogical and textural features of the iron-titanium oxides, as well as their relationship with other mineral phases. geological setting and petrography of the investigated dikes the northwestern region of uruguay is geologically underlain by the southern extreme of the paraná basin, with ca 100,000 km2 of exposed surface (figure 1a) and sedimentary infill since the devonian (fúlfaro et al., 1997). the sedimentary pile, which has been identified in both outcrops and boreholes, is represented by 1) marine devonian sediments (durazno group; bossi, 1966); 2) a late carboniferous to late permian sequence composed, from base to top, of glacial sediments, deltaic to intra-cratonic marine sandstones, organic-rich claystones and aeolian sandstones (de santa ana et al., 2006); and 3) a late jurassic intra-continental sedimentary sequence composed of siltstones and aeolian sandstones (bossi et al., 1998). figure 1 a) regional distribution of the paraná basin; b) location of the study area in uruguay. references: 1 – crystalline basement; 2 – sedimentary infill of the uruguayan extension of the paraná basin; 3 – intrusions (sills) related to the paraná magmatic province (cuaró fm); 4 – early cretaceous rhyolitic magmatism; 5 – tholeiitic basalt flows of the paraná magmatic province (arapey fm); 6 late cretaceous sediments (cuenca del litoral del rio uruguay); c – study area, detailed in figure 1c; c) simplified geological framework of the san gregorio de polanco area. rossana muzio, fernando scaglia and henri masquelin 153 these units are covered by jurassic – cretaceous paraná basalt flows of the pmp, which are grouped under the name of the arapey formation (bossi, 1966) and are over 1000 m thick (de santa ana et al., 2006). later, sedimentation continued until the late cretaceous, forming alluvial and fluvial siltstones and sandstones that are locally referred to as the cuenca litoral del río uruguay (goso and perea, 2003). based on geophysical and structural geological surveys, the regional structural framework of the paraná basin is dominated by nw and ew alignments that controlled the depositional history of the basin and the associated magmatism in the early stages of gondwana break up (almeida, 1972; fúlfaro et al., 1982; hasui et al., 1993). the target area is located near the geographic center of uruguay (figure 1b and c) in the southern portion of tacuarembó department and the northern portion of durazno department (between latitude 32º24’02’’–32º51’07’’s and longitude 56º05’52’’–55º28’55’’w), bordering the southern extreme of the basin. the sgpds consists of doleritic dikes trending n140º–n160º that cross-cut devonian, lower permian and lower jurassic sediments, as well as coeval basalts of the pmp. it follows the same general direction (n100º–n140º) as the eastern paraguay and ponta grossa dike swarms, which are also part of the pmp (deckart et al., 1998; raposo and ernesto, 1995). good exposures are infrequent and occur mainly near the village of san gregorio de polanco (figure 1c). this paper focuses on 17 dike segments identified by masquelin et al. (2009) and scaglia (2010). they correspond to five major dikes classified as macro-dikes (after rickwood, 1990). their thicknesses vary from 5 to 20 m, and they are 15 to 60 km in figure 2 – photomicrographs showing the textural arrangement and the mineral content of the dikes. a and b) skeletal crystals of the first population of titanomagnetite (reflected light microscopy); c) distribution of titanomagnetite with mainly skeletal shapes (first population). d) titaniferous magnetite (small euhedral crystals of the second population) distributed in intersertal groundmass. abbreviations: timgt 1 – titanomagnetite (first population); timgt 2 – titanomagnetite (second population); gl glass. titaniferous magnetite and barite from the san gregorio de polanco dike swarm, paraná magmatic province, uruguay 154 length. field observations and thin-section descriptions of specimens from the sgpds reveal a broad petrographic similarity. they crop out as typically grey, moderately altered dolerites characterized by glomeroporphyritic textures, with clusters of plagioclase up to 5 mm long and occasional clinopyroxene (augite), set in a fine-grained groundmass. the matrix frequently grades from a medium-fine grain in the center of the dikes to aphyric near the wall contacts, depending on the dike’s thickness. no evidence of multiple intrusions has been observed in the field or in the thin sections (such as internal chilled margins), despite evidence of this process in some similar mesozoic dike swarms to the northeast near the city of melo (masquelin et al., 2009). a zigzag emplacement pattern has been observed for most of the dikes, with minor dike segments trending n110º–120º to e-w and nne-ssw. regionally, the basement fractures and major lineaments comprise two main structural trends: n140º–n170º and n50º–n80º. both are vertical to sub-vertical and affect some of the sedimentary rocks intruded by the dikes. the low dispersion of the dike segments relative to the surrounding material adjacent to the dikes suggests that the main orientation (n140º–n170º) is coincident with previous fracturing that was reactivated during the late jurassic early cretaceous extensional events (almeida, 1983; hoeck, 1994 in castaño and druguet, 2008). most of the structures found both in the host rocks and dike walls and the general orientation of the dike arrays suggest a ns to nne–ssw finite extension direction, synchronous with diking, as postulated by masquelin et al. (2009) for similar structural patterns in the northeastern part of the basin. silicified sandstones near the dike walls that were produced by contact metamorphism, as well as chilled margins, have been identified in many dike segments. hydrothermal veining is also observed, based on the presence of hematite, calcite or silica veinlets within the dikes and their host rocks. methodology the objective of this paper is to deepen petrographic studies, and the results obtained from the various analytical techniques are discussed in separate sections. analytical techniques the samples for petrographic descriptions and subsequent studies using scanning electron microscopy – energy dispersive spectrometry (sem – eds) techniques were selected from 37 outcrops (figure 1c). whenever possible, samples were collected at the center and edge of the dikes, depending on the state of weathering and fracturing. petrographic descriptions of 60 thin sections (textural arrangement and mineralogical content) were studied using a nikon eclipse – 50i polarizing microscope at the faculty of sciences, uruguay. to avoid textural or mineral relationships that could be susceptible to misinterpretation, each sample was cut in different directions in order to locate a three-dimensional view of the texture in a single thin section. the textural interpretation criteria follow those of hibbard (1995). later, 12 representative samples were analyzed and described using sem – eds (jeol 5900 low vacuum equipment at the faculty of sciences, uruguay), with particular emphasis on the opaque minerals. analytical techniques that are routine for this type of equipment (reed, 2005) were used. due to the ubiquitous presence of similar mineralogical arrangements of interest, two representative samples were selected for compositional mapping either in the middle or close to the walls of the dikes. figure 3 – (a) well-shaped titanomagnetite (first population), partially surrounding an augite phenocryst; bse image with eds spectra. (b) euhedral titanomagnetite with corroded borders, partially including an augite phenocryst; bse image. (c) skeletal titanomagnetite; bse image. (d) second population of titaniferous magnetite. abbreviations: px pyroxene; timgt titanomagnetite. rossana muzio, fernando scaglia and henri masquelin 155 petrography and sem – eds results the mineral assemblage of the groundmass consists of plagioclase (an55), clinopyroxene (augite), and relicts of olivine (iddingsitized, with celadonite also present) and opaque minerals. the presence of some interstitial glass has been observed, which causes the matrix mineral array to vary from sub-ophitic to intersertal textures (figure 2). comparison with previous petrographic descriptions in peate (1997), deckart et al. (1998), bossi and schipilov (2007) and masquelin et al. (2009) indicates that similar dolerites are common throughout the pmp. two populations of opaque minerals have been noted by scaglia (2010). sem-eds analysis shows that the opaque minerals are fe–ti oxides, with a homogeneous distribution in the matrix that corresponds to titaniferous magnetite (also known as ulvospinel) and a chemical composition of fe2tio4 (figure 3; sial and mcreath, 1984). these iron-titanium oxides represent approximately 10% of the modal mineralogical composition, and they are characterized by different petrographic features. one population occurs as euhedral crystals, with a mean size of 0.5 mm and frequently embayed borders leading to skeletal shapes. the second population consists of euhedral grains ranging in size from 0.1 to 0.3 mm. the ti – magnetite crystals from the centers of the dikes mainly have skeletal shapes, most likely as a result of dissolution processes. they have rhombic, rectangular, square and skeletal shapes, and they sometimes include pyroxene crystals. on one hand, the presence of two populations of titaniferous magnetite indicates that precipitation occurred at different stages during magmatic crystallization. on the other hand, the presence of ti-magnetite crystals along the cleavage of plagioclase and clinopyroxene phenocrysts is indicative of relatively early crystallization. the geometry of the opaque minerals described here is found both in the center of the dikes and on the borders, with no significant differences. however, the titanomagnetites in the centers of the dikes (mainly from the earlier population), are skeletal but partially preserve the original shape (rhombic, square or prismatic; figure 3a – c). this may be due to interaction and reabsorption processes between early ti-magnetite crystals and the residual magmatic fluid (hibbard, 1995). interstitial light-colored (bright white) xenomorphic crystals were also identified. further eds-bse (backscattered electrons) analyses indicate that they are barite crystals (baso 4 ), figure 4a. the barite crystals in the centers of the dikes are up to 50 µm in diameter, and they have not been identified near the dike walls. samples from the contact between the dolerite and the host rock contain sandstones of the tacuarembó formation (bossi, 1966), and they have also been analyzed to observe possible metamorphic assemblages. the petrographic studies revealed the presence of fine grains of illite between quartz and feldspar, implying a very low degree of contact metamorphism. the analyses and interpretations of sem images and x-ray spectra also indicate the penetration of magma through the host rock due to the porosity of the sandstone, which is verified by the crystallization of pyroxenes and titanomagnetite crystals in the sandstone. compositional mappings the petrographic study using sem analysis was complemented with compositional mappings (sem – bse images). these images show the chemical distribution based on the main element abundances in the analyzed section, in this case, si, ca, fe and ti. these results were used to complement the interpretation of the petrogenetic processes. two refigure 4 – (a) pyroxene and titanomagnetite crystals (second population), including barite crystals; bse image and eds spectrum. (b, c and d) interstitial barite between plagioclase and pyroxene crystals; bse images. abbreviations: ba barite; plg plagioclase; px pyroxene. titaniferous magnetite and barite from the san gregorio de polanco dike swarm, paraná magmatic province, uruguay 156 presentative samples were selected based on conventional petrography and sem studies: one corresponding to the center of the dike and one near the contact with the doleritic host rock. the compositional maps are shown in figures 5 and 6. the main mineral phases were identified in the compositional images, and the main concentration of si is observed to be distributed in plagioclase crystals, with minor amounts in olivine and pyroxene. areas with no si correspond to the ti-magnetite crystals (figure 5b). figure 5c shows a similar distribution for ca, with the highest concentrations in plagioclase and pyroxene crystals. figure 5e and f shows that the highest concentrations of ti are limited to some portions of the sample, coincident with the sites of titanomagnetites. the distribution of ti is also concordant with the distribution of fe (figure 5d). although most of the fe and ti corresponds to the fe-ti oxides, some is also present in olivine and pyroxene. the distributions of ti, fe and si observed in figure 6 (c, d and e, respectively) are similar to those in figure 6a. however, this sample corresponds to the center of the dike and has some areas that are not well defined by bse images, indicated by bright white (figure 6a). these areas correspond to high ba concentrations, described previously as barite. discussion the presence of barite, the high percentage of titanomagnetite crystals with skeletal shapes, the existence of reabsorption borders in plagioclases and olivines and the occurrence of oscillatory zoning in plagioclase phenocrysts, partially altered to sericite, are clear evidence of the concentration of fluids in the centers of the dikes, which is most likely driven by differentiated flow processes (hibbard, 1995; rickwood, 1990). this would create a “wall effect” with low permeability that causes the migration of residual fluids to the center of the dike. consequently, the high concentration of fluids in the centers of the dikes over a long time period would have allowed the reaction with early formed crystals. after a gradual decrease in temperature, the second population of titaniferous magnetite would have crystallized in the late stages of magmatic crystallization. textural variations of the dikes can also be attributed to the disparity in the rate of flow, as proposed by komar (1976). as the viscosity increased, phenocrysts would not have been displaced, but would rather have clumped together in a hypocrystalline matrix. these variations, as well as differences in phenocryst abundances, can be observed in several samples. additionally, unmixing processes affecting augite-pigeonite are common in plutonic and hypabyssal rocks (sial and mcreath, 1984). their presence as exsolution lamellae, both in the groundmass and phenocrysts, suggest the presence of metastable solid solutions under well-constrained temperature and pressure conditions. therefore, the presence of augite-pigeonite can be used as a geothermometer, indicating that unmixing processes occurred at 950º–1100ºc (brown, 1967). finally, the genesis of fe-ti oxides may have occurred as magmatic segregation, partially crystallizing figure 5 (a) compositional mapping of a dike; bse images. (b, c, e and f) distribution of the main elements, si, ca, ti and fe, in different mineral phases in the studied section. (d) distribution of fe and ti using superimposed bse images. rossana muzio, fernando scaglia and henri masquelin figure 6 – (a) bse image of the central portion of the dike. (b, c, d and e) compositional mapping of ba, ti, fe and si, respectively. abbreviations: plg plagioclase; px pyroxene; ol olivine; ba barite; timgt titanomagnetite. 157 with the other silicate phases (guilbert and park, 1986). crystallization temperatures of 590ºc and 450ºc are estimated for the first and for the second population, respectively. as noted previously, sem-eds analyses also highlight the presence of barite (baso4) as interstitial xenomorphic crystals, ranging in size from 10 to 50 µm. as crystallization continued, the residual liquid enriched in sulfate and ba, either as a product of crustal assimilation or from the contribution of hydrothermal fluids crystallizing as barite at low p – t conditions. the source of these hydrothermal fluids can either be magmatic (juvenile waters) or related to connate waters and juvenile waters (guilbert and park, 1986). according to our field data and as noted by montaño et al. (2006), these barite-bearing dikes are mainly hosted in the san gregorio formation, which is characterized by so4 -2, cl-, fand ba+2 anomalies. hence, the s content could be magmatic, in association with the groundwater contribution and/ or assimilation processes. however, hydrothermal fluids from mafic magmas usually have low ph values, but they progressively increase while the magma rises and assimilation processes occur (hibbard, 1995). these ph variations in the sgds could have been controlled by the oxidation of the organic matter available in devonian sediments (3 to 3.6% organic matter; bossi, 1966). according to pons et al. (2009), the organic content may change the ph in fluids, favoring the precipitation of dissolved metals (barite, in this case). furthermore, this ph variation in the residual fluids may have also favored the crystallization of the second population of ti-magnetite. for the samples in this study, there would have been reducing conditions with temperatures of approximately 450ºc, while the temperature would have varied between 350ºc–100ºc for barite precipitation (guilbert and park, 1986). conclusions the information obtained by sem–eds/bse analysis indicates that both populations of opaque minerals correspond to titaniferous magnetite. the mineralogical and textural features observed suggest that fluids were concentrated in the centers of the dikes. sem-eds analyses indicate the presence of interstitial xenomorphic barite crystals, which were produced by the crystallization of hydrothermal fluids. the source of these hydrothermal fluids is postulated to be either magmatic or related to the mixing of connate waters with juvenile waters. a crystallization sequence for the sgds is proposed, inferred from textural observations and mineralogical relationships: a) crystallization of olivine, plagioclase and calcium-rich pyroxene phenocrysts; b) precipitation of the first population of titaniferous magnetite at approximately 600ºc; c) crystallization of plagioclase and pyroxene from residual magmatic fluids, developing the groundmass; d) partial dissolution of the first population of titaniferous magnetite crystals (resulting in skeletal shapes) through the reaction with magmatic residual fluid; e) crystallization of the second population of titaniferous magnetite from ti present in the residual magmatic fluid, whose source was the previous dissolution of the first population of ti-magnetite; the low fugacity of oxygen and a temperature close to 450ºc would have been necessary for the second generation of titanomagnetite; and f ) crystallization of barite from hydrothermal fluids rich in s and ba. the precipitation of barite indicates stable reducing conditions and temperatures between 100ºc and 350ºc. acknowledgments we are grateful to the anii (agencia nacional de investigación e innovación) for the financial support of project fce2007 – 038. we would especially like to thank the editor and anonymous reviewers for helpful comments. references almeida, f.f. 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(1938). eruptivos afro-brasileño-argentinos de edad permo – triásico-eo-jurásico: la supuesta uniformidad del magma ‘‘melafídico’’ y sus relaciones con la parentela alcalinítica, bol. instituto geológico del uruguay. 24, 117–167. rossana muzio, fernando scaglia and henri masquelin earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016) letter from the editor as we are sure you’ve noticed, the earth sciences research journal has undergone some substantial changes this year. the hard work of the last few years has resulted in a some major steps forward for this journal, and now we want to announce our four volumes to be published since 2016 every year. our goal is to become a premier journal, and it is to this end that we work day in and day out. recently several conferences have turned to us for special issues, and we expect this to be an ongoing relationship where we are able to provide readers with novel articles. by supplying conferences, we are able to broaden the pool of scientific papers by giving those who might otherwise have been denied it the opportunity of publishing to a worldwide audience. it is not just earth sciences research journal that is changing, so too is the global petroleum market. the fluctuating price of oil and technological advancements have played a role in upturning the previous status quo. many previously unknown actors are now operating on the global stage in this new emerging situation. management of resources is in the political spotlight; energy and water use, as well as environmental concerns, have been thrust to the forefront of discussion. fracking and shale extraction, as well as well as the stretching of exploration into more and more delicate environments have caused substantial damage, and pose the risk of causing immeasurable destruction in the future. this new reality has brought a whole new focus to a segment of the scientific community, and our journal is poised to accept these new articles. we want to encourage everyone involved in the scientific research on earth sciences to submit their papers. we have substantially modernized our processing, revising all submitted works, and otherwise improving the experience for the authors involved. the review process has undergone marked update, reducing wait times while improving quality. in the future, we intend to collaborate with other organizations in the earth sciences field. in us, you will find a friendly partner with whom to collaborate and bring our field into a new age. for the volume 20, we are presenting 17 selected papers in different areas of study; original contents we hope will be of interest and use. comments, reviews, and suggestions are always welcomed in earth sciences research journal through the mail account earthjour_fcbog@unal.edu.co. thanks for reading and submitting your valuable scientific papers! carlos alberto vargas jiménez. editor in chief, earth sciences research journal le tt er f r o m t h e ed it o r the aim of this study is to present the evaluations based on comparisons of geoid heights that are computed from several global geopotential models (ggms) and the gnss/levelling data. in this application framework, differences between geoid heights obtained by ggms and gnss/levelling were computed. then, the availability of geoid heights calculated by ggms for engineering applications were investigated. the konya-polatli (ankara) express train project as a strip area project was chosen as the study area. the length of the project is approximately 210 km and consists of 110 benchmarks that belong to the turkish national triangulation network. in this study a total of 69 ggms were compared. in order to examine more detail, these models were classified as three groups based on champ, grace and goce. each group was evaluated separately and the results were obtained. according to results, the best five models were detected for geoid height differences (ngnss/lev-nggm) in terms of standard deviation. these are eigen-6c4, eigen-grace01s, egm2008, eigen-6c3stat and eigen-6c2, respectively. also, geoid heights were obtained using different parametric models. these parametric models were used in order to minimize the impact of the terms of bias, tilt etc. generally, three, four, five and seven parametric models are used for the least-squares adjustment of the geoid height differences in the literature. therefore, in this study the geoid heights were calculated for such different parametric models. after the geoid height values were computed from the parametric models, the best global geopotential models in terms of standard deviation were obtained as eigen-6c2, eigen-6c3stat, egm2008, eigen-6c4 and eigen-grace01s, respectively. el propósito de este estudio es presentar las evaluaciones comparativas de alturas geoidales que fueron computadas a partir de varios modelos geopotenciales globales (ggm, del inglés global geopotential models) y la nivelación de información del sistema global de navegación por satélite. luego se investigó la disposición para aplicaciones de ingeniería de las alturas geoidales calculadas por los modelos ggm. se seleccionó el proyecto del tren expreso konya-polatli (ankara) como el área de estudio por ser un terreno lineal. la longitud del proyecto es de 210 kilómetros y consiste de 110 puntos de referencia que pertenecen a la red de triangulación nacional de turquía. en este estudio se compararon 69 modelos ggm. para un mejor examen, estos modelos se clasificaron en tres grupos basados en champ (challenging minisatellite payload), grace (gravity recovery and climate experiment) y goce (gravity field and steady-state ocean circulation explorer). cada grupo se evaluó por separado. de acuerdo con los resultados, se detectaron los cinco modelos mejores para las diferencias de alturas geoidales (ngnss/lev-nggm) en términos de desviación estándar. estos son eigen-6c4, eigengrace01s, egm2008, eigen-6c3stat, y eigen-6c2. también se obtuvieron las alturas geoide a través de diferentes modelos paramétricos. este mecanismo se utilizo para minimizar el impacto en términos de inclinación y declive. generalmente, se utilizan tres, cuatro, cinco, y siete modelos paramétricos para el ajuste por mínimos cuadrados de las diferencias de alturas geoide, según la literatura. por lo tanto, en este estudio se calcularon las alturas geoide con estos modelos paramétricos. después de que se computaron los valores de altura geoide desde los modelos paramétricos, se obtuvieron los mejores modelos geopotenciales globales en términos de desviación estándar, estos son el eigen-6c2, eigen-6c3stat, egm2008, eigen-6c4 y eigen-grace01s, respectivamente. earth sciences research journal earth sci. res. j. vol. 20, no. 3 ( september, 2016 ) : c1 c10 abstract resumen keywords:global geopotential model, gnss/ levelling, geoid undulations, orthometric and ellipsoidal heights. palabras clave: modelo geopotencial global, nivelación gnss, ondulaciones geoidales, alturas geoide y ortométricas. an evaluation of recent global geopotential models for strip area project in turkey record manuscript received: 28/01/2016 accepted for publication: 01/07/2016 how to cite item doganalp, s. (2016). an evaluation of recent global geopotential models for strip area project in turkey. earth sciences research journal, 20(3), c1-c10. doi:http://dx.doi.org/10.15446/esrj.v20n3.55440 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n3.55440 serkan doganalp necmettin erbakan university, department of geomatics engineering, konya, turkey g e o d e sy evaluación de modelos geopotenciales globales recientes para un proyecto de área lineal en turquía 1. introduction determining the gravity field with as much accuracy as possible is significant for earth sciences such as geodesy and geophysics, and the gravity field is instrumental in many natural incidents related to earth dynamics, primarily mass transportation. in recent years, research conducted to determine the earth’s gravity field has gained speed thanks to low earth orbits (leo). various global geopotential models (ggms) have been released using in particular champ (challenging minisatellite payload), grace (gravity recovery and climate experiment) and goce (gravity field and steady-state ocean circulation explorer) orbit data. these models have a positive impact on determining geoid changes. launched from russia’s plesetsk spaceport on 15 july 2000, champ was the first low earth satellite launched in order to determine gravity field. the champ mission was carried out by the potsdam (germany) earth research center (geoforschungszentrum-gfz). the satellite is almost circular and located close to the pole, with 454 km starting height and 87.3º inclination degree. although its work period was planned to be five years, it operated until 19 september 2010. the core aims to be reached through the champ satellite are to specify long wavelength features of the statical earth’s gravity field (and partially time changes) with the greatest possible accuracy, mapping the global magnetic field, which means specifying the earth’s primary magnetic field and its time changes, and to reveal ionosphere and troposphere profiles, respectively (seeber 2003; hofmann-wellenhof and moritz 2005). champ satellite was groundbreaking in specifying long wavelength components of the gravity field. compared to grim5-s1 and egm96s models produced by several observations and satellites, it has been found that gravity field resolution is higher specified with a couple of month’s old champ orbit tracking data (reigber et al. 2003). grace mission is the continuing part of the champ satellite. unlike champ, it consists of two identical satellites following each other on the same orbit with 220 km ± 50 km distance between them. both satellites were launched simultaneously from the russian plesetsk spaceport on 17 march, 2002. grace mission is a product of a joint project of dlr (deutsches zentrum für luftund raumfahrt) and nasa (u.s. national aeronautics and space administration). university of texas, center for space research (csr) was assigned as the project principal. like champ, data are needed that are homogeneously distributed and surrounding the earth entirely, in order to obtain a sensitive projection of gravity potential. therefore, the grace mission is close to the pole and almost circular. initial orbit height was selected at around 500 km, and the orbit tendency 89º. aim of grace mission is to determine the earth’s high resolution global gravity field and time changes in this field. these satellites are also used in order to map total electron content (tec) as in the champ satellite mission (hofmann-wellenhof and moritz 2005; jäggi 2007). resolution of the gravity field based on grace data is better than the previous ones. according to tapley et al. (2004), it was found that improvements in the global gravity field were significant compared to ggm01s earth gravity model egm96 determined by 110-day grace data. goce mission is the core project of the living planet programme of the european space agency (esa). goce satellite is the last in the satellite series launched to determine gravity field. orbit of goce satellite was launched from the russian plesetsk spaceport on 17 march 2009 as almost circular and solar-synchronized. orbit height was selected as 250 km in order to get a stronger and more sensitive gravity signal. monitoring and controlling the satellite were carried out by esa/esoc in collaboration with earth stations kiruna in sweden, and svalbard in norway. main objective of goce is to participate in measuring the earth’s gravity field and modelling the geoid with perfect accuracy and spatial resolution. anticipated accuracies are determining gravity-field anomalies with 1mgal (10-5 ms-2), geoid with 1-2 cm accuracy, and reaching a spatial grid resolutions better than 100 km. many researchers have investigated the contribution of ggms produced from champ, grace, and goce satellites in local and global geoid models for various areas of the earth (amos and featherstone 2003; kiamehr and sjöberg 2005; rodriguez-caderot et al. 2006; benahmed dahoa et al. 2006; ustun and demirel 2006; erol et al. 2009; kotsakis and katsambalos 2010; yilmaz et al. 2010; janak and pitonak 2011; gikas et al. 2013; godah and krynski 2013; soycan 2014). various methods and approaches have been suggested as a result of the above-mentioned research. one of the most frequently employed methods in the literature to determine the best ggm for an area’s gravity field is comparing ggms and revealing their performance through independent datasets (gnss/ levelling, gravity etc). in this research, the geoid undulation values are calculated through ggms (69 ggms at the time of writing this paper) for a 210km long strip project. then these values are compared to accurate geoid undulation values calculated with gnss/levelling method. as a result, performance of ggms is revealed statistically and the best ggm is suggested. moreover, investigation of possible maximum geoid accuracy to be obtained from ggms is aimed at in this research employing only geographical latitude and longitude information of points without any geodetic measuring in the field. 2. theoretical background 2.1. global geopotential models (ggms) and gnss/levelling the sum of centrifugal and gravity forces on an object is defined as gravity force. determining the earth’s gravity field is the same as determining its potential. as this potential is harmonical out of masses that form the earth, spherical harmonic series are generally used for determining the gravity field (kaula 1966; heiskanen and moritz 1984; rummel et al. 2002; seeber 2003; hofmann-wellenhof and moritz 2005). global geopotential models (ggms) are defined as spherical harmonic coefficients representing the earth’s gravity field in various wavelengths. these coefficients are obtained from satellite orbit deviation analyses, satellite altimeter data, gravity gradiometer data, and gravimeter data. ggms are divided into three groups in this context (vaniček and featherstone 1998; featherstone 2002). • satellite-only models: coefficients of these ggms are derived from orbit deviation analyses of artificial earth satellites. degree of these models is low, and resolution is insufficient. • combined models: the combined ggms are produced by combining gravity data derived from satellite data, terrestrial gravity observations, airborne gravimetry and satellite altimeter data in marine areas. therefore, integrated models have higher degrees and produce more accurate results compared to satellite-only models. • tailored models: these models are generated as a result of improving harmonic coefficients of ggms using special mathematical techniques within the first and second groups. the main aim of these models is to increase the degree of the model. improving measurement, calculation and evaluation techniques have contributed to constant improvement of ggms. this improvement causes enhanced geoid resolution. in other words, when the degree of ggms increased, errors of the deflection of the plumb line, gravity anomaly, and height anomaly are decreased, and the better the determination of the geoid (wenzel 1998). ggms point out the gravity field of the earth with spherical harmonic series. according to this the geoid height of a point the spherical geocentric coordinates of which are known might be calculated with eq. (1) (hofmann-wellenhof and moritz 2005). 1 serkan doganalpc2 an evaluation of recent global geopotential models for strip area project in turkey c3 2 3 here n stands for geoid height (undulation), h for ellipsoidal height obtained from gnss measurements, and h for orthometric height calculated after levelling measurements. to evaluate the quality of ggm-derived geoid undulation values (nggm) many independent ngnss/lev values could be used that are spread over the project area. therefore, absolute geoid height differences ∆n might be calculated with eq. (3) (kotsakis and sideris 1999; fotopoulos et al. 2003; huang and véronneau 2004; gikas et al. 2013). 3. applications 3.1. study area and data study field is the high-speed train route between konya-polatli (ankara), which is 210 km long. there are 110 gnss/levelling points (benchmarks) within the project field. orthometric heights (h) of the points were determined with the geometric levelling method in the datum of the turkey national vertical network (tudka). the geographic coordinates including ellipsoidal heights (h) were determined in static positioning mode and referred to the turkish national fundamental gps network (tutga). the orthometric and ellipsoidal height of the points within the study field are varying between 696 – 1198 m and 733 – 1234 m respectively. also, the geoid heights are varying between 36.72 and 37.70 m (fig. 1). the minimum, maximum and mean values of distance between sequential points of the route were observed as 1.05, 5.53 and 1.94 km respectively. srtm-3 data and the generic mapping tools (gmt) software were used to draw figure 1 (wessel and smith 1998). the shuttle radar topography mission (srtm) contains elevation data with 3 arcsecond resolution and 16 m absolute height error (90 percent confidence level). these data are available with free of charge via the internet for approximately 80 percent of the earth’s land mass (bildirici et al. 2009). fig. 1. study area, konya-polatli express train project 3.2 evaluation procedure a total of 69 models were used to investigate the quality of ggms. in order to display the results from those models clearly, ggms are classified in three models derived from champ, grace, and goce satellites. spherical harmonic coefficients of all models were obtained from the international centre for global earth models (icgem) web page. in addition, geoid heights based on ggms can be calculated using coefficients and software on this website. within the scope of implementation, in order to include all of turkey, separate grid networks (grid step is 0.1o x 0.1o) were established for each ggm. grs80 elipsoid parameters were used to establish grids. a correction needs to be applied on the difference between the true potential of the geoid and the normal potential of grs80 when calculating geoid height, when using grs80 as the chosen reference field. the reason for this correction is the difference between the gm value estimated for earth and the gm value for grs80 elipsoid. therefore, the “zero-degree” term based on different gm values needs to be considered in geoid height computation (smith 1998). in addition, while the shape of the geoid depends on the permanent tide system type (mean, zero and tide-free), the values of true gravity potential do not change when one system transform another system. therefore, the tide-free geoid model is suggested to be used in conversion of the gnss heights (smith 1998). thus, correction term (zero degree term) has been applied to all calculations within the context of this application, and tide-free model is used as the tide system. in each of the ggm grid nets established, geoid heights of project points are interpolated using a script. where r, θ, λ are the spherical geocentric coordinates of the computation point: radial distance, co-latitude and longitude, respectively, gm is the gravitational constant (g) times mass (m) of the earth, γ is mean normal gravity of the reference ellipsoid, r is the mean earth’s equatorial radius, cnm' snm are fully normalized geopotential coefficients with degree n and order m, pnm(cos θ) fully normalized associated legendre functions, and nmax is the maximum degree of the ggm. in order to determine the optimum ggm for an area’s gravity field, geoid heights derived from the gnss/levelling method could be compared to the geoid heights calculated by ggms (amos and featherstone 2003; kılıçoğlu et al. 2009; yilmaz et al. 2010; hirt 2011; guimarães et al. 2012). in many gnss applications users feel a need for transformation between ellipsoidal height and orthometric height. the main reason of this is using orthometric heights in engineering, determined by levelling, and the difficulty of making levelling measurements for each point. highly precision geoid models are needed for this type of implementation. geoid height (undulation) values produced based on gnss/levelling measurements are necessary to determine sensitively to this end (doganalp and selvi 2015). according to the gnss/levelling method, geoid undulation value at a point is calculated with eq. (2) (heiskanen and moritz 1984). serkan doganalpc4 ggms based on champ in this section, the results on ggms for 110 points of the study area produced based on champ are addressed. totally 17 champ-based global geopotential models are used. names, production years, degrees and the data used to produce them are given in table 1. geoid undulations within study area are estimated using these models. the statistics of the differences between the gnss-based and the ggm-based geoid undulations are given in table 2 and figure 2. table 1. global geopotential models based on champ (s: satellite, g: gravity, a: altimetry) table 2. statistics of geoid height differences between ggm-derived (nggm) and gnss/levelling (ngnss/lev) data, units in meters (based on champ) fig. 2. the standard deviations of geoid undulation differences between gnss/levelling and ggms (based on champ) table 2 and figure 2 show the standard deviation values range between 0.28 – 1.03 m with regard to geoid undulation differences. moreover, the best five models for champ-based ggms are aiub-champ03s, eigencg01c, eigen-cg03c, eigen-champ05s and ulux_champ2013s, respectively. ggms based on grace in this section, 24 global geopotential model evaluations have been done using grace-based models. information on ggms is given in table 3. performance of ggms determined after evaluation of 110 points within the project field is shown in table 4. moreover, the diagram displaying the differences between geoid heights calculated by ggms and the known heights of gnss/levelling points is given in figure 3. table 3. global geopotential models based on grace (s: satellite, g: gravity, a: altimetry) table 4. statistics of geoid height differences between ggm-derived (nggm) and gnss/levelling (ngnss/lev) data, units in meters (based on grace) an evaluation of recent global geopotential models for strip area project in turkey c5 fig. 3. the standard deviations of geoid undulation differences between gnss/levelling and ggms (based on grace) table 4 and figure 3 show the standard deviation values range between 0.13 – 1.24 m according to geoid undulation differences. lower standard deviation values are reached compared to champ-based models. in addition, the best first five models for grace-based ggms are eigengrace01s, egm2008, gif48, aiub-grace01s and eigen-51c, respectively. ggms based on goce in this section, 28 global geopotential models are evaluated based on the goce satellite. information on goce-based ggms is given in table 5. performance of ggms within the study area is shown in table 6. moreover, the diagram displaying the standard deviation of differences between geoid heights calculated by ggms, and the known heights of gnss/levelling points is given in figure 4. table 5. global geopotential models based on goce (s: satellite, g: gravity, a: altimetry) table 6. statistics of geoid height differences between ggm-derived (nggm) and gnss/levelling (ngnss/lev) data, units in meters (based on goce) fig. 4. the standard deviations of geoid undulation differences between gnss/levelling and ggms (based on goce) table 6 and figure 4 show the standard deviation values range between 0.12 – 0.49 m with regard to geoid undulation differences. these values might be considered good compared to champ and grace-based models. moreover, the best first five models for goce-based ggms are eigen-6c4, eigen-6c3stat, eigen-6c2, eigen-6c and go_cons_gcf_2_tim_r5, respectively. best ggms in the study area in order to summarize the results of all the implementations, five ggms obtained from implementations conducted in three classes are evaluated. table 7 contains the best five ggms according to geoid undulation differences within every class. figure 5 shows the performance of ggms using the standard deviation values calculated in table 7. table 7. statistics of geoid height differences between ggm-derived (nggm) and gnss/levelling (ngnss/lev) data, units in meters. serkan doganalpc6 fig. 5. the standard deviations of geoid undulation differences between gnss/levelling and ggms table 7 and figure 5 show the best ggm for the study area is eigen6c4. this model is followed by eigen-grace01s, egm2008, eigen-6c3stat and eigen-6c2, respectively. considering the performance of these models, there is no large difference between them with regard to standard deviation values. in addition to this, general characteristics of models show that all the models except eigen-grace01s have a considerable degree, and gravity and altimeter data make positive contributions to the model (table 8). differences between gnss/levelling-based geoid heights and ggmbased geoid heights are influenced by datum inconsistencies and systematic errors. therefore, different parametric models could be used in order to minimize these values (bias and tilt) (kotsakis and katsambalos 2010; yilmaz et al. 2010). generally, three, four, five and seven parametric models are used for the least-squares adjustment of the geoid height differences in the literature. therefore, these parametric models were tested in this paper. table 8. the best five global geopotential models in the study area (s: satellite, g: gravity, a: altimetry) geoid heights determination using different parametric models systematic errors such as datum shifts and distortion among height systems might be minimized using the least squares adjustment (lsa) model. if eq. (3) is edited with this aim 4 10 11 12 9 is obtained (kotsakis and sideris 1999; kiamehr and sjöberg 2005). here ai stands for known coefficients vector, x unknown parameters vector, and νi a residual random noise term. four different parametric models (3, 4, 5 and 7-parameters) are used within the context of application. mathematical statements for these models are as below, respectively (kotsakis and katsambalos 2010). where xi transformation parameters between two datums, vi residual random noise term, (φi, λi) geodetic coordinates of points, e first eccentricity of the reference elipsoid, and . the matrix form is created as follows for the equations where ai is the coefficients vector and x is unknown’s vector for model 4. for other parametric models, coefficients and unknowns vector might be formed as in eq. (9). if the formed coefficients are expanded for each gnss point, a coefficients matrix (design matrix) would be obtained to be used in lsa (eq. 10). afterwards, unknown parameters can be solved with lsa. table 9 and figure 6 show the statistical values obtained with 3, 4, 5, and 7-parameter lsa implementation of the best five ggms reached as a result of application. table 9 and figure 6 also show the best results obtained from the eigen-6c2 model. moreover, the 7-parameter transformation model gives results that are the most compatible with minimum standard deviation values when compared to other parametric models. figures 7-11 show the geoid heights after implementing transformation and geoid heights obtained from ggms with gnss/levelling geoid heights. table 9. statistics of the differences between geoid heights obtained from ggms and from gnss/levelling data after applying the 3-, 4-, 5and 7-parameter transformation, units in meters an evaluation of recent global geopotential models for strip area project in turkey c7 fig. 6. the standard deviations of geoid undulation differences between gnss/levelling and ggms after transformations fig. 7. undulation values according to eigen-6c2 model fig. 8. undulation values according to eigen-6c3stat model fig. 9. undulation values according to egm2008 model fig. 10. undulation values according to eigen-6c4 model fig. 11. undulation values according to eigen-grace-01s model when the figures 7-11 were examined, it has seen that there were a jump for the first geoid height values calculated from ggms and the datum shift. the reason for these effects is thought to be systematic errors. therefore, different parametric models could be used in order to minimize these effects. if the figures are reviewed, it can be seen that these effects are minimized when the degree of parametric transformation is increased. the parametric models describing the systematic errors and datum shifts in the different height data sets are used in the study. when the application results and the figures were examined, the 7-parameter transformation model gave results that are the most compatible with minimum standard deviation values when compared to other parametric models. also, results show that eigen-6c2, eigen-6c3stat, eigen-6c4 and egm2008 global geopotential models produce approximately ±3 cm accuracy when compared to gnss/levelling. especially, 7-parameter transformation of eigen-6c2 and eigen-6c3stat models produces geoid height information with ±2.7 cm accuracy for 110 points within this test field. this accuracy obtained from ggms can be seen adequate for several engineering applications. when the contributions of the new satellites going to be launched in the near future are considered, the accuracies obtained from ggms will increase with each passing day. thus, the use of ggms for determining values such as the geoid height, the height anomaly, the gravity field etc. will play an important role in the earth sciences. 4. conclusion and suggestions the aim of this study is to investigate the performance of ggms produced by champ, grace, and goce as well as comparing ggms with gnss/levelling data, and eventually to determine the best model. within the scope of this study, the 210-km long konya-ankara high-speed train project was selected in order to investigate ggms performance. when the study area is examined, it can be stated that the route has a flat ground and there is not a sudden height-change structure. while the orthometric height values of the points in the study area range between 696 and 1198 m, the ellipsoidal height serkan doganalpc8 733 1234 m. 110 gnss/levelling (benchmarks) points were included in the study area. geoid heights of these points were calculated for all ggms using only latitude and longitude of the points. then these values were compared to true geoid heights and the performances of ggms were examined. according to results, the standard deviation was obtained as approximately 12 cm after comparing with true values of the geoid heights obtained from the best match ggms. the best ggms are obtained eigen-6c4, eigen-grace01s, egm2008, eigen-6c3stat, and eigen-6c2, respectively. high degrees of the ggm of all the models except for eigen-grace01s along with its different data variety (gravity and altimeter) contributed to improving geoid height information obtained from the models. systematic errors in ggms such as datum shifts and distortions can be minimized using different parametric models. therefore, these errors are minimized using four different parametric (3, 4, 5, and 7-parametric) models in the scope of the implementation. when were examined the results after transformation, 7-parameter model compared to other models were produced the better results. according to 7-parameter model results, the best five ggms are eigen-6c2, eigen-6c3stat, egm2008, eigen-6c4 and eigen-grace01s. standard deviation value obtained from ggms is approximately 3 cm for the first four models and 8 cm for the eigen-grace01s model. standard deviation value obtained from the eigen-grace01s is significantly higher compared to the other models. this is most probably related to the data, including only satellite observations, and lower degree (nmax = 140) of the model. according to the application results, it may be possible to use 3 cm-geoid height information for various engineering applications. it is obvious that geoid heights obtained from ggms will be improved through developing technology and new satellite missions to be launched in the near future. it is also possible to conclude that after reaching 1-2 cm geoid height accuracy, height of points will be obtained with high accuracy using ggms without the need for the levelling process in many engineering applications. acknowledgement the author thanks the coordinatorship of necmettin erbakan university scientific research projects for supporting this work. references amos, m. j., & 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1st general assmbly, nice/france. wessel p, smith whf (1998) new improved version of generic mapping tools released. eos trans. amer. geophys. u., 79 (47), pp. 579. yi w, rummel r, gruber t (2013) gravity eld contribution analysis of goce gravitational gradient components. studia geophysica et geodaetica vol. 57, no. 2, p. 174-202, issn (online) 1573-1626, doi: 10.1007/s11200-011-1178-8 yilmaz i, yilmaz m, güllü m, turgut b (2010) evaluation of recent global geopotential models based on gps/levelling data over afyonkarahisar (turkey). scienti c research and essays vol. 5(5), pp. 484-493. we carried out electromagnetic measurements in the vicinity of the near surface molybdenum contamination observed in the “blue lagoon” plum located in the north-east of hungary. the aim of the investigation was to find the origin of the molybdenum pollution, situated in the area, which could be a container or other infrastructure leading to the source of contamination. the field study was conducted in two stages. in the first phase, we gathered electromagnetic data by using gem-2 type equipment. we derived conductivity and susceptibility maps for each acquired frequency from the electromagnetic data. in the second stage of the field study, gpr data were obtained from 50 mhz and 450 mhz antennas. in the gpr measurements, we gathered detailed information from gpr depth slices from very shallow depth ranges to deeper parts of the investigated area (from 50cm to 8 m depths). in all results from the measurements executed by the gem-2 and (50 mhz and 450 mhz) gpr equipment, there are several clear anomalies are observed in the data for distribution of molybdenum contamination. although there are several contaminated zones observed, no clear evidence of the source of contamination was found in the vicinity of the molybdenum pollution peak. en este estudio se llevaron a cabo mediciones electromagnéticas en las inmediaciones de un foco de contaminación de molibdeno hallado en el "blue lagoon", al noreste de hungría. el propósito de la investigación fue encontrar el origen de la contaminación de molibdeno, detectada en el área, y que podría ser el contenedor u otra infraestructura que lleve a la fuente de polución. el trabajo de campo se llevó a cabo en dos partes. en la primera fase se reunió la información electromagnética a través del equipo gem-2. de esta forma se obtuvieron mapas de conductividad y susceptibilidad para cada frecuencia adquirida de la información electromagnética. en la segunda fase del estudio de campo se obtuvieron datos del radar de penetración terrestre (gpr, en inglés) con antenas de 50 y de 450 megahercios (mhz). en las mediciones gpr se recopiló información detallada de las muestras de penetración en un rango desde la superficie a la profundidad en el área de estudio (desde 50 cm a 8 m de profundidad). en todos los resultados de las medidas hechas con los equipos gem-2 y gpr se encontraron varias anomalías en la distribución de datos de la contaminación por molibdeno. a pesar de que se observaron varias zonas contaminadas, no hay evidencia clara de la fuente de polución encontrada alrededor del pico de contaminación por molibdeno. abstract resumen keywords: electomagnetic, gpr, molybdenum, contamination. palabras clave: electromagnetismo, gpr, molibdeno, contaminación. record manuscript received: 12/08/2014 accepted for publication: 14/08/2015 how to cite item kanli, a.i. & neducza, b. (2015). electromagnetic measurements for monitoring molybdenum contamination in near-surface survey. earth sciences research journal, 19(2), 107-111. doi: http://dx.doi.org/10.15446/esrj.v19n2.44897 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.44897 electromagnetic measurements for monitoring molybdenum contamination in near-surface survey ali ismet kanli1* and boriszlav neducza2 1*istanbul university, faculty of engineering, department of geophysical engineering, 34320, avcilar campus, istanbul, turkey. e-mail: kanli@istanbul.edu.tr 2 mingeo ltd., h 1142 budapest, ráskai lea u. 20., hungary. mediciones electromagnéticas para rastrear la contaminación de molibdeno en un estudio de superficies earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 107 111 el ec tr o m a g n et ic m et h o d s 108 ali ismet kanli and boriszlav neducza introduction: molybdenum does not occur naturally as a free metal on earth, but rather in various oxidation states in minerals. it readily forms hard, stable carbides in alloys, and for this reason most of world production of the element (about 80%) is in making many types of steel alloys, including high strength alloys and superalloys. most molybdenum compounds have low solubility in water, but the molybdate ion moo42− is soluble and forms when molybdenum-containing minerals are in contact with oxygen and water. industrially, molybdenum compounds are used in high-pressure and high-temperature applications, as pigments and catalysts. about 86% of molybdenum produced is used in metallurgical applications such as alloys, with the rest of molybdenum used as compounds in chemical applications (smallwood, 1984). a congenital molybdenum cofactor deficiency disease, seen in infants, results in interference with the ability of the body to use molybdenum in enzymes. it causes high levels of sulfite and urate, and neurological damage. the cause is the inability of the body to synthesize molybdenum cofactor, a heterocyclic molecule that binds molybdenum at the active site in all known human enzymes that use molybdenum (smolinsky et al. 2008, reiss, 2000). although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation, infertility, low birth weight and gout; it can also affect the lungs, kidneys and liver (coughlan, 1983). our study area is located nearby both agriculture and industrial environment having some problems about molybdenum contaminations. therefore, we have conducted electromagnetic based geophysical surveys that are adequately used in environmental studies in the area (figure 1). figure 1. the study area located at north-east of hungary. the monitoring wells can be seen on the field. in the figure, some monitoring wells located in the survey area can be seen. these wells have been monitored to acquire information on molybdenum contamination with hydrogeological conditions of the investigated area. electromagnetic survey and measurements: electromagnetic based survey methods have been successfully applied to large-scale groundwater prospecting, hydrogeological studies, engineering and environmental research, monitoring and imaging subsurface. there have been several important studies and results on the success of gpr, electrical and electro-magnetic based research used in contamination surveys (cassidy, n.j., 2007, daniels et al., 1995, olhoeft, 1992, nyari and kanli, 2007, nyari et.al., 2010). combining cutting-edge technology with the simplicity of use, the gem-2 is the finest electromagnetic geophysical method for geological, environmental, and geotechnical shallow-earth surveys. the geophex gem-2 electromagnetic (em) instrument used in our investigation provides a rapid multi-frequency technique for shallow geophysical exploration (figure 2). gem-2 electromagnetic (em) instrument that uses an active em signal to detect variations in subsurface conductivity can provide typical 1.5 ground point spacing and allows a surveyor to collect around 20.000 data points per hour over five frequencies when linked with active gps tracking. it operates in a frequency range of 330 hz to 24 khz, and can transmit an arbitrary waveform containing multiple frequencies. em currents result in a secondary magnetic field that is measured together with the original transmitted signal by using a receiver coil on the em instrument. the penetration depth is dependent on various factors but mostly the ground conductivity and the em wave frequencies (yaccup and brabham, 2012), when the quality of data acquired is influenced by the ambient electromagnetic noise. based on many analyses and field data, we estimate the gem-2 should be able to see about 30-50 m in very high resistive areas (>1000 ohmm) and some metres or 10 metres in conductive areas (<100 ohmm). the noise level is high in urban areas and low in rural areas. for typical applications, we do not recommend the gem-2 for depths deeper than 20 m. the gem2 instrument outputs the ratio of the secondary to the primary field (hs/hp) in ppm. figure 2. electromagnetic measurements in the study area using gem-2 instrument. ground penetrating radar (gpr) is the general term applied to techniques that employ radio waves, typically in the 1 to 1000 mhz frequency range, to map structure and features buried in the ground (or in man-made structures). historically, gpr was primarily focused on mapping structures in the ground; more recently gpr has been used in nondestructive testing of non-metallic structures. the concept of applying radio waves to probe the internal structure of the ground is not new. radar systems can be deployed in three basic modes that are referred to as reflection, transillumination and velocity sounding. the most common mode of operation is single-fold, fixed-offset reflection profiling as illustrated in figure 3 (annan, 2001). this mode of operation gives rise to data such as shown conceptually in the figure. a detailed description of gpr systems, data processing and interpretation can be found in daniels, 1996, reynolds, 1997; daniels, 2004, jol, 2009, miller et. al. 2010. 109electromagnetic measurements for monitoring molybdenum contamination in near-surface survey we have carried out electromagnetic measurements in the vicinity of the near surface molybdenum contamination observed in the “blue lagoon” plum located in the north-east of hungary. the aim of the investigations was to delineate and find the origin of the molybdenum pollution, situated in the area, which could be a container or other infrastructure leading to the source of contamination. figure 3. single-fold, fixed-offset reflection profiling is given at the left and the measured gpr profile on the structure is given at the right. the field study: the field survey was carried out in two stages. in the first phase of measurements, gem-2 type electromagnetic equipment was used. the data acquisition was in a 100x50 m area that was chosen around the polluted shallow boreholes, parallel to the railway track (figure 1). in electromagnetic measurements, conductivity and susceptibility maps are produced for each acquired frequency. data were gathered at all available frequencies, but below 1 khz too noisy data were collected. therefore, we omitted them for reliable interpretation. the interpreted maps, in the frequency range of 1.1 42.2 khz, are given in figure 4. the conductivity and susceptibility values are given for different frequencies. in the maps, the location of the temporary and the monitoring wells can also be seen. although we were not given the detailed data from the wells, only high content of molybdenum pollution were reported. when the acquired maps are interpreted, an “l” shaped (reversed) anomaly can be identified at the upper end of the maps, along the railway line (on the left), which has high conductivity (low resistivity) and low susceptibility. on the other hand, when compared to their surroundings, three higher conductivity and lower susceptibility anomalies can be identified about ~2 m long stripes that showing up at different frequencies on the maps at near the well no. 13. detailed interpretation results also show us that there is a shapeless, low conductivity zone, observed on conductivity maps that can not be seen on the susceptibility maps (figure 5). the maximum of this zone is located near the high molybdenum concentration. however, there is no any indication that it is connected to some infrastructure. in the second stage of measurements, gpr surveys were carried out in the study area. the aim of this investigation was to find a container or other any geological unit that can be a source of contamination. in geo-radar measurements, two different type of antennas were used, 450 mhz and 50 mhz frequency respectively. during the measurements, the attenuation of the waves was very high with the maximum penetration depth was about ~1.5 m. the data acquisition was carried out along profiles with 1 m interval. the data was collected in the area among the wells numbered 34, 31, 32 and 33. the spatial sampling rate was less than 10 cm within the sections. in radar measurements, we get some reflections from the border of the materials with different conductivity properties due to the existence of plastic and metal tubes. we processed the sections concerning reflection strength, and the results are sliced at several depth levels. the maps at different depth levels from 50cm to 150cm are given in figure 6. figure 4. gem-2 measurement results over the shallow (near-surface) molybdenum contamination. in the maps, apparent electrical conductivity and magnetic susceptibility values are given. light blue circles indicate the monitoring well locations and red crosses show temporary borehole locations. 110 ali ismet kanli and boriszlav neducza figure 5. contour map of the low conductivity zone. figure 6. depth slices of processed 450mhz gpr data measured above the shallow molybdenum contamination. the important anomaly locations highlighted on the slices were searched in details on the individual records to understand the causes of the reflections. as seen on the 1.5 m depth slice, location of the significant anomaly can be recognized. the depth slice with the recorded gpr data is displayed in figure 7. the reflections, observed on the record, are coloured by transparent red. in the figure, a dashed red line that indicates an anomaly can be observed on all the sections independently. figure 7. a depth slice from 150 cm processed with 450 mhz gpr data is given on the left and the gpr section from the relatively high reflectivity zone is given on the right. to increase the penetration depth, we recorded gpr data at 50 mhz lower frequency. due to the lower resolution at this frequency, the attenuation of the electromagnetic waves is also reduced. this low-frequency data give us to investigate deeper strata and subsurface of the study area. since there are unshielded antennas at these frequencies, the outside effects are more dominant on the records. the hyperbolas appearing deeper than 4 m are obviously the results of external effects given in the profile in figure 8. we calculated depth slices from the 50 mhz gpr data from 70 cm to 2.5 m depths. the depth slices are given in figure 9. after detailed analysing the individual records, based on the higher amplitude reflections observed on the slices, none of them were found to be the outcome of human activities or the effects of manmade objects. figure 8. gpr depth section measured by 50 mhz antennae. 0 2 4 6 8 10 12 14 16 12 14 16 18 20 22 24 26 28 33 32 8 34 31 150cm distanceh[m] de pt h [m ] depth [m ] 111electromagnetic measurements for monitoring molybdenum contamination in near-surface survey figure 9. depth slices of processed 50mhz gpr data above the shallow molybdenum contamination. conclusions: we have carried out electromagnetic measurements in the vicinity of the near surface molybdenum contamination observed in the “blue lagoon” plum located in the north-east of hungary. the aim of the investigations is to delineate and find the origin of the molybdenum pollution leading to the source of contamination. in electromagnetic measurements recorded by gem-2 equipment, conductivity and susceptibility maps are produced for each acquired frequency. the results of the measurements by gem-2 data, several anomalies are observed indicating the polluted area. the gpr profiles measured by the 450 mhz antenna have high-resolution images and the data gathered with 50 mhz gpr have given us useful information from the deeper parts of investigated area. verifying the gpr anomalies no buried container or clear manmade sources of molybdenum contamination were indicated in the vicinity of the pollution peak. acknowledgments: the author, ali ismet kanli, was supported by the division of scientific research projects of istanbul university, international research projects, project number: irp-50242. the authors would like to thank anonymous reviewers for their constructive remarks in the preparation of the final form of the paper. references: annan, p. (2001). ground penetrating radar workshop notes, sensors & software inc., 192 pages. cassidy, n. j. (2007). evaluating lnapl contamination using gpr signal attenuation analysis and dielectric property measurements: practical implications for hydrological studies, journal of contaminant hydrology 94, pp. 49–75. coughlan, m. p. (1983). the role of molybdenum in human biology, journal of inherited metabolic disease, 6: 70–77. doi:10.1007/bf01811327. daniels, j. j., roberts, r. and vendl, m. (1995). ground penetrating radar for the detection of liquid contaminants, journal of applied geophysics 33, 195–207. daniels, d. j. (1996). surface penetrating radar, radar, sonar, navigation and avionics series 6. the institute of electrical engineers, london, u.k. daniels, d. j. (ed.). (2004). ground penetrating radar, radar, sonar, navigation and avionics series 15. the institute of electrical engineers, london, u.k. jol, h. m. (ed.). (2009). ground penetrating radar: theory and applications, elsevier, isbn 978-0-444-53348-7, 524 pages. miller, r. d., bradford, j. d. and holliger, k. (ed.). (2010). advances in nearsurface seismology and ground-penetrating radar, geophysical developments series no. 15, seg reference publications program, tulsa, oklahoma-usa. , isbn 978-1-56080-224-2, 487 pages. nyari, z. and kanlı, a. i. (2007). imaging of buried 3d objects by using electrical profiling methods with gpr and 3d geoelectrical measurements, journal of geophysics and engineering, vol.4, p.83-93 nyari, z., kanlı, a. i., stickel, j. and tillmann, a. (2010). the use of nonconventional cpte data in determination of 3-d electrical resistivity distribution, journal of applied geophysics, vol. 70, issue 3, p. 255-265. olhoeft, g. r. (1992). geophysical detection of hydrocarbon and organic chemical contamination. in: bell, r. s. (ed.), proceedings on application of geophysics to engineering, and environmental problems. society of engineering and mining exploration geophysics, golden, co, oakbrook, il, pp. 587–595. reiss, j. (2000). genetics of molybdenum cofactor deficiency, human genetics 106(2): 157–63. doi:10.1007/s004390051023. reynolds, j. m. (1997). an introduction to applied and environmental geophysics. john wiley & sons, chichester, england. smallwood, robert e. (1984). tzm moly alloy, astm special technical publication, 849: refractory metals and their industrial applications: a symposium. astm international. p. 9. , isbn 9780803102033. smolinsky, b., eichler, s. a., buchmeier, s., meier, j. c. and schwarz, g. (2008). splice-specific functions of gephyrin in molybdenum cofactor biosynthesis, journal of biological chemistry 283 (25): 17370–9. doi:10.1074/jbc.m800985200. yaccup, r. and brabham, p. (2012). ground electromagnetic survey (gem2) technique to map the hydrocarbon contaminant dispersion in the subsurface at barry docks, wales, uk, awam international conference on civil engineering (aicce12) and geohazard information zonation (giz 12), penang, malaysia, 28-30 august 2012. the anomalies detected in phosphatic series of sidi chennane, one of phosphatic basins in morocco, hinder the proper exploitation of phosphate levels and the assessing phosphate reserves seems incorrect. the purpose of this study was the evaluation of geostatistical methods for delimitation of these disturbances. to cover all the zones being able to be disturbed, we carried out, during the geophysical prospection in a parcel of 50 ha, 5151 resistivity measurements as horizontal profiling using the well-known schlumberger array, in order to map the spatial distribution of the sterile hardpan inclusions. geostatistical tools were used to quantify the spatial correlation between apparent resistivity data. semivariograms were obtained using a classical matheron semivariogram estimator and fit to the experimental semivariograms obtained. we have selected those with the best fit in terms of sum of squared residuals (ssr). geostatistical analysis was performed using the software vesper 1.63. spatial distribution maps were made by ordinary kriging, the qualitative interpretation of these maps reflects that the exponential model is found to be the best model representing the spatial variability of our geoelectric data. the qualitative interpretation of the kriged resistivity maps allows defining resistivity contrast, consequently we have delimited the crossing dominate area from a “normal” into a “disturbed” area. models of the geology were successfully obtained from geostatical method, which help mapping the phosphate deposit inclusions and the estimations of phosphate reserves were improved and better constrained. las anomalías detectadas en las series fosfóricas de sidi chennane, una de las cuencas fosfóricas de marruecos, dificultan la explotación apropiada de los niveles de fosfato y hacen parecer incorrectos los cálculos de las reservas. el propósito de este estudio es la evaluación de los métodos geoestadísticos para la delimitación de estas anomalías. para cubrir todas las zonas donde se pueden presentar estas alteraciones se llevaron a cabo, durante la exploración geofísica en una parcela de 50 hectáreas, 5151 medidas de resistividad con perfiles horizontales a través del conocido sondeo schlumberger, con el fin de mapear la distribución espacial de las inclusiones estériles de la capa sólida. se utilizaron herramientas geoestadísticas para cuantificar la correlación espacial entre los datos de resistividad. se obtuvieron semivariogramas a través del tradicional estimador de semivariogramas matheron y se adecuaron a los semivariogramas experimentales obtenidos. la selección se basó en aquellos que mejor se acoplaban en términos de la suma de cuadrados residuales (sce). los análisis geoestadísticos se realizaron con el programa vesper 1.63. los mapas de distribución espacial se hicieron por kriging regular, y la interpretación cualitativa de estos mapas refleja que el modelo exponencial es el que mejor representa la variablilidad espacial de estos datos geoeléctricos. la interpretación cualitativa de los mapas de resistividad obtenidos por la técnica del kriging permite definir el contraste de resistencia, lo que fija el área entre los estándares de “normal” y “con alteraciones”. los modelos geológicos fueron obtenidos del método geoestadístico, lo que ayudó a mapear los depósitos de inclusiones de fosfato y mejoró las estimaciones de las reservas a través de una mejor definición de estas. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 143 148 abstract resumen key words: moroccan phosphate, geoelectric prospecting, geostatistical, interpolation, kriging. palabras clave: fosfato marroquí, exploración geoeléctrica, geoestadística, interpolación, kriging. record manuscript received: 12/12/2013 accepted for publication: 21/09/2014 geostatistical analysis for delineating sterile inclusions in sidi chennane' phosphatic series, morrocco khadija baba¹, lahcen bahi¹, latifa ouadif¹ ¹gie laboratory, mohammadia engineering school, mohammed vagdal university, rabat, morocco) email: baba.khadija@gmail.com, dircedoc@emi.ac.ma, ouadif@gmail.com. geostatistics issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.41173 http://dx.doi.org/10.15446/esrj.v18n2.41173 144 khadija baba, lahcen bahi, latifa ouadif introduction morocco is the world’s third largest phosphate producer, after the usa and china. total mine production recorded by the ministry of energy and mines in 2013 was 30 mt so more than 75% of world reserves. four major phosphate basins are now known and are being exploited, three of which are located in central-northern morocco. the four main deposits of phosphate are: the oued eddahab basin situated in sahara, the central ganntour basin near youssoufia, the meskala basin at east of essaouira and the oulad abdoun basin situated near khouribga. the existence of morocco sedimentary phosphate rock has been known since 1908 in the meskala basin, but it had not generated significant interest until the discovery, in 1917, of the oulad abdoun basin. the geological investigations carried out in sidi chennane phosphatic deposit in the oulad abdoun basin revealed a phenomenon of sterile hardpan [bakkali 2005, baba & al. 2012; 2014a and 2014b] these sterile bodies are formed by accumulations of silicified limestones or by limestone blocks within an argillaceous matrix which interfere with phosphate extraction and their resistivity, above 200 ohm.m, is higher than the phosphate-rich mineral resistivity wich is near 120 ohm.m [bakkali and amrani 2008; baba & al. 2012]. the application of the electric prospection methods constitutes a suitable means to map these sterile bodies [baba & al. 2012, 2014a and 2014b]. one of tools for mapping of electrical resistivity data is geostatistical methods. resistivity data were analyzed using geostatistics (variogram) in order to describe and quantify the spatial continuity. this study has been carried out with the aim of evaluation geostatistical methods for estimation of subsurface layers delimitation and specified their spatial variability in order to establish a model of sterile bodies’ distribution and would permit the definition of these structures before the mining front reaches them. material and methods data acquisition electrical and electromagnetic (em) methods have been important in the field of applied geophysics for about a century, particularly for shallow and near-surface investigations. geoelectrical resistivity surveying has become an important and useful tool in hydrogeological studies, mineral prospecting and mining, as well as in environmental and engineering applications [griffiths & al. 1990; griffiths and barker, 1993; dahlin and loke, 1998; olayinka, 1999; olayinka and yaramanci, 1999; amidu and olayinka, 2006; baba & al. 2012; ouadif & al. 2012]. electrical sounding is a method to investigate the change in earth resistivity with depth at a particular location. traditionally, arrangements using four electrodes (two current-transmitting electrodes and two voltagesensing electrodes) are used for either vertical soundings or horizontal profiling. for vertical soundings, the electrodes are arranged symmetrically according to a center, with increasing distances between electrodes used to explore deeper depths [baba & al. 2012; ouadif & al. 2012 and 2014a]. when using geoelectrical surveying techniques, an estimate of the earth resistivity is calculated by using the well-known relation between resistivity, an electric field, current density (called ohm’s law), the geometry and spacing of the current and potential electrodes. when the earth is not homogeneous neither isotropic, this estimated value is called the apparent resistivity, p_app, which is an average of the true resistivity in the measured section of the earth. figure 1: the basic principles of schlumberger device profiling resistivity surveys can be employed to detect lateral variations in resistivity. this type of survey provides estimates of the spatial variation in resistivity at some fixed electrode spacing [chapellier 2000; dabas and caraire 2012]. to determine the intrinsic resistivity for the different terms of the phosphatic series, we carried out, during the geophysical prospection in a parcel of 50 ha,5151 resistivity measurements as horizontal profiling (syscal-r2 resistivity instrument iris instruments) using the wellknown schlumberger array, in order to map the spatial distribution of the sterile hardpan inclusions. the 5151 stations of the resistivity survey is a compilation of 51 profiles spaced at 20 m. for every profile, there were 101 stations separated by 5 m using the well-known schlumberger array. 1.1 geostatistics geostatistics, started with the work in geology and mining of krige (1951) then developed by matheron (1963) with his theory of regionalized variables, are methods of interpolation which predict unknown values from data observed at known locations, and it minimizes the error of predicted values which are estimated by spatial distribution of the predicted values. as variables are supposed having an effect on each other, the first concept is related to the variogram of values we want to estimate. it relates the similarity or difference, expressed as the semi-variance, between values at different places to their separation distance (lag) and direction. distance that variables have an effect on each other is called range of a variogram and is an important concept because after that distance, variables have no effect on each other. the plot of the semivariance versus the lag h is called semivariogram. in some cases, it increases with h and reaches an asymptotic value, a sill. range is the value of h at which the semivariogram asymptotes. values of the properties at sites separated by a distance shorter than the range will be spatially correlated. the range depends on the scale of the study and on the property evaluated. figure 2: typical semivariogram and its components. the technique of variogram (or semi-variogram) quantifies the spatial variability of a regionalized variable, and provides the input parameters for the spatial interpolation of kriging [krige, 1951; webster and oliver, 2001]. let be r(x) and r(x+h) the values of electrical resistivity sampled at. 145geostatistical analysis for delineating sterile inclusions in sidi chennane' phosphatic series, morrocco positions x and x+h, with h a distance, known as lag, which separates both positions, semi-variances can be estimated as: (1) for discrete sampling sites, such as earthen material samples, the function is estimated as: (2) where r(xi) : the value of electrical resistivityr at location of xi n(h) : the number of point pairs for distance class h. the five models most frequently used are: nugget: spherical: (3) exponential: (4) gaussian: (5) power: (6) with h: lag distance c: sill a: range nugget effect is essential to optimize the process under study: the higher its difference for the semivariogram sill, the higher the phenomenon continuity, the lower the estimation variance, or the higher the estimation confidence. the variogram plot is fitted with a theoretical model which provides information about the spatial structure as well as the input parameters for kriging interpolation. kriging algorithms (krige, 1951) are described as a best linear unbiased estimator (blue), which is a process of a theoretical weighted moving average: (7) where r(x0) is the value to be estimated at the location of x0, r(xi) is the known value at the sampling site xi. and li denotes the weight of the ith observation, they are calculated searching an unbiased estimator of z with minimum variance [webster, 1985]. the estimation errors (or kriging variances) need to be minimised. to ensure that the estimate is unbiased, the weights need to sum to one: (8) i. study area and data processing 1.1. geological framework and methodology of work the ouled abdoun basin is the largest phosphate basin in morocco. it’s located about 100 km in south-east of casablanca. the phosphate deposits of ouled abdoun area belongs to the western moroccan meseta, commonly considered being stable. the local sedimentary deposits resulting from a large transgression occurred in mid-cretaceous. it consists of :marly limestone and gypsum of cenomanian, turonian white limestones, senonian marl and yellow marly limestones, phosphatic series dated from maastrichtian to ypresian and lutetian calcareous thersitean slab. the neogene continental deposits cover locally the marine series. figure 3: localization map of the study area [yans & al. 2014]. 1.2 statistical analysis the classical statistical procedures assume that variations are randomly distributed within each class of observations and independent of sample location [mohmmadi, 2006]. the mean is therefore used for the estimation of properties for unsampled locations within each class. statistics of dispersion (e.g. variance, standard deviation, coefficient of variability, confidence limits) are used to show the precision of the mean as an estimator. table.1: summary statistics of random samples statistics ab40 ab80 ab120 number of sample 5151 5151 5151 sum 808077,22 503542,9 383859,2 mean 156,877736 97,7563386 74,5212968 max 659 400 250 min 9,2 7,1 4,1 variance 10597,0331 2299,88124 1160,37271 standard deviation 102,951886 47,9617328 34,067551 95% confidence limit 0,08995038 0,04190478 0,02975391 the resistivity distributions are not normal, which is the particularity of the anomalous bearing population. variation between maximum and minimum for these data shows a wide range. figure 4.a: histogram of the resistivity data for ab=40m 146 khadija baba, lahcen bahi, latifa ouadif figure 4.b: histogram of the resistivity data for ab=80m figure 4.c: histogram of the resistivity data for ab=120m assuming the samples were collected randomly from different locations in the field, we can conclude from these statistics that resistivity at the upper sublayers is more variable on the field than that at the lower layers. 2.3 geostatistical methods vesper 1.6 software (acpa, university of sydney) developed by the australian center for precision farming [minasny & al. 2002] was used to calculate experimental electrical resistivity semivariograms of experimental fields in order to obtain the parameters: range, sill and nugget variance for evaluating the spatial correlation of these variables. semivariograms were obtained using a classical matheron semivariogram estimator. the experimental semivariograms were fitted with various theoretical models like spherical, exponential, gaussian, linear and power by an automatic (least-squares) estimation of semivariogram parameters. indicator variograms were built adopting a lag of 50 m. the theoretical model that gave minimum standard error is chosen for further analysis. table 2 shows semivariogram models and parameters. the attributes were fitted to the exponential model. table 2: summary details of fitted exponential models c0 (m 2) c1 (m 2) a1(m) sdd1 ab40 555,0 13714 33,35 mod.2 ab80 570,0 1991,0 137,3 strong ab120 444,0 812,0 76,75 mod. c0= nugget effect c1= sill nugget effect a1= range 1spatial dependence degree 2moderate the ratio of nugget to total semivariance, expressed as a percentage, was used to classify spatial dependence [cambardella & al. 1994], a ratio less than 25% indicated strong spatial dependence, between 25 and 75% indicated moderate spatial dependence and more than 75% indicated weak spatial dependence. figure 5.a: experimental semivariogram and fitted models for ab=40m figure 5.b: experimental semivariogram and fitted models for ab=80m figure 5.c: experimental semivariogram and fitted models for ab=120m spatial interpolation accuracy and precision were evaluated through 147geostatistical analysis for delineating sterile inclusions in sidi chennane' phosphatic series, morrocco spatial interpolation accuracy and precision were evaluated through crossvalidation approach. most important criteria include: mean bias error (mbe), mean absolute error (mae), mean squared error (mse) and root mean squared error (rmse). mae and rmse are among the “best” overall measures of model performance [willmott, 1982, webster and oliver 2001]. rmse provides a measure of error size, but it is sensitive to outliers as it places a lot of weight on large errors [chai and draxler 2014]. mae provides an absolute measure of the size of the error and it is less sensitive to extreme values [delbari & al. 2013]. the definitions of mae, mbe and rmse are as follows: (9) (10) (11) where z(xi) and z*(xi) are the observed and predicted value respectively. to see how predicted values match closely the observed values, the rmse can be used: the smaller the rmse the better. figure 6.a: cross validation graphs for ab40 figure 6.b: cross validation graphs for ab80 figure 6.c: cross validation graphs for ab120 we used the rmse to evaluate model performances in cross-validation mode. the smallest rmse indicate the most accurate predictions. cross-validation of the model are shown in figures 6.a, 6.b and 6.c for the variables ab40, ab80 and ab120. because the rmse are closer to zero, fitted exponential models yield better results and are adopted in this study. after analysis of spatial dependence, we evaluated sites that had not been sampled in order to produce surface maps by ordinary kriging. figures 7.a, 7.b and 7.c show isoresistivity maps carried out at the nodes of the square grid of 1km x 1km with the exponential model of variogram. figure 7.a: isoresistivity map for ab40 figure 7.b: isoresistivity map for ab80 figure 7.c: isoresistivity map for ab120 the comparison of the maps of the phosphate deposit anomalous zones corresponding to the three schlumberger devices improved the quality of subsurface layers delimitation and specified their spatial variability: (1) for ab=40 and ab=80 the contour maps show anomalous zones located at an average depth of 20m (figures 7.a and 7.b). (2) for ab=120 (figure 7.c) the disturbances as detected from surface measurements, which present depth higher than 20 m, seem to be not very significant regarding to surface study area. thus, the majority of anomalies affect only the subsurface of the phosphate series 148 khadija baba, lahcen bahi, latifa ouadif conclusion in this work ordinary kriging, a type of geostatistical techniques is applied for mapping apparent resistivity at sidi chennane, one of phosphatic basins in morocco, which is characterized by the presence of sterile hardpan -so-called “disturbances”hard to detect. a detailed geoelectric resistivity survey was executed using the horizontal electrical sounding approach with shlumberger array, in order to study the lateral variations in the geoelectric behavior. the data process was executed using the vesper 1.6 software (acpa, university of sydney). the exponential model is found to be the best model representing the spatial variability of the resistivity, for this reason, it is used to plot resistivity map by kriging method. these maps permitted us to identify disturbed zones in this area; the qualitative interpretation of these maps reflects that the disturbances as detected from surface measurements which present depth higher than 20 m seem to be not very significant. the study showed the benefits of the using geostatistical methods in the assessment of data, sampling strategy, and estimation of values at unsampled areas of the sub-surface to delineate anomalies. our results suggest that geostatistical methods are potentially very useful for building a model in the area of complicated geological structures. bibliography amidu s.a., olayinka a.i. 2006. environmental assessment of sewage disposal systems using 2d electrical resistivity imaging and geochemical analysis: a case study from ibadan, southwestern nigeria. environ. eng. geosci., vol.7, n°3, pp. 261-272. baba, k., bahi, l., ouadif, l. & akhssas, a. 2012. mapping sterile bodies in the sidi chennane phosphatic deposit (morocco) using geoelectrical investigations. international journal of engineering research and applications (ijera), vol. 2, n°5, pp. 2132-2136. baba, k., bahi l. and ouadif, l. 2014a. enhancing geophysical signals through the use of savitzky-golay filtering method. geofísica internacional. vol.53, n°4: pp. 153-162. baba, k., bahi, l., ouadif, l. & cherradi, c. 2014b. application des méthodes d’analyses statistiques multivariées à la délimitation des anomalies de sidi chennane. journal des matériaux et des sciences de l’environnement vol.5, n°4, pp. 1005-1012. bakkali, s. 2005. analysis of phosphate deposit disturbances using the horizontal gradient responses of resistivity data (oulad abdoune morocco). earth sci. res. j., vol. 9, n°2, pp. 123-131. bakkali, s. and amrani, m. 2008. denoising resistivity phosphate “disturbances” using haar mother wavelet transform (sidi chennane, morocco). earth sci. res. j., vol. 12, n°1, pp. 62-71. cambardella, c.a.; moorman, t.b.; novark, j.m.; parkin, t.b.; karlen, d.l.; turco, r.f.; konopka, a.e. 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vol.63, pp. 1309–1313. yans, j., amaghzaz, m., bouya, b., cappetta, h., iacumin, p., mouflih, m., selloum, o., sen, s., storme, j.y., kocsis, l., and gheerbrant. e. 2014. first carbon isotope chemostratigraphy of the ouled abdoun phosphate basin, morocco; implications for dating and evolution of earliest african placental mammals. gondwana research 25 pp. 257–269. 169 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 169 181 the assessment of slope stability and rock excavatability in a limestone quarry kadir karaman, bayram ercikdi, ayhan kesimal department of mining engineering, karadeniz technical university, 61080, trabzon, turkey geotechnics the aim of this study is to evaluate the stability and excavatability of newly stripped rock slopes (slope 1 (sn–1), slope 2 (sn–2), and slope 3 (sn–3)) in a limestone quarry. these are new production sites with comparable geological formations along the southern part of the quarry where three planar failures were previously observed. for this reason, detailed fieldwork was performed to determine the properties (spacing, roughness, etc.) of the discontinuities of the rock slopes in the study area. the shear strength parameters of the discontinuities and the point load strength index (is(5o)) and the uniaxial compressive strength (ucs) of rock samples obtained from the study area were tested in the laboratory. the stability of the slopes was assessed using kinematic analysis and an orientation–dependent and orientation–independent slope stability probability classification (sspc) system. the results of the sspc system analyses were compared with those of the slope mass rating (smr). the kinematic analysis shows that planar, wedge and toppling failures are unlikely in the slopes of the study area. the orientation–dependent sspc analysis revealed that sn–2 would experience sliding failure if its dip angle is greater than 66°. the slopes were shown to have a stability probability of ≥80%, provided that a pneumatic hammer or blasting methods are used for the excavation. however, the maximum slope height (hmax) in blasting operations is required to be lower for durable slope faces. furthermore, the smr analysis has indicated that sn–1 and sn–2 will most likely lose their stability when blasting is used as an excavation method. the rock slopes could be excavated by pneumatic hammer because the category of excavatability of the rock was determined to be ‘easy ripping.’ based on the kinematic, sspc and smr analyses, the angles for a safe slope are proposed to be 70°, 66° and 75° for sn–1, sn–2 and sn–3, respectively, with a slope height of 8 m. el objeto de este estudio es evaluar la estabilidad y la medida de excavación de nuevas pendientes rocosas descubiertas (slope 1 (sn–1), slope 2 (sn–2), y slope 3 (sn–3)) en una cantera de caliza. estos son nuevos sitios de producción con formaciones geológicas similares en la parte sur de la mina. por esta razón se llevó a cabo un trabajo de campo detallado para determinar las propiedades (espaciado, irregularidades, etc.) de las discontinuidades en las pendientes rocosas del área de estudio. en un laboratorio se evaluaron los parámetros de fuerza de las discontinuidades, el índice de punto de carga (is(5o)) y la fuerza de comprensión monoaxial (ucs) de las muestras de rocas tomadas en el área de estudio. la estabilidad de las pendientes fueron medidas a través de un análisis kinemático y de un sistema de clasificación de estabilidad probable (sspc) en orientación-dependiente y orientaciónindependiente. los resultados del análisis al sistema sspc fueron cotejados con las clasificaciones de masa de las pendientes (smr). el análisis kinemático muestra que las fallas planas, de bloque y de tope son improbables en las pendientes del área de estudio. los análisis sspc de orientación-dependiente muestran que la sn-2 podría presentar fallas de deslizamiento ya que el ángulo de la cuesta es mayor a 66°. las pendientes evidencian una probabilidad estable de ≥80% si se utiliza un martillo neumático o métodos de explosión para la excavación. sin embargo, la pendiente de máxima altura (hmax) en operaciones de explosión debe ser menor para garantizar la duración de las fachadas de las pendientes. además, los análisis smr indicaron que la sn-1 y la sn-2 son más propensas a perder su estabilidad si se utilizan métodos de excavación con explosivos. las pendientes rocosas podrán ser excavadas con palabras clave: estabilidad de pendientes rocosas; análisis kinemático; sistema de clasificación de estabilidad probable (sspc); clasificaciones de masa de las pendientes (smr); excavabilidad. abstract resumen key words: rock slope stability; kinematic analysis; slope stability probability classification (sspc); slope mass rating (smr); excavatability 170 introduction the slope stability analysis of rocks is vital for designing safe slopes in open–pit mines. a proper slope design not only leads to improvements in slope stability and safety but also reduces costs, extends the life of mines and decreases the stripping ratio (bye and bell, 2001, naghadehi et al., 2011). there are different methods used to evaluate the slope stability of rocks. the assessment of slope stability in rocks is usually performed by kinematic analysis, limit equilibrium analysis and a rock mass classification system, such as the sspc system (ulusay et al., 2001; pantelidis, 2009; alejano, 2011). kinematic analysis is based on the motion of bodies without consideration of the forces that cause the motion (kliche, 1999; kulatilake et al., 2011). however, kinematic analysis does not consider the forces acting on a slope or important geotechnical parameters, such as cohesion and unit weight. the other method, known as limit equilibrium analysis, considers shear strength along a failure surface, the effects of pore water pressure and the influence of external forces, such as reinforcing elements or seismic accelerations (kentli and topal, 2004; gürocak et al., 2008). although limit equilibrium analysis is widely used and is a simple method to assess the stability of the slopes, it is often inadequate if the slope fails via complex mechanisms (e.g., discontinuity orientation, progressive weathering, excavation disturbances, etc.) (eberhardt, 2003). the sspc system was designed by hack (1998) specifically for slopes to classify rock masses and assess the in situ stability and probability of failure of an engineered slope constructed in that rock mass. hack (1998) developed the sspc by analyzing the stability of 286 engineered rock slopes, which varied from 5 to 45 m in height. the excavations were performed by varying means, such as manual, explosives or excavators. the lithologic units were highly varied and ranged from gypsiferous shales to limestones, sandstones and granodiorites. the sspc system considers both the orientation–dependent and orientation–independent stability of the slope and thus gives a better assessment of slope stability (hack, 1998; lindsay et al., 2001). the orientation–dependent sspc analysis is related to the orientation of the discontinuities and the slope, whereas the orientation–independent sspc analysis considers the strength, cohesion and angle of internal friction of the rock mass in which the slope is located, independent of the orientation of both the discontinuities and the slope. hack (1998) reported that a large number of slopes were found to be stable using the orientation–dependent analysis, although they were visually assessed in the field as unstable. for these slopes, the orientation–independent analysis based on the rock mass friction angle and cohesion and the slope height and dip has been reported to produce an estimate of the probability of failure, expressing the stability as a percentage probability (lindsay et al., 2001; hack et al., 2003). lindsay et al. (2001) indicated that, in addition to the design parameters, including rock mass cohesion, rock mass friction angle, rock mass strength and maximum stable bench angle, sspc also provides kinematic analysis and a prediction of optimum height for a stable slope. the sspc system is a rock mass classification system based on a three– step approach and on the probabilistic assessment of different independent failure mechanisms in a slope (hack and price, 1995). first, the exposed rock mass (exposure rock mass, erm) is characterized and is corrected for weathering and excavation disturbance to derive the parameters critical to the geomechanical behavior of the slope in its imaginary unweathered and undisturbed state (reference rock mass, rrm). then, the design slope (slope rock mass, srm) is assessed based on the derived reference rock mass parameters, taking into consideration both the method of excavation and the effect of weathering (hack, 1998; lindsay et al., 2001; hack, 2002; hack et al., 2003). the excavatability of rocks is of importance for the selection of suitable and cost–effective excavation methods not only in mining and quarrying but also in the construction of tunnels, subways, highways and dams. a number of different methods have been proposed to evaluate the excavatability of rocks based on their geotechnical properties, such as (is(5o)), discontinuity spacing and seismic velocity (weaver, 1975; scoble and muftuoglu, 1984; macgregor et al., 1994; başarir and karpuz; 2004). it has been reported that the type of equipment used and the method of working also affect the excavatability of rocks (pettifer and fookies, 1994). 2. the araklı-taşönü limestone quarry the araklı–taşönü limestone quarry is situated approximately 40 km from the city of trabzon in northeast turkey (fig. 1). the quarry is operated by the askale cement factory and exploits the limestone-rich kirechane formation. the quarry has experienced three separate planar failures, which occurred between 2005 and 2006. kesimal et al. (2008) suggested that uncontrolled blasting activities and heavy rainfall, as well as very steep slopes, are responsible for the failures. the failures resulted in the demolition of houses, a mosque and a school, and the failures adversely affected the road and farm fields surrounding the quarry (fig. 2a, b). raw material production came to a halt due to the environmental issues associated with these failures. figure 1. location of the araklı-taşönü limestone quarry (ceryan, 2009) a new production site with a similar geological formation at the southern part of the landslide area was then selected to produce limestone (figs. 3 and 4a). the aim of this study is to evaluate the stability of the slopes at this new production site. furthermore, the excavatability of the martillo neumático ya que la medida de excavación de la roca fue determinada como de "fácil explotación". basado en los análisis kinemático, sspc y smr, los ángulos propuestos para una pendiente segura son de 70°, 66° y 75° para las pendientes sn–1, sn–2 y sn–3, respectivamente, con una pendiente de altura de 8 m. record manuscript received: 04/12/2012 accepted for publication: 04/11/2013 kadir karaman, bayram ercikdi, ayhan kesimal 171 rocks in the site was also assessed using the (is(5o)) and discontinuity spacing index proposed by pettifier and fookes (1994). the mechanical properties of the limestone, such as ucs, (is(5o)) and the friction angle of the discontinuity surfaces measured using a direct shear test, were determined. additionally, the main orientation, spacing, aperture, filling, weathering and roughness of the discontinuities were described using scan–line mapping (isrm, 1981). 3. geology in the quarry and surrounding area, the basement is composed of rhyolitic and rhyodacitic pyroclastics and basaltic volcanics. the quarry mined the various subfacies of the late campanian–paleocene–aged limestone, which appears as red–grey–colored limestone intercalated with claystone, sandstone and marl, conformably overlying the late santonian– figure 2. a general view (a) and a closer view (b) of the failures along the northern parts of the arakli–taşönü limestone quarry (a) (b) figure 3. geological cross section of the study and landslide area the assessment of slope stability and rock excavatability in a limestone quarry 172 early campanian volcanics (ercikdi, 2004; karaman, 2011). due to the composition of the red micrite and pelagic fauna, the studied limestones are concluded to have formed under shallow marine environmental conditions (ceryan, 2009). the geological units of the study area are characterized by intercalations of light grey, dark grey, red and yellow limestones. bioclasts are composed of benthic foraminifera, various-sized bivalve fragments, radiolaria, echinoid fragments, bryozoa and pelagic fossils. the broken macrofossils in the limestone units exposed in the study area imply that the unit transitioned from a primary sedimentation basin. based on the microfossil investigations of the biomicritic limestone, all of the samples have the same age and microfossil content in spite of the color differences (ceryan, 2009; karaman, 2011). in the lower part of the study area, shelly limestone outcrops with abundant karstic voids are observed. all of the rocks from the study area are weathered to various degrees. the main lithological units, starting with the lowest, are rhyolite, rhyodacitic pyroclastics and basaltic volcanics; volcanic tuff; shelly limestone; mudstone; sandy–clayey limestone and marl (ceryan, 2009; karaman, 2011). the stratigraphy of the study area and descriptions of the lithological units are shown in fig. 5. all of the sedimentary units are highly stratified, and their dips change figure 4. a general view of the study area (a) and a closer view of sn–1 (b), sn–2 (c) and sn–3 (d) (a) (b) (c) (d) sn 1 sn 2 landslide area sn 3 new open pit area (study area) kadir karaman, bayram ercikdi, ayhan kesimal 173 figure 5. generalized stratigraphic column of the study area (modified after ceryan, 2009) the assessment of slope stability and rock excavatability in a limestone quarry 174 from 4° to 25° inclined to the se, sw and s. the depth of the open pit is approximate 70 m, and its length is 500 m, ranging from unweathered to moderately weathered zones. the average thickness of bedding planes of the limestones varies between 7 and 35 cm. 4. field and laboratory studies the geotechnical properties of the rock mass exposed in the study area were investigated using both field observations/measurements and laboratory tests. three different slope faces (called sn–1, sn–2 and sn–3) were defined for the study based on their discontinuity properties (figs 4b, c, d). the smalland large-scale roughness, infilling material and karstic conditions of the discontinuities of these slopes were determined according to the sspc system. the orientation and spacing of the discontinuities were described using the scanline mapping method, following the isrm description criteria (isrm, 1981). the discontinuity orientation analyses were performed using commercially available stereographic projection software, dips 5.0 (rocscience, 1999), where the major joint sets for each slope were distinguished. in the study area, a total of 396 joint measurements were taken. the properties of sn–1, sn–2 and sn–3 are given below (table 1). a number of core (50), unshaped (97) and oriented block (16) samples were collected from the field for laboratory testing. laboratory-based direct shear tests were performed on the oriented block samples. the description of rock material and mass characteristics, such as ucs, (is(5o)), shear strength parameters of the discontinuities, spacing, roughness, and aperture of discontinuities, were determined in accordance with the isrm methods (isrm, 1981, 1985). the limestone has an average ucs of 11.40 mpa, an average is(5o) of 1.07 mpa, and an average friction angle of 30.8º (table 2). 5. assessment of slope stability in this study, kinematic, sspc and slope mass rating (smr) analyses were performed to assess the stability of the slopes and therefore to determine safe slope angles and heights for the study area. limit equilibrium analysis was not performed for the slopes, which were kinematically stable. table 1. joint properties of the designated sectors properties of discontinuities sn–1 sn–2 sn–3 j1 j2 j1 j2 j1 j2 large-scale roughness slightly curved slightly wavy straight curved slightly wavy wavy small-scale roughness smooth undulating rough stepped polish undulating smooth undulating rough stepped rough stepped infilling no infill–surface staining no infill–surface staining generally open–without filling (1–3 mm) occasionally open with calcite and clay filling generally open–without filling (1–3 mm) occasionally open with calcite and local clay filling without filling (1–3 mm) occasionally open with calcite and clay filling without filling (1–3 mm) occasionally open with calcite and clay filling weathering degree unweathered unweathered slightly weathered slightly weathered slightly weathered slightly weathered groundwater conditions dry joint surfaces dry joint surfaces dry joint surfaces dry joint surfaces dry joint sufaces dry joint surfaces karst condition available available available available available available 5.1. kinematic analysis the kinematic analysis method, which was first described by hoek and bray (1981), developed by goodman (1989) and modified by wyllie and mah (2004), allows the investigation of potential planar, wedge and toppling failure modes of rock slopes. this method assumes that the shear strength of the sliding surface is composed only of friction and that the cohesion is zero. wyllie and mah (2004) indicated that it is possible to determine the stability conditions of a slope on the same stereonet by analyzing daylight envelopes (i.e., a combined kinematic analysis). for a stable condition, the force vector, which is normal to the plane, must lie within the friction cone. when gravity is the only force acting on a block, the pole to the plane is in the same direction as the normal force. therefore, the block will be stable, provided that the pole point lies within the friction circle. it is also possible to determine the stability conditions according to the relationship between daylight envelopes and pole points (gischig et al., 2011). in this study, a combined kinematic analysis was performed to plot and analyze joint orientation data for each slope in the limestone quarry. the dip and dip directions of the slopes were determined to draw the daylight envelopes. two major joint sets exist in the rock mass, and the dip directions of the slopes vary between ne and nw. the internal friction angles of the discontinuity planes vary from 24º to 33º, which were obtained from the laboratory tests for each slope, and these angles were used for the kinematic analyses (table 3). 5.2. orientation–dependent sspc analysis the orientation–dependent stability analysis identifies the sliding and the toppling mode of failure within a rock mass and assesses the probability of failure based on the condition of the discontinuity, which controls the failure (hack, 1998; lindsay et al., 2001). in the current study, the condition factor (tc) for a discontinuity set was calculated using the equation by hack et al. (2003): tc = rl rs im kc (1) kadir karaman, bayram ercikdi, ayhan kesimal 175 where rl and rs are the large–scale and small–scale roughness of the discontinuity planes, respectively, im is the infilling material in the discontinuities and kc is the presence of karst along the discontinuities. the properties of the joint sets used in the calculation of the condition factor (tc) are shown in table 4. table 4. properties of the joint sets used in the calculation of the condition factor (tc) slope no joint set (j1) joint set (j2) rl rs im kc rl rs im kc 1 0.80 0.75 1.00 0.92 0.95 0.95 1.00 0.92 2 0.75 0.70 0.75 0.92 0.85 0.75 0.65 0.92 3 0.95 0.95 0.90 0.92 1.00 0.95 0.90 0.92 rl: large-scale roughness; rs: small-scale roughness; im: infill material; kc: karstic condition the following prior criteria were taken into account as the boundary conditions for sliding and toppling failure, respectively (hack and price, 1995; hack et al., 2003). tc < 0.0113 ap (2) tc < 0.0087 (–90º– ap+dipdiscontinuity) (3) where tc is the condition factor and ap is the apparent angle of the dip of the discontinuity plane in the direction of the slope dip. ap is quoted by hack (1998) as: ap= arctan (cosδ tandipdiscontinuity) (4) if ap > 0° → ap = apparent discontinuity dip in the direction of the slope dip if ap < 0° → │ap│= apparent discontinuity dip in the direction opposite the slope dip δ =dipdirectionslope – dipdirectiondiscontinuity in the orientation–dependent analysis, additional conditions (table 5), which consider the relationship between ap and slope dip, were also considered for the evaluation of toppling and sliding failure (hack, 1998; lindsay et al., 2001; hack et al., 2003). table 5. additional conditions considered for sliding and toppling modes in the orientation–dependent analysis (hack, 1998; lindsay et al., 2001; hack et al., 2003) assessment of stability sliding toppling ap> 84o or ap< -84 %100 %100 slope dip+5<ap<84 %100 %100 slope dip-5<ap<slope dip+5 %100 %100 0o<ap< (slope dip–5) use graph sliding %100 ap<0 and -90-ap+slope dip<0 %100 %100 ap<0 and -90-ap+slope dip>0 %100 use graph toppling table 2. material properties of the limestone and joints properties number of test mean max. min. std. dev. uniaxial compressive strength (ucs, mpa) 50 11.40 20.89 5.58 4.35 point load strength index (is(50), mpa) 97 1.07 2.10 0.41 0.44 unit weight (γ, kn/m3) 22 22.37 23.25 20.99 0.07 joint set number two major joint sets spacing of discontinuities (cm) 109–1 (average 17.34) aperture of discontinuities (mm) generally 1–3 mm roughness of discontinuities generally smooth undulating, occasionally rough stepped infilling generally without filling, occasionally calcite, clay+silt, silt+sand weathering degree generally slightly weathered, occasionally moderately weathered groundwater conditions generally dry, occasionally damp table 3. the parameters used in the kinematic analyses slope no jı (dip direction/dip) j2 (dip direction/dip) slope (dip direction/dip) ф (°) 1 243/5 140/71 012/75 24 2 180/4 007/70 320/62 31 3 116/15 043/74 340/75 33 the assessment of slope stability and rock excavatability in a limestone quarry 176 5.3. orientation–independent sspc stability in the orientation–independent analysis, srm properties were used to assess the stability of slopes because this analysis considers both the excavation disturbance to which the slope will be subjected and the slope’s susceptibility to weathering. the rrm parameters (rspa, rcd and rucs) were first derived from the erm properties (table 6) and subsequently used in the srm calculations (sspa, scd and sucs) using equations 5 and 6, respectively. it should be noted that rrm is the rock mass in an imaginary, unweathered and undisturbed condition prior to excavation and that the srm is the rock mass in which the existing or new slope is to be situated. rspa= spa/(we me) rcd= cd/we (5) rucs= ucs/we where rspa, rcd and rucs represent the spacing factor, condition of the discontinuities and unconfined compressive strength parameters for the reference rock mass (rrm), respectively. sspa= rspa swe sme scd=rcd swe (6) sucs=rucs swe where sspa, scd and sucs are the spacing factor, condition of the discontinuities and unconfined compressive strength parameters for slope rock mass (srm), respectively. sme is the excavation method, which depends upon the degree of disturbance to which the rock mass is subjected by blasting and pneumatic hammering. in this study, the sme value was considered to be 0.76 and 0.62 for pneumatic hammering and blasting, respectively. swe represents the future weathering conditions and is considered to be between 0.90 and 0.95 (moderate to slight).cd is the condition of the discontinuities (cd) and was calculated from the following equation suggested by hack et al. (2003): where tc1,2 are the condition factors and ds1,2 are the average spacings of the joint sets for each slope (table 6,7). the internal friction angle (φ′mass ) and cohesion (coh′mass ) of the rock mass were obtained from the following equations utilizing the ucs, cd and spacing parameter (spa) (table 6), which were proposed by hack et al. (2003): φ′mass = 0.2417 ucs + 52.12 spa + 5.779 cd (8) coh′mass = 94.27 ucs + 28629 spa + 3593 cd (9) because the dip angle (62–75°) of the slopes was higher than the friction angle (26–32°) of the rock mass, the maximum slope heights (hmax ) were computed using the following equation: where hmax is the maximum possible height of the slope, βs is the dip of the slope, φ′mass is the friction angle of the rock mass, and coh′mass represents the rock mass cohesion. further details of orientation–dependent and orientation–independent analyses of stability can be found elsewhere (hack, 1998; lindsay et al., 2001; hack et al., 2003). 5.4. slope mass rating (smr) the slope mass rating (smr) is a geomechanical classification that is mostly used for the characterization of rock slopes (romana, 1985). the smr is derived from the basic rock mass rating (rmr, bieniawski, 1989) using four factors that take into account the geometrical relationship bettable 6. the erm parameters used in the orientation–independent sspc analysis slope no ucs (mpa) we me spa cd φmass (deg.) cohm (pa) average spacing (m) (ds1) average spacing (m) (ds2) hslope (m) 1 18.85 1.00 0.76 0.42 0.69 30.43 16280.34 0.17 0.16 4.30 2 7.65 0.95 0.76 0.46 0.36 27.91 15183.99 0.08 0.31 5.16 3 11.62 0.95 0.76 0.50 0.77 33.32 18176.53 0.20 0.24 4.30 figure 6. excavatability assessment chart (pettifer and fookes, 1994) of the rocks in the study area (7) (10) kadir karaman, bayram ercikdi, ayhan kesimal 177 ween the slope face and the discontinuities affecting the rock mass as well as the excavation method. the smr is obtained using the following equation: smr = rmrb + (f1 f2 f3) + f4 (11) where rmrb is the basic rmr index resulting from bieniawski’s rock mass classification without any correction. therefore, smr is calculated according to rmr classification parameters (bieniawski, 1989). f1 depends on the parallelism between discontinuity dip direction and slope dip; f2 depends on the discontinuity dip in the planar and toppling mode of failure; f3 depends on the relationship between slope and discontinuity dips; and f4 is a correction factor that depends on the excavation method used. the detailed figure 7. the combined kinematic analyses of sn–1 (a), sn–2 (b) and sn–3 (c) explanations of the factors and the smr can be found elsewhere (romana, 1985; calcaterra et al., 1998; romana et al., 2003; thomas et al., 2012). 6. excavatability assessment in the present study, the assessment of rock excavatability was performed based on the work of pettifer and fookes (1994), which takes into account the types of excavating equipment and geotechnical parameters, such as the discontinuity spacing index (if) and the (is(50)), which are easily obtained through field and laboratory studies. for the (is(50)), a total of 97 samples were collected from the slopes and tested to determine the excavatability of the limestones in the study area. discontinuity spacing index (if) and (is(50)) were calculated based on the procedure recommended by isrm (1981, 1985). the average (is(5o)) for the slopes varied from 0.41 to 2.11 mpa, and the discontinuity spacing index was determined to be between 0.13–0.22 m. the plotting of the data in the excavatability chart for the different limestone formations is shown in fig. 6. the analysis revealed that the excavatability of these formations can be categorized as easy ripping. 7. results and discussion fig. 7 illustrates that the pole points for the joint sets (j1 and j2) and the intersection point of j1–j2 are not located inside the daylight envelope (a) (b) (c) the assessment of slope stability and rock excavatability in a limestone quarry 178 joint sets (j1 and j2) (the current 62° slope dip angle), no risk for failure is expected (sliding and toppling). however, when the dip angle of sn-2 is over 66°, the second discontinuity set will daylight in the slope face of sn-2, in accordance with the additional conditions (0o<ap< slope dip–5). therefore, there appears to be a risk for sliding failure (according to eq. 2) in the second discontinuity set. this could be related to the roughness and infill properties of the discontinuities, which lower the value of tc, increasing the likelihood of sliding failure (table 4) in the second discontinuity set. according to the orientation–dependent stability analyses, sn–2 will experience a sliding failure only if its dip angle is greater than 66°, whereas sn–1 and sn–3 will be stable at all dip angles (0–90°). for any modes of failure. the pole points indicate that all major joint sets on the slopes are not critical for slope stability. planar failure does not occur on the slopes of the studied area because the pole points of the discontinuities are not located inside the planar daylight envelope (fig. 7). the pole of intersection point of the joints lies within the friction cone or safe region because the dip angle of j1 is very small. this suggests that wedge failure does not occur. similarly, toppling failure is not expected to occur because of the orientation of the discontinuities and the slopes in the studied area. the results of the kinematic analyses indicate that no failure (e.g., planar, wedge or toppling) would occur in the slopes. the dip direction (s and se) of the dominant joint set (j1), which has a very small dip angle and is opposite to the direction of the slope angle (n, ne and nw) (table 3), contributes to the stability of the slopes. kesimal et al. (2008) claimed that uncontrolled blasting and high slope angles were the primary reasons for the earlier landslide in the quarry. field studies also suggest that the similarity in the dip directions for the slopes and the discontinuities could have led to the instability of the slope. in this regard, slopes with dip directions opposite those of the discontinuities are expected to have greater stability in the new study area. the kinematic analyses have revealed that the slopes (sn–1, sn–2 and sn–3) with dip angles (0–90°) would not impose planar, toppling or wedge failures. the orientation–dependent analysis with the sspc system based on the pre–conditions (eq. 2 and 3) showed that the sliding and toppling failures would not occur for the second discontinuity sets of sn–1 and sn–3. the probability of stability for the second discontinuity sets for these slopes is over 95% (fig. 8). in accordance with the additional conditions (table 5), the first set of discontinuities for sn–1 and sn–3 (in their current slope dip angles, 75°) possess a 100% stability against sliding and toppling failures (table 7). furthermore, there will be no sliding and toppling failure risk (the probability of stability is over 95%) for the first joint sets of sn–1 and sn–3, even though the dip angles of these slopes are over 86° and 79°, respectively. when additional conditions (table 5) are taken into account for the sn–2 figure 8. orientation–dependent stability with respect to sliding (a) and toppling failure design charts (b) for the second discontinuity sets of sn–3 and sn–1 table 7. assessment of orientation–dependent stability analyses according to additional conditions slope no j1 j2 slope definition of stabilitytc ap sld % tpl % tc ap sld% tpl % sld % tpl % 1 0.55 -3.14 100 100 0.83 -60.78 100 >95 100 >95 stable 2 0.36 -3.07 100 100 0.38 61.91 100 100 100 100 stable 3 0.75 -10.91 100 100 0.79 57.72 >95 100 >95 100 stable sld: sliding; tpl: toppling figure 9. effect of excavation method on the orientation–independent stability of slopes (a) sn3-j2 sn1-j2 • • (b) pneumatic hammer blasting sn1 sn2 sn3 kadir karaman, bayram ercikdi, ayhan kesimal 179 based on the current dip angle of the slopes (75°, 62° and 75° for sn–1, 2 and 3, respectively), the orientation–independent stability analysis for these slopes was performed for instances of excavation by pneumatic hammering and blasting (fig. 9). the slopes were determined to have a probability of ≥80% stability for both excavation methods. however, the probability for stability was 6–14% lower using blasting compared with pneumatic hammering. blasting can create adverse rock mass disturbances. the calculated hmax values (figs. 10a, b) for varying slope angles (60–90°) suggest that the hmax is required to be low for blasting to ensure slope stability. given the excavatability of rocks was classified as easy ripping (fig. 6), the most suitable excavation method is determined to be pneumatic hammering for the new production site. in the orientation–independent stability analysis, the hmax of sn–2 for stability was calculated to be 8.22 m for a dip angle of 66º. for a 90º slope angle, the hmax of sn–1 and sn–3 would be 4.19 and 4.93 m, respectively (fig. 10a). however, the orientation–independent stability figure 10. the maximum slope heights (hmax) according to the means of excavation: pneumatic hammering (a) and blasting (b). analysis at this angle for sn–1 and sn–3 predicts a 40-50% reduction in stability compared with the current situation. rmrb values, which were calculated based on the described parameters (ucs, rock quality designation (rqd), discontinuity spacing, discontinuity condition and groundwater condition), were found to be 53, 48 and 59 for sn–1, sn–2 and sn–3, respectively (table 8). these data suggest that the rocks in these slopes can be classified as ‘fair rock,’ according to bieniawski (1989). the smr values derived from eq. 11 for the current orientation of the discontinuities (j1 and j2) and slopes showed that the slopes are ‘partially stable (some joints and many wedge failures)’ under mechanical excavation, according to romana (1985). however, the sspc system produces a more distinctive differentiation between stable and unstable conditions than the smr systems. sn–1 and sn–2 will be ‘unstable (planar or big wedge failures)’ when blasting is selected as the excavation method (table 8). this is consistent with the results of the sspc that suggest that blasting reduces the probable stability of the slopes. the stability table 8. assessment of the stability of slopes based on the excavation methods according to the smr slope no rmrb failure mode smr (j1) smr (j2) stability mech. excavation poor blasting mech. excavation poor blasting mech. excavation poor blasting sn–1 53 planar 52 44 46 38 partially stable unstable toppling 53 45 49 41 partially stablewedge 52 44 52 44 sn–2 48 planar 47 39 47 39 partially stable unstabletoppling 48 40 44 36 wedge 47 39 47 39 sn–3 59 planar 58 50 52 44 partially stable partially stable toppling 59 51 55 47 wedge 58 50 58 50 the assessment of slope stability and rock excavatability in a limestone quarry 180 of sn–3 seems to be unaffected by different excavation methods, which is most likely due to its high rmr values. there will be no failure risk (planar or big wedges) for the sn–1 and sn–3 joint sets (j1 and j2), even if the dip angles of the slopes are 90°. however, the orientation–independent sspc analysis indicated the reduction (40–50%) in the stabilities of these slopes at this angle. the disparity could be related to the orientation–independent stability analysis (slope height and future weathering), which are not taken into account in the smr classification system (romana et al., 2003; lindsay et al., 2001). sn–2 will be unstable (planar or big wedge failures) if its dip angle is greater than 80°. based on the slope stability analyses (kinematic, sspc and smr), the proposed safe slope angles for the pneumatic excavation of sn–1, 2 and 3 can be taken as 70°, 66° and 75°, respectively, at a slope height of 8 m. 8. conclusion in this study, kinematic, sspc (orientation–dependent and orientation–independent) and smr analyses were performed to evaluate the stability of rock slopes in a limestone quarry. the excavatability of the limestones was also assessed in association with the slope stability analyses. the geotechnical parameters (discontinuity properties, ucs, friction angle, etc.) used in the assessment of the stability and excavatability of the rock slopes were obtained from field and laboratory studies. the kinematic analyses of the slopes indicated that no failure is expected to occur for the slopes, even for high dip angles of the slopes. the kinematic analyses have also shown that the discontinuities dip in opposite directions to the slopes, which improves the stability of slopes. the orientation–dependent sspc analysis revealed that no failure will occur for sn–1 and sn–3 at a dip angle of 0–90°. however, a sliding failure is most likely to occur for sn–2 if the dip angle is greater than 66°. the orientation–independent stability analysis has shown that the slopes have a stability probability of ≥80% if pneumatic and blasting methods are used for excavation. however, the blasting method appeared to reduce the stability probability of the slopes as well as the hmax. the excavatability of rocks was determined to be ‘easy ripping’ and, as the orientation–independent stability analysis suggests, the pneumatic hammer method is recommended as the means of excavation. the orientation–independent sspc analysis also indicated that the internal friction and cohesion of rock mass, as well as the slope height and dip angle, are the important parameters in the stability of slopes. the smr analysis has shown that sn–1, sn–2 and sn–3 are ‘partially stable (some joints and many wedge failures)’ when mechanical excavation is used. it was deduced from the kinematic, sspc and smr analyses that the safe dip angles of sn–1, sn–2 and sn–3 should be 70°, 66° and 75°, respectively, with a maximum height of 8 m to ensure the stability of these slopes. acknowledgements the authors would like to acknowledge karadeniz technical university (ktu) for funding this work through research project no. 2008.112.008.1. we would like to express our sincere thanks and appreciation to the askale cement company for providing help during the study, assistant prof. dr. emel abdioglu for the geological evaluation and dr. ayberk kaya, şener aliyazıcıoğlu and assoc. prof. dr. hacı deveci for improving the quality of the manuscript. references alejano, l.r., ferrero, a.m., oyanguren, p.r., fernandes, m.i.a. 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(2004). rock slope engineering civil and mining. 4th ed. new york, spon pres. 431 pp. the assessment of slope stability and rock excavatability in a limestone quarry debris flows in tributaries rush into and block the main branches of rivers and often result in serious hazards. dam failures cause large floods in the downstream area and can lead to fatalities and property damage. this study proposes an assessment method to evaluate the formation of a debris flow dam, which includes two conditions: (1) the sediment transported by the debris flow must reach across the river; and (2) the thickness of the deposit by the debris flow must be higher than the in situ water depth. this methodology was used to study the case of a debris flow dam caused by debris flow across the er river in taiwan, which blocked the chishan river and led to the formation of the namasha debris flow dam. this methodology can also be applied to identify the formation of debris flow dams. abstract keywords: debris flow dam; dam failure; assessment method an assessment method for debris flow dam formation in taiwan issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.62389 kun-ting chen1, chia-hsing lin2, xiao-qing chen1*, gui-sheng hu1, xiao-jun guo1 and chjeng-lun shieh2 1 key laboratory of mountain hazards and earth surface processes, institute of mountain hazards and environment, chinese academy of sciences, chengdu, 610041, china. 2 department of hydraulic and ocean engineering, national cheng kung university, tainan, 70101, chinese taipei. *corresponding author: xqchen@imde.ac.cn earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 37-43 el flujo de detritos que cae en los tributarios de los ríos puede bloquear los ramales principales y eventualmente convertirse en un riesgo. el rompimiento de uno de estos represamientos de agua puede causar inundaciones en las zonas de la corriente, además de víctimas y daños a propiedades. este estudio propone un método para evaluar la formación de represamientos de agua por flujo de detritos bajo dos condiciones: (1) los sedimentos transportados por el flujo de detritos deben alcanzar el lecho del río; (2) el grosor de los depósitos por el flujo de detritos debe ser mayor que la profundidad de agua in situ. esta metodología se utilizó para estudiar el caso de represamiento por el flujo de detritos en el río er de taiwán, el cual bloqueó el río chishan y que condujo a la formación de la presa namasha. esta metodología también puede aplicarse para identificar la formación de represamientos por flujo de detritos. resumen palabras clave: represamiento por flujo de detritos; rompimiento de represamientos; método de evaluación record manuscript received: 02/02/2017 accepted for publication: 12/01/2018 how to cite item: chen, k. t., lin, c. h., chen, x. q., hu, g. s., guo, x. j., & shieh, c. l. (2018). an assessment method for debris flow dam formation in taiwan. earth sciences research journal, 21(2), 37-43. doi: http://dx.doi.org/10.15446/esrj.v22n1.62389 método de evaluación en la formación de represamientos por el flujo de detritos en taiwán en g in ee r in g g eo lo g y 38 kun-ting chen, chia-hsing lin, xiao-qing chen, gui-sheng hu, xiao-jun guo and chjeng-lun shieh 1. introduction fragile geological conditions, frequent and intensive typhoon attacks and heavy rainfall can cause debris flows to transport large amounts of sediment, which can lead to the formation of dams on rivers in taiwan (chen, 1999; kuo et al. 2011; zhou, et al. 2012; dong et al. 2014). approximately 40-50% of the dams fail within 1 week, and only 10-15% remain for one year or longer (costa and schuster, 1988; peng and zhang, 2012). when a dam fails, the materials of the dam are rapidly transported by the river flow to the downstream area and can cause serious debris flow, river bank erosion and flooding hazards downstream of the catchment (ermini and casagli, 2003; korup, 2004; dong et al. 2011; li et al. 2011; peng and zhang, 2012; chen et al. 2014; shrestha and nakagawa, 2016). typhoon morakot (2009/08/08) brought 1,911 mm of precipitation to southern taiwan via heavy rainfall and caused serious compound disasters. the maximum hourly rainfall, 24-h rainfall, and 48-h rainfall were recorded as 94.5, 1,077.5 and 1,601 mm, respectively. compound disasters included landslides, landslide dams, debris flows and floods (chen et al. 2011; dong et al. 2011; lee et al. 2013). the extreme continuous rainfall triggered 20 debris flows (jan et al. 2011), the majority of which blocked main branches of rivers. dam formations are difficult to evaluate because short-lived dams typically fail before an investigation can be conducted. therefore, a theoretical method to evaluate dam formation is important for disaster prevention. three compound relations between the debris flow and the main branch of the river are typically important for dam formation: (1) the deposition volume of debris flow that crosses the river is larger than the minimum river-blocking volume; (2) the deposition thickness is greater than the water depth of the main branch of the river; and (3) the amount of sediment supplied by the debris flow is larger than the amount of erosion by the river flow (zhou et al. 1991; wu et al. 2005; cheng et al. 2005). these conditions are affected by the confluence angles, the discharge ratio, the velocity ratio between the tributary and main branch of the river, the initial shear stress of the debris flow, the bulk density of the debris flow, and the grain sizes. these key factors were used to establish the blockage index or critical index based on experiment results (cui et al. 2006; cheng et al. 2007; dang et al. 2009; du et al. 2014). although the criteria were established, some important issues were not considered, namely, (1) whether the debris flow can run across the main branch of the river and (2) whether the minimum deposition thickness of the debris flow is higher than the water depth of the main branch of the river. in addition, the discharge ratios and velocity ratios considered in previous experiments were relatively large and may not be suitable in certain regions, such as taiwan. this study aimed to develop an assessment method to evaluate debris flow dam formation considering the debris flow volume, velocity, deposition slope angle, possible run-out distance, and deposition depth. the assessment method can evaluate debris flows that dam rivers at different watershed scales. 2. study area the namasha debris flow dam is located at the upstream cishan river in kaohsiung county, taiwan (figure 1; twd67, e: 223973 and n: 2579888). according to satellite images and field investigations, the dam area is approximately 0.27 km2, the backwater length is approximately 1.7 km, and the water storage volume is approximately 1,850,000 m3 (figure 2). in 2009, typhoon morakot brought 1,911 mm of rainfall over a three-day period in southern taiwan. the total accumulated rainfall was more than 2,000 mm, which triggered a large-scale landslide in the upstream of the cishan river watershed. the main rainfall on aug. 7-9 brought rainfall with a maximum peak hourly intensity of 94.5 mm/h, and the duration was 99 h (dong et al. 2014). the debris flow from the er river catchment flowed to the main branch of the cishan river with a confluence angle of approximately 90º and formed a large deposition fan at the junction of the cishan river and er river. the debris flow fan formed the dam, and the dam height was approximately 9.3 m. when the main branch of the cishan river was blocked by the debris flow fan, the dam was formed and the flow storage behind the dam gradually increased. the water level increased rapidly and began to flow over the top of the dam. the debris flow dam was overtopped and formed a 20-m-wide spillway. during the typhoons of fanapi (2010), the maximum peak hourly rainfall intensity was 34 mm/h, which triggered a debris flow from the er river and blocked the main branch of cishan river again. the flooding caused the overtopping erosion of the debris flow dam. the pingtung forest district office of the forestry bureau performed flow-divergent engineering treatment to divert the flow of the river back to its original direction (taiwan forestry bureau, 2013). for the namasha debris flow dam, the tributary watershed area of the er river upstream is approximately 6.7 km2. the mean channel width, upstream slope and downstream slope of the er river are 10 m, 17.8º, and 0º, respectively. these parameters can be evaluated using gis. evaluations of river blocking by debris flow using several empirical formulas (cui et al. 2006; cheng et al. 2007; dang et al. 2009) indicated that damming of the river did not occur because of the confluence of the cishan river and er river. the results of these empirical formulas were unsatisfactory because of several reasons. first, for different watershed scales, a tributary debris flow rushes into the main branch of the river and forms a landslide dam; a smaller discharge-velocity ratio may easily form a landslide dam under a smaller watershed. second, the initial shear stress can affect the assessment result for the debris flow dam formation and requires a precise value. the initial shear stress is not easily obtained or assumed when evaluating the formation of a debris flow dam. third, the topographical effect plays an important role in debris flow dam formation; steep narrow valleys require relatively small volumes or discharge of debris flow to block the main branch of the river and form a dam (costa and schuster, 1988). the relationships between the run-out distance and deposition thickness of a debris flow must be considered when analyzing debris flow dam formation. du et al. (2014) proposed that if the run-out distance of a debris flow was larger than the width of the main branch of the river, the debris flow might block the river and form a landslide dam. however, this result did not consider the relationship between the deposited thickness of the debris flow and the water depth of the main branch of the river. therefore, a more appropriate assessment methodology is necessary in this study. figure 1 . location of the cishan river watershed in taiwan 39an assessment method for debris flow dam formation in taiwan figure 2. satellite image of the namasha landslide dam after typhoon morakot 3. assessment method the method for evaluating the formation of a debris flow dam, based on the two characteristics of river blocking, is described in figure 3. the potential for the formation of a debris flow dam relies on whether the run-out distance lc is larger than the river width b and whether the deposited thickness zmin by the debris flow is larger than the in situ water depth hw. to rapidly assess the debris flow dam that has not occurred, these parameters of the assessment method can be evaluated based on the satellite images, topographical maps (1/25,000), dtm and assumed conditions. two dam formation conditions under a debris flow event were established based on these parameters. figure 3. flowchart for evaluating debris flow dam formation 3.1 sediment transported by debris flow the sediment from the tributary deposits at the confluence to form a debris flow fan. the debris flow fan must reach across the river. if the confluence angle is 90º, the run-out distance of the debris flow lc must be greater than the average width b of the main branch of the river. the run-out distance was estimated by takahashi and yoshida (1979), as shown in equations (1), (2) and (3). if the confluence angle is larger than 90°, lc should be replaced by lc sin α, where α is the confluence angle of the tributary and main branch of the river (du et al. 2014). the momentum conservation of the debris flow is (1) (2) (3) (4) (5) (6) where ơ is the density of the particle; ρ is the density of the fluid; g is the acceleration due to gravity; θu is the upstream slope of the tributary; θd is the downstream slope of the tributary; and ϕk is the kinematic friction angle (tsai (1999) proposed that ϕk =31º ); ka= tan2 (45ºϕ /2) is the coefficient of active earth pressure ( ϕ is the internal friction angle; tsai (1999) proposed that ϕ=37º); hd is the mean depth of the debris flow; u is the mean velocity of the debris flow; and cd is the volume concentration of the debris flow, as shown in figure 4. the volume concentration of the debris flow cd can be substituted with the equilibrium concentration cd∞. takahashi (1977) derived the equilibrium concentration as follows: the debris flow is assumed to be a uniform flow based on the conservation of mass of water and sediment. the debris flow discharge qd is calculated in eq. 5. where bd is the mean channel width of the tributary. uhbq ddd = figure 4. run-out distance of the debris flow tsai (1999) derived the following relation between the debris flow discharge qd and the rainfall intensity i in taiwan: 40 kun-ting chen, chia-hsing lin, xiao-qing chen, gui-sheng hu, xiao-jun guo and chjeng-lun shieh where c* is the volume concentration of the debris flow in deposition (tsai (1999) proposed that c* = 0.67); c is the runoff coefficient (assumed to be 0.8); i is the rainfall intensity; and a is the watershed area of the tributary. based on the constitutive relationship (bagnold, 1954), takahashi (1977) evaluated the mean velocity of the debris flow u using eq. (7). (7) (8) (9) (10) (11) (12) where d is the particle diameter and can be represented by the mean particle diameter d50; and a is the bagnold number, which is equal to 0.042 (bagnold, 1954). substituting eqs. (6) and (7) into eq. (5), we obtain the mean depth hd and mean velocity u of the debris flow. finally, the run-out distance lc can be estimated. 3.2 deposition thickness by debris flow the configurations of debris flow fans were studied in a series of experiments and field investigations (tsai, 1999). takahashi (1991) proposed the following equation for the deposition slope of a debris flow fan tanγ : where q is the unit width discharge of the debris flow (q = uhd). the deposition thickness of a debris flow zmax is provided by the geometric relationship between the deposition slope of the debris flow fan tanγ and the run-out distance of the debris flow lc as follows: we suppose that the minimum deposition thickness of the debris flow zmin must be higher than the water level of the main branch of the river hw. otherwise, the debris flow fan is submerged. in addition, the deposition gradient of the debris flow fan tanγ remains constant. the minimum deposition thickness of the debris flow zmin is given by the following geometric relationship (figure 5): to obtain the water depth hw of the main branch of the river, the discharge of the main branch of the river at the debris flow dam must be estimated. the area ratio method can be used to estimate the discharge of the main branch of the river at the debris flow dam as follows: where qw1 and qw2 are the discharge of the main branch of the river at the debris flow dam and downstream gauging station, respectively. a1 and a2 are the upstream watershed areas of the debris flow dam and downstream gauging station, respectively. substituting the discharge of the main branch of the river at the debris flow dam qw1 into manning’s formula, we obtain the mean depth of the main branch of the river hw as follows: where aw is the cross-sectional area of the flow; uw is the mean velocity of the flow; n is manning’s coefficient; and s is the mean slope of the main branch of the river. figure 5. deposition thickness and minimum deposition thickness formed by the debris flow 4. validation of the assessment method 4.1 parameters of the namasha debris flow dam in this study, typhoon morakot and typhoon fanapi were adopted to evaluate the formation of namasha debris flow dam. for the namasha debris flow dam in typhoon morakot, the volume concentration of the debris flow cd can be calculated using eq. (4); the value is approximately 0.451. according to the calculation, the debris flow discharge during typhoon morakot approached 431 m3/s, and the mean velocity u and mean depth hd of the debris flow were 35.7 m/s and 1.21 m, respectively. from the field investigations and photographic techniques used to estimate the grain size distribution (casagli et al. 2003), the mean particle diameter d50 is approximately 20 mm. the density of water is 1, the density of particle is 2.65, and we assume that the internal friction angle ϕ , kinematic friction angle ϕk, and volume concentration of the debris flow being deposited c* are 37º, 31º, and 0.67, respectively. the watershed area of the confluence upstream for the cishan river and er river is approximately 212 km2, and the mean river width b and the mean river slope s are approximately 40 m and 2.04º, respectively. during typhoon morakot, the peak discharge in the recorded data was 3,870 m3/s at the shanlin bridge. the area ratio method can be used to obtain the cishan river flow discharge, which was approximately 1,630 m3/s at the namasha landslide dam with a water depth of 3.6 m. in typhoon fanapi event, the mean channel width, upstream slope and downstream slope of the er river had been changed to 22 m, 18.3º, and 0º, respectively. calculating by eq. (4), the volume concentration of debris flow cd is approximately 0.474. according to the calculation, the debris flow discharge approached 173 m3/s, and the mean velocity u and mean depth hd of the debris flow were 17.7 m/s and 0.44 m, respectively. in the research report (taiwan forestry bureau, 2013), the mean particle diameter d50, the mean river width b, and mean river slope s of cishan river are 8mm, 20 m, and 0.9º, respectively. the peak discharge in the recorded data was 860 m3/s at the shanlin bridge during typhoon fanapi. the area ratio method can be used to estimate the cishan river flow discharge, which was approximately 362 m3/s at the namasha landslide dam with a water depth of 2.93 m. to understand the formation mechanics of the namasha debris flow dam in the events of typhoon morakot and typhoon fanapi, the criteria of debris flow dam formation must be calculated using the assessment method. 41an assessment method for debris flow dam formation in taiwan 4.2 evaluation of the run-out distance and minimum deposited thickness of debris flow the run-out distance of the debris flow lc for typhoon morakot and typhoon fanapi events, which can be estimated by eq. (1) for the er river, is approximately 463 m and 111 m, respectively. and the mean river width of cishan river b is approximately 40 m and 20 m, respectively. these values satisfy the lc>b condition, which indicates that the debris flow from er river can reach across the main branch of the cishan river in typhoon morakot and typhoon fanapi events (table 1). in addition, the deposition slope of the debris flow fan tanγ and the minimum deposition thickness of the debris flow zmin must be estimated. the deposition slope of the debris flow fan from the er river tanγ is approximately 0.048 and 0.074. and the minimum deposition thickness of the debris flow zmin is 20 m and 8 m, respectively. the estimated mean water depth of the cishan river hw is approximately 3.6 m and 2.9 m in typhoon morakot and typhoon fanapi events by means of manning’s formula. these results indicate that the deposition thickness are higher than the water depth (zmin > hw ) and will thus form the landslide dam in typhoon morakot and typhoon fanapi events. all of the parameters are listed in table 2. table 1. parameters of the debris flow run-out distance table 2. parameters of the minimum deposition thickness and gradient 5. discussion the formation conditions of debris flow dams must be considered to ensure that the evaluation method is applicable. the assessment method results show that our prediction method can be applied in our study area. indeed, some parameters cannot be obtained, such as the initial shear stress, the volume concentration of the debris flow in deposition, and the internal friction angle; thus, the appropriate assumptions are necessary. notably, we use two evaluation conditions to predict the formation of the landslide dam resulting from the debris flow based on debris flow theory, at least in the debris flow that had not occurred. the debris flow theory parameters can be estimated from historical data in our study; therefore, the prediction method is more useful for pre-event prediction. for the accurate pre-event prediction of landslide dam formation, the in situ landslide dam parameters are hard to obtain. therefore, the parameter sensitivity need to be scrutinized for finding out the most sensitive parameters (alejano et al. 2010; gonzález-ollauri and mickovski, 2014). the prediction result of typhoon morakot case is used to illustrate the sensitivity of input parameters. one parameter will be varied at a time while holding the others fixed. base on debris flow discharge is about 431 m3/s in typhoon morakot event, the sensitivity analysis of the density of the particle ơ, the kinematic friction angle ϕk, the volume concentration of the debris flow in deposition c* and the internal friction angle ϕ were (13) (14) assessed by the sensitivity index si and the percentage of variation pv (daniel, 1973) and changing the parameter’s base value by ± 10 %. where lc is run-out distance of the debris flow in typhoon morakot event; lci is run-out distance of the debris flow at changed value; lm is average between lc and lci; pc is parameter’s base value; pci is parameter’s changed value; pm is average between pc and pci. table 3 and figure 6 present the parameter sensitivity analysis results. the internal friction angle ϕ is a most sensitive parameter for the assessment method. the base value of 10% change gets a percentage of variation of 62%. this range of change is considerably more than other parameter. it should be cautiously used for accurate estimation of internal friction angle ϕ in assessment method. the assessment method to evaluate the formation of debris flow dams has limitations. this method cannot react to the effects of the water flow action to form a dam, such as the water static pressure, drag force and buoyancy. we will continue to develop and improve the assessment method in future studies. table 3. parameters of 10% change and sensitivity analysis pv pv 42 kun-ting chen, chia-hsing lin, xiao-qing chen, gui-sheng hu, xiao-jun guo and chjeng-lun shieh figure 6. sensitivity analysis of parameters by the sensitivity index si and the percentage of variation pv 6. conclusions this study presents an assessment method to evaluate the formation of debris flow dams, which can be described under two conditions: (1) the sediment transported by the debris flow must reach across the river; 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(2012). experimental study on cascading landslide dam failures by upstream flows, landslides 10(5), 633-643. doi: 10.1007/s10346-012-0352-6 hua deng1,2,*, yan li1,2, yingchao zhang2, hou zhou1, peipei cheng1, jia wang3, muhammad aqeel ashraf4 1key laboratory of meteorological disaster of ministry of education, nanjing university of information science and technology, nanjing 210044, china 2collaborative innovation center on forecast and evaluation of meteorological disasters, nanjing 210044, china 3inner mongolia meteorological service center, huhehaote 010051, china 4international water, air & soil conservation society 59200 kuala lumpur, malaysia *email of corresponding author: denghua@nuist.edu.cn; dengnll@hotmail.com the forecast of wind energy is closely linked to the prediction of the variation of winds over very short time intervals. four wind towers located in the inner mongolia were selected to understand wind power resources in the compound plateau region. the mesoscale weather research and forecasting combining yonsei university scheme and noah land surface model (wrf/ysu/noah) with 1-km horizontal resolution and 10-min time resolution were used to be as the wind numerical weather prediction (nwp) model. three statistical techniques, persistence, back-propagation artificial neural network (bp-ann), and least square support vector machine (ls-svm) were used to improve the wind speed forecasts at a typical wind turbine hub height (70 m) along with the wrf/ysu/noah output. the current physicalstatistical forecasting techniques exhibit good skill in three different time scales: (1) short-term (dayahead); (2) immediate-short-term (6-h ahead); and (3) nowcasting (1-h ahead). the forecast method, which combined wrf/ysu/noah outputs, persistence, and ls-svm methods, increases the forecast skill by 26.3-49.4% compared to the direct outputs of numerical wrf/ysu/noah model. also, this approach captures well the diurnal cycle and seasonal variability of wind speeds, as well as wind direction. la estimación de la energía eólica está relacionada con la predicción en la variación de los vientos en pequeños intervalos de tiempo. se seleccionaron cuatro torres eólicas ubicadas al interior de mongolia para estudiar los recursos eólicos en la complejidad de un altiplano. se utilizó la investigación climática a mesoscala y la combinación del esquema de la universidad yonsei con el modelo de superficie terrestre noah (wrf/ysu/noah), con resolución de 1km horizontal y 10 minutos, como el modelo numérico de predicción meteorológica (nwp, del inglés numerical weather prediction). se utilizaron tres técnicas estadísticas, persistencia, propagación hacia atrás en redes neuronales artificiales y máquina de vectores de soporte-mínimos cuadrados (ls-svm, del inglés least square support vector machine), para mejorar la predicción de la velocidad del viento en una turbina con la altura del eje a 70 metros y se complementó con los resultados del wrf/ysu/noah. las técnicas de predicción físico-estadísticas actuales tienen un buen desempeo en tres escalas de tiempo: (1) corto plazo, un día en adelante; (2) mediano plazo, de seis días en adelante; (3) cercano, una hora en adelante. este método de predicción, que combina los resultados wrf/ysu/noah con los métodos de persistencia y ls-svm incrementa la precisión de predicción entre 26,3 y 49,4 por ciento, comparado con los resultados directos del modelo numérico wrf/ysu/noah. además, este método diferencia la variabilidad de las estaciones y el ciclo diurno en la velocidad y la dirección del viento. abstract resumen keywords: wind forecast, wrf/ysu/noah, bp-ann, ls-svm palabras clave: predicción del viento, esquema de la universidad yonsei combinado con el modelo de superficie terrestre noah (wrf/ ysu/noah), propagación hacia atrás en redes neuronales artificiales, máquina de vectores de soporte-mínimos cuadrados. prediction of hub height winds over the plateau terrain by using wrf /ysu/noah and statistical forecast record manuscript received: 01/03/2017 accepted for publication: 31/03/2017 how to cite item deng, h., li, y., zhang, y., zhou. h., cheng, p., wang, j., ashraf, m. a. (2017). prediction of hub height winds over the plateau terrain by using wrf /ysu/noah and statistical forecast. earth sciences research journal, 21(1). 37-43. doi:http://dx.doi.org/10.15446/esrj.v21n1.63004 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.63004 predicción de vientos en una altiplanicie a la altura del eje con el esquema de la universidad yonsei/modelo superficie terrestre noah y la predicción estadística earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 37 43 m e t e o r o l o g y 38 hua deng, yan li, yingchao zhang, hou zhou, peipei cheng, jia wang and muhammad aqeel ashraf 1. introduction the introduction of significant amounts of wind energy into power systems makes accurate wind power forecasting as a crucial element of modern electrical grids. power systems require forecasts with temporal scales of tens of minutes to a few days at wind farm locations. traditionally these forecasts predict the wind at turbine hub heights, and then they are converted into power output predictions at wind farms. since the available wind power is proportional to the wind speed cubed, even small wind forecast errors result in significant power prediction errors. accurate wind forecasts with high temporal scales and spatial resolution at the location of wind farms are significant and essential components in the integration of wind power into power systems. numerous efforts have been undertaken to predict the wind at a hub height of large wind farms using numerical weather prediction (nwp) models (asis et al., 2017; giebel et al. 2011) or statistical models. nwp models focus on the thermodynamic characteristics of the atmosphere coupling with the complex topography and land cover, which have the temporal scale of a few minutes to a day and horizontal spatial scale of a few kilometers to several hundred kilometers (lindang, 2017; pielke, 2002). however, nwp models systematic errors arise from errors in the initial conditions or parameterization. the statistical approach is based on training with measurement data and uses the difference between the predicted and the actual wind speeds in immediate past to tune model parameters, which are more economical regarding computer resources. usually, statistical methods forecast for lead times shorter than 6 hours and the mathematical relationships should be formulated from the past observation as long as possible. therefore, nwp model and statistical post-processing models can be combined to accurately forecast the hub-height winds at wind farms in complex terrain (landberg 2001). the physical models try to use environmental considerations as long as possible to reach the best possible estimate of the local wind speed. statistical models try to find the relationships between nwp results and observations to reduce the remaining error. the physical methods (nwp model) differ widely in their model formulations, spatial and temporal resolutions, and parameterizations. since wind turbines are situated in the planetary boundary layer (pbl), physics options including the pbl scheme and land surface model taking the variation of the wind with height into account are proposed. wind power statistical models, such as calman filter algorithm, artificial neural network (ann, carolin and fernandez 2007; li et al. 2009) and support vector machine (svm, yang et al. 2009; zhang and zeng 2010) models, have been shown to outperform the conventional statistical models as artificial intelligence methods. the combined statistical models, including 2 or more statistical models, overcomes the limitations of one statistical model and improve the accuracy effectively (peng et al. 2011). then, which mesoscale nwp model should be proposed to predict the hub-height winds, how to set up the nwp model, and which post-processing statistical modules could correct the errors, are significant problems and need more study. this study aims to accurately predict the hub heights (70 m) winds at a wind farm on a plateau located in the inner mongolia using a hybrid approach, which combines physical and statistical approach. the weather research and forecasting (wrf) model was used as the mesoscale nwp because wrf is evidenced to have the ability to simulate the near-surface wind even in complex terrain (lai et al., 2017; shimada et al. 2011; deppe et al. 2012). the most widely used planetary boundary layer (pbl) schemes, the yonsei university scheme (ysu), and the noah land surface model were combined with the wrf model. the statistical post-processing models were selected to accurate the nwp model output, including persistence (p), back-propagation artificial neural network (bp-ann) and least square support vector machine (ls-svm) models. the short-term forecasting (day-ahead), immediate-short-term forecasting (6-h ahead) and nowcasting (1-h ahead) were selected to verify the above dynamicalstatistical forecast. this work contributes to a better understanding of the wind conditions and the predictability of the hub-height winds for wind farms. in section 2, we describe the wind data, the wrf/ysu/noah, and the statistical models. section 3 presents the results, which are summarized and discussed in section 4. 2. data and methodology 2.1. data the wind observations are taken from the four meteorological towers (05009, 05010, 05011, 05012; see table a2) on a wind farm in inner mongolia at different height (10 m, 30 m, 50 m, 70 m, and 100 m) with a 10-min time step from 2 january to 31 december in 2010. based on the homogeneity test, the observations from 2 january to 22 march in 05010 and from 17 march to 22 march in 05011 are removed (li et al. 2015). table 1. the four meteorological towers used in this study. 2.2. wrf /ysu /noah the mesoscale model used in this study is the non-hydrostatic wrf model version 3.3.1 (skamarock et al. 2008). there are 34 vertical levels with higher resolution in pbl. the top of the model is 50 hpa, and the lowest is 10 m above ground level (agl). there are 12 levels in the lowest 1 km agl to improve the vertical resolution in pbl. wrf model is configured with four two-way nested domains with horizontal grid spacing of 27 km (d1), 9 km (d2), 3 km (d3), and 1 km (d4), respectively (fig. b1). the outermost domain covers the most of china, the south china sea and some parts of the pacific ocean, which provides the background circulation for the inner grid. the private domain covers the research region to simulate the local circulation system (yu & rahim, 2017). model physics options include the wrf single-moment 5-class (wsm5) scheme for microphysical parameterization (hong et al. 2004), the kain-fritsch cumulus scheme (kain 2004) only for the utmost two domains, the dudhia (1989) shortwave radiation scheme, and the rapid radioactive transfer model (rrtm) for radiation (mlawer et al. 1997). we found pbl schemes and land surface models are important for the reasonable prediction of the near-surface winds. then the ysu pbl parameterization with the monin-obukhow surface layer scheme (hong et al. 2006) and noah land surface model (ek et al. 2003) were selected after some examinations (li et al. 2015). table 2. root mean square error (rmse) for different prediction methods and different forecast lengths to 10 day test period from the 20th to 30th of each month in 2010. (unit: m·s-1) 39prediction of hub height winds over the plateau terrain by using wrf /ysu/noah and statistical forecast the national centers for environmental prediction (ncep) final operational global analysis (fnl) data on 1.0 ×1.0 grids at every six hours are used to initialize the wrf model, which are available on the surface and at 26 pressure levels from 1000 to 10 hpa. the moderate-resolution imaging spectroradiometer (modis) land data is adopted which is more close to the actual land surface features. the wrf output is archived at 10min intervals for analysis. d1, d2, d3, and d4 are activated at 0000 utc, 1200 utc, 1800 utc, and 0000 utc in the next day, respectively. then the model integral time of d1, d2, d3, and d4 are 48 h, 36 h, 30 h, and 24 h, respectively. the first 24 h of d1 runs as a spin-up period. the finest domain simulations are used to forecast the turbine hub winds. this forecast experiment covered the time from 2 january to 31 december in 2010. figure 1. the 3 km and 4 km model domains and topography (shaded, unit: m) used in this study, with a black dot showing the location of four meteorological towers. 2.3. statistical models (a) persistence method persistence is one of the simplest prediction models and most frequently used in wind energy forecasting. in this model, the forecast for all times ahead is set to the value it has now. the mean relative errors of persistence model for the four sites in 1-hour forecast horizon are shown as figure b2, which were calculated from the observed 70-m winds during 2 january to 31 december in 2010. the results indicate persistence has a good forecast in a few minutes to several hours ahead. this is because the dominant time scales of large synoptic scale changes in the atmosphere are in several days; then the pressure systems and winds would be changed on the same time scales. because the relative error of forecast winds is required to be smaller than 20%, the persistence model was used in immediate short-term forecasting and nowcasting. (b) bp-ann the back-propagation algorithm is the most common artificial neural network (ann), and the learning algorithm is the steepest descent algorithm that minimizes the errors between the produced output and the desired output by adjusting the weights. bp-ann is based on the error back-propagation learning algorithm, which has the distributed information storage and processing structure, and is suited for shortterm forecasting (carolin and fernandez 2007; li et al. 2009). threelayered bp-ann can approximate any nonlinear function and shows good performance on handling complex nonlinear problems. two normalized functions were used in this paper. one is premnmx, which is mainly used to normalize the training set, and another is tramnmx, which is used to normalize the prediction set. the anti-normalized function used in this paper is postmnmx. table 3. rmse of wind speeds intervals associated with different forecast methods and different forecast lengths to 10 day test period from the 20th to 30th of each month in 2010. (unit: m·s-1). (c) ls-svm support vector machine (svm) is a statistical learning theory system based on the structural risk minimization principle and vapnikchervonenkis dimension theory (vapnik 1995), which is to map the input data into a higher-dimensional feature space via a kernel function and to construct the optimal hyperplane for classification and regression. lssvm works with a sum squared error cost function, uses the equality constraints instead of inequality constraints in conventional svm and transforms the quadratic programming problem into linear equations, which simplifies and speeds up the calculation and enhances the accuracy of convergence. in this paper, intelligent search of genetic algorithm was employed in the process of choosing parameters of ls-svm, and to find the optimal model parameters of ls-svm for different data sets. compared with the exhaustive search, the performance is improved with less searching times in larger parameter space (rahim & usli, 2017). 2.4. hybrid structure bp-ann and ls-svm were used in this paper as statistical correction model for short-term forecasting. persistence combined with bp-ann and persistence combined with ls-svm were used for immediate short-term forecasting (referred to p6-bp and p6svm, respectively) and nowcasting (referred to p1-bp and p1-svm, respectively). to eliminate the seasonal variation of wind speeds, the statistical relationships are trained for each month. thus the each monthly wind speeds are divided into training (the first 2/3 time series) and validation sets (the last 1/3 time series). the statistical models are trained using the first 2/3 data to learn the relationship between the nwp output wind speeds, and the observations, then use the model parameters to forecast the 70 m winds for the last 10 or 11 days with a time step of 10 min (updates every 10 min). during the training processes, the training vectors are the wrf output wind speeds and the target vectors are the observations, which are both from the 1st to 20th days in each month with a time step of 10 min and the sample number is 2880. during the predicting processes, the testing and target vectors are from the wrf outputs and observations during the 20th to 30th/31th of each month in 1440 sample-numbers, respectively. the combined technique focuses on the variable weight coefficient, which is chosen based on the lowest root mean square error (rmse) of prediction. 40 hua deng, yan li, yingchao zhang, hou zhou, peipei cheng, jia wang and muhammad aqeel ashraf figure 2. the mean relative error (%) of persistence model for the four meteorological towers, which calculated from the observed 70-m winds during 2 january to 31 december in 2010. figure 3. a 10-day time series at 1-hour increments (first 20 days not show due to use as statistical training sets) of annual average wind speed (m·s-1) at four sites associated with seven forecast approaches compared to observational data. (a) 05009, (b) 05010, (c) 05011, (d) 05012 3. evaluation of 70 m wind forecasts to test the hybrid approach for 70 m wind forecasts, the wrf/ ysu/noah wind output, the short-term forecasting (including bp-ann, ls-svm), the immediate short-term forecasting (including p6-bp, p6svm), as well as the nowcasting (including p1-bp, p1-svm) were compared using the rmse as the error measure. four bias-correction approaches were examined: one based on wind speed only, one using wind speed along with the direction, one based on the diurnal cycle of wind speeds, and the last examine the seasonal variability of wind speeds. the test set using only the validation sets from the 20th to 30th of each month in 2010 was used to determine which bias-correction approach resulted in the lowest rmse. figure 4. a 24-hour time series of rmse (unit: m·s-1) for wind speed at 70 m agl associated with different forecast approaches, which calculated from the 20th to 30th of each month in 2010. (a) 05009, (b) 05010, (c) 05011, (d) 05012 3.1. wind speed evolution figure b3 shows the 10 day time series at 1 hour increments of annual average wind speed which were calculated from the 20th to 30th of each month in 2010 based on the wrf/ysu/noah, bp-ann, and ls-svm for short-term forecasting, p6-bp and p6-svm for immediate short-term forecasting, as well as p1-bp and p1-svm for nowcasting. the six forecast approaches were found to have underprediction of the wind speed, and the accuracy of wind speed prediction was decreasing along with prediction time. the wrf/ysu/noah outputs without any post-processing show the lowest average 70-m wind speeds, which indicates that the nwp model outputs combined with statistical techniques are necessary for wind speed forecast. the six hybrid approaches, bp-ann, ls-svm, p6-bp, p6-svm, p1-bp, and p1-svm, forecast the 70-m wind speeds in the rmse of 2.89, 2.46, 2.76, 2.43, 1.75, 1.67, respectively (table a2). the quality of short-term forecasting, including bp-ann and ls-svm, is better than the wrf/ysu/noah model outputs, which improved the day-ahead forecast in rmse of 12.3% and 26.3%, respectively. the immediate short-term forecasting shows a 16.3% and 26.9% improvement over nwp output in 70-m wind speed prediction using p6-bp and p6-svm methods respectively. the nowcasting approaches using p1-bp and p1-svm exhibits a significant degree of improvement in 70-m wind speed forecasting over the other approaches tested, and the lowest rmse is 1.75 and 1.67 m·s-1 respectively. it is noted that the ls-svm technique predicts much better than bp-ann after they were combined with persistence model for the three kinds of 70-m wind forecasting. the available wind power is proportional to the wind speed cubed, hence the power curve is non-linearity. the available wind speed for a turbine is between the cut-in and rated wind speed (3-25 m·s-1). when the wind speed is in the range of 3-8 m·s-1 the wind turbine is gradually running, and the production power rises slowly; when it reaches 8-14 m·s-1, the production power increases rapidly; and if it reaches 14-25 m·s-1, the wind energy is stable. therefore, to understand the different issues involved in 70-m wind forecasting it is useful to divide the wind speed into three distinct groups: (1) 3-8 m·s-1; (2) 8-14 m·s-1; (3) 14-25 m·s-1. table a3 shows the rmse of wind speed groups associated with different forecast methods and different forecast lengths, which indicates the rmse of wind speed prediction is increasing along with stronger wind speed. the improvement in rmse of the 70-m wind speed forecast is 41prediction of hub height winds over the plateau terrain by using wrf /ysu/noah and statistical forecast in the range of 9.4% to 32.6% in short-term forecasting, 12.8% to 33.9% in immediate short-term forecasting, and 39.7% to 53.7% in nowcasting. it should be noted that such forecast technique significantly improves the wind speed forecast in the range of 8-14 m·s-1, which is just associated with the fastest growth of turbine output along with the wind speed. figure 5. the monthly rmse (unit: m·s-1) for wind speeds at 70 m agl associated with different forecast approaches, which calculated from the 20th to 30th of each month in 2010. (a) 05009, (b) 05010, (c) 05011, (d) 05012 figure 6. wind roses of hourly winds (unit: m·s-1) at 70 m agl at four sites associated with different forecast approaches, which calculated from the 20th to 30th of each month in 2010. the wind speed interval is 5 m·s1. (a) 05009, (b) 05010, (c) 05011, (d) 05012 3.2. diurnal cycle of wind speeds figure b4 shows a 24-hour time series of rms for wind speed at 70 m agl associated with different forecast approaches in the training sets. the mountainous terrain of the inner mongolia plateau is complex with a strong turbulence, then the daily variation of the wind speed is different for different sites. but even that, all physical-statistical approaches exhibit a significant improvement in wind forecast and present the similar error in the diurnal cycle. the wrf/ysu/noah outputs have the largest error in rmse, and the nowcasting have the lowest. for the short-term forecasting, wrf/ysu/ noah coupling bp-ann and ls-svm has the similar bias-correction on the station 05009, and 05010, whose error curve in rmse is consistent with the cure of wrf/ysu/noah output. but for the station 05011 and 05012, the forecast ability of wrf/ysu/noah coupling ls-svm is better than coupling bp-ann obviously. the immediate short-term wind speed forecastings also develop the forecast error compared to the short-term forecasting, and the two error cures (p6-bp, p6-svm) are almost the same. 3.3. seasonal variability of wind speeds to test the forecast skill on seasonal variability, the monthly rmse for 70-m wind speeds of different forecast approaches is shown in figure b5. what the results also agree with the conclusion that the accuracy of wind speed prediction is decreasing along with prediction time and the now casting is better than other time-scale forecasts (shuib, et al., 2017). the three timescale wind speed prediction in rmse is between 1.2 to 4.9 m·s-1. also, the accuracy of wind speed prediction is smaller in autumn and larger in spring and summer. the relative largest rmse is in july and the smallest in october. furthermore, for the same time-scale forecasts, the method using ls-svm forecast wind speed is better than that using bp-ann. 42 hua deng, yan li, yingchao zhang, hou zhou, peipei cheng, jia wang and muhammad aqeel ashraf 3.4. wind speed rose diagram the wind direction is an important factor to be considered in wind power generation. the observation and forecast wind data were averaged over the 15day test period to generate the wind speed rose, plotted at a 22.5-degree angular resolution and in 5 m·s-1 intervals in wind speed, illustrated in figure b6. the value represents the average wind speed in the 16 different wind directions. as can be seen that all the wind speed roses illustrate a valley in the about northerly direction. the wrf/ysu/noah outputs exhibit the lower 70-m wind speed at the four sites than other forecasts (roslee, et al., 2017; ridzuan et al., 2017). the three time-scale forecasting successfully forecast the westerly (from ssw to nnw) speed at 05012, and the nowcasting shows a significant degree of improvement in wind speed forecasting at all four sites. the error of immediate short-term forecasting is between nowcasting and short-term forecasting. although the different approaches have different improvement on the different stations, the hybrid forecasting with physical forecasting and statistical postprocessing could predict the wind much better. 4. summary the wind energy forecasting problem is closely linked to the problem of forecasting the variation of winds over very short time intervals because the wind is variable and intermittent over various time-scales. to understand the different issues involved in 70-m wind forecasting, the four wind towers in the inner mongolia were used to represent the complex plateau terrain, and it was useful to divide the problem into three difference time scales: short-term, immediate-short-term, and nowcasting. the mesoscale wrf/noah/ysu as the nwp model performed 24-h wind forecast with 1-km horizontal resolution and 10-min time resolution. bp-ann and ls-svm were used to short-term forecasting. persistence method was combined with bp-ann and ls-svm to forecast the immediate short-term forecasting and nowcasting, respectively. the results indicate that the current forecasting techniques exhibit considerable skill in short-term, immediate-short-term, and nowcasting forecasting. short-term forecasts typically outperform an nwp forecast by 12% to 26%; nowcasting forecasts usually outperform nwp forecasts by 47% to 50%. this paper also demonstrates that the approaches using ls-svm model is more accurate than the approaches using bp-ann model over the different time-scales tested. the possible reasons are as follows: 1, svm is proposed for problems with limited samples to obtain the global optimum solution. 2, svm algorithm can transform the problem into a quadratic programming problem to obtain the global optimum solution which bpann cannot achieve. 3, svm uses the nonlinear transformation to map the original variables to the higher dimensional feature space to construct the linear classification function, thus ensuring the good generalization ability of the model, and then avoids the curse of dimensionality. acknowledgments this work is supported by national natural science foundation of china (40805039, 20110138, 41575056, and 41675156), key laboratory of meteorological disaster of ministry of education program (klme1303) in nanjing university of information science and technology, the research innovation program for college graduates of jiangsu province under grant no. sjzz16_0155 and sjzz16_0156, national public welfare industry (meteorological) scientific research, no. gyhy20110604, the open research fund of chongqing meteorological bureau, and the priority academic program development of jiangsu higher education institutions (papd). references asis, j., tahir, s. h., rahim, a. r., konjing, z., kob, r. c., & tjia, h.d. 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(2010). wind speed forecasting model study based on support vector machine. acta energiae solaris sinica, 31(7), 928–932. the elastic structure beneath greenland is shown by means of s-velocity maps for depths ranging from zero to 350 km, determined by the regionalization and inversion of rayleigh-wave dispersion. the traces of 50 earthquakes, occurring from 1990 to 2011, have been used to obtain rayleigh-wave dispersion data. these earthquakes were registered by 21 seismic station located in greenland and the surrounding area. the dispersion curves were obtained for periods between 5 and 200 s, by digital filtering with a combination of mft (multiple filter technique) and tvf (time variable filtering). later, all seismic events (and some stations) were grouped to obtain a dispersion curve for each source-station path. these dispersion curves were regionalized and inverted according to the generalized inversion theory, to obtain shear-wave velocity models for a rectangular grid of 16x20 points. the shear-velocity structure obtained through this procedure is shown in the s-velocity maps plotted for several depths. these results agree well with the geology and other geophysical results previously obtained. the obtained s-velocity models suggest the existence of lateral and vertical heterogeneity. the zones with consolidated and old structures present greater s-velocity values than the other zones, although this difference can be very little or negligible in some case. nevertheless, in the depth range of 15 to 45 km, the different moho depths present in the study area generate the principal variation of s-velocity. a similar behaviour is found for the depth range from 80 to 230 km, in which the lithosphere-asthenosphere boundary (lab) generates the principal variations of s-velocity. finally, the new and interesting feature obtained in this study: the definition of the base of the asthenosphere (for the whole study area and for depths ranging from 130 to 280 km, respectively) should be highlighted. la estructura elástica bajo groenlandia es mostrada por medio de mapas de velocidad de onda para profundidades variando desde cero a 350 km, determinada por la regionalización e inversión de la dispersión de onda rayleigh. las trazas de 50 terremotos, ocurridos desde 1990 hasta 2011, han sido usados para obtener datos de dispersión de onda rayleigh. estos terremotos fueron registrados por 21 estaciones sísmicas localizadas en groenlandia y el área circundante. las curvas de dispersión fueron obtenidas para periodos entre 5 y 200 s, por filtrado digital con una combinación de mft (técnica de filtrado múltiple) y tvf (filtrado en tiempo variable). después, todos los eventos sísmicos (y algunas estaciones) fueron agrupados para obtener una curva de dispersión para cada trayecto fuente-estación. estas curvas de dispersión fueron regionalizadas e invertidas de acuerdo con la teoría de la inversión generalizada, para obtener modelos de velocidad de cizalla para una rejilla rectangular de 16x20 puntos. la estructura de velocidad de cizalla obtenida a través de este procedimiento es mostrada in los mapas de velocidad de onda s representados para varias profundidades. estos resultados muestran buen acuerdo con la geología y con otros resultados geofísicos obtenidos previamente. los modelos de velocidad de onda s obtenidos sugieren la existencia de heterogeneidad lateral y vertical. las zonas con estructuras antiguas y consolidadas presentan mayores valores de velocidad de onda s que las otras zonas, aunque esta diferencia puede ser muy pequeña o despreciable en algún caso. no obstante, en el rango de profundidad de 15 a 45 km, las diferentes profundidades del moho presentes en el área de estudio generan la principal variación de velocidad de onda s. un comportamiento similar es encontrado para el rango de profundidad desde 80 a 230 km, en el cual la frontera litosfera-astenosfera (lab) genera las principales variaciones de velocidad de onda s. finalmente, debería ser destacada la nueva e interesante característica obtenida en este estudio: la definición de la base de la astenosfera (para el área de estudio completa y para profundidades variando desde 130 a 280 km, respectivamente). earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): a1-a11 abstract resumen keywords: rayleigh wave, shear wave, crust, upper mantle, greenland. palabras clave: onda rayleigh, onda de cizalla, corteza, manto superior, groenlandia. shear-wave velocity structure of greenland from rayleigh-wave analysis estructura de velocidad de cizalla de groenlandia obtenida de análisis de onda rayleigh record manuscript received: 05/12/2013 accepted for publication: 30/11/2015 how to cite item corchete, v. (2016). shear-wave velocity structure of greenland from rayleighwave analysis. earth sciences research journal, 20(1), a1-a11. doi:http://dx.doi.org/10.15446/esrj.v20n1.41079 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.41079 se is m o l o g y v. corchete* higher polytechnic school, university of almeria, 04120 almeria, spain *corresponding author. fax +34 950 015477; e-mail: corchete@ual.es a2 v. corchete 1. introduction the great ability of the rayleigh-wave dispersion analysis to delineate the earth’s deep structure is well known (corchete, 2013a). this methodology allows the features of the earth’s structure to be determined, because it is well known that the surface-wave dispersion (rayleigh-wave dispersion) is related to the earth’s structure crossed by the waves (represented by the s-velocity structure). thus, the features of the elastic structure for any region of earth can be studied through the analysis of rayleigh-wave dispersion. in this paper, the features of the elastic structure beneath greenland (and its surrounding area) will be revealed through this analysis. greenland is one of the earth regions for which there are relatively few seismic studies (it is among the most poorly studied regions of earth), due to the particular conditions of this region (it is covered by a large and thick ice sheet), which have made difficult the development and maintenance of high-quality seismic networks. nevertheless, since the knowledge of the crust and upper-mantle structure of this region can be the key to answer essential questions about the pre-history of earth, the determination of a detailed seismic structure of this region could be of great importance. fortunately, in the last years, international agencies (as gsn, iris and geofon) joint to local agencies (as canadian, norwegian and danish seismograph networks or the international glisn project) have been able to develop and support new and permanent broad-band digital seismic stations. as a consequence of this, in the last twenty years high-quality seismic records have been made available from seismographic databases for greenland and its surrounding area (although the best seismic records are only available for the last 12 years). in pioneer studies, gregersen (1970, 1982) investigated crust structure in greenland using surface waves. they used periods of the fundamental modes ranged from 15 to 55 s, obtaining shear-velocity profiles from 0 to 50 km of depth. these very old studies are relevant, in spite of the very limited data used. later, dahl-jensen et al. (2003) provided information about the moho depth, in many locations in greenland, calculated from receiver-function analysis. these depths have later been confirmed by other authors as braun et al. (2007), but they didn’t perform any surface-wave tomography. the first attempt of tomography for greenland was performed by darbyshire et al. (2004), who investigated the crust and upper mantle structure using surfacewave tomography by first time. they used periods of the fundamental modes of rayleigh-wave phase-velocity from 30 to 185 s, performing maps of regionalized phase velocity for some selected periods (from 30 to 146 s), but no s-velocity mapping was obtained with depth. they only performed some s-wave velocity profiles (from 50 to 350 km of depth). the lithosphere-asthenosphere boundary (lab), inferred from these s-velocity profiles agree well with the lab calculated by kumar et al. (2005), from receiver-function analysis. finally, pilidou et al. (2005) performed a rayleigh-wave tomography for the north atlantic area, in which they obtained s-velocity maps from 75 to 350 km of depth (although the resolution decreases drastically with depth from 200 km). nevertheless, in the study performed by pilidou et al. (2005), the region of greenland is very small in size comparing with the wide study area considered by them. as a consequence, a very poor resolution is achieved by this s-velocity mapping for greenland and its surrounding area. it should be noted that no 3d s-velocity structure has been obtained for greenland and its surrounding area, for depths ranging from 0 to 75 km. furthermore, the s-velocity structure obtained in this previous study shows a poor resolution for the depth range from 200 to 350 km. also, it should be noted that the greenland crust and upper-mantle structure never has been the subject of a surface-wave tomography survey, only it has been investigated as part of a bigger area (the north atlantic region). the goal of the present study is the determination of the elastic s-velocity structure beneath greenland and the surrounding area, from rayleigh-wave analysis, for the whole depth range from 0 to 350 km. this rayleigh-wave analysis will consist of filtering of rayleigh waves, to obtain dispersion curves from 5 to 200 s. this dispersion analysis will be performed in several steps. first, all seismic events will be grouped in source zones to get an average dispersion curve for each source-station path. then, the dispersion curves will be obtained by digital filtering with a combination of the multiple filter technique (mft) and time variable filtering (tvf). thus, a set of source-station averaged dispersion curves will be calculated. this set of dispersion curves will be regionalized and then inverted according to the generalized inversion theory, to get s-wave velocity models for a rectangular grid defined in the study area. finally, these models will be plotted to obtain a 2d mapping of the 3d s-wave velocity structure, for the study area. the period range to be obtained in the present study, by digital filtering, will be wider than the period range measured in the previous studies. thus, the determination of the 3d s-velocity structure, in the whole depth range from 0 to 350 km, will be more accurate in the present study than the previous studies, in which no surface-wave dispersion was measured in the period ranges from 5 to 30 s and from 146 to 200 s. 2. data set in this study, 50 earthquakes occurring from 1990 to 2011 in the neighbour of greenland have been considered (supplement 1). these earthquakes have been registered by 21 seismic stations located in this region (supplement 2). from these stations digital data are available recorded with a different instrument for each available channel (supplement 6). from all these channels, only the records for the channels: bhz, lhz, blz, llz, bh1, lh1, bh2, lh2; have been considered as the most suitable data for this study, because the period interval of best registration for these channels is between 1 or 5 to 200 or 250 s (period interval in which the frequency response is almost flat). this period interval (from 5 to 200 s) is more suitable for exploring the elastic structure of the earth, for a depth range of 0 to 350 km of depth. also, to ensure the reliability of the results, only the earthquake traces in which a well-developed rayleigh wave train is present, with a very clear dispersion, have been considered. thus, many records have been discarded. the instrumental response must be taken into account to avoid the time lag introduced by the seismograph system and all distortions produced by the instrument. this correction recovers the true amplitude and phase of the ground motion, allowing the analysis of the true dispersion of rayleigh waves. for this reason, all the traces considered in this study were corrected for instrument response. 3. pre-processing and filtering for the pre-processing and calculation of dispersion curves, the computation method detailed by corchete et al. (2007) was followed. in this paper, only a brief review of the principal concepts of this methodology will be presented. grouping of stations and seismic events. it is well known that rayleigh waves propagated along near epicentre-station paths show similar dispersion curves, because they cross the same earth structure and sample the same elastic properties of the medium. as a consequence of this, it is possible to group all seismic events listed in supplement 1 in source zones, as listed in supplement 3, to get source-station paths from epicentre-station paths. these source zones are defined as a location at which seismic events with similar epicentre coordinates have occurred. the coordinate differences for a group of events must be less than or equal to 1 degree in latitude and longitude, to be able to group them in the same source zone. then, an average dispersion curve for each source-station path can be obtained, when the dispersion curves calculated for the traces of the events of this source zone (recorded at the same station) are averaged. in this way, any small deviations obtained are considered errors, which can be described by the standard deviation. it should be noted that some stations considered in this study also show very similar coordinates. thus, they have been also grouped using the same criteria described above for the events, defining new station codes to denote these average stations. these new stations are listed in supplement 4. figure 1 shows the path coverage obtained for the study area (given by the source-station paths). a3shear-wave velocity structure of greenland from rayleigh-wave analysis dispersion analysis. the rayleigh-wave group velocity for the trace of each event registered has been measured by means of the combination of digital filtering techniques: multiple filter technique (dziewonski et al., 1969) and time variable filtering (cara, 1973), as shown in the flow chart displayed in figure 2. as an example of this filtering process, the application of this combination of filtering techniques to the trace of the event 6 (recorded at frb station) will be detailed. the mft (multiple filter technique) has been applied firstly to this trace as it is shown in figure 3. this trace and the dispersion curve obtained are used to compute the digital filtered signal by using the tvf (time variable filtering). figure 4 shows the time-variable filtered signal resulting from the tvf. finally, figure 5 shows the final dispersion curve obtained after application of the mft to the filtered signal. a comparison between figures 3 and 5 shows the benefits of the proposed filtering process. it should be noted that a combination of mft and tvf works better than the application of the mft only, because the signal/noise ratio is highly increased. figure 1. path coverage of the rayleigh waves (104 paths). figure 2. steps followed in the filtering process of each event-station seismogram to obtain its dispersion curve. the circles are used to denote the application of a digital filtering technique and the rectangles are used to denote the results obtained. the enhanced rectangles are used to show the initial data and the final result. preprocessed signal (observed seismogram with instrumental correction) mft group velocity tvf filtered signal mft group velocity (final dispersion curve) 0 25 50 75 100 125 150 175 200 2.0 2.5 3.0 3.5 4.0 4.5 0 25 50 75 100 125 150 175 200 600 900 1200 1500 600 900 1200 1500 0 25 50 75 100 125 150 175 200 600 900 1200 1500 period (s) g ro up t im e ( s) 89 90 91 92 93 94 95 96 97 98 99 db (d) g ro up v el oc ity (k m /s) period (s) (c)g ro up t im e ( s) period (s) (b) time (s) (a) figure 3. (a) observed seismogram corresponding to the event 6 (supplement 1) recorded at frb station (supplement 2), instrument corrected. (b) contour map of relative energy normalized to 99 decibels, as a function of the period and the group time (white points denote the group times inferred from the energy map). (c) group time curve inferred from the energy map. (d) group velocities calculated from the group times and the epicentral distance (group velocity is the epicentral distance divided by the group time for each period). figure 4. (a) observed seismogram corresponding to the event shown in figure 3a. (b) group velocity dispersion curve obtained after application of the mft (as shown in figure 3). (c) time-variable filtered seismogram. figure 5. (a) time-variable filtered seismogram obtained after application of the tvf, as shown in figure 4. (b) contour map of relative energy normalized to 99 decibels as a function of the period and the group time (white points denote the group times inferred from the energy map). (c) group time curve inferred from the energy map. (d) group velocities calculated from the group times and the epicentral distance. 0 25 50 75 100 125 150 175 200 2.0 2.5 3.0 3.5 4.0 4.5 0 25 50 75 100 125 150 175 200 600 900 1200 1500 600 900 1200 1500 0 25 50 75 100 125 150 175 200 600 900 1200 1500 period (s) g ro up t im e ( s) 89 90 91 92 93 94 95 96 97 98 99 db (d) g ro up v el oc ity (k m /s) period (s) (c)g ro up t im e ( s) period (s) (b) time (s) (a) a4 v. corchete then, equations (4) can be expressed as a system of linear equations, for each period t, in the form formula (5) is a set of m linear equations, for a given period t, with n unknowns xj (with j =1, 2, …, n), where n = 4(ny+1)(nx+1). it should be noted that there is a forward problem posed for each period t, for which the equations (5) can be written. in each forward problem, the number of equations m can take different values depending on the considered period t, because the path coverage can be different for each period (supplement 5). as a consequence of this, the number of unknowns n that can be properly determined may be also different in each problem, because always n must be less than or equals to m. otherwise, formula (5) will be a set of equations with more unknowns n than equations m, being very difficult or impossible the determination of the unknowns xj or equivalently the bnm coefficients of formula (3). once the forward problem has been defined by formula (5), for each period t, the regionalization of the surface-wave velocities is performed by the inversion of formula (5), or equivalently inverting the matrix a. the inverse matrix of a can be obtained using the inverse method detailed by corchete and chourak (2010). this inverse matrix is given by b=vpʌp(vp 2+αi)-1upt the matrix b defined by formula (6) is called the damped generalized inverse. then, the linear equations given by formula (5) are inverted to obtain x by means of x̅ = by (7) average dispersion curves and estimation of the error. when the dispersion curves of the traces for each event involved in a source-station path have been calculated. the average group velocity for this path can be obtained as a mean of the group-velocity values calculated for each period. an estimation of the error for this media is calculated by the standard deviation (1-s error). for paths with only one event involved, a mean of the standard deviations obtained for the other near paths (which have more than one event involved) can be considered as a good estimation of the error. the average group velocities obtained for the 104 source-station paths considered in this study are listed in supplement 5 for several periods. the periods for these dispersion curves range from 5 to 200 s. it should be noted that the period range obtained in the present study exceeds the period range obtained in previous studies (darbyshire et al., 2004; pilidou et al., 2005). this fact points to a better determination of the elastic structure beneath greenland for shallow and deeper depths, if the present dispersion data are used. 4. regionalization of group velocities the above-described group velocities constitute the dispersion curves to be considered for regionalization. the period range of these curves goes from 5 to 200 s (supplement 5) and there are 104 dispersion curves corresponding to the source-station paths shown in figure 1, a dispersion curve for each source-station path. thus, for each abovementioned path, the forward problem in the regionalization of the surfacewave velocities is expressed as (keilis-borok, 1989) where ues(t) is the group velocity (dispersion curve) observed between the source and the station along a great-circle path connecting those two points (the source-station path), δes is the source-station distance (δes =fds) and u(t, p) is the local velocity at a period t and a point p of the earth surface (on the great-circle path that connects the source and the station). the inverse problem is posed to calculate the local surface-wave velocity u(t, p), at each period t and at each point p, from the average group velocity ues(t) previously measured for a number m of source-station paths. the inverse problem defined in these terms is known as regionalization of the surface-wave velocities. this inverse problem can be simplified, if the reciprocal of the local group velocity u(t, p), or equivalently the slowness s(ϕ, λ), is firstly represented in a general stereographic polar projection (rotated 45 degrees on longitude) by the equations where (ϕ, λ) are the geographical latitude and longitude of the point p (on the earth surface considered as a sphere). later, s(ξ , η) is expanded into a fourier series by the formula where x is ξ ξ0 and y is η η0, (ξ0, η0) are the coordinates of the point at the bottom left corner of the study area (figure 1), x is ξmax ξ0 and y is ηmax η0, (ξmax, ηmax) are the coordinates of the point at the top right corner of the study area (figure 1) and (anm, bnm, cnm, dnm, ny and nx) are constants. it should be noted that u(x, y) takes the velocity values of u(t, p), i. e., u(x, y) = u(ϕ, λ) and s(x, y) = s(ϕ, λ), for each period t, because each point p (on the earth surface considered as a sphere) has coordinates (x, y) and (ϕ, λ) related by the equations (2). if formula (3) is introduced in the relationship (1) the group time can be expressed as where ti is the group time of the ith source-station path (with i =1, 2, …, m), ci is the great-circle that connects the source and station points of the ith source-station path. it should be noted that m is the number of source-stations paths measured for the period t (104 paths maximum for this study). equations (4) also can be written in the form where bnm is (anm; bnm; cnm; dnm) and (4) a5shear-wave velocity structure of greenland from rayleigh-wave analysis (8) where it should be noted that x̅ = bax the product of matrixes b and a is called resolution matrix and it is obtained through then if the resolution matrix given by formula (8) is the identity matrix, the obtained solution x̅ is unique and the resolution is perfect. nevertheless, the addition of the factor α in the main diagonal makes that the equality ba = i is only reached when the damping factor α is zero. the introduction of the damping parameter degrades resolution, but stabilizes the obtained solution by means of the reduction of the variance (corchete and chourak, 2010). the error δx̅ in the model parameters (or equivalently the error in the solution x̅ ) due to the error δy in the data (i. e. the error in the group times), is given by the corresponding covariance matrix ‹ δx̅, δx̅t › as (aki and richards, 1980) ‹ δx̅, δx̅t › = b ‹δy, δy t› bt (9) where ‹ › indicates averaging, b is given by (6) and the covariance matrix for δx̅ is calculated from the covariance matrix of the error in data δy. assuming that all the components of the data vector y are statistically independent (or assuming an independent error for each individual measurement), i. e. assuming that the fluctuation in any data is entirely independent of the fluctuation of other data, the off diagonal elements are made zero. then, the main diagonal elements of the data covariance matrix ‹ δx̅, δx̅t ›are the variances of the data. if a similar behaviour is assumed for the model parameters x (i. e. assuming that all the components of the vector x are statistically independent), the off diagonal elements are made zero. then, the main diagonal elements of the model covariance matrix ‹ δx̅, δx̅t › are the variances of the model parameters. the obtained solution x̅ (or equivalently the set of coefficients bnm) is calculated introducing the relation (6) in the equations (7). after, the slowness s(x, y), or equivalently its reciprocal u(x, y), is computed from these coefficients bnm using formula (3). the errors in the model parameters (or equivalently the errors in the coefficients bnm) are obtained by means of the relation (9). later, the error in the slowness s(x, y) is computed from the errors in the coefficients bnm using the partial derivative of formula (3) given by finally, the error in the local group velocity u(x, y) is computed from the error in the slowness s(x, y), using also the partial derivative of the formula figure 6 shows the regionalized group velocity u(x, y) obtained following the above-described method for the periods 10, 100 and 150 s. figure 7 shows the errors of the regionalized group velocity du(x, y) for the same periods. 2.5 2.5 2.7 2.4 2.3 2.3 2.8 2.5 2.9 2.4 3.0 2.8 2.3 2.3 2.9 2.0 2.5 3.0 3.5 4.0 4.5 u (km/s)10 s10 s 3.7 3.8 3.9 3.6 3.8 3.5 2.0 2.5 3.0 3.5 4.0 4.5 u (km/s)100 s100 s 3.7 3.6 3.5 3.8 3.8 3.6 2.0 2.5 3.0 3.5 4.0 4.5 u (km/s)150 s150 s figure 6. rayleigh-wave group velocity u(x, y) computed by means of formula (3) at the periods 10, 100 and 150 s. the isolines interval is 0.1 km/s. a6 v. corchete selection of an initial earth model. the selection of an initial earth model is a preceding step before the inversion process. the initial model must be prepared considering all information available for the study area, with respect to the s-wave, p-wave and density distributions with depth. for the present study, this geophysical and geological information has been obtained from previous studies developed in the study area, which are mentioned in the introduction section of this paper. this information has been incorporated to the initial models, for the above-mentioned grid points (or blocks). particularly, the moho depths calculated by dahl-jensen et al. (2003), for many places in greenland, has been used to provide the moho depth in the initial models, as well as the moho depths predicted by the moho map performed by braun et al. (2007). thus, an initial model has been prepared for each grid point (or block) for depths ranging from 0 to 400 km. for depths deeper than 400 km, the prem model developed by dziewonski and anderson (1981) has been used. these models must provide a theoretical dispersion curve as close as possible to the observed curve, because the inversion scheme to be used only considers small perturbations for the wave velocities involved. as an example, the initial model considered for the inversion of the dispersion curve (group velocity), associated with the grid point located at the centre of the block shown in figure 8, is listed in table 1 and its shear-wave velocity distribution with depth is plotted in figure 9a (dashed line). the theoretical group velocity shown in figure 9c (dashed line) is calculated by forward modelling, from the starting model listed in table 1. 0.08 0.08 0.08 0.09 0.07 0.09 0.08 0.08 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 du (km/s)10 s10 s 0.08 0.07 0.08 0.07 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 du (km/s)100 s100 s 0.07 0.07 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 du (km/s)150 s150 s figure 7. error in rayleigh-wave group velocity du(x, y) calculated by means of formula (10) at the periods 10, 100 and 150 s. the isolines interval is 0.01 km/s. figure 8. topography under ice sheet and bathymetry of the study area, obtained from etopo1 database. the black-line rectangle denotes a block area, for which a sample of the inversion process will be performed. table 1. initial earth model (a: p-wave velocity, b: s-wave velocity and r: density) considered for the inversion of the group velocities (obtained by regionalization), for the block area shown in figure 8. 5. inversion of regionalized dispersion curves the above-described group velocity distributions with period must be inverted to obtain the shear-wave velocity distribution with depth (the 3d s-velocity structure for greenland and its surrounding area). for it, these group velocity surfaces u(x, y) calculated for each period, from 5 to 200 s, are sampled in a rectangular grid of 16x20 points. really, these grid points are the centres of a grid with 16x20 rectangular blocks, as the block shown in figure 8. these grid data can be inverted obtaining a shear velocity model (a shear velocity distribution with depth) for each grid point (or block) of the study area, achieving thus a 3d s-velocity model that represents the 3d elastic structure beneath greenland and its surrounding area. the inversion of the group velocity, for each grid point of the study area, will be performed using the computation method described in detail by corchete et al. (2007). here only a brief review of the principal concepts of this methodology is presented. -5300 -4000 -3000 -2000 -1000 -500 -250 0 250 500 1000 1500 2000 2800 270 ºe 0 ºe 60 ºn 65 ºn elevation (m) a7shear-wave velocity structure of greenland from rayleigh-wave analysis inversion. when the starting model is ready, the group velocity can be inverted to obtain the corresponding s-velocity distribution with depth. figure 9 show the results of an example of such inversion. in this example, the dispersion curve corresponding to the block area shown in figure 8 has been inverted. the s-velocity model shown in figure 9a (continuous line) is the final model obtained for this block. the model improvement iteratively obtained, is checked by the comparison between the observed group velocity (considered as observed data) and the theoretical group velocity (calculated from the actual model by forward modelling and considered as theoretical data). when the theoretical curve falls within the vertical error bars of the observed curve, as shown in figure 9c, the inversion process is finished, because the obtained s-velocity model describes the observed curve within its experimental error. resolution of the final model. the resolving kernels shown in figure 9b indicate the reliability of the solution obtained for the inverse problem posed. a good engagement between the calculated solution and the true solution (which implies the reliability of the estimated solution), is obtained when the absolute maxima of these functions fall over the reference depths. this reliability is also related to the width of these absolute maxima. the solution for the inversion problem is more reliable when the maxima of these resolving kernels are narrower. it should be noted that the s-velocity models (figure 9a) and the resolving kernels (figure 9b), are plotted only for depths above 800 km. this fact is due to the bad resolution obtained for depths deeper than 350 km. figure 9b shows that the deeper layers (below 350 km) have a poor resolution. this is shown in the lack of coincidence between the resolving-kernel absolute maxima and their reference depths, for depths down to 350 km. therefore, the results obtained for depths below 350 km have been left out, because the resolution at these deeper depths is poor. for depths of 0 to 350 km, the s-velocity models must be considered as valid models. thus, the s-velocity final model shown in figure 9a (continuous line), from 0 to 350 km of depth, is the definitive s-velocity distribution with depth calculated for the above-mentioned block area. this s-velocity distribution agrees well with the results obtained by darbyshire et al. (2004), for the greenland lithosphere. nevertheless, this comparison only can be approximately done, because they didn’t calculate s-velocity profiles in the southernmost part of greenland. moreover, they didn’t perform s-wave velocity profiles from 0 to 50 km of depth. for it, at this depth range, it is not possible any comparison with the s-velocity profile shown in figure 9a. this profile also can be compared with the mapping performed by pilidou et al. (2005), for the north atlantic area, in which the region of greenland is included, but this region is very small in size comparing with the big size of the wide study area considered by them. as a consequence of this, a very poor resolution is achieved by this s-velocity mapping for greenland and its surrounding area, which makes difficult the precise comparison between this mapping and the s-velocity profile shown in figure 9a. logically, the information about the greenland structure that can be discerned from global mappings will have always a poor resolution. on the other hand, the s-velocities shown in figure 9a present high values in general, as expected for the block area shown in figure 8, which is inside of an archean block that hosts some of the oldest rocks on earth and therefore has a high rigidity (braun et al., 2007). shear velocity mapping. the above-described s-velocity distributions with depth, obtained for each grid point (or rectangular block) of the above-mentioned grid defined in the study area, are plotted with contour maps. this 3d s-velocity structure is mapped with contours by depth intervals, obtaining 2-d images of shear velocity for the study area, as shown in figure 10. such 2-d images are suitable for an easy interpretation and correlation, with other geophysical and geological information available for the study area. the 1-sigma errors in the s-wave velocities are also represented in 2-d images. these errors are plotted in figure 11, for the some depth intervals, to check the error spatial distribution of the s-wave velocities mapped in figure 10. 800 700 600 500 400 300 200 100 0 2 3 4 5 6 0 25 50 75 100 125 150 175 200 2.0 2.5 3.0 3.5 4.0 4.5 starting final (a) shear velocity (km/s) de pt h (k m ) observed starting final (c)g ro up v elo cit y (k m /s) period (s) 0 100 200 300 400 500 600 700 800 (b) depth (km) resolving kernels figure 9. (a) shear-wave velocity (final model) obtained after the inversion process for the block area shown in figure 8, plotted with continuous line only from 0 to 800 km of depth. the horizontal bars show standard deviation for each layer considered in this inversion process. the shear velocity distribution of the initial model listed in table 1 is plotted with dashed line, only from 0 to 800 km of depth. (b) resolving kernels of the inversion problem posed (plotted only from 0 to 800 km of depth). the reference depths are marked by vertical bars for the media depth of each layer considered. (c) the theoretical group velocity obtained from the final model plotted in figure 9a, is shown with continuous line. the theoretical group velocity obtained from the initial model listed in table 1 (and plotted also in figure 9a), by means of forward modelling, is plotted with dashed line. the dots line denotes the average group velocity, calculated by regionalization, for the above-mentioned block area (considered as observed data). the vertical bars show the standard deviation in group velocities at each period (1-s errors in the observed data). a8 v. corchete figure 10. geographical distribution of the s-velocity as a function of depth. the interval between isolines is 0.1 km/s. white lines are used to plot contours associated to s-velocities greater than or equals to 4 km/s. 0.09 0.09 0.09 0.08 0.09 0.09 0.08 0.08 0.08 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 db (km/s)0 5 km 0.07 0.08 0.07 0.07 0.07 0.08 0.07 0.07 0.07 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 db (km/s)35 40 km 0.07 0.07 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 db (km/s)280 350 km figure 11. geographical distribution of the 1-sigma errors arisen in computation of the s-velocities, for the depth ranges: 0-5 km, 35-40 km and 280-350 km. the interval between isolines is 0.01 km/s. figure 12. geologic map of greenland with definitions of the units (braun et al.,2007). 6. results and discussion in this section, the results obtained in this study and shown in the s-velocity mapping of figure 10, will be presented and discussed. the structural features shown by these images will be also correlated with other geophysical studies (developed in the study area) and correlated with the geology (figure 12). a9shear-wave velocity structure of greenland from rayleigh-wave analysis figure 13. locations of the iceland and jan mayen hotspots (small black circles). the thin solid white line defines the plate boundaries and the thick solid white lines refine the canadian and east-european craton boundaries (pilidou et al., 2005). depth range: 0 – 5 km. the s-velocity map shows clearly the distribution of sedimentary basins of the study area, located between the principal chains of mountains (figure 8). the s-velocity presents the lowest values for the basins and the highest values for the consolidated and old structures (figure 12). unfortunately, the obtained results for this depth range don’t can be correlated with the results obtained by other authors, because any seismic tomography (or even any s-velocity profile) never has been obtained in the depth range from 0 to 50 km, for greenland and its surrounding area, until now. this fact is due to the absence of precise surface-wave dispersion measurements, in the period ranges from 5 to 30 s, in the previous studies (darbyshire et al., 2004; pilidou et al., 2005). this lack of information in these above-mentioned previous studies caused the lack of a 3d s-velocity structure (or even any s-velocity profile), for the depth range from 0 to 50 km. thus, figure 10 shows the first s-velocity values obtained for this study area, from 0 to 50 km of depth, and the first s-velocity mapping performed from 0 to 75 km of depth. depth range: 5 – 15 km. the geographical distribution of the s-velocity values shows an s-velocity pattern different to the s-velocity pattern obtained for the previous depth range. for this depth range, the lowest s-velocity values are not correlated with the location of the sedimentary basins. the s-velocity presents the highest values for the southern part of greenland. nevertheless, the s-velocity values are high in general, because greenland’s crust is mostly of precambrian age and therefore is an old and very rigid structure, which propagates the seismic waves very well (the seismic waves travel fast and their velocities are high). the s-velocity presents the highest values for the consolidated and older structures (located mostly at the southern part of greenland), while the s-velocity values at the north are the lowest ones for the greenland region (figure 12). the s-velocity shows the highest values for the archean blocks, north and east of the ketilidian and nagssugtoqidian blocks, respectively. these areas of high velocity are separated by a zone of slightly lower s-velocity, associated to the nagssugtoqidian block. the s-velocity values in the zone of the ketilidian block also are slightly lower than those for the above-mentioned archean blocks. the s-velocity shown for the area of the nagssugtoqidian and rinkian blocks, presents very similar values for both blocks, because these blocks are quite similar geologically. the s-velocity shows the lowest values of the greenland region, in the area of the east and west greenland tertiary basalts, as expected for the youngest and less consolidated areas of greenland. the s-velocity values for the area of the ellesmere island and the caledonian and north greenland foldbelts, are similar to those shown for the northern part of the north greenland block. it should be noted that the s-velocity shows sligthtly different values for the northern and southern parts of the north greenland block, the s-velocities are higher for the southern part, because the southern part of north greenland belongs to the greenland shield, which is an older and more consolidated structure than the northern part, therefore it is a more rigid structure that propagates the seismic waves better than the northern part. in the whole study area, the lowest s-velocities are shown in the off-shore areas. for these areas, the zone of the reykjanes and kolbeinsey ridges shows the lowest s-velocity values, as expected for a zone of younger crust and lithosphere (figure 13). for this low velocity zone, it should be noted that a small area of slightly lower s-velocity is just located at the iceland hotspot, for the depth range from 10 to 15 km. for the jan mayen hotspot has not associated a similar low s-velocity area, probably due to an inadequate rayleigh-wave path coverage for this area (figure 1). depth range: 15 – 45 km. the geographical distribution of the s-velocity values for this depth range shows higher values (4 km/s and higher shown in figure 10 by contours with white line) in zones of the study area in which the moho depth is overcome (i. e. zones which the depth range considered is located below the moho discontinuity). for each depth range shown in figure 10 (between 15 and 45 km), the moho depth is overcome in different areas. the moho depth is firstly overcome in off-shore areas and lastly in on-shore and continental areas. in general, the s-velocity increases with depth for the whole study area, for each depth range between 15 and 45 km. again, the s-velocity presents the lowest values at the north of greenland, while the highest ones are present at the south. the s-velocity presents higher values for the consolidated and old structures (figure 12). nevertheless, the s-velocity difference associated to this structural difference is minor than the s-velocity difference due to the moho depth overcoming, because the s-velocities associated with the crust are much minor than those associated with the upper mantle. thus, if the s-velocity value of 4 km/s is used to find the moho depth in the whole study area, considering that an s-velocity value greater than 4 km/s is associated with upper-mantle structure, then it is possible to plot a map of the moho discontinuity, such as the map shown in figure 14 (top), from the s-velocity mapping shown in figure 10 (which is in fact an ensemble of 2d plots for a 3d s-velocity distribution). the moho map shown in figure 14 (top) agrees well with the moho map calculated by braun et al. (2007), for greenland and its surrounding area. also, a good agreement is found with the moho depths calculated by dahl-jensen et al. (2003), from receiver-function analysis. depth range: 45 – 80 km. again, the s-velocity presents the highest values at the south of greenland, while the s-velocity values at the northern part are the lower. the s-velocity increases with depth for the whole study area, for each depth range between 45 and 80 km. once again, the s-velocity presents higher values for the consolidated and old structures, which allows the identification, in terms of s-velocity, of the principal units that compose greenland (figure 12). the s-velocity shows the highest values for the archean blocks, north and east of the ketilidian and nagssugtoqidian blocks, respectively. these areas of high velocity are separated by a zone of slightly lower s-velocity, associated to the nagssugtoqidian and ketilidian blocks. again, the s-velocity shown for the area of the nagssugtoqidian and rinkian blocks, presents very similar values for both blocks, as in the previous depth ranges. however, the s-velocity doesn’t show low values, in the area of the east and west greenland tertiary basalts, opposite to the previous depth ranges. also again, the s-velocity values for the area of the ellesmere island and the caledonian and north greenland foldbelts, are similar to those shown for the northern part of the north greenland block, as in the previous depth ranges. it should be noted that the difference in s-velocity values for the northern and southern parts of the north greenland block, shown for the previous depth ranges, is very small for this depth range. in the whole study area, the lowest s-velocities are shown in the off-shore areas. for these areas, the zone of the reykjanes and kolbeinsey ridges (figure 13) shows the lowest s-velocity values, as in the previous depth ranges. it should be noted that a small area of slightly low s-velocity, just located at the iceland hotspot (figure 13), is also shown for this depth range. this pattern of low velocities is typical for young oceans (evans and sacks, 1979; sigmundsson, 1991). on the other hand, the s-velocity values calculated in the present study agree well with those obtained by darbyshire et al. (2004) and pilidou et al. (2005). nevertheless, this comparison only can be approximately done, because darbyshire et al. (2004) didn’t perform any s-velocity mapping, they only performed some s-wave velocity profiles. respect to the s-velocity mapping performed by pilidou et al. (2005) for the north atlantic area (in which the region of greenland is included), it should be noted that greenland is a very small region, comparing with the big size of the north atlantic area. as a consequence of this, a very poor resolution is achieved by their s-velocity mapping for greenland and its surrounding area. logically, the information about the greenland structure, which can be discerned from global mappings as the mapping performed by pilidou et al. (2005), will have always a poorer resolution than that obtained from the present study. a10 v. corchete depth range: 80 – 130 km. the s-velocity pattern shown for the whole study area is very similar to that shown for the previous depth range, except for the study area at the south of the davis strait, in which the s-velocity shows lower values respect to those in the previous depth range, because the asthenosphere is reached at this depth range and for this area, as shown in figure 14 (middle). the lithosphere-asthenosphere boundary (or equivalently the lithosphere thickness) shown in figure 14 (middle), has been computed from the 3d s-velocity structure, calculated in the present study and represented by the 2d s-velocity mapping shown in figure 10, considering for each point of the study area the depth in which the s-velocity starts to decrease with depth. the map of the lithosphere-asthenosphere boundary (lab), shown in figure 14 (middle), is a very interesting feature obtained in the present study, which agrees well with that calculated by kumar et al. (2005), from receiver function analysis. the lab shown in figure 14 (middle) is more detailed than that calculated by kumar et al. (2005), covering greenland and its surrounding area more efficiently. also, it should be noted that the lithosphere thickness, for areas in which are present consolidated and old structures, shows values very similar to those calculated for areas with consolidated and old structures in other continents as south america (corchete, 2012), antarctica (corchete, 2013a) or africa (corchete, 2013b). on the other hand, the s-velocity values calculated in this study agree well with those obtained by darbyshire et al. (2004) and pilidou et al. (2005), also for the present depth range. the s-velocity mapping performed by pilidou et al. (2005) shows similar s-velocity pattern for greenland and its surrounding areas. nevertheless, the identification of the principal units that compose greenland, in terms of s-velocity, is clearer in the s-velocity maps of the present study than in those performed by pilidou et al. (2005), due to the poorer resolution achieved by their global mapping for a small area as greenland. figure 14. map of the moho depth (top), the lithospheric thickness (middle) and the base of the asthenosphere (bottom); obtained from the 3d s-velocity structure beneath greenland and its surrounding area, shown in the 2d s-velocity mapping of figure 10. the moho discontinuity is located, for any point of the study area, at the depth in which the s-velocity value of 4 km/s is overcome by first time. the lithospheric thickness is defined, for any point of the study area, as the depth in which the boundary lithosphereasthenosphere (the top of the asthenosphere) is located. the base of the asthenosphere is defined, for any point of the study area, as the depth in which the low s-velocity of the asthenosphere increases to higher values. the interval between isolines is 5 km for the moho surface representation and 10 km for the other surfaces. depth range: 130 – 180 km. the s-velocity pattern shown for the whole study area is very similar to that shown for the previous depth range, except for the study area at the south of the davis strait, because the asthenosphere is overcome, at this depth range and for this area, as shown in figure 14 (bottom). the boundary of the asthenosphere base (or equivalently the lithosphere-asthenosphere system thickness) shown in figure 14 (bottom), has been computed from the 3d s-velocity structure, calculated in the present study and represented by the 2d s-velocity mapping shown in figure 10, considering for each point of the study area the depth in which the s-velocity starts to increase with depth below the lab. it should be noted that the map of the asthenosphere base shown in figure 14 (bottom), is a new and very interesting feature never obtained before the present study. on the other hand, the s-velocity mapping performed by pilidou et al. (2005) shows an s-velocity pattern similar to that obtained in this study, at this depth range. nevertheless, the poorer resolution achieved for their global mapping, in the greenland region, makes difficult the precise comparison with the present study. depth range: 180 – 230 km. the s-velocity pattern shown for the whole study area is very similar to that shown for the previous depth range, except for the southern part of greenland (in the area of the archean, ketilidian, nagssugtoqidian and rinkian blocks), in which the s-velocity shows lower values respect to those in the previous depth range, because the asthenosphere is reached at this depth range and for this area, as shown in figure 14 (middle). this feature is also observed in the s-velocity mapping of pilidou et al. (2005), although the poor resolution of this global mapping for greenland makes difficult the precise comparison with the present study. the improvement in resolution achieved in this study at the deeper depths, is also related to the wide period range in which the rayleigh-wave group velocities has been measured. in the present study, dispersion curves (group velocities) until 200 s of period have been determined, providing observed data to fit the deep s-velocity structure with more accuracy than any previous study. depth range: 230 – 280 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the north and eastern part of greenland (in the area of the ellesmere island and the caledonian, north greenland foldbelt and north greenland blocks) and for the major part of the off-shore areas which are surrounding greenland (in the north atlantic area of the reykjanes and kolbeinsey ridges, in the area of the wandel and labrador seas and in the off-shore area of north greenland and the ellesmere island), in which the s-velocity shows lower values, because the asthenosphere is reached, at this depth range and for this area, as shown in figure 14 (middle). thus, the asthenosphere is reached beneath the whole greenland and the major part of the study area, at this depth range. for this reason, the s-velocity presents the lowest values shown for the upper mantle, as a consequence of the asthenosphere presence in the major part of the study area, at this depth range, as shown in figure 14 (middle). once again, this feature is also observed in the s-velocity mapping of pilidou et al. (2005), although the poor resolution of their global mapping for this study area and this depth range (their resolution decreases drastically with depth from 200 km), makes difficult the precise comparison with the results achieved in the present study. depth range: 280 – 350 km. the s-velocity values shown for this depth range jump respect to those shown in the previous depth range, from low values (4.3 – 4.5 km/s) to high values (4.8 – 5.0 km/s). obviously, the low-velocity channel associated to the asthenosphere, shown in the previous depth range, has been overcome for the whole study area in the present depth range. also for this depth range, the s-velocity presents the highest values at the south of greenland, while the s-velocity values are lower at the north of greenland. the lowest s-velocity values are shown for the off-shore areas that surround greenland. the identification, in terms of s-velocity, of the principal units that compose greenland is more difficult for this deeper depth range, because the s-velocity differences are very small for this depth range. the s-velocity contrast decreases with depth, while the s-velocity values increase with depth. the result is an s-velocity pattern of similar high s-velocity values, for the whole study area in this depth range. a11shear-wave velocity structure of greenland from rayleigh-wave analysis 7. conclusions the results presented in this paper show that the techniques used here are a powerful tool for investigating the elastic structure of earth, through the dispersion analysis, regionalization and inversion to obtain shear-velocity maps for several depths. this methodology is a good tool to investigate the physical properties of the earth structure, in any region of earth with a variety of tectonic features, as in the present study. by means of this analysis, the principal structural features beneath greenland and its surrounding area have been revealed. such features include the existence of lateral and vertical heterogeneity in the whole study area, as concluded from figure 10. the zones with consolidated and old structures present higher s-velocity values than the other zones, although this difference can be negligible in some cases. nevertheless, the identification, in terms of s-velocity, of the principal units that compose greenland (figure 12) has been possible for many depth ranges. on the other hand, in the depth range of 15 to 45 km, the different moho depths present in the study area generate the principal variation of s-velocity, because the s-velocities associated to the crust are lower than those associated to the upper mantle. a similar behaviour is found for the lab, which generates the principal variations of s-velocities in the depth range from 80 to 230 km, because the s-velocities associated to the lithosphere are higher than those associated to the asthenosphere, as is well known. finally, a new and interesting feature obtained in this study is the definition of the base of the asthenosphere (for the whole study area), for depths ranging from 130 to 280 km, respectively. acknowledgements. the author is grateful to the national geophysical data center (ngdc), the incorporated research institutions for seismology (iris) and the glisn group, for providing data used in this study. the elevation data has been retrieved from ngdc, through the database: etopo1. the seismograms and the instrumental response have been retrieved from iris (through the internet utility wilber ii) and from the database: iris dmc metadata aggregator, respectively. references aki k. and richards p. g., 1980. quantitative seismology. theory and methods. w. h. freeman and company, san francisco. braun a., kim h. r, csatho b. and von frese r. r. b., 2007. gravityinferred crustal thickness of greenland. earth and planetary science letters, 262, 138-158. cara m., 1973. filtering dispersed wavetrains. geophys. j. r. astr. soc., 33, 65-80. corchete v., chourak m. and hussein h. 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larsen t. b., mosegaard k., dahl-jensen t., gudmundsson o., bach t., gregersen s., pedersen h. a. and hanka w., 2004. a first detailed look at the greenland lithosphere and upper mantle, using rayleigh wave tomography. geophys. j. int., 158, 267-286. dziewonski a., bloch s. and landisman m., 1969. a technique for the analysis of transient seismic signals. bulletin of the seismological society of america, 59, 427-444. dziewonski a. m. and anderson d. l., 1981. preliminary reference earth model. phys. earth planet. inter., 25, 297-356. evans j. r. and sacks i. s., 1979. deep structure of the iceland plateau. j. geophys. res., 84, 6859-6866. gregersen s., 1970. surface wave dispersion and crust structure in greenland. geophys. j. r. astr. soc., 22, 29-39. gregersen s., 1970. seismicity and observations of lg wave attenuation in greenland. tectonophysics, 89, 77-93. keilis-borok v. i., 1989. seismic surface waves in a laterally inhomogeneus earth. kluwer academic publishers, london, uk. kumar p., kind r., hanka w., wylegalla k., reigber ch., yuan x., woelbern i., schwintzer p., fleming k., dahl-jensen t., larsen t. b., schwintzer j., priestley k., gudmundsson o. and wolf d., 2005. the lithosphere-asthenosphere boundary in the north-west atlantic region. earth and planetary science letters, 236, 249-257. pilidou s., priestley k., debayle e. and gudmundsson o., 2005. rayleigh wave tomography in the north atlantic: high resolution images of the iceland, azores and eifel mantle plumes. lithos, 79, 453-474. sigmundsson f., 1991. post-glacial rebound and asthenosphere viscosity in iceland. geophys. res. lett., 18, 1131-1134. geomorphic information is omitted from many existing methods of generating gridded digital elevation models (dems) from contour lines, resulting in significant errors during interpolation. here, we present an advanced schema for improvement of the comprehensive regionalized method of linear interpolation. this approach uses a moving fitting method for an interpolated point and selects elevation points that are representative of geomorphic features as a whole to improve interpolation quality. a total of 16 points are selected, according to certain criteria, in eight directions surrounding the interpolated point; thus, there are two points in each direction, which is sufficient to provide an accurate representation of the geomorphic features of the dem. our method introduces virtual control points to prevent sudden changes in the interpolation results, which helps to overcome problems related to the distortion of the local geospatial distribution in areas where feature geomorphic information is inadequate. we construct the spline interpolation function using intersection points and virtual control points, all of which are applied to compute the point elevation. moreover, we index all elevation values and spatial points of linear features using the r-tree method to ensure that points related to an interpolated position can be retrieved as quickly as possible. finally, we test our method using a coal mine elevation dataset. the results confirm that our proposed method can generate dems smoothly and, in particular, avoid problems related to local distortion. la información geomórfica se omite en muchos de los métodos de generación de modelos digitales de elevación (dem, en inglés) que se elaboran a partir de líneas de contorno, lo que resulta en errores significativos durante la interpolación. en este trabajo se presenta un esquema avanzado para el mejoramiento del método comprensivo regionalizado de interpolación lineal. esta aproximación utiliza un método de ajuste movil para un punto interpolado y selecciona puntos de elevación representativos o características geomórficas como un todo para mejorar la calidad de la interpolación. se seleccionaron 16 puntos de acuerdo con ciertos criterios, en ocho direcciones alrededor del punto interpolado; por lo tanto, hay dos puntos en cada dirección, lo que es suficiente para proveer una representación precisa de las características geomórficas del dem. el método propuesto consta de puntos virtuales de control para prevenir cambios repentinos en los resultados de la interpolación, lo que ayuda a vencer los problemas relacionados a la distorsión de la distribución geoespacial local en áreas donde la información de las características geomórficas es inadecuada. se utilizó una función de interpolación spline con puntos de intersección y puntos de control virtual, que fueron utilizados para calcular el punto de elevación. además, se indexaron todos los valores de elevación y los puntos espaciales de las características lineales con el método de árbol-r para asegurar que los puntos relacionados a una posición interpolada pueden ser recuperados tan rápido como sea posible. finalmente, el método fue evaluado con la configuración de elevación de una mina de carbón. los resultados confirman que el método propuesto puede generar modelos sin problemas y, en particular, evitar complicaciones relacionadas a distorsión local. abstract resumen keywords: digital elevation models (dem), interpolation points, contour lines map, geomorphic features. palabras clave: modelos de evaluación digital, puntos de interpolación, mapas lineales de contorno, características geomórficas. an algorithm for generation of dems from contour lines considering geomorphic features algoritmo para la generación de modelos digitales de elevación considerando líneas de contorno y características geomórficas record manuscript received: 25/01/2016 accepted for publication: 15/07/2016 how to cite item rui, x. p., yu, x. t., lu, j., ashraf, m. a., & song, x. f. (2016). an algorithm for generation of dems from contour lines considering geomorphic features. earth sciences research journal, 20(2), g1-g9. doi:http://dx.doi.org/10.15446/esrj.v20n2.55348 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.55348 xiao-ping rui1, xue-tao yu2, 3, jin lu4, muhammad aqeel ashraf5, xian-feng song1* 1college of resources and environment, university of chinese academy of sciences, beijing 100049, china; 2school of traffic and transportation, shijiazhuang tiedao university, shijiazhuang 050043, china; 3traffic safety and control lab in hebei province, shijiazhuang 050043, china; 4china institute of water resources and hydropower research, beijing 100038, china; 5faculty of science and natural resurces, university malaysia sabah 88400 kota kinabalu, sabah, malaysia; * corresponding author, email: song7537338@163.com earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : g1-g9. g e o m o r ph o l o g y g2 xiao-ping rui, xue-tao yu, jin lu, muhammad aqeel ashraf and xian-feng songg2 1. introduction digital elevation models (dems) are imperative in the field of geosciences and have been used widely in the generation and display of three-dimensional terrain. for example, dems have been applied to investigate slope and aspect, construct terrain profiles between selected points, and delineate other significant surface features (kweon and kanade, 1994; zhou and liu, 2004; zhou and chen, 2011; hickey et al., 1994; jones, 1998; oky et al., 2002). many methods have been proposed to generate dem data, including photogrammetry techniques, radar interferometry, laser altimetry, and interpolation (wood and fisher, 1993; wood, 1994; hearnshaw and unwin, 1994). although elevation mapping technology now allows production of dems directly from measured points, accurate interpolation from contours is still important in areas where high-resolution dems are not available (taud et al., 1999; werbrouck et al., 2011) or in cases where historical contour maps are used to evaluate topographic change (carley et al., 2012). for the generation of regional or continental dems, interpolation methods using fine scale contours are useful owing to their low cost and the relative ease of obtaining contour maps in this way (doytsher and hall, 1997; heritage et al., 2009; wise, 2011). accordingly, various interpolation algorithms have been developed to generate dems from contours. the contours, which are in vector or raster format, consist of a large number of elevation points. moreover, the shape of (and geometric relationships between) the contours can be indicative of certain geomorphic features. to determine the elevation at a given point, interpolation algorithms often adopt a sampling strategy that selects representative elevation points from contours in order to reduce computation time, subsequently conducting interpolation using functions such as the inverse distance weight function or surface spline function (huang et al., 2011; maekawa et al., 2007; gofuku et al., 2009). here, we describe several of the most popular interpolation algorithms. the weighted moving average technique interpolates by making use of the geometric relationships between adjacent contour lines (watson, 1992). for a given grid, the elevation at each cell is computed as follows. first, geometrical rays are drawn from the cell center following both cardinal and intermediate directions. then, eight corresponding intersection points between those rays and their closest adjacent contour lines are identified, and the elevation value f(n) is computed from these intersection points using the inverse distance weight method (figure 1) based on these eight crossing points (c1, c2… c8). however, this approach cannot reflect the topography of mountaintops and depressions with sufficient resolution for most purposes (figure 2) (rvachev et al., 2001; dinis et al., 2007; watson, 1999). figure 1. weighted moving average technique. c1, c2, and so on are intersection points in eight directions; p is an interpolated grid point. f(n) is elevation computation function of point p. figure 2. an example to illustrate the drawbacks of the weighted moving average technique: it is hard to determine whether p is in a valley or on top of a hill. taud et al. (1999) developed a raster-based interpolation method to generate digital elevation models through the dilation of contour lines stored in a raster grid. this method uses an iterative procedure to produce an extension of contour lines by applying alternate fourand eight-connected erosions of the background until the generated surfaces become contiguous. this process stops when no new contour lines can be generated. the contour line dilation algorithm can prevent any crossing, especially when the isoline contours are adjacent. this method is simple to apply and has a low computational cost (oky et al., 2002; gousie and franklin, 2003). however, it is clear that, when the contour line is closed, the interpolated data in the inner exhibits the same elevation value as the closed contour line, which may result in error. to insert the height of steep slope areas, xie et al. (2003) developed a special kernel filter that can interpolate a grid cell through which multiple contour lines pass. the contour lines are first rasterized as a grid. then, three cell types are defined concerning the relationships between contours and cells: no-value (nvc), single-value (svc), and multi-value (mvc) cells, which indicate no contour lines, a single contour line, or multiple contour lines passing through the cell, respectively. the elevations of mvcs are computed using an mvc filter, whereas those of svcs are directly assigned based on the appropriate contour elevation. the value of a given nvc is interpolated from its neighboring svcs or mvcs in both the cardinal and intermediate directions. wang et al. (2005) further improved the interpolation of the elevation of svcs by considering the spatial relationship between svc-associated contours and their neighboring contours. in this modified method, a triangulated irregular network is constructed using two adjacent contours; then, the triangle within which the svc center falls is identified and its three vertices used to interpolate the elevation of the svc. clarke et al. (1982) evaluated three interpolation methods: linear interpolation with four data points found in the grid axis directions (lixy), linear interpolation between two data points found in the approximate direction of steepest slope (liss), and cubic interpolation using four data points found in the approximate direction of steepest slope (ciss). their results indicated that dems generated using methods that consider terrain features such as break lines, ridges, and spot heights are more accurate than those generated using methods that do not take such features into account. however, the methods introduced by clarke et al. (1982) use only a few points to infer geomorphic trends; thus, they are successful in improving the quality of dems but are incapable of delineating specific patterns. g3an algorithm for generation of dems from contour lines considering geomorphic features skeleton lines (i.e., ridge and drainage lines) are essential for the description of terrain surfaces. aumann et al. (1991) developed two methods to achieve automatic derivation of skeleton lines from digitized contours but were unable to apply their results to generate dems from contours. conversely, a hydrologically correct grid of elevation can be produced using hutchinson’s algorithm, which takes into account information inherent to the contours and uses them to build a generalized drainage model. then, this information can be applied to form constraints for the interpolation process to ensure that the output dem displays appropriate hydrogeomorphic properties (hutchinson 1988, 1996, 2008). using this method, a network of streams and ridges can be built by identifying areas of local maximum curvature in each contour and, thus, the field of steepest slope. the interpolation procedure uses a discretized thin plate spline technique, where the roughness penalty has been modified to allow the fitted dem to follow abrupt changes in terrain (e.g., streams and ridges). the method uses a maximum of 50 data points from these contours within each cell, which improves computational efficiency. this algorithm has a well-known implementation of topogrid in arcgis, and the drainage conditions imposed by the algorithm produce higher accuracy surfaces with fewer input data (peralvo and maident, 2008; binh and thuy, 2008). to enable interpolation in the regions surrounding mountaintops or depressions, yu and tan (2002) proposed a regionalized comprehensive interpolation method to produce dems using contour lines. based on an interpolated point, geometrical rays in the horizontal and vertical directions are used to intersect with two neighboring contour lines, thus producing a total of eight intersection points in four directions (figure 3). these intersection points are then used to calculate the point elevation using a surface spline function (yu, 2001). this system represents a considerable advantage over the above methods, in that the method of yu and tan (2002) can predict the elevations of mountaintops or depressions using splines because the sample points along each ray direction have different height values. however, points obtained from only four fixed cardinal directions may not be representative; that is, the points on contour lines that are close to the interpolated cell may not be selected, but the points obtained from the intersection points between contour lines and rays that are far from the interpolated cell are selected (figure 4). thus, this method often leads to serious errors, particularly in narrow ridge or valley areas with steep slopes or cliffs. figure 3. regional interpolation. red dots indicate eight intersection points from four directions; blue dot represents p; f(n) represents the elevation computation function of point p. the quality of the derived dem depends greatly on the data source and interpolation technique used (heritage et al., 2009; bouillon et al., 2006; wise, 2011, 2012). vieux (1993) was the first to suggest that the shannon–weaver information statistic, often referred to as entropy, might offer potential as a measure of some characteristics of dem error. li (1994) and carrara et al. (1997) evaluated the quality of dem results derived from digital contour lines using different algorithms. however, their analysis evaluated errors primarily through comparison with the contour interval (ci), and they did not discuss the causes of errors introduced through the use of various algorithms. in summary, it is clear that dems generated using methods that consider terrain features such as break lines, ridges, and spot heights are better than dems produced using methods that do not take such features into account. moreover, of the methods that do consider all available geomorphic information, many are still unable to process cells in the region of mountaintops or depressions, which are often enclosed by only one contour line (xiao and liu, 2012; xie et al., 2003). although hutchinson’s algorithm works well in most cases, it may also introduce some errors in regions of complex geomorphology. however, geomorphic information is included within existing contour maps, and information relating to mountaintops and depressions can be achieved by studying the contour lines of these maps. using this information when interpolating gridded dem data from contour lines can preserve the precision of the results (bonin and rousseaux, 2005). therefore, this paper introduces a grid-based dem interpolation method that considers the geomorphic information hidden in contour maps to avoid the interpolation error that typically occurs in traditional methods. it was achieved by adopting the positive aspects of both the weighted moving average technique and the comprehensive regionalized method. this approach utilizes eight-directional ray sampling method to obtain necessary geomorphic information from a contour map. for areas such as mountaintops, depressions, boundaries, or corners, some virtual control points to control the precision of interpolation were added. then, the obtained results were compared with those obtained using the comprehensive regionalized method of yu and tan (2002) and associated arcgis functions. 2. an interpolation method considering geomorphic information 2.1. basic premise the fundamental step in this process is finding the contour line nearest to an interpolated point p and obtaining a perpendicular foot b from point p to this figure 4. an example to illustrate the drawbacks of regional interpolation. red dots represent eight virtual control points from four directions. the blue dot represents p. the intersection point a is far from the interpolated point p and the information on the adjacent contour line is not used. g4 xiao-ping rui, xue-tao yu, jin lu, muhammad aqeel ashraf and xian-feng song contour line. then, using p–b as the base direction, eight-half lines are drawn at intervals of 45° to produce rays in eight directions. these half lines intersect with two adjacent contour lines around p, resulting in a maximum of 16 intersection points denoted by p1, p2, p3, and so on. if the number of sample points, n, is less than 16, the results will reflect the geomorphic features of the terrain adequately. conversely, if n > 16, the efficiency of the method will be impacted negatively. figure 5 presents an example in which 16 intersection points are obtained for the interpolated p. we use a spline function to construct an interpolation function, as described by curry and schoenberg (1996) and yu (2001): where x and y are coordinates, w (x,y) is the elevation value of interpolated point p, and a0, a1, a2, and fi (i = 1,2,...,n) are undetermined coefficients, of which there are n + 3 in total. r 2 i = (x-xi)2 + (y-yi)2 and e are empirical parameters that allow for the adjustment of surface curvature. for large (small) changes in the surface shape, ɛ should be small (large); values of e range from 10-2 to 1 for flat surfaces and 10-6 to 10-5 for singularity surfaces. figure 5. using eight directions to obtain intersection points. red dots indicate 16 intersection points from eight directions; blue dot represents p. the n + 3 unknowns are determined according to equation 2. the coefficient cj, which is in units of length squared, is equal to 16πd/kj, where d is the plate rigidity and kj the spring constant associated with the jth point. during interpolation, kj = ∞ and cj = 0. 2.2. introduction of virtual control points to ensure that the interpolated results can be constrained by the values of the contour lines, virtual control points were added to control the precision of interpolation. it was designated en as the value of a point on the contour line near p and eci as the contour interval. from the features of the contour map, it is clear that, if p is located between two contour lines, w (x,y) should lie between the values of these two contour lines. similarly, if p is within a closed contour line, w (x,y) should be less than en + eci or larger than en − eci. if not, then this may lead to wrong results. therefore, to prevent sudden changes from occurring in complex areas, we add a virtual control point v for the four intersection points. then, our algorithm calculates the elevation value at point v based on the heights of the four surrounding points, according to the following rules. the elevation of v is zv, which is calculated according to equation 3. where δ is a coefficient between 0 and 1 and is used to control the elevation of v. the adding and subtracting operations were used for dems covering mountaintops and depressions. figure 6. the procedure for adding a virtual control point. the top part of this figure illustrates two neighboring contour lines that indicate a mountaintop area; points a, b, c, and d are four intersection points on the contour line. the bottom of the figure illustrates the corresponding sections of lines ab and cd. figure 6 illustrates a typical example of the addition of a virtual control point in one direction for a case in which the interpolated point lies along an adjacent contour line. in this case, the elevation value of v should be larger than hbc (where hbc is the height of the contour line located using b and c) but lower than hbc + δh (where δh is the interval of the contour lines). θ1 and θ2 denote the inclinations of lines ab and cd, respectively, and δ can be set according to the geomorphic feature studied: δ ranges from 0 to 1 and is equal to 1 when θ1 = θ2. based on the features of the contour line map, it is clear that zv should be less than hbc + δh; however, when δ = 1, zv will be equal to hbc + δh. to avoid zv reaching this value, a small perturbation (ɛ, equal to 0.01) is added to δ such that δ = 0.99. when θ1 ≠ θ2, δ will be greater than 0 and less than 1 and equation 3 can be rewritten as follows: g5an algorithm for generation of dems from contour lines considering geomorphic features δ ranges from 0 to 1 and is equal to 1 when θ1 = θ2. based on the features of the contour line map, it is clear that zv should be less than hbc + δh; however, when δ = 1, zv will be equal to hbc + δh. to avoid zv reaching this value, a small perturbation (ɛ, equal to 0.01) is added to δ such that δ = 0.99. when θ1 ≠ θ2, δ will be greater than 0 and less than 1 and equation 3 can be rewritten as follows: subsequently, the coordinates and elevation of the virtual control point can be determined. here, e donates the intersection of lines ab and cd, and its coordinates can be obtained by solving equations 6 and 7. then, the value of sin(θ1) can be obtained as follows. sin(θ2) can be calculated in a similar manner. thus, when both sin(θ1) and sin(θ2) are known, the elevation of the virtual control point can be obtained from equation 3. as discussed above, rays in eight directions are used to query the intersections with the contour lines. however, two intersection points are not present in all directions, e.g., if the interpolated point is at a boundary or corner of the map. therefore, geomorphic information is missing in such directions. moreover, it is not necessary to add a virtual control point in cases where there are no intersection points in a particular direction because this virtual point may introduce obvious error. if there is only one intersection point in a particular direction, two intersection points are used on the symmetrical contour lines; this produces three crossing points in total, which is sufficient to determine whether a virtual control point is required based on the geomorphic trend inferred from the three points. there are three possible situations in cases with three points (figure 7). figure 7. three intersection points in one direction. 1) when the elevation of the intersection point is higher than that of either a or b (e.g., d1 in figure 7), the geomorphic trend gets higher from a to d1, so the interpolated point is just between b and d1, and no virtual control point is required. 2) when the elevation of the intersection point is equal to that of a or b or lower than that of b (e.g., d2 in figure 7), it can be assumed that the local maximum occurs at b (or in its vicinity) and that the elevation of the local maximum will never exceed hbc + δh. a virtual control point must be added in such cases. for example, line ab could be extended to intersect the constraint contour (i.e., point c in figure 8). here, c acts to limit the elevation of the virtual control point. then, the method described above can be applied to compute the coordinates and elevations of virtual points using the points a, b, c, and d. figure 8. the procedure for adding a virtual control point for three intersection points. 3) when the height of the intersection is lower than that of b (e.g., d3 in figure 7), it can be assumed that b represents the local maximum; in such cases, the geomorphic trend is clear, and no control point is required. figure 9. other geomorphic features expressed in contour line maps. in other cases, such as where the topography is terraced or serrated (figure 9), the elevation and location of v reflect the average of the neighboring intersection points and the midpoint of line bc, respectively. the distances between all of the virtual control points were calculated adding and removing the control point if the distance is less than a threshold value. then, all of the remaining distances between virtual control points are above the threshold value. different threshold values were used to test the algorithm; according to the experimental results, the threshold value should be set to 2–3 times the interpolation accuracy. finally, it was constructed the interpolation equation using all of the virtual control points and intersection points and calculate the value of point p based on the interpolation equation. 2.3. algorithm workflow here, it was introduced an efficient algorithm that achieves the interpolation of dems from contour maps, as shown in figure 10. the main steps in this algorithm workflow can be summarized as follows. first, the contour lines of the map were traversed and decomposed, before dividing them into line segments and saving them. this procedure allows obtaining the average length of the line segments. then, the r-tree indexes for these line segments were built and stored the elevation in the r-tree (huang, p. lin and h. lin, 2001; corral g6 xiao-ping rui, xue-tao yu, jin lu, muhammad aqeel ashraf and xian-feng song and almendros-jiménez, 2007; zhu, gong and zhang, 2007). finally, each interpolated point a was processed as follows. 1) the r-tree was used to query the closest line segment and point (i.e., b) to point a on the line segment. 2) a step k was set, whose value is equal to the average length of the line segments, and extend line ab in two directions at intervals of k. in this manner, it was assessed whether line ab intersects with the contour line, find the intersection points in both directions along line ab, and add a virtual control point if the interpolated position is on a closed contour line (figure 11). 3) there were constructed eight lines at intervals of 45°, find the associated intersection points, and add virtual control points where appropriate. 4) it was filtered the control points according to the distance between virtual control points based on a filtering rule, which states that the distances between the added virtual control points must be above a threshold value, i.e., 2–3 times the interpolation accuracy. 5) there were calculated the coefficients of the spline interpolation function, which are determined according to the control points, and calculated the interpolated point value based on the spline interpolation feature. (6) finally, it was output the value of the interpolated point. figure 10. flowchart illustrating the interpolation of dem data. figure 11. intersection point finding method in programming. 3. experiments and discussion 3.1. experimental data and preprocessing in the present study, it was validated a method using a surface contour line map for the dayang coal mine in shanxi province, china. the original contour map for this area is shown in figure 12. the original contour interval is 5 m, and the output dem data is in the form of an ascii grid text file with a grid cell size of 2 m. the original contour map was cut into 16 sub-maps to test the results of the proposed method (figure 12b), and those maps were used to verify the accuracy of interpolation. then, it was generated the dem data for each subarea using the “topo to raster” function of arcgis, the regional interpolation method of yu and tan (2002), and the method proposed in this study. g7an algorithm for generation of dems from contour lines considering geomorphic features figure 12. the original contours. 3.2. results and discussion it was used a stretched color ramp to display the dem results. obvious errors (i.e., abnormal colors, indicated by red color in figure 13) were found in the results achieved using the method of yu and tan (2002). however, neither the proposed method, not the arcgis method produce obvious errors. in fact, the results obtained using the “topo to raster” function of arcgis are very similar to the results obtained with this method; thus, they are illustrated in figure 13. figure 13. comparison of dems generated using the method of yu and tan (2002) and our method in arcmap. red color indicates the obvious error. the dem results were transformed into contour line maps to compare the results in more detail, and to set the intervals and baselines of these maps to be the same as those of the original contour map (figure 14). it was found that, in most cases, the contour lines generated by the proposed dem fit the original contour lines very well. moreover, in some areas, the results agree more closely with the original contour lines than the contours generated using the arcgis method. in fact, the arcgis method is found to produce obvious errors (e.g., figure 14d, in which a closed contour line was generated according to the arcgis method), suggesting that the proposed method provides more accurate dems that can reflect geomorphic features more closely. it was noted that the smoothing effect of the contour lines is less developed in the proposed method than in the arcgis’s method (figure 14). furthermore, the proposed method generates more isolated points on the contour lines than other methods, because each dem grid value is interpolated dynamically using intersection points and virtual control points in this approach, then smoothness is not so good. figure 14. comparison of contours generated using the proposed method and the arcgis method with original contours. finally, the transformed contour line maps were compared with the original map using the mean hausdorff distance (mhd; chen, ma, xu, paul, 2010; hung and yang, 2004). the hausdorff distance is a measure of the similarities between two nonempty compact (closed and bounded) sets a and b in a metric space s on their positions (nadler, 1978). the mhd between the original and transformed contour line maps was calculated for each submap and the mhds obtained were compared between the proposed method and the arcgis method (figure 15). the results illustrate that the errors for each algorithm are similar; therefore, it was concluded that the proposed method exhibits similar accuracy to the arcgis process. figure figure 15. the difference in hausdorff distance calculated using the two different algorithms. 4. conclusions here, it was presented an improved method of generating gridded dems from contour line maps while taking geomorphic information into account. first, it was identified the point on a contour line that is nearest to the interpolated point; the choice of this position plays a critical role in interpolation accuracy. then, 16 points on neighboring contour lines were obtained in eight directions from the interpolated point; these points can reflect the basic geomorphic features of the resulting dem. for regions of complex geomorphology, it was inserted a virtual control point at a specified place in each direction according to various geomorphic features identified g8 xiao-ping rui, xue-tao yu, jin lu, muhammad aqeel ashraf and xian-feng song from the original map, which prevents the interpolation result from exceeding the contour interval. finally, the points with feature geomorphic information were used to construct local surface spline functions for interpolation. the method was tested using a surface contour line map of the dayang coal mine in shanxi province, china, and the results were compared with those obtained using existing methods. it was concluded that the proposed method is at least as accurate as the arcgis “topo to raster” function and is more accurate than the method described by yu and tan (2002). in this method, each dem grid value is interpolated based on intersection points and virtual control points, and the spline interpolation function was constructed dynamically. therefore, the method requires a greater analysis time than the arcgis method and the dems generated using it are not as smooth as those generated using the arcgis method. improvements in this regard will form the basis of future work. the interpretation of real complex geomorphic features from contour maps remains challenging, and the objective of this study is to continue improving the methods for interpolation of gridded dem data based on such maps. funding this work was supported by the national science and technology major project of china [no. 2011zx05039-004], the national natural science foundation of china [no. 40901191], the strategic priority research program of the chinese academy of sciences [no. xda05030200], the hebei province natural science fund [no. d2016210008], the social science foundation project of hebei province [no. hb15sh015], and the scientific research foundation for doctor of shijiazhuang tiedao university. references aumann, g., ebner, h. & tang, l. 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(2007). an efficient 3d r-tree spatial index method for virtual geographic environments. isprs journal of photogrammetry and remote sensing, 62(3), 217–224. doi: 10.1016/j.isprsjprs.2007.05.007 the north anatolian fault zone (nafz) is one of the most important fault zones of turkey and the world. it has produced several high magnitude earthquakes that have resulted in massive loss of lives and resources. national and international research on the north anatolian fault zone that turkey resides on have been realized to better understand and predict the earthquakes produced by it. this study focuses on the çankırı – bingöl segment of the nafz. the aim of this study is to calculate the strain and latent earthquake potential of the studied area. for this purpose, geodetic data coming from several individual projects have been merged. strain values have been calculated from the combined data and regions on the fault zone, and strain accumulations have been presented graphically. after calculation, çankırı, amasya and kelkit regions were analyzed. the compressional and extensional deformation has been shown in north and south part of çankırı basin, respectively. eastern adjacent area of the çankırı basin, amasya region, has the primary branch of the naf and its subbranches. in the amasya region, the deformation is mostly on the main branch and the earthquake potential has risen to it. the kelkit valley has complex structures and inhomogeneous dispersion. southeastern and northwestern part of the kelkit valley has varied deformation in micro scale. consequently, the study results indicate that strain accumulation is concentrated on areas such as the çankırı basin, amasya region, and various areas in the kelkit valley from west to east. la zona de la falla de anatolia del norte (nafz, del inglés north anatolian fault zone) es una de las zonas de fallas más importantes de turquía y del mundo. esta falla ha generado varios terremotos de gran magnitud que han resultado en pérdidas humanas y de recursos. la investigación nacional e internacional de la zona de la falla de anatolia del norte, que atraviesa turquía, se ha realizado con el fin de un mejor entendimiento y predicción de los terremotos que allí se originan. este análisis se enfoca en el segmento çankırı-bingöl de la nafz. el objetivo es calcular la tensión y el potencial de terremoto del área de estudio. con este propósito se recopiló la información geodésica de varios proyectos individuales. los valores de tensión se calcularon de la información combinada de las regiones que componen la zona de falla y se presentan gráficamente las acumulaciones de tensión. tras el cálculo de estos valores se analizaron las regiones çankırı, amasya y kelkit. la deformación de compresión y la de extensión aparecen al norte y al sur de la cuenca çankırı, respectivamente. el área ubicada al este de la cuenca çankırı, la región de amasya, posee la rama principal de la nafz y sus subdivisiones. en la región de amasya la deformación se presenta en la rama principal de la nafz, donde se eleva el potencial de movimientos sísmicos. el valle de kelkit tiene estructuras complejas y dispersión no homogénea. el sudeste y el noroeste del valle kelkit muestran una deformación variada a microescala. los resultados de este estudio indican que la acumulación de tensión se concentra en la cuenca çankırı, la región amasya y varias áreas del valle kelkit desde el oeste hacia el este. abstract resumen keywords: gps, strain rate, earthquake, deformation, geodynamics. palabras clave: gps, velocidad de deformación, terremoto, deformación, geodinámica. strain rate analysis on the çankiri-bingöl segment of the north anatolian fault in turkey hakan yavasoglu istanbul technical university insaat fakultesi, geomatik muhendisligi bolumu, maslak, istanbul, 34469, turkey yavasoglu@itu.edu.tr record manuscript received: 11/02/2015 accepted for publication: 28/07/2015 how to cite item yavasoglu, h. (2015). strain rate analysis on the çankırı-bingöl segment of the north anatolian fault in turkey. earth sciences research journal, 19(2), 121-127. http://dx.doi.org/10.15446/esrj.v19n2.49063 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.49063 análisis de la velocidad de deformación en el segmento çankırı-bingöl de la falla de anatolia del norte, turquía earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 121 127 se is m o t e c to n ic s 122 hakan yavasoglu introduction the nafz is one of the most offensive fault system all over the world and approximately 1500km long strike-slip fault system delineating the boundary between eurasia and anatolia plates (barka and kadinsky-cade, 1988). the nafz runs along the northern part of turkey, from karliova in the east to the gulf of saros in the west and connects the east anatolian compressional regime to the aegean extensional regime. the study area is bordered by the coast of the black sea in the north, çankırı – ilgaz (kastamonu) in the west, the sungurlu residential area in the south and bingöl – karlıova in the east. the main and secondary branches of the north anatolian fault have meant that the study area has been dissected into several continental blocks (fig 1). figure 1. reverse triangles are gps sites, reds from yavasoglu et al., 2011, blues from tatar et al., 2012, greens from ozener et al., 2010 and magentas from reilinger et al. 2006. the tectonic map of study area, nafznorth anatolian fault zone, sf-sungurlu (ezinepazar) fault, of-ovacık fault, pf-pülümür fault and yf-yedisu fault. the focal mechanism of earthquakes (beach balls) were obtained from globalcmt web page. according to recent research (i̇şseven and tüysüz, 2006; kozacı et al., 2007; tatar et al., 2012), the continental blocks that are bordered by faults move independently in different directions around a vertical axis. also, it is important to understand how the potential energy of deformation accumulates and releases regarding spatial and time variables. spacebased geodetic technology enables us to determine the degree of crustal deformation with millimeter accuracy. so, it is possible to estimate present-day tectonic strain accumulation on the nafz using these geodetic technologies and fault mechanism incorporating variables such as slip rates, locking depths, fault geometry, etc. (straub et al., 1997; armijo et al., 1999; mcclusky et al., 2000; reilinger et al. 2006; ozener et al. 2010; yavasoglu et al. 2011; tatar et al. 2012; peyret et al., 2013). since 1990 many pieces of research have been undertaken in the area. these were mostly on a micro scale geodetic network or a global scale, but with an inadequate density of geodetic stations (mcclusky et al., 2000; hubert-ferrari et al., 2002; hartleb et al., 2003; reilinger et al. 2006; and kozacı et al., 2007). velocity field derivatives from an inadequate density of networks affect the results. additionally, in micro-scale studies, tectonic activity bordering the surveyed area cannot be distinguished sufficiently. therefore, an analysis incorporating both a large scale study (covering all the area) and adequate localization resolution is needed. in this study, the aim is to combine previous micro-scale geodetic studies and an estimate of the earthquake potential of the region using strain analysis computation (write et al., 2001; reilinger et al., 2006, ozener et al. 2010; yavasoglu et al. 2011; tatar et al. 2012). the variations of the slip/strain rate and deformations in the study areas were investigated through a rigorous combination of the published micro scale geodetic data as a primary and new contribution. moreover, with this study, the nafz segment located between longitudes e33 and e41 degrees was investigated. there are three subsegments in this area. the first section runs from kastamonu to amasya, the second segment from amasya to erzincan, and the last part from erzincan to bingöl. historical and instrumental records indicate that they are active (ambraseys, 1970, 2009; barka, 1992, 1996; barka et al., 2000). another important aspect of this region is the fault behavior. what are the general characteristics of tectonic loading in this region? this is the main question concerning how loading is accommodated by the nafz. to answer these issues, firstly the tectonic and the seismic settings of the study area are summarized, and then the combined gps velocity field is presented, and finally, the model results obtained from gps velocities are discussed. tectonic settings the north anatolian fault zone (nafz) which is the best-known dextral strike-slip faults in the world because of its remarkable seismic activity, separates the anatolian plate from the eurasia plate. the anatolia plate escapes to the west about the eurasia plate along the nafz. the nafz system has a strike-slip and right lateral tectonic settings because of two important mechanisms: the first is the arabian plate push it in the east, and the second, it escape to the west along the hellenic arc. (mcclusky et al., 2000; bozkurt, 2001; sengor et al., 2004; bayrak et al., 2009 and 2011). in the 20th century, many destructive earthquakes happened on the nafz, affecting the lives of millions and causing damage regarding billions of dollars in the study area (table 1). besides the 1999 earthquakes affecting izmit and düzce, most of the destructive earthquakes happened on the central and east segments, from kastamonu to bingöl (table 1, tan et al., 2008). table 1. major earthquakes of last century in the area. (url1) the nafz extends to the karlıova (bingöl) triple junction in the east and the aegean sea in the west. it formed about 13 to 11 ma in the east and propagated westward at about 11 cm/yr according to geological studies (sengor et al., 2004). however, the nafz was formed in early pliocene according to several scientist like barka and kadinsky-cade, (1988) and bozkurt (2001). the segmentation of the naf and the structure of its splines are still in conflict. according to sengor et al., (2004), the nafz bounds the anatolian plate to the north. its width, which is about 100 km, steadily increases from east to west, even though some pinched or swollen zones exist and the fault is located along an interface that separates subduction-accretion material to the south, from the older and stiffer continental basement to the north. the nafz has been known to incorporate a uniform and homogenous 123strain rate analysis on the cankiri-bingol segment of the north anatolian fault in turkey structure in many segments. however, present day gps data and strain analysis show us that it is not strictly uniform and homogenous from east to west. many studies prove that the geological settings are different for each segment (bozkurt, 2001; sengor et al., 2004; bayrak et al., 2009 and 2011; peyret et al., 2013). therefore, the velocity and seismicity of each segment of the nafz is also different. in this area, the type of deformation is strike-slip along the fault. on the other hand, the central part of the nafz fault deformation has a standard component, because the fault is parallel to the black sea coastline. additionally, geological evidence indicates compressive strain near the ilgaz mountains (piper et al., 2010). the main offsets on the nafz are in the pontide suture which is located close to the city of erzincan (longitude e39◦20’) (sengor et al., 1985), around the sea of marmara (armijo et al., 1999), and in the western part of the central bend (hubertferrari et al., 2002). the cumulative displacement of the nafz is about 80 km as has been indicated by evidence obtained from river deflection such as appertain to the yeşilırmak, kızılırmak and gerede rivers (hubert-ferrari et al., 2002; sengor et al., 2004; peyret et al., 2013). moreover, previously estimated geological slip rates of ±18 mm/yr (hubert-ferrari et al., 2002) to ±20.5 mm/yr (kozacı et al., 2007) are in consensus with present day gps derivative slip rates as determinated by block modeling that ranges from 17 to 25 mm/yr (mcclusky et al., 2000; reilinger et al., 2006; yavasoglu et al., 2011). the aim of the earth science studies on the nafz is to understand the large-scale behavior of the nafz zone. for this purpose, geodetic networks have been established on the nafz segments (mcclusky et al., 2000; reilinger et al., 2006; ozener et al., 2010; yavasoglu et al., 2011; tatar et al., 2012). in this study, the horizontal gps velocity fields published by reilinger et al. (2006), ozener et al. (2010), yavasoglu et al. (2011) and tatar et al. (2012) will be used as a reference. these velocities have been estimated from at least 3 gps campaigns and have been computed by using geodetic gps process software such as gamit/globk and bernese. therefore, the velocities are accurate to the sub-millimeter level. geodetic studies today, insar (synthetic aperture radar interferometry) and gps (global positioning system) are the most common methods used to observe tectonic movements. during the last decades, applications of such usage have been expanded, and precision of calculation has been increased. in this research, gps data that has been gathered from research published between 2006 and 2012, have been included in the analysis. geodetic studies have been carried out concerning local regions. although global scale measurements have been performed in previous studies (mcclusky et al., 2000; and reilinger et al., 2006), their cover area does not represent the entire fault zone, and the number of geodetic points was limited. therefore, it is once more expressed that geodetic studies should be merged and analyzed accordingly (table 2). table 2. gps sites velocities used in strain rate computation. 124 hakan yavasoglu strain rate calculation (modeling) the gps data obtained from different projects (papers) have been transformed into the same datum. then, the eurasia fixed velocity field has been calculated. the data used in this study have been processed using geodetic software gamit/globk, which can to provide velocity vectors with sub-millimeter accuracy. the velocity field is necessary to show the movement of anatolian plate. but it is usually not sufficient for earth science purposes. velocity field data gathered from gps data is meaningful when translated into strain and slip values using block modeling or elastic half-space modeling. the general approach associated with this method is to obtain a continuous strain field via a different interpolation of the east and north velocity components (wessel and bercovici, 1998) on a regular grid using the splines in tension algorithm using only geodetic data. a factor (t) controls the tension. t=0 is the minimum curvature, and t=1 is the maximum curvature. also, the gps sites must be sufficiently distributed to cover all the area under consideration, and they must be distributed in such a way as to cover an area bigger than the grid size. it was tested the t values and set to t=0.3 as recommended by hackl et al. (2009). the area was divided into cells using the grid size. they contain more than one observation. the median of all the included data was computed for each, with the regions having a large number of comments in need of being averaged. in this way, outliers and bias can be removed. the interpolation will give two continuous scalar fields from east and north velocities are independent for the interseismic period. in this way, the spline interpolation function can be applied to calculate the strain rate tensor for two components of the velocity fields. the elements of the strain rate tensor are defined in hackl et al. (2009) as; (1) where i, j substitute east and north. in a similar way, it is possible to compute the antisymmetric rotation rate tensor: (2) at this point, any tensorial analysis can be performed. the eigenspace analysis of the tensor is the starting point for the full description of deformation at every grid point, providing different aspects of the strain rate. the eigenvectors of the strain rate, for example, represent the direction of maximum and minimum strain rates, while their associated real eigenvalues λ1 and λ2 represent the magnitude (note that the notation that positive values indicate extension and negative values stand for compression was followed) (hackl et al., 2009). the maximum shear strain rate and its direction might provide a tool to identify active faults since motion along faults is related to shear on that structure. faults oriented in this direction are the ones most likely to rupture in a seismic event. the maximum shear strain rate at every grid point can be obtained by a linear combination of the maximum and minimum eigenvalues: (3) while the direction of maximum shear is oriented 45◦ from the direction of the eigenvector corresponding to the largest eigenvalue: (4) note that eq. (4) corresponds to two conjugate perpendicular directions that cannot be distinguished without further constraint. the trace of the tensor, (5) 125strain rate analysis on the cankiri-bingol segment of the north anatolian fault in turkey corresponds to the relative variation rate of surface area (dilatation) and thus can indicate regions of thrusting or normal faulting (hackl et al., 2009). therefore, strain rates, the variation rate of the area under consideration, maximum and minimum share and strain rate tensor were calculated from the velocity field obtained from gps data using a method published in hackl et al. (2009) in detail, (figs. 2, 3). figure 2. the map represents strain rate distribution, green regions are the most active areas, the numbers (1, 2, 3, 4, 5) are shown çankırı, amasya, kelkit, erzincan and karlıova regions, respectively. figure 3. the map represents strain rate distribution, red reverse triangles are the location of the gps sites. based on the allocation of the gps observations, different grid sizes were tested to identify the best resolution. the ideal situation would be to have a grid with at least one observation per cell. after various tests, it was found that a regular grid with a cell size of 0.03◦ is most suitable for the interpolation of the horizontal velocity field components for this region. using gmt routines (wessel and smith, 1998), the strain rate tensor was calculated. figures 2, 3 shows the three components of the strain rate tensor. this method is more suitable to determine relative strain and strain rate changes (hackl et al., 2009). the plate boundary along the nafz is mainly of strike-slip nature. thus, the direction and magnitude of the maximum shear strain rate are good scalar fields to represent the strain rate tensor. these two parameters are suitable to characterize the amount of localization of the shear deformation and the direction along which strike-slip faulting is more favorable. in figures 2, 3, the color scale indicates the magnitude of the maximum shear strain rate, while the crosses indicate the two conjugate maximum shear directions. the maximum shear strain rate is highest in the southeast (region-4 and 5) along the nafz (max 0.44 µstrain/yr) and along the central section of the nafz (region-2 and 3) (max 0.33 µstrain/ yr). this matter can partially be a consequence of the fact that in these regions, the deformation can better localize along the major segments of the fault especially region-2 and 3. the maximum shear strain rate is less in region-1 according to the other four regions, but it has significant values (max 0.28 µstrain/yr) in çankırı basin. discussion biryol et al. (2010) suggest that comparison of fast polarization directions with plate motion directions requires selection of a reference frame that will yield true absolute plate velocities. there exist multiple reference frames for plate motions, based on different assumptions, and each of these has different motion directions and speeds. one of the most commonly used reference frames for our study area regards eurasia as fixed and focuses on the relative motions of the surrounding plates (i.e. anatolia) on fixed eurasia (mcclusky et al. 2000). in this case, the direction of lithospheric motion depends strictly on the selection of the fixed plate (i.e. eurasia) and does not necessarily represent an absolute plate motion that can be used for comparison with mantle anisotropy measurements. regional strain rates for anatolia indicate variations in the principal compressional and extensional strain axes from east to west, following the pattern of the counter-clockwise rotation of the anatolian plate. this variation in direction for maximum compression and extension is also in agreement with the structural features of the anatolian crust (biryol et al., 2010). regarding the results of this study, there is a high degree of consistency between the results obtained with geodetic methods and geological–geophysical methods (biryol et al., 2010). therefore, the data used in this study, the modeling calculations, and the computed strain rates can be accepted. the study of regional strain rates is crucial for any seismic hazard assessment. in this study, the velocity fields of the middle and eastern parts of the nafz have been merged to calculate the strain accumulation in a greater area. similar studies have been realized previously in local areas. however, none of them either covered the focus area of this research nor were as large in scope. a previous study by mcclusky et al. (2000) and reilinger et al. (2006) should be noted, however, since they cover the same area of focus but with limited geodetic data. the çankırı basin (region-1) that is located in different tectonic regimes is an active seismic region (kaymakci et al., 2003; yavasoglu et al., 2011; and peyret et al., 2013). the strain accumulation in the çankırı basin has been discussed in kaymakci et al. (2003) and peyret et al. (2013) concerning the possibility that it is a post-seismic strain that occurred after the duzce (1999) earthquake, and the effects were thought to be improbable regarding the postseismic activity. however, strain accumulation in region-1 can be seen to be associated with a right lateral slip rate (fig. 3). in this study, for the çankırı basin, the northern side of the çankırı basin indicates compressional deformation, and the southern side indicates extensional deformation. besides, the rotational displacement is also shown (cinku et al., 2011). in amasya (region-2), the nafz exhibits a horse tail structure with the main branch and several secondary branches that extend into anatolia. the most important and well-known of these branches is the sungurlu (ezinepazar) fault, on which deformation signs were not detected in this study, in concordance with yavasoglu et al. (2011). however, it is known that the 1939 erzincan earthquake also fractured the sungurlu fault (i̇şseven and tüysüz, 2006). despite this, there is a concentrated strain accumulation in region-2, where the sungurlu fault and the nafz main branch merge. between the main branch and the sungurlu fault, a structure of normal and reverse faults have triggered the extension. due to this extension, the strain has built up in the northern parts of the region-2, and the earthquake potential has risen accordingly. kelkit valley (region-3) and erzincan (region-4) exhibit a very active setting. seismicity is noteworthy in this region as is reported in tatar et al. (2012). while region-4 shows signs of compression, region-3 shows signs of extension. in region-3, the nafz is wider (sengor et al., 2004). the tension that builds up in region-2 also affects region-3, which in turn continues the extension. karlıova (region-5) is a very complex zone (barka et al., 1987; sengor and yılmaz, 1981). many physical elements contribute to the deformation where the right lateral nafz and the left lateral east anatolian fault zone 126 hakan yavasoglu (eafz) merge. the earthquakes of march 12 and 14, 2005 in karlıova (table 1) were the results of an extended period of seismic inactivity. the seismic gap that lasted from the year 1784 has been ended to a degree in this region. however, strain accumulation in the area can be postulated to be still building up. this complex structure also affects the results of this study. the tension in region-5 does not exhibit a homogeneous dispersion. the fault segments found in the yedisu, ovacık and pülümür faults should be investigated separately. conclusion gps velocity field data has been modeled for the strain rates in middle and eastern parts of the nafz, using published gps derivative data (reilinger et al., 2006, ozoner et al., 2010, yavasoglu et al., 2011, tatar et al., 2012) and the mathematical model published in hackl et al. (2009). the presence of earthquake potential and strain accumulation in 5 regions in the middle and eastern parts of the nafz (figs. 2, 3) have been found in this study. with the help of this new modeling method, the results are free of ambiguity regarding parameters such as fault locking depth, fault geometry, and are calculated to show present day activity using only geodetic data. between region-1 and region-2, there is no significant strike-slip or dip-slip on the sungurlu fault that is the spline of the nafz. the sudden decrease in strain accumulation from the east (the kelkit valley) to the west (south of the çankırı basin) reveals that all the strain is sufficiently accommodated by the main branch of the nafz. region-5 indicates a high strike-slip rate. the last rupture in the region-5 was approximately 250 years ago (ambraseys, 1970). the accumulated slip deficit is about 3m, corresponding to an earthquake potential of between mw 7 and 7.7. there is great risk concerning the five regions focused on in this study. extensive and detailed seismic records are needed to estimate earthquake times and magnitude. also, micro-geodetical research in the region should be increased. multi-disciplinary studies on both the global and the local scale should be increased, especially in the regions where the nafz shows a very complex structure. acknowledgements i would like to thank m. hackle, r. malservisi and s. widowinski for their gmt scripts. i am grateful to t. aykan kepekli, huseyin a. yavasoglu and ali goksenli for their comments. this paper has been supported by tubitak (2219 scholarship) and itu. maps and figures were drawn using gmt (wessel and smith, 1998). references ambraseys, n.n. 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(2011). determining and modelling tectonic movements along the central part of the north anatolian fault (turkey) using geodetic measurements. journal of geodynamics, 51, 339–343 groundwater is vulnerable and more susceptible to contamination from various anthropogenic elements. various steps are taken to measure the groundwater vulnerability for a sustainable groundwater development. the present study estimates the aquifer vulnerability by applying drastic model in the geographic information system (gis) environment. the drastic model uses seven hydrological parameters which include depth to water level, net recharge, aquifer media, soil media, topography, the impact of vadose zone and hydraulic conductivity. drastic index was calculated from drastic model that ranged from 31 to 154. all these parameters characterize the hydrological setting for evaluating aquifer vulnerability. sensitivity analyses have also been performed to determine the sensitivity of every individual drastic parameter towards the aquifer vulnerability. sensitivity analysis indicated that all the parameters have an almost similar influence on vulnerability index. depth to water parameter inflicts larger impact on aquifer vulnerability followed by recharge, topography and soil media. the whole of kodaganar basin is classified into very low, low, moderate and high vulnerable zones. nearly threefourth of the basin has very low and low vulnerability. incorporating drastic model in the gis environment has proved efficient in handling large volumes of data and in determining the groundwater vulnerability. el agua subterránea es vulnerable y más susceptible a la contaminación de varios elementos antropogénicos. se midió la vulnerabilidad del agua subterránea en varias etapas para establecer el desarrollo sustentable de la fuente acuífera. este trabajo estima la vulnerabilidad del agua subterránea por la aplicación del método drastic en el entorno del sistema de información geográfica (gis, en inglés). el método drastic utiliza siete parámetros hidrológicos: profundidad del agua subterránea, recarga neta, litología del acuífero, tipo de suelo, topografía naturaleza de la zona no saturada y conductividad hidráulica del acuífero. el índice drastic fue calculado a través de este método y que oscila entre 31 y 154 unidades. estos parámetros caracterizan la configuración hidrológica para la evaluación de vulnerabilidad del acuífero. también se realizaron los análisis de susceptibilidad para determinar la respuesta de cada parámetro drastic frente a la vulnerabilidad del agua subterránea. el análisis de susceptibilidad indicó que todos los parámetros tienen una influencia similar en el índice de vulnerabilidad. el parámetro de profundidad ocasiona un mayor impacto en el índice de vulnerabilidad, seguido por la recarga, la topografía y el tipo de suelo. toda la cuenca de kodaganar se clasifica en zonas de vulnerabilidad muy baja, baja, moderada y alta. la incorporación del método drastic en el entorno gis prueba la eficiencia en el manejo de grandes volúmenes de información y en la evaluación de vulnerabilidad de aguas subterráneas. abstract resumen keywords: aquifer vulnerability, drastic, gis, sensitivity analysis, kodaganar basin. palabras clave: vulnerabilidad de acuíferos, drastic, sistema de información geográfica, análisis de sensibilidad, cuenca kodaganar. record manuscript received:12/08/2015 accepted for publication: 04/12/2016 how to cite item colins, j., sashikkumar, m. c., anas, p. a., & kirubakaran, m. (2016). gis-based assessment of aquifer vulnerability using drastic model: a case study on kodaganar basin. earth sciences research journal 20(1), h1-h8. doi: http://dx.doi.org/10.15446/esrj.v20n1.52469 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.52469 evaluación basada en el sistema de información geográfica a la vulnerabilidad de un acuífero a partir del método drastic: caso de estudio en la cuenca kodaganar earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): h1 h8 h y d r o g e o l o g y gis-based assessment of aquifer vulnerability using drastic model: a case study on kodaganar basin johnny colins1, m.c. sashikkumar1, p.a. anas2, m. kirubakaran2 1. department of civil engineering, regional office of anna university, tirunelveli, tamil nadu, india – 627007 2. department of civil engineering, regional centre of anna university, tirunelveli, tamil nadu, india – 627007. corresponding author: colinsjohnny@gmail.com h2 johnny colins, m.c. sashikkumar, p.a. anas and m. kirubakaran introduction: the socioeconomic development of any region relies on the fresh and quality of water available in the area. groundwater is a better resource for water requirement due to its economic and hygienic reasons. it is an important natural resource that mainly depends on the subsurface and various hydrological characteristics. the demand for the fresh water has been increased in the semiarid regions of tamil nadu in the recent years due to the poor availability of surface water and increase in the population. according to voudouris et al. (2010) the most effective method of combating the groundwater pollution is its initial prevention. prevention of groundwater contamination is essential for effective groundwater resource management. people living in dindigul district mainly depends on groundwater for various purposes due to the limited availability of surface water. for an effective groundwater resource management and a sustainable development, determination of aquifer vulnerability is essential. it is assumed that the physical environment might provide certain degree of protection to groundwater against the contaminants entering the subsurface environment. contamination of groundwater by anthropogenic activities is a slow but harmful phenomenon which may result in adverse effects. scientists and resource managers have sought to develop techniques for predicting areas that are more likely than others to become contaminated as a result of activities at or near the land surface (nrc 1993). intrinsic vulnerability is considered as a property of groundwater system that also depends on the impact of anthropogenic activities. integrated vulnerability is a modified method that combines risk of loading of pollutants with the intrinsic vulnerability. the intrinsic method applied in this case study is drastic which is a parametric system method developed by aller et al. (1987) for the u.s. environmental protection agency (epa). it is most widely used in united states for the groundwater vulnerability assessment. drastic is an index model designed to produce vulnerability scores for different locations by combining several thematic layers (babiker et al. 2005). the drastic method requires data that are easily available and it suits for regional scale assessment. a high number of input data layers are included which in turn reduces the impacts of individual parameters on the vulnerability index. drastic deals only with hydro-geological setting and does not describe the pollutant influence. this method uses seven environmental parameters like depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity. all these parameters characterize the hydrogeological setting and in evaluating aquifer intrinsic vulnerability. vulnerability assessment delineates areas that are more likely than others to get contaminated from natural or anthropogenic causes. overlay and index method combine factors controlling the movement of pollutants from ground surface into saturated zone which results in groundwater contamination. gis tools are used to create the vulnerability index map which depicts the pollution potential for the aquifer by overlaying various map layers. the vulnerability results obtained through drastic model are considered unreliable due to the adopted and selected feature weights and ratings used by the model (aminreza neshat et al. 2014). therefore, the drastic model is a potential victim for criticism. hence, we attempt to evaluate whether it is really necessary to use all the seven drastic parameters for determining aquifer vulnerability. the rated drastic parameters were first evaluated for interdependence and variability. the concept of sensitivity analysis helps in determining the sensitivity of individual drastic parameters to aquifer vulnerability. the influence of rating values and weights assigned to each parameter are provided by sensitivity analysis. when two sensitivity analyses tests namely the map removal and single-parameter sensitivity analysis were performed that causes variation in vulnerability index, it becomes more effective in the assessment of aquifer vulnerability. the main objective of this study is to demonstrate drastic incorporated in gis environment is an efficient method for assessing groundwater vulnerability and water resource management. study area: figure 1. study area the study area chosen for evaluating the intrinsic vulnerability is kodaganar basin which is located in dindigul district of south india is shown in figure 1. it covers an area of 1540 km2. the basin lies in 10°18’n to 10°57’n latitude and 77º37’e to 78°12’e longitude. it is located between kodaikanal hills, sirumalai hills, karanthamalai hills and thoppasamimalai hills. the average elevation in the basin is 418 m with lowest and highest elevations as 207 m and 1846 m respectively. the kodaganar basin is drained by river kodaganar which flows northward and join river amaravati. the tributaries of kodaganar river originate from the hills which encloses the basin from three sides. it is almost surrounded by structural hills and entire runoff drains into river amaravati. the larger part of the basin is occupied by the metamorphic, crystalline rocks which are highly folded, fractured and jointed. the major geographic units found in the basin are shallow pediments and pediments. most of the rainfall occurs between september and december. the average annual rainfall recorded in the dindigul district is about 828 mm. the basin has plain and hilly area and encounters modera te variations in climatic conditions. in the summer, temperature reaches a maximum of 37°c and minimum of 29°c. during winter the temperature ranges from 20°c to 26°c. data and method: in this paper, the geographic information system (gis) based drastic model was used to evaluate the groundwater vulnerability of the kodaganar basin. the drastic model was developed by aller et al. (1987) for the united states environmental protection agency (epa), with the purpose of creating a methodology that would permit a systematic evaluation of groundwater pollution potential of any hydrological setting. the various hydrological parameters used in the drastic model include depth-to-water, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity. the model yields a numerical index which is derived from the ratings and weights assigned to the seven model parameters. each parameter is further subdivided into ranges or significant media types that are rated between 1 and 10 according to their relative impact on the pollution potential. the seven parameters are assigned weights ranging from 1 to 5 based on its relative importance. the drastic index is then computed by applying a linear combination of all factors according to the following equation. drastic index = drdw + rrrw + araw + srsw + trtw + iriw+ crcw (1) h3gis-based assessment of aquifer vulnerability using drastic model: a case study on kodaganar basin )where d, r, a, s, t, i, c are the seven parameters and the subscripts r and w refers to rating and weights respectively. the drastic index is determined by multiplying each parameter rating by its weight and adding together the resulting values. then all parameters are weighted to express their relative importance with respect to each other and two types of sensitivity tests were also carried out. spatial data analysis and computerized mapping are done in the gis environment preparation of parameters maps: the seven model parameters are prepared as thematic maps which are obtained from several types of data and sources. the depth to water (d) represents the depth from the ground surface to the water. more deep in the level of water table inflicts less chance for contamination to occur. depth to water parameters has been assigned with a relative weight of 5. depth to water affects the time for a contaminant to undergo chemical and biological reaction. with a decrease in depth of water, the potential of groundwater contaminant increases. this parameter was derived from water level data of 19 control wells in and around the study area from the public works department (pwd). the well location vector layer was prepared based on gps survey and spatial distribution map of water table was obtained by interpolation through idw technique. net recharge (r) is the amount of water which penetrates the ground surface and reaches water table. it is the total quantity of water (in mm) which infiltrates into aquifer on an annual basis. the recharge value ranges from a minimum of 5 cm/year to a maximum of 11 cm/year in the study area. it acts as a significant medium for transporting the contaminants. it transports the contaminant to the water table vertically. the total recharge for the study area was estimated by adopting piscopo method (kaliraj et al. 2015) which is considered a better field based method. the relative weight is assigned as 4 for net recharge. aquifer media (a) refers to the saturated zone material properties that control the pollutant attenuation process. it has the potential to store water. the attenuation characteristic of the aquifer material is reflected by the mobility of the contaminants through aquifer media. it includes the pore spaces and fractures of the media where the water is held. a longer travel time will result in more attenuation of the pollutant. its relative weight is assigned as 3. soil media (s) refers to the uppermost weathered portion of the unsaturated zone and controls the amount of recharge that can infiltrate downward. it transports the contaminant and water from soil surface to the aquifer. soils with clay and silts increase travel time of pollutant (sinan et al. 2009). digital soil map was prepared from soil maps obtained from the tamil nadu public works department. based on the permeability the rating values of soils are assigned. its relative weight is assigned as 2. topography (t) refers to the slope of terrain which is an input in drastic method. the flow rate at the surface that influences biodegradation and attenuation is expressed by this parameter. the slope parameter dictates whether the water will remain on surface to allow contaminant percolation to saturated zone. water stagnates in areas of low slope, increases the infiltration and a greater possibility for contaminant migration. slope map was prepared from the srtm dem (shuttle radar topographic mission-digital elevation model) data. it is expressed in percentage and its relative weight is 1. impact of vadose zone (i) is defined as the unsaturated zone material lying below the typical soil horizon and above the water table. this is unsaturated or discontinuously saturated. the movement of the contaminants to the saturated zone is controlled by this parameter. many processes that influence the pollution potential take place in vadose zones and control the passage and attenuation of the contaminated material to the aquifer (yin et al. 2012). the parameter map was prepared from well lithology data obtained from pwd. the relative weight of impact of vadose zone is 5. hydraulic conductivity (c) indicates the ability of aquifer to transmit water which determines the rate of flow of contaminant material within the groundwater system. the hydraulic conductivity data was obtained from the pwd. the parameters relative weight in drastic method is 3. the hydraulic conductivity map was spatially registered and conductivity zones in the area were resampled into different zones. all the weights were assigned based on literatures. sensitivity analysis: the sensitivity analysis of the drastic parameters has been carried out to evaluate the sensitivity of individual parameter in assessing the intrinsic vulnerability of groundwater. the weights of the drastic parameters are used to compute the vulnerability index values. hence it is attempted to evaluate necessity to use all the seven drastic parameters in determining the intrinsic vulnerability. two sensitivity tests performed are the map removal and single parameter sensitivity analysis. the map removal sensitivity analysis represents the sensitivity associated with removing one or more layer at the time of model execution. in this analysis, the map s = |v/n – v’/n |*100 (2) removal sensitivity analysis is performed by the following equation. where s -sensitivity measurement of particular parameter v -total vulnerability index v’-vulnerability index of the individual parameter n -total no. of parameters involved in computation of v n -no. of parameters involved in calculating v’ here v and v’ are the unperturbed and perturbed vulnerability indices respectively. the actual vulnerability index obtained from using all the seven parameters was considered as an unperturbed vulnerability. the perturbed vulnerability is the one which is computed by lower number of data layers. the measurement represents the relationship between unperturbed and perturbed vulnerability conditions. single parameters sensitivity measure was developed to determine and assess the impact of seven parameters of drastic method on the vulnerability index. it evaluates the degree of influence of an individual parameter for the vulnerability of groundwater. the effective weight of w = prpw * 100 (3) each feature class is found by the below formula where w – effective weight of each parameter pr, pw – rating and weight of each parameter v overall vulnerability index results and discussion: the total range of vulnerability index values ranges from 23 to 230. according to drastic model, the vulnerability index map is classified into four classes such as very low, low, moderate and high vulnerable zones. the range and ratings for the study area is shown in table-1. h4 johnny colins, m.c. sashikkumar, p.a. anas and m. kirubakaran table 1. range and rating of the drastic parameters for kodaganar basin drastic parameters: the depth to water is an important parameter in assessing the groundwater contamination and evaluating the intrinsic vulnerability. it is a significant parameter that controls the ability of contaminants to reach the groundwater .the spatial variation of depth to groundwater table in the region was interpolated from 19 wells. the depth to groundwater level in the study area varies from 2.33 to 19.50 m. the assigned rating varies from 1 to 9 and the index value ranges from 5 to 45. around 44 % of the basin has depth in the range of 6.6 m – 10.9 m. 32 % of the study area falls under 10.9 m – 15.2 m of depth. less than 1 % has depth between 15.2 m and 19.5 m. the groundwater contaminant potential decreases with increase in the depth. hence low ratings are assigned to areas with high water table depth. depth to water parameter map is shown in figure-2. figure 2. depth to water map the net recharge or effective infiltration is the amount of entering the ground per unit area of land which percolates and reaches water table. the study area is characterized by a moderate rainfall. infiltration parameter is mainly controlled by land use on the surface. the vulnerability index rises with increase in rank of net recharge. 41 % of the study area has net recharge value in the range of 7 to 9 cm/year. the highest rating 10 is assigned only to 4 % of study area that has a recharge value more than 11 cm/year. recharge parameter map is shown in figure-3. figure 3. net recharge map the aquifer media map is shown in figure-4 which was prepared from subsurface geology map and lineament map. it consists of consolidated and unconsolidated rocks. the ratings are assigned from 1 to 7. the various types of aquifers found in the study area are charnockite, h5gis-based assessment of aquifer vulnerability using drastic model: a case study on kodaganar basin anorthosite, fissile horneblonde biotite gneiss (fhbg) and migmatite gneiss. the lineaments are found in migmatite gneiss and fhbg regions. this factor is highly controlled by degree of fracture and the weathered nature of rocks. majority of kodaganar basin (around 65%) has gneissic rock. anorthosite, migmatite gneiss with lineament and fhbg with lineament are found in negligible amount in the study area. charnockite covers 27.5% of the study area and it has a low rank of 1. figure 4. aquifer media map the textural properties of the soil media control the groundwater vulnerability. it has a significant impact on the amount of recharge and act as a contaminant aquifer. the soil media is usually variable and it plays an important role in the evaluation of intrinsic vulnerability. the ranks 1, 2, 3, 5 and 6 are assigned to forest unsurvey, vertisol, alfisol, inceptisol and entisol respectively. fine soil and organic matter decrease permeability and retard the migration of contaminant. various types of soils like entisol, inceptisol, alfisol, vertisol and forest unsurvey covers 13.7%, 33%, 36.2%, 4.9% and 12.2% respectively. high ratings are assigned for entisol and inceptisol. forest unsurvey is assigned 1 because it is restricted to development. five types of soil media present in the study area are shown in figure-5. figure 5. soil media map topography map was generated from srtm dem data. areas with low slope tend to retain water for a longer period of time and support infiltration. hence high rating values are assigned to such areas where infiltration increases the migration of contaminants. the topographic thematic layer displayed a gentle slope (0 to 6 %) over most of the study area, hence scored ranks of 9 and 10. slope range of 0 to 2 % occupies less than 1 % of study area and its rating is 10. a rank of 9 is assigned for 24.6% study area which has 2 to 6% slope. places with steep slopes (>18%) are assigned with rank 1 which indicates its minimal effect on the aquifer vulnerability. the decrease in slope angle generally results in the increase in aquifer vulnerability. topography map is shown in figure-6. figure 6. topography map the unsaturated or vadose zone plays a prominent role in the assessment of groundwater vulnerability. this zone protects the aquifer from pollutants for an extent. the ratings 1, 3, 4 and 5 are assigned to various lithology like charnockite, anorthosite, migmatite gneiss and fhbg respectively. the fhbg has been assigned with a rank of 5 as it influences more on intrinsic vulnerability. figure-7 depicts the impact of vadose zone map. the hydraulic conductivity test is obtained from the pwd and cross verified with pumping tests conducted in the basin. the hydraulic conductivity ranges from 0.04 to 29 m/day. this is a critical factor to control the contaminant migration and dispersion from injection point of contaminant source to the saturated zone (rahman 2008). the potential for groundwater pollution is more for areas with high hydraulic conductivity zones. more than half of the study area has hydraulic conductivity in the range of 4.1 to 12.3 m/day. figure 7. vadose zone map around 29.5 % of study area has 0.04 to 4.41 m/day with a low rating of 1. the values in the range of 12.3 to 28.7 m/day isfound in 15 % of the kodaganar basin. the ranks 1, 2 and 4 are assigned to the different zones of hydraulic conductivity. hydraulic conductivity map is shown in figure-8 which is generated by idw technique in gis environment. h6 johnny colins, m.c. sashikkumar, p.a. anas and m. kirubakaran figure 8. hydraulic conductivity map drastic vulnerability index: the drastic aquifer vulnerability map is shown in figure 9. in the study area the vulnerability falls under very low, low, moderate and high vulnerable zones. drastic index is highly sensitive to the parameter scores, weightings and the numerical values assigned. the drastic vulnerability for the study area is shown in table-2. the drastic index in the range of 31-56 and 56-91 comes under very low and low vulnerable zones. very low and low vulnerable zones cover 29.5% and 45% of the kodaganar basin respectively. this is mainly due to deep water table depth and relatively low permeability of vadose zone in such specific sites. around 25 % of the study area has a moderate groundwater pollution potential. this does not mean that such zones are free from contamination but it is relatively less susceptible to contamination when compared with other areas. very high vulnerability is found in less than 1% of the study area. figure 9. drastic vulnerability map table 2. drastic vulnerability index for study area sensitivity of the drastic model: table-3 provides the statistical summary of the drastic parameters. the net recharge parameter induces the highest risk of contamination with a high mean value of 5.6. the parameters depth to water level, topography, aquifer media, soil media and vadose zone are influenced with moderate risk of contamination with mean values 5, 5, 4, 4 and 4 respectively. the hydraulic conductivity imposes a low risk of contamination with a mean value of 2. the topography parameter is highly variable as its coefficient of variation (cv) value is 66.6%. the depth to water, aquifer media, soil media, vadose zone and hydraulic conductivity are moderately variable with cv values 52%, 50%, 50%, 50% and 50% respectively. the net recharge parameter is least variable parameter (cv= 35.7%). the sensitivity is measured based on the rating and weights assigned to the feature classes of each parameter. the process of sensitivity analysis is mainly carried out to evaluate the sensitivity of seven parameters in determining the aquifer vulnerability. table 3. a statistical summary of drastic parameters map removal sensitivity analysis: the map removal sensitivity analysis is performed by removing one or more thematic maps at a time as shown in table 4 & 5. it is stated that a high variation in vulnerability index is obtained upon the removal of depth to water parameter whose mean variation index is 13.55%. this could be due to the high theoretical weight assigned to this parameter. the vulnerability index seems to be moderate sensitive upon the removal of recharge and topography. the removal of map was based on the map removal sensitivity measure which is shown in table-4. the variation of aquifer vulnerability due to removal of one or more layers at a time is presented in table-5. the least mean variation index was calculated after removing hydraulic conductivity layer (9.54%). the adequate information on the most effective parameter involved is provided by computation of variation of vulnerability index. table 4. statistics of map removal sensitivity analysis h7gis-based assessment of aquifer vulnerability using drastic model: a case study on kodaganar basin table 5. statistics of map removal sensitivity analysis single parameter sensitivity analysis: for the assessment of impact of individual parameters towards aquifer vulnerability, the single parameter sensitivity analysis is performed. the statistical summary of single parameter sensitivity analysis is shown in table-6. the single parameter sensitivity analysis compares the effective weights and theoretical weights. the mean value of the effective weight of net recharge parameter is 32.34% and its theoretical weight is 17.4%. this indicates that this parameter is more effective in vulnerability assessment. the depth-to-water table and impact of vadose zone have high theoretical weights (21.7%). they have been assigned with an effective weight with mean value such as 22.67% and 16.18%. the remaining parameters like aquifer media, soil media, topography and hydraulic conductivity exhibits effective weights such as 11.65%, 7.12%, 9.71% and 8.73% respectively. the theoretical weights assigned to the parameters such as net recharge, topography, impact of vadoze zone and hydraulic conductivity are not in agreement with the effective weight. the net recharge has a greater influence in the vulnerability study followed by depth to water table, impact of vadoze zone, aquifer media, and topography and soil parameter. the mean value arrived for the input layer of the model is good representation of the weight for the respective parameter. table 6. statistics of single parameter sensitivity analysis conclusions: the evaluation of vulnerability index for the kodaganar basin was done using the geographic information system (gis) based drastic model. the seven hydrogeological parameters are used in determining the vulnerability. from the index values assigned, it was found that 74.85% of the study area is under very low and low vulnerability of groundwater contamination. these zones have a minimum susceptibility to intrinsic vulnerability. it is noticed that the parts of kodaikanal hills, sirumalai hills, karanthamalai hills and thoppasamimalai hills comes under very low vulnerability. this is due to steep slopes which causes more runoff in such hilly areas. nearly 24.85% of the study area has moderate vulnerability and it is found in northern part of the basin. the villages in the blocks such as vadamadurai, vedasandur, dindigul and oddanchatram are found to be moderately vulnerable region. less than 1% of the study area comes under high vulnerable zone. the industrial activities were witnessed in the dindigul, vedasandur and vadamadurai region. the agricultural activities are found to be a major threat in the region of moderately vulnerable zone. the drastic index value ranged from 217-280 for the high vulnerability zones. the study signifies that net recharge parameter induces high risk to contamination whereas hydraulic conductivity induces a low risk of contamination with a mean value of 2. sensitivity analyses helped to validate and evaluate the consistency of analytical results. with an efficient database in gis environment, drastic is an effective tool for analyzing the groundwater vulnerability. the study suggests that this model can be used as a better tool for the local authorities and water resource managers who are responsible for groundwater management. references: akhavan, s., mousavi, s.f., abbaspour, k.c. (2011). conditioning drastic model to stimulate nitrate pollution case study: hamadan-bahar plain, environ earth sci. 63, 1155-1167. al hallaq, a.h., elaish, b.s.a. (2012). assessment of aquifer vulnerability to contamination in khanyounis governorate, gaza strippalestine, using the drastic model within gis environment, arab j geosci. 5, 833-847. alam, f., umar, r., ahmed, s., dar, f.a. (2012). a new model (drasticlu) for evaluating groundwater vulnerability in parts of central ganga plain, india, arab j geosci. aller, l., bennet, t., leher, j.h., petty, r.j., hackett, g. (1987). drastic: a standardized system for evaluating groundwater pollution potential using hydro-geological settings. epa 600/2-87-035:622. babiker, i.s., mohamed, a.a., hiyama, t., kato, k. (2005). a gisbased drastic model for assessing aquifer vulnerability in kakamigahara heights, gifu prefecture, central japan, science of the total environment. 345, 127-140. chitsazan, m., akhtari, y. (2009). a gis-based drastic model for assessing aquifer vulnerability in kherran plain, khuzestan, iran, water resour manage. 23, 1137-1155. kaliraj, s., chandrasekar, n., peter, t.s., selvakumar, s., magesh, n.s. (2015). mapping of coastal aquifer vulnerable zone in the south west coast of kanyakumari, south india, using gis-based drastic model, environ monit assess. 187:4073, 1-27. national research council (nrc). (1993). groundwater vulnerability assessment: predictive relative contamination potential under conditions of uncertainty. national academy, washington. neshat, a., pradhan, b., dadras, m. (2014). groundwater vulnerability assessment using an improved drastic method in gis. resource, conservation and recycling. 86, 74-86. pathak, d.r., hiratsuka, a., awata, i., chen, l. (2009). groundwater vulnerability assessment in shallow aquifer of kathmandu valley using gis-based drastic model, environ geol. 57, 1569-1578. prasad, r.k., singh, v.s., krishnamacharyulu, s.k.g., banerjee, p. (2011). application of drastic model and gis: for assessing vulnerability in hard rock granitic aquifer, environ monit assess. 176, 143-155. rahman, a. (2008). a gis-based drastic model for assessing groundwater vulnerability in shallow aquifer in aligarh, india, applied geography. 28, 32-53. saidi, s., bouri, s., ben dhia, h. (2010). groundwater vulnerability and risk mapping of the hajeb-jelma aquifer (central tunisia) using a gis-based drastic model, environ earth sci. 59, 1579-1588. h8 johnny colins, m.c. sashikkumar, p.a. anas and m. kirubakaran santha sophia, m., syed, t.h. (2013). assessment of vulnerability to seawater intrusion and potential remediation measures for coastal aquifers: a case study from eastern india, environ earth sci. 70, 1197-1209. sinan, m., razack, m. (2009). an extension to the drastic model to assess groundwater vulnerability to pollution: application to the haouz aquifer of marrakech (morocco), environ geol. 57, 349-363. srinivasamoorthy, k., vijayaraghavan, k., vasanthavigar, m., rajivgandhi, r., sarma, v.s. (2011). integrated techniques to identify groundwater vulnerability to pollution in a highly industrialized terrain, tamilnadu, india, environ monit assess. 182, 47-60. voudouris, k., kazakis, n., polemio, m., kareklas, k. (2010). assessment of intrinsic vulnerability using the drastic model and gis in the kiti aquifer, cyprus, european water. 30, 13-24. wen, x., wu, j., si, j. (2009). a gis-based drastic model for assessing shallow groundwater vulnerability in the zhangye basin, northwestern china, environ geol. 57, 1435-1442. yin, l., zhang, e., wang, x., wenninger, j., dong, j., guo, l., huang, j. (2012). a gis-based drastic model for assessing groundwater vulnerability in the ordos plateau, china, environ earth sci. geofisica colombrana pp. 73·81 bogota 0 cn° 5 diciembre de 200( issn 0121 2974 diseiio e adqulstctcn implementacion de un de datos stsrnotoqtcos sistema de en tiempo real pedro a. vega v. pav@terra.com pav@tutopia com universidad nacional de colombia, bogota resumen ei objetivo propuesto fue el diserio, caracterizaci6n y puesta en operaci6n de un sistema de adquisici6n de datos en tiempo real. para lograrlo se parti6 de un sism6metro existente tipo l-4c con base en el cual se implementaron el software y hardware requeridos para obtener la estaci6n de registro sismol6gico. ei hardware seleccionado fue una tarjeta de adquisici6n de datos fabricada por national instruments, referencia daq-516, la cual entre sus principales caracterfsticas cuenta con ocho canales de entrada anal6gica, y una rata de muestreo de 50.000 muestras por segundo. ei software utilizado fue el labview 6.0, tam bien de national instruments, un software de programaci6n grafica que controla todas las posibilidades de la tarjeta de adquisici6n de datos y cuenta adicionalmente con poderosas herramientas de tratamiento de datos. los otros componentes del sistema son un computador portatil y una tarjeta de acondicionamiento de senales. una vez implementado y probado el sistema de captura de datos, se procedi6 a realizar la caracterizaci6n del sistema. aquf se determin6 el ruido instrumental y se caracteriz6 el sensor, para 10 cual se disefio y construy6 una mesa vibratoria. mediante una conexi6n a la red internet y un software del tipo atomtime 98, se realiza la sincronizaci6n del reloj del sistema, abstract palabaas clave: adquisicion en tiempo real, intrumentacion sismica, software de oiseno en instrumentacion the proposed objective was the design, characterization and setting in operation of a real time data acquisition system. starting with lac type existent seismometer for which was implemented the software and hardware required to obtain the seismological registration station. the selected hardware was a national instruments data acquisition card daq-516, this board has 8 analog input channels (16 bit) and 50 ks/ s. the used software labview 6.0 was also national instruments which is a graphic programming software that controls all card's data acquisition possibilities and additionally has powerful data handling tools. the other system components are a portable computer and a signal conditioning card. once implemented and proven the data acquisition system, we proceeded to carry out the characterization of the system. here the instrumental noise was determined, and the sensor was "characterized for this a shaking table was designed and built. by means of an internet connection, the system's internal clock synchronization is carried out by means of atomtime 98 software. key words: real time acquisition, seismical instrumentation, software for design of instruments introduccion las ciencias naturales en general, yen especialla geoffsica, estan fundamentadas en las medici ones realizadas de los parametres que caracterizan los fen6menos que nos rodean, para, a partir de estos datos, inferir la realidad que se oculta a nuestros sentidos, es por esto que para obtener conclusiones acertadas se debe prestar la mayor atenci6n al dato mismo, ya que partiendo de datos err6neos se llega a conclusiones alejadas de la realidad. por tanto, en el disefio de un sistema de medici6n es vital conocer principalmente la variable por medir, sus efectos, 6rdenes de magnitud y factores que la afectan. con esta informacion se pueden disefiar los sensores adecuados, filtrar los efectos que generen distorsion y obtener la seiial rna pura posible. aqui se trata principal mente del disefio, caracterizaci6n e implementaci6n de tti1 sistema de medici6n de movimientos de la tierra, paniendo de un sisrnometro tipo l-4c, y el diseno, construccion e implementaci6n de un prototipo de mesa vibradora para la calibraci6n de sensores sfsmicos. geoff sica colombiana, 3, junio de 2002 conceptos baslcos sistemas de adquisicion de datos en las ultimas decadas, la tecnologia ha desarrollado sistemas complejos de adquisici6n de datos, compuestos por subsistemas que interacnian coordinadamente, implementados tanto en software como en hardware (national instruments corporation, 2000). un sistema de adquisici6n de datos esta conforrnado basicamente por tres subsistemas: subsistema de adquisici6n de datos subsistema de c6mputo subsistema de interfaz figura i. composici6n basica de un sistema de adquisicion de datos 73 pedro a vega v. el sistema de adquisici6n de datos incluye los sensores, equipos y circuitos para adecuaci6n de sefiales y transmisi6n de datos hacia el sistema de compute. el sistema de c6mputo realiza el procesamiento de los datos recolectados, ya que aquf se ejecutan rutinas de adquisici6n, proceso, presentaci6n y registro de los datos. el sismometro un sism6metro es un instrumento que permite medir vibraciones de la tierra, causadas por una gran variedad de procesos, principal mente los terremotos. en general consta de: un sistema inercial, como por ejemplo una masa m6vil, acoplada a una cubierta rigida mediante un resorte y un elemento amortiguador. un sistema transductor, que produce una sefial de salida, correspondiente ai movimiento relativo de la cubierta con respecto a la masa, y que sirve como un punto de referencia debido a su gran inercia. w rue figura 2. diagrama esquemarico de un sism6metro (keller, fritz, 1997) para "rnuy altas frecuencias" del movirniento del suelo (comparado con la frecuencia natural del sistema), la masa suspend ida tiende a mantener su posici6n original, y su desplazamiento relativo a la cubierta representa el desplazamiento del suelo. para "muy bajas frecuencias" del rnovimienro del suelo, la masa tiende a seguir el movimiento de la cubierta y el desplazarniento relativo tiende a ser cero. una descripci6n cuantitativa de esta caracteristica pasa altos, que relaciona el desplazamiento de la masa con el desplazamiento del suelo ~ especialrnente entre los extremos "muy baja" y "muy alta" frecuenciapuede deducirse a partir del equilibrio de todas las fuerzas intemas yextemas que actuan sobre la masa, y que conduce a la ecuaci6n diferencial de segundo orden (keller, 1997): m ·lx" (t )+ w" (t )jdx (t )c . x(t) = 0 que relaciona: xtr), x '(t) y x"(t) desplazarniento relativo de la masa, velocidad y aceleraci6n y w"(t) aceleraci6n del suelo (= cubierta) 74 con los parametres del sistema rnecanico: m [kg] masa suspendida, resultante de una fuerza inercial, proporcional a la aceleraci6n total x"(t) + w"(t) d [ns/m] con stante de amortiguarniento, que produce una fuerza proporcional a la velocidad x'(t) y que refleja las perdidas internas. c [n/m] constante del resorte, resultante en una fuerza proporcional al desplazamiento de la masa de su punto de equilibrio (= posici6n cero), y proporcional al desplazamiento relativo de la masa x(t). instrumento virtual !.que es un instrumento virtual? figura 3. instrumento virtual es la combinaci6n de hardware y software, usados poi'un computador personal (pc), para cumplir la funci6n de un instrumento tradicional. en general es una aplicaci6n particular de un sistema de adquisici6n de datos. el software es la clave del instrumento virtual, ya que este es el que sustituye aj instrumento tradicional. ei software desempefia un papel vital en el desarrollo de sistemas de adquisici6n de datos y control, adem as de dirigir la interacci6n de las especificaciones de hardware. el software se elige de acuerdo con las necesidades y preferencias del usuario. muchos factores afectan la elecci6n del software, incluidos aplicaciones, requerirnientos, el hardware del computador, el sistema operativo y el hardware de instrumentaci6n. ei software que el usuario escoge debe ser versatil, para adaptarse a diversas arquitecturas de computadores, a instrumentos y dispositivos diferentes de adquisici6n de datos. la tarjeta de interfaz con el pc puede ser de dos tipos: tarjetas insertadas dentro del computador (plug in) 0 instrumentos con puertos de comunicaci6n que operan independientemente (stand alone). tiempo real ei terrnino "tiernpo real" es uno de los mas ampliamente usados en la industria actual mente; sin embargo, existen al respecto algunas concepciones err6neas. una definicion ampliamente aceptada de sistema de tiempo real es la siguiente: "aquellos sistemas en los cuales las correcciones del sistema dependen no s610 de los resultados 16gicos de los calculos sino tarnbien geofisica colombiana, 5, diciembre de 2001 diseno e implementacion de un sistema de adquisicion de datos sismolocicos en tiempo real del tiempo en el cual se producen los resultados. la distinci6n entre tiernpo real y no tiempo real es que los sistemas de tiempo real deben cumplir con restricciones de tiempo". los sistemas de tiempo real se c1asifican frecuentemente en duros (hard) y blandos (soft). los sistemas duros se precisan libres de fallas para satisfacer todos los requerimientos de respuestas de tiempo en forma constante, en cualquier circunstancia y ante cualquier combinaci6n de eventos, como ocurre en los sistemas de control de reactores nucleares en las plantas generadoras de energfa. un sistema blando esta mas relacionado con el desempefio promedio; de este se requiere solamente que en la mayorfa de los casos la respuesta de tiempo caiga dentro de un margen admisible. estos sistemas toleran salidas de funcionamiento ocasionales, como ocurre en el caso de los cajeros automaticos. desarrollo del sistema usando el software labview se realiz6 la siguiente rutina de adquisici6n, con el cual se capturan las sefiales provenientes del sensor y se almacenan en un archivo de texto que graba la fecha, hora y magnitud en voltios para posterior analisis, rj:·~;::·,;;·m <, l"'n·--. .. -•• .. ---1 l ~~~-~---------,~jq-fr: ----.,----f: ~ 0 rlt ~·~'i..,,~,#~__.:~.~~,d,'.. !,.,' i ""mt'm ijl-~ ! 1i> ;,.~. iz1 ll0 •• ,,,." l ~' i. ••••••••••••••••.••..•.••.•••••.•••••.•••••••.. j 1ft;..1 ·..·-1 ~figura 4. c6digo grafico para el sistema de adquisici6n de datos sismicos igualmente se disefi6 en el panel un frontal como el que se muestra en la figura 5. figura 5. panel frontal del sistema de adquisici6n de datos geofisica colombiana, 5, diciembre de 2001 cuando se esta ejecutando este programa, se sensa perrnanentemente la entrada del primer canal de la tarjeta, comparandolo con un valor predefinido en una constante nurnerica; este valor debe corresponder al maximo nivel de ruido del sitio donde se instala el sistema, y, a pesar de ser arbitrario, es el que deterrnina cual es la amplitud de entrada a partir de la cual consideramos que se presenta un evento de interes. una vez la rutina de arranque detect a a la entrada del canal analogo una tensi6n mayor al valor predefinido en la constante numerica, se inicia un loop case, el cual ejecuta las siguientes funciones: leer datos de la tarjeta de entrada anal6gica leer la hora del computador. concatenar las variables string para formar la cadena con la fecha que se registrars en el archivo. concatenar la cadena de fecha y hora con el dato leido desde el primer canal analogo, crear un archivo con los datos que se leen, como el de la figura 6. detener la grabaci6n despues de transcurrido el mimero de lecturas que se ha predefinido. _1.","101 ~1.n:'1 _l'>:l!1 -1.lw71 _1_t;ll)~1 -i.~:-: -i_"'~ --'.l:itw _l!::~~ 1:>ln:j('l "'.",l'ioi}li/cltij«o;. 01).::./= r.>"'.1"'=-1~; tll)llfot. ()ri;}"1:j~ u o,,j.:}q7. c".,j~=js~. oljl:.:/ot (l1::'3:971~ ... "'lj;u/a: (lll"~""~~ .... r:ujlhaz ot:,i<j.1s u ol,)::j= £lm."',.l .... ~ cujj.l!ol f.l,.;~~.-:t~l~. o\.il gi./'::j!a:. .:"",, ,~<] ~ .... !jjj.!:jaz ot.,m:l.~ .u 1;lij:;ig:, q,.l~;l:; 1.11 l)ij22/0l 0li~~51~1!.iii l:t .::zio: oll'::>':l':' m !uj~:}~ i:ji.,i",'!..l.ft ~<j:;,tql, qs;·;l;?;h ...~ <sljuj~l:s:;l<'f','! .u iu.,t:vw lq1in3.1':' '.a l~nj:j}o:1. i:jt.,i~"'!. u al.}nlcjl.qt.:j"i':l:i.u '".11 .......()~ ()l.}~1/o: ;ai,':;',l!. .u {'!-j, _i,"le 1l1.tli/0l ol\":i!:l=t.... c.i-4 _ •• u~ dl1,j::/= £l",:,.=]~ ... ()ljzljmljt.:j:lil:l:i u gij.:::/a;: o",,"',]~". <:,_" illjll/m (l(':l:i!,]~.1.1 (li~ • ..,")=,=o ..:''':''~ ..~~~~~~==;:::~~~~~::jf?jw.,.~n.-:.;; .i::f"?tf:~ '-!!likir'tij ill""' i; .....! t!"""i~_':>""~~1l"'" figura 6. ejemplo de archive de datos creado por la rutina caracterizacion del sistema ruido instrumental todos los sism6grafos modemos usan amplificadores semiconductores, los cuales --{;omootros componentes electr6nicosproducen mido electr6nico continuo cuyo oligen se disuibuye por el equipo. lo u'iulsductoreselecu'omagneticos, tales como los usados en ge6fonos, tambien producen ruido electr6nico termico (ruido resistivo). las conuibuciones del mido de los semiconductores y delruido resistivo es frecuentemente comparable, yjuntos limitan la sensibilidad del sistema. los sism6grafos pueden tambien sufrir penurbaciones transitorias oliginadas en semiconductores ligeran1ente defectuosos 0 en paites mecanicas cuando esuin sujetas a esfuerzcs. los sism6meu'os y ge6fonos elecu'omagneticos son sensores pasivos cuyo auto-ruido es de oligen puramente tel1nico, y no aumenta a bajas fi'ecuencias como oculte con elementos activos (disipadores de potencia). el nivel de sefial a la salida es, sin embai'go, compai'ativarnente baja, pol'10 que se debe insenar un preai11plificadorde bajo ruido entre el sensor y el regisu'ador (wielai1dt,200 l). 75 pedro a. vega v. inforrunadamenre, el ruido del preamplificador aumenta a bajas frecuencias y limita la sensibilidad general. la sensibilidad esta normalmente limitada por el ruido del amplificador; sin embargo, este ruido no depende del amplificador solarnente sino de la impedancia del transductor. hasta cierta irnpedancia el ruido del amplificador es aproximadamente constante, pero luego aumenta linealmente con la impedancia, debido a una corriente de ruido que fluye desde la entrada del amplificador. por otro lado, la seiial de voltaje aumenta conla raiz cuadrada de la impedancia. la mejor relacion de seiial a ruido se obtiene por tanto con una impedancia optima definida por la esquina entre el voltaje y el ruido de corriente, la cual es diferente para cad a tipo de amplificador, y depende tambien de la frecuencia, y viceversa: cuando el transductor es dado (nuestro caso) el amplificador debe ser seleccionado para bajo ruido a la impedancia y a la frecuencia de trabajo (wielandt, 2001). el ruido electronico puede predecirse cuando se conocen los datos tecnicos del sensor y del amplificador, ei ruido semiconductor aumenta a bajas frecuencias, por 10 que se deben aplicar las especificaciones del amplificador a las frecuencias sfsmicas mas que a las frecuencias de audio. el ruido del amplificador puede observarse bloqueando el sensor 0 inclinandolo hasta que la masa este detenida firmemente, 0 sustituyendolo con una resistencia que tenga un valor igual a la resistencia de la bobina, si estas manipulaciones no reducen significativamente el ruido, entonces el sismografo no resolvera el ruido sismico. un sensor bloqueado 0 una resistencia no representan exactarnente la impedancia electrica de un sensor desbloqueado. para caracterizar el ruido del amplificador de nuestro sistema, utilizamos un potenciornetro de 5k lineal, con el cual ajustamos una resistencia igual a la resistencia vista des de el amplificador -y que corresponde a la resultante de la intema de la bobina de seiial del sensor y la de amortiguamiento, es decir 3,9 kn-, se realize un registro de prueba utilizando la rutina del labyiew descrita antes y se obtuvo el resultado mostrado en la figura 7. de la grafica observamos que el ruido instrumental de nuestro sistema es de +/50 my, 10 que involucra la tarjeta de adquisicion de datos, el pc y el software. este ruido se redujo instalando un filtro capacitivo de 50 pf a la entrada del canal. los resultados fueron los mostrados en la figura 8. 0"1 figura 7. ruido instrumental registrado con el sistema de adquisici6n de datos antes de filtro 76 figura 8. selia i de ruido con el condensador de filtro a la entrada del canal adicionalmente se realizaron medici ones de ruido con el sensor conectado, es decir, tomando registros del sistema en ellaboratorio. i "". ~ 1"". figura 9. serial de ruido con el sensor conectado en el caso con el sensor conectado, hay que tener en cuenta que aqui se esta registrando el ruido ambiente, es decir, se esta registrando todo aquello que influye sobre el sensor en el lugar en donde esta instalado. aquf se observan amplitudes de +/0.15 voltios. en los registros tomados luego de instalar el condensador de entrada, se puede observar que las amplitudes de las seiiales de ruido se reducen hasta ± 0.001 voltio. calibracion con ondas seno con una entrada senoidal, la salida de un sistema lineal es tam bien senoidal, y la relacion de las dos amplitudes es el valor absoluto de la funci6n de transferencia. por tanto, un experimento con ondas seno permite un chequeo inmediato de la funci6n de transferencia, sin ningun conocimiento a priori de su forma matematica y sin modelamiento de la forma de onda; este es frecuentemente el primer paso en la identificaci6n de un sistema desconocido. cuando solamente se tiene disponible equipo analogo, la bobina de calibraci6n 0 la mesa agitadora deberan ser alirnentadas con una seiial geofisica colombiana, 5, diciembre de 200"1 diseno e implementacion de un sistema de adquisicion de datos sismolqcicos en tiempo real de prueba senoidal, y se deben registrar las seiiales de entrada y salida en un registrador xy. posteriormente, las seiiales de deberan ser graficadas como una elipse de lissajous, a partir de la cual tanto la amplitud como la fase se pueden leer con buena precision. los resultados obtenidos para nuestro sistema, y a una frecuencia de i hz, fueron los siguientes: figura 10.grafico para medida de ia fase en el sism6metro l4c para una frecuencia de 1 hz a partir de este grafico se puede calcular el desfase mediante la formula: d, ( 2.1 j<p = ±2arctan g(-) = ±2 * arctang ~ = ±22.62s, 10.5 para la realizacion de la verificacion de la respuesta en frecuencia del sensor, se alimento una seiial senoidal suministrada con un generador marca hewlett packard, modelo 8116a por la bobina de cali brae ion del sensor, y se registraron tanto la entrada como la salida, tomando un registro de 30.000 lecturas para cada una de las siguientes frecuencias: 0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9, 1,2, 3,4,5,6,7, 8, 9, 10,20, 30,40, 50, 60, 70, 80, 90, 100, 120, 140, 150 y 200 hz. con los datos asf obtenidos, se realize una curva de respuesta en frecuencia en la cual se grafico la amplitud de salida contra la frecuencia, y a partir de la cual se puede establecer que la frecuencia de resonancia de este sensor es de i hz. una vez determinada mediante este procedimiento la frecuencia natural, se procedio en esta frecuencia a realizar pruebas de entrada! salida a diferentes niveles de voltaje, con 10 cual se pudo establecer la respuesta del sensor en amplitud, la cual tiene una forma funcional como la que se rnuestra en la figura 12. respuesta escalon y prueba de levantamiento de peso las forma mas simple pero solamente moderadamente precisa de calibrae ion es observar la respuesta del sistema a una seiial de entrada escalon. esta puede ser generada interrumpiendo una corriente a traves de la bobina de calibracion, 0 aplicando y removiendo una fuerza mecanica constante sobre la masa sismica. geofisica colombiana, 5, diciembre de 2001 rj'.~ •• ",~"" ~,.'"'''''''' rui\_ i " \, figura 11. respuesra en amplitud del sensor l-4c en el conrexto de calibracion relativa, el metodo de la respuesta escalon es todavia iiril como una prueba rapida e inruitiva, y tiene la ventaja de que puede evaluarse manualmente. la figura 13 muestra la respuesta escalon caracreristica de filtros de segundo orden pas a alto, pasa banda y / figura 12. respuesta en amplitud del sensor l-4c 77 pedro a. vega v. pasa bajo con un amortiguamiento enrico de 1/12 cad a respuesta es una oscilaci6n fuertemente amortiguada alrededor de su vajor asint6tico. con el arnortiguarniento especificado, los sistemas son filtros butterworth, y la amplitud decae hasta e -ff 04.3% dentro de un semiciclo. la relaci6n de dos amplitudes subsecuentes de polaridad opuesta es conocida como raz6n de overshoot, y puede ser evajuada para el amortiguamienro nurnerico h, donde xi y xi+n son dos amplitudes n perfodos ajejada, con n entero 0 semientero, entonces (wielandt, 200 i): ~=1+(-2~ j2 112 inxi -in xi+1i el perfodo libre puede en principio tarnbien ser determinado a partir de la respuesta de impulso 0 escal6n de un sistema amortiguado, pero debe preferiblemente ser medido sin amortiguamiento electrico de tal forma que se puedan observar mas oscilaciones. un sistema con perfodo libre to y amortiguamiento numerico h, oscila con un perfodo to / -,'1= h2 (wiejandt, 2001). ~ y la rejaci6n de overshoot e-ffh / ,1 h2 figura 13. respuestas escal6n normatizadas de filtros de segundo orden pasa altos, pasa banda y pasa bajo en este caso se utilize una onda cuadrada de 0.1 hz, con la cual se prob6 la respuesta a un impulso. la figura 14 muestra la res~uesta a esta onda, a partir de la cuaj, y utilizando la f6rmula antes descrita, permiti6 cajcular el arnortiguarniento h, y posteriormente el perfodo libre. nr"" ... a .... u... ~"" c .... r .... d~ d.l111 ~"+------------------lt-----j i{ \i ii figura 14. respuesta a una onda cuadrada de 0.1 hz 78 a partir de lafigura se midi61a arnplirud de dos maxim os consecutivos maxi = -4.24 max2 = +0.27 max3 = -0.08 max4 = +0.02 aplicando ja formula se obtiene h = oao midiendo ej perfodo de oscilaci6n en la figura se obtiene t = 1.021 s pol' tanto to = 0.99 segundos cajculando la relaci6n de overshoot mediante la f6rmula se obtiene: -ot-l: / 1.1 f 2e "v 1 relaci6n de overshoot = 0.34 haciendo uso de la formula del manual meq800-b se puede calcular entonces el amortiguamiento del sensor: coeficiente de arnortiguamiento total: b, = bo +bc donde bo = 0.28 para shunt abierto, es decir sin resistencia de amortiguamiento y r, = resistencia de la bobina r, = resistencia del shunt aplicando estas f6rmulas se tiene un arnortiguarniento de 0.61 (utilizando los valores obtenidos para r, y rs)' calibraci6n sobre mesa vibraoora usar la mesa vibradora es la forma mas directa de obtener una calibraci6n. en la practica, sin embargo, la precisi6n es usualmente pobre por fuera de la banda de frecuencias comprendidas entre 0.5 y 5 hz. a frecuencias mayores, una mesa vibradora cargada con un sism6metro puede desarrollar resonancias parasitas, y las fuerzas inerciales pueden causar movimientos no deseados de la mesa. a bajas frecuencias, los maximos desplazamientos ocasionan que la relaci6n serial-ruido sea insuficiente, y el movimiento puede ser no uniforme debido a fricci6n 0 rugosidad en los rodamientos. aiin peor, la mayoria de las mesas vibradoras no producen un movirniento puramente translacional sino tambien alguna inclinacion. esto tiene dos efectos indeseables: la aceleracion angular puede ser sensada por el sismornerro, y la gravedad puede estar acoplada dentro de la seiial sismica, la inclinaci6n puede ser catastrofica para las componentes horizontajes a perfodos largos, ya que el error aumenta con el cuadrado del perfodo de la sefial. se puede pensar que una inclinaci6n de to rnrad por rnm de movimiento lineal no tiene importancia; sin embargo, una inclinaci6n asf inducira a perfodos de 30 segundos, seiiajes sfsmicas el doble de largas que las originadas por el rnovimiento lineal. a perfodos de is, el efecto de la misrna inclinaci6n podrfa despreciarse. mediciones de perfodos largos en una mesa vibratoria, si fueran posibles, requeririan cuidados extremos (wielandt, 200 i). para realizar la caracterizaci6n absoluta del sensor e diseii6 una mesa de agitacicn basad a en una bobina m6vil a frecuencia y amplitud variable, con la cual se pudo encontrar la respuesta del sensor en un rango de frecuencias entre 0.1 y 100 hz. geofisica colombiana, 5, diciembre de 200"1 diseno e implementacion de un sistema de adquisicion de datos sismolocicos en tiempo real la parte basica es una bob ina m6vil que se encuentra en un campo rnagnetico con stante generado por un iman permanente. esta bobina es alimentada con una serial de corriente, a la cual se ie puede controlar la frecuencia, la amplitud y la forma. dependiendo de la forma de la sefial recibida, esta bobina se desplazara desde su posici6n de equilibrio, siguiendo la forma de la sefial, figura /5. mesa vibradora disefiada para calibrar el sensor figura /6. di su-ibucicn de elementos del mecanisme impulsor la mesa vibradora esta compuesta por dos partes principales: el servornecanismo (bobina, iman permanente y gufa) y la parte electronica de control (generador de frecuencia, fuente de alimentaci6n, etapa de control y etapa de potencia). ei servornecanismo controla el rnovimiento del sensor mediante una guaya de acero. geoflslca colombiana, 5. diciembre de 2001 figura j 7. distribucion de elementos dentro del modulo de control figura j 8. unidad de control la fuente de alirnentacion esta constituida principal mente par dos partes; una regulada (compuesta por dos reguladores 7815 y 7915) que suministra la potencia regulada de + 15 y -15 voltios d.c. a la parte de control, y una no regulada que proporciona el voltaje que mantendra la corriente de alta en la bobina; esta constituida por un transformadar y un puente de diodos como rectificador de onda completa. la etapa de generaci6n de frecuencia esta basada en un circuito integrado lm 8038, que mediante la conmutaci6n de tres condensadores permite variar la frecuencia entre 0.1 hz y 100 hz aproximadamenre. este circuito tambien hace po sible ajustar la simetria de la onda, amplitud y tipo (permite generar ondas cuadradas, triangulares y senoidales). la etapa de control se basa en el circuito integrado lm324, el cual alberga cuatro amplificadores operacionales independientes. en esta erapa se controla la serial que se aplicara a la bobina mediante una etapa pid; adernas recibe la retroalimentaci6n de un potenciometro ubicado 79 pedro a. vega v. en el camino del sensor, el cual garantiza que el servomecanismo siga exactamente el movimiento de la serial aplicada. en esta etapa se utiliza un amplificador operacional en configuraci6n seguidor, que pennite el acople de impedancia y amplifica la corriente proveniente del generador de sefiales; aquf la ganancia es l. esto se encuentra en paralelo con otro amplificador operacional, tambien con configuraci6n seguidor, que permite ajustar el recorrido de la bobina. la sefial se aplica a un amplificador operacional en configuraci6n inversor, la cual, junto con la sefial de retroalimentaci6n tomada de un potenciometro movido poria guaya de tracci6n es enviada a un amplificador que amplifica el error que existe entre el movimiento del sensor y la serial aplicada; esta sefial es enviada a la etapa proporcional-integral, que a la vez ie envfa la sefial de correccion a la bobina a traves de la etapa de amplificacion de potencia. para entregarle la corriente a la bobina se diseiio una etapa de potencia, la cual esta basada en dos pares de transistores complementarios: ecg280-ecg281 (cuatro transistores) en paralelo; estes son amplificadores de potencia de proposito general cada uno con una ganancia de 70 (hfe = 70) yuna corriente maxima de colector de 12 amperios. figura 19. erapa de ampjificaci6n de porencia el conjunto esta conectado a un pc equipado con una tarjeta de adquisicion de datos, a la cual se conectan las sefiales: tanto !a generada poria mesa vibradora como la respuesta del sensor. para registrar las dos seriales se utiliz6 el software del labview que se muestra en la figura 20. esta rutina registra dos canales en sendos archivos y permite modificar la rata de muestreo y el mimero de muestras tomadas, con 10 que se puede seleccionar el niimero de ciclos de la serial que se requiera registrar, ahorrando asi espacio en el disco. en la figura 21 se muestran las curvas de respuesta obtenidas en la mesa vibradora, para posicion vertical y para inc1inaciones de 60 y 75. se puede observar que para estas inclinaciones ja respuesta del sensor es sobreamortiguada, ya que las amplitudes caen vertical mente y no perrniten oscilaciones, debido a que la masa sismica se frena con la cubierta. 80 ~""""j')" m... '.e> 0:0: r;r1 ,,""~i ':.i.c.. ......... ::v: i ~ .. tooa:oj 'r ~'~j"~~~ ! i:!el 1,0000:1@} figura 20. subrutina de labvtew utilizada en las pruebas y tomas de datos de la mesa vibradora :1 on' "" figura 21. respuesta a la inclinaci6n obtenida en la mesa vibradora sincronizacion de tiempo como se dijo al comienzo, los sistemas de tiempo real pueden ser hard 0 soft, y dado que este equipo tolera salidas de operacion transitorias, se clasifica como soft. lo que se pretende es que la etiqueta de tiempo de los eventos este 10 mas cerca posible del tiempo real 0 correcto en el momenta del evento, es decir que el tiempo del computador que adquiere los datos y que genera la etiqueta de tiempo este sincronizado con el tiempo astral, de tal manera que los datos puedan servir en las redes sfsmicas como apoyo para el calculo de los parametres del terremoto. con esto en mente, se investigaron formas posibles de sincronizar el reloj del computador con el tiempo astral. la mas precisa es adquirir un reloj gps, el cua! calcula el tiempo y la posicion, bas ado en la red de satelites de geo-posicionamiento, y genera un tren de pulsos que ie entrega al computador un c6digo con la fecha y hora exactas; con este dato se sincroniza el reloj interno del sistema. otra forma no tan precisa, pero mucho mas barata, es como 10 hacemos en este sistema: utilizar programas de distribucion a bajo costa (usd$50), a traves de la red internet, que sincronizan el tiempo del c<twe putador con el tiempo lefdo de un servidor de tiempo (time server) geoff sica colombiana, 5, diciembae de 2001 diseno e implementacion de un sistema de adquisicion de datos sismolocicos en tiempo real esta generando permanentemente los pulsos de sincronizaci6n. estos servidores de tiempo son sistemas basados en un reloj at6mico que envia sefiales con la fecha y hora exactas a traves de la red. ei software utilizado se denomina atomtime 98 versi6n 2,5, que cuando se instala y ejecuta presenta una ventana en la cual se puede ver el tiempo del computador en ese momento, y algunos botones que permiten configurar la conexi6n, el chequeo del tiempo y una casilia donde indica el tiempo del time server. este software hace posible ademas tener pulsos autornaticos a pedodos fijos preestablecidos por el usuario para realizar la verificaci6n y el ajuste del reloj del sistema, como se puede ver en la figura 21. este tipo de sistema de sincronizaci6n tiene el inconveniente de los retrasos en las redes de comunicaciones, que dependen de la congesti6n en las lfneas de transmisi6n de datos y de las distancias a que se encuentren los time server; sin embargo, el hecho de estar permanentemente recibiendo el pulso de sincronizaci6n garantiza un reloj del sistema confiable. aunque este software permite un intervalo minimo de tres horas entre chequeo y correcci6n del reloj, existen otros que permiten tener pulsos cada segundo, con 10cual se tendrfa una correcci6n mas cercana, pero el precio se incrementa considerablemente. sintesis se concluye el capitulo sintetizando los experimentos realizados en la tabla i, que muestra los resultados obtenidos y que caracterizan el sistema de adquisici6n de datos realizado. s610 se trascribiran algunos de los resultados con el objeto de no caer en repeticiones inecesarias. tabla 1. resumen de caracter1stica5 obten1da$ para el sistema 1 rango entrada amplificador ± 5 v d.c. 2 rata de muestreo maxima 6250 sis 3 resolucion teo rica 7.631lv 4 conversion aid 16 bits 5 sensor mark products 6 relacion de overshoot 0.34 7 periodo libre to 0.99seg 8 amortiguamiento 061 9 masa 9652 gr 10 frecuencia natural 1 hz 11 respuesta en amplitud figura 11 12 respuesta en trecuencia figura 11 conclusiones se disefio, construy6 e implement6 un sistema de adquisici6n de datos sismol6gicos portatil, debidamente caracterizado, que permite el almacenamiento en disco de los eventos sfsmicos con una etiqueta de tiempo sincronizada con el tiempo astral. se caracteriz6 el sensor mark products lac para ser usado como detector de actividad sismica, y se puede garantizar dentro de los lfmites establecidos en las curvas de calibraci6n. geoflslca colombiana, 5, diciembre de 2001 se ie entrega a la universidad un equipo caracterizado con el cual se pueden realizar practicas academicas, como parte del proyecto, se disefio, construy6 y prob6 una mesa vibratoria que permite realizar calibraciones de sensores sisrnicos. se ha demostrado que se pueden desarrollar instrumentos y equipos con elementos que se pueden conseguir en el mercado nacional. es posible aplicar esta tecnica a otro tipo de instrumentos en el area geoffsica, con 10 cual se pueden crear nuevos instrumentos y recuperar los que se encuentren deteriorados. referencias bibliograficas asea. 1990: coursec4 remote terminal unit, sindac training. dewey, james. (to 1900). 2001: usgs national earthquake information center, the earl y history of seismometry. havskov,jens, 2001: institute of solid hera! physics, instrumentation of eartquake seismology. keller, fritz. 1997: institut fuer geophysik der tu clausthal, seismometer documentation. landtech enterprises. 1999: instructions on how to calibrate a seismograph, modified from: manual of seismological observatory practice. national instruments corporation. 1996: cb-27 connector block installation guide. national instrumentscorporation. 1996: daqcard-500 user manual. national instrumentscorporation. 1997: labview quickstart guide. national instruments corporation. 1998: developing a labview instrument driver. national instrumentscorporation. 1999: data adquisition fundamentals. national instrumentscorporation. 2000: data acquisition basics manual. rodgers,peter w 1999: signal coil calibration of electro-magnetic seismometers. sam, dekey. 1998: making windows nt a real-time solution (a technical overview). soli':, enric figueras. 1999: disefio de aplicaciones scada con lab view sprengnether instruments inc. 1986: microearthquake system meq-800-b, technical manual. wielandt, erhard. 200 i: institute of geophysics, university of stuttgart, seismic sensors and their calibration. 81 earthquake light (eql) mainly blue-white flashes from the ground, was observed coseismically during a new zealand m7.1 earthquake, (4.47 am 4-sep-2010 local time). a local production mechanism is most probable. the blue-white flash observations support the freund et al. mechanism, i.e. shockwave disturbance creating electronic “holes” from ruptured peroxy bonds in quartz, and subsequent surface positive charge from the ground, followed by light emission during neutralization. examination of video records shows the flash median length is about half a second and needs to be differentiated from the light during electricity supply short circuits. observed ground-level white colors appear to result from very intense non-specific air ionization. blue colors seem to be nitrogen emission with a short lifetime, succeeded by green oxygen emissions with longer lifetime followed by much lower intensity red. these were created by transient low-to-moderate voltages and probably include significant uv production. the maximum likely radiation dose is small, restricted to the skin, and equivalent at most to a few months natural background radiation. calculations confirm the release of 222rn is not the major mechanism for creating earthquake light, and would contribute minimal radiation dose. other unique observations are: streamers of light changing from blue to green as they passed from west to east, vertical sheets of blue-green light from cracks in an asphalt road surface, created by local shockwaves and shearing forces, daylight observation by fishermen of rapid linear undersea travel of blue light seconds before a 2011 m6.0 aftershock, and a mid-day green-blue glow over nearby hills containing a fault zone, shortly before the further destructive m6.3 earthquake, (12.51 pm 22-feb-2011). las luces de terremoto o eql (del inglés "earth quake light") son principalmente azul blancas y fueron cosísmicamente observadas durante un terremoto de magnitud 7.1 en nueva zelanda (4:47 a.m., 4 de septiembre de 2010, hora local). es muy probable que el fenómeno haya sido producto de un mecanismo local. las observaciones del destello azul blanco coinciden con el mecanismo de freund y otros, esto es, la alteración de un movimiento sísmico que crea "agujeros" electrónicos a partir de lazos de ácido peroxi rotos en cuarzos, con carga positiva superficial consecuente del suelo, y seguida por la emisión de luz durante la neutralización. la revisión de grabaciones de video muestra que la duración del destello es de cerca de medio segundo y debe ser diferenciado de la luz generada por los cortos circuitos del fluido eléctrico. los colores blancos observados a nivel del terreno parecen resultar de una intensa ionización aérea no específica. los colores azules serían emisiones de nitrógeno con un corto período de duración, sucedidas por emisiones de oxígeno verde de mayor duración y seguidas por un rojo mucho menos intenso. estos colores fueron creados por voltajes transitorios de bajo a moderados y probablemente incluyen una producción significativa de radiación ultravioleta. la cantidad de radiación máxima probable es pequeña, restringida a la piel, y equivalente a unos pocos meses de radiación natural regular. los cálculos confirman que la liberación de 222rn (radón) no es el principal mecanismo para crear las luces de terremoto y que este contribuye con una mínima cantidad de radiación. otras observaciones únicas hablan de rayos de luz que cambian de azul a verde mientras van de oeste a este, de cortinas verticales de luz azul verdosa que emergen de grietas en la carretera, creadas por movimientos sísmicos y fuerzas de corte; de observaciones de pescadores a luz del día del trayecto subacuático, rápido y lineal, de una luz azul antes del movimiento sísmico de magnitud 6.0 de 2011, y de un resplandor verde azuloso a mediodía sobre las colinas cercanas a una zona de fallas momentos antes del destructivo terremoto de magnitud 6.3 (12:51 p.m., 22 de febrero de 2011). abstract resumen keywords: : earthquake light, earthquake precursors. palabras clave: luces de terremoto, precursores de terremoto. origin of earthquake light associated with earthquakes in christchurch, new zealand, 2010-2011 record manuscript received: 02/11/2014 accepted for publication: 06/08/2015 how to cite item whitehead, n. e., and ulusoy, u. (2015). origin of earthquake light associated with earthquakes in christchurch, new zealand, 2010-2011. earth sciences research journal, 19(2), 113-120. http://dx.doi.org/10.15446/esrj.v19n2.47000 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.47000 n.e.whitehead1 and ű.ulusoy2 1corresponding author: whitehead associates, 54 redvers drive, lower hutt, new zealand 5010 64-4-5650263, whiteh@paradise.net.nz 2department of physics, engineering faculty of engineering, hacettepe university, 06800 beytepe, ankara, turkey origen de luces de terremoto asociadas con los terremotos de christchurch, nueva zelanda, 2010-2011 earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 113 120 se is m o l o g y 114 n.e.whitehead and ű.ulusoy introduction eql is mainly flashes of light or glows associated with earthquakes, but because of its transient nature has been treated sceptically by scientists. following capture on film and webcam/security cameras, it is now accepted as real by most seismologists, as judged by the published literature referenced in this paper. there are, by our count, about half a dozen videos available on the internet, (e.g. an aftershock reported by the bbc in sendai, japan: http://www.youtube.com/watch?v=5hrgqlu3y_y, and in lima, peru: http://www.pucp.edu.pe/inras/peru-magneto/mod-3cam-3.exe). most show a white spherical ground-based core surrounded by radiating blue-white light, fading the further it is from the core. there are now thought to be numerous valid records throughout history (derr, 1973), (stothers, 2004), (thériault, st-laurent, freund, and derr, 2014), though the origin is still argued, (straser, 2007), (fidani, 2010), (fidani, 2012), (freund, 2011), and may be multiple. eql has been observed before in new zealand ((aimuri, hutton, 1888); (reefton, anonymous, 1888a; anonymous, 1888b); wellington, 1943, green aerial explosions over the sea (personal communication edith costello, 2012), (grayland, 1957); inangahua 1968, coseismic orange glow, less than 15 min (personal communication w. inwood, 2012)) the references give only superficial descriptions; locations are found in figures 1-3. however, we are still at an early stage, where many more observations should be collected and analysed. this paper attempts to add to the collection and supply some interpretation. two large earthquakes in christchurch, new zealand contributed four unique observations that support some explanations of eql origins in the literature, particularly work by freund et al. (2009), thériault et al. (2014). new zealand is earthquake prone because of its tectonic setting on the “ring of fire” (stirling, 2002). christchurch is a city of 377,000 people on the east coast of the south island of new zealand at 43º s (figure 1). an m7.0 event (4-sep-2010 at 4:35:47 am local time, 16:35:47 ut), had an epicenter 12 km deep, near darfield, 42 km west of christchurch, on the greendale fault (quigley et al., 2010). this main event produced most of the reported eql. there were many thousand aftershocks (bannister and gledhill, 2012). a later event triggered by the first earthquake (stramondo et al., 2011), was on 22-feb-2011, at 12:51:42 pm, local time, 23:51:42 ut, m6.1, 6 km deep (usgs, 2011) and very close to christchurch, on the lyttelton or port hills fault (increasing detail in figures 1, 2, 3). figure 1. location map of new zealand, with christchurch and faults. asterisks mark epicentres. underlined named locations are of eql, some historic. more detail in figures 2,3. figure 2. christchurch environs. black dots mark greendale fault (buried and detected mainly through seismology, with no published defined line at the time of writing). figure 3. christchurch localities. port hills fault zone in black dots (from seismology). black-type localities are those mentioned in text. observer positions asterisked. observations:. observer #1 (typical of many) blue flashes near airport; observer #2; blue/green curtain emissions from cracks in road; observer #3 submarine blue beams; observer #4 blue-green glow above port hills. the city is mostly on a broad river plain of sedimentary greywacke gravel and silt (suggate, 1958). the only igneous rocks are the 500m high port hills eroded remnants of a 10-12 ma-old basalt volcano (stipp and mcdougall, 1968), outside the city to the southeast, within which is a 15km long inlet from the northeast, parallel to the lyttelton/port hills fault (figure 3). the basalt is underlain by torlesse terrane, which is a new zealand mixed sedimentary and metamorphic rock (forsyth et al. 2008). method an appeal was made for earthquake experiences via a then-existing current affairs program (“close-up”) on national tv. the wording was “many people have reported events they think are associated with a later earthquake. for example from animals, birds, plants or in the ground, water, or the air, sky, clouds, electrical appliances or from other events. if you experienced something like this, please visit the website whose address is on the screen”. results were mostly collected on a website specializing in surveys called surveymonkey. for other details including data on animal response, see the parallel paper (whitehead and ulusoy, 2013). for some of the observations, obvious questions arise, but most participants did not 115origin of earthquake light associated with earthquakes in christchurch, new zealand, 2010-2011 supply email addresses for follow-up questions. the sample was obviously biased towards the mentioned categories, but many coseismic accounts were supplied most of which mentioned eql and were used in this paper. the sample will be biased towards the young and computer-literate, and is clearly not representative, but this will not matter for the phenomena described. consideration of reported daylight rainbow segments in clouds was deferred and is not included. results there were nearly 400 usable replies given to the survey, of which 42 mentioned eql for the september 2010 event. eql descriptions were also collected from an unrelated website specializing in meteorology. most people in the city saw brief blue/white coseismic flashes of light surrounding a white core, somewhere near the airport, and a minority, further away, reported a few red colors. the most intense light was a large white flash after tens of seconds of shaking, near the city, rather than at the epicenter 40 km distant. many similar reports were given for aftershocks. the light was captured on security cameras at the christchurch/lyttelton road tunnel (figure 3), princess margaret hospital, (central christchurch), and one less official video, incorporated in a subsequent film documentary, when a city falls, distributed by frank films (new zealand film commission, 2011) the latter available for time analysis, frame by frame. for four flashes, the median length was 0.12 sec (see discussion section). in the following eyewitness summaries, unique observations are marked by asterisk: 1. blue-white lightning from darfield, 5-10 seconds before p-waves at christchurch 2. co-seismic ground lightning near the west of christchurch, mostly blue-white flashes with final intense white flash 3. *mainly coseismic: rivulets of light on the ground, changing from blue to green 4. *mainly coseismic: vertical sheets of blue/green light from road surface (same event as 3). 5. *preseismic: submarine blue streaks of light 6. *preseismic: blue-green haze on hills in daylight (feb-2011 only). selected eyewitness accounts detailed accounts are available from the first author or at: www.chchquake.co.nz in the following, similarly in appendix 1 with representative accounts, some spelling has been regularized. editorial comments are in square brackets. a typical description is first, then the four unusual observations. typical observation #1 the colour of the flashes was a blue-white, definitely white like lightning, but more like the sheet lightning we occasionally see in the mountains. there did not appear to be a specific source like a concentrated point or jagged lightning flash, more of a bright round glow in the sky. the position of the glow changed with each flash. our arc of view was south around to west and the variation was in this range. also the elevation would have been below 55 degrees…….. the duration of the flashes was confined to the period of the earthquake and the number would have been at least six and each lit up a large part of the sky. in other words each flash was not confined to a small circle……. unique observations a. sheets of light from cracks in the road/colour changes [this account by observer #2 (figure 3) is significant for both reported colour change and the sheets of light] i was driving into christchurch on the morning of the quake. [southwards] …..4.35am. the sky lit up with an incredible lighting display. i knew it was an aurora. to my left, east, …[ see figures 2 and 3] flashing dark green to light green as it moved away from me. to my right, west, light blues to dark blue. the display was ‘sheeting’. i have seen auroras on tv, and that is what it was. this was not lightning. my car clock clicked over 4.36am, just seconds into the aurora, and the car lurched and tipped and shuddered. the force was enough to have tipped my car over, but due to experience i managed to steer into the shudders. the lights of the motorway were still on. [i.e. this was not yet power-line arcing] ‘rivulets’, or wriggles, were flashing towards me from the south west, like a giant spider web, or ‘radials’, … i would guess that the centre of the spiderweb would have been …. […darfield direction, figure 2, but about 20 km further away]. two ‘rivulets’ flashed passed me on either side, straight into kaiapoi which was on my immediate left, east. the lighting display suddenly became incredibly intense, and was not ‘in the distance’, it was all around me. then with the violent shuddering of the car, there was a huge flash of brilliant white light, and the display vanished, and the whole city sank into blackness [the car functioning was not affected. kaiapoi (figure 3) is 17 km n. of christchurch.]. and further (recapitulation): the light display appeared first …. as the colours changed they moved away, or, as they moved away the colours changed. greens on my left, east, blues on my right, west. the brilliant white light was directly over the city. the range of the light extended right out to sea, east, into the whole distance i could see. i was ‘inside’ the light [and it was hugging the ground, about car height]. …if you could imagine a flat concertina. the radials [= rivulets, earlier] advanced across the road in front of me, from southwest to northeast, and they moved as fast as i could take in the spectacle. the radials were not uniform in that i saw some which appeared to be behind/ ahead of others. each radial, … was each, ker-bump, and a ker-bump, with me travelling south at 100ks per hour. on each side of each radial, the road moved in opposite directions, like a push-pull effect [shearing forces in the asphalt]. as i passed over each radial, the car lurched the other way. each radial looked strangely like the pictures of gophers burrowing along, where they leave a little upraised trail. but there were no open cracks as appeared later. i would guess that the motorway was swaying inside a channel of about one meter overall in the distance i could see. the lights of the motorway were still on until the last tremor. the lights of the city vanished basically simultaneously with the aurora. …., in this instance, the radials shot through like the main radials of a web, and they did not cut through at the same rate. some appeared in front of me, and i ran over them, and some appeared behind me. the light appeared above the ground in sheets. the lights were high in the sky, and also appeared above the cracks. i think one of your correspondents said the light came from multiple sources. i think the light came from the thousands of points where the ground cracked. the impression i have is that gas was forced from the ground, and ‘something’ caused it to appear in colour, …. the lowest point of the appearance of the light, would have been about level with my windsceen. it was fleeting, like a rainbow which disappears when one gets closer…. this correspondent also reported feeling for a few days as though he had been sunburned (discussed below). b. lyttelton submarine observation [from fisherman, observer #3, figure 3, 4 km from the port of lyttelton from the open sea, early afternoon, 11-july-20 11.] “i was looking out the front of the boat and i saw a couple of blue flashes down in the water, was definitely a flash, a dim blue flash. “the blue flashes were about 100 meters to the side of the vessel and shot from left to right in a straight line across us. then the next minute it was like the boat hit something.”[earthquake shock wave from a m6 116 n.e.whitehead and ű.ulusoy116 aftershock (gorman, 2011)]. the flashes were also described as rather milky blue, like the subaerial september events, but visible in daylight. there were two “rays” about 10 m apart which vanished into the distance. the depth was less than 12.4 m, (port company, 2013) and the surface is soft sediment. the time between the observation and the shock was a few seconds. the earthquake was m6.0 and 10 km under lyttelton, on the port hills fault zone . c. blue-green glow on hills observer #4 there was a blueish green glow in the sky just before the quake started [over the port hills to the southeast and above the fault zone. about 30 minutes before the m6.1 quake (12:51, 22-feb-2011), glow visible even though it was midday; no duration noted. weather cloudy]. discussion credibility of reports in the present paper, we have accounts of at least four unique and remarkable observations. however, there is no reason to doubt their content; observers gained little or nothing by reporting them, and in some cases were reluctant to report them because of their strangeness, until they realized that there was historical precedent. related events in the literature were unknown to the observers. most colors at christchurch had a white core (200m?) fading to blue-white further out, as described in the literature (briggs, 1999), (freund, 2003), (ikeya, 2004), with a few reports of green and red (only in the sky). other colors figured much less prominently. in general for the m7.0 quake the bluer colors were reported as being towards darfield, the epicenter, any green tending to be well away, and possibly over the sea to the east. from eyewitness reports, there were perhaps as many as six blue flashes, culminating in an intense white flash of large but uncertain extent (probably a larger version of the blue-white flashes), then a general electricity shutdown. another christchurch report was of a coseismic flash color like tungsten (incandescent) lighting. there were also reported preseismic glows, perhaps hours long, mostly on hills at night, as reported in the previous literature for other earthquakes (tsukuda, 1997). as for many new zealand earthquakes, there was a train-like rumble heard well before the first p-wave, and even before the earthquake origin (figure 4) which is puzzling. this also happened before aftershocks. this paper endorses a previous explanation (gordienko et al., 2008); pre-seismic rumble may originate from very numerous microcracks produced by pre-earthquake strain over a large area. video evidence will be biased in favor of recording the blue-white flashes because they have the highest intensity. glows are much less intense and will require long exposures to register (about 30 seconds, see long (2014) for similar difficulties with aurora detection). neither video facilities nor security cameras will record them, so eyewitness accounts will continue to be important. difficulties in differentiation from transformer arcing the christchurch median flash length was 0.12 s from a frameby-frame analysis, using the 7 movie maker program, compared with similarly derived medians of 0.56 s for an event in lima, peru (hyperlink in heraud and lira (2011)) and 0.22 s for sendai (bbc, 2011). chile data (taylerburdeen, 2010) appears similar. all are much longer than conventional lightning (10-50 µs) and a good rule of thumb is about 0.5 s. since some earthquake lights are several seconds in length, they are perhaps best described as short glows. for luenser (2013), and anonymous (2013) which were superficially similar, but were known to be electrical flashes rather than earthquake light, the analysis gave respective medians of 0.43 s (115 points) and 0.89 s (12 points). using montecarlo statistical methods (e.g. robert and casella, 2005) it was shown the electrical and genuinely earthquake data could not be distinguished. the possibility of transformer arcing must, therefore, be checked for all assertions of earthquake light observations. for the september quake in christchurch (see figure 4), the electrical distribution network was operating a logger, monitoring line voltage. collating eye-witness accounts, the eql appeared at least 10 seconds before the slightest preliminary line voltage fluctuations and were therefore not power supply arcing. no destroyed transformers were reported, which implies no destructive arcing during the whole episode. according to two good independent eyewitness accounts, flashes of light appeared 5-10 seconds in darfield before the first main tremors (p-waves) in christchurch. these flashes continued, but much nearer the city, until the power failure, and strong white flash (again much nearer christchurch than darfield). this confirms that eql may be at least tens of kilometers distant from an epicenter. figure 4. the timing of reported events at christchurch city, september 4, 2010. the epicenter was near darfield, depth 12 km, and 41.9 km distant. the accelerometer trace is from an instrument in the basement of the christchurch westpac trust building (gns, 2011), voltage changes and power failure are from logger data. the eql were coseismic, or a few seconds later than the first p-waves both for darfield and christchurch. for the strongest white flash, it could be a coincidence that this immediately preceded the power shut-down, but the simplest hypothesis is that an intense electromagnetic pulse contributed. more importantly this figure confirms some eql need not be associated with voltage fluctuations in power lines, or arcing. ionization involved. this paper suggests blue, green and red colors arise from ionization levels in nitrogen and oxygen which are relatively long-lived and remain, though only for a few seconds, after the less specific ionizations have disappeared. the white core of the “lightning” looks identical to white air discharges caused by arcing, or malfunctioning in high voltage electrical sources (usually accompanied by some ultraviolet). this is non-specific ionization of air molecules. individual colors are harder to explain, but this paper suggests they are from ionization events visible only because they are relatively long-lived. the blue colors at atmospheric pressure are known to arise naturally through de-excitation of nitrogen molecules, n2 +1, to neutral nitrogen molecules (wiberg, wiberg, and holleman, 2001) but have a lifetime of tens of nanoseconds only. the rarer green colors are due to ionization of oxygen and have a life of about 0.75 s, following which they are in a low but still excited energy level (wiberg et al., 2001). see also gartlein, kimball, and sprague, (1965). as oxygen passes to its lowest energy level with a very long 110 second lifetime, it emits red light. these data may be found in various standard compilations (e.g. wiese and martin, 1986). the colors are also seen in auroras (sorokin and chmyrev, 2014) but at ground level one would need a very intense primary ionization event of perhaps many tens of kilowatts per square meter. could this give a significant radiation dose? radiation dose? the eql themselves must correspond in energy terms only to ev, in the visible to ultraviolet region, and would deliver a dose only to the skin. earth movement could not create higher energy radiation such as gamma rays. observer #2 entirely surrounded by the white flash reported feeling sunburnt, possible if the blue-white flashes had strong uv output. ionized air has a 117origin of earthquake light associated with earthquakes in christchurch, new zealand, 2010-2011 strong uv emission component from atomic transitions in ionized nitrogen (anonymous, 2012), and electric arc welders must use uv protection. conventionally (unscear, 1993), the penetrating radiation dose from inhalation of radon is 1 msv y-1. if we assume that excess radon is released by pre-quake stresses and reaches 10x normal for 10 days before the quake (e.g. igarashi et al., 1995), the dose might be 10/365*1 or 0.027 msv. this is slight compared with the normal yearly radon exposure, and both these doses are negligible in exposure radiation terms. in contrast, normal levels of radon in cities caused through common atmospheric meteorological inversions are about 10 times higher than those likely from earthquakes (ikeya and whitehead, 2013). different analytical treatment, (freund, 2011) also concludes radon is not a factor in eql. thus, although ionizing events are occurring, the radiation dose is not critical. associated electrical phenomena during the week before 4-sep, static electricity was frequently reported, with local tv interference and other electronic glitches (whitehead and ulusoy, 2013). before and after the 22-feb earthquake, one respondent reported 5 minutes of local radio/tv static, though the broadcast signal was unchanged and continuous (personal communication nztv1). another respondent reported this static for 4-sep-2010, 22feb-2011, and 24-mar-2012 quakes. this has been reported for other earthquakes (wadatsumi, 1995), (st-laurent, 2000), (nagao et al., 2002), implying electromagnetic disturbance. however most importantly no thunder ever accompanied the eql, implying quite low voltages compared with conventional lightning, i.e. much less than 30kv cm-1 (lowke, 1992). coseismic dimming and brightening of domestic lights was reported, and noted in available video material, which must be very local changes of electric transmission line voltages, and it seems at least some of this was before the line-voltage fluctuations recorded with the logger. electric appliances not blown (observer #6), suggests a maximum of several thousand kilowatts power variation associated with the earthquake light. otherwise, the common 50 amp domestic electric circuit breakers would be activated. this means electromagnetic effects were involved, whether primary or secondary and were possible at christchurch. this erratic dimming has been previously recorded for incandescent light (terada, 1931). he also reported voltage fluctuations in a submarine cable during the idu earthquake. mechanisms unusual auroral activity was absent. the account from observer #2 described light that occurred on asphalt and seems to have been generated by shearing forces in the asphalt itself or more probably from directly below the cracks. notable is the description of physical forces on the car, coincident with the light, which does not seem to have been recorded elsewhere. this suggests very local seismic forces are strongly associated with these eql. although the previous literature suggests eql are much more frequent near the igneous rock, the present observations show it can occur in asphalt, sedimentary rocks and unconsolidated gravels. however like igneous rocks, the gravels contain small crystals of quartz (e.g. beggs, 1980) which is important for the most likely mechanism suggested below. the furthest observation (coseismic) was 65km north in the hills and above the hills of scargill valley (figure 2), which again means local shockwaves or strain can generate this light. most proposed mechanisms for the creation of high voltage potentials, and emission of ions apply only at the fault; (terada, 1931; lockner, johnston, and byerlee 1983; hedervari and noszticzius 1985). a piezoelectric origin for more distant influence has been considered (derr, 1973; takaki and ikeya, 1998; ikeya, 2004). the theory proposed slightly aligned quartz grains subjected to slowly varying pressure give rise to ulf waves that are one of the very few physical influences that can reach the surface from a deeply buried epicenter. at the surface, electrical fields were produced by these waves, and caused acceleration of local free electrons, creating the light, because ulf itself does not cause ionization. however (st-laurent, derr, and freund, 2006), there are major difficulties with the model, i.e. two mirrorimage forms of quartz will always occur in a 1:1 ratio and their piezoelectric effects cancel when there are many small crystals. laboratory experiments (bishop, 1981; sasaoka et al., 1998), showed that rocks containing quartz gave a weak piezoelectric-like effect proportional to stress. since under their test conditions, other rocks did not, their conclusion was that quartz was somehow involved, but the following work suggests that they were observing a unique mechanism in quartz that is called here “freundtype”. this model (e.g. freund et al., 2009; st-laurent et al., 2006) is based on extensive laboratory experiments in which high pressure on gabbro and diorite produced a flow of current to the surface due to rapid movement of positively charged “holes” in a quartz matrix from breaking peroxy-bonds that are a natural trace impurity (the balancing excess electrons produced do not move). the speed of movement of the holes may be 200 m sec-1. currents were calculated to be in the range 5-100ka per km-2, and many thousand cubic kilometers of rock are probably involved in an earthquake. it is calculated that this creates an intense positive electric field at the surface, air breakdown/positive ionization, then dissipation of the charge (e.g. freund (2011) and references therein). the recombination could give rise to the blue/white flashes observed, and uv. this model has experimental evidence from freund for the creation of these voltages even in sand and soil, so this mechanism could, therefore, apply to the christchurch gravels. it requires a return path for the positive charge to the trapped electrons under the ground, and the flashes in the air are part of this. why was the largest white flash in the city itself, not at the fault, and 30 seconds into the decreasing shaking? this could be because of the severe soil liquefaction in many prone areas in the city, providing a sudden, aqueous, highly conducting return path for electrical charge. individual light sources at christchurch may have been 200 m in diameter, but if they were apparently generated 40 km away in darfield, the implied very narrow beam at the epicenter seems quite improbable. most physical mechanisms should have created wide-beam light spreading from the epicenter, particularly if it is ulf. much more plausible is local stress in christchurch creating local light sources. ulf with its widespread nature may be a better though indirect explanation for long-lived diffuse glows in the sky. ground forces in the “ground aurora” accounts like the above “ground aurora” of observer #2, exist (terada, 1931), (derr, 1973), (tsukuda, 1997), (st-laurent et al., 2006), (fidani, 2010) but mostly with scanty detail or with “streamers”, rather than curtains. a closer parallel is a quebec observation (st-laurent, 2000). the colors produced at christchurch had perhaps a few seconds lifetime and possibly a height of tens of meters, which could be consistent with other literature (st-laurent et al., 2006). the sheets of light emerged from slight ground cracks, like a curtain. this sounds very much like the freund mechanism and may be an example of it. the observer was in the middle of the white flash and not burnt or shocked electrically, but the faraday cage effect from the car body makes magnitude estimation difficult. the lyttelton sub-sea observation at lyttelton, there was an appearance of light before the main shock. in this case, initiation was 10 km deep, but the resulting blue streaks were only 10 m apart, and at most several meters thick–perhaps a few percent of subaerial heights. the effect producing light has apparently penetrated upward through several meters of soft sediment. the milky-blue color again suggests ionization of nitrogen; the solubility of about 20 cm3 l-1 is not impossibly low compared with the 800 cm3 l-1 in the air. however a liquid medium would lead to very brief light compared to air, and since the light was a few seconds before the shockwave, unlike for the blue-white flashes, another mechanism may be responsible. there are other descriptions of marine light: (derr, 1973; kilbourne 118 n.e.whitehead and ű.ulusoy and saucedo, 1981; tsukuda, 1997; soter, 1999; stothers, 2004; ulusoy and ikeya, 2008; granin et al., 2012) differing from each other and with little detail. (heraud and lira, 2011) described surface bluish-white vertical pillars of light from low-lying rocky islets but generation under the sea seems less commonly reported. blue-green haze on hills. the reported color corresponds with ionization of oxygen and nitrogen giving blue-green light. longer-lasting pre-seismic night glows on or above hills have been reported for various earthquakes (e.g. heraud & lira, 2011). if these are coronal lasting glows they would depend on an unprecedentedly high dielectric constant in the earth at these extremely low frequencies (i.e. ulf), and this is uncertain, although the dielectric constant does increase very markedly as frequency decreases (lockner and byerlee, 1985; ikeya, 2004). glow characteristics are so different from the flash-type eql that the mechanism is probably different. the hills supporting the glow were basalt (low quartz), so the underlying torlesse terrane sedimentary/metamorphic rock could be more likely as a source. utility for pre-quake warning? the most possibly useful premonitory earthquake light was diffuse glows on hills the night before the september event giving perhaps several hours warning. unfortunately, this is a night phenomenon and relies on informed observers who are rare at present. however following such an apparent precursor it could be prudent to avoid old and glass-fronted buildings for a day or so. this needs many confirming instances before it is conclusively known to be useful. conclusions the present observations show four types of observation, production of light in a flash on the surface of the ground, change of rivulet/ray color from blue to green, production of light by earthquake impulses, leading to vertical curtains of light above shearing zones, and production without the impulses but due to moderate voltage phenomena which precede p-waves, and may be attributable to local preseismic strain. eql does seem to involve ionization of nitrogen/oxygen which accounts for the colors seen. the actual dose of ionizing radiation is calculated to be very low. there may be some uv emission associated with the flashes, but electronic interference appears to be mainly radio/tv static. possible mechanisms supported include the freund model, particularly for ground-origin light, and ulf for glows in the air. general non-specific rather low voltage ionization gives a white color in the air and corresponding to atomic excited lifetimes, blue, green and red colors follow. preseismic rumbles are consistent with the generation of preseismic strain. acknowledgements sincere thanks to respondents to the survey for their accounts of eql, and orion network personnel for the voltage-logger data and discussion. gerard smythe, for video analysis permission. alan wright for a detailed account of driving through the eql, numerous correspondents for other historical accounts within new zealand, particularly edith costello, dr. martin reyners, warren inwood. thanks to gary vallance for his account of the undersea light, and gps coordinates. thanks to dr. kassym zhumadilov for some tests and discussion, mrs. b.k. whitehead for graphics assistance, dr. y. ikeya for assistance with the japanese literature, mr. d. chord for assistance with copying a christchurch video eql segment. references anonymous. 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(2001). inorganic chemistry (1656pp). berlin: associated press. appendix 1 contemporary new zealand eql reports #1. driving to chch [christchurch] from darfield at 9.30pm on friday 03/09/10 [3september-2010, the night of the quake, but about 7 hours before]. saw a strange halo/arc of light over the city of chch. it was blueish i guess. don’t know how long it went on as i was concentrating on driving… [the location of this halo is ambiguous. although, over the city, this could include the port hills.] #2. [rolleston 6-9pm 3september-2010] blue light [seen] towards darfield [this was for an extended time, at least many minutes, not a flash]. #3. i have seen exactly that, [blue light flashes during the september quake] at least 3 all in about 2 kilometers of [from?] each other and maybe 5 kilometers west of parklands [figure 3, tens of km from darfield]. i was deciding whether i should jump out the window when i looked up from the first bright flash and observed the next two, one of which i swore had an arc of lightning leading to it. [this must be forked or arc lighting, rather than sheet lightning] … all the light came from the ground. the one i got a good look at made an amazing amount of light in the form of a sphere on the ground. #4. …there were no clouds, and the light seemed to originate from the ground. we live on dyers pass [eastern edge of the city] and could see flash after flash in the direction of the airport [west of the city and several kilometers away]. #5 i definitely saw the lights (looked a lot like lightning) … we live in broadoaks, cashmere [eastern edge of the city] and what i saw was a blue-red color flash backwards & forwards across the sky. #6 we live in redcliffs [eastern edge of city] and also saw the bluish-white flashes in the sky … what we couldn't understand though is that our lights also flashed like crazy with blue/white flashes as if it was a power surge but all our appliances are ok, which would indicate that wasn't the case at all? #7 the other thing we observed on the cctv footage was what appeared to be a type of lightning (earthquake lightning)? only it appeared to be coming out of the ground. now that sounds weird i know, but members of the police on duty that night also reported it. [in the new zealand context this would more usually mean sheet lightning rather than forked lightning.] #8 [church bay, 50 km south from darfield]. as the earthquake began the sky was lit up with bright blue flashing light... similar to sheet lightning. it appeared 'streaky' like the sun shining through clouds #9 the other scary occurrence i would like to mention is, i awoke before the shaking, to the sound of what i can only describe as 'amplified static 120 n.e.whitehead and ű.ulusoy electricity' coming from my roof (it is iron), which at the time i retorted to my wife 'jeez, what's that, hail?'. we then jumped out of bed and heard the 'train like rumble' people often now compare the eq noise to. my son, downstairs, had his curtains drawn and was awake prior to the quake, he reported seeing blue flashes of light in the sky prior to the quake. the earthquake light was also seen associated with the aftershocks: #10 on saturday night of the big quake my wife and i spoke with a work friend who said he was out driving at 4:35 am when he saw numerous blue flashes in the sky. i was skeptical about what he had seen until the following night. my wife and i were driving back from blenheim home to christchurch [southwards] when just south of amberley [60km n of christchurch] we both saw two blue flashes in the sky ahead of us. one slightly to the left and the later one to the right. a definitive earthquake phenomenon in our opinion. #11 purple light glow the night before 4-september. another witness reported coseismic near-indigo color towards the east, and another, coseismic purple clouds. in this study, site-specific earthquake spectra for bitlis province in lake van basin has been obtained. it is noteworthy that, in probabilistic seismic hazard assessment, as a first stage data from geological studies and records from the instrumental period were compiled to make a seismic source characterization for the study region. the probabilistic seismic hazard curves for bitlis were developed based on selected appropriate attenuation relationships, at rock sites, with a probability of exceedance 2%, 10% and 50% in 50 year periods. the obtained results were compared with the spectral responses proposed for seismic evaluation and retrofit of the building structure in turkish earthquake code, section 2. at the end of this study, it is apprehended that the code proposed earthquake response spectra are not sufficient for the performance evaluation of the existing structures and the current estimations show that the potential seismic hazard research of the turkey is underestimated in the code. therefore, sitespecific design spectra for the region should be developed, which reflect the characteristics of local sites. en este estudio se obtuvieron espectros de terremoto de sitio específico para la cuenca del lago de van, en la provincia de bitlis, al este de turquía. la primera fase del trabajo consistió en una evaluación probabilística de riesgo sísmico donde se compilaron los estudios geológicos y registros del período instrumental para hacer una caracterización de fuente sísmica en la región de estudio. las curvas de amenaza sísmica para la provincia de bitlis se desarrollaron con base en las relaciones de atenuación apropiada seleccionadas en los sitios rocosos, con una probabilidad de exceso de 2 %, 10 % y 50 % durante 50 años. los resultados obtenidos se compararon con las respuestas de espectro propuestas para la evaluación sísmica y modernización de estructuras contempladas en el código de terremoto de turquía, en la sección 2. en la parte final de este trabajo se comprende que las respuestas de espectros de terremoto propuestos en el código no son suficientes para la evaluación de desempeño de las estructuras existentes y que las estimaciones actuales muestran que la investigación de amenazas potenciales sísmicas en turquía está subestimada en el código. por lo tanto, el diseño de espectros de sitio específico para la región se debe desarrollar, ya que permitiría conocer las singularidades locales. abstract resumen keywords: site-specific, earthquake spectra, site response analysis, codes, probabilistic. palabras clave: sitio específico, espectros de terremoto, análisis de respuesta de sitio, códigos, probabilística. record manuscript received: 12/04/2015 accepted for publication: 17/06/2015 how to cite item isik, e. and kutanis, m. (2015). determination of local site-specific spectra using probabilistic seismic hazard analysis for bitlis province, turkey. earth sciences research journal, 19(2), 129-134. http://dx.doi.org/10.15446/esrj.v19n2.50101 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.50101 determinación de espectros de sitio específico locales a través del análisis probabilístico de amenazas sísmicas para la provincia de bitlis, turquía earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 129 134 se is m o l o g y determination of local site-specific spectra using probabilistic seismic hazard analysis for bitlis province, turkey ercan işık1*, mustafa kutanis2 1 bitlis eren university, civil engineering dept. tr-13100 bitlisturkey 2 sakarya university, civil engineering dept. tr-54100 sakaryaturkey * correspondence author: ercan işık, phone:+90 (434) 222 00 97 ext:9701; fax: +90 (434) 222 91 01; e-mail: ercanbitliseren@gmail.com 130 ercan işık, mustafa kutanis introduction seismic hazard analysis of the earthquake-prone eastern anatolia region of turkey has become more important due to its growing strategic importance as a global energy corridor and closer integration with the european union. in this study, bitlis province is selected as the study area. the town of bitlis has a population of 70,000 (including the surroundings) as of the year 2000. the town is located 15 km from lake van along the steep slopes of the bitlis river valley at an elevation of 1,400m. the seismicity of bitlis has been evaluated using a performancebased earthquake engineering approach in this study. performance-based earthquake engineering seeks to improve seismic risk decision-making through assessment and design methods that have a strong scientific basis and that reveal options in terms that enable stakeholders to make informed decisions. given the inherent uncertainty and variability in seismic response, it follows that a performance-based methodology should be formalized on a probabilistic basis. the framework has four main analysis steps: hazard analysis, structural/nonstructural analysis, damage analysis, and loss analysis. the first assessment step entails a hazard analysis, through which one evaluates one or more ground motion intensity measures (im). for standard earthquake intensity measures (such as peak ground acceleration or spectral acceleration) is obtained through conventional probabilistic seismic hazard analysis. typically, im is described as a mean annual probability of exceedance, which is specific to the location and design characteristics of the facility (moehle and deirlein, 2004). in performance-based design and assessment method, it is possible to determine in quantities the damage levels that may arise from the design ground motion within the system structural system elements. it is checked whether this damage stays under the acceptable damage levels for each related component. acceptable damage limits are defined in a way to be consistent with the foreseen performance targets at various earthquake levels (aydınoğlu, 2007; doran et al. 2011; kutanis and boru, 2014). sitespecific design spectra for the region have great importance to determined building’s performance under an earthquake hazard. according to section 2 of the turkish earthquake code (tec’07), the demand spectra used to determine seismic performance of an existing building based on a probability of exceedance of 10% in 50 years (fig 1). figure 1. the earthquake spectrum curves for different exceedance probability the assessment procedure aims to estimate the earthquake force demand at which the building would sustain the performance objectives. demand spectrum, which is used in determining the performance of the building’s system, shows the maximum response that a building gives against seismic activities during an earthquake (i̇lki and celep, 2011). the assessment calculations were done based on a simple technique called the “equivalent displacement rule”. the equivalent displacement approximation is based on the assumption that inelastic spectral displacement is the same as that which would occur if the structure remained perfectly elastic (atc40, 1996). for the flexible structures, where the natural vibrational periods are greater than the corner periods, this rule yields acceptable results. in other cases, particularly in short period (rigid) structures, where the natural vibrational periods are shorter than the corner periods, the displacements obtained from this approximation method might be significantly different from the actual results (fig. 2 and fig. 3). in such cases, elastic spectral displacement is modified by multiplying it by a spectral displacement amplification factor (cr1) to obtain inelastic spectral displacement. figure 2. equivalent displacement rule: flexible structures figure 3. equivalent displacement rule: rigid structures 2. local geology soil conditions change the characteristics of surface seismic response. it is a known fact that this may cause damage to the existing structures built on these grounds (borcherdt, 1990). the lake van basin, which contains bitlis, is located in the region known as the bitlis thrust zone. it is a collapsed tectonic basin which is related to the eastern taurus region (özkaymak et al., 2003). orogenic movements have occurred in the field until the third phase of miocene. volcanic and tectonic events have caused many faults to form, as well as depressions and large lakes in this period (facenna et al., 2006; köse, 2004). metamorphic rock in the region belonging to the bitlis massif include the upper cretaceous ahlatadilcevaz mélange and ahlat conglomerate, miocene adilcevaz limestone, pliocene-quaternary volcanic rocks and alluvial deposits from the surface in bitlis and surrounding region (report 1, report 2). a geological map of lake van basin is shown in figure 4. ki spectral displacement, sd sp ec tra l a cc ele rat ion , s a capacity curve elastic spectrum with 5% damping sde sdi d(p) sae where d(p), target displacement sdi , inelastic displacement sde , elastic displacement ki, initial stiffnes cr, spectral amplification factor (cr=1.0) d(p)=sdi=1.0·sde ki spectral displacement, sd sp ec tra l a cc ele rat ion , s a elastic spectrum with 5% damping sde sdi d(p) where d(p), target displacement sdi , inelastic displacement sde , elastic displacement ki, initial stiffnes cr, spectral amplification factor (cr>1.0) d(p)=sdi=cr·sde sae sde capacity curve 131determination of local site-specific spectra using probabilistic seismic hazard analysis for bitlis province, turkey figure 4. geological map of the lake van region. n – nemrut volcano, s – süphan volcano near the lake (litt et al., 2009). 3. tectonic setting and seismicity of bitlis and surrounding areas the general tectonic setting of eastern anatolia is mainly controlled by the collision of the northerly movable arabian plate against anatolian plate along a deformation zone known as bitlis thrust zone (fig. 5). the collision leads to the westward extrusion of the anatolian plate along the two notorious transform faults with a different sense of slip, the dextral north anatolian fault, and the sinistral east anatolian fault zones, which join each other in karlıova triple junction (ktj) in the eastern anatolia (fig. 5). in the eastern side of ktj: however, the collision deformation is largely accommodated within the eastern anatolian block through distributed nwse trending dextral faults and ne–sw trending sinistral faults representing escape tectonics, and shortening of the continental lithosphere along the caucasus thrust zone. east–west trending mush-lake van and pasinler ramp basins constitute other conspicuous tectonic properties within the eastern anatolian border (şengör et al. 1985; barka and kadinsky, 1988; mc clusky et al. 2000; reilinger et al. 2006; utkucu et al. 2013). the east anatolian fault zone is a 550 km-long, approximately northeast-trending, sinistral strike-slip fault zone (fig. 5) that comprises a series of faults arranged parallelly, sub-parallelly or obliquely to the general trend. the bitlis suture is a complex continent-continent and continent-ocean collisional boundary that lies north of fold-and-thrust belt of the arabian platform and extends from south-eastern turkey to the zagros mountains in iran (homke, 2007; bonnin et al. 1988; piper et al. 2008; stern et al. 2008 and lyberis et al. 1992). the area to the east of karlıova triple junction is characterized by an n–s compressional tectonic regime (fig. 6). conjugate strike-slip faults of dextral and sinistral character paralleling to north and east anatolian fault zones are the dominant structural elements of the region. some of these structures include ağrı fault, bulanık fault, çaldıran fault, erciş¸ fault, horasan fault, iğdır fault, malazgirt fault, süphan fault, balıklıgölü fault zone, başkale fault, çobandede fault zone, dumlu fault zone, hasan timur fault zone, kavakbaşı fault, kağızman fault zone, doğubayazıt fault zone, karayazı fault, tutak fault zone, yüksekova–şemdinli fault zone and the northeast anatolian fault zone (fig. 6) (bozkurt, 2001). aydemir et al. (2014) investigated the faults and possible structural elements that may have caused the devastating earthquake that occurred on october 23, 2011. this potential extended fault zone starting from the nemrut mountain in the west may exist through to the east of lake van (aydemir et al. 2014). the faults are seismically active and form the source for many earthquakes. some of the major earthquakes in the 20th century are 13 september 1924 pasinler (m = 6.8), 1975 lice (m = 6.6), 24 november 1976 çaldıran (m = 7.3), 1988 van (m= 5.0), 2000 van (m=5.7), 23 october 2011 van (m=7.2) and 09 november 2011 van (m=5.6) earthquakes. figure 5. tectonic map of turkey including major structural features (from bozkurt, 2001) figure 6. active faults of eastern anatolian province (bozkurt, 2001). the examination of historical and instrumental earthquakes in bitlis and its surroundings proves that this region is constantly under the influence of micro and macro earthquakes. thus, bitlis remains under an enormous influence of micro and macro earthquakes (işık et al., 2012). therefore, its buildings must be constructed, especially in lake van basin where earthquake resistant design has always been neglected, according to earthquake codes. 4. seismicity parameters on any given fault within any particular region, earthquakes occur at irregular intervals in time, and one of the primary activities in seismology has long been the search for meaningful patterns in the time sequences of earthquake occurrence (dowrick, 2003). among some recurrence laws have been proposed, in this study, gutenberg and richter’s law was used because there is no available evidence to determine whether the gutenberg –richter or some other recurrence laws are correct. during any given interval in time, the general underlying pattern or distribution of size of events is that first described by gutenberg and richter (1944), who derived an empirical relationship between magnitude and frequency of the form log n = a − b.m (1) where n is the number of shocks of magnitude at least m per unit time and unit area, and a and b are seismic constants for any given region (dowrick, 2003). in a seismic hazard modeling study of bitlis, recurrence rates are 132 ercan işık, mustafa kutanis estimated by using historical and digital records. after the compilation of collected data, a plot of “m” against “log n” was constructed and the bestfit line of the form of eqn. 1 was determined by regression analysis (fig. 7). in probabilistic seismic hazard analysis, beside magnitude–frequency relationship that is calculated for bitlis province as logn = 5.6247 – 0.7794 m. figure 7. gutenberg-richter magnitude–frequency relationship for earthquakes from bitlis and surrounding data 5. site-specific design spectra for bitlis province the seismic hazard analysis approach is based on the model developed originally by cornell (1968) who quantified it regarding the probability of exceedance of the design level peak ground acceleration (pga). the procedure for conducting a probabilistic seismic hazard analysis includes seismic source characterization, size distribution and rate of occurrence determination for the source, ground motion estimation and, lastly, probability analysis. in the current study, since the neotectonic faults are not identified in the research area clearly, earthquake sources are characterized as area source zones. area seismic sources are often defined where specific fault data are not known, but seismicity does exist. area sources assume that the rate of occurrence is uniform throughout. therefore, every location within the area has equal probability that an event will occur (ez-frisk; anton and gibson, 2008). all seismic sources, that can generate strong ground shaking in bitlis and surroundings, are classified into 7 areal seismic zones (fig. 8): (1) bitlis-zagros suture zone; (2) northern bitlis thrust fault zone (dhont and chorowicz, 2006); (3) kavakbaşı fault zone; (4) malazgirt fault zone; (5) ahlat and surrounding fault zone; (6) suphan fault zone ; and (7) southern van faults (erçek fault, kalecik fault, edremit fault and southern boundary fault (utkucu, 2006). figure 8. earthquake areal zones (bitlis suture, van south, bitlis north suture, kavakbaşı, ahlat, malazgirt and suphan) in bitlis and surroundings in eastern anatolia region, previously recorded strong ground motion acceleration records are limited. therefore, in the current analysis, worldwide applicable three empirical attenuation relationships are utilized to perform the seismic hazard analysis. attenuation relationships for rock sites employed in this study are abrahamson-silva (1997), ambraseys et al. (2005), boore et al. (1997), campell (2003) and idris (2008) (fig. 9). figure 9. abrahamson-silva (1997), ambraseys et al. (2005), boore et al. (1997), campell (2003) and idriss (2008) attenuation relationships for rock sites. after the compilation of the seismic hazard analysis data, the procedure for conducting a probabilistic seismic hazard analysis, by using ez-frisk (mcguire, 1995). the software was employed to produce the pga as a function of return periods (fig. 10) and uniform probability response spectra for selected return periods (fig. 11). the results of probabilistic seismic hazard analysis for bitlis are presented regarding spectral responses at 5% damping for the return periods of 72, 474.6 and 2474.9 years (fig. 11). the results are compared with the spectral responses proposed for seismic evaluation and retrofit of building structure in turkey earthquake code (2007) section 7. figure 10. peak ground acceleration (pga) at bitlis with varying return periods. 133determination of local site-specific spectra using probabilistic seismic hazard analysis for bitlis province, turkey figure 11. spectral responses at 5% damping for the return period of 72, 474.6 and 2474.9 years the results of probabilistic seismic hazard analysis revealed peak acceleration values for a typical rock site as 0.76g for 50% probability of exceedance in 50 years, 1.61g for 10% probability of exceedance in 50 years and 2.68g for 2% probability of exceedance in 50 years. the obtained results are compared with the spectral responses proposed for seismic evaluation and retrofit of building structure in turkey earthquake code, section 7 (fig. 12). figure 12a. comparison of spectral responses at 5% damping for the return period of 2474.9 years in bitlis figure 12b. comparison of spectral responses at 5% damping for the return period of 474.6 years in bitlis figure 12c. comparison of spectral responses at 5% damping for the return period of 72 years in bitlis 6. results and conclusions by utilizing available data and the use of improved methods, a probabilistic seismic hazard of bitlis province in turkey was determined. as a first step of the performance-based earthquake engineering, it is well understood that the code proposed spectra are not sufficient to represent earthquake demand in the performance evaluation. the results of this work will form the basis for the replacement of the existing earthquake design spectra in the assessment of earthquake performances of the existing buildings in bitlis province. in this study, since active faults are not identified clearly, regional areas were used as an earthquake source zones. future work will increase the resolution of the seismotectonic model by adding specific active faults. the obtained results are compared with the spectral responses proposed for seismic evaluation and retrofit of building structure in turkish earthquake code, section 7, and the amplitude and frequency range was different from each other (fig.13). using the response spectrum obtained from probabilistic seismic hazard analysis will make obtained data for bitlis and other regions which are under a threat of earthquakes. figure13. comparison of demand displacement of the earthquake therefore, sitespecific design spectra for the region should be developed, which reflect the characteristics of local sites. references abrahamson, n.a. and silva. 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(2013). coulomb static stress changes before and after the 23 october 2011 van, eastern turkey, earthquake (mw = 7.1): implications for the earthquake hazard mitigation, natural hazards and earth system science, 13(7), 1889–1902. utkucu, m. (2006). implications for the level change triggered moderate ( m≥4.0) eartquakes in lake van basin, eastern turkey, journal of seismology, 10, 105-117. palabras clave: gradiente gravitacional; modificación de modelos; gradiente gravitacional radial; procesamiento de ruido. how to cite item wan, x., & ran, j. (2018). an alternative method to improve gravity field models by incorporating goce gradient data, colombia. earth sciences research journal, 22(3), 187-193. doi: https://doi.org/10.15446/esrj.v22n3.64666 the aim of this paper is to present an alternative method that can be used to improve existing gravity field models via the application of gradient data from gravity field and ocean circulation explorer (goce). first, the proposed algorithm used to construct the observation equation is presented. then methods for noise processing in both time and space domains aimed at reducing noises are introduced. as an example, the european improved gravity model of the earth by new techniques (eigen5c) is modified with gradient observations over the whole lifetime of the goce, leading to a new gravity field model named as egmgoce (earth gravitational model of goce). the results show that the cumulative geoid difference between egmgoce and egm08 is reduced by 4 centimeters compared with that between eigen5c and earth gravitational model 2008 (egm08) up to 200 degrees. the large geoid differences between egmgoce and eigen5c mainly exist in africa, south america, antarctica and himalaya, which indicates the contribution from goce. compared to the newest goce gravity field model resolved by direct method from european space agency (esa), the cumulative geoid difference is reduced by 7 centimeters up to 200 degrees. abstract keywords: gravity gradients, model modification, radial gravity gradient, noise processing. an alternative method to improve gravity field models by incorporating goce gradient data método alternativo para mejorar los modelos de campo gravitacional al incorporar información del satélite explorador de la circulación oceánica y de gravedad issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.64666 el objetivo de este trabajo es presentar un método alternativo que sea usable para mejorar los modelos de campo gravitacional existentes a través de la aplicación de la información de gradiente ofrecida por el satélite explorador de la circulación oceánica y de gravedad (goce, del inglés gravity field and ocean circulation explorer). primero, se presenta el algoritmo propuesto para la construcción de la ecuación de observación. luego se introducen los métodos para el procesamiento y reducción del ruido tanto en el tiempo como en el espacio. por ejemplo, el modelo europeo mejorado de gravedad a través de técnicas nuevas (eigen5c, del inglés european improved gravity model of the earth by new techniques) se modificó con las observaciones de gradiente del satélite goce, en órbita desde marzo de 2009, lo que condujo a un nuevo modelo gravitacional llamado egmgoce (modelo gravitacional terrestre del goce). los resultados muestran que la diferencia geoidal acumulada entre el modelo egmgoce y el modelo gravitacional terrestre de 2008 (egm08) se reduce en cuatro centímetros a comparación de la diferencia geoidal entre el eigen5c y el egm08, por encima de los 200 grados. las mayores diferencias geoidales entre el egmgoce y el eigen5c se presentan principalmente en áfrica, suramérica, la antártica y el himalaya, lo que muestra la contribución del goce. comparado con el más reciente modelo gravitacional goce resuelto a través del método directo por la agencia espacial europea (esa), la diferencia geoidal se reduce en 7 centímetros por encima de 200 grados. resumen record manuscript received: 07/05/2017 accepted for publication: 25/05/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 187-193 xiaoyun wan1 & jiangjun ran2* 1 school of land science and technology, china university of geosciences (beijing) 2* delft university of technology, stevinweg 1, 2628 cn, delft, the netherlands *corresponding author: jiangjunran@gmail.com g e o d e sy 188 xiaoyun wan & jiangjun ran introduction the gravity field and steady-state ocean circulation explorer (goce) satellite (european space agency, 1999) was launched on march 17th, 2009 and was terminated on november 11th, 2013. by combining the gradient and orbit data from goce, three groups of gravity field models with increasingly high accuracies have been derived by the esa (european space agency) goce level 2 processing facility. indeed, in addition to goce data, a number of other kinds of observations can also be used to derive gravity field models, including observations from altimeter satellites (marchenko, 2003a; flechtner et al., 2006), the challenging minisatellite payload (champ) (reigber et al., 1996) and the gravity recovery and climate experiment (grace) (tapley et al., 2004), as well as some ground observations (marchenko, 2003a; pavlis et al., 2012). the use of such data has enabled a range of institutes to resolve more than 100 gravity field models, released by the international centre for global earth models operated by the german research centre for geosciences. in comparison to other kinds of gravity information, goce data are of higher accuracy in some special frequency bands, and the spatial consistency of the accuracy is superior to that of ground observations (marchenko et al., 2016). nevertheless, deriving an accurate gravity field model using goce gravity gradient observations in combination with other data remains an important issue (gilardoni et al., 2013). one possible approach to this question is to initially construct a series of normal equations based on all the observations (pail et al., 2010) and then resolve them using least square estimation method (lsm). however, as the large quantity of data makes the computational cost of this process prohibitive, an alternative method is to utilize goce gravity gradient data to improve existing gravity field models that have been derived from other kinds of observations. if this approach is adopted, there is no need to process the other (non-goce) observation data over again. in this study, we adopt the latter method in which the existing gravity field models are selected as reference models to be improved. to resolve the gravity field model, two approaches can be used, i.e., lsm and spherical harmonic analysis. currently, the former is more commonly applied for processing goce gradient data for gravity field recovery, for example, the resolving of time-wise solution (pail et al., 2011) and direct solution of the goce gravity field models (bruinsma et al., 2010) by esa, dgm-1s (hashemi et al., 2013), etc. one of the main advantages of the lsm approach is the ease with which a gravity field model can be resolved using gradient and orbit data in combination. considering that the existing gravity field models are improved mainly by gradient data in this study, we adopt spherical harmonic analysis to recover the gravity field model. resolving of a gravity field model via the use of spherical harmonics can be treated as a boundary value problem (bvp) (rummel, et al., 1993), the first step of which is to construct a so-called boundary value condition based on the observation equation. a great deal of research has been conducted in this area. rummel and van gelderen (1992) analyzed the spectral characteristics of different gravity gradient components and generated a series of two-dimensional fourier expressions. van gelderen and rummel (2001) summarized the solution of a general geodetic bvp under gravity gradient boundary conditions. martinec (2003) discussed green function to gradiometric bvp. in addition, invariants of gradient tensors (rummel, 1986, sacerdote & sanso, 1989, vermeer, 1990, marchenko, 2003b, baur et al., 2008) were discussed to recover gravity field models by lsm. holota (1989) and yu and zhao (2010) constructed the boundary conditions using invariants of a gravity gradient tensor. those conditions contain few attitude errors but create linear errors. marchenko et al.(2016) investigated a special version of the space-wise method based on the second degree radial derivatives of goce missions. their results show that the second order derivatives of egg_trf_2 data can improve the middle and long waves of the geo-potential (marchenko et al., 2016). the radial gravity gradients are utilized in this study to construct the boundary condition. it is noteworthy that fuchs et al. (2013) proposed a superior and highly accurate method for vertical gravity gradient computation, which they successfully applied to a discussion of the tokyo earthquake. by this method, bouman et al.(2016) calculated satellite gravity gradient grids for geophysics. meng et al.(2016) proposed an alternative combination of vxx , vyy and vzz to compute a new vzz which then is used for computing the radial gravity gradient. since accurate radial gravity gradients can be obtained, we believe that they can be used as a boundary condition to improve some existing gravity field model and to obtain new ones. this is the starting point of this paper. methods radial gravity gradient computation goce measurements include the vxx , vyy , and vzz components of the gravity gradient tensors (stummer et al., 2012). thus, if no noise is observed, the laplace equation can be applied, as follows: v v vzz xx yy ' ( )=− + (1) this leads us to obtain two vertical gravity gradient values ( i.e., vzz and vzz ' ) that are, however, unequal to one another because of noise. fuchs et al. (2013) addressed this by using mean values of vzz and vzz ' to improve the accuracy of vertical gravity gradients. meng et al. (2016) proposed the following combination to compute new vertical gravity gradients, v v v vzz c zz xx yy, = − +( )1 3 2 3 (2) compared to vzz, noises of vzz c, are reduced about 42.3% (meng et al., 2016). however, vzz c, are the vertical components of gravity gradient tensors in gradiometer reference frame (grf) and not the radial gravity gradients, hence the coordinate transformation is needed. although fuchs et al. (2013) and meng et al. (2016) proposed new methods for computing new vertical gravity gradients, horizontal gravity gradiens (ie., vxx and vyy) are also needed for the data rotation. however, no literature discussed how to compute high accurate vxx and vyy. indeed, according to the laplace equation, we can construct the following equations, v v v v v v xx zz yy yy zz xx ' ' = − = −      (3) this leads us to obtain two x-components (i.e., vxx and vxx ' ) and two y-components ( i.e., vyy and vyy ' ) of a gravity gradient tensor. we provide combinations for these values by solving the following equations: v av b v v a b f a b b ab xx c xx zz yy xx zz yy , min min = + −( ) + = = + + + 1 22 2 2 2 2 2     xx zz xx yy yy zzab b, , , 2 2 2 22 2− −( )      (4) v cv d v v c d f c d d cd yy c yy zz xx yy zz xx , min min = + −( ) + = = + + + 1 22 2 2 2 2 2     yy zz xx yy xx zzcd d, , , 2 2 2 22 2− −( )      (5) in these expressions, a and b are the combination factors used for computing the new x-component of the gravity gradient tensor; c and d are the combination factors used for computing the new y-component of the gravity gradient tensor, while ii i x y z2 =( ), , are the variance of vii noises, and ii jj i x y z, , ,2 =( ) refer to the co-variances between different vii noises . the solutions of these expressions are therefore as follows: a yy zz xx yy xx zz yy zz xx yy zz xx yy = + + − − + + +          2 2 2 2 2 2 2 2 2 2 , , , , 22 2 2 2 2 2 2 2 2 2 2 2 − − = + − + + +         xx zz yy zz xx xx yy xx zz xx yy zz b , , , ,   xx yy xx zz yy zz, , , 2 2 22 2− −      (6) 189an alternative method to improve gravity field models by incorporating goce gradient data c xx zz xx yy yy zz xx zz xx yy zz xx yy = + + − − + + +          2 2 2 2 2 2 2 2 2 2 , , , , 22 2 2 2 2 2 2 2 2 2 2 2 − − = + − + + +         xx zz yy zz yy xx yy yy zz xx yy zz d , , , ,   xx yy xx zz yy zz, , , 2 2 22 2− −      (7) thus, as long as ii jj i x y z, , ,2 =( ) are known, it is possible to obtain optimal factors from equation (6) and (7). indeed, if the components of the gradient tensor are independent, equation (6) and (7) can be simplified as follows: a b yy zz xx yy zz xx xx yy zz = + + + = + +               2 2 2 2 2 2 2 2 2 (8) c d xx zz xx yy zz yy xx yy zz = + + + = + +               2 2 2 2 2 2 2 2 2 (9) the error distribution of the goce gradient tensor applied by fuchs et al. (2013) is used here, as follows: δ δ δ δ σzz xz xx yy= = = =2 2 2 0 (10) solutions for a and b , c and d can therefore be derived from equation (8) and equation (9), as follows: a b = =      5 6 1 6 (11) c d = =      5 6 1 6 (12) thus, v v v v v v v v xx c xx zz yy yy c yy zz xx , , = + −( ) = + −( )      5 6 1 6 5 6 1 6 (13) hence,  vxx c, .» 0 91 0 and  vyy c, .» 0 91 0. the noises of horizontal gravity gradients are reduced by about 8.9%. in this paper, besides vxx c, and vyy c, expressed by equation (13), vzz c, expressed by equation (2) are used to compute the radial gravity gradients by the following coordinate transformation, egg r v u v u v u v u v lonf t xx c xy xz xy yy c yz xz yz zz c g =               , , , rrf r (14) where r denotes the transformation matrix from the grf coordinate system to the local north oriented frame (lnof) (gruber et al., 2010) coordinate system. in lnof the z direction is same as the radial direction of the earth. it should be noted that, owing to the poor accuracy of vxy and vyz , we replace them with uxy and uyz respectively, that were calculated using the reference model (i.e., the model to be modified). since accuracies vxx c, , vyy c, , and vzz c, are higher than those of vxx, vyy, and vzz, this means that the accuracy of vrr can be improved via the use of vxx c, , vyy c, , and vzz c, . residual gradient data were used in this study for data rotation as follows: eggt r t t t t t t t t t lonf t xx c xy xz xy yy c yz xz yz zz c =               , , , ggrf r (15) where t t t txx c xx zz yy, = + − 5 6 1 6 1 6 , t t t txx c yy zz xx, = + − 5 6 1 6 1 6 , t t t tzz c zz yy xx, = − − 1 3 2 3 2 3 t t t tzz c zz yy xx, = − − 1 3 2 3 2 3 ; t t txx yy zz, , and txz denote the respective differences between v v v vxx yy zz xz, , , v v v vxx yy zz xz, , , and u u u uxx yy zz xz, , , which are obtained from the reference model. txy and txz are both equal to zero because vxy and vyz are both replaced by uxy and uyz, respectively. the use of this rotation approach enables us to obtain trr . the variable trr, obtained from equation (15), was then used to construct the following boundary value problem ( wan & yu, 2013), as follows: ∆ ο t t r f t r s = ∂ ∂ = =      − 0 2 2 1( ), at infinity (16) where f denotes trr, and s refers to the mean surface of goce orbits. thus, the solution of the boundary value problem (16) provides the spherical harmonic coefficients of t at satellite altitude. by solving this bvp, we can obtain solutions of ( , )d dc snm nm , as follows: ∆ θ λ σc n n r gm r a t p mnm n rr nm= + +       ⋅ 1 4 1 2 3 π σ( )( ) (cos )cos d∫∫∫ = + +       ⋅∆ θ λs n n r gm r a t p mnm n rr nm 1 4 1 2 3 π( )( ) (cos )sin ddσ σ∫∫      (17) where gm denotes the gravitational constant of the earth, a is the semimajor axis of the earth, r, ,θ λ( ) denote the spherical coordinates, and pnm cos( ) is the fully normalized associated legendre functions with degree n and order m. the final solutions of the gravity field model are as follows: c c c s s s nm nm nm nm nm nm = =     , , 0 0 +∆ +∆ (18) where ( )c snm nm, ,,0 0 denote the coefficients of the initial model, i.e., eigen5c (förste et al., 2008) applied in this study. ( )c snm nm, are the coefficients of the final model. noise processing of goce gradient data before using goce gradient observations to recover a gravity field model, the observed noise should be processed first (schuh, 2003; reguzzoni & tselfes, 2009; bruinsma et al., 2010; pail et al., 2011; gatti et al., 2014). three types of noise should be considered. the first one is low frequency noise. as implied by the name, this type of noise is located in the low frequency band (i.e., lower than 0.005 hz in this study) and constitutes the main type of noise in goce gradient data. this low frequency noise is large in magnitude, and within a known frequency band. the second type of noise is random noise. this noise is smaller in magnitude and is distributed throughout the full frequency band. the combinations presented in equations (2) and (13) are used to reduce this random noise. hence, the techniques of white noise processing are not discussed here. the third one is gross error. their magnitude is very large and its distribution is scattered. gross error is found in all forms of observational data used in the gravity field recovery, including gradient, orbit, and attitude data. gross error can also be created by a kind of computing, e.g., the warm effect of filtering. 190 xiaoyun wan & jiangjun ran a. time domain processing forward and backward finite impulse response (fir) filtering was initially applied to the residual gravity gradient data, which are the difference values of the actual goce gradient data and the simulated gradient data from the reference gravity field model (i.e., the reference model to be modified). as noted, eigen5c model was selected here as the reference model, which was derived by gfz and cnes ( national centre for space studies) using five years of grace data as well as some ground observations. we only selected 0-300 degrees / orders of eigen5c as the reference model, considering the limits of measurement band wise (mbw) and the accuracy of the gradient observations. to show the filtering effect, we processed the gradient data provided by egg_nom2 from esa over the short time interval between 0:0:1.6 on october 17th , 2010 and 23:59:59.6 on october 26th , 2010, using a band-pass fir filter. the pass band frequency is 0.005~0.1 hz. fig. 1 shows the power spectral density (psd) of the data after filtering. it is clearly that signals in the pass band remained unchanged by this filtering processing, while those beyond the pass band were largely removed and only occupy a very small proportion in the final data obtained from the filtering. this filtering step therefore enabled us to remove low frequency noise while saving the signals during the pass band. it is also noteworthy that no phase drift occurred (e.g., yu & wan, 2011); thus, the data after filtering can still be considered as gravity gradient signals. in order to verify the effects of fir processing, we perform a statistical analysis to the absolute values of laplace equation. table 1 gives the results. smaller values of this statistic indicate a higher accuracy; thus, according to table 1, the accuracy was improved by a factor of 159172 by fir filtering in terms of the mean value. table 1. laplace equation statistics raw data fir number 863999 858500 min(me) 789524.4 2.036e-05 max(me) 800881.4 957.125 mean (me) 795860.75 5.617 std (me) 2128.44 5.334 b. space domain processing although the dominant noises have been removed in the time domain via fir filtering, we still need to remove the residual gross errors. these errors were processed in the space domain. when the radial gradients of the disturbing gravity were obtained using equation (15), we were able to perform grid processing on the satellite orbit surface. we first divided the surface into girds, such as 20’ × 20’ and then conducted statistical processing. mean values, variances, and standard errors of the data in each grid were then obtained, and we removed the data that deviated from the mean value by more than twice the standard deviation. this processing led to the removal of approximately 3.84% of the data. processing was carried out in the space domain because gross errors are sometimes stable over short time periods, perhaps because of an abnormal satellite situation, making these errors difficult to remove in the time domain. however, even though the data in the same grid are from different points in time, their values should be the same or close to each other. if some abnormal values exist, there must be gross errors that can be isolated. results and discussion all the gradient data obtained by goce throughout the lifetime of the project are processed in this paper. as described above, we first processed the noises in both time and space domains. radial disturbing gravity gradients were then calculated using equation (15). in this section, the eigen5c is taken as an example to be improved with the proposed method. the final solution is derived by integration based on the orthogonality of spherical harmonic functions given by equation (16) and equation (17). the new model is named as the earth gravitational model of gravity field and ocean circulation explorer (egmgoce). comparisons in spherical domain the degree variances of egmgoce and egm08 (pavlis et al., 2012), are compared in fig. 2 and the geoid heights are compared in fig. 3. in fig. 2, variance_eigen5c denotes the degree variance differences between egm08 and eigen5c, and variance_egmgoce denotes the differences between egm08 and egmgoce. in fig. 3, eigen5c-egm08 and egmgoce-egm08 indicate the cumulative geoid differences between figure 1. comparison of psd before and after fir filtering 191an alternative method to improve gravity field models by incorporating goce gradient data eigen5c, egmgoce and egm08 respectively. these two figures illustrate that the initial model was mainly improved in the degrees from 100 to 200. if egm08 is considered as the true model, the geoid accuracy of the new model is higher than that of the initial model by 4 cm up to 200 degree. for a comparison with goce models, egmd5 from esa is selected, which is the newest goce gravity field model derived by the direct method. the degree variance differences and cumulative geoid differences between egmgoce, eigen5c and egmd5 are shown in fig. 4 and fig. 5, respectively. the new model is clearly much closer to egmd5 than eigen5c, particularly at degrees from 80 to 230. if egmd5 is considered as the true model, the geoid accuracy of the new model is higher than that of the initial model by 7 cm up to 200 degree. these results indicated that the extraction of the goce gravity gradient signals and modification of eigen5c were successful. comparisons in space domain the differences between egmgoce and eigen5c can be seen as the contribution from goce gradient observation. in order to present goce contribution in space domain, we selected 0-200 degrees / orders of eigen5c, egmgoce, egm08 and egmd5 to compute geoid values at mean surface of the earth. the data were calculated at grids of 1˚×1˚. statistics of their differences are given in table 2 and table 3. according to table 2, if egm08 is the true model, the accuracy of eigen5c is improved by 4.6 cm because of the use of goce gradient data. according to table 3, if egmd5 is the true model, the accuracy of eigen5c is improved by 7 cm. in order to show the differences of the geoids between eigen5c and egmgoce, fig. 6 and fig. 7 show the geoids calculated by eigen5c and egm08 respectively, and fig. 8 shows their differences. table 4 gives the statistics of absolute values of the differences. according to this table, mean value of the difference is about 5 cm and the standard deviation is about 9 cm. according to fig. 8, large differences mainly occur in africa, south america, antarctica and himalaya (yi, 2001; rummel & gruber, 2012). this indicates that the data used to derive eigen5c has a little poorer accuracy in these areas, since accuracy of goce gradient data has a high space consistence, so that accuracy of eigen5c can be improved by goce gradient observations. conclusion this paper discusses the method for modifying existing gravity field models using goce gradient observations. we used combinations of vxx, vyy and vzz to compute new horizontal and vertical gravity gradients, which can reduce white noises of them by 8.9%, 8.9% and 42.3% respectively. compared with the conventional method, accuracies of the radial gravity gradients can be improved correspondingly. strategies for processing noise within goce gradient observation data in both time and space domains are also discussed in this study. band pass fir filtering is adopted in the time domain and grid processing based on the statistical properties of each grid is implemented in the space domain. these strategies enable the removal of low frequency noises and gross errors. the combinations given by equations (2) and (13) can reduce white noises. the eigen5c model was improved as an example in this study using all available goce gradient data, and a new gravity field model named as egmgoce was derived. compared with egm08, modifications using goce gradient data are mainly reflected in the degrees from 100 to 200. up to the 200th degree, the cumulative geoid difference is reduced by 4cm. compared with egmd5, modifications from goce data are mainly reflected in degrees from 80 to 230, and up to the 200th degree, the cumulative geoid difference is reduced by 7 cm. these results showed that the goce gradient observations can contribute meaningfully to the development of egmgoce model and demonstrate the effectiveness of the methods adopted in this paper. other gravity field models can also be processed similarly to derive new models. figure 2. differences in the degree of variance between eigen5c, egmgoce and egm08 figure 3 cumulative geoid height differences. red line: differences between eigen5c and egm08; green line: differences between egmgoce and egm08. figure 4. differences in the degree of variance between eigen5c, egmgoce and egmd5 192 xiaoyun wan & jiangjun ran acknowledgments we thank the esa for providing goce data. we also thank the reviewers for their helpful suggestions. this paper is funded by the national natural science foundation of china (nos. 41674026,41404019) and the open fund of key laboratory of space utilization, chinese academy of sciences (csu-wx-a-kj-2016-044). table 2. statistical geoid differences between eigen5c, egmgoce and egm08 eigen5c-08 egmgoce-08 num 64800 64800 sum(m) 72.4087 122.1064 minimum(m) -3.9080 -2.3716 maximum(m) 3.2736 3.0994 mean(m) 0.0011 0.0019 std(m) 0.2000 0.1538 table 3. statistical geoid differences between eigen5c, egmgoce and egmd5 eigen5c-d5 egmgoce-d5 num 64800 64800 sum(m) -906.3277 -856.6300 minimum(m) -3.6455 -1.7668 maximum(m) 3.8129 2.0641 mean(m) -0.0140 -0.0132 std(m) 0.2272 0.1570 table 4. statistical absolute values of geoid differences between eigen5c and egmgoce num min (m) max (m) mean (m) std (m) 64800 4.40e-07 2.2011 0.0523 0.0911 figure 5. cumulative geoid height differences. red line: differences between eigen5c and egmd5; green line: differences between egmgoce and egmd5. figure 6. geoid calculated by 0-200 degrees and orders of eigen5c figure 7. geoid calculated by 0-200 degrees and orders of egmgoce figure 8. geoid differences between eigen5c and egmgoce since the radial gravity gradients as well as the horizontal components of gravity gradients can be derived with high accuracy, they could not only be utilized to recover gravity field models, but also be used to extract and interpret geophysical signals directly, such as moho geometry (ye et al., 2016 ), earthquake (fuchs, et al., 2013), etc. hence, the methods proposed in this paper can also be powerful tools for researches in geophysics. 193an alternative method to improve gravity field models by incorporating goce gradient data references 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(2010). the gravitational gradient tensor’s invariants and the related boundary conditions. science in china series d-earth sciences 53(5), 781–790. articulo 3c.indd earth sciences research journal seismology earth sci. res. sj. vol. 16, no. 2 (december, 2012): 103 108 introduction since fractal description was developed for the geometry of natural objects (mandelbrot b.b., 1982), it has been recognised that many complex space-time phenomena, such as seismicity, may be described and interpreted in terms of fractal distributions with power-law scaling (e.g. hirata t., 1989; öncel a.o. et al., 1995; caneva a., smirnov v., 2004; öncel a.o., wilson t.h., 2007; roy s. et al., 2011). however, studies of possible correlation between seismicity and fault distribution are limited. applying seismic hazard assessment to fractal association between seismotectonic variables requires distinguishing normal and anomalous correlations between such data sets’ fractal attributes. since the statistical behaviour of seismicity may potentially represent sensitive short-term predictors of major earthquakes, this study’s main aim was to find an empirical relationship between seismic b-values and the fractal distribution of epicentres for turkish earthquakes. data regarding earthquakes and tectonic zoning several earthquake catalogues are available for turkey from both national and international sources. earthquakes occurring between 1970 and 2006 were taken from öztürk’s catalogue of instrumental data statistical correlation between b-value and fractal dimension regarding turkish epicentre distribution serkan öztürk gümüşhane university. turkey. e-mail: serkanozturk@gumushane.edu.tr abstract this study was aimed at analysing the relationship between seismic b-value and fractal dimension dcvalue for turkish epicentres. the earthquake catalogue consisting of 99,737 instrumentally-registered turkish events was analysed for the period between 1970 and 2011; turkey was divided into 55 tectonic zones for making a detailed comparison. the b-values were calculated by the maximum likelihood method and the dc-values were obtained with 95% confidence limits by linear regression. the results showed that higher dc-values were associated with lower b-values and this could have been an indication of relatively high stress intensity and stronger epicentre clustering in these regions. orthogonal regression was used to estimate a suitable statistical correlation between two seismotectonic parameters; the dc = 2.44 0.30*b relationship was obtained with a strong negative correlation (r = -0.82) between b-value and dc-value for turkish earthquake distribution. this seemed to agree with other regional results obtained for different parts of turkey and the rest of the world. resumen este estudio tuvo como objetivo analizar la relación entre el valor b y la dimensión del valor fractal dc para los epicentros de turquia. el catálogo sísmico que consiste en 99.737 eventos registrados instrumentalmente fue analizado para el período comprendido entre 1970 y 2011; turquía se dividió en 55 zonas tectónicas para hacer una comparación detallada. los valores de b se calcularon por el método de máxima verosimilitud y la dc-valores se obtuvieron con límites de confianza del 95% mediante regresión lineal. los resultados mostraron que el aumento del valor fractal dc se asocia con menores valores b y esto podría haber sido un indicador de intensidad de tensiones relativamente altas y una más fuerte agrupación de epicentro en estas regiones. se utilizó una regresión ortogonal para estimar la correlación estadística adecuada entre dos parámetros sismotectónicos la relación de la dimensión fractal dc = 2,44 a 0,30*b, se obtuvo con una fuerte correlación negativa (r = -0,82) entre valores b y valores fractales dc para la distribución de terremotos en turquía. esto parece estar de acuerdo con otros resultados regionales obtenidos para las diferentes partes de turquía y el resto del mundo. palabras claves: terremoto de turquía, dimensión fractal, valor-b, correlación. keywords: turkish earthquake, fractal dimension, b-value, correlation. record manuscript received: 13/02/2012 accepted for publications: 06/05/2012 serkan öztürk104 (2009), including duration magnitude (md) for 73,530 earthquakes occurring in turkey between 1970 and 2006; öztürk used empirical relationships to compile a homogenous and complete earthquake catalogue (1970 to 2006). the bogazici university’s kandilli observatory and research institute (koeri) catalogue was also used for 2006 to 2011; koeri usually gives local magnitudes (ml) for local earthquakes having missing md. if an md was unknown in the koeri catalogue for 2006 to 2011, then md were calculated using the relationships given in öztürk (2009). 26,207 earthquakes in and around turkey were thus selected for the aforementioned period. this earthquake catalogue was homogeneous for md and contained 99,737 earthquakes occurring between 1970 and 2011. many authors have suggested tectonic zoning as a widely-used methodology for evaluating the hazard of an earthquake occurring (e.g. erdik m. et al., 1999; jiménez m. et al., 2001; bayrak y. et al., 2009). the present study’s new seismic source zones for turkey and its adjacent areas were based on tectonic zoning studies by erdik (1999) and bayrak (2009), the existing tectonic structure and earthquake epicentre distribution (figure 1). turkey was divided into 55 different zones, as shown in figure 1. new smaller zones were selected to compare such tectonic zoning in detail, in the same regions. figure 1 shows turkey’s active tectonics and many details regarding turkey’s tectonic structure can be found in şaroğlu (1992), mcclusky (2000) and bozkurt (2001). the numbers of earthquakes occurring in each zone were sufficient for analysis; table 1 shows the seismic zones in this study (numbered 1 to 55) with their tectonic environments. earthquakes’ frequency-magnitude distribution (seismic b-value) earthquakes’ magnitude distribution (md) is usually parameterised using gutenberg-richter’s (g-r) power law relationship (gutenberg b., richter c.f., 1944); such frequency-magnitude relationship has been found to be applicable (in simplified form) as follows: log10 n(m) = a – bm (1) where n(m) is the cumulative number of events having a magnitude greater than m, b describes the slope of the size distribution of events and a is proportional to the productivity of a volume, or the seismicity rate. the b-value is one of the most important statistical parameters for describing the size scaling properties of seismicity; b-values change figure 1. tectonic areas and active fault systems in turkey. tectonic structures have been modified from şaroğlu f. et al., (1992) and bozkurt e. (2001). roughly in the range of 0.3 to 2.0, depending on the different regions. however, average regional scale estimates of b-value are usually equal to 1 (frohlich c., davis s., 1993). many factors can cause perturbation of average b-value (b~1.0); regions having lower b-values are probably regions subjected to higher applied shear stress after the main shock, whereas regions having higher b-values are areas which have experienced slip. high b-values are reported from areas having increased geological complexity, indicating the importance of multi-fracture areas; a low bvalue is thus related to a low degree of heterogeneity of cracked medium, large stress and strain, high deformation speed and large faults (bayrak y., öztürk s., 2004). many methods can be used for calculating any region’s b-value but the maximum likelihood method is the most robust and widely-accepted method for estimating b-values (aki k., 1965): b = 2.303 /(m — – mmin + 0.005) (2) where m — is average magnitude value and mmin is minimum magnitude value. 0.05 in equation (2) is a correction constant. if b=1 and mmin=1 is used, maverage=3.25 will be obtained. however, this is an extreme mmin value. mmin=1 did not occur in previous turkish earthquake catalogues; mmin in turkish earthquake catalogues was around 3.0 until the beginning of the 2000s. many stations have been built in turkey, especially after two destructive earthquakes in 1999, minimum earthquake magnitude now being seen to be around 1.5 (average value may sometimes be higher than 3.25 which is not high value for turkish earthquakes). the standard deviation of seismic b-value (95% confidence limit) may be determined using the equation suggested by aki (1965) as  1.96b / √n, where n is the number of earthquakes used to make the estimate. this would yield ±0.1–0.2 confidence limits regarding b-value for a typical sample consisting of n=100 earthquakes. such sample consisting of n=100 events represents a specific calculation. the number of earthquakes in this study was 97,737; table 1 shows that sometimes 110 earthquakes were used (in region 13) or 10,328 earthquakes on other occasions (in region 34). this value for n=100 earthquakes is thus a typical sample for the aforementioned calculation. fractal dimension (correlation dimension, dc) of epicentres’ spatial distribution earthquake distribution spatial patterns and temporal patterns of occurrence were demonstrated to be fractal using a two-point correlation dimension (dc). analysing correlation dimension is a powerful tool for quantifying a geometrical object’s self-similarity. grassberger and procaccia (1983) defined dc and correlation sum c(r) as follows: dc = lim[logc(r)/logr] (3) r  0 c(r) = 2nr<r / n(n – 1) (4) where c(r) is the correlation function, r is the distance between two epicentres and n is the number of pairs of events separated by distance r<r. if the epicentre distribution has a fractal structure, the following relationship would be obtained: c(r)  rdc (5) where dc is the fractal dimension (more strictly, the correlation dimension). distance r between two earthquakes would be calculated (in degrees) from: r = cos–1(cosi cosj + sini sinj cos (fi – fj)) (6) statistical correlation between b-value and fractal dimension regarding turkish epicentre distribution 105 table 1. seismotectonic parameters b-value and dc-value, as well as the number of earthquakes occurring in entire seismic areas of turkey. region tectonics earthquake number b-value dc-value 1 north east anatolian fault zone (neafz) (horosan fault, dumlu and çobandede fault zones) 338 0.62±0.03 2.26±0.09 2 1234 1.05±0.03 2.08±0.07 3 ağrı, tutak, balıklıgöl and kağızman faults (atbkf) 219 0.86±0.06 2.30±0.04 4 iğdır, doğubeyazıt and çaldıran faults (idçf) 268 0.63±0.03 2.23±0.02 5 başkale, erciş, muradiye and süphan faults (bemsf) and hasan timur fault zone (htfz) 159 0.98±0.09 2.09±0.03 6 350 0.95±0.05 2.12±0.03 7 malazgirt and bulanık faults (mbf) 320 1.13±0.07 2.08±0.04 8 bitlis-zagros thrust zone (bztz) (kavakbaşı fault, muş thrust zone and yüksekova-şemdinli fault zone) 442 1.05±0.06 2.08±0.04 9 238 0.77±0.05 2.28±0.01 10 116 1.21±0.10 2.03±0.01 11 254 0.63±0.04 2.28±0.05 12 251 0.83±0.06 2.25±0.05 13 karacadağ extension zone (kez) 110 1.17±0.04 2.00±0.03 14 510 1.49±0.08 1.94±0.04 15 east anatolian fault zone (eafz) 2272 0.96±0.02 2.11±0.06 16 junction of a part of dead sea fault and eafz 298 0.84±0.05 2.31±0.06 17 1118 0.93±0.04 2.12±0.03 18 northern part of cyprus 179 1.61±0.05 2.00±0.03 19 southern part of cyprus, including eastern part of cyprus arc 369 1.20±0.07 2.07±0.02 20 340 1.28±0.05 1.99±0.04 21 western part of cyprus arc 350 1.05±0.07 2.10±0.03 22 1161 1.15±0.05 2.10±0.02 23 acıgöl, dinar and çivril faults (adçf) 2154 1.36±0.09 2.06±0.04 24 muğla and rhodes region 1534 1.64±0.09 1.94±0.04 25 5392 1.38±0.09 1.97±0.03 26 aegean arc 1914 1.38±0.09 2.21±0.03 27 burdur fault zone (bfz) 1787 1.01±0.02 2.15±0.05 28 büyük and küçük menderes grabens (bkmg) 3428 1.24±0.08 2.02±0.04 29 1408 1.34±0.07 2.08±0.04 30 aliağa and dumlupınar faults (adf) and zeytindağ-bergama faults (zbf) 926 1.30±0.07 2.08±0.04 31 3770 1.13±0.02 2.10±0.07 32 alaşehir and gediz grabens (agg) 1782 1.24±0.06 2.03±0.02 33 sultandağı, beyşehir and tatarlı faults (sbtf) 3252 1.35±0.05 2.05±0.04 34 kütahya and simav grabens (ksg) 10328 1.15±0.01 2.08±0.05 35 8036 1.62±0.04 2.03±0.05 36 soma and bakırçay grabens (sbg) 6422 1.93±0.05 1.95±0.07 37 1710 1.65±0.08 2.04±0.05 38 1168 1.18±0.04 2.06±0.05 39 eskişehir, i̇nönü-dodurga and kaymaz faults (ei̇dkf) 2527 1.41±0.03 1.98±0.05 40 manyas and ulubat faults (muf) 1432 1.44±0.03 2.04±0.04 41 yenice-gönen and sarıköy faults (ygsf) 4220 1.41±0.04 2.03±0.04 42 etili fault (ef) 1398 1.65±0.07 1.92±0.04 serkan öztürk106 where (i,fi) and (j,fj) are the latitudes and longitudes of the ith and jth events, respectively (hirata t., 1989). by plotting c(r) against r on a double logarithmic coordinate, fractal dimension dc would be obtained from the slope of the graph. fractal analysis is often used to quantify size scaling attributes and seismotectonic variables’ clustering properties. fractal dimension dc can be calculated to determine possible unbroken sites and such unbroken sites have been suggested as being potential seismic gaps to be broken in the future. variations in fractal correlation dimension mainly depend on the complexity or quantitative measurement of the degree of heterogeneity of seismic activity in a fault system. in areas of increased complexity in an active fault system (higher dc) associated with lower b-value, stress release occurs on fault planes having smaller surface area (öncel a.o., wilson t.h., 2002). results and discussion an investigation of seismotectonic parameters in and around turkey involved using the b-value of the g-r relationship (equation 1) and fractal correlation dimension dc-value of turkish earthquake distribution from 1970 to 2011; turkey was divided into 55 seismic source zones in an attempt to find an empirical relationship between b-value and dc-value. zmap software was used for calculating b and dc-values for each region (wiemer s., 2001). the b-values were calculated by using the maximum likelihood method (95% confidence limit) because it yields a more robust estimate than the least-square regression method (aki k., 1965). the dcvalues for all turkish areas were obtained with 95% confidence limits by linear regression. table 1 shows the results for two seismotectonic parameters with their standard deviations as well as the number of earthquakes for entire tectonic areas. the first seismic parameter b-value ranged from 0.62 to 1.93 for 55 turkish areas. b-values smaller than 0.8 were found in regions 1, 4, 9 and 11, including the north-eastern anatolian fault zone (neafz) including the dumlu and çobandede fault zones, iğdır, doğubeyazıt and çaldıran faults (idçf) and two parts of the bitlis-zagros thrust zone (bztz). such low values may have depended on the position of these regions neighbouring the easternmost part of the north anatolian fault and the bztz. the conjugate strike-slip fault system related to these areas dominates eastern anatolia’s active tectonics and thrust faults can produce destructive earthquakes in the bitlis thrust which forms a complex continent-continent and continent-ocean collision boundary (bozkurt e, 2001). these systems generate major earthquakes, such as patnos in 1903 (ms=6.3), pasinler in 1924 (ms=6.8), lice-diyarbakır in 1975 (ms=6.6), çaldıran in 1976 (ms=7.5) and horasan in 1983 (ms=6.8). global positioning system (gps) data gives 10±2 mm/yr for total shortening between the strike slip faults in eastern turkey and thrusting along the caucasus. the bztz’s north-western motion is 18±2 mm/yr and such motion is related to that of eurasia (mcclusky s. et al., 2000); small b-values in these regions are thus related to a low degree of heterogeneity and large faults resulting in strong earthquakes. the other small b-values changing between 0.8 and 1.0 were observed in regions 3, 5, 6, 12, 15, 16, 17 and 50 covering the ağrı, tutak, balıklıgöl and kağızman faults (atbkf), the başkale, erciş, muradiye and süphan faults (bemsf) and the hasan timur fault zone (htfz), the western part of bztz, the east anatolian fault zone (eafz), the junction of a part of the dead sea fault and eafz, the eastern part of the north anatolian fault zone (enafz) including the bingöl-karakoçan and sancakuzunpınar fault zones. table 1 shows that regions 15, 16 and 17 are connected with the east eafz which is not as seismically active as the nafz. unlike nafz, the eafz in region 15 has been relatively quiescent in the instrumental period compared to the historical period (mcclusky s. et al., 2000). the data used in this study only included the instrumental period for earthquakes occurring from 1970 to 2011. according to the fault slip rate and observed seismicity during recent years, relatively larger b-values were observed in these regions than in regions 1, 4, 9 and 11. according to scholz (1968), low b-values indicate that the state of stress is high. special interest should thus be given to whole regions where low b-values have been observed, especially after the occurrence of the mw=6.0 elazığ earthquake (in region 15) on 8th march 2010 and the mw=7.2 van lake earthquake (in region 6) on 23rd september 2011. regions such as 2, 8, 21, 27, 46, 48, 49 and 52 have almost the similar b-values (very close to 1.0); these regions are related to the neafz, the eastern part of bztz, the western part of the cyprus arc, the burdur fault zone (bfz), the düzce fault (df), the yağmurlu-ezinepazarı fault zone (yefz) the another part of the enafz and sürgü fault. the maximum b-value was estimated as being 1.93 in region 36, such region lying around the soma and bakırçay grabens. other largest b-values greater than 1.5 were calculated in regions 18, 24, 35, 37 and 42; these zones were covered by the northern part of cyprus, the muğla and rhodes region, the kütahya and simav grabens (ksg) and the etili fault (ef). the b-values obtained for the rest of the zones were between 1.1 and 1.5. the nafz is a very active structure and, according to geodesy, accommodates 24±2 mm/yr of dextral motion (mcclusky s. et al., 2000). two large earthquakes occurred in regions 44 and 46 during august and november 1999 and, of course, the stress level in these region tectonics earthquake number b-value dc-value 43 marmara part of north anatolian fault zone (mnafz) 1378 1.10±0.07 2.10±0.06 44 7859 1.26±0.02 2.05±0.03 45 1430 1.35±0.07 2.05±0.03 46 düzce fault (df) 2011 1.02±0.03 2.14±0.05 47 ismetpaşa segment (is) 1684 1.38±0.09 2.04±0.05 48 yağmurlu-ezinepazarı fault zone (yefz) 1369 1.04±0.03 2.10±0.04 49 eastern part of north anatolian fault zone (enafz) (bingöl-karakoçan, sancak-uzunpınar fault zones) 794 1.03±0.05 2.13±0.03 50 2236 0.87±0.07 2.28±0.06 51 ovacık and malatya fault zone (omfz) 1340 1.20±0.04 2.05±0.03 52 sürgü fault (sf) 577 1.06±0.05 2.06±0.03 53 central anatolian fault zone (cafz) (yakapınar-göksun and yıldızeli fault zones ) 659 1.27±0.05 1.99±0.03 54 738 1.25±0.04 2.02±0.03 55 tuzgölü fault zone (tfz) 2178 1.19±0.03 2.03±0.06 statistical correlation between b-value and fractal dimension regarding turkish epicentre distribution 107 regions would thus not have been so high in recent years. the regions characterised by large b-values had a greater proportion of low magnitude earthquakes whereas regions having low b-values represented areas in which large magnitude earthquakes occurred. b-values estimated using the maximum likelihood approach for the g-r method seemed to have a good relationship to the tectonics and level of seismicity. the second seismotectonic parameter dc-value ranged from 1.92-2.31 for the 55 tectonic source regions in turkey and the surrounding areas. dcvalues smaller than 2.0 were estimated in regions 14, 20, 24, 25, 36, 39, 42 and 53. these zones were related to the karacadağ extension zone (kez), the southern part of cyprus including the eastern part of the cyprus arc, the muğla and rhodes region, the sbg, the eskişehir, i̇nönü-dodurga and kaymaz faults (ei̇dkf), the ef, the central anatolian fault zone (cafz) including the yıldızeli fault zone. dc-values greater than 2.2 were observed in regions 1, 3, 4, 9, 11, 12, 16, 26 and 50 which were related to the neafz including the dumlu and çobandede fault zones, the atbkf, the idçf, the bztz, the junction of part of the dead sea fault and the eafz, the aegean arc and the enafz. dc-values ranged from 2.0 to 2.2 in the rest of the seismic source areas. the results obtained from analysis of data along the nafz (regions 43-49) suggested that epicentre distribution became less clustered (higher dc) as the probability of larger magnitude earthquakes became smaller (larger b-value). this implied greater fracture toughness in the central portions of the nafz. as stated in öncel and wilson (2002), stress becomes released on fault planes having smaller surface area in regions of increased complexity in an active fault system (higher dc) associated with lower b-values. this means that areas in a fault network characterised by higher dc-values would be associated with greater complexity in a fault pattern and the persistence of such complexity on smaller scales. higher order fractal dimension (especially greater than 2.0) is increasingly sensitive to heterogeneity in magnitude distribution, suggesting that seismicity is more clustered at larger scales (or in smaller areas) in these fault zones. it may also be related to the number of events to a certain extent. since the uniform distribution of earthquakes decreases with an increase in the clustering of events, it is reasonable to assume that higher dc values (≥2.2) and lower bvalues (0.8≤) are the dominant structural feature in the study area and may have arisen due to clusters. correlation between b and dc-value this study was aimed at determining a statistical relationship between seismic b-values and fractal dimension dc-value for turkish epicentres. the orthogonal regression (or) method (e.g, carroll r.j., ruppert d., 1996) was used to find the most suitable correlation between both seismotectonic parameters selected here. as the standard least squares method figure 2. dc-value compared to b-value for turkish earthquakes occurring between 1970 and 2011. the statistical relationship (with its coefficient of correlation) is also given is based on the assumption that horizontal axis values are estimated without error (carroll r.j., ruppert d., 1996), or was applied in fitting the relationship. figure 2 gives a graphical representations of or fit between b and dc-values for turkish earthquake epicentres. figure 2 shows that the correlation coefficient of regression fit was very strong (r=-0.82). linear fit was used for regression and the following equation was derived: dc = 2.44 – 0.30*b (7) the correlation between fractal dimension and b-value has been studied in several parts of the world (e.g. aki k., 1981; hirata t., 1989; henderson j. et al., 1992; caneva a., smirnov v., 2004; roy s. et al., 2011) and particularly the turkish region (e.g. öncel a.o. et al., 1995 and 1996; öncel a.o., wilson t.h., 2002 and 2007). since aki (1981) proposed a simple relationship between b-value and fractal dimension having d=2b positive co-relationship, both positive (e.g. öncel a.o., wilson t.h., 2004; roy s. et al., 2011) and negative co-relationships (e.g. hirata t., 1989; henderson j. et al., 1992; öncel a.o. et al., 1995 and 1996) between these two scaling exponents have been reported and debated in the pertinent literature. such co-relationships could even change from negative to positive (e.g. öncel a.o., wilson t.h., 2002 and 2007). hirata’s results (1989) did not support aki’s assumption that d=2b but showed, on the contrary, a negative co-relationship dc=2.3-0.73*b (r=-0.77) between b and fractal dimension of epicentres in the tohoku region of japan. henderson (1992) obtained a similar result for the riverside catalogue in southern california. similarly, a study of seismicity in the nafz, turkey, revealed a long-term negative co-relationship between b and dc (öncel a.o. et al., 1995). the b-value was found to be weakly negatively correlated with fractal dimension dc=2.74-1.52*b (r=-0.56) for the nafz (including the northern aegean sea) by öncel (1995). öncel (1996) has also observed a strong negative correlation (r=0.85) between dc and b-values as dc=2.32-1.09*b was associated with the nafz. by contrast, öncel and wilson (2002) found weak positive correlation (r=0.48) between variations in b and dc in the western nafz. however, variability between b and dc along the length of the fault zone in this study revealed divergence between b and dc in the central nafz and spatial variation yielded a strong negative correlation (r=-0.85) between b and dc. analysis presented in öncel and wilson (2004) revealed a strong positive correlation (r=0.81) between dc and b-values along the nafz preceding the 1999 izmit earthquake. öncel and wilson (2007) observed a strong positive co-relationship between dc and b-value during 1992-1994 (r=0.84) and 1996-1998 (r=0.94) and negative correlation (r=-0.71) extending from mid-1994 to mid-1996 in north-western turkey. table 2 gives previous studies on the b-value and dc-value relationship. reference relation aki k., (1981) d=2*b hirata t., (1989) dc=2.3-0.73*b öncel a.o. et al., (1995) dc=2.74-1.52*b öncel a.o. et al., (1996) dc=2.32-1.09*b öncel a.o., wilson t.h., (2002) positive correlation öncel a.o., wilson t.h., (2004 positive correlation öncel a.o., wilson t.h., (2007) positive and negative correlation table 2. some examples of previous studies of the relationship between b-value and dc-value. serkan öztürk108 conclusions this study tried to estimate a suitable and reliable correlation between two seismotectonic parameters: b and dc-values for turkish earthquakes. statistical analysis of the available data included 99,737 earthquakes from 1970 to 2011. the maximum likelihood method was used for calculating b-values and the linear regression technique for obtaining dc-values (95% confidence limit). it was observed that the largest events were associated with low b and high dc-values, respectively, implying relatively high stress intensity and stronger epicentre clustering. orthogonal regression was used for correlating the chosen seismotectonic parameters. the results showed strong negative correlation between b-value and fractal dimension dc-value for turkish earthquakes dc = 2.44 0.30*b (r=-0.82) given by or. the results had good agreement with previous studies for different parts of turkey and the rest of the world. acknowledgements the author would like to thank to prof. dr. stefan weimer for providing zmap software and anonymous reviewers for their useful and constructive suggestions in improving this paper. i am grateful to dr. doğan kalafat (koeri) for providing us with the earthquake catalogue. i would also like to thank koeri for providing an earthquake database via internet. references aki k. 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(2001): “a software package to analyze seismicity: zmap”. seismol res lett. 72(3), 373-382. the estimation of caving depth is of particular interest in the oil industry. during the drilling process, the rock classification problem is studied to analyze the concentration of cuttings at the vibrating shale shakers through the classification of caving images. to date, depth estimation based on caving rock images has not been treated in the literature. this paper presents a new depth caving estimation system based on the classification of caving images through feature extraction. to extract the texture descriptors, the cutting images are first mapped on a common space where they can be easily compared. then, textural features are obtained by applying a multi-scale and multiorientation approach through the use of gabor transformations. two different depth classifiers are developed; the first separates the textural features by using a soft decision based on the euclidean distance, and the second performs a hard decision classification by applying a thresholding procedure. a detailed mathematical formulation of the developed classifiers is presented. the developed estimation system is verified using real data from rock cutting images in petroleum wells. several simulations illustrate the performance of the proposed model using real images from a wellbore in a colombian basin. the correct classification rate of a database containing 17 depth estimates is 91.2%. la estimación de la profundidad de la que provienen los derrumbes que usualmente se presentan en las caras del pozo o también llamados cavings es de gran interés en la industria petrolera. durante el proceso de perforación de un pozo, el problema de clasificación de rocas ha sido estudiado con el fin de analizar la concentración de recortes o ripios de perforación en las zarandas vibratorias a través de la clasificación de imágenes de cavings. sin embargo, la estimación de la profundidad de los derrumbes basada en la utilización de imágenes de los mismos no ha sido tratada en la literatura. este artículo presenta un nuevo modelo para la estimación de la profundidad de derrumbes a través de extracción de características. para la extracción de estas características o descriptores de textura, imágenes de recortes son transformadas en un espacio común, el cual permite su comparación. luego, las características se obtienen aplicando la transformación de gabor, un enfoque que se caracteriza por proporcionar un análisis multi-escala y multi-orientación. se desarrollaron dos clasificadores, el primero separa las características de textura usando un enfoque basado en la norma euclideana y el segundo basado en decisiones por umbral. la formulación matemática detallada de los clasificadores desarrollados se presenta en este artículo. el sistema de estimación desarrollado se evalúa usando datos reales de imágenes de derrumbes pertenecientes a un pozo petrolero. simulaciones muestran el rendimiento del modelo propuesto usando imágenes reales de un pozo perteneciente a una cuenca colombiana. la correcta clasificación para una base de datos de imágenes que contiene 17 clases o profundidades es de 91.2%. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 157 163 abstract resumen key words: caving classification. cuttings. wellbore. rock images. caving depth. palabras clave: clasificación de cavings. ripios de perforación. pozo petrolero. imágenes de roca. profundidad de derrumbes. record manuscript received: 31/08/2013 accepted for publication: 25/09/2014 caving depth classification by feature extraction in cuttings images laura viviana galvis c.¹˴³, reinel corzo rueda², henry arguello fuentes³ ¹ grupo de investigación estabilidad de pozo (giep). universidad industrial de santander, bucaramanga, colombia. ² ecopetrol-instituto colombiano del petróleo. grupo de investigación estabilidad de pozo, piedecuesta, colombia. ³ escuela ingeniería de sistemas e informática. universidad industrial de santander, bucaramanga, colombia. geotechnics issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.39716 http://dx.doi.org/10.15446/esrj.v18n2.39716 158 laura viviana galvis c., reinel corzo rueda, henry arguello fuentes 1. introduction a crucial problem in the oil industry is the collapse of the wellbore during the drilling process. the collapse of the wellbore occurs when pieces of rock fall from the walls of the borehole, which is usually known as caving (aldred, et al., 1999). the costs associated with the drilling process are in the millions, and between 15% and 30% of the total resources dedicated to an oil project are designated to cover losses, including material and drilling equipment and continuity of the drilling process; these losses are known as non-productive time (aldred, et al., 1999). hence, classifying the rocks associated with the collapse of the wellbore is an important area of research in this industry. in the oil industry, the estimation of the caving depth is of particular interest. conventional techniques used in geology do not solve this problem because they require excessive processing time. specifically, the geological age of the caving materials provides information about their approximate depth. however, this method requires a micro-paleontological analysis that is not immediately available (aldred, et al., 1999). despite extensive work in pattern recognition and rock classification (crida & de jager, 1996), (mengko, susilowati, mengko, & leksono, 2000), (thompson, niekum, smith, & wettergreen, 2005), (wang & lin, 2005), (galvis, ochoa, arguello, carvajal, & calderón, 2011), (marmo, amodio, tagliaferri, ferreri, & longo, 2005), (bajwa & choudhary, 2006), (kachanuban & udomhunsakul, 2007), research regarding depth estimation based on caving rock images is extremely limited. the problem of rock classification during the drilling process is investigated by analyzing the cutting concentration at the vibrating shale shakers by classifying caving images (marana, et al., 2009). the detection of a high caving concentration indicates problems such as the collapse of the well borehole walls (castillo & moos, 2000). the cutting concentration has been addressed using several supervised classifiers, such as the optimum-path forest (opf), artificial neural networks with multi layer perceptron (ann-mlp), support vector machines (svm) and bayesian classifiers (bc) (marana, et al., 2009). artificial neural networks and svm are the most used and result in up to 93.5% correct classification when the rocks texture is analyzed (singh, singh, tiwary, & sarkar, 2010), (marmo, amodio, tagliaferri, ferreri, & longo, 2005). the opf technique offers a lower computational complexity than other methods (marana, et al., 2009). the bayesian classifier is the most traditional classification method, and its principal advantages include its simplicity, better performance when more data are available, and self-correction, which means that the result changes for different data (islam, wu, ahmadi, & sid-ahmed). however, those classification methods have only a few well-established classes. despite the diverse research into cutting concentration, research on depth estimation using caving images is lacking in the literature (galvis, arguello, & tarazona, 2011). this paper presents a new system for estimating caving depth based on features extracted from rock images. given a set of textural rock features, the estimation system divides the caving depth into a set of depth ranges previously established as classes in the classification system. the depth classifier aims at separating the textural features by using a soft decision based on the euclidean distance and a hard decision by using a threshold constant. the developed estimation system is verified using real data from rock cutting images in petroleum wells. additionally, the system for estimating the depth of caving can be extended to other areas, such as mining, where problems associated with caving in a specific region of the wellbore can be avoided or addressed. this is the first work applying a classification system to estimate the depth of the caving. the principal contributions of this paper include the establishment of an extensive mathematical model for the rock classification problem and the verification of the developed model using images from real caving rocks. an important set of caving rocks, called cuttings, are rocks whose depth origin is previously known. each cutting rock image is considered a mosaic of different textural regions. to extract these textural regions, the cutting images are first mapped onto a common space where they can be easily compared. this mapping reduces the dimensionality of the images by retaining only the relevant information. a two-dimensional (2d) gabor transformation is used to map the rock images as this transform emphasizes the textural features. texture plays a particularly important role in the composition of natural images, and its analysis and classification are areas of continued research (kachanuban & udomhunsakul, 2007). depth classification is a multi-step process. in the first stage, a large database of rock images is acquired from a real wellbore in a colombian basin. the sensed gray-scale images are divided into training and testing sets. the training set is denoted as c=[c1, …, cd, …, cd], and the testing set is denoted as x=[xd1, …, xdl] where d and l indicate the number of images in each group and, traditionally, d » l. the images cd and xdℓ are n m matrices representing the dth training and testing images, respectively. figure 1 shows the representation of the data sets. the rock images in the training set belong to rocks that have a known depth d, and therefore, they are called cuttings. conversely, images in the testing set belong to regions of caving, whose depth d must be estimated. each image cd and xdℓ is again divided into h non-overlapping p p subimages to work with small image sections and non-repeated information. the next step involves the extraction of the textural features from the training and test subsets. the selected method of texture analysis for feature extraction is critical to the success of the texture classification. a multichannel gabor function is used for the feature extraction. this function has been recognized as a very useful tool in computer vision and image processing, especially for texture analysis (shen, jia, & chen, 2011) (clausi & jernigan, 2000). the gabor transformation provides a multi-scale and multi-orientation representation of the underlying images (clausi & jernigan, 2000). the features are extracted from the gabor transformations and through the co-occurrence matrix (marana, et al., 2009) (tou, tay, & lau, 2009). the set of features is represented as a matrix of features fc for the training set and fx for the test set. specifically, the κ-extracted features correspond to the mean µ, the standard deviation σ, the contrast α, the homogeneity ß, the energy ɣ, and the correlation ρ. figure 1. data representation in the model. there are two sets of images; each image in a set is divided in subimages, and then, the gabor transformations are obtained. in the classification stage, a multiclass approach is applied. in this approach, a number of binary classifiers is developed with the set of image features in the training set fc. then, given a caving image from the test set, its set of features fx is used to test the classifier. to make the decision about the class or depth to which the caving belongs, the classifier uses two methods: one method based on rules that make the decision using a hard thresholding approach and another method based on the ℓ2-norm. the number of binary classifiers depends on the number of classes or, in this case, the number of known depths d. the mathematical model for classification and the approaches presented here correctly classify depths up to 91% of the time. the paper is organized as follows. section 2 describes the data and process methodology, including the division of the data, the feature extraction step, the gabor transformation used, and the classification. section 3 presents the experimental setup. section 4 presents the results of the simulations using data from a real wellbore in a colombian basin. 159caving depth classification by feature extraction in cuttings images 2. data and methodology let cd and xdℓ be n m matrices representing cutting and caving images, where d and dℓ represent the image depth, and let c and x be the ensemble of images cd and xdℓ, respectively. thus, c=[c1, …, cd, …, cd] and x=[xd1, …, xdℓ, …, xdl]. note that c represents the training image set, x represents the test image set, and d is the number of depths. these values are associated with the number of classes in the system. note also that c1 is the image from the first depth and that cd is the image from the depth d. the images cd and xdℓ are divided into h non-overlapping p×p subimages, such that h={h1}{h2}, where h1=[n/p] and h2=[m/p]. the jth subimage of cd is denoted by cdj, and the jth subimage of xdℓ is denoted by xdℓj. the image cd can be expressed as a function of its subimages: cd=[cd1, cd2, …, cdh]. similarly, the image xdℓ can be expressed as a function of its subimages: xdℓ=[xdℓ1, …, xdℓh]. figure 1 shows this representation of the data sets. let cdj and xdℓj be the vector representations of cdj and xdℓj, such that cdj = vec(cdj) and xdℓj = vec(xdℓj). more specifically, the vector representation of the jth subimage is given by (cdj)ℓ = (cdj) (ℓ-rp)r´ for ℓ = 0, …, p2 1 and r = [ℓp ]. 2.1. gabor representation the vectors cdj and xdℓj are first transformed to extract their texture features. a two-dimensional gabor function is used because it provides a multi-scale and multi-orientation representation of the underlying signals (clausi & jernigan, 2000). in the spatial domain, a 2d gabor function is a sinusoidal plane wave modulated by a gaussian function. more specifically, the 2d gabor function is given by (1) where u0 is the frequency and θ is the anti-clockwise rotation of the gaussian function and the plane wave. the values σx and σy represent the size of the gaussian function or the scale in the x and y dimensions, respectively. the gabor function in (1) can be expressed in matrix notation as: (2) where wfu θv represents the gabor filter bank at frequency u and rotation v. using w, a discrete set of transformations is obtained. this set with different frequencies and orientations is used to extract features from the subimages. selection of the frequencies emphasizes the intermediate frequency band as the most significant information about a texture often appears in the middle frequency channels (zhang, tan, & ma, 2002), (chang & kuo, 1993). the frequencies are given by (3) where the set of frequencies is fu = { fl, fh } and p is the size of the subimages. however, the literature indicates that a finer quantization of orientation (larger number of rotations) is needed. the restriction on the decision about the number v of rotations, θ, is based on the computational efficiency. given a training vector cdj, the respective training gabor transformation can be written in matrix notation as (4) where u and v indicate the frequency and the rotation, respectively. analogously, the gabor transformation of the test vectors xdℓ j can be written as (5) figure 1 shows the representation of the data sets and the process used to obtain the respective transformations. 2.2. feature extraction two features, the mean and the standard deviation of the classification system, can be directly estimated using the gabor transformation in (4) and (5); the index c represents the set of images to which the transformation belongs. these features are calculated for each subimage of the training set using the transformation in (4) as (6) (7) where p is the dimension of the subimage and u is a p2-long one-valued vector. equivalently, the mean and standard deviation are calculated for the test images by using the respective in (5) as (8) (9) the energy, contrast, homogeneity and correlation are also calculated for the subimages, in addition to the mean and the standard deviation given in (6), (7), (8) and (9). the co-occurrence matrix is defined as a matrix or distribution of co-occurring values at a given offset. the use of this matrix is based on the assumption that the texture information on the image vector or is contained in the overall or average spatial relationships of these vectors (haralick, shanmugan, & dinstein, 1973). the co-occurrence matrix is defined as g = q ( ) where q is an operator that defines the position of two pixels relative to each other. let the elements of g be gij. then, gij represents the number of times that pixel pairs with intensities i and j are present in an image in the position specified by q. the size of this co-occurrence matrix is determined by the number of possible intensity levels in the image. for an 8-bit (256 possible levels) image, g will be 256x256. to reduce computation load, a frequently used approach is to quantize the intensities into a few bands to keep the size of the matrix g treatable. in the case of the 256 intensities, a quantization on b levels, where l = 8 results in a co-occurrence matrix of size b x b (gonzalez & woods, 2008). the co-occurrence matrix associated with the training image vector is given by . (10) accordingly, the co-occurrence matrix for the image vector is . (11) 160 laura viviana galvis c., reinel corzo rueda, henry arguello fuentes using the co-occurrence matrices, the energy, contrast, homogeneity and correlation are calculated. the angular second moment or energy for the training set is calculated by . (12) the contrast or the measure of the amount of local variation is given by . (13) the direct measure of the local homogeneity is calculated by (14) finally, the correlation or the measure of linear dependency ρ of neighboring image pixels is given by (15) where µk, µr, σk and σr are the means and standard deviations of gc and gc, respectively. similarly, textural features are calculated for the testing image vector . the 6 characteristics for the training set in (6), (7) and (12)-(15) are combined into the matrix fc shown in equation (16) for the h sub-images and the uv gabor transformations of each of those sub-images. similarly, the matrix fx is constructed using the characteristics for the testing set. the columns of this matrix represent the h subimages, and the rows represent the filtered images uv for each of the k features. a flowchart of the procedure used to obtain the matrix of features of any image and to make a final decision about its classification is presented in figure 2. (16) figure 2. flowchart of the process used to estimate the matrix of features. the matrices of features fc and fx represent the texture at determined frequencies u and orientations v after filtering by the gabor transformation. some of the frequency-orientation combinations will be more representative than others, taking into account that the real frequency-rotation of the texture is adjusted to the combination given by the gabor function. the analysis of the data allows an image to be characterized using only a small set of the images filtered by gabor filters. thus, a portion of the data extracted from and can be used for the classification while the other portion is noise, making the classification more difficult. for this reason, a classification based on distances is used in this model, following heuristic rules to use the most representative features. two approaches for the estimation of the depth d by the classification of a caving are proposed. 2.3. classification based on a hard thresholding in this approach, the goal is to find the subset of feature vectors that determines the texture of an image. each feature matrix of the training classes is compared with the feature matrix of a test image. a sum of the differences of the matrices for small distances is calculated. to calculate those distances and to make a decision related to the similarity among vectors, a threshold is included in the model. this threshold τ is estimated using the training set of images. the distances in this approach are given by: (17) for h = 0, …, h ‒ 1; ℓ = 0, …, uvh ‒ 1 and . then, the maximum value for similar features present in is used for the classification, and this selection can be represented as rc = maxh maxℓ (sc )ℓ,h (18) to make the decision, the training set class with the greater number of similar feature sets is established as the class to which the test image belongs. this decision is given by the comparison among the rc of the possible classes and is defined as: class = maxc r c (19) 2.4. classification based on the euclidean norm (ℓ2) instead of the sum presented with the previous approach, this method uses the distances between vectors. to calculate those distances, is calculated as: (20) for h = 0, …, h ‒ 1; ℓ = 0, …, uvh ‒ 1 and . the matrix contains the minimum distances between the features in the training and test images, and those minimum distances characterize each subimage and image, respectively. to use the most representative distances, a distance per class is obtained by rc=minh minℓ(sc )ℓ,h (21) for comparison among classes, the distances between the classes are evaluated and the class with the minimum distance is selected as the class to which the test image belongs. this decision is given by the evaluation of the distances rc as: class = minc r c (22) finally, weight can be applied in the model to give more importance to some of the features. to apply this weight, a vector ω with the weights is applied to the feature vector before the comparisons in equations (17) and (20). then, the vector ω can be expressed as a function of the weights, ω = [ω1, …, ωk]. given the images of the test set, the accuracy of each of the approaches is calculated as the sum of the successful classifications over the number of evaluations, which consists of the evaluation of the l images in the test set in this case. the equations for the calculation of the performance are (23) 161caving depth classification by feature extraction in cuttings images 3. experimental setup to evaluate the efficiency of the presented model, rock samples extracted from a wellbore in a colombian basin were used. cuttings from depths between 1371.6 and 3825.54 meters (4500-12550 ft) were acquired via realtime monitoring by the colombian petroleum institute (icp). to facilitate the process of acquisition and reduce uncertainty, specimens were created in an icp laboratory containing samples of cuttings and caving materials. images of some samples are presented in figure 3. figure 3. real cuttings samples selected for creation of specimens. for the preparation of the specimens, the rock samples were placed in a metallic mold lined with vacuum grease or acrylic. a mixture of resin and hardening in a ratio of 1 to 2 grams was added and allowed to dry. when the specimen was completely hardened, it was extracted from the mold. finally, the surface was polished until the visible area of the sample surface was maximized. figure 4 shows the process used to generate specimens. figure 4. specimen preparation. the rock samples are placed into a mold lined with acrylic, and a resin-hardener is added and allowed to dry. the specimen is extracted from the mold when it is consistent and polished to obtain the maximum visible area of the samples. a set of images was acquired using a reflected-light microscope. figure 5 presents some of these images, and it should be noted that different textures are present. training set c and testing set x were thus randomly created. to simulate the classification process, a set of d = 17 classes was chosen, each representing a depth in a well as illustrated in table 1. then, the mathematical model presented in section 2 was applied. class depth in meters (ft) class depth in meters (ft) (ft) 1 1371.60 (4500) 10 3111.70 (10209) 2 1463.04 (4800) 11 3232.40 (10605) 3 1572.46 (5159) 12 3358.90 (11020) 4 1750.16 (5742) 13 3441.50 (11291) 5 2083.31 (6835) 14 3538.73 (11610) 6 2508.81 (8231) 15 3658.51 (12003) 7 2706.32 (8879) 16 3758.18 (12330) 8 2798.37 (9181) 17 3825.54 (12551) 9 2944.98 (9662) table 1. training classes. figure 5. rock images for classification. note the different textures present in the set. a subsection of each of the images in c and x was extracted to eliminate the blurred borders that were present. then, each image cd and xdℓ was divided into 12 non-overlapping subimages. the preprocessing was performed using a bank of gabor filters w, as presented in equation (2). for each subimage of cd and xdℓ, a set of u = 14 frequencies and v=6 rotations were applied, for a total of uv = 84 gabor filters. figure 6. comparison of histograms for different classes. the descriptors for different classes are concentrated in the same value ranges. for each of the transformations and , k = 6 texture descriptors were calculated. an analysis of the descriptors among the different d classes allowed determining the difficulty of separation. this is the main reason to use this approach instead of traditional classification methods, given that the classes are not linearly separable. to show the non-separable characteristic, the following figures present the histograms and some examples of the features. the histograms in figure 6 show the concentration of the descriptors in the same value ranges for two randomly selected classes. it is important to clarify that the domain does not significantly change between classes and that the overlapping behavior is similar for any pair of selected classes. 162 laura viviana galvis c., reinel corzo rueda, henry arguello fuentes figure 7 shows samples of two randomly selected features from the data set, and again, the overlap is similar for any pair of selected classes. each data point is labeled according to the classes it belongs to. here, the classes are depicted in green and blue, and the goal is to use these data as a training set to classify a new observation according to the feature classes. note that class overlap is present in the projected space. in evaluating the computational complexity of the algorithms, we consider two stages: feature extraction and classification. the two stages can be performed efficiently, requiring ơ(uvh + d2) operations per image, where h is the number of subimages (12 in our experiments), u and v are the frequency and rotation of the gabor filter bank, respectively, and d is the number of classes. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 feature 1 fe at ur e 6 class 1 mean: class 1 class 2 mean: class 2 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 feature 2 fe at ur e 5 class 1 mean: class 1 class 2 mean: class 2 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 feature 2 fe at ur e 6 class 1 mean: class 1 class 2 mean: class 2 0 0.5 1 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 feature 3 fe at ur e 5 class 1 mean: class 1 class 2 mean: class 2 figure 7. scatter plot of depth features for two randomly chosen features represented by green and blue. there is a class overlap in the projected space showing that the classes are not linearly separable. 4. simulations and results the classification was performed following the mathematical model in section 2. to calculate the performance of this method, the correct classification of each of the binary classifiers must be considered. the general performance calculated using equation (23) for each of the approaches—hard thresholding and ℓ2 -norm—is presented in figure 8. the performance results obtained using real data and the two approaches are 91.3% and 87.5%, respectively, for the hard thresholding and ℓ2-norm approach. to highlight the influence of the different features, the performance was calculated using different numbers and combinations of features. figure 9 shows the performance using 4,5 and 6 features, and each of the bars represents a different combination of features. the comparison shows that the performance increases with the number of features and becomes more precise for k = 4,5,6. the best performance is obtained using the complete set of features. figure 8. overall performance evaluated using hard thresholding and ℓ2norm distances. given the variability in the performance, especially when 4 features were used, an analysis was performed to determine whether some features are dominant over the others following the weighting method in the mathematical model. simulations involving the total number of features and assigning weights were performed by applying the vector ω. improvements in the performance obtained using the hard thresholding classification resulted in 92%, 93% and 94% success when the weight values ω2, ω4 and ω5 were respectively applied to the standard deviation, contrast and homogeneity with greater consideration compared with the other features. the weight values considered in the analysis were ω2 = 1.2, ω4 = 1.6 and ω5 = 1.6. figure 9. comparison of the performance using different values of k. different settings were tested in the hard thresholding classification, which includes a threshold value τ. figure 10 shows the variation in classification accuracy for 13 different values of τ for the proposed κ features and distance measures. figure 10 shows the independence of the performance from the ℓ2norm distance, represented by the red line, and the best performance achieved was 91.3% correct classification using the hard thresholding approach and a τ value of 0.008. 0.007 0.0072 0.0074 0.0076 0.0078 0.008 0.0082 0.0084 0.0086 0.0088 0.0090 0.0092 0.0094 60 70 80 90 100 tau values pe rfo rm an ce (% ) figure 10. performance for different τ values represented by the blue bars. the performance using the ℓ2-norm distances is shown in red and is independent of the τ values. the performance of the binary classifiers is presented in figure 11; values of 0 (blue) above the diagonal represent classification errors and values of 1 (red) represent a successful classification. the point highlighted in figure 11a represents a classification error in the binary classifier among classes 2 and 4 for the hard thresholding classification. 163caving depth classification by feature extraction in cuttings images (a) (b) figure 11. binary classifiers using the (a) heuristic and the (b) ℓ2-norm decisions. in both figures, the 0 (blue) values represent errors and the 1 (red) values represent successful classifications. in figure 11b, some of the classifier errors are present in binary classifiers involving the same class. for instance, the errors in classes 2 16, 11 16 and, 12 – 16 indicate that some classes as the 16 in this case present very similar features compared to other classes, which makes the classification more difficult. conclusions the classification of caving depth estimation using hard thresholding and the ℓ2-norm has been developed in this work. the developed methods and a complete mathematical model are then used to classify caving depths by features extracted from cutting images. the multi-scale and multi-orientation representation obtained using gabor filters permit a much more complex characterization of the texture due to the independence of the rotation used in the acquisition process. the performed analysis precisely extracts the texture features for different rotations, which allows an optimal comparison among the images in a training set and a testing set. the size of the images and the number of gabor filters applied are important because these parameters allow a complete characterization of the texture. however, as a consequence, additional information is included. this information is noise and should be removed for the classification process. the approaches in this work treat this problem by searching a reduced and a representative set. additionally, simulations using different numbers of feature combinations reveal variation in the performance of these methods. however, none of the combinations with fewer features than the established κ achieved a greater performance. the performance achieved with the hard thresholding classification is 91.3%, whereas the ℓ2-norm based classification achieved 87.5% success. these values demonstrate the applicability of the methods and techniques developed in this work. in addition, these methods can be applied directly in rock classification problems. references aldred, w., plumb, d., bradford, i., cook, j., gholkar, v., cousins, l., et al. 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(2002). invariant texture segmentation via circular gabor filters. 16th international conference on pattern recognition, 901-904. earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 95-96 letter from editor earth sciences research journal presents its latest issue, volume 19, number 2, with a positive balance in the eve of 2016 and with the challenges for the future editions. during the last two decades, we have published more than 200 papers in all the spectrum of earth sciences. we expect we will be here for many years more. esrj is now part of international databases, and we hope to improve our numbers thanks to the tireless word we do every day for our readers. we have accomplished the principal objectives for an international publication -punctuality in our volumes, respect for the authors and the spotlight on the scientific quality. for a fair editorial process, we are part of the open access and its policies of pair blind-review. the time of the operations has been reduced, and the international participation increased. for the coming years, esrj would reach a better visibility thanks to the publication of top-quality scientific articles, which allows us to be part of the central positions in international databases and providing researchers with fundamental elements of their work. to make possible this goal, we want to call authors to put lights on scientific quality and editorial quality. your help is essential for us. we want to encourage researchers to pick up esrj as a platform for their works, but also to check the authors' guidelines as a first step to saving efforts and time during processes. needles to say the importance of manuscript parts, english level and images readability to the acceptation of submissions. as a complementary strategy, we would like to invite our authors to create profiles in one of the available platforms, not only as an introduction to the esrj editorial team but also as a promotion of your work. several challenges we have gripped during the work of the last years, for that reason we have adopted the principles of ethic and best practices in our procedure protocols. this decision would be a tool giving papers’ authors a warranty of complete respect for their work and to readers a quality signature that our contents have been verified for their use. in this adoption of best-practices procedures, many submitted papers have been rejected for having a not proper citation; this fault needs to be eradicated from all journals promoting scientific contents. a glimpse of the numbers of esrj points the international collaboration as one of the biggest challenges for the future issues. international collaboration accounts for the articles that have been produced by researchers from several countries. it's not a call to improve the rates, but the scientific quality would be feed for the interaction of researchers facing different realities. early 2016, esrj will publish a special issue with the selected papers of the international conference on earth and environmental sciences by the university malaya. this special issue on frontiers in earth sciences will publish articles on the most outstanding discoveries across a broad research spectrum in the field. the mission of frontiers in earth sciences is to provide an introduction to some of the current advanced concepts, probb lems, and controversies that are at the forefront of earth sciences and to obtain a view of how the different parts of the earth behave and interact. as it has been a costume in this letter from the editor, the following paragraphs will introduce the works of the current issue. our first paper deals with the problem of recovering moho surface from satellite gradiometry data considering the non-isostatic corrections. the second manuscript presents a case study on the application of electromagnetic and gpr measurements to a suspicious metalcontaminated site. while subsurface pollution is very harmful to nature-beings and finally human, detection of its extent in surface and more in depth is considerably challenging. the following work analyzes the earthquake light phenomenon based in the christchurch earthquakes occurred in 2010 and 2011, with testimonies related from people who observed or recorded the lights. in the seismotectonic field is presented a paper showing the strain rate in the north anatolian fault zone (nafz), that is one of the most important fault zones of turkey. another turkish submission tried to determine seismic specifications of the bitlis province in lake van basin and used probabilistic seismic hazard analysis for the determination of local site-specific spectra. also from turkey, a paper examines the performance of a single base rtk station compared with similar systems and presenting a complete exposure of values and challenges of the system. a contribution from iran analyzes the magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the manganese mineralization in a mine located in central iran. also from in iran, a paper assesses the stability of the rock slopes with various rock mass qualities. the manuscript discusses the combination of rmr and smr systems to evaluate rock slope stability. from latin america, a paper makes a contribution to the knowledge of the tatui formation, in brazil, emphasized in paleontology and paleoenvironmental considerations. we hope you find useful these contents. carlos vargas editor in chief, earth sciences research journal keywords: burial history; carboniferous-permian; jiyang sub-basin; maturation evolution; secondary hydrocarbon generation. palabras clave: registro de enterramiento; carbonífero-pérmico; subcuenca jiyang; evolución de desarrollo; generación secundaria de hidrocarburos. how to cite item sijie, h., shuxun, s., peiming, z., jinlong, j., & jingjing, l. (2020). burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china. earth sciences research journal, 24(4), 397-408. doi: https://doi. org/10.15446/esrj.v24n4.63220 in the jiyang sub-basin, carboniferous-permian (c-p) coal-measure source rocks have experienced complex multistage tectonics and therefore have a complex history of hydrocarbon generation. because these coal measures underwent multi-stage burial and exhumation, they are characterized by various burial depths. in this study, we used the basin modeling technique to analyze the relationship between burial history and hydrocarbon generation evolution. the burial, thermal and maturity histories of c-p coals were reconstructed, including primary hydrocarbon generation, stagnation, re-initiation, and peak secondary hydrocarbon generation. the secondary hydrocarbon generation stage within this reconstruction was characterized by discontinuous generation and geographical differences in maturity due to the coupled effects of depth and a delay of hydrocarbon generation. according to the maturity history and the delay effect on secondary hydrocarbon generation, we concluded that the threshold depth of secondary hydrocarbon generation in the jiyang sub-basin occurred at 2,100 m during the yanshan epoch (from 205 ma to 65 ma) and at 3,200 m during the himalayan period (from 65 ma to present). based on depth, residual thickness, maturity, and hydrocarbongenerating intensity, five favorable areas of secondary hydrocarbon generation in the jiyang sub-basin were identified, including the chexi areas, gubei-luojia areas, yangxin areas, the southern slope of the huimin depression and southwest of the dongying depression. the maximum vro/burial depth (%/km) occurred in the indosinian epoch as the maximum vro/time (%/100ma) happened in the himalayan period, indicating that the coupling controls of temperature and subsidence rate on maturation evolution play a significant role in the hydrocarbon generation evolution. a higher temperature and subsidence rate can both enhance the hydrocarbon generation evolution. abstract burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china registro de enterramiento de carbones del carbonífero-pérmico y su impacto en la generación secundaria de hidrocarburos en la subcuenca de jiyang, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.63220 en la subcuenca de jiyang, rocas madre del carbonífero tardío y del pérmico temprano (c-p) han experimentado multifases tectónicas complejas que además tienen una historia de generación de hidrocarburos. debido a que estas rocas estuvieron sometidas a un enterramiento multifásico y a exhumación, estas se caracterizan por una variedad de profundidades de enterramiento. en este estudio se usó la técnica de modelado de cuenca para analizar la relación entre la historia de enterramiento y la evolución de generación de hidrocarburos. se reconstruyeron el enterramiento y los registros termal y de desarrollo de las rocas fuente, incluida la generación de hidrocarburos primarios, el estancamiento, el reinicio, y el pico secundario de la generación de hidrocarburos. la fase secundaria de generación de hidrocarburos en esta reconstrucción se caracterizó por la generación discontinua y las diferencias geográficas de desarrollo debido a los efectos acoplados de profundidad y un retraso en la generación de hidrocarburos. de acuerdo con el registro de desarrollo y el efecto de retraso en la segunda fase de generación de hidrocarburos, se concluyo que el umbral de profundidad de esta fase en la subcuenca jiyang ocurre a 2100 metros durante la época yanshan (de 205 ma a 65 ma) y a 3200 metros durante la época himalaya (de 65 ma al presente). con base en la profundidad, el grosor residual, el desarrollo y la intensidad de generación de hidrocarburos, se identificaron cinco áreas favorables de la segunda fase de generación en la subcuenca jiyang, que incluye las áreas de chexi, gubei-luojia, y yangxin, la pendiente al sur de la depresión de huimin y el suroccidente de la depresión de dongying. la profundidad máxima vro/enterramiento (%/km) ocurrió en la época indosina, mientras que el máximo vro/tiempo (%/100 ma) ocurrió en la época del himalaya, lo que indica que el acoplamiento de controles de temperatura e índice de subsidencia en la evolución del desarrollo juegan un rol significante en la evolución de generación de hidrocarburos. una mayor temperatura y un mayor índice de subsidencia pueden mejorar la evolución de generación de hidrocarburos. resumen record manuscript received: 10/03/2018 accepted for publication: 14/08/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 397-408 han sijie1, sang shuxn1,2, zhou peiming3, jia jinlong4, liang jingjing4 1low carbon energy research institute, china university of mining and technology, xuzhou 221008, china 2the education ministry’s key laboratory of the cbm resources & accumulation process, china university of mining and technology, xuzhou 221008, china 3guizhou provincial institute of petroleum exploration and development engineering, guiyang 550081, china 4school of resources and earth sciences, china university of mining and technology, xuzhou, china *corresponding author: hsj_cumt@126.com g eo lo g y https://doi.org/10.15446/esrj.v24n4.63220 https://doi.org/10.15446/esrj.v24n4.63220 https://doi.org/10.15446/esrj.v24n4.63220 398 han sijie, sang shuxun, zhou peiming, jia jinlong, liang jingjing introduction the jiyang sub-basin is a secondary structural unit in the bohai bay basin, which contains rich oil and gas resources; therefore, it has significant potential for exploration and development (pang et al., 2003; li et al., 2006; jin et al., 2009). recent studies investigating hydrocarbon accumulation in the jiyang sub-basin have focused on palaeogene and neogene sandmudstone reservoir systems and associated cenozoic source rocks that include multilayer organic-rich argillaceous strata (zhu et al., 2005, 2007; zhang et al., 2016). however, the deeper carboniferous-permian (c-p) coal measures have received little attention (hou et al., 2008; zhang, 2012). owing to the presence of faults and surface unconformities, the formation of a petroleum system consisting of cenozoic oil and gas reservoirs from palaeozoic source rocks remains problematic unless structural conditions are favorable (zhang et al., 2004, 2009b). previous studies have shown that coal-measure source rocks have a high potential for gas generation, and coal-derived gas has thus become a key research focus for future oil and gas exploration and development in china (dai, 2007, 2009; yang and liu, 2014; liang et al., 2015). previous works have demonstrated that c-p coal-measure source rocks from the jiyang sub-basin possess favorable characteristics for secondary hydrocarbon generation because of their complex, multi-stage tectonic history (zhang et al., 2005; hu et al., 2006, 2008). variations in the tectonic thermal evolution during different periods have impacted organic matter maturity, while secondary hydrocarbon generation is common in the study area (li et al., 2001; li, 2006; fan et al., 2008). however, to date, few specialized quantitative studies have investigated hydrocarbon generation thresholds and the relationship between subsidence rates and the evolution of vitrinite reflectance (vro) in the jiyang sub-basin. this study initially reconstructed the burial and thermal histories of c-p coals using petromod software modeling. secondary hydrocarbon generation for c-p coals was evaluated in terms of these events in an attempt to understand the relationships between burial, thermal history, and the evolution of hydrocarbon generation. a series of threshold depths and favorable areas were then proposed based on geographical differences as well as the delay effect in secondary hydrocarbon generation, and the quantitative relationship between vro and the subsidence rate was discussed according to changes in vro and burial depths. the aim of this study was to confirm secondary hydrocarbon generation occurred and establish a relationship between burial history and the evolution of source rock maturity in the jiyang sub-basin. regional geological setting the jiyang sub-basin is located within the southern part of the bohai bay basin, china (fig. 1a). affected by multi-stage tectonic activity, this sub-basin has a complex structural framework (he et al., 1998; zong et al., 1999). the residual carboniferous and permian strata in the jiyang sub-basin is distributed from northeast to southwest, matching the regional tectonic framework, and the jiyang sub-basin is bounded by the chengning uplift to the northwest and the luxi uplift to the south (fig. 1b). the basic tectonic framework includes an extensional opening in the northeast and convergence in the southwest. the c-p coal-bearing sequences that occur within the jiyang sub-basin likely formed in a peat-swamp environment that included both littoral and paralic transitional facies (zhang et al., 2009a). following deposition of these coalbearing sequences, convergence of the yangtze and north china plates led to tectonic inversion during the early yanshan epoch. after uplift and denudation, extension started during the middle and late mesozoic, while magmatic activity was widespread across the jiyang sub-basin (ren et al., 2002; yang et al., 2008). thus, c-p coal-bearing sequences were subject to large changes in temperature due to the hydrothermal influence of magmatic activity (li et al., 2001, yang et al., 2008). the embryonic jiyang sub-basin formed during the late jurassic and early cretaceous when this region underwent a slight uplift, followed by polycyclic alternation of extension and sharp thermal subsidence (he and wang, 2004). the jiyang sub-basin was affected by both the yanshan (from 205 ma to 65 ma) and himalayan (from 65 ma to the present) tectonic episodes, leading to the formation of a mesozoic-cenozoic rift-depression sub-basin (zong et al., 1999). secondary tectonics within the jiyang sub-basin, including highs and depressions that formed during the late mesozoic and cenozoic, can be divided into southern and northern sections by the wudi-binxian-chenjiazhuang highs (fig. 1b). the four dominant secondary tectonic units within this region are the chezhen, zhanhua, huimin, and dongying depressions. the basic tectonic attributes of these depressions include a northern fault and southern overlap (wang et al., 2002). it is also worth noting that several c-p strata in the jiyang sub-basin were exhumed and eroded during the later phases of mesozoic-cenozoic tectonic activity. residual strata within these coal-bearing sequences are bounded at the north and south and are unevenly distributed in segments of the sub-basin (fig. 2). drilling data and previous studies (li et al., 2004) showed that the depth of the c-p coal-bearing sequences generally exceeds 2,000 m, with an average between 3,000 and 5,000 m (fig. 1b). coal is preserved in the shanxi formation (fig. 1c) and is considered to be main source rock in the coal-bearing sequences. the vro of the c-p coals shows a scattered distribution and is restrained by c-p residual strata. areas with coals with high vro are distributed on the south slope of the dongying depression, in the gubei district in the zhanhua depression, and south of the huimin depression. the vro of coals in these areas exceeds 1.0%, and even exceeds 1.5% in the center of these areas (han et al., 2017). several measured vro values provided by the geological scientific research institute, shengli petroleum administration (gsrispa) from different wells are reported in table 1. reconstruction of burial, thermal, and maturity histories: methods and input data burial, thermal, and maturity histories were reconstructed by applying a basin modeling technique (using the commercial modeling software package petromod, 2010 version). the purpose of this evaluation was to reconstruct thickness and temperature conditions during the geological history of the jiyang sub-basin on the basis of observed present-day strata thickness data and the geothermal field (shi, 2000; pang, 2003). input data consisted of formation depths (m) and associated lithologies, numerical ages (ma), well temperatures (°c), eroded time (ma) and depths (m), petroleum system element (as source, reservoir, or seal), total organic carbon (toc, %), source rock kinetics, and the initial hydrogen index (hi). this method yielded reconstructions of table 1. measured vro values for carboniferous-permian (c-p) coal measures within the jiyang sub-basin depth (m) well sample mean vro (%) standard deviation lithology 2890.4 gbg1 core 0.82 0.03 coal 3014.34 gbg1 core 0.74 0.03 coal 3032.5 gbg1 core 0.83 0.05 coal 3325 wg7 core 1.14 0.15 coal 3636 wg7 core 1.32 0.09 coal 4305.6 bg403 core 6.52* 0.25 coal 4313.1 bg403 core 6.6* 0.22 coal 4369 bg403 core 1.05 0.06 mudstone 4473 hg1 cuttings 2.19 0.14 mudstone 4480 hg1 cuttings 2.22 0.11 coal 4504 l28 cuttings 2.23 0.14 mudstone 4515 l28 core 1.09 0.08 coal 4539 l28 cuttings 2.73 0.1 coal values marked with * are abnormally high because the c-p coal measures were heated by magma. at the same time, vro values for cuttings are also high because these are mixed with cavings, impacted by contact metamorphism from igneous bodies with high vro values. 399burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china this method is the establishment of relatively accurate sandstone-mudstone compaction curves. because the porosities of sandstone and mudstone affected by their depths, it is possible to establish compaction curves that reflect the relationship between porosity and burial depth. these relationships can be observed in the logging results (fig. 3). the calculation of eroded thickness in different erosion events are based on sandstone and mudstone acoustic burial, thermal, and maturity histories of the target source rocks. before basin modeling, a back-stripping method (steckler and watts, 1978) was used to obtain the eroded thickness in different geological periods. the back-stripping method is based on sediment compaction theory, and decreases in the thickness of strata are a result of increases in burial depth because the overlying load causes a decrease in porosity (sclater and christie, 1980). thus, the basis for figure 1. a. tectonic framework of the bohai bay basin, china (han et al., 2019). b. burial depths of carboniferous-permian (c-p) coal-bearing sequences within the jiyang sub-basin (li et al., 2004). c. typical lithologic and electric logs of c-p strata, including coal measures and overlying interbedded sandstones and mudstones. 400 han sijie, sang shuxun, zhou peiming, jia jinlong, liang jingjing travel time. the values of sandstone and mudstone acoustic travel time reflect rock compaction, i.e. porosity. the amount of erosion can be determined by discerning gap of acoustic travel time and calculating depth difference. a detailed method for calculating erosion amounts in geological periods was proposed by magara (1976). initially, the presence of two main erosion surfaces on compaction curves was confirmed by logging observations of the erosion surfaces. one of these was in the palaeozoic-mesozoic and the other was in the mesozoic-cenozoic. eroded thicknesses could then be calculated from logging results provided by gsrispa (table 2), except for palaeogene and triassic erosions. in addition, according to calculations by du (2005), there is a erosion between the paleogene and neogene and the eroded thickness of this period is 150 m. owing to the lack of triassic stratigraphic information, this study referred to the result reported by zhu et al. (2001), which suggests that the erosion amount at this time was 1800 m in the southern jiyang sub-basin. although the data in this modeling were from different sources, they had a credible matching. this is because the non-well drilling data, like hf value figure 2. thickness distribution of residual c-p strata within the jiyang sub-basin (unpublished data from the geological scientific research institute, shengli petroleum administration, china). table 2. key input parameters for the basin model associated with c-p coal measures in the jiyang sub-basin chrono-stratigraphy lithology present thickness (m) eroded thickness (m) toc (%) hi (mg/g) deposition age (ma) erosion age (ma) hf (mw/m2) q soil 232 2–0 70.50 nm sandstone 203 6–2 70.75 ng sandstone 688 14–6 71 e 150 24.6–14 71.5 mudstone + sandstone 1443 48.1–24.6 72 j+k 450 98–48.1 82.25 mudstone 630 208–98 76 t 1800 225–208 73.5 0 250–225 71 p 800 258–225 70.5 coal + sandstone 434 65 179 290–258 70 c coal + mudstone 220 74 203 340–290 70 401burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china and ages, had a high regional consistency. additionally, the toc and hi values were directly obtained from gsrispa and the coal sampling sites were located in the uplift in the jiyang sub-basin. therefore these coals could represent the low rank coals which were preserved at deep depth in the depression. the detailed input parameters used for this model are listed in table 2. the chronostratigraphic units are based on results from actual logging, while event ages and heat flow (hf) values were taken from the work by du (2005). the designated land surface temperature for this work was 14°c and was considered to have been constant throughout geological history. the easy%ro vro evolution algorithm of sweeney and burnham (1990) was applied to model thermal maturation. the detailed modeling procedure using petromod 1d was reported in previous studies (yu et al., 2018, 2020). the modeling results obtained correlate very well with the current measured temperature and vro data (fig. 4b). the toc and hi values for c-p coals were directly obtained from gsrispa. the permian coal-bearing strata are major reservoirs due to a high adsorption capacity of coal and pore space of sandstone. results and discussion reconstruction of burial, thermal, and maturity histories as discussed, the modeling here was based on measured present-day thicknesses and partially eroded thicknesses from well wg7 and others, as presented in previous works (zhu et al., 2001; du, 2005). the depth of c-p coal measures in well wg7 has a similar depth with their average depth in the study area (3500-4000m). the logging observations and previous studies figure 3. mudstone and sandstone porosities for different depths from well wg7 in the jiyang sub-basin. the unconformity between triassic and jurassic strata cannot be discerned in that triassic strata were missing in well wg7. figure 4. a. wg7 well modeling results for burial, thermal, and maturity histories. b. calibration curve for reconstruction of temperature and vro (data were obtained from gsrispa). 402 han sijie, sang shuxun, zhou peiming, jia jinlong, liang jingjing suggest complete erosion events after the deposition of c-p coal measures and the source rocks has not been affected by a direct magmatic intrusion. the thickness of coal is equivalent to the average value of coals in the study area (10-20m) and the vitrinite reflectance of coals in well wg7 is about 1.3%, which also represents the deep c-p coals in the jiyang sub-basin. therefore, our modeling results can represent the burial, thermal and maturity histories of deep c-p coal measures in the study area. the modeling results, including burial depth and temperature data (fig. 4a), allowed for the determination of vro throughout geological history and suggested that the c-p coals experienced four developmental stages in the jiyang sub-basin. (1) alternating patterns of subsidence during the indosinian epoch (from 290 ma to 225 ma) the deepest burial depth recorded during the indosinian epoch exceeded 2,200 m and the temperature in the center of this depression exceeded 105°c. modeling results show that the temperature of the primary hydrocarbon generation was 80°c, and that during initial burial to 1,700 m at 260 ma, the vro value for the c-p coal-measure source rocks approached 0.6%, which is the primary hydrocarbon generation threshold. during the late permian, c-p coal strata experienced a short period of uplift and exhumation. following a period of exhumation (which ended at 250 ma), re-burial to 2,300 m led to an increase in vro to 0.75%. (2) uplift in the early yanshan epoch (from 225 ma to 200 ma) during the early yanshan epoch, c-p coals were exhumed again by a north-to-south compressional stress field, which resulted in the denudation and stagnation of hydrocarbon generation. at this time, significant hydrocarbon generation did not occur in source rocks despite being within the early gas generation window (vro = 0.7–0.8%) for humic coals (price, 1989; zhao et al., 2005). vro values remained at around 0.75%, but favorable conditions for secondary hydrocarbon generation nevertheless occurred. (3) subsidence and small-scale exhumation during the middle to late yanshan epoch (from 200 ma to 40 ma) the maximum burial depth throughout the entire yanshan epoch did not approach that of the indosinian epoch. the maximum burial depth of around 2,100 m was attained at about 100 ma, while the maximum temperature remained close to 120°c. these results show that regional differences in the amount of subsidence led to variation in thermal maturation. the vro value reached as high as 1.0% in the depocenter, where source rocks approached peak hydrocarbon generation; however, in specific areas, source rock maturity (heated by magmatism) increased above the window for oil generation (vro > 1.8%, affected by igneous heat). these results are similar to those reported for the gudao area in the zhanhua depression, although the distribution area for secondary hydrocarbon generation is relatively limited (li et al., 2006; fan et al., 2008). (4) rapid subsidence during the himalayan epoch (from 40 ma to the present) the c-p coals developed into their present state as a result of more or less continuous subsidence over the last 50 ma. the burial depth in the center of the jiyang sub-basin commonly exceeded 3,500 m and the highest temperature exceeded 165°c. this means that the burial depth increased by more than 2,000 m over just 40 ma. at the same time, the geothermal gradient increased substantially (from 90°c to 170°c), and vro attained a high maturity stage (vro > 1.3%). this indicates that the vro distribution areas were also consistent with those of secondary structures (fig. 5). the distribution of vro values that reached high maturity (vro > 1.5%) are approximately annular in the zhanhua and dongying depressions; however, in the chezhen depression, vro values cluster around 1.0%, and form a banded distribution. in contrast, close to the fault zone of the north–south partition in the huimin depression, vro values are relatively high (vro > 1.2%). this maturity level corresponds with the peak for secondary hydrocarbon generation within the jiyang sub-basin (li, 2006; zhu et al., 2010). the c-p coals in the jiyang sub-basin are currently at the peak stage of secondary hydrocarbon generation and thus contain a considerable proportion of gaseous hydrocarbons. these source rocks are promising for exploitation of coal-derived gas if the preservation conditions and gas migration pathways are well matched. evolution of hydrocarbon generation in c-p coals source-rock kerogen types are determined based on the sedimentary environment. observations showed that terrestrial forest bogs were widely figure 5. present-day vro isogram for c-p coals in the jiyang sub-basin (measured vro values were provided by gsrispa). file:///e:/trabajos_2020/un/fc/revistas/esrj_24(4)/txt/javascript:; 403burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china distributed over the deltaic plain, corresponding to a weak reducing, or weakly oxidizing, environment (zhang et al., 2009). in this context, higher plant remains tended to form humic organic matter following deposition and coalification. coal petrography observations identified vitrinite as the dominant source-rock maceral component (>70%), while the inertinite content was less than 30% alongside a limited amount of exinite (song and qin, 2005). organic geochemical results for c-p coal-related source rocks also demonstrated a type iii kerogen with a low h/c ratio but intermediate o/c ratio (yu et al., 2003). thus, humic kerogen (type iii), which leads to the generation of natural gas, forms the main source rock type within c-p coal measures (ai and li, 1999). minimal differences in source-rock maceral content across all depressions imply that c-p coals in the jiyang sub-basin are products of a similar depositional setting. however, the cyclic subsidence–uplift pattern indicates that the tectonic framework of this sub-basin is very complex, resulting in the uneven distribution of c-p coals and regional differences in the evolution of hydrocarbon generation. relationship between temperature and vro values in organic matter following the deposition of c-p coals in the jiyang sub-basin, these units were subject to multiple phases of tectonic activity that resulted in a number of different characteristics during the indosinian, yanshan, and himalayan epochs. temperature data and curves that illustrate the evolution of vro for c-p coals in the jiyang sub-basin are presented in fig. 6 and identify four hydrocarbongeneration stages. the divisions between these stages are based on burial history and maturity variations and include the maximum depth for primary hydrocarbon generation, recovery of maturity following extended stagnation, and accelerated increase in maturity due to rapid subsidence. the temperature curve is highly variable but nevertheless conforms to a generally increasing trend, while the vro curve exhibits a smooth and uniformly increasing trend. the vro values tend to increase, consistent with temperature, while the temperature decrease led to cessation of vro evolution. thus, both decreasing and recovering temperature stages can be considered as stagnation phases in the evolution of hydrocarbon generation. the modeling results also show that the initial temperature for secondary hydrocarbon generation was lower than the maximum temperature for primary hydrocarbon generation (zou et al., 2000). this was possibly due to the low degree of initial evolution, with vro only reaching 0.8%. this “stagnation” stage resulted in residual organic matter retaining a high potential for secondary hydrocarbon generation (gao et al., 2007). characteristic features and the delay effect for secondary hydrocarbon generation as a result of the complex tectonic evolution of the jiyang sub-basin, c-p coal-bearing sequences have generally experienced secondary hydrocarbon generation, controlled by temperature and initial source rock maturity (hunt, 1991; zou et al., 2000). two features of secondary hydrocarbon generation characterize the study area. the first is discontinuities in the history of hydrocarbon generation due to changes in temperature that resulted from burial, exhumation, and re-burial. notable gaps between primary and secondary generation are evident in figure 6. the second characteristic feature is regional differences in maturity controlled by burial depth across the study area (fig. 5). thus, based on the results in fig. 6, two distinct phases of hydrocarbon generation can be identified encompassing the yanshan (from 170 ma to 100 ma) and himalayan stages (from 40 ma to the present). secondary hydrocarbon generation ceased in the jiyang sub-basin between 100 ma and 40 ma because burial depths were too shallow at this time and the temperature decreased to less than 80°c. from a spatial perspective, source rocks at different burial depths differ in their hydrocarbon-generating intensity, as indicated by experiments modeling secondary hydrocarbon generation (jin et al., 2009) because different temperatures lead to differences in organic-matter maturation. the experimental samples used for this study were immature coals from the jiyang sub-basin, and a quantitative relationship between vro and temperature and hydrocarbon productivity was obtained. thus, using the hydrocarbon productivity modeled by jin et al. (2009) (fig. 7) and current vro values (fig. 5), the intensity of hydrocarbon generation could be calculated (fig. 8). using data for thickness (fig. 2), burial depth (fig. 1b), and the distribution of c-p coal-bearing sequences, it was possible to determine that c-p coals from burial depths of more than 4,000 m with a thickness of more than 600 m had the highest hydrocarbon-generating potential (fig. 8). the largest secondary hydrocarbon intensity of each center exceeded 400 kt/km2. figure 6. geothermal and vro curves depict the evolutionary history of c-p coals in the jiyang sub-basin. i: primary hydrocarbon generation, ii: stagnation of hydrocarbon generation, iii: initiation of secondary hydrocarbon generation, and iv: peak of secondary hydrocarbon generation. 404 han sijie, sang shuxun, zhou peiming, jia jinlong, liang jingjing the maturity evolution history shows that vro values were approximately 0.75% following the stagnation of primary hydrocarbon generation in the c-p coals (fig. 6). previous studies showed that in late palaeozoic coals of the bohai bay basin, secondary hydrocarbon generation included a delay phenomenon (guan et al., 2003). this delay in vro with respect to the initial vro (i.e., the abscissa) for secondary hydrocarbon generation (fig. 9) conforms to a flexural change rule. in other words, the degree of delay (approximately 0.1% in this case) will be lowest when vro is about 0.9%. data showed that during the yanshan epoch, the delay in the primary maturity of secondary hydrocarbon generation was approximately 0.15%, while during the himalayan epoch, this delay was approximately 0.1% (fig. 9) (qin et al., 2000; zhang et al., 2002). thus, notable secondary hydrocarbon-generating maturities (i.e., generated hydrocarbon quantities of more than 10 mg/g) within the yanshan and himalayan epochs were 0.9% and 1.1%, respectively. threshold depths and favorable areas for secondary hydrocarbon generation in coal-measure source rocks according to the thermal history, delay effect, and hydrocarbongenerating maturity, the threshold depth for secondary hydrocarbon generation was approximately 2,100 m at a vro value of 0.9%. these c-p coal-bearing sequences experienced uplift during the middle cretaceous (i.e., the late yanshan epoch), resulting in stagnation of secondary hydrocarbon generation. the source rocks could not reach peak hydrocarbon generation (i.e., vro value of 1.0%) and thus areas of hydrocarbon generation were limited, for example, to the depocenter during the yanshan epoch. in contrast, during the cenozoic, an increase in burial depth resulted in an increase in vro for c-p coals from 0.9% to 1.5%. a significant transition in the vro evolution history for c-p coals was observed in the cenozoic (fig. 6). this curve suggests that the rate of vro evolution increased during this stage. when vro reached 1.1%, the source rocks entered the peak stage of hydrocarbon generation, corresponding to a threshold depth of 3200 m. by combining the results shown in figs. 1b, 2, and 5, favorable areas for secondary hydrocarbon generation in the jiyang sub-basin could be identified (fig. 10) on the basis of burial depth (i.e., exceeding 4,000 m), residual thicknesses (i.e., exceeding 400 m), and vro value (i.e., exceeding 1.0%). according to these data, favorable areas include the chexi region in the chezhen depression, the gubei-luojia region in the zhanhua depression, the yangxin area, the southern slope in the huimin depression, and the area southwest of the dongxing depression. this subdivision of favorable areas for secondary hydrocarbon generation is also indicative of current hydrocarbongenerating intensity (i.e. exceeding 200 kt/km2; fig. 8). high potential for secondary hydrocarbon generation provides the conditions necessary for the formation of gas reservoirs, and these areas may be suitable for coal-derived gas exploration and development. relationship between burial history and the evolution of hydrocarbon generation during the indosinian epoch, initial burial depths were as deep as 1,500 m and were controlled by deposition, while the temperature reached almost 90°c, and vro values exceeded 0.6%. at this time, the vro value passed the threshold for hydrocarbon generation and increased further to approximately 0.75% as burial depths increased to approximately 2,500 m. during the late indosinian and early yanshan epochs, c-p coal-bearing sequences were uplifted until they reached the surface, while the temperature decreased sharply. this resulted in stagnation of hydrocarbon generation and constant vro values at around 0.75%. finally, during the late yanshan epoch, burial depths did not figure 7. secondary hydrocarbon productivity of c-p coals from the jiyang sub-basin obtained from laboratory simulation experiments (jin et al., 2009). figure 8. present-day hydrocarbon-generating intensities of c-p coals in the jiyang sub-basin (unpublished data from gsrispa). 405burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china attain the maxima that occurred during the indosinian epoch, but source rock evolution continued and vro values approached 0.9%. this period of evolution was the result of a magmatic thermal anomaly that occurred during the yanshan epoch. because the overall paleogeothermal gradient during the mesozoic ranged between 4.33°c/100 m and 4.67°c/100 m (li et al., 2001), these strata underwent a long period (>100 ma) of heating. this contributed to an increase in vro values by approximately 0.15%. during the himalayan epoch, the basin subsided substantially, which resulted in burial depths exceeding 3,500 m, rapid subsidence rates of 40 m/ma, and an increase in temperature from 105°c to 165°c. this rapid temperature increase resulted in a corresponding increase in the rate of vro evolution, and vro values increased to their current level of 1.3%. in the center of the zhuahua and dongying depressions, the evolution of hydrocarbon generation reached the post-mature stage, but vro values were generally lower than expected at other areas in the jiyang sub-basin. the strata also attained a relatively high temperature; however, the short duration of heating suggests that the vro value of the source rocks was not as high as it would have been after a longer time at this temperature. on the basis of the discussion presented above, we observed that burial history determines the evolution of temperature and controls the evolution of hydrocarbon generation in source rocks. the hydrocarbon maturity history was consistent with the burial history. of the four stages for burial depth and vro evolution (table 3), the maturity evolution of the second stage was unchanged and characterized by a decrease in burial depth. therefore, burial depth and vro values did not correlate well during this stage. data showed that the change in vro with respect to burial depth was 0.24%/km during the first stage and 0.125%/km during the third stage. the third stage could therefore be defined as a recovery stage, characterized by abnormally high temperatures. the fourth stage was characterized by a strong subsidence process and heating conditions but had a relatively low rate of vro change compared with the first stage. the vro rate decreased to 0.22 %/km during this period due to the short heating duration. the fourth stage had the shortest time but the highest vro/time (1.0%/100 ma), suggesting that the subsidence rate played an important role in the maturation of organic matter. maximum subsidence rates were discerned in the himalayan epoch, while maximum vro/burial depths occurred in the indosinian epoch. this suggests that simple growth in the subsidence rate in the absence of a temperature increase could not significantly promote hydrocarbon generation. evaluation of the evolutionary progress of c-p coals in the jiyang sub-basin indicated that both the subsidence rate and temperature controlled the rate of vro change (fig. 11). during the middle to late yanshan epoch, because c-p coals were heated by magmatic hydrothermal activity, the rate of change in vro was relatively high (fig. 11a). this result suggests that temperature played a direct role in the vro change, and the temperature increase caused by subsidence had a weaker effect on the vro change compared with magmatic activity. these rocks experienced identical temperature changes (55°c) during the middle to late yanshan and the himalayan epochs. however, temperatures during the middle to late yanshan epoch were controlled by a magmatic thermal anomaly, while temperatures during the himalayan epoch were controlled by burial depth. as shown in fig. 11b, when temperature effects were removed, the rate of vro change was strongly positively correlated with the subsidence rate. this result suggests that the subsidence rate exerted a positive impact on the change in vro. the evolution of organic matter and increases in vro are controlled by temperature and time (ritter, 1984; ismail and shamsuddin, 1991). a high subsidence rate indicates a greater depth during the same period, and a greater depth indicates a higher temperature. a higher temperature during organic matter evolution can improve maturity at a faster rate. thus, a higher subsidence rate contributed to a stronger increase in organic matter maturation. figure 9. quantitative plots depicting delayed secondary hydrocarbon generation according to qin et al. (2000). figure 10. secondary hydrocarbon generation in favorable areas of c-p coals in the jiyang sub-basin according to thermal history and burial depth (burial depth exceeding 4,000 m, residual thickness exceeding 400 m, and vro exceeding 1.0%). 406 han sijie, sang shuxun, zhou peiming, jia jinlong, liang jingjing a previous study also suggested that a higher subsidence rate results in higher organic matter maturation and stronger hydrocarbon generation over a short time (lei et al., 2014). the calculated results show that although the maximum change in temperature occurred during the indosinian epoch, the highest increase in vro was during the himalayan epoch (1.0%/100 ma), suggesting a positive effect of the subsidence rate. because the subsidence rate remained relatively high between the late palaeozoic and the present time in the jiyang sub-basin, c-p coals were subjected to a relatively greater burial depth over a short time. this resulted in greater heating compared with a normal or low subsidence rate. the resulting elevated temperatures accelerated the maturation of organic matter, which eventually led to greater vro change over the course of hydrocarbon generation. during the middle to late yanshan period, the temperature did not return to the maximum temperature of the indosinian period, but rather, vro increased by 0.15% at a low rate of increase (0.09%/100 ma). this suggests that the initiating temperature for secondary hydrocarbon generation was lower than the highest temperature during primary hydrocarbon generation. however, a significant subsidence in the himalayan epoch caused a higher increase in temperature and vro. this indicates that a higher temperature accelerated the evolution of hydrocarbon generation. a higher subsidence rate contributed to a stronger heating effect on the organic matter present and this increase in temperature enhanced the evolution of hydrocarbon generation. therefore, both the subsidence rate and temperature played important roles in the evolution of hydrocarbon generation. conclusion (1) the burial, thermal, and maturity histories of c-p coal measures in the jiyang sub-basin were reconstructed using petromod modeling. the results show that the c-p coals experienced four stages of hydrocarbon generation: primary hydrocarbon generation, stagnation, initiation of secondary hydrocarbon generation, and peak secondary hydrocarbon generation. (2) the secondary hydrocarbon generation of c-p coals could be characterized in terms of discontinuous hydrocarbon generation and geographical differences in vro distribution. the process of secondary hydrocarbon generation could be divided into two phases: the yanshan stage and the himalayan stage. threshold maturities for secondary hydrocarbon generation within these stages were 0.9% and 1.1%, respectively, which were both impacted by a delay effect. (3) the threshold depth for initial secondary hydrocarbon generation was 2,100 m during the yanshan epoch, while peak hydrocarbon generation occurred at a depth of 3,200 m during the himalayan epoch. observations showed that favorable areas for secondary hydrocarbon generation consist of the chexi, gubei-luojia, and yangxin areas, as well as the southern slope of the huimin depression and the southwestern section of the dongying depression. (4) the vro change rate associated with burial depth in c-p coals in the jiyang sub-basin was 0.24%/km during the indosinian epoch, 0.125%/km during the middle to late yanshan epoch, and 0.22%/km during the himalayan epoch. a higher subsidence rate contributed to a stronger heating effect on the organic matter and this increase in temperature enhanced the evolution of hydrocarbon generation. acknowledgements this research was financially supported by the natural science foundation of china (grant nos. 41727801, 41802182, and 41272154), geological table 3. relationship between burial depth and vro. the stages for vro values are derived from the division of burial history stage 1 2 3 4 time 290–225 ma 225–200 ma 200–40 ma 40 ma–present epoch indosinian early yanshan middle-late yanshan himalayan burial depth range (m) 400–2300 2300–800 800–2000 2000–3800 subsidence rate (m/ma) 29.3 denudation 7.5 45 temperature range (°c) 35–115 115–55 55–110 110–165 vro range (%) 0.3–0.75 0.75 0.75–0.9 0.9–1.3 vro/burial depth (%/km) 0.24 0 0.125 0.22 vro/time (%/100 ma) 0.69 0 0.09 1.0 vro/burial depth = (difference between the maximum and minimum values for vro)/(difference between the maximum and minimum values for burial depth). figure 11. rate of vro change in relation to (a) temperature and (b) the subsidence rate. 407burial history of carboniferous-permian coals and its impact on secondary hydrocarbon generation in the jiyang sub-basin, china exploration foundation of guizhou province (208-9912-jbn-uts0), the key laboratory of coalbed methane resource & reservoir formation process, ministry of education, china university of mining and technology (2018-004), and the priority academic program development of jiangsu higher education institutions (papd). we thank the geological scientific research institute, shengli petroleum administration (gsrispa) for providing measured data and logging results. the authors are grateful to reviewers for their constructive comments and suggestions, which helped to improve the manuscript significantly. references ai, t. & li, r. 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(2000). maturity control on the patterns of hydrocarbon regeneration from coal. acta geology sinica, 74(2), 370–374. ilgin lies on newly formed, loose, granular deposits, and there is substantial risk for surface liquefaction and foundation settlements due to the seismic effects resulting from groundwater close to the surface. this study evaluates potential settlement due to seismic effects in the residential areas of ilgin using the standard penetration test (spt) performed on 45 geotechnical bores. in turkey, where earthquakes occur frequently, the selection of residential areas is of great importance. in this research, the number of settlements was calculated considering an earthquake having a local magnitude of 6 (i.e., ml ≥ 6.0 and a ≥ 0.4 g) under a 0.4 g seismic force, and a potential settlement map of the residential area was prepared. the amount of settlement exceeds 20 cm at locations near ilgin lake and in the northern section of ilgin residential areas; downtown, the settlement ranges from 10-20 cm. the settlements presented here exceed the allowable threshold limits for structures constructed using adobe and brick in this district. thus, improvements to minimise earthquake-induced damages are required for structures in ilgin. moreover, the selection of new residential areas, along with the proper design of the structures before construction, should be examined further to avoid ground liquefaction and structure damage due to settlement. la localidad de ilgin está ubicada sobre depósitos recién formados, granulares y no compactos, por lo que existe un riesgo sustancial de licuefacción de la superficie y la creación de asentamientos o deslizamientos debido a los efectos sísmicos resultantes del agua subterránea poco profunda. este artículo evalúa el potencial de asentamiento debido a los efectos sísmicos en las áreas residenciales de ilgin a través del ensayo de penetración estándar (spt, en inglés) realizado en 45 perforaciones geotécnicas. en turquía, donde los terremotos ocurren frecuentemente, la selección de áreas residenciales es de gran importancia. en esta investigación, se calculó el número de asentamientos ante un terremoto con magnitud local (ml) de 6 y con una fuerza sísmica de 0.4 g para preparar un mapa de asentamientos en el área residencial. la cantidad de asentamientos supera los 20 centímetros en lugares cercanos al lago ilgin y en la sección norte del área residencial; en el centro, los rangos de asentamiento van de 10 a 20 cm. los asentamientos presentados exceden los límites de lo permitido para estructuras construidas en adobe y ladrillo en este distrito. por esto, se requieren mejoras para minimizar los daños inducidos por terremotos en las estructuras de ilgin. además, la selección de nuevas áreas residenciales, junto con el diseño apropiado de las estructuras antes de la construcción, debe ser revisado atentamente para evitar la licuefacción del suelo y el daño de las estructuras debido al asentamiento. abstract resumen keywords: soil settlement, soil dynamic, liquefaction, earthquake, ilgin. palabras clave: asentamiento del suelo, dinámica de suelos, licuefacción, terremoto, ilgin. potential settlement due to seismic effects in the residential area of ilgin (konya, turkey) record manuscript received: 10/04/2015 accepted for publication: 11/03/2016 how to cite item ozdemir, a., & nalbantcilar, m. h. (2016). potential settlement due to seismic effects in the residential area of ilgin (konya, turkey). earth sciences research journal, 20(2), f1-f9. doi:http://dx.doi.org/10.15446/esrj.v20n2.50062 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.50062 adnan ozdemir, necmettin erbakan university, department of civil engineering, konya, turkey. m. tahir nalbantcilar, batman university, department of geological engineering, batman, turkey. earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : f1-f9. asentamiento potencial del suelo debido a efectos sísmicos en el área residencial de ilgin (konya, turquía) n a t u r a l h a z a r d s f2 adnan ozdemir and m. tahir nalbantcilarf2 1. introduction ground oscillation, liquefaction, flows, slides, and settlements caused by seismic effects result in damage or destruction of transportation infrastructure, energy transmission lines, municipal water infrastructure, wastewater facilities, communication lines, and other engineered structures. earthquakes can also lead to significant economic losses and loss of human lives (bardet and kapuskar, 1993; clough et al., 1994; kimura, 1996; holzer, 1998). displacements, which occur in various forms due to the variety of flows, slides, falls, precipitations, and settlements, may exceed 10 m at times (zhang et al., 2004). porewater pressure, which appears to increase during seismic cycles in water-absorbable sandy soils, gradually dissipates after an earthquake, and the settlement, which is the vertical component of the volumetric deformations to the ground, can significantly damage nearby engineered structures. settlements can even result in the destruction of structures. furthermore, differential settlement caused by the nonuniformity of structures can result in increased structural damage. currently, the selection of residential areas and the implementation of earthquake-design constructions are of high importance. recent studies have been focused on determining the most appropriate residential areas and designing earthquake-resistant structures (bartlett and youd, 1995; hamada et al., 1996; seed, 1979; ishihara and yoshimine, 1992; o’rourke and pease, 1992; stewart et al., 2002; stewart and whang, 2003; zhang et al., 2004; kayabali and beyaz, 2011). the selection of residential areas is particularly important in turkey, where earthquakes occur frequently. earthquake-induced damages can be devastating due to the limited number of earthquake-resistant structures that can tolerate both seismic settlements and differential ground settlements. (kayabali, 1997; ulusay et al., 2000; cetin et al., 2002; mollamahmutoglu et al., 2003). thus, some researchers have investigated liquefaction and shear failures in soils. zhang et al. (2002) examined the estimation of liquefactioninduced ground settlements from cpt data, while andrus et al. (2004) compared the liquefaction evaluation methods using the penetration-vs relations. tatsuoka et al. (1987) observed settlement in saturated sand due to cyclic undrained simple shear. yoshimine et al. (2006) showed the flow deformation of a liquefied sand layer under a constant shear load and analysed the slide flow of an infinite slope. this study focuses on the determination of ground settlement potentials in the residential areas of ilgin (figure 1), which is located in the first-degree earthquake zone of the region of lakes (i.e., the centralsouthwest of turkey). in situ test data were obtained by the standard penetration test (spt). although there is much interest, previous studies have not studied ground settlement due to seismic effects. generating fundamental data to inform the planning of residential areas (in both ilgin and other regions) and the design of earthquake-resistant structures requires determining the ground deformations due to the seismic movements, as well as liquefaction susceptibility of mediumhigh and low resistance areas (ozdemir and ince, 2005). this particular study determines the level of the settlements in ilgin and similar areas by performing a full evaluation of the morphology, geology, seismotectonics, and liquefaction potential of the residential areas in ilgin. in the following sections of the study, the morphology, geology, and hydrogeology after seismo-tectonics, as well as the liquefaction potential of the study area, are examined. furthermore, the calculation details for saturated and unsaturated soil settlements with seismic effects are given. figure 1. map of the study area 2. materials and methods 2.1 location information, morphology, geology, and hydrogeology ilgin is located 90 km northwest of konya in the south-central section of central anatolia (figure 1). ilgin is bordered by hamam sirti to the southwest and sivri tepe to the northeast. the town is located on a flat area, and the study area has an altitude between 1025–1035 m. cavuscu lake is approximately 5 km northeast of ilgin. the study area is located to the south and north of the central section, while the overall topographical inclination is from west to east due to the major hills in the region. the topographical inclination in the residential areas varies between 1 and 3%. there are irrigation canals in the western and northern sections that run from south to north and from west to east (figures 1 & 2, respectively). figure 2. geological map of the ilgin residential area and vicinity (ozdemir and ince, 2005) f3potential settlement due to seismic effects in the residential area of ilgin (konya, turkey) the geology and hydrogeology of ilgin and its vicinity has been previously researched (boray et al., 1985; saroglu et al., 1987; barka et al., 1995; kocyigit et al., 2000; canik, 1981). the regional geology, hydrogeology, tectonics, historical earthquakes, and potential for liquefaction of the study area has also been previously investigated in detail (ozdemir and ince, 2005). data from previous studies that is relevant to this study is summarised in the next paragraph. the region consists of neogene and pre-neogene aged rock units along with quaternary units. triassic-aged marble (trm) and schist (trsc) are present in the region. these units do not show liquefactionsettlement potential resulting from seismic effects. additionally, there are units such as tertiary-aged conglomerate, limestone, sandstone, and marl (nc-kg-s) that are not susceptible to liquefaction-induced settlement in the residential areas. such units are unconformably overlain by quaternary gravel (qg), sand (qs), silt (qm), and clay (qc). 2.2 tectonics, seismo-tectonic, and ground liquefaction potential the thicknesses of quaternary-aged units, which have liquefaction and settlement potential, vary from 20-50 m and 0-20 m north of the selected study location and at the other locations, respectively. vertical and lateral transitions at short distances are present within the quaternaryaged units. areas in ilgin are underlain by loose quaternary units, and liquefaction susceptibility of such loose precipitation is medium to high (ozdemir and ince, 2005). a groundwater map was drawn from data obtained by water depths measurements in 45 boreholes during july of 2000 (figure 3). the groundwater table depth ranges from -2 m to -4 m. figure 3. locations and values of groundwater level measurements (ozdemir and ince, 2005) ilgin is located at the intersection of the following four active major faults: the southwest trending sultandagi (boray et al., 1985; saroglu et al., 1987), the e-w trending argithani fault (kocyigit et al., 2000), the nne-wws trending mecidiye fault, and the n-s trending cavuscu fault. there are also numerous secondary faults in the region. earthquakes ranging in magnitudes from 5.5-6.5 have been recorded. as ilgin is located on recently formed and loose precipitations, there is a high potential for liquefaction following a seismic event (figures 2 & 3). 2.3 ground settlement due to earthquakes calculations of expected seismic settlements based on the unsaturated (i.e., dry) and/or saturated soil settlement resulting from seismic movements have been extensively investigated (pyke et al., 1975; tokimatsu and seed, 1987; ishihara and yoshimine, 1992; shamoto et al., 1998; darendeli and stokoe, 2001; liu et al., 2001; seed et al., 2001; stewart et al., 2001; stewart et al., 2002). in this study, seismic settlements were calculated using the most commonly used method, the standard penetration test (spt), developed by tokimatsu and seed (1987). the spt blow counts obtained from the 45 borehole logs were used for the calculations. the correction factor of transferred energy for the spt test was taken as 0.75 (seed et al., 1985), and the correction coefficient for the diameter of the borehole was taken as 1 (robertson, 1994; robertson and fear, 1996) to obtain the corrected blow counts n1(60) from the raw spt blow counts (equation 2.1). the correction factor for the effective overburden stress was determined according to liao and whitman (1986) (equation 2.2). the cyclic stress ratio (csr) was determined according to seed and idriss (1971) (equation 2.3), and the stress reduction factor was determined according to nceer (1997) (equations, 2.4-2.7). where: (n1)60 is the corrected spt blow count, cer is the energy ratio in percent, and cn is the overburden stress correction factor (nceer, 1997). where: σ0’ is the effective vertical overburden stress and n is the raw spt blow count. where: csr is the cyclic stress ratio induced by a given earthquake, σ0 is the total vertical overburden stress, σ’0 is the effective vertical overburden stress, amax is the peak horizontal ground acceleration (pga) in g, and rd is an estimated stress reduction coefficient (nceer, 1997). the volumetric deformations were estimated according to the corrected spt blow counts and the cyclic stress rates (csr) using the curves given by tokimatsu and seed (1987) (figure 4). the settlement (ssat) of each layer (i) was calculated as the product of the volumetric deformation and the thickness (hi) of the layer (equation 2.8; tokimatsu and seed, 1987). figure 4. volumetric strain versus corrected standard penetration blow count [(n1)60] and cyclic stress ratio (csr) (tokimatsu and seed, 1987) f4 adnan ozdemir and m. tahir nalbantcilarf4 where: ssas is the settlement of saturated soil; i is the number of layers; n is the total number of layers; εc, is the volumetric deformation; and hi is the thickness of layer i. because the minimum peak horizontal ground acceleration (pga) of 0.4 g should be used when determining structural designs in ilgin, which is located in the most dangerous first-degree earthquake zone (aydinoglu, 1998), the pga in this study was taken as 0.4 g when calculating the settlements. the settlements on dry soil are calculated according to tokimatsu and seed (1987), using an estimated earthquake magnitude (richter magnitude, ml) of 6. for dry soil settlement calculations, the procedure is the same but uses the corresponding spt input data. the dry soil settlement is calculated by the following six steps: i. gmax, the shear modules of the soil at small strain, is estimated (equation 2.9; seed and idriss, 1970). where: (n1)60 is the normalised spt blow count (unitless) and σm is the mean normal total stress in kpa, which is approximated as 0.65 σv (σv is vertical stress). ii. shear strain-modulus ratio, is determined to evaluate the cyclic shear strain (equation 2.10) by the shear modulus, gmax: where: is a hypothetical effective shear strainmodulus ratio, amax is the peak ground acceleration, and csrm=7.5 is the cyclic stress ratio for an earthquake with a magnitude of 7.5. iii. effective shear strain (γeff) is estimated using the previously calculated shear strain – shear modulus ratio (figure 5). figure 5. plot for determining earthquake-induced effective shear strain in deposits (tokimatsu and seed, 1987) iv. the volumetric strain due to compaction, εc, for an earthquake with a magnitude of 7.5 (15 cycles) is determined (figure 6), and the shear strain is evaluated (step 3; assuming that γeff ≈ γc (cyclic shear strain)). figure 6. relation between volumetric strain (εc), cyclic shear strain (γc), and corrected spt blow count for (n1) unsaturated sands (tokimatsu and seed, 1987) v. figure 6 corresponds to an earthquake with a magnitude of 7.5, with approximately 15 representative cycles. magnitude correction of the volumetric strain was employed because the figures used above were developed for a magnitude 7.5 earthquake. for earthquakes with magnitudes other than 7.5, the volumetric strain (εc) is obtained from the volumetric strain scaling factor rm (equation 2.11). values for rm are obtained from a plot of magnitude correction factor versus magnitude: where: εc,m is calculated by multiplying εc m=7.5 with the magnitude strain ratio (rm). figure 7. magnitude correction factor versus magnitude (tokimatsu and seed, 1987) vi. dry soil settlement is calculated using the volumetric strain (equation 2.1): 2.11 f5potential settlement due to seismic effects in the residential area of ilgin (konya, turkey) f5 where: sdry is the settlement of dry soil; εc,m is the volumetric deformation [magnitude (ml) = 6]; hj is the thickness of the unsaturated or dry soil layer j; and n is the total number of soil layers. then, the total settlement (stotal) at any specified depth, d, in the saturated and unsaturated (ssat) or in the dry soil (sdry) is determined (equation 2.13): where: stotal is the earthquake induced total settlement at a certain depth (d); gwt is the depth of groundwater level; and d is the certain depth. the spt blow counts used in this study, along with the other relevant data, are provided in table 1. the settlements calculated for ground thicknesses of less than 10 m are presented in table 2, and the map generated for the potential settlements according to the existing settlements at the depth of 2 m is shown in figure 8. table 1. data used for the calculation of seismically affected settlements (mercan-su, 2000) f6 adnan ozdemir and m. tahir nalbantcilar table 2. calculated settlements 3. results and discussion ilgin and its vicinity are located in a first-degree earthquake zone; are at the intersection of the argithani, mecidiye, and cavuscu faults; are on the southeastern section of the active sultandagi fault; and experience seismic movements in the range of 5.5≤m≤6.5. earthquakes that hit sultandagi and cay towns, which are 75 km to the northwest of the centre of ilgin, between 2000-2003 were measured in the range of 6-6.2, based on local magnitude. they caused significant damage and deaths in the region. however, it is well-known that seismic energy on the northeast section of the fault (i.e., ilgin and its surroundings) has not yet been discharged (kocyigit et al., 2002). studies indicate that there is a seismic gap around the argithani fault (demirtas and yilmaz, 1996). the extent of the damage to structures is moderate, and damage was observed in the centre of ilgin due to the effects of the recent earthquakes in sultandagi and cay towns. these data indicate that any future earthquake hitting the ilgin epicentre or its proximity will cause considerable damage to structures, and the degree of damage largely depends on the settlements and ground liquefaction. any earthquake of 6.0≤m≤6.5 would inevitably lead to fatal damage. pre-quaternary rocky units and quaternary-aged and loose deposits are present in the residential areas of ilgin. there is a potential for liquefaction and settlement of the quaternary units due to seismic effects. quaternary-aged and loose precipitations not only have the potential for liquefaction but also the potential for settlement due to seismic effects. ozdemir and ince (2005) showed that the ilgin soils have medium to high liquefaction potentials (i.e., 70-80%). in general, the groundwater flows from the southwest to the northeast. the groundwater level depth is between 1.5 m to 6 m in most f7potential settlement due to seismic effects in the residential area of ilgin (konya, turkey) sections except for hamam sirti and sivri tepe; however, in several sections the water level is as high as 2-4 m. groundwater levels near the surface contribute to the risk of liquefaction and oversettlement of the ground (dependent on the degree of liquefaction). statistical analysis of the settlement potential for a ground thickness of 10 m under the seismic movement caused by an earthquake (6 on the local magnitude), examined using the results of the spt test (table 3), shows that the maximum settlement is 2 m. settlement varies from 0-22.5 cm for different depths, with an average settlement of 10.3 cm. at a depth of 4 m, settlement ranges from 0-22 cm, with an average settlement of 9 cm. however, the standard deviations of settlements are large and reflect the complexity of the ground. table 3. statistical analysis of the calculated settlements settlements larger than 20 cm are found at borehole 70 to the north of ilgin, near the region of the ilgin lake, and settlements between 15 cm and 20 cm are observed in the western, northern, and central sections of ilgin (figure 5); settlements are to the lower south of ilgin. when settlements were evaluated in conjunction with the geological map, the maximum settlement is observed around the borehole number 70; settlements between 15 cm and 20 cm are found in the sandy alluvium area (qs). south of ilgin, where the potential for settlement is relatively low, gravel and clayish alluvium are found together with sandy alluvium. sandy alluvium is between gravel and clayish alluvium, indicating that sandy alluvium is largely comprised of clay and gravel. in this particular section, the lower settlement is partially due to deeper groundwater levels than those found in other sections. when the geological, hydrogeological, and settlement maps are combined, it is clear that the groundwater level is in close proximity to the surface, and in sections containing sandy alluvium, intense settlements can follow an earthquake. the map showing the settlement potential indicates numerous areas with settlements in the range of 10-15 cm. figure 8. a map showing the settlement potential due to seismic effects in the residential area of ilgin the indications on the ground due to the settlement effect and their relation with the expected damage to the structures are presented in table 4 (ishihara and yoshimine, 1992). table 4. relation between settlement and degree of damage (ishihara and yoshimine, 1992) in general, the settlements due to seismic effects near ilgin are between 10-20 cm. small cracks and sand penetration to the surface of the earth can also be expected in conjunction with the medium-degree damage to structures. it should be noted that the structures in ilgin and the vicinity are in the form of heaps, and the major structural materials are adobe and briquettes. a few of these structures have had engineering services provided because their seismic resistance is quite low. even an earthquake on the order of 5≤m≤ 5.5 in the region could cause substantial damage and a large loss of life due to the poor materials used in the structures. the maximum allowable settlement for structures made of adobe and briquette is 8-10 cm (whals, 1981). settlements exceeding 10 cm were predicted in sections of ilgin and the vicinity, indicating that settlements due to seismic effects have the potential to exceed the permissible threshold values for structures in the area. the settlements in the northern section of the istasyon quarter of ilgin (15 cm 22.54 cm) exceed the maximum permissible limit for structural reinforced concrete (15.2 cm). during the 2000 and 2003 earthquakes in sultandagi and afyon, respectively, cracks in ilgin structures became visible, depending on the amount of the settlement. however, no systemmatical data were recorded for these settlements. seismic activity above ml= 6 within 75 km of the centre of ilgin would likely increase damage to a large extent due to settlement. to avoid damage, studies focused on earthquake damage minimisation should be conducted. in addition, recommendations of the best available planning/ designing activities should be made and implemented for residential areas to avoid ground liquefaction and damage arising from settlement caused by earthquakes. 4. conclusion the following conclusions are made based on the outcome of this research. (1) the expected settlements of the residential areas in ilgin, which bear medium and high degrees of liquefaction potential, were determined to exceed the allowable settlement limits of the structures. (2) settlements of 5 cm were observed to the south of ilgın, while in the northern and north-eastern areas, the settlement amounts reached 15-20 cm. (3) north of the selected area, at the sections near the region of the lake of ilgin, the settlements were observed to be 23 cm. (4) the structures in the region are not earthquake-resistant structures. 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(in turkish). seed, h.b., idriss, i.m., 1971. simplified procedure for evaluating soil liquefaction potential. journal of the soil mechanics and foundations division, asce 97 (sm 9), 1249-1273. seed, h.b., tokimatsu, k., harder, l.f., chung, r.m., 1985. the influence of spt procedures in soil liquefaction resistance evaluations. j. geotech. eng., 111(12), 1425-1445. seed, h.b., 1979. soil liquefaction and cyclic mobility evaluation for level ground during eartquakes. j. geotech. engrg. div., asce 105(2), 201-255. seed, r.b., cetin, k.o., moss, r.e.s., kammerer, a.m., wu, j., pestana, j.m., riemer, m.f., 2001. recent advances in soil liquefaction engineering and seismic site response evaluation. proc. 4th int. conf. on recent advances in geotech. eqk. engrg. soil dyn., paper no. spl-2. shamoto, y., zhang, j., tokimatsu, k., 1998. new charts for predicting large residual post-liquefaction ground deformations. soil dyn earthquake engrg, 17: 427-438. stewart, j.p., bray, j.d., mcmahon, d.j., smith, p.m., kropp, a.l., 2001. seismic performance of hillside fills. j. geotech. & geoenv. eng., 127(11), 905-919. stewart, j.p., smith, p.m., whang, d.h., bray, j.d., 2002. documentation and analysis of field case histories of seismic compression during the 1994 northridge, california earthquake: report no. peer-2002/09, pacific earthquake engineering research center, u.c. berkeley, october. weathering involves important processes that alter the original structure, texture and chemical components of rocks and minerals. bulk changes produced by weathering in granitic rocks have been studied by several methods, including chemical weathering indices. they are based on the assumption that some ions are more easily leached from minerals in relation to others. such methods have also been briefly tested on building stone but not in urban environments, where fast stone weathering rates are typically observed, mainly due to interaction of minerals with several pollutants and where other specific processes, such as salt weathering, can occur. the aim of this work is to discuss the use of weathering indices in the study of weathering rates on granitic stones applied in four historical buildings of an urban area. results suggest that some factors can cause scatter of results in the relation indices vs. exposition age in the built environment, namely previous weathering degree in rock massifs and the quarry, different orientation of façades and different exposure to urban pollutants. indices that consider some highly mobile cations that are present in other building materials (namely ca that is related to leaching from mortar joints) should be avoided due to the uptake of such elements by the stone pores or used to assess this some types of weathering. moreover, the use of some indices shows more reliable results. la meteorización implica importantes procesos que alteran la estructura, textura y componentes químicos originales de las rocas y minerales. los cambios producidos por la meteorización en rocas graníticas han sido estudiados por varios métodos, incluyendo los índices de meteorización química. éstos se basan en la suposición de que algunos iones son lavados de los minerales más fácilmente que otros. tales métodos también se han probado, aunque en pocos casos, en rocas de edificios, pero no de ambientes urbanos, en donde se han observado típicamente elevadas tasas de meteorización, principalmente por la interacción de los minerales con varios compuestos contaminantes y en dónde otros procesos específicos, tales como la erosión por sales, pueden producirse. el objetivo de este trabajo es discutir el uso de índices de meteorización en el estudio de las tasas de meteorización de rocas usadas en cuatro edificios históricos de un área urbana. los resultados sugieren que algunos factores pueden causar la dispersión de los resultados obtenidos a través de los índices en relación al tiempo de exposición en el ambiente construido, principalmente por el grado de meteorización previo de la roca en los macizos rocosos y en la cantera, la orientación de las fachadas y la diferente exposición a compuestos contaminantes. los índices que consideran algunos cationes altamente móviles que están presentes en otros materiales de construcción (principalmente el ca que se relaciona con la meteorización de morteros en las juntas) deben ser evitados debido a que penetran en los poros de las rocas de edificios o deben ser usados para evaluar algunos tipos de meteorización. por otra parte, el uso de algunos índices muestra resultados más fiables. earth sciences research journal earth sci. res. j. vol. 20, no. 3 ( september, 2016 ) : f1f13 abstract resumen keywords: weathering indices, weathering rates, urban environment, granitic rocks, historical buildings palabras clave: índices de meteorización, tasas de meteorización, ambientes urbanos, rocas graníticas, edificios históricos. assessing the weathering of granitic stones on historical urban buildings by geochemical indices evaluación de la meteorización de rocas graníticas en edificios históricos urbanos por medio de indices geoquímicos record manuscript received: 09/03/2015 accepted for publication: 27/05/2016 how to cite item sanjurjo-sánchez, j., alves, c., vidalromaní, j.r. (2016). assessing the weathering of granitic stones on historical urban buildings by geochemical indices. earth sciences research journal, 20(3), f1-f13 doi:http://dx.doi.org/10.15446/esrj.v20n2.49560 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.49560 jorge sanjurjo sánchez1, carlos alves2 and juan ramón vidal romaní1 1instituto universitario de xeoloxía “isidro parga pondal”, campus de elviña, universidade da coruña, 15071 a coruña, spain. e-mail:jsanjurjo@udc.es 2lab2pt-landscape, heritage and territory laboratory school of sciences, university of minho, braga, portugal. g e o c h e m is t r y jorge sanjurjo sánchez, carlos alves and juan ramón vidal romaníf2 1. introduction stone surfaces suffer alteration processes when exposed to surface agents (water, atmospheric chemical components, etc) resulting in the formation of coatings and/or weathering. such alteration occurs at variable rates on natural outcrops but faster on urban environments (amoroso and fassina, 1983; sanjurjo-sánchez et al., 2012; sanjurjo-sánchez and alves, 2012a. 2012b), being necessary to take conservation measures (alves and sanjurjosánchez, 2015). weathering produces modifications in the structure, texture, mineral and chemical components of rocks. it results in increasing stone porosity (and weakening) and the disintegration of the rock and minerals. these changes have been extensively observed by petrological, physical and chemical studies (meunier et al., 2007; moses et al., 2014). chemical weathering leads to the decomposition of the original minerals of fresh (unweathered) rocks. some authors (topal, 2002) have postulated that weathering can occur on the earth surface in geologically short time periods (even decades or centuries). depending on the rock composition, environmental factors (temperature, ph, eh, ionic potential) and the previous weathering degree, some ions can be leached from minerals preferably in relation to others. the leaching processes are linked to other effects such as mineral transformations, porosity increase and mechanical strength decrease (meunier et al., 2007; chiu and ng, 2014; moses et al., 2014). chemical variations have been considered by many researchers in the definition of chemical weathering indices to assess quantitatively the degree of weathering (reiche, 1943; ruxton, 1968; irfan, 1996; ng et al., 2001; duzgoren-aydin et al., 2002; otha and arai, 2007). according to gerrard (1998), miller proposed in 1961 the following mobility sequence for ions of igneous rocks: ca>mg>na>ba>k>si>-fe=mn>ti>al. however, other different sequences have been proposed for granites as that of gupta and rao (2001): na>ca>k>mn>mg>si>fe>al>ti. several weathering indices have been successfully applied to assess the weathering degree on igneous rocks (ng et al., 2001). some indices have also been tested on building stones (topal and sözmen, 2003; lee and yi, 2007; lee et al., 2009), where surface weathering occurs and can be assessed for periods of decades or centuries (meunier et al., 2007; smith, 2003; moses et al., 2014). however, to our knowledge, such indices have not been tested on stones applied in buildings located in the urban environment, despite the fast decay observed on them mainly due to interaction of minerals with different pollutants (amoroso and fassina, 1983; sanjurjo-sánchez and alves, 2012a). fast decay in buildings is observed in urban areas as well as high weathering rates that could result in chemical and physical erosive processes. in addition, the built environment entails specific weathering processes namely salt weathering. contributing sources to salt contamination and weathering are variable leading to different geochemical signatures (sanjurjo-sánchez and alves, 2012a). the possible sources can be related to surrounding natural sources such as sea spray (whose impact is enhanced in some portions of the built environment according to erosion) and human-related pollution either from the ground (capillary rising), atmospheric (air pollution) and other building materials such as joint mortars (sanjurjo-sánchez et al., 2010). these sources can contribute diverse substances such as carbonates, sulphates, chlorides and nitrates or alkaline and earth alkaline elements (sanjurjo-sánchez and alves, 2012a). previous studies on buildings regarding chemical weathering indices have been focused on the surface and subsurface weathering (comparing weathering vs. depth), but not in the weathering rates (comparing weathering vs. time). to assess such rates, it is important to check the suitability of different weathering indices to assess the relation age/weathering for these stones and buildings. for this purpose, the behaviour of major and minor components of rock minerals was previously studied on building granite in an urban area (sanjurjo-sánchez et al., 2011a). depending on the considered elements, different weathering indices provide variable results in the leaching of some mobile elements. the aim of this paper is to compare the suitability of weathering indices to study the weathering rates on granite surfaces in a polluted environment and to assess them (relation age/weathering) for granitic stones taken from different parts of four historical buildings (built in different historical time periods) of an urban area. such comparison must be taken carefully as the weathering process could be variable in different time periods. in this sense, the exposition of the rock ashlars to strong air pollution conditions has been limited, above all, to the last century or half-century. 2. determination of weathering indices several chemical weathering indices have been proposed for quantification of weathering (table 1) and tested to assess weathering depth profiles in natural outcrops or weathering rates after exposure to artificial weathering tests. in general, a good result has been observed for most indices to assess weathering (irfan, 1996; kim and park, 2003), but they usually provide scattered data depending on the type of weathering (chemical or not), heterogeneity of the studied rocks and the rate of weathering. the indices that have been proposed can be classified as either absolute or relative. relative indices allow determining the weathering degree of different samples from the same rock type by comparing the proportion of some mobile and immobile oxides. absolute indices are based on the assumption that the fresh rock composition is known. they assume that some elements (i.e. al, ti) remain constant during the weathering process. thus, the degree of weathering can be measured by the loss of mobile oxides from the analysis of a reference non-weathered rock sample (kim and park, 2003). figure 1. pictures of the studied granitic rocks: (a) san pedro leucogranite quarry front (left fresh rock. right weathered rock). (b) w façade of colegiata de santa maría built with san pedro leucogranite. (c) detail of a wall of the s façade of colegiata de santa maría built with san pedro leucogranite (bottom) and bregua granodiorite (top). (d) bregua granodiorite ashlars of casas de paredes (note erosive features on the window lintel). assessing the weathering of granitic stones on historical urban buildings by geochemical indices f3 multiple relative indices have been proposed and tested from the first insight of reiche (1943) who proposed several weathering indices to assess weathering, taking into account a decrease in mobile cations. relative indices are based on the decrease of some mobile cations (ca, na, k) compared with immobile ones (al, fe, si), expressed as ratios (parker, 1970; irfan, 1996; gupta and rao, 2001; ng et al., 2001; arel and tugrul, 2001; kim and park, 2003). in other cases, the indices compare the loss of major cations (harnois and moore, 1988; jayaverdena and izawa, 1994; sing et al., 1998). alternative indices consider the loss on ignition (loi) or the water content, reflecting the increase of weathering caused by hydration of clays, and they are considered as sensitive to weathering under humid conditions (ng et al., 2001; kim and park, 2003). other authors have proposed absolute indices such as the lixiviation index (β), insensitive to mild chemical weathering processes, but useful to compare the evolution of mobile cations to al (ng et al., 2001). irfan (1996) identified the different behaviour of mobile and immobile elements during weathering and devised an index that is considered a good indicator of weathering, known as the mobiles index (imob) (guan et al., 2001). the use of weathering index on building stones has been scarce. esaki and jiang (2000) used the loi index in the study of the weathered condition of tuffs (vs. depth) applied in historical bridges, inferring that chemical weathering attained around 10 cm of depth. topal and sözmen (2003), in a study of tuffs in the midas monument (carved in tuffs in a little village of central anatolia, turkey) compared diverse chemical indices and refer that loi and wpi indices were good indicators for weathering depth quantification. lee and yi (2007) studied the weathering degree of diverse igneous rocks (tuffs, granites, andesitic and basaltic rocks) used in a stone pagoda located in a rural area of south korea (near a medium-size city), concluding that the chemical index values were high for all the rock types. they also observed that weathering indices showed very active potential for studying chemical weathering in all the stone materials presented (basaltic and andesitic rocks being the most sensitive). lee et al. (2009), in a study of granite stones in an historical building (located 10 km far from the city of gyeongju, south korea), stated that the weathered stones presented slightly higher weathering index than the fresh granite. the scarcity of studies performed on historical buildings is due to the difficulties to get and analyse samples from materials considered of high cultural value. in the last years, non-destructive techniques such as portable xrf have allowed obtaining chemical analyses on the surface of stones table 1. summary of chemical weathering indices used in the study. of historical buildings. however, the non-destructive techniques do not allow obtaining information in depth or they can be difficult to apply in heterogeneous rocks. to consider reference values (unweathered samples) to compare the indices, we have used quarry samples as an alternative to surface-depth comparison. thus, the study of quarry samples provides essential information to assess the weathering degree of building stones. despite some weathering indices were tested on building rock surfaces some aspects must be considered in urban environments that not occur either on natural outcrops or buildings in non-urban environments. these are pollution sources and façade orientations. several pollution sources lead to different geochemical signatures (sanjurjo-sánchez and alves, 2012a) being the sources correlated to natural sources, such as sea spray or anthropic sources such as air pollution due to human activities (e.g., traffic exhaust, industrial emissions, heating system emissions). moreover, water inputs in building materials can come either from the ground (capillary rising), atmospheric water (air pollution) or other building materials, being the pollutants present in the water different, but dissolving and transporting carbonates, sulphates, chlorides or nitrates to the building rocks (sanjurjo-sánchez and alves 2012a, 2012b). these effects must be considered in the interpretation of the results of the use of weathering indices. moreover, some aspects must be taken into account referring rock weathering in buildings. the direct or not direct exposure to sunlight of could result in more or less marked thermal and wetting/drying cycles (wendler et al., 1990). wetter f4 conditions can cause stronger dissolution and leaching of some mobile elements in the rock while more marked thermal cycles can cause more physical weathering. besides, the exposure of façades to pollutants can be variable depending on their distance and orientation to air pollution sources (e.g. sea spray, atmospheric pollution) as it has been observed (sanjurjo-sánchez et al., 2009, 2011a, 2012). 3. study area the studied building stones are located in a coruña, a town on the nw coast of spain with approximately 250 000 inhabitants and about 600 000 in the metropolitan area. the downtown is located on a peninsula with the seacoast to the north, east and west and dominant winds came from the ssw and n. thus, the climate of the city is mild, sub-humid mediterranean with atlantic trend and strong seaside influence, average annual temperatures of 13.9°c and average annual rainfall of 1000 mm. air pollution in the town is not very high but important in some urban areas due to industrial activities and car traffic. previous studies have referred a marked occurrence of gypsum-rich black crusts (sanjurjo-sánchez et al., 2009, 2011b), a feature generally considered as indicator of the effect of air pollution. heritage buildings of the city have been constructed with two types of granitic rocks from local quarries: san pedro leucogranite and bregua granodiorite. the san pedro leucogranite is the most used rock in the historic buildings of the town. it is a white-yellowish fine-grained granite with quartz (forming heterogranular aggregates), feldspars (with plagioclase > microcline) and muscovite, while biotite is absent (igme, 1975). feldspars and muscovite grains are slightly deformed and oriented. in the quarry, near the town centre (3 km w) and abandoned in the middle 20th century, two granitic varieties can be observed in relation to natural rock weathering. in the nw part of the quarry, the leucogranite is white and hard (fresh leucogranite) and few joints are observed. in the se part, the granite is white-yellowish, weathered and joints are very frequent. both parts of the quarry were historically used for buildings. it has been observed that weathering sharply decreases the mechanical strength of the rock surface and increases the porosity from ~1.1% in the non-weathered leucogranite to ~4% in weathered specimens (sanjurjo-sánchez and alves, 2006). the bregua granodiorite (igme. 1975) is a milky gray coarse-grain rock with biotite and muscovite. plagioclase is dominant but it shows major microcline megacrystals. it also shows biotite as well as muscovite, apatite, zircon, epidote and opaque minerals. when weathered, the granodiorite shows a reddish colour. this can be observed in most ashlars of buildings and most areas of the present quarry front, located about 8 km s from the town centre, and abandoned in the early 20th century. the mechanical strength of the weathered rock is lower than that of the weathered leucogranite (in both cases building ashlars and quarry surfaces) and porosity is around ~4% (sanjurjo-sánchez and alves. 2006). the studied buildings are las capuchinas church (cap), the colegiata de santa maria del campo (csm), the church of santiago (sc) and the casas de paredes (p). the first one was built using only san pedro leucogranite ashlars and the last one only using bregua granodiorite, while the others were built with both rock types. the studied buildings were constructed in different historic periods and re-buildings were made with the same rock type (table 2). this fact allows us to get rock samples exposed to surface weathering during different periods from one to more than seven centuries. they are also exposed to different air pollution intensities, at least in the last decades, due to traffic exhaust and industrial air pollution, as data from previous studies have shown (sanjurjosánchez et al., 2009, 2011b, 2012). exposure to sea spray should be similar as the studied buildings are between 50 and 500 m from the seaside. to define a weathering geochemical baseline, samples from both quarries were taken. samples csp-1 and csp-2 correspond to the fresh (unweathered) and weathered rock of san pedro leucogranite in the quarry, respectively. the sample bre-1 corresponds to the bregua granodiorite quarry. in this last quarry the rock shows a red pale colour indicating some weathering effects. table 2. studied samples and orientation of the façades and time exposure in the built environment (a coruña. spain). jorge sanjurjo sánchez, carlos alves and juan ramón vidal romaní assessing the weathering of granitic stones on historical urban buildings by geochemical indices f5 ta bl e 3 . x rf re su lts (i n % ) o f t he sa m pl es . f6 4. methods to compare the weathering indices and the age of exposition of building stones, samples were taken from façades of known age (construction period) and this information is presented in table 1. for our study, samples taken from the quarries are considered as no exposed in the built environment (that is zero age). sampling was carefully carried out on the studied buildings causing the least possible damage to the façades. material was removed from the surface of ashlars (about 1 cm in depth) with a chisel by gentle hammering. quarry samples were taken by taking rock blocks in order to use the sample for other analyses (water absorption) being results previously published (sanjurjosánchez and alves, 2006). the samples were taken at least 10 cm far from ashlars joints, to minimise contamination from nearby joint mortars. about 40 samples were taken from the façades. major and trace elements were estimated by x-ray fluorescence (xrf). thirty grams of every sample were taken and two 10 g aliquots were powdered and analyzed in a fluorescence spectrometer s4 pioneer of wavelength dispersion bruker-nonius. two other 5 g aliquots were fired to 975°c to obtain loss on ignition (loi). 5. results the results obtained must be analysed considering the behaviour of all the analysed elements with increasing weathering and the suitability of the used indices to assess such weathering. values of different oxides (table 3) show table 4. pearson’s correlation coefficient (r) of the elements in oxides for samples with the time exposure in the building environment. table 5. weathering indices calculated for the samples. jorge sanjurjo sánchez, carlos alves and juan ramón vidal romaní assessing the weathering of granitic stones on historical urban buildings by geochemical indices f7 some overlapping between samples of both considered lithologies, bregua samples present higher values of iron, calcium, titanium, potassium, zirconium and magnesium (which has null results in most of the san pedro samples) while san pedro samples present higher sodium and rubidium content. these differences might be explained by primary mineralogical differences between the considered rock types, with bregua being richer in mafic minerals while san pedro having higher albite content as is frequent in leucogranitic rocks. the oxide with higher variation is so3 (between 0% and 4.59%), probably reflecting the effects of weathering processes in the built environment. the possible correlations between oxides and the time elapsed since the sampled ashlars have been placed in the building façades are discussed considering the plots presented in figures 2 and 3 as well as tables 3 and 4. for the san pedro leucogranite changes in the content in most of the studied oxides did not showed correlation with the exposure time. thus, no clear correlation between oxides and the exposition time in the built environment is observed to assess if leaching progress with time at a more or less constant rate. however, a decrease in si is observed with time for san pedro samples (correlation coefficient of -0.56), being clearer in façades not oriented to south (table 3 and figure 2). there seems to be some positive trend with time in relation to so3 (correlation coefficient = table 6. pearson's correlation coefficient (r) and squared coefficient (r2) of the weathering indices for the san pedro leucogranite. table 7. pearson’s correlation coefficient (r) and squared coefficient (r²) of the weathering indices for the bregua granodiorite. 0.49), a result that might represent the effect of sulphate contamination in more recent works. soluble elements such as cl and ca seem to show increase with time, above all in façades not oriented to south. it must be considered that, depending on the orientation of the studied façades, different erosive patterns are commonly observed on buildings due to the different weathering rates because the different exposure to possible leaching effects by rainwater (and/ or capillary rise), wind (with the consequent transport of pollutants), hydrationdehydration and thermal cycles (depending on direct exposure to sunlight). in relation to the bregua granodiorite samples, it is observed that some oxides, such as al2o3 and p2o5, show a negative correlation with age (correlation coefficients of -0.85 and -0.81, respectively), that is, it seems that there is a leaching of such cations with increasing exposure and weathering. in this case, this seems not to be correlated with the orientation of façades. however, the correlation of other ions with age is positive, as occurs again in the case of cl, so3 and this time with mno (correlation coefficients of 0.61, 0.45 and 0.81, respectively). some scatter can be observed in most of these correlations, significantly increased due to the deviation of some samples (above all the quarry sample bre-1 and the sample from casas de paredes, p-08), but f8 evident in the figure 3 and table 3. despite the low number of samples in relation to the considered orientations of façades, it is noteworthy the good correlation of some ions with age in both rocks, such as cl and so3, as this could result from salts typically found in the building environment (arnold and zehnder. 1989), namely in coastal regions and in the urban environment (and uptake with time). as expected, the calculated weathering indices showed different trends. there is some overlapping of values between the two rock types and the only one that shows a clear separation is the alk index for which san pedro presents lower values (between 45.7 and 57.1) than the bregua type (between 59.00 and 69.23). regarding the san pedro leucogranite samples, it is observed a good correlation (correlation coefficients > 0.90) for some indices (figure 4, tables 5 and 6), especially because several of them consider some same elements. the observed differences can be attributed to the variation in the concentration of some oxides. for the bregua granodiorite (tables 5 and 7. figure 5), absolute weathering indices (imob and lc) show a good correlation with most relative weathering indices as the pearson’s correlation coefficient shows (it can achieve 0.99 between imob and lc). thus, absolute indices seem to be more consistent and considerable to assess weathering in such complex environment. the relative indices based on the consideration of al2o3 and/or loi as reference parameter (hc, acn, akn, loi, vr and cia) show several low correlation values in this rock type. this can be indicative of leaching of some elements in the built environment (and not previously in the quarry). moreover, the results indicate that hydration effects related to weathering of minerals are negligible. regarding the possible correlation between weathering degree and exposure time in the built environment, only some indices show a weak correlation (r=0.48 to +0.43) for the san pedro leucogranite (imob. mwpi. lc and sio2/r2o3 index). in addition, the rate of variation of the indices observed in south façades seems to be lower than for other façades, indicating a slower weathering rate and leaching of some elements (figure 6). if we consider that south façades are more exposed to direct sunlight and dry environments (and water remains in the porous systems of the rock surface and subsurface) than north façades, this weaker trend would indicate that south façades could be affected to lower weathering rates. moreover, it must be remarked that the granite of this buildings comes at least from two varieties of the rock with different weathering degrees. this certainly affects to the obtained result and it can be observed by visual inspection of the ashlars surfaces on the buildings that show very variable states of decay. for the bregua granodiorite, a very good positive correlation (correlation coefficient of 0.90) is found for alk (it considers k2o and na2o) and a weak (more scattered) correlation for pi. sa. soc hc. loi. rr. r2o3 and sio2/r2o3 index. the correlation of some indices is also different if we consider south and other façades (figure 7), as occurs with the leucogranite. thus, this consideration must be taken into account for the discussion of the results. it is remarkable that the number of samples considered for this granodiorite is lower (n=13) than the number of samples used for the san pedro leucogranite (n=21). in this case, the use of an only rock type (an only weathering degree observed in the quarry) is an important factor for the final results that provide the weathering indices. from these, index that consider k, al and h2o seem to provide a weathering vs. age correlation. 6. discussion 6.1. variability of the chemical composition of samples although some substances are not typically considered to assess most weathering indices, it is very interesting the apparent correlation between cl and so3 with age in the bregua granodiorite samples (and weaker for cao). for the san pedro leucogranite, a good correlation is observed for cao and so3, and weaker for cl. this cannot be correlated with the distance to the seaside, as can be observed in the table 1. in the case of the built environment, such substances can be considered indicators of weathering effects, and thus should be included in the weathering assessment, as they are due to uptake from the atmospheric environment (seaside air and air pollution in the case of cl and so3, respectively) or other sources (joint mortars and capillarity rise for so3). in the case of building walls made with the san pedro leucogranite it has been observed that gypsum coatings are present sometimes due to the migration of calcium from mortars (sanjurjo-sánchez et al., 2008; sanjurjo-sánchez et al., 2009). considering that the porosity of the weathered granite increases about four times (with reference to the fresh granite), the uptake of calcium and sulphate from joint mortars is possible through the porous system. air pollution in the area also supports the presence of sulphates (sajurjo-sánchez et al., 2011). thus, it is plausible that joint mortars are an important source of salts and weathering, but difficult to assess by most weathering indices. it would be considered that the ca and s content must be considered in similar cases (building stones in urban environments). the porosity of the bregua granodiorite is similar to the weathered leucogranite. thus, these elements (ca and s) could also be used to assess the uptake of pollutants in granite rocks in buildings and to assess the risk of damage due to salt growth, responsible for some typical decay features in these kinds of stone and environments (amoroso and fassina, 1983; sanjurjo-sánchez et al., 2009; sanjurjo-sánchez and alves, 2012b). 6.2. variability of the chemical indices the weathering indices calculated for a same rock type should reflect the degree of weathering of every sample. comparing the most common indices in the studied samples, a good correlation (among them) is observed for the most part (tables 6 and 7). however, an important scatter is observed. for the san pedro leucogranite, we cannot dismiss the leaching of some elements due to in situ chemical weathering (as observed in the case of si) and two probable causes are considered for the scatter. the first one is related to the previous weathering of the rock in the massif, as can be observed in the quarry. as referred above this rock quarry shows two varieties regarding weathering (fresh and weathered). both varieties show slight differences in some minor and major components (table 3, csp-1 and csp-2). this makes difficult the identification of both types within the buildings. however, the porosity observed in the studied samples implies that some of the sampled rock ashlars probably come from the weathered variety of the quarry and some others from the fresh variety (sanjurjo-sánchez and alves, 2006; sanjurjo-sánchez and vidal vázquez, 2013). thus, differences would be more significant in the weathering indices than in the oxides. this is due to the different starting point of the weathering process when the rock ashlars are placed in the façades. the higher differences found in the relationship between weathering indices and age for this rock type could be due to the rate of weathering not being high enough to affect significantly the weathering state of the rock in comparison with the quarry rock (either for an initial weathered or fresh variety). moreover, the samples taken from both las capuchinas church (cap) and from the oldest part of the colegiata de santa maria de campo (csm) are more scattered than the others. this may be attributed to two reasons. on one hand, the two different varieties found in the quarry and the different weathering progress, as related above. this can be observed in the present state of conservation of las capuchinas church, with strong decay of most ashlars and as the building was plastered with gypsum after construction as it has been demonstrated (sanjurjosánchez et al., 2009). on the other hand, some samples have been taken from façades of different orientation. while the samples taken from younger façades (one to five centuries) come from north façades, the older samples come from south (las capuchinas church and some colegiata de santa maria del campo samples) or east (only the csm-13 sample) façades and this is a typical cause of different rates of decay (sanjurjo-sánchez and alves, 2012a, 2012b). the most important source of scatter for bregua granodiorite samples are the samples taken from the quarry (bre-1) and from casas de paredes (p-08). in the first case, the quarry front shows that the rock has suffered some weathering, as can be observed in its pale red colour. the casas de paredes were built some years after the other considered buildings, and thus some scatter can be expected as the rock used was taken from other parts of the quarry, where some jorge sanjurjo sánchez, carlos alves and juan ramón vidal romaní assessing the weathering of granitic stones on historical urban buildings by geochemical indices f9 figure 2. plots of oxides content vs. exposition time in the built environment for san pedro leucogranite. key: open circles (south façades). filled squares (other façades). filled circles (quarry). differences in the weathering processes could occur. moreover, the weathering of a façade can also vary depending on its relation to the sources of air pollution. in this sense, p-08 corresponds to a building located in a street which supports intense car traffic (sanjurjo-sánchez et al., 2009, 2011b), being more exposed to some pollutants (higher concentration of air pollutants) that cause decay (amoroso and fassina, 1983; sanjurjo sánchez and alves. 2012a, 2012b). this is the most probable cause of the gypsum crusts observed in that building (sanjurjo-sánchez et al., 2008), despite analysed sample p-08) does not show an important uptake of ca and s. however, another source of scatter is the different orientation of façades. the weathering degree seems to occur at different rates depending on the orientation of façades that is different in several studied buildings (figures 2 and 3). despite this, some of the used indices provided good results (figure 4) and a good weathering vs. exposure age correlation, due to the leaching of some elements (al, k) and the increase of porosity (h2o content). thus, these elements must be considered to choose reliable indices to assess weathering on building rock surfaces in urban environments. in buildings, the direct exposition to the sun of south façades could result in marked wetting/drying cycles (wendler et al., 1990). wetter conditions can cause stronger dissolution and leaching of some mobile elements in the rock. however, the concentration of some mobile elements, such as ca, increase with exposition time to weathering (figure 2). the increase of cao with exposition time could be due to the dissolution of ca from lime mortars (joint mortars) and uptake within the rock ashlars’ porous system. this hypothesis is also supported by the uptake of so3 observed on samples, as the lime of mortars easily gives origin to the neoformation of gypsum under polluted air conditions in buildings of a coruña (sanjurjo-sánchez et al., 2009). therefore, indices that do not consider cao as a mobile oxide would be independent from influence of dissolution of ca form mortars. other mobile elements do not show this or any other clear trend. f10 figure 3. plots of oxides content vs. exposition time in the built environment for the bregua granodiorite. key: open circles (south façades). filled squares (other façades). filled circles (quarry). figure 4. plots of selected weathering indices (indices that show similar trend) for the san pedro leucogranite. key: open circles (south façades). filled squares (other façades). filled circles (quarry). jorge sanjurjo sánchez, carlos alves and juan ramón vidal romaní assessing the weathering of granitic stones on historical urban buildings by geochemical indices f11 figure 5. plots of selected weathering indices (indices that show similar trend) for the bregua granodiorite. key: open circles (south façades). filled squares (other façades). filled circles (quarry). figure 6. plots of weathering indices vs. exposition time in the built environment for the san pedro leucogranite. open circles (south façades). filled squares (other façades). filled circles (quarry). f12 figure 7. plots of weathering indices vs. exposition time in the built environment for the bregua granodiorite. open circles (south façades). filled squares (other façades). filled circles (quarry). regarding again the scatter of indices, despite the considered questions it seems that relative weathering indices using a high number of elements will usually be more consistent than those that consider a few oxides (tables 6 and 7). moreover, indices that consider the leaching of some elements (si, al, k) or the water content (that can be directly correlated with the increase of porosity) seem to be more consistent. thus, it must be considered in future studies that the use of weathering indices to assess weathering in buildings constructed with granite rocks in urban environments should consider: (i) any causes of the scatter observed in this study (façade orientation, exposure to direct air pollution, inherited weathering of rock in the quarry) previously to sampling, (ii) indices that include alkaline elements (mainly, ca, s and cl) will show the effects of pollution and interaction with building materials that will cause some physical and chemical weathering processes (e.g. salt weathering), and (iii) indices that include some elements (such as si, al and k) seem to be generally more consistent in these complex environments. 7. conclusions the applicability of weathering indices to the weathering of rocks used as building materials of historic buildings in urban environments has been assessed considering two igneous rock types in buildings of a coastal region with a certain amount of atmospheric pollution. theoretically, the progress of weathering on the stone surfaces can be assessed due to the leaching of some mobile elements. however, this effect is not clearly observed in the studied granite rocks from the considered urban environment in most elements. this is due to different factors that cause scatter of results. in this case, previous rock weathering in the quarry, the different orientation of façades and the different exposure to pollutants are proposed as the cause of such scatter. also, at the considered timescale (from one to seven centuries), weathering rates in south and other façades seem to be different. thus, future studies should consider these questions before sampling. if possible it is also recommended the use of a significant number of samples, as the use of 13-21 samples in this case is not very conclusive due to the referred scattering factors. in addition, the interference of some mobile ions that migrate from other building materials, as occurs with ca dissolved from lime joint mortars, does not advise the use of weathering indices that consider cao for the assessment of weathering losses. some other mobile oxides, such as so3 or cl, commonly related with other building materials, air pollution or other environmental factors should be monitored and used to assess the influence of such factors on the decay and weathering in the built environment. despite scattering, the use of weathering indices can be useful to assess the degree of surface weathering of granitic rocks used as construction materials in historical buildings of urban environments, as it has been observed in one of the studied rocks. moreover, indices that include some elements (such as si, al and k) seem to be more consistent. acknowledgments this work is included in the activities of the project lab2pt landscapes, heritage and territory laboratory aur/04509, which has financial support of the fct through national funds and when applicable of the feder co-financing, in the aim of the new partnership agreement pt2020 and compete2020 poci 01 0145 feder 007528. the authors are also grateful for funding from xunta de galicia, spain through projects r2014/001 and gpc2015/024. 8. references alves, c., sanjurjo-sánchez, j. 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(1990). contour scaling on building facades dependence on stone type and environmental conditions. advanced workshop analytical methods for the investigation of damaged stone, pavia. the purpose of this research is to assess the suitability of dune sands as construction materials, moreover, such a purpose is considered beneficial in assessing appropriate methods for soil stabilization or ground improvement and to assess the suitability of dune sands as subgrade layer for carrying roads and rail foundation. dune sand samples were collected from a region in baiji area in salah-aldeen governorate, north of iraq. a grey-colored densified silica fume (sf) and lime (l) are used. three percentages are used for lime (3%, 6% and 9%) and four percentages are used for silica fume (3%, 6%, 9% and 12%) and the optimum percentage of silica fume is mixed with the percentages of lime. unsoaked cbr on compacted dune sands treated dune sands with l-sf by mixing and cured for a period of 1 day. the increasing in cbr ranged between 443 – 707% at 2.54 mm penetration and 345 – 410% at 5.08 mm penetration. el propósito de esta investigación es evaluar el uso de arena de dunas como materiales de construcción. además, este objetivo permite determinar los métodos apropiados para la estabilización del suelo, el mejoramiento del terreno y la evaluación de pertinencia de la arena de dunas en capas subbase para carreteras y cimientos férreos. se recolectaron muestras de arena de dunas en el área de baiji, del comisionado salah-aldeen, al norte de irak. se utilizó vapor de óxido de silicio (sf, en inglés), grisáceo y densificado, y óxido de calcio (l). se utilizaron tres porcentajes para el óxido de calcio (3 %, 6 % y 9 %), y cuatro para el óxido de silicio (3 %, 6 %, 9% y 12%) y el máximo porcentaje del óxido de silicio se mezcló con las proporciones de óxido de calcio. se realizó en seco el ensayo de relación de soporte de california (del inglés california bearing ratio, cbr) en arena de dunas compactada y tratada con la mezcla l-sf curada durante un día. el incremento en el ensayo cbr osciló entre 443-707 % en la penetración de 2.5 mm y 345-410 % en la penetración de 5.08 mm. abstract resumen keywords: dune sand, lime, silica fume, stabilization, cbr, strength. palabras clave: arena de dunas, óxido de silicio, óxido de calcio, estabilización, ensayo de relación de soporte de california. compaction and collapse characteristics of dune sand stabilized with lime-silica fume mix record manuscript received: 20/05/2015 accepted for publication:13/05/2016 how to cite item fattah, m. y., joni, h. h., & al-dulaimy, a. s. a. (2016). compaction and collapse characteristics of dune sand stabilized with lime-silica fume mix. earth sciences research journal, 20(2), i1-i8. doi:http://dx.doi.org/10.15446/esrj.v20n2.50724 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.50724 mohammed y. fattah1 hasan h. joni2 and ahmed s. a. al-dulaimy3 1 professor, building and construction engineering department, university of technology, baghdad, iraq. 2 assistant professor, building and construction engineering department, university of technology, baghdad, iraq. 3 formerly graduate student, building and construction engineering department, university of technology, baghdad, iraq. email: myf_1968@yahoo.com earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : i1 i8. características de compactación y de colapso de arena de dunas estabilizada con una mezcla de óxido de calcio y óxido de silicio g e o t h e c n ic s i2 mohammed y. fattah, hasan h. joni, and ahmed s. a. al-dulaimy. introduction: ground improvement refers to any procedure undertaken to increase the shear strength, decrease the permeability and compressibility, or otherwise render the physical properties of soil more suitable for projected engineering use. the improvement may be accomplished by drainage, compaction, preloading, reinforcement, and grouting, electrical, chemical or thermal methods. among the various soil stabilization procedures, the most suitable one is selected depending upon the type of soil available, time, cost involved etc. (kumar, 2010). one of the most dramatic depositional landforms, which occupy large areas all over the world, is sand dune. sand dunes in various forms cover large areas of the world. extensive efforts have been made to understand their origin (shearzid, 1989) physical and mineralogical properties, environmental impacts. some studies have also been directed towards dune sand stabilization (al-soud, 2000). there is, however, scarcity in the literature concerning the geotechnical characteristics of these soils. a single dune can be defined as a mound or hill of sand, which rises to a single summit. they are accumulations of windblown sand, which change their position or their shape due to wind action as long as their surface consists of loose granular material of appropriate size. some of typical properties of sand dunes are as follows (das, 2007): 1. grain size distribution of the sands at any particular location is uniform. the uniformity can be attributed to the sorting action of the winds. 2. the general grain size percentages decrease with distance from the source, because the wind carries the small particles farther than the large ones. 3. the relative density of sand deposited on the windward side of the dunes is as high as (50-65) % and decreases to about (0-15) % on the leeward. collapse is a typical feature of unsaturated, loose and low plasticity soils, which are typical features of dune sand. the behavior of collapsible soil is often quantified by conducting a collapse potential test. however, these test interpret the soil behavior in terms of total stress, while it is well known that the behavior of soil is governed by its effective stress. also the collapse is defined as the significant volume reduction observed when wetting an unsaturated sample under load. the collapsing behavior of this type of soil generally occurs when the water is introduced into the soil. collapsing cause damage too many civil engineering structures such as: spread footing, buildings, roads, highways, and earth dams leading to high economic losses. there are many factors affecting collapse behavior of compacted and soils which are: initial dry unit weight, initial water content, percentage of fines, and the method used in compaction, (al-juari, 2009). to measure the collapsing potential of this type of soil, it may use an oedometer/consolidation ring to prepare an undisturbed sample (similar preparation as consolidation test). when water is introduced to this specimen, the sample may start to collapse, ie. change in height(dh/h) or change in initial void ratio (e/eo) may be used as the degree of collapse of the soil. janning and knight discussed the problem in 1957. then, in 1963, they proposed a test method by which they determined the collapse potential for one specimen under an applied vertical stress of 200kpa, after one day of soaking, (albusoda and salem, 2012 and mansour et al., 2008). abbeche et al., 2010 have studied the soil collapse and listed the factors contributing to the occurrence of this phenomenon. the main factors are summarized as follows: 1. an open unstable structure and partially saturated. 2. a high total stress is applied. 3. the presences of cementing agent in the soil, for example calcium carbonate (caco3), in unsaturated conditions contribute to stabilize the soil. a moistening the soil will reduce the suction and destroy the liaison officer. albusoda and salem (2012) used cement kiln dust (ckd) to stabilize affek dune sand the effectiveness of adding different percentages of (ckd) was studied. time of curing effect, was also considered. it was found that stabilization of collapsible soils with ckd can provide tremendous advantages. ading of 8% ckd, for example, increased the ultimate bearing capacity of shallow foundation to 250%. the variation in shear strength parameters became almost constant after fourteen days of curing. vegetation cover on sand dunes mainly depends on wind power (drift potential—dp) and precipitation. when this cover decreases below a minimal percentage, dunes will start moving. it is therefore necessary to study the effects of dp and precipitation on contemporary dune activity in order to predict likely future dune mobility in the coming decades. ashkenazy et al. (2012) concentrated on the future activity of the currently fixed dune fields of the kalahari and the australian deserts. these sand seas include the largest areas of stabilized dunes in the world, and changes in their mobility have significant economic implications. global maps of dp are introduced, based on real and reanalysis data. analyses of two global circulation models (gfdl and cgcm3.1) provide future predictions under the sres-a1b ipcc scenario, which is a moderate global warming scenario. according to the gfdl model, both the australian and kalahari basin dunes will apparently remain stable towards the end of the 21st century because the dp will stay small, while the rate of precipitation was expected to remain much above the minimal threshold necessary for the vegetative growth that leads to dune stabilization. the cgcm model predicts insignificant changes in dps and shows that the precipitation rate is above 500 mm/year for almost the entire kalahari basin. the central-northern part of australia is predicted to have larger dps and greater precipitation than the southern part. since the predicted changes in dp and precipitation are generally not drastic, both the australian desert and kalahari basin dunes are not likely to become active. still, the australian dunes are more likely to remobilize than the kalahari ones due to some decrease in precipitation and an increase in wind power. the research work carried out by bouziani et al. (2012) dealt with the effect of dune sand, available in huge quantities in these regions, on the properties of flowing sand-concrete (fsc) prepared with different proportions of dune and river sands. mini-cone slump test, v-funnel flow-time test and viscosity measurements were used to characterize the behaviour of fsc in fresh state. the 28-day compressive strength was also determined. test results showed that an optimal content of dune sand, which makes satisfied fresh and hardened properties of fsc, is obtained. moreover, the obtained flow index (constant b) calculated by the help of power-law viscosity model is successfully correlated to the experimental results of v-funnel flow time. jackson et al. (2014) stated that seven types of coastal dune are identified on kwazulu-natal coast: estuary barrier dunes; mainland beach foredunes; prograded beachridge dunes; headland bypass dunes; transverse dune fields; parabolic/migrating dunefield; and climbing dunes. the development and distribution of the various dune forms is more strongly controlled by the geological setting, than the aerodynamic regime. ciccarelli (2014) tried to assess the conservation status of coastal dune systems in tuscany (italy). emphasis was given to the presence and abundance of plant communities identified as habitat in accordance with the directive 92/43/eec. twenty transects perpendicular to the shoreline were randomly positioned on the whole coastal area (30 km in length) in order to sample the full spectrum of plant communities. vegetation zonation and relationships with the most frequent disturbance factors in the study area—beach cleaning, coastline erosion, presence of paths and roads, bathing settlements and trampling—were investigated through principal coordinate analysis and canonical correspondence analysis. natural factors, such as distance from the sea and total length, were also considered. differences in the conservation status of the sites were found, ranging from the total disappearance of the foredune habitats to the presence of the complete psammophilous (sand-loving) plant communities. erosion, trampling, and paths were found to be closely correlated with degradation and habitat loss. furthermore, the overall plant species diversity of dunes was measured with nhdune, a modified version of the shannon index; while the incidence of invasive taxa was calculated using n, a naturalness index. however, these diversity indices proved to be a weaker bioindicator of ecosystem integrity than habitat composition along transects. a possible strategy for the conservation and management of these coastal areas could be i3compaction and collapse characteristics of dune sand stabilized with lime-silica fume mix to protect the foredunes from erosion and limit trampling through the installation of footbridges or the use of appropriate fences. the purpose of this research is to assess the suitability of dune sands as construction materials. the tests include field density, moisture content, classification, compaction, relative density and cbr test. the effectiveness of adding different percentages of lime-silica fume mix was studied. experimental work: dune sand samples were obtained from baiji area in salah-aldeen governorate – north of iraq. three disturbed samples were taken from the surface at a depth of 0.4 m. the consistency and other physical properties of the dune sand were studied by a series of tests. lime-silica fume mix is suggested to overcome the problems of dune sands. the physical and chemical properties of these materials are determined. the specification of each material is as follows: in situ field density (rfield) was obtained by sand-cone test according to (astm d 1556-00). the moisture content (%w.c) for the soil studied was determined according to (astm d 2216-00). grain size distribution was determined by sieve analysis and hydrometer tests conducted on dune sands samples in accordance with (astm d 422-00). the grain size distribution of the soil used reveals that the soil is composed of 91% sand, 3% silt and 6% clay. table 1 summarizes the physical properties of dune sand. the minimum and maximum dry densities, rd.min and rd.max, were determined according to (astm d 4254-00) and (astm d 4253-00), respectively. table 1. summary of baiji dune sand properties. the lime material (l): calcium hydroxide (slaked lime) is most widely used for stabilization. calcium oxide (hydrated lime) may be more effective in some cases. chemical and physical tests were carried on the lime material in the national center for construction laboratories and research (ncclr) in iraq. the results of chemical and physical properties of the lime used are shown in table 2. table 2. results of hydrated lime tests. silica fume material (sf): a grey-colored densified silica fume is used. it is a pozzolanic material which has a high content of amorphous silicon dioxide and consists of very fine spherical particles. silica fume was used as an additive material to improve soil properties. the presence of this substance gives greatly improved internal cohesion, water retention and increased density when set. chemical and physical tests were carried on the silica fume material in the national center for construction laboratories and research (ncclr) in iraq. the results of the chemical analysis of this material are presented in table 3. table 3. results of silica fume tests. compaction test: moisture-density relationship was determined by means of the standard and modified proctor compaction test. the tests were performed in accordance with (astm d 698-00, method a) and (astm d 1557-00, method a) standards, respectively. the moisture density relationships are shown in figure 1. i4 mohammed y. fattah, hasan h. joni, and ahmed s. a. al-dulaimy. figure 1. standard and modified compaction curves for the dune sand. california bearing ratio (cbr) test: cbr-value is used as an index of soil strength and bearing capacity. a series of tests were conducted on each dune sand sample as unsoaked samples. the preparation of specimens and testing procedure were generally in accordance with astm d 1883-99 and astm d 1557-00. the specimens were prepared at 95% of maximum dry density of the modified compaction test, and compacted in five layers using 4.5 kg hammer dropped from a height of 45 cm. fifty six blows per layer were used for compacting the two specimens. california bearing ratio values were determined and calculated as follows (astm d 1883-99): (1) cbr value was measured after 1 day curing for the soil stabilized with l-sf. the test was performed on samples prepared at 95% of optimum moisture content and compacted to various densities. collapse test: collapse test of soil the soil studied was determined according to (astm-d5333-03). the magnitude of one-dimensional collapse that occurs when the soil inundated with water. the test method consists of placing a compacted soil specimen at the desired water content in an oedometer, applying a predetermined applied vertical stress to the specimen and inundating the specimen with distilled deionized water to induce the potential collapse in the soil specimen. the test method follows these steps. the one-dimensional response-to-wetting test, which is performed using conventional consolidation equipment represents the frequently used laboratory collapse test for determining the collapse potential of the soil. pressure was applied until the stress on the sample was equal to (or greater than) that expected in the field or up to 200 kpa as suggested by jennings and knight (1975) and as standardized by astm d-5333 (2003). according to astm d5333 (2003), collapse: indicates a decrease in the height of confined soil following wetting at a constant applied vertical stress. a standard oedometer ring of 75 mm diameter specimen was prepared in steel ring. after careful placing. the effective vertical stresses used were: (25, 50, 100, 200 and 400) kpa. the collapse potential (ic) denotes the percent-relative magnitude of collapse determined at any stress level as follows (astm d-5333-03): (2) where: dh= the change in specimen height resulting from wetting, mm, and ho= the initial specimen height, mm. equation (2) may be rewritten in terms of void ratio as follows: (3) where: eb, e1= the void ratio at the appropriate stress level before wetting, ei , e2 = the void ratio at the appropriate stress level after wetting, and eo= the initial void ratio. the collapse index (ie), which is the wetting induced strain measured at a reference stress level of 200 kpa. the classification of collapse index is listed in table 4, (astm d5333-03). table 4. classification of collapse index (ie), (astm d5333-03, table 1). results and discussion: table 5 shows the maximum density, minimum density and relative density values of the soil mixed with different percentages of lime-silica fume mix. the table shows the increase in relative density of soil with increasing of l-sf content due to the low values of the density of lime (0.60 gm/cm3) and silica fume (0.75 gm/ cm3). this indicates that the additives (lime and silica fume) are fine materials and these materials relatively filled the voids of dune sands about on executing maximum density test (astm d 4253), which led to the increase in maximum density for soil and thereby increase the relative density. this difference may be attributed to the round shape of sand particles tested, (lambe and whitmen, 1979). table 5. maximum and minimum dry densities and relative densities of soil mixed with different percentages of l-sf. results of compaction test: table 6 and figure 2 show the moisture-dry density relationship for stabilized soil. by examining the results, the following points could be noticed: 1. for small percentage of silica fume, (3%) and any percentage of lime, there is a decrease in the optimum water content with slight increase in maximum dry density. i5compaction and collapse characteristics of dune sand stabilized with lime-silica fume mix 2. the soils stabilized with (6%, 9% and 12%) of silica fume and any percentage of lime (which are fine materials) were compacted at lower maximum dry density and higher optimum water content. the increase in optimum moisture content due to addition of lime and silica fume may be caused by the absorption of water by l-sf mix. 3. the percentage (6% l + 3% sf) revealed the maximum dry density. 4. the shape of curves became more flat with increase of the l-sf percentage. similar finding was reported by parsons et al., (2004) and al-jobouri, (2013). table 6. results of proctor compaction tests of soil mixed with different percentages of l-sf. figure 2. variation of the dry density with moisture content of standard proctor compaction for soil mixed with different percentages of l-sf. results of california bearing ratio (cbr) tests: the values of stresses are plotted against depth of penetration in figure 3. stress and penetration curves for baiji dune sands consist of an approximately straight portion for about 3 mm penetration in which stress and penetration are directly proportional, then tend to become a horizontal at further penetration. figure 3. stress–penetration curves from cbr test for natural dune sand and selected soils stabilized with different percentages of l-sf by mixing. unsoaked cbr values at 2.54 and 5.08 mm penetration are plotted in figure 4 and summarized in table 7. the higher cbr values are obtained at 5.08 mm penetration for baiji dune sands, where the soil is sand and containing lower percentage of material passing the no. 200 sieve. table 7. results of cbr test a chosen soils stabilized by mixing with l-sf. the results of figure 4 show that when stabilizing the soil with l-sf after curing for 1 day, there is an increase in cbr due to the increase in the angle of internal friction (f) or cohesion (c) or both. the added materials of l-sf play the major role in stabilization. using this type of material is reflected in the improvements in terms of bearing capacity which is directly proportional to cbr. cbr-value increase with increasing silica fume but this increasing reduces with increasing lime. the increase in cbr ranged between (443–707)% at 2.54 mm penetration and (345–410) % at 5.08 mm penetration as illustrated in table 7 and this increase is calculated as follows: (4) figure 4. cbr-value of natural dune sand compacted and stabilized with different percentages of l-sf by mixing at different penetrations. results of collapse test: single oedometer test is selected for the collapse test for disturbed and undisturbed samples. this test is carried out according to the procedure recommended by astm d5333. collapse potential (ic) and collapse index (ie) magnitude are determined according astm d5333-03 by using the conventional oedometer device in a constant temperature and humidity environmental at different stress level. it shows high values when flooding the specimens by water during primary consolidation. it is concluded that there is an increases in collapsibility due to removal of bonding between particles upon wetting due to stress release and leaching of soil during i6 mohammed y. fattah, hasan h. joni, and ahmed s. a. al-dulaimy. a. natural soil b. treated soil water infiltration leading to softening due to rearrangement of soil particles. moreover, the collapse potential decreases with the increase in the relative density. this is due to the reduction in void ratio of the specimens with increasing the relative density. table 8 . results of collapse potential test a chosen soils stabilized by mixing with l-sf.£ plate 1. collapse of dune sands after the test for natural and treated soils. figure 5. single oedometercollapse test result for natural dune sands. figure 6. single oedometer collapse test result for dune sands stabilized by mixing of l-sf, (3%l + 12%sf). figure 7. single oedometercollapse test result for dune sands stabilized by mixing of l-sf, (6%l + 3%sf). figure 8. single oedometer collapse test result for dune sands stabilized by mixing of l-sf, (6%l + 12%sf). figure 9. single oedometer collapse test result for dune sands stabilized by mixing of l-sf, (9%l + 6%sf). figures 5 to 9 and table 8 show the collapse potential decreases slightly with increasing the percentage of l-sf mix for soil samples tested by single collapse. the collapse index and potential of treated soil are found less than that of natural soil and decrease with increasing stabilizer content. collapsing increased continuously with applied stresses. on the other hand, the added silica content to the treated soil has an effect of higher than lime in reducing the collapse index at different stresses. plate 1 shows the collapse of natural and treated dune sands after test. the decreasing in collapse index ranged between (17.2–36)% at 200 kpa as illustrated in table 8 and this decrease is calculated as follows: (5) i7compaction and collapse characteristics of dune sand stabilized with lime-silica fume mix the calculated collapse index (ie)gives an indication thatdegree of specimen collapse for soil. baiji dune sands undergo only small collapse. the degree of collapse of this natural soil is classified as “slight” under different soaking pressure. this result coincides with the results found by (al-taieet al., 2013). also degree of collapse for treated soils by mixing with l-sf mix are classified as “slight” as shown in table 8. table 8. results of collapse potential test a chosen soils stabilized by mixing with l-sf.£ figure 10 gives the collapse percent versus percent of lime and silica fume, diagrams obtained from one dimensional consolidation. it is clear that there is a slightly decrease in collapse with increasing stabilizers content. l-sf is effective in reducing the collapse potential of the soil. on the other hand, add l-sf decrease void ratio of soil. this indicates that the additives (lime and silica fume) are fine materials and these materials relatively filled he voids of dune sands about on executing compaction, which led to the increase in relative density for soil and thereby decrease the collapse index. figure 10. effect of additives (lime and silica fume) on collapse potential for natural dune sand and selected soils stabilized with different percentages of l-sf by mixing. conclusions: 1. the mix of l-sf allowed compaction of the soil at higher maximum dry density. the treatment with l-sf shows a general increase in the maximum dry density from (1.609 to 1.722) gm/cm3 of standard compaction, and from (1.609 to 1.834) gm/cm3 of modified compaction at (6% l + 3% sf). 2. the optimum water content increases from 13.6% to 15.5% at 12% sf of standard compaction, the increase in the optimum moisture content is due, in spite of the reduced surface area caused by flocculation and agglomeration, to the additional fine contents to the samples which requires more water in addition to the free lime and silica fume that needed more water for the pozzolanic reactions to take place. 3. unsoaked cbr on compacted dune sands treated with l-sf by mixing and cured for a period of 1 day, exhibited higher cbr values than compacted untreated dune sands. moreover, the 5.08 mm penetration cbr of the treated dune sands is higher than the 2.54 mm penetration cbr. the increasing in cbr ranged between 443 – 707% at 2.54 mm penetration and 345 – 410% at 5.08 mm penetration. 4. the degree of collapse of natural baiji dune sands and treated soils by mixing with l-sf mix are classified as “slight” under different soaking pressure. 5. the difference in collapse potential between some percent of l-sf becomes very little. this means that any additional amount of l-sf does not improve considerably the control of collapse. collapse potential and collapse index decreased with stabilizers content and drop to (ie= 1.06%) at (6%l + 12%sf). finally, l-sf treatment in dune sands generally reduces collapse. the decreasing in collapse index ranged between (17.2–36)% at 200 kpa for treated soils. references: abbech, e. k., laouar, m. s., & messaoud, f. (2010). prediction of collapsible soils by cone penetrometer and ultra-sonic tests. studia geotechnica et mechanica, vol. xxxii, no. 2. albusoda, b. s. & salem, l. a. (2012). stabilization of dune sand by using cement kiln dust (ckd). journal of earth sciences and geotechnical engineering, 2(1), 131-143. al-jobouri m. m. (2013). strength and compressibility characteristics of soft soil stabilized with lime silica fume mix. m.sc. thesis, civil engineering department, university of baghdad, iraq. al-juarikh a. (2009). volume change measurement of collapsible soil stabilized with lime and waste lime. civil engineering, tikrit journal of engineering sciences, 16(3), 38-54. al-soud, m. s. (2000). stabilization of baiji sand dunes by petroleum residues. m.sc. thesis, civil engineering department, university of baghdad, iraq. al-taie, a. j., al-shakarchi, y. j., & mohammed, a. a. (2013). investigation of geotechnical specifications of sand dune soil. a case study around baiji iraq. civil engineering, international journal of advanced research, 1(6), 208-215. ashkenazy, y., yizhaq, h., & tsoar, h. (2012). sand dune mobility under climate change in the kalahari and australian deserts. climatic change, 112, 901–923, doi: 10.1007/s10584-011-0264-9. astm d1883-99 (1999). standard test methods for cbr (california bearing ratio) of laboratory-compacted soils. american society for testing and materials. astm d5333-03 (2003). standard test methods for measurement of collapse potential of soils. american society for testing and materials. bouziani, t., bederina, m., hadjoudja, m. (2012). effect of dune sand on the properties of flowing sand-concrete (fsc). international journal of concrete structures and materials, 6(1), 59-64. doi 10.1007/s40069-012-0006-z. ciccarelli, d. (2014). mediterranean coastal sand dune vegetation: influence of natural and anthropogenic factors. environmental management, 54, 194–204. doi: 10.1007/s00267-014-0290-2. cooke, r. u., & warren, a. (1973). desert geomorphology. london: b. t. batsford ltd., 374 pp. 1st. edition. das, b. m. (2007). principles of foundation engineering. adapted international student edition, nelson a division of thomson canada limited. sixth edition. jackson, d. w. t., cooper, j. a. g., & green, a. n. (2014). a preliminary classification of sand dunes of the kwazulu-natal coast. in: green, a. n. & cooper, j. a. g. (eds.), proceedings 13th international coastal symposium (durban, south africa), journal of coastal research, special issue 70, 9, pp. 718-722, issn 0749-0208. doi: 10.2112/si70-121.1. i8 mohammed y. fattah, hasan h. joni, and ahmed s. a. al-dulaimy. jannings, j. e., & knight, k. (1975). a guide to truction on or with materials exhibiting additional settlement due to collapse of grain structure. proceedings, sixth regional conference for africa on soil mechanics and foundation engineering johannesburg, south affrica, pp. 99-105. kumar, s. (2010). a study on the engineering behaviour of grouted loose sandy soils. ph.d. thesis, division of civil engineering, cochin university of science and technology, india, 2010. lambe, t. w. & whitman, r. v. (1979). soil mechanics. john wiley and sons inc., new york. mansour, z., chik, z., & taha, m. r. (2008). on soil collapse potential evaluation. university kebangsaan malaysia, iccbt 2008 -e(03), pp. 21-32. parsons, r. l., kneebone, e., & milburn, j. p. (2004). use of cement kiln dust for subgrade stabilization. final report, kansas department of transportation, usa. shearzid, m. f. (1989). sediment wind and its impact on the design of human settlements. m.sc. thesis, department of architectural engineering, university of technology, baghdad, iraq. v16n1c.indb earth sciences research journal geotechnical analysis earth sci res sj vol 16, no 1 (june, 2012): 65 74 multicriteria decision-making analysis based methodology for predicting carbonate rocks’ uniaxial compressive strength hakan ersoy and derya kanik karadeniz technical university, geological engineering department, 61080, trabzon, turkey, phone: + 90 462 377 35 06, fax: +90 462 325 7405 e-mail: blavetirraa@hotmail com; kanik@ktu edu tr abstract uniaxial compressive strength (ucs) deals with materials’ to ability to withstand axially-directed pushing forces and especially considered to be rock materials’ most important mechanical properties however, the ucs test is an expensive, very time-consuming test to perform in the laboratory and requires high-quality core samples having regular geometry empirical equations were thus proposed for predicting ucs as a function of rocks’ index properties analytical hierarchy process and multiple regression analysis based methodology were used (as opposed to traditional linear regression methods) on data-sets obtained from carbonate rocks in ne turkey limestone samples ranging from devonian to late cretaceous ages were chosen; travertine-onyx samples were selected from morphological environments considering their surface environmental conditions test results from experiments carried out on about 250 carbonate rock samples were used in deriving the model while the hierarchy model focused on determining the most important index properties affecting on ucs, regression analysis established meaningful relationships between ucs and index properties; 0 85 and 0 83 positive coefficient correlations between the variables were determined by regression analysis the methodology provided an appropriate alternative to quantitative estimation of ucs and avoided the need for tedious and time consuming laboratory testing resumen la  resistencia a la compresión uniaxial  (rcu) trata con la capacidad de los materiales para soportar fuerzas empujantes dirigidas axialmente y, especialmente, es considerada ser uno de las más importantes propiedades mecánicas de los materiales rocosos sin embargo, una prueba de rcu es costosa, lleva mucho tiempo para hacerlo en el laboratorio y  requiere muestras de núcleos de alta calidad que tienen una geometría regular por lo tanto, ecuaciones empíricas fueron propuestas para la predicción de rcu como una función de las propiedades índice de las rocas las metodologías de proceso analítico jerárquico (paj) y análisis de regresión múltiple fueron utilizados (en vez de los métodos tradicionales de regresión lineal) en conjuntos de datos obtenidos de las rocas carbonatadas en el noreste de turquía muestras de rocas calizas que van desde el devónico hasta finales del cretácico fueron escogidas;  muestras de travertino y  ónix fueron seleccionadas de ambientes morfológicos teniendo en cuenta sus  condiciones ambientales de superficie los resultados de los experimentos llevados a cabo en alrededor de 250 muestras de rocas carbonatadas fueron utilizados para derivar un modelo mientras que el modelo de jerarquía se centró en determinar las propiedades índice más importantes afectados por la rcu, el análisis de regresión establece relaciones significativas entre la rcu y las propiedades del índice; coeficientes de correlación positivas de 0,85 y 0,83 fueron determinadas por análisis de regresión entre las variables la metodología proporciona una alternativa adecuada para la estimación cuantitativa de la rcu y evita la necesidad de realizar pruebas del laboratorio las cuales son tediosas y dispendiosas palabras claves: resistencia a la compresión, ecuaciones empíricas, rocas carbonatadas, jerarquía analítica keywords: compressive strength, empirical equations, carbonate rock, analytical hierarchy record manuscript received: 06/02/2012 accepted for publications: 01/06/2012 hakan ersoy and derya kanik66 introduction intact rocks’ uniaxial compressive strength (ucs) is the main parameter used for almost all rock mechanical studies in most civil, geological and mining projects (bieniawski 1974; cargill and shakoor 1990); however, regular geometry, high-quality core samples are necessary for determining ucs standard cores cannot always be extracted from weak, highly-fractured, thinly-bedded, foliated rocks this test is thus expensive, time-consuming and requires well-prepared rock samples resent trends in estimating ucs from simple laboratory index tests has been improved to overcome such difficulties, and simple prediction models have become attractive for engineering geologists using non-destructive and easily-applied techniques such as rocks’ ultrasonic wave velocity (uwv) and index properties many attempts have been made to predict intact rocks’ ucs (kahraman 2001; katz et al., 2000; koncagul and santi 1999; chau and wong 1996) some researchers have found that sound velocity is closely related to rock properties (gaviglio 1989; chang et al., 2006; yalçınalp et al., 2008; babacan et al., 2009; moradian and behnia, 2009) whilst other have correlated ucs with index properties such as porosity, density and uwv (ramana and venkatanarayana 1973; yasar and erdogan 2004; kanik 2010) simple statistical methods-based multiple regression techniques have been used to establish predictive models; new techniques such as artificial neural networks and fuzzy inference systems have been used for developing predictive models for estimating the required parameters during recent years (grima and babuska 1999; kayabasi et al., 2002, gokceoglu and zorlu 2004; sonmez et al., 2003; karakus and tutmez 2006; dehfigure 1. map showing the location of the area being studied multicriteria decision-making analysis based methodology for predicting carbonate rocks’ uniaxial compressive strength 67 ghan et al., 2010, yagiz et al., 2011) different evaluation methods have been developed to establish predictive models; such methods include linear vector approach, matrix method, fuzzy set theory, checklist methods, parametric ranking methods and multi criteria decision analysis based methodologies analytic hierarchy process (ahp), was developed as a type of multi-criteria analysis, to standardise multi-criteria decision-making (saaty 1980) however, ahp uses a quantitative comparison method based on pair-wise comparisons of decision-making criteria, rather than utility and weighting functions many engineering geological projects have adopted such as approach (cook et al., 1984; siddiqui et al., 1996; ersoy and bulut 2009) the paper presents multi-criteria decision-making and multiple regression analysis-based methodology for predicting ucs regarding carbonate rocks’ index properties the model is based on non-destructive and relatively easy to apply laboratory tests research methodology sampling location and geology the morphology of mountainous region being studied in north-eastern turkey (figure 1) consist of rough, irregular land having steep slopes and peaks reflecting the eastern black sea region’s geology and tectonic features the main morphological units (mainly faults and folds) have been shaped by structural elements in the region trending ne-sw the region is drained by the harşit and çoruh rivers, forming the most significant fluvial system deep incision forms like v-shaped valleys are characterised by deep gorges and steep slopes in these drainage systems turkey is located in the alpine-himalayan orogenic belt having the world’s richest natural stone formations; it is the country having 5 billion tons of reserves and almost 35% of the world’s natural stone reserves the eastern black sea region has rich potential in terms of the variety of carbonatebearing rocks the region has around 450 million tons natural stone reserves almost 70% of production today consists of travertine and limestone, the remaining 30% being volcanic rocks the rock samples for this study were collected from 5000 km2 in the southern zone of the eastern pontides (ne turkey) the eastern pontide belt is a major metallogenetic province in the eastern black sea coastal region and forms a 500 km long and 100 km wide mountain chain along the black sea coast the eastern pontides may be subdivided into northern and eastern zones on structural and lithological differences (özsayar et al., 1981; okay and sahinturk 1997) the northern zone is dominated by late cretaceous and middle eocene volcanic and volcaniclastic rocks, whereas pre-late cretaceous sedimentary rocks are widely exposed in the southern zone (arslan et al., 1997; eyuboglu 2006; sen 2007) (figure 2) the limestone samples were selected from pre-late cretaceous platform carbonates which are widely exposed in the area and liassic-aged formations; travertine and onyx samples were selected from the gümüşhane, bayburt and giresun area three rock types from ten rock formations were sampled and tested for this study table 1 gives the rock type, age, location and description of the samples sample preparation and laboratory tests several carbonate rock samples were collected in this study to determine physical and strength properties seven oriented block samples for each group of carbonates were collected from the field for laboratory testing; each block was represented by at least ten core specimens 225 nxsize core specimens having a 2 5:1 length to diameter ratio were prepared from the block samples rock samples were carefully inspected before testing, to obtain the most representative stone samples for performing laboratory tests figure 2. main tectonic units and zones in the eastern pontides naf: north anatolian fault; neaf: northeast anatolian fault (eyüboğlu 2006) hakan ersoy and derya kanik68 uwv, unit weight, water absorption and content are the rock materials’ most important index properties and they are often related to porosity porosity is the ratio of the non-solid volume to the total volume of material and it also describes how densely the material is packed in the study, firstly unit weight, water content, apparent porosity and water absorption by weight were determined with respect to the description criteria of isrm (1981) a pundit plus ultra-sonic pulse (usp) instrument giving more precise rock sample measurements and two 54 khz transducers having piezoelectric properties were used in this study to calculate ultrasonic longitudinal wave velocity the transducers were located parallel to the surface of the sample, transit time was measured and ultrasonic wave velocities were calculated from transit time ucs was determined for the study according to isrm (1981) description criteria core surface flatness was supplied for the ucs test to obtain an even load distribution; specimens were loaded axially up to failure establishing decision-making methods regression analysis includes any techniques for modelling and analysing several variables when the focus is on the relationship between a dependent variable and one or more independent variables (freedman 2005) multiple regression is aimed at learning more about the relationship between several independent or predictable variables and a dependent or criterion (davis 1986) regression analysis was used in this with analytic hierarchy process (ahp), a widely accepted decision making method constraints are compared to each other in ahp, to designate each variable’s relative importance in accomplishing an overall goal numerical values were assigned to each pair of constraints using established guidelines and a constrained matrix is built the sum of each column within the matrix was then normalised and weighting was calculated mathematic formulation (simple additive weighting) was defined following an equation for calculating final grading values in multiple criteria problems (saaty 1980); where, v(y) was area’s suitability index, w was a criterion’s weighting or importance factor of a, y was a criterion’s degree or compliance level, i was the criterion number and q was the number of criteria pairwise comparisons were used to determine each criterion’s relative importance; ahp is based on such approach decision makers can quantify their opinions about the criteria magnitude by using a verbal scale when comparing pairs of criteria the pairwise comparison matrix (pcm) constructed by decision-makers in the previous had to have the following attributes; wij = wji -1 the next step was to calculate the criteria’s relative importance weighting implied by previous comparisons saaty (1980) proposed estimating pcm’s right principal eigenvector which can be approximated using the geometric mean for each row of the pcm (by multiplying the elements in each row and then taking the nth root, where n is the number of criteria) this mode is known as multiplicative ahp (saaty and millet 2000) and was used in the present work the calculated geometric means were then normalised and relative importance weighting extracted (2) table 1. location and general properties of the carbonate rocks rock type rock description age location 1a. travertine (28 samples) porous and soft white-pale cream in colour quaternary bahçecik (gümüshane) 1b. travertine (25 samples) porous and soft pale cream in colour quaternary kalecik (gümüshane) 1c. travertine (25 samples) porous and soft white-pale cream in colour quaternary çamoluk (giresun) 1d. travertine (10 samples) compact and soft white-pale cream in colour quaternary çamoluk (giresun) 1e. travertine (13 samples) compact and soft white-pale cream in colour quaternary gölova (giresun) 2a. onyx (23 samples) compact and soft white-pale grey and cream in colour contained cream colored band quaternary yaylapınar (bayburt) 2b. onyx (15 samples) compact and soft light green, pale cream in colour contained green colored band quaternary yaylapınar (bayburt) 3a. limestone (20 samples) hard and compact light grey in colour contained some micro fossils l. cretaceous dogger-malm kelkit (gümüshane) 3b. limestone (24 samples) fractured and slightly metamorphosed reddish in colour contained a number of fossils liassic esenyurt (gümüshane) 3c. limestone (20 samples) hard and compact light grey and white in colour cretaceous jurassic kemah (erzincan) 3d. limestone (20 samples) hard and slightly metamorphosed light purple and white in colour contained light brown colored band cretaceous devonian tercan (erzincan) v(y)= wj yij q ∑ j-1 (1) multicriteria decision-making analysis based methodology for predicting carbonate rocks’ uniaxial compressive strength 69 table 2. water absorption, water content and apparent porosity values of the rock groups studied table 3. unit weight, p wave velocity and ucs values for the rock groups being studied sample location and type water content (%) water absorption by weight (%) apparent porosity (%) max min sd mean max min sd mean max min sd mean kalecik travertine (1a: 30 samples) 0.20 0.08 0.04 0.13 3.33 0.88 0.73 1.39 7.37 2.16 1.36 3.29 bahçecik travertine (1b: 15 samples) 0.77 0.12 0.15 0.34 1.20 0.48 0.24 0.77 2.88 1.18 0.57 1.90 çamoluk travertine (1c: 25 samples) 2.48 0.49 0.43 1.40 2.90 0.98 0.48 1.88 6.65 2.37 1.01 4.28 çamoluk (2) (1d: 10 samples) 0.62 0.45 0.06 0.59 1.30 0.91 0.12 1.07 3.03 2.44 0.28 2.61 gölova travertine (1e: 13 samples) 0.31 0.18 0.04 0.23 0.62 0.37 0.063 0.47 1.55 0.94 0.15 1.19 bayburt onyx (2a and b: 21) 0.79 0.01 0.18 0.07 0.72 0.02 0.02 0.10 1.95 0.06 0.32 0.26 kelkit limestone (3a: 12 samples) 0.06 0.03 0.01 0.05 0.18 0.13 0.01 0.15 0.48 0.34 0.04 0.40 esenyurt limestone (3b: 15 samples) 0.46 0.04 0.09 0.14 0.31 0.06 0.07 0.19 0.08 0.16 0.19 0.51 kemah limestone (3c: 21 samples) 0.14 0.01 0.03 0.03 0.07 0.02 0.01 0.04 0.18 0.05 0.03 0.11 tercan limestone (3d: 25 samples) 0.09 0.01 0.01 0.06 0.12 0.04 0.02 0.09 0.31 0.12 0.05 0.24 sample location and type unit weight (kn/m3) velocity (m/s) p wave ucs (mpa) max min sd mean max min sd mean max min sd mean kalecik travertine (1a: 30 samples) 24.7 22.2 0.8 24.0 4541 3813 220 4050 65 34 8 53 bahçecik travertine (1b: 15 samples) 25.1 24.1 0.3 24.6 5072 4091 297 4598 44 25 5 37 çamoluk travertine (1c: 25 samples) 24.6 24.1 0.2 24.4 3693 3506 61 3590 44 30 5 38 çamoluk (2) travertine (1d: 10 samples) 25.4 24.9 0.2 25.2 4727 3913 268 4323 43 39 1.2 41 gölova travertine (1e: 13 samples) 29.2 22.2 0.9 25.0 5072 3813 374 4430 66 20 10 40 bayburt onyx (2a and b: 21 samples) 27.4 26.5 0.3 27.1 6800 4121 805 5240 70 20 13 45 kelkit limestone (3a: 12 samples) 27.1 26.8 0.1 27.0 6321 5988 98 6200 154 30 38 80 esenyurt limestone (3b: 15 samples) 27.4 26.6 0.2 27.0 6103 5562 169 5760 117 40 24 70 kemah limestone (3c: 21 samples) 27.1 26.8 0.1 27.0 6353 6127 59 6250 162 25 39 90 tercan limestone (3d: 25 samples) 27.3 25.3 0.6 26.8 6057 4327 371 5414 101 79 9 85 hakan ersoy and derya kanik70 integrating site selection criteria was based on multi-criteria assessment methods (eastmen, 1995): = ∑ (sk*wk) where, sk was the land’s suitability for landfill for objective k (priority groups), (fi)k was factor i (discriminating features) for objective k, (wi)k was the weighting for factor i (score given experimental studies) for objective k, (rj)k was constraint j for objective k (value 0 or 1), s was multi-objective suitability and wk was the weighting for objective k results the rocks’ index and strength properties ucs is an expensive and very time-consuming test the core surfaces have to be ground down to make them parallel at a specified tolerance and a high load capacity loading frame is usually required thus, in the study, ten groups of carbonate rock samples were collected in the southern zone of the eastern pontides (ne turkey) to establish some relationships between ucs and index properties in the study the index and strength properties of the rocks were determined in accordance with isrm (1981) the results of the laboratory studies are given table 2 and table 3 when taking into consideration the mean values of the water content, water absorption by weight, apparent porosity, p wave velocity and ucs value data, the highest values were observed for erzincan limestone and the lowest values were observed for gümüşhane travertines however, the values of the ucs were obtained between 20 and 66 mpa for travertines, 20 and 70 mpa for onyx and 25 and 162 for different limestones depending on the mineral shapes, weathering features and occurrence of the microcracks, the ucs values varied in a wide range as found in the study empirical relationship between index and strength properties rock materials’ uniaxial compressive strength greatly depends on their index properties such as porosity, water content, water absorption and mineral composition p wave velocity and porosity are the most preferred properties for predicting ucs amongst such properties (sachpazis 1990; tugrul and zarif 1999; palchik 1999) regression analysis is usually preferred for establishing statistical relationships between different variables relationship intensity of between variables is defined by regression values and correlation coefficients; explanatory variables must thus be weighted regarding dependent variables before mulp w av e ve lo ci ty (m /s ) 6500 r2=0 70 6000 5500 5000 4500 4000 3500 0 50 100 ucs (mpa) a pp ea ra nc e po ro si ty (% ) 5 r2=0 45 4 3 2 1 0 0 50 100 ucs (mpa) w at er a bs ro rb ti on b y w ei gh t ( % ) 2 0 r2=0 45 1 5 1 0 0 5 0 0 50 100 ucs (mpa) u ni t w ei gh t (g r/ cm ) 2 8 r2=0 40 2 7 2 6 2 5 2 4 0 50 100 ucs (mpa) w at er c on te nt (% ) 1 5 r2=0 32 1 0 0 5 0 0 50 100 ucs (mpa) figure 3. the empirical relationship between ucs and index parameters for different carbonate rock groups table 4. pairwise comparison matrix and relative importance weighting for index properties a b c d e eigenvector weighting a 1 2 3 3 4 3.23 0.45 b 1/2 1 2 3 3 2.34 0.33 c 1/3 1/2 1 2 3 0.73 0.10 d 1/3 1/3 1/2 1 2 0.60 0.08 e 1/4 1/3 1/3 1/2 1 0.30 0.04 a: ultrasonic wave velocity, b: apparent porosity, c: water absorption by weight, d: water content, e: unit weight 1: equal importance, 2: weak or slight (two parameters contributed equally to the objective) 3: moderate importance, 4: moderate plus experience and/or analysis slightly favoured one parameter over another sk = ( )k∑ fi*wi i s ∏ rj j (3) (4) multicriteria decision-making analysis based methodology for predicting carbonate rocks’ uniaxial compressive strength 71 figure 4. the empirical relationship between ucs and vp/n for different travertines and onyx (a: bayburt onyx, b: bahçecik and kalecik, c: çamoluk and d: gölova travertines) figure 5. the empirical relationship between ucs and vp/n for the different limestones (a: esenyurt, b: kelkit, c: tercan and d: kemah) u c s (m pa ) 80 70 60 50 40 30 20 10 0 25000 50000 75000 100000 vp/n u c s (m pa ) 160 140 120 100 80 60 40 20 14000 15000 16000 17000 18000 vp/n u c s (m pa ) 60 55 50 45 40 35 30 25 2600 3000 3400 3800 4000 vp/n u c s (m pa ) 70 60 50 40 30 20 10 500 1500 2500 3500 4500 5500 vp/n u c s (m pa ) 160 120 60 40 30000 50000 70000 90000 110000 130000 vp/n u c s (m pa ) 85 75 65 55 45 10000 20000 30000 40000 50000 vp/n u c s (m pa ) 110 90 70 50 30 6000 10000 14000 18000 22000 vp/n u c s (m pa ) 50 40 30 20 10 500 750 1000 1250 1500 1750 vp/n ucs = 14 in (vp/n)-96 r2=0 83 n: 20 ucs = 0 0184 (vp/n)-22 r2=0 80 n: 12 ucs = 0 0049 (vp/n)+12 r2=0 84 n: 14 ucs = 0 001 (vp/n)+30 r2=0 71 n: 25 ucs = 0 001 (vp/n)+26 r2=0 81 n: 21 ucs = 0 0032 (vp/n)-427 r2=0 86 n: 12 ucs = 0 022 (vp/n)+0 r2=0 84 n: 33 ucs = 19 5 in (vp/n)-107 r2=0 87 n: 35 (a) (a) (c) (c) (b) (b) (d) (d) hakan ersoy and derya kanik72 rock description formulas r2 onyx (21 samples) ucs= 14 ln (vp/n) 96 83 travertine (76 samples) ucs= 15 ln (vp/n) 73 86 limestone (60 samples) ucs= 0.0009 (vp/n) + 38 83 table 5. the formulas used for estimating ucs in this study tiple regression analysis to establish an empirical model ahp used the data obtained from linear regression analysis results concerning ten rock groups (table 1 and figure 3) to decide which material properties should be selected as explanatory variables from rock materials’ index properties this represents a tool which reduces time and the cost involved in many engineering models table 4 lists the priority vectors for all criteria; relative importance weightings are included in the final column of this table ahp parameters are also shown in the table, indicating that the judgments (and therefore final relative importance weightings) seemed to be reasonable the method needed a scale of numbers indicating how many times more important or dominant one element was over another figure 6. the empirical relationship between ucs and vp/n for all travertine (a) and limestone (b) samples figure 7. line-scatter plot diagrams for measured compared to calculated ucs values and histograms showing frequency versus difference in measured and calculated ucs (a and b: travertines, c and d: limestones) u c s (m pa ) 60 50 40 30 20 10 0 1000 2000 3000 4000 5000 vp/n u c s (m pa ) 140 120 100 80 60 40 20 0 0 20000 40000 60000 80000 100000 vp/n ucs = 15 in (vp/n)-73 r2=0 8671 n: 276 ucs = 0 0009 (vp/n)+38 r2=0 83 n: 60 (a) (a) (b) (c) (d) (b) c al cu la te d u c s (m pa ) 70 60 50 40 30 20 10 10 20 30 40 50 60 70 measured ucs (mpa) fr eq ue nc ie (n um be r) 25 20 15 10 5 0 -9 -6 -3 0 3 6 9 measured ucs calculated ucs (mpa) fr eq ue nc ie (n um be r) 16 14 12 10 8 6 4 2 0 -16 -12 -8 -4 0 4 8 12 16 measured ucs calculated ucs (mpa) c al cu la te d u c s (m pa ) 140 120 100 80 60 40 20 20 40 60 80 100 1200 140 measured ucs (mpa) multicriteria decision-making analysis based methodology for predicting carbonate rocks’ uniaxial compressive strength 73 element regarding the criterion or property to which they were being compared the ahp analysis showed that the most important criteria affecting on ucs for the carbonate rocks being studied were uwv (45% weighting) and apparent porosity (33 % weighting) the other index properties’ weighting did not surpase %10 as regression analysis provides a means of summarising the relationship between variables, multiple regression analysis-based methodology was used to establish some numerical relationships between rock materials’ ultrasonic wave velocity, apparent porosity and ucs furthermore it is known that vp decreases with increased porosity vp and n-1 were thus used in the equations for predicting ucs in this study index properties were considered to be explanatory variables, and the ucs a depending variable figure 4 and 5 show multiple regression analysis results, involving some positive correlations between uwv rate regarding apparent porosity and ucs: such relationships were characterised by 0 71 and 0 87 regression coefficients however, figure 6 indicates meaningful empirical relationships for all travertines and limestones and the relationships are represented by the formulas given in table 5 a goodness of fit test establishes whether an observed frequency distribution differs from a theoretical distribution; however, a model’s suitability is tested using the difference between observed and expected values normal distribution is a continuous probability distribution which is often used as a first approach for describing real-valued random variables tending to cluster around a single mean value a normal distribution would thus be expected in the histograms showing difference between observed and expected values line-scatter plot diagrams of observed-expected values (figure 7a and c) and histograms of difference between observed and expected values (figure 7b and d) were prepared and normal distributions were observed histograms having a normal distribution showed that these equations did fairly well in estimating ucs using vp/n the results showed that the model being tested was suitable for the study conclusions this study involved three types of carbonate rocks being collected from ten rock formations in the north-eastern turkey laboratory studies were conducted in line with isrm (1980) to establish some relationships between p wave velocity, apparent porosity and ucs and ahp-based multiple regression analyses methodology was used for statistically analyzing the suggested methods and results ahp analysis indicated that uwv (45% weighting) and apparent porosity (33% weighting) were the most important index properties affecting on ucs regarding the carbonatebearing rocks being studied multiple regression analysis correlations indicated by the 0 81-0 87 regression coefficient were determined for uwv rate regarding apparent porosity and ucs the equations obtained by such analysis results were practical, simple and accurate enough to apply and may be recommended for use in practice acknowledgments this study was partly supported by the karadeniz technical university scientific research fund (project no: 2008 112 005 10) the authors would like to thank bülent yalçınalp and ali babacan from karadeniz technical university in trabzon (turkey) for field and laboratory work the authors would also like to thank nazmiye yazıcı from güvencem marble limited company for helping during all stages of the study references arslan, m , tuysuz, n , korkmaz, s , kurt, h (1997) geochemistry and petrogenesis of the eastern pontide volcanic rocks, northeast turkey chemie der erde, 57: 157–187 babacan, a e , ersoy, h , gelisli, k (2009) determination of physical and mechanic properties of rocks with direct and indirect methods: a case study on the beige limestones in the eastern pontides, proceedings of the 21st international mining congress and exhibition of turkey, antalya, 123-130 bieniawski, z t (1974) estimating the strength of rock materials journal of the south african institute mining metallurgy, 74: 312-320 cargill, j s , shakoor, a (1990) 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and analytical methods in geomechanics, doi: 10 1002/nag 1066 yasar, e , erdogan, y (2004) estimation of rock physicomechanical properties using hardness methods engineering geology, 71: 281–288 earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016) letter from the editor at the end of the 2016 and early 2017, earth sciences research journal wants to thank you for making this project a successful journal. four numbers in one year were our principal milestone for 2016. in total, we issued 44 papers expecting to sum our efforts in the earth sciences' state of the art. comparing to recent years, we have increased our figures in publishing and citation close to the 200 percent. for 2017, the commitment is the same: hard-working for the best of your papers and the journal. as part of our mission, we'd like to make a brief list of the highlighted events for 2017 around the world on geophysics and geology, as invitation letter to pay attention them. for march is scheduled in cyprus the fifth international conference on remote sensing and geo-information of the environment. the conference sessions and workshops include presentations regarding the state-of-the-art methods and innovative projects in various areas of remote sensing and geoinformation. one of the most important meetings in the field will be celebrated in april, the association of petroleum geologists annual convention and exhibition 2017, in houston. the aapg is positioned to attract a focused audience of geoscience professionals. the event has an average of 6900 global attendees from 78 countries over the las five years, according to organizers reports. same on april is scheduled the european geoscience union (egu) general assembly, in viena, austria. the egu aims to provide a forum where scientists, especially early career researchers, can present their work and discuss their ideas with experts in all fields of geoscience. in calgary, canada, between 15 and 19 may 2017, will take place the geoconvention. the event expects going beyond the traditional technical sessions, geoconvention 2017 will take a holistic approach to a vast spectrum of topics including financial market and long-term strategy, infrastructures, regulations, safety and environmental protection, among others. for those how are students with innovation ideas, paris will be the host of the european association of geoscientists and engineers 79 conference, student programme. organizers are expecting 6000 experts attending, more than 1000 presentations in facilities over the 11.000 square meters. le tt er f r o m t h e ed it o r for september 2017, in australia, will be held the 18th annual conference of international association for mathematical geosciences. the meeting will provide a venue for the presentation of research and developments in mathematical geosciences in the form of oral and poster presentations, in the historic port city of fremantle in western australia. in october is expected the joint meeting of the international sedimentological association (ias) and the association des sédimentologistes français (asf). this event will bring the opportunity to gather the sedimentological and associated geological research communities to share experiences, to discuss new challenges, and to stimulate future rewarding collaborations. in turkey, a country from many papers are coming from to earth sciences research journal, will celebrate the fourth international conference on earthquake engineering and seismology, on october. the principal aim of the conference is to bring all national and international researchers within the earthquake engineering and seismology disciplines together and to facilitate their participation in a scientific environment. in this same month will be celebrated the american association of petroleum geologists international conference and exhibition 2017, in london, and the geological society of america (gsa) 2017 annual meeting, in seattle, the united states. we hope all this information is usable for you and expect the papers we are presenting can attract your attention. our next volume will be ready soon! carlos alberto vargas jiménez. editor in chief, earth sciences research journal to analyze the effects of the leakage recharge of the aquifer on the initial dewatering of coalbed methane wells, the mathematical seepage model of water in the coalbed considering the aquifer leakage was established by using the leakage coefficient according to the unsteady seepage theory. the model was solved after laplace transform and the stehfest numerical reverse inversion was used to obtain the solution in right space. then, the log-log type curves of pressure and pressure derivative were created with new combinations of parameters. based on the natural seepage mechanism, the influence of aquifer leakage on curve shape was judged. it is found that the radial flow ends earlier as the leakage coefficient increases. moreover, it was proposed to obtain reservoir permeability, skin factor, and leakage coefficient by using type curve matching. the type curves are useful for quantitatively evaluating the level of leakage, thereby guiding the adjustment of the following production system for cbm wells. este estudio estableció el modelo matemático de filtración de agua en una capa carbonífera al estimar la salida acuífera con el uso del coeficiente de fuga, de acuerdo con la teoría de filtración inestable, para analizar los efectos en la recarga de pérdida de fluidos de un acuífero en el drenado inicial para pozos de gas metano. el modelo se resolvió tras usar la transformación laplace y la inversión numérica stehfest para encontrar la respuesta en el lugar indicado. luego, se creó la representación algorítmica de la presión y la presión derivativa con nuevas combinaciones de parámetros. se evaluó la influencia de la pérdida de fluido del acuífero en la forma de la curva con base al mecanismo físico de filtración. se estableció que el flujo radial finaliza antes de que el coeficiente de pérdida de fluido se incremente. además, se propone el uso de la curva tipo correspondiente para obtener la permeabilidad del reservorio, el factor de daño y el coeficiente de pérdida de fluido. las curvas tipo son útiles para evaluar cuantitativamente el nivel de la pérdida de fluido, y de esta manera guiar el ajuste de un sistema de producción consecuente para pozos de gas metano de carbón. earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 1721. abstract resumen keywords: coalbed methane (cbm); formation water; leakage; well test analysis palabras clave: gas metano de carbón (cbm); formación de agua; pérdida de fluido; análisis de pozo. solution and type curves for the seepage model of the water-bearing coalbed with leakage recharge curvas de solución y tipo para el modelo de filtración de capas carboníferas acuíferas con recarga de fugas record manuscript received: 01/03/2017 accepted for publication: 31/03/2017 how to cite item cheng, w., yang, y., sun, h., zhang, c., wen, q., & li, y. (2017). solution and type curves for the seepage model of the waterbearing coalbed with leakage recharge. earth sciences research journal, 21(1), 17-21. http://dx.doi.org/10.15446/esrj.v21n1.63003 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.63003 c b m wangang chen 1, yu yang2*, hansen sun 1, chengwei zhang 2, qin wen 2, yanyan li 2 1research center, china united coalbed methane co. ltd., beijing 100011, china 2college of energy, chengdu university of technology, chengdu 610059, china *email of corresponding author: zjstxz@163.com 18 wangang chen, yu yang, hansen sun, chengwei zhang, qin wen and yanyan li 1. introduction the coalbed methane (cbm) exists, in the form of adsorbed gas, in the coal reservoir. to recover the cbm, the coalbed is firstly dewatered and depressurized. only when the coal seam pressure drops to the critical desorption pressure, can the methane be desorbed and diffuse from matrix pores to fracture network. in the wyodak anderson coalbed (onsager and cox, 2000; cox and onsager, 2002) of fort union formation in the powder river basin, u.s., the 15# coalbed in shouyang-yangquan of the qinshui basin, china, and other coal measure strata (meng 2010), water bodies exist in the upper or lower coalbed, in addition to the coalbed cleat system. the silty or muddy aquitard with a certain thickness of the coalbed and the aquifer has a certain permeability. during the coalbed dewatering, the pressure difference between the aquifer and the coalbed may drive the interlayer leakage recharge, and accordingly water production of the gas well may change, thus impressing the coalbed dewatering and depressurizing. the scholars in china and abroad have frequently studied interlayer leakage of the reservoir. in the 1960s and 1970s, some scientist elsewhere investigated the reservoir seepage in the case of leakage (katz and tek, 1962; russell and prats, 1962). in china, jia yonglu established the dynamic model of double-layer reservoir with inter-layer leakage in 1997 (jia 1997). then proposed the double-layer dual-porosity model, double-layer triple-porosity model and triple-layer confined model with inter-layer leakage (zhang et al., 2009; chen et al., 2011; huo et al., 2006). sun hedong, gao chengtai, et al. established the dynamic model of infinite triple-layer oil and gas reservoir with inter-layer leakage. wu yilu analyzed the causes for the leakage in the multi-layer gas reservoir and established the mathematical seepage model for the multi-layer gas reservoir with leakage [12]. for the models established in previous studies, all layers were perforated for exploitation, and horizontal radial seepage occurred in each layer, except for inter-layer leakage. however, when the coalbed is perforated for production, the aquifers in the roof and floor are selectively avoided, so horizontal radial flow only occurs in the coalbed. therefore, the existing models for multi-layer reservoir with leakage are not suitable for the analysis of inter-layer leakage in the cbm wells. according to the unsteady seepage theory, the infiltration model for cbm wells considering the inter-layer leakage can be established to obtain the analytical solution of bottom hole flowing pressure under laplace space. after the solution in true space is obtained by stehfest numerical inversion, new type curves can be established. then, these type curves are used to judge whether any leakage recharge exists in the coalbed during dewatering, so as to determine the formation parameters and leakage parameters and analyze the level of leakage recharge. 2. methods 2.1. physical model as shown in figure 1, for a cbm well in the infinite formation center, only the coalbed is perforated for dewatering and gas production. compared with the water content in the coalbed, the water body of the aquifer in the roof and floor can be regarded as infinite water body with a great volume (su and lin, 2009). aquitard with low permeability exists between coalbed and water body. based on the data statistics of the powder river basin in the united states, the vertical permeability of such aquitard is 0.003-0.03 md, suggesting a certain impermeable capacity (onsager and cox, 2000). according to the research on the leakage process of the underground water by hantush and jacob, et al., the model is based on the following assumptions (hantush and jacob, 1955a; hantush 1955b): 1). each layer is homogeneous and isotropic, neglecting the effect of capillary pressure and gravity; 2). each layer has an infinite boundary; 3). only the coalbed is perforated, and the formation water in the coalbed flows into the wellbore in the form of horizontal radial flow; 4). the formation water in the floor recharges the coalbed through the aquitard in the manner of vertical seepage and then the inter-layer leakage occurs; 5). the compressibility of the rock in the aquitard is not considered; 6). due to the significant volume, the water body in the aquifer can be regarded as the constant pressure recharge boundary for the coalbed water. 2.2. mathematical model and solution in the coalbed with leakage recharge, the basic mathematic equation for water seepage is expressed as (streltsova, 1988): (1) the aquifer is regarded as the infinite water body, so the boundary between the aquifer and the aquitard can be treated as the constant pressure boundary. the flow rate recharged into the coalbed by the aquifer through the aquitard is (2) based on the definition of leakage coefficient , the dimensionless leakage coefficient is defined as: (3) equation (2) is substituted into equation (1). according to the definitions of dimensionless pressure, dimensionless time and dimensionless radius, the dimensionless partial differential equation is obtained after simplification. (4) initial condition: ; outer boundary condition: ; inner boundary condition: 19solution and type curves for the seepage model of the water-bearing coalbed with leakage recharge according to the definition of laplace transform: v equation (4) is transformed to ordinary differential equation: (5) similarly, the expressions of the stablish condition in laplace space are as follows: (6) (7) (8) (9) equation (5) is zero-order bessel equation with imaginary argument, and its laplace spatial solution is (10) due to , where is the euler constant and 1.781ã = , equation (10) is deformed as: (11) 3. results and discussion in the traditional well testing analysis, the reservoir permeability, skin factor, and other formation parameters can be determined, with the measured data in type curve matching. for the coalbed with leakage recharge, the leakage coefficient should also be determined and the level of interlayer leakage recharge be analyzed. next, considering the dewatering test with continuous production and water injection test with constant flow rate, the corresponding type curves are derived. 3.1. type curves of the bottom hole pressure in the dewatering with constant production the drawdown well test is conducted to measure the change of the bottom hole fluid pressure with time in the process of dewatering with constant production. according to the ideas of ramey, gringarten, and bourdet to make type curves (liu, 2003). new type curves applicable to dewatering test analysis with constant production can be derived by establishing a new combination of parameters. the dynamic change of the bottom-hole pressure tested is always susceptible to the wellbore storage effect. even before the wellbore storage effect ends, the leakage effect of the aquifer has superseded the influence of after flow. to avoid the interference of wellbore storage effect, the well should be opened or shut in by using downhole control valve in the test process (yang et al., 2015). the wellbore storage coefficient is minuscle and can be ignored, i.e. 0cd = . let , then . equation (11) can be simplified as: (12) it is proved below that is the laplace transformation result of dp based on . (13) equation (12) can be finally written as: (14) the solution in right space can be calculated by using stehfest numerical inversion. the basic equation is [19]: (15) where (16) (17) in the log-log coordinate system, a group of dimensionless pressure type curves (figure 2) are prepared, with as ordinate against as abscissa and -2s deb as the curve parameter. each type curve corresponds to a value of -2s deb . similarly, in the log-log coordinate system, is calculated based on the definition of dimensionless pressure derivative . a group of pressure derivative type curves (figure 3) are drawn, with as ordinate against as abscissa and -2s deb as the curve parameter. each type curve corresponds to a value of -2s deb . in figure 3, when the inter-layer leakage does not happen, i.e. 0=-2s deb , the model only has radial seepage, and the derivative curve of the bottom-hole pressure is a horizontal line, which conforms to the study results of gringarten, bourdet, et al. (yang et al., 2015). according to the definition of db , the effect of aquitard thickness confh and vertical permeability confk ,v on the formation water leakage 20 wangang chen, yu yang, hansen sun, chengwei zhang, qin wen and yanyan li is comprehensively considered. from the underground fluid seepage law, we can see that the smaller the aquitard thickness confh is, or the bigger the vertical permeability confk ,v of the aquitard is, the weaker the barrier action of the aquitard on the vertical flow of the formation water is and the stronger the effect of the formation water leakage on coalbed seepage is. figures 2-3 indicates that the bigger the value of db is (i.e. the bigger the value of -2s deb is), the earlier the leakage affects the coalbed water seepage and the earlier the coalbed radial flow stage ends. in the actual dewatering and production of the coalbed, it is also proved that when the surrounding rock of the coalbed roof and floor has high permeability and strong leakage effect, desorption of cbm can only occur in the small range near the borehole, which is manifested as the deficiency of gas supply (zhang et al., 2013). according to the definition of dimensionless pressure it is shown that the smaller dp is, the higher the bottom-hole flowing pressure is. in other words, the inter-layer leakage may increase the bottomhole pressure and affect desorption of the methane. according to the characteristics of the type curves, the existence of leakage recharge in the aquifer during testing can be qualitatively judged. figure 2 type curves of dimensionless pressure in case of inter-layer leakage. figure 3 type curves of dimensionless pressure derivative in case of inter-layer leakage. then, the formation parameters can be obtained by using the type curves shown in figures 2-3. 3.1.1. formation parameters matching by using the type curves first, the actual testing data obtained from the well site is processed. on the log-log coordinate with the same size of type curves, the curve is drawn with actual differential pressure as ordinate and actual dewatering time t as abscissa and then matched with the type curves in figure 2. permeability can be obtained through the pressure matching value. (18) the skin coefficient can be obtained by time matching value. (19) the leakage coefficient can be obtained by curve matching value. (20) 3.1.2. formation parameters matching by using the type curves of pressure derivative similarly, on the log-log coordinate with the same size of pressure derivative type curves, the curve is drawn with the product of actually measured differential pressure derivative and time t as ordinate against actual dewatering time t as abscissa, and then matched with the interpretation chart. permeability can be obtained through the pressure derivative matching value. (21) by time matching, the skin coefficient can be obtained and its calculation equation is same with equation (19). by curve value matching, the leakage coefficient can be obtained and its calculation equation is same with equation (20). 3.2. type curves of the bottom-hole pressure in the water injection test with constant production in the process of dewatering test, the bottom-hole pressure may drop below the critical desorption pressure, resulting in the gas and water (two-phase) flow, which may interfere the well test interpretation. in order to prevent such two-phase flow, the coalbed can be tested by using the method of water injection with constant flow rate. in the process of water injection, the added value of the bottom-hole pressure is (22) 21solution and type curves for the seepage model of the water-bearing coalbed with leakage recharge the differential pressure is transformed to , then all the parameters may be determined and the effect of leakage on gas well production can be analyzed by using the type curves in figure.2 and figure.3 directly and matching of equations (18)-(21). 4. conclusions (1) the mathematical model for inter-layer leakage in cbm wells is established. under bottom hole open and shut-in condition, the true spatial solution of the mathematical model is obtained by using laplace transformation and stehfest reverse inversion. (2) new combination of parameters is developed under the precondition of dewatering or water injection with constant production, respectively. on the log-log coordinate, the type curves of and , and are drawn, respectively. so the existence of leakage recharge in the aquifer in the process of testing can be judged. the characteristics of the type curves show that the bigger the leakage efficient is, the earlier the leakage recharge is and the higher the bottom-hole pressure is. (3) by type curve matching, the coalbed permeability, skin factor, and leakage coefficient can be determined. the curve matching contributes to quantitatively evaluating the level of leakage recharge, and judging the effect of the aquifer on the coalbed dewatering and production, so that subsequent dewatering and production can be adjusted timely and properly. explanation of symbols p indicates the pressure, mpa; r indicates the radius, m; indicates the pressure transmitting coefficient, cm2/s; t indicates the production time, h; indicates the fluid viscosity, mpa·s; zv indicates the seepage velocity of aquitard, m/s; k indicates the permeability of coalbed, md; h indicates the coalbed thickness, m; indicates the porosity, decimal; tc indicates the comprehensive compressibility, m3/m3; indicates the permeability of aquitard, md; indicates the aquifer pressure, with value equal to initial formation pressure, mpa; confh indicates the aquitard thickness, m; b indicates the leakage coefficient, md/m; db indicates the dimensionless leakage coefficient, dimensionless; wr indicates the borehole radius, m; dp indicates the dimensionless pressure, dimensionless; dr indicates the dimensionless radius, dimensionless; dt indicates the dimensionless time, dimensionless; dc indicates the dimensionless wellbore storage coefficient, dimensionless; indicates the dimensionless bottomhole flowing pressure, dimensionless; s indicates the skin factor, dimensionless; z indicates the laplace transform variable based on dt , dimensionless; dp indicates the image function of laplace transform about dimensionless bottom-hole pressure, dimensionless; indicates zero-order second class modified bessel function, dimensionless; indicates first-order second class modified bessel function, dimensionless; indicates euler’s constant, ; z' indicates the laplace transform variable based on , dimensionless; i indicates the natural number, dimensionless; n indicates the incremental number; indicates the dimensionless pressure derivative, dimensionless; q indicates daily water production, m3/d; b indicates the water volume factor, m3/m3; indicates the producing pressure drop, mpa; ip indicates the initial formation pressure, mpa; indicates the bottom-hole flowing pressure, mpa; indicates the drawdown pressure derivative. acknowledgements this research was financially supported by china’s national major and special project in the 12th five-year plan “development demonstration project for the coalbed methane vertical wells in the south of shanxi qinshui basin (phase ii)” (grant no. 2011zx05060) and 13th fiveyear plan “combined demonstration project for the coalbed methane, tight gas and shale gas in linxing-shenfu area” (grant no. 2016zx05066). references chen, f. f., jia, y. l., & zhang, f. x. 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(2009). coalbed methane geology. coal industry press, beijing. streltsova, t. d. (1988). well testing in heterogenous formations. john wiley & sons inc., new york. pp. 313-314. stehfest, h. (1970). remark on algorithm 368 numerical inversion of laplace transforms. communications of acm, 13(10), 624-625. sun, h. d., liu, l., & zhou, f. d. (2003). exact solution and type curve for infinite three layers reservoir with leakage. mineralogy and petrology, 23(1), 101-104. wu, y. l. (2004). pressure behavioral characteristics and well testing analysis theoretical research of stratified gas reservoir. southwest petroleum institute, 17-39. yang, y., zhang, f. d., & sun, h. s. (2015). well test analysis. geological publishing house, beijing. zhang, l. h., wang, h. t., & jia, y. l. (2009). seepage model of dualporosity media double-layer reservoir with inter-layer leakage. journal of southwest petroleum university (edition of natural science), 31(5), 178-182. zhang, s. b., sun x. b., guo h. y. (2013). identifying method of leakage recharge in cbm wells. coal geology & exploration, 41(5), 29-32. the eastern margin of the qinghai–tibet plateau (qtp) is the focus of studies on eastward lateral extrusion of the latter’s crustal material. this study aims to explore the structural response of the qtp’s eastern crust–mantle to the extrusion, and the basis for the latter’s geological structure. data on the long-period magnetotelluric sounding of cross-tectonic units and bouguer gravity were used to determine the physical structure of the crust–mantle at the plateau’s eastern margin. the findings are as follows: (i) the apparent density structure indicates the large presence of a low-density material in the middle–lower crusts of the songpan and sichuan–yunnan blocks at the qtp’s eastern margin. on the other hand, the yangtze cratonic block (sichuan basin) contains a material with a significantly higher density. to the west of the longmenshan–panxi tectonic zones, and along the lower crust at 40–50 km depth, is an obvious low-density zone aligned in a northeast–southwest orientation. (ii) the electrical structural model spanning songpan block–longmenshan tectonic zone–yangtze block reveals three distinct electrical structural units along the cross-section bounded by the longmenshan tectonic zone. the first is the songpan block, which has high and low resistivity at the superficial layer and middle–lower crusts, respectively. next is the yangtze craton, which has a low and relatively higher resistivity at the superficial layer and middle–lower crusts, respectively. the third is the longmenshan transitional tectonic zone, whose shallow layer and deep structure are characterized by an electrical structure with a thrust nappe towards the east, and a high-conductivity material extending to the lithospheric mantle, respectively. (iii) the apparent density and electrical structures indicate that the panxi tectonic zone has a weakened structure in the lower crust; and (iv) physical properties of the qtp’s deep structure show that its eastern margin may contain a middle–lower crustal fluid material with the attributes of high conductivity and low density. its distribution is interrelated to the uplift mechanism and deep seismogenic activities at the qtp’s eastern margin. abstract keywords: eastern margin of qinghai–tibet plateau, bouguer gravity, long period magnetotellurics, crustal fluid, apparent density deep physical structure and geotectonic implications of the eastern margin of the qinghai–tibet plateau issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.61632 g e o d y n a m ic s earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): a1 -a8 jun lia,b*, xuben wang a,b, qingyan qin c, gang zhang d, dahu lie, jun zhou a,b a key lab. of geo-detection & information techniques of ministry of education, chengdu, china b college of geophysics, chengdu university of technology, chengdu, china c college of resources and environmental engineering, guizhou institute of technology, guiyang, china d school of environment and resource, southwest university of science and technology, mianyang, china e sichuan earthquake administration, chengdu, china *corresponding author: lijun3@cdut.cn el margen oriental del altiplano qinghai-tíbet (qtp, del inglés qinghai-tibet plateau) es el área de la extrusión lateral hacia el este de material cortical. este trabajo se enfoca en explorar la respuesta estructural de las capas superiores en el altiplano y las bases para su estructuracón geológica. se utilizó información magnetotelúria y anomalías de bouguer para determinar la respuesta geofísica de las capas superiores en el margen occidental del altiplano. dentro de los principales resultados se tiene: (i) la distribución de la densidad aparente indica la presencia de material de baja densidad en las capas medias y bajas de los bloques songpan y sichuan-yunnan en el este del qtp. por otro lado, el bloque cratónico yangtze (en la cuenca sichuan) contiene material con una mayor densidad. al oeste de las zonas tectónicas longmeshan-panxi, y a lo largo de las capas inferiores, entre 40 y 50 kilómetros de profundidad, hay una zona de baja densidad con orientación noreste-suroeste. (ii) el modelo eléctrico que abarca el bloque songpan, la zona tectónica longmeshan y el bloque yangtze, revela tres unidades a lo largo de la sección cruzada subordinada a la zona tectónica longmenshan. la primera unidad está en el bloque songpan, con alta resistividad en la capa superficial y baja en las capas media e inferiores. luego aparece el cratón yangtze, con baja resistividad en la superficie y resistividad media en las capas media e inferiores. la tercera unidad es la zona tectónica transicional de longmenshan, cuya estructura superficial y profunda está caracterizada por una estructura eléctrica asociada a una falla de cabalgamiento hacia el este y alta conductividad de material que se extiende hacia el manto litosférico. (iii) la densidad aparente y las estructuras eléctricas indican que la zona tectónica de panxi está debilitada en las capas inferiores. (iv) las propiedades geofísicas de la estructura profunda del altiplano qinghai-tíbet muestran que su margen oriental puede contener un fluido de material en las capas bajas y medias con características de alta conductividad y baja densidad. su distribución está interrelacionada con el mecanismo de elevación y las actividades sismogénicas profundas en el margen oriental del altiplano. resumen palabras clave: margen oriental del altiplano qinghai-tíbet; anomalía de bouguer, anomalias magnetotelúricas de largo período, fluidos de la corteza, densidad aparente. record manuscript received: 17/09/2016 accepted for publication: 07/01/2017 how to cite item li, j., wang, x., qin, q., zhang, g., li, d., & zhou, j. (2016). deep physical structure and geotectonic implications of the eastern margin of the qinghai– tibet plateau. earth sciences research journal, 20(4), a1-a8 doi:http://dx.doi.org/10.15446/esrj.v20n4.61632 estructura profunda e implicaciones geotectónicas del margen oriental del altiplano qinghai-tíbet a2 jun li, xuben wang, qingyan qin, gang zhang, dahu li and jun zhou 1. introduction the eastern margin of the qinghai–tibet plateau (qtp) comprises two main geometrical blocks (fig. 1). one is the triangular-shaped songpan block, which is bounded by the xianshuihe and kunlunshan faults to the north and south, respectively, and sandwiched between the qinghai– 80 85 90 95 100 105 110 80 85 90 95 100 105 110 20 25 30 35 40 20 25 30 35 40t a t i m b a s i n north china s i c h u a n b a s i n south china india i n d o c h i n a ~50mm/yr ~40mm/yr 0 250 500 km 98 100 102 104 106 108 98 100 102 104 106 108 24 26 28 30 32 34 36 24 26 28 30 32 34 36 （a） a b c chengdu chongqing kunming guiyang (b) 0 200 400 km-0 1000 2000 3000 4000 5000 6000 [m] tibet internal block and sichuan basin. the other is the rhombic-shaped sichuan–yunnan block, which has the xianshuihe–xiaojiang and jinshajiang–honghe fault systems as its eastern and western boundaries, respectively. the qtp’s eastern margin is the focus of studies on eastward lateral extrusion of the former’s crustal material (tapponnier et al., 1982). figure 1. schematic diagram of the location and geotectonic of the qtp’s eastern margin. (a) sketched tectonic map of the qinghai–tibet plateau (qtp); (modified from tapponnier et al., 2001); red box is the location of the study area; (b) tectonic and topographic map of the eastern margin of the tibetan plateau; red lines a and b show lmt measuring lines; line c indicates the cross-section of residual gravity; l-j c z=longmenshan-jinpinshan construction zone, surrounded by the blue dashed box; p-x c z=panxi construction zone. over the years, researchers have produced tomographic imaging results for the qtp’s crust–mantle structure (wang, 2007; wu et al., 2009; wang et al., 2005; zhang et al., 2012), as well as geophysical detection results on a semi-molten, lowvelocity, and high-conductivity layer in the region’s middle and lower crust (sun et al., 2003; xu et al., 2007; zhao et al., 2008; wang et al., 2009, 2013, 2014). with these results as the basis, royden et al. (1997) proposed a lower-crustal channel flow model to explain the rapid uplift movements at the qtp’s eastern margin. this model has been widely proven and recognized. the results of deep geophysical detection indicate that the crustal and upper mantle structures of the qtp’s eastern margin are significantly different from that of the sichuan basin and south china block, which are located at its periphery. depth gradients for the mohorovicic (moho) surface, crustal resistivity, and seismic wave velocity are greater in the border areas between the qtp and its surrounding blocks. the distribution of crustal material at the border areas is also inhomogeneous. this is especially the case for the songpan block and interior of the sichuan–yunnan block, where there is an extensive distribution of a low-velocity, high-conductivity viscous layer within the lower crust in many regions. the physical structure of this layer is distinct from that of the brittle upper crustal layer. located to the north of the xianshuihe fault zone, the longmenshan zone was impeded by sichuan basin’s crust, which has greater rigidity. this resulted in middle–lower crustal accumulation, as well as rapid crustal thickening and significant uplift of the ground surface, leading to the formation of the soaring longmen mountains. clark and royden (2000) proposed that the rigid sichuan basin had obstructed the migration channels for the plateau material, resulting in its diversion. this directional change for material flow has a significant relationship to the active tectonics at the qtp’s eastern margin. at the region south of the xianshuihe fault zone, crustal strength is relatively weak. consequently, channels for material flow in the middle–lower crusts are wider (clark et al., 2005). the results of deep geophysical detection can provide substantial evidence for asthenospheric material flow within the lower crust. however, disagreements remain over the understanding of many fundamental theoretical issues relating to the qtp’s eastern margin. a consensus has yet to be reached on matters such as structural properties of the earth’s crust at the qtp’s eastern margin and the characteristics of its crustal movements, uplift mechanism of the eastern margin, and the characteristics of activities at the regional fault zones. substantial amounts of observations at natural seismic stations and magnetotelluric (mt) probing of the deep structure were carried out at the qtp’s eastern margin following the 2008 wenchuan earthquake. although the findings are remarkable and have been recognized, limitations remain when these are applied to research on the qtp’s complicated deep structure and mechanism of dynamic uplift. this is because of the limited number of natural seismic stations, and the scattered bathymetric profiles did not adequately cover the vast expanse and complex geology and geomorphology of the qtp’s eastern margin. in this study, an analysis of the deep physical structure of the qtp’s eastern margin was attempted. it was done via high-resolution apparent density imaging of bouguer gravity anomalies within the study region, combined with long-period magnetotelluric (lmt) sounding results. the density and electrical parameters were then used for a comprehensive analysis of the profound structural differences and their geotectonic implications. 2. tectonic background qtp’s eastern margin refers mainly to the region between the east himalayan syntaxis and west qinling orogenic belt (fig. 1). its northern side is connected to west qinling, which is bounded by the east kunlun a3deep physical structure and geotectonic implications of the eastern margin of the qinghai–tibet plateau fault system. the longmenshan tectonic zone forms its eastern boundary, where it is adjacent to the yangzi block. the sichuan–yunnan block constitutes the bulk of its southwestern section. convergence of the indian and eurasian tectonic plates, which commenced at approximately 50 ma, resulted in the closure of the neo-tethys ocean and the qtp’s rapid uplift. these processes had profound impacts on the geological and geomorphological changes that occurred at the qtp’s eastern margin. on the one hand, northward advancement of the east himalayan syntaxis generated robust extrusion forces towards the east, forming nappe structures in an eastward direction. on the contrary, the qtp’s rapid uplift and gravitational effect caused the material in the upper section of the lower crust to creep and spread towards the east, generating horizontal extrusion forces. under the combined action of both forces, the longmenshan–jinpingshan nappe tectonic zone was eventually formed and positioned, resulting in an approximately 3,000 m difference in terrain elevation between the two sides. separately, a large-scale arcuate strike– slip fault system was formed at the eastern margin. this indicates the presence of a principal horizontal stress field, which overcame the confines of the original tectonic system to form a new geotectonic unit. specifically, this refers to the southeast–east divergence of the songpan block, and the south–southeast lateral slipping of the sichuan–yunnan block. the honghe fault, which is the boundary between the indochina and sichuan–yunnan blocks, underwent dextral strike–slip movements at approximately 13–15 ma (tapponnier et al., 1990). for the xianshuihe fault, which divides the songpan and sichuan–yunnan blocks, sinistral strike–slip movements occurred at approximately 15–20 ma (xu et al., 2006). the maximum horizontal sliding distance between the two faults was 100 km (wang et al., 2000; zhang, 2008). this critical mode of movement not only affected the eastern margin’s geology and geomorphology but also had constraining effects over the occurrence of seismic activities. 3. physical structure 3.1. apparent density structure data on the apparent density structure were acquired from the studies by xu 2006, xu et al. 2007 and yang et al., 2008. using their findings as the basis, we developed a set of high-speed imaging techniques based on apparent density and multi-scale separation of gravity anomalies (li et al. 2015). the main thought process was as follows: first, anomalies were separated by interpolating and splicing bouguer gravity anomalies using various cutting radii to obtain the residual gravity anomalies for different dimensions. second, the iterative downward extrapolation method proposed by xu (2006) was used to obtain residual anomalies for the various dimensions, thereby improving data resolution; and finally, extrapolation depth was defined as the depth of the field source, and residual anomalies for that depth were subjected to apparent density inversion. the resultant apparent density has better capabilities regarding lateral resolution because unlike traditional density inversions, the proposed method carries out inversion based on gravity anomalies located nearer the field source. the bouguer gravity anomalies were separated with cutting radii at increments of 10 km, which gave the residual gravity anomalies for the various cutting dimensions. iterative downward extrapolations were carried out using the latter to obtain the residual apparent density distribution for the qtp’s eastern margin at depths of 10–100 km (fig. 2). the middle–lower crusts of the songpan and sichuan–yunnan blocks located at the qtp’s eastern margin were found to contain a lowdensity material, which is distributed extensively along the western side of the longmenshan–jinpingshan tectonic zone. this material accumulated was has been circulated along a northeast orientation, reaching vertically upwards from the upper mantle to the lower crust and forming an open density gradient zone with the high-density yangzi block in the eastern section. figure 2. apparent density distribution at 10–100 km depths. a4 jun li, xuben wang, qingyan qin, gang zhang, dahu li and jun zhou after the low-density material of the northern longmenshan transition zone had been obstructed by the cold, rigid, and highdensity block in the east, it made a noticeable nne directional change in the lower crust at 40 km depth, before extending northwards and crossing over the west qinling tectonic structure. mo et al. (2006), yu et al. (2009) believed that a lot of cenozoic volcanic rocks formed with material originating from the mantle were related to this low-density material. for the sichuan–yunnan block, the low-density material spread out in an arc along the northeast–southwest orientation within the middle crust at 20 km depth. after being bounded by the panxi tectonic structure at 50 km depth, it extended in a northeast–southsouthwest direction at the western end and a southeast direction within the daliangshan region at the eastern end. this could be due to a weakening of structural density in the middle–lower crusts of the panxi tectonic structure. within the middle–lower crusts at the eastern end, the low-density material crossed the weakened panxi tectonic structure to reach the daliangshan tectonic region. zhao (2008) interpreted the mt data and postulated that the low-density, the high-conductivity material might be the channel flow of the middle–lower crusts. 3.2. electrical resistivity structure there were two issues for consideration in respect of mt survey: (i) the limited signal frequency band (1/320–2000 s) of data acquired with mt equipment (v8); and (ii) inadequate understanding of the electrical structure of the region’s mantle. mt and lmt observations were separately made at each measuring point when acquiring electrical data to address these issues. both sets of data were then merged, and the resultant data set was subjected to inversion (wang et al., 2013, 2014). the outcome was the deep electrical structure for cross-sections a and b shown in figure 1. cross-section a has a total length of approximately 620 km and straddles the songpan block, the longmenshan tectonic zone, and the sichuan basin. electrical imaging (fig. 3a) indicates that the songpan block’s lithosphere has a lower resistivity than that of the overall structure. vertical segregation of the lithosphere is as follows: high-resistivity upper crust; low-resistivity middle–lower crusts; top of the upper mantle, which has weak characteristics of high resistivity; and bottom boundary of the lithosphere at 130 km depth, where resistivity becomes low again. this resulted in an alternating high–low–high–low layout. lithospheric resistivity is significantly high at the yangzi block (sichuan basin). vertical segregation of the lithosphere, which is thin in the east and thick in the west, is as follows: low resistivity for the middle–upper crusts, high resistivity for the middle–lower crusts, steadily high resistivity before reaching the bottom boundary of the lithosphere, low resistivity again at the lithosphere’s bottom edge. the longmenshan tectonic zone is sandwiched between the yangzi and songpan blocks. the middle–lower crusts of its transition zone has the characteristic of prominently high resistivity that extends downwards into the lithosphere’s upper mantle. figure 3. resistivity model (a) and (b) derived by two-dimensional inversion of the merged mt–lmt data. hcl: high-conductivity layer; hrb: a high-resistivity body; red triangle: measuring station. cross-section b, with a total length of approximately 240 km, straddles three tectonic zones: jinpingshan, panxi, and daliangshan. the middle–lower crustal structure at the eastern and western sides of the panxi tectonic zone have significantly high conductivity, with its deep structure showing a trend of linkage to the panxi tectonic structure. the resistivity of the middle–upper crusts is prominently high. for the daliangshan tectonic zone, the lithosphere’s upper mantle exhibits the weak characteristic of high conductivity. in contrast, the lithosphere of the panxi tectonic zone has relatively high resistivity. 4. discussion on tectonic implications 4.1. discussion on uplift mechanism there are two major views regarding the mechanism for cenozoic uplift at the qtp’s eastern margin. one is the plate extrusion model, which postulates that the crustal uplift process was caused by deformation of brittle material within the crust that arose from the thrust nappe effect (tapponnier et al., 1986, 2001). however, the model’s assumptions are not supported by the absence of a quaternary foreland basin in front of the longmenshan fault zone (burchfiel et al., 1995; jia et al., 2006), as well as a crustal shortening rate that is less than 3 mm/a (chen et al., 2000). the other is the lower-crustal channel flow model, which proposes the existence of a low-viscosity asthenosphere in the lower crust of the qtp’s eastern margin. the asthenosphere slipped eastwards under the effect of plate movements. when it was obstructed by the sichuan basin, the low-velocity, high-conductivity lower-crustal material accumulated in front of the rigid yangzi structure and was uplifted. this implies that the crustal uplift process was mainly caused by the accumulation of the middle–lower crustal material flow within the qtp’s interior (royden et al., 1997; clark et al., 2000). a5deep physical structure and geotectonic implications of the eastern margin of the qinghai–tibet plateau figure 4. geotectonic characteristics of the qtp’s eastern margin: (a) isobaths of moho surface; and (b) horizontal gradient modulus distribution characteristics of physical properties show the existence of a low-density, high-conductivity material that is extensively distributed within the middle–lower crusts of the qtp’s eastern margin. seismic data analysis revealed that this material has a relatively high poisson’s ratio of > 0.25 (xu, 2007), which might be related to the crust having been partially molten. this high-conductivity, low-density molten material might be the basis for the material extruded from the qtp’s eastern margin, as well as the dynamic basis for the qtp’s uplift. the low-density, high-conductivity geologic body in the middle–lower crusts is also weak, easily deformed, and has a fast creep rate, thereby facilitating adjustments to crustal thickness. as evidenced by seismic data (gao et al., 2009) and the moho surface (fig. 4a) obtained from inversion of the bouguer gravity interface (parker 1972), a significant trend of crustal thickening exists for both the songpan and sichuan– yunnan blocks. however, the horizontal gradient modulus for bouguer gravity anomalies (fig. 4b) indicates the presence of multiple arcuate tectonic structures aligned in a northwest–southeast orientation within the sichuan–yunnan block. the earth’s surface reveals these arcuate tectonic structures to be mostly sinistral strike– slip faults. it’s hard to imagine such arcuate tectonic structures being able to explain the overall extrusion of rigid material at the qtp’s eastern margin. driven by the effect of plate movements, the low-density, highconductivity fluid material that is widespread in the middle–lower crusts of the qtp’s eastern margin began migrating eastwards to the longmenshan– jinpingshan tectonic zone. it was then obstructed by the yangzi block’s lithosphere, which has the characteristics of high density and resistivity. consequently, it gathered and accumulated along the front edge of the obstruction, forming an open density gradient zone along the northeast direction. the action of isostatic correction caused the middle–lower crustal fluid to be uplifted vertically and extended horizontally. on the one hand, the rate of geomorphic uplift since the quaternary reached approximately 3 mm/a. this vertical uplift resulted in a vertical height difference of more than 3,000 m between the geomorphology of the longmenshan tectonic transition zone and that of the sichuan basin. the amount of uplift for the sichuan–yunnan block reached 1,180 m (zhang, 2004), with many highly-angular thrust nappe tectonic structures appearing in the longmenshan–jinpingshan orogenic zone. on the other hand, the middle– lower crustal fluid of the sichuan–yunnan block spread extensively towards the southeast and south–southeast directions. arising from the northeast–north northeast extension of the songpan block, the longmenshan fault zone within the longmenshan tectonic zone had dextral strike–slip characteristics. after the 2008 wenchuan earthquake with a surface wave magnitude (ms) of 8.0, surface rupture of the central longmenshan fault took the form of vertical uplift with thrusting circulation, as well as slipping movement in the northeast direction. the maximum amount of dextral horizontal displacement resulting from the movements was 6.2 and 4.9 m, respectively. average offset along the entire rupture zone was 2–3 m (xu et al., 2009). the sinistrally-extruded xianshuihe fault had an apparent directional change at the eastern section, connecting at an obtuse angle with the north–south aligned anning river and the daliangshan fault. as such, the sinistral strike–slip movement of xianshuihe fault should have been broken down and absorbed by the blame with a near north–south alignment. this would cause east–west extrusion and crustal shortening. however, surface investigations show that the north–south aligned fault did not have any large thrust shortening components. similar to the xianshuihe fault, this fault also has sinistral strike–slip shearing, which is its adjustment to the eastward movement of the qtp’s crustal material (zhang, 2008). this change forced the deep crustal fluid in the daliangshan tectonic zone to extend towards the south–southeast and southeast directions. this manifested on the surface as the sinistral strike–slip faults in the daliangshan series, which has similar south–southeast and southeast alignments. since the crustal strength of the sichuan–yunnan block is relatively weak, the middle–lower crusts of the panxi tectonic zone with their relatively higher conductivities and lower densities provide the open and broad channels for the fluid’s southeast extension. gradual thickening of the earth’s crust thus occurred at the 1,000 m dimension and more, causing surface uplift over extensive areas and formation of the yunnan–guizhou plateau in southwest china. 4.2. dynamic mechanism of the 2008 wenchuan earthquake given the differing views on the plateau’s uplift mechanism, two different models were used to explain seismogenesis of the wenchuan earthquake, involving shortening and extrusion of the brittle crust versus expansion of the ductile lower crust (hubbard and shaw, 2009). after analyzing the physical properties of the crust at the qtp’s eastern margin, we believe that its uplift mechanism was mainly caused by the fluid that exists extensively in the middle– lower crusts. a similar perspective was applied to the seismogenesis of the wenchuan earthquake. in this study, a cross-section of residual gravity anomalies was selected to analyze the deep crustal structure of the wenchuan earthquake better. the cross-section passes through the earthquake’s epicenter and straddles four tectonic units: the east kunlun tectonic zone, songpan block, longmenshan tectonic zone, and sichuan basin. imaging of the cross-section and its anomalies was done after regularization and a6 jun li, xuben wang, qingyan qin, gang zhang, dahu li and jun zhou downward extrapolation. the image (fig. 5) reveals the extensive presence of negative gravity anomalies in the middle–lower crusts of the songpan block and eastern margin of the longmenshan tectonic zone, at a depth of 20–40 km. at the longmenshan fault zone, a prominent positive gravity anomaly (enclosed by red dashed lines in fig. 5) was found at similar depths as the epicenter. the deep structure is connected to the sichuan basin’s rigid lithosphere. figure 5. residual gravity anomalies after regularization and downward extrapolation. the area enclosed by red dashed lines: high-density rock mass. based on the apparent density distribution within a similar depth range, the variation in apparent density for the positive anomaly was up to 0.01 g/cm3. wang (2014) carried out mt cross-sectional imaging that passed through the epicenter and found the defect to be a high-resistivity geologic body. this cold and rigid geologic body was deduced to be located at a depth of 20–40 km and acted as a significant obstruction to the lower crustal fluid from the west, which has a negative gravity anomaly and low density. what is the relationship between the unusual characteristic of this deep geologic body and seismogenesis of the wenchuan earthquake? the china crustal movement observation network has a gps network in the study area, which implemented four periods of mobile gps observations between 1999 and 2007. the results indicate that before the earthquake, horizontal shortening and slip components were within the range of 2 mm/a for an area wider than 200 km and straddling the entire longmenshan fault zone (shen et al., 2005; meade, 2007). this is consistent with the seismic and geological survey results, confirming that the rate of movement for the longmenshan fault zone was slower. separately, the surface rupture zone of the wenchuan earthquake was inclined up to 70°–80° near-surface. precise positioning of the aftershocks indicates that the fault’s inclination near the hypocenter (15–19 km) was steep (huang et al., 2008). retest results for seismic expedition levels further showed that the ratio of coseismic vertical displacement between the fault’s upper and lower plates was as high as 8:1 (wang et al., 2008). it was, therefore, elementary to conclude that horizontal crustal shortening did not contribute significantly to the wenchuan earthquake. when the indian plate collided with the eurasian plate, the subduction effect caused the middle–lower crustal fluid of the songpan block to migrate eastwards and then accumulate after being obstructed by a high-density rigid body located at similar depths underneath the longmenshan tectonic zone. the effect of isostatic movement caused the accumulated crustal fluid to act in two ways. first, it extended in a northeast direction, leading to dextral strike–slip of longmenshan fault zone’s northern section. second, it was uplifted at the rate of 1.8 mm/a. with this vertical uplift and eastward extrusion, the accumulated fluid and rigid body generated eastward movement and uplift about each other, forming an angular thrust fault on the upper crust. simultaneously, strain energy was being produced, and accumulated by the fault. the wenchuan earthquake occurred when the accumulated strain energy exceeded the rupture strength of the geologic body (fig. 6). the ms 7.9 mainshock actually triggered the aftershock at longmenshan fault zone’s northern section. it was mainly a manifestation of the cumulative effect of the rigid body’s shear strain energy, which was produced by the coupled movement of the upper and lower crusts of the songpan block when the latter extended towards the northeast. 40-50km 60-70km songpan block crustal fluid m=7.9 figure 6. the conceptual model for deep seismogenesis of the wenchuan earthquake (modified from hubbard and shaw, 2009). the epicenters of earthquakes registered at ms 4.0 and above, which have occurred in this tectonic region since january 1, 2008, were projected onto the density distribution map of the middle–upper crusts at 20 km depth (fig. 7). it was found that the majority of epicenters are located in such abrupt high–low-density zones, or in areas where tectonic structures converged. further studies can be carried out to determine whether the wenchuan earthquake had a similar deep seismogenic process. figure 7. seismic distribution and density distribution characteristics. 5. conclusion in this study, the density structure of the deep crust–mantle structure at the qtp’s eastern margin was determined based on gravity anomaly data, while its electrical structure was determined using lmt sounding data. the uplift mechanism for the qtp’s eastern margin and deep seismogenesis of the wenchuan earthquake were then analyzed and discussed, based on distribution characteristics of the physical properties. the findings provide us with the following understanding: (i) a low-density and high-conductivity material is extensively distributed within the middle–lower crusts of the qtp’s eastern margin. after combining the material’s geophysical evidence with available new tectonic dynamic characteristics, it is deemed to be a molten fluid material a7deep physical structure and geotectonic implications of the eastern margin of the qinghai–tibet plateau located in the middle–lower crust. (ii) the apparent density distribution features and structure of the electrical cross-section indicate accumulation of the middle–lower crustal fluid material at the western side of longmenshan–jinpingshan. this accumulated material is the material basis and key factor behind the tectonics of the qtp’s eastern margin, which includes crustal thickening, geomorphic uplift, and formation of thrust nappe structures in the orogenic tectonic zone. 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(2008). evidence of crustal ‘channel flow’ in the eastern margin of tibetan plateau from mt measurements (in chinese). chinese science bulletin, 53(12), 1887-1893. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 99 105 abstract resumen key words: acceleration data, shear wave velocity, antakya, soil conditions, 1d velocity model. palabras clave: información de aceleración; ondas de corte; antakya; condiciones de suelo, modelos de velocidad 1d. record manuscript received: 30/01/2014 accepted for publication: 23/09/2014 seismology this manuscript presents a site response analysis and an estimation of s-wave velocity that are dependent on acceleration data. first, existing data, such as density, seismic wave velocity, and soil cross-sections, are obtained from previous seismic microzonation studies and used to prepare input data for a suite of matlab routines, which are referred to as sua software. acceleration data are obtained from four free-field strong-motion stations of the seramar project, which was conducted between 2006 and 2009 in conjunction with a turkish-german joint research project, and inputted into the software as basic data. the results include a 1d velocity cross-section versus depth and an amplification model of the site. three different depth levels can be determined for the ranges of 0-5 m, 5-15 m and 15-25 m. the seismic velocities vary between 380 and 470 m s-1 for the first 5 m; 320 and 480 m s-1 for 5-15 m; and 470 and 750 m s-1 for 15-25 m. these results are comparable with the amplification values from the microtremor data from previous studies. the 1d velocity models are appropriate for the soil conditions. este trabajo presenta el análisis a una respuesta de sitio y una estimación de la velocidad de la onda de corte que son dependientes de la información de aceleración. los datos adicionales como la densidad, la velocidad de onda sísmica y los cortes transversales de suelo, se obtuvieron de estudios previos de microzonificación sísmica y se utilizaron para preparar el registro de datos en una plataforma de rutinas matlab, que se refieren al software sua. los datos de información de la aceleración se tomaron de cuatro estaciones de monitoreo de movimientos fuertes a campo abierto del proyecto seramar, que se realizó entre 2006 y 2009 en una investigación conjunta turco-alemana, y se ingresaron en el programa como la información básica. los resultados incluyen una sección cruzada de velocidad 1d versus profundidad y el modelo amplificado del sitio. se pudieron determinar tres niveles diferentes a partir de los rangos de 0-5 m, 5-15 m y 15-25 m. las velocidades sísmicas pueden variar entre 380 y 470 m s-1 para los primeros 5 metros; 320 y 480 m s-1 para el rango 5-15 m, y 450 y 750 m s-1 para el rango 15-25 m. estos resultados son comparables con los valores de amplificación del perfil microtemor de estudios previos. los modelos de velocidad 1d son apropiados para las condiciones del suelo. estimating shear wave velocity using acceleration data in antakya (turkey) aydın büyüksaraç1, semir över2, m. cemal geneş3, murat bikçe4, selçuk kaçın4, özcan bektaş5 1. bitlis eren university, civil engineering dept., tr-13000 bitlis-turkey 2. mustafa kemal university, geophysical engineering dept. tr-31200 iskederun/antakyaturkey 3. zirve university, civil engineering dept. tr-27260 gaziantepturkey 4. mustafa kemal university, civil engineering dept. tr-31200 iskederun/antakyaturkey 5. cumhuriyet university, geophysical engineering dept. tr-58140 sivasturkey correspondence author: aydın büyüksaraç. e-mail: absarac@comu.edu.tr issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.41810 http://dx.doi.org/10.15446/esrj.v18n2.41810 100 aydın büyüksaraç, semir över, m. cemal geneş, murat bikçe, selçuk kaçın, özcan bektaş introduction site characteristics of urban areas are primarily estimated by analyses of recorded ground motions. the estimation of site characterization is very important for disaster mitigation planning. however, the types of recorded ground motions differ. because largeor moderate-sized earthquakes do not occur on a frequent basis, they cannot be considered as a source of ground motion. in this case, the site characteristics may be estimated from microtremors or small-sized frequent earthquakes (shrikhande and basu, 2004), which are divided into zones according to their similarities after estimation of the site characteristics. this process is referred to as microzonation. microzonation establishes several parameters that are associated with disasters such as earthquakes and landslides. however, microzonation provides a detailed description of earthquake hazards for the selected locality in a seismic microzone. this process is known as seismic microzonation. data from seismic microzonation includes the intensity of shaking at the bedrock level, the site characteristics, the predominant period and the spectral amplification of surface motion with respect to bedrock motion (shrikhande and basu, 2004). the distribution of damage caused by earthquake ground shaking commonly reflects regional areal differences in local soil conditions. earthquake damage is significantly greater on thick deposits of unconsolidated sediments compared with nearby bedrock locations. as a result, the site conditions should be considered as part of the assessment of ground motion hazards. studies indicate that shear-wave velocity is a critical factor in determining the intensity of ground shaking and is a useful parameter for characterizing local soil conditions in ground motion studies (over et al., 2011). the behavior of soil layers at large strains associated with strong earthquakes is normally nonlinear; therefore, it should be considered in seismic microzonation studies. the majority of researchers currently prefer to use microtremor measurements to estimate seismic microzonation maps. however, to estimate site characteristics from microtremors or weak motion data, all techniques are based on linear system theory (shrikhande and basu, 2004). microtremors have been used to estimate the resonant period of a site since 1957; the results were used to specify the earthquake design forces in various zones in japan depending on the predominant period (kanai, 1957). the characteristics of microtremors were compared with strong earthquake accelerograms recorded in el centro, california by udwadia and trifunac (1973). they concluded that the characteristics of microtremors significantly differ from the characteristics of strong earthquake accelerograms. many researchers have investigated ground motion behavior to establish a correlation between average shear wave velocity and the site amplification factors. surface wave recordings are very useful for seismic response analyses of the sites. this approach is very important to understand the behavior of soils because the soil conditions are related to the mechanical characteristics of surface soils, such as the slope of the soils, and the depth of the ground water table. the estimation of these conditions, which are related to the soils, increases the cost of the site investigations. the soil behavior during earthquakes or microtremors reflects its conditions. seismic hazard analysis provides a framework for which uncertainty in the size, location and likelihood of future earthquakes can be incorporated to model earthquake hazard. in this paper, a simple approach was applied to estimate the level of uncertainty in the modelled amplification factors due to variations in alluvial thickness and velocity structure. matlab routines, which are referred to as sua, are provided to implement an equivalent linear site-response analysis with the option of an assessment of uncertainty (robinson et al., 2006). the presence of soils, geological sediments and weathered rocks can amplify the level of ground shaking experienced during an earthquake. consequently, the effect of the type of soil on earthquake ground shaking is an important component for a seismic hazard analysis. this paper provides a detailed and comprehensive description of an equivalent linear site-response analysis, which is a technique for modelling the amplification of seismic waves due to propagation through alluvial soil in antakya. the description includes a theoretical solution for the wave equation, derivation of a transfer function that relates bedrock acceleration to surface acceleration, calculation of a response spectral acceleration and computation of an amplification factor. 1. overview of the geology, seismicity and soil conditions of antakya in the eastern mediterranean, plate motions occur between the arabia/ anatolia, africa/arabia and anatolia/africa boundaries along the left lateral amanos fault, the dead sea fault and the cyprus arc, and the extension of the latter on land [named the cyprus-antakya transform fault by over et al. (2004a)]. active faults that are associated with the previously mentioned structural zones join to form a triple junction at the quaternary amik basin near antakya (over et al., 2004a). the seismicity and filling of the quaternary age in antakya and its surrounding area have been controlled by these active major faults. the length of the nne-trending segment is approximately 145 km and is known as the amanos fault (lyberis et al., 1992) or karasu fault (westaway, 1994), which has been considered to be a southern continuation of the east anatolian fault zone (eafz) (over et al., 2002; over et al., 2004a). it is located along the western flanks of the karasu valley and the amik basin, which has a width of approximately 30 km, and is filled with plio-quaternary sediments to create a thickness greater than 1000 m (perincek and eren, 1990). some isotopic dating studies suggest a quaternary to recent age for the basaltic rocks along the karasu valley (rojay et al., 2001). plio-quaternary sediments and quaternary volcanics unconformably overlie miocene beds (fig. 1). several authors (ergin et al., 1967; ambraseys, 1970; mckenzie, 1972, 1978; soysal et al., 1981; rotstein and kafka, 1982) have described the general trends of seismicity in the eastern mediterranean, anatolia and levant countries. during this century, this region has been characterized by a low seismicity (jackson and mckenzie, 1988). however, it is known to have been seismically active based on historical catalogues (poirier and taher, 1980; ambraseys and barazangi, 1989). two large historical earthquakes (denoted by stars in fig. 2) destroyed the town of antakya on august 13, 1822 (ms=7.4) and april 13, 1872 (ms=7.2) (ergin et al., 1967), which caused more than 20,000 deaths in the region (kalafat and bagci, 2001). these historical events caused surface ruptures with lengths of 20 km and 30 km along directions of n30° and n20°e, respectively (ambraseys and jackson, 1998). figure 1: geological map of antakya. inset map shows the location and main tectonic units. nafz: north anatolian fault zone, eafz: east anatolian fault zone (over et al., 2011). the most important earthquake during the instrumental period occurred on january 22, 1997; it had a body-wave magnitude (mb) of 5.5 and aftershocks with mb of 5.2 and 5.3 and maximum intensity of io=vi-vii, which caused damage in certain buildings (erdik et al., 1997). 101estimating shear wave velocity using acceleration data in antakya (turkey) figure 2: seismicity and seismotectonic map of the hatay region and surroundings, and some focal mechanisms of shallow earthquakes. the red points represent the earthquakes in 1997. eafz: east anatolian fault zone, dsfz: dead sea fault zone, kofz: karataş osmaniye fault zone, rf: reyhanli fault, and af: amanos fault. data on the near-surface geological conditions are important for understanding the site amplifications that were experimentally measured using the earthquakes. two deep borehole logs provide a vertical profile of the surface deposits up to 60 m (w1) and 100 m (w2), respectively (table 1). antakya municipality housing is situated along the two sides of the asi river, which originates in syria and runs to the mediterranean sea through antakya city. the sediments within the antakya region are primarily composed of quaternary alluvial fill, which consists of clay, silt, sand and gravel. the soil description in the borehole logs show that the surface soil consists of quaternary materials composed of clay, gravel detritic formations of conglomerates and alluvial sands. the average ground water level in the area is 3 m according to the deep boreholes. table 1: deep boreholes in the study area (w1=60 m and w2=100 m) (over et al., 2008) w1 w2 depth (m) formation depth (m) formation 0 1 agricultural soil table 0 7 gravel with clay 1 7 gravel 7 11 clay stone 7 – 26 clay 11 18 clay stone with gravel 18 22 gravel 22 26 conglomerate 26 29 conglomerate with clay 26 36 gravel 29 33 conglomerate 33 53 conglomerate with clay 36 76 clay 53 60 clay stone 76 – 80 76 80 gravel 80 90 clay 90 100 gravel with clay 2. site effects from h/v ratios and vs30 analysis the fundamental frequencies of the near-surface deposits at a site are a valuable tool for developing land-use plans for urban regions that consider earthquake disaster mitigation. here, the resonant periods and site amplifications are determined by dividing the smoothed fourier amplitude spectrum of the horizontal component by the smoothed spectrum of the vertical component that was recorded at the same site, which eliminates the need for a reference site. the spectral ratio technique for estimating the transfer function for the site response, involves the ratio of the fourier amplitude spectrum of the motion recorded at the site to the fourier amplitude spectrum of the same motion recorded at a nearby reference site, has been proposed to estimate the resonant periods of the site (edac, 2004). over et al. (2011) analyzed the microtremor data for antakya. the h/v spectra were obtained for all observation sites, and the predominant periods of all sites were identified. the microtremor measurements show that the predominant period for the sites in antakya range from approximately 0.2 s to 0.8 s (fig. 3). figure 3: locations of sections from combination of microtremor and remi measurement positions. p represents microtremor observations and ë represents remi observations. 3. data 3.1. strong-motion stations in close collaboration with mustafa kemal university (mku) and other local partners, the earthquake damage analysis center (edac) at bauhaus-universität weimar initiated a turkish-german joint research project on seismic risk assessment and mitigation in the antakya-maras-region (seramar) (edac 2004). in the framework of the seramar project, three reinforced concrete structures were equipped with a building monitoring system manufactured by syscom instruments of switzerland. in this building monitoring system, three or four sensors were installed on the structure and one sensor was installed on the free field (schwarz et al., 2007). the sensors were mr2002+ triaxial accelerometers with a linear 0 to 150 hz frequency response range and a ±2 g measuring range. the free-field station was installed at a distance that was equivalent to the minimum height of one instrumented or neighboring building (schwarz et al., 2007). data obtained from the fourth station, which were instrumented by the general directory of disaster affairs of turkey, were employed in this study (fig. 4). the instruments consisted of kinemetrics etna with three channels and an internal triaxial episensor force balance accelerometer. 102 aydın büyüksaraç, semir över, m. cemal geneş, murat bikçe, selçuk kaçın, özcan bektaş 3.2. acceleration data in the considered time-window between 2006 and 2009, approximately 30 earthquakes were recorded and identified at these stations (abrahamczyk et al., 2008; schwarz et al., 2009). all earthquake parameters were derived from the website of the kandilli observatory and the earthquake research institute (koeri) bulletins, as shown in table 2. figure 4: locations of accelerometer stations: stations i, ii and iii are free-field stations of the building monitoring systems, whereas station iv is an independent free-field station. table 2: list of the measured earthquakes in the project area. no. date location h (km) md ml recorder lat. long. sta-ii sta-i sta-iii sta-iv 1 19/09/2006 35.95 35.77 22 3.8 ● ● ● 2 09/10/2006 35.82 35.60 39 4.3 ● ● ● ● 3 07/01/2007 37.13 36.09 20 3.9 ● 4 25/01/2007 36.12 35.90 17 3.0 ● ● 5 01/02/2007 35.84 35.90 12 3.6 ● ● ● 6 03/02/2007 36.95 35.42 27 3.6 ● ● 7 11/03/2007 36.56 35.88 6 3.8 ● 8 14/03/2007 36.47 35.86 4 3.2 ● 9 12/05/2007 36.39 35.63 8 3.8 ● ● 10 15/05/2007 35.97 35.83 5 3.1 ● 11 21.01.2008 37.00 36.12 7.0 2.9 ● ● 12 07.03.2008 36.78 36.42 8.7 3.2 ● ● 13 10/03/2008 36.61 36.47 6.8 2.8 ● ● 14 17/04/2008 36.28 36.13 7.3 3.3 ● ● ● 15 03/05/2008 36.73 36.62 7.1 3.9 ● 16 12/06/2008 36.25 36.06 7.4 3.0 ● ● 17 19/06/2008 36.63 36.05 6.8 3.7 ● 18 01/11/2008 36.06 35.86 24.2 3.3 ● 20 18/11/2008 36.22 36.98 7.0 2.9 ● ● 21 30/11/2008 36.24 36.11 7.0 2.7 ● ● 22 04/12/2008 36.18 36.02 7.2 2.9 ● 23 10/12/2008 36.04 36.25 8.4 3.2 ● 24 19/12/2008 36.21 36.29 7.0 3.0 ● 25 19/12/2008 35.93 36.36 7.0 2.8 ● 26 17/01/2009 37.16 36.31 7.5 4.6 ● ● 27 22/01/2009 36.11 35.88 24.6 3.4 ● ● 28 22/01/2009 36.09 35.83 22.8 3.1 ● ● 29 05/02/2009 36.20 36.09 7.4 3.1 ● ● 30 09/02/2009 36.09 36.05 2.1 3.3 ● the magnitudes of the earthquakes (md) ranged from 2.7 4.3. a total of 30 acceleration data were obtained from the time histories of this area. several earthquakes with magnitudes (md) that ranged from 3.0 4.3 were measured. examples of recorded acceleration time histories are given in figure 5 (schwarz et al., 2007). the response spectrum is the most suitable tool for expressing the excitation response relation in earthquake engineering and seismic design. although it is an indirect measure of ground intensity, it directly expresses the maximum response, which is a major concern in structural design. if sets of response spectra for ground motions recorded at different locations during previous earthquakes are generated, a large variation would be observed in both the response spectral values and the shape of the spectra will vary among the sets. figure 5: recorded building response due to a magnitude ml = 3.1 earthquake (march 3, 2008); sensor: free-field of the building (a) recorded acceleration time histories. these variations are dependent on many factors, such as the energy release mechanism in the vicinity of the focus or the hypocenter and along the fault interfaces, the epicentral distance and the focal depth, the geology and its variations, the energy transmission paths, the richter magnitude and the local soil conditions at the recording station. thus, the response spectral values s (spv, spa and sd) (eq. 1) for earthquake ground motion should be considered to be a function expressed in the following form (clough and penzien, 1993; tehranizadeh and hamedi, 2002) s = s(sm, ed, fd, gc, m, sc, ξ ,t) (1) s=s(sc,ξ) (2) where the independent variables include sm: source mechanism, ed: epicentral distance, fd: focal depth, gc: geological conditions, m: richter magnitude, sc: soil conditions, x: damping ratio and t: period. the effects of sm and gc on both the spectral values and the shapes of the response spectra are not well understood; therefore, these effects cannot be quantified when defining response spectra for design purposes. the effects of ed, fd and m are usually considered when specifying the intensity levels of the design response spectra; however, they are frequently disregarded during the specification of the shape of these spectra due to a lack of knowledge regarding their influences. the effect of sc on both the intensities and the shapes of the response spectra are considered in the definition of the design response spectra (tehranizadeh and hamedi, 2002). in many cases, the ratio of the spectral acceleration to the peak ground acceleration (acceleration amplification) is plotted as a function of either frequency or period (mohraz and elghadamsi, 1989). studies have shown that the response spectra from accelerograms recorded on similar soil conditions reflect similarities in shape and amplification. for this reason, the response spectra from records with common characteristics are averaged and smoothed before they are used in design (mohraz and elghadamsi, 1989). 103estimating shear wave velocity using acceleration data in antakya (turkey) 4. antakya case study robinson et al. (2006) collectively described the presence of soils, geological sediments and weathered rock as regolith and developed matlab routines, which are referred to as sua, to model the amplification of seismic waves due to propagation through regolith from sydney, australia. antakya exhibits high risk due to earthquakes. the approach of robinson et al. (2006) was employed with available geotechnical information to investigate the potential for ground motion amplification in antakya. antakya is located within the amik basin, which comprises a sequence of plio-quaternary sediments with a minimum thickness of 1000 m from a variety of geological environments, including sand, gravel, clay and silt. five site classes were identified in antakya using microtremor data and seismic velocities obtained by over et al. (2011). the surface acceleration time history and the response spectral acceleration for the recorded earthquakes in the stations (table 2) were obtained using sua software (robinson et al., 2006). the methodology enabled a broad-scale site-response analysis despite limited geotechnical data. the analysis provided an indication of the level of uncertainty associated with the two geotechnical parameters layer thickness and seismic velocity, for which the amplification factors are the most sensitive. the sua software packages are designed for individual site-response analysis and do not define the site response for a larger region. accelerometers were installed in two different soil types to record multiple earthquake data in the framework of the seramer project. we prepared input data for sua software for each of the four locations using vs data on the densities of shallow wells drilled by private firms. one-dimensional velocity versus depth was calculated for all four locations by controlling the acceleration data for each station (figures 6a, b, c, d). the cross-sections were simulated from the site classes using the approach suggested by robinson et al. (2006) to produce amplification factors for acceleration time history combinations. figure 6: 1-d velocity models generated from acceleration data using input data site class 4 for station i and site class 2 for stations ii, iii and iv. (a) station i, (b) station ii, (c) station iii, and (d) station iv. gray colored drilling logs show the soil condition at the station. lf: landfill, css: clay with silt and sand, ssc: silt with sand and clay, sss: sand with silt and sand, sgcs: sand with gravel, clay and silt, gcs: gravel with clay and sand, sc: sand with clay, and cs: clay with sand. 104 aydın büyüksaraç, semir över, m. cemal geneş, murat bikçe, selçuk kaçın, özcan bektaş amplification factors are assumed to be log-normally distributed. note that the mean in log-space is equivalent to the median in normal space. the median amplification factors are shown within the 16th and 84th percentiles in figures 7a, b, c, d. figure 7: median amplification factor (solid line) and 16th and 84th percentiles (dashed lines) (a) station i, (b) station ii, (c) station iii, and (d) station iv. 5. discussion and conclusions this paper describes the site response and velocity changes using acceleration data with inversion for four acceleration stations in antakya. the study also uses the microzonation results of over et al. (2011). 1 the median amplification factor indicated the following maximum levels of amplification: 1.5 at 0.4 s periods for station i, 1.5 at 0.5 s periods for station ii, 2 at approximately 0.25 s periods for station iii, and 2 at 0.4 s periods for station iv, which approximately corresponds to the natural period calculated by the microtremors. 2 the methodology enabled a broad-scale site-response analysis to be conducted despite limited geotechnical data. o analysis provided an indication of the level of uncertainty associated with the two geotechnical parameters soil thickness and seismic velocity, for which the amplification factors are the most sensitive. the seismic velocities are comparable with the drilling results. 3 the soil in the study area presented different velocity structures. three different depth levels were determined: 0-5 m, 5-15 m and 15-25 m. the seismic velocities varied between 380 and 470 m s-1 for the first 5 m; 320 and 480 m s-1 for 5-15 m; and 470 and 750 m s-1 for 15-25 m. however, stations 1 and 3 exhibit a similar seismic velocity structure. station 2 exhibits different velocity characteristics due to improvements in seismic velocity values. station 4 presents a similar soil structure along the deep profile. 4 our analysis determined the correlations, calculated periods and amplifications from microtremor and acceleration data. the periods were approximately identical (fig. 8a). the shape and amplitude of the h/v spectral ratio of the microtremor for each site were similar to the shape and amplitude of the h/v spectral ratio for the seismic motion. the amplifications showed differences due to different sources of the data (fig. 8b). this difference can be explained by the effects of the local topography. the study indicated that the alluvial soils in antakya can significantly amplify the ground shaking associated with an earthquake. the incorporation of uncertainty revealed that a range of possible amplification factors can be expected. the method assumed that the velocity in each soil model unit can be represented by a linear gradient. figure 8: comparison of acceleration and microtremor data (a) period changes and (b) amplification changes. 105estimating shear wave velocity using acceleration data in antakya (turkey) acknowledgements the authors are grateful to dr.-ing. jochen schwarz and dipl.-ing. lars abrahamczyk from the earthquake damage analysis center (edac) of bauhaus university in weimar, germany for the permission to use the recorded data provided by the collaborate seramar project . the instruments donated by syscom inc., switzerland and the financial support from the scientific and technological research council of turkey (tubitak) under the project grant no. 106m420 are also appreciated. this paper has been reviewed by dr. hakan cinar of karadeniz technical university, turkey, which has significantly improved its quality. references abrahamczyk, l., schwarz, j., lang, d.h., leipold, m., golbs, ch., genes, m.c., bikce, m., kacin, s., and gülkan, p. 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(2009). building typology for risk assessment: case study antakya (hatay). earthquake and tsunami, istanbul, turkey. schwarz, j., lang, d.h., abrahamczyk, l., bikce, m., genes, m.c. and kacin, s. (2007). seismische instrumentierung mehrgeschossiger stahlbetonbauwerke-ein beitrag zum seramar project. der österreichischen gesellschaft für erdbebeningenieurwesen und baudynamik (d-a-ch tagung 2007), 28 september 2007, wien. shrikhande, m. and basu, s. (2004). strong motion v/s weak motion: implications for seismic microzonation. journal of earthquake engineering, 8, 159-173. soysal, h., sipahioglu, s., kolcak, d. and altinok, y. (1981). turkiye tarihsel deprem katalogu. tubitak project (no: tbag-341), ankara. tehranizadeh, m. and hamedi, f. (2002). engineering structures, 24, 933-943. udwadia, f.e. and trifunac, m.d. (1973). comparison of earthquake and microtremor ground motion in el centro, california. bulletin of the seismological society of america, 63, 1227-1253. westaway, r. (1994). present-day kinematics of the middle east and eastern mediterranean. journal of geophysical research, 99 (b6), 12071-12090. geofisica colombiana pp.65-70 bogota, d.c.n° 7 diciembre de 2003 issn 0121 2974 analisis de la amplitud referenciada con trayectorias de rayo y su aplicaci6n en avo jesus g. bohorquez q. luis a. montes departamento de geociencias, facuitad de ciencias, grupo de investigaci6n en geofisica, universidad nacional de colombia, lamontesv@unal.edu.co resumen ei analisis avo se basa en la variaci6n de amplitudes de ondas con el offset, determinada entre otras causas por las propiedades elasticas de las rocas. las variables necesarias para caracterizar el comportamiento elastico de las rocas, como la velocidad de ondas-s y atenuaci6n, se llegan a determinar mediante analisis avo. la valoraci6n de velocidades resulta menos costosa en unos casos, cuando el avo remplaza la utilizaci6n de multicomponentes en adquisiciones terrestres, y en otros casos posible, cuando se utiliza a partir de adquisiciones marinas. el presente articulo describe una metodologia diferente para la utilizaci6n avo (ava), que consiste en analizar los cambios relativos de amplitud de onda en frecuencia entre dos reflectores pr6ximos, corrigiendo as! perdidas de energia de la onda por efectos de transmisi6n y conversi6n que dependen del angulo de incidencia. el metoda se aplic6 a datos sinteticos de modelos de capas con interfaces planas y paralelas asociados a los ambientes de dep6sito principalmente marinos de baja actividad tect6nica con cambios principalmente estratigraficos, como por ejemplo en zonas de los llanos orientales de colombia. palabras clave: avo, rayo, recuperaci0n de amplitudes abstract the avo analysis is based in variation of amplitude of waves with offset, due mainly to elastic properties of rocks. the variables characterizing the elastic behavior ofrocks, licke velocity of s-waves and attenuation, can be estimated by avo analysis. this can be considered cheap in case ofland acquisition and possible in case of maxime acquisition. this paper describes a different methodology to use avo, analyzing the relative change ofthe amplitude use with frequency between two reflectors, recovering the last energy of wave due transmission are conversion of wares affected by the angle of incidence. the method was applica to synthetic data of models with homogeneus and isotropic layers separated by plan-paralled interfaces, models associated to maxime depositation environment with low tectonic activity, with stratigraphic features as seen in wt plans in colombia. keywdrd: avo, ray, true amplitude recdvery introduccion ei analisis avo presenta dificultad al discriminar variaciones debidas a contrastes de velocidad, densidad, atenuaci6n, angulo de incidencia, m6dulos de elasticidad. dentro de los divers os factores que influyen para que se presente este tipo de dificultad, el mas significativo y objeto de analisis en este articulo, es el de perdidas por transmisi6n (gassaway, 1984). manuscrito recibido para evaluaclon ells de junio de 2003. articulo aceptado para publcacion por el cornite editorial el 30 de octubre de 2003. geoflslca cdlombiana, 7, diciembre de 2003 actualmente estas perdidas se corrigen mediante la normalizaci6n del reflector objeto de analisis avo con respecto a un reflector de referencia (chiburis, 1987): " ...esta normalizaci6n es usualmente hecha empiricamente mas que deterministicamente mediante la remoci6n aproximada de todos los efectos de capas que suprayacen el objetivo mediante la consideraci6n unica de diferencias entre el objeto de reflexi6n y una reflexi6n cercana de referencia (chiburis, 1984, 1987, 1993). aunque esto no garantiza el exito porque la capa de referencia puede por si misma generar conversiones de onda que suprayacen y distorsionan la reflexi6n objeto (e. g. ross, 1991, referido por adriansyah and george a. macmechan, geophysics, 1998). 65 boh6rquez et al. las perdidas por transmision y conversion debidas al angulo de incidencia dependen de la complejidad estructural del modelo de velocidad, razon por la cual se debe ser cuidadoso al hacer analisis conciuyentes, y recordar que la mayoria de reflexiones observadas son superposiciones de eventos de una serie de interfaces suprayacentes, haciendo el analisis ayo mas dificil. ei ayo inciuye de manera integral diferentes tecnicas 0 procesos para su estudio que son utiles para identificar caracteristicas o factores fisicos como los modules y constantes elasticas de las rocas en profundidad. la determinacion de .an malias producto de estos factores fisicos de las rocas que intervienen en el comportamiento ayo son ciaves en la exploracion minera del subsuelo. para que estas anomalias sean mas significativas -es decir, que nos sirvan como indicadores indirectos de un objetivo minerose hace necesario crear modelos cada vez mas integrales a partir de otros modelos puntuales ya estudiados. asi, en el presente articulo no solo se analiza el ayo para un solo reflector sismico, sino tambien las variaciones relativas de ayo con respecto a otro reflector cercano. la metodologia que permite analizar deterministicamente estas variaciones en conjunto es diferente a otras, y constituye un punto de partida para estudios de este estilo con modelos mas complejos. la tecnica de norrnalizacion utilizada por chiburis para corregir perdidas por transrnision se basa en la consideracion unica de las diferencias de amplitud de onda, asumiendo eventos de incidencia normal entre dos reflectores sismicos proximos, resultando con poco fundamento teorico (chiburis e. f., 1984, 1987). en este estudio se corrige estas perdidas considerando tanto las diferencias de amplitud a traves de trayectorias de rayo, como los efectos par conversion-transmision en las amplitudes de onda de los dos reflectores. , en el articulo se resume como se realiza una recuperacion de amplitudes relativa de dos interfaces, analizando variaciones de amplitud de onda-p en frecuencia para cuantificar fenomenos de absorcion, a diferencia de las tecnicas convencionales en donde se analizan las variaciones de amplitudes de onda-p en tiempo de manera directa mediante la seleccion y suavizado de las mayores amplitudes dentro de la interface sismica. teoria la reflexi6n y transmision de ondas planas en un limite plano entre dos medios isotropicos son dos de los temas de estudio mas fundamentales en la propagaci6n de ondas, la leyes que gobieman la reflexion y transmision de onda en el limite de dos medios se enfocan desde el punto de vista geometrico 0 ley de snell, dadas las condiciones de continuidadestablecidas en las ecuaciones de zoeppritz (1919). 66 inicialmente se pretendio trabajar con un modelo ideal basado en las siguientes ecuaciones: por definicion, el coeficiente de reflexion es arpi rppi(0)=-. aipi (i) y el coeficiente de transmision es: alpi tppi(0)=alpi (2) siendo: rppi el coeficiente de reflexion en el primer reflector, tpp i el coeficiente de transmision en ei primer reflector, aip 1 la amplitud de la onda-p incidente, arpl la amplitud de la onda-p reflejada, atplla arnplitud de la onda-p transmitida (ver figura i). la amplitud de ia onda que proviene de una fuente de energia (s) y sigue una determinada trayectoria de rayo (i) que se refleja sobre la primera interface (in), arp ln.i, tiene una equivalencia proporcional con ia amplitud de ia onda que se refleja sobre la segunda interface (n) (ver figura 1) arpn,i, y se puede expresar como: n arpn; -k rppn t. it. x lnx in arpin.; rpan (3) siendo: arpn,i -arp ln.i las amplitudes que se reflejan de las dos interfaces sismicas en frecuencia, k una relacion de proporcionalidad entre los coeficientes de reflexion, rppn-rppln, y los coeficientes de transmision tin-itin. estos coeficientes en general tienen dimensiones de: velocidad yin, vn, win, wn para ondas p y s, respectivamente (ver figura i), y angulos de incidencia tin, tn de ia onda p y fln, fn para los angulos de ia onda convertida s con ia normal. con ei modelo de ia ecuacion (3), para trayectorias de rayo, se trabajo y combino con las curvas teoricas para cada reflector de los coeficientes reflex ion y transmision sismica que varian con el offset de acuerdo con la ecuacion zoeppritz para ondas planas, dadas par cerveny y ravindra (1971). de este modo se llego a saber la relacion de velocidades de las ondas p y s que deterrninan, en las interfaces, contrastes elasticos. si se observa de nuevo la ecuacion (3), esta incluye una normalizacion adecuada entre dos reflectores (proximos) de las amplitudes de reflexion (arp), y una constante (k) que es proporcional a la divergencia geometrica de la onda, y a ia atenuacion del medio comprendido entre las dos interfaces del modelo de capas deterrninado a su vez por un factor q de calidad. n es la curva de normalizaci6n que se obtiene directamente de los datos sismicos y fue 10 que se llarno el referente empirico, el cual se analiza por ajuste de minimos cuadrados con la expresion de la derecha de la ecuacion anterior. mediante un truncamiento geofislca colombiana. 7, oiciembre oe 2003 analisis de la amplitud referenciada con tra yecforias de rayo y su apllcacl6n en avo de las series de taylor alrededor de un punto de la funcion para cada factor de la ecuacion (3), podemos escribir que: 1 arp".l x" ,t,,) arp".i( x~ ,tn 1_ arpl".i(x,,,,tl,,) arp,,,.i(xi~,tl~)wi" wi~ o w" o will el modelo de ajuste de la ecuacion (4) cuantifica y discrimina la velocidad de ondas-s, w, densidad, r, atenuacion por absorci6n y divergencia geometrica, k, y angulos de incidencia, t, dado un modelo inicial propuesto, que se calculo en este proyecto usando relaciones petrograficas empiricas. a) (0,0) i .". i i xo to i l~li , ?~i i frp1n'ir~ i . coltimiemo de rase b) '"f:! v1n 0v1~1d ij nn vn (wn) (~ rn i:§)vn1 rn1 figura 1. represenraci6n esquematica de corrimientos de fase, teniendo como base trayectorias de rayo, previa analisis frecuencial f-k. a) secci6n sismica: b) modelo de velocidades. ei manejo de los coeficientes de reflexion y transmision, resultantes de las ecuaciones de zoeppritz (1919) se hace utilizando geofislca colombiana. 7. oiciembre oe 2003 expresiones en forma parametrica (cerveny y ravindra, 1971), que resultan practicas cuando se aplican en el analisis de amplitudes de onda con el offset para resolver un problema inverso. con sus cambios de nomenclatura, los coeficientes de transmision y reflexion para la interface superior e inferio son rppln, tppin, itpp in, rppn y tppn siendo el coeficiente de reflexion escrito como: (4) para la interface superior, y rppin=-i+ 2* nrtn» dln2 * cos (tin) para la interface inferior. mientras que los coeficientes de transrnision son: tin=2* ntin* d in,l* cos (tin) itin =2* nt in* id in'j -cos (tin) para la interface superior en don de nrln, ntin, din, idln son funciones de las velocidades de ondas p y s, la densidad de los medios, los angulos de incidencia y parametres de rayo asociados a la interface superior nrn, ntn, dn son funciones de las velocidades de ondas p y s, la densidad de los medios, los angulos de incidencia y parametres de rayo, para la interface inferior. los estudios hechos de propagacion a traves de una interface de velocidades para frentes de onda pianos son validos tambien para frentes de onda esfericas, ya que localmente estas ondas se pueden descomponer en frentes pianos: r: [ i ]~"'r(8)+-n(g) , r kr n(8)=(1/2)[ dr(8) ctang (8)+ d 2 r(g)] d8 d82 (5) siendo rte) el coeficiente de reflexion para ondas planas y 0 el coeficiente para ondas esfericas. brekhovskikh (1960) implemento la ecuacion (5) para el casu acustico, se puede aplicar esta ecuacion para el casu elastico, por ejemplo con los coeficientes de zoeppritz. de la estructura de esta ecuacion esto parece ser valido dado que kr es apreciablemente mayor que 1. ei descomponer estas ondas permite aplicar la extrapolacion del campo de onda utilizando corrimientos de fase de acuerdo con trayectorias de rayo en ondas generadas por fuentes puntuales, para norrnalizar las amplitudes de acuerdo con la posicion de la reflexion en las interfaces (ver figura 6). el objetivo general de este estudio fue hacer una recuperacion de amplitudes entre dos interfaces y deducir propiedades 67 boh6rquez et al. elastodinamicas de la roca a partir de variaciones relativas de amplitud debidas a coeficientes de reflexi6n-transmisi6n. datos para obtener un registro sismica, se asumi6 un modelo elastica de tres capas planas paralelas y horizontales cuyos medios son homcgeneos e isotr6picos, can iimites 0 interfaces continuas, porque son condiciones en las cuales se cumple la ley de snell y las ecuaciones de zoeppritz. por medio de parametres fisicos de velocidad de ondas p y s , absorcion y densidad, se pueden caracterizar estos medios para representar rocas como caliza, arenisca, shale, etc. (ver tabla i y figu a 5). tabla 1. val ores de los parametros del modelo inical que se obtendran can el analisis de la curva de normalizaci6n i~ i s i i idr~ ~ ~ i ecuencia velocidad velocidad q calidad / a profundidad itologica de ondas de ondas atenuaclon base m el modelo pm/s sm/s (db/m-rad)'---1 i jale 2000 551.8 i 60/0.023 890.95 renisca 2500 982.85 43/0.0025 1357.6 aliza _~_ 3000 1413.9 ~ql_indeterminad~d la velocidad de onda s (vs) se relaciona con la de la onda p (vp) mediante la expresi6n de castagna: vs = 0.862 i *vp-i 172.4, conocida tambien como la linea del mudrock, y la densidad asume el valor de i gr/cc para efectos practices. ei conocimiento de las velocidades de onda p que caracteriza a cada una de las capas 2121..32 2546.58 o figura 2. relacion del modelo de rayo teniendo como base el modelo de condiciones de velocidad con la respuesta sismica en superficie. 68 permite establecer trazados de rayo que representan las trayectorias de un frente de onda plano. si se tiene en cuenta el comportamiento d in am ico (divergencia geornetr ica, absorci6n y reflexi6n) de una ondicula a traves de las trayectorias de rayo en funcion del tiempo offset y profundidad se obtiene un registro sismico sintetico (ver figura 2). resultados obtencioti de la curva de normelizscion que relaciona los dos reflectores sfsmicos finalmente se normalizan los datos de amplitud para una determinada frecuencia, dada una trayectoria de rayo, de tal forma que la amplitud perteneciente a la interface mas profunda se divide entre la amplitud, a la que igualmente pertenece la respuesta de la amplitud en frecuencia, de la interface menos profunda (0 reflector de referencia). asi sucesivamente se hace para cada una de las catorce trayectorias de rayo hasta completar los catorce datos que conforman la curva de normalizacion (ver figuras 3 y 4) como punta de partida 0 dato empirico que sintetiza la operacionalizacion del marco teo rico y el disefio concreto 0 metodologia. es de anotar que en las variaciones de la curva de norrnalizacion se condensan todos los efectos de las condiciones iniciales del modelo como un indicador que es punto de partida para su analisis. anelisls de datos de la curva de normeltzecion a partir de esta curva de normalizacion, como ya se ha mencionado, se efectua el anal isis cualitativo por medio de nuestro modelo iniciai, y cuantitativo de acuerdo con el ajuste de las variables por minimos cuadrados. figura 3. a) modelo de rayo inicial; b) curva inicial de normalizacion (color rojo) dado un modelo de rayo, de un modelo aproximado; valores de las condiciones 0 variables iniciales por ser ajustadas con la curva real de norrnalizacion (color negro); c) y d) coeficientes de reflexion; interfaces superior (color azul) e inferior (color verde). geofislca colombiana. 7. oiciembre de 2003 analisis de la amplitud referenciada con tra yectorias de rayo y su aplicaci6n en ava figura 4. valores ajustados al modelo real cuyas variables son las velocidades de onda-s y la atenuaci6n. este permite entonces determinar variables importantes como las velocidades de ondas-s para las tres interfaces (vn, vl n, vn i), necesarias para el calculo de las constantes elasticas (ver figura 6) que estan determinando el comportamiento a yo, y adem as permite ex traer tambien la variacion de atenuacion con respecto al offset (a2), de la capa que se encuentra entre las dos interfaces objeto de analisis, conclusiones es ventajoso hacer analisis de ayo teniendo en cuenta las frecuencias y trayectorias de rayo ya que cada unidad de dato en la curva de norrnalizacion guarda informacion de manera condensada que se puede utilizar dando un mayor rango de informacion. se cornprobo que se puede utilizar una relacion mas sintetica a partir de definiciones puntuales, como es el caso de combinar las formulas de zopepritz para reflexion y transrnision dada una sola interface, relaciones de snell para trayectorias de rayo, i oilio-s"",dlol=!>!il.ll~ iii!s i -s _io2-902.9344 mi. i _i03=1413.8922 oils pesormedio 2=466.6634 m ,ale cilin .rdiu 2-=o.oo2~6 flb/. ~ ir d. ajost.= 1.111m •• ooq • efici .. te tie ""isson tiel ...,dill =0.45881 . .eticie.'e tie ""isson tiel medii 2 =0.4086 oeticie.te tie hisson *i.e_ 3-o.js723 figura 5. muestra de los valores de las variables, que sirvieron como estimulo al rnodelo, resultado del ajuste por tecnicas de minimos cuadrados, y los coeficientes elasticos resultantes de este ajuste. geoflslca colombip.na, 7, oiciembre de 2003 , ~ veloli~s coei de~~ u t1rj , , figura 6. correlaci6n en profundidad de los valores obtenidos de las caracteristicas medibles 0 variables como condiciones iniciales de cada perfil rocoso, y como resultado externo el calculo de los coeficientes de poisson. definiciones de sommerfiel en las que se presentan aproximaciones validas para coeficientes de reflexion de ondas esfericas en terminos de ondas planas. se puede establecer una relacion de forma indirecta tal que permita calcular variables como las velocidades de onda s que son necesarias a la hora de determinar el comportamiento elastico de las racas. la integracion de varios facto res en un modelo fisico que permita estudiar las relaciones de tales variables se hace ventajoso ya que por metodos deterministicos nos podemos aproximar mas al modelo real. se puede extender el estudio, utilizando directamente las relaciones de coeficientes de reflexion y transmision para ondas esfericas, hasta obtener una mayor aproxirnacion, tarnbien se pueden hacer analisis a traves de la atenuacion inelastica, dado que nos puede ayudar a determinar litologias de las rocas y permitiria un mejor analisis en cuanto a zonas en que se presenta una mayor atenuacion, sirviendo entonces como indicador de variaciones en las condiciones geologicas que potencialmente sefialan esa variaci6n. por ultimo, se establecio una metodologia para corregir las perdidas por transrnision de manera deterministica, mediante una norrnalizacion 0 ecualizacion de tales trazas de acuerdo con un modelo de trazado de rayo . seria provechoso aplicar el presente trabajo utilizando coordenadas t-p, ya que su estructura de analisis encaja perfectamente con las relaciones en este proyecto, puesto que se bas a tarnbien en el analisis de trazado de rayo y utiliza ondas planas para su representacion, 69 bohorquezit al. aunque el trabajo se contempla como experimental en el cual se controlan las condiciones iniciales (problema directo), su aplicacion es un acercamiento mas, como 10 serian todos los trabajos de esta naturaleza, a hechos reales, y en este caso se identificarian anomalias. esto es debido a que son multiples los factores que acompafian un medio geologico real, y la discriminacion de estos factores requeriria una hipotesis mucho mas compleja que tal vez se incluiria dentro de la componente k de la presente relacion inicial de hipotesis, agradecimientos los agradecimientos son para todos los profesores que confonnan el posgrado de geofisica de la universidad nacional de colombia, en especial al grupo de investigacion en geofisica. referencias adriansyah,mcmechan, g., a. 1999, efect of attenuation and scattering on avo measurement: j. geophysics,vol. 63, 6, 2025-2034. ak1,k., and richard,p. g. 1980. quantitative seismology: theory and methods. w. h. freeman and co. ___ .2002. quantitive seismology, 2nd ed, university science books. bortfeld,r. 1961. aproximation to the refletion and transmission coeficients of plane longitudinal and transverse waves. geophys. prosp., 9, 485-503. brekhovshikh,l. 1960, waves in layered media, academic press. castagna,1. p., batzle,m. l., and kant,t. k. 1994. rock physics: the link between rock properties and avo response in castagna, j. p., and backus, m. m., eds., offset dependent reflectivity theory and practice of avo analysis. soc. expl. geophys, 135-171. castagna,1. p., herbert,w.s., and douglasj. foster.1998. framwork for avo gradient and intersept interpretation. soc. expl. geophys, 948-955. 70 cerveny,v., and ravindra,r. 1971. theory of seismic head waves. univ. of toronto press. chacko,s. 1989. porosoty identification using amplitud variation with offset: examples from south sumatra. geophisycs, 54, 942-951. chiburise. f., 1984. analysis of amplitud versus offset to detect gas-oil contat in saudi arabia. 57 ann. internat. mtg., soc. expl. geophys, expanded astracts, 669-670. ___ . 1987. studies of amplitude versus offset to detect gas-oil contacts in the arabia gulf. 57 ann. internat. mtg., soc. expl. geophys, expanded astracts, 614-616. ___ . 1993. avo aplications in saudi arabia, avo current status and the future, texaco inc. ecgo. carcioneet ai, 1998. effects of attenuation and scattering on avo mesurement. geophysics 63, 1652-1658. danielc. houston,milo m. backus.1998. offset-dependent mistie analysis at seismic line intersection. geophysics, vol. 54, no.8 (august), 962-972. duren,r. e., 1992. range --equation weights for avo. geophisics, 57, 1203-1208. foster,d.j., and mosher,c. c. 1992. supresion of multiple reflexion using radon transform. geophysics, 57, 386-395. gassaway,g. s. 1984. effects ofshalow reflector on amplitude versus offset (seismic lithology) analysis. 54, ann. internat. mtg., soc. expl. geophys, expanded astracts, 665-669. greenberg,m.l., and castagna,j. p. 1993. shear-wave velocity estimation in porous rocks: theorical formulation, preliminary verification and aplication. geophys. prosp., 40, 195-209. hilterman,f. 1989. is avo the seismic signature of rock properties: 57 ann. internat. mtg., soc. expl. geophys, expanded astracts, 559. knott,c., g. 1899. refletion and refraction of elastic waves with seisrnological application. phil. mag., 48, 64-97. geofislca colombiana, 7, oiciembre oe 2003 this paper applies for cluster analysis and factor analysis theory to statistically analyze environmental isotope (δ18o,δ2h, 3h, 14c) and water chemistry (k+, na+, ca2+, mg2+, hco3 -, so4 2-, cl-) test data from different water bodies in the coal-mining exploring district. the result shows that groundwater can be clustered into four categories, namely ga, gb, gc and gd classes. deep karst groundwater and spring were grouped into ga class, and the contour map of the second-factor scores shows that karst water and spring of the ga group is in the same area, indicating the same recharging source from the northern mountainous area. deep fissure water was clustered into gc class with the lowest second-factor scores, and cation exchange plays a central role, then did not detect tritium with 14c of lower levels, indicating the late pleistocene rainfall recharging. shallow pore water and surface water were clustered into gb class with the high third factors scores, indicating surface water leakage recharging. the water samples of gd class have the highest three factors score, pointing out that the shallow pore water and surface water were polluted. the results of this study provide a scientific basis for assessing groundwater circulation mechanism in the coal-mining exploring district. este estudio utiliza la teoría del análisis de grupos y del análisis factorial para examinar estadísticamente la información de pruebas al isótopo ambiental (δ18o,δ2h, 3h, 14c) y a la química del agua (k+, na+, ca2+, mg2+, hco3 -, so4 2-, cl-) en diferentes cuerpos de agua en el distrito de exploración carbonífera. el resultado muestra que el agua subterránea puede ser agrupada en cuatro categorías, nombradas clase ga, clase gb, clase gc y clase gd. el agua subterránea del karst profundo y el agua de manantial fueron agrupadas en la clase ga; el mapa topográfico de los marcadores de segundo factor muestra que el agua del karst y el agua de manantial del grupo ga se encuentran en la misma área, lo que indica que tienen la misma fuente de recarga, en la región montañosa al norte del distrito. el agua de las fisuras profundas fue agrupada en la clase gc con los marcadores más bajos de segundo factor y donde el intercambio de cationes es determinante; no se detectó tritio con los bajos niveles de 14c, lo que indica una recarga por lluvia en el pleistoceno tardío. el agua poco profunda y el agua superficial fueron agrupadas en la clase gb, con los mayores marcadores de tercer factor, lo que indica una recarga por vertido superficial. las muestras de agua de la clase gd tienen los mayores marcadores de los tres factores, lo que señala que las aguas poco profundas y las superficiales están contaminadas. los resultados de este estudio proveen una base científica para la evaluación del mecanismo de circulación del agua subterránea en el distrito de exploración carbonífera. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march 2016): g1 g8 abstract resumen keywords: cluster analysis, factor analysis, isotope geochemistry, groundwater circulation, coal mining district. palabras clave: análisis de grupos, análisis factorial, geoquímica isotópica, circulación de agua subterránea, distrito de exploración carbonífera. assessment by multivariate analysis of groundwater–surface water interactions in the coal-mining exploring district, china record manuscript received: 26/03/2013 accepted for publication: 13/11/2015 how to cite item huang, p. & han, s. (2016). assessment by multivariate analysis of groundwatersurface water interactions in the coal-mining exploring district, china. earth sciences research journal, 20(1), g1-g8. doi: http://dx.doi.org/10.15446/esrj.v20n1.37601 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.37601 h y d r o g e o l o g y pinghua huang1*, sumin han2 1. institute of resources and environment, henan polytechnic university, jiaozuo, 454000, china. * corresponding author. e-mail: hph2001@hpu.edu.cn.tel: +86-15893039167 2. school of electrical engineering and automation, henan polytechnic university, jiaozuo, 454000, china. e-mail: hansumin@hpu.edu.cn evaluación de las interacciones entre agua superficial y agua subterránea a través del análisis multivariante en el distrito de exploración carbonífera en china g2 pinghua huang and sumin han 1 introduction over the last few decades, as china’s exploitation of underground coal has gone deeper and deeper, mine displacement also enhances gradually, the groundwater table assumes coming down in the successive years. for example, groundwater was already dredged trunk in some shallow aquifers in the coal-mining district of jiaozuo, one of the famous karst mining areas in china, the result is of which amount of salinity was accumulated. on the other hand, besides part of pit groundwater was exploited by the factories, the majority of which were untreated and directly discharged to rivers. the paper beliefs that contaminated river supplies the shallow aquifer in the leakage area, and severely affects irrigation and domestic water. to provide a scientific basis for groundwater security assessment in coalmining districts, it has become necessary to probe into the hydraulic connections between groundwater-surface water and the sources of groundwater recharge. several studies using hydrochemistry and stable isotopes of water have not only already been undertaken to characterize recharge processes in similar karst-dominated geological settings (marifia et al., 2003; long and putnam, 2004; barbieri et al., 2005; aravena and suzuki, 1990; harrington et al., 2002), but they have also determined the main mechanisms of the water cycle, through study of the dynamic processes of different water bodies, such as their formation, movement, mixture and so on, in coal-mining districts (duan et al., 1994; gui et al., 2005). in this study, multivariate statistical methods have been used to solve the hydrochemical classification and evolutionary laws of the groundwater in the coal-mining district of jiaozuo. multivariate analyses have been established as efficient tools in the analysis of hydrochemical data. examples of the successful use of multivariate statistical methods in hydrochemical studies are contained in steinhorst and williams (1985), farnham et al. (2003), farnham et al. (2000) and steinhorst et al (2001). multivariate statistical methods do not indicate cause-and-effect relationships but they do provide information from which such relationships can be inferred. the tracer technique of isotope and water chemistry, combined with multivariate statistical analysis theory, not only can describe the impact of different water bodies, but also can assess the uncertainty of the mixed model and the relationship of hydrogeochemical data in time and space during hydrological processes. the method plays a key role in the assessment of surface water-groundwater interaction (farnham et al., 2000; farnham et al., 2003; steinhorst et al., 2001; steinhorst and williams, 1985; aruga et al., 1995; ashley and lloyd, 1978; bengraı et al., 2003; helena et al., 2000; hernandez et al., 1991; join et al., 1997; liedholz and schafmeister, 1998; meng and maynard, 2001; seyhan et al., 1985; wang et al., 2001). the purpose of this study is that is reasonably assessed under the environment of coal mining being serious, and the groundwater circulation mechanisms of the main aquifers are reasonable judgment in the coal-mining district of jiaozuo. 2 hydrogeological setting the jiaozuo coal-mining district is situated on the southeast edge of the mount taihang uplift in china. the district is part of the tectonic system of new china and qinling mountains and the east wing of the frontal arc of the epsilontype structural system in northwest henan province (fig. 1). due to the control of the mount taihang uplift, this place has mainly fault structure, folded structures being less developed, and the overall tendency is a monoclinic structure from south to east. the fault structures are mainly high-angle normal faults, which develop into three groups with the trend of e–w, nw–se and ne–sw. the water samples correspond to location in the study area. the jiaozuo coal-mining district is situated on the southeast edge of the mount taihang uplift in china. the district is part of the tectonic system of new china and qinling mountains and the east wing of the frontal arc of the epsilon-type structural system in northwest henan province. jiaozuo coal-mining district is situated on the southeast edge of the mount taihang uplift in china. the district is part of the tectonic system of new china and qinling mountains and the east wing of the frontal arc of the epsilon-type structural system in northwest henan province. figure 1. hydrogeological map of the jiaozuo coal mining district, china; the water samples correspond to location in the study area. the according to the lithology, thickness, water features and burial conditions of the stratum, the main groundwater aquifers from top to bottom, can be divided into three categories. firstly is the shallow pore aquifer, made up of the quaternary sandstone, clay, the thickness of which is 10-15m with permeability (k) 0.1 0.3 m/d. secondly is the deep fissure aquifer, consisting of the sandstone, siltstone, the thickness of which is 30 40m with the permeability of the aquifer is about 1 3 m/d. thirdly is deep karst groundwater aquifer, made up of the carboniferous limestone with thickness 8 12m and the ordovician limestone with thickness about 400m, the permeability of which is about 1 30 m/d. the aquifers have complex boundary position and conditions with heterogeneous spatial distribution, anisotropic characteristics and irregular. pumping (injecting) water test can form complete and unified funnel. the karst groundwater with uniform flow field and similar water level dynamic. karst water flow from north to south and from west to east in the mining area. the water level is 240 2000 m in the wet season, and water level is 130 -160m in the dry season. the amplitude of annual water level ranged from 10 to 12m.the main aquitard among the aquifers are composed of alluvial mudstone. the sampling points in the coal-mining district were selected where groundwater leakage has been detected from the shallow pore aquifer, the deep fissure aquifer, and deep karst groundwater aquifer. the study area has a semi-arid and semi-continental monsoon climate, being located in the warm temperate zone, with average annual rainfall of 600 mm, total average annual evaporation 2048 mm (data from 1952 to 2001). north of the coal-mining district is the southern foothill of mount taihang, which is a tectonic denudation landform of low mountains with monoclinic structure. three rivers originate from the southern foothill of mount taihang, the source of which is spring, extending in northward and westward directions and basically vertical to the anticline of mount taihang (fig 1). 3 materials and methods 3.1. monitored parameters and analytical methods 71 groups of water samples were collected from the coal-mining district in 2008, 2010 and 2011, including surface water samples, spring samples of mount taihang, deep karst groundwater, deep fissure water and shallow pore water. water chemical and isotopic of all water samples were analyzed for tds, ph, ca2+, mg2+, na+, k+, hco3-, so42-, cl-, f-, δ18o, δ2h, 3h, 14c (table 1). g3assessment by multivariate analysis of groundwater–surface water interactions in the coal-mining exploring district, china 3 3.2 cluster analysis cluster analysis theory studies how the object has been a comprehensive classification in accordance with many aspects characteristics, which can effectively solve the classification about the multi-factor study, the multiindex, which has extensive application in earth science (steinhorst and williams, 1985; schot and wal, 1992; ribeiro and macedo, 1995). in practical applications, hierarchical clustering (hca) is the most commonly used (steinhorst and hodge, 2001), by which all samples were clustered in this paper. in the clustering process, all parameters of the samples were normalized by z-score method and sample spacing was defined by the euclidean distance. the euclidean distance usually gives the similarity between two samples and a distance can be represented by the difference between analytical values from the samples (otto, 1998). samples were classified according to square sum of deviations (wards method). the ward’s method uses an analysis of variance approach to evaluate the distances between clusters in an attempt to minimize the sum of squares (ss) of any two clusters that can be formed at each step. the results are displayed with clustering pedigree chart (mckenna, 2003). table 1. chemical and isotopic composition of all water samples in the study area g4 pinghua huang and sumin han 3.3 factor analysis factor analysis method is a multivariate statistical analysis technique, which is used to concentrate the data , study dependencies between the variables, and explore the basic structure of the observational data which are expressed by several factors that can reflect main message which most of the observed variables contain, and that explain the interdependent relationship between these observed variables (brown, 1998).in this paper, factor analysis method is applied by principal component analysis technique, which is the most commonly used (welch and lico, 1998; ashley and lloyd, 1978; tao, 1998). when the eigen value is usually greater than 1 and the cumulative contribution rate is as large as possible, the common factors can be determined. actual meaning of common factors is determined according to the loading of the observations which represent the correlations coefficient between analysis variables and the common factors. 4 results 4.1 hierarchical clustering results pedigree fig 2 shows that the water samples were clustered into four categories (ga, gb, gc and gd groups), they can be clearly distinguished. water chemistry types are classified by the shug kalev method (wang, 1995). the euclidean distance in the ga group which is made of spring and deep karst water with ca-mg-hco3 type is the shortest. the gb group is mainly composed of surface water and shallow pore water. the so4 2concentrations of water samples up to average 114.7 mg / l in the gb group with ca-mghco3-so4 type are more than the other groups. the gc group with na-hco3 type which has high na + concentration, low ca2 + concentration is mainly composed of deep fissure water. the tds, cl-and so4 2contents of the gd group including three water samples (16 #, 18 # and 37 #) are significantly high. water chemical type is na-cl-so4 in the gd group. 16# and 18# water samples from shallow pore water are polluted by nearby factories, yet # 37 water samples from surface water is serious by man-made pollution. figure 2. dendrogam for the groundwater samples, showing the division into four clusters and the median concentration stiff diagram of each cluster g5assessment by multivariate analysis of groundwater–surface water interactions in the coal-mining exploring district, china 5 the euclidean distance in the ga group, which is made of spring and deep karst water with ca-mg-hco3 type is the shortest. the gb group is mainly composed of surface water and shallow pore water. the so4 2concentrations of water samples up to average 114.7 mg / l in the gb group with ca-mghco3-so4 type. the gc group with na-hco3 type is mainly composed of deep fissure water. the tds, cl-and so4 2contents of the gd group including three water samples (16 #, 18 # and 37 #) are significantly high. 4.2 statistical characteristics standard deviation such as tds, hco3-, na+, clof water samples is greater than 70, reflecting the obvious differences of water quality characteristics (table 1). box and whisker plots show the median of water quality parameters in the four clusters (ga gb gc gd) (fig 3).it shows that ions concentrations of the gd group are significantly higher than the others, reflecting the pollution characteristics, yet the na+, ca2+, clconcentrations of the gc group reflects the characteristics of the cation exchange. figure 3. box and whisker plots show the median of water quality parameters in the four clusters (ga gb gc gd). the ions concentrations of the gd group are significantly higher than the others, yet the na+, ca2+, clconcentrations of the gc group reflects the characteristics of the cation exchange. 4.3 factor analysis results r factor analysis was executed to all parameters of the water samples. use principal component analysis to estimate the factor loadings matrix to obtain factors eigen values of more than 1. the cumulative contribution rate of the first four factors which can explain the most original data information is 84.04% (table 2). table 2. principal component loadings and explained variance for the fore components extraction method: principal component analysis. rotation method: varimax with kaiser normalization. bold values: loadings>0.6. so the extracted four factors (factor 1, factor 2, factor 3 and factor 4) are more scientific and reasonable. table 2 shows that hco3 -, na+ load of the first factor is more than 0.6 , reflecting groundwater alkalinity distribution, which is defined as the “alkalinity” factor (fig 4). the δ2h and δ18o load of the second factor is more than 0.9, indicating the information of groundwater recharging source. the third factor has high positive load of ca2+ and mg2+, reflecting groundwater hardness distribution, which is defined as “hardness” factor. the fourth factor has only high correlation coefficient with ph which explains the distribution characteristics of groundwater acidic and alkaline. the actual means of the above four factors are very clear; therefore, all factor scores can be used to analyze groundwater space evolution. figure 4. the loadings for the first and third factors with varimax normalized rotation. g6 pinghua huang and sumin han the factor 1 is defined as the “alkalinity” factor. the factor 3 has high positive load of ca2+ and mg2+, reflecting groundwater hardness distribution which is defined as “hardness” factor. 5 discussion 5.1 groundwater level and water quality changing in the coal-mining exploring district the variation range of local annual rainfall is stable (450~660mm), yet coal mine displacement has increased year by year since 1966. shallow porous aquifer is the state of dewatering or dewatering in half. the buried depths of groundwater are more than 30m and deep karst water level also decrease in the mining district, indicating that the mine drainage leads to decreasing continuously groundwater level (fig 5 ). 0 20 40 60 80 100 120 1 31 61 91 121 151 181 211 241 271 months 1964-1986 l e v e l （ m ） 0 100 200 300 400 500 600 700 r a i n f a l l （ m m ） a n d d i s p l a c e m e n t （ m 3 / m i n ） rainfall karst level displacement figure 5. the relationship among the mine displacement, rainfall and deep karst water level in the coal-mining district. the variation range of local annual rainfall is stable (450 ~ 660mm), yet coal mine displacement has increased year by year since 1966. shallow porous aquifer is the state of dewatering or dewatering in half. the buried depths of groundwater are more than 30m and deep karst water level also decrease in the mining district. 0 100 200 300 400 500 600 700 1 2 3 4 5 6 7 8 years,1995-2002 h a r d n e s s ( m g / l ) 0 500 1000 1500 2000 2500 c l ( m g / l ) clconcents of deep karst groundwater shallow pore water hardness deep karst groundwater hardness clconcents of shallow pore water figure 6. the clcontents and hardness of karst groundwater and shallow pore groundwater changing characteristic in the coal mining district. clconcentrations and total hardness of shallow pore water and deep karst water had increased year by year. from 1995 to 2002 years, the cl contents of shallow pore water had increased from 13mg / l to 2135mg / l, and the clcontents of deep karst water had increased from 12mg / l to 112 mg / l, yet the total hardness had increased from 303mg / l to 403 mg / l. clconcentrations and total hardness of shallow pore water and deep karst water had increased year by year. from 1995 to 2002 years, the cl contents of shallow pore water had increased from 13mg / l to 2135mg / l, and the clcontents of deep karst water had increased from 12mg / l to 112 mg / l, yet the total hardness had increased from 303mg / l to 403 mg / l (fig 6). the above data shows that the shallow and deep groundwater quality has become worse in the mining district, especially the clcontents of shallow pore water have increased significantly. the coal mining has brought about that the groundwater balance has been severely broken and the condition of groundwater recharging, runoff and excretion has changed. 5.2 groundwater circulation mechanism in the coal-mining exploring district from mount taihang to coal mining district, the first factor scores, the third factor scores and clcontents of karst water which belongs to the ga group increases gradually (fig 7), indicating salt accumulating gradually and groundwater lateral recharging process. groundwater lies the starting end of runoff evolution, which is often considered as recharging source (lloyd and heathcoat, 1985). the contour map of the second factor scores shows that karst water and spring of the ga group is at the same area, indicating that karst water mainly accept recharging from spring (fig 8 (b)).the euclidean distance in the ga group clustering is the shortest, indicating that deep karst water and spring experienced the roughly same hydrogeochemical environment. fig 9 (a) shows that the correlation coefficients of about cl, na, f and between mg to hco3 are more than 0.5, indicating that the rock salt and dolomite dissolving are the dominant groundwater evolution process in the ga group. the durov diagram shows that hco3 -and ca2+ are dominations in the ga group, further confirming the above conclusions (fig 10). figure 7. the loadings of the first and third factors for the four clusters (ga, gb, gc, gd). from mount taihang to coal mining district, the first factor scores, the third factor scores of karst water which belongs to the a group increases gradually (fig 7). the second factor scores variation ranges of shallow pore water in the gb group are from -0.3 to -0.9, and the dominant anion and cation gradually become obvious along the x-axis direction in figure 10, indicating the groundwater mixing process. shallow pore water has the high second factor scores in the gb group, indicating that the δ18o and δ2h values are more positive, which reflect surface water recharging characteristic. there are excellent correlations among k, na, ca,18o (fig 9 (b)), indicating that the runoff process of shallow pore water in the gb group, because the fes oxidized of carboniferous-permian coal seams bring about large amounts of sulfuric acid which take part in dissoluting carbonate rock. g7assessment by multivariate analysis of groundwater–surface water interactions in the coal-mining exploring district, china 7 figure 8. regional distribution of factors scores of water samples for (a), (b) with the geology shown in background. plot (a) shows the first factor scores contour map. plot (b) shows the second factor scores contour map. figure 9. correlation between the ionic species in the ga, gb, gc, gd clusters. the correlation coefficients of about cl, na, f and between mg to hco3 are more than 0.5, indicating that the rock salt and dolomite dissolving are the dominant groundwater evolution process in the ga group. the first and second factor scores of deep fissure water in the gc group are the lowest in the coal mining district (fig 8(a) and fig 8(b)). the tritium was not detected and 14c contents were low in all deep fissure water samples, indicating the late pleistocene rainfall recharging characteristic. fig 9 (c) shows that there are good correlations about hco3, na, ca, mg, δ18o and no correlations between ca, na in the gc group, indicating that the ca2+, na+ exchanging interaction play a key role, leading the balance equation of carbonate dissolving to increasing hco3 direction pan. the correlation coefficients of about mg2+, hco3 -, so4 2-, fare more than 0.5, indicating that sulfuric acid may take part in the dissolution action in the gd group (fig 9 (d)). figure 10. the durov diagram shows that hco3-and ca2+ are dominant ions in the ga group the dominant anion and cation gradually become obvious along the x-axis direction in the gb group, indicating the groundwater mixing process. 5.3 conceptual model in the coal-mining exploring district, runoff recharging path of groundwater can be described by conceptual model (fig 11). shallow pore water to be contaminated in a certain range accepts surface water leakage recharging in the coal mining district. the deep karst water is mainly supplied from mount taihang. deep fissure water accepted rainfall recharging in the late pleistocene. figure 11. conceptual model of groundwater flow path in the study area. the shallow pore water to be contaminated in a certain range accepts surface water leakage recharging in the coal mining district. the deep karst water in coal mining district mainly originates from the recharge of groundwater from northern mountainous area. the deep fissure water a mainly originates from the recharge of rainfall in the late pleistocene. g8 pinghua huang and sumin han 6 conclusions coal mining has destroyed groundwater equilibrium states, so groundwater quality became worse and water inrush accident occurred more frequently. based on mathematical statistical analysis theory, this paper studies groundwater isotope hydrogeochemical evolution in the coalmining exploring district to determine shallow pore water, deep fissure water and deep karst water recharging process and evolution mechanism in order to assess water quality and water inrush prevention providing scientific basis in the coal mining district. 7 acknowledgements the work was financially supported by science and technology key research project of 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(1998). factors controlling as and u in shallow groundwater, southern carson desert, nevada. applied geochemistry, 13, 521-539. 77 the present research work examines three aspects of great importance for the future supply of oil and gas in colombia. 1) the geographic and geological distribution of hydrocarbon resources for use in estimations of the contribution of different field sizes to the total supply of oil and gas in colombia. 2) the current state of the hydrocarbon reserves in colombia and factors that influence their increase or decrease; statistical tools are used to calculate the volume of hydrocarbons that may be found in the future. 3) the observed decline of hydrocarbon production in colombia, which is used to estimate the amount of resources that may be extracted in the future once peak production has been reached. based on these three aspects, we constructed different scenarios to forecast the potential production of oil and gas in colombia. in addition, we calculated the amount of hydrocarbon reserves in these scenarios. el presente trabajo resume tres aspectos considerados de gran importancia para el futuro suministro de petróleo y gas en colombia. estos son: 1) la distribución geográfica y geológica de los recursos hidrocarburiferos como herramienta para conocer la contribución de los diferentes tamaños de campo a la producción total de hidrocarburos en colombia; 2) el estado actual de las reservas de hidrocarburos en colombia y los factores que inciden en el incremento o disminución de las reservas, que son dos asuntos temas que inciden en el futuro suministro de estos recursos. al respecto, por medio de herramientas estadísticas se calcula el volumen de hidrocarburo que puede ser encontrado en el futuro. 3) las tasas de declinación en la producción de hidrocarburos observados observadas en colombia, permiten modelar los recursos que pueden ser recuperados en el futuro una vez alcanzados los picos de producción. bajo esta hipótesis, se construyen diferentes escenarios que resumen pronostican la posible producción de petróleo y gas en colombia para los próximos años, además se calculan las reservas de hidrocarburos para los mismos escenarios. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 77 83 abstract resumen key words: ultimately recoverable resources, oil production, gas production, production peak, oil reserves, gas reserves, decline rates. palabras clave: rfr, producción de petróleo, producción de gas, pico de producción, reservas de petróleo, reservas de gas, tasas de declinación. hydrocarbon production scenarios in colombia. review of field sizes, hydrocarbon reserves and expectations of conventional and unconventional resources. beltrán rivas julian daniel y vargas jiménez carlos alberto universidad nacional de colombia mail: jdbeltranr@unal.edu.co cvargas@ciencias.unal.edu.co record manuscript received: 30/09/2013 accepted for publication: 31/05/2014 hydrocarbon potential 78 beltrán rivas julian daniel & vargas jiménez carlos alberto introduction hydrocarbons currently represent the primary source of energy in the world and are expected to remain the primary energy source despite discoveries of alternative energy sources. therefore, the supply of hydrocarbons is a topic of great political, economic and academic interest. hydrocarbon reserves are determined by a variety of geological, technological, economic and political factors that create uncertainty for the future of hydrocarbon supply and demand. the goal of this work is to present the current state of three aspects that will influence the supply of hydrocarbons in colombia in the coming years. 1 the size distribution of hydrocarbon fields, which is important for determining the distribution of hydrocarbon resources in colombia and studying the relative importance of large and small fields for the production of oil and gas in the country. the state of hydrocarbon reserves, which is a fundamental topic for the prediction of oil and gas supplies. an estimation of the size of hydrocarbon reserves in colombia must consider all factors that may influence increases or decreases of these reserves. 2 an analysis of the decline in hydrocarbon production. the methodology developed in this article allowed us to estimate oil and gas production for specific fields or regions over time. 2. field size distribution the methods for evaluating recoverable resources and hydrocarbon supplies develop possible future scenarios that are usually based on the size distribution of oil and gas fields for a given region. the field sizes to which we refer in this article correspond to the ultimately recoverable resources (urr) of each field. urr = accumulated production + recoverable reserves. the calculation of oil and gas urr is conducted by adding the accumulated production and recoverable reserves. most urr in a region are concentrated in a small number of large fields and are typically discovered in the early stages of exploration. subsequent discoveries usually correspond to smaller fields that are more difficult to find. table 1 shows the top 5 largest oil fields in the world up to the year 2008 and their year of discovery, which indicates their advanced productive age. only 1% of the oil fields represent 75% of the accumulated discovered resources in the world. table 1. top five largest oil fields in the world up to 2008 and their year of discovery. note their advanced productive age (ivanhoe and luckie, 1993). field year of discovery ghawar 1948 cantarell 1977 safaniyah 1951 rumaila 1953 greater burgan 1938 2.1 the importance of large fields in colombia the size distribution of oil and gas fields in colombia has been established based on the proposal by ivanhoe and luckie (1993) in which fields are grouped into ten categories according to their estimated urr (tables 2 and 3). table 2. size distribution of oil fields in colombia by urr. category estimated urr number in colombia (mmbbl) number in colombia 0 super giant 5.000 50.000 0 giant 500 5.000 5 very large 100 500 20 large 50 100 11 medium 25 -50 23 small 10 25 52 very small 1 – 10 131 minuscule 0.1 – 1 103 insignificant < 0.1 101 as shown in table 2, large fields dominate the supply of hydrocarbons in colombia2 (acp, 2012), figs. 4 and 5, which is consistent with global trends. the five fields with a urr larger than or equal to 500 million barrels (mmbbl) are only 1% of the total fields, but they represent 35% of all accumulated discoveries. the fields with urr between 100 mmbbl and 500 mmbbl correspond to 5% of all fields and represent 70% of the accumulated discoveries. table 3. size distribution of gas fields in colombia by urr (acp, 2012). category estimated urr (mmcf) number in colombia very large 1,000,000 (tera)-10,000,000 4 large 100,000 – 1,000,000 15 medium 10,000 – 100,000 50 small 1,000 – 10,000 74 very small 100 – 1,000 63 minuscule 10 – 100 54 insignificant < 10 42 for gas reserves, there are four fields classified as “very large” (table 3) that exceed a urr of one tera-cubic feet (tcf), which represent 1% of all fields and are responsible for 75% of the accumulated resources. during the development of this work, we primarily relied on demonstrated oil and gas reserves with the exception of a few fields for which the number of probable reserves was used. figure 1. estimated contribution of the giant and very large fields to the total colombian oil production for the last two decades. the definitions of giant and very large fields are presented in table 2. the giant and very large oil fields in colombia contribute on average 60% of the daily oil production in the country (fig. 1). regarding gas production, 88% of the gas delivered to the pipeline originates in large fields (fig. 5), which indicates the strong production control exerted by large fields in colombia. 0   100.000   200.000   300.000   400.000   19 90   19 95   20 00   20 05   20 10   m bb l   daily  oil  produc/on  (1990  -­‐  2012)     producción  total   super  grande   gigantes   79hydrocarbon production scenarios in colombia. review of field sizes, hydrocarbon reserves and expectations of conventional and unconventional resources. figure 2. estimated contribution from very large and large gas fields to the total colombian gas production for the last 25 years. the definition of very large and large fields is presented in table 3. 3. oil and gas reserves in colombia the increase in oil and gas reserves is a fundamental topic for the prediction of future hydrocarbon supplies. therefore, a review of the following concepts is necessary. reserves are the amounts of oil and/or gas whose extraction is considered to be technologically accessible and economically feasible under given conditions. the estimation of reserves is susceptible to change because of accumulated production, revisions to the geological scenario, improvements of extraction techniques, the introduction of new technologies, or the modification of social, political and economic conditions in a given region. considering these variables, the reserves are presented within different confidence levels, which are explained below (salager, 2005). • demonstrated reserves (1p) are the amounts of oil and/or gas that have been determined (by the analysis of geological, geophysical and engineering data) to be commercially recoverable from a given date, from known deposits and under well-defined economic conditions, operational methods and government regulations. in probabilistic terms, there must be a probability of at least 90% that the real extracted quantities will be equal to or superior than the estimates. • probable reserves (2p) are additional reserves whose extraction has been determined (by the analysis of geological and engineering data) to be less likely than that of 1p but more likely than that of possible reserves. in probabilistic terms, there must be at least a 50% probability that the real extracted quantities will be equal to or superior than the estimates. • possible reserves (3p) are reserves that are less likely to be extracted than 2p. there must be a probability of at least 10% that the real extracted quantities will be equal to or superior than the estimates. (anh, 2008). 3.1 sources of reserve increases or decreases the factors influencing the increase or decrease of hydrocarbon reserves can be grouped into three categories. geological factors: these factors represent an increase or decrease in the estimations of oil originally in place (ooip) and gas originally in place (goip) because of increased knowledge of the geology of a field or region. new geological knowledge may lead to the discovery of new reservoirs or extensions of known reservoirs and directly affects the volume of ooip. technological factors: these factors represent activities that increase the rate of extraction of ooip and goip, increasing the feasibility and economic viability of extractions from certain fields. two techniques that stand out are related to increases the effective porosity and the injection of pressurized fluids for the mobilization of hydrocarbons, which concentrate hydrocarbons in a given zone and facilitate their extraction (salager, 2005). definition factors: this term encompasses all legal definitions, economic and political factors that are independent of the hydrocarbons present at the site and extraction rates. these factors include changes in the classification schemes for the reserves, calculation of the reserves by different institutions, calculation methods and oil prices. figure 3. cumulative oil discoveries from 1921 to 2012, which indicate an increase during the 1980s and 1990s (vargas, 2012). ecopetrol s.a. (2005) the increase in oil cumulative discoveries (mmbbl) between the years 1980 and 2000 corresponds to the discovery of fields such as caño limón (1986), cusiana (1988), cupiagua (1993) and rubiales (1988) (fig. 6). the fields of cusiana, cupiagua, chuchupa and ballenas (1973) represent significant gas reserves in colombia, which also increased the cumulative discoveries in the country. 3.2 contribution of colombian oil and gas reserves to the world reserves the hydrocarbon industry in colombia represents close to 60% of all export revenue thus, it is of great importance for the country’s economy (dane, 2012). the columbian hydrocarbon reserves relative to the world’s reserves are shown in table 4. table 4. comparison between the world oil reserves (anh, 2009; bp, 2012), and colombian oil reserves (anh, 2008; acp, 2012) for the last three decades. decade world reserves (mmbbl) colombian reserves (mmbbl) 1980 683.4 0.554 1990 1027.5 1.991 2000 1257.9 1.972 2010 1622.0 2.058 2012 2000.0 2.377 in the last three decades, colombia’s contribution to the world’s oil reserves has been 0.17% on average. the maximum contribution occurred in the 1990s when colombia’s reserves were 0.25% of the total world oil reserves (table 4). the contribution by colombian gas reserves was lower, and over the last 25 years, colombia’s gas reserves have amounted to 0.11% on average of the world’s reserves. this percentage does not vary significantly for the considered time interval (table 5). table 5. comparison between the world’s gas reserves ( ) british petroleum (2012)and colombian gas reserves acp (2012) for the last 25 years. decade world reserves (tcf) colombian reserves (tcf) 1987 3687 3.426 1990 4438 3.577 2000 5447 6.235 2010 6926 5.405 2012 7815 6.460 0   200   400   600   800   1.000   1.200   1.400   19 87   19 92   19 97   20 02   20 07   20 12   m m pc   daily  gas  produc0on  (1987  -­‐  2012  )   total  produc5on   large  +  very  large   very  large   0   2.000   4.000   6.000   8.000   10.000   1. 90 0   1. 92 0   1. 94 0   1. 96 0   1. 98 0   2. 00 0   2. 02 0   m m bb l   año   cumula,ve  oil  discoveries     cumula/ve  discoveries  (mmbbl)   80 beltrán rivas julian daniel & vargas jiménez carlos alberto 3.3 estimation and forecast of the oil and gas reserves in colombia in this section, we discuss a fractal model used to estimate the oil and gas reserves that could potentially be found in colombia. with this model, we are able to forecast the hydrocarbon reserves that may be extracted based on data for the current reserves and production. in addition, this model allows us to project the amount of hydrocarbons that may be found under a given set of technological, political, social and environmental conditions (vargas, 2012) the fractal method assumes that the reserves of oil and gas fields are described by an ideal distribution, given by a parabolic or linear function at a log-log scale. this function is determined from a trend analysis that considers the field sizes and ranges to group a number of major fields in a given size [mmbbl and billions of cubic feet (bcf)]. under these conditions, the function that best fits the observed data for the size and field reserves suggests the total amount of resources that may be discovered. based on the urr distribution, it is possible to estimate the “amount of hydrocarbon to be discovered,” which corresponds to the total amount of resources still underground, defined by the fitting function minus the accumulated resources found to date, or the urr (fig. 4, vargas, 2012) thus, the reserves to be discovered in a region are estimated by the difference between the curve of the observed data and their theoretical fit (parabolic or linear). we used annual data on production and reserves from oil and gas fields to forecast the discovery of reserves using the fractal method. these data correspond mainly to fields located near the basins of the upper magdalena valley (umv) and middle magdalena valley (mmv), the eastern llanos and the caguán-putumayo, catatumbo and guajira offshore basins. this information was provided by companies operating in colombia as well as several institutions, including the colombian association of petroleum engineering (acipet), colombian petroleum association (acp), ecopetrol, and ministry of mines and energy. fields located at other basins from which lower densities of data are available were also included in this study. figure 4. fractal representation for undiscovered resources in a region. the blue line represents the distribution of fields with a given urr at a given time. the green line represents the data fit. the gray region is the difference between these two curves and represents the resources to be discovered (vargas, 2012). 1. 4 methodology for reserve calculation from the fractal method 1 the data were organized into different ranges, and each range corresponds to the number of fields with urr larger than or equal to a given number of interest. 2 the ranges are plotted on the horizontal axis, and their corresponding urr are plotted on the vertical axis (in mmbbl for oil and bcf for gas) at a loglog scale, thus providing the size distribution of the oil and gas fields in colombia. 3 the function that best fit the data was estimated. figure 5. fractal analysis of the undiscovered oil resources in colombia. the blue line represents the distribution of oil fields in colombia in 2012; the green line is the data fit; and the difference between the two curves corresponds to the undiscovered reserves. according to the production and reserve data from the basins mentioned above and additional data from other basins, it is estimated that under the current technological, political, social and environmental conditions, the total volume of oil to be discovered is 21.486 mmbbl (fig. 5). it is important to clarify that these values can be interpreted as new discoveries or reevaluations of currently known fields. the calculation of the gas volume to be discovered was conducted following the same methodology. according to the production and reserve data from the basins mentioned above and data from other basins, it is estimated that under the current technological, political, social and environmental conditions, the total volume of gas to be discovered is approximately 49,834 bcf (fig. 6). figure 6. the blue line represents the observed field distribution; the red line represents a parabolic fit to this data; and the green line represents a linear fit. 4. decline rates the production behavior of the fields over time is the determining factor for the trends of hydrocarbon supply. the hydrocarbon supply can be forecast from the decline rates observed in production. in this section, we examine the decline in hydrocarbon production, and we use this information as a tool for the prediction of future production behavior in colombia. 4.1 analysis of the decline in the hydrocarbon production decline rate: by “decline”, we are referring to the decrease in the production capabilities of a field. the decline rate represents the percentile decrease in production at a given time with respect to an earlier time period (fig. 8). for colombia, a decline rate of 9% has been proposed for oil, whereas the corresponding number for gas is 15% (anh, 2009) the estimated productive life of a field and the expected production during that time are two questions that can be answered in terms of volumetric calculations. however, the necessary information for an accurate prediction may or may not be available for a given field. therefore, generating empirical models for the extrapolation of variables, such as the production levels of a 1   10   100   1000   10000   100000   1   10   100   1000   rf r   (m m bb l)   rango   observed  data   data  fit   1   10   100   1000   10000   100000   1   10   100   1000   fr f   (b pc )   rango   observed  data   parabolic  fit   linear  fit   81hydrocarbon production scenarios in colombia. review of field sizes, hydrocarbon reserves and expectations of conventional and unconventional resources. given resource in time, can provide answers to these questions. one of the simplest and most readily available characteristics of a well is its production rate. forecasts of production can be extrapolated from a plot of known production vs. time by means of an empirical equation. from this extrapolation, it is possible to predict the hydrocarbon production at a given time in the future. the intersection of the extrapolated curve and economic limit indicates the remaining productive life of a field or region; in addition, it provides information on the volume of oil that may be extracted in the future. in other words, an equation can be generated that predicts the future hydrocarbon production trends of a region from known production data under the statistical principles of extrapolation. however, this empirical method may underestimate the values predicted by volumetric methods. the hydrocarbon production of a field may vary according to the geological characteristics, region and technology employed for its exploitation. nonetheless, the production cycle of a field or region can be described as a function of time, which is described below. when extraction begins, there is a rapid increase in production until peak production is reached. the peak production is then sustained for a given time period referred to as the plateau. the duration of this plateau is proportional to the field size and may be extended or reduced by the incorporation of new extraction technologies. at a certain point, the period of peak production comes to an end, and production rates begin to decline (fig. 7; höök, 2009). initially, the decline in production is very steep, but it gradually slows as time passes. figure 7. stylised production cycle of an oil field empirical equations that model hydrocarbon production within a field or region after their production peak show that production decreases geometrically at a constant rate (fig. 8). this statistical model assumes that the production rate at any given time is a fraction of its value at a previous instant. therefore, the production rate forms a geometric progression at regular intervals ( arps, 1944). a geometric progression includes a constant ratio between successive terms. (swokowsky, 1989) this also explains why the decrease in production during a given time interval is given by a fixed fraction or percentage of its value at the time immediately before. this model is used for the estimation of oil and gas production in colombia for the coming years. figure 8. production decline for different decline rates. at first, a large decline in production is observed, which tends to decrease with time. 4.2 empirical model for hydrocarbon production decline the analysis of decline curves is a method of predicting the future hydrocarbon production of a well, field or region based on production records. (arps, 1944) introduced empirical curves based on the following principal equation: , equation 1 where q(t) is the production of oil or gas at time t during the decline stage. the production peak is given by q(t0), where t0 is the year corresponding to peak production. the decline rate is given by λ. 4.3 model for the estimation of future oil and gas production in colombia the model described above is applicable for the stage after the production peak. therefore, the periods of peak oil and gas production in colombia must be established. the unit for mining and energetic planning (upme) has proposed three scenarios for hydrocarbon supply in colombia based on five sources for the projected incorporation of reserves and the development of production profiles for the period 2012 to 2030 (upme, 2012). for the construction of these scenarios, the upme used variables specific to every aspect (technical, operational, economic, political, social and environmental) that has an impact on hydrocarbon exploration and the prospective extraction of resources. description of scenarios the proposed scenarios are scarcity, base and abundance. for the construction of these three scenarios, five sources of projected incorporation of reserves and production profile development were considered. the factors to be considered are listed below. • demonstrated reserves currently producing: these are existing fields where oil and gas are currently being extracted. • additional reserves resulting from improved extraction: added reserves and increased production because of improvements in the extraction rate resulting from the application of new technologies. • undeveloped reserves: currently known resources, re-estimations of specific fields, and probable reserves in other fields. • conventional resources: incorporation of other potential conventional resources, including heavy crude oil and off-shore gas. • unconventional resources: incorporation of resources such as shale oil, shale gas, tar sands and coal bed methane (cbm). it is important to clarify that the abundance and base scenarios encompass all five items, whereas the scarcity scenario does not consider unconventional resources. with the aforementioned factors, the upme estimated a time of peak production for oil and gas for each of the proposed scenarios (table 6). these production peaks are of vital importance for the development of decline models. table 6. estimation of the production peaks of oil and gas for each of the proposed scenarios. scenario hydrocarbon abundance base scarcity oil 2037 2018 2016 gas 2028 2027 2016 once the peak production period has been established (table 6), the parameter t0 of eq. (1) is known (fig. 9) for each scenario. the production magnitude at peak production corresponds to q(t0) and is known because of the scenarios constructed by upme. therefore, we have all the information needed to develop the production model. after the results are obtained, a production vs. time plot is constructed (figs. 9 and 10), where the results are presented graphically so that the three 82 beltrán rivas julian daniel & vargas jiménez carlos alberto scenarios can be compared. this type of plot also allows us to evaluate the oil and gas production models for colombia in the coming years. figure 9. daily production of crude oil in colombia for different scenarios. the dashed curves represent the scenarios proposed by the upme, and the solid curves represent the scenarios proposed in the present work. these scenarios include production between the years 2000 and 2012. 4.4 results the average oil production in colombia during 2012 was 944 mmbbl/ day (acp, 2012) and the base and scarcity scenarios show a rapid increase to peak production. the scarcity scenario reaches peak production in 2014, with a net production of 1,022.8 mmbbl/day, which is predicted to be sustained for a period of 3 years and will subsequently decline. in 2027, the production will have been reduced to half the value at peak production, and the estimated production for 2050 is 241.8 mmbbl/day. during the first 10 years of the decline stage, the scarcity scenario proposed by the upme fits well to the model proposed in the present work. however, the upme proposes an increase in crude oil production after 2022 based on the discovery and development of yet-to-find (ytf) oil fields. our prediction model forecasts a continued decline after that year. the base scenario reaches peak production in 2016, with a production of 1,273.5 mmbbl/day. no significant difference is observed with respect to the scarcity scenario once the decline stage has begun. the production in 2030 will have been reduced to half its peak value, and the estimated production for 2050 is 315.2 mmbbl/day, which is only 70 mmbbl/day better than in hat the scarcity scenario. the proposal by the upme shows a correlation with the base scenario proposed in this work up to 2022. at that point, the prognoses diverge because of the increase in crude oil production proposed by the upme. the abundance scenario considers unconventional resources, and it presents us with a different projection compared with the base and scarcity scenarios. in this case, a growth in the production rate is predicted up to 2033, where a peak production of 3,296.2 mmbbl/day will be reached and sustained for 5 years. after this stable stage ends and the decline stage begins, the production will not return to current values for nearly three decades. the production in the year 2070 is estimated to be 797.1 mmbbl/day under this scenario, which is much larger than the value obtained in the scarcity and base scenarios for the previous 20 years. for the abundance scenario, the upme only presents results up to 2035; therefore, our results cannot be compared. the average decline rate used for modeling the oil production in all three proposed scenarios in this work is 9% (anh, 2009) and a similar number is expected from the upme. we now present our results for the developed model applied to the production of gas in colombia (fig. 10). the average gas production in colombia during 2012 was 1,169.2 mmcf/ day (millions of cubic feet per day (acp, 2012) the scarcity scenario shows that the production peak of 1,322.8 mmcf/day is quickly reached in 2014 and sustained for 3 years. production then drops to half its peak value in 2023, and in 2050, the production is estimated to be 216.9 mmcf/day. the production proposed by the upme for the scarcity scenario once peak production has been reached shows production up to 2029 that is larger than what we found in the current work. subsequently, however, the situation is inverted, and our decline model shows higher production levels than those predicted by the upme. the base scenario shows an increase in production up to 2025, when a peak production of 1,743.8 mmcf is reached and sustained for 3 years. once the decline has begun, it is predicted that production will be reduced to half the peak value after only 7 years, and in 2060, the production estimate is 293.1 mmcf/day. for the base scenario, the gas production proposed by the upme after peak production is always smaller than the estimates from the decline model presented here. the abundance scenario, which considers unconventional resources, presents production growth up to 2025, when a peak production of 3,209.0 mmcf/day is reached and sustained for 4 years. both the base and abundance scenarios reach peak production in the same year; however, they differ because the growth rate for the abundance scenario is on average twice that of the base scenario. once the 4-year period has passed and the decline stage has begun, the production predicted for 2040 shows similar numbers to current production levels. by 2070, the production is estimated to be 439.6 mmcf/day. the decline rate for gas production in colombia used for the production model is 15% (anh, 2009). the proposal by the upme shows a larger decline rate; thus, the production values predicted by the upme decrease faster than our results. according to the upme, gas production will be exhausted much sooner. figure 10. daily gas production in millions of cubic feet under each scenario. the dashed curves correspond to the scenarios proposed by the upme, and the solid lines correspond to the scenarios proposed in the present work. these scenarios include production for the period from 2000 to 2012. once these production curves were constructed, it was possible to estimate the oil and gas reserves in colombia according to the decline model presented in this work. this estimation is illustrated by the area under the curve corresponding to each production scenario. conclusions the results and forecasted data presented in this research work correspond to state-of-the-art and highly relevant topics in the hydrocarbon industry. in addition, we presented results obtained from the application of various statistical techniques and models, which allowed us to generate an estimation of the hydrocarbon resources in colombia. in colombia, 0.1% of all oil fields contain 35% of the total exploitable resources, and only 0.5% of all fields represent 70% of all ultimately recoverable oil resources. in the case of gas, 0.1% of all fields contain 75% of all exploitable resources. these numbers confirm that colombia follows the same trend as the rest of the world, where most resources are concentrated in a small number of fields. for colombia, the hydrocarbon industry represents a significant economic resource, although colombia’s contribution to the world’s reserves is only 0.17% for oil and 0.11% for gas. these numbers show that colombia does not have a significant effect on the world’s reserves and supply of hydrocarbons. from decline models and based on the production curves constructed for this work, colombian oil reserves were calculated for each scenario. for the scarcity scenario, a total reserve of 7,018.0 mmbbl up to 2050 was calculated. 83hydrocarbon production scenarios in colombia. review of field sizes, hydrocarbon reserves and expectations of conventional and unconventional resources. for the base scenario, the reserves calculated for the same year were 9,315.0 mmbbl, and for the abundance scenario, our calculations predicted reserves of 39,046.0 mmbbl up to 2070. using the aforementioned modeling methodology, colombian gas reserves were estimated for each scenario. for the scarcity scenario, the total gas reserves calculated up to 2050 were 7.9 tcf. for the base scenario, total reserves of 15.4 tcf were calculated for 2060, and for the abundance model, the estimation was 27.6 tcf up to 2070. the forecasted yet-to-find oil volume, which was calculated in section 3.3 using the fractal method, was 21,486.0 mmbbl. if we add to this the 2,377.0 mmbbl corresponding to the demonstrated oil reserves up to december 31, 2012, we obtain a total of 23,863.00 mmbbl. this number lies within the range generated by the maximal reserves calculated by the base and abundance scenarios, which are 9,315 mmbbl and 39,046 mmbbl, respectively. for the case of gas exploitation, the estimated yet-to-find volume was 49.834 tcf. if we add to this the 6.760 tcf corresponding to the demonstrated gas reserves up to december 31, 2012, we obtain a total volume of 56.6 tcf. this value is double that of the reserves estimated under the abundance scenario and much larger than the gas reserves calculated for the base and scarcity scenarios. finally, the exploitation of hydrocarbons in colombia must be incentivized, and the implementation and development of new technologies must occur so that significant increases in extraction rates can be achieved. acknowledgments the authors would like to thank the institutions that provided valuable information for the development of the present work: the colombian association of petroleum engineering (acipet), colombian association of petroleum (acp), ecopetrol, and ministry of mines and energy. references agencia nacional de hidrocarburos anh (2008): sistema gerencial para el manejo de los recursos petroleros [management system for the management of oil resources] – acuerdo 11 de 2008 de la anh, 19 – 20, bogotá. agencia nacional de hidrocarburos -( anh) (2009): colombia petróleo y futuro [colombia oil and future]. 37p., bogotá. arps, .j.j., (1944): analysis of decline curves. 229 – 230 p. asociación colombiana del petróleo (acp), (2012): informe estadístico petrolero [statistical review on oil], bogotá. british petroleum. 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(1989): calculo (cálculo) con geometría analítica [calculus (calculus) with analytic geometry] 132p., grupo editorial iberoamericana. upme unidad de planeación minero energética (unidad de planeación minero energética upme) (2012). escenarios de oferta y demanda de hidrocarburos en colombia [scenarios for supply and demand for hydrocarbons in colombia]. 9 -216p., la imprenta editores s.a, bogotá. colombia, exportaciones de café, carbón, petróleo y sus derivados, ferroníquel y no tradicionales según valores y pesos netos. [colombia, coffee exports, coal, petroleum and petroleum products, ferronickel and non-traditional products according to their values and net weights] departamento administrativo nacional de estadística. dane (2013) vargas, c. a., 2012. evaluating total yet-to-find hydrocarbon volume in colombia. earth sci. res. j., vol. 16, special issue (april, 2012): 1-246. agencia nacional de hidrocarburos (anh) (2009): colombia petróleo y futuro [colombia oil and future]. 37p., bogotá. höök, m (2009: (2009) depletion and decline curve analysis in crude oil production (licentiate thesis, uppsala university) 112p. arps .j.j. (1944): analysis of decline curves. 229 – 230 p. earl w. swokowsky (1989): calculo (cálculo) con geometría analítica [calculus (calculus) with analytic geometry] 132p., grupo editorial iberoamericana. unidad de planeación minero energética (upme) (2012). escenarios de oferta y demanda de hidrocarburos en colombia [scenarios for supply and demand for hydrocarbons in colombia]. 9 -216p., la imprenta editores s.a, bogotá. seonstca colombiana n°.7 pp. 30-34 dictembre de 2003 bogota, d.c. issn 0121 2974 efectos de las capas someras de alta velocidad en los datos sismicos aplicados a la detecci6n de hidrocarburos saul guevara trino salinas carlds piedrahita e-mail: tsalinas@ecopetrol.com.co grupo de geofisica, ecopetrol-instituto colombiano del petr6leo. a.a. 4185, bucaramanga, santander, e-mail: sguerra@ecopetrol.com.co resumen las ondas elasticas son una importante fuente de informaci6n de las caracteristicas del interior de la tierra, necesarios en la busqueda de hidrocarburos y otros recursos minerales. esta informaci6n es afectada generalmente por las capas someras, cuyas caracteristicas distorsionan los datos provenientes de las capas de interes, usualmente mas profundas. un caso, frecuente en la geologia colombiana es el de las capas someras de alta velocidad. en este caso las ondas siguen direcciones an6malas, y con frecuencia la informaci6n es muy dificil de identificar, ya que se generan eventos que la contaminan y probablemente la atenuan, aunque se sabe que existe este efecto, muchos de sus detalles son desconocidos. aqui se presenta uno de estos casos, se ilustra con modelamiento sismico con datos reales y se presenta una interpretaci6n de sus caracteristicas. palabras clave: metodo sismico de exploracion, zonas geologicamente complejas, estratos someros, afloramientos de rocas de alta velocidao, modelamiento sismico, ruioo coherente abstract elastic waves provide characteristics of earth interior, which are necessary in hydrocarbon and mineral prospect ions. this information is affected by shallow layers distorting the data coming from deep layers. in the colombian geology are fragment shallow layer with high velocity. in this case waves travel by anomalous paths being difficult to identity them, because these events contaminate and alternate the information considerated usefull. the effects associated to this phenomena are unknown with details. in this paper, one of these case is presented througen sismic modeling and vial data with an interpretation about its characteristics. keywords: seismic exploration, zones geologically complex, shallow strata, high shallow velocity layer, seismic modeling. coherent noise introduccion las ondas sismicas elasticas son utilizadas en la exploraci6n de hidrocarburos para obtener imagenes de detalles geol6gicos dentro de la tierra. una tecnologia avanzada, el metodo sismico de exploraci6n, se ha generado con este fin, y notables exitos se han obtenido por este medio. tipicamente en el piedemonte de las cordilleras se encuentran zonas de interes petrolifero en colombia. en este caso, es importante considerar el efecto de la abrupta geografia y compleja geologia que caracteriza a la presencia de la cordillera de los andes. manuscrito recibido para evaluacon el 24 de julio de 2003. articulo aceptaoo para publicaci6n por el comtte edttorial el 30 de octubre de 2003. 30 en general, las caracteristicas de los registros sismicos generados en estas regiones con fines exploratorios se salen de los estandares encontrados en otras zonas menos complejas, y la informaci6n objetivo es mas dificil de recuperar. uno de los obstaculos que hay que superar para obtener imageries de la geologia es el efecto de las capas someras. normalmente estas son capas de baja velocidad, y con propiedades heterogeneas, afectadas por efectos de la meteorizaci6n y depositaci6n, 10 que contribuye a distorsionar la imagen de los estratos de interes, mas profundos. dentro del metodo sismico de exploraci6n se han generado tecnicas para contrarrestar estos efectos, conocidas en la industria como correccion estatica (cox, 1999). sin embargo, en el piedemonte colombiano, y en otras partes del mundo, con frecuencia geofisica colombiana, 7, diciembre de 2003 ot.st.bt'i':;_ <no!) -4 . .j2 -2.5 0.0:!.5 5.0 1.5 to.o 12.5 efectos de las capas someras de alta velocidad en los datos sismicos aplicados a la deteccion de iddrocarburos afloran rocas caracterizadas por ser altamente compactas, y por tanto con alta velocidad de propagacion de las ondas. es el caso de rocas de alta antiguedad 0 rocas de origen igneo, que afloran a la superficie y suprayacen sobre rocas sedimentarias con potencial petrolifero. varios autores han estudiado los efectos de este tipo de caracteristicas de las capas someras en piedemonte. estos efectos se pueden dividir en varias partes; por ejemplo, uno son las caracteristicas geometricas (topografia y estructura geologica) y otro la estratificacion, que incluye estratos de alta velocidad, junto con otros de menor velocidad. por ejemplo la direcci6n de propagacion de la onda tiende a no ser perpendicular a la superficie, como sucede en el caso de capas superficiales de baja velocidad, sino a tener un fuerte componente horizontal. ademas, se pueden generar varios tipos de eventos que producen interferencia, como las ondas de superficie 0 las reverberaciones. para estos casos se han propuesto diversos modelos y soluciones. las tecnicas de migraci6n preapilado en profundidad (basadas en los principios de propagaci6n de las ondas) son las favoritas para obtener imagenes de zonas altamente plegadas, pero dependen de una buena correccion de los efectos de las capas someras para que sean efectivas. se han desarrollado metodos que extrapolan la propagaci6n de onda hasta la superficie, teniendo en cuenta caracteristicas como la abrupta variaci6n de la topografia y las altas velocidades (salinas y lamer, 1997), que son extensiones de tecnicas de migraci6n. en otro enfoque, fliedner y white (2001) aplican un metodo que integra diversas tecnicas como la refracci6n, el modelamiento y los datos sismicos de reflecci6n, para obtener imagenes del campo de velocidades bajo capas basalticas, esta tecnica implica la obtenci6n de datos sismicos de gran separaci6n entre fuente y receptor (offset), que llega a 15 ian en el caso de dicho articulo. otros metodos, por ejemplo el de jones y gaiser (1999) incluyen la utilizaci6n de ondas s u ondas convertidas, aparentemente menos afectadas por las capas someras de alta velocidad. sin embargo, en pocos trabajos publicados se trata el tema de la generaci6n de ruido coherente debido a las caracteristicas estratigraficas. en este trabajo se tratan algunos aspectos de las caracteristicas de la propagaci6n de ondas en este medio, gracias al modelamiento de la propagaci6n de ondas sismicas, buscando contribuir a la explicaci6n de las caracteristicas de los registros sismicos afectados por afloramientos de roc as de alta velocidad. metodologia en este caso se estudi6 un modelo tipico de las montafias colombianas, en particular de las montafias del valle alto del rio magdalena, en los que las capas de rocas de alta velocidad, frecuentemente igneas, yacen sobre capas sedimentarias altamente plegadas. la figura i muestra un perfil aproximado de las principales caracteristicas de un caso tipico de este terreno. la figura 2a muestra un registro sismico, producto de un disparo detectado por geofislca colombiana, 7, oiciembre oe 2003 '1.9 -1.0 2.0 3.0 'i' '.0"i 5.0 5 6.0 11.0 3.0 , 3.5 5.0 5.'4.' figura i. seccion geologica que ilustra caracterfsticas tipicas de algunas zonas montafiosas de colombia. notese el estrato superior de alta velocidad. figura 2a. ejemplo de un registro de disparo que ilustra las caracrerfsticas de los registros de disparo en zonas con afloramienros de alta velocidad. la coordenada vertical corresponde a tiempos de arriba, y la horizontal a localizaciones de los ge6fonos en la superficie del terreno. figura 2b. registro de disparo en una zona no compleja. la t1echa sefiala la refleccion proveniente de reflectores de interes, 31 vp=4800 rnls y=2.0 r>=2.6 1000 rn _____ v_p_=_3_8_0_0_rn_i_s__ y_=_2_.0__ r>_=_2_a i~o m vp=4200 rnls y=2.0 r>=2a figura 3. modelo geologico a, con el afloramiento de un estrato de alta velocidad. vp=1250 rnls 1'=2.0 p=2.1 t 30 rn vp=4800 rn/s 1'=2.0 f'=2.6 1000 rn vp=3800 rn/s 1'=2.0 f'=2.4 500 r-rt vp=4200 rnls y=2.0 r>=2a figura 4. modelo geologico, b can una capa somera de baja velocidad sabre un estrato de alta velocidad. una serie geofonos (sensores 0 sism6grafos) en una zona compleja y con afloramiento de rocas igneas. la figura 2b muestra un registro resultante de un experimento sismico en una zona de caracteristicas mas normales, es decir sin topografia compleja 0 afloramientos de alta velocidad. n6tese que en este ultimo caso es mucho mas facil identificar los eventos correspondientes a las interfaces geol6gicas de interes, un modelo simplificado se estudia con el objeto de identificar caracteristicas de la propagaci6n de las. ondas en este tipo de terreno. en esta aproximaci6n se dejaron de lado efectos de la topografia y de la heterogeneidad lateral de los estratos, para concentrarse en los eventos sismicos generados por la variaci6n vertical de velocidades y comparar estos efectos con 10 que se observa en los datos reales. se utilizaron dos modelos geologicos, uno con un estrato espeso de alta velocidad (modelo a, figura 3) y otro con una cap a de baja velocidad sobre un estrato espeso de alta velocidad (modelo b, figura 4). el estrato de alta velocidad simula rocas igneas. para el modelamiento de propagaci6n de las ondas se utilizaron dos rnetodos que se pueden considerar complementarios entre si: 32 guevara et al. diferencias finitas y trazado de rayos. ei metoda de diferencias finitas utiliza una sirnulacion basada en aproximaciones nurnericas a las ecuaciones diferenciales parciales de la propagaci6n de las ondas elasticas. una caracteristica muy importante de este metodo es que genera todos los eventos de 1a propagacion de ondas en medios elasticos, en el trazado de rayos, los frentes de onda son remplazados por sus normales, conocidas como rayos. este metodo tiene flexibilidad para mostrar varios tipos de caracteristicas de los datos e identificar diferentes eventos y, ademas, es muy eficiente computacionalmente. para el metoda de diferencias finitas se utilize el algoritmo elastico de dos dimensiones desarrollado por levander (1988). este tipo de algoritmo establece limitaciones a las dimensiones y caracteristicas del tipo de modelo que se puede utilizar, relacionados con los recursos computacionales necesarios y con los posibles problemas numericos que se generan en el proceso de calculo. estos factores influyeron en el modelo de velocidadcs considerado, asi como en el contenido de frecuencias elf' la fuente de energia (con una frecuencia dominante de 15 hz). para el modelamiento por trazado de rayos se utilize el software norsa?m 2-d. resultados la figura 5 muestra el resultado de la componente vertical para el modelo a, en el que aflora un estrato superior de alta velocidad, segun el metodo de diferencias finitas. la fuente de energia utilizada tiene una frecuencia dominante de 15 hz. se observan reflecciones tipicas de las interfaces del modelo, para varios tipos de onda. la identificaci6n de los tipos de onda se puede confirmar por medio de la instantanea 0 snapshot de la figura 5b, correspondiente disparo a o 1000 2000 3000 4000 5000 figura 5a. resultados del modelamiento de diferencias finitas sabre el modelo a. observese que las reflecciones se distinguen facilrnente, geofislca colombiana, 7, oiciembre de 2003 efectos de las capas someras de alta velocidad en los datos sismlcos aplicados a la deteccion de hldrocarburos figura 5b. instantanea 0 snapshot de la propagaci6n de las ondas a traves del modelo a para un tiempo de 0,6 segundos. dispero b 0.5 500 1000 1500 2500 3000 3500 4000 4500 figura 6a. resultados del modelamiento de diferencias finitas sobre el modele b. observese la dificultad para distinguir las reflecciones y la presencia de varios eventos que interfieren (ruido). o sod 1000 1sod 500 louo 1500 2000 2500 3000 3500 4000 4500 figura 6b. instantanea 0 snapshot de la propagaci6n de las ondas a travcs del modelo b para un tiempo de 0,6 segundos. a 0,6 segundos. en esta figura tam bien se nota que las ondas reflejadas (modos pp y ps) inciden en angulo agudo con la superficie del terreno. tambien se pueden observar algunos "artefactos" debidos al calculo numerico y no a la propagacion de la onda, como la dispersion en el even to gr y la refleccion de borde r. geoflslca colombiana, 7, oiciembre de 2003 las figuras 6 y 7 muestran los resultados del modelamiento del modelo b, que tiene una delgada capa de baja velocidad sobre el estrato de alta velocidad. la figura 6 corresponde al resultado del rnetodo de diferencias finitas, y la figura 7 al trazado de rayos, las reflecciones con informacion de las interfaces de interes se pueden identificar en la figura 7. en la figura 6 se muestra el predominio de varios tipos de eventos, que hacen dificil la identificaciori de las reflecciones provenientes de las interfaces de interes, para este modelamiento se utilize una fuente de energia con frecuencia dominante de 12 hz. se pueden notar dos tipos de eventos lineales predominantes, con caracteristicas diferentes entre sf y que aparentemente se repiten varias veces. las velocidades aparentes de los dos eventos son de 4,800 y 2.000 m/s; sin embargo se puede notar cierta dispersion, dado que la velocidad muestra cierta disrninucion en los eventos mas tardios. estas velocidades corresponden, la primera, a la de las ondas p del estrato de alta velocidad, y la segunda, aparentemente a una velocidad menor que la de la onda s del mismo estrato. este ultimo hecho coincide con una caracterfstica de las ondas de superficie (rayleigh), cuya velocidad suele ser menor que la de las ondas s. adernas este segundo evento muestra mayor contenido de energia, de un rapido analisis de polarizacion del segundo evento se observe que muestra una tendencia al movimiento eliptico tipico de las ondas de superficie. las instantaneas de la figura 6b presentan caracterfsticas de los diferentes eventos. aquf tarnbien se observan los numerosos eventos repetidos. tarnbien se puede ver que la longitud de onda es mayor al espesor de la capa de baja velocidad, y que en este estrato se generan muchos eventos inexistentes en el modele a. trazado de rayos modelo b 0.5 1.5 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 figura 7. resultado del modelamiento con trazado de rayos en el modele b. s6lo aparecen las reflecciones provenientes de los estratos de interes y arribos directos. 33 guevara et al. discusion al comparar los resultados anteriores con los datos reales de la figura 2, y teniendo en cuenta la simplicidad de este modelo numerico comparado con el caso real, se observan similitudes entre los datos de la figura 2a y los resultados de la figura 6. en la figura 2a, los eventos reflejados son aparentemente imposibles de identificar, y 10 mas coherente que se observa son eventos aproximadamente paralelos con dos direcciones predominantes, como en la figura 6. los datos de modelamiento parecen mostrar que, en el caso de capas de alta velocidad cubiertas por una delgada capa de baja velocidad, varios eventos diferentes a las ondas sismicas compresionales provenientes de las capas profundas se generan y predominan en el campo de onda detectado en la superficie. segun estos datos, los eventos repetidos que arriban primero parecen corresponder a reverberaciones de refracciones en la interface de alta velocidad. ei grupo que arriba mas tarde parece corresponder a ondas de superficie, perc que a su vez genera un tipo de reverberacion relacionada con ella. este resultado corresponde y puede dar una explicacion a los datos reales, como por ejemplo los observados en la figura 2. sin embargo, es necesario tener en cuenta varias caracteristicas de los datos reales, asi como limitaciones del modelamiento para interpretar mas acertadamente estos resultados. una de elias es que el modelamiento esta basado en un modelo elastico e isotropico y referido a un medio homogeneo, y la tierra tiene comportamientos anelasticos (atenuacion), cambio de propiedades con diferentes direcciones y cambios de propiedades dentro de los estratos. ademas, el modelamiento tiene sus propios requisitos, que en este caso se traduce en una relativa baja frecuencia en la fuente de energia, 10 cual puede contribuir a efectos como la dispersion. conclusiones en este trabajo se muestra la factibilidad de que la interaccion entre una capa muy somera de baja velocidad (capa meteorizada 0 weathering) y un estrato rocoso de velocidad muy alta genere un 34 alto nivel de ruido coherente que contribuye a la dificultad para obtener informacion de estratos sedimentarios mas profundos, de interes en la industria petrolera. experimentos posteriores pueden contribuir a una caracterizacion detail ada de propagacion de las ondas en este tipo de medios, y, con base en eso, a la busqueda de metodos que permitan contrarrestar su efecto pemicioso en la obtencion de informacion geologica del subsuelo. filtramientos de estos efectos implican una diferenciacion de las caracteristicas de estos eventos de ruido coherente comparado con los datos de reflecciones que queremos resaltar. agradecimientos este trabajo se debe a ecopetrol, empresa que facilito los medios para su realizacion, tarnbien se conto con la contribucion de hector alfonso, de exploracion ecopetrol, en el planteamiento de este tipo de problemas y su relacion con ejemplos de casos reales. se agradece al proyecto crewes de la universidad de calgary y a gerald schuster y su gropo de la universidad de utah que nos facilitaron herramientas de software y el codigo de modelamiento pot diferencias finitas, asi como a la compafiia norsar que facilito el software de modelamiento pottrazado de rayos. referencias cox, m. (1999). static corrections for seismic reflection surveys. seg geophysical references. fliedner,m. y white,r. (2001). "seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling". geophysics 66, 1925-1936. jones,n, y gaiser,1. (1999). "imaging beneath high-velocity layers". mlh seg meeting abstracts. levander,a. (1988). "fourth-order finite difference p-sy seismograms". geophysics, 53,1425-1436. saunast. y larner,k. (1997). "comparison of datuming alternatives in areas of rough terrain". seg 6th annual meeting expandedabstracts. geoflslca colombiana, 7, diciembre de 2003 135 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 135 140 detecting ionospheric precursors of a deep earthquake (378.8 km) on 7 july 2013, wm =7.2, in papua new guinea under a geomagnetic storm: two-dimensional principal component analysis jyh-woei, lin dept. of earth science, national cheng kung university, no. 1 university road, tainan city, taiwan, tel: +886-6-2757575 ext 65430. pgjwl1966@gmail.com seismology two-dimensional ionospheric total electron content (tec) data were collected during the time period from 00:00 on 2 july to 12:00 ut on 08 july 2013. this period spanned 5 days before to 1 day after a deep earthquake (378.8 km) in papua new guinea at 18:35:30 on 7 july 2013 ut ( wm =7.2). data were examined by two-dimensional principal component analysis (2dpca) to detect tec precursors related to the earthquake because tec precursors have usually appeared in earlier time periods (liu et al. 2006). a tec precursor was highly localized around the epicenter on 6 july for 5 minutes, from 06:00 to 06:05. ionizing radiation from radon gas release could possibly have caused the anomalous tec fluctuation through, for example, a density variance. the plasma might have experienced large damping to cause short-term tec fluctuations, and the gas released in a small amount in a short time period. 2dpca can also identify short-term tec fluctuations, but this fluctuation lasted for a considerable length of time. other background tec anomalies caused by the geomagnetic storm, small earthquakes and non-earthquake activities, e.g., equatorial ionization anomaly (eia), resulted in small principal eigenvalues. therefore, the detection of tec precursors through large eigenvalues was not due to these background tec anomalies. datos del contenido total de electrones ionosféricos en dos dimensiones (tec) fueron medidos durante el período del 2 de julio de 2013, a las 0:00:00 horas gmt., hasta las 12:00 gmt. del 8 de julio. en este lapso se abarcan cinco días antes y un día después de un terremoto profundo (378,8 kilómetros) en papúa nueva guinea, que se presentó a las 18:35:30 del 7 de julio ( wm =7.2). los datos fueron examinados a través de los componentes principales en dos dimensiones (2dpca) para detectar los precursores tec relacionados al terremoto (liu et al. 2006). un precursor de los tec fue localizado alrededor del epicentro el 6 de julio durante 5 minutos, desde las 06:00 a las 06:05. la radiación ionizada por la liberación de radón podría haber causado la fluctuación anómala de los tec a través de, por ejemplo, una variación de densidad. el plasma podría haber experimentado una fuerte humectación lo que causaría fluctuaciones cortas de los tec y la liberación de gas en pequeñas cantidades en poco tiempo. a través de los 2dpca también se pueden identificar fluctuaciones cortas de los tec, pero estas últimas duraron por un tiempo considerable. otras anomalías en el entorno de los tec causadas por tormentas geomagnéticas, pequeños terremotos y actividades no sísmicas, por ejemplo la anomalía ecuatorial de ionización (eia, en inglés), resultaron en pequeños autovalores principales. por lo tanto, la detección de precursores tec a través de grandes valores propios no se debe al entorno de estas anomalías en los tec. palabras clave: contenido total de electrones ionosféricos en dos dimensiones, papúa nueva guinea, anomalías tec, análisis de componentes principales en dos dimensiones, fluctuaciones tec anómalas, anomalía ecuatorial de ionización). record manuscript received: 24/07/2013 accepted for publication: 02/12/2013 abstract resumen key words: ionospheric two-dimensional total electron content, papua new guinea, tec anomaly, twodimensional principal component analysis, anomalous tec fluctuations; equatorial ionization anomaly (eia)). introduction recent studies have shown that principal component analysis (pca), which is a technique for mapping multidimensional data into lower dimensions with less loss of information (kramer, 1991), can distinguish between the earthquake-related tec anomalies and other possible causes of tec disturbance, such as tec long-term variance, solar flare and geomagnetic storm activity (lin 2010; 2011). when applied to earthquake-related tec anomalies, pca has been able to detect and even describe the spatial pattern or physical shape of earthquake-related tec anomalies (lin 2011). tec is an important quantity for describing the ionosphere of the earth. lin’s work in taiwan (2010) used the one-dimensional tec data 136 from ground-based receiving stations near the epicenters of earthquakes to detect the earthquake-related tec anomalies. the global ionospheric maps (gims) were decoded by image processing, and then the earthquakerelated tec anomalies were found by pca (2011). in the current study, two-dimensional principal component analysis (2dpca) was used to detect ionospheric tec precursors related to an earthquake that occurred at 18: 35:30 on 7 july 2013 (ut) ( wm =7.2), with an epicenter of 3.939° s, 153.882 ° e in papua new guinea. interestingly, the epicenter was 378.8 km below the surface. global ionospheric two-dimensional tec data were collected from 2 july to 12:00 ut on 08 july 2013. this period began 5 days before this earthquake, providing time for any tec precursors to develop and be revealed (liu et al. 2006). tec data were examined to detect tec precursors, while gims were only used to observe the tec situation. gps users with single-frequency receivers require ionospheric electron content information to achieve positioning accuracy that is similar to that of dual-frequency receivers. the gdgps system provides a global real-time map of ionospheric electron content (currently producing a map every 5 minutes). these maps are also of value in monitoring the effect of the ionosphere on radio signals, power grids, and space weather. the maps are derived using data from approximately 100 real-time gdgps tracking sites. the integrated electron density data along each receiver-to-gps satellite link are processed through a kalman filter to produce the global maps of tec every 5 minutes. the maps are available redundantly from multiple gdgps operations centers (gocs) as images, as text files containing the gridded tec values, or as a binary data stream containing the gridded tec values (twodimensional) (http://www.gdgps.net/products/tec-maps.html). the kalman filter is performed to estimate both the differential orbit between the two satellites as well as a reference orbit, together with their associating dynamics parameters. the basic assumptions for performing a kalman filter are that the two satellites are separated by a moderately long baseline (hundreds of km or less) and that they are of roughly similar shape. the differential dynamics, therefore, can be tightly constrained, strengthening the orbit determination. without explicit differencing of gps data, double-differenced phase biases are formed by a special transformation matrix. integer-valued fixing of these biases is then performed, greatly improving the orbit estimation (wu and bar-sever, 2005; kechine et al 2004; muellerschoen et al 2004; ouyang et al 2008). the tec data need to be corrected because of some biases or delays during the measurements of dual-frequency (l1 = 1575.42 mhz and l2 = 1227.60 mhz) delays of gps signals, e.g., carrier phase biases, satellite state (orbit) corrections, ionospheric delay corrections and troposphere, which need to be removed using ground-based post-processing software (raman and garin., 2005; wu and bar-sever, 2005). 2. methods 2. 1 2dpca 2dpca is a procedure that can detect anomalies in two-dimensional data. let the data be represented by a matrix b with dimensions m x n, where m, n>1 (for a map with m x n pixels of gray intensity). the linear projection of matrix b is considered as follows (sanguansat 2012): here, x is a projection axis with dimensions of n x 1, and y is the projected feature of these data on x with dimensions of m x 1 and is called the principal component vector. e is the ensemble average of the elements of a vector. the covariance matrix for 2dpca is defined as follows: the trace of w is defined as: , where the vector x maximizing eq. 3 corresponds to the largest (principal) eigenvalue of w , and the largest eigenvalue is the most dominant component of the data. therefore, the largest eigenvalue represents the principal characteristics of the data (kong et al 2005; sanguansat 2012, jeong et al. 2009). 2dpca can remove the small sample signal size (sss) problem for two-dimensional tec data (fukunnaga, 1991). traditional pca converts the measurements into one-dimensional data before calculation of the covariance matrix (yang et al. 2004). the covariance matrix of pca is based figure 1 (a) shows the gim at the time 06:00 on 06 july 2013. the red spot indicates the epicenter of this earthquake. jyh-woei, lin (1) (2) (3) 137 on an input matrix with dimensions of m x n, which is reshaped from onedimensional data (length of m multiplying n). reshaping data will cause computational errors because pca is a tool to address one-dimensional data. therefore, when using pca, the spatial structure information cannot be preserved well without some original information loss when inverting back to the original dimension when the reshaped matrix has a small sample size (sss) (kramer, 1991). such information loss is called an sss problem. however, the covariance matrix in 2dpca is full rank for a matrix (two-dimensional data) without reshaping the data. therefore, the challenges of dimensionality and the sss problem can be avoided using 2dpca (kong et al 2005; sanguansat 2012). 2.2 data processing using 2dpca the tec data for the mentioned time period were examined using 2dpca. however, after the data processing, only one tec precursor related to this earthquake was detected during the time period from 06:00 to 06:05 ut on 6 july 2012, which is approximately 1 day before the earthquake. therefore, this study also processed the tec data for this five-minute time period. the tec data in other time periods were examined using the same data analysis, but these results are not shown because other earthquake-related anomalies were not detected. figure 1 (a) shows the gim at the time 06:00 ut. the red spot indicates the epicenter of this earthquake. this global region is divided into 600 smaller areas, 12° in longitude and 9° in latitude, to detect more detailed tec situations. the spatial resolution of the tec data for gdgps system is 5 and 2.5 degrees in longitude and latitude, respectively (hernández-pajares et al. 2009; chen and gao 2005; gao and chen 2006) (http://www.gdgps.net/system-desc/references.html). therefore, 4 tec data (two-dimensional data) are collected in each area because the size of each small area is 12° in longitude and 9° in latitude. the 4 tec data form the matrix b (which belongs to sss data) with dimensions 2 x 2 for eq. (1) to detect a tec precursor related to the earthquake, which allows principal eigenvalues to be computed for each of the 600 smaller areas. 3. results figure 1(b) presents a color-coded scale of the magnitudes of principal eigenvalues corresponding to figure 1 (a). the color intensity denotes figure 1 (b) shows a color-coded scale of the magnitudes of the principal eigenvalues corresponding to figure 1(a). detecting ionospheric precursors of a deep earthquake (378.8 km) on 7 july 2013, wm =7.2, in papua new guinea under a geomagnetic storm: two-dimensional principal component analysi 138 the magnitude. from this figure, it can be observed that 600 principal eigenvalues are assigned. each principal eigenvalue represents the tec characteristic for each area. figures 2 (a) and (b) show similar results at time 06:05 ut. the high intensity here shows the existence of a tec precursor with a large principal eigenvalue calculated in the region of the epicenter for this large earthquake. this precursor had a duration of at least 5 minutes, from 06:00 to 06:05 ut. tec anomalies from other small earthquakes and non-earthquake activities, e.g., equatorial ionization anomaly (eia), in the same time period are not detectable. a large geomagnetic storm occurred during the examined time period, according to the dst indices in figure 3. tec anomalies caused by the storm activity resulted in the small magnitude principal eigenvalues. therefore, the geomagnetic storm activity and previous background anomalies can be eliminated as potential causes when detecting the tec precursor of the earthquake. 4. discussion 2dpca was able to detect a tec precursor around the epicenter of this earthquake. the precursor was detected from 06:00 to 06:05 ut with a duration of at least 5 minutes. another earthquake-associated tec anomaly after china’s wenchuan earthquake on 12 may 2008 (ut) ( wm =7.9) has been identified using pca and image processing (lin 2011). however, unlike the earthquake studied here, the wenchuan earthquake was not a deep earthquake. nevertheless, 2dpca has detected and located a tec precursor apparently related to such a deep earthquake. the physical reasons for this relation will now be considered. accordingly, studies of tec disturbance suggest three possible explanations for earthquake-associated anomalies. one is shock waves (jin et al., 2010). because the earthquake was very deep, it is not likely those acoustic shock waves from topside vibrations would be responsible for the tec anomaly. the other possibility is the presence of an electric field creating large-scale, ionospheric density irregularities, e.g., p-type semiconductor effects due to stress variance in rocks near the focus of the earthquake (pulinets and legen’ka, 2003; pulinets, 2004; freund, 2003; bošková et al. 1994; rothkaehl et al. 2006). a more likely possibility is that tec anomalies were being generated by radon release in the lower atmosphere rather than p-type semiconductor effects. p-type semiconductor effects would not be able to travel through the heterogeneous rocks that exist between the surface and 378.8 km down, and p and s type waves would have attenuated so greatly they would not have created adequate rock compression at the surface to generate the p-type semiconductor effect. radon gas release, on the other hand, could occur through micro-cracks formed in the crust and the earth’s surface. radon gas can lead to lower-atmosphere electric fields, and these can travel unimpeded into the ionosphere along geomagnetic lines (pulinets, 2004). this appears to be the most reasonable explanation for the tec precursor. it implies that radon gas release might have caused the anomalous tec fluctuation through, for example, a density variance. 5. conclusion 2dpca was used to detect a highly localized tec precursor in the time period from 06:00 to 06:05 ut on 6 july 2013, which occurred approximately 1 day before the main shock of the july 7 2013 papua new guinea earthquake. the duration of the tec precursor was at least 5 minutes. radon gas release was a possible cause of the anomalous tec fluctuations through, for example, a density variance. acknowledgements the author is grateful to the nasa global differential gps system (gdgps) for its useful references and tec data and to the memory of his own life in germany as a student from 1996-2000. references: bošková, j., j., šmilauer, p. tříska, and k. kudela, 1994, anomalous behaviour of plasma parameters as observed by the intercosmos 24 satellite prior to the iranian earthquake of 20 june 1990, studia geophysica et geodaetica, vol. 38, issue 2, pp 213-220 figure 2 (a) shows the gim at the time 06:05 on 06 july 2013. the red spot indicates the epicenter of this earthquake. jyh-woei, lin 139 chen, k and y. gao, 2005, real-time precise point positioning using single frequency data. ion gnss 2005, long beach, ca, usa, sept.2005 freund, f. t., 2003, rocks that crackle and sparkle and glow strange pre earthquake phenomena, j. sci. explor., 17(1), 37–71. gao, y and k. chen, 2006, development of a real-time single-frequency precise point positioning system and test results. ion gnss 19th international technical meeting of satellite division, 26-29 september 2006, fort worth, tx hernández-pajares, m., j. m. juan, j. sanz, r. orus, a. garcia-rigo · j. feltens, a. komjathy, s. c. schaer and a. krankowski, 2009, the igs vtec maps: a reliable source of ionospheric information since 1998. j geod, 83:263–275. doi 10.1007/s00190-008-0266-1 jeong, d.h., c. ziemkiewicz, w. ribarsky and r. chang, 2009, understanding principal component analysis using a visual analytics tool. charlotte visualization center, unc charlotte jin, s., w. zhu, and e. afraimovich, 2010, co-seismic ionospheric and deformation signals on the 2008 magnitude 8.0 wenchuan earthquake from gps observations, int. j. remote sens., 31(13), 3535–3543, doi:10.1080/01431161003727739. kramer, m. a., 1991, nonlinear principal component analysis using autoassociative neural networks. aiche journal, vol. 37, no. 2, pp. 233-243 kechine, m. o., c.c.j.m. tiberius, h. van der marel, 2004, real-time kinematic positioning with nasa’s global differential gps system. gnss conference, st. petersburg, russia kong, h., l. wang, e. k. teoh, x. li., j. g. wang, r. venkateswarlu, 2005, generalized 2d principal component analysis for face image representation and recognition. neural networks, 18, 585–594 figure 2 (b) shows a color-coded scale of the magnitudes of the principal eigenvalues corresponding to figure 2(a). figure 3 shows the dst indices during the time period from 01 july to 12:00 ut on 08 july 2013 (wdc for geomagnetism, kyoto). detecting ionospheric precursors of a deep earthquake (378.8 km) on 7 july 2013, wm =7.2, in papua new guinea under a geomagnetic storm: two-dimensional principal component analysi 140 lin, j.w., 2010, ionospheric total electron content (tec) anomalies associated with earthquakes 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association of geodesy symposia, 2008, volume 133, part 4, 751-758, doi: 10.1007/978-3-540-85426-5_86 pulinets, s. a., 2004, ionospheric precursors of earthquakes; recent advances in theory and practical applications, terr. atmos. oceanic sci., 15(3), 413–435. pulinets, s. a., and a. d. legen’ka, 2003, spatial-temporal characteristics of the large scale disturbances of electron concentration observed in the f region of the ionosphere before strong earthquakes, cosmic res., engl. transl., 41, 221–230, doi:10.1023/a:1024046814173. sanguansat, p, 2012, principal component analysis, published by intech, isbn 978-953-51-0195-6, 1-23, pp.300 raman, s. and l. garin., 2005, performance evaluation of global differential gps (gdgps) for single frequency c/a code receivers. sirf technology, inc. rothkaehl, h., r. bucik and k. kudela, 2006, ionospheric plasma response to the seismic activity, physics and chemistry of the earth, vol. 31, issue 4-9, pp. 473-481 wu, s and y. bar-sever, 2005, real-time sub-cm differential orbit determination of two low-earth orbiters with gps bias fixing jet propulsion laboratory, california institute of technology. tx, usa, sept.2005 yang, j., d. zhang, a.f. frangi., j.y yang, 2004, two-dimensional pca: a new approach to appearance-based face representation and recognition. ieee transactions on pattern analysis and machine intelligence, vol. 26, no. 1, pp.131-137. jyh-woei, lin deflections of the vertical(dov) are essential data in the geodetic observation reduce, the earth gravity field and geoid model refinement, and the mass earth change. the meridional and prime vertical components of global oceanic dovs are estimated from altimetry data of topex/poseidon, jason-1, and jason-2, through 1992 to 2013 with the crossover method, to analyze the temporal-spatial distribution of marine dovs. comparing with the egm2008-modelled dovs, precisions of meridional and prime vertical components can be up to 0.98” and 1.02”, respectively. the time series of annual mean dovs from 1992 to 2013 are studied to get the annual variation law. annual changes of meridional and prime vertical components are small over most oceans. but the annual changes are significantover oceans with large submarine topography undulations.the spatial distribution of oceanic dovs is also analyzed. the absolute dovs over oceans around lands, islands, and large oceanic trenches are greater than those over the other oceans. the meridional and prime vertical components are consistent withthe longitude and latitude directions, respectively. las desviaciones de la vertical (dov, del inglés deflections of the vertical) son información esencial en la simplificación de la observación geodésica, el campo gravitatorio terrestre y el perfeccionamiento del modelo geoide, y el cambio de la masa terrestre. se estimaron los componentes meridional y principal vertical de las desviaciones de la vertical oceánica global con la información altimétrica de topex/poseidón, jason 1 y jason 2, de 1992 a 2013, con un método de cruce para analizar la distribución espacio-temporal de las desviaciones de la vertical marinas. comparadas con las desviaciones de la vertical obtenidas con el modelo egm2008 (modelo de gravitación de la tierra), la precisión de los componentes meridional y principal vertical puede aumentarse 0.98" y 1.02", respectivamente. se estudiaron las series temporales de la media anual de las desviaciones de la vertical desde 1992 a 2013 para obtener la ley anual de variación. los cambios anuales de los componentes meridional y principal vertical son muy pequeñas en la mayoría de los océanos. sin embargo, los cambios anuales son mayores en aquellos océanos con una topografía submarina de grandes ondulaciones. también se analizó la distribución espacial de las desviaciones de la vertical oceánicas. las desviaciones de la vertical absolutas alrededor de los continentes, islas y fosas oceánicas mayores son superiores que aquellas sobre los otros océanos. los componentes meridional y vertical principal son consistentes con las direcciones de la longitud y la latitud, respectivamente. abstract resumen keywords: deflection of the vertical (dov), satellite altimetry, temporal-spatial distribution, crossover point, marine gravity field. palabras clave: desviación de la vertical (dov), altimetría satelital, distribución espacio-temporal, punto de cruce, campo de gravitación marino. temporal-spatial distribution of oceanic vertical deflections determined by topex/poseidon and jason-1/2 missions record manuscript received: 26/11/2015 accepted for publication: 06/03/2016 how to cite item guo, j. y., shen, y., zhang, k., liu, x., kong, q. and xie f. (2016). temporal-spatial distribution of oceanic vertical deflections determined by topex/poseidon and jason-1/2 missions. earth sciences research journal, 20(2), h1-h5. doi:http://dx.doi.org/10.15446/esrj.v20n2.54402 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.54402 jin yun guo1,2*, yi shen1, kaihua zhang1,3, xin liu1, qiaoli kong1,2, feifei xie1,2 1college of geodesy and geomatics, shandong university of science and technology, qingdao 266590, china 2 state key laboratory of mining disaster prevention and control,co-founded by shandong province and ministry of science & technology, qingdao 266590, china 3tianjin survey and design institute for water transport engineering,tianjin 300456, china * corresponding author: jinyunguo, e-mail: jinyunguo1@126.com earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : h1-h5 m e t e o r o l o g y h2 jin yun guo, yi shen, kaihua zhang, xin liu, qiaoli kong and feifei xie 1. introduction the deflection of the vertical (dov)as one basic research topic in geodesy and geophysics can characterize thedirection of gravity vector and provide one tight local tie among space geodetic techniques (shen et al., 2015). the traditional methods to measure dov includethe astronomical geodetic surveying, gravimetry, combination of astronomical geodesy and gravimetry, and global navigation satellite system (gnss). based on the astronomical geodetic principle, the automatic measurement system integrating gnss and ccd zenith telescope can precisely measure dovs (guo, song, chang, &liu,2011; hirt&seeber, 2008; tian et al., 2014, peng&xia,2015).these measurement methods of the vertical deflectionare applicable to lands (ning,guo, b. wang,& h. m. wang, 2006), but are unable to measure practically the oceanic vertical deflection due to limitations of these techniques under effects of ocean dynamic environment over open oceans (guo,chen,liu, zhong,&mai, 2013;guo, liu, chen, wang, &li,2014). dovs over open oceans are commonly calculated from the earth gravity field model at present (jekeli, 1999; pavlis, holmes, kenyon, & factor,2012). ship-borne gravity data can also be used to estimate the oceanic vertical deflections. but the ship-borne gravimetry can mostly be carried out over coastal seas, and most of theoceans are short of gravity field data. with the rapid development of satellite altimetry, altimeter data can widely be used to estimate the oceanic vertical deflection (fu and cazenave, 2000; guo, hu, wang, chang,&li, 2015). with the development of satellite altimetry technique, oceanic dovs are mainly utilized to determineoceanic gravity field model with high precision. dovs on crossover points can be calculated from altimeter data by the crossover method (sandwell, 1984). altimeter data of ers-1 and geosat were processed to estimate dovs of crossover points from which gravity field over antarctic oceans was estimated by the laplace equation (sandwell, 1992). sandwelland smith (1997) used ers-1 and geosat altimetry data to compute dovs of crossover points which are interpolated to get dovs of grids and then estimated the gravity anomalies over oceans with the fast fourier transformation (fft).single differences of sea surface height along thetrack and corresponding track azimuths are used to calculate oceanic dovs along thetrack (olgiati, balmino, sarrailh,& green, 1995). altimetry data from multi-satellite altimetry missions can be integrally processed to get dovs along tracks which are interpolated to calculate dovs of grids used to estimate marine gravity anomalies. gravity anomalies along tracks with the alongtrack dovs are more accurate than those determined by fft with the inverse stokes formula. the inverse vening meinesz formula is derived to calculate dovs with the grid method (hwang, 1998). the geoid and gravity anomalies over china seashas been calculated from oceanic dovs with the crossover method (li,ning, chen,& chao, 2003). dovs from the crossover method are more accurate than those from the grid method and the along track method, and the grid method is optimal to determine the high-resolution precise marine gravity field using altimeter data (peng and xia, 2004). dovs over china coastal seas are calculated from geophysical data records (gdr) of envisat with the grid method (xing, li &liu, 2006). comparing with egm96 (lemoine et al., 1998), precisions of the prime vertical and meridional components are 6″ and 3″, respectively. multi-altimeter data are processed to calculate global dovs with the grid method. comparing with egm96, precisions of the prime vertical and meridional components over global oceans are 1.97” and 1.12”, respectively. topex/poseidon (t/p) launched in 1992, jason-1 initiated in 2002 and jason-2 initiatedin 2008 can together measure oceans. jason-2 as jason-1 follow on runs the same track of t/p and continuously surveys oceans. theprecision of single observation is estimated to be near to 3 cm (beckley et al., 2010; fu and cazenave, 2000; tunini et al., 2010).t/p and jason-1/2 providealtimetry data with high precision which are processed by the crossover method. global oceanic dovs are estimated and then the temporal-spatial distribution of global oceanic dovs is studied in the paper. 2. satellite altimetry data gdrs version c of t/p, jason-1, and jason-2 released by the physical oceanography distributed active archive center(po.daac) are processed in the paper. time spans of t/p, jason-1, and jason-2 data are from october 1992 to august 2002 with cycles 2 to 364, from august 2002 to january 2009 with cycles 22 to 259, and from january 2009 to may 2013 with cycles 21 to 177, respectively. t/p starts the precise satellite altimetry era, and jason-1 and jason-2 follow t/p. these three altimetry satellites are in the same exact repeated orbit. figure 1 shows global tracks and crossover points of t/p cycle 17. the radial precision of t/p orbit determination is better than 3 cm, and the radial precisions of jason-1 and jason-2 orbit determination can be up to 1 cm based on the on-board satellite tracking systems. sea surface height accuracyfrom these three altimeter data can be lower than 3 cm. to get more precise sea surface heights, altimetry data should be edited based on the criterions provided by aviso (aviso/altimetry, 1996; dumont et al., 2011; picot, case, desai, vincent, &bronner, 2012; shah, sajeev, & gopika, 2015). figure 1. t/p tracks and crossover pointsfor cycle 17 3. determination of oceanic dovs the crossover method is one of thebest waysto determine precise oceanic dovs from satellite altimetry data. altimetry profiles of ascending and descending arcs can be determined by the geoid gradient or the single difference of dov along thetrack, and then the prime vertical and meridional components of dov can be solved by combining thetrack gradients. the meridional componentand ξ the prime vertical component η of dov are calculated from the two directions of geoid gradients (heiskanen&moritz,1967), that is (1) (2) where r is the average radius of the earth. ∂n ⁄ ∂φ =1⁄ 2|φ̇ | (n ̇a-n ̇d)and ∂n ⁄ ∂ λ=1⁄ 2|λ ̇ | (n ̇a+n ̇d) and, in which φ andare the directions of prime vertical and meridional speedsof nadir pointrespectively, is the derivative of geoid heightwith respect to time on thealtimetry data point relative,and subscripts and denotethe ascending and descending arcs respectively. ,, and can be obtained by the time and location information in the altimetric data (hwang& parsons,1995; sandwell& smith,1997).the detailed data processing procedure is following. 1) altimetry data preprocessing. these three-altimeter data are read with the corresponding program based on data formats. these data are edited according to the relatedcriteria published by the data providers. all geophysical corrections are applied to altimetry data. these data including latitude, longitude, sea surface height and time are saved based on the ascending and descending arcs. h3temporal-spatial distribution of oceanic vertical deflections determined by topex/poseidon and jason-1/2 missions 2) determination of crossover point. the quadratic polynomials are used to fit the ascending and descending arcs on geodetic latitude and longitude based on the positioning information in altimetry data, respectively. the quadratic equations of ascending and descending arcs are combined to solve coordinates of thecrossover point. 3) unification of time and space datum. there exist systematic biases between different altimeter data under effects of a different ellipsoid, time-varying sea surface height, satellite orbit determination error, altimeter error and geophysical correction error. corrections of coordinate frame biases for different altimetry data are calculated to unify the time and space datum for t/p, jason-1, and jason-2. 4) sea surface height of crossover point. sea surface height of crossover point is interpolated from the ascending and descending arcs’ data based on coordinates with the distance weight method. 5) dov determination of crossover point. 8-10 altimetry points around crossover point are selected based on the position of thecrossover point. the quadratic polynomial on latitude and longitude, and the quadratic curves of latitude and longitude with respect to time are fitted based on information about sea surface height, latitude, longitude and time of altimetry point, respectively. then derivatives of these quadratic polynomials are made to get , andwhich are substituted for equations (1) and (2) to calculate the prime vertical and meridional components of dov on crossover point. 6) repeat step 5) to calculate all dovs of all crossover points. the same pass may be different for all cycles and the farthest distance of crossover points for the same ascending and descending arcs may be up to 1 km for t/p, jason-1 and jason-2 (fu and cazenave, 2000; luz clara, simionato, d’onofrio, & moreira, 2015). so the mean position of crossover point for the same ascending and descending arcs should be determined. 7) reduce dovs of crossover points to the mean position with the inverse distance weighted interpolation method within the studied time span. we can get oceanic dovs of crossover pointsfrom altimetry data from 1992 to 2013. so the temporal-spatial distribution of oceanic dovs can be obtained. altimetry data should be edited and corrected before dov calculation because of contamination of many kinds of errors. the same editing criteria are used to modifyand correct altimetry data to get precise results. to check the precision of oceanic dovs from altimetry data, itwasselected t/p data of cycle 17 to compute oceanic dovs of crossover points in the global area (-60°s~60°n, 0°~360°), which are compared with the modeled dovs from egm2008 (pavlis, holmes, kenyon,& factor,2012). table 1 lists the statistical results. from table 1, it can be seen that theprecisions of prime vertical and meridional components are 1.02″ and 0.98″ comparing with egm2008-modelled components, respectively, which indicates that oceanic dovs of crossover points from altimetry data are reliable. table 1. statistical results of oceanic dovs from t/p, jason-1, and jason-2 4.temporal variations of oceanic dovs the earth gravity field not only changes in space but also changes in time. the temporal-spatial variation of the earth gravity field is one important part of geodynamics (ding, li, &ding, 2006). the gravity field change containing the abundant geophysical information of interior earth can be studied throughthe dov variations. oceanic dovs of crossover points for each cycle are calculated from altimetry data of t/p, jason-1, and jason-2 with the crossover method. the mean position of crossover point is determined by averaging positions of crossover points from the same ascending and descending arcs in one year. dovs from the altimetry data of about 37 cycles in one year are used to get the mean dov of the averageposition with the inverse distance weighted interpolation method. so itispossibletoget a group of dovs of annual mean crossover pointson a global scale, which can make up a time series of annual mean dovs from 1992 to 2013. the time series can be analyzed to study the interannual variations of oceanic dovs. figures 2 and 3 show annual variations of meridional and prime vertical components of oceanic dovs determined by the crossover method, respectively. figure 2.annual change rate ofmeridional componentof dov figure 3.annual change rate ofprime verticalcomponent of dov from figure 2, it can be found that the annual change of meridional component over most oceans is slight in 1992-2013, and the annual change in most oceans in the north hemisphere is greater than that in the south region. the annual change over open oceans far away from islands and lands is small and apt to convergence, such as the middle pacific ocean and the middle indian ocean. the annual variation over coastal seas is great, such as the bering sea, the indonesian sea, the new zealand sea, and the caribbean sea. on the one hand, the mass over these coastal seas changes greatly. on the contrary, the variation can reflect the lower altimetry data quality over coastal seas than that over open oceans. figure 3 shows the annualchangesin prime vertical components. annual variations are small and apt to convergence over most oceans except for the eastern oceans of the north american, the bengal bay, and the china coastal seas. also, the annual variation is significantover some oceans around islands in the pacific ocean. 5. spatial distribution of oceanic dovs the marine gravity field is mainly determined by the oceanic lithosphere and the deep mass distribution and geological structure. it shows the basic geographic information of oceanic lithosphere structure and the submarine topography (chao, yao, li,&xu, 2002). the uneven distribution of vast oceanic gravity affects the direction of themarine plumb line, which makes the spatial distribution of marine dovs inconsistent. the marine gravity field can be analyzed by studying the spatial distribution of global oceanic dovs. dovs of crossover point for the same ascending and descending arcs in all cycles are interpolated to the mean position with the inverse distance weighted interpolation method. so the averagedov of each crossover point in 1992-2013 can be obtained. figures 4 and 5 show the global distribution of oceanic dovs. h4 jin yun guo, yi shen, kaihua zhang, xin liu, qiaoli kong and feifei xie figure 4.spatial distribution of meridional component of dov figure 5. spatial distribution of prime vertical component of dov from figure 4, it can be foundthat the absolute meridional components of dov are comparatively large over the coastal oceans. the meridional components are large over the west pacific ocean and the middle indian ocean. it is also large over the seas around islands and large oceanic trenches, such as the bering sea, the japan seas, the indonesian seas, and the caribbean sea. the significant meridional components over these oceans are mainly caused by the obvious marine mass changes. the meridional components are positive over the most indian ocean, the northcentral pacific ocean, and the caribbean sea. most meridional components over the other oceans are negative. the meridional components tend to be consistent along the longitude direction over most oceans. from figure 5, it can be found that the spatial distribution of prime vertical components of oceanic dovs is approximately consistent with the distribution of meridional components over most oceans. the absolute prime vertical components are large over the china seas, the west pacific ocean, the indian ocean, and the northwest atlantic ocean. it is also large over the oceans around islands and oceanic trenches, like the bering sea, the japan seas, the indonesian seas, the caribbean sea, and the ocean around the mariana trench. the large prime vertical components over these oceans may be mainly caused by the visiblechange of marine gravity field. the prime vertical components over the most indian ocean, the north-central pacific ocean and the caribbean sea are positive, and these are more negative over the other oceans. in contrast to the meridional component, the prime vertical components tend to be consistent along the latitudedirectionover most oceans. 6. conclusions marine dovs are determined from altimetry data of t/p, jason-1, and jason-2 in 1992-2013 with the crossover method. the temporalspatial distribution of oceanic dovs is studied. comparing with the egm2008-modelled dovs, the precisions of prime vertical and meridional components determined from one-cycle altimetry data of t/p, jason-1, and jason-2 can be up 0.98″ and 1.02″, respectively, which indicates that dovs of crossover pointsestimated from altimetry data are very reliable. annual changes of meridional and prime vertical components of dovs over most open oceans are small. but the annual variations over most coastal seas are significant because mass changes over coastal ocean may be large and thealtimetry precision over coastal seas may be lower. absolute dovs over oceans around lands, islands and vastocean trenches are biggerthan those over the other oceans. this is mainly due to the obvious gravity change of interior earth and the severe submarine topography undulation. the meridional and prime vertical components of dovs are consistent withthe longitude and latitude directions, respectively. the spatial resolution of oceanic dovs determined from altimetry data with the crossover method is limited by theexact repeating period of altimetry satellite. thecombination of more satellite altimetry missions can improve the resolution of oceanic dovs. acknowledgments: this study is supported by the national natural science foundation of china (grant no. 41374009), the shandong natural science 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(2006). study on ocean vertical deflection of envisat satellite altimetry along-track geoid gradient(in chinese). science of surveying and mapping, 31(5), 48-49. articulo 8c.indd earth sciences research journal geochemistry earth sci. res. sj. vol. 16, no. 2 (december, 2012): 151 164 the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria jude e. ogala geology department, delta state university, p.m.b.1, abraka, nigeria. e-mail: etunimogala@yahoo.com abstract major and trace element compositions of lignite from the tertiary ogwashi-asaba formation, southern nigeria, have been investigated to determine the prevailing environmental conditions which controlled their formation. seven samples were obtained from outcrops along river valleys, streams and springs in seven localities; these samples were subsequently analysed using fusion combined with inductively-coupled plasma (fus-icp) and total digestion with inductively-coupled plasma (td-icp). geochemical analysis revealed the following concentration range in weight percent (wt. %) for major oxides: sio2 (0.04-9.78); al2o3 (0.56-6.40), fe2o3 (0.05-1.22), mno (0.001-0.004), mgo (0.02-0.11), cao (0.05-0.15), k2o (0.01 0.04), tio2 (0.016-0.299) and s (0.08-0.39). trace elements indicated the following concentration range in parts per million (ppm): be (2-5), zr (4-63), sr (5-22), y (13-68), ba (9-89), v (4-216), zn (17176), ni (8-28), co (5-13), cr (2-31), cu (1-22) and ga (1-14). the low silica (<10 wt. %) and alumina contents (<7 wt. %) were explained by very limited detrital input during coal formation. magnesium oxide and cao content were relatively low thereby confirming the continental nature of the peat-forming environment. redox-sensitive trace element ratios (ni/co, v/cr and v/v+ni) indicated predominantly oxic environments for coal deposition. the low trace element concentrations determined in the lignite samples did not point to any severe environmental impact from coal use. resumen las composiciones de elementos mayores y trazas de lignita de la formación terciaria ogwashi-asaba, en el sur de nigeria, han sido investigadas para determinar las condiciones ambientales prevalecientes que controlaron su formación. siete muestras fueron tomadas de afloramientos a lo largo de valles de ríos, arroyos y manantiales en siete localidades; estas muestras se analizaron posteriormente mediante fusión combinada (fus-icp) y la digestión total con plasma de acoplamiento inductivo (icp-td). el análisis geoquímico reveló el siguiente intervalo de concentración en porcentaje en peso (% en peso) para los óxidos mayores: sio2 (0,04 a 9,78); al2o3 (0,56-6,40), fe2o3 (0,05-1,22), mno (0.001-0.004), mgo (0,02 -0,11), cao (0.05-0.15), k2o (0,01 0,04), tio2 (0.016-0,299) y s (0,08 a 0,39). los elementos traza indican el siguiente rango de concentración en partes por millón (ppm): be (2-5), zr (4-63), sr (5-22), y (13-68), ba (9-89), v (4-216), zn (17-176), ni (8-28), co (5-13), cr (2-31), cu (1-22) y ga (1-14). el bajo contenido de sílice (<10 en peso.%) y el contenido de aluminio (<7 wt.%) fueron explicados por la entrada muy limitado de detrítos durante la formación de carbón. el óxido de magnesio y el contenido de cao son relativamente bajos, lo que confirma el carácter continental del entorno de formación de turba. la relación de elementos traza redox-sensibles (ni/co, v/ cr y v/v + ni) indicó ambientes predominantemente óxicos para la deposición de carbón. las bajas concentraciones de trazas de elementos hallados en las muestras de lignito no indican un impacto ambiental severo de la utilización del carbón. palabras claves: geoquímica, lignito, formación ogwashiasaba, ambientte continental, nigeria keywords: geochemistry, lignite, ogwashi-asaba formation, continental environment, nigeria. record manuscript received: 12/01/2012 accepted for publications: 06/05/2012 introduction coal accumulation in nigeria took place mainly during upper cretaceous and tertiary times, resulting in the formation of extensive lignite, sub-bituminous and bituminous coal deposits in the benue trough and anambra basin (simpson, 1954; reyment, 1965; akande et al., 1992; obaje and ligouis, 1996; obaje and hamza, 2000; obaje, 2009). the ogwashi-asaba formation occurs extensively within the niger delta basin jude e. ogala152 in southern nigeria (figure 1); it was first described by wilson (1925) and named the “lignite group.” lithologically, it consists of a sequence of coarse-grained sandstones, light-coloured clays and carbonaceous shales, within which are continental lignite seam intercalations (reyment, 1965; kogbe, 1976; jan du chene et al., 1978). nigeria has the largest known lignite deposits in africa, proven reserves exceeding 300 million tons (orajaka et al., 1990; momsd, 2007). lignite is mined in open pits in nigeria and is used mainly as a substitute for wood as fuel in domestic cooking. the pertinent literature on nigerian lignite deposits’ major and trace element composition is scarce (adedosu et al., 2007; olobaniyi and ogala, 2011). coal is heterogeneous sediment having both organic and inorganic fractions; it contains variable amounts of almost all the elements included in the periodic table (christanis et al., 1998; orem and finkelman, 2003). most elements contained in coal usually show a close association with organic matter or with the coal’s inorganic fraction. changes in the elements’ affinity with either the organic or inorganic fraction may also occur during coalification (christanis et al., 1998). several researchers have attempted to study the geochemical features of trace elements contained in coal to understand and evaluate trace elements’ mode of occurrence as well as their behaviour during combustion (gluskoter et al., 1977; swaine, 1990; finkelman, 1994; meij, 1995; spears and zheng, 1999; davidson, 2000; vassilev et al., 2001). previous studies on trace elements in nigerian coal have revealed their distribution (olajire et al., 2007; ogala et al., 2010a), composition (ndiokwere et al., 1983; olabanji, 1991; ewa et al., 1996; adedosu et al., 2007; ogala et al., 2010b), characteristics (nwadinigwe, 1992; sonibare et al., 2005) and association ( ewa, 2004; ogala et al., 2009). several researchers have studied trace element content in lignite and their behaviour during combustion (foscolos et al., 1989; goodarzi et al., figure 1. map of nigeria showing the location of ogwashi-asaba formation and adjacent units. the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria 153 1990; georgakopoulos et al., 1994; filippidis et al., 1996; gentzis et al., 1990; gerouki et al., 1996; sakorafa et al., 1996; christanis et al., 1998; chatziapostolou et al., 2006; adamidou et al., 2007). trace elements are of great importance in ascertaining the depositional environment and maturity of geological samples within a particular basin (adedosu et al., 2007). the present study was aimed at investigating the chemical composition (major and trace elements) of lignite samples from the ogwashiasaba formation, southern nigeria, to predict the prevailing geochemical environmental conditions as well as the processes responsible for their distribution. the study area’s geology and stratigraphy the ogwashi-asaba formation (reyment, 1965) occurs extensively within the niger delta basin in southern nigeria, covering a 4,900 km2 area (fig. 1). the formation was originally referred to as a “lignite group” (wilson, 1925; wilson and bain, 1928), “lignite series” (simpson, 1949; 1954), and “lignite formation” (de swardt and casey, 1963). lignite layers have also been encountered in the lowermost strata of the ameki group (fig. 1) and the uppermost strata of the benin formation in drill holes, streams/river banks and road-cut outcrops (okezie and onuogu, 1985). the southern nigerian sedimentary basin’s formation began during the early cretaceous period (albian) following basement subsidence along the benue and niger troughs (nwachukwu, 1972; olade, 1975). folding and uplift occurred during the santonian along a northeast-southwest axis in the abakaliki-benue area. the anambra platform, lying to the west and southwest of the abakaliki folded belts, subsided to form the anambra basin (reyment, 1965; short and stauble, 1967; murat, 1972; benkhelil, 1989). the upper cretaceous stratigraphic succession in the anambra basin began with the deposition of sediment from the marine campanian-maastrichtian enugu/nkporo shales and their lateral equivalent deltaic owelli sandstone (fig. 2). these base units were successively overlain by the lower-middle maastrichtian deltaic coal-bearing mamu formation (lower coal measures) and middle maastrichtian tidal ajali sandstone (false-bedded sandstones) and overlain by the fluvial-deltaic nsukka formation (upper coal measures) . the imo and nsukka formations marine shales were deposited during the paleocene era and overlain by the regressive ameki formation (eocene); the paralic ogwashi-asaba formation (oligocenemiocene) was capped by the continental benin formation (miocene-recent) constituting the tertiary succession (figures 1 and 2). the ogwashiasaba formation consists of a succession of coarse-grained sandstone, clay and carbonaceous shale, containing continental lignite seam intercalations (kogbe, 1976; jan du chene et al., 1978). reyment (1965) suggested an oligocene-miocene age for the ogwashi-asaba formation, but palynological study by jan du chene et al. (1978) proposed a late eocene age for the base part. the lignite seams found within the ogwashi-asaba formation are usually brownish to black, varying in thickness from a few millimetres to a maximum of 6 meters. they are thinly laminated and fissile, having leaf and woody fragments on fresh cleats. the average overburden to figure 2. shows the stratigraphy of three basins in southern nigeriabenue trough, anambra basin and niger delta basin (modified from reyment, 1965). age formation lithology depositional environment basin quaternary benin formation ogwashi asaba formation ameki formation/nanka sand imo formation nsukka formation sandstones, clay, shales clay, shales, sandstones, lignite sandstones, cl ay, shales, limestone clay, shales, limestone, sandstone, marl sandstones, clays, shales, coals, limestone continental continental estuarine, shallow marine shallow marine, deltaic fluvio deltaic n ig er d el ta b as in te rt ia ry pliocene miocene oligocene eocene paleocene u pp er c re ta ce ou s maastrichtian ajali formation sandstones, claystones fluvio – deltaic mamu formation enugu/nkporo/owelli formation sandstones, clays, coals shales, sandstones, clay, ironstones, siltstones. shallow marine, deltaic shallow marine, deltaic campanian major unconformity santonian awgu formation sandstones, limestones, clays, coals, siltstones shallow marine, deltaic conician m id dl e c re t. turonian eze -aku formation shales, limestones, sandstones shallow marine cenomanian odukpani formation sandstones, limestones, shales shallow marine lo w er c re t. albian asu river group shales, limestones, sandstones shal low marine aptian major unconformity basement complex granites, gneisses, schists, migmatites igneous, metamorphic a na m br a b as in b en ue t ro ug h jude e. ogala154 and okpuzu rivers in ibusa and at the mgbala and ekete springs in okpanam and nnewi, respectively. lignite seams are also exposed along river valleys at the nkpunkpa and iyiodo springs as well as nnem-agadi river, all occurring in obomkpa. the lignite beds are brownish to black and vary in thickness from a few millimetres to a maximum of 2 meters. they are thinly laminated and fissile with leaf and woody fragments on fresh cleats. the lithological sequence of lignite seam outcropping along the nnem-agadi river was logged (figure 3); seven lignite samples were collected: three samples from obomkpa, two from ibusa and one each from okpanam and nnewi. analytical procedure samples (0.25 g) were powdered in an agate mortar and determined by a combination of methods to investigate lignite chemical composition (major and trace elements), i.e. fusion with inductively-coupled plasma (fus-icp) and total digestion with inductively-coupled plasma (tdicp). loss on ignition (loi) caused by escaping volatiles was measured after heating to about 1,050oc. limits of detectable measurement (ldm) for major elements were 0.01 for sio2, al2o3, fe2o3(t), mgo, cao, na2o, k2o and p2o5, 0.001 for mno and tio2 and 0.01% for s. ldm for trace elements were 2 ppm for ba, sr, zr, bi, te, 1 ppm for y, sc, be, co, cr, cu, ga, hg, ni and zn, 5 ppm for v, sb, tl and w, 0.3 ppm for ag and cd, 10 ppm for u, 3 ppm for as and pb and 4 ppm for sc coal data was calculated on an ash basis (value in ash = element value in coal × 100/ash yield, where ash yield = 100 – loi). major oxides and selected trace elements were analysed by the fusion technique (table 1). samples were mixed with a flux of lithium metaborate and lithium tetraborate and fused in an induction furnace. the molten melt was immediately poured into a 5% nitric acid solution containing an internal standard and mixed continuously until becoming completely dissolved (c. 30 minutes). the samples were run for major oxides and selected trace elements using a combination fusion technique (fus-icp) followed by inductively-coupled plasma-optical emission spectrometry (icp-oes) analysis using simultaneous/sequential thermo jarrell-ash enviro ii icp. a 0.25 g sample was digested with four acids for trace element analysis (table 2), beginning with hydrofluoric acid (hf), followed by a mixture of nitric (hno3) and perchloric acid (hclo4), heated using precise figure 3. lithologic column of outcrop along nnem-agadi river at obomkpa. lignite ratio is 6:1, thereby appearing to rule out open-cast mining (da swardt and piper, 1957). ogwashi-asaba lignite outcrops along river valleys and streams/ springs in ibusa (okpunzu and mgbiligba river), okpanam (mgbala spring), obomkpa (nnem-agadi river, iyiodo and nkpunkpa springs) and nnewi (ekete spring) (fig. 1). analysis methods field work and sampling field work for the present study took place in outcrops along river valleys, streams and springs in seven localities (fig. 1). lignite seams do not easily outcrop in the field but good exposure was found along the mgbiligba element detection limit analysis method location 1 location 2 location 3 location 4 location 5 location 6 location 7 sio2 0.01 fus-icp 0.39 0.04 3.94 1.04 9.38 9.78 2.99 al2o3 0.01 fus-icp 0.56 0.65 2.47 1.31 6.4 2.13 2.7 fe2o3t 0.01 fus-icp 0.05 1.22 0.13 0.21 0.42 0.38 0.17 mno 0.001 fus-icp 0.001 0.004 0.002 0.004 0.002 0.004 0.001 mgo 0.01 fus-icp 0.02 0.02 0.04 0.04 0.04 0.11 0.02 cao 0.01 fus-icp 0.08 0.08 0.09 0.15 0.05 0.43 0.05 na2o 0.01 fus-icp < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 k2o 0.01 fus-icp < 0.01 < 0.01 0.02 0.01 0.04 0.03 0.02 tio2 0.001 fus-icp 0.016 0.017 0.095 0.025 0.299 0.07 0.095 p2o5 0.01 fus-icp < 0.01 < 0.01 < 0.01 < 0.01 0.02 < 0.01 < 0.01 loi fus-icp 97.45 96.84 91.02 95.92 82.8 87 92.8 total 0.01 fus-icp 98.6 98.88 97.82 98.71 99.45 99.94 98.86 ash yield fus-icp 2.55 3.16 8.98 4.08 17.2 13 7.2 table 1. major element concentration in weight percent (wt. %) for lignites of the ogwashi-asaba formation. the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria 155 programmer-controlled heating in several ramping and holding cycles thus drying the samples. after dryness was attained, samples were brought back into solution using hydrochloric acid. the sample solution was then analysed for element concentration using varian vista pro inductively-coupled plasma-optical emission spectrometry (icp-oes). usgs and canmet certified reference materials were used for calibration. the activation laboratories in ancaster, canada, performed the chemical analysis. statistical analysis the elements were subjected to univariate (minimum, maximum, mean and standard deviation, correlation coefficient) and multivariate statistical analysis (factor and principal component) using statistical package for social sciences (spss) software (version 16.0). since most parameters measured here were not normally distributed, spearman’s rank correlation was used to examine the correlation between elements (the correlation coefficient matrix measures how well each constituent’s variance can be explained by its relationship to the others). factor analysis (fa)/principal component analysis (pca) was applied to the experimental data standardised through z-scale transformation to avoid misclassification due to differences in the units of measurement. standardisation tends to increase the influence of variables whose variance is small and reduce the influence of variables whose variance is large (liu et al., 2003). factor analysis was applied to the data set to study the association of the trace elements and table 2. trace elements and sulphur concentration for lignites of the ogwashi-asaba formation (values are in parts per million except for s which is in weight percent); 1: low-rank coal world average after seredin and finkelman (2008). elements detection limit analysis method location 1 location 2 location 3 location 4 location 5 location 6 location 7 lrcwa1 ba 2 fus-icp 17 44 54 27 60 93 23 150.0 sr 2 fus-icp 5 6 12 7 22 20 10 110.0 y 1 fus-icp 34 68 57 42 33 18 13 8.6 sc 1 fus-icp < 1 < 1 4 2 7 6 4 4.1 zr 2 fus-icp 4 6 31 10 63 32 24 36.0 be 1 fus-icp 2 4 3 4 5 3 3 1.6 v 5 fus-icp < 5 < 5 47 23 197 45 33 25.0 ag 0.3 td-icp < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 0.1 as 3 td-icp < 3 < 3 < 3 < 3 < 3 < 3 < 3 8.3 bi 2 td-icp < 2 < 2 < 2 < 2 < 2 < 2 < 2 cd 0.3 td-icp 0.4 < 0.3 < 0.3 0.3 < 0.3 < 0.3 < 0.3 0.2 co 1 td-icp 7 5 13 8 9 6 6 5.1 cr 1 td-icp 3 2 12 6 31 16 15 16.0 cu 1 td-icp 1 1 21 3 22 12 5 16.0 ga 1 td-icp 2 1 5 2 14 2 5 5.8 hg 1 td-icp < 1 < 1 < 1 < 1 < 1 < 1 < 1 0.10 mo 1 td-icp < 1 < 1 1 < 1 2 < 1 < 1 2.2 ni 1 td-icp 17 8 28 22 21 17 10 13.0 pb 3 td-icp < 3 < 3 5 < 3 8 3 < 3 7.8 sb 5 td-icp < 5 < 5 < 5 < 5 < 5 < 5 < 5 0.84 s 0.01 td-icp 0.12 0.19 0.14 0.16 0.08 0.34 0.11 te 2 td-icp < 2 < 2 < 2 < 2 < 2 < 2 < 2 tl 5 td-icp < 5 < 5 < 5 < 5 < 5 < 5 < 5 0.68 u 10 td-icp < 10 < 10 < 10 < 10 < 10 < 10 < 10 2.4 w 5 td-icp < 5 < 5 < 5 < 5 < 5 < 5 < 5 1.1 zn 1 td-icp 176 52 105 54 29 36 17 23.0 v/(v+ni) 0.23 0.38 0.63 0.51 0.9 0.73 0.77 ni/co 2.43 1.6 2.15 2.75 2.33 2.83 1.67 v/cr 1.66 2.5 3.92 3.83 6.36 2.81 2.2 jude e. ogala156 extract the principal factors governing trace element distribution (lu et al., 1995). components having > 1 eigenvalue were selected to explain association amongst the measured variables. cluster analysis (ca) established site segregation and desegregation; such analysis is an unsupervised pattern recognition technique which uncovers a data set’s intrinsic structure or underlying pattern without previous knowledge concerning the data being available. this is to enable classification being made based on measured objects’ nearness or similarity, thereby helping to establish relationships among sites in the form of a dendrogram. hierarchical agglomerative ca involved normalising the data set by means of ward’s method (i.e. using euclidean distances as a measurement of similarity). ca was applied to the data set to group similar sampling sites (spatial variation) spread over the region. results and discussion the chemical composition of lignite the chemical composition of the lignite samples is presented in tables 1 and 2. thirty-seven parameters were measured, consisting of ten major elements, loss on ignition (loi) and 26 trace elements. sio2 (0.04-9.78%), al2o3 (0.56-6.40%) and fe2o3 (0.05-1.22%) were the dominant oxides in most lignite samples; mgo, mno, cao, tio2, na2o, k2o and p2o5 all occurred in traces. loi had high values (82.8097.45%). the low to moderate sio2, al2o3 and fe2o3 concentration was due to quartz, feldspar and pyrite constituting the lignite samples’ main mineral phase (adamidou et al., 2007; kalaitzidis et al., 2010). the low sio2 (< 10%) and al2o3 content (< 7%) suggested very limited detrital input during coal formation. the relatively low cao (0.05-0.43%) and mgo content (0.02-0.11%) implied a continental origin for the lignite (olobaniyi and ogala, 2011); this agreed with the ogwashi-asaba formation’s continental setting (reyment, 1965; kogbe, 1976; jan du chene et al., 1978). the most abundant trace element was zn (average 67 ppm), followed by v, ba, y and zr (50.71 ppm, 45.43 ppm, 37.86 ppm and 24.29 ppm average values, respectively) (tables 2 and 6). the average mean values for sr, sc, be, cd, co, cr, cu, ga, mo, ni and pb ranged from 0.2 ppm to 18 ppm, ag, as, bi, hg, sb, te, tl, u and w concentration being below the detection limit in all samples. compared to average worldwide coal concentration (seredin and finkelman, 2008), most trace element content was lower than world average values for low-ranking coal due to low ash yield (table 2). y, be, v, co and zn were the only trace elements having relatively higher concentrations in the coal samples used here than the world coal average. redox-sensitive trace element ratios (ni/co, v/cr and v/v+ni) are usually considered powerful geochemical indicators for environmental disv /c r 5 8 6 4 2 0 0 5 10 15 20 ni/co v /( v + n i) 1 0,9 0,8 0,7 0,6 0,5 0 5 10 15 20 ni/co oxic oxicdysoxic dysoxic su bo xi c/ an ox ic suboxic/anoxic suboxic/anoxic d ys ox ic o xi c eu xi ni c an ox ic d ys ox ic (a) (b) figure 4. cross plots of redox-sensitive trace metal ratios (a) v/cr vs. ni/co; (b) v/(v+ni) vs. ni/co. ranges for v/cr and ni/co are from jones and manning (1994); ranges for v/(v+ni) are from hatch and leventhal (1992). element n=7 range minimum maximum mean standard deviation sio2 9.74 .04 9.78 3.937 4.099 al2o3 5.84 .56 6.40 2.317 1.990 fe2o3t 1.17 .05 1.22 .369 .398 mno .003 .001 .004 .003 .001 mgo .09 .02 .11 .041 .032 cao .38 .05 .43 .133 .135 na2o .000 < 0.01 < 0.01 < 0.01 .000 k2o .031 < 0.01 .040 .020 .012 loi 14.65 82.80 97.45 91.976 5.473 table 3. compositional average and standard deviation of major oxides for lignites of the ogwashi-asaba formation. the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria 157 crimination (lewan, 1984; hatch and leventhal, 1992; jones and manning, 1994; hoffman et al., 1998; rimmer, 2004; rimmer et al., 2004; algeo and maynard, 2004; johnson et al., 2010; saez et al., 2011). jones and manning (1994) suggested that < 5 ni/co ratios inferred oxic conditions, 5-7 dysoxic conditions and > 7 suboxic to anoxic conditions. they also used < 2 v/cr ratios to infer oxic conditions, 2-4.25 for dysoxic conditions and > 4.25 for suboxic to anoxic conditions. lewan (1984) showed that v/v+ni should be greater than 0.5 for organic matter accumulated in euxinic conditions. hatch and leventhal (1992) compared v/v+ni ratios to other geochemical redox indicators, including degree of pyritisation, and suggested that ratios greater than 0.84 showed euxinic conditions, 0.54-0.82 anoxic water and 0.46-0.60 for dysoxic conditions. according to calvert and pedersen (1993), jones and manning (1994), hoffman et al (1998) and algeo and maynard (2004) anoxic environments are characterised by high v/cr ratios and v/v+ni values lying between 0.5 and 0.9 because of the disparate behaviour of v, ni and cr during redox processes in marine environments characterised by fine-grained detritic sedimentation. all the samples analysed and shown on the v/cr cf ni/co and v/ v+ni cf ni/co diagram (figure 4) plotted within the oxic ranges for the four redox-sensitive trace element ratios proposed by jones and manning (1994). figure 5. box plots showing the distributions of major elements. c on ce nt ra ti on (% ) 0,020 0,015 0,010 0,005 0 mno p2o5 ly io do _s pr in g c on ce nt ra ti on (% ) oxides 10 8 6 4 2 0 sio2 al2o3 fe2o3t r iv er _m gb ili gb a ly io do _s pr in g c on ce nt ra ti on (% ) oxides 0,5 0,4 0,3 0,2 0,1 0 mgo cao k2o tio2 r iv er _m ne m _a ga di r iv er _m ne m _a ga di ly io do _s pr in g table 4. correlation coefficients between the major elements in lignite samples from ogwashi-asaba formation sio2 al2o3 fe2o3t mno mgo cao k2o tio2 p2o5 ash_yield sio2 1.000 al2o3 .744 1.000 fe2o3(t) -.122 -.127 1.000 mno .075 -.261 .577 1.000 mgo .736 .126 -.077 .503 1.000 cao .513 -.166 -.016 .581 .951 1.000 k2o .955 .902 -.111 -.099 .505 .240 1.000 tio2 .717 .993 -.084 -.288 .077 -.214 .885 1.000 p2o5 .585 .905 .057 -.180 -.020 -.270 .755 .937 1.000 ash_yield .963 .889 -.100 -.030 .547 .276 .994 .870 .739 1.000 jude e. ogala158 table 5. unrotated component matrix from loadings of 9 major elements and loi on three significant factors. elements factor 1 factor 2 factor 3 sio2 0.929 0.341 -0.105 al2o3 0.935 -0.316 0.080 fe2o3t -0.127 0.193 0.932 mno -0.103 0.764 0.544 mgo 0.451 0.868 -0.203 cao 0.175 0.956 -0.204 k2o 0.991 0.063 -0.028 tio2 0.923 -0.361 0.123 p2o5 0.823 -0.386 0.301 loi -0.988 -0.114 0.011 eigen values 5.485 2.801 1.372 % of variance 54.847 28.008 13.716 cumulative % 54.847 82.855 96.571 major element geochemistry-correlation matrix table 3 shows the composition average and standard deviation for the lignite samples from the ogwashi-asaba formation analysed here. the data set was screened to identify outliers (figure 5). the other elements were not normally distributed, except for sio2, k2o and mno; a non-parametric spearman test was thus used which applies variables’ ranking order in determining the correlation coefficient. table 4 shows major elements’ correlation coefficients. there was a strong positive relationship between mgo, k2o, tio2 and p2o5 with sio2, al2o3 and ash yield, suggesting a common detrital source for these elements; the strong positive statistical correlation between al2o3 and tio2 (r= 0.993) was typical of elements which probably came from clastic rocks. factor analysis of the major elements studied here gained statistical information on the grouping and hence the processes responsible for their formation. the variance/covariance and factor loadings of the variables having eigenvalues were computed; factor analysis results are summarized in a factor matrix (table 5). a 3-factor model was adopted, covering 96.6% of total variance. the first factor expresses 54.9% of the total data variance and showed high positive loading for elements sio2, al2o3, k2o, tio2, and p2o5 and negative loading for loi (table 6). the strong positive correlations between sio2, al2o3, and tio2 were typical of clay minerals. the loadings in factor 1 reflected sediments believed to have their source probably in the mafic basement complex (oban massif ) on the north-eastern flank of the niger delta basin (edet et al., 2003; elueze et al., 2009). the presence of tio2 and p2o5 was indicative of basaltic rocks of oceanic environment (ogala et al., 2009); such interpretation is supported by the strong positive correlation between tio2 and p2o5 (r= 0.937). the second factor expressed 28% of the total variance and showed high positive loadings with mno, mgo and cao. the grouping in factor 2 suggests a lithogeochemical input representative of weathering and composition of carbonate sediments. the close proximity of the limestone bands/nodules in the ameki formation within the niger delta basin in close proximity may be the possible source. the strong positive correlation between cao and mgo c om po ne nt 2 component 1 component 3 1,0 0,5 0,0 -0,5 -1,0 mno cao p2o5 sio2 k2o mgo loi ii i iii fe2o3t al2o3 -1,0 -1,0 -0,5 -0,5 0,0 0,0 0,5 0,5 1,0 1,0 figure 6. principal component analysis for the first 3 components for oxides. figure 7. the results of cluster analyses on the major elements contained in the lignite samples from ogwashi-asaba formation. the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria 159 trace elements trace elements ba bezr pby cdcr ssr gacu cosc scni zn river_okpuzu lyiodo_spring lyiodo_spring lyiodo_spring mgbala_spring river_nnem_agadiriver_okpuzu figure 8. box plots showing the distribution of trace elements. 200 150 100 50 0 12.5 10.0 7.5 5.0 2.5 0.0 c on ce nt ra ti on (p pm ) c on ce nt ra ti on (p pm ) element n=7 range minimum maximum mean s.d. ba 76.00 17.00 93.00 45.43 26.46 sr 17.00 5.00 22.00 11.71 6.80 y 55.00 13.00 68.00 37.86 19.76 sc 6.01 .99 7.00 3.57 2.37 zr 59.00 4.00 63.00 24.29 20.63 be 3.00 2.00 5.00 3.43 .98 v 192.01 < 5 197.00 50.71 66.72 ag .00 < 0.3 < 0.3 < 0.3 .00 as .00 < 3 < 3 < 3 .00 bi .00 < 2 < 2 < 2 .00 cd .11 .29 .40 .31 .04 co 8.00 5.00 13.00 7.71 2.69 cr 29.00 2.00 31.00 12.14 10.02 cu 21.00 1.00 22.00 9.29 9.14 ga 13.00 1.00 14.00 4.43 4.50 hg .00 < 1 < 1 < 1 .00 mo 1.01 < 1 2.00 1.14 .38 ni 20.00 8.00 28.00 17.57 6.95 pb 5.01 < 3 8.00 3.99 1.92 sb .00 < 5 < 5 < 5 .00 s .26 .08 .34 .16 .09 te .00 < 2 < 2 < 2 .00 tl .00 < 5 < 5 < 5 .00 u .00 < 10 < 10 < 10 .00 w .00 < 5 < 5 < 5 .00 zn 159.00 17.00 176.00 67.00 55.71 table 6. compositional average and standard deviation of trace elements for lignites of the ogwashi-asaba formation. jude e. ogala160 table 7. correlation coefficients between trace elements and ash yield in lignite samples from ogwashi-asaba formation. table 8. unrotated component matrix from loadings of 17 trace elements on four significant factors. table 9. varimax rotated component loadings of 17 trace elements on four significant components explaining 93.80% of the total variance. ba sr y sc zr be v cd co cr cu ga mo ni pb s zn ash_ yield ba 1.000 sr .807 1.000 y -.097 -.412 1.000 sc .705 -973 -.503 1.000 zr .568 .918 -.302 .940 1.000 be .243 .423 .246 .381 .539 1.000 v .387 .803 -.195 .798 .940 .678 1.000 cd -.507 -.468 -.079 -.511 -.467 -.629 -.322 1.000 co .093 .204 .294 .266 .398 .054 .332 -.114 1.000 cr .490 .903 -.453 .943 .981 .521 .932 -.432 .274 1.000 cu .602 .795 -.026 .814 .886 .358 .767 -.432 .729 .791 1.000 ga .188 .679 -.198 .722 .901 .596 .963 -.262 .397 .903 .741 1.000 mo .245 .668 -.104 .639 .830 .709 .968 -.186 .220 .831 .620 .939 1.000 ni .209 .297 .139 .311 .377 .056 .338 -.011 .902 .274 .663 .316 .224 1.000 pb .298 .667 .061 .662 .874 .624 .945 .255 .551 .817 .829 .947 .925 .477 1.000 s .293 .242 -.125 .130 -.147 -.196 -.330 -.224 -.350 -.173 -.099 -.539 -.428 -.132 -.465 1.000 zn -.386 -.500 .279 -.546 -.463 -.653 -.384 .863 .278 -.522 -.198 -.317 -.298 .295 -.178 -.196 1.000 ash_ yield .728 .987 -.380 .981 .969 .469 .870 -.475 .291 .952 .851 .780 .741 .338 .761 .091 -.491 1.000 elements factor 1 factor 2 factor 3 factor 4 ba .557 -.536 .536 .114 sr .897 -.323 .218 -.172 y -.216 .395 .060 .817 sc .909 -.288 .182 -.219 zr .990 -.026 .028 -.105 be .636 -.089 -.453 .548 v .957 .130 -.197 -.075 cd -.508 .498 .007 -.596 co .421 .660 .511 .217 cr .963 -.091 -.087 -.224 cu .882 .185 .390 .073 ga .907 .277 -.286 -.123 mo .858 .209 -.363 -.068 ni .412 .547 .618 .094 pb .900 .384 -.106 .063 s -.165 -.763 .557 .057 zn -.487 .686 .342 -.276 eigen values 9.230 3.002 2.071 1.644 % of variance 54.295 17.660 12.183 9.669 cumulative % 54.295 71.954 84.138 93.806 elements component 1 component 2 component 3 component 4 ba 0.276 0.872 0.164 0.185 sr 0.779 0.606 0.084 0.070 y -0.437 -0.316 0.500 0.577 sc 0.818 0.560 0.072 0.035 zr 0.920 0.292 0.195 0.152 be 0.507 -0.070 -0.040 0.809 v 0.954 0.025 0.151 0.209 cd -0.220 -0.478 0.028 -0.764 co 0.242 -0.048 0.928 0.008 cr 0.958 0.260 0.034 0.088 cu 0.691 0.340 0.600 0.129 ga 0.961 -0.151 0.162 0.152 mo 0.913 -0.164 0.074 0.226 ni 0.235 0.102 0.883 -0.114 pb 0.859 -0.117 0.406 0.239 s -0.386 0.866 -0.152 -0.009 zn -0.374 -0.399 0.474 -0.614 eigen values 7.898 3.028 2.798 2.223 % of variance 46.46 17.81 16.45 13.07 cumulative % 46.46 64.27 80.72 93.80 the geochemistry of lignite from the neogene ogwashi-asaba formation, niger delta basin, southern nigeria 161 (r= 0.951) indicates the presence of dolomite in the lignite samples (yazdi and shiravani, 2004). finally, the third factor expresses 13.7% of the total variance and shows high positive loadings with fe2o3. the high positive loading of fe2o3 in factor 3 confirms the presence of iron bearing minerals such as pyrite. also the moderate positive correlation between fe2o3 and mno (r= 0.577) indicates that the element is present in carbonates, probably siderite. principal component analysis extraction method was further applied to characterize the geochemistry of major elements. the elements were plotted in rotated space yielding three components (fig. 6) which explained 97% of the total data variance. the chemical compositions were categorized to three groups (fig. 7) by cluster analysis. according to horner and krissek (1992), cluster analysis is a powerful tool that helps in the identification of groups with similar samples, while principal component analysis aids in the identification of elements (variables) that are responsible for the similarities or differences between groups of samples. the three components derived from factor analysis (fig. 6) correspond to the three cluster groups of major elements (fig. 7). cluster i comprises of al2o3, tio2, p2o5, sio2, and k2o, indicating that the majority of these elements were mainly of terrigenous origin. cluster ii comprises of mgo and cao, indicating sediments of carbonate origin, while cluster iii comprises of fe2o3, mno and loi indicating affiliation to epigenetic minerals such as pyrite and siderite. trace element geochemistry-correlation matrix table 6 shows the average composition and standard deviation of the trace elements in the lignite sample. the data set was screened to identify outliers (figure 8). among the trace elements analysed, zn, cd, ga, mo, pb, s and co were not normally distributed. spearman’s rank correlation determined the correlation coefficient between trace elements and ash yield (table 8). most trace elements had strong positive correlation coefficients with ash yield and with each other (table 7); this indicated a common detrital source for most of these elements. y, be, cd, co, ni, s and zn poorly correlated with ash yield (0.380 < r < 0.469) which could be explained by organic affinity and these elements’ association with sulphide figure 9. principal component analysis for the first 3 components for trace elements. figure 10. the results of cluster analyses on the trace elements contained in the lignite samples from ogwashi-asaba formation. c om po ne nt 2 component 1 component 3 1,0 0,5 0,0 -0,5 -1,0 ni cd co y zn ii i iii -1,0 -1,0 -0,5 -0,5 0,0 0,0 0,5 0,5 1,0 1,0 s ba sr cr sc cu zr be mo pb ga phases. the poor correlation of s with cu (r = 0.099), zn (r = 0.196), cd (r = 0.224), ga (r = 0.539), pb (r = 0.465), sr (r = 0.242), sc (r = 0.130), y (r = -0.125), zr (r = 0.147), be (r = 0.196), v (r = 0.330), co (r = 0.350), cr (r = 0.173), ni (r = 0.132) and mo (r = 0.428) indicated that sulphur was not only present in its sulphide form but also as an organic form (finkelman, 1995). there were highly significant correlations (r ≥ 0.90) between the elements in the following couples: sr and sc, sr and zr, sr and cr, sc and zr, sc and cr, zr and v, zr and cr, zr and ga, v and cr, v and ga, v and mo, v and pb, co and ni, cr and ga, ga and mo, ga and pb and mo and pb (table 7). this result was consistent with that recorded by spears and zheng (1999) and was interpreted as being essentially associated with detrital minerals and clay minerals. fa applied to the correlation matrix initially produced overlaps in ba, be, co, ni and cd (table 8), thereby necessitating further analysis. jude e. ogala162 this was achieved by rotating the axis, thereby producing a new set of factors; this was attained after 11 iterations (table 9). each factor involved a sub-set of the original variables with as much minimal overlap as possible (table 9). since the differences between the unrotated and rotated components were significantly different, the rotated option was used for further analysis. a 4-factor model having 93.8% total cumulative variance was obtained (table 9). principal component 1 (pc1) expressed 46.5% of total data variance and showed high positive loadings with sr, sc, zr, v, cr, cu, ga, mo and pb (table 9). the inferred host phases for pc1 were a mixture of lithophile (sr, sc, zr, v and cr; r = 0.803-0.973) and chalcophile (cu, ga, mo and pb; r = 0.620-0.829). the high positive loading of pb in pc1 may have resulted from processes which led to the emplacement of pb-zn mineralisation in the lower benue trough (akande and erdtmann, 1998; ogala et al., 2010b). the positive loading of cu in both pc1 and pc3 indicated dual sources contributing to the presence of cu. further analysis using ca (figure 10) confirmed that cu belonged to pc1. pc1 had a high positive loading with zr (table 9), indicating a concentration of resistant heavy minerals, such as zircon. principal component 2 (pc2) expressed 17.8% of total data variance and had the highest loadings with ba, sr and s. according to finkelman (1995) and christanis et al., (1998), ba and s have a strong organic affiliation, while sr indicates a carbonate origin. principal component 3 (pc3) expressed 16.5% of total data variance and showed positive loading with co, cu and ni. it could be inferred from the data obtained that this group of elements was associated with fe-sulphide (pyrite). the reported occurrence of chalcopyrite (cufes2) in the abakaliki area of the lower benue trough cannot be ruled out due to the presence of cu (elueze et al., 2009). principal component 4 (pc4) expressed 13% of total variance and showed positive loading with y and be and negative loading for cd and zn. pc4 was made up of a mixture of lithophile (be and y) and chalcophile (cd and zn), suggesting that the elements were associated with silicates and sulphides, respectively. the trace elements were plotted in rotated space yielding three components (fig. 9). ca established trace element association and origin (fig. 10); this clustering (fig. 10) coincided with the results obtained from studying the correlation coefficient matrix (table 7) and pca (fig. 9). cluster 1 consisted of elements associated with detrital input (i.e. zr, cr, sr, sc, cu, v, mo, ga, pb and be) including quartz, clay and accessory minerals such as zircon and rutile commonly representing inorganic constituents (finkelman, 1995). these elements’ strong positive correlation with each other (table 7) and with ash yield indicated a common terrigenous origin. cluster 2 was made up of ba and s, suggesting a strong organic association. the positive statistical correlation between ba and s (r = 0.693) also supported an organic association for these elements. cluster 3 consisting of cd, zn, co, ni and y, indicated a strong association with sulphides such as pyrite (fes2), sphalerite (zns), chalcopyrite (cufes2) and galena (pbs). the occurrence of polymetallic sulphide lodes of pbs, zns and cufes2 within the lower benue trough cannot be ruled out for these elements (elueze et al., 2009). conclusion the geochemical characteristics of the ignite seams within the ogwashi-asaba formation were investigated; the main mineral phases contained in the lignite were quartz, feldspar and pyrite. the relatively low silica (sio2) and alumina (al2o3) content suggested very limited detrital input during coal 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(1993, 1995), from the analysis of rayleigh-wave dispersion. this analysis consisted of filtering and inverting the rayleigh-wave dispersion, to obtain the variation in the shear-wave velocity versus depth. nevertheless, the rayleigh-wave dispersion is dependent upon the propagation path of these waves, and it does not depend (primarily) upon the local crustal structure beneath the recording station (seismic station). as a consequence of this, the rayleigh-wave dispersion analysis could not be used to determine earth models (crustal models) beneath local areas (small areas), such as the area in the vicinity of the seismic station (malischewsky et al., 2010; tuan et al., 2011). at this point, the analysis (filtering and inversion) of the rayleighwave ellipticity gain importance, because sexton et al. (1977) have shown that the rayleigh-wave ellipticity depends primarily upon the local crust structure and that it does not exhibit azimuthal dependence, for a period range of 10 to 50 s. thus, for this period range, the observed ellipticity is primarily controlled by the local crustal geology beneath the seismic station, it does not depend upon the propagation path of the rayleigh waves. this basic property of the ellipticity means that the ellipticity analysis is a very useful tool for obtaining local crustal models, which can be used to determine the site and/or local effects in studies of seismic risk and/or seismic design (lee et al., 2003; rastogi et al., 2011). for this reason, it is very desirable to develop a methodology for the analysis of the rayleigh-wave ellipticity that can be used to determine the local crustal structure beneath the recording station (seismic station). this analysis consists of filtering and inverting rayleigh-wave ellipticity curves to obtain s-velocity models (variations in shear-wave velocity versus depth). thus, the goal of this paper is to provide a comprehensive explanation of the analysis (filtering and inversion) of the rayleigh-wave ellipticity, devoting particular attention to the inversion problem and the assumptions necessary to solve it. 128 table 1. near events recorded at the tol station (latitude 39.881 ºn, longitude -4.049 ºe). event (nº) date (day month year) origin time (hour min. sec.) latitude (ºn) longitude (ºe) magnitude (mb) 1 09 05 80 09 21 36.7 35.43 1.10 6.0 2 13 10 80 14 33 48.2 36.28 1.29 5.2 3 14 10 80 17 35 02.1 36.22 1.33 7.1 4 15 10 80 03 17 24.7 36.19 1.29 6.0 5 19 10 80 21 47 24.2 35.51 1.20 5.8 6 22 10 80 16 23 15.8 36.22 1.28 5.4 7 23 10 80 09 57 56.7 36.35 1.32 5.4 8 30 10 80 23 38 14.9 36.18 1.34 5.3 9 01 02 81 23 00 42.0 36.37 1.64 5.8 10 05 02 81 09 12 33.2 36.35 1.44 5.4 11 14 02 81 13 15 18.0 35.85 1.22 5.3 12 20 02 81 20 41 02.9 35.40 1.18 5.0 13 05 03 81 01 21 52.3 38.47 0.00 5.5 14 11 09 82 21 35 51.9 36.32 1.60 5.6 15 11 09 82 04 48 41.6 35.56 1.31 5.5 16 15 11 82 20 07 52.4 35.59 1.25 5.7 17 31 03 84 03 31 56.0 36.41 1.52 4.9 18 12 02 89 12 02 21.6 36.33 2.60 5.3 19 09 05 80 09 21 36.7 35.43 1.10 5.0 20 13 10 80 14 33 48.2 36.28 1.29 5.4 21 14 10 80 17 35 02.1 36.22 1.33 5.2 22 15 10 80 03 17 24.7 36.19 1.29 5.1 figure 1. frequency response of the wwssn seismograph. v. corchete 129 2. data in this type of study, the primary data used are seismograms. the seismograms used in this study correspond to 22 earthquakes (table 1) that occurred in the neighboring region of the iberian peninsula. these earthquakes were registered by the wwssn station tol located in iberia (latitude 39.881 ºn, longitude -4.049 ºe). the records of this station are considered to be suitable for this study, because the period range of best registration for the wwssn seismograph is between 5 and 50 s (the period range in which the amplitude spectrum is maximized), as seen in figure 1 (kulhánek, 1990). this period range (from 5 to 50 s) is most suitable for exploring the elastic structure of the earth for a depth range from 0 to 50 km, which coincides with the goal of this study. only earthquake traces in which there exist well-developed rayleigh wave trains with a very clear dispersion are considered to ensure the reliability of the results. logically, instrumental correction must be taken into account to eliminate the time lag introduced by the seismograph system and all distortions produced by the instrument (bath, 1982; kulhánek, 1990). this correction recovers the true amplitude and phase of the ground motion, allowing the analysis of the true ground motion attributable to the rayleigh wave. for this reason, all traces considered in this study have been corrected for the instrument response. 3. computation of the observed ellipticity it is known that the observed ellipticity of a rayleigh wave can be calculated using the formula (aki and richards, 1980; ben-menahem and singh, 1981) where and are the instrument-corrected spectral amplitudes of the longitudinal (also called radial) and vertical seismograms for the period t (sexton et al., 1977). nevertheless, the use of equation (1) poses some practical difficulties. the first difficulty of formula (1) is that the ellipticity values can be calculated only for the periods generated by the fast fourier transform (fft). thus, many values can be computed for short periods, but only a few values can be computed for long periods. the second important difficulty of formula (1) is the presence of spectral holes in , the spectral amplitude of the vertical seismogram. a spectral hole is a small value (near zero) that can exist, for some period t, in the spectral amplitude. this small value, when introduced into the denominator of a spectral ratio such as the ratio given by formula (1), produces a very large value (near infinite) for this ratio ( (t) → ∞ for some period t in which → 0). in a spectral hole, the formula (1) has no physical meaning and it cannot be applied. spectral holes can be produced by noise or other such undesirable perturbations that typically are present in all seismograms. because of the above-mentioned difficulties, the use of equation (1) directly is not suitable for a systematic computation; instead, a time-domain method, based on the multiple filter technique (mft) of dziewonski et al. (1969), is more suitable for the systematic computation of the observed ellipticity. thus, the time-domain method developed by sexton et al. (1977) is used here for the computation of the observed ellipticity, instead of the spectral ratio given by formula (1). figure 2 shows the seismograms recorded at the tol station for event 3 (listed in table 1). these records are used to illustrate the time-domain method applied in this study for the computation of the observed ellipticity. figure 3 shows the results obtained by applying the method developed by sexton et al. (1977), to the seismograms shown in figure 2. it should be noted that noise and other undesirable perturbations, which are always present in the data (figure 3c), can introduce errors into the computation of the observed ellipticity. figure 2. observed (instrument-corrected) seismograms corresponding to event 3 of table 1 recorded at the tol station. the three components are: north-south (n), east-west (e) and vertical (z). figure 4. (a) observed instrument-corrected seismogram (vertical component) corresponding to event 3 of table 1 recorded at the tol station. (b) group velocity dispersion curve obtained after the application of the mft to the seismogram shown in figure 4a. (c) time-variable filtered seismogram. figure 3. (a) observed (instrument corrected) seismograms corresponding to event 3 of table 1 recorded at the tol station. the longitudinal (radial) seismogram (l component) is computed from the horizontal seismograms (north-south and east-west seismograms) shown in figure 2. (b) observed ellipticity computed using the timedomain method developed by sexton et al. (1977). (c) rayleigh-wave particle motion. a methodology for the analysis of the rayleigh-wave ellipticity (1) 130 the majority of this noise and undesirable perturbations can be removed from the data (seismograms) using the time variable filtering (tvf) technique. tvf is a filtering technique that produces a smooth seismogram (a time-variable-filtered seismogram), in which all effects of noise, higher modes and other undesirable perturbations have been removed (cara, 1973). this technique can be used in combination with the mft, as described by corchete et al. (2007), to obtain better results than those obtained using the mft only. the use of tvf in this study is illustrated in figure 4, in which the vertical seismogram shown in figure 2 has been time-variable filtered. this technique was applied to all seismograms used in this study to compute the observed ellipticity, as shown in figure 5. a comparison between figures 3 and 5 illustrates the improvement achieved by the combination of the mft and tvf (figure 5) versus the application of the mft only (figure 3). the particle motion (ground motion) attributable to the rayleigh wave is much clearer in figure 5c than in figure 3c, which confirms that the observed ellipticity presented in figure 5b is more reliable than the one presented in figure 3b. once the ellipticity curve (observed ellipticity) for the seismograms of each event listed in table 1 has been calculated using the above-described combination of filtering techniques (mft and tvf), an average ellipticity curve (figure 8c) can be obtained by computing the median of the 22 ellipticity curves calculated for the 22 events listed in table 1. an estimation of the error on this average ellipticity curve is obtained by computing the standard deviations (1-s error) of the values involved in the calculation of this median. 4. inversion of the observed ellipticity theoretical problem. once the rayleigh-wave ellipticity (observed ellipticity) has been obtained, as described in the section above, a relation between the ellipticity and the earth model (given by its characteristic parameters d: layer thickness, a: p-wave velocity, b: s-wave velocity and r: density) must be established. fortunately, this relation exists (and is well known) through the rayleigh-wave amplitudes in the form (ben-menahem and singh, 1981) = u(z = 0) /w(z = 0) where u(z) and w(z) are the horizontal (also called radial or longitudinal) and vertical amplitudes, respectively. these amplitudes are calculated from an earth model as functions of the depth z, for a given period t, using the forward modelling method described by abo-zena (1979) and aki and richards (1980). thus, the theoretical ellipticity (known as the surface ellipticity) defined at z = 0 by formula (2) can be related to an earth model through the general relation i = 1, 2, …, m where i is the ith value of the period t (in which the observed ellipticity is calculated), m is the number of values for the period t and n is the number of layers for the earth model under consideration. this layered model is defined by the following parameters: (layer thickness), (pwave velocity), (s-wave velocity) and (density), where the subscript j corresponds to the jth layer of this model. linearization of the problem. equation (3) is a set of m no-linear equations with 4*n unknowns that express the existing relation between the ellipticity and the earth model. nevertheless, the inverse relation of equations (3) is needed to obtain the values of the model parameters ( , , and for j varying from 1 to n) from the observed data ( i for i varying from 1 to m). this inverse relation can be very difficult to obtain from equations (3) in absence of any simplification. the simplification approach used in this study is the method developed by dorman and ewing (1962). this method allows the linearization of equations (3) using the partial derivatives of the ellipticity. these partial derivatives are introduced into the problem through the definition of the ellipticity differential, given by i = 1, 2, …, m where n (= 4*n) is the total numbers of model parameters and pj is a model parameter (with value ). then, if an initial model is prepared to produce a theoretical ellipticity curve that is as close as possible to the observed curve, the difference between the theoretical and observed ellipticities can be written as i = 1, 2, …, m where is the observed ellipticity obtained for the ith value of the period t, is the theoretical ellipticity calculated using formula (3) and dei is computed using formula (4). it should be noted that equations (4) and (5) provide an approximate linear relation between the ellipticity and figure 5. (a) time-variable-filtered seismograms computed from the seismograms shown in figure 2, as illustrated in figure 4. the longitudinal (radial) seismogram (l component) is computed from the time-variable filtered seismograms (horizontal seismograms). (b) observed ellipticity computed using the time-domain method developed by sexton et al. (1977). (c) rayleigh-wave particle motion. figure 6. (a) shear velocity distribution as a function of depth for the initial model described in table 2. (b) theoretical ellipticity calculated from the initial model (described in table 2), by means of forward modelling. v. corchete (2) (3) (4) (5) 131 the earth model, when only small perturbations ( ) of model parameters are considered. in this case, a linear relation between the values of the model parameters ( , , and for j varying from 1 to n) and the observed data (ei for i varying from 1 to m) can be written in the form i = 1, 2, …, m equations (6) is a set of linear equations that can be inverted to obtain the values of the model parameters from the observed data. nevertheless, when the total number of parameters n that are taken into consideration is very large, a precise solution for the inverse problem can be very difficult to obtain. for this reason, the total number of parameters must be reduced to the minimum necessary parameters. the total number of parameters can be reduced when only the s-wave velocities are considered in formula (6), i. e. and n = n. thus, the perturbations ( ) for the p-wave velocity and density are not considered in formula (6) because surface waves are not very sensitive to them (dorman and ewing, 1962). under this final assumption, equation (6) can be inverted if the partial derivatives are evaluated numerically. this numerical evaluation of the partial derivatives can be performed using the approximate relation given by where i is calculated using formula (3) and is a small perturbation ( << ). inversion scheme. once the forward problem has been linearized using (6) and (7), the inversion of (6) or, equivalently, the inversion of the matrix relation y = ax must be performed, where y = , a is given by (7) and x = figure 7. steps followed in the process of inverting the observed ellipticity to obtain the s-velocity model. the circles are used to denote computation steps, and the rectangles are used to denote the results obtained. the triple-outlined rectangles are used to indicate the initial data and the final result. a methodology for the analysis of the rayleigh-wave ellipticity (7)(6) (8) 132 . the inverse of the matrix a in relation (8) can be calculated using (dimri, 1992) where a is the damping parameter or damping factor and i is the identity matrix. the addition of this factor to the main diagonal of matrix ata increases the magnitude of its eigenvalues, thereby avoiding the problems caused by small or zero eigenvalues, which can cause the matrix ata to be singular or nearly singular. when the relation called the singular value decomposition (svd) of a a = uplpvp t is substituted into formula (9), b can be written as where the matrices and are described in detail by aki and richards (1980) and dimri (1992). the matrix b defined by formula (10) is called the damped generalized inverse. then, the linear equations given by (8) can be inverted to obtain x using the expression given in (10) as follows where it should be noted that the product of matrices b and a is called the resolution matrix, and it is obtained as follows if the resolution matrix given by (12) is the identity matrix, then the solution obtained in this manner is unique and the resolution is perfect. nevertheless, it should be noted that the addition of the factor a to the main diagonal ensures that the equality ba = i is never reached when the damping factor a is not zero. therefore, the particular solution that is obtained is never the true solution. the obtained solution is instead a smoothed solution because each model parameter is a weighted mean of the parameters x that define the true model. for this reason, the factor a must be as small as possible to yield the best possible solution, i.e., the particular solution that is as near as possible to the true solution. the error on the model parameters is given by the corresponding variance (assuming that the data are statistically independent and that the same is true for the model parameters) as follows where and are the variances of the model parameters and the data, respectively. the inversion scheme proposed in this paper is defined by (10) and it introduces the damping parameter, which degrades the resolution but stabilizes the obtained solution by reducing the variance, as seen in (12) and (13). thus, an increase in the value of a reduces the error in the determination of the model parameters at the expense of the degradation of the resolution. on the other hand, the inverse given by (10) uses the svd method instead of the least-squares method, which is used by dorman and ewing (1962), because the svd method provides higher computation accuracy (keilis-borok, 1989). inversion process. a necessary step that must precede the inversion process is the preparation of an initial model that contains all available information for the study region, including the s-wave, p-wave and density distributions as functions of depth. in this study, this information was obtained from the references provided and is described in section 1. all this information was considered in the preparation of the initial model used to invert the ellipticity curve shown in figure 8c, which was obtained as described in section 3. the initial model must be prepared to produce a theoretical ellipticity curve that is as close as possible to the observed curve, because the inversion scheme described above considers only small perturbations in the model parameters involved. this method simply conditions the initial model, which is chosen to be as similar as possible to the true earth structure, as represented by all geophysical information available for the study region. the initial model prepared here is described in table 2, and its shear-wave velocity distribution as a function of depth is plotted in figure 6a. the calculated theoretical ellipticity is shown in figure 6b. this curve was obtained using the forward-modelling method described by abo-zena (1979) and aki and richards (1980). table 2. initial earth model considered in this study (a: p-wave velocity, b: s-wave velocity, r: density). thickness (km) α (km/s) β (km/s) ρ (g/cm3) 2 3.30 2.50 2.28 3 5.60 3.19 2.74 5 6.10 3.48 2.79 5 5.60 3.18 2.79 10 6.40 3.58 2.80 5 6.85 3.90 3.05 60 8.00 4.50 3.20 80 8.15 4.40 3.40 130 8.49 4.77 3.53 8.81 4.89 3.60 once the initial model has been prepared, the inversion of the observed ellipticity curve (figure 8c) can be performed to obtain the corresponding s-velocity model. a computer program based on the above-described inversion scheme, which performs the computation steps shown in the flow chart displayed in figure 7, was used for this task. the results of this computer program are shown in figure 8. the s-velocity model shown in figure 8a is the final obtained model, which was improved iteratively. the iterative improvement in the model is assessed by comparison between the observed ellipticity, considered to be the observed data, and the theoretical ellipticity calculated from the actual model via forward modelling. if each successive curve is closer to the observed ellipticity curve than the previous iteration, the process converges. when the theoretical curve falls within the vertical error bars of the observed data, as seen in figure 8c, the inversion process is considered to be finished because the obtained shear-velocity model describes the observed curve within its experimental error. the resolving kernels shown in figure 8b indicate the reliability of the solution obtained for the considered inverse problem. the resolving kernels are the row vectors of the resolution matrix given by formula (12), and therefore, they are a measure of the exactness of the solution of the inverse problem. good agreement between the calculated solution and the true solution (which implies the reliability of the estimated solution) is obtained when the absolute maxima of these functions lie at the reference depths. these reference depths are defined as the median depths of each layer considered in the model. the reliability of the solution is also related to the widths of these absolute maxima. the solution identified for the inversion problem is more reliable when the maxima of these resolving kernels are narrower. to v. corchete (9) (10) (11) (12) (13) 133 achieve this goal and improve the resolution, the number of model layers considered for the inversion must be reduced to be the minimum number of layers that are necessary to obtain a detailed shear-velocity distribution as function of depth, with sufficient precision to describe the observed data within the experimental error (singh et al., 2011). in this case, the number of layer considered for the inversion (i. e., the total number of model parameters to be perturbed) is n = 9. if a large number of layers are considered, then the number of unknowns is increased but the amount of data remains the same, and the resolution is degraded as a result (corchete et al., 2007). this effect manifests as resolving kernels with wider absolute maxima and in the lack of coincidence between the maxima of these functions and the reference depths. another method of improving the resolution is to consider layers with thicknesses that increase with increasing depth, as the resolution worsens. this effect is apparent in figure 8b: the resolving kernels worsen as the depth increases. the layer thickness considered in our model increases with increasing depth to avoid this effect as much as possible. thus, the shallow layers are thinner than the deeper layers, which are thicker (figure 8a). figure 8. (a) shear-wave velocity model obtained after the inversion of the observed ellipticity curve. the horizontal bars represent the standard deviation computed for each layer considered in this inversion process (9 layers). (b) resolving kernels of the inversion problem posed. the reference depths are indicated with vertical bars at the median depth of each layer considered. (c) theoretical ellipticity curve calculated from the final model plotted in figure 8a (continuous line). the dots and line denotes the observed ellipticity, and the vertical bars represent the standard deviation of the ellipticity (1-s errors). 5. conclusions the results presented in this paper demonstrate that the analysis of rayleigh-wave ellipticity is a powerful tool for investigating the local structure of the crust and upper mantle directly beneath the recording station. this technique can reveal the principal structural features beneath the seismic station because the rayleigh-wave ellipticity depends primarily upon the local crust structure and does not exhibit azimuthal dependence in the period range of 10 to 50 s. thus, a general picture of the lithospheric structure beneath the station, in a depth range from 0 to 50 km, can be obtained using this type of analysis. therefore, the methodology presented in this paper, which consist of filtering and inverting rayleigh-wave ellipticity curves to obtain s-velocity models (variations in shear-wave velocity versus depth), can be a very useful tool in studies of seismic risk and/or seismic design because it provides local earth models in the range of crustal depths. acknowledgements the author is grateful to instituto geográfico nacional (madrid, spain), which provided the seismic data used in this study. references abo-zena a., 1979. dispersion function computations for unlimited frequency values. geophys. j. r. astr. soc., 58, 91-105. aki k. and richards p. g., 1980. quantitative seismology. theory and methods. w. h. freeman and company, san francisco. bath m., 1982. spectral analysis in geophysics. elsevier science publishers. amsterdam. ben-menahem a. and singh s. j., 1981. seismic waves and sources. springer-verlag. new york. cara m., 1973. filtering dispersed wavetrains. geophys. j. r. astr. soc., 33, 65-80. corchete v., badal j., pujades l. and canas j. a., 1993. shear velocity structure beneath the iberian massif from broadband rayleigh wave data. phys. earth planet. inter., 79, 349-365. corchete v., badal j., serón f. j. and soria a., 1995. tomographic images of the iberian subcrustal lithosphere and asthenosphere. journal of geophysical research, 100, 24133-24146. corchete v., chourak m. and hussein h. m., 2007. shear wave velocity structure of the sinai peninsula from rayleigh wave analysis. surveys in geophysics, 28, 299-324. dimri v., 1992. deconvolution and inverse theory. application to geophysical problems. methods in geochemistry and geophysics, 29. elsevier science publishers, amsterdam. dorman j. and ewing m., 1962. numerical inversion of seismic surface wave dispersion data and crust-mantle structure in the new yorkpennsylvania area. journal of geophysical research, 67, 5227-5241. dziewonski a., bloch s. and landisman m., 1969. a technique for the analysis of transient seismic signals. bull. seism. soc. am., 59, 427444. kulhánek o., 1990. anatomy of seismograms. elsevier science publishers. amsterdam. lee w., kanamori h., jennings p. and kisslinger c., 2003. international handbook of earthquake and engineering seismology. elsevier, new york. malischewsky p. g., zaslavsky y., gorstein m., pinsky v., tran t. t., scherbaum f. and flores-estrella h., 2010. some new theoretical considerations about the ellipticity of rayleigh waves in the light of site-effect studies in israel and mexico. geofísica internacional, 49, 141-152. rastogi b., k., a. p. singh, b. sairam, s. k. jain, f. kaneko, s. segawa and j matsuo, 2011. the possibility of site effects: the anjar case, following the past earthquakes in the gujarat, india. seismological research letters, 82(1): 692-701; doi 10.1785/ gssrl.82.1.692. sexton j. l., rudman a. j. and mead j., 1977. ellipticity of rayleigh waves recorded in the midwest. bull. seism. soc. am., 67, 369-382. singh, a., p., o. p. mishra, b. k. rastogi and dinesh kumar, 2011. 3-d seismic structure of the kachchh, gujarat and its implications for the earthquake hazard mitigation. natural hazards, 57(1): 83-105; doi 10.1007/s11069-010-9707-2. tuan t. t., scherbaum f. and malischewsky p. g., 2011. on the relationship of peaks and troughs of the ellipticity (h/v) of rayleigh waves and the transmission response of single layer over half-space models. geophys. j. int., 184, 793-800. a methodology for the analysis of the rayleigh-wave ellipticity 97 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 97 103 a multivariate statistical approaches on physicochemical characteristics of ground water in and around nagapattinam district, cauvery deltaic region of tamil nadu, india s. venkatramanan1, 2, s. y. chung1*, t. ramkumar2, g. gnanachandrasamy2, s. vasudevan2 1 institute of environmental geosciences, department of earth & environmental sciences, pukyong national university, 599-1 daeyeon-dong nam-gu, busan 608-737, south korea 2department of earth sciences, annamalai university, annamalai nagar 608-502, tamil nadu, india *corresponding author email: chungsy@pknu.ac.kr, venkatramanansenapathi@gmail.com hydrogeology ground water samples collected at different locations in and around the nagapattinam district were analyzed for their physicochemical characteristics. the ground water samples were collected from fifty two dug and deep wells during the monsoon and summer seasons in june and december, 2011. the present investigation is focused on the determination of physico-chemical parameters such as ph, ec, tds, ca, mg, na, k, hco3, so4 and cl. ground water suitability for drinking, domestic and agricultural purposes was examined by using who standards. correlation, factor and cluster analyses were applied to classify the ground water qualities and to categorize the geochemical processes controlling ground water geochemistry. factor analysis indicates that seawater intrusion and agriculture runoff are dominant factors controlling the hydrogeochemistry of ground water in the study area. cluster analysis was helpful for the classification on the basis of contamination characteristics of ground water quality. this study also elucidates that multivariate statistical analyses can be used to improve the understanding of ground water status and assessment of ground water quality. en este estudio se analizan las características fisicoquímicas de muestras de aguas subterráneas tomadas en diferentes locaciones en y alrededor del distrito de nagapattinam. las muestras se recolectaron en 52 pozos cavados y perforaciones profundas durante las subestaciones del monzón y el verano, en los meses de junio y diciembre (2011). la presente investigación está enfocada en la determinación de parametros fisicoquímicos como ph, ec, tds, ca, mg, na, k, hco3, so4 y cl. se examinó la pertinencia de estas aguas para consumo y para irrigación a la luz de los estándares de la organización mundial de la salud. se aplicaron análisis de correlación, factores y cúmulos para clasificar las muestras y categorizar los procesos geoquímicos que controlan las aguas subterráneas. el análisis de factores indica que la intrusión del agua marina y los vertidos de la agricultura son los principales agentes que intervienen la hidroquímica de las aguas subterráneas en el área de estudio. el análisis de cúmulos ayudó a la clasificación de las bases de contaminación características de las aguas subterráneas. este estudio, además, dilucida que los estudios estadísticos multivariantes pueden ser utilizados para mejorar el conocimiento y la evaluación de la calidad de las aguas subterráneas. palabras clave: calidad de aguas subterráneas, análisis estadístico, incursión de agua marina, nagapattinam, costa este de la india. record manuscript received: 03/12/2012 accepted for publication: 01/11/2013 abstract resumen key words: ground water quality, multivariate statistical analyses, ground water suitability, correlation, factor and cluster analyses. introduction one of the most severe problems concerning ground water quality is the rapid salinization of water resources. the increase in ground water salinity, particularly in coastal areas, may be due to the inflow of natural saline water, such as sea water intrusion, dissolution of soluble salts in the unsaturated zone, and saline water naturally derived from adjacent aquifers (vengosh and rosenthal, 1994, vengosh et al., 1999, maslia and prowell, 1990, jones et al., 1999, rosenthal et al., 1992, glynn and plummer 2005, cloutier et al., 2008, brinda and elango, 2011). anthropogenic contamination is another major cause of salinization and water-quality degradation. ground water irrigation causes salts recycling and accumulation in aquifers. the salts are accumulated in the soil and flushed through the unsaturated zone to the aquifer. moreover, irrigation with wastewater, which is generally more saline than regional ground water, increases the salinization rate of shallow ground water. this problem is more conspicuous 98 in arid and semi-arid zones, where potable water is replaced by wastewater for irrigation in order to save the depleting water resources. ground water salinization and degradation is usually observed through the sampling of pumping wells. in these wells, the screens are located in the deeper parts of the aquifer, far below the water table. in the light of this fact, one of the management strategies for ground water resources especially in the coastal belt is to ensure that ground water extraction does not lead to the intrusion of saline seawater into the aquifers. however, due to the relatively low elevations in the area and the lack of strict regulation, ground water salinization continues to be a problem in some of the coastal areas. on the other hand, this resource is the only source of water for agricultural activities in view of the erratic nature of rainfalls in recent times. the high salinity of ground water and the heavy reliance on ground water for irrigation necessitate the overall assessment of ground water quality for irrigation purposes. (hussein, 2004, ramkumar et al. 2011, anithamary et al. 2012, venkatramanan et al. 2012, gnanachandrasamy et al. 2012). thus, the suitability of ground water for irrigation and drinking purposes thus had to determined based on the presence of major ions in the ground water of this coastal region. this study proposes using multivariate statistical analysis to identify a correlation between fresh and saline water incursion. description of the study area the study area is located in east coast of india, and its stretches between coleroon in the north and kodikarai in the south (fig. 1). the district has a coastline stretching of about 190 km. the nagapattinam district water quality parameters in mg/l who standard (1993) monsoon season summer season ph 6.5-8.5 7.4-8.2 7.2-7.8 electrical conductivity (μs/cm) (ec) 846.8-2476.3 987.6-2576.6 total dissolved solidsc(tds) 500-1500 912-1875 846-1973 calcium (ca) 200 32-312 28-322 magnesium (mg) 150 35.4-318 38.5-321 sodium (na) 200 109.3-358.6 118.8-375.1 potassium (k) 14.3-312.4 18.9-324.9 bicarbonate (hco3) 97.3-326.2 107.5-324.9 sulphate(so4) 250 52.7-378.2 58.6-398.6 chloride(cl) 250-600 354.8-821.9 355.4-845.2 table. 1. physico-chemical parameters of ground water in the study area table. 2. correlation coefficients of ground water chemical components in monsoon and summer seasons monsoon ph ec tds ca mg na k hco3 so4 cl ph 1.00 ec -0.01 1.00 tds -0.05 0.84 1.00 ca -0.13 0.18 0.26 1.00 mg 0.13 -0.20 -0.28 0.06 1.00 na -0.03 0.74 0.68 0.14 0.17 1.00 k -0.14 0.00 -0.04 -0.10 0.24 -0.15 1.00 hco3 0.10 -0.15 -0.07 0.13 0.10 0.70 -0.15 1.00 so4 0.36 0.54 0.64 -0.12 0.45 0.70 -0.27 -0.03 1.00 cl -0.15 0.76 0.84 -0.37 0.16 0.76 0.12 0.11 0.34 1.00 summer ph 1.00 ec -0.12 1.00 tds -0.13 0.94 1.00 ca 0.18 0.17 0.24 1.00 mg 0.13 -0.21 -0.18 0.07 1.00 na 0.29 -0.07 0.84 0.11 0.18 1.00 k -0.27 -0.02 -0.03 -0.11 0.25 -0.13 1.00 hco3 0.24 -0.14 -0.14 0.13 0.07 0.70 -0.16 1.00 so4 0.31 0.65 0.72 -0.11 0.45 0.78 -0.27 -0.03 1.00 cl -0.11 0.88 0.95 -0.37 0.12 0.82 0.14 0.06 0.68 1.00 s. venkatramanan, s. y. chung, t. ramkumar, g. gnanachandrasamy, s. vasudevan 99 physico-chemical parameters monsoon season summer season factor 1 factor-2 communality factor 1 factor-2 communality ph 0.193 0.096 0.343 0.140 0.629 0.284 electrical conductivity (μs/cm) (ec) -0.772 -0.083 0.759 -0.854 -0.090 0.900 total dissolved solids (tds) -0.807 -0.018 0.776 -0.840 -0.074 0.902 calcium (ca) -0.479 0.345 0.289 -0.438 0.441 0.293 magnesium (mg) 0.514 0.200 0.525 0.450 0.152 0.464 sodium (na) 0.040 0.879 0.540 0.062 0.780 0.558 potassium (k) 0.105 -0.344 0.406 0.113 -0.441 0.388 bicarbonate (hco3) 0.062 0.878 0.552 0.113 0.771 0.517 sulphate (so4) 0.532 0.092 0.652 0.469 0.140 0.574 chlordie (cl) 0.536 0.054 0.469 0.480 -0.163 0.386 eigenvalues 2.41 1.801 2.417 24.179 total % of variance 24.152 18.011 1.982 19.826 cumulative % of variance 24.152 42.16 24.17 44.00 table. 3. varimax rotated factor loadings in monsoon and summer seasons mainly comprises of the quaternary sediments, which increases towards the southern part of coleroon river. these sediments have been delineated as alluvial plain deposits of the cauvery river and its tributaries, narrow fluvio-marine deltaic plain deposits and marine coastal plain deposits (east coast formation). the fluvial deposits comprise of flood plain, flood basin, point bar, channel bar and palaeo channels with admixtures of sand, silt and clay. the deltaic plain includes palaeo tidal flats with clays, and sands and sand ridges or gray brown sand. the marine coastal plains include beach, tidal flats, salt marsh, mangrove swamps, and deposits of sand and clay. the cretaceous formations of the coastal track of the cauvery basin consist of faunal, rich marine sedimentary rocks, such as limestones, sandstones, clays and sandy beds, etc. the mouth of the river comprises of alluvial deposits including clays and silts. ground water occurs in these formations and is extracted by filter point wells, tube wells, shallow boreholes and infiltration wells, especially from the sandy aquifers. the pliocene and quaternary shallow aquifers consists of sand, gravel and clay. the aquifer is more clayey towards the east and south eastern part of the district, except the coastal stretch of beach sands. the depth to shallow aquifers varies between 3 m and 35 m, whereas the depth to deep aquifers ranges from 80 m to 100 m. the average annual rainfall in the monsoon season is about 451.48 mm, whereas during that of summer, rainfall averages is about 43.12 mm. method of study fifty two ground water samples were collected from di fferent water sources such as dug and bore wells in june and december, 2011, using high-density polythe ne containers. the water samples were analyzed for major physico-chemical parameters adopting standard procedures (apha 1995). ph was measured by a ph pen (eco tester ph) and electrical conductivity (ec), total dissolved solids (tds), ca, na, mg, k, hco3, cl, and so4 were measured using a multi-parameter ion analyzer (hanna, hi83099). all concentrations are expressed in milligrams per litre (mg/l), except ph and ec. multivariate statistical analyses were performed to find out determine the relationship between ground water parameters using statistica software (ver.8). the results were evaluated in accordance with the norms prescribed under ‘who’ (world health organization, 1993) (table 2). results and discussion summary of ground water analysis the water quality analysis of different ground water samples were carried out to determine different physicochemical parameters such as for ph, ec, tds, ca, mg, na, k, hco3, so4, and cl; and the results are presented in table 1 and figure 2. ph value of ground water samples between 7.4-8.2 and 7.2-7.8 during the monsoon and summer seasons, respectively. ground water is a little alkaline, but the quality is within the limiting value of the drinking water standard. the general increase of ph in a sedimentary terrain is related to weathering of plagioclase feldspar in sediments. this is aided by dissolved atmospheric carbon dioxide resulting in the release of sodium and calcium, which progressively increases ph and alkalinity of the ground water. ec varied to 846.8-2476.3 and 987.62576.6 μs/cm, and tds changed to 912-1875 mg/l and 846-1973 mg/l during monsoon and summer seasons, respectively. ec and tds exceed who (1993) permissible limits. the summer season recorded high levels of ec and tds compared to the monsoon season. the reasons are the intrusion of residual solids into the aquifer, the movement of water through sediments containing higher soluble mineral matter, and the influx of industrial and municipal wastes with incursion of seawater. the calcium and magnesium concentrations are generally derived from the leaching of limestone, dolomites, gypsum and anhydrites (garrels 1976, ramkumar et al. 2010, anithamary et al. 2012). calcium and magnesium ranged between 32-312 mg/l and 28-322 mg/l, respectively during the monsoon sub season, and 35.4-318 mg/l and 38.5-321 mg/l, respectively during the sum mer season. most of the ground water samples exceed who (1993) permissible limits in ca and mg. the increased concentration in ca and mg in ground water samples is due to the leaching of limestone, dolomites, gypsum and anhydrites (aiuppa et al., 2003, banaszuk et al., 2005, samsudin et al., 2008). the ionic concentration ranged between 109.3358.6 mg/l and 118.8-375.1 mg/l for so dium, and 354.8-821.9 mg/l and between 355.4-845.2 mg/l for chloride, respectively. thus, sodium might have been derived from the dissolution of evaporated minerals and its subsequent mixing with ground water. the higher na and cl concentration ground water in the coastal region is attributed to the influence of seawater on the coastal aquifer, which was highly visible during the summer due to a decline in water table (einsiedl 2012, kouzana et al. 2010, yidana, and yidana, 2009). ionic concentrations were 14.3-312.4 mg/l for potassium, and 18.9-324.9 mg/l for bicarbonate, and 97.3-326.2 mg/l for sulphate during the monsoon season; and 107.5-324.9 mg/l for potassium, 52.7378.2 mg/l for bicarbonate, and 58.6-398.6 mg/l for sulphate during the summer season, respectively. the large portion of potassium is due to the intrusion of saline water as well as the fertilizers of agricultural activity. a multivariate statistical approaches on physicochemical characteristics of ground water in and around nagapattinam district, cauvery deltaic region of tamil nadu, india 100 in this study, it was observed that na, k, ca, and mg were higher in summer than in monsoon season. na and mg were lower, but k and ca were higher during monsoon. it indicates that the seasonal variation of cation concentrations was very conspicuous in this environment. the relative abundance of sodium and magnesium during the summer revealed the influence of monsoon rainwater on the ground water. it led to an increase in these elements from irrigation runoff, thereby increasing their concentration in the ground water during summer season. the weathering of plagioclase feldspar and the nature of dissolved atmospheric carbon dioxide releases sodium and calcium, which progressively increases the sodium and calcium concentrations. chloride and sulfate were dominated during summer season, but chloride showed more variations than sulfate during two seasons. it is resulted from agricultural activity and seawater incursion in the shallow aquifer system. correlation between variables the correlations between major cations and anions were carried out using pearson’s correlation. a correlation analysis is a bivariate method applied to describe the degree of relation between hydrochemical parameters. associated the variable representing with correlation coefficient is r, and the multiple correlations, which is the percentage of variance in fig.1. location map of the study area s. venkatramanan, s. y. chung, t. ramkumar, g. gnanachandrasamy, s. vasudevan 101 the dependent variable, is explained collectively by all of the independent variables. the results of the correlation analysis are considered in the subsequent interpretation section. a high correlation coefficient (near 1 or -1) means implies a good strong relationship between the two variables, and a correlation coefficient around of approximately zero means implies that there is no relationship between them two variables, at a significance level of, 0.05. more precisely, it can be said that parameters showing r > 0.7 are considered to be strongly correlated, whereas when r has a value between 0.5 and 0.7, a moderate correlation is shown is said to exist (kim et al. 2002; helena et al. 2000, shammas and jacks, 2007). positive values of r indicate a positive relationship, while negative values indicate an inverse relationship. the correlation coefficients of the studied parameters are shown in table 2. in the present study, ec and tds are strongly correlated with na, cl and so4 during the monsoon, and similar type of result was observed during summer. they thus indicate seawater incursion and leaching of secondary salts. the lower concentration of calcium compared with sodium and chloride is a result of the cation exchange process that occurs naturally when seawater is introduced into the freshwater aquifer system (appelo and postma 2005, zeng and rasmussen, 2005, venkatramana et al., 2009). furthermore, during the summer season, sodium is positively correlated with cl, ec, and tds, indicating seawater incursion due to continuous pumping activities in the study area. the ca shows significant correlation with ec and tds and good correlation was noted in hco3. the na was strongly correlated with cl, clearly indicating that the seawater incursion plays a vital role in this present study (ramkumar et al., 2011 and anithamary et al 2012). varimax rotation factor analysis factor analysis is a frequent common statistical technique employed in hydrochemical studies (subbarao et al., 1996, adami et al., 1997, davis, 1986). the main goals of factor analysis are: (1) to reduce the number of ‘variables’and (2) to detect structure in the relationships between ‘variables’(analytical parameters) and/or ‘cases’(sampling points in our study). the mathematical base of factor analysis and a review of the chemical aspects can be found in mermet et al, (1998). currently, the calculations of factor analysis calculations and their graphical results performed using by davis (1986). consequently during the summer season, the first factor accounts for 24.17% whereas the second factor accounts for 44.00% (table 3). factors 1 and 2 show a good strong relationship with ec, tds, hco3, na, cl and mg, which is indicative of seawater incursion and agricultural runoff incursions in the study area. the concentration of na and cl can be ascribed to the incursion of seawater into the aquifer systems. furthermore, during the monsoon season, the first factor accounts for 24.15% and second factor accounts for 42.16%. the factor analyses performed in this study has clearly indicated that seawater incursion into the coastal aquifer as is the primary source of ground water pollution, the similar observations was made by venkatramanan et al. (2012). cluster analysis cluster analysis is used for grouping the cases based on the similarity of the responses to several variables. on the basis of the connecting distances between parameters and their positions on the dendrogram, distinctive clusters of the variables were defined for each of three locations along the basin. though this procedure is subjective, the distinction between clusters in this analysis is quite clear from the dendrogram. cluster analysis is a useful method for combining ground water wells into homogenous groups according to their water quality. in this study, three types of clustering nature was observed during the monsoon and summer seasons. the first clusters are ph, k, ca, na, hco3 and so4, whereas the second cluster is cl accompanied by ph, k, ca, na, hco3 and so4. the third cluster is accompanied by ec and tds. these variables would be found in natural mineralization and anthropogenic processes. the non-homogenous nature of the aquifer led to different hydrogeochemical facies for ground water such as na and cl. a close relationship between na and cl, may be originated from sea water incursion into coastal aquifers (figs. 3a, b). fig.2. distributions of physico-chemical parameters of ground water in the study area fig.3a. dendrogram of 10 chemical parameters for 10 cases (monsoon) fig.3b. dendrogram of 10 chemical parameters for 10 cases (summer) a multivariate statistical approaches on physicochemical characteristics of ground water in and around nagapattinam district, cauvery deltaic region of tamil nadu, india 102 conclusions the problem of ground water in nagapattinan district coastal region is rather complicated, and is derived from excessive exploitation of the aquifer systems. the study area depends totally on the water derived from the aquifer for its irrigation use. the overpumping of fresh water caused the deterioration of the water quality by the upwelling of deep saline water. fifty two ground water sam ples were collected near the coastal village of the nagapattinam district during monsoon and summer seasons, and physico-chemical characteristics of the samples were studied to determine the quality and the suitability of the ground water for various purposes as well as the characteristics of ground water quality. the maximum concentrations were observed near the coastal area because of sea water incursion. in addition, irrigation return flow also plays a major role in controlling ground water quality in the nagapattinam district. a dendrogram of the 10 cases and 10 variables are plotted and grouped into three main clusters. the values of correlation coefficients and their significance levels contributed to the selection of proper treatments to minimize the contamination of the ground water in the study area. the factor analysis reveals that the ground water from nagapattinam area has been greatly influenced by lithologic and environmental events in the area. a continuous monitoring program of the water quality will help to avoid further deterioration of the ground water quality in this coastal region. acknowledgements the authors are grateful to two anonymous referees for their constructive comments and suggestions which led to significant improvements to the manuscript. we also thank for the valuable suggestions given by carlos alberto vargas jimenez, editor, which greatly helped in the final presentation of the paper. this research was supported by a grant (code 13awmpb066761-01) from awmp program funded by the ministry of land, infrastructure and transport of korean government and department of science and technology (project no: dst/war-w/wsi/02/2010), new delhi, india. references adami, g., barbieri, p., picciotto, a. and reisenhofer, e. 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(1993). fluorine and fluorides, environmental health criteria, 36. world health organization, geneva. a multivariate statistical approaches on physicochemical characteristics of ground water in and around nagapattinam district, cauvery deltaic region of tamil nadu, india v16n1c.indb earth sciences research journal geotechnical earth sci res sj vol 16, no 1 (june, 2012): 55 63 experimental assessment of the liquefaction resistance of calcareous biogenous sands eimar a sandoval1 and miguel a pando2 1 assistant professor, school of civil engineering and geomatics, university of valle, cali, colombia e-mail: eimar sandoval@correounivalle edu co 2 associate professor, civil and environmental engineering department, university of north carolina at charlotte, nc, usa 28233 e-mail: mpando@uncc edu abstract liquefaction is a phenomenon in which soils, typically sands, suddenly loose a substantial amount of their shear strength and stiffness, this often triggered by large-magnitude earthquakes most liquefaction research has focused on silicate-based sands and not on other sand types, such as calcareous biogenous sands calcareous sands are usually composed of skeletal or non-skeletal remains of marine organisms, with unique characteristics in terms of their mineralogy surface roughness, particle shape, crushability, and intraparticle porosity the unique characteristics of calcareous sands suggest that their geotechnical engineering behaviour can be substantially different compared to that of terrigenous sands, including their behaviour under seismic loading, which have not been very well studied this paper presents the results of an experimental programme aimed at studying the cyclic liquefaction resistance of uncemented calcareous biogenous sands retrieved from south-western puerto rico evaluation of liquefaction potential involved a comprehensive set of isotropically consolidated undrained cyclic triaxial tests on reconstituted samples of this calcareous sand the programme also included tests on ottawa terrigenous silica sand samples prepared and tested in similar conditions for comparison purposes in general, the experimental results showed that cabo rojo calcareous sands had higher liquefaction resistance compared to ottawa silica sands tested under similar conditions important differences between calcareous and silica sands regarding pore pressure generation characteristics and axial strain accumulation were also observed resumen la licuación es un fenómeno en el cual los suelos, típicamente arenas, pierden una cantidad importante de su rigidez y resistencia al esfuerzo cortante, generalmente por causa de sismos de gran magnitud la mayoría de investigaciones sobre licuación se han enfocado en arenas terrígenas con base de sílice o cuarzo, y no sobre otro tipo de arenas, como las arenas calcáreas biogénicas las arenas calcáreas están usualmente compuestas por residuos de organismos marinos esqueléticos y no esqueléticos, con características específicas en términos de su mineralogía, rugosidad superficial, forma de partículas, susceptibilidad al rompimiento y porosidad interna de los granos las características particulares de las arenas calcáreas, hacen suponer que su comportamiento ingenieril geotécnico puede ser considerablemente diferente comparado con el de arenas terrígenas, incluyendo su comportamiento bajo carga sísmica, el cual no ha sido estudiado en detalle este artículo presenta los resultados de un programa experimental desarrollado con el fin de estudiar la resistencia a licuación cíclica de una arena calcárea biogénica no cementada, extraída del suroeste de puerto rico la evaluación del potencial de licuación incluyó una extensa cantidad de ensayos triaxiales cíclicos no drenados consolidados isotrópicamente, sobre muestras no cementadas de esta arena calcárea para fines de comparación, el programa también incluyó ensayos sobre la arena terrígena de ottawa bajo condiciones de prueba similares en general, los resultados experimentales mostraron que las arenas calcáreas de cabo rojo tuvieron mayor resistencia a licuación, comparada con las arenas de ottawa para condiciones de prueba similares también se observaron diferencias importantes en cuanto a las características de generación de presiones de poros y acumulación de deformaciones axiales palabras claves: arenas calcáreas, deformación axial cíclica, presión de poros cíclica, resistencia a licuación keywords: calcareous sands, liquefaction resistance, cyclic pore pressure, cyclic axial strain record manuscript received: 15/11/2011 accepted for publications: 25/05/2012 eimar a sandoval and miguel a pando56 introduction liquefaction is a serious phenomenon which often manifests itself in areas having loose, saturated, sandy soils following strong earthquakes or vibrations liquefaction is manifested as a drastic loss of shear strength and stiffness of the affected soil, resulting from a sharp increase in internal pore water pressure which builds up during load cycles imparted by earthquakes or vibrations if excess pore pressures increase enough, the soil may lose all its strength and rigidity, leading eventually to behaviour similar to that of liquid, hence the term liquefaction due to soil liquefaction, there may be ground subsidence and settlements, sand boils, lateral spreading, slope failures and damage or even collapse of structures the significance of damage which can be caused by liquefaction has been recognised for many years (e g yoshimi et al , 1977; seed, 1979; seed & idriss, 1982) the 1964 earthquakes in alaska, u s a and niigata, japan, represent two major seismic events where extensive damage was reported due to liquefaction thereby highlighting its importance extensive work has been carried out worldwide since these two earthquakes to better understand soil liquefaction progress in this area has been periodically summarised in state-of-the-art reports and monographs such as that published by the earthquake engineering research institute (eeri) (seed & idriss, 1982), the proceedings of a workshop run by the us national research council (nrc, 1985), the us national center for earthquake engineering research (nceer) (youd et al , 2001) and a very recent monograph by idriss & boulanger (2010) even though much progress has been made in understanding the complexity of the phenomenon, many aspects still lack important knowledge requiring ongoing research most engineering procedures for assessing soil liquefaction potential have been based on field data gathered over the years from sites providing evidence of liquefaction (i e empirical) most case histories used in developing such empirical liquefaction screening methods have been based on sites where liquefied deposits have primarily consisted of silica sands the following section provides background information regarding one of the most commonly-used empirical procedures for assessing liquefaction potential based on the geotechnical standard penetration test (spt) field test besides providing valuable background information for readers new to the subject, the material also shows that the empirical methods used in practice are mostly based on liquefaction case histories involving silicate-based terrigenous sandy soil deposits (i e transported from land) containing mineral grains such as quartz and feldspar there is thus an important gap in current knowledge as few or no liquefaction case figure 1. an example of the spt-based database used for estimating liquefaction resistance (crr) for silica-based sand deposits (adapted from idriss & boulanger, 2010) c yc lic st re ss r at io , c sr =  cy c /  ' v (m = 7 .5 ;  ' v = 1 a tm ) 0,6 0,5 0,4 0,3 0,2 0,1 0 0 10 20 30 40 50 corrected spt blow count (n1) 60cs liquefaction no liquefaction empirical line as per idriss & boulander, 2010 histories involving marine biogenous calcareous sands (uncemented or cemented) have been included in current liquefaction case history databases (e g idriss & boulanger, 2010) this paper is focused on calcareous biogenous marine sands, thus hopes to motivate research on non-silica sand liquefaction and to assess the influence of factors such as origin, genesis, deposition history, and mineralogy it presents a summary of results from an experimental research programme aimed at studying the liquefaction resistance of calcareous sands retrieved from surficial beach deposits at cabo rojo, south-western puerto rico the experimental programme involved mineralogical and grain characterisation, critical state line (csl) evaluation, and 31 undrained cyclic triaxial tests on isotropically-consolidated reconstituted samples of uncemented calcareous sands the triaxial samples were tested at three relative densities and three isotropic consolidation stress levels the experimental programme included similar tests carried out on ottawa terrigenous silica sand samples for comparison purposes; these were prepared using similar techniques and tested under the same experimental conditions background spt-based simplified liquefaction evaluation methodology this section provides background on how liquefaction potential of sandy deposits is commonly assessed in practice the methodology presented was based on an extensive database of case histories of sites where evidence of cyclic liquefaction has been reported following a major earthquake this methodology is based on classical work by seed & idriss (1971) and multiples updates including the recent work by idriss & boulanger (2010) the field data regarding each case history in the spt-based liquefaction database has been interpreted and analysed to assess the cyclic stresses levels which would induce cyclic liquefaction for the spt-based methodology, these interpreted cyclic stresses levels were compared to field spt blow counts figure 1 gives a typical graph summarising empirical data from the spt-based liquefaction case history database this plot shows normalised cyclic stresses (presented as cyclic stress ratio csr) compared to corresponding normalised spt blow counts (n1,60) the solid circles represent points where cyclic liquefaction occurred and the open circles the data points where no liquefaction was observed the solid black line represents an empirically based line that can be used to estimate the cyclic stresses that would trigger cyclic liquefaction for a certain spt blow count several versions of this curve have been presented over the years, changing as the spt-based cyclic liquefaction database gains additional case histories versions of this line also differ based on differences in case history interpretations existing procedures for estimating liquefaction potential are currently based on empirical data gathered from sites primarily involving silica sandy soils; a review of the relevant liquefaction literature did not reveal a specific procedure for sandy soil having other mineralogy types or origins such as the calcareous sands investigated in this article the definition of liquefaction used in this study as mentioned earlier, soil liquefaction is a complex phenomenon, involving different soil responses depending on the material stress-strain behaviour, type of loading, initial stress, etc for the purposes of this paper, liquefaction refers to “cyclic liquefaction” involving cyclic softening of the soil due to cyclic loading (e g earthquake loading) having sufficient intensity (i e cyclic stress levels and/ or duration) to allow enough build up of excess pore pressures to make the sandy soil reach a state of zero effective stress (robertson, 1994) robertson & wride (1998) have provided a definition for different types of liquefaction, including cyclic liquefaction most liquefaction research, including the spt-based simplified method presented above, has been reexperimental assessment of the liquefaction resistance of calcareous biogenous sands 57 figure 2. micrographs of cabo rojo uncemented calcareous sand lated to cyclic liquefaction cyclic liquefaction usually applies to level or gently sloping ground where shear stress reversal occurs during earthquake loading (robertson & wride, 1998; pando & robertson, 1995) marine biogenous sediments and calcareous sands marine biogenous sediments may be siliceous or calcareous and are formed from planktonic organisms, the most abundant organisms in the ocean phytoplankton (having plant-like photosynthesis) and zooplankton (grazing like animals) are the two main types of plankton the insoluble shells of these creatures can be formed of calcite or silica calcareous biogenous sediments (the focus of this paper) consist of large accumulations of the skeletal remains of plants and animal life skeletal grains are formed as internal or external skeletal units of marine organisms, including crustose and articulate corallines, mollusc shells, forams, gastropods, algae, and echinoid fragments the calcium carbonate content in skeletal grains is primarily determined by the marine organism from which the grains were derived, in turn, being dependent on local water temperatures and nutrient availability and type (morelock & ramirez, 2004) the most common biogenic calcareous particles are shells of foraminifera-micro-organisms, usually being less than 1 mm in diameter (silva, 1974) calcite is the main component of calcareous sand (usually greater than 90% calcium carbonate content) calcareous sands are unique in terms of particle characteristics, including mineralogy, shape, surface texture, high void ratio, and intra-porosity calcareous sands have variable cementation, ranging from non-cemented to strongly-cemented state these sands are more crushable, more contractive, and less stiff than terrigenous silica sands due to their unique particle properties and carbonate content (cataño & pando, 2010) methodology the experimental programme used in this study primarily involved sand mineralogical and grain characterisation, characterising the csl and consolidated undrained cyclic triaxial testing of specimens prepared using cabo rojo calcareous sands and ottawa silica sands thirty-nine consolidated undrained cyclic triaxial tests were performed: 31 of them on cabo rojo calcareous sands and 8 on ottawa silica sands description of test sand materials used the calcareous sand being studied was collected from a beach in puerto real, cabo rojo, south-western puerto rico; it was retrieved from the near surface beach using an ordinary shovel and buckets this sand was uncemented and classified as being white to yellow, fineto medium-grained, poorly-graded, having sub-angular to angular grains and high internal poeimar a sandoval and miguel a pando58 rosity (figure 2 shows grain characteristics for this sand) this calcareous sand had at least 91% calcium carbonate content ottawa silica sand was also tested; this is white, fineto medium-grained, poorly-graded, having rounded grains and is a terrigenous quartzitic sand having no calcium carbonate content table 1 lists the main index properties for both test sands and figure 3 shows their grain size distribution besides having different mineralogy, these two sands also have important differences in terms of grain characteristics and particle size distribution for example, table 1 shows that silica sand is coarser than calcareous sand and its specific gravity is lower cabo rojo calcareous grain characteristics cabo rojo calcareous sands are sedimentary carbonates for which diagenesis and resulting grain mineralogy are influenced by the depth and temperature of the water column (morse & mackenzie 1990, chaney et al 1982, morelock & ramirez 2004) carbonates are usually dominated by aragonite in shallow waters, followed by magnesium-rich calcites, whereas in deep waters carbonates are composed of calcite having low magnesium content parameter cabo rojo calcareous ottawa silica sand astm standard d10 (mm) 0.24 0.65 astm d422-463 d30 (mm) 0.30 0.71 d50 (mm) 0.37 0.75 d60 (mm) 0.41 0.78 cu 1.75 2.1 cc 0.94 1.1 gs 2.84 2.65 astm d854 min (kn/m3) 9.10 14.6 astm d4254emax 2.07 0.78 max (kn/m3) 11.1 17.3 alternative methodemin 1.51 0.50 table 1. summary of index properties for both sands tested a unique characteristic of calcareous sands is their intra-particle void structure which, combined with the skeletal void ratio, can result in total void ratios being quite higher than void ratios for silica sands for example, emax and emin values listed in table 1 were found to be 2 07, 1 51 and 0 78, 0 50 for the calcareous and silica sands respectively cabo rojo sands void ratio values are almost 300% higher than otawa sands furthermore, due to their mineralogical composition, calcareous sands can have considerably higher specific gravity values than typical values for terrigenous silica sands ( 2 65) cabo rojo calcareous sands had an average specific gravity value of 2 84 this higher specific gravity value is associated to the unique biogenic characteristics of these sands, and to their mineralogical composition which usually includes heavier minerals than quartz such as calcite and aragonite the crushing susceptibility of the cabo rojo calcareous sand grains during cyclic triaxial testing was checked by comparing gradation curves before and after selected cyclic triaxial tests crushing potential was found to be very low to low, at least for the range of isotropic effective consolidation stresses considered, ranging from 50 to 200 kpa cataño & pando (2010) have also reported low to moderate particle crushability for monotonic isotropic consolidated undrained triaxial compression tests and torsional resonant column tests, including tests with up to 500 kpa effective isotropic consolidation stress crushing potential values for the cabo rojo calcareous sand experiments reported here thus indicated little to no particle crushing as mentioned earlier, cabo rojo sands were extracted in an uncemented state and then reconstituted in the laboratory without cementation critical state line evaluation of cabo rojo calcareous sands sands under shear loading may be contractile or dilatant, based on their initial state regarding csl the initial state of a sand sample is usually defined in terms of its initial void ratio (e) and effective stress level, such as the effective minor principal stress (´3) or the effective mean principal stress (p´) this initial state is important for liquefaction susceptibility analysis because samples above or to the right of the csl have a contractile behaviour where the skeleton of particles would tend to compress, and thus develop positive excess pore pressures which would make the sand more susceptible to undergoing cyclic liquefaction by contrast, sand samples having an initial state below or to the left of the csl will have a dilatant behaviour according to critical soil mechanics, and will not develop large positive excess pore pressure which would make them less susceptible to cyclic liquefaction many authors have proposed parameter variables for quantifying liquefaction potential of sandy soils based on the position of their initial state regarding the csl for example, been & jefferies (1985) proposed using state parameter y defined as the difference between the initial void ratio of the sandy soil sample and the void ratio at the csl for the same effective mean pressure p’ a positive state parameter value would thus represent an initial state of the sandy soil sample on the contractile side of the csl line, while a negative value would represent a dilatant initial state part of this study involved characterising the csl for the cabo rojo calcareous sands (figure 4) the lower portion of the csl was obtained from monotonic isotropically-consolidated undrained (ciu) triaxial compression tests reported by catano & pando (2010) the upper portion of the csl was obtained as part of this research using the simplified method proposed by santamarina & cho (2001) figure 4 also shows the average position for the eight groups of calcareous sand samples from cabo rojo regarding the csl each group corresponded to the same effective consolidation stress and relative density after consolidation, just before the undrained cyclic loading phase figure 4 shows that six of the eight groups of samples were located above the csl, i e on the contractile side, therefore expected to exhibit a higher liquefaction potential compared to the two other groups of samples located below the csl line (i e expected dilatant behaviour) standard sieve size pe rc en t fi ne r by w ei gh t ( % ) 100 80 60 40 20 0 10 1 0 1 0 01 grain size (mm) figure 3. grain size distribution curves for both sands tested in this study 1/4" #4 #10 #16 #20 #30 #40 #60 #100 #200 cabo rojo calcareous sand ottawa # 20-30 silica sand cabo rojo calcareous sand ottawa silica sand experimental assessment of the liquefaction resistance of calcareous biogenous sands 59 cyclic triaxial testing procedure used to assess liquefaction resistance the liquefaction resistance for both test sands was evaluated by means of consolidated undrained cyclic triaxial tests, which were carried out in general accordance with the procedure described in astm standard d5311 (astm 2004) a cyclic triaxial system (geocomp corp) was used for the cyclic triaxial tests figure 5 shows a photo of the cyclic triaxial equipment used in this research; it shows a loading frame with a hydraulic actuator (loadtrac ii unit), two pressure/volume control units (flowtrac ii) and a pc-based data-acquisition (daq) system cyclic triaxial test specimens were prepared using the “moist tamping” technique using five layers and the under-compaction method suggested by ladd (1976) prepared samples were about 102 mm high, having 51 mm diameter the moist tamping technique was carried out with 4% to 25% moisture content for cabo rojo calcareous sand and 2% to 8% for ottawa silica sand lower moisture content values were used to produce looser specimens for both test sands while higher moisture content values were used to prepare the denser samples relative densities ranged from 20% to 78% these relative densities corresponded to compacted samples prior to consolidation samples were saturated until greater than 0 95 skempton b-values were achieved (skempton, 1954) after saturation, samples were isotropically consolidated at three different effective stress levels (50, 100 and 200 kpa) the soil samples underwent densification during isotropic consolidation, resulting in 26% to 83% final relative density specimens were allowed to stabilise once consolidation stress had been reached for at least 30 minutes prior to beginning the cyclic loading phase the isotropicallyconsolidated samples were subjected to stress-controlled cyclic triaxial tests at 1 hz frequency results and discussion figure 6 shows a typical set of results from this research for a ciu cyclic triaxial tests data regarding on an ottawa sand sample; this figure shows three plots representing deviator stress, excess pore pressure, and axial strain measurement as a function of number of load cycles initial deviator stress (before the cyclic loading) was zero for the isotropically consolidated tests presented here deviator stress was cycled ± d once a second (1 hz frequency) in sinusoidal form during cyclic loading (figure 6a) two important things occurred to samples during undrained cyclic loading: i) excess pore pressure figure 4. critical state line (csl) for the cabo rojo calcareous sands figure 5. cyclic triaxial system equipment figure 6. an example of test results for an isotropically-consolidated undrained cyclic triaxial test on a reconstituted ottawa silica sand sample tested at csr=0 10, ’ 3cons =50 kpa, and 1hz 2,4 2,2 2,0 1,8 1,6 1,4 1,2 10 100 1000 minor principal effective stress, '3 (kpa) '3cons = 50kpa, dr = 26% '3cons = 100kpa, dr = 27% '3cons = 100kpa, dr = 45% '3cons = 100kpa, dr = 64% '3cons = 100kpa, dr = 80% '3cons = 200kpa, dr = 50% '3cons = 200kpa, dr = 65% '3cons = 200kpa, dr = 82% 20 0 -20 0 5 10 15 20 25 number of load cycles (a) deviator stress vs load cycle (b) excess pore pressure vs load cycle (c) axial strain vs load cycle 60 50 40 30 20 10 0 0 5 10 15 20 25 number of load cycles 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0 5 10 15 20 25 number of load cycles vo id r at io (e ) c yc lic d ev ia to r st re ss (k pa ) ex ce ss p or e pr es su re (k pa ) a xi al s tr ai n (% ) eimar a sandoval and miguel a pando60 started to increase and, as load cycles increased, it approached the consolidation effective stress value (liquefaction is often defined as the condition when excess pore water pressure first equals initial consolidation effective stress, as in figure 6b) and, ii) axial strain was low in the sample at the beginning but then started to increase rapidly after some load cycles (indicating the initiation of liquefaction, see figure 6c) the results are presented here in terms of the number of load cycles to liquefaction in response to the cyclic stress ratio (csr) however, since these curves represent the ability of the soil to resist liquefaction, these curves are often called cyclic resistance ratio (crr) curves to differentiate them from seismic application on a soil layer, expressed in terms of cyclic stress ratio (csr) induced by an earthquake the crr for cyclic triaxial tests is defined as the ratio between maximum applied cyclic shear stress and effective consolidation stress, as follows: three to five cyclic triaxial tests were performed at different crrtx levels for each relative density and effective consolidation stress level the number of cycles required to produce liquefaction in the sample was recorded for each crrtx level subscript (tx) mean that crr was based on cyclic triaxial tests and not on field estimates based on correlations (as in figure 1) cyclic liquefaction was considered to have occurred when zero effective stress was reached (i e excess pore pressure equalled the consolidation effective stress) the systematic test programme used (involving samples of each sand type) led to obtaining crr curves for different relative densities and effective consolidation stress levels figure 7 summarises the crr curves obtained for the cabo rojo calcareous sand these curves represent eight test conditions in terms of relative density and consolidation effective stress, six of which (figure 4) were on the contractive side of the csl before cyclic loading for comparison purposes, figure 8 shows crr curves obtained from ottawa silica sand samples prepared and tested in similar conditions comparing crr curves for cabo rojo calcareous and ottawa silica sand figure 9 compares the liquefaction resistance of cabo rojo calcareous sand and ottawa silica sand for samples prepared in loose state (dr: 23%– 27%) and isotropically-consolidated at 50 kpa effective stress a similar comparison is presented in figure 10 for medium to dense state reconstituted samples (dr: 64%–68%) consolidated at 100 kpa effective stress figures 9 and 10 show that cabo rojo calcareous sands had much higher liquefaction resistance than ottawa silica sand, for samples prepared and tested at similar relative densities and effective consolidation stresses excess pore pressure generation figure 11 summarises the excess pore pressure produced during triaxial test undrained cyclic loading phase, showing excess pore pressure generation curves for loose state (dr= 23%–27%) for both test sands consolidated at 50 kpa effective stress 0,6 0,5 0,4 0,3 0,2 0,1 0 10 100 1000 10000 number of cycles to liquefaction (nf) cabo rojo ('3cons = 50kpa, dr = 26% cabo rojo ('3cons = 100kpa, dr = 27% cabo rojo ('3cons = 100kpa, dr = 45% cabo rojo ('3cons = 200kpa, dr = 50% cabo rojo ('3cons = 100kpa, dr = 64% cabo rojo ('3cons = 200kpa, dr = 65% cabo rojo ('3cons = 100kpa, dr = 80% cabo rojo ('3cons = 200kpa, dr = 82% figure 7. cyclic resistance curves for cabo rojo calcareous sands figure 8. cyclic resistance curves for ottawa silica sands figure 9. cyclic resistance curves for loose relative density cabo rojo and ottawa sand 0,4 0,3 0,2 0,1 0 10 100 1000 10000 number of cycles to liquefaction (nf) ottawa #20-30 ('3cons = 50kpa, dr = 25% ottawa #20-30 ('3cons = 100kpa, dr = 68% 0,30 0,25 0,20 0,15 0,10 0,05 0 10 100 1000 10000 number of cycles to liquefaction (nf) all samples issotropically consolidated to '3cons = 50kpa numbers by data point refer to after consolidation relative density (dr) 26 1 26 0 26 7 24 5 23 4 25 6 25 6 calcareous sand silica sand c yc lic re si st an ce r at io , c r r t x (  d/ 2 ' 3c on s ) c yc lic re si st an ce r at io , c r r t x (  d/ 2 ' 3c on s ) c yc lic re si st an ce r at io , c r r t x (  d/ 2 ' 3c on s ) crrtx= d 23cons (1) experimental assessment of the liquefaction resistance of calcareous biogenous sands 61 figure 12 gives a similar comparison for medium to dense state samples (dr: 64%–68%), for both test sands consolidated at 100 kpa figures 11 and 12 present normalised residual excess pore pressure (dures) regarding consolidation stress (s’3cons) as a function of the normalised number of cycles for reaching liquefaction (i e n/nf) residual excess pore pressure (dures) was defined as excess pore pressure value when deviator stress was zero during each load cycle (seed & lee, 1966) figures 11 and 12 highlight differences found regarding excess pore pressure generation characteristics between cabo rojo calcareous sand and ottawa silica sand calcareous sand developed larger excess pore pressures than the silica sand during the earlier stages of the cyclic loading phase calcareous sands had greater fluctuations of excess pore pressures between loading cycles, suggesting some sort of stress relaxation between loading cycles silica sands had slow, gradual excess pore pressure generation during the initial test cyclic loading phase and very small pore pressure fluctuations between loading cycles another important difference was that silica sands typically showed a sudden or abrupt increase in excess pore pressure towards the end of the tests (i e as the sample approached liquefaction) while the calcareous sands showed a more gradual or incremental increase in excess pore pressure as it reached liquefaction the large fluctuations in excess pore pressure exhibited by calcareous sands during loading cycles could have been due to particle rearrangement arising from the characteristics concerning contact areas between figure 10. cyclic resistance curves for medium to dense relative density cabo rojo and ottawa sand figure 11. excess pore pressure generation curves for loose state cabo rojo and ottawa sands, isotropically-consolidated at 50 kpa effective stress figure 12. excess pore pressure generation curves for medium to dense state cabo rojo and ottawa sands, isotropically-consolidated at 100 kpa effective stress 0,4 0,3 0,2 0,1 0 10 100 1000 10000 number of cycles to liquefaction (nf) 69 5 70 0 66 8 66 8 64 4 63 5 64 5 62 2 silica sand calcareous sand all samples issotropically consolidated to '3cons = 100kpa numbers by data point refer to after consolidation relative density (dr) grains, including unique particle shapes, surface roughness, intra-grain porosity, etc the occurrence of large pore pressure relaxation between loading cycles could contribute towards the greater cyclic resistance exhibited by the cabo rojo calcareous sands compared to ottawa silica sand cabo rojo sand excess pore pressure generation characteristics were similar to that reported by morioka & nicholson (2000) and ross & nicholson (1995) for two calcareous sands from hawaii development of axial strains samples in cyclic triaxial testing experienced a marked increase in accumulated axial strain when approaching liquefaction state; this is due to large soil stiffness loses as the effective stress in the sample decreases and eventually reaches zero at cyclic liquefaction how axial strains are accumulated in a sample to reach liquefaction depended largely on the initial position of the sample regarding the csl (i e if the initial state before loading was contractile or dilatant) the individual characteristics of the sand grains are another important factor on how permanent axial strains are developed and accumulated similar to the finding for excess pore pressures, the unique properties of calcareous sands had a great influence on how axial strains were developed during cyclic triaxial tests figure 13 illustrates simultaneously the axial strain accumulation of calcareous and silica sand to loose state samples (dr= 23%-27%) consolidated under an effective stress of 50 kpa a similar comparison is presented in figure 14 for samples in medium to dense state (dr= 64%-68%) consolidated to 100 kpa it may be noted in these figures that calcareous sand experienced an increase in axial strain amplitude since the start of cyclic loading, gradually increasing until liquefaction calcareous sands underwent no dramatic increase in axial deformation, even to reach liquefaction state moreover, silica sands were deformed very little during the cyclic test, but experienced a sudden increase in axial strain just before liquefaction, when pore pressure increased axial strain amplitude for both sands increased with increasing relative density summary and conclusions this paper has presented a summary of an experimental study for assessing the liquefaction resistance of uncemented calcareous sands from cabo rojo in south-western puerto rico the cyclic triaxial tests revealed important differences between the response of calcareous sands and samples prepared with silica sand tested under similar conditions the calcare100 80 60 40 20 0 0 0 0 2 0 4 0 6 0 8 1 0 normalized number of cycles (n/nf) 100 80 60 40 20 0 0 0 0 2 0 4 0 6 0 8 1 0 normalized number of cycles (n/nf) calcareous sand calcareous sand silica sand silica sand c yc lic re si st an ce r at io , c r r t x (  d/ 2 ' 3c on s ) n or m al . r es id . p or e pr es su re (% ) ( δ u re s/ ' 3c on s ) n or m al . r es id . p or e pr es su re (% ) ( δ u re s/ ' 3c on s ) eimar a sandoval and miguel a pando62 ous sand had greater liquefaction resistance than ottawa silica sand tested at similar relative densities and effective consolidation stress levels the cabo rojo calcareous sand also had unique characteristics regarding their excess pore pressure generation curves which showed large fluctuations between loading cycles these fluctuations could be attributed to stress relaxations due to the unique characteristics of this sand, such as particle shape, roughness and intra-particle porosity resulting in important differences in the characteristics of particle contact in samples of this sand compared to ottawa silica sand samples in terms of axial strain development, calcareous sand also had a unique response under cyclic triaxial loading; axial strain gradually increased until cyclic liquefaction was reached whereas for the ottawa silica sand a sudden marked increase in axial strains was consistently observed just before samples reaching a state of cyclic liquefaction it may thus be affirmed that based on the important differences observed in terms of cyclic triaxial testing, between the cabo rojo calcareous sands and the quartzitic terrigenous ottawa sands, conventional empirical in-situ based liquefaction methods (e g the spt based simplified liquefaction method) not provide good support for evaluating liquefaction resistance for calcareous sands, due to their particular mineralogy, particle characteristics and associated geotechnical behaviour acknowledgements the authors would like to acknowledge the financial support provided by the puerto rico strong motion program and the pr sea grant which partially funded the assistantship for the first author additional funding for this research project came from a nehrp-usgs 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press, 45-76 skempton, a w (1954) the pore pressure coefficients a and b, geotechnique. 4, 143-147 yoshimi, y , richart, f e , prakash, s , balkan, d d , and llyichev, v a (1977) soil dynamics and its application to foundation engineering, 9th international conference on soil and mechanics and foundation engineering, tokyo, japan, july, 605-650 youd, t l , idriss, i m , andrus, r d , arango, i , castro, g , christian, j t , dobry, r , finn, w d l , harder, l f , hynes, m e , ishihara, k , koester, j , liao, s , marcuson iii, w f , martin, g r , mitchell, j k , moriwaki, y , power, m s , robertson, p k , seed, r , and stokoe, k h (2001), liquefaction resistance of soils, summary report from the 1996 nceer and 1998 nceer/nsf workshops on evaluation of liquefaction resistance of soils, journal of geotechnical and geoenvironmental engineering, asce, 127, no 10, 817-833 energy resources in addition to water resources are the most limited resources in jordan, being one of the fourth poorest countries in water resources, and limitation of surface water resources put huge pressure on groundwater which is the main resource there. high expenses and the increasing prices of oil over all worlds increase the feasibility to mining the oil shale that exists in southern jordan area, siwaqa. this study took place to clarify the possible effects of mining and energy production activities on the water resources in that area. groundwater vulnerability mapping was done for many areas all over the country, including this part. the initiative of this work is to determine the vulnerability under the conditions of removing the bedrock of the oil shale which is described as a confining layer. results that are obtained by this work conclude that the oil shale area becomes highly vulnerable to the human activities because of the existing geological structures while it is small and medium vulnerable in the fields in which there are no geological structures. in addition to the structural features and adding the possibility of the oil shale mining from the outcropped areas which will decrease the depth to water table and hence will affect the vulnerability values. las fuentes de energía y agua son las más limitadas en jordania, uno de los cuatro países más pobres en recursos hídricos; además, las limitadas fuentes superficiales hacen de las aguas subterráneas las más importantes. las ganancias y el precio del petróleo, por su parte, incrementan la viabilidad de la minería de esquistos bituminosos en el sur de jordania, en la región de siwaqa. este estudio se realiza con el fin de establecer los posibles efectos de las actividades de producción de energía y minería en las fuentes hídricas de la zona. se elaboró un mapa de vulnerabilidad para las fuentes de agua subterránea en varias regiones del país, incluida el área de estudio. la iniciativa de este trabajo es determinar la vulnerabilidad si se remueve el lecho rocoso del esquisto bituminoso, que funciona como una capa de confinamiento. los resultados obtenidos evidencian que el esquisto bituminoso se torna bastante vulnerable a las actividades humanas debido a la presencia de estructuras geológicas, mientras que su vulnerabilidad es media o baja en aquellos lugares donde no hay estructuras geológicas. además de los rasgos estructurales, existe la posibilidad de que la minería del esquisto bituminoso en las áreas de afloramiento podría descender hasta la capa freática y allí afectar los valores de vulnerabilidad. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): f1f8 abstract resumen keywords: oil shale, ex-situ mining, groundwater, vulnerability, drastic. palabras clave: esquisto bituminoso, minería ex situ, aguas subterráneas, vulnerabilidad, drastic. the effects of ex-situ oil shale mining on groundwater resources in siwaqa area, southern jordan, using drastic index and hydrochemical water assessment record manuscript received: 24/042014 accepted for publication:08/01/2016 how to cite item mohammad, a. h., alkurdi, o., & salameh, e. (2016). the effects of ex-situ oil shale mining on groundwater resources in siwaqa area, southern jordan, using drastic index and hydrochemical water assessment. earth sciences research journal, 20(1), f1-f8. doi: http://dx.doi.org/10.15446/esrj.v20n1.43128 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.43128 h y d r o g e o l o g y efectos en las fuentes de agua subterránea de la minería ex situ de esquistos bituminosos, en el área de siwaqa, al sur de jordania, a través del índice drastic y la evaluación hidroquímica del agua alsharifa hind mohammad, omid alkurdi, elias salameh the university of jordan, jordan. s.jasem@ju.edu.jo f2 alsharifa hind mohammad, omid alkurdi and elias salameh introduction: having independent energy resources increases the stability of a country and lessens its dependence on exterior resources and boosts the economy. jordan, aiming for such goals, tries to enter a new phase of energy development and raw material acquisition which can be seen in the increased number of mining concessions granted to numerous major companies throughout the country, especially in the central part of jordan, to extract oil shale from their deposits there, figure 1. the main issue which faced and is still facing jordan during the last decades is the urgent need to protect the groundwater resources in quality and quantity. vulnerability studies represent the most important protection actions to sustain these resources for present and future generations. (mohamad; 2014) figure 1: location map of the study area. the increasing interest in new sources of energy both renewable and non-renewable has also hastened the process of explorations and also mining for these resources. mining processes, products and byproducts could have serious repercussions on the environment if these issues are not studied and treated accordingly. jordan, in the most part, is considered to have arid climate with little influence of semi-arid climate in small parts of the country, making the country short in water resources much like other countries that fall under the same climatic conditions. being a country that is poor in water resources there must be a need to be aware of any developments in the water situation, and water characteristics, and quality changes of both surface and ground water which are of high importance. protecting these resources from all kinds of pollution becomes an important task. therefore, the new mining oilshale and other resources must be adequately studied on their effects on water resources both from qualitative and quantitative aspects (luck, 2008). the rocks found in the area belong to the most upper deposits of the upper cretaceous of jordan. following is a brief description of the geologic formations cropping out in the study area and of relevance to the present study (table1 and figure 2). the ajlun group overlies the kurnub group, and includes all the marine sediments which mainly consists of limestone, dolomite, marl, shale and sandstone and is subdivided into five formations; na`ur formation which consists mainly of limestone, gray and pink in color, hard crystalline coarse-grained and fractured, and is considered as a poorly developed aquifer with a thickness ranging from 200 to 230 m (bender, 1968). fuhais formations which consists of marl, limestone, marly limestone and shale, and is considered as a semi aquifer with a thickness of 60-80 m (mc donalds, 1965). hummar formation; this formation forms an important lower good aquifer generally consisting of limestone and dolomitic limestone. generally, the thickness of this formation is between 30 and 60 m. shueib formation this formation acts as an aquiclude between hummar and wadi as-sir aquifers in the groundwater basin with a thickness of about 200 m, mainly consisting of marl and marly limestone (mc donalds, 1965). wadi as-sir formation; this formation acts as a good aquifer. it covers the minor part of the study area and consists of about 100 m thick bedded limestone with chalky limestone interactions and occasional chert beds and nodules (mc donalds & partners, 1965). belqa group was firstly named by quennell (1951) and was divided into five formations of predominantly limestones and chalks showing partly high amounts of chert and phosphate; wadi umm ghudran formation which is considered as an aquitard, it is composed of white chalk, chert, and microcrystalline limestone concretions with phosphatic chert, with an average thickness of about 20 m (mc donalds, 1965). in places, this formation is missing and the overlaying formation amman silicified limestone directly overlays the underlying formation of wadi assir formation; then amman silicified limestone formation is found; lithologically, this formation is composed of dark gray to brown chert intercalated with limestone. the chert is micro fractured and brittle deformation is found in this formation. marl and chalky dolomite laminas are found an addition to some phosphate granules in the most upper parts (mc donalds, 1965). this formation is exposed in the central parts of the area and in a remarkable outcrop to the north of the swaqa town. the thickness of the formation is around 50 m. this formation forms a good aquifer in the northwestern part of the country including the study area.after that; al hasa phosphorite formation which is composed of phosphatic chert, phosphatic limestone, phosphate, chalky limestone, micritic limestone, marl, with the presence of oysters shells. it is widely exposed in the northern parts of the area, and extends to most eastern and western parts of the northern part of the area. the formation thickness is around 60 m in the study area. together with the overlying amman silicified it forms a good aquifer. muwaqqar (chalk marl) formation this is composed of marl, chalky limestone, micritic limestone and chert. oil shale forms the lower parts of this formation. it crops out in some central parts of the area in addition to the southern parts of the study area. generally it underlies the whole study area with a thickness ranging from 200 to 300 m. then; umm rijam formation this formation consists mainly of brown chert, chalk, and chalky limestone. it is exposed in a small part of the study area to the east, and has a thickness of about 70 m in the study area. overthat, superficial deposits are exists due to the relatively high relief in the study area the activities of surface water widely deposited quaternary superficial deposits including the following types fluviatile and lacustrine gravels; these sediments comprise coarse grained sand and sub angular to sub rounded gravels of chert, limestone, dolomite, dolomitic limestone, sandstone and basalt (shawabekeh 1998). these sediments are found as cemented layers or friable sediments and can be found widely distributed in most parts of the study area where the topography is gentle. and calcrete (caliche) which covers some parts of the study area, calcrete developed preferentially over the calcareous rocks in the presence of soil moisture evaporating and depositing the caliche. the thickness of caliche over the study area is generally less than or around 1m (alali, 2006). ground water vulnerability assessment was carried out for the wadi assir aquifer unit, to provide further understanding of the environmental risks and probability of pollution for this aquifer, the following is a brief summary of for the index that was used in this study. f3the effects of ex-situe oil shale mining on groundwater resources in siwaqa area, southern jordan, using drastic index and hydrochemical water assessment table 1. geologic formations in the study area (nra, open files). figure 2. geologic map of the study area (after nra, open files). methodology and data processing: groundwater vulnerability is an overlay method that is used to determine the ability of pollutants to penetrate to the target aquifer and to harm it. this method helps decision makers by shedding light on pollution areas expected to pollute groundwater aquifers as caused by human activities on the ground surface. in jordan, groundwater is the main water resource the country uses to match its demand, (mohammad, 2015). drastic is an overlay and indexing method which is widely used to assess groundwater vulnerability to a wide range of potential contaminants. merchant (1994) argued that drastic has been used throughout the world with exceptional frequency. in this model, spatial datasets on depth to groundwater recharge by rainfall, aquifer type, soil properties, topography, impact of the vadose zone and the hydraulic conductivity of the aquifer are combined (engel et al., 1996). according to aller et al., (1987) the di vulnerability index can be calculated according to the following equation: di = dwdr + rwrr + awar + swsr + twtr + iwir + cwcr where: dr = rating to the depth to water table, dw = weight assigned to the depth to water table. rr = rating for ranges of aquifer recharge, rw = weights for the aquifer recharge. ar = rating assigned to aquifer media, aw = weight assigned to aquifer media. sr = rating for the soil media, sw = weight for soil media. tr = rating for topography (slope), tw = weight assigned to topography. ir = rating assigned to vadose zone, iw = weight assigned to vadose zone. cr = rating for rates of hydraulic conductivity, cw = weight given to hydraulic conductivity. f4 alsharifa hind mohammad, omid alkurdi and elias salameh table 2. drastic index vulnerability index model; aller et al., 1987). f5the effects of ex-situe oil shale mining on groundwater resources in siwaqa area, southern jordan, using drastic index and hydrochemical water assessment the d factor; depth to water table: depth to the water is the distance between the ground surface and the groundwater table. the water level map is prepared by interpolating well information obtained from the water authority of jordan. these data present a d factor map indicating the water table. the rating for d decrease with increasing depth. the most vulnerable d rates are pixels which are located in areas with the lowest depth to water table and therefore higher potential to contamination. the rating of different depth intervals are shown in table 2. net recharge – the “r” factor: precipitation is the primary source of groundwater because priciptation rates ifiltrates through the ground surface and percolates down to the water table. net recharge represents the amount of water per unit area of land which penetrates the soil cover and reaches the water table. recharge water is the principle vehicle for leaching and transporting contaminants vertically into the groundwater and horizantally within the aquifer. arears with higher rainfall amounts pose higher potential and more vulnerable effects on the aquifer. many parameters affect the r factor. these parameters are rainfall and evaporation and finally permeability, soil retention and thickness of the soil (piscopo, 2001). components and there corresponding values and standardized values are shown in tables 2. aquifer media – the “a” factor: this factor refers to the type of the layer that contains water whether it is consolidated or unconsolidated rock serving as aquifers (sand and gravel or limestone). the large grain size and the presence of fractures or openings within the aquifer lead to higher permeability and lower attenuation capacity of the aquifer media; for aquifer media e.g. massive shale (low permeability) or sand and gravel (very high permeability) (table 2). topography – the “t” factor: topographic digitized maps of a scale of 1:50000 were used to produce digital version which were then converted into tin feature and slope maps. table 6 shows the values of slopes interval, according to aller et al., 1987. impact of the vadose zone – the “i” factor: this is the zone above the water table. it is unsaturated or discontinuously saturated with water, and determines the attenuation characteristics of the material below the typical soil horizon and above the water table. it is the region where pores and fractures are partially filled with water and partly by air and where also water droplets are generally in vertical a downward movement.table 2 shows the ratings for each vadose zone. hydraulic conductivity – the “c” factor: this is the ability of the aquifer materials to transmit water. it controls the rate at which groundwater flows under a given hydraulic gradient. the rate at which the groundwater flows also controls the rate at which a contaminant moves away from the source. table 2 shows the ratings of the hydraulic conductivity. values for the hydraulic conductivity are calculated and modeled from aquifer pumping tests. final vulnerability classes: the final vulnerability map is determined and classified into four legend categories following fritch et al (2000). this study divides the land into: low, moderate, high or very high vulnerability as shown in table 3. table 3. values assigned to vulnerability classes (aller et al., 1987) results and discussion: oil shale is a general term indicating a sedimentary fine grained rock containing a fossilized mixture of organic material that when heated breaks down –like oilto gas and other oil products after processing. difference between oil shale and normal oil is the burial time, which indicates the process at which both formed and the depth at which burial took place, in the case of the oil shale the burial was not deep enough or not for long enough time to transform into oil. a conservative estimate of oil shale reserves in the center of jordan is 47 billion tons, although the reserve estimates are not a direct indicator it gives a general idea of the situation (nra, open files). off-site processing of the oil shale generally referred to as exsitue. this is the widest spread processing technology. since oil shale processing entails mining the ore and shipping to fit facilities to undergo, processing in order to produce a more productive material. two primary methods are used to process the oil shale into synthetic crude oil: 1. retorting: retorting starts with drying and heating the oil shale at 500 600 degrees. then the heated shale passes to a gasification stage, where the residual organic carbon is oxidized at temperatures of less than 1000 degrees; to prevent its fusion which forms masses of in usable slag. finally, the spent shale is cooled to 80 – 100 degrees and passed through the lower parts of the retort and removed using a water seal. the heat required to retort the shale is supplied by recycled fuel gas resulting from the retorting process or combusting conventional fossil fuels (bsieso. m, 2006). 2. circulating fluidized bed: in this method, fine oil shale particles of a diameter less than 10 mm are introduced into a fluidized bed, these particles are separately heated to about 800 degrees. after that it mixed with the raw oil shale, causing it to break at 500 degrees. oil vapor and shale gas are separated and cooled to condense and collect the resulting oil. applying drastic index for the current situation in sewaqa area; the d factor (depth to water table) is determined from wells data collected from the ministry of water and irrigation (mwi) and analyzed for all the aquifers in the study area. after that, the data was drawn to give the final depth to water map. for the main aquifer in the study area, depth to water table is more than 30 meters; hence according to the drastic index the d value for this depth is 1 which means there are a little to no effects of the depth to the water level in the vulnerability ratings. for producing the r factor map; which consists of the sub factors of rainfall amounts, the slope of the topography and the soil permeability; the study area is with rainfall rates less that 500 mm/yr , which gives the value 1 for the building of the r map, in addition to the rainfall, the soil types map according to the soil permeability which reflects the values 1 and 2 according to the differences of the soil cover in the study area, the most effective factor in building the r map in the study area was the slope factor which vary from 1 to 3 in the located aquifer. applying that the r factor ranges from the value 1 to three and distributed in the study area as shown in figure 3. f6 alsharifa hind mohammad, omid alkurdi and elias salameh figure 3. r -factor values and recharge zone distribution in the study area. the information about the afactor, which is the aquifer media types was obtained from the natural resources authority (nra) the geological maps with 1:50000 scale, from well data jordan water authority, (open files )and own observation during the field work. the information was combined to determine the aquifers properties and hence the a factor values. the study area aquifer media is bedded and karstic limestone, the common media of the b2a7 aquifer, since the study includes only 1 aquifer the map will also show a solid color and a uniform value of 6 according to the used index. for finding the s factor map; a soil map of jordan which prepared by moa in 1993 was used as a base and along with field observations using the grain size analysis, most of the soils in the study area are of the loam type; these reflect the value of 6 after aller et al., 1987. producing the t factor map, the topographical setting of the study area was done using digitized maps of a scale of 1:50000 to produce digital version which were then converted into slope maps using the arc gis.results obtained from this factor reflects t values range from 1 in the areas with steep slopes and up-to 7 in the gentle slopes areas as shown in figure 4. figure 4. slopes and t factor distribution in the study area. for the i factor; the impact of vadose zone, the national water master plan (1977), geologic maps nra and field visits were used to obtain the information of the vadose zone in the study area, many types of rocks and sediments lay over the targeted aquifer which reflect many values of i factor, the distribution of these values are shown in figure 5. the final factor of the drastic index is the c map which is the hydraulic conductivity factor, this was produced by bundesanstalt für geowissenschaften und rohstoffe bgr (1997) which is the federal institute for geosciences and natural resources, they calculated the hydraulic conductivity of the aquifer rocks and b2a7 conductivity was found to be 2*10-6 -1*10 -4 m/s; which reflects the value of 1 according to drastic. the final drastic vulnerability thematic layer is generated by summing each drastic thematic parameter. each of these parameters according to be already multiplied by its designated weight. figure 6 shows the final vulnerability classes in the study area, dividing it into 2 classes of low and medium vulnerability classes (alkurdi, 2013). figure 5. i factor distribution in the study area. but considering the geological structure in the area, which mainly consisting of extensional faults (figure7), makes the groundwater quite vulnerable to human activities, especially along those faults. overlaying the final vulnerability map with the existing faults, shows that the groundwater in the b2/a7 aquifer system will be endangered by human activities including oil shale mining (alkurdi, 2013). figure 6. drastic vulnerability classes in b2a7 aquifer for the study area f7the effects of ex-situe oil shale mining on groundwater resources in siwaqa area, southern jordan, using drastic index and hydrochemical water assessment figure 7. drastic vulnerability classes in b2a7 aquifer after ading the faults for the study area in the case of the ex-situe oil shale mining; the oil shale in the study area froms from the muwaqqar (chalk marl) formation; this formation is composed of marl, chalky limestone, micritic limestone and chert and overlaid the main aquifer in the study area. oil shale forms the lower parts of this formation. it crops out in some central parts of the area in addition to the northern parts of the study area (figure 8). generally it underlies the whole study area with a thickness ranging from 200 to 300 m. removing this amounts of rocks from the outcropped areas will change the depth to water table in that areas and so will increase the vulnerability values there. figure 9 shows the new vulnerability map in which the areas that supposed to be mined in order to extract the oil shale with its new vulnerability classes with extremely high value in some of the mining areas which need high protection parameters to be done in that areas during any activities to be done in the future. figure 8. mwaqqar chalk marl formation outcropped in the study area. figure 9. expected groundwater vulnerability in the study area in the case of the exsitue oil shale minig. hydrochemical analysis: ground water samples from seven wells and 10 surface water samples were collected from the study area and were analyzed for their major, minor, and trace elements. electric conductivity (ec), ph value, and temperature were measured directly on site using specialized digital equipment. the temperature of the collected samples ranged from 25 c° to 28 c° for the surface water which is alongside the normal natural temperature for the area. however, it was from 30.5 c° to 41.5 c° for the samples collected from various wells in the study area. electrical conductivity, which reflects the total dissolved solids in the target sample, was measured directly in the field for better accuracy of the readings; it ranged from 148µs/cm to 180µs/cm for surface collected samples, and from 1980 µs/cm to 2380 µs/cm for the ground water samples. sample ph ranged from 8 to 8.29 for the ground water samples and from 8.41 to 9.2 for the surface water samples. calcium and sodium are the dominant cations in the samples of the ground water. the calcium concentrations range from 136.8 mg/l to 453.3 mg/l for and sodium from 156.5 mg/l to 228.7 mg/l. for the surface water samples calcium ranged from 33.48 mg/l to 525 mg/l and sodium from 8.48 mg/l to 51.2 mg/l. chloride and sulfate are the anions dominating the samples in terms of measured concentration, while for the ground water samples the concentrations of chloride ranged from 125.5 mg/l to 183.4 mg/l for sulfate from 242 mg/l to 321 mg/l, the concentrations in surface water ranged from 6.8 mg/l to 31.6 mg/l and 1.2 mg/l to 1155 mg/l for both chloride and sulfate respectively. furthermore, the other concentrations are represented in tables 4 and 5. trace elements group consists of boron (b), barium (ba), iron (fe), manganese (mn), strontium (sr), phosphorus (p), and silicon (si) were determined in all of the samples. the concentrations were in ranges less than 10 mg/l with sr and si being the major contributors with concentrations of sr from 2.75 mg/l to 4.55 mg/l for well samples and from 0.16 mg/l up to 2.34 mg/l for surface water samples, si ranged from 5.06 mg/l to 9.84 mg/l in well retrieved samples and 2.71 mg/l up to 11.65 mg/l in surface water samples, results of these analysis are provided in table 10. f8 alsharifa hind mohammad, omid alkurdi and elias salameh table 4. concentrations of anions in analyzed water samples, (mg/l). table 5. major cations and trace elements in the collected samples (mg/l) conclusion: results that are obtained by this work concluded by that the study area by applying the normal drastic index shows two vulnerability classes which are low and moderate because of the deep water table and the limited recharge ability in the area in addition to other drastic factors, but in the case of applying the modified drastic index which considers the geological features into account increase the vulnerability classes by converting some areas to highly vulnerable areas to the human activities because of the existing geological structures which are weak zones that may allow pollutants to reach easily to the water table. in addition to the structural features; adding the possibility to the oil shale mining from the outcropped areas in the study area decrease the depth to water table by removing the vadose zone that is existed over the targeted aquifer and which will directly affect the vulnerability values in that areas. this work in very important to be a reference point to the option of using the ex-situe oilshale mining; this will shed the light on the groundwater vulnerability of the areas that was covered by the oilshale bed and that will be removed after the mining process. references: abed, a., (2000). the geology, environment and water of jordan. amman: the jordanian association of geologists. al-adamat, r. a. n., foster, i. d. l., baban, s. m. j. 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(1996): water quality degradation in jordan (impacts on environment,economy generation resources base), final report. friedrich-ebert-stiftung and royal society for the conservation of nature, higher council of science and technology. the national library, deposit no. 1134/8/1996, amman luck, t (2008-08-07). “jordan set to tap oil shale potential”. the jordan times. retrieved 2008-10-25. the german council of environmental advisors (sru) (1998): special report on groundwater protection.http://www.umweltrat.de/son98en1.htm water authority of jordan (waj), open files. in this paper, the test-bench for sonic logging tool is proposed and designed to realize automatic calibration and testing of the sonic logging tool. the test-bench system consists of host computer, embedded controlling board, and functional boards. the host computer serves as the human machine interface (hmi) and processes uploaded data. the software running on host computer is designed on vc++, which is developed based on multithreading, dynamic linkable library (dll) and multiple document interface (mdi) techniques. the embedded controlling board uses arm7 as the microcontroller and communicates with host computer via ethernet. the embedded controlling board software is realized based on embedded uclinux operating system with a layered architecture. the functional boards are designed based on field programmable gate array (fpga) and provide test interfaces for the logging tool. the functional board software is divided into independent sub-modules that can repeatedly be used by various functional boards and then integrated those sub-modules in the top layer. with the layered architecture and modularized design, the software system is highly reliable and extensible. with the help of designed system, a test has been conducted quickly and successfully on the electronic receiving cabin of the sonic logging tool. it demonstrated that the system could greatly improve the production efficiency of the sonic logging tool. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): k1 k4 abstract keywords: sonic logging tool, test-bench, embedded controlling board, modularization, layered architecture. research on test-bench for sonic logging tool issn 1794-6190 e-issn 2339-3459 doi:http://dx.doi.org/10.15446/esrj.v20n1.54141 g e o ph y si c s xianping liu, xiaodong ju, wenxiao qiao, junqiang lu, baiyong men, kai zhang, yongchao yao (state key laboratory of petroleum resources and prospecting, china university of petroleum, beijing 102249, p. r. china) email: 2005xinchen2006@163.com record manuscript received: 12/11/2015 accepted for publication: 05/02/2016 how to cite item liu, x., ju, x., qiao, w., lu, j., men, b., zhang, k., & yao, y. (2016). research on test-bench for sonic logging tool. earth sciences research journal, 20(1), k1k4. doi:http://dx.doi.org/10.15446/esrj.v20n1.54140 introduction the test equipment is an essential tool that plays a role in both production and maintenance of corresponding logging tool. in this paper, the test-bench for sonic logging tool is proposed and designed to realize automatic calibration and testing of the sonic logging tool. due to the various structures and functions of different types of logging tools, the test equipment for them are quite differently and must be designed separately. during the past years, early researchers in china has developed several test equipment for logging tools, such as multiple function test tables for well-logging instruments (lu et al., 2005), development of the self-test unit for eilog complete logging equipment (yan et al., 2008). however, this early test equipment is designed to check simply the functioning of corresponding logging tool as a whole, not test for the inner modules of it. with the development of electronic technologies, the functional modules of logging tools are becoming more and more complex. a typical example is the imaging logging tool. therefore, it has become a necessity to develop a new test-bench that verifies not only the tool as a whole but also the various internal modules. against this background, researchers have proposed a new method to develop test-bench systems for logging tools the one that is based on the idea of c/s architecture and building blocks layout (ju et al., 2009). so far, test-bench systems have already been developed in this way for multipole acoustic array logging tool (lu et al., 2010; lu et al., 2012), array induction logging tool (men et al., 2011), micro-resistivity scanning imaging logging tool (yang et al., 2012), and have successfully applied to the test and calibration of tool modules and subs. in this article, the test-bench for sonic logging tool is also designed based on the c/s architecture. the software system for test-bench is developed with highly extensible functional modules considering the hardware configuration of the sonic logging tool and the test items required. 1. structure of the test-bench system as shown in figure 1, the test-bench for logging tools mainly consists of a host computer, an embedded controlling board, and some functional boards. the embedded controlling board is connected to the host computer via ethernet, while to the various functional boards via extended i/o bus (eiob) (ju et al., 2009). in this paper, the functional boards mainly refer to the telemetry communication board, the bus interface board, and the signal acquisition board. among them, the telemetry communication board is provided with two can interfaces, the bus interface board is integrated with the high-speed interconnection k2 xianping liu, xiaodong ju, wenxiao qiao, junqiang lu, baiyong men, kai zhang and yongchao yao tool module bus (tmb) and a signal generator for acoustic signal simulation, the signal acquisition board realizes analog signal acquisition and processing in eight channels. according to the structure of hardware system, the software system for the test-bench is also divided into three parts: the host computer software, the embedded controlling board software, and the functional board software. the host computer software is aimed at managing the internetworking with embedded controlling board and performing a series of routine tasks such as data processing, graphic displaying, data storage. the embedded controlling board software communicates with the host computer and in charges of the control of various functional boards, by the way of command forwarding and data uploading. the functional board software controls the interface circuit of the corresponding board directly according to test assignments. figure 1. structure of the test-bench 2. system design 2.1 software design of host computer the host computer is a high-performance notebook pc, which is connected to the embedded controlling board via ethernet. the host computer sets test parameters and mode by sending commands to the embedded controlling board. by employing microsoft visual c++ 6.0 as the software developing platform, taking advantage of the object-oriented programming model, and using the technique of multiple document interface (mdi), the software for host computer has been developed to include functional modules such as hmi, graphic displaying, network communicating, data processing, plotting and testing. functional modules of the software are shown in figure 2. own to the sophisticated built-in functions for network applications in windows, there is no need to develop tcp/ip protocol stack or network drivers anymore. when testing a certain instrument sub, the software can be switched to corresponding functional module and operation interface by making a choice in the menu. testing yes no yes system initialization establishing a network connection connection successful? data accepted? no start to receive data no yes data upload？ create sending thread sending data distinguish frame type figure 2. functional modules of host computer software 2.2 design of embedded controlling board the embedded controlling board realizes functions such as bottom hardware management, test control, and network communication. it uses arm chip s3c44b0x as the microcontroller, a 10m network interface with isolation in which the ethernet controller rtl8019as is integrated; the 16-bit eiob is designed based on the bank8 memory space of s3c44b0x. the software system of the board mainly consists of hardware drivers, real-time operating system (rtos) based on uclinux (wu et al., 2011) and applications. hardware drivers are used to controlling hardware devices directly; rtos realizes multi-task scheduling, inter-process communication management and provides applications with system functions; applications realize various test functions based on the rtos. the software design for embedded controlling board mainly refers to the transplantation of embedded operating system and the development of drivers and applications. hardware drivers are independent of operating system and stored with system files. upon function calls, they are dynamically loaded into memory in the form of modules. the flow chart of the application program is shown in figure 3. after the system is powered on, global variables are initialized, at first; then device drivers are called, and bottom layer circuit boards are initialized. the network communication module is used to realize socket connection with host computer, where the socket works in the non-block mode and data is sent to host computer by a separate thread. the transmitted data frame is defined to consist of a frame head and data segment. upon arrival of data, the program determines which testing routine the data is sent for by reading and decoding the frame head. then the data frame is sent to target routine to be processed. once the test is completed, the program determines whether there is any data to be returned to the host computer. if no data is to be uploaded, the program returns and waits for new information. otherwise, a new thread will be created to send data to the host computer and after that, the program returns to wait for new information. the newly created thread will run independently to finish data uploading and then exit. figure 3. flow chart of application program for embedded controlling board 2.3 design of functional boards the functional boards provide circuit interfaces for logging tool or subs. figure 4 is the hardware structural of functional boards. all the three functional boards are developed based on fpga and integrated with eiob for communication with embedded controlling board. the telemetry communication board uses cyclone ep1c12q240 by altera as the controller and is integrated with controller area network (can) bus interface. the bus interface board and the signal acquisition board both adopt ep2c20q240c8 as the microcontroller. the bus interface board provides tmb circuit interface and the simulation signal. the signal acquisition board can perform synchronous data acquisition and processing in up to eight channels at the same time. all the functional boards software is designed with vhsic hardware description language (vhdl). from modularization and top-down design strategy, the software is divided into several functional modules that are integrated with the top layer of schematic, making it provided with a simple and intuitional architecture that is easy for maintenance. k3 research on test-bench for sonic logging tool figure 4. hardware structural of functional boards the telemetry communication board, which is integrated with two can interfaces and can be used to control logging tool or electronic master sub in replace of telemetry sub, realizes functions of responding to commands from host computer and storing the uploaded data. figure 5 gives the structural modules of telemetry communication board software. the main functional modules of the software are bus interface logic module, interruption management unit, and can interface module. the bus interface logic module makes an address assignment for the 16-bit eiob and generates the control bus that performs interface module selection as well as read-write control. coordinated by the control bus, the internal multiplexer will connect the chosen module to the eiob. the interruption management unit is used to enable, screen or clear the interrupt request (irq) from can interfaces. the can interface modules convert the bus control timing into the address and data bus multiplex timing of sja1000. besides, they will respond to interruptions from sja1000 and issue irq to the interruption management unit. t_data r_data d_wave switching control eiob bus interface logic module transmitting fifo receiving fifo transmitting-receiving controller dac module serial transmitter serial receiver waveform storage module figure 5. structural modules of telemetry communication board software the bus interface board is integrated with tmb interface for highspeed data transmission between internal modules of the downhole instrument, and an acoustic signal generator. the tmb can be used to simulate master or slave node of internal instrument modules by switching the working mode in software. it mainly performs three tasks. first, it receives and analyzes the serial output command from the electronic master sub and then returns the given data. second, it sends commands to sonic receiving sonde or electronic receiving cabin and returns the acquired data. third, it provides command strings for testing of sonic transmitting sonde where serial commands are needed. the acoustic signal generator uses the digital-to-analog converter (dac) ad5541 as waveform generator and produces simulated acoustic signal for testing of electronic receiving cabin (zhang et al., 2014). the waveform data is stored in fpga in advance. the analog signal generated by the dac is provided to corresponding test module after the processing of filtering and impedance matching. figure 6 gives the software structure of bus interface board, which mainly consists of a bus interface logic module, with functions such as transmitting fifo, receiving fifo, transmitting-receiving controller, serial transmitter, serial receiver, waveform storage module, dac module. the bus interface logic module is mainly used to write in the data to be transmitted, read the received data and control data transmitting and receiving. the transmitting and receiving fifos are configured within fpga, serving as a cache for transmitted or received data storage. the transmitting-receiving controller switches the working mode of the board to make it work in transmitting or receiving mode in turn. the serial transmitter or receiver is constructed of counter and shift register. they perform serial data transmitting and receiving in the charge of the transmitting-receiving controller. the waveform storage module is used to store the simulated waveforms that are prepared in advance. the dac module governs the digital-to-analog conversion process according to the timing of dac chip as well as the clock. figure 6. structural modules of bus interface board software the signal acquisition board can to acquire and process analog signals in the eight channels synchronously, and it is used for voltage acquisition of logging tool power supply in this study. after signals are put in, one of them will be selected by the multi-channel time-sharing multiplexer adg408 to be amplified and filtered and then converted by the analog to digital converter (adc) ad7677. after the conversion, fpga stores the data and uploads it if necessary. as shown in figure 7, the signal acquisition board software consists of bus interface logic module, adc controlling module, fifo. the bus interface logic module receives and analyzes the command string from embedded controlling board, breaking it into information pieces such as channel selection, gain factor, sampling control. the channel selection command is used to specify a channel among the eight channels so as to perform signal acquisition in. the gain factor is the number of times signal being amplified before acquisition begins and it ranges from 0 to 90db with a step of 6db. the sampling control command is sent to the adc controlling module to control the sampling process, e.g. initiating or ceasing the process, using setting sampling depth and interval. in particular, the adc controlling module will assign sampling tasks to ad7677 according to the command received. the fifo is a 16-bit first-infirst-out register that is configured in the fpga. it is used for temporary storage of the acquired data, which will then be uploaded to the embedded controlling board by the bus interface logic module via eiob. adc controlling module fifo eiob . . . signal 2 signal n signal 1 bus interface logic module channel selection gain factor sampling control collected data singal conditioning figure 7. structural modules of signal acquisition board software k4 xianping liu, xiaodong ju, wenxiao qiao, junqiang lu, baiyong men, kai zhang and yongchao yao4 3. test set-up and results the electronic receiving cabin is a critical as well as a widely used module in sonic logging tool, with its performance significantly affects the accuracy of signal acquisition. in the production of the module, the module test procedure should be too much complex and time-consuming if it is performed only by using basic instruments such as an oscilloscope, multimeter, which obviously is not beneficial to the scale production. therefore, in this paper, the test-bench is proposed and developed to realize automatic detection of the sonic logging tool. the cabin is power supplied by the test-bench and provided with simulated acoustic signal and tmb interface by the bus interface board. during the test, the host computer configures test parameters and issues commands to the embedded controlling board via ethernet. after receiving the command, the embedded controlling board forwards it to bus interface board via the eiob. according to the command, the bus interface board will send a simulated acoustic signal to the electronic receiving cabin, read the returned data via tmb interface and finally upload the data to host computer by way of embedded controlling board. figure 8 exhibits the waveform displaying interface of the host computer software that shows the test result of electronic receiving cabin. the waveform tagged d_ wave is the simulated acoustic signal while r_wave the signal returned by the electronic receiving cabin. from the figure, it can be seen that the two waveforms are almost the same in shape despite the existing of a time lag. therefore, the electronic receiving cabin module is considered as qualified, and the user may proceed with other tests. figure 8. test result of electronic receiving cabin conclusions in this paper, a brief introduction is given to the architecture of the test-bench before the system design is discussed in detail. the test-bench system design for sonic logging tool is conducted for the three parts respectively: the host computer software, the embedded controlling board, and functional boards. the host computer software is written in vc++, which is an object-oriented programming language. the embedded controlling board uses arm chip as the microcontroller, its software is designed based on a layered architecture with the uclinux operating system serving as the system kernel. the functional boards are designed based on fpga. the software modules on each board are developed in vhdl language and finally integrated with the top layer of schematic. with the layered architecture and modularized design, the software system is highly reliable and extensible. with the help of designed test-bench, a quick test has been conducted successfully on the particular logging tool module of electronic receiving cabin. to conclude, the test-bench for sonic logging tool has made it easy and time-saving to perform tests on the logging tool or subs, which has a significant meaning in laying foundations for industrial production of the sonic logging tool. acknowledgments: this work is supported by national natural science foundation of china (61102102,11204380,11374371,11134011), national science and technology major project (2011zx05020-009), petrochina innovation foundation (2014d-5006-0307), china national petroleum corporation (2014b-4011,2014d-4105,2014a-3912) and science foundation of china university of petroleum, beijing (2462015yq0516). references: ju, x.d., cheng, x.y., lu, j.q., xu, w., men, b.y. and wu, w.h. (2009). design of test-bench system for logging tools based on embedded structures. well logging technology, 33(3), 270-274. lu, b.p., qin, l. and zhang, q.j. (2005). multiple function test tables for well logging instruments. petroleum instruments, 19(6), 17-19. lu, j.q., ju, x.d., men, b.y. (2012). design of test platform for receiver mandrel of multi-pole array acoustic logging tool. in: fuzzy systems and knowledge discovery (fskd), 2012 9th international conference on ieee, 2660-2663. lu, j.q., ju, x.d., men, b.y., and wu, w.h. (2010). study on test method for transmitter mandrel of acoustic logging tool based on arm. well logging technology, 34(4), 389-392. men, b.y., ju, x.d., qiao, w.x., cheng, x.y. and lu, j.q. (2011). design of test-bench system for the multi-frequency array induction imaging logging tool based on embedded structures. science technology and engineering, 11(11), 2450-2454. wu, w.h., ju, x.d., cheng, x.y., lu, j.q. and deng, l. (2011). design of front-end computer software for logging tool test-bench based on μclinux. journal of china university of petroleum, 35(3), 63-66. yan, z.g., zhang, j.t., qin, l. and zhang, q.j. (2008). development of the self-test unit for eilog complete logging equipment. petroleum instruments, 22(1), 12-14. yang, x.f., ju, x.d. and wu, w.h. (2012). design of the communication software for logging tool test-bench based on b/s model. journal of china university of petroleum, 36(3), 88-92. zhang, k., ju, x.d., lu, j.q. and men, b.y. (2014). design of acoustic logging signal source of imitation based on field programmable gate array. journal of geophysics and engineering, v.11(4), 4500845013. doi: http://dx.doi.org/10.1088/1742-2132/11/4/045008 earth sciences research journal earth sci. res. j. vol. 20, no. 2 (june, 2016) letter from the editor dear readers. the earth sciences research journal has been recently flooded with dozens of good papers expecting to be published. thanks to you, readers, reviewers, authors for your trust. we are continuously working to improve our editorial processes. the trend topics of those papers go from gas and petroleum exploitation, land use, geological hazards, to sustainable development, environmental protection and optimization of operation techniques. without forgetting the basic research. all those subjects are welcome in earth sciences research journal. proudly, we remark our journal, your journal, as the natural stage for the debate between economic development and sustainable development. there is no a better referee for this discussion than education entities. if your manuscript put lights on this important subjects, do not hesitate to submit it. is this not a minor topic! nowadays, every political decision, every policy designed, every industrial and human activity are seen through the lens of the environmental awareness. we can not quit on this responsibility. as an example of how important those topics are, during the coming days, us president barack obama is going to enact a law expanding the protection areas in hawaii in close four times the current size. it means longlines fishing operators will not be permitted in the 582581 square miles; neither the seabed mining in a region with rich deposits of manganese, nickel, zinc, cobalt, and titanium. similar decisions have been taken in chile and costa rica, this last one with a 26% of its land territory protected as a natural park. just talking about some cases in this side of the world, where we are more familiar with. the next meeting of geological society of america, taking place in denver, on september, will hand on this same topics. are the extracting and exploitation of resource prohibitions the solution to environmentalism? welcome all your papers cracking this nut. carlos alberto vargas jiménez. editor in chief, earth sciences research journal le tt er f r o m t h e ed it o r 119 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 119 126 nonlinear forecasting of stream flows using a chaotic approach and artificial neural networks hakan tongal engineering faculty, civil engineering department, süleyman demirel university, turkey fluid dynamics this paper evaluates the forecasting performance of two nonlinear models, k-nearest neighbor (knn) and feed-forward neural networks (ffnn), using stream flow data of the kızılırmak river, the longest river in turkey. for the knn model, the required parameters are delay time, number of nearest neighbors and embedding dimension. the optimal delay time was obtained with the mutual information function; the number of nearest neighbors was obtained with the optimization process that minimizes rmse as a function of the neighbor number and the embedding dimension was obtained with the correlation dimension method. the correlation dimension of the kızılırmak river was 2.702d = , which was used in forming the input structure of the ffnn. the nearest integer above the correlation dimension (i.e., 3) provided the minimal number of required variables to characterize the system, and the maximum number of required variables was obtained with the nearest integer above the value 2 1d + (takens, 1981) (i.e., 7). two ffnn models were developed that incorporate 3 and 7 lagged discharge values and the predicted performance compared to that of the knn model. the results showed that the knn model was superior to the ffnn model in stream flow forecasting. however, as a result from the knn model structure, the model failed in the prediction of peak values. additionally, it was found that the correlation dimension (if it existed) could successfully be used in time series where the determination of the input structure is difficult because of high inter-dependency, as in stream flow time series. este trabajo evalúa el desempeño de pronóstico de dos modelos no lineares, de método de clasificación no paramétrico knn y de redes neuronales con alimentación avanzada (fnnn), usando datos de flujo del río kizilirmak, el mayor de turquía. para el modelo knn, los parámetros requeridos son tiempo de retraso, número de vecindarios cercanos y dimensión de encrustamiento. el tiempo óptimo de retraso fue obtenido con la función de información mutua; el número de vecindarios cercanos fue obtenido con la optimización de procesos que minimizan el rmse como una función del número de vecindarios y la dimensión de incrustación fue obtenida con el método de dimensión correlativa. la dimensión de correlación del río kizilirmak fue utilizado en la formación de la estructura de ingreso de las redes ffnn. la integración cercana sobre la dimensión de correlación proveyó el número mínimo de variables requeridas para caracterizar el sistema y el número máximo de variables requeridas fue obtenido con el número entero por encima del valor (takens, 1981). se desarrollaron dos modelos de redes fnnn que incorporan 3 y 7 valores de descargas retrasadas y el desempeño de predicción comparado con el modelo knn. los resultados muestran que el modelo knn fue superior al modelo de redes ffnn en el flujo de pronósticos. sin embargo, como un resultado del modelo de estructura knn, el modelo falla en los valores pico. adicionalmente, se encontró que la dimensión de correlación (de existir) podría ser usada eficientemente en series temporales donde la determinación de estructura de ingreso es difícil por la gran interdependencia, como en las series temporales de flujo. palabras clave: kizilirmak, método de clasificación no paramétrico knn, redes neuronales con alimentación avanzada, funciones de información mutua, dimensión de correlación. record manuscript received: 11/02/2013 accepted for publication: 01/11/2013 abstract resumen key words: kızılırmak, k-nearest neighbor, feedforward neural networks, mutual information function, correlation dimension 1. introduction reliable and accurate stream flow forecasting is essential for water resources management. stream flow simulations and forecasts are also important for optimization of water resources planning and allocation. therefore, in addition to many other reasons, understanding stream flow dynamics constitutes one of the most important problems in hydrology and water resources. for this purpose, many data-driven models have been developed, including linear, nonlinear, parametric and nonparametric models for hydrologic time series prediction in the past decades (marques et 120 al., 2006). generally, in regard to system dynamics, there are two basic assumptions that underlie different modeling techniques, stochastic and chaotic dynamics. regarding the former assumption, the observed hydrologic time series originated from a stochastic process with an infinite number of degrees of freedom. to this assumption, the mean behavior of a time series could be captured with linear models such as autoregressive, autoregressive moving-average (al-awadhi and jolliffe, 1998; toth et al., 2000), autoregressive integrated moving-average (chang et al., 2002; lisi and villi, 2001) and seasonal autoregressive integrated moving-average (modarres, 2007; ooms and franses, 2001; pekárová et al., 2009), from which great results have been obtained. however, to utilize these models, a priori assumptions are required, such as stationarity and gaussian distribution. as most hydrological time series involve non-stationarity and non-gaussian conditions, the data should be transformed before it can be used in stochastic models. however, after this transformation, nonlinearity still exists, and time series are processed with a linearity assumption (chen et al., 2008). recent studies have shown that the complex nonlinear behavior of stream flow dynamics should not necessarily be the outcome of a stochastic process. with the advent of deterministic chaos theory, a few hydrologic modeling studies have suggested irregular behavior could be the outcome of simple deterministic systems influenced by a few nonlinear interdependent variables (khokhlov et al., 2008; sivakumar, 2003; yu et al., 2011). numerous researchers have tried to reveal chaotic behavior in hydrologic time series, such as river flow (ng et al., 2007; sivakumar, 2007; sivakumar et al., 2001), lake level (frison et al., 1999; khokhlov et al., 2008) and rainfall (dhanya and kumar, 2011; sivakumar et al., 2006). because the studies revealed possible chaotic behavior in stream flow dynamics, the requisition for chaotic prediction methods in stream flow was obvious. the most employed prediction method for chaotic hydrological time series is the k-nearest neighbor (k-nn), which was used in this study (elshorbagy et al., 2002; liu et al., 1998; sivakumar, 2003; wu et al., 2009). however, in the hydrology literature, the studies that compare the modeling capability of the k-nn approach with other nonlinear modeling techniques are limited. to obtain more insight into the modeling capability of k-nn, another widely used nonlinear modeling technique the feed-forward neural network (ffnn) (araghinejad et al., 2011; deka et al., 2012; kuo-lin, 2011; vafakhah, 2012; wu and chau, 2010) was employed. additionally, in determining the number of input parameters for ffnn, to the author’s knowledge, the chaotic procedure in this study is the first proposed. the values for the dominant variables obtained from the chaos analysis were used as the minimum and maximum input parameters in the ffnn. generally, the prediction techniques for a dynamic system can generally be divided into two approaches: local and global (wu and chau, 2010). because the local approach uses only nearby states to make predictions, the k-nearest neighbor can be included in this class, and ffnn can be classified as the latter one. therefore, in this study, we preliminarily evaluate which nonlinear approach, local or global, is more efficient in forecasting stream flow time series. the two nonlinear approaches’ modeling capabilities are compared for the kızılırmak river, the longest river in turkey. the paper is organized as follows. section 2 introduces the study area and the stream flow time series that are used. in section 3, the basic principles of the k-nn and the feed-forward neural network as a sub-class of ann are described. following this, in section 4, the obtained results are given with a detailed discussion. the conclusions of the paper are presented in section 5. 2. study area and data the kızılırmak river basin is located between 37°58'-41°44' north latitudes and 32°48'-38°22' east longitudes. the kızılırmak river flows through a 1,355 km long course, the longest in turkey. in the basin, the continental climate is dominant, for which summers are characterized with moderate precipitation and winters are characterized with severe cold. the annual mean precipitation and temperature are 446.1 mm and 13.7°c, respectively. the flow regime of the river is irregular resulting from rainfall and snow-melt. the lowest discharges are observed between july and february, and the river starts to rise in the beginning of march and reaches its highest level in april (çakmak et al., 2007). the data that were used cover the period of january 1960 to september 2004 (figure 1), and the main statistical parameters of the stream flow time series are given in table 1. figure 1. daily discharge data at the 15-1501 (yamula) gauge station of the kızılırmak river 3. methodology 3.1. k-nearest neighbor approach the temporal evolution of a system can be described by a multi-dimensional phase space. the most frequently used reconstruction method for a univariate or multivariate time series is the delay time method which was developed by packard et al. (1980) and takens (1981). the main idea behind phase-space reconstruction is that the system is characterized by selfinteraction, and the observed time series can hold the information about the dynamics of the entire system (sivakumar et al., 2002). the past observations can be embedded in an m-dimensional state space according to: where , m is the embedding dimension of the vector and is the delay time. to characterize a dynamic system with an attractor dimension d, ( )-dimensional space is required (takens, table 1. the statistics of the kızılırmak river discharge data station no/name obs. period drainage area (km2) elevation above sea (m) mean (m3/s) std. dev. (m3/s) median (m3/s) coeff. of variation skewness kurtosis 151501/yamula 19602004 15182 990 66.47 87.66 31.20 1.32 2.94 14.59 hakan tongal (1) 121 1981). however, abarbanel et al. (1990) proposed that m d> would be sufficient. as it is seen in eq. (1), the main parameters that should be determined in the phase-space reconstruction method are the delay and the embedding dimension parameters. the most commonly used methods for determining these parameters are the mutual information (mi) and the correlation dimension methods, respectively. another commonly used method for determining the delay parameter is the autocorrelation function (acf). however, because the acf does not measure the nonlinear dependence, in this study, we employed a mi function that was based on joint probabilities that enabled us to measure the nonlinear dependency beyond linear correlation. the mutual information was computed according to: where and are successive values, and are the individual probabilities of and , respectively and is the joint probability density. to determine the optimal delay time parameter, it is advised to use the local minimum of the mi (frazer and swinney, 1986), i.e., no increase or decrease in the mutual information function’s successive values for a specific lag time. however, in some mi functions, it is rather difficult to determine a local minimum. therefore, to determine the optimal delay time, we recommend using the following formula to amplify the successive differences between the calculated mi function values (tongal et al., 2013): where currentmi is the mutual information value of which relative change will be calculated, nextmi is the successive value of currentmi the first local minimum could be taken where the relative change of mutual information as a function of delay time started to become constant with lag time. with a properly selected time delay, the considered time series can be reconstructed in the m-dimensional phase space by calculating the correlation exponent from the correlation integral ( ( )c r ) as follows: where h is the heaviside step function with ( ) 1h u = for 0u > , and ( ) 0h u = for , , r is the radius of a sphere centered on iy or jy , is the euclidean norm, and n is the number of data points. ( )c r gives the probability of two randomly selected vectors that lie within a certain distance (ng et al., 2007). the dimension d of the state space is related to the correlation integral as: additionally, the logarithm of both sides of eq. (5) gives a linear relationship where the slope equals to the correlation exponent d. if the correlation exponents increase as a function of embedding dimension, then the considered system can be thought of as stochastic, otherwise it is chaotic. in the latter situation, the correlation exponents reach a saturation value, which provides the correlation dimension of the system. the correlation dimension is a parameter that gives the number of dominant processes that are acting in the system dynamics (sivakumar and jayawardena, 2002). with the determination of the optimal delay time and embedding dimension, the state space of the system can be constructed, from which interpreting the dynamics of an m-dimensional map is possible using the following equation: where te is a noise term and and are vectors of dimension m that describe the state of the system at times t (current state) and t t+ (future state), respectively. if the function is known, it is possible to predict the future trajectory of a system. the function can be found with the k-nearest neighbor (k-nn) approach which was proposed by farmer and sidorowich (1987). in this approach, the nearby states are used to obtain future forecasts. one of the most commonly used functions for k-nn is the weighted form: where ( )ia t are the nearest neighbors of the last observed value (i.e., prediction starting point) and are the weights to be adjusted using the information from the n-nearest points. l denotes that these weights are different for each forecasted point. for more details, please refer to wu and chau (2010), ng et al. (2007), laio et al., (2003) and liu et al. (1998). 3.2. feed-forward neural networks artificial neural networks (ann) use a multilayered approach that approximates complex mathematical functions to process data. an ann is a system that is composed of discrete layers with each layer including at least one neuron. with a connection weight, each node of a layer is connected to the node(s) of the preceding layer but not to nodes of the same layer (sahoo et al., 2009). considering the feed forward neural network architecture in figure 2, if the layers and the neurons in the layers increase, the architecture of the ann becomes more complex, which complicates the solution process. therefore, it is important to select an optimal ann architecture. in this study, the numbers of neurons in the layers were determined with a trial and error process that minimized the root mean square error calculated from the differences between the predicted and observed values. figure 2. a typical feed forward neural network architecture in figure 2, the input signal propagates from layer to layer through the network in a forward direction. the weight vectors between the layers ( ijw and jkw ) were randomly generated in the range between -1 and 1. the total output of a jth hidden neuron was computed according to: where iq is the value of the ith input parameter to the hidden layer neurons, jb is the bias for the jth hidden layer neuron and n is the total number of input neurons. the calculated total input signal, js , that received the jth hidden layer neuron was converted to an output signal using an activation function . the output signal of the jth hidden layer neuron was for the n th pattern of the training data set. nonlinear forecasting of stream flows using a chaotic approach and artificial neural networks (2) (3) (4) (5) (6) (7) (8) 122 the output neuron received signals from all hidden-layer neurons and converted them to a single signal as output using an activation function . thus, the input and output signal of the kth output neuron were, respectively: where kb is the bias and n is the total number of neurons in the hidden layer (sahoo et al., 2009). by comparing the estimated output with the desired output, the weights were adjusted with well-known error propagation algorithms including levenberg-marquardt (lm), bayesian regularization (br) and gradient descent with momentum and adaptive learning rate back-propagation algorithm (gdx). in this study, we employed the lm algorithm as a learning algorithm. detailed information about the lm algorithm can be found elsewhere (aqil et al., 2007; daliakopoulos et al., 2005). in this study, the tangent sigmoid transfer function, of which the validity has been proven in hydrological applications, was used: where u is the js and ks in the hidden layer and output layer, respectively. 3.3. performance indices to evaluate the model performances, the following performance indices were used; nash-sutcliffe coefficient of efficiency (ce), mean absolute error (mae), persistence index (pi), root mean square error (rmse), relative volume error (rve) and coefficient of determination (r2). nash and sutcliffe (1970) proposed the coefficient of efficiency in the following form: where io and ip are the observed and predicted values, respectively, and o is the mean value of the observed values. mean absolute error is a measure that evaluates the absolute deviation of the predicted values from the observed ones. it is calculated as: the persistence index (pi) proposed by kitanidis and bras (1980), compares the predictions of a model with the best estimate for the future, which is given by the last observation (randrianasolo et al., 2011). the index has the following form: where is the last observed value at time i minus the lead time l. rmse is one of the most widely used criterions to assess model efficiency, and it evaluates the forecast errors with the following form: because this criterion is sensitive to large forecast errors (i.e., the errors are amplified by squaring), it provides a good measure for the goodness-of-fit at high flows. additionally, it has the same units as the observed values; thus, it enables the interpretation of the magnitude of error. the rve criterion shows the total relative error resulting from the model predictions as: r2 describes the proportion of the total variance in the observed data that can be explained by the model and is formulated as below: r2 changes between 0 (no relation) and 1 (perfect fit), which describes how much of the observed dispersion is explained by the model. 4. results and discussion to determine the chaotic dynamics within the stream flows, the correlation dimension method was applied. to calculate the correlation integrals, the delay time ( ) was computed using the mutual information function and its relative change with lag time (figure 3). figure 3. mutual information function and its relative change with lag hakan tongal (9) (10) (11) (12) (13) (14) (15) (16) (17) 123 in figure 3, it was difficult to select the first local minimum from the original mi function. the amplified differences (i.e., the relative change of the mi) between the successive values were more informative about where the first local minimum could be found. in the mi relative change, the mutual information function was stable and fluctuated around a constant value after 30 days. by determining the delay value, the correlation integrals were computed by the grassberger-procaccia algorithm for different embedding dimensions (m) from 1m = to 25m = (figure 4a). (a) (b) figure 4. (a) correlation integrals as a function of different embedding dimensions (m) (b) correlation exponents obtained from correlation integrals as a function of embedding dimensions from the scaling regions (i.e., nearly linear sections of each integral plots) of the calculated correlation integrals, the correlation exponents were calculated with the least squares estimation method and each calculated correlation exponent was plotted against its embedding dimension as seen in figure 4b. because the correlation exponent values increased with the embedding dimension up to a certain value (i.e. 2.702d = ) and then fluctuated around this value, chaotic behavior was indicated. this value was the correlation dimension calculated for the kızılırmak river and the nearest integer above this value provided the minimum embedding dimension for reconstructing the phase-space or the number of variables (i.e., the number of dominant variables) necessary to model the dynamics of the system (khokhlov et al., 2008; sivakumar and jayawardena, 2002; stehlik, 1999). thus, the results from these analyses showed that the required minimum number of variables to model the system dynamics was 3 (3 2.702> ) and the maximum number of variables to model the system dynamics was 7 ( ). in the k-nn model development, the embedding dimension was taken as 3 by considering the required minimum embedding dimension obtained from the above analysis. the required nearest neighbor numbers for k-nn analysis was determined with a trial and error process that minimized rmse as a function of nearest neighbor number. figure 5 shows that the rmse decreased as a function of nearest neighbor number until nearest neighbor number equaled 33 days and after this value, rmse started to increase. therefore, the optimal nearest neighbor number was selected as 33 days. figure 5. determination of the nearest neighbor number as a function of rmse criterion with these results, the required parameters for the k-nn model were obtained. to determine whether the obtained correlation dimension could be used as the lag value for the discharges, the following model structures were built. to the author’ knowledge, this study was the first to take the correlation dimension as the of required lag value for the discharges. by considering the correlation dimension value as the required number of variables that characterize the system, the following ffnn model structures were constructed that incorporate a minimum of 3 (3 2.702> ) and maximum of 7 ( ) lagged values. typically, the training data set is selected as 70%-80% of a time series and the remaining part is used as the calibration and test period (banerjee et al., 2011; daliakopoulos et al., 2005; riad et al., 2004). in this study, approximately 78% (36 years) of the entire data set was selected as the training period, and the remaining part, approximately 22% (10 years), was selected for the test period. table 2. the model structures that considered minimum and maximum lagged discharges ffnn model structures with minimum and maximum variables variable numbers ffnn-i model 3 ffnn-ii model 7 nonlinear forecasting of stream flows using a chaotic approach and artificial neural networks 124 (a) (b) figure 6. determination of the number of neurons in the hidden layers for the (a) ffnn-i and (b) ffnn-ii models as a function of rmse the ffnn model structures contained one input layer, two hidden layers and one output layer. the number of neurons in the hidden layers was determined using an optimization process that minimized rmse as a function of the number of neurons (figure 6). the results from these models are given in figure 7. from figure 7, the inadequacy of the k-nn model is obvious; failing to capture the peak flows. the reason for this is that the k-nn model predicted the next value by considering the past observed values as much as the number of nearest neighbors. to obtain accurate peak flow prediction, the number of peak values in the past observed period should be as much as the nearest neighbor number. if the number of observations of peak flow in the training period is smaller than the nearest neighbor number, than the model fails in the accurate prediction of peak flows. this is also valid for low flow predictions. from figure 7, there is a section at the end of the test period in which no flow was observed. the k-nn model constantly over-predicted this period. however, the ffnn models performed better in peak flow predictions than the k-nn model. to acquire more insight into the models’ performance, the performance indices were calculated and are given in table 3. (a) (b) (c) figure 7. the observed river flow and predicted river flow for the entire test period for the (a) k-nn (b) ffnn-i and (c) ffnn-ii models by means of the performance indices, the best model was selected as the k-nn model and the worst model was selected as the ffnn-i model. the highest pi, ce and r2 and the lowest rmse, rve and mae values were obtained with the k-nn model. as the results showed, there was not much difference between the ce and r2 values, and stand-alone evaluation of these performance indices does not give much insight into the model comparison. in addition to these criteria, rmse, rve, mae and pi demonstrated the clear superiority (nearly twice as much) of the k-nn model over the ffnn model. therefore, in the model comparison, it was important to take into account other performance indices that emphasized different features of the predicted values. table 3. forecasting performance of the nonlinear models kızılırmak river performance indices rmse rve mae pi ce r2 k-nn model 4.4776 10,634 1.6686 0.8803 0.9965 0.9970 ffnn-i model (3-7-3-1) 13.2685 23,571 4.3124 -0.0511 0.9689 0.9699 ffnn-ii model (7-6-7-1) 9.8451 20,838 3.4546 0.4219 0.9829 0.9829 hakan tongal 125 these results showed that the correlation dimension could be used in the determination of the ffnn model structure by taking the lagged values as the minimum and maximum dominant variable number. to the authors’ knowledge, this study is the first to show that the correlation dimension could be used in determining of number of lagged values of the discharges. this is important where the autocorrelation function remains high for higher lags such as in this case (figure 8). figure 8. autocorrelation function of the kızılırmak river it is difficult to determine the optimal lag values in the model structure where the autocorrelation function values start to become statistically insignificant. for instance, in figure 8, the autocorrelation function becomes statistically insignificant on the 90th day. to determine the optimal lag values that will be considered in the model structure, various models should be considered that incorporate combinations of lag values up to 90. obviously, this is quite time consuming. however, by considering the correlation dimension of the examined system, it is possible to construct two models that incorporate minimum and maximum lagged values that are determined from the correlation dimension. this result is evidence that chaos theory could be used in simplifying the modeling procedure, in which the determination of the input structure is rather difficult. 5. conclusions hydrological systems are complex and dynamic in nature as their current and future states depend on numerous variables (tongal et al., 2013). therefore, it is important to determine the number of dominant variables acting within the system dynamics. in regards to this, the methods from chaos theory provided us a proper framework. in this study, one of the chaotic forecasting methods, the k-nn method, was employed for the kızılırmak river, the longest river in turkey. the necessary parameters for this method are the delay time, the embedding dimension and the nearest neighbor number. the optimal delay time was determined from the mutual information function and the nearest neighbor number was determined from the optimization process that minimized rmse as a function of the nearest neighbor number. in determining the optimal delay time from the mi function, we calculated the relative differences between the successive values of the mi function. the optimal delay time was selected as 30 days. to determine the embedding dimension for the k-nn method, the correlation integrals were calculated for various embedding dimensions, i.e., 1m = to 25m = . when the obtained correlation exponents from the correlation integrals were plotted as a function of the embedding dimension, the correlation exponents reached a value (correlation dimension, 2.702d = ), which gave us the dimension of the system. the dimension of the system shows the number of dominant variables that are acting within the system dynamics. for this system, the minimal number of dominant variables was 3 (3 2.702> ) and the maximum was 7 ( ). to the author’ knowledge, this study is the first to examine whether the obtained correlation dimension could be used in the model development phase. in feed-forward neural network input parameter determination, the two models (ffnn-i and ffnn-ii) that were described incorporated lagged discharge values of 3 and 7, respectively. the number of hidden layer neurons was determined from a trial and error process that minimized rmse. the predictions obtained from the models showed that the k-nn model, which is one of the most commonly used chaotic prediction approaches, was superior to the ffnn models, which are a sub-class of another nonlinear prediction approach, the artificial neural networks. however, the k-nn model failed to predict 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(editor), dynamical systems and turbulence, lecture notes in mathematics. springer-verlag, berlin, pp. 366-381. tongal, h., demirel, m.c. and booij, m.j., (2013). seasonality of low flows and dominant processes in the rhine river. stochastic environmental research and risk assessment, 27(2): 489-503. toth, e., brath, a. and montanari, a., (2000). comparison of short-term rainfall prediction models for real-time flood forecasting. journal of hydrology, 239(1–4): 132-147. vafakhah, m., (2012). application of artificial neural networks and adaptive neuro-fuzzy inference system models to short-term streamflow forecasting. canadian journal of civil engineering, 39(4): 402-414. wu, c.l. and chau, k.w., (2010). data-driven models for monthly streamflow time series prediction. engineering applications of artificial intelligence, 23: 1350-1367. wu, c.l., chau, k.w. and li, y.s., (2009). predicting monthly streamflow using data-driven models coupled with data-preprocessing techniques. water resour. res., 45(8): w08432. yu, b., huang, c., liu, z., wang, h. and wang, l., (2011). a chaotic analysis on air pollution index change over past 10 years in lanzhou, northwest china. stochastic environmental research and risk assessment, 25: 643-653. hakan tongal this paper aims at the problem that the migration quality is poor in deep water. it analyzed the influence that velocity model accuracy had on migration and studied the impact that variable seawater velocity makes on movement effect. at first, variable seawater velocity affected by temperature, pressure, and salinity is defined to replace the actual seawater velocity. then variable seawater velocity’s influence on interface migration location, layer sickness and movement energy focusing degree are analyzed in theory. moreover, finally, a deep water layered medium model containing variable seawater velocity, a syncline wedge shape model, and a complex seafloor velocity model are constructed. by changing the seawater velocity of each model and comparing migration results of constant seawater velocity model and variable seawater-velocity model, it was found the conclusion. under the condition of deep water, variable seawater velocity’s impact on the quality of seismic migration is significant, which not only can change the location of geologic body migration result but also can influence the resolution of the geologic interface in the movement section and maybe can cause migration illusion. este artículo se enfoca en el problema de la baja calidad de la migración en aguas profundas. se analiza la influencia que tiene el modelo de precisión de velocidad en la migración y se estudia el impacto que la variación de velocidad del agua marina tiene en el efecto de movimiento. en primera instancia, se define la variación de la velocidad del agua marina afectada por la temperatura, la presión y la salinidad para reemplazar la velocidad del agua marina actual. luego se analiza la teoría de la influencia de la velocidad del agua marina sobre la interfaz de la ubicación de migración, el grosor de la capa y la energía de movimiento de acuerdo con la inclinación. además, finalmente, se construyó un modelo medio por capas de aguas profundas que contiene las variaciones de velocidad del agua marina, un modelo de forma de cuña sinclinal, y un modelo de complejo de velocidades del lecho marino. al cambiar la velocidad del agua marina en cada modelo y comparar los resultados de migración del modelo de velocidad de agua marina constante con los del modelo modelo de velocidad variable se encontró la conclusión. bajo las condiciones de agua profunda, el impacto de la variación de la velocidad del agua es significativo en la migración sísmica, lo que no solo puede cambiar el resultado de migración en la ubicación del cuerpo geológico sino que además puede influir en la resolución de la interconexión geológica en la sección de movimiento y podría causar la ilusión de migración. earth sciences research journal earth sci. res. j. vol. 20, no. 3 ( september, 2016 ) : h1-h7 abstract resumen keywords:seawater velocity, deep-water geology, migration imaging, migration velocity palabras clave: velocidad del agua marina, geología de aguas profundas, imágenes de migración, velocidad de migración. research into the effects of seawater velocity variation on migration imaging in deep-water geology record manuscript received: 22/03/2016 accepted for publication: 23/11/2016 how to cite item sun, h., sun, j., han, f., sun, z., liu, z., liu, m., & huang, x. (2016). research into the effects of seawater velocity variation on migration imaging in deep-water geology. earth sciences research journal, 20(3), j1-j6. doi:http://dx.doi.org/10.15446/esrj.v20n3.56382 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n3.56382 hui sun1, jianguo sun2, fuxing han1, zhangqing sun1, zhiqiang liu1, mingchen liu1, xingguo huang1 1. college for geoexploration science technology, jilin university, changchun, 130026 correspondence should be addressed to jianguo sun, e-mail: sun_jg@jlu.edu.cn se is m ic investigación de los efectos de la variación en la velocidad del agua marina sobre las imágenes de migración en la geología de aguas profundas hui sun, jianguo sun, fuxing han, zhangqing sun, zhiqiang liu, mingchen liu, xingguo huangh2 introduction as more and more offshore oil and gas fields have been discovered recently, the exploration difficulty for new ones increases sharply. thus, growing number of human and financial resources has been invested into deep-sea oil and gas exploration worldwide (barely, 1999). the south china sea is rich in oil resource while 70% of that located in the deep sea (li, 2006). during latest ten years the south china sea oil and gas exploration experienced both more and more attention from the government and also a series of problems of which one exists in migration processing on marine seismic data. the problem can be described as the movement energy not high enough in deep water so that the real structure and stratum could not be recognized clearly and exactly. the paper aims at solving it, thus analyzes the affection of variable seawater-velocity on migration in deep water from the perspective of velocity model’s accuracy. physical oceanography study reveals that the main influencing factors on the acoustic velocity of seawater are temperature, pressure, and salinity (feng, 1999; jones, 1999) which are also correlated with depth. therefore, seawater acoustic velocity is a function of depth as well. it is well known that pre-stack depth migration is so sensitive to velocity model that even a tiny error could change the final result (versteeg, 1993; herron, 2000). the seawater velocity is always processed as constant speed during marine seismic data processing. such processing has little affection on seismic data of shallow seawater but a big one on that of deep water which could not be neglected. the relationship between seawater velocity and wave field propagation has been studied and concluded by many researchers as follows. dyk and swainson (1953) drew the conclusion from the perspective of kinematics that the seawater acoustic velocity which varies as depth has a great effect on ray path. barley (1999) pointed out in his paper that seawater velocity variation is one of the important reasons that reduce the efficiency of marine seismic data processing. mackey and fried (2003) revealed further that it not only affects the event continuity of marine seismic data but also influences stack section showing inaccuracy amplitude information and wrong submarine tectonics. the similar question is put forward by lacombe et al. (2006), that seawater velocity variation can cause seismic event dislocation. xu and pham (2003) and jones (2010) studied seismic imaging and achieved the conclusion that variable seawater velocity sets back seabed imaging. papenberg et al. (2010) and song et al. (2010), who used seismic data to achieve the temperature and salinity variation curve form with the depth of seawater, provide the theory basis of variable seawater velocity employed in this paper. the influence of seawater velocity on seismic travel time, ray path, and amplitude is revealed based on empirical formulas of temperature, depth, and salinity (han et al., 2012). the paper firstly studies the practical method of seawater acoustic velocity and draws the variation curve of acoustic velocity with depth. then based on that method, the effect of the speed is researched theoretically on interface deviation, layer thickness, and focusing of migration energy. finally, a layered model, a syncline model, and a complex model are built to contrast movement results between variable seawater-velocity models and constant seawater-velocity models and reveal the effect of variable velocity on migration imaging in deepwater geology structure. 2. methodology 2.1 construction of seawater acoustic velocity model seawater velocity is a function of temperature, salinity, and pressure and always described in empirical formula. the one it was applied in this paper is the wilson empirical formula simplified by frye and pughfrom (feng, 1999). (1) in the above formulas c represents seawater velocity, t expresses time， s means salinity and p describes pressure. variable seawater-velocity model intuitively reflects the position relationship between seawater velocity and depth. thus temperature, pressure and salinity’s relationship with depth are needed. the changed law of pressure with depth is quite simple in linear form as is shown in figure 1a. temperature and salinity variation with depth are rather complicated because of sun radiation, ocean current, water masses and so on. the temperature and salinity are assumed to change evenly with depth in different extreme sections. then approximate variation curve of temperature and salinity with depth is formed in consideration of physical oceanography knowledge and temperature and salinity data (jones, 1999; chen and wen, 2010; zhao, 1985) collected in a region of south china sea in summer as is shown in figure 1b and figure 1c. the variable seawater velocity curve with depth is obtained after the equal pressure, temperature and salinity are calculated in equation 1. figure 1. the changing curve of pressure a, temperature b, salinity c and seawater velocity d in depth 2.2 effect of seawater velocity error on pre-stack depth migration the accuracy of velocity has a direct influence on migration and even stronger impact on pre-stack depth migration. in this section, the effect of seawater velocity on seawater movement is analyzed theoretically. in migration, imaging mv is assumed to be constant seawater movement velocity and ( )v z is the function of actual seawater velocity with depth. in seawater velocity model including horizontally stratified medium, each interface can still be recognized in migration section even when there is a significant difference between mv and true seawater velocity. however, the position of each interface changes as well as the layer thickness. the following part will discuss the influence of seawater velocity on interface position and layer thickness quantitatively. figure 2. the sketch map of layers the figure 2 shows the seawater velocity model including the horizontally stratified medium in which 1h describes seawater layer’s thickness while 1nh + represents the thn horizontal stratified medium’s thickness. zero offset time of geophone through single pass from interface one can be described as follows. research into the effects of seawater velocity variation on migration imaging in deep-water geology h3 (2) seawatert and vaverage respectively represents zero offset time of single pass and average seawater velocity. the whole layer position will climb when migration velocity mv is lower than vaverage . meanwhile, each layer is assumed to be thick enough to acquire the exact position and climbing height of each one as is shown below. (3) in the function, ' nh means the migration depth of the thn interface and nh� demonstrates the difference between migration depth and real depth. as is shown in equation 3 migration position error of the thn interface correlates with seawater migration velocity mv and velocity in the overlying layer nv , which decreases with mv and increases with nv . because of different speed in overlaying layers for a different interface, thus the climbing height of each layer is different in migration section. therefore, movement thickness is different from the true layer thickness. (4) in the above function, , 1n nh � means the thickness of the horizontal layer between thn and 1thn� interface. the average geological structure enjoys bigger velocity in the downside strata. when mv is lower than vaverage , the thickness of horizontal stratified medium decreases. furthermore, the more significant the overlying stratum velocity is, the less the migration thickness of the layer is. when mv is higher than vaverage , migration location of each interface will fall. (5) it can be seen from the functions that migration position error is directly proportional to movement velocity in seawater. different from the condition when movement velocity in seawater is lower than average speed, it is velocity in the strata below the interface that affects migration location error as well as the thickness of each horizontal layers. (6) the equation 6 shows that when migration velocity is higher in seawater, the movement thickness of the horizontal layer will increase. figure 3. the sketch map of simple layers in the figure 3, the horizontal strata below the seawater have the velocity of 2500m/s and the thickness of 1000m while the second horizontal stratified medium enjoys the velocity of 3500m/s. velocity in seawater varies with depth and velocities at different depth can be matched in figure 1d. table 1 shows the location error of interface one and thickness error of layer one in corresponding migration velocity at a different depth. table 1: the location error of interface one (m) and thickness error of layer one (m) remarks: + means the falling of migration location and the increase of migration thickness of the 1th layer. otherwise, − shows the climbing of migration area and the decrease of migration thickness of 1st layer the data given in table 1 are consistent with the analysis conclude before. the effect of variable seawater velocity on interface location and layer thickness has been taken into consideration. however, its influence on migration energy focusing can not be neglected as well. supposing v represents seawater acoustic velocity and mv is the applied migration velocity, the function of migration depth is obtained of common imaging point gathers by zhu (1998). (7) in equation 7, mz acts as migration depth and /mv vb = exists. when migration velocity is smaller, b is less than 1, otherwise b is more than 1. after then /mv vd = � and mz z z= + � are calculated in equation 7, the equation 8 is acquired. (8) in marine data processing, is less than z and velocity variation is much lower than true velocity. the equation 9 can be obtained after second order term of and are omitted. (9) based on functions above, the cigs of an impulse response was drawn in figure 4 to illustrate velocity error’s impact on cigs. figure 4 shows that when seawater migration velocity is quite low, the orbit of pulse imaging points is the one above the true orbit. when the seawater movement speed is higher, the orbit of pulse imaging points is the one below the true orbit. what’s more, depth error of the migration will increase with the growth of horizontal distance to object point. the velocity will affect the focusing of the movement energy and lower the effectiveness of migration imaging of marine geology structure. figure 4. the sketch map of velocity error’s impact on cigs hui sun, jianguo sun, fuxing han, zhangqing sun, zhiqiang liu, mingchen liu, xingguo huangh4 3. examples and results a layered model, a syncline model, and a complex model were built and then it was simulated to acquiring the seismic record. the effect of variable velocity on geology structure migration in deep water is achieved after the contrast between true velocity model and different constant seawater velocity model. seawater velocity in the three models all takes temperature, pressure, and salinity into consideration. the figure 1d describes variable seawater velocity curve with depth. 3.1 influence of variable seawater velocity on migration of horizontally stratified medium the figure 5 demonstrates the horizontally stratified medium model in deep water with the size of a 1501 x 1601, grid spacing of 4m x 4m and seawater depth of 3500m at which the data is collected in a region of south china sea. the horizontally stratified medium can be divided into three layers whose velocity are respectively 3250m/s , 4000m/s , and 4750m/s from up to down. figure 5 layered model in deep water firstly, the migration of true velocity model is obtained and then the seawater velocity is substituted with different constant seawater velocities, 1450m/s, 1470m/s, 1500m/s, 1520m/s, 1570m/s, to get similar constant velocity model and migration section of each one as is shown in figure 6. the contrast of movement results in different models shows that when seawater velocity is 1450m/s the lower the layer is, the higher the horizontal interface climbs. though three interfaces can be demonstrated in migration section, movement energy can not be entirely focused on each interface but both on the interface and the area above it. the more shallow the interface location is, the less focused the energy is in the interface. as the increase of migration velocity of seawater, the upward movement of each interface decreases and migration energy focus strengthens. when seawater velocity is 1500m/s, the movement result which still has some difference with the one achieved at variable velocity is most similar to the original model. when seawater velocity is 1500m/s, the migration result which still has some difference with the one achieved at variable velocity is most similar to the original model. as the increase of migration velocity of seawater, the downward movement of each interface increases and is directly proportional to the depth of interface. movement energy mainly distributes on the interface and in the region below it. figure 6. migration results of layered model in deep water, the corresponding seawater velocity of a, b, c, d, e, f is true seawater velocity 1450m/s, 1470m/s, 1500m/s, 1520m/s, 1570m/s research into the effects of seawater velocity variation on migration imaging in deep-water geology h5 figure 8. migration results of syncline model in deep water, the corresponding seawater velocity of a, b, c, d, e, f is true seawater velocity, 1450m/s, 1470m/s, 1500m/s, 1520m/s, 1570m/s 3.2 influence of variable seawater velocity on migration of syncline model the figure 7 shows the syncline model in deep water with the size of 1501 1601� , grid spacing off 4m 4m� and seawater depth of 3500m. the horizontally stratified medium can be divided into three layers whose velocity are respectively 2500m/s , 3000m/s , and 3500m/s . the velocity of wedge geologic body is 5500m/s . figure 7. syncline model in deep water the figure 8 demonstrates the variable seawater velocity model of syncline and migration corresponding to constant seawater velocity model at the speed of 1450m/s, 1470m/s, 1500m/s, 1520m/s and 1570m/s. the contrast of migration results in different models reveals that when seawater velocity is 1450m/s all movement location are higher than true interface position. the distance between two horizontal layers decreases as well as the angle of wedging geology body. migration interface of syncline is steeper than original interface. movement energy is not well focused on interfaces but also distributed in the region above migration interface. as the increase of seawater velocity, the location of movement section and radian of syncline interface decreases. the distance between two layers increases and the focus of migration energy is improved. when seawater velocity is 1520m/s, the movement result is most similar to the one applying real seawater velocity. when seawater velocity is as high as 1570m/s, radian of syncline interface and distance between two horizontal layers decreases and migration section moves downside. meanwhile, energy is scattered both above and under movement interface. therefore, the shape of movement interface is a quite different from that of the true interface. hui sun, jianguo sun, fuxing han, zhangqing sun, zhiqiang liu, mingchen liu, xingguo huangh6 3.3 effect of variable seawater velocity on complex seabed geology structure model in deep water the figure 9 displays the complex seabed geology structure model in deep water with the size of 1501 1601� and grid spacing of 4m 4m� . velocities of layer in the model are respectively 2500m/s, 2700m/s, 2900m/s, 3000m/s, 3100m/s, 3300m/s and 3500m/s from up to down. the velocity of embed body on the right side is 3300m/s. figure 9. complex model in deep water figure 10 illustrates migration results of complex model in deep water at constant seawater velocity of 1450m/s, 1470m/s, 1500m/s, 1520m/s, 1570m/s. it can be concluded from the contrast of these migration sections that when seawater velocity is 1450m/s all interfaces move upward in the migration section. at different depth, different horizontal position in migration section moves upwards in different distances which increase with deeper depth of the sea. furthermore, movement energy of interfaces cannot be well focused mainly on interface two and interface three whose interfaces can not be recognized clearly in migration section. meanwhile, the up and down interfaces are also unable to be connected to embed layer. when seawater velocity is 1470m/s or 1500m/s, the problem is partially improved in contrast with the condition when the velocity is 1450m/s but still having a severe influence on migration results. then when the seawater velocity is as high as 1520m/s, movement achieves the best result among ones of constant seawater velocity. however, migration energy still can not be well focused. afterward, when the velocity is 1570m/s, the up and down interfaces can not be connected to embed layer. interfaces can be recognized reluctantly but in low resolution. in comparison with a true condition, there is a marked difference in space position and geometry. figure 10. migration results of complex model in deep water, the corresponding seawater velocity of a, b, c, d, e, f is true seawater velocity 1450m/s, 1470m/s, 1500m/s, 1520m/s, 1570m/s. research into the effects of seawater velocity variation on migration imaging in deep-water geology h7 4. conclusions acoustic velocity of seawater is mainly influenced by temperature, pressure, and salinity and is also correlated with depth. the seawater acoustic velocity is always processed as constant velocity during marine seismic data processing. such processing has a severe effect on seismic data of deep seawater which can not be neglected. meanwhile, the accuracy of velocity model can influence accuracy and results of migration. this paper begins with improving the weaker movement field and worse migration results in deep water region of south china sea and then analyzes effects of seawater velocity on migration results. it can be concluded from layer model and syncline including wedge model that variable velocity has an adverse influence on interface location, the shape of geology body and focusing on migration energy. the analysis of complex seabed geology model reveals that variable seawater velocity can make interfaces of geology body hard to be distinguished in movement section. furthermore, variable speed even leads movement section to show wrong geology structure. therefore, seawater velocity should not be only taken as constant in deep-water seismic data processing. 5. acknowledgments this study was supported financially by the national natural science foundation of china (42174120, 41404085, 41504084). we are grateful to danian huang of jilin university, changchun, for his kind computer supporting. references barley, b. 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(2010). seawater thermohaline structure inversion from seismic data. chinese journal of geophysics, 53(11), 2696-2702. doi: 10.1002/cjg2.1569 versteeg, r. j. (1993). sensitivity of prestack depth migration to the velocity model. geophysics, 58(6), 873-882. doi: 10.1190/1.1443471 xu, s., & pham, d. (2003). global solution to water column statics: a new approach to an old problem. 73rd ann. internat. mtg.: soc. expl. geophys. zhao, x. y. (1985). the comprehensive research report of the south china sea. science press, beijing. zhu, j., lines, l., & gray, s. (1998). smiles and frowns in migration/ velocity analysis. geophysics, 63(4), 1200-1209. the concentration and distribution of aromatic biomarkers in coals and shales from five boreholes penetrating the maastrichtian mamu formation of the anambra basin, southeastern nigeria, were investigated by gas chromatography-mass spectrometry to assess the thermal maturity and organic matter input. the study focused on the variations of the relative abundances of naphthalenes, phenanthrenes, and monaromatic and triaromatic steroids identified on the mass fragmentograms. trimethylnaphthalene (tmn) is the most abundant member of the naphthalene family while methylphenanthrene (mp) is the most abundant phenanthrene family member. the total of phenanthrenes and their isomers was greater than that of naphthalenes. the distribution of these aromatic hydrocarbons and their akyl derivatives was strongly controlled by a selective expulsion mechanism and thermal maturation of organic matter. the low dibenzothiophene/phenanthrene (dbt/phen) ratios (0.01-0.06), as well as the enhanced concentrations of 1,2,5-tmn relative to 1,2,7tmn, indicates organic matter derived mainly from higher plants, and the extract ternary plot of c27, c28 and c29 monoaromatic steroids suggests a type iii and mixed type ii/iii kerogen. the calculated mean vitrinite reflectance (%rm), determined from the distributions of the isomers of methyldibenzothiophene ratio (mdr) in the rock extracts, ranged from 0.51 to 1.43. these maturity values indicate that the coal and shale extracts are marginally mature for hydrocarbon generation. la concentración y distribución de biomarcadores aromáticos en carbones y esquistos de cinco perforaciones en la formación maastrichtian mamu de la cuenca de anambra, en el sureste de nigeria, fueron analizados a través de un estudio de espectometría cromatográfico y de masa del gas para medir la madurez termal y la entrada de material orgánico. el estudio está enfocado en las variaciones de la abundancia relativa de naftalinas y fenantrenos, y en los esteroides monoaromáticos y triaromáticos identificados en los fragmentogramas de masas. la trimetinaftalina (tmn) es la más abundante de la familia de las naftalinas mientras el metilfenantreno (mp) es el más abundante de los fenantrenos. el tota de los fenantrenos y sus isómeros fue mayor que el de las naftalinas. la distribución de estos hidrocarbones aromáticos y sus alquilos derivados fue controlada ampliamente por un mecanismo de expulsión selectiva y de la maduración térmica de material orgánico. la baja proporción dibenziotofeno/fenantreno (dbt/ phen) (0.01-0.06), al igual que las concentraciones mejoradas de 1,2,5-tmn relativas de 1,2,7-tmn indican que la materia orgánica se deriva principalmente de plantas mayores, y del diagrama terniario de los esteroides monoaromáticos c27, c28 y c29 sugiere un tipo iii mezclado con tipos ii/iii de querógenos. el valor calculado de la reflectancia de vitrinita (%rm) determinado de la proporción de isómeros de metildibenziotofeno (mdr) en los extractos rocosos oscila de 0.51 a 1.43. estos valores de madurez indican que los extractos de carbones y esquistos son marginalmente maduros para la generación de hidrocarbono. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 51 62 abstract resumen key words: aromatic biomarkers, maturity, mamu formation, anambra basin palabras clave: biomarcadores aromáticos; madurez geológica; formación mamu, cuenca de anambra. using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation, southeastern nigeria 1. jude e ogala, delta state university, nigeria. email: etunimogala@yahoo.com 2. mike i akaegbobi, university of ibadan, nigeria. email: izumike20022002@yahoo.com petroleum geology record manuscript received: 06/11/2012 accepted for publication: 29/04/2014 52 jude e. ogala & mike i. akaegbobi introduction aromatic biomarkers and their structural isomers have been found useful for the maturity assessment of oils, source rocks and coals (radke et al., 1982a; radke and welte, 1983; alexander et al., 1985; radke et al., 1986; garrigues et al., 1988; strachan et al., 1988; akaegbobi et al., 2000; akinlua et al., 2007; sonibare et al., 2008). complex mixtures of aromatic hydrocarbons have been identified in crude oils (cumbers et al., 1986), organic extracts of coals (radke et al., 1982a, b; puttman and villar, 1987) and ancient sediments (alexander et al., 1985; garrigues et al., 1988, 1990). these complex mixtures of polyaromatic hydrocarbons (pahs) are usually not generated through biosynthetic processes by living organisms in the sediments. however, some living organisms may synthesize benzoyl-derivatives in large quantities (akaegbobi et al., 2000). aromatic compounds found in crude oils and ancient sediments are believed to have been derived from modification through degradation of biologically produced compounds such as steroids and terpenoids (hall et al., 1984; strachan et al., 1988). they also observed that steroids give rise to substituted phenanthrenes, and terpenoids produce alkylnaphthalenes. other potential sources of alkylnaphthalene include the thermal degradation of spores, coals, and sporopollenin (allan and larter, 1981), and of kerogens and cyclic sesquiterpenoids present in resinous components (anderson et al., 1992). for the alkylated phenanthrenes, the terpenoids derived from higher plant constituents such as resins and waxes seem to be one major source. radke et al. (1982a) suggested that direct methylation of phenanthrene during catagenesis could be a substantial source of alkyl phenanthrene. furthermore, the conversion of higher-plant triterpenoids in sediments into aromatic hydrocarbons is through the release of oxygen functional groups and subsequent aromatization of the rings (johns, 1986; radke, 1987; rullkotter et al., 1994; akaegbobi et al., 2000). the ultimate products of such processes are therefore tetracyclic and pentacyclic aromatic hydrocarbons. through cleavages of bonds within the rings coupled with photochemical and acid catalyzed processes during diagenesis/ catagenesis, these tetracyclic and pentacyclic aromatic hydrocarbons could be broken into methyl-substituted naphthalenes and phenanthrenes, respectively. therefore, relative variations in the concentrations of akylnaphthalenes and alkylphenanthrenes in the sediments could supply information on the maturity of organic matter, organic facies input and depositional environment (seifert and moldowan, 1978, 1986; radke et al., 1982a, b; radke, 1988). the aim of this paper is to examine the ratios of aromatic compounds in coal and shale extracts from the maastrichtian mamu formation in the anambra basin, southeastern nigeria, and to assess the organic matter input and thermal maturity for the source rocks. geological setting the anambra basin, constituting the southwestern portion of the benue trough (fig. 1), is a roughly triangular-shaped sedimentary depression covering an area of approximately 40,000 km2 (akaegbobi et al., 2000). the present study area lies within latitudes 6º20/ to 7º55/ n and longitudes 7º10/ to 8º00/ e (fig. 2). the anambra basin was initiated in the early cretaceous with the formation of the benue-abakaliki trough as a failed arm of a rift triple junction associated with the separation of the african and south american continents and the subsequent opening of the south atlantic ocean (murat, 1972).the ne-sw trending benue-abakaliki trough is believed to be the result of a pre-albian rifting of the african shield prior to the opening of the south atlantic ocean (uzuakpunwa, 1974). the movements were reactivated by further plate activity in the lower tertiary soon after the intermittent upper cretaceous rifting (petters, 1978). the anambra basin contains over 6 km thick cretaceous/tertiary sediments and provides a structural link between the cretaceous benue trough and the tertiary niger delta (whiteman, 1982; onuoha, 1986; mohammed, 2005). figure 1: geologic map of the anambra basin and adjoining areas in southeastern nigeria showing sample locations (modified from akande et al., 2007). figure 2: location of study area with sampling sites. 53using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation, southeastern nigeria figure 3: stratigraphic subdivision of southern benue trough. the stratigraphic succession ranges from late santonian to eocene (fig. 3), with sediments of fluvial, deltaic, pro-delta, and shelf facies origins. accumulation of sediments in the anambra basin commenced with the campano-maastrichtian marine and paralic shales of the enugu and nkporo formations, along with the deltaic owelli sandstone equivalent. these basal units are overlain successively by the coal measures of the mamu formation (lower coal measures), the ajali sandstone (middle coal measures) and the nsukka formations (upper coal measures).the nsukka formation, which overlies the ajali sandstone, begins with coarseto medium-grained sandstones and passes upward into well-bedded blue clays, fine-grained sandstones, and carbonaceous shales with thin bands of limestone. the marine shales of the imo formation were deposited in the paleocene, and overlain by the ameki formation (lateral equivalents of the tidal nanka sandstones) of eocene age which constitutes the tertiary succession (petters and ekweozor, 1982)(fig. 3). figure 4a: m/z 156, m/z 170 and m/z 184 mass chromatograms showing the distributions of naphthalenes and its alkyl derivatives of representative coal sample (og/1267/03) from the anambra basin figure 4b: m/z 156, m/z 170 and m/z 184 mass chromatograms showing the distributions of naphthalenes and its alkyl derivatives of representative shale sample (eng/1001/02) from the anambra basin materials and methods seven core samples consisting of coal (4) and shale (3) having total organic carbon (toc) values ranging from 3.35 to 55.07 wt.% were carefully selected at various depth intervals (75-221 m) from five boreholes penetrating the mamu formation in the anambra basin in three coal fields at enugu, ezimo and ogboyoga (fig. 2). pulverized samples were exhaustively extracted by soxhlet extraction using dichloromethane as solvent. all samples were prepared according to standard organic geochemical sample preparation techniques (radke et al., 1980, 1984). aliquots of the extracts were separated into saturated and aromatic hydrocarbons and heterocyclic compounds by liquid chromatography using a dual column system and a back-flushing technique as a preparatory step for biomarker analysis. the asphaltene fraction of the sample was recovered by precipitation in pentane overnight at room temperature. the pentane soluble portion of the sample (i.e., the maltenes) was further separated by liquid chromatography into saturated, aromatic and resin fractions using a large column containing silica gel with fractions quantified by gravimetric analysis. aromatic hydrocarbons were eluted using 40 ml of 90:10 (v/v) pentane/dichloromethane mixtures. the aromatic fractions were diluted with 10 µl of carbon disulfide (cs2) per mg of fraction and analyzed by gas chromatography-mass spectrometry using a hewlett packard 6890 gc/5973 msd system (agilent technologies, california, u.s.a.) equipped with a highresolution column (60 m, db-1 phase, 0.2 mm i.d., 0.2 µm film). a constant flow of hydrogen carrier gas was used during the entire gas chromatographic run.the gc was temperature programmed from 80 °c (2 minutes hold) to 3200c at 3.5 0c/minute with a final hold time of 20 minutes. there was a 7.5-minute solvent delay on acquisition, and selected ion monitoring (sim) was the standard method for analysis. results biomarker chemistry the mass chromatograms of naphthalenes, phenanthrenes and akyl derivatives of representative coal and shale extracts from the maastrichtian mamu formation in the anambra basin are shown in figures 4a-b and 5a-b, respectively. the identification of the peaks labeled in figures 4a-b and 5a-b are reported in tables 1 and 2. two methylnaphthalenes (mn), two ethylnaphthalenes (en), nine dimethylnaphthalenes (dmn), ten trimethylnaphthalenes (tmn), eleven tetramethylnaphthalenes (temn) and two ethyl-methyl-naphthalenes (emn) isomers were identified in the aromatic fractions of the coal and shale extracts (figs. 4a-b). among the tricyclic aromatic hydrocarbons, four methylphenanthrenes (mp), and fourteen dimethylphenanthrene isomers were identified (figs. 5a-b). dimethylnaphthalene was the most abundant among the naphthalene family (fig. 6). all samples have similar distributions of phenanthrenes (m/z 178). 54 jude e. ogala & mike i. akaegbobi figure 5a: m/z 178, m/z 192 and m/z 206 mass chromatograms showing the distributions of phenanthrene and its alkyl derivatives of coal sample (og/1267/03) from the anambra basin. figure 5b: m/z 178, m/z 192 and m/z 206 mass chromatograms showing the distributions of phenanthrene and its alkyl derivatives of shale sample (eng/1001/02) from the anambra basin table 1: peak identification in the m/z 156, 170, 184, 198, 154, 168, 178 and 192 mass chromatograms ion peak label compound name 128 naph naphthalene 142 2mn 2-methylnaphthalene 142 1mn 1-methylnaphthalene 156 2en 2-ethylnaphthalene 156 1en 1-ethylnaphthalene 156 26dmn 2,6-dimethylnaphthalene 156 27dmn 2,7-dimethylnaphthalene 156 13+17dmn 1,3 & 1,7-dimethylnaphthalenes 156 16dmn 1,6-dimethylnaphthalene 156 23dmn 2,3-dimethylnaphthalene 156 14dmn 1,4-dimethylnaphthalene 156 15dmn 1,5-dimethylnaphthalene 156 12dmn 1,2-dimethylnaphthalene 170 bb_emn ethyl-methyl-naphthalene 170 ab_emn ethyl-methyl-naphthalene 170 137tmn 1,3,7-trimethylnaphthalene 170 136tmn 1,3,6-trimethylnaphthalene 170 146+135t (1,4,6+1,3,5)-trimethylnaphthalenes 170 236tmn 2,3,6-trimethylnaphthalene 170 127tmn 1,2,7-trimethylnaphthalene 170 167+126t (1,6,7+1,2,6)-trimethylnaphthalenes 170 124tmn 1,2,4-trimethylnaphthalene 170 125tmn 1,2,5-trimethylnaphthalene 184 1357 1,3,5,7-tetramethylnaphthalene 184 1367 1,3,6,7-tetramethylnaphthalene 184 1247 (1,2,4,7+1,2,4,6+1,4,6,7)-tetramns 184 1257 1,2,5,7-tetramethylnaphthalene 184 2367 2,3,6,7-tetramethylnaphthalene 184 1267 1,2,6,7-tetramethylnaphthalene 184 1237 1,2,3,7-tetramethylnaphthalene 184 1236 1,2,3,6-tetramethylnaphthalene 184 1256 1,2,5,6-tetramethylnaphthalene 184 dbt dibenzothiophene 198 4mdbt 4 methyl dibenzothiophene 198 2+3mdbt 2 & 3 methyl dibenzothiophenes 198 1mdbt 1 methyl dibenzothiophene 154 bp biphenyl 168 2mbp 2-methylbiphenyl 168 dpm diphenylmethane 168 3mbp 3-methylbiphenyl 168 4mbp 4-methylbiphenyl 168 dbf dibenzofuran 178 phen phenanthrene 192 3mp 3-methylphenanthrene 192 2mp 2-methylphenanthrene 192 9mp 9-methylphenanthrene 192 1mp 1-methylphenanthrene 55using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation, southeastern nigeria figure 6: relative distribution (concentration) of naphthalene homologues in rock extracts from the anambra basin. see table 3 caption for abbreviations. figure 7: relative distribution (concentration) of phenanthrene homologues in rock extracts from the anambra basin. see table 3 caption for abbreviations. dimethylphenanthrene was the least abundant of the phenanthrene homologues (fig. 7). the distribution of total naphthalenes and phenanthrenes in the coal and shale extracts from the mamu formation indicates that the total sum of the phenanthrenes and their isomers was greater than that of the naphthalenes in all the samples analyzed (fig.8). the distribution of these aromatic hydrocarbons and their akyl derivatives was strongly controlled by a selective expulsion mechanism and thermal maturation of organic matter (akaegbobi et al., 2000). in the present study, the relative abundance of the phenanthrenes and their akyl derivatives was greater than that of the naphthalenes. radke et al. (1982a) suggested that a substantial source of alkyl phenanthrene could be the direct methylation of phenanthrene during catagenesis. the results obtained in this study corroborate with those reported by akinlua et al. (2007).these isomers were applied in the calculation of temperature-sensitive maturity parameters such as methylphenanthrene indices (mpi-1 and mpi-2), methyldibenzothiophene ratio (mdr), dimethylnaphthalene ratios (dnr-1 and dnr-2) and trimethylnaphthalene ratio (tnr-1). 0 50000 100000 150000 200000 250000 300000 350000 400000 t-naph t-phen s1 s2 s3 s4 s5 s6 s7 figure 8: relative concentrations of the sum total of naphthalenes (t-naph) and phenanthrenes (t-phen) in rock extracts from the anambra basin. see table 3 caption for abbreviations. table 2: peak identification in the m/z 206, m/z 191, m/z 231 and m/z 253 mass chromatograms ion peak label compound name 206 36dmp 3,6-dimethylphenanthrene 206 26dmp 2,6-dimethylphenanthrene 206 27dmp 2,7-dimethylphenanthrene 206 39dmp (3,9+3,10+2,10+1,3)-dimethylphenanthrenes 206 29dmp (2,9+1,6)-dimethylphenanthrenes 206 17dmp 1,7-dimethylphenanthrene 206 23dmp 2,3-dimethylphenanthrene 206 19dmp 1,9-dimethylphenanthrene 206 18dmp 1,8-dimethylphenanthrene 206 12dmp 1,2-dimethylphenanthrene 191 bh32 c32 benzohopane 191 bh33 c33 benzohopane 191 bh34 c34 benzohopane 191 bh35 c35 benzohopane 231 1ta20 c20 triaromatic steroid 231 2ta21 c21 triaromatic 231 3ta26 c26 20s triaromatic 231 4ta26+27 c27 20s & c26 20r triaromatic 231 5ta28 c28 20s triaromatic 231 6ta27 c27 20r triaromatic 231 7ta28 c28 20r triaromatic 231 8ta29 c29 20r triaromatic (24 n-propyl) 253 1ma21 c21 ring-c monoaromatic steroid 253 2ma22 c22 monoaromatic steroid 253 3ma27 c27 reg 5b(h),10b(ch3) 20s 253 4ma27 c27 dia 10b(h),5b(ch3) 20s 253 5ma27 c27 dia10bh,5bch3 20r+reg5bh,10bch3 20r 253 6ma27 c27 reg 5a(h),10b(ch3) 20s 253 7ma28 c28 dia 10ah,5ach3 20s+reg5bh,10bch3 20s 253 8ma27 c27 reg 5a(h),10b(ch3) 20r 253 9ma28 c28 reg 5a(h),10b(ch3) 20s 253 10ma28 c28 dia 10ah,5ach3 20r+reg5bh,10bch3 20r 253 11ma29 c29 dia 10bh,5bch3 20s+reg5bh,10bch3 20s 253 12ma29 c29 reg 5a(h),10b(ch3) 20s 253 13ma28 c28 reg 5a(h),10b(ch3) 20r 253 14ma29 c29 dia 10bh,5bch3 20r+reg5bh,10bch3 20r 253 15ma29 c29 reg 5a(h),10b(ch3) 20r 56 jude e. ogala & mike i. akaegbobi figure 9: relative distribution (concentration) of aromatic hydrocarbons in rock extracts from the anambra basin. see table 3 caption for abbreviations. discussion thermal maturity the concentration of aromatic compounds, such as naphthalenes, phenanthrenes, dibenzothiophenes and their structural isomers are at present attracting increasing attention as maturity indicators for coals and shale source rocks. these indicators rely either on an increase in maturity with the degree of alkylation of a given parent compound, or a shift in the isomer distribution of alkylaromatic homologues increasing towards isomers with greater thermal stability (radke, 1988). the aromatic isomer ratios and indexes (dnr-1, dnr-2, tnr-1, tde-1, tde-2, mpi-1, mpi-2, and mdr) used in this study are reported in table 5. the total concentration of naphthalene and its isomers is greater than that of the phenanthrenes. the higher abundances of naphthalenes relative to phenanthrenes can be attributed to selective expulsion mechanisms and direct methylation during diagenesis/catagenesis (akaegbobi et al., 2000; radke et al., 1982a). the increase in the concentrations of naphthalenes along with an increase in the number of alkyl substitutes indicates that the oils are mature. this increase in the number of methyl substituents of naphthalene in mature oils occurs because of the thermal rearrangement of methylnaphthalene with table 3: abundance of aromatic hydrocarbons in rock extracts (concentrations are in ppm) s a m p l e number lithology n mn en dmn emn tmn temn p mp dmp t-naph t-phen og/1267/03 shale nd 16.70 9.40 532.90 65.60 1077.30 499.80 293865.20 52542.10 15581.00 2201.70 361988.30 eng/1001/02 shale nd 759.50 153.10 8496.00 292.60 3016.60 583.70 5554.50 2403.90 2799.50 13301.50 10757.90 eng/1001/12 coal 358.50 505.00 20.90 859.60 27.40 301.00 357.30 1280.10 1157.40 1275.00 2429.70 3712.50 eng/1001/16 coal nd 14.80 11.10 858.20 86.80 1703.10 1053.80 14007.80 8875.70 3444.00 3727.80 26327.50 eng/1002/06 shale nd 21.30 22.5.00 1479.40 230.90 2852.50 657.60 33841.70 3946.90 1240.40 5264.20 39029.00 eng/1008/13 coal 39.50 402.20 25.00 1175.80 49.30 581.90 287.20 2397.80 686.10 889.70 2560.90 3973.60 ez/1219/08 coal 372.50 793.00 62.20 1991.50 76.30 1157.80 526.30 63560.40 7983.40 2703.20 4979.60 74247.00 abbreviations: n – naphthalene, mn – methylnaphthalene, en – ethylnaphthalene, dmn – dimethylnaphthalene, emn – ethylmethylnaphthalene, tmn – trimethylnaphthalene, temn – tetramethylnaphthalene, p – phenanthrene, mp – methylphenenthrene, dmp dimethylphenenthrene, t-naph – total naphthalene, t-phen – total phenanthrene, nd – not detected table 4: abundance of isomers of trimethylnaphthalenes and methylphenanthrenes in rock extracts (concentrations are in ppm) sample number depth (m) lithology 2-mp 3-mp 1-mp 9-mp 125tmn 127tmn og/1267/03 75-76 shale 19203.20 13913.70 10818.90 8606.30 173.50 44.20 eng/1001/02 157 shale 519.90 369.80 595.40 918.80 389.90 159.40 eng/1001/12 186-187 coal 144.90 184.80 179.00 648.70 34.90 16.50 eng/1001/16 205 coal 1797.30 1065.80 2640.50 3372.10 418.20 144.80 eng/1002/06 167-178 shale 1446.70 1079.20 701.20 719.80 325.90 176.50 eng/1008/13 220-221 coal 144.50 120.40 108.90 312.30 62.50 30.60 ez/1219/08 80-82 coal 2567.10 1784.30 2113.60 1518.40 166.80 50.20 abbreviations: 1-mp – 1-methylphenanthrene, 2-mp – 2-methylphenanthrene, 3-mp – 3methylphenanthrene, 9-mp – 9methylphenanthrene, 125tmn – 1, 2, 5-trimethylnaphthalene, 127tmn – 1, 2, 7-trimethylnaphthalene 57using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation, southeastern nigeria increasing thermal stress (akaegbobi et al., 2000). furthermore, the coal and shale extracts contain enhanced concentrations of 1, 2, 5-trimethylnaphthalene relative to 1, 2, 7-trimethylnaphthalene (table 4). this is typical of mature oils (strachan et al., 1988; akinlua et al., 2007). however, in the present study, the relative abundance of the total of phenanthrene and its isomers was greater than that of naphthalenes (table 3, fig.9). the abundance of phenanthrenes relative to naphthalenes suggests that the rock extracts are mature (akinlua et al., 2007). figure 10a: m/z 231 and 253 mass chromatograms showing the distributions of triaromatic steranes and monoaromatic steranes in representative shale sample og/1267/03 from the anambra basin. figure 10b: m/z 231 and 253 mass chromatograms showing the distributions of triaromatic steranes and monoaromatic steranes in representative coal sample eng/1001/12 from the anambra basin. table 5: thermal maturity parameters computed from the aromatic biomarkers and aromatic sulphur compounds in the coal and shale extracts interpretative ratios og/1267/03 eng/1001/02 eng/1001/12 eng/1001/16 eng/1002/06 eng/1008/13 ez/1219/08 naphthalenes dnr-1=(2,6+2,7)/(1,5)dmn 2.08 3.93 4.13 2.04 2.71 6.03 4.93 dnr-2=(2,6+2,7)/(1,4+2,3)dmn 1.21 2.52 3.37 1.35 1.72 2.93 1.91 tnr1=(2,3,6)/(1,4,6+1,3,5)tmn 1.12 0.86 0.73 0.75 0.95 0.87 1.22 tde-1=1,2,5-tmn/1,2,4-tmn 10.43 10.81 4.41 15.21 7.22 7.22 11.51 tde-2=1,2,7-tmn/1,2,6-tmn 0.20 0.30 0.33 0.53 0.33 0.31 0.20 1,2,7-tmn/1,3,7-tmn 0.45 0.46 0.44 1.07 0.54 0.39 0.41 phenanthrenes mpi-2=3(2mp)/(phen+1mp+9mp) 0.18 0.22 0.21 0.27 0.12 0.15 0.11 mpi-1=1.5(2mp+3mp)/(phen+1mp+9mp) 0.16 0.19 0.23 0.21 0.11 0.14 0.10 rc(a)=0.6(mpi-1)+0.37 (for ro<1.3) 0.47 0.48 0.51 0.50 0.43 0.45 0.43 rc(b)=0.6(mpi-1)+2.3 (for ro>1.3) 2.40 2.41 2.44 2.43 2.36 2.38 2.36 mdr=4mdbt/1mdbt 6.56 1.69 0.40 0.72 3.13 0.88 0.62 rm=0.40+0.30(mdr)-0.094(mdr)2+0.011(mdr)3 1.43 0.69 0.51 0.57 0.76 0.60 0.55 mdr23=23mdbt/dbt 0.42 0.31 0.65 0.32 0.21 0.75 0.06 mdr1=1mdbt/dbt 0.07 0.23 0.66 0.75 0.13 0.67 0.14 dbt/phenanthrene 0.06 0.02 0.04 0.01 0.01 0.02 0.01 triaromatic steroids tas c20+c21/all tas (20r and 20s) 0.52 0.13 0.06 0.11 0.21 0.09 0.13 tas c20/c20+c27 0.70 0.22 0.12 0.19 0.31 0.20 0.20 tas c20/c20+c28 (20r) 0.72 0.26 0.08 0.20 0.42 0.17 0.35 tas c21/c21+c28 0.15 0.08 0.04 0.07 0.13 0.05 0.10 tas c28/c26 (20s) 3.62 3.89 12.71 4.04 2.09 17.11 5.6 tas c28/c27 (20r) 2.12 2.16 3.96 2.92 2.27 2.87 1.33 % c26 tas 5.30 5.70 2.30 6.70 8.20 2.30 4.70 % c27 tas 36.70 35.90 23.70 32.20 40.5 23.9 35.80 % c28 tas 52.80 50.70 68.90 57.70 42.70 70.10 43.70 % c29 tas 5.20 7.60 5.10 3.40 8.60 3.60 15.70 monoaromatic steroids mas dia/reg c27 0.00 0.69 0.00 1.21 0.91 1.17 2.19 mas c21+c22/all mas 0.05 0.04 0.04 0.04 0.05 0.04 0.06 % c27 ma 50.10 43.00 11.00 26.50 46.30 14.20 11.60 % c28 ma 29.30 21.70 14.50 21.80 21.00 20.90 22.60 % c29 ma 20.60 35.20 74.50 51.70 32.70 65.00 65.70 tas/(tas+mas) 0.22 0.38 0.59 0.46 0.28 0.57 0.23 tas c28/(ta c28+ma c29) 0.58 0.79 0.87 0.87 0.69 0.88 0.39 58 jude e. ogala & mike i. akaegbobi a number of aromatic maturity parameters based on the distribution of naphthalene, phenanthrene and alkyl isomers were calculated for the study samples after radke et al. (1982a), radke et al. (1986) and alexander et al. (1985, 1986). radke (1988) observed that aromatic hydrocarbons do change in a regular fashion with increasing maturity and thus developed maturity parameters based on the distributions of alkylnaphthalene and alkylphenanthrene.the maturity parameters computed from the aromatic biomarker distributions in the coal and shale extracts are listed in table 5. the maturity parameters computed from naphthalene and its isomers, the dimethylnaphthalene ratios (dnr-1 and dnr-2) and trimethylnaphthalene ratio (tnr-1) ranged from 2.04 to 6.03, 1.21 to 3.37 and 0.73 to 1.22, respectively (table 5). the methylphenanthrene index (mpi-1) is one of the most widely used maturity parameters based on aromatic hydrocarbons (radke et al., 1982a, 1982b; radke, 1988). the parameter relies on a shift with maturity in the methylphenanthrene distribution towards a preponderance of β-type isomers. the methylphenanthrene indices (mpi-1 and mpi-2) were computed based on the relative concentrations of the 1-mp, 2-mp, 3-mp, 9-mp and phenanthrene (p). this was based on the assumption that the 2-mp and 3-mp are derived from the rearrangement of the 1-mp and 9-mp, and from phenanthrene (p) through methylation reactions. the relative abundances of the alkyl homologs of the aromatic hydrocarbons (phenanthrenes) were used to calculate the vitrinite reflectance (% rc) after the method of radke et al., (1982a, b) and radke (1988). the mpi-1 is often used for estimating the equivalent vitrinite reflectance values (% rc) for crude oils and source rocks. the mpi-1 and % rc values calculated for the coal and shale extracts range from 0.10 to 0.23 and 0.43 to 0.51, respectively (table 5). mpi-1 calculated reflectance (%rc) was carried out because of insufficient and good reflecting vitrinite in the samples. maturity parameters based on the isomers of the methyldibenzothiophene (mdbt) were calculated (radke et al., 1986). this ratio (mdr) generally increases with increasing maturity and correlates well with vitrinite reflectance and the rock-eval tmax of source rock (radke, 1988). the ratio is also useful in calculating mean vitrinite reflectance (% rm). the methyldibenzothiophene ratio (mdr) and mean vitrinite reflectance (%rm) for the coal and shale extracts ranged from 0.40 to 6.56 and 0.51 to 1.43, respectively (table 5). the high mdr exhibited by some of the samples may be a result of the effect of heating rates or thermal stress (akaegbobi et al., 2000). the % rm values show that the coal and shale extracts range from immature to mature (table 5). monoaromatic and triaromatic steroids were detected in all samples in the m/z 253 and m/z 231 fragmentograms (figs. 10a-b). both 20r and 20s homologues of c26-c28, c20 and c21 triaromatic compounds are present (figs. 10a-b). figure 12a: m/z 184 and 198 mass chromatograms showing the distributions of aromatic sulfur compounds of representative shale sample in well eng/1001/02 from the anambra basin. figure 12b: mass chromatograms showing the distributions of aromatic sulfur compounds of representative coal sample in well eng/1001/12 from the anambra basin. peters and moldowan (1993) and el-gayar (2005) suggested that the extent of cracking in the side chains of triaromatic steroids can be used to provide information about petroleum maturity. the triaromatic steroid maturity parameters c20/c20+c28 (20r) and (c20+c21)/ (c20+c21+c26 to c28 20r and 20s) were used to evaluate the thermal maturity of the coal and shale extracts in this study (peters and moldowan, 1993; peters et al., 2005). the maturity parameters computed from the peak areas of the m/z 231 ranged from 0.08 to 0.72 and 0.09 to 0.52, respectively (table 5). these values indicate an immature to marginally mature status of the source rock. the monoaromatic steroid ratios (mas dia/reg c27 and mas c21+c22/ all mas) were measured from the m/z 253 fragmentograms (table 5) and used to evaluate the source-rock depositional environment and thermal maturity (peters and moldowan, 1993). both of these ratios increase with higher thermal maturity (mackenzi et al., 1981; moldowan and fago, 1986). the mas dia/ reg c27 sterane ratio ranges from 0.69 to 2.19 (table 5). the coal and shale extracts in the mamu formation contain significant amounts of diasteranes, which appear to form through the interaction of steranes with clay mineral surfaces in source rocks (rubinstein et al., 1975; sieskind et al., 1979). thus, the presence of significant amounts of diasteranes relative to regular steranes in oils has been used as evidence of petroleum generation from argillaceous source rocks, whereas low concentrations are considered an indication of a source rock lacking in clay minerals (hughes, 1984; mello et al., 1988a, 1988b; hill et al., 2007). the calculated monoaromatic maturity ratio (mas c21+c22/ all mas) ranges from 0.04 to 0.05 (table 5). thermal maturity was also evaluated on the basis of monoaromatic and triaromatic steroids because they are more resistant to the effects of biodegradation than alkane-type biological markers (volkman et al., 1983). according to tissot and welte (1984), aromatization of c-ring steroid hydrocarbons occurs in mature oils. with an increase in thermal maturation, there is a conversion of c-ring monoaromatic steroid hydrocarbons to abcring triaromatic steroid hydrocarbons (akinlua and ajayi, 2009). the tas/ (tas+mas) and tas 28/ (tas 28+mas 29) ratios ranged from 0.22 to 0.59 and 0.39 to 0.88, respectively (table 5), indicating an immature to marginally mature status of the rock extracts. source of organic matter the trimethylnaphthalene is dominated by 1, 2, 5tmn in all the samples analyzed (figs. 4a-b and table 4). the high abundance of the naphthalenes, especially the dimethyland trimethyl-isomers, indicates organic matter derived 59using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation, southeastern nigeria mainly from higher plants (strachan et al., 1988). high concentrations of 1, 2, 5and 1, 2, 7trimethylnaphthalene (tmn) in oils have been attributed to aromatization and cleavage of the ring-c of β-amyrin in angiosperms (strachan et al., 1988). 1, 2, 5and 1, 2, 7trimethylnaphthalenes can form as diagenetic products of oleanane-type triterpenoids (chaffee and johns, 1983; chaffee et al., 1984; sonibare et al., 2008). the presence of 1, 2, 5-tmn in the studied samples could be linked to aromatization of oleanane, as indicated by the absence of appreciable amounts of oleanane in the m/z 191 fragmentograms. this compound can also be formed from gymnosperm resins (van aarssen et al., 1999) or from hopanoid precursors (villar et al., 1988). among the tetramethylnaphthalene family, 1, 2, 5, 6tetramethylnaphthalene is the most abundant (figs. 4a-b). strachan et al. (1986) used two related ratios (tde-1 and tde-2) of trimethylnaphthalenes to differentiate coal swamps from marine, lacustrine, and deltaic environments. 1, 2, 7trimethylnaphthalene (tmn) has been used as a marker of angiosperm input. 1, 2, 5-trimethylnaphthalene and 1, 2, 7trimethylnaphthalene can form as diagenetic products of oleanane-type triterpenoids (chaffee and johns, 1983; chaffee et al., 1984). strachan et al. (1988) compared the relative abundance of trimethylnaphthalene isomers in southeast asian samples ranging in age from permian to tertiary. they found that the relative concentrations of the 1, 2, 7-trimethylnaphthalenes were appreciably higher in most oils derived from post-cretaceous source rocks dominated by the input of higher-plants, and in extracts from cretaceous and younger samples, than in samples of older age or with little or no terrigenous plant input. oils of this type yielded ratios of 1, 2, 7-/1, 3, 7-trimethylnaphthalenes from 0.46 to 1.36, while oils from older source rocks containing higher-plant material yielded ratios from 0.34 to 0.16, and oils from marine source rocks yielded ratios from 0.15 to 0.32. in this study, the rock extracts yielded ratios of 1, 2, 7-/1, 3, 7-trimethylnaphthalenes ranging from 0.40 to 1.07 (table 5). the computed tde-1 and tde-2 ratios for the study samples range from 4.41 15.21 and 0.20 0.53, respectively (table 5). the presence of 1, 2, 5and 1, 2, 7-tmn in the rock extracts (figs. 4a-b; table 4) indicate both angiosperm and gymnosperm material contribution to the organic matter that formed the coals and shales (killops and killops, 2005; adedosu et al., 2012). furthermore, the occurrence of appreciable amounts of 1, 6and 2, 6-dmns in all the samples indicate terrestrial organic matter input (achari et al., 1973). additionally, the occurrence of 1, 7-dmp in the analyzed samples (figs. 5a-b) is attributed to terrestrial organic matter (simoneit et al., 1986), while plots of c27,c28 and c29 monoaromatic steranes (fig. 11) indicate that the kerogens were derived from mixed terrestrial and marine organic matter. figure 11: ternary diagram showing the relative contributions of c27, c28 and c29 monoaromatic steranes for rock extracts from the anambra basin. the distribution of the aromatic sulfur compounds including dibenzothiophenes (dbts) and their methyl homologues in the coal and shale extracts are reported in figures 12a-b. the dibenzothiophene/phenanthrene (dbt/phen) ratio together with the pr/ph ratio can be used to infer source rock depositional environments and lithologies (hughes et al., 1995). high dbt/ phen ratios are found in oils from marine carbonates, and low ratios are found in oils from marine shales and most lacustrine rocks (petersen et al., 2007). the coal and shale extracts (table 5) have low dbt/phen ratios (0.01 – 0.06), indicating a strong input of terrigenous type iii organic matter. however, dbt/phen (table 5) and pr/ph ratios only reflect the eh and ph irrespective of depositional environment, and a cross-plot of the two ratios by itself cannot be used to distinguish lacustrine from marine oils (petersen et al., 2007). the presence of organic sulfur compounds and sulfur-containing aromatics in rock extracts and crude oils indicates an anoxic depositional environment (sinninghe damste et al., 1988, 1989a, 1989b; hughes et al., 1995). 1-methyldibenzothiophene (1-mdbt), 2+3-methyldibenzothiophene (2+3-mdbt) and 4-methyldibenzothiophene (4mdbt) were detected in all the samples analyzed (figs. 12a-b; table 6). the high relative abundance of dibenzothiophenes (dbts) in all of the samples is indicative of probable oxic/anoxic conditions prevailing during deposition (table 6). the distribution of dbts has been proposed as an indicator of crude oils derived from carbonate versus siliciclastic sources (hughes, 1984; hegazi and el-gayar, 2009). the presence of a definite v pattern (4-methyl > 2+3-methyl < 1-methyl) in the methyldibenzothiophenes is generally associated with oils from predominantly carbonate source rocks, while a stair-step pattern (4-methyl > 2+3-methyl > 1-methyl) is associated with predominantly siliciclastic source rocks, or oils of advanced maturity (lateto post-oil window) from carbonate sources (hegazi and el-gayar, 2009). furthermore, oils from carbonate sources are characterized by an abundance of benzothiophenes (bts) and a fairly equal distribution of substituted dbts, while siliciclastic oils exhibit low concentrations of bts and decreasing amounts of dimethyldibenzothiophenes and trimethyldibenzothiophenes relative to methyldibenzothiophenes (hughes, 1984; hegazi and el-gayar, 2009). the results obtained in this study indicate that the 4-methyldibenzothiophenes (24.0%) are more abundant than the 2+3-methyldibenzothiophenes (20.4%) and 1-methyldibenzothiophenes (4.4%) (table 6), indicating a siliciclastic source rock input for the organic matter (hegazi and el-gayar, 2009). (a) (b) figure 13a-b: m/z 191 mass chromatogram showing the distributions of benzohopanes in representative shale (a) og/1267/03 and coal (b) eng/1001/12 from the anambra basin. 60 jude e. ogala & mike i. akaegbobi the mass chromatograms (m/z 184 and m/z 198) of the extracts representing the dbts in this study show a high concentration of dbts (51.2%) and decreasing amounts of dimethyldibenzothiophenes (24.0%) and trimethyldibenzothiophene (20.4%) relative to methyldibenzothiophenes (4.4%) (figs. 12a-b and table 6). these results are consistent with previous analyses that indicate that the coal and shale extracts in this study were generated by a siliciclastic source rock (hegazi and el-gayar, 2009). benzohopanes are formed by the cyclization of extended hopanoid side chains followed by aromatization (hussler et al., 1984a, 1984b). benzohopanes range in carbon number from c32 to c35. he and lu (1990) observed that oils and bitumens from evaporitic and carbonate source rocks have the highest concentrations of benzohopanes, which occur in trace amounts in most source rocks and crude oils. benzohopanes were identified in all the rock extracts, but concentrations are low (figs. 13a-b). all samples have similar distributions of benzohopanes (m/z 191). the distribution of benzohopanes (figs. 13a-b) shows that c32 benzohopane is the most abundant member of the bezohopane family. the relative concentrations of the various members of the aromatic hopane series seem to follow that of the regular hopanes in the saturated hydrocarbon fraction, and like the saturated hopanes, they most likely have a bacterial origin (petersen et al., 2007). conclusion the results of this study indicated that the aromatic hydrocarbons contained in the coal and shale samples from the campano-maastrichtian mamu formation of the anambra basin are marginally mature. the kerogens were formed from organic matter of mixed origin (terrestrial and marine). the distributions/concentrations of naphthalenes and phenanthrenes show that trimethylnaphthalene is the most abundant member of the naphthalene family and methylphenanthrene is the most abundant phenanthrene family member. all samples have similar distributions of phenanthrenes. the relative abundance of the total of phenanthrene and its isomers was greater than that of the naphthalenes. thus, the distributions of aromatic hydrocarbons in the rock extracts are influenced by methylation reactions at elevated maturity. the slightly higher concentrations of c29 monoaromatic steranes (49%), compared with c28 (22%) and c27 (29%) monoaromatic steranes, suggests a mixed input of terrestrial and marine organic matter. acknowledgement we are grateful to daniel m. jarvie and the staff of humble geochemical laboratory, texas, usa, for the assistance rendered during the geochemical analyses. we gratefully acknowledge the nigerian geological survey agency (ngsa) for providing the core samples used in this study, and the anonymous reviewers for their helpful comments. table 6: abundance of dibenzothiophenes and their alkyl homologues in rock extracts (concentrations are in ppm) sample number lithology dbt 4mdbt 2+3mdbt 1mdbt og/1267/03 shale 17410.70 8454.20 7230.60 1287.90 eng/1001/02 shale 106.20 40.80 32.60 24.10 eng/1001/12 coal 48.80 13.10 32.00 32.40 eng/1001/16 coal 137.00 74.40 43.80 103.40 eng/1002/06 shale 281.80 116.50 59.00 37.20 eng/1008/13 coal 49.00 28.80 37.00 32.80 ez/1219/08 coal 726.40 63.50 41.60 102.90 abbreviations: dbt – dibenzothiophene, 4mdbt – 4-methyldibenzothiophene, 2+3mdbt – 2, 3methyldibenzothiophene, 1 methyldibenzothiophene references achari, r.g., shaw, g., and holleyhead, r., 1973. identification of ionene and other carotenoid degradation products from the pyrolysis of sporopollenins derived from some pollen exines, a spore coal and the green river shale. chemical geology, volume 12, pp. 229-234. adedosu, t.a., sonibare, o.o., tuo, j., and ekundayo, o., 2012. biomarkers, carbon 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iran. e-mail: behnaz_dahrazma@yahoo.com. 2 mining department, islamic azad university shahrood branch, shahrood, iran. e-mail: kharghani.m@gmail.com. * corresponding author: behnaz dahrazma, environmental geology department, earth science faculty, shahrood university of technology, p.o. box 316, shahrood, 36199-95161, iran. phone: +98 (912) 373 9562 fax: +98 (273) 339 6007. e-mail: behnaz_dahrazma@yahoo.com abstract the release of heavy metals into the environment represents one of the most important environmental effects involved in extracting coal; it needs to be studied more fully. the present research investigated the effects of coal-mining in an alkaline environment and alkaline mine drainage in the takht coal mine regarding the distribution of selected heavy metals (zn, pb, ni, cr and ba) on the region’s surface and ground water . the mine is located 12 km southeast of minoodasht, in golestan province in northern iran. samples were collected from groundwater and surface water resources upstream and downstream of the mine. the elements’ concentrations were measured by the inductively-coupled mass spectrometry (icpms) method. the results showed that an alkaline environment was responsible for producing alkaline mine drainage due to the presence of limestone; this caused high ph (8.41) in the area’s groundwater resources. mining activities increased ba, cr, ni, pb and zn concentration in the groundwater from 3.39, 0.5, 0.2, 0.5, 9.2 ppb to 83.52, 2.2, 0.6, 2.6, 48.3 ppb and from 68.7, 0.5, 1.3, 0.8, 172.6 ppb to 91, 1.2, 4.5, 1.3, 27.6 ppb in surface water, respectively. due to the basic environment, heavy metal accumulation in the bed sediment for both tunnel effluents and runoffs was higher than during the soluble phase. ph was the main controlling factor in elements’ solubility and their distribution in the environment. increased ba concentration in water resources was due to high ba concentration in the coal, coal tailing and in quarry tailings. resumen la liberación de metales pesados en el medio ambiente representa uno de los efectos ambientales más importantes en la extracción de carbón, lo cual necesita ser estudiado con mayor profundidad. el presente estudio investigó los efectos de la minería del carbón en un ambiente alcalino y en el drenaje de la mina de carbón takht, con respecto a la distribución de determinados metales pesados (zn, pb, ni, cr y ba) en superficie y en las aguas subterráneas de la región. la mina está ubicada a 12 km al sureste de minoodasht, en la provincia de golestán en el norte de irán. se recogieron muestras de aguas subterráneas y aguas superficiales aguas arriba y aguas abajo de la mina. las concentraciones de los elementos se midieron mediante la espectrometría de masa de acoplamiento inductivo (icp-ms). los resultados indican que un ambiente alcalino produjo drenaje de minas alcalinas debido a la presencia de la piedra caliza; esto causo altos niveles de ph (8,41) en las fuentes de aguas subterráneas de la zona. la actividad minera incrementó los niveles de concentración de ba, cr, ni, pb y zn en las aguas subterráneas de 3,39, 0,5, 0,2, 0,5, 9,2 ppb a 83,52, 2,2, 0,6, 2,6, 48,3 ppb y de 68,7, 0,5, 1,3, 0,8, 172,6 ppb a 91, 1,2, 4,5, 1,3, 27,6 ppb en el agua de la superficie. debido al ambiente básico, la acumulación de metales pesados en el lecho sedimentario, tanto para los efluentes del tunel como para las aguas de escorrentia, fueron mayores que durante la fase soluble. el ph fue el principal determinante en la solubilidad de los elementos y su distribución en el medio ambiente. el aumento de la concentración de ba en los recursos hídricos se debió a la alta concentración de ba en el carbón, carbón de relaves y colas en cantera. palabras claves: metales pesados , aguas subterráneas, aguas superficiales, sedimentos, drenaje alcalino en minería, minería de carbón. keywords: heavy metals, groundwater, surface water, sediment, alkaline mine drainage, coal-mining. record manuscript received: 17/02/2012 accepted for publications: 06/05/2012 behnaz dahrazma and mehdi kharghani28 introduction coal is a heterogeneous material which is formed through plant decomposition in a variety of conditions regarding moisture, temperature and pressure. the diversity of elements in coal’s compositions as well as the complex process of the coal forming are responsible for the existence and accumulation of light elements (e.g. li, be), non-metallic elements (e.g. se, as, bi) and heavy metals (e.g. cu, pb, co, cr) in coal. these elements can be released into the environment during coal extraction, its preparation, processing, transportation and combustion (mainly in power plants) thereby causing environmental pollution (finkelman et al., 2002; swaine and goodarzi, 1995; yiwei et al., 2007). geochemical characteristics of coal in the alborz region of iran have been thoroughly investigated. the findings have shown that aluminium silicates and sulphide minerals are heavy metals’ main hosts. according to the chemical and physical weathering of excavated materials (coal and its wastes), elements are released from various complexes and then transported or precipitated due to their geochemical properties. after tunnel effluent transportation from a reduced to an oxidised environment, the elements’ oxidation state becomes changed and some heavy metals become precipitated (i.e. pb, ni and zn). reaction oxidation and reduction potential, ph, colloid materials in transporting media and transferred materials’ ion potential are the main factors controlling materials’ transportation and precipitation (northeast alborz mining co., 2009). the takht coal mine is located at 37o 2’ 10” n and 37o 10’ 35” n latitude and 55o 22’ 30” e and 55o 27’ 40” e longitude, lying 12 km southeast of minoodasht, golestan, in iran (figure 1). extraction started from 2001 onwards with horizontal tunnels being bored on the south side of the area (minoodasht water office, 2003a). summer in the area is warm and humid whilst the winter involves a large amount of precipitation, mostly in the form of snow in the mountainous areas. the temperature varies from -10oc to 35oc. the basin’s annual average rainfall is about 500 mm. runoff feeds the chehel-chay river which is located downstream of the mine (minoodasht water office, 2003b). this research investigated the effects of alkaline mine drainage on heavy metals’ distribution (namely ba, cr, ni, pb and zn) on groundwater and surface water in the vicinity of the takht coal mine in minoodasht, iran, regarding environmental concerns. materials and methods triplicate samples were collected from coal, coal and quarry tailings tunnel effluent, tailing leachate and surface and ground water as well as bed sediments from tunnel effluent and runoff to assess the effects of coal minparameter from to coal moisture 1.2% 1.8% ash content 11.2% 44.7% volatile compounds 34.2% 37.5% heat value 7.985 kcal/kg 8.132 kcal/kg carbon content 79.99% 84.24% hydrogen content 4.7% 5.77% oxygen + nitrogen content 10.72% 15.34% phosphorus content 0.04% 0.31%. sulfur content 0.56% 1.3% (mostly in the form of pyrite) table 1. coal sample characteristics (northeast alborz coal mining co. 2009). table 2. sampling locations and the samples’ ec and ph. no. place of sampling longitude latitude ph ec(μs/cm) 1 tunnel effluent 361608 4112469 8.41 1451 2 tailing leachate 361244 4113631 8.12 2620 3 upstream groundwater 361940 4111490 7.23 219 4 downstream groundwater 356526 4115781 7.58 726 5 upstream runoff 361494 4112509 8.21 536 6 downstream runoff 361451 4112539 8.29 828 7 downstream river 357480 4114509 8.31 654 8 tunnel effluents' bed sediments 361509 4112506 7.63 9 upstream runoff bed sediments 361494 4112509 7.53 10 downstream runoff bed sediments 361451 4112539 7.77 11 downstream river bed sediments 357480 4114509 7.37 the impacts of alkaline mine drainage on ba, cr, ni, pb and zn concentration in the water resources of the takht coal mine, iran 29 ing on heavy metals’ distribution in surface and ground water. sampling was done in october 2009 at the end of the dry season to find the worst conditions. table 1 gives the coal samples’ characteristics. sediments were air dried at room temperature and passed through #80 mesh (180 mm opening size) to measure the heavy metals’ concentrations. samples were analysed at the acme lab, vancouver, canada using inductively-coupled mass spectrometry (icp-ms) techniques. the water’s ph and ec were measured on site using a jenway ph meter (model 3510) and jenway ec meter, respectively. for the sediments, ph was measured referring to the epa sw-846 method 9045. table 2 shows sampling locations, ph and ec. results and discussion alkaline mine drainage has resulted from the neutralisation of acidic mine drainage (amd) by carbonate minerals and low grade pyrite rocks. the basic tunnel effluent (ph 8.4) indicated the presence of alkaline mine drainage (table 2) due to the highly alkaline environment. this alkalinity was responsible for the sedimentation of most elements in tunnel effluent bed sediments. ec tended to increase downstream of the mine which could have been due to the high concentration of heavy metals and ions. sample 4 (taken from groundwater 7 km downstream of the mine) had an ec which was 3.3 times higher than that of sample 3 (taken from groundwater upstream of the mine). this increase could have resulted from tunnel effluent and tailing leachate entering the environment (1,451 and 2,620 µs/cm ec, respectively) and causing an increased ec in surface water (from 536 µs/cm in upstream runoff to 828 µs/cm in downstream runoff). a rise in surface figure 1. overall view of the study area’s location. figure 3. heavy metals’ concentration in tunnel effluent bed sediment. figure 2. heavy metals’ concentrations in tunnel effluent and tailing leachate. table 3. the heavy metals’ concentration in coal and tailings (in mg/kg). figure 4. heavy metals’ concentration in bed sediment samples (sample 9: upstream runoff sediments, sample 10: downstream runoff sediments, sample 11: downstream river sediments) sample zn pb ni cr ba coal 33.8 21.76 28.4 18.1 118.8 coal tailing 128.4 46.66 70.8 43.3 54.8 quarry tailing 80.3 32.95 60.5 42.8 146.4 and ground water ph was also observed (table 2). the results indicated that sulphate concentration in surface water increased from zero to 48 mg/l due to pyrite oxidation and sulphate complex entering the environment which facilitated the retention of heavy metals in solution phase. the present research’s findings agreed with anderson et al., (2005) who noted that concentrations of sulphate, iron, manganese and other constituents can increase, even in the presence of alkaline mine drainage. coal, coal tailing and quarry tailing analysis revealed high concentrations of heavy metals (especially ba) in all three contaminant sources. comparing these three contaminant sources indicated that coal tailings had the maximum zn, pb, ni and cr concentrations while maximum ba concentration occurred in quarry tailings (table 3). washing and the consequent leaching of coal, coal tailings and quarry tailings were the main factors for contaminants entering water. tunnel effluent and drainage of tailing washing were the main sources of pollution, containing heavy metals ba, cr, ni, pb and zn. ba had the highest concentration due to its high solubility in an alkaline environment (figure 2). barium also had the maximum concentration in tunnel effluent behnaz dahrazma and mehdi kharghani30 bed sediment compared to the other heavy metals (figure 3). regarding ph 8.4, the heavy metals in the bed sediment had much higher concentrations than in the tunnel effluent. in other words, heavy metals’ concentrations in solid phase were higher than in the aqua phase when ph was 8.4 (figures 2 and 3, table 2). figure 4 shows that heavy metals’ concentration in bed sediments from upstream and downstream runoff had the following order: ba > zn > ni > cr > pb. ion exchange, adsorption and complexation are the main known mechanisms for heavy metals’ accumulation in sediment (dahrazma and mulligan, 2007). smith (1999) mentioned that increased ph enhances cation sorption and decreases anion sorption. the high ba concentration in upstream runoff bed sediment could have been due to a carbonated environment and ca exchange with ba; this finding agreed with that of hem (1985). concentrations of heavy metals in downstream river bed sediments (except for pb) became decreased, due to drained water becoming diluted in the river. contaminants entering the environment through mining activities (as mentioned above) changed the area’s heavy metals’ distribution in groundwater and surface water (table 4). ba concentration in downstream groundwater was 24.6 times greater than upstream due to high ba concentration in tunnel effluent (142.6 mg/l) and tailing leachate (90.8 mg/l). mining activities also caused increased ba concentration in downstream runoff compared to upstream (table 4). the results showed that cr, ni, pb and zn concentrations in groundwater increased 4.4, 2, 5.3, and 5.5 times, respectively, from upstream to downstream. in terms of runoff, cr, ni and pb concentrations from 0.5, 1.3 and 0.8 upstream reached 1.2, 4.5 and 1.3 (all in mg/l) downstream, respectively. zinc did not increase in downstream runoff (comparing figure 2 and table 4) since its concentration in tunnel effluent and tailing leachate (62.3 and 64.5 mg/l, respectively) was lower than its concentration in upstream runoff (172.6 mg/l). it should be noted that the concentration of heavy metals in the river was generally lower than in downstream runoff due to dilution. this was clearly an indication of the adverse effect of mining activities on the region’s groundwater and surface water. it can thus be concluded that alkaline mine drainage being released into the environment increased ba, zn, pb, cr and ni concentration (in descending order) in groundwater and ni, cr, pb and ba in surface water. conclusion alkaline mine drainage rarely occurs in coal mines throughout the world. the takht coal mine is one of the few coal mines where alkaline mine drainage occurs. the mining activities started in 2002 and have already had adverse effects on the quality of water resources in the area since the concentration of heavy metals in downstream groundwater is higher than in upstream water. tunnel effluent and tailing leachate entering the environment were the main causes of such increases. an alkaline environment was responsible for the heavy metals’ solubility and distribution in surrounding water resources. heavy metals’ concentration was much higher in tunnel effluent bed sediments and upstream and downstream runoff than in the aqua phase due to the alkaline environment reducing solubility. mining activities increased ba, cr, ni and pb concentration in the area’s surface water and groundwater. alkaline mine drainage has led to increased zn concentration in downstream groundwater compared to upstream groundwater (5.2 times greater) while it decreased zn concentration in surface water. the rate of coal-mining-related alkaline mine drainage effects on the environment should be investigated more fully to prevent further damage in the foreseeable future. references anderson, r. m., beer, k. m., buckwalter, t. f., clark, m. e., mcauley, s. d., sams, j. i., iii, williams, d. r., 2005. water quality in the allegheny and monongahela river basins pennsylvania, west virginia, new york, and maryland, 1996-98. u.s. geological survey circular 1202. dahrazma, b., mulligan, c. n. (2007). investigation of the removal of heavy metals from sediments using rhamnolipid in a continuous flow configuration, chemosphere, 69, 705-711. finkelman r. b., orem w., castranova v., tatu c. a., belkin h. e., zheng b., lerch h. e., maharaj s. v., bates a. l. (2002). health impacts of coal and coal use: possible solutions, international journal of coal geology, 50, 425-443. hem, j. d. (1985). study and interpretation of the chemical characteristics of natural water, 3rd ed. water supply, us geological survey, paper 2254, 263pp. minoodasht water office. (2003a). quality control report on minoodasht water samples (in persian). minoodasht water office. (2003b). takht basin report (in persian). northeast alborz mining co. (2009). detailed exploration of takht region, in annual exploration report (in persian). smith, k. s. (1999). sorption of trace elements by earth materials—an overview with examples relating to mineral deposits. in: plumlee, g. s., logsdon, m. j. (eds). the environmental geochemistry of mineral deposits. part a. processes, techniques, and health issues: society of economic geologists, reviews in economic geology 6a, 61–182. swaine, d. j., goodarzi, f. (1995). environmental aspects of trace elements in coal. kluwer academic publishers, dordrecht. yiwei, c., guijian, l., yanming, g., jianli, y., cuicui, q., lianfei, l. (2007). release and enrichment of 44 elements during coal pyrolysis of yima coal, china, journal of analytical and applied pyrolysis, 80(2), 283-288. sample place ba cr ni pb zn 3 upstream groundwater 3.4 0.5 0.2 0.5 9.2 4 downstream groundwater 83.5 2.2 0.6 2.6 48.3 5 upstream runoff 68.7 0.5 1.3 0.8 172.6 6 downstream runoff 91 1.2 4.5 1.3 27.6 7 downstream river 63.1 1.1 0.2 1 9.4 min 3.4 0.5 0.2 0.5 9.2 max 91,0 2.2 4.5 2.6 172.6 mean 61.94 1.1 1.36 1.24 53.42 standard deviation 34.6 0.7 1.8 0.8 68.5 table 4. the heavy metals’ concentration in the region’s water resources (in μg/l). in this study, a statistical analysis is carried out to reveal the current earthquake potential in the eastern anatolia region of turkey. for this purpose, a space-time assessing based on the recent changes in gutenbergrichter b-value, seismic quiescence z-value, cumulative moment, annual probability and recurrence time of earthquakes is achieved. temporal variation of b-value indicates that average b-value decreased from 1.25±0.02 between 2002 and 2010 to 1.08±0.02 between 2011 and 2015. a clear quiescence in the seismic moment in recent years may be a clue foretelling the occurrence of an intense event. annual probability of the earthquakes between 3.5 and 4.0 magnitude levels exhibits a value equal to and larger than 30. recurrence time of the earthquakes has a value between 2.0 and 3.0 years for magnitude level of 6.0, and a value between 6.0 and 7.0 years for magnitude level of 7.0. a significant decrease in b-value and clear quiescence anomalies in z-value at the beginning of 2015 are observed in the several same regions. these areas cover the central anatolia fault zone, malatya and ovacık faults, the southeastern part of the eastern anatolia fault zone, and the junction of the eastern anatolia and dead sea fault zones. in these anomaly regions, a decreasing trend in b-value may be an indicator of stress increases and an increasing trend in z-value may show the quiescence areas before the next earthquake occurrences. as a remarkable fact, these areas in the eastern anatolia region may be interpreted as one of the most likely locations for the major earthquakes in the future. abstract keywords: eastern anatolia region; b-value; z-value; annual probability; recurrence time; earthquake potential; space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n2.50889 earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): 67 75 s. öztürk1,* 1gümüşhane university, department of geophysics, tr-29100, gümüşhane, turkey serkanozturk@gumushane.edu.tr en este estudio se realizó un análisis estadístico para evidenciar el potencial actual de terremotos en la región de anatolia, al oriente de turquía. con este objetivo, se obtuvieron una evaluación espacio-temporal basada en los cambios recientes del valor b en la ley gutenberg-richter, el valor z de inactividad sísmica, el momento acumulado, la probabilidad anual y el tiempo recurrente de los terremotos. la variación temporal del valor b indica que este factor cayo desde 1.25±0.02, entre 2002 y 2010, a 1.08±0.02 entre 2011 y 2015. la clara inactividad en el momento sísmico de los años recientes podría ser una clave predictiva de la ocurrencia de un terremoto intenso. la probabilidad anual de terremotos de magnitud entre 3.5 y 4.0 muestra un valor igual o mayor a 30. el tiempo de recurrencia de terremotos tiene un valor de 2.0 y 3.0 años en la magnitud de nivel 6.0 y un valor entre 6.0 y 7.0 años para la magnitud de nivel 7.0. a comienzos de 2015 se observó una caída significante del valor b y claras anomalías en el valor z de inactividad en varias regiones. estas áreas cubren la zona de la falla de anatolia central, las fallas malatya y ovacık, la parte sureste de la falla este de anatolia, y las zonas de intersección de la falla este de anatolia con la falla del mar muerto. en estas regiones de anomalías, una tendencia a la baja en el valor b podría ser el indicador de un aumento de tensión, y una tendencia al alza del valor z podría mostrar las zonas de inactividad antes de la ocurrencia de próximos terremotos. como un hecho remarcable, estas áreas en la región este de anatolia pueden ser calificadas como unas de las más propensas a sufrir terremotos mayores en el futuro. resumen palabras clave: región este de anatolia; valor b; valor z; probabilidad anual; tiempo de recurrencia; potencial de terremoto. record manuscript received: 28/05/2017 accepted for publication: 11/05/2017 how to cite item ozturk, s. (2017). space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey. earth sciences research journal, 21(2). 67 75 doi:http://dx.doi.org/10.15446/esrj.v21n2.50889 evaluación espacio-temporal del potencial de terremotos en los años recientes para la región de anatolia, en el este de turquía se is m o l o g y 68 s. öztürk 1. introduction the space-time assessment of earthquake activity is an important tool in seismicity studies and some valuable results have been provided to the literature by different authors. many researchers prefer to use different tools in practical applications of seismicity analyses, such as physical models, scaling laws and several different seismic parameters such as seismic b-value, fractal dimension, precursory seismic quiescence, annual probability, recurrence time, moment and energy releases. the magnitude-frequency distribution, known as the b-value of gutenberg-richter relation (gutenberg and richter, 1944), is one of the most commonly used as the power-law distribution of earthquakes. estimating the variations of b-value may supply some useful relations between the frequency of earthquakes, seismic moment or energy. the b-value reflects the relative numbers of both large and small earthquakes, and is related to the properties of the seismotectonic structures, and stress distributions in time and space (scholz, 1968). when b-value shows a decrease for a given region, one can make an assessment that there is a possibility of an earthquake occurrence. in addition to the b-value, the evaluation of the seismicity rate changes by considering the precursory seismic quiescence may supply remarkable outcomes. wyss and habermann (1988) made a definition of seismic quiescence phenomenon in the following way: “the seismic quiescence means that an important decrease in the average earthquake activity rate as compared to declustered background activity rate in the same crustal volume may be observed before the occurrences of some main shocks. this decrease in earthquake activity may occur within part, or all of the source volume of the subsequent main shock. also, this decreasing trend may continue to the main shock time, or may be separated from it by a relatively short period of increasing trend in earthquake activity”. since the duration of quiescence depends strongly on tectonic structure and probably on the loading rate, the measurement of earthquake activity rate may be important to reveal the future earthquake potential. these types of parameters have been used in many seismicity studies for different parts of the world and also turkey (e.g., katsumata and kasahara, 1999; cao and gao, 2002; öztürk et al. 2008; polat et al. 2008; joseph et al. 2011; öztürk, 2011; 2015a; öztürk and bayrak, 2012; rehman et al. 2015; negi and paul, 2015; yadav et al. 2015; singh, 2016; tsapanos et al. 2014; 2016). 2. seismic and tectonic structures of the eastern anatolia region it is well-known that the eastern anatolia (ea) area is one of the most seismically active regions in turkey, since many strong and destructive earthquakes have occurred in the past, such as on december, 26 1939 in erzincan (m8.0), august 17, 1949 in elmalıdere-bingöl (m7.1), march 13, 1992 in erzincan (m6.8), january 27, 2003 in tunceli (m6.1), may 1, 2003 in bingöl (m6.4), august 11, 2004 in elazığ (m5.7), january 25, 2005 in hakkari (m5.9), march 12, and 14, 2005 in bingöl (m5.7 and m5.9). in addition, because of the occurrence of three recent earthquakes “mw = 6.0 in elazığ, march 8, 2010; mw = 5.5 in erzincan, september 22, 2011; and mw = 7.2 in lake van, october 23, 2011” -, studies of earthquake hazard potential for this region have become more important. some details of these mentioned earthquakes are also given in table 1. the ea part of turkey has tectonically a very complex structure. the main tectonic regimes in this part of turkey are the eastern anatolia fault zone (eafz), the dead sea fault zone (dsfz), the bitlis thrust zone (btz) and the karlıova triple junction (ktj). the eafz is a transform fault forming parts of the boundaries between the arabian and african plates, and between the anatolian and the eurasian plates (westeway, 1994). this zone is a sinistral strike-slip fault zone and is about 550 km long. the eafz is thought to be a conjugate structure to the north anatolian fault zone (nafz). it meets and forms triple junctions with the nafz and the dead sea fault zone (dsfz), respectively (bozkurt 2001). the dsfz is a sinistral intraplate strike-slip fault zone, approximately north-south direction, and measures approximately 1000 km in length. it separates the arabian plate to the east and the african plate to the west, and it is considered as a plate boundary of transform type (şengör and yılmaz, 1981). the african plate moves northward slower than the arabian plate and the dsfz takes up this dissimilar motion between these plates. thus, the dsfz and eafz meet in a triple junction between the anatolian, african and arabian plates. the bztz is a complex collisional boundary of continent-ocean and continent-continent. the eurasian and arabian plates come into collision along the bztz and this clash results in the uplift of mountains along the suture zone. this boundary reaches to the north of fold-and-thrust belt of the arabian platform and it extends from southeastern turkey to the zagros mountains in iran (şengör and yılmaz, 1981). a north-south compressional tectonic regime is dominant in the area to the east of the ktj. this region is characterized by two combined strike-slip faults of sinistral and dextral character which is parallel to the nafz and eafz (bozkurt, 2001). compressional basins with east-west trending give shape to the most spectacular structures in the region. however, the major tectonics of ea are formed by the conjugate strike-slip fault systems. table 1. a list of some great and destructive earthquakes occurred in the eastern anatolia region of turkey. figure 1. a) main tectonic structures from şaroğlu et al. (1992) and bozkurt (2001). names of the faults: eafz: eastern anatolia fault zone, neafz: north east anatolia fault zone, çfz: çobandede fault zone, akf: aşkale fault, ezf: erzurum fault, kf: kağızman fault, af: ağrı fault, if: iğdır fault, dfz: doğubeyazıt fault zone, bfz: balıklıgölü fault zone, çf: çaldıran fault, erf: erciş fault, htf: hasan-timur fault, bf: başkale fault, ysfz: yüksekova-şemdinli fault zone, sf: süphan fault, mf: malazgirt fault, sufz: sancak-uzunpınar fault zone, gfz: göynük fault zone, pf: pülümür fault, bkkf: bingöl-karakoçan fault zone, gf: genç fault, mtz: muş thrust zone, kbf: kavakbaşı fault, kez: karacadağ extension zone, bzf: bozova fault, srf: sürgü fault, mlf: malatya fault, of: ovacık fault, cafz: central anatolia fault zone. b) epicenter distributions of whole catalog including 33,865 shallow earthquakes with md≥1.0 between 1970 and 2015 for the eastern anatolia part of turkey. magnitude distribution of earthquakes are represented with different symbols. 69space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey figure 1a shows the principal faults in the ea. a large detail on the tectonic structure of the ea can be found in some studies such as şengör and yılmaz (1981), hempton (1987) and bozkurt (2001). also, there are several studies (e.g., sayıl 2005; bayrak et al. 2008; öztürk, 2009; öztürk and bayrak, 2012; tsapanos et al. 2014; öztürk 2015b; maden and öztürk, 2015) for turkey including the ea. these studies generally consider the regional and temporal variations of seismotectonic b-value, seismic quiescence z-value, and some other statistical parameters for the analysis of earthquake behaviors, and their possible usage as precursors. for this reason, seismic and tectonic assessments of temporal changes in b and z-values, cumulative seismic moment, annual probability and recurrence time of the earthquakes are evaluated in this study and it is aimed to present some useful clues for the future earthquake potential in the ea. 3. earthquake database and brief description of methods of analysis the earthquake database used in the scope of this analysis is obtained from bayrak et al. (2009) for the time interval of january 6, 1970 to december 31, 2005. in addition, the catalog from january 1, 2006 to december 31, 2014 is prepared using the database of bogazici university and kandilli observatory and research institute (koeri). in total, a catalog of 33,865 earthquakes (original catalog) whose magnitude is equal to or greater than 1.0 with a depth <70 km is prepared between the coordinates 36°n and 42°n latitude, and 36°e and 45°e longitude. duration magnitude, md, is used in analysis and epicenter distributions of all 33,865 earthquakes with main tectonics drawn from şaroğlu et al., (1992) and bozkurt (2001) are plotted in figure 1b. in order to decluster the earthquake catalog into main and secondary events, reasenberg’s (1985) algorithm is used in this study because the exclusion of dependent events from the catalog is an important part of an earthquake hazard analysis. all dependent events are separated from independent ones, and these dependent events are substituted with a unique event by removing each cluster. in all, 9,515 events (nearly 28 %) were removed by the declustering method, and the declustered catalog of 24,350 earthquakes is obtained. also, completeness magnitude, mc, is also a very important parameter for many seismicity studies, and temporal changes in mc-value can affect the results of the seismicity parameters, especially in b-value. thus, the maximum number of earthquakes in the catalog is tried to be used for high-quality results for the analysis of all statistical parameters. average mc-value for the earthquake catalog from 1970 to 2015 is estimated as 2.9. there are totally 33,865 shallow earthquakes with magnitudes larger than or equal to 1.0 in the catalog. as stated above, the reasenberg’s declustering algorithm eliminated 9515 earthquakes. after the estimation of mc-value as 2.9, the earthquakes with md<2.9 are removed from the original catalog and the number of earthquakes with md<2.9 is 13,793. totally, about 68.83% of all events are subtracted from the original catalog and the number of events for z-test is reduced to 10,557. 3.1. gutenberg-richter relation, b-value gutenberg and richter (1944) proposed an empirical relation for the frequency-magnitude distribution of earthquake occurrences. this power-law for the size distribution of earthquakes was given in the following form: log 10 n (m) = a bm (1) where n(m) is the expected number of events in a specific time interval with magnitudes greater than or equal to m, and a and b are constants. the a-value is proportional to the seismic activity rate, and variations in a-value depend on the length of the study area, observation period, and also number of events. the b-value is the slope of the magnitude-frequency distribution and b-value can be thought as one of the most important parameters for geotectonic and rheological characteristics in a given region. according to the results of laboratory experiments for rock fracture, a decrease in the confining pressure and an increase in the applied shear stress cause a reduction in b-value (scholz, 1968). however, an increase in the thermal gradient, fracture density, or material heterogeneity in the geological complexity (mogi, 1962) is related to high b-values. many studies show that b-value of g-r is a scale invariant and it is related to the clustering and spacing properties of epicenters or distribution of fault segments. one of the most fundamental observations in earthquake physics is the gutenberg-richter (g-r) relation. ishimoto and iida (1939) and gutenberg and richter (1944) suggested the linear equation (1) and this equation is also called as frequency-magnitude relation. it plays an important role in the formulation of realistic design earthquakes for a given region. in many cases, g-r relation for one and the same region or time interval follows the different regimes for small and large events (olsson, 1999). an estimation of the maximum b-value in g-r relation was carried out by olsson (1999) and the deduced maximum b-value was given as 1.5 (or with the error limits, between 1.30 and 1.64). however, utsu (1971) suggested that b-value varies from 0.3 to 2.0 from place to place in each seismic region in the world. the choice of the statistical method used for to estimate the b-value is an important and a critical point. several algorithms can be used to estimate the b-value, but the maximum likelihood method by aki (1965) is the most widely used and the most robust method to estimate the b-value. traditionally, the b-value was estimated graphically, but more efficient estimation is carried out by the maximum likelihood method. it was derived from the quality of the first order moment, and later aki (1965) showed that the maximum likelihood estimate maximize the likelihood function. thus, it is given as in the following form: (2) where mmean is the mean magnitude of the earthquakes, and mmin is the minimum magnitude of completeness in the earthquake catalog. a numerical value of 0.05 is a correction constant. correct estimates of local variations in mmin can be done if relatively large numbers (100 or so) of local observations are available for analysis. confidence limits of 95 % on the calculations of b-value are , where n is the number of events used to make the estimation. this calculation gives the confidence limits of ±0.1-0.2 concerning b-value for a characteristic sample of n=100 earthquakes. thus, the number of earthquakes in this work was 24,350 and the value of n=100 events is an ordinary sample for calculations and represent a specific calculation. 3.2. precursory seismic quiescence, z-value the phenomenon of precursory seismic quiescence was originally committed by wyss and habermann (1988). wiemer and wyss (1994) introduced a methodology that can be performed in zmap software (wiemer, 2001) and the recognizing of the seismic quiescence for different parts of the world has been reported in many papers. so, a brief description of the method will be given here. a continuous image of rate changes in the earthquake activity and plotting the areas showing seismic quiescence in space and time can be provided by zmap by plotting in geographical coordinates. the standard normal deviate z-test is one of the most common technique generally used for detecting of precursory seismic quiescence. in this study, z-test is applied to detect the anomaly regions of precursory quiescence and log term average (lta) function is generated for the statistical assessment of confidence level in standard deviation units: (3) where r1 is the mean rate in the overall background period, r2 is the average earthquake activity rate in the foreground window, s1 and s2 are the standard deviations in these periods, and n1 and n2 are the number of 70 s. öztürk samples with a measured earthquake activity rate. z-value is estimated as a function of time and named lta. 4. results the cumulative number of earthquakes with time both the original catalog including all shallow earthquakes with md≥1.0 (33,865 events) and declustered data including the earthquakes with md≥2.9 (10,557 events) is shown in figure 2. any significant seismic activity is not observed from 1970 to 1995 and there is a little variation between 1995 and 2000. conversely, there is a significant earthquake activity after 2000. however, it can be said that the database is separately homogeneous between 1970 and 1995, between 1995 and 2002, and between 2002 and 2015. koeri and the other observatories have provided the real time data, especially after the construction of many stations in this area in recent years. since this analysis focuses on the recent earthquake activity, completeness magnitude can be taken as 2.9 for all catalog as given in öztürk and bayrak (2012). some authors such as katsumata and kasahara (1999), and joseph et al. (2011) made detailed analysis on earthquake behaviors and they pointed out that the selection of the completeness magnitude and declustering of the catalog are an important stage for the analysis of earthquake occurrences. katsumata and kasahara (1999) made a seismic quiescence analysis for the 1994 kurile earthquake. they made a completeness analysis and they used the declustered the catalog for their statistical estimations. joseph et al. (2011) stated that just the main earthquakes must be considered in the recurrence analysis of earthquakes and all dependent events such as foreshocks and aftershocks must be excluded from the catalog before starting any calculations. as shown in figure 2, the slope of cumulative number curve of the declustered data is smoother than that of the original catalog. it is a remarkable fact that these two processes removed the dependent events from the original data set. as an important result, after completing of the declustering process and completeness analysis, a more reliable, homogeneous and robust earthquake data is provided for the statistical estimations. magnitude and time histograms of the study region are given in figure 3. as stated above, magnitudes of the earthquakes are between 1.0 and 6.6 and show an exponential decay in their numbers from the smaller to larger magnitudes. as shown in figure 3a, the size of the many earthquakes varies from 2.0 to 4.0 and a maximum is observed for md=2.8. the number of earthquakes with 2.0≤md<4.0 are 30,591. however, there are 2119 evens with 1.0≤md<2.0, 20,149 events with 2.0≤md<3.0, 10,442 events with 3.0≤md<4.0, 1074 events with 4.0≤md<5.0, and 81 events with 5.0≤md. thus, the earthquake occurrences with magnitudes between 2.0 and 4.0 are more dominant than those of the others in the study region. it is clear that this increasing tendency in the number of relatively small events may be a sign of increasing stress in recent years in the ea. also, time histogram from 1970 to 2015 is given in figure 3b. there is a little change in the earthquake activity between 1970 and 2002, and the number of earthquakes including all magnitude levels in this time interval is 2191. there is also a stableness between 2003 and 2009, and the number of earthquakes in this period is 8879. however, an increasing trend in the number of earthquakes have started after 2010 and it has continued until 2012. the number of earthquakes between 2010 and 2012 is 15,128 and the maximum increasing is reported in 2012. the systematic increase in seismic activity shows a decreasing trend after 2012, and the number of earthquakes between 2012 and 2015 is 7667. as a remarkable fact, such kind of analyses can supply a useful perspective for the evaluation of seismicity rate changes which is associated with the temporal variations of precursory seismic quiescence in the ea. figure 4 shows the annual probabilities and recurrence times for different magnitude levels. annual probabilities of earthquake occurrences for different sizes show relatively higher (30 and above) values between 3.5 and 4.0 magnitude levels, intermediate values (between 1 and 10) from 4.5 to 5.5 magnitude levels, and quite smaller values (<1.0) between 5.5 and 7.0 magnitude levels. recurrence times of earthquake occurrences for different magnitude levels are also shown in figure 4b. it is observed quite smaller (<1.0) years for magnitudes from 3.5 to 5.5. for magnitude level of 6.0, the values between 2.0 and 3.0 years are estimated while the values between 6.0 and 7.0 years are estimated for magnitude level of 6.5. it can be concluded that the occurrences of earthquakes between 3.5 and 4.0 magnitude level are more likely than those of the others, and an earthquake occurrence for a magnitude size larger than 6.0 can be expected in every seven years. these results can also be seen from table 1 and the results of the recurrence times analysis support the existing earthquake potential in the ea. consequently, such kind of analyses on the probabilities and recurrence times of earthquake occurrences for specific magnitude levels show that the ea has an earthquake risk potential for the possibility of strong earthquake occurrence. figure 2. temporal changes of the cumulative number of earthquakes for the original catalog including all md≥1.0 earthquakes (33,865 events) and for the declustered data set with md≥2.9 (10,557 events). figure 3. a) magnitude histogram and b) time histogram of the earthquake activity between 1970 and 2015. 71space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey figure 4. a) annual probability and b) recurrence time of the earthquakes for different magnitude levels in the ea region. figure 5. gutenberg-richter relation and b-value with its standard deviation. declustered catalog is used for calculations and mc-value is also given. figure 6. changes in b-value and its standard deviation (dashed line, δb) as a function of time, from 1970 to 2015. thick horizontal lines show the average b-values, and the upper and lower thin horizontal lines shows their standard deviations. declustered catalog is used for analysis. as stated above, the maximum likelihood method is preferred to estimate the b-value since this algorithm gives a more robust estimate than the least-square regression method (aki, 1965). figure 5 shows the cumulative number of events versus the magnitude of earthquakes. as shown in figure 5, average mc for the whole catalog from 1970 to 2015 is taken as 2.9 and b-value is estimated as 1.02±0.01. on global scale, b-value varies from 0.3 to 2.0 depending on region (utsu, 1971) and is equal to 1.0 on average (frohlich and davis, 1993). as a result, both the mc and b-value estimations in this study are similar to the results of the analysis by öztürk and bayrak (2012), and the magnitude-frequency distribution of the earthquakes in the ea is well represented by the g-r power law distribution with the b-value close to 1.0. changes in b-value as a function of time are plotted in figure 6. temporal variations of b-value are estimated for overlapping samples of 350 earthquakes per window. a systematic decrease can be seen until 1996, and the average b-value from 1970 to 1995 (464 earthquakes) is calculated as 1.28±0.1 with mc=4.3. the b-value does not show any significant changes between 1996 and 2001 (1,168 earthquakes), and the average b-value is found to be 0.99±0.04, a lower value compared to that of the previous period, with mc=3.4 for this period. however, there are great fluctuations and a clear increase in b-value between 2002 and 2010 (9,067 earthquakes), and the average b-value is computed as 1.25±0.02 with mc=2.9 for this time interval, a larger value that that of the previous period. for the last time interval of 2011-2015 (13,651 earthquakes), there is a clear decrease in b-value and the average b-value is estimated as 1.08±0.01 with mc=2.5. as seen in figure 6, there is a clear increase from 2002 to 2011, whereas there is a clear decrease after 2011. in addition, one can see many fluctuations which are related to the period before the occurrence of large earthquakes between 2002 and 2015. there are 81 events whose magnitude is larger than or equal to 5.0 (figure 1b), and the b-value shows a clear tendency of decrease before some of these strong main shocks in this time interval. öztürk (2011) observed these types of decrease before some major earthquakes, such as the 1999 i̇zmit, 2003 tunceli and 2003 bingöl earthquakes. as mentioned above, many variables can affect the variations of b-value. it is thought that the decrease in b-value before the main shocks may be the result of increasing shear stress, and thus this tendency of decrease in b-value may be interpreted as an indicator of a possible earthquake in the ea. figure 7 shows the magnitude variations of earthquakes as a function of time. in order to see the fluctuations in magnitude levels in the ea from 1970 to 2015, temporal distribution of earthquakes with md≥4.0 is plotted. as shown in table 2, the average magnitude is 4.49±0.33 for the time interval between 1970 and 1995, 4.29±0.22 between 1996 and 2001, 4.35±0.31 from 2002 to 2010, and 4.34±0.32 from 2011 until 2015. there are 22 events whose magnitude is equal to or larger than 5.0 (the largest magnitude among them is 6.5) between 1970 and 1995; 9 events from 1996 to 2001 (the largest among them is 5.5); 28 events from 2002 until 2010 (the largest has a magnitude of 6.2); and 22 earthquakes between 2011 and 2015 (the largest magnitude is 6.6). magnitude-time histogram shows significant variations in the number of large events from 1995 but further on especially after 2002. the earthquake activity which is related to the clustering characteristics is obviously observable and it may correspond to a main earthquake in study region. also, temporal clustering properties of the earthquake activity related to the earthquakes with md≥5.5 is strong enough for many earthquakes which occurred in 1975, 1976, 1983, 1992, 1995, 1997, 2001, 2002, 2003, 2004, 2005, 2007, 2010, 2011. figure 7. temporal changes in earthquake magnitudes with md≥4.0 including 1155 events between 1970 and 2015. whole earthquake catalog is used for plotting. thick horizontal lines show the average magnitude and the upper and lower thin horizontal lines shows standard deviations for these periods. 72 s. öztürk figure 8. cumulative moment release as a function of time from 1970 to 2015. arrows indicate the occurrence of large earthquakes in study region. whole earthquake catalog is used for analysis. table 3. cumulative moment increases after some great earthquakes. figure 9. regional variations of temporal b-value changes between 2002-2010 and 2011-2015 in the eastern anatolia region of turkey. white dots show the declustered earthquakes used in the analysis. main tectonics are also given. figure 10. regional variations of z-value in the beginning of 2015 with t w (iwl)=5.5 years in the eastern anatolia region of turkey. white dots show the declustered earthquakes with md≥2.9 used in the analysis. major tectonics are also given. table 2. several statistics for the earthquakes with m≥4.0 between 1970 and 2015. cumulative moment variations (cmv) versus time from the declustered catalog are plotted in figure 8. cmv is calculated as about 1.36*1026 nm for the 1992 erzincan, cmv~1.86*1026 nm for the 2003 tunceli, cmv~2.26*1026 for the 2003 bingöl, cmv~2.43*1026 for the 2005 bingöl, cmv~3.06*1026 for the 2010 elazığ, and cmv~5.93*1026 for the 2011 lake van earthquakes (table 3). high values of moment release represent a high seismic energy release, and consequently the occurrence of strong earthquakes. it can be interpreted that temporal and regional changes in seismic moment are partly due to the variations in tectonic stress. the stress conditions in a given region after the occurrence of a strong earthquake will change, so the cumulative moment is expected to increase in proportion to the great energy released as the consequence of stress modification produced by the dynamic ruptures of the main shock. the total stress drop and cumulative moment depending on these conditions are associated with the seismicity of the focal region (marcellini, 1995). as shown in figure 8, sudden and high increases in cumulative moment are observed at the time when the main shocks occur. these large variations in cumulative moment are also in accordance with the temporal variations of magnitude as given in figure 7, and there are clear uniformities in moment variations before the occurrence of great earthquakes as seen in figure 8. this means that there are constant moment values in the period in which seismic activity is stationary, so it can be said that variations in cumulative moment are related to the changes of seismic activity. it can be seen from figure 8 that there is a clear quiescence in the temporal variation of seismic moment after 2011, and thus if these kinds of behavior in the seismic moment can be described as a reliable indicator of future earthquakes, then these variations can give us significant evidence about the forthcoming earthquake hazard potential in the ea. regional variations in b-value as a function of time in the ea are given in figure 9. this figure shows the changes of b-value between 2002-2010 and 2011-2015. in order to map these variations, the study region is divided into spatial grids of points with a size of 0.1° in latitude and longitude. there are six regions exhibiting large decrease (between -0.4 and -1.0 units) in b-value. these regions are centered at 39.03°n-37.19°e (region a, covering the cafz, mlf and of), at 39.46°n-41.80°e (region b, the southwest of af and kf), at 38.96°n-42.50°e (region c, the north of lake van, including mf), at 36.85°n-38.64°e (region d, the south of bzf), at 36.84°n-37.72°e (region e, the southeast part of the eafz), and at 36.35°n-36.29°e (region f, in the junction of the eafz and dsfz). regional variations in z-value in the ea are shown in figure 10. this demonstration supplies the spatial image of z-value estimation at the beginning of 2015. in order to establish the distribution of precursory seismic quiescence, the study region is divided into spatial grids of points with a size of 0.1° in longitude and latitude. there are several areas exhibiting seismic quiescence at the beginning of 2015. these anomaly areas are observed on the north western part of the nafz, between akf and ezf, on the pf, bkkf, sufz and gfz, in and around the gf, between bf and ysfz, on a part of the eafz, in the eastern and western parts of the mlf, in the southeastern part of the eafz, in the junction of the eafz and dsfz, and in the southern part of the cafz. as shown in figures 9 and 10, there are clear decreases in b-value and seismic activity rate z-value in the several same areas: the cafz, mf, of, the southeastern part of the eafz, and the junction of the eafz and dsfz. as a remarkable fact, such kind of combination of these seismotectonic parameters may supply significant clues to reveal the seismic potential in the ea and thus, special interest needs to be paid to these anomaly regions. 73space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey table 4. return periods for some regions of ea part of turkey. these values are taken from öztürk et al. (2008). 5. discussions the ea has many important tectonic faults and fault systems and also is one of the most seismically active parts of turkey. many large and destructive main shocks have occurred in the last five years in the ea and so, the statistically analysis of earthquake behaviors would be important. in recent years, the space-time assessments of different statistical parameters have been made in order to put forth the earthquake hazard potential in the ea (öztürk et al. 2008; bayrak et al. 2008; öztürk, 2009; öztürk and bayrak, 2012; tsapanos et al. 2014; öztürk 2015b; maden and öztürk, 2015). also, several valuable investigations which are focused on the such kind of assessments, especially on b-value and seismic quiescence analysis, have been achieved by different authors for different parts of the world (e.g., cao and gao, 2002; polat et al. 2008; öztürk, 2011; 2015a; rehman et al. 2015; negi and paul, 2015; yadav et al. 2015; singh, 2016; tsapanos et al. 2016). polat et al. (2008) made a statistical analysis for the aegean extension part of turkey in order to evaluate the earthquake potential by using the seismotectonic b-value, fractal dimension dc-value and standard normal deviate z-value. their results show that the regions with smaller b-values and larger z-values may be interpreted as the possible areas for the large earthquakes. öztürk (2011) and öztürk (2015a) made a similar study on the space-time relationships between several seismotectonic parameters such as b-value, dc-value and z-value for the north anatolia fault zone and the western anatolia region of turkey, respectively. in these studies, it is suggested that the correlations between these three seismic and tectonic parameters (for example, the regions with the lower b-value, larger dc-value and higher z-value) may give significant evidences for the future earthquake potential. rehman et al. (2015) made an analysis on the spatial and temporal changes of b-value in and around the north pakistan. they used different histograms, time series distribution of earthquakes and b-value map in order to characterize the seismic behaviors of different parts of pakistan. their results on the temporal distribution of b-value for hazara region show a remarkable decrease before the occurrence of october 8, 2005 kashmir earthquake. negi and paul (2015) made a comprehensive study for the spatial and temporal analysis of earthquake activity for garhwal-kumaun himalaya region. they used b-value and dcvalue in order to explain the spatio-temporal characteristic of seismicity. they stated that the lowest b-value for the south east garhwal indicates high stress conditions and also means that the number of small earthquakes in this region decreases relatively due to the stress accumulation. as a remarkable fact that, they suggested a temporal observation of b-value is necessary for the possible earthquake risk assessment. yadav et al. (2015) achieved a probabilistic estimation for the most perceptible earthquake magnitudes in and around the nw himalaya. they used two earthquake recurrence models of kijko and sellevoll and gumbel’s third asymptotic distribution for different 28 subregions. they stated that spatial distribution of these parameters presents a good relation with high seismic hazard regions in previously reported hazard maps, the maximum expected magnitudes and acceleration. they suggested that such kinds of maps may give useful information in the selection of preparing earthquake criteria for the antiseismic design of non-critical structures. singh (2016) made a spatial analysis of seismic b-value for all nw himalaya segment between 1964 and 2010 in order to understand the tectonic implications. their results show that garhwal-kumaun segment with small b-value has a large seismic potential and indicates a high stress distribution. they suggested that b-value is a one of the most significant parameters for the seismic hazard assessment in a given region since the b-value is directly related to the changes of stress accumulation in the crust. tsapanos et al. (2016) made an assessment of the relative largest earthquake hazard level in 28 subregions in and around the nw himalaya. they used the gumbel’s third asymptotic distribution for the estimation of the largest earthquake in their study region. they suggested that spatial variations of their parameters such as b-value (upper bound magnitude) and k-index (relative seismic hazard level) may give useful clues for scientific goals along with the designation of priority areas for earthquake resistant design. as mentioned above, some authors used different parameters to make an earthquake risk assessment in the ea. an estimation of the earthquake hazard parameters for different parts of turkey is made based on gumbel’s i distribution by öztürk et al. (2008). in their study, a few parameters such as the mean return period, the probable maximum magnitude and the probability of a great earthquake occurrence for some magnitudes and periods are estimated for different 24 regions in and around turkey (table 4). their results show that the mean return period for ms≥5.0 is calculated as 10.23±0.47 years in their region 2 including af and kf in this study. the last earthquake greater than 5.0 was occurred in 2007 in this region. the mean return period for ms≥5.0 was obtained as 13.49±0.63 years for their region 3 covering the lake van, and mf in this study. the last earthquake larger than 5.0 in this area was occurred in 2011. the mean return period for ms≥5.0 was found as 45.71±4.22 years for their region 5 covering the bzf in this study. the last earthquake larger than 5.0 was occurred in 2012 in this zone. for their region 6, covering the eafz in this study, the mean return period for ms≥5.0 was estimated as 11.75±0.54 years. the last earthquake larger than 5.0 in this zone was occurred in 2012. the mean return period for ms≥5.0 was obtained as 10.23±0.71 years for their region 7 covering the junction of the ea and dsfz in this study. the last earthquake larger than 5.0 was occurred in 2002 in this region. the mean return period for ms≥5.0 was calculated as 8.91±0.41 years for their region 22 covering cafz in this study. for their region 23, including mlf and of in this study the mean return period for ms≥5.0 was estimated as 13.49±0.62 years. the last earthquake larger than 5.0 was occurred in 2012 in regions of 22 and 23. the mean return period for ms≥5.5 was found as 7.50±0.09 years for their region 24 including the eastern part of the nafz in this study. the last earthquake with magnitude 5.5 was occurred in 2011 in this zone. taking into consideration the mean return period values for strong events whose magnitudes are larger or equal to 5.0 from öztürk et al. (2008), the areas covering af and kf may have a next earthquake potential between 2017 and 2018. for the other anomaly regions, however, the future time of a strong earthquake can be expected after 2020. bayrak et al. (2008) made a seismicity analysis for different 24 seismogenic zones using gumbel first asymptotic distribution and gutenbergrichter relation. they stated that their estimations of b-values with the maximum likelihood method for the gutenberg-richter relation have a good consistency with the tectonics. they calculated similar b-values for their region 1 (b=0.74, in neafz), region 2 (b=0.76, in af and kf), region 3 (b=0.78, including lake van, and mf) and region 7 (b=0.79, in a part of dsfz). as given in bayrak et al. (2008), these regions include the earthquakes such as 1924 pasinler, 1983 horasan, 1976 çaldıran and 1903 patnos, and these authors stated that these low b-values are related to the low heterogeneity degree and great faults causing destructive earthquakes. they also calculated b-values smaller than 1.0 in their region 4 (b=0.96, in bitlis thrust zone), region 5 (b=0.94, including bzf and kez), region 6 (b=0.91, in eafz). according to the gps (global positioning system) data, there is a total shortening of 10±2 mm/yr between the strike slip faults in eastern turkey and thrusting along the caucasus (mcclusky et al. 2000). also, sinistral motion in the eafz is given as 9±1 mm/yr (mcclusky et al. 2000). because of the seismic activity rate and the fault slip rate, the b-values in regions 1, 2, 3 and 7 are smaller than those of regions 4, 5 and 6 (bayrak et al. 2008). the b-values are estimated as 0.62 for their region 22 (including cafz, 0.58 for their region 23 (covering mlf and of), and 0.68 for their region 24 (including the eastern part of the nafz), respectively. these resulting values show that these areas are not multifractal regions and have a large homogeneity degree (bayrak et al. 2008). also, many researchers such as öztürk (2011) and 74 s. öztürk öztürk (2015a) observed a decreasing trend in the b-value before the occurrence of a large earthquakes. consequently, as seen from the results, the regions with small b-values in their study and the regions showing great decrease in b-value in this study cover the same regions: cafz, mlf, of, af, kf, lake van, mf, bzf, eafz and the junction of the eafz and dsfz. as a remarkable fact that, the data, estimations and anomaly regions in this study are more up-to-date and the similar with those of bayrak et al. (2008). an earthquake hazard evaluation in the ea in terms of seismic quiescence z-value was made by öztürk (2009). in this study, he aimed to estimate the duration of precursory quiescence for the strong earthquakes occurred in ea and used a few strong earthquakes greater than 5.0 between 1970 and 2005. in öztürk (2009), average precursory quiescence duration before a strong earthquake in the ea is estimated about 5.0±1.5 years. then, regional distribution of precursory seismic quiescence in the ea at the beginning of 2009 is analyzed by öztürk and bayrak (2012). they observed four anomaly regions around aşkale (erzurum), ovacık (tunceli), elazığ and bingöl and, van lake. in two of these areas, 2010 elazığ and 2011 lake van earthquakes are occurred, but not aşkale and ovacık. however, a similar anomaly is observed in ovacık region. as a result, this common area in which both seismic quiescence and low b-value are observed may be considered as a potential earthquake zone. therefore, these kinds of seismic behaviors can be an important clue for the region of next earthquake occurrences. öztürk and bayrak (2012), tsapanos et al. (2014), maden and öztürk (2015), and öztürk (2015b) made a detailed evaluation of earthquake hazard potential for including the ea by using different statistical parameters. öztürk and bayrak (2012) achieved a statistical analysis for the eastern part of turkey by using the gutenberg-richter b-value and by applying the standard normal deviate z-test. the b-value is calculated as 1.07±0.09 with a completeness magnitude of 3.0. their results indicate four quiescence regions (with high z-value) at the beginning of 2009 and these anomaly regions are observed around erzurum (aşkale), tunceli (ovacık), elazığ, bingöl and lake van. the b-value estimated in this study (b=1.02 with mc=2.9) is smaller than that of öztürk and bayrak (2012). also, several seismic quiescence anomalies estimated in this study are the same regions with öztürk and bayrak (2012) and these regions cover erzurum, tunceli, elazığ, bingöl cities including akf, pf, bkkf, sufz, gfz, in and around the gf, between bf and ysfz. a significant result of öztürk and bayrak (2012) is that march 8, 2010 elazığ, september 22, 2011 erzincan and october 23, 2011 lake van earthquakes occurred in the seismic quiescence areas which are detected by them. when compared the results in both studies, it is clear that b-value is in a decreasing trend in recent years and this decrease can be also seen in figure 6 in this study. also, such kinds of assessments which is made on the seismicity rate changes can play an important role in the estimation of possible location of next earthquakes and in the earthquake hazard potential. tsapanos et al. (2014) made a study including regional changes of seismicity parameters in and around turkey. for this purpose, they used the gumbel’s third asymptotic distribution of extreme values in order to define the seismicity pattern. they suggested that there is a good relation between their seismicity parameters and the tectonic structures for turkey and surrounding area. they calculated a high value of upper bound magnitude w-value in the eastern part of turkey. according to their earthquake hazard maps (figures 2 and 3 in their study), the larger w-values greater than 7.0 can be seen from the geographical distribution of the parameter of the 50-years mode in the nafz, in the ea and in the western part of turkey. they related these large and very large values with the seismotectonically active zones and, they stated that large values in the eastern part of turkey are related to the btz. consequently, the higher w-values estimated by tsapanos et al. (2014) in the ea cover the anomaly areas of b-value and z-value found in this study. spatial distribution of b-value for the ea is also mapped by maden and öztürk (2015). they searched the relations between b-value, bouger gravity and heat flow data for tectonic implications. they calculated the b-value as 1.11±0.08 with a completeness magnitude of 2.8. one can clearly see that calculated b-value in this study (b=1.02 with mc=2.9) is smaller than that of maden and öztürk (2015). so, it can be said that b-value decreases at the beginning of 2015 (see figure 6). according to their spatial results, low b-values are shown in and around çfz, af, kf, the north, south and east of lake van, the west of akf, on the eafz and btz, between mlf-sf-bzf, between mtz and kez, in the south east part of the eafz, and in the junction of the eafz and dsfz. thus, the areas having small b-value estimated by maden and öztürk (2015) and the areas exhibiting great decrease in b-value in this study include several same regions: af, kf, the north of lake van, in the south east part of the eafz, and in the junction of the eafz and dsfz. such decreases can imply an increase in stress relaxation in the mentioned parts of the ea. by contrast, relatively small variations in b-value during the post-2011 epoch may be related to a relatively small stress distribution in which energy can be spent along with time during the earthquakes. thus, the earthquake dataset and anomaly regions of b-value in this study are more up-to-date and the similar with those of maden and öztürk (2015). öztürk (2015b) made a comprehensive statistical analysis for the ea. for this purpose, a modelling on the fractal dimension of earthquakes and intermediateterm forecasting for the location of expected earthquakes were achieved. the ea was divided into 19 seismogenic zones for a detailed evaluation. for each region, b-values and dc-values were estimated and a relationship between seismotectonic b-value and fractal dimension dc-value was determined by comparing different regression methods. it is also generated a composite forecast map based on the relative intensities (ri method) and pattern informatics (pi method). according to the results of öztürk (2015b), small b-values and large dc-values are observed in the neafz, af, kf, dfz, bfz, çf, bf, erf, sf, btz, bkkf, sufz and the eastern part of the nafz. according to the composite forecast map by öztürk (2015b), several hazardous regions were identified for the possible locations of the next earthquake occurrences: in and around akf, the west part of the lake van (between sf and erf), around ysfz, around and in the north part of of, on the eafz, on a part of nafz (between mtz and pf). when compared the results of this study and öztürk (2015b), it can be clearly seen that the results of spatial distributions of b-value and z-value in this study are suitable with the results of öztürk (2015b). 6. conclusions a space-time assessing in the eastern anatolia part of turkey is achieved by evaluating several seismotectonic parameters. seismic activity in this region show an important increase in recent years and 50 earthquakes with md³5.0 occurred after 2002. average value of completeness magnitude is calculated as 2.9 and the b-value is estimated as 1.02±0.01. important fluctuations and increases are observed in b-value (1.25±0.02) between 2002 and 2010. however, a clear decrease is observed in b-value from 2011 to 2015 and the b-value is calculated as 1.08±0.01. cumulative moment releases are estimated as about 1.36*1026 nm after the 1992 erzincan, ~1.86*1026 nm after the 2003 tunceli, ~2.26*1026 after the 2003 bingöl, ~2.43*1026 after the 2005 bingöl, ~3.06*1026 after the 2010 elazığ, and ~5.93*1026 after the 2011 lake van earthquakes. however, a quite clear quiescence in temporal changes of cumulative seismic moment is detected after 2011. an increasing trend in the number of earthquakes until 2012 tends to decrease after 2012. analyses of probability and recurrence time of the earthquakes suggested that the eastern anatolia region of turkey has an earthquake risk potential for the probability of large earthquake occurrence in the future. an important decrease is observed in temporal variations of b-values for the time interval of 2011 to 2015 in comparison with those for the time period of 2002 to 2010. also, there are several regions exhibiting seismic quiescence at the beginning of 2015. the regions having remarkable decreases in b-value and z-value at the beginning of 2015 cover the central anatolia fault zone, malatya and ovacık faults, the southeastern part of the eastern anatolia fault zone, and the junction of the eastern anatolia and dead sea fault zones. as a remarkable fact, the decreases in the number of earthquakes after 2012 support the temporal variations of b-value and seismic quiescence z-value. consequently, a decrease in the b-value, seismic quiescence anomalies, recurrence times of earthquakes and seismic moment variation may be an indicator of high stress release, and these changes may be interpreted as an evidence of strong earthquake occurrences in the next future in the eastern anatolia region of turkey. 75space-time assessing of the earthquake potential in recent years in the eastern anatolia region of turkey acknowledgements the author would like to thank to the reviewers for their useful and constructive suggestions in improving this paper. i thank to koeri for providing free earthquake database via internet. references aki, k. 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(2015). a probabilistic estimate of most perceptible earthquake magnitudes in the nw himalaya and adjoining regions. pure applied geophysics 172(2), 197-212. logging observations of auger profiles (tarmizi, 2014) indicate a vertical, downwards, general decrease of peat humification levels with depth in a tropical lowland peat forest in the kota samarahan-asajaya área in the región of west sarawak (malaysia). based on pollen analyses and field observations, the studied peat profiles can be interpreted as part of a progradation deltaic succession. continued regression of sea levels, gave rise to the development of peat in a transitional mangrove to floodplain/floodbasin environment, followed by a shallow, topogenic peat depositional environment with riparian influence at approximately 2420 ± 30 years b.p. (until present time). the inferred peat vegetational succession reached phasic community i at approximately 2380 ± 30 years b.p. and followed by phasic community ii at approximately 1780 ± 30 years b.p., towards the upper part of the present, ombrogenic, peat profile. observations of the presence of large, hollow, shorea type trees, supports that successive vegetational zonation of the tropical lowland peat dome may have reached phasic community ii. some pollen types were found that are also known to occur in the inferred vegetational zonation of phasic community iii and iv or higher. pollen analyses indicates that estuarine and deltaic, brackish to saline water influence may have gradually ceased at approximately 0.5 m below the lithological boundary between peat and underlying soil (floodplain deposit) in the tropical lowland peat basin. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): o1 o10 abstract keywords: lowland peat swamp, phasic community, mangrove swamp, riparian environment, pollen diagram, vegetation succession. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.53670 en g in ee r in g g eo lo g y mohamad tarmizi mohamad zulkifley1, ng tham fatt1, zainey konjing2, muhammad aqeel ashraf3,4* 1. department of geology, faculty of science, university of malaya, kuala lumpur, malaysia 2. biostratex sendirian berhad, batu caves, gombak, malaysia. 3. department of environmental science and engineering, school of environmental studies, china university of geosciences, 430074 wuhan, p. r. china 4. faculty of science & natural resources, university malaysia sabah88400 kota kinabalu, sabah, malaysia. corresponding authors: muhammad aqeel ashraffaculty of science & natural resources university malaysia sabah, 88400, kota kinabalu, sabah. (ashraf@ums.edu.my) development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia record manuscript received: 20/10/2015 accepted for publication: 12/02/2016 how to cite item mohamad, m. t., fatt, n. t., & ashraf, m. a. development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia. earth sciences research journal, (20)1, o1-o10. doi: http://dx.doi.org/10.15446/esrj.v20n1.53670 1. introduction organic materials only accumulate to form peat under certain conditions, whereby it is essential that the production of biomass (organic materials) be greater than its chemical breakdown to form peat (andriesse, 1988). anaerobic, muddy conditions, which prevent the microbiological activity needed for the chemical breakdown of organic materials, are assumed to be largely responsible for the accumulation of partly decomposed biomass/organic matter in the form of peat. the anaerobic conditions are created by a particular hydro-topography whether marsh, swamp, bog or mire. properties of such hydrotopographic units depend on many environmental factors, including climate, landform, local geology and hydrology (andriesse, 1988). the definition of peat also refers to the net accumulation of purely one hundred percent organic matter and the difference between soil and organic or vegetative accumulation varies (murtedza et al., 2002) probably due to varying definitions from the different academic fields associated with the practical study of peat and its’ properties (agriculture, botany, geology and engineering, for instance). ‘peat’ has been alternately referred to as ‘organic soils’ and histosols. peat is referred to as organic soils by mass composition (murtedza et al., 2002), i.e. soils that contain at least 65% organic matter or conversely, less than 35% mineral content. the soil division of sarawak (malaysia) has adopted a definition for organic soil that is based on profile partition, i.e. soils that have 50 cm or more organic soil matter within 100 cm or more than twice that of mineral soil materials overlying bedrock within 50 cm (murtedza et al., 2002). on the other hand, the united states department of agriculture (usda) has defined soil types as organic soils (or histosols) if more than half of the upper 80 cm of the soil is organic or if organic soil material of any thickness rests on rock or fragmental material having interstices filled with organic materials (united states department of agriculture, 1998; murtedza et al., 2002). peats, as we now know, are formed by the limited decomposition and hence, the accumulation of organic soil materials and these organic materials can further consist of undecomposed, partially decomposed and highly decomposed plant remains. tropical lowland peats usually have undecomposed and partly decomposed branches, logs or twigs. tropical lowland peats form a fragile ecosystem because of its’ domed shape and is almost purely organic (paramananthan, 2011). o2 mohamad tarmizi mohamad zulkifley, ng tham fatt, zainey konjing and muhammad aqeel ashraf 2. origin and characteristics of peat deposits the paludal deposits of sarawak occur in large basin swamps and in small interior valleys that have developed in mostly near coastal areas in relatively recent times (murtedza et al., 2002).14c dating from baram in northern sarawak indicates that the sea was at the inland margin of the peat swamp about 5400 years ago (wilford, 1960; murtedza et al., 2002). peat basin swamps are dome-shaped with ‘ombrogenous peat’ occupying the central portion of the peat dome (murtedza et al., 2002; tie, 1990). ‘ombrogenous peat’ or ombrogenic peats (paramananthan, 2011), comprises mainly of broken tree trunks, branches, leaves, roots and fruits (yogeswaran, 1995; huat, 2004; anderson and muller, 1975, anderson, 1976) and are relatively mineral or ash free. however, the marginal peats surrounding the base and along fringes or margins of the peat dome and lower bank of streams draining the peat swamps are dominated by ‘clayey or topogenic peat’, which consists of mainly slightly to moderately decomposed plant matter and fine clastic mineral-sediments (moderate to high ash content). 3. description of site studied the study area covers approximately 25 km2, is located between longitudes 01º 26’ 30’’n and 01º 29’46’’n (north and south of sungai tuang) and latitudes 110º 27’44’’e and 110º 30’58’’e (east and west of batang samarahan). it is located about 30 km from kuching city and is accessible by sealed road. within the study area (figure 1), accessibility is possible using gravel, bund roads or sealed roads. figure 1. location map of study area. physiography-topography, drainage, climate and vegetation the topography of the kota samarahan-asajaya area is flat and low-lying with isolated hills standing out of the floodplains of the batang samarahan and sungai tuang rivers. the general elevation of the flood basin is low and seldom higher than 5 m above sea level. the majority of these areas are prone to high tide flooding except few localities on the natural levees along rivers and man-made bunds. the major rivers draining the study area into the south china sea are the batang samarahan and its distributary, sungai tuang. the meandering batang samarahan flows in a northerly direction while the sungai tuang flows eastward to join the batang samarahan. the area experiences an equatorial climate which is characterized by hot and humid conditions with a drier period during the middle of the year and high rainfalls at the end of the year. 4. objective of study the initial purpose of this study was to identify, classify and map the distribution and natural occurrence of peat deposits in the kota samarahan-asajaya study area. a further objective was to determine the depth and thickness of peat encountered in the project area. the stratigraphy of a tropical lowland peat swamp and some of its related palynological characteristics were studied using samples collected from auger holes. the palynological investigation was performed to identify the peat swamp phasic communities’ vegetation succession that had occurred from basal clay to present-day tropical lowland peat swamp forest based on pollen analyses of augered core samples. also, this study aims to determine the initial age of peat deposition by 14c dating method. 5. materials and methods field methods augering was done using hand augers. augered holes and sampling points were located using a garmin gps device (models gps 76 and gpsmap 76csx). hand augered core samples were extracted, photographed and logged in the field. peat classification was carried out on a scale of h1 to h10 in the field (mohamad tarmizi, 2014) using the von post classification method (engineering geology working group, 2007; von post, 1922). peat sampler the peat or russian-type sampler was mainly used for peat sampling. it consists of an anchored fin and a movable sampling chamber. samples collected using the peat sampler is not disturbed compared to those collected with the gouge auger (guts auger). this is because the core sample remains enclosed within the sampling chamber throughout the process of sampling. field description peat and organic soils for field description, slightly organic silts or clays will appear as fine-grained inorganic soils, probably black or dark brown in colour, with an organic odour and possibly some visible organic remains. peat, on the other hand, may well appear to be completely organic, contains many recognizable plants remains, have a low density and also being black or dark brown in colour. the peat material was described using the von post squeeze test and results were provided according to the von post degree of humification with values between h1 and h10. soft soils classification and description in the field, the cores were logged and described by visual observation and feel . the samples were described according to the malaysian soil classification system for engineering purposes and field identification (engineering geology working group, 2007; jarret, 1995; bujang, 2004) which includes the factors of organic content and degree of humification for classification of peat. soils that have organic content from 3 to 20% are termed slightly organic soils, and soils with organic content in the range of 20 to 75% are classified as organic soils. finally, peats have more than 75% organic content. fibric or fibrous peats (ptf) have a humification range of h1 to h3 whereas hemic or moderately decomposed peats (pth) have a range of h4 to h6. sapric or amorphous peats range from h7 to h10 on the humification scale (jarret, 1995; bujang, 2004). general geology soft, quaternary sediments underlie the study area. two sedimentary facies observed to be exposed in the study area are the peat (paludal) o3development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia deposits and floodplain deposits. from logging observations and pollen analyses, these two facies were observed and concluded to be deposited over the estuarine/deltaic sediments (mohamad tarmizi, 2014 and mohamad tarmizi et al., 2013(a), 2013(b) and 2013(c)). peat (paludal) deposits the term paludal deposits has been introduced to define a holocene unit mainly made up of peat (lam, 1989). peat covers a small portion of the western part of the study area (also referred here as ‘plaie area’). the thickness of the peat in the western part/plaie area ranges from 0.2 to 2.0 m. the types of peat mapped and encountered in the study area are (a) topogenous peat: also known as clayey peat, composed of plant remains and clastic sediments (with ash content) and formed in topographic depressions by plants that extracted nutrients from the mineral subsoil. topogenous peat is influenced by flood waters which supply the mineral components (mainly clays and silts). (b) ombrogenous peat: plant remains formed by plants growing solely on nutrient cycled through vegetation and peat (no mineral subsoil or ash content) and is fed by rain water only. the type of peat produced is composed of plant remains without any clastic sediments from the mineral subsoil. it is usually formed beyond the flood levels and away from mineral deposition by flooding rivers. the topogenous peat is composed of slightly to moderately decomposed plant matters. topogenous peat with a humification range of h3 to h8 on the von post humification scale was augered, logged and classified at the peat forest, in the plaie area, on the western part of the study area (figure 5). ombrogenous peat is characterized by a high water content, an extremely low bulk density, a relatively higher decomposition rate and a relatively lower ph, and is composed of mainly loose trunks, branches, roots, fruits and leaves (paramananthan, 2011). ombrogenous peat encountered in the study area is moderate to highly decomposed and ranges from h4 (hemic) to h7 (sapric) on the von post scale of humification. the munsell colour value is 7.5yr 2.5/2 (very dark brown) depending on the degree of humification. peat in the western peat forest of study area (plaie area) peat in the plaie area is composed of slightly to moderately decomposed plant material ranging from h4 to h8 on the von post degree of humification scale. the peat that was encountered vertically at location ks.tp.10 can be classified as hemic to sapric, hemic and fibric to hemic peat. the thickness of the peat layers from the margin (ks.tp.02) towards the basin centre (ks.tp.10) ranges from 0.2 to 2.3-m thick, getting thicker westwards. the peat encountered at locations ks.tp.09 and ks.tp.10 vertically has a dark brown to very dark brown colour ranging from 7.5 yr 3/3 and 7.5 yr 3/2 (dark brown) to 7.5 yr 2.5/2 (very dark brown) according to the munsell soil colour chart (table 1). the groundwater levels in the western part of kota samarahan (plaie) peat area are approximately 0.3 to 0.5-m below ground surface (mohamad tarmizi, 2014) at augered locations ks.tp.09 and ks.tp.10 respectively (figure 2). field identification and classification (von post) of the top to the bottom peat layer of the tropical lowland peat dome shows that there is a vertical variation of peat humification levels (von post) with every 0.5-m interval of depth in the form of dominantly occurring hemic to sapric (h6 to h7), hemic (h5 to h6), hemic (h4 to h6) and fibric to hemic (h3 to h5) humification levels, respectively, at auger hole location ks.tp.10 , and sapric (h7 to h8), hemic to sapric (h6 to h7) and hemic (h5 to h6) humification levels, respectively, at auger location ks.tp.09 , with both profiles indicating a vertical decrease of peat humification levels with peat depth. floodplain deposits the exposed floodplain deposits cover the northern, eastern and southern parts of the study area, including the cross river asajaya areas clockwise from kg. sui, kg. tanjung parang, kg. tanjung tuang to the sg. paloh area and westwards. the deposits are observed to be overlain by peat deposits (mohamad tarmizi, 2014 and mohamad tarmizi et al., 2013) in the western plaie area and a portion of the kg. sui area (figure 2). figure 2. location of peat cores ks.tp.02, ks.tp.07, ks.tp.08, ks.tp.09 and ks.tp.10 within the peat basin/dome/deposit and depth or thickness of peat (lithological depth concluded from field observations and pollen analyses) table 1. thickness/depth of sampled peats occurring from the peat basin margin (near location ks.tp.02) towards the approximate centre of the peat dome (near location ks.tp.10) in the western peat forest of the kota samarahan-asajaya study area o4 mohamad tarmizi mohamad zulkifley, ng tham fatt, zainey konjing and muhammad aqeel ashraf floodplain deposits, also known as riverine deposits, were formed by the deposition of river sediments during floods in flood basins (backswamp deposits) or on levees along the river banks (levee deposits). these floodplain deposits are confined to river valleys upstream of the estuarine/deltaic system and away from tidal marine or estuarine influences. the levee floodplain deposits form narrow belts along the banks of the rivers and streams and consist mainly of silt and fine sand, rare gravel and minor to abundant plant remains. the backswamp floodplain deposits which are behind the levees consists predominantly of clays and silts with minor to abundant plant remains (lam 1989). lithology the lithology of the quaternary riverine or floodplain (backswamp) deposits observed to occur in the area consists mainly of clay, silt and fine sand with minor to abundant plant matters (wooden fragments 0.5 to 4-cm thick). plant matters are observed to occur as wood fragments ranging from less than 5% to 50 %. these deposits are usually pale brown in colour and sometimes greenish gray to dark bluish gray or bluish gray in colour with munsell colour values of 10yr 3/3 to 10yr 6/3, gley 2 6/1, gley 2 5/1, gley 2 4/1 and 10yr 4/2. typical auger log sections of the floodplain deposits are as shown in figure 2 (log no ks.tp.02 to ks.tp.10). for the profile section of ks.tp.10 and the related pollen diagram (figure 5), from the beginning of the interval 3.5 to 2.5 m, begins the rise in pollen abundance followed by peaking at 2.5 m for the elaeocarpus angustifolius pollen. the interval 2.5 to 2.0 m shows an increase for eugenia grandis type pollen and abundance peaks at 2.0 m. thus, it is interpreted here that an influx of pollen from riparian type vegetation had occurred due to increasing freshwater influence from 2.5 to 2.0 m and that this interval represents an increasingly riverine/floodplain depositional environment. from the profile section of ks.tp.09 (figure 4), at the interval 2.0 to 1.4 m, the presence of the riparian type pollen of both eugenia and elaeocarpus again supports the deposition of a layer of floodplain/floodbasin deposits (approximately 0.5m thick) immediately below the peat layer and overlying the estuarine and deltaic deposits. estuarine/deltaic deposits the estuarine/deltaic deposits are found to underlie the floodplain and peat deposits in the study area (tarmizi, 2014). from pollen analyses and the pollen diagram of the soil samples at interval 2.5 to 2.0 m of the profile section at ks. tp.10, the pollen abundance of rhizophora type pollen starts to decline from 2.5 to 2.0 m and disappears at 2.0 m, which marks the possible end or dominance of mangrove type vegetation and related mangrove depositional environment. from the beginning of the interval 3.5 to 2.5 m, begins the rise in pollen abundance followed by peaking at 2.5 m for the elaeocarpus pollen. the interval 2.5 to 2.0 m shows an increase for eugenia type pollen and abundance peaks at 2.0 m. hence, it is interpreted from pollen abundance of riparian type vegetation that freshwater influence had risen from 2.5 to 2.0 m and that this interval probably represents an increasing floodplain depositional environment with lesser mangrove/ estuarine or marine influence. thus, it is interpreted from this part of the profile section that the estuarine/mangrove type sediment depositional environment had decreased, and saline or brackish water influence has begun to lessen due to continued sea level regression from 2.5 to 2.0 m and, that the mangrove/estuarine or deltaic depositional environment is more predominant from 2.5 m and below the mentioned profile (ks.tp.10). likewise, from pollen analyses and related pollen diagram of the soil samples at interval 2.0 to 1.4 m of the profile section at ks.tp.09, the pollen abundance of rhizophora type pollen starts to decline from 2.0 to 1.4 m and disappears at 1.4 m, signifying the gradual cessation of mangrove type vegetation and related mangrove type depositional environment from 2.0 to 1.4 m. thus, estuarine and deltaic brackish to saline water influence may have gradually ceased at approximately 0.5 m below the peat-soil boundary as indicated by the core auger profiles of both ks.tp.10 and ks.tp.09. as a result, this approximately 0.5 m thick layer consisting of the floodplain, riverine or floodbasin deposits was deposited by flooding river waters after or with the gradual decline of mangrove influence due to receding sea levels. lithology the underlying estuarine/deltaic deposits are composed of mainly soft to very soft, marine clayey silts and silt, clay and sometimes thin layers of fine sand accompanied with shell debris/fragments as observed in augered holes ks.ts.05 and ks.ts.06. these deposits are greenish gray to dark bluish gray or bluish gray in colour with munsell colour values of gley 2, 6/1, gley 2, 5/1, gley 2, 4/1 and 10yr, 4/2. the estuarine and deltaic deposits are probably marine in origin and, in the field, these deposits can usually be differentiated from the overlying floodplain deposits because the former has a very soft consistency, has a gritty feel when rubbed between the fingers, greenish gray to dark bluish gray or bluish gray in colour, is saturated or very wet in appearance and is sometimes accompanied by marine shell fragments (lam, 1989). these deposits are observed, in the field, to have little or no wooden fragments when compared to the overlying floodplain deposits. they are usually sulphidic (acidic) and produce yellow coloured minerals known as jarosite after oxidation and exposure to the atmosphere (fitzpatrick et al., 1998). 6. results peat characterization characterization of logged peat samples, at augered holes ks.tp.09 and ks.tp.10 from locations near and towards the centre of the peat dome, in the western peat forest of the kota samarahan-asajaya study area are as described in table 2 (tarmizi, 2014; tarmizi et al., 2013(a), 2013(b), 2013(c). table 2. description of sampled peats occurring near the approximate centre of the peat dome (near locations ks.tp.09 and ks.tp.10), in the western peat forest of the kota samarahan-asajaya study area o5development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia the pollen diagram pollen analyses: methods a total of 8 samples were collected for every 0.5-m interval from the surface to a depth of 5.5 m for borehole ks.tp.10, and four samples were collected from the surface to a depth of 2.0 m for ks.tp.09 (mohamad tarmizi, 2014). these samples for pollen analyses were taken every 10 cm along each core for regular core sampling intervals such as shown in table 3 except for surface samples and samples for both radiocarbon dating and pollen analyses (mohamad tarmizi, 2014) which was taken at 180 to 200 cm (regular interval 2.0 to 1.5 cm) from borehole location ks.tp.10. figure 3 geological map of the kota samarahan-asajaya study area, west sarawak showing sampling location in the western plaie peat forest area. the enlarged inverted section (top left-hand corner) are the locations of the major peat cores selected for this study. the samples were treated in the laboratory with 10% koh, sieved to remove larger pieces of plant material, undergone acetolysis and heavy liquid separation in zinc bromide/hydrochloric acid for the samples in which some mineral residue was present especially in some clayey peat or organic soil samples (wood et al. 1996). the samples revealed considerable quantities of pollen grains and spores and counts varied from 26 to 266 grains per slide for ks.tp.10 and from 51 to 110 grains per slide for ks.tp.09. figure 4. pollen and spores assemblage (ks.tp.09). table 3. general and exact sampling intervals of auger cores from locations ks.tp.10 and ks.tp.09 o6 mohamad tarmizi mohamad zulkifley, ng tham fatt, zainey konjing and muhammad aqeel ashraf the results of the pollen analysis are as shown in the diagram figures 4 and 5 inclusive of all pollen and pteridophyte spores . pollen types which could not be identified with any certainty were excluded from the pollen sum. in the analyses of the peat borehole profile of ks.tp.10, no extreme over-representation was observed and hence all pollen, and spore types have been included in the pollen sum. however, the over-representation of some peat pollen types may occur (for e.g. the cephalomappa pollen) in peat swamps due low pollen dispersal when only a single prolific tree may produce the maximum pollen representation (anderson and muller, 1975). also, misrepresentation or misinterpretation due to the absence of shorea type pollen may also occur as shorea type trees were observed present in the field at the locations of the studied augered holes (ks. tp.09 and ks.tp.10). the absence of pollen from shorea type trees maybe due to the low pollen producing capability of these tree species. the affinity of the types recognized is indicated on the pollen diagram either by reference to a living taxon or, if the type occurs in more than one taxon, by adding “type”. the comparison was mainly limited to taxa known to occur in peat swamp forest, and in adjacent vegetational environments such as mangrove and riverine (or riparian) vegetation. description of the diagram from the pollen diagram, it is possible to interpret the inferred succession of some forest types present in the locality of the augered holes (ks.tp.09 and ks.tp.10), and that these can be used to assess the past and current depositional environments represented by different sampling intervals from the augered hole profiles mentioned. the inferred succession of various forest types is described and discussed in the following sections. description of pollen diagram for borehole ks.tp.09. a total of 4 samples were analyzed from the locality of borehole ks.tp.09 (figure 3 2) (tarmizi, 2014). based on the pollen diagram (figure 4) for the core profile from this augered hole and from the regular interval of 2.0 to 1.5 m, the rhizophora and oncosperma pollen is dominant along with a few grains of the excoecaria aggulocha pollen. this range is also well marked by the occurrence of pollen originating from riparian vegetation which is elaeocarpus and eugenia. the asplenium type spores with acrostichum aureum and stenochlaena palustris are also present, which represents non-arboreal pollen and spores. the sampling from 1.4 to 1.0 m shows a significant change in pollen assemblage, characterized by a decrease in the mangrove elements of oncosperma and rhizophora. this decrease continues with only 2 grains of oncosperma pollen occurring at 0.5 to 1.0 m and the marked disappearance of the rhizophora pollen from 1.4 to 1.0 m. from 2.0 to 1.4 m, there is a sharp increase of peat swamp pollen type campnosperma and a decrease of the mangrove elements. at 1.4 m and the actual beginning of initial peat deposition, there is a prominent peak for campnosperma. from 1.4 to 1.0 m, there is a marked decrease of campnosperma and is accompanied by other peat swamp species which, however, are only present in low numbers such as gonystylus, cyrtostachys, cephalomappa and zalacca. the upper section between 1.4 to 0 m shows a decreasing trend of campnosperma abundance but still shows dominance compared to other pollen types. the other types of peat swamp pollen present from 1.4 to 0 m in the peat section of the profile are alangium, calophyllum, dillenia, durio, parastemon, timonius, shorea, melanorrhea and stemonurus. the pollen types casuarina and podocarpus are also recorded in this interval. the non-arboreal pollen and spores which characterize this range are represented by asplenium type, figure 5. pollen and spores assemblage (ks.tp.10). o7development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia stenochlaena palustris, polypodium, pteris type and lygodium microphyllum. stenochlaena palustris has shown a significant trend as this spore is highly abundant from 1.4 to 1.0 m and shows a decreasing trend from 1.0 to 0 m. from 1.0 to 0.5 m, the pollen types found to include cephallomappa, callophyllum, campnosperma, palaquim, parastemon, zalacca followed by oncosperma, casuarina, magnolia, polypodium, stenochlaena palustris, durio and eugenia. from 0.5 to 0 m, the significant pollen types that make up the peat swamp catena, as reported by anderson and muller (1975) and observed to occur here, are stemonurus, campnosperma, palaquim and dactylocladus. these are followed by less significant types such as cephallomappa, callophylum and blumeodendron. the abundance of rhizophora and oncosperma along with minor amounts of excoecaria aggulocha between 2.0 to 1.5 m has revealed that the lower part of the inferred succession represents a former mangrove or estuarine swamp vegetational environment. the other important non-mangrove pollen types which are present in this zone are elaeocarpus and eugenia, which are locally subject to transportation by tidal action and are common species of riparian vegetation in sarawak (malaysia). these riparian type pollens may indicate and support the presence of a floodplain or a riverine depositional environment within this 2.0 to 1.4 m interval. a sharp floristic boundary is marked at 1.4 m. the zone between 2.0 to 1.4 m locally shows a sudden decrease in mangrove pollens such as oncosperma and rhizophora with the latter virtually disappearing at 1.4 m. this sharp decrease strongly indicates a transition zone from a mangrove to floodplain/riverine, to a peat swamp depositional environment. this zone may be comparable to the inferred succession of the peat swamp catena as studied and described by anderson and muller (1975) because it is characterized by the presence of significant pollen types of campnosperma, cyrtostachys and zalacca. hence, may strongly be comparable to the campnosperma-cyrtostachys-zalacca sub-association characteristic of transitional mangrove to shallow peats occurring before the vegetational succession of phasic community i (anderson and muller, 1975). this is further supported by the abundance of typical climbing ferns, of the spore type stenochlaena palustris especially at the lower part of the succession (1.5 m); this spore is common during the early development of peat swamps and is widely confined to the transitional mangrove-shallow peats (campnospermacyrtostachys-zalacca sub-association), which occurs prior to succession of phasic community i (anderson and muller, 1975). description of pollen diagram for borehole ks.tp.10. a total of 8 samples were analyzed from the locality of borehole ks.tp.10 (figures 2 and 3). from 5.5 to 3.5 m, the rhizophora pollen is present in relatively high percentages along with relatively high counts of the oncosperma pollen. the rhizophora pollens are still dominant from 3.5 to 2.0 m. however, this interval is marked by an increase of campnosperma, along with minor proportions of other mangrove pollens such as nypa fruticans, avicennia and excoecaria aggulocha. this range also records some prominent peaks from the riparian type pollens which are represented by elaeocarpus and eugenia. towards the top of the succession between 1.5 to 1.0 m, the mangrove elements show sharp decreasing percentages, especially the oncosperma and the rhizophora pollen with the latter virtually disappearing (at 2.0 m). this interval represents a transitional assemblage and is characterized by a significant change in pollen assemblage which is then dominated by peat swamp taxa. the campnosperma pollen is relatively dominant and is associated with typical peat swamp pollens such as blumeodendron, cyrtostachys, zalacca and parastemon. in the upper part of the section from 1.0 to 0 m, the campnosperma pollen is still dominant and is associated with other pollen types such as gonystylus, dillenia, cephalomappa and eugeissona. a peak from the dactylocladus pollen was recorded at 1.0 m. in the uppermost part of the assemblage, from 0.5 to 0 m, the dominant pollen types observed to occur are campnosperma followed by the presence of blumeodendron, gonostylus. parastemon and stemonurus the dominant non-arboreal pollens are represented by asplenium type spores throughout the succession especially from 0.5 to 0 m, with some minor occurrences of polypodium and stenochlaena palustris. the stenochlaena palustris is relatively more prominent from 2.0 to 1.0 m and shows an increasing trend before it virtually disappears in the interval 0.5 to 0 m. based on the analyses of the pollen assemblage from this locality, the assemblage can be divided to represent four main vegetational phases or environments of deposition. at the interval 5.5 to 3.5 m, the dominance of rhizophora and oncosperma clearly demonstrates a former mangrove or estuarine swamp environment . this range is locally overlaid by the interval 3.5 to 2.0 m that is characterized by the abundant occurrence of oncosperma, which is associated with the nypa fruticans pollen. the dominance of oncosperma and nypa fruticans are widely confined to the inland zone of mangrove or estuarine vegetation. the peak of elaeocarpus within this interval is probably related to tidal action or flooding which has brought an influx of this pollen type from riparian vegetational areas, and this relates to and supports a floodplain or riverine depositional environment. this zone may perhaps be comparable to the campnospermacyrtostachys-zalacca sub-association zone as studied by anderson and muller (1975) and is interpreted to occur within the 2.0 to 1.0 m interval. this zone which is locally defined as shallow, topogenic peat is interpreted and derived from the association of the campnosperma, cyrtostachys and zalacca pollen which is related to the initial development of peat or transitional phase from mangrove-floodplain to peat swamp environment. this is further supported by the abundance of stenochlaena palustris peaking at 1.0 m. at the interval of 1.0 to 0.5 m, approximately 2380 ± 30 years b.p.(mohamad tarmizi, 2014) , it is still interpreted as a combination of shallow peat, and probably begins to change to phasic community i (anderson and muller, 1975) as this zone is locally characterized by the peak of dactylocladus and the appearance of single pollen grain of gonystylus. from 0.5 to 0 m near the surface, or approximately 1780 ± 30 years b.p. until present time (mohamad tarmizi, 2014), the pollens of stemonurus and gonostylus are observed present indicating that vegetational successive zonation of phasic community ii has developed and may have been reached, but without the appearance of the shorea type pollen (however, the large, shorea type trees have been observed present in the field near to locations ks.tp.09 and ks.tp.10). 14c age of peat samples peat starts developing just above 200 cm from the profile at location ks.tp.10 which is near the centre of the peat basin and radiocarbon (14c) dating was attempted on a sample taken from 180 and 200 cm. an age of 2420 ± 30 years b.p. for the regular sampling interval 1.5 to 2 m (180 to 200 cm) was obtained for the age of initial deposition of peat into the tropical lowland peat basin studied (table 4). table 4. 14c age of peat sampling intervals (quoted in conventional years b.p. (before 1950 ad) o8 mohamad tarmizi mohamad zulkifley, ng tham fatt, zainey konjing and muhammad aqeel ashraf 7. discussion horizontal or lateral zonation in tropical lowland peat domes anderson (1961, 1964, 1976, 1983) and anderson and muller (1975) studied the domed topography of tropical peat deposits and the relationship between the concentric zonation of surface vegetation and increasing peat thickness, acidity and decreasing nutrient availability with horizontal distance between margin and centre of peat basin. variations that occur in peat type within the deposits reflect the succession and lateral migration of the surface vegetation and the associated environment concurrent with coastal progradation (murtedza et al. 2002, anderson and muller, 1975). buwalda (1940) in paramanathan (2011) earlier did work in sumatra, which supported the hypotheses/idea of horizontal zonality occurring in tropical lowland peats. he reported that different plant communities exist in the peat swamp forest depending on the thickness of the peat and the distance from the river (peat basin margin). in the central part of the peat forest, where the thickest peat deposits occurred, he observed that vegetation was poorly developed, with twisted and stunted trees and scattered pools of deep, acidic (ph value range of 3.0 to 3.5), brown water compared to vegetation occurring at shallower depths of peat (or peat deposited near or at the margin/boundary of peat dome/basin). however, on the peat deposits shallower than three meters deep, the soils had a ph of 3.5 to 4.5. buwalda (1940) in paramanathan (2011) reported six different vegetation types and zones occurring in the indragiri area in sumatra. anderson (1961, 1963, 1964) also described six vegetation zones occurring in the lowland peat forests of borneo and brunei. anderson (1961, 1963, 1983) did a comprehensive study of the ecology of the tropical lowland peat swamp forest. he observed and recorded 253 tree species that are mostly confined to the periphery of the peat swamp forest. according to anderson (1963), most of the plant species that grow in the forests at the centre of the peat domes are mostly those that are usually found on nutrient poorer soils, such as podzols of the heath forest (anderson, 1963). the tropical lowland peat swamp forests in sarawak, malaysia and adjacent brunei show lateral or horizontal changes in vegetation types from its periphery to the centre of the dome-shaped peat swamps and each of the six dominant lateral vegetation zone was designated “phasic community’’ by anderson (1961). six distinct phasic communities or successive lateral vegetation zones were recognized based on their floristic composition and structure of vegetation in each phasic zone (anderson, 1961; paramananthan, 2011) and were numbered phasic community i at the margin to phasic community vi in the centre of the peat swamp (table 5). table 5. characteristics of the six phasic communities occurring as horizontal or lateral zonation (adapted from anderson, 1961, 1963, 1983; paramananthan, 2011) o9development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia discussion on palynological analysis in general, palynological analysis has provided information regarding the locally inferred succession of a tropical lowland peat swamp vegetation. the pollen succession interpreted from the location at ks.tp.10 near the lowland tropical peat dome centre is comparable to a normal peat development which starts on mangrove clay. based on the pollen analysis and comparison to the inferred plant succession as studied and published by anderson and muller (1975), the peat deposit is concluded as local and autothochthonous and includes the phasic communities of i to ii. field observations have noted the presence of large shorea type species of trees with wide girths (1 to 3 m) along the traverse section from ks.tp.09 to ks.tp.10 which further supports that the vegetational zonation has reached phasic community ii (at an interval of 0.5 to 0 m) even though the pollen of shorea type is scarce and not observed in the studied profile of ks.tp.10. from 5.5 to 2.0 m, it is clear that mangrove or estuarine elements dominate. from 5.5 to 3.5 m, the dominance of rhizophora and oncosperma (plate 1) is evidence of a former mangrove swamp environment (anderson and muller, 1975). from 3.5 to 2.0 m there is the abundant occurrence and dominance of the oncosperma and nypa fruticans pollen which largely represents the inland zone of mangrove vegetation. the peak of elaeocarpus at 2.5 m in the 3.0 to 2.5 m interval is probably related to an influx of pollen from riparian type vegetation due to a freshwater influx or floodwaters, and the interval may thus represent and support a riverine floodplain depositional environment setting. overall, the analyses of the pollen succession at 2.0 to 1.5 (or at approximately 2420 ± 30 years b.p.) is indicative of a mangrove to shallow peat swamp transition zone interpreted to have developed near to the coast and which usually occurs before phasic community i (anderson and muller, 1975) and is indicated by the campnosperma-cyrtostachyszalacca sub-association (anderson and muller, 1975). from 1.5 to 1.0 m, the presence of peat pollen types that may support the vegetational successions or zones ranging from phasic community i and phasic community iv to vi is noted (anderson and muller, 1975). pollen types such as blumeodendron that are known to be abundant in phasic community i (anderson and muller, 1975), campnosperma (abundant in pc i), parastemon (abundant in pc i and pc iv to vi) and stenochlaena palustris (abundant in mangrove to shallow peat transition zones and pc i) are also present (anderson and muller, 1975). however, peak abundance of stenochlaena palustris occurs at this interval, thus signifying vegetation succession of phasic community i. from 1.0 to 0.5 m, at approximately 2380 ± 30 years b.p., the presence of peat pollen types that may support the vegetational succession of phasic community zones that range from phasic community i to vi is observed (anderson and muller, 1975). pollen types like dactylocladus (abundant in pc i to vi), pandanus (abundant in pc i to v) and parastemon (abundant in pc i and pc iv to vi) are also present in this interval as observed by anderson and muller (1975). however, stenochlaena palustris and zalacca (plate 1) is still present, which implies that the zonation may still be only in the phasic community i. from 0.5 to 0 m at location ks.tp.10, or approximately at 1780 ± 30 years b.p. until the present time, is noted the presence of peat pollen types that may support the occurrence of vegetational successions or zones ranging pc i to pc vi. pollen types like callophylum (abundant in pc i to pc iv), gonostylus (abundant in pc iv), blumeodendron (abundant in pc i), campnosperma (locally abundant near coastal margin of pc i), cephallomappa (abundant in pc ii to iii), eleocarpus (abundant in pc i to iv) and parastemon (abundant in pc i and pc iv to vi) are present (anderson and muller, 1975). in this interval, however, the stenochlaena palustris and zalacca pollen were observed not to be present, which implies that the zonation may no longer be phasic community i. hence, the zonation may have reached to be at least phasic community ii (or higher). as mentioned earlier, field observations support and have noted the presence of large, hollow, scattered shorea type (table 5) species of trees (abundant in pc ii to iv according to anderson and muller, 1975). 8. conclusion from field identification and von post classification as well as pollen analysis of the augered peat samples (ks.tp.09 and ks.tp.10) at the western peat forest of the study area, the findings could be summarized as follows: 1) field identification and classification (von post) of the vertical, top to the bottom peat layer on the tropical lowland peat dome in the western part of the study area shows that there is a vertical variation of peat humification levels for every 0.5 m interval, in the form of dominantly occurring decomposition levels of hemic to sapric (h6 to h7), hemic (h5 to h6), hemic (h4 to h6) and fibric to hemic (h3 to h5), respectively, at ks.tp.10; and sapric (h7 to h8), hemic to sapric (h6 to h7) and hemic (h5 to h6), respectively, at location ks.tp.09, with both profiles generally indicating a vertical decrease of humification levels with peat depth. 2) the proposed phasic communities inferred vegetational zonation that were observed to occur vertically, from the top to the bottom of the studied augered peat profiles near the centre of the tropical lowland peat dome are phasic community ii (alan swamp forest), phasic community i (mixed peat swamp forest) followed by the underlying transitional mangrove to peat swamp shallow/marginal peats, respectively. 3) based on pollen analyses and field observations, it can be concluded that most of the studied peat profiles from locations ks.tp.10 and ks.tp.09 were part of a progradation deltaic succession whereby a floodplain depositional environment is progressively replacing a mangrove swamp environment followed by a peat swamp environment. the peat swamp phasic communities inferred succession (bottom to top of peat profile) interpreted in this study is similar to that as studied by anderson and muller (1975) in the sense that it ranges from a transitional mangrove to peat swamp environment (or shallow/marginal peats) to a normal open swamp phasic community i (mixed peat swamp forest) that may have developed and extends up to at least phasic community ii (alan swamp forest) at present. also, field observations and the presence of large, hollow, scattered shorea type trees, supports that the surface, vegetational zonation of the studied tropical lowland peat dome may have reached phasic community ii, at the least. also, there are pollen types that are found to occur or are also abundant in pc iii and pc iv or more from pollen evidence and analysis, as previously reported by anderson and muller (1975). 4) it is interpreted that the depositional environment at the location of the profile seems to commence on a mangrove swamp environment which, due to a continued regression of sea levels, gave rise to a more inland mangrove swamp environment with an increased riparian influence. peat then starts to develop in a transitional mangrove to floodplain (or floodbasin), and followed by a shallow, topogenic peat depositional environment at approximately 2420 ± 30 years b.p. this shallow/marginal topogenic peat swamp assemblage with riparian influence then starts to develop with an inferred peat vegetational succession (anderson and muller, 1975) reaching phasic community i (at approximately 2380 ± 30 years b.p.) and phasic community ii (at approximately 1780 ± 30 years b.p. until present time), towards the upper part of the now ombrogenic peat profile. 5) it is concluded from pollen analyses of the augered core profiles of both ks.tp.10 and ks.tp.09, that estuarine and deltaic brackish to saline water influence may have gradually ceased at approximately 0.5 m below the lithological boundary between peat and soil. as a result, this approximately 0.5 m thick soil or slightly organic to organic soil layer consisting of floodplain or floodbasin deposits and laying between the peat and estuarine/deltaic deposits was increasingly deposited by flooding river waters after or with the gradual decline of mangrove or estuarine influence due to receding sea levels. acknowledgements the authors have no conflict of interests. the authors are thankful to all anonymous reviewers that have contributed constructive comments to improve this manuscript considerably. the authors also acknowledge the ippp grant pv018-(2011a) and bantuan khas penyelidikan (bkp) grant no o10 mohamad tarmizi mohamad zulkifley, ng tham fatt, zainey konjing and muhammad aqeel ashraf bk064-2015 for financial support from the university of malaya. the authors are grateful, appreciate and acknowledge the assistance and scholarship/ study leave (hlp) from the minerals and geoscience department of malaysia (jmg), public services department of malaysia, forestry department of malaysia and the ministry of natural resources and environment of malaysia. references anderson, j.a.r. 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(1995). geological considerations in the development of kuching area dialogue session. geological and geotechnical considerations in civil works, geological survey of malaysia, kuala lumpur. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 41 51 metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria olu-owolabi bamidele i.1, agunbiade foluso o.2 and adebowale kayode o.3 1 department of chemistry, university of ibadan. email: iromidayobamidele@yahoo.co.uk 2 redeemer’s university, nigeria. email: foagunbiade@gmail.com 3 department of chemistry, university of ibadan. email: adebowale2003@yahooi.com oil prospecting information obtainable from metal speciation is far more valuable regarding the bioavailability, toxicity and fate of metals than information from total metal data. metal speciation on sediment was thus carried out in this study to assess the bioavailability, fate and mobility of as, cd, cr, fe, mn, ni, pb, and v in the crude oil prospecting area of ondo state in nigeria. five operationally-defined metal species were obtained using the sequential extraction method: exchangeable species, carbonate-bound species, iron/manganese-bound species, organic-bound species and residual species. species’ concentration, spread and relative abundance were used to assess their fate, mobility and potential toxicity. the crude oil exploration site and the mouth of the estuary had the highest concentrations of these metals compared to other sites. the exchangeable and the organic-bound high risk species were most predominant in the crude oil exploration sites but their concentration became reduced downstream away from these sites. the residual fraction (the least in the crude oil prospecting site) was the most predominant in the most distant sites downstream. there were indications of self-management of the metals in the coastal system through favourable biogeochemical reactions partitioning metals from high risk species from low risk ones and the non-toxic residual fraction. it could, however, be concluded that sediment from the crude oil prospecting area may serve as non-point sources for metals contaminating the coastal system, they have higher metal bioavailability and higher toxicity risk potential than other sites (which should be curtailed). la información obtenida a través de la especiación de metales es mucho más valiosa en términos de biodisponibilidad, toxicidad y destino que aquella información de sus datos generales. en este trabajo se llevó a cabo una especiación de metales en sedimentos para evaluar la biodisponiblidad, destino y movilidad de as, cd, cr, fe, mn, ni, pb y v en el área de explotación de crudo del estado de ondo, nigeria. se obtuvieron cinco especies de metales operacionalmente definidas con el método de extracción secuencial: especies intercambiables, especies vinculadas al carbono, especies vinculadas al hierro y al manganeso, especies relacionadas a lo orgánico y especies residuales. la relativa abundancia, propagación y concentración de las especies fue utilizada para evaluar su destino, movilidad y riesgo de toxicidad. el sitio de la explotación de crudo y la boca del estuario tienen la mayor concentración de estos metales en comparación con otros lugares. las especies intercambiables y relacionadas a lo orgánico de alto riesgo fueron más predominantes en los sitios de exploración de crudo, pero su concentración se reduce río abajo de estos lugares. la fracción residual (menos en el sitio de exploración de crudo) fue la más predominante en los lugares más distantes aguas abajo. hubo señales de automanejo de los metales en el sistema costero a través de reacciones biogeoquímicas con particiones de metales desde especies de mayor riesgo hacia las de menor riesgo y con la fracción residual no tóxica. podría concluirse que los sedimentos del área de explotación de crudo podrían ser fuentes no puntuales de metales contaminantes en el sistema costero, ya que tienen mayores potenciales de riesgo en biodisponibilidad y alta toxicidad a comparación de otros lugares (lo que debería ser reducido). key words: metal, fate, speciation, biogeochemical reaction, coastal sediment. palabras clave: metales, destino, especiación, reacción biogeoquímica, sedimento costero. record manuscript received: 24/10/2012 accepted for publication: 04/04/2013 abstract resumen introduction sediment contamination poses one of the worst environmental problems in marine ecosystems around the world (chapman et al., 1996; sadiq, 2002; bay et al., 2003; valdes et al., 2005). sediments act as sinks and sources for contaminants in aquatic systems (adams et al., 1992; rowlatt and lovell, 1994; mucha et al., 2003). contaminants may sink in aquatic systems through the precipitation, complexation, adsorption and bioaccumulation of such contaminants which then settle by gravitational force (james, 2002). sediments having high organic matter and fine-grained particles have been reported as showing high affinity for metal sorption, which may serve as better sinks (ip et al., 2006; pekey et al., 2007). however, changes in the environment’s biogeochemical and physical factors can reverse reactions and cause contaminants to be released from sediments. high magnitude contaminant concentrations and low variation regarding sediment composition have been documented in the pertinent literature (shine et al., 1995, beiras et al., 2003, caccia et al., 2003; pekey et al., 2006). these qualities have enhanced the use of sediments as preferred tools for environmental pollution monitoring in aquatic systems (liu et al., 2003). such monitoring always involves investigating metals, organic pollutants and other toxic substances on surface sediments or by coring through sediment depth (szefer and kaliszan 1993; atgın et al., 2000). studies of metals in sediments abound in the literature (vogt, 1990; zwolsman et al., 1996; shin and lam, 2001; korfali and davies, 2004). however, most studies have investigated total metal concentrations in these sediments. this approach has recently been regarded as less appropriate since not all total metal concentrations are bioavailable; only bioavailable metals are regarded as toxic in both soil and sediment (ritchie et al., 2001; korfali and davies, 2004). these bioavailable metal species are the labile ones which are affected by changes in the environment’s biochemistry, thereby representing a departure from using total metals in monitoring the environmental pollution of soil and sediments. the use of metal species in now being adopted (ianni et al., 2000; korfali and davies, 2004). speciation studies of metals have been undertaken by sequential extraction, as initially proposed by tessier et al., (1979), several modifications having mentioned in the literature. the operationally-defined species are: exchangeable species, carbonate-bound species, iron/manganese-bound species, organic-bound species and residual species. the outcome of metal speciation from sequential extraction gives the true pictures of metals’ bioavailability, their potentially adverse effects on the environment, the fate of the metals discharged into the environment and the degree of such metals’ mobility. this study was therefore aimed at evaluating the different species into which metals in coastal sediments from the crude oil exploration area of ondo state have been partitioned. this will serve for measuring the metals’ bioavailability, potential risk, degree of mobility and fate. methodology background this study was conducted in mahin, in the ilaje area of ondo state (lat. 5o 50'n – 6o 09'n and long. 4o 45'e – 5o 05'e); it is an estuary adjacent to the atlantic ocean. the area covered by the study has emerging communities dispersed along throughout the coastal area. the population of the entire state is about 3 million, population growth rate being 2.2% annually. the communities are dependent on water for their economic activities, being a fishing community. several settlements are scattered around the river tributaries that empty into the ocean through this estuary. the area also falls within nigeria’s oil-prospecting states, the so-called the niger delta region, because site 1 (ukua) is an oil-prospecting site hosting the crude oil exploration platform of one of the operators of the nigerian national petroleum corporation (nnpc) joint venture. diverse river courses and streams traverse different settlements and discharge into the ocean (bright of benin, atlantic ocean) through this estuary. ten sampling points were selected (figure 1) along the estuarine part of the coast for evaluation in this study; sediment samples were collected at these sampling points. sampling points were chosen after considering anthropogenic activities in the area representing potential sources of contaminants having a negative impact on the zone. site 2 (awoye), which is the estuary discharge point, also plays a significant role in the chemistry of the coast. several crude oil exploration platforms are located within the vicinity of the discharge point but located in the atlantic ocean. the other eight sites were close to different settlements, at increasing distances from sites 1 and 2. trading and fishing are the major occupations of the people along the coast. all domestic waste is directly discharged via river tributaries onto the coast; there is no waste treatment system in the area. transportation of heavy goods and people in the area and fishing activities are carried out by motorboat, which is a potential non-point source of pollution. sampling and chemical analysis a van veen grab sampler was used for randomly sampling sediments at each sampling site before being composited and kept in nylon. samples were spread on plastic bags and air dried at the laboratory. the samples were ground after air drying and sieved through a 2mm sieve and analysed for as, cd, cr, fe, mn, ni, v and pb. figure 2 gives a schematic representation of the sequential extraction used for this analysis modified from the method proposed by rauret et al., (1989). fractions were shaken in conditions specified in the schematic diagram for each extraction (figure 2) and centrifuged before decanting the supernatants. the exchangeable fraction was obtained after equilibrating for 1 hr at 25oc, after which the residue was used for extracting carbonate-bound metals by shaking the reaction mixture at 25oc for 5 hrs. fe/mn oxide-bound metals were then extracted from the residue with 6 hr equilibration of extractants and sediment samples at 100oc in a hot water-bath shaker. after extracting fe/mn oxide-bound metal species, the organic-bound fractions were obtained by shaking the reaction mixture at 25oc for 30 min. the final fraction (i.e. residual fraction) was obtained by wet oxidation at 100oc for 2hours. the chemicals used for the extraction are represented in the flow-chart (figure 2). all reagent used were analytical grade and the necessary quality assurance steps were observed. the supernatants so obtained were quantitatively transferred into volumetric flasks and made up to 25ml each with 1m hno3 before analysing for heavy metals using an atomic absorption spectrophotometer (aas, buck scientific model 205a). analysis was carried out in triplicate and the mean values calculated. results and discussion metal concentration in sediments table 1 presents total metal species concentration results for the sediment samples from the ten sites in the coastal area. the results showed mn to be the most abundant metal followed by fe, this being consistent with earlier studies carried out along this coast (adebowale et al., 2008a; agunbiade et al., 2010). ni and cr ranked next in concentration. metal concentrations had the highest values at the discharge point into the atlantic ocean at awoye, followed by the crude-oil prospecting site (ukpua) which had the highest as and ni concentration. this trend may have been attributed to crude oil exploration in this region. studies have shown that metal concentrations in the mouth of an estuary are generally higher than in the middle estuary (grabemann et al., 1997; guieu et al., 1998; ip et al., 2007). the spread of metals in the different species obtained from sequential extraction is useful in understanding coastal sediments’ potential toxicity and the impact of activities along the coast, the fate and mobility of metals rather than information olu-owolabi bamidele i., agunbiade foluso o. and adebowale kayode o. sites total metal concentration (µg/g) as cd cr fe mn ni pb v ukpa 0.84±0.06 1.26±0.22 16.4±1.1 349±10 705±28 15.8±0.9 5.46±0.4 5.04±0.6 awoye 0.77±0.05 1.73±0.21 18.7±2.1 418±22 818±33 15.5±0.6 6.81±0.8 6.42±0.7 odonla 0.64±0.07 0.89±0.11 14.4±0.9 343±16 713±34 13.2±1.1 5.04±0.4 3.69±0.4 ikorigho 0.77±0.06 0.89±0.10 14.7±1.5 322±18 749±25 14.5±1.2 4.24±0.3 4.19±0.3 ojumole 0.66±0.09 0.71±0.06 14.1±1.3 296±15 714±21 13.5±0.8 3.66±0.2 4.43±0.1 obenla 0.76±0.10 0.92±0.12 16.8±1.1 287±8 675±27 13.5±1.0 4.14±0.4 5.14±0.5 ilepete 0.62±0.05 0.79±0.11 15.4±1.2 299±11 656±33 14.3±1.2 3.32±0.1 4.16±0.2 ilowo 0.63±0.07 0.86±0.07 14.5±2.0 294±14 659±35 14.3±1.3 3.34±0.2 3.56±0.1 ibijinmi 0.69±0.08 0.99±0.08 16.0±1.9 328±15 727±40 14.6±0.9 4.59±0.1 4.14±0.3 idi ogba 0.74±0.05 0.99±0.07 16.1±1.8 356±19 723±38 15.6±1.1 4.49±0.3 4.24±0.4 table 1: total concentration of metal species in sediment from the ondo coastal area figure 1. map of the coastal area of ondo state and the sampling points. the sequence and labelling of the sites is as follows: site 1 (ukuakaba); site 2 (awoye); site 3 (odonla); site 4 (ikorigho); site 5 (ojumole); site 6 (obenla); site 7 (ilepete); site 8 (ilowo); site 9 (ibijinmi); site 10 (idi ogba). metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria obtainable from total metal concentrations. box and whisker presentation of data was used for assessing the spread of the following operationally-defined species: exchangeable-species, carbonate-bound species, fe/mn-bound species, organic-bound species and residual species. exchangeable metal species exchangeable metal species as presented by the extraction from sediment method was the most loosely-bound and mobile metal species. it could be extracted from samples having aqueous solutions at ph 7 with only a change in extractant ionic strength; it was therefore the most bioavailable one. at high concentration it is an indicator of high impact, fresh discharge and increased environmental risk for the ecosystem (álvarez et al., 2002). the box and whisker presentation of this specie is presented in figure 3. as and cd average concentration were almost equal, but their spread was different; cd had the wider spread. the two metals ranged from 0.4µg/g to 0.1µg/g in this fraction. ni and cr were other metals having close concentration ranges and nearly equal median values. ni varied more widely than cr, both metals having values ranging from above 2µg/g to less than 3.5µg. the plot for pb and v also showed nearly equal median values for both metals. pb plots skewed towards the lower concentration and had a clearly wider range than v. the plot for fe and mn indicated that their concentration and variation was high in the exchangeable fractions in the coastal sediments from the coastal sediments. exchangeable fraction concentrations of the studied metals indicated increased exposure risk potential from the sediments. the spread was wide and caused by high values at crude oil exploration sites, estuary discharged points and the reduction as distance increased away from these points. this was consistent with the results of earlier studies revealing the role of the environment’s biogeochemistry which has selfpurification ability (agunbiade et al., 2010). such self-purification lowers the concentration of the highly mobile, more toxic fractions from the sediments as distance increases away from the aforementioned sites. carbonate-bound species the extraction procedure for carbonate species in sediments requires a reduction in ph to 5 from the residue after exchangeable species extraction. ph reduction liberates co2 and enhances metal species’ mobility and bioavailability in this matrix. it implies that the fate of carbonatebound species metals is dependent on an ecosystem’s prevailing ph. jarvie et al., (1997) documented that partitioning metals in the carbonate phase is enhanced by increased ph and the possibility of having a river with underlying carbonate, sedimentary rock. reduced ecosystem ph will increase this species’ mobility and bioavailability and the risk of affecting an ecosystem without which a metal is safe. jain (2004) reported that metals in carbonate species are in the medium risk range compared to high risk metal species. the outcome of carbonate-bound species extraction and their spread across this coast is presented in figure 4. as and cd results were closely related, as witnessed in exchangeable fractions, only their concentration and spread being lower than the result for exchangeable species. the lower concentration implied that as and cd were less scavenged by the carbonate. outlier values were observed at awoye (site 2) for the metals which were in the upper extreme, indicating the estuary discharge point’s positive role in cleaning-up metals from the coastal region by partitioning cd discharge into the carbonate fraction and reducing its bioavailability. carbonate species in sediment were less affected by environmental changes such as naturally and commonly occurring microbial activities (korfali and davies, 2004). this species was therefore scavenging some metal contaminants and would stabilise them for a long time once the system’s ph was not lowered. comparing the carbonate species (figure 4) to the exchangeable species (figure 3) showed lower concentrations (median value) and spread of carbonate species than exchangeable ones. however, upper extreme outliers witnessed for the carbonate bound metals at awoye (site 2) represent positive results. it is therefore very evident that the chemistry of the awoye coastal environment (the biogeochemical reactions in it) significantly influenced contaminant status by events occurring in this estuary and were favourable for reducing metal mobility, their consequent bioavailability and toxicity, thereby further confirming this coastal environment’s self-purification ability. fe/mn-bound species metal affinity for fe and mn oxides by the adsorption of such metals to precipitate fe and mn oxides has been documented in the pertinent literature (dekov et al., 1997; guieu et al., 1998; korfali and davies, 2004). fe and mn oxide precipitates have active sites adsorbing metals and reducing their mobility and availability for living tissues (ip et al., 2007). the extraction procedure also reflected a condition which may not occur naturally and frequently, indicating this species’ degree of stability in the ecosystem, medium range exposure risk and their reduced toxicity. the fe/mn-bound metal species trend in this study area’s coastal sediment is presented in figure 5. extreme upper concentration outliers were witnessed for most metals at site 6 (obenla), except for mn, pb and ni. arsenic and nickel had a high concentration outlier at site 9 (ibijinmi), therefore implying that the geochemistry for sites 6 and 9 was enhancing the scavenging of these metals from the ecosystem into this less bioavailable and relatively safe fe/mn bound fraction. however, fe/ figure 2: a schematic diagram of the method used for metal speciation in sediment samples, modified from rauret et al., 1989 olu-owolabi bamidele i., agunbiade foluso o. and adebowale kayode o. 1111n = mnfe 200 180 160 140 120 100 80 60 40 11111111n = vpbnicr 3.5 3.0 2.5 2.0 1.5 1.0 .5 0.0 1111n = cdas .4 .3 .2 .1 0.0 figure 3: box and whisker presentation of 8 exchangeable metal species in sediment samples from all sites. the units of the metals are in µg/g. the vertical bars in the box are median values, the ends of the whiskers are the extreme data while the edges of the boxes present half that data point between the medians and extremes. 1111n = cdas .6 .5 .4 .3 .2 .1 -.0 11111111n = vpbnicr 6 5 4 3 2 1 0 2 2 2 1111n = mnfe 300 200 100 0 2 2 figure 4: box and whisker presentation of 8 carbonate-bound metal species in sediment samples from all sites. the units of the metals are in µg/g. the plot description is as that presented in figure 3. metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria mn bound metal species and the organic-bound ones in sediment could be remobilised by microbial activity (jones and turki, 1997; korfali and davies, 2004). micro-organisms having reducing ability in sediment were able to reduce insoluble fe (iii) oxides to soluble fe (ii) oxides, thereby releasing the associated metals into solution; this metal fraction’s fate and mobility and its resulting toxicity was thus determined by these microbes’ activities and other prevailing biogeochemical reactions. the concentration and spread of the metal followed a trend similar to that of other fractions but geochemical activity at site 6 and 9 complimented the role played by the estuary discharge point to ensure contamination management in this coastal zone. organic-bound specie figure 6 presents the outcome for organic-bound metal species in coastal sediments. concentration and variation were higher than that for the other species studied thus far. the highest set of organic species was observed at site 1 (ukpa) and site 2 (awoye), such wide variation being due to the reduction in value with increasing distance at these two sites. this would imply that metals discharging into the coast may have formed chelates with organic ligands possibly arising from ukpa crude oil exploration and partition into the sediment phase as organic-bound metal species. the bioavailability of metals bound to organic complexes is affected by microbial activity in the sediment. oxidising micro-organisms can oxidise the organic moiety into carbon dioxide and release associated metals, thereby increasing their mobility and toxicity. likewise, organic complex formation by metals can increase their solubility in a lipophilic system and enhance their transport into living tissues for bioactivity. organic-bound species’ concentration and spread pattern are thus indicators of high risk factors associated with anthropogenic activities in an area, especially in regions associated with the discharge of organic ligands. these organic and exchangeable fractions should be controlled, regardless of a coastal environment’s absorbing capacity so as not to over-burden it. residual fraction the residual metal species is an indicator of metals natural to a site and is neither bioavailable nor mobile; it is thus not a toxic fraction. this fraction is stable and can only be obtained from samples by rigorous acid oxidation which does not occur in nature. this species’ high value relative to that of others is a positive development, indicating safe, non-toxic environmental media and could also be an expression of long-term metal deposition. metals in the non-residual fraction may be indicators of temporary fresh discharge as against distant time release (ip et al., 2007). the results of residual fraction for metals in ondo coastal sediment is presented in figure 7. the concentrations of the metals’ residual fractions increased away from sites 1 and 2. the plot shows higher concentration outliers at sites 9 and 10 except for v and mn. it is evident that the metals in distant sites 9 and 10 were not freshly discharged and were less associated with anthropogenic activity. site 9 and 10 were relatively safer than other sites studied. this was reflected in earlier studies on this coast (adebowale et al., 2008a, adebowale et al., 2008b). the increasing concentration of residual fraction away from sites 1 and 2 lend supported the reported self-management structure found in the coastal environment. metal species’ relative abundance the trend of metal species relative to each other at the sites was evaluated with percentage-based bar charts. figure 8a – 8j presents the results for the relative abundance of metal species within the five fractions investiga1111n = cdas .3 .2 .1 0.0 6 9 6 11111111n = vpbnicr 5 4 3 2 1 0 6 2 8 6 1111n = mnfe 180 160 140 120 100 80 60 40 6 fe/mn-bound figure 5: box and whisker presentation of 8 fe/mn-bound metal species in sediment samples from all sites. the units of the metals are in µg/g. plot description is as presented in figure 3. olu-owolabi bamidele i., agunbiade foluso o. and adebowale kayode o. organic-bound figure 6: box and whisker presentation of 8 organic-bound metal species in sediment samples from all sites. the units of the metals are in µg/g. plot description is as presented in figure 3. 1111n = cdas .7 .6 .5 .4 .3 .2 .1 0.0 11111111n = vpbnicr 6 5 4 3 2 1 0 1111n = mnfe 200 180 160 140 120 100 80 60 40 10 211 residual-bound figure 7: box and whisker presentation of 8 residual metal species in sediment samples from all sites. the units of the metals are in µg/g. plot description is as presented in figure 3. 1111n = cdas .6 .5 .4 .3 .2 .1 9 10 9 10 11111111n = vpbnicr 6 5 4 3 2 1 0 9 10 9 10 9 10 1111n = mnfe 200 180 160 140 120 100 80 60 40 9 10 metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8c: relative abundance of metal species in odo nla overall, the relative abundance results for metal species at sites 1 to 3 revealed that the sediment from these sites had a negative impact on the coastal area. despite this result, however, notable amounts of carbonate-bound fractions of v, cd, cr and mn were witnessed at site 2. this would definitely have reduce these metals’ mobility, bioavailability and resultant toxicity. by contrast, carbonate species of extremely toxic metals like as, cd, and pb were least common at site 1 followed by their fe/mn-bound species. these three metals being toxic could have been preferred in carbonateand fe/mn-bound species, especially at site 1 for discharged metal management. site 3 witnessed a slight increase in residual fraction compared to other sites, thus far discussed particularly for as, cd, cr, fe and ni which were above 25% of total metal concentration. the exchangeable fraction predominated at site 4 (figure 8d), especially for metals like as, cd, fe and pb while the organic fraction was more reduced than the ratio obtained in earlier sites but relatively higher than that for other fractions. these trends regarding exchangeable and organic fractions were both risk-creating situations for this site. the residual fraction in this site was further reduced than that at earlier sites. there was a very high likelihood that there were recent discharges of metals from domestic waste disposal or other related anthropogenic activities. closer distribution of relative abundance of other metal species was witnessed for other fractions at this site than at other sites. a similarly close relationship concerning the relative abundance of metal species was witnessed at site 5 (figure 8e) where the five metals species were closely related for all the metals. the residual fraction however was more increased than at the preceding site (site 4) and was so at all other sites as the distance increased away from the crude-oil exploration site and the estuary discharge point. 0% 20% 40% 60% 80% 100% r el at iv e pe rc en ta ge (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8d: relative abundance of metal species in ikorigho ted at the ten sites. it is evident that the organic bound (oxidisable species) and the exchangeable fraction were the most dominant species relative to others in sites 1 to 3 (figure 8a – 8c) while the residual was the lowest in sites 1 and 2 but increased slightly at site 3. this would imply that metals in sediment from sites 1 to site 3 were highly mobile and could cause a high risk to the ecosystem. it indicated fresh or recent discharge of the metals. these two sites have been reported as playing significant roles in the chemistry of the area and the coast’s environmental status (adebowale et al., 2008b). crude oil exploration at site 1 (ukpa) was therefore established as point source of contaminants affecting the coastal environment and the metals from this particular site were found in the free ion state in the water column while the sediment was the readily available exchangeable fraction. all the above increased the toxicity tendency of the metals at this site. the organic fraction was found abundant in these three locations, even the exchangeable fraction which was likely associated with formation of complexes (ligands) between the organic compounds from the crude-oil with metals. the organic fraction was also a potentially risky species to the biota because the formation of complexes by metals would increase the metals’ solubility and their mobility into lipophilic systems which are common in living tissues and enhance the formation of bonds in living tissues by the complexes so formed. thus, organic fraction formation increases metal mobility into living tissues, the food chain and consequently their bioaccumulation and bioactivity. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% as cd cr fe mn ni pb v r el at iv e p er ce nt ag es (% ) metal species exchangeable carbonate fe/mn bound organic residual figure 8a: relative abundance of metal species in ukpua 0% 20% 40% 60% 80% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8b: relative abundance of metal species in awoye olu-owolabi bamidele i., agunbiade foluso o. and adebowale kayode o. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8g: relative abundance of metal species in ilepete the reduction in the trend of exchangeable fraction concentration witnessed at site 7 became further reduced at site 8 (figure 8h) except for pb as was the case with site 7. the residual fraction also increased especially for as, cd, cr, and ni. pb exhibited a high preference for the reducible fraction similar to that obtained at site 7 while the carbonate specie of as in this site was similar to that of site 7. it was evident that this coastal site’s alkalinity was essential for management of as present in the area. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8h: relative abundance of metal species in ilowo the relative percentage abundance of the metal species at sites 9 and 10 (figure 8i and 8j respectively) clearly revealed the favourable roles played by the biogeochemical reactions natural to this environment. the metals discharged into the environment have been demobilised into the residual fraction at these two sites. this showed why the safe, non-bioavailable residual fraction predominated at these coastal sites. the trends observed for all the metal fractions in the sediment from these sites agreed perfectly with the results expressed earlier in this study which supported the selfpurification hypothesis. of particular interest was cd and pb management at these two sites as they have been found in high quantities in other sites referred to by previously discussed results. the organic and exchangeable fractions which were higher at other sites reduced in these sites while the carbonateand fe/mn-bound species maintained their relative abundance, except in cd. thus, the relative abundance of the metals at these two sites 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8e: relative abundance of metal species in ojumole the trend of the species at site 6 (figure 8f ) was completely different from all other sites. the less common fraction at most other sites (fe/mn bound fraction) was the predominant specie at this site. the relative abundance of as, cd, pb and v was notably high in this fraction while that of cr, mn, fe and ni increased more than at other sites discussed earlier. it was therefore evident that this site’s biogeochemistry favoured these metals precipitation into the reducible fraction with resultant favourable reduction of metal mobility, bioavailability and toxicity. the organic and the exchangeable fractions reduced at this site while the carbonate-bound fraction was the least observed for most metals. the residual fraction only increased slightly in the site to more than 20%, except for v. this residual quantity was maintained in site 7 (figure 8g) with increased residual fraction for v over the previous site. organic fractions for cd, cr, fe, mn and ni predominated at the site while as and v existed in the carbonate bound fraction which was positive for managing these metals at that site. the exchangeable fraction in site 7 was more reduced than that observed in earlier sites except for pb which increased more than the preceding site. pb in this site exhibited a preference for the fe/mn-bound at this site while the other metals had the lowest concentration in the fe/mn-bound fraction. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8f: relative abundance of metal species in obenla metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria indicated that the sites were more risk-averse zones on the entire coast and less stressed in the ecosystem. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% r el at iv e p er ce nt ag e (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8i: relative abundance of metal species in ibijinmi 0% 20% 40% 60% 80% 100% r el at iv e p ec en ta ge (% ) as cd cr fe mn ni pb v metal species exchangeable carbonate fe/mn bound organic residual figure 8j: relative abundance of metal species in idi ogba conclusion this study has assessed five operationally-defined metal species: exchangeable species, carbonate-bound species, iron/manganese-bound species, organic-bound species and the residual species in sediments from the ondo coastal area using sequential extraction. the crude oil exploration site and the mouth of the estuary had the highest concentrations of these metals compared to other sites. mn and fe had the highest concentrations of all the eight metals investigated. the high risk, exchangeable and organic-bound species were the most predominant metal species in the crude oil exploration and estuary discharge sites. there were reductions in the concentrations of highly mobile species away from these two sites to other sites downstream. the reduction was complimented by increased low risk species. the residual fraction occurring least in the crude oil prospecting site was the most predominant in the farthest downstream sites. this would thus indicate self-management of metals in some of the sites within the costal system through favourable biogeochemical reactions partitioning the metals from the high risk species to the low risk ones and the non-toxic residual fraction. it could thus be concluded that the 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ecotoxicological mapping of the relationships between sediment chemical composition and fauna diversity. the sci. total environ. 90, 149–161. zwolsman, j. j., van eck, g. t. m. and burger, g. 1996. spatial and temporal distribution of trace metals in sediments from the scheldt estuary, south-west netherlands. estuarine, coastal and shelf science 43, 55 – 79. metal speciation in sediments from crude oil prospecting in the coastal area of ondo state, nigeria hydrogeochemical studies were carried out in and around udayagiri area of andhra pradesh in order to assess the chemistry of the groundwater and to identify the dominant hydrogeochemical processes and mechanisms responsible for the evolution of the chemical composition of the groundwater. descriptive statistics, correlation matrices, principal component analysis (pca), together with cluster analysis (ca) were used to gain an understanding of the hydrogeochemical processes in the study area. pca has identified 4 main processes influencing the groundwater chemistry viz., mineral precipitation and dissolution, seawater intrusion, cation exchange, and carbonate balance. further, three clusters c1, c2 and c3 were obtained. samples from c1 contain high level of cl− and may be due to the intensive evaporation and contamination from landfill leachate. most of the samples from c2 are located closer to the sea and the high level of na++k+ in these samples may be attributed to seawater intrusion. the geochemistry of water samples in c3 are more likely to originate from rock weathering. this has been supported by gibbs diagram. the groundwater geochemistry in the study area is mostly of natural origin, but is influenced to some degree by human activity. se realizaron estudios hidrogeoquímicos en y alrededor del área udayagiri de andhra pradesh para evaluar la química del agua subterránea e identificar los procesos hidrogeoquímicos dominantes y los mecanismos responsables de la evolución en la composición química del agua subterránea. se utilizaron estadísticas descriptivas, matrices de correlación, análisis de componentes principales, al igual que análisis de grupos, para obtener y entender los procesos hidrogeoquímicos en el área de estudio. los análisis de componentes principales identificaron cuatro procesos determinantes que influenciaron la química del agua subterránea, estos son, la precipitación y disolución de minerales, l intrusión de agua marina, el intercambio de cationes y el equilibrio de carbonatos. de esta forma se obtuvieron tres grupos, c1, c2, y c3. las muestras del grupo c1 contienen un alto nivel de cl-, lo que podría deberse a la intensa evaporación y contaminación de los lixiviados de rellenos sanitarios. muchas de las muestras del grupo c2 se ubican cerca del mar y el alto nivel de na++k+ podría atriburise a la intrusión de agua marina. la geoquímica de las muestras de agua en el grupo c3 probablemente se desprende de la meteorización de rocas. el diagrama de gibbs valida estos resultados. la geoquímica del agua subterránea en el área de estudio es principalmente de origen natural pero también está influenciado por la actividad humana en algún grado. earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): e1-e7 abstract resumen keywords: hydrogeochemistry, multivariate analysis, pca, ca, udayagiri palabras clave: hidrogeoquímica, análisis multivariado, análisis de componentes principales, análisis de grupo, udayagiri. assessment of groundwater quality of udayagiri area, nellore district, andhra pradesh, south india using multivariate statistical techniques evaluación de la calidad del agua subterránea a través de técnicas estadísticas multivariadas en el área udayagiri, distrito nellore, andhra pradesh, en el sur de india record manuscript received: 04/12/2015 accepted for publication: 18/11/2016 how to cite item nagaraju, a., thejaswi a. and sreedhar y (2016). assessment of groundwater quality of udayagiri area, nellore district, andhra pradesh, south india using multivariate statistical techniques. earth sciences research journal, 20(4), e1-e7 http://dx.doi.org/10.15446/esrj.v20n4.54555 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.54555 h y d r o g e o l o g y arveti nagaraju, arveti thejaswi 2 and yenamala sreedhar 1 1sri venkateswara university, india. arveti9@gmail.com 2department of environmental sciences, sdlcku, kakatiya university, warangal – 506 009, a. p. india e2 arveti nagaraju, arveti thejaswi and yenamala sreedhar 1. introduction water is a solvent and dissolves minerals from the rocks with which it comes in contact. groundwater may contain dissolved minerals and gases that give the taste being enjoyed by many people. however, over 97% of the water on earth is salty and most of the remaining 3% is frozen in the polar ice caps (bhattacharya et al. 2012). the atmosphere, river, lakes, and underground stores hold less than 1% of all the fresh water needed to support the earth’s population (venkateswara rao, 2011). the investigation and assessment of groundwater chemistry and quality is a hot topic worldwide, triggering international attention from both researchers and policy makers due to its relevant link with environmental objectives and regional development. the quality of water is of vital concern for mankind since it is directly linked with human health, protection of the environment, plant growth and sustainable development.groundwater is one of the earth’s most important resources for human life. the quality of groundwater depends on various chemical constituents and their concentration, which are mostly derived from the geological data of the particular region (nagaraju et al. 2006; suresh et al. 2007; gupta and saharan, 2009; tripathi et al. 2012; gnanachandrasamy, et al.2015; nagaraju et al. 2015). groundwater chemistry has been extensively studied, and a great deal of literature can be found in various scientific journals and databases. geochemical analysis is an approach for defining groundwater baseline concentrations using geochemical and statistical tools (edmunds et al. 2003). it is of most importance for thorough examination of spatial and temporal groundwater chemistry in aquifer while interpreting other possible contributing sources and processes of groundwater. earlier workers have applied different methodologies to assess variations of groundwater geochemistry (panagopoulos et al. 2012). amongst these methods are multivariate statistical analysis are important (cloutier et al. 2008; razmkhah et al. 2010; nagaraju et al. 2016a, b, c). the multivariate statistical analysis such as principal component analysis (pca) and hierarchical cluster analysis (hca) is an approach to classify groundwater samples and make correlation between variables (hussain et al. 2008). pca attempts to identify underlying variables, or factors that explain the pattern of correlations within a set of observed variables. hca helps to identify relatively homogeneous groups of cases (or variables) based on the selected characteristics by an algorithm that starts with each case (or variable) in a separate cluster and combines clusters until only one is left (massart and kaufman, 1983). this technique can be used to group the commonly collected water quality data, in which each cluster indicates water with particular quality (belkhiri et al. 2010; nagaraju et al. 2014a, b, c). in the present study, principal component analysis (pca) and cluster analysis (ca) techniques have been applied to study the water quality characteristics of udayagiri area. to identify the rock–water interaction processes and groundwater redox conditions. all the hydrogeochemical studies mentioned in this study will show that multivariate statistical analyses significantly help to classify groundwater and identify major mechanisms influencing groundwater chemistry. hence, the main objective of this study is to enumerate the usage of factor score in identification of the hydrogeochemically active regimes represented by the major factors. relationship among dissolved ions present in water may vary with seasons and different lithologies. this study also illustrates the usefulness of statistical analysis to improve the understanding of groundwater systems. study area location and climate the study area is located between longitudes 79°17'00" and 79°26'30" e and latitudes 14°51'00" and 15°00'00" n (figure 1). the climate of the area enjoys a temperature ranging from the highest mean maximum of 44°c in april to lowest mean maximum of 32°c in january. this area receives major rainfall from north-east monsoon season starting from october and end by decemberjanuary months. this period forms the main cropping season receiving 66.7% of the annual rainfall through ne monsoon, while drought conditions generally prevail during south-west monsoon season with roughly 33.3% of the annual rainfall. farmers take up cropping if monsoon rainfall is good in kharif season (rainy season in india, which lasts between april and october). figure 1. geological and hydrogeological map of study area with water sample location geology the study area is underlain by variety of geological formations comprising from the oldest archaeans to recent alluvium (figure 1). the formations include mainly garnetiferous amphibolites and pelitic schist of nellore schist belt, granitic gneiss and cuddapahs (quartzites and shales) of pre-cambrian period. among these consolidated formations, schists (amphibolites and pelitic schists) and gneisses occupy most of the study area. hydrogeology. hydrogeologically this area is classified as consolidated (hard), semiconsolidated (soft) and unconsolidated (soft) formations. groundwater occurs in all most all geological formations and its potential depends upon the nature of geological formations, geographical setup, incidence of rainfall, recharge and other hydrogeological characters of the aquifer. among the consolidated formations gneisses are relatively good aquifers. schistose formations also form potential aquifers when the wells tapping contact zones with intrusives. quartzites and shales of cuddapah group are of little significance from the groundwater point of view as they are restricted to the hilly terrain. groundwater is developed in these formations by dug wells, dug cum bore wells and bore wells tapping weathered and fractured zones. the yield of the dug wells are in the range e3assessment of groundwater quality of udayagiri area, nellore district, andhra pradesh, south india using multivariate statistical techniques of 15 to 35 m3/day. the depth of dug wells ranges from 5 to 12 m, while the depth of filter point wells generally varies from 6 to 11 m. the yields generally range in this aquifer between 500 to 1000m3/day.hh materials and methods sample collection and analysis a total of 30 groundwater samples were collected from udayagiri area during september 2014. sampling locations were recorded using a potable gps device and they were shown in figure 1. the water samples were collected from bore wells after 10 minutes of pumping and transferred into precleaned polyethylene bottles. electrical conductivity and ph were measured in the field immediately after sampling, while the determination of major cations (na+, k+, ca2+, mg2+ and anions (f–, cl–,so42– hco3 -) were carried out in the laboratory using the standard analytical procedures recommended by apha (2005).minimum, maximum, average values and se values are presented in table 1. table 1. minimum, maximum, average and standard deviation values of different constituents of water samples sd: standard deviation; se: standard error multivariate techniques multivariate techniques such as principal component analysis (pca) and hierarchical cluster analysis (hca) were useful in analyzing hydrochemical data (wu et al. 2014). in this study, the ibm® spss v.20 software was used to analyze the dimensional groundwater quality data of the samples including ec, ph, tds, ca2+, mg2+, na+, k+, hco3 −, co3 2−, f−, so4 2−, cl−, hardness, alkalinity. pca can determine the principal factors corresponding to both natural and anthropogenic processes that may have significant influences on the chemical composition of groundwater, while hca can determine the number of groups that the hydrochemical data can be classified (voutsis et al. 2015). two types of hierarchical cluster analysis (hca) are usually used: q mode hca and r mode hca. q mode hca can be used to highlight the spatial relationships among the sample points, while r mode hca is usually applied to analyse the parameters by clustering them into groups or facies based on their similarity (wu et al. 2014). in this study, q mode hca was adopted. before clustering, raw data was standardized using the z score approach. a combination of the squared euclidean as a similarity measure and the ward’s agglomeration scheme to link clusters was determined to yield optimal results in the hca. determining the number of principal components is important in pca. it can be determined either by the total variance for which the total variance explained by the principal components should be higher than 85% or by the kaiser criterion for which only the components with eigenvalues greater than one can be retained(cloutier et al. 2006; wu et al. 2014). in this study, the kaiser criterion was adopted to determine the number of components. results and discussion from the geochemical analyses of groundwater samples as depicted in table 1, the ph is in range of 6.98 to 7.70 indicates its suitability for irrigation.the ec is ranging from 100 to 272 µmhos/cm. ca is varying from 14 to 144 mg/l and mg is ranging from 33 to 122 mg/l; na is between 31 to 312 mg/l and k is varying from 2 to 56 mg/l. the fluoride concentration is ranging from 0.09 to 1.05 mg/l. the total dissolved solids lie between 65 to 177 mg/l, in which most of the samples are within desirable limit. the concentration of sulphate varies between 11 to 120 mg/l. the hco3 and co3 concentrations in groundwater range from 168 to 827 mg/l and 39 to 181 mg/l, respectively. the chloride content varies from 28 to 228 mg/l/. the alkalinity varies from 567 to 1771 mg/l and the hardness varies from 148 to 540 mg/l. the abundance of the major ions in groundwater is in following order: na >mg >ca >k > and cl> hco3 − > co3 2− > so4 2− multivariate statistical analyses multivariate statistical analyses are useful tool solving multivariate problems (wu et al. 2014). in this study, pearson correlation analysis was first conducted to reveal the hidden relationship between each pair of the physiochemical parameters, and then principal component analysis (pca) and hierarchical cluster analysis (hca) were carried out in spss 20 to analyse the hydrogeochemical data for a better understanding of the origin and evolution of groundwater chemistry. pearson correlation analysis correlation analysis is useful in understanding the chemical reactions occurring in groundwater system (wu et al. 2015). in this study, pearson correlation coefficients were calculated using spss 20 and were shown in table 2. ec is positively correlation with tds (r=1.000, p<0.01), because both ec and tds represent the salinity of groundwater due to dissolved ions. furthermore, these two parameters are positively correlated with major ions except f− and co3 2−, indicating that the dissolved ions are the major factors influencing the values of ec and tds. both ca2+ and mg2+ show significantly negative correlations with ph (r=-0.518, p<0.01 and r=-0.668, p<0.01, respectively), which suggests that low ph value favours the occurrence of ca2+ and mg2+. considering the significantly positive correlation between ca2+ and mg2+, it is reasonable to conclude that the dissolution of dolomite is a possible origin of ca2+ and mg2+ (r1), as these two minerals are prevalent in shale and the quaternary formations. the positive correlation between mg2+ and hco3 − is an additional evidence of this conclusion. (r1) co32−is negatively correlated withca2+ and hco3− (r=-0.403, p<0.05 and r=-0.419, p<0.05, respectively), which can be explained as follows: in a balanced co3 2−−hco3− system, the addition of ca2+ in this system will incur the decrease of hco3− (the reverse reaction of r1), which will further promote the reaction of r2, resulting in the decrease of co3 2−. (r2) e4 arveti nagaraju, arveti thejaswi and yenamala sreedhar table 3. total variance explained by each pc and the loading matrix of pcs ** correlation is significant at the 0.01 level (2-tailed). * correlation is significant at the 0.05 level (2-tailed). hardness shows significantly positive correlations with ca2+ and mg2+ (r=0.823, p<0.01, and r=1.000, p<0.01, respectively), because hardness is an approximate measure of ca2+ and mg2+ (li et al. 2013). cl− and na+ usually show a positive correlation if they originate merely from the weathering of halite r3 (li et al. 2011, 2014a). but some other reactions such as cation exchange and weathering of silicate may change their relationship (li et al. 2014b). in the present study, the cation exchange and sea water intrusion may be reasonable explanations to this. as the study area is located near the sea, sea water intrusion will alter the na-ca balance in the fresh water system, promoting cation exchange (r4). the negative correlation between ca2+ and na+ (r=0.361, p<0.05) can be indicative of the cation exchange. r(3) r(4) correlation analysis is helpful to provide a preliminary understanding of the processes controlling groundwater chemistry. however, it should be noted that these reactions induced by correlation analysis are with great uncertainty (li et al. 2011). geological and hydrogeological conditions must be taken fully into consideration and other techniques must be applied to verify its results. principal component analysis according to the kaiser criterion, the first four principal components (pcs) which represent 84.531 % of the total variance in the hydrochemistry were extracted (table 3). the component loadings which represent the importance of the variables for the components (values greater than ±0.532 are in bold) (table 4). table 2. pearson correlation coefficients of physiochemical parameters e5assessment of groundwater quality of udayagiri area, nellore district, andhra pradesh, south india using multivariate statistical techniques pc1 explains the greatest of the variance (41.654%) and has significantly positive loadings by cl−, tds, ec, so 4 2−, hardness, mg2+, ca2+, and a negative loading by ph (table 3), which may represent the primary impacts from the dissolution/precipitation processes of minerals such as dolomite, calcite, silicates and gypsum. considering the origin of cl−and so4 2-, as well as the location of samples, it is reasonable to guess that intensive evaporation is also responsible for this component. therefore, pc1 represents a mixed origin of these parameters from natural processes including mineral weathering and groundwater evaporations, and it is the major factors influencing the characteristics of groundwater chemistry. pc2 explains 23.209% of the variance and has positive loadings in na+, alkalinity and k+, which is indicative of cation exchange triggered by salt water intrusion to a great degree. this is also an important process regulating the evolution of groundwater chemistry (wu et al. 2014, 2015). pc3 and pc4 explain 12.127 and 7.541% of the variance, respectively, which shows that they are relatively more local or specific effects than pc1 and pc2. pc3 is negatively correlated with co3 2− and has a positive loading in f−, indicating the dissolution of fluorite that introduces more f− into the groundwater but constrains the dissolution of carbonate minerals such as dolomite and figure 2. calculated illustration of factor scores of each sample calcite. pc4 has negative loading in hco3 −, which may be explained by the transformation between hco3 − and co3 2− (carbonate balance). factor scores of each sample have been calculated and illustrated in figure 2. from this diagram it is clear that samples v1, v2 and m3 are governed by factors represented by pc1, pc2 and pc3, i.e. influenced by natural water-rock interactions, seawater intrusion and some local fluorite dissolution. m3 is also influenced by carbonate balance represented by pc4. samples v3, v4 and v5 are influenced by pc2 and pc3, indicating the impacts from seawater intrusion and local fluorite dissolution. samples g8, g9, g10, m6, m7, m8 and m10 have positive loading on pc3 and pc4, indicating that these samples are governed mainly by local fluorite dissolution and carbonate balance. m2 is governed by pc1 and pc2, while v8 is influenced by pc2 and pc4. actually, most of the samples are associated with pc1, pc2 and/or pc3, indicating that natural processes such as mineral weathering and ion exchange are responsible for the groundwater chemsitry in the study area, although pc1 represents some effects of local groundwater evaporation. therefore, the results from correlation analysis can be regarded logical evidenced by pca analysis. table 4. mean concentrations of parameters from different clusters hierarchical cluster analysis (hca) figure 3 shows the dendrogram of q mode hca. three distinct clusters can be divided by hierarchical cluster analysis as shown in (figure 3).c1 includes 11 samples (m4, m5, m6, m7, m8, m9, m10, v9, v10, g2 and g3), c2 and c3 with a lower linkage distance, indicating that they have greater similarity.c2 includes 8 samples (v1, v2, v3, v4, v5, v8, m2 and m3) and c3 consists of 11 samples (g1, g4, g5, g6, g7, g8, g9, g10, m1, v6 and v7). the mean values of geochemical and physical data were presented in table 4 to describe the characteristics of each cluster of samples. the stiff diagrams (figure 3) and the data (table 4) indicate that the three clusters are geochemically independent groups.c1 is represented by high concentrations of hco3 −+co3 2− and cl− for anion and mg2+ for cation, c2 is characterized by high levels of hco3 −+co3 2− for anion and high concentrations of na++k+ and mg2+ for cation, and c3 is characterized by high concentrations of hco3−+co3 2− for anion and mg2+ for cation (figure 3). c1, indicated by table 3, is characterized by highest salinity and hardness and medium f− and alkalinity among the three clusters, while c3 possesses the lowest salinity, f− and alkalinity, and medium hardness, and c2 has medium salinity and hardness, highest alkalinity and f−. e6 arveti nagaraju, arveti thejaswi and yenamala sreedhar figure 3. dendrogram of q mode hca for water quality parameters the distinct geochemical characteristics of the clusters indicate that they have undergone geochemical evolution of different types and intensities. samples from the three clusters are all characterized by high concentration of hco3 −+co3 2−, but samples in c2 have more na++k+ than those from c1 and c3. considering that most of the samples in c2 are located closer to the sea than those from c1 and c3, it is possible that seawater intrusion may be a significant factor impacting the groundwater chemistry. the geochemistry of water samples in c3 are more likely to originate from rock weathering, but rock weathering is of different intensities for different samples in this cluster. samples in c1 are also characterized by high cl−, which may be due to intensive evaporation. mechanism controlling the groundwater geochemistry the gibbs diagram can evaluate the hydrochemistry of groundwater in the study area. mechanism controlling groundwater geochemistry a reaction between groundwater and aquifer minerals has a significant role in water quality which is useful to understand the genesis of water (gibbs 1970; subramani et al. 2009; vasanthavigar et al. 2012). the chemical data of water samples of the area are plotted in gibbs diagrams (figure 4).in the present study, the gibbs ratio 1 values vary from 0.11 to 0.48 and gibbs ratio 2 values ranges from 0.34 to 0.94. the majority of the samples irrespective of the formation are falling in the rock weathering region. this is due to the chemical weathering with the dissolution with rock forming minerals. figure 4. mechanism controlling the chemistry of groundwater (after gibbs 1970) conclusions the gibbs diagram revealed that the hydrochemistry of groundwater falls in the rock weathering region and is due to dissolution with rock forming minerals. pearson correlation analysis reveals that natural processes such as mineral dissolution/precipitation and cation exchange are dominant factors influencing the groundwater chemistry. four components are extracted by the pca. pc1 which explains the greatest of the variance represent a mixed influence of the dissolution/precipitation processes and salt water intrusion on the groundwater chemistry.pc2 is indicative of cation exchange triggered by salt water intrusion. pc3 and pc4 indicate relatively more local or specific effects such as the dissolution of fluorite and carbonate balance. three distinct clusters (c1-c3) are obtained through cluster analysis. samples from c1 contain high level of cl− that may be due to intensive evaporation and contamination from landfill leachate. most of the samples from c2 are located closer to the sea and the high level of 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(2007). effects of barite mine on groundwater quality in andhra pradesh, india. mine water and env., 26: 119-123. tripathi, a.k., mishra u.k., mishra ajay., dubey, p.(2012). assessment of groundwater quality gurh tehseel, rewa district madhya pradesh, india. international journal of scientific and engineering research, 3(9): 1-12. vasanthavigar, m., srinivasamoorthy. k., prasanna. m. v. (2012). evaluation of ground water suitability for domestic, irrigational, and industrial purposes: a case study from thirumanimuttar river basin, tamilnadu, india, journal of environmental monitoring assessment, 184: 405 420. voutsis, n.,kelepertzis, e.,tziritis, e.,kelepertsis, a. (2015). assessing the hydrogeochemistry of groundwaters in ophiolite areas of euboea island, greece, using multivariate statistical methods. journal of geochemical exploration, 159:79–92. wu, j., li, p., qian, h., duan, z., zhang, x. (2014). using correlation and multivariate statistical analysis to identify hydrogeochemical processes affecting the major ion chemistry of waters: a case study in laoheba phosphorite mine in sichuan, china. arab j geosci, 7:3973–3982. doi:10.1007/s12517-013-1057-4 wu, j., li, p., qian, h. (2015). hydrochemical characterization of drinking groundwater with special reference to fluoride in an arid area of china and the control of aquifer leakage on its concentrations. environ earth sci, 73:8575–8588. divination and determination of catchment surface runoff are the most important contestable process of hydrology. soil conservation service curve number (scs – cn) method is employed to estimate the runoff. it is one of the physical based and spatially distributed hydrological models. in this model, the curve number is a primary factor used for runoff calculation. the selection of curve number is based on the land use pattern and hsg (hydrological soil group) present in the study area. since the spatial distribution of cn estimation by the conventional way is very difficult and time consuming, the gis (geographic information system) based cn method is generated for kundapallam watershed. with the combination of land use and hsg the estimated composite cn for amc (antecedent moisture condition) i, amc ii and amc iii for the entire watershed was about 48, 68 and 83 respectively. the average annual runoff depth estimated by scs-cn method for the average annual rainfall of 173.5 mm was found to be 72.5 mm. the obtained results were comparable to measured runoff in the watershed. la predicción y la determinación del caudal de escorrentía de una cuenca son procesos de amplio debate en la hidrología. el método coeficiente de escurrimiento, del servicio de conservación de suelos (scs-cn, inglés) fue utilizado en este trabajo para estimar la escorrentía. este es uno de los modelos hidrológicos basados en conceptos físicos y distribución espacial. en este modelo el coeficiente de escurrimiento es un factor de relevancia para el cálculo de la escorrentía. la selección del coeficiente de escurrimiento está basada en los patrones del uso de la tierra y del grupo de suelos hidrológicos (hsg, inglés) relativos a esta área de estudio. debido a que la estimación del coeficiente de escurrimiento en la distribución espacial es compleja, para la cuenca kundapallam se implementó un método a partir de un sistema de información geográfica (gis, inglés), y basado en el coeficiente de escurrimiento. con la combinación del uso de suelos y el hsg, la estimación compuesta del coeficiente de escurrimiento para el antecedente de condición de humedad amci, amcii y amciii para toda la cuenca fue de 48, 68 y 83. el promedio anual de escorrentía profunda estimada por el método scs-cn con una media anual de lluvia de 173,5 mm fue de 72,5 mm. los resultados fueron comparados con la escorrentía medida en la cuenca. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 5964 abstract resumen key words: hydrological modeling, land use and hydrological soil group, geographic information system, scs-cn, surface runoff. palabras clave: modelamiento hidrológico, uso del suelo, grupo de suelos hidrológicos, sistema de información geográfica, scs-cn, caudal de escorrentía. record manuscript received: 01/08/2014 accepted for publication: 15/01/2015 gis based scs cn method for estimating runoff in kundahpalam watershed, nilgries district, tamilnadu r. viji¹, p. rajesh prasanna², r. ilangovan³ ¹assistant professor, department of civil engineering bit, anna university chennai, tiruchirappalli ²,³professor, department of civil engineering bit, anna university chennai, tiruchirappalli email:¹rajaviji_23@yahoo.co.in, ²rajeshprasanna71@yahoo.com hydrology issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.44714 60 r. viji, p. rajesh prasanna, r. ilangovan introduction water is one of the most vital requirements for economic and social development. the nilgiris is an upstream catchment district, and the bulk of its water resources are dedicated to power generation for the state. over the years, the natural systems and watersheds have seen much change in the nilgiris. building a series of hydroelectric projects, tunnelling the water to different areas of irrigation, building reservoirs, planting the upper areas with commercial pulpwood species and replacing natural grasslands with tea and marshes with vegetables has changed the district’s water regime. overall, water resources are said to be abundant in the district but about a third of the sources are seasonal, more than 80 percent rural settlements get less than 40 lpcd (liters per capita per day) of water available, and shortages are also common in urban locations (nilgries water portal). this prompted the study of water resources regimes in the nilgries. for a better water management in the watershed, the relationship between rainfall and runoff should be established. rainfall and runoff are the main components of the hydrological process and contribute a major source of water to the upstream and downstream of the watershed. physically based and spatially distributed hydrologic computer models are in existence to calculate the phases of runoff generation for a given rainfall event (beven and kirkby 1979, murari paudel et al 2006, ratika et al.2010, jaehak et al 2010). the key benefit of these models is the precision of their predictions. their major weakness is that they require considerable expertise, time and effort to be used effectively. in between the boundaries there are methods like the scs-cn (soil conservation service curve number) method that are relatively easy to use and yield acceptable results (schulze et al. 1992). in this approach a simple empirical formula and readily available tables and curves are used. the cn is a crucial factor to consider for runoff estimation (bonta1997). a high curve number means high runoff and low infiltration; whereas a low curve number means little runoff and high infiltration (xiaoyong zhan and min-lang huang 2004). the curve number is a function of land use and hydrologic soil group (hsg). it is a method that can incorporate the land use for the computation of runoff from rainfall. the scs-cn method provides a rapid way to estimate runoff change due to land use change (zhan and huang 2004). there are certain advantages of a fully distributed model and that prompted its selection for this study. first, it enables to divide any significant catchment into a number of grids and accordingly runoff can be generated over each grid (dubayah and lerrenmaier1997). runoff can be routed from each pixel to the outlet of the whole basin for the purpose of calibration and validation. secondly, once the model is developed, calibrated and validated, the same can be applied to any grid in that basin to calculate runoff, even if they are ungauged at sub-catchment or sub-basin level. there is a certain limitation to the present study, i.e., the point that the rainfall data have been applied uniformly to the entire grid area. the principle of the present study is to build up a fully distributed gis-based hydrologic model considering physiographic heterogeneity in terms of soil and land use to estimate the daily run-off. keeping these points in view, a fully distributed rainfall –runoff model has been developed based on the widely accepted, scscn method in gis environment (jain 1996; nash and sutcliffe 1970; sameer shaded and mohammad almasri 2010). this provides basic information for managers to assess runoff generation volumes within the region and thus supports the decision-making process for future development of water resources and hydrologic structures in the area. description of the study area kundahpalam micro watershed number 12 c/75 with an area of 14.37 km2 lies in the nilgiris district of tamil nadu. it is a hilly area located in the fragile environment of the western ghats with an elevation ranging from 1560 m in the moyar gorge to 2410 m above mean sea level at doddabetta peak. kundahpalam micro watershed is bounded by 11º14’17”11º 16’ 54” north latitude and 76º 34’ 45” 76 º 39’ 7” east longitudes. the name of the river in the study area is kundah, and the catchment for the drainage towards the east is larger in the area compared to those of the west. the drainage is influenced mainly by joint patterns and the foliation trends of the rock. the area is fully covered by charnockite rock. a very important hydel power project, kundah is located in the study area. the major problems of the study area are the improper land use practice and the degradation of shola forest which has caused severe soil erosion and damage to the fragile environment. the wind velocity and relative humidity of the kundahpalam watershed are e 0.3 m/s and 61%. the annual rainfall of the area varies from 1300mm to 2000mm. the maximum and minimum temperature of the watershed are 21º c during the summer season and 10.5º c during the winter season. the figure 1 shows the location map of the kundahpalam watershed. figure 1 the location map of the kundahpalam watershed data and methodology the study area was delineated from survey of india (soi) toposheet no.58a/11 of scale 1:50000 of 1972 and the base map was prepared. the daily meteorological and rainfall data for five years were collected from the isro automatic weather station (aws) no 175. the soil survey reports from the agricultural department were used for preparing the detailed soil map and texture map. the altitude values were obtained from the contour lines available in the soi toposheets, and the contour map was made from cartosat 1 dem. the digital elevation model (dem) map and the base map of the study area are shown in figure 2, and 3.awifs satellite data were used to prepare the land use map. the figure 4 shows the methodology adopted for runoff generation from the various theme layer and meteorological data. 61gis based scs cn method for estimating runoff in kundahpalam watershed, nilgries district, tamilnadu. figure 2 the dem map figure 3 the base map of the study area figure 4 the methodology adopted for runoff generation 62 r. viji, p. rajesh prasanna, r. ilangovan land use/land cover map after merging of the four bands of resourcesat -1 awifs satellite image of the study area, the unsupervised land use classification would be generated by using a composite tool in the idrisi software. with unsupervised land use classification, the visual interpretation of the study area was done using image characteristics and finally the land use/ land cover map would be prepared. the accuracy of the land use/land cover was verified by correlating ground truth information. the study area was classified into eight classes such as agrohorticultural plantation, built-up land, dense forest, forest plantation, water body/lake/reservoir, land with scrub, open forest and tea plantation. the detailed classification of the land use map is shown in figure 5. figure 5 land use map soil map the soil map of kundahpalam watershed was prepared after a detailed study of the soil profiles in the watershed. soil types were defined on the basis of physiographic units homogenous in terms of geomorphology, topography and land use/cover, which ultimately resulted for eight different soil types such as coarse loamy typic dystropepts, coarse loamy typic humitropepts, fine loamy lithic troporthents, fine loamy typic dystropepts, sandy loam, typic tropudalf, ultic dystropepts, ultic tropudalf. the soil map was then classified into hydrological soil groups (hsgs) that referred to the infiltration potential of the soil after prolonged wetting. the three hsgs that existed in the kundahpalam watershed is shown in figure 6. figure 6 hydrological soil group map calculation of surface runoff the curve number (cn) is a function of land use and hsg. cn values range between 0 and 100, with higher cn values associated with higher runoff potential. traditionally, an area weighted average curve number is used for the entire watershed to study the runoff of a watershed. a fully distributed rainfall-runoff model with a fixed grid size was constructed in the present work. appropriate curve number values for each grid element were assigned based on standard scs curve number tables (scs, 1975) considering antecedent moisture conditions (amc). runoff calculations were done for each sub grid. the quality of this model was improved by incorporating the spatial variation of watershed characteristics using remote sensing and gis. the cn2 values arrived at based on the combination of land use, and hydrologic soil group were meant for amc-ii condition. cn values for amc-i (dry condition) and amc-iii (wet condition) conditions were calculated using conversion equations are given below. the recharge capacity ‘s’ was calculated by substituting the value of weighted cn in the equation daily rainfall data (p) collected for a period of 2008 to 2012 from the isro automatic weather station (aws) was used as input. program in micro soft excel was developed to calculate daily runoff for each grid using scs-cn method. the daily direct runoff (q) of each grid was computed by the following equation where, q = runoff depth in mm, p = rainfall in mm, s = maximum recharge capacity of watershed after five days rainfall antecedent ia = 0.3s (for indian conditions) in this study, the curve numbers were weighed with respect to the watershed area by using the following equation: where cn= the area-weighted curve number for the drainage basin; cni= the curve number for each land use-soil group polygon; ai = the area of each land use-soil group polygon; n = the number of land use-soil polygons in each drainage basin. the average rainfall data from 2008 to 2012 of the watershed and the curve numbers corresponding to different land use and hydrological soil cover complex are given as inputs for each sub grid and daily runoff results were obtained for each sub-grid. the cumulative runoff from the basin outlet was computed. finally, the runoff zonation map was generated by using a runoff coefficient. the runoff coefficient is the ratio between runoff and rainfall. 63gis based scs cn method for estimating runoff in kundahpalam watershed, nilgries district, tamilnadu. results and discussion generation of runoff model for the kundahpalam watershed, the land use and soil maps were processed using gis techniques. based on the usda soil classification the hsg of the study area could be determined according to the percentage of sand, silt, and clay. table1 shows the classification hsg of the study area based on usda soil classification. table 1 hsg based on usda soil classification based on the u.s. soil taxonomical classification, soil texture was defined for the different soil types of kundahpalam watershed. table 2 shows the soil texture corresponding to different soil types. table 2 soil types according to soil texture the hsg and land use map of kundahpalam watershed was intersected using gis according to the data shown in tables 1 and 2. this step retains all details of the spatial variation of soil and land use. due to the humid nature of kundahpalam watershed, the produced theme and its related attributes table were used to determine the cnii value for each grid. cni and cniii were computed using equations 1 and 2 to adjust for antecedent moisture conditions (amci or amciii). the weighted curve numbers calculated for the study area under amci, amcii and amciii were 48, 68 and 83 respectively. the spatial distribution of cnii of the kundahpalam watershed for amcii is shown in figure 7. the hydrologic soil cover group and cn used for different land use under amcii condition are given in table 3. the runoff depth calculated for various land use patterns and hsg of the study area is shown in figure 8 . figure 7 the spatial distribution of cnii table 3 hydrological soil group, land use and cn 64 r. viji, p. rajesh prasanna, r. ilangovan figure 8 the runoff depth for various land use patterns and hsg the runoff zonation map was developed for the study area is shown in figure 9. the 30% of the study area fell under very high runoff potential, and 2% of study area fell under high runoff potential. the moderate and low runoff potential classes occupied 35% and 33% of the study area. figure 9 the runoff zonation map conclusion gis based scs cn model will be helpful to the researchers for the protection of water resources and water quality in watersheds. this method makes the runoff estimation more accurate and fast. in this study, the average annual precipitation of 1780.143 mm from 2008 to 2012 was used for the runoff calculation. the generated runoff from the precipitation was 1591.8 mm. this result shows that 11% of rainfall would be infiltrated into the ground and remaining 89% of rainfall is converted in to runoff. this occurs due to the hilly terrain of the watershed, i.e., the elevation of the watershed ranges from 1560 to 2410 m above mean sea level. the spatial distribution of cn value varies from 46.25 to 100. here the values of 46.25 and 100 are in correspondence with to dense forest and water. another highest cn value 86.75 corresponds to the built-up land. the higher cn value has a high runoff and lower cn value has a little runoff. due to the high infiltration capacity of the dense forest, the runoff was low and in the hard surface of the built up land the runoff was high. in the water body, 100% of rainfall is converted into runoff. in horticulture plantation, agricultural land and degraded forest the infiltration is very less, and surface runoff will be more and join to the stream at the base that may cause top-soil loss. the estimated runoff showed that the watershed had a very good surface runoff potential. hence, the surface water can be recharged into the ground by constructing suitable artificial ground water recharge structures. reference 1. beven k.j., kirkby m.j. (1979). a physically based variable contributing area model of basin hydrology. hydrological sciences, bulletin, 24(1), 43-69. 2. bonta j. v. (1997). determination of watershed curve number using derived distributions. journal of irrigation and drainage engineering, 123(1), 28-36. 3. dubayah r., lerrenmaier d. (1997). combing remote sensing and hydrological modeling for applied water and energy balance studies. nasa eos interdisciplinary working group meeting, san diego, ca. 4. http://nilgirihillswater.pantoto.org/nilgiri/index.html, 2007. 5. jain m.k. (1996). gis based rainfall, runoff modelling for hemavathi catchment, nih report cs/ar-22/96-97. national institute of hydrology, roorkee. 6. nash j. e., sutcliffe j. v. (1970). river flow forecasting through conceptual models. j. hydrol. 10, 282–290. 7. sameer shadeed, mohammad almasri (2010). application of gisbased scs-cn method in west bank catchments. palestine water science and engineering, 3(1), 1-13. 8. schultz g.a. (1993). hydrological modeling based remote sensing information. adv. space res., 13(5). 9. soil conservation service (1985). hydrology, national engineering handbook, washington, usda. 10. xiaoyong zhan , min-lang huang (2004). arccn-runoff: an arcgis tool for generating curve number and runoff maps, environmental modelling & software, 27-04-04 13:11:52 3b2 ver.: 7.51c/w model: 5 enso1500. this study applies the ecological green equivalent approach to evaluating the land use structure of mt. yuelu scenic area in hunan province, china. a mathematical model is established based on land use and land cover data, and then arcgis used to extract the spatial extent of the ecological green equivalent within each of the relevant elements. results show that the area has a relatively reasonable land use structure even though the forest coverage rate is slight below the optimum. the overall green equivalent (1.13) was higher than the optimum forest coverage ecological green equivalent (1.00). the distribution of forest was uneven, with most of the forest within the site; the area’s land use structure could thus be improved by extending the green area outside of mt. yuelu. we conclude by reiterating that landscape and infrastructure development should consider ecological system conditions. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): n1 -n5 abstract keywords: ecological green equivalent, land use structure, landscape, scenic area, forest, china. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54138 e n v i r o n m e n t a l g e o l o g y xi chen 1,2,3, ke-lin wang 2, wei-jun zhou 4, hong-bin li 4, muhammad aqeel ashraf 5 1. college of bioscience and biotechnology, hunan agricultural university, changsha, hunan 410128, china 2. key laboratory of agro-ecological processes in subtropical region, institute of subtropical agriculture, chinese academy of sciences, changsha, hunan 410125, china; kelin@isa.ac.cn; (for correspondence) 3. biological research institute of chenzhou, chenzhou, hunan 423000, china 4. college of resources and environment, hunan agricultural university, changsha, hunan 410128, china 5. faculty of science and natural resources, university malaysia sabah 88400 kota kinabalu sabah, malaysia optimization of land use at mt. yuelu scenic area: an analysis using the ecological green equivalent record manuscript received: 12/11/2015 accepted for publication: 27/02/2016 how to cite item chen, x., wang, k., zhou, w., li, h., & ashraf, m. a. (2016). optimization of land use at mt. yuelu scenic area: an analysis using the ecological green equivalent. earth sciences research journal, 20(1), n1-n5. doi: http://dx.doi.org/10.15446/esrj.v20n1.54138 introduction many different methods presently exist to evaluate the utilization of ecotourism resources. focused on the environmental impact of tourism, the ecological footprint of tourism and other concepts assess the environmental carrying capacity related to tourism (wu, 2013). such information provides an essential basis for the sustainable development of ecotourism, especially in china, but these methods have limitations and shortcomings (gu and he, 2014). for example, the ecological footprint of tourism is hard to calculate. given the inherent mobility involved in tourism, particularly regional tourism, it is hard to define the ecological footprint accurately. moreover, tourism ecological footprint calculations often focus on analyzing the current multi-part footprint of energy, shopping, construction land, food, and other items, but the consumption footprint related to contaminated waste in environmental systems is not considered (hu and zheng, 2008; yang et al, 2012). in this study, we try to develop a new evaluation method by using ecological green equivalent analysis to analyze and evaluate land use at hunan province’s mt. yuelu scenic ecotourism area. this work should provide a scientific basis for the further development and utilization of ecotourism zones. the rest of the paper proceeds as follows. section 1 offers an overview of the green equivalent concept while section 2 describes the research methods. section 3 then presents results, which are used to inform the conclusions and recommendations of the final section. 1. the green equivalent concept and land type classification forests, the largest ecosystem on land, carry out key ecosystem services including climate regulation, soil and water conservation, and air purification. in the process of developing and using tourism resources, it is often difficult to avoid damaging forest ecosystems; such damage, in turn, affects the sustainable development of a region’s ecotourism industry. some scholars have thus suggested providing compensation for the destruction of or harm to regional forest ecosystems and their accompanying services (hu and zheng, 2008; hardin and jensen, 2007). developing efficient and practical compensation measures, however, requires a means of estimating comparable ecological function “equivalents.” the concept of “green equivalent” has been put forward to meet this need (li and bian, 2012). n2 xi chen, ke-lin wang, wei-jun zhou, hong-bin li and muhammad aqeel ashraf the “green equivalent” is defined as the minimum amount of green space required to ensure sufficient green biomass to deliver the same ecosystem services essentially—that is, the ecological function “equivalent” (mao, 1998). this biomass may occur in the form of a variety of green vegetation types, such as lawns, gardens, and farmland. the green equivalent is thus the ratio of the amount of other green vegetation to the green forest area that can deliver the same services (zhang et al., 2012). currently, ecological green equivalent analysis is mainly used to optimize land use structure when constructing regional development resources because land is typically the basis of the ecosystems on which tourism relies. planners can thus analyze the ecological green equivalent to optimizing the use of tourism land resources or use it to evaluate the green resources available within the land use structure of the tourism ecosystem after changes are made (sun, 2013). under the ecological green equivalent concept, traditional land use patterns can be divided into three categories (zhao et al., 2011; liu et al., 2002). first, there is ecological green equivalent land, characterized by its ability to be quantified regarding ecological mechanisms carried out by forest ecosystems. this includes farmland, woodland, orchards, and certain types of unused land. second is the implicit green equivalent land. quantification of this is difficult and must be done via qualitative research. the third is the non-green equivalent land (table 1). table 1. land classification under the ecological green equivalent approach. 2 research methods 2.1 description of the study area mt. yuelu scenic area is located in the changsha city of hunan province (longitude 112º44’-112º48’, latitude 28º20’-28º27’). it is about 4 km long in the north-south direction and about 1.5-2 km wide in the eastwest direction; the main mountain area is 8 km2, and the main mountain peak, bi xu, has an altitude of 300.8 m above sea level. the zone includes several scenic areas, including yuelu mountain, tianma mountain, orange isle, peach blossom mountain, shi jialing, zhai zi mountain, and xianjia lake. these combine with the surrounding protection zone for a total area of 38.4 km2. the area enjoys a humid subtropical monsoon climate, with an average annual temperature of 17ºc, rainfall of 1200-1400 mm, and relative humidity of 80%. it is one of china’s rainier zones (yang, 2004). the weather is damp or rainy and hot in spring and summer: july average and higher temperatures are 28.6ºc and 40.6ºc, respectively. in late autumn and early winter there is less rainfall, and january temperatures average 4.6ºc, plummeting to extreme lows of -11.3ºc. vegetation types in mt. yuelu scenic area are dominated by typical subtropical evergreen broadleaf forest and subtropical warm coniferous forest. according to species lists, the mountain hosts 285 kinds of forest plants (excluding foreign species) from 68 families and 159 genera; this accounts for 59.7%, 28.9%, and 14.3% of the respective families, genera, and species of hunan province’s native forest plants (yang, 2004). 2.2 methods of ecological green equivalent calculation and ecosystem analysis 2.2.1 spatial analysis of the study area in this study, we use arcgis 9.3 to extract the relevant factors under the spatial extent of ecological green equivalent given by the urban land use map (scale 1:10000, 2005); we combine this with the national land classification (trial) to classify the area of each land use type (zhao et al., 2011; xiao, 2013). using forest ecosystems as a standard, we then calculate the total ecological service function scores for all kinds of green land cover to obtain the green equivalent for each type of ecological system. by estimating the actual total area of green equivalent, we identify the lowest ecological optimization criterion to meet the minimum needs of the research area. finally, the structure of the ecological land can be adjusted within the research area to achieve ultimately a reasonable regional land resource utilization structure and promote the sound development of ecotourism resources (luo and zhang, 2007). 2.2.2 calculate total ecological service function score land use patterns change, as do ecosystem processes and functions. changes in land use patterns can have profound impacts on regional climate, soil, water, and biological processes. therefore, considering the regulation and protection of natural, economic, and social subsystems, criteria must be used to select typical, representative, and systematic indicators. table 2 lists 17 kinds of ecological functions of ecosystems, encompassing five aspects: the atmosphere, water, soil, biological processes, and natural disasters. we used the evaluations of japanese experts, obtained via surveys (mao, 1998), combined with the actual mt. yuelu context, to determine assessment scores of the various environmental features in the ecosystem (mao, 1998). the scores in table 2 take into consideration the environmental sub-functions with appropriate weights to determine the cumulative summation of the total value of ecosystem services that derive from each sub-function (or index value): in the formula, the total value of ecosystem services is p, f is an index value, and the number of indicators in the index system is i. based on this, the ecological services value of forestland is estimated as 169.1, considering the full year and area. under the same circumstances, the ecological services value of natural grasslands is 132.26, that of grasslands is 121.84, that of wetlands is 127.56, that of dry land is 113.55, and the garden plots is 124.53 (zhao, 2006). n3optimization of land use at mt. yuelu scenic area: an analysis using the ecological green equivalent table 2. the scores of different ecological system by environmental protection features notes: evaluation significance: optimal is 10, acceptable is 7.5, and minimal is 5. “atmospheric composition improvement -1” indicates co2 sequestration functions. “atmospheric composition improvement -2” indicates o2 sequestration functions. “atmosphere purification -1” and “atmosphere purification -2” indicate dust capture and toxic gas cleansing ecological service functions, respectively. 2.2.3 determining average ecological green equivalents in the ecosystem according to the different estimated land ecosystem service values, we assume that optimal woodland forest coverage corresponds to green equivalent 1, under the premise of year-round, wholly green coverage. it follows that xi= fi /fl the ecological services value score for woodland is fl and the total score for all ecological services of the green surface cover of class i is fi. thus, the surface green cover of class i regarding ecological green equivalent in the ecosystem is fi. based on zhao et al. (2006), the green equivalent of garden plots is 0.73, that of the natural meadow is 0.76, grassland is 0.73, paddy land is 0.77, and ordinary upland is 0.68. however, due to climatic differences between regions, there are both interregional and intertemporal differences in green equivalents. given the actual circumstances in each region, the above green equivalent results must be multiplied by a coefficient representing the growing period about a one-year-period. according to expert-recommended values (table 3), when three crops are grown in a year in subtropical regions, the growing period parameter is 0.83 (liu et al., 2002). table 3. average green equivalent of cultivated land and grassland notes: relative growing period parameter = (length of growing period (months) / 12). the harvest frequency and growing period are based on expert advice. 2.3 model for optimizing the land utilization structure based on the ecological green equivalent we use the ecological green equivalent of optimal forest cover as the basis for ecosystem optimization. the total area of the research is st, the optimal forest coverage is rmax, regional forest area under the optimal forest coverage requirements is sf, actual forest area is srf, si is the area of land used for land use type i (i is an integer, i ϵ [1, 4]), and ecological green equivalent is gi. we can then use soil and precipitation data from the meteorological department and the agricultural sector to consider the benefits arising from ecological protection and apply the method of yuanbo (zuo and ruan, 2009; gong et al., 1996) to determine the optimal forest cover rate (rmax) (liu eat al., 2009): rmax% =(p × s1) / (w × st) × 100% in this formula, the region’s total land area is st, the daily maximum precipitation (t/hm2) within a year is p. s1 = st -(surface area of urban, mining, transport, and paddy field land) (hm2), and the forest soil’s saturated water storage capacity per unit area is w (t/hm2) (zuo and ruan, 2009). sf = st × rmax, and the corresponding ecological green equivalent is =1. the ecological green equivalent of the actual woodland area is given by: the region’s green equivalent x is calculated as follows: comparing the sizes of x and a determines whether the measure is on target. if ≥ , then the standard is met. if < , the areas of various types of green space must be adjusted, with the index recalculated for verification (liu et al., 2002). 3. results the tourist ecosystem is shaped by a combination of factors involving land, the environment, technology, policy, human needs and behaviors, and ecology. changes to any one factor will lead to changes in the existing ecosystem, altering its balance. although the tourism ecosystem itself has a certain ability to recover, this is limited. therefore, if the tourism industry engages in the excessive pursuit of economic benefits, the extent of land use for economic purposes will be overly emphasized. such a process ignores environmental protection, eventually harming tourism itself. through calculating the green equivalent of the tourism ecosystem (wu et al., 2011; n4 xi chen, ke-lin wang, wei-jun zhou, hong-bin li and muhammad aqeel ashraf xiang and meng, 2013), we will be able to address the problem of properly using tourism ecosystem land resources (via structural optimization) through a combination of qualitative and quantitative analysis. according to the region’s endowment of natural resources and its environment, we determine the ecological minimum standards; we then use ecological green equivalent calculations for the area to protect the lowest stock of land required to maintain ecosystem functions. this procedure ensures that the degree of regulated land use does not exceed the capacity of the ecosystem, thereby achieving the coordination between ecological, social, and economic motivations that is the goal of sustainable development (lu et al., 2013). 3.1 ecological green equivalent by drawing on previous research defining forest ecosystem services values, we quantitatively evaluate the ecological service value of various types of green spaces in the ecosystem (hardin and jensen, 2007). to investigate based on a “considerable green amount” of land area regarding the relationship between forest and farmland and gardens and lawns, we measure the ecological green equivalent of cultivated land, grasslands, and garden plots. this quantifies, compared to forests, the ecological functions of a given area of agricultural land (including cropping land, grassland, and garden plots) (sun, 2013). calculating the green coverage within the scenic area, we can thus explore how to optimize the structure of ecological standards within the scenic area. this insight can strengthen the construction of scenic environments (zhang et al., 2011; zhang et al., 2012; li et al., 2014). based on the above approach, we determine that mt. yuelu scenic area has an ecological green equivalent of x = 1.13; as this is greater than 1.00, it implies a proper structure of land use. among the contributors, the ecological green equivalent of cultivated land is 0.20, that of garden plots is 0.10, and that of forest land is 0.84. table 4. green equivalents in mt. yuelu scenic area despite this positive indication, to accelerate the development of the area in recent years, a large number of human landscapes have been built, causing massive destruction to forest vegetation. the woodland area and the total ecological green equivalent have both fallen. as a result, habitats have been destroyed for key species in the biological community and biodiversity has been reduced. community characteristics and ornamental values have also declined, with inadequate connections within the area’s greenbelt: some roads lack roadside greenery and proper green infrastructure or rely on solely a single species. 3.2 land use structure land resources support various types of ecotourism resources. therefore, this study seeks to apply the concept of ecological green equivalent to optimizing the allocation of ecotourism land use structures. this represents an application of systems theory thinking and the principles of ecosystem ecology. we thus ecologically optimize land use for ecotourism in the mt. yuelu scenic area to safeguard the ecological environment while balancing economic, social, and ecological benefits. according to a mapping survey of land use within the scenic area, which covers an area of 38.49 km2, the land use structure is as given in table 5. table 5. land use in the mt. yuelu scenic area using this current land use structure for mt. yuelu scenic area and basic data on the area (e.g., p = 1165 t/hm2, w = 2500 t/hm2, and s1 = 2500 hm2), we calculate that optimal ecological protection benefits are achieved when there is 30.3% forest cover. although mt. yuelu scenic area’s forest coverage rate was 25.4%, lower than optimal, results show that its overall green equivalent (1.13) is higher than the green equivalent of the optimal forest cover rate (1.00), indicating that the scenic area currently has an extremely reasonable land resource utilization structure. however, mt. yuelu’s woodlands are unevenly distributed; on the mountain itself, for example, total forest area is 5.33 km2 of an area of only 8.00 km2; 20.8% of the total scenic land area thus accounted for 54.5% of the woodland area. 4 conclusions and recommendations 4.1 conclusions the essence of landscape pattern optimization is using landscape ecology principles to achieve reasonable land use by jointly considering natural and social data. by analyzing the appropriate and reasonable distribution pattern of landscape types, one can adjust the landscape distribution (in both location and scope) to maximize the total ecological value of the landscape. the results of this research indicate that the mt. yuelu scenic area has a proper land resource utilization structure but with less-than-optimal forest cover. this suggests a need for the area’s managers to optimize land use through structural adjustment. this study investigated ecological green equivalent to considering ecotourism resource allocation. further research could consider combined optimization of both the land quantity resources for ecotourism and the spatial structure based on a comprehensive model of quantity optimization and spatial pattern analysis. to facilitate realtime monitoring of mt. yuelu’s ecotourism resources and environment and provide dynamic warnings on ecological security, research should also take advantage of “3s” technology for analysis and monitoring. 4.2 recommendations to promote the ecological health of the mt. yuelu scenic area, we suggest four areas for further optimization based on the problems found in this research. first, this study is the first to apply the green equivalent analytical model to ecotourism resources for the purpose of determining scientific and operable land use planning guidance (jim, 2001; li et al., 2012). results indicated that the area’s land resource utilization structure was reasonable, but the area’s woodlands are unevenly distributed. their scope should thus be increased outside of the core scenic area to support the forest coverage rate to reach the desired level. moreover, the use of the forest ecological environment quality as the overall situational indicator reflects regional limitations; there is thus a need for continued research to improve the computational model and select better indicators (hu and zheng, 2008; yang et al, 2012). it should also be noted that the quality of woodland is not entirely reflected in the regional green equivalent, highlighting a further need to improve the index method as part of future research efforts (xiao, 2013). n5development of tropical lowland peat forest phasic community zonations in the kota samarahan-asajaya area, west sarawak, malaysia second, optimization of land use structures helps improve ecosystem functions and services, thereby realizing regional ecological security objectives. as the scenic area’s woodlands are unevenly distributed, woodland resources should be reallocated, and the green area outside of the mt. yuelu scenic area should be increased. by optimizing the structure of construction land and safeguarding the ecological health and regional ecological security through the land use pattern (yang et al, 2012), the scenic area’s ecological green equivalent can be targeted to meet requirements while maximizing eco-efficiency in line with the needs of economic and social development. third, we note that, in recent years, the mt. yuelu area’s environment has been damaged, impinging upon the sustainable social, economic, and environmental development. the development of the area’s cultural landscape through the construction of infrastructure such as roads must thus be done in compliance with ecological priorities (jim, 2001). this should be based on existing roads and should aim to widen green spaces, optimize species distributions, and increase the scenic area’s green space. finally, these results have also revealed that there is scope for improving the consistency of land use and ecological landscape functions in the mt. yuelu area. the landscape spatial pattern should be aligned to local conditions, with space allocation made in a manner that is reasonable for a given project, fully aligning the project’s properties and functions with the functions and spatial characteristics of the land, thus achieving ecological coordination, system harmony, and scenario blending (hardin and jensen, 2007). future work in the area should also consider the renewal and transformation of the area’s tree species distributions to include some ornamental (e.g., flower, foliage, and concept tree) species, such as magnolia, ginkgo, and elaeocarpus decipiens. the goal of this work should be to enhance the natural colors of the forest and thus achieve coordination between ecological functions of the forest and its landscaping and greening structure, thereby improving the area’s landscape efficiency. simultaneously, it may be appropriate to develop additional tourism options, such as agriculture and horticulture tours (e.g., tourist farms), and to adjust the industrial structure actively to promote the balanced development of the ecosystem (ye et al., 2012; 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(2009). the application of ecological green equivalent on land use structure optimization and example. heilongjiang environmental journal, 33(3), 11-13. v16n1c.indb earth sciences research journal electrical and electromagnetic methods earth sci res sj vol 16, no 1 (june, 2012): 75 82 subsurface cavity detection in a karst environment using electrical resistivity (er): a case study from yongweol-ri, south korea muhammad farooq1*, samgyu park2, young soo song3, jung ho kim2, mohammad tariq1 and adepelumi adekunle abraham4 1 department of earth sciences, comsats institute of information technology, abbottabad, pakistan 2 mineral resources research division, korea institute of geoscience and mineral resources, south korea 3 department of mineral resources and energy engineering, chonbuk national university, south korea 4 department of geology, obafemi awolowo university, nigeria * corresponding author e-mail: farooq@ciit net pk abstract karst voids were encountered during road construction in yongweol-ri, south korea attempts to determine the extent of the karst voids through drilling was expensive, time-consuming and too dangerous an electrical resistivity survey was thus carried out to investigate current subsurface geology beneath the proposed road network this investigation was aimed at imaging karstic voids and detecting areas prone to ground subsidence through the collapse of cavities beneath a road segment overlying such features a numerical modelling study preceded the field survey to determine whether the electrical resistivity method could identify such features the field data set consisted of eleven electric profiles acquired using dipoledipole array; electrical resistivity profiles (100 m and 300 m length) were measured using a dipole-dipole electrode configuration and electrode spacing was kept at 5 m to ensure good coverage of the study area the resistivity data was inverted using 2d and 3d algorithms based on the least squares smoothness constraint technique the inverted resistivity images provide a clear view of weathered soils, the distribution of weak areas or karst voids and bed rock several low resistivity areas were identified and the subsequent drilling of such anomalous areas led to the discovery of several weak zones or clayfilled underground cavities; the drilling results had excellent correlation with the resistivity images the electrical resistivity imaging result proved precise and extremely efficient in delineating the karstic void investigation and should be taken into account when choosing an investigation technique to be used at complex geological sites a remedial action plan involving consolidation grouting work has been suggested prior to road construction resumen cuevas kársticas fueron encontrados durante la construcción de unas carreteras en yongweol-ri, corea del sur los intentos para  determinar el grado/extensión de las cuevas kársticas a través de perforaciones eran costosos, lentos y muy peligrosos así, un estudio de resistividad eléctrica (re) se llevó a cabo para investigar la geología actual del subsuelo por debajo de la red de carreteras propuesta la presente investigación fue dirigida a obtener imagen de las cuevas kársticas y la detección de zonas propensas al hundimiento del  terreno a través del colapso de las cavidades debajo de un segmento de una carretera encima de las mismas un estudio de modelación numérica precedió a la investigación de campo para determinar si el método re podría identificar tales características el conjunto de datos de campo constaban de once perfiles eléctricos adquiridos con arreglo dipolo-dipolo; perfiles re (100 m y 300 m de longitud) fueron medidos usando una configuración de electrodos dipolo-dipolo y la separación entre electrodos se mantuvo a 5 m para garantizar una buena cobertura del área de estudio los datos de resistividad fueron invertidos utilizando algoritmos 2d y 3d basados en la técnica mínimos cuadrados amortiguados las imágenes de resistividad invertidas proporcionan una vista clara de los suelos erosionados, la distribución de las áreas débiles o de cuevas  kársticas y  de lecho de roca varias zonas de baja resistividad fueron identificadas y la perforación posterior de dichas áreas anómalas llevó al descubrimiento de varias palabras claves: cavidad, dipole-dipolo, cuevas kársticas, modelo de resistividad, hundimiento keywords: cavity, dipole-dipole, karst void, resistivity modelling, subsidence record manuscript received: 19/10/2011 accepted for publications: 04/05/2012 muhammad farooq, samgyu park, young soo song, jung ho kim, mohammad tariq and adepelumi adekunle abraham76 figure 1. cavities at a field test site in a karst area in yongweol-ri, muan-gun zonas débiles o de cavidades subterráneas llenas de arcilla; los resultados de las perforaciones tuvieron una excelente correlación con las imágenes de resistividad el resultado del mapa de re demostró ser precisa y extremadamente eficiente en la delimitación de la investigación de cuevas kársticas y debería tomarse en cuenta cuando se escoja una técnica de investigación para ser usada en sitios geológicos complejos ha sido sugerido un plan de acción de medidas correctivas que implica el trabajo de consolidación de anclajes antes de la construcción de carreteras introduction karst voids are often a major engineering risk associated with construction in areas underlain by carbonate rocks several problems are associated with subsurface karstic voids, such as road and highway subsidence, building foundation collapse and dam leakage large void formation in a karst environment may lead to sudden and catastrophic pavement failure, whilst fine particles’ slow migration from the sub-base may cause gradual ground subsidence any of the above situations in a karst environment can be expensive for engineering structure design boring within karst areas often does not meet areas of concern in the subsurface mislaid borings can provide insufficient data and can misrepresent the subsurface system examining aerial photographs and satellite images is a classical karst detection technique which has led to reasonably good maps being produced over the past decade however, such dataset is spatially and temporally discontinuous, representing only immediate snapshots geological and geomorphological methods cannot be directly applied in many urban areas where rapid development has concealed surface features reliable hazard mapping, especially for detailed civil engineering studies, cannot rely solely on a limited source of information; other techniques are thus required for accurate measurement subsurface voids represent disturbances of the close subsurface layered system in karst areas; such disturbances can be identified using geophysical techniques karst void characterisation using geophysical methods has gained wide interest in the past few decades a variety of geophysical techniques can be used for detecting karst voids below the surface all of them are based on the physical contrast between a void and the surrounding rocks geophysical techniques allow large areas to be covered in a short period of time and represent an efficient way of detecting subsurface heterogeneities including voids and subsidence areas (i e seismic reflection, microgravity, ground penetrating radar and electrical resistivity) all such techniques have shown great potential for accurately mapping subsurface conditions; however, geo-electrical imaging is considered one of the most promising techniques for karst voids (roth and nyquist, 2003) promising results have been obtained by researchers around the world during the last two decades involving electrical resistivity surveys in karst subsurface cavity detection in a karst environment using electrical resistivity (er): a case study from yongweol-ri, south korea 77 terrains their objectives were to map karst structures such as voids, sinkholes and weathered areas (gautman et al , 2000; van schoor, 2002; zhou et al , 2002; gibson et al , 2004) these investigations have demonstrated the electric resistivity method’s efficiency regarding mapping karst structures due to the large contrast in resistivity between filled cavities (low resistivity) and limestone bedrock however, karst areas represent a very unfriendly environment for 2d geophysical exploration due to large subsurface heterogeneity an engineering firm encountered subsidence during road construction in the yongweol-ri area (figure1); the project was thus halted and a geophysical investigation was embarked on to understand such subsidence research was thus designed to use electrical resistivity (er) and drilling boreholes to detect and investigate the extent of karst voids and underground cavities in the subsurface of the route to be taken by the road in question er was chosen due to previous researchers’ promising results ground penetrating radar could not be applied in the study area due to high salinity 2d and 3d er surveys were thus carried out to evaluate these methods and data regarding eight boreholes acquired in the area was analysed site descriptions the field test site was located in yongweol-ri muan-gun, southwestern korea; ground subsidence has been reported in nearby rice fields the area is underlain by limestone bedrock overlain by alluvial deposits ubiquitous bedrock underground cavities are known to exist within the area structural damage resulting from ground subsidence has already affected some road networks and buildings in this area; the risk of future damage resulting from subsidence is thus considered high regarding future development programmes the origin of the network of cavities in the area has been ascribed to bedrock limestone dissolution by groundwater flowing through a network of fractures associated with a large fault zone near the site known as the kwangju fault all the boreholes drilled for agricultural purposes in the area have had very high groundwater yield, possibly due to such cavities and fractures (kim et al , 2007; park et al , 2009) the sinkhole collapse mechanism at the test site involves happening during the farming off-season, the water-table lying above the regolithlimestone boundary (conversely, when the water-table drops into or below cavities due to a large volume of groundwater pumped for irrigation during the farming season this causes regolith arches, spanning the openings in the bedrock) and collapse occurring due to loss of buoyant support for regolith arches, attributed to downward moving groundwater the area’s overall geology consists of pre-cambrian basement complex rocks (i e schist and granitic gneiss) usually overlain by unknown metasediments whose exact age has not been determined; part of the site is covered by cretaceous volcanic sediments, basal quartzite and schist intercalated with thinly-bedded, lensshaped limestone and quartzite beds (park et al , 2009; park et al , 2010) d ep th (m et er ) 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 distance (meter) 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 distance (meter) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) distance (meter) distance (meter) 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 (a) (b) (c) (d) figure 3. synthetic resistivity inversion results obtained using (a) wenner array, (b) pole-pole array, (c) dipole-dipole array and (d) pole-dipole array figure 2. diagram showing a subsurface cavity embedded within bedrock bedrock resistivity was 500 ω-m and subsurface clay/water filled cavity resistivity was 20 ω-m muhammad farooq, samgyu park, young soo song, jung ho kim, mohammad tariq and adepelumi adekunle abraham78 the electrical resistivity (er) method the er method involves determining subsurface resistivity distribution by taking ground surface measurements; it is based on ohm’s law current is usually introduced into the ground by one pair of electrodes and a second pair is then used to measure the resulting potential difference an apparent resistivity a is determined by measuring potential difference δv at a different couple of electrodes’ spacing: where k is a geometric constant depending on the four electrodes’ reciprocal positions and is injected current intensity however, data is acquired simultaneously if multiple electrodes are used and the area will be efficiently covered though probes at various depths a fast numerical approach is then required to optimise an initial multi-layer model constructed from such apparent resistivity values this data can be converted to a true geological model of the subsurface through inversion techniques a finite difference or finite element method (fem) is usually used to calculate the model’s 2d or 3d forward response successive iteration will ensure that the model will improve until a minimum root mean square 168050 168000 167950 167900 167850 167800 167750 152400 152450 152500 152550 152600 152650 figure 4. diagram showing the study area, its location, resistivity survey traverse orientation and borehole drilling figure 5. 2d inverted resistivity images obtained beneath lines ‘a’ and ‘b’ d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) a= k δv i (1) subsurface cavity detection in a karst environment using electrical resistivity (er): a case study from yongweol-ri, south korea 79 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) distance (meter) distance (meter)distance (meter) distance (meter) 10 20 30 40 50 60 70 8010 20 30 40 50 60 70 80 10 20 30 40 50 60 70 8010 20 30 40 50 60 70 80 figure 6(a). inverted field resistivity data obtained from survey lines 1 to 4 line 1 line 2 line 3 line 4 (rms) misfit between observed and model pseudosection is achieved resistivity differences normally correspond to changes in subsurface materials or pore fluids synthetic modelling synthetic modelling was carried out prior to field data acquisition in this study for deciding on a suitable electrode array to be used during data acquisition em2dmodel numerical forward model software (south korean institute of geosciences and mineral resources’ geo-electrical imaging laboratory) was used to simulate subsurface cavity electrical response a finite element algorithm was used to calculate direct response (apparent resistivity) from a set of resistivity models intended to reflect cavity area pattern the resulting apparent resistivity from forward modelling was input into dipro software and inverted into earth resistivity models the forward model tested (figure 2) had three layers having 20 -m, 70 -m and 500 -m resistivity; 70 -m was selected to represent the top layer of overburden material (weathered soils), 20 -m a subsurface cavity in the bedrock and 500 -m bedrock resistivity (i e limestone) figure 3 shows inverted resistivity images obtained for wenner, polepole, dipole-dipole and pole-dipole arrays (a, b, c & d, respectively) to determine which array best imaged the subsurface cavity; inverted resistivity representing the arrays were compared figure 3a shows an inverted model obtained using a wenner array; the image shows that inverted resistivity was underestimated and the model did not resolve the embedded 20 -m area (subsurface cavity) and thus its image was not a true representation of the subsurface model used the estimated pole-pole array model shown in figure 3b overestimated subsurface cavity location whilst the dipole-dipole array model (figure 3c) revealed a subsurface cavity giving the best approximation to the initial model (its resistivity and thickness were well resolved) the pole-dipole array (figure 3d) was similar to that of the dipole-dipole model but the pole-dipole inverted model gave an asymmetrical array; this gave the dipole-dipole array a slight advantage over the pole-dipole array the dipole-dipole array was thus adopted for this study as it reproduced subsurface cavity images more precisely than other electrode array types data acquisition and processing supersting r8/ip was used for data acquisition in profiling mode, using the dipole-dipole array; data acquisition involved two phases profile lines “a” and “b” (300 m length) were acquired with 5 m electrode spacing on both sides of the road during phase 1 a small portion of the road was selected for detailed investigation based on the results for lines a and b nine traverses (lines 1 to 9, 100 m length) were acquired using 5 m electrode spacing along the road during phase 2 these lines’ orientation and extent and borehole drilling locations are shown on the location map (figure 4) the geoelectrical data was processed and inverted using dipro resistivity software the inversion routines were based on the smoothness constrained least squares optimisation technique (yi et al , 2001; yi et al , 2003); forward resistivity calculations involved applying an fem-based iterative algorithm inversion divided the subsurface into a number of small rectangular blocks; the blocks’ resistivity values were determined results and interpretation figure 5 shows the inverted resistivity sections for the two parallel traverses (a and b); two distinct resistivity areas could be noted the first had weathered, low conductivity soils occurring near the surface, beginning from 0 to 150 m (horizontal distance) the second area had high conductivity within weathered soils; the highly conductive area occurred between 150 to 250 m horizontal distances the former could have be due to seasonal variation whilst the other area was probably due to the presence of highly weathered limestone depth in this area appeared to be about 5m to 15 m this area’s resistivity values ranged from 20 to 90 -m increased weathering suggested increased clay content, indicating a less resistive environment (increased conductivity) this area was interpreted as being the subsurface cavity area nine parallel resistivity profiles were acquired on both sides of the road to obtain more reliable and realistic subsurface images of the risky area (cavity area) (figure 4) figure 6 (a & b) shows the nine parallel traverses’ inverted resistivity sections the whole area was divided into two regions (lines 1 to 4 are shown in figure 6a and lines 5 to 9 in figure 6b) muhammad farooq, samgyu park, young soo song, jung ho kim, mohammad tariq and adepelumi adekunle abraham80 the first region’s images (figure 6a) were interpreted to represent two main areas having moderate and high resistivity; the first extended from ground level to a depth of 5-13 m and had moderate resistivity and the second (figure 6b) had three distinct resistivity areas that characterised lines 5 to 9 the uppermost part extended beyond 13 m and had higher resistivity than the previous area the shallow area having moderate resistivity values (50-70 -m) was interpreted to represent unconsolidated material consisting of silty and/or sandy gravel lenses (conductive material) the 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) d ep th (m et er ) distance (meter) distance (meter) distance (meter)distance (meter) distance (meter) 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 8010 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 figure 6(b). inverted field resistivity data obtained from survey lines 5 to 9 figure 7. depth-slice images of resistivity distribution reconstructed from 3d resistivity inversion line 5 line 6 line 7 line 9 line 8 deeper area having higher resistivity values (>200 -m) was interpreted to represent weathered limestone the very low resistivity anomalies observed within this layer could have represented incompetent rock or possibly clay/ water-filled cavities the result obtained along lines 5 to 9 (figure 6b) was considerably different from the results for lines 1 to 4 (figure 6a) this difference was attributed to contrasting resistivity between shallow, moderate and deeper resistivity areas the dipole-dipole resistivity data was further subjected to advanced 3d resistivity processing using an algorithm developed by yi et al , (2001) to obtain a more reliable and realistic representation of subsurface structure the subsurface 3d images obtained were represented as horizontal slices of the subsurface at different depths (figure 7) figure 7 shows resistivity slices at various depths these images clearly showed that the lateral extent and depth of the subsurface cavities beneath the survey area had been effectively mapped core drilling results after processing and interpreting the resistivity data, exploratory drilling confirmed whether areas delineated as having low resistivity were actually cavities borehole drilling was carried out along lines 1 to 9 and eight locations were selected for coring along these lines the borehole drilling area was selected based on the strength of the anomalies figure 8 shows a comparison of the geoelectrical cross-section for line 5 and bh 106 and borehole drilling results from other boreholes figure 8 clearly shows that the subsurface consisted of limestone cavities; each borehole’s results were comparable to delineated low resistivity areas along survey lines it also shows weak rock and/or clay-filled cavities and that the corresponding resistivity images identified low resistivity areas within the study region subsurface cavity detection in a karst environment using electrical resistivity (er): a case study from yongweol-ri, south korea 81 0 5 10 15 20 25 line 5 bh 106 d ep th (m et er ) distance (meter) 10 20 30 40 50 60 70 80 figure 8. comparing geoelectrical cross-section and bh 106; all borehole lithological logs acquired in the survey area are depicted conclusions and recommendations the er survey of a proposed road construction site revealed several low resistivity regions which were interpreted as clay/water-filled voids in a karst terrain a shallow boring programme involving the anomalous portion of the surveyed area revealed numerous cavities and verified the resistivity measurements’ overall accuracy the synthetic model and the field survey results demonstrated the efficacy of dipole-dipole array er method in subsurface cavity mapping the drilling result validated cavity location and existence delineated by the er technique and lent credence to its applicability in karst terrain it is recommended that cement-based grout should be injected into the subsurface cavities to improve the strength of subsurface materials and reduce permeability acknowledgments this research work has been supported by the korean institute of geosciences and mineral resources’ basic research project funded by the korean ministry of knowledge economy (mke) references adepelumi, a a , solanke, a a , sanusi, o b and shallangwa a m (2006) model tank electrical resistivity characterization of lnapl migration in a clayey-sand formation environmental geology 50, 1221-1233 adepelumi a, yi mj, kim jh, ako bd, son js (2006) integration of surface geophysical methods for fracture detection in crystalline bedrocks of south-western nigeria hydrogeology journal 14, 12841306 gautman, p , pant, s r , and ando, h (2000) mapping of subsurface karst structure with gamma ray and electrical resistivity profiles: a case study from pokhara valley, central nepal journal of applied geophysics 45, 2, 97-110 gibson, p j , lyle, p and george, d m (2004) application of resistivity and magnetometry geophysical techniques for near-surface investigations in karstic terranes in ireland journal of cave and karst studies, 66, 2, 35-38 kim, j , yi, m , hwang, s , song, y , cho, s and synn, j (2007) integrated geophysical surveys for the safety evaluation of a ground subsidence zone in a small city journal of geophysics and engineering, 4, 332-347 park, g , park, s , yi, m j , rim, h , cho, s j , and kim, j h (2010) geostatistical integration using 2-d electrical resistivity and 3-d gravity methods for detecting cavities in a karst area, environmental earth sciences 60, 965-974 park, s , kim c , son j s , yi, m j , and kim, h h (2009) detection of cavities in a karst area by means of a 3d electrical resistivity technique exploration geophysics 40, 27-32 103 104bh# 106 107 108 109 110 111 5 10 15 20 25 30 35 40 d ep th (m et er ) topsoil and weathered bedrock cavity filledwith clay / water limestone bedrock muhammad farooq, samgyu park, young soo song, jung ho kim, mohammad tariq and adepelumi adekunle abraham82 roth, m j s and nyquist, j e (2003) evaluation of multi-electrode earth resistivity testing in karst geotechnical testing journal, astm, 26, 167-178 van schoor, m (2002) detection of sinkholes using 2d electrical resistivity imaging: applied geophysics journal, 50, 4, 393-399 yi, m j , kim, j h , and chung, s h (2003) enhancing the resolving power of least-squares inversion with active constraint balancing geophysics, 68, 931-941 yi, m j , kim, j h , song, y h , cho, s j , chung, s h , and suh, j h (2001) three dimensional imaging of subsurface structures using resistivity data geophysical prospecting, 49, 4, 483-497 zhou, w f , beck, b f , and adams, a l (2002) effective electrode array in mapping karst hazards in electrical resistivity tomography environmental geology, 42, 8, 922-928 159 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 159 167 debris flow susceptibility mapping in a portion of the andes and preandes of san juan, argentina using frequency ratio and logistic regression models. esper angillieri, m.y.1 1conicetgabinete de neotectónica y geomorfología, ingeo. facultad de ciencias exactas, físicas y naturales, universidad nacional de san juan. e-mail: yaninaesper@gmail.com geotechnics in this study, the frequency ratio and logistic regression models are applied and verified for the analysis of debris flow susceptibility in a portion of the dry frontal andes and occidental preandes of san juan at approximately 30°s latitude, through aninvestigation based on a geographic information system (gis). the site under study covers an area of 2175.9 km2 with a debris flow area of 42.45 km2. for this purpose, thematic layers including debris flow inventory, lithology, elevation, slope, aspect, and solar radiation were used. the debris flow inventory map was prepared by interpreting aerial photographs and satellite images and was supported by field surveys. lithology was extracted from an existing geological map. slope, aspect and solar radiation were calculated from a digital elevation model created from srtm (shuttle radar topographic mission) and topographical maps. the relationship between the variables and the debris flow inventory was calculated using the frequency ratio and logistic regression models. both models helped to produce debris flow susceptibility maps that classified susceptibility into five categories: very low, low, moderate, high and very high. subsequently, each debris flow susceptibility map was compared with known debris flow locations and tested. the frequency ratio model (accuracy is 82.71%) was more accurate than the logistic regression model (accuracy is 75.64%) for predictons of the high and very high categories. en este trabajo se aplican, mediante el uso de sistemas de información geográfica, dos modelos estadísticos en la evaluación de la susceptibilidad del terreno a la ocurrencia de flujos de detritos, la relación de frecuencias (fr) y la regresión logística. el área de estudio comprende un sector de cordillera frontal y de precordillera occidental a los 30°s de latitud media. se crearon mapas de elevación, pendiente, insolación, orientaciones, estratigrafía y un inventario de flujos de detritos. este último, a partir de la interpretación y análisis digital de fotografías aéreas e imágenes satelitales. la estratigrafía fue obtenida a partir de cartas geologicas preexistentes. las pendientes, orientaciones e insolación fueron calculadas, a partir de un modelo digital de elevaciones. los mapas de susceptibilidad generados han sido reclasificados en cinco categorías: muy baja, baja, moderada, alta y muy alta. finalmente, estos mapas, fueron validados espacialmente y como resultado se observa que el modelo fr predice mejor (82,71%) que la regresión logística (75,64%) para las clases alta y muy alta. palabras clave: susceptibilidad, flujos de detritos, sig, san juan, argentina. record manuscript received: 22/07/2013 accepted for publication: 01/11/2013 abstract resumen key words: debris flow susceptibility, gis, san juan, argentina introduction a debris flow is a flow of sediment and water mixture that acts as a continuous fluid driven by gravity, and it attains large mobility from the enlarged void space saturated with water or slurry (takahashi, 2007)it is. debris flows are widely recognized as one of the dominant geomorphic processes in the central andesis and are strongly correlated with heavy or prolonged rainstorms, often in association with intense snowmelt. debris flows cause loss of life and property and damage to natural resources and hamper developmental projects, such as roads and communication lines. because the study area is lowly populated or, in most locations, uninhabited, these events have hardly been documented and rarely damage property. however, most of the small towns in the department of iglesia (figure 1) are situated downstream of river basins from the frontal andes (malimán, colangüil, angualasto) or occidental preandes (buena esperanza). these villages have suffered flash floods and debris flows during every summer period, which has caused material suffered damage, and several casualties have resulted, as evident by events that occurredfor example, during 160 occurrences in 1913, 1944 and 2007 (angualasto, buena esperanza and malimán). the elongated morphology of the main watersheds, coupled with their significant height difference (promoting rapid runoff), are the most important variables that control flow generation. several authors have studied landslides in argentina (fauqué et al., 2000; moreiras, 2006; fernández, 2005; penna et al., 2008; gonzález díaz, 2009; gonzález díaz and folguera, 2009; perucca and esper, 2008, 2009a,b). debris flow susceptibility mapping aims to differentiate a land surface in homogeneous areas according to the probability of occurrence in an specific area on the basis of local terrain conditions (varnes, 1978; brabb, 1984). the debris flow susceptibility analysis was performed by applying the frequency ratio and logistic regression models. in this study, debris flow susceptibility assessment in a gis environment is based on a comparison of a debris flow inventory map and conditioning variables (lithology, elevation, slope, aspect, and solar radiation). the debris flow inventory map was prepared by interpreting aerial photographs and satellite images and was supported by field surveys. slope, aspect and solar radiation were calculated from a digital elevation model created from srtm (shuttle radar topographic mission) and topographical maps. in various papers, models such as frequency ratio and logistic regression have been used for landslide susceptibility. these include studies by atkinson and massari (1998); süzen and doyuran (2004); lee (2005); ayalew and yamagishi (2005); lee and sambath (2006); nefeslioglu et al. (2008); kincal et al. (2009); nandi and shakoor (2010); bai et al. (2010, 2011); chauhan et al. (2010); das et al. (2010); oh and lee (2010); ercanoglu and temiz (2011); and yalcin et al. (2011). the aim of this study is to prepare debris flow susceptibility maps using the frequency ratio and logistic regression models for a sector of the andes and preandes of san juan, argentina. the resulting maps can provide a basis for urbanism, land use planning and would be utilized by public administration offices and insurance companies. regional setting study area the territory of interest covers an area of 2175.9 km2 in which debris flows have a spatial distribution of 42.45 km2. it is located in the middle west of argentina in northern san juan province, iglesia department, between latitudes 29°39′–30°9′s and longitudes 68°47′-69°41′w, on the northeast side of the central frontal andes and north of the occidental preandes, which are characterized by high relief with peaks up to 5000 m a.s.l. (figure 1). the hillsides rise from 1587 m a.s.l. to 5639 m a.s.l. with average slope angles of 12°. the slopes are very steep in the upper portion (~45°) and are rocky and poorly vegetated. the lower slopes are comprised of talus and relict debris flow deposits. climate the area is characterized by semi-arid conditions with a dry climate, short summers and severe winters with very low temperatures (−18°c to 0°c) and strong winds, and the rainy season normally lasts from november to march. however, the rainfall regime is irregular, with periods of little precipitation and some with lengthy heavy rains. above 4000 m a.s.l., the precipitation is mainly snow and hail; precipitation events are irregular. the largest amount of rainfall measured during one day was 52 mm on 26 december 1967 with an intensity of 104 mm/h, and the largest amount of rainfall measured in one month was 62.3 mm in january 1993. the annual precipitation of the town of rodeo is 44.9 mm, but precipitation varies notably with elevation; the annual precipitation for most of the study area is between 100 and 200 mm (minetti et al., 1986). according to observations recorded over 42 years in rodeo, july is the coldest month, with an average temperature of 7.91°c, and the hottest month is january, with temperatures averaging 23.1°c. from 3300 to 4300 m a.s.l., the temperatures are between −18°c to 10°c. above 4300 m a.s.l., the average temperature in january is below 0°c, and the formation of permafrost is likely. geological setting the area is dconsists of the cordillera frontal ranges and the precordillera occidental geological provinces. the cordillera frontal is a mountainous system generally running in the north–south direction with peaks up to 5000 m a.s.l. and is composed of a suite of upper paleozoic rocks which unconformably overlie a middle proterozoic substratum (ramos, 1999) and highly deformed lower paleozoic sedimentary rocks. the oldest stratigraphic unit in the area is a sedimentary unit mainly composed of upper carboniferous–lower permian dark green mudstones interbedded with sandstones and conglomerates. this unit is overlain unconformably by a permian–lower triassic mesosilicic and silicic volcanic and igneous complex, including pyroclastic, subvolcanic and intrusive rocks, that consists of a lower andesite to dacite section and an upper rhyolitic section. both units are intruded by medium-grained triassic grayish granodiorites, which in turn are intruded by pink-red rhyolitic bodies. the sequence continues with an eocene-oligocene unit composed of conglomerates, sandstones, multicolor tuffs, andesites, breccia and ignimbrite that rests unconformably on the permo-lower triassic volcanic complex and is intruded by miocene intrusive rocks of varied composition (granodioritic, andesitic, dacitic, dioritic, and granitic). the precordillera, an older orographic system with lower topographical hierarchy, has a general north–south direction and rises up to 3000 m a.s.l. lower ordovician sedimentary rocks are the oldest stratigraphic units, composed of limestones, sandstones and lutites, and are observed in the eastern parts of the study area. devonian conglomerates composed of sandstones and pelites appear in the area, and carboniferous marine formations (lutites, wakes and conglomerates) are unconformably overlain in sequence. along the iglesia valley, the lithology is represented by paleogene, neogene and quaternary deposits. the paleogene–neogene deposits are represented by sedimentary units composed of alluvial and colluvial deposits as well as miocene argillites. the quaternary deposits are formed by gravels, sands, marls, and clays and are restricted to narrow river channels and valleys. was data were acquired from geologic sheets published by the servicio geológico minero argentino (argentine mining geologic service) on a 1:250 000 scale. structurally, the eastern border of the cordillera frontal ranges and the precordillera occidental geological provinces is characterized by a system of faults with an eastern vergence (allmendinger et al., 1990). the lithology map of the study area is shown in figure 2. materials and methods the fundamental principle of this approach to debris flow susceptibility mapping is to use the characteristics of existing debris flows to evaluate possible areas of future debris flows. for this reason, debris flow and debris flow conditioning factor databases were constructed. to store the information of these parameter maps in a uniform thematic database, the size of each pixel or cell for all of the products was set to 15×15 m. an inventory of debris flows in the study area was prepared (esper angillieri, 2010). the debris flows were identified using aerial photographs and digital satellite images (spot 5 and landsat 7 etm), which were georeferenced using a geographical information system (gis), and the debris flow locations were verified by fieldwork. esper angillieri, m.y. 161 geologic sheets published by the servicio geológico minero argentino (argentine mining geologic service) on a 1:250000 scale were used to determine the lithology (figure 2). elevations were obtained from 1:100000 topographic sheets with a 50-m contour interval published by the instituto geográfico militar (argentine military geographic institute) and from topographical information obtained from the radar shuttle topographical mission (usgs, 2000). then, a digital elevation model (dem) with a 15-m grid spacing was interpolated (figure 2). using the dem, the slope angle, slope aspect, and the amount of solar radiation were calculated. solar radiation (in wh m−2; equal to 0.001 h of sun) in january and february was estimated using the hemispherical viewshed algorithm figure 1. location of study area and spatial distribution of debris flows, showing the regional context in san juan province and iglesia department in argentina. debris flow susceptibility mapping in a portion of the andes and preandes of san juan, argentina using frequency ratio and logistic regression models 162 developed by rich et al. (1994), fu and rich (2000, 2002) and rich and fu (2000). see figure 2. the spatial relationships between the landslide location and each debris flow-related factor were analyzed using a frequency ratio and a logistic regression model (lee and sambath, 2006; lee and pradhan, 2007) the frequency ratio is the ratio of the probability of an occurrence to the probability of a non-occurrence for given attributes (bonham carter, 1994). therefore, the frequency ratio (fr) can be calculated according to equation 1: where s is the total number of pixels, n is the number of pixels with debris flow occurrences, si is the number of pixels of the i variable, and ni is the number of pixels in which the debris flows occurred in the i variable (table 1). if fr is greater than 1, there is a high correlation; a value smaller than 1 aindicates low correlation. the algorithm of logistic regression applies a maximum likelihood estimation after transforming the dependent variable into a logic variable (the natural log of the odds of the dependent occurring or not). in this manner, logistic regression estimates the probability of a certain event occurring (atkinson and massari, 1998) in the presence or absence of debris flows. logistic regression is useful for predicting the presence (1) or absence (0) of a process based on a set of predictive variables. the advantages of logistic regression are that 1) each variable used can be either continuous or discrete, and 2) there is not necessarily a normal distribution. in the present situation, the binary dependent variable represents the presence or absence of debris flows. in a logistic regression analysis, it is preferable that the number of pixels representing areas with or without a process should be the same (e.g., süzen and doyuran, 2004; ayalew and yamagishi, 2005; nefeslioglu et al., 2008); therefore, 10000 pixels representing the occurrence of debris flows and 10000 pixels without debris flows were randomly selected for logistic regression. the lithologic unit classes were considered to be categorical variables, whereas slope, elevation and solar radiation were considered to be continuous variables. because semantically distinct parameters may have a strong correlation, the pearson correlation coefficient was used to check the correlations between variables. finally, the susceptibility maps were verified and compared using known debris flow locations that were randomly selected and had not been previously used in the calculation of the models. figure 2. thematic data and maps, including the elevation, slope, aspect, solar radiation and lithology, used in the frequency ratio and logistic regression analyses. (1) esper angillieri, m.y. 163 results and discussion the variables were converted to a raster grid with 15×15 m cells for the application of the logistic regression and frequency ratio models. the grid area was 3271 rows by 5144 columns (i.e., the total number is 9670614), and 155058 cells had debris flow occurrences. a total of 346 debris flows were identified (figure 1) and covered an area of 42.45 km2, accounting for 1.95% of the study area (2175.9 km2). only 214 debris flows were used for susceptibility analysis; the rest were reserved for verification purposes. on table 1, the debris flow percentages that were used for the susceptibility analysis are shown, whileand the percentage distribution of all debris flows within each conditioning factor are presented in figure 3. the pearson correlations (table 2) show that the variables used in the present study are only weakly correlated with each other. the highest correlation was found between solar radiation and elevation (0.78). in the study area, the outcropping lithologies were grouped into nineteen classes: 1lower ordovician sedimentary rocks, 2devonian conglomerates, sandstones and pelites, 3,4,5 and 6carboniferous marine formations, 7, 10 and 11granites, 8granodiorites, 9rhyolites, 12mesosilicic and silicic volcanic and igneous complex, 13multicolor tuff, 14miocene intrusive rocks, 15miocene argillites, 16 and 17pleistocene deposits, 18silt, and 19recent deposits. (figure 2). the pleistocene deposits, which represent 40% of the total area of the studied territory, are involved in more than 44% of the total area of debris flows identified (figure 3). figure 3. distribution of all debris flows within each conditioning factor. debris flow susceptibility mapping in a portion of the andes and preandes of san juan, argentina using frequency ratio and logistic regression models 164 factor class nº de pixels showing debris flows occurrencea % of pixels showing debris flows occurrenceb pixels in domainc píxels %d frequency ratioe coefficients of logistic regression lithology 1 378 0.24 506132 5.234 0.047 -1.977 2 0 0.00 37688 0.390 0.000 -3.499 3 0 0.00 213316 2.206 0.000 -22.109 4 112 0.07 389213 4.025 0.018 -21.246 5 572 0.37 19065 0.197 1.871 -4.213 6 31407 20.26 1227249 12.690 1.596 -0.215 7 4150 2.68 118083 1.221 2.192 0.036 8 501 0.32 75035 0.776 0.416 0.479 9 0 0.00 49029 0.507 0.000 0.338 10 3977 2.56 1347213 13.931 0.184 -20.064 11 1099 0.71 129083 1.335 0.531 -1.545 12 4650 3.00 750112 7.757 0.387 0.824 13 0 0.00 63345 0.655 0.000 -0.411 14 3909 2.52 58263 0.602 4.184 -20.584 15 13260 8.55 202490 2.094 4.084 1.405 16 51924 33.49 2454464 25.381 1.319 0.167 17 17237 11.12 1543984 15.966 0.696 -0.627 18 242 0.16 10919 0.113 1.382 -1.156 19 21640 13.96 475931 4.921 2.836 -0.824 elevation [m. asl] 15852000 39543 25.50 1278115 13.216 1.930 -0.0002 20003000 72790 46.94 3591097 37.134 1.264 30004000 36573 23.59 3145416 32.526 0.725 40005000 6152 3.97 1468588 15.186 0.261 50005643 0 0.00 187398 1.938 0.000 slope angle [degree] 0-5 87351 56.33 3157910 32.655 1.725 -0.087 5-15 53350 34.41 3667061 37.920 0.907 15-30 12949 8.35 2443179 25.264 0.331 30-49 1408 0.91 402464 4.162 0.218 slope aspect [degree] -1 0 0.00 396 0.004 0.000 0.001 315-45 16207 10.45 1613220 16.682 0.627 45-135 46808 30.19 3162587 32.703 0.923 135-225 67039 43.23 3297291 34.096 1.268 225-315 25004 16.13 1597120 16.515 0.976 solar radiation x10-5 [wh/m2] 2.97-3.97 43995.000 28.37 2015051 20.837 1.362 -0.005 3.97-4.13 31192.000 20.12 1946419 20.127 0.999 4.13-4.29 39799.000 25.67 2070447 21.410 1.199 4.29-4.48 32434.000 20.92 2369679 24.504 0.854 4.48-5.40 7638.000 4.93 1269018 13.122 0.375 total number of pixels showing debris flows occurrence (n) = 155058 total number of pixels in domain (s) = 9670614 b = (a/155058)*100 d= (c/9670614)*100 e= b/d table 1. frequency ratios and logistic regression coefficients. esper angillieri, m.y. 165 the elevation map (figure 2) of the study area was divided into five elevation classes; the elevation ranges from 1585 to 5643 m a.s.l. a total of 72.44% of the debris flows are dominant in the elevation interval from 1585 to 3000 m a.s.l. (figure 3). the slope map of the study area was divided into four slope categories (figure 2). the result shows that most of the debris flows occur at a slope angle less than 15° (figure 3). aspect regions were classified according to the aspect class as flat (−1°), north (315°–360°, 0°–45°), east (45°–135°), south (135°–225°) and west (225°–315°). this factor indicates that debris flows occur on east-facing (30.19%) and south-facing (43.23%) slopes (figure 3). the solar radiation map indicates that debris flows occur in almost all classes with similar percentages (figure 3). the observed distribution of the debris flows, the frequency ratio and the logistic regression coefficients are shown in table 1. according to the logistic regression method, the aspect coefficient is positive, while the elevation, slope, and solar radiation coefficients are negative. this means that the aspect is positively related to the occurrence of debris flows, whereas elevation, slope, and solar radiation have an inverse relationship with debris flow occurrence in the study area. the variables chosen and the system constructed were found to be valid; 70.4% of the pixels were correctly predicted (76.7% of the pixels of the debris flows and 64.1% of the pixels of non-debris flows). furthermore, the logistic regression analysis revealed that the distributions of the debris flows are mainly controlled by lithology and slope, as 82.1% and 75.7%, respectively, of the pixels with debris flows beingwere correctly classified. the frequency ratios of each variable’s type or class were added to calculate the debris flow susceptibility index (dfsi), according to equation 2: dfsi = lithologyr + elevationr + sloper + aspectr + solar radiationr (where, lithologyr is the frequency ratio of lithology, elevationr is the frequency ratio of elevation, sloper is the frequency ratio of slope, aspectr is the frequency ratio of aspect, and table 2. pearson’s correlation coefficients between variables. frequency ratio logistic regression area (%) % debris flows area area (%) % debris flows area very low 4.60 0.18 20.77 4.99 low 29.20 7.26 7.30 0.29 moderate 19.28 9.85 36.69 19.07 high 39.35 50.09 20.49 53.41 very hig 7.57 32.62 14.76 22.23 figure 4. debris flow susceptibility maps based on logistic regression and frequency ratio. solar radiationr is the frequency ratio of solar radiation) (2) a logistic regression formula was created, as shown in equation 3, from table 1 data: z = lithologyb + (-0.0002 x elevation) + (0.001 x aspect) + (-0.087 x slope) + (-0.005 x solar radiation) + z (3) (where, elevation is the elevation value, slope is the slope value, aspectr is the aspect value, solar radiationr is the solar radiation;, and lithologyb are the logistic regression coefficient values listed in table 1. z = 2.333 and is a value determined by the program). according to the debris flow susceptibility map produced from the frequency ratio method, 4.60% of the total area was haveshowed very low debris flow susceptibility. low, moderate, high and very high susceptible zones represent 29.20%, 19.28%, 39.35% and 7.57% of the total area, respectively (figure 4). table 3. verification values of the debris flows susceptibility maps of frequency ratio and logistic regression models.   lithology elevation slope angle slope aspect solar radiation lithology 1.000 -0.603 -0.584 0.105 -0.390 elevation -0.603 1.000 0.683 -0.152 0.780 slope angle -0.584 0.683 1.000 -0.133 0.197 slope aspect 0.105 -0.152 -0.133 1.000 -0.167 solar radiation -0.390 0.780 0.197 -0.167 1.000 debris flow susceptibility mapping in a portion of the andes and preandes of san juan, argentina using frequency ratio and logistic regression models 166 the logistic regression method showed different results with very low, low, moderate, high and very high susceptibility zones representing 20.77%, 7.30%, 36.69%, 20.49% and 14.74% of the total area, respectively. the testing validation performed by comparing the known debris flow location data, which were not included in the susceptibility analyses, is shown in table 3. as a result, the frequency ratio model (accuracy of 82.71%) is better for prediction than the logistic regression model (accuracy of 75.64%) for the high and very high susceptibility zones. the precision of the susceptibility maps produced here is related to correct identification of the debris flows on the field, the interpretation of digital images and dem resolution (90 m) with itsrespect to the derived variables (slope angle, slope aspect and solar radiation). for the method to be generally applicable, more high quality debris flowrelated digital variables are necessary (dem of major resolution of approximately 5 or 10 m). however, the susceptibility maps produced were positively tested on january 23, 2010 at 19:00, when a sizeable debris flow affected the town of malimán de arriba as a result of heavy rainfall. several homes were damaged and most of the crops and farm animals were lost due to flash flooding. this validates the applicability of the methods used because the total area affected was located on high and very high susceptibility zones. conclusions in this study, the frequency ratio and logistic regression models were applied to generate debris flow susceptibility maps, using the same conditioning factor database. through fieldwork, aerial photographs, and digital satellite image interpretation, a debris flow inventory map was created. of the 346 debris flows identified, 214 locations were used for model training, and the remaining 132 cases were used for model validation. the logistic regression method revealed that the most important conditioning factor was lithology, and the rate of success was 82.1%. the validation results show that the frequency ratio model has an a improved prediction accuracy of 82.71%, which is better than that for the logistic regression model (82.71%). thus, the results of the frequency ratio model showed the 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(1978). slope movement types and processes. r.l. schuster and r.l. krizek (editors.), landslides: analysis and control, special report, vol. 176, transportation research board, national academy of sciences, washington, dc, 11–33. yalcin, a., reis, s., aydinoglu, a.c. and yomralioglu, t. (2011). a gis-based comparative study of frequency ratio, analytical hierarchy process, bivariate statistics and logistics regression methods for landslide susceptibility mapping in trabzon, ne turkey, catena. 85, 274–287. debris flow susceptibility mapping in a portion of the andes and preandes of san juan, argentina using frequency ratio and logistic regression models physical phenomena, such as attenuation of high frequency components and velocity dispersion, deteriorate seismic images. to enhance seismic resolution, q filtering is usually applied, where the accurate estimation of q is the core of this approach. the matching pursuit (mp) approach is an instantaneous spectral analysis method that overcomes windowing problems caused by decomposing a seismic trace, providing a frequency spectrum for each time sample of the trace. by changing variables, the spectra is transformed into a new domain where q is accurately measured. this value is input to a stable inverse q filter, which solely compensates for the amplitude without distorting the phase, and as a result, an improved time-resolution image is obtained. mp, windowed fourier (wf), and gabor transforms (gt) were tested on synthetic seismograms providing q values that indicated the best performance of mp. applied to a migrated offshore colombian seismic line, the entire procedure coded in matlab compensated for the energy losses and increased the s/n ratio. the trace-by-trace approach guarantees the reliability of the improvement in the continuity of seismic events and is applicable to pre-stack data as well. los fenómenos físicos de atenuación de los componentes de alta frecuencia y dispersión por velocidad de fase deterioran la imagen sísmica. para incrementar la resolución sísmica usualmente se aplican filtros q inversos, por lo que la estimación precisa del factor de calidad q es el problema central de este método. el “matching pursuit” (mp) es un método de análisis espectral instantáneo que supera los problemas generados al usar ventanas para descomponer una traza, generando un espectro de frecuencia por cada muestra de la traza. mediante un cambio de variables el espectro se transforma a un nuevo dominio donde se estima q con precisión. éste valor se alimenta a un filtro q inverso estabilizado, el cual compensa solamente la amplitud sin distorsionar la fase obteniendo como resultado se logra una imagen con mejor resolución. los métodos de mp, transformada de fourier con ventanas y transformada de gabor se ensayaron con datos sintéticos suministrando valores de q que señalaban el mejor desempeño de mp. el proceso completo fue codificado en matlab y aplicado a una sección marina colombiana migrada, compensando la perdida de energía e incrementando la relación señal/ruido. el enfoque de traza por traza garantiza la confianza en el realce de la continuidad de los eventos sísmicos, siendo aplicable a datos pre-apilados. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 63 67 abstract resumen key words: inverse filter, quality factor, spectral analysis, instantaneous spectrum, attenuation. palabras clave: filtro inverso, factor de calidad, análisis espectral, espectro instantáneo, atenuación. enhancing time resolution by stabilized inverse filter and q estimated on instantaneous spectra 1. álvaro corrales, delta state university, nigeria. email: etunimogala@yahoo.com 2. francisco cabrera, universidad de pamplona, departamento de física, pamplona, norte de santander. email: fcabrera61@yahoo.com.br 3. luis montes, universidad nacional de colombia, departamento de geociencias, of 326, bogotá. email: lamontesv@unal.edu.co signal processing record manuscript received: 17/12/2012 accepted for publication: 29/04/2014 64 álvaro corrales, francisco cabrera and luís montes. 1 1 notwithstanding, this model does not satisfy the minimum delay condition of dispersive medium, rigorously stated by the kramers-krönig relation. to correct phase distortions, and considering the narrow seismic broadband, equation 1 is rewritten as (wang and guo, 2004). (2) here, ωh is the maximum frequency of the seismic broadband and ᵧ= qr equation 2 leads to k(ω) = 1− (3) hence, wave equation ( ∂u (ω, r) ⁄ ∂r −ik(ω)u(ω, r) = 0 admits as a solution u(ω, r + ∆r) = u(ω, r)exp exp i (4) the first exponential compensates for the amplitude spectra, and the second one the k(ω) phase spectra, and downward after continuing from the surface t = 0 to t u(ω, t) = u(ω, 0)exp (5) in consequence β (ω, t) u (ω, t) = u (ω, 0) exp i (6) where β (ω, t) = exp − equation 5 can be solved like an inversion problem with a stabilization factor o introduced through u(ω, t) = i (7) the final solution is obtained by integrating equation 13 along the broadband u (t) = (8) for a horizontal-layer model, the interval quality factor of the nth-layer and the average quality factor of n layers have a cumulative relation given by: (9) with the above equation, qn is estimated where tn and tn-1 are the times where the average quality factors q n and q n-1 are measured. matching pursuit decomposition the mp algorithm (mallat and zhang, 1993) decomposes a signal into a linear superposition of wavelets selected from a redundant functions dictionary. the dictionary is built with previously calculated wavelets that are estimated and chosen because of their best fitting of the seismic trace. a time-frequency wavelet family is generated by scaling, translating, and modulating a continuous window function m(t) l² (r) such that ‖m‖ = 1,m(0) ≠ 0, ∫ m(t)dt ≠ 0 defined by mᵧ = s ͞ exp(iƒmt) (10) with scale factor s, modulating frequency ƒm and translation td. due to m(t) being even mᵧ is centered at the td abscissa concentrating the energy in the vicinity of td. by fourier transforming equation 10 becomes introduction a seismic wave that propagates in inelastic media is transformed attenuating its amplitude due to frequency-dependent absorption and delaying its phase due to different phase velocities. many approaches have been applied to increase time resolution using spectral inversion (castaño et al., 2011) or by spectral blueing with relative success (kazemeini at al., 2010). on the other hand, the quality factor q quantifies the attenuation as the percentage of energy lost in a cycle at a certain frequency (kjartansson, 1979), and methods to apply inverse q filtering have been developed and applied to seismic data (margrave, 1998; wang, 2002), and their performances compared (montana and margrave, 2005). the spectral ratio method (srm) is the most common approach used to calculate the interval attenuation factor. initially, it was widely used to estimate attenuation in acoustic vsp data, and recently in shear vsp data (chuandong & stewart, 2006). in addition, new methodologies have been implemented to calculate p-wave and s-wave attenuation (qp and qs) from standard well log data, relating these to the presence of oil or natural gas (walls et al., 2006). it is known that attenuation in seismic data is related to gas-bearing reservoirs, which can cause significant changes in seismic amplitude with offset providing erroneous avo results in deep targets. in this case, attenuation is a valuable alternative tool as a direct indicator of hydrocarbons (kumar et al, 2003). seismic attenuation has been used to distinguish changes in spectra in order to detect the presence of fluids and fractures in rocks (ramirez-cruz et al., 2005). therefore, a stable inverse q filter might compensate attenuation in seismic data if a reliable pseudo q log was available, otherwise trying to recover amplitude spectra and correct phase spectra by q filtering could lead to possible artifacts on seismic data. the stable inverse q filter must achieve the previous goals without boosting ambient noise and increasing s/n ratio. the accurate determination of q may provide a more accurate amplitudeoffset analysis and reliable seismic inversion for reservoir characterization. therefore, the accurate estimation of q is a core challenge to overcome before applying inverse q filtering, considering that q does not depend linearly on frequency (kolsky, 1956). the robustness of q estimation through transforms that use fixed and variable time-frequency windows have been analyzed, pointing out that the precision and accuracy achieved depends on the transform used to supply the spectrum (reine et al, 2009). the wft cannot describe signal structures with varying size, because all wavelets have a constant scale that is proportional to the window size, whereas in mp, the signal structures are represented by wavelets that match their time frequency signature (wang, 2007). in this case, the mpd uses a morlet wavelet dictionary to decompose a seismic trace into a time-frequency spectrum with a higher resolution than those ones provided by gt and wft. the matching algorithm is based in morlet wavelets because they have shown high performance along with computational advantages (liu & castagna, 2005). mp, windowed fourier (wf) and gabor transforms (gt) were coded in matlab and tested on synthetic seismograms, which pointed out the best performance of mp to provide accurate q logs. the t-ω spectrum supplied by mp is brought by a change of variable, in which q is calculated using a linear regression. finally, the estimated q log is input to a matlab program that implements the stable inverse q filtering (wang, 2006). applied to a stacked offshore colombian seismic line, the above two applications corrected the wavelet distortion, compensated the energy losses, and increased the s/n ratio. the trace-by-trace approach guarantees the reliability of the improvement in continuity of seismic events, being applicable to pre-stack data as well. attenuation according to kolsky´s model (1956) a plane wave travels in a viscoelastic medium with wave number k(ω) = ω ⁄ v(ω) iα (ω) that includes an attenuation factor α(ω)=|ω| ⁄ ((2v(ω)q(ω)) and phase velocity v(ω) that depend linearly with ω frequency. when ω is much greater than ωr (an arbitrarily low reference frequency), the following expression becomes true vᵣ |(ω)| ω ω ω (τ) (τ) ω ω∆τ ω∆τ ω∆τ ω∆τ 2q(ω) 2q(τ) 2q(τ) vᵣ2q(ω) i ω ω (τ) dτ exp i dτ ωdτ ω (τ) ωdτ u (ω, 0 ) exp u (ω, 0 ) exp i ω (τ) ωdτ ω (τ) ωdτ 65enhancing time resolution by stabilized inverse filter and q estimated on instantaneous spectra mᵧ (ω) = √sm (s(ω − ωm )) exp −i (ω − ωm ) td (11) where, m(ω) is the fourier transform of m(t) with its energy concentrated around ƒm. henceforth a trace x(t) might be expressed by a linear combination of wavelets mᵧ which belong to the redundant mᵧ -family dictionary. the morlet wavelet was selected due to its best fit of the seismic wavelets and may include quantitative attenuation and dispersion (wang & pann, 1996). in each step, the mp algorithm looks for the wavelet that provides most energy to the signal and removes this energy from the signal. the energy of the signal is iteratively reduced until reaching the threshold energy previously established the remainder energy is considered noise. methodology and results a synthetic seismogram was created by superposing three traces. the first trace by convolving a 0°20 hz morlet wavelet with 1 and -1 reflection coefficients at 0.3 and 0.9 s, the second by convolving a 45°30 hz morlet with 1, 1 and 1 reflection coefficients at 0.45, 0.93 and 1.2 s, and the third by convolving a 90°50 hz morlet with 1 and -1 reflection coefficients at 0.6 and 1.23 s. in figure 1a, three morlet wavelets at 0.3, 0.45 and 0.6 s are clearly separated, and also two wavelets at approximately 0.9 and 1.2 s where it is hard to discern the two close events, at 0.9 and 0.93 s, and 1.2 and 1.23 s. to compare the performances of matching pursuit (mp), continuous wavelet (cw), gabor (g) and windowed fourier (wf) transforms, the synthetic seismogram was spectrally decomposed by the four methods. the supplied mp, cw, wf and g spectra are shown in figures 1b, 1c, 1d and 1e. as a result, the 20, 30 and 50 hz bandwidths are best defined in the mp spectrum, besides a better time resolution of the reflectors at 0.3, 0.45, 0.6, 0.9, 0.93, 1.2 and 1.23 s. on the other hand, the wf spectrum strongly depicts gibbs phenomena as a consequence of the windowed effects, even at 0.3, 0.45 and 0.6 s. figure 1 a) synthetic trace and spectra obtained by b) matching pursuit c) continuous wavelet transform d) gabor and e) windowed fourier transforms. next, a blocky model with q = 100 constant in depth was made, along which a plane wave traveling generates the attenuated synthetic seismogram of figure 2a. this seismogram was mp decomposed, providing the t-ω spectrum shown in figure 2b. considering exclusively the attenuation phenomena, the wave amplitude decays as a (ω, t ) = a0 exp [ − ωt / 2q ] (18) a0 is the highest amplitude. equation 18 is equivalent to lna² (ω, t) − lna0² = ¯ / q (19) by replacing ωt by the new variable χ the equation 19 turns into 2 [lna(χ) / lna(χₐ)] = − (χ − χₐ) / q (20) χₐ is associated with the highest energy value in the new χ −spectrum, on which amplitudes decrease along the χ axis with a slope equal to qֿֿֿ ¹ . a map of isovalue χ-curves over the t-ω spectrum is shown in figure 2b. the amplitudes along each χ curve are summed and associated to a (χ) in the χ spectrum, as shown in figure 2c. as a clarification, the abscissa of figure 2b represents frequency (hz) and not angular frequency (ω) and χ represents ft instead of ωt, so the χ =10 curve cuts the abscissa at 2.0 s and 5 hz and achieves 100 hz at 0.1 s. so henceforth, the χ abscissa, as in the case of figure 2c, must be scaled by a factor of 2π. figure 2 a) a trace propagated along a simple q=100 model, b) the χ curve map superimposing the t − f spectrum provided by mp and c) the χ spectrum obtained by replacing ft with χ and summing along constant χ curves in the t − f spectrum. figure 3 normalized χ – spectrum with χb limiting the zone where q is estimated through a linear regression. figure 4 the χ – spectrum supplied by wft and b) zooming of the interval along which the linear regression provides q estimation. ωt 66 álvaro corrales, francisco cabrera and luís montes. figure 5 the χ – spectrum supplied by gt and b) zooming of the interval along which q is estimated. the maximum amplitude in the spectrum of figure 2c is set to χ0 = 4. ahead of the highest peak of the χ spectrum, a very low amplitude zone around χ =10 is observed, which agrees with the low energy content in the vicinity of the χ =10 curve as is shown by the t−ω spectrum of figure 2b. the χ spectrum is normalized in a new one, see figure 3, where all values are negative and with the highest value around χ =5. the slope is steepest after χb = 80, a value that defines the superior limit of the interval along which q is estimated. tiny values after χb = 80, are rejected because they are associated to numerical noise observed in figure 2c. a linear regression provided a value of 16.1 for the inverse of the slope that scaled by 2π gives a q value of 101.16, which is close to 100. the same seismogram was decomposed by wft and gt supplying the respective χ spectra, observed in figures 4 and 5. wft and gt χ spectra depict a similar pattern that differs from the mp χ spectrum. in both new spectra, the low amplitude zone around χ =10 is not observed, and instead is full of spurious amplitudes. in addition, the absence of a linear decreasing trend in both spectra increases the uncertainty to estimate q. in an attempt to estimate q, some χ values were excluded to achieve a linear trend. in the wft case, a q value of 193.9 was obtained, and in the gt case a value of 134, both considered very inaccurate. figure 6 a) the mp spectrum of a stacked trace of the stacked section rc2103129, b) the 0-2 s spectrum, c) the 0-3.5 s spectrum and d) the 0-5 s spectrum. figure 7 a) three χ – spectra used to calculate q at 2, 3.5 and 5 seconds, b) stacked section of the colombia basin, c) stacked section after q estimation and q filtering. the seismic events look sharper than before q filtering. the preceding procedure was applied to the migrated offshore section rc2103-129 of the caribbean colombia basin, formed by a siliciclastic neogene sequence that overlays a carbonate and siliciclastic paleogene sequence. a trace of this section was decomposed by mp, and its spectrum shown in figure 6a. the 0-2s, 0-3.5 s and 0-5 s intervals were considered to estimate their associated average quality factors. their respective χ – spectra are shown in figures 6b, 6c and 6d, on which linear regressions provided ‹q› values of 39.9, 55.2 and 73.1, respectively. using equation 9, interval q values of 39.9, 113.5 and 296.3 related to the 0-2s, 2-3.5 and 3.5-5s intervals were obtained, with these values being in agreement with q measurements reported for this lithology. next, q values were estimated at 0.5, 1.5, 2.5, 3.0, 3.5, and 4.5 seconds in the migrated section, and in order to consider the q lateral changes it was estimated in positions separated forty cdps. the final q model was achieved by smoothing laterally the obtained attenuation factors. an inverse q-filter was developed and implemented in matlab, which includes a stabilization factor σ compensating for the amplitudes according to equation 13. the filter was applied to the above section, and as a result, a higher resolution image was achieved, as observed in figure 7b. the seismic events in the q filtered image look sharper, with a better lateral continuity than those in the former section and are reliable due to the trace by trace approach of the applied inverse q filter. an area, from 1000 to 1300 ms and between the cdps 1870 and 1890, was selected to observe the q-filtering performance. figures 8a and 8b show the zoomed areas before and after applying the inverse filter. the seismic events between 1050-1100 ms and 1150-1200 ms look clearly separated after q inversion, whereas the weak event between 1100-1150 ms becomes stronger with an observable lateral continuity not visible before. after q filtering, the image depicts higher amplitudes and better lateral continuity than before q filtering. a spectral analysis was performed in the limited area. figures 9a and 9b show the amplitude’s spectra before and after applying the q filter. after q filtering, the spectrum increased in one order of magnitude, and the dominant amplitude moved from 10 hz in the stacked section, to 28 hz. moreover, frequency content above 30 hz was recovered increasing the frequency bandwidth. 67enhancing time resolution by stabilized inverse filter and q estimated on instantaneous spectra conclusions q filters, which are widely used to recover seismic frequency bandwidth, may create artifacts in seismic images due to phase distortions, so to increase s/n ratio, an inverse q filter that compensates for the amplitude spectrum without introducing phase distortion was implemented. therefore, the strict use of the q filter implies an accurate estimation of the quality factor. to accomplish this, q must be estimated on the instantaneous spectra provided by different methods. the attenuation factors are estimated in a normalized spectrum making the achieved value reliable. spectral decompositions supplied by matching pursuit, windowed fourier, and gabor transforms were compared, and the analysis of the estimated attenuation factors noted the high performance achieved by the matching pursuit method. applied to a stacked offshore seismic line off the colombia basin, the inverse q filtering provided an enhanced image, where the reflectors depict sharp reflectors with better continuity than the stacked image. a zoomed zone of the inverted image showed separated events, not clearly seen in the stacked section. an analysis of the spectra before and after inversion indicated a one-order increase in the amplitude scale, and the increase of the frequency bandwidth. the trace-by-trace approach makes the method suitable to pre-stack inversion. all the decomposition methods and q filter algorithms were specially implemented for this research. figure 8 a) the zoomed area before q filtering, and b) the same area after applying q filtering with lateral continuity increased and new discerned reflectors. figure 9 spectral analysis of the zoomed area a) before and b) after q filtering where the spectrum is improved at least in one order of magnitude and the dominant frequency moves from 10 to 25 hz, increasing the frequency bandwidth. acknowledgements the authors wish to thank the institute of geophysics university of houston for permission to use the seismic data of the project southern margin of the caribbean, and also universidad nacional de colombia for the support given. bibliography castaño, k., ojeda, g., & montes, l., 2011, thin-layer detection using spectral inversion and a genetic algorithm. earth sci. res. j., 15, 121-128. hargreaves, n., 1998, theory of nonstationary linear filtering in the fourier domain with applications to time-varyiant filtering: geophysics, 63, 244-259. kjartansson e., 1979, constant q wave propagation and attenuation, j. geophysics. res., 84, 4737-4748. kazemeini s., yang c., juhlin c., & formel s., 2010, enhancing seismic data resolution using the prestack blueing technique: an example from the ketzin co2 injection site, germany. geophysics 75, 101-110. kolsky h, 1956, the propagation of stress pulses in viscoelastic solids: philosophical magazine, 1, 693710. kumar, g. batzle, m. & hofmann, r., 2003, effects of fluids on the attenuation of elastic waves. expanded abstracts, 73st. annual international meeting, society of exploration geophysicists, pp. 1592–1595. liu j., and marfurt k., 2005, matching pursuit decomposition using morlet wavelets: seg/houston 2005 annual meeting. 786-789 mallat s., and zhang z., 1993, matching pursuit with time-frequency dictionaries: ieee transactions on signal processing, 41, 3397–3415. montana c., margrave g., 2005, comparing three methods for inverse q filtering, crewes news, volume 16, no. 4-5 apr-may 2005 ramirez-cruz l., vale-garcia r., urrutia j., 2005, enhanced oil production in a mature field assisted by spectral attenuation analysis, j. geophys. eng, 2, 48-53. reine, c., van der baan m., and clark r., 2009, the robustness of seismic attenuation measurements using fixedand variable-window timefrequency transforms: geophysics, 74, 123–135. wang, y., 2002, a stable and efficient approach of inverse q-filtering: geophysics, 67, 657-663. wang y. 2006, inverse q-filter for seismic resolution enhancement: geophysics, 71, v651–v660. wang y., 2007, seismic time-frequency spectral decomposition by matching pursuit: geophysics: 72, v13–v20. earth sciences research journal earth sci. res. sj. vol. 16, no. 2 (dcember, 2012): 83 corrigendum presentamos excusas a nuestros lectores y al antropólogo javier andrés sandoval andrade puesto que en el vol 15 no 2 del 2011 de la revista earth science research journal (esrj) se publicó el articulo titulado "quaternary dating by electron spin resonance (esr) applied to human tooth enamel" en el cual no se hace un correcto pie de página de la figura 1, ya que no es tomada de (groot, 1992) como reza la misma, sino que es tomada de la tesis de pregrado titulada datación de restos humanos prehispánicos a través de esmalte dental usando resonancia paramagnética electrónica (epr) (2010). asimismo, no corresponde al sitio arqueológico aguazuque, sino que corresponde al sitio arqueológico checua. carlos vargas editor en jefe earth science research journal corrigendum we apologize to our readers and the antropology javier andres sandoval andrade because in the vol 15 issue 2 year 2011 in the earth science research journal (esrj) published the article entitled "quaternary dating by electron spin resonance (esr) applied to human tooth enamel" which there is an incorrect footnote in the figure 1 as it is not quoted from (groot, 1992), but it is taken from the undergraduate thesis entitled datacion de restos humanos prehispanicos a traves de esmalte dental usando resonancia paramagnetica electronica (epr) (2010). also, this figure do not correspond with the aguazuque arqueological site, but it is from the checua arquelogical site. carlos vargas editor-in-chief earth science research journal palabras clave: galería en muro; análisis numérico; estabilidad subterránea. how to cite item guo, j., fan, h., wang, x., zhang, l., ren, l., he, y., ma, f., meng, w., bai, x., & yin, j. (2019). integrating geochemical anomaly and remote sensing methods to predict oiland gas-bearing areas in the yanchang oil field, ordos basin, china. earth sciences research journal, 23(1), 79-86. doi: https://doi.org/10.15446/esrj.v23n1.57544 oil seepage is one of the most important characteristics of hydrocarbon formation, and understanding oil seepage is crucial for oil-gas exploration and the assessment of petroleum resources. remote sensing and geochemical methods have the same material and theoretical bases for extracting oil and gas information from underlying strata and the identification of media features. as an emerging exploration method, hyperspectral remote sensing is efficient compared with traditional geochemistry because it is a finer, and sometimes more directly quantitative method for determining the specific mineral anomaly content. hence, the use of both methods together is important. this paper describes the analysis of hyperspectral remote sensing data and the extraction of abnormal index information, including the level of carbonate alteration and the content of acidolytical hydrocarbons, pyrolysis hydrocarbons, headspace gas, and ferric and ferrous ions. the two methods have mutual authentication, and they are complementary and are useful in oil-bearing areas. when these methods are integrated, the acidolytical hydrocarbon index is the most effective geochemical index and is better at characterizing the oil field distribution than other indices. also, hydrocarbon geochemical anomalies occurring in oil fields generally show continuous distribution points and are consistent with oil reservoirs. consequently, a 3d model was established to comprehensively utilize hyperspectral remote sensing and geochemical data to determine the distribution of petroleum reservoirs efficiently as well as to delineate oiland gas-bearing prospects. there is great potential for determining oiland gas-bearing fields through the integration of hyperspectral and geochemical data. abstract keywords: oil seepage; hyperspectral remote sensing; geochemistry; anomaly; hydrocarbon; prospecting area. integrating geochemical anomaly and remote sensing methods to predict oiland gas-bearing areas in the yanchang oil field, ordos basin, china integración de métodos de anomalías geoquímicas y de teledetección para predecir las áreas gasíferas y petrolíferas en el campo yanchang, en la cuenca de ordos, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.57544 la filtración de crudo es una de las características más importantes de la formación de hidrocarburos y entender este fenómeno es crucial para la exploración petrogasífera y la evaluación de recursos petrolíferos. los métodos de teledetección y geoquímicos tienen las mismas bases teóricas y el mismo material de información para la extracción de gas y petróleo de las capas subyacentes y de identificación de las características del medio. como un método emergente de exploración, la teledetección hiperespectral es eficiente comparada con la geoquímica tradicional, ya que es un método más fino y, en algunos casos, más directamente cuantitativo en la determinación del contenido de anomalías minerales específicas. por consiguiente, el uso conjunto de ambos métodos es importante. este artículo describe el análisis de información de la teledetección hiperespectral y la extracción de un índice de información anormal que incluye el nivel de alteración de carbonato y el contenido de hidrocarburos acidolíticos, hidrocarburos pirolíticos, espacios de gas, e iones férricos y ferrosos. los dos métodos tienen una autenticación mutua, son complementarios y útiles en las areas petrolíferas. cuando estos métodos se integran, el índice de hidrocarburos acidolíticos es el índice geoquímico más efectivo y es el mejor para caracterizar la distribución del campo petrolífero. además, las anomalías geoquímicas de hidrocarburos que ocurren en los campos generalmente muestran los puntos de distribución y coinciden con las reservas de crudo. por esto se estableció un modelo 3d para utilizar la teledetección hiperespectral integralmente con la información geoquímica para determinar la distribución de las reservas petrolíferas eficientemente, al igual que para delinear los prospectos gasíferos y petrolíferos. existe un gran potencial para determinar los campos gasíferos y petrolíferos a través de la integración de información hiperespectral y geoquímica. resumen record manuscript received: 20/05/2016 accepted for publication: 21/09/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 79-86 jianming guo1,#,*, hailong fan1,2,#, xiangzeng wang3,#, lixia zhang3, laiyi ren3, yonghong he3, fangxia ma3, wangcai meng3, xiaoyin bai3, jintao yin3 1key laboratory of petroleum resources, gansu province/key laboratory of petroleum resources research, institute of geology and geophysics, chinese academy of sciences, lanzhou 730000, china. 2university of chinese academy of sciences, beijing 100049, china. 3yanchang petroleum co. ltd., xian 710075, china. #these authors contributed equally to this work. * corresponding author: gjm2001cn@yahoo.com g eo ch em is tr y 80 jianming guo, hailong fan, xiangzeng wang, lixia zhang, laiyi ren, yonghong he, fangxia ma, wangcai meng, xiaoyin bai, jintao yin introduction in general, oil and gas traps are not completely sealed, so their accumulation will cause a release of some of the oil and gas to the surface. when oil and gas are clearly visible on the surface, macroseepage traces will give invisible clues of oil and gas traps in the near-surface soil and sedimentary rocks. historically, the accumulation of seepage is closely related to the presence of oil and gas traps. therefore, a lot of oil and gas fields in the world have been drilled in leaky areas. in modern exploration projects, macroseeps are often seen as direct clues of source rocks and the formation of convincing evidence of a petroleum system in a given basin. the use of remote sensing for oil and gas exploration is an extraordinarily complex and systematic undertaking (panciera et al., 2009; van der kwast et al., 2009). a large amount of remote sensing data is required to interpret and analyze the micro-percolation, alteration, and geological basin structures, such as regional structures, strata, and tectonic belts, of oil-bearing areas (jiang et al., 2011; wang et al., 2015). geological, geophysical, and geochemical exploration methods are combined to detect oiland gas-bearing exploring prospects under the guidance of modern geological petroleum theory (yi et al., 2016; cheng and tong, 2015; kandpal et al., 2016; petrovic et al., 2012; sun, 2012). hydrocarbon percolation is a phenomenon in which petroleum and natural gas are accompanied by a dense layer of penetrating rocks (bateson et al., 2000; asadzadeh et al., 2016; cui et al., 2013). the hyperspectral trap area itself is a dynamic balance system (zhu et al., 2008; zheng et al., 2015; de carolis et al., 2014). hydrocarbon molecules penetrate through the overlying rock stratum in the process of migration under the influence of concentration gradients. under the effects of pressure gradients, hydrocarbons migrate toward the surface along pathways such as joints, porosities, and cracks (svejkovsky et al., 2012; clark et al., 2003; otremba et al., 2013). hyperspectral remote sensing exploration technology is based on reflectance spectrometry and oil and gas percolation theory, and is a straightforward approach to detecting oil and gas reservoirs. since the 1990s, it has been widely used in oil exploration in china and overseas. many studies in hyperspectral remote sensing have been conducted, and applied models have been established. the pioneering work in the early 1970s paved the way for the interpretation of remote sensing data using quantum mechanical concepts (wu et al., 2005). their work established a link between the reflectance change and the chemical and physical properties of minerals, focusing on its potential use in remote sensing (hunt et al., 1977; 1979). in 1998, researchers at west virginia university made use of a mineral alteration anomaly and successfully discovered an oil and gas percolation field (sabins et al., 1999; eismann et al., 2007; heylen et al., 2014). in 1999, utilizing hyperspectral instruments, german researchers identified an anomaly and determined the position of oil reservoirs from the spectral radiation characteristics of high signal-to-noise (snr) hymap pixels (goetz and rowan, 1981; gregory and moore, 1975; sabins et al., 1999). spanish researchers sought out underground oil-bearing areas in 2000 via the delineation of lithologic alterations and thermal anomalies caused by oil percolation (gu et al., 2010). chinese scientists successfully developed airborne hyperspectral the remote sensing phi (push-broom hyperspectral imager) and omis (operative modular imaging spectrometer) instruments in the 1990s, which have also been used in research applications (yang et al., 2000; zhou, 2014; liu et al., 2016). regarding soil adsorption, geochemistry, geothermal anomalies, and other factors, chinese researcher sought remote sensing indications after systematically summarizing hydrocarbon percolation (wang, 2010; chen et al., 2007). tian et al. determined the spatial distribution of oil percolation based on the oil and gas percolation of inner mongolia’s dongsheng area (tian et al., 2000; richards, 2005; liu et al., 2011; liu et al., 2015), using the diagnostic of mineral alteration absorption characteristics and field measurements of spectral curves (kruse et al., 2012). ni et al. used hyperspectral data to classify the natural gas alteration anomalies in an area of china and effectively extracted regional surface gas alteration characteristics with feature extraction based on the principal component analysis (pca) wavelet method (xu et al., 2007; ni et al., 2007). the general development of oil and gas applications of remote sensing technology in china and overseas indicates that hyperspectral remote sensing may be a favorable method for exploring new types of oil and gas pools (goetz, 2009; white et al., 2009; agar and coulter, 2007). research has demonstrated many reliable means of identifying additional oil reservoirs based on reflectance spectrometry and oil percolation theory, which have great significance for oil exploration (wang et al., 2011). regional geological conditions the ordos basin is a large oil and gas field, enriched in paleozoic coalderived gas (zhang et al., 2009; wang et al., 2014; zheng et al., 2006). sugeli, yulin, and other massive large-scale coal-derived gas fields have been identified (yao et al., 2013; zhu et al., 2013; wang and wang, 2013). the basin covers areas of the shaanxi, shanxi, and gansu provinces and two autonomous regions, ningxia and inner mongolia, covering an area of approximately 37×104 km2 in total (wang et al., 2013; li et al., 2010; liu et al., 2008). the basement is a metamorphic rock series formed in the archean to paleoproterozoic. in the mesoproterozoic to neoproterozoic, it changed to a faulted trough that was filled with shallow sea classic rocks and carbonate sediments (wang et al., 2008; hao et al., 2012). the distribution of oil and gas and the structure of sedimentary caprock were controlled directly by the pattern of the basement tectonics (liu et al., 2008; luo, 2008; duan et al., 2008). then, in the early paleozoic, it evolved to an infracratonic basin, and the sediment derived from an epicontinental carbonate platform. since the late paleozoic, a craton depression basin and a large inland depression basin successively formed due to lake and river sediment and eventually deposited thick loess in the cenozoic era (wu et al., 2015; liu et al., 2006; ren et al., 2006; zhang et al., 2013). as a multicycle superimposed basin, the ample petroleum resources of the ordos basin provide a wide realm of bright prospects for oil and gas exploration (zhao et al., 2006; wang et al., 2015). the general tectonic framework controls the formation of the depression and the distribution of depositional systems that are fundamental for reservoir formation conditions (liu, 2007; zhao et al., 2012). the southern part of the basin primarily produces petroleum, and the northern part produces gas (figure 1). oil production occurs in shallow areas of the basin, whereas gas production occurs in the deeper parts. four sets of oil-bearing systems that belong to the stratigraphic-lithologic reservoir have been exploited (deng et al., 2005). the oil-bearing reservoir systems are the sandstone of the triassic yanchang formation, the lower jurassic yan’an formation, the ordovician majiagou carbonate group, and the permocarboniferous coal-gas bearing system. the basin is ideal for the realization of the coordinated development of oil and gas in china (liu et al., 2003). there are many instances of oil and gas leakage in the ordos basin. the leakage above the oil and gas reservoir is in the form of a continuous or intermittent belt and in the area of fault development, the anomaly is distributed along the extension direction of the fault as a strip or string of beads. the landform in the survey area belongs to the loess plateau, providing a high degree of rich exploration, geological and geophysical data that can be used. the behavior of geochemical indices is unique in that area, so the experience gained from studying other areas can be put to full use. geochemical indices include: acid-hydrolysis methane, acid-hydrolysis heavy hydrocarbon, heat release methane, carbonate, altered carbonate, fe2+, fe3+, and some other indices. we fully excavate the information of hydrocarbon information indicated by hydrocarbons under different surface occurrences, verify the hyperspectral anomalies, and provide detailed geochemical data for further air and ground integrated oil and gas exploration. methods remote sensing technology with imaging spectrometry creates an image with spatial, spectral, and radiation information, thus allowing spectral analysis technology to reveal features directly (chen et al., 2016). using hyperspectral remote sensing technology can extract hydrocarbon percolation information such as vegetation lesions, mineral alteration, and micro-seepage from hyperfine spectra (ellis et al., 2001; lennon et al., 2006). in this study, the philosophies of remote sensing and petroleum exploration are combined to thoroughly understand oil and gas accumulation for comprehensive evaluation and full-scale planning. the core technology of the study includes remote sensing data processing and analysis, interpretation of geological information, and geochemical analysis. this study uses hyperspectral remote sensing images 81integrating geochemical anomaly and remote sensing methods to predict oiland gas-bearing areas in the yanchang oil field, ordos basin, china to comprehensively extract oil and gas percolation in the work area in addition to other methods indicated below. abnormal surface mineral spectral results were synthetically determined by collecting hyperspectral remote sensing images and extracting percolation information through preprocessing based on the surface spectral characteristic identification method of alteration minerals and comparative analysis. upon the completion of geochemical testing of typical areas and response relationship analysis between remote sensing and surface geochemical abnormalities, the study examined the accuracy of hyperspectral remote sensing. together, geochemical methods and remote sensing establish an integrated system that provides comparable means for oil and gas exploration. wave spectrum testing and surface matter analysis includes spectral measurements, filtering, principal component analysis, maximum noise component transformation, and curve filtering of transect data analysis (cao et al., 2003; lei et al., 2008; clark et al., 2003). according to the hyperspectral anomalies, typical areas were selected for geochemical testing; primarily, the tests involved microbial geochemical exploration, headspace gas examination, acid and alkali hydrocarbon methods, and fluorescent spectrometry (lo et al., 2015). the specific technical concept is as follows: starting from the geochemical field of the survey area, the geochemical behavior, the background field characteristics and the anomaly manifestations of the research methods in the adjacent area and other similar areas were compared. by using multivariate statistical processing and anomaly separation technology, we extract the concentration anomaly, fabric anomaly, and variation characteristic information of geochemical indicators, while removing the false anomalies formed by interference factors, and conduct comprehensive anomaly analysis. according to the petroleum geological significance and multivariate statistical analysis results of each index (table 1), we selected five indices—acid hydrolysis heavy hydrocarbon, alteration carbonate, heat release heavy hydrocarbon, top air heavy hydrocarbon, and fe2+/fe3+ as the main geochemical indicators. main research content typical regional geochemical testing the study area is located in the minhe basin, the juncture of fuxian and ganquan counties of shaanxi province. the area has unique topographic and geomorphic conditions and extensive vegetation (chang, 2013). the geochemical exploration deployed two study blocks, a and b, the former in fuxian county and the latter in ganquan county. this study focuses on the fuxian area. the main methods used include the analysis of acidolytical hydrocarbons, pyrolysis hydrocarbon carbonate, carbonate alteration, and ferric ions. after analyzing the validity of the geochemistry index, the study focused on the capacity of bearing oil and gas in key target areas and anomaly points, and verifying the hyperspectral anomalies. all of those indices are needed to protect and explore oiland gas-bearing areas. moreover, it is necessary to fully tap the potential indicated by hydrocarbons near the ground surface for oil and gas resources and to verify the data obtained from hyperspectral remote sensing images. after comparing the geochemical field of the study area and adjacent areas, the geochemistry index, background field characteristics, and abnormal appearances were studied. the analysis included multivariate statistical methods and other technologies that have been developed in recent years and have been used for anomaly separation. conjunctive use of these technologies can lead to accurate assessments of favorable oiland gas-bearing sectors while eliminating false anomalies caused by interference factors. after a study of the geochemical anomalies over oil and gas reservoirs, the response relationship between the oil field and geochemical anomalies was estimated, and the geochemical correspondence and distinction marks of oil and gas reservoirs were established both inside and outside the reservoirs. figure 1. geological map of the study area (wang et al., 2013) table 1. matrix of 12 geochemical index factors index f1 f2 f3 f4 f5 carbonate 0.2683 -0.5048 0.407 0.5443 -0.0169 altered carbonate 0.0565 -0.6889 0.1419 0.5719 -0.0001 acid-hydrolysis methane 0.2486 0.4562 0.6668 0.4156 -0.0181 acid-hydrolysis heavy hydrocarbon 0.0089 0.6924 0.4962 0.3984 0.0319 heat release methane -0.041 0.3971 -0.6297 0.4678 0.009 heat release heavy hydrocarbon -0.0132 0.5062 -0.4384 0.5588 -0.0026 fe2+ 0.8253 0.0433 -0.0507 0.1707 -0.0183 fe3+ -0.8717 0.0907 -0.1308 0.0761 -0.1115 top air methane 0.1758 -0.0629 -0.0865 0.1053 0.6646 top air heavy hydrocarbon -0.0407 0.0782 0.0833 -0.1113 0.7991 fe2+/fe3+ 0.9569 -0.0293 0.053 0.0319 0.0926 fe3+/fe2+ -0.948 0.0087 -0.0984 -0.0327 -0.0398 cumulative variance contribution 0.2856 0.4399 0.5659 0.6947 0.7868 82 jianming guo, hailong fan, xiangzeng wang, lixia zhang, laiyi ren, yonghong he, fangxia ma, wangcai meng, xiaoyin bai, jintao yin geochemical characteristics the characteristics of those indices are displayed in figure 2. the carbonate geochemical index presents a chaotic distribution compared with the distribution of a known oil field (jiang et al., 2013) (figure 2a). the methane and heavy hydrocarbon acidolytical indices mainly show a distribution area in the northeastern part of the study area. the pyrolysis hydrocarbon also reflects a high value in the northeastern part, consistent with the distribution of a known oil field (figure 2b, 2c). the headspace index of the geochemical field of methane and heavy hydrocarbons have a different distribution, with high values of methane mainly occurring in a circular formation in the northeastern part of the study area; the other concentration is in the southern part, with a sequential circular formation, while the northern part also contains high points (figure 2d). the geochemical characteristics of ferrous and ferric iron show a distribution pattern around the known site of an oil field (figure 2e). through analysis of various indicators of geochemical fields, it is apparent that the index follows a certain regular variation. for instance, the pyrolysis and headspace heavy hydrocarbons show a high dispersion before iterations were conducted because of the multi-source superimposition of the latter stage. to highlight the geochemical anomalies and dispose of the interfering factors of non-oil and gas composition, pyrolysis and headspace heavy hydrocarbons were adopted as trend values, the values for acidolytical heavy hydrocarbon and carbonate alteration were used as concentration values, and single-index geochemical anomalies were applied to determine the ferric iron and ferrous iron values. hyperspectral remote sensing interpretation the response of remote sensing and geochemical information compared with the proportion of acidolytical heavy hydrocarbons, the pyrolysis heavy hydrocarbon, headspace heavy hydrocarbon, and hyperspectral anomalies are less than that for geochemical acidolytical heavy hydrocarbon in the study field, but remote sensing data has better correspondence with oiland gas-bearing reservoirs (figure 3c). the carbonate anomalies of the remote sensing data and the carbonate alteration of the geochemical method are basically consistent in southwestern fuxian and show poor correspondence in the northeast (figure 3a). the percent of coverage area for iron anomalies is good in the northeast (figure 3b).. using special data processing technology, information regarding carbonate alteration, clay, iron-bearing mineral, and soil-absorbed hydrocarbons can be properly extracted, thus identifying abnormal areas quickly and providing a scientific basis for oil and gas exploration. in this study, different anomalies have different corresponding weight values. the proportion for the hydrocarbon anomaly is 0.3, clay is 0.25, carbonate is 0.25, and iron is 0.2. the value of the first-grade anomaly is greater than 0.7, the second-grade anomaly ranges from 0.5 to 0.7, and the third-grade anomaly ranges from 0.3 to 0.5. the study primarily considered first-grade remote sensing anomalies as the basis for assessment to determine prospective oiland gas-bearing areas. comprehensive anomaly analysis indicates that in southern fuxian, the response relationship of geochemistry and remote sensing is not as ideal as expected, while the anastomosis degree is good in the north. comprehensive abnormality index due to numerous index properties and other interaction factors, different indices have their own genetic types, and the position of geochemical anomalies may be different. to improve the reliability of using geochemical methods to estimate oiland gas-bearing reservoirs, it is necessary to use various methods and develop a comprehensive system that combines geochemical characteristics, the distribution pattern of high-value points, oil and gas traps, and other factors. the pi (polarization index) uses average values, maximum values, and the key index effectively in abnormality analysis and assessment. it involves many factor estimates and an enormous amount of information; thus, comprehensive evaluation using the pi index better reflects actual geological conditions in anomaly analysis. for instance, it can indicate oil and gas information objectively (yang et al., 2015). figure 2. geochemical anomalies (a. carbonate alteration, b. acidolytical hydrocarbon, c. pyrolysis hydrocarbon, d. headspace gas, and e. geochemical iron) figure 3. hyperspectral remote sensing anomalies (a. carbonate alteration, b. iron anomalies, and c. hydrocarbon index) 83integrating geochemical anomaly and remote sensing methods to predict oiland gas-bearing areas in the yanchang oil field, ordos basin, china a certain weight was assigned to each anomaly index: hydrocarbon anomaly 0.5, carbonate anomaly 0.3, iron anomaly 0.2. values > 0.7 belonged to the primary anomaly, 0.5 to 0.7 to the second-grade anomaly, and 0.3 to 0.5 to the third-grade anomaly. delineation of the far oil and gas scenic spots are mainly determined by the primary anomaly. the results show that in the area of the known reservoir, an anomaly belt shows an annular pattern with high strength and better continuity with a high degree of coincidence with a known reservoir (figure 4a), and in another area, the anomaly points are located around the reservoir (figure 4b). the interpreted results have good correspondence with the exploration results at this stage (figure 4). remote sensing and geochemical comprehensive model various models for oil and gas exploration have been put forth based on dispersion migration mechanisms and oil and gas micro-percolation (turner, 1980; ungerer et al., 1984; tissot et al., 2012; blake, 2013). the reservoir hydrocarbons generate negative ions in the process of migrating upward along with fractions and produce a redox battery effect with anodic areas and anode surfaces because of the potential difference (he et al., 2013; zhang et al., 2005; lu et al., 2013). the hydrocarbon migration to the surface ground causes changes in the redox environment of different types of anomalies; for example, remote sensing images exhibited fading when the ferrous iron had been transformed to ferric iron (wang et al., 2015; luo, 2011). a wealth of data has demonstrated that geochemical anomalies are not controlled by a single factor because of the complexity of petroleum dispersity migration, and investigators have established a vertical percolation model of hydrocarbons that considers the factors of the migration mechanism, impetus, channels, and mineral alterations. the research achievements above reflect the processes and results of hydrocarbon micro-percolation. along with the migration path of fracture, joints, and bedding planes, light hydrocarbons migrate toward the surface in a vertical direction driven by many types of action, occurring in a series of geochemical, geophysical, topographical and landform anomalies in annular patterns above oil reservoirs. using the available information, this study develops the oil and gas vertical percolation model in combination with comprehensive geochemical and remote sensing data (figure 5). discussion and conclusions by processing and analyzing hyperspectral remote sensing and geochemical data, this study extracted percolation information concerning oil and gas reservoir exploration, and established a comprehensive model. the main research findings are as follows: (1) hyperspectral remote sensing anomaly extracting technology was established for petroleum geology and surface conditions through the collection of abnormal information of carbonate alteration, hydrocarbons, and ironbearing minerals to ascertain abnormal prospective areas accurately. (2) geochemical blocks were deployed to collect samples for analysis of hydrocarbon percolation in the study area, and then, the response relationship with the reservoir was analyzed. the results demonstrated that the acidolytical hydrocarbon index is the most effective geochemical index, with a better correspondence with the oil field distribution compared with the headspace and carbonate alteration indices. generally, hydrocarbon geochemical anomalies occur above the oil field as a continuous distribution of points that is consistent with oil reservoirs. figure 4. comprehensive anomalies of the geochemical and remote sensing data figure 5. mechanism of oil (gas) reservoir migration and permeation. oil and gas seepage model, b. the surface of remote sensing anomalies, c. the surface of geochemical anomalies) 84 jianming guo, hailong fan, xiangzeng wang, lixia zhang, laiyi ren, yonghong he, fangxia ma, wangcai meng, xiaoyin bai, jintao yin (3) geochemical index and remote sensing data show that the hyperspectral exploration area coincides with the geochemical analysis at the surface and in the vertical profile. comprehensive anomaly analysis indicates that in southern fuxian, the response relationship of geochemistry and remote sensing are not as ideal as expected, while in the north, the degree of anastomosis is good. (4) an oil and gas vertical percolation model was developed with a comprehensive combination of geochemistry and remote sensing data on the basis of available acidolytical hydrocarbon, pyrolysis hydrocarbon, carbonate alteration, and ferric and ferrous ion information. the superposition of the geochemical index and remote sensing hydrocarbon anomaly shows that most of the planar hyperspectral remote sensing data are coincident with the geochemical exploration work area, and the longitudinal profile correspondence is more significant. however, there are also some regions, remote sensing anomalies, and geochemical indicators that do not show abnormal characteristics, so further studies are needed. combining remote sensing results with other non-seismic techniques not only provides new insight into the microleakage phenomenon, but also adds even more value to existing exploration data and increases the success rate of subsequent drilling. a complicated composite relationship of remote sensing hydrocarbon anomalies with known oil and gas reservoirs was developed. in some regions, good anastomosis indicates that hyperspectral data provides great practicability for oil exploration. however, in other remote sensing anomaly areas, oil and gas reservoirs were lacking; therefore, further investigation is necessary. acknowledgments we are grateful to two anonymous reviewers for their constructive reviews on the manuscript. this study is financially supported by the important direction project of knowledge innovation in resource and environment field and the chinese academy of sciences (kzcx2-ew-qn112). funding was provided by the key laboratory of petroleum resources, gansu province, institute of geology and geophysics, chinese academy of sciences (no. 135ccjj20160517; no. szdkfjj20170604). references agar, b. & coulter, d. 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(2013). sedimentary characteristics and model of shallow braided delta in large-scale lacustrine: an example from triassic yanchang formation in ordos basin. earth science frontiers, 20(2), 19-28. the study present the use of geographic information system (gis) to assess the geotectonic regimen of the sabinas basin. initially, the gis database was designed using geological and geophysical information (total magnetic field reduced to the pole). subsequently, 2d models of the basin were obtained along 4 profiles running on a north-south direction. the implementation of gis enables a geological-geophysical interpretation generating various thematic maps that overlap the magnetic map during interpretation. the analysis of overlapping maps enables the identification of areas with different geotectonic regimes in the sabinas basin, as well as the relationship between these environments and mineralization. the qualitative and quantitative interpretation of the aeromagnetic data obtained for the total magnetic field reduced to the pole delineate uplifted basement areas as well as shallower blocks within them. this paper presents the location of both the basin´s deepest zones and its probable faults, which can be linked to the block boundaries. the faults zones and block boundaries correspond to the main mineral deposits found in the basin. finally, the geotectonic regime of some sectors of the basin is detailed in order to reaffirm the results obtained by the interpretation of aeromagnetic data. abstract keywords:geotectonic regime; gis;aeromagnetic data; northeastern mexico; sabinas basin. interpretation of aeromagnetic data using gis to evaluate the geotectonic regime of the sabinas basin issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n4.57924 earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017): 175-181 josé a. batista rodríguez, yuri almaguer carmenates, josue d. martínez gonzález universidad autónoma de coahuila. méxico josebatista@uadec.edu.mx se presenta el uso de un sistemas de información geográfica para evaluar el régimen geotectónico de la cuenca de sabinas. inicialmente se llevó a cabo el diseño de la base de datos del sistema de información geográfica, teniendo en cuenta las características de la información geológica y geofísica (campo magnético total reducido al polo). posteriormente se obtienen modelos 2d de la cuenca a lo largo de 4 perfiles con direcciones norte-sur. a partir de la implementación del sistema de información geográfica, se logró una interpretación geológica-geofísica, generando diversos mapas temáticos que se superpusieron al mapa magnético durante dicha interpretación. el análisis de los mapas superpuestos permitió identificar áreas con diferente régimen geotectónico dentro de la cuenca de sabinas, así como la relación de esas áreas con mineralizaciones. a partir de la interpretación cualitativa y cuantitativa de los datos del campo magnético total reducido al polo se delimitan áreas levantadas del basamento de la cuenca y los bloques más someros dentro de las mismas. también se localizan las zonas más profundas de la cuenca y probables fallas, que pueden estar vinculadas con los límites de los bloques. las zonas de fallas y los límites de bloques están relacionados con los principales depósitos minerales de la cuenca. por último, se detalla el régimen geotectónico de algunos sectores de la cuenca, para reafirmar los resultados de la interpretación de los datos aeromagnéticos. resumen palabras clave: régimen geotectónico; sig; datos aeromagnéticos; noreste de méxico; cuenca de sabinas. record manuscript received: 08/06/2016 accepted for publication: 23/08/2017 how to cite item: batista, j. a., almaguer, y., & martínez j. d. (2017). interpretation of aeromagnetic data using gis to evaluate the geotectonic regime of the sabinas basin. earth sciences research journal, 21(4). 175-181. doi: http://dx.doi.org/10.15446/esrj.v21n4.57924 interpretación de datos aeromagnéticos usando sig para evaluar el regimen geotectónico de la cuenca de sabinas a pp l ie d g e o ph y si c s 176 josé a. batista rodríguez, yuri almaguer carmenates, josue d. martínez gonzález 1. introduction the efficient interpretation of geophysical data requires the knowledge and appropriate use of geological information, especially information represented on geological maps. magnetometry is a geophysical method usually used in regional geological studies. the surface and subterranean distribution of different rocks types and geological structures provide the basis for the behavior of magnetic field (telford et al., 1990). in sedimentary basins, the magnetic field is principally related to the crystalline basement, which is generally composed of igneous and metamorphic rocks which have a very high magnetic effect (vázquez et al., 1990; gunn, 1997). deformations in the sedimentary infill of the basin may be related to their basement dynamics, which are linked to faults that cause uplifted or depressed blocks (gunn, 1997). the magnetic method has been used for the mineral exploration in the sabinas basin, which is located in northeastern mexico (figure 1) (pascacio-toledo, 2001). the deposits of metallic and nonmetallic minerals found in this basin, comprise mainly coal, fluorite, celestine, pbzn-ag and hydrocarbons (eguiluz de antuñano, 2001; corona-esquivel et al., 2006; gonzález-sánchez et al., 2007, 2015). in the interpretation of magnetic data, it is very important to have the geological maps to identify the distribution of igneous rocks, geological structures and mineralized areas which can be linked to positive anomalies in the magnetic field. these geological maps also show the surface distribution of rocks that can be classified as diamagnetic, paramagnetic or ferromagnetic, according to their magnetic susceptibility values. once those geological characteristics are known, they can be the behavior of the magnetic fields. gis techniques are widely used to integrate geological and geophysical data, because they provide effective tools for managing geo-referenced information by enabling the correlation and comparison of different types of information. the graphical, numerical and text information can be combined using these tools (ziaii et al., 2010; khemiri et al., 2013). for example, in this study, fault locations (graphical information) with their respective names (text information), with values of magnetic anomalies (numerical data) can be combined, generating several maps of interpretation. this research presents an example of using gis, combined with other tools, to use maps to interpret geophysical data, taking as an example, the aeromagnetic data reduced to pole obtained for the nueva rosita area, in order to assess the geotectonic regime in this section of the sabinas basin, located in northeastern mexico (figure 1). 2. geological setting the study region is located in the sabinas basin in northeastern mexico (figure 1). this basin forms part of a rift associated with the opening of the gulf of mexico (eguiluz de antuñano, 2001). this structure originated as a figure 1. location of the study area (a section of the geological map of nueva rosita). light gray line indicates the border with the sabinas basin. modified from martínez-rodríguez et al. (2008). figure 2. structural configuration and tectonic features of northeastern mexico (coahuila and adjacent areas). rectangle indicates the study area. taken from chávez-cabello et al. (2005). rift-type graben limited by paleotectonic and paleogeographic blocks, such as the coahuila block to the southwest (sw), the tamaulipas block to the northeast (ne), and the burro-peyotes blocks to the north (n) (figure 2). the san marcos and la babia faults are deep regional structures that separate the blocks of the basin (chávez-cabello et al., 2005). the sabinas basin is filled with different types of sedimentary rock, principally limestone, sandstone, shale, conglomerate, limolite and gypsum, accumulated during the mesozoic and cenozoic (eguiluz de antuñano, 2001; gonzález-sánchez et al., 2007). in the late cretaceous, the laramide orogeny altered the sedimentary regime in the region and caused deformation in all sedimentary sequences (eguiluz de antuñano, 2001). this orogeny was also accompanied by various magmatic events which generated different types of igneous rocks during the middle eocene (basic, intermediate and acidic rocks), constituting some intrusive bodies that outcrop in the candela-monclova intrusive belt located to the southeast of the sabinas basin (chávez-cabello et al., 2011). the geological map of nueva rosita (martínez-rodríguez et al., 2008) is located in the sabinas basin (figure 2 and 3). outcroppings of sedimentary rocks are found in the entire area covered by the map, aside from the central-north part of the map, named esperanzas-kakanapo lava , in which volcanic rocks outcrop (martínez-rodríguez et al., 2008). various anticline and syncline folds are observed in this region, whose axes lie northwest-southeast (nw-se). also there are also several fault systems following the same direction. some of the identified anticline folds being heavily eroded, such as the obayos anticline (figure 3). a high level of erosion in this structure causes outcropping in the majority of the sedimentary mesozoic sequence (martínez-rodríguez et al., 2008). metallic and nonmetallic mineral deposits can also be found in this area (figure 3). the nonmetallic mineralization mainly comprises coal (corona-esquivel et al., 2006), barite, celestine, fluorite and hydrocarbons, whereas ag-pb-zn comprises the most significant metallic mineralization (gonzález-sánchez et al., 2007, 2015). the deposits of barite, celestine, fluorite and ag-pb-zn are mississippi valley-type (mvt) mineral deposits (gonzález-sánchez et al., 2007). 177interpretation of aeromagnetic data using gis to evaluate the geotectonic regime of the sabinas basin figure 3. simplified geochronological map of nueva rosita (modified from martínez-rodríguez et al., 2008). the black symbol indicates the location of mineral deposits of coal, barite, celestine and fluorite. the letters a and b indicate the uplifted blocks of the basement that correspond to surface deformations. figure 4. magnetic anomaly map of nueva rosita. black line indicates the location of 2d profiles (p1, p2, p3 and p4). the magnetic field is expressed in nanotesla (nt). 3. methodology the gis database structure was designed to include geological and geophysical information from previous research undertaken by martínezrodríguez et al. (2008) and sgm (2000), as well as, results of the interpretation of geophysical data. the structure of the database included the objects considered (geological information, tectonic, aeromagnetic data, etc.), as well as the type of object (areal, vector or point) its attributes (name, rock types, mineralization, values of the magnetic field, etc.) and type of field (integer, logical and character). subsequently, the input of geological and geophysical data was performed under the categories of object type, attribute and field type. the representation and management of all the information was made the geographical coordinates system. the geological and geophysical interpretation was carried out in the final stage of gis management, overlapping and comparing the set of geological and geophysical information used. the geological information is based on the geological map of nueva rosita to a scale of 1:250, 000 (martínezrodríguez et al., 2008). this geological map provided information pertaining to outcrops of geological formations, tectonic features and other mining-related information. the geophysical data includes total aeromagnetic field data taken on a 1:250,000 scale by the sgm (2000) using a g-b22a optically pumped cesium vapor magnetometer with a sensitivity of 0.01nt. the flight lines were measured on a north-south direction, with a spacing of 1000m and a height above average terrain of 300m. the geomagnetic field in the center of the measured area had an intensity of 47303nt, an inclination of 56o19´ and declination of 7o31´. initially aeromagnetic data were reduced to the pole using the methodology of both baranov (1957) and baranov and naudy (1964). once the gis design had been performed, the qualitative and quantitative interpretation of the aeromagnetic data and their results were also included in the gis (e.g. magnetic alignments). the limits of the stratigraphic formations and the location of faults, fold axes and mines were digitized on the geological map, using didger 3.02 and mapinfo professional 6.5 software. mapinfo and arcgis software were used to assemble and manage the gis. information was added to each of the attributes that characterize the objects of the gis during the assembly process. for example, the object geology formation was added information to its attributes, such as names, types of rocks, age, mineralization, etc. before beginning the process of managing the gis, the description and analysis of the magnetic map was performed using surfer software. a quantitative interpretation of aeromagnetic data using 2d forward modeling was performed during this first stage. this interpretation was carried out along 4 profiles on a north-south direction, thus located in the area in which the main anomalies are found, in order to calculate the depths of the basement and determine any possible subsurface structures. forward modeling was undertaken using mag2dc software (cooper, 2003) which calculates the magnetic field anomalies by means of the talwani method (talwani, 1965). this method calculates magnetic anomalies produced by arbitrarily twodimensional bodies, represented by irregularly shaped polygons in a crosssection. two groups of rocks were used in the modeling, the first composed by igneous rocks taken from the basement of the basin, with an average magnetic susceptibility of 13x10-3 si, while the second group includes the sedimentary rocks taken from the basin presented an average magnetic susceptibility of 0 si. subsequently, several graphical entities were defined on the magnetic map (contour lines, areas, alignments and points) entities which were exported into the mapinfo and arcgis software for inclusion in the gis database. the entities were then compared with surface geological information to determine their probable relationship with the behavior of the magnetic field. various thematic maps (e.g. rock types, geological age and tectonics regime) were also generated in the mapinfo and arcgis software. these maps were exported to the surfer software for comparison with the magnetic map. two sectors of the study area were selected in order to assess the detail of the geotectonic regime, using the results of the interpretation of the aeromagnetic data. the first sector is located in an area of uplifted blocks in the basement, while the second sector is located inside an area of depressed blocks in the basement. 4. results and discussion 4.1. use of gis to assess the geotectonic regime using aeromagnetic data reduced to the pole from the description and analysis of the aeromagnetic data reduced to the pole, positive anomalous zones are shown as elongated mainly on a nw-se direction (figure 4). this zone occupies the biggest area of the map (bounded by the contour line with the value 0 nt). as it is considered that the behavior of the magnetic field mainly correspond to the basement rocks of the sabinas basin (pascacio-toledo, 2001), this area is characterized by uplifted basament blocks (vázquez et al., 1990; gunn, 1997) composed of igneous and metamorphic rocks (pascaciotoledo, 2001; jacobo-albarrán et al., 2011). 178 josé a. batista rodríguez, yuri almaguer carmenates, josue d. martínez gonzález the 2d models obtained from the 4 profiles show the depths of the basin as varying from 1.5 and 9 km. the deepest zone of the basin is located at the southern end of the p4 profile, while the shallower zone is located in the p2 profile, in the obayos area. the basement geometry of the basin suggests several faults, which suggest the probable presence of graben and horst. this later structure may be related to the uplifted basement blocks (figure 5). these results suggest that the 160 nt are contour lines delimit the areas of the basement where the blocks themselves are presumed to be shallower; whereas the contour lines with the value 240 nt indicate the shallower parts of these blocks. theoretically, the negative values for the magnetic field should correspond to with the deep zones of the basin (vázquez et al., 1990), namely the depressed blocks of the basement. using the 2d models (figure 5) revealed that the areas with negative values were delimited by isolines of -80 nt, which should define the deepest parts of the basin (figure 4). within these areas, the isolines with a value of -100 nt indicate the areas of the basin with the greatest thickness of sedimentary rock, thus indicating that the basement rock is deeper. figure 5. 2d-models of the sabinas basin. arrows indicate the position of the isolines of magnetic field. location of p1, p2, p3 and p4 are indicated in figure 4. figure 7. map of rocks type (sedimentary and igneous rocks) in the nueva rosita region. light gray color represents sedimentary rocks and the dark gray color represents igneous rocks. lines indicate the contour lines for the magnetic field (black is positive and gray is negative). black symbols indicate the location of coal mines and gray symbols indicate the location of barite, manganese and pb-zn mines. figure 6. shaded relief map of the aeromagnetic data for nueva rosita. dashed lines indicate magnetic alignments. black lines indicate faults and axes of fold (see the legend for figure 3). black symbols indicate the location of coal mines and white symbols indicate the location of barite, manganese and pb-zn mines. p1, p2, p3 and p4 indicate position of 2d profiles. the shaded relief map of the aeromagnetic data shows various alignments along the nw-se and ne-sw trajectories (figure 6). most alignments can also be identified in the 2d models (figure 5), alignments which have been associated with deep faults delimiting the basement blocks (cruz-falcón et al., 2010). contour lines and magnetic alignments were exported to the gis and subsequently compared with geological information (e.g., outcrops of different types of rocks, faults and location of mineral deposits). the mapinfo software shows the exported vectors overlapping on the geological map (figure 3). the comparison of the geological and geophysical information allows the causes for the behavior of the magnetic field to be inferred. the shallower blocks of the basement sometimes cause changes in the sedimentary cover, as seen in the blocks located in the central and nw end of the study region and identified by the letters a and b in figure 3. both of these blocks form part of the obayos anticline and the santa rosa range, with the shape and direction of block a forming part of the obayos anticline and following the direction of the fold axis. the nw end of this geological structure is truncated by block b, which follows a different direction (ne-sw) similar to the santa rosa range. the relationship between the characteristics of the magnetic field described here and the geological structures suggest a remobilization of the basement during the laramide orogeny, in which various deep fault systems were reactivated, thus dividing the basement into blocks. some of these blocks were uplifted, causing deformations in the sedimentary filling of the basin. the deepest zones of the basin are located south of obayos and on the se and nw edges. the areas delimiting these zones and the uplifted blocks of the basement correspond to magnetic alignments. these alignments suggest the probable location of deep fault systems which are likely to be the cause of the geotectonic environment described here. generally, surface indications of the presence of these faults cannot be found. it is likely that these deep structures affect the sedimentary cover at its lower and medium levels. while these structures could perhaps affect all sedimentary sequences, later surface indications were covered by more recent sedimentary deposits. the comparison of the magnetic map with the map of rock types shows that the basalts have little influence on the behavior of the magnetic field (figure 7)as they do not generate a magnetic anomaly due to their very low thickness (an average of approximately 3 meters). these outcrops are located on the edges of uplifted blocks and apparently correspond to faults that follow the alignments of the magnetic field (figure 3 and 6). such faults may constitute pathways through which lava spills occurred in this area, volcanic events which occurred during the pliocene-pleistocene (martínez-rodríguez et al., 2008) and indicate the recent active nature of these presumed faults. 179interpretation of aeromagnetic data using gis to evaluate the geotectonic regime of the sabinas basin figure 8. map of mesozoic and cenozoic rocks of the nueva rosita region. dark gray color represents mesozoic rocks and light gray color represents cenozoic rocks. gray and black lines indicate the contour lines for the magnetic field (black is positive and gray is negative). table 1. classes of the magnetic field map. figure 9. geotectonic regime map of the sabinas basin. rectangles delimit the zones for examples 1 and 2. table 2. spatial distribution of the shallower area of uplifted blocks in the sector for example 1. according to the locations of the main geological structures (folds and faults), the axes of the big anticlines are located on the edges of what appear to be uplifted blocks (figure 6). sometimes these axes of folds and small sections of faults correspond to the nw-se magnetic alignments. with figure 6 showing all barite, manganese and pb-zn mines as located on the edges of the supposed blocks and apparently corresponding to the faults, the magnetic map shows one important feature of these mvt-type mineral deposits, namely their proximity to large fault systems (gonzález-sánchez et al., 2007). furthermore, most of the coal mines are located on the edges of these blocks or in areas of the basin of intermediate depth. this result demonstrates the effect of the remobilization of the basement on the structural configuration of the region, a remobilization which caused shallow coal seams to be formed. comparing the ages of the rock with the magnetic map (figure 8) reaffirms the relationship between the basement and the tectonic deformations, as most of the areas with high magnetic field values and corresponding to the uplifted blocks of the basement are surrounded by outcrops of mesozoic rocks, generally very folded and eroded. 4.2. details of the geotectonic regime in two sectors of the sabinas basin. using the above results, this section presents the details of the geotectonic regime in two sectors of the sabinas basin, sectors referred to hereafter as example 1 and example 2 (figure 9). detailed analysis was undertaken in these sectors to confirm the results of the interpretation of the aeromagnetic data, which suggested uplifted and depressed basement blocks. the following analytical steps and results are described below. 4.2.1. management of databases initially the magnetic field data were interpolated to create a map in raster format, which was classified into five classes of relevance taking into account the different ranges of the magnetic map indicate by the contour lines in figure 4. these contour lines delineate the areas with different levels of the tectonic regime of the basin. the map depicts each of the classes and the different attributes of the tectonic regime, as well as the corresponding effect on the basin (table 1). classes 1 and 2 present negative values for the magnetic field and include the zones of the basin in which predominates the tectonic regime of subsidence, which is recorded at great depths, mainly in the zones of corresponding to class 1. the zones presenting an uprising tectonic regime are indicated in three classes, with positive values for the magnetic field that indicate a different order of uprising for the basement blocks. the characteristics of some of these areas are detailed in the two examples shown below. 4.2.2. example 1. the geotectonic regime map in the sabinas basin (figure 10) indicate that the sector represented by example 1 is located within a positive magnetic field anomaly (figures 3 and 9) representing one of the sectors of more uplifted blocks of the basement in the basin. in the inside of this sector is also delimited the shallower area of the block. the spatial analysis results for the shallower area of the uplifted block in this sector are shown in table 2. the combination of geotectonic regime map with the geological map indicates that in this area predominate cretaceous rocks on the surface (figure 10), with mainly late cretaceous covering more than 69% of the area (table 2). this indicates uplift and the erosion of the youngest geological formations, according to the results of other studies (buccolini et al., 2014; wu et al., 2014). this uprising regimen is corroborated by the presence of holocene colluvial sediments formed by piedmont deposits covering 27% of the area, with the few alluvial deposits identified in the area revealing a sector of uprising, where erosion processes predominate compared to cumulative processes. this approach is validated by other research results (shikakura, 2012; medvedev and harst, 2015). 180 josé a. batista rodríguez, yuri almaguer carmenates, josue d. martínez gonzález 4.2.3. example 2. the sector illustrated by example 2 is located in a negative anomaly of the magnetic field (figures 3 and 9), associated with zones of subsidence of the basin. in this sector the magnetic field values below of -100 nt delimit the deeper area of the sector, i.e. larger thick sediments. table 3 shows the results of the spatial analysis conducted on the deeper area found within the sector. according to the combination of geotectonic regime map with the geological map it is noted that 91 % of the area is covered by holocene deposits, i.e., alluvium, colluvium and conglomerates, as well as a small area of cretaceous rock (figure 10). these attribute indicate a regime of tectonic stability with a prevalence of cumulative erosion processes (shikakura, 2012; medvedev and harst, 2015). figure 10. combination of geotectonic regime map with the geological map in the sabinas basin. the legend of the geological map is shown in figure 3. rectangles delimit the zones for examples 1 and 2. table 3. spatial distribution of the area with greater thicknesses of sediments in the sector for example 2. 5. conclusions the analysis carried out in this study highlights the importance of the use of gis for the interpretation of geophysical data, particularly data pertaining to the total magnetic field as used to characterize the geotectonic regime of a basin. the interpretation of the magnetic data was carried out both qualitatively and quantitatively, with the latter interpretation made using 2d models. the results of this study enabled the following: an analysis of the magnetic field; the generation of graphical entities; the definition of areas with certain geological significance, such as areas thought to correspond to the uplifted blocks of the basement in the sabinas basin and its deeper areas; and, the identification of the alignment of the magnetic field that probably correspond to fault systems. the nature of the magnetic anomalies was assessed by comparing these graphical entities with geological information by means of gis. a remobilization of the basement in the sabinas basin during the laramide orogeny is evidenced from the relationship between the magnetic field and the geological structures. this relationship suggests that, during the orogeny, various deep fault systems were reactivated, thus dividing the basement into blocks which were uplifted, causing deformations in the sedimentary filling of the basin. the development of thematic maps using gis, such as maps drawn based on rock type, age and tectonics, enables an accurate and precise comparison with the aeromagnetic data. the main structural deformations observed on the surface, as well as the principal areas with mineral deposits and lava spills, correspond to the edges of the presumed uplifted blocks of the basement. the results enabled researchers to detail the geotectonic regime in two sectors within the study area, which are linked to uprisings and subsidence in the basin. a prevalence of erosion processes were identified in the sector where uprisings were recorded in the basin whereas the cumulative processes predominate in the sector showing subsidence. the analysis conducted here on the tectonic regime in two sectors supports the concept pertaining to the dynamics identified in the basement blocks, which indicated uplifted and depressed blocks, suggested by the interpretation of the aeromagnetic data. acknowledgements we thank to the autonomous university of coahuila for the support it has provided for this research. references baranov, v. 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(2014). impact of wind erosion on detecting active tectonics from geomorphic indexes in extremely arid areas: a case study from the hero range, qaidam basin, nw china, geomorphology. 224, no. 1, 39-54. ziaii, m., abedi, a., ziaei, m., kamkar-rouhani, a. and zendahdel, a. (2010). gis modelling for au-pb-zn potential mapping in torud-chah shirin areairan, international journal of mining & environmental issues. 1, no. 1, 17-27. issn-0121-2974 la funcion caracteristica de punto de hamilton y la funcion de tiempos de difraccion de la sismica geometrica en la migracion con amplitudes verdaderas luis alfredo montes vides profesor asociado, departamento de geociencias-universidad nacional de colombia montes l. (2000): la funci6n caracteristica de punto de hamilton y la funci6n de tiempos de difracci6n de la sismica qeometnca en la migraci6n con amplitudes verdaderas. geofis. colomb. 4:67-71. issn 0121-2974. santa fe de bogota, d.c. colombia resumen usando la teoria del rayo paraxial en la aproximaci6n de segundo de los tiempos de transite, podemos estimar la funci6n de tiempos de transito asociada a un frente de onda proveniente de un reflector, y en consecuencia calcular la funci6n de tiempos de difracci6n de un frente de onda generado por un punto difractor situado en el reflector. ei conocimiento numerico de estas funciones hace posible un proceso de migraci6n en tiempo de secciones apiladas que generan irnaqenes de secciones sismicas con amplitudes verdaderas. la restituci6n de la amplitud de la onda se realiza mediante un operador que es funci6n de los parametres de la funci6n tiempo de transito. a las secciones migradas se les aplica un filtro de diferenciaci6n para obtener las fases iniciales del campo de onda. se usan modelos de capas separadas por interfaces suaves. abstract using the paraxial ray theory in the second order approximation of the traveltimes, we can estimate the traveltime function of a wavefront coming from a reflector, and in consequence calculate the diffraction traveltime function associated with a diffractor point located on the reflector. the numerical knowledge of those two functions makes possible a process to migrate stacked sections in time, process which generate images of seismic sections with true amplitudes. the restitution of the wave amplitude is made using an operator which depends of the parameters of the traveltime function a differentiation filter is applied to the migrated sections in order to obtain the original wave phase. layered models with smooth interfaces are used. 1. introduccion del tiempo, en la cual no se requiere conocer el campo de velocidades, y en su lugar se precisa calcular la funci6n caracteristica de punto de hamilton para la sismica. esta funci6n puede ser estimada registrando al menos nueve (9) tiempos de transito en la cercania de un rayo normal (montes, 2000). a la secci6n migrada se ie aplica un operador que permite remover del campo de onda los efectos por dispersi6n qeornetrica, la cual representa el factor mas importante en la modificaci6n de la amplitud y fase. en el proceso de secciones sismicas, la estimaci6n de las velocidades para las diferentes etapas es una labor dispendiosa; sin embargo, es de vital importancia para lievar a cabo la migraci6n tanto en tiempo como en profundidad. usando la aproximaci6n de segundo orden en los tiempos de transite, desarrollada por bortfeld (1989) y usando las ideas planteadas por bortfeld & kiehn (1992) se presenta una' metodologia para migrar secciones sismicas apiladas (post-stack) en el dominic geofisical:oi.lombiana\n°4/marz~;e) 2. funciones de tiempos de transito ei sistema sismico se define como un apilado de capas hornoqeneas e isotropicas separadas por interfaces suaves. se llama superficie anterior a la interface limitrofe superior del modele donde se situan fuentes y geofonos, coincidiendo con la superficie terrestre que se considera plana. la interface mas profunda donde tiene lugar la reflexion se llama superficie posterior, y establece un limite inferior del modelo. se identifica un rayo que al atravesar el sistema sismico incida normalmente sobre la superficie reflectora (rayo normal), el cual se denomina rayo central (axial). este rayo central establece el sistema de referencia de los otros rayos en su vecindad (rayos paraxiales). ei sistema de coordenadas cartesiano xyz tiene origen en el punta f;" el plano xy de este sistema es tangente a la superficie anterior en el punta de origen f;,. la interseccion del rayo central con la superficie posterior define en el reflector el nuevo sistema de coordenadas cartesiano x'v z', con el plano x'y' tangente a la superficie reflectora en el punta p,; . un rayo paraxial en la proximidad del rayo central, parte del punta p indicado por el vector posicion x en la superficie anterior, y viaja hasta el punta p' indicado por el vector posicion x' sobre el reflector. ei rayo paraxial sobre la superficie anterior en x se describe por el vector de lentitud p (slowness vector) y en x'sobre la superficie posterior por el vector de lentitud p'. cualquier rayo paraxial queda definido en la superficie anterior por la pareja de vectores (x, p ), y sobre la superficie posterior por la pareja ( x' ,pi ) para la slmplificacion de los calculos, se usan las proyecciones de los vectores posicion y lentitud sobre los pianos xy en la superficie anterior y x'y' en la posterior. por c1aridad los vectores 20 se denotaran en minuscula y los vectores 3d en rnayuscula. en la superficie limitadora superior el vector x corresponde a la proyecci6n de x sobre el plano xy. para obtener p se proyecta primero p sobre el plano tangente a la superficie anterior en el punta p, y el vector resultante se proyecta luego sobre el plano xy en la superficie posterior se obtienen x' y p' al proyectar del mismo modo x' y p sobre el plano xy. ei rayo transmitido se refleja sobre la interface posterior en la posicion final x' con vector lentitud p' y de acuerdo a la aproximaci6n de segundo orden de los tiempos de transite (bortfeld, 1989), estos vectores estan relacionados con los vectores posicion inicial x, lentitud inicial del rayo paraxial p y lentitud inicial del rayo central po ' en la superficie anterior a traves de la ecuacion: x' .l'l.x 0 + 1l (p po) lx 0 + d (p po) (1) p' 11 = 3(.~,y') =,.r = 3(x',y') , 3(x,y) 3(p,q) con: c = 3(p',q') y d = ~(p',q') 3(x,y) 3(p,q)' matrices jacobianas estimadas en el origen del sistema de coordenadas sobre la superficie anterior. la aproximaci6n es valida unicarnente dentro del range de distancias para correcci6n de tiempo en la aproximaci6n hiperb61ica (normal move out) usado en el apilamiento (stacking) de secciones sisrnicas. las ecuaciones (1) describen cualquier rayo que viaje desde la superficie anterior hasta la posterior, en la vecindad del rayo central. ei tiempo de transite para un rayo transmitido desde x en la superficie anterior hasta x' en el reflector viene dad~ por: t(x,x') = to po . x + t-\"· db.-ix' + tx . .b.-i & -»«'r (2) ei tiempo de transite para un evento de reflexi6n sera la suma del tiempo de viaje del rayo transmitido desde .x hasta ;' y del tiempo del rayo reflejado desde x' hasta el punta x" , siendo esta suma: " i" -1 i "l(x,x)=!q-2po·xo+2(x +x)·d c2(x +x) 1" -i i "+2(x -x)·jl. a"2(x -x), (3) donde to = 2to es el tiempo de transite de reflexi6n del rayo central (bortfeld, 1989). la expresi6n (3) corresponde a la funci6n caracteristica de punta de hamilton en optica, liamada asi porque suministra el tiempo de trans ito desde un punta en la superficie anterior hasta otro cualquiera sobre la interface reflectora. las matrices producto ji-i il y d-ic son sirnetricas (una demostraci6n de esta propiedad puede verse en bortfeld, 1989); la primera representa la matriz de curvatura de la onda nip (normal incidence point) y la segunda la matriz de curvatura de la onda normal (hubral, 1983). por tal simetria la ecuaci6n (3) tiene solo nueve incognitas: una en to, dos en po' tres e b-ia y tres d-ie; las cuales pueden estimarse registrando al menos nueve tiempos de transite especialmente seleccionados (montes, 1998). ei tiempo de transito desde un punta difractor en x' = 0 hasta un detector situado en x sobre la superficie anterior viene dada por el doble del tiempo calculado por la ecuaci6n (2): y se denomina funci6n tiempo para una difracci6n zerooffset. esta ecuaci6n es importante para la surnatoria en el proceso de migraci6n. sequn los tiempos dados por la curva de tiempos de migraci6n (4) (hiperbola en fig.1), se seleccionan las amplitudes de la secci6n que seran sumadas. t / ...... x figura 1. la curva de tiempos zero-offset tiene la forma del reflector y la hiperbola representa la funcion de tiempos de difraccion. en la posicion donde emerge el rayo normal al reflector (central) las curvas son tangentes. el rayo imagen (normal a superficie) emerge en la posicion del apice de la hiperbola. de acuerdo al modelo de reflector explosivo (loewenthal et ai., 1976), cada punta del horizonte reflector se comporta como un punta difractor en concordancia con el modele de transmisi6n de ondas de huygens. la envolvente de los eventos difractores constituye el frente de onda definido por la funci6n tiempo zero-offset. en la fig.1, se representa por la curva de punta y trazo. ei resultado de la suma se coloca finalmente en el apice de la curva de migraci6n (minimo de la hiperbola). cuya posici6n coincide con el rayo imagen que emerge perpendicular a la superficie limitrofe superior. a traves de este procedimiento se lieva a cabo la transformaci6n del tiempo de transite del rayo normal (central) a tiempo de transite del rayo imagen. la superficie definida por la funci6n tiempo de difracci6n (4) es tangente a la superficie definida por la funci6n tiempo zero-offset en la posici6n del rayo central. en la fig.2, la hiperbola es tangente a la curva zero-offset. si se migrase hacia una posici6n arbitraria x, sequn (4) el tiempo de migraci6n sera: u s , = to po 'x (5) donde x = (b 0 a t i (p 0 p x) (6) y p debe satisfacer v'tj)(x,o) = -2px' la migraci6n en el apice de la superficie de tiempo de difracci6n corresponde al rayo imagen definido por hubral & krey (1980), el cual satisface la donde px debe satisfacer condici6n px = 0 (el rayo emerge perpendicular a la superficie limitrofe superior). en el apice el tiempo de migraci6n queda definido sequn la ecuaci6n (5) par: t(xa)=t()-pu'x (7) donde x .1 3. migracion en tiempo para comprender la migraci6n en tiempo se usara la fig.2, que muestra la funci6n tf) (x m) de tiempos de transito para una secci6n zero-offset no migrada, asi como la funci6n de tiempos de difracci6n tho (xm), sus curvas son tangentes en el punta 0 que corresponde a la posici6n donde emerge el rayo normal (central). las amplitudes de las trazas de la secci6n zero-offset coincidentes con la curva tf) (xm) se seleccionan y adicionan, y la suma se coloca en el apice de la curva 0', ei campo de onda de una secci6n zero-offset sequn bortfeld & kiehn (1992) se expresa por: rc: coeficiente de reflexi6n, d, ti,: coeficientes de transmisi6n de las ondas descendentes y ascendentes respectivarnente. muestrear las trazas de fa secci6n zero-offset con la funci6n de difracci6n, implica que t = tj) (xm) en (8), asi sine (tf) (x iii) t i/o (x iii)) tornara un valor significativo cuando el argumento de la funci6n sea cero, v.g. donde ambas funciones coinciden, indicando que la muestra de la traza pertenece a un punta de reflexi6n, fig.2. j{) distancia figura 2, muestra la curva de tiempos de transito de reflexion tro, tangente a la curva de tiempo de difraccion to en el punto d donde emerge el rayo central. la traza sera migrada de dad', se observa la curva de tiempo de difraccion desfasada tor' en configuraci6n zero-offset el tiempo de reflexi6n vendra dado por: (9) sustituyendo (3) y (9) en el argumento de la funci6n sine incluyendo un desfase t se obtiene: ei resultado de la expresi6n matricial en (10) es una matriz sirnetrica de la forma: (11) ai sustituir(11) y xm = (x,y) en (10): n--: {n 2 2)u(xm,r)= diuiisin /)"t(r+x mll+2xym12+y mn) (12) para migrar un evento de x m hasta s a se suma el campo de onda de todas muestras de las trazas contenidas en una ventana definida en plano xy: r n m u(xa,-r)= i1d;u;_c i i sinc(~(r+x2mll l ,=-n)=-m !'!..t + 2xy~2 + y2m22)i!'!..xj!'!..y (13) l1x, /)"y : intervalos de muestreo en direcci6n x,y. aplicando la transformada de fourier en (13) para lievar al dominic de la frecuencia y tomando la expresi6n integral de la misma se tiene: eo co -co-co (14) para simplificar (14) se aplica transformaci6n de los ejes hacia los ejes principales (x',y) u(xa,f) = ndiu, rc /)"t ilexp(i2nf(ax' + fjy'))dx'dy' l -00-00 (15) por la notaci6n de euler (15) es: u(xa,!) = ti diu; ~2fj"t * l j {j[cos( 2nfax' ) + i sen( 2"jax" )]dx')}' [cas( 2;rr!fjy'2) + i sene 2;rr!fjy'2 )]dy' (16) r 2 r 2 1 r;las integrales cosy dy = seny dy = 2" ~"2 son conocidas como integrales de fresnel, por 10 cual la expresi6n (16) se transforma en: (17) la ecuaci6n (17) representa el campo de onda migrado, al cual no se ie ha hecho correcci6n por dispersi6n qeometrica, ni por cambios en la fase. de acuerdo a los resultados obtenidos por bo rtfe i & kiehn (1992), la dispersi6n geometrica viene dado por el factor: l = 2cosfjo vo~det(b-1 a d-1c) (18) con v0 velocidad de la capa superior del sistema sismico, po anqulo entre la normal a la superficie superior y el rayo central. reemplazando (18) en (17) y usando el hecho que ap = det!b-1 a d-1ci = mil ml2 mf2 resulta en: v(x i) = od.u.r ( vo!1~ sgn(f)] a' i i c 8cosfjo il/l (19) en la expresi6n (19), el termino entre parentesis representa un operador que representa el factor de dispersi6n geometrica. ei inverse del operador, el cual permitira recuperar la amplitud de la serial por efecto exclusivo de la mencionada dispersi6n, es sequn ('19): fdg1 8cosfjo ( j 4i (j))= -ii; isgn v/:::r (20) en la ecuaci6n (20), la expresi6n entre parentesis corresponde al inverso del operador: sgn( i)¢ = illi (21) que introduce un desfase en el campo de onda y por tanto modifica el espectro de fase. su aparici6n es consecuencia de la integraci6n en el proceso de migraci6n.!(t) = jf(w)e-iw1dw por 10que para remover su efecto se requiere aplicar su operador inverso, es decir, el operador de diferenciaci6n i!f1 ' dado a traves de: !f(t)=f-iwf(w)e-iwldw. -i=eos~-isen3'-, dt 2 2 entonces il/l sgn(f) corrige la fase del campo de onda migrado en ~. como la parte real del espectro de fase 2 es nula: a(f) = ~ r2(f) + 12(f) = )12(f) = :/(f} y su espectro de fase es entonces: fj(f) = arctan( l(f») = _ 1r sgn(f) r(f) 2 (22) donde i r son las partes imaginaria y real del campo de onda. ei operador de restituci6n del campo de onda migrado consta del filtro de diferenciaci6n multiplicado por un factor de peso, que sequn la ecuaci6n (20) es 8eos po l(vo8.t), el efecto debido a la dispersi6n qeornetrica se remueve de la secci6n sismica multiplicando el operador en el dominic de la frecuencia antes 0 despues de la migraci6n. conclusiones usando ta teoria del rayo paraxial sobre modelos con capas hornoqeneas e isotr6picas separas por interfaces suaves, las secciones sismicas apiladas pueden ser migradas en el tiempo utilizando un novedoso enfoque que no necesita saber el valor del campo de velocidad. como es sabido del proceso de secciones sismicas, la estimaci6n del campo de velocidad conlleva una gran incertidumbre sobre los valores obtenidos, por 10 cual las irnaqenes de secciones migradas tienen un nivel de confiabilidad acorde con los niveles de incertidumbre valores estimados de la velocidad. la estimaci6n de la funci6n de tiempos de transite asociadas a eventos de reflexi6n permite estimar la funci6n de difracci6n y aplicar un proceso de migraci6n al que adicionalmente se ie incluye la restauraci6n del campo de onda por el efecto de dispersi6n qeometrica, el cual es el factor que mas influye en la modificaci6n de la amplitud y fase de las senai registrada. la migraci6n a traves de este proceso seria de utilidad para verificar y refinar las irnaqenes de secciones previa mente apiladas y migradas en tiempo, obtenidas a partir de un campo de velocidad de migraci6n estimado con incertidumbre. referencias bibliograficas bortfeld, r., (1989): geometrical ray theory: rays and traveltimes in seismic systems: geophysics. 54:342-349. bortfeld, r., & m. kiehn (1992): reflection amplitudes and migration amplitudes (zero-offset situation): geophysical prospecting. 40:873-884. hubral, p. & krey, th., (1980): interval velocities from seismic reflection time measurements. seg.tulsa. ok. hubral, p., (1983):computing true amplitude reflections in a laterally inhomogeneous earth. geophysics. 48: 10511062. loewewthal, d., l. lu, r. robertson, & j. sherwood, w., c. 1976: the wave equation applied to migration: geophysical prospecting. 24:380-399. montes, l. a. (1998): deterrninacao das velocidades intervalares usando a teoria paraxial do raio: aproxirnacao de segunda ordem dos tempos de transite. ph.d. thesis. ufpa, brazil. montes, l. a. (2000): un analisis de sensibilidad en la estimaci6n de la funci6n caracteristica. de punta de hamilton. geofis. colomb. 4:31-36. santa fe de bogota. geofisica colombiana n°.7 pp 49-54 diciembre de 2003 bogota, d.c. issn 0121 2974 casos hlstorlcos colombianos del registro de anomalias del radon-222 antes de eventos sismicos de naturaleza tectonlca doris yaneth serna valencia, estela catalina moran rooriguez, gustavo garzon valencia instituto de investigaci6n e informaci6n geocientifica, mineroambiental y nuclear -ingeominasobservatorio vulcanol6gico y sismol6gico de manizales monica lucia hernandez silva departamento de quimica, universidad de narino resumen desde el afio de 1995 se han estado realizando medici ones continuas de las emisiones del isotope radiactivo radon-222 en los suelos de los volcanes galeras y nevado del ruiz, y sobre fallas activas en los departamentos de narifio, cauca y caldas. en el monitoreo del gas radon-zz? se ha utilizado el sistema de medicion de camaras ionizantes e-perm, atrapando los gases en el horizonte b de los suelos. se ha encontrado que en zonas de fallamiento activo las emisiones de este gas estan entre los 1000 y2500 pcill. en algunos cruces de fallas activas se han medido niveles hasta de 25000 pci/l. en el presente trabajo se presenta una cornpilacion de ejemplos de las emisiones anornalas registradas en varias estaciones antes de sismos de caracter tectonico. se muestran anomalias del radon registradas antes de: i) eventos de magnitudes entre 2 y 4; 2) el terremoto del quindio de enero de 1999, y 3) la ocurrencia de enjambres sismicos registrados por las redes sismologicas de los observatorios vulcanologicos y sismologicos de pasto y manizales. palabras clave: isotopo, radiactividad, raoon-222, sis mo. tectonica abstract since 1995 it had been making continuous measurements of the radioactive isotope radon-222 emissions in soils at galeras and nevado del ruiz volcanoes; and on active geological faults in narifio, cauca and caldas districts. the e-perm ionizing chamber system has been used in the radon-222 monitoring, trapping this gas in the b horizont of soils. in zones of active faulting it has been stablished radon soil emissions between 1000 and 2500 pci/l. in some crossings of active faults it has been measured levels of25000 pcill. in the present work appears a compilation of examples of the registered anomalous emissions in several stations before earthquakes of tectonic character. examples of registered radon anomalies before: i) events of magnitudes between 2 and 4; 2) the quindio earthquake of january 1999; and 3) the occurrence of seismic swarms registered by the seismological networks of the volcanological and seismological observatories at pasto and manizales, are described. keywords: isotope. radioactivity, radon-222, earthquake. tectonics introduccion desde hace algunas decadas se han incorporado los estudios isotopicos en el estudio de los trazos de falla y en la investigacion del estado de actividad tectonica de las mismas. ei isotope que mas se ha estado investigando con este fin es el radon-222. paralelamente se realizan mediciones de otras gases como el helio, hidrogeno, dioxido de carbona (garzon, 1996), y otras isotopes como el toron (rn-220), radio-226 y carbono-13 (heiligmann et al., 1997). las concentraciones de los gases emitidos desde el interior de la tierra estan contralados par la litologia y la estructura de la zona de estudio (williams-jones et al., 2000). pero sus emisiones pueden variar en el tiempo por influencias geodinamicas 0 meteorologicas. las variables meteorol6gicas reportadas a nivel mundial como las que mayor influencia presentan sobre las emisiones de los gases son: i) las lluvias; 2) los cambios de temperatura del aire, y 3) los gradientes de la presion atmosfericamanuscr~o recibido para evaluaclon el 30 de octubre de 2002. articulo aceptado para publicacion por el cornte editorial el 30 de agosto de 2003. geofislca colombiana. 7. oiciembre de 2003 49 (schumann et al., (992). de acuerdo con trabajos realizados con anterioridad (gonzalez & garzon, 200 i) se ha observado que en nuestro medio tropical las variaciones de la temperatura del aire no superan los 8°c en un afio; de la misma manera, las variaciones de la presion atmosferica en el mismo periodo no son mayores a los 5 mbar. por el contrario, se ha observado que en periodos de incremento de las precipitaciones por lluvias, las emisiones de los gases pueden resultar afectadas negativamente, especialmente en aquellos ambientes donde preferencialmente los gases se transportan por mecanismo de difusion. ei radon-zz? esra en constante ernision desde el interior de la tierra hacia la atmosfera. hace varias decadas se ha estado planteando: i) la existencia de emisiones anornalas de este gas sobre las fallas activas; 2) las variaciones temporales relacionadas con los cam bios de las condiciones atmosfericas, y 3) su posible manifestaci6n premonitoria antes de un evento sismico (ulornov & mavashev, 1971; williams-jones et ai., 2000; toutain & baubron, 1999; moran et al., 2001). desde el afio de 1995 se han implementado algunas redes de estaciones isotopicas cerca del municipio de san juan de pasto, departamento de narifio (figura i); cerca del municipio de purace, en el departamento del cauca, y cerca del municipio de manizales, departamento de caldas (figura 2). figura i. localizacion de algunas estaciones isot6picas de rad6n-222, cerca del municipio de san juan de pasto, departamento de narifio. (imagen satelital tomada de: nasa, sir-c). metodologia ei radon-222 es uno de los tres isotopes naturales del gas noble radon. la naturaleza ionica de las particulas alfa que el radon-222 produce en su desintegracion natural permite utilizar metodos basados en cuantificar las huellas dejadas por el bombardeo de estas particulas sobre superficies sensibles. en los observatorios vulcanologicos y sisrnologicos del instituto de investigacion e informacion geocientifica, mineroambiental y 50 sernaetal. figura 2. locahzaci6n de algunas estaciones isot6picas de rad6n-222, cerca del municipio de manizales, departamento de caldas. nuclear -ingeominaspara medir el radon-222 en zonas de fallamiento activo es utilizado el sistema e-perm (electret passive j;,nvironmental radon monitor) propuesto por kotrappa et al. (1988). solamente en el observatorio vulcanologico y sismologico de pasto, en desarrollo de una estacion multiparametro se viene utiiizando el sistema pylon para la medici on de los niveles del radon-222 en fumarola (faber et al., 2002). figura 3. sistema de instalaci6n de una estacion isot6pica de rad6n-222. tornado de: salazar (2002). cuando se realizan estudios sisrnotectonicos son muy importantes los criterios a seguir para la seleccion de los sitios de instalac ion de las estaciones isotopicas, tales como: i) geologia y estructura del area, 2) topografia y 3) facil acceso al sitio. antes de instalar las estaciones isotopicas es aconsejable realizar un estudio de los perfiles del suelo, de tal manera que en la estacion, el tuba de geofislca colomb lana. 7, oiciembre oe 2003 casos hist6ricos colombianos del registro de anomallas del raoon-222 antes de eventos sismicos de naturaleza tect6nlca pvc que servira de trampa para los gases, quede enterrado hasta el suelo natural (salazar, 2002), tal como se muestra en la figura 3. con el fin de acercarse mas al conocimiento de los mecanismos de transporte de los gases en los sue los proximos a la estacion de monitoreo, es importante realizar medici ones del dioxido de carbono 0 helio 0 mercurio, paralelamente a las mediciones del gas radon. ei sistema e-perm se compone de tres elementos principales: 1) un disco de teflon du-pont cargado electrostaticamente, llamado electreto, el cual colecta iones en su superficie; 2) una camara fabricada con un material de polipropileno conductor de la electricidad, dentro de la cual un electreto puede ser descargado, y 3) un medidor del potencial de superficie del electreto (garzon, 1998). r,,222 filtro /\ rn2 " i..-tetrafluoroetilieno + c=== figura 4. procesos que tienen lugar dentro del sistema camara-electreto. tornado de: diago et al., 200 i. la configuracion 0 combinacion camara-electreto se utiliza para permitir la desintegracion del radon-222 en su interior. en la figura 4 se muestra una combinacion de camara con electreto, donde se indican todos los posibles procesos que dentro de ella pueden ocurrir, tales como: 1) desintegracion radiactiva del radon-222 y de sus descendientes; 2) adicion de los descendientes sobre aerosoles; 3) formacion de descendientes con carga electrica; 4) electrificacion de superficies; 5) deposicion de descendientes sobre superficies (plate-out); 6) filtracion del radon a traves de membranas selectivas. la concentracion activa del radon-222 se calcula de acuerdo con la siguiente expresion: [rn] = [(vi vf)/(fcxta)] dg don de, [rn] es la concentracion del radon-222, expresada en pci/l; vi es el potencial de superficie inicial del electreto, expresado en v; vfes el potencial de superficie final del electreto, expresado en v; fc es un factor de conversion para ajustar el metodo a la norma nist; t a es la duracion del analisis, expresado en dias y do son las dosis de radiacion gamma ambiental, expresada en pci/l. geofislca colomb lana. 7. olclembre de 2003 resultados y discusion en el afio de 1995 se inicio la instalacion de estaciones isotopicas en los volcanes galeras y nevado del ruiz, y sobre algunos trazos de fallas activas. algunas estaciones estuvieron en funcionamiento por algunos meses, mientras que otras contiruian funcionando en forma regular. en la actualidad, funciona una red de 22 estaciones cerca del municipio de pasto, cuatro estaciones cerca del municipio de purace, y seis estaciones entre el municipio de manizales y el volcan nevado del ruiz. las anomalias mas destacadas de incremento de las emisiones del gas radon antes de eventos sismicos son: 1. garzon (1996) reporto el incremento de las emisiones del gas radon-zz? durante un periodo de rnedicion de diez meses en la estacion barranco de la red isotopica instalada por el ovspasto. de acuerdo con la figura 5, el incremento de las ernisiones del radon-222 fue de 350 pci/l, 10 cual aparentemente precedio la ocurrencia de microsismos de magnitudes hasta de 2,1 en ja escala de richter y a una distancia menor a los 6,0 km de la localizacion de la estacion isotopica. 2. se realizaron estudios comparativos entre las emisiones del radon-222 y el nurnero de sismos volcano-tectonicos de magnitudes superiores a 1,5, registrados por la red sisrnologica del ovspasto. en la misma estacion barranco y para una ventana de tiempo de cinco afios, se obtuvo la figura 6, de acuerdo con la cual se observan incrementos de las emisiones del radon durante y antes de la ocurrencia de los principales eventos volcano-tectonicos. 2.5 2.0 t 1400 ! {1200 "" i 1000i i i-·-~ 800 ~" \ i \ ]:i, " i)' ""~" t 400 f200 /"v\-"..." ;"" ~j.5 '".. '~ . 11.0 \ _magnilud -.~ figura 5. emisiones del isotope rad6n-222 vs. magnitud de microsismos. estaci6n barranco (agosto 1995-junio 1996). 3. para una ventana de tiempo de un afio, garzon & serna (2001) reportaron una cornparacion entre las emisiones del radon-222 en la estacion sismo i de la red isotopica del ovspa to y dos eventos sismicos de naturaleza tectonica de magnitudes 2,3 y 3,2, tal como se muestra en la figura 7. 51 6000 sernaetal. " .1500 i, 16 3000,. 2,00 " '"'0 ~ 0 20(1)c, z <'; n z ~ 1500 i '", 51}oo + 3001.1 -» 0 .~ .c ~ -o ~ c.c. o, -» o; ~;0 0 j. ~.j, j v l " ~ = ~. " ~ c~ c, ~ ~ o~ ~ a, c. ~ ~ "! 1,q 0 0 ~ ~ 0 j ~ 1, 0 g ~ ~. 1 .; j~ ~' ;.=; -r o. ;:t j;-c figura 6. emisiones de rad6n en la estaci6n barranco (1995 2000) ynumero de sismos vt. figura 7. emisiones del rad6n en 1aestaci6n sismo i (enero-diciembre. 1998). 4. previo a la ocurrencia del terremoto del quindio (mw = 6,2) del 25 de enero de 1999, se registraron anomalias de incremento de las emisiones del radon en la estacion ovsm de la red isotopica instalada por el observatorio vulcanologi~o y sismologico de manizales-ingeominas. en la figura 8 se muestran las emisiones del radon-222, y con una flecha se indica el dia que ocurrio el terremoto del quindio. hoy el comportamiento de las emisiones del radon-222 en la estacion ovsm, como se muestra en la figura 9, ha regresado a sus niveles cercanos a los 500 pci/l que se estuvieron registrando entre 1995 y mediados e 1998. 5. la estacion isotopica ubicada en los predios del hotel termales del ruiz ha presentado anomalias en las emisiones del radon de! suelo, antes de la ocurrencia de enjambres sismicos vt, tal como se muestra en la figura 10. 52 '" -c cc r-: r-, co 00 c'.o~ co; c.' o; c.' ~~ f ~ ~ ~ ~e " ~ ~~ -6 -6 ~ -c -6 r-t n <'i -c r, -c n n n <', --~h i i i figura 8. incremento de las emisiones del rad6n en la estaci6n ovsm (septiembre 1995-marzo 1999). '" co r-, 00 co c~ ~ c-; 000 [ ~ if ~ t 6"" ~ ~ ~ '" ~ ~ -6 ~n <', figura 9. emisiones del rad6n en la estaci6n ovsm (1995-2002). 6000 t i 4000 'i i ' ~ -, i • , " 2()of~ . j • .;" ,. 20 18 16 14 12 ~ 10 v. ",~, , 6 ~.. _z figura j o.emisiones de rad6n vs. enjambres sismicos. estaci6n hotel (1999-2000). .~ ··\~·f·:~ :. ~~••••.•• :".~,t\,~...~ geofislca colombiana. 7. oiciembre de 2003 casos historicos colombianos del registro de anomalias del radon-222 antes de eventos sismicos de naturaleza tectonica desde hace varios afios se ha planteado la pregunta respecto a la efectividad del metodo de rnedicion de las anomalias del isotope radon, para su utilizacion como precursor de la actividad sismica. sirnultanearnente, ha surgido la inquietud de si todos los sismos podrian ser pronosticados y si todas las estaciones isotopicas instaladas resultarian efectivas en el monitoreo de la actividad sismica. al respecto, y con base en los monitoreos y en las investigaciones desarrolladas en los ultimos siete aiios, se ha llegado a concluir que hasta el momenta se tienen evidencias de la posibilidad de monitorear los eventos sismicos de caracter tectonico solamente en aquellas estaciones isotopicas que se localicen dentro de trazos de fallas activas. los gases en general y el gas radon-222 en particular provenientes del interior del subsuelo, pueden arribar a la superficie por accion de los gradientes de presion que se crean entre la presion en el subsuelo y la presion atmosferica, algunas variables como las precipitaciones por lluvias han surgido como los mayores facto res meteorologicos en ambientes tropicales, que influyen sobre los niveles de desgasificacion desde el suelo y el subsuelo (moran et al., 2001). estos factores, junto con las caracteristicas estructurales y mineralogicas del suelo y del subsuelo desempefian un papel muy importante en la determinacion de los niveles base de las emisiones gaseosas en una estacion isotopica. par otro lado, los incrementos de las emisiones gaseosas que superan 1 de desviacion estandar podrian ser el producto de perturbaciones surgidas durante los procesos de formacion de un sismo, donde los esfuerzos creados alrededor de un futuro foco sismico produce un paulatino resquebrajamiento de las rocas. investigaciones desarrolladas en lab oratorio (goodman, 1963) han demostrado que la destruccion de una roca es precedida por la formacion de numerosas y muy pequefias fisuras, las cuales van acornpafiadas de la emision de pulsos de variadas frecuencias. los pulsos de frecuencias ultrasonicas podrian ser los perturbadores del equilibrio mineral-gas y, por tanto, catalizadores del incremento de las emisiones del isotope radon-222, antes del registro de eventos sismicos de naturaleza tectonica. los ejemplos arriba presentados de anomalias del radon previas a la ocurrencia de un evento sismico se resumen en un as pocas estaciones isotopicas. generalmente, no en todas las estaciones instaladas se han observado anomalias premonitorias de un evento sismico. en varias estaciones isotopicas, en las cuales no se observaban posibles respuestas premonitorias, se realizaron estudios de los perfiles de los suelos y se establecio que dichas estaciones habian sido mal instaladas. esto significa que los tub os de pvc utilizados como trampas de gases no estaban enterrados hasta el suelo natural, sino que habian quedado en la capa organica. los materiales de naturaleza organica presentan altos contenidos de moleculas del elemento carbono, el cual es utilizado mundialmente como un material absorbente (retenedor) de gases. esto significa que asi exista un factor perturbador del equilibrio parcial en la superficie de los geofislca colombiana, 7, diciembre de 2003 minerales del suelo y del subsuelo, los gases en general permaneceran retenidos en la superficie de los materiales de la cap a organica del suelo, desempefiando una func icn sellante. para confirmar esta hipotesis, se han realizado estudios de la genesis de los gases, en particular del dioxide de carbona en las estaciones isotopicas, los resultados obtenidos de los anal isis de los contenidos del isotopo carbono-13 en las rnoleculas del dioxide de carbono, para cad a una de las estaciones se muestra en la tabla i. con base en los datos presentados en la tabla se construyo la figura 11. en esta figura se muestra que la proporcion de carbono-13/carbono-12 en las moleculas del co2 producidas por las plantas terrestres son menores que en las moleculas del co2 tabla 1. valor promedio de los contenidos de dioxide de carbona y carbono-13 en las estaciones isotopicas de los observatorios vulcanologicos y sismologicos de ingeominas. aiio de i muestreo i c02 [% v/v) d13c(c02) i sede de [%0) iingeominas estacion isotopica tabio 3,6 -14,6 polli!xan 2001 i barranco 3 6 -9 0 pasto 1995 2001 i sismol 49 -14,4 pasto t1995-2001 sismo 2 3,1 -123 r--easto __ 122~ 200lj sismo 5 5 2 ---=.11,0 pasto t1995 ~ san juan 1 1,0 r--~l pasto_20qi __j meneses 4 il--134 pasto 200~ ovsm 1 6 -13 6 manizales i 1999 i i florida 4 0 -13 5 manizales' 2001 i rargiiel!o alto 04 -253 pasto 2001 anil2 20 -206 pasto 2001 san francisco 0 4 -=.25~..l±_pasto --t--_4qq.l_san antonio 1 0,3 -27 6 easjq 2'p~ san antonio 2 03 i -283 pasto 2001 isan antonio 3 i 1,4 -23 3 pasto 200 i _ ~jon 1 i 0 9 -211 pastq_t__ 2q.ql_l:i'o 2 ~ 4 -21 9 ?"to 2001. zanjon3 09 -184 pasto ~~ ihuecada 1 0 2 -28 i pasto ~~ i huecada 3 0 8 -24,0 pasto i 200 i ~ :~~~:;:3__~_:-~:~~+~~:~i ~::=j san rafael 0 5 -225 polli!y~ 2001 i 'bedon 02 -31,5 i polli!y~1 2001 \ inueve i 0,5 -24y manizales i 2001 i ila play~1 -246 i manizales i 2001 : 53 provenientes de los fosiles y los carbonatos no marinos; yestos ultirnos a su vez producen co2 con menores contenidos de carbono-13, si los comparamos con el co2 proveniente del manto. en la misma figura se destacan las estaciones que han presentado anomalias de las emisiones del radon antes de eventos sismotectonicos, como barranco, sismo 5, sismo 1, oysm. estas estaciones, marcadas en la figura ii con un cuadrado presentan relaciones de carbono-13/carbono-12 en las moleculas de co2 propias de ambientes profundos, 10 que indica que se encuentran ubicadas sobre trazos de fallas activas donde la alta permeabilidad facilita el transporte de los gases hacia la superficie a traves del mecanismo de adveccion, la mayoria de las estaciones de radon, las cuales se indican con circulos en la figura ii, muestran firmas isotopicas que indican la procedencia somera de los gases alli atrapados, y por tanto su posible erronea instalacion en suelo adsorbentes de los gases. conclusiones de acuerdo con los monitoreos e investigaciones realizadas, se establece que: 1. las estaciones de radon con firmas isotopicas de c-13 en dioxide de carbono mayores a los -16%0 contra el estandar pdb, son las que han presentado anomalias antes de la ocurrencia de eventos sismotectonicos. 2. la mayo ria de las estaciones de las redes isotopicas de losobservatorios de ingeominas estan mal instaladas, y por esta razon se recomienda iniciar con un proceso de evaluacion de los perfiles de los sue los donde se encuentran instaladas. 3. cuando se desee instalar una estacion de radon con el fin de realizar estudios de la actividad sismotectonica, se recomienda realizar un detail ado estudio de: 3.1 los perfiles de los suelos 3.2 los mecanismos de transporte de los gases 3.3 la genesis de los gases por medio de estud{os de carbono-13. co2 de fosiles 0 carbonatos no marinas 6,0 fsisrtioil (s1sffiotl-----------t:=~~:h •.--:-.-=~_~::j_--__+5,0 iflorida' co, del manto --------'i'f;;);w,."t""ab.,.io~-·,---~ ib--~i--+ 4,0 l.:..::.::.j. .1 arranco co2 de plantas tcrrestres o u -----------""-.!!..."..,------__+ 3,0 ~ -----~--o___;i==i--c=1 ==""1---1 2,0ovsm. san juan i --------:-<,----0-,,-=-.:....,.,----.-----+ 1,0 o~ 9jg ?o: ~_~~~_,___;~~~~~~~~~~~~_i 0,0 0,035,0 30,0 25,0 20,0 15,0 10,0 5"qco,) [per mil] 5,0 figura ii. dioxide de carbona vs. delta 13 de carbona. 54 sernaetal. bibliografia diago, j., salazar, s., garz6n, g. (2001). "metodologia de monitoreo del rad6n-222 en sismotect6nica". revista horizontes naturales. universidad de caldas, facultad de ciencias exactas y naturales, manizales,3:37-42. faber, e., moran, c., poggenburg, j., garz6n, g., teschner, m. (2002). gas monitoring at the galeras volcano, colombia. submitted at journal of volcanology and geothermal research. 16 pp. (en prensa) garz6n, g. (1996). "mediciones del rn222 y co2 en el suelo del volcan galeras (colombia) y ocurrencia de sismos locales". ii seminario latinoamericano: volcanes, sismos y prevenci6n, lima y arequipa (peru), 115-118. ___ . (1998). "ei radon, manual teorico-practico", ingeominas (informe interno), 55 pp. ___ & serna, d. (200 i). "anomalias de las emisiones de rad6n-222 antes de un sismo tectonico", revista horizontes naturales. universidad de caldas, facultad de ciencias exactas y naturales, manizales, 3:43-51. gonzalez, l. & garz6n, g. (2001). "estudios isot6picos en la falla viliamaria-termales, departamento de caldas". revista horizontes naturales. universidad de caldas, facultad de ciencias exactas y naturales, manizales, 4: 17-30. goodman, r. (1963). "subanclibe noise during compression of rocks". bull. geol. soc. america, 4:487-490. heiligmann, m., stix, j., williams-jones, g., sherwood lollar, b., garz6n, g. (1997). "distal degassing of radon and carbon dioxide on galeras volcano, colombia". j. volcanol. and geotherm. res. 77:267-283. kotrappa, p., dempsey, j., hickey, j., stieff, l. (1988). "an electret passive environmetal 222rn monitor based on ionization measurements". health phys. 54:47-56. moran, c; chica, a., garz6n, g. (2001). "estudio de gases en seis falias geol6gicas en el departamento de narifio". revista horizontes naturales. universidad de caldas, facultad de ciencias exactas y naturales, manizales, 4:53-62. salazar, s. (2002). "mapa geoquimico del gas rad6n para el suroriente del municipio de manizales, colombia". tesis de pregrado. universidad de caldas, facultad de ciencias exactas y naturales, programa de geologia y minas, manizales, p. 17. schumann, r., owen, d., bolinder, s. (1992). "effects of weather and soil characteristics on temporal variations in soil gas radon concentrations". in: gates, a. and gundersen, l., geologic controls on radon: boulder, colorado, geological society of america special paper, 271 :65-72. toutain, j-p. & baubron, j-c. (1999). "gas geochemistry and seismotectonics: a review". tectonophysics, 304: 1-27. ul6mov, v. & mavashev, b. (197 i). "forerunners of the tashkent earthquake". izv. akad. nauk uzb. ssr, 188-200. williams-jones, g., stix, j., heiligmann, m., charland, a., sherwood lollar, b., arner, n., garz6n, g., barquero, 1., fernandez, e. (2000). "a model of diffuse degassing at three subductionrelated volcanoes". bull. volcanol.62:130-142. geoflslca colombiana, 7. oiciembre de 2003 the fenghuangshan copper deposit, in the tongling area of anhui province, china, is genetically associated with the fenghuangshan granodiorite, which contains amphiboles that can be used to determine the pressure and temperature history of the magmas that formed this intrusion. here, we investigate the petrography and mineralogy of the fenghuangshan granodiorite, focusing on variations in amphibole composition and determining the conditions of amphibole crystallization. the data are used to determine the petrogenetic history and metallogenic significance of the hosting granodiorite. the amphiboles within the intrusion can be divided by texture into phenocryst, matrix, and poikilocrystal amphiboles. electro model, combined with the barometry and thermometry and the stratigraphy of the study area, indicates that the fenghuangshan copper deposit has not undergone any significant changes since its formation, thereby indicating that both the granodiorite and the associated mineralization are well preserved. this also suggests that the final depth of emplacement of the magma (3–6 km) represents the depth of formation of the associated copper deposit. this inference, combined with the fact that the current depth of exploration and mining in the study area is <1 km, indicates the great potential for discovering significant copper mineralization by deep exploration in this area. el depósito cuprífero de fenghuangshan, en el área de tongling, provincia de anhui, china, está genéticamente asociado con la granodiorita de fenghuangshan, que contiene anfíboles que pueden ser usados para determinar la presión y la temperatura histórica de los magmas que formaron esta intrusión. en este trabajo se investigo la petrografía y la mineralogía de la granodiorita de fenghuangshan enfocados en las variaciones de composición de anfíboles y en determinar las condiciones de cristalización de estos. los datos se utilizaron para determinar la historia petrogenética y la significación metalogénica de la granodiorita anfitriona. los anfíboles al interior de la intrusión pueden dividirse por texturas en fenocristales, cristales en matriz y cristales moteados. los análisis de microsonda electrónica indican que los anfíboles son de características magnesiohornblenda y tschermakita. las plagioclasas en la granodiorita son andesinas y oligoclasas. la barometría del aluminio en los anfíboles y la termometría de los anfíboles plagioclásticos indican que los anfíboles moteados cristalizados se fundieron a presiones entre 4,46 y 4,74 kbar y temperaturas de 1066 °c y 1071 °c, y los anfíboles en la matriz se formaron bajo condiciones de 1,00-1,91 kbar y 784-823 °c. los datos indican que el magma que formó la granodiorita fue generado dentro del manto superior y migró a través de fracturas antes de ubicarse en la corteza media-baja a una profundidad de ~15 km, lo que formó una profunda cámara magmática donde el magma sufrió una cristalización y asimilación fraccionada. el magma migró luego a través de estructuras geológicas (como fallas y fracturas) antes de ubicarse en la corteza a poca profundidad, entre 3 y 6 km, y formando eventualmente la granodiorita fenghuangshan. este modelo combinado con la barimetría, la termometría y con la estratigrafía del área de estudio indica que el depósito cuprífero de fenghuangshan no ha experimentado cambios sustanciales desde su formación, lo que indica que tanto la granodiorita como la mineralización asociada están bien preservadas. esta condición también nos sugiere que la profundidad final de la ubicación del magma (3-6 km) representa la profundidad de formación del depósito cuprífero asociado. esta inferencia, combinada con el hecho de que la actual exploración y explotación en el área de estudio es de <1 km, señala que el gran potencial para descubrir mineralización cuprífera con una exploración profunda en el área. petrology and mineralogy of the fenghuanshan granodiorite, anhui province, china: implications for petrogenesis and metallogenesis implication aiping zhang, yangsong du, yi cao, zhenshan pang² , gan luo¹ , kangkang xu¹ 1. school of the earth sciences and resources, china university of geosciences, beijing 100083, china 2. development and research center of china geological survey, beijing 100037, china correspondence should be addressed to aiping zhang, e-mail: zhangaiping_cugb@163.com earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 31 37 abstract resumen key words: fenghuangshan granodiorite, mineralogy, geothermometry–barometry, emplacement depth, denudation, tongling, anhui province, china. palabras clave: granodiorita de fenghuangshan, mineralogia, geotermometría-barometría, profundidad de emplazamiento, denudación, tongling, provincia de anhui, china. petrology record manuscript received: 25/08/2014 accepted for publication: 04/02/2015 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.45202 32 aiping zhang, yangsong du, yi cao, zhenshan pang , gan luo , kangkang xu introduction amphibole is widespread in igneous rocks, can form in a number of tectonic settings, and plays an important role in the magmatic evolution of igneous systems. many studies have indicated that al substitution in amphibole is controlled by both temperature and pressure (e.g., anderson, 1980; hammarstrom et al., 1986; johnson et al., 1989; schmidt, 1992; ernst, 1999; davidson et al., 2007; martin, 2007; larocque et al., 2010; ridolfi et al., 2010; dessimoz et al., 2012; krawczynski et al., 2012), indicating that amphiboles can accurately record the pressure and temperature evolution of a magma (zen, 1989; tulloch et al., 2000; stein et al., 2001; lu et al., 2011). the middle–lower yangtze river polymetallic belt (mlyrb) is one of the most economically important metallogenic belts in china, and contains more than 200 cu, fe, au, mo, zn, pb, and ag deposits (li, 1989; chang et al., 1991; pan and dong, 1999). the tongling ore district is located within anhui province, within the central mlymb (fig. 1), and hosts widespread igneous rocks that temporally and spatially related to mineralization. dozens of metallic ore deposits have been discovered in this ore district, including the tongguanshan, fenghuangshan, baocun, chaoshan, jiguanshan, and dongguashan deposits, among others. the fenghuangshan deposit is a skarn cu deposit that is typical of such mineralization in this area (chang et al., 1991; zhai et al., 1992; fig. 1). the copper mineralization within this deposit is closely related to the fenghuangshan intrusion, with mineralization hosted by both the intrusion and the contact between the intrusion and the surrounding triassic limestone. in addition, the mineralization in this area (141.71 ± 0.82 ma) was formed at the same time as the fenghuangshan intrusion (143.1 ± 1.6 ma; li et al., 2014). this means that research into the fenghuangshan intrusion can provide insights into the process associated with the formation of magma and mineralization in this district. previous geochemical and geochronological research in this area has focused on the origin of the magma that formed the fenghuangshan intrusion, the processes involved in the emplacement of the magma, and the ages of intrusion and mineralization. however, little research has been undertaken on the crystallization conditions of minerals within the granodiorite, in turn meaning that little is known about this crucial mineralogical evidence of the magmatic conditions and ore-forming processes associated with the fenghuangshan deposit. here, we describe variations in amphibole composition within the fenghuangshan granodiorite and use these data to estimate the magmatic conditions and reveal the petrogenetic and metallogenic significance of the fenghuangshan granodiorite. these data enable the determination of the depth of emplacement and the degree of denudation that this area has undergone, providing useful information on the preservation history of the fenghuangshan copper deposit and the prospectivity of the surrounding region. fig.1 sketch map of the middle–lower yangtze river polymetallic belt, east china. the inset is a simplified structural map of china. tlf-tancheng-lujiang fault, xgf-xiangfan-guangji fault, ycf-yangxingchangzhou fault. modified from chang et al. (1991), mao et al. (2011), tang et al. 1998), and zhai et al. (1992). 2. geological setting the fenghuangshan deposit is located in the eastern part of the tongling ore district, 35 km southeast of tongling. the geology of this area is dominated by the fenghuangshan granodiorite, which intruded the core of a ne–sw-trending synclinorium (fig. 2). the country rocks are dominantly triassic limestones that have metamorphosed into marble, forming a 400 to 1200-m-wide thermal metamorphic aureole around the intrusion. the fenghuangshan intrusion formed during the late yanshanian, is high-k and calc-alkaline, and is dominated by a granodiorite phase (qu et al., 2010; li et al., 2014). the geochemistry of the granodiorite indicates that it formed from a deeply sourced alkaline magma that assimilated crustal material; this crustal contamination may have occurred during partial melting (i.e., contamination of the source) or during ascent and the emplacement of the magma. as such, this suggests the magma that formed the intrusion underwent typical assimilation and fractional crystallization (afc) processes (liu et al., 2002; mao et al., 2004; wang et al., 2008; qu et al., 2010; li et al., 2014). the fenghuangshan granodiorite, which is the largest intrusion in the area, is circular in cross-section, and is exposed over an area of about 10 km2. it is dominated by a granodiorite phase and is cross-cut by some later trachytic, doleritic, and lamprophyre dikes. six cu–fe–au deposits are located near the contact zone of the fenghuangshan granodiorite: the fenghuangshan, jiangjiachong, qingshuitang, xianrenchong, tieshantou, and baoshantao deposits (fig. 2). the granodiorite samples analyzed in this study were collected from the –440 m level of the fenghuangshan copper deposit. fig.2 location of the fenghuangshan ore field in tongling, anhui province (a) and a sketch geological map of the ore field (b). modified from 321 geological team of bureau of geology and mineral exploration of anhui province (1989). 33petrology and mineralogy of the fenghuanshan granodiorite, anhui province, china: implications for petrogenesis and metallogenesis sample fh011 fh100 spots 1a 2a 3a 4a 5a 1b 2b sio2 49.56 48.32 48.11 49.21 48.30 43.32 43.00 al2o3 5.38 6.40 6.40 5.31 6.09 9.44 10.84 feo* 12.79 12.96 13.53 14.84 15.32 19.95 17.74 mno 0.49 0.50 0.45 0.50 0.58 0.55 0.63 mgo 16.37 15.88 16.15 15.10 14.33 11.44 11.95 cao 11.66 11.59 11.36 11.51 11.84 11.58 11.44 na2o 1.26 1.59 1.60 0.91 1.15 1.61 1.83 k2o 0.05 0.07 0.07 0.04 0.05 0.13 0.12 total 97.56 97.31 97.67 97.42 97.66 98.02 97.55 sit 7.15 7.02 6.97 7.16 7.05 6.43 6.37 al (ⅳ)t 0.85 0.98 1.03 0.84 0.95 1.57 1.63 total t 8.00 8.00 8.00 8.00 8.00 8.00 8.00 al (ⅳ)b+c 0.07 0.12 0.06 0.07 0.09 0.08 0.27 fe3+ b+c 0.41 0.40 0.51 0.51 0.53 1.00 0.81 fe2+ b+c 1.13 1.18 1.12 1.29 1.34 1.48 1.39 mnb+c 0.06 0.06 0.06 0.06 0.07 0.07 0.08 mgb+c 3.52 3.44 3.49 3.27 3.12 2.53 2.64 cab+c 1.80 1.80 1.76 1.79 1.85 1.84 1.82 total c + b 7.00 7.00 7.00 7.00 7.00 7.00 7.00 naa 0.35 0.45 0.45 0.26 0.33 0.46 0.53 ka 0.01 0.01 0.01 0.01 0.01 0.02 0.02 total a 0.36 0.46 0.46 0.26 0.33 0.49 0.55 p(kbar) 1.01 1.64 1.91 1.00 1.53 4.46 4.74 d(km) 3.35 5.42 6.31 3.32 5.05 14.72 15.63 3. petrology the fenghuangshan intrusion is granodioritic and has a porphyraceous texture (fig. 2a) comprising phenocrysts of plagioclase (30%), amphibole (10%), quartz (15%), k-feldspar (10%), and biotite (3%) in a fine-grained matrix of the same minerals with accessory zircon, apatite, and sphene (fig. 3a, b). the amphiboles within the intrusion can be texturally divided into phenocryst, matrix, and poikilocrystal amphiboles, and plagioclase within the intrusion is divided into phenocryst and matrix groups. amphibole phenocrysts are 1–3 mm in size, are euhedral, have rhombus-shaped cross-sections, and are rimmed by opaque magnetite (fig. 3c, d). these phenocrysts coexist with plagioclase phenocrysts (fig. 3c), with the latter having oscillatory zoned overgrowths and partially resorbed textures (fig. 3a, b). matrix amphibole within the intrusion is present as small (<0.5 mm) subhedral–anhedral crystals (fig. 3b, d), some of which are optically twinned. nearly all of the matrix amphibole also coexists with matrix plagioclase (fig. 3d). the amphibole poikilocrystals within the intrusion are small (<0.2 mm) and euhedral, and are enclosed within the cores of plagioclase phenocrysts (fig. 3b), indicating that these poikilocrystals crystalized earlier than or at the same time as the amphibole phenocrysts. this indicates that the amphibole poikilocrystals were the first amphiboles to crystalize from the magma, whereas the matrix amphibole was the last amphibole to crystalize out, indicating that these differing amphiboles represent two different phases of the evolution of the magma that formed the fenghuangshan intrusion. fig. 3 microphotographs of amphibole and plagioclase from the fenghuangshan granodiorite. (a)-granodiorite showing typical porphyraceous texture (crossed polarized light). (b)-phenocryst of zoned plagioclase containing an inclusion of amphibole (crossed polarized light). (c)-phenocryst of euhedral amphibole with a hexagonal cross-section and with nearby magnetite grains (plane polarized light); (d)-amphibole and plagioclase in the matrix are <0.2 mm in size (plane polarized light); pl-plagioclase, am-amphibole, bt-biotite, q-quartz; mt-magnetite. 4. mineralogy electron microprobe analyses (epma) were undertaken at the beijing research institute of uranium geology, beijing, china, using a jxa-8100 instrument operated at a 20 kv accelerating voltage, 10 na beam current, and using a 10 mm beam diameter, giving a detection limit of 0.002 wt%. analyses were calibrated using albite (na), sanidine (si, al, and k), diopside (ca and mg), almandine (fe), rutile (ti), fluorapatite (p), and rhodonite (mn) standards. table 1 results of compositional analyses (wt%) of amphiboles fig.4 classification of amphibole (a) and plagioclase (b) in the fenghuangshan granodiorites after the international mineralogical association (leake et al., 1997) and smith (1974), respectively. a-amphibole in the matrix, b-amphibole poikilocrystals 34 aiping zhang, yangsong du, yi cao, zhenshan pang , gan luo , kangkang xu 34 sample fh011 spots 1c 2c 3c 4c 5c 1d 2d sio2 58.73 58.26 58.49 61.68 60.37 59.15 60.60 tio2 0.07 0.08 0.08 0.00 0.00 0.00 0.00 al2o3 25.08 25.31 25.48 23.38 24.51 25.78 25.31 feo 0.19 0.33 0.24 0.27 0.24 0.20 0.24 mgo 0.00 0.00 0.00 0.00 0.00 0.00 0.02 cao 7.15 7.11 7.48 4.98 6.07 6.61 6.16 na2o 7.39 7.90 7.28 9.19 8.49 7.49 7.63 k2o 0.06 0.04 0.03 0.02 0.02 0.27 0.42 total 98.67 99.06 99.08 99.52 98.28 99.52 100.39 si 2.66 2.63 2.64 2.75 2.70 2.65 2.69 ti 0.00 0.00 0.00 0.00 0.00 al 1.34 1.35 1.35 1.23 1.29 1.36 1.32 fe2+ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ca 0.35 0.34 0.36 0.24 0.29 0.32 0.29 na 0.65 0.69 0.64 0.80 0.74 0.65 0.66 k 0.00 0.00 0.00 0.00 0.00 0.02 0.02 cr 0.00 0.00 0.00 0.00 0.00 ab 64.94 66.64 63.67 76.87 71.60 66.15 67.42 an 34.72 33.14 36.15 23.02 28.29 32.26 30.12 or 0.35 0.22 0.17 0.11 0.11 1.59 2.47 p(kbar) 1.01 1.64 1.91 1.00 1.53 4.46 4.74 t(°c) 798.50 812.29 822.73 784.26 803.41 1066.39 1070.55 fh100 table 2 results of compositional analyses (wt%) of plagioclase c-plagioclase in the matrix; d-core of plagioclase phenocrysts coexisting with amphibole poikilocrystals the classification of amphiboles is based on the composition of the standard amphibole formula ab � ivc5 ivt8 o � � (oh) � . the composition and stoichiometrically calculated chemical formulae of representative amphiboles from the fenghuangshan granodiorite are given in table 1, with mineral formula calculations based on 23 oxygens and standardized on 13 cations. the calculation method of droop gtr (1987) was used to adjust fe²+ and fe³+ concentrations, and the electrovalency balance principle was used to calculate crystal formulae. amphibole stoichiometry and nomenclature followed the international mineralogical association (ima; leake et al., 1997; fig. 4). all of the amphiboles analyzed during this study are calcic amphiboles, defined by (ca + na)b values of ≥1.00, with na values of <0.50 and cab values of ≥1.50 (leake et al., 1997). consequently, they are classified as magnesiohornblende and tschermakite amphiboles, with the matrix amphiboles being classified as magnesiohornblende and the lower-si amphibole poikilocrystals being classified as tschermakite. magnesiohornblende within the intrusion has si values of 6.97– 7.16, al(iv)t contents of 0.84–1.03, cab values of 1.96–1.85, (na + k)˄ values of 0.27–0.46, and ca˄ values of <0.5. in comparison, tschermakite within the intrusion has si values of 6.37–6.43, al(iv)t values of 1.57– 1.63, cab values of 1.82–1.84, (na + k)˄ values of 0.48–0.55, ca˄ values of <0.5, and mg/(fe2+ + mg) values of 0.63–0.66 (table 1). the compositions and calculated formulae of the plagioclase analyzed during this study are given in table 2, with structural plagioclase formulae calculated on the basis of 8 oxygens and 5 cations. these data indicate that the plagioclase has an values of 23.02–36.15 and that the granodiorite contains oligoclase (an = 23–28) and andesine (an = 30–36) plagioclase (fig. 4). both matrix and phenocryst plagioclase have similar compositions, with na2o and cao concentrations increasing from 7.49 to 8.49 wt% and decreasing from 6.61 to 6.07 wt%, respectively, from core to rim in zoned plagioclase. 5. barometry and thermometry amphibole is stable over a wide range of pressure (p) and temperature (t), making it useful for determining the geothermometry and geobarometry of calc-alkaline intrusive magmatic systems (blundy et al., 1990). the total al content of amphibole can be used to estimate the crystallization pressure of a magma (hammarstrom et al., 1986; hollister et al., 1987; johnson et al., 1989; ghent et al., 1991; vyhnal et al., 1991; ague et al., 1992; schmidt, 1992). the first empirical al-in-amphibole barometer was proposed by hammarstrom (1986; eq. 1), who used this barometer with hornblende compositions to estimate the pressure of formation of intermediate calc-alkaline plutons. hollister (1987) reduced the error of this barometer by using a larger dataset for calibration (eq. 2) with a similar correlation. johnson and rutherford’s (1989) calibra tion (eq. 3) was obtained by performing experiments on natural samples under set co² + h � o volatile pressures, with further experimental calibration of this barometer under water-saturated conditions (eq. 4) by schmidt (1992). p (+3 kbar) = –3.92 + 5.03 a1t (1) p (+1 kbar) = –4.76 + 5.64 a1t (2) p (±0.5 kbar) = –3.46 + 4.23 a1t (3) p (±0.6 kbar) = –3.01 + 4.76 a1t (4) the last two calibrations are more commonly used because they are experimentally rather than empirically derived and have lower uncertainties that the first two calibrations. here, we use the calibration of schmidt (1992; eq. 4) to calculate the pressures obtained from amphibole analyses. the present amphiboles yield pressures of 1.00–4.76 kbar, with the highest values (4.46–4.74 kbar) being derived from the amphibole poikilocrystals. blundy (1990) published an empirical thermometer based on the edenite– tremolite reaction that can only be used on quartz-bearing, intermediate to felsic igneous rocks containing plagioclase with compositions of an ≤92% and amphibole with si values of ≤7.8, as follows: where k = xab, xab = the composition of plagioclase coexisting with amphibole, and using the pressure values derived from the al-inamphibole barometry described above. the fenghuangshan granodiorite has sio2 concentrations that vary between 60.82 wt% and 65.97 wt% (shao et al., 2003; li et al., 2014), contains plagioclase with an values between 23.02 and 36.15, and contains amphibole with si values of 6.37–7.16. all of these features mean that these samples are suitable for amphibole–plagioclase thermometry (blundy et al., 1990), yielding crystallization temperatures of 784°c–1071°c, with amphibole poikilocrystals yielding both higher pressures (as detailed above) and temperatures (1066°c–1071°c). 6. discussion 6.1 crystallization conditions the fact that poikilocrystals and matrix amphiboles within the fenghuangshan granodiorite have different compositions suggests that they formed in two different magmatic environments. the poikilocrystals most likely formed in a deeper and hotter setting, in a deep magma chamber, whereas the matrix amphiboles probably formed within a shallow magma chamber under the conditions of the final emplacement of the magma, and these amphiboles therefore record the depth and temperature of emplacement. the barometry and thermometry undertaken during this study indicate that the amphibole t= 35petrology and mineralogy of the fenghuanshan granodiorite, anhui province, china: implications for petrogenesis and metallogenesis poikilocrystals crystallized from the melt at pressures of 4.46–4.74 kbar and temperatures of 1066°c–1071°c, whereas matrix amphiboles formed at 1.00–1.91 kbar and 784°c–823°c. the pressures are probably equivalent to the depth of emplacement of the magma, which, when combined with the fact that the average crustal density of the mlyrb is 3.3 g/cm3, yields a depth of 14.72–15.63 km (average of 15.16 km) and a temperature of 1066°c–1071°c (average of 1068°c) for the deep magma chamber, and an emplacement depth and temperature for the shallow magmatic chamber of 3.32–6.32 km (average of 4.7 km) and 784°c–823°c (average of 804°c), respectively. the geochemistry of the fenghuangshan granodiorite suggests that the intrusion has an adakitic affinity similar to that of other intrusions in the tongling area (du et al., 2004, 2007; cao et al., 2008, 2010 ; wang et al., 2008; mao et al., 2009; zhai et al., 2010; wu et al., 2014), which have initial 87sr/86sr ratios of 0.709074 – 0.709965 and end(t) values of –9.26 to –16.25 (xing et al., 1996). this suggests that the magma that formed the fenghuangshan granodiorite originated from the upper mantle, and assimilated crustal material during its evolution. these data have allowed the development of the following model for the fenghuangshan granodiorite: an upper-mantle-derived magma migrated along faults and was emplaced within the middle–lower crust (at a depth of ~15 km), forming a deep magma chamber where the magma underwent assimilation and fractionation crystallization processes. the region then underwent tectonic deformation, causing the magma within the deep magma chamber to migrate along tectonic fractures to the shallow crust (depths of 3–6 km), forming the fenghuangshan granodiorite. 6.2 estimate of denudation amount ore deposits undergo significant changes after formation, and post-formational changes and preservation of orebodies provide vital information for targeting during mineral exploration (harrison, 1994; kesler et al., 2006; uchida et al., 2007; wilkinson et al., 2007; wang et al., 2008; kirstein, 2011; lu et al., 2011; zheng et al., 2011). the degree of denudation of an ore deposit is an important factor in determining the preservation potential of orebodies and hence the prospectivity of an area. the copper mineralization in the present study area is closely related to the fenghuangshan granodiorite, indicating that the denudation of the deposit can be inferred from the emplacement depth of the intrusion, the timing of mineralization in the area, and the thickness of overlying sediments. the samples analyzed in this study were obtained from the –440 m level of the fenghuangshan deposit. this, combined with the shallowest emplacement depth obtained during this study (3.23 km), suggests that this area has experienced at least ~2.8 km of uplift since 141 ma. the intrusion in the study area was emplaced into sediments at 141 ma; the total thickness of all of the sediments overlying the intrusion is >2.6 km (321 geological team, 1989). this indicates that all of these sediments must have been eroded away, although the intrusion itself has not undergone significant erosion. as such, the fenghuangshan copper ore deposit, which is closely related to the fenghuangshan granodiorite, is well preserved and has undergone few postmineralization changes. this suggests that the emplacement depth (3.32–6.32 km) of the intrusion could also represent the depth of exhumation of the copper mineralization in this area. therefore, exploration for copper mineralization at depths of <1 km, which is the current limit of exploration and mining in this area, may identify significant amounts of previously unknown mineralization . 7. conclusions analyses of minerals from the fenghuangshan granodiorite yielded the following conclusions. amphibole poikilocrystals contain lower amounts of si than other amphiboles in the intrusion and are classified as tschermakite, whereas matrix amphibole in the intrusion is magnesiohornblende. plagioclase within the deposit has oligoclase and andesine compositions. amphibole poikilocrystals crystallized from the melt at pressures of 4.46– 4.74 kbar and temperatures of 1066°c–1071°c, whereas matrix amphibole formed at pressures of 1.00–1.91 kbar and 784°c–823°c, representing conditions within deep and shallow magma chambers, respectively. the fenghuangshan granodiorite formed from upper-mantlederived magmas that initially migrated along fault zones before being emplaced in the middle–lower crust (at a depth of ~15 km), forming a deep magma chamber where these magmas underwent fractionation crystallization and assimilation. tectonism then caused the magma within the deep magma chamber to migrate along faults to the shallow crust (depths of 3–6 km), forming the fenghuangshan granodiorite. the barometry and thermometry undertaken during this study, combined with the stratigraphy of this region, indicates that the fenghuangshan copper deposit is well preserved and has undergone few changes since formation. this indicates that the emplacement depth of the intrusion (3.32–6.32 km) also represents the initial burial depth of the copper deposit. the current depth of exploration and mining in this area is <1 km, meaning that this area is highly prospective for deeper exploration for copper mineralization similar to that within the fenghuangshan copper deposit. acknowledgements this study was supported financially by the china geological survey (grant 12120113069900) and the national natural science foundation of china (grant 40672045). senior engineers chenghuo zhang and shuping tang provided valuable assistance in the field. we are grateful to huiyan zhu and li liu of china universitiy of geosciences, beijing, and jingqiu ren of peking universitiy,for their kind help during this work. we thank ruiping liu, qiang fu, and two anonymous reviewers for their critical and constructive reviews, which greatly improved the paper. 36 aiping zhang, yangsong du, yi cao, zhenshan pang , gan luo , kangkang xu references ague, j.j.; 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yao, s.z.; lin, x.d.; zhou, x.n.; wan, t.f.; jin, f.q.; zhou, z.g.( 1992). fe-cu (au) metallogeny of the middle-lower changjiang region. geological publishing house, beijing, 235pp. zheng, y.c.; gu, l.x.; tang, x.q.; liu, s.h.; wu, c.z.; li c.h.(2011). oxygen isotope characteristics of the footwall alteration zones in the hongtoushan volcanogenic massive sulfide deposit, liaoning province, china and restoration of their formation temperatures. acta geologica sinica-english edition. 85(3): 683-693. the wenchuan earthquake of may 12, 2008, generated a significant amount of loose solid material that can produce devastating debris flows. in the five years since the earthquake, there have been many large-scale individual and group catastrophic debris flows that have caused lots of damage to the resettled population and the reconstruction efforts. the reconstructed towns of yingxiu, yinxing and miansi have suffered debris flows and other secondary disasters in the past five years and are still not out of danger in the future. a debris-flow catastrophic event hit four towns of wenchuan county along the duwen highway, part of china’s national highway 213, at midnight on july 10, 2013, following a local extreme rainfall of 176.2mm 24h-1. the debris flows occurred simultaneously along seven gullies. a total of 15000 people were affected due to the destruction of resettlement areas, factories, power stations, and houses. because of this devastating event, traffic along the duwen highway was completely disrupted during the disaster and recovery period. the present study focuses on the lianshan bridge debris flow gully; the disaster characteristics and cause of the debris flow were analyzed based on field investigations, remote sensing interpretation, and laboratory experiments. the particular dynamic parameters of the debris flow were calculated and analyzed including density, velocity, discharge, total volume and impact force. also, the dynamic processes and changes that occurred in the debris flow were examined, and the block and burst characteristics of debris flow were studied based on statistical calculation and analysis dynamic characteristic parameters of debris flow. finally, a program to prevent further debris flow was proposed according to the on-site investigation and based on the analysis of the features and dynamic processes of the debris flow. el terremoto de wenchuan, el 12 de mayo de 2008, generó una gran cantidad de material sólido suelto que puede producir flujos de detritos devastadores. en los años posteriores al terremoto han ocurrido deslizamientos a gran escala individuales y simultáneos que han causado daño a los habitantes reubicados y a los esfuerzos de reconstrucción. las ciudades reconstruidas de yingxiu, yinxing y miansi han sufrido flujos de detritos y otros desastres secundarios desde el terremoto, y no están exentas de eventos futuros. un evento simultáneo de flujo de detritos afectó a cuatro localidades del condado de wenchuan, a lo largo de la autopista de duwen, parte de la carretera nacional 213, en la medianoche del 10 de julio de 2013, después de una lluvia extrema de 176,2 mm 24h-1. los movimientos de detritos ocurrieron en siete pendientes. un total de 1500 personas resultaron afectadas debido a la destrucción en áreas de reasentamiento, fábricas, estaciones eléctricas y viviendas. debido a este devastador hecho, el tráfico de la autopista duwen estuvo interrumpido durante el período del desastre y mientras se recuperaba la zona. este estudio se enfoca en el deslizamiento del puente lianshan, donde se analizaron las características y las causas del flujo de detritos basados en investigaciones de campo, interpretación de detección remota y experimentos de laboratorio. se calcularon y analizaron los parámetros dinámicos particulares del flujo de detritos como la densidad, velocidad, descarga, volumen total y fuerza de impacto. también se analizaron los procesos dinámicos y los cambios que ocurrieron en el flujo de detritos, al igual que se estudiaron las características de bloqueo y ruptura del flujo con base en cálculos estadísticos y análisis de los parámetros dinámicos característicos. finalmente, se propone un programa para prevenir mayores movimientos de detritos de acuerdo con la investigación de campo y basado en los análisis de las características y procesos dinámicos del flujo de material sólido suelto. abstract resumen keywords: debris flow, dynamic process, the block and burst characteristics, prevention and cure measures. palabras clave: flujo de detritos, procesos dinámicos, características de bloqueo y ruptura, medidas de prevención. case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china record manuscript received: 23/12/2014 accepted for publication: 13/05/2016 how to cite item hu, g. s., chen, n. s., tanoli, j. i., you, y., & li, j. (2016). case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china. earth sciences research journal, 20(2), e1-e13. doi:http://dx.doi.org/10.15446/esrj.v20n2.48026 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.48026 gui-sheng hu1, ning-sheng chen1,*, javed iqbal tanoli1,2, yong you1 and jun li1 1 key lab of mountain hazards and surface processes, institute of mountain hazards and environment, chinese academy of sciences, 610041, chengdu, china. tel: +86-18683259117. email: huguisheng@imde.ac.cn. 2department of earth sciences, comsats institute of information technology, abbottabad, 22060, pakistan *corresponding author: chennsh@imde.ac.cn earth sciences research journal earth sci. res. j. vol. 20, no. 2 ( june, 2016 ) : e1 e13. características y procesos dinámicos de los catastróficos movimientos de detritos en el condado de wenchuan, china, el 10 de julio de 2013 caso de estudio n a t u r a l h a z a r d s e2 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li 1. introduction numerous landslides have been triggered in earthquake-affected areas and the largest ones have been located close to the rupture faults (chen et al. 2009; huang et al. 2012). abundant loose solid erodible material from co-seismic rock falls and landslides are then deposited in gullies, which form the source material for debris flows (shieh et al. 2009). numerous debris flows have been triggered during the rainy season in the years following an earthquake (yu et al. 2014). the frequency and magnitude of debris flows were thus increased significantly after the wenchuan earthquake of 2008 (cui et al. 2011; chen et al. 2013); during the rainy seasons in the past five years, over 800 debris flows have occurred and this has caused great damage to the resettled population and the reconstruction efforts (zhang et al. 2013). although mitigation measures were constructed in a few gullies such as the wenjia and bayi gullies, disastrous debris flows still occurred; in some small gullies the mitigation measures installed proved to be inadequate and incomplete (wang et al. 2012, 2013). for example, a catastrophic debris-flow event hit four towns in wenchuan county along the duwen highway, part of the china national highway g213, at midnight on july 10, 2013; this event broke out simultaneously in seven debris flow gullies. a total of 15000 people were affected; 18 people were reported dead or missing and there was significant destruction of resettlement areas, factories, power stations and houses (table 1). these debris-flow gullies located along the duwen highway have seen reoccurring debris flow disasters in the geological history, and it is likely that some part of these gullies will have another debris flow disaster in the near future. debris flow activity of these small gullies is still very active and poses a serious threat to the local population, property, traffic safety, and the development of the local social economy. thus, debris flow mitigation measures should be installed in a timely fashion (xu et al. 2012). using the lianshan bridge debris flow gully as an example, an analysis was performed on the disaster characteristics, cause of the debris flow, the dynamic processes and the changes in the debris flow through field measurements, remote sensing interpretation, and laboratory experiments. it is essential to provide a comprehensive debris flow control scheme which will lay the foundation for future research on regional debris flows. table 1. summary of destruction caused by a group of catastrophic debris flows that occurred on july 10, 2013. 2. study area 2.1 location of the study area the lianshan bridge debris flow gully is located in yinxing town, wenchuan county, sichuan province of china. it is the first order and left branch of the minjiang river (figure 1), with coordinates 103°29′49″e, 31°11′24″n at the gully mouth. the catchment covers an area of 0.7 km2 and the main channel is 1.34km long with a 48.3% longitudinal slope. the flow direction of the gully is from west to east. the dimensions of the watershed from east to west and south to north approximately 3.0 and 4.0 km, respectively. the gully is accessible to traffic via the duwen highway that passes through its deposition area. 2.2 landform conditions the study area, located on the eastern edge of the qinghai–tibetan plateau, has a complex terrain characterized by considerable elevation differences and steep slopes (zhang et al. 2006). high mountains, deepcut rivers, and steep slopes are the main landform characteristics of this area. the elevation difference of the gully is 1159 m from the maximum elevation of 2184m in the west of the catchment to the minimum elevation of 1025 m at the gully mouth. figure 1. location of the study area. e3case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china 2.3 geological conditions the geological conditions of the study area are complex; earthquakes occur frequently and cause significant destruction to mountain surface. the “5.12” wenchuan earthquake that occurred in this area was due to the longmen mountain fault zone. the gully is in the cathaysian tectonic belt, very close to the deep fracture zone of the maoxian-wenchuan reverse fault (longmen mountain fault zone), which lies in the southwest to northeast direction (zhou et al. 2000). the strata exposed in the gully include diorite (δ23) in the upper reaches, plagioclase granite (γo24) in the middle reaches, and granodiorite (γδ23) in the lower reaches. the rock mass of the study area belongs to the magmatic rock formed in the jinning-chengjing period with structural characteristics of high strength, joint fissures, and strongly weathered layers at the surface. consequently, it is prone to instability, collapse and landslides. the lianshan bridge gully is located in the area of the active fault. furthermore, the gully is situated about 5 km from the epicenter of the wenchuan earthquake. the earthquake caused widespread destruction in the catchment of lianshan bridge gully and formed a large number of landslips and landslides, providing a rich source of loose solid material for the debris flows. based on “the ground motion parameter zoning map of the wenchuan earthquake” (gb18306-2001), the study area belongs to the high-intensity seismic region (viii degrees) with peak ground acceleration of 0.2 g and characteristic period of seismic response spectrum of 0.35 s. 2.4. rainfall the study area is in the subtropical humid monsoon climate zone of the sichuan basin border and it is situated within the rainstorm center of the west sichuan region. according to the observation data from the wenchuan meteorological station, the average annual rainfall is 932.6mm, and the maximum and the minimum annual rainfalls are 1277 and 754 mm, occurring in 1992 and 1998, respectively. the maximum daily rainfall can be up to 166.7mm mm. precipitation over the year is unevenly distributed, concentrated mainly in the period may to september (figure 2), which accounts for 79% of the annual rainfall. this period usually has a large number of collapses, landslides, debris flows, and other geological disasters (liu et al. 2010). in short, the water source condition is favorable to the formation of debris flows in the lianshan bridge gully. figure 2. the average monthly rainfall of yinxing town, wenchuan (data from 1990 to 2010). 3. methodology 3.1 field investigations this research used field investigations, remote sensing interpretation and model calculation to determine the dynamic characteristics of the debris flow. the steps taken to collect and analyze the data are shown in figure 3. field work was conducted from july 12 to august 28, 2013, by interviewing local residents to ascertain the debris flow history in lianshan bridge gully (we will visit and inquiry many local residents of different ages in near lianshan bridge gully. some older local residents can remember their grandparents, parents who tell them the history of debris flow disasters. we ascertain the frequency of the debris flow disasters based on investigative information) and show that the “7.10” debris flow was the largest debris flow in the past century in this gully. a 1:1000 topography map of the deposition area and a 1:200 cross-section map of the downstream channels are given in figure 4a and b. as shown in figure 4, nine test pits in the downstream channel and debris flow samples at six sites (s1–s6) from the downstream channel and debris fan were collected for sieving and particle gradation analysis (figure 4a and c). field and laboratory dry sieving tests were conducted following the british standards (chen et al. 2012). the seven materials identified (smaller than 2 mm diameter) were tested further using laboratory dry sieving tests. grain sizes were determined by using field weighing and laboratory particle size analysis techniques, from which the debris flow bulk density was then obtained. figure 3. schematics of investigation methods for debris flow dynamic characteristics e4 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li figure 5. the field weighing of debris flow density. figure 4. field investigation methods of debris flow gully. 3.2 parameter calculating 3.2.1 density of debris flow density of debris flow was calculated using field matching and statistical analysis methods. a. the field matching method in the field matching method, first of all, we measured the size of bucket before sampling, and then we can calculate the volume of samples. secondly, soil samples were uniformly mixed in the field in order to help local residents to compare it with 7.10 debris flow (figure 5). finally, we got the weight of the soil samples, and at the same time we can use these data to calculate the mean value of debris flow density. we select five soil samples to calculate debris flow density; the results of debris flow density are shown in table 2. table 2. the calculation results of debris flow density. b. the statistical analysis method. the density of debris flow was calculated using the formula 1 (chen et al. 2003). this formula was obtained from data collected from a large number of debris flow gullies located in the southwest region of china; this reflected the relationship between the percentage of clay particles in the debris flow (< 0.005mm) and the density of the debris flow. (1) where γ ϲ is the density of the debris flow (kg/m³), and ˟ is the percentage of clay particles in the debris flow. formula 1 is mainly suitable for the calculation of the viscous debris flow density that clay content of debris flow is less than 10%. the percentage of debris flow clay particles was obtained by particle size analysis tests conducted in the laboratory, and the results of debris flow density with clay content are shown in table 3. the debris flow density of lianshan bridge debris flow gully is 1940kg/m³ according to the calculation results of the above two methods. table 3. the results of debris flow density with clay content. 3.2.2 velocity of debris flow the velocity of the viscous debris flow was calculated using the experiential formula 2 (chen et al. 2011a), which was obtained from statistical data of a large number of debris flow gullies located in the southwest region of china. (2) where vc is velocity of the viscous debris flow (m/s); nc is the roughness values of debris flow gully, the nc is empirical coefficient that it was determined by the hydraulic radius of the debris flow; hc is the hydraulic radius of the debris flow (m); ic is the longitudinal gradient of debris flow gully bed (‰). the hydraulic radius and longitudinal slope of 10 debris flow sections were measured from the top of gully to the gully mouth (figure 6). the velocity of 10 debris flow sections were calculated through these acquired parameters, and the results are showed on the table 4. e5case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china figure 6. the field investigation of debris flow section. table 4. the calculation of debris flow velocity. e6 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li 3.2.3 the discharge of debris flow the peak discharge of debris flow qc was calculated by both the morphological investigation method and the hydrological manual method. a. the morphological investigation method the discharge of debris flow based on the morphological investigation method was calculated using the area of each debris flow section and the corresponding velocity of debris flow. the results are showed in table 5. table 5 the calculation of discharge using the morphological investigation method b. the hydrological manual method. firstly, the flood peak discharge (qp) of lianshan bridge gully was determined in order to calculate the debris flow peak discharge (qc). the flood peak discharge (qp) can be calculated by formula 3 that the flood peak discharge calculated from this formula is more suitable for the southwest region of china (ni et al. 2012), and the results are given in table 6. (3) where a is the watershed area of debris flow gully; ψ is the runoff coefficient of the flood peak; s is the rainfall intensity; τ is the runoff confluence time of the rainstorm, and n is the attenuation index of the rainstorm; l is the length of debris flow gully; j is the mean gradient of debris flow gully bed; p is the rainstorm frequency, for example, p=1% means that the rainstorm was a once-in-a-century. table 6. the calculation of the flood peak discharge. the debris flow peak discharge (qc) can be calculated using the formula 4. (4) where du is the blockage coefficient in the debris flow gully. usually, the degree of blockage is classified as very serious blockage (du=3.0–2.5), serious blockage (du=2.5–2.0), normal blockage (du=2.0–1.5) and minor blockage (du=1.5–1.0). for the situation in the wenchuan earthquake area, the values were calculated to be 5.5–4.5, 4.5–3.5, 3.5–2.5 and 2.5–1 (you et al. 2012). based on the field investigation and the channel blockage as identified from aerial photography, the blockage coefficient for the lianshan bridge gully is considered to be 2 and 3. is the increase coefficient of debris flow peak discharge, which can be calculated (formula 5) as follows. (5) where γw is the density of water (kg/m3), usually determined as 1000 kg/m3; γs is the density of the solid material (kg/m3), usually determined as 2650kg/m3, and γc is the density of debris flow (kg/m3). 3.2.4 the total and solid volume of debris flow the total and solid volumes of debris flow were calculated using the experiential formulae 6 and 7(chen et al. 2011b). the results are given in table 7. (6) (7) where wc is the total volume of debris flow (m³); tc is duration of the debris flow (s); qc is the debris flow peak discharge (m³/s); γw is the density of water (kg/m³); γc is the density of the debris flow (kg/m³); γs is the density of the solid material (kg/m³); is the solid volume of the debris flow. table 7. the total volume of debris flow in different frequency.m 3.2.5 the impact force of the debris flow the impact force of debris flow is an important parameter in debris flow control engineering design that includes the impact force of the debris flow due to the dynamic pressure and the maximum stone size. a. the impact force of debris flow dynamic pressure the impact force of the debris flow due to the dynamic pressure was calculated using the formula 8 (he and wu 2009). the results are shown in table 8. (8) where σ is the impact force of the debris flow due to the dynamic pressure (kg/m²); ᵧc is the density of the debris flow (kg/m³); λ is the building shape coefficient, usually determined as 1.33 when the building is rectangular; vc is velocity of debris flow (m/s); σ is the angle between the buildings and the stress surface pressure direction, which is usually determined as 90°. e7case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china 4.1.2 characteristics of 24 hour rainfall 24 hour rainfall data from july 5th to july 10th were collected from taoguan and dongjialaonao village meteorological stations. (table 11 and figure 7). table 11 indicates that from 1hr to 5hr, rainfall has a maximum intensity which reaches up to 88.1mm. the maximum 1 hour rainfall is 21mm on july 10th and lianshan bridge debris flow triggered at the same time around 3 am. table 8. the results of the impact of the debris flow. b. the impact force of the maximum stone size the impact force of debris flow dynamic pressure resulting from the maximum size of individual stones was calculated by the formula 9 (he and wu, 2009). the results are shown in the table 9. (9) where f is the impact force of the maximum article size in the debris flow (kn); kc is the correction coefficient and usually determined as 0.1; rs2 is the radius of the maximum stone size and determined as 2.9m; v1 is the poisson’s ratio of material and v2 is poisson’s ratio for the maximum stone size; m2 is quality of the maximum stone (kg) in the gully; v is relative velocity of the stones (m/s) and determined as the 70% of vc ; is the modulus of elasticity of binding material and is modulus of elasticity of the maximum stones (kg/m²). table 9. the impact of debris flow calculated using the elastic impact method. 4. disaster characteristics 4.1 triggering rainfall 4.1.1 the previous rainfall amount the previous rainfall amount is a very important factor in calculating potential debris flow. the daily precipitation from july 5 to july 10, 2013 at the lianshan bridge debris flow gully was collected through the taoguan village meteorological station (e103°29′13″, n31°15′05″, h: 1113m) located 100m upstream of the gully and the dongjielaonao village meteorological station (e103°29′13″, n31°15′05″, h: 955m) located 50m downstream of the gully. the previous rainfall amounts are given in table 10. there has been rainfall within 3 days before debris flow triggered. taoguan village meteorological station showed that maximum daily rainfall is up to 57.5 mm and the cumulative rainfall is 102.5mm within 3 days before debris flow triggered, the dongjielaonao village meteorological station showed that maximum daily rainfall is up to 185.5mm and the cumulative rainfall is 240.9 mm within 3 days before debris flow occurred. table 10. daily rainfall occurring before july 10, 2013 (mm). figure 7. 24 hour rainfall curve. e8 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li table 11. 24 hour rainfall data from july 5 to 10. 4.2 the loose solid material the wenchuan earthquake of may 12, 2008 produced a great deal of loose solid material that can lead to the development of devastating debris flows (tang et al. 2009, 2011). shieh et al (2009) pointed out that the loose sediments, deposited by the co-seismic landslides, were the main reason for a lowering of the meteorological thresholds for debris flows. the debris flow activity in these earthquake-affected areas is very high in the first 5–10 years after an earthquake, while the total period with an increased potential for debris flow activity can last 20 to 40 years (tang et al. 2012). the distribution of loose solid materials in lianshan bridge debris flow gully was mapped through post-disaster field investigations and remote sensing interpretation (figure 8). pit tests were used to determine the thickness of solid materials, and the total volume of loose solid materials was calculated through field measurements. radar mapping was used to determine the thickness of the solid materials in some areas of the gully bed where pit tests were difficult to undertake, and the volume of solid materials calculated (figure 9). the area and thickness of old debris flow deposits and landslide deposits were obtained by remote sensing, which measured the area of colluvial deposits; the function (guzzetti et al. 2008) was used to calculate the volume of the colluvial deposits. the loose solid material in the lianshan bridge debris flow gully was 140.5×104 m3 based on the field investigation and remote sensing interpretation, including collapse sediments 197,000 m3, landslide 721,000 m3, bed sediments 57,000m3, slope erosion loose solid materials 325,000 m3, old debris flow sediments 105,000m3. such abundant loose solid material provided source conditions for the large-scale debris flow outbreak. figure 8. field measurement and radar test. e9case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china figure 9. the distribution map of loose solid materials. 4.3 deposition characteristics 4.3.1 the characteristics of particle size along the gully the gully is divided into 6 parts based on the changes in the particle size characteristics (figure 10). these are: (a) from the top of gully to qidaoyan, the particle size is mostly gravel-cobble, containing angular diorite and granite, without sorting. the gravel size is generally in the 3-10cm range, and the gravel content is 40-50%. the stone size is generally 20-40cm, and the content is about 30%. the largest stone size is up to 3m. (b) from the right branch ditch to the entrance of the main gully, the particle distribution is mostly stone, containing angular diorite and granite, without sorting. the gravel size is generally in the 5-15cm range, and the gravel content is 30-40%. the sizes of the stones are generally in the 30-60cm range, and the stone content is approximately 40%, with the largest stone size up to 2.5m. (c) from qidaoyan to shipizi, the particles are mostly stone and soil; the stone is angular diorite and granite, without sorting. the gravel size is 6-16cm, and the gravel content is 20-30%. the stone size is generally in the 20-60cm range, and the stone content is about 50%. the largest stones are up to 2m. (d) in the shipizi gully, the characteristic particle size shows mostly gravel and soil, containing angular diorite and granite, without sorting. the gravel size is generally 5-15cm, and the gravel content is 40-50%. the stone size is mostly in the 20-40cm range, and the content of stone is about 25%; the stone size can be up to 4m. (e) from shipizi to an altitude of 1100m, the particles are mostly gravel and soil; the gravel is angular diorite and granite, without sorting. the gravel size is 5-20cm, and the content is 30-40%. the stone size is in the 20-60cm range, and the stone content makes up about 20%, with the largest stones up to 2m. (f) from an altitude 1100m to the mouth of gully, the particles are mostly gravel and soil; the gravel is angular diorite and granite, without sorting. the gravel size is generally in the 5-15cm range making up 10-15% of the whole. the stone size is mostly 20-60cm, and the stone content is about 5%; the largest stones can be up to 1.5m. figure 10. the changed characteristics of particle size along the gully. e10 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li 4.3.2 the particle characteristics on debris flow deposition the debris flow partition accumulation characteristic of the gully is significant because of the gradient of the gully. the debris flow deposition is mainly distributed on the lower slopes and wider regions of gully downstream. from the field investigation, it was found that the composition of the material has a wide grading of soil, and the clay fraction is less. the majority of the stone is angular, and the stone content is about 50-60%. lithology varies between mostly anorthosite and granodiorite (table 12). table 12. statistics of particle survey on the gully mouth（site: n31°11′26.2″e103°29′57.5″ h:1068m）. 5. the block and burst characteristics of a debris flow gully it is unusual that such a large-scale debris flow outbreak should occur in such a small debris flow gully. blockage of debris flow gully is the main reason for large-scale debris flow outbreaks in small debris flow gullies in earthquake-affected areas. in lianshan bridge debris flow discharge case, it was found that there were two discharge alteration points, in section no. 2 to no. 3 (the discharge increased from 33.6m3/s to 62.4m3/s, an increase of nearly 100%) and in section no. 4 to no. 5, where the discharge increased from 73.2m3/s to 157.8m3/s, a discharge increase of over 100% (figure 11). e11case study of the characteristics and dynamic process of july 10, 2013, catastrophic debris flows in wenchuan county, china figure 11. the block and burst characteristics of debris flow gully. site investigations make it apparent that large stones (greater than 1.5m in length) in the lianshan bridge channel were considered to have a statistical effect on the debris flow discharge (table 13). because the shape of large stone is irregular, the volume of stone is calculated based on the cuboids, and multiplied by the experiential coefficient 0.7 for china’s western mountains (hu et al.2011; chen et al. 2005). the channel distribution of these large stones is shown in figure 9 and 11. the main reason for the change in debris flow discharge is these large landslides and massive stones. two sections 3 and 5 block the movement of debris flow. because of higher hydraulic gradient and strong impact of landslide around this section, it offers more resistance as compared to section 3 (figure 11 and 12). table 13. statistics of the large stones size on the channel (units: m and m3). 6. prevention and control measures strong earthquakes often induce a large number of collapses, landslides and unstable slopes and also generate abundant unconsolidated material that contributes to debris flow. debris flows will be a major hazard in the wenchuan earthquake area for many years (cui et al. 2014). characteristics of debris flow in the wenchuan earthquake area compel us to take engineering measures before the active period of debris flows. new engineering structures are proposed in this paper to mitigate debris flow hazards along some small gullies and slopes. the appropriate engineering measures include the buildup of one or more block dams, to stop the huge stones, and a fence wall to resist the loose solid materials to reach on the outside of gully. a block dam engineering measure has already been applied in the circulation area of debris flow which makes the steady flow of the loose solid materials. one horizontal grilled dam was built in the accumulation area to intercept large stones. the debris flow will then be channeled into a drainage groove, and finally drained into the minjiang river (figure 12). e12 gui-sheng hu, ning-sheng chen, javed iqbal tanoli, yong you and jun li figure 12. aerial photograph of the lianshan bridge gully (a: shoot on august, 2013; b: november, 2014). 7. summary after the wenchuan earthquake, debris flows from yinxing to miansi town in wenchuan county along the g213 road occurred often, resulting in frequent interruption of traffic on both the g213 road and duwen highway. using field investigations, remote sensing interpretation, laboratory experiments, model calculation and interviewing local residents, it was concluded that the factors influencing the debris flow were the formation processes and dynamic characteristics of “7.10” lianshan bridge debris flow. the following conclusions were drawn from the available information. (a) in areas with long term seismic activity, intense rainfall can easily induce debris flow. (b) bank collapse and large rocks also contributed to an increase in debris flow scale and discharge, and eventually led to the “7.10” large scale debris flow in the lianshan bridge gully watershed. (c) currently, there is still considerable loose solid material available for debris flow formation in these gullies. landslide deposition, channel deposition, and soil erosion sediment are three kinds of sediment that supply material for debris flow formation. the total volume of loose solid material is 140.5× 104 m3. such large amounts of solid loose sediment could generate debris flows easily given adequate rainfall. (d) reliable prevention and control measures should be put forward with the purpose of reducing debris flow disasters in these debris flow gullies (e) monitoring and early warning systems should be set up and government should strengthen propaganda on disaster prevention and mitigation to improve residents’ consciousness of debris flow disasters, which could effectively reduce the human and infrastructure losses caused by debris flow hazards. acknowledgements this research was supported by the national natural science foundation of china (grant no. 41501012 and 41661134012) and foundation for selected young scientists, institute of mountain hazards and environment, cas (no.sds-qn-1306, y3l1340340, sds-135-1202-02). references chen, h.x., zhang, l.m., chang, d.s., and zhang, s. 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(2014). preliminary study of the effect of earthquakes on the rainfall threshold of debris flows, engineering geology. 182, 130–135. zhang, h.p., yang, n., and zhang, y.q. (2006). geomorphology of the minjiang drainage system (sichuan, china) and its structural implications, quat. sci. 26, 126–135 (in chinese with english abstract). zhang, s., zhang, l.m., chen, h.x., yuan, q., and pan, h. (2013). changes in runout distances of debris flows over time in the wenchuan earthquake zone. journal of mountain science. 10 (2), 281–292. zhou, r.j., pu, x.h., and he, y.l. (2000). recent activity of minjiang fault zone, uplift of minshan block and their relationship with seismicity of sichuan, seismol. geol. 22, 285–294 (in chinese with english abstract). palabras clave: inclusiones fluidas, microtermometría, espectroscopía raman, mineralizaciones auríferas, distrito de el vapor, fluidos reducidos, orogénico. how to cite item dorado, c. e., & molano, j. c. (2018). microthermometry and raman spectroscopy of fluid inclusions from el vapor gold mineralizations, colombia. earth sciences research journal, 22(3), 151-158. doi: https://doi.org/10.15446/esrj.v22n3.63442 epigenetic el vapor gold mineralization is hosted by the segovia batholith and sedimentary rocks at both sides of el nús fault, in the eastern flank of the colombian andes central cordillera. gold mineralization is composed by continuous and discontinuous sigmoidal and stockwork veins and veinlets from a few centimeters to two meters of thickness and by hydrothermal breccias. ore mineralogy includes pyrite + galena + sphalerite + chalcopyrite + pyrrhotite + proustite pyrargyrite. gold occurs as inclusions within pyrite, between quartz crystals and filling fractures within pyrite at a late stage of mineralization. the fluid inclusions in quartz veins occur as clusters of primary inclusions or in alignments of secondary and pseudo-secondary inclusions. based on petrography and raman spectroscopy, four types of fluid inclusions could be recognized: (1) type i are primary biphasic, liquid-rich, with co2(v) + n2(v) + ch4(v) + kcl(aq) + nacl(aq) + h2o(l), (2) type ii are primary or pseudosecondary, multi-volatile presenting co2(v) + co2(l) + n2(v) + ch4(v) + kcl(aq) + nacl(aq) + h2o(l), (3) type iii are secondary, two-phase, liquid-rich inclusions, composed of h2o(v) + h2o(l) + kcl(aq) + nacl(aq) and have two different times of formation (iiia and iiib), (4) type iv are very rare, three-phasic, secondary inclusions and composed of s + h2o(v) + h2o(l) + kcl(aq) + nacl(aq). the microthermometric and spectroscopic analysis of fluid inclusions indicates that the fluid associated with the first event of mineralization identified in el vapor have low to moderate salinities (3.5-9.2 wt% nacl equiv.), with trapping temperatures between 214°c and 350°c and pressures between 0.5kbar and 2.9kbar. due to the nonexistence of fluid inclusions with variable degree of filling with opposite homogenization and because of the variations of salinity in a restricted homogenization temperatures range, it is proposed an isothermal fluid mixture process, which together with fo2 changes, generated by fluid reaction with the carbonaceous shales of the segovia sedimentary rocks, could influence the process of gold deposition. hydrothermal fluids from el vapor were near neutral and reduced; simlar features have been found in orogenic deposits hosted by turbidite sequences around the world. abstract keywords: fluid inclusions, microthermometry, raman spectroscopy, gold mineralizations, el vapor district, reduced fluids, orogenic. microthermometry and raman spectroscopy of fluid inclusions from el vapor gold mineralizations, colombia microtermometría y espectroscopía raman de inclusiones fluidas de las mineralizaciones auríferas de el vapor, colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.63442 las mineralizaciones epigenéticas de el vapor se encuentran hospedadas en el batolito de segovia y rocas sedimentarias a ambos lados de la falla de el nús, en el flanco oriental de la cordillera central de los andes colombianos. las mineralizaciones auríferas están compuestas por venas y vetillas continuas y discontinuas, sigmoidales y en stockwork desde pocos centímetros hasta dos metros de espesor, así como por brechas hidrotermales. los minerales de mena incluyen pirita + galena + esfalerita + calcopirita + pirrotina + proustita – pirargirita. oro se presenta como inclusiones dentro de pirita, entre cristales de cuarzo y rellenando fracturas dentro de pirita, en un estadio tardío de mineralización. las inclusiones fluidas en las venas de cuarzo se presentan como agrupaciones de inclusiones primarias como alineaciones de inclusiones secundarias y pseudo-secundarias. con base en petrografía y espectroscopía raman, se identificaron cuatro tipos de inclusiones: (1) tipo i, primarias, bifásicas, ricas en líquido, con co2(v) + n2(v) + ch4(v) + kcl(aq) + nacl(aq) + h2o(l), (2) tipo ii, primarias o pseudo-secundarias, multivolátiles, con presencia de co2(v) + co2(l) + resumen record manuscript received:22/03/2017 accepted for publication: 01/06/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 151-158 camilo e. dorado & juan c. molano universidad nacional de colombia departamento de geociencias cedoradom@unal.edu.co, jcmolanom@unal.edu.co g e o l o g y 152 camilo e. dorado & juan c. molano figure 2. photos showing different morphologies of quartz veins. a. anastomosing veins in black shales, la fortuna mine. b. hydraulic breccia cemented by milky quartz, el infierno mine. c. discontinuous quartz veins with abundant sulfides, los misteriosos mine. d. zoned vein, la argentina mine. introduction el vapor gold deposit is located at the eastern flank of the central cordillera of colombia. the area is dominated by deep cortical, strike-slip structures with a general strike n-s related to the el nús fault, el bagre fault and palestina fault and with a crystalline basement characterized by multi-strained, paleozoic and neoproterozoic metamorphic rocks. this basement has been intruded by jurassic and meso-cenozoic granitic bodies and has acted as a rigid block facing the marked strain produced by the nazca plate convergence, subduction from the west and the collision of the western cordillera and the choco-panama block. although el vapor gold zone has experienced craft mining for over forty years, no published information about its characteristics or its possible origin exists. this article describes for the first time the features of el vapor gold deposit, specifically focusing on the hydrothermal conditions of its formation. a morphological, mineralogical and paragenetic description of quartz veins is presented, a detailed fluid inclusion study is reported, and an estimation of the conditions related to temperature, pressure, ph and oxygen fugacity (fo2) is performed. also, the transport and deposit mechanisms of gold are discussed, to provide a better understanding of the formation conditions of this gold deposit. geological setting three main geological units lie in the area of el vapor district, (fig 1): (i) san lucas quartz-feldspathic gneisses (gonzález, 2001), which contains lenses of amphibolite and marble and are proterozoic (feininger et al., 1972); (ii) segovia batholith of jurassic age (160 ± 7 m.a., k/ar in hornblende, feininger et al., 1972), consists of a diorite with textural and compositional variations of quartz diorite and locally hornblende gabbro; and finally, (iii) segovia sedimentary rocks in the west of segovia mainly composed of black shales interbedded with siltstones, sandstones and intraformational conglomerates, which in the north of the area have been dated between the upper aptian and the lower albian (feininger et al., 1972) based on macro-fauna dating. vapor gold mineralization is mainly hosted on segovia sedimentary rocks, but is also seen in the rocks of segovia batholith, in proximity to distention zones and el nús fault with ne strike, related to the conjugate movement of el bagre and el nús faults. bedding planes in segovia sedimentary rocks work as a first order controller of the arrangement of the mineralized zones. the mineralized zones are veins and veinlets with thickness varying from one centimeter to two meters , and constitute a mineralization trend of approximately 2km long, with variable strikes between n20°e and n40°w, with dips between 45° and 85°. to the east of el nús fault, gold mineralization is composed by sigmoidal, discontinuous, anastomosed and stockwork veins and veinlets with brecciated texture (fig 2a and 2c). the veins have incipient zonation with thin alteration halos and fine-grained pyrite into the backrests. towards the central part they have coarse-grained sulfides with aggregates of massive milky quartz and, at the center appear druses filled by quartz crystals with ‘comb’ texture. in some places, hydraulic breccias are observed, in which angular fragments of the host rock are cemented by aggregates of quartz with extensive surface. to the east of el nús fault, el vapor veins and veinlets present a zonation (fig 2d), with massive milky quartz in the borders, sulfides towards the middle zone and in some cases quartz druses with ‘comb’ texture in the center. figure 1. geological map and location of el vapor district, showing the location of the samples used in this study. n2(v) + ch4(v) + kcl(aq) + nacl(aq) + h2o(l), (3) tipo iii, secundarias, bifásicas, ricas en líquido, compuestas por h2o(v) + h2o(l) + kcl(aq) + nacl(aq), con dos temporalidades diferentes (iiia y iiib), (4) tipo iv, muy escasas, de tres fases, secundarias y compuestas por s + h2o(v) + h2o(l) + kcl(aq) + nacl(aq). los análisis microtermométricos y espectroscópicos de las inclusiones fluidas indican que el fluido hidrotermal relacionado con el primer evento de mineralización identificado en el el vapor, presenta una salinidad baja a moderada (3.5-9.2 wt% nacl equiv.), temperaturas de entrampamiento entre 214°c y 350°c y presiones mínimas de entrampamiento entre 0.5kbar y 2.9kbar. debido a la inexistencia de inclusiones fluidas con grados variables de relleno y modos diferentes de homogenización, y teniendo en cuenta las variaciones de salinidad observadas en un rango restringido de temperaturas de homogenización, se propone que un proceso de mezcla isotermal de fluidos, junto con cambios en la fugacidad de oxígeno, generadas por la reacción de los fluidos con los shales carbonosos de las rocas sedimentarias de segovia, pudieron influenciar el proceso de depósito del oro. los fluidos hidrotermales en el vapor fueron reducidos y cercanos a la neutralidad, características similares han sido encontradas en depósitos orogénicos hospedados en secuencias turbidíticas alrededor del mundo. 153microthermometry and raman spectroscopy of fluid inclusions from el vapor gold mineralizations, colombia el vapor veins and veinlets show narrow alteration halos (0.1m to 0.5m thick). this pervasive alteration is moderate to strong and suggests channeled fluid circulation. the alteration halos are composed of chlorite + calcite + epidote, in the diorites and quartz diorites of segovia batholith and of muscovite + illite + quartz + pyrite in the black shales of segovia sedimentary rocks. mineralogy and paragenesis el vapor veins and veinlets are mainly composed of milky quartz and present sulfides in small proportions. locally it is found concentrations of sulfides exceeding 15% or disseminated fine-grained pyrite at the veins borders. based on textural patterns, three different types of quartz could be identified in el vapor district, (fig 3): 1. quartz with massive composition represents more than 80% of the modal content of el vapor minerals. this texture is characterized by aggregates of sub to euhedral crystals with straight edges, ranging in size from 120µm to 5mm, and is associated with coarse-grained pyrite crystals. this texture tends to be located in the middle and central part of the mineralized zones at both sides of the el nús fault. 2. quartz with mosaic texture appears as small aggregates of single crystals (between 10µm and 70µm), with sutured edges and undulatory extinction. this quartz occurs (1) at the borders of mineralized zones, together with sericite/muscovite and fine-grained pyrite and (2) at the borders of the massive quartz crystals, or filling cavities and micro-cracks in coarse-grained pyrite crystals, that occur associated with galena, chalcopyrite, sphalerite and gold. 3. quartz with ‘comb’ texture appears as sub to euhedral crystals with size varying from 200µm to 5cm. in exceptional cases, this texture is observed filling veinlets and druzy vugs into the central part of the mineralized zones, and in some cases is associated with pyrite and light colored sphalerite. based on mineralogical associations, textural differences and crosscutting relationships between different ore, gangue and alteration minerals it was possible to identify the following sequence of geological events: (1) hydrothermal alteration event in a early mineralization stage observed in the contact of quartz veins and host rocks, which has fine-grained pyrite, sericite, and quartz with mosaic texture and is associated with fine-grained calcite, chlorite and epidote. (2) the first event of mineralization that presents quartz with massive texture, and coarse-grained pyrite with small inclusions of gold and is associated with galena, sphalerite, chalcopyrite and pyrrhotite. (3) the second event of mineralization is characterized by quartz with mosaic texture, gold, galena, chalcopyrite, sphalerite, and proustite-pirargirita filling fractures and cracks. (4) the third event of mineralization is comprised of quartz with ‘comb’ texture, pyrite and light colored sphalerite. (5) supergene alteration with covellite and bornite observed at the borders of chalcopyrite crystals. gold is occurs: (1) as small inclusions (between 7µm to 100µm) hosted by coarse-grained pyrite crystals and sphalerite (fig 4a); (2) as grains between massive quartz crystals; (3) as irregular, small to large (20µm 250µm) grains into micro-fractures within coarse-grained pyrite crystals (fig 4b), associated with galena and chalcopyrite crystals. figure 3. microphotographs of different types of quartz in el vapor district. a. quartz crystals with ‘comb’ texture. b. an aggregate of small quartz crystals with mosaic composition filling spaces. c. and d. subhedral to euhedral crystals with straight edges and massive texture. figure 4. microphotographs of textural relationships in el vapor district. a. gold inclusions inside sphalerite with chalcopyrite inclusions and pyrite. b. gold filling fractures inside pyrite. methodology rock samples six (6) rock samples of mineralized milky quartz veins located in the west to the east of el nús fault were chosen for this fluid inclusion research (fig 1). these veins present subhedral to euhedral quartz crystals with massive texture and are representative samples of the first mineralization event. due to the size of the quartz crystals with mosaic texture and their intense internal deformation, it was not possible to make microthermometric determinations of the second mineralization event. from the selected rock samples, five (5), 1414, 1418, 1419, 1428(b) and 1443, are veins with concentrations of fine-granular pyrite to the borders and coarse-grained sulfides toward the middle. however, one sample, 1268, is on a hydraulic brecciated zone with ‘cockade’ texture where fragments of black shales from segovia sedimentary rocks are cemented by crystals aggregates of massive texture. analytical techniques the petrography and microthermometry were carried out in doubly polished thin sections with average thickness between 110µm and 130µm, using traditional methods (goldstein, 2003) and prepared using representative fragments of rock samples. there were used the principles given by roedder (1984) and the concept of fia (‘fluid inclusion assemblage’), defined by goldstein (2003) as well as the number of phases, their proportions at ambient temperature (25°c) and the spectroscopic data for the recognition of primary pseudo-secondary and secondary inclusions. a total of ninety-six (96) microthermometric measurements in fluid inclusions were performed using a linkam thsm 600 stage coupled to an olympus bx41 petrographic microscope. two standards of synthetic inclusions from the university of leoben fluid inclusions laboratory, containing h2o (final ice melting temperature: 0ºc; homogenization temperature: 374.1ºc), and co2 (co2 melting temperature: -56.6ºc) were 154 camilo e. dorado & juan c. molano used for calibration. the accuracy of the equipment was estimated at +1°c to the lowest temperature and +4.7°c for the highest. the microthermometric analyzes were carried out in the laboratories of the national university of colombia, bogotá. six phase changes in the fluid inclusions were recorded during cooling/heating routines: (1) co2 melting temperature (tmco2), (2) eutectic temperature (te), (3) final ice melting temperature (tmice), (4) clathrates melting temperature (tmclath), (5) co2 homogenization temperature (thco2) and (6) total homogenization temperature (th). raman spectroscopy analysis for the liquid and vapor phase were made using a nicolet almega equipment, which uses a laser with a wavelength of 532nm and is located at the technological development center of the colombian emerald, bogota. some measurements were carried out at the laboratories of the united states geological survey (usgs) in denver, colorado. the identification of the molecular species contained in the fluid inclusions was conducted according to the offset of values determined to each gas and liquid (burke, 2001; frezzotti et al., 2012). fluid inclusion characteristics quartz crystals with massive texture are full of fluid inclusions, assembled into inclusion ‘clusters’ of variable sizes (5-40 µm) (fig5a-b). there are also restricted fluid inclusions alignments at growth zones (fig 5c) and into aligned trans-granular cracks which crosscut the crystals (fig 5d). it was possible to establish the occurrence of four fluid inclusion types in el vapor district (table 1). type i inclusions: rare, primary fluid inclusions, restricted to growth zones in crystals. although, some samples occur as isolated groups. they are two-phase and liquid-rich fluid inclusions (co2 + n2 + ch4)v / (h2o)l, with a v/l ratio between 0.06 and 0.21. this type of inclusions present natural contours with ovoid and rounded shapes, with size between 4µm and 30µm (fig 6a). these fluid inclusions coexist para-genetically with type ii inclusions. type ii inclusions: primary to pseudo-secondary inclusions, settled in alignments that do not touch the edges of the crystals and organized as randomly isolated groups in three-dimensional arrangements in quartz crystals. they are multi-volatile inclusions with three phases at ambient temperature (co2 + n2 + ch4)v + (co2 + h2o)l + (h2o)l and a v/l ratio between 0.05 and 0.38. they have different shapes, from regular with soft edges to irregular with jagged edges. their size is between 5µm and 50µm (fig 6b). type iii inclusions: they are the most abundant inclusions in the samples and are easily identified as secondary inclusions aligned in trans-granular cracks. they are two-phase, liquid-rich inclusions (h2ov/ h2ol), subdivided in iiia and iiib considering their temporality. type iiia inclusions have a v/l ratio between 0.08 and 0.18 percent, while in type iiib inclusions, the v/l ratio varies between 0.07 and 0.11. their size is between 6µm and 26 µm and between 5µm and 16µm respectively. they have regular shapes, which appears as ovoids with smooth edges (fig 6c). type iv inclusions: they are rare in el vapor district, they were observed filling fractures associated with type iii inclusions, and classified as secondary inclusions. they have three phases at ambient temperature (h2ov + h2ol + s), the solid phase could not be identified by petrographic methods (fig 6d). their v/l ratio is between 0.04 and 0.11 and its size between 12µm and 13µm. they have regular, ovoid shapes with smooth contours. figure 5. fluid inclusions occurrences in quartz crystals from el vapor district. a-b. clusters of fluid inclusions. c. small primary fluid inclusions along the edge of a quartz crystal. d. multitude of small secondary fluid inclusions aligned along trans-granular cracks. figure 6. fluid inclusions types in el vapor district. a. small type i inclusions, rock sample 1443. b. multi-volatile, type ii inclusions, rock sample 1418. c. small biphasic secondary type iii inclusions, rock sample 1414. d. type iv inclusion with three-phase, rock sample 1414. table 1. types of fluid inclusions identified in el vapor district. type size (µm) origin composition (25°c) v/l ratio thtotal (°c) i 4-30 primary (co 2 +n 2 (?) +ch 4 (?) ) v + (h 2 o) l 0.06-0.21 236.2 349.8 (l) ii 5-42 primary to pseudo secondary (co 2 +n 2 + ch 4 ) v + (co 2 + h 2 o) l + (h 2 o) l 0.05-0.38 214 -306 (l) iiia 6-26 secondary h 2 o v + h 2 o l 0.08-0.18 159 – 209 (l) iiib 5-16 secondary h 2 o v + h 2 o l 0.07-0.11 131 – 168 (l) iv 12-13 secondary (h 2 o v +h 2 o l + s) 0.04-0.11 -------------155microthermometry and raman spectroscopy of fluid inclusions from el vapor gold mineralizations, colombia microthermometric results the microthermometric results are separately presented for each fluid inclusion type: type i fluid inclusions: these type of inclusions do not exhibit a melting temperature of solid co2 and the first observed change of phase corresponds to the first melting temperature of ice (te) ranging from -21°c and -22.8 °c. ice final melting temperature (tmice) is between -2.8°c and -10.2°c. a vigorous shaking of the vapor bubble between +4.5°c and +8.8°c indicate melting temperature of clathrates (tmclath), revealing the presence of co2, confirmed by raman spectroscopy. it is also probable the existence of n2 + ch4 in the vapor phase. all type i inclusions completely homogenize (th) by the disappearance of the vapor phase (l+vl) in a temperature range between 236.2°c and 349.8°c (fig 7). type ii fluid inclusions: type ii fluid inclusions present co2 melting temperature (tmco2) between -53.4°c and -60.3°c, with high concentration of data at -57.5°c (fig 7). the eutectic temperature of the aqueous subsystem (te) varies between -18.9°c and -22.8°c. ice final melting temperature (tmice) is between -1.9ºc and -8°c, with a greater data concentration from -4°c to -5°c and in -7°c. clathrate melting temperature occurs in almost all samples in one of the quadruple equilibrium points (q2) from the system, this temperature varies between +5.1°c and +8.2°c, with a data concentration + 6.5°c. (fig 7) co2 homogenization temperature, was observed between +23.6ºc and +30.9°c with high data concentration at +30°c. total homogenization temperature varies between 214°c and 306°c, with a high data concentration at 250°c (fig 7). type iii fluid inclusions: eutectic temperature (te) of type iiia fluid inclusions varies from 22.1°c to -22.8°c, and type iiib fluid inclusions eutectic temperature ranges from -10°c to -22.2°c. ice final melting temperature (tmice) varies between -2.8°c and 7.6°c for type iiia inclusions and from -2ºc to and -6.4°c for type iiib. all observed inclusions of type iii homogenize to a supercritical fluid by vapor phase contraction. the homogenization temperature range for type iiia fluid inclusions is from 159°c to 209°c and from 131°c to 168°c for type iiib fluid inclusions (fig 7). type iv fluid inclusions: the type iv inclusions are very few; cooling/ heating routines were only performed in three fluid inclusions. the first melting temperature of ice (te), varied between -23.1 and -23.5°c. ice final melting temperature (tmice), is between -4.1 and -7.7°c. the maximum temperatures reached during microthermometric routines for type iv inclusions correspond to the homogenization of the fluid phases, between 131.5ºc and 179.2°c (fig 7). the disappearance of the solid phase was not observed. raman spectroscopy the spectroscopic analysis in type i inclusions confirms the presence of vapor co2 and probably of n2, which was responsible for the gas hydrates (clathrates) formation in the cooling routines (fig 8a). type ii inclusions analyses confirm the presence of n2 and ch4 and co2 in the vapor phase (fig 8b-c). the composition of these components in the vapor phase varies from 21 to 97 mol% for the co2, 3 and 65 mol% for n2 and 0 to 14 mol% for ch4. composition of mineralizing fluids the composition, salinity and molar volume of type i inclusions were modeled on the ice program (bakker, 1997; bakker & brown, 2003), using the state equation of peng & robinson (1976). the program uses an h2o+nacl+co2 system (based on tmclath, tmice) and assumes an xco2 = 1 composition for vapor step (according to the raman spectroscopy analysis) as well as a volume estimation of each phase after melting of clathrates. due to the phase volumes were calculated based on two-dimensional data, there is an inherent error in the calculations (bakker & diamond, 2000). however, considering that type i inclusions have a small vapor phase compared to the liquid phase, the effects of error propagation in the estimation of the composition are considered relatively low. in addition to information obtained, type i inclusions indicate a different salinity between 4.1 and 4.9 wt % nacl equiv., a density between 0.82 and 0.93g/cc, a molar volume between 20.7 and 23.4cc/mol and molar fraction of co2 from 0.029 to 0.039 and of na+from=0.013-0.015. figure 7. a. type ii inclusions co2 melting temperatures. b. type i and ii fluid inclusions clathrates melting temperatures. c. type i, ii and iii fluid inclusions salinities. d. total homogenization temperature of fluid inclusions in el vapor district. 156 camilo e. dorado & juan c. molano type ii inclusions were modeled by the q2 program (bakker, 1997; bakker & brown, 2003) using the state equation of peng and robinson (1976). the program uses values tmclath and thco2, the composition of the non-aqueous phases from raman spectroscopy analysis (xco2= 0.96, xn2=0.04) and the proportion of the phases at ambient temperature. the results indicate that type ii inclusions have salinities between 3.5 and 9.2wt % nacl equiv (6.4 on average), a density between 0.75 and 0.97g/ cc (average 0.94), a molar volume between 19.3 and 37.9cc/mol (21 on average) and molar fraction of co2 from 0.04 to 0.08, of n2 from 0.001 to 0.026 and of na+ from 0.001to 0.0275. furthermore, the type iii inclusions characteristics were modeled by the bulk program from fluids package (bakker, 1997; bakker & brown, 2003), using empirical equations for salinity calculation, given by bodnar (1993) and the state equation of zhang & frantz (1987) for estimating the amount of components in the inclusions. salinity values for type iiia inclusions are between 4.6 and 11.2 wt % nacl equiv and for iiib inclusion are between 3.4 and 9.7 wt % nacl equiv. the density values range between 1.07 and 1.08g/cc and between 1.02 and 1.07g/cc respectively. the molar volume for type iiia inclusions is in a range of 17.4 to 17.8cc/mol, and for type iiib inclusions are from 17.5 to 17.8cc/mol. discussion and interpretation of microthermometric data hydrothermal fluid composition microthermometric analysis for all types of fluid inclusions in el vapor district indicated the hydrothermal fluids presents a relatively simple composition, containing a mixture of nacl and kcl as the dissolved salts in the aqueous system and different proportions of co2, ch4 and n2 in the vapor phase for type i and type ii fluid inclusions, indicated by the co2 melting temperatures and confirmed by raman spectroscopic analysis. these fluids presents low to moderate salinity and are similar to ore fluids of orogenic gold deposits (groves et al., 1998; wilkinson, 2001; bodnar et al., 2014). immiscibility or fluid mixtures the occurrence of aqueous fluid inclusions (type i) and aqueouscarbonic fluid inclusions (type ii) para-genetically coexisting in the same rock sample in el vapor district can be explained by three main processes (zoheir et al., 2008): 1. deformation post trapping (‘stretching’, ‘necking down’) producing partial loss of water, with an enrichment of some inclusions with volatiles (co2, ch4, n2, etc.) and the aqueous phase trapping in liquid-rich fluid inclusions. 2. phase separation (immiscibility), followed by a trapping process of carbonic components (co2+ch4) and liquidrich fluid inclusions simultaneously (ramboz et al., 1982, diamond, 1990). 3. mixing of two fluids from different sources, followed by trapping of representative fluid inclusions of each fluid and/or trapping of various inclusions at different times. although there are local evidences of a post trapping modification process by fragile deformation in some of the analyzed rock samples in el vapor, especially in type ii inclusions with a bigger size, the narrow range of homogenization temperatures obtained for fluid inclusions with similar phase relationships allows us to interpret that the trapping modification process can be ruled out in most of the cases. in practical terms, an immiscibility or effervescence process can be explained due to the coexistence of inclusions with a variable degree of filling with opposite homogenization (l and v) in the same temperature range. although in el vapor, there were found locally some fluid inclusions figure 8. a. raman spectra for type i fluid inclusions in el vapor. b-c. raman spectra for multi-volatile type ii fluid inclusions. 157microthermometry and raman spectroscopy of fluid inclusions from el vapor gold mineralizations, colombia the value variations of ƒo2 indicate that the mineralizing fluids in el vapor district probably had a complicated story. the co2/ch4 variable relationship is related to the presence of reduced phases (with a greater abundance of ch4), and is a product of its reaction with host rocks, particularly with black shales of the segovia sedimentary rocks. gold transport and deposit some authors have explored the transport mechanisms in relatively reduced hydrothermal solutions in equilibrium with pyrite/pyrrhotite. these solutions contain chlorides and sulfides (seward, 1973; seward, 1984; hayashi & ohmoto, 1991) in a temperature range between 160ºc and 350°c and a ph between 1.4 and 9.5. the experimental results indicate that in ph conditions that are close to neutrality, and with a temperature range of 250°c to 350°c (similar to those established in the el vapor district), the gold is mainly mobilized as bi-sulphide [hau(hs)2 0, au(hs)2-; auhs(h2s3) 0]. this process neglects the use of chlorine complexes as a method of fluid transport, except if the fluid is low in h2s, rich in chloride and the ph is less than 4.5 (hayashi & ohmoto, 1991). according to the recognized conditions of ph and oxygen fugacity for the el vapor district and the frequent association of gold crystals with sulphides, the most likely mechanism of gold transport is in the form of auhs(h2s3) 0, similar to the established conditions for other similar deposits emplaced in sedimentary sequences (jia et al., 2000). because the lack of evidence supporting a process of fluid immiscibility, a change in ph, or the loss of h2s in the el vapor district and because the microthermometric and spectroscopic analyses supports instead a fluid mixture and fluid reduction processes, we propose these mechanism as responsible for gold deposition from the hydrothermal solutions. with variable phase proportions (v/l ratios between 0.06 and 0.21 for type i and between 0.05 to 0.38 for type ii), different densities and different co2 proportions, the nonexistence of inclusions with contrasting homogenization, suggests that an immiscibility process is not a responsible for the formation of inclusions. finally, the presence of coexisting type i and type ii inclusions, can be explained by an isothermal fluid mixture process based on the fact that two-phase, liquid-rich fluid inclusions (type i) and primary inclusions in all cases have a restricted salinity range (4.1 and 4.9 wt between nacl equiv %). while type ii inclusions suggest, in a similar range of trapping temperatures (th), a bigger variability in the calculated salinity (3.5 to 9.2wt % nacl equiv) (fig 9). trapping temperature and pressure homogenization temperatures represents minimum trapping temperatures, except when there is evidence of well-supported immiscibility (roedder & bodnar, 1980). in el vapor district due to the lack of conclusive evidences for an immiscibility process, the calculated p-t conditions represents a minimum value. the calculated isochores for type ii inclusions using the ‘isochore’ program (bakker, 1997; bakker & brown, 2003) indicate a pressure range between 0.591kbar and 2.996kbar. on the other hand, the calculated isochores for type i fluid inclusions indicate a pressure range between 0.129kbar and 1,452kbar. due to the lack of other geo-barometers in el vapor district, the results of this study should be understood as the only available source of information about the depth to which the ore deposit may have been formed. ph and oxidation state of hydrothermal fluids in el vapor the mineralogical alteration associations (carbonate + epidote + chlorite and muscovite + illite + quartz) indicate a fluid near to neutrality, with a range of ph between 5 and 6 (phillips & groves, 1984) in el vapor district. meanwhile, the co2, ch4 and n2 variable content, identified in the microthermometric and raman studies, shows a reduced oxygen fugacity (ƒo2) in the hydrothermal fluid, consistent with the ore mineralogy: abundant pyrite, presence of pyrrhotite, and the absence of arsenic in the system, confirmed by the nonexistence of arsenopyrite. the presence of ch4 and n2 in fluid inclusions as well as the association observed of carbonic material in the mineralized zones can be explained by the reaction of mineralizing fluids with host rocks (johnson et al., 1995; cox et al., 1991; 1995), particularly with carbonaceous shales of segovia sedimentary rocks. the values of oxygen fugacity, calculated to ph=5.5 (determined by the association of minerals alteration) at a pressure of 300mpa (comparable to the maximum trapping temperature obtained), is between 10-30 and 10-38 bar and is considered with th achieved values (fig 10). figure 9. diagram showing the relationship between total homogenization temperature and salinity for type i, ii and iii inclusions in el vapor district. figure 10. diagram showing the relationship between temperature and oxygen fugacity for mineralizing fluids in el vapor district, at ph = 5.5 and p = 300 mpa. modified from jia et al., 2000. 158 camilo e. dorado & juan c. molano conclusions based on the microthermometric and raman analysis in this research, it can be concluded that in el vapor district, hydrothermal fluids were responsible for the transport and deposition of gold in the first mineralization event. in addition these fluids are related to the co2 + ch4 + n2 + h2o + nacl system, with low to moderate salinity, reduced ph near neutrality, with a trapping temperature between 213ºc and 340°c, a trapping pressure between 50 and 300mpa, and a depth of formation between 2 and 10km. the destabilization of bi-sulphide complexes in which gold was transported, was produced by an isothermal fluid mixture process, as well as the reduction with host rocks. the characteristics in el vapor ore deposit are remarkably similar to characteristics described for orogenic deposits (groves et al., 1998), particularly those deposits emplaced in turbidite sequences around the world, as muruntao (drew et al., 1996; graupner et al., 2001), north deborah in bendigo belt (jia et al., 2000), wattle gully in victoria (cox et al., 1995) and dungash (zoheir et al., 2008). acknowledgments this research was undertaken by the project called ‘anteproyecto de investigación metalográfica, microtermométrica, geoquímica e isotópica para algunos yacimientos minerales en colombia', signed between national university of colombia and colciencias. in the beginning, this research was supported by the project called ‘asesoría y ejecución de ensayos de laboratorio para la elaboración del mapa metalogénico de colombia’ signed between the national university of colombia and colombia geological survey (ingeominas). the authors would like to thank the united states geological survey and the technological development center of the colombian emerald (cdtec) for allowing the development of raman spectroscopy analysis and bibiana rodriguez for their suggestions and contributions to the discussion. references bakker, r. j. 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(1987). determination of the homogenization temperatures and densities of supercritical fluids in the system nacl–kcl–cacl2–h2o using synthetic fluid inclusions. chemical geology, 64, 335-350. zoheir, b. a., el-shazly, a. k., helba, b., khalil, k. i., & bodnar, r. j. (2008). origin and evolution of the um egat and dungash orogenic gold deposits, egyptian eastern desert: evidence from fluid inclusions in quartz. economic geology, 103, 405-424. remote sensing quantitative retrieval of ecological water (eco-water) has been foundational in systemic and quantitative research for water resources. eco-water resource levels indicate conservation ability for the eco-water layer and influence of this on precipitation transformation and runoff regulation. the remote sensing quantitative inversion retrieved the mec (modulus of eco-water conservation) of the upper minjiang river basin study area in 1994 and 2001, and combined with climate data between 1990 and 2005, the influence of conservation water on the eco-water layer on runoff was then analyzed. results revealed significant efficacy for flood control and water supply during the drought from the hydrologic cycle of ecowater. thus protection and restoration of the eco-water layer for flood and drought prevention are crucial. el sondeo remoto del agua ecológica (del inglés eco-water, agua conservada en la superficie terrestre) es indispensable en la investigación sistemática y cuantitativa de las fuentes de agua. los niveles de suministros de agua ecológica indican la capacidad de conservación de la capa de agua ecológica y la influencia de esta en la transformación de precipitación y la regulación de escorrentía. la inversión cuantitativa por sondeo remoto estableció el módulo de conservación de agua ecológica (mec, del inglés modulus of eco-water conservation) para el área de estudio en la cuenca alta del río minjiang entre 1994 y 2001, y combinada con la información climática de entre 1990 y 2005, se analizó la influencia de conservacion de agua en la capa ecoacuática. los resultados mostraron una gran eficacia en el control de inundaciones y en el suministro de agua durante la sequía a lo largo del ciclo hidrológico. por esta razón, la protección y restauración de la capa de agua ecológica para la prevención de inundaciones y sequía es necesaria. earth sciences research journal earth sci. res. j. vol. 20, no. 3 ( september, 2016 ) : e1 e6 abstract resumen keywords: eco-water, remote sensing quantitative retrieval, upper minjiang river basin, runoff palabras clave: agua ecológica, sondeo remoto cuantitativo, cuenca alta del río minjiang, escorrentía. the influence of eco-water retrieved by quantitative remote sensing on runoff in upper minjiang river basin record manuscript received: 15/01/2016 accepted for publication: 21/11/2016 how to cite item huang, j., yang, w., yang, x., & deng, b. (2016). the influence of eco-water retrieved by quantitative remote sensing on runoff in upper minjiang river basin. earth sciences research journal, 20 (2), e1-e6. doi:http://dx.doi.org/10.15446/esrj.v20n2.55177 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.55177 jin huang, wunian yang*, xin yang*, bing deng key laboratory of geoscience spatial information technology, ministry of land and resources of the p.r. china, chengdu university of technology, chengdu, sichuan, 610059, p. r. china correspondent authors: wunian yang and xin yang equally contributed to this work e-mail: jin huang (first author) jinjin0522@163.com wunian yang (correspondent author) ywn@cdut.edu.cn xin yang (correspondent author) johnny0701@qq.com bing deng dengbing2000@163.com h y d r o l o g y influencia del agua ecológica en la escorrentía de la cuenca alta del río minjiang medida a través de teledetección cuantitativa jin huang, wunian yang, xin yang, bing denge2 1 introduction the concept of “eco-water” was introduced by yang wunian et al. in 2001, to investigate the impact of water conserved on the earth’s surface in forest vegetation on the hydrologic cycle and the local ecological environment (yang et al., 2001; wan et al., 2004). eco-water is water closely related to the earth’s surface vegetation and is conserved by the vegetation, the vegetation humus layer and the root soil layer after conversion of the atmospheric precipitation forms the “ecological water layer” centered on vegetation. ecowater is not directly involved in the hydrological cycle process yet significantly contributes as a buffer and runoff regulator as rainfall is intercepted and groundwater recharged in the eco-water layer. eco-water resource levels are difficult to extract and quantify with conventional methods. quantitative remote sensing inversion theory and algorithm, however, facilitate quantification as developments in high sensitivity remote sensing imaging related to vegetation and soil have occurred in recent years (li et al., 2009; jian et al., 2012). runoff influence resulting from the vegetation canopy, vegetation humus layer, and the soil layer has consistently presented challenges in hydrological research. extensive hydrological effect experimentation has followed involving respective sample regions producing substantial runoff information (liu et al., 1989; ogee and brunet, 2002; rao et al., 2005; hu et al., 2014). causative mechanisms from the influence of vegetation canopy, vegetation humus layer and soil layer on runoff are exhibited independently, yet interaction effectiveness is the same—conserving water, buffering and adjusting flow. the eco-water layer then utilizes the three combined as an “independent converter” in the hydrologic cycle. the capability of converting eco-water quantitatively, as expressed by the remote sensing quantitative retrieval method, simplifies water resource quantification while concurrently contributing to increased practicality for determining the influence of eco-water on runoff. 2 eco-water remote sensing quantitative retrieval 2.1 study area and data upper minjiang river basin, located in the transition region from eastern qinghai-tibet plateau to sichuan basin, is an essential tributary to the upper yangtze river and also a vital water conservation area for the chengdu plain. vegetation of the upper minjiang river has been referred to as the “natural barrier and green reservoir of the land of abundance”, but due to massive deforestation for hundreds of years, deterioration of this ecological environment has created severe reductions in water conservation capacity as forest vegetation has decreased and lead to reduction in dry season runoff. the relation between spectral data and biochemical parameters of vegetation and soil should be determined respectively for the mec modeling. different plants and soil specimens were collected and biochemical parameters then measured. the water content of vegetation and soil was computed from the biochemical parameters, while the spectral data was acquired with field spec pro fr, the full spectrum portable spectrometer of asd (analytical spectral devices). landsat5 tm images were utilized in this study with the product type referred to as “l1t”, provides systematic radiometric and geometric accuracy by incorporating group control points while employing a dem (digital elevation model) for topographic accuracy. the study area consists of three scenes; the ranks numbers are 13037, 13038 and 13039. preprocessing is required before quantitative image retrieval: mosaic, subset, radiation calibration and atmospheric correction. image quality and imaging time for eco-water remote sensing quantitative retrieval are critical factors for accuracy. cloud, snow, and fog appearance in images will, for example, significantly interfere with the ecological water quantitative inversion if not controlled within 10%. consistent seasons should be compared to the amount of eco-water resources in different years as vegetation and precipitation vary among seasons producing large differences in vegetation and soil water content. images for mec retrieval were acquired on june 26, 1994, and on june 13, 2001, in this study. two phase images are with high quality and similar imaging time that makes inversion results more accurate and comparable. 2.2 modulus of eco-water conservation modeling the amount of water conserved by the eco-water layer was represented by the modulus of eco-water conservation (mec), an integral parameter of ecological water. mec is related to vegetation and soil water content and represents the water conservation ability of the earth’s surface based on vegetation. remote sensing technology with high sensitivity and accessibility allowed to extract the vegetation and soil information. thus quantification of each was a precondition for calculating mec. the basic research unit of remote sensing image digital processing is a single pixel with mec following by also utilizing a single pixel as the primary research unit. according to the definition of mec that the mec model of pixel (i,j) is as follows: (1) where ewtc(i,j) is vegetation water content of the pixel(i,j), w(t)(i,j) is soil water content of the pixel(i,j). if there are m lines and n rows of the remote sensing image of the study area, the amount of ecological water resource is: (2) the principle of remote sensing quantitative inversion is the reflection spectrums of surface features exhibit particular characteristics (zhang et al., 2010). vegetation water content has an obvious response to spectral characteristics in the wavelength range from near infrared to intermediate infrared. 1600nm indicates a high moisture absorption band while 820nm indicates a weak moisture absorption band, allowing the vegetation index sr (simple ratio) established by the ratio effectively to highlight the vegetation water content information (zhang et al., 2010; wang et al., 2011; yi et al., 2012). the modeling process of vegetation water content is demonstrated in figure 1. advantages and disadvantages of soil moisture estimation models with optical sensor differ, and after analyzing the characteristics and regression effects with actual soil water content measurement, smmrs (soil moisture monitoring by remote sensing) based on soil line was chosen. this is a straight line in a twodimensional spectral space of red and near infrared generated from a series of pure soil pixels ranged by reflectance. the line is a comprehensive reflection of the soil’s characteristic in various water conditions (zhan et al., 2007; qin et al., 2012). the modeling process of soil water content is demonstrated in figure 2. the influence of eco-water retrieved by quantitative remote sensing on runoff in upper minjiang river basin e3 figure 1. the vegetation water content modeling flow figure 2. the soil water content modeling flow 2.3 results of eco-water remote sensing quantitative retrieval sr is the ratio of infrared wavelengths (center wavelength of 1676 nm) and nearly red band (center wavelength of 840 nm) in remote sensing image inversion of ewtc. the soil line of the image must be derived initially to compute smmrs for the remote sensing image reversal of w(t). the modulus of eco-water conservation of the study area was then retrieved according to the mec model based on the already retrieved ewtc and w(t). ecological water-related parameters inversion results are displayed in table 1. table 1. eco-water remote sensing inversion results jin huang, wunian yang, xin yang, bing denge4 3 the influence analysis of eco-water on runoff 3.1 change trend of runoff and climate weather conditions during the same period should also be considered when studying the influence of ecological water on runoff. climate data collected for this study was related to precipitation and temperature of year and month in the upper minjiang river basin between 1990 and 2005. precipitation and temperature data were collected from 9 national surface meteorological stations (songpan, zoige, barkam, hongyuan, xiaojin, dujiangyan, pingwu, mianyang, ya'an) which were within or adjacent to the river basin and then was interpolated to derive raster data of the study area. runoff data of year and month in the upper minjiang river basin between 1990 and 2005 was collected from zipingpu hydrologic stations. according to the data analysis results, during the period 1990-2005 in upper minjiang river basin, the annual runoff depth, and the annual precipitation both revealed a decreasing trend (figure 3); annual average temperature was on the rise (figure 4); minimum monthly average depth in dry season appeared nearly consistent in february; and february monthly runoff depth exhibited a decreasing trend (figure 5). figure 3. annual precipitation and runoff depth change trend between 1990 and 2005 of upper minjiang river basin figure 4. annual average temperature change trend of between 1990 and 2005 of upper minjiang river basin figure 5. monthly runoff depth of february change trend between 1990 and 2005 of upper minjiang river basin 3.2 variation of eco-water two phases of mec were retrieved with quantitative remote sensing method. the mean of mec in 1994 was 1.6206 g/cm2, converted to a depth of 16.206 mm. in 2001 was 1.4635 g/cm2, converted to a depth of 14.635 mm. the depth of eco-water was 1.6mm less in 2001 than in 1994; thus water conservation capability decreased in 2001 from 1994 levels. subtraction of the two phases of mec may reveal the change in eco-water levels, extent and spatial distribution of recovery or degradation, and the impact of eco-water on runoff. a high ratio overlap of the mec maximum reduction area and the deforestation area occurred (figure 6), revealing that deforestation likely resulted in a reduction of water conservation capability in the basin. various levels of mec reduction took place in the lower altitude valley area, with slight increases in the relatively smooth terrain areas of songpan and heishui counties. the mec changes closely correlated to the vegetation level with reduction of mec due to vegetation degradation resulting from both natural and human factors. temperature rise and decreasing precipitation caused aridification of the river valley, leading to vegetation degradation (wu et al., 2003), while deforestation and overgrazing generated the same result (sansom, 1999). measures for plant protection and restoration were implemented to achieve an increase in mec as deforestation have been prohibited since 1999 and farmland has been converted back to forests or grasslands with mitigative forest planting continuously and actively promoted. recovery of mec remains much less than a reduction, as the mec recovery area was significantly less than the reduction area with the greatest degree of improvement at 0.6178 g/ cm2, much less than the most significant reduction level of 2.6820 g/cm2. the influence of eco-water retrieved by quantitative remote sensing on runoff in upper minjiang river basin e5 table 2. depth contrast of precipitation, runoff, and eco-water eco-water is a vegetation-centered water resource that decreases with vegetation destruction and increases with vegetation restoration. the year 1999 was a turning point for vegetation change in the upper minjiang river basin as deforestation practices were prohibited and implementation of plant protection measures followed. the changing trend is illustrated as eco-water was monitored between 1990 and 2005, first decreasing until 1999 and then slightly increasing. change direction of february runoff depth during the same period was analyzed and found to exhibit an obvious decreasing trend between 1990 and 1999 as the correlation coefficient r2 reached 0.7037 (figure 7a). the declining trend was weakened between 1999 and 2005 as the correlation figure 6. eco-water mec changes and comparison with deforestation area between 1994 and 2001 of upper minjiang river basin 3.3 the influence analysis of eco-water on runoff a comparison of mean mec with annual precipitation depth, annual runoff depth and runoff depth in the dry season was conducted to determine the eco-water influence on runoff. the effect of eco-water on runoff is displayed more clearly by water recharging abilities in the dry season as the formation of flood peak is so closely aligned with precipitation. influence of eco-water on runoff focuses on exploring water recharging capability for a runoff in the dry season in this study. comparing the data from the year 2001 with the year 1994 (table 2), the annual precipitation depth had increased by 8.7% and the annual runoff depth had increased by 9%, but the runoff depth in dry season had decreased by 5.6%, indicating transformation of precipitation directly into surface runoff increased, while intercepted and conserved precipitation fell. the mean of mec depth had dropped8.9%, explaining the reduction in water conservation capacity of the basin. comparing the two phases for the depth of runoff and eco-water reveals the vital influence of eco-water on runoff as when the amount of eco-water was reduced, the water supply capacity of eco-water to flow in dry season also decreased. jin huang, wunian yang, xin yang, bing denge6 coefficient r2 was only 0.1116 and the decreasing slope increased from -0.3358 to -0.1952 (figure 7b). the trend change comparison further proved that ecowater exerted significant influence on runoff in the upper minjiang river basin. trends demonstrated a pattern showing that when the amount of ecowater decreased, eco-water water recharge capability to a runoff in dry season also decreased, while under eco-water recovery conditions, water recharge capability also exhibited a degree of recovery. figure 7a. february runoff depth change trend between 1990 and 1999 of upper minjiang river basin figure 7b. february runoff depth change trend between 1999 and 2005 of upper minjiang river basin 4 conclusion and prospect (1) as a product of vegetation canopy, vegetation humus layer, and the soil layer, eco-water may be assumed to be independent of the hydrologic cycle, allowing the study of water storage and recharge capability from the three sources to be systematic and quantitative and simplifying the study of the related hydrological effects. (2) eco-water is related to vegetation at the earth’s surface. thus changes of eco-water resource amounts are closely related to vegetation alterations. deforestation is central to decreases in the supply of eco-water resources making plant protection and restoration for the eco-water layer protection and restoration crucial. (3) the comparison of change revealed that eco-water exerted influence on runoff in the upper minjiang river basin. a vast supply of eco-water increased capability of water recharge for a runoff in the dry season and a decreasing eco-water supply weakened the capacity. eco-water is integral to the hydrological cycle in determining the allocation of water resources. water resources will attain a dynamic and natural balance in well-developed eco-water areas; however, when the eco-water layer suffers damage, precipitation will begin to transform into surface runoff directly and instantaneously, contributing to an imbalanced setting off flooding, droughts, and ecological deterioration. further research may produce sufficient eco-water data retrieved over time to be analyzed with precipitation and temperature for specifying more accurate effect and action mechanisms of eco-water on runoff and for contributing to improved flood and drought management. references jian, j., yang, w. n., jiang, h., wan, x. n., li, y. x., & peng, l. (2012). a model for retrieving soil moisture saturation with landsat remotely sensed data. international journal of remote sensing, 33(14), 4553-4566. doi: http://dx.doi.org/10.1080/01431161.2011.652313 li, y. x., yang, w. n., tong, l., jian, j., & gu, x. f. (2009). remote sensing quantitative monitoring and analysis fuel moisture content based on spectral index. acta optica sinica, 29(5), 14031407. doi: 10.3788/aos20092905.1403 liu, x. d., wu, q. x., & su, n. h. (1989). studies on rainfall interception in canopy, litter and soil hydrological characteristics of forests in liupanshan mountains. scientia silvae sinicae, 25(3), 220-227. ogee, j., & brunet, y. (2002). a forest floor model for heat and moisture including a litter layer. journal of hydrology, 255(1), 212-233. doi: http://dx.doi.org/10.1016/s0022-1694(01)00515-7 qin, q. m., you, l., zhao, y., zhao, s. h., & yao, y. j. (2012). soil line automatic identification algorithm based on two-dimensional feature space. transactions of the csae, 28(3), 167-171. rao, l. y., zhu, j. z., & bi, h. x. (2005). hydrological effects of forest litters and soil in the simian mountain of chongqing city. journal of beijing forestry university, 27(1), 33-37. sansom, a. l. (1999). upland vegetation management: the impacts of overstocking. water science and technology, 39(12), 85-92. wan, x. n., yang, w. n., wu, b f., sun, w. d., & huang, q. (2004). conception of eco-water sphere and its application. advance in earth sciences, 6(19), 117–121. wang, l. t., wang, s. x., zhou, y., liu, w. l., & wang, f. (2011). vegetation water content retrieval and application of drought monitoring using multi spectral remote sensing. spectroscopy and spectral analysis, 31(10), 2804-2808. wu, y., su, z. x., & fang, j. y. (2003). study on causes and ecological renewal of arid and warm valley of upper minjiang river. journal of china west normal university (natural sciences), 24(3), 276-281. yang, w. n., yuan, p. x., & wan, x. n. (2001). the study report: the ecological environment integrative survey and evaluation in the minjiang upper river based on remote sensing. chengdu: chengdu university of technology archives, 863-308-21(6). yi, q. x., bao, a. m., luo, y., & zhao, j. (2012). measuring cotton water status using water-related vegetation indices at leaf and canopy levels. journal of arid land, 4(3), 310-319. zhang, j. h., xu, y., yao, f. m., wang, p. j., guo, w. j., li, l. & yang, l. m. (2010). advances in estimation methods of vegetation water content based on optical remote sensing techniques. science china technological sciences, 53(5), 1159-1167. zhang, r. h., tian, j., li, z. l., su, h. b., chen, s. h., & tang, x. z. (2010). principles and methods for the validation of quantitative remote sensing products. science china (earth sciences), 53(5), 741-751 zhan, z. m., qin, q. m., ghulan, a., & wang, d. d. (2007). nir-red spectral space based new method swelling behavior of clays is of great importance for numerous engineering applications due to the prevalence of expansive clays around the world. expansive clays are present in mexico city, united states, australia, africa and south america, among others. in some cases, these types of clays could present swell pressure values greater than 400 kpa. in this technical note, correlation equations are developed to estimate the swell pressure of clays using laboratory tests performed on swelling clays in the city of barranquilla, colombia. correlations are based on atterberg limits and water content among other soil properties. equations with statistically significant coefficients were selected and compared with equations found in the literature. developed correlations evidenced that swell pressure decays quickly as water content increases. it was found that for the studied soils, atterberg limits did not present statistical significance in the estimation of swell pressure. abstract keywords: expansive clays, swell pressure, atterberg limits, water content. empirical correlations for the swelling pressure of expansive clays in the city of barranquilla, colombia correlaciones empíricas para la determinación de la presión de expansión de arcillas expansivas en barranquilla, colombia issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.60226 earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 45 49 víctor cantillo, vicente mercado, césar pájaro* universidad del norte, colombia *capajaro@uninorte.edu.co el comportamiento expansivo de las arcillas presenta vital importancia alrededor del mundo, ya que este tipo de arcillas se encuentras en diversas y extensas zonas del planeta. arcillas con este tipo de comportamiento se encuentran presentes en diversas regiones del mundo como la ciudad de méxico, los estados unidos, australia, algunas zonas de áfrica y sudamérica, entre otras. este tipo de arcillas pueden presentar en algunos casos presiones de expansión superiores a los 400 kpa. en la presente nota técnica se desarrollan correlaciones para estimar la presión de expansión de arcillas con base en ensayos de laboratorio practicados a arcillas con comportamiento expansivo presentes en la ciudad de barranquilla, colombia. las correlaciones desarrolladas se basan en los límites de atterberg y contenido de humedad, entre otras propiedades de los suelos. se seleccionaron aquellas ecuaciones que presentaban coeficientes estadísticamente significativos y se realizaron comparaciones con correlaciones presentes en la literatura. las correlaciones mostraron que la presión de expansión decae rápidamente a medida que el contenido de humedad aumenta. además, se observó que para los suelos estudiados los límites de atterberg no son estadísticamente significativos para la estimación de la presión de expansión. resumen palabras clave: arcillas expansivas, presión de expansión, límites de atterberg, contenido de humedad. record manuscript received: 20/09/2016 accepted for publication: 10/02/2017 how to cite item cantillo, v., mercado v., & pajaro, c. (2017). empirical correlations for the swelling pressure of expansive clays in the city of barranquilla, colombia. earth sciences research journal, 21(1). 45-49. doi:http://dx.doi.org/10.15446/esrj.v21n1.60226 g e o t h e c n ic s 46 víctor cantillo, vicente mercado and césar pájaro 1. introduction the soils of barranquilla, a city of colombia, in south america, are mainly composed of layers of sand and gravel (moreno, 2013). however, the western and southwestern part of the city (figure 1) present extensive clay layers with swelling behavior (geotecnología ltda., 2006). in this part of barranquilla, the overall type of construction is essentially residential housing. for this reason, the demographic density in this sector of the city is high. since the mid-1970s, the western and southwestern zone of barranquilla have presented large landslides and slopes instability problems (figure 2) as a consequence of both anthropic and natural factors, and these phenomena have affected around two thousand lowincome families (guardo, 2000). figure 1. location of the study area. (ingeominas, 2011) some of the geotechnical problems in barranquilla´s western and southwestern zone are due to swell behavior of underlying clays (geotecnología ltda, 2006). swelling of clays affects the structural stability of the overlying infrastructure; structural problems are often accentuated due to the topographical characteristics of the terrain, which is constituted by inclined slopes, leading to the collapse of structures, as illustrated in figure 2. studies developed to assess and understand the swelling behavior of these clays have been carried out by geotecnología ltda. (2006), and by edgar rodríguez (2014). the main goal of the work undertaken by geotecnología ltda. (2006) was to increase and improve the geological information regarding the western and southwestern part of the city. on the other hand, rodríguez (2014) aimed to study the geological structure of clay materials in the zone. the results of rodríguez (2014), geotecnología ltda. (2006), and other researchers revealed that most of the geotechnical problems of the area, are related to the water seepage through the clay layers. geotecnología ltda. (2006) explains these phenomena in detail in their work. they state that the upper soil layers in this zone are constituted by sands and gravels with high permeability, followed by barranquilla’s marine clays. during rainy seasons, the granular soils act as huge water retainers in the interface between these soils and the clays beneath. this accumulated water causes clays to swell and at the same time, it causes the loss of slope stability (geotecnología ltda., 2006). a typical soil profile for the study area is presented in figure 3. another important cause of landslides is that earth fills’ inclination in some cases is greater than the clay’s effective friction angle; for instance, some slopes have inclinations between 16° and 25° (guardo, 2000), while the internal effective angle of friction for these materials ranges between 5° and 12° (rodríguez, 2014). figure 2. consequences of landslides at western and south western of barranquilla (angulo, 2014). figure 3. typical soil profile of the study area. (after geotecnología ltda., 2006) clays in barranquilla´s western and southwestern zone present prominent swelling behavior which is mainly produced by the montmorillonite clay mineral content (feferbaum and beltran, 1974). soils with similar characteristics can be found in different zones around the world, for example in the united states, south-america, africa, australia, china and india (murthy, 2011). swell phenomena in clayey soils have been studied by komornik and david (1969), hensen and smith (2002), day (1991, 1992), among others. several authors presented a variety of correlation equations to determine swell pressure of expansive clays, and examples of these equations are present in erguler and ulusay (2003), sridharan and gurtug (2004) and phanikumar (2006). these investigations define the swell pressure value as a function of water content, atterberg limits, dry unit weight and other parameters proposed by the authors, with laboratory tests designed by them. erguler and ulusay (2003), for instance, suggest the “maximum water content in 24 hours” and the “maximum water content in 72 hours” as predictive parameters, while phanikumar (2006) introduces a parameter called free swell index. there is still a great necessity for civil and geotechnical engineering practice in the city to deepen the understanding of the mechanical properties of the soils in this zone of the city. in an effort to extend the knowledge about the mechanical behavior of clays in this part of barranquilla, this technical note presents correlation equations to estimate swell pressure as a function of water content and atterberg limits. correlations found in this study could be useful in the study of expansive clays around with similar characteristics in different zones around the world. predictive equations presented in this technical note are based on the water contents and atterberg limits of clays in barranquilla’s western and southwestern zone as explanatory variables. these properties were selected 47empirical correlations for the swelling pressure of expansive clays in the city of barranquilla, colombia because they are widely used to develop swell pressure and swell index correlations in the literature, and at the same time, these measurements are perhaps the most prevalent in most geotechnical reports. the associated laboratory tests were performed following the procedures outlined by the technical specifications of invias (colombian national institute of roads): inv e-125-07 (corresponding to astm d 4318-00) to determine liquid limit, inv e-126-07 (corresponding to astm d 4318-00) to determine plastic limit, and inv e-122-07 (corresponding to astm d 2216-98) to determine water content. 2. measurement of swell pressure there are three standardized methodologies which determine swell pressure using laboratory testing (sridharan, 2009): swell – load method (curve 1 on figure 4), swell under load method (curve 2 on figure 4), and constant volume method (curve 3 on figure 4). it should be mentioned that, as illustrated in figure 4, each laboratory methodology to determine swell pressure yields different swell pressure values (thompson et al., 2006; kayabali and demir, 2011). figure 4. typical stress-strain curve in laboratory methodologies to determine swell pressure. (after sridharan et al., 2009) in this study, the constant volume method was used to determine swell pressure values implemented to develop the correlations equations. the constant volume method, usually called strain controlled method, can be conducted by taking a specimen in a consolidation ring and applying a pressure equal to the result of the addition of the effective overburden pressure and the approximate anticipated surcharge caused by the foundation. water is then added to the specimen to start swelling. as the specimen starts to swell, a small pressure increment is applied to prevent swelling. when the total stress causes specimen consolidation the swell pressure is attained. more details on the implemented constant volume method can be found in das (2015). 3. correlation equations. correlation equations presented in this section, are based on test results of barranquilla´s western and southwestern zone clays. samples were obtained mainly from the area of campo alegre, because this zone is perhaps the most affected in the city as a consequence of landslides and swelling behavior of clays. a total of 38 samples were used in this study. for each sample, water content, liquid limit, plastic limit, and void ratio were measured. table 1 summarizes the maximum and minimum values for each soil property within the collected samples. the complete water content, liquid limit, plastic limit, plasticity index and void ratio information for each of the 38 samples is summarized in appendix 1. table 1. measurement variables ranges. logarithmic models ( y = a + b ln x ), and exponential models (ln y = a + bx) were used to model swell pressure (dependent variable) as a function of the selected properties. only two models presented correlation coefficients and determination coefficients higher than 0.5; furthermore, only water content coefficients and models using water content as unique dependent variable were statistically significant. correlation equations, determination coefficients ( r2 ) and correlation coefficients (r) of the developed models are presented in table 2. table 2. correlation equation. as can be seen in table 2, statistically, the best fit and correlation equation to predict swell pressure is equation 2. table 3 summarizes goodness of fit parameters for the correlation equations. figure 5(a) shows swell pressure-water content pairs measured for each specimen and swell pressure predictions using equations 1 and 2 from table 2. among the different functional forms used to develop correlation equations between swell pressure and other soils properties, only a third equation (equation 3) which incorporated plasticity index (pi) and water content as explanatory variables, presented high determination coefficient and correlation coefficient ( r2=0.86 and r=0.93 ): ln (( sp( kpa )) = 7.77-0.12w (%) + 0.0054 pi (%) (3); however, the plasticity index was not statistically significant using a statistical significance of 0.05, having a p-value of 0.28 using a t-student distribution (table 3). figure 5(b) shows the correlation from equation 3 along the samples data; it results evident that the plasticity index does not have a major impact on swell pressure for the studied soil samples. figure 5. developed correlation models. 48 víctor cantillo, vicente mercado and césar pájaro table 3. goodness of fit parameters. 4. discussion a broader revision of swell pressure correlation equations can be found in al-rawas and goosen (2006). table 4 presents some relevant correlation equations and corresponding correlation coefficients reported by the authors. table 4. correlation equations found in the literature. w is water content; pl is plastic limit; ll is liquid limit; il is liquidity index; γdry is dry unit weight. table 4, shows that for most of the presented correlation equations, correlation coefficient values are relatively high, which means that the equations could adequately predict the trend of the swell pressure measurements. although correlation coefficients are high, it should be emphasized that the equations presented above be only valid in the measurement range of properties used to develop the equations. another important fact is that the equations are only valid for the soils for which they were developed because swell properties vary among different clays. however, all the equations present similar trends among the various properties and swell pressure values. similarly to the trend presented in this work, some of the proposed correlations yield a decrease in the logarithm of the swell pressure as a function of the water content. erguler and ulusay (2003), for instance, present the variation of swell pressure as a function of water content for soils samples collected at three different sites. in their work, swell pressure decays quickly as water content increases, reaching values of nearly 0 for water contents higher than 30%. such behavior is similar to that shown in figure 5 of this study. 5. conclusions this research presents correlation equations developed to estimate swell pressure of clays using laboratory tests performed on swelling clays in the city of barranquilla, colombia. swell pressure values were estimated using the constant volume method. estimated pressures reached values of more than 600 kpa for low levels of water content. swell pressure values decayed quickly as water content increased, reaching values of nearly 0 for water contents higher than 40%. for the tested soils, it was found that swell pressure could be best modeled as an exponential function of the water content. while previous works by other authors use several additional soil properties (e.g., liquid limit, plastic limit, plasticity index or unit weight), it was found that for the studied soils adequate swell pressure estimates were obtained based solely on water content. moreover, unlike previous works, it was found that atterberg limits did not have statistical significance in the estimation of swell pressure for the studied samples. appendix 1. database of material parameters obtained experimentally. 49empirical correlations for the swelling pressure of expansive clays in the city of barranquilla, colombia references. al-rawas, a. a., & goosen, m. f. 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(1974). the prediction of soil heave and swell pressures developed by an expansive clay. university of texas. sridharan, a. (2009). critical evaluation of determining swelling pressure by swell-load method and constant volume method. sridharan, a., & gurtug, y. (2004). swelling behavior of compacted finegrained soils. engineering geology, 72(1), 9-18. thompson, r. w., perko, h. a., & rethamel, w. d. (2006). comparison of constant volume swell pressure and odometer load-back pressure. in unsaturated soils 2006 (pp. 1787-1798). asce. the yangtze river crossing project, which is part of the chinese gas pipeline trunk network and aims to transmit the natural gas to the yangtze river delta area, sets up a record for the length of horizontal directional drilling(hdd) in the world. the term of pipeline installed is 3300m, and the diameter is 40in. many technical difficulties have been inevitably encountered in a large diameter hole reaming in fine sand, the calculation of section scour depth which determines the minimum buried depth of the pipe, protection of outside coating and the method of long distance pilot hole drilling, such other challenges. in view of this,it was recommended that two pipelines of 711mm should be installed to replace the existing one of 1016mm in diameter and 1% flooding scour depth obtained by reliable statistical analysis. in period of installation, application glass fiber reinforced epoxy resin as a protective coating, executing the pilot hole by intersect method.as a result, this super project is finished successfully on may 21.2013. the experience can be applied to any pipeline engineering the installed by horizontal directional drilling or similar works. earth sciences research journal earth sci. res. j. vol.20, no. 1 (march, 2016): p1 p5 abstract keywords: engineering innovation, long distance, large diameter, gas pipeline, horizontal directional drilling (hdd), starting velocity. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54504 li guohui1, ma xiaocheng2, yang chunling3 1. doctoral students/vice chief engineer, southwest jiaotong university/china petroleum pipeline engineering corp. email cppe_ligh@cnpc.com.cn 2.manager of crossing department, china petroleum pipeline engineering corp. email: maxiaocheng@cnpc.com.cn 3. engineer, china petroleum pipeline engineering corp. email: yang-chl@cnpc.com.cn. engineering innovation of a length of nearly 3300m large diameter pipeline installed by hdd record manuscript received: 02/12/2015 accepted for publication: 22/03/2016 how to cite item guohui, l., xiaocheng, ma, & chunling, y. (2016). engineering innovation of a length of nearly 3300mlarge diameter pipeline installed by hdd. earth sciences research journal, 20(1), p1-p5. doi: http://dx.doi.org/10.15446/esrj.v20n1.54504 introduction to improve the energy instruction in the yangtze river delta area by using clean fuels instead of coal, china national petroleum corporation(cnpc)executes jiangdu-rudong gas pipeline project. this pipeline aims to transmit the natural gas of jiangsu lng terminal to the delta area where is home to one-third of china’s population and generates an estimated 20% of the country’s annual gdp (carlin,etc.2014). yangtze river crossing is an essential part of the whole project,with the total length of 3300m,design pressure of 10.0mpa,the diameter of 1016 mm,pipe material of api 5l x70 and external coating of 3lpe. the crossing point is located at the downstream of yangtze river, and the topography is flat, which the feature land unit is over bank alluvial plain and new delta alluvial plain. the width of this location is about 3000m with active water transportation,and the average of water depth ranges between 20m to 30m..the stratumunder quaternary overburden is devonian, carboniferous, permian, triassic, jurassic and cretaceous along with the time sequence. 33 bores were drilled on both sides of crossing axis to make clear the geological condition. relying on properties like geological time, source, lithology, physical and mechanical properties, the field stratum was divided into four parts from top to the bottom. main properties are as follows: layer 1 is muddy, silty clay which contains a single thin layer of silt with a thickness of 2-5 mm. the silty clay was interbedded with silt locally. layer 2 is silty clay and the soil is uniform,containing little calcareous concretions with a diameter of 5-50mm, partially. this layer was exposed by all the borehole and it was found stable at the crossing site. layer 3is silty (dense). this layer was exposed by all the borehole,and it is evidenced stableat the crossing site. the buried depth on the roof is 10.70-43.70m and 28.40m on average. layer4 is medium sand, containinglittle like-round gravel particle and pebble. the overall diameter is 2-10mm and 20-40mm, individually.this layer was exposed to most of the borehole and distributed widely at the crossing site. the geotechnical characterization profile (gcp) is shown in figure 1. en g in ee r in g g eo lo g y p2 li guohui, ma xiaocheng and yang chunling al t i t ude( m) 51. 00 39. 00 27. 00 15. 00 3. 00 61. 00 sout h bank 51. 00 39. 00 27. 00 61. 00 15. 00 3. 00 1 2 3 4 ri ver bed figure 1. geotechnical characterization profile cppe (china petroleum pipeline engineering corp.), which is a branch company of cnpc, served as a design contractor. engineers selected the scheme of installing by hdd rather than shield tunnel. however, it is a significant challenge: the crossing length is nearly 3300m, and the pipeline diameter is 40 inches. the critical problems of this project are 1) large diameter hole reaming in the fine sand; 2) the calculation of section scour depth, which determines the minimum buried depth of the pipe; 3) protection of outside coating, and 4) the method of long distance pilot hole drilling. 1. two small diameter pipeline replace a large one avoiding to reach large diameter hole yangtze river is the longest river in china.there are rare hdd crossings in yangtze river before this project.for example,lanzhouzhengzhou-changshan production oil pipeline project uses hdd crossing in yangtze river, with a pipe diameter of 610mm, and a length of 2100m (li guohui,etc.2012).the crossing construction confronted with many difficulties like bore collapse,drillpipefracture,and finally it was finished after two difficult years..other projects that adopt big diameter and long distance hdd crossing are also rare. in 2007, hangzhou-yongning pipelineprojectcrossed qiantang river,with a diameter of 813mm,crossing a distance of 2454m(tan mingxing,etc.2008).in2008, zhuhai-zhongshan gas pipeline project crossed the modaomen river,with a diameter of 660mmand a length of 2630m(yin gangqian,etc.2008).in the recent two years,pipe diameter enlarges continually.in 2011,china-myanmar gas pipeline crossed at irrawaddy river,using thehdd crossing method,with a pipe diameter of 1016mm, and a crossing length of 1760m. a final example is the second west-east natural gas pipeline crossingpengjiawan sluice,with a pipe diameter of 1219mm, and a length of 1397m. in others countries, the record-setting bore came during the drilling of a second 10,000-foot bore and pull-in of a 30-inch pipe near al jabayl for the bern causeway pipeline project. a new global horizontal directional drill (hdd) undersea crossing record has been set regarding pipe diameter, pipe weight and length of crossing, in saudi arabia, in 2007 (share, 2009). southern gas networks (sgn) is currently reinforcing the gassupply infrastructure on the isle of wight, located off the south coast of england. part of this process required the replacement of the pipelines supplying gas from the uk mainland to the island. two 324mm-diameter steel pipelines were installed through separate holes some 50m below the bed of the solent using horizontal directional drilling (hdd). each 3,930m-long crossing is believed to be some 20% longer than any previously completed using hdd(seamans,2011). analyzing the cases mentioned regarding pipe diameter, pipe weight and length of the crossing, the case of utilizing a 1016mm diameter,with 3300 lengths and a weight reaching 2096tons pipe cannot complete pullback.take into consideration adopting two pipes of the same diameter as a replacement.comparing different diameters ranging from 610mm to 914mm,the sectional area sum for two pipe are as followsin table 1. table 1. different diameters pipe sectional area. concluding from the table 1,there is little difference between two pipes with a diameter of 711mm and one pipe with a diameterof1016mmconcerning the medium through capacity.as a consequence, adopting hdd crossing will reduce the difficulty in comparison with the use of a previous scheme. meanwhile,set two pigging station at both sides, named jingjiang station and jiangyin station.it is required that the outlet temperature is no less than 20°cand outlet pressure is no less than 8.8 mpa in jiangyin station to satisfy the transmission to downstream.we should analyze processing influence for the new scheme, which includes two stations and two 711mm diameter pipelines.the analysis of the fundamental dataconsists oftwo pipes with a diameter of 711mm and pipe thickness of 19mm, the inlet pressure of 9.48mpa, the rate of flow 139.85×104nm3/h, 3pe reinforced protecting coating thickness of 3.8mm,no thermal insulation,no inner coating, earth temperature of 25°c. the total length of the pipe is 4600m. the two pipes processing result is shown in table 2. table 2. two pipes processing result through technology analysis,the pressure of transmission pigging station is 9.18 mpa, and the temperature is 21°c, having no influence on the feasibility ofdownstream and pipe.hence,this scheme does not affect the whole design. 2. 1% flooding scour depth obtained by reliable statistical analysis the yangtze river scour primarily controls the depth of penetration because of the flood peak. according to code gb50423(china petroleum pipeline engineering co., ltd., 2007), the minimum depth of penetration should not be less than 6m from the design flood scour line.the project is defined on a large scale, and we should choose 1% (100-year return period) flood frequency. the chosenrule is shown in table 3. the design flood frequency of channel erosion calculation is 1%,and the peak flood is 108800 m3/s. table 3. engineering scale for hdd project through water area p3engineering innovation of a length of nearly 3300mlarge diameter pipeline installed by hdd since 1960, the crossing region varies relatively small.one main reason is that this project section is influenced by the dual role of runoff and tidal current, so the riverbed maintains a certain amplitude balance of erosion and deposition.the frontier waters riverbed of proposed construction suffers interactive effects of upstream inflow and reciprocating tidal current. the shoreline varies little and keeps relatively stable for years. measures have to be taken to protect the shoreline of proposed project so that the scheme has strong anti-erosion features.on condition that yangtze river regime does not change greatly,the riverbed condition can keep stable over the long run. the change of riverbed is shown in figure 2. 20 10 0 10 20 30 al t i t ude ( m) 0 500 1000 1500 2000 2500 3000 f r om nor t h bank t o sout h( m) 1966. 04 1976. 05 1986. 12 1993. 12 1998. 11 2001. 09 2003. 01 2006. 05 2007. 08 2009. 01 r i ver bed figure 2. change of riverbed from 1960 to 2009 observing the 50 years scour condition,the river regime of pipe crossing region is stable;a deep groove(-15m) in the north bank varies a lot over the years, and the south bank keeps slightly stagnant.also,the axis of the deep groove in the south bank and the deep groove itself(-20m)remains stable. the envelope line for depth scour of 50 years is shown in figure 3. figure3. envelope line for depth scour of 50 years the commonly used methods for river scour depth calculations are samov formula,xie jianheng formula,zhang ruijin formula and sha yuqing formula(zhang fengliang,etc.2013).consider samov formula and xie jianheng formula are only adapted to coarse granular sediment which satisfied the condition d>0.15~0.20mm, and they are not suited to fine sand and silty clay particles.as a consequence,scour calculation in this article uses zhang ruijin formula and sha yuqing formula which are adapted to fine cohesive sediment to figure out the starting velocity. zhang ruijin starting velocity formula professor zhang ruijin from water conservancy and electric power institute wuhan university started from rolling mode of sediment and gave the equilibrium equations of force,then derived the zhang ruijin starting velocity formula. where the units of depth h,diameter d are both m.zhang ruijin starting velocity formula is not only adapted to fine cohesive sediment but also the cohesionless coarse sediment.it is a general formula. sha yuqing starting velocity formula where ε means porosity,stable data is 0.4; the units of d and h are mm and m,respectively;ucmeans starting velocity, and the unit is m/s. the maximum scour depth always arises in the main channel.when the main channel velocity is larger than sand starting velocity,the riverbed stops scouring until the main channel velocity is smaller than starting velocity, and then the scouring process stops. the maximum rate of flow per unit width multiples the maximum vertical average velocity can be derived according to the result of 2-d tidal flow mathematical model. then by the riverbed property,use the formula below to calculate the maximum depth after scouring. if the riverbed consists of cohesive soil,the highest depth after scouring is: where uc’ means the non-scouring velocity of cohesive soil in corresponding depth. if the riverbed consists cohesionless soil,use the formula below to calculate the maximum depth: where vc is the sediment starting velocity,which can be calculated by corresponding velocity formula. when the diameter satisfies the condition d<0.15mm,the sediment particle possesses certain viscosity,the starting velocity increases according to the d decreasing,the fine particle size should be replaced by the equivalent diameter in the same starting velocity or utilizing cohesive sediment starting velocity formula to calculate. the calculated result of scour depth by sha ruiqing formula and zhang ruijin formula is shown intable 4.the scour curve by calculation is shown in figure4. p4 li guohui, ma xiaocheng and yang chunling table 4.calculation of scour depth (m) figure 4. scour curve by calculation superimpose previous monitoring envelop curve for scour line and scour line by calculation, obtaining scour line for engineering. the scour curve for engineering is shown in figure 5. figure 5.scour curve for engineering figure 5 implies that we should lay the pipeline in layer 3 fine sand. 3. application a new type protective coating 3lpe is the most commonly used as outside coating for long distance pipelines(j.d.hair,etc.2000). strengthen 3lpe are coated on hdd pipes and dedicated filed joint coating system are used for 3lpe which consists of heat shrinkable sleeve (hss) and another protective sleeve installed in the front end of the drilling direction, with 100mm overlap on hss and 3lpe, respectively. an uneven seam, or step, would be inevitably formed between hss and 3lpe after the completion of field joint coating system installation, resulting hss dragged off from the pipe during hdd pulling, even extensive damage of 3lpe coating in case of harsh geological conditions. this condition would consequently affect the integrity performance of coating due to lots of hole or damages. in a worst-case scenario, the hss might slip off from the pipe causing sticking accidents and hence making the crossing fail. cppe’ engineers applied a modified liquid solvent-free, glass fiber reinforced epoxy resin (gfrer) coating to protect the anticorrosion coating on hdd pipes. as showed in figure 6, gfrer is coated outside completely encasing the 3lpe layer and its hss field joint coating system, obtaining 3lpe/hss/gfrer system with a smooth surface for the all hdd pipeline to favor pipe pullback. figure 6. 3lpe/hss/gfrer system gfrer protective coating was first applied to the hdd project. the coating thickness is 3.7mm. using a glass fiber reinforced epoxy resin as an armoring coating over the corrosion coating preserve the integrity of the coating and minimize damage when installation. figure 7 shows the glass fiber reinforced epoxy resin painting. figure 7. photos of gfrer protective coating on completion 4. hdd design entry and exit point. it is same geotechnical for drilling effectiveness at south and north bank. so we should select the availability of workspace for pull section fabrication at the exit point. the north bank side is flatness and has enough length for pipeline welding before pulling. it is properly to clarify that the north side is exit point, and the south is the entry. input and output angle. hdd input and output angle should be determined based on crossing length, burial pipeline depth, diameter, elastic laying conditions, terrain conditions. entry angle is limited by equipment capabilities and should be 8° ~ 20°; the exit angle should be 4° ~ 12°.choose lower exit angle when pipeline diameter is large. according to the condition of surface and subsurface, in this case, the selected entry angle is 10°, and the exit is 8°. design radius of curvature. the curvature radius of hdd elastic laying section should not be less than 1500 times the diameter of the pipe to be installed, and must not be less than the 1200d. in this case,the radius is 1067m,which is 1500d. the hdd crossing horizontal distance between yangtze river buried and unearthed point is 3290m and actual length is 3300m. the hdd curve is shown in figure 8. p5engineering innovation of a length of nearly 3300mlarge diameter pipeline installed by hdd figure 8. hdd curve 5.executing the pilot hole by intersect method magnetic signal submarine cable laying.in this project, prepare to lay armored cable used to provide artificial magnetic field strength in crossing area. rent dedicated laying barge to bury the submarine cable while laying.employ the dedicated submarine measurement company to measure the cable coordinate. parameters of measurement direction control.calibrate guide hole crossing paratrack ii direction control system and mark direction control parameter.to ensure the accuracy of data,measure at different locations across the center line to compare and make a record. intersect area.combine paratrack ii direction control system and magnetic signal cable,main rig drills along with the design crossing curve and the auxiliary rig drills at the same time.two sets of rigs coordinate each other while drilling,reaching the intersect area simultaneously. in the intersect area,start axial magnet which is installed in the assistant rig drill bit,leading drill bit of main rig drilling until two drill bits combined, that is drill bit of main rig drills into the guide hole of the auxiliary rig. auxiliary rig pulls back the drill pipe, the main rig pushes on the drill pipe and ultimately unearths along with guide hole dug by the auxiliary. mud sensor, which is installed behind the main rig drill bit, will supervise the mud pressure in holes during the entire processing in pilot hole construction procedure.if the mud pressure increases unusually,pull back the drill bit and go on drilling after hole flushing. pilot holeworks last 20 days only, from march 22, 2013, to april12, 2013. 6. result once the pilot hole was successfully completed, the following reaming operations were executed using new φ168mm s-135 drill pipe: stage one: φ610mm (24”) barrel reamer pre-reaming; stage two: φ762mm (30”) barrel reamer pre-reaming; stage three: φ924mm (36”) barrel reamer pre-reaming; stage four: φ1,118mm (44”) barrel reamer pre-reaming followed by washing of the hole. the pullback was the last step in the crossing process. the pullback was executed by φ168mm drill pipe, a φ1,067mm (42”) barrel reamer, and 3,300m of steel pipeline. all drill parameters and fluid parameters were closely monitored to ensure the smooth pullback and avoid damage to the outside coating (fig. 9). figure 9. pullback operations the original project included 274 working days. the actual work was completed within this time frame. overall, the drilling operations were successful at steering along the design profile. 7. conclusion the following conclusions were reached through the hdd project: 1. it is reasonable that two 711-mm-diameter pipelines replace one 1016-mm-diameter pipeline installation by hdd. this scheme is not only guaranteeing success installation but also reduce thedifficulty. no any contractor can finish the construction if we use one 1016-mm-diameter pipeline, and we should select the shield tunnel which is more expensive than hdd. 2. it is most important that confirm the depth of penetrationaccording to river scour depth analysis. in this project, we analyze the scour depthorganizing the 50 years river scour condition and calculation by zhang ruijin formula and sha yuqing formula. 3. the success of the crossing of yangtze river has established a new record of diameter * length in oil and gas pipeline installation by hdd. references carlin, m., ariaratnam, s., ma, b.s., and zeng, c., 2014. “record setting yangtze river hdd crossing of three parallel pipelines over a length of nearly 3300m,” proc. 2014 nastt no-dig conference, north american soc. for trenchless technology, ny. gb 50423-2007: code for design of oil and gas transportation pipeline. (2008). china plans to press, beijing, china. j.d. hair & associates, inc., 2000. “coating requirements for pipelines installed by horizontal directional drilling and slip boring,” pipeline research council international, inc., houston, united states. li guohui, wei.yang, feng.qian and xc. ma,2012. “high-risk, 610mm diameter, 2,090m hdd projecta case study”, icptt 2012, american society of civil engineers (asce), reston, united states seamans, j., 2011. “hdd in record solent crossing,” world tunnelling, 2011 (jun.):46-49. share, j., 2009. “challenging hdd project provides 10,000-foot recordsetting crossing,” pipeline and gas journal, 2009, 236(5):40-42. tan,mingxing.,liu,yufeng,yang fan,.zeng qiang, lei jie, xie qishuai, 2008. “design of hdd engineering in qiantang rover”, natural gas and oil, natural gas and oil press, chengdu, china yin gangqian.,and tang x.f.,2008. “the construction technology of horizontal directional drilling crossing in modaomen river,” petroleum engineering construction vol.34 (6), petroleum engineering construction press, tianjin, china. zhang fengliang and wang xintao,2013. “discussion on the calculation of the scour depth in the flood impact assessment”, china flood & drought management,2013(5), china flood & drought management press, beijing, china. v16n1c.indb earth sciences research journal earth sci res sj vol 16, no 1 (june, 2012): 35 40 introduction the 1,100 km long strike-slip gulf of aqaba-dead sea transform fault is a major active tectonic feature linking the southern turkey-western iran taurus-zagros area with the red sea rift it has been suggested that its left-lateral strike-slip motion is a result of relative oblique left-hand movement between arabia and africa which opened up the red sea (quennell 1959; freund et al , 1970) lyberis (1988) indicated that movement in the southern dead sea transform fault, representing the gulf of aqaba, started in late miocene and was associated with a strike-slip stress pattern (40° extension associated with 130° compression) this movement produced the left lateral motion between the arabian plate and sinai peninsula and faulting has been the result of an e-w extension since the end of the miocene, indicating a rotation of the regional stress pattern in the vicinity of the transform fault al-arifi (1996) found the main maximum compressive stress (l) direction to be 137° and the main minimum stress direction (3) to be 222° using the focal mechanism solution for 53 recent earthquakes in the gulf of aqaba, the gulf ’s structure is dominated by en-echelon normal faults delimiting three elongated basins (ben-avraham et al , 1979, reiss and hottinger 1984) the northern basin has a simple bathymetry and structure, dominated by the flat-bottomed eilat deep (<900 m) the central basin consists of two deeps: aragonese deep (<1,850m), and arnona deep (<1,550m); the southern basin includes the dakar (<1,400m) and tiran (<l,300m) deeps (figure 1) migration of local earthquakes in the gulf of aqaba, north red sea nassir s al-arifi1, aref a lashin1,2 and saad al-humidan1,3 1 geology department, king saud university, p o box 2455, riyadh 11451, saudi arabia email: nalarifi@ksu edu sa 2 benha university, science faculty, geology department, p o box 13518, benha, egypt 3 sgs research chair, king saud university, p o box 2455, riyadh 11451, saudi arabia email: salhumidan@ksu edu sa abstract the gulf of aqabah has been considered one of the most seismically-active regions in the middle east during the last 15 years; a catalogue of 1,415 earthquakes (duration magnitude md ≥ 2 8) during 19851995, compiled by al-arifi (1996) and al-shaabi (1998), mainly based on data from king saud university seismic studies center (ssc), saudi arabia, for 28°30°n and 30°-36°e, has been used for studying local aftershock migration aftershocks migrated northwards about 60 km for the 1993 sequence and about 70 km for the 1995 sequence and also to shallow focal depths (15 km focal depth for 1993 main-shock) depths reduced as time elapsed until reaching 2 km for the last strong aftershock (md=5 2) which occurred during the late stage of the sequence the 1993 main-shock caused a redistribution of stresses to the parallel faults’ segment where the largest 1993 aftershock occurred triggering these faults’ segment to become an area of stress nucleation and generated the 1995 main-shock resumen el golfo de aqabah ha sido considerada una de las regiones con más actividad sísmica del medio oriente durante los últimos 15 años un catalogo de 1415 terremotos (magnitud de duración md≥2 8) entre 1985-1995, compilado por al-arifi (1996) y al-shaabi (1998), basado principalmente en datos de la universidad king saud, centro de estudios sísmicos (ssc), arabia saudita, para la zona ubicada entre 28°30°n y 30°-36°e, ha sido usado para estudiar la migración de replicas locales las replicas migraron unos 60 km al norte para la secuencia correspondiente al año 1993 y unos 70 km para la secuencia por el año 1995 y también hacia bajas profundidades focales (15km de profundidad focal para el evento principal en el año 1993) las profundidades se redujeron con el tiempo hasta alcanzar 2 km para la ultima replica fuerte (md=5 2) que ocurrió durante la última etapa de la secuencia el principal evento del año 1993 causo una redistribución de los esfuerzos hacia el segmento de las fallas paralelas donde la mayor replica en el año 1993 ocurrió, desencadenando el movimiento de estas fallas para llegar a ser un área de nucleación y generando el principal evento en el año 1995 palabras claves: golfo de aqabah, migración local terremotos, sismicidad, mar rojo keywords: gulf of aqabah, local earthquake migration, seismicity, red sea record manuscript received: 14/10/2011 accepted for publications: 06/05/2012 seismology nassir s al-arifi, aref a lashin and saad al-humidan36 this paper considers aftershocks’ vertical and horizontal local migration for the most recent sequences (i e the 1993 and the 1995 sequence) in the gulf of aqaba and presents some observations regarding the stress migration which has caused large earthquakes in the gulf data the gulf of aqaba has been considered one of the most seismicallyactive regions in the middle east during the last 15 years a catalogue of 1,415 earthquakes (duration magnitude ≥ 2 8 md ) for 1985-1995 has been compiled by al-arifi (1996) and a1-shaabi (1998), based mainly on seismic studies centre (ssc) data, king saud university, saudi arabia, for 28° 30°n and 30°-36°e historical seismicity (1068-1964) has shown that the region has suffered at least 18 moderate to large earthquakes instrumental seismicity (1965-1984) has included 284 events, 244 of them related to the 1983 sequence and centred on the eilat deep in the northern gulf recent seismicity (1985-1995) has shown that seismicity in the gulf of aqaba has been episodic (figure 2) an earthquake sequence in july 1993 began with foreshocks, followed by a mainshock (6 0 md) on august 3rd 1993, and then 403 aftershocks (md≥2 8) during the next four months (md 5 6 for the largfigure 1. index map showing the location of the study area in addition to a bathymetric chart of the gulf of aqaba (bathymetric map after hall and ben-avraham, 1978) figure 2. a histogram showing the frequency of occurrence (number of events/day) in the gulf of aqaba from 1985 to 1995 0 100 200 300 400 500 600 700 800 19 85 19 86 n um be r o f e ar th qu ak es year 19 87 19 88 19 89 19 90 19 91 19 92 19 93 19 94 19 95 migration of local earthquakes in the gulf of aqaba, north red sea 37 figure 3. epicentral distributions of earthquakes (md>3 5) for two sequences where a is the 1993 sequence and b is the 1995 sequence small and large arrows indicate the direction of the minimum and maximum compressive stresses respectively, according to small scale structures study (reches 1987), and focal mechanism study (al-arifi 1996) figure 4. (a) shows the local migration of strong aftershocks of the 1993 sequence in the gulf of aqaba (b) shows profiles for jumping of strong aftershocks of the 1993 gulf of aqaba sequence the arrows indicate the direction of the local migration and jumping est aftershock) (figure 3); the region experienced a widely-felt earthquake on november 22nd 1995, located at 28 81°n, 34 75°e, having 12 km focal depth the mainshock (md=6 2) was followed by 733 aftershocks (md≥2 8) over a 40-day period (md 5 3 for the largest aftershock) (figure 3) the 1983 sequence was concentrated in the northern part of the gulf of aqaba (28 8°29 4°n and 34 335 l°e) most of this sequence was offshore and coincided with the eilat deep although determining depth was difficult (due to a lack of stations before ssc stations were established), el-isa et al , (1984) observed that surface waves had been clearly reported in jordan university seismic station records, suggesting a very shallow depth for the 1983 sequence their observation was supported by recent ssc recorded earthquakes located in the same area that suffered from the 1983 sequence; these earthquakes were located at a shallow depth not exceeding 10 km (a1-shaabi 1998) the 1993 sequence was concentrated in the dakar and tiran deeps in the southern part of the gulf having focal depths not exceeding 26 km the 1995 sequence was distributed into two clusters, the more northerly of which was again concentrated in the eilat deep, whereas the southern cluster was in the aragonese and arnona deeps in the central gulf the 1995 sequence depth was less than the 1993 sequence; this may have been due to the aftershock area location where the 1993 aftershock zone was concentrated in the southern part (a) (b) (a) (b) nassir s al-arifi, aref a lashin and saad al-humidan38 of the gulf whereas the 1995 aftershock area spread along the gulf and aftershock density was greater in the northern than the southern part this may have indicated that earthquakes occurring in the northern part were shallower than those in the southern part of the gulf and were related to the brittle-ductile transition zone (alshaabi, 1998) this observation was supported by the 1995 aftershocks where the events in the northern cluster were shallower than those in the southern cluster the surface waves appeared clearly when the earthquakes were located in the north of the gulf of aqaba (a1-shaabi 1998); however, they disappeared from the records when they were located in the south method hypo71 software (lee and valdes, 1985) was used to relocate all events in this study (±1 0 km horizontal component error and ±2 0 depth error) of aftershocks’ local migration, both vertically with depth and horizontally in a ne-sw direction corresponding to the general state of extensional tectonic stress in the gulf of aqaba and the dead sea transform, according to small-scale structure study (richard 1987) and focal mechanism study (al-arifi 1996) the term ‘local migration’ has been used to denote a strong aftershock’s horizontal movement during a sequence; the term ‘jumping’ has been used to denote a strong aftershock’s vertical movement the term strong aftershock in this paper means the aftershocks which were felt in at least three towns in the study area because all strong aftershocks occurred at a shallow depth not exceeding 20 km, all aftershocks having md 4 9 and over were felt in at least three towns the 1993 mainshock occurred at the aftershock area’s extreme southern end: 28 45°n and 34 87°e (figure 3) the aftershocks moved northward figure 4 shows strong aftershocks’ local migration regarding the 1993 sequence on the same day in which the mainshock occurred, activity was concentrated in the north between 28 70-28 85° n and 34 75-34 9° e where two strong aftershocks (md 5 6 and 4 9) occurred 80 days after the mainshock the activity moved northwards again where another strong aftershock occurred at 29 0°n and 34 83°e activity leaped to the southwest (still north of the mainshock) on the 3rd of november 1993 with another strong shock that occurred at 28 62°n and 34 6°e; activity then moved northwards where the last strong aftershock occurred on the 4th of december 1993 at 28 86°n and 34 41°e figure 4 shows that the 1993 focal depth started with more than 10 km for foreshocks, no foreshock having less than 10 km depth the mainshock had the depth of 15 km and then the depth reduced with time from 11-12 km for the first two strong aftershocks until the last strong aftershock was at a depth of 2 km the 1995 sequence was similar, aftershock focal depth becoming reduced as time elapsed such ‘jumping’ could be explained by earthquakes migrating northwards into an area where all seismicity was shallower than in the south of the gulf as regards 1995 sequence local migration it was difficult to determine a specific direction for a strong aftershock (even though the aftershocks generally moved northwards) results the focal mechanism solution for the mainshocks and the largest aftershocks for both sequences (1993 and 1995, figure 5) included the flowing: 1) the 1993 mainshock indicated normal and strike-slip fault dipping 34° to the northwest and striking n 21°e; figure 5. shows the focal mechanism solution for the mainshocks and the largest aftershocks for both sequence of the 1993 and 1995 migration of local earthquakes in the gulf of aqaba, north red sea 39 figure 6. generalized model for the migration of large earthquakes in the gulf of aqaba 2) the largest 1993 aftershock occurring 3 hours and 50 minutes after the main shock indicated left-lateral strike-slip fault dipping 64° to the north and striking e 04°s; and 3) the 1995 mainshock indicated left-lateral strike-slip fault dipping 43° to the north and striking n 09°e; and 4) the largest 1995aftershock occurring 35 hours after the mainshock indicated normal and strike-slip fault dipping 31ρ to the northwest and striking 198° observations regarding these focal mechanism solutions, mainshock locations and the largest aftershocks for both gulf of aqaba sequences (i e 1993 and 1995) indicated the following: 1) the seismic source for the main shocks for both sequences and their largest aftershocks were completely different (figure 5); 2) the seismic source of the largest 1993 aftershock and the 1995 mainshock were located in the same fault zone (figure 5); and 3) there was systematic south to north migration of the earthquakes’ seismic sources (figures 4 and 5) such systematic northward migration was influenced by the gulf of aqaba’s main maximum tectonic stress direction (137° according to al-arifi, 1996) discussion it has been observed in many cases that mainshock epicentres are very often located at one end of an aftershock zone (matsuzawa, 1979) this happened in the gulf of aqaba with the 1993 and 1995 sequences where the 1993 mainshock was located in the south end of the aftershock zone and below the bottom depth of the aftershock zone this also applied to the 1995 sequence where the mainshock was located near the southern end of the aftershock zone at a depth representing the bottom of the aftershock zone most aftershocks were concentrated between the mainshock and the largest aftershock for both sequences (figure 3) aftershock characteristics give clues regarding the nature of the relatively long-term processes redistributing stress following instantaneous stress changes associated with a mainshock (wesson, 1987) it seems likely that this was responsible for the concentration of stress in the hypocentral region of the largest 1993 aftershocks prior to and resulting in the nucleation of the 1995 mainshock such systematic northward migration led to suggesting an initial model for earthquake migration in the gulf of aqaba (figure 6), although real migration would likely have been more complicated however, the largest aftershock’s epicentre in this model represented the stresses nucleation area and location of the next large mainshock (figure 6) conclusions an earthquake mechanism for the gulf of aqaba fault system has proposed that seismic energy becomes gradually accumulated until it reaches failure point; a large drop in stress during a mainshock (the 1993 mainshock) then caused stress redistribution to the location of the largest aftershock thereby triggering the area to become an area of stress nucleation producing another large mainshock (the 1995 earthquake) again, the last mainshock’s largest aftershock area could have received the next large mainshock because a future large earthquake could easily nucleate in the gulf of aqaba, more effort is needed to fully understand stress migration and seismic behaviour in this critical set of the southern dead sea fault system however, it should be mentioned here that triggering may have played an important role in the 1995 sequence even though further evidence is needed acknowledgments this project was financed by king saud university’s college of science research centre deanship of scientific research references al-arifi, n s , (1996) micro-seismicity and lineament study of the eastern side of the gulf of aqaba nw saudi arabia (1986-1994), ph d thesis, university of manchester, uk al-shaabi, s , (1998) the seismicity of the gulf of aqaba, northern red sea (1985-1995), master thesis, university of leeds, uk ben-avraham, z , garfunkel, z , almagor, g , and hall, j k (1979) continental breakup by a leaky transform; the gulf of elat (aqaba), science, 206, 214–216 el-isa, z , h merghelani and m bazzari (1984) the gulf of aqaba earthquake swarm of 1983, january-april, geophysical journal of the royal astronomical society, 78, 711-722 freund, r , garfunkel, z , zak, i , goldberg, i , weisbrod, t , and derin, b (1970) the shear along the dead sea rift, royal society of london philosophical transactions, 267, 107–130 nassir s al-arifi, aref a lashin and saad al-humidan40 hall, j and ben-avraham, z (1978) new bathymetric map of the gulf of eilat (aqaba) tenth international congress on sedimentology, abstract, 1:285, scale 1:250000 lee, w h k and c m valdes (1985) hyp071pc: a personal computer version of the hypo71 earthquake location program, u s geological survey open file report 85-749, 43 pp lyberis n (1988) tectonic evolution of the gulf of suez and the gulf of aqaba, tectonophysics , 153, 209-220 quennell, a m (1959) tectonics of the dead sea rift, international geological congress, 20th, mexico, 385–403 reches, z , (1987) determination of the tectonic stress tensor from slip along faults that obey the coulomb yield criterion tectonics, 6, 849±861 reiss, z , and hottinger, l (1984) the gulf of aqaba: ecological micropaleontology ecological studies, 50 springer-verlag, berlin wesson, r l (1987) modeling aftershock migration and afterslip of the san juan bautistia, california, earthquake of october 3, 1972, tectonophysics, 144, 215-229 this paper proposes the concept of variable-permeability effect and sets up the one-dimensional and two-dimensional non-darcy well testing models. the finite difference algorithm is employed to solve the differential equations of the variable-permeability model, and the non-convergence of the numerical solutions is solved by using the geometric mean of permeability. the type curves of pressure and pressure derivative with variable-permeability effect are obtained, and sensitivity analysis is conducted. the results show that the type curves upturn in the middle and late sections, and the curves turn more upward with the severer of the variable-permeability effect. the severer the non-darcy effect is, the less obviously the curve upturns caused by boundary effect. furthermore, the boundary effect is increased by increasing the number of impermeable boundaries or decreasing the distance between the well and boundary. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): j1 j6 abstract keywords: variable-permeability effect, lowpermeability reservoirs, non-darcy flow, well testing, pressure gradient, boundary effect. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.54144 h y d r o l o g y naichao feng1, shiqing cheng1*, weixing lan2, guoquan mu3, yao peng4, haiyang yu1 1. moe key laboratory of petroleum engineering china university of petroleum beijing beijing 102249, china 2. e&d research institute of liao he oil field company, panjin, liaoning 124010, china 3. e&d institute of changqing oil company, xi’an, shanxi 710021, china 4. department of petroleum engineering, university of texas at austin, austin 78712, usa *corresponding author shiqing cheng. e-mail: chengsq973@163.com variable-permeability well-testing models and pressure response in low-permeability reservoirs with non-darcy flow record manuscript received: 12/11/2015 accepted for publication: 05/02/2016 how to cite item feng, n., cheng, s., lan, w., mu, g., peng, y., & yu, h. (2016). variable-permeability well-testing models and pressure response in low-permeability reservoirs with non-darcy flow. earth sciences research journal, 20(1). j1 j6. doi: http://dx.doi.org/10.15446/esrj.v20n1.54144 1. introduction to date, darcy’s law (darcy, 1856) is fundamentally used in petroleum engineering, soil science, underground hydrology and hydrotechnies to mathematically describe the flow through porous media. for decades, the classic percolation theory based on darcy’s law has been developing and becomes an important branch of fluid mechanics. however, limitations of darcy’s law were found both from the laboratory experiments and also reported in field observations. many investigators have reported the presence of a threshold gradient for certain reservoir formations, so that there is no flow through porous media when the pressure gradient is below this value, and the relationship between flow rate and pressure gradient was found to be approximately a straight line that does not pass through the origin. when water flows through porous media under small pressure gradient, the connate water will not flow in narrow pores and will block the flow of free water in the adjacent bigger pore. the block of the connate water will be removed, and the free water will become to flow, only when the pressure gradient increased to a critical value, that is, threshold pressure gradient. the threshold pressure gradient effect in the non-steady flow through a porous media was investigated, and the effect on the pressure and flow rate distributions in a flow system was evaluated in oil reservoir (pascal, 1981). the existence of threshold pressure gradient during fluid transport in a porous medium has also been proved by other researchers (yan et al., 1990; das, 1997), and there is a need for correcting darcy’s law for other conditions. the threshold pressure gradient may be related to the flow boundary layer (huang, 1998). in low-permeability reservoirs, the differences of pore sizes and interfacial forces at solid-liquid interface affect boundary layers, resulting in different threshold pressure gradient. its value is inversely proportional to formation permeability, that is, lower permeability leads to higher threshold pressure gradient. however, the power law correction was obtained by regression of three core flood experiments by using brine (prada and civan, 1999). in 1973, the consolidation problem with initial gradient was investigated, and the approximate solution of the moving boundary model with threshold pressure gradient was obtained (schmidt and westman, 1973; pascal, 1973). then several modified expressions of darcy’s law were developed (huang, 1998), based on the flow rate-pressure gradient curves of core flood experiments. feng and ge (1985, 1988) investigated the low-velocity non-darcy flow mechanisms in single-porosity and dualporosity reservoirs and derived the well testing solution with wellbore storage and skin effect based on regular constant-production model of j2 naichao feng, shiqing cheng, weixing lan, guoquan mu, yao peng and haiyang yu single phase flow. by employing the moving boundary method of the thermal conduction, wu (1990) presented the second order integral solution of bingham fluid flow with threshold pressure gradient. based on the gringarten type curves, cheng and xu (1996) further introduced the solution and type curves of apparent wellbore radius model. many other researchers have studied on well testing problem and got many modified mathematical models, solutions and interpretation methods (li and liu, 1997; warren, 1993; hegeman et al., 1993; liao and lee, 1993; luo and wang, 2011; guo et al., 2005; liu et al., 2014). all the researchers discussed above considered the existence of threshold pressure gradient in low-permeability reservoirs. however, the practical experiments indicate that the flow curves are non-linear at very low velocity, and the existing low-velocity non-darcy model is only an approximation, and it cannot accurately represent the flow behavior in low permeability reservoirs (zhou et al., 2002). so developing a well testing interpretation model and the analytical method, by actual flow, will improve the accuracy of the well testing analytical interpretations significantly. this paper presents the concept of variable-permeability in the well testing of low-permeability reservoirs; meanwhile, wellbore storage effect and skin effect are taken into consideration. 2. one-dimensional variable-permeability well-testing model 2.1. variable-permeability effect the non-darcy flow process in low permeability reservoir is shown in figure 1. the linear section (above point jn) obeys darcy’s law, and there are mainly three methods to describe the curved section (huang, 1998): (1) simplifying into a straight line passing through the origin with a slope different from the straight line above point jn, which is simple but cannot reflect the flow accurately especially in low permeability reservoirs; (2) power law expression, which is accurate but difficult for mathematical simulation; (3) converting into linear relationship with threshold pressure gradient, reflecting the threshold pressure but resulting in lower velocity in big pores under low pressure gradient condition. the third method, threshold pressure gradient model, is used in most cases in recent years; however, there exist some deviations based on this model. this paper develops a variable-permeability model by dividing the curved section from j1 to jn into many linear sections with different slopes and assuming that darcy’s law can be applied in these linear sections. in this way, the simulation can describe the percolation more accurately, which is close to power law method but easy to calculate. according to the definition of permeability, the slope of the curve at each point stands for the permeability at the corresponding pressure gradient. the permeability rises with the increase of the pressure gradient under a low pressure gradient condition while it maintains constant when the pressure gradient is higher than a critical value. this phenomenon is called the variable-permeability effect. figure 1. percolation curve of low-velocity non-darcy flow in figure 1, the slope at any point stands for the permeability at the correlated pressure gradient. in this way, the relationship between permeability and pressure gradient can be obtained, as shown in figure 2. thus, permeability is converted from a physical meaning into a mathematical meaning, and the flow simulation can be solved by the mathematical method. the processing procedures are as follows: figure 2. permeability curve of low-velocity non-darcy flow (i) through the initial experimental data, plot the flow velocity versus pressure gradient curve v-∆p, then plot the k-∆p curve presenting the relationship between permeability and pressure gradient; (ii) divide the curve into several linear sections, and consider the permeability of each linear section as a constant; (iii) the finite difference algorithm is employed to discretize the nonlinear flow differential equations, and then set up a non-linear equation system in which the pressure is the unknown, and the permeability is a function of pressure gradient, k= k( dp/dr). an alternating iterative method is used to solve the equations: during the iteration process, initialize the permeability first and make it linear, obtain the pressure solution using iteration method to solve the equations. get the pressure gradient in each section, and then obtain the permeability of each section according to the plot of permeability versus pressure gradient. substitute this permeability into the non-linear flow equations and make it linear again. circulate the alternating iterative process until the pressure tends to be stable and satisfies the accuracy requirement. this result is the pressure at the correlated time step, and then go to calculate the next time step. 2.2. well testing model 2.2.1 assumptions (i) a well is located in the center of an infinite, horizontal, homogeneous, isopachous and low-permeability reservoir with constant production rate; (ii) single phase compressible fluid flow in the low-permeability reservoir. the flow does not obey darcy’s law, and variable-permeability effect is taken into consideration; (iii) wellbore storage effect and skin effect are considered; (iv) the effect of gravity and capillary pressure is negligible; (v) the total compressibility is constant. 2.2.2 mathematical model according to the assumptions discussed above, the condition equations are as follows: (1) j3variable-permeability well-testing models and pressure response in low-permeability reservoirs with non-darcy flow state equations: the flow equation was discretized as: where where where inner boundary condition was discretized as: external boundary condition was discretized as:mass conservation equation: substitute equations 1-3 into equation 4: by considering the effects of wellbore storage effect and sk in effect, the correlated initial condition, and boundary conditions are: initial condition: external boundary condition: inner boundary conditions: kinematic equation: equations 5-9 compose the general non-darcy flow well-testing model in the low-permeability reservoir with variable-permeability effect under radial coordinate. 2.3. numerical simulation 2.3.1 model discretization for one-dimensional radial flow, the pressure gradient is higher near the wellbore. so, non-uniform grid, with denser grids near the wellbore, is employed for precise calculation, which can solve the non-convergence problem and decrease iteration time. set r = rwex , time step ∆t = t /(m-1), and (7) (8) (9) (10) (11) (12) (13) (2) (3) (4) (5) (6) implicit difference method is employed to discretize equations 5-9. forward elimination and backward substitution algorithm were employed to solve the non-linear equation system composed by equation 10-13, and pressure distribution and bottom hole pressure (bhp) in reservoirs with time were then obtained. 2.3.2 iterated misconvergence solution during numerical simulation, the approximate solution may not converge and may oscillate near the exact solution. the main reason for the iterated misconvergence is that the non-linear flow curve is not divided into proper sections. at tn, the pressure gradient is ∆pi(tn) , and the correlated permeability obtained is ki+1(tn), then pi+1(tn) and ∆pi+1(tn) can be calculated. after that, calculate the new permeability ki+2(tn) , similarly ki+3(tn), ki+4(tn),..., kn+1(tn) can be calculated. however, some problems are likely to appear: the oscillation of the solution makes it difficult to solve the variablepermeability model, and the geometric mean of permeability is used to solve this problem. calculate the permeability ki+1(tn) using the pressure gradient ∆p i+1(tn) obtained after the ith iteration. (tn) for next iteration is: then keep doing this iteration process until max | km+1 (tn) k (tn)| ≤ ε. 2.4. well testing analysis and interpretation the relationship between flow velocity and the pressure gradient is presented based on three groups of experimental data to investigate the influence of non-darcy effect (fig. 3a), where the non-darcy effect increases gradually in curve 1, 2, 3. figure 3b shows the relationship between the corresponding permeability and pressure gradient. the model discussed above is used to interpret the field test data (table 1), and the type curves are further obtained, as shown in figure 4. table 1. field test data and reservoir parameters 1 ' 1 1 ' 2 ( ) ( )( ) ( ) ( ) i i i n n n i i n n k t k tk t k t k t + + + = + j4 naichao feng, shiqing cheng, weixing lan, guoquan mu, yao peng and haiyang yu figure 3. darcy and non-darcy flow curves: flow velocity/permeability vs. pressure gradient figure 4. influence of non-darcy effect on non-darcy type curves in the early section of figure 4, the type curves in log-log scale coincide as a 45o line, indicating the influence of wellbore storage effect. in the transition section, the pressure derivative curves drop down after reaching the peak values. the peak value depends on the value of cde2s. larger value of cde2s results in larger peak value and later appearance of the peak. in the later section, the type curves upturn to various degrees. the longer duration of the variable-permeability effect, the higher the type curves upturn; meanwhile, the pressure derivative curve achieves horizontal for darcy flow since there is no variable-permeability effect. compare the following three kinds of type curves: (i) the gringarten type curves of darcy flow (gringarten et al., 1979); (ii) the type curves with threshold gradient model (cheng and xu, 1996); (iii) the type curves based on the variable-permeability model of this paper, using the data in curve 1 in figure 3a. the comparison of the type curves of these three different models is shown in figure 5. horizontal sections appear in the pressure derivative curves of infinite and homogeneous reservoirs (type i), and this is the feature of typical radial flow. compared with the type curve based on gradient threshold model (type ii), the type curves based on variable-permeability model (type iii) result in gentle upturn in the later section, and its calculation method is more stable than that based on moving boundary model with threshold pressure gradient. figure 5. type curves based on three methods 3. two-dimensional variable-permeability well-testing model to take the boundary effect into consideration, we discuss the solution of the percolation problem in two-dimensional reservoirs. 3.1. well testing model and numerical simulation taking boundary conditions into account, and the two-dimensional well testing model with variable-permeability effect is as follows: control equation: initial condition: inner boundary conditions: (14) (15) (16) (17) external boundary condition with one impermeable boundary: flow control in equation14 was discretized as: where where (18) (19) (20) other boundary conditions corresponding to other forms can be obtained successively: qi,j=q for well point grids, and qi,j=0 for non-well-point grids. consider the grids within wells as sink & source items. since the pressure gradient near the well is large, linearize the non-boundary grids. the inner boundary condition was discretized as: for rectangle grids, and re = 0.208∆x for square grids. j5variable-permeability well-testing models and pressure response in low-permeability reservoirs with non-darcy flow (21) (22) (23) (24) external boundary conditions were discretized as: pressure gradients were discretized as: a non-linear system is formed by equations 19-22. gauss-seidel method was employed to solve the above equations, and pressure distribution and bottom hole pressure in reservoirs with time were further obtained. 3.2. type curves of well testing models with boundaries considering different boundary types, we obtained the type curves based on the model discussed above, as shown in figures 6-8. figure 6 demonstrates the influence of non-darcy effect on type curves in reservoirs with one impermeable boundary and shows that the pressure derivative curve will become horizontal with a value of 0.5 before reaching the impermeable boundary for darcy flow. after reaching the impermeable boundary, the pressure derivative curve initially upturns and then become horizontal again with a value of 1. for non-darcy flow, the pressure derivative curves upturn during radial flow period. the severer the non-darcy effect results in the larger amplitude of the ‘concave’, the earlier the derivative curves upturn, the longer duration of the upturned radial non-darcy section, and the weaker the effect of the boundary (nondarcy effect covers the boundary effect). figure 6. influence of non-darcy effect on type curves with one impermeable boundary the impact of impermeable boundaries with different shapes and distances on type curves are shown in figure 7, including parallel faults, orthogonal faults, three faults, and closed boundaries (the dimensionless distances from the well to the faults all equal 500). the four darcy flow type curves at the bottom of figure 7 reveal that how fast the pressure derivative curves upturn in the later section is related to the number of the faults. if the distances from the well to the faults are equal, the more the boundaries are, the earlier the curves upturn and the larger the amplitudes are. the pressure derivative curves of closed boundary reservoirs rise with a slope unity line in the later section. if there are two faults near the well, the orthogonal faults show the second radial flow plateau and the pressure derivative curve of the parallel faults rise with a 0.5 slope line. the non-darcy flow well testing derivative curves show that pressure derivative curves in the radial flow section upturn due to non-darcy effect, lying above the 0.5 line of darcy flow. in the later section, the pressure derivative curves upturn further due to the impact of the faults, later than the darcy flow. more faults result in larger amplitude upturn. if there are closed boundaries near the well, the derivative curves of darcy flow and non-darcy flow coincide with each other and show a slope unity line. the variable-permeability well testing curves of four different faults reveal that more faults lead to the earlier upturn of the non-darcy well testing curves. the non-darcy pressure derivative curve of orthogonal faults upturns less obviously. therefore, to detect the distance from the well to the faults, the testing time should be appropriately extended when there are fewer faults near the well. figure 7. influence of different boundaries on type curves figure 8 demonstrates the influence of non-darcy effect on type curves in a reservoir with a constant pressure boundary. in the later section, the pressure derivative curves initially upturn and then drop down after the second hump. it drops down faster and faster until the decline rates reach that of darcy flow. the larger the amplitudes of the concave parts are (the more severe the non-darcy effect is), the higher the second humps are and the larger the rate of decline of the derivative is at the end of the curves. j6 naichao feng, shiqing cheng, weixing lan, guoquan mu, yao peng and haiyang yu figure 8. influence of non-darcy effect on type curves with a constant pressure boundary 4. conclusions this paper employs the concept of variable-permeability effect and develops one-dimensional and two-dimensional non-darcy well testing models with variable-permeability and boundaries in low permeability reservoirs. forward elimination and backward substitution algorithm are used to address the difference equation system of the models. the geometric mean of permeability was employed to solve the oscillation of the solution, and the reliable solutions are obtained. the general non-darcy flow well-testing model based on variable-permeability effect and its calculation method are more stable and more practical than those based on threshold pressure gradient. the type curves of well testing with variable-permeability effect are further investigated. in the transition section, the pressure derivative curves drop down after reaching the peak values, and the peak value depends on the value of 2c s de . a larger value of 2c s de results in larger peak value, sharper dropping down rate and later appearance of the peak. in the later section, the type curves upturn. the longer duration of the variablepermeability effect, the higher the type curves upturn. for the well-testing model with one impermeable boundary, the radial flow section upturns again after first upturning under the influence of boundary. the pressure derivative curves upturn along with the increasing of the number of boundaries under the influence of non-darcy effect. when the impermeable boundary is far away from the well, the curves are close to the non-darcy type curves of the infinite reservoir and the boundary effect is not evident. the more the boundaries are, and the less the distance from the well to the boundary is, the earlier the non-darcy curves upturn and the large the amplitudes are. for constant pressure boundary, the fast the derivative curves drop down after the second hump. acknowledgements this work was sponsored by national science and technology major projects (no. 2016zx05009-004, no. 2016zx05013002-005) and national natural science foundation of china (no. 51304223). references cheng, s. q. and xu, l. x. 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(2002). fully implicit simulation model for low-velocity non-darcy flow (in chinese). petroleum exploration & development, 29(2), 90-93. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 74 letter from editor welcome to earth sciences research journal, volume 18, number 1. more than 20 years have passed since we decided to be part of the scientific world in geology, seismology, geophysics, geodesy, meteorology, oceanography, planetary sciences, and so many other fields. we raised our voice to say to the world that in colombia, there is a journal committed to providing the highest level of manuscript in earth sciences. it has been not an easy road, but we are here. this 2014 has been full of challenges moving us to find the best ways to present new manuscripts. our volume 18 was made possible by the non-profit cooperation of authors and reviewers working hand-inhand with us. in this volume, we are presenting to the eyes of scientific world nine papers coming from three different continents, with contributions by people from seven countries. our first paper comes from egypt, about an aeromagnetic data analysis in the region of fayoum-cairo. this work regarding the october 12, 1992 earthquake, suggests how the new tectonics of africa’s northeast is predominant in this area. from turkey, our second manuscript obtained a new, important, more up-to-date and more reliable relationship between surface wave magnitude and rupture length for earthquakes in that country. the results obtained in this study will give some useful and up-to-date information to authors whose studies are focused in this area. in the field of hydrogeology, the third article analyzes the hydrogeochemistry of shallow groundwater in the yellow river alluvial plain. the results suggest that the shallow groundwater in the study area is fresh to moderately mineralized water. the assessment of samples shows how this water is acceptable for agricultural use but is not potable. from china, too, an interesting paper about the petrography and mineralogy of biotites within the wushan granodiorite is presented. the formation conditions of these biotites are also determined and the significance of these minerals in terms of petrogenesis and mineralization is discussed. from india, where climate change is a topic trend, a study attempts to provide information on climate variability and its impacts on runoff in the kosasthaliyar sub-basin. from nigeria, a work related to aromatic biological markers are used for assessing thermal maturity of source rocks in the campano-maastrichtian mamu formation. results of this study show how these maturity values indicate that the coal and shale extracts are marginally mature for hydrocarbon generation. we invite you to read also three south american papers. from colombia, the manuscripts titled “enhancing time resolution by stabilized inverse filter and q estimated on instantaneous spectra” and “hydrocarbon production scenarios in colombia. review of field sizes, hydrocarbon reserves and expectations of conventional and unconventional resources” are presented. from chile comes the paper “uncertainty in rainfall input data in a conceptual water balance model: effects on outputs and implications for predictability”. thanks to every one of you and enjoy this issue of the esrj. carlos a. vargas. editor with the continuous improvement of precision requirements for borehole geophysical exploration, the application of transient electromagnetic method (from now on referred to as tem) in a borehole has become a hot spot. the conventional borehole tem can only determine the longitudinal depth of the geological anomaly, the radial azimuth and depth cannot be resolved. a double-transmitting and sextuple-receiving borehole tem is proposed, through which the radial anomaly is excited by the electromagnetic field generated by the double-emitting loops, and the azimuth and depth of the anomaly will be identified by the difference characteristics of the six receiving loops signals. in this paper, the response equations of the transmitting-receiving mode of double-transmitting and sextuple-receiving borehole tem are deduced, and the response characteristics of the induction segment and the attenuation segment of the receiving loops are obtained based on the response equations under ramp function turnoff condition, providing the basis for theoretical analysis. due to the negative value of the double-transmitting and sextuple-receiving transient electromagnetic response signals, a negative transformation algorithm under the double logarithmic coordinate system is proposed to provide the essential method for the analysis of two kinds of physical simulation experimental data of the radial azimuth and radial depth detection of the anomaly. the results show that the double-transmitting and sextuple-receiving borehole tem has decent resolution ability in detecting the radial azimuth of the anomaly, and the effective resolution is 30°. the geometric difference among induced voltages of different measuring points can be used to evaluate the radial depth of the anomaly qualitatively. it is expected that the double-transmitting and sextuple-receiving borehole tem can provide technical guidance for little borehole geophysical exploration in the fields of oil, natural gas, coal and basic engineering construction. abstract keywords: borehole transient electromagnetic method; double-transmitting and sextuplereceiving; negative transformation algorithm; radial azimuth; radial depth. double-transmitting and sextuple-receiving borehole transient electromagnetic method and experimental study issn 1794-6190 e-issn 2339-3459 earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): 77 83 bo wang1, shengdong liu1*, shining li3, fubao zhou2 1state key laboratory of deep geomechanics & underground engineering and school of resource and earth science, china university of mining and technology, xuzhou 221116, china 2school of safety engineering, china university of mining and technology, xuzhou 221116, china 3hebei research institute of investigation and design of water conservancy and hydropower, tianjin 300143, china *email of corresponding author: wbsyes@126.com con la necesidad continua de mejorar la precisión en la exploración de perforaciones geofísicas, la aplicación del método electromagnético transitorio (tem, del inglés transient electromagnetic method) se ha convertido en un tema de constantes estudios. la aplicación del tem en perforaciones convencionales solo puede determinar la profundidad longitudinal de las anomalías geológicas, pero no puede resolver el acimut radial y la profundidad. este estudio propone la aplicación del método tem con transmisión doble y recepción séxtuple a través de la cual se altera la anomalía radial con el campo electromagnético generado por la doble emisión de ondas, mientras el acimut y la profundidad de las anomalías se identifican por las características de las señales de las seis ondas de recepción. en este trabajo se dedujo la respuesta de las ecuaciones del modo transmisión-recepción para la transmisión doble y la recepción séxtuple en las perforaciones con el método tem, y se obtuvieron las respuestas características del segmento de inducción y el segmento de atenuación de las ondas de recepción basados en la solución de las ecuaciones en condición de apagado de la gráfica de función, lo que proporciona las bases para el análisis teórico. debido al valor negativo en las señales de respuesta de la transmisión doble y recepción séxtuple del electromagnetismo transitorio, se propone un algoritmo de transformación negativa bajo el sistema de coordenadas logarítmicas dobles para proveer el método esencial del análisis de dos clases de información de simulación física experimental del acimut radial y la detección de la profundidad radial de la anomalía. los resultados muestran que el tem con transmisión doble y recepción séxtuple en perforaciones tiene una buena capacidad de resolución para detectar el acimut radial de la anomalía, con una resolución efectiva de 30 grados. la diferencia geométrica de los voltajes inducidos desde los diferentes puntos de medida se puede utilizar para evaluar cualitativamente la profundidad radial de la anomalía. la expectativa es que el método tem con transmisión doble y recepción séxtuple en perforaciones puede proveer orientación técnica para pequeñas exploraciones de perforación geofísica en los campos de petróleo, gas natural, carbón y construcciones básicas de ingeniería. resumen palabras clave: método electromagnético transitorio; transmisión doble y recepción séxtuple; algoritmo de transformación negativa; acimut radial; profundidad radial. record manuscript received: 01/03/2017 accepted for publication: 01/07/2017 how to cite item: wang, b., liu, s., li, s., zhou, f. (2017). double-transmitting and sextuple-receiving borehole transient electromagnetic method and experimental study. earth sciences research journal, 21(2), 77 83. doi:http://dx.doi.org/10.15446/esrj.v21n2. 63006 estudio experimental del método electromagnético transitorio con transmisión doble y recepción séxtuple http://dx.doi.org/10.15446/esrj.v21n2.63006 po t e n t ia l m e t h o d s 78 bo wang, shengdong liu, shining li and fubao zhou 1. introduction with the continuous improvement of precision requirements for borehole geophysical exploration, the application of tem in a borehole has become a hot spot (strack, 2014; gan et al., 2016; khoshbakht et al., 2014; karinskiy et al., 2014; gorbatenko et al., 2014). in the study of tem in borehole and well drilling, stolz (2003) examined the borehole electromagnetic method of the gold-bearing tectonic zone, and the borehole electromagnetic method can detect significant anomalies in the shear zone. wilkinson et al., (2005) studied the underground distribution of saline-alkali materials using the aeronautical electromagnetic method, ground electromagnetic methods, and borehole tem, combined with soil, subsurface materials, and groundwater data. school and edwards et al. (2007) studied the resolution of the electromagnetic method and high-resistance targets in submarine borehole dipole-dipole devices. ren et al., (2011) studied the response characteristics of the transient electromagnetic signal in production wells by mean of numerical simulations. wang (2012) supplemented the forward and inverse models of cased well logging and introduced the overall design scheme of the cased well logging system. zhang (2012) studied the design and implementation of high-power electromagnetic pulse source, and detailed description of the proper logging instrument structure and the particular circuit implementation method. guo and song (2012) designed the parameters of the tem receiving loops of the production well by studying the signal response characteristics of the receiving loops. zang et al. (2014) used the open-hole model to study the propagation characteristics of electromagnetic waves in the well and gave a twodimensional spectrum and its dispersion curve describing the propagation of electromagnetic waves in the well. shen et al. (2016) proposed an induction logging method based on transient electromagnetic excitation. borehole tem reduces the size of the excitation source and receiving loops and places them in the borehole for excitation and reception, achieving better coupling between the device and the underground medium and more evident abnormal response. for roslee et al. (2017), the burial depth of the anomaly can be more accurately distinguished by increasing measure points. however, the conventional borehole tem can only determine the longitudinal burial depth of the geological anomaly, the radial azimuth and depth cannot be resolved (liu et al., 2014). to solve this problem, in this paper the response equations of the transmitting-receiving mode of double-transmitting and sextuple-receiving borehole tem will be deduced. a negative transformation algorithm will be proposed, and the borehole tem instrument will be created, and two kinds of physical simulation experiment study of the radial azimuth and radial depth detection of the anomaly will be carried out. 2. principle as shown in figure 1, the probe detects signals with measuring points in the borehole, and it can be seen from the enlarged view that the entire probe consists of two transmitting loops and six receiving loops. the transmitting circuit conveys electromagnetic field with the same value and opposite direction so that the electromagnetic field diffuses to the radial azimuth from the middle of the probe. lindang et al. (2017), as shown in figure 2, the six receiving loops produce a positive primary field response to the magnetic field excited by the transmitting loops. kamsani et al., (2017) the response to the electromagnetic signals generated from the outside anomaly of the borehole is different: for the loops directly facing anomaly, the signal of reply value is positive; for the loops reversely facing anomaly, the induced voltage is negative. figure 2. schematic diagram of borehole tem signal response. when excitation source is ramp function turn-off source (mcneil et al., 1984; fitterman and anderson, 1987), the expression of the ramp function turn-off transient electromagnetic field response can be obtained according to the duhamel integral (zhong and zheng, 1986). induction segment: (1) attenuation segment: where t0 is the turn-off time; r (r) is the transient response excited by the ramp function turn-off source; r = t -s is a replacement function. under the excitation of ramp function turn-off source, the horizontal magnetic component of the single magnetic dipole in the whole space is (yang, 2009): (3) where is the vacuum permeability, , m is the magnetic moment of the single transmitting loop; a is the distance from the center of receiving loop to transmitting loop axis; 2l is the distance between the centers of the two transmitting loops; r0 is the absolute distance from the midpoint of the receiving loop to that of the transmitting loop, . it can be seen from figure 2 that the two transmitting circuits produce an electromagnetic field with the same value and opposite direction. lai et al., (2017) according to the symmetry of the electromagnetic field, the 79double-transmitting and sextuple-receiving borehole transient electromagnetic method and experimental study vertical components cancel each other, and the horizontal components are doubled, so the magnetic field response at the midpoint of the receiving loop of the transmitting-receiving device is: (4) take equation (4) into equations (1) and (2). induction segment: (5) attenuation segment: (6) the induced voltage curve of the receiving loop can be obtained by . the layered medium is a standard medium for studying the propagation of transient fields. the particular case is that there is a small plate in the layered strata with high conductivity, and the response equation of the magnetic field of receiving point at any position in the thin plate: where b = 2/ μs, is the height of the dipole source above the horizontal plate s, x is the horizontal projection distance between the field source and the receiving point, z is the vertical projection distance between the point of receipt and the field source, m is the magnetic moment of the double magnetic dipole source, m=2m. when a receiving loop is directly facing the thin plate, the receiving point receives only the signal of vertical component, and the loop of receipt is coaxial with the dual transmitting source , and x =0 , substitute in equation (7): obtain the time derivative: (10) as shown in figure 2, the transient response of the dual transmitting system shows that the primary response to the six receiving loops is the same, and the secondary response is different. the secondary field response of the loop directly confronting the thin plate is positive, while the secondary field response of the channel reversely facing the thin plate is negative. lai et al. (2017), considering the 2a space between the two faceto-face receiving loops in the device, the secondary field response values are different when the distance between the plates is close, and the value of reply of the reversely facing the plate is: (11) 3. negative transformation algorithm as mentioned above, the response signal value received by some receiving loops is negative when the probe detects a low-resistance anomaly, and the negative value usually cannot be expressed in the double logarithmic coordinate system. lai et al., (2017) it is necessary to use an algorithm to express negative values in the correct geometric form in the double logarithmic coordinate system. the transformed value is obtained through the following transformations of the original data: where n is a symmetric coefficient. taking the induced voltage data of the receiving loop rx4 as an example, the symmetry coefficient n=0. the horizontal axis is chosen as the axis of symmetry before and after transformation. as shown in figure 3, the curve with positive values did not change before, and after the rx4 data transformation, the absolute value of the negative value data is symmetrical to the transformed value along horizontal axis v=1, and the curve attenuation is normal. figure 3. comparison of rx4 measuring data before and after data transformation. 4. physical simulation of abnormal azimuth recognition the transmitting-receiving unit is placed in an open space, and the plate low-resistance anomaly situated on one side of the probe. roslee et al., (2017) the borehole tem instrument is independently created by our research group, and the instrument and the probe structure are shown in figures 4-5 respectively. the physical simulation experiment is shown in figure 6, and the first measuring point is the anomaly directly facing 1# receiving loop. for the following measuring points, the distance between the anomaly and the probe remains the same, and every time the probe is rotated by 5° when each measuring point is set for detection. 80 bo wang, shengdong liu, shining li and fubao zhou figure 5. schematic diagram of probe structure. figure 6. schematic diagram of physical simulation. from the probe structure in figure 5, it is known that the receiving loops are combined to form a regular hexagonal prism structure. since the structure is centrosymmetric and the minimum symmetry angle is 30°, the rotation angle selects: 0°, 5°, 10°, 15°, 30° and 60°, and the measuring data are shown in figure 7. the induced electromotive force of receiving loops rx3, rx4 and rx5 are negative values, and the data is calculated by equation (11) with symmetric coefficient n=0, and transformed and drawn in the double logarithmic coordinate system. as can be seen from the figure 7-a, the secondary field response value of the receiving loop rx1 directly facing anomaly is positive and maximum, and the raw values of the secondary field response of receiving loops rx3, rx4, and rx5 reversely facing the anomaly are negative. kamsani et al., (2017) according to the law of attenuation, the attenuation curves of each receiving loop eventually converge to zero. since the data is transformed, its value should converge to the symmetry axis v=10n, and the curves converge to one in the graph, and the curve configuration of negative values conform to abnormal response characteristics. it can be seen from figure 7 that the response value is the highest when the anomaly directly faces rx1 receiving loop, the response signals of rx2 and rx6 loops are the same and are positive; while the response values of rx3, rx4, and rx5 loops reversely face rx1 loop are negative, and it is shown as the characteristics of the six curves in figure 7-a. when the anomaly is within the normal angle between two adjacent receiving loops, the abnormal response value of the two loops is positive, and the response value of the loop is higher when the anomaly is closer to a certain loop; as shown in figure 7-b and figure 7-c, the anomaly is closer to rx1 loop. when the defect is located on the angular bisector of the two adjacent loops, the responses of the two loops are positive, and the values are the same, as shown in figure 7-e. by comparing figure 7-a and figure 7-f, when the anomaly directly faces a certain loop, the response of the loop is the highest, and the value of the loops on both sides are positive and approximately same, and the remaining three loops have a negative response. from the above phenomenon, the following four laws can be summarized: (1) when there is a lowresistance anomaly, there exist at least two measuring curves with the positive secondary field. (2) the normal direction of the several receiving loops with the positive secondary field of measuring curves must be the azimuth of the anomaly. (3) if there is only one highest curve among the measuring curves with the positive secondary field, then the anomaly is near the normal direction of the receiving loop. (4) if there are two higher curves among the measuring curves with positive secondary field and coincide, the anomaly is near angular bisector of the two receiving loops. therefore, the measuring curves obtained by the borehole transient electromagnetic system can identify the azimuth of the radial anomaly, and the resolution is 30°. it can be seen from figure 7-b, 7-c and 7-d that when the anomaly is at different angles within the range of 30°, the curves are also different, but cannot be directly distinguished from the significant difference among the curves. figure 4. borehole tem instrument. 81double-transmitting and sextuple-receiving borehole transient electromagnetic method and experimental study figure 7. response attenuation curves of the receiving loops at different rotation angles. 5. physical simulation of anomalies with different radial depth from the above theoretical derivation, it is known that the borehole tem has different response signals for different anomalies, and the physical simulation experiments were carried out for lowresistance anomaly with different radial depths of the same azimuth (roslee et al., 2017; lindang et al., 2017; kamsani et al., 2017; lai et al., 2017). as shown in figure 8, the probe moves along a measuring line, the direction of the probe remains the same, and the low-resistance anomalies of different radial depths are placed in the same direction directly facing rx1 receiving loop. there are seven measuring points in the measuring line, and the 1# measuring point is no anomaly detection, and the radial depths of 2#-7# measuring points are from h to 6 h. in the experiment, the anomaly directly faces rx1 receiving loop, so rx1 gets a positive response signal, rx4 gets a negative response signal. the experimental data of the two receiving loops were extracted to obtain the response curves shown in figures 9-10, and the symmetry coefficient n=-1 in negative value transformation algorithm. 82 bo wang, shengdong liu, shining li and fubao zhou figure 8. schematic diagram of physical simulation experiment of different radial depths. figure 9. contrast curve for each measuring point of rx1. figure 10. contrast curve for each measuring point of rx4. it can be seen from figure 9 that when the anomaly directly faces a receiving loop, the secondary field response signal of the anomaly decreases with the increase of the radial depth of the transmitting-receiving device and the low-resistance defect. the secondary field segments of curves are parallel to each other, which is in line with the correct attenuation law. at the same time, the transient time (zhong and zheng, 1986; jiang, 1988) of each curve increases with distance, which accords with the time law of abnormal response. it can be seen from figure 10 that when the anomaly faces the rx4 receiving loop reversely, the anomaly response is negative. the amplitude of the secondary field response signal decreases with the increase of radial depth between the transmitting-receiving device and the abnormality, and the subsequent field segments of curves are parallel to each other, which accords with the correct attenuation law. figures 9-10 show that there is a same law between the receiving loop directly facing the anomaly and reversely facing anomaly. the change of the anomaly depths can be determined by the induced voltage difference of fixed time window. through data analysis, the best resolution window ranges from 138μs to 975μs. the geometric distance between the two curves under the double logarithmic coordinate system can be solved by the following equation: (12) the equation shows the geometric induced voltage difference of the curve distance in the double logarithmic coordinate system, v1 is the transformed value in a fixed time window when receiving loop faces the anomaly directly, and v2 is the transformed value in a fixed time window when receiving loop reversely faces the anomaly. the response values of t=136.8μs, 231.2μs, 390.4μs, 659.2μs and 856μs in the response signals of rx1 and rx4 receiving loops are calculated in equation (8) to obtain the curve graph for the geometric difference of different fixed time windows shown in figure 11, where the length of a standard grid in the vertical axis represents the length of the 10 times of data difference in the double logarithmic coordinate system. it can be seen from figure 11 that the geometric distance between the positive and negative signals decreases as the anomaly depth increases, so the geometric difference between the response signals of different measuring points can be used to evaluate the radial depth of the anomaly qualitatively. two curves of rx1 and rx4 are coincident with zero geometric distance when there is no anomaly. in the fixed time window, the geometric difference is the lower when the time is larger. from the curve comparison, the curves of several different windows are parallel, and the radial depth does not affect the attenuation speed of its secondary field response signal when the same low-resistance anomaly is detected. figure 11. curve graph for the geometric difference of different measuring points. 5. conclusion borehole tem is a kind of fine geophysical method which effectively utilizes the drilling space. in this paper, through theoretical derivation and physical simulation experiment, the response of borehole tem double transmitting loops is analyzed and studied. through the negative transformation algorithm, the negative values obtained by double-transmitting and sextuplereceiving borehole tem can be plotted correctly in the double logarithmic coordinate system, which provides an effective method for the radial direction and depth experimental data analysis. from the analysis of the electromagnetic principle, it can be seen that the borehole tem has efficient resolution ability for the radial azimuth of low-resistance anomaly. by physical simulation experiment, the effective resolve of the double-transmitting and sextuplereceiving borehole tem is 30°. at the same time, the borehole tem can qualitatively analyze the radial depth between the low-resistance anomaly and borehole tem probe by the geometric difference, so the double-transmitting and sextuple-receiving borehole tem is an efficient way to identify the radial azimuth and depth of low-resistance anomaly. 83double-transmitting and sextuple-receiving borehole transient electromagnetic method and experimental study since each measuring point of the double-transmitting and sextuplereceiving borehole tem can obtain different pattern signals of six receiving loops, which has the advantages of less time consuming and a large amount of data. therefore, the next step in research should take advantage of the multi-parameter advantage of double-transmitting and sextuple-receiving borehole tem to achieve high-resolution quantitative interpretation. acknowledgements this research has been performed by the national natural 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technology, xuzhou, china. zang, d.f., zhu, l.f., zhang, f.m., shen, j.g., shen, y.j., wang, z.l., 2014. theory study for transient electromagnetic logging: electromagnetic wave. well logging technology, 5, 530-534. zhang, h., 2012. design of downhole transient electromagnetic large power source. yangtze university, jingzhou, china. zhong. s.m., zheng, d.z., 1986. transient process analysis. tsinghua university press, beijing. palabras clave: imagen sísmica; inversión de onda completa; ecuación de onda acústica; modelo de velocidad marmousi; desenfoque gausiano. how to cite item singh, s., kanli, a. i., & mukhopadhyay, s. (2018). full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code. earth sciences research journal, 22(4), 291-300. doi: https://doi.org/10.15446/esrj.v22n4.59640 this paper investigates the capability of acoustic full waveform inversion (fwi) in building marmousi velocity model, in time and frequency domain. fwi is an iterative minimization of misfit between observed and calculated data which is generally solved in three segments, i.e., forward modeling, which numerically solves the wave equation with an initial model, gradient computation of the objective function, and updating the model parameters, with a valid optimization method. fwi codes developed in matlab herein fwisimat (full waveform inversion in seismic imaging using matlab) are successfully implemented using the marmousi velocity model as the true model. an initial model is obtained by smoothing the true model to initiate fwi procedure. smoothing ensures an adequate starting model for fwi, as the fwi procedure is known to be sensitive on the starting model. the final model is compared with the true model to review the amount of recovered velocities. abstract keywords: seismic imaging, full-waveform inversion, acoustic wave equation, gaussian smoother, marmousi velocity model. full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code inversión de onda completa en el campo de frecuencia y tiempo del modelo de velocidad en imagenes sísmicas: el código matlab fwisimat issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.59640 este trabajo investiga la capacidad de la inversión de onda completa (fwi, del inglés full waveform inversion) en la construcción del modelo de velocidad marmousi, en el campo de frecuencia y tiempo. la inversión de onda completa es una minimización repetitiva compleja entre la información observada y la calculada que generalmente se resuelve en tres segmentos: modelado directo, que numéricamente resuelve la ecuación de onda con un modelo inicial; la computación de gradiente de la función objetiva, y la actualización de los parámetros del modelo, con un método de optimización válido. los códigos de la inversión de onda completa desarrollados en matlab (fwisimat) fueron implementados éxitosamente con el modelo de velocidad marmousi como modelo verdadero. se obtuvo un modelo inicial tras igualar el modelo verdadero para iniciar el procedimiento de inversión de onda completa. la igualación asegura un modelo inicial adecuado para la inversión de onda completa, mientras que este procedimiento se reconoce por ser sensitivo al modelo inicial. el modelo final se compara con el modelo verdadero para revisar el número de velocidades recuperadas. resumen record manuscript received: 18/08/2017 accepted for publication: 29/06/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 291-300 sagar singha, ali ismet kanlib*, sagarika mukhopadhyaya adepartment of earth sciences, indian institute of technology roorkee, 247667, roorkee, india. b* istanbul university-cerrahpasa, faculty of engineering, department of geophysical engineering,34320 avcilar campus, istanbul-turkey e-mail: kanli@istanbul.edu.tr g e o ph y si c s 292 sagar singh, ali ismet kanli, sagarika mukhopadhyay 1. introduction exploration industries commonly use seismic imaging tools, which allow waves to propagate through subsurface rock structure and reveal possible crude oil and natural gas bearing formations. first arrival traveltime tomography is a classical seismic imaging method that for many years has proven to be stable, gives good results, even in lateral heterogeneous media (chabert, 2010). the data set for the inversion is small (consisting only first arrival travel times), so the method is fast and efficient. however, the first arrival travel times are only sensitive to the long wavelengths of the medium (chabert, 2010) which limits the possible resolution of the final estimated model. another disadvantage of the method is the picking of the first arrivals from the seismic data, which is both time consuming, and introduces the possibility of picking errors (improta et al., 2002). on the other hand, fwi, uses amplitude, travel time and phase information simultaneously to invert the model parameters. fwi is an inverse method that generally employ the least squares objective function to minimize the difference between observed seismic data and synthetic seismic gathers by updating the initial model parameters until convergence (margrave et al., 2011). the model updates are generated by reverse time migration of the data residual. the main obstacles that have prevented fwi’s common application in exploration seismology are its computational cost and need for a good initial model (ma, 2010). various efforts from different perspectives have been developed to reduce the computational costs associated with gradient computation and seismic data generation. for example, sirgue and pratt (2004) and operto et al. (2007) limited the number of frequencies in updating the model. vigh and starr (2008) used plane wave method for three-dimensional problems and margrave et al. (2010) used phase shift plus interpolation one-way wave gradient calculations. in this study, fwisimat is developed in both time and frequency domain based on acoustic wave equation. the frequency-domain inversion approach is equivalent to the time-domain inversion approach if all of the frequency data components are used in the inversion process (hu et al., 2009). fwisimat use non-linear conjugate gradient (nlcg) (dai and yuan, 1999; hanger and zhang, 2005) and broyden-fletcher-glodfarbshanno (bfgs) optimization procedure in time and frequency domain algorithms respectively. as to the forward modelling, second order finite difference method is employed due to its high efficiency. this discretization scheme includes absorbing boundary conditions applied on the edges of the velocity medium. however, in order to initiate any iterative scheme an initial model is required (kanlı, 2009), and is derived from smoothing the true velocity model using a gaussian kernel. 2. fwisimat for time domain full waveform inversion is an iterative minimization technique to update the initial model parameters, which is described at kth iteration as follows: m m pk k k k+ = +1  (1) where, m is the model parameter, p is the direction of update and  is the step length. fwi is composed of forward modelling computation, gradient computation, and valid optimization technique (cao and liao, 2013) 2.1 forward modelling forward modelling is a necessary step for solving any inverse problem. the governing equation for time domain fwi is non-homogeneous 2d acoustic wave equation (eq. 2). 1 2 2 2 2 v u u f x z x z t t x z t t x x z zs s , , , , , ( ) ( ) − ∇ ( ) = ( ) −( ) −( )∂ ∂ δ δ (2) in equation 2, u(x, z, t) is the source wave-field, v(x, z, t) is the velocity, f(t) is the source time function and (xs, zs) is the source location in space. let un m l , be the wave-field at the time l t∆ and spatial position n x m z∆ ∆,( ) , vn m, be the velocity at n x m z∆ ∆,( ) then equation 2 can be discretized using central difference scheme as follows: u u u v u u u n m l n m l n m l n m n m l n m l n m l t x , , , , , , , + − + − = − + − + + 1 1 2 2 1 1 2 2 2 ∆ ∆ ∆ uu u u f n m l n m l n m l s z l t n n m m , , ,+ −− +               ( ) −( ) − 1 1 2 2 ∆ ∆+ δ δ ss( ) (3) with the initial conditions u � un m n m, ,,− = =1 00 0 (4) the numerical stability conditions for equations 3 and 4 is as follows: ∆ ∆ ∆ t x z max ≤ ( )1 2 min , � v (5) the unsuitable choice of time and spatial steps may cause severe dispersion and wave distortion in wave simulation. in order to suppress numerical dispersion, it is usually required that there are 10 sampling points per wavelength in equation 2, min ,∆ ∆x z f min max ( ) < v 10 (6) for the same frequency, the numerical dispersion increases as the spatial steps increase. however, the dispersion can be depressed if the higher point approximation schemes are used. in numerical computations, the computational domain is usually truncated to a finite region. so, absorbing boundary conditions (abcs) are included in order to reduce reflection from the edges of the grid. the absorbing boundary conditions are constructed from paraxial approximations of the wave equation (clayton and enquist, 1977). figure 1 illustrates the effect of boundary conditions when the wave-field hits the edges of the medium. wave-fields are recorded for each time at each receiver positions per source. these records are called as synthetic seismograms (fig. 2). 2.2 gradient computation: reverse time migration the objective function o v( )( ) that would be minimized is given as follows: o v d d( ) = −1 2 2 obs cal (7) where, dobs is the observed data and dcal is the calculated data. the gradient of the objective function is given as follows: ∇ ( ) = − ( ) ( )∑∫o v q us s0 0 0 s t ttx z t x z t dt, , , ,, (8) where, qs represents the back-propagated wave-field from receivers to source. qs can be computed from equation 9 backward in time (demanet, 2015). 293full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code 1 0 2 2 2 2 v q r x z t x z t x ts s , , , , ( ) − ∇         ( ) = ( )  (9) in equation 9, r is the residual field (eq. 10). r d dx t x t x tobs cal, , , , , ,( ) = ( ) − ( ) ( ) = ( ) − ( )( )r x t d x t d x tobs cal 8 (10) the sum over in equation 10 is sometimes called a stack (fig. 3). stack uses the redundancy in the data to bring out the information and reveal more details (demanet, 2015). a systematic way for solving back-propagated wave-field (eq. 9) is to follow the following sequence of three steps: (1) time reverse the data residual r x t,( ) at each receiver position, (2) solve the wave equation forward in time with this new right hand side, and (3) time reverse the result at each receiver position in x. 2.3 optimization: non-linear conjugate gradient method conjugate gradient optimization method updates the velocity model according to the descent direction pk (eq. 11). v v pk k k k+ = +1  (11) figure 1. effect of boundary conditions. (a) wave-field at time 0.17 sec., (b) wave-field at time t = 0.42 sec (no boundary condition is used), (c) wave-field at time t = 0.42 sec (absorbing boundary conditions is used). figure 2. synthetic shot data for true model at shot location (a) x=50-m (b) x=360-m (c) x= 670-m. 294 sagar singh, ali ismet kanli, sagarika mukhopadhyay according to pica et al., (1990) step length is estimated as follows: k k k obs cal k k k k = −j p d d j p j p , , � (12) where, jk stands for jacobian matrix and .,. denotes inner product. for computing the direction pk , nlcg optimization process is used (hanger and zhang, 2005), which decreases the objective function along the conjugate gradient direction (eq. 13). p v v pk k k k o k o k = − ∇ ( ) = −∇ ( ) + >     − 0 1 1 1 ,  (13) in equation 13, k k is computed according to equation 14   k k hs k dy= ( )( )max ,min ,0 (14) in which γ ∇ ∇ −∇ ∇ −∇ γ ∇ − − k hs k k k k k k k dy k = ( ) ( ) ( ) ( ) ( ) = ( ) − o v o v o v p o v o v o v , , 1 1 1 ,,∇ ∇ −∇, − o v p o v o v k k k k ( ) ( ) ( )        −1 1 (15) this provides an automatic direction reset while over-correction of k in conjugate gradient iterations. it reduces to steepest descent method when the subsequent search directions lose conjugacy. 2.4 numerical computation: marmousi velocity model the marmousi velocity model is a benchmark model for testing the ability of migration or inversion methods (zhang, 2009). the original velocity model is shown in figure 4. however, for this study a resampled velocity model (true velocity model) is used as illustrated in figure 5. the initial model is generated by smoothing the true model using a gaussian kernel (fig. 6). there were total 31 sources at a depth of 10-m and 720 receivers at a depth of 2-m equally spaced in the medium. absorbing boundary condition figure 3. an example of gradient computation using reverse time migration method, where (a) is a theoretical model (b) is initial model and (c) is migrated image. 295full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code is applied over all four edges of the medium. grid spacing is kept at ∆ ∆x z� =( ) =0.01-m, sampling is taken over ∆t = 0 0014. with number of sample point set as nt=890. to save computational, we use the whole frequency band (0-20hz) for the inversion (in frequency domain, this implementation is different). in practice, multiscaling approach is employed to reconstruct the model from low to high frequency fashion (as discussed later in the paper). this implementation generally cause the difference of the resolution in different inversion algorithms. however, the time-domain fwi is still able to provide good estimation of the acoustic parameter without sacrificing the high resolution. figure 7 and figure 8 represent the inversion result after 200 iterations and the value of the objective function after each iteration. a more qualified analysis of the velocities is easier when looking at the velocity profiles in fig. 9. velocity logs at three different locations are extracted (at 200, 400 and 600 m) from true, initial and final model. in all three logs, the final velocities from fwi result matches with the true velocities very well. 3. fwisimat in frequency domain frequency domain waveform inversion, which includes solving the wave equation forward modelling and inversion, is similar to time domain waveform inversion. the efficiency of frequency domain fwi depends on the number of frequencies used for the inversion and is independent of the number of sources used for inversion. 3.1 forwardmodelling for frequency domain forward modelling equation 2 must be converted into frequency domain (eq. 16).     2 2 2 v u u s x z x z x z x z , , , , , , , ( ) ( ) + ∇ ( ) = ( ) (16) figure 4. original marmousi model. figure 5. true model. figure 6. initial velocity model for inversion figure 7. inversion result after 200 iteration figure 8. objective function value in logarithmic scale at each iteration. figure 9. velocity log extracted at horizontal distance x = (a) 200 m (b) 400 m (c) 600 m 296 sagar singh, ali ismet kanli, sagarika mukhopadhyay in equation 16,  is the frequency, v x z,( ) is the velocity in the medium, u x z, , ( ) is the source wave-field, and s x z, , ( ) is source in freuency domain (fourier-transform of time-domain source wavelet). the boundary condition in frequency domain is given as follows:   u n u− ( ) =ih x y, 0 (17) where, nn is the outward normal of the boundary, h x z= ( ) / ,v , and i is the imaginary unit (erlangga, 2008; zhang and dai, 2013). by means of the finite difference technology along with the boundary condition, the wave equation (eq. 16 and 17) can be recast into matrix form as follows: au s= (18) in equation 18, a is a complex-valued, nx x nz (discretization along xx and z direction) order impedance matrix and it depends on frequency, medium properties, finite difference format, and boundary conditions (wang et al., 2012). solving the linear system in equation 18 wave-field can be obtained (fig. 10). wave-fields are recorded for each frequency at each receiver positions per source. in fwisimat the above mentioned process is implementation as follows: d r * a q= ( )′ \ (19) in equation 19, d =( )dcal , \ is the matrix inversion operator and r´ is the transpose of the vector consisting receivers at spatial positions and q is the source vector. figure 11 illustrate the data obtained from a same source but for different frequencies. 3.2 gradient computation: reverse time migration the gradient of the objective function in frequency domain is given as follows, ∇ ( ) = − ( ) ( )∫o v q u0 2 02π ω ω ω ω r s sx z x z d� �, , , ,, (20) where, u0, , ,s x z ( ) represent the complex conjugate operator (demanent, 2015). the integral in  in above equation is over r and should be truncated to the number of frequencies used in the process. equation 20 represents the frequency domain version of equation 10. in fwisimat the back-propagated wave-field qs x z , , ( ) is computed as follows: figure 10. wave-field recorded at frequency (a) 5 hz (b) 12 hz (c) 20 hz. 297full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code figure 11. frequency data for true model at, (a) 5 hz frequency component, real (b) 5 hz frequency component, imaginary (c) 12 hz frequency component, real (d) 12 hz frequency component, imaginary (e) 20 hz frequency component, real (f) 20 hz frequency component, imaginary 298 sagar singh, ali ismet kanli, sagarika mukhopadhyay figure 14 shows the velocity comparison among the final, true model and initial model at different horizontal distances. after frequency domain waveform inversion algorithm, the final result is obtained close to the true velocity model, especially that the near-surface speed is almost identical to the theoretical model. the results confirm the validity of the algorithm. 4. conclusions the full waveform inversion method and its capabilities, when applied to a complex geological model, have been presented in this study. the fwisimat algorithm used in the study, performs an iterative search for a velocity model that minimizes the residuals between the data computed in the velocity model and the observed data, i.e. the final result is a “best fit” model. the whole wave-field, including both waveform and phase, is being used as data. fwisimat computes in the time and frequency domain. comparing with the frequency domain inversion method, the time domain method has the advantage of high efficiency in forward modelling as the wave modelling in frequency domain requires solving a large-scale system at each iteration which is time consuming for large scale problems. apart from this, time domain modelling provides the most flexible framework to apply time windowing of arbitrary geometry. in the frequency domain, the computational cost is only proportional to the number of frequencies used in the inversion, and not the number of sources. if the seismic data contains wide angle components, there will be a redundancy in the wave number spectrum present in the data, such a redundancy can be exploited in the frequency domain fwi by inverting fewer frequencies (and hence save computational costs) because it is possible to invert for single, discrete frequencies one at the time. another great advantage with the frequency domain implementation is the ease and efficiency when adding multiple sources. the method is very sensitive to the non-linearity of the problem, v a r* d r * a sobs= − ( )( )′ ′\ ( \ (21) where, v ( )= qs  is back-propagated wave-field. dobs can be obtained from equation 19 if applied on true model. 3.3 optimization: broyden-fletcher-goldfarb-shanno (bfgs) in time domain fwi, nlcg optimization method is used. nlcg consumes a substantial time to converge. according to zhang and luo (2013), the bfgs based method has higher inversion accuracy and converge faster than the conjugate gradient based algorithm because the later uses the second order derivative information in computation. the bfgs method is a quasi-newton method which proposed dependently by broyden (1970), fletcher (1970), goldfarb (1970), shanno (1970). let hk is the inverse hessian approximation then the next iterate will be given as follows: h h bb g b g h g g b g b bg h h gbk k t t t k t t t k k t + = + +         − +   1 1 1 (22) where, b m mk k k= −+1 and g o m o mk k k= ∇ ( ) − ∇ ( )+1 . for k = 1, h1 can be approximated by an identity matrix. the next update on the model parameter is given as follows: m m pk k k k+ = +1  (23) where, p v h vk k k i o k o k = − ∇ ( ) = − ∇ ( ) >     0 1 1 , (24) bfgs method does not require exact line search in order to converge, therefore, wolfe line search algorithm (wolfe, 1969; 1971) is acquired to find step length k. the strong wolfe line search algorithm requires the following conditions to be satisfied. o o c ok k k k k k t km p m m p+( ) ≤ ( ) + ∇ ( ) 1 (25) ∇ +( ) ≤ ∇ ( )o c ok k k t k k t km p p m p 2 (26) in equation 26 0 11 2< < <c c . 3.4 numerical computation: marmousi velocity model true and initial velocity model for frequency domain fwisimat are kept same as figure 5 and figure 6 respectively (ensuring 10 gridpoints per wavelength). the number of sources and receivers are 30 and 90 which are equally spaced in the medium at a depth of 20-m. the absorbing boundary conditions are applied on all edges of the medium. the inversion result after 25 iterations is shown in figure 12. the inversion is from low to high frequency, and choice of which is usually related to the model. the low frequencies are more linear related to the model perturbations than the high frequencies (sirgue, 2003). therefore, the inversion process should start with the low frequency components and progressively add higher frequencies. this will help mitigate some of the non-linearity of the problem, hence increasing the chances of finding the global minimum. it also ensures that progressively higher wavenumbers are recovered (ravaut et al., 2004). according to the experience and experiments, twelve frequencies are selected, not only to obtain good results, but also for faster computation. the twelve frequencies from low towards high are 1.00 2.73 4.45 6.18 7.91 9.64 11.36 13.09 14.82 16.55 18.27 20.00. figure 13 shows the objective function value in logarithmic scale during the inversion process. figure 12. inversion result figure 13. objective function value in logarithmic scale at each iteration. 299full waveform inversion in time and frequency domain of velocity modeling in seismic imaging: fwisimat a matlab code and proper input parameters, such as an accurate starting model and an initial frequency as low as possible, is important for obtaining optimum results. in this work, it is shown that full waveform inversion has a great potential 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(2013). finite-difference solution of the helmholtz equation based on two domain decomposition algorithm. journal of applied mathematics and physics, 1(4), 18-24. keywords: tarim basin; tadong area; carbonate; hydrothermal process; hydrothermal dolomite. palabras clave: cuenca del tarim; área de tadong; carbonato; proceso hidrotermal; dolomita hidrotermica. how to cite item tan, w., meng, y., feng, z., meng, q., li, q., wang, x., li, g., & li, y. (2019). analysis of the ordovician carbonate hydrothermal process in tadong area, xinjiang, china. earth sciences research journal, 23(2), 127-132. doi: https:// doi.org/10.15446/esrj.v23n2.80525 hydrothermal activity played an important part in the formation of the ordovician dolostone reservoirs in tadong area (eastern tarim basin). by applying the technologies of thin rock section microscopic identification, cathodoluminescence analysis, carbon and oxygen isotope analysis, trace element, rare earth element analysis, among others, the mineralogical characteristics and geochemical characteristics of the hydrothermal process are studied. it can seen that the ordovician hydrothermal process has represented by dolomite in various forms in the aspect of petrology and mineralogy. it is mainly composed of medium-megacryst crystalline dirty dolomite and powdermegacryst crystalline cleaner dolomite. the geochemical characteristics are the hydrothermal origin, mediummegacryst crystalline dirty dolomite with lower δ18o value, high mn-fe content, and positive eu anomaly. the powder-megacryst crystalline has cleaner dolomite, with higher δ18o value, the mn content low, and the negative eu anomaly. provide a reference for the ordovician carbonate in tadong area oil and gas exploration in the study. abstract analysis of the ordovician carbonate hydrothermal process in tadong area, xinjiang, china análisis del proceso hidrotermal del carbonato ordovícico en el área de tadong, xinjiang, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.80525 la actividad hidrotermal desempeñó un papel importante en la formación de los depósitos de roca dolomítica durante el período del ordovícico en el área de tadong (cuenca oriental de tarim). al aplicar las tecnologías de identificación microscópica con pruebas de láminas delgadas, análisis de catodoluminiscencia, análisis de isótopos de carbono y oxígeno, análisis de elementos traza, análisis de elementos de tierras raras, entre otros, se estudian las características mineralógicas y las características geoquímicas del proceso hidrotérmico. se puede ver que el proceso hidrotermal del ordovícico representa diversas formas en el aspecto de petrología y mineralogía de la dolomita. este se compone principalmente de megacristales medios de dolomita cristalina sucia y megacristales de polvo de dolomita limpiadora cristalina. las características geoquímicas son de origen hidrotermal; los megacristales medios de dolomita cristalizada sucia tienen un valor inferior a δ18o; el contenido de mn-fe es alto mientras que la anomalía eu es positiva. el cristalino de megacristales en polvo tiene una dolomita más limpia, con un valor mayor a δ18o, un bajo contenido de mn y una anomalía de eu negativa. este estudio proporciona una referencia para el carbonato ordovícico en la exploración de petróleo y gas del área de tadong. resumen record manuscript received: 05/02/2019 accepted for publication: 18/05/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 127-132 wancang tan1,2, yuanlin meng1*, zihui feng2, qi’an meng2, qiang li2, xiandong wang2, guorong li3, and yaguang li2 1college of earth science, northeast petroleum university, daqing, china; 2the exploration & development institute of daqing oil field ltd. company, daqing, china; 3college of energy resources, chengdu university of technology, chengdu, china; * corresponding author: yuanlinmeng_nepu@163.com g eo lo g y 128 wancang tan, yuanlin meng, zihui feng, qi’an meng, qiang li, xiandong wang, guorong li, and yaguang li introduction the hydrothermal process is an important geological function for the formation of a carbonate reservoir. hydrothermal fluids change the characteristics of carbonate reservoir through dissolution, replacement, and filling (davies & smith, 2006; luczaj, harrison, & williams, 2006; smith, 2006; sibley & gregg, 1987). the typical hydrothermal carbonate reservoir has been found in the sichuan basin and tarim basin (jiao & xing, 2011; gu, 2000; zhao et al., 2014; huang & liu, 2013; zhang et al., 2009). at present in tadong area ordovician has become a key area of oil and gas exploration and development, the rock core reveals the influence of thermal fluid on the ordovician reservoir in this area. the predecessors attached great importance to the hydrothermal activity in tadong area. it is considered that the hydrothermal dolomitization and hydrothermal dissolution have important control effect on the transformation of carbonate reservoir (ma et al., 2009; shen et al. 2009; zou et al., 2009; jiao et al., 2011; jin & yu, 2011; yu & jin, 2010; liu et al., 2008; jin et al., 2012; pan et al., 2009; hu et al., 2009). in the process of evolution in tarim basin mainly experienced four important geological thermal events, respectively in the sinian to cambrian, ordovician, permian, and cretaceous (li et al., 2005; chen et al., 1997; wen & wang, 2005; yang & xu, 2014). at present, the study of the hydrothermal effect on ordovician carbonate in tadong area is still in the initial stage, while the research of the hydrothermal activity patterns and mechanism is not deep enough. based on the thin section, cathodoluminescence, carbon and oxygen isotope, trace element and rare earth elements analysis, this paper studies the hydrothermal process of ordovician carbonate in tadong area, establishes litho mineralogical and geochemical identification marks of the hydrothermal process. also, it analyzes the mechanism of the hydrothermal process and its influence on the formation of reservoir space, which is significant for the understanding of the ordovician dolomite and its reservoir genesis mechanism. regional geological background the tadong area is located in the eastern tarim basin, controlled by the kongquehe fault and cheerchen fault, divided into three tectonic units, namely northern depression, tadong uplift, southeast depression. the scope of this study area is gucheng low uplift, lop nur salient, and tadong uplift (figure 1). there are cambrian to ordovician carbonate in the area, according to the drilling results, lithology of the lower ordovician penglai dam group is grey limestone, and limestone-bearing dolomite, lithology of yingshan section is gray dolomitic limestone and limestone bearing dolomite, with thin dolomite. the lithology of upper yingshan to yijianfang formation is gray granular limestone, tumuxiuke formation is gray-brown argillaceous limestone. charchag formation is a large set of grey mudstone, silty mudstone, and siltstone. the tadong area has complex tectonic evolution history, there are multi-period big faults, as the main channel of hydrothermal fluid migration and diffusion, has a certain control effect on the hydrothermal dolomite and its dissolution transformation (wu et al., 2011; lu et al., 2006; wu et al., 2009; wu, he, & sun, 2015; han et al., 2009; li et al, 2014; jia et al., 1995). in recent years, oil and gas exploration in the tadong area have a breakthrough; many exploration wells have obtained industrial gas flow in the lower palaeozoic ordovician dolomite sewn and cavern reservoirs. this paper reveals that the ordovician carbonates are the potential oil and gas exploration strata and fields in the study area, and the hydrothermal dolomite provides an important reservoir space for oil and gas. identification mark of hydrothermal process petrology and mineralogy marks according to the thin section and cathodoluminescence analysis technology, in tadong area more than 200 thin section samples of ordovician, 13 wells have analyzed and found that the ordovician hydrothermal action formed various forms of dolomite; they can be medium-megacryst crystalline dirty dolomite (figure 2a). the powder-megacryst crystalline cleaner dolomite (figure 2b), medium-megacryst crystalline dirty dolomite patches and medium-megacryst crystalline cleaner dolomite patches in limestone. under the cathode ray, the ordovician hydrothermal dolomite is not luminescent or dark. it can be specific that medium-megacryst crystalline dirty dolomite dark rosy light (figure 2a). powder-megacryst crystalline cleaner dolomite is near no light (figure 2b). no luminescence of medium-megacryst crystalline dolomite figure 1. tectonic location in the study area 129analysis of the ordovician carbonate hydrothermal process in tadong area, xinjiang, china (a) (b) (c) (d) (e) (f) figure 2. mineralogy forms and cathodoluminescence features of the ordovician hydrothermal process in tadong area. (a) gc7, 6079m, medium-megacryst crystalline dirty dolomite, under the cathode ray, it is dark rosy light. (b) gc7, 6510m, powder-megacryst crystalline cleaner dolomite, under the cathode ray, it is near no light. (c)gc8,6065m, medium-megacryst crystalline dolomite formed by recrystallization, under the cathode ray, it is not luminescent. (d) yd1, 4980m, hydrothermal fracture pore cave and a saddle like dolomites, under the cathode ray, it is dark rosy light. (e) yd2,4390m, dolomite of the edge of the fracture pore cave, under the cathode ray, it is dark rosy light. (f)gc7, 6087m, the saddle like dolomite in structural fractures, under the cathode ray, it is near to no light. formed by recrystallization (figure 2c); early high salinity formation dolomites, hydrothermal fracture pore cave and a saddle like dolomites dark rosy light (figure 2d); dark rosy light from dolomite of the edge of the fracture pore cave (figure 2e). the saddle-like dolomite in structural fractures is near to no light (figure 2f). geochemical characteristics and recovery of hydrothermal fluid properties subsubsection characteristics of carbon and oxygen stable isotopes the carbon and oxygen isotope analysis of 56 ordovician dolomite samples in the study area shows that the δ13c value and the δ18o value are relatively high. the δ13c value is -1.63~0.62 ‰ pdb, the average value is -0.70‰pdb, the δ18o value is -8.21~-4.69‰pdb, and the average value is -6.48‰ (figure 3). medium-megacryst crystalline dirty dolomite and powdermegacrysts crystalline cleaner dolomite is different, the former showed slightly lower δ18o values, δ18o values of -8.21~-6.25‰pdb, with an average of -7.05‰pdb; the latter showed slightly higher values of δ18o, δ18o value is -7.96~-4.69‰pdb, with an average of -5.94‰pdb (figure 3). compared with the region and adjacent areas of ordovician carbonate minerals δ18o values found that δ18o hydrothermal dolomite of ordovician in tadong was higher than that δ18o value of the nonreformed limestone in the region (-7.92~-6.48‰pdb, and the average value is 6.89‰pdb)(figure 4). it was also higher than the region ordovician evaporation pump, seepage reflux, shallow burial of microcrystalline dolomite, powder-fine crystalline anhedral dirty dolomite, powder-fine crystalline euhedral dolomite δ18o values (figure 4). it was significantly higher than that of shallow buried powderfine crystalline euhedral dolomite plaque δ18o value of ordovician in northern tarim basin (-9.47~-6.80‰pdb，and the average value is -8.14‰pdb). obviously, the composition characteristics of ordovician carbonate δ18o value in tadong cannot be explained by the process that ordovician formation water increases with burial deepened temperature. it is revealed that the source of the formation or plainlight plainlight plainlight cathodeluminescenc cathodeluminescenc cathodeluminescenc plainlight plainlight plainlight cathodeluminescenc cathodeluminescenc cathodeluminescenc 130 wancang tan, yuanlin meng, zihui feng, qi’an meng, qiang li, xiandong wang, guorong li, and yaguang li transformation of dolomite is not the ordovician, but it is related to the action of the external fluid. through the test of homogenization temperature of fluid inclusions, homogenization temperature of salt-water inclusion of medium-megacryst crystalline dirty dolomite in the study area is 125.9~136.4℃; the average value is 131.15℃, the δ18o value is -8.21~6.25‰pdb. while homogenization temperature of salt-water inclusion of powder-megacrysts crystalline cleaner dolomite is 137.5~137.9℃, the average value is 137.7℃; the δ18o value is -7.96~-4.69‰pdb. by the formula proposed in document (land, 1983), the oxygen isotope value (smow) of the fluid can be calculated. 1000ln[（1000+δ18od）/（1000+δ18of）]=3.14×106t-2-2.0， in the formula, t—absolute temperature, k, δ18od—the oxygen isotope value (smow) of dolomite, δ18of—the oxygen isotope value (smow) of the fluid. the oxygen isotope value (smow) in formula 1 is the standard average seawater international standard. according to the formula recommended by the us investigation department (δ18osmow=1.03086δ18opdb+30.86), it can be converted to the isotope pdb standard used in the above study. the calculated results show that the δ18osmow value of the mediummegacryst crystalline dirty dolomite hydrothermal fluid is 4.95~6.94‰, and the δ18osmow value of the powdermegacrysts crystalline cleaner dolomite is 5.81~9.13‰. the comparative analysis shows that they reveal that the precipitated dolomite fluid is not brine derived from seawater or seawater. the δ18o value of brine is about -10% smow. the δ18o value of the sedimentary fluid forming medium-megacryst crystalline dirty dolomite is similar to the δ18o value (6.0±1.0‰smow) of the primary water derived from the mantlederived from the taylor (1979) mantle, indicating that its diagenetic fluid may be the source of the basic magmatic-hydrothermal fluid. the δ18o value of the sedimentary fluid forming powder-megacrysts crystalline cleaner dolomite is similar to the δ18o value of granitic magma (6~ 12‰ smow), revealing that its diagenetic fluid may be the source of acid magmatic-hydrothermal fluid. composition characteristics of trace elements the analysis of trace elements in 23 ordovician hydrothermal dolomite samples shows that there are obvious differences in trace element compositions between two dolomites, such as medium-megacryst crystalline dirty dolomite and powder-megacryst crystalline cleaner dolomite. medium-megacryst crystalline dirty dolomite shows high mn content, low sr content, and high mn/sr ratio. the content of mn is 134.1~255.1ppm; the average value is 183.72ppm, the sr content is 74.89~106.3ppm; the average value is 87.11ppm, the mn/ sr ratio is 1.783~3.237; the average value is 2.132. the powder-megacryst crystalline cleaner dolomite and a saddle like dolomite showed low mn content, high sr content, and low mn/sr ratio. the content of mn is 8.167~140.6ppm, the average value is 57.61ppm; the sr content is 91.5~263.8ppm, the average value is 170.85ppm; the mn/sr ratio is 0.089~1.282, the average value is 0.36. the comparison of their trace elements with other types of dolomites is shown in figure 5. combined with the characteristics of luminescent dark under the cathode ray of the hydrothermal dolomite, the mn content of medium-megacryst crystalline dirty dolomite is high, suggesting that the content of mn and fe in the diagenetic fluid is high. it may have revealed that its diagenetic fluid has the properties of the basic magmatic-hydrothermal fluid. the mn content of powder-megacryst crystalline cleaner dolomite is low, suggesting that the content of mn in the diagenetic fluid is low. it may have revealed that the diagenetic fluids have the properties of acid magmatic hydrothermal fluids. this is in agreement with the results calculated by the δ18o value of the front fluid. the characteristics of the composition of rare earth elements the ree analysis of 20 ordovician hydrothermal dolomites in the study area revealed that the composition of the rare earth elements of two types of dolomite in the area has formed. the total amount of rare earth elements in the medium-megacryst crystalline dirty dolomite is slightly higher than the total amount of the rare earth elements in the normal marine limestone. ree average figure 3. carbon and oxygen isotopic characteristics of the ordovician hydrothermal dolomite in tadong area figure 4. carbon and oxygen stable isotopic composition characteristics of the ordovician carbonate minerals in tadong area figure 5. mn/sr distribution characteristics of the ordovician hydrothermal dolomite in tadong area (1) 131analysis of the ordovician carbonate hydrothermal process in tadong area, xinjiang, china value is about 1.63ppm. there is an obvious positive eu anomaly, and the mean value of the δeu value is up to 1.445 (figure 6). the powdermegacryst crystalline cleaner dolomite has higher total rare earth elements. ree between 6.737~23ppm. relatively rich in light rare-earth elements and relatively poor in heavy rare earth elements, positive ce anomaly, and negative eu anomaly. the average value of the ratio of light rare earth elements to the content of heavy rare earth elements is 5.o. the δce average value is about 1.264ppm (figure 7). the characteristics of the saddle like dolomite and powder-megacryst crystalline cleaner dolomite are the same. comparative analysis shows that the rare earth elements composition and distribution pattern of ordovician medium-megacryst crystalline dirty dolomite in tadong is similar to mafic anorthosite, and the composition and distribution pattern of the rare earth element in powder-megacrysts cleaner dolomite has certain similarities with acidic granite. combined with the analysis of carbon and oxygen stable isotopes and trace element mn and fe contents, the geochemical characteristics of two types of dolomite in the ordovician hydrothermal area are very well matched. the medium-megacryst crystalline dirty dolomite with lower δ18o value, high mn-fe content, and positive eu anomaly. the powder-megacryst crystalline cleaner dolomite, with higher δ18o value, the mn content low, and the negative eu anomaly. combined with the tectonic background of the middle late ordovician sedimentary basin[19], rare earth elements composition characteristics in medium-megacryst crystalline dirty dolomite reflect basic magmatic hydrothermal fluid; ree composition characteristics in powdermegacryst crystalline cleaner dolomite reflect the acidic granitic magmatic hydrothermal fluid. mechanism of hydrothermal process and its effect on reservoir after the middle-lower ordovician carbonate deposition, during the late ordovician period, the area entered the trench-arc-basin system, with the development of structure and fracture, and the development of basic and intermediate acid magmatic activities, and entered the intense period of hydrothermal geology. early in the middle-lower ordovician, the rock has not completely consolidated into rock, and its plasticity is stronger. with structural fracture, the cracks are not developed, but the micropores are still preserved in large quantities. therefore, the magmatic hydrothermal rise to the middlelower ordovician with fracture, mainly through the micropore diffusion flow, occurrence of hydrothermal dolomitization, which is resulting in mediummegacryst crystalline dirty dolomite or dolomite plaque, powder-megacryst crystalline cleaner dolomite or medium-megacryst crystalline cleaner dolomite plaque. during the action of the completely hydrothermal process, the ascending magma hydrothermal fluid has an important control effect on the movement and circulation of the fluid. however, because the middle-lower middle ordovician of ordovician has not completely consolidated into rock, the compaction effect also affects the driving effect of the fluid. the combined action of them can drive the fluid far beyond the fracture system and flow motion in a wider area, resulting in hydrothermal action occurring in a large area, rather than just near the fault. at the same time, a large number of hydrothermal dissolution fracture pore cave formed, which constitute the main storage space for the accumulation of natural gas in the region. conclusion (1) the rock mineralogical characteristics of the ordovician hydrothermal process have manifested in various forms of dolomite development; it is mainly composed of medium-megacryst crystalline dirty dolomite and powder-megacryst crystalline cleaner dolomite. under the cathode ray, the hydrothermal dolomite is not luminescent or dark. 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(2014). the porosity origin of dolostone reservoirs in the tarim, sichuan and ordos basins and its implication to reservoir prediction. science china earth sciences, 57(10), 2498-2511 zou, c., li, q., & wu, g. (2009). characteristics and exploration direction of cambrian ordovician carbonate rocks in the tarim basin. xinjiang petroleum geology, 30(4), 450-453. keywords: benin; scaling exponent; mfdfa; singularity spectrum; temperature; nonlinear dynamics. palabras clave: benín; exponente de escalado; mfdfa; espectro de singularidad; temperatura; dinámica no lineal. how to cite item agbazo, m., koto n'gobi, g., alamou, e., kounouhewa, b., afouda, a., & kounkonnou, n. (2019). multifractal behaviors of daily temperature time series observed over benin synoptic stations (west africa). earth sciences research journal, 23(4), 365-370. doi: https:// doi.org/10.15446/esrj.v23n4.51863 in this study, multifractal detrended fluctuation analysis (mfdfa) is applied to daily temperature time series in benin synoptic stations from 1967 to 2012. the purpose is to examine the degree of multifractality and compare multifractal characteristics of temperature in various climatic areas (subequatorial and sudanian). the analysis reveals that the daily temperature time series shows a multifractal behavior that is sensitive to the station’s geographic position and presents a positive long-term correlation. weather conditions and the geographic position of the synoptic stations affect the shape and the characteristics of temperature spectrum. the multifractality is stronger in subequatorial region than the sudanian. the major source of multifractality in temperature series is found to be the fat-tailed probability density function. however, long-range correlations also partly contribute to the multifractal features. except kandi’s synoptic station, the spectrum is left-skewed. the findings indicate the usefulness of the non-linear analysis in climate research due to the complex interactions among the natural processes. it can help understand the mechanisms governing the dynamics of temperature time series in benin. abstract multifractal behaviors of daily temperature time series observed over benin synoptic stations (west africa) comportamientos multifractales de las series temporales de temperatura observadas en las estaciones sinópticas de benin (africa occidental) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.51863 en este estudio, el análisis de la fluctuación multifractal descendente es aplicado a diversos tiempos de temperatura cotidiana en estaciones sinópticas en benín de 1967 a 2012. el objetivo es examinar el nivel de multifractalidad y comparar las características multifractales de temperatura en varias zonas climáticas (subecuatorial y sudanés). el análisis muestra que los diversos tiempos de temperatura diaria presentan un comportamiento multifractal, que son sensitivos a la posición geográfica de la estación y que hay una correlación positiva a largo plazo. las condiciones climáticas y la posición geográfica de la estación sinóptica afectan la forma y la característica del espectro de la temperatura. la multifractalidad es más fuerte en la región subecuatorial que en la sudanesa. se encontró que la mayor fuente de multifractalidad en las diversas temperaturas consiste en una función probabilística de colas pesadas. sin embargo, correlaciones de gran autonomía también se corresponden con las características multifractales. excepto la estación sinóptica de kandi, el espectro inclina a la izquierda la asimetría estadística. los hallazgos señalan la utilidad del análisis no lineal en la investigación climática debido a las interacciones complejas entre los procesos naturales. este estudio permite comprender los mecanismos que manejan la dinámica de las series temporales de la temperatura en benin. resumen record manuscript received: 14/07/2016 accepted for publication: 28/06/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 365-370 m. agbazo1*, g. koto n’gobi2, e. alamou3, b. kounouhewa2, a. afouda3, & n. kounkonnou1 1international chair in physics mathematics and applications (cipma-chair unesco), university of abomey-calavi, benin bp: 526 uac benin 2laboratoire de physique du rayonnement, university of abomey-calavi, bp: 526 uac benin 3applied hydrology laboratory (lha), university of abomey-calavi, bp: 526 uac benin * corresponding author: agbazomedard@yahoo.fr m et eo ro lo g y 366 m. agbazo, g. koto n’gobi, e. alamou, b. kounouhewa, a. afouda, & n. kounkonnou introduction according to several studies, linear statistical methods are insufficient for a complete analysis of meteorological parameters time series. researchers have explained these findings by the fact that the atmosphere and the natural processes taking place in it are a complex natural system (kantz and schreiber, 2004; peters and neelin, 2006; vassoler and zebende, 2012; shi, 2014). therefore, meteorological parameters are characterized by a high degree of nonlinearity, non-stationarity and complexity (he, 2015; agbazo et al., 2019a; philippopoulos et al., 2019; kalamaras et al., 2017, 2019; jiang et al., 2016; burgueño et al., 2014 and dong et al., 2016). in this context, multifractal methods are suitable to analyze processes that obey to nonlinearity characteristics. nowadays, the multifractal detrended fluctuation analysis (mfdfa) introduced by kantelhardt et al. (kantelhardt et al., 2002) has been used widely to examine the intrinsic behavior of meteorological parameters in order to explain climate change, to assess climate change impact by improved methods and to do their prediction (kalamaras et al., 2019). for example, recently, mfdfa has been applied to time series of: precipitation by efstathiou and varotsos (2012); agbazo et al. (2019a); temperature by kalamaras et al. (2017); kalamaras et al. (2019); jiang et al. (2016); burgueño et al. (2014); dong et al. (2016); wind speed by kavasseri and nagarajan (2005); feng et al. (2009), and many others. the authors found that mfdfa methods can help reveal some properties, which could not be detected by linear methods (kantelhardt et al., 2002; kalamaras et al., 2017, 2019; philippopoulos et al., 2019; jiang et al., 2016). variations in temperature and precipitations’ records are important indicators of climate change (jiang, 2017). agbazo et al. (2019a) have studied the fractal analysis of the long-term memory in precipitation in benin (west africa). they have shown, (i) the existence of positive long-term memory characteristic in the rainfall of area and found that (ii) fractal theory reveals benin climatic characteristics better. therefore, in order to generalise the differences in the dynamics of meteorological processes and to assess climate impacts, it is important to compare multifractal properties of temperature in various climate conditions. however, long-term memory of temperature has not yet been studied in a fractal framework in benin, one of the west african countries affected by the effects of climate change (ipcc, 2007). thus, the aim of this work is to analyse spatial and time variation of multifractal properties of daily temperature time series observed in benin synoptic stations in order to examine the basic features of their multifractal behavior. materials and methods materials site description the study covers all the synoptic stations of benin (figure 1), the geographical positions of which are presented in table 1 below. according to boko et al. (1988), benin is characterized from the south to the north by two main climatic zones in which synoptic stations are located: cotonou, bohicon, and save are located in the subequatorial region where march is the hottest month (~26°c), while august is the coldest month (~24°c). the daily and annual thermal amplitudes are, respectively, ~10°c and ~5°c. the relative humidity ranges between 70% and 95% because of the proximity to the atlantic ocean. the subequatorial climate has four seasons in a year: a long rainy season (april to july) followed by a short dry season (august to september) and a short rainy season (october to november) followed by long dry season (december to march). however, the stations of parakou, kandi, and natitingou are located in sudanian region in the northern part of the country. the daily mean of air temperatures in natitingou, parakou, and kandi are, respectively, ~25°c, ~27°c, and ~35°c. table 1. geographical coordinates of synoptic stations stations latitude (°n) longitude (°e) altitude (m) cotonou 6.21 2.23 3.9 bohicon 7.10 2.03 166.00 save 7.59 2.26 198.51 parakou 9.21 2.37 392.00 natitingou 10.19 1.23 460.00 kandi 11.08 2.52 289.7 data records data were provided by the agency for aerial navigation safety in africa and madagascar (asecna). daily temperature data from benin synoptic stations: cotonou (co), bohicon (bo), save (sa), parakou (pa), kandi (ka) and natitingou (na) are used for the period from 1967 to 2012. figure 2 shows the mustache boxes of temperature. the figure shows that the smallest and largest values of minimum temperature are observed respectively in save (~ 3°c) and cotonou (~ 39°c). while, the smallest and largest values of maximum temperature are observed respectively in parakou and cotonou. the temperature distribution is asymmetrical. methods description of mfdfa method to investigate multifractal characteristics of non-stationary, non-linear, natural and complex meteorological phenomenon time series (shen et al., 2015, 2017; wan et al., 2016; jiang et al., 2017), the mfdfa method was proposed figure 1. study sites location: (a) benin’s location in west africa, (b) synoptic stations’ location in benin. 367multifractal behaviors of daily temperature time series observed over benin synoptic stations (west africa) if the time series follow the power law, then we can obtain the scaling function: f s sq h q( ) ∝ ( )� (4) where h(q) is the generalized hurst scaling function (kantelhardt et al., 2002; telesca et al., 2004). for monofractal time series, h(q) is independent of q, whereas, for a multifractal time series, h(q) varies with q. to be specific, the scaling exponent h(2) provides information about the average fluctuation of the series. the series can be categorized into one of the following three types depending on the h(2) values. these are: (i) 0 < h(2) < 0.5 for an anti-persistent type long range correlated process (negative long-term memory) where large values (compared to the average) are more likely followed by small values and vice versa, (ii) h(2) = 0.5 for an entirely random uncorrelated distribution, and (iii) 0.5 < h(2) < 1 for a persistent and long range correlated process (positive long-term memory) where large values are more likely to be followed by large values and vice versa. as király and jánosi (2005) suggest, the term “persistent” is used in the sense that an increasing trend in the past implies an increasing trend in the future. thus, it lightly differs from “persistence” in climatology, defined as the continuance of a specific pattern. this dependence is considered to be a characteristic of multifractal process (kantelhardt et al., 2002). in order to avoid a divergence of moments in the fat tails of the fluctuation distribution as mentioned by some authors (wang et al., 2013; ihlen et al., 2012), we restrict the order q within the range -5 ≤ q ≤ 5 as in (wang et al., 2013). multifractal spectrum in order to get more information about the temperature data and to characterize the strength of the multifractality, two functions and few parameters are deduced from h(q) via a legendre transform (feder et al.,1988; kantelhardt et al., 2002; shi et al., 2008; liu et al., 2014a, 2015; jiang et al., 2016). these functions are defined by following equations:    q h q q dh q dq f q h q ( ) = ( ) + ( ) ( ) = − ( )  +       1 (5)  q( ) and f ( ) are respectively singularity strength (or hölder exponent) and multifractal spectrum. these parameters are defined as followed: ∆  = −max min (6) and ∆f f fmin max= −( ) ( )  (7) where max and min are obtained from the relation f ( ) = 0. f fmax0( ) = (8) if the observed time series is a single-scale fractal series, the function f () is a constant. f () generally, if the time series is a multiscale series, the function has a bell-like shape. the first important property of the spectrum is its width.  is multifractal spectrum width or spectrum width. the parameter  describes the inhomogeneity of the distribution of probability measured on the overall fractal structure, which has been identified as the intermittency and degree of multifractality (liu et al., 2014a). the smaller  is, the more uniformly distributed the fractal region is; while, the bigger  is, the more heterogeneous the probability distribution is. in others words, if a spectrum has a broad width,  is indicative of a strong multifractality (i.e., it has a ‘fine’ structure). if the width becomes smaller, then the time series tends to be a monofractal one. the spectrum width of a pure monofractal time series is equal to zero. f > 0 shows that the number of largest subsets is greater than the minimal one in probability measures, so, multiscale fractal spectrum shows a “left hook” shape; f < 0 shows a contrary condition, and the multiscale fractal spectrum is “right hook” shaped. designing 0 as the singularity strength with maximum spectrum, a small value of 0 means that the underlying process “loses fine-structure”; while a large value of 0 ensures larger complexity. by kantelhardt et al. (2002). it can be briefly described according to shen et al. (2017), as followed: let x t nt | = …{ }1 2, , , be an original time series of n equidistant measurements to which the procedure of the mfdfa method is applied. first, a new series named “profile” is determined as followed: y k x x t k t( ) = −( ) = ∑ 1 (1) where x is the mean value of xt and k = 1, 2, …, n. the profile is then divided into n = int(n/s) equal-sized nonoverlapping windows with a length s. since n is not the integral multiple of s in most cases, there might be a short part at the end of the profile that remains uncovered. to take full account of the series, the same procedure can be repeated starting from the end of the series. hence, we obtain 2ns segments altogether. we then calculate the variance of each window as follows: f s s y y for n i s s i m s 2 1 1 21 1 2    , , , , ( ) = − ( )      = … = −( ) +  ( )∑ i 11 1 2 1 2 s y y for n n i s n n s i m s s = − −( ) +  ( )∑ − ( )      = +  i ( ),.., ss          (2) it should be noted that linear (i.e., polynomial order m=1), quadratic (i.e., m=2), cubic (i.e., m=3), or higher-order polynomials y m ( ) ( )i can be used to fit the local trend. in mfdfam, possible m order trends are eliminated in the profile. the order of the polynomial has been found not to alter the results, with the order varying from 2 to 5 (koscielny-bundea et al., 2006). in this study, a third-order polynomial has been adopted as shown by agbazo et al. (2019a) in benin for the precipitation time series. by averaging over all windows, we obtain the fluctuation as follows: f s n f s for q exp n q s n q q s s ( ) = ( )            ≠ = ∑1 2 0 1 4 1 2 2 2 1  , / /   = ∑ ( )            =           1 2 2 0 ns f s for qln , (3) figure 2. mustache boxes of temperature data observed at each synoptic station from 1967 to 2012. cotonou (co), bohicon (bo), save (sa), parakou (pa), kandi (ka) and natitingou (na). 368 m. agbazo, g. koto n’gobi, e. alamou, b. kounouhewa, a. afouda, & n. kounkonnou according to shimizu et al. (2006), a measure of the width of the multifractal spectrum can be obtained by fitting a second-order equation to the curve of the spectrum around 0 f a b c    ( ) = −( ) − −( ) +0 2 0 (9) among the three constants a, b, c, the most important is b, which is an asymmetry parameter. coefficient b indicates the asymmetry of the spectrum. according to burgueño et al. (2014) and kalamaras et al. (2019), when b being zero for a symmetric spectrum, the shape of the spectrum is symmetrical. for positive b values, the spectrum is right-skewed and for negative left-skewed (burgueño et al., 2014). a right-skewed spectrum is related to relatively strongly weighted high fractal exponents (with “fine-structure”), while a leftskewed spectrum is indicative of low fractal exponents (a more ‘regular’ time series). c is an additive constant equal to 1. origins of multifractality generally, there are two major types of sources for multifractality in time series: (a) different temporal correlations for small and large fluctuations and (b) a fat-tailed probability distribution for the values of the time series (rak and zieba 2015). the main methods to find the contributions of the two sources of multifractality are the shuffling procedure and the surrogating procedure, respectively (kwapien et al., 2005). indeed, to test the first source of multifractality, we randomly shuffle the series to remove any temporal correlations. thus, if no multifractal feature remains after we conduct the shuffling procedure on the original multifractal series, we can conclude that long-range correlation dominates the multifractality in the original series. the shuffling procedure consists in generating a random permutation of the array elements of time series. in contrast, surrogate data is used to check whether the multifractality comes from fat-tailed probability distribution, because it can eliminate any sort of non-linearities in original series and weaken the nongaussianity of the distributions. in this paper, we use the common method of amplitude adjusted fourier transform (aaft) developed in (theiler et al., 1992; dong et al., 2016; wu et al., 2018) to obtain surrogate data. aaft can be summarized according to dong et al. (2016) as follows: 1) a discrete fourier transform of the original series is conducted, 2) the discrete fourier transform of the data is multiplied by random phases, and 3) an inverse fourier transform is performed to generate a phase-randomized surrogate. to further compare the contributions of multifractality from the two sources, the corresponding multifractal spectrum width is computed for the original series, shuffled series and surrogate series. to better compare the impact of both types of multifractality in time, the following computations are done: (1) the ratio to , which can reflect the contribution of the long-range correlations’ multifractal strength to the whole multifractal strength (shen et al., 2016); (2) the ratio ∆shuf to , which indicates the proportion of the fat-tailed probability distribution multifractality. results and discussion the fluctuation function log f sq ( )( ) versus log s( ) of the original temperature time series is plotted for different statistical moments q= (-5, -2, 2, 5) and shown in figure 3. it is observed that the linear relationship between the mfdfa fluctuation factor fq(s) and the time scale s is obvious, which implies the existence of power-law relationship. in addition, a phenomenon appears in some of the relationships shown in figure 3: the regression lines with different orders of q tend to converge, which indicates the multifractal nature of the original series of temperature records. moreover, the scaling behavior of fq(s) (i.e. slope) for positive and negative q are not the same at each synoptic station. this behavior reveals the existence of different degrees of multifractality at benin synoptic stations. in order to verify if these apparently converging lines are multifractal or monofractal, the plot of the generalized hurst exponent h(q) versus q obtained at each synoptic station for original temperature times series are shown in figure 4. to further compare the contributions of multifractality from the two sources, the corresponding multifractal spectrum f () is computed. in figure 3. log-log plots of the mfdfa fluctuation factor fq(s) versus the time scale s for specific moments q for the temperature time series observed at (a) cotonou, (b) bohicon, (c) save, (d) parakou, (e) natitingou and (f) kandi synoptic stations. figure 4. generalized hurst exponent h(q) as a function of q for the original temperature at all synoptic stations it is observed that multifractal behavior for temperature at each synoptic station is clearly shown in figure 4 because all the curves have obvious negative slopes. to be specific, all the h(2) values are greater than 0.5. one can conclude that the original temperature series is nonstationary signals with long range power-law correlations (shi, 2014). therefore, the fluctuations of the temperature time series are positively correlated in a power-law fashion. 369multifractal behaviors of daily temperature time series observed over benin synoptic stations (west africa) figure 5 the multifractal spectrum is presented as a function of the singularity strength  for the original series, shuffled series and surrogate series. in all cases, the spectrum shape is a humped curve and it is obviously unsymmetrical about its axis. moreover, the spectrum has a left truncation and a long right tail. this is attributed to multifractal structure that is insensitive to the local fluctuations with large magnitudes. the south-north distributions of the main multifractal parameters are presented in figure 6. it is observed in figure 6a that except cotonou and bohicon stations, 0 values increase with the station’s latitude. however, highest and lowest values are respectively observed at cotonou and save stations, which implies that the underlying mechanism governing daily temperatures has larger complexity at cotonou than save. figure 6a reveals that spectrum width  increases with the latitude in subequatorial and sudanian regions. however,  values in the subequatorial region are systematically greater than those of sudanian region. these findings indicate that in sudanian region the fractal range is more uniformly distributed; while in subequatorial region the probability distribution is more heterogeneous. in others words, the multifractality is stronger in subequatorial region. the highest value of  is obtained at parakou station, which implies a stochastic dynamic character and strong fluctuations of the original series at this station as compared to the others (xue et al., 2015). the ratio ∆surr to , and ratio ∆shuf to  are presented in figure 6b. it is shown that no gradient is obvious in their south-north distribution. it is clearly observed that, whatever the station, the contribution of the longrange correlations’ multifractal strength to the whole multifractal strength is systematically greater than the proportion of the fat-tailed probability distribution multifractality. it can be noticed that parakou temperature series have the highest value of the ratio ∆surr to  among all the benin synoptic stations. regarding the spectral asymmetry spatial distribution (figure 6c), it is observed that except kandi, the spectrum asymmetry values are negative; therefore, the spectrum is left-skewed, which is indicative of low fractal exponents (a more ‘regular’ time series). kandi’s spectra are with higher spectrum asymmetry values, therefore, contain a greater number of high fractal components. as shown in figure 6d, f > 0 for the original, shuffled and surrogate temperature times series over all synoptic stations. these findings show that the number of largest subsets is greater than the minimal one in probability measures, so, multiscale fractal spectrum shows a “left hook” shape. however, f values are systematically different for original, shuffled and surrogate temperature times series. therefore, the source of multifractality in temperature depends on long-range temporal correlations and a fat-tailed probability distribution. however, both of them influence the multifractality. moreover, the multifractality in temperature depends on the geographical position and the type of climate of the studied station. the value of f is not systematically reduced after the original series is shuffled and surrogated. therefore, the shuffling and the surrogating procedure do not systematically imply the reduction in the degree of multifractality. conclusions in this paper, the multifractal behavior of daily temperature time series for benin synoptic stations from 1967 to 2012 is investigated by multifractal detrended fluctuation analysis (mfdfa) method. this is achieved through five multifractal parameters (hurst exponent, hölder exponent with maximum spectrum, spectrum asymmetry, spectrum width and singularity spectrum). the main and nontrivial findings obtained are the following: 1. temperature time series exhibit long-range temporal correlation, which cannot be fully described by a single scaling exponent. they are long-term positively correlated, meaning that an increase in temperature is more likely to be followed by another increase. 2. temperature time series have a multifractal nature; however, weather conditions and the geographic position strongly affect the strength of their multifractality. multifractality is stronger in subequatorial region. figure 5. singularity spectrum f () as a function of the singularity strength function  for the temperature of original, shuffled and surrogated series at (a) cotonou, (b) bohicon, (c) save, (d) parakou, (e) natitingou and (f) kandi synoptic stations. figure 6. south-north distribution of the main multifractal parameters obtained for the original, shuffled and surrogate temperature times series. (a) 0 and the spectrum width, ; (b) the ratio ∆surrogated to �∆original and the ratio ∆shuffled to �∆original; (c) spectrum asymmetry and (d) f values. 370 m. agbazo, g. koto n’gobi, e. alamou, b. kounouhewa, a. afouda, & n. kounkonnou 3. regarding the spectral asymmetry spatial distribution, it is observed that except in kandi, the spectral asymmetry values are negative, therefore the spectrum is left-skewed. 4. long-range temporal correlations for small and large fluctuations and a fat-tailed probability distribution are two major sources for multifractality of temperature; however, fat-tailed probability distribution contributes more to multifractality. future works will be focused on the investigation of the relationship between the predictability of the meteorological time series and their multifractal nature in different climatic conditions. acknowledgments we, the authors, would like to thank the agency for aerial navigation safety in africa and madagascar (asecna) for providing the data that we used to conduct this study. we also thank moudachirou language institute (mli) for reviewing the english structure in the text. references agbazo, m. n., koto n’gobi, g., alamou, e., kounouhewa, b. & afouda a. 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(2015). multifractal nature of particulate matters (pms) in hong kong urban air. science of the total environment, 532, 744-751. earth sciences research journal geochemestry earth sci. res. sj. vol. 16, no. 2 (december, 2012): 39 56 investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria bassey edem ephraim department of geology, university of calabar, p. m. b. 1115 calabar – nigeria. phone: +2348035510115. e-mail: basifrem@yahoo.com abstract hitherto unknown metacarbonate deposits constitute parts of the cretaceous mamfe embayment which straddles the border between south-eastern nigeria and western cameroon. the rock is characterised by a high concentration of loi, cao and mgo and depleted content of various insoluble components. assuming all the cao and mgo content of the rock were related to calcite and dolomite phases, these two minerals would account for around 22.5 wt% and 76.3 wt% on average, respectively. among the trace elements investigated, only ba, cs, rb, sr, nb, pb, zr, cd, cu, ni, u, y and zn display concentrations beyond their detection limits. chondrite normalised rare earth element patterns show that the rock under investigation show moderate to strong fractionation of light rare earth elements (lreen) over heavy rare earth elements (hreen) and distinct negative eu anomaly. multivariate statistical treatment and variation plots revealed several geochemical interrelationships, among which are the sio2 – al2o3 – k2o – tio2 –fe2o3 – ba – nb – rb – zr links which is associated with the rock’s silicate fraction. the carbonate fraction comprises cao, mgo, sr, pb and cu. the overall geochemical signatures support development of the metacarbonate deposit from sedimentary carbonate materials that was deposited in a saline, shallow-marine, low-energy seawater environment. the consistency of the rock’s chemical properties can be attributed to the relative stability experienced during the parent sedimentary materials’ deposition. resumen depósitos metacarbonatadas desconocidos hasta la fecha, parecen constituir partes de la bahía de mamfe, de edad cretácica, ubicada en la frontera entre el sudeste de nigeria y camerún occidental. la roca se caracteriza por una alta concentración de loi, cao y mgo, así como el contenido empobrecido de varios componentes insolubles. suponemos que todo el contenido de cao y mgo de la roca estaban relacionados con las fases calcita y dolomita, minerales que representan aproximadamente el 22,5% en peso y 76,3% en peso promedio, respectivamente. entre los elementos traza investigados, sólo las concentraciones de ba, cs, rb, sr, nb, pb, zr, cd, cu, ni, u, y y zn se encuentran por encima de los límites de detección. elementos de tierras raras normalizadops con patrones de condrita muestran que la roca bajo investigación presenta un moderado a fuerte fraccionamiento de elementos de tierras raras ligeros (lreen) sobre los elementos de tierras raras pesados (hreen) y una anomalía negativa distintiva de eu. el tratamiento estadístico multivariado y las proyecciones variacionales revelaron varias interrelaciones geoquímicos, entre ellos el sio2 al2o3 k2o tio2 fe2o3 ba nb rb zr, asociados con la fracción de silicato de la roca. la fracción carbonatada comprende cao, mgo, sr, pb y cu. las firmas geoquímicas sugieren el desarrollo del depósito a partir de materiales sedimentarios carbonatados que se depositaron en una solución salina, de poca profundidad marina, y en un medio con agua de mar de baja energía. la consistencia de las propiedades químicas de la roca puede ser atribuida a la estabilidad relativa, experimentado durante la deposición de los materiales sedimentarias parentales. palabras claves: geoquímica, ambiente de deposito, bahía de mamfe. keywords: metacarbonate, geochemistry, depositional environment, mamfe embayment. record manuscript received: 16/01/2012 accepted for publications: 06/05/2012 bassey edem ephraim40 introduction nigeria lies within an area of the ancient african shield, in-between the archaean to early proterozoic west african craton and the congogabon craton (figure 1). the south-eastern region is characterised by several megastructural features, notably the calabar flank, the mamfe embayment, the anambra basin, the afikpo syncline, the abakaliki anticlinorium, the niger delta, the oban massif and the obudu plateau (figure 2). megastructural features, such as the mamfe embayment, straddle the border between south-eastern nigeria and western cameroon (figure 3). while the cameroon sector of the basin has benefited from diverse and repetitive studies (for instance, collignon, 1968; dumort 1968; eben, 1984; eyong, 2003; hell et al., 2000; kande, 2000; kangkolo, 2002; kangkolo and ojo, 1995; ndougsa-mbarga 2004; ndougsa-mbarga et al., 2004, 2007; ndougsa-mbarga and ntep–gweth, 2005; ngando et al., 2004; njieatih, 1997; nouayou, 2005; tabod, 2008; tokam et al., 2010), the nigerian segment is yet to be investigated in sufficient detail. the few available works on the nigerian segment of the mamfe basin, to the best of my knowledge, include those of fairhead and okereke (1987, 1988), fairhead (1991), olade (1975), petters (1987 and 2004) and reyment (1965). the paucity of geological information on the nigerian segment of the embayment has previously led to over-generalisation, some of which may subsequently be proven to be incorrect. a recent survey by the nigerian geological survey agency (ngsa) revealed mappable deposits of metacarbonate rock, which had been hitherto unknown in the south-eastern nigerian sector of the mamfe embayment. this discovery calls for re-examination of earlier paleo-environmental interpretations of the mamfe basin. of particular concern is an earlier viewpoint by petters et al. (1987) that apart from brine seepages which are common, there is no substantial evidence of marine influence during the deposition of the mamfe formation’s cretaceous fluviatile sequences. the present study which is focused on the investigation of the geochemical features of the newly-revealed metacarbonate deposits is intended to provide invaluable insight into the nature and processes which can be reasonably associated with the conditions of formation of the carbonate deposits. this work represents an effort towards revisitfigure 1. a generalised geotectonic map of africa, showing the location of nsofang (modified from affaton et al., 1991). –25˚e –25˚e –25˚n –25˚n 0˚n 0˚n 25˚n 25˚n 0˚e 0˚e 25˚e 25˚e investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 41 ing issues related to paleo-environmental interpretations for the basin. the present study also constitutes an important approach towards the in-depth exploration of the hitherto sparsely-known geology of the nigerian sector of the mamfe embayment. geological background the mamfe embayment which is located roughly between latitudes 5° n and 6° n and longitudes 8°45’ e and 10° e is a coastal sedimentary basin that straddles the federal republic of nigeria and the republic of cameroon (figure 3). despite the fact that the sedimentary infill of the aptian to albian (hell et al., 2000; ndougsa et al., 2004) mamfe basin is largely covered by dense forestation, the type locality on the bank of the cross river at mamfe town in the republic of cameroun reveals thickly folded and faulted series of massive arkosic sandstone and grit having intercalations of marl, arenaceous limestone and shale (reyment, 1965). metacarbonate deposits, which constitute the focus of the present study, are now revealed to be parts of the nigerian sector of the mamfe embayment (figure 3). exposure of mostly the clastic infillings in the embayment show fining upward cycles, which is characteristic of fluvial channel fill, with point bar deposits and over bank siltstone/mudrock. a sedimentary deposit comprising sandstone, mudstone, shale, limestone, microconglomerates and polygenic conglomerates having about 2,000 m thickness has been reported in the lower benue trough section (olade, 1975). these deposits narrow towards the east until it disappear under tertiary and recent rocks of the cameroon volcanic axis. thus, the cameroon figure 2. a geological sketch map of south-eastern nigeria, showing the various megastructural features characterising the region (modified from ofomata, 1973). figure 3. a geological map of south-eastern nigeria and western cameroon, showing the location of nsofang marble within the mamfe embayment of south-eastern nigeria bassey edem ephraim42 volcanic axis, comprising deposits such as basalt, trachyte and rhyolite, uncomformably overlies the mamfe formation (fitton, 1980). metamorphism has so far not been reported in the nigerian sector of the mamfe embayment, but the close association of sedimentary deposits with postdated volcanic rocks appear to provide a good setting for contact metamorphism in the region. field and petrographic aspects observation of the metacarbonate deposits (generally consisting of low-lying deposits having several slightly elevated portions) showed that they are distributed within low to very low grade metamorphosed and un-metamorphosed sedimentary sequences in very close association with the basement and volcanic rocks units. evidence of this association is provided by the close proximity of the units (figure 3) and the ubiquitous basement and volcanic rock fragments abounding in the vicinity of the deposits. generally, exposures of the metacarbonate have steep dips of about 26o–40o and consistent ne–sw strike orientation. joints and fractures having variable orientation are also common features of these outcrops. millimetre–scale laminations, small centimetre-sized voids/ vugs and caves of up to tens of centimetres wide and 1 to 2 metres high are also predominantly conspicuously displayed on the outcrops (figure 4). the fine laminations ran parallel to sub-parallel to the bedding in most cases (figure 4a and 4b) and the voids are irregular but occasionally occluded by mostly laminated buff coloured material (figure 4c). the caves are likewise irregular in shape, usually having solution sculpted walls (figure 4d). the metacarbonate rocks are dense and somehow uniform, having rugged surfaces that possibly indicate the relevance of earlier organic activity and/or weathering in the rocks’ evolutionary history. hand specimen investigation showed that the rocks are characterised by a fairly homogeneous texture, uniform hardness and good resistance to abrasion. also displayed are whitish or greyish colour, polymodal grainsized distribution pattern of mostly fine to medium grains with occasional porphyroblasts and distinct linear and planar fabric, highlighted by the laminar structures. the microscopic features (as observed by optical microscopy) show a predominantly carbonate mineralogy and the textural framework could best be described as heteroblastic. medium grained (~ 1.0–2.0 mm) carbonate minerals are frequently surrounded by finer (<1.0 mm) grain crystals of similar mineral. the boundaries of the medium crystals are often sutured to embayed and the grains are interlocked. unfortunately, the main carbonate minerals could not be differentiated during optical microscope examinations because the thin sections investigated were not stained with alizarin or any other relevant chemical. also, much of the silicate mineralogy is so fine-grained that identification by optical means is difficult to impossible, hence the need for using an x-ray diffraction (xrd) technique which is particularly suitable for discerning individual mineral components of the cryptocrystalline phases and for proper differentiation of carbonate minerals into calcite and dolomite. xrd analysis was carried out on powdered samples under the following conditions: cuk ∝1 graphite monochromatic radiation (1.54059 å) at 30 kv, 30 ma energy. table 1 gives the results of semi-quantitative xrd studies of representative samples of the metacarbonate rocks. a cursory appraisal of the results (table 1) indicates that the metacarbonates are (a) (b) (c) (d) figure 4. nsofang metacarbonates’ outcrop features. (a) distinct millimeter–scale laminations running parallel to the bedding. note cavities that are outlined with the laminates. (b) lamination comparable to algal laminates normally produced on tidal flats by colonies of blue green algae. (c) small centimeter-sized voids/vugs. (d) caves developed within the metacarbonates. investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 43 predominantly dolomitic with calcite as subordinate, while quartz, talc, phlogopite and probably muscovite constitute the accessory phases. geochemistry sampling and analytical procedure several unweathered rock samples weighing 1 to 2 kg were collected during fieldwork and traversing of various metacarbonate rock outcrops in nsofang and its environs in the ikom area of south-eastern nigeria. systematic sampling was hampered by the rocks’ irregular exposure and the thick forestation cover. the samples so collected were cleaned to remove evident allogenic material and/or observed weathered phases in situ before being transported to the laboratory for further investigation. eleven representative metacarbonate rock samples were used for the geochemical analysis which was performed at the acme analytical laboratories in vancouver bc, canada. the geochemical analysis involved measuring major, trace and rare earth element abundance as well as determining the total carbon, sulphur and loss on ignition. prior to the geochemical analysis, about 1 kg of representative rock sample was broken to thumbnail-sized pieces with a hardened-steel hammer. these pieces were crushed and pulverised to particle size as fine as –60 mesh with the aid of a “jawcrusher”. the samples were powdered in an agate mortar to –200 mesh after coning and quartering and thoroughly homogenised. every possible precaution was taken to minimise cross-contamination between samples, including cleaning all the crushing, grinding and homogenisation equipment with a brush, compressed-air, distilled water and acetone to remove possible remains from previously crushed samples. sample preparation and treatment was carried out at the thin–section workshop of department of geology, university of calabar, calabar, nigeria. loss on ignition (loi) was determined by igniting 400 mg of each sample split at 1,000°c and then measuring the weight loss. total carbon and total sulphur concentration was determined with the aid of a leco carbon–sulphur analyser, after sample ignition at >800oc. two instrumentation techniques were used for whole–rock geochemical analysis, namely inductively-coupled plasma-emission spectrophotometry (icpes) and inductively-coupled plasma-mass spectrophotometry (icp-ms). the lithium metaborate-tetraborate fusion digestion technique was found most appropriate for these instrumentation methods. each sample solution was analysed in duplicate in each analytical run and reproducibility was found to be within ± 2%. the detection limit for all the major and minor element oxides was 0.01%, the only exceptions being fe2o3 and k2o (0.04%. detection limits). the trace elements’ detection limit came within the 0.01 to 1 ppm range. figure 5. comparison of the calcium and magnesium contents of the nsofang marble with the ratio characteristics of stoichiometric dolomite (modified from johnson, 2010). sample # dolomite calcite quartz talc phlogopite muscovite l1 d sd ftr tr tr ftr l2 cd cd ac ftr ftr ftr l3 d sd tr ftr ftr ftr d = dominant (> 50%) cd = codominant (subequal abundance of major components) sd = subdominant (20%-50%) ac = accessory (5%-20%) tr = trace « 5%) ftr = faint trace «1%) l1 = outcrop location 1, where l11, l12, l13 geochemical samples were also collected l2 = outcrop location 2, where l21, l22, l23 geochemical samples were also collected l3 = outcrop location 3, where l31, l32, l41, l42, l43 geochemical samples were also collected table 1. semi-quantitative x-ray diffraction data of representative samples of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. line depicting stoichiometric dolomite geochemical data and interpretation major element oxides and relevant data the concentrations of major element oxides and other related chemical data of the metacarbonate rocks are presented in table 2. a cursory appraisal of the data (table 2) revealed that loi, cao and mgo content frequently constituted more than 95 wt% of the rock composition and this corroborate mineralogical observations (table 1) that the carbonate phases are the predominating phases in the rock. the high loi values (table 2) possibly reflect the low silica composition of the rocks (table 1). qadhi (2008) observed that increase of silica and other silicate constituents in marble reduces the loi value. apart from loi, cao constitute the dominant constituent, with concentrations that ranges from 29.85 to 51.67 wt% (35.79 wt% mean value). this is closely followed by mgo which has concentration varying from 3.88 to 21.25 wt% and 16.70 wt% mean value (table 2). assuming that all cao and mgo content is related to calcite and dolomite phases, these two minerals would account for around 22.5 wt% and 76.3 wt % on average (table 2). however, the true picture may be slightly different as the non–correspondence of the metacarbonate rocks’ sample plots with the line depicting stoichiometric bassey edem ephraim44 dolomite on the ca cf mg biplot (figure 5), similar to that of johnson et al., (2010), is an indication that dolomite may not be the dominant host mineral for cao and mgo. it is possible that some mgo are also admixed in the calcite’s structural lattices, as observed in the jabal farasan marble of central-western saudi arabia (qadhi, 2008). both cao and mgo are also possibly bound in the structure of the small and probably insignificant silicate phases, which is represented by quartz, talc, phlogopite and muscovite that constitute parts of the rock’s modal mineralogy (table 1). the insoluble residues, notably, sio2 (0.82–6.21 wt%), al2o3 (0.003–1.01 wt%) and k2o (0.01–0.49 wt%) have considerably low abundance. ignoring sample l11, fe2o3 is frequently less than 0.08 wt% while tio2, mno and na2o concentration are negligible. both k2o and p2o5 have 0.07 wt% mean concentration (table 2). the relatively lower abundance of fe, mn and p in the samples are probably the reflection of low detrital and organic effects relative to the inorganic chemical carbonate precipitate (tucker, 1983). the total carbon values are considerably high (11.31–12.97 wt%, av. 12.28 wt%) as expected for carbonate-bearing rocks, while the same could not be said for total sulphur concentration which are generally below the 0.02 wt% detection limit. trace element composition the trace elements geochemical data of the metacarbonate rocks are presented in table 3. as shown in table 3, only ba, cs, rb, sr, nb, pb, zr, cd, cu, ni, u, y and zn have concentrations beyond their detection limits since many trace elementsconcentrations were below the relevant detection limits (table 4). the rock’s trace element concentrations are not as low as expected and sr and ba values are highly variable (table 3), possibly suggesting a complex distribution of the element (georgieva, 2009). light ion lithophile element (lile) concentration, notably ba (55 ppm average), sr (145.1 ppm average) and probably rb (2.7 ppm average) are considered moderate. sr concentration particularly appears to be approtable 2. concentrations of the major element oxides and related chemical data of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. l11 l12 l13 l21 l22 l23 l31 l32 l41 l42 l43 major elements oxide, total carbon, total sulphurs and loi compositions (%) sio2 6.21 4.08 3.26 1.08 2.74 0.82 1.6 1.43 1.03 1.37 1.82 tio2 0.06 <0.01 <0.01 0.01 <0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01 al2o3 1.01 0.03 0.03 0.24 0.15 0.15 0.3 0.25 0.14 0.06 0.06 fe2o3 0.36 <0.04 <0.04 0.08 0.08 0.08 0.08 <0.04 <0.04 <0.04 <0.04 mno 0.01 0.01 <0.01 0.01 0.02 0.02 0.02 <0.01 0.01 <0.01 <0.01 mgo 20.42 21.25 20.56 12.6 21.09 20.14 18.43 11.43 3.88 16.35 17.52 cao 30.26 30.1 30.94 41.09 29.85 31.67 33.78 41.88 51.67 36.94 35.55 na2o <0.01 <0.01 <0.01 0.01 0.01 0.01 0.04 0.02 <0.01 <0.01 <0.01 k2o 0.49 <0.01 <0.01 0.03 0.06 0.06 0.03 0.02 0.02 0.03 0.03 p2o5 0.05 0.05 0.06 0.02 0.15 0.14 0.14 0.13 0.02 0.04 0.02 loi 40.8 44.1 44.8 44.6 45.4 46.5 45.2 44.5 43.1 44.9 44.6 total 99.68 99.69 99.73 99.77 99.56 99.6 99.63 99.72 99.93 99.76 99.67 total carbon 11.31 12.16 12.17 12.46 12.18 12.97 12.37 12.61 12.14 12.45 12.21 total sulphur <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 elemental abundance (ppm) si 29,028 19,072 15,239 5,048 12,808 3,833 7,479 6,684 4,815 6,404 8,507 ti 360 <60 <60 60 <60 60 <60 60 <60 <60 <60 al 5,345 159 159 1,270 794 794 1,588 1,323 741 318 318 fe 2,518 <280 <280 560 560 560 560 <280 <280 <280 <280 mn 77 77 <77 77 155 155 155 <77 77 <77 <77 mg 123,153 128,159 123,997 75,991 127,194 121464 111,151 68,934 23,400 98,607 105,663 ca 216,268 215,125 221,128 293,670 213,338 226,345 241,426 299,316 369,285 264,010 254,076 na <74 <74 <74 74 74 74 297 148 <74 <74 <74 k 4,068 <83 <83 249 498 498 249 166 166 249 249 p 218 218 262 87 655 611 611 567 87 175 87 computed abundance of carbonate minerals calcite 3 1 4 42 1 7 15 46 83 25 20 dolomite 93 97 94 58 96 92 84 52 18 75 80 investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 45 priate, given the fact that sr content of recent carbonates are expected to range from 30 ppm to 200 ppm (shearman and shirmohammadi, 1969). among the high-charged cations, zr has concentration ranging from 4.1 to 35.7 ppm (11.5 ppm mean value), nb concentration ranging from 0.1 to 1.5 ppm (0.3 ppm mean value) and u content varying from 0.3 and 3.0 ppm. y concentration ranged from 0.6 to 5.7 ppm; zn (5.6 ppm average.), cu (2.0 average) and ni (1.6 ppm average) display moderate concentration, while cd (0.3 ppm average), pb (0.6 ppm average) and cs concentration (0.6 ppm average) appear low. figure 6 illustrates the composition of the trace elements of the metacarbonate rock normalised to the average upper continental crust of taylor and mclennan (1981). rare earth element geochemistry table 5 gives the rare earth elements (la, ce, pr, nd, sm, eu, gd, tb, dy, ho, er, tm, yb and lu) abundance in the rock , together with other relevant data. the ree values shown in table 5 are quite low (∑ree = 1.33–6.12 ppm, excluding sample l11 having an abnormal 15.41 ppm concentration) and the concen tration of most components from locations l22, l23, l42 and l43 are below detection limits (table 5). the tabulated data (table 5) clearly show that lree components dominate over hree, and that total lree and hree concentrations decrease from 12.53 to 1.1 ppm and from 2.88 to 0.27 ppm, respectively (table 5). identification of ree fractionation in the metacarbonate rocks was carried out by normalising (haskin et al, 1968) the concentration of the rare earth elements to average chondritic meteorites, and the result for samples having concentrations beyond detection limits is presented as ree pattern (figure 7). a cursory appraisal of the chondrite–normalised rare earth element (reen) plot show that all samples exhibit similar reen patterns, moderate to strong fractionation of light rare earth elements (lree) over heavy ree (hree) (lan/ybn = 0.98 – 9.35) and distinct negative eu anomaly, eu/eu* ranging from 0.48 to 0.60. it is also obvious that la – nd – sm – eu appear to define an inclined straight line and the heavy rare earth elements (hree) exhibit uncoordinated zigzags patterns in the gd – dy – ho – er – yb – lu spans (figure 7). table 3. trace element composition and relevant ratios of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. table 4. measured trace elements occurring below detection limits in metacarbonate rocks of nsofang, ikom area of southeastern nigeria l11 l12 l13 l21 l22 l23 l31 l32 l41 l42 l43 ba 190 22 6 33 78 70 37 37 19 57 52 cs 3.9 0.2 <0.1 0.2 0.4 0.3 0.2 0.2 0.1 0.3 0.4 rb 17.8 0.5 0.3 0.8 2.7 2.2 0.9 0.7 0.6 1.2 1.5 sr 59.1 53.8 60.1 167.7 97.4 95.6 100.1 181.4 552.3 115.8 112.6 nb 1.5 0.2 0.1 0.3 0.2 0.3 0.2 0.2 0.1 <0.1 <0.1 pb 0.9 0.4 0.5 0.7 0.6 0.5 0.7 0.8 1.1 0.3 0.5 zr 35.7 4.1 6.7 7.7 14.9 15.1 7.8 12.4 3.8 5.4 12.8 cd 0.2 0.2 0.7 0.3 0.2 0.1 0.3 0.3 0.1 0.3 0.3 cu 0.9 0.7 1.6 0.6 1 0.6 2.7 1.5 12.1 0.2 0.2 ni 0.3 <0.1 0.8 3.9 1.2 1.7 1.3 3.5 1.6 1.1 1 u 0.5 1.3 3 0.8 0.3 0.3 0.6 0.6 2.3 2.3 1.8 y 5.7 2.7 2 1.8 0.6 0.7 1.4 2 1 0.7 0.6 zn 9 5 6 8 4 5 7 6 3 5 4 mn/sr 1.30 1.43 1.28 0.46 1.59 1.62 1.55 0.42 0.14 0.66 0.68 mg/ca 0.57 0.60 0.56 0.26 0.60 0.54 0.46 0.23 0.06 0.37 0.42 detection limits (in ppm except otherwise stated) comments ag <0.1 no sample revealed measureable amounts be <1 no sample revealed measureable amounts bi <0.1 no sample revealed measureable amounts co <0.2 2 samples showed traces of 0.3 – 0.4 ppm ga <0.5 1 sample showed traces of 1.4 ppm hf <0.1 2 sample showed traces of 0.2 – 0.8 ppm mo <0.1 3 samples showed traces of 1 – 2 ppm sb <0.1 no sample revealed measureable amounts sc <1 1 sample showed traces of 1ppm se <0.5 1 sample showed traces of 0.6 ppm sn <1 no sample revealed measureable amounts ta <0.1 1 sample showed traces of 0.1ppm th <0.2 4 samples showed traces of 0.2 – 0.6 ppm tl <0.1 no sample revealed measureable amounts v <8 5 samples showed traces of 9 22 ppm w <0.5 1 sample showed traces of 1.6ppm hg <0.01 (ppb) no sample revealed measureable amounts au <0.5 (ppb) no sample revealed measureable amounts bassey edem ephraim46 figure 6. selected trace element concentrations of nsofang metacarbonate rock normalized to the composition of average upper continental crust of taylor and mclennan (1981). l11 l12 l13 l21 l22 l23 l31 l32 l41 l42 l43 la 2.7 0.3 0.6 1 0.7 0.6 0.8 1.1 0.9 0.3 0.2 ce 5.9 1 1.7 2.2 1.2 1.2 1.9 2.6 1.8 0.5 0.5 pr 0.7 0.1 0.16 0.25 0.12 0.13 0.19 0.26 0.18 0.05 0.05 nd 2.6 0.5 0.7 1 0.6 0.7 1 1.1 0.7 <0.3 <0.3 sm 0.63 0.11 0.13 0.23 0.09 0.11 0.14 0.23 0.16 0.05 <0.05 eu 0.1 0.02 0.04 0.04 <0.02 <0.02 0.02 0.05 0.03 <0.02 <0.02 gd 0.66 0.15 0.13 0.18 0.08 0.09 0.12 0.2 0.13 <0.05 <0.05 tb 0.12 0.03 0.03 0.03 0.02 0.01 0.02 0.04 0.02 <0.01 <0.01 dy 0.73 0.21 0.14 0.21 0.08 0.08 0.12 0.22 0.14 0.05 <0.05 ho 0.15 0.05 0.05 0.05 <0.02 <0.02 0.03 0.04 0.02 <0.02 <0.02 er 0.48 0.21 0.16 0.14 0.07 <0.03 0.08 0.09 0.11 0.05 0.05 tm 0.08 0.03 0.02 0.02 0.01 <0.01 0.01 0.02 0.01 <0.01 <0.01 yb 0.49 0.19 0.15 0.14 <0.05 0.06 0.08 0.15 0.06 <0.05 <0.05 lu 0.07 0.03 0.03 0.02 <0.01 <0.01 0.01 0.02 <0.01 <0.01 <0.01 ∑ree 15.41 2.93 4.04 5.51 3.07 3.07 4.52 6.12 4.27 1.47 1.37 ∑lree 12.53 2.01 3.29 4.68 2.71 2.74 4.03 5.29 3.74 1.2 1.1 ∑hree 2.88 0.92 0.75 0.83 0.36 0.33 0.49 0.83 0.53 0.27 0.27 table 5. rare earth element composition of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 47 figure 7. chondrite normalised (haskin, 1968, 71) rare earth element patterns of the nsofang metacarbonate rock. table 6. correlation coefficients of major element components and selected trace elements of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. sio2 tio2 al2o3 fe2o3 mno mgo cao na2o k2o p2o5 loi total c ba nb ni pb rb sr zr sio2 1 tio2 0.78 1 al2o3 0.62 0.94 1 fe2o3 0.73 0.98 0.96 1 mno -0.23 -0.19 -0.05 -0.02 1 mgo 0.53 0.23 0.11 0.28 0.38 1 cao -0.57 -0.27 -0.16 -0.32 -0.38 -1.00 1 na2o -0.20 -0.13 0.11 -0.06 0.44 0.00 0.00 1 k2o 0.75 0.99 0.95 0.99 -0.10 0.26 -0.30 -0.14 1 p2o5 -0.13 -0.15 0.02 -0.04 0.82 0.35 -0.37 0.51 -0.08 1 loi -0.69 -0.82 -0.74 -0.73 0.57 0.19 -0.15 0.18 -0.76 0.50 1 total c -0.85 -0.79 -0.66 -0.72 0.36 -0.15 0.18 0.16 -0.75 0.39 0.88 1 ba 0.62 0.90 0.87 0.93 0.09 0.34 -0.38 -0.15 0.94 0.07 -0.57 -0.59 1 nb 0.74 0.98 0.96 0.99 -0.11 0.26 -0.30 -0.11 0.99 -0.07 -0.75 -0.70 0.91 1 ni -0.63 -0.33 -0.09 -0.27 -0.05 -0.56 0.56 0.13 -0.32 0.10 0.27 0.54 -0.29 -0.24 1 pb 0.05 0.37 0.53 0.38 -0.10 -0.65 0.60 0.16 0.36 -0.05 -0.58 -0.36 0.21 0.37 0.29 1 rb 0.76 0.99 0.94 0.98 -0.09 0.28 -0.32 -0.15 1.00 -0.08 -0.76 -0.75 0.95 0.98 -0.34 0.34 1 sr -0.44 -0.20 -0.13 -0.24 -0.22 -0.92 0.91 -0.08 -0.22 -0.30 -0.21 0.05 -0.30 -0.25 0.30 0.69 -0.23 1 zr 0.63 0.89 0.88 0.91 0.08 0.35 -0.39 -0.12 0.92 0.15 -0.55 -0.56 0.95 0.91 -0.20 0.29 0.93 -0.33 1 bassey edem ephraim48 ponents. the tio2, fe2o3 and k2o concentration appear consistent with those of sio2 and al2o3 (table 5). furthermore, there appear to be no correlation between cao and na2o (table 6, figure 8). examination of variations of trace element in the rock reveals pb, ni, sr and cu enrichment with increasing cao values while ba, nb, rb and zr display weak negative correlation with cao (table 6, figure 9). it is also interesting to observe that most trace elements display positive interrelationships with the major insoluble residue components of the rock (table 6) and prominent among these are the sio2 – al2o3 – k2o – tio2 –fe2o3 – ba – nb – rb – zr links (table 6, figure 10). in the rotated factor matrix (table 7), loading greater than ± 0.55 are highlighted and considered significant members of each of the 3 factor score groupings. as shown in table 7, factor 1 (comprising significant loadings of si, ti, al, fe, k, ba cs, rb, nb, zr, y and zn) account for 38.59% of total data variance while factor 2 (high loadings of ca, sr, pb and cu) account for 17.15% of total data variance. factor 3 (with high loadings of mn, ba, u and y) account for 12.68% of total data variance. interpretation and discussion evaluation of consistency of primary chemical signatures a number of geochemical parameters have been proposed for testing the degree of preservation of primary chemical signatures in carbonatebearing rocks (kaufman et al., 1993; narbonne et al., 1994; pandit et al, 2003; veizer, 1983). in particular, the mn/sr ratio has been accepted as a definitive indicator for the degree of preservation and/or post-depositional alteration (brand and veizer, 1980; derry et al., 1992; veizer et al., 1989, 1992). the mn/sr ratio of the nsofang marble varies between 0.14 and 1.62 with an average of 1.01 ppm. these values are considered to fall within acceptable limits for carbonate rocks that are well-preserved and unaffected by post-depositional alteration (derry et al., 1992; kaufman et al., 1992, 1993; kaufman and knoll, 1995). moreover, brand (1983) has suggested that trace element signatures can aid in interpretation of depositional environment conditions despite post-digenetic alteration. the chemical signatures of the marble can therefore be taken as representative of the precursor rock. distribution of chemical species the reduction in sio2 content compared to increasing cao concentration (figure 8) agree with the fact that the rock comprises distinct silicate and carbonate fractions, having contrasting variations. the carbonate fraction increases at the expense of the silicate fraction and vice versa. the positive relationship between sio2 and mgo (table 6, figure 8) most likely confirm earlier observations that mgo constitutes parts of the silicate fraction. the positive relationship existing between sio2 and mgo (table 6, fig. 8) most likely confirm earlier observations that mgo also constitute parts of the silicate fraction. the positive relationship displayed between sio2 and al2o3 (fig. 8), together with the consistency observed in the variation of tio2, fe2o3 and k2o with those of sio2 and al2o3, (table 6) and the weak negative correlation existing between cao and the various insoluble residues (fig. 8) suggests that the non-carbonate or silicate fractions are mainly aluminosilicates. however, where al2o3 is extremely low (as in sample l12, l13, l42 and l43), clearly the sio2 cannot have been introduced into the original sediment in aluminosilicate phases and was probably therefore either in traces of detrital quartz or in siliceous organisms. among the trace elements investigated, sr, pb and ni appear to be enriched in the carbonate fractions as implied by the consistency that these components display with cao, which is an important member of the carbonate fraction. the enrichment of sr in carbonates is often accounted for by the fact that sr2+ readily substitutes for ca2+ in calcium – bearing strucfactor 1 factor 2 factor 3 log si 0.58 -0.34 -0.53 log ti 0.94 0.01 -0.18 log al 0.75 0.51 0.36 log fe 0.95 0.08 0.17 log mn 0.03 0.01 0.78 log mg -0.09 0.00 0.00 log ca 0.00 0.71 -0.06 log na 0.00 0.41 0.18 log k 0.88 -0.04 0.36 log p 0.15 0.03 0.48 log ba 0.69 -0.21 0.57 log cs 0.91 -0.30 0.14 log rb 0.86 -0.20 0.38 log sr -0.42 0.67 0.22 log nb 0.95 0.11 0.08 log pb 0.36 0.82 0.05 log zr 0.79 -0.18 0.39 log cd -0.10 -0.25 -0.53 log cu -0.09 0.86 -0.10 log ni -0.31 0.47 0.48 log u -0.52 -0.10 -0.70 log y 0.64 0.27 -0.65 log zn 0.65 0.08 -0.25 eigenvalue 8.88 3.95 2.92 percent of total variance 38.59 17.15 12.68 cumulative percentage 38.59 55.75 68.43 table 7. varimax rotated r – mode factor analysis of bulk rock composition of metacarbonate rocks of nsofang, ikom area of southeastern nigeria. the highlighted loadings are greater than ±5.5 geochemical interrelationships the interrelationships between the major element oxides and trace elements of the metacarbonate rock have been investigated and the results presented as correlation coefficients (table 6) and co-variation plots (figures 8–11). also, following the procedure adopted by veizer et al (1992), r–mode factor analysis have also been employed to reveal any communality of chemical elements and, indeed, the most likely identity of the host minerals. the rotated factor matrix is presented in table 7, together with eigenvalues, total variance percentage and cumulative percentage. examination of changes of the major elements oxides with respect to cao concentration reveal that sio2 and mgo are significantly decreased with increasing cao content in the rock (table 6, figure 8). cao composition also display an expected weak negative correlation with most of the insoluble residues, notably tio2, al2o3, fe2o3, mno, k2o and p2o5 (table 6, figure 8), and both sio2 cf mgo and sio2 cf al2o3 cross-plots (figure 8) show significant positive correlation between the involved cominvestigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 49 ture. however, in the nsofang metacarbonates, both ca and sr display very strong positive correlation (figure 9) which rather suggests that both components are probably bound together in the same calcium – bearing lattice structures. this is consistent with the position of dissanayake (1981) that sr being the main trace elements in limestone and dolomites is often presume to be located in the lattices of carbonate minerals. the observed lack of significant positive correlation between sr and na (table 6; figure 11) is an indication that na2o is not controlled by the carbonate phases, and consequently does not form parts of the lattices of the ca – mg carbonates nor occur as inclusions in the carbonate phases (fritz and katz, 1972; land and hoops, 1973). the lack of strong relationship between sr and na, together with the absence of significant positive correlation between na/ca versus mg/ca or sr/ca (figure 11), also precludes the consideration of an evaporitic hypersaline (sass and katz, 1982) conditions as existing during developments of the protoliths of the metacarbonate. other trace elements, notably ba, nb and rb, tend to concentrate in the silicate fractions as inferred from the weak negative correlation they displays with cao (figure 9) and the positive interrelationships they exude with major insoluble residue components of the rock (figure 10). arising from the marked correlation exhibited between zr and al2o3 (figure 10), it thus seems reasonable to suggest that much of the rock’s zr composition was probably not introduced as detrital zircon but also formed parts of the rock’s silicate fraction. the strong positive interrelationships amongst the various trace elements and major insoluble residues (figure 10) underline the relevance figure 8. the degree of correspondence amongst the major element oxides. bassey edem ephraim50 of the minor aluminosilicate phase in controlling the distribution of both insoluble components and most trace elements. accordingly, the positive correlation among al2o3 and tio2, fe2o3, k2o, ba, nb, rb and zr (figure 10) is taken to imply that all these components are contained in different proportions of aluminosilicate phases, as appropriately reflected in the factor 1 loadings (table 7). the strong positive interrelationship amongst ba, rb and k2o (table 6, figure 10) underline the relevance of k-bearing minerals in the aluminosilicate phases. it has long been known that ba and rb are especially retained in k-bearing minerals, and common k – bearing minerals are mostly the feldspars. however, the non–linear interrelationships involving na, pb, sr and al2o3 (table 6, figure 11) and na2o and cao (figure 8) corroborate petrographic observations (table 1) that feldspar does not constitute a significant phase in the modal mineralogy of the rock, rather quartz, talc, and phlogopite or muscovite are the main aluminosilicate phases in the rock. the high loadings of ca, sr, pb and cu of factor 2 (table 7) most likely reflect the composition of the carbonate fraction, while the high loadings of mn, ba, u and y recorded for factor 3 could not be readily interpreted. the exclusion of mgo as a significant component in all three factor score groupings could be explained by considering diverse sourcing of the mgo. conditions of formation of metacarbonate deposit in an effort to gain insight into the pre-metamorphic conditions of the parent materials of the metacarbonate deposits, the chemical data of the rock were plotted on discrimination diagrams like garrels & mckfigure 9. the degree of correspondence of selected trace elements with cao composition of the nsofang metacarbonate rock. investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 51 enzie’s na2o/al2o3 cf k2o/al2o3 diagram (1971) and leyreloup et al’s mgo–cao–al2o3 discrimination diagram (1977). the samples dominantly plotted in the sedimentary field on the na2o/al2o3 cf k2o/ al2o3 diagram (figure 12), thereby confirming that the rock is largely of sedimentary origin. on the mgo–cao–al2o3 discrimination diagram (following leyreloup et al., 1977) (figure 13), the metacarbonate rock samples plotted outside the magmatic funnel, thereby supporting the sedimentary antecedents of the rock. the sedimentary origin of the precursor rocks is also clearly observed in the bedded nature of the metacarbonate rocks (figure 4) and the fact that sr and total ree abundance of the rock rarely exceed 200 ppm and 25 ppm, respectively, thereby agreeing with observations that sedimentary carbonates are usually characterised by low sr and total ree content (nothdurft et al., 2004; veizer et al., 1992). additionally, interrelationships observed earlier between major insoluble residue components and selected trace elements (notably the sio2 – al2o3 – k2o – tio2 – fe2o3 – ba – nb – rb – zr links) (figure 10) points to the close association of the parent carbonate rocks with aluminosilicate phases, which is a diagnostic sedimentary feature. having confirmed the sedimentary nature of the protoliths, it become necessary to investigate the nature and characteristics of the depositional environment of the parent sedimentary carbonate, and for this, the trace element composition of the rock, especially the rare earth elements are involved. according to tanaka and kawabe (2006), unaltered marine sedimentary carbonates are expected to preserve the chemical characteristics (i.e. ree features) of the medium in which the carbonates precipitated. however, care must be taken when making such an interpretation figure 10. cross–plots showing the varying degrees of positive inter-relationships displayed between various trace elements and the insoluble components of the nsofang metacarbonate rock bassey edem ephraim52 in outlining the ree pattern differences between shallow coastal waters and deep waters, tanaka et al., (1990) observed that data points for la, nd, sm, and eu for coastal waters fall on a nearly straight line, and their ree patterns are characterised by marked discontinuity between eu and gd. similar observations have been made on the ree patterns of the nsofang metacarbonate rock (figure 7), and the indication is that the protoliths of the rock was deposited in a shallow, near-shore marine environment. besides, the observed uncoordinated zigzag patterns in the gd – dy – ho – er – yb – lu spans of the heavy rare earth elements (hree) of the nsofang metacarbonate (figure 7) require further investigation, as such features can also be associated with a surface or shallow water environment (masuda and ikeuche, 1979; tanaka et al., 1990). interpretation of a surface or shallow marine depositional environment for the parent carbonate material of the nsofang rocks is in agreement with the reported negligible na2o, fe2o3, mno (table 3) and sr concentration (table 4) (clarke, 1911; frank 1975; turekiah and wedepohl, 1961). davou and ashano (2009) considered low concentration of na2o to imply a shallow, near-shore, low energy “clean water” marine environment of deposition for the parent carbonate materials of the marble deposit occurring east of the federal capital territory (fct) of nigeria. moreover, most sedimentary carbonates in nigeria’s cretaceous basins have been interpreted as product of shallow marine environment of deposition. enu and adegoke (1988) used microfacies and microfaunal evidence to interpret the ewekoro limestone occurring within the eastern dahomey basin of south-western nigeria as products of deposition in shallow marine environment. adekeye and akande (2002) integrated lithological, paleontological and sedimentary data to reveal that the sedimentary sequence of the yandev area in the benue trough (consisting of a limestone–shale sequence) were deposited in a shallow marine shelf lagoonal environment. figure 11. cross–plots showing vague and inconsistent inter-relationships displayed among various chemical components of nsofang metacarbonate rock figure 12. composition of the nsofang metacarbonate rock on the na2o/al2o3 cf k2o/al2o3 discrimination diagram of garrels and mackenzie (1971). because carbonate ree content could also be influenced by non-carbonate materials, such as fine-grained siliciclastic material in excess of 5% (banner et al., 1988). fortunately, in the case of the nsofang metacarbonate rocks, petrographic and geochemical observations have shown that noncarbonate phases are negligible, and this precludes the consideration of an additional influence on the ree chemistry of the rock by non-carbonates. investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the mamfe embayment of south-eastern nigeria 53 among the sedimentary cycles delineated for the mfamosong limestone of the calabar flank, is an initial transgressive cycle that occurred and resulted in a normal shallow marine environment (fayose, 1978). akande et al., (1988) concluded that shallow water epicontinental seas, flanked by fault–bounded basement uplands seems to have been the primary environments of deposition of carbonate materials in nigeria. following ikhane et al., (2009) the low al2o3 concentration measured in nsofang rock is interpreted as being indicative of a low energy environment. when the relevant chemical data of the nsofang metacarbonate deposit are projected on the caco3 – mgco3 – others (insoluble component) diagram (figure 14) adapted from harker (1939), carr and rooney (1985) and davou and ashano (2009), indication is that the metacarbonate rock is dolomitic. the occurrence of lamination, vug and cavernfigure 13. composition of the nsofang metacarbonate rock on the mgo – cao – al2o3 diagram by leyreloup et al., (1977). figure 14. composition of the nsofang metacarbonate rock on the caco3 – mgco3 – others (insoluble components) classification diagram, adapted from harker (1939), carr and rooney (1985) and davou and ashano (2009). ous structures as part of the structural features (figure 4) of the nsofang metacarbonates possibly reflects the relevance of biogenic activity at some point in the evolutionary history of the rock. the lamination probably represents relic organic imprints similar to the type produced on tidal flat environments by colonies of blue green algae, while the irregular shape of the caves and small cavities with their sculptured walls suggest formation through dissolution of lithified carbonate material. there is therefore every indication that the metacarbonate rocks most likely began as calcitic material before conversion to dolomitic prior to metamorphism. dolomitisation of sedimentary carbonate material is a widely-reported process for many nigerian carbonates and metacarbonates (e.g. davou and ashano, 2009; odigi and amajor, 2008; oti, 1983; petters, 1978). the prior demonstrated existence of precursor carbonate sedimentary material most likely provided the required setting for extensive dolomitisation of the precursor rock of the nsofang metacarbonate. however, as already known (e.g. deelman, 2008; purser et al., 1994; welch, 2001), the condition of formation of dolomite in the natural environment is frequently different from synthesis of ordered dolomite, accomplished at temperature of over 100°c and 20 atm pressure in the laboratory. the source of high mgo is of considerable importance in the conversion of the sedimentary carbonate to dolomitic rock. according to akcay et al., (2003), possible mgo sources for dolomitic carbonate–bearing rocks include pore-water, seawater and clay minerals. pore-water trapped in limestone may promote dolomitisation (kupecz and land, 1991) because these waters are typically ca-enriched and their molar ca/mg ratio increases with increasing temperature (kupecz and land, 1991). however, a pore-water mgo source for the nsofang metacarbonates is not likely because pore-water is usually characterised by moderate to high salinity (10–25 wt% nacl equiv.; gomez-fernandez et al., 2000; yao and domicco, 1997) and na2o and k2o concentration in carbonate tends to decrease with increasing salinity but appears to increase with depth (clarke, 1911). the metacarbonate rocks of nsofang are characterised by low na2o values and shallow depth of deposition, which is at variance with considbassey edem ephraim54 eration of the pore-water source. seawater, another major source of mgo for sedimentary dolomitic rocks, contains significant mg concentration (~3.7%) (taylor and sibley, 1986; tucker and wright, 1992). land (1985) concluded that seawater is the only widely-available fluid having sufficient magnesium to cause massive dolomitisation, and diagenetic dolomitisation of calcite-rich carbonates requires large volumes of water to supply the required mg (land, 1992). the low na2o and k2o concentration in the metacarbonate, combined with the moderate sr content, support the seawater source of mgo and, with seawater as the major source of mgo, the low pb concentration in the rock (table 6) can be readily explained. wedepohl et al., (1974) advanced the view that calcite and dolomite cannot incorporate appreciable concentrations of pb since sea and interstitial water usually contains very little pb. clay minerals represent another important major source of mgo, which is frequently regarded as a source of mgo for dolomitisation, especially for late-stage dolomite formations (morteani et al., 1982; kupecz and land, 1991; warren, 2000). given that nsofang metacarbonate contains a quantity of quartz, talc, phlogopite and probably muscovite (table 1), it is highly probable that transformation of these materials may release additional (though negligible) mgo for the carbonates. detailed analysis of the textural and structural features preserved in the nsofang metacarbonate rocks reveal that the precursor sedimentary materials were affected by several widespread, laminae-parallel, diagenetic processes, most likely controlled by fine-scale layer-parallel differences in initial porosity and permeability. xu (2011) observed that fenestral fabrics such as lamination and open space structures like vug and cavernous structures are usually formed in shallow, near-coast supratidal and upper intertidal carbonate environments (flügel, 2004). the fenestral fabrics, formed in tidal flats, are generally attributable to diagenetic processes, associated with the degradation of microbial mats (hofmann et al., 1980). summary and conclusions the hitherto unknown metacarbonate deposits of the cretaceous mamfe embayment comprised distinct carbonate and minor silicate fractions. while the carbonate fraction include ca, mg, sr, pb and cu, the silicate fraction are associated with si, ti, al, fe, k, ba cs, rb, nb, zr, y and zn. geochemical data, integrated with field information, have been used to infer a metasedimentary petrogenetic affiliation for the rock. indication is that the metacarbonate deposits were formed from sedimentary carbonate materials that were deposited in a saline, shallow-marine environment, possibly in a supratidal and/or upper intertidal environments. seawater seemingly constituted a major source of mgo. generally, the consistency in the chemical properties of the rock most likely reflects relative stability during deposition of the initial sediments. future studies, possibly including isotopic and geochronological data, would shed more light on the evolutionary history of the metacarbonate deposit. however, the present study provides a basis for revisiting earlier paleo-environmental interpretations and intense exploration of the geology of the nigerian sector of the mamfe embayment as this could be very rewarding for the country. acknowledgements i would like to acknowledge the support and unparalleled hospitality received from the chiefs and people of nsofang community in present– day etung local government area in cross river state of nigeria. my heartfelt appreciation also goes to my graduate students, charles umagu, columbus edet and festus udumu, for the roles they played in the successful completion of this research. furthermore, i give special thanks to all the people who were involved in sample preparation and analysis stages of the work at both the department of geology, university of calabar, calabar, nigeria and acme analytical laboratories, vancouver bc, canada. last, but not least, is the anonymous peer reviewer whose review greatly added to the value of the final work. references adekeye o. a. and akande s. o. 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(1997). dolomitization of the cambrian carbonate platform, southern canadian rocky mountains: dolomite from geometry, fluid inclusion geochemistry, isotopic signature, and hydrogeologic modeling studies. american journal of science 297: 892 – 938. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 173 179 spatial distribution of sediment pollution in the khajeh kory river using kriging and gis maryam zare khosh eghbal department of mathematics and science, astara branch, islamic azad university, astara, iran e-mail: m.zare@iau-astara.ac.ir gis the aim of this research was to study of sediment pollution conditions in the river bed of khajeh kory, which passes from the southern border of the caspian sea in astara city. to determine the pollution of river bed sediment along the route, from the river source in the heights to estuary in the caspian sea, sampling was conducted at 10 stations. afterward, each sample was divided into two groups based on size (>63 µm and <63 µm). the concentration of heavy metals cu, cd, co, zn, pb, ni, fe, and mn in each group was measured using icp-oes. in the next stage, the enrichment of samples was determined by normalization with aluminum. the data were then interpolated using the kriging method and various models (including spherical, circular, exponential, gaussian) were fitted to the data, and the best method was selected using the cross validation method. using the obtained enrichment outcomes, an information layer was produced for each element in the gis environment utilizing the kriging method. the layers were aggregated and the compiled layer was classified into 4 layers again. using this method, the length of the river from its source to the sea estuary was characterized based on the contamination of heavy metals, and contaminated and unpolluted areas in river sediments could be observed. the source of this pollution, either natural or manmade, was revealed through this method. el objeto de esta investigación fue estudiar las condiciones de contaminación sedimentaria en el lecho del río khajeh kory, que se extiende desde el borde sur del mar caspio en la ciudad de astara. para determinar la contaminación del lecho sedimentario, se recolectaron muestras en 10 estaciones establecidas entre la fuente del río en sus partes altas hasta los estuarios en el mar caspio. después, cada muestra fue dividida en dos grupos de acuerdo con el tamaño (>63 µm and <63 µm). la concentración de metales pesados cu, cd, co, zn, pb, ni, fe y mn en cada grupo se midió con la técnica icp-oes. en la siguiente etapa, se determinó el enriquecimiento de las muestras por la normalización con aluminio. los datos se interpolaron luego con el método kriging y varios modelos (incluidos el esférico, el circular, el exponencial, el gaussiano) se ajustaron a la información y se seleccionó el mejor método a través de la validación cruzada. con los resultados obtenidos del enriquecimiento se produjo una capa de información para cada elemento en el ambiente gis a través del método kriging. las capas fueron agregadas y la compilación de estas se clasificó nuevamente en otras cuatro capas. de esta forma se caracterizó el río a lo largo, desde su nacimiento hasta el estuario marino, con base en la contaminación de metales pesados y se pudo distinguir entre las zonas contaminadas y las áreas limpias en sedimentos. las fuentes de esta contaminación, sean naturales o humanas, también se identificaron con este método. abstract resumen key words: sediment pollution, khajeh kory river, kriging, gis palabras clave: sedimentos contaminados; río khajeh kory; kriging; gis record manuscript received: 19/07/2014 accepted for publication: 16/09/2014 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.44491 http://dx.doi.org/10.15446/esrj.v18n2.44491 174 maryam zare khosh eghbal, shahriar sobh zahedi introduction: rivers are an essential aquatic ecosystems, and rivers are polluted by various contaminants (bednarova et al., 2013). one class of most important pollutants is heavy metals. heavy metals are ecologically impactful because they experience high resistance in the environment, accumulate in natural resources, and ultimately enter the food cycle (varol & şen, 2012). river sediments are the ultimate destination of these metals (maanan et al, 2013). geochemistry-based methods have been considered appropriate in determining the contaminated condition of these deposits (meybeck, horowitz, & grosbois, 2004). the guilan province is especially notable for contamination as various rivers directly or indirectly pour into the caspian sea. within this province a number of urban areas, urban sewage, domestic wastewater, and industrial sewage play important roles in the contamination of rivers (gheshlagh, ziarati, & bidgoli, 2013). domestic wastewaters are poured into special attraction wells within each residential unit in rural areas. however, various agricultural fertilizers and pesticides are common in this area because of rice cultivation and the use of other agricultural lands. these materials enter the river along with surface drainage and cause pollution. additionally, the land may also contain high levels of heavy metals due to special lithology and geology, naturally causing contamination without human interference (salati & moore, 2010). the khajeh kory river is located in the western part of guilan and passes along forest, agricultural, and residential areas upstream and crosses astara downstream to pour into the caspian sea. the aim of this study is to investigate contamination conditions in this river from different sections from its source (areas without agricultural lands and human sewage) to river estuaries at the sea (areas that receive urban and rural sewage, as well as agricultural land surface drainage). one problem in measuring the contamination of an area or river involves the limited sampling possible within an area or along the river length; to overcome this problem, interpolation methods should be used. interpolation analysis is in line with the spatial characteristics of methods that are common in spatial statistics studies (xie et al., 2011) and is based on the principle that points that are closer in space are also likely to be more similar to each other than to points that are further apart. there are various spatial interpolation algorithms that are based on geostatistical and geometric methods. recently, the use of gis software has been utilized in the spatial modeling arena and a variety of studies have been conducted in interpolation analyses for zoning contamination sites (mcgrath, zhang, &carton, 2004). validation of these methods requires precision and various factors are recommended. in the present study, geographic information system (gis) and standard kriging methods have been used to prepare interpolation maps to examine heavy metal contamination in the khajeh kory river. materials and methods: in the studied area, the sediments of the khajeh kory river covers a watershed area of 10233 hectare, located in the guilan province, which ranges between 38° 20΄ 45˝ and 38° 26΄ 45˝ latitude and 48° 42΄ 22˝ and 48° 52΄ 28˝ longitude. this watershed is covered with forest (63.12%), agricultural and garden lands(12.71%) and 2.47% rural and urban areas (figure 1). geologically, the basin is covered with structures from the mesozoic and cenozoic ages. the main part of basin (44.5%) is composed of pevet units from the paleocene age with alternation of tuff ,tuffereous sandstone with pyroxin.   caspian  sea figure 1: land use map in the khajeh kory watershed area. figure 2: geological map of the khajeh kory watershed area. this special lithology can cause an increase in the measurements of elements in the area and is given special attention. qcs units encompassing the quaternary age cover 27.33% of the total area and constitute the second most prevalent the type of sediment in the caspian sea coastal plain (figure 2). figure 2 is a geological map of the area and was prepared using arc gis 10 software and georeferencing a 1:100000 map of astara geology. sampling stations were selected to cover a range of various sites depending on the land use and type of lithology in the area. the pattern of sampling and the condition of each area are represented in figure 1. after sampling stations were determined (singh, mohan, singh, & malik, 2005), trapped sediments were sampled from each station (horowitz, 2009). sediments were dried at ambient air temperature in laboratory conditions. dried sediments were powdered and divided by size into two groups (>63 µm (muddy part) and <63 µm), and each group was subjected to icp-oes (inductively coupled plasma optical emission spectrometry) to determine the concentration of elements, including cu, cd, co, zn, pb, ni, fe, and mn. icp-oes uses plasma to prepare qualitative and quantitative analyses of elements. in our analysis, the argon gas was ionized using a magnetic field (radio frequency of 27-40 mh) and the temperature was ~10,000 k. the sample was infused into the plasma using a nebulizer, and the high temperature sample was converted to ions and diffused through the sediments. the rate of diffusion was measured by the apparatus. after determining the elemental concentrations, a method must be applied for quantitative measurement of elemental contamination. one such method uses aluminum as a standard in normalizing sediments samples (choi, kim, hong, & kim, 2013). aluminum is present in high amounts as a result of erosion in stones and its presence is sediment is not affected by human activities. conversely, aluminum has variable solubility, can chemically react in soil, and migrates after being deposited in sediment. normalization based on aluminum   175spatial distribution of sediment pollution in the khajeh kory river using kriging and gis content occurs in two stages. first, the concentrations of each element in a sample are divided by the concentration of aluminum, and the sample with the least variation is selected as the reference sample. second, the amount of pollution is obtained by the ratio of a given element to aluminum in the sample and compared to an inner reference sample using the following formula: ef = [x / al] sample /[x / al] ref in the following method, the descriptive statistics of analysis and geostatistics will be used to fit the best data to an enrichment model. finally, an enrichment map is prepared for each element, and after merging the elements, a cumulative map of pollution weight in river foot can be produced. statistical methods: 1. descriptive statistics of data: to investigate the distribution of element enrichment and to summarize the statistical information of contamination, concentration of each element in ppm was evaluated for maximum, minimum, mean, standard deviation and variance using spss software. 2. geostatistical analyses: geostatistics is capable of modeling uncertain temporal and spatial phenomena. basically, geostatistics estimates each unknown value as a random number within a known probable distribution across an understudied space (dao, morrison, kiely, & zhang, 2013). before using the data, the normality of data needs to be determined (kroulík, mimra, kumhála, & prošek, 2006.) and both kolmogorov and smirnov tests are used to test the distribution of data with a reliability of 5% and 1%, respectively. after determining the normality of the data, geostatistics is used to correlate information of a point to the information of that area on a map. the kriging method of geostatistics was used to interpolate, zonate, and forecast the probability of data correlation. 3. analyzing variograms: after checking the normality of the data, kriging measurements were implemented in calculating a variogram. a variogram is a function that measures the variability of spatial data; it is extremely important in spatial data correlation and is the base of geostatistics (kaushik, kansal, kumari, & kaushik, 2009; maanan, et al., 2013). dividing a variogram in two, we can obtain easily understood semivariograms, which is the practical use in geostatistical methods. semivariograms are represented by γ (h) and can be calculated according to the following relation: in this equation, n is the number of sample couples per distance, h; zi (x) is the value of variance in i point; and indicates the value of variance in a point that is h distance from point i. increasing h, the value of semivariogram is increased to a certain distance and then fixed at a threshold level, sill. the distance between samples at which variable values have minimal effect on each other and the value of semivariogram does not significantly differ is called the range of influence. the value of the semivariogram when h=0 is called the nugget effect (figure 3) (liu, wu, & xu, 2006). the nugget effect usually results from errors in data sampling, measurement and, analysis. the less the nugget effect, the less the estimation error will be. variogram modeling is an iterative process. in all geostatistical interpolation methods, including kriging, unknown values are estimated using the following equation: figure 3: schematic image of the variogram and its parameters. γ (h) = 1 2n zi (x)− zi (x + h)[ ] i=1 n ∑ 2 zi (x + h)   sill range of influence nugget effect structured  variance   z *(x) = λiz(xi ) i=1 n ∑ in this equation, z(xi) indicates the value of observable z variable at xi, λi indicates the weight of a given ratio to z variable at xi, and n is the number of observations. after drawing an experimental variogram, four models (including circular, spherical, exponential, and gaussian) were fitted for each data set. 4. assessment methods, criterion, and validation models: in the present study, the methods used to control and validate parameters used in the estimation will be assessed as a method of cross-validation. this method requires deleting samples and re-estimating them using the kriging method, followed by using other samplings and fitted models in an experimental variogram (angulo-martínez, lópez-vicente, vicente-serrano, & beguería, 2009). following the subtraction of real and estimated values, the estimations are statistically evaluated (castrignanò, giugliarini, risaliti, & martinelli, 2000). in the present research, validation models and estimations were evaluated by calculating the root mean square (rms) criteria and the standard root mean square (srms) values using the following formulas: in these formula, n is the number of points zi,act indicates the value of i at a known point, zi,est is the estimation of i known point, and s indicates error variation. the best estimation should have a minimal rms, and srms should approximate 1. if srms equals 1, it means that rms is equal to s (angulo-martínez, et al., 2009). generally, the smaller the rms, the more precise the method will be. theoretically, if this criterion equals zero, the method precision is one hundred percent and the estimated value of a quantity is equal to its real value. discussions and results: table 1 details the changes in the elemental concentrations in the sediments from the khajeh kory river. these results demonstrate that the heavy metal concentrations were mainly found in fine-grain deposits (fang, jiang, wang, & xie, 2010; kaushik, et al., 2009). additionally, the highest concentrations of iron were found in station s10; the maximum concentrations of manganese, cobalt, and cadmium were found in s3; the maximum concentration of nickel was found in s9; the maximum concentration of lead was found in s1; and the maximum concentration of zinc was found in s7. in regard to sampling station placements by geological units and land use, station 1 (s1) reveals high values of lead, which result from urban life and petrol use in that area. the abundance of iron in s10 comes from natural, geological sources as there are no contamination factors from agriculture, urban and rural sewage, or industry. conversely, constitutive rocks in this area are mainly composed of pyroxene, which contain iron and magnesium components. three stations, including s9, s8, and s7, have similar compositions. therefore, the noted frequency of zinc in s7 is of non-geological origins and likely originates from the neighboring besat residential suburb, which utilizes gable roofs that are composed of galvanized zinc. this zinc can decompose due to rainfall and enter surface water, resulting in higher concentrations in sediments. abundance of manganese, cobalt, and cadmium in s3 among agricultural lands is likely a result of chemical fertilizers and pesticides used in rice fields, which are common in the area. rms = 1 n (zi,act i=1 n ∑ − zi.est ) 2 srms = 1 n (zi,act − zi,est ) 2 s2i=1 n ∑ = rms s 176 maryam zare khosh eghbal, shahriar sobh zahedi table 1: concentrations of selected elements of sample sediments from sampling stations. f * indicates fine-grain (smaller than 63 micron) and c indicated coarse-grain (greater than 63 μm). the enrichment factor values and use of normality method by al in various stations is described in table 2. enrichment factors are used in this analysis instead of absolute elemental concentrations because of differences in densities between elements. for instance some elements, including iron and manganese, are more common in the crust and are measured in percent, whereas other elements are less abundant and are measured in ppm. using enrichment as an index without dimension is effective and efficient because it allows for different analyses to be conducted without needing to consider the differences between the amounts of individual elements. these results indicate that the pollution is mostly in fine-grain sediment (rodríguez, ruiz, alonsoazcárate, & rincón, 2009; seshan, natesan, & deepthi, 2010). additionally, s3 stations contained the highest amounts of mn, co, and cu. the level of zn enrichment was found to be 30 times higher in this area. after the river crosses astara city, where it begins to pour into the caspian sea, there is an 18.73-enrichment of zinc, which demonstrates that there is a relationship with residential and agricultural areas (bai et al., 2011). table 2: the results of element enrichment factors in sediment samples. sample x y fe mn cd co cu ni pb zn s1 f 313741 4255467 0.61 0.76 0.7 0.67 0.97 0.72 3.79 15.92 s1 c 313741 4255467 0.89 0.9 1.03 0.95 0.85 1.03 0.89 1.01 s2 f 313277 4255522 0.56 1.28 0.69 0.56 0.9 0.77 2.08 3.03 s2 c 313277 4255522 1 1 1 1 1 1 1 1 s3 f 309388 4256292 0.7 2.22 1.03 0.8 1.15 0.76 1.13 0.85 s3 c 309388 4256292 0.9 0.82 0.97 0.82 1.22 0.82 0.85 0.93 s4 f 308955 4255522 0.62 0.58 0.72 0.59 0.9 0.72 1.09 4.68 s4 c 308955 4255522 0.87 0.81 0.99 0.91 0.87 1.03 1.23 1.06 s5 f 308627 4255966 0.6 0.53 0.65 0.6 0.91 0.71 1.01 12.91 s5 c 308627 4255966 1.22 0.99 1.03 1.12 1.04 1.17 1.93 1.15 s6 f 307798 4253708 0.65 0.85 0.67 0.7 0.91 0.82 1.25 5.25 sample fe 7.14 cd co cu ni pb zn s1 f 4.86 3.76 0.27 25 63 50 117 2116 s1 c 4.64 5.37 0.26 23 36 47 18 88 s2 f 4.48 0.79 0.27 21 59 54 65 407 s2 c 5.16 0.63 0.25 24 42 45 20 86 s3 f 5.59 7.14 0.4 30 75 53 35 114 s3 c 5.14 3.76 0.27 22 57 41 19 89 s4 f 5.03 5.37 0.28 22 59 51 34 630 s4 c 4.56 0.79 0.25 22 37 47 25 92 s5 f 5.37 0.63 0.28 25 66 55 35 1920 s5 c 6.35 7.14 0.26 27 44 53 39 100 s6 f 5.36 3.76 0.27 27 61 59 40 723 s6 c 6.03 5.37 0.26 27 51 58 29 104 s7 f 4.95 0.79 0.27 24 57 53 37 4529 s7 c 5.94 0.63 0.24 26 48 57 32 119 s8 f 4.85 7.14 0.8 27 64 60 51 201 s8 c 6.42 3.76 0.25 26 47 61 21 100 s9 f 3.76 5.37 0.28 17 60 44 100 2696 s9 c 5.75 0.79 0.23 23 40 56 19 96 s10 f 5.93 0.63 0.26 26 69 45 28 345 s10 c 7.14 7.14 0.25 30 47 60 48 113 max 7.14 3.76 0.8 30 75 61 117 4529 min 3.76 5.37 0.23 17 36 41 18 86 average 5.37 0.79 0.3 24.7 54.1 52.45 40.6 733.4 standard deviation 0.79 0.63 0.12 3.11 11.1 6 26.27 1182 variance 0.63 7.14 0.02 9.69 123.15 36.05 689.94 1396988.9 177spatial distribution of sediment pollution in the khajeh kory river using kriging and gis table 3: the results of kolmogorov – smirnov in enrichment rate of river sediments. fe mn cd co cu ni pb zn n 10 10 10 10 10 10 10 10 normal parameters a, b mean 0.61 0.99 0.85 0.63 0.94 0.73 1.74 10.26 std. deviation 0.05 0.51 0.42 0.09 0.08 0.09 1.07 11.06 most extreme differences absolute 0.16 0.21 0.36 0.14 0.26 0.22 0.28 0.28 positive 0.15 0.21 0.36 0.14 0.26 0.13 0.28 0.28 negative -0.16 -0.18 -0.23 -0.10 -0.16 -0.22 -0.18 -0.20 kolmogorov-smirnov z 0.52 0.65 1.125 0.45 0.819 0.684 0.873 0.87 asymp. sig. (2-tailed) 0.95 0.79 0.16 0.99 0.51 0.74 0.43 0.44 s6 c 307798 4253708 0.89 0.76 0.79 0.85 0.92 0.98 1.1 0.92 s7 f 308689 4254667 0.63 1.01 0.7 0.65 0.89 0.77 1.21 34.35 s7 c 308689 4254667 0.98 0.77 0.81 0.92 0.97 1.08 1.36 1.17 s8 f 306606 4252861 0.58 1.24 1.99 0.7 0.95 0.83 1.58 1.45 s8 c 306606 4252861 1.1 0.81 0.89 0.96 0.99 1.2 0.93 1.03 s9 f 305939 4252099 0.51 0.77 0.79 0.5 1.01 0.69 3.53 22.1 s9 c 305939 4252099 1.27 0.93 1.05 1.09 1.08 1.42 1.08 1.27 s10 f 305527 4251633 0.6 0.62 0.54 0.57 0.86 0.52 0.73 2.1 s10 c 305527 4251633 1.22 0.88 0.88 1.1 0.99 1.18 2.12 1.16 parameter model nugget effect partial sill nugget/ partial sill rms srms fe circular 0.00151 0.00135 1.1156 0.05047 1.022 spherical 0.00150 0.00137 1.0950 0.05028 1.017 exponential 0.00146 0.00153 0.9517 0.05119 1.023 gaussian 0.00167 0.00127 1.3165 0.05061 1.031 circular 0 0.36795 0 0.5347 0.8304 mn spherical 0 0.36795 0 0.5399 0.8376 exponential 0.01416 0.36795 0.0385 0.554 0.8569 gaussian 0.00500 0.36795 0.0136 0.5352 0.8459 circular 0 0.24437 0 0.4221 0.8868 cd spherical 0 0.24816 0 0.4274 0.8901 exponential 0 0.29234 0 0.4552 0.9356 gaussian 0.00028 0.27828 0 0.4142 0.9015 circular 0 0.00867 0 0.08893 0.9204 co spherical 0 0.00867 0 0.08956 0.9236 exponential 0 0.03115 0 0.09237 0.9404 gaussian 8.98e-06 0.00898 0 0.08999 0.9269 circular 0 0.01064 0 0.09753 0.8729 cu spherical 0.00262 0.00936 0.2794 0.09714 0.9326 exponential 0 0.01237 0 0.09722 0.8957 gaussian 1.09e-05 0.01090 0.0010 0.09552 0.8687 to process geostatistics, the normality of the data must first be evaluated, and the kolmogorov-smirnov test is used to distribute normal data (dao, et al., 2013). the results of this test are illustrated in table 3. this test indicated that all parameters have a normal distribution at a 0.05 level. after calculating and drawing experimental variograms, four models (including spherical, circular, exponential, gaussian) were fitted using experimental semi-variograms for each data set; the parameters for each one are displayed in table 4. table 4: variogram parameters for each of the elements. 178 maryam zare khosh eghbal, shahriar sobh zahedi as indicated in table 4, the nugget effects of zn, co, and cd elements for three models (including spherical, circular, and exponential) equals zero. low ratios of nugget/partial sill represent the spatial coherence of variables and the lesser this ratio, the greater the spatial coherence of the data will be (angulo-martínez, et al., 2009). the smallest ratios of this value were observed for cadmium, cobalt, and zinc in the three models. it is not possible to comment on the best data fit model because of the low nugget/sill for zn, cd, and co in the three different models; more calculations are required. cross-validation was conducted to validate the data using standard root mean square (srms) value and root mean square (rms) values. the calculated srms and rms values fitted from various models on different semi-variograms are given in table 4. the variogram with the smallest rms indicates the best model for the sample. the best models for each element were as follows: the exponential model for zn; the gaussian model for pb, cu, cd, and ni; the spherical model for fe; and the circular model for co and mn. in regards to the srms values, the use of kriging in spatial changes of selected data indicated that optimal estimation had been performed. preparing the map of contamination zoning: as described using the kriging method, the smallest rms establishes the criterion for selecting the best model. with this consideration, the best model was obtained for enrichment values of each elements. high precision estimation of the parameters in this and other studies (angulo-martínez, et al., 2009; bai, et al., 2011; bednarova, et al., 2013; castrignanò, et al., 2000; dao, et al., 2013) indicates that geostatistical methods are appropriate for studying spatial changes in data, using the necessary calculations when there is a lack of data. to obtain the best kriging models in gis, which are used to determine and interpret pollution in river sediments, raster layer were first generated for each element based on the best selected model during the kriging application stage (table 4). then, all raster layers were compiled, the resulting map was grouped into four classes, and the pollution zoning map was used to interpret pollution deposits in the khajeh kory river. figure 4: map of the superficial sediments accumulating in river pollution.   sample  location   to decrease uneven weighing effects by various factors, the raster map for each parameter was divided into four classes. these four class maps were collected, and the cumulative map was divided into four classes. the final map indicates that much of the river length contains high accumulations of contamination and that upstream parts of the river basin have been mainly formed from pyroxene deposits from the besat suburb, which contains the most pollution and largely depends on the igneous lithology of area (figure 4). as heavy metals exclusively demonstrate different contamination levels along the route and because cumulative contamination relies on multiple factors that are unique to each heavy metal, this implies different responses in the soil contamination of an element. conclusion: the conducted studies revealed that geochemical characteristics and river contamination differ at various points. the reason for this difference is based on the concentration of population centers, the way of life within these centers, land uses, and geology. urban population primarily concentrates in the eastern part of the drainage basin and decreases from east to west. the results demonstrate that elemental concentrations, excluding iron, increase as the river approaches the sea. iron concentration are higher upstream, as in station s10, despite the lack of contaminating agricultural land and residential areas; this is due to the ferruginous lithology of the area. additionally, the zn concentrations do not follow a clear pattern as the concentration fluctuates among different basin points and is at high concentrations after crossing all the basins and prior to entering to the sea. data from fine-grain samples from each station (s1f, s2f, s3f, s4f, s5f, s6f, s7f, s8f, s9f, s10f) were normalized, using aluminum, to determine enrichment. pollution can concentrate in fine-grain texture, and the data from this texture indicates area contamination. then, using the kriging method, the enrichment factor of the samples was converted to a map in an gis environment. to validate this map, rms factors were calculated and the most validated model was determined to fit each sample. next the data were converted to a surface map based on the best interpolation model as determined using kriging method. the obtained maps for each element were then converted to raster layer and these layers were collected, which may provide evidence of pollution. the results of classification indicate that much of the river length experiences low values of contamination. in addition, major parts of the river route contain low values of contaminants. the rest of the river route contains medium to high contamination. the highest amounts of contamination were observed between where the rivers exits the city and enters the sea, as well as in the areas crossing rice fields. in addition, in the heights with igneous lithology, high levels of contamination were observed around station s7 next to the besat residential suburb. as the river passes among pyroxene masses in the heights, the pollution is observed to be andesitic. this work highlights that the high rate of contamination in the khajeh kory drainage basin is from either natural or manmade sources. in the heights, the residential areas do not have considerable concentrations of heavy metals. additionally, there are no rice fields that can circular 0.00167 0.00554 0.3016 0.08104 0.9855 ni spherical 0.00179 0.00546 0.3282 0.0809 0.9866 exponential 0 0.00726 0 0.08408 1.005 gaussian 0.00290 0.00439 0.66013 0.0807 0.9896 circular 0.74358 0.59072 1.2588 1.158 1.096 pb spherical 0.73107 0.55505 1.3171 1.155 1.098 exponential 0.82256 0.34810 2.3630 1.154 1.07 gaussian 0.65856 0.94277 0.6985 0.4142 0.9015 circular 0 296.44 0 15.22 1.09 zn spherical 0 267.18 0 14.97 1.046 exponential 0 239.63 0 13.75 0.922 gaussian 15.83 243.24 0.0651 15.17 1.063 179spatial distribution of sediment pollution in the khajeh kory river using kriging and gis cause contamination, and the dominant coverage is natural rainforest. therefore, the existence of contamination in the heights is definitely related to the geology of the area. abundance of iron in river sediment after passing through the mass confirms its role in increasing elemental concentrations. the main reason for manmade contamination in the basin, excepting residential areas, are fertilizers and pesticides used in rice fields and the use of highly enriched lead, petrol and worn tires of transportation vehicles in urban areas. additionally, in urban areas, the main cause of contamination is domestic and urban sewage pouring into the river and the accumulation of waste on its banks. recommendations: contamination is one of the most important factors threatening the khajeh kory river ecosystem. as noted previously, wastes and effluents are considered to be the main sources of contamination. drainage water from agricultural lands contains high percentages of chemical fertilizers and materials that pour into the river. it is recommended that rivers should be managed to prevent solid waste accumulation and promote recycling and that the concentration of heavy metals in sewage feeding into the river should be monitored. additionally, sampling time intervals should be reduced and conducted in different seasons of the year to better and precisely analyze contamination in future research. references: angulo-martínez, m., lópez-vicente, m., vicente-serrano, s. m., & beguería, s. 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(2011). spatial distribution of soil heavy metal pollution estimated by different interpolation methods: accuracy and uncertainty analysis. chemosphere, 82(3), 468-476. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 67 71 late permian brachiopoda fauna in north-western iran mostafa yousefirad1, touran khamooshi2 and rahim shaabanian2 1 department of geology, faculty of earth sciences, payame noor university,tehran, iran 2 department of geology, faculty of earth sciences, payame noor university,tabriz, iran radyousefi@yahoo.com mobile: 00989183638957 paleontology the late permian marine sequence in the northwest of iran, in eastern azerbaijan province in the zal stratigraphic section, was selected for studying brachiopods. samples were collected from the ali bashi and jolfa formation.s. twenty-seven species from 13 brachiopoda genera were recognised in this study. the recognised fossil community was compared to brachiopod communities in some regions of iran and the tethyan region, suggesting the late dzhulfian period as the age for the deposits being studied. recognized brachiopods belonge to the orders athyris, rhynconellida, productida and strophomenida. keywords: brachiopoda, late permian, iran. la secuencia marina del pérmico superior en el noroeste de irán y el este de azerbaiyán, sección estatigráfica de zal, fue seleccionada para este estudio de braquiópodos. las muestras fueron tomadas en las formaciones ali bashi y jolfa. se reconocieron 27 especies de 13 géneros de braquiópodos en este análisis. las muestras recolectadas fueron comparadas con sus similares de otras regiones de irán y de la región del tetis, lo que sugirió el período de tardío de dzhulfan como la proveniencia de estos depósitos. los braquiópodos reconocidos en este trabajo pertenecen a las familias de athyris, rhynconellida, productida y strophomenida. key words: brachiopoda, late permian, iran palabras clave: braquiópodos, pérmico superior, irán record manuscript received: 16/11/2012 accepted for publication: 04/04/2013 abstract resumen 1. introduction marine permian deposits are exposed in many parts of eastern and western azarbaijan, in north-western iran (shabanian and bagheri 2008) .the zal stratigraphic section, ( 40,45’ eastern longitude and 38,43’ latitude), is located in north -eastern azerbaijan (figure 1);. this section is part of the abade-jolfa deposit sequence, having a general northwest-southeast trend.. the permian deposit sequence in the region being studied consisteds of the vajnan formation (baghbani, 1997; shabanian and bagheri, 2008 ) (or doroud formation), the surmaq formation (iranian-japanese research group, 1981), the jolfa formation (parto azar, 1997) and ali bashi formation (parto azar, 1997; tiechert, 1973). the permian deposit in the zal section,( having igneous nonconformity), is located on an igneous rock series before the permian age and is itself covered by constant transition to clararia limestone, vermiculite and the elika formation,caused by middle and early trassiac aging. the 46-meter-thick ali bashi formation was examined in this research to enable a paleontological study of brachiopods. the permian sequence in southwest jolfa was comprehensively studied for the first time by stepanone et al., in 1969. they divided the permian and triassic beds in the ali bashi mountain section in to 8 separate stone units;. unitse a and b were from the guadalupian age, c-d from the dzulfian age, e transitional stages, f a paratirolites horizon with early triassic age and h and g introduced the elika formation in to the section being studied. tiechert et al., then examined the ali bashi section and made seminal changes to the divisions of stepanov et al., (1967);. the according to them, e and f belonged to the highest part of the permian division, i.e. the dorashamian stage and they called it the ali bashi formation. re-examining the section, parto azar made some changes in permian borders and lithostratigraphic unites names. according to him, the lower part of unit a was the surmaq formation, the upper part and all of unit b consisted of thes jolfa formation and the 52 meters thickness c,d,e, and f were introduced as the ali bashi formation . based upon brachiopod fossils, (cephalopods and condonts), the age of the jolfa formation has been classified as being late dzhulfian in terms of permian stages in the tethyans region (chen et al., 2005). several reports have been concerned with the palaeontology of foraminifera, corals, brachiopods, conodonts and cephalopods of the ali bashi formation and formations from the same stage in alborz and abade. first information to be published on iran’s brachiopods dates back to 1916, (stayonav). stayonav et al., (1916) released a comprehensive report on the permian and permo-triassic border in the ali bashi section mostafa yousefirad, touran khamooshi and rahim shaabanian in southern jolfa and studied its brachiopods. restudying the ali bashi section, tiechert et al., (1973) and baud (2008) introduced many of its macrofossils, including brachiopods. an iranian-japanese research group (iranian-japanese research group, 1981) introduced brachiopods from the hambast formation in abade; and research into late permian brachiopods was published by fantini-sistin, in1965, fantini-sistini & glaus in, 1966 and angiolini in, 2010. biostratigraphy and event stratigraphy in iran around the permian-triassic boundary (ptb) has been published by kozur (2007). figure 1: position of study area the present research was aimed at introducing late permian brachiopods in stratigraphic section so as to determine their age and stratigraphic correlation as well as stratigraphically examining the value of recognising brachiopod fossils (figure 2). methodology systematic sampling was used for recognising and studying permian brachiopods in the zal stratigraphic section,; 60 samples were collected, along with corals and bryozoans, 13 genera and 27 species being recognised in the 15-meter thicksection. an attempt was made to determine change in their age and size, based on brachiopods. discussion the ali bashi formation in the zal stratigraphic section is 50 meters thicks and includes thin layers of limestone, shale, marl and middle layers of limestone (khamooshi 2012). an accurate field study has revealsed brachiopod, separate rugose and colonial coral , bryozoan, crinoids and cephalopod macrofossil frequency;. 27 species f rom 13 genera regarding late permian brachiopod s were recognized in the present research work, theost significant brachiopod families, in terms of frequency and variety, included productida, strophomenida, rhynconellida and athyris. studies were based upon external shell features , including shell, length, thickness and width, examining features such as pedicle valve, brachial valve, hinge line, foramen development or non-development, delthyrium (notch) status , shell shape, umbo (posterior extremity) shape, commissure line shape, existence or non-existence of fold and sulcus , their development and type of decoration . according to brachiopod stratigraphic dispersion in the zal stratigraphic section and comparison to a recognized late permian fossil community in iran and other parts of the tethyan region , late dzhulfian to dorashamian has been introduced as the age of the ali bashi formation. this research resulted in 13 genera and 27 species belonging to 5 different families being recognized, as follows: spinomarginifera helica(abich), s.spinicilicata(arthaber), orthotetina cf. persica (schewien) orthotetina cf. persica (schewien), o.eusarkos, o. ex.gr eusarkos, araxilevis intermedius (abich) tschernyschewia typica (stoyanow), compresoproductus dzhulfensis (stoyanow), permophricodothyris ovata (pavlova), araxathyris protea (abich), a . quadrilobata (abich) a. lata (grunt), a. lata minor (grunt), a. araxensis (grunt), a. araxensis minor (grunt), a. aff. ogbinensis, , wellerella dorashamensis,( sokolskaja ), psedowellerella araxensis (sokolskaja) , haydenella sp. figure 2: a chart of brachiopod biozonation in the zal section and stratigraphic column for the zal section brachiopods, in terms of biostratigraphy value, can be categorized into two assemblage areas (figure 2): figure 2.stratigraphic column of the zal section the araxilevis – orthotetina assemblage biozone this biota association was 10 metrs thick. the zone began with the appearance of araxilevis, tschernyschewia, spinomarginifera and end with appearance of compresoproductus, orthotetina. the biozons had many huge brachiopods, among which productidae and arthotetins were remarkable. some of the most frequently recognized productidae are given below: spinomarginifera helica(abich , 1878), araxilevis intermedius (abich, 1878), tschernyschewia typica (stoyanow, 1910), haydenella sp., compressoproductus dzhulfensis (stoyanow,..). huge (araxilevis) and moderate productida, such as tschernyschewia, compresoproductus, and huge orthotetina such as orthotetina eusarkos (abich), along with the coral plentamplexus leptoconicus (abich 1878) and the crinoid erisocrinus araxensis (yakovlev and ivanov, 1965) represent significant fossils from this biota association. permophricodothyris – araxathyrisa assemblage biozone: this biota association was 12 metres thick. this biozone began with the appearance of compresoproductus dzhulfensis (stoyanow), orthotetina eusarkos (abich) other brachiopods from this biota association were: compresoproductus dzhulfensis (stoyanow), araxilevis intermedius (abich), tschernyschychewia typica (abich), permophricodothyris ovate (pavlova). spinomarginifera helica (abich, araxathyris araxensis minor (grunt), araxathyris cf. protea (abich) and araxathyris quadriloba (abich; coral type pleramplexus leptoconicus (abich) has also been found in this biota association(figure 3). figure 3.biozonation of the zal section based on brachiopod fauna systematic classification of brachiopods 13 genera and 27 species were recognized in this research in the zal section (plates 1-5). comparing the recognized fossil community with other brachiopod communities other parts of iran and tethyan region suggested late dzulfian as of the age of deposits being studied. differen late permian brachiopod genera were found in the zal section. some of the recognized species and genera were as followi (based on moore, 1965): phylum brachiopoda class articulata huxley , 1869 order spiriferida waagen .1883 suborder athyrididina boucot, johnson&staton,1964 superfamily athyridacea mcoy, 1844 subfamily spirigerellinae grunt, 1955 genus araxathyris quadriloba, abich,1878. description: shell shape was convex-convex, dimensions beng almost the same, its width a little being a little bit more than its length, the pedicle valve had a sulcus lying from the umbo to the front side, and the brachial valve had a huge, liftedup fold. the commissure line was uniplicate. the most sulcus depression was in the front 1/3 of the shell. it was 3.1 cm thick, 3.9 cm long and 3.2 cm wide. geographical spread: shahreza, caucus, alborz – balade. order strophomenida opik, 1934 suborder strophomenidina opik ,1934 superfamily davidsonicea king, 1850 family meekellidae stehli, 1954(lamp shell) subfamily meekellinae stehli, 1954 genus orthotetina eusarkos abich, 1878 description: shell shape was convex – convex, with direct hinge line. the delthyrium coud be seen. interarea was obvious. the commissure line was plain. decorations were thorny. it was 2.3 cm thick, 4 cm long and 5 cm wide. suborder productidina waagen, 1883 superfamily productacea gray, 1840 subfamily marginiferidae stehli, 1954 subfamily marginiferinae stehli, 1954 genus spinomarginifera helica abich,1878 pl1, ig 2a-2e. description: shell shape was concave – convex, its width beinggreater more than its length.raised umbo.pedicle was little and compressed and the ears not distinct. sulcus waas replaced, brachial valve depressed and edged at the front. rear decoration was thorny. the shell was 1 cm thick, 1.7 cm long and 1.8 cm wide. geographical spread: shahreza, jolfa, balade, zal suborder productidina waagen, 1883 superfamily productacea gray, 1840 subfamily marginiferidae stehli, 1954 subfamily marginiferinae stehli, 1954 genus araxilevis intermedius. abich, 1878 description: shell shape was convex – convex. pedicle valve had sulcus from umbo to the end of the shell. brachial valve was convex, hinge line curved, commissure line antiplicate, umbo raised, development line delicate and unicentric. the shell was 2.5 thick, 4.4 long and 4.2 wide. suborder productidina waagen, 1883 superfamily productacea gray, 1840 family productidae gray, 1840 genus: tschernyschewia typical stoyanow, 1910 late permian brachiopoda fauna in north-western iran description: the shell was convex – concave and large. umbo was convex and raised. hinge line was direct, the beak obvious, and decorations had radial lines and were thorny. the shell is 2 cm thick, 3.5 long and 4 cm wide. suborder productidina waagen , 1883 superfamily productacea gray , 1840 subfamily marginiferidae stehli , 1954 subfamily marginiferinae stehli , 1954 genus: araxilevis intermedius, (abich,1878) description: the shell was convex – convex. pedicle valve had sulcus from umbo to the end of the shell. brachial valve was convex, hinge line curved, commissure line antiplicate, umbo raised and , development line unicentric and delicate. the shell was 2.5 cm thick, 4.4 cm long and 4.2 cm wide. plate 1 cl bl al el dl c2 b2 a2 e2 d2 1: araxathyris quadrilobata( abich, 1878). 1a: dorsal, 1b: posterior, 1c: ventral, 1d: lateral, 1e: anterior. 2: araxathyris cf. protea (abich, 1878). 2a: ventral, 2b: posterior, 2c: dorsal, 2d: lateral, 2e: anterior. plate 2 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e 1.araxathyris araxensis minor (grunt 1965).1a:ventral, 1b:dorsal, 1c:lateral, 1e:posterior. 2.orthotetina eusarkos (abich 1878). 2a:ventral, 2b:posterior, 2c:anterior, 2d:lateral, 2e:dorsal. plate 3 2a 2b 2c 2d 2e 1.: spinomarginifera helica (abich, 1878).1a: dorsal, 1b: lateral, 1c: ventral, 1d: anterior, 1e: posterior. 2.: araxilevis intermedius (abich, 1878). 2a: anterior, 2b: lateral, 2c: venteral,,2d: dorsal, 2e: posterior. plate 4 cl bl al el dl c2 b2 a2 1.: tschernyschewia typica (stoyanow,1910), 1a: anterior, 1b: posterior, 1c: lateral, 1d: ventral, 1e:dorsal. 2.: haydenella sp,. 2a: posterior, 2b: dorsal, 2c: lateral. plate 5 mostafa yousefirad, touran khamooshi and rahim shaabanian cl bl al cd b2 a2 2d 2e 1.: sarytchevinella diulfensis (or compresoproductus dahulfensis lamp shell) (stoyanow,1910). 1a: dorsal, 1b: venteral, 2c: posterior. 2.: permophricodothyris ovate (pavlova,1965). 2a: lateral, 2b: dorsal, 2c : posterior, 2d: anterior, 2e: ventral conclusion field study led to 27 species from 13 genera being examined, thereby revealsing that brachiopods havingglarge shells, belonging to the productidae and spiriferida families, likepermiansphaeroidothyris, araxilevis and orthotetina, were more frequent and the shells were bigger in the lower and middle parts of the sequence being studied. the shells become smaller towards the headof the sequence, that shapes having small shells belonging to the athyrisd and rhynconellida families (like werella and psedowerella) were more frequent during the late dzhulfian and dorashamian ages. the studies indicated that changes in brachiopod frequency and dimensions suggested their dependency upon deep environmental changes. based upon brachiopods, stratigraphic dispersion, the sequence being studied was determind to be late permian to dorashamian in terms of tethyans permian stage. acknowledgement the authors gratefully acknowledge the financial and other support of this research, provided by payame noor university,tehran, iran. authors would like to thank reviewers for their structured comments which improved the manuscript. references angiolini,l.,carabelli,l.,nicora,a.,crasquin-soleau,s.,marcoux,j. and rettori,r.2005, brachiopoda and other fossils from the permiantriassic boundary beds of the antalya nappes(sw taurus,turkey). geobios,40:715-729. baghbani, d. 1997. correlation chart of selected permian strata from iran. pemophiles, 30:24-25. baud, a. 2008. correlation of upper permian localities in the kuh-e-ali bashi area, nw iran: old collections, old and new data. permophiles 52,6–11. chen, z.,campia,m.,j.,shi,r.g.and kaiho ,k.,2005, post-extinction brachiopoda fauna from the late permian wuchiapingian coal series of south china ,acta paloentologica polanica, 50(2):343-363. fantini sestini ,n., 1965,the geology of the upper djaerud and lar valley (north iran); ii.paleontology;bryozoans , brachiopods and mollusks form ruteh limestone (permian). riv.ital .paleont .stratiger.,71(3):773-787. fantini sestini ,n.1966,milano-brachiopods from the upperpermian nessen formation(north iran)-v.72-n.4. fantini sestini,n.and glaus,m.,1966, brachiopods from the upper permian nessen formation (north iran):riv.ital.paleont stratigr.v.72,n4,p.887.923. iranian-japanes reserch group,1981, the permian and the lower triassic systems in abadeh region.central iran-mem.fac.sci.,kyoto uni v.,so .p.61-133. khamooshi, t,2012, biostratigaphy and chemostratigraphy of permotriassic sequence in the zal section in the south julfa,estherrn azarbaijan province,payame noor university,pp 182. kozur, h.w. 2007. biostratigraphy and event stratigraphy in iran around the permian-triassic boundary (ptb); implications for the causes of the ptb biotic crisis; environmental and biotic changes during the paleozoic-mesozoic transition. global and planetary change 55, 155–176. partoazar, h.,1997, permian system in iran.geological survey of iran,p.1-340. shabanian. r. and bagheri, m., 2008, permian in northwest of iran, permophiles, n.51, p 28-31. stoyanow,a.a.,1910,on the character of the boundary of paiaeozoic and mesozoic near djulfa:zapiski peterb.mineral. obshch.,ser.2,ch.47,p.61-135. stepanov ,l.d.,golshani, f.and stoklin ,j. 1967, upper permian and permian-triassic boundary in north iran.geological survey of iran ,report,12:72+xvplate. teichert,c.,kummel,b. and sweet, w., 1973, permian – triassic strata, kuhe-ali bashi , north western iran .bull.mus.zool.,145(8):359-472. late permian brachiopoda fauna in north-western iran geofisica colombian a pp. 55-60 bogota, d.c.n°.7 diciembre de 2003 issn 0121 2974 detecci6n en linea de onda-p en sismogramas 3-d utilizando redes neuronales carlos allate german castellanos grupo de control y procesamiento digital de seriales. universidad nacional de colombia sede manizales, canalza@epm.net.co resumen se desarrolla un detector en linea del tiempo de arribo de ondas p sobre registros electronicos de tres componentes de eventos tectonicos. se lleva a cabo la deteccion en linea de la onda-p empleando redes neuronales (rn), utilizando dos diferentes formas: en la primera se sintetiza el detector de lrv, mientras en la segunda se desarrolla un clasificador estadistico con las respectivas clases (seiial con onda-p presente y sefial con solo ruido presente). la deteccion se realiza empleando redes neuronales de tipo perceptron multi-capa, en las cuales se aprovechan las siguientes cualidades: su capacidad de mapeo no lineal de entrada-salida, buena generalizacion, bajo costo computacional, entre otras. las anteriores cualidades hacen que las rn sean apropiadas para el proceso de sefiales en tiempo real. las entradas a la rn corresponden a datos normalizados del registro sismico y las caracteristicas que miden el grado de polarizacion de la onda. los registros procesados se obtuvieron de la base de datos del instituto iris y de ingeominas, y su analisis da como resultado un mejor desernpefio del sistema en la deteccion del tiempo de arribo, aunque no presenta una exactitud aceptable en la estimacion del tiempo de arribo de la onda p. palabras clave: redes neuronales, perceptron muln-capa. deteccion onda p abstract we have developed a p-wave arrival time detector of tectonic events over digital three-component records online. this process uses two different methods: the first one works with a lvr detector, and the second one uses a statistical classifier of classes (signal with p-wave and signal with seismic noise). detection uses multi-layer perceptron neural network where is taken advantage of non-lineal input-output, relation, well-generalization, low computational demands, etc. these appropriate qualities for real time signal processing. the input data for the system becomes a normalized vector of seismic signal and its polarization arguments. testes were done with data from iris and ingeominas database. although the system has a good perforance, its accuracy is not acceptable. keywords: neural network, perception, mulllcapa, detecting p-ware introduccion la detecci6n de la onda-p es referida a la tarea de deteccion de cambios abruptos, la cual consiste en identificar el instante del tiempo en el que las propiedades del proceso cambian internpestivamente, considerando que antes y despues del momento del cambio, las propiedades del proceso soninvariantes en el tiempo. tal planteamiento permite que la derivacion formal de algoritmos de detecci6n de cambios abruptos se realice dentro de la c1ase de metodos estadisticos de analisis de procesos aleatorios estacionarios, para los cuales se consideran dos modelos de procesos manuscrito recibido para evauaclon el 30 de octubre de 2002. articulo aceptaco para publicacion par el comte editorial el 30 de octubre de 2003. geofislca colombiana. 7, diciembre de 2003 aleatorios: completamente independientes y los basados en modelos regresivos (ar). la prueba de hip6tesis mas empleada corresponde allogaritmo de relacion de verosimilitud (lrv), de la cual se derivan los algoritmos analizados: la suma acumulativa (cusum), el lrv generalizado (glrv), x2 cusum ycusum bilateral, entre otros. sin embargo, en caso de tener ruido no gaussiano a la entrada del detector, la funci6n lrv tiene un caracter no lineal con la entrada y, practicarnente, es muy dificil hallar unaexpresion explicita con el vector de entrada, 10 que dificulta la sintesis practica del detector. asi, el detector basado en el lrv para ruido no gaussiano debe considerarse como un dispositivo conmapeo no lineal. por cuanto las redes neuronales pueden aproximar unagran c1asede mapeo no lineal, su aplicacion se ha considerado en la detecci6n de sefiales (lippmann et al., 1989). 55 en el presente articulo se lleva a cabo la deteccion en linea de la onda-p empleando rn, utilizando dos diferentes formas: en la primera se sintetiza el detector de lry, mientras en la segunda se desarrolla un clasificador estadistico con las respectivas clases (sefial con onda-p presente y sefial con solo ruido presente). la deteccion en linea se realiza deslizando una ventana de n muestras a traves de todo el registro sismico de tres componentes. en cada instante de tiempo se obtienen las funciones de caracterizacion que corresponden a las entradas de la red neuronal, se realiza la propagacion y se decide si en la ventana actual esta el tiempo de arribo de la onda-p. deteccion en linea de onda-p utilizando redes neuronales detector de lr v. la deteccion de sefiales implica decidir si una sefiai objetivo esta presente 0 no, a partir de datos observados. existen dos criterios comunes para el disefio e implementacion de un detector de sefiales: el criterio de bayes y el criterio de neyman-pearson. ambos criterios dan como resultado el detector de lry (luo et ai., 1997), el cual esta compuesto de dos partes: la primera determina la lry de la funcion densidad de probabilidad (fdp) en presencia de la sefial objetivo y de la fdp en ausencia en la sefial objetivo (ruido). la segunda parte determina la deteccion (salida del detector igual a uno) si la razon de verosimilitud es mayor que un umbra i 0 , figura 1 la deteccion de un cambio abrupto de la onda-p, se puede simplificar al caso de analisis cuando se tiene un solo patron dado s(k), 11m, que representa el cambio abrupto mas ruido blanco gaussiano (k), 1:-::;k s; n, con media e[(ll)]=o y varianza e[1l2(k)]=cr2ei algoritmo de deteccion puede ser simplificado hasta el correlador que calcule el producto escalar de y con s [4]: z(n)=(y,s) = !s(k)x(k) > ao, k=1 acordes con la estructura del correlador de la figura i, se sugiere una red de tres capas con n neuronas en la capa de entrada, m neuronas en la capa oculta y funcion de transferencia fi (-)y una neurona como capa de salida con funcion de transferencia sr-), en principio, la estructura del detector secuencial descrito puede ser 56 alzate et al. implementado con una red neuronal del tipo retropropagacion (figura 2). los pesos w./l, l:-::;i:-::;ndenotan la conexion entre la neurona i en la capa de entrada y la neuronaj en la capa oculta; mientras yjck) yhjck) , 1:-::;j:-::;mcorresponden a su salida y funcion de activacion, respectivamente; wjpj denota la conexion entre la neurona j de la capa oculta y la neurona en la capa de salida. se define respectivamente z(k),h(k) y .y como la salida, la funcion de activacion, y el umbral para la neurona en la capa de salida. sin embargo, en caso de tener ruido no gaussiano a la entrada del detector, la funcion de verosimilitud tiene un caracter no lineal con la entrada y, practicamente, es muy dificil hallar una expresion explicita con el vector de entrada, 10que dificulta la sintesis del detector. ei detector basado en la relacion de verosimilitud para ruido no gaussiano debe considerarse por tanto, como un dispositivo con mapeo no lineal. por cuanto las redes neuronales del tipo mlp pueden aproximar una gran clase de mapeo no lineal, su aplicacion se ha considerado en la deteccion de sefiales. en luo et ai., (1997) se sugiere la realizacion de la funcion de verosimilitud para sefiales de entrada con diferente fdp a la gaussiana, entrenando adecuadamente los pesos de la rn para obtener el nivel de no linealidad requerido por el detector. detector basado en la clasificacion simple de eventos: en este caso, se desarrolla un clasificador estadistico con las respectivas clases (sefial con onda-p presente y serial con solo ruido presente). la rn identifica la clase a la cual el patron pertenece debido a su capacidad d~ extraccion de caracteristicas importantes a partir del conjunto de entrenamiento. la clasificacion es una transformacion que mapea los puntos intermedios obtenidos de la extraccion de caracteristicas a puntos correspondientes a cada clase en un espacio de decision n-dimensional, donde n es el numero de clases por reconocer. en este esquema los perceptrones multicapa realizan reconocimiento de patrones debido a que poseen un sistema no supervisado para extraccion de caracteristicas (capas ocultas) y un sistema supervisado para clasificacion (capa de salida) .• entrenamiento del sistema funciones caracterfsticas con el objeto de aumentar la efectividad del estimador durante la extraccion de parametres, se emplean las funciones caracteristicas. entre las mas empleadas estan las siguientes: razon de energia. en las sefiales sismicas de tres componentes, se puede construir una funcion para detectar movimiento sismico coherente en un plano bidimensional. la razon entre la energia en el plano horizontal y la energia total en el instante de tiempo i esta dado: er i+n-li (xl(n)2 +x2(n)2) 1/=; (1)i+-n ii(xl (n)2 +x2(n)2 +xj (nf) t1=1 geofislca colombiana, 7, diciembr de 2003 detecci6n en linea de onda.p en sismogramas 3·d utilizando redes neuronales donde x" x2 yx3 son las tres componentes de la sefial sismica, n es el tamafio de la ventana. er tiende a i si la mayoria de la energia sismica esta concentrada en el plano xy, y tiende a 0 si esta concentrada en la componente vertical. short term average/long term average (sta/lta). se calcula la razon entre el valor medio de la amplitud absoluta de dos ventanas de tiempo consecutivas, una corta (sta) y una larga (lta), y se compara con un umbral predeterminado que declara la presencia del evento sismico. la determinacion del tamafio optimo de las ventanas depende del contenido frecuencial del sismo (oonicx, 1998). se considera una sefial x con n muestras, con la que se construye una funcion caracteristica dada por: para i ~ i, con c(i) como parametro dependiente del tiempo. se define a 0 (0) =x(0)2, y en forma recursiva los promedios sobre las ventanas cortas y largas se establecen como: sta(i) = c\ 0 (i) + (1 ci)sta(i 1) lta(i) = c2 0 (i) + (1 c2)sta(i 1) para i ~ 1, y valores iniciales st a(o) = lta(o) = 0 (0). los valores de las constantes ci y c2 estan entre 0 y i, valores que se deb en sintonizar en la practica segun la aplicacion, la determinacion del cambio es declarada como: ip = inf {i en i sta(i) i lta(i) > c3} siendo c3 el umbral de decision. funcion rectilineal f. esta mide el grado de polarizacion lineal de una sefial sismica de tres componentes, la cual aumenta si la energia sismica se concentra en una direccion principal. la onda-p esta generalmente polarizada de forma lineal, de esta forma se espera el maximo valor de f en el tiempo de arribo de la onda-p (an ant et ai., 1995). f=ia2 a, donde ai es el mayor valor propio y 2 es el segundo mayor valor propio de la matriz de covarianza: [ var(xl) cov(xi,x2) cov(x1 ,xj)] m= cov(xi,x2) var(x2) cov(x2,xj) cov(xi,xj) cov(x2,xj) var(xj) donde x], x2 y x3, y son series de tiempo correspondientes a los tres componentes de una sefial sismica. var(xj) es la varianza de y cov(xi ,x2) es la covarianza entre xi y x2: geofislca colombiana, 7, diciembre de 2d03 donde 111 y 112 son los val ores medios de xi y x2 respectivamente y n, es el numero de muestras. la funcion recti lineal f es igual a i cuando una onda esta total mente polarizada de forma lineal (arribo de la onda-p) y a 0 cuando no existe polarizacion. funcion grado de polarizacion do? mide el grado de polarizacion de una sefial de tres componentes: dop= (a,a2i + (a2 aj)2 + (aj a,)2 2(11., + 11.2 + as (9) (2) donde ai, a2, a3 son los valores propios de la matriz de covarianza m. esta funcion caracteristica puede ser usada para detectar ondas-p y ondas-s debido a que estas poseen alto grado de polarizacion (oonincx, 1998). funcion valor absoluto. definida de la siguiente forma: fabs =abs {x (i)},'if i= 1,2, ... , n , (10) (3) (4) don de n es el mimero de muestras y x es una sefial sismica. esta funcion tiene alta fidelidad y velocidad de proceso (dai et al., 1997). funcion envolvente analitica compleja. usada ampliamente en sistemas de deteccion con el algoritmo de stall t a y esta definida como (kuang et al., 1998): (ii) (5) donde xu) es la transformada hilbert de la sefial sismica xu). funcion de planaridad. registra el grado de polarizaci6n de una sefial de tres componentes en un plano, y esta definida por (oonincx, 1998): p= a, + 11.2 2a.j a, + a2 (12) (6) donde 11.\, 11.2, 11.3 son los valores propios de la matriz de covarianza m. la onda-s esta polarizada de tal forma que el movimiento de sus particulas esta confinado a un plano; por tanto se espera el maximo valor de p en el tiempo de arribo de esta onda. (7) topofogfa de fa rn la funcion valor absoluto tiene alta fidelidad y velocidad de proceso (dai et. al., 1996), mientras la funcion rectilineal (6) y el grado de polarizacion (do?) (9) son medidas discriminantes debido a que miden el grado de polarizacion de una serie de tiempo, y la onda-p es unaonda compresional que general mente esta polarizada de forma lineal. de esta forma, el espacio de entrada 10 conforman la funcion valor absoluto, la funcion rectilineal y la funcion do? numero de neuronas de entrada. esta determinado por el mimero de muestras de la ventana correspondiente a la funcion valor absoluto, la funcion rectilineal y la funcion do? el numero (8) 57 de muestras de la ventana es determinado a traves de la funcion recti lineal y la norma varimax (anant et. al., 1995): 1.50\-~-t,;-~-;;,.--....-~-,i;o~-,i;,,,,-~,,,,~--,\;;~--.i m (numero de muestras en ia ...entaroa) figura 3. norma varimax promedio. lx(i)4 v = ---;-,'c-' ~~,c (~x(i)2) donde xes la serie de tiempo correspondiente a la funcion rectiftmcion reclilineal (n = , 50) figura 4. ejernplo de funcion rectilineal. alzate it al. en las figuras 4 y 5 se muestra la funcion rectilineal y la funcion dop, respectivamente, de la sefial de tres componentes de la figura 6 para una ventana de 150 muestras con 5 de traslape. ei maximo de las funciones corresponde a la ventana que contiene el tiempo de arribo de la onda-p. numero de capas ocultas. cualquier espacio de decision arbitrario puede ser aproximado por una red neuronal de perceptrones multicapa con una capa oculta de neuronas no lineales (haykin, 1999). de esta forma es necesaria solo una capa oculta para separar el espacio de entrada que corresponde al conjunto de entrenamiento, pero esto no implica que sea el valor optimo en el sentido de generalizacion. funci6n dop (n ;< 150) (13) figura 5. ejemplo de funcion dop. numero de neuronas ocultas. este mirnero determina la cantidad de hiperplanos que van a formar el espacio de decision. en general, la cantidad de neuronas ocultas debe ser la minima posible para evitar sobreentrenamiento y mala generalizacion. se realizaron pruebas empezando con dos neuronas ocultas y entrenando con el algoritmo de levenberg-marquardt y el criterio de x 10g corrccoeote n , il ~"w, __ ,___", j .. , 0 500 1000 1500 2000 2500 3000 3500 .000 'f corrccoeree ee~~"-'-j ·4 0 500 1000 1500 2000 2500 3000 3500 '000 if componente z:.if----. 1 0 500 1000 1500 2000 2500 3000 3500 .000 mu.,~s{ll) lineal para ventanas deslizantes de m muestras, m = 2,3,4, ...mmax' la idea es obtener el valor optirno de m para que la funcion rectilineal tenga solo un pico prominente que corresponde a la ventana donde esta la onda-p, es decir, minimizar la entropia de la funcion recti linea!. la norma varimax mide la entropia de una serie de tiempo; mientras mas alta es, menor numero de picos tiene la sefial, se determine que el valor optimo promedio de m es de 150 muestras en cada ventana como se muestra en la figura 3; por tanto, el nurnero de neuronas de entradas es 150 para la funcion valor absoluto, una para la funcion rectilineal y otra para la funcion do? en total, el numero de neuron as de entrada es 152. figura 6. sefial de prueba de tres componentes. 58 geofislca colombiana, 7, diciembae de 2003 detecci6n en linea de onda-p en sismogramas 3-d utilizando redes neuronales early stopping [14]. los resultados mostraron que el nurnero minimo de neuronas para obtener una buena generalizacion fue de 5 neuronas en la capa oculta. conjunto de entrenamiento. inicialmente se utilizaron conjuntos pequefios de entrenamiento correspondientes a 10 datos de onda-p y 10 datos de ruido. ei desernpefio del sistema fue pobre, con una elevada tasa de falsas alarmas y falso rechazo. posteriormente, se aumento de forma significativa el nurnero de patrones de ruido. finalmente, el sistema fue entrenado con 465 patrones de ruido y 13 patrones de onda-p. algoritmo de entrenamiento. este corresponde al de levenberg-marquardt, el cual es una modificacion del metodo de minimizacion de gauss-jordan. este algoritmo tiene menor tiempo de convergencia que la mayoria de los metodos clasicos como la regia delta generalizada. se utilizo el criterio de early stopping para detener el entrenamiento cuando el error de validacion alcanzo el minimo. este metodo garantiza que el sistema entrenado tiene buena generalizacion para un conjunto predeterminado de patrones de validacion. el error de entrenamiento fue del orden de 10-5• el esquema del sistema neuronal se ilustra en la figura 7: en el caso del detector neuronal de razon de verosimilitud, la sintesis se realize con los siguientes parametres: tamaiio de la ventana (n). debe ser determinado a partir del tipo y del contenido frecuencial de la sefial sismica (oonincx, 1998). se determino el valor de n a partir del contenido frecuencial de las sefiales sismicas de tipo regional y local, 10 que resulta en un valor de n=80 muestras, que corresponde a 4s para sefiales muestreadas a 20 hz. numero de neuronas ocultas. se determine empezando con dos neuronas y probando el desempefio de la red, para un resultado final de 25 neuronas en la capa oculta. en cuanto al conjunto de entrenamiento, este fue determinado de la misma forma que el clasificador antes analizado, empezando figura 7. diagrama del detector basado en el clasificador. geofislca colombiana. 7. oiciembre oe 2003 con conjuntos pequefios y luego incrementando dependiendo del desernpefio del sistema. al final de las pruebas el conjunto de entrenamiento estuvo compuesto de 2300 patrones de ruido y 48 patrones de onda-p. el algoritmo de entrenamiento fue una modificacion de la regia delta generalizada descrito en luo et al. (1977). resultados y conclusiones los sistemas fueron probados utilizando 77 sefiales sisrnicas de tres componentes muestradas a 20 hz, que corresponden a sismos regionales y locales de la zona de california y japon. adicionalmente se agrego ruido blanco gaussiano a las sefiales para determinar el desempefio de los algoritmos frente a condiciones de ruido. la relacion sefial a ruido se calculo hallando la varianza del ruido antes de la onda-p, y hallando la varianza de la senal-ruido despues de la onda-p en una ventana estacionaria: snr= 1olog,o(crlr -ij los resultados del tiempo de proceso se representan en la tabla 1, los cuales se midieron para los algoritmos en la estacion sun enterprise 3500, como tambien en matlab y en fortran 95 en un pc sobre pentium 4. tabla 1 i i ! ~-l tiempo proccsil tiempo proceso !, tiempo proceso detector (sun) (pc f95) i (pc matlab) f-------------------------"---------clasificador i 212 ms 950 ms i 4 s lrv i 59 ms i 400 ms lis en el caso del detector neuronal de razon de verosimilitud, el desempefio dio como resultado un error promedio de 9,1% para una relacion serial a ruido minima promedio de 15db. mientras, para el clasificador, el desernpefio del detector resulto en un error promedio de 1,3 % y la relacion sefial a ruido minima promedio fue de 9,67db. como se observa el trabajo del clasificador es superior al detector basado en la lry. se presentan ejemplos de deteccion con el clasificador neuronal sobre sefiales sismicas de tres componentes con alta snr (figura 8) y baja snr (figura 9). el recuadro indica la ventana donde se encuentra el tiempo de arribo de la onda-p. bibliografia kuang, c. and mendel, j. (1994). first break refraction event picking using fuzzy logic systems, ieee transactions on fuzzy systems, vol 2, no.4. november 1994. 59 luo, f. and unbehauen, r. (1997). applied neural networks for signal processing, university of erlangen-nuremberg, 1997. dar, h. and macbeth, c. (1997). the application of back-propagation neural network to automatic picking seismic arrivals from single-component recordings, british geological survey, edinburgh, scotland. anant, k. s. and dowla, f. a. (1995). wavelet transform methods for phase identification in three-component seismograms, university of california, davis and lawrence livermore national laboratory. 60 alzateet al. oonincx, p. 1. (1998). automatic phase detection in seismic data using the discrete wavelet transform, centrum voor wiskunde en informatica. ha ykin, s. (1999). neural networks: a comprehensivefoundation, prenticehal!. lippmann, r. and bekman p. 1989. "adaptive neural ney preprocessing for signal detection in non gaussian noise", adv. neural information processing system, vol i, pp. 124-132. geoflslca colombiana. 7, diciembre de 2003 keywords: liquefaction; variable permeability; numerical simulation; cyclic loading. palabras clave: licuefacción; variable de permeabilidad; simulación numérica; carga cíclica. how to cite item ghassemi, a., seyfi, s., & shahir, h. (2020). evaluation of variable permeability model in simulation of seismic behavior of uniform level and gently sloping sand layers. earth sciences research journal, 24(3), 335-343. doi: https:// doi.org/10.15446/esrj.v24n3.60654 in this study, a fully coupled dynamic finite element model was employed for numerical simulation of the response of level to gently sloping saturated sand layers subjected to cyclic loading. this model utilized a critical state two-surface-plasticity constitutive model to simulate the cyclic behavior of sandy soil. moreover, a recently proposed variable permeability function was implemented in the numerical model to reflect the effects of soil permeability variations during the liquefaction phenomenon. the numerical model was validated by simulating a number of well-documented geotechnical centrifuge tests with different relative density of sand, base acceleration time history, and surface slope of the sand layer. the obtained results confirmed that the developed model was capable of simulating the behavior of saturated sand under cyclic loading for both level and gently sloping conditions. abstract evaluation of variable permeability model in simulation of seismic behavior of uniform level and gently sloping sand layers evaluación del modelo variable de permeabilidad en la simulación del comportamiento sísmico de capas arenosas planas y ligeramente inclinadas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.60654 en este estudio se presenta un modelo parejo dinámico de los elementos finitos para la simulación numérica del nivel de respuesta de las capas de una cuesta arenosa saturada objeto de una carga cíclica. este mecanismo utiliza un modelo constitutivo de plasticidad de dos superficies en estado crítico para simular el comportamiento cíclico de los suelos arenosos. además, se implementó una función de permeabilidad variable recientemente propuesta en el modelo numérico para reflejar los efectos de estas variaciones durante el fenómeno de licuefacción. el modelo numérico ha sido validado al simular varias pruebas bien documentadas de centrifugación geotécnica con diferentes densidades relativas de arena, línea de tiempo de aceleración base, e inclinación superficial de la capa arenosa. los resultados obtenidos confirman que el modelo desarrollado es capaz de simular el comportamiento de las capas arenosas bajo cargas cíclicas en condiciones planas y de ligeramente empinadas. resumen record manuscript received: 22/10/2017 accepted for publication: 28/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 335-343 a. ghassemi1, s. seyfi2, h. shahir3 1assistant professor, civil engineering department, qazvin branch, islamic azad university, qazvin, iran. 2m.sc. graduate, sharif university of technology, tehran, iran. 3assistant professor, civil engineering department, faculty of engineering, kharazmi university, tehran, iran. *corresponding author: a_ghassemi@qiau.ac.ir se is m o lo g y en g in ee ri ng https://doi.org/10.15446/esrj.v24n3.60654 https://doi.org/10.15446/esrj.v24n3.60654 https://doi.org/10.15446/esrj.v24n3.60654 2m.sc 336 a. ghassemi, s. seyfi, h. shahir introduction in recent decades, the simulation of the seismic behavior of saturated sand layers has been vastly extended through different methodologies based on analytical, empirical, experimental, and numerical studies. among the different approaches, numerical simulation has the advantages of investigating the influence of individual parameters independently. following the pioneering numerical study of liquefaction by zienkiewicz et al. (1978), the behavior of saturated sand layer under seismic loading has been investigated in a number of studies using numerical models (prevost (1981), finn et al. (1986), hamada (1999), byrne et al. (2004), taiebat et al. (2007), valsamis et al. (2010), shahir et al. (2012), pak et al. (2013), karimi and dashti (2015), dobry et al. (2018), and yang et al. (2020)). the majority of the previous studies have focused on a specific geometrical condition. only in a small number of numerical studies have both level and gently sloping ground been considered (elgamal et al. (2002) and andrianopoulos et al. (2007)). elgamal et al. (2002) used an elastic–plastic model for simulating the essential response characteristics of pressure-sensitive geo-materials under general loading conditions. they applied this elastoplastic model to simulate experiments no. 1 and no. 2 from the verification of liquefaction analysis by centrifuge studies (velacs) project. the velacs project is a well-known dynamic centrifuge testing program consisting of nine geotechnical models constructed from or founded on liquefiable soil deposits. this project offers an effective opportunity to verify the accuracy of the different analytical procedures for the simulation of saturated sand response to cyclic loading. andrianopoulos et al. (2007) presented a numerical methodology for the simulation of non-cohesive soil response under small, medium, and large cyclic shear strains using an elastoplastic bounding surface model implemented in a finite difference code. their model was validated by simulating centrifuge model tests from the velacs project. although the results of such studies demonstrate the capacity of numerical modeling in the simulation of the seismic response of saturated sand layers, the accurate estimation of the liquefactioninduced settlement remains a challenge. this can be attributed to the unrealistic modeling of the water discharge from the soil mass as a consequence of not considering the variation of the soil permeability during the seismic loading in the numerical modeling (shahir et al. (2012) and, to a large extent, shahir et al. (2014)). the variation of saturated soil permeability during cyclic loading has been observed in several experimental studies (arulanandan and sybico (1992), jafarzadeh and yanagisawa (1996), su et al. (2009), and ueng et al. (2015)). shahir et al. (2012) proposed a general function relating the soil permeability to the excess pore pressure ratio (which is defined as the ratio of excess pore pressure to initial vertical effective stress of the soil). shahir et al. (2014) used the variable permeability model to simulate several centrifuge tests conducted on a level-ground sand layer. they compared the settlement and excess pore pressure obtained from the numerical model with the results of the centrifuge tests to suggest proper values for the constants of the variable permeability function. in this study, a fully coupled dynamic finite element model adopting a two-surface plasticity constitutive law was employed to investigate the response of level and gently sloping saturated sand layers to cyclic loading. the variable permeability function first proposed by shahir et al. (2012) and later modified by shahir et al. (2014) was implemented in this model to incorporate the variation of the soil permeability during liquefaction. the numerical model equipped with the proposed variable permeability function was validated by the numerical simulation of a number of well-documented geotechnical centrifuge tests to investigate the capability of the model for simulation of saturated soil response to cycling loading under different ground surface conditions. numerical model modeling framework in this study, a fully coupled u p formulation was applied for modeling the two-phase porous medium of saturated sand. the primary unknowns in this formulation were the displacement of the solid phase (u) and pore fluid pressure (p). the fully coupled effective stress u p formulation is a simplified case of the general set of equations governing the behavior of saturated porous media. this formulation is applicable for dynamic problems where high-frequency oscillations are not important, such as a soil deposit under earthquake loading. using the finite element method for spatial discretization, the u p formulation is as follows (zienkiewicz and shiomi (1984)): mu b dv q ft p v + ′ =( )∫ σ − − 1 0 (1) q u h s ft p p + + =( ) 2 0 (2) where m is the mass matrix, u is the solid displacement vector, b is the strain-displacement matrix, ′ is the effective stress tensor, q indicates the discrete gradient operator coupling the motion and flow equations, p is the pore pressure vector, s is the compressibility matrix, and h is the permeability matrix. the vectors f (1) and f (2) include the effects of body forces and external loads, and fluid flux, respectively. in equation (1), which is the equation of continuity of motion, the first term represents the inertia force of the mixture, followed by the internal force due to soil skeleton deformation, and then by the internal force due to pore-fluid pressure. in equation (2), which is the equation for continuity of fluid flow, the first and third terms represent the rate of volume change for the soil skeleton and fluid phase, respectively; the second term is the rate of pore fluid seepage. constitutive law a critical state two-surface plasticity model was employed for modeling of the sand behavior. this model was originally developed by manzari and dafalias (1997) and refined later by dafalias and manzari (2004). the formulation of the model is based on the bounding surface plasticity theory (dafalias (1986)) within the critical state soil mechanics framework to yield a comprehensive multi-axial constitutive model for simulating the monotonic and cyclic behavior of sand. the dafalias and manzari constitutive model is well-known and adequately verified in simulating the monotonic and cyclic behavior of sands (taiebat et al. (2007), jeremic et al. (2008), shahir et al. (2012), and rahmani et al. (2012)). the formulation of the model is defined by introducing bounding, dilatancy, and critical surfaces in stress space. a schematic representation of these surfaces in the stress ratio π-plane is illustrated in fig. 1. in this model, the isotropic hypoelasticity assumption is adopted with the elastic moduli as functions of the current pressure and void ratio. the yield surface is a circular cone with its apex at the origin. the size of yield surface is normally considered a constant (no isotropic hardening) and is rather small value in majority of the applications. the critical surface is in direct correspondence to the critical stress ratio in the triaxial space. the critical state of a soil is attained when the stress ratio  = q/p equals the critical stress ratio (m), which is a material constant. in a similar manner, the bounding and dilatancy surfaces are introduced by defining the peak (bounding) and dilatancy stress ratios, mb and md. in the current model, the bounding and dilatancy stress ratios are related to the critical stress ratio via the “state parameter” as follows: m m nb b= ( )exp − ψ and m m nd d= ( )exp − ψ , (3) where nb and nd are positive material constants.  = ee is the “state parameter” proposed by been and jefferies (1985), where e is the current void ratio of the soil element and ec is the critical void ratio corresponding to the existing confining stress. in the current model, the power relation was used for defining the critical void ratio as follows: e e p pc c c at =    0−λ ζ (4) where e0, c, and  are the critical state constants. in fig. 1, the bounding and dilatancy surfaces are specified by dashed lines indicating their change with , and the critical surface by a solid line. all surfaces are fully determined by the value of the state parameter , increasing the numerical efficiency of the model. moreover, the state parameter includes the combined effect of the density (void ratio) and confining stress. thus, the current constitutive model can be employed to simulate the cyclic behavior of sand with different densities and confining pressures using a single (sand-specific) set of material constants. 337evaluation of variable permeability model in simulation of seismic behavior of uniform level and gently sloping sand layers where ki is the initial (static) coefficient of permeability and , 1, and 2 are constants. several experimental studies can be found in the literature where the increase of saturated soil permeability during cyclic loading has been evaluated quantitatively. for example, arulanandan and sybico (1992) studied the variation of the permeability coefficient by measuring the changes in the electrical resistance of the saturated sand deposit during centrifuge tests. they attempted to introduce pore shape factor and tortuosity as the mainspring causes of the growth in the permeability coefficient during liquefaction regarding the kozeny–carman equation. according to their study, the average value of the increased permeability coefficient of the sand was approximately 6–7 times its initial value. jafarzadeh and yanagisawa (1996) conducted a number of shaking table tests to examine the settlement of saturated sand columns under cyclic loading. assuming a laminar flow through the soil mass during the application of the input motion, they employed darcy’s law to calculate the increased permeability coefficient by measuring the cumulative water flowing upward through the sand column. they determined that the average increased permeability coefficient was 5–6 times greater than what had been measured in the static condition. su et al. (2009) attempted to estimate the increase of the permeability coefficient of saturated sand subjected to earthquake loading using the results of a centrifuge test. based on the law of conservation of mass, they obtained the average discharge velocity from the rate of the surface settlement. by introducing the obtained discharge velocity and hydraulic gradient of the flow calculated from the recorded excess pore pressures into darcy’s law, the increased permeability coefficient was backcalculated; it was found that its value could be as large as six times its static value. recently, a laboratory experiment system was developed by ueng et al. (2015) to conduct tests for evaluating the water movements and permeability in a sand column before, during, and after liquefaction. their results indicated that the permeability of fine vietnam sand during liquefaction was approximately four times the initial value before liquefaction, and reduced to 0.9 times the initial value after full dissipation of the excess pore pressures. the abovementioned experimental studies indicate that the “average” increase of the permeability coefficient during the experiments was in the range of 4–7 times of its initial value. during each experiment, the permeability coefficient gradually increased from its initial (static) value to a maximum value (in-flight permeability), which corresponded to the liquefaction condition. at the end of the shaking, the dissipation of the excess pore pressure commenced and simultaneously, the decrease of the permeability coefficient continued until it returned to its initial value. therefore, it is clear that the value of “in-flight” permeability is significantly greater than the value of “average” permeability. based on selected ideal assumptions (e.g., triangular variation of permeability coefficient against time), shahir et al. (2014) proposed the value of the permeability coefficient during a complete liquefaction state ru =1( ) to be ten the plastic modulus (kp) and dilatancy coefficient (d) are related to the distance from the bounding and dilatancy surfaces as follows: k hb np m´ := 2 3  (5) d a d nd= : (6) the vectors b and d displayed in fig. 1 are defined as the vectors connecting the current stress state to its image on the bounding and dilatancy surfaces, respectively. m is the mean effective stress and h is a positive scalarvalued function. ad is a function including the effects of the “fabric change phenomenon” arising during stress increment reversal after a dilative plastic volumetric strain occurrence. the “distance dependent plastic modulus” is the main feature of the classical bounding surface model (dafalias (1986)). in the current model, the dilatancy coefficient is also defined based on the basic idea of “distance dependency” in the bounding surface model. in this constitutive model, 17 model parameters were divided into 6 categories based on their functions (elasticity, critical state, yield surface, plastic modulus, dilatancy, and fabric-dilatancy tensor parameters). dafalias and manzari (2004) presented the proper values of these constants for toyoura sand by comparison with laboratory data over an extremely wide range of pressures and densities. shahir and pak (2010) calibrated the model constants for nevada sand using the monotonic and cyclic triaxial test data performed by arulmoli et al. (1992). the parameter values of the model for nevada sand are shown in table 1. figure 1. schematic representation of two-surface model in π-plane variable permeability model changes in the effective (interconnected) porosity of the soil mass and tortuosity of the flow paths through the porous medium have the main roles in altering the permeability of the saturated sand during earthquake loading. as the rate of these changes directly depends on the contact between the soil particles (macroscopically, on the effective stress state), relating the permeability coefficient to the excess pore pressure ratio can represent the realities of microscopic events in macroscopic analyses. shahir et al. (2012) suggested a general variable permeability function with respect to excess pore pressure ratio (ru). their equation shows a gradual increase in permeability up to the onset of liquefaction when the permeability coefficient becomes equal to “” times the initial permeability (ki): k k rb i u= + ( )( )1 1 1α− β during pore water pressure build up phase ru 1( ), k ki1 =  during liquefaction state ru =1( ), (7) k k rd i u= + ( )( )1 1α− β2 during pore water pressure dissipation phase ru 1( ), table 1. dafalias–manzari material parameters for nevada sand (shahir and pak, 2010) parameter function parameter index value elasticity g0 v 150 0.05 critical state m c λc e0 ξ 1.14 0.78 0.027 0.83 0.45 yield surface m 0.02 plastic modulus h0 ch nb 9.7 1.02 2.56 dilatancy a0 nd 0.81 1.05 fabric dilatancy zmax cz 5 800 338 a. ghassemi, s. seyfi, h. shahir times the initial (static) permeability coefficient (i.e.,  = 10). an appropriate value of constant 1 is critical for the accurate simulation of the settlement and rate of excess pore pressure buildup; 2 is a key parameter for the accurate simulation of excess pore pressure variation during the dissipation period. in their study, parameters 1 and 2 were calibrated such that the results of the numerical simulations provided an acceptable match to the experimental records of the centrifuge tests including a leveled saturated sand layer subjected to seismic loading. using the obtained values in the general variable permeability function, shahir et al. (2014) proposed the following form of equation to be applied for numerical simulations: k k rb i u= +( )1 9 2 during pore water pressure build up phase ru 1( ), k ki1 10= during liquefaction state ru =( )1 , (8) k k rd i u= +( )1 9 10 during pore water pressure dissipation phase ru 1( ). herein, the above variable permeability function is applied in the numerical model to simulate the behavior of a saturated sand layer with either level ground or gently sloping surface conditions. numerical simulation of centrifuge tests in this study, six well-documented centrifuge experiments (experiments no. 1 to no. 6) were considered for simulation using the previously described numerical model. the selected benchmark tests addressed different settings of the important influencing factors such as relative density (30–55%), maximum base acceleration (0.15–0.55 g), slope angle (0–4°), height of sand layer (3–24 m), and duration of cyclic loading (11–35 s). centrifuge experiments benchmark experiment no. 1: the centrifuge model test no. 1 from the velacs project was conducted by taboada and dobry (1993) at rensselaer polytechnic institute (rpi). in this test, a laminar box containing a uniform leveled layer of nevada sand with a relative density of approximately 40%, which had been fully saturated with water, was subjected to a centrifugal acceleration of 50 g. this centrifugal acceleration led to a prototype soil permeability 50 times greater than the permeability of the soil. the height of the sand layer was 10 m in the prototype scale. the laminar box was excited horizontally at the base using the target prototype accelerogram with a peak acceleration of 0.2 g. benchmark experiments no. 2 and no. 3: a series of four centrifuge experiments involving buildings situated atop a liquefiable layer were performed using the large centrifuge facility at the center for geotechnical modeling (cgm) at the university of california at davis by dashti et al. (2010). free field results related to two of these four models were used in this study to validate the numerical model (herein called experiments no. 2 and no. 3). experiment no. 2 included a liquefiable soil layer (nevada sand with relative density of 30%) with a prototype thickness of 3 m located beneath a 2 m thick monterey sand layer that was located there to minimize capillary rise during liquefaction. the models were saturated with a fluid having a viscosity 22 times greater than the viscosity of water. the actual earthquake ground motion, recorded at the kobe port island station during the 1995 kobe earthquake with peak base accelerations of approximately 0.55 g (large kobe) was applied to the base of the model. the water level was 1.1 m below the surface. in experiment no. 3, the thickness of liquefiable layer was the same as the previous model; however, the relative density of the sand was 50% and the specimen was subjected to a moderate port island event with a peak acceleration 0.15 g. benchmark experiments no. 4 and no. 5: taboada and dobry (1998) reported a series of centrifuge tests including two categories corresponding to horizontal and sloping ground. in all tests, a 10 m prototype nevada sand deposit submerged in water was simulated d e gs10 0 08 0 887 2 67= = =( ). , . , .min . two of these tests were used in this study to validate the numerical model (experiments no. 4 and no. 5). both experiments were conducted on a sloping sand layer with relative density of approximately 40% subjected to 50 g centrifugal acceleration. the slope of the ground surface was 2° and 4° in experiments no. 4 and no. 5, respectively. the consequence of using water as the pore fluid was that the actual prototype permeability was 50 times greater, corresponding to coarser sand. the base of the laminar box was excited horizontally by a sinusoidal acceleration with prototype frequency of 2 hz and peak acceleration of 0.18 g and 0.17 g for experiments no. 4 and no. 5, respectively. benchmark experiment no. 6: sharp et al. (2003) presented a series of conducted centrifuge tests at rpi utilizing the same sand, laminar box, shaker, and centrifuge as used by taboada and dobry. however, they used a viscous pore fluid instead of water, and different relative densities and seismic loading characteristics. all tests were conducted at 50 g centrifugal acceleration. in this study, one of these centrifuge tests considered a laminar box containing a soil thickness of 20 cm (simulating a 10 m thick prototype soil deposit) tilted at an angle of 2°. during this test, the base of the box was excited by 22 cycles of a sinusoidal acceleration with prototype frequency of 2 hz and horizontal peak acceleration of 0.23 g. a brief description of these experiments is presented in table 2. model description in all benchmark experiments considered in this study, a uniform sand column placed in a laminar box either with a level or gently sloped surface was subjected to a one-directional earthquake excitation. in the numerical model, a column of 8-noded cubic elements with u p formulation was applied. each node had three displacement and one pore-water pressure degree of freedom. the mesh base nodes were fixed in all directions. nodes at equal depths were tied together in the direction of the excitation, and were fixed perpendicular to the direction of the excitation. all features of the numerical analyses were the same for level ground and gently sloping ground models, excluding the inclination of the soil layer. the water level was located at the ground surface for experiments no. 1, no. 4, no. 5, and no. 6, and 1.1 m below the ground surface for experiments no. 2 and no. 3. in the finite element model, pore table 2. benchmark experiments used for numerical simulations in this study benchmark no. model characteristics input motion thickness of liquefiable layer (m) spinning acceleration fluid viscosity ground slope (degree) relative density (%) type amax(g) duration (s) 1 10 50 1×µwater 0° 40 earthquake 0.23 15 2 3 55 22×µwater 0° 30 earthquake 0.55 35 3 3 55 22×µwater 0° 50 earthquake 0.15 35 4 10 50 1×µwater 2° 40 sinusoidal 0.18 11 5 10 50 1×µwater 4° 40 sinusoidal 0.17 10 6 10 55 50×µwater 2° 45 sinusoidal 0.23 10 339evaluation of variable permeability model in simulation of seismic behavior of uniform level and gently sloping sand layers water pressure was maintained at zero for nodes located on the water level. for nodes under water level, the pore water pressures were free to change. because nevada sand was used in all the considered benchmark tests, the constants of the employed constitutive model were selected as proposed in table 1. results and discussion level-ground sand layer the results of the numerical analysis conducted to simulate experiment no. 1 are displayed in figures 2 and 3. figure 2 indicates the variation of generated excess pore pressure with time at two different depths. for both considered depths, the maximum excess pore pressure was marginally underpredicted by the numerical model. in fig. 3, the time history of the surface settlement obtained from the numerical simulation is compared with the experimental time history. the results indicate that the predicted settlement was approximately 15 cm, which was less than the recorded value (approximately 20 cm). although the results indicated that the numerical ultimate settlement was less than the recorded value, the numerical model significantly improved the results of the conventional numerical modeling using the constant permeability coefficient (shahir et al. (2014)). furthermore, the obtained settlement was noticeably superior to the small number of available studies involving numerical simulation of both velacs experiments no. 1 and no. 2 with the same set of modeling constants. for example, the reported settlement by elgamal et al. (2002) was approximately 5 cm, and the numerical estimate of the settlement reported by pak et al. (2013) was approximately 12.5 cm. an unrealistic modeling of the water discharge from the soil mass during earthquake loading, which is responsible for the reduction in saturated mass volume, is one of the main reasons for the under-prediction of the settlement by the mentioned numerical models (shahir et al. (2012)). (a) (b) figure 2. evolution of excess pore pressure with time at depths: (a) 2.5 m, (b) 7.5 m (benchmark experiment no. 1) the results of the numerical simulation of benchmark experiment no. 2 are depicted in figs. 4 and 5 in terms of variation of excess pore pressure and settlement with time, respectively. although the numerical simulation did not represent the abnormal fluctuations of the settlement measured in the centrifuge test, the surface settlement at the end of the excitation was admissibly predicted by the numerical model. moreover, the numerically obtained maximum excess pore pressure at the middle of the liquefiable sand layer was in acceptable agreement with the experimental record. figure 4. evolution of excess pore pressure with time at middle of liquefiable sand layer (benchmark experiment no. 2) figure 5. measured and computed time history of settlement (benchmark experiment no. 2) similar results for benchmark experiment no. 3 are presented in figs. 6 and 7. for this model, the rate of settlement was identical for the numerical and experimental results. the predicted final settlement and maximum excess pore pressure by the developed model was approximately 10% less than that measured in the experiment. gently sloping sand layer figure 8 displays the evolution of the excess pore pressure with time at different depths for experiment no. 4. both the numerical and experimental figure 3. measured and computed time history of settlement (benchmark experiment no. 1) 340 a. ghassemi, s. seyfi, h. shahir results indicated that initial liquefaction ru =( )1 was more probable at lower depths. the results also indicated that the trend of the generation and dissipation of the excess pore pressure at the middle and deepest depths (5.0 and 7.5 m) obtained from the numerical simulation was in a better agreement with the experimental observations compared to the depth of 2.5 m. at this depth, the numerical results indicated an increase in the excess pore pressure commenced immediately after the beginning of the cyclic loading. however, the experimental excess pore water pressure began to increase after approximately two seconds from the beginning of the loading, with a higher rate compared to the numerical results. variations in soil displacements over time at the surface of the gently sloping layer are displayed in figure 9. a comparison of the predicted and measured settlement versus time indicates that the discrepancy between the predicted and measured settlement increased after 8 s; hence, the final amount of the numerical settlement was approximately 3 cm higher than the experimental settlement. conversely, the lateral displacement of the surface was under-predicted by the numerical model at the end of the seismic loading. in figure 10, a comparison is made between the buildup of excess pore pressure observed in experiment no. 5 with the results of the numerical simulation. the figure indicates that the numerical curves obtained from the numerical model followed the trend of the experimental time history at all considered depths. similar to experiment no. 4, the largest discrepancy between the numerical and experimental results occurred at the shallower depth of 2.5 m. moreover, the predicted excess pore pressure curve indicates reduced oscillation amplitude compared to the measured values. figure 11 presents the variation of the vertical and lateral displacement of the soil layer at the ground surface. figure 11a indicates an excellent agreement between the numerical and experimental results. however, the numerical model predicted a lateral spreading of the layer surface with less than a 15% error at the end of shaking. the results of the variations in the excess pore pressure and lateral displacement versus time at different depths of experiment no. 6 are displayed in figures 12 and 13. similar to two other gently sloping experiments (no. 4 and no. 5), the numerical model under-predicted the values of the maximum excess pore pressure with a maximum error of approximately 10% and 20% at figure 6. evolution of excess pore pressure with time at middle of liquefiable sand layer (benchmark experiment no. 3) figure 7. measured and computed time history of settlement (benchmark experiment no. 3) (a) 2.5m (b) 5 m (c) 7.5 m figure 8. evolution of excess pore pressure with time: (a) 2.5 m, (b) 5.0 m, (c) 7.5 m (benchmark experiment no. 4) (a) (b) figure 9. variation of soil displacements at surface of gently sloping layer: (a) settlement, (b) lateral displacement (benchmark experiment no. 4) 341evaluation of variable permeability model in simulation of seismic behavior of uniform level and gently sloping sand layers (a) (b) figure 12. evolution of excess pore pressure with time: (a) 2.5 m, (b) 7.5 m(benchmark experiment no. 6) (a) (b) (c) figure 13. variation of lateral soil displacements with time at different depths: (a) 1.25 m, (b) 2.5 m, (c) 5.0 m (benchmark experiment no. 6) (a) (b) (c) figure 10. evolution of excess pore pressure with time: (a) 2.5 m, (b) 5.0 m, (c) 7.5 m (benchmark experiment no. 5) (a) (b) figure 11. variation of soil displacements at surface of gently sloping layer: (a) settlement, (b) lateral displacement (benchmark experiment no. 5) depths 2.5 and 7.5 m, respectively. fig. 13 indicates that the difference between final lateral displacement of the layer obtained by the numerical model and measured in the experiment was less than 5% for the depth of 1.25 m, with the discrepancy increasing to approximately 15% for the depths of 2.5 m and 5 m. 342 a. ghassemi, s. seyfi, h. shahir figure 14 represents a comparison between the average volumetric and shear strain predicted by the numerical model and experimental results. figure 14a indicates a relatively acceptable agreement between the numerical and experimental average volumetric strains for both cases of level and gently sloping ground. for the majority of the considered cases, the numerical simulations marginally under-predicted the average volumetric strains (consequently the settlements). figure 14b implies that the average shear strain of the soil layer obtained by the numerical model for one of the studied cases was in excellent agreement with the experimental records. however, for the other two cases, the difference between the numerical and experimental results was approximately 20%. there are several reasons that can be considered for the deviation of the numerical results from those observed in the experiments. because no vertical acceleration input was applied in the numerical models in this study, a part of the difference between the numerical and experimental results can be attributed to the assumption made in the numerical model, which did not completely correspond to the real condition of the tests (e.g., the measured vertical accelerations in benchmark experiment no. 1 is approximately 0.04g). moreover, employing the same set of constitutive model constants for the different nevada sands used in the considered experiments could possibly not be in complete accordance with the real physics of the problem. finally, the values of the actual modeling parameters such as fluid compressibility coefficient, which were not measured in the experiments, could have had a significant effect on the obtained results. (a) (b) figure 14. comparison between average volumetric and shear strain predicted by numerical model and experimental results conclusion in this study, a fully coupled finite element model was applied to investigate the response of a saturated sand layer, either level or with a gently sloping ground, subjected to seismic loading. for this purpose, a well-known critical state two-surface plasticity constitutive law adequately verified in simulating cyclic behavior of sands was employed in the numerical analyses. furthermore, applying a recently proposed variable permeability function, which relates the permeability coefficient to the excess pore pressure ratio, was used to enhance the model to simulate the gradual variation of saturated soil permeability during seismic loading. in this function, the maximum permeability coefficient occurring during the liquefaction of the sand was considered to be ten times the initial (static) permeability coefficient, which is in agreement with several experimental observations and can be justified by theoretical evidence. the focus of the study was to validate the applicability of the proposed variable permeability function in numerical simulations of different geometrical condition of ground surface by comparing the results of the numerical modeling with measurements of other creditable centrifuge experiments. for this purpose, six centrifuge experiments involving three tests on a uniform leveled sand layer and three tests on a uniform sloping layer with different conditions of relative density, base acceleration time history, and slope angle were simulated using the developed numerical model. in general, the results of the numerical simulations, including excess pore pressure time histories at different depths, settlement of ground surface, and lateral displacement of soil layer at different depths were in agreement with the experimental measurements. although in specific cases, the predictions by the numerical model indicated relatively considerable discrepancy from the experimental records, up to 20%, the more realistic modeling of the water discharge from the soil mass during earthquake loading using the variable permeability function significantly improved the simulation results compared to the previous numerical models using a constant permeability coefficient. the deviation of the numerical results from the experimental data could be attributed to the assumptions made in the numerical model that do not completely correspond to the condition of the experiments. for example, the vertical acceleration and fluid compressibility coefficients that were not measured or reported in the experiments could influence the obtained results significantly. moreover, using the same set of constants for the constitutive model and variable permeability functions for the different conditions could possibly not be in complete accordance with real physics of the problem. references andrianopoulos, k. i., papadimitriou, a. g., & bouckovalas, g. d. 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(1984). dynamic behaviour of saturated porous media; the generalized biot formulation and its numerical solution. international journal for numerical and analytical methods in geomechanics, 8(1), 71-96. palabras clave: magma; actividad térmica; calefacción magmática; movimiento de agua en canales de fractura-grieta; sistema de red; configuración jerárquica. how to cite item sun, k., tang, s., zhang, s., xi, z., & li, j. (2019). the magma thermal field and the shallow-level gas accumulation of tight gas reservoirs in the middle-eastern parts of the qinshui basin. earth sciences research journal, 23(1), 27-34. doi: https://doi.org/10.15446/esrj.v23n1.78805 in this study, the metal-non-metal mineral gas-water hydrothermal concept is used to analyze the movement channels of magmatic heating water. further, the concept of fractures-faults-cracks microfissures hierarchical configuration of the movement channels of magmatic heating water is proposed. in addition, the magma thermal field formed by magmatic heating water movement is studied and analyzed. based on the basin simulation method, which is combined with the paleo-tectonic evolution analysis and restoration of the ancient burial depth in the middle-eastern parts of the qinshui basin, the tectonic evolution history, thermal evolution history, and hydrocarbon generation and exhaustion history of tight gas reservoirs in the yushe-wuxiang block in the middle-eastern parts of the qinshui basin have been investigated. on the basis of the theories and methods that are proposed in this study, the hierarchical configuration of fractures-faults-cracks microfissures movement channels of magmatic heating water in the yushe-wuxiang block in the middle-eastern parts of the qinshui basin was studied and analyzed. it is observed that the magmatic heating water rises to the source formation through the movement channels of hierarchical configuration, heats the source rocks, accelerates the evolution of the source rock in the shallow layer, and forms a tight gas reservoir abstract keywords: magma; thermal activity; magmatic heating; water movement channel fracture-crack; network system; hierarchical configuration. the magma thermal field and the shallow-level gas accumulation of tight gas reservoirs in the middle-eastern parts of the qinshui basin el campo térmico magma y la acumulación de gas a nivel superficial de reservorios de gas en las partes del medio oriente de la cuenca de qinshui issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.78805 en este estudio, el concepto de mineral metal-no-metal hidrotermal gas-agua se utiliza para analizar los canales de movimiento del agua de calentamiento magmático. también se propone el concepto de configuración jerárquica de las microfisuras fracturas-fallas-grietas de los canales de movimiento del agua de calentamiento magmático. además, se estudia y analiza el campo térmico de magma formado por el movimiento de agua de calentamiento magmático. basado en el método de simulación de la cuenca, que se combina con el análisis de la evolución paleotectónica y la restauración de la profundidad de enterramiento en las partes del medio oriente de la cuenca qinshui, se ha investigado la historia de evolución tectónica, la historia de evolución térmica y la historia de generación y agotamiento de hidrocarburos en los depósitos de gases en el bloque yushe-wuxiang en las partes del medio oriente de la cuenca qinshui. sobre la base de las teorías y métodos que se proponen en este estudio, se analizó la configuración jerárquica de los canales de movimiento de microfisuras fracturas-fallas-grietas del agua de calentamiento magmática en el bloque yushe-wuxiang en las partes medio-orientales de la cuenca qinshui. se observa que el agua de calentamiento magmático sube a la formación de la fuente a través de los canales de movimiento de configuración jerárquica, calienta las rocas de origen, acelera la evolución de la roca de origen en la capa superficial y forma un depósito de gas estrecho. resumen record manuscript received: 12/12/2018 accepted for publication: 05/02/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 27-34 ke sun1,2,3, shuheng tang1,2,3*, songhang zhang1,2,3, zhaodong xi1,2,3, jun li4 1school of energy resource, china university of geosciences(beijing), beijing, 100083, china 2beijing key laboratory of unconventional natural gas geological evaluation and development engineering, beijing, 100083, china 3key laboratory of strategy evaluation for shale gas, ministry of land and resources, beijing, 100083, china 4college of geoscience and surveying engineering, china university of mining and technology (beijing), beijing, 100083, china. * corresponding author: tangsh@cugb.edu.cn g eo th er m al e ne rg y 28 ke sun, shuheng tang, songhang zhang, zhaodong xi, jun li introduction for a long time, scholars at home and abroad generally believe that the magmatic and volcanic activities of oil-bearing basins depict a considerable influence on the generation, migration, and accumulation of oil and gas. however, it is only during the previous decade or so that some scholars have paid attention to this phenomenon. magma can invade the sedimentary formations in sedimentary basins in the form of dikes or caprocks and can heat the rocks that are located within a certain distance from the dikes (zhang, guan, & jian, 2014; li et al., 2016). if the sedimentary (argillaceous) shales are abundant in organic matter, they will accelerate the maturation of the source rock formations (rodriguez, villar, & baudino, 2007; polteau, el at., 2016; peace el at., 2017; alalade & tyson, 2013; hakimi et al., 2017). if magma invades in the form of dikes, either underneath or on both the sides of the dikes, there may be different heat conduction conditions that are caused due to the different heating effects; consequently, the source rocks will exhibit different maturity and heating ranges (zhu, jin, hu, song, & gao, 2007). the distance from the source rocks to the dikes is also an important factor that affects their maturity. due to the different heating effects, halos with different degrees of maturity can be formed around the dikes (muirhead, bowden, parnell & schofield, 2017). under special conditions, even asphalt veins that are parallel to the dikes will be formed (zanella, cobbold, ruffet, & leanza, 2015). in the atlantic research drilling project, it was observed that the intrusion of dikes significantly affects the maturity of the organic matter. magmatic intrusions and volcanism affect the thermal history of the sedimentary basins and the maturation history of the source rocks (spacapan et al., 2018). the magmatic intrusion thickness, duration of intrusion, and distance from the rock mass affects the hydrocarbon generation (aarnes, planke, trulsvik, & svensen, 2015). in addition, polyansky, reverdatto, khomenko & kuznetsovab (2009) postulated that the flow of the formation water is an auxiliary heat transfer method. several scholars have used various modeling methods to simulate magmatic intrusion and hydrocarbon generation after heating the source rocks (sydnes, fjeldskaar, løtveit, grunnaleite, & cardozo, 2018; iyer, svensen, & schmid, 2018). guangwen et al. conducted numerical thermal simulation studies on the heat dissipation process of invasive magma and classified the evolution of the source rocks into three modes: near, middle, and far (tian, jiang, pang, meng, & fu, 2005). several scholars have observed methane gas emissions while studying magmatic and volcanic activities (iyer, schmid, planke, & millett, 2017; frieling et al., 2016). evidence not only proves that magmatic and volcanic activities are conducive to generate hydrocarbons, such as light hydrocarbons and methane, but also proves that the generation of h2s in certain oil and gas fields is closely related with the magmatic intrusions (alberdi-genolet, cavallaro, hernandez, crosta, & martinez, 2013). the “magmatic thermal field” that is formed by magmatic activities not only heats the source rocks and promotes hydrocarbon generation but also exhibits a certain transformation effect on reservoirs (zhang, jin, li, & jiao, 2014; zhang et al., 2016; senger, planke, polteau, ogata, & svensen, 2014); magmatic and volcanic activities may also play a positive role in the migrations of oil and gas and the formation of hydrocarbon traps (rateau, schofield, & smith, 2013; cukur et al., 2010; xu, fang, zhang, hu, & zhu, 2006). the magmatic ascending channels of magmatic and volcanic activities can later become hydrocarbon migration channels (schofield et al., 2017) the magmatic intrusion into salt rock formations can form hydrocarbon migration channels (blažić & moreau, 2016); and a magmatic intrusion can form hydrocarbon storage spaces (witte, bonora, carbone, & oncken, 2012). the intrusion of caprocks deforms the strata and forms a trap structure that is similar to that of salt rock diapirism. after weathering the volcanic structure, it can also form a storage space that can be used as both oil and gas reservoirs (lee, kwon, yoon, kim, & yoo, 2006). additionally, magmatic activities exhibit a destructive effect on the existing reservoirs and can damage pores as well as fracture systems (salmachi, rajabi, reynolds, yarmohammadtooski, wainman, 2016; gao & liu, 2003). previous researches about the effect of magmatic activities on hydrocarbon accumulation mainly focused on the mass of a rock or the effect of the volcano itself on hydrocarbon generation in source rocks as well as hydrocarbon migration and accumulation. this study intends to use the metalnon-metal mineral gas-water hydrothermal concept to deeply analyze and study the movement of magmatic heating water that was formed in magmatic activities in the eastern qinshui basin and the effect of the formed magmatic thermal field on hydrocarbon accumulation, which can give rise to new ideas and methods concerning hydrocarbon accumulation due to magmatic activities. movement of magmatic heating water in the formation and the formed magma thermal field in ore-deposit, hydrothermal gas-water rises along a certain channel after being heated by magmatic rocks. mineralization is observed at the appropriate formational sites with surrounding rocks, which forms useful metal and nonmetallic deposits. inspired by this concept, the authors of this study believe that the hydrothermal gas-water heated by magma depicts a positive effect on hydrocarbon accumulation. the movement channel of magmatic heating water after the emplacement of magma, because the temperature of magma is as high as 800°c to 1300°c, the water in the surrounding formation is heated by magma to form magmatic heating water. the magmatic heating water flows in the strata and it not only carries the ore-forming material that can form ore deposits but also carries a large amount of heat in the strata that is located far from the magmatic emplacement location. when the water is heated, the physical properties are observed to alter. the most significant change is the increase in volume and the decrease in density; the water with less density will move through the appropriate channels to the shallow parts of the stratum under the action of buoyancy. at the macro level, various fracture systems developed in the basin include large-scale fractures and faults; at the micro level, the various cracks and microfissures that are observed to develop in the formation rocks are good magmatic heating water movement channels. additionally, all kinds of rock and debris particles will expand and become brittle when the temperature increases. it is easier to form tensile or extrusion cracks under the influence of structural movement stress and to improve the thermal water passing capacity of micro cracks and micro cracks. micro-level cracks and microfissures are extensively distributed in the strata, and their effects on thermal water movement cannot be ignored. the imbibition is a phenomenon of surface action that occurs when two media are in contact with each other. under normal circumstances, all the rocks in the formation are mostly characterized by water wetting and strong hydrophilia. therefore, magmatic heating water will enter the fracture along the walls of cracks or microfissures through the action of imbibition, thereby heating the formation and increasing the temperature of the formation. this is beneficial to improve the organic matter maturity in source rocks. in addition to fractures, faults, cracks, and microfissures of various sizes, the unconformity between formations and permeable sandstone formations are also good passageways for magmatic water heating, and play a role in communicating different fractures and faults systems. hierarchical configuration of magmatic heating water movement channel level configuration those who have studied the geological structure of the oil field have learned that the development of fractures and faults is mainly formed by the action of squeezing or tensile stress. this kind of squeezing or tensile stress is a kind of regional tectonic stress; that is, the tectonic stress in a certain area acts on the whole area; therefore, in the same area, the development of fractures and faults does not develop an isolated fracture or a fault, but a number of fractures and faults form a three-dimensional network structure at different levels and scales. the three-dimensional network structure composed of fractures and faults is a good magmatic heating water movement channel, and the magmatic heating water can be diverted from the vicinity of the magmatic emplacement position to a farther and more extensive range, thus heating source rocks in farther and wider areas. 29the magma thermal field and the shallow-level gas accumulation of tight gas reservoirs in the middle-eastern parts of the qinshui basin the distribution of fractures-faults-cracksmicrofissures in the strata is not an unordered combination, but there are certain rules to follow. large fractures and large faults develop earlier, with longer extension distances and deeper cuts through the formation. these fractures may have developed earlier than the magmation period, and may also develop in the same period as the magmation. the magma will also intrude into the shallower parts of the strata along the fractures, which is more conducive to the magmatic heating of the formation water and achieve better heating effects on source rocks. although the small faults, cracks, and microfissures are developed on a small scale, the development quantity is large and the distribution range is wide; hence, the magmatic heating water can heat source rocks across a wide range. therefore, fractures-faults-cracks-microfissures of this hierarchical configuration formed a good magmatic heating water movement network system (figure 1). in addition, the unconformity surface and the permeable sandstone formation interact between fractures and faults, allowing the magmatic heating water to move to fracture systems and formations further away from the magma. 1 magmatic heating water moves up along the deep fractures; 2 magmatic heating water enters large faults from deep fractures and moves up along the faults; 3 magmatic heating water moves up along sub-lever faults; 4 magmatic heating water enters cracks-microfissures along small faults; 5 magmatic heating water enters another fracture (fault) system from a fracture system along the sandstone permeable layer or unconformable surface magma thermal field formed by magmatic heating water magmatic heating water moves in a hierarchical configuration fracturescracks network system and is superimposed on the background of normal geothermal gradients; formations have higher temperatures in the deeper parts and lower temperatures in the shallow parts of the strata; heating water has higher temperatures in the deeper parts of the strata and the temperatures in the shallow parts of the strata are low. and the temperatures will not suddenly drop during the process of the magmatic heating water continuously moving to the shallow parts of the strata; the process of moving the magmatic heating water from the deep parts of strata to the shallow part of strata is a process by which the temperature is successively lowered. the heating effects of magmatic heating water on the formation can gradually reach the shallow parts of the strata. although the tectonic stress acts on the entire area, the development of fractures-cracks is not uniform throughout the region; in the tectonic stress accumulation area, the fractures-cracks system is well developed and the movement of magmatic heating water from the inside layers of the region to the shallow parts of the strata is not balanced; moreover, the heating effects of source rocks of the formation are also not balanced. due to the existence of the fractures-cracks network system, heating water can reach areas farther away from the magma; magmatic activities are regional activities controlled by the tectonic setting of the earth and are usually active during a geological period, such as the active period of yanshan magma in the qinshui basin, which is very favorable for the maturity of source rocks. the main sources of magmatic heating water are magma separation water, stratigraphic water, rock metamorphic water, and sea water. tectonic evolution and thermal evolution in the middle-eastern parts of the qinshui basin the characteristics of tight gas reservoirs in the middle-eastern parts of the qinshui basin the yushe-wuxiang coalfield block in the qinshui basin is located in the middle-eastern parts of the qinshui basin and has good exploration prospects for coalbed methane and tight sandstone gas. at present, 48 exploratory wells have been drilled for carboniferous-permian coalbed gas exploration, while only four exploration parameter wells have been drilled for the exploration of tight sandstone gas reservoirs. the research and analysis of the data and cores obtained from these four parameter wells are the key to a breakthrough in the exploration for tight sandstone gas reservoirs in the middle-eastern parts of the qinshui basin. the sandstone rock mass in the yushe-wuxiang block is relatively developed. the total thickness of the sand bodies such as rivers, deltas, tidal flats, and barrier islands have been deposited is 50 to 140m. tight sandstones are mainly composed of quartz, feldspar, and rock debris; cements are mainly siliceous cements, clay minerals, and carbonate cements. the pores of tight sandstone reservoirs are mainly secondary pores, and the primary pores basically disappear due to compaction and transformation; there are three types of secondary pores, namely, dissolved pores, hetero-micro pores, and microfissures. the physical properties of the tight sandstone in the yushe-wuxiang block are very poor. the porosity of the reservoir sample is 1.4%~11.9%, the average porosity is 3.74%, the lowest permeability is less than 0.02 md, the highest is 0.66 md, and the average permeability is less than 0.1md. in general, the overall layer is an ultra-low porosity and ultra-low permeability reservoir. the structural features of the middle-eastern parts of the qinshui basin the north china area is a stable craton basin; in the paleozoic, the degree of basal consolidation was high; in the early paleozoic, it was a stable neritic continental shelf-plateau facies carbonate sedimentation; in the middle ordovician-early carboniferous, it was overall uplift; and in the late paleozoic middle carboniferous-permian, it was a paralic coal-bearing deposit. in the north china areas, the mesozoic influence of the surrounding orogenic belt was in an extrusion environment, and there were magmatic activities in the deep crust. in the late mesozoic, a large mesozoic basin was formed in the north china areas. with the increase of the compressive force, a series of complex anticline and complex synclinal structures were formed in the eastern uplift of the qinshui basin, accompanied by intense magmatic activities and fractures. after the cenozoic era, the western parts of the north china areas (ordos, qinshui basin) were uplifted, which were the opposite of the rise of eastern regions and the fall of the western regions in the late mesozoic era. in the eastern margin of the yanshan qinshui basin, the taihang fault was thrust towards the east, and the eastern qinshui basin was strongly pressed. a series of thrust faults formed in the eastern part of the qinshui basin (right of figure 2). the burial and thermal evolution of middle-eastern parts of qinshui basin in this paper, we use the geochemical data of wells zk03-2 and zk091 to carry out basin simulations; based on results from comprehensive basin figure 1. diagram of fractures-faults-cracks-microfractures hierarchical configuration. 30 ke sun, shuheng tang, songhang zhang, zhaodong xi, jun li simulation, tectonic movement analysis, formation thickness, and ancient burial depth restoration, the evolution of the basin after late carboniferous can be divided into five stages, which are: i. rapid settlement stage from late carboniferous to the end late triassic; ii. the overall crustal uplifting stage of early jurassic; iii. secondary settlement stage of middle jurassic; iv. the uplift and magmatic activity stage of late jurassic-early cretaceous; v. the stage of uplift subjected to denudation after the mid-cretaceous (figure 3). different stages of basin development, with different characteristics of subsidence and different thermal evolution characteristics, play different roles in hydrocarbon generation and hydrocarbon expulsion of source rocks. the characteristics of the basin development and the geothermal evolution in the five stages are as follows: i. rapid settlement stage from late carboniferous to the end of late triassic: the maximum sedimentation rate at this stage was nearly 90m/ma, and reached the maximum depth at the end of the triassic, with a depth of about 4000m. at this stage, the yushe-wuxiang block was located in a normal paleogeothermal field. the paleogeothermal gradient was about 3°c/100m, and the formation temperature increased with the depth of the buried depth. ii. the overall crustal uplifting stage of early jurassic; in this stage, the entire crust was uplifted, the depth of burial was shallow, and the temperature was lowered. iii. secondary settlement stage of middle jurassic: the settlement rate at this stage was relatively small, around 15m/ma, and the temperature increased slowly. iv. the uplift and magmatic activity stage of late jurassic-early cretaceous: at this stage, the crustal body was uplifted, magmatic activities in the yanshan period were strongly affected, and the geothermal gradient was abnormal. the qinshui basin was in an anomalous paleogeothermal stage, and the paleogeothermal gradient was close to 6°c/100m, although the earth’s crust uplifted as a whole, but the temperature of the formation had increased significantly. in the slow uplift stage of the mid-cretaceous, the strata were lifted at a relatively low rate, the strata suffered erosion, and the temperature decreased. the paleogeothermal gradient was 3~4°c/100m, close to the normal paleo geotemperature. figure 2. the characteristics of structural location and faults of yushe-wuxiang block (the left picture is based on the study by wei shuhong et al.) figure 3. the burial evolution of the middle-eastern parts of the qinshui basin 31the magma thermal field and the shallow-level gas accumulation of tight gas reservoirs in the middle-eastern parts of the qinshui basin analysis of hydrocarbon generation and expulsion in middle-eastern parts of the qinshui basin temperature and pressure are the key factors in controlling the maturation of organic matter. the maturation of organic matter in the middle-eastern regions of the qinshui basin mainly occurs in stages i and iv (figure 4). during the period of rapid deposition from late carboniferous to the end of late triassic, the basin rapidly settled and the depth of the formation increased rapidly. as the depth of burial depth increased and the temperature increased, the organic matter became deeply metamorphic and the maturity gradually increased. during the middle stage of the triassic, organic matter has entered the mature stage. at the end of the triassic, the temperature was about 140°c and the ro% reached 0.9%±. the source rock transited from the immature stage to the mature stage and reached the first hydrocarbon generation peak. in the ii. early jurassic and iii. middle jurassic stage, the whole basin was uplifted and then slowly settled, organic matter heating temperature gradually decreased, temperature increased slowly, and maturation evolution of the organic matter of source rock was weak. in the iv. late jurassic-early cretaceous, due to the influence of magmatic events of the yanshan period, the thermal evolution of organic matter greatly improved from maturity to over-maturation, and gas began to form in large quantities. v. after the middle cretaceous, the organic matter heating temperature reduced, and there was little progress in the evolution of source rocks. the middle-eastern parts of the qinshui basin experienced two stages of hydrocarbon generation and expulsion: the first hydrocarbon generation stage was the late paleozoic to the end of the triassic, and the early strata slowly settled and the organic matter was in an immature stage. during the triassic period, the sedimentation rate was relatively fast, and the organic matter entered a deep metamorphic stage. at the end of the triassic, the maximum depth of the formation was attained, the temperature increased, and the organic matter entered the mature stage (ro was 0.7%~1.0%) and hydrocarbon production began. the first hydrocarbon expulsion occurred in the early jurassic period. at this point, the first stage of hydrocarbon generation ended, the basin was uplifted and denuded, and the pressure on the formation reduced, resulting in the desorption of oil and gas and expulsion of hydrocarbons to adjacent reservoirs. the second hydrocarbon generation stage occurred in the earlymiddle cretaceous. because of the magmatic activity in the late yanshan movement, the temperature of the coal-bearing formation increased rapidly and was higher than the temperature at the end of the triassic period, and the organic matter reached the over-mature stage (ro was greater than 1.3%) and the secondary hydrocarbon generation occurred. the secondary hydrocarbon expulsion began in the secondary main hydrocarbon generation period. a large amount of gas was generated in the hydrocarbon source rock formation. after the formation was saturated with gas, hydrocarbons were discharged to the adjacent reservoirs. through comprehensive analysis of the basin’s tectonic evolution, simulation, and inclusions, the main controlling factors, preservation conditions, hydrocarbon generation and expulsion history, and trap formation period in the qishe-wuxiang block constituted an effective match; the tectonic development history and the preservation period had formed a trap before the jurassic formation, and the preservation conditions were good. combined with the dynamic progress of hydrocarbon accumulation, it was known that the depth of the buried deep stratum at the end of the triassic strata was at its maximum, and the organic matter reached the hydrocarbon generation threshold and thus hydrocarbon generation began. however, because of the low maturation of organic matter, the amount of hydrocarbons generated was limited; also, because of the destruction of the tectonic movement, such as the uplift of the earth’s crust in the later period, the accumulation of organic matter had not been found on a large scale. magmatic activity occurred in the early cretaceous study area, which further increased the maturity of organic matter and thus the secondary hydrocarbon generation began. as all the elements of reservoir forming were available and well matched, large-scale gas reservoirs began to form. magmatic activity in qinshui basin the lithosphere has been thinning in the whole area since the mesozoic of north china platform, and the lithosphere is also significantly thinner in the qinshui basin which was characterized by the active mantle thermal in the deep figure 4. hydrocarbon generation history of well zk03-2 in the north of the yushe-wujie block. 32 ke sun, shuheng tang, songhang zhang, zhaodong xi, jun li qinshui basin during the late yanshan period as well as the upwelling of the mantle, thereby resulting in high geothermal background values. the north china platform was strongly transformed in the late mesozoic and formed extensive fold structures and thrust faults; the magmatic activity was strong and the activities after the orogenic period of the orogenic belt were evident. intense magmatic intrusion occurred on the margin of the qinshui basin, and many mesozoic intrusive rocks have been found on the margin of the qinshui basin. inside the qinshui basin, there were also magmatic intrusions in the same period. in the west, there were granite bodies in the erfengshan and tarshan between xiangfen, fushan, and yicheng. the jinzhong mantle has the qixian diorite body, the huyan granite body to the west of the xishan coalfield in taiyuan, and the zishan granite body in linxian. there were magmatic intrusions of the yanshan period in the eastern jincheng, gaoping, lingchuan pingshun, and xigou. in addition, magmatic hydrothermal dykes were found in jincheng, yangcheng, and yangquan. on the aeromagnetic map, intermittent positive magnetic aerodynamic anomalies in the east-west direction of the south end of the qinshui basin were observed. these positive magnetic anomalies may be a reflection in the magmatic rock mass. magma thermal field and shallow-level formation in middleeastern region of qinshui basin in the late jurassic-early cretaceous, magmatic activities were active during the yanshanian period. although no wells encountered magmatic rocks in the yushe-wuxiang block, the basin simulation results from the geochemical data of wells zk03-2 and zk09-1 show that the middle-eastern regions of the qinshui basin were obviously affected by magmatic activities and were in an anomalous paleo geotherm stage, with the paleogeothermal gradient approaching 6°c/100m. although the earth’s crust rose as a whole, the temperature of the earth’s stratum rose dramatically; the maximum temperature reached about 200°c, and the thermal evolution of organic matter also increased greatly from the mature stage to the overmaturation stage (ro% reaches 2.0%), and the gas began to form in large quantities. according to the analysis, the yushe-wuxiang block is located far from the taihang fault and the drilling did not penetrate the magmatic rock mass. therefore, there is no direct baking effect of the magmatic body; instead, due to the deeper and more distant magmatic heating water which reaches the source rocks’ strata through the three-dimensional network channel formed by fractures-faults-cracks microfissures arranged by stages and the consequent heating of the source rocks, the thermal evolution of organic matter reaches the over mature stage from the mature stage. under the microscope, the fluid inclusions were found in the microfissures of the quartz particles and they were all secondary two-phase fluid inclusions (fig. 5). the analysis shows that these quartz particles are brittle after being heated by magmatic heating water, causing the quartz particles to break under the action of diagenesis or multi-phase tectonic stress. the magmatic heating water carries fluids such as ch4, co2, and carbonaceous asphalt into the cracks; then, fill up, precipitation, and cementation cause the fractures to shrink, forming microfractures in the quartz grains; the trapped gas-liquid fluid forms secondary fluid inclusions in the microfractures. microscopically, it was also found that there was a small amount of carbonate cements in the detrital grain cements. these carbonate cements may be dissolved by magmatic water from the lower carboniferous-permian marine formations and be carried through the hierarchical configuration of fracturesfaults, which is also indirect evidence of the magmatic heating water movement. conclusions and recommendations in this paper, the tectonic evolution history, thermal evolution history and hydrocarbon generation and expulsion history of tight gas reservoirs was studied in the yushe-wuxiang block in the middle-eastern regions of the qinshui basin. this paper also puts forward the idea and method of magmatic heating water moving from deep to shallow strata, thereby heating the source rocks in the strata and improving the maturity of hydrocarbon source rocks and promoting hydrocarbon generation. also, applying this idea and method to study and analyze the magma thermal fields and characteristics of shallow-level oil and gas accumulation formed by the magmatic heating water in the tight gas reservoirs of the yushe-wuxiang block in the eastern parts of the qinshui basin has been discussed. based on these studies, the following conclusions are drawn: the three-dimensional network of fractures-faults-cracksmicrofissures in a hierarchical configuration is a good channel for magmatic heating water to rise from the deeper to the shallower parts of the formation, and the formed magma thermal field promotes the maturity of the source rocks. because the three-dimensional network of fractures-faults-cracksmicrofissures is unevenly distributed throughout the strata, the heating effects of magmatic heating water on the strata are not balanced. the development and evolution of the middle-eastern part of the qinshui basin after the carboniferous can be divided into five stages, namely: i. rapid settlement stage of the late carboniferous to the end of late triassic; ii. overall uplift stage of the crust of early jurassic; iii. secondary settlement stage of middle jurassic; iv. magmatic activity stage of late jurassic-early cretaceous; v. slow uplift stage of middle cretaceous; the hydrocarbon generation and expulsion mainly occurs in stages i and iv. the middle-eastern part of qinshui basin of the early cretaceous further increased the maturity of organic matter and secondary hydrocarbon generation began. the elements for hydrocarbon accumulation were available and wellmatched, large-scale gas reservoirs began to form, and magmatic activities of the yanshan period constitute the most critical factor in the formation of tight sandstone gas reservoirs in the middle-eastern parts of the qinshui basin. figure 5. the fluid inclusions in microfractures of quartz particle in well zk03-2. 33the magma thermal field and the shallow-level gas accumulation of tight gas reservoirs in the middle-eastern parts of the qinshui basin the middle-eastern parts of the qinshui basin were strongly squeezed in the late mesozoic and developed an overthrusting fracture system that cooperated with the fractures and microfissures to form the magmatic heating water movement channel of hierarchical configuration fractures-faults-cracks microfissures, heating the source rock and thereby facilitating the thermal evolution of organic matter from its mature stage to a stage of higher maturity. at present, the research on the role of magmatic activities in hydrocarbon accumulation in oil and gas accumulation basins is not deep enough and has not attracted widespread attention of scholars. however, in some basins, magmatic activities play key roles in hydrocarbon accumulation. the effective study of magmatic activities of oil and gas accumulation can be a breakthrough in the associated theory, and in the future, the study of this phenomenon can lead to new developments in oil and gas exploration. references aarnes, i., planke, s., trulsvik, m., & svensen, h. 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(2007). effect of igneous activity on hydrocarbon source rocks in jiyang sub-basin, eastern china. journal of petroleum science & engineering, 59(3-4), 309-320. doi: 10.1016/j.petrol.2007.05.002. the late permian thermal events related to emeishan basalt has made a great impact on the underlying carbonate rock properties in the western margin of the yangtze platform. in this paper, we investigate the carbon and oxygen isotopic composition of the qixia formation carbonates from two sections: the qiaoting section in the northeastern part of the sichuan basin and the changjianggou section at the northwestern edge of the basin. the data reveal that: (i) samples from qiaoting section show a relatively narrow range of δ13c and δ18o, varying from 2.7‰ to 5.2‰ with an average of 4.2‰, and -3.8‰ to -7.8‰ with an average of -5.4‰, respectively. in contrast, samples from changjianggou section exhibit larger magnitude of variation in δ13c and δ18o, ranging from -1‰ to 3.8‰ with an average of 1.5‰, and -2.1 to -9.2‰, with an average of -6.0‰ respectively; (ii) δ13c and δ18o records in carbonates from qiaoting section are similar to those of middle permian seawater whereas carbonates from the changjianggou section are depleted in 13c and 18o compared to contemporary seawater; (iii) on the basis of combined petrographic and paleo-heat flow evidence, the lower carbon and oxygen isotopic composition of the carbonates from the changjianggou section are interpreted to be the results of thermal effects of emeishan basalt because of its proximity to the eruption center of the basalt. the high temperature reduced the δ18o values of the carbonates and forced the organic matter to mature at an early stage, thus producing 13c-enriched carbon dioxide to participate in the formation of carbonates. los eventos térmicos del pérmico tardío relacionados con el basalto emeishan han tenido un gran impacto en las propiedades de las rocas de carbonato subyacentes en el margen occidental de la plataforma yangtze. en este artículo se investiga la composición isotópica de carbonatos de la formación qixia en dos secciones: la sección de qiaoting en la parte nororiental de la cuenca sichuan y la sección changjianggou, en el noroccidente de la cuenca. la información evidencia que, (i) las muestras de la sección qiaoting tienen un espectro más angosto que va de δ13c a δ18o, con variación de 2.7 ‰ a 5,2 ‰, con un promedio de 4.2 ‰, y -3.8 ‰ a -7.8 ‰, con una media de -5.4 ‰, respectivamente. en contraste, las muestras de la sección changjianggou tienen una variación de mayor magnitud en δ13c a δ18o que va desde -1 ‰ hasta 3.8 ‰, con un promedio de 1.5 ‰, y de -2.1 a -9.2 ‰, con una media de -6.0 ‰, respectivamente; (ii) los registros de δ13c y δ18o en carbonatos son similares a los de agua marina del pérmico medio, mientras que los carbonatos de la sección changjianggou se ven empobrecidos en 13c y 18o comparados al agua marina de esta edad; con base en la petrografía combinada y la evidencia del flujo paleotérmico, la baja composición isotópica de oxígeno y carbón de los carbonatos en la sección de changjianggou se interpretó como resultado de los efectos térmicos del basalto emeishan debido a la proximidad con el centro de erupción. la alta temperatura redujo los valores de δ18o de los carbonatos y llevó a la maduración temprana de la materia orgánica, lo que produjo dióxido de carbono enriquecido en 13c durante la formación de los carbonatos. abstract resumen keywords: sichuan basin, qixia formation, carbon and oxygen isotopic composition, thermal effect palabras clave: cuenca de sichuan, formación qixia, composición isotópica de carbón y oxígeno, efecto térmico. record manuscript received: 08/08/2015 accepted for publication: 09/10/2015 how to cite item huang, k.k., zhong, y.j., li, x.n., & hu, z.w. (2016). oxygen and carbon isotopic composition of carbonate rocks of the permian qixia formation, sichuan basin: thermal effects of emeishan basalt. earth sciences research journal, 20(1). b 1b9. doi: http://dx.doi.org/10.15446/esrj.v20n1.52403 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.52403 oxygen and carbon isotopic composition of carbonate rocks of the permian qixia formation, sichuan basin: thermal effects of emeishan basalt earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): b1 b9 g e o c h e m is t r y huang keke1,2, zhong yijiang1,2*, li xiaoning1,2 and hu zuowei1,2 1 state key laboratory of oil and gas reservoir geology and exploitation, cdut, chengdu 610059 china 2 institute of sedimentary geology, chengdu university of technology, chengdu 610059, china * corresponding author. e-mail:zhongyijiang2012@cdut.cn composición isotópica de oxígeno y carbón en rocas de carbonato de la formación de edad pérmica qixia, en la cuenca de sichuan: efectos térmicos del basalto emeishan b2 huang keke, zhong yijiang, li xiaoning and hu zuowei 1. introduction the middle permian qixia formation is widely exposed on the margin of the sichuan basin. as a typical lithostratigraphic unit of the upper paleozoic marine strata, this formation can be used for stratigraphic correlation. the carbonate rocks of the qixia formation gradually change from “black qixia” to “white qixia” from the east to the west of the sichuan basin. the black qixia is mainly comprised of medium-thick strata of dark grey limestone interbedded with shale and siliceous rocks, and it generally has dark colors and high contents of organic carbon. it is considered to be one of the four sets of marine source rocks in south china (chen et al., 2010; lv et al., 2010; chen et al., 2012; liu et al., 2014). the white qixia is comprised of light grey blocks of limestone containing dolomitic limestone and dolomite, and it is mainly distributed throughout the northern sections of the micang mountain and the longmen mountain (sichuan provincial bureau of geology and mineral resources, 1991). this finding may reduce the significance of the traditional opinion that “the qixia formation is black and the maokou formation is while” in the case of western sichuan basin. for example, the qixia formation in the qiaoting section in nanjiang area, northeastern sichuan, is made of overall grey-black limestone strata (fig. 1a). moreover, the qixia formation in the changjianggou section in jian’ge area, northwestern sichuan, is generally of dark grey limestone, with grey dolomitic limestone forming the upper part (fig. 1b). it is widely thought that the black qixia was deposited in a deep water environment while the white qixia was deposited in shallow water within a carbon platform (hu et al., 2010; chen, 2009). previous work has shown qixia formation has the potential to become a hydrocarbon reservoir rock (zeng et al., 2010; hao et al., 2013; tian et al., 2014). thus, this regional variation of lithological features have elicited strong academic interest for deciphering the nature and development mechanism of this formation. several studies have noticed abundant saddle dolomite and other types of dolomite that are characterized by curved-face and xenomorphic crystals with high homogenization temperatures (max. > 200°c) were found to be associated with dissolution pores, vugs and factures in the western part of the sichuan basin. the impact of geothermal activity has been invoked to explain the dolomitization mechanism (he and feng, 1996; wang and jin, 1997; chen et al., 2012; hao et al., 2012; li et al., 2012; shu et al., 2012; huang et al., 2013; tian et al., 2014). in this paper, we focus on the influence of the related thermal events on the carbon and oxygen isotopic compositions of the marine carbonates of the qixia formation. the global permian carbonate rocks have stable and relatively high carbon isotopic compositions (hermann et al., 2010; saltzman and thomas, 2012; laya et al., 2013). in the carbon isotope curve of the phanerozoic carbonates (veizer et al., 1999), the central values of δ13c of the permian rocks were concentrated in the range of 2.5-4.5‰. in the carbon isotope record of china’s upper yangtze permian, which had a higher resolution than that of the curve reported by veizer et al. (1999), the δ13c values were also mostly in this range (fig. 2a, huang, 1997). a decline in δ13c, resulting from extinctions and replacements of life, was found near the permiantriassic boundary (hiete et al., 2013;tohver et al., 2013), indicating a high content and rapid burial of organic carbon in the global permian. figure 1. outcrop photos of carbonates of qixia formation in in sichuan basin in (a) the qiaoting section and (b) the changjianggou section. the δ18o values of the global permian carbonate rocks are mostly in the range of -3~6.5‰, and the corresponding curve is lower than that of the carboniferous and triassic (veizer et al., 1999). this could have occurred because the permian seawater had a relatively high temperature, or because the thermal events after the depositional stage had an impact on the permian carbonate rocks. the oxygen isotopic composition of the upper yangtze permian (huang, 1997) was lower than that of the same period in the curve reported by veizer et al. (1999). there are two possible reasons for this. one, the samples used by veizer et al. were more representative of seawater, as carbon isotopes are more easily preserved in carbonates than oxygen isotopes. the other possible reason is that the carbon isotope curve of china’s upper yangtze permian had a higher resolution than that of the curve reported by veizer et al. (1999). as shown in the previously reported curve, the lower δ18o values of the permian strata from upper yangtze were mainly occurred in: (1) the upper part of the qixia formation, (2) the top of the maokou formation, and (3) the top of the permian. these three excursion may be associated with the thermal events related to emeishan basalt eruption, the karstification related to the dongwu movement, and the increase in seawater temperature during the late permian, respectively (sun et al., 2012). figure 2. curve of carbon isotope of marine carbonate rock in upper yangtze permian. the curve was based on the related data in oxygen isotope data (huang, 1997) and carbon isotope data (huang, 1997). though the curve of carbon isotope in china’s upper yangtze permian had a relatively high resolution, only 8 samples of the qixia formation were used in the research (fig. 2a). for further understanding of the composition and variation in carbon and oxygen isotopes in the qixia formation, the research group conducted a study on the carbonate rocks of this formation in two sections, which were located in the northeast and northwest of the sichuan basin. a total of 94 groups of carbon and oxygen isotope data were collected and studied through a comparative analysis. moreover, the impact of the thermal events related to the emeishan basalt erruption on the carbon and oxygen isotopic compositions of this formation was discussed. 2. geological setting and sampling samples were collected from the qiaoting section in the northeast of the sichuan basin (nanjiang county) and from the changjianggou section in the northwest of the basin (jian-ge county). according to the tectonic division of the sichuan basin (liu et al., 2000), the two sections are parts of the fault zones of the micang—daba the longmen mountain—panxi, respectively (fig. 3). currently, the linear distance between the two sections is about 130 km in a north-west direction. the initial distance between the b3oxygen and carbon isotopic composition of carbonate rocks of the permian qixia formation, sichuan basin: thermal effects of emeishan basalt 3 sections was supposed to be significantly greater than 130 km, because the tectonic activities following the depositional stage, especially those related to the himalayan orogeny, could affect the distance between the sections. for instance, the nappe belt on the front edge of the longmen mountain, where the changjianggou section is located, might decrease this distance. figure 3. geological scheme of the study area, modified after the 1:1,000,000 geological map of sichuan province, sichuan geology and mineral bureau, 1991. the map shows the locations of the two sections studied in the research, which are respectively parts of the micang-daba mountains fault zone and the longmen mountain-panxi fault zone. the qixia formation occurs in the western and northern edges of the sichuan basin. the triple division of the permian has been widely used in stratigraphic studies in china and other countries (e.g. li et al., 2005; shen et al., 2005). in this paper, the qixia and maokou formations were included in the middle permian in accordance with the triple divisions of the permian marine facies in the sichuan basin (jin et al.,1999; li et al.,2005) as well as the stratigraphic classification in international stratigraphic chart (2008) (zhang et al., 2009). in these division plans, however, the lower permian was poorly developed or even absent in sichuan basin. the lower permian, comprised of the liangshan formation, is about 1m thick in the changjianggou section and less than 1m in the qiaoting section. due to very thin-bedded layers, this formation was not marked in the maps in this paper. the large area of late paleozoic basalt (emeishan basalt) sits along the western margin of the yangtze block (huang and opdyke,1998 ) and is supposed to have thermal effect on its underlying carbonate rocks. zhu et al., (2010) has demonstrated that the paleo-heat flow in the west of the sichuan basin reached a maximum at the end of the middle permian at about 259 ma (fig. 4a). in this period, the paleo-heat flows measured in most wells were 60-80 mw/m2, while the values in a few wells exceeded 100 mw/m2. the paleo-heat flow characteristics reflected the thermal effect of the emeishan basalt eruption during the late paleozoic. when the paleo-heat flow peaked, the permian qixia formation was at the shallow burial stage (fig. 4b). figure 4. (a) a synthetic map of burial history and paleo-heat flow distribution for well hs1 in western sichuan basin; (b) a paleo-heat flow distribution map for representative wells in the sichuan basin (modified after zhu et al., 2009, 2010) 3. methods samples used in the carbon and oxygen isotopic analysis were taken from the limestone matrix and dolomite matrix, avoiding the vugor vein-filling calcite and dolomite crystals. previous research indicated that the carbon and oxygen isotopic compositions of calcite and dolomite in vugs are lower than those of carbonate matrix (huang et al., 2014). this research mainly involved the use of some geochemical methods, including carbon and oxygen isotopic analysis, as well as elemental analysis of some samples. the carbon and oxygen isotopic analysis was carried out at nanjing institute of geology and palaeontology, chinese academy of sciences, and the cnnc beijing research institute of uranium geology, with the use of mat-252 mass spectrometry. the latter was responsible for conducting the test in accordance with the standard of application of phosphoric acid method to the measurement of carbon and oxygen isotopic compositions of carbonate minerals or rocks (dz/t 0184.17-1997). the former was responsible for analyzing the accuracy of the test results using the kiel iv carbonate device sampling system in accordance with the gbw-04405 standard. the standard deviations of the δ13c (pdb) and δ18o (pdb) values were smaller than 0.040 and 0.080, respectively. the analysis of the contents of cao, mgo, mn, sr, and fe contents was performed at the geological and mineralogical testing center of huayang in sichuan province. the cao and mgo contents were measured using normal chemical analysis, with the limit of detection set at 1%. the relative errors of cao and mgo measurements were 2% and 5%, respectively. the contents of mn and sr were determined by atomic absorption spectrometry, with the limits of detection set at 5×10-6 and 42×10-6, respectively. the relative errors of the measurements were 13% and 14%, respectively. the fe content was determined by colorimetry, with the limit of detection set at 0.01%. the relative error of the result was less than 8% 4. results and discussion table 1 exhibits the analysis results pertaining to the carbon and oxygen isotopes in 94 carbonate samples from the qixia formation in qiaoting, nanjiang, and changjianggou, jian’ge. the data of 22 samples marked with ‘δ’ in table 1 have been published by lv (2013). in addition to the isotopic data, the contents of ca, mg, sr, mn, and fe and the corresponding mn/sr ratios of 69 of the 94 samples are also given for an understanding of the mineral compositions (relative contents of calcite and dolomite), representativeness, and diagenetic alteration of the samples. 4.1 elemental compositions and their implication for diagenetic alteration according to the elemental analysis, the average values of mn, fe, and sr contents in the 69 samples were 48.8 ppm, 729.1 ppm, and 331.5ppm, respectively. kaufman et al. (1992; 1993) employed mn/sr ratio<2~3 as criterion for selecting carbonate samples in which the isotopic values are considered to be proxy for those of seawater. most of our samples have rather low mn and fe contents and relatively high sr content and the average mn/sr ratio was very low, of only 0.31. however, among the aforementioned 69 samples, there were four samples with mn content higher than 200ppm, having values at 818 ppm, 278 ppm, 406 ppm and 326ppm. three of these four samples had very high fe content as well, having values at 10442 ppm, 2724 ppm, and 14987 ppm, corresponding to the first three high mn values listed above. the curves of carbon and oxygen isotopes in the subsequent discussion indicated that, among the three samples with both high mn and fe contents, two samples exhibited very low δ13c and δ18o values, which deviated from the overall trend. these two samples were collected from the bottom of the qixia formation in the qiaoting section, which was adjacent to the liangshan formation. it was speculated that these samples were affected by some 12cand 16o-rich fluids that were associated with the clastic rock strata. b4 huang keke, zhong yijiang, li xiaoning and hu zuowei table 1. carbon and oxygen isotopic compositions of carbonates of the qixia formation in qiaoting, nanjiang and changjianggou, jian’ge, and contents of ca, mg, sr, mn and fe in the samples and corresponding mn/sr ratio b5oxygen and carbon isotopic composition of carbonate rocks of the permian qixia formation, sichuan basin: thermal effects of emeishan basalt note: blank cells in the table represent without detection; ng=below the limit of detection; 22 samples marked with ‘δ’ have been published by lv, 2013; 3 samples marked with * have both high mn content (>200ppm) and high fe content (>2000ppm). b6 huang keke, zhong yijiang, li xiaoning and hu zuowei the carbonates of the qixia formation in the qiaoting section and the changjianggou section did not vary substantially in fe and mn contents, except for the aforementioned three samples with high mn and fe contents. the average mn contents of the two sections were 47.3 ppm and 52.1 ppm, and their average fe contents were 322.7 ppm and 344.2 ppm, respectively. the sr content in the qixia formation was significantly higher in the qiaoting section than in the changjianggou section. their average sr contents were 421.0 ppm and 140.2 ppm, respectively. the mn/ sr ratio of the qixia formation in the qiaoting section was 0.05, which is much lower than the ratio of 0.74 in the changjianggou section. overall, it can be concluded that geochemical information of these samples are not significantly affected by non-seawater-like fluids such as meteoric fluids, which would lead to enrichment of mn, fe and higher mn/sr ratio. 4.2 carbon and oxygen isotopes though the carbonates of the qixia formation in the two sections did not vary substantially in terms of elemental composition, their carbon and oxygen isotopic compositions varied greatly from one another (table 1, fig. 5, and fig. 6). selecting the boundary of carboniferous-permian and boundary of qixia and maokou formation as joint, by which the δ13c and δ18o profile of sections in this research and previous study could be compiled. the carbon isotopic compositions of carbonates of the qixia formation in the qiaoting section were relatively stable, except for two samples from the bottom that were affected by the clastic rocks of terrigenous origin in the liangshan formation. the δ13c values of the samples ranged from 2.7‰ to 5.2‰, with the average being 4.2‰. this carbon isotopic composition was close to that of the global seawater of the period (veizer et al., 1999). the high and stable δ13c values of the marine carbonates in the qixia formation reflected a good ecological status of the earth, stable species and populations of marine invertebrates with calcareous shells, and fast and continuous burial of organic carbon during this period. in contrast, the qixia formation in the changjianggou section exhibited significant variations and negative excursions in δ13c and δ18o records. the δ13c values were between -1‰ and 3.8‰, and the average was 1.5‰, which was significantly lower than that of the qixia formation in the qiaoting section and the global seawater of the same period (veizer et al., 1999). the low δ13c values were mainly distributed throughout the middle part of the qixia formation (the bottom of the second member of the formation); the δ13c values of samples from this part were mostly between -1‰ and 1‰. despite qixia formation in the changjianggou section and qiaoting section was deposited in a shallow-water platform facies and an open platform facies, respectively (huang et al., 2004), such difference could not lead to the observed variation in δ13c values due to its insensitive to spatial factor. it is more difficult to measure the oxygen isotopic compositions of carbonates that can represent seawater in geologic history. in the curve of oxygen isotope in phanerozoic carbonates reported by veizer et al. (1999), the oxygen isotopic compositions of marine carbonates showed an overall upward trend over time in the geologic history. in the qiaoting section, however, the oxygen isotopic compositions of the carbonates of the qixia formation were relatively stable overall, ranging from -3.8‰ to 7.8‰ with an average of -5.4‰. this was slightly lower than that of the global seawater of the same period (veizer et al., 1999). in contrast, the carbonates of the qixia formation in the changjianggou section showed a significant variation and a negative excursion in oxygen isotopic composition. the δ18o values mostly ranged from -2.1‰ to -9.2‰, and the average was -6.0‰, which is significantly lower than that of both the qixia formation in the qiaoting section and the global seawater of this period reported by veizer et al. (1999). figure 5. the cross-plots of carbon isotopic compositions and the thicknesses of carbonate rocks of qixia formation in qiaoting and changjianggou sections. the carbon isotopic compositions of the permian and adjacent formation are given. the data sources are the same as in fig. 2. the grey circles and dark-blue rhombus are symbols for data from this study and huang (1997). the red squares in fig. 5b refer to the three samples marked with * in table 1. the liangshan formation of the lower permian in the changjianggou and qiaoting sections are not shown in the figure due to their negligible thickness (<1m). marine carbonates are the largest inorganic carbon pool in nature. the carbon contents of most diagenetic fluids are relatively low compared to carbonate rock itself, indicating that carbon sources are buffered by the carbonates rather than fluid during diagenesis. as a result, the information about carbon isotope in seawater can be well preserved in carbonate rocks, especially the micritic carbonates that are characterized by low porosity and permeability. because the possibility of meteoric influence has been ruled out during the sampling and data selecting process, the distinctly low δ13c values of the carbonates of the qixia formation in the changjianggou section indicated that carbon sources with low δ13c values participated in carbonate formation when the rocks still had relatively high porosity and permeability, which lead to high water/rock ratio. such process could not be involved in the normal thermal evolution of burial diagenesis sequence, because the rock would be highly compacted after high volumes of co2 have been released by the organic matter. in this case, the water/rock ratios were relatively low thus organic carbon’s input was not available. davies (2004) proposed the term ‘‘forced maturation’’ for the alteration of kerogen in host limestones during transient thermal anomalies. davies and smith jr. (2006) pointed out that this process precedes regional burial maturation of organic material. therefore, we suggested certain unusual thermal events took place in the early stage of diagenesis and caused early forced maturation of organic matter, subsequently decreasing the δ13c values of carbonates by the entrance of carbon sources with low δ13c values. in summary, the lower b7oxygen and carbon isotopic composition of carbonate rocks of the permian qixia formation, sichuan basin: thermal effects of emeishan basalt carbon & isotopic composition of the carbonates of qixia formation in the changjianggou section suggested that the thermal events in the early stage of diagenesis had a greater influence on the changjianggou section. figure 6. cross-plots of oxygen isotopic compositions and thicknesses of carbonate rocks of qixia formation in qiaoting and changjianggou sections. the oxygen isotopic compositions of the permian and adjacent formation are shown. the data sources are the same as in fig. 2. the grey circles and dark-blue rhombus are symbols for data from this study and huang (1997).the red boxes in fig. 6b refer to the three samples marked with * in table 1. the liangshan formation of the lower permian in the changjianggou and qiaoting sections are not shown in the figure due to their negligible thickness (<1m). the oxygen isotopic composition of carbonates is quite sensitive to temperature, and the decrease in the δ18o values of carbonates is usually caused by isotope fractionation resulting from rising temperature. the lower oxygen isotopic composition of the carbonates of the qixia formation in the changjianggou section suggested that the qixia formation in this section had experienced higher temperatures during diagenesis. due to the influence of the thermal events related to the eruption of the emeishan basalt, the paleoheat flow in the western sichuan basin reached the highest level during the end of the middle permian. the maximum paleo heat flows experienced by most wells were between 60 and 80 mw/m2, and a few wells have exhibited maximum paleo heat flows higher than 100 mw/m2 (zhu et al., 2010). during this period, the qixia formation was at the stage of shallow burial (depth=500~100m) (fig. 1b). the thermal events associated with the emeishan basalt eruption could reduce the oxygen isotopic composition of the carbonates in the areas affected by the events and cause forced evolution and maturation of the organic matter. this was especially important to the qixia formation because of the extremely rapid burial of organic carbon in this formation. moreover, the release of co2 by organic carbon sources during the process can further change the carbon isotopic composition of the affected carbonates. therefore, the qiaoting section in nanjiang, which is farther from the basalt area, was less affected by basalt eruption. this could be the major reason for the lower carbon and oxygen isotopic compositions of the carbonates of the qixia formation in the changjianggou section in the west of the sichuan basin. numerous studies suggest that dolomite formed from seawater would have higher δ18o than calcite (eg. fouke,1994). several experimental determinations have been carried out for measuring carbonate-water oxygen isotopes fractionation factor and concluded that the fractionation factor of oxygen will increase during ca substitution by mg (eg. tarutani et al., 1969; jiménez-lópez et al., 2004). the rate of change between the δ18o of dolomite and mg content has been estimated from the difference in δ18o between coprecipitated dolomite and calcite (vahrenkamp and swart, 1994). such estimates for δδ18odolo-cal at 25°c, based on experiments and theoretical calculations, include 4 to 7‰ (northrop and clayton, 1966; o’neil et al., 1969; matthews and katz, 1977; clayton et al., 1989), 2.6 to 4‰ (fritz and smith, 1970; vahrenkamp and swart, 1994; schmidt et al., 2005; vasconcelos et al., 2005;chacko and deines, 2008), 3‰ (land, 1980). although such values can be translated to 0.05–0.14‰ increase in δ18o per 1%mg increase, the magnitude of that increase for dolomite is poorly constrained for different temperature condition. therefore, dolomitization effect are not likely to have significantly contributed to the overall patterns of oxygen isotopic records thus it is not quantified in the current discussion. in addition, given the fact that dolomitization degree in carbonates from changjianggou section is generally higher than that from qiaoting section (li et al., 2015), the difference of temperature effects between these two sections would be higher if we take the effect of δδ18o between dolomite and calcite into account. the discussion above should lead to the conclusion that, in the areas heavily influenced by the eruption of emeishan basalt, such as the western part of the sichuan basin and some regions in yunnan and guizhou provinces, the organic carbon has experienced forced maturation and oxidation in the early stage of diagenesis due to the thermal effects of the basalt eruption. this might have an adverse impact on hydrocarbon generation in the later stage. huang and wang (2008) have noticed that the thermal maturity of bitumen veins in the kuangshanliang area in northwestern sichuan, which is near the changjianggou section, had high thermal maturities. they proposed that this was caused by thermal alteration and diffusion during deep burial rather than by biodegradation. the thermal events associated with the emeishan basalt eruption could be another cause of the high thermal maturities of the bitumen veins. furthermore, the hydrocarbon exploration potential of the areas under the influence of the thermal events may have been reduced due to the early forced maturation and oxidation of organic matter. 5. conclusions the marine carbonates of the global permian qixia formation have very high carbon isotopic composition, indicating an ecological prosperous status, and fast and continuous burial of organic carbon during this period. however, qixia formation in two sections, the qiaoting section in the northeast and the changjianggou section in the northwest of the sichuan basin, varied significantly in carbon and oxygen isotopic compositions. the carbon and oxygen isotopic compositions of the carbonates of the qixia formation in the qiaoting section vary relatively stable; the δ13c values ranged from 2.7‰ to 5.2‰ (mostly around 4‰), with an average of 4.2‰, and the δ18o values ranged from -3.8‰ to -7.8‰, with an average of -5.4‰. both the carbon and oxygen isotopic compositions of the carbonates were close to those of the global seawater of this period. the carbonates of the qixia formation in the changjianggou section exhibited significant variations and negative excursions in δ13c and δ18o records. the δ13c values were between -1‰ and 3.8‰, and the average was 1.5‰, which is significantly lower than that of the global seawater of the period. the δ18o values were mostly in the -2.1 to -9.2‰ range, and the average was -6.0‰, which is also significantly lower than that of the global seawater of the period. most of the carbonate samples from the changjianggou and qiaoting sections had very low mn and fe contents and a relatively high sr content. the elemental composition characteristics indicated that the samples from both sections have been diagenetically altered to a small degree and thus b8 huang keke, zhong yijiang, li xiaoning and hu zuowei were representative of seawater of that period. therefore, it is unreasonable to attribute the differences between the carbon and isotopic compositions in the two sections of the qixia formation to only diagenetic alteration. the low carbon and oxygen isotopic compositions of the samples from the changjianggou section were associated with the proximity to the eruption center of the emeishan basalt. the thermal effect of the related thermal events reduced the oxygen isotopic composition of the carbonates. the high temperature forced the organic matter to mature rapidly, and the co2 released by organic carbon sources during the thermal evolution entered the carbonates, thus resulting in a decline in the δ13c values. the early forced maturation and oxidation of organic matter may have reduced the hydrocarbon exploration potential of the areas heavily influenced by the eruption of emeishan basalt, such as western sichuan basin and some regions in yunnan and guizhou provinces. acknowledgments this study was supported by the national natural science foundation of china (41272130, 41172099). we also wish to acknowledge the support provided by key laboratory for sedimentary basin and oil and gas resources of mlr, grant zdsys2014003 as well as state key laboratory of oil and gas reservoir geology and exploitation, grant plc201308. in addition, lv jie and lan yefang are thanked for helpful discussion during field work. references chacko, t. and deines, p. 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(2010). palaeo-geothermal response and record of the effusing of emeishan basalts in sichuan basin. chinese science bulletin. 55(6), 474–482 (in chinese with english abstract) . the reserves of coalbed methane (cbm) in qinshui basin are quite promising, but the outputs from cbm wells are quite small even after massive hydraulic fracturing. herein the fracture system with #3 and #15 coal seams in qinshui basin was analyzed, and it was found that both of the macro-scale fractures and micro-scale fractures are filled with clay and carbonate minerals, which explains the low productivity of cbm wells after conventional hydraulic fracturing. acid fracturing has long been an effective method for carbonate gas reservoir to improve the gas well production. however, there were few reports about the application of acid fracturing in coal bed methane field. based on the mineral identification and acid sensitivity test, the feasibility of acid fracturing demonstrated that the acid does more help than damage to increase the permeability of coal seams in qinshui basin. onsite operations have shown that acid fracturing is applicable for the cbm wells in jincheng mining area. it was also observed from the microseismic survey that when applying the acid fracturing treatment, the stimulated reservoir area depends on the acid volume pumped in the first stage, which is crucial to the success of the stimulation. abstract keywords: coalbed methane, acid fracturing, acid sensitivity test, stimulated reservoir area. the pilot appraisal of acid fracturing of coalbed methane reservoir in southeast qinshui basin, china issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.61633 c m bearth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): c1 c6 yu yanga*, chengwei zhangb, huijun tianb, wangang chenc, xiaodong pengd, hao zhanga a. state key laboratory of oil and gas reservoir geology and exploitation ( chengdu university of technology ), chengdu 610059, china b. college of energy, chengdu university of technology, chengdu 610059, china c. china united coalbed methane co. ltd, beijing 100011, china d. cnooc ltd. zhanjiang, zhanjiang 524057, china *corresponding author: yangyu@cdut.cn las reservas de gas metano de carbón (cbm, del inglés coalbed methane) en la cuenca qinshui son más que prometedoras, pero la producción en los pozos es muy pequeña, incluso después de fracturas hidráulicas masivas. en este trabajo se analizaron los sistemas de fractura de las vetas de carbón #3 y #15 de la cuenca qinshui y se encontró que tanto las fracturas a macroescala como aquellas a microescala están cubiertas con arcillas y minerales carbonatos, lo que explica la baja productividad de los pozos de gas metano de carbón después de la fractura hidráulica convencional. la fractura ácida ha sido un método efectivo en los depósitos de gas carbonato para mejorar la producción en el pozo de gas. sin embargo, existen pocos informes sobre la aplicación de la fractura ácida en el campo del gas metano de carbón. de acuerdo con la identificación mineral y las pruebas de sensibilidad ácida, la factibilidad de la fractura ácida demostró que el ácido es reparador en el incremento de la permeabilidad en las vetas de carbón de la cuenca qinshui. las operaciones in situ han demostrado que la fractura ácida es aplicable para los pozos de gas metano de carbón en el área minera de jincheng. también se observó en el sondeo microsísmico que cuando se aplica un tratamiento de fractura ácida, el área del depósito estimulada depende del volumen de ácido bombeado en primera instancia, lo que es determinante en el éxito de la estimulación. resumen palabras clave: gas metano de carbón, fractura ácida, pruebas de sensibilidad ácida, área del reservorio estimulada. record manuscript received: 16/09/2016 accepted for publication: 07/01/2017 how to cite item yang, y., zhang, c., tian, h., chen, w., peng, x., zhang, h. (2016). the pilot appraisal of acid fracturing of coalbed methane reservoir in southeast qinshui basin, china. earth sciences research journal, 20(4), c1-c6. doi:http://dx.doi.org/10.15446/esrj.v20n4.61633 evaluación piloto de fractura ácida en depósitos de gas metano de carbón en el suroeste de la cuenca qinshui, china c2 yu yang, chengwei zhang, huijun tian, wangang chen, xiaodong peng, hao zhang 1. introduction current statistical data shows that the total global cbm is more than 50 billion m3 of great exploitation potential (al-jubori et al., 2009). in the dual-porosity system of coalbed methane reservoir, the methane is adsorbed in the pore of the matrix (rogers, r. e., ramurthy, k., rodvelt, g., & mullen, 2007). when the fluid pressure in nature fracture system decreases to some degree, the adsorbed methane will begin to desorb from the surface of micro-pore, diffuse to nature split, and seepage to near well bore area (schein & mack, 2007). hydraulic fracturing with gel should be applied to cbm reservoir as routine, which helps to stimulate the flow channel in the near wellbore zone, to make the gas production commercially viable (mccabe et al., 1999; cramer, 2008). the reserves of cbm in qinshui basin of china are quite promising (wei et al, 2007), but most of the productions of cbm wells are quite small even after massive hydraulic fracturing (chen et al., 2009). the measured permeability of the core plugs from qinshui basin is minuscule, most of which being less than one md measured by injection tests (wang, tang, wei, xu, & leng, 2006). the clay and calcite mineral observed in the natural fractures of coal seam are to explain the low productivity of gas wells (puri, king, & palmer). therefore, increasing the permeability of natural cleat is the important target for petroleum engineers who should be stimulating the coal bed methane in qinshui basin. despite the fact that acid fracturing has been proved as an important way to improve oil and gas production for carbonate reservoir with natural fractures (hill & schechter, 2000; perex, d, huidobro, e., & avendano, 1998), it has not been applied universally in the stimulation of cbm wells. it was reported that small volume of acid spearhead ahead of the main fracturing had been implemented to clean out the cement deeper into near well bore area and reduce the breakdown pressure of a few cbm wells (adil, sharma, & bhat, 2012). the acid spotting /socking with 1000 gal of inhibited 15% hcl and 10% acetic acid mix, which react with perforation debris and remove cement invasion, has also been proved successful in india (pooniwala, 2012). however, it is said in the literature that the coal formations are rather clean with minimum natural deposits in the cleat system, which is entirely different from those in the qinshui basin. to evaluate the feasibility of acid fracturing in qinshui basin, herein a series of tests have been designed to verify that the acid is beneficial to dissolve the carbonate fillings in the coal core of both macroscale and microscale, and the pilot acid fracturing treatments have been performed to demonstrate that the hcl acid pad, the volume of which being much larger than that in literature (adil et al., 2012; pooniwala, 2012; tamayo, lee, & taylor, 2007) can remove the carbonate cement of the natural fracture net, and lower the breakdown pressure efficiently as expected. it is learned from the microseismic survey that the stimulated reservoir volume (srv) is dependent on the application of acid pad before the proppant laden stage, and it is suggested that larger volume of the acid pad is crucial to the success of the stimulation operation in cbm wells with carbonate filling in cleats. 2. natural fracture net and mineral contents 2.1 observation of macro-scale nature fracture the main pay zones of qinshui basin coal bed methane are named as 3# and 15# coal seams. from the observation of working tunnels of down-hole coal mines located in qinshui basin, it is found that there are two groups of cleats. the strike of the first group ranges from 20 to 50°, the density of which being unevenly distributed, with the strike of the second group being between 276 and 355°. the dip angle of cleats are more than 70° and can be taken as vertical braces (liu, kang, wang, & deng, 2008). the mineral identification with hcl acid shows that most of the fillings in cleats are carbonate type, which agrees with the observation of wei et al. table 1. statistics of coal seams in qinshui basin 2.2 sem analysis of filling in micro-scale fracture scanning electron microscope (sem) is an effective way to observe the occurrence of micro fractures, such as the dimension, filling, and connection. since the filling in micropores and cracks is the resistance for the gas to flow, the discrimination of mineral type, current status, and the content can be the theoretical foundation for acid fracturing in the coal seam. the qunta250 feg scanning electron microscope made by fei co. and incax-max20 energy dispersive spectrometers made by oxford co. of the national key laboratory of oil and gas reservoir geology and exploitation are applied to describe the thin sections of the coal seam. the analysis of 3# and 15# coal in jincheng mine are as follows: clay mineral clay mineral is the most widely distributed and abundant mineral in china’s coal. it is also found in this investigation that kaolinite, illite, chlorite, mixed layer of smectite and illite are abundant in jincheng mining area. figure 1. flake type illite and smectite mixed layer in microfracture carbonate mineral most of the carbonate minerals in jincheng cbm mine area are identified as epigenetic origin (liu et al., 2008). figure 2. the calcite filling in microfracture c3the pilot apprisal of acid fracturing of coalbed methane reservoir in southeast qinshui basin, china sulphide mineral even if the content of sulphide minerals in coal is quite small, this kind of mineral could be the origin of insoluble precipitate of acid reaction, which can plug the fracture. the common sulphide mineral in coal is identified as pyrite (fe2s). figure 3. blackberry type pyrite aggregation in microfracture 2.3 x-ray diffraction analysis of filling in micro-scale fracture it is hard for sem to discriminate every clay mineral sometimes. x-ray diffraction method analyzes diffraction pattern on the photographic plate by making use of x-ray diffraction formed by crystal, so as to clarify the crystalline structures and shapes of minerals, and further analysis can tell the relative content of minerals by quantity. table 2. x-ray diffraction analysis results from part of coal samples according to table 2, most of the clay minerals in coal rock are illite and kaolinite, with few illite/smectite mixed layer. it is found from more than 230 reports in jincheng mine that the average amount of illite is 68.7%, and that of kaolinite is 24%. 3 feasibility of acid fracturing stimulation in coal bed methane coal bed methane reservoir is typical of the dual-porosity media system; the methane is adsorbed in matrix pore. when coal seam pressure decreases to a certain degree, the adsorbed methane begins to desorb from the surface of matrix pores, and diffuse to macro fracture network through pores and microfracture, and flow to well bore via hydraulic fracture. through downhole observations, sem and x-ray diffraction analysis, we can see fillings in coal fractures as that most of the clay minerals are kaolinite or illite, and the carbonate fillings in both macroscale fracture and miro-scale fracture are also abundant. mingshou wang et al. measured the temperature of inclusions of fillings existing in coal seam cleat systems of qinshui basin and suggested that the filling sequence of minerals is that pyrite precipitates first, then quartz and clay minerals, finally carbonate minerals (calcite and fe-dolomite). so that is to explain why the carbonate minerals are filled in the center of fractures. from the 1990s to 2010s, more than 50 cbm wells were tested with the injection-drawdown methods to measure the permeability, and the results showed that the permeability of most of the pay zones is less than 1md. because of the relatively low permeability of the natural fractures in 3# and 15# coal seams of qinshui basin, the productivity of cbm wells are quite low even if the wells have been stimulated using massive hydraulic fracturing. therefore, by injecting some kinds of acids into the formation, to get the carbonate mineral in the center of natural fractures resolved, the conductivity of reservoir fractures could be upgraded. the hydrochloric acid and mud acid are screened at first to simplify the feasibility of acid applied in cbm. at the early stage of the experiment, it is found that the calcium fluoride precipitation would follow the reaction between hydrofluoric acid and carbonate. however, the permeability of coal sample soaked in hydrochloric acid increases significantly. carbonate minerals react with hcl acid as follows: caco3 + hcl = cacl2 +co2 +h2o (1) 3.1 test preparation the coal samples used in the experiment are taken from 3# coal seam of jincheng mine. figure 4. core samples prepared from coal seam 3.2 porosity change of coal core there are various methods to test the porosity of coal core, and the most widely used one is using nitrogen flooding. because nitrogen is inert, the molecule diameter of which is only 0.265 nm and can enter micro-pore of coals, it can cover the micropore and fracture. three different concentrations of 6 %, 9 %, and 15 % hydrochloric acid are used respectively. coal samples are taken from 3# coal seam in jincheng mine in qinshui basin. coal cores are divided into three groups, each group with ten cores, to compare the porosity changes before and after treatment with acids. experiment steps 1. when drying the coal samples before the test, caution should be taken to avoid the destruction of coal plug due to high temperature, and the temperature is suggested as 80 °c (dabbous, reznik, taber, & fulton, 1974.) 2. test the porosity of coal samples of each group before acid treatment. 3. have coal samples soaked 6 hours in hydrochloric acid of 6 %, 9 %, and 15 % concentration, respectively. 4. test porosity coal samples after treatment. c4 yu yang, chengwei zhang, huijun tian, wangang chen, xiaodong peng, hao zhang 5. porosity changes are compared in table 3. it is found that the average porosity increase after the acid treatment. according to the table 3, if the acid concentration increases, the average porosity will increase accordingly. table 3. porosity changes of coal samples 3.3 evaluation of acid sensitivity acid sensitivity is the permeability decline due to the reaction between an acid and some minerals (hsi, 1984). the minerals sensitive to hcl are iron calcite, iron dolomite, hematite, pyrite, magnesium iron ore, glauconite, hydrobiotite. the minerals sensitive to hf are calcite, dolomite, calcium feldspar, zeolites, and various types of clay minerals. since pyrite was observed in sem, the acid sensibility of coal seam samples is measured. in the experiment, permeability is measured before and after injecting hcl of 15% concentration. the acid-sensitive index is calculated by formula 2. (2) where: k0 is the permeability by distilled water flooding before acidification, k1 is the permeability by distilled water flooding after acidification, is the acid sensitivity index. table 4. part of acid sensitivity analysis of coal samples experiments suggest that acid treatment can improve coal seam porosity, and since there are few minerals sensitive to hcl acid in coal seams, the acid sensitive index is relatively low. 3.4 geo-mechanic property changes of coal samples after acid reaction because the cement in a fracture in jincheng mine can be resolved in acid, the geo-mechanic properties would change accordingly (huang, wang, c., wang, t., & zhang, z., 2015). before this experiment, when preparing the core sample, care should be taken to avoid turbulence which may destroy the core sample because of the fragile nature of coal. the samples listed are numbered from 1 to 10. half of the samples are soaked in acid for 2 hours before the test, while the others are tested after acid treatment. it can be seen that after acid treatment, the cores are much easier to break down. figure 5. triaxial destruction experiment of coal sample. table 5. geo-mechanic parameters of coal samples before acidizing table 6. geo-mechanic parameters of part of coal samples after acidizing 3.5 sem observations before and after acidizing through sem experiment, the changes of minerals in the microfractures before and after acid treatment are studied, and the result is shown in figure 6. figure 6. the change of carbonate minerals in micro fractures before and after acidizing it can be learned from an experiment in the laboratory that, carbonate minerals in natural fractures are dissolved almost thoroughly, the porosity and its connectivity of coal sample increase significantly, and permeability is improved obviously after the acid reaction. 4. field application of acid fracturing engineering 4.1. case of applying acid fracturing treatment from 2011 to 2012, ten coal bed methane wells were stimulated with acid fracturing in qinshui basin ([19]. herein the fracturing treatment of well tsd1 is given as an example. before the operation of tsd1 well, the hydraulic of the abnormally high breakdown pressure. the initial suggestion was to inject hcl for tsd1 well to decrease the breakdown pressure of 3# coal seam, whose measured depth was from 1031.1mracturing stimulation of two nearby wells, tsd5, and tdd3 had been proved as a failure because of o to 1037.1m. the operation was carried out on november 22, 2012. the total volume of proppant laden fluid was 452m3, and the pump rate was 6.5m3/min. before the proppant laden stage, 50 m3 volume of hcl acid was performed as a pad to not only clear the debris of perforation but also remove the carbonate cement of the natural fracture net. the operation curve is shown in figure 7. c5the pilot apprisal of acid fracturing of coalbed methane reservoir in southeast qinshui basin, china figure 7. fracturing curve of well tsd1 the operation of ts06 cbm well was compared, which was stimulated with guar based polymers, to show the advantage of acid fracturing. the target zone of ts06 well was also 3# coal seam, and the perforation interval depth was 691.6m-698.2m. instead of the acid pad before the proppant-laden stage, 2% kcl water was pumped as a pad to break down the pay zone for the ts06 cbm well. figure 8. fracturing curve of well ts06 it can be learned from the comparison of the curves from both well tsd1 and well ts06 that, since the hcl acid was applied in the pad stage of fracturing for tsd1 well, the peak pressure in proppantladen stage was 1.5 mpa lower than that in the pad stage, which is the evidence that the hcl acid must have effectively resolved the fillings in natural fracture system. while in the proppant-laden stage of well ts06, the peak fracturing pressure was almost 2mpa higher than that in pad stage, illustrating that the permeability of natural fracture system in well ts06 has not been improved effectively with kcl pad. 4.2. acid fracturing monitoring analysis to monitor the acid fracturing situation of tsd1 well, pinnacle company was authorized to conduct micro-seismic monitoring during the fracturing operation. ten three-component geophones, combined with the optical fiber transmission technique and mathematical analysis, were installed in monitoring well ts86 which is about 300 meters away as shown in figure 9 to measure the position and length of the fracture. figure 9. location map of ts86 and tsd1 well after the acid fracturing pad, it is shown in figure 10 that the injected acid has filtrated from the hydraulic fracture to the natural cleat net, the stimulated area being approximately 140m wide and 150m long. figure 10. microseismic monitoring curve at the end of the pad stage the variation of downhole microseismic events with time is showed in the figure11. the bluepoints labeled as the early stage is representing the srv after the acid pad stage, and red points labeled as the late stage is representing the srv after the proppant laden stage. it is also learned that the extension of stimulated reservoir volume has almost stopped after the acid pad stage, which is showed as figure 11. the total srv depends on the scale of the early acid pad operation. figure 11. downhole microseismic events of well tsd1 in the early stage and the final stage 5. conclusions 1. fillings in 3# coal seam, or the primary production zone of qinshui basin, is predominated by carbonate minerals with clay minerals. through lab tests, it is learned that the carbonate minerals in the center of natural cleat system of coal seam can explain the feasibility of acid fracturing stimulation. if the filling in natural cleat net is clay or quartz with less carbonate, the acid will not do any good to the fracturing stimulation and should be avoided. c6 yu yang, chengwei zhang, huijun tian, wangang chen, xiaodong peng, hao zhang 2. the hydraulic fracturing can create a high conductivity fracture initiated from the wellbore, but the productivity of well would be quite low if the conductivity of nature fracture has not been increased. acid fracturing pad in cbm reservoir can increase the permeability of natural fracture net with carbonate filling, and improve the production. 3. it is learned from the microseismic survey that when fracturing the cbm well, the stimulated reservoir volume is dependent on the applied volume of the acid pad, and it is suggested that larger acid fracturing pad is crucial to the success of the whole operation. 4. massive acid fracturing treatment may contribute to some kinds of contamination in the reservoir, changing the properties of the formation. thus a series of tests as illustrated herein should be designed before onsite treatment to avoid these damages. it is verified that careful preparation can ensure the success application of large volume of the acid pad in stimulation treatment. acknowledgements this paper is supported by the national science and technology major project of china (2011zx05014-004-004hx). references adil, f., sharma, a., & bhat, s. (2012). hydraulic fracturing of cbm wells in india using a unique fracturing-service technologyoperational and technological lessons. society of petroleum engineers. doi: http://dx.doi.org/10.2118/153144-ms al-jubori, a., johnston, s., boyer, c., lambert, s. w., bustos, o. a., pashin, j. c., & wray, a. (2009). coalbed methane: clean energy for the world. oilfield review, 21(2): 4-13. chen, z. h., wang, y. b., yang, j. s., wang, x., chen, y., & zhao, q. (2009). influencing factors on coal-bed methane production of single well: a case of fanzhuang block in the south part of qinshui basin. acta petrolei sinica, 30(3), 409-416. cramer, d.d. 2008. stimulating unconventional reservoirs: lessons learned, successful practices, areas for improvement. society of petroleum engineers. doi: http://dx.doi.org/10.2118/114172-ms dabbous, m. k., reznik, a. a., taber, j. j., & fulton, p. f. (1974). the permeability of coal to gas and water. society of petroleum engineers journal, 14(6), 563-572. doi: http://dx.doi.org/10.2118/4711-a hill, a. d., & schechter, r. s. (2000). fundamentals of acid stimulation. in economides, m. j. & nolte, k. g. (eds), reservoir simulation, third. edition. john wiley & sons ltd, england. hsi, c. d. (1984). evaluation of clay control additives for matrix acidizing operations. paper presented at spe annual technical conference and exhibition, houston, texas, 16-19 september. doi: 10.2118/13086-ms huang, x. h., wang c. m., wang, t. z., & zhang, z. (2015). quantification of geological strength index based on discontinuity volume desity of rock masses. international journal of heat and technology, 4(33): 255-261. doi: 10.18280/ijht.330434 liu, h. l., kang, y. s., wang, f., & deng, z. (2008). coal cleat system characteristics and formation mechanisms in the qinshui basin. acta geologica sinica, 82(10): 1377-1381. ma, z. w., & chen, c. (2015). research on the overlapping rights of coalbed methane in china. environmental and earth sciences research journal, 2(1): 21-26. doi: http://dx.doi.org/10.18280/eesrj.020105 mccabe, m. a., robert, l. m., blauch, m. e., terracina, j. m., lehman, l. v., & bowles, b. (1999). investigation of a new fracturing fluid and conductivity enhancement technology on coalbed methane production. society of petroleum engineers. doi: http:// dx.doi.org/10.2118/52193-ms perex, d, huidobro, e., & avendano, j. (1998). applications of acid fracturing technique to improve gas production in naturally fractured carbonate formations, veracruz field, mexico. society of petroleum engineers. doi: http://dx.doi.org/10.2118/47820-ms pooniwala, s.a. (2012). stimulation unlocks coalbed methane: lessons learned in india. society of petroleum engineers. doi: http:// dx.doi.org/10.2118/149872-ms puri, r., king, g. e., & palmer, i. d., (1991). damage to coal permeability during hydraulic fracturing. society of petroleum engineers. doi: http://dx.doi.org/10.2118/21813-ms rogers, r. e., ramurthy, k., rodvelt, g., & mullen, m. (2007). coalbed methane: principles and practices. oktibbeha publishing company. schein, g. w., & mack, d. j. (2007). unconventional gas. in: economides, m. j. & martin, t. (eds), modern fracturing: enhancing natural gas production. energy tribune publishing inc., houston, 387-398. tamayo, h. c., lee, k. j., & taylor, r. s. (2007). enhanced aqueous fracturing fluid recovery from tight gas formations: foamed co2 pre-pad fracturing fluid and more effective surfactant systems. journal of canadian petroleum technology, 47(10). doi: http://dx.doi.org/10.2118/08-10-33 wang, m. s., tang, d. z., wei, y. p., xu, w., & leng, x. (2006). reservoir characteristics and enrichment mechanism of the coal-bed gas in the north of qinshui basin. petroleum geology & experiment, 28(5), 440-444. wei, c., qin, y., wang, g. g. x., fu, x., bo, j., & zhang, z. (2007). simulation study on evolution of coalbed methane reservoir in qinshui basin, china. international journal of coal geology, 72(1): 53-69. doi: http://dx.doi.org/10.1016/j.coal.2006.12.001 medium grade metapelites of the silgará formation at the santander massif (colombian andes) have been affected by a medium-pressure/high-temperature barrovian type of metamorphism, developing a sequence of metamorphic zones (biotite, garnet, staurolite and sillimanite). these rocks record a complex tectonometamorphic evolution and reaction history. metapelitic rocks from the staurolite zone are typically foliated, mediumto coarse-grained, pelitic to semipelitic schists that contain the mineral assemblage biotite + garnet + staurolite ± kyanite; all contain muscovite + quartz + plagioclase with minor k-feldspar, tourmaline, apatite, zircon, epidote, calcite, and fe–ti oxides. field and microscopic evidences reveal that fe-rich staurolite in pelitic schists is involved in several chemical reactions, which explains its formation and transformation to other minerals, which are very important to elucidate the reaction history of the silgará formation metapelites. medium grade metapelites of the silgará formation en el macizo de santander (andes colombianos) han sido afectadas por un metamorfismo de tipo barroviense, el cual se ha producido en condiciones de media presión y alta temperatura, desarrollando una secuencia de zonas metamórficas (biotita, granate, estaurolita y silimanita). estas rocas registran una evolución tectono-metamórfico e historia reacción compleja. las metapelitas de la zona de la estaurolita están representadas por esquistos pelíticos a semipelíticos de grano medio a grueso típicamente foliados que contienen la paragénesis mineral biotita + granate + estaurolita ± cianita; todos contienen moscovita + cuarzo + plagioclasa con menor feldespato potásico, turmalina, apatito, zircón, epidota, calcita, y óxidos de fe-ti. evidencias de campo y microscópicas revelan que la estaurolita rica en fe en esquistos pelíticos se involucró en varias reacciones químicas a partir de las cuales esta se formó o transformó a otra fase mineral, lo cual es muy importante para dilucidar la historia de reacción de los metapelitas de la formación silgará. abstract resumen keywords: staurolite, pelitic schists, silgará formation, metamorphism, santander massif. palabras clave: estaurolita, esquistos pelíticos, formación silgará, metamorfismo, macizo de santander. petrologic significance of fe-rich staurolite in pelitic schists of the silgará formation, santander massif record manuscript received: 13/11/2013 accepted for publication: 15/10/2015 how to cite item ríos, c.a., & castellanos, o.m. (2016). petrologic significance of fe-rich staurolite in pelitic schists of the silgar formation, santander massif. earth sciences research journal, 20(1), c1-c7. doi: http://dx.doi.org/10.15446/esrj.v20n1.40847 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.40847 carlos alberto ríos r.1*, oscar mauricio castellanos a.2 1*. escuela de geología, universidad industrial de santander, bucaramanga, colombia, e-mail: carios@uis.edu.co 2. programa de geología, universidad de pamplona, colombia significado petrológico de estaurolita rica en fe en esquistos pelíticos de la formación silgará, macizo de santander pe t r o l o g y earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): c1 c7 c2 carlos alberto ríos r. and oscar mauricio castellanos a 1. introduction the metamorphic history of the santander massif is well documented by ríos and co-workers (e.g., ríos et al., 2003; garcía et al., 2005; ríos et al., 2005; castellanos et al., 2008), and only a brief overview of events is presented here. two distinct geological domains constitute the crystalline basement of the santander massif (castellanos et al., 2008): (1) the deformed and metamorphosed rocks of the precambrian bucaramanga gneiss complex, the lower paleozoic silgará formation and the ordovician orthogneiss and (2) a triassic-jurassic post-orogenic magmatism with calc-alkaline affinity which developed after the paleozoic syn-orogenic magmatism with alkaline affinity (e.g., goldsmith et al., 1971; banks et al., 1985; dörr et al., 1995; ordoñez, 2003). the metamorphic rocks of the silgará formation in the santander massif (figure 1) were metamorphosed to upper amphibolites facies during the caledonian orogeny, developing a sequence of metamorphic zones (biotite, garnet, staurolite and sillimanite) that defines the regional thermal structure. the silgará formation consists of predominantly metapelitic to semipelitic rocks with interlayered mafic rocks on scales ranging from a few centimeters to several meters. these rocks have been intruded by granitoids and gabbros during the triassic-jurassic (goldsmith et al., 1971). however, the occurrence of a thermal event superimposed to the regional metamorphism should be considered, which is evidenced by the emplacement of granitoids associated to a late magmatism of the silurian fammatinian minor orogenic event proposed by mantilla et al. (2015). metamorphic rocks of the silgará formation show evidence of a complex metamorphic history characterized by crustal thickening during heating and a regional retrograde metamorphism after the peak metamorphic temperature (ríos et al., 2003). the rocks of interest in this study correspond to staurolite-bearing metapelitic rocks, which form part of the barrow’s staurolite zone, and record conditions of 590–612 oc and 6.6–7.5 kbar (ríos et al., 2003). garnet can grow from a variety of reactions over a large range of p-t conditions and so the recognition of multiple phases of growth in rocks of different compositions is well documented (e.g., bell and welch, 2002). in contrast, staurolite forming reactions typically have a lower variance than garnet forming reactions (sanislav and shah, 2010). multiple steps of staurolite growth in metapelitic rocks of the silgará formation with little or no change in bulk composition can be recognized, which, according to sanislav and shah (2010) has considerable implications for the common assumption that mineral reactions are controlled by the whole rock bulk chemistry. they demonstrate that mineral reactions are controlled by sites of strain localization rather than whole rock reaction. in this work we document the occurrence of fe-rich staurolite in pelitic schists of the silgará formation (santander massif). from the staurolite’s chemistry, in combination with textural observations, phase equilibria, and evidence for early crystallization and later destruction of ordinary staurolite, we infer that this metamorphic unit has undergone a polymetamorphic history. figure 1. generalized geological map of the santander massif modified after ward et al. (1970, 1973). open stars indicate the location of staurolite-bearing metapelitic rocks. 2. field sampling and analytical methods a research team from the university industrial de santander was involved in reconnaissance fieldwork on the santander massif. fieldwork was primarily focused on localities with staurolite in pelitic schists from the silgará formation as previously reported (ríos et al., 2003; garcía et al., 2005) and the sampling strategy focused on taking staurolitebearing samples from several outcrops. detailed petrographic studies of thin sections were carried out on all specimens using a trinocular nikon (labophot2-pol) transmitted light microscope of the school of geology, university industrial de santander. mineral abbreviations are after kretz (1983). electron microprobe analyses were performed using a jeol jxa 8800m electron probe microanalyzer at the research center for coastal lagoon environments, shimane university (japan), under the following analytical conditions: accelerating voltage 15 kv and probe current 2.5 x 10-8 å. data acquisition and reduction were carried out using zaf correction procedures. natural and synthetic minerals were used as standards. mineral compositions were determined by multiple spot analyses. 3. field occurrence the metamorphic rocks of the silgará formation display a welldeveloped schistosity and has been divided by ríos et al. (2003) into dominantly metapelitic (to semipelitic) and with minor intercalations of metamafic rocks (figure 2). the lithology changes northeastward from quartzrich pelitic schists (southwest) to feldspar-rich semipelitic schists (northeast). the staurolite zone can easily be recognized because it forms a wide zone and because staurolite only grows within the most pelitic horizons of the silgará formation. pelitic schists from this metamorphic zone have a lustrous silvery aspect and are coarser grained than those from the garnet zone. staurolite forms honey-brown large prismatic porphyroblasts which quite frequently reach up to 1.2 cm across and 12 cm in length, elongated along the schistosity and usually displaying reaction rims. it is usually abundant along with garnet, particularly in the transition between the garnet and staurolite metamorphic zones. figure 2. field and hand-specimen photographs of staurolite-bearing metapelitic rocks of the silgará formation at the santander massif. (a) outcrop of metapelitic rocks interlayered with quartzites. (b) pelitic schist displaying a tabular geometry and schistosity. (c) close-up of an outcrop showing the occurrence of a staurolite-bearing pelitic schist. (d)-(i) several examples of the occurrence of garnet-staurolite mica schists; the luster is from large muscovite grains; the equidimensional porphyroblasts are garnet; untwinned staurolite porphyroblasts are larger and darker than garnet. the orange-reddish color of muscovite is due to fe-oxide/hydroxide staining. c3petrologic significance of fe-rich staurolite in pelitic schists of the silgará formation, santander massif 4. petrography staurolite-bearing pelitic schists are typically foliated, medium to coarse-grained, that contain graphite and fe-ti oxides. in general, they are represented by the mineral assemblage muscovite + biotite + quartz + plagioclase + garnet + staurolite ± kyanite. they also contain minor k-feldspar, tourmaline, apatite, zircon, epidote, calcite, and fe–ti oxides). they display a well-developed schistosity defined by the preferred orientation of muscovite and biotite flakes and crystals of graphite and fe-ti oxides. biotite is not only present as small flakes underlying the planar structure but also as lozenge-shaped crystals which represent early porphyroblasts and their asymmetry is sometimes useful to determine sense of shear. a last generation of biotite appears as undeformed crystals with their cleavage randomly oriented to the schistosity and can be interpreted as due to a postkinematic thermal event. quartz and plagioclase form domains between the mica-domains. garnet, usually equigranular, occurs as euhedral to subhedral large porphyroblasts (up to 1.2 cm diameter), with cores frequently rich in minute inclusions of quartz, plagioclase, graphite and feti oxides, and their rims are poorer in inclusions. most of the garnets do however contain inclusion trails of quartz and opaque minerals. the inclusion trails either form straight lines or sigmoidal structures within the garnet poikiloblasts. the garnets with straight inclusion trails can be restricted to the upper structural part of the staurolite zone, whereas the garnets with sigmoidal inclusion trails appear in the lower structural part of the staurolite zone. depending on their structural position, the garnets are thus either pre-, syn or post-tectonic with respect to d3 (s3). pre-tectonic garnet develops shattered and partially chloritized porphyroblasts, with muscovite and ilmenite parallel to the rock’s dominant foliation (se) wrapping around garnet and mica and quartz occurring in strain shadows. garnet sometimes occurs as elongated grains parallel to se. inclusion patterns in syn-tectonic garnet define an internal foliation (si), usually discordant to se, and sometimes developing a pattern of sigmoidal inclusions. post-tectonic garnet has overgrown si or se, with quartz and ilmenite as the main inclusions. another characteristic texture is large web-like garnet crystals that contain so many quartz inclusions that the quartz predominates. staurolite commonly forms euhedral poikiloblastic porphyroblasts with elongated prismatic shapes of various aspect ratios (1.2 cm across), which can contain inclusion trails that preserve earlier fabrics. it grew late after garnet. it displays a moderate preferred orientation of elongate crystals (contain quartz and ilmenite inclusions parallel to se) parallel to se, indicating that it formed during the late regional metamorphism. on the other hand, in the direction of shear, staurolite porphyroblasts with high aspect ratios tend to lie with their long axis parallel to se. straight inclusion trails oriented at various angles with respect to se reveals that staurolite porphyroblasts rotated with respect to rock’s dominant foliation. various angles of the internal foliation (si), represented by straight quartz inclusion trails, with the external foliation (se) indicate that rotation of staurolite porphyroblasts was not uniform and depended on their aspect ratios and their orientation in space with respect to the kinematic frame of the bulk flow prior to the onset of deformation (e.g., johnson, 2009; mezger, 2010). staurolite overgrew micaand quartz-feldspar-domains, which have been preserved as inclusion-poor and inclusion-rich zones, respectively, that give porphyroblasts a striped appearance (mezger, 2010). it also shows sigmoidal patterns of inclusions that indicate syn-kinematic crystallization. however, it can be also oriented across se indicating post-kinematic crystallization. both generations of staurolite are rich in inclusions of quartz, fe-ti oxides, and minor graphite. staurolite is commonly retrogressed to sericite, although chlorite is also a common retrograde alteration product of this mineral. small-scale folding of the schistosity planes has produced a crenulation cleavage, accompanied by segregation of quartz into layers. deformation structures in the mylonitic rocks from the staurolite zone is very interesting as it reveals that these rocks have undergone two major phases of ductile deformation showing opposite sense of shear. the oldest phase of deformation is preserved as inclusion trails within the poikiloblastic garnet or staurolite porphyroblasts, and shear sense criteria for this phase are given by the syn-tectonic sigmoidal garnet or staurolite. from the whole story of polymetamorphism it seems that kyanite belongs to the early events with deformation and andalusite and sillimanite (fibrolite) to the late (contact) metamorphism. these rocks frequently contain evidence of retrograde metamorphism, involving hydration processes, including partial replacement of garnet, biotite and staurolite by chlorite, staurolite by muscovite and plagioclase and staurolite by sericite. figure 3 illustrates the main petrographical aspects of the investigated staurolite-bearing pelitic schists. figure 3. (a)-(b) staurolite in a muscovite-biotite schist, displaying parallel extinction, one cleavage parallel to its length, and characteristic pale to darker golden yellow pleochroism; this example has a twin, highlighted by different color and birefringence caused by its different orientation. (c)-(d) staurolite showing inclusion patterns defining si discordant to se, and inclusion-free rim. (e)-(h) relicts of garnet included in late staurolite porphyroblasts. (i)-(j) occurrence of staurolite in a garnet-staurolite schist containing kyanite, andalusite and sillimanite. (k)-(p) resorption of staurolite by sericite and muscovite, developing reaction rims around staurolite. 5. staurolite crystal chemistry representative analyses of staurolite are given in table 1. staurolite is fe rich (xfe = 0.68-0.90), with 9.22-14.97 wt.% feo and 0.71-2.42 wt.% mgo, and characteristically higher in xfe than coexisting minerals, and contains up to 0.81 wt.% tio2 and up to 3.70 wt.% zno. the zno content in staurolite reveals the highest values (2.37-3.70 wt.% zno) for samples pcm-49 and pcm-361, and the lowest values (0.05 wt.% zno) for samples pcm-701 and pcm-905. c4 carlos alberto ríos r. and oscar mauricio castellanos a table 1. representative chemical compositions of staurolite-bearing pelitic rocks of the silgará formation at the santander massif. in general, staurolite shows slight compositional variation, which can be related to coexisting minerals, becoming more fe rich toward the rim of individual porphyroblasts, although with no appreciable crystal to crystal variations in chemical composition (figure 4). sample no. pcm-20 pcm-21 pcm-28 pcm-47 pcm-49 pcm-361 pcm-403 pcm-413 pcm-473 pcm-511 pcm-618 pcm-651 pcm-700 pcm-701 pcm-702 pcm-905 pcm-907 pcm-940 pcm-971 weight % sio2 26.95 27.94 26.45 27.86 27.74 27.02 27.22 27.34 27.26 27.20 26.55 27.56 26.44 26.97 28.62 27.31 27.11 26.54 26.68 tio2 0.46 0.46 0.64 0.52 0.40 0.44 0.60 0.64 0.69 0.67 0.52 0.63 0.64 0.70 0.55 0.58 0.52 0.81 0.60 al2o3 54.44 52.81 53.75 53.17 52.77 54.43 54.52 52.10 54.87 55.13 54.48 52.68 53.64 53.78 51.85 52.97 53.33 53.76 53.17 feo* 13.43 12.79 14.03 13.80 9.22 11.86 13.13 13.84 12.49 11.89 13.19 14.50 13.94 14.76 14.97 13.79 13.84 13.16 14.05 mno 0.02 0.50 0.06 0.05 1.24 0.14 0.23 0.00 0.17 0.71 0.01 0.05 0.01 0.03 0.03 0.29 0.41 0.64 0.15 mgo 2.42 2.27 1.87 1.81 2.39 0.94 1.11 2.23 1.00 0.71 1.21 1.55 0.92 1.52 1.45 1.94 1.89 1.54 1.28 cao 0.03 0.00 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.02 0.00 0.01 0.00 na2o 0.02 0.00 0.03 0.02 0.14 0.11 0.01 0.05 0.05 0.05 0.08 0.01 0.01 0.00 0.00 0.03 0.00 0.07 0.07 k2o 0.01 0.03 0.03 0.03 0.03 0.07 0.00 0.03 0.02 0.02 0.04 0.02 0.01 0.06 0.03 0.03 0.03 0.06 0.04 zno 0.08 0.61 0.56 0.15 3.70 2.37 1.10 0.71 1.70 0.85 1.34 0.50 0.62 0.05 0.12 0.05 0.30 1.16 1.36 cr2o3 0.01 0.04 0.04 0.04 0.03 0.01 0.02 0.00 0.02 0.01 0.04 0.08 0.04 0.05 0.05 0.06 0.04 0.05 0.00 total 97.87 97.44 97.46 97.44 97.67 97.40 97.93 96.94 98.28 97.24 97.47 97.58 96.27 97.92 97.67 97.07 97.47 97.81 97.39 numbers of ions on the basis of 23 oxygens si 3.738 3.896 3.710 3.884 3.876 3.783 3.786 3.852 3.777 3.787 3.720 3.862 3.748 3.763 3.999 3.831 3.795 3.717 3.761 aliv 0.262 0.104 0.290 0.116 0.124 0.217 0.214 0.148 0.223 0.213 0.280 0.138 0.252 0.237 0.001 0.169 0.205 0.283 0.239 sum t 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 alvi 8.638 8.575 8.596 8.620 8.565 8.763 8.721 8.504 8.739 8.835 8.715 8.561 8.711 8.607 8.538 8.587 8.593 8.591 8.596 cr 0.001 0.005 0.004 0.005 0.003 0.001 0.002 0.000 0.002 0.002 0.004 0.009 0.005 0.006 0.005 0.006 0.005 0.006 0.000 ti 0.048 0.048 0.068 0.054 0.042 0.046 0.063 0.068 0.072 0.070 0.055 0.066 0.069 0.073 0.057 0.061 0.055 0.086 0.064 fe2+ 1.558 1.491 1.646 1.609 1.077 1.388 1.526 1.630 1.447 1.385 1.545 1.698 1.653 1.722 1.749 1.617 1.620 1.542 1.657 mn 0.002 0.059 0.007 0.006 0.146 0.017 0.027 0.000 0.020 0.084 0.001 0.005 0.001 0.004 0.003 0.035 0.048 0.076 0.017 zn 0.008 0.062 0.058 0.015 0.382 0.245 0.113 0.073 0.174 0.088 0.139 0.051 0.065 0.005 0.012 0.005 0.031 0.120 0.141 mg 0.500 0.471 0.391 0.377 0.498 0.195 0.230 0.468 0.207 0.147 0.253 0.325 0.194 0.316 0.301 0.406 0.394 0.322 0.268 sum y 10.756 10.712 10.770 10.687 10.712 10.656 10.683 10.744 10.661 10.609 10.712 10.715 10.698 10.733 10.666 10.718 10.746 10.741 10.744 ca 0.004 0.000 0.000 0.000 0.003 0.003 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.003 0.003 0.000 0.001 0.000 na 0.005 0.000 0.009 0.005 0.039 0.030 0.001 0.014 0.014 0.014 0.021 0.002 0.003 0.000 0.000 0.008 0.001 0.019 0.019 k 0.002 0.005 0.006 0.005 0.005 0.012 0.000 0.006 0.004 0.004 0.007 0.004 0.001 0.011 0.005 0.005 0.005 0.011 0.007 sum x 0.012 0.005 0.015 0.009 0.047 0.044 0.001 0.019 0.018 0.018 0.028 0.006 0.004 0.011 0.008 0.015 0.006 0.031 0.027 total 14.767 14.716 14.784 14.696 14.759 14.701 14.684 14.764 14.679 14.627 14.740 14.721 14.702 14.744 14.674 14.733 14.752 14.772 14.771 *total fe as feo xfe 0.76 0.76 0.81 0.81 0.68 0.88 0.87 0.78 0.87 0.90 0.86 0.84 0.89 0.84 0.85 0.80 0.80 0.83 0.86 figure 4. textural features of skeletal (above) and twinned (below) staurolite and the mg and zn color maps of the skeletal staurolite and mg and fe color maps of the twinned staurolite. an idealized formulae of staurolite, such as (fe2+,mg,zn) vi 2(al,fe3+,ti)vi 9o6[(si,al)o4]4(o,oh)4 does not consider the complex crystal chemistry of this metamorphic mineral (smith, 1968). griffen and ribbe (1973) inferred from a statistical study of staurolite analyses that fe, mg and al are each distributed over both the tetrahedral fe and the octahedral al sites, with fe preferring the tetrahedral occupancy and mg and al the octahedral occupancy. therefore, fe and mg are not related by the simple onefor-one substitution scheme implied by the formulae above. staurolite analyses were normalized on a 46-oxygen basis, assuming 4 h per formula unit. on this basis the total of mg+fe cations is unusually high, up to 4.197, which is lower than results reported in previous studies (e.g., gritren and ribbe, 1973; ward, 1984). cation totals in the range 29.348-29.569 are similar to those obtained by gritren and ribbe (1973) and lower than unusually high cation totals reported by ward (1984). figure 5 illustrates plots of mg+fe vs. al, and mg+fe vs. al′, where al′ is al+si-8 (that is, al cations per unit cell after subtracting al to fill up the tetrahedral si site to 8). different to what is suggested by ward (1984), there is no strong correlation after this adjustment, and therefore we cannot assume that al fills up the si site to essentially full occupancy (griffen et al., 1982). the extent of the adjustment is considerable because of the variable and low si content. the high si content (7.420-7.997) compared with values (7.374-7.538) reported by ward (1984) reveals a high silica activity of the rock, which is manifested by the occurrence of abundant quartz. there is no good correlation of mg+fe vs. al′ (with considerable variation of mg), and, therefore, we cannot support the idea of an important (mg,fe) = al substitution in the octahedral sites as revealed by gritren and ribbe (1973). staurolite is the only common silicate phase, which can incorporate significant amounts of zn in rocks of the amphibolite facies. zn substitutes for tetrahedrally coordinated divalent fe (deer et al., 1982), and partitions strongly into staurolite due to the lack of other suitable cation sites. at higher metamorphic grade, as staurolite begins to react out, zn is increasingly concentrated in the remaining staurolite. figure 5. mg+fe vs. al (left) and mg+fe vs. al′ (right) plots for staurolite from pelitic rocks. least squares regression lines are shown. 6. petrologic significance metapelitic rocks containing staurolite commonly occur in medium-p regional metamorphism in the staurolite zone of the amphibolite facies, where staurolite is associated with almandine, muscovite, quartz and kyanite. however, staurolite-pelitic schists are not always encountered even though the p-t conditions lie within the stability field of staurolite, which led several petrologists to suggest that the bulk chemical composition plays an important role in the formation of staurolite in pelitic schists that are rich in al2o3 and feo and poor in k2o (e.g., turner, 1968). in the rocks of interest of this study, staurolite mainly occurs in highly aluminous bulk composition layers, showing strong alteration, and numerous ilmenite or quartz inclusions. c5petrologic significance of fe-rich staurolite in pelitic schists of the silgará formation, santander massif on the other hand, many petrologists would attribute the rare development of staurolite as compared with garnet, biotite and kyanite to a more restricted p-t range (deer et al., 1992) or a limited range of oxygen fugacity (ganguli, 1968). the stability field of staurolite is restricted roughly to medium p-t conditions commonly associated with regional metamorphism resulting from crustal thickening during earlier orogenic stages (mezger, 2010). although staurolite-bearing rocks have a general composition field in the akf diagram, the various mineral assemblages are better represented in afm diagrams in which molecular proportions of feo and mgo are considered as separate components (lal and shukla, 1970). to calculate the plotting parameters for the afm diagram, we assume a = [al2o3-3k2o], f = [feo], and m = [mgo]. using these parameters, it is possible to grid off the afm diagram with the vertical scale represented by the normalized values for the a parameter as [al2o3-3 k2o]/[al2o33k2o+feo+mgo] and the horizontal position based on the ratio of mgo/ (feo+mgo). these values were obtained after converting the bulk rock chemistry data to molecular proportions (figure 6). figure 6. bulk rock chemistry of staurolite-bearing rocks plotted on a afm projection plane as black triangles. adapted and modified after http://www.tulane.edu/~sanelson/eens212/triangular_plots_metamophic_petrology.htm the staurolite-bearing metapelitic rocks of the silgará formation, which contain garnet, fall close to the three-phase field of biotite-garnetstaurolite (+quartz+muscovite) and have only a small range of feo/mgo ratios. plagioclase is ignored by the diagram (cao is not plotted), but we can resolve biotite and garnet because they clearly have different compositions in the afm plot. because the garnet structure readily takes in ca, and especially mn, almandine garnet is stabilized beyond the range of conditions of bulk composition represented in the diagram. it is thus possible that the presence of sufficient mn stabilizes garnet as a very common phase in the assemblage biotitestaurolite-al2sio5, and then the occurrence of garnet in the staurolitebearing pelites may be related to high mn content of the rock. in the afm diagram, the analyses of these rocks show that they have a high al2o3/ (feo+mgo) ratio (between 0.2 and 0.8). further, they show a relatively variable feo/mgo ratio, although they are restricted to low feo/mgo ratio (between 0.0 and 0.4). therefore, the bulk composition field of these rocks migrates towards the lower feo/mgo side in the afm diagram. 7. discussion on staurolite-forming and -breakdown reactions (figure 7) mineral chemistry and textural evidence shows that the fe-rich staurolite in the metapelitic rocks of the silgará formation are involved in producing and consuming reactions. as follows, we discuss several reactions: according to bucher and frey (1994), staurolite forms under greenschist facies conditions by continuous reactions such as: chlorite + muscovite = staurolite + biotite + quartz + h2o and discontinuous reactions such as: chlorite + muscovite + garnet = staurolite + biotite + quartz + h2o however, spear (1993) does not include quartz in the reaction products and, therefore, the growth of staurolite was accompanied by decomposition of garnet and continued decomposition of chlorite probably occurred by the reaction: garnet + chlorite + muscovite = staurolite + biotite + h2o ríos and takasu (1999) consider that at the staurolite isograd, xgrs increases with increasing temperature, which is difficult to determine taking into account that garnet tends to be resorbed back to higher xgrs concentrations as staurolite grows. however, according to ríos et al. (2003) after the maximum pressure conditions (ppeak), pressure does not remain constant, according to the chemical zoning of xgrs in garnet from the staurolite zone, which suggests a ppeak of metamorphism during garnet growth as suggested by ríos and takasu (1999). the diagnostic mineral assemblage for the staurolite metamorphic zone is staurolite + garnet + biotite + muscovite + chlorite + plagioclase + quartz. staurolite growth is thus either synor post-tectonic with respect to the tectonic transport of the silgará formation metapelitic rocks. garnet is sometimes found as inclusions within the staurolite poikiloblasts, which indicates that the rock from the staurolite zone first crossed a garnet-in reaction. with increasing temperature, staurolite becomes unstable and reacts to produce garnet, biotite and kyanite by the following reaction staurolite + muscovite + quartz = garnet + biotite + al2sio5 (kyanite) + h2o however, this reaction, which promotes the disappearance of staurolite also coincides with the appearance of kyanite (at pressure greater than about 2.5 kbar), and if we were to encounter this reaction in the field, it could be used to define the kyanite isograd. the occurrence of andalusite and sillimanite in these rocks can be associated to a superimposed low-p/high-t thermal event, although the presence of these polymorphs implies pressure below the al2sio5 triple point, itself placed at 3.8 kbar (holdaway, 1971). kyanite can be partly replaced by andalusite following the reaction al2sio5 (kyanite) = al2sio5 (andalusite) according to yardley (1989), staurolite can be consumed by reactions, which generate sillimanite, such as staurolite + muscovite + quartz = al2sio5 (sillimanite) + biotite + h2o and staurolite + muscovite + quartz = al2sio5 (sillimanite) + garnet + biotite + h2o according to spear (1993), the staurolite-out reaction occurs near the muscovite melting reaction: staurolite = al2sio5 (sillimanite) + garnet + biotite and, in this case, muscovite and quartz are not considered as reactants. this reaction would explain the highly resorbed staurolite. the occurrence of sillimanite and biotite rimming garnet suggest the dissolution of garnet by the continuous reaction: garnet + kfeldspar + melt = biotite + al2sio5 (sillimanite) (spear et al.,1999) this reaction also helps to explain the absence of k-feldspar. from these constraints, it is proposed that pelitic schists reached peak p-t conditions of 675-850 oc and 4-8 kbar. in the absence of muscovite, yardley (1989) considers the following reaction staurolite + quartz = al2sio5 (sillimanite) + garnet + h2o the reactions involving muscovite take place at a lower grade than the staurolite + quartz reaction (bucher and frey, 1994), so staurolite can persist to higher grades in muscovite-free assemblages, such as those in the silgará formation pelitic schists. within the staurolite-bearing rocks of the santander massif, domains with abundant sillimanite have very little staurolite, with the relics that remain containing less zn than in the sillimanite-free domains. reactions such as those above have presumably consumed the znpoor staurolite and generated sillimanite. at this respect, we consider that the formation of sillimanite can be probably associated to a thermal event which has been recently registered the authors in several parts of the santander massif due to the emplacement of intrusive rocks. according to foster and dutrow (2010), c6 carlos alberto ríos r. and oscar mauricio castellanos a6 the nucleation of staurolite is controlled by two fundamental reactions: the stable reaction garnet + chlorite + muscovite = staurolite + biotite in regions that have equilibrated with garnet, and the metastable reaction chlorite + muscovite = staurolite + biotite in regions that have not equilibrated with garnet, and at 4 kbar, this reaction is overstepped first about 10 oc below the stable reaction for a typical pelite composition, suggesting the metastable reaction may be an important control on staurolite nucleation and growth in these environments. in this study, we have observed textural features involving garnet and staurolite that can support this hypothesis. staurolite porphyroblasts (with inclusions of quartz and ilmenite) are corroded by fine grained aggregates of muscovite or muscovite + biotite that form thick coronas. the replacements also progress along fractures within staurolite, suggesting the presence of a fluid phase during replacement. foster (1981) determined that muscovite and biotite pseudomorphs after staurolite formed as a result of sillimanite nucleation that sets up chemical potential gradients between the growing sillimanite, pre-existing staurolite, and the rock matrix of biotite, muscovite, plagioclase, quartz, ilmenite, and garnet. staurolite is replaced by aggregates of muscovite. muscovite replacement of staurolite becomes more prominent approaching the sillimanite zone, as the staurolite becomes unstable concurrent with the nucleation and growth of sillimanite (whittington, 2006). however, a much simpler explanation is a retrograde reaction. within these replacements, staurolite appears as fine xenomorphic grains that are often arranged in optical continuity, indicating that they are relicts of larger porphyroblasts. figure 7. reaction textures involving staurolite. (a)-(c) relicts of garnet included in late staurolite porphyroblasts. (d) highly corroded relicts of staurolite in contact with muscovite of the matrix. (e) resorption of staurolite by sericite and muscovite, developing reaction rims around staurolite. (f) highly corroded relict of staurolite in prograde muscovite. (g) several corroded relict of staurolite enclosed by a very fine-grained aggregate of sericite (h) poikiloblastic staurolite porphyroblast with oriented inclusions of quartz and ilmenite. (i) relicts of staurolite in garnet porphyroblast. 8. conclusions fe-rich staurolite in pelitic schists of the silgará formation at the santander massif (colombian andes) represent a very interesting index mineral for the reconstruction of the tectono-metamorphic evalution of this geological unit. field occurrence and textural relations of staurolite with other mineral phases, as well as microstructural evidence observed in the investigated metapelites, reveal that these rocks underwent (1) prograde metamorphism during a complex polyphase deformation suggested by several foliations, as well as inclusion trails in staurolite porphyroblasts and (2) extensive retrograde metamorphism after the ppeak. with increasing metamorphic conditions, the protoliths of the staurolite-bearing metapelites were tectonically transported and buried to its maximum depth at the ppeak, (in the staurolite zone) which is attributed to crustal shortening and thickening accompanied by deformation, which produce, respectively, a slaty cleavage si and a crenulation cleavage se associated with thrusting. mineral chemistry along with textural relations was very useful to determine the staurolite-forming reactions in the silgará formation metapelites. initially, staurolite (+ biotite + h2o) formed as a product of the breakdown of the assemblage garnet + chlorite + muscovite. after the ppeak, these rocks experienced decompression, new mineral growth and movement strain-slip cleavages during a late deformation. in this study, we consider that the occurrence of andalusite and sillimanite (fibrolitic variety) as late mineral phases can be attributed to a thermal event superimposed to the regional metamorphism. then, metapelites were affected by cooling accompanied by retrograde metamorphism as suggested by several rehydration reactions that often do not terminate and preserved relic mineral assemblages from the least hydrated states. the retrograde reactions may be associated to the infiltration of h2o-rich fluids circulated along planes of discontinuity in metapelites of the silgará formation. staurolite probably was consumed by reactions which generate al2sio5 (+biotite ± garnet + h2o), which include as reactants staurolite + muscovite + quartz. acknowledgments the authors are indebted to universidad industrial de santander and instituto colombiano para el desarrollo de la ciencia y la tecnología “francisco josé de caldas” colciencias through grant no. 1102-05-083-95 financially supported fieldwork (research project: “metamorphism and associated metallogeny of the santander massif, eastern cordillera, colombian andes”). we thank to department of geoscience of shimane university. the authors are most grateful to the above-named people and institutions for support. references banks, p., vargas, r., rodríguez, g.i., and shagam, r. 1985. zircon u-pb ages from orthogneiss, pamplona, colombia. vi congreso latinoamericano de geología, bogotá. resúmenes. bell, t.h., and welch, p.w., 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the silgará formation, santander massif university, matsue (japan), 150pp. ríos, c., and takasu, a. 1999. chemical zoning of garnet from the lowgrade metamorphic rocks of the silgará formation, santander massif, eastern cordillera (colombian andes). geosciences reports of shimane university, 18: 97-107. ríos, c.a., garcía, c.a., and takasu, a. 2003. tectono-metamorphic evolution of the silgará formation metamorphic rocks in the southwestern santander massif, colombian andes. journal of south american earth sciences, 16: 133-154. ríos, c.a., castellanos, o.m., gómez, s., and avila, g. 2008. petrogenesis of the metacarbonate and related rocks of the silgará formation, central santander massif, colombian andes: an overview of a “reaction calcic exoscarn”. earth sciences research journal (universidad nacional de colombia), 12: 72-106. sanislav, i.v., and shah, a.a. 2010. the problem, significance and implications for metamorphism of 60 million years of multiple phases of staurolite growth. journal of the geological society of india, 76: 384-398. spear, f. 1993. metamorphic phase equilibria and pressure-temperaturetime paths, monograph series, mineralogical society of america, washington, dc, 799pp. spear, f.s., kohn, m.j., and cheney, j.t. 1999. p-t paths from anatectic pelites. contributions to mineralogy and petrology, 134: 17-32. turner, f.j. 1968. metamorphic petrology, mcgraw-hill, new york, 403p. ward, c.m. 1984. magnesium staurolite and green chromian staurolite from fiordland, new zealand. american mineralogist, 69: 531-540. ward, d.e., goldsmith, r., cruz, b.j., jaramillo, c.l., and vargas, l.r., 1970. mapa geológico del cuadrángulo h-13, pamplona, colombia. ingeominas. ward, d.e., goldsmith, r., cruz, b.j., jaramillo, c.l., and restrepo, h., 1973. geología de los cuadrángulos h-12, bucaramanga y h-13, pamplona, departamento de santander. u.s. geological survey e ingeominas. boletín geológico xxi(1-3), 1-132. whittington, j.k. 2006. muscovite pseudomorphs after staurolite as a record of fluid infiltrating during prograde metamorphism. unpublished msc thesis, louisiana state university, baton rouge (usa), 119pp. yardley, b. 1989. an introduction to metamorphic petrology, longman, harlow, 248p. earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 25 32 applying dc resistivity imaging to investigating the relationship between water quality and transpiration beneath circular islands in the okavango delta, botswana: a case study of thata island *e.m. shemang1 l. n. molwalefhe2 and k. mickus3 1 department of earth and environmental sciences, botswana international university of science and technology, p. bag bo 041 gaborone, botswana 2 department of geology, university of botswana, p. bag ub 00704, gaborone, botswana 3 department of geography, geology and planning, missouri state university springfield, mo 65897 missouri usa e-mail: shemange@biust.ac.bw; eshemang@yahoo.co.uk hydrogeology an electrical resistivity survey was carried out on thata island, one of the numerous circular islands in the okavango delta, to investigate the mechanism governing interactions between surface water, vegetation and groundwater. seven data profiles were collected across the island and modelling results indicated that the centre of the island had low resistivity values (less than 10 ohm-m), while resistivity values increased laterally outside the island. such lateral resistivity zoning responded to variation in shallow groundwater chemistry below the islands having a high concentration of solutes inside and the presence of fresh water outside the islands. borehole to surface resistivity imaging results for the island indicated a plume of saline water sinking to depths of 60 m. groundwater salinity below the island ranged from 11.7 g/l from the margin of the island to 122 g/l at the centre of the island. beyond the 60m depth, groundwater salinity dropped to about 0.33 g/l at the centre of the island. lateral and down-hole imaging results, as well as water salinity values, showed a migrating plume of high salinity groundwater from the surface of the island invading a relatively deeper low-density fresh groundwater environment. combining geochemical, isotopic and electrical resistivity data indicated that capillary evaporation may have been the most important mechanism in creating high saline water under the centre of the island; however, high transpiration rates along the edges of the island may have been important in producing salts which could migrate toward the island’s centre. the results of this study showed how high salinity levels may have formed in near surface aquifers in natural inland delta environments in semi-arid areas. este estudio sobre la resistividad eléctrica fue llevado a cabo en thata, una de las numerosas islas circulares en el delta del okavango, para analizar los mecanismos que gobiernan las interacciones entre la superficie acuática, la vegetación y las aguas subterráneas. siete perfiles de datos fueron recolectados y los resultados modelados indicaron que el centro de la isla tiene bajos valores de resistividad (menos de 10 ohm-m), mientras que estos niveles se aumentan lateralmente fuera de la isla. esta zonificación lateral de resistividad responde a la química de las aguas subterráneas poco profundas debajo de la isla que tienen una gran concentración de disoluciones al interior y presencia de agua fresca fuera del territorio. perforaciones para medir los resultados de la resistividad en la isla mostraron una columna de agua salina que se hunde hasta 60 metros de profundidad. la salinidad de las aguas subterráneas en la isla oscila entre 11.7 g/l en el margen y 122 g/l en el centro. debajo de los 60 metros de profundidad, la salinidad de las aguas profundas cae hasta el 0.33 g/l en el centro de la isla. los resultados del sondeo lateral y debajo de las perforaciones, además de los valores de salinidad en el agua, mostraron una columna de alta salinidad en aguas subterráneas que se desplaza desde la superficie de la isla y que invade un ambiente de aguas subterráneas medianamente profundo de baja densidad. la combinación de datos geoquímicos, isotópicos y de resistividad eléctrica key words: seismic resistivity imaging, salinity, circular island, okavango delta. abstract resumen palabras clave: resistividad sísmica, salinidad, isla circular, delta del okavango introduction surface and groundwater salinisation presents a formidable threat to water supply and remains a major problem for the global environment, especially in semi-arid regions. water quality issues in botswana’s okavango delta, specifically high salinity and nutrients and arsenic pollution, have limited water being used for rural water supply. solutes can enter the delta system through several pathways. solutes resulting from dust and ash fires (garstang e.m. shemang l. n. molwalefhe and k. mickus indican que la evaporación capilar podría ser el mecanismo más importante en la creación de la alta salinidad en el centro de la isla; sin embargo, la alta tasa de transpiración en los extremos de la isla podría ser importante en la producción de sal que se desplazaría hacia el centro. los resultados de este estudio muestran cómo los niveles de alta salinidad podrían haberse formado en acuíferos superficiales de ambientes naturales al interior del delta, en áreas semiáridas. record manuscript received: 08/10/2012 accepted for publication: 20/06/2013 figure1: location of and main hydro-morphological features regarding the okavango delta, botswana. also shown are botswana and the okavango delta in africa and southern africa. the kalahari basin is outlined in southern africa inset et al., 1998) and chemical weathering reactions are typically derived from atmospheric deposition in the headwater region in angola. during transit both natural (saline groundwater encroachment, hydrothermal springs and evaporite deposit dissolution) and anthropogenic (agricultural return flow and domestic/industrial effluent waste water) sources may add solutes to the river system. these increases are magnified by evapotranspiration from open water, agriculture involving irrigation or natural riparian areas, thus influencing the chemistry of ground and surface waters in stream ecosystems. nutrient concentration often alters when subsurface water flows through riparian areas. therefore, groundwater-surface water interaction dynamics can affect riparian vegetation ecology (hughes, 1990) and can influence wetland hydrology through transpiration and uptake of groundwater (doss, 1993; sacks et al., 1992; winter and rosenberry, 1995). the sensitive links between water quality, sediment, flora and channel hydraulics have already been established for the okavango delta (mccarthy and ellery, 1994; gieske, 1996, 1997; ellery et al., 2003; wolski and murray-hudson, 2006). a growing population, along with associated activities and shifts in consumption patterns contributing to water quality deterioration, and pressure being exerted on the okavango’s water resource, makes it become necessary to understand the delta’s ecology in terms of solute sources and recycling element and nutrient pathways within the delta and associated environments on a local and basin scale. within this framework lies the answer to sustainable wetland management and maintenance of ecosystem integrity while undertaking economic activities such as the thriving tourism industry, agriculture and drinking water supply without causing dire consequences regarding the needs of the ecosystem, people, and wildlife. the present study was therefore aimed at applying electrical and borehole resistivity to surface electrical resistivity imaging (eri) techniques for providing accurate areal information regarding the subsurface of the thata island area, in terms of subsurface electrical conductivity within thata island and the surrounding area. the study was also aimed at mapping and delineating the lateral and vertical distribution of low electrical resistivity zones which might be linked to areas where high salinity occurs and becomes accumulated, as well as investigating the pattern and associated processes concerning salt accumulation within the island. 1.1 the okavango delta the okavango delta (figure 1) is southern africa’s largest inland wetland (i.e. ~12,000 km2, mccarthy et al., 2003) and is located on the northern fringes of the kalahari desert (figure 1). the delta is part of the okavango river basin (orb), which spans the three riparian states of angola, namibia and botswana. the okavango delta receives the annual arrival of a flood wave from the angolan highlands bringing large volumes of water (gieske, 1996, 1997; wolski and murray-hudson, 2006) with lesser input coming from local rainfall. both rainfall and flood pulse are temporally out of phase with rainfall occurring between november and march while the flood-cycle arrives in april, reaching its peak during august/september. runoff from the catchment in angola, some 600 km to the north, brings an estimated 11 cubic kilometres of water into the okavango delta (16 billion m3/yr of flood and rainfall; gieske, 1996), carrying at least 300,000 tons (bauer, 2004) of dissolved salts every year. within this flood-pulsed wetland with a mean surface gradient of approximately 1:3700, water is normally spread across through a complex system of meandering distributary branches, swamps and floodplains (mccarthy, 1992). the surface water flowing out of the delta has a dissolved solid concentration (tds) of about 90mg/l, only three times more concentrated (30mg/l) than the flows entering the delta (bauer, 2004). the electrical conductivity of the groundwater beneath the centre of the islands may reach 40 ms/cm while that in the floodplain water ranges from 35.9 to 75.2 μs/ cm, this being more than 500 times higher (bauer, 2004). total nitrogen in the surface water is less than 1mg/l while it may be around 5.2mg/l in the groundwater at the island’s centre. total phosphorus concentration in surface water is around 0.05 mg/l and about 22mg/l in the island’s groundwater. the okavango delta has been described as a mosaic of flat broad floodplains and round to shapeless islands (figure 2); such islands vary in shape and size from several square metres to more than 500 km2 (gieske, 1996; wolski and savenije, 2006). irrespective of these morphological characteristics, all islands are usually fringed by a zone of broad leafed evergreen trees (phreatophytic) giving way to deciduous trees and eventually palms and open grassland. such zoning results from a concentration gradient in the groundwater beneath the islands, due to evaporative uptake and reflects faunal species’ differing degrees of salt tolerance (mccarthy and ellery, 1994). figure2: typical islands within the okavango delta showing characteristic hummocky topography along one of the distributary channels figure 3: cross-section through an island in the okavango delta at the interface between permanent and seasonal wetlands, (a) showing the distribution of vegetation, (b) island topography, (c) groundwater electrical conductivity and variation in water table depth over a flood-cycle (mccarthy and ellery, 1994) applying dc resistivity imaging to investigating the relationship between water quality and transpiration beneath circular islands in the okavango delta, botswana: a case study of thata island the depressed interior of the islands consists of bare soil having crusts of trona at the centre, precipitating from island groundwater because of evapotranspiration enrichment (mccarthy et al., 1991; mccarthy and ellery, 1994) and may thus support only high salinity tolerant grasses. the water table below the islands (figure 3) in the delta generally ranges from 0.5 m (gieske, 1996) to almost 4 m below ground surface (wolski and savenije, 2006), depending on the time of the year and floodplain flood levels. water table elevation usually decreases during the summer due to low rainfall and high evapotranspiration and increases rapidly early in the winter in response to the incoming floods. groundwater level fluctuations are larger in the riparian belt (2 m) while being more stable at the centre of an island (0.5 m) where the groundwater table is normally depressed, so that groundwater continues to flow from the floodplains towards the islands throughout the year, i.e., no groundwater gradient reversal occurs during the flood subsidence phase (gieske, 1996; wolski and savenije, 2006). groundwater salinity (determined by electrical conductivity) beneath islands in the downstream swamps is 50-100 times more concentrated than the surface water in the surrounding floodplains (gieske, 1996; bauer et al., 2006; wolski et al., 2006). it also contains up to 400 times more phosphorus and five times more nitrogen inside the islands than in the floodplains, as wolski has shown that nutrients and other inorganic ions accumulate underneath the islands (wolski et al., 2006). the preceding statements show that the existence and occurrence of shallow saline groundwater systems in the okavango delta’s infill sediment provinces will indisputably be controlled by the delta’s riparian structure and the history of flooding. however, more information is needed on this topic and this study was aimed at collecting additional electrical resistivity data regarding thata island (figure 4: aerial photo) and our interpretation was aimed at supplementing bauer et al.,’s electrical resistivity investigation (bauer et al., 2006); they collected a series of 2-d electrical resistivity profiles across islands within the okavango delta, including thata island. they emphasised how near surface saline waters interact with deeper aquifers. 3-d inversion of surface resistivity data indicated that instability (called density fingers) brought the saline water to the deeper fresh water aquifers. thata island is remotely located within one of the okavango delta’s more extensive floodplains. this remote location makes it unaffected by all major anthropogenic sources of salt and nutrients which could interfere with its natural environment. the additional data collected by this study assisted in understanding the dynamic relationships between surface water, groundwater and vegetation regarding discrete salt build-up in a deltaic environment’s water resources and conceptualising aquifer systems. figure 4: thata island, showing the characteristic rim of vegetation around the fringes found in the other islands in the okavango delta 2.0 data acquisition, processing and interpretation the present study used eri for studying surface and subsurface water interactions within islands in the okavango delta. eri (also known as dc resistivity imaging or electrical resistivity tomography (ert)) has become an increasingly popular method over the last decade for mapping subsurface electrical conductivity (griffiths and barker, 1993). it allows imaging groundwater salinity with relatively high spatial resolution, since formation resistivity as determined ert strongly depends on groundwater salinity. ert has been applied to mapping salt tracer movements in the subsurface (white, 1988; bevc and morrison, 1991; kemna et al., 2002), as well as monitoring seawater intrusion (gondwe, 1991; kruse et al., 1998; nowroozi et al., 1999). in addition to the geophysical experiment, groundwater sampling determined salinity and stable oxygen isotope content and depth to the water table. 2.1 groundwater level measurement the depth to the groundwater table was determined through a network of deep boreholes and shallow piezometers located around thata island. piezometer depth varied from 3-73 meters and the deepest borehole was 96 meters. the bottom part of the pipes was slotted, covered with 3 m long filter pack and sealed using bentonite (figure 5). all boreholes were cased in steel, depth depending on the desired aquifer level for monitoring. figure 5: diagram showing the different borehole designs used to discretise sections of the aquifer for groundwater level monitoring on thata island 2.2 electrical resistivity imaging (eri) eri in two-dimensional (2-d) profile mode was used for studying vertical and horizontal variation in electrical conductivity (resistivity) within and outside the thata island area. seven data profiles were collected across the island in an east-west direction. abem lund imaging system equipment was used for the survey as well as an iris syscal r2 instrument having automatically-selected electrodes (dahlin, 1996). the wenner-schlumberger array technique was e.m. shemang l. n. molwalefhe and k. mickus used, current electrode spacing varying between 5 and 7.5 m. the profiles were arranged in such a way that the centre of each spread coincided with the centre of the island (figure 6). one profile was situated outside the island in the swamps. surface-to-borehole eri was also carried out to increase surface imaging resolution using one of the boreholes drilled at the centre of the island. one current-potential pair of electrodes was positioned in the borehole and one pair on the surface for surface-borehole imaging (pure hole-surface configurations). the potential electrode in the hole was kept in a constant position to minimise the number of possible configurations. a radial electrode spread was used, such spread consisting of four profiles crossing each other at the centre of the salt accumulation zone (figure 6) where the borehole was located. each profile was then separated into two sections, incorporating 56 surface electrodes using 2m spacing and 24 borehole electrodes spaced at 4 m. after measurements for each section had been completed, the borehole cable was shifted 1 m and the measurements repeated. after completing two shifts and the second part of the profile measured in the same way, the combined data set was equal to a borehole-to-surface section having 112 surface and 96 borehole electrodes with the borehole in the centre. a typical data set for a profile consisted of around 6,000 single measurements. figure 6: locations of the 2-d resistivity surface and borehole to surface profiles and the location of the monitoring borehole within the island 2.3 inverting 2-d eri data resistivity data were inverted using a 2d smoothness-constrained inversion software (res2dinv version 3.54) using a quasi-newton technique to reduce calculations (loke and barker 1996). the inversion programme is a forward modelling method consisting of a number of rectangular blocks, block arrangement being loosely tied to data distribution in the pseudosection. the forward modelling subroutine calculated apparent resistivity values and a non-linear least-squares optimisation technique was used for the inversion routine (degroot-hedlin and constable 1990; loke and barker 1996). surface-borehole resistivity data were inverted as individual 2d vertical cross-sections. 3.0 results and discussions 3.1 groundwater level measurement groundwater levels beneath thata island varied between 2.79-2.82 m below the ground surface outside the riparian rim (figure 7) and in the central parts of the island. hydraulic pressure in each borehole for the deeper sections of the aquifer was higher than that measured in the shallow piezometers (i.e. 3 m depth). the foregoing may have indicated that the aquifer system had different hydraulic layers at depth and may have been (semi)confined at higher hydraulic pressure. 3.2 groundwater salinity and δ18o groundwater salinity was measured for shallow piezometers and deep boreholes (figure 7). electrical conductivity was also much higher for areas lying directly beneath the centre of the island and was low in areas occupied by the swamp around the island (figure 8). this showed that ions were being accumulated at the island centre and closer to the surface than at depth, because the groundwater became less saline with depth. isotope measurements of groundwater δ18o were spatially out of phase with high salinity areas and showed enriched values of about +0.6 ‰ at the island’s margin directly beneath the vegetation rim, indicating intense transpiration activity in that area (figure 9) compared to -3.17 ‰ in the surrounding swamps. the fact that this area did not coincide exactly with areas of highest salinity indicated that transpiration was not the only process responsible for inorganic salt enrichment in the subsurface. capillary evaporation was also important and was more prominent at the centre of the island; however, isotope results did not reveal which was more important. figure 7: groundwater levels and salinity measurements (g/l) in the boreholes and piezometers around thata island (blue dashed-lines indicate water table; red dashed-lines indicate peizeometric levels from deep borehole. values in blue represent salinity for shallow water table and values in red indicate salinity for deeper sections of the aquifer) 3.3 interpreting 2-d surface and surface-to-borehole resistivity sections 2-d resistivity profile inversion results are shown in figure 10. as seen from the images in figure 10, the central part of the profile consisted of low resistivity materials (about 10 ωm) having increasing resistivity values (up to 150 ωm) towards the edge of the profiles. the low resistivity seen in the central part of the island clearly suggested that the area was more conductive than other areas. moving away from the centre of the island (towards the vegetation), imaged section resistivity values were seen be higher (about 50 ωm). resistivity values increased across the vegetation and into the swamps (up to 150 ωm) being indicative of low ground conductance. low resistivity values for the central part of the island suggested that there may have been conductive fluids (saline water) associated with the fine clay-like materials characterising the centre of the island. the high salinity environment observed on the resistivity images was seen to extend radially into the relatively fresh environment in the surrounding swamps. the saline horizon depth beneath the centre of the island (as interpreted applying dc resistivity imaging to investigating the relationship between water quality and transpiration beneath circular islands in the okavango delta, botswana: a case study of thata island from the 2-d models) was seen to persist up to 15 m below ground surface on all models, except 10 (l7). figure 10 (l7) shows the 2d inversion model of the profile collected outside the island. the model indicated that fresh water overlay saline water within the swamps and that such freshwater area persisted from the surface up to depths of about 25 m. surface to borehole imaging results (figure 11) indicated that the low resistivity areas were mainly beneath the island and they existed mainly as plumes persisting to depths as great as 60 m. lower conductivity, indicating fresh water was encountered below and next to these plumes. the aforementioned modelling of electrical resistivity data indicated that thata island was underlain by saline water in its centre with less saline on its outer edges. however, oxygen isotopic data indicated that transpiration was highest along the island’s edge, suggesting that salt accumulation may have been partially occurring due to some other mechanism, the most likely mechanism being capillary evaporation strongly occurring in winter months having little rain, high temperatures and low humidity. the isotopic data could not determine which mechanism was correct in the island’s centre but low δ18o along its edges did show that capillary evaporation was important in the islands. the resistivity data did not suggest high salinity e.m. shemang l. n. molwalefhe and k. mickus figure 10: 2-d inversion models in the thata island region. models a-f run across the island into the adjacent swamps, whilst model g shows the profile collected outside the island (in the swamps) figure 8: map (colour figure superimposed on an aerial photo) of electrical conductivity (μs/cm) regarding shallow groundwater, indicating concentric salinity areas around thata island figure 9: map (colour figure superimposed on an aerial photo) of the δ18o values for shallow groundwater at thata island groundwater at the edges, but instead showed high salinity at the centre resulting in vertical migration due to density contrasts and horizontal migration away from the centre due to hydrodynamic dispersion. this might have suggested that capillary evaporation at the centre is more important than evapotranspirative concentration at the islands’ margins; however more detailed geochemical studies are needed. 4.0 conclusions this study has shown that the riparian areas of islands within the okavango delta were hotspots and warehouses for the discrete concentration of inorganic salts and other elements in the groundwater because of increased salinity as determined by surface and borehole-surface eri data. the islands supported a variety of tree types and trees were potent transpiration pumps releasing water to the atmosphere at a high rate, as determined by δ18o measurements and by the depressed water table below the islands as this was always below the level of the surrounding swamps. water was constantly lost beneath the islands, through evapotranspiration and groundwater became enriched in dissolved solutes, so that island centres were the final repositories of dissolved salts. the groundwater table was heavily depressed in the island’s centre, resistivity values were low and δ18o values were enriched as salinity (electrical conductivity) gradually became accumulated. eri results revealed that fresh water overlay saline water in the swamps. this suggested that salinity beneath the islands or the saline water within the islands migrated below the fresh water in the swamps. the denser groundwater migrated downwards and eventually linked up with saline water from neighbouring islands displacing the fresh water and forming a saline layer at the bottom. this was believed to persist throughout the okavango delta and was responsible for both discrete salinity buildup below the islands and widespread shallow saline aquifers in the delta. whether evapotranspiration salt concentration, as indicated by enrichment of stable isotopes (coplen et al., 2000), was sufficient to account for the high salinity in groundwater beneath the delta islands or whether there are other contributions from the terrestrial environment to these aquatic systems, remains enigmatic and requires further experimentation and research. the results of this study indicated that none of these methods (geochemistry, isotope or electrical resistivity) could uniquely determine the natural processes involved in creating high salinity environments; however, a unique model could be inferred when interpreted together. whether, such model is unique to thata island is unknown as similar studies are needed at other islands within the okavango delta region. our interpretation of the origin of localised high saline surface can be used as a first order model for similar regions worldwide. 5.0 references bauer, p. 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(1988). measurement of groundwater parameters using saltwater injection and surface resistivity. ground water 26, 179–186. doi: 10.1111/j.1745-6584.1988.tb00381.x winter, t.c. and rosenberry, d.o. (1995). the interaction of groundwater with prairie pothole wetlands in cottonwood lake area, east-central north dakota. 1979–1990, wetlands, 15, 193–211. 10.1007/bf03160700 wolski, p., and murray-hudson, m. (2006). flooding dynamics in a large low-gradient alluvial fan, the okavango delta, botswana, from analysis and interpretation of a 30 year hydrometric record. hydrology and earth system sciences, 10, 127-137. doi:10.5194/ hess-10-127-2006 wolski, p., and savenije, h.h.g. (2006). dynamics of floodplain-island flow in the okavango delta, botswana. journal of hydrology, 320, 283-301. doi:10.1016/j.jhydrol.2005.07.027 wolski, p., murray-hudson, m., fernkvist, p., liden, a., hundsmann-mapila, p., and ramberg, l. (2006). islands in the okavango delta as sinks of water-borne nutrients. botswana notes and records, 37, 253-263. e.m. shemang l. n. molwalefhe and k. mickus keywords: máximo urban area, loss estimation, kırşehir, turkey, earthquake. palabras clave: área urbana; estimación de perdidas; kirsehir; turquía; terremotos. how to cite item isik, e. sagir, c., tozlu, z., & ustaoglu, u. s. (2019). determination of urban earthquake risk for kırşehir, turkey. earth sciences research journal, 23(3), 237-247. doi: https://doi.org/10.15446/esrj.v23n3.60255 predicting the outcomes of earthquakes before they occur is one of the fundamental components of modern disaster management. loss estimation analyses have an important place at the assessment stage of earthquakes and in estimation of losses that earthquakes may lead to. with these analyses, it is possible to access information that is relevant to potential damages and losses. in this paper, loss estimation analyses were carried out by using the earthquake scenario which foresaw a previous earthquake that was experienced in an around kırşehir which is seismically active and located in the central anatolia region in turkey. the 1938 akpınar earthquake which occurred in and around the province of kırşehir was taken into consideration as an earthquak escenario, and loss estimation analyses were conducted for this earthquake scenario. in this paper, significant contributions will be made for preparation of an earthquake master plan and risk management plan for kırşehir. besides, studies on reduction of earthquake losses in the region may utilise these results. abstract determination of urban earthquake risk for kırşehir, turkey determinación del riesgo urbano en caso de un terremoto en kirsehir, turquía issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.60255 la predicción de los efectos de los terremotos es uno de los componentes principales de la administración moderna de desastres. los análisis de estimación de perdidas son fundamentales en la etapa de evaluación y en la valoración de los daños que un terremoto podría ocasionar. con estos análisis es posible evaluar la información que es relevante para los daños y las perdidas potenciales. en este artículo, los análisis de estimación de perdidas se realizaron en el escenario de un terremoto previo en el área de kirsehir, que es sismicamente activa y se localiza en la región central de anatolia, en turquía. el terremoto de akpinar, en 1938, que ocurrió la zona de estudio, se consideró como el escenario del terremoto, y los análisis de estimación de perdidas se realizaron con base a este escenario. con este estudio, se hace contribuciones significativas para la preparación de un plan maestro de terremotos y un plan de manejo del riesgo para kirsehir. también los estudios de reducción de perdidas en la región podrían utilizar estos resultados. resumen record manuscript received: 15/05/2018 accepted for publication: 29/03/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 237-247 ercan işık1, çoşkun sağır2, zuhal tozlu2, ümit salim ustaoğlu2 1bitlis eren university, turkey 2ministry of environment and urbanisation * corresponding author: eisik@beu.edu.tr g eo lo g ic al h az ar ds 238 ercan işık, çoşkun sağır, zuhal tozlu, ümit salim ustaoğlu introduction the destructive earthquakes which have occurred in the world and in turkey especially in recent years and large-scale losses of life and property as a result these earthquakes have necessitated studies to be carried out and precautions to be taken in relevance to earthquakes as an agenda. despite the catastrophic seismic events of the past, the loss estimation and seismic risk assessment studies regarding turkey generally focused on the north-western part of the country because of the higher concentration of population (bommer et al., 2002; strasser et al., 2008; bal et al., 2008; erdik et al., 2003; erdik and durukal, 2008; ansal et al., 2009). this is rather understandable due to the high population density and economical activities; however, findings from the earthquakes that occur in other parts of the country with relatively less impact are still significant in terms of both the hazard and vulnerability points of view. the fairly frequent midto large-magnitude tremors that strike the region provide useful insights for i) the characteristics of the earthquakes and ii) the taxonomy and seismic response of the buildings in the region. the potential damage of an earthquake will be directly influenced by the magnitude of it, buildings where proper safety measures are not taken and structures that were not built in compliance with the requirements stated in the regulations. it is not possible to prevent earthquakes or forewarn with the present technology. however, it is possible to take precautions reduce losses in the case of an earthquake. in determination of these precautions to be taken, earthquake scenarios have a significant place. the objective in earthquake scenarios is to numerically estimate the damage which will be created by an earthquake estimated for any region. minimisation of losses may be achieved by estimating losses of life and property and taking necessary precautions. this is one of the basic elements of modern disaster management approaches (işık, 2010; işık et al., 2017; lednická et al., 2006). when an earthquake scenario is created to make these calculations for a region; it is aimed to determine the damage which may occur in the buildings in the relevant region. based on this, social and economic losses may also be calculated. it is required to get ready for earthquakes which are expected to occur and calculate the areas and buildings which are foreseen to be affected by potential disasters, as well as economic losses. the most significant factor required for earthquake damage estimation is software which may apply the scenarios covering the whole region and conduct risk analyses based on these scenarios. there are various types of software developed for such analyses. in this paper, the earthquake loss estimation routine (eler) software was used, and damage determination analyses were carried out for the province of kırşehir in turkey. kırşehir was selected as a pilot province so that it could pose as an example for other provinces within the scope of the “research and development project of automation system based geography information system at which data of local authorities for determination of urban transformation areas and creation of scenario disasters.” only the province of kırşehir was considered within the scope of this project. this research that was conducted for kırşehir may be applicable for other residential areas. kırşehir was selected not only because of its geometric location but also due to its seismic characteristics. the study firstly provides information about loss estimation analyses. information about the earthquakes and elements of earthquakes that have been experienced in an around the province of kırşehir is provided. the characteristics of the akpınar fault line and the akpınar earthquake, which constitute the subject matter of the study, are explained. considering the possibility of the akpınar earthquake to occur today, structural and economic losses were estimated. while this process was being carried out, calculations were made based on two different methods that are found in the software that was used in the study. material and method earthquake loss estimations may differ depending on the types of losses considered, the extent of the geographical area involved and the types of buildings and other structures including bridges, utility stations and water distribution networks. loss estimation analysis has two main components as seismic hazard analysis and vulnerability analysis. a seismic hazard analysis involves the identification and quantitative description of the earthquake to be used as a basis for evaluating losses, namely the earthquake scenario. the second component entails the analysis of the vulnerability of buildings and other facilities to earthquake damage and the losses that may result out of this damage (fema-177, 1989). in order to prepare for a possible earthquake in the best way, it is required to estimate which regions the earthquake will affect, which type of buildings it will affect at the utmost extent and what the earthquake will result in beforehand. loss estimation analyses have a significant place at the assessment stage of earthquakes and estimation of losses that earthquakes may lead to. studies on various scientific models have had the aim of determining the performance that a dwelling unit will show in the case of a possible earthquake, structural damage status and losses of life, injuries and losses of property depending on these (crowley and bommer, 2006; bal et al., 2007; bal et al., 2008a; center m.a.e, 2006; zülfikar et al., 2017; bommer et al., 2002; molina et al., 2010; erdik et al., 2010; hancılar et al., 2010; eler, 2010; zamora-hernández et al., 2013). one of these studies was on the software named eler, which is a research project covering the physical, social and economic outcomes of earthquakes on the community within the scope of the european union’s the network of research infrastructures for european seismology (neries) project, and it was developed by boğaziçi university department of earthquake engineering (demircioğlu et al., 2009). it is not possible to prevent earthquakes or forewarn with the existing technology. however, it is possible to take precautions to decrease losses created by earthquakes. in determination of these precautions to be taken, earthquake scenarios havea significant place (işık, 2016). the magnitude of an earthquake that may occur in the future is aimed to be determined by seismic hazard studies, and the extent to which settlement areas will be affected from a possible earthquake is attempted to be determined with earthquake scenarios (özmen and can, 2016). the possibility of occurrence of a ground motion caused by an earthquake which may lead to damage and loss of life in a specific area and specific time period is defined as earthquake hazard. earthquake risk may be defined as the possibility of damage and loss of property and life due to an earthquake (eyidoğan, 2003; özmen and can, 2016; işık and kutanis, 2015). earthquakes may lead to very significant destructionin all types of social settlements and economic structures, particularly human life. the damage levels which occur immediately after anearthquake are expressed with the loss of life and injuries. the most noticeable effect of an earthquake is the destruction of the environment which has completed its formation process. an earthquake may lead to damages in all structures. from time to time, it may lead such structures to lose all their functions. these damages will ruin the economic structures of a region. calculation of losses that occur depending on earthquakes in settlement areas that are under the danger of an earthquake is named as the “loss estimation models”. in loss estimation models, the objective is to calculate the losses which may occur in any region depending on the damage an earthquake will create. loss estimation models are defined as the most significant tool that is required for risk mitigation. loss estimation models may access key information concerning potential damages and losses (işık, 2010). studies on various scientific models have had the aim of determining the performance that a dwelling unit will show in case of a possible earthquake, structural damage status and the losses of life, injuries and losses of property depending on these. in general terms, such a study should cover the studies mentioned below (figure 1). • seismic hazard analysis of settlement area based on probabilities (probabilistic analysis); the location, time of occurrence, magnitude and other characteristics of future earthquakes are estimated based on probability calculations. • obtaining design spectrums or stimulated strong ground motions • structural analyses with calculation methods based on displacement (deformation) • loss estimation depending on the performance levels of structures the province of kırşehir is located in the middle of the central anatolia region in turkey. the province of kırşehir is located in middle kızılırmak 239determination of urban earthquake risk for kırşehir, turkey basin at 39°41’39°48’ northern latitudes and 33°25’-34°43’ eastern longitudes. kırşehir is surrounded with nevşehir in the east and south-east, aksaray in the south, kırıkkale in the north-west, yozgat in the north-east and east and ankara in the west (figure 2a). kırşehir, which is located in the central anatolia region, has an interesting location in its immediate vicinity in terms of earthquake potential. in this region, earthquakes occur depending on strike slip faults. the city centre of kırşehir is located within the 1st degree seismic hazard belt in turkey’s earthquake regions map (figure 2b). historical and some of earthquakes with an epicentre about 100km of kırşehir that are recorded in an instrumental catalogue which have caused destructive and significant damages that occurred in and surrounding kırşehir are given in table 1. the neo-tectonic map of central anatolia where the province of kırşehir is located and its immediate vicinity isgiven in figure 3. in general, the fault lineswhich may be effective in the province of kırşehir are the north anatolian fault, tuz gölü fault zone, akpınar fault, salanda fault, manahözü fault, tosunburnu and çoğun fault, boztepe fault, gümüşkümbet, kırşehir, karıncalı and gülütarla fault lines, seyfe çek – ayır basin (ataman et al., 1975; bozkurt, 2001; yıldırım, 2014; aydemir, 2009; ketin, 1976; köksal and göncüoğlu, 1996; kürçer and gökten, 2014; maden et al., 2014; şengör et al., 2005; temiz, 2004; yıldız, 2008; koçyiğit and doğan, 2016). in particular, the northern anatolia fault zone (nafz) is one of the most significant fault zones in turkey. it has led to several earthquakes with high magnitudes that have caused large losses of life and resources (yavaşoğlu, 2015). some fault lines and fault groups that areconsidered while working for earthquake risk of kırşehir is shown in figure 3. this study focused on the akpınar earthquake in 1938 which caused significant loses for the province of kırşehir. it is known that the akpınar fault line caused this earthquake. seismic sources play an important role in loss estimation analyses.the fault line which leads to the earthquake scenario that is used in this study was the akpınar fault line located within the provincial borders of kırşehir. this fault line is approximately nw-se extensionally and it is located between keskin in the northern and southern parts of the lake seyfe in the south. the fault line between the villages of akpınar and taşkovan is 14km-long, the value of figure 1. analysis phases of loss estimation figure 2. a) location map of kırşehir; b) earthquake risk map of kırşehir (deprem.gov.tr, 2016) 240 ercan işık, çoşkun sağır, zuhal tozlu, ümit salim ustaoğlu its horizontal slide is 30-100 cm, and it cuts rocks such as marble and granite (ketin, 1969). the fault line which created an earthquake at the magnitude of 6.8 on 19.04.1938 and started from approximately from 2km northwest of the akpınar district was a right lateral strike slip fault within the akpınar fault line which extends up to the village of sofrazlı and then continues along the village of taşkovan (yıldız, 2008; temiz, 2004; temiz and gökten, 2011). the earthquake was effective in an area of 226 km2 which covers the provinces of kırşehir, yozgat and ankara. the area of the region constituting its second degree of effect was 1550 km2 (özer, 2016).the aftershocks of this earthquake are given in table 2. table 2. the aftershocks that occurred after the 19.04.1938 akpınar earthquake (yıldız, 2008). no date time latitude longitude depth (km) magnitude 1 19.04.1938 23:11 39.65 33.87 30 5 2 27.04.1938 10:40 39.89 34.10 10 4.6 3 14.05.1938 04:45 39.74 33.55 10 4.8 4 14.05.1938 06:55 39.50 33.70 0 4.7 5 28.05.1938 00:05 39.40 33.81 30 4.9 6 21.07.1938 21:56 39.56 33.68 10 5.0 7 16.12.1938 11:03 39.52 33.91 10 4.8 8 23.12.1938 01:32 39.50 33.50 0 4.2 table 1. major earthquakes which occurred in and around kırşehir before and after the 20th century (koeri, 2016; temiz, 2004; yıldız, 2008) no date region intensity magnitude 1 240 kayserisivas ix 2 1104 niğde ix 3 1190 karaman-konya viii 4 1205 kayseri viii 5 12.08.1168 beypazarı 6 15.08.1168 ankara 7 01.01.1695 sivas 8 09.06.1704 kayseri 9 26.12.1706 konya 10 1714 kayseri vii 11 09.05.1717 kayseri viii 12 16.09.1754 kangal -sivas vii 13 23.08.1835 develi-kayseri viii 14 1845 çankırı v 15 1866 ilgın konya vi 16 1871 konya vi 17 1888 çankırı v 18 02.07.1897 beylikahır eskişehir v 19 05.07.1928 kozaklı (nevsehir) 4.7 20 09.04.1930 akcakent (kırşehir) 5.3 21 19.04.1938 kaman (kırşehir) 6.6 22 27.04.1938 yerkoy (yozgat) 4.8 23 14.05.1938 kaman (kırşehir) 4.9 24 14.05.1938 keskin(kırıkkale) 4.9 25 28.05.1938 kaman (kırşehir) 5.2 26 21.07.1938 keskin (kırıkkale) 5.3 27 16.12.1938 akpınar (kırşehir) 4.9 28 13.04.1940 sorgun (yozgat) 5.7 29 30.07.1940 sorgun (yozgat) 6.2 30 27.04.1941 sorgun (yozgat) 5.8 31 26.10.1975 alaca (çorum) 5.0 32 22.05.1985 keskin (kırıkkale) 4.0 33 30.01.1991 delice (kırıkkale) 4.0 34 14.02.1992 delice (kırıkkale) 4.2 35 29.02.2000 delice (kırıkkale) 4.1 36 21.07.2004 akpınar (kırşehir) 4.4 37 05.10.2012 kaman (kırşehir) 4.5 38 10.01.2016 çicekdağı (kırşehir) 5.2 figure 3. tectonic map of the central anatolia region and its surroundings (koçyiğit and doğan, 2016). 241determination of urban earthquake risk for kırşehir, turkey the map of the aftershocks of the 1938 akpınar earthquake is shown in figure 4. analysis results the data required for this study were primarily supplied from the relevant public enterprises and institutions. the data supply and collection were achieved within the provincial borders of kırşehir. after determining the study area, the area was divided into 0.002° grid data in order to describe the buildings’locations. the 0.002° grid data that were produced for the province of kırşehir are shown in figure 5. the locations of the buildings in the province of kırşehir based on their geometric locations are given in figure 6. additionally, for each grid that was produced, the buildings in the grid area were entered into the system. the building location distribution map of the province of kırşehir based on the grid in the system is presented in figure 7. vulnerability may simply be defined as the sensitivity of exposure to seismic hazard(s). the vulnerability of an element is usually expressed as a percentage loss (or as a value between zero and one) for a given hazard’s level of severity (coburn and spence, 2002). vulnerability is also defined as “a state combining the physical, economic and environmental factors which increase a system’s sensibility to danger. earthquake vulnerability is defined as a risk factor that may occur as a risk element or risk group if a predicted earthquake hazard occurs. the evaluation of regional earthquake vulnerability is a basic concept in hazards research and a critical step in earthquake planning, prevention and mitigation. assessing regional vulnerability to earthquakes is a complex problem and involves many factors such as population density, economic development, the average age of buildings, the average height of buildings, the intensity of the earthquake, the quality of the building materials and lifeline systems (peng, 2015; hancılar et al., 2014; moradi et al., 2017; hadzima-nyarko et al., 2015; calvi et al., 2006; düzgün et al., 2011; hadzimanyarko et al., 2016; ademovic et al., 2013). within the confines of the project, the characteristics of the buildings to be recorded from the field were determined based on the parameters that would be needed in the loss estimation analyses. a database was established for these parameters that were developed in the office environment. firstly, training was provided to the personnel that were employed in the field regarding the building parameters that are needed for the software that was used in the study. the parameters that were used as inputs were separately introduced to the system. as a result of the field studies, these data that were related to buildings were transferred to the database via tablets. in the study, the results of the field work were recorded in the database by using the forms that were suitably created for the software that was used in the province of kırşehir. the building data that were recorded into the database were checked in the office environment by using building visuals and statistics. repeated examinations were made in the field for buildings with records of missing or inaccurate data.type of the structural system, number of stories, seismicity of the region and local soil conditions, current situation and visual quality, year of building, night/daytime population, usage of building, building type and number of independent sections are the parameters that are used for loss estimation analysis.the grid-based building inventory in the light of the data that were added to the database included a total of 90337 building entries. the building inventory for the province of kırşehir was classified in a way that was suitable for the earthquake damage estimation software. the total numbers of the buildings in each class are given in table 3. figure 4. akpınar earthquake and aftershocks of this earthquake 242 ercan işık, çoşkun sağır, zuhal tozlu, ümit salim ustaoğlu the population data were calculated based on the data of the turkish statistical institute (tüi̇k) and the building data that were collected from the field. according to the tüi̇k data that was published by the governorate of kırşehir (www.kirsehir.gov.tr/nufus), the population of kırşehir is 225197, and the average household size is 3.2 persons. these values were applied to the buildings that were seen as residential or workplace areas by considering the population distributions in the neighbourhoods and villages for the province of kırşehir, and accordingly, population distribution was calculated for each building. knowing about the local surface characteristics has an important place in the selection of new settlement locations and assessment of existing buildings. knowing about the surface characteristics will allow researchers to prevent structural problems that are based on the territory (ozdemir and nalbantcilar, 2016; büyüksaraç et al., 2014; işık et al., 2016).within the scope of this study, the local soil conditions of the region were determined by geological maps of turkey with a scale of 1/500000 prepared based on geological ages determinedby the general directorate of mineral research and exploration (mta). the eler software calculates the selected ground motion prediction models for the magnitude and fault tear associated with the specified earthquake scenario and the ground motion distribution using vs30 maps. subsequently, the obtained average ground motion distributions were assigned to geographical grids containing building distributions for use in risk analysis. in this part, the capacity spectrum method (csm) and coefficient method (cm) models were used separately for an earthquake scenario and the estimations for building damage and loss of life were submitted in the form of tables. in both methods, the structures under the effect of earthquake were classified by into five different classes as collapse, heavy damage, moderate damage, slightdamage and no damage. after the calculations were made for both methods, the average values for these two methods were calculated. the building damage results that were obtained for the earthquake scenario are shown in table 4. figure 5. the 0.002˚spaced grid data produced for kırşehir figure 6. building location distribution map of kırşehir 243determination of urban earthquake risk for kırşehir, turkey figure 7. grid-based building inventory distribution map for kırşehir table 3. building inventory classification for the province of kırşehir type of load-bearing system number of storeys date of construction number of buildings masonry 1-4 1979 and before 17676 masonry 1-4 1980-2000 24548 masonry 1-4 2001 and later 4 precast 1-4 1979 and before 27 precast 1-4 1980-2000 59 reinforced frame 1-4 1979 and before 2487 reinforced frame 1-4 1980-2000 9540 reinforced frame 1-4 2001 and after 32843 reinforced frame 5-8 1979 and before 205 reinforced frame 5-8 1980-2000 1354 reinforced frame 5-8 2001 and after 1471 reinforced frame 9-19 1980-2000 11 reinforced frame 9-19 2001 and after 29 reinforced shear wall 1-4 1979 and before 8 reinforced shear wall 1-4 1980-2000 25 reinforced shear wall 1-4 2001 and after 50 total number of buildings 90337 244 ercan işık, çoşkun sağır, zuhal tozlu, ümit salim ustaoğlu table 4. damages estimated results for buildings for scenario earthquake damage level method mean ratio to the total buildingscsm cm number of building collapsed 2033 786 1410 2% severely damaged 2965 2400 2683 3% moderately damaged 7742 8404 8073 9% slightly damaged 14698 16796 15747 17% no damage 62899 61951 62425 69% the maps that were produced in accordance with the damage levels are shown in the figures below. the figures were obtained by using the average values. figure 8 shows the distribution map of the number of very heavily damaged buildings; figure 9 shows the distribution map of the number of heavily damaged buildings; figure 10 shows the distribution map of the number of moderately damaged buildings; figure 11 shows the distribution map of the number of lightly damaged buildings. the calculations were made for two models with the eler software. the estimations of loss of life and injuries were calculated with hazus-mh (2003) and koeri (2002) models according to the distributions of the buildings. in the koeri (2002) model, the earthquakes that occurred in turkey were accounted for, and the matrix of injury rates for reinforced-concrete and masonry buildings were described. the rates that were obtained with the help of these matrices were multiplied bythe number of people in the building at the time of the earthquake. the difference between the rates that are given under the definition of very heavy damage is relevant to the way that the building collapses. the first rate in this section is relevant to the buildings which were not damaged completely (not collapsed). the second rate is the rate of injuries in the buildings which were completely damaged (collapsed). the buildings which were completely damaged (collapsed) determined total number of deaths in an earthquake. the hazus-mh (2003) model directly correlates deaths and injuries with building damage. in the areas where structural damage was lower, injuri es mostly resulted from adamage which was not structural and in the areas where structural damage was heavy.there is apossibility of a large number of deaths. however, as there are no data concerning the deaths in the types of buildings and the types of damages that caused these deaths, in the statistical information concerning the deaths and injuries in earthquakes, attention should be paid to the fact that the injury rates that were used here were close. the estimations of loss of life were calculated below by using two different estimation models of loss of life for the earthquake scenario that was used in the study. the estimation results of loss of life and injury which may occur for the earthquake scenario that was used in the study are given in table 5. there are differences between the results that were obtained from the koeri (2002) and hazus-mh (2003) models. the reason for this was that, while the koeri (2002) model was being prepared, the damage and loss data from the past earthquakes in turkey were used. it may be stated that there was a difference between the results of the two models as a result of using data specific to turkey (hancılar et al., 2010). this is why the average values of the results that were obtained in the study by the two methods were taken for the analyses. the comparison of the loss ratios for the two methods that were used in the study is given in table 6 for reinforced-concrete buildings and table 7 for masonry buildings. as it may be understood from the two tables, the ratios in the koeri (2002) model were higher. due to this issue, there were differences between the two methods. the most distinct effect of earthquakes is the destruction of the environment which has completed its structuring process. an earthquake may lead to damage in all structures. the construction areas and structural damage rates according to the damage levels in buildings as a result of the earthquake scenario are shown in table 8. in financial loss calculations, the total areas of damaged buildings are separately obtained with the help of the data that are input in regard to the buildings in the software that was used in the study. in the study, the average area of a storey in the software was assumed to be 100m2. the financial costs for any building could be calculated by approximation. for this, the class of the building based on the architecture services in turkey was considered, and approximate costs of buildings are calculated each year by including non-tax general costs (15%) and contractor profit (10%) for different groups of buildings. while making this classification, the building is categorised in terms of the architectural services that are provided. an approximate m2 unit price is determined for each building class. these prices are published annually as a notice. as it was not a practical solution to calculate costs for each building in the study, the building class of iii-a was considered to cover all the buildings. in the “notification concerning the 2019 structure approximate cost per unit which will be used in the calculation of architecture and engineering service fees” published in the official gazette by the ministry of environment and urban planning, the price of class iii– group a structures was 980 tl/m2. according to the currency values provided by the turkish central bank, this value is approximately180$/m2 today. the financial losses that result from structural damage were estimated by multiplying the total area of the damaged construction and unit cost. the construction areas obtained according to the damage types in conformity with the earthquake scenario that was used in the study are presented in table 6. figure 8. distribution map of collapsed buildings figure 9. distribution map of severely damaged buildings 245determination of urban earthquake risk for kırşehir, turkey of the project, only the province of kırşehir and its districts were considered to provide an example for other residential areas. the study provides information on the seismicity of the province of kırşehir. the earthquake of 1938 that inflicted significant effects in and around the province of kırşehir was selected as the earthquake scenario, and loss estimation analyses were carried out accordingly. in t his context, cost calculations were made on building damages, losses of life, rates of injury and structural damages. the study also compared the loss rates that were calculated by the koeri (2002) and hazus-mh (2003) models that were used to calculate the rates of losses. there were differences between the models. it is required to estimate that the regions and types of structures that an earthquake will affect the most and there sults it will create should be considered before the earthquake occurs to be prepared for a possible earthquake in the best way. loss estimation analyses play a significant role at the assessment stage of earthquakes and estimation of damages that earthquakes may lead to. with these analyses, information concerning the potential damages and losses may be accessed. natural disasters result in a substantial amount of loss of national resources in ways such as destroying a large number of animals, collapsing buildings, extinction of forests, destruction of electricity and communication lines, closing roads, collapsing bridges, blocking streams and creating risks of flood, as well as losses of life. determination of the regions that are under natural disaster risk is not adequate in decreasing and/or eliminating losses. in order to reduce disaster risk, raising awareness among the community is of the utmost importance. loss estimation models are defined as the most significant tools required for risk mitigation. thanks to loss estimation models, urban planners and disaster managers will access key information on potential damages and losses. it was determined that there are 90337 buildings of which approximately 55% are reinforced-concrete within the provincial borders of kırşehir. approximately 93% of the reinforced-concrete buildings in the study are 1-4 storied. on average, 4093 (approximately 5% of all buildings) buildings were figure 10. distribution map of moderately damaged buildings figure 11. distribution map of slightly damaged buildings table 5. loss of life and injury to forecast results injury level method meancsm cm koeri hazus koeri hazus number of people level 4 (loss of life) 130 28 57 12 57 level 3 (seriously injured) 130 14 57 6 52 level 2 (hospital treatment) 274 92 137 47 138 level 1 (slightly injured) 495 349 281 224 337 table 6. comparison of casualty rates for rc structures injury severity casualty rates for reinforced concrete structures (%) slight damage moderate damage severely damaged complete damage (collapsed) koeri (2002) hazus-mh (2003) koeri (2002) hazus-mh (2003) koeri (2002) hazus-mh (2003) koeri (2002) hazus-mh (2003) severity 1 0.05 0.05 0.2 0.25 1 1 10-50 5-40 severity 2 0.005 --0.02 0.03 0.5 0.10 8-15 1-20 severity 3 --------0.01 0.001 4-10 0.01-5 severity 4 --------0.01 0.001 4-10 0.01-10 results and conclusions this study has significance in terms of constituting an example implementation of loss estimation analyses. the application principles of loss estimation analyses are expressed. the parameters that were required for the loss estimation analysis software that were used in the study were collected from the field studies and checked in the office environment. the results of such studies may be used in other residential areas. this study chose kırşehir, which is prominent in terms of its geometrical position in turkey and seismicity, as the pilot province in turkey. in the scope 246 ercan işık, çoşkun sağır, zuhal tozlu, ümit salim ustaoğlu damaged to an irreparable extent (heavy + very heavy damage) depending on the earthquake scenario. the number of the moderately and slightly damaged buildings was estimated to be 23820 (26% of all buildings) depending on the earthquake scenario. the population within the borders of kırşehir was determined as 225197. the estimation of loss of life determined by using various methods was 57 persons on average depending on the scenario. the number of people for which hospital care was required was 138 persons on average depending on the earthquake scenario. the financial cost of the structural damage that was calculated for the case of the occurrence of the earthquake scenario that was considered in the study was 274 million $. this is an economically significant value. this damage represents the economic dimension of direct damages. in addition to this, the economic dimension of the damage will increase even further if indirect economic losses are also included. the fact that the data on damage assessment after an earthquake for the province of kırşehir is missing made it impossible to compare the results of the study to others. this is why it is important to keep damage assessment operations after earthquakes in a database are important for the evaluation of such studies. acknowledgments this study was carried out as a result of a project which was carried out within the scope of the “research and development project of 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(2017). real-time earthquake shake, damage, and loss mapping for istanbul metropolitan area. earth, planets and space, 69(1), 9. earth sciences research journal geochemistry earth sci. res. sj. vol. 16, no. 2 (december, 2012): 57 67 the geochemistry of claystone-shale deposits from the maastritchian patti formation, southern bida basin, nigeria olugbenga a okunlola and olubunmi idowu geology department, university of ibadan, ibadan, nigeria. e-mail: gbengaokunlola@yahoo.co.uk abstract an inorganic geochemical study of a claystone and shale sequence from the patti formation around ahoko in the southern bida basin, nigeria, was carried out to determine the basin’s depositional conditions, provenance and tectonics. representative samples underwent mineralogical and geochemical analysis involving major, trace and rare earth element analysis. mineralogical studies using x-ray diffraction analysis revealed prominent kaolinite, dickite and illite peaks; accessory minerals included quartz and microcline. major element abundance showed that the shale samples had sio2 (61.26%), al2o3 (16.88%) and fe2o3 (3.75%) constituting more than 79% of bulk chemical composition whilst claystone samples contained sio2 (67.79%), al2o3 (17.81%) and fe2o3 (1.67%). higher sio2, ba, sr, v, and nb concentrations were observed in claystone samples rather than shale whereas the shale samples were observed to be more zr-, niand zn-enriched than the claystone ones. the shale and claystone samples showed slightly light rare earth enrichment and slightly flat heavy rare earth depleted patterns having a negative eu and tm anomaly. high tio2 and rb/k2o values also indicated that the shale and claystone samples were matured. geochemical parameters such as u, u/th, ni/co and cu/zn ratios indicated that these shales were deposited in oxic conditions; the al2o3/tio2 ratio suggested that intermediate igneous rocks were probable source rocks for the shales, while mafic rocks were suggested as being source rocks for the claystone. however, the la/sc, th/sc, th/ co ratios and shales and claystone plots revealed that they came within the range given for felsic rocks as provenance, thereby suggesting mixed provenace for the sediments. a passive-margin tectonic setting was adduced for the sedimentary sequences. resumen se realizó un estudio de geoquímica inorgánica de una secuencia de arcillolitas y limolitas de la formación patti en ahoko, cuenca sur de bida, nigeria, con el fin de determinar las condiciones de sedimentación de la cuenca, la procedencia de los sedimentos y la tectónica de la zona. a las muestras representativas se les realizaron análisis mineralógicos y geoquímicos: estudio de trasas y análisis de tierras raras. los análisis de difracción de rayos x revelaron importantes niveles de caolinita, dickita e ilita y, minerales accesorios incluido cuarzo y microclina. en las muestras de limolita se encontró sio2 (61,26%), al2o3 (16,88%) y fe2o3 (3,75%) como constituyentes de más del 79% de la composición química, mientras que las muestras de arcillolita contienen sio2 (67,79%), al2o3 (17.81%) y fe2o3 (1,67%). las concentraciones más altas de sio2, ba, sr, v, nb se observaron en las muestras de lutita mientras que en las muestras de arcillolita se observó una mayor concentración de zr, niy enriquecimiento de znque en las lutitas. las muestras de lutitas y arcillolitas presentan un ligero enriquecimiento de tierras raras y algo de tierras raras pesadas teniendo patrones de anomalía negativa de eu y tm. los altos valores de tio2 y rb/k2o también indican que las muestras de limolita y arcillolita fueron maduradas. los parámetros geoquímicos como la relación de u, u/th, ni/co y cu/zn indican que las limolitas y arcillolitas fueron depositadas en condiciones aeróbicas, la relación al2o3/tio2 sugiere que las rocas ígneas intermedias fueron probablemente la roca fuente de las limolitas, mientras que se sugiere que las rocas máficas son las rocas fuente de las arcillolitas. sin embargo, la relación palabras claves: geoquímica, inorgánica, esquisto,arcillas, procedencia, tectónica. keywords: geochemical, inorganic, shale, claystone, provenance, tectonic. record manuscript received: 29/09/2010 accepted for publications: 06/05/2012 olugbenga a okunlola and olubunmi idowu58 introduction shale represents the most abundant type of sediment in sedimentary basins worldwide (pettijohn, 1975) and are considered to represent average crustal provenance composition much better than any other detrital sedimentary rocks (mcculloch and wasserburg, 1978). some researchers have suggested that sedimentary rocks’ major element geochemistry is more useful in discriminating a tectonic setting (bhatia, 1983; roser and korsch, 1986); however, major trace elements, such as la, y, sc, cr, th, zr, hf and nb, particularly in combination withtio2, are best suited for provenance and tectonic setting determination studies because of their relatively low mobility during sedimentary deposition (mclennan et al., 1983). the relative distribution of immobile elements differing in concentration in felsic and basic rocks such as la and th (felsic rock-enriched) and sc, cr and co (enriched in basic rocks relative to felsic rocks) has been used to infer felsic and basic sources’ relative contribution in shales from different tectonic environments (wronkiewicz and condie, 1990). several authors have used geochemical parameters to understand ancient sediments’ paleo-oxygenation conditions (calvert and pedersen, 1993; jones and manning, 1994; nath et al., 1997; cullers, 2002; armstrong-altrin et al., 2003; dobrzinski et al., 2004). this study was therefore aimed at identifying patti formation claystone-shale sequence source rock characteristics, tectonic setting and paleo-environmental conditions in the bida basin around ahoko in central nigeria using their major, trace and rare earth elements’ geochemistry. geology and stratigraphy the bida basin is a northwest-southeast-trending embayment, lying perpendicular to the main axis of the benue trough (figure 1). the south atlantic–tethys seaway was routed through this basin during the campanian to maastrichtian age. the basin is usually regarded as a north-western extension of the anambra basin (whiteman, 1982; ladipo et al., 1994; akande and ojo, 2002; akande et al., 2005), both of them being major depocenters during this period. adeleye (1975a,b) viewed the bida basin as a gently down-warped trough, whose genesis may be closely connected with south-eastern nigeria and the benue valley’s santonian orogenic movements, while whiteman (1982) has suggested that the basin was formed from simple cratonic sag. agyingi (1991) has proposed a post-santonian origin for the bida basin; he reported tension build-up in the benue trough during africa’s separation from south-america (aptian-albian). the adjacent area probably sagged along a northwest-southeast orientated fracture during this period to ease tensional build-up, resulting in the nupe depression forming a side basin. he stated that the sediments in the basin were generally undisturbed as in the case of the post-santonian sediment of the adjacent sokoto and figure 1. map of the study area and sampling locations. longitude n008.21 e06.5 e06.51 e06.52 e06.53 e06.54 e06.55 e06.5 e06.51 e06.52 e06.53 e06.54 e06.55 n008.2 n008.19 n008.18 n008.17 n008.16 n008.15 n008.21 n008.2 n008.19 n008.18 n008.17 n008.16 n008.15 longitude la t it u d e la t it u d e la/sc, th/sc, th/co y limolitas indica que la zona de procedencia está dada en el rango de rocas félsicas, lo que sugiere una fuente mixta para los sedimentos. un ambiente tectónico de margen pasivo se adujo para las secuencias sedimentarias. the geochemistry of claystone-shale deposits from the maastritchian patti formation, southern bida basin, nigeria 59 figure 2. stratigraphic successions in the mid-niger basin (taken from akande et al.). figure 3. sections of the patti formation showing dark grey-black shales and claystone inter-bedded with siltstone and sandstone. anambra basins. a maximum sedimentary pile of up to 3,300m has been recorded from aeromagnetic interpretation, suggesting a post-santonian origin (agyingi, 1991). the bida basin’s stratigraphy varies, depending on the area being considered; several authors have discussed this basin’s stratigraphy in two sectors: the northern bida basin (adeleye, 1971) and the southern bida basin (agyingi, 1991; akande and ojo, 2002). the northern bida basin is successively made up of bida sandstone, sakpe ironstone, enagi siltstone and batati ironstone (figure 2) whereas the southern bida basin lithostratigraphy successively consists of lokoja sandstone and the patti and agbaja ironstone formation. this study has concentrated on the southern bida basin’s patti formation. methodology the twenty-five samples used for this study were taken from the exposed shale section in road cuttings at ahoko, along the koton-karifi/ abaji highway. the location being studied lies within the patti formation in the southern bida basin. representative shale and claystone samples were collected from different vertical sections of the outcrops (figure 3). efforts were made to avoid weathered horizons. representative whole rock shale samples were studied by x-ray diffraction (phillips – pw 1011 diffractometer). the diffractograms were recorded using a 1o 2/min/cm scanning rate with ni-filtered fe–k alpha radiation. x -ray diffraction curves were interpreted by comparing notable intensity peaks with those for standard minerals established by carrol (1971). quantitative determination was made by a real method; the shale samples’ clay fraction was not separated for speciation evaluation in this study because, initially, it was necessary to establish the shale sequence’s holistic compositional features to have an unobliterated pattern of its evolution within the basin. geochemical characterisation involved analysing thirteen shale and two claystone samples at the acme analytical laboratories ltd for major, minor and trace element geochemistry using an icp mass spectrometer (perkinelmer, elan 6000) and inductively coupled plasma mass spectrograph on powdered, pressed pellets prepared from 3-5g samples. it was digested by weighing 0.2g aliquot in a graphite crucible mixed with 1.5g libo2/lib4o7 flux. the crucibles were placed in an oven and heated at 980ºc for 30 minutes. the cooled bead was dissolved in 5% hno3 (acs grade nitric acid diluted in demineralised water). calibration standards and reagent blanks were added to sample sequences. the basic package consisting of thirty-four elements (ba, co, cs, ga, hf, nb, rb, sn, sr, ta, th, u, v, y, zr, la, ce pr, nd, sm, eu, gd, lu, etc) was determined for the shale and claystone samples. a second 0.5g split sample was digested in aqua regia and analysed by icp-ms to determine au, ag, as, bi, cd, cu, hg, mo, ni, pb, sb, se and zn. an icp emission spectrograph (spectro ciros vision or varian 735) was used for determining major oxides and some trace elements (i.e. sio2, al2o3, fe2o3, cao, mgo, tio2, p2o5, cr2o5, ba, nb, ni, sr, sc, y and zr). loss on ignition (loi) was determined for both packages by measuring the weight loss after heating a 1g split sample at 95ºc for 90 minutes. results and discussion mineralogy the x-ray diffractograms (xrd) for the whole rock bida shale samples from location a revealed minerals such as kaolinite, quartz, mordenite, microcline, zircon and gamma alumina (figure 4a); kaolinite was the dominant clay mineral. the xrd results for shale in location b also revealed the predominance of kaolinite, quartz, microcline, pyrite and zircon present in subordinate amounts (figure 4b) and whole at location c. kaolinite, illite, quartz, microcline, dickite, mordenite, gamma alumina and iron rich minerals, such as pyrite and hematite, constituted the mineral makeup. the percentage of kaolinite in the shale in location c was 46.91%. typical diffractograms of the claystone revealed kaolinite as the dominant mineral (66%), while illite, dickite, quartz, microcline and zircon were present in subordinate amounts (figure 4d). age northrn bida basin southern bida basin m a st r ic h t ia n batati formation agbaja ironsstone formation enagi formation patti formationsakpe formation c a m pa n ia n jima member claystone (member) bida formation lokoja formation sandstone (member) doko member basal conglomerate (member) lo w er pa le o zo ic location c olugbenga a okunlola and olubunmi idowu60 figure 4. (a) x-ray diffractogram for the shale samples from location a. (b) x-ray diffractogram for the shale samples from location b. (c) x-ray diffractogram for the shale samples from location c. (d) x-ray diffractogram for the claystone samples. k clay mineral (probably kaolinite) qz quartz mo mordenite m microcline ga gamma alumina z zircon k clay mineral (probably kaolinite) m microcline qz quartz il clay mineral (probably illite) mo mordenite d dickite h hermatite p pyrite ga gamma alumina k clay mineral (probably kaolinite) qz quartz m microcline p pyrite ga gamma alumina z zircon loi loss on ignition *a4 & *a5 claystone samples tfe2o3 as total iron k clay mineral (probably kaolinite) qz quartz il clay mineral (probably illite) m microcline ga gamma alumina a1 a2 a3 *a4 *a5 b1 b2 b3 b4 b5 c1 c2 c3 c4 c5 sio2 56.78 57.84 56.91 67.97 67.6 64.08 65.22 64.42 65.2 65.43 59 60.55 59.95 60.6 60.41 al2o3 18.11 18.61 18.74 17.95 17.67 15.29 14.42 15.42 13.94 14.27 18.21 17.85 18.32 17.76 18.53 tfe2o3 5.09 4.4 4.5 1.35 1.98 2.9 3.69 2.95 4.12 3.99 3.76 3.16 3.64 3.3 3.18 mgo 0.18 0.18 0.18 0.13 0.13 0.18 0.15 0.18 0.17 0.18 0.13 0.13 0.13 0.12 0.13 cao 0.06 0.06 0.04 0.08 0.07 0.04 0.05 0.05 0.04 0.05 0.04 0.05 0.05 0.04 0.04 na2o 0.05 0.05 0.05 0.07 0.07 0.06 0.07 0.07 0.07 0.07 0.06 0.06 0.06 0.06 0.06 k2o 1.13 1.14 1.12 1.64 1.61 1.49 1.5 1.53 1.53 1.5 1.39 1.43 1.44 1.43 1.45 tio2 1.67 1.76 1.73 2.41 2.43 1.8 1.56 1.84 1.58 1.57 1.8 1.81 1.81 1.82 1.83 p2o5 0.06 0.07 0.07 0.16 0.15 0.07 0.07 0.1 0.07 0.06 0.08 0.08 0.08 0.08 0.09 mno 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 cr2o3 0.013 0.014 0.013 0.015 0.014 0.011 0.011 0.013 0.01 0.01 0.014 0.014 0.014 0.014 0.014 loi 16.5 15.6 16.3 7.8 7.9 13.7 12.8 13 12.8 12.4 15.2 14.6 14.2 14.5 14 total 99.66 99.74 99.67 99.57 99.64 99.64 99.56 99.59 99.56 99.52 99.70 99.75 99.71 99.74 99.74 table 1. major element concentration of the bida shale and claystone samples. (a) (b) (c) (d) the geochemistry of claystone-shale deposits from the maastritchian patti formation, southern bida basin, nigeria 61 geochemistry major element composition table 1 gives the major oxide analysis of thirteen shale and two claystone samples; table 2 summarises the average major element oxide (wt %) data for the samples being studied. these were compared to average shales worldwide (pettijohn, 1957), nasc (gromet et al., 1984), turekan & wedephol (1961) and shales from other parts of nigeria (table 3). it was apparent that most shale samples in table 1 varied slightly regarding sio2 (56.78-65.43%), had moderately high al2o3 (13.94 -18.74%) and a small variation in fe2o3 values (2.90%-5.09%); they were, however, low in tio2, cao, na2o and k2o. such low k2o content indicated the low amount of illite or k-feldspar present (akpokodje et al., 1991). the claystone was relatively rich in al2o3, tio2 and sio2 compared to the shale, even though the shale had higher fe2o3 content than the claystone. the table 3. comparing average chemical composition of the bida shale studied here to published average shales table 2. average chemical composition of bida shale compared to shale from other sedimentary basins in nigeria oxide present study asu river group (amajor,1987) ezeaku shale (amajor,1987) auchi shale ifon shale (ajayi et al, 1989) bida shale bida claystone sio2 61.26 67.79 69.94 44.91 51.68 63.3 tio2 1.74 2.42 0.52 0.65 1.95 1.02 al2o3 16.88 17.81 10 15.71 18.76 18.47 fe2o3 3.75 1.67 4.04 6.24 4.67 1.26 mno 0.02 0.02 0.04 0.06 0.06 0.01 mgo 0.16 0.13 0.87 2.58 4.39 0.82 cao 0.05 0.08 3.38 15.42 1.9 0.09 na2o 0.06 0.07 0.4 0.42 0.93 0.42 k2o 1.39 1.63 1.15 2.36 1.16 2.36 p2o5 0.08 0.16 0.17 0.46 0.25 0.46 loi 14.2 7.8 9.21 11.1 14.05 11.6 total 99.59 99.58 99.69 99.91 99.87 99.81 oxide average bida shale average shale (pettijohn, 1957) turekan & wedephol (1961) paas nasc (gromet et al., 1984) sio2 61.26 58.1 58.5 62.40 64.82 tio2 1.74 0.6 0.77 0.99 0.8 al2o3 16.88 15.4 15 18.78 17.05 fe2o3 3.75 6.9 4.72 7.18 5.7 mno 0.02 trace _ _ mgo 0.16 2.4 2.5 2.19 2.83 cao 0.05 3.1 3.1 1.29 3.51 na2o 0.06 1.3 1.3 1.19 1.13 k2o 1.39 3.2 3.1 3.68 3.97 p2o5 0.08 0.2 0.16 0.16 0.15 sio2/al2o3 3.63 k2o/na2o 23.16 k2o/al2o3 0.08 al2o3/tio2 9.70 cu/zn 0.12 ni/co 0.58 olugbenga a okunlola and olubunmi idowu62 claystone (17.67-17.95%) had less variation in al2o3 content than the shale (13.94-18.74%). most samples had low p2o5 content; p2o5 depletion could have been due to the lower amount of accessory phases, such as apatite and monazite, compared to paas (ramasamy et al., 2007). the mgo, mno and na2o content values were very low in both samples, collectively only accounting for less than 2%. the shale and claystone mgo and cao content was low; the samples thus did not indicate associated carbonates or dolomitisation. the shale and claystone alumina to silica ratio was low (0.27). the 14.28% average loss on ignition (loi) for shale was high, showing great shale potential for carbonaceous compounds. the 3.63 silica-alumina ratio showed that that shale and claystone samples were highly siliceous. the average sio2 (61.26%) and al2o3 (16.88%) chemical composition in shale constituted about 78% of the samples’ total chemical composition. the other chemical impurities in shale samples from the patti formation were fe2o3 (3.75%) and tio2 (1.74%). the patti formation shale and claystone samples had higher tio2 values than post-archean australian average shale (paas; taylor and mclennan, 1985). sediments’ k2o/al2o3 ratio can be used as an indicator of ancient sediments’ original composition. the k2o/al2o3 ratios for clay minerals and feldspars are different (0.0 to 0.3, 0.3 to 0.9, respectively) according to cox ( cox et al., 1995). the average k2o/al2o3 ratio for shale varies from 0.06 to 0.1 and is 0.09 for claystone. the k2o/al2o3 ratios were closer to the lower clay mineral range limit in both the shale and claystone samples. comparing the chemical composition of the shale and claystone samples from bida with that of shales from other parts of nigeria revealed that the shale and claystone samples were relatively rich in sio2, al2o3, tio2 and fe2o3 comparable with black lignite shale from ifon and shale from the auchi area (akpokodje et al.,). the present study’s cao and mno values also had values comparable with ifon shale. the samples were rich in sio2, al2o3 and tio2 compared to ezeaku shales; however, the ezeaku shale had higher fe2o3 and cao compared to bida shale and claystone. the asu river group had higher sio2, fe2o3 and cao values but lower tio2 and fe2o3 values compared to the bida shale and claystone samples from the a1 a2 a3 *a4 *a5 b1 b2 b3 b4 b5 c1 c2 c3 c4 c5 ba 334 343 340 630 563 462 460 465 468 430 356 373 364 359 371 sr 54.2 49.6 51.1 77 73.3 59 59.9 58.4 56.7 53.2 60 66.8 64.8 65.3 73.1 ni 31.7 37.6 24.9 <0.1 0.3 14.5 18.4 47.8 14 18.2 14.5 7.5 10.4 6.3 6 co 49.4 56.5 35.7 4.1 4.2 27.3 28.7 71 24.7 27.8 35.2 19.9 23.8 18.3 16.1 cu 14.1 12 14.2 6 5.1 12.8 10.3 12.6 10.5 10.9 17 17.1 19.8 18.4 18.2 zn 214 177 180 3 3 117 307 159 110 117 34 24 40 16 18 v 126 137 134 111 116 109 76 105 72 69 117 113 123 118 115 y 53.4 52.4 54.9 71.7 78.1 78.3 83.5 76.2 84.2 85.3 77.5 68.4 69.4 66.1 69.4 zr 922.4 829.2 838.4 1358.3 1432.4 1270.5 1735.9 1252.5 1731.4 1702.9 927.5 1001 928.8 977.9 916.6 mo 1.2 1.1 1 0.08 0.2 0.3 1.2 0.6 0.9 1.2 0.4 0.3 0.3 0.4 0.4 nb 48.2 50.3 49.1 70.8 71.3 54.5 49.2 57.6 49 48 55.2 56.3 55 54.8 54.8 pb 28.8 28.8 28.6 14.6 11.6 21.7 22.4 22.5 21.1 22.1 18.3 16.6 21.2 18.1 19.4 rb 36.8 34.2 35.6 47.4 47.3 53 49.4 52.1 50.2 47 48.8 51 47 46.8 48.5 th 24.7 28.3 26.6 32 29.9 28.5 28.9 29.3 33.1 29.9 30.8 29.1 28.9 29.5 32.2 u 10.5 10.6 10.6 13 13 13 11.1 13.4 11.5 11.1 14.5 16.8 15.3 15.8 15.7 table 4. trace element distribution of shales and claystone samples from ahoko. *a4 & a5 claystone samples a1-a3: shale from location a a4-a5: claystone from locations b1-b5: shale from location b c1-c4: shale from location c figure 5. chondrite normalised rare earth element plot for shale and claystone samples from the patti formation. chondrite normalisation values were taken from wakita et al., (1971) patti formation. the shale’s mgo and cao contents were generally low by contrast with values reported by emofurieta et al., (1994) for ewekoro and gombe shales. when compared to shales from other parts of the world (table 4), the bida basin shale had similar averages to those reported by pettijohn (1957), turekian and wedephol (1961) and nasc (gromet et al., 1984) in terms of sio2, al2o3 and fe2o3; the tio2 composition of the bida shale was perhaps slightly higher than that of the other shales. the bida shale samples were, on average, relatively rich in sio2, al2o3, tio2 the geochemistry of claystone-shale deposits from the maastritchian patti formation, southern bida basin, nigeria 63 and p2o5 comparable with paas and nasc (gromet et al.,1984). however, the bida shale had depleted mgo, cao, na2o, k2o composition compared to world shale averages. provenance, tectonic setting and depositional environment the geochemical signatures of clastic sediments have been used to ascertain provenance characteristics (taylor and mclennan, 1985; condie et al., 1992; cullers, 1995; armstrong-altrin el al., 2004). most clastic rocks’ al2o3/tio2 ratios are essentially used to infer source rock composition because al2o3/tio2 ratios increase from 3 to 8 for mafic igneous rocks, from 8 to 21 for intermediate rocks and from 21 to 70 for felsic igneous rocks (hayashi et al., 1997). the al2o3/tio2 ranged from 8.38 to 10.84 in the ahoko shales (patti formation) and 7.27 to 7.44 in the claystone samples; hence, this study’s al2o3/tio2 ratio suggested intermediate rocks as being probable source rocks for the shale samples while mafic rocks were suggested as being source rocks for the claystone samples. cr and ni abundance in siliciclastic sediments was considered a useful provenance tool. cr and ni concentration was low in the shale and claystone samples; a low cr concentration indicates felsic provenance, according to wrafter and graham (1989), and high cr and ni content is mainly found in ultramafic rock-derived sediment (armstrong-altrin et al., 2004). the cr and ni content in these shales was low, therefore suggesting felsic provenance. ratios such as la/sc, th/sc and th/co are significantly different in felsic and basic rocks and may lead to constraints on average provenance composition (wronkiewicz and condie, 1990; cox et al., 1995; cullers, 1995). the th/co, th/sc and la/sc ratios for shale and claystone samples from this study were compared to those of felsic and basic rock-derived sediment (fine fraction) upper continental crust (ucc) and paas values (table 8). these comparisons also indicated that such ratios came within the range of felsic source rocks. the th/co compared to la/sc plot (figure 5; cullers, 2002) also suggested that the patti formation shale and claystone samples were both derived from felsic source rocks. table 7 shows the shale and claystone samples’ rare earth element (ree) concentration; a slight variation in ∑ree content was observed between the shale (361.18 to 422.75) and claystone samples (365.83 to 371.04). bulk ree normally reside in the fine fraction (silt or clay) and it has also been inferred that trivalent ree is readily accommodated in most clay minerals enriched with alumina and ferric iron (cullers et al., 1987, 1988). the chondrite-normalised ree plot (figure 5) showed that shale and claystone samples from the patti formation were lreeenriched and had an almost flat hree pattern, with negative eu and tm anomalies. table 5. average trace element chemical composition compared to bida shale and shale from other sedimentary basins. *paas post archean austrialian average shale *nasc – north-american shale composite n.a. not analysed present study levinson (1974) vine & tourtelot (1970) turekan & wedephol (1961) *paas *nasc (gromet et al., 1984)bida shale claystone average ba 394.23 596.5 300 700 580 650 636 sr 59.39 75.15 200 300 300 200 142 ni 19.37 0.19 50 70 68 55 58 co 33.42 4.15 10 20 _ 23 n.a cu 14.45 5.55 70 50 45 50 n.a zn 116.39 3 300 100 95 85 n.a v 108.77 113.5 150 130 130 150 130 y 70.69 74.9 30 25 _ n.a zr 1156.54 1395.35 70 160 160 210 200 mo 0.72 0.14 10 3 _ n.a nb 52.46 71.05 20 20 n.a 1.90 n.a pb 22.28 13.1 20 n.a n.a 20 n.a rb 46.19 47.35 140 n.a n.a 160 n.a th 29.22 30.95 12 n.a n.a 14.60 n.a u 13.07 13 4 n.a n.a 3.10 n.a cu/zn 0.12 1.85 (cu +mo)/zn 0.13 1.90 ni/co 0.58 0.05 rb/k2o 33.23 29.05 u/th 0.45 0.42 olugbenga a okunlola and olubunmi idowu64 present study paas codo shale (mclennan 1980) average shale (levinson,1974)bida shale claystone la 77.40 76.35 38.2 29.7 121 ce 170.42 160.45 79.6 63.4 50 pr 18.71 17.62 8.83 _ _ nd 67.66 57.75 33.9 27.9 24 sm 12.46 10.78 5.55 _ _ eu 2.25 1.78 1.08 _ _ gd 11.01 8.91 4.66 _ _ tb 1.98 1.74 0.744 _ _ dy 11.79 11.13 4.68 _ _ ho 2.48 2.52 0.991 _ _ er 7.69 8.32 2.85 _ _ tm 1.14 1.24 0.405 _ _ lu 1.18 1.31 0.433 _ _ table 6. average rare earth elements of bida shale compared to world averages. table 7. rare earth element distribution of the shale and claystone samples from the location studied here. *paas post archean australia shale a1 a2 a3 *a4 *a5 b1 b2 b3 b 4 b 5 c1 c2 c3 c4 c5 la 71.10 74.30 75.3 77.7 75 77.3 79.1 73.5 76.6 77.1 78.7 78.3 80.2 80.6 84.1 ce 160.7 172.5 168.7 163 157.9 162.5 171.7 162.3 166.7 167 170.8 172.7 176.1 177.6 186.2 pr 16.58 18 17.58 17.73 17.51 18.37 19.17 18.57 18.86 18.99 19.17 19.07 19.34 19.11 20.36 nd 59.6 68.1 65.9 57.8 57.7 75.6 67.1 66.6 64.7 65.7 65.1 68.3 70.7 68.9 73.3 sm 11.65 12.22 12.4 10.67 10.89 13.73 12.24 12.79 12.1 12.35 12.51 12.72 12.4 12.06 12.86 eu 2.15 2.35 2.42 1.81 1.75 2.24 1.98 2.29 1.95 2.04 2.42 2.31 2.35 2.3 2.41 gd 10.53 10.19 10.37 8.59 9.22 12.02 10.9 11.09 11.45 11.71 11.32 10.92 10.99 11.02 11.52 tb 1.78 1.82 1.83 1.73 1.74 2.09 2.1 2.04 2.12 2.17 2.01 1.94 1.93 1.89 1.96 dy 10.22 10.06 10.99 10.67 11.59 12.49 13.06 12.19 12.87 13.49 12.34 11.69 11.21 11.29 11.42 ho 2.22 2.07 2.24 2.4 2.63 2.76 2.87 2.58 2.84 2.83 2.57 2.37 2.32 2.23 2.36 er 6.34 6.5 6.54 8.11 8.53 7.87 9.42 8.12 9.26 9.24 8.18 7.39 6.97 6.87 7.2 tm 0.94 0.89 0.92 1.2 1.27 1.24 1.35 1.23 1.4 1.35 1.16 1.11 1.07 1.06 1.11 yb 6.41 6.19 6.41 8.37 8.74 8.85 9.11 8.17 9.18 9.1 7.67 7.42 6.93 6.92 6.84 lu 0.96 0.96 0.97 1.26 1.36 1.33 1.44 1.26 1.46 1.41 1.17 1.11 1.09 1.11 1.11 the sedimentary rocks’ ree pattern and eu anomaly also help in providing important clues regarding source rock characteristics (taylor and mclennan, 1985). higher lree/hree ratios and negative eu anomalies are generally found in felsic rocks, whereas mafic rocks have lower lree/ hree ratios and no or small eu anomalies (cullers, 1995). higher lree/ hree ratios and negative eu anomalies further confirmed the felsic source rock characteristics of the shale and claystone samples studied here. bhatia and crook (1986) identified la, th, zr, nb, y, sc, co, and ti as being the most useful elements in discriminating different tectonic settings. distinctive field for four environments (oceanic island arc, continental island arc, active continental margin and passive margin) are recognised on la-th-sc and th-sc-zr/10 trivariate plots. the tectonic setting identified in the present study was passive-margin type. figure 7 shows the shale and claystone units plot within the passive margin using a discriminant function diagram (bhatia 1983). the roser and korsch (1986) plot of log (k2o/na2o) cf sio2 discrimination diagram and th-sc-zr/10 greywackes discrimination diagram (after bhatia and crook, 1986) indicated a passive-margin tectonic setting (figures 8 and 9). the passive-margin comthe geochemistry of claystone-shale deposits from the maastritchian patti formation, southern bida basin, nigeria 65 figure 6. th/co cf la/sc for shale and claystone samples from the patti formation (fields taken from cullers, 2002). shale clay 10 1 th /c o la/sc 0,1 0,01 0,01 basic rocks silic rocks 0,1 1 10 table 8. range of claystone and shale element ratios in this study compared to ratios for similar fractions derived from felsic rocks, mafic rocks, upper continental crust and post-archean australia shale prised atlantic-type rifted continental margins developed along the edges of the continent, remnant, ocean basins adjacent to collision orogens, and inactive or extinct convergent margins. intra-cratonic and rift-bounded grabens (e.g. the benue trough) were formed on a thick continental crust which are included in the passive-margin type tectonic setting. the bida basin is an intra-cratonic sedimentary basin. passive-margin type sandstone is generally enriched in sio2 and depleted in na2o, cao and tio2, suggesting their highly recycled and mature nature (bhatia, 1983). major element analysis of the studied shale and claystone samples confirmed this, as the samples were also enriched in sio2 but depleted in na2o, cao and tio2, respectively. paleo-oxygenation condition cu/zn and (cu+mo)/zn ratios have been put forward by hallberg (1976) as redox parameters. according to hallberg (1976), high cu/zn ratios indicate reducing depositional conditions, while low cu/zn ratios suggest oxidising conditions; he showed that high values should indicate more reducing conditions in the depositional basins than lower values, which may indicate more oxidising conditions. in this present study, the shale and claystone samples had ~0.03-1.15 and ~1.7-2.0 cu/zn ratios, respectively (table 5), indicating more oxidising conditions. wedephol (1968) noted that average shales reflect shallow marine sediments accumulating in oxidising conditions. the u/th ratio may be used as a redox indicator with u/th ratio being higher in organic-rich mudstones (jones and manning, 1994). a u/th ratio below 1.25 suggests oxic deposition conditions whereas values above 1.25 suggest suboxic and anoxic conditions (nath et al., 1997). in this study, the shale and claystone samples had low u/th ratios (table 5: ~0.37-0.58, ~0.41-0.44, respectively), indicating that both shale and claystone samples were deposited in an oxic environment. dypvik (1984) and dill (1986) have used the ni/co ratio as a redox indicator. jones and manning (1994) have suggested that ni/co ratios below 5 indicate oxic environments, whereas ratios above 5 indicate suboxic and anoxic environments. thus, the low ni/co ratio for shale (~0.340.70) and claystone (~0.01-0.07) samples indicate that patti formation shale and claystone was deposited in well-oxidising conditions. conclusions field studies have indicated that patti formation shales are found at the base of an outcrop section at ahoko along the koton-karfi and abaji axis, occurring alongside siltstones, claystones and sandstone inter-bedded with bioturbated ironstone. the mineralogical composition of the whole rock shale samples based on x-ray diffraction analysis identified clay mineral constituents as probably being kaolinite and illite, although, subsequent detailed clay fraction determination is needed to further confirm this. other non-clay minerals included microcline, quartz, zircon, pyrite and hematite. the 1 this study, 2 cullers (1994, 2000), cullers and podkovyrov (2000), cullers et al., (1988), 3 taylor and mclennan (1985). element ratio 1shale 1claystone range of sediments2 upper continental crust3 post-archean average shale3 felsic rocks mafic rocks th/sc 1.37-2.36 1.57-1.60 0.84-20.5 0.05-0.22 0.79 0.9 th/co 0.5-2.0 7.11-7.81 0.67-19.4 0.04-1 0.63 0.63 la/sc 3.95-5.65 3.89-3.95 2.5-16.3 0.43-0.86 2.21 2.4 figure 7. a discriminant function diagram for sandstone (taken from bhatia, 1983) showing fields for shale samples from passive continental margins, oceanic island arcs, continental island arc and active continental margins. the discriminant function was as follows: discriminant function 1 = -0.447 sio2 + 0.972 tio2 + 0.008 al2o3 -0.267 fe2o3 + 0.208 feo -3.082 mno + 0.140 mgo + 0.195 cao + 0.719 na2o – 0.032 k2o + 7.510 p2o5+0.303 discriminant function 2 = -0.421 sio2 + 1.988 tio2 0.526 al2o3 -0.551 fe2o3 -1.610 feo + 2.720 mno + 0.881 mgo – 0.907 cao 0.177 na2o – 1.840 k2o + 7.244 p2o5+ 43.57 -6 -4 -2 0 2 4 -6 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 discriminant function 1 d is cr im in an t f un ct io n 2 passive margin samples oceanic island-arc continental island-arc active continental margin olugbenga a okunlola and olubunmi idowu66 proportions of minerals identified in the shale and claystone samples varied. the patti formation’s claystone and shale samples showed considerable variation regarding their major, trace and rare earth elements. major element abundance showed that shale samples had sio2 (61.26%), al2o3 (16.88%) and fe2o3 (3.75%), constituting more than 79% of the bulk chemical composition while claystone samples contained sio2 (67.79%), al2o3 (17.81%) and fe2o3 (1.67%). there was close correlation between the major element and rare earth element composition of shale and claystone samples in the bida basin. higher sio2, ba, sr, v and nb concentrations were observed in claystone than shale, whereas shale samples were more zr-, niand zn-enriched than the claystone samples. zr was extremely high in both samples. the discrimination diagrams (bhatia, 1983) for distinguishing the tectonic setting mostly indicated a passive-margin type. the provenance indicated a non-homogenous source rock. analysis and interpretation using geochemical parameters, such as la/sc, th/sc and th/co ratios and la/sc cf th/co, suggested that the shale and claystone samples might have been derived from felsic source rock rather than intermediate or mafic source rocks, such provenance being indicated from interpretation using the al2o3/ tio2 ratio. geochemical parameters like u, u/th, ni/co and cu/zn ratios strongly implied that these shales were deposited 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co-occurrence region; remote sensing image retrieval. a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.66107 rui zeng1, 2, *, yingyan wang1, wanliang wang2 1school of electro-mechanical and information technology, yi wu industrial &commercial college, yiwu 322000, china 2college of computer science & technology, zhejiang university of technology, hangzhou 310014, china * email of corresponding author: jamse007@126.com earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 29-35 a pesar del gran número de trabajos en recuperación de imagenes basadas en contenido, poco se ha avanzado en la recuperación de imágenes por teledetección. tanto la teoría como la aplicación de los sistemas son inmaduros. ya que las imágenes por teledetección se caracterizan por un gran volumen de información, amplia cobertura, temas difusos y semántica abundante, los resultados de las investigaciones en imágenes naturales e imágenes médicas no pueden ser usados directamente en la recuperación de imágenes por teledetección. incluso en una consulta perfecta de imágenes basadas en contenido, los sistemas tienen muchas dificultades con la organización de información, almacenamiento y manejo, descripción de características y extracción, medición de similitudes, retroalimentación relevante, modo de servicio de red y diseño e implementación del sistema estructural. este artículo propone un algoritmo de recuperación de imágenes por teledetección que combina la co-ocurrencia local de una red bayesiana de recuperación de imagénes con el promedio de potencia de la señal de alta frecuencia. por las redes bayesianas, se establecen las relaciones de correspondencia entre imágenes y semántica, además de permitir la recuperación de imágenes de teledetección a través de la semántica. mientras tanto, se desarrolló el módulo de región integrada para la recuperación repetitiva, lo que mejora efectivamente la precisión de la recuperación semántica. resumen palabras clave: red bayesiana; región de coocurrencia; recuperación de imágenes por teledetección. record manuscript received: 05/07/2017 accepted for publication: 19/01/2018 how to cite item zeng, r., wang, y., & wang, w. (2018). a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm. earth sciences research journal, 21(1), 29-35 doi: http://dx.doi.org/10.15446/esrj.v22n1.66107 región de coocurrencia basada en una red bayesiana gradual hacia un algoritmo de recuperación de imágenes por teledetección e n g in e e r in g g e o l o g y 30 rui zeng, yingyan wang, wanliang wang 1. introduction studies on natural and medial image retrieval have achieved remarkable results. meanwhile, there have been numerous research projects designed specifically for remote sensing image retrieval. however, both theoretical and application systems are immature and need to be further researched (samadzadegan et al., 2012; hejazi et al., 2017). for example, the kim system developed by dlr explores implied semantic features of images by bayesian networks. nanyang technological university conducts multi-sensor (rs)2i thoroughly research feature description and extraction, multidimensional indexing technology and system structure design (mountrakis, 2012). the study compares existing image retrieval systems and focuses on image storage, network transmission model, feature extraction and description, segmentation algorithm, reasonable segmentation, similarity measurement and relevance feedback. the results show that studies on remote sensing image retrieval have some deficiencies, which are mainly manifested as follows: (1) although studies on content-based image retrieval have achieved brilliant results, they are mostly focused on natural and medical images. rarely do scholars study remote sensing images (han, 2012; xu, 2015; radan et al., 2017). (2) currently, remote sensing image retrieval systems are mainly based on texts, strip numbers, latitudes, and longitudes, rather than contents (abdi, 2013). (3) scholars have made great use of bayesian networks to study semantic-based remote sensing image retrieval. retrieval precision of existing algorithms can be further improved. (4) the retrieval precision is inversely related to time efficiency for most image retrieval algorithms. in other words, high-precision image retrieval tends to consume lots of time (zhai, 2014; simon et al., 2017). this paper proposes a stepwise bayesian network algorithm for retrieving remote sensing images, aim to address these problems; the schema combines co-occurrence region-based bayesian network image retrieval with average high-frequency signal strength, and adopts integrated region matching for iterative retrieval, thereby efficiently improving the precision of semantic retrieval and significantly reducing the retrieval time. in the meantime, semantic-based feature vectors are introduced to enhance the precision of image retrieval. 2. material and methods in content-based image retrieval, scientific and reasonable similarity measurement models are the keys to achieving the precision of the entire retrieval system (zhong et al., 2012; bata et al., 2017). bayesian network image retrieval systems consist of three parts (boser et al., 1992; roslee et al., 2017): (1) segment images with a simple image segmentation algorithm and extract color or texture features of each sub-image to describe image contents, which is referred to as image segmentation and feature extraction. (2) employ unsupervised classification to classify the extracted features, generate codebooks and encode images following the codebooks, thereby producing encoded image library. (3) obtain the final retrieval results by selecting training samples and calculating related probabilities, which is known as semantic inference. the paper proposes two concepts: code co-occurrence matrix and semantic score function to simplify the process of semantic inference. 2.1 co-occurrence region based bayesian network algorithms for remote sensing images retrieval 2.1.1 features of ikonos images ikonos is the world’s first high-resolution commercial satellite, with an orbit altitude of 681km, a revisit period of 3 days and a nadir image swath width of 11.3km. it has one panchromatic band and four multi-spectral bands (pedergnana, 2013; rahman et al., 2017). wherein, the spectral resolution of the panchromatic band and multispectral bands is 1m and 4m respectively. fundamental parameters of wave bands are shown in table 1. table 1. wave bands of iknos imagesz2.1.2 spectral feature extraction this study adopts a 256-dimensional color histogram to describe spectral features of images. in fact, swain et al. proposed to describe color features of images with color histograms in 1991. since color histogram, which presents the number of image pixels of each color afterimage colors is quantified, has transition invariance and strong robustness to rotation and scale changes, it is extensively used (stumpf, 2014; ruiz, 2014; maleki-ghelichi and sharifi, 2017). normalization processing shall be adopted, and color features shall be described by the proportion of (1) different colors to keep the scale invariant; as shown in formula (1). in the formula, n is the total number of image pixels; l is the dimension of the color histogram; nk is the number of image pixels in the k-th color dimension. 2.1.3 image segmentation in the experiment, multi-spectral konos images are segmented into non-repetitive sub-images, and 256-dimensional color histograms of the sub-images are extracted and considered as their spectral features. however, the sub-images are in large quantities, and objects with similar characteristics are segmented into different sub-images, which is not conducive to image retrieval (amiri et al., 2017). to better describe major features of images and reduce the number of feature vectors, the k-means algorithm is introduced to cluster segmented sub-images. the clustering results are deemed as the final segmentation results. sub-image is taken as the minimum unit to judge whether image regions are spatially connected, preventing segmented regions from being spatially unconnected (tariq et al., 2017). if not, the clustering results shall be segmented so that the images are spatially combined after segmentation. 2.2 codebook and image coding 2.2.1 codebook image features are accurately extracted for quantification and classified by the k-means algorithm to realize many-to-many matching between image features and semantic meanings. assuming that all image areas in the database are expressed as , n is the total number of image segmentation regions, f(d) is the region feature extraction function and f(d) represents features of all image areas in the database. f(d) is classified into l categories by the k-means algorithm, and each cluster is a set of features. let cu be the cluster center of λu, then there is a mapping relation . wherein, cbi is referred to as codebook and the index number between cu and λu is referred to as code. for the image it, its region code rt can be expressed as ci(rt ). wherein, ci is the code function of the region. since codebooks generated by the k-means algorithm are all of the fixed length and appropriate codebook sizes are difficult to be identified, this paper adopts tree structured coding to solve the problem. tree-structured codebooks are composed of different levels of (n represents the number of codebook levels). figure 1 demonstrates a schematic drawing 31a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm of structured tree coding. figure 1. a schematic drawing of three-level tree structured codebooks according to figure 1, top-level codes have more regional features, which flexibly and efficiently allows users to select codebooks required at different levels of . based on the features of image areas, the chosen codebooks shall be used to encode images, thereby generating image library (encoded image library). 2.2.2 image coding it is necessary to select proper codebooks from tree-structured codebooks to encode images. the coding function of image areas is defined as (n represents the number of tree-structured codebook level), and the process of image coding is described as follows: step 1: select a m-level tree-structured codebook for in it and its region features , and let the optimal codebook level l be 1. step 2: traverse each cluster λu, identify the cluster center cu that contains clusters of regional features and encode the region as u, then, . step 3: if , the program stops. otherwise, it will go to step 2. step 4: encode into a code strand (each code represents clusters of image areas at different codebook levels), encode each image area and obtain encoded images, thereby forming encoded image library. 2.3 semantic inference 2.3.1 mathematical models , and coexist in the image it , and are referred to as cooccurrence regions of . semantic inference is based on codes of and its co-occurrence regions ( and ). spectral or texture features of the triad constituted by , and are described by corresponding code triad (region code). assuming that the codebook has l codes, it may generate l3 code combinations from the perspective of the combination. in this way, spectral or texture feature symbols of the region increased from l to l3, thereby improving the precision of semantic inference. when inferring semantic meanings of images, the correlation between it and is measured by the posterior probability of cooccurrence region based bayesian networks. co-occurrence region based bayesian networks are shown in figure 2: figure 2. co-occurrence region based bayesian networks formula (2) is the formula for calculating the posterior probability of co-occurrence region based bayesian networks. (2) (5) (7) (6) (3) according to bayesian formula, formula (2) can be expressed as: where in, is the prior probability of ; the conditional probability reflects random links between codes and semantic concepts and can be obtained from user feedback or training samples. assuming that the prior probability follows a uniform distribution, it can be ignored when calculating . the calculation formula is as follows: when interpreting the image , importance of the code triad can be described with and calculated by the product of region importance function and co-occurrence region importance function . assuming that and its co-occurrence regions are equally important to interpreting , then and . where in, the function is element data in the set. if all co-occurrence regions are taken into account, the following formula is valid: formula (6) prevents the bias toward regions with more co-occurrence regions when calculating . based on formula (5) and the calculation formula of , formula (4) can be simplified as: 2.3.2 code co-occurrence matrix the codes are defined to describe the importance and generate code cooccurrence matrix to simplify the calculation of , the correlation between and is reflected by the code triad importance function . it is possible to determine the importance of the code triad to by virtue of prior probability of . the calculation formula is as follows: (8) wherein, u, v and w are codes that jointly constitute code cooccurrence matrix mk. assuming that the prior probability follows a uniform distribution, formula (7) can be simplified as: (4) 32 rui zeng, yingyan wang, wanliang wang 2.3.3 semantic score function semantic scores can be divided into region semantic scores and semantic image scores. region semantic score function is used to calculate the correlation between and : (9) (10) (12) (13) (14) (15) (17) (18) (19) (16) (11) the normalization coefficient prevents bias of semantic scores toward regions with more co-occurrence regions. according to the region semantic score function, the image semantic score function can be defined as: the image semantic score function is used to measure the correlation between it and . if is greater than the predefined thresholds, it is deemed to contain the semantic concept at least. according to formula (9) and formula (11), it should be noted that this is a special form of formula (3), which indicates that the image semantic score function is consistent with cooccurrence region based bayesian networks. 2.3.4 learning with bayesian networks the prior probability can be obtained from a group of training samples that reflect semantic concept and consist of and its cooccurrence regions to calculate the semantic score of images. the probability of the code triad can be calculated through formula (13): since and may not belong to tk, the choice function ε is defined as: the prior probability can be approximately calculated through formula (15): if users submit a new training sample and merge it with the original training sample tk, the prior probability shall be recalculated through formula (16): if there are intersections between the new training sample and the original training sample, the calculation formula of the prior probability is . 2.4 stepwise remote sensing image retrieval traditional content-based remote sensing image retrieval systems tend to calculate the distance between low-level feature vectors. due to semantic gaps, however, images with similar features and irrelevant semantics are always retrieved (ali et al., 2017). in the process of image retrieval, time efficiency is also a key factor that determines the performance of the retrieval system. users require not only high precision and recall but also demand high time efficiency. 2.4.1 integrated region matching integrated region matching (irm), which has the strong robustness to possible segmentation errors, determines the overall similarity of images by comprehensively calculating the similarity of all image regions. mathematically speaking, image similarity can be determined by calculating the distance between two point sets in high-dimension space (feature space). although there are many methods of calculating the distance, such as euclidean distance, between points in high-dimension space, they cannot be used to calculate the distance between two point sets. the biggest problem of defining the distance between two point sets in feature space is to maximize the difference between overall similarity and people’s subjective feelings of image similarity. by comprehensively considering significance and distance between image regions, integrated region matching allows one region to match multiple regions, thereby minimizing retrieval errors caused by inaccurate image segmentation (yasin et al., 2017). according to the research results of wang et al.(2016), the algorithm is described as follows: image 1 and image 2 are represented by and , and feature vectors of the segmentation region i and j are represented by and respectively. also, the distance between and is .when calculating the distance between r1 and r2, it is necessary to match all regions of the two images, calculate the distance between all regions and give corresponding weights to significance factors si,j. since significance factor represents the matching degree between and , is referred to as significance matrix. the significance matrix s of integrated region matching is determined by significance factors si,j – the area ratio between one region and the whole image. assuming that the area ratio of in image 1 and in image 2 is and separately, then under standard conditions, . a reasonable matching mechanism is required to involve all image regions and give priority to the matching of the most similar regions. under this matching mechanism, regions in image 1 will only match with similar regions in image 2. in other words, the distance between the two images is 0. by distributing significance factors, integrated region matching connects the most similar regions in two images with the shortest distance, which is referred to as the most similar highest priority (mshp). it is assumed that d(i’,j’) is the minimum distance, and to obtain significance matrix with an iterative method. when , i’ and j’, which are regions with the minimum matching distance, are successfully matched together by significance factors of i’. the significance factor of j’ is calculated through . according to mshp, the matching problem of can be solved when the following four formula requirements are met. 33a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm (20) (21) (22) (23) (24) (25) the iteration stops when all significant factors are calculated. by co-occurrence region based bayesian networks, the algorithm selects a group of candidate images that are highly correlated with the query semantics from large-scale remote sensing image database and adopts integrated region matching to reorder candidate images and return top ranking images, thereby realizing rapid image retrieval. since co-occurrence region based bayesian network image retrieval is characterized by low time complexity, it can retrieve a group of candidate images that are highly correlated with the query semantics within a short time (shahzad et al., 2017; basarian and tahir, 2017). integrated region matching has high time complexity. in comparison with the entire database, candidate images are small in quantities, which effectively reduce image retrieval time. flowchart of the whole process is shown in figure 3. figure 3. a flowchart of stepwise region retrieval stepwise region retrieval is mathematically described as follows: step 1: select a group of candidate images that are highly correlated with the query semantics. there are two alternatives: (1) users choose candidate images by searching images by the image semantic score function trained from co-occurrence region based bayesian networks. let be the query image and be the threshold of semantic image score; then the candidate images can be expressed as: (2) users select candidate images by searching semantic information of pictures by semantic score function. assuming that has two semantic meanings, and are the two semantic score functions, and and are thresholds of the two functions, then the candidate images selected can be expressed as: under normal circumstances, the threshold shall be set to 0, to ensure the precision of image retrieval. step 2: adopt integrated region matching and query to reorder the candidate image gallery a, return top ranking images and finish the search. the more the returned images are similar to the query images in spectral and texture features, the more top-ranked the images will be. 3. results this experiment involves a total of 28 multi-spectral remote sensing images photographed by ikonos and synthesizes standard true-color images with blue, green and red wave bands. each image is segmented into 256x256 sub-images, and the maximum repeatability between two subimages is 50%. after deleting unqualified images, the database has a total of 12,000 sub-images, covering nearly 40,000km2 earth surfaces.images in the database are classified into six categories following semantic meanings: farmland (644), city (3,547), water body (3,243), vegetation (2,937), bare earth (1,033) and rock (1,730). wherein, the figures in brackets represent numbers of images related to corresponding semantic concepts, and one image may be correlated with multiple semantic concepts. this paper measures performance of the retrieval system with precision and recall. in the experiment, the author segments images into 32x32 subimages extracts color histograms of regions with spectral features and employs the k-means algorithm for image clustering. when generating encoded image library, the color feature vectors are classified into tree codes by the k-means algorithm, thereby generating codebooks (hussin et al., 2017). each image area is encoded into corresponding codes following its feature vectors, thereby making encoded image library. this experiment creates three-level codebooks: 150, 300 and 600. experiments are designed to verify the retrieval algorithm proposed, which focuses on co-occurrence region based bayesian network stepwise remote sensing image retrieval, verifies precision and time efficiency of the algorithm. 3.1 experiment design and results considering that the retrieval time of integrated region matching may be influenced by the segmentation region numbers of query images, the experiment selects images with 4 to 13 image regions, with integrated region matching and stepwise image retrieval as controlled trails. each retrieval experiment is repeated for three times, to ensure the accuracy of time, and the average of the retrieval time is deemed as the experiment result. since stepwise image retrieval experiment records the time consumed in step 1 and step 2, it is possible to obtain the total retrieval time. this experiment measures performances of retrieval systems by retrieving time and precision. the retrieval time of integrated region matching and stepwise image retrieval is shown in table 2, and the comparison results are shown in figure 4(a). since integrated region matching belongs to sorting algorithm, rather than classification algorithm, it selects the first 100 images for precision calculation. as for stepwise image retrieval, it will fetch the first 100 images if it retrieves more than 100 images; otherwise, it will be subject to the number of images returned. the final retrieval precision is demonstrated in figure 4(b). table 2. a comparison of retrieval time between integrated region matching and stepwise image retrieval 34 rui zeng, yingyan wang, wanliang wang figure 4. a comparison of between integrated region matching and stepwise image retrieval 4. discussion 4.1 time efficiency analysis according to table 2, the retrieval time of integrated region matching increases gradually with the increase in region numbers, which is mainly because the number increase adds difficulties to the computation. nevertheless, the increase in region numbers has little impact on the retrieval time of stepwise image retrieval. it fluctuates around 1.3s, 1.4s and 2.35s at 150, 300 and 600 yards respectively. with the increase of codebook numbers, however, the retrieval time increases accordingly. on average, the retrieve time rises by 0.126s from 150 yards to 300 yards and by 0.943s from 300 yards to 600 yards. due to the increase of codebook numbers, code co-occurrence matrices appear. the codebooks at 300 yards are eight times those at 150 yards, and the codebooks at 600 yards are eight times those at 300 yards, which indicates that the ratio of retrieval time difference is 1:8. through experiment, the ratio is proved to be 1:7.4841, which is consistent with the speculation. the retrieval time of step 2 is related to the number of candidate images returned in step 1. it can be seen from figure 4(a) that the retrieval time of stepwise image retrieval is significantly shorter than that of integrated region matching, which is because co-occurrence region based remote sensing image retrieval has lower time complexity. 4.2 recall analysis according to figure 4(b), the precision of integrated region matching is 0.609, which is significantly higher than that of stepwise image retrieval (0.853, 0.926 and 0.93 at 150, 300 and 600 yards respectively). the reasons are as follows: images retrieved by integrated region matching are merely similar to each other in respect to lowlevel features, without considering semantic similarity. stepwise image retrieval selects candidate images highly correlated with query images and retrieves images with similar characteristics to query images by integrated region matching, which effectively improves the precision of image retrieval. 4.3 retrieval cases this paper selects an image of city region for retrieval analysis and 600 yards for stepwise image retrieval. the first ten images retrieved are shown in table 3: arranged from left to right and from top to bottom. table 3. a comparison of retrieval results between stepwise image retrieval and integrated region matching 35a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm 5. conclusions by considering remote sensing image features and remote sensing image retrieval, the paper proposes a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm that takes into account both time efficiency and retrieval precision. this algorithm mainly draws reference from stepwise remote sensing image retrieval proposed by moustakidis et al.(2012) and based on csbn and integrated region matching. it consists of two parts: co-occurrence region based remote sensing image retrieval and integrated region matching. select a group of candidate images that are highly correlated with query images in semantics by co-occurrence region based remote sensing image retrieval and reorder the candidate images with integrated region matching, thereby obtaining the final retrieval results. from the perspective of time efficiency, stepwise remote sensing image retrieval consumes 1.5171s, 1.5425s and 2.4454s at 150, 300 and 600 yards respectively. regarding the precision, integrated region matching enjoys an accuracy of 0.609 while stepwise image retrieval significantly improves the precision to 0.853, 0.926 and 0.93 at 150, 300 and 600 yards respectively. acknowledgments this research was financially supported by science and technology project of zhejiang province (grant no. 2015c31171) and national education information technology research plan project(grant no. 146241806) (grant no. 136241559). references abdi, m. j., & giveki, d. 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(2018). on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures. earth sciences research journal, 22(4), 239-249. doi: https://doi.org/10.15446/esrj.v22n4.69332 sand-clay mixtures are one of the most usual types of soils in geotechnical engineering. these soils present a hydraulic conductivity which highly depends on the fine content. in this work, it will be shown, that not only the mean fine content of a soil sample affects its hydraulic conductivity, but also its spatial distribution within the sample. for this purpose, a set of hydraulic conductivity tests with remolded samples of sand-clay mixtures have been conducted to adjust a regression for the hydraulic conductivity depending on the fine content. then, a finite element (fe) model simulating a large scaled hydraulic conductivity test is constructed. in this fe-model, the heterogeneity of the fine content is simulated following a gaussian distribution. the equivalent hydraulic conductivity resulting of the whole fe-model is then computed and the influence of the spatial distribution of the fine content is evaluated. variations of the mean fine content and the standard deviation have been considered to analyze the behavior of the equivalent hydraulic conductivity. the results indicate that the equivalent hydraulic conductivity is not only related to the mean fine content, but also on its heterogeneity. at the end, a simulation example of a water flow around a sheet pile, showed also that the flow rate depends on the fine content heterogeneity in a similar fashion. abstract keywords: sand -clay mixtures; permeability; groundwater flow. on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures sobre la influencia de la distribución espacial del contenido de finos en la conductividad hidráulica de mezclas areno-arcillosas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.69332 las mezclas areno-arcillosas son uno de los suelos más típicos en la ingeniería geotécnica. estos suelos presentan una conductividad hidráulica que dependen fuertemente de su contenido de finos. en este trabajo, se demostrará que la conductividad hidráulica no solo depende del contenido promedio de finos sino también de su distribución espacial. para tal fin, se ejecutó un set experimental de mezclas areno-arcillosas para medir su conductividad hidráulica y correlacionarla con el contenido de finos. luego, se construyó un modelo numérico de un ensayo de permeabilidad a grande escala. en este modelo se consideró una muestra con variaciones internas del contenido de finos siguiendo una distribución gaussiana. se calculó la conductividad hidráulica equivalente de todo el modelo y se analizaron sus resultados. los resultados indican que la conductividad hidráulica no solo dependen del contenido promedio de finos, sino también de su heterogeneidad. resumen record manuscript received: 09/12/2017 accepted for publication: 09/07/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 239-249 william mario fuentes1*, carolina hurtado1, carlos lascarro1 1universidad del norte, colombia. * corresponding author: fuenteslacouture1@gmail.com g eo te ch ni ca l en g in ee ri ng 240 william mario fuentes, carolina hurtado, carlos lascarro introduction the design of some geotechnical structures requires the estimation of the hydraulic conductivity on soils. sand-clay mixtures are one of the most common soils involved in these structures, either as the surrounding natural soil, or as fill material. for both cases, the estimation of the hydraulic conductivity allows to quantify infiltration flow rates, infiltration forces, duration of consolidation, among others. the literature shows that for saturated sand-clay mixtures, the hydraulic conductivity depends on the relative proportions between the two materials (yang & aplin, 1998; schneider, flemings, day-stirrat, & germaine, 2011; indrawan, rahardjo, & leong, 2006; shafiee, 2008; shakoor & cook, 1990; shelley & daniel, 1993), the overall density or porosity (alyamani & şen, 1993; chapius, 1990; hazen, 1911; kenney, lau, & ofoegbu, 1984; loudon, 1952; odong, 2007), the mineralogy or material type (deng, wu, cui, liu, & wang, 2017) and the interconnection between pores (belkhatir, schanz, arab, & della, 2014). a vast number of correlations to estimate the hydraulic conductivity has been proposed depending on the mentioned properties. authors report correlations to estimate the hydraulic conductivity of clean sands (amer, asce, amin, & awad, 1974; biernatowski, e. dembicki, dzierżawski, & wolski, 1987; kollis, 1966; pazdro, 1983), clean clays (mesri & olson, 1971; al-tabbaa & wood, 1987; nagaraj, pandian, & raju, 1994; samarasinghe, huang, & drnevich, 1982; tavenas, leblond, jean, & leroueil, 1983), kaolin clays (mesri & olson, 1971; hamidon, 1994), and sand-clay mixtures (chapius, 1990; kenney, van veen, swallow, & sungaila, 1992; kumar, 1996; shafiee, 2008; luijendijk & gleeson, 2015). correlations for the estimation of hydraulic conductivity are mostly calibrated on experimental results using homogeneous soil samples (altabbaa & wood, 1987; amer, asce, amin, & awad, 1974; biernatowski, e. dembicki, dzierżawski, & wolski, 1987; chapius, 1990; kollis, 1966; kumar, 1996; mesri & olson, 1971; nagaraj, pandian, & raju, 1994; pazdro, 1983; samarasinghe, huang, & drnevich, 1982). to achieve homogeneity, for example in sand-clay mixtures, researchers often mix the two different materials by hand, until a homogeneous soil mass is obtained (koltermann & goerlick, 1995; revil & catchles, 1999). disappointingly, the reality shows something different: the experience tells us that samples obtained in a single layer of a sand-clay soil, present similar (but not equal) fine content with a certain dispersion (gomez-hernandez & gorelick, 1989; de dreuzy, de boiry, pichot , & davy, 2010). in the geotechnical practice, the mean value of fine content is often used to classify the mixed soil of a single layer, and with this, some properties such as the hydraulic conductivity are estimated. this suggests that the influence of the spatial distribution of the fine content is in some soil properties disregarded. this fact is disappointing considering the existing numerical tools which allow to investigate this influence (dassault systèmes, 2016). definitely, more investigation is in this direction recommended. in this work, the influence of the spatial distribution of the fine content in sand-clay mixtures is investigated. for this purpose, some experiments are conducted to study the behavior of the hydraulic conductivity of a sandclay mixture. characterization tests and permeability tests are systematically performed to that end. a kaolin clay and the santo tomas sand are mixed with different proportions to obtain sand-clay mixtures. the hydraulic conductivity results are carefully examined to calibrate an exponential regression curve. having this, the influence of the spatial distribution of the fine content is investigated through numerical simulations. a boundary value problem with finite elements is constructed simulating a constant head hydraulic conductivity test with large sample dimensions. for the proposed model, different spatial distributions of the fine content are considered following a gaussian distribution. the influence of the gaussian distribution parameters, namely the mean value and the standard deviation, is carefully studied. at the end, a boundary value problem of a flow around a sheet pile wall is simulated for comparison purposes. finally, some concluding remarks are given. material characterization in this section, the testing soils are characterized. a set of experiments including sieve analysis, hydrometer analysis, specific gravity and atterberg limits have been conducted on these materials. the testing clay corresponds to a kaolin clay from the northern region of colombia. similarly, the testing sand corresponds to the santo tomás sand, very often used for construction purposes in colombia. characterization tests have been conducted on the kaolin clay with the following results. it presents a specific gravity of gs = 2.66, a fine content (percent passing sieve #200) of 95%, a liquid limit of ll = 27%, a plastic limit of pl = 17% and plasticity index of pi = 10% according to the unified soil classification system uscs, the kaolin clay is classified as a low plasticity clay cl. results are summarized in table 1. table 1. characterization test results of the kaolin clay. gs: specific gravity, ll: liquid limit, pl: plastic limit, pi: plasticity index. parameter value gs 2.66 fine content (%) 95 ll (%) 27 pl (%) 17 pi (%) 10 uscs classification cl the santo tomás sand is a uniform sand with a fine content of 1.21%, uniformity coefficient of cu = 4.08, curvature coefficient of cc = 0.98, effective diameter of d10 = 0.16 mm, specific gravity of gs = 2.65, maximum void ratio emax = 0.91 and minimum void ratio emin = 0.54. the latter values are related to maximum and minimum dry densities of dmax = 1.72 g/cm3 and dmin = 1.39 g/cm3 respectively. according to the unified soil classification system uscs, the santo tomás sand is classified as a poorly graded sand sp. the results are shown in table 2. the grain size distributions for the santo tomás sand and the kaolin clay, obtained by sieve analysis and hydrometer analysis respectively, are shown in figure 1. table 2. characterization test results of the santo tomás sand. gs: specific gravity, d10: effective diameter, cu:uniformity coefficient, cc:curvature coefficient, emax: maximum void ratio, emin: minimum void ratio parameter value gs 2.65 fine content (%) 1.21 d10 (mm) 0.16 cu (-) 4.08 cc (-) 0.98 emax 0.91 emin 0.54 uscs classification sp results of hydraulic conductivity tests of the sand-clay mixtures a number of permeability tests were conducted on the different testing materials, namely the santo tomás sand, the kaolin clay, and the sandclay mixtures with different proportions. constant head permeability tests were conducted on all samples (invias, 2013), except for the one of the kaolin clay, in which considering its low hydraulic conductivity, a variable 241on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures head permeability test was performed. samples were prepared with initially ovendried santo tomas sand and dried kaolin clay powder. a small amount of water was mixed thoroughly until reaching a homogeneous soil mass with a water content of approximately w  10% the process of homogenization was performed by hand on an external recipient. the wet tamping method, with three layers of approximately 2.0 cm, was used to produce samples within the permeameter. the followed procedure was as follows: for the first layer, the permeameter cell was filled with 2 centimeters of water. then, the soil was gently poured till reaching approximately half centimeter above the water surface. a special cylindrical tamper with diameter of 5 cm was used to compact the first layer, by providing 25 blows on the surface. the procedure was repeated for the second and third layer. special care was taken by controlling the blow intensity and by keeping the number of blows constant, in order to produce samples with similar (but not equal) void ratios. the void ratio for the clean sand sample was e = 0.72 which corresponds to a relative density of dr = 51.3% (medium dense sand) and a dry density of d = 1540 kg\m3 for the case of mixed sand-clay samples, void ratios within the range of e = {0.65 0.72} were always obtained, corresponding to dry densities of d = {1610 1540} kg\m3 the void ratios were always checked at the end of the preparation process, and in case that the resulting one was not within the expected range, the procedure was then repeated. top and bottom porous stones, covered by filter papers were included in the assembly. a standard spring for permeameters was fixed on the top porous stone to provide a small vertical stress to all samples (bardet, 1997). then, the sample was continuously flooded with distilled water during two days to guarantee its complete saturation (degree of saturation equal to one). detailed description of the followed test procedure can be found in the literature (bardet, 1997; invias, 2013). the prepared sand-clay mixtures include fine contents of {0%, 4%, 8%, 12%, 16%, 20%, 100%}. the case of 0% corresponds to clean santo tomas sand (without fines), and 100% to clean kaolin clay. the results of the hydraulic conductivity tests are compiled in table 3, including a correction due to temperature according to the method by (invias, 2013). a suitable exponential interpolation function depending on the fine content fc is employed to adjust the results, which considers the hydraulic conductivity of clay, denoted by k100, and the hydraulic conductivity of clean sand, denoted with k0 and reads: k k exp c fc k k= + − −( )( ) −( )0 100 01 * * (1) where c is a material constant to be calibrated. notice that for fc = 0, the proposed relation renders k = k0, while for fc = 100, a value of k  k100 is obtained. for the reported results, a value of c = 0.28 has been found to reproduce well the observed behavior. figure 2 includes the experimental results in conjunction with the proposed relation, and an accurate estimation. figure 3 plots once more the obtained results of the permeability tests, and compares it with the adjusted regression and other correlations reported in the literature for sand-clay mixtures. it includes the relation by (warren & price, 1961), (cardwell & parsons, 1945), and others as the arithmetic mean and harmonic mean as a function of the fine content fc. a brief summary of these relations is given in appendix b, while their analysis may be found in other works, e.g. (luijendijk & gleeson, 2015). notice that the relation by (cardwell & parsons, 1945) includes parameter , which must be adjusted. the cases p = 1 and p = -1 correspond to the arithmetic and harmonic means respectively. for the particular correlation by (cardwell & parsons, 1945), it has been found that a value of p = -0.4 matches well the obtained results. the fact that parameter p is lower than zero, indicates that the hydraulic conductivity is mostly dominated by the clay portion (luijendijk & gleeson, 2015). 0 10 20 30 40 50 60 70 80 90 100 0,0000,0010,0100,1001,000 pe rc en t f in er [ % ] particle size d [mm] santo tomas sand kaolin figure 1. grain size distribution for santo tomás sand and the kaolin clay. test method: sieve analysis for santo tomas sand, hydrometer analysis for kaolin clay. table 3. results of hydraulic conductivity test of the sand-clay mixtures. kaolin content (%) hydraulic conductivity cm s     hydraulic conductivity 20º c cm s     0 1.08e-02 1.04e-02 4 2.57e-03 2.48e-03 8 9.34e-04 9.01e-04 12 3.18e-04 3.07e-04 16 1.64e-04 1.59e-04 20 4.19e-05 4.19e-05 100 1.90e-06 1.90e-06 0,e+00 2,e-03 4,e-03 6,e-03 8,e-03 1,e-02 1,e-02 0 20 40 60 80 100 h yd ra ul ic c on du ct iv ity [ cm /s ] fine content [%] hydraulic conductivity test regression figure 2. results and adjusted regression of hydraulic conductivity test for different fine contents (t=20ºc) 242 william mario fuentes, carolina hurtado, carlos lascarro numerical simulations to evaluate the effect of the spatial distribution of the fine content on the equivalent hydraulic conductivity for a non-homogenous soil sample, i.e. a sample presenting spatial variations of the fine content and/or other variables, the equivalent hydraulic conductivity keq is the one resulting from the homogenization of a representative volume. with simpler words, if we perform a constant head hydraulic conductivity test, the equivalent hydraulic conductivity keq is the one resulting from the test, independently of the sample inner heterogeneities. in this section, the influence of the fine content heterogeneity is evaluated through numerical simulations. for that end, boundary value problems bvps with finite elements are constructed. the bvps simulate sand-clay mixtures subjected to a steady state water flow. the simulations account for spatial distributions of the fine content with a certain statistical distribution. details of the creation of this heterogeneity will be explained later on. with this procedure, different points within the problem may have different fine contents and therefore different hydraulic conductivities. the hydraulic conductivity has been correlated to the fine content according to equation 1. the results will show whether the equivalent hydraulic conductivity resulting from the overall problem depends or not of the fine content spatial distribution. the bvps assume 2d conditions under plane strain assumptions. the commercial software abaqus standard v6.16 (dassault systèmes, 2016) is used for these simulations. the simulations are based on the steady state equation for underground water flow, which using indicial notation reads:   w x i w i = 0 (2) where x x xi = { }1 2, are the coordinates and wi w is the darcy flow velocity vector defined as: w k g p x g gi w w ij w w j j= − −      δ ρ ∂ ∂ 1 � (3) where ij is the kronecker delta operator, kw is the hydraulic conductivity coefficient, pw is the water pore pressure, w is the water density, g is the gravity magnitude and gj is the unit vector pointing in the gravity direction. the hydraulic coefficient k can be also related to the material’s permeability k through the relation k k gw= ( )µ ρ/ , where  is the dynamic viscosity of the fluid. notice that we have considered isotropic hydraulic conductivity. steady state analysis of groundwater flow does not depend on the material’s mechanical behavior, and therefore no constitutive model for the soil is in the present analysis required. different spatial distributions of fine content are considered for the analysis. specifically, random values of fine content saved at each element gauss point are simulated. these values follow a gaussian distribution with mean value σ µ= f � and standard deviation . variation of these variables {µ σ, } were considered for the analysis, including the fine content mean values of σ µ= f � 2.0%, 5% y 10%} and standard deviations computed as a fraction of the mean value, i.e. σ µ= f � , where f takes different values between 0 and 1. the non-homogenous fields of fine content were simulated through a special fortran subroutine (intel(r) , 2007) compatible with abaqus. the software provides the user subroutine voidri which enables to create an initial field, usually the void ratio, and permits to establish a direct relation between this variable and the hydraulic conductivity. hence, we have used this user subroutine to write a fortran code able to produce a random value for the fine content following the aforementioned gaussian distribution. the code is given in appendix a. the subroutine inputs correspond to the mean value and standard deviation , which are controlled for each simulation. three different boundary value problems bvps were constructed: the first simulates a constant head hydraulic conductivity test with a large soil simple, with dimensions of 1 m times 1m. this bvp allows to compute an equivalent hydraulic conductivity for a given heterogeneous field of fine content. the second corresponds to the same problem under the threedimensional (3d) case. the last problem simulates a bidimensional flow around a sheet pile. in the following lines, a brief description of the problems is given and their results are carefully analyzed. simulation of a constant head hydraulic conductivity test with heterogeneous soil the simulation considers a squared domain of a large soil sample, with dimensions of 1 m wide times 1 m high. the aim is to simulate the test depicted in figure 4, wherein a vertical upward flow is experienced by the soil due to an imposed hydraulic gradient. small sized finite elements were used (of 0.01 mx 0.01 m) for the bvp. at the top boundary, a pore water pressure equal to pw = 0 kpa is fixed as boundary condition to allow its drainage. at the bottom, a water pressure of pw =50.0 kpa is given to produce the pressure differential. the geometry, mesh and boundary conditions are depicted in figure 5. 0,000001 0,00001 0,0001 0,001 0,01 0,1 0% 50% 100% 150% h yd ra ul ic c on du ct iv ity k [c m /s ] fine content [%] experiment regression warren and price (1961) cardwell and parsons (1945) arithmetic mean harmonic mean figure 3. comparison of different correlations to estimate the hydraulic conductivity of sand-clay mixtures. correlations are given in appendix b. figure 4. schematic model of hydraulic conductivity test. the model geometry is not scaled. the water flows upward. 243on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures the developed fortran subroutine, described in the previous section, was used to generate the random field of fine content following a gaussian distribution. as an example, figure 6 presents the fine content contours for a mean value of  = 5% and standard deviation of . the computation of the equivalent hydraulic conductivity is as follows. according to the darcy’s law (darcy, 1856), the water outflow q obtained at the top boundary is computed with: q k iaeq= (4) where keq is the equivalent hydraulic conductivity, i h h= ∆ / is the hydraulic gradient, ∆h is the water head differential, h is the height of the sample and a is the transversal section area of the sample. for the given geometry, ∆h w= =50 5kpa m/  , h=1 m and a = 1m x 1m. substitution of these values yields to the following simplified relation: q k meq= × ( )5 2 (5) on the other hand, the flowrate is computed with: q va= � (6) where v is the mean velocity of the water outflow. the latter is simply computed by averaging this variable at the top boundary nodes. substitution of equation 6 in equation 5 yields to the following relation for the equivalent hydraulic conductivity: k v h eq = ∆ (7) results of simulations for mean fine contents of with different values of the standard deviation  are plotted from figure 7 to figure 12. figure 7 presents the resulting outflow velocity  for each node located at the top boundary for . as expected, scattered values of the outflow velocity  are obtained due to the fine content heterogeneity. notice that the dispersion increases for higher values of f, where f is the factor controlling the standard deviation , see figure 7. the resulting mean outflow velocity, denoted by v , is plotted in figure 8. it is reminded that the mean outflow velocity results from averaging the outflow velocities . the results clearly indicate an increasing behavior of the mean outflow velocity v for increasing values of f. the resulting equivalent hydraulic conductivity keq, computed with equation 7, is plotted in figure 9 and presents the same trend. hence, one may conclude that the equivalent hydraulic conductivity keq depends on the spatial distribution of the fine content. similar results were obtained for a mean fine content of as shown in figure 10, figure 11 and figure 12. we are now interested to extend the analysis for the three dimensional (3d) case. for this purpose, a 3d fe-model has been constructed. the geometry consists of a cube of 1m x 1m x 1m. while displacements are in all directions restricted, a pore pressure gradient is similarly produced by imposing a value of 50 kpa at the bottom, and 0 kpa at the top. the random field of fine content is generated in the same way as in the 2d model. figure 13 shows an example of the fine content fc heterogeneity figure 5. geometry, mesh and boundary conditions of the simulation of hydraulic conductivity test. figure 6. simulation of hydraulic conductivity test. random field of fine content with mean value of  = 5% and standard deviation of . -2,00e-02 -1,00e-02 0,00e+00 1,00e-02 2,00e-02 3,00e-02 4,00e-02 5,00e-02 6,00e-02 7,00e-02 0 50 100 150 o ut flo w v el oc ity v [m /s ] top boundary nodes (from leftt to right) [ -] f=0.05 f=0.3 f=0.1 f=0.5 = 5% figure 7. outflow velocity at the top boundary nodes. mean fine content of . the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. 244 william mario fuentes, carolina hurtado, carlos lascarro for the mean value of  =10% and standard deviation of  = 4%. figure 14 shows the results for the equivalent hydraulic conductivity keq of the 2d and 3d models, whereby very small discrepancies are observed. hence, one may expect very similar conclusions on the 3d case. simulation of a flow around a sheet pile wall with heterogeneous soil the influence of the fine content heterogeneity is now analyzed in a simulation of a water flow around a sheet pile. similar to the previous simulations, a distribution of the fine content is given following a gaussian distribution. the sheet pile wall is submerged in the soil with a depth of 3 m and a thickness of 0.2 m, as depicted in figure 15. the problem is 20 m wide and 6 m high. lateral and bottom boundaries are considered as impervious. the sheet pile itself is considered as impervious as well. in order to generate a water flow around the sheet pile, a water pressure of pw = 0 kpa is fixed on the left top boundary, while a water pressure of pw = 100 kpa is set on the right top boundary. the geometry, mesh and boundary conditions are depicted in figure 15. [m /s ] 0,00e+00 2,00e-03 4,00e-03 6,00e-03 8,00e-03 1,00e-02 1,20e-02 1,40e-02 1,60e-02 1,80e-02 0 0,2 0,4 0,6 0,8 m ea n ou tfl ow v el oc ity v ̅ factor f [-] = 5% figure 8. mean outflow velocity vs. factor f. mean fine content of . the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. 0,00e+00 5,00e-04 1,00e-03 1,50e-03 2,00e-03 2,50e-03 3,00e-03 3,50e-03 0 0,2 0,4 0,6 0,8 eq ui va le nt h yd ra ul ic c on du ct iv ity k e q [m /s ] factor f [-] = 5% figure 9. equivalent hydraulic conductivity vs. factor f. mean fine content . the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. -2,00e-02 -1,00e-02 0,00e+00 1,00e-02 2,00e-02 3,00e-02 4,00e-02 0 20 40 60 80 100 120o ut flo w v el oc ity v [m /s ] top boundary nodes (from leftt to right) [-] f=0.05 f=0.3 f=0.1 f=0.5 = 10 % figure 10. outflow velocity at the top boundary nodes. mean fine content of = 10%. the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. 0,00e+00 1,00e-03 2,00e-03 3,00e-03 4,00e-03 5,00e-03 6,00e-03 7,00e-03 8,00e-03 0 0,2 0,4 0,6 0,8 m ea n ou tfl ow v el oc ity v ̅ [m /s ] factor f [-] = 10 % figure 11. mean outflow velocity vs. factor f. mean fine content of = 10%. the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. 0,00e+00 2,00e-04 4,00e-04 6,00e-04 8,00e-04 1,00e-03 1,20e-03 1,40e-03 1,60e-03 0 0,2 0,4 0,6 0,8 eq ui va le nt h yd ra ul ic c on du ct iv ity k e q [m /s ] factor f [-] = 10% figure 12. equivalent hydraulic conductivity vs. factor f. mean fine content of = 10%. the standard deviation is computed as , where f is a factor. bvp of hydraulic conductivity test. 245on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures as an example, the spatial distribution of the fine content for a mean value of with a factor of f = 0.9 is plotted in figure 16. the figure shows the initial heterogeneity of the fine content. the contours of the pore water pressure are shown in figure 17 and the excess of pore water pressure, computed as ∆p p pw w w= − 0, where pw0 is the initial pore water pressure, is plotted in figure 18. in general, these plots show the typical flow net of a water flow around a sheet pile wall. once more, the nodal outflow velocities, from the left top boundary, are plotted in figure 19. the results show that for nodes closer to the sheet pile, the outflow velocity increases. dispersion is again obtained in these results due to the heterogeneity of the fine content. their mean values are plotted in figure 20 and similar to the hydraulic conductivity test simulation, it shows an increasing behavior for increasing standard deviation. for comparison purposes, we have now plotted in figure 21 the response of the normalized velocity v v/ 0, where v0 is the velocity for the homogeneous case  = 0, for a mean fine content of . surprisingly, both results show a similar response despite they were computed on very different boundary value problems. final remarks in the present work, the influence of the fine content heterogeneity on the hydraulic conductivity was evaluated. for this purpose, a set of fe simulations of a large scaled permeability test with constant head was performed. the simulations considered the behavior of the hydraulic conductivity of the santo tomas sand, mixed on different proportions with a kaolin clay, according to some experiments. the results showed a clear dependence of the resulting equivalent hydraulic conductivity with the spatial distribution of the fine content. specifically, it was shown, that for a given mean fine content, increasing standard deviations  are related to higher equivalent hydraulic conductivity values keq. this trend is at least true for mean values of fine contents of for the fe models figure 13. simulation of hydraulic conductivity test, 3d fe model. random field of fine content with mean value of  =10% and standard deviation of . 0,00e+00 2,00e-04 4,00e-04 6,00e-04 8,00e-04 1,00e-03 1,20e-03 1,40e-03 1,60e-03 1,80e-03 0 0,2 0,4 0,6 0,8 eq ui va le nt hy dr au lic c on du ct iv ity k eq [ m /s ] factor f [-] 2d model 3d model = 10% figure 14. comparison between the 2d and 3d model, simulation of hydraulic conductivity test. mean fine content of = 10%. the standard deviation is computed as , where is a factor. figure 16. spatial distribution of the fine content for a mean value of 5% and a factor of f = 0.9. the standard deviation is computed as , where is a factor. bvp of flow around sheet pile wall figure 15. mesh, geometry and boundary conditions for bvp of a sheet pile wall. water flows around the sheet pile, from the left side to the right side. 246 william mario fuentes, carolina hurtado, carlos lascarro figure 17. contours of pore water pressure pw. mean fine content of and f = 0.9. the standard deviation is computed as . bvp of flow around sheet pile wall figure 18. contours of excess of pore water pressure ∆p p pw w w= − 0, where pw0 is the initial pore water pressure. mean fine content of and f = 0.9. the standard deviation is computed as . bvp of flow around sheet pile wall -2,00e-03 -1,00e-03 0,00e+00 1,00e-03 2,00e-03 3,00e-03 4,00e-03 5,00e-03 6,00e-03 7,00e-03 0 10 20 30 40 top-left boundary nodes (from left to right) [-] f=0.05 f=0.3 f=0.1 = 5% o ut flo w v el oc ity v [m \s ] figure 19. outflow velocity at the top left boundary nodes. mean fine content of . bvp of flow around sheet pile wall m ea n ou tfl ow v el oc ity v 0,00e+00 5,00e-04 1,00e-03 1,50e-03 2,00e-03 2,50e-03 0 0,2 0,4 0,6 0,8 ̅ [m /s ] factor f [-] = 5% figure 20. mean outflow velocity vs. factor f. mean fine content of . the standard deviation is computed as , where f is a factor. bvp of flow around sheet pile wall 247on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures 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(2000). undrained anisotropy of k0-consolidated silt. canadian geotechnical journal, 37(1), 178-200. 249on the influence of the spatial distribution of fine content in the hydraulic conductivity of sand-clay mixtures appendix: appendix a it is desired to simulate the dependency of the hydraulic conductivity k with the fine content fc. in addition, a spatial heterogeneous field of the fine content fc is required. abaqus allows to provide an equation between the hydraulic conductivity and a field variable, which is internally called as voidri. for our convenience, we assume that the variable voidri corresponds to the fine content fc, i.e. voidri=fc. the equation relating the hydraulic conductivity and voidri is provided through a tabular method, i.e. a set of solution points of at different fc. the spatial distribution of the fine content fc is provided through the user subroutine voidri, fully compatible with abaqus. the subroutine must be written in fortran code, and according to our work, follows from a gaussian distribution with mean value of fc =  and a standard deviation of  = f x , where f is a factor. in the following lines, the programming lines of the subroutine voidri is briefly described. steps of the subroutine voidri 1. define factors f and  as input variables. 2. compute the standard deviation  = f x  3. generate a random value fcx, between 0 to 1, with the fortran subtoutine rgauss 4. compute the fine content random value fc = + fcx 5. in case of a negative value fc < 0 correct to fc = 0. in the following lines, the programming lines of the user subroutines voidri and rgauss are given. subroutine voidri(ezero,coords,noel) use ifport include ‘aba_param.inc’ integer noel real*8 ezero, aux1, aux2, dif, factor real*8 coords(3) average=2.0d0 ! (input, defined by the user for each analysis) factor=0.5 ! (input, defined by the user for each analysis) sigma=factor* average call rgauss(sigma, aux1,aux2) ezero= average +aux1 dif= average if (ezero.le.( average -dif)) then ezero= average -dif+0.01 endif if (ezero.ge.( average +dif)) then ezero= average +dif-0.01 endif end subroutine subroutine rgauss(sigma, y1,y2) use ifport real*8 x1, x2, w, y1, y2, sigma do while ( (w .ge. 1.0d0).or.(w.eq.0.0d0) ) x1 = 2.0d0 * rand(0) 1.0d0 x2 = 2.0d0 * rand(0) 1.0d0 w = x1 * x1 + x2 * x2 end do w = sigma*sqrt( (-2.0d0 * log( w ) ) / w ) y1 = x1 * w y2 = x2 * w end subroutine appendix b the following correlations for the hydraulic permeability of sandclay mixtures have been employed. the correlation proposed by (warren & price, 1961) reads: log logk fc k fc k= ( ) + −( ) ( )100 01 log equation 8 where k is the hydraulic conductivity, fc is the fine content, k100 is the hydraulic conductivity of the clay and k0 is the hydraulic conductivity of the sand. the correlation proposed by (cardwell & parsons, 1945) is: k fc fc kp p p= + −( )( )� �k100 0 1 1 equation 9 where p is an exponenet which should be adjusted. for the present case, a value of p = -0.4 has been found to match well the experiments. the last equation yields to the arithmetic mean by setting p = 1, and reads: k fc k fc k= + −( )� �100 01 equation 10 the particular case of the harmonic mean can be also obtained by setting p = 1, and reads: 1 1 100 0k fc k fc k = + −( )� equation 11 after the late 1990s, gnss/gps network rtk technology has become the preferred technique in the world, today widely used in various applications such as monitoring, early warning systems, and mapping and engineering applications. by the end of 2008, the cors-tr network was being used for engineering applications to determine 3d positions across the whole of turkey. on the other hand, single base rtk stations are being operated in turkey. the yldz is one of these types of stations. it was established at the yıldız technical university by the department of geomatic engineering in 2012. in this paper, the performance of the yldz station is examined by use of measurements in 5 istanbul gps triangulation network (igna) benchmark points, with five repeatability measurements up to 50 km from the station. the cors-tr network rtk solution is used at the same points, and the results are compared. the measured coordinates produced by the two rtk solutions are compared with the igna point coordinates. satellite geometry, initialisation time and the standard deviation of repeatability are also determined. the differences between measured and known coordinates are calculated as 1-2 cm horizontally and close to 3 cm vertically using the yldz single base rtk method. después de la década de los 90, el sistema global de navegación por satélite/sistema de posicionamiento global (gnss/gps, en inglés), basado en tecnología de navegación cinética satelital en tiempo real (rtk, del inglés real time kinematic) se convirtió en la técnica preferida en el mundo y que todavía se utiliza en varias aplicaciones como el monitoreo, sistemas de alerta temprana y mapeo e ingeniería de aplicativos. para finales de 2008, los aplicativos de ingeniería utilizan la red de estaciones de referencia de funcionamiento continuo (cors-tr, un proyecto establecido en turquía) para determinar posiciones en tres dimensiones a lo largo del territorio turco. por otro lado, las estaciones rtk de una sola base han funcionado en turquía. la yldz es una estación de este tipo que fue establecida en la universidad técnica de yildiz por el departamento de ingeniería geomática en 2012. en este trabajo se examina el desempeño de la estación yldz a través de la medición de cinco puntos de referencia del sistema de triangulación gps de estambul (igna, en inglés) con cinco mediciones repetitivas a más de 50 km de la estación. similares medidas rtk se hicieron con la red cors-tr y se compararon los resultados. las dos mediciones producidas por las soluciones rtk se compararon con los puntos de coordenadas igna. también se determinó la geometría satelital, el tiempo de inicialización y la desviación estándar de la repetibilidad. las diferencias entre las coordenadas medidas y las coordenadas conocidas se calcularon en un rango de 1-2 centímetros horizontalmente y cerca de 3 centímetros verticalmente para la estación yldz con el método rtk. abstract resumen keywords: rtk gnss, cors, single base rtk palabras clave: navegación cinética satelital en tiempo real, sistema global de navegación por satélite, estaciones de referencia de funcionamiento continuo, base única rtk. record manuscript received: 12/06/2015 accepted for publication: 28/07/2015 how to cite item aykut, o.a., gülal e., and akpinar b. (2015). performance of single base rtk gnss method versus network rtk. earth sciences research journal, 19(2) 135 139. http://dx.doi.org/10.15446/esrj.v19n2.51218 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.51218 desempeño de un método de navegación cinética satelital en tiempo real (rtk) de una sola base en comparación de un sistema rtk earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 135 139 g e o d e sy performance of single base rtk gnss method versus network rtk nedı̇m onur aykut*1, engı̇n gülal1, burak akpinar1 1yıldız technical univesity, department of geomatics engineering, esenler, istanbul, turkey 136 nedi̇m onur aykut, engi̇n gülal and burak akpinar http://www.ggntr.com/ link. on the other hand, as of august 2014, the yldz station is a euref proposed station with a 20815m001 number. the coordinates of the yldz station are determined by processing the gnss data by gamit/globk software in irtf2008 datum, using 38 reference stations in the igs network. the velocities of these points are millimetre sensitive for calculating orbit information and earth rotation parameters spread over a 1000 km diameter of the yldz point. in this data process, the igs final ephemeris data was used to calculate precise coordinates and velocities. the coordinates and standard deviation of yldz station were then transformed to itfr96 datum for national applications (table 1). table 1. cartesian coordinates and standard deviation of yldz station introduction global navigation satellite systems (gnss) provides various kinematic and post processing methods of positioning solutions (feng and wang 2008). different real-time kinematic (rtk) gnss methods have been used for determining the position of any point on the globe. the differential gnss method has been used since 1980, although rather for navigation applications, using pseudo-range data with the position accuracy of submetres. carrier phase data has been used in real time kinematic applications since 1990 to increase the position accuracy at a centimetre level (langley 1998). both of these methods require at least two receivers. one receiver is the reference, the other is the rover. the correction data is transmitted by radio wave or satellite communication. a typical rtk gps accuracy may be given as 8 mm + 1 ppm horizontally and 15 mm + 1 ppm vertically (spectra precision, manual). this accuracy varies depending upon the distance between the reference and moving receiver. the limitation of rtk is due to distance-dependent biases, such as orbit bias, ionosphere bias and troposphere bias (rizos and han 2003). this restricts the distance to 10 kilometres or less. the network rtk (nrtk) or continuously operating reference stations (cors) has been developed to overcome the distance dependency by using a network of gnss reference stations spread over a wide area (rizos 2003). in turkey, the cors tr network has been used in the operation of 150 reference stations since 2008. single base rtk gnss at least two receivers are required for classical or single base rtk survey. the first receiver is the base receiver, which is fixed to a point with known coordinates. the other receiver is the rover, used for measuring the unknown coordinates of the points in real time. there is a communication link between both receivers to send corrections from base to the rover receiver by radio waves or gsm links. the real time kinematic gnss employs a method of carrier phase differential gnss positioning, whereby users can obtain centimetre level position accuracy in real time (langley 1998). the number of full waves between the receiver and satellite cannot be accounted for at the initial signal acquisition (xu 2007). the ambiguous number of these waves in carrier phase measurement is resolved by on-the-fly (otf) techniques in real time (hu et al. 2003). the pseudo-range phase is measured at the base station a at epoch t0 may be represented by the equation given below (hoffmann et al. 2008): (1) is the geometric range, is the satellite-receiver dependent bias, is satellite dependent only, is receiver dependent only. is the phase ambiguity. (2) described in equation 2 are the results for corrected phase ranges where and is the difference of phase ambiguities. the point positioning at the rover site b is performed with the corrected pseudorange phase (hoffmann et al. 2008). the yldz single base rtk station is operated by the department of geomatic engineering, located in yildiz technical university in istanbul, turkey. the aerat1675_120 spke gnss antenna is located on the roof of the civil faculty. gnss data is collected by the ashtech proflex pf800 receiver located in the geodetic metrology laboratory of the department. the proflex web server is used for the ntrip caster and for monitoring the base station. the corrections are sent for real time applications via internet communication. the yldz single base rtk system ip is 193.255.212.232 and a 2101 port. users can access the web page of the station using the network rtk the limitation of the single base rtk technique is the distance between the base receiver and the rover receiver due to distancedependent biases; namely, orbit bias, ionosphere bias and troposphere bias. techniques have been developed to overcome this distance dependence using a network of gnss reference stations spread over a wide geographic area (hu et al. 2003; rizos and han 2003). the aim of network rtk (nrtk) is to compute network corrections from the observations of several reference stations and send them to the user (wegener and wanninger,2005). nrtk requires an ambiguity resolution ‘engine’ which will fix the integer ambiguities between the static multiple reference receivers, located at stations of known position, and so make up the network (rizos and han 2003). the reference station separation is usually restricted to less than 100 km to allow for quick and reliable ambiguity resolution. as the number of stations increases, better corrections can be attained (ahmed el-mowafy 2012). there are several correction computing solutions, such as the vrs (virtual reference stations), mac (master auxiliary concept), prs (pseudo reference stations) and fkp (flächenkorrektur parameter) methods. the vrs technique is currently the most popular and efficient method of transmitting corrections through a data link to the network users for rtk positioning (hu et al., 2003). the basic theory of the vrs method is to transform measurements made at real reference stations to the location of the vrs and, therefore, to a different location at the same epoch. the pseudo-range phase is described by equation 3 after considering a substitution of f s=c/λs. on the left side, is the measured carried phase which is modeled by , the geometric distance between receiver and satellite, integer ambiguity and , the combined receiver and satellite clock bias (hoffmann et al. 2008). (3) the terms and are changed in accordance with varying locations in this equation. the receiver r is once located at the real reference station a, represented by the coordinate factor xa, and once at the vrs, represented by xv. the results of these two locations from (3) are the two following equations (hoffmann et al. 2008). (4) (5) 137performance of single base rtk gnss method versus network rtk satellite orbit errors, ionospheric and tropospheric refraction errors are included in term, which is modelled or solved by all baselines of the reference network. calculating the differences of these equations gives the following result. (6) turkey has had all of the covered network, named cors-tr or tusaga active, which currently has 150 reference stations (4 of them are located in north cyprus) providing 24/7 rtk positioning applications, geodetic/cadastral surveys, modelling the atmosphere researches, monitoring plate tectonics, earthquake prediction early warning systems and gis applications (gülal et al., 2013) in itrf96 datum and 2005.0 epoch. the positioning accuracy of real-time measurements is under 3 cm horizontally and 5 cm vertically (eren et al. 2009). the master and auxiliary control centres broadcast the coordinate correction using vrs cmr+, vrs rtcm 3.1, sapos fkp 2.3, rtcm3net (mac) and dgps techniques (yildirim et al. 2011). the cors-tr network was established at the end of 2008 with the support of the tubitak (the scientific and technological research council of turkey) project. the istanbul kültür university has taken over the responsibility of making the network fully operational under the supervision of the general command of mapping and general directorate of registration and cadastre. this network was built with the benefit of some tutga (turkish national fundamental gps network) points which serve as a reference to geographic information, established with the cooperation of the general command of mapping and general directorate of land registry and cadaster (kurt and cingoz 2013). performance test of the yldz single base rtk station the performance tests of the yldz station were carried on in two stages. in the first stage, the coordinates of yldz station were determined in the cors-tr network by real-time measurements using the vrs method. as a result of this measurement, the precision of coordinates and standard deviations of the yldz point was calculated in the cors-tr network system datum. the ashtech proflex 800 gnss receiver and aerat1675_120 spke geodetic antenna were used. in total, 6500 epoch by epoch coordinate data with an interval of 10 seconds was collected in approximately 18 hours. the average value of the measured coordinates and standard deviations was calculated. the differences in these coordinates are given in fig. 1, while the differences percentage of all the surveyed epochs are given in table 2. -2 -1 0 1 2 -2 -1 0 1 2 3 -4 -2 0 2 4 6 north (cm)east (cm) up (c m ) figure 1. the deviation of rtk coordinates from average value table 2. differences of the rtk coordinates the average value of yldz coordinates of 6500 epochs is determined by the cors-tr network, while the differences between north east up coordinates are calculated from static and real-time observations. the post processed coordinates of yldz station were entered into the ashtech web server software to be used in real-time measurements in order to determine the coordinates of benchmark points in the next stage. on the other hand, the yldz point was determined by cors-tr network in real time to calculate the coordinate differences between these two systems for precision of the yldz single base rtk solution versus the cors-tr solution. the differences between the north, east and up coordinates are close to 2 cm. in the second stage, test surveys were carried out in 5 pillar-constructed benchmarks with a 5 repeatability up to 50 kilometres from yldz single base station, in distances of 10, 15, 25, 40 and 50 kilometres respectively. real-time surveys were carried out with the spectra precision sp80 gnss receiver, which has an 8 mm + 1 ppm horizontal, 15 mm + 1 ppm vertical accuracy. at each benchmark point, the coordinates were determined by the yldz single base rtk method, the cors-tr network and the vrs method. an average of 5 epochs of north, east, up coordinates, initiation time, measuring time, pdop, observed satellite numbers, horizontal rms and vertical rms values of coordinates were recorded at all 5 benchmark points with 5 repeatability in order to determine the precise coordinates and standard deviation between the sessions. benchmark points are istanbul gps triangulation network (igna) points which are in the itrf96 datum 2005.0 epoch. the igna network was constituted in 2005 by processing gnss data through the fixing of 11 tutga points close to istanbul city. rtk measurements were carried out in the itrf96 and 2005.0 beginning epoch at the benchmark points with 5 repeatability in order that the standard deviation values between sessions be calculated. the coordinate differences between known and measured coordinates are examined in fig. 2 for two rtk solutions. figure 2 (a). coordinate differences (cors-tr network vrs solution) 138 nedi̇m onur aykut, engi̇n gülal and burak akpinar figure 2 (b). coordinate differences (yldz single base rtk solution) in figure 2 (a), the deviation between known and measured coordinates by the cors-tr network was under 4 cm both horizontally and vertically. the cors-tr solution is not due to the distance from yldz single base station but the cors-tr network points are far away from the measured points. the distance from reference points to measured benchmarks is about 50 km. in particular, the last of two test results have high coordinate differences from the known values, perhaps due to the distances from the reference points of cors-tr network. in fig.2 (b), the results of the yldz single base solution are given. the deviation of the coordinates is under 2-3 cm horizontally and vertically up to 50 km from the yldz station. during the test survey, the repeatability of the rtk solutions was also tested. five sets of rtk measurements were attempted. at each of the benchmark points, the gnss receiver was initiated to the cors-tr and yldz casters one by one. when the gnss receiver was initiated to cors-tr, measurements were taken by calculating the mean value of 5 surveyed epochs. this constituted a first set of rtk measurements. the gnss receiver was then initiated to the yldz caster and the first set of the measurements was completed. this operation was continued until 5 sets of rtk measurements were completed. we now had 5 by 5 north, east and up coordinates of each benchmark points solved by the cors-tr and yldz solutions. the standard deviation between these measurements was calculated to describe the measurement differences in respect to their average value. if the deviation of the sets were at a high value, it showed us that the measurements were far from the average value. if the standard deviation value was then small, the measurements were close to the mean value and so the measurements were coherent in relation to each other. figure 3 (a). standard deviation of 5 rtk measurement sets (cors-tr network solution) figure 3 (b). standard deviation of 5 rtk measurement sets (yldz single base rtk solution) looking at figure 3, it can be said that the standard deviation of the coordinates is small and close to zero value. hence, the measurement quality was good. only in the third benchmark point were the values higher in contrast to others, but still under 4 cm for both cors-tr and yldz solution. the reason might be poor satellite geometry. figure 4 shows the satellite numbers which were observed by both the cors-tr and yldz solutions during benchmark tests with 5 repeatability. figure 4. satellite numbers at the benchmark points initialisation time is another important criterion in network rtk solutions (eren 2009). fig. 5 shows the average of the initialisation times of 5 rtk sets for all the benchmarks. the yldz rtk solution initialisation time was sometimes close to the cors-tr solution. figure 5. initialisation time of rtk solutions 139performance of single base rtk gnss method versus network rtk conclusion network rtk solutions are useful across the world for real time positioning. in turkey, the cors-tr rtk network has operated since 2008 for real time applications in itrf96 datum. as a result of the measurements carried with the cors-tr network at the benchmark points, the differences between the known and measured coordinates are under 3-4 cm horizontally and vertically (fig. 2a). the results of the measurements with 5 repeatability are close to each other at the same benchmark points. the standard deviations of these are near/under 1 cm horizontally and 3-4 cm vertically (fig. 3a). the coordinate differences of 4 and 5 points are higher than the others. at the 4th point, the observed satellites are 13-14 numbers. the measurements made at the 4th point are the same because the standard deviation of these measurements is 0.5 cm horizontally and almost 2 cm vertically (fig.3a). at the 5th point, the observed satellite numbers are 10 (fig. 4). besides, the initialisation time of this solution is approximately 40 seconds; therefore higher than the other values. satellite geometry may be a reason for dropping positioning accuracy. in addition to these, at the 3rd point, the differences of north, east and up coordinates are under 2 cm but the standard deviation of the up coordinates is 4 cm (fig. 3a). the observed satellites are 12. cors-tr network positioning accuracy, which was tested at 5 benchmark points, was calculated as 3-4 cm horizontally and vertically (as given in the literature). this value depends on satellite geometry and therefore on the initialisation time and the distances to reference points. when examining the yldz single base rtk solution, the coordinate differences of known and measured are both near and under 1-2 cm horizontally and near 3 cm vertically (fig. 2b). the standard deviations of 5 repeatability of the measurements are an average of 1 cm horizontally and 3 cm vertically. the higher value is 4 cm with the 3rd test point at the up coordinate, just as with the cors-tr network solution. however, at the benchmark points 4 and 5, the coordinate differences calculated by yldz solution are under 1 cm and 2 cm respectively (fig. 2b). the observed satellites in these points 14 and 15 (fig. 4). the average initialisation time of yldz single base solution is under 20 seconds. the differences of known and measured coordinates change from 0.5 cm to 2 cm horizontally and from 0.5 to 3 cm vertically. the distance between base and rover receiver does not change perceptibly for the coordinate differences up to 50 km. however, at the 5th point, the differences of north coordinates are about 2 cm and higher than the other points (fig. 2b). judging from this result, it may be said that the distance between the receivers is important for coordinate precision. in addition to this claim, the initialisation time (fig. 5) is changed with the distance between the base and the rover receiver. the coordinate differences determined by the two rtk solutions were under 1.5 cm with 99% of measured epochs (table 2). going by these measurements, it can be said that the yldz single base rtk solution is a choice for users up to 50 km away from base station. the distance among base and rover receiver may therefore be efficient for coordinate accuracy. in addition to this conclusion, the satellite geometry has been shown to be important for rtk measurements when determining precise coordinates. being that the initialisation time is dependent on the satellite geometry, this is important for the accuracy of positioning. acknowledgments this research has been supported by the yildiz technical university scientific research projects coordination department (project number: 2012-05-03-kap01). the authors wish to thank geomatics ankara-turkey for gnss equipment and technical support. references ahmed el-mowafy 2012. global navigation satellite systems – signal, theory 162 and applications, part 7, precise real-time positioning using network rtk, pages 161-164. eren, k., uzel, t., gülal, e., yıldırım, ö, cingöz, a. 2009. results from a comprehensive global navigation satellite system test in the cors-tr network: case study, journal of surveying engineering © asce / february 2009, pages 10-18. feng, y. and wang, j. 2008. gps rtk performance characteristics and analysis, journal of global positioning systems (2008) vol. 7, no. 1, pages 1-8. gülal, e., erdoğan, h., tiryakioğlu, i̇. 2013. research on the stability analysis of gnss reference stations network by time series analysis, digital signal processing 23 (2013) pages 1945–1957. hofmann-wellenhof, b., hoffmann, et al,, h., wasle, e. 2008. gnss – global navigation satellite systems, gps, glonass, galileo, and more, springerwiennewyork, isbn 978-3-211-73012-6. hu, g.r., khoo, h.s., goh, p.c., law, c.l. 2003. development and assessment of gps virtual reference stations for rtk positioning, journal of geodesy (2003) 77: pages 292–302. kurt, a.i̇, cingöz, a. 2013. birlikte çalışabilirlik kapsamında türkiye ulusal sabit gnss i̇stasyonları ağı (tusaga-akti̇f), tmmob coğrafi̇ bi̇lgi̇ si̇stemleri̇ kongresi̇, ankara. landau, h., chen, x., kipka, a., vollarth, u. 2007. network rtk, getting ready for gnss modernization, gps world, april 2007, pages 50-55. langley 1998. rtk gps, gps world; sep 1998; 9, 9; proquest science journals, pages 70-76. rizos c. and han, s..2003. reference station network based rtk systemsconcepts and progress, wuhan university journal of natural sciences vol. 8 no. 2b, 2003, pages 566-574 rizos, h. 2003. network rtk research and implementation – a geodetic perspective. spectra precision sp-80 gnss receiver manual. volker wegener and lambert wanninger 2005. communication options for network rtk / sapos® realization, proceedings of the 2nd workshop on positioning, navigation and communication (wpnc’05) & 1st ultra-wideband expert talk (uet’05), pages 115-118. yildirim, ö., salgin ö., bakici, s. 2011. the turkish cors network (tusaga-aktif), fig working week 2011, bridging the gap between cultures, marrakech, morocco. xu, g. 2007. gps, theory, algorithms and applications, 2nd edition, springer, isbn:978-3-540-72714-9. earth sciences research journal earth sci. res. sj. vol. 16, no. 2 (dcember, 2012): 3 letter from the editor, greatly appreciated readers of esrj, i am pleased to bring you vol. 16(2) 2012 which contains interesting articles related to applying methods regarding seismic prospecting, geotechnics, geomorphology, seismology and geochemistry in such varied countries as japan, turkey, iran, nigeria and cameroon. procedures for suitably treating seismic signals by using discrete wavelet transform (dwt), or treating clays aimed at modifying their physical attributes form part of the basic contributions regarding seismic engineering and geotechnics’ immense usefulness. this issue also contains the results of geomorphological analysis carried out on the nw coast of kyushu, japan, inferring paleoclimatic conditions and the configuration of the paleoshoreline during the holocene age. such information contributes towards knowledge concerning coastal dynamics and its relationship with ancient populations’ evolution in the japanese archipelago. a systematic analysis concerning instrumental seismicity led to inferring conditions regarding the turkish peninsula’s fracture regime. such work illustrates the concentration of events and their fractal characterisation, possibly being related to this active region of the world’s seismotectonic conditions. four pieces of geochemical work, one of them related to the effects of runoff from a coal mine in iran and the other three related to the environmental characterisation of several mesozoic and palaeozoic formations in nigeria and cameroon, emphasise new geological knowledge emerging from asia and africa. we would like to thank all our authors who submitted work for making their contributions and their willingness to support esrj, and invite the whole community to continue sending us new research work coming within the scope of our geoscientific journal. on a different matter, i would like to take this opportunity to invite the geoscientific community to participate in two interesting events to be held in colombia during 2013: the 14th colombian congress of geology to be held in bogota (july 31st to august 2nd 2013) will offer an appropriate scenario for presenting and discussing recent results regarding knowledge of colombian geology, associated with this traditional meeting, will be held the simposio de exploradores, that will allow to identify new exploration horizons in o&g and mining in latin america and caribbean. and the aapg ice will be held in cartagena (8-11th september) where topics related to latinamerican basins and petroleum systems, unconventional resources, challenges concerning heavy oil, mature fields, deep water exploration and production, and environmental geology will be dealt with within the framework of the o&g industry, as well new academic foci and government interests. the aforementioned events will surely enable new advances to be made which will be of interest to all. we hope to see you in colombia during these events and the scientific workshop and commemorating of the first 100 years of san calixto observatory in la paz, bolivia. other important events in latin america will be: the meeting of the americas, a joint assembly that covers topics in all areas of the geophysical sciences. the meeting will be held at the cancun center in cancun, mexico, 14–17 may 2013. this important meeting is leadership by the agu and also sponsored by institutions like: asociación argentina de geofísicos y geodestas (aagg), asociación latinoamericana de geofísica espacial (alage), asociación latinoamericana de paleomagnetismo y geomagnetismo (latinmag), chilean geological society (cgs), centro regional de sismología para américa del sur (ceresis), geochemical society (gs), mexican association of exploration geophysicists (amge), mexican geological society (sgm), mexican union for quaternary studies (umec), mineralogical society (ms), sociedade brasileira de geofísica (sbgf), society of exploration geophysicists (seg) and unión geofísica mexicana (ugm). finally i shall mention the official opening of the national laboratories network of geosciences of colombia, an initiative sponsored by the colombian state through the national hydrocarbons agency, colciencias (national science foundation of colombia), five colombian universities (national university of colombia, caldas university, universidad industrial de santander, uptc, eafit) as well other local institutions associated to the gesociences (the colombian geologic survey, the cartographic institute and the international center of physics). a large, complete networking, of last generation, distributed countrywide, that support specialized analyses for the o&g and mining industries, academy and research centers. many of our lectors may find in this networking an interesting source of mediums for new studies along the worldwide. carlos a. vargas editor research on the effect of climate variability/climate change on runoff is limited in humid tropical regions. climate change has implications beyond the water resources sector, such as effects on agriculture and fisheries. hence, such studies are becoming increasingly important. this study uses both historical field data and future climate forecasts from general circulation model hadley centre coupled model, version 3 gcm hadcm3. these data are further downscaled using third generation of the hadley centre’s regional climate model (hadrm3), with providing regional climates for impacts studies (precis) software under the three quantifying uncertainties in model projections (qumps) with a horizontal resolution of 25 km x 25 km by the centre for climate change and adaptation research, anna university chennai, for the state of tamil nadu. these downscaled data are used to study runoff changes due to climate change for the kosasthaliyar sub-basin in south india. a trend analysis of the hydro-meteorological data for the sub-basin indicates that future rainfall is expected to decrease by approximately 10%, while the mean temperatures will increase by the year 2100. the runoff changes from 2011 to 2040 are not perceptible when compared to the historical period of 1971 to 2000. this study attempts to provide information on climate variability and its impacts on runoff in the kosasthaliyar sub-basin. la investigación del efecto variabilidad climática/cambio climático en el modelo pluviosidad/escorrentía es limitada en regiones tropicales húmedas, donde el cambio climático tiene implicaciones tanto en los recursos acuíferos, como en la agricultura y la pesca. por lo tanto este tipo de estudios han incrementado su importancia. este estudio utiliza tanto los datos adquiridos en este campo como las predicciones climáticas del modelo general de circulación de la célula de hadley, en su versión 3gcm hadcm3. estos datos fueron reducidos luego al utilizar la tercera generación del modelo climático regional del centro hadley (hadrm3), con el programa de estipulación de climas regionales para estudios de impacto (precis, en inglés), bajo los tres modelos de cuantificación de la incertidumbre en proyecciones (qumps, en inglés). el centro para el cambio climático y la investigación de adaptación de la universidad anna, de chennai, en el estado tamil nadu, utiliza una escala horizontal de 25 kilómetros por 25 kilómetros. esta reducción de datos se utiliza para estudiar los cambios de escorrentía por el cambio climático en la subcuenca de kosasthaliyar, al sur de la india. un análisis de tendencia de los datos hidrometeorológicos en la subcuenca indica que en el futuro la pluviosidad caerá en un 10 %, mientras que la temperatura media se incrementará para el año 2100. los cambios de escorrentía para el período 2011-2040 no difieren de los del período 1971-2000. este estudio es uno de los primeros acercamientos para proveer información sobre la variabilidad climática y sus impactos en la escorrentía de la subcuenca de khosastaliyar. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 45 49 abstract resumen key words: gcm, hadcm3, hadrm3, qump, runoff palabras clave: gcm; hadcm3; hadcm3; qump; escorrentía. climate variability and its impacts on runoff in the kosasthaliyar sub-basin, india balambal usha1 and b v mudgal2 1. research scholar, centre for water resources, anna university chennai. email: u_seshadri@rediffmail.com 2. professor, centre for water resources, anna university chennai. introduction evidence is mounting that climate change is occurring (alavian et al (2009)). every region of the world is expected to experience climate change. the impact of climate change on water resources may be positive or negative, depending on the geographical region (rao et al (2012)). each river basin faces a unique set of climate-related water challenges, depending on the hydrological regime. the extent that each water system will be affected by climate change will depend on the area’s degree of vulnerability, including its internal capacity to adapt to such change. internal capacity refers to the ability or resilience of the system to cope with this externality. in general, warming increases the variability in precipitation, i.e., the number of rainy days may decrease, while the intensity of rainfall may increase (mukerji 2009). this trend is likely to create excessive runoff within a short period (mall et al, 2006). additionally, as the atmosphere warms, the evaporation from soil moisture increases (chattopadhyay and hulme 1997; mall et al 2006). runoff is decreasing in some regions and increasing in other regions. whether runoff increases or decreases in a region is basin-specific within the context of climate change. meteorological data are fundamental inputs for hydrological investigations. a critical issue is that “due to changing climate past data and hydrological experience may no longer be a guide for decision making”, i.e., the stationarity of climate, which is a key assumption in planning for water resources, is no meteorology record manuscript received: 19/09/2013 accepted for publication: 31/05/2014 46 balambal usha and b v mudgal longer valid [ebnflo environmental aqua resource inc.(2010)]. hence, all impact assessments of climate change will need to incorporate a wide range of climate scenarios to study the potential of future water resources potential. the study was conducted by considering the above factors. the main aim is to assess the impacts of climate variability on the reservoir inflows of the major reservoirs located within the sub-basin of kosasthaliyar. a number of authors have used weap (water evaluation and planning system) to study stream flow impacts under climate change. a notable study was conducted by harma (2010) on the okanagan basin in british columbia. in this study, land use and climate, which are changing simultaneously, were considered for modeling stream flows using weap. the results demonstrate that all future climate conditions will critically reduce streamflows relative to the societal and ecological demands in at least a few months of “normal” and “dry” years. in all cases, the combination of demand, reservoir operations and climate variability will result in less than optimal conditions for instream flow needs. in another study by yates et al (2009), the sacramento basin in california was chosen to study the effects of climate change on reservoir inflows. the sacramento basin was divided into more than 100 sub-basins. a 37-year monthly climate time series from 1962 to 1998 was applied to a distributed hydrological model in weap. the results showed that the weap model was able to simulate the inflows well for this period. the problem the ipcc (2007) ar4 states that india could suffer from water stress – an annual water availability of less than 1000 cubic meters per capita – by 2025, and groundwater availability could decrease as much as 37 percent by mid-century. chennai basin is situated in a humid tropical climate in south india. water availability in this region depends on rainfall, particularly its spatial and temporal distribution. the basin receives approximately 1300 mm of rainfall annually. approximately 90 percent of this annual rainfall is received from two monsoons spread over a period of six months from june to december. the remaining rainfall occurs during a six-month period encompassing the summer months. most of the reservoirs in the basin are sources of domestic water for the city (indicated in the chennai basin micro level report). hence, the inflow into these reservoirs is very important because it dictates the supply of water for the basin. groundwater, which is a major water source, has been subjected to large withdrawals. therefore, surface water resources must be judiciously conserved and managed. quantifying hydrological responses to climate change is extremely important for proper water resources management (ipcc 2007). global climate models (gcms) are the best tools to study climatic variations at the global scale. however, gcms do not capture local details, which are necessary for impact assessments at the regional level. hence, there is a need to link the output from gcms by further downscaling so that they can be used as input for hydrological modeling at basin level . so far, such studies have apparently not been reported at the regional level for chennai basin. study area and methodology 3.1 study area chennai basin is situated in south india. the kosasthaliyar subbasin is the part of chennai basin located between 12°40’n-13°40’n and 79°10’e-80°25’e at the northeast corner of tamil nadu. the total area of the basin is 7282 km², 5542 km² of which is in tamil nadu; the remaining area is in andhra pradesh. the kosasthaliyar sub-basin covers an area of 2013.58 km². the poondi reservoir is a single purpose reservoir that was constructed in 1944 as a source of drinking water for the city of chennai. cholavaram and red hills are the other two reservoirs of the kosasthaliyar sub-basin. the map of the study area is given in figure 1. figure 1 study area (source: water resources department, tamil nadu) 3.2 trend analysis the non-parametric mann-kendall (mk) test (mann, (1945); kendall, (1975)) has been commonly used to assess the significance of trends in hydrometeorological series (chiew and mcmohan 1993 and others). first, the presence of a monotonic increase or decrease has been tested using the slope of the linear trend estimated from regression analysis. second, the significance of the trend is estimated using the mann –kendall method. 3.3 weap model the water evaluation and planning system (weap) is a physically based, semi-distributed river basin model developed by the stockholm environment institute. weap was developed to assess water resource allocation problems, and it is also used to study the impacts of climate change on hydrology. it is a complex integrated river basin model that operates with a flexible time step, i.e., hourly, daily, etc. the weap rainfall runoff model requires data on soil, land use, climate, etc., for the assessment of water resource availability at desired locations of a drainage basin. 3.4 research stages the methodology flowchart is given in figure 2 below. figure 2 methodology flowchart trend and variability analysis (1971-2000) projection (2011-2098) climate change impact assessment on water resources of chennai basin hydrometeorolical parameters of rainfall and temperature runoff changes due to changes in hydrometeorological parameters inferences based on results from above 47climate variability and its impacts on runoff in the kosasthaliyar sub-basin, india to conduct this research, rainfall and temperature field data of the past 30 years were analyzed; subsequently, the virgin flows for the same 30-year historical period (1971-2000) were used to develop a hydrological model. the rainfall and temperature data are used to drive the model to study past runoff impacts. along with this analysis, rcm data of rainfall and temperature for the same period are used to simulate the runoff. then, the results from the two simulations are compared. furthermore, rcm data are used for predicting future flows for the period of 2011-2040. for past impact analysis, field data of monthly rainfall and virgin flows were obtained from the institute for water studies, taramani, chennai. data for temperature were obtained from the imd (india meteorological department). to study the future changes in the basin, 120 years of daily climatic data were obtained by dynamically downscaling gcm hadcm3 output using precis software. the a1b emission scenario under the three qump (quantifying uncertainties in model projections) was considered. the spatial resolution of hadrm3 (regional climate model) was 25 km x 25 km. a land use map from satellite images was obtained from the institute of remote sensing (irs) , anna university, chennai. the land use map of the catchment of poondi reservoir is given in figure 3. figure 3 land use map of the catchment of poondi reservoir irs 1997. data for the weap input parameters, such as soil water capacity, hydraulic conductivity, crop coefficients, net irrigated area, deep water capacity and deep conductivity, were obtained from the institute for water studies, chennai. the leaf area index (lai) was calculated by using the fao method. the leaf area index is obtained from crop coefficients of both the mid-season and harvesting season. the initial value of the crop coefficient is 0.1. to assess the effects of climate change on future flows, weap was used to first calibrate and validate the model for the kosasthaliyar reservoirs with virgin flow data. a monthly time step was selected, as climate change is a longterm phenomenon. hence, monthly flows aggregated to annual flows were considered more representative of stream flows than shorter time steps. upon consultation with a few field experts, it was found that the catchment of the red hills is a highly urbanized sub-watershed. the virgin flows into both the red hills reservoir and the cholavaram tank are negligible. the runoff from kosasthaliyar is stored in poondi reservoir. therefore, only the virgin flows of poondi were considered, and the watershed was considered a single reservoir system. the stream flows were modeled accordingly. results and discussion 4.1 trend analysis – past (1971-2000) the main aim of this paper is to study the climate variability impacts on runoff. therefore, a detailed description of the work conducted on trend analysis is not presented. instead, a snapshot of the results of the trend analysis is shown in figure 4. an initial analysis indicates increases for both rainfall and temperature during 1971-2000 (control climate). the mk test at a significance level of 0.85 showed that these trends are statistically significant. further rainfall and runoff trends for the sub-basin of kosasthaliyar showed an increase. 4.1 trend analysis – future (2011-2100) the graphs show that rainfall will decrease in the coming years, whereas temperatures will increase. the maximum mean temperature (during may) is likely to increase from 34.5 degrees celsius in 1970-2000 to 37.5 degrees celsius by 2098. 4a 4b figure 4(a-b) chennai basin climatology – past and future monthly rainfall and temperature trends 4.3 weap model the upstream catchment of the poondi reservoir was considered for the analysis. the same period of analysis was taken into consideration, where1971-2000 is the control period and 2011 – 2040 is the future climate. the runoff in relation to the rainfall for the kosasthaliyar basin was plotted. the runoff increases in accordance with the rainfall for this sub-basin. to test the statistical significance of this trend, the mann-kendall test was performed 48 balambal usha and b v mudgal using xlstat. the trend is statistically significant at the significance level of alpha equal to 0.3 with a p-value of 0.288. to assess the effects of climate change on future flows, weap was used to first calibrate and validate the model for the poondi reservoir with virgin flow data. for rainfall data that used field data for the first simulation run, the mean of five relevant rain gauge stations was considered for the analysis. for the second simulation run using rcm data, a monthly bias correction factor was applied to the rcm rainfall data for the average rainfall of the nearby grids because there were seasonal biases compared to the field data. literature on rcm studies indicates that rcm data should never be used in hydrological studies without bias correction (maraun et al 2010). the details of the calibration and validation periods are given in table 1. table 1 model testing period for the calibration and validation reservoir calibration validation poondi 1971-1998 1998-2000 the model performance was assessed using nse and other indices, as shown in table 2.the nash sutcliffe efficiency (nse) produced very high values for both the calibration and validation of the field data. for the rcmderived runoff, the nse values were within an acceptable range. per the performance rating recommended by moriasi et al (2007) for the monthly time step, this model is very good for chennai basin. table 2 indices for testing the model efficiency nse calibration validation field data 0.92 0.85 rcm bias corrected 0.26 0.4 4.4 future impacts on runoff of the kosasthaliyar sub-basin the weap model was used to predict inflows under climate change. to build a future scenario, a single land use change was considered for each decade i.e., 2011-2020, 2021-2030 and 2031-2040. for poondi, the change in land use was an increase in the wasteland, which leads to a reduction in water bodies, as shown in figure 5. it might be noted that there were no major perceptible changes in the land use extrapolated for the future period. this process, coupled with climate change under the a1b emission scenario, was used to model future inflows. these results are graphically represented in figure 6. 5a figure 5 (a-c) future land use changes/land cover for the poondi reservoir 6a 6b 6c figure 6(a-c) climate change effects on runoff – baseline and future periods the graphs show that overall, there will be no significant change in the runoff in the coming years. 49climate variability and its impacts on runoff in the kosasthaliyar sub-basin, india conclusions in this study, runoff changes due to climate change were investigated for the kosasthaliyar sub-basin, south india. both past and future impacts on runoff were mapped using field data and climate model data, i.e., data from rcm precis using a horizontal resolution of 25 km x 25 km. the climatic data obtained were superimposed on a hydrological model using weap software. from a trend analysis of the climatic data and runoff data, the following are observed: approximately a 3°c rise in temperature is expected by the end of 2100. this is consistent with other studies on chennai basin using the same a1b scenario (nic report). additionally, the rainfall projections for the same scenario in the regionindicate a 10 percent reduction for the basin. this result is also consistent with the cited literature. runoff is mainly influenced by rainfall in the area. the weap model is able to simulate the hydrology of chennai basin under changing climatic conditions. this study attempts to estimate runoff changes due to climate change in the study area using the very high spatial resolution rcm hadrm3, the a1b scenario, and qump projections. notably, a larger set of climate simulations is necessary to improve the robustness of the conclusion that there is no significant change in the future runoff when compared to the baseline period for this sub-basin. this conclusion is based on the rcm data, which was derived from one ensemble run. this limitation was mainly due to the run-time of the simulations (approximately six months). using simulations from multiple runs could provide a better estimate of the range of variability in the future stream flows. references alavian, v., qaddumi, h. m., dickson, e., diez, s. m., danilenko, a. v., hirji, r. f., and blankespoor, b. 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(2009). climate driven water resources model of the sacramento basin, california. journal of water resources planning and management, 135(5), 303-313. palabras clave: piritas; cuarzos; tierras raras; mineralogía genética; yacimiento aurífero de gaosongshan; china. how to cite item geng, h., gu, x., & zhang, y. (2018). characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china. earth sciences research journal, 22(4), 301-318. doi: https://doi.org/10.15446/esrj.v22n4.57512 the gaosongshan epithermal gold deposit in heilongjiang, northeast china, is hosted by the lower cretaceous intermediate-basic volcanic rocks. three auriferous quartz veins including eleven gold orebodies were all discovered in tectonoclastic zones. genetic mineralogy study including the thermoelectricity, rare earth elements and trace elements of pyrite and rare earth elements of quartz were carried out. thermoelectric conductive type of pyrite is mainly n-p type. calculating the thermoelectric parameters xnp and denudation percentage γ of pyrites from orebodies 1-i, 2-ii and 2-iv, suggests that gold orebodies are all eroded to their middle-lower parts. the variable range of co concentrations (51.3-264.0ppm) and ni concentrations (68.9-258.0ppm) and co: ni ratio (0.31-1.90), together with relatively small sr/ba ratio in ore-bearing pyrites (0.11-0.50), supports a hydrothermal origin of mineralization at gaosongshan gold deposit. compared with volcanic rocks, the chondrite-normalized ree patterns of ore-bearing pyrites and quartz are all lree enriched with similar σlree/σhree ratio ranging from 7.37-13.68 in ore-bearing pyrites, 4.74-15.37 in ore-bearing quartz and no ce anomalies. δeu values in ore-bearing pyrites and quartz are 0.651.66 (average=0.93) and 0.66-1.62 (average=1.03), respectively. δeu values of volcanic rocks are 0.86-1.07 (average 0.94), suggesting no obvious negative eu anomalies. similar ree characteristics of ore-bearing pyrites and quartz and volcanic rocks, together with previous oxygen and hydrogen isotope studies of quartz, suggest that the ore-forming fluids of the gaosongshan gold deposit were mainly magmatic origin which was associated with andesitic magma and was partly mixed with atmospheric water. comparing trace elements characteristics of ore-bearing pyrites with volcanic rocks, together with previous s isotopic studies, it is concluded that the ore-forming materials were derived from the surrounding rocks. slight changes of y/ho (23.80-27.28), zr/hf (35.41-47.83), nb/ta (10.96-18.52) in orebearing pyrites indicate that the ore-forming fluid system is relatively stable during the ore-forming process abstract keywords: pyrite; quartz; rare earth elements; genetic mineralogy; gaosongshan gold deposit; china. characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china características de mineralogía genética de piritas y cuarzos y su importancia en el yacimiento aurífero de gaosongshan, provincia de heilongjiang, al noreste de china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.57512 el yacimiento aurífero epitermal de gaosongshan en heilongjiang, al noreste de china, está incrustado en una formación de rocas volcánicas en el cretácico inferior. tres vetas de cuarzo aurífero que incluyen once yacimientos de oro se descubrieron en zonas tectonoclásticas. se realizaron análisis de mineralogía genética, entre estos de termoelectricidad, de tierras raras, y de trazas de piritas y tierras raras. el tipo de conductividad termoeléctrica de la pirita es principalmente tipo n-p. el cálculo de los parámetros xnp de termoelectricidad y el porcentaje de denudación γ de piritas en los yacimientos 1-i, 2-ii y 2-iv sugiere que los yacimientos auríferos están erosionados en sus partes medias y bajas. el rango variable de las concentraciones de co (51.3-264.0ppm) y de ni (68.9-258.0ppm), y el índice co: ni (0.311.90), junto con un índice pequeño de sr/ba de piritas mineralizadas (0.11-0.50) suponen un origen hidrotérmico de mineralización en el yacimiento aurífero de gaosongshan. comparado con rocas volcánicas, los patrones de tierras raras en las condritas normalizadas de piritas y cuarzos son todos tierras raras ligeros enriquecidos con índices σlree/σhree similares y que se ubican en el rango 7.37-13.68 en las piritas mineralizadas, 4.74-15.37 en los cuarzos mineralizados y sin anomalías de cerio. los valores δeu en piritas mineralizadas y cuarzos son de 0.65-1.66 (con promedio de 0.93) y 0.66-1.62 (con promedio de 1.03), respectivamente. los valores δeu de las rocas volcánicas son 0.861.07 (con promedio de 0.94), lo que sugiere que no hay anomalías negativas de europio. las características similares de tierras raras entre piritas mineralizadas, cuarzos y rocas volcánicas, al igual que estudios previos de isotopos de oxígeno e hidrógeno en cuarzos, sugieren que los fluidos hidrotermales del yacimiento de gaosongshan son principalmente de origen magmático, el cual está asociado con el magma andesítico, y está parcialmente mezclado con agua atmosférica. resumen record manuscript received: 22/05/2017 accepted for publication: 27/07/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 301-318 huiqing geng1,2, xuexiang gu2, yongmei zhang2 1 school of resources, hebei geosciences university, shijiazhuang 050031, china. qingqing_heb@126.com 2 school of earth sciences and resources, china university of geosciences, beijing 100083, china m in e r a l o g y 302 huiqing geng, xuexiang gu, yongmei zhang introduction the epithermal deposit is originally defined by lindgren (1933). after decades of research (heald et al., 1987; hedenquist and lowenstern, 1994; cooke and simmons, 2000; chen et al., 2001), the terminology epithermal is still used but its definition has been broadened. it is considered that epithermal deposits formed at relatively low temperatures (100-320°c) and shallow depths (surface to 1-2km) (white and hedenquist 1995; chen et al., 2012). according to their geological environments, altered minerals and fluid chemistry, the epithermal deposits can be classified into two types: low-sulfidation type and high-sulfidation type (bonham, 1986; white and hedenquist, 1990, 1995; hedenquist et al., 1998; hedenquist et al., 2000), also known as adularia-sericite type and alunite-kaolinite type, respectively (heald et al., 1987; arribas, 1995; hedenquist et al., 1998). as one of important type of gold deposits in the world, epithermal gold deposits have provided more than 6000 tons of gold reserve (kerrich et al., 2000), which are mainly located in the metallogenic domain of the circum-pacific, mediterraneanhimalaya and paleo-asian (chen et al., 2001; jiang et al., 2004; sillitoe, 2008; hao et al., 2016). many deposits of this type developed in china’s northeast region, which is not only one part of pacific rim metallogenic domain, but also one of paleo-asian metallogenic domain. these deposits are mostly distributed in four ore concentrating districts: derbugan, huma, lesser hinggan range, eastern part of jilin (qi et al., 2005). gaosongshan deposit is a newly discovered large epithermal gold deposit which is located in the lesser hinggan range ore concentrating district. the electrical properties of pyrite have been studied since the beginning of the 19th century (hill and green, 1962; zhao, 1990; yan et al., 2012). as an important physical property, the thermoelectricity of pyrite has been widely applied in gold deposits research (ying et al., 2001; ji et al., 2013; xue et al., 2014). the change rule of the conduction type and thermoelectric in the vertical direction is an important sign denoting the orebody denudation level (li et al., 1996; chen, 1987; shao et al., 1990). with detailed genetic mineralogy data of pyrite, the prospects of gold deposits can get better evaluated. nowadays, trace elements and rare earth elements (ree) determinations of pyrites and quartz are carried out by different in situ techniques such as laser ablation-inductively coupled plasma-mass spectrometry (la-icp-ms), secondary ion mass spectrometry (sims) and electron probe microanalysis (epma). as one of the most sensitive methods, inductively coupled plasma mass spectrometry (icp-ms) has been widely utilized in recent years (date and gray, 1985; lichte et al., 1987; kantipuly and westland, 1988; liu, 1995; fan et al., 2000; ghazi et al., 1993; su et al., 1998; bie et al., 2000; li et al., 2003; li et al., 2004). the trace elements and the rees are valuable in identifying the source of ore-forming materials and fluid, tracing the evolution of hydrothermal systems, constraining the conditions of ore formation, and evaluating the genesis of ore deposits (dennen, 1967; henderson, 1984; michard and albarède, 1986; shao, 1988; lottermoser, 1992; klinkhammer et al., 1994; franchini et al., 2015). previous studies about the gaosongshan gold deposit focusing on metallogenic geological conditions, the origin of metallogenic materials, petrology and geochemistry of ore-bearing volcanic rocks, diagenetic tectonic setting, gold enrichment mechanism and geochemical studies of primary halos, have been published (lv, 2012; wang et al., 2014; zheng et al., 2014; hao et al., 2014; hao et al., 2016). no work about the thermoelectric properties of pyrites, ree characteristics of pyrites and quartz and their significant geological significance has yet been published in english. in this study, new ree data of pyrites and quartz from the gaosongshan gold deposit, analyzed with icp-ms, are presented to further constrain the source of hydrothermal fluid and ore-forming materials. measured with bhte08 thermoelectric coefficient instrument, the thermoelectric properties of pyrites are analyzed to provide information about the orebody denudation level and the medium conditions during the formation of deposits, and thus giving suggestions on finding blind orebodies. regional geology the gaosongshan gold deposit is located in the northeastern part of the east xingmeng orogenic belt (exob) in ne china (fig. 1a). the exob, which is separated from the north china craton by the chifengkaiyuan fault, located between the siberian and the north china craton, comprises a series of micro-continents (sengör et al., 1993; zeng et al., 2012; ouyang et al., 2013): the eerguna block in the northeast, the xing’an and songliao blocks in the center, the liaoyuan terrane in the southeast and the jiamusi block in the east, which is separated from the songliao block by the mudanjiang fault. tectonism and magmatism in this area were controlled by the evolution of the paleo-asian ocean in the paleozoic. final closure of the paleo-asian ocean which is characterized by suturing between the songliao block and the liaoyuan block is believed to have taken place along the solonker-xar moron-changchun suture zone during the late permian to the early triassic (wu et al., 2011; ouyang et al., 2013). according to previous works (e.g., zhou et al., 2009; wilde et al., 2010; zhang et al., 2010), the exposed stratigraphy in this study area comprises precambrian metamorphic basement, cambrian and permian submarine sedimentary cover, jurassic to cretaceous terrigenous clastic and volcanic rocks and cenozoic clastic rocks and basalt. the precambrian metamorphic rocks are composed of gold-bearing silicon-iron formation from the dongfengshan group and schist from the heilongjiang group. cambrian rocks consist of dolomite, dolomite limestone and marble, unconformably overlying the precambrian metamorphic rocks. permian metamorphic clastic rocks comprise metasandstone, slate and phyllite, which discordantly overlie the cambrian strata. meanwhile, the permian strata are discordantly overlain by the jurassic terrigenous clastic rocks, consisting of sandstone and sandy conglomerate. the cretaceous volcanic rocks, widely distributed throughout this area, are composed of basalt, trachyandesite, andesite, tuff, rhyolite and rhyolitic tuff. the oligocene clast rocks and the quaternary unconsolidated sediment are also extensively distributed in this study area. complex geological evolution from the early jurassic has resulted in extensive precious and nonferrous metal mineralization in this area, especially epithermal gold mineralization. a number of deposits including gaosongshan, dong’an, and tuanjiegou (zhou et al., 2002; zhang et al., 2010; liu et al., 2015; hao et al., 2016; wang et al., 2016;) have been discovered in recent years in heilongjiang province. the oldest formations in this area are proterozoic greenschist facies metamorphic rocks of the heilongjiang group. gold mineralization was controlled by many faults and its secondary faults. major structures are dominated by the nw-setrending shaaqi fault and a number of associated fractures trending nee, nww and ew (fig. 1c). deposit geology strata the gaosongshan gold deposit, one of the epithermal gold deposits in heilongjiang province, discovered in 2000, is located in the northern part of the yichun-yanshou fold belt, which is situated between the xing’ansongnen block and the jiamusi block (wilde et al., 2010; zhou et al., 2010) (fig. 1b). the strata exposed are mainly intermediate-basic volcanic rocks, consist of the lower cretaceous banzifang formation (k1b) and the ningyuancun formation (k1n) (fig. 2), hosting the mineralization at al comparar las características de los elementos traza en las piritas mineralizadas con las rocas volcánicas, y basados en estudios isotópicos del azufre, se puede concluir que los materiales hidrotermales se derivan de las rocas circundantes. pequeños cambios de las piritas mineralizadas en y/ho (23.8027.28), zr/hf (35.4147.83), nb/ta (10.96-1852) indican que el sistema hidrotermal del depósito es relativamente estabe durante los procesos hidrotérmicos. 303characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china gaosongshan gold deposit. the banzifang formation, the main wall rock of veins i and ii, is widespread in this area and consists of andesite, basaltic andesite, dacite, trachyandesite, andesitic volcanic breccia, and tufflava. the ningyuancun formation, the major host rock of vein iii, is composed of tuff, rhyolite, and volcaniclastic rock. magmatic rocks although the scale of intrusive rocks in this area is relatively small, the types of intrusive rocks are complex, mainly intermediate-acid rocks. granite and alaskite from the late variscan, located in the southern and eastern of the mining area, mainly occur as batholite or stock. biotite granite, granodiorite and granite porphyry from the early yanshanian, distributed in the eastern part of the mining area, occur as batholite, stock or minor intrusive bodies. structure there are many well-developed fault structures, such as the shaaqi fracture, providing channels for ore solutions, which is nw-se trending, and its associated fracture, the main host structure, trending nee, nww or ew (fig. 2). the wuqiganxiao-gaosongshan fault and xueshuiwen fault are ne-trending extensional structures controlling the emplacement of early mesozoic magmatic rocks and the deposition of mesozoic volcanic rocks (fig. 1c). figure 1. (a) tectonic framework of northeastern china (modified from wu et al., 2000; wu et al., 2011; imbgmr, 1991). (b) the distribution of the gold deposits in heilongjiang province (after hbgmr, 1993). (c) regional geological map of the gaosongshan gold deposit (modified after hao et al., 2016) 304 huiqing geng, xuexiang gu, yongmei zhang orebodies four auriferous quartz veins have been found in gaosongshan gold deposit, which are subdivided into 11 mineralized zones or orebodies, trending nee, nww and approximately ew. gold orebodies discovered in the gaosongshan gold deposit are all situated in tectonoclastic zones and mainly occur as vein type bodies (fig. 2), and their wall rocks primarily consist of andesite, volcanic breccia, tuff, and basalt. among all the gold ore bodies, 1-i, 2-i and 2-ii are nww-trending to nw-trending; 1-ii, 1-iii, 2-ii-1, 2-iii, and 2-iv are close to ew-trending; and 1-iv, 3-i, and 3-ii are nee-trending. the 1-i gold ore body, the biggest one, is ~2 km long. it is controlled by different sets of fractures in different directions that the west part of this body is ew-trending and the east part is nww-trending to see-trending (fig. 2). it strikes ~56-368m long with the strike direction of 86-115° and the dip angle 55°-72°. the largest thickness is 7.90m, while the average thickness is 1.72m. the average grade of au is 5.47g/t. the content of au in this ore body accounts for 76.8% of the total resources in this mining area. total measured, indicated, and inferred resources for the gaosongshan gold deposit are 21.7tonnes of gold with the grade of 6.3g/t au, making it one of the largest epithermal gold deposits in china. ore ores are generally divided into breccia type, quartz-vein type and altered rock type (fig. 3a, b, c). for the first type, the breccia fragments are composed of poorly sorted angular clasts of altered andesite, volcanic breccia and tectonic breccia, which were cemented by quartz-sulfide veins and the breccia partly contains chlorite and illite alteration minerals and pyrite (fig. 3a, g). commonly, anhedral granular pyrite and lesser amount of chalcopyrite are presented in this siliceous cement (fig. 3f). after oxidation and leaching, pyrite were oxidized into limonite, which occurs as veinlets or stockworks, meanwhile, part of the pyrites, are distributed as sparse disseminations or cloddy. quartz-vein type can occur in many kinds of volcanic rocks, mainly andesite and volcanic breccia. these rocks were crosscut by quartz-sulfide vein, with pyrite occurring as fine-grained. native gold, distributed in quartz vein, can also be observed in this ore type (fig. 3b) and gold mineralization mainly occurs with silicification and pyritization. altered rock type is strongly associated with intense silicification, pyritization, chloritization and illitization and its protolith are commonly volcanic breccia and tuff (fig. 3c). pyrite occurs as massive, dense disseminated in the gray quartz vein together with chlorite (fig. 3d). mineralogy pyrite with minor chalcopyrite and native gold are present in the ores. most of ore minerals are predominantly subhedral grains, although some displays euhedral, cubic or anhedral textures (fig. 3c, f). quartz, chlorite, illite, adularia, sericite, calcite, fluorite are gangue minerals and commonly occur as fine-grained assemblages, although some are mediumand micrograined assemblages. the ore minerals are distributed as sparse disseminated, locally as dense disseminated or scattered (fig. 3c, e). conversely, the gangue minerals occur as massive, veinlets or drusy structures (fig. 3b, fig. 4a, d). sulfides are the major ore minerals, which generally account for less than 1vol. %. pyrite is the most abundant sulfide coexisting with minor anhedral chalcopyrite, galena and sphalerite (fig. 3f). quartz is believed to be the most extensive gangue minerals in ores, accounting for more than 60% of the ores by volume. it occurs predominately as veinlets with sulfides or as stockworks individually. gold grains occur as irregular, sheet or filiform and range in size from 0.1 to 10mm in diameter (hao et al., 2014). they are distributed predominately along quartz crystal boundaries in the form of native gold. secondly, lesser amounts of gold are enclosed in pyrite, in the form of native gold in the early stage and invisible cationic gold in the late stage, respectively. adularia can be occasionally seen under the microscope with its typical rhombic section and is commonly closely associated with quartz and sericite (fig. 3h). adularia forms euhedral-subhedral platy and rhombic crystals with a diameter of <0.1mm. calcite is a common mineral, developed as veins with quartz and chlorite (fig. 3i, fig. 4i). fluorite is purple in color associated with quartz and chlorite and it occurs as anhedral assemblages (fig. 4f). alteration according to previous works (zheng et al., 2015) and our field observations, the wall rock alteration minerals in the gaosongshan gold deposit comprise quartz, adularia, chlorite, sericite, illite, fluorite, calcite and lesser amounts of pyrite. the quartz, adularia and pyrite alteration is spatially closely associated with gold mineralization. this hydrothermal alteration is mainly developed along faults and in volcanic rocks and breccia (fig. 4a, b, c and e), showing an alteration zone, from the proximal mines with silicification, pyritization and adularization to the distal mines with sericitization, illitization and propylitization. figure 2. geological sketch map of the gaosongshan gold deposit (cited from zheng, 2013). 305characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china figure 3. photographs of representative hand specimens showing the types of gold ores (a, b, c) and microphotographs showing some ore minerals and gangue minerals in the gaosongshan gold deposit. (a) breccia type: strongly altered volcanic breccia cemented by gray quartz-sulfide stockworks, most of the pyrite were oxidized into limonite; (b) quartz-vein type: breccia fragments were cemented by wide milky quartz with native gold; (c) altered rock type: volcanic breccia with intense silicification, pyritization, chloritization, pyrite sparsely disseminated in the gray quartz vein together with chlorite; (d) breccia fragments were cemented by quartz-pyrite-chlorite veins, with pyrite being cubic or subhedral-anhedral granular; (e) cubic pyrite disseminated in tectonic breccia; (f) anhedral chalcopyrite in quartz vein; (g) plagioclase in andesite altered into sericite, thin chlorite vein grow with quartz edge in amydaloidal structure; (h) adularia can be seen under the microscope with its typical rhombic section, part of adularia were replaced by quartz; (i) calcite vein with its typical rhombic cleavage. photographs c and f were taken under reflected plane-polarized light, while figures g, h and i were taken under reflected crossed-polarized light. qtz-quartz, lm-limonite, py-pyrite, ccp-chalcopyrite, chl-chlorite, adl-adularia, pl-plagioclase, au-native gold, ser-sericite, cal-calcite figure 4. photographs of mineralization and alteration styles. (a) extensive gray quartz veinlets, stockworks and milky quartz vein with drusy quartz developed in tectonic breccia with disseminated and massive pyrite; (b) purple volcanic breccia with strong chloritization and epidotization; (c) quartz-pyrite vein: cubic and euhedral-subhedral pyrite being massive distributed in milky quartz; (d) andesite with strong and extensive kaolinization and chloritization crosscut by 3cm quartz-calcite vein; (e) strongly kaolinization with some fine grained pyrite; (f) pruple-colored fluorite associated with white quartz (iv); (g) second-stage quartz vein cut by two third-stage quartz vein developed in silicified volcanic breccia with scattered xenomorphic granular pyrite; (h) second-stage quartz veinlet crosscut by fourth-stage quartz vein; (i) fourth-stage quartz-calcite vein, white calcite overgrows within quartz. qtz-quartz, py-pyrite, chl-chlorite, ep-epidote, kln-kaolinite, cal-calcite, fl-fluorite 306 huiqing geng, xuexiang gu, yongmei zhang based on ore textures, crosscutting relationships, and mineral assemblages, the hydrothermal mineralization process can be divided into four stages: chlorite/ epidote + pyrite + sericite + illite + adularia (stage i), quartz + pyrite (stage ii), quartz + pyrite + au (stage iii), and quartz + carbonate + fluorite (stage iv). the quartz in stage ii occur commonly as milky white stock-work vein or gray microcrystal veins (fig. 4g). stage iii is considered to be the main stage for ore formation and is characterized by the occurrence of au. pyrite is scattered or disseminated in thin smoky gray quartz veins (fig. 4h). the quartz in stage iv is characterized by milky veins or miarolitic cavities, associated with white calcite and purple fluorite (fig. 4d, i). all above suggests that the gaosongshan gold deposit is a typical low-sulfidation epithermal gold deposit (zheng et al., 2014; hao et al., 2016). sampling and analytical methods samples including quartz of stage ii to stage iv, pyrites of stage ii and iii (fig. 4), were collected from different ore bodies. in the preparation of samples, quartz and pyrite-bearing samples were crushed into a grain size of approximately 300 to 500μm using a pre-cleaned griddle and pestle. about 2g pure quartz grains and 1g pure pyrite grains were hand-picked using a binocular microscope to eliminate visible impurities. the calcite impurities on the surface were readily removed after 24hours of 10% hcl etching. then all samples were washed with ultrapure water and then finally dried. the thermoelectricity of pyrites was measured by bhte-08 thermoelectric coefficient instrument (made by beijing university of aeronautics and astronautics) at the laboratory of genetic mineralogy, china university of geosciences (beijing). parameters of this measuring instrument are as follows: reading accuracy is 0.1μv/°c, and its measuring accuracy is beyond 1%fs. activation temperature was picked at 60°c with indication error small than 1°c. each sample was randomly prepared 50 granules for testing. 100mg of each sample used in trace elements analysis were crushed again to a grain size of approximately less than 70μm using a carefully precleaned agate mortar and pestle. tests on the mortar and the pestle showed that contamination introduced in this way could be neglected. the analyses were carried out at the ore deposit geochemistry microanalysis room, the state key laboratory of geological processes and mineral resources, china university of geosciences (beijing). 50±0.3 mg of powdered sample was weighed into a teflon digestion vial, and 1 ml of hcl, 1 ml of hno3, 3 ml of hf, and less than 0.5 ml of concentrated hclo4 were added. screw the cap tightly and then it was heated for 48 hours at temperature 170°c on a hot plate. unscrew the cap and until it was evaporated to saltlike residue. then the residue was added with 2.5 ml hno3 and 2.5 ml ultrapure water. screw the cap tightly and then it was heated for 12 hours at temperature 170°c on a hot plate. after cooling, the solution in the each vial was subsequently transferred into teflon plastic bottles and made up with ultrapure water to a volume of 50 ml. icp-ms analysis was carried out with thermo fisher x series ii quadrupole plasma mass spectrometer of america. the analytical accuracy is higher than 5% for element contents>10ppm, and higher than 10% for element contents<10ppm. in order to ensure data accuracy, standard samples, parallel samples and blank samples (2% hno3) were used during the experiment. 10ppb rh solution was the internal standard for the icpms measurements. part of the analysis of pyrites was accomplished at analytical laboratory of beijing research institute of uranium geology, and the icp-ms instrument is a high resolution one (model element i, finnigan mat). experiments were conducted in accordance with general rules of dz/t0223-2001 icp-ms methods under the following conditions: temperature of 20°c, relative humidity of 30%. characteristics of genetic mineralogy of pyrite and quartz thermoelectricity twenty-six pyrite samples, 1300 granules in total, were collected at different bodies in the gaosongshan gold deposit, and the results are listed in table 1. based on pyroelectric coefficient, pyrite is divided into two types: (1) n-type (α < 0 μv·ºc-1), and (2) p-type (α > 0 μv·ºc-1). conductive types of pyrites in this deposit belong to mixed type, with thermoelectric coefficients ranging from -262.6 μv·°c-1 to 335.6 μv·°c-1. it can be concluded from figure 5 that pyroelectric coefficient of n-type ranges from -262.6 μv·°c-1 to -1.6 μv·°c-1 with pronounced intervals at -161.7 μv·°c-1 to -15.1 μv·°c-1. and those of p-type range from 1.6 μv·°c-1 to 335.6 μv·°c-1 with two respective pronounced intervals at 29.4-140.8 μv·°c-1 and 182.5 335.6 μv·°c-1. figure 5. frequency histogram of pyrites’ thermoelectric coefficient rare earth elements pyrite the measured ree concentrations of thirteen gold-bearing and nineteen au-free pyrites in the gaosongshan gold deposit are presented in tables 2 and 3, and the chondrite-normalized curves are shown in figure 6. the total ree concentrations in ore-bearing pyrites are in the range of 9.75-139.43 ppm. the chondrite-normalized ree patterns are characterized by lree enrichment. σlree are 9.09-124.48 ppm, and σhree are 0.67 14.95 ppm. the lree/hree ratios are 7.37-13.68 (mean= 10.39). (la/sm) n are 0.71-16.75 (mean= 4.77), and (gd/yb) n are 2.1912.75 (mean= 8.69). two samples, 1-ii zk1502-06 and 1-izk8001-11/12, have positive eu anomalies and the rest have obvious or weak negative eu anomalies. generally, δeu= 0.65-1.66 (mean= 0.93). δce are 0.98-1.11, and the average value is 1.02, for which no ce anomaly is observed. the chondrite-normalized ree patterns for au-free pyrites are similar to those of gold-bearing pyrites. σree ranges from 4.75-123.91 ppm, σlree are 4.07-115.47 ppm, σhree are 0.69-9.16 ppm, and the σlree/σhree ratios are 5.93 13.77 (average: 8.85). (la/sm)n are 2.259.92 (mean= 4.77) and (gd/yb)n are 1.67-11.16 (mean= 5.42). unlike the gold-bearing pyrites, au-free pyrites display an obvious negative eu anomaly and no ce anomaly with δeu= 0.55-1.11 (mean= 0.82) and δce = 0.92-1.10 (mean= 1.00). quartz the ree concentrations and related parameters of quartz are given in table 4 and the chondrite-normalized ree patterns are shown in figure 7. ree concentrations in ore-bearing quartz are lower than those in orebearing pyrites, ranging from 3.56 ppm to 10.08 ppm and those in barren quartz ranging from 0.47 ppm to 6.50 ppm, which are much smaller. the lree and hree contents in ore-bearing quartz are 3.27-9.37 ppm and 0.29 1.32 ppm, respectively, while those in barren quartz are 0.41-6.17 ppm and 0.06 0.33 ppm, respectively. the lree/hree ratios of ore-bearing quartz and barren quartz are 4.74-15.37 and 5.62-18.85, respectively. (la/ yb)n ratios of ore-bearing quartz and barren quartz are 7.59-50.22 and 5.36-75.53, respectively. the lree enrichment chondrite-normalized ree patterns are characteristic of quartz in the gaosongshan gold deposit (fig. 307characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china 7). the (la/sm)n ratios and (gd/yb)n ratios for ore-bearing quartz are 2.857.41 and 2.19-5.97, respectively. those for barren quartz are 3.06-13.29 and 1.12 10.13. all the quartz samples are characterized by no ce anomalies. δce values range from 0.88 to 1.00 (average 0.96) in ore-bearing quartz and those range from 0.84 to 1.07, with an average of 0.96 in barren quartz. barren quartz is characterized with obvious negative eu anomalies (δeu = 0.45-0.95, average: 0.80). meanwhile, the values of δeu are 0.66-1.62 (average: 1.03) in ore-bearing quartz in general, showing obvious negative eu anomalies in samples 1-ii zk1502-07 and 2-ii zk9601-04\05, no eu anomalies in samples 1-i zk8001-05 and 1-i zk9604-12, while sample 2-ii zk8004-03-2 exhibits a positive eu anomaly. figure 7. rare earth element chondrite-normalized patterns of quartz from the gaosongshan gold deposit. trace elements the trace elements contents of thirteen ore-bearing pyrites in the gaosongshan gold deposit are listed in table 5, and those of the eighteen au-free pyrites are listed in table 6. a spider diagram of the portions of rare earth elements, lithophile elements, and high-field-strength elements with respect to the chinese continental crust trace elements is shown in figure 8. pyrite commonly contains a high level of trace elements, either hosted by substitution within the crystal lattice, such as co and ni replace fe, and as, se, and te replace s, or as nanoparticles or micro-inclusions, such as au, ag, cu, pb, zn, sb, bi, and w. co and ni are the common and abundant trace elements in pyrite, with co contents of ore-bearing pyrites ranging from 51.3 264.0ppm, with an average of 157.1ppm and ni contents ranging from 68.9 258.0ppm, with an average of 225.7ppm. the ratios of co/ni are 0.31 1.90, with an average of 0.92. the contents of co in au-free pyrites are 63.4 426.0ppm, with an average of 170.6ppm. the contents of ni are 16.0 421.7ppm, with an average value of 126.9ppm. the ratios of co/ni are 0.33 5.36, with a mean value of 2.26. the ratios of sr/ba in ore-bearing pyrites are 0.11 0.50, with an average value of 0.24, and that of au-free pyrites are 0.09 1.11, with an average value of 0.39. the hf/sm ratios in ore-bearing pyrites are 0.28-0.94. the nb/la and th/la ratios are 0.06-0.28 and 0.05-0.22, respectively. the y/ho ratios are 23.80-27.28, except for that of one sample (1-izk8001-11/12), the zr/hf ratios are 35.41 47.83, and the nb/ta ratios are 10.96-18.52. the ratios of hf/ sm, nb/la, th/la, y/ho, zr/hf, and nb/ta in ore-free pyrite are 0.39-1.17, 0.07-0.45, 0.06-0.28, 23.51-28.42, 33.99-48.87, and 12.61-22.84, respectively. ore-bearing pyrites and ore-free pyrites in the spider diagram of the trace elements generally exhibit similar characteristics, compared with the chinese continental crust (fig. 8). pyrites consist of enriched co, ni, cu, zn, pb, with enrichment coefficients higher than 2 and also comprise strongly depleted elements with enrichment coefficients less than 1, including sr, ba, rb, and other large-ion lithophile elements, as well as high-field-strength elements (hfse) such as nb, ta, zr and hf. discussion the origin of pyrite trace element ratios, especially those for co and ni in pyrite appear to have been used to distinguish magmatic, hydrothermal and sedimentary origins of pyrites in ore deposition (loftus-hills and solomon, 1967; bralia et al., 1979; bajwah et al., 1987; brill, 1989). the co/ni ratio for magmatic pyrites is greater than 5 in most cases, and mostly greater than 10 (bralia et al., 1979; bajwah et al., 1987). the co and ni contents and the co/ni ratios in hydrothermal pyrites vary greatly; the co and ni contents can reach up to 1000ppm, and the co/ni ratios are generally greater than 1 and can show much variation. sedimentary pyrites are generally characterized by low contents of co and ni and a co/ni ratio<1. generally, co contents, which figure 6. chondrite-normalized ree patterns of pyrites from the gaosongshan gold deposit. the chondrite values for the figures in this paper are from sun and mcdonough (1989). 308 huiqing geng, xuexiang gu, yongmei zhang ta bl e 1 . t he rm oe le ct ric p ar am et er s o f p yr ite s i n th e g ao so ng sh an g ol d de po sit sa m pl e nu m be r g eo lo gy e le va tio n (m ) pty pe (μ v ·º c –1 ) n -t yp e( μv ·º c –1 ) fr eq ue nc y of pty pe (% ) g ol d gr ad e( μg /g ) α av er ag e α av er ag e 2ii zk 72 01 -1 vo lc an ic b re cc ia 40 1. 61 4 11 .6 ~2 15 .5 11 4. 8 -1 23 .1 ~1. 7 -5 6. 9 48 2ii zk 72 01 -1 4 a nd es ite 30 0. 10 1. 7~ 23 7. 9 88 .7 -1 31 .4 ~10 .1 -6 1. 5 50 <0 .1 2ii zk 96 03 -1 9 tu ff 10 8. 17 9 -2 31 .4 ~10 -1 22 .7 0 0. 2 2ii zk 96 03 -2 0 tu ff 10 7. 22 9 -1 43 .3 ~36 .5 -1 02 .6 0 <0 .1 2ii zk 10 40 120 a nd es ite 30 3. 19 -2 06 .4 ~54 .7 -1 36 .9 0 0. 31 2ii zk 10 40 139 a nd es ite 29 7. 89 1 -2 62 .6 ~18 .5 -1 29 .9 0 <0 .1 2ii zk 19 20 108 vo lc an ic b re cc ia , o re b od y 30 7. 07 6. 6~ 61 .8 32 .6 -1 70 .3 ~1. 7 -7 3. 9 10 10 .9 2ii zk 19 20 3b 12 tu ff 21 7. 38 -2 00 .7 ~25 .1 -1 31 .4 0 <0 .1 3ii zk 11 10 114 vo lc an ic b re cc ia 26 2. 54 52 .0 ~2 13 .6 13 3. 5 -1 60 ~18 .3 -7 6. 9 18 <0 .1 2iv fq 217 vo lc an ic b re cc ia 24 0. 8~ 33 5. 6 31 3. 0 — 10 0 2iv fq 219 vo lc an ic b re cc ia 6. 8~ 30 2. 2 11 1. 7 -1 86 .7 ~5. 1 -8 8. 2 60 2iv fq 223 vo lc an ic b re cc ia 66 .9 ~3 35 .0 21 0. 8 -9 5. 2~ -1 1. 7 -4 2. 1 92 2iv fq 225 tu ff 31 .5 ~2 89 .3 14 8. 9 -8 6. 1~ -2 8. 6 -5 1. 7 84 2ii zk 20 00 114 tu ff, o re b od y 28 1. 33 1. 7~ 10 0. 3 50 .5 -1 3. 4~ -1 35 .2 -5 9. 7 16 2ii zk 20 00 118 vo lc an ic b re cc ia 27 6. 43 5. 0~ 19 7. 7 58 .1 -6 .7 ~13 8. 8 -7 8. 0 20 2ii zk 20 00 125 vo lc an ic b re cc ia 21 7. 93 10 .1 ~2 75 .5 64 .8 -2 5. 1~ -1 53 .5 -8 2. 7 32 2ii zk 20 00 127 vo lc an ic b re cc ia 20 3. 03 18 .4 ~8 8. 5 53 .1 -1 0~ -1 48 .3 -9 0. 8 10 2iz k 48 01 -0 608 a nd es ite , o re b od y 5. 0~ 20 7. 2 72 .0 -2 05 .6 ~16 .3 -1 10 .1 14 3. 59 1iv tk 11 a nd es ite 36 0 22 6. 8~ 29 6. 1 25 9. 7 10 0 1iz k 00 01 -0 9. 10 a nd es ite 32 2. 22 5 1. 6~ 98 .0 43 .5 -1 17 .4 ~4. 8 -6 1. 1 14 0. 7 1ii zk 15 02 -0 6 a nd es ite , o re b od y 28 2. 47 43 .6 ~2 34 .0 12 1. 8 -5 8. 2~ -1 1. 6 -3 4. 9 96 14 .7 8 1iz k 23 02 -2 0 te ct on ic b re cc ia , o re b od y 20 4. 75 1. 6~ 31 4. 0 11 6. 2 -1 18 .9 ~1. 6 -6 0. 3 96 2. 15 1iz k 24 02 -0 5 vo lc an ic b re cc ia , m in er al iz ed b od y 11 9. 58 11 7. 7~ 31 8. 7 24 2. 6 10 0 0. 59 1iz k 39 02 -1 5 vo lc an ic b re cc ia 8. 2~ 18 2. 6 69 .2 -1 50 .7 ~4. 8 -4 5. 3 64 0. 32 1iz k 47 01 -2 0. 28 te ct on ic b re cc ia 12 2. 03 0~ 16 7. 2 65 .2 -1 62 .4 ~3. 2 -4 8. 7 62 0. 18 1iz k 47 03 -1 0 te ct on ic b re cc ia , o re b od y 17 9. 65 15 .9 ~3 13 .0 13 8. 7 -8 3. 7~ -1 4. 4 -4 7. 4 88 1. 19 n ot e: α in th e ta bl e re pr es en ts th e th er m oe le ct ric c oe ffi ci en t r an ge o f e ac h sa m pl e; n um be rs a fte r s ym bo l” <“ ar e de te ct io n lim its ; s ym bo l” -“ re pr es en ts n o da ta . 309characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china ta bl e 2 . r ar e e ar th el em en t c on ce nt ra tio ns (p pm ) a nd re la te d pa ra m et er s o f o re -b ea rin g py rit es fr om th e g ao so ng sh an g ol d de po sit sa mp le r nu mb er 2-i iz k2 00 01 -14 2-i iz k1 92 01 -8 1-i i z k1 50 2-0 6 2-i iz k1 08 01 -16 1-i zk 80 01 -11 , 1 2 2-i i z k9 60 2-1 3 2-i iz k7 20 1-1 4 1-i zk 31 02 -10 1-i zk 23 02 -20 2-i z k4 80 1-0 6, 08 1-i zk 47 03 -10 1-i zk 39 02 -08 3-i iz k7 90 1-1 6 la 12 .9 8 7. 87 10 .1 0 26 .8 0 2. 43 4. 52 20 .2 1 9. 80 16 .9 0 11 .3 0 11 .0 0 5. 60 18 .2 0 c e 25 .8 0 15 .6 3 17 .0 0 54 .0 0 4. 16 9. 06 35 .1 9 18 .4 0 28 .8 0 22 .5 0 19 .5 0 11 .3 0 29 .0 0 pr 2. 51 1. 64 1. 69 6. 77 0. 43 1. 02 3. 44 1. 91 3. 09 2. 65 2. 09 1. 35 2. 90 n d 12 .4 9 8. 01 6. 65 28 .9 0 1. 65 4. 39 15 .9 1 7. 31 11 .6 0 11 .3 0 7. 72 5. 38 10 .2 0 sm 2. 46 1. 59 1. 25 6. 23 0. 28 0. 80 2. 89 1. 22 1. 73 2. 32 1. 21 1. 05 1. 57 eu 0. 56 0. 41 0. 44 1. 78 0. 14 0. 26 0. 66 0. 26 0. 38 0. 60 0. 38 0. 22 0. 41 g d 2. 14 1. 37 0. 96 5. 33 0. 23 0. 76 2. 39 1. 09 1. 45 2. 09 0. 90 0. 97 1. 51 tb 0. 37 0. 24 0. 19 0. 89 0. 04 0. 14 0. 39 0. 17 0. 22 0. 39 0. 12 0. 16 0. 23 d y 1. 88 1. 20 0. 85 4. 28 0. 21 0. 73 1. 85 0. 86 1. 14 2. 01 0. 60 0. 81 1. 30 h o 0. 34 0. 21 0. 15 0. 69 0. 03 0. 12 0. 31 0. 16 0. 23 0. 35 0. 09 0. 15 0. 24 er 0. 97 0. 57 0. 42 1. 88 0. 07 0. 32 0. 83 0. 50 0. 68 1. 01 0. 25 0. 47 0. 69 tm 0. 16 0. 10 0. 06 0. 24 0. 01 0. 04 0. 14 0. 07 0. 11 0. 14 0. 03 0. 07 0. 11 y b 0. 81 0. 48 0. 31 1. 45 0. 07 0. 26 0. 70 0. 47 0. 65 0. 77 0. 23 0. 41 0. 67 lu 0. 13 0. 08 0. 05 0. 20 0. 01 0. 04 0. 10 0. 07 0. 10 0. 12 0. 03 0. 06 0. 11 y 8. 71 5. 17 3. 75 17 .9 0 0. 71 3. 21 7. 50 4. 42 5. 67 8. 81 2. 36 4. 17 6. 38 σr ee 63 .6 0 39 .4 1 40 .1 2 13 9. 43 9. 75 22 .4 6 85 .0 3 42 .2 9 67 .0 7 57 .5 4 44 .1 4 27 .9 9 67 .1 4 lr ee 56 .7 9 35 .1 6 37 .1 3 12 4. 48 9. 09 20 .0 5 78 .3 0 38 .9 0 62 .5 0 50 .6 7 41 .9 0 24 .9 0 62 .2 8 h r ee 6. 80 4. 25 2. 99 14 .9 5 0. 67 2. 41 6. 73 3. 39 4. 57 6. 88 2. 24 3. 10 4. 86 lr ee / h r ee 8. 35 8. 26 12 .4 2 8. 33 13 .6 4 8. 31 11 .6 4 11 .4 9 13 .6 8 7. 37 18 .6 7 8. 04 12 .8 2 (l a/ sm ) n 3. 41 3. 19 3. 23 16 .7 5 0. 71 2. 70 4. 51 3. 68 4. 90 7. 81 2. 36 3. 48 5. 24 (g d/ y b) n 2. 19 2. 35 11 .5 0 7. 43 12 .7 1 7. 41 2. 81 10 .5 7 12 .7 5 6. 49 17 .7 2 7. 15 11 .8 9 (l a/ y b) n 11 .5 4 11 .6 7 23 .0 7 13 .2 6 24 .9 0 12 .4 2 20 .6 0 15 .0 5 18 .7 1 10 .5 7 34 .3 1 9. 87 19 .5 4 δe u 0. 74 0. 85 1. 22 0. 94 1. 66 1. 02 0. 77 0. 69 0. 73 0. 83 1. 10 0. 65 0. 81 δc e 1. 11 1. 07 1. 01 0. 98 1. 00 1. 03 1. 03 1. 04 0. 98 1. 01 1. 00 1. 01 0. 98 g ra de (μ g/ g) 10 .9 14 .7 8 1. 09 1. 01 1. 61 1. 98 2. 15 3. 59 1. 19 1. 13 5 1. 08 n ot es : δ eu =( eu ) n /(( sm ) n ×( g d) n )½ , δ c e= (c e) n /(( la ) n ×( pr ) n )½ ; σ lr ee /σ h r ee =σ (l asm )/ σ (g dlu ); sy m bo l” -“ re pr es en ts n o da ta . 310 huiqing geng, xuexiang gu, yongmei zhang ta bl e 3 . r ar e e ar th el em en t c on ce nt ra tio ns (p pm ) a nd re la te d pa ra m et er s o f a ufr ee p yr ite s f ro m th e g ao so ng sh an g ol d de po sit sa mp le 2-i iz k1 04 01 -20 1-i iz k1 50 1 -07 1-i zk 88 01 -02 1-i zk 00 01 -09 , 1 0 1-i vt k 1 -1 1-i vt k 1 -2 2-i i z k9 60 1 -04 2-i iz k1 92 03 -b 12 2-i iz k2 00 01 -1 2-i vf q2 -25 2-i iz k2 00 01 -25 2-i iz k2 00 01 -27 2-i vf q2 -19 1-i zk 39 02 -15 1-i zk 31 01 -13 1-i zk 24 02 -05 1-i zk 47 01 -20 , 2 8 2-i vf q2 -17 3-i iz k7 90 1 -08 la 8. 95 2. 67 13 .6 0 9. 22 13 .1 0 14 .1 0 4. 81 21 .0 3 8. 24 34 .0 7 24 .3 9 8. 60 9. 68 10 .8 0 11 .7 0 26 .5 3. 59 0. 91 5. 81 c e 18 .7 1 6. 00 25 .6 0 17 .6 0 24 .0 0 25 .1 0 8. 20 34 .8 0 16 .0 3 51 .5 2 34 .3 6 19 .1 9 19 .3 6 20 .7 0 23 .1 0 49 .4 6. 14 1. 61 11 .3 0 pr 2. 05 0. 77 2. 83 2. 04 2. 79 2. 87 0. 89 3. 46 1. 73 4. 74 3. 12 2. 13 2. 08 2. 39 2. 83 5. 86 0. 65 0. 20 1. 28 n d 10 .3 7 3. 32 10 .9 0 8. 21 10 .9 0 10 .9 0 3. 67 16 .1 6 8. 31 20 .8 1 13 .9 6 10 .4 5 9. 90 9. 75 12 .0 0 23 .6 2. 45 1. 04 5. 27 sm 2. 20 0. 72 1. 95 1. 63 2. 07 1. 98 0. 70 3. 04 1. 67 3. 47 2. 51 2. 05 1. 95 1. 73 2. 49 4. 22 0. 48 0. 23 1. 04 eu 0. 59 0. 23 0. 42 0. 52 0. 62 0. 56 0. 18 0. 51 0. 42 0. 87 0. 59 0. 42 0. 46 0. 37 0. 43 1. 09 0. 11 0. 08 0. 29 g d 1. 90 0. 66 1. 76 1. 47 1. 94 1. 81 0. 59 2. 64 1. 43 3. 06 2. 06 1. 75 1. 60 1. 65 2. 28 3. 4 0. 47 0. 18 1. 01 tb 0. 35 0. 11 0. 32 0. 27 0. 34 0. 31 0. 11 0. 47 0. 25 0. 48 0. 33 0. 31 0. 29 0. 29 0. 39 0. 55 5 0. 09 0. 04 0. 18 d y 1. 72 0. 54 1. 65 1. 31 1. 71 1. 65 0. 61 2. 44 1. 23 2. 29 1. 57 1. 52 1. 46 1. 49 2. 18 2. 53 0. 53 0. 16 1. 00 h o 0. 31 0. 09 0. 29 0. 22 0. 31 0. 29 0. 09 0. 46 0. 22 0. 39 0. 26 0. 27 0. 27 0. 26 0. 35 0. 40 4 0. 12 0. 04 0. 19 er 0. 83 0. 28 0. 79 0. 62 0. 76 0. 74 0. 30 1. 34 0. 56 1. 04 0. 71 0. 73 0. 75 0. 72 0. 98 1. 12 0. 41 0. 10 0. 54 tm 0. 14 0. 03 0. 11 0. 08 0. 12 0. 12 0. 05 0. 23 0. 10 0. 17 0. 12 0. 12 0. 13 0. 10 0. 15 0. 15 8 0. 06 0. 03 0. 08 y b 0. 68 0. 22 0. 67 0. 45 0. 71 0. 69 0. 26 1. 28 0. 47 0. 87 0. 60 0. 59 0. 65 0. 64 0. 76 0. 86 5 0. 46 0. 09 0. 51 lu 0. 11 0. 04 0. 10 0. 05 0. 12 0. 11 0. 04 0. 21 0. 08 0. 13 0. 09 0. 09 0. 11 0. 09 0. 12 0. 12 6 0. 08 0. 03 0. 08 y 7. 30 2. 48 7. 28 5. 53 8. 36 7. 69 2. 45 11 .7 0 5. 18 9. 33 6. 55 6. 59 6. 35 7. 04 9. 86 11 3. 21 0. 71 4. 57 σr ee 48 .9 0 15 .6 7 60 .9 9 43 .6 8 59 .4 8 61 .2 2 20 .4 8 88 .0 5 40 .7 4 12 3. 91 84 .6 6 48 .2 3 48 .6 7 50 .9 9 59 .7 6 11 9. 83 15 .6 3 4. 75 28 .5 7 lr ee 42 .8 8 13 .7 0 55 .3 0 39 .2 2 53 .4 8 55 .5 1 18 .4 5 78 .9 9 36 .4 0 11 5. 47 78 .9 3 42 .8 4 43 .4 2 45 .7 4 52 .5 5 11 0. 67 13 .4 2 4. 07 24 .9 9 h r ee 6. 02 1. 96 5. 69 4. 46 6. 01 5. 71 2. 03 9. 05 4. 34 8. 43 5. 73 5. 39 5. 25 5. 24 7. 21 9. 16 2. 22 0. 69 3. 58 lr ee / h r ee 7. 13 6. 98 9. 71 8. 79 8. 91 9. 72 9. 10 8. 72 8. 39 13 .7 0 13 .7 7 7. 94 8. 27 8. 73 7. 29 12 .0 8 6. 06 5. 93 6. 98 (l a/ sm ) n 2. 62 2. 25 6. 28 4. 97 6. 68 6. 30 2. 25 4. 46 3. 18 6. 34 6. 26 2. 70 3. 21 5. 84 8. 20 9. 92 2. 58 2. 52 4. 09 (g d/ y b) n 2. 31 6. 10 8. 81 7. 90 8. 01 8. 81 8. 20 1. 71 2. 53 2. 91 2. 84 2. 44 2. 03 7. 83 6. 41 11 .1 6 5. 20 1. 67 6. 11 (l a/ y b) n 9. 48 8. 79 14 .4 7 14 .6 6 13 .2 9 14 .6 8 13 .4 2 11 .7 9 12 .6 3 28 .1 0 29 .1 9 10 .4 2 10 .6 2 12 .1 4 11 .1 0 21 .9 8 5. 57 7. 19 8. 25 δe u 0. 88 1. 01 0. 69 1. 02 0. 94 0. 90 0. 86 0. 55 0. 82 0. 81 0. 80 0. 68 0. 79 0. 68 0. 55 0. 88 0. 73 1. 11 0. 87 δc e 1. 07 1. 03 1. 01 0. 99 0. 97 0. 97 0. 97 1. 00 1. 04 0. 99 0. 97 1. 10 1. 06 1. 00 0. 98 0. 97 0. 99 0. 92 1. 02 g ra de (μ g/ g) 0. 31 0. 57 <0 .1 0. 7 0. 65 0. 32 0. 55 0. 59 0. 18 0. 13 n ot e: δ eu =( eu ) n /(( sm ) n ×( g d) n )½ , δ c e= (c e) n /(( la ) n ×( pr ) n )½ , σ lr ee /σ h r ee =σ (l asm )/σ (g dlu ). ““m ea ns n o va lu e. 311characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china ta bl e 4 . r ar e e ar th el em en t c on ce nt ra tio ns (p pm ) a nd re la te d pa ra m et er s o f q ua rtz fr om th e g ao so ng sh an g ol d de po sit sa m pl e o re b od y n on -o re b od y 1i zk 80 01 -0 5 1iz k 96 04 -1 2 1ii zk 15 02 -0 7 2iiz k 96 01 -0 4\0 5 2iiz k 80 04 -0 32 2iiz k 96 03 -0 51 2ii tc 96 01 -11 1it c1 61 1it c5 61 fq 233 la 1. 32 1. 51 2. 53 2. 58 1. 09 1. 55 0. 14 0. 10 0. 10 1. 53 c e 2. 41 2. 56 4. 05 3. 86 1. 40 1. 56 0. 22 0. 19 0. 17 2. 72 pr 0. 26 0. 28 0. 40 0. 45 0. 12 0. 11 0. 03 0. 02 0. 02 0. 29 n d 1. 27 1. 43 1. 90 2. 03 0. 51 0. 48 0. 13 0. 11 0. 09 1. 33 sm 0. 25 0. 34 0. 34 0. 38 0. 09 0. 08 0. 03 0. 02 0. 02 0. 24 eu 0. 08 0. 13 0. 08 0. 07 0. 05 0. 02 0. 01 0. 00 0. 01 0. 06 g d 0. 23 0. 39 0. 30 0. 29 0. 10 0. 08 0. 02 0. 02 0. 02 0. 18 tb 0. 04 0. 08 0. 04 0. 04 0. 02 0. 01 0. 00 0. 00 0. 00 0. 02 d y 0. 18 0. 41 0. 21 0. 16 0. 10 0. 06 0. 02 0. 02 0. 02 0. 08 h o 0. 03 0. 07 0. 03 0. 02 0. 01 0. 01 0. 00 0. 00 0. 00 0. 01 er 0. 09 0. 18 0. 09 0. 06 0. 03 0. 02 0. 01 0. 02 0. 01 0. 02 tm 0. 01 0. 03 0. 01 0. 01 0. 00 0. 00 0. 00 0. 00 0. 00 0. 00 y b 0. 09 0. 14 0. 07 0. 04 0. 02 0. 02 0. 01 0. 01 0. 01 0. 01 lu 0. 01 0. 02 0. 01 0. 01 0. 00 0. 00 0. 00 0. 00 0. 00 0. 00 σr ee 6. 29 7. 57 10 .0 8 9. 98 3. 56 4. 01 0. 62 0. 53 0. 47 6. 50 σl r ee 5. 60 6. 25 9. 31 9. 37 3. 27 3. 80 0. 55 0. 45 0. 41 6. 17 σh r ee 0. 69 1. 32 0. 77 0. 61 0. 29 0. 21 0. 07 0. 08 0. 06 0. 33 σl r ee /σ h r ee 8. 10 4. 74 12 .1 3 15 .3 7 11 .1 7 18 .5 0 8. 15 5. 62 6. 76 18 .8 5 (l a/ sm ) n 3. 35 2. 85 4. 76 4. 34 7. 41 13 .2 9 3. 45 3. 75 3. 06 4. 11 (g d/ y b) n 2. 19 2. 28 3. 39 5. 97 5. 44 4. 01 1. 64 1. 12 1. 31 10 .1 3 (l a/ y b )n 10 .9 7 7. 59 24 .9 9 46 .5 3 50 .2 2 67 .4 7 10 .5 2 5. 36 7. 73 75 .5 3 δe u 1. 00 1. 10 0. 75 0. 66 1. 62 0. 87 0. 95 0. 45 0. 88 0. 87 δc e 1. 00 0. 97 0. 98 0. 88 0. 96 0. 91 0. 84 1. 07 0. 98 1. 01 n ot e: δ eu =( eu ) n /(( sm ) n ×( g d) n )½ , δ c e= (c e) n /(( la ) n ×( pr ) n )½ , σ lr ee /σ h r ee =σ (l asm )/ σ( g dlu ). c ho nd rit e no rm al iz ed v al ue s c ite d fr om s un a nd m cd on ou gh ,1 98 9. 312 huiqing geng, xuexiang gu, yongmei zhang ta bl e 5 . t ra ce el em en t a bu nd an ce s ( pp m ) a nd re la te d pa ra m et er s o f o re -b ea rin g py rit es fr om th e g ao so ng sh an g ol d de po sit sa m pl e nu m be r 2ii zk 20 00 114 2ii zk 19 20 18 1ii zk 15 02 -0 6 2ii zk 10 80 116 1i zk 80 01 -11 /12 2ii zk 96 02 -1 3 2ii zk 72 01 -1 4 1i zk 31 02 -1 0 1i zk 23 02 -2 0 2i z k 48 01 -0 6/0 8 1i zk 47 03 -1 0 1i zk 39 02 -0 8 3ii zk 79 01 -1 6 ch in es e co nt in en ta l c ru st li 1. 28 0. 90 1. 31 80 .0 0 0. 57 1. 40 1. 44 1. 42 0. 74 1. 40 0. 87 0. 70 1. 93 44 sc 4. 56 1. 97 1. 14 10 .8 0 0. 31 1. 37 4. 26 0. 59 0. 72 1. 82 0. 48 1. 05 1. 49 11 v 7. 12 12 .6 6 6. 74 95 .7 0 6. 13 5. 90 11 .4 7 2. 42 2. 09 13 .0 0 1. 59 3. 69 9. 70 99 c r 4. 33 2. 71 17 .8 0 17 0. 00 4. 42 12 .4 0 0. 96 5. 30 9. 33 16 .8 0 6. 79 6. 92 6. 00 63 c o 15 9. 2 13 7. 6 24 7. 0 51 .3 14 0. 0 22 0. 0 15 8. 0 10 2. 0 14 5. 0 26 4. 0 18 4. 0 15 2. 0 82 .2 32 n i 10 1. 0 13 5. 3 25 8. 0 16 7. 0 24 9. 0 11 6. 0 14 4. 9 98 .6 15 2. 0 42 3. 0 70 9. 0 31 2. 0 68 .9 57 c u 20 4 61 92 92 9 38 9 18 6 18 1 18 1 11 8 13 1 23 1 79 29 8 38 zn 22 16 37 99 94 10 4 12 96 10 5 80 24 7 61 42 1 86 g a 0. 78 0. 51 0. 97 19 .1 0 0. 53 1. 20 1. 10 0. 98 1. 03 1. 08 0. 83 0. 72 1. 41 20 r b 7. 95 3. 34 6. 45 14 2. 00 2. 85 5. 51 5. 43 2. 81 2. 31 3. 92 4. 72 1. 40 5. 92 15 0 sr 4. 89 1. 60 4. 67 15 6. 00 1. 89 3. 79 8. 68 7. 73 3. 4 8. 62 4. 06 4. 82 6. 98 69 0 y 8. 71 5. 17 3. 75 17 .9 0 0. 71 3. 21 7. 50 4. 42 5. 67 8. 81 2. 36 4. 17 6. 38 27 n b 2. 18 2. 14 0. 83 2. 91 0. 63 0. 88 2. 32 1. 19 1. 63 1. 71 0. 70 1. 56 1. 62 34 b a 22 .4 11 .0 20 .1 40 7. 0 6. 9 11 .9 17 .5 45 .9 31 .5 58 .6 14 .0 43 .0 24 .9 61 0 ta 0. 20 0. 15 0. 05 0. 24 0. 01 0. 05 0. 15 0. 08 0. 09 0. 10 0. 05 0. 09 0. 09 3. 5 pb 20 7 23 4 20 7 65 16 1 32 2 37 2 44 5 37 8 22 6 29 3 25 1 43 15 15 th 2. 46 1. 67 0. 50 3. 54 0. 13 0. 52 2. 20 1. 58 1. 72 2. 28 1. 11 1. 24 2. 18 17 u 0. 92 0. 58 0. 16 0. 91 0. 14 0. 14 0. 63 0. 47 0. 59 0. 64 0. 37 0. 44 0. 69 5. 6 zr 79 .9 2 55 .8 1 12 .5 0 95 .1 0 9. 24 14 .4 76 .9 3 41 .4 0 46 .8 0 54 .6 0 30 .3 0 39 .5 0 65 .2 0 16 0 h f 1. 99 1. 39 0. 35 2. 47 0. 15 0. 40 1. 61 1. 02 1. 15 1. 39 0. 75 0. 99 1. 48 5. 1 c o/ n i 1. 58 1. 02 0. 96 0. 31 0. 56 1. 90 1. 09 1. 03 0. 95 0. 62 0. 26 0. 49 1. 19 0. 56 h f/ sm 0. 81 0. 87 0. 28 0. 40 0. 53 0. 51 0. 56 0. 84 0. 67 0. 60 0. 62 0. 94 0. 94 0. 70 n b/ la 0. 17 0. 27 0. 08 0. 11 0. 26 0. 19 0. 12 0. 12 0. 10 0. 15 0. 06 0. 28 0. 09 0. 81 n b/ ta 10 .9 6 14 .1 9 18 .0 9 12 .2 3 57 .5 5 17 .5 6 15 .6 1 14 .8 8 18 .5 2 18 .0 0 15 .1 7 17 .1 4 18 .0 0 9. 71 th /l a 0. 19 0. 21 0. 05 0. 13 0. 05 0. 12 0. 11 0. 16 0. 10 0. 20 0. 10 0. 22 0. 12 0. 41 y /h o 25 .2 6 24 .6 0 24 .8 3 25 .9 8 23 .8 0 26 .3 1 23 .8 8 27 .2 8 25 .2 0 25 .3 9 26 .8 2 27 .2 6 26 .3 6 28 .1 2 zr /h f 40 .1 2 40 .1 7 35 .4 1 38 .5 0 62 .0 1 35 .7 3 47 .8 3 40 .5 9 40 .7 0 39 .2 8 40 .2 9 39 .8 6 44 .0 5 31 .3 7 sr /b a 0. 22 0. 15 0. 23 0. 38 0. 28 0. 32 0. 50 0. 17 0. 11 0. 15 0. 29 0. 11 0. 28 1. 13 n ot e: e le m en t a bu nd an ce s o f c hi ne se c on tin en ta l c ru st a re fr om l i a nd n i, 19 90 . 313characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china characteristics of ore-forming fluid minerals crystallizing in the same hydrothermal ore-forming evolutionary process have similar ree characteristics, which depend on the features and forming environments of the ree ore-forming fluids in the sedimentary process (yuan et al., 1998). lanthanide elements in quartz are prioritized to occurrence in fluid inclusions (ghazi et al., 1993; monecke et al., 2002; götze, 2009). meanwhile, the ionic radius of ree3+ and fe2+ differ from each other, so it is difficult for ree3+ to replace the cations in the pyrite lattice; therefore, it is speculated that rees in pyrite are mainly present in fluid inclusions. so ree characteristics of quartz reflect that of ore-forming fluid in equilibrium with quartz. then ree compositions of pyrite coexist with quartz also show ree characteristic of ore-forming fluid. therefore, ree characteristics of quartz and pyrite formed during main mineralization stage in the gaosongshan gold deposit are representative of mineralization hydrothermal fluid ree compositions. figure 6 shows no distinctive differences in ree chondrite-normalized patterns of two types of pyrite, indicating that they may have the same origin. similar results can be obtained from comparison between those of ore-bearing and barren quartz (fig. 7). pyrites and quartz show almost no ce anomalies, no matter in ore-bearing samples or in ore-barren samples. however, obvious negative eu anomalies are typical of ore-free pyrites (δeu= 0.55-1.11) and barren quartz (δeu= 0.45-0.95). eu anomalies show a little complexity either in ore-bearing pyrites or in ore-bearing quartz. except one sample shows a positive eu anomaly (δeu= 1.62), the rest quartz samples show weak negative eu anomalies. similarly, with the exception of two ore-bearing pyrite samples showing positive eu anomalies (δeu=1.22 and 1.66), the rest show significant or weak negative eu anomalies. eu anomalies in ore-bearing quartz and ore-bearing pyrites suggest that those anomalies partly reflect partitioning of eu2+ into plagioclase during magmatic evolution. the two special pyrites and one quartz-sample showing positive anomalies were all sampled from altered andesite, in which contact with hydrothermal fluids, plagioclase alteration may release some eu2+ into hydrothermal fluids causing eu anomalies to be positive. the (la/yb)n ratios of quartz show a negative correlation with formation depth of deposit (zhao, 1997), indicating that the gaosongshan gold deposit is a hypabyssal one, of which the (la/yb)n is much higher than 1. wang et al. (2014) has analyzed the ree concentrations of orebearing volcanic rocks in the lower cretaceous banzifang formation, consisting of basaltic andesite, hornblende andesite, trachyandensite and andesitic volcanoclastic rock. σree ranges from 114.46 to 166.16ppm with an average of 130.60ppm. (la/yb)n are 5.41-7.11. δeu values are 0.861.07 with an average value of 0.94 and no obvious negative eu anomalies. the chondrite-normalized ree patterns are characterized by lreeenrichment. despite somewhat variable ree concentrations, the shapes of the chondrite-normalized ree patterns of ore-bearing quartz, pyrites and volcanic rocks are quite similar with no obvious negative eu anomalies, are less than 100ppm, are less than ni contents (chen et al., 1987). the co and ni contents of ore-bearing pyrites from the gaosongshan gold deposit range from 51.3-264.0ppm and 68.9-258.0ppm, respectively. average contents of co and ni are 157.1ppm and 225.7ppm, respectively. the co/ ni ratios of ore-bearing pyrites are 0.31-1.90, with an average of 0.92. ore-bearing and au-free pyrites exhibit similar characteristics in the co/ni distribution diagram (fig. 9). the co contents hardly changes, while the ni contents change considerably; the date points both cross the hydrothermal and sedimentary origin zones, while most data fall in the hydrothermal origin zone. previous researches indicated that the sr/ba ratio in pyrite can be used as an indicator of the hydrothermal process and sedimentation (ge and han, 1987). a smaller ratio usually reflects a more explicit hydrothermal factor. the sr/ba ratios in ore-bearing pyrites from the gaosongshan gold deposit range from 0.11 to 0.50, with an average of 0.24, while those in orefree pyrites are 0.09-1.11, with an average of 0.39. in general, co and ni concentrations and co/ni ratio, the relatively small sr/ba ratio in pyrites, together with the obvious field characteristics of pyrites mentioned above, indicate that the pyrites are of hydrothermal origin. figure 9. co/ni distribution diagram of pyrites from the gaosongshan gold deposit. boundaries of different geological settings are defined after bajwah et al., 1987 and brill, 1989. figure 8. spider diagram of trace elements of pyrites from the gaosongshan gold deposit. 314 huiqing geng, xuexiang gu, yongmei zhang ta bl e 6 . t ra ce el em en t a bu nd an ce s ( pp m ) a nd re la te d pa ra m et er s o f b ar re n py rit es fr om th e g ao so ng sh an g ol d de po sit sa mp le nu mb er 2-i iz k1 04 01 -20 1-i iz k1 50 1 -07 1-i zk 88 01 -02 1-i zk 00 01 -09 , 1 0 1-i vt k 1 -1 1-i vt k 1 -2 2-i i z k9 60 1 -04 2-i iz k1 92 03 -b 12 2-i iz k2 00 01 -1 2-i vf q2 -25 2-i iz k2 00 01 -25 2-i iz k2 00 01 -27 2-i vf q2 -19 1-i zk 39 02 -15 1-i zk 31 01 -13 1-i zk 24 02 -05 1-i zk 47 01 -20 , 2 8 3-i iz k7 90 1 -08 li 2. 83 0. 66 1. 69 0. 53 1. 01 2. 67 1. 03 0. 88 0. 96 2. 26 0. 51 0. 42 0. 93 1. 02 0. 52 0. 61 0. 98 2. 57 sc 5. 43 0. 63 1. 39 1. 10 0. 76 0. 94 0. 63 4. 48 3. 50 4. 12 4. 32 4. 17 2. 56 0. 99 0. 93 0. 73 1. 17 1. 02 v 22 .1 9 4. 64 12 .8 0 7. 40 15 .3 0 16 .6 0 7. 30 10 .4 7 13 .1 4 15 .7 9 10 .1 6 10 .4 7 11 .3 6 5. 13 2. 01 1. 16 4. 18 18 .2 0 c r 59 .1 7 8. 01 5. 20 8. 40 7. 44 7. 53 3. 39 0. 96 4. 15 0. 18 3. 27 0. 44 n. d . 8. 46 5. 89 4. 73 6. 61 14 .7 0 c o 13 7. 4 42 6. 0 15 6. 0 26 8. 0 16 5. 0 23 5. 0 63 .4 89 .9 14 5. 9 20 1. 29 18 8. 4 15 1. 3 13 2. 4 19 1. 0 15 1. 0 85 .7 78 .3 20 4. 0 n i 42 1. 7 12 7. 0 57 .9 36 4. 0 76 .9 83 .1 44 .2 63 .8 48 .4 76 .6 74 .9 53 .6 28 .3 11 7. 0 71 .6 16 .0 23 7. 0 32 3. 0 c u 12 7 27 2 24 1 12 9 13 7 27 6 18 1 68 6 23 2 25 6 17 6 45 6 10 5 72 3 60 86 67 21 8 zn 21 10 4 84 49 34 17 3 53 20 17 38 44 46 68 75 26 1 13 3 39 9 78 g a 1. 18 0. 78 1. 22 0. 94 0. 94 1. 33 0. 54 0. 96 0. 70 1. 16 0. 59 0. 52 0. 66 1. 22 1. 21 1. 56 0. 86 1. 09 r b 7. 36 2. 84 3. 16 1. 65 7. 83 10 .9 0 0. 98 2. 62 4. 19 12 .2 4 1. 28 1. 38 5. 51 3. 10 4. 00 3. 02 0. 33 8. 86 sr 6. 97 18 .2 0 5. 08 7. 55 4. 91 6. 68 2. 05 14 .9 7 4. 53 12 .1 5 2. 93 3. 67 2. 65 13 .1 0 9. 17 3. 38 1. 40 3. 99 y 7. 30 2. 48 7. 28 5. 53 8. 36 7. 69 2. 45 11 .7 0 5. 18 9. 33 6. 55 6. 60 6. 35 7. 04 9. 86 11 .0 0 3. 21 4. 57 n b 3. 3 0. 7 2. 3 2. 3 1. 8 2. 9 1. 5 3. 4 3. 1 5. 7 3. 1 3. 3 4. 4 1. 7 2. 1 1. 9 0. 4 0. 6 b a 25 .7 16 .4 25 .6 10 .4 24 .4 73 .6 5. 6 22 .1 13 .9 43 .6 11 .5 6. 9 15 .7 35 .4 38 .8 24 .8 1. 9 15 .0 ta 0. 24 0. 03 0. 16 0. 12 0. 12 0. 13 0. 06 0. 25 0. 25 0. 45 0. 19 0. 25 0. 30 0. 12 0. 11 0. 10 0. 03 0. 04 pb 24 2 29 2 83 2 16 7 43 6 61 6 21 2 24 1 20 1 53 9 27 7 22 2 49 7 30 8 20 3 45 5 86 . 35 8 th 2. 15 0. 39 2. 31 1. 05 2. 70 2. 38 0. 61 3. 11 1. 55 3. 18 1. 90 2. 16 2. 75 2. 22 1. 40 1. 48 0. 64 1. 25 u 0. 90 0. 14 0. 76 0. 34 1. 53 1. 02 0. 42 1. 10 0. 59 1. 60 0. 97 0. 85 0. 76 0. 70 0. 46 0. 46 0. 35 0. 49 zr 80 .7 1 10 .4 0 59 .5 0 38 .9 0 76 .1 0 70 .5 0 26 .4 0 84 .4 2 61 .6 2 98 .9 8 67 .9 5 72 .4 2 81 .1 9 60 .4 0 52 .9 0 67 .6 0 23 .8 0 23 .4 0 h f 2. 06 0. 31 1. 53 1. 04 1. 71 1. 56 0. 60 2. 14 1. 77 2. 73 1. 87 2. 04 2. 29 1. 52 1. 29 1. 66 0. 49 0. 56 c o/ n i 0. 33 3. 35 2. 69 0. 74 2. 15 2. 83 1. 43 1. 41 3. 02 2. 63 2. 52 2. 82 4. 68 1. 63 2. 11 5. 36 0. 33 0. 63 h f/ sm 0. 93 0. 42 0. 78 0. 64 0. 83 0. 79 0. 86 0. 70 1. 05 0. 79 0. 74 0. 99 1. 17 0. 88 0. 52 0. 39 1. 02 0. 54 n b/ la 0. 37 0. 24 0. 17 0. 25 0. 14 0. 21 0. 31 0. 16 0. 38 0. 17 0. 13 0. 39 0. 45 0. 16 0. 18 0. 07 0. 12 0. 11 n b/ ta 13 .6 4 19 .1 2 15 .0 0 18 .8 4 15 .0 0 22 .8 3 25 .1 7 13 .5 2 12 .6 1 12 .7 7 16 .3 6 13 .3 5 14 .6 3 15 .1 3 20 .1 9 18 .5 0 13 .8 4 14 .6 7 th /l a 0. 24 0. 14 0. 17 0. 11 0. 21 0. 17 0. 13 0. 15 0. 19 0. 09 0. 08 0. 25 0. 28 0. 21 0. 12 0. 06 0. 18 0. 22 y /h o 23 .5 3 26 .3 8 25 .2 8 24 .9 1 26 .8 8 26 .6 1 26 .0 6 25 .6 2 23 .9 0 23 .9 6 25 .0 6 24 .1 7 23 .5 1 26 .9 7 28 .4 1 27 .2 3 27 .6 7 24 .5 7 zr /h f 39 .2 5 33 .9 9 38 .8 9 37 .4 0 44 .5 0 45 .1 9 43 .8 5 39 .4 0 34 .9 2 36 .2 1 36 .3 5 35 .4 8 35 .5 2 39 .7 4 41 .0 1 40 .7 2 48 .8 7 41 .7 9 sr /b a 0. 27 1. 11 0. 20 0. 73 0. 20 0. 09 0. 37 0. 68 0. 33 0. 28 0. 26 0. 53 0. 17 0. 37 0. 24 0. 14 0. 73 0. 27 n ot e: n . d .-n ot d et ec te d. 315characteristics of genetic mineralogy of pyrite and quartz and their indicating significance in the gaosongshan gold deposit, heilongjiang province, ne china which suggest that the partial melting processes played an important role in the rock formation and the plagioclase fractionation played a small role during magma evolution. it is likely that the ore-forming fluid was related to the andesitic magma. the genetic relationship between different objects can be interpreted by using a correlating concentration of two trace elements to establish trace elements covariance diagram (henderson, 1984). geochemical parameters such as δeu and δce can reflect the process of ree fractionation. this paper chose the δeu-δce to plot the covariance diagram, which is obviously shown that the dot of ore-bearing quartz and pyrites are almost in the same region with volcanic rocks (fig. 10), which indicates that ore-bearing quartz veins with pyrite have close relationship to volcanic rocks, reflecting that the ore-forming fluids are mainly from magmatic hydrothermal fluids. the stable isotopic compositions of oxygen and hydrogen studies of quartz in the gaosongshan gold deposit find that δ18o=1.7 to 7.76‰, with an average of 3.5‰, while δd=–115 to –133 (zheng et al., 2013); such data are different from both typical magmatic water and metamorphic water, but are much related to magmatic water and they are located in the right side of the meteoric water line, indicating that the ore-forming fluids originated from magmatic fluids mixed with atmospheric water. compared to cl chondrite (sun & mcdonough, 1989), lree is more enriched than hree in pyrites; high field strength elements are strongly depleted; the hf/sm, nb/la, and th/la ratios are almost smaller than 1. some trace element ratios in pyrites, such as hf/sm, nb/la, th/la, y/ho, zr/hf, nb/ta, have important significance in demonstrating the properties of ore-forming fluids (oreskes and einaudi, 1990; yaxley et al., 1998). recent studies showed that cl-rich hydrothermal liquids are enriched in lree, and the hf/sm, nb/la, and th/la ratios are less than 1 while the f-rich hydrothermal liquids are simultaneously enriched in lree and hfse with the hf/sm, nb/la, and th/la ratios generally larger than 1 (oreskes and einaudi, 1990; bi et al., 2004; mao et al., 2006). the quartz and pyrites are all enriched in lree and the hf/sm, nb/la, and th/la ratios in pyrites are smaller than 1, indicating that the c1contents in the ore-forming fluids of the gaosongshan gold deposit may be slightly higher than the fcontents, which was proven by the liquid-phase compositions analysis of the fluid inclusions in quartz (liu, 2015). figure 10. bivariate scatter plot shows the relationship between δeu and δce of ore-bearing quartz and pyrites and volcanic rocks from the gaosongshan gold deposit. (data points of volcanic rocks cited from wang et al., 2014) the ree contents of pyrite and the fractionation degree of lree and hree can be used to determine the depth of ore-forming fluids. in general, ore-forming fluids derived from mantle source materials are characterized by low ree content and slight differentiation of lree and hree (zhao, 1997; li et al., 2007). in this deposit, the differentiation of lree and hree in pyrite is large, with a relatively high ree content, which suggests that the ore-forming fluids are derived from crust source materials. some trace elements in pyrite are similar to those in ore-bearing volcanic rocks, i.e., the cr, co, ni, cu and zn contents are high (wang et al., 2014), showing the extraction of some metal components from the surrounding rocks into the ore-forming fluids. then, metallogenetic elements are released during fluidwall rock interaction and join the mineralization processes. this conclusion is also consistent with the result indicated by s isotopes of pyrite that metal components originate from ore-bearing volcanic rocks (zheng et al., 2013). previous research on s isotopes in pyrites of this deposit show that the δ34s values ranging from -4.3‰~2.8‰ with an average of -1.1‰ (zheng et al., 2013). by combining the geological facts, i.e., that the sulfate minerals are not observed and the sulfides compositions are simplex in the deposit, it also can be stated that the ph and f (o2) of the fluid had no obvious change during the ore-forming process. y-ho, zr-hf and nb-ta separately have the similar ionic radii and electrovalence, and thus, they often show the same geochemical characteristics, respectively. in many geological processes, y/ho, zr/hf, and nb/ta ratios are stable in one hydrothermal system; however, when the system is disturbed by hydrothermal activities or metasomatism, these element pairs will change significantly, and are presented as a large variation range among different samples (bau et al., 1995; yaxley et al., 1998). y/ho, zr/hf, and nb/ta ratios vary slightly (excluding one sample) in pyrites from the gaosongshan gold deposit, showing that the hydrothermal system is relatively stable in the ore-forming process. application for prospecting analysis of thermoelectric properties of pyrites can provide information about depth of ore-forming, mineralization degree, and the medium conditions during the formation of deposits, and thus giving help on finding blind orebodies (ji et al., 2013). thermoelectric conductive type of pyrite has certain instructive significance to the erosion depth of the orebody. based on the thermoelectric coefficient values, the thermoelectric parameters xnp of pyrites are calculated by using the following equation (xue et al., 2014), and can be qualitatively identify the denudation level of gold deposits. χnp= (2ƒi +ƒii)-(ƒiv +2ƒv) (1) ƒi represents the percentage of pyrite in the samples corresponding to the compensation pyroelectric coefficient: ƒi means α>400μv·ºc -1; ƒii refers to α=200400μv·ºc-1; ƒiv refers to α=0 to -200μv·ºc-1; and ƒv means α < -200μv·ºc-1. the xnp value ranges from 200 to 100 for the uppermost orebody, 100-50 for the upper part of the orebody, 50 to -50 for the central orebody, and -50 to -100 for the lower orebody. the xnp values of different ore bodies in the gaosongshan gold deposit are calculated: 1-i orebody, xnp=-86 to 94, with an average of -10; 2-ii orebody, xnp=-106 to -46, with an average of -84.7; 2-iv orebody, xnp=-32 to 50, with an average of 19. according to the above standard, the gold orebodies are eroded to their middle-lower part in different ore veins. the denudation percentage γ (γ=50-χnp/4) of the orebody with respect to the total length of mineralization can be calculated using the thermoelectricity parameter: 1-i orebody, 26.5% to 71.5%; 2-ii orebody, 61.5% to 76.5%; 2-iv orebody, 37.5% to 58%. these results are in accordance with those obtained by thermoelectric parameters xnp. differences in the thermoelectric properties of pyrite essentially reflect physical and chemical changes during the migration of the oreforming solution (li et al., 2009; zhai et al., 2013). according to the vertical zoning sequence of the primary halo in general hydrothermal deposits, halos of co and ni often exist in the lower part of the orebody while those 316 huiqing geng, xuexiang gu, yongmei zhang of s and as exist in the upper part. co and ni are present as isomorphism substituting for fe2+, resulting in n-type conductivity of pyrite. meanwhile, replacement of s with as leads to p-type conductivity. in general, the zoning of thermoelectric conductivity in the vertical direction is as follows: upper part of the orebody, p-type; central part, p+n type, and lower part, n-type. from the relationship between the occurrence rate of the p-type pyrite and the elevation (fig. 11), it is concluded that the p-type pyrite occurrence rate and elevation do not have a clear linear relation no matter what its wall rock is. probably because the vertical spacing between sample collection points is no more than about 300 m, which is very small in the scope of ore formation. the non-linear relationship between the sample points may suggest that the pre-formed orebodies were superimposed and modified by the post-formed orebodies, resulting in pyrite with mixed thermoelectric conductivity at the identical depth. researches on geochemistry of primary halo also indicate that in the longitudinal direction, the superimposition of ore-forming elements, the front halo elements, halo elements near the ore, and tail halo elements show superposition characteristics of multi-stage mineralization (lian et al., 2010; lv, 2012). figure 11. the relationship between the occurrence rate of p-type pyrites and its elevations in summary, from 1-i orebody to 2-ii body, the occurrence rate of p-type pyrite declines from 70.7% to 15.7%, that is to say, from the southwest to the northeast of this mining area, the denudation degree of orebody becomes increasingly strong. the electronic conductivity type of pyrite in orebody i is p-type to p-n type with fairly good ore-bearing potential. those in orebody 2-ii is transferred from the mix type to n-type from the west to the east direction, indicating that gold mineralization gradually weakens from west to east. the southeast part of this mining area may be more valuable for finding orebodies. conclusions 1. the chondrite-normalized ree patterns of pyrites and quartz are lree-enriched. barren quartz and pyrites are characterized by obvious negative eu anomalies, while ore-bearing quartz and pyrite generally exhibit no eu anomalies. similar ree characteristics of ore-bearing quartz and pyrites and volcanic rocks, together with oxygen and hydrogen isotope studies of quartz, suggest that the ore-forming fluids of the gaosongshan gold deposit were mainly magmatic origin which was associated with andesitic magma and was partly mixed with atmospheric water. trace elements characteristics of ore-bearing pyrites and quartz, together with those of volcanic rocks indicate that the ore-forming materials were derived from the surrounding rocks. 2. variable concentrations range of co and ni and co/ni ratio (0.311.90, average 0.92), together with relatively small sr/ba ratio in ore-bearing pyrites, support a hydrothermal origin of mineralization at gaosongshan gold deposit. slight changes of y/ho, zr/hf, nb/ta in ore-bearing pyrites indicate that the ore-forming hydrothermal solution has no obvious change in ph and f (o2) during the ore-forming process. 3. pyrites are enriched in co, ni, cu, zn, pb, with enrichment coefficients higher than 2. large-ion lithophile elements (such as, sr, ba, rb), as well as high-field-strength elements, such as nb, ta, zr and hf are strongly depleted with enrichment coefficients less than 1. 4. the electronic conductivity of pyrites in the gaosongshan gold deposit is hybrid type. thermoelectric parameters xnp and denudation percentage γ of pyrites from orebodies 1-i, 2-ii and 2-iv all indicate that the gold orebodies were eroded to their middle-lower part. from 1-i orebody in the southeast of the mining zone to 2-ii orebody in the northwest, the p-type occurrence rate of the pyrite gradually decreases, suggesting that orebody erosion is increasingly intense. the southeast part of this mining area may be more valuable for finding orebodies. acknowledgements this research was supported by geological survey projects of china geological survey (grant no. 1212011120686). we thank our colleagues for their great support during the geological and deposit investigations at the gaosongshan gold mine and for their assistance in genetic mineralogy analyses. we are grateful to the technicians, authors of exploration reports and previously published papers whose work contributed to this article. references arribas, a. jr. 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(2002). tectonics and distribution of gold deposits in china-an overview. mineralium deposita, 37(3-4), 249-282. in the lower benue trough of southeastern nigeria, lead-zinc-barium mineralization occurs as widely distributed epigenetic fracture-controlled vein deposits which are restricted to albian – turonian sediments. detailed field studies carried out in ishiagu, enyigba-ameki-ameri, wanikande-wanakom, and gabu-oshina which together constitute the four main areas of mineralization in the lower benue trough, show that mineralization appears restricted to nw-se and n-s fractures while the more common ne-sw fractures are barren. apart from the enyigba area, igneous bodies are found in the vicinity of the ore deposits while in the wanikande area, barite veins and veinlets were observed to be closely interwoven with intrusive bodies. the host lithologies are highly varied, ranging from shales to siltstones, sandstones and occasionally igneous bodies. the ore assemblage also varies remarkably, with lead:zinc:barium ratios ranging from approximately 3:1:0 at ishiagu, to 2:1:0 at enyigba, 1:0:2 at wanikande and nearly 100% barite at gabu-oshina. thus, there is a remarkable increase in barite content from the southwest (ishiagu) to the northeast (gabu). the characteristics of the ore deposits roughly fit the base metal type mineralization known as clastic dominated lead-zinc-barium deposits. this study forms part of a mineral potential database which is invaluable for any detailed exploration venture in the area. abstract keywords: lead-zinc-barium mineralization, benue trough, epigenetic, vein deposits. contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n1.39703 o r e d e po si t s earth sciences research journal earth sci. res. j. vol. 21, no. 1 (march, 2017): 7 16 ifeanyi andrew oha*,1, kalu mosto onuoha1, silas sunday dada2 1department of geology, university of nigeria, nsukka. 2kwara state university malete *ifeanyi.oha@unn.edu.ng en la parte baja de la formación benue trough, en el sudeste de nigeria, la mineralización de plomo-zinc-bario ocurre en depósitos epigenéticos de fractura controlada ampliamente distribuidos pero que se restringen a sedimentos albianturonian. estudios de campo detallados realizados en ishiagu, enyigba-ameki-ameri, wanikande-wanakom y gabuoshina (que conforman las cuatro áreas principales de mineralización en la parte baja de benue trough) muestran que la mineralización aparece restringida a fracturas nw-se y n-s, mientras que las fracturas ne-sw, que son más comunes, aparecen estériles. aparte del área enyigba, los cuerpos ígneos se encontraron junto a los depósitos de minerales, mientras en el área wanikande se observaron vetas y vetillas de baritina que están prácticamente entretejidas con cuerpos intrusivos. las litologías sede son ampliamente variadas, y van desde esquistos a limolitas, areniscas y ocasionalmente cuerpos ígneos. el ensamble de la mena también varía con proporciones de plomo:zinc:bario que van aproximadamente desde 3:1:0 en ishiagu, 2:1:0 en enyigba, 1:0:2 en wanikande y cerca del 100 % de baritina en gabu-oshina. por ende, hay un incremento importante en el contenido de baritina desde el suroeste (ishiagu) hacia el noroeste (gabu). las características de los depósitos minerales se enmarcan aproximadamente en el tipo de mineralización de base metálica, conocido como depósitos plomo-zinc-bario de dominación clástica. este estudio hace parte de una base de datos de minerales potenciales que es invaluable para una exploración detallada en el área. resumen palabras clave: mineralización plomo-zinc-bario, benue trough, epigénesis, vetas. record manuscript received: 30/08/2014 accepted for publication: 30/12/2016 how to cite item oha, i. a., onuoha, k. m., & dada, s. s. (2017). contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria. earth sciences research journal, 21(1). 7 16. doi:http://dx.doi.org/10.15446/esrj.v21n1.39703 contraste de estilos de mineralización de plomo-zinc-bario en la parte baja de la formación benue trough, en el sudeste de nigeria 8 ifeanyi andrew oha, kalu mosto onuoha and silas sunday dada8 introduction the lead-zinc-barium deposits of the benue trough have been known since colonial times. initial exploration efforts mainly by geologists from the geological survey of nigeria (gsn) between the late 1930’s and the early 80’s led to the discovery of some small to medium-sized deposits in a fairly extensive belt stretching from the southeastern part of the trough to the northeastern parts. tattam 1930 made the earliest report on lead-zincbarite mineralization in the lower benue trough, highlighting the mode of occurrence and mineralogy of the lead-zinc deposits in the abakaliki area. this was followed by extensive surveys to determine the extent of mineralization in the abakaliki and ishiagu areas, (mcconnel, 1949; mckay, 1950; farrington, 1952; orajaka, 1965). nwachukwu (1974) determined the temperature of formation of the abakaliki deposits from fluid inclusion studies and suggested a magmatic origin for the deposits. the origin based on circulating connate waters proposed by olade (1976) was supported by fluid inclusion studies and trace element geochemistry (olade and morton, 1985). ezepue (1984) described the geologic setting of the lead-zinc deposits around ishiagu, with emphasis on how tectonic fissures influenced the loci of mineralization. lead isotope (maurin and lancelot, 1987) and geophysical studies (etim et al., 1988) suggested that the most credible lead source for the abakaliki mineralization corresponds to detrital feldspars which were eroded from the basement and re-concentrated within the sandstone-shale series. the latter supported this claim from 48 electric soundings with correlation from 2 well logs within the vicinity. the results of this survey revealed the presence of a sandstone formation probably more than 300m thick at the core of the ne-sw anticlinal structure and the absence of an evaporitic sequence or salt dome up to the depth of 275m. akande et al. (1988, 1989) used a combination of field evidence, ore microscopy, electron microprobe, fluid inclusion and stable isotope studies on the lead-zinc deposits of the middle and lower benue trough, and suggested a basinal-brine expulsion model for the benue ore fluids. the distribution of the lead-zinc-barium deposits in the benue trough of nigeria are restricted to the albian – turonian sediments but more widely distributed spatially. they occur along the entire stretch of the basin, but as secluded and widely separated bodies. sediment-hosted lead-zinc-barium deposits can be of the sedimentary exhalative (sedex), also known as clastic-rock dominated sedimentary sequences (leach et al., 2010) or of the carbonate-dominated sequences also known as the mississippi valley type (mvt). the most important characteristics of these two main deposit types are found in gustafson and williams (1981), large (1983), sangster (1990), lydon (1996), sangster and hillary (1998) and leach et al. (2005). the general characteristics of the benue trough lead-zinc-barium deposits, therefore, are: (1) the ores are epigenetic. (2) they are hosted in carbonaceous shales and siltstones of albian – turonian age. (3) they are closely associated (spatially) with igneous intrusions. (4) ores occur as veins which may extend up to 2km in length. (5) veins are structurally controlled, and trend nw-se and n-s. (6) the temperature of ore formation from fluid inclusion studies is between 90oc and 200oc. (7) they are closely associated with saline springs. (8) the principal mode of occurrence is open space filling (rather than replacement). (9) wall rock alteration is minimal or non-existent, suggesting that temperature of mineralizing fluid was low and (10). the ores are localized in a variety of widely separated rocks, which implies that ore formation is related to regional rather than local processes. in the present work, four mineralized areas were identified and studied in greater detail. these include: (1) the ishiagu area; (2) the abakaliki area; (3) the wanikande-wanakom area; (4) the gabu – oshina area. this paper presents the results of lithologic and structural studies carried out around quarry sites and mining pits in these four mineralized areas. regional geologic setting the benue trough is a major sedimentary basin in africa, stretching over 1000 km in length, with a width ranging between 150 and 250 km. it is part of the cretaceous west african rift system (wars, fig. 1), which can be traced along a distance of about 4000km from nigeria, running northwards into the neighboring republic of niger and terminates in libya (binks and fairhead, 1992). geographically, the trough is divided into three regions, from southwest to northeast; the lower, middle and upper benue trough, which correspond to different geologic features (maurin et.al, 1986; benkhelil, 1989; rebelle, 1990). figure 1. location of the benue trough in the west african rift system (wars) and central african rift system (cars) (modified from binks and fairhead, 1992). in the lower benue trough, basin formation was as a result of at least two tectonic phases. the first tectonic phase was assigned aptian – santonian by short and stauble (1967) and albian – santonian by murat (1970). simpson (1954) recorded that the sediments of the abakalikibenue trough underwent a minor folding phase during the cenomanian and a major one accompanied by igneous intrusions during the santonian. the folding has an ne-sw trend, plunging southeastwards and allowing the younger formations to overstep into the older beds. a second tectonic phase formed the anambra basin and afikpo syncline (santonian – maastrichtian). the down-faulting of the anambra platform to form the anambra basin, and the down warping in the east to create the afikpo syncline coincide with the structural deformation of the abakaliki-benue trough. the sedimentary fills of the basins in the study area were controlled by cycles of transgressions and regressions and local tectonics. the transgressive phases were characterized by deposition of shales with shoal carbonates developed on submerged structural highs (platforms, horsts). the regressive phases were characterized by extensive deltaic sediments and by predominantly fine clastics (shallow marine shales) deposited over the structural highs. three cycles of basin fill have been recorded in the lower benue trough: the neocomian – cenomanian asu-river group, the early to late turonian eze-aku group and the coniacian – santonian awgu group. the first cycle was deposited during the aptian – cenomanian. the asu river group was described as the initial clastic fill of the lower benue trough (petters and ekweozor, 1982). it occupies the core of the abakaliki anticlinorium and consists of about 3,000m thick basal arkosic sandstones (awi and mamfe formations of aptian – albian age) and middle marine shales sandstones (awe formation of early cenomanian to early turonian age). 9contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria the gboko formation has been described as the oldest lithologic unit in the lower benue trough, (reyment, 1965; ramanathan and nair, 1984). the gboko formation rests directly on the precambrian granitic basement, which had been dated 539 ma old (umeji and caen-vachette, 1983). the general sequence of this formation from the oldest to the youngest units has been described by umeji (2000) as consisting of a basal sandstonesiltstone unit with limestone bands overlain by alternations of limestone with siltstone-shale units. the environment of deposition ranges from hyposaline to marginal hypersaline lagoon (ramanathan and nair, 1984). in the abakaliki-ogoja region, the sediments were subdivided into the middle albian sandy unit at the base (ekebeligwe formation), the shaly upper unit of late albian age (abakaliki shale) and the early cenomanian ibir and agala sandstones (ojoh, 1992). overlying the asu-river group is the turonian eze-aku group which consists of fossiliferous calcareous sandstones, shales, and limestones. occasionally, thick sandstone or limestone units alternate with the calcareous shales. sediments of this age have been mapped along the entire benue trough and are noted as the most extensive marine sedimentation deposited when the mediterranean or tethys sea linked up with the atlantic across the sahara. this marine sedimentation probably started during the late cenomanian and continued into the early turonian without a break. the main shale unit in this group has its type locality at the eze-aku river, near aka-eze in the southern part of the basin. it has been observed that in this section the cenomanian continues up the stratigraphic sequence without a break into the turonian, (ojoh, 1992). thick sequences of alternating pyritic, black, calcareous shales and limestones in isinkpume, ezillo, and nkalagu areas were described as nkalagu formation (petters, 1978). northeast-southwest trending sandstone ridges in the amasiri and ugep areas were described as amasiri formation within the eze-aku group. the sandstone ridges are 20-30m thick and up to 20m higher than the surroundings (banerjee, 1980). these sandstones which are commonly bioturbated grade from fine and wave-rippled at the base to coarse, pebbly and trough cross-bedded at the top. the rippled basal part had been described as offshore mud and sand deposit whereas the trough cross-bedded upper part was regarded as tidal and subtidal bars (banerjee, 1980). the cenomanian to lower turonian sediments were estimated to be 2,100m thick (cratchley and jones, 1965). figure 2. stratigraphic succession of southeastern nigerian sedimentary basins (modified from nwajide, 1990). the overlying awgu group (coniacian – santonian) consists of the grey-blue awgu-ndeaboh shale and the medium to coarse-grained bioturbated agbani sandstone. large scale cross-stratification and herringbone cross-bedding are common primary sedimentary structures in the agbani sandstone. reyment (1965) considered the agbani sandstone as a time-equivalent of the awgu shale. conversely, cratchley and jones (1965) thought that it is a later deposit of thick sandstone beds which accompanied the shallowing of the coniacian sea. cratchley and jones (1965) also thought that the sandstone beds of the agbani sandstone appeared to herald the period of uplift, renewed folding and intense local erosion, which is assigned to the santonian sub-stage because of the absence of fossiliferous materials of santonian age. the awgu shale is estimated to be about 900m thick (benkhelil, 1986). the santonian represented a period of tectonic and igneous activity, when the sediments of the abakaliki – benue trough were folded, uplifted and intruded by igneous rocks, leading to low-grade metamorphism in some cases (ojoh, 1992). the sediments of the benue trough were eroded and variously overstepped by the campanian and maastrichtian beds of the anambra basin. the horst and graben structure in the calabar flank were initiated at this time (whiteman, 1982; reijers, 1996). figure 3. geologic map of the study area showing positions of mineralized areas (white frames). a: ishiagu area; b: abakaliki area; c: wanikandewanakom area; d: gabu-oshina area. 10 ifeanyi andrew oha, kalu mosto onuoha and silas sunday dada characteristics of the ore deposits the ishiagu mineralization the ishiagu lead-zinc deposits are located in the southwestern tip of the abakaliki anticlinorium (fig. 2). the deposits are hosted in albian sediments which have been folded during the santonian, producing numerous anticlinal and synclinal features. the anticlines are transected by several northwesterly and northerly trending tensional faults and fractures. around the ishiagu area, lead-zinc deposits are hosted mainly by shales of the asu-river group and occasionally in sandstone facies of the group figure 4. geologic map of the ishiagu area showing locations of lead-zinc quarries. the shales are dark gray, finely laminated, carbonaceous and in places hard and splintery due to effects of burial. some exposed sections contain beds of laminated, medium to coarse grained poorly sorted sandstones which represent sandstone facies of the asu-river group. the overlying turonian sediments belonging to the eze-aku group are exposed both to the north and south of the underlying shales. they consist of both sandstone and shale members. the sandstone units exposed mainly to the north (fig. 4) are pinkish, but weathered and ferruginized in some places; they are fine – medium grained, moderately to poorly sorted and highly indurated sandstone units with a general ene-wsw strike and dips of 25o – 40o to the nw and se. the shale units are exposed mainly to the south, and are generally dark gray, carbonaceous, thinly bedded, flaggy and fissile on weathering. they strike between ne-sw to enewsw trends with dips of 30o – 40o nw and se. igneous bodies around the ishiagu area are mainly intrusive and hyperbysal rocks, whereas volcanic rocks are minor and relatively less extensive. the results of petrographic studies indicate that the rocks are mostly of doleritic to dioritic compositions characterized by vein and cavity fillings of calcite. the crush rock quarry at ngwogwa exposed a large section of these bodies (fig. 5b) emplaced concordantly to the bedding planes of the host pelitic rocks, indicating that they are sills. adjacent to the quarry, dioritic rocks occur as large boulders (diameter 2 – 5m) and forms a slightly elevated hill. the dioritic rocks are characterized by zoned plagioclase of andesine composition in a sub-ophitic relationship with augite (fig. 5c). the doleritic rocks are fine – medium grained with similar mineralogic composition with the diorites. figure 5. (a) boulders of dioritic rocks at ugwu chime. (b) a section of the crush rock quarry at ngwogwa, ishiagu exposing thick sills of doleritic to dioritic rocks beneath an overburden of approximately 3m. (c) view of doleritic sills in ngwogwa. (d) nw-se trending fracture (shown by the direction of the arrow) on the shale-mudrock sequence in an abandoned lead-zinc quarry at ugwutangela. (e) the lead-zinc vein in sandstone at green field mine (broken line shows the width of vein approx. 35cm). (f) galena and sphalerite ready for market. figure 6. photomicrographs of doleritic and dioritic rocks around the ishiagu area (x-nicols x25). (a) phenocryst of clinopyroxene (cpx) surrounded by groundmass of plagioclase (pl) in diorite. (b) well developed crystal of calcite (ca) showing rhombohedral cleavage in dolerite. (c) clinopyroxene (cpx) and plagioclase (pl) in a sub-ophitic relationship in dolerite. (d) large crystals of clinopyroxene (cpx) in intersertal relationship with plagioclase (pl) and calcite (ca) in dolerite. (e) zoned clinopyroxenes in diorites. (f) stilbite (st) exhibiting a typical bow tie form in dolerite. 11contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria the imprints of the santonian event in this area are characterized by both ductile and brittle deformation. these features may have controlled the emplacement of the lead-zinc deposits, which is observed to be restricted to nw-se and n-s trending fractures. a rose diagram (fig. 7) of field measurements for fracture orientations taken from both the intrusives and albian sediments shows a predominance of nw-se trends and minor ne-sw trends. crosscutting relationships suggest that the ne-sw fractures predate the nw-se fractures. the ne-sw fractures are generally barren and are more extensive than the nw-se fractures. it is observed that the mineralized nw-se fractures consist of a series of closely spaced, steeply dipping sub-parallel features, typically forming en-echelon patterns (ezepue, 1984; benkhelil, 1986). figure 7. rose diagram for fractures around the ishiagu area, showing the dominance of nw-se trends. the ishiagu mineralization consists principally of lead-zinc in the ratio of approximately 2:1, barite is conspicuously absent. other vein constituents include pyrite, siderite, chalcopyrite, marcasite, bornite, and quartz. the veins are generally 0.25 – 5m wide and 20 – 150m long. wall rock alteration is either absent or minimal, suggesting a low temperature for mineralizing fluids. the abakaliki mineralization the abakaliki mineralization consists of a series of fracture filling sub-parallel en-echelon mineralized lodes occurring around enyigba, ameri, ameka, and ndiagu areas. they appear to be the largest and most extensive of the mineralized areas within the study area, in addition to possessing a long history of mining. between 1905 and 1909, the leadzinc mineralization was already known and investigated by the mineral survey of southern nigeria. by the period from 1925 – 1929 the output in the abakaliki area increased but declined rapidly around 1937 due to a global fall in price. mining activity picked up in the 1950’s with the establishment of the nigerian lead-zinc mining company. in recent times exploration and mining have been restricted to the activities of artisanal miners and a few indigenous mining companies. figure 8. geologic map of the abakaliki area showing locations of lead-zinc quarries sedimentary infilling comprises of albian to coniacian formations which include the asu-river group and the eze-aku group. the asuriver group in this area is composed mainly of dark carbonaceous shales, which appear baked in some places accompanied by a mixture of mudstone and shale (fig. 9a, b). some sections include thin beds of highly indurated siltstone which have undergone both ductile and brittle deformation (fig. 9c). the turonian eze-aku formation is exposed within 2-3kms southeast of the main mineralization centers in the study area and is composed of dark gray carbonaceous shales which grade further south into a series of highly indurated feldspathic sandstones. the sandstone bodies form a set of sub-parallel approximately ene-wsw trending ridges which stand out relative to the more easily eroded shale intercalations. figure 9. (a) mudstone facies in association with dark gray shale around enyigba. (b) dotted lines show contact between mudrock and shale. (c) folded beds of highly indurated siltstone intercalations with shale. (d) well-jointed basaltic exposures around obubra. (e) lead-zinc artisanal mine pit in enyigba. (f) abandoned mine shaft at ameri. 12 ifeanyi andrew oha, kalu mosto onuoha and silas sunday dada the absence of igneous bodies in the vicinity of the mineralized area is peculiar only to the abakaliki area. the closest igneous body outcrops around obubra, about 10km southeast of the deposit and is composed of basaltic bodies, exposed by the cross river. around abakaliki town (approximately 30km north of the mineralized area) a number of volcanic and intrusive rocks consisting mainly of pyroclastics form small reliefs corresponding to a heterogeneous group. benkhelil (1986) has given a detailed account of structural features associated with the abakaliki mineralization. like the ishiagu area, mineralization here is also related to nw-se and n-s trending fractures which are transected in some places by n-s trending sinistral faults. the fractures are less conspicuous than those around ishiagu; however, it is clear that like in the ishiagu area, the mineralization follows the trend of the dominant fracture trend. the abakaliki mineralization is considered the largest in the study area based on the size and density of lodes. the enyigba lode, for example, has been traced for almost 2km and can be up to 30m wide. the ore assemblage consists mainly of galena, sphalerite, siderite, chalcopyrite, pyrite, bornite, and quartz. like in the ishiagu case, barite is conspicuously absent. field evidence, ore microscopy, and electron microprobe analyses of the lodes suggest that the vein mineralization commenced with pre-ore precipitation of diagenetic marcasite, pyrite, and minor gel-like colloform textured schalenblende and galena particularly in the vein – wall contacts (akande and mucke, 1993). the wanikande-wanakom mineralization the wanikande-wanakom barium-lead-zinc mineralization is not well known. the area is best known as a magmatic center (benkhelil, 1986; obiora, 2002). detailed work carried out by benkhelil (1986) around the area focused on tectonic features. obiora, (2002) concentrated on the detailed description of the composition and general disposition of the igneous bodies found in the area. between 2005 and 2008, increased local demand for barite led to the opening of some mining pits/quarries in this area. this development facilitated the gathering of field data for this study. figure 10. the geologic map showing the positions of barium-lead-zinc quarries around the wanikande-wanakom area. yellow frame illustrates the extent of mineralization. figure 11. (a) synclinal fold of shales of the asu-river group around ameka, about 3km north of wanikande. (b) folded unit of mudstones around wanihem. (c) view of baked shales of the asu-river group. (d) well-jointed baked shale of the asu-river group exposed in a road cut along wanikande-wanakom road. igneous bodies in the wanikande-wanakom area include dioritic rocks, micrograbbros, nepheline syenites, dolerites, basaltic sills, nephelinites, trachytes, and pyroclastics. the diorites occur as boulders, are mesocratic, course grained with the interlocking mineral arrangement. the light colored minerals are dominantly plagioclase of andesine composition and may include sodalite, and nepheline. prismatic lensoid hornblende and subordinate short, pyroxenes are also a major part of the essential mineral constituents, whereas biotite, apatite, sphene and opaques occur as accessories. the syenites are commonly nepheline syenites, and are leucocratic, medium to coarse grained, and form large plutons. figure 12. (a) boulder of pyroclastics on the wanudu hills containing large xenoliths of lithic fragments. (b) an outcrop of dioritic rock along the wanikande-wanakom road. (c) exposure of nepheline syenite around an abandoned barite mine at wanikande. (d) abandoned barite mine at wanikande. (e) a sample of barite mined from syenites. (f) samples of low-grade barites around abandoned quarry at wanikande. 13contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria figure 13. photomicrographs of igneous rocks around wanikande. (x-nicols x25) (a) calcite veinlet in alkali basalts. observe the distinct rhombohedral cleavages in the calcite (ca). (b) phenocryst of sodalite (s) in a groundmass of predominantly plagioclase (pl). (c) lithic fragments (lf) in pyroclastics, tiny laths of plagioclase (pl) are observed. (d) large grain of plagioclase (pl) of oligoclase composition found in syenites. (e) euhedral grain of sodalite (s) in syenite. (f) disoriented crystals of plagioclase (pl) in alkali trachyte. chlorite (cl) may have resulted as an alteration product of feldspars. at the uganga hill, nepheline syenite exposures are quite extensive and host the barite veins around the area. obiora (2002) described the nepheline syenite body as having an oblate shape and aligned in an essentially n-s orientation with dimensions of 3.5 x 2.0 km. the body exhibits a compositional zonation from the center towards the periphery. the core consists of a kaersutite-hastingsite bearing rock with abundant alkali feldspar and diopside. the presence of clinopyroxene, hastingsite, with or without paragsite, characterizes the intermediate zone, while only clinopyroxene with minor arfvedsonite were noted in the periphery. a leucocratic, medium-grained rock suspected to be nephelinite was also encountered around uganga hill. the outcrop is extensive and of about 100m long and 10m wide. the rock is composed predominantly of feldspathoidal minerals with appreciable feldspars and subordinate hornblende, biotite, and pyroxene. under thin section, the rock exhibits trachytic texture. over 20 outcrops of pyroclastics clustering around the wanadu hills were encountered, they consist of vent-tuffs and lapilli tuffs, agglomerates, and breccias, with widespread occurrence of lithic fragments which constitute shales and fossils. the prominent structural features of this area include tight folding, fracturing, and crenulation cleavage. benkhelil (1986) reported the existence of a set of large scale faults in this area. the faults are a combination of dip slip and strike slip faults, and are well displayed because they are oblique to the bedding planes; they are often mineralized with quartz and have traces of slickenside striations along the walls. a series of tight anticlinal/synclinal features were observed (fig. 11a) with fold axes approximately oriented in the e-w direction. the fractures manifest mainly as tensional joints and are more prominent in the igneous rocks than the sediments. hundreds of joint orientations were taken from the shales and the different igneous bodies in the area and the data plotted on rose diagrams (fig. 14). the rose plots show that the dominant trend for joints in the syenites and the nepheline syenites is nw-se. in the syenites, out of the 85 data samples plotted, 23 of them plotted in the nw-se direction, 18 plotted in the nnw-sse direction and 9 in the nne-ssw direction. in the nepheline syenites, the wnw-ese trend predominates containing about 45 data out of the 98 data plotted. the pyroclastics and the albian shales contain predominantly ne-sw fractures. for the pyroclastics, 21 data out of the 88 plotted were in the ne-sw direction followed closely by 19 in the nnw-sse direction, while for the albian shales 29 out of the 66 data plotted in the ne-sw direction. field relationships show that the nw-se/ nnw-ssw fracture trends are younger than the ne-sw trends. most of the nw-se fractures are filled with barite and less commonly, lead-zinc ores, and their predominance in the intrusive rocks suggests that they are probably post-santonian in age. figure 14. rose diagram showing the dominant trends of joints in the albian shales and some of the intrusive rocks around the wanikande-wanakom area. mineralization in the wanikande-wanakom area represents a marked deviation from the ishiagu and abakaliki mineralizations. barite appears and predominates in most of the veins studied. it is observed that the barium:lead:zinc ratio increases northwards from 2:2:1 near adum (wanakom) to 100% barite at the wanudu hills (wanikande). the mineralized veins seem to be interwoven with the igneous bodies and in some places are hosted by the igneous bodies, generally having a width of between 5cm – 200cm and length of between 2m – 30m. the intimate relationship between the veins and the igneous bodies presents an enormous exploitation challenge to artisanal miners, consequently, at the time of fieldwork (2010), the mines were all abandoned. 14 ifeanyi andrew oha, kalu mosto onuoha and silas sunday dada the gabu-oshina mineralization the gabu-oshina mineralization occurs approximately 25km nnw of ogoja town. like the wanikande-wanakom area, this area is relatively unknown from literature; however, it possesses great potential for barium-lead-zinc mineralization. over the past eight years, a number of medium to large veins (mainly barite) were discovered in this area, making it one of the most mineralized areas in the entire study area. the activities of a mining company, emo-ashapura, which is involved in semi-mechanized open cast barite mining in the area since 2008, led to the discovery of other veins in the area. the gabu-oshina area consists mainly of turonian sediments deformed along northeasterly trending axes producing numerous gentle folds. the lithostratigraphic units include highly indurated sandstones, shales, and mudstones of the eze-aku group. the shales are exposed mainly along stream channels; they are dark, highly fissile and sometimes flaky with dips of 30o – 40o (s or sse). the sandstones comprise of highly indurated poorly sorted, laminated arkosic units, which sometimes display a fining upward sequence. the sandstone bodies are also tilted with strikes of n30oe – n60oe and dips of 20o – 30o to the southwest. around the barite quarry at oshina, dips greater than 50o were observed on the sandstones. some of the barite quarries expose small bodies of basaltic – doleritic sills. the sills are usually thin (0.4m – 2m), consequently they do not constitute any serious barrier to mining activities. figure 15. geological map of ogoja sheet showing the location of barite-lead-zinc quarries in the gabu-oshina area. structural features within the area are restricted to gentle folds and randomly oriented fractures. the folds are reflected by tilting of the beds, although the dips can be high (30o – 60o), the wavelengths of the folds are naturally long (kilometric) and so unlike in the areas around the workum hills, where the profile of tight folds can be traced on outcrop scale, the folds in the gabu area manifest from opposite dips over long distances. fracture orientations were taken mainly from sandstones around the barite quarries. the readings were plotted on a rose diagram (fig 18) which reveals that the dominant trend is oriented in the ene-wsw direction and accounts for about 14% of the data plotted. nw-se, nne-ssw, and n-s trends are prominent and are related closely and intimately to each other. cross-cutting relationships suggest that the n150oe (nw-se) fractures predate the n80oe (ene-wsw) fractures. although the n80oe joints are more in number, they are conspicuously barren of barium-lead-zinc minerals relative to the n150oe fractures which are mineralized. figure 16. (a, b, c) fractured and highly indurated turonian sandstones observed around the gabu – oshina area. (d) planar lamination in highly indurated sandstone. (e) shale beds exposed in a valley around otukpo in the gabu-oshina area. (f) highly tilted beds exposed adjacent the oshina mining pits. figure 17. (a) abandoned barite mine pit at oshina. (b) nodules of barite and galena found in old mine pits at gabu. (c) barite vein in weathered sandstones. (d) heaps of mined barite crystals ready for market. (e) artisanal miners in one of the mining pits around gabu. (f) salt pond at gabu, at approximately 50m from a mine pit. the yellow circle indicates the position of seepage. 15contrasting styles of lead-zinc-barium mineralization in the lower benue trough, southeastern nigeria figure 18. rose diagram showing the dominant trends of fractures in the sandstones and shales around the gabu-oshina barium-lead-zinc quarries. the gabu-oshina barium-lead-zinc mineralization consists of an array of closely related veins arranged en-echelon along an approximately ne-sw direction. the mineralized veins are predominantly oriented in the nw-se and n-s directions and extend for up to 1.5 km in length. the width of the veins ranges from a few centimeters to tens of meters and are composed predominantly of barite. galena and sphalerite are rare forming minor constituents of the mineralization and are conspicuously absent in some of the principal lodes. discussion the distribution of existing lead-zinc-barium mineralized locations around the study area (fig. 3), do not show any definite trend (at least locally) however, an approximate ne-sw trend which coincides with the main axis of the basin can be assumed on a regional scale. associated host rocks include albian shales belonging to the asu-river group which are baked in some places, turonian sandstones and shales belonging to the eze-aku group and post-turonian to coniacian intrusives ranging from diorites, syenites, nepheline syenites, which are of highly variable composition. these suggest that the mineralization is devoid of any major lithologic affinity. the deposits are associated at least spatially with igneous bodies and saline springs. the absence of igneous bodies in the vicinity of the quarries around enyigba-ameri-ameka and of saline springs around the wanakomwanakonde area is highly notable. due to the widespread distribution of intrusive rocks exposed in the study area, the authors are of the opinion that the observed close spatial association between mineralized veins and intermediate to mafic intrusive rocks in the study area does not have genetic implication. furthermore, it is evident from field observations that the intrusives predate the mineralization. in the wanakonde area, barite-dominated veinlets are seen cross-cutting late cenomanian to santonian intrusives. it is clear from field evidence that the mineralization is structurally controlled. without exception, mineralization appears to be restricted to parallel or sub-parallel en-echelon nw-se trending fractures. this trend, although subordinate to the dominant ne-sw trend observed in most locations, is prominent and appears younger than the more dominant trend. mineralized veins range from a few centimeters to tens of meters in width and from a few meters to over 2km in length. the broad variation in size and geometry is more because of the non-restriction of the veins to a lithologic type than it is to spatial location. ore assemblage varies from one location to the other; there is a marked dominance of galena and sphalerite in the southern parts of the study area with barite conspicuously absent. barite appears around wanakom and continues to increase in dominance northwards. also, the southern deposits are more polymetallic in composition, including such minerals as galena, sphalerite, pyrite, siderite, chalcopyrite, marcasite, bornite, and quartz. except for galena and sphalerite, all other ores occur only in minute quantities. although the benue trough lead-zinc-barium deposits may be considered a clastic rock dominated lead-zinc-barium deposit, they lack the essential characteristics of sedex deposits and hence do not fit properly into any of leach et al.’s (2005) grouping of sediment-hosted lead-zinc deposit. the suggestion by olade and morton (1985) from mineralization features, that the benue trough lead-zinc deposits be assigned a distinct category (riftogenic) is considered inappropriate for the following reasons. (1) the term riftogenic derived from “rift” has a genetically tectonic affiliation and falls under the category of deposits (mvt) formed in continental rift tectonic setting described by leach et al. (2010). (2) there are insufficient data on sulfur, lead and oxygen isotopes to precisely characterize the deposits and compare with other classical lead-zinc-barium deposits in other parts of the globe. the association of the well-known lead-zinc deposits in the lower benue trough around ishiagu and enyigba with barites around wanakom, wanikande and gabu-oshina is well established. the observation that the ores co-exist in the same veins around wanakom suggests that they belong to the same metallogenic province and epoch. it is also interesting to note the variation in ore composition from predominantly lead-zinc deposits in the south to lead-zinc-barium in the middle and predominantly barium in gabu-oshina (northern part of the study area). conclusion the benue trough of nigeria is host to numerous lead-zinc-barium veins trending in predominantly nw-se trends. mineralization appears to be associated at least spatially with igneous intrusions and saline springs. the widespread occurrence of intrusive igneous rocks of varying mineralogical composition in the lower benue trough coupled with observed crosscutting relationships with the mineralized veins and absence of igneous bodies in the vicinity of lead-zinc quarries around enyigba suggest that the deposits are not genetically related to the intrusions. host rocks range from shales, siltstones, sandstones, and intrusives hence there is no distinct lithologic affinity. the ore assemblage varies remarkably from the southwestern to the northeastern part of the study area, with lead:zinc:barium ratios ranging from approximately 3:1:0 at ishiagu, to 2:1:0 at enyigba, 1:0:2 at wanikande and nearly 100% barite at gabu-oshina. the ore deposits have been characterized to fit roughly into the base metal type mineralization known as clastic-rock dominated lead-zinc-barium deposits. acknowledgments the authors wish to thank the department of geology, university of nigeria, nsukka for providing logistical support during the field work at gabu-oshina. chidozie dim, onyekachi ayogu, and nneka osuji assisted in part of the fieldwork. rose diagrams were plotted using georient academic version, provided by dr. rod holcombe of the university of queensland. emmanuel ugwuonah helped in the production of the photomicrographs. references akande, s. o., horn, e. e. & reutel, c. 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(2020). group velocity measurements of earthquake rayleigh wave by s transform and comparison with mft. earth sciences research journal, 24(1), 91-95. doi: https://doi.org/10.15446/ esrj.v24n1.85366 based upon the synthetic rayleigh wave at different epicentral distances and real earthquake rayleigh wave, s transform is used to measure their group velocities, compared with the multiple filter technique (mft) which is the most commonly used method for group-velocity measurements. when the period is greater than 15 s, especially than 40 s, s transform has higher accuracy than mft at all epicenter distances. when the period is less than or equal to 15 s, the accuracy of s transform is lower than that of mft at epicentral distances of 1000 km and 8000 km (especially 8000 km), and the accuracy of such two methods is similar at the other epicentral distances. on the whole, s transform is more accurate than mft. furthermore, mft is dominantly dependent on the value of the gaussian filter parameter α , but s transform is self-adaptive. therefore, s transform is a more stable and accurate method than mft for group velocity measurement of earthquake rayleigh waves. abstract group velocity measurements of earthquake rayleigh wave by s transform and comparison with mft mediciones de la velocidad de grupo de las ondas sísmicas rayleigh mediante la transformada s y comparación con la técnica de filtrado múltiple (mft) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85366 con base en ondas sintéticas rayleigh a diferentes distancias epicentrales y en ondas sísmicas rayleigh reales, en este trabajo se emplea la transformada s para medir la velocidad de grupo de este tipo de ondas y se compara con la técnica de filtrado múltiple (mft, por sus siglas del inglés, del nomre multiple filter technique), que es el método más utilizado para las mediciones de velocidad de grupo. cuando el periodo es mayor a 15 s, especialmente de 40 s, la transformada s tiene mayor precisión que mft, para todas las distancias epicentrales. cuando el periodo es menor o igual a 15 s, la precisión de la transformada s es menor que mft para distancias epicentrales de 1000 km y 8000 km (especialmente 8000 km). la precisión de estos dos métodos es similar para otras distancias epicentrales. en general, la transformada s es más precisa que mft. además, mft depende predominantemente del valor del parámetro de filtro gaussiano α, pero la transformada s es autoadaptativa. por lo tanto, la transformada s es un método más estable y preciso que mft para la medición de la velocidad de grupo de las ondas sísmicas rayleigh. resumen record manuscript received: 12/03/2019 accepted for publication: 17/12/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 91-95 chanjun jianga,b,c, youxue wanga,b*, gaofu zengd aschool of earth science, guilin university of technology, guilin 541006, china. bguangxi key laboratory of hidden metallic ore deposits exploration, guilin 541006, china. cbowen college of management, guilin university of technology, guilin 541006, china. dchina nonferrous metals (guilin) geology and mining co,. ltd, guilin 541006, china. * corresponding author: uxue.wang@glut.edu.cn g eo ph ys ic s 92 chanjun jiang, youxue wang, gaofu zeng introduction earthquake surface wave tomography has become a powerful tool to image the deep structures of the crust and upper mantle due to its dispersion characteristics. measuring high-quality group velocities or phase velocities of surface wave, such as rayleigh wave, is a key foundation for surface wave tomography. currently, there are some methods to measure group velocities of surface wave, such as moving window analysis (mwa) (landisman et al., 1969), multiple filter technique (mft) (dziewonski et al., 1969) and continuous wavelet transform (cwt) (jiang et al., 2017), of which mft is the most commonly used one (ritzwoller & levshin, 1998; zhu et al., 2002; huang et al., 2003; cho et al., 2007; li et al., 2009; saygin & kennet, 2010; rindraharisaona et al., 2017; tang et al., 2018; lu et al., 2018; mechie et al., 2019). in 1969, mwa was proposed by landisman et al. using rectangular window in time domain. in the same year, mft was presented by dziewonski et al. using gaussian filter in frequency domain. when taking gaussian filter instead of rectangular window in time domain, mwa is equivalent to mft in frequency domain. mft, a commonly used method, is dominantly dependent on the value of gaussian filter parameter α, and much research has been done (inston et al., 1971; cara, 1973; nyman & landisman, 1977; herrmann & ammon, 2004; kolínsky, 2004; zhu et al., 2007; chen et al., 2014; jiang et al., 2019). especially, herrmann and ammon (2004) gave recommended values of gaussian filter parameter α with distance variation when the period is from 4 s to100 s. cwt was used to measure group velocities of earthquake rayleigh wave, compared to mft (jiang et al., 2017). in 1996, s transform was proposed by stockwell et al., which is an extension of cwt based on morlet wavelet (stockwell et al.,1996). s-transform combines advantages of short-time fourier transform (stft) and wavelet transform (wt). for instance, inverse s transform is directly related to fourier transform and it is a lossless transformation, linear transformation has no cross-term and high time-frequency resolution, the time-frequency resolution is related to the frequency of the signal, and the basic wavelet need not satisfy the admissibility condition (pinnegar & mansinha, 2003a, 2003b; zheng & wang, 2015). in recent years, s-transform has been widely used in geophysics (pinnegar & eaton, 2003, 2006; askari & siahkoohi, 2008; parolai, 2009; askari et al., 2011; tselentis et al., 2012; askari & ferguson, 2012; tu et al., 2013; zheng & wang, 2015). in this paper, s transform is introduced to measure group velocities of earthquake rayleigh wave and is compared with the commonly used mft. s transform if x(t) is a surface wave signal generated by natural earthquake, then the s transform of x(t) is defined as s f x t f t f i ft dtτ − −τ − −∞ ∞ , exp exp( ) = ( ) ( )        ( ) + ∫ 2 2 2 2 2 π π (1) where t is time, τ is the central point of the window function and can control the position of window function along time axis, and f is frequency. in s transform, gaussian function and basic wavelet are respectively defined as g t f t f f ( ) =       2 2 2 2 π exp − (2) w t f t f i ft g t i ftf f( ) =       = ( ) ( ) 2 2 2 2 2 2 π π πexp exp− − − (3) the basic wavelet in s-transform is the product of simple harmonics only scaled in time domain and gauss function scaled and shifted. time window width of s-transform varies inversely with frequency, i.e. the time window is wider in low frequency band, which leads to higher frequency resolution, while the time window in high frequency band is narrower, which leads to higher time resolution. therefore s transform is self-adaptive. inverse s transform is written as x t s f d e dfi ft( ) = ( )           ++ ∫∫ τ τ −∞ ∞ −∞ ∞ , 2π (4) comparison with mft at present, mft is the most commonly used method to measure group velocities of earthquake rayleigh wave. based upon the synthetic rayleigh wave at different epicentral distances and real earthquake rayleigh wave, s transform is compared with mft. mft choosing appropriate value of gauss filter parameter α can reduce the group-velocity measurement error of surface wave. the recommended values of α with epicentral distance variation in the period range from 4 s to 100 s given by herrmann and ammon (2004) are listed in table 1, in which the value of α at epicentral distance of 3 000 km is originally not available but is obtained by interpolation of the values at epicenter distances of 2 000 km and 4 000 km. table 1. values of gaussian filter parameter α epicentral distance (km) values of α 1000 25 2000 50 3000 75 4000 100 8000 200 synthetic rayleigh waves in order to check the effectivity of s transform in the measurement of group velocities and compare s transform with mft, synthetic data of rayleigh waves were generated by modal summation method (herrmann & ammon, 2004) based upon ak135 earth model (kennett et al., 1995), and the source parameters are listed in table 2. there are five stations with epicentral distances of 1 000 km, 2 000 km, 3 000 km, 4 000 km and 8 000 km, respectively. the azimuth angles of such stations are 45º and the sampling interval of data is 1 s. synthetic seismogram of rayleigh wave at epicentral distance of 3000 km and its amplitude spectra are shown in figure 1. table 2. the source parameters surface-wave-modes strike dip slip angle magnitude depth 5 0º 45º 0º 6.0 20 km and then s transform and mft are respectively applied to measure group velocities of the synthetic data . from the results by such two methods (fig. 2), the following conclusions can be drawn: 1) when the epicentral distance is 1 000 km, the accuracy of groupvelocity curve obtained by s-transform is slightly lower than that by mft in the period range of less than or equal to 15 s but much higher in the other period range, in the period range of greater than 40 s groupvelocity curves measured by s transform and mft both have large error and therefore the period to be measured should not be large when the epicentral distance is small; 2) when the epicentral distance is 2 000 km, 3 000 km or 4 000 km, the accuracy of group-velocity curve obtained by s-transform is similar as that by mft in the period range of less than or equal to 15 s but much higher in the other period range, especially when the period is greater than 40 s the group-velocity curve obtained by mft fluctuates obviously and has large error with the theoretical one while that by s transform fits well with the theoretical one; 93group velocity measurements of earthquake rayleigh wave by s transform and comparison with mft 3) when the epicentral distance is 8 000 km, the accuracy of group-velocity curve obtained by s-transform is lower than that by mft in the period range of less than or equal to 15 s but higher in the other period range. on the whole, the group-velocity curve gained by s transform fits better with the theoretical one than mft, therefore its accuracy is higher than that of mft. real earthquake rayleigh wave the real seismogram of earthquake rayleigh wave acquired from mobile station in guangxi of china is shown in fig. 3 and the parameters of the earthquake event are listed in table 3. the coordinates of the station are (110.50°e, 24.09°n) and epicentral distance is 3 551.22 km. for mft, the value of gauss filter parameter α at such epicentral distance is 87.5 by interpolation (table 1). figure 3. real seismogram of earthquake rayleigh wave acquired in guangxi of china. table 3. parameters of the earthquake event magnitude/mw greenwich mean time longitude/°e latitude/°n depth/km 7.1 2013/10/25 17:10:18.43 144.68 37.17 25.67 then s transform and mft are respectively applied to measure group velocities of such real data and the results are shown in fig.4. overall the group-velocity curves obtained by s transform and mft are similar. but when the period is greater than 40 s, the group-velocity curve obtained by mft fluctuates obviously while the curve by s transform is much smoother, hence s transform is more reliable than mft. conclusions in this paper, based upon the synthetic rayleigh wave at different epicentral distances and real earthquake rayleigh wave, the group velocities are measured by cwt and mft respectively. from the analysis and comparison, the conclusions can be drawn generally as following: 1) when the epicentral distance is small, such as 1 000 km, in the period range of greater than 40 s group velocities measured by s transform and figure 1. synthetic seismogram of rayleigh wave at distance of 3000 km (a) and its amplitude spectra (b) figure 2. comparison between s transform and mft based on synthetic data 94 chanjun jiang, youxue wang, gaofu zeng mft both have large error and are both not reliable and therefore the period to be measured should not be large in such case; 2) when the period is greater than 15 s, s transform has higher accuracy than mft at all epicenter distances, especially when the period is greater than 40 s the group-velocity curve obtained by mft fluctuates obviously and its accuracy is much lower than that by s transform; 3) when the period is less than or equal to 15 s, the accuracy of s transform is lower than that of mft at epicental distances of 1000 km and 8000 km (especially 8000 km), and the accuracy of such two method is similar at the other epicental distances. on the whole, s transform is more accurate than mft. furthermore, mft is dominantly dependent on the value of the gaussian filter parameter α, but s transform is self-adaptive. therefore, s transform is a more stable and accurate method than mft for group velocity measurement of earthquake rayleigh waves. acknowledgment funded by project 41574039 from nsf, key project 2016gxnsfda380014 from guangxi scientific foundation and project 2018ky0857 from guangxi education department. references askari, r., ferguson, r. j., & demeersman, k. 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(2013). real-time coseismic velocity and displacements retrieving and de-noising processs by high-rate gnss. china satellite navigation conference(csnc), proceedings, lecture notes in electrical engineering, 244, 523-537. zheng, c. l., & wang, b. s. (2015). applications of s transform in seismic data processing. progress in geophysics, 30(4), 1580-1591. zhu, j. s., cao, j. m., cai, x. l., yan, z. q., & cao, x. l. (2002). high resolution surface wave tomography in east asia and west pacific marginal seas. chinese journal of geophysics, 45(5), 646-661. seismotectonic-geologic hazards zoning map of iran is drawn based on deterministic seismic hazards evaluation using the seismicity records, structural trends, tectonic settings, fault ruptures and neotectonics activities in iran. this map has been prepared to indicate the seismotectonic-geologic hazards of iran. it contains the earthquake hazards parameters such as b value and mmax for the nineteen seismotectonic provinces. furthermore, moho discontinuity depth, seismogenic layer depths and seismic rates for all provinces are investigated. the majority of deformation in iran has been concentrated in the continental crust of the country. zagros and alborz experience deep earthquakes that are an indication of existence of the thick-skinned tectonics. finally, some provinces such as piranshahrborojen, east iran, naienrafsanjan and east alborz, which include main suture zones of iran, have faced high seismic hazards. este estudio se realizó con el fin de preparar un mapa zonal de amenazas seismotectónicas y geológicas de irán con base en la evaluación de amenazas sismícas deterministas a través de los registros sísmicos, las tendencias estructurales, las configuraciones tectónicas, las rupturas de fallas y la actividad neotectónica en irán. este mapa contiene los parámetros de amenaza sismológica como los valores b y mmax para las 19 provincias sismotectónicas del país. además, se investigó la profundidad de la discontinuidad de mohorovičić, las capas sismogénicas profundas y las clasificaciomes sísmicas para todas las provincias. la mayoría de la deformación de irán está concentrada en la corteza continental del país. las provincias de zagros y alborz experimentan terremotos profundos que indican la existencia de una configuración tectónica de capa gruesa. finalmente, algunas provincias como piranshahr-borojen, este de irán, naien-rafsanjan y este de alborz, que incluyen las principales zonas de sutura de irán, han enfrentado grandes riesgos sísmicos. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 7 13 abstract resumen key words: seismotectonic, hazards, zoning map, iran, seismicity, earthquake palabras clave: sismotectónica, amenazas, mapa zonal, irán, sismicidad, terremoto. seismotectonic-geologic hazards zoning of iran mehran arian department of geology, science and research branch, islamic azad university, tehraniran; mehranarian@yahoo.com seismology record manuscript received: 5/11/2013 accepted for publication: 14/01/2015 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.40664 8 mehran arian introduction the deterministic assessment of the seismotectonic-geologic hazards of iran involves specifying the magnitude ranges and dominant mechanism of earthquakes that might bring about destructive effects on the regions and main cities. a seismotectonic province is a homogenous statistical unit that has distinguishable time, location and magnitude ranges for expected seismic events (arian, 2010). each seismotectonic province has a range of seismogenic layer depths and dominant mechanism of earthquakes, as these are related to tectonic settings, neotectonics activities, mechanical stratigraphy and deformational models of crust. therefore, earthquake hazard parameters can be estimated for them. in this study, regional tectonics (qorashi and arian, 2011) was integrated with seismologic data to prepare the seismotectonic-geologic hazards of iran. the main steps in this study have been summarised as follows: -seismic records such as archaeological, historical (berberian and yeats, 1999, 2001) and instrumental seismic data up to the year 2014 which have provided information on the seismic sources have been gathered from different sources. the seismotectonicgeologic hazards map is identified based on the physiographic-tectonic zoning map of iran’s sedimentary basins (arian, 2013). these basins described the locations of different structural units of iran (fig. 1). -seismicity parameters were estimated for each seismotectonic province. materials and methods at the first time, ambraseys and melville (1982) provided iran’s historical earthquakes and, subsequently, earthquake catalogue of iran was compiled by berberian (1994). investigation of seismicity of iran reveals that significant destructive earthquakes are related to the primary active faults of different tectonics settings (engdahl, et al., 2006). iran is one of the seismically active areas of the alpinehimalayan orogenic belt. based on previous work on neotectonics regime in iran, zagros in south iran is the most active zone (arian et al., 2002 and 2003; arian and qorashi 2006; arian and hashemi, 2008; rahimi and arian, 2014; arian and aram, 2014). then, alborz in north iran (arian and pourkermani 2004; feizi et al., 2007; khavari et al., 2009 and 2010; sorbi et al., 2011; poroohan et al., 2009 and 2013; arian et al., 2011a and 2012; arian and bagha, 2012;nouri et al,3013; bagha et al., 2014; moghimi et al., 2015) and central iran (arian, 2010; housini toudeshki et al., 2011;houseni toudeshki and arian, 2011; arian et al., 2011b, c, d; pourkermani and arian, 2001 have been situated in the next orders. the seismotectonic provinces of iran were formerly studied by several researchers such as abdalian (1963), berberian (1976), nowroozi (1976), and tavakoli and ghafory-ashtiany (1999). in this study, based on updated catalogue of significant and damaging iranian earthquakes, a new seismotectonic -geologic hazards zoning map has been proposed. the boundaries of the provinces are established through consideration of geologic materials, seismic sources, tectonic settings, and major faults and physiographic divisions (fig. 2). earthquake hazard parameters seismic hazard analysis requires an assessment of earthquake hazard parameters and the future earthquake potential in a region. earthquake hazard parameters such as, maximum expected magnitude, mmax, seismic rate and b value have been evaluated for each seismotectonic province of iran by kijko and selevoll (1992) method. the results, which are shown in table 1, were estimated for each seismotectonic provinces of iran. result and discussion zagros province the dominant structural trends in zagros province (fig. 2) are nw-se in north-western part and e-w in the south-eastern section. from tectonics figure 1. physiographic-tectonic zoning map of iran’s sedimentary basins iran, modified from arian (2013). numbers in this figure are; 1: zagros-east taurus hinterland, 2: persian gulf mesopotamian foreland basin, 3: makran accretionary prism, 4: bashagard mountains, 5: jazmorianmashkel fore arc basin, 6: shahsavaran -soltan magmatic arc, 7: south lut-south helmand back arc basin, 8: east iran mountain belt, 9: westcentral alborz and lesser caucasus hinterland, 10: great kavir-northen urmieh lake foreland basin, 11: south great kavir fold and thrust belt, 12: south caspian-black sea foreland basin, 13: urmiehdokhtar magmatic arc, 14: naien-kerman retro arc foreland basin, 15: sanandaj-sirjan overthrust belts, 16: east alborz or binalod hinterland, 17: torbat-e am-neyshabour retro arc foreland basin, 18: kopet dagh hinterland, 19: south caspian remnant basin, 20: maiamay -taibad inverted back arc basin, 21: khafkavir plain magmatic arc, 22: lut plain-gonabad back arc basin, 23: tabas hinterland, 24:yazd-khour piggy back basin. figure 2. the seismotectonic-geologic hazards zoning map of iran. 9seismotectonic-geologic hazards zoning of iran point of view, this province contains the overthrust and simple folded belts of zagros which has been formed in the north-eastern part of arabian plate’s passive margin. zagros mountains extend to east taurus mountains in turkey and are called zagros-east taurus hinterland (fig.1). focal mechanisms of numerous earthquakes are reversed and thrust (beberian, 1995) such as naghan and ardal (ms=6.1, 1977) and/ or right slip in related to n-s strike slip faults such as kazeron (se boshehr ms=6.2, 2013) or kareh bas (firozabad-fars ms=6.1, 1994, arian et al, 2006). zagros province experiences low to moderate earthquakes with high frequency, short repeat time and 10-20km focal depth. the intensity of earthquakes is in middle levels in which there are several ductile rocks (thick evaporates) in sedimentary covers such as hormuz, dashtak, and gachsaran formations. sometimes in the northern margin of this province, focal depths exceed to 70km (hatzfeld et al.2003) that is an indication of initial stages of thick-skinned tectonics. the most severe seismic hazards in zagros province, including large cities such as ilam, ahvaz, yasoj, boshehr and shiraz, are landslides in high regions of northern margins, settlements in some lowlands and coastal areas and tsunami in southern margins. piranshahrborojen province the dominant structural trend in piranshahrborojen province (fig. 2) is nw-se. from tectonics point of view, this province contains the north margin of overthrust belts of zagros formed on the northeastern part of arabian plate’s passive margin. focal mechanisms of several earthquakes are a right slip about main recent faults of zagros such as doroud (ms=6.1, 2006). piranshahrborojen province is struck by moderate to high earthquakes with low frequency, long repeat time and 10-15km focal depth. the intensity of earthquakes is at high levels. sometimes, focal depths exceed to 70km which is an indication of initial stages of thick-skinned tectonics. the most severe seismic hazards in piranshahrborojen province, including cities such as kermanshah, khoramabad, borojerd and shahrekod, are landslides in high regions of northern margins, settlements in some lowlands and surface faulting. bandar-e abbas province the dominant structural trend in bandar-e abbas province (fig. 2) is nesw. from tectonics point of view, this province contains the south-eastern part of overthrust and simple folded belts of zagros. focal mechanisms of several earthquakes are reversed and thrust such as khorgo (ms=7, 1977). bandar-e abbas province is hit by moderate to high earthquakes with low frequency, short repeat time and 10-20km focal depth. the intensity of earthquakes is in middle levels in which there are ductile rocks in sedimentary covers. the most severe seismic hazards in bandar-e abbas province, containing large cities such as bandar-e abbas, gheshm, bandar-e lengeh and hajiabad, are rapid uplifting in northern margins, settlements in some lowlands and coastal areas and tsunami in southern margins. makran province the dominant structural trend in makran province (fig. 2) is e-w. from tectonics point of view, this province contains the accretionary prism on trench of oman sea subduction that extends to pakistan (fig. 1). focal mechanisms of a vast number of earthquakes are reversed and thrust such as pasni in pakistan (ms=8.3, 1945). makran province experiences low to moderate earthquakes with low frequency, very long repeat time and down to 20km focal depth. the intensity of earthquakes is in the middle to high levels. the most severe seismic hazards in makran province, which contains large cities such as bandar-e chabahar and nikshahr, are rapid uplifting in northern margins, settlements in some lowlands and coastal area and tsunami in southern margins. jazmorian province the dominant structural trend in jazmorian province (fig. 2) is e-w. from tectonics point of view, this province contains a fore arc basin (jazmorian plain) which extends to mashkel plain in pakistan (fig. 1). focal mechanisms of many earthquakes are reversed and thrustand sometimes normal (in relation to bending of subducting oceanic lithosphere) such as nw saravan with 69 km focal depth (ms=5.2, 2009) and se khash with 82 km focal depth (ms=7.7, 2013). jazmorian province experiences moderate earthquakes with middle frequency, medium repeat time and down to 150km focal depth. the intensity of earthquakes is in middle levels by very deep hypocenters. the most important seismic hazards in the jazmorian province, containing large cities such as saravan and iranshahr, are rapid uplifting and settlements in some lowlands. it should be noted that low seismicity in makran and jazmorian provinces is related to slow subduction rate and steeper dip of benioff zone. current subduction rate of oceanic crust (related to late cretaceous) is 19 millimetres per year (vernant et al. 2004). urmiehsirjan province the dominant structural trend in urmiehsirjan province (fig. 2) is nwse. from tectonics point of view, this province contains two different units: a) urmieh-dokhtar magmatic arc (fig. 1) which is the result of subduction beneath the southern active continental margin of cimmerian plate. b) sanandajsirjan overthrust belts (fig. 1) is formed by metamorphic rocks of the northeastern part of arabian plate (alavi, 1994). focal mechanisms of several earthquakes are reversed and thrust such as changureh (ms=6.4, 2002) the urmieh-sirjan province is struck by low to moderate earthquakes with low frequency, medium repeat time and down to 10km focal depth. the intensity of earthquakes is in high levels in which there are cold igneous rocks. sometimes, focal depths exceed to 70km that it is an indicator for initial stages of thick-skinned tectonics. the most serious seismic hazards in urmiehsirjan province, which contains large cities such as urmieh, sirjan, sanandaj, hamadan, arak, ghom and isfahan, are landslides in high regions of northern margins, settlements in some lowlands and surface faulting. lut province the dominant structural trend in lut province (fig. 2) is nw-se. from tectonics point of view, this province contains a retro arc (back arc) basin (lut plain) which extends to helmand plain in pakistan (fig. 1). focal mechanisms of several earthquakes are reversed and thrust and/ or right slip about n-s strike slip faults such as nayband and nehbandan. lut province experiences low earthquakes with low frequency, medium repeat time and down to 10km focal depth. the intensity of earthquakes is in high levels in which there are hot igneous rocks. the most serious seismic hazards in lut province, which contains lut plain, are settlements, surface faulting and volcanic hazards in southern and western parts of the plain. east iran province the dominant structural trend in east iran province (fig. 2) is n-s. from tectonics point of view, this province contains a collision belt brought about by eastward subduction of east-central iran miniplate (fig. 1) beneath central blocks of afghanistan. this collision belt has been formed by a dextral transpression zone. focal mechanisms of many earthquakes are right slip in relation to n-s strike slip faulting such as zirkuh (berberian et al.,1999) in qa’enat (ms=7.2, 1997) and thrust such as west zabol (ms=6.2, 1993). east iran province is hit by moderate to high earthquakes with middle to high (on fault terminations) frequency, medium repeat time and down to 15km focal depth. the intensity of earthquakes is at high levels. the most serious seismic hazards in east iran province, which contains large cities such as nehbandan, zahedan and khash, are landslides in high regions, settlements in some lowlands, surface faulting and dispersed volcanic hazards. gonabadbirjand province the dominant structural trend in gonabadbirjand province (fig. 2) is nw-se. from tectonics point of view, this province contains the northern part 10 mehran arian of east-central iran miniplate (fig. 1). it has been impressed by splays at the north termination of nehbandan fault system. focal mechanisms of numerous earthquakes are left slip in relation to w-e strike slip faulting such as dasht-e bayaz (ms=7.4, 1968) and right slip in relation to nw-se strike slip with thrust component along longitudinal faults such as south ghaen (ms=5.6, 2012). gonabadbirjand province experiences moderate to high earthquakes with low frequency, long repeat time and down to 10km focal depth. the intensity of earthquakes is at high levels. the most serious seismic hazards in gonabad birjand province, which contains large cities such as gonabad and birjand, are landslides in high regions, settlements in some lowlands, and surface faulting. tabas-minab province the dominant structural trend in tabas-minab province (fig. 2) is n-s. from tectonics point of view, this province contains the tabas, nayband, kuh banan, gowk, bam, sazvaran and minab fault systems. focal mechanisms of several earthquakes are dextral strike-slip faulting with reverse component in southern part such as bam (ms=6.6, 2003) and reverse faulting with dextral strike slip component in northern part such as tabas (ms=7.4, 1978) and western part such as zarand (ms=6.5, 2004). tabas-minab province experiences moderate to high earthquakes with low frequency, long repeat time and down to 15km focal depth. the intensity of earthquakes is at high levels. southward earthquake migration is index characteristic in this province. the most severe seismic hazards in tabas-minab province, containing large cities such as tabas, minab, kerman, bam, jiroft and jask, are landslides in high regions, settlements in some lowlands, surface faulting. kashmaranarakbafgh province the dominant structural trend in kashmaranarakbafgh province (fig. 2) is ne-sw. from tectonics point of view, this province contains the northern and western part of east-central iran miniplate (fig. 1). focal mechanisms of several earthquakes are sinistral strike-slip faulting and thrusting. kashmaranarakbafgh province experiences low to moderate earthquakes with low frequency, medium repeat time and down to 20km focal depth. the intensity of earthquakes is at low levels. the most serious seismic hazards in kashmaranarakbafgh province, which contains large cities such as kashmar, anarak and bafgh, are settlements in some lowlands and surface faulting. naienrafsanjan province the dominant structural trend in naienrafsanjan province (fig. 2) is nw-se. from tectonics point of view, this province contains the extensive part of naien-kerman retro arc foreland basin (fig. 1). focal mechanisms of several earthquakes are thrusting. naienrafsanjan province experiences low to moderate earthquakes with low frequency, medium repeat time and down to 20km focal depth. the intensity of earthquakes is at low levels. the most serious seismic hazards in naienrafsanjan province, which contains large cities such as naien, rafsanjan and yazd, are settlements in some lowlands and surface faulting. great kavirtaibad province the dominant structural trend in great kavirtaibad province (fig. 2) is nw-se in the eastern part and ne-sw in the western part. from tectonics point of view, this province contains the maiamaytaibad marginal back arc basin, southeastern part of great kavirurmieh lake foreland basin and south great kavir folded and thrust belt (fig. 1). the latter case is the result of the collision between the northern part of east-central iran miniplate and southeastern part of north-central iran miniplate.there are many salt diapirs that have been formed by tectonic forces (arian, 2012, asadian and arian 2009, pourkermani and arian, 1997) focal mechanisms of several earthquakes are thrusting and dextral strike-slip faulting in the eastern part and sinistral strike slip faulting and thrusting in western part such as garmsar (ms=5.1, 2007). great kavirtaibad province is hit by low to moderate earthquakes with low frequency, medium repeat time and down to 10km focal depth. the intensity of earthquakes is at low levels. the most serious seismic hazards in great kavir taibad province, which contains large cities such as semnan, sabzehvar and torbat-e heydarieh, are settlements in some lowlands and surface faulting. east alborz province the dominant structural trend in east alborz province (fig. 2) is nw-se in the eastern part and ne-sw in the western part. from tectonics point of view, this province contains the torbat-e jam-neyshabour retro arc foreland basin and east alborz (fig. 1) hinterland formed in a fore arc basin on the north-eastern part of cimmerian plate. an overthrust belt with sw vergence has been formed. focal mechanisms of several earthquakes are thrusting such as neyshabour (ms=7.4, 1405). east alborz province experiences low to moderate earthquakes with high frequency, short repeat time and down to 20km focal depth. the intensity of earthquakes is at high levels. the most serious seismic hazards in east alborz province, which contains large cities such as neyshabour, esfaraien and shahrod, are landslides in high regions, settlements in some lowlands, surface faulting. kopet dagh-middle caspian province the dominant structural trend in kopet dagh-middle caspian province (fig. 2) is nw-se. from tectonics point of view, this province contains the kopet dagh hinterland (fig. 1) or kopet dagh folded and thrust belt that has been formed in the passive margin of eurasia plate. focal mechanisms of several earthquakes are dextral strike slip faulting such as garmkhan-e bojnourd (ms=6.4, 1996) and thrusting. dagh-middle caspian province experiences low to moderate earthquakes with high frequency, short repeat time and down to 20km focal depth. sometimes in the northern margin of south caspian basin, focal depths exceed to 70km which is a depiction of its subduction. the intensity of earthquakes is at low levels. the most serious seismic hazards in dagh-middle caspian province, which contains large cities such as mashhad, ghochan and bojnourd, are landslides in high regions, settlements in some lowlands, and surface faulting. south caspian province the dominant structural trend in south caspian province (fig. 2) is nw-se in the eastern part and n-s in the western part. from tectonics point of view, this province contains the south caspian remnant oceanic basin (berberian, 1983) and northern foreland basin of central alborz hinterland (fig. 1). focal mechanisms of several earthquakes are reversed to thrust (jackson et al., 2002) such as astara (ms=6.2, 1980). south caspian province is struck by low to moderate earthquakes with middle frequency, short repeat time and down to 20km focal depth. the intensity of earthquakes is at low levels. the most serious seismic hazards in south caspian province, which contains large cities such as gorgan, sary and rasht, are settlements in some lowlands and tsunami. central alborz province the dominant structural trend in central alborz province (fig. 2) is nesw in the eastern part and nw-se in the western part. from tectonics point of view, this province contains the middle part of great kavirurmieh lake foreland basin and central alborz hinterland (fig. 1) that has been formed in the northern active margin of the cimmerian plate. focal mechanisms of several earthquakes are sinistral strike-slip faulting such as manjil (ms=7.3, 1990) and reversed with a sinistral strike-slip component such as boien zahra (ms=7.2, 1962). central alborz province experiences moderate to high earthquakes with low frequency, long repeat time and down to 20km focal depth. the intensity of earthquakes is at high levels. the most serious seismic hazards in central alborz province, which contains large cities such as tehran, 11seismotectonic-geologic hazards zoning of iran ghazvin and zanjan, are landslides in high regions, settlements in some lowlands, surface faulting and volcanic hazards around damavand cone. azerbaijan province the dominant structural trend in azerbaijan province (fig. 2) is nwse. from tectonics point of view, this province contains the north-western part of great kavirurmieh lake foreland basin and west alborz – lesser caucasus hinterland (fig. 1). focal mechanisms of several earthquakes are dextral strike slip faulting such as salmas (ms=7.3, 1930) and reversed such as varzaghan (ms=6.2, 2012), that the latter case was an index case of sympathetic fault segments in iran. this segment has become active only 9 minutes after an earthquake (ms=6.4) with the strike-slip dominant mechanism in another segment of the same fault. azerbaijan province experiences moderate to high earthquakes with high frequency, medium repeat time and down to 10km focal depth. the intensity of earthquakes is at high levels. the most serious seismic hazards in azerbaijan province, which contains large cities such as tabriz, and ardebil, are landslides in high regions, settlements in some lowlands, surface faulting and volcanic hazards around sahand and sebalan cones. persian gulf province the dominant structural trend in persian gulf province (fig. 2) is nwse. from tectonics point of view, this province contains the foreland basin of zagros hinterland (fig. 1) which has been formed on the north-eastern passive margin of the arabian plate. it extends to mesopotamian terrain in iraq. focal mechanisms of several earthquakes are reversed or thrust. persian gulf province experiences low earthquakes with low frequency, medium repeat time and 10 to 20km focal depth. the intensity of earthquakes is at middle levels in which there are several ductile rocks in sedimentary covers such as hormuz and dashtak formations. the most serious seismic hazards in persian gulf province, which contains large cities such as khorramshahr, abadan, mahshahr and iranian islands, are settlements in some lowlands and coastal areas and tsunami. table 1. estimated seismic hazard parameters for seismotectonic provinces. 12 mehran arian conclusions seismotectonic-geologic hazards zoning map has been prepared to indicate the seismic hazard in iran. characteristics of this map are as follows: the map is intended as a pattern for the assessment of the geologic hazards in iran. some provinces such as zagros, persian gulf, kopet dagh-middle caspian and south caspian have rarely been surface faulted. zagros, persian gulf, kopet dagh-middle caspian, south caspian makran and bandar-e abbas provinces face tsunami hazards. the principal part of deformation in iran has been concentrated in the continental crust of the country. zagros and alborz experience deep earthquakes which is an indication of the existence of thick-skinned tectonics. jazmorian and southern margin of kopet dagh-middle caspian provinces are struck by deep earthquakes which is an indication of slow subduction of oceanic lithosphere. some provinces such as piranshahrborojen, east iran, naien rafsanjan and east alborz, which include main suture zones of iran, have faced high seismic hazards. acknowledgements this work is funded by the department of geology, islamic azad university, science and research branch, tehran, iran. also, special thanks to vice-president for research in science and research branch, tehran and dr.g.razaghian from islamic azad university, damavand branch. 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(2004). present – day crustal deformation and plate kinematics in the middle east constrained by gps measurements in iran and northern oman, geophysical journal international.157, 381-398. keywords: volcanic fault basins; wenxiang granite; la-icp-ms; isotopic dating; late early jurassic. palabras clave: cuencas de fallas volcánicas; granito wenxiang; la-icp-ms; datación isotópica; jurásico temprano tardío. how to cite item tang, j., & lou, f. (2020). la-icp-ms zircon u-pb dating of early jurassic granite basement in chagan area of the northeast guangdong and its geological implications. earth sciences research journal, 24(2), 133-140. doi: https:// doi.org/10.15446/esrj.v24n2.87364 the rencha basin is one of the typical mesozoic-cenozoic volcanic fault basins in northeastern guangdong. the borehole data indicate that the basement is mainly composed of paleozoic metamorphic rocks and wenxiang granite. at present, the basement granite has been lacking in precise years. then, to discuss its formation age and improve the understanding of the geological evolution of the area, la-icp-ms zircon u-pb isotopic dating of wenxiang granite cores from two boreholes in the chagan area of the basin is carried out. the two harmonic ages are 179+1 ma and 186 ma, respectively. the results show that the basement wenxiang granite in rencha basin was formed in late early jurassic, and it is the product of yanshan's first volcanic activity. this age not only deepens the understanding of the formation and evolution of rencha basin but also provides new chronological data for the lack of magmatic activity in the early jurassic in northeastern guangdong, china. abstract la-icp-ms zircon u-pb dating of early jurassic granite basement in chagan area of the northeast guangdong and its geological implications aplicación del método la-icp-ms y datación u-pb en granitos de basamento de principios del jurásico en el área de chagan, en el noreste de guangdong (china) y sus implicaciones geológicas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87364 la cuenca de rencha es una de las cuencas típicas de fallas volcánicas mesozoico-cenozoicas en el noreste de guangdong, china. los datos del pozo indican que el basamento está compuesto principalmente por rocas metamórficas paleozoicas y por granito wenxiang. en la actualidad, no se conocen datos específicos del granito del basamento. para discutir su edad de formación y mejorar la comprensión de la evolución geológica del área se aplicó el método laicp-ms y datación u-pb a dos muestras de granito wenxiang de dos pozos en el área de la cuenca de chagan. las dos edades armónicas son 179+1 ma y 186 ma, respectivamente. los resultados muestran que el granito wenxiang del basamento en la cuenca rencha se formó a finales del jurásico temprano, y que es el producto de la primera actividad volcánica de yanshan. esta edad no solo profundiza la comprensión de la formación y evolución de la cuenca rencha, sino que también proporciona nuevos datos cronológicos para la falta de actividad magmática en el jurásico temprano en el noreste de guangdong. resumen record manuscript received: 13/11/2019 accepted for publication: 03/02/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 133-140 jinhui tang1, 2*, feng lou2 1school of geophysics and measurement-control technology, east china university of technology, nanchang, 330013, china 2party 292, geological bureau for nuclear industry of guangdong, heyuan, 517001, china * corresponding author: amjiangbo@163.com g eo lo g y https://doi.org/10.15446/esrj.v24n2.87364 https://doi.org/10.15446/esrj.v24n2.87364 https://doi.org/10.15446/esrj.v24n2.87364 mailto:amjiangbo@163.com 134 jinhui tang, feng lou introduction the renzi basin is located at the intersection of the eastern end of the east-west nanling polymetallic metallogenic belt and the northwestern wuyi polymetallic metallogenic belt (fig. 1-b). the caprock is mainly composed of the late cretaceous yetang formation volcanic rocks, followed by the paleogene paleocene and eocene sedimentary rocks. the formation is relatively flat, forming an open syncline basin. according to a large number of field geological surveys and borehole control, the basement of the basin is mainly composed of cambrian, devonian-carboniferous metamorphic rocks and wenxiang granite, namely “double base”. the cap layer is not integrated on the basement, forming a pattern of the upper collapsed basin and the red basin of the brazier. at present, many uranium, molybdenum, lead, zinc, gold, silver and rare earth polymetallic deposits and mineralization have been found in renchao fault basin (liu et al., 2007; xu et al., 2010). according to dating data, the cover volcanic rocks in rencha basin were mainly formed at 90 main late cretaceous. the dashenba rock mass (135 ma) of early cretaceous, guikeng granite of indosinian-yanshanian period and eight-foot gneissic granite (165 +1 ma) were widely exposed on both sides of the basin (liu, et al., 2007). so far, no plutonic rocks corresponding to the caprock volcanic series have been found in the basin. in addition, the cover of the basin is thick and the granite is less exposed. at present, the granites exposed by the borehole are mainly composed of fine-grained zeolitic granites, which are in contact with the overlying paleozoic strata. at present, the igneous rocks of these basements have been lacking in precise dating. the formation of the la-icp-ms zircon u-pb age was studied by geological exploration of the bored area of the rencha basin. survey of research areas renchao basin is located in the caledonian fold system of south china, the late paleozoic depression belt of yongan-meizhou, and the northeastern part of the heyuan deep fault zone. it belongs to pingyuan county, meizhou city, guangdong province. it is about 15 km in length from north to south, 10-15 km in width from east to west, and has an area of about 200 km 2m2. the basin is a typical volcanic faulted basin, which consists of a set of rhyolitic and acidic rock-dominated complex formations. the lower part of the strata is breccia tuff and rhyolite, the middle part is pyroclastic rock and lava tuff, and the upper part is pyroclastic rock (song et al., 2010). the basin is nnetrending and belongs to mesozoic-cenozoic continental volcanic faulted basin (k2-e). the basement of the basin is composed of cambrian, devoniancarboniferous epimetamorphic rocks and early yanshanian wenxiang granites, which mainly distribute in the margin and outside of the basin. the caprock is mainly composed of cretaceous volcanic rocks, followed by late cretaceoustertiary volcanic-clastic sedimentary rocks. the basin structure is dominated by faults, mainly nne, followed by ew and nww. the nne on the west side of the basin to the zhegulong fault and the east side nne to the zhumaba basin is the basin control structure, and the near ew to malou fault constitutes the southern boundary of the basin. several nne-ne-trending secondary faults are also developed in the central part of the basin. in addition, the nwwtrending structures developed in the basin are filled with medium-basic dikes. the granite is mainly distributed in the east and west sides and the margin of the basin. the east side is the early dashenba rock mass in the yanshan mountains, and the west side is the indosin yanshan period guikeng rock mass. the lithology is medium-grained porphyritic biotite granite. in the western part of the basin, there are also medium-fine-grained mica granites and fine-grained biotite granites. in the south, there are sub-flow porphyries with narrow strips and other outputs (li et al., 1998). samples and analytical methods in this work, samples of granite cores from zk1702 (760 m) and zk1401 (610 m) drilling holes in chagan area, northern renchao basin are collected and zircons are separated for la-icp-ms zircon u-pb dating. the sample number collected in zk1702 is b-1 (north latitude 24°53’07′′; east longitude 115°57′33′′), and the sample number in zk1401 is b-2 (north latitude 24°53′26′′; east longitude 115°57′ 07′′). the core of the collected sample is preserved intact, and the rock is fresh and not weathered. the rock hand specimen is fine grained granitic granite (fig. 2-a~b). the sample is patchy, the matrix is microscopic, and the image symbiote is between 0.05 mm and 0.15 mm. there is a local mosaic structure (fig. 2-c~d). the size of automorphic quartz phenocrysts and potassium feldspar (diorite) phenocrysts is mostly 1-2mm, and the maximum is 5mm. firstly, the core samples weighing about 1 kg are cut up and crushed, 80-100 meshes are screened, the rock powder washed out. the obtained heavy sand is electromagnetically sorted, the non-magnetic part is separated by heavy liquid, and the heavy part is taken. finally, zircon crystals are manually selected. the zircon crystal is made into an epoxy sample target and polished. zircon transflective and cathodoluminescence (cl) photography is performed at the laboratory of geological mineralization and process center of sun yat-sen university. the target of zircon sample is observed by optical microscopy and scanning electron microscopy cathodoluminescence (cl). zircon crystals with good crystal shape, no cracks and undeveloped inclusions are selected for determination (figure 3). la-icp-ms zircon u-pb dating is completed online by icp-ms and laser denudation system at the state key laboratory of geological processes and mineral resources, china university of geosciences (wuhan). icp-ms is agilent 7500a produced by agilent company in the united states, and laser denudation system is geolaspro produced in the united states. the laser spot size of sample melted by laser is 18μm. icpmsdatacal software is used for data processing (liu et al., 2010; liu et al., 2008; gao et al., 2020). u-pb isotope dating uses standard zircon 91500 as external standard for isotope fractionation and u content correction. recommended values for u-th-pb isotope ratios for zircon standard 91500 according to wiedenbeck et al. (1995). using standard zircon plesovice as a monitoring sample, the tims measured at 206 pb / 238 u was 337.13 ± 0.37 ma (2σ) (slama et al., 2008). sample u-pb harmonic plotting and weighted average calculations are performed using isoplot/ex_ver3 (ludwig, 2003). normal pb correction is performed using the compbcorr #3.18 program (andersen, 2002). thirty zircons are tested in two samples. a group of figure 1. (a) simplified geological map of rencha volcanic basin area (b) tectonic location of the studying area 135la-icp-ms zircon u-pb dating of early jurassic granite basement in chagan area of the northeast guangdong and its geological implications the two samples is not obvious, considering the th/u ratio of zircon, the zircon selected in this test has typical magmatic zircon characteristics, and the u-pb age of the main group zircon can represent the age of magmatic crystallization. a total of 30 data of 30 particles were obtained from sample b-1. the weighted average age of 206pb/238u is186+1ma (mswd=0.084). one of the analysis points (point 9 has a harmonicity of 7%) has a low degree of harmonicity and is eliminated when recalculating the age. the remaining 28 analysis points have an age of 206pb/238u between 184ma and 187ma. the weighted mean age of the remaining 29 analysis points is calculated by isoplot software to be 186. ma (mswd = 0.082), which represents the crystalline age of b-1 fine grained granite (figures 4 and 5). a total of 2830 data are measured for sample b-2 (points 6 and 7 and points 11 and 12 belong to the same zircon). in the analysis point, the age of point 3 is 519 ma, which may be the early magmatic zircon captured in the process of rock formation. the ages of point 9 and 21 deviate from the mode obviously. therefore, the three analysis points do not participate in the calculation of weighted age. the weighted average value of the remaining 27 206pb/238u ages obtained by isoplot software is 179+1ma (mswd=0.18), which represents the crystallization age of fine grained granite from b-2 samples. the above la-icp-ms zircon u-pb dating data show that the mswd values of the basement fine-grained hieroglyphic granites in renchao basin are small and reliable. the two diagenetic ages are 179±1 ma and 186 ma, samples are determined for 60 seconds, in which the first 20 seconds turned off the laser and the last 40 seconds turned on the laser. because apatite contains more common lead, if the content of la or p is higher at the analytic point, it indicates that the laser melting point contains apatite inclusions, these analytic data points should be excluded (liang et al., 2006; harris et al., 2004). u-pb dating of zircon the results of la-icp-ms zircon u-pb isotope dating parameters of fine grained wenxiang granite in rencha basin are shown in tables 1 to 10, respectively. the results of optical microscopy show that the zircon grains in the samples are small, mostly self-shaped or semi-self-shaped columnar, with good crystal shape, light yellow, light rose or colorless. the polymorphism is composed of cylindrical (110) (100) and conical (111) (131). the lengthwise direction of the zircon grains is 80-200μm, and the aspect ratio is between 1 and 3. the th/u ratio of zircon can indicate metamorphism or magmatic origin to some extent. the th/u ratio of metamorphic zircon is usually less than 0.1, and that of magmatic zircon is generally greater than 0.4. th/u ratios at all points measured are greater than 0.4, averaging 0.76 (b-1) and 0.69 (b-2), with obvious magmatic zircon characteristics. although a small part of zircon oscillating zoning or fan-shaped zoning of fine-grained wenxiang granite in (a) (b) (c) (d) figure 2. (a) sample hand specimen and micrograph in chagan area. a, b-fine grain granitic granite hand specimen; c, d-text image granite microphotograph (orthogonal polarized light) 136 jinhui tang, feng lou table 1. la-icp-ms zircon (b-1) uth-pb dating results (01-06) point number 01 02 03 04 05 06 pb ppm 8.6 42.3 21.1 17.1 30.3 22.3 th ppm 119 962 323 312 545 600 u ppm 192 856 525 433 730 523 th/u 0.61 1.12 0.61 0.71 0.74 1.14 207pb / 206pb ratio 0.074 0.0661 0.0526 0.0524 0.0539 0.049 δ 0.004 0.0022 0.0014 0.0018 0.0014 0.0013 207pb/ 235u ratio 0.2905 0.276 0.2122 0.2131 0.2197 0.1996 δ 0.0125 0.0104 0.0055 0.0075 0.006 0.0052 206pb/ 238u ratio 0.0295 0.0294 0.0294 0.0293 0.0294 0.0295 δ 0.0006 0.0003 0.0003 0.0004 0.0003 0.0003 208pb/232th ratio 0.0115 0.009 0.0101 0.0082 0.01 0.0086 δ 0.0004 0.0001 0.0002 0.0002 0.0002 0.0002 207pb /235u age(ma) 258.92 247.5 195.38 196.14 201.67 184.82 δ 9.79 8.28 4.56 6.29 5.03 4.38 206pb/ 238u age(ma) 187.52 186.83 186.74 186.37 186.5 187.29 δ 3.46 2.12 1.92 2.25 1.98 2.17 208pb /232th age (ma) 232 180 203 165 201 174 δ 8 3 4 4 4 5 harmonious degree % 68 72 95 94 92 98 figure 3. cl images of analyzed zircon from graphic granite (b1, b2) basement in chagan area (the figures in the figure represent the point number and zircon age, which correspond to tables 1 and 2) table 2. la-icp-ms zircon (b-1) uth-pb dating results (07-11) point number 07 08 09 10 11 12 pb ppm 9.9 7.7 51.6 48.3 20.9 14 th ppm 162 125 938 897 322 217 u ppm 217 181 848 1096 526 335 th/u 0.74 0.69 1.1 0.81 0.61 0.64 207pb / 206pb ratio 0.0692 0.0639 0.1419 0.0642 0.0582 0.0596 δ 0.0027 0.0025 0.0111 0.0014 0.0016 0.0022 207pb/ 235u ratio 0.2722 0.2518 0.6446 0.263 0.2361 0.241 δ 0.0094 0.0091 0.0655 0.0061 0.0067 0.0089 206pb/ 238u ratio 0.0293 0.0294 0.0291 0.0294 0.0294 0.0293 δ 0.0005 0.0005 0.0009 0.0003 0.0004 0.0004 208pb/232th ratio 0.0109 0.0106 0.0155 0.0103 0.0102 0.0106 δ 0.0003 0.0003 0.0015 0.0002 0.0002 0.0003 207pb /235u age(ma) 244.44 228.04 505.13 237.09 215.19 219.22 δ 7.49 7.38 40.48 4.91 5.53 7.3 206pb/ 238u age(ma) 186.03 186.91 184.98 186.51 186.6 186.36 δ 3.4 3.09 5.5 1.99 2.47 2.55 208pb /232th age (ma) 220 214 310 207 206 213 δ 7 7 30 4 5 6 harmonious degree % 72 80 7 76 85 83 137la-icp-ms zircon u-pb dating of early jurassic granite basement in chagan area of the northeast guangdong and its geological implications table 3. la-icp-ms zircon (b-1) uth-pb dating results (13-18) point number 13 14 15 16 17 18 pb ppm 24.5 15.4 9.1 21.1 28.2 30.5 th ppm 379 212 155 392 493 539 u ppm 601 285 215 471 665 725 th/u 0.63 0.74 0.72 0.83 0.74 0.74 207pb / 206pb ratio 0.0537 0.097 0.0576 0.0553 0.0504 0.0511 δ 0.0012 0.0073 0.0019 0.0016 0.0011 0.0013 207pb/ 235u ratio 0.2158 0.4463 0.2293 0.2248 0.2047 0.2059 δ 0.0048 0.0414 0.0073 0.0068 0.0045 0.0051 206pb/ 238u ratio 0.0293 0.0295 0.0292 0.0293 0.0293 0.0292 δ 0.0004 0.0006 0.0004 0.0004 0.0003 0.0003 208pb/232th ratio 0.0099 0.0141 0.0092 0.0095 0.0089 0.0097 δ 0.0002 0.0009 0.0003 0.0002 0.0002 0.0002 207pb /235u age(ma) 198.44 374.68 209.66 205.85 189.08 190.06 δ 4.01 29.08 6.01 5.59 3.82 4.27 206pb/ 238u age(ma) 185.9 187.33 185.73 185.98 186.12 185.83 δ 2.35 3.48 2.47 2.23 1.71 2.17 208pb /232th age (ma) 198 283 185 191 180 195 δ 4 17 5 5 3 4 harmonious degree % 93 33 87 89 98 97 table 4. la-icp-ms zircon (b-1) uth-pb dating results (19-24) point number 19 20 21 22 23 24 pb ppm 44.7 12.3 25.1 14.8 10.1 39.6 th ppm 731 183 491 267 180 670 u ppm 1020 294 610 328 228 897 th/u 0.71 0.62 0.8 0.81 0.78 0.74 207pb / 206pb ratio 0.0569 0.0605 0.0608 0.0645 0.0528 0.0615 δ 0.0011 0.0022 0.0019 0.0021 0.0019 0.0015 207pb/ 235u ratio 0.2313 0.2449 0.25 0.2588 0.212 0.256 δ 0.0045 0.0087 0.0094 0.0078 0.0074 0.0085 206pb/ 238u ratio 0.0292 0.0293 0.0294 0.0294 0.0293 0.0295 δ 0.0003 0.0004 0.0005 0.0004 0.0004 0.0003 208pb/232th ratio 0.0104 0.0106 0.0098 0.0096 0.01 0.0103 δ 0.0002 0.0003 0.0003 0.0002 0.0003 0.0002 207pb /235u age(ma) 211.3 222.44 226.58 233.72 195.2 231.4 δ 3.67 7.13 7.66 6.28 6.2 6.88 206pb/ 238u age(ma) 185.73 186.21 186.8 186.53 186.13 187.15 δ 1.81 2.31 2.86 2.39 2.32 2.11 208pb /232th age (ma) 210 212 198 194 200 207 δ 4 6 6 5 6 5 harmonious degree % 87 82 80 77 95 78 table 5. la-icp-ms zircon (b-1) uth-pb dating results (25-30) point number 25 26 27 28 29 30 pb ppm 14.2 12.8 40.5 31.1 28.3 21.5 th ppm 231 223 737 355 456 403 u ppm 289 272 913 595 658 492 th/u 0.8 0.82 0.8 0.59 0.69 0.81 207pb / 206pb ratio 0.0829 0.0664 0.0645 0.1131 0.0653 0.0563 δ 0.004 0.0025 0.003 0.0086 0.0024 0.0015 207pb/ 235u ratio 0.3286 0.2637 0.2743 0.5127 0.2764 0.2277 δ 0.0148 0.0084 0.0158 0.0501 0.0129 0.0055 206pb/ 238u ratio 0.029 0.0295 0.0294 0.0292 0.0293 0.0292 δ 0.0004 0.0004 0.0004 0.0007 0.0004 0.0003 208pb/232th ratio 0.0116 0.0104 0.0096 0.018 0.0109 0.0087 δ 0.0003 0.0002 0.0002 0.0014 0.0003 0.0002 207pb /235u age(ma) 288.49 237.68 246.09 420.24 247.76 208.33 δ 11.32 6.73 12.6 33.67 10.25 4.57 206pb/ 238u age(ma) 184.1 187.3 186.49 185.4 186.09 185.8 δ 2.22 2.66 2.21 4.57 2.58 2.05 208pb /232th age (ma) 234 210 193 360 220 174 δ 7 5 4 28 7 4 harmonious degree % 55 76 72 22 71 88 table 6. la-icp-ms zircon (b-2) u-th-pbdating results (01-06) point number 01 02 03 04 05 06 pb ppm 11.8 20.6 24.7 11.3 13.2 8 th ppm 176 335 148 162 132 137 u ppm 285 429 209 290 219 204 th/u 0.62 0.78 0.71 0.56 0.6 0.67 207pb / 206pb ratio 0.073 0.0834 0.0669 0.0632 0.1616 0.0569 δ 0.0039 0.0049 0.002 0.005 0.0073 0.0025 207pb/ 235u ratio 0.281 0.318 0.77 0.236 0.6465 0.2215 δ 0.0134 0.0175 0.031 0.0113 0.0337 0.0096 206pb/ 238u ratio 0.028 0.028 0.0839 0.0283 0.0281 0.0283 δ 0.0004 0.0003 0.0028 0.0005 0.0005 0.0004 208pb/232th ratio 0.0113 0.0138 0.0279 0.0106 0.0205 0.0092 δ 0.0004 0.003 0.0009 0.0004 0.001 0.0003 207pb /235u age(ma) 251 280.35 579.74 215.11 506.32 203.12 δ 11 13.49 17.78 9.26 20.76 7.96 206pb/ 238u age(ma) 178.21 178.24 519.53 180 178.7 179.61 δ 3 2.06 16.43 3.32 3.31 2.49 208pb /232th age (ma) 226 278 556 213 411 186 δ 8 59 18 8 19 6 harmonious degree % 65 55 89 82 4 87 138 jinhui tang, feng lou table 7. la-icp-ms zircon (b-2) u-th-pbdating results (07-12) point number 07 08 09 10 11 12 pb ppm 8.3 8.1 21.9 13.9 13.4 9.3 th ppm 150 129 434 194 239 130 u ppm 217 202 589 326 348 245 th/u 0.69 0.64 0.73 0.59 0.68 0.53 207pb / 206pb ratio 0.0538 0.0635 0.0664 0.0763 0.055 0.0542 δ 0.0025 0.0035 0.0023 0.0038 0.0029 0.0027 207pb/ 235u ratio 0.2045 0.2477 0.2321 0.3083 0.2149 0.2056 δ 0.0089 0.0134 0.0078 0.0181 0.0109 0.009 206pb/ 238u ratio 0.028 0.0285 0.0252 0.0281 0.0284 0.0284 δ 0.0004 0.0005 0.0003 0.0004 0.0004 0.0006 208pb/232th ratio 0.0086 0.0113 0.009 0.0121 0.0088 0.0107 δ 0.0003 0.0005 0.0003 0.0005 0.0003 0.0004 207pb /235u age(ma) 188.92 224.75 211.94 272.87 197.68 189.82 δ 7.51 10.94 6.44 14.07 9.07 7.6 206pb/ 238u age(ma) 177.91 180.91 160.6 178.87 180.56 180.3 δ 2.66 2.99 1.98 2.6 2.63 3.73 208pb /232th age (ma) 174 226 181 243 178 216 δ 5 11 5 10 5 7 harmonious degree % 93 78 7 58 90 94 table 8. la-icp-ms zircon (b-2) u-th-pbdating results (13-18) point number 13 14 15 16 17 18 pb ppm 11.7 19.4 12.8 9.8 8 15.9 th ppm 179 405 188 170 125 262 u ppm 296 454 298 249 210 411 th/u 0.6 0.89 0.63 0.68 0.59 0.63 207pb / 206pb ratio 0.0578 0.0545 0.0793 0.058 0.0585 0.0577 δ 0.0032 0.0023 0.0026 0.0024 0.0028 0.0028 207pb/ 235u ratio 0.2292 0.2146 0.3069 0.2229 0.223 0.2195 δ 0.0124 0.0103 0.0097 0.0087 0.0091 0.0072 206pb/ 238u ratio 0.0284 0.0283 0.0282 0.0282 0.0282 0.0281 δ 0.0004 0.0004 0.0004 0.0004 0.0004 0.0003 208pb/232th ratio 0.0099 0.0095 0.012 0.0097 0.0097 0.0093 δ 0.0004 0.0002 0.0003 0.0003 0.0003 0.0002 207pb /235u age(ma) 209.55 197.39 271.81 204.31 204.37 201.52 δ 10.22 8.61 7.5 7.22 7.52 5.99 206pb/ 238u age(ma) 180.75 179.74 179.17 179.43 179.39 178.88 δ 2.61 2.3 2.39 2.75 2.64 2.1 208pb /232th age (ma) 199 192 242 195 196 187 δ 8 5 6 6 6 5 harmonious degree % 85 90 58 87 86 88 table 9. la-icp-ms zircon (b-2) u-th-pbdating results (19-24) point number 19 20 21 22 23 24 pb ppm 17.7 9.9 18.8 14 18 13.6 th ppm 294 171 700 195 306 260 u ppm 466 252 745 326 439 336 th/u 0.63 0.67 0.93 0.59 0.69 0.77 207pb / 206pb ratio 0.051 0.0552 0.0873 0.0836 0.079 0.0494 δ 0.0015 0.0021 0.0034 0.0044 0.0037 0.0017 207pb/ 235u ratio 0.1991 0.2114 0.1983 0.3455 0.3045 0.1914 δ 0.0057 0.0069 0.0087 0.0229 0.0146 0.0058 206pb/ 238u ratio 0.0282 0.0282 0.0163 0.0282 0.0283 0.0283 δ 0.0003 0.0004 0.0003 0.0005 0.0006 0.0004 208pb/232th ratio 0.0091 0.0092 0.0058 0.014 0.0131 0.009 δ 0.0002 0.0002 0.0002 0.0008 0.0006 0.0002 207pb /235u age(ma) 184.41 194.74 183.69 301.35 269.93 177.87 δ 4.84 5.81 7.36 17.3 11.33 4.92 206pb/ 238u age(ma) 179.43 179.52 104.45 179.46 179.94 179.83 δ 1.81 2.65 1.98 2.85 3.6 2.24 208pb /232th age (ma) 182 186 117 282 263 181 δ 4 5 4 16 12 5 harmonious degree % 97 91 44 49 59 98 table 10. la-icp-ms zircon (b-2) u-th-pbdating results (25-30) point number 25 26 27 28 29 30 pb ppm 14.9 10.7 17.8 23.8 15.4 11.7 th ppm 273 162 324 436 279 214 u ppm 351 249 439 550 386 279 th/u 0.77 0.65 0.73 0.79 0.72 0.76 207pb / 206pb ratio 0.0635 0.1233 0.0516 0.0635 0.0537 0.0574 δ 0.0025 0.065 0.0017 0.0022 0.0017 0.003 207pb/ 235u ratio 0.2468 0.5182 0.1996 0.2543 0.2048 0.2235 δ 0.0094 0.2958 0.0056 0.0104 0.0061 0.0113 206pb/ 238u ratio 0.0281 0.0282 0.0281 0.0283 0.0277 0.0282 δ 0.0004 0.0005 0.0003 0.0004 0.0003 0.0005 208pb/232th ratio 0.0105 0.0099 0.0091 0.0109 0.0093 0.0095 δ 0.0003 0.0003 0.0002 0.0003 0.0002 0.0003 207pb /235u age(ma) 223.95 423.93 184.78 230.07 189.16 204.8 δ 7.65 200.37 4.75 8.42 5.18 9.34 206pb/ 238u age(ma) 178.88 179.56 178.91 179.72 176.12 179.54 δ 2.43 2.93 1.86 2.67 2.17 2.9 208pb /232th age (ma) 212 199 183 220 187 192 δ 6 6 4 7 5 6 harmonious degree % 77 19 96 75 92 86 139la-icp-ms zircon u-pb dating of early jurassic granite basement in chagan area of the northeast guangdong and its geological implications respectively. the ages are late early jurassic. because of the age difference between magmatism and consolidation diagenesis, the basement granite in renchao basin should be the product of early jurassic magmatism. discussion the early yanshanian in southeastern china is generally considered to be a critical period for the transition from the paleo-tethys tectonic domain to the pacific tectonic domain (shu & zhou, 2002; chen et al., 2002). before the middle jurassic was affected by the tethys tectonic domain and the indosinian collision, the late jurassic has been mainly constrained by the combination of pacific tectonic and intracontinental deep structures. however, during the transition period of the two tectonic domains (ie, the early yanshanian), the south china provinces lacke the isotopic age values (200-180 ma) of the early jurassic igneous rocks (j1). therefore, it is generally believed that j1 is the intermittent period of magmatic activity (yu et al., 2009). although the geological work in southeastern china has been relatively high in the past, it mainly focuses on the late mesozoic igneous rocks and cenozoic volcanic rocks. the study on the geological characteristics and tectonic setting of early mesozoic igneous rocks is not in-depth, especially the isotopic age of early jurassic igneous rocks. chen et al. (2002) made rb-sr isochron dating of bimodal volcanic rocks in the xunwu-baimianshi basin and dongkeng-linjiang basin in southern jiangxi province. the values of 178±7ma for basalts in dongkeng basin, 173±6ma for basalts in baimianshi basin and 165±2ma for rhyolite are equivalent to those of late early jurassic to early middle jurassic, which changed the predecessors’opinions on the formation of igneous rocks in these areas in late jurassic. xu (1992)proposed for the first time that the volcanic rocks in fankeng formation in yongding, southwest fujian province were bimodal assemblages of early jurassic (rb-sr age 179 ma), formed in intracontinental extensional environment, which aroused scholars’ attention to the early jurassic igneous rocks in the southeastern continent. zhou and chen (2001) obtained the rb-sr isochron age of the rhyolite in the yongding hulei basin again. li and li (2007) have made shrimp zircon u-pb dating of the keshubei intrusion in southern jiangxi, and obtained an isotopic age of 189±3ma, which coincides with the age of 187±4 ma (zircon u-pb) measured by ding et al. (2005) of the xiangzikou intrusion in the middle part of nanling mountains, hunan province. xing et al. (2002) made rb-sr isochron dating of gabbro in meizhou xialan complex, and obtained 179 ma age. again, shrimp zircon u-pb dating of granite (1196±2ma)) and gabbro (1195±1ma) in the intrusion proves that magmatic activity also existed in the early jurassic in northern guangdong (yu et al., 2009), which is also the earliest granite age of yanshanian period up to now. it is believed that the pacific tectonic domain has begun to act on south china (chen et al., 1999) at least around 195ma. the yanshanian magmatic activity along the southeastern coast of china is a part of the tectonic-magmatic cycle in the western pacific ocean. compared figure 4. sample number b-1 weighted average age figure 5. sample number b-2 weighted average age 140 jinhui tang, feng lou with the unprecedented magnitude and intensity of magmatic activity in the late jurassic (~140 ma), the signs of early jurassic magmatic activity are not obvious. however, the above age data indicate that the pacific plate subducted to the eurasian plate in the early mesozoic (~200ma), which has a precise chronological record in the middle and eastern part of nanling mountains. the u-pb zircon ages of the two groups of fine grained wenxiang granite in the renchao basin are 179±1ma and 186 ma, which provide new information for the lack of reports of magmatic activity during the j1 period in southeastern china. the origin of the u-pb zircon may be related to plate subduction, mantle material upwelling or heating of crustal material and the introduction of some mantle-derived material. although the early yanshanian granites are mostly the products of magmatic emplacement under extensional tectonic environment, the alkaline rock-syenite-granite assemblages formed in the typical extensional tectonic environment of the same period have not been found yet because of the intense volcanic activity in the late yanshanian period of renchao basin, the thick coverage of volcanic caprock and the few outcrops on the surface. in addition, the study area is located at the junction of nanling and wuyi metallogenic structural belts, and its tectonic significance is particularly special (zhao et al., 2008). therefore, the geological work in this area needs to be further strengthened. conclusion the la-icp-ms zircon u-pb dating of the basal-image granite in the poor dry area of the renzi basin yielded the isotopic ages of 179±1ma and 186ma, respectively. it indicates that the formation of the basement lithology granite in the yanshen basin is in the late early jurassic period and is the product of the first magma activity of yanshan. this age provides new chronological data for reports of the lack of magmatic rock activity during the j1 period in northeastern guangdong. the subduction of the pacific plate to the asian plate in the early mesozoic (~200ma) resulted in the warming of the crust, the melting of crustal materials into magma, and the compression of the crust resulted in a series of ne-trending sag basins. the basement hieroglyphic granites in the renchao basin were formed at this stage. during this period, the dading iron ore back rock mass (176±4m a) in lianping and zijinshan rock mass in southwestern fujian were (168±4ma) (wu & zheng, 2004), both of which developed granite basement. although they were formed later, they should be the product of magmatic activity in the same period regionally. acknowledgments: central geological exploration fund project (2007441004) reference andersen, t. 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(2001). late mesozoic crustal slow-acting geochemistry in southeastern fujian. geochemistry, 30(6), 547-558. this paper investigates the potential of data mining techniques to predict daily soil temperatures at 5-100 cm depths for agricultural purposes. climatic and soil temperature data from isfahan province located in central iran with a semi-arid climate was used for the modeling process. a subtractive clustering approach was used to identify the structure of the adaptive neuro-fuzzy inference system (anfis), and the result of the proposed approach was compared with artificial neural networks (anns) and an m5 tree model. result suggests an improved performance using the anfis approach in predicting soil temperatures at various soil depths except at 100 cm. the performance of the anns and m5 tree models were found to be similar. however, the m5 tree model provides a simple linear relation to predicting the soil temperature for the data ranges used in this study. error analyses of the predicted values at various depths show that the estimation error tends to increase with the depth. abstract keywords: soil temperature; data mining; m5 tree model; anfis; ann. estimation of daily soil temperature via data mining techniques in semi-arid climate conditions issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n2.49829 earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): 85 93 m. taghi sattari1*, esmaeel dodangeh2, john abraham3 *1department of water engineering, faculty of agriculture, university of tabriz, tabriz, iran. email: mtsattar@gmail.com (corresponding author), 2department of natural resources, sari university of agriculture and natural resources, sari, iran. email: smaeel.dodangeh@gmail.com 3university of st. thomas, minnesota, school of engineering 2115 summit avenue st. paul, minnesota 55105, usa. jpabraham@stthomas.edu este artículo investiga el potencial de las técnicas de búsqueda y procesamiento de datos para pronosticar las temperaturas diarias del suelo a profundidades que van de los 5 a los 100 cm con propósitos agrícolas. se utilizó la información climática y de temperatura del suelo de la provincia ishafan, ubicada en el centro de irán y de clima semiárido, para el proceso de modelamiento. se usó un enfoque de agrupamiento sustractivo para identificar la estructura del sistema de inferencia neuronal difuso adaptado (anfis, del inglés adaptive neuro-fuzzy inference system) y el resultado del acercamiento propuesto se comparó con redes artificiales neuronales (ann) y el modelo tipo árbol m5. los resultados sugieren un desempeño mejorado al usar el enfoque anfis en la predicción de las temperaturas del suelo en varios puntos de profundidad, excepto en los 100 cm. el desempeño de las redes artificiales neuronales y los modelos de árbol m5 fueron similares. sin embargo, el modelo tipo árbol m5 provee una relación linear simple para predecir los rangos de datos de la temperatura del suelo utilizados en este estudio. los análisis de error de los valores predichos a varias profundidades muestran que la estimación de error tiende a incrementarse con la profundidad. resumen palabras clave: temperatura del suelo; minería de datos; modelo tipo árbol m5; anfis; ann record manuscript received: 26/03/2015 accepted for publication: 31/05/2017 how to cite item: sattari, m. t., dodangeh, e., & abraham, j. (2017). estimation of daily soil temperature via data mining techniques in semi-arid climate conditions. earth sciences research journal, 21(2), 85 93. doi:http://dx.doi.org/10.15446/esrj.v21n2. 49829 estimación de la temperatura diaria del suelo a través de técnicas de búsqueda y procesamiento de datos en condiciones climáticas semiáridas m e t e o r o l o g y 86 m. taghi sattari, esmaeel dodangeh, john abraham 1. introduction soil temperature prediction is important for various agricultural purposes, especially in arid and semi-arid regions, such as iran. temporal patterns of soil temperatures in these regions show large seasonal and daily fluctuations. these variations in soil temperature affect plant growth directly through their effect on physiological activities and indirectly through the effect on soil nutrient availability (tuntiwaranurk et al., 2006). for example, root growth and biological soil activity are directly influenced by soil temperature (kang et al., 2000). soil temperature fluctuations affect various processes within the soil such as microbial decomposition, p and k absorption, soil-moisture content (elshorbagy and parasuraman, 2008) and soil respiration (gaumont-guay et al., 2006). the success of seeding efforts depends greatly on the spatial and temporal distribution of soil temperature. thus, the temperature predictions help to improve the understanding of the dynamics of vegetation (kang et al., 2000). it also helps agronomists and engineers to decide the proper plantation date, design drainage and irrigation systems, and to optimize the application of pesticides and fertilizers to reduce chemical pollution of soils and groundwater. for these reasons, understanding of the variation in soil temperature is vital. however there are very few climatologic stations in arid and semi-arid regions where soil temperatures are recorded at various depths. it is usually quite difficult to measure temperature at depth. to aid in this measurement process, application of data-driven models are used to estimate daily soil temperatures in ungaged homogeneous regions. temperatures in soil are influenced by a number of factors, such as meteorological conditions (i.e. solar radiation and air temperature), site topography, soil water content, and whether the surface is covered by litter and canopies of plants. many conceptual models have been proposed to model soil temperature based upon the meteorological parameters such as surface global radiation and air temperature (kang et al., 2000; shannon et al., 2000; timlin et al., 2002), soil physical parameters, such as water content and texture, topographical variables such as elevation, slope and aspect (kang et al., 2000), and other surface characteristics such as leaf area index (lai) and ground litter stores. other models such as multiple regression and fourier analysis have also been suggested with some modification. most of these models are based on several assumptions and boundary conditions resulting in the limitations of their use in practice. for instance, kang et al. (2000) developed a hybrid soil temperature model based on heat transfer physics and a relationship between air and soil temperature to predict daily spatial patterns of soil temperature in a forested landscape. they incorporated the effects of topography, canopy and ground litter. despite of the availability of different models to predict the soil temperature, these models are found to work well only for specific climatic and agronomic conditions under which they were originally developed. over past decades, remote sensing techniques have also been utilized to measure and predict soil temperature over large areas but a major drawback of this approach is the availability of soil temperature data in the top few centimeters (elshorbagy and parasuraman, 2008). the temperature of the soil profile with increasing depth is difficult to predict so that the abovementioned techniques are limited to shallow soils (tyronese et al., 2008). within the last decade, artificial intelligent (ai) systems such as fuzzy logic (fl) and artificial neural networks (anns) have effectively been used to model nonlinear and non-stationary process (shiri and kisi, 2011). anns are mathematical models consisting of a network of computation nodes called neurons with established connections between them. fuzzy logic is an alternative technique capable of generating models that incorporate expert knowledge and available measurements for a system by using a set of easily comprehensible rules in the form of a fuzzy inference system (fis) (zadeh, 1965). a fis is a nonlinear mapping of a given input vector to an output using fuzzy logic based on a set of membership functions and rules. improved performance can be obtained by integrating fuzzy systems and the ann approach to deal with large and imprecisely defined complex systems. an adaptive neuro-fuzzy inference system (anfis) is one of the most successful schemes which combine the benefits of these two powerful paradigms into a single model. the goal of the anfis is to find a model or mapping that will correctly associate the inputs with the output. data mining refers to the process of searching for and discovering various patterns in data and of summarizing a set of known values to obtain the most important information (quinlan, 1992). tree-based methods are one data mining technique and their output is a model having the structure of a tree with input and output data. the m5 model tree was introduced by quinlan in 1992 and is a subset of data mining methods. the m5 algorithm is the most common classification used in the family of decision-making tree models. a decision tree model is essentially a decision-making tree in which linear regression equations at the leaves replace terminal class values. within the last decade, several studies reported the use of data mining techniques such as the m5 tree model for water resource issues applications (solomantine and dulal, 2003; bhattacharya and solomatine, 2005; stravs and brilly, 2007; pal et al., 2012; sattari et al., 2013a; sattari et al., 2013b; sattari, et al., 2014; esmailzadeh and sattari, 2015; biabani et al., 2016; shortridge et al., 2016; adnan et al., 2017; sayagavi et al., 2016; schnier, 2016). however to the best knowledge of the authors, there has not been an application of the m5 tree model to predict soil temperatures at various soil depths, especially in arid and semi-arid climates. our primary motivation in this study is to simulate soil temperatures at various depths. in this manner, it is possible to predict future soil temperatures by simply acquiring the climatic data from the meteorological stations. this is particularly important when planning future agriculture practices. this study will evaluate the performance of various data mining techniques such as anfis, anns and the m5 tree model for estimating daily soil temperature at different depths in semi arid regions such as iran. 2. materials and methods 2.1 study area and data used located in the central arid region of the country, the isfahan province of iran lies between 30o 42’ to 34o 30’n and 49o 36’ to 55o e (figure 1). the altitude in this area varies from 707 to 4000 m. the significant change in altitude and its effect on climate provide various habitats and diverse plant species. the experimentally obtained daily soil temperature and other meteorological parameters were measured at the weather station of isfahan from 1992 to 2005. the soil temperature data were obtained for soil profiles at various depths (i.e. 5, 10, 20, 30, 50 and 100 cm). meteorological parameters including: daily mean, minimum and maximum air temperature (ave t, min t, max t), evaporation (ev), daily sunshine hours (sunh) and radiation (ra) were also considered as inputs. figure 1. location of the study area 87estimation of daily soil temperature via data mining techniques in semi-arid climate conditions the descriptive statistics min, max, mean, standard deviation, coefficient of skewness (cs), coefficient of kurtosis (ck) and coefficient of variation (cv) of the soil temperature and meteorological data time series, are provided in table 1. table 1. descriptive statistics of the soil temperature data the available data were split into two parts. the first part consists of 3028 samples between june 1992 and october 2004 and was used to train different models. the second part, consisting of 335 samples between october 2004 and december 2005 was used for testing. the coefficient of determination (r2) and root mean square error (rmse) statistics were used to compare the performance of various modeling approaches used in this study. it is often useful to scale the input and output parameters before using them with anns. in the present work, input and output data were scaled to a range from -1 to +1, which is preferable when tan-sigmoid activation functions are used with the neural network. the following normalization equation was used (1) where z is standardized input values lying in the range of [-1, +1], and xmin and xmax are minimum and maximum input values, respectively. 2.2 adaptive neuro-fuzzy inference system (anfis) fuzzy logic represents knowledge using if–then rules in the form of “if x and y then z” (zadeh, 1965). fis mainly consists of fuzzy rules and membership functions and fuzzification and de-fuzzification operations (jang, 1993). figure 2 shows a typical architecture of an anfis. in this figure, circles represents fixed nodes, whereas squares indicate adaptive nodes. the input and output nodes represent the meteorological parameters and soil temperature, respectively. the nodes in the hidden layers act as membership functions (mfs) and rules. for simplicity, it is assumed that the examined fis has two inputs and one output. for a first-order sugeno fuzzy model, a typical rule set with two fuzzy ‘‘if-then’’ rules can be expressed as follows: (2) (3) where x and y are two crisp inputs, and ai and bi are the linguistic labels associated with the node function. the anfis has the multiple layers, as displayed in figure 2. figure 2. architecture of the anfis layer 1: all the nodes in this layer are adaptive nodes which mean that the outputs of the nodes depend on the parameters pertaining to these nodes. each node corresponds to a linguistic label which has a membership function that may be gaussian or any other mf. layer 2: every node in this layer is a fixed node labeled as ii, representing the firing strength of each rule. layer 3: every node in this layer is a fixed node labeled as n, representing the normalized firing strength of each rule. layer 4: every node i in this layer is an adaptive node with a node function defined as: (4) where oi4 is node output, wi is the normalizing firing strength from layer 3 and {pi, qi, ri} are the parameter set of this node. layer 5: the single node in this layer is a fixed node labeled σ which computes the overall output by summing all incoming signals and is the last step of the anfis. the output of the system is calculated as: (5) where o15( node output), is the weighted sum of right hand side polynomials in equation 5. the implementation of anfis consists of two major phases; the structure identification phase and the parameter identification phase. the anfis parameter estimation can be carried out by training the 88 m. taghi sattari, esmaeel dodangeh, john abraham anfis system by using a hybrid learning algorithm. the hybrid learning algorithm of anfis consists of the two parts: (a) the learning of the premise parameters by back-propagation and (b) the learning of the consequence parameters by least-squares estimation. in the forward pass of the hybrid learning algorithm, functional signals move forward to layer 4 to calculate each node output. the premise parameters in layer 2 remain fixed in this pass. the consequent parameters are then identified by the least-squares estimate. in the backward pass, the error rates propagate backward from the output towards the input, and the premise parameters are updated by the gradient descent (shu, et al., 2008). 2.2.1 subtractive clustering subtractive clustering is an automated data-driven approach to generate primary fuzzy models. the subtractive clustering algorithm is used in case the number of clusters is not clear. this algorithm depends on the structure of the data and can be used as a dimensionality reduction tool. this algorithm can also be used to generate a fuzzy system with the minimum number of rules required to distinguish the fuzzy qualities associated with each of the clusters. subtractive clustering is based on a measure of the density of data points in the feature space. the idea here is to find regions in the feature space with high densities of data points. consider a collection of n data points {x1,…,xn}, subtractive clustering algorithm assumes each data point as a potential cluster center. a density measure at a data point xi is then defined as: (6) where di is the density measure and the cluster radius rα is a positive constant (rα > 0) defining the neighborhood radius for each cluster center. thus, a data point that has many neighboring data points will have a high potential of being a cluster center. data points existing outside of this radius have little or no effect on the density measure. the choice of rα is crucial in determining the cluster numbers. large value of rα will generate a limited number of clusters, while small values of rα will generate a large number of clusters. after calculation of the potential of each vector, the one with the highest potential is selected as the first cluster center. suppose xc1 is the point selected and dc1 is its density measure. the density measure for each data point xi is revised by the formula: (7) where rb is a positive constant (rb > 0) that represents the radius of the neighborhood for which considerable potential reduction will happen in density measure. in order to avoid obtaining closely spaced cluster centers, the constant rb is usually 1.5 times that of rα. finally, the clusters’ information is used to determine the initial number of rules and antecedent membership function that is used for identifying the fis (chiu, 1994). 2.3. artificial neural networks (anns) the anns are alternative artificial intelligent (ai) methods and employed in this study to predict soil temperature. a number of network and training algorithms are reported in the literature. the multi-layer perceptron (mlp) is one of the mostly used ann in many research areas. this study uses mlp possessing a three-layer learning network consisting of an input layer, one hidden layer, and one output layer. the input layer accepts values of the input variables and the output layer provides estimations. the hidden layer which lies between the input and output layers contains the processing elements called as neurons. the hidden layer and nodes play very important roles for successful application of the neural network. the nodes in the hidden layer allow neural networks to detect the feature, to capture the pattern in the data, and to perform complicated non-linear mapping between input and output variables. it has been suggested that only one hidden layer is sufficient for anns to approximate any complex nonlinear function within desired degree of accuracy. in the case of the hidden layer, many studies suggest "2m+1" (hecht-nielsen, 1990; lippmann, 1987), "2m" (wong, 1991) and "m" (tang and fishwick, 1993) hidden neurons for better forecasting accuracy, where m is the number of input nodes. in the current study, a large number of trials were carried out to select the optimal number of nodes in the hidden layer. the transfer functions used in this study include the tan-sigmoid in the hidden layer and the linear transfer function in the output layer. the levenbergmarquardt training algorithm was used for the ann models because this technique is more powerful and faster than the gradient descent algorithm. 2.4 m5 model tree model trees generalize the concepts of regression trees and are analogous to piece-wise linear functions. a m5 model tree is a binary decision tree having linear regression function at the terminal nodes, which can predict continuous numerical attributes (quinlan, 1992). the m5 model tree is an algorithm for making numerical predictions, and the selected tree nodes have the attribute of maximum expected error that is a function of the standard deviation in the output parameters. a model tree based regression approach works in two different stages. in the first stage, a splitting criterion is used to create a decision tree. the splitting criterion for the m5 model tree algorithm is based on treating the standard deviation of the class values that reach a node as a measure of the error at that node and calculating the expected reduction in this error as a result of testing each attribute at that node. the formula for computing the standard deviation reduction (sdr) is: (8) (9) and t is a set of samples entering each node. the symbol ti represents a subset of the samples that have the ith result of the potentiality test, sd is the standard deviation, yi is the numerical value of the target attribute of sample i, and n the total number of data points (alberg et al., 2012). the splitting process allows the data in child nodes to have a lower standard deviation compared to their parent node and can be considered as more pure. after examining all the possible splits, m5 chooses the one that maximizes the expected error reduction. the division of training data with a m5 model tree produces a large tree-like structure which may cause overfitting of the data. a pruning algorithm is used to prune back the tree, for example by replacing a sub tree with a leaf in order to remove the problem of overfitting. thus, the second stage in the design of the model tree involves pruning the overgrown tree and replacing the sub trees with linear regression functions. this technique of generating the model tree splits the parameter space into areas (subspaces) and builds in each of them a linear regression model. for further details of m5 model tree, readers are referred to related studies (pal and surinder, 2009; quinlan, 1992). 3. results and discussion analysis of table 1 suggests a noticeable change in coefficient of variation (cv) value of soil temperature from the surface to the depth of 100 cm. the highest value of the coefficient of variation was observed at a 5 cm soil depth with a continuous decline in its value with increasing depth of the soil. other statistical properties such as the maximum and mean soil temperatures are also decreasing with increasing soil depth. 89estimation of daily soil temperature via data mining techniques in semi-arid climate conditions it can be observed that the variability in thermal behavior of the soil profile can be a cause of this change. the higher values of coefficients of variation in the surface layers indicate that higher variability of the soil temperature at this level is possibly due to the variety of causal mechanisms influencing soil temperature. figure 3 shows the variation of soil temperature at different soil depths in the study area. as can be seen from the figure, a wide range of fluctuations exists for the surface layers which decreases with increasing depth. 0 500 1000 1500 2000 2500 3000 3500 0 10 20 30 40 50 time (day) so il te m pe ra tu re (c o ) depth 5cm depth 10cm depth 20cm depth 30cm depth 50cm depth 100cm figure 3. time series of observed soil temperatures at different soil depths (5-100 cm) the three modeling techniques; anfis, ann and m5 tree model were used to predict the soil temperature at varying depths. to obtain optimal network architecture in anns, the number of neurons in the hidden layer was determined by trial and error. based on the number of input neurons (m = 6 input nodes are included here), the number of hidden nodes were varied from 1 to 2m+1 to find optimal number of nodes in hidden layer. the network was trained for 300 epochs using the levenberg-marquardt training algorithm. based on a trial and error process (by exploring 1 to 2m+1 hidden neurons), hidden layers with 3, 4, 5, 4, 5 and 3 neurons were found to be optimal for the 5, 10, 20, 30, 50 and 100 cm soil depths, respectively (see table 2 for the 10 and 50 cm soil depths). table 2. tested model structures and rmse values of the anns models. the parameters of the subtractive clustering algorithm should be specified in anfis models to predict soil temperatures at various depths. the clustering radius (ra) is the most important parameter in the subtractive clustering algorithm and is optimally determined through a trial-and-error procedure. the values of ra ranging between 0.2 and 1 with a step size of 0.01 were examined to minimize the root mean squared error (table 3). for the values below 0.2, network training was found to be difficult and for the values above 1, no remarkable change in rmse values was achieved. clustering radius rb was selected as 1.5ra and default values of the other parameters given in the matlab were used. table 3. resulting model structures and rmse values of the anfis models. *shows optimal clustering radius (ra) gaussian membership functions were used for each fuzzy set in the fuzzy system. the number of membership functions and fuzzy rules required for a particular anfis model were determined through the subtractive clustering algorithm. parameters of the gaussian membership function were optimally determined using the hybrid learning algorithm. each anfis model was trained for 100 epochs. the test results of the anns, anfis and m5 model trees are compared with respect to r2 and rmse in table 4. table 4. the test results of the optimal anns, anfis and m5 model trees. results indicates a high correlation (r2 value changes from 0.98 at 5 cm depth to 0.80 at 100 cm depth) between the actual and predicted soil temperatures suggesting that all three models achieved acceptable results in predicting the soil temperatures at varying depths. time variation and scatterplots of the test results obtained by m5 tree model, ann and anfis models are illustrated in figures 4-6. 90 m. taghi sattari, esmaeel dodangeh, john abraham plot of observed and predicted soil temperature values suggests that all three models are suitable in modeling soil temperatures. figure 5. performance of ann model at soil depths; 5 cm and 100 cm, with test data set 91estimation of daily soil temperature via data mining techniques in semi-arid climate conditions as can be seen from the table 4, the rmse values also increase with the increasing depth. the lowest and highest rmse values achieved by the anfis, ann and m5 tree model are 1.86, 1.95, 1.97 (observed at 5 cm soil depths) and 2.39, 2.44, 2.37 (observed at 100 cm soil depths) respectively. the plots of actual and predicted soil temperature in figures 4, 5 and 6 as well as table 4 suggest a slightly improved performance by anfis in predicting soil temperature in comparison to ann and m5 models except at the depth of 100 cm. a comparison of results from table 4 also suggests that both the ann and anfis perform better than the m5 model tree approach in predicting the surface soil temperature, however at depth of 100 cm, m5 tree model seems to be most robust. figure 6. performance of m5 model tree at soil depths; 5 cm and 100 cm, with test data set these figures depict good agreement between the actual and predicted soil temperature values of the surface layers in comparison to the layers at increasing depths. all three models tend to provide less biased estimates for the surface layers in comparison to the predictions for deeper layers (figure 7). figure 7. rmse values of the evaluated models for different soil depths analysis of predicted soil temperature values by different modeling approaches (figure 7) shows increasing values of rmse with increasing soil depth, thus indicates that the soil temperatures predicted by anfis, ann and m5 models are more accurate for the surface temperatures. the reason for increasing rmse value with increasing depth may be mainly due to the reduction in correlation between the input climatic variables and the soil temperature at increasing depth. the superior performance of the anfis and ann in modeling the surface soil temperatures may be attributed to the increase in the network nonlinearity and the better correlation between input and output values (gao et al., 2007). moreover it may be noted that a trial-anderror procedure has to be adopted to select suitable user-defined parameters for the ann model; this process is time consuming. on the other hand, no such procedure is required to develop an anfis model. although error analysis of predicted values confirmed better performance for anfis and ann approaches for the surface soil temperatures, the m5 model tree had slightly better results with increasing depth. it is worth noting that m5 model trees being analogous to piecewise linear functions, provides a simple linear relation to model the soil temperatures, as described mathematically in figure 8. 92 m. taghi sattari, esmaeel dodangeh, john abraham figure 8. linear functions for predicting temperature at 5 cm soil depth based on m5 tree model. 4. conclusions the anfis, anns and m5 tree model approaches have been used to predict the daily soil temperature with increasing depth in this study. the results presented here are quite encouraging and confirm that all three approaches work well in predicting soil temperatures at different depths. a comparison among the models indicates that the anfis model provides more accurate estimates of soil temperature than the anns and m5 tree model. the results also suggests that both ann and m5 tree model approaches work well in predicting daily soil temperatures, however, m5 tree model has simple linear relations that can be easily used to predict the daily soil temperature data by field engineers. error analysis of temperature predictions at different soil depths indicates that all three models perform well in predicting surface soil temperature data rather than at deeper depths. the reason behind this may be the fact that there is a strong relationship between climatic parameters and surface soil temperature. the results of the present study demonstrate that the proposed anfis model is quite efficient in predicting soil temperature for the surface layers however further investigation is needed with different data sets to compare proposed approach and other mathematical methods such as time series modeling to model soil temperatures in deeper layers. 5. references adnan, r. m., yuan, x., kisi, o., & anam, r. 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(1965). fuzzy sets. information control, 8, 338–353. in this study, an effective and time-saving strategy for groundwater exploration in alluvium basins in arid-semiarid lands was applied by integrating gis and geophysics. three steps were followed to locate productive local groundwater aquifers prone to annual recharge in the region of petra, which covers an area of 800 km². in the first step, geological, structural and geomorphological knowledge was applied to determine the parts of the sedimentary basin having potentials of being local groundwater aquifers. some locality of the selected area with suitable hydrological conditions for recharge was chosen for an optimization process using gis. in the second step, the selected locality of the basin was given emphasize on its detailed geology and hydrology to choose suitable locality for applying gravity visualization. evaluation of this data leads to step number three. now at localities with promising conditions vertical electrical soundings was applied to enable the authors to determine productive and renewable groundwater aquifers. as it is known that geophysical methods are considered as expensive and time-consuming work, therefore, in this strategy of groundwater surveying a special attention is given to budgets of time, efforts and funds constraints. en este estudio se aplicó una estrategia efectiva y eficaz para la exploración de aguas subterráneas en cuencas con depósitos aluviales para terrenos áridos y semiáridos a través de la integración del sistema de información geográfica (gis, inglés) y herramientas geofísicas. se siguieron tres pasos para localizar acuíferos locales de aguas subterráneas propensos a las recargas anuales en la región de petra, que cubre un área de 800 km2. en el primer paso se aplicó el conocimiento teórico para determinar las partes de la cuenca sedimentaria con el potencial de ser un acuífero local de aguas subterráneas. se escogió una de las localidades del área seleccionada con condiciones hidrológicas apropiadas de recarga para un proceso de optimización con el gis. en el segundo paso se hizo énfasis en la geología e hidrología detallada de la localidad seleccionada de la cuenca para así escoger un lugar apropiado para la aplicación del método de visualización gravitacional. la evaluación de los datos condujo el estudio al tercer paso. se realizaron sondeos eléctricos en los lugares con evidencias para permitirle a los autores determinar los depósitos de aguas subterráneas productivos y renovables. si bien los métodos geofísicos son considerados costosos y demandantes de tiempo, con esta estrategia de exploración de aguas subterráneas se hizo un énfasis especial a los presupuestos de tiempo, esfuerzo y limitaciones de fondos. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 39 50 abstract resumen key words:petra, baida basin, gravity method, vertical resistivity method, groundwater, survey strategy palabras clave: petra, cuenca baida, método gravitacional, método de resistividad vertical, aguas subterráneas, estrategias de exploración. using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan mohammad al farajat¹, bernhard schaefers², hussein al hassanat³ nedal al atteyat4, nidal al jahed4, jamal khataibeh4 ¹alhussein bin talal university, email: alfarajat@hotmail.com ²beratende geoingenieure gbr (bgi)/wuerzburg, germany ³petra development and tourism region authority/jordan 4natural resources authority/jordan applied geophysics record manuscript received: 18/01/2015 accepted for publication: 31/03/2015 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.48357 40 mohammad al farajat, bernhard schaefers, hussein al hassanat, nedal al atteyat, nidal al jahed, jamal khataibeh introduction: geophysical exploration for groundwater is an indirect geological investigation method that was developed over the past century. active geophysical methods such as resistivity and seismic methods can investigate the succession of the geological layers in mineralogy and thickness, the fact that resistivity can reveal water-saturated layers. inactive methods like gravity and geomagnetic can map the topography of the bedrock, reveal the thickness of the alluviums above the basement and can assist in delineating the internal structures of the basins. as a result, the majority of the geophysical methods can emphasize on the results of each other. the methods were developed over time according to the equipment, field survey methods and interpretation software, al farajat (2008). despite the advancement of the means of geophysical technology in the exploration of groundwater, still they consume time, efforts and money; thing emphasizes on that the field surveys should be applied in promising areas. gis can be used in determining promising areas when the geology and the geomorphology of an area are taken into consideration. in 2007, petra was ranked the second wonder of the seven world wonders. since then, petra region witnessed an accelerating prosperity associated with land use expansion and rapid population growth. petra has a local population of thirty thousand persons and is visited by around couple thousand international tourists daily. in the summer, tourism boosts cause serious pressure on the drinking water resources. therefore, in 2009 the central government of jordan allocated a local authority for petra “petra development and tourism region authority” to develop the region and search for solutions to problems caused by tourism boost, especially on the water supply impact. the following facts clarify the challenges the water supply faces: • jordan was ranked four on the international water poverty index, salameh (1996). • the country with predominantly arid climate mainly depends on groundwater resources for its water supply, ministry of water and irrigation of jordan (2014). • some of the groundwater basins were depleted because they are nonrenewable (salameh, 2001). • tourism sector forms one of the most important economic engines in jordan. about 50,000 persons are working in this industry. in addition, the country’s dependent on the tourism sector to secure its needs from hard currencies. however, tourism industry consumes high amounts of water and increases the pressure on the water-supply system. for abstraction purposes, environmental sustainability of groundwater in dry lands calls for dealing with rechargeable/renewable aquifers as much as possible, salameh (1996). to secure more drinking water, especially in the summer session, for the petra region, which covers an area of 800 km2, it is essential to locate productive local groundwater aquifers prone for recharge. this study used an effective, sustainable strategy for locating productive local groundwater aquifers in alluvial sedimentary basins. in groundwater hydro-geophysics and in the lands having mediterranean type of climate (dry, hot summer and cold, wet winter) alluvium basins surrounded by highlands form productive aquifers with good groundwater potentials, al farajat and et al (2005). the presence of faults within the beds of the alluviums enhances percolation of rainwater into the depths of the aquifers, al farajat (2008). 1.2 methodology in this study, three logical phases were followed to locate productive local groundwater aquifers in alluvium sedimentary basins that are prone for recharge in the petra region: phase 1: geological, structural and geomorphological knowledge will be used in selecting suitable sedimentary basins with potentials for being local groundwater aquifers. a small number of the selected sedimentary basin with appropriate hydrological conditions for recharge will be chosen after running an optimization process by using gis. phase 2: the selected basin will be emphasized on its detailed geologic and hydrologic aspects. gravity visualization of the chosen part of the sedimentary basin will be implemented. phase 3: parts of the basin with suitable conditions to contain groundwater (concerning to the gravity visualization) will be chosen to locate and apply vertical electrical soundings (ves). 1.3 literatures review near surface geophysics for groundwater exploration is usually limited to depths of less than 250 m under the ground surface. groundwater applications of near-surface geophysics include mapping the depths and thickness of an aquifer, mapping water migration paths such as fractures, joints, cracks, bedding planes and fault zones, and mapping pollution of the groundwater aquifers. geoelectric methods depend on the correlation of the subsurface electrical properties, especially the resistivity (ves) of layers varying extremely with lithology. the vertical resistivity method proved to be a very important method of groundwater prospecting. coker et al (2009) investigated geophysical potentials of groundwater of oke-badan estate, ibadan, and southwestern, nigeria. in sweden, wattanasen & elming (2008) used this method to detect groundwater. gowd (2004) used electrical resistivity surveys to delineate groundwater potential aquifers in peddavanka watershed, anantapur district in india. edet & okereke (2002) and urish & frohlich (1990) used the ves method for the same purpose. babu (1991) used the geomagnetic method to map the topography of the bedrock. carmichael et al (1977) used gravity to explore for groundwater and bedrock topography in glaciated areas. bose et al. (1973) used this method to explore groundwater potentials in india. in jordan, al farajat (2008) modeled the geometry of the southern basin of aqaba in jordan to delineate groundwater rich layers, and to determine the topology and structures of the granite basement and the thickness of the alluviums. in the model he built gravity and resistivity methods were used. in 2009, founding on the scientific findings of this article – a study for the suitability of the area of this basin as a location for the proposed jordanian atomic energy plant took place. al farajat (2006) modeled geometrical properties in the southern basin of aqaba for groundwater potentials, using resistivity method. al farajat and et al. (2005) investigated the potentials of groundwater in the alluvium aquifer of titin basin using gravity and resistivity method. the gravity study led to the conclusion that the basin is an asymmetrical graben and the help of the resistivity method the groundwater rich layers have been found in the deepest limp of the graben with values less than 100 ω m. al farajat et al (2005) investigated the hydrogeophysical and environmental aspects of the groundwater potentials in al sokhna alluvium aquifer in zarqa jordan. in this study, the gravity method was used to delineate depth to bedrock under the alluviums. al farajat et al (2004) delineated the hydrodynamic interface between fresh and seawater in the coasts of gulf of aqaba. in a ph.d. thesis, al farajat (2002) studied the hydro geoecosystems in aqaba coasts and nearby region for topics such as natural settings, impacts of land use, spatial vulnerability to pollution and sustainable management. in the study, al farajat modeled and sliced a groundwater aquifer in this coastal city using direct measurements in groundwater wells besides vertical electrical soundings. al farajat (2002) in an ms.sc. thesis delineated subsurface karstic holes using winner array in the northern part of jordan. 1.4 study area the petra region is located in the southern part of jordan, about 280 kilometers to the south of amman. the study area wadi al baida is located about 8 kilometers to the northeast of petra archeological park (pap) (figure 1a). the following coordinates define the center of the area: easting: 194616, northing: 975746, while its elevation is around 1100 m. the area overlooks the dead sea valley (central part of the jordan valley). the majority of the geologic structures of the area are attributed to the dead sea fault (figure 1b). 41using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan figure 1a: detailed location map of the petra region (in pink color). borders of pdtra are shown in red, borders of baida area in yellow, surface water border lines in light-blue and drainage nets in dark-blue lines. figure 1b: location map of the study area on a structural map of jordan. 2. optimization of the most promising areas for exploration to optimize promising areas for groundwater exploration in the petra region, geological and surface modeling is necessary to extract the most suitable sites from the region that are characterized by good groundwater potentials and recharge abilities. consequently, geophysical exploration will be implemented. in the first step, two models were produced using arcgis (figure 2): 1. the digital geologic map of the region was classified into two classes with areas consisting of bare rocks (all types of hard rock) and areas consisting of alluviums. 2. structural map of the region with the major faults. with these models, it is hoped they will help in locating areas consisting of alluvium basins and surrounded by highlands, and crossed by major faults. in the second step, arcgis with a high-resolution digital elevation model (dem) was used to build a topographic model for the petra region. drainage nets, surface water basins, and topographic slope models were derived by using surface modeling option (fig. 3a, 3b). the selected basin had a relatively flat topography (0-5%) and drainage system and located exactly on the periphery of hisheh highlands (fig. 3a). the basin is located in the lowest topographic zones of the region. hisheh highlands are well known to be swept annually in the winter by snow and heavy rainfall events. 42 mohammad al farajat, bernhard schaefers, hussein al hassanat, nedal al atteyat, nidal al jahed, jamal khataibeh figure 2: the digital geologic map of the region “right” was classified into two classes; areas consisting of bare rocks “all types of hard rock” and areas consisting of alluviums. the map on the left side is the structural map of the region with major faults.       hisheh highlands figure 3a: map of the study area “in yellow line” with drainage system nets shown in blue, surface water basins in light blue and the borders of the region of petra in red, hisheh highlands are overlooking the basin of baida from the eastern part. “scale 1:150.000”. figure 3b: slope model derived from dem of the region of petra, showing the different categories of the percent slope. the slope of the study area is 0-5%. 43using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan 3. detailed geological settings of the selected locality wadi al baida forms from an escarpment trending n-s and consisting of the following lithological and structural features (figure 4): 3.1. stratigraphy • the umm ishrin sandstone formation (in) (middle-late cambrian) the formation forms the oldest rock unit and covers the lowest points in the study area especially along wadi courses. it can be found in the southwestern parts of the study area as massive, steep slopes and cliffs. it consists of redbrown to yellow-brown and mauve-red, medium to coarse-grained massive weathered quartzose sandstone with thin beds of micaceous sub-arkosic sandstone. the color banding was caused by rhythmic deposition of iron and manganese oxide and hydroxide minerals within the sandstone. • disi sandstone formation (di) (upper cambrian-lower ordovician). the formation overlies the umm ishrin formation and can be found in the southwestern part of the study area. its thickness reaches up to 250 m and consists of white, grey, yellowish-grey, medium to coarse-grained sandstone. • the kurnub sandstone group (ks) (aptain to early cenomanian) the group is unconformably overly the disi formation and reaches about 160 m in thickness. • na’ur limestone formation (nl)(cenomanian). this is the first carbonate marine formation. it follows the siliciclastic rocks of about 60-75 m in thickness. • fuhays/ hummar/ shu’ayb(f/h/s) (undifferentiated) (cenomanianlower turonian) the formation is about 80-110 m thick. it consists of grey-green, pink, red calcareous siltstone-mudstone, thin-bedded yellow-buff fossiliferous marl, marly limestone, banded gypsum with green and red mudstone and multicolored sandstone beds. • wadi as sir limestone formation (wsl) (turonian). it is about 120 m thick and consists of massive, buff and grey dolomitic limestone, which at the base is overlain by massive sandy-dolomitic carbonate beds intercalated with marly limestone, silty mudstone and nodules of grey chert. • pleistocene sediments (pl) (pleistocene) they are the youngest sediments in the study area consisting of poorlygood sorted gravel, sand, silt and fine loess materials. it is mainly covering the down slopes and sliding blocks. it covers a large area in the northern part of the study area especially down the escarpment where the erosional process is concentrated in addition to the effect of gravitational force. 3.2. detailed structural settings of the selected locality the region where the study area is located is overlooking the dead sea transform fault system as shown in figure 1a and figure 4 affecting the area and leading to the formation of active faults extended both east-west and south-north. wadi al baida area is cross-cut by a major sinistral strike-slip fault (ras mulayh fault) trending about 35o. this is sub-parallel to the wadi musa fault system that is located very close to the eastern boundary and caused the main disturbance in the study area. a faulted and shifted block in the southern part of the study area is a clear evidence for the tectonic influence of the fault. secondary faults cut the major one, and other sub-parallel faults are found cross-cutting the study area with different trends and throws. a series of ne-sw trending folds of synclines and anticlines are documented in the southern part of the study area especially within the wadi as sir formation. figure 4: geological map of wadi al baida area and its surrounding, the symbols below point to the rock formations. nra (2000). 3.3 detailed hydrological settings of the selected locality: figure 1b illustrates the hydrological settings of the study area. the petra area is located on the western slopes of the north-south sharah mountain range, which extends from showbak to ras al naqab. this part of western highland range forms a continuous elongated recharge mound. the crest of this mound attains its highest elevation between wadi musa and ras al naqab. a part of the recharge flows westwards and gives rise to springs along the escarpment in the showbak wadi musa areas. a large proportion of the recharge discharges eastwards into the jafer basin. since the ras mulayh fault occurs between the kurnub sandstone and the disi sandstone, it acts as a conduit for groundwater movement because both sandstones are aquifers. figure 5 illustrates the monthly rainfall amounts distribution in mm from 1980 to 2007 in the area of wadi musa. 44 mohammad al farajat, bernhard schaefers, hussein al hassanat, nedal al atteyat, nidal al jahed, jamal khataibeh figure 5: monthly rainfall amounts distribution in mm from 1980 to 2007 of the central part of the petra region. 4. geophysical measurements and data 4.1. gravity method the gravity measurement in this study utilizing forty-three gravity stations (figure 6, black dots) for the period of december 2012 to january 2013. the stations were placed at random positions (figure 6) considering the rugged surface. the coordinates and altitudes for the gravity stations were obtained from the survey division, a part of the natural resources authority. the survey incorporated very precise benchmarks using the transit and rod surveying method. the gravity measurements were performed by using a lacoste romberg g880 gravity meter. each day of the data collection began and ends with a measurement taken at the gravity base station called “ma’an 901690”, which belongs to the jordan gravity standardization network (jgsn), established in nra (1990). the location and absolute gravity value of this station are: latitude 30° 12.90 ‘ longitude 35° 43.22 ‘ elevation of 1110.88 m the observed gravity at this base station is 979075.193 mgal. the above information from the base station were needed, because the used gravimeter cannot determine the absolute gravity, but only the difference in their readings at two stations, which is proportional to the difference between the gravity values at the two stations. to convert these differences or relative gravities into absolute gravity values, one station at least is needed, called the gravity base station, where the absolute gravity field was already determined earlier by tying it to the national network. 4.1.2 data processing the gravity values, as well as the complete bouguer anomaly (cbog) for each gravity station, were computed utilizing raw gravimeter readings on a portable computer. oasis montaj software from geosoft was used for data reductions; the program computes all the necessary corrections to the ideal earth model gravity. in addition, it makes the conversion from gravimeter readings to observed gravity and applies the earth tide and instrument drift corrections. moreover, the program calculates the gravity effect of a homogeneous layered spinning earth, its elevation correction or “free air effect”, the bouguer slab effect and its curvature correction, and the total terrain correction to generate the gravity effect of the ideal earth model. the bouguer density used for the data reduction was 2.67 g/cm3. however, the program also computed the gravity anomaly associated with any regional densities. the program then converts each gravimeter reading into gravity value and computes the earth tide and instrument drift corrections from the base station data applying them to produce the true observed gravity value for each station. finally, the program subtracted the earth model gravity from the observed gravity to give the complete bouguer gravity anomaly (cbog) for each station. figure 6: computed gravity anomaly map of the study area, palestine coordinate system. the white line shows the cross-section in figure 8. 4.1.3 analysis results the complete bouguer gravity anomaly map (figure 6) was generated from the data generated from the data processed in section 4.1.2. for contouring, the oasis montaj software was used. the contour interval was drawn with 1mgal with a grid cell size of 200 meters. the calculated minimum value was -70.65 mgal, the maximum value was -56.74 mgal, the mean value was -62.00 mgal and the standard deviation was 3.74 mgal. the calculated depth to the basement by werner deconvolution is shown in the figure 7. a 2d gravity model (figure 8) was constructed across the study area (figure 6, white line). 45using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan figure 7: depth to the basement from ground surface obtained from gravity modeling. figure 8: 2d gravity model across the study area showing the thickness of the alluviums, the topography of the basement and the faulting in its surface. the results have been correlated in order to determine the depth of the basement; results were correlated in order to determine the depth of the basement, to develop a possible geological scenario and to visualize a threedimensional view for its site (figure 9). figure 9: three-dimensional view of the topography of the basement rock surface. 4.2. vertical electrical sounding (ves) method 4.2.1 data acquisition in the selected site of the basin the use of gravity method enables us to visualize the basin in 3-d. with this, it is possible to choose areas having promising groundwater potentials. in those areas, the resistivity survey was applied. four schlumberger electrical resistivity depth soundings were performed in the study area utilizing syscal r2 resistivity meter (iris instruments. ltd.). the maximum electrode spacing was 2000 m. the electrodes were configurated in a north-south direction, which is the most suitable direction to have the electrical current passing -as much as possiblethrough the same geology. the selected ves sites are shown in figure 10; with coordinates that resemble the palestine coordinate system of these sites are shown in table 1. figure 10: vertical electric soundings (ves) location map. 46 mohammad al farajat, bernhard schaefers, hussein al hassanat, nedal al atteyat, nidal al jahed, jamal khataibeh table 1: the coordinates of the ves sites (palestine coordinate system). during the fieldwork, data quality was controlled by exchanging the current and potential electrodes and simultaneously checking the results obtained on site by plotting the values on a logarithmic scale to ensure smooth operations. field measurements of apparent resistivity values are shown in table 2. table 2: apparent resistivity from field measurements. interpretation of the geoelectric results was executed by automatic techniques. the sounding curves were interpreted by using computer resistivity programs, ix1dv3, and ipi2win; the interpretation proceduresuse forward modeling. a hypothetical layered earth model is generated to compute theoretical response with the observed or measured field response. then a model refinement is made by using automated inversion. the electrical resistivity distribution of the subsurface can be obtained from an inversion of the full dataset (roy & apparao, 1971; koefoed, 1979). 5. interpretation and discussion by the aid of gis, it was possible to determine a suitable area to apply the geophysical survey. baida basin was found to be one of the flattest and lowest areas in the petra region with a topographic location that can drain the surrounding highlands; thing which is typical for indirect types of groundwater recharge, and a slope less than 5%, which makes the basin an optimized one for direct groundwater recharge. hydrologically, it is surrounded with side wadis that drain highland. from the geological point of view, it consists of alluviums (pleistocene sediments) with poor good sorting where elongated active faults are cutting the basin. the faults in the basin and due to their direction (parallel to the dead sea transform fault system) and due to their clear extension on the space photos (despite the continuous erosion processes) seem to be active faults, thing makes rainwater to percolates easier into the depths of the basin making additional direct recharge. in the winter, the basin receives both direct and indirect types of recharge into those layers able to store water; indirect type of recharge is coming from the side wadis surrounding the basin especially in the eastern parts. the highlands of hisheh receive annual precipitation more than 300 mm. snow storage also plays its role in these highlands when the snow is melting a flowing down in stream flow through the mentioned wadis. surface runoff along the wadis with the presence of some active faults (due to the impact of the dead sea transform fault that is close to the basin) can easily percolate and reach the groundwater-bearing layers. the basin itself receives annual rainfall amounts of about 150 mm giving a good chance for annual direct recharge, especially with the relatively accepted amounts of actual evaporation (around 80%) and proper ratios of infiltration. the gravity anomaly map (figure 6) shows high negative values of bouguer anomaly at the west and low negative values at the east of the map. this anomaly reflects the gradient of the basement rocks, which are characterized by densities higher than the densities of the overlying sedimentary rocks. values that are more negative indicate the deeper are the basement rocks. the gravity, modeling the bulk density of the alluvium was given 2.4 g/cm3, and the basement density was given 2.67 g/cm3. the depth of the basement surface was found to range between 300 m in the west and southwest of the study area to 1300 m under the sedimentary cover at the far east of it. with the aid of the gravity-2d and 3d-models it was possible to visualize the basin of baida. the fault systems (extending north-south) were found to show vertical continuity through the sediments into the basement (also emphasized on the surface geology mapping of the area). those faults should enhance and improve recharge processes of groundwater. figure 7 shows that the basement surface is tending in general from the west to east, where the sedimentary cover is thicker. these sedimentary rocks are forming high mountains. the depth of the surface basement ranges between 300 m in the western area of the map to 1300 m under the sedimentary cover in the eastern part of the study area. in order to test this aspect of the data analysis results, the 2d gravity model (fig. 8) was constructed across the study area (figure 6, white line). this model shows an abrupt change in thickness in the middle and the east of the profile. this can be interpreted as a result of series of faults with an orientation in north-southward direction. these faults are according to the faults shown in the geological (figure 4) and tectonic map (figure 2, left side). the model also shows the thickness of the sedimentary rocks; it ranges from 300 m in the west to 1000 m in the east over the basement rocks. the coincidence with werner technique result map (figure 7) is good. in both figures, the general inclination of the basement from west to east is visible as well as the thickening of the sedimentary coverage. the findings of the gravity also are pointing to that baida basin is a structural basin and more likely to be a graben structure. the inferring elongated and vertically extended faults in the gravity modeling extending parallel to each other are emphasizing on that. the gravity models allowed the location of the ves points in promising areas with good potentials to contain groundwater. in ves 1, the first resistivity horizon from surface to about 1.5 m depth with resistivity values from 112 to 193 ωm (layers 1 and 2 in table 3) indicates surface materials like sand, clay, gravel and boulders of sandstone. the underlying horizon (ranges from 1.5 m to about 15 m in depth, layers 3 and 4 in table 3) consists of wet sand, gravel, limestone and clay with a low resistivity (47-76 ωm). the high resistivity values of the third horizon (ranges 47using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan from 17-137 m in depth, layers 5, and 6 in table 3) can be interpreted as hard, dry and fractured sandstone. at a depth of about 137.5 m the low resistivity value (35 ωm) indicates saturated sandstone (figure 11, layer 7 in table 3). figure 11: measured and calculated data (left) and layered model of ves1 (right). table 3: results of ves1. in ves 2, the first resistivity horizon from surface to 55 m depth (layer 1 to 4 in table 4) had moderate to high resistivity values indicating surface materials consisting of sand, gravel, boulders of sandstone, limestone and clay. the second resistivity horizon (layer 5 in table 4) is characterized by low resistivity value (24.3 ωm) indicating wet or saturated sandstone supposed to be umm ishrin sandstone. the third resistivity horizon (layer 6 in table 4) starts at a depth of about 121.5 m and reaches 194 m with high resistivity values. these data allowed the interpretation of this layer as saturated sandstone (figure 12, table 4). figure 12: measured and calculated data (left) and layered model of ves2 (right). table 4: results of ves2. in ves 3, the different resistivity values ranging from 32 ωm to 147 ωm from the surface to about 6 m depth (layer 1 to 3 in table 5) are interpreted as surface materials. the underlying layer with a depth of nearly 36 m (layer 4 in table 5) with a resistivity value of 447 ωm is supposed to be the top of the sandstone. the lowest resistivity value (27.8 ωm) appears at 36 to 150 m depth (layer 5 in table 5). this result of measurement indicates high iron-oxide concentration known from umm ishrin sandstone. saturated sandstone was found at depths starting at 150 m with a depth of 45 m having a resistivity value of 150 ωm (figure 13, layer 6 in table 5). figure 13: measured and calculated data (left) and layered model of ves3 (right). table 5: results of ves3. in ves 4, the surface layers appear down to a depth of 6.7 m consisting of sand, clay, gravel and boulders of sandstone (layers 1 to 3 in table 6). the resistivity values are ranging from 397 to 28.3 ωm. the top sandstone layer starts at a depth of 6.7 m to 41.5 m with a resistivity value of 192 ωm (layer 4 in table 6). the lowest resistivity value (25.7 ωm) appears at 41.5 to 158 m depth (layer 5 in table 6) according to iron-oxide-rich umm ishrin sandstone. water-saturated sandstone starts at the depth of 158 m with a resistivity value of 89.5 ωm (figure 14, layer 6 in table 6). 48 mohammad al farajat, bernhard schaefers, hussein al hassanat, nedal al atteyat, nidal al jahed, jamal khataibeh figure 14: measured and calculated data (left) and layered model of ves4 (right). table 6: results of ves4. 6. conclusions baida basin was found to be a structural basin, it was proved by this study to be most likely a graben structure. the inferred elongated and vertically extended faults extending parallel to each other from its eastern and western parts are emphasizing on that. the basin is composed of successive layers of alluvium fillings with thicknesses ranging between 300 meters in the west to 1300 meters in the east. the sediments have been transported through wadis eastern wards and westwards of the basin. those alluviums cover beds of sandstone from ordovician age. a good correlation was found between the gravity derived cross section and the geoelectric. both methods showed very similar depths of the basement..with the gravity method, it was possible to visualize the basin in 3-d. this helps to find promising areas to have groundwater potentials. due to the lack of direct groundwater data about the area and depending on the previous data and results, the proposed location for an exploration well should be located at the measurement points ves1 or ves2 site. the proposed drilling depth should be about 190 m. figure 15 illustrates the steps to be followed in groundwater exploration using ves method if there are available gravity readings of the basins and if there is a good correlation between the gravity and geoelectric data. as results of this study, the following conclusions can be made: 1. promising areas in this context mean low topographic and flat geological basins, filled with alluviums, surrounded by side wadis that draining their floods into the basin from the surrounding highlands. 2. in arid and semi-arid areas, the described strategy can be followed in looking for promising areas that contain rechargeable groundwater aquifers of alluvium sedimentary types, especially for alluvium-filled sedimentary basins. therefore, it is possible to neglect the majority of the unwanted areas. this contributes to saving time, money and efforts in the exploration for groundwater. 3. the modern software with the presence of digital spatial models makes easier to reach the promising areas for groundwater exploration using geophysics. where the surface modeling using gis with the aid of dems is of a paramount importance in identifying areas prone for groundwater recharge, while regional geology and structures enrich the hydrogeological conceptuality of the studied basin to neglect unpromising sites. 4. conditions are typical when the basin contains faults. active types of faults can keep the connection with depths of the basin, which means rapid groundwater recharge and percolation. 5. details of geology and structures of the site can emphasize on resulted geophysical visualization models. 6. using gravity in visualizing sedimentary basins is an excellent step locating ves sites, to enable application of resistivity investigations in promising sites with good water potentials. where the gravity results can be directly modeled to reveal the basin’s geometry, bedrock topography, and subsurface structures. also, gravity method is of a great importance in understanding the internal behaviors of the faults that can be inferred from the surface. 7. a good correlation was found between the gravity derived cross section and the geoelectric results. this is obvious from the obtained depths to the basement of the two methods. consequently, besides its importance in studying stratigraphic sequences and delineating water-rich strata, results of the vertical electrical sounding method can emphasize on gravity models. 2-d gravity modeling on the depths of the basin 3-d gravity modeling on the depths of the basin choosing suitable locations of ves in promising sites of the basin building 2-d cross section of the geology and depths of the basin from gravity results delineating depth to groundwaterim pl em nt in g ve rti ca l e le ct ric al s ou nd in gs in th e pr om is in g si te s of th e ba si n 49using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan 2-d gravity modeling on the depths of the basin 3-d gravity modeling on the depths of the basin choosing suitable locations of ves in promising sites of the basin building 2-d cross section of the geology and depths of the basin from gravity results delineating depth to groundwaterim pl em nt in g ve rti ca l e le ct ric al s ou nd in gs in th e pr om is in g si te s of th e ba si n figure 15: illustration of the strategy to be followed in groundwater exploration using ves method, for fast and efficient results. acknowledgment alhussein bin talal university is highly recognized for making the software and hardware of the study available. pdtra is highly acknowledged for their continuous support and choosing scientific methods in decisionmaking. nra is also highly acknowledged for the continuous follow up with pdtra during the phases of field survey in both gravity and resistivity methods, and in sharing data processing and interpretation with directorate of studies and planning in pdtra. eng. adel hasant (m.sc. holder in software development) is highly acknowledged for his technical assistance. references apparao a., roy a. 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(2001). water shortages and environmental degradation. in s. m. j. baban, & n. a. alansari (eds.), living with water scarcity: water resources in the jordan badia region, the way forward pp. 71–87. jordan: al al-bayt university. smith d., (1986). application of the pole-dipole resistivity technique to the detection of solution cavities beneath highways: geophysics, 51, 833-837. spiegel r., sturdivant v., and owen t., (1980). modeling resistivity anomalies from localized voids under irregular terrain: geophysics, 45, 1164-1183 telford w.m., geldart, l.p., sheriff, r.e., and keys, d.a., (1978). applied geophysics: cambridge university press, 1976. urish d.w., and r.k. frohlich (1990). surface electrical resistivity in coastal groundwater exploration, geoexploration 26, 4, 267-289, doi: 10.1016/ 0016-7142(90)90008-g. wattanasen k., and s., -a., elming (2008). direct and indirect methods for groundwater investigations: a case-study of mrs and ves in the southern part of sweden, j. appl. geophys. 66, 3-4, 104-117, doi: 10.1016/j.jappgeo.2008. 04. 005. william jess ellett (1994). geologic controls on the occurrence and movement of water in the lower cihnega creek basin, department of hydrology and water resources,. 31 the manganese (mn) deposits of iran date from late proterozoic to pliocene but the major high-grade mn ore deposits are associated with tertiary basic to intermediate volcanic rocks. the venarch mn deposit is located in the sw qom city and considered as one of the largest in iran. the mn mineralization is mostly hosted by eocene high-k volcanic and pyroclastic rocks of urmia-dokhtar magmatic arc (udma). the study area mainly consists of basalt, trachybasalt, andesite, and basaltic-andesite that are intruded by eocene-oligocene monzodiorite, monzonite, and granodiorite intrusions. the venarch mn ore bodies are mostly hosted by shale beds and wrapped by altered mega-porphyritic andesites and spilitic lavas both in the foot and hanging walls. petrographic studies show that the principal ore minerals are pyrolusite, psilomelane, and other mnoxides, and quartz and calcite were determined as the gang minerals. integration of field geology, petrography, and geochemical results suggest a hydrothermal origin for the venarch mn deposit. our geochemical data furthermore suggest the volcanic host rocks to be comparable to high-k series of subduction zones. los yacimientos de manganeso (mn) en irán datan del proterozoico tardío hasta el plioceno, pero el mayor grado de los depósitos de la mena de manganeso se asocian a rocas volcánicas básicas a intermedias de edad terciaria. el yacimiento de manganeso en la mina de venarch se localiza al suroeste de la ciudad de qom y está considerado como uno de los más grandes de irán. la mineralización de manganeso se presenta en su mayoría en las rocas piroclásticas y volcánicas de alto contenido de k del eoceno en el arco magmático urmia-dokhtar (udma, en inglés). el área de estudio presenta basaltos, traquibasaltos, andesitas y andesitas basálticas que están intruidas por monzodioritas, monzonitas y granodioritas del eoceno-oligoceno. los cuerpos de mineral de manganeso en venarch en su mayoría están dentro de capas de pizarra y envueltos por lavas espiliticas y andesitas megaporfiroides tanto en las paredes del piso como en las colgantes. los estudios petrográficos muestran que los minerales son pirolusitas, silomelanas y otros óxidos de manganeso, mientras que los cuarzos y las calcitas constituyen los minerales de ganga. la integración de los trabajos de geología de campo, petrográficos y geoquímicos sugieren un origen hidrotérmal del yacimiento de manganeso de venarch. los datos geoquímicos de este estudio también sugieren que la roca volcánica anfitriona es comparable con las series ricas en k de las zonas de subducción. abstract resumen keywords: geochemistry, mn mineralization, petrogenesis, venarch, central iran. palabras clave: geoquímica, mineralización de manganeso, petrogénesis, venarch, irán central. record manuscript received: 26/02/2015 accepted for publication: 20/06/2015 how to cite item mahdavi, m, dabiri, r. and hosseini e.s. (2015). magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the venarch manganese mineralization, sw qom, central iran. earth sciences research journal, 19(2) 141 145 http://dx.doi.org/10.15446/esrj.v19n2.49372 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.49372 evolución magmática y características de composición en rocas volcánicas terciarias asociadas con la mineralización de manganeso en la mina de venarch, al suroeste de qom, en la región central de irán earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 141 145 o r e d e po si t s magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the venarch manganese mineralization, sw qom, central iran mansoureh mahdavi1, rahim dabiri2*, elham shah hosseini3 1department of geology, science and research branch, islamic azad university, tehran, iran 2 department of geology, mashhad branch, islamic azad university, mashhad, iran 3department of geology, north tehran branch, islamic azad university, tehran, iran. *corresponding author: rahim dabiri (rahimdabiri@yahoo.com) 142 mansoureh mahdavi, rahim dabiri and elham shah hosseini introduction numerous manganese deposits have been found in different parts of iran date from late proterozoic to pliocene (förster, 1978; laznicka, 1992; roy, 2012; samani, 1988). these deposits are widespread in the country but mostly occurred in the central and northern part of iran (shahabpour, 2007) (fig. 1). the major manganese deposits of iran are tertiary in age and can be classified into three broad genetic types: sedimentary, volcano-sedimentary, and hydrothermal. the mn mineralization is mainly occurred in the eocene volcanic rocks, particularly in the andesitic-dacitic lavas. however, mn mineralization is also reported in some of post-eocene granitic intrusions (förster, 1978). the formation of manganiferous iron ores in the central iran are thought to be associated with the pan-african metallogeny, and volcano-sedimentary deposits of posht badam area are proposed to be formed in a rift zone (amidi et al., 1984; foerster and jafarzadeh, 1994; moore and modabberi, 2003). however subduction-related mn deposits are considered as the most important manganese deposits in iran such as venarch, robat karim, chah basheh, bozni (ghorbani, 2013). the venarch manganese mine production is about 85000-100000 tons per year and is considered as the major producer of manganese in iran (tahmasbipoor and fazeli, 2010). the mn-rich layers and bands of the venarch deposit contains 21.5-28.5% mn. this deposit is genetically classified as one of the hydrothermal manganese deposits in a volcano-sedimentary environment (makvandi et al., 2010). in this paper, we study the magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the venarch manganese mineralization. fig. 1 a) location of the study area on the structural subdivision map of iran (after shah hosseini et al., 2014), b) geologic map of the study area and venarch manganese mine position (galamghash and emami, 1996). analytical methods a suite of 60 samples was collected from the mineralized zones and associated volcanic rocks representing different lithologies in the area. among them, 24 samples from the least altered or deformed rocks were selected for petrographic studies, and 20 polished thin sections prepared for ore microscopy. after petrographic studies, a suite of 17 samples from the volcanic host rocks was analyzed for major and trace elements using x-ray fluorescence (xrf) and inductively coupled emission spectrometry (icpes), respectively (table 1). the xrf and icp-es analyses were performed at the zarazma laboratory ltd. in tehran (iran). a suite of 17 samples was analyzed for the mineralogical purposes using xrd machine at the kansaran binalood in pardis science and technology park (iran). table 1: major (wt.%), trace, and rare earth element (ppm) compositions of some volcanic rocks from the venarch area. geological setting the study area is located in the middle of urmia-dokhtar magmatic arc (udma), and located about 27 km in the southwestern qom city, and situated within longitudes 50º, 47', 57'' to 50º, 47', 10'' and latitudes 34º, 23', 54'' to 34º, 27', 35''. this area mainly consists of eocene basaltic-andesite to andesitic volcanic rocks with tuff and other pyroclastic rocks with minor intercalated sedimentary lenses such as shale, gypsiferous marl, and shallow depth limestone (aftabi and atapour, 2000). tectonic movements of upper eocene caused the deposition of lower red formation (lrf) which mainly consists of red beds of sandstone and siltstone intercalating the volcanic rocks (reuter et al., 2009). the lrf itself overlies by marine oligo-miocene qom formation. the entire volcano-sedimentary strata are intruded by postlower miocene granitoides and syenites intrusions (amidi et al., 1984). the volcanic and volcano-sedimentary units in which mn mineralization occurred mostly consist of mega-porphyritic andesite, spilitic lavas, tuffite, and tuff. in the venarch deposit, the mn mineralization forms a few centimeters to 1.5-meter lenses and alternating bands within shale, tuffite, and tuff layers. the mega-porphyritic andesites with large and white plagioclases set in a green to gray groundmass are also present in the venarch area. 143magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the venarch manganese mineralization, sw qom, central iran fig. 2 a) magnetite (mag) and hematite (hem) (hematite is white and brighter compared to the magnetite minerals. b) oscillatory zoning in plagioclase (pl) with alteration in andesite (xpl). c) iddingsite alteration of olivine (ol) and formation of secondary minerals and opaque minerals (ppl). d) skeletal magnetite in trachyandesite (xpl). geochemistry of host rocks in the zr/tio2×0.0001 versus nb/y diagram (winchester and floyd, 1977), which uses only relatively for immobile elements, the studied volcanic rocks plot in the fields of basalt, andesite, trachyandesite (fig. 3a). the na2o + k2o versus sio2 diagram (middlemost, 1994) shows the plutonic samples ploted in the monzodiorite to quartz monzonite (figure 3b). by using the irvine and baragar (1971) boundary line, it is clear that the samples have a subalkaline affinity (figure 3b). on the ta versus yb discrimination diagrams of pearce et al. (1984), the rocks define a volcanic arc to syn-collision tectonic setting (fig. 3c). discussion alteration the venarch manganese mine is associated with late eocene basic to intermediate volcanic rocks (tahmasbipoor and fazeli, 2010). the basaltic and andesitic volcanic lavas and pyroclastic rocks are deposited in the sedimentary basin in which magmas are enriched in iron and to some extent manganese. due to mafic magmatism and subsequent heat exchange, high-temperature hydrothermal fluids pass through the volcano-sedimentary rocks in the sedimentary basin, and elements such as mn, fe, ca, and si are therefore leached from the mafic rocks into the water. these elements are then precipitated as femn oxides and hydroxides (such as goethite, hematite, magnetite, and manganite), and carbonates during the magmatic lull. iron-manganese precipitation in the shallow depositional environment in the venarch area has formed the bands of manganese, hematite, silica, and calcium carbonates. the mn mineralization in venarch deposit has occurred as bands, laminates, and manganese layers concurrent with the other volcano-sedimentary rocks. the manganese mineralization in the area is easily distinguished by their black color in the field. however, the secondary mobilization of the fe-mn oxides, silica, and carbonates is also evident in the area, which might happen during the final stages of mn mineralization. during the later stages of ore formation, mn filled veins and veinlets of fractures and joints of the host rocks. the mineralized horizons (laminate and bands) are locally boudinaged and cut by the veins and veinlets, indicating that the role of tectonic activities and fault rupturing during the late stage of mn mineralization in the area. however, emplacement of eocene-oligocene intrusions have also affected the ore bodies in the venarch mine deposit and produced different alteration minerals such as chlorite, epidote, serecite, and hematite. mn mineralization the venarch mn mineralization consists of two different mineralized horizons called a and b. the mn mineralization in the horizon a of the venarch deposit is mostly formed as beds of hematitic shale in which there is no observed anomaly for the trace elements. this zone is si-enriched, but recognized at he low-grade horizon, perhaps as a result of its far distance from the both mega-porphyritic andesite (as a source for mn), and also far from the granitoides as a heat engine for hydrothermal fluids circulation. the petrography of host rocks the venarch manganese deposit is accommodated by volcanic and plutonic rocks. the volcanic strata mainly consist of basalt, trachybasalt, andesite, and basaltic andesite that are intruded by monzonite, monzodiorite, granodiorite, and syenite. the plutonic rocks are mostly granular in texture; however other textures such as graphic and poikilitic textures are present. the dominant minerals are plagioclase phenocrysts, quartz, k-feldspar, and hornblende and accessory minerals such as biotite, pyroxene, apatite, sphene, and opaque minerals can be seen in some of these rocks. the venarch volcanic rocks are mostly porphyritic (the main texture in the samples), but other textures such as glomeroporphyritic, microlitic, and amygdaloidal textures are also present (fig. 2a). volcanic rocks consist of plagioclase, olivine, augitic pyroxene, and amphibole (fig. 2). figure 3a) fig. 3. the plot of nb/y versus zr/tio2 (after winchester and floyd (1977)) showing the classification of the venarch volcanic assemblage. b) total alkalis vs. silica (tas) diagram after middlemost (1994) for classification of intrusive rocks. c) tectonic discrimination diagrams of pearce et al. (1984) fields for syn-colg (syncollisional), vag (volcanic arc), wpg (within-plate) and org (ocean-ridge) granites are indicated. 144 mansoureh mahdavi, rahim dabiri and elham shah hosseini ore geochemistry we used different variable diagrams (e.g. fe-mn and fe-al-mn) to define the ore type and to understand the geodynamic setting of the sedimentary environment in which the venarch deposit has occurred. the geochemical data show that the venarch deposit samples plot in the field of freshwater and shallow marine continental margins (fig. 4). moreover, in the fe-al-mn diagram the samples are manganese rich and hydrothermal (fig. 5). fig. 4 fe versus mn diagram for discrimination of sedimentary basin (pack et al., 2000). fig. 6 co/zn versus co+ni+cu diagram of toth (1980). the venarch samples plot in the field of deposits with the hydrothermal origin. fig.5 fe-al-mn triangular diagram of name, (adachi et al., 1986) to define the venarch mn ore type. we also used the co/zn versus co+cu+ni diagram of toth (1980) to discriminate the venarch samples, and results show that the samples plot in the field of hydrothermal deposits (fig.6). co/zn ratio in the hydrothermal deposits is 0.15 and for hydrogenous deposits is above 2.5. this ratio for the venarch samples varies between 0.003 and 0.11 and clearly represents their hydrothermal origin. co, ni, and cu values are usually used to distinguish the hydrothermal from the hydrogenous deposits (deep marine manganese nodules), and zn values are mainly sources from continents (bonatti et al., 1972). the data indicate that the venarch samples have lower values for co, ni, and cu, suggesting their hydrothermal origin. titanium is an immobile element during hydrothermal alteration and can be used as an indicator of the amount of detrital sediments from continents in deposits (sugisaki, 1984). the higher values of ti in the mn deposits represent the higher amount of detrital sediments in the sedimentary rocks. accordingly, the venarch deposit sample has lower content of tio2, which is correlated with the lower amount of al2o3, implying that samples hydrothermal origin (fig. 7). b horizon is the lower mineralized horizon in the venarch mine that exceed up to 580 m in thickness, and is made of two main thin and stretched mineralized lenses with the barren strata in between. this central horizon (b) is the economically important one in the venarch manganese mine with a thickness of 800 meters of mineralized lenses conformable with the bedding and the maximum thicknesses between 2.1m and 3m. sericitization, hematization, argillitization and chloritization alterations are observed in wall rocks around mineralizations (table 2). the ore horizons in the venarch area are hosted by alternating shale beds and tuffs as laminates, layers, and bands. however, in some parts mn mineralization presents as small black nodules in shale beds. our field studies show that the mn mineralization occurred conformable with volcano-sedimentary strata, and the high-grade horizons have taken place in the vicinity of volcanic lavas. in particularly, the shale beds close to the mega-porphyritic andesites and spilitic basalts show the highest grade, suggesting that the source rocks in the area controlled the degree of mineralization. in this regard, the deposit characteristics in the venarch area and its formation in a shallow sedimentary basin is highly likely similar to the greenstone subgroup of mn-rich formations. for example, the olympic island mn deposit in the washington (us) is one of the major mn deposits, hosted by volcano-sedimentary strata of spilite, diabase, tuff, greywacke, limestone, and red shale and argillites. table 2: xrd analysis results of the venarch samples from altered volcanic rocks for recognition of different types of alteration 145magmatic evolution and compositional characteristics of tertiary volcanic rocks associated with the venarch manganese mineralization, sw qom, central iran fig. 7 tio2 versus al2o3 diagram of choi and hariya (1992). conclusions the venarch mn deposits are associated with late eocene basalticandesitic volcanic rocks and formed in a shallow sedimentary basin in an intra-arc or retroarc tectonic setting. the venarch mn deposit host rocks are volcanic and plutonic rocks. volcanic rocks include basalt, trachybasalt, basaltic andesite and andesite and plutonic rocks consist of monzonite, granodiorite, monzodiorite, and quartz monzonite. the dominant minerals are plagioclase phenocrysts, quartz, k-feldspar, and hornblende and accessory minerals such as biotite, pyroxene, apatite, sphene, and opaque minerals can be seen in some of these rocks. mn mineralization occurred as bands, laminates, layers, and small nodules in the shale and tuff beds conformable with volcanosedimentary strata and the high-grade horizons have taken place in the vicinity of mega-porphyritic volcanic lavas. according to the geochemical results for the major and trace elements of the study area, volcanic rocks are subalkaline and calc-alkaline and belong to subduction-related volcanic arcs, but plutonic rocks are formed in a volcanic arc and or syn-collisional setting. the ore geochemistry of the venarch samples suggest that they have formed by hydrothermal alteration and is similar to marine hydrothermal mn deposits. references: adachi, m., yamamoto, k., and sugisaki, r. 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(1977). geochemical discrimination of different magma series and their differentiation products using immobile elements. chemical geology, v. 20, p. 325-343. 105 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 105 114 spatio-temporal variability of groundwater depth in the eghlid aquifer in southern iran masoomeh delbari1*, masoud bahraini motlagh2 and meysam amiri3 1 associate professor in the department of water engineering, faculty of water and soil, university of zabol, zabol, iran, tel: +98/542/2232113-8, fax: +98/542/2255133, *corresponding author email: mas_delbari@yahoo.com 2 graduated m.sc. student of water resources engineering, department of water engineering, faculty of water and soil, university of zabol, zabol, iran 3 scientific staff in the department of water resources, international hamoun wetland research institute, university of zabol, zabol, iran hydrogeology groundwater is the main water source for domestic and agricultural use in eghlid, a city located in fars province in southern iran. here, spatial and temporal changes in groundwater depth were monitored by using geostatistical methods at 41 observation wells in eghlid during the wet and dry seasons of 1997, 2003 and 2010. experimental semivariograms were calculated and modeled with the gs+ (gamma design software, plainwell, michigan usa),and groundwater depth was interpolated by using the ordinary kriging (ok), simple kriging (sk) and inverse distance weighting (idw) methods within the gis environment. moreover, groundwater depth fluctuations over 13 years (from 1997 to 2010) were calculated and mapped for wet and dry periods. the groundwater depth in the eghlid aquifer exhibited a strong spatial correlation that followed a spherical model for three years. however, the spatial correlation distance was larger for both seasons in 1997 (greater than 27 km) than in 2003 (22 to 27 km) and 2010 (23 to 25 km). the cross-validation results indicated that ok resulted in the lowest root mean square error (rmse) and mean error (me) and was the most appropriate method for interpolating groundwater depth. therefore, ok was used to map the spatial distribution of the groundwater depth over the study area. the resulting maps indicated that the spatial variability of groundwater depth was greater during the wet season than during the dry season over three years. in addition, changes in the depth to groundwater occurred more slowly during the wet seasons than during the dry seasons. furthermore, the ground water depth decreased slightly from 1997 to 2003 and decreased considerably (2-13 m) from 2003 to 2010. moreover, the decrease in the groundwater depth was more notable in the central to west and southern regions of the aquifer. thus, these regions are critical and should be managed carefully to optimize groundwater resource exploitation. las aguas subterráneas son la principal fuente hídrica para el uso doméstico y agrícola en eghlid, una ciudad ubicada en la provincia de fars, al sur de irán. aquí, los cambios temporales y espaciales en la profundidad de las aguas subterráneas se sondean a través de métodos geoestadísticos en 41 pozos de observación en eghlid durante las estaciones de verano e invierno de 1997, 2003 y 2010. se calcularon semivariogramas experimentales y se modelaron con el programa informático gs+, y la profundidad de las aguas subterráneas se interpolaron por el uso de krigging ordinario (ok), krigging simple (sk) y métodos de ponderación de distancia inversa (idw) dentro del ambiente gis. sin embargo, la fluctuación de profundidad de las aguas subterráneas en cerca de 13 años (de 1997 a 2010) se calculó y se mapeó para las temporadas húmeda y seca. la profundidad de las aguas subterráneas en el acuífero de eghlid muestra una fuerte correlación que sigue un modelo esférico de tres años. sin embargo, la distancia de correlación espacial fue mayor para ambas temporadas en 1997 (mayor que 27 km.) y en 2003 (22 a 27 km.) y 2010 (23 a 25 km.). los resultados de validación cruzada indicaron que el producto ok en la raíz cuadrada media de desviación más baja (rmse) y error medio (em) fueron los métodos más apropiados de interpolación de la profundidad de aguas subterráneas. por lo tanto, el ok fue usado para mapear la distribución espacial de la profundidad de las aguas profundas en el área de estudio. el resultado de los palabras clave: estimación; geoestadística; profundidad de aguas subterráneas; variación espacial; gis. abstract resumen key words: estimation, geostatistics, groundwater depth, spatial variation, gis 106 mapas indican que la variabilidad espacial de la profundidad de aguas subterráneas fue mayor durante la temporada húmeda que durante la temporada seca en tres años. adicionalmente, la profundidad de las aguas subterráneas fue menor durante la temporada húmeda que durante la temporada seca. además, el nivel de las aguas profundas cedió ligeramente desde 1997 a 2003 y cayó considerablemente (de 2 a 13 metros). además, el decrecimiento en el nivel de las aguas profundas fue más notable en las regiones de oeste, centro y sur del acuífero. por ende, estas regiones son críticas y se deben manejar cuidadosamente para optimizar el recurso de explotación de las aguas subterráneas. record manuscript received: 20/12/2012 accepted for publication: 18/11/2013 introduction in iran, which is located in an arid and semiarid region, surface water bodies are scarce. consequently, groundwater is the main water source for domestic and irrigation purposes. approximately 95 % of the fresh water resources in iran are used for agricultural production, of which 80 % are obtained from groundwater (ahmadi and sedghamiz, 2008). thus, groundwater resources are important for sustainable agriculture. in recent years, water table levels have dropped from 0.5 to 15 m beneath many agricultural lands, which has resulted in unproductive wells (ahmadi and sedghamiz, 2008). therefore, to optimize groundwater resource exploitation, a detailed study regarding the spatial and temporal fluctuations of groundwater in this region is needed. in addition to water supply studies, knowledge regarding the spatio-temporal variability of groundwater depth is important for landfill characterization, management of agricultural salinity and chemical seepage (buchanan and triantafilis, 2009). however, the groundwater depth is generally measured at a small number of random locations. to obtain the required information on a dense grid, several interpolators should be employed. the common interpolation methods of classical statistics are not appropriate because they neglect the geographical positions of wells. in addition, it is well known that environmental variables are often spatially dependent. geostatistics is a branch of statistics in which the spatial continuity behavior of the groundwater data is incorporated into the estimation process (isaaks and srivastava, 1989). many researchers have mapped groundwater levels or optimized groundwater monitoring networks with geostatistical approaches (delhomme, 1978; sophocleous et al., 1982; virdee and kottegoda 1984; tonkin and larson, 2001; nunes et al., 2004; and delbari, 2013). geostatistics can be used to improve the management and protection of water resources (kumar and sondhi, 2005; reghunath et al., 2005). for example, delbari (2013) compared several univariate and multivariate interpolation approaches for estimating groundwater levels in iran. kumar and ahmad (2003) used geostatistical methods to estimate the groundwater depth in india. furthermore, desbarats et al. (2002) used a digital elevation model (dem) to provide auxiliary data for estimating groundwater levels by using kriging with external drift. cameron and hunter (2002) determined an optimum sampling network by investigating the spatial and temporal variations of groundwater quality. in addifigure 1. location of the study area and observation wells masoomeh delbari, masoud bahraini motlagh and meysam amiri 107 tion, hu et al. (2005) investigated the spatial variability of groundwater levels, salinity and nitrate in northern china. these authors found that indicator kriging (ik) was more appropriate than ordinary kriging (ok) for mapping nitrate contamination risk. kumar and remadevi (2006) performed a spatial variability analysis of groundwater depth using data that were provided from more than 60 observation wells in india. these authors fit spherical, gaussian and exponential models to the experimental semivariograms and compared kriging estimates to inverse square distance (isd) estimates. in this case, the kriging estimator with the gaussian semivariogram model yielded the smallest error when estimating groundwater depth. theodossiou and latinopoulos (2007) used the kriging method to estimate groundwater levels and to optimize a groundwater-monitoring network. furthermore, ahmadi and sedghamiz (2007, 2008) used geostatistics to map groundwater depth in iran and ta’any et al. (2009) investigated groundwater level fluctuations between 2001 and 2005 in the amman zarqa basin in jordan. these authors used ok to interpolate groundwater levels by using data from 31 wells. sunet al.(2009) compared different methods for modeling groundwater level changes in 48 piezometric wells from 1981 to 2003 in china by using inverse distance weighting (idw), radial basis function (rbf) and three kriging variants (the ok, simple kriging (sk) and universal kriging (uk) variants). these authors showed that the sk method was the most appropriate method among those tested. this study aimed to investigate the spatial and temporal changes of the groundwater depth in an aquifer near eghlid city, which is located in fars province in southern iran. groundwater depth data were collected during dry and wet seasons in1997, 2003 and 2010. the ok, sk and idw methods were used. methodology study area and groundwater depth the city of eghlid is located in fars province in southern iran, covers an area of 7054 sq. km and is located at 31° 13’ n latitude and 52° 55’ e longitude. eghlid is allocated in a mountainous and highland region. the city of eghlid follows the zagross mountains and bell mountains to the south with a maximum elevation of 3943 m asl. this area is characterized by cold winters and mild summers with minimum and maximum temperatures of -22 and 37 °c, respectively. the average annual rainfall levels in the city and highland villages are 300-330 mm and 400-600 mm, respectively. the study area includes an aquifer that covers an area of approximately 716.2 sq. km (figure 1). the elevation in this region fluctuates from 2068 m west of the aquifer to 2690 m southeast of the aquifer. however, several small regions northwest of the study area are characterized by high elevation. furthermore, eghlid is an important location for agricultural production, especially for wheat, barley, potato and fruit production (including grapes, walnuts, apples and pears). groundwater resources provide an important water source for drinking water and water for agricultural purposes. here, the spatiotemporal variability of the groundwater depth was determined by using data from 41 observation wells that were placed throughout the study area. the geographical locations of the wells are shown in figure 1. for these observation wells, the monthly averages of groundwater depth data for march (wet season) and september (dry season) were monitored between 1997 and 2010.these data were used for the geostatistical analysis. to consider temporal fluctuations, data were analyzed from 1997, 2003 and 2010. geostatistics the theory of geostatistics has been expressed in many textbooks, including those of isaaks and srivastava (1989) and goovaerts (1997). here, a brief description of the geostatistical methodsthat were used in this study is presented. in geostatistics, a semivariogram is used to quantify the differences between sampled data values as a function of their separation distance, h. in practice, the experimental semivariogram, *(h), is calculated as follows: where n(h) is the number of sample pairs that are separated by a vector h, and z(xi) and z(xi+h) are the values of the variable z at locations of xi and xi+h, respectively. however, for kriging analysis, an appropriate theoretical model should be used to fit the experimental data. the most widely used models include the spherical, exponential and gaussian models. the spherical model used in this study is defined as follows: where c0 is the y-axis intercept (the “nugget effect”), c0+c is the “sill”, which is near the sample variance, and a represents the range of influence. ordinary kriging ok assumes that the mean is stationary but unknown. in addition, the ok estimator is known as the best linear unbiased estimator (blue) and is defined as follows (journel and huijbregts, 1978): where z*(x0) is the ok estimator at location x0, z(xi) is the observed value of the variable at location xi, λi is the weight assigned to the known values near the location to be estimated and n(u) is the number of neighboring observations. the values of λi are weighted to obtain a sum of unity, and the error variance is minimized as follows: where μ is the lagrange coefficient for minimizing the ok estimation variance, γ(xi, xj) is the average semivariogram value between the observed values and γ(xi, x0) represents the average semivariogram value between the location xi and the location to be estimated (i.e., x0). the ok estimation variance (or standard deviation) can be used as a measure of the estimation uncertainty as follows: simple kriging for example, ok and sk assume a stationary but known mean. in addition, ok uses local averages and sk uses entire observation averages. the sk estimator is given as follows: where m is the mean value of the variable z and λi is the weight assigned to the residual value of z(xi) from the mean. this estimate is unbiased because e [z(xi)-m]=0 and e [z*(x0)]=m=e [z(x0)]. spatio-temporal variability of groundwater depth in the eghlid aquifer in southern iran (1) (2) (3) (4) (5) (6) 108 inverse distance weighting when using idw, the unknown value at location x0, z *(x0) is estimated from the linear weighted average of several neighboring observations as follows: where z(xi) is the observed value at location xi, di is the distance between the interpolated and observed values and a is an exponent. in idw, the weights are inversely proportional to a power of the distance between the observations and the location x0. as the value of power aincreases, the effect of the farthest observations on the estimated value decreases. in here, values of a were set to 1, 2 and 3. comparison of the interpolation methods here, the cross-validation technique (isaaks and srivastava 1989) was used to compare different interpolation approaches. the evaluation criteria that were used included the root mean square error (rmse) and the mean error (me), which are defined as follows: where z*(xi) and z(xi) are the estimated and measured values at location xi, respectively, and n is the number of observations. the most appropriate method has the smallest rmse. in addition, the best estimator has an me near zero. here, the semivariogram calculations and groundwater depth modeling were performed in gs+ (robertson, 2000). interpolation and mapping were performed by using the arcgis geostatistical analyst. results and discussion statistical analyses a summary of the groundwater depth data is presented in table 1. the mean groundwater depth in 2010 was greater than its corresponding value in 1997 and 2003. in contrast, the groundwater depth did not fluctuate much between the two seasons between 1997 and2003. however, the groundwater depth significantly decreased (approximately 4.5 m) between 2003 and 2010. the groundwater depth fluctuations of four randomly selected wells (wells number 13, 19, 29, and 36) during the dry (september) and wet (march) seasons of 1997 and 2010 are shown in figure 2. the groundwater depth fluctuations were minimal until 2006 (for wet season) and 2007 (for dry season) (figure 2). however, the groundwater depth significantly decreased after 2007. the increases and decreases in groundwater depth for all 41 wells between 1997 and 2010 are presented in table 2. in september, the largest decrease occurred at well number 24 (18.47 m).in march, the largest decrease (10.14 m) occurred at well number 29. both of these wells are located near the center of the study area (figure 1). as shown in table 2, in majority of wells, the drop in march was less than the drop in september, which could be attributed to changes in aquifer storage. recharge generally occurs in the late winter and spring seasons, when precipitation is generally the greatest and the evapotranspiration rate is low. in figure 3, the amounts of annual rainfall during 1997, 2003 and 2010 are shown. a drastic reduction in rainfall occurred between 2003 and 2010. this decrease potentially resulted in the deeper groundwater depth between 2003 and 2010. table 1. summary statistics of the groundwater depth data from 41 observation wells year month mean (m) standard deviation (m) maximum (m) minimum (m) 1997 september march 14.05 11.49 13.44 12.88 1.15 0.51 46.85 42.90 2003 september march 14.40 11.39 13.01 11.76 1.18 0.68 46.11 40.97 2010 september march 18.76 15.94 14.29 14.38 1.63 0.33 54.62 55.26 table 2. groundwater depth fluctuations (m) from 41 observation wells between 1997 and 2010 well number wet season dry season well number wet season dry season 1 -3.73 -5.98 22 -3.44 -8.81 2 -4.7 -3.95 23 -0.4 -2.57 3 1.22 4.3 24 -5.21 -18.47 4 -0.61 -2.86 25 -7.33 -17.52 5 0.5 4.89 26 -1.63 -2.72 6 -6.66 -9.23 27 -6.77 -15.9 7 -4.82 -7.53 28 1.38 0.21 8 -2.7 -4.74 29 -10.14 -13.2 9 -0.78 -1.37 30 -4.64 -9.19 10 0.87 -1.18 31 -7.7 -10.33 11 -1.75 -2.75 32 -6.45 -15.07 12 -1.91 -6.66 33 -8.18 -9.54 13 -2.31 -3.66 34 -1.38 -1.76 14 5.67 4.97 35 -1.48 -2.83 15 -8.07 -13.63 36 -3.57 -10.76 16 -2.02 -14.69 37 -2.06 -3.2 17 0.42 0.13 38 -2.15 -3.59 18 -3.61 -4.2 39 -7.34 -9.16 19 -2.85 -7.56 40 -3.72 -8.75 20 1.18 -2 41 -0.96 -3.14 21 -1.8 -4.2       table 3. semivariogram parameters of the groundwater depth data model type c0 (m 2) c0+c (m2) a (m) c/(c0+c) r2 rss sep 1997 spherical 50.4 250.2 27340 0.799 0.857 6018 mar 1997 spherical 43.1 234.7 27170 0.816 0.860 5408 sep 2003 spherical 50.2 222.1 22410 0.774 0.803 5576 mar 2003 spherical 41.5 195.3 26500 0.788 0.851 3705 sep 2010 spherical 59.6 280.7 23430 0.788 0.852 7330 mar 2010 spherical 52.1 297.6 25000 0.825 0.873 7726 masoomeh delbari, masoud bahraini motlagh and meysam amiri (7) (8) (9) 109 table 4. cross-validation results from estimating the groundwater depth with the different interpolation methods in 1997, 2003 and 2010 year estimation method season rmse (m) me (m) 1997 ok dry 2.69 -0.4 wet 1.80 -0.37 sk dry 2.81 -1.1 wet 2.20 -0.94 idw-1 dry 4.15 1.16 wet 3.26 -1.27 idw-2 dry 4.15 -1.73 wet 2.06 -1.55 idw-3 dry 3.59 -2.07 wet 2.25 -1.82 2003 ok dry 1.40 -0.36 wet 1.43 -1.23 sk dry 2.65 -0.99 wet 1.54 .1.09 idw-1 dry 4.31 -1.24 wet 3.87 -1.11 idw-2 dry 3.39 -1.64 wet 1.53 -1.43 idw-3 dry 3.44 -2.00 wet 1.74 -1.74 2010 ok dry 2.10 -0.36 wet 1.69 -1.45 sk dry 2.44 -1.51 wet 2.15 -1.55 idw-1 dry 3.80 -1.32 wet 3.80 -1.32 idw-2 dry 2.28 -1.72 wet 2.17 -1.72 idw-3 dry 2.98 -2.06 wet 2.60 -2.06 figure 2. groundwater depth fluctuations (m) for wet (up) and dry (bottom) seasons from 1997 to 2010 figure 3. annual precipitation (mm) across the study area in 1997, 2003 and 2010 geostatistical analyses to investigate the spatial autocorrelations of the groundwater depth for selected years, the omnidirectional experimental semivariogram was calculated (figure 4). the best theoretical model of the semivariogram is shown in figure 4. to determine the directional behavior of the spatial variability of the groundwater depth, the experimental semivariogram of groundwater depth was calculated for four directions (0, 45, 90 and 135 degrees) with an angle tolerance of 22.5°.these results did not show any significance differences. thus, the groundwater depth was assumed as isotropic and the semivariograms shown in figure 4 were considered for further analysis. the groundwater depth during the wet and dry seasons in 1997, 2003 and 2010 was strongly spatially correlated because the ratio of the nugget effect to the sill of the semivariograms was small (figure 4). the spatial structure of the groundwater depth during the wet and dry seasons in 1997, 2003 and 2010 followed a spherical model. similarly, kumar and ahmad (2003) indicated that the spherical model was the best for modeling the spatial structure of groundwater depth. however, delbari et al. (2010) and ahmadi and sedghamiz (2007) indicated that the spherical and exponential models were the best theoretical semivariogram models. ta’any at el. (2009) and kumar and remadevi (2006) introduced the gaussian model and mendes and lorandi (2008) introduced the exponential model as the best fitted semivariogram models for predicting groundwater levels. as shown in table 4, the smallest and largest nugget effects (random variance) were found for groundwater depths in march 2003 and september 2010, respectively. the influence range was greatest (approximately 27 km) during september 1997 and smallest (approximately 22 km) during september 2003. for the interpolation of groundwater depth, the ok, sk and idw methods were used with exponents of 1, 2 and 3. the rmse and me values that were associated with each method were calculated through cross-validation and are presented in tables 4 for 1997, 2003 and 2010. these results indicated that the ok approach is the most appropriate interpolation approach for estimating groundwater depth because it yielded the lowest rmse for all three years and all seasons and resulted in the smallest bias (me) in most cases. the idw method, which does not consider the spatial correlation of the observations during the estimation processes, often resulted in the highest error. in the study by kumar and remadevi (2006), ok was more accurate than idw for groundwater level interpolation. figure 5 shows the maps of interpolated groundwater depth during the wet and dry seasons of 1997, 2003 and 2010 that were obtained by ok. greater spatial variability occurred for the groundwater depth during the wet seasons than during the dry seasons. this finding potentially resulted from the spatial variations of the groundwater recharge and discharge. the groundwater depth was greater from the center to the west of the aquifer, which potentially resulted from the lower topographical elevation and greater groundwater resource exploitation. a drastic decrease in groundwater depth occurred between 1997 and 2010 throughout the study area. this decrease resulted from the excessive exploitation of groundwater resources for drinking and agricultural purposes and from the lack of precipitation during the most recent spatio-temporal variability of groundwater depth in the eghlid aquifer in southern iran 110 figure 4. experimental semivariograms and the best-fit groundwater depth models for the wet and dry seasons in 1997, 2003 and 2010 years. moreover, as shown in figure 5, the groundwater depth significantly increased between march and september each year. for example, in march 2010, the groundwater depths in many areas in the southern, southeastern and upper portions of the study area were approximately 2 m below the surface. however, in september these groundwater depths increased from 2 to 8 m. this change resulted from increasing water levels during the late winter and spring and decreasing water levels through the summer and fall. as previously indicated, the estimated standard error of ok can be used to measure the uncertainty of groundwater depth estimates. the maps of the ok estimation error (figure 6) show that a smaller uncertainty occurs near the observation wells (i.e., the upper half (except northwest) and southeastern regions of the study area). larger uncertainty regarding groundwater depth estimates can be observed in regions where more variability in the spatial distribution of groundwater depths (e.g., northwest) occurs and where the sample points are farther apart (e.g., near the center of the study area). in addition, larger uncertainty can be observed around the borders of the study area, especially in the lower half, which has no observation wells. these locations are primary candidates for digging additional wells in the future. in addition, groundwater depth fluctuation maps were obtained from 1997 to 2010 during the wet and dry seasons (figure 7). during the wet season, the groundwater depth decreased by7 to 10 m in the central regions of the study area and by 3 to 5 m in the southern regions of the study area. during the dry season, the groundwater depth decreased by approximately 8 to12 m in the central regions of the study area and by 5 to8 m in the southern regions of the study area. this significant decrease in groundwater level potentially resulted from (1) lower rainfall during recent years and from (2) high groundwater resource exploitation. overall, these results could be used in future macropolicies, e.g., using modern irrigation systems rather than conventional irrigation methods or enhancing the efficiency of current irrigation systems. masoomeh delbari, masoud bahraini motlagh and meysam amiri 111 figure 5. groundwater depth (m) maps for the wet and dry seasons in 1997, 2003 and 2010 (obtained by ok) conclusions here, the spatiotemporal variability of groundwater depth was investigated. three interpolation methods, including ok, sk and idw, were applied to estimate the groundwater depth throughout the study area. the groundwater depth is strongly spatially correlated and its spatial structure follows a spherical model. the ok method was the most appropriate method for the three investigated years. the spatial distribution maps of spatio-temporal variability of groundwater depth in the eghlid aquifer in southern iran 112 groundwater depth showed that groundwater depth in eghlid aquifer did not change much between 1997and 2003, but decreased in 2010. this decrease potentially resulted from the lower annual rainfall in 2010 relative to 2003. moreover, the generated maps clearly indicated that the groundwater depth decreased during the wet seasons relative to the dry seasons. the groundwater depth fluctuations maps indicated that the groundwater decrease was more noticeable in the central and southern regions of the aquifer than in the northern regions, which potentially resulted from the figure 6. estimation error (m) maps for groundwater depth during the wet and dry seasons of 1997, 2003 and 2010 (obtained by ok) masoomeh delbari, masoud bahraini motlagh and meysam amiri 113 higher 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(2008) groundwater quality evaluation for productive uses— the afram plains area, ghana. j. irrig. and drain. engrg., 134(2), 222–22 masoomeh delbari, masoud bahraini motlagh and meysam amiri the superlarge dongfeng gold deposit is located in the potouqing faults-alteration belt of the eastern part of the ‘zhao-lai-gold ore belt’, which belongs to the northwestern part of the jiaodong area. tectonically, ore bodies are controlled by faults and gold mainly occurs in the pyrite and polymetallic sulfide-bearing quartz vein. in this paper, rb–sr isotopic analysis is carried out with the beresite, which formed by hydrothermal metasomatism, and the rb–sr isochron age is 125.5±6.7ma, indicating this deposit set up in the early cretaceous of the late yanshanian. based on the relationship between the dongfeng gold deposit and the mesozoic granite, it is suggested that the formation of the gold deposit is a complex geological process of gradual enrichment and precipitation of the ore-forming elements. the initial 87sr/86sr ratio of the beresite is 0.711502±0.000069, which indicates the ore-forming materials mainly come from the crust. combined with the complex mineralization process of the dongfeng gold deposit and the reported h-o isotopic data, it is suggested that the ore-forming materials are mainly derived from the crust with some mantle materials, while the oreforming fluids are originated primarily from magmatic hydrothermal and mantle with some precipitate water. earth sciences research journal earth sci. res. j. vol. 20, no. 1 (march, 2016): i1 i7 abstract keywords: rb–sr isochron, (87sr/86sr) ratio, mineralization, dongfeng gold deposit, jiaodong gold-centralized area. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n1.55178 o r e d e po si t s wang zongyong1*, lv guxian2, zhang xunyu1, han xin1, zhang yingchun1, fan xiao1, huo qinglong1, xu yaqing 1.school of earth sciences and resources, china university of geosciences, beijing 100083, china 2.institute of geomechanics, chinese academy of geological sciences, beijing 100081, china * e-mail: hawks_wzy@163.com rb-sr isotopic geochronology and geological implications of dongfeng gold deposit in jiaodong area record manuscript received: 15/01/2016 accepted for publication: 19/04/2016 how to cite item zongyong, w., xin, h., guxian, l. v., xunyu, z., yingchun, z., xiao, f., qinglong, h., & yaqing, x. (2016). rb-sr isotopic geochronology and geological implications of dongfeng gold deposit in jiaodong area. earth sciences research journal, 20(1),i1 -i7. doi: http://dx.doi.org/10.15446/esrj.v20n1.55178 1. introduction as one of the most significant gold concentration areas in china, jiaodong area has special metallization background and metallogenesis (goldfarb and santosh, 2013). the great linglong gold ore field is located in the northwest of the jiaodong area, the eastern part of the ‘zhao-lai-gold ore belt’, including linglong, jiuqu, dakaitou, dongfeng and dongshan ore blocks or fields. previous researchers have carried out many studies of all types of gold deposits in the jiaodong area. yiao et al. (1990), miao et al. (1999) and hu et al. (1998) suggested the gold mineralization was closely correlated with the mesozoic granites. based on the studies of the gold deposit geochronology, yang et al. (2000), wang et al. (2000) and chen et al. (2004) implied that gold mineralization occurred in the mesozoic and concentrated from 128 to 115 ma. regarding the ore-forming fluids, fan et al. (2005) suggested that ore-forming fluids of gold deposits were consistent throughout the whole jiaodong area under similar mineralizing temperature and pressure conditions and the fluids were characterized by h2o-co2-nacl±ch4, with low salinity and low-to-moderate temperature. moreover, mao et al. (2005) pointed out that mantle fluids were associated with the gold mineralization. sun et al. (2000), wan et al. (2000), zhai et al. (2002), zhai et al. (2004), lv et al. (2007), song (2008), song et al. (2013) and yang et al. (2014a) have comprehensively studied and reviewed the mineralizing setting and mineralization model of the gold deposits in the jiaodong area. since the dongfeng deposit has been found at the end of 1980’s, some studies have been implemented in this reservoir. xu et al. (2013) established the deposit database and ore grade distribution model for the dongfeng deposit based on the surpac software and applied the ordinary kriging to estimate the orebody resources. feng et al. (2009) studied the ore-forming conditions, ore body characteristics and wall rock alteration about the no.171 gold vein of dongfeng gold deposit. jiang et al. (2011) primarily discussed the metallogenic regularity of the dongfeng gold deposit. however, the research of the metallogenic geochronology and the source of minerogenic substances is relatively weak. in this paper, rb and sr isotopic analysis has been carried out with the beresite, which has a close relationship with the mineralization of the dongfeng gold deposit. the purpose of this study is to determine the mineralization age and discuss the source of oreforming materials of the dongfeng gold deposit. this study also put forward a new understanding of the mineralization for the dongfeng gold deposit. i2 wang zongyong, lv guxian, zhang xunyu, han xin, zhang yingchun, fan xiao, huo qinglong, xu yaqing 2. geological background and deposit geological characters the dongfeng gold deposit is located in the uplift of the northwestern part of jiaodong area, which belongs to north china craton. the regional strata in the studying area consist mainly of metamorphic rocks in jiaodong rock group, in archaean era and quaternary. the metamorphic rocks are composed of biotite-granulite and amphibolite. faults are well-developed in the dongfeng area. the faults are mainly neand nne-trending. the ne-trending faults are present at the potouqing fault. the ne-trending faults are represented by the luanjiahe fault. the intersection of two sets of faults controls the morphology and distribution of the dongfeng gold deposits (fig 1). the intrusions in the studying area mainly include the linglong biotite adamellite, guojialing granodiorite, and luanjiahe monzonitic granite. the linglong biotite adamellite occurs as batholiths or stocks, with shrimp zircon u-pb age of 160.45±0.83ma (wan, 2014). the guojialing granodiorite occurs as batholiths or stocks, with shrimp zircon u-pb age of 131.86±0.99ma (wan, 2014). the luanjiahe monzonitic granite also occurs as batholiths or stocks, with shrimp zircon u-pb age of 154~152ma (luo and miao, 2002). the recent data of geophysical prospecting, drilling project, and deep exploration reveals that there is a giant guojialing granodiorite batholith in the deep of the studying area, overlain by the linglong biotite adamellite (feng et al., 2009). figure 1. regional geological sketch map of dongfeng deposit in jiaodong area 1. quaternary; 2. jiaodong group; 3. linglong granite; 4. guojialing granite; 5. luanjiahe granite; 6. faults; 7. fractured belt; 8. gold body; 9. gold deposit range; f1. jiuqujiangjia faults; f2. potouqing faults; f3. luanjiahe faults. the main vein of the dongfeng gold deposits is no. 171 vein. the shape of the vein is simple, the vein strikes 60° 70°ne and dips se. (fig. 2). the ore minerals are mainly pyrite, galena, and sphalerite. the ores mainly comprise cataclastic, idiomorphic or hypidiomorphic granular, and gric textures. the structures of the ores can be disseminated, massive, vein, or veinlet (fig. 3). the alteration assemblage includes beresitization, potash feldspathization, chloritization, and carbonatization, occurring as bands in the ore-bearing faults and pervasive alteration of nearby wall rocks. the dominant alteration related to the au mineralization is beresitization. gold mainly exists in polymetallic sulfide. figure 2. no. 160 vertical cross section of the no.171 vein of dongfeng gold deposit. 3. sampling and analytical methods the five samples for the rb and sr isotopic analysis were beresite which formed by hydrothermal metasomatism and closely related to metallogenesis. the samples were collected at the -350m level in orebody no. 171 of the dongfeng gold deposits. the sampling location of each sample and detailed characteristics of the five samples are shown in the table 1 and figure 3. table 1. the sampling locations and the characteristics of five samples the crushing work of the samples was carried out in the analytical laboratory of cnnc beijing research institute of uranium geology. the rb and sr isotopic analysis of 5 samples were performed at the igge of chinese academy of geological sciences. the steps of the rb and sr isotopic analysis are as follows: 1) according to “the specification of testing quality management for geological laboratories” (dz/t01302006), the beresite samples were crushed up to 200 meshes without contamination. 2) the sample powder (ca. 0.1–0.2 g) was separated for the rb-sr analysis. samples were dissolved in teflon bombs with hf+hno3+hclo4 acid (24h). 3) after completely dissolved, the samples were followed by drying.4) then, the samples were treated with 6mol/l hcl and completely converted to chlorate, then dried. 5) samples were dissolved by 0.5mol/l hcl. following the centrifugal separation, the solution has been into cation exchange column and cleared by 1.75mol/l hcl with rb. then, the drying was carried out; the dissolve was cleared by 2.5mol/l hcl with sr; at last, the drying was carried out. the rb and sr isotopic analysis were performed on a phoenix isoprobe-t thermal ionization mass spectrometer. the relative i3rb-sr isotopic geochronology and geological implications of dongfeng gold deposit in jiaodong area figure 3. the characteristics of beresite samples of the dongfeng gold deposit acelandine green beresite; bmassive beresite ores, pyrite distributes in the quartz veins, with the fine vein and dissemination structure; cquartz closely coexists with sericite under the cross-polarized light; qtzquartz; srt-sericite; py-pyrite 4 analytical results the rb and sr isotopic compositions of the five beresite samples in the dongfeng deposit are presented in table 2. the isochron age was calculated with the isoplot program (ludwing, 1998). the rb and sr concentrations of 5 beresite samples range from 71 to 171 ug/g and 114 to 1269 ug/g, respectively. the isotope ratios of 87rb/86sr range from 0.3891 to 3.4915, averaging 1.76702, and the isotope ratios of 87sr/86sr vary from 0.712201 to 0.718079, with an average of 0.7146836. five beresite samples yield a rb–sr isochron age of 125.5±6.7 ma, with an initial 87sr/86sr ratio of 0.711502±0.00006 and an mswd of 0.96 (fig. 4). these results indicate that the dongfeng gold deposit was formed at 125.5±6.7 ma. table 2. rb–sr isotopic analysis of beresite from the dongfeng gold deposit. figure 4. rb–sr isochron for the beresite from the dongfeng gold deposit. 5 discussion 5.1 mineralization age endogenic gold deposits were developed among hydrothermal metasomatic alteration, granulating relatively small alteration minerals which were in a fair preservation of rb and sr. moreover, rb-sr isochron method of altered minerals has been widely used in the study of the mineralization age of gold deposits (jager, 1979; andre and deutsch, 1986; wei et al., 1997; yang and zhou, 1999; xie and hu, 2000; wang et al., 2002). however, as we know, under the effect of the crystal chemical conditions, rb and sr occur as the isomorph in the crystal lattice of the potassium-bearing and calcium-bearing minerals, respectively. therefore, coinstantaneous enrichment of rb and sr in the same kind of mineral seems impossible. also, a part of the sr will be lost in the process of single mineral selection. meanwhile, the purity of the single mineral can notbe ensured because of the tedious separation work. therefore, the hydrothermal metasomatic system of total rocks, with good sealing ability, has been used in the rb-sr isotopic analysis. based on the above theoretical foundation, the beresite which formed by strong hydrothermal metasomatism was collected to be employed in the rb-sr isotopic analysis to obtain the formation of the mineralization age of the dongfeng gold deposit. humidity and temperature are 45% and 20cº respectively. during the analysis, reproducibility and accuracy of sr isotope running have been periodically checked by running the standard reference material nbs 987, with a measured 87sr/86sr ratio of 0.710250±7 (2 σ mean). both of the total procedure blanks for rb and sr were 2×10-10 g. i4 wang zongyong, lv guxian, zhang xunyu, han xin, zhang yingchun, fan xiao, huo qinglong, xu yaqing in recent studies, by using rb-sr, ar-ar, and shrimp u-pb isotope system of ore minerals, the ages of the representative deposits in the jiaodong gold-centralized area have been obtained (table 3). the statistical results show that the ages of representative deposits are similar and range from 128 to 115ma, indicating these deposits mainly formed from the late jurassic to early cretaceous, which is consistent with the previous conclusions of the time of the large scale gold metallogeny in the jiaodong area (fan et al., 2005; chen et al., 2004). the rb-sr isochron age of the beresite from the dongfeng gold deposit in this study is 125.5±6.7 ma. this result is consistent within the age range showed in table 3, indicating the mineralization age of the dongfeng gold deposit is similar to other gold deposits in the jiaodong area. this fact suggests that the gold deposits in the jiaodong area are formed in a relatively short time and under the same mineralization background, and related with the mesozoic tectonic transition. thus, this age (125.5±6.7ma) can be considered as the mineralization age of the dongfeng gold deposit. table 3. representative isotopic ages of the gold deposits in the jiaodong gold-centralized area since 2000 5.2 relationship between the mesozoic magmatism and the dongfeng gold metallization the mesozoic linglong biotite monzonitic granite, guojialing granodiorite, and luanjiahe monzodiorite are extensively distributed in the jiaodong gold concentration area, which was closely related to the gold deposit mineralization. therefore, research on isotope chronology for these mesozoic granites is introduced in great detail. based on the reported zircon shrimp dating results of granites (table 3), combined with the rb-sr isochron ages of the dongfeng gold deposit, this paper discussed the relationship between the mesozoic magmatism and the dongfeng gold mineralization. as shown in table 3, linglong biotite monzonitic granite, luanjiahe monzodiorite and guojialing granodiorite formed from 160 to 153 ma, 154 to 152 ma and 131 to 126 ma, respectively, which indicate that mesozoic granites are mainly formed in two phases: 160 to 152 ma and 131 to 126 ma. these results are a coincidence with the mineralization ages (128 to 115 ma) of the representative gold deposits of the jiaodong gold concentration area and the ore-forming age (125.5±6.7m) of the dongfeng gold deposit. furthermore, the mineralization of the dongfeng gold deposit is contemporaneous with the formation of the guojialing granodiorite, but later than the formation of the linglong biotite monzonitic granite and luanjiahe monzodiorite (fig. 5). there is 155±3ma inherited zircon in the guojialing granodiorite, which indicates that the guojialing granodiorite is considered to be the partial melting product of the jiaodong group and early granites. also, the δ34s values of the goldbearing pyrite in the dongfeng gold deposit and the guojialing granodiorite are consistent, while the pb isotopic values（208pb/206pb） show an increasing trend from linglong biotite monzonitic granite to guojialing granodiorite, then to gold deposits, which reveals an inheritance evolution trend (wan, 2014). therefore, it could be ascertained that the dongfeng gold deposit should go hand in hand with the mesozoic magmatism. this can be embodied in the following respects: (1) during the emplacement of the linglong granite during the late paleoproterozoic–neoproterozoic period, the rock mass not only carried a gold element of itself but also gave rise to the activation and migration of the ore-forming elements disseminated in the archean metamorphic rocks in jiaodong area. then these elements could be enriched in the favorable place for mineralization. (2) during the indosinian-early yanshanian period, the tectonicmagmatic activity was extensively active, the intense interaction between crust and mantle materials occurred, with the formation of the guojialing granodiorite. during the emplacement, the guojialing granodiorite further activated and extracted the gold elements disseminated in the early metamorphic basement and granites. also, the neand nnetrending brittle fracture activities were intense, which provided the migration pathway and ore-forming space for the ore-forming fluids, while the ore fluids continuously interacted with the wall rocks. under a favorable physicochemical condition, the ore-forming materials could precipitate and form the deposit in the convenient place. as a result, the dongfeng gold mineralization was eventually completed. i5rb-sr isotopic geochronology and geological implications of dongfeng gold deposit in jiaodong area table 4. the shrimp zircon u-pb ages of mesozoic granites in gold concentration area of jiaodong note: “——” means no data; data in brackets mean the number of analysis spots. figure 5. the collective age diagram of mesozoic granites diagenesis and dongfeng gold mineralization in jiaodong area. 5.3 sources of ore-forming materials and ore fluids the rb and sr isotopic analysis are an effective tool to discuss the source of ore-forming materials (fan et al., 2005; medford et al., 1983; bell et al., 1989; darbyshire et al., 1996). also, the isr value is considered as an important indicator to discriminate the properties of the source of ore-forming materials (tu et al., 1982; wang and zhou, 2002); the isr value is less than 0.705 for the mantle source, the isr value is more than 0.709 for the crustal source. while the isr value is between 0.705 and 0.709 for the mixture of the mantle and crust sources. however, the indicators of different areas are variable due to the different geological background. in this study, we apply this indicator to discuss the origin of ore-forming materials. the（87sr/86sr）i ratio of beresite in this paper is 0.711502±0.000069（fig. 4); it is thought to be reliable due to the reliability of the rb-sr isochron age of beresite. as mentioned above, the （87sr/86sr）i ratio of beresite is more than 0.709, appearing to indicate that the ore-forming materials are mainly from crust or dominant crust. according to the results of previous researchers, the(87sr/86sr)i ratios of the linglong granite were from 0.710498 to 0.712286 (deng et al., 2011), the (87sr/86sr)i ratios of the guojiaing granodiorite were from 0.71071 to 0.71172 (wang et al., 2014). by the above discriminant indicator, the linglong granite and the guojialing granodiorite were both from the crust. moreover, there was inherited zircon (155±3ma) in the guojialing granodiorite; the age of the inherited zircon was consistent with the diagenetic age of early granites. so we can infer that the guojialing granodiorite is the partial melting product of the early linglong granite, which indirectly supports the view that the two kinds of granites are both from crust sources. as discussed above, the formation of the gold deposit is a complex geological process of gradual enrichment and precipitation for the ore-forming elements. especially, during the indosinian-early yanshanian period, tectonicmagmatic activities caused the upwelling of the mantle materials, the mantle materials interacted with the previous crust materials (such as the rock series of archean jiaodong group and neoproterozoic linglong granite) existing in the lithosphere, then formed a crust-mantle mixed-source type of magma, i.e. guojialing granodiorite. based on the above evidence, it is inevitable that there will always be the mantle material involved in the ore-forming materials and ore fluids. therefore, this article suggests that the ore-forming materials of the dongfeng gold deposit are mainly from the crust. meanwhile, some mantle materials also participate in mineralization of gold deposit. the previous studies also confirmed this judgment (zhou et al., 2002; liu et al., 2003). the rb and sr isotopic analysis of the gold-bearing mineral (pyrite), the carbonate minerals in gold ores, the contemporary magmatic rocks with gold mineralization and basement metamorphic rocks from the typical gold deposits, suggested that the rb and sr isotopic compositions of the gold ore minerals were similar with that of the mantle-derived magmatic rocks, and had a certain i6 wang zongyong, lv guxian, zhang xunyu, han xin, zhang yingchun, fan xiao, huo qinglong, xu yaqing connection with that of the basement metamorphic rocks and the contemporary magmatic rocks with gold mineralization, which indicated that the ore-forming materials of the gold deposits were of multi-sources, including contemporary magmatic rocks, basement metamorphic rocks, and mantle-derived magma. also, based on the hydrogen and oxygen isotopic data of the fluid inclusions in quartz of the dongfeng deposits (wan, 2014), this article uses sheppard ‘s diagram to discriminate the sources of ore-forming fluids (fig. 5). in the hydrogen and oxygen isotopic diagram, the plotted points of the linglong granite and guojialing granodiorite samples are partly located in the usual range of magmatic water. however, another part of the plotted points is close to the range of water in the mantle, which implies that there may be some mantle materials participated in the process of granites diagenesis. moreover, the plotted points of the ore samples of dongfeng deposit are on the left of the range of magmatic water, and near the range of precipitation water, indicating that the ore-forming fluids are possibly mainly from magmatic hydrothermal and mantle with some precipitate water. figure 6. the hydrogen and oxygen isotopic diagram (base map from sheppard, 1977, and data from wan, 2014) 6 conclusions 1) the rb–sr isochron age of beresites for the dongfeng gold deposit in jiaodong gold concentration area is 125.5±6.7ma, indicating this deposit formed in the early cretaceous of the late yanshanian. 2) it could be ascertained that the dongfeng gold deposit should go hand in hand with the mesozoic magmatism. the formation of the gold deposit is a complex geological process of gradual enrichment and precipitation for the ore-forming elements. 3) the ore-forming materials of the dongfeng gold deposit are mainly from the crust. meanwhile, some mantle materials also participate in mineralization of gold deposit. the ore-forming materials of the gold deposits are of multi-sources, including contemporary magmatic rocks, metamorphic basement rocks, and mantle-derived magma. moreover, the ore-forming fluids are possibly mainly from magmatic hydrothermal and mantle with some precipitate water. acknowledgements sincerely give thanks to prof. guxian lv, dr. xin han, dr. tiegang li, dr. jingjing liu for their constructive comments in the process of writing the paper. we also thank xunyu zhang, yingchun zhang, xiao fan, qinglong huo, yaqing xu, jiancheng ren and zhifang liu for their help offered in the field work. this research was financially supported by the national natural science foundation of china (no. 40972061), and the research program of the shandong gold mining (ling long) co., ltd. references andre, l. and deutsch, s. 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(2003). metallogenesis of superlarge gold deposits in jiaodong region and deep processes of subcontinental lithosphere beneath north china craton in mesozoic. science in china series d: earth sciences 46, supplement 1, 14–25. 141 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 141 149 pressure-temperature path of arquía group rocks (nw colombia): a petrographic analysis from mineral assemblages yuly tatiana valencia-morales, luz mary toro toro, elvira cristina ruiz-jiménez, mario moreno-sánchez geological sciences department of universidad de caldas, calle 65 # 26-10, manizales, colombia. petrology based on the type locality mineral associations of the arquía group metamorphic rocks in colombia, the pressure-temperature conditions of metamorphism were obtained and the associated p-t path was built. the analysis of the mineral assemblages indicates a prograde regional metamorphism, which varies from low to medium grade and is represented by greenschist, epidote-amphibolite and amphibolite facies. the results that are based on the prograde mineralogy [actinolite + chlorite + albite + quartz + muscovite + epidote minerals for greenschist facies and hornblende + epidote minerals + garnet + albite ± quartz for epidote-amphibolite and amphibolite facies] indicate a constant increase of metamorphic conditions from 433°c/11.8 kbar to 696°c/14.4 kbar in the metamorphic peak. retrograde mineral assemblages [muscovite + chlorite + epidote minerals + albite + actinolite] indicate new metamorphic conditions between 417-357°c and 8.1-6.5 kbar. in addition, four deformation events and evidence of shear zone and intracrystalline deformation were found in these rocks. con base en las asociaciones minerales existentes en las rocas metamórficas del grupo arquía en su sección tipo en colombia, se determinaron las condiciones de presión-temperatura del metamorfismo y se construyó la trayectoria p-t que siguieron estas rocas. el análisis de las asociaciones minerales es consistente con un metamorfismo regional progrado de bajo a medio grado, representado por facies de esquistos verdes, facies epidota-anfibolita y facies anfibolita. las asociaciones minerales progradas [actinolita + clorita + albita + cuarzo + moscovita + epidota en facies de esquistos verdes y hornblenda + epidota + granate + albita ± cuarzo en facies epidota-anfibolita and anfibolita] indican un aumento constante en las condiciones del metamorfismo, desde 433°c/11.8 kbar hasta 696°c/14.4 kbar en el pico metamórfico. las asociaciones minerales retrogradas [moscovita + clorita + epidota + albita + actinolita] señalan nuevas condiciones metamórficas entre 417-357°c y 8.1-6.5 kbar. adicionalmente, estas rocas evidencian cuatro eventos de deformación, presencia de zonas de cizalla y deformación cristalina. palabras clave: complejo arquía, grupo arquía, asociaciones minerales, metamorfismo, trayectoria p-t. record manuscript received: 17/10/2012 accepted for publication: 14/11/2013 abstract resumen key words: arquía complex, arquía group, mineral assemblages, metamorphism, p-t path. introduction the arquía complex is a suite of lithodemes formed from rocks with different origins and diverse ages, forming a narrow and elongated belt that extends from the departamento de antioquia in colombia (north) to the provincia de oro in ecuador (south) (moreno-sánchez & pardo-trujillo, 2002, 2003; moreno-sánchez et al., 2008). in colombia, the complex is located at the western flank of the cordillera central, being bounded by the silvia-pijao fault in the east and by the cauca-almaguer fault in the west (maya & gonzález, 1995; pardo-trujillo & moreno-sanchez, 2001; moreno-sánchez & pardo-trujillo, 2002, 2003; nivia et al., 2006; moreno-sánchez et al., 2008). these authors proposed that the arquía complex is formed by medium to high pressure metamorphic rocks from igneous, sedimentary and metamorphic protoliths, although its origin, age and evolution is still unclear. because of the mentioned above and the dense system of faults that juxtapose the lithodemes, this complex has been the object of a wide discussion (moreno-sánchez & pardo-trujillo, 2003). with the aim of clarifying what the arquía complex is, in the latter study, it is proposed to be composed by paleozoic, mesozoic and cenozoic blocks that were affected by subduction, magmatism and shear processes. the lithodeme was first defined as an independent group by restrepo & toussaint (1976) and arias & caballero (1978); it was later redefined as part of the arquía complex by moreno-sánchez & pardo-trujillo (2003). the arquía group is formed by garnet amphibolites, greenschists and mica schists, from basic, volcano-sedimentary and pelitic protoliths, respectively, metamorphosed under greenschist and amphibolite facies conditions (restrepo & toussaint, 1976; arias & caballero, 1978; sánchez, 1988; ríos142 reyes, et al., 2008; marín, 2009; ruiz-jiménez, et al., 2012). a k/ar age analysis on a hornblende from an amphibolite by restrepo & toussaint (1976) indicating 110 ± 5 ma is interpreted as the age of metamorphism. the metamorphism is related not only to overthrusting and the obduction of oceanic crust (restrepo & toussaint, 1976; arias & caballero, 1978) but also to a subduction zone setting (sánchez, 1988). in a recent study by ruizjiménez, et al. (2012) based on geochemical analyses, it was concluded that these rocks originated from morb basalt protoliths and were formed in the early/middle cretaceous in a supra-subduction marginal ocean basin. in this study, it was analyzed the type section of one of the lithodemes of the arquía complex, called the arquía group, which is located in the valley of the arquía river between la pintada village (departamento de antioquia) and la felisa village (departamento de caldas, colombia; figure 1) (restrepo & toussaint, 1976; arias & caballero, 1978). here, it is presented the results obtained from a detailed mineral assemblage analysis of the different rocks of arquía group, based on optical microscopy. this work also provides the deduced metamorphic conditions and its associated p-t path. geology of the area the lithology of the area consists of greenschists, mica schists, quartzites, hornblende schists, amphibolites, garnet-amphibolites, serpentinites and porphyritic hypabyssal rocks with andesitic to dacitic composition (figure 1). metamorphic rocks show variable schistosity with westward dips and some local folds. these rocks also show shear zones, characterized by mylonitic foliation. from east to west, rocks appear in the area as follows: the eastern rocks correspond to greenschists, which have a well-developed schistosity s1 (figure 2a), with strikes from n45°e to n60°w, and dips westward, with angles ranging from 29° to 85°. the greenschists in their middle section are intercalated with layers of mica schists, such intercalations vary in thickness from millimeters to meters. in addition, the local intrusions of andesitic and dacitic subvolcanic bodies can be observed in these lithologies (figure 2b). at the western section of the greenschists, these are in transitional contact with hornblende schists. this contact is represented by a small increase in grain size and a slight variation in color. foliation in these rocks shows strikes ranging from n15 to 22°e and dips either 60°-80°nw or verticals (figure 2c). at their western part, the hornblende schists are in contact with serpentinites (figure 1). although it was not specifically observed, it is inferred to be a fault contact due to the presence of mylonitic foliation (n10°e/90°), which is defined by quartz and plagioclase porphyroclasts in the hornblende schists (figure 2d). marín (2009) also described a fault contact between these lithologies, with n15°e/54°nw fault plane, although the type of movement was not defined. at their western section, serpentinites are in contact with amphibolites, separated by a normal fault with a n36°e/65°nw plane (figure 2e). the amphibolites have a well-developed schistosity with a n31°e/52nw trend. in hand specimen, it is possible to observe red garnets up to 6 mm in size (figure 2f). these rocks are in contact with mica schists through a n35°w/75°sw sinistral fault (marín, 2009) (figure 2g) at their western part. mica schist are green or black (figure 2h) in color, depending on the modal percentage of graphite. these rocks have a well-developed schistosity (n35°w-n28°e/50-69°w). quartzites and anthophyllite schists are also present in this zone, although they do not appear on the map (figure 1) because of the scale. quartzites outcrop as schistosity-concordant layers up to 15 cm in thickness and alternate with mica schists. anthophyllite schists are restricted to hornblende schists, and they appear as anastomosed irregular lenses, with thicknesses between 4 and 11 cm, composed of translucent and fibrous minerals parallel to schistosity (figure 2i). methodology petrographic observations and thermodynamic modeling have made it figure 1. lithological units of arquía group in its type section. note the contacts among them. anthophyllite schists and quartzites are much small to appear in this scale. yuly tatiana valencia-morales, luz mary toro toro, elvira cristina ruiz-jiménez, mario moreno-sánchez 143 possible to determine the pressure and temperature conditions that occurred at the time t of the metamorphic processes and the construction of p-t paths from the mineral assemblages found in metamorphic rocks at equilibrium (e.g., apted & liou, 1983; guiraud et al., 1990; pattison, 2002; tibaldi et al., 2007; abbott & bandy, 2008; zhang et al., 2009; goswami et al., 2009; aoki et al., 2009; blanco-quintero et al., 2010). in this study, 18 thin sections from the arquía group were analyzed with the aim of determining the metamorphism conditions and the p-t paths of their rocks. the interpretations are based on the following: 1. the mineral assemblages at equilibrium found in greenschists, hornblende schists, garnet amphibolites, and amphibolites , and 2. the temperatures and pressures reached for similar mineral assemblages, as reported by blanco-quintero et al., 2010 (table 1). the petrographic analysis was performed using a nikon eclipse e200 trinocular polarizing microscope with transmitted illumination. the mineral abbreviations correspond to those recommended by siivola & schmid (2007). petrography this section describes the mineral assemblages and the textures observed in the metamorphic rocks of the arquía group as well as a likely protolith for each one. the modal percentages and mineral assemblages are shown in tables 2 and 3, respectively. greenschists the rocks correspond to actinolite schists and chlorite schists (figure 3a and 3b) with mineral assemblages as follows: actinolite + chlorite + quartz + plagioclase (albite) + epidote minerals ± muscovite ± calcite. titanite, zircon, opaque minerals and fe and ti oxides are also present as accessories. these rocks were formed in greenschist facies. their protoliths correspond to volcano-sedimentary rocks and marls. the textures in the rocks are nematoblastic and granoblastic. the former is defined by an actinolite orientation and the latter by an inequigranular mosaic of epidote, plagioclase and quartz (figure 3a and 3b). hornblende schists the rocks have two different mineral assemblages. the first one corresponds to epidote-amphibolite facies, represented by hornblende figure 2. rocks and contact among them in arquía group type section. a. greenschist hand sample. b. hypabyssal body intruding mica schists. c. hornblende schist hand sample. d. plagioclase and quartz porphyroblasts defining mylonitic foliation (n10°e/90°) in hornblende-schist. e. faulted contact between the serpentinites to the east and the amphibolites to the west (fault plane n36°e/65°nw). f. garnet amphibolite hand sample. g. faulted contact between the amphibolites to the east and the mica schists to the west. h. mica schist hand sample. i. irregular lens of anthophyllite schists into hornblende schists. pressure-temperature path of arquía group rocks (nw colombia): a petrographic analysis from mineral assemblages 144 + epidote group minerals + quartz + albite ± garnet (figure 3c). the second one shows the greenschist facies, represented by chlorite + actinolite + epidotes + quartz (figure 3d). zircon, opaque minerals, titanite and fe oxides are present in the rock as accessory minerals. protoliths of hornblende schists correspond to basic and volcano-sedimentary rocks. the textures found in the rocks are nematoblastic, granoblastic and porphyroblastic. the first texture is defined by hornblendes, the second one by plagioclase, quartz, and the epidote group, and the last one by garnet (figure 3c). amphibolites and garnet amphibolites amphibolites and garnet amphibolites present two types of paragenesis. the first one corresponds to amphibolite facies (figure 3e), defined by hornblende + plagioclase ± garnet ± quartz. the second one corresponds to greenschist facies (figure 3f ), represented by chlorite + muscovite + epidotes+ plagioclase (albite) + quartz + actinolite. calcite, fe oxides and opaque minerals are present in the rocks as accessory minerals. the protolith of these rocks corresyuly tatiana valencia-morales, luz mary toro toro, elvira cristina ruiz-jiménez, mario moreno-sánchez 145 ponds to basic rocks. the existent textures include nematoblastic, granoblastic, porphyroblastic and poikiloblastic. the nematoblastic texture is defined by its preferential orientation of hornblende (figure 3e). the granoblastic texture is determined by plagioclase. the porphyroblastic texture is described by garnet and some plagioclase (figure 3e). the poikiloblastic texture is defined by the inclusion of epidote minerals, titanite, muscovite, hornblende and garnet in the plagioclase (figure 3g). mica schists these rocks correspond to those whose principal components are mica minerals in the absence of actinolite and epidote. they are classified as quartz-muscovite-chlorite schists. their mineral assemblage is quartz + graphite + muscovite + chlorite + plagioclase (albite) ± biotite. tourmaline, calcite, opaque minerals and fe oxides are also present as accessory minerals. this mineral association is typical of greenschist facies (figure 3h and 3i). these rocks have a pelitic protolith. additionally, muscovite, chlorite, and graphite minerals define the lepidoblastic texture. quartz, plagioclase and calcite define the granoblastic texture. anthophyllite schists these rocks were classified as anthophyllite-quartz-hornblende schists. the mineral assemblage corresponds to anthophyllite + hornblende + quartz + plagioclase + chlorite + epidote minerals, with titanite and calcite as accessory minerals. these rocks likely have a basic protolith. textures in these schists are nematoblastic, defined by anthophyllite and hornblende, and granoblastic, defined by quartz, plagioclase and epidotes (figure 3j). quartzites the mineral assemblage in these rocks corresponds to quartz + plagioclase ± muscovite ± chlorite, with biotite and zircon as accessory minerals. these rocks have a granoblastic texture with a domain of quartz and plagioclase of different shapes and sizes, showing evidence of recrystallization. the lepidoblastic texture is also observed due to the orientation of mica minerals. the possible protolith of these rocks is a quartz-feldspathic sedimentary rock, likely a sandstone. microstructures the mineral assemblages and textures in the rocks allow for the interfigure 3. photomicrographs of the rocks present in the area. a. actinolite schist with nematoblastic and granoblastic textures. b. chlorite schist with lepidoblastic and granoblastic textures. c. prograde mineralogy (hda+ep+qtz+ab+grt) in hornblende schist and nematoblastic, granoblastic and porphyroblastic textures. d. hornblende schist with retrograde mineralogy (chl+act+ep+qtz). e. prograde mineralogy (hda+grt+pl±qtz) of garnet amphibolites and nematoblastic and porphyroblastic textures. f. retrograde mineralogy (chl+ms+ep+pl[albita]+qtz+act) in garnet amphibolites. g. poikiloblastic texture in garnet amphibolites. h. anthophyllite schist, nematoblastic and granoblastic textures. i. mica schist and lepidoblastic and granoblastic textures. j. three deformation phases that affected mica schists. pressure-temperature path of arquía group rocks (nw colombia): a petrographic analysis from mineral assemblages 146 pretation of the relative sequence of metamorphic and deformation events. consequently, four deformation phases were found using the relationship between the fabric elements and the minerals that define them. deformation phases: four deformation phases with the same number of deformation events were identified. three phases are compressional, and one is extensional. the first deformation phase (s1) is indicated by schistosity (it is dipping to the west). this deformation phase is defined by actinolite, chlorite and muscovite in the greenschists (figures 3a y 3b), by hornblende in the hornblende schists (figure 3c) and the amphibolites (figure 3e), by chlorite, muscovite and graphite in quartz-muscovite-chlorite schists or mica schists (figures 3h and 3i), and by hornblende and anthophyllite in the anthophyllite schists (figure 3j). in addition, some minerals of columnar habit (epidote group minerals) and elongated quartz crystals are parallel to schistosity, suggesting that their growth happened simultaneously with the deformation. the second deformation phase (s2) is manifested in all the rocks by the folding of schistosity, which is macroscopically observed (see figures 2f and 2h). microscopically, this phase is also well-recorded in quartz-muscovite-chlorite schists (mica schists). in these rocks, s2 is observed as crenulation foliation and/or fracture cleavage that developed from s1 (figure 3i). notably, the quartz is found in the hinges and not in the flanks of the folds developed by s2, suggesting that this deformation phase was accompanied by the dissolution and recrystallization of this mineral. moreover, muscovites are bent in the hinges of folds. microsopically, this deformation phase also was recorded in the hornblende schists. the development of a third deformation phase (s3) is observed in quartz-muscovite-chlorite schists (mica schists). s3 is represented by the folding of flanks of crenulation foliation (s2) and defined by graphite and bent muscovites (figure 3j). a fourth deformation phase (s4) is observed in hand specimen of greenschists by the presence of quartz-plagioclase boudins; this deformation event is extensional in character. textures of shear zone and other intracrystalline deformation the arquía group rocks indicates that they were affected by shearing, as is evidenced by the presence of brittle shear zones manifested by microfaults, intensely sheared zones, broken crystals, and s-c fabrics. these rocks also show ductile shear zones, which are indicated by the formation of porphyroclasts with pressure shadows. the rocks present undulose extinction, deformation twinning, and evidence of dynamic recrystallization processes, including grain boundary migration, subgrain rotation recrystallization in quartz and plagioclase, and stylolitic surfaces in calcite. discussion prograde and retrograde metamorphism the metamorphic rocks in the arquía valley were affected by prograde metamorphism, which was followed by a retrograde event after reaching maximum temperature and pressure conditions. this statement is based on the following observations: the relationships among the various types of rocks that form the arquía group are consistent with prograde metamorphism. this is shown by an increase in the metamorphic grade from east to west (figure 1). thereby, in the arquía group type section, the greenschists change in a transitional form to hornblende schists westward, and the amphibolites appear further to the west. it should be noted that a serpentinized peridotites body is located between the hornblende schists and the amphibolites; however, these three rock-bodies are separated by faults. (figures 1 and 2e). then, a progressive metamorphism in these rocks is marked by the passing of greenschist facies mineralogy [actinolite + chlorite + quartz + plagioclase (albite) + epidote minerals ± muscovite ± calcite] to epidote-amphibolite facies mineralogy [hornblende + epidote minerals + quartz + albite ± garnet], and finally to amphibolite facies mineralogy [hornblende + garnet + plagioclase ± quartz]. the prograde metamorphism was followed by a retrograde metamorphic event that affected the mineral associations formed at the metamorphic peak. this is supportd by the mineralogy of greenschist facies that is found among the mineral assemblages of the amphibolite and epidote-amphibolite facies, such as chlorite + actinolite + epidote minerals + quartz (mineralogy of greenschist facies), between hornblende + epidote minerals + quartz + albite ± garnet (mineralogy of epidote-amphibolite facies) in the hornblende schists (figures 4d), and the chlorite + muscovite + epidote minerals + plagioclase (albite) + quartz (mineralogy of greenschist facies) between the hornblende + garnet + plagioclase ± quartz (amphibolite facies) in the garnet amphibolites (figure 3f ). in addition, there is textural evidence of retrograde metamorphism in amphibolites (sample arq59a1 in figure 1), such as the presence of muscovite, epidote minerals, hornblende and garnet as inclusions into the plagioclase (figure 3g). these inclusions could represent mineral phases in formation [muscovite and minerals of epidote group] and previous mineral phases that were not fully consumed [garnets with very rounded (figure 3g) and/or completely irregular boundaries]. figure 4. pressure-temperature path diagram of arquía group in its type section. diagram adapted from blanco-quintero et al. (2010). metamorphic conditions to define the intensive variable temperature and pressure values that affect the arquía group rocks, mineral assemblages of these rocks were compared with data obtained by blanco-quintero et al. (2010) (table 1). yuly tatiana valencia-morales, luz mary toro toro, elvira cristina ruiz-jiménez, mario moreno-sánchez 147 metamorphic phases metamorphic assemblages t(°c) p(kbar) greenschist facies prograde metamorphism act+chl+ep+ms+ab+qtz 433 11.8 amphibolite facies grt+amp+ep+pl+qtz 696 14.3 greenschist facies retrograde metamorphism act+chl+ep+ms+qtz 417 8.1 act+chl+ms+ab+qtz 357 6.5 table 1. mineral assemblages and temperatures and pressures calculated for la corea mélange rocks by blanco-quintero et al. (2010). table 2. location of samples taken in the type section of arquía group and mineral assemblages for each one. lithology sample location mineral assemblage n w prograde retrograde g re en sc hi st s arq 48a 5° 31’ 1,52’’ 75° 34’ 45,73’’ act+qtz+chl+ab +ep. arq 49 5° 31’ 2,6’’ 75° 34’ 48,18’’ act+chl+qtz+ab +ep. arq 51 5° 31’ 3,86’’ 75° 34’ 48,76’’ act+chl+qtz+ab+ep. arq 52 5° 31’ 6,13’’ 75° 34’ 58,48’’ chl+ms+qtz+ab+ +ep. arq 53 5° 31’ 7,43’’ 75° 35’ 0,35’’ act+qtz+chl+ab+ep. h or nb le nd e sc hi st s arq54 5° 31’ 7,14’’ 75° 35’ 1,68’’ hbl+ep+grt+qtz+ab chl+ep+qtz arq55 5° 31’ 7,68’’ 75° 35’ 2,47’’ hbl+ep+ab act+chl+ep+qtz. arq55a 5° 31’ 7,68’’ 75° 35’ 2,47’’ hbl+ep+ab act+chl+czo+ qtz. arq56 5° 31’ 6,53’’ 75° 35’ 2,33’’ hbl+ep+ab act+ep+chl. a m ph ib ol it es a nd g ar ne t am ph ib ol it es arq59a 5° 31’ 5,84’’ 75° 35’ 6,65’’ hbl+grt+pl ms+chl+ ep+ab+act. arq59a1 5° 31’ 5,84’’ 75° 35’ 6,65’’ hbl+grt+pl ms+chl+ep+ab. arq59b 5° 31’ 5,84’’ 75° 35’ 6,65’’ hbl+grt+pl ms+chl+ep+ab. arq60 5° 31’ 4,63’’ 75° 35’ 10,42’’ hbl+grt+pl arq61 5° 31’ 4,70’’ 75° 35’ 10,50’’ hbl+pl ep anthophyllite schist arq57 5° 31’ 7,32’’ 75° 35’ 2,36’’ ath+hbl+chl+qtz quartzite arq60b 5° 31’ 4,63’’ 75° 35’ 1,42’’ qtz+ab+ms+chl+bt m ic a sc hi st s o r qu ar tz -m us co vi te ch lo ri te sc hi st s arq48 5° 31’ 1,52’’ 75° 34’ 45,73’’ qtz+ms+chl+gr+ab. arq50 5° 31’ 2,53’’ 75° 34’ 48,25’’ ms+gr+chl+qtz+ms+ab arq60a 5° 31’ 4,63’’ 75° 35’ 10,42’’ qtz+ms+chl+bt+gr+ab. arq62,0 5° 31’ 5,41’’ 75° 35’ 13,31’’ qtz+ms+chl+cal+ab. arq62 5° 31’ 5,66’’ 75° 35’ 13,49’’ qtz+ms+chl+ms+ab+gr. pressure-temperature path of arquía group rocks (nw colombia): a petrographic analysis from mineral assemblages 148 p-t conditions of prograde metamorphism: blanco-quintero et al. (2010) (table 1) considered the equilibrium point of the act+chl+ep+ms+ab+qtz association at 433°c/11.8 kbar. this mineral assemblage was observed in the greenschists (sample arq53; figure 1; table 2) with an act+qtz+chl+ab+ep±ms association. in addition, these authors considered the grt+amp+ep+pl+qtz mineral assemblage equilibrium point to be at 696°c/14.4 kbar. this assemblage was observed in hornblende schists and garnet amphibolites with the grt+hbl+ep+pl+qtz association (sample arq54; figure 1; table 2), whereas the amphibole is represented in the arquía group rocks by hornblende. p-t conditions of retrograde metamorphism: mineral assemblages act+chl+ep+ms+qtz and act+chl+ms+ab+qtz (blanco-quintero et al., 2010) are in equilibrium at 417°c/8.1 kbar and 357°c/6.5 kbar, respectively. these correspond to retrograde mineral assemblages that are present in hornblende schists and garnet amphibolites (sample arq59a; figure 1), where the retrograde mineral association is ms+chl+ep+ab+act. accordingly, during the prograde metamorphism that affected the arquía group, the rocks with mineral assemblages formed under greenschist facies conditions reached temperatures of 433°c and pressures of 11.8 kbar, whereas those with mineral assemblages formed under epidote-amphibolite and amphibolite facies reached values of 696°c and 14.5 kbar; the latter values correspond to metamorphic peak conditions. the garnet amphibolites and hornblende schists showed a retrograde metamorphic event at the greenschist facies. this event reached temperatures between 417 and 357°c and pressures between 8.1 and 6.5 kbar. plotting the values mentioned above in a p-t diagram (figure 4), the arquía group rocks follow a clockwise trajectory. this graphic shows a constant increase in temperature and pressure until the metamorphic peak at 696°c and 14.5 kbar. the maximum conditions are followed by a decrease in temperature and pressure, which is shown by mineral assemblages that were retrograded from amphibolites and epidote-amphibolite facies towards greenschist facies. conclusions the rocks of the arquía group at its type section show an increase of metamorphic grade from east to west. this is manifested by transitional changes from greenschists to hornblende schists and subsequently to amphibolites. the mineral assemblages that record s1 show a prograde metamorphism event with a continuous increase in temperature and pressure. this behavior is initially expressed in the mineral assemblages of greenschist facies passing for epidote-amphibolite facies, and continues until the maximum pressure and temperature conditions for amphibolite facies are reached. the metamorphic peak was followed by one retrograde event, which manifested with the decrease in metamorphic conditions. this caused the development of greenschist facies assemblages from epidote-amphibolite and amphibolite facies mineralogy. the metamorphic rocks of the arquía group record a clockwise p-t path, with initial conditions of 433°c and 11.8 kbar and a constant increase in temperature and pressure until values of 696°c and 14.4 kbar were reached. after this, there was a period of decompression and constant cooling until conditions of 417–357°c and 8.1-6.5 kbar were reached. the p-t path shows that the conditions that followed to the metamorphic peak caused a retrograde metamorphic event. the arquía group in the arquía river valley recorded four deformation events with the same number of deformation phases. three of these events are compressional, and the last one is extensional. in addition, evidence of brittle and ductile shear zones and intracrystalline deformation is commonly found in these rocks. acknowledgments this study was carried out on the framework of a project between universidad de caldas and colciencias (administrative department of science, technology and innovation in colombia), called petrography and geochemistry of the arquía complex rocks between antioquia and caldas departments, colombia. comments from hugo murcia are gratefully acknowledged. melody humphreys, from the university of auckland, revised the english. comments from two anonymous reviewers are greatly appreciated. references arias, l. a. and caballero, j. h. (1978). petrología metamórfica del grupo arquía, bsc thesis, departamento de ciencias de la tierra, universidad nacional de colombia, medellín, colombia. lithology modal percentages (%) greenschists act (22-26%) + qtz (21-30%) + chl (17-26%) + ab (2-5%) + czo (4-14%) + ep (1-5%) + zo (3-11) ± ms (2-20%) ± cal (4-15%). hornblende schists hbl (29-48%) + czo (2-11%) + zo (5-13%) + ep (2-8%) ± grt (0-16%) + qtz (3-29%) + ab (532%) + chl (5-11%) + act (1-8%). amphibolites and garnet amphibolites hbl (27-44%) + grt (0-14%) + pl (18-23%) ± qtz (4-12%) ± chl (4-19%) ± ms (0-2%) ± ep (0-2%) ± zo (0-2%) ± czo (1-6%) ± act (0-2%) ± ttn (2-5%). anthophyllite schist ath (33%) + hbl (12%) + czo (7%) +zo (6%) + chl (6%) + qtz (29%)+ pl (6%). quartzite qtz (82%) + ab (5%) + ms (8%) + chl (3%) +bt (2%) mica schists qtz (22-38%) + ms (16-28%) + chl (5-26%) + gr (2-32%) + ab (2-7%) ± bt (0-7%). table 3. modal percentages in the rocks of the arquía group in its type section. yuly tatiana valencia-morales, luz mary toro toro, elvira cristina ruiz-jiménez, mario moreno-sánchez 149 abbott, r. and bandy, b. 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(2008). una contribución al estudio de la evolución tectono-metamórfica de las rocas de alta presión del complejo arquía, cordillera central, andes colombianos, geología colombiana. 33, 3-22. ruiz-jiménez, e. c., blanco-quintero, i. f., toro. l., moreno-sánchez, m., vinasco, c. j., garcía-casco, a., morata, d. and gomez-cruz, a. (2012). geoquímica y petrología de las metabasitas del complejo arquía (municipio de santa fé de antioquía y río arquía, colombia): implicaciones geodinámicas. sánchez, l. (1988). nuevos aspectos petrológicos del grupo arquía. universidad de caldas, departamento de ciencia geológicas. special publication no. 2. siivola, j. and schmid, r. (2007). systematic nomenclature for metamorphic rocks: list of mineral abbreviations, recommendations by the iugs subcommission on the systematics of metamorphic rocks, http://www.bgs.ac.uk/scmr/docs/papers/paper_12.pdf, web version 01/02/07 (last accessed june 2012). tibaldi, a., otamendi, j. and demichelis, a. (2007). evolución metamórfica de granulitas piroxénicas asociadas a los complejos máficos sol de mayo y suya taco, norte de la sierra de comechingones, córdoba, revista de la asociación geológica argentina. 62, no. 2, 175-186. zhang, l., wang, q. and song, s. (2009). lawsonite blueschist in northern qilian, nw china: p-t pseudosections and petrologic implications, journal of asian earth sciences. 35, 354-366. pressure-temperature path of arquía group rocks (nw colombia): a petrographic analysis from mineral assemblages palabras clave: roca fracturada; carga dinámica; vida a la fatiga; curva s-n. how to cite item li, r., jian, w., kang, r. (2018). study on fatigue life of fractured rock under constant amplitude cyclic loading. earth sciences research journal, 22(4), 275-280. doi: https://doi.org/10.15446/esrj.v22n4.77379 the electro-hydraulic servo loading system was applied to make cyclic loading tests for fractured rock-like specimens. according to the results of the fatigue test, the power function model was used to fit and analyze the test data, and the fitting degree of the curve was determined by the correlation coefficient, and the result was fitted well. the main factors influencing the fatigue life of specimens were analyzed. the upper limit stress was the most important factor affecting the fatigue life of specimens. under the same conditions, the specimen had a shorter fatigue life when being applied by higher upper limit stress; the rock of different types and strengths not only had different fatigue life, but also had the different sensitive degrees between the fatigue life and the upper limit stress; the fatigue life of specimens was significantly influenced due to different initial damages. abstract keywords: fractured rock; dynamic loading; fatigue life; s-n curve. study on fatigue life of fractured rock under constant amplitude cyclic loading estudio sobre la vida a la fatiga de rocas fracturadas bajo cargas cíclicas de amplitud constante issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.77379 el sistema electrohidráulico de carga servo se aplicó para realizar pruebas de carga cíclicas para muestras de roca fracturada. de acuerdo con los resultados de la prueba de fatiga (resistencia a la fatiga), el modelo de función de potencia se utilizó para ajustar y analizar los datos de la prueba; el grado de ajuste de la curva se determinó mediante el coeficiente de correlación, y se ajustó el resultado. se analizaron los principales factores que influyen en la resistencia a la fatiga de las muestras. la tensión límite superior fue el factor más importante que afectó la vida a la fatiga (resistencia de fatiga) de las muestras. en las mismas condiciones, el espécimen tuvo una vida a la fatiga más corta cuando se aplicó por un límite superior de tensión; la roca de diferentes tipos y fuerzas no solo tuvo una vida a la fatiga diferente, sino que también tuvo diferentes grados de sensibilidad entre la vida a la fatiga y la tensión límite superior; la vida a la fatiga de los especímenes fue significativamente influenciada debido a diferentes daños iniciales. resumen record manuscript received: 01/08/2018 accepted for publication: 08/10/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 275-280 run li1*, wenbin jian2, rongtao kang3 1engineering college, fujian jiangxia university, fuzhou 350108, china; 2college of environment and resources, fuzhou university, fuzhou 350116, china; 3fuzhou architecture design institute, fuzhou 350011, china *corresponding author: lr52tkd25@126.com g e o t e c h n ic a l e n g in e e r in g 276 run li, wenbin jian, rongtao kang introduction many defects exist in the natural rock masses, such as: fractures, joints, etc. in the process of rock engineering construction and service, there are not only static loading, but also dynamic loading, especially the rock engineering along the highway, railway and bridge. due to the long-term impact from traffic fatigue loading, the internal stress field of rock is changing, the rock strength is continuously attenuated with the accumulation of fatigue damage, and long-term safety and stability of rock engineering are greatly impacted. with the improvement of people’s requirements on the safety and durability of rock engineering, the scholars at home and abroad conducted relevant researches on the fatigue of rock. xiurun (1987, 1992, 2003) researched the irreversible deformation and fatigue property of rock under cyclic loading, and put forward fatigue threshold value at the same time. yanbo, xing and guojie (2017)researched the stress-strain entire process curve, micro structure change, macro fracture characteristics and other fatigue damage characteristics of gypsum rocks under the cyclic loading by using acoustic emission system and environmental scanning electron microscope. erarslan and williams (2012) researched the fatigue damage mechanism of tuff under two different types of cyclic loading methods by using sem. momeni, karakus and khanlari (2015) researched the mechanical property of granite under cyclic loading. according to the existing researches, the strength, deformation, fracture characteristics and failure mechanism of rock materials under cyclic loading are obviously different from that of static loading. compared with other materials, the researches on rock fatigue are relatively limited at home and abroad, and rock fatigue is still one of the important lucubrate problems in rock engineering. in this paper, the constant amplitude fatigue test was applied to research the fatigue life of rocklike fractured specimens, and the suitable fatigue life models were selected. meanwhile, the main influencing factors of fatigue life were analyzed. the research results were helpful to the prediction of rock engineering life and design of fatigue resistance under fatigue loading. test overview rocklike specimens as the natural rock has defects without specific laws, there are great dispersion and uncertainty in the rock fatigue test, and thus, it is difficult to achieve the purpose of test. through the indoor similar tests, the way of researching the strength, deformation and failure mechanism of rocks under certain structural characteristics is very effective to study the fractured rocks (haeri, shahriar and marji, 2014a, b; cao, liu and pu, 2015; zhou, cheng and feng, 2014). in recent years, indoor simulation tests have been widely used (cheng-zhi and ping, 2012; jinjin, ping and chengzhi, 2014; shilei, junsheng and feng, 2016). in this test, barite powder, river sand (particle size was less than 0.5 mm), 32.5rcomposite portland cement, model gypsum, borax solution with concentration of 1% were used as similar materials, and the rocklike specimens were made according to the weight ratio, barite powder: river sand, cement, gypsum, borax solution = 42.0:21.5:8.5:12.7:15.3. the dimensions of specimens were 70mm×70mm×140mm, while the thin metal plate was used to make pre-facture, the length of fracture was 20mm, and the angles of fracture were set to 0°, 30°, 45°, 60° and 90°. the fractured specimens were shown in figure 1. test methods the electro-hydraulic servo fatigue testing machine was applied to load the fractured specimens, and constant amplitude loading control method was used in the test. the loading frequency was 5hz, and sine wave was used as loading waveform. the characteristic parameters of loading waveform were shown in figure 2. of which, was the upper limit stress of cyclic loading; was lower limit stress of cyclic loading; , was the loading amplitude; t was cycle time, , was the frequency. the amplitude range of loading was 1.9 ~ 2.4 mpa according to the uniaxial compressive strength of specimens. fatigue curve analysis s-n curve fatigue life s-n curve (also called as wohler curve) refers to the relationship curve between stress level sand material fatigue life n, which is mainly used to estimate and evaluate the fatigue life or fatigue strength of materials. in practical engineering, the stress level should be the upper limit of stress smax, n represents fatigue life, the absolute fatigue life log n or logarithmic fatigue life log n could be chosen. the fatigue life mentioned here refers to the required stress cycle-index n when the materials reach the fatigue failure under the cyclic loading. according to the characteristics of the relationship between stress level and material fatigue life of s-n curve, the domestic and oversea researchers put forward many empirical models on the basis of experiments, and there are many commonly used forms as follows: exponential function model: n e cs⋅ = (1) figure 1. fractured specimens 277study on fatigue life of fractured rock under constant amplitude cyclic loading in equation,  and c are material constants. both sides of equation (1) take the logarithm, then: lg lgn s cα+ = (2) a c= lg / , b = l/  are set, the equation (2) can be simplified as: s a b n=  lg (3) therefore, s-n exponential function model represented by equation (3) is a straight line in semi-logarithmic coordinate system. power function model: s n c = (4) in equation,  and c are material constants. both sides of equation (4) take the logarithm, then: α +lg lg lgs n c= (5) a c= lg / , b = l/  are set, the equation (5) can be simplified as: lg lgs a b n=  (6) therefore, s-n power function model represented by equation (6) is a straight line in double logarithmic coordinate system. (3) power function model of three parameters: s s n cf− α 1( ) = (7) in equation,  and c are material constants. s f 1 represents the normalized value of rock fatigue limit, and it is obtained according to the ratio between the fatigue limit and the static strength of rock. s f 1 is the material constant related to the rock property. both sides of equation (7) take the logarithm, then: α − +lg lg lgs s n cf 1( ) = (8) a c= lg / , b = l/  are set, the equation (8) can be simplified as: lg lgs s a b nf 1( ) = (9) in practical engineering, the s-n curve is generally fitted to the indoor fatigue test data according to the corresponding model. fatigue curve fitting and analysis of rocklike specimens in this paper, the test data was fitted according to equation (6), the fitting degree of curve was judged by the correlation coefficient r2 of the fitting curve. the fitting results are shown in figure 3, and the curve data is shown in table 1. figure 3 and table 1 show: (1) under the fatigue loading, if the upper limit stress of complete and fractured rocklike specimens was higher, the fatigue life of specimens was shorter; if the upper limit stress was lower, the fatigue life of specimens was longer. (2) in terms of the slope of the s-n fitted curve, when the slope was smaller, the specimen fatigue life affected by the loading stress was more sensitive, the significant change of fatigue life was caused by the slight change of loading stress. therefore, if the slope of s-n fitted curve was smaller, the deviation range in the prediction of fatigue life should be larger; if the slope was larger, the deviation range in the prediction of fatigue life should be smaller. (3) in the process of specimens preparation and test, errors were inevitable. therefore, under the same upper limit stress conditions, the fatigue life of rocklike specimens with same fracture dip was not exactly the same. the fatigue life discreteness of some rocklike specimens was relatively large, but from the fitting results of the test data, the correlation coefficient r2 was 0.85 above. it indicated that the fitting degree of s-n model and test data was relatively good. in practical engineering, s-n fitted curves are widely used in the researches of material fatigue life due to its clear concept, simple form and convenient measurement. under the condition of constant amplitude loading, the fatigue life of rock can be estimated by using interpolation method in s-n curves. influence factors of fatigue life on the one hand, the fatigue damage of the rock under fatigue loading is related with rock itself (rock type, strength, defects, etc.); on the other hand, it is affected by test conditions (stress amplitude, waveform, frequency, figure 2. schematic diagram of periodic loading waveform table 1. fatigue life s-n curve table of rocklike specimens fractured dip curve parameters s-n curve correlation coefficient r2 a b complete 0.75798 0.09615 lg s = 0.75798 0.096151g n 0.90071 0° 0.73887 0.09489 lg s = 0.73887 0.094891g n 0.91066 30° 0.63917 0.07931 lg s = 0.63917 0.079311g n 0.85416 45° 0.55511 0.06348 lg s = 0.55511 0.063481g n 0.90898 60° 0.62014 0.07422 lg s = 0.62014 0.074221g n 0.85370 90° 0.76277 0.10267 lg s = 0.76277 0.102671g n 0.91967 278 run li, wenbin jian, rongtao kang etc.). due to the complex geological conditions and different environment of natural rock mass, the influence factors of fatigue life are more complicated, and there is relatively large discreteness in fatigue life. in this paper, only the main influencing factors involved in the test were analyzed. influence of upper limit stress the fatigue life of specimens is greatly affected by the upper limit stress. the previous studies have shown that the upper limit stress is the first factor influencing the fatigue life (xiufeng, 2008). the cyclic loadings of different upper limit stress on the rocklike specimens were made, and the fatigue life of typical specimens was shown in table 2. it was indicated in table 2 that under the same conditions, if the upper limit stress of fatigue test was larger, the fatigue life of specimens was shorter; conversely, if the upper limit stress was smaller, the fatigue life of specimens was longer. generally speaking, if the strength of the material is higher, the external force is lower and the fatigue life of specimens is longer (yu, 2003). here, the fatigue threshold value of the material should be considered, the fatigue threshold value refers to the stress threshold of the fatigue failure of rock. ge xiurun put forward, whether the rock was damaged or not, it was related to the stress threshold value, and he also thought, rock fatigue threshold value was close to conventional yield value (xiurun, 1987). in the fatigue test, if the upper limit stress is lower than the threshold of fatigue, the specimens could not reach the fatigue failure. if the upper limit stress is too close to the peak stress and the properties of (a) complete specimen (b)  = 0° (c)  = 30° (d)  = 45° (e)  = 60° (f)  = 90° figure 3. s-n fitted curve of typical complete specimens and fractured specimens. 279study on fatigue life of fractured rock under constant amplitude cyclic loading the specimen are unstable, the failure rate of test will be increased. in this test, the upper limit stress ratio 0.76 ~ 0.90 was selected, and after the test proved, this range was relatively suitable. influence of rock type and strength due to different formation causes of rocks, the mineral composition, content and other aspects are different, so that different types of rocks have differences in their natures. even the same type of rocks, their material composition, structure and construction are different under different internal and external dynamic geological processes, and then, the rock strengths are different. xiufeng (2008)made fatigue test researches on grayish yellow sandstone, and concluded the s-n relationship of grayish yellow sandstone fatigue strength under cyclic loading: smax = 1.326 0.109 log nf r = -0.997 in equation, smax = max/fc was upper limit stress ratio; nf was fatigue life. the fitted curve s-n was shown in figure 4. yu (2003) conducted a fatigue test study on red sandstone and concluded the regression equation: smax = 1.04333 0.05978 log nf r = -0.931 in equation, s = max/c was upper limit stress ratio; n was fatigue life. the fitted curve s-n was shown in figure 5. figure 4. grayish yellow sandstone fatigue s-n curve under cyclic loading (xiufeng, 2008) figure 5. red sandstone fatigue s-n curve under cyclic loading (yu, 2003) figure 4 and figure 5 showed both grayish yellow sandstone and red sandstone were called as sandstone; however, due to different internal mineral composition, content and strength of rocks, the slopes of fatigue life s-n curves were different. the slope of fatigue life s-n curves reflected the sensitive relationship between the fatigue life of specimens and upper limit stress ratio. it was indicated, if the slope was smaller, the change of fatigue life to upper limit stress ratio was more sensitive; on the contrary, it was less sensitive. figure 4 indicated that the slope of grayish yellow sandstone s-n curve was relatively large, but the figure 5 illustrated that the slope of red sandstone was relatively small. it was indicated, with the change of stress, the sensitive degree of fatigue life of grayish yellow sandstone was lower than that of red sandstone. it could be seen that different types and strengths of rocks not only had different fatigue life, but also had different sensitive degree between fatigue life and maximum stress. impact of initial damage the initial damage indicates the loading history of the rock, and it is the damage state of the rock before fatigue loading. even the same type of rock, if the environment is different, the initial damage is also different. in this test, the preparation process and curing conditions of each specimen were consistent, the small discrepancies in the specimen preparation were not considered as the key points; therefore, pre-fracture was used as the initial damage of rocklike specimen. it was found in the test, different dips of pre-factures had the significant effect on the fatigue life of rocklike specimens. figure 6 illustrated the fatigue life changes of rocklike specimens with different dips when the loading amplitude, the curves were the average fatigue life line of rocklike specimens with different dips. table 2. constant amplitude fatigue life of typical specimens facture dip upper limit stress smax / mpa rmax fatigue life nf / time average life n f /time 30° 2.0 0.81 13100 15500 18200 19000 20800 17320 2.1 0.85 8100 8600 9100 10300 13100 9840 2.2 0.89 5100 5200 5800 6600 8000 6140 45° 1.9 0.79 14300 19100 21200 24400 27000 21200 2.0 0.83 8900 9500 10200 11800 12300 10540 2.1 0.88 3700 4500 4900 5600 6100 4960 60° 2.0 0.80 14600 16800 17000 20700 21100 18040 2.1 0.84 8500 9500 11600 12000 13100 10940 2.2 0.88 3900 5800 6300 6600 7500 6020 note: rmaxwas upper limit stress ratio, equal to the upper limit stress to the uniaxial compressive strength of the specimens, rmax = smax/c 280 run li, wenbin jian, rongtao kang figure 6. curves of rocklike specimen fatigue life under same loading amplitude with the change of fracture dips figure 6 illustrated: according to the fatigue life data of specimens tested in this paper, under the same loading amplitude, the fatigue life of complete specimens was largest. when the dip of fracture  < 45º, the fatigue life of rocklike specimens was reduced with the increase of fracture dip; when  < 45º, the fatigue life was increased with the increase of fracture dip. conclusion (1) according to the fatigue test results of the rocklike fracture test, the power function model was adopted to fit the test data, and the fitting degree of the curve was determined by the correlation coefficient , and the result was fitted well. (2) s-n fitted curves had clear concept, simple form and convenient measurement; under the constant amplitude loading condition, the interpolation method was applied in the s-n curves to estimate the fatigue life of rock. (3) the upper limit stress was the most important factor affecting the fatigue life of specimens. under the same conditions, the specimen had a shorter fatigue life when being applied by higher upper limit stress, whereas the specimen had a longer fatigue life when being applied by lower upper limit stress. (4) the slope of fatigue life s-n curves reflected the sensitive relationship between the fatigue life of specimens and upper limit stress ratio. it was indicated, if the slope was smaller, the change of fatigue life to upper limit stress ratio was more sensitive; on the contrary, it was less sensitive. the rock of different types and strengths not only had different fatigue life, but also had the different sensitive degrees between the fatigue life and the maximum cycling stress. (5) the initial damage reflected the loading history of the rock, and different dips of pre-factures (initial damage) had the significant effect on the fatigue life of rocklike specimens. acknowledgements this work was kindly supported by natural science foundation of fujian province (no. 2015j05077), young and middle-aged education scientific research project of fujian province (ja15533), china scholarship council (csc, no. 201608350057), and national natural science foundation of china (no. 41072232). references cao, p., liu, t. & pu, c. (2015). crack propagation and coalescence of brittle rock-like specimens with pre-existing cracks in compression. engineering geology, 187, 113-121. erarslan, n. & williams, d. (2012). the damage mechanism of rock fatigue and its relationship to the fracture toughness of rocks. international journal of rock mechanics and mining sciences, 56, 15-26. haeri, h., shahriar, k. & marji, m. f. (2014a). experimental and numerical study of crack propagation and coalescence in pre-cracked rocklike disks. international journal of rock mechanics and mining sciences, 67, 20-28. haeri, h., shahriar, k. & marji, m. f. (2014b). on the strength and crack propagation process of the pre-cracked rock-like specimens under uniaxial compression. strength of materials, 46, 140-152. jinjin, cao, p. & pu. c. (2014). influence of flaw parameters on damage mode and strength of rock-like materials (in chinese). changsha, china: journal of central south university (science and technology), 45(2), 529-535. momeni, a., karakus, m., & khanlari, g. r. (2015). effects of cyclic loading on the mechanical properties of a granite. international journal of rock mechanics and mining sciences, 77, 89-96. pu, c., & cao, p. (2012). failure characteristics and its influencing factors of rock-like material with multi-fissures under uniaxial compression. transactions of nonferrous metals society of china, 22, 185-191. shilei, k., junsheng, y., & feng, y., (2016). macro-and-meso analysis of failure behavior of rock-like samples with flaws using a numerical method (in chinese). journal of yangtze river scientific research institute, 33(4), 71-77. xiufeng, f. (2008). study on the damage evolution of rock fatigue and the response of rock slope under cyclic loading (in chinese). fuzhou, china: fuzhou university. xiurun, g. (1987). study on deformation and strength behaviour of the large-sized triaxial rock samples under cyclic loading (in chinese). rock and soil mechanics, 8(2), 11-19. xiurun, g. & yingfa, l. (1992). discussion on the problem of rock fatigue failure and irreversible deformation under cyclic loading (in chinese). chinese journal of geotechnical engineering, 14(3), 56-60. xiurun, g., yu, j. & yunde, l. (2003). testing study on fatigue deformation law of rock under cyclic loading (in chinese). chinese journal of rock mechanics and engineering, 22(10), 1581-1585. yanbo, z., xing, h. & guo, j. (2017). experimental study of fatigue characteristics of gypsum rock under cyclic loading (in chinese). chinese journal of rock mechanics and engineering, 36(4), 940-952. yu, j. (2003). fatigue failure and deformation development law of rock under cyclic load (in chinese). shanghai, china: shanghai jiaotong university. zhou, x. p., cheng, h. & feng y. f. (2014). an experimental study of crack coalescence behaviour in rock-like materials containing multiple flaws under uniaxial compression. rock mechanics and rock engineering, 47, 1961-1986. keywords: fluid inclusions; el bagre mining district; shear zone; lode gold deposits. palabras clave: inclusiones fluidas; distrito minero el bagre; zona de cizalla; depósitos vetiformes. how to cite item naranjo sierra, e., & alvaran echeverri, m. (2020). fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia. earth sciences research journal, 24(3), 245-257. doi: https:// doi.org/10.15446/esrj.v24n3.80653 the el bagre mining district is located in the northeastern zone of the antioquia department of colombia, in the lower cauca-nechi river basin. the shear zone hosted lode-gold type deposits are located in the northern part of the el bagre mining district. the characteristics of ore-forming fluids were investigated using fluid inclusion petrography and microthermometry analysis. two stages, were identified in this study, namely a quartz-pyrite premineralization stage (1) and a reactivation-sulfide-telluride mineralization stage (2). microthermometry analysis revealed the presence of two types of fluid inclusions. primary aqueous-carbonic inclusions (type i) are trapped in massive quartz, are characterized by the presence of clathrate, with salinities between 1.5 and 8.3 wt% nacl equiv. and homogenization temperatures (to liquid) between 238.1° and 297.1°c. secondary aqueous saline inclusions (type ii) are trapped in reactivated quartz (type iia) and crosscutting calcite veins (type iib). the salinity displays a positive correlation trend from low to relatively high salinity, with 3.82 wt% nacl equiv. (type iib) to 9.21 wt% nacl equiv. (type iia). homogenization to a liquid phase occurred between 150.8° and 210.6°c for the type iia inclusions and between 130.3° and 190.4°c for the type iib inclusions. based on these results, the shear zone hosted lode-gold type deposits from the el bagre mining district, show characteristics of orogenic gold deposits. abstract fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia estudio de inclusiones fluidas de los depósitos vetiformes de oro hospedados en zona de cizalla: distrito minero el bagre, antioquia-colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.80653 el distrito minero de el bagre está localizado al noreste del departamento de antioquia-colombia, en la cuenca baja de los ríos cauca-nechi. los depósitos vetiformes hospedados en zonas de cizalla están localizados en el sector norte del distrito minero el bagre. las características de los fluidos mineralizantes se investigaron con un análisis petrográfico y microtermométrico de inclusiones fluidas. se identificaron dos etapas en este estudio, nombradas cuarzo-pirita premineralización (etapa 1) y reactivación-sulfuro-telururos mineralizante (etapa 2). el análisis microtermométrico reveló la presencia de dos tipos de inclusiones fluidas. inclusiones acuo-carbónicas primarias (tipo i) atrapadas en cuarzo masivo, caracterizadas por la presencia de clatratos, con salinidades entre 1.5 y 8.3 w%t nacl equiv. y temperaturas de homogenización entre 238.1° y 297.1°c. las inclusiones acuosas secundarias (tipo ii) están atrapadas en cuarzo reactivado (tipo iia) y venas de calcita (tipo iib). las estimaciones de salinidad muestran una tendencia de correlación positiva con miembros de baja a relativamente alta salinidad, con 3.82 wt% nacl equiv. (tipo iib) a 9.21 wt% nacl equiv. (tipo iia). la homogenización a una fase líquida ocurrió entre 150.8°c y 210.6°c para las inclusiones tipo iia y entre 130.3° y 190.4°c para el tipo iib. con base en estos resultados, los depósitos vetiformes hospedados en zonas de cizalla al interior del distrito el bagre, muestran características de depósitos de oro orogénico. resumen record manuscript received: 25/06/2019 accepted for publication: 01/05/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 245-257 edwin naranjo sierra1 and mauricio alvaran echeverri2 1geólogo msc. operadora minera s.a.s. cra. 43a n° 14-109, edificio nova tempo. medellín, colombia 2geólogo msc. departamento de ciencias geológicas. universidad de caldas. cl. 65 n°26-10, manizales, colombia *corresponding author: edwin.naranjo@operadoraminera.com.co; mauricio_alvaran@ucaldas.edu.co; g eo lo g y https://doi.org/10.15446/esrj.v24n3.80653 https://doi.org/10.15446/esrj.v24n3.80653 https://doi.org/10.15446/esrj.v24n3.80653 mailto:edwin.naranjo@operadoraminera.com.co mailto:mauricio_alvaran@ucaldas.edu.co 246 edwin naranjo sierra and mauricio alvaran echeverri introduction the el bagre mining district is located in the ne part of the antioquia department of colombia (figure 1a), in the region known as bajo cauca in the cauca-nechí river basin between zaragoza and el bagre municipalities (figure 1b). this region has long been recognized as an important historic alluvial gold district in colombia (shaw, 2000; sillitoe, 2008; londoño et al., 2009; leal-mejía, 2011). lode gold type mineralization is hosted in shear zones with fragile-ductile deformation within carboniferous granitic rocks (diorite and tonalite) of the el carmen stock (leal-mejía, 2011; naranjo-sierra et al., 2016). few studies (londoño et al., 2009; leal-mejía, 2011; naranjo-sierra et al., 2016; naranjo-sierra and alvaran-echeverri, 2018; naranjo-sierra, 2019) related to geological characteristics, structural controls, ore-forming fluids, and metallogenic processes regarded the el bagre area, have been reported. londoño et al., (2009), highlighted the marked structural control of vein-type mineralization and the displacement associated with dextral inverse faults. lealmejía (2011), demonstrated according to sericite k-ar dating of 280±6 ma and isotope geochemistry data (208pb/204pb, 207pb/204pb and 206pb/204pb) that vein-type mineralization at el bagre pre-dates the jurassic intrusions of segovia batholith (188.9±2.0 ma., u/pb in zircon, spikings et al., 2015), and proposed an intimate spatial relationship between gold vein mineralization and the carboniferous el carmen stock. the sericite k-ar dating of 280±6 ma reported by leal-mejía (2011) from the la ye mine and sericite ar-ar dating of 276±5 ma reported by naranjo-sierra (2019) from the los mangos mine, are the only evidence of a permian gold mineralization to date in colombian andes. based on δ18o, δd and δ34s isotopes, naranjo-sierra et al., (2016) proposed that the ore fluids from the la ye shear-zone lode-gold-type deposit at el bagre are compatible with magmatic and/or metamorphic origin. although the fluid inclusion characteristics of the gold lodes were hitherto unknown, the authors proposed h2s or hs ligand species as the predominant sulfur species, indicating that au(hs)2 was likely the gold-transporting complex. this paper presents the first microthermometric data of the main lodes from the el bagre figure 1. a). simplified map showing the location of colombia and antioquia department. b). regional map of the ne region of the antioquia department, showing the main major faults and (in green) the region of bajo cauca. c). simplified geological map of the study area based on gomez-tapias et al., (2015). 247fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia et al., 2015). the western limit of the segovia batholith and the el carmen stock is marked by the n to nnw trending otu-pericos east fault (figure 1c). the kinematics of this regional fault are not fully understood in detail; however, right-lateral as well as left-lateral faulting and thrusting seem to have occurred (horner, 2018). to the east, along the el bagre reverse fault system, san lucas precambrian metamorphic rocks make contact with the jurassic granitic rocks of the segovia batholith (figure 1c). this fault system forms branches of n to nne trending palestina fault system, which has been recognized as a regional fault with right and left-lateral movement (horner, 2018). lode gold deposits at the el bagre mining district are best described as structurally controlled mesothermal gold-sulphide-telluride-bearing quartz vein deposits, with veining emplacement in the ns shear zones within the competent intrusive rocks of the el carmen stock (figure 2). these shear zones have undergone brittle-ductile deformation and are usually mylonitized around their contacts. boudinage and vein off-sets are mining district, with the aim of characterizing the thermal and compositional evolution of the hydrothermal fluids and discuss their implications for gold mineralization. the evolution of the hydrothermal system is discussed in terms of fluid composition and temperature, with emphasis on fluid sources, gold transportation, pressure estimates, and depositional mechanisms. geological setting based on restrepo and toussaint (1988) and cediel et al. (2003), the el bagre mining district is characterized by accreted terranes (chibcha and tahamí terranes). major strike-slip otu-pericos fault system (figure 1b and 1c) is the dominant structure in the area. this major fault system separates the triassic low to medium grade metamorphic rocks associated to the cajamarca complex from the carboniferous intrusive rocks of the el carmen stock (gómez-tapias figure 2. local geological map of the el bagre mining district, showing the main lode gold deposits and the samples used in this study. 248 edwin naranjo sierra and mauricio alvaran echeverri common associated structures in these deformed rocks. according to starling (2015), four deformation events can be recognized at the el bagre mining district. the d1 deformation that formed the ns-nnw structural corridor. this stage covers all the pre-mineralization events. the d2 deformation event reactivated the ns-nnw corridor as a dextral transpressional shear structure, generating wnw low-angle thrust faults and introducing significant volumes of quartz with low contents of sulphide (mainly pyrite) and gold (figure 3a). the d3 deformation reactivated the main veins as a sinistral transpressional shear with nw-trending riedel cross faults controlling the main phase of high-grade gold-base metal-telluride mineralization (figure 3b). the d4 deformation covers all the post-mineralization events. the veins consist of milky white quartz containing native gold and up to 20% mixed sulfides, dominated by pyrite with occasional galena and chalcopyrite. sulphide and native gold distribution within the veins is patchy. gold mineralization is restricted to massive quartz veins with pyrite, chalcopyrite, galena and tellurides. the thickness of the veins averages approximately one meter. chloritization halos are generally restricted to the carboniferous rock near the veins contacts. sericite and carbonates are the most common alteration minerals within the auriferous quartz veins. sericite occur as fine-grained flakes filling fractures of massive quartz (qv1) and reactivated quartz (qv2). calcite occurs as fine-grained crystals or formed narrow sheeted veins cutting massive quartz (qv1). methods twenty quartz veins samples of different types (massive and reactivated) and at different mineralization stages were sampled from several locations within the el bagre mining district (figure 2). doubly polished thin sections (80-100μm) of these samples were prepared by minerlab, bogotá, colombia. all the sections were studied first for petrography to define the relative timing of the fluid inclusions using a nikon 200i pol microscope in the laboratory of mineral deposits at universidad de caldas (manizales). there were used the principles given by roedder (1984) and the concept of fluid inclusion assemblage (fia), defined by goldstein, (2003). ten samples were selected for the microthermometry; measurements were performed using the linkam thms 600 with maximum and minimum temperature limits of 600°c and -196 c̊, coupled to an olympus bx41 petrographic microscope in the fluid inclusion laboratories at the universidad nacional (bogotá). according to the methods described by roedder (1984), goldstein (2003) and lai et al., (2015), the linkam stage was calibrated using two synthetic fluid inclusions from the university of leoben, containing h2o (final ice melting temperature of 0.5°c and critical point of 374.1°c) and co2 (co2 melting temperature of -56.6°c), the estimated precisions of the measurements were ±0.1°c and ±1°c for freezing and heating, respectively. the heating/freezing rate generally ranged from 0.2°c/min to 5°c/min but decreased to <0.5°c/min near phase transformation. based on roedder (1984), goldstein (2003) and lai et al., (2015), the measurements at this stage, include eutectic temperature (te), ice melting temperatures (tm-ice), clathrate melting temperatures (tm-cla) and total homogenization temperatures (th). fluid inclusions salinities were estimated by the final ice melting and final clathrate melting temperatures respectively. the results were complemented with raman spectroscopy for the determination of the vapor phase composition, using a renishaw invia centrus 0281qi equipment which uses a laser with a wavelength of 532nm. this equipment is located in the fluid inclusion laboratories at the universidad nacional (bogotá). the fluids software of bakker (2003) was used for the calculations. microthermometric measurements were performed on small groups or trails of fluid inclusions, which have constant liquid to vapor ratios. the fluid inclusions groups or trails that have experienced leakage or decrepitation were discarded. salinity estimates were calculated using the equations summarized in diamond (1992) for the h2o-co2-nacl±ch4 system and the equations of bodnar and vityk (1995) for the h2o-nacl system. in this study, the equation of state proposed by brown and lamb (1989) for the h2oco2-nacl±ch4 system and the equation of state from bodnar and vityk (1995) for h2o-nacl system were used to estimate the trapping pressure. sample description quartz veins at el bagre can be divided into two types: (1) massive quartz veins (qv1) mostly related to dextral shear zones with low gold grades, and (2) massive reactivated quartz veins (qv2) related to nnw riedel shearing with high gold grades. based on these observations and previous studies (londoño et al., 2009; leal-mejía, 2011; naranjo-sierra et al., 2016; naranjo-sierra and alvaran-echeverri, 2018) mineralization processes could be divided into two stages (figure 4): the quartz-pyrite stage (1) and the reactivation-sulfide (galena-chalcopyrite)-telluride stage (2). stage (1) is characterized by the development of massive quartz with coarse-grained euhedral to subhedral pyrite deposited as a clusters and scarce gold mineralization (figure 5a). gold occurrence (figure 5b) in this stage are mainly as small inclusions (<10µm) in coarse-grained pyrite (py1), attached to pyrite crystals surfaces and as free-milling particles in massive quartz (qv1). this early stage was significant in terms of quartz volume (see figure 3a picture #3). this stage is associated with dextral shear zones due to d2 deformation. sericite is present in this stage mainly as small flakes filling fractures in massive quartz (qv1). confident assignment of calcite to this stage is possible only where this carbonate mineral is associated to coarse-grained pyrite (figure 5a). stage (2) is the main mineralization stage and is characterized by modification of existing massive quartz veins (qv1) by deformation (reactivation) associated with d3 deformation due to sinistral shearing (figure 5c). this stage consists of an assemblage of fine-grained quartz (qv2) with irregular-shapes and recrystallized crystals, fine-grained pyrite (py2), base metals sulfides (chalcopyrite and galena) and tellurides. under the microscope, the quartz crystal shows undulate extinction, deformation lamellae and dynamic recrystallization (figure 5d). significant gold mineralization is observed in this stage. gold are spatially associated with (figure 5e) chalcopyrite, galena and tellurides (calaverite, sylvanite and altaite) sealing fractures in early pyrite crystals (py1). coarse pyrite is also replaced by chalcopyrite and galena (figure 5f). galena show deformation in their pits (figure 5g). major sericitic alteration is related to this stage (figure 5h). paragenetic relationships between tellurides is not well known, although it is likely that calaverite, sylvanite and altaite were cogenetic with gold and base metals sulfides. calcite sealed interstitial voids around quartz crystals or formed narrow (width 1-5 mm) sheeted veins, cutting massive quartz (qv1). fluid inclusion characteristics well-developed primary and secondary fluid inclusions are observed in quartz veins from stages (1) and (2). the analyzed fluid inclusions are trapped in quartz and calcite (figure 6). their paragenetic chronology was established using the criteria of primary, secondary and pseudosecondary, defined by roedder (1984). according to the nature of phase relationships at room temperature (20°c) and phase transitions during heating and cooling, two types of fluid inclusions were recognized: aqueous-carbonic inclusions (type i) mostly related to massive, low-grade quartz veins and aqueous-saline inclusions (type ii) related to main stage of mineralization (stage 2). all the fluid inclusion types are two-phase with a l/v ratio between 0.82 to 0.97. aqueous-carbonic inclusions [(co2±ch4)v+(h2o)l] are volumetrically liquid-dominant, with l/v ratio between 0.93 to 0.97. these type of inclusions present natural contours with ovoid and irregular shapes. they occur as isolated clusters or fia (figure 6a and 6b) in deformed quartz samples, and are therefore, interpreted as primary. laser raman spectroscopy study indicated a low content of co2, and detected ch4 in minor quantities (figure 6c). type (ii) fluid inclusions [(h2o)v+(h2o)l] are the most common inclusions observed in the studied plates and are easily identified as secondary inclusions aligned as trails crossing boundaries grains (figure 4d and 4e). they are liquid-rich inclusions with liquid ratios between 0.92 and 0.82. they have different shapes, from regular with soft edges to irregular with jagged edges. microthermometric properties type i fluid inclusions do not exhibit a melting temperature of solid co2. these fluid inclusions are characterized by the presence of clathrate, which melts between 5.0 and 9.1°c (figure 7a), revealing the presence of co2, confirmed by raman spectroscopy (figure 6c). a minor quantity of ch4 was also detected by raman spectroscopy (figure 6c). first melting temperature 249fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia figure 3. principal deformation events defined by starling (2015) at el bagre mining district. a). d2 deformation related to dextral shearing due to nne/ssw compression. for figure 3a: picture #1 showing tension gash veins indicating d2 dextral shearing along vein margins, picture #2 showing wnw thrust fault, picture #3 showing massive low-grade quartz vein and picture #4 showing domino faulting and block rotation within the d2 shear. b). d3 deformation related to sinistral shearing due to nnw/sse compression. for figure 3b: picture #1 showing strong nnw fracturing due to low-angle shears and step joints, picture #2 showing domino faulting and block rotation within the d3 shear, picture #3 showing overlap or duplication of vein due to vein-parallel sinistral shearing during d3 deformation and picture #4 showing massive quartz reactivation due to d3 deformation. note: views are up-dip in roof of mine and so apparent shear sense is opposite of that in plan. 250 edwin naranjo sierra and mauricio alvaran echeverri of ice range between -20.0°c and -22.6°c (±0.1°c), indicating predominance nacl in solution (figure 7b). total homogenization (to liquid) temperatures occurred between 238.1°c and 297.1°c (±1°c) (figure 7c). molar fraction of co2 range between 0.042 and 0.079 and molar volume range between 18.72 to 20.23 cc/mol. this relatively complex composition (h2o-co2-nacl±ch4) and the non-determination of the co2 homogenization temperature make the bulk salinity estimates difficult. using the equation by diamond (1992) based on clathrate melting, the estimates of salinities range between 1.5 and 8.3 wt% nacl equiv. (figure 7d). for the primary aqueous-carbonic inclusions, the estimation of trapping pressure was calculated ignoring the ch4 content in the fluid and the non-observation of separated vapor to liquid co2-phases is indicative of a low density of the co2 phase (diamond, 1992; diamond, 2001; wilkinson, 2000; fall, 2008). tm-cla allows the appropriate figure to be chosen based on salinity (brown and lamb 1989). in the absence of co2 homogenization temperatures, the calculations can be made using a specific co2 density of 0.75gcm-3 (brown and lamb 1989; diamond, 1992; wilkinson, 2000; diamond, 2001; haeberlin, 2002). based on the above simplifications, the estimated trapping pressures for aqueous-carbonic inclusions (type i) show relatively high pressures conditions of <5kbar. the microthermometric properties of the analyzed fluid inclusions were summarized in table 1. secondary aqueous inclusions were trapped in reactivated quartz (type iia) and cross-cutting calcite veins (type iib). both types yielded significant variations in their melting and homogenization temperatures. according to eutectic temperatures measured (-22.5°c and -19.8°c), nacl is the main salt in solution (see figure 7b). homogenization to a liquid phase occurs between 150.8°c and 210.6°c (±1°c) for the type iia inclusions and between 130.3°c to 190.4°c (±1°c) for type iib (see figure 7c). the salinity estimates were determined from ice final melting temperatures, which ranged between -7.5° and -5.0°c for type iia inclusions and-3.5° and -0.9°c for type iib. the salinity estimates displayed a positive correlation trend (see figure 7d) from low salinity, with a media of 3.82 wt% nacl equiv. (type iib) to relatively intermediate salinity, with a media of 9.21 wt% nacl equiv. (type iia). the density values range between 1.05 to 1.07 g/cc for type iia fluid inclusions and 1.00 to 1.03 g/cc for type iib fluid inclusions. the molar volume for type iia inclusions is in a range of 17.40 to 17.70cc/mol and 17.72 to 17.95 cc/mol for type iib discussion modelling of the formation conditions of the shear zone hosted lodegold type deposits at el bagre was carried out based on the fluid inclusion results, ore minerals present, alteration mineralogy, and the sulfur, oxygen and deuterium isotopes reported by naranjo et al. (2016) and naranjo-sierra and alvaran-echeverri (2018). pressure estimates pressure were estimated from the salinities and homogenization temperatures of the fluid inclusions (diamond, 1992 and haeberlin, 2002). despite the absence of geothermometric isotopic data to constraint the independent trapping temperature and the lack of conclusive evidence for an immiscibility process, the homogenization temperature is considered the minimum value for the temperature of fluid inclusion trapping (roedder and bodnar, 1980; randive et al., 2014). homogenization temperatures for primary aqueous-carbonic (type i) have a range between 238°c and 297°c and for the secondary aqueous inclusions (type ii) a range between 130°c and 210°c. due to the phase volume were calculated based on two-dimensional data, there is an inherent error in the calculations (bakker and diamond, 2000). however, considering that type i fluid inclusions have l/v ratios of 0.93 to 0.97 (liquid dominant), the effects of error propagation are considered relatively low. figure 4. paragenetic sequence of ore, gangue and alteration minerals. significant volumes of quartz, pyrite and low content of gold were introduced during stage (1). main gold mineralization was during stage (2). table 1. summary of the descriptive properties and thermometric measurements of type i, type iia and type iib fluid inclusion populations. microthermometric data type composition n° phases form l/v* classification host n clathrate melt t° eutectic t° ice melt t° homog. t° salinities i h2o-co2-nacl±ch4 two irregular isolated 0.93-0.97 primary quartz 23 5.0 to 9.1°c -20 to -22.6°c 238.1 to 297.1°c 1.5 to 8.3 wt% nacl equiv. iia h2o-nacl two irregular trails 0.82-0.91 secondary quartz 38 -22.5 to -19.8°c -7.5 to -5.0°c 150.8 to 210.6°c 7.86 to 11.09 wt% nacl equiv. iib h2o-nacl two irregular trails 0.82-0.92 secondary calcite 15 -21.9° to -20.0°c -3.5 to -0.9°c 130.3 to 190.4°c 1.5 to 5.7 wt% nacl equiv. *liquid to vapor ratios of the two-phase inclusions at room temperature (20°c) 17.70cc/mol 251fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia figure 5. general aspects of shear zone hosted lode gold type deposits at the el bagre mining district. a). low gold grade massive quartz vein with coarse pyrite crystals associated to clusters of calcite. b). photomicrograph (reflected light) of gold attached to coarse pyrite crystals and gold as a free-milling particle. c). high gold grade quartz vein with fragments of qv1 and pyrite (py1), sheeted veins of calcite and crystals of chalcopyrite. d). photomicrograph (transmitted light) of sample show in figure 5c. note the sericite aggregates associated to quartz crystals showing undulose extinction, deformation lamellae and dynamic recrystallization e). photomicrograph (transmitted light) of sample show in figure 5c. note gold, galena, altaite, sylvanite and calaverite filling fracture in coarse pyrite crystals (py1). f). chalcopyrite and galena sealing fractures and partially replaced coarse pyrite (py1) crystals. g) deformation of the characteristic triangular pits of galena. h). sericitic alteration related to stage 2. 252 edwin naranjo sierra and mauricio alvaran echeverri figure 6. general aspects of the fluid inclusion observed. a). isolated clusters (fia) of primary (p) aqueous-carbonic fluid inclusions and secondary (s) fluid inclusions cross-cutting grain boundaries. b). primary fluid inclusion trapped in a massive quartz vein (qv1). c). raman spectra for type i fluid inclusions. d). crosscutting secondary (s) fluid inclusions. e). secondary fluid inclusions trapped in a reactivated quartz vein (qv2). l=liquid; v=vapor. 253fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia figure 7. a). type i fluid inclusions clathrates melting temperatures (tm-cla). b). type ii fluid inclusions eutectic temperatures (te). c). homogenization temperature (th) histogram of type i, type iia, and type iib fluid inclusions. d). homogenization temperature (th) versus salinity (wt% nacl equiv.). 254 edwin naranjo sierra and mauricio alvaran echeverri the results obtained in this study, indicate that the stage of massive, low gold-grade quartz veins (stage 1) emplacement seems to be occurred at crustal levels (±18km, assuming near lithostatic regime) (figure 8). however, as predicted by the variations in salinities and homogenization temperatures, there is a wide range of trapping pressures for the secondary inclusions. type iia fluid inclusions show trapping pressures of <2.0kbar, while type iib fluid inclusions show pressures estimates lower than 0.5kbar. therefore, the fluid pressures for main gold mineralization (stage 2) are mostly between 2.0kbar and 0.5kbar corresponding to depths of ±7.58km to ±1.8km, assuming a density of 2.7gcm-3 and near lithostatic regime (figure 8). these results are consistent with orogenic gold deposits type (moritz, 2000; ridley and diamond, 2000; groves et al., 2003; goldfarb et al., 2005). this p-t evolution can be explained in terms of: (1) depth changes related to a rapid uplifting of the host unit, although the tectonic regimen in the area to support this rapid uplifting remains unclear, (2) upward migration and cooling and/or (3) the influx of a second and different fluid in the hydrothermal system. these values should be viewed as semi-quantitative, given the difficulties in accurately constraining the fluid system of primary inclusion and the absence of geothermometric data to constraint the trapping temperature. ore fluid evolution the fluid inclusions data suggest that at least two fluids inputs and circulation systems were involved in the formation of the shear zone hosted lode-gold type deposits at el bagre. two fluid types were determined through the ore deposition sequence: (i) an early relatively hot h2o-co2-nacl±ch4 fluid broadly coeval with massive quartz (qv1) formation that is related to dextral shearing during d2 deformation, and (ii) a relatively low temperature h2onacl fluid displaying a positive correlation trend from a low salinity member with 3.82 %wt nacl (type iib) to relatively saline one with 9.21 %wt nacl (type iia), related to sinistral shearing during d3 deformation. fluid inclusions described here share common microthermometric properties: (1) composition (figure 9a), (2) salinity and (3) homogenization temperatures (figure 9b), with the fluid inclusions reported in a large number of orogenic gold deposits: patazparcoy, cachoeira, charters towers, pogo, bendigo, linglong, muruntau, juneau and jiadong (moritz, 2000; ridley and diamond, 2000; groves et al., 2003; goldfarb et al., 2005; golbfarb and groves, 2015). although some similarity with epithermal deposits (low sulfidation) is discernible in figure 9a and 9b, some key factors preclude this observation: (i) absence of adularia, chalcedony and marcasite-arsenopyrite (ii) depth of formation (iii) absence of crustiform and colloform quartz textures and (iv) structural settings. the transition from an aqueous-carbonic fluid, presumably deepsourced (±18km), to an aqueous-saline co2-free fluid seems to be related to the beginning of sulfide deposition (drummond and ohmoto, 1985; haeberlin, 2002; saunders et al., 2014). however, it is unclear whether the co2 loss and later salt enrichment simply result from the progressive evolution of the early carbonic fluid or reflect the influx of a second and different fluid in the hydrothermal system. possible explanation for the first scenario of progressive evolution include: (1) loss of volatile phases after pressure drop and (2) fluidrock interaction with the wall-rocks. there is no evidence of phase separation figure 8. pressure-temperature versus pressure diagram showing isochores covering the range of composition and densities of type i (solid lines) and type ii (dotted lines) fluid inclusions. vertical lines limit the homogenization temperature (th) range for type i and type ii fluid inclusions. schematic trend of different fluid evolution processes (wilkinson, 2001). 255fluid inclusion study of shear zone hosted lode gold type deposits: el bagre mining district, antioquia-colombia precipitation with calcite and quartz precipitated by different fluids; with quartz derived from a more saline fluid and calcite precipitated just after from the remaining lower salinity fluid. the influx of this second fluid seems to coincide with late deformations stage (d3) that opened crack within coarse-grained pyrite (py1) and with their filling with the main sulfide-telluride-gold (stage 2). the source whether magmatic and/or metamorphic (leal-mejía, 2011; naranjo-sierra et al., 2016; naranjo-sierra and alvaran-echeverri, 2018) provoked the release of low-salinity aqueous-carbonic fluid from presumably from lower crustal levels, and potentially gold-bearing fluids were able to begin their upward migration. these early fluids deposited significant quartz volumes, carbonates (calcite), sulfides (mainly pyrite) and low contents of gold. as upward migration continues, a second influx of aqueous-saline fluid were incorporated to the system, as a result the ore-related fluid became more dilute and cooler and the sulfur activity in the system dropped, allowing gold, sulfides and tellurides to precipitate in microfractures within the massive quartz-sulfide veins. transport and deposition of gold fluid inclusion, alteration mineralogy, ore mineral and isotope results provide direct evidence of the ph and redox conditions of ore-forming fluid (klein et al., 2005). dominant sericite alteration, the absence of hematite, the coexistence of pyrite and chlorite, and the δ34s values (+1.0 to +5.0%o) indicate relatively reduced conditions (naranjo et al., 2016) for the ore-related fluid at el bagre. moreover, the presence of calcite and sericite as the dominant alteration minerals also indicates neutral to slightly alkaline conditions (klein et al., 2005). the majority of the fluid inclusions presented here, have low-salinity, which under the aforementioned neutral ph conditions, suggest that h2s or hsligand species were the predominant sulfur species in ore fluids. in consequence, au(hs)2was likely the gold-transporting complex; a dominant transporting mechanism in orogenic gold deposits (moritz, 2000; ridley and diamond, 2000; groves et al., 2003; goldfarb et al., 2005; phillips and powell, 2010; seward et al., 2014 and saunders et al., 2014). at the el bagre mining district, gold occurs as small inclusions in pyrite (py1), as free-milling particles associated to massive quartz (qv1) and as sealing of fractures with tellurides and base metal sulfides. this might indicate different mechanisms of precipitation. the presence of co2-bearing inclusions and of calcite shows that the early fluid was rich in co2. removal co2 from the fluid to form fluid inclusions and by dissociations to form carbonate minerals would rise the ph of the solution and the activity of the carbonate ion the solution favoring the precipitation of calcite (rimstidt, 1997). the removal of co2 from the fluid will cause an increase in ph and will decrease the temperature and activity of the bisulfide complex (drummond and ohmoto, 1985; seward et al, 2014; seward et al., 2014; ridley and diamond, 2000). at the same time, the total s decrease due to the volatilization of h2s and the precipitation of sulfides. the reducing process of the total s has an ultimate effect of approaching the au super saturation and precipitation. this would effectively destabilize the au bisulphide complex and lead to au deposition (ridley and diamond, 2000; phillips and powell, 2010; seward et al, 2014; saunders et al., 2014). conclusions from this study of fluid inclusions of shear zone hosted lode gold type deposits at the el bagre mining district, we draw the following conclusions: 1there were two mineralization stages. stage (1) mostly related to massive, low gold-grade quartz veins (qv1) and stage (2) directly related to gold mineralization, tellurides and base metal sulfides. 2|the fluid responsible for main gold mineralization are related to h2onacl system, with low to moderate salinity (9.21 %wt nacl), with a trapping temperature between 150.8°c and 210.6°c and a trapping pressure between 2.0kbar and 0.5kbar, corresponding to depths of ±7.5km to ±1.8km. whereas those of massive low gold-grade quartz veins are characterized by the h2o-co2-nacl±ch4 system, with trapping temperature between 238.1°c and 297.1°c and pressure estimates <5kbar, corresponding to crustal levels (<18km). 3the el bagre ore-forming fluids were neutral to slightly alkaline, as indicated by the mineral assemblages and relatively reduced according to figure 9. a). plot showing the general compositions of orogenic gold deposits and other deposits (ridley and diamond, 2000) and the results of this study (red square). ls= low sulfidation; hs= high sulfidation b). homogenization temperature-salinity diagram illustrating typical ranges for fluid inclusions from different deposits type (wilkinson, 2001) and the result of this study (red square). (drummond and ohmoto, 1985; ridley and diamond, 2000; haeberlin, 2022) in the fluid inclusions. this observation precludes the progressive evolution scenario. therefore, this fluid transition seems to be the result of two different fluids incorporated into the hydrothermal system, and suggest that the conditions for gold precipitation were reached locally during the circulation of co2-bearing fluid. during this early stage (1) co2-bearing fluid bring au to the system and during stage (2) gold concentration occurred. the aqueous saline inclusions (type 2) represent younger fluid that are synchronous with the main gold stage. the positive correlation discernible in figure 7d between type iia and type iib fluid inclusions can be explained by dilution trough the mineral 256 edwin naranjo sierra and mauricio alvaran echeverri the absence of hematite, the coexistence of pyrite and chlorite, and the δ34s values. in consequence, au(hs)2was likely the gold-transporting complex. 4-| the characteristics at the el bagre district share similar characteristics with orogenic deposits type. acknowledgment we would like to thank mineros s.a and operadora minera s.a.s for the opportunity to present this document. we also thank the brownfield and greenfield exploration teams for their support and comments on the first editions of this document. thank you to juan carlos molano and ruben gaitan for their observations on the freezing/heating stages. finally, we thank the anonymous reviewers for pointing out points of muddled thinking and for constructive comments. references bakker, r. j. 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(2001). fluid inclusions in hydrothermal ore deposits. lithos 55, 229-272. _goback the global gravity and crustal models are used in this study to determine the regional moho model. for this purpose, we solve the vening meinesz-moritz’s (vmm) inverse problem of isostasy defined in terms of the isostatic gravity gradient. the functional relation between the moho depth and the second-order radial derivative of the vmm isostatic potential is formulated by means of the (linearized) fredholm integral equation of the first kind. methods for a spherical harmonic analysis and synthesis of the gravity field and crustal structure models are applied to evaluate the gravity gradient corrections and the respective corrected gravity gradient, taking into consideration major known density structures within the earth’s crust (while mantle heterogeneities are disregarded). the resulting gravity gradient is compensated isostatically based on applying the vmm scheme. the vmm inverse problem for finding the moho depths is solved iteratively. the regularization is applied to stabilize the ill-posed solution. the global geopotential model goco-03s, the global topographic/bathymetric model dtm2006.0 and the global crustal model crust1.0 are used to generate the vmm isostatic gravity gradient with a spectral resolution complete to a spherical harmonic degree of 250. the vmm inverse scheme is used to determine the regional isostatic crustal thickness beneath the tibetan plateau and himalayas (compiled on a 1x1 arc-deg grid). the differences between the isostatic and seismic moho models are modeled and subsequently corrected for by applying the non-isostatic correction. our results show that the regional gravity gradient inversion can model realistically the relative moho geometry, while the solution contains a systematic bias. we explain this bias by more localized information on the earth’s inner structure in the gravity gradient field compared to the potential or gravity fields. este estudio utiliza los modelos globales de gravedad y de espesor de la corteza para determinar un modelo regional de la discontinuidad de mohorovičić (moho). con este fin se resolvió el problema inverso de isostasia vening meinesz-moritz (vmm) definido en términos de gradiente gravitatoria isostática. la relación funcional entre la profundidad de la moho y la derivación radial de segundo orden del potencial isostático vmm fue formulado a través de la ecuación integral fredholm de primera clase. se aplicaron métodos para el análisis esférico armónico y para la síntesis del campo gravitacional, y los modelos de estructura de corteza para evaluar las correcciones de gradiente gravitatoria y el respectivo gradiente gravitatorio corregido, considerando el conocimiento de las principales densidades de la estructura al interior de la corteza de la tierra (las heterogenidades del manto fueron ignoradas). el gradiente gravitatorio resultante se compensó isostáticamente con la aplicación del esquema vvm. se resolvió reiterativamente el problema inverso vvm para encontrar las profundidades de la discontinuidad moho. se aplicó la regularización para estabilizar la solución planteada. el modelo geopotencial global goco-03s, el modelo global topográfico/batimetrico dtm2006.0 y el modelo global de la corteza crust 1.0 permitieron generar el gradiente gravitacional isostático vvm con una resolución espectral completa a un grado esférico armonioso de 250. a través del esquema inverso vmm se determinó el espesor isostático regional bajo la meseta tibetana y los himalayas (compilada en una cuadrícula de 1x1 grados sexagesimales). las diferencias entre los modelos isostático y sísmico de la moho fueron modeladas y corregidas con la aplicación de la corrección no isostática. los resultados muestran que la inversión del gradiente gravitatorio puede modelar realísticamente la geometría de la moho, a pesar que la solución contiene una desviación sistemática. esta inclinación se explica por la información estructural interna de la tierra en el campo del gradiente gravitatorio comparado con el potencial gravitatorio. earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 97 106 abstract resumen keywords: density, gravity gradiometry, isostasy, moho interface. palabras clave: densidad, gradiometria gravitatoria, isostasia, discontinuidad de mohorovičić. isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data espesor isostático de la corteza bajo la meseta tibetana y los himalayas a partir de datos satelitales de gradiente gravitatoria record manuscript received: 23/07/2014 accepted for publication: 27/07/2015 how to cite item tenzer, r., bagherbandi, m., sjöberg, l.e. & novák, p. (2015). isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data. earth sciences research journal, 19(2), 97-106. doi: http://dx.doi.org/10.15446/esrj.v19n2.44574 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.44574 robert tenzer ¹, mohammad bagherbandi ², lars e sjöberg 2, pavel novák 3 ¹ wuhan university, china ² royal institute of technology, sweden 3 university of west bohemia, czech republic g r av it y 98 robert tenzer, mohammad bagherbandi, lars e sjöberg, pavel novák 98 introduction gravimetric methods for the moho depth determination have been developed and applied in global and regional studies. examples of the gravimetric methods include, but are not limited to, studies by čadek and martinec (1991), braitenberg and zadro (1999), braitenberg et al. (2000a, 2000b, 2006), arabelos et al. (2007), and sjöberg (2009). gravimetric methods should optimally combine the gravity and seismic data (if available). braitenberg and zadro (1999) proposed a method based on the iterative 3-d gravity inversion with integrated seismic data. sjöberg and bagherbandi (2011) developed and applied a least-squares approach based on solving the vening meinesz-moritz (vmm) inverse problem of isostasy (sjöberg 2009). this gravimetric method used the constraining information from the seismic model. they also presented a method of modeling the differences between isostatic and seismic models by applying the nonisostatic correction (bagherbandi and sjöberg 2012). the gravimetric determination of the moho depth has been typically realized using the isostatic gravity (or potential) field quantities. sampietro (2011) discussed possibilities of using the gravity gradiometry data provided by the gravity field and steady-state ocean circulation explorer (goce). bagherbandi (2011) developed an iterative method to recover the moho geometry using directly the goce gravity gradiometry data. his formulation of the isostatic scheme for the gravity gradient field was based on solving the vmm problem of isostasy. bagherbandi and eshagh (2012) applied this theory to the real gradiometry data of goce. their simulation study revealed a relatively good agreement between their global solution and the crust2.0 moho seismic model (bassin et al. 2000); the rms of differences was found to be ῀5 km. reguzzoni and sampietro (2012) presented a case study to assess the inversion algorithm based on the linearization of newton's gravitational law by exploiting the goce observables. they proposed a possible solution using the (local) fourier analysis and the wiener deconvolution of satellite data. sampietro et al. (2013) presented the goce estimated moho depths beneath the tibetan plateau and himalayas. they showed that the expected moho geometry has a good agreement with existing local seismic profiles. the goce-only solution was improved by using seismic profiles as additional observations; the rms of residuals with respect to the seismic profiles was found to be ῀4 km. eshagh (2014) developed a linear approach to determine the moho geometry from satellite gradiometry data and investigated some numerical aspects of the gravity gradiometry inversion. in the studies mentioned above, the moho determination was realized using only the gravity gradiometry data (bagherbandi and eshagh 2012, eshagh 2014). sampietro et al. (2013) used the seismic data to improve the gravimetric moho solution. in this study, we incorporate the information from seismic data directly to the vmm gravimetric solution for finding the moho depths. this is done by combing the gravity and seismic crustal structure models in computing the isostatic gravity gradient and solving the vmm inverse problem of isostasy. for this purpose, we combine three recently developed numerical methods for a regional determination of the moho geometry from the isostatic gravity gradient. the computation of the gravity gradient is realized according to the approach introduced by tenzer et al. (2012a). they presented a uniform mathematical formalism of computing the gravity corrections and respective corrected gravity field. novák and tenzer (2013) applied this method to evaluate the diagonal elements of the (second-order) marussi gravity gradient tensor corrected for major known anomalous density structures within the earth’s crust. we further apply the vmm scheme to evaluate the isostatic gravity gradient and to estimate the isostatic moho model according to the procedure of bagherbandi (2011). regarding to bagherbandi and sjöberg (2012), we finally define and apply the non-isostatic gravity gradient correction for combining the isostatic and seismic models. the vmm isostatic model and its gravity gradient modification are briefly recapitulated in sections 2 and 3. the non-isostatic gravity gradient correction is defined in section 3. the numerical experiment is conducted in the study area covering the tibetan plateau and the himalayas (characterized by the largest continental crustal thickness). summary of published seismic and gravimetric studies of the moho geometry at the study area can be found in tenzer et al. (2013). results are presented and discussed in section 5. the summary and concluding remarks are given in section 6. vening meinesz-moritz problem of isostasy sjöberg (2009, 2013) formulated the vmm problem of isostasy in the following generic form (1) where g = 6.674 × 10 ¯¹¹ m³ kg¯¹ s¯² is the newton’s gravitational constant, r= 6371 × 10³ m is the earth’s mean radius, ∆ρc/m is the moho density contrast (in kg/m³), and f is the isostatic gravity functional. the 3-d position is defined in the spherical coordinate system (r,ω) ; where r is the spherical radius, and ω = (ϕ,λ) is the spherical direction with the spherical latitude ϕ and longitude λ . the infinitesimal surface element on the unit sphere is denoted as: d ω' = cosϕ' dϕ' dλ', and φ {ω' = (ϕ', λ'): ϕ' ∈ [-π/2, π/2] ˄λ' ∈ [0,2 π)} is the full spatial angle. the integral kernel k in eq. (1) is a function of the spherical distance ψ and the moho-depth parameter s= 1-τ , with d'/ r and d' is the moho depth. the spectral representation of k reads (cf. sjöberg 2009) (2) where pn is the legendre polynomial of degree n for the argument of t = cosψ. the integral equation in eq. (1) functionally relates the (unknown) moho depths with the (given) isostatic gravity data (i.e., the isostatic gravity disturbances or anomalies). for the isostatic gravity disturbance δgᵢ, the isostatic gravity functional f on the right-hand side of eq. (1) becomes (cf. tenzer and bagherbandi 2012) f (r, ω) ≡ δgi (r, ω) = δgcs (r, ω) + gc(r, ω) = 0, (3) where gc is the isostatic compensation attraction (moritz 1990, sjöberg 2009), and δgcs is the consolidated crust-stripped gravity disturbance (tenzer et al. 2009). the evaluation of δgcs from the gravity disturbance δg is based on applying the topographic and stripping gravity corrections due to major known anomalous crustal density structures. these refined gravity data should have (theoretically) a maximum correlation with the moho geometry (tenzer et al. 2012b). however, these gravity data still comprise the long-wavelength gravity signal of unmodelled heterogeneities within the lithospheric mantle and the asthenosphere (including the core-mantle boundary zone) as well as uncertainties within the crustal model. the isostatic compensation attraction gc in eq. (3) is given by (cf. sjöberg 2009) , (4) where d0 is the nominal (mean) value of the moho depth, ℓ is the euclidean spatial distance of two points (r, ω) and (r', ω'), and ψ is the respective spherical distance. the formula in eq. (3) defines the condition of the vmm isostatic equilibrium by means of a fully-compensated crustal model. the expression in eq. (1) is a (non-linear) fredholm integral equation of the first kind. its direct solution was given by sjöberg (2009) in the form of a second-order approximation , (5) 99isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data the term d1 is computed approximately using the following spectral expansion , (6) where yn,m are the (fully-normalized) surface spherical harmonics of degree n and order m, fn,m are the numerical coefficients of the functional f, and n is the maximum degree of spherical harmonics. the third constituent on the right-hand side of eq. (5) has a singularity for ψ →0 . sjöberg (2009) solved this integral singularity by applying a planar approximation to the inner-zone contribution. this constituent is computed by implementing the surface integration over the inner zone (while disregarding the distant-zone contribution). the system of integral equations for finding the moho depths is then solved directly without applying an iterative procedure. vening meinesz-moritz isostatic model for the gravity gradient bagherbandi (2011) reformulated the vmm inverse problem of isostasy for the second-order radial derivative of the isostatic potential v i; i.e., v irr = ∂ ²v i/∂ r². by analogy with eq. (3), he established the isostatic condition for the component v i rr as follows (see also bagherbandi and eshagh 2012) v irr (r , ω) = t (r , ω) + v (r , ω) = 0, (7) where the second-order radial derivatives of the consolidated crust-stripped disturbing potential t csand the isostatic compensation potential v c are denoted as t and v , respectively. the computation of the consolidated crust-stripped gravity gradient t is summarized in appendices a and b. bagherbandi (2011) applied a decomposition of the isostatic compensation gradient v c into the nominal and residual parts: v (r , ω) = v (r , ω) + dv (r , ω). (8) the nominal part v is computed from the following formula , (9) where τ0 = d0 / r. with reference to eq. (1), the residual part dv is given by , (10) where the integral kernel function krr reads . , (11) bagherbandi (2011) derived the solution to the integral equation in eq. (10) in the following form , (12) where ds = 1dτ is a function of the differential moho-depth correction (i.e., dτ = dd'/r ), and the isostatic gravity gradient functional f is defined as . , (13) 99 krr r,ψ, s( ) = r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+3 n+1( ) n+ 2( ) n+3n=0 ∞ ∑ 1− sn+3( ) pn t( ) the moho-depth corrections dd' are found based on solving the vmm inverse problem of isostasy formed for the given values of the isostatic gravity gradient. the final moho depths d' are obtained by adding these corrections dd' to the nominal moho depth d0 ;i.e.,d'= d0+dd'. from eq. (11), the partial derivative of the kernel function krr with respect to the argument s was found to be , (14) the corresponding closed analytical form of eq. (14) was given by bagherbandi (2011). the linearization of krr by means of the taylor series yields , (15) where s1 is an approximate value to the argument s. non-isostatic correction the isostatic mass balance depends on loading and effective elastic thickness, rigidity, rheology of the lithosphere and viscosity of the asthenosphere (e.g., watts 2001). moreover, the glacial isostatic adjustment, present-day glacial melting, plate motion, mantle convection and other geodynamic processes contribute to the overall isostatic balance, which are not modeled realistically by the adopted isostatic model. it imply that: δgi ≠ 0. to fulfill the condition of the isostatic equilibrium of f = δgi = 0 in eq. (3), bagherbandi and sjöberg (2012) introduced the non-isostatic correction term. by analogy with their definition, we apply the non-isostatic correction v to the isostatic gravity gradient functional f in eq. (13). hence . , (16) the non-isostatic gravity gradient correction v is given by , (17) the coefficients are generated by applying the following integral convolution , (18) the non-isostatic gravitational attraction gnie is computed approximately from (cf. sjöberg 2009) , (19) where is the seismic moho depth, and the nominal compensation attraction (of zero-degree) c 0 ~g stipulated at the sphere of radius r is computed as (20) results the study area of the tibetan plateau and himalayas is bounded by the parallels of 25 and 40 arc-deg northern latitudes and the meridians of 70 and 110 arc-deg eastern longitudes. the solid topography of the study area is shown in fig. 1. the solid topography was computed on a 1×1 arc-deg grid using the coefficients of the dtm2006.0 global topographic/ bathymetric model (pavlis et al. 2007) with a spectral resolution complete to degree/order 250. the maximum bathymetric depth is 0.3 km, and the maximum topographic heights reach 5.8 km. f r,ω( ) = trrcs r,ω( )+vrrc,0 r,ω( ) s=s1 + dvrr c r,ω( ) s=s1 ∂krr r,ψ, s( ) ∂s = − r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+3 n+1( ) n+ 2( ) n=0 ∞ ∑ sn+2 pn t( ) krr r,ψ, s( ) ≅krr r,ψ, s1( )+ ∂krr r,ψ, s1( ) ∂s s=s1 ds vrr nie r,ω( ) = 4πg r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+3 n+1( ) n+ 2( ) 2n+1n=0 ∞ ∑ cn,m nie m=−n n ∑ yn,m ω( ) cn,m nie cn,m nie m=−n n ∑ yn,m ω( ) = 2n+1 4π gnie r,ω( ) pn t( ) d ʹ′ω ∈φ ∫∫ ʹ′ω d f r,ω( ) = trrcs r,ω( )+vrrc,0 r,ω( ) s=s1 + dvrr c r,ω( ) s=s1 100 robert tenzer, mohammad bagherbandi, lars e sjöberg, pavel novák 100 1. isostatic gravity gradients we applied the gravimetric forward modeling technique to compute the vmm isostatic gravity gradient with a spectral resolution complete to degree 250 of spherical harmonics. all gravity gradient-related computations were realized on a 1×1 arc-deg surface grid. the computation consists of three numerical steps. the spherical harmonic analysis of the crustal density structures (eqs. a.4-a7) was first applied to generate the coefficients ln,m and un,m , which describe the mass density (or the density contrast) distribution within the topography, seawater (bathymetry), sediments and remaining crustal structures. the spherical harmonic synthesis (eqs. a.1-a.3 and eqs. b.1-b3) was then applied to compile the respective gravity gradient corrections and the consolidated crust-stripped gravity gradient. the ice stripping correction due to glacial cover of the tibetan plateau and himalayas was not applied due to the absence of reliable data of the ice thickness. the vmm isostatic gravity gradient was finally obtained from the consolidated crust-stripped gravity gradient after applying the isostatic compensation gradient (see section 3). the gravity gradient was generated using the goco-03s coefficients (mayer-guerr et al. 2012). the parameters of grs-80 (moritz 2000) were used to define normal gravity gradient. the topographic and bathymetric stripping gravity gradient corrections were calculated using the dtm2006.0 coefficients (pavlis et al. 2007). the average density of the upper continental crust of 2670 kg m-3 (see hinze 2003) was adopted as the topographic density. the same value was used to define the reference crustal density. the bathymetric stripping gravity gradient correction was evaluated utilizing the seawater density-depth equation (tenzer et al. 2011). the 1×1 arc-deg data of sediments and consolidated crustal layers from the global crustal model crust1.0 were used to evaluate the respective stripping gravity gradient corrections. note that these two corrections we computed with a spectral resolution only up to degree 180 of spherical harmonics. the goco-03s gravity gradient over the study area is shown in fig. 2. it ranges from -1.4 to 1.4 e (e = eötvös), with a mean of -0.04 e and a standard deviation of 0.6 e. the regional map of the consolidated cruststripped gravity gradient is shown in fig. 3. this refined gravity gradient is everywhere negative and varies between -11.9 and -0.5 e, with a mean of -5.7 e and a standard deviation of 3.5 e. the absolute gravity gradient maxima are situated in central tibet, while the corresponding minima are in the sichuan, tarim, qaidam and indo-gangetic basins. the vmm isostatic gravity gradient is shown in fig. 4. it varies between 14.2 and 2.3 e, with a mean of 8.6 e and a standard deviation of 3.4 e. the pattern of the isostatic gravity gradient is very similar to the consolidated crust-stripped gravity gradient, while having opposite sign. 2. vmm moho model the values of the isostatic gravity gradient (shown in fig. 4) were used to solve the vmm inverse problem for finding the moho depth within the study area on a 1×1 arc-deg grid. the system of observation equations was formed by discretizing the integral equation in eq. (12). the zero-order tikhonov (1963) regularization was applied to stabilize the solution. the inversion scheme with the regularization parameter α was described in the following vector-matrix form δŝ = ( at a+α i)-1 at1, (21) where a is the design matrix of the discretized integrals (i.e., the righthand side of eq. 12); δŝ is the parameter vector of the unknown mohodepth parameters ds ; the regularization matrix r is the identity matrix i , i.e. r≡i ; and i is the vector of observables consisting of values of the isostatic gravity gradient. the solution to the system of observation equations in eq. (21) for finding the unknown moho-depth parameters ds was carried out iteratively using the gauss-seidel method (e.g., young 1971). a priori error model was not applied. the stopping criterion of the convergence between the results of two successive steps (k and k+1) was set based on the following condition:||s(k+1) s(k)||2 = ||dŝ (k+1)||2 ≤ c, where c is a limit of convergence. the iteration stopped when the relative differences between two successive solutions reached 0.1%. the choice of the regularization parameter was based on minimizing the rms of differences between the gravimetric and seismic models. in this way, we selected the regularization parameter for the gravimetric solution which had the best rms fit with the crust1.0 moho depths. the result is shown in fig. 5. the vmm moho depths, determined within the study area, vary between 44.0 and 66.9 km, with a mean of 56.2 km and a standard deviation of 5.8 km. the most pronounced pattern of the moho geometry is a significant contrast between the crustal thickness of orogenic formations compared to continental sedimentary basins. the largest crustal thickness is detected beneath himalayas and tibetan plateau; the moho depths there typically exceed 60 km. the moho depths substantially decrease to less than 50 km in the sichuan, tarim, qaidam and indo-gangetic basins. to model the differences between the isostatic and seismic moho models, we further applied the non-isostatic correction into the isostatic gravity gradient. the non-isostatically corrected gravity gradient data were then used to refine the vmm moho solution. the non-isostatic corrections to the isostatic gravity gradient and to the moho depths are shown in fig. 6. the former varies between -2.4 and 3.1 e, with a mean of 0.3 e and a standard deviation of 1.0 e (see fig. 6a). the latter is between -6.8 and 8.0 km, with a mean of 2.3 km and a standard deviation of 3.2 km (see fig. 6b). the vmm moho depths corrected for the non-isostatic effect are shown in fig. 7. these moho depths vary between 31.1 and 67.9 km, with a mean of 54.2 km and a standard deviation of 7.5 km. the application of the non-isostatic correction changed significantly a spatial pattern of the moho geometry (compare figs. 5 and 7). the most noticeable changes are related to the maximum moho depths in central tibet and less pronounced contours of the crustal thickness between orogens and sedimentary basins. 3. correlation analysis we investigated the spatial correlation of the consolidated cruststripped gravity gradient with the non-isostatically corrected and uncorrected moho geometry. the scatter plots are shown in fig. 8. as seen in fig. 8a, the consolidated crust-stripped gravity gradient has a systematic trend with respect to the vmm moho depths. the largest (absolute) values of this gravity gradient correspond with the largest spatial variations of the moho geometry under orogens attributed to a high correlation of the consolidated crust-stripped gravity disturbances with the moho geometry (cf. tenzer et al. 2012c). the application of the non-isostatic correction to the vmm moho depths changed the spatial correlation between investigated quantities, especially under orogens (cf. fig. 8b). to explain this spatial behavior, we further investigated the relation between the non-isostatic correction with the nonisostatically corrected and uncorrected moho geometry. the scatter plots are shown in fig. 9. whereas there is no apparent correlation of this correction with the uncorrected moho depths (see fig. 9a), the non-isostatic correction is highly corrected with the corrected moho geometry (see fig. 9b). the application of this correction thus significantly changed the spatial pattern of the moho geometry; it increased the crustal thickness in central tibet and decreased beneath most of sedimentary basins. this is also evident from the relation between the non-isostatic gravity gradient correction and the corresponding changes of the moho geometry (see fig. 10). the largest (absolute) values of the non-isostatic gravity gradient correction typically correspond with the largest changes of the moho geometry. from these findings we can conclude that the purely isostatic model significantly underestimates the crustal thickness of the orogenic formations, while overestimates the crustal thickness of continental sedimentary basins. it is worth mentioning that the errors in the gravimetrically determined moho depths attributed to the commission and omission errors of the global geopotential model are much smaller than the moho errors caused by the uncertainties in the crust1.0 crustal structure model 101isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data 101 (bagherbandi et al. 2014). the constraining information on the crustal density structure from seismic data is thus essential for the combined gravimetric-seismic moho recovery, while the accuracy of the currently available global gravity models is sufficient. 4. validation of results both regional moho solutions were compared with the eucrust07 (tesauro et al. 2008, stolk et al. 2013), crust1.0 (bassin et al. 2000) and tibxs13 (tenzer et al. 2013) moho models. the eucrust07, crust1.0 and tibxs13 moho models at the study area are shown in figs. 10-13; statistics are summarized in table 1. the differences between the vmm moho solution (shown in fig. 5) and these three moho models are shown in fig. 14; statistics of differences are given in table 2. the same compassion was done for the non-isostatically corrected vmm moho solution (shown in fig. 7). the moho depth differences are shown in fig. 15 and the corresponding statistics are summarized in table 3. as seen from the comparison in tables 2 and 3, the application of the non-isostatic correction slightly improved the rms fit of the vmm moho model with the crust1.0 and eucrust07, while decreased significantly when compared with tibxs13. the most significant change after applying the non-isostatic correction is seen in a substantial reduction of the systematic bias of the vmm moho solution relative to the crust1.0 and tibxs13 moho models. the systematic bias between the vmm and eucrust07 moho models increased in the absolute sense. summary and concluding remarks we have utilized the vmm isostatic scheme for a determination of the moho geometry from the gravity gradient data. the gravimetric forward modeling of the isostatic gravity gradient was realized in a frequency domain based on applying the spherical harmonic analysis and synthesis of the global gravity and crustal structure models. the regional moho inversion was realized in a frequency domain. the non-isostatic correction was applied to combine the isostatic and seismic moho models. the results of the gravimetric forward modeling showed that the maxima of the consolidated crust-stripped gravity gradient correspond with the largest spatial variations of the moho geometry, revealing mainly the contrast in the crustal thickness between the orogens and continental sedimentary basins. the vmm isostatic gravity gradient correction has a similar magnitude as the consolidated crust-stripped gravity gradient, while opposite sing. both gravity gradient quantities have a similar spatial distribution. the range of the consolidated crust-stripped gravity gradient is 11.4 e. the isostatic gravity gradient has a similar range of values of 11.9 e. this indicates that the gravity gradient field approximates realistically a relative geometry of the moho interface. hoverer, the refined gravity gradient field is systematically biased compared to the isostatic gravity gradient field; the difference between (absolute) mean values is 2.9 e. the results of the gravimetric moho inversion revealed how the systematic bias in the gravity gradient data propagates into the vmm moho solution. to remove the systematic bias in the regional moho model, we introduced and applied the non-isostatic gravity gradient correction for combining the isostatic and seismic moho models. this correction also accounts for the unmodelled heterogeneous sub-crustal density structures, uncertainties of applied crustal model and geodynamic processes which cannot be modeled by the adopted isostatic scheme. the application of the non-isostatic correction changed significantly the moho geometry. the spatial geometry of the non-isostatic correction indicates that the largest discrepancies between the vmm isostatic and seismic moho models at the study area are likely attributed to the active tectonic and crustal flexure. acknowledgments: the swedish national space board (snsb) is cordially acknowledged for a financial support of m. bagherbandi and l.e. sjöberg by project no. 76/10:1 and 116/12. p. novák was supported by the project 209/12/1929 of the czech science foundation. we also acknowledge the research founding for foreign experts provided to r. tenzer by the chinese 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4, 609-626. tenzer, r., and m. bagherbandi, 2012: reformulation of the veningmeinesz moritz inverse problem of isostasy for isostatic gravity disturbances. inter. j. geoscien., 3, 5, 918-929. tenzer, r., p. novák, p. vajda, v. gladkikh, and hamayun, 2012a: spectral harmonic analysis and synthesis of earth’s crust gravity field. comput. geosc., 16, 1, 193-207. tenzer, r., v. gladkikh, p. vajda, and p. novák, 2012b: spatial and spectral analysis of refined gravity data for modelling the crust-mantle interface and mantle-lithosphere structure. surv. geoph., 33, 5, 817-839. tenzer, r., m. bagherbandi, ch. hwang, and e. t. y. chang, 2013: moho interface modeling beneath himalayas, tibet and central siberia using goco02s and dtm2006.0. in: special issue on geophysical and climate change studies in tibet, xinjiang, and siberia from satellite geodesy. terr. atm. ocean. scie., doi: 10.3319/ tao.2012.11.01.02 (tibxs). appendix a: gravimetric forward modeling tenzer et al. (2012a) developed and applied a uniform mathematical formalism of computing the gravity corrections. it utilizes the expression for the gravitational potential v generated by an arbitrary volumetric mass layer with a variable depth and thickness while having laterally distributed vertical mass density variations. for a computation point (r,ω), it reads v r,ω( ) = gm r r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+1 vn,m yn,m ω( ) m=−n n ∑ n=0 n ∑ . (a.1) where gm=3986005×108 m3 s-2 is the geocentric gravitational constant. the potential coefficients vn,m in eq. (a.1) are defined as , (a.2) where ρearth =5500kg/m3 is the earth’s mean density (cf. novák 2010). the numerical coefficients { } are given by (a.3) the terms and un,m yn,m m=−n n ∑ define the spherical lower-bound and upper-bound laterally distributed radial density variation functions ln and un of degree n. these numerical coefficients combine the information on the geometry and mass density (or mass density contrast) distribution within a volumetric layer. the computation of these coefficients is realized to a certain degree of spherical harmonics using discrete data of the spatial density distribution (typically provided by means of density, depth and thickness data) of a particular structural component of the earth’s interior. the spherical functions ln and un, and their higher-order terms {lnk+1+i( ),un k+1+i( ) : k = 0,1,... ; i =1,2, ..., i } are generated using the following integral convolutions (a.4) (a.5) the heights/depths du and dl of the upper-bound and lower-bound of the volumetric mass density layer are stipulated with respect to the sphere of radius r. for a specific volumetric layer, the mass density ρ is either constant ρ, laterally-varying ρ (ω') , or – in the most general case – approximated by a 3-d density model. for each lateral column ʹ′ω : ʹ′ω ∈φ{ } , we have (a.6) the nominal value of the lateral density ρ(du,ω') is stipulated at du. this density distribution model describes the radial density variation (by means of the coefficients { α:i = 1,2,...,i} and β ) within the volumetric mass layer at a location ω' . alternatively, when modeling the gravitational field of anomalous density structures, the density contrast δρ of a volumetric mass layer relative to the reference density ρc is defined as (a.7) where δρ(du,ω') is the nominal value of the lateral density contrast stipulated at du. here the reference density ρc of the homogenous crust was used. fln,m (i) , fun,m (i) : i = 0,1, ..., i ln,m yn,m m=−n n ∑ 103isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data 103 appendix b: consolidated crust-stripped gravity gradient the consolidated crust-stripped gravity t gradient is obtained from the corresponding gravity gradient component trr after applying the topographic and crust density contrasts stripping gravity gradient corrections. the computation is realized according to the following scheme (tenzer et al. 2012b) (b.1) where and v are, respectively, the gravitational gradients generated by the topography and density contrasts of the ocean (bathymetry), ice, sediments and remaining anomalous density structures within the consolidated (crystalline) crust. the radial gravity gradient vrr at a point (r, ω) is computed as , (b.2) where tn,m are the (fully-normalized) numerical coefficients which describe the disturbing gravity potential t (i.e., the difference between the earth’s and normal gravity potentials). the coefficients tn,m are obtained from the coefficients of a global geopotential model (describing the earth’s gravity field) after subtracting the spherical harmonic coefficients of the normal gravity field. the computation of the gravity gradient corrections in terms of the potential coefficients vn,m (see eq. a.2) for a particular structural component of the earth’s crust is realized using the following expression , (b.3)vrr = ∂2v ∂r2 = gm r3 r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+3 n+1( ) n+ 2( ) vn,m yn,m ω( ) m=−n n ∑ n=0 n ∑ table 1: statistics of the moho models. table 2: statistics of the differences between the vmm moho solution and the eucrust-07, crust1.0, and tibxs13 moho models. table 3: statistics of the differences between the non-isostatically corrected vmm moho solution and the eucrust-07, crust1.0, and tibxs13 moho models. fig.1 solid topography of the study area (in km). fig.2 regional map of the goco-03s gravity gradient (in e). fig.3 regional map of the consolidated crust-stripped gravity gradient (in e). fig.4 regional map of the isostatic gravity gradient (in e). trr r,ω( ) = ∂ 2t ∂r2 = gm r3 r r ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ n+3 n+1( ) n+ 2( )tn,m yn,m ω( ) m=−n n ∑ n=0 n ∑ 104 robert tenzer, mohammad bagherbandi, lars e sjöberg, pavel novák fig.5 vmm moho depths (in km) based on the isostatic gravity gradient inversion. a. b. fig.6 the non-isostatic correction to: (a) the gravity gradient (in e), and (b) the moho depths (in km). fig.7 vmm moho depths (in km) corrected for the non-isostatic effect. a. b. fig. 8 scatter plots of the consolidated crust-stripped gravity gradients (shown in fig. 3) versus: (a) the vmm moho depths (shown in fig. 5), and (b) the non-isostatically corrected vmm moho depths (shown in fig. 7). a. 105isostatic crustal thickness under the tibetan plateau and himalayas from satellite gravity gradiometry data b. fig. 9 scatter plots of the non-isostatic correction to the gravity gradient (shown in fig. 6a) versus: (a) the isostatic moho depths (shown in fig. 5), and (b) the nonisostatically corrected moho depths (shown in fig. 7). fig. 10 relation between the non-isostatic correction to the gravity gradient and moho depts. fig. 11 eucrust07 moho model (in km). fig. 12 crust1.0 moho model (in km). fig. 13 tibxs13 moho model (in km). a. b. 106 robert tenzer, mohammad bagherbandi, lars e sjöberg, pavel novák 106 c. fig. 14 comparison of the vmm moho solution with: (a) eucrust-07, (b) crust1.0, and (c) tibxs13 moho models (in km). a. b. c. fig. 15 comparison of the non-isostatically corrected vmm moho solution (nie-vmm) with: (a) eucrust-07, (b) crust1.0, and (c) tibxs13 moho models (in km). more than 40% area of the xixian new area is a loess deposit region, and most of the loess landform is tableland and terrace where the thickness of loess is considerable large. therefore, loess collapsibility will be the most important geotechnical problem in future foundation investigation and construction. to explore loess collapsibility in the xixian new area, we conducted the k0 compression test, based on the collapsibility mechanism, which has different combinations of pressure (0~1.2 mpa) and water content (4%~sat). based on the σ-ε curve under different water content, we calculated the generalized collapse settlement and collapsibility coefficient of every water content under every pressure by subtracting the relevant curve from the saturated curve and analyzed the cross action of pressure and water on loess compressibility. the results show that the average collapsibility level of the northern xixian new area is self-weight collapsible level ii, with a lower limit of 14 m. the compressibility of loess is proportional to pressure and water content. under low water content, the collapsibility coefficient δs increased while the pressure increased, but under medium and high water content, δs will reach a peak with increasing pressure and after that, δs will decrease until its value is close to constant. when under the same pressure, δs decreases when water content increases. if set the additional strain 1.5% as collapse start criterion, then the first collapse pressure pi will linear proportional to water content. the initial collapse water content wi will increase sharply when pressure raises under low pressure, but wi will reach a constant value of 26% when pressure is more than 200 kpa. this consequence will be meaningful for future geotechnical investigation and design in the xixian new area. abstract keywords: xixian new area; loess collapsibility; wetting; pressure; initial collapse water content influence of pressure and water content on loess collapsibility of the xixian new area in shaanxi province, china issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n4.66106 earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017): 197-202. jiading wang*, yan ma, qianyi guo, di chu state key laboratory of continental dynamics, northwest university, xi’an 710069, china *corresponding author of email: wangjd@nwu.edu.c para estimar la colapsabilidad en xixian se realizó el test de compresión (k0), de acuerdo con el mecanismo de colapsabilidad, con diferentes combinaciones de presión (0~1.2 mpa) y contenido de agua (4%~sat). basado en la curva σ-ε con diferentes contenidos de agua se calcularon los coeficientes de asentamiento y colapsabilidad a partir de substracción de la curva en condiciones de saturación y analizando los efectos de presión y contenido de agua. los resultados muestran que el promedio de colapsabilidad por su propio peso para el área de xixian es de nivel ii, con un límite bajo de hasta 14 m. la compresibilidad de loess es proporcional a la presión y al contenido de agua. con un bajo contenido de agua, el coeficiente de colapsabilidad (δs) se incrementó mientras la presión se aumentó; pero, con contenidos medios y altos de agua, el coeficiente de colapsabilidad alcanzó su pico con el incremento de la presión, y luego bajó para prácticamente estabilizarse. bajo presión constante, la colapsabilidad descendió al aumentar el contenido de agua. si se asume como criterio inicial de colapso (pi) una deformación adicional de 1.5%, pi es linealmente proporcional al contenido de agua. el contenido de agua inicial de colapso (wi) se incrementa drásticamente cuando la presión se eleva a baja presión, sin embargo, el contenido de agua alcanza un valor constante de 26 % cuando la presión es de más de 200 kpa. estos resultados son significativos para la investigación geotécnica y el diseño estructural en xixian. resumen palabras clave: distrito especial de xixian (china); colapsabilidad de loess; humectación; presión; contenido de agua incial de colapso. record manuscript received: 05/07/2017 accepted for publication: 16/11/2017 how to cite item: wang, j., ma, y., guo, q., & chu, d. (2017). influence of pressure and water content on loess collapsibility of the xixian new area in shaanxi province, china. earth sciences research journal, 21(4). 197-202. doi: http://dx.doi.org/10.15446/esrj.v21n4.66106 influencia de la presión y el contenido de agua sobre el índice de colapsabilidad de loess, en el distrito especial xixian, provincia de shaonxi, china h y d r o l o g y 198 jiading wang, yan ma, qianyi guo, di chu 1. introduction china’s loess plateau is a vast territory with a large population. however, since the late quaternary, the climate and ecological environment in this area have been deteriorating, resulting in frequent loess disasters, which seriously restrict economic development and urban construction (zhang, 1980; abbasi et al., 2017). loess is a special soil that has a loose structure, high porosity and collapse and vibration sensitivity (derbyshire, 1995). therefore, the study of collapsible loess plays an important role in urban construction in the loess area. because of its special overhead pore structure and high sensitivity of its microstructure to interactions between water and force, aeolian loess has significant collapsibility. previous researchers (rabinovich & urinov, 1974; kozubal & steshenko, 2015) have studied the collapsibility of loess. gao (1980) and lei (1987) came up with a classification for loess microstructure that achieved good results, which has been used up to today. many scholars (zhenghan & zudian, 1986; yang, 1988; miao, 1999; wang & gu, 2013, zhang, 2000; bakar et al., 2017) conducted an in-depth study of the mechanism of collapsible deformation of loess. chinese studies (zhang, 1995; liu, 1997; zhang, 2000; ma, 2014; usman et al., 2017) developed research on engineering properties of loess in china. in engineering practice, a standardized evaluation and treatment system has been established by collapsibility and lower limit depth (people’s republic of china national standard for building construction in collapsible loess area gb 50025-2004). however, from the above common features, the engineering properties of loess in china have regional characteristics (bata et al., 2017). differences in spatial distribution of loess deposits, provenance differences, and a series of physical and chemical reactions that differ from region to region after deposition affect the microstructure characteristics of loess and change its macroscopic physical and mechanical properties. more than 40% area of the xixian new area is a loess deposit region. most of the loess landforms are tableland and terrace where loess thickness is very large. therefore, loess collapsibility is the most important geotechnical problem in future foundation investigation and construction. as a new state-level development area, the xixian new area is an important part of the western development and is the integration of xi’an and xianyang. it is also an important link in the strategy of building an international metropolis. the loess plateau area in the northern part of the xixian new area will allow the development of logistics and cultural tourism industries around the existing xianyang airport (airport metro) and historical and cultural scenic spots (qin and han dynasties). the scale and span of construction in this area will expand rapidly with an influx of investment over a short period. the number and scale of original construction located in the loess plateau area in the northern part of the xixian new area is small and large portions to be developed are still farmland; therefore, we have accumulated less basic engineering-related data in this area. meanwhile, there is little research on the engineering properties of loess in this area. also, the engineering properties of loess vary in space. therefore, studying the collapsibility of loess in the xixian new area will have significant application value and scientific significance (gao et al., 2017). the traditional collapsibility of loess is the nature of the additional settlement caused by the saturation of saturated loess when the deformation of loess under certain pressure is stable. to deepen the understanding of the mechanism of loess collapsibility, some researchers (zhang, 1990, 1992; sun & liu, 2000, francisca, 2007; yasin et al., 2017) have studied the collapsible deformation of loess during the processes of humidification and dehumidification. the definition of collapsibility is extended to the nature of additional settlement of loess by humidification under a certain pressure to form a stable deformation (yew & rahim, 2017). the collapsible deformation of loess is considered a kind of plastic deformation, and the amount of collapsible deformation is related to the state of humidification (hussin et al., 2017). it is independent of the infiltration path, which indicates that loess has a memory of moistening deformation under certain pressures. however, the amount of humidification deformation and its variation law in loess are both different from the soil, so it is impossible to establish a relational expression for calculating deformation (ismail et al., 2017; sarkar et al., 2017). based on this, we carried out a pre-investigation of loess in the xixian new area, and the k0 test for the typical horizon under different water content and pressure. the purpose of the study is to provide reference and scientific guidance for future engineering investigation and design in the loess distribution area in the northern part of the xixian new area (tariq et al., 2017; tunggolou & payus, 2017). 2. test schemes 2.1 physical parameters of soil foundation the study area is located between the cities of xian and xianyang. the location of this area in shaanxi province is shown in figure 1. five exploratory wells as the cross-shaped of the five endpoints were placed on the large thickness loess plateau in the north of xixian new area (as fig. 1), and the depth of exploratory wells is 30 m. the sampling (fig. 2) and testing of the t1, t2, t3, t4 and t5 exploration wells were completed by the state key laboratory of continental dynamics in the northwest university of china (fig. 3). t1, t2, t3 and t5 exploration wells were located in the loess plateau, and t4 was located in the second terrace of the weihe river. by comparing the five groups of experimental data, we found that the t4 site was a site of first-grade self-weight collapsibility. also, the four sites in the loess plateau are sites of second-grade selfweight collapsibility. moreover, the closer a sampling point is to the center of the loess plateau, the greater the amount of collapsibility and self-weight collapsibility and the deeper the depth of collapse. although the amount of collapsibility and self-weight collapsibility in each sampling site are different, the homogeneity of soil samples at a depth of 5 m is higher, which is shown in table 1. therefore, selected the malan loess located 5 m deep at the t3 site near the xianyang airport as the representative layer. we further studied the influence of pressure and water content on the collapsibility of loess in the xixian new area. figure 1. the sketch of location, landform, and sample diagrammatic in xixian new area 199influence of pressure and water content on loess collapsibility of the xixian new area in shaanxi province, china figure 2. sampling in site exploration wells figure 3. automatic air pressure consolidation instrument (gzq-1) at t3, two 6-m-deep exploratory wells were re-excavated, and 40 soil samples were taken from the borehole wall at 5 m. in the field, the soil samples were sealed with plastic wrap to keep moisture content unchanged and then wrapped in bubble film as a buffer. basic physical and mechanical parameters tests were carried out immediately after samples were transported to the laboratory. these tests measured the natural density, dry density, plastic limit, liquid limit, plastic index, liquid index, void ratio, water content, compression coefficient, compression modulus, the coefficient of collapsibility and coefficient of self-weight collapsibility of soil. the collapsibility test used the single line method. table 1. physical and mechanical properties of ma lan loess at a depth of 5 m in each sampling well 2.2 high-pressure k0 compression test scheme for loess with different water content to explore the effects of interaction between water content and pressure on the compressibility of undisturbed loess in the xixian new area, we designed a k0 compression test scheme with ten kinds of water content and nine levels of pressure (see table 2). the preparation methods of soil samples with different water content are as follows. first, we cut ring-knife samples from massive soil samples at the site. each sample was 2 cm high and had a surface area of 50 cm2. we tested soil moisture content in the upper and lower parts of the ring-knife sample and took the average value. based on the moisture content of each sample, we calculated the amount of water that needed to be increased according to the target moisture content. to ensure uniform moisture content, we dropped water into soil samples that needed wetting, with 6 g/d as an upper limit. soil samples that are necessary to be dehumidified were uniformly dried to a lower water content (about 3.5%). then we calculated the current water content according to the sample’s measured mass. finally, we prepared soil samples using the same humidification method. we prepared saturated soil samples by saturating the soil sample with the natural water content after the first stage pressure compression. to prevent a humidity difference between permeable stone, filter paper and soil samples from affecting humidity test results, we put the permeable stone and filter paper into the soil, which was mixed with water to the same water content as in table 2 after being dried to reach a humidity balance. the test instrument was an automatic air pressure consolidation apparatus (gzq-1 type) produced by nanjing soil instrument factory (fig. 3), and the test methods refer to ‘the standard of soil test methods’ (highway research institute of ministry of communications gb/t 50123 1999 geotechnical test method standard). table 2. k0 compression test scheme 3. test results and analysis 3.1 effect of pressure and water content on compressive deformation properties of loess in the xixian new area the relation curves of pressure and strain at different levels of water content (fig. 4) and the curves of water content and strain under different levels of loess pressure in the xixian new area (fig. 5) were all obtained using the k0 test with 10 kinds of water content and 9 levels of pressure. the bivariate trend surface of compressibility coefficient, water content and pressure are plotted based on the two sets of data 200 jiading wang, yan ma, qianyi guo, di chu figure 4. p-ε curves of loess under different water content figure 5. w-ε curves of loess under pressure at all levels figure 7. p-ε curves of loess under different water content figure 8. δs-w curves of loess under different pressures figure 6. bivariate trend surface of compressive strain and water content, pressure of loess as figure 4 suggests, compressive deformation increases with the increase of pressure at all levels of water content, but the increasing regularity with increasing water content is different. under the low water content conditions (4%, 7%, and 10%) the pε curves of loess in the xixian new area are linear. as the water content increased to 13%, the p ε curve began to show an inflection point, which indicated the axial strain increased significantly with the increase of pressure, and the shape of the curve changed from linear to s-sharped. the position of the pressure value corresponding to the inflection point decreased with increased water content. this phenomenon reflects a law that the mechanical stability of loess microstructure decreases as water content increases. as figure 5 suggests, compressive deformation increases with the increase of water content at all pressure levels, but the increasing regularity with increasing pressure is different. under low-pressure conditions (50, 100 and 150 kpa), when moisture content increases from 4% to 40%, strain increases linearly. with the increase of pressure, the wε curve, similar to the p-ε curve, also began to show an inflection point. the water content corresponding to the inflection point decreases as pressure increases, and the rise of water content means a decrease in suction. this phenomenon reflects a law that the water stability of loess microstructure decreases with increasing pressure. as figure 6 suggests, pressure and water content affect the compression deformation of loess. in general, there is a positive correlation between water content, pressure and compression deformation. these two factors and the compression coefficient form a three-dimensional trend surface similar to a ‘loess hill’ in which the lower part is gentle, the middle is steep, and the top is smooth again. the gentle lower part indicates that humidification under low-pressure conditions and compression under low water content conditions will not cause a large compressive deformation. the steep middle shows that humidification under middle-pressure conditions and compression under middle water content conditions will cause compressive deformation to increase significantly. the recovery of the smooth slope at the top indicates that the growth of this compressive strain was restored to a gentle trend under high pressure and high-water content. 3.2 effect of pressure and water content on compressive deformation properties of loess in xian new area the data in figure 4 are equivalent to carry out nine sets of double wire test of soil samples with different initial water content. the difference of strain was obtained by subtracting the compression curves of unsaturated soil samples under nine levels of water content from the full compression that is loess collapsibility. the relation curves of loess collapsibility coefficient and pressure at different levels of water content are plotted in figure 7, which shows relation curves between collapsibility coefficient and water content of loess under different pressure levels 201influence of pressure and water content on loess collapsibility of the xixian new area in shaanxi province, china as figure 7 suggests, under low water content conditions (4%, 7%, 10%, and 13%), as pressure increases, the loess collapsibility coefficient in the xixian new area increases slowly at first, then decreases gradually after reaching a particular pressure, and finally, the growth rate gradually slows down. the δs-p curve is a sigmoid. under medium-high water content conditions (16%, 19%, 22%, 25% and 28%), as pressure increases, the collapsibility coefficient increases to a peak value and then decreases, eventually becoming constant. the reason for this phenomenon is that the strength of the cement in the skeleton and particles of loess is larger under low water content conditions, and the effective stress caused by matrix suction on the particles is larger, which leads to an increase in friction between particles. therefore, the strength and stiffness of soil microstructure are lower under low water content conditions, and an increase of pressure cannot do a large amount of damage to loess pore structure. finally, the compression deformation is small before soaking. if the deformation of saturated soil under pressure at all levels is the total macroscopic deformation of the deformable microstructure under that pressure, the amount of deformation before saturation will be smaller, the amount of collapsibility after saturation will be greater, and the corresponding coefficient of collapsibility will also be greater. for soil samples under high water content conditions, as water content increases, the infiltration of water will weaken the contribution of the cement and matrix suction to the stable state of the particles. the collapsibility coefficient increases as pressure increases before the pressure has not reached the microstructural strength at this saturation state. however, when the pressure exceeds the micro structural strength at this saturation state, the large amount of deformation of the compression stage is caused by a large amount of destroyed overhead pore structure. once the structural strength is exceeded, deformation increases as pressure increases and the proportion of the total deformation is larger. finally, the collapsibility coefficient decreases as pressure increases. as figure 8 suggests, the collapsibility coefficient decreases with the increase of water content at all pressure levels. when loess is under the combined action of certain pressure and soaking saturation, the adjustable upper limit of pore structure is the total strain of saturated soil under this pressure. the whole strain is composed of compression deformation before saturation and collapsible deformation after saturation. the reasons for the decrease of the collapsibility coefficient with the increase of water content are as mentioned earlier. under constant pressure, microstructure strength decreases as soil moisture content increases, and the deformation is greater in the pre-compression stage. after that, the proportion of the deformation in the collapse stage (i.e. the collapsibility coefficient) will be reduced accordingly. the initial collapse pressure is a crucial parameter in the investigation and design of a collapsible loess foundation and takes an additional settlement caused by saturation of more than 1.5% as the criterion of collapsibility. based on the data in figure 7, we used an interpolation method to calculate the initial pressure, pi, of the collapsible loess at all levels of water content and then draw the curve of pi with water content (fig. 9). meanwhile, based on the data in figure 8, we used an interpolation method to calculate the initial water content, wi, of the collapsible loess at all levels of pressure and then draw the curve of wi with pressure (fig. 10). as figure 9 suggests, the initial pressure of loess collapsibility in the xixian new area increases linearly as water content increases. this phenomenon is due to the compressibility of soil rising as water content increases. therefore, with the increase of water content, a part of the adjustable overhead pore structure can be compressed before it reaches saturation, and to produce more than 1.5% of the collapsible strain after soaking, a greater axial pressure is required. as figure 10 suggests, under low-pressure conditions (100 and 150 kpa), the initial moisture content of loess in the xixian new area increased sharply with the increase of pressure. this shows that the microstructure of loess is more sensitive to humidification under low pressure. but after the pressure reached 200 kpa, and up to 1.2 mpa, the moisture content of collapsibility was stable at about 26%. the results show that loess is not sensitive to the lower level of humidity under high pressure. this is because porous loess structure appears to be micro arch, and the normal stress between particles increases as pressure increases before reaching the structural strength under the initial water content. accordingly, friction resistance is also stronger, and the suction drop effect caused by low-level humidification is offset. the moisture content of 26% is the critical value of moistening deformation of loess in the xixian new area under pressures of more than 200 kpa. when the moisture content does not exceed the critical value, it will not produce significant collapsibility. 4 conclusion based on five representative sites in the loess plateau and the high order of loess deposition area reconnaissance, digging and indoor tests, we evaluated collapsibility grade of loess in the northern part of the xixian new area. at the same time, we carried out a special experimental study on the collapsibility of typical loess stratum. conclusions are as follows: 1. we found that the t4 site was a site of first-grade self-weight collapsibility. in addition, the four sites in the loess plateau are sites of second-grade self-weight collapsibility. meanwhile the closer to the loess plateau hinterland, the larger the collapsibility of the site, which makes for a lower depth of collapsibility. 2. pressure and water content affect the compression deformation of loess. in general, there is a positive correlation between water content and pressure. these two factors and the compression coefficient form a surface with an initial gentle slope, a steep slope in the middle, and return to gentle slope surface at the top. the surface has a shape similar the ‘loess hill’ three-dimensional trend surface. figure 9. relationship between the initial pressure of collapsible loess and moisture water figure 10. relationship between initial water content of collapsible loess and pressure 202 jiading wang, yan ma, qianyi guo, di chu 3. under the same water content, when wi was lower (4%, 7%, 10% and 13%), the coefficient of collapsibility of loess in the xixian new area increased slowly as pressure increased, and then the coefficient of collapsibility increased sharply. finally, the growth rate slowed down gradually. at the same time, the δs -p curve was s shaped. for middle and high water content (16%, 19%, 22%, 25%, and 28%), the collapsibility coefficient increased to a peak value as pressure increased. then it began to decrease and tended to increase with increasing pressure. under the same pressure, the collapsibility coefficient decreases gradually as moisture content increased. 4. if the additional settlement is greater than 1.5% for collapsibility criterion, the initial pressure of loess collapsibility increases linearly with the increase of water content in the xixian new area. however, the initial moisture content of collapsibility increases sharply with the rise of pressure at low pressure (100 and 150 kpa). additionally, when the pressure is between 200 and 1.2 mpa, the moisture content of collapsibility is stable at about 26%. acknowledgement the paper is mainly funded by the state key program of national natural science of china (41630639) and national natural science foundation of china (41372269) and the state key laboratory of continental dynamics foundation (2014). herein, i express my deepest gratitude to them and those who have given me further help during the completion of this paper. references abbasi, n., khan, m.s., mughal, m.s., & yasin, m. 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(in chinese) keywords: qinghai-tibet plateau; eastern margin area; geological structure; basic characteristics; evolution research; crustal movement. palabras clave: meseta qinghai-tibet; área del margen oriental; estructura geológica; características básicas; investigación de la evolución; movimiento de la corteza. how to cite item deng, d., wang, c., & peng, p. (2019). basic characteristics and evolution of geological structures in the eastern margin of the qinghaitibet plateau earth sciences research journal, 23(4), 283-291. doi: https://doi.org/10.15446/ esrj.v23n4.84000 based on field geological survey and stratigraphic profile survey in the eastern margin of the qinghai-tibet plateau, the basic characteristics and evolution of geological structure in the eastern margin of the qinghai-tibet plateau are studied. the dongyuan area of the qinghai-tibet plateau is divided into the late cenozoic period and the current period. during the late cenozoic, the pliocene xigeda lacustrine deposits develops from 4.2 mabp to 2.6 mabp, with 9 coldwarm climate change stages. there are 4.3 mabp old glacial period in this area, and 5 extreme paleoclimate events in quaternary. at present, the horizontal movement intensity and mode of different tectonic zones are determined by the northward extrusion, eastward extrusion and rotation around the eastern tectonic junction in the study area, and the stages of the movement state changing with time are related to the gestation and occurrence of extra-large earthquakes. at present, the three-dimensional crustal movement shows that the tectonic activity differentiation of mountain and basin, which is related to tectonic dynamic environment and deep material activity, is related to the compression, shortening and uplift of plateau mountain and the extension and subsidence of basin, reflecting the inheritance of neotectonic activity. through practical analysis, it is found that the eastern margin of the qinghai-tibet plateau is composed of minshan fault block and longmenshan structural belt. the left-lateral dislocation of minjiang fault is roughly the same as the vertical dislocation. in longmenshan tectonic belt, the right-lateral dislocation of maowenwenchuan fault, beichuan-yingxiu fault and other main faults is the same as the vertical dislocation. abstract basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau características básicas y evolución de las estructuras geológicas en el margen oriental de la meseta qinghai-tíbet issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84000 en este trabajo se estudian las características básicas y la evolución de la estructura geológica en el margen oriental de la meseta qinghai-tíbet con base en el análisis geológico de campo y el estudio del perfil estratigráfico de la zona de estudio. el área de dongyuan de la meseta qinghai-tíbet se divide en el período cenozoico tardío y el período actual. durante el cenozoico tardío, los depósitos lacustres del plioceno xigeda se desarrollaron de 4.2 mabp a 2.6 mabp, con 9 etapas de cambio climático frío-cálido. hay 4.3 mabp viejos períodos glaciales en esta área, y cinco eventos paleoclimáticos extremos en el cuaternario. en la actualidad, la intensidad del movimiento horizontal y el modo de las diferentes zonas tectónicas están determinados por la extrusión hacia el norte, la extrusión hacia el este y la rotación alrededor de la unión tectónica oriental en el área de estudio, y las etapas del cambio del estado del movimiento con el tiempo están relacionadas con la gestación y ocurrencia de terremotos extragrandes. en la actualidad, el movimiento cortical tridimensional muestra que la diferenciación de la actividad tectónica de la montaña y la cuenca, que está relacionada con el entorno dinámico tectónico y la actividad del material profundo, está relacionada con la compresión, el acortamiento y la elevación de la meseta de montaña y la extensión y hundimiento de cuenca, que refleja la herencia de la actividad neotectónica. a través del análisis práctico, se descubre que el margen oriental de la meseta qinghaitíbet está compuesto por el bloque de falla minshan y el cinturón estructural longmenshan. la dislocación lateral izquierda de la falla de minjiang es aproximadamente la misma que la dislocación vertical. en el cinturón tectónico de longmenshan, la dislocación lateral derecha de la falla de maowen-wenchuan, la falla de beichuan-yingxiu y otras fallas principales es la misma que la dislocación vertical. resumen record manuscript received: 27/04/2019 accepted for publication: 25/09/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 283-291 dejie deng1, changliu wang2*, peihao peng3 1college of earth sciences, chengdu university of technology, chengdu, 610059, china 2architecture and urban planning college, southwest minzu university, chengdu, 610041, china 3college of tourism and urban-rural planning, chengdu university of technology, chengdu, 610059, china * corresponding author: clwang19@163.com g eo lo g y 284 dejie deng, changliu wang, peihao peng introduction the eastern margin of the qinghai-tibet plateau mainly refers to the region between the eastern himalayan tectonic knot and the western qinling orogenic belt. the eastern kunlun fault system borders the western qinling mountains in the north, the longmenshan tectonic belt borders the yangtze block in the east, and the sichuan-yunnan block in the southwest, i.e. the southeastern margin of the qinghai-tibet plateau (including eastern tibet, western sichuan, yunnan, central and eastern myanmar, china-myanmar-lao-vietnam border region, but mainly sichuan-yunnan region) bounded by the sn-oriented allright-lateral strike-slip fault system and the yushu-xianshuihe-xiaojiang arc fault system (wang, tang, & wang, 2017). geotectonically, the area is the convergence zone of yangtze block, songpan-ganzi block, lanping-simao block, baoshan block, lushui-longling block, gangdise-tengchong block and other micro-blocks. during cenozoic, the area underwent multi-stage magmatic activities and strong tectonic deformation, which resulted in many fault structure with different scales and properties (liang et al. 2017). during the neotectonic period, especially during the quaternary or late quaternary, under the dynamic background of the continuous collision between the indian plate and the eurasian plate, the crustal movement in this area is very active. there are many active faults with different scales, types and activities, which are also the most prominent strong earthquake active zones in china. activities are characterized by high frequency and intensity, and are accompanied by quaternary volcanic activities, such as the famous tengchong volcanic area (jiang, ruan, & chen, 2016). the process of neotectonic deformation and its dynamic mechanism in the eastern margin of the qinghai-tibet plateau, the present pattern of active tectonic deformation, and the relationship with large earthquakes are all issues of great concern in the field of geological science at home and abroad (yang, miao, dan, liu, & wang, 2017). because these problems are closely related to the present crustal deformation mode and its dynamic mechanism of the qinghai-tibet plateau and even the whole east asian continent, and to the uplift process of the qinghai-tibet plateau and its genetic mechanism, the correct understanding of the neotectonic movement law and the characteristics of active tectonic system on the eastern margin of the qinghai-tibet plateau is also the correct establishment of kinematic model for crustal deformation process and the key to exploring its dynamic mechanism in the present qinghaitibet plateau (feng, gao, yang, hui, & zhou, 2017). therefore, this area has been the key area to deeply understand the current crustal deformation mode and its dynamic mechanism in the qinghai-tibet plateau and east asia, and to explore the eastward extrusion of material in the plateau and its regulation mode (zhang et al., 2016). around the above problems, predecessors have done a lot of work in neotectonics and active structures, seismogeology, geophysics and so on, and have put forward many models or viewpoints on the latest structural deformation mode and its dynamic mechanism in this area. among them, the most representative ones are the outlaw-shaped tectonic system, the extrusion or rotation mode of sichuan-yunnan rhombic block, the continental escape mode, the rotation movement mode of fault block and the lower crustal flow mode. the above viewpoints have important influence on the deep understanding of neotectonic movement and active tectonic deformation mode in this area. however, with the emergence of new data, especially new achievements in active fault research, geophysical exploration and gps observation, it is difficult to fully explain the current mode of crustal activity, dynamic mechanism and its relationship with strong earthquake activity and migration in this area due to the limitation of technical means and research level at that time (wei, guo, yao, zhu, & wang, 2016). therefore, by studying the basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau, we can systematically obtain the latest crustal deformation process, the temporal and spatial framework of neotectonic evolution and other important continental dynamic background information, reconstruct the active tectonic system and kinematic geological model of the area, and consolidate the seismogenic area of the area. the basis of geological work is not only helpful to further understand the latest crustal deformation process and mechanism of the qinghai-tibet plateau, to scientifically evaluate the future trend of large earthquakes in the region, but also to provide a more reliable geological basis for the rational planning, development and utilization of regional territory and the evaluation of crustal stability of major engineering projects (hu et al., 2016). materials and methods late cenozoic tectonics age and environment of pliocene xigeda lake facies deposition on the eastern margin of the qinghai-tibet plateau a set of fluvial and lacustrine deposits were widely developed in the dadu river and anning river basins during the pliocene, indicating that there was a lake development period in the eastern margin of the qinghai-tibet plateau, which was the pliocene pan-lake period. this study is of great significance in discussing the coupling relationship between pliocene paleoenvironmental changes and uplift of the plateau. haiziping section in luding and zhoujiagou section in mianning are typical sections of xigeda river lacustrine deposits in the western sichuan plateau (yi & wang, 2016). the lake facies stratigraphic section of haiziping lake in luding begins in ganhaizi and ends at wantou and dengzhangwo through jindongzi. the total thickness is 440.5m, which is divided into 15 layers. the zhoujiadagou section in mianning is located in zhoujiadagou, fengcao village, about 7 km north of the county town. the total thickness of the section is 300.7m. it can be roughly divided into two sections according to lithology, namely, the yellow layer with 103.5m thickness and the grey layer with 197.2m thickness. it can be divided into 18 layers. previous scholars have studied the xigeda formation in western sichuan. there have been different opinions on the sedimentary age of the xigeda formation, which directly affects the study of the paleoenvironment of the fluvial and lacustrine facies strata. in this paper, high resolution magnetic stratigraphic samples of typical sections in this area with spacing of 25 cm to 50 cm are sampled and tested on the superconducting magnetometer of institute of geomechanics and institute of geology and geophysics, chinese academy of sciences. the results of magnetic strata and environmental records obtained by superconducting magnetometer show that the haiziping section is mainly formed in the gauss period, with the development ages ranging from 4.2 mabp to 2.6 mabp. the geological age belongs to the middle and late pliocene. the xigeda formation in luding is the earliest section in the xigeda lacustrine facies. the zhoujiagou section of the anning river basin is also formed in the gauss period. the boundary between gauss positive polarity and gilbert negative polarity is 297.2m in the mianning xigeda formation, and the sedimentation time is about 3.58ma-2.6ma. according to the above results, the preliminary results of magnetic stratigraphy of xigeda formation distributed in dadu river, anning river and jinsha river valley are very consistent and can be well compared. the xigeda formation is mainly formed during the gauss positive polarity period, probably beginning at about 4.2 ma and ending at 2.6 ma at the earliest (xiao, tan, hu, zhou, & he, 2017). the results of magnetostratigraphy provide a chronological guarantee for the study of long-time-scale paleoenvironmental changes recorded in xigeda lacustrine deposits. environmental indicators such as organic matter content, grain size and sporopollen in the time frame provide information on pliocene environmental changes in the eastern margin of the plateau. the environmental records of organic carbon content in haiziping profile show that the lowest value of organic carbon is 0.14%, the highest value is 1.04%, and the average value is 0.38%. organic carbon records show that the palaeoenvironmental changes in haiziping area of luding have undergone nine stages of palaeoclimate change: cool→ warm →cold →warm →cold→ cold →cold →cool. six warm→ cool cycles have appeared in the xigeda deposit of mianning. pollen and isotope analysis also recorded the periodic variation of paleoclimate during the period of 4.2 mabp to 2.6 mabp (chen et al., 2016). based on the preliminary study of the evolution pattern of paleoenvironment during pliocene in western sichuan, a new understanding of the relationship between paleoenvironment changes in the eastern margin of the qinghai-tibet plateau and the stage uplift of the whole plateau has been obtained. geological characteristics of quaternary glaciers on the eastern margin of the qinghai-tibet plateau the eastern margin of the qinghai-tibet plateau is a part of the eastern extension of the plateau, with an average altitude of over 3500 m. the eastwest mountain ranges on the plateau have turned into nearly north-south mountain ranges in this area, which is a transitional area from the first step 285basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau nuclide exposure is 17 kabp-7 kabp, and the penultimate glacial period is also developed. there are two glacial stages in zheduoshan area. the last glacial exposure age is 19 kabp to 24 kabp. the older glacial moraines advance to zheduotang, kangding. there are three grade overlying loess moraine platforms on the north bank of zagunao in the east of lixian county, western sichuan province. the geographic coordinates are 31°26′48.7″ in the north latitude and 103°10′10.8″ in the east longitude. the esr age of the gravel layer in the high platform is 691,000 years, that in the middle platform is 613,000 years, and that in the low platform is 79,000 years. the gravel layer in the high platform and the middle platform represents the two stages of large-scale glacial development in this area. in the northern part of the shaluli mountains, the ancient ice caps of nearly 2000 km2 have been developed in the haizi mountains of ganzi and baiyu. the geographic coordinates of haizi mountains in the core area are 31°13′48.7″ in the north latitude and 99°51′12.6″. the typical glacial remains such as glacial lakes, glacial troughs and valleys, side till dikes, terminal till dikes, polished surfaces, sheep’s back stones and glacial debris have been developed in the area. preliminary studies show that there are two glacial stages in this area. six complete arc-shaped end-moraine dykes have been developed in nalengcaogou valley. the cosmic nuclide exposure age of the glacial debris in the central part is 20,000 years. many moraine dykes represent different stages of glacial retrogression since the last glacial period. the lateral moraine dike on the west bank of amara trough valley is earlier than the last moraine dike in nalengtao, which is the penultimate glacial period. with further study, this area will become one of the typical quaternary glacier areas in the eastern margin of the qinghai-tibet plateau. in a word, the moraine layer at the bottom of xigeda formation in haiziping, luding represents the old glacial period 4ma ago in the eastern margin of the qinghai-tibet plateau. there are five glacial periods in the quaternary period. the oldest one is the moraine outcropped from daqiangliangzi on the eastern side of luoji mountain. it is 2.2 million years ago. the second one is the ancient glacier developed from 1 million to 1.1 million years ago in the source area of anning river. the second one is the ancient glacier from 600,000 to 700,000 years ago, with large scale. it corresponds to the deep-sea o isotope stage 16-18, represented by the high and middle moraine platforms of zagunaohe river in lixian county. another stage is about 200,000 years ago, which is equivalent to stage 6-8 of deepsea o isotope, represented by moraines on the southern slope of tanggula mountains. the last stage is 20,000 to 70,000 years ago, which corresponds to two to four stages of deep-sea o isotope, represented by moraines in litang haizi mountain, kangding zheduoshan mountain and baiyu haizi mountain in western sichuan. current construction spatial difference and time stage characteristics of horizontal motion velocity field in region in view of the fact that no major earthquakes occurred in the eastern border area of the qinghai-tibet plateau between 1999 and 2009 since the establishment of the “network project”, we can approximately regard the long period of gps horizontal motion velocity field from 1999 to 2009 before the wenchuan earthquake as the background trend pattern of horizontal motion in the study area to analyze the spatial zoning differences. the temporal characteristics of horizontal motion changes related to the occurrence of large earthquakes (bu et al., 2016) are analyzed by using the recent motion state from 2009 to 2017 after the earthquake. the horizontal velocity field of the eastern margin of the qinghai-tibet plateau from 1999 to 2009 is shown in fig. 1. movement are mainly reflected in the differences of intensity and mode of movement. (1) the difference of movement intensity between different tectonic parts is mainly manifested in the lower horizontal movement rate of the qinghai-tibet plateau and the south china block, the ordos block and the alxa block, while the internal movement rate of the plateau increases significantly, reflecting the difference between the intensive activity of the qinghai-tibet plateau and the activity of the relatively stable blocks around it. for example, the gps velocity vector in qaidam-qilian mountains in the northeastern margin of the qinghai-tibet plateau has been reduced from 15mm/a to 20mm/a in the interior of the plateau to 5mm/a to 10mm/a in the boundary zone of the plateau, even smaller, and the crust has been shortened obviously. the horizontal velocity of gps stations near the seismogenic to the second step of chinese geomorphology. late cenozoic glacial deposits are well developed in the eastern part of the qinghai-tibet plateau, which is a typical area for the development of quaternary paleoglaciers in the transitional zone between the eastern and western parts of the mainland of china. many scholars have studied the quaternary glaciers in this area and discussed the glacial evolution and global change of the qinghai-tibet plateau, which has a good working basis. several paleoglaciers have been discovered in recent years. the discovered glacial remains have been chronologically studied. the regional quaternary glacial sequence has been established and the regional paleoclimate and paleoenvironment changes in the late cenozoic have been discussed (jie, ya, de, & gao, 2017). a set of 43 m gravel beds is developed at the lacustrine bottom of xigeda formation in luding, western sichuan, with the geographic coordinates of 29°54′50.4″ in the north latitude, 102°12′51.0″in the east longitude, and elevation of 1940 m. according to the analysis of gravel sedimentary characteristics and surface morphology of quartz sand by scanning electron microscopy, it is considered that it belongs to a set of ancient moraine beds. the results of magnetostratigraphic study show that the moraine is formed in the kirchati positive polarity event of the gilbert polar epoch, about 4.3 ma from now on, which is the oldest cenozoic glacial age ever discovered in east asia. at about 10 km in qionghai, xichang, a 15 m gravel layer is developed on the eastern side of the luoji river. its geographic coordinates are n27°42′42″in the north latitude and 102°21′33″ in the east longitude. its elevation is 2575 m. it is distributed in the watershed of zemu river and ezhang river. the gravel layer is a set of purple-grey mud-gravel layer overlying the daqing liangzihe lacustrine sedimentary stratum. the gravel in the gravel layer mainly comes from luoji mountain in the west. the gravel is poorly sorted and has no directional arrangement. the sedimentary characteristics of gravel layer and the results of sem analysis of quartz sand show that this layer is a typical moraine layer. the daqing liangzi moraine layer corrodes with the overlying daqing liangzi lake facies deposits. based on the detailed study of the magnetic strata of the river and lake facies layers, the paleomagnetic calculation shows that the age of the bottom boundary of the daqing liangzi formation is about 2.14 ma. therefore, the daqing liangzi moraine is deposited in the early pleistocene, and its absolute age should be earlier than 2.14 million years ago. a set of gravel layers is developed in the back hill of jiancao village, 7 km from the north of mianning county. the geographic coordinates are 28 33.738’ in the north latitude and 102 12.610’ in the east longitude, and the altitude is 1972 m. the gravel layers extend southward to the canteen where 108 national highway meets the anning river. the thickness of gravel layer in jiancao is about 20m. it is a mixed accumulation of gray-white and grayyellow clay, silt and gravel. the gravel weathers strongly, and its separation and grinding are poor. there are fractured glaciers and streaks which have not been completely disconnected. there are many groups of scratches on the gravel. the bottom of the gravel layer is grey-black claystone and siltstone of xigeda formation in gauss period, which are unconformity contact relations. the discovery of sedimentary fabrics, fractured rocks and streaked rocks and the occurrence of distant granite drift gravel indicate that the maple channel gravel layer should be a typical moraine layer (wang, tang, & wang, 2017). at the same time, the results of scanning electron microscopy of quartz sand particles in gravel layer show that the gravel layer has the characteristics of shell fracture, parallel cleavage and parallel scratch, which further proves that the gravel layer is glacial origin. in the study of the age of the moraine gravel layer in jiancao, esr dating samples are collected from the upper, middle and lower parts of the gravel layer. the ages of the upper, middle and lower samples are 822,000 years, 1012,000 years and 128,800 years, respectively. they are deposited in the middle and late pleistocene, roughly in the same period as the thick loess l9-l15 in the loess plateau. the quaternary glacier development sequence in the eastern part of the qinghai-tibet plateau can be established based on the determination of cosmic nuclide age of moraines in tanggula mountain, the north-central section of shaluli mountain and the zheduo mountain and the study of 21ne, 10be and 26al ages of glacial debris. the results of radionuclide dating show that there are three glacial stages in tanggula mountains, and one glacial stage develops before 180kabp. the smaller glacial advance occurs before 68kabp. the oldest glacial moraine in tanggula mountains has a high degree of weathering and distributes on the front plateau of tanggula mountains. the last glacial period developed in litang-yidun haizishan area. the time window of cosmic 286 dejie deng, changliu wang, peihao peng structure of the longmenshan fault zone before the wenchuan earthquake is smaller, except the horizontal velocity of internal gps stations in bayan hara block is 1/3 or 1/2, especially the relative velocity of the trans-longmenshan fault zone is less than 1 mm/a. the wenchuan m8 earthquake in 2008 occurred in the background of the eastern movement of the bayan hara massif in the northeastern qinghai-tibet plateau, which was blocked by the south china massif for a long time, and the honghe fault in southwestern yunnan province. the horizontal velocity of clockwise rotation from the southwestern side of the rift to the border between china and myanmar decreases significantly from north to south. (2) the difference of movement modes is mainly manifested in the clockwise slow torsional movement from ne, nee and gradually to e-s in the northern part of the eastern margin of the qinghai-tibet plateau. if the qilian-haiyuan main fault zone is taken as the boundary, the torsional movement in the southern side is significantly larger than that in the northern side, resulting in significant intense compression and left-lateral shear. in the shape region, the motion is generally characterized by extrusion, thrust and torsion. besides the eastward movement of the bayan hara block hindered by the south china block and characterized by both compression and strikeslip, the central and southern parts of the study area generally exhibit se-sse horizontal slip and clockwise rotation around the vertical axis with the sichuanyunnan rhombic block as the main body. the sichuan-yunnan rhombic block is ne-oriented in jianchuan and lijiang—xiaojinhe fault is bounded by se to sse. in southwestern sichuan-yunnan, the nw-oriented honghe fault is the northern boundary, and its movement rate is obviously weaker than that of sichuan-yunnan rhombic block. the direction of movement is gradually shifted from sse to s. the nw-yongde longling and setang-mengzha faults near ns in the western margin play a certain role in shielding and absorbing the clockwise movement rate in this area, and the gps stations in the western side even have the effect of absorbing the clockwise movement rate. the motion in the direction of ssw is presented. these are consistent with the inheritance trend of tectonic movement in the study area since the neotectonic period, reflecting that the present horizontal movement is controlled by the northward compression, eastward extrusion and rotation around the eastern tectonic junction of the qinghai-tibet plateau. the horizontal velocity field images of the eastern margin of the qinghai-tibet plateau from 2009 to 2017 are shown in fig. 2. from figure 2, it can be seen that the overall shape of horizontal movement in the large area during this period seems to be similar to that in the earlier period, but there are still some inconsistencies in some tectonic locations, reflecting that the changes of current horizontal movement in the region have a temporal stage related to large earthquake events. (1) from 2009 to 2017, compared with 1999-2009, the overall movement rate of the bayan hara block towards e-e shifted to s increased, especially in the wenchuan earthquake area and nearby gps stations. in addition, the horizontal velocity projection of gps stations across longmenshan tectonic belt from the eastern margin of the bayan hara massif to the chengdu plain shows that the se-sse horizontal velocity of gps stations in wenchuan earthquake-prone zone of longmenshan fault and its western side is about 20 mm/a from 2009 to 2017, while the velocity of gps stations in chengdu plain is still about the same as the previous period. similarly, the amplitudes of motion on both sides of the fault are quite different (zhang, liu, & huang, 2017). these reflect the dislocation of seismogenic tectonic rupture in wenchuan earthquake. the large-scale thrust-dominated crustal extensional movement on the west side of the seismic rupture zone releases the strain energy accumulated by long-term compression and blockade, and is in the state of adjustment relaxation after the earthquake. (2) the movement rate of gps stations at the southern end of longmenshan mountains and the intersection area of anning river fault, the eastern part of the sichuan-yunnan border and the western qinling tectonic belt to the south of longmenshan mountains are relatively reduced, reflecting that these zones are in a state of sustained compression. (3) the clockwise rotational velocity of the sichuan-yunnan rhombic block is also increased, and the difference between the strength of the block and that of the outer boundary is more obvious. (4) the se-oriented motion of gps stations on the southern side of the ganzi-yushu fault, where the seismogenic structure of the yushu m7.1 earthquake occurred in april 2010, increased, reflecting the influence of left-lateral strike-slip dislocation of the seismogenic structure of the yushu earthquake on the tectonic activity in this area. analysis of 3-d crustal movement characteristics in this area in order to analyze and compare the current crustal horizontal and vertical motion reflected by gps and leveling observations in the eastern margin of the qinghai-tibet plateau, we first use the leveling network in the study area as the starting data, and use the least squares collocation method to calculate the surface fitting, so that the vertical velocity surface data corresponding to the continuous spatial distribution of the region are obtained (liu, cai, & chen, 2017). the vertical motion rate color maps of different regions are drawn with different colors. the rising and falling rate values in the color maps are marked at the bottom of the map with color scale. the red-yellow color represents the area with higher rising rate, and the blue-light blue color represents the area with larger falling rate. secondly, the horizontal motion velocities of gps stations in two periods are separately plotted. vector labeling is used in the color map of regional vertical motion rate, and a more intuitive regional horizontal and vertical three-dimensional crustal motion image is obtained. the image is shown in figure 3. by analyzing and comparing the present three-dimensional horizontal and vertical crustal motion images in the eastern margin of the qinghaitibet plateau shown in figure 3, it can be seen that the crustal velocity field of regional horizontal motion is relative to the background field of longfigure 1. image of horizontal motion velocity field from 1999 to 2009 figure 2. image of horizontal motion velocity field from 2009 to 2017 287basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau time-scale vertical motion, whether in the period of 1999-2009 before the wenchuan earthquake or in the period of 2009-2017 after the recent earthquake. the vertical uplift movement of the zones with strong horizontal compressionshortening movement is also remarkable, i.e. the red and yellow range in fig. 3. they have certain symbiotic characteristics, such as the western qinling mountains and the vicinity of liupanshan mountains in the north of the region, the eastern part of the western sichuan plateau in the south of the region, and the northeastern mountain area of the sichuan-yunnan rhombic block, etc. similarly, basins with horizontal extensional movement are mainly sub-ducted vertically, such as zhongdian-lijiang area in the south of sichuanyunnan rhombic block and yinchuan basin in the north of the study area. these reflect that the three-dimensional crustal movement in the eastern margin of the qinghai-tibet plateau under the dynamic environment of extrusion, extrusion and rotation presents the inherited general features and general trend of crustal compression and shortening to form uplift of mountain areas and extensional subsidence of basins. the western qinling mountains, liupanshan mountains, qilian mountains and helan mountains in the northern part of the eastern margin of the qinghai-tibet plateau are extruded and uplifted, and the basins are relatively subsided, forming a vertical differential movement gradient zone between the mountains and basins. in the south-central part of the eastern margin of the qinghai-tibet plateau, besides the overall uplift of the western sichuan plateau to the west of the longmenshan fault, the north-central rise of the sichuan-yunnan rhombic block is also significantly different from the south relative subsidence, showing a general trend of east-west extension subsidence and edge compression in the sichuan-yunnan block. on the basis of the above, the zonal areas of strong vertical differential movement and relatively significant horizontal compression movement in the study area are selected, which are se-oriented across longmenshan structural belt, ne-oriented across west qinling fault zone and sse-oriented across anning river fault zone on the eastern boundary of sichuan-yunnan rhombic block. they are also the seismogenic faults of wenchuan earthquake and their surrounding structures, shown as the pink rectangular box in fig. 3. projection of horizontal motion velocity across fault zone along the direction of dominant velocity of gps station is obtained. firstly, from the point of view of the above, the movement state changes of wenchuan earthquake area and its peripheral related tectonic fault area before and after wenchuan earthquake are obtained. the horizontal velocity projection of gps station across wenchuan seismogenic structure from the eastern margin of the bayan hara block to the chengdu plain has a dominant direction of se. from 1999 to 2009 before the wenchuan earthquake, the relative horizontal velocity near longmenshan fault is small and the difference between the two sides of the fault is weak. from 2009 to 2017, the se-see horizontal velocity of gps stations near longmenshan fault zone and its west side is about 20 mm/a, while the velocity of gps stations in chengdu plain is still the same as that in the previous period. the great difference in the amplitudes of motion between the two sides of the fissure reflects the dislocation of the seismogenic tectonic rupture of the wenchuan earthquake. the crustal movement of the western side of the seismic rupture zone, which is dominated by huge thrust, releases the strain energy accumulated by long-term compression and blockade, and is in the post-earthquake adjustment state. secondly, from the projection maps of horizontal velocity of gps stations crossing the western qinling fault zone and the anning river fault zone crossing the eastern boundary of sichuanyunnan rhombic block in the periphery of the earthquake area, it can be seen that during the long period from 1999 to 2009 before wenchuan earthquake, the horizontal velocity of gps stations on both sides of the fault shows quasilinear trend distribution; from 2009 to 2017 after wenchuan earthquake, the quasi-linear distribution trend of velocity values across faults changes, and the difference of horizontal motion amplitude between the two sides of the fault increases (li, zheng, & peng, 2018). the horizontal motion rate in the south side of the fault in the northern margin of the west qinling mountains is obviously higher than that in the north side of the fault, while the horizontal movement in the east and west side of the anning river fault is stronger and weaker in the east side and the difference is more significant. the former reflects the strong thrust and dextral dislocation of the seismogenic structure of the wenchuan earthquake, which results in the intensification of compression movement and the possible locking of faults in the western qinling tectonic belt. the latter may reflect the strong left-lateral dislocation of the yushu earthquake with m 7.1, triggered by the wenchuan earthquake, which occurred at the southern boundary of the bayan-hara block (ganzi-yushu fault). the clockwise rotational acceleration of the sichuan-yunnan rhombic block strengthens the compression in the eastern boundary-anninghe fault zone. further analysis of the strong vertical uplift and remarkable horizontal compression movement of the two sections shown in fig. 4 shows that the strain accumulation in the fault zone crossed by the section after the wenchuan earthquake is still in progress. this is consistent with previous scholars’ understanding that coulomb rupture stress of wenchuan earthquake has a role in strengthening stress accumulation in the eastern boundary tectonic areas of the west qinling and sichuan-yunnan blocks. results minshan fault block the minshan fault block originated from the gonggaling mountains in the north and passed southward through hongxingyan, xuebaoding, xueguzhai and maobaoshan, disappearing in the longmenshan tectonic belt north of maoxian county. it lies between the near east-west motianling structural belt (a) 1999 to 2009 (b) 2009 to 2017 figure 3. crustal movement image 288 dejie deng, changliu wang, peihao peng and the north-east longmenshan structural belt, forming an eastern boundary of the qinghai-tibet plateau. from the deep tectonic background, minshan fault block is situated in the steep change zone of crustal thickness in the east and west of china, constituting the boundary of two first-order neotectonic units in the strong uplift area in western china and the weak uplift area in eastern china, and the middle section of the north-south seismic belt. minshan fault block is a north-south trending neotectonic uplift, which is quite different from the eastern and western landforms. its formation mechanism is closely related to the movement characteristics of the eastern and western boundary faults since the neotectonic period. huya fault is the eastern boundary of minshan fault block, which extends in nnw direction, inclines westward with uncertain dip angle. the eastern part of the fault is middle-low mountain area with planation elevation of about 3200 m-3500 m and the western part of the fault is minshan, with planation elevation of about 4200 m-4500 m. therefore, the huya fault should be characterized by a thrusting from west to east, and the planation planes on both sides of the east and west should be broken perpendicularly by about 1000m. considering the fact that the latest flattened strata in this area are pliocene laterite slope formation (n2h) and the oldest one after the disintegration of planation plane is early pleistocene guanyinshan formation (wan & du, 2018), the average vertical slip rate of huya fault since quaternary should be about 015 mm/a. the new activity of huya fault shows clear linear characteristics in aerial and satellite photographs. along the fault line, fault ridges, fault scarps, fault-fault gullies, alluvial fans and river terraces can be seen. at about 1 km in xiaohebei, huya fault cut out of date from the alluvial fan and forms a relatively obvious fault scarp, which results in a left-lateral displacement of the gully forming the alluvial fan of 47 m, as shown in fig. 4. figure 4. new active trace and fault geomorphology map of tiger tooth fault near xiaohe it can be seen from fig. 4 that the front of the alluvial fan and the fujiang’s secondary river terrace are in a gradual transition state, and the analysis should belong to synchronous heterogeneous deposits. at the top of the second terrace, the age of sub-sandy soil determined by thermoluminescence method is 32700a ±2600a. based on this, the average horizontal slip rate of huya fault since late quaternary is estimated to be about 114mm/a. based on this, the average horizontal slip rate of huya fault since late quaternary is estimated to be about 114 mm/a. it is interesting to note that the huya fracture cut a small dry ditch on the alluvial fan at a depth of only 0.3m-0.5m and a width of about 4m-5m with a left-handed dislocation of 413m, as shown in figure 5. from figure 5, it can be seen that the lateral edge walls and pipelines on both sides of xiaogangou are synchronously left-handed staggered, with good consistency. it is very likely that they are the co-seismic dislocations of the songpan-pingwu earthquake of m 7.2 in 1976. minjiang fault is the western boundary of minshan fault block, which constitutes the boundary between songpan-ganzi orogenic belt and motianling block. the fault originates from the north of gonggaling mountains and is intercepted by the east kunlun fault. after extending southward through zhangla to songpan, it continues to extend southward along the west bank of minjiang river to jiaochang and ma laoding, and disappears in the area of animal husbandry shops north of maoxian county. the minjiang fault can be divided into three sections: north, middle and south, roughly bounded by the main temple of sichuan and jiaochang. in the southern section of jiaochang, we can see the slope ridge landform formed by the thrust of the west wall of the fault on the quaternary strata, and cut the holocene slope diluvium staggeringly. in history, there were the earthquakes of m 7 in diexi in 1713 and m 7.5 in diexi in 1933. the active tectonic landform of chuanzhusibianchang section is slightly weaker, but new fault phenomena of quaternary can still be seen, especially in the vicinity of muerzhai and songpannan. the new activity of minjiang fault north of chuanzhusi is more distinct. it is composed of three fault segments with different lengths and strikes, which are featherlike. the right-step feather-like zone between gami temple and chuanpan is 3 km apart, which leads to the deformation of minjiang river terrace. near hanpan-shuijing, the minjiang fault forms obvious fault scarps on minjiang terrace and alluvial fan. the height of the fault scarp on the terrace surface ranges from 6 m to 10 m, and that on the alluvial fan is about 16 m. through trench excavation, the steep fault ridge near chuanpan reveals a thrust zone with a width of about 4 m formed by four to five reverse faults. the occurrence of the main fault is n45°e, inclined to nw and dip angle is 40°. the gravels in the thrust zone are obviously arranged in a directional direction and have the general characteristics of overthrust faults. the top of the second terrace is a calcareous cemented gravel layer (the tl age of the top is 27000a±2100a), so the displacement marks are clear. the average vertical slip rate of the minjiang fault since the late pleistocene should be greater than and equal to 0 137 mm/a, because the displacement of the fault cannot be completely preserved until the river is cut down. the fault scarps on the hanpancun alluvial fan have obvious retrogressive characteristics, with slopes ranging from 20° to 30°. fine sand beds with rhythmic characteristics are deposited in the lower wall of the scarps (the tl age at the top is 30200a±2300a). considering the general process of the steep ridge development of the reverse fault, the average vertical slip rate of the minjiang fault since the late pleistocene should be less than 0153mm/a. near shuijing, the minjiang fault passes through the surface of the minjiang river terrace on the west bank of the minjiang river, forming two to three fault scarps which are nearly parallel and of varying height. the lowest fault scarp is 1 m-2 m high, the highest is 7 m-8 m, and the total height is about 10 m. in the gully of shuijing, under the steep fault ridge, a reverse fault can be seen in the quaternary gravel layer. the gravel layer on the upper wall of the fault bends slightly at the cross section, forming a towed anticline, and the gravel layer along the fault surface has obvious directional arrangement. in addition, several small gullies on the second-order surface are left-handedly dislocated by faults with a dislocation of about 10 m. the above phenomena indicate that the minjiang fault shows the nature of reverse-strike-slip movement, and the horizontal dislocation is approximately equal to the vertical dislocation. based on the activity of the two boundary faults since the neotectonic period, it is likely that the minshan fault block is formed by the differential movement of the minjiang fault and huya fault caused by the westwardeastward thrust. the starting time should be roughly from the end of miocene to the beginning of pliocene, which is related to the slip of the chuanqing block towards the south-east-east direction. geological evidence and modern figure 5. rupture horizontal dislocation dry ditch 289basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau geodetic deformation data show that the slip process of the sichuan-qinghai block towards the southeast-east direction is still continuing, resulting in strong modern seismic activity in the minshan fault block area. longmenshan tectonic belt maowen-wenchuan fault in order to study the maowen-wenchuan fault, it is necessary to study the maowen-wenchuan fault in maowen and wenchuan counties. firstly, the maowen-wenchuan fault in maowen county is analyzed. the section of maowen-wenchuan fault in north brick factory of maowen county is shown in figure 6. figure 6. maowen-wenchuan fault profile in figure 6,  shows phyllite and slate schist,  shows bedrock fracture zone,  shows river facies gravel bed, and  shows directional arrangement of gravel. through investigation and analysis, it is found that the right-handed dislocation of the first tributary of minjiang river on the maowen-wenchuan fault reaches hundreds of meters. although it is not yet possible to determine the starting time of faults after the formation of the river system and to exclude the influence of erosion, the synchronous dextral bending of the river system should be the geomorphological evidence of the dextral shear movement of the maowen-wenchuan fault and probably the cumulative displacement since the quaternary. in the brick and tile factory north of maowen county, we can see the fault river terrace of maowen-wenchuan fault. the bedrock fracture zone here is metamorphic phyllite, slate and schist of the silurian maoxian group, with a visible width of 20 m-30 m (sun, zhao, & wang, 2018). it directly thrusts over the sandy gravel layer of the third-grade river terrace. the occurrence of the fault is n35°e, with an inclination of se and an inclination angle of 63°. a gravel orientation zone with a width of about 0.3 m to 0.5 m is formed along the section. in the middle of the terrace, the age of the sandy gravel layer determined by tl method is 23700a±1900a. based on this, the average vertical slip rate of the fault since late pleistocene is estimated to be about 0184mm/a. secondly, the maowen-wenchuan fault in wenchuan county area is studied. the paleoearthquake sand veins in wenchuan county area are shown in figure 7. figure 7. paleo-seismic sand veins of sand gravel layer in jiangwei city, wenchuan river terraces in figure 7,  shows the gray-yellow slope alluvial gravel layer,  represents the brown-gray sand with sub-prismatic breccia,  represents the coarse sand,  represents the brown-gray gravel,  represents the yellowishbrown fine sand,  represents the yellowish-yellow fine sand, and  represents the green-gray fine-silt. in jiangweicheng on the south bank of minjiang river in wenchuan county, there are compressive-torsional faults in alluvial-diluvial strata with 120 m high river bed. the faults strike n30°e, with an inclination of se and an inclination angle of 70° and form “entry” structure with the main faults, indicating the right-lateral dislocation of the faults. in addition, there is a group of sand veins in alluvial-diluvial gravel strata corresponding to the elevation of river terraces in jiangweicheng. the width of sand veins is generally 0.5 cm-1 cm, and the maximum width is 3 cm-5 cm, which may be two paleoearthquake events. beichuan-yingxiu fault beichuan-yingxiu fault is mainly studied in yingxiu area and dongxujiagou of baishuihe river. firstly, beichuan-yingxiu fault near yingxiu is studied. the section of north east wall of beichuan-yingxiu fault exploration trough in this area is shown in fig. 8. figure 8. profile of the north east wall of the beichuan-yingxiu fault exploration trough in yingxiu area in figure 8,  is brown red clay with limestone fragments,  is yellowish clay,  is black clay with peat, and  is modern humus layer. it can be seen from the graph that the beichuan-yingxiu fault near yingxiu extends in the direction of n60-70°e. some scholars have studied the phenomenon of pengguan complex thrusting over the quaternary fluvial sandy gravel bed in the north-west wall of the fault. in yingxiu substation, a ne trench with a length of about 100 m, a width of about 20 m and a depth of about 5 m is formed on the terrace surface of the minjiang river, and a steep ridge with a height of about 40 m is formed on the terrace surface, which is suspected to be caused by the vertical fault of the fault. for this reason, tl samples are collected on both sides of suspected faults. the age values measured are 76360a±6490a and 73000a±6200a, which are close to each other. it is confirmed that the same terrain is the fault scarp caused by differential movement of faults. this section can be interpreted by the model of structural deformation of thrust faults: the upward thrust of the nw wall of the beichuan-yingxiu fault forms a fault ridge on the fourth-order surface, while the bending fault is formed on the upper wall of the thrust faults due to local tension, and the trench in the form of graben is formed by the downward action of the bending fault. the average vertical slip rate of beichuan-yingxiu fault is estimated to be about 0154mm/a according to the height of the fault scarp and the age of terrace surface. secondly, the beichuan-yingxiu fault in xujiagou area in the east of baishui river is studied. the beichuan-yingxiu fault presents better linear image characteristics in aerial photographs. the slope of slope-cut mountain forms a typical landform of slope trough. the northwest plate of the fault declines relatively to form a fault-plug pond, which strictly restricts several smaller holocene pluvial fans. on the north-west side of the fault, there are some abandoned gullies in the south-east wall. it is estimated that the average 290 dejie deng, changliu wang, peihao peng horizontal slip rate of the fault since late pleistocene should be between 0182 mm/a and 113 mm/a, and the average value is about 1.1 mm/a. in addition, near fengyanzi, a small gully in beichuan-yingxiu fault which will develop on the third terrace surface has a dextral dislocation of 10 m, but the starting time of the dislocation is unknown. a groove about 7m in length is laid in a broken plug pond. the structure of the groove is shown in figure 9. in figure 9,  is brown-black gravel with clay and charcoal wood,  is grey-black clayey silt,  is grey-white coarse sand with gravel,  is grey-black sand with gravel and charcoal,  is grey-white clay,  is brown-yellow sandy clay,  is brown sand with gravel,  is blue-gray coarse sand with gravel, and  is yellow-black gravel with clay. two paleo-seismic events can be identified from the profile of the trough. seismic fault cutting events occurs in event 1 and seismic fault cutting events occurring in event 2 cut layer ,  and , resulting in vertical faults of about 800mm, which are sealed by layer . sedimentary characteristics of layer 5 are obviously controlled by the fault scarp shape at that time, and under the fault. the dish is gray-white clay and the upper dish is gravel with sandy soil. the time of the earthquake is after 21910a±290a. event 2 results in the liquefaction of grey-black clayey silt of layer , which penetrates upward into the overlying stratum (pan, yang, & zhu, 2018). the upper stratum of vein sand penetrated upward is the middle part of the brown-yellow sandy clay of layer . it is estimated that the layer  has been deposited and is in a saturated state at that time, so when the earthquake occurs, the interval is after 11770a±360a. discussion through the analysis of cenozoic structures in the eastern margin of the qinghai-tibet plateau, it is found that xigeda lacustrine facies deposits in the eastern margin of the qinghai-tibet plateau are distributed in the dadu river, anning river and jinsha river basins. detailed magnetostratigraphic studies show that the main body of the strata is gaussian deposits, which is a good record carrier of paleoenvironmental change information in the p sedimentation. xigeda formation in luding is the earliest, about 4.2 ma to present, and that of mianning and honggexigda formations is about 0.6 ma later. the sedimentary records of xigeda formation have many regular change periods. the sedimentary records of mianning section show that the sedimentary environment of 3.2 mabp has changed greatly. the sedimentary records of luding section show that the strong uplift of qinghai-tibet plateau has begun at 2.8 mabp. the xigeda ancient lake is a water-crossing lake, and the xigeda ancient lake completely disappears at 2.6 mabp. the pan-lake period on the eastern margin of the qinghai-tibet plateau is ended at the end of pliocene. in the early quaternary, the river-lake facies strata only appears in daqingliangzi, xichang. glacier and river deposits are the main deposits in the region, and the paleoclimatic environment changes greatly. at this time, typical wind-dust deposits appears on the loess plateau, which is closely related to the uplift of the qinghai-tibet plateau. in the eastern margin of the qinghai-tibet plateau, there are many glacial periods in the late cenozoic. in addition to the three glacial periods since the penultimate glacial period, three sets of moraines are developed earlier. haiziping moraine layer in luding represents the 4.3 ma old glacial age in the eastern margin of the qinghai-tibet plateau. during the quaternary period, there are five glacial periods, forming at 2.2 mabp, 1.1 mabp, 0.6-0.7 mabp, 0.2 mabp, 0.01 mabp-0.07 mabp, respectively. the paleoglacial sequence in the eastern part of the qinghai-tibet plateau is preliminarily established, which deepens the understanding of the late cenozoic glaciers in the “third pole of the earth”. with the continuous change of time, the structure of the eastern margin of the qinghai-tibet plateau is analyzed. current horizontal movement reflected by regional gps observations has the difference of spatial distribution and timevarying stages. the difference of horizontal movement intensity and mode in different tectonic zones is controlled by ne-oriented compression, e-oriented extrusion and rotation around the eastern tectonic junction of the qinghai-tibet plateau, and the periodicity of movement state changes with time (especially in the eastern tectonic junction). the strain energy accumulated in the wenchuan earthquake-generating section of longmenshan tectonic belt under long-term compression and locking is in a state of relaxation and adjustment after the concentrated release of large earthquake of m 8.0, but the tectonic location associated with it and the western qin dynasty in the periphery are in a state of relaxation and adjustment. the strain accumulation in the tectonic zones such as the eastern boundary of the western qinling and sichuan-yunnan rhombic blocks may continue. based on the above data, several main faults in longmenshan tectonic belt have shown thrust movement from nw to se since late quaternary, and have significant dextral strike-slip components. the average vertical slip rate of a single fault is about 1 mm/a, and the horizontal dislocation is approximately equal to the vertical dislocation, which is consistent with the conclusion that the average crustal shortening rate in longmenshan area measured by gps is less than 3 mm/a. according to the available paleoseismic data, the beichuanyingxiu fault and the maowen-wenchuan fault both have a history of strong earthquake activity before history. although the current data are not enough to directly determine the magnitude of paleoearthquakes, the magnitudes of earthquakes causing surface ruptures and dislocations in western china are generally above 6 or 7, it can be roughly determined that these two main faults have the ability to produce earthquakes of about 7 magnitudes. according to the research results of beichuan-yingxiu fault, it can be judged that the last strong earthquake occurred not long after 3830a±200a, based on the historical seismic records of chengdu area for more than two thousand years. the recurrence interval of strong earthquakes on the single fault of these two main faults should be at least 2000-3000a. conclusions the eastern margin of the qinghai-tibet plateau is one of the areas with the strongest neotectonic movement, strong earthquake activity and internal and external dynamic coupling. this is not only an important window to deeply understand the uplift process of the qinghai-tibet plateau and its environmental disaster effects and to understand the crustal deformation process and its dynamic mechanism of the east asian continent, but also a bridgehead for the development of the western part of china. it is the most densely populated area in western china, among which many major national transportation and hydropower projects are being implemented and will be implemented. therefore, a comprehensive understanding of the basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau, a detailed identification of the distribution of major active faults and their geometric and kinematic characteristics, and a thorough exploration of the glacier geological characteristics and three-dimensional crustal movement characteristics in the area are all very significant geological foundations of theoretical significance and application value. through theoretical analysis and practical investigation, it is found that the eastern margin of the qinghaitibet plateau has been characterized by typical tectonic deformation of the intracontinental regenerative orogenic belt since quaternary. several preexisting main faults are reversed-strike-slip faults, indicating both crustal figure 9. cross-section of the north-east wall of the north-east wall of the north-to-the-show break in the vicinity of the jiajiagou 291basic characteristics and evolution of geological structures in the eastern margin of the qinghai-tibet plateau shortening and horizontal slip. from this point of view, the eastward escape of crustal material in the qinghai-tibet plateau does exist. however, from the slip rate values of several main faults since late quaternary, it can be judged that the crustal material escape rate is much less than the estimated 20 mm/a, which is roughly consistent with the results of less than 3 mm/a measured by gps nowadays. acknowledgement the study was supported by “national natural science foundation of china (no.51508484)” reference bu, h. y., wang, x. j., zhou, x. h., qi, w., liu, k., ge, w. j., … zhang, s. t. 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(2017). influence of cross section shape of electromagnetic track launcher on emission performance. journal of china academy of electronics and information technology, 12(5), 513-517. v16n1c.indb earth sciences research journal petrology earth sci res sj vol 16, no 1 (june, 2012): 41 53 introduction zeolites are formed during aqueous fluids reaction with rocks in a variety of geological environments (breck, 1974; gottardi and galli, 1985; weisenberger, 2009) most zeolite became formed during diagenetic processes in sedimentary rocks (including volcaniclastic deposits) which can be grouped into several geological environments or hydrological systems (hay and sheppard, 2001), such as hydrologically open systems (hay and sheppard, 2001), hydrologically closed systems (langella et al , 2001), soil and surficial deposits (ming and mumpton, 1989), deep marine sediments (boles and coombs, 1977) and marine sediments from arc-source terrains (boles and coombs, 1977) zeolites occurring in volcanic lava flow cavities are formed either during lava pile burial metamorphism (neuhoff et al , 1999), continental basalts’ hydrothermal alteration (walker, 1960) or diagenesis in areas of high heat flow caused by active geothermal systems (weisenberger and selbekk, 2008) zeolites, as products of hydrothermal crystallisation, are generally known from active volcanic rockassociated geothermal systems very little work has been published on zeolite occurrences related to late stage pegmatite crystallisation (orlandi and scortecci, 1985) in hydrothermal ore veins (deer et al , 2004) as alteration products along fault planes (vincent and ehlig, 1988) and in hydrothermal fractures and veins in granites and gneisses (weisenberger and bucher, 2010) zeolites are crystalline, microporous, hydrated aluminosilicates of alkaline or alkaline earth metals the framework consists of [sio4] 4− and [alo4] 5− tetrahedra, which corner-share to form open structures; such tetrahedra are linked to each other by sharing all of the oxygen to form interconnected cages and channels containing mobile water molecules and alkali and/or alkaline earth cations (breck, 1974; barrer, 1978; gottardi natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland juan manuel triana r 1, javier francisco herrera r 1, carlos alberto ríos r 1*, oscar mauricio castellanos a 2, josé antonio henao m 3, craig d williams4 and clive l roberts4 1* escuela de geología, universidad industrial de santander, bucaramanga, colombia 2 programa de geología, universidad de pamplona, colombia 3 grupo de investigación en química estructural (gique), escuela de química universidad industrial de santander 4 school of applied sciences, university of wolverhampton, england * corresponding author e-mail: carios@uis edu co abstract natural zeolites filling amygdales and veins in tertiary basaltic host rocks on the isle of skye (nw scotland) have been studied in some detail by transmitted light microscopy, scanning electron microscopy and x-ray diffraction techniques the zeolites and associated minerals so identified in order of their relative time of formation from early to late were nontronite, amorphous silica phases, carbonate phases, chabazite, phillipsite, wairakite, thomsonite, analcime, natrolite and stilbite-type minerals zeolite formation in the skye basalts began with low si/al ratio na zeolites and a gradual increase in ca content and si/al ratio, ending up as ca zeolites they were probably formed as a consequence of late-stage hydrothermal activity, although, locally, contact metamorphism may control the process of zeolite formation resumen las zeolitas naturales como relleno en amígdalas y venas que ocurren en rocas basálticas terciarias de la isla de skye (nw escocia) han sido estudiadas en detalle por técnicas de microscopía de luz transmitida, microscopía electrónica de barrido y difracción de rayos x las zeolitas y los minerales asociados identificados, en orden de su tiempo relativo de formación, son nontronita, fases de sílice amorfa, carbonatos, chabazita, filipsita, wairakita, thomsonita, analcima, natrolita y minerales tipo estilbita la formación de zeolitas en los basaltos de skye comenzó con una zeolita rica en na de baja relación si/al y con el aumento gradual en el contenido de ca y si/al, terminó con una zeolita rica en ca estas probablemente se formaron como consecuencia de una actividad hidrotermal tardía, aunque localmente un metamorfismo de contacto pudo controlar el proceso de formación de las zeolitas palabras claves: zeolita natural, amígdala, vena, roca basáltica, isla de skye keywords: natural zeolite, amygdale, vein, basaltic rock, isle of skye record manuscript received: 16/12/2011 accepted for publications: 25/05/2012 juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts42 and galli, 1985; szoztak, 1998) zeolites have been widely used as catalysts, adsorbents and ion exchangers in many technical applications due to their exceptional properties (breck, 1974; booker et al , 1996; dixit and prasada, 1998; misaelides, 2011; loiola et al , 2012) they have become worthy of being called the mineral of the future several countries around the world have made significant progress in the exploring and exploiting of this mineral however, only a few of the natural zeolites in the world are found in sufficient quantities and having the purity required by industry the u s geological survey has reported natural zeolites’ worldwide occurrence in the usa, japan, korea, bulgaria, czechoslovakia, romania, hungary, russia, yugoslavia, south africa, italy, germany, turkey and china, the latter having the greatest worldwide production clinoptilolite, mordenite, heulandite, chabazite, phillipsite and laumontite are some of the more than 40 known natural zeolites occurring in reasonably high quantities and purity this study was aimed at recognising natural zeolites filling amygdales and veins in tertiary basaltic host rocks on the isle of skye (nw scotland) for their mineralogical characterisation and determining the sequence of zeolite formation of and that of associated minerals field sampling and analytical methods a university of wolverhampton (england) research team was involved in reconnaissance fieldwork on the isle of skye (scotland) during autumn 2008 fieldwork was primarily focused on localities presenting excellent examples of natural zeolites in spillites from the skye main lava series (thompson, 1982; stuart et al , 2000; fowler et al , 2004) the sampling strategy consisted of taking samples containing natural zeolites and other secondary minerals filling in amygdales and veins from several outcrops in fresh road cuts and new quarries along the roads petrographical and microscopy studies were made of specimens of zeolite-bearing rocks to identify mineral species; this led to clarifying and confirming field observations a trinocular nikon (labophot2-pol) transmitted light microscope was used to observe the minerals’ optical properties in thin sections the mineral phases’ morphological features were examined in a scanning electron microscope (zeiss evo50), using the following analytical conditions: 1 na i probe, 20 00 kv eht, 100 μa beam current, signal a=se1, wd=8 0 mm natural zeolites’ x-ray diffraction (xrd) patterns were recorded with a rigaku d/max 3b diffractometer operating in braggbrentano geometry with cu-1 radiation ( = 1 5406 å) at 40 kv and 20 ma and a graphite monochromator scan parameters were 0 02o step size, 12 s dwell time and 2–70o 2 range the hanawalt method was used for phase identification using the international centre for diffraction data’s (icdd) crystallographic database powder diffraction file (pdf-2) geological setting of the isle of skye the isle of skye in scotland covering 1,656 km2 is the largest, most northerly island in the inner hebrides it is located on the north-west coast of the scottish highlands, bounded on the north by the north minch (an atlantic sea channel between the outer hebrides islands to the west and the figure 1. left, general sketch where the isle of skye is located right, simplified geological map of the isle of skye (adapted and modified from emeleus and gyopari, 1992) the sampling sites are indicated by black circles natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland 43 mainland of scotland to the east), on the west by the little minch (a channel between the isle of skye and the outer hebrides), to the south by the sea of the hebrides and to the east by scotland (figure 1) the isle of skye has a series of peninsulas trotternish to the north-east, sleat to the south-west and strathraird, minginish, duirinish and waternish to the west britain’s best ancient landscape is exposed in the trotternish peninsula fractures in the basalt blocks allowed the fall of the main body of lava to slip and rotate on the soft sedimentary rocks below, eventually sliding down the hill to form the quiraing, old man of storr, the needle and many other features in a quite particular landscape geologically, the isle of skye is divided into three provinces: the south-east sleat peninsula, the north-west peninsulas and the red hills and cuillin hills (figure 1) the oldest rocks (3 000 ma old lewisian complex) on skye are found on the sleat peninsula (goodenough and bradwell, 2004) the lewisian complex mainly consists of grayish gneiss although it also encompasses quartzites, marbles, graphitic schists and amphibolites, which are thought have been formed from a wide variety of even older igneous and sedimentary rocks whose original composition and structure became modified by the effects of heat and intense pressure deep in the earth’s crust (gibbons and harris, 1994) lewisian complex’ rocks were caught up during the caledonian orogeny, and they appear in the hanging walls of the thrust faults formed during the late stages of this tectonic event (whitehouse and bridgwater, 2001) these rocks form the basement on which the moine supergroup and torridonian sediments were deposited the moine supergroup consists of a thick and extensive sequence of neoproterozoic siliciclastic metasedimentary rocks, dominated by psammites with subordinate pelites (mendum et al , 2008) the complete lack of fossils is consistent with a precambrian age for the sequence; this is also shown by the age of the earliest known intrusions that cut the sequence, dated at c 870 ma there is also no evidence of the sequence being affected by the 13001000 ma grenville orogeny (strachan et al , 2002), suggesting an early neoproterozoic sedimentation age radiometric dating of micas from deformed pegmatites provided the first evidence of a precambrian metamorphic event of 690-750 ma (giletti et al , 1961) the torridonian sequence describes a series of neoproterozoic sedimentary rocks, including sandstones, arkoses and shales having coarse conglomerates locally at the base radiometric ages from the torridonian sequence itself give ages ranging from c 1200-950 ma (park et al , 2010) the north-western duirinish, waternish and trotternish peninsulas are composed of jurassic sediments covered by plateau basalts which erupted from dikes and fissures during the earliest tertiary time, c 62 ma ago (drury et al , 1976) the latter can be classified as continental flood basalts (cfbs), which appear to have been erupted in a subaerial environment, as indicated by the tectonic environment of continental flood basalt provinces and characteristic continental flood basalt lava flows (garner, 1996) white and mckenzie (1989) have stated that the eruption of continental flood basalts requires both a mantle source having abnormally high potential temperatures, together with high stretching factors typical of advanced stages of continental extension horizontal flows built up on top of each other to a height of around 2,000 ft the rocks have since been eroded by rivers and ice, leaving flat-capped hills and stepped plateau in the north-west spectacular landslides in trotternish caused by paleogene age lava falling away from weaker sediments underneath have produced a unique landscape the spectacular alpine scenery results from the exposed plutons of red granite (red hills) having 54±3 ma rb-sr ages (moorbath and bell, 1965) and dark gabbro (the cuillin) intruded 54-62 ma ago (ferry et al , 1987) such intrusions are thus slightly younger than the plateau basalts these rocks form the highest and most rugged mountain scenery on figure 2. the isle of skye’s geomorphological features (see text for details) (a) (d) (g) (b) (e) (h) (c) (f ) (h) juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts44 the island gillen (2003) has provided a comprehensive treatment of the geology of the fascinating scenery of scotland, including the isle of skye, having a landscape that has been studied by geologists and geographers from the beginnings of geology as a science skye has periglacial cliffs, remote beaches, lakes, irregular rock peaks, steep rocky hills, high peaks and varied geomorphology erosion has been the main force in shaping the current landscape, particularly erosion by ice when glaciers covered most of skye it highlights the presence of the most rugged mountains in britain, the imposing cuillin more gentle hills are scattered across the island and the long trotternish summit in the north has a series of landslides on its eastern flank the coast has steep cliffs caused by weathering (perhaps the most impressive in britain), sea stacks, caves, arches and waterfalls (townsend, 2001) figure 2 illustrates characteristic landforms on the isle of skye figure 2a shows the typical morphology of a glacial valley at loch slapin bay macleod’s tables are two curious flat-topped hills which are prominent in views from many parts of skye (figure 2b); they consist of two plateaus, healabhal mhor (macleod’s plateau north) and healabhal bheag (macleod’ s plateau south) figure 2c shows the waterfall at the kilt rock which is the geomorphological expression of the little minch sill complex on the top of the skye main lava series the view is of the cliff face to the north which shows good columnar jointing and there are also complete columns which have fallen on to the beach a typical depositional coastal landform is represented by a sandy beach (figure 2d) talisker bay is one of the most long-standing and best-known sites on the isle of skye, partly due to being one of the more accessible of the skye main lava series and, more importantly, to the bay’s sea stack, the heddle’s stack (figure 2e) the old man of storr is a slipped rock pinnacle (figure 2f); the basalt lavas that make up the old man and the rock face behind it are clearly in layers representing different lava flows the cuillin represents the deeply eroded roots of large volcanoes active some 60 ma ago (figure 2g) sgurr alasdair is the highest point of the cuilli hills and the isle of skye (3,255 ft) the quiraing is a landslip on the eastern face of meall na suiramach, the northernmost summit of the trotternish ridge on the isle of skye (figure 2h) distinctive features of this landscape include the needle (a jagged 120 ft high landmark pinnacle), the table (a flat grassy area slipped down from the summit plateau) and the prison (a pyramidal rocky peak which can look like a medieval keep when viewed from the right angle) loch dunvegan is a sea loch on the west coast of the isle of skye (figure 2i) natural zeolite-bearing rocks at the skye igneous centre the skye igneous centre forms part of the british tertiary igneous province manifest as a series of voluminous mafic to silicic magmatism in mainland western scotland and the offshore hebridean islands associated with the opening of the north atlantic ocean (saunders et al , 1997) the plutonic complexes and associated lava fields were produced between 58 and 62 ma ago (hamilton et al , 1998) the igneous rocks exposed on the isle of skye represent the eroded remains of one of the major igneous centres along the western shore of scotland stuart et al , (2000) have described the tertiary igneous rocks on the isle of skye in detail, including the following units: little minch sill complex, skye main lava series, red hills granites and cuillin complex and broadford gabbro the little minch sill complex became emplaced into middle jurassic sediments underlying the skye main lava series, although intrusive relationships suggest that it is marginally younger than the lavas (gibson, 1990) gibson and jones (1991) have stated that is composed of both single and multiple sills, represented by three main, genetically related units: picrite, picrodolerite and crinanite (analcime-rich dolerite); these are the result of the differentiation of an alkali-olivine basalt magma in an upper-crustal magma chamber the eastern side of trotternish peninsula is characterised by a series of dolerite sills; the best known being the kilt rock the skye main lava series crops out in north and west-central skye, covering 1500 km2 (over 1200 m and with a cumulative thickness) each lava field’s stratigraphy has been established by using deep weathering profiles and fluvial to lacustrine sedimentary intercalations which developed during hiatuses in volcanism (thompson, 1982) these rocks predominantly consist of transitional to alkaline and tholeiitic lava flows on top of the sequence erupted c 58 ma ago (bell and jolley, 1997; hamilton et al , 1998) the importance of the town of portree is its exposure through the beinn edra group which forms the basis of the skye main lava series (emeleus and gyopari, 1992) at least 24 lava flows have been recognised by anderson and dunham (1966), which accumulated after being subjected to deep weathering in wet, warm tropical or subtropical conditions (emeleus and gyopari, 1992) a central intrusive complex having four major centres of activity represented by multiple intrusions developed in succession as the focus of igneous activity moved progressively eastward at the end of the period of flood basalt volcanism (fowler et al , 2004) from west to east, these are the cuillin centre, the srath na creitheach centre, and the western and eastern red hills centres (bell, 1976; emeleus and gyopari, 1992) the cuillin centre is a composite confluent intrusion ranging in composition from peridotite to gabbro (stuart et al , 2000) although they post-date most basalts, the cuillin intrusive complex is most likely coeval with the latest skye main lava series tholeiitic basalt flows dickin and exley (1981) reported an age of 59 3±0 07 ma for the youngest cuillin centre intrusion, the coire uaigneich granophyre the srath na creitheach centre is small and poorly documented and consists of volcaniclastic breccias, post-dating the cuillin centre but pre-dating the red hills granites (trewin, 2003) the southern part of the isle of skye is dominated by the red hills alkali granites age determinations have been reported as 58 7±0,9 ma for the loch ainort granite (western red hills centre) and 53 5±0,9 ma for the beinn an dubhaich granite (eastern red hills centre) (dickin, 1981) the presence of hybrid bodies in the western and eastern red hills centres suggests that basaltic magma was available during granitic centre formation (fowler et al , 2004) further evidence of this is the intrusion of mafic to intermediate cone sheets and dykes throughout the history of the central complex, which are coeval with many of the above cuillin rocks (emeleus and gyopari, 1992) according to emeleus and gyopari (1992), the cuillin centre’s gabbros and peridotites were cut by numerous basaltic cone-sheets and subsequently by red hills’ granites the central complexes thus record varied intrusive sequences in which basaltic and granitic magmas have been intimately associated field occurrence of natural zeolites in spillites from the skye main lava series the occurrence of different zeolites and other secondary minerals from the skye main lava series and their frequency are not homogeneous and their spatial distribution is directly controlled by host rock’s chemical composition and local hydrothermal conditions (weisenberger and spürgin, 2009) host rock porosity controls the fluid/rock interaction descriptions of zeolite appearance, occurrence and textural relationships are given below some very small amounts of zeolite-like species still require further investigation regarding their characterisation outcrops containing zeolites mostly correspond to transitional to alkaline basalts differing in colour, degree of weathering and content and distribution of amygdales and veins now filled with zeolites spillitic basalts (making up most outcrops) are dark gray with zeolites developed in randomly arranged veins and cavities a zeolite occurrence was found in a basalt quarry near dunvegan where zeolites occur in scarce, elongated amygdales having considerable sizes up to 10 cm (figure 3a) hosted by columnar basalts usually showing a redbrown weathering surface (figure 3b), as observed in the small village of dunans gray spillitic basalts with zeolites developed in irregularly shaped cavities in contact with a floor of red laterite overlain by highly vesicular basalts without zeolites having a light brown weathering surface occur in the same village (figure 3c) the red laterite is formed by the deep weath(2) natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland 45 ering of the upper layers of lava flows before the next one had erupted in some cases, contact between spillitic lava flows marks a contrast in the degree of weathering, colour and development of layering, reddish-brown meaning a greater degree of weathering and vesicular random distribution, which apparently contain a smaller amount of zeolite amygdales a typical example of zeolite occurrence in amygdales can be observed at talisker point (figure 3d) the degree of these dark gray spillitic basalts’ weathering is low near the quiraing, eventually reaching cavities containing zeolites to become concentrated so that they develop a banding (figure 3e) zeolites occur in veins at treaslane, a small remote scattered crofting hamlet overlooking the western entrance to loch treaslane; they may be parallel or develop stockworks (figure 3f ) sometimes massive green basalts outcrop in contact with highly weathered gray to red spillitic basalts having spheroidal weathering can be observed near the quiraing (figure 3g) one of these contacts in particular shows a dolerite sill along the plane of contact veins containing zeolite material appear in some surfaces of spillitic basalts parallel to the lava flow (up to 3 cm thickness) or transverse to it in several directions when there are many veins they develop as stockworks (rock masses very interpenetrated by small veins) which, in some sectors, have a brecciated appearance veins thickness is not uniform the veins described above are present in basalts having spheroidal weathering, produced through perpendicular joint systems columnar basalts falling right to the sea in horizontal rock formations resulting from the lava flows’ rapid cooling may be observed at allt luig mhóir (figure 3h) locally, dolerite dikes cutting spillitic basalts appear smaller and are easily visible at allt coir’ a ghabhainn (figure 3i) no discernible chilling is seen towards their margins, but the presence of zeolite-filled cavities (amygdales) is common in a number of the dykes (bell, 1984) petrographical analysis figure 4 shows zeolite’s general macroscopic characteristics, as seen in the field zeolites appear filling cavities in a non-uniform way; sometimes they just line cavities and sometimes they fill them completely they are associated with other secondary minerals such as amorphous silica, calcite and nontronite amygdales have a spheroid, ellipsoidal or irregular shape; they usually appear in separate units but locally coalesce with each other (figures 4a-4b) they are usually about 0 5-1 5 cm in diameter but could locally reach up to 10 cm drusy amygdales, having a partial filling, may exhibit various types of zeolite material, a certain amount of crystals having well-defined faces in the centre of the cavity and massive granular aggregates with vitreous luster covering the walls of the cavity (figures 4c4f ) a combination of irregular and large amygdales ranging from 1 0 mm to 1 0 cm and having a massive appearance contrasts with vesicles lacking any kind of padding radial aggregates of zeolites with vitreous luster usually occur in irregular amygdales of variable shape and size, some up to 5 0 mm in diameter being semi-spherical and others up to 1 2 cm being spherical (figures 4g-4h) zeolites also occur in fibrous aggregates, partly or completely filling spheroid, ellipsoidal or irregular amygdales (figure 4i) irregular to ellipsoidal shaped amygdales (up to 2 0 cm) may be completely filled by crystals, revealing the occurrence of at least two zones: a fibrous lining in the outer part of the cavity wall and a fibrous interior padding moreover, amygdales whether spheroid, elongated or irregular shaped may have a well-defined layer of fibrous material that only occurs in the outer part of the cavity wall in some cases, a zoning zeolite distribution can be distinguished; fibrous zeolite phases displaying a silky luster in the outer part of the cavity wall and granular zeolite phases having a glassy (a) (d) (g) (e) (h) (c)(b) (f ) (i) figure 3. outcrop-scale features illustrating the occurrence of zeolite-bearing basalts and associated rocks from the skye main lava series (see text for details) juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts46 luster as an interior padding a different zeolite occurrence can be observed in veins traversing basalts; these veins are up to several centimetres thick and can be as much as a few meters long zeolites are white with silky luster and show a predominance of fibrous aggregates although these can intergrow with acicular and radial aggregates (figures 4j-4l) it is sometimes difficult to identify the type of aggregate due to the diversity of varyingly shaped and sized minerals cross-linked aggregates intergrown with tabular aggregates can be distinguished in some veins, whereas only tabular, radiated and yellowish-white aggregates can be recognized in others most zeolites are relatively coarse-grained and easily identifiable however, they sometimes occur as fine-grained aggregates, which are very difficult to identify zeolytic phases and associated secondary minerals in basalts were sequentially deposited from the outer wall to the centre of amygdales and veins chabazite is usually vitreous and colourless to pink it is cubic to rhombohedral however, other more complex forms also occur, mainly developing regularly twinned crystal intergrowths phillipsite is uncommon and is normally the sole occupant of vesicles wairakite is generally vitreous with well-formed pseudo-octahedral to pseudo-icositetrahedral colourless crystals which can be confused with analcime thomsonite varies in habit showing bladed crystals, spherulites, radial crystal groups and rich coatings mainly made up of small crystals, although it sometimes has a fibrous nature analcime appears as vitreous and granular well-formed trapezohedral crystals which are often colourless and sometimes white natrolite is generally beige to white, either massive or forming radiating fibres and shows a more glassy brightness than mesolite, which is generally milky white and fibrous, having larger fibres than those of natrolite stilbite commonly occurs as vitreous sheaves or plates, ranging from colourless to green to white figure 5 illustrates the main microscopic features of the zeolites investigated here and associated mineral phases the vesicles and fractures in basalts were gradually filled up with secondary minerals, developing amygdales and veins usually showing mineral zoning distribution though, in some cases, they were not completely filled figure 5a gives an example of the occurrence of a mixed layer of celadonite (fe-rich mica) and nontronite (fe-rich smectite) and vesicles commonly lined by these mineral phases celadonite is usually green and botroidal and has fibrous sphefigure 4. macroscopic scale features illustrating the occurrence of several types of zeolites in amygdales and veins (see text for details) (a) (d) (g) (j) (b) (e) (h) (k) (c) (f ) (i) (l) natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland 47 roidal morphology, having detectable striated birefringence under crossed nicols nontronite is typically brown in transmitted light and does not have a discernable structure except for banding parallel to the cavity wall it occasionally exhibits geopetal features the vesicles are sometimes lined by fe-oxyhydroxides (figure 5b), displaying similar morphology to that described for the celadonite-nontronite mixed layer figure 5c shows a composite vein characterized by the occurrence of analcimeand thomsonite-associated quartz and calcite chalcedony shows a banded structure, consisting of innumerable cryptocrystalline silica grains it occurs as parallel fibres or in radial or spherulytic aggregates chabazite shows a typical rhombohedral habit, sometimes having angles about 90° between their faces it is colourless and presents dark-grey almost black interference colours and symmetrical extinction (figure 5d) analcime occurs as white trapezohedral crystals, low birrefringence, optical zoning and penetration twinning, and it sometimes occurs along with fibrous natrolite radial aggregates (figure 5e) figure 5f shows an intergrowth between large laumontite crystals and twinned stilbite crystals laumontite was determined taking into account the following optical properties: prismatic crystals with acute ends and diamond-shaped cross-section, having low relief and low refraction index stilbite occurs as idiomorphic, tabular crystals forming sheaf-shaped aggregates it is colourless and shows interference colours of the middle of the first order natrolite associated to chabazite occurs toward the centre of an irregular amygdale (figure 5g) it develops fan-like aggregates of acicular crystals having a moderate yellowish birefringence the optical properties making it clearly distinguishable from other zeolites are the extinction of the fibres parallel to its length between crossed nicols and the absence of an optical figure in convergent polarised light mesolite, scolecite and thomsonite can occur as overgrowths on natrolite figure 5h illustrates the association between thomsonite and chabazite thomsonite occurs as fibrous radial aggregates, which are colourless in plane polarised light and show first order gray in crossed nicols mesolite occurs as long fibrous to acicular crystals grouped in radial aggregates scolecite usually occurs as acicular (needle-like) and fibrous aggregates at least two varieties of fan or radial aggregates of fibrous zeolites occur together as illustrated by figure 5i (natrolite epitaxial growth on thomsonite) and figure 5j (natrolite and gonnardite epitaxial growth on thomsonite) figure 5. cross-polarised light (xpl) photomicrographs of the occurrence of several types of zeolites in amygdales and veins (see text for details) (a) (d) (g) (j) (b) (e) (h) (k) (c) (f ) (i) (l) juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts48 figure 6. scanning electron microscope images illustrating morphological features regarding several types of zeolites filling amygdales and veins (see text for details) ana, analcime; cha, chabazite; nat, natrolite; non, nontronite; tho, thomsonite figure 5k shows the occurrence of neddle-like crystals of thomsonite along with botroidal and fibrous spheroidal fe-oxyhydroxides a fibrous and reticulate aggregate which is transparent to translucent having a variety of colours ranging from white to yellow to reddish gray, can be attributed to levyne (figure 5l) this zeolite has a moderate relief, low refractive index and is presented as the only infill in certain amygdales scanning electron microscopy figure 6 illustrates sem images of the natural zeolites analcime crystals displaying intergrowth and typical isometric morphology (up to 1 8 mm) can be clearly seen in figures 6a (trapezohedral habit) and 6b (pentagonal dodecahedron habit) the mixed nontronite-celadonite layer appears as tiny plates or flakes by sem chabazite has a variable morphology, reaching up to 800 μm it usually occurs as rhombohedral crystals developing interpenetration twinning (figures 6c-6d) in other cases, it has a pseudocubic habit in which angles between their faces are closer to right angles (figures 6e) figure 6f gives an example of complexly-twinned (phacolitic) chabazite crystals, showing streaks parallel to the edges of the crystal natrolite occurring as groups of randomly oriented acicular crystals associated with trapezohedral analcime crystals is illustrated in figure 6g nontronite occurs as fine-grained random aggregates of flaky crystals along with trapezohedral analcime crystals (figure 6h) thomsonite appears as long tabular crystals (up to 1 5 mm in length) arranged in fan-shaped or radial divergent aggregates (can be completely closed taking on a spherical appearance) as shown in figures 6i and 6j however, it also occurs as acicular to elongated tabular crystals (up to 650 μm in length) developing radial aggregates figure 6k illustrates an example of zeotypes’ spatial distribution in amygdules, thomsonite displaying the typical radial fibrous aggregates in the other part of the cavity wall, followed by natrolite occurring as fine fibres forming clusters sometimes growing on typical trapezohedral analcime crystals growing in the centre of the cavity needle-like and pointed natrolite crystals (up to 2 mm in length) are wildly dispersed among pseudocubic chabazite crystals (figure 6l) (a) (d) (g) (j) (b) (e) (h) (k) (c) (f ) (i) (l) natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland 49 010-88-1263 chabazite-c a 010-83-1734 analcime 000-58-2033 nontronite, heated 000 500 000 500 500 000 500 000 500 0 000-41-1478 analcime-c 010-78-0296 �omsonite-(ca) 010-89-1998 greig eite 000-58-2027 n ontronite º2� (cuk�) º2� (cuk�) º2� (cuk�) º2� (cuk�) º2� (cuk�) º2� (cuk�) 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) 010-75-4067 potassium c alcium sodium silicon aluminum o xide h ydrate 010-83-1655 wairakite 000-22-0339 koninckite 000-58-2027 n ontronite 000-34-0137 c habazite-c a b 9 z q . d a t 0 1 0 -8 8 -1 2 6 1 c h a b a zit e -n a m 4 -1 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) c dat 000-35-0498 �omsonite 010-88-1261 c habazite-n a 010-73-6618 quartz, low, syn 000-05-0622 d olomite 010-74-3097 n atrolite 000-46-1389 laumontite 00 00 00 00 00 00 00 00 9 dat 00 00 00 00 00 00 00 00 0 000-26-1382 steller ite-(n a) 000-24-0894 stilbite-c a 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) 0 10 20 30 40 50 60 70 in te ns id ad (u ni da de s a rb itr ar ia s) chabazite-nachabazite-ca nontronite koninckite wairakite knaca-saoh laumontite natrolite dolomite quartz chabazite-na �omsonite-ca stilbite-ca stellerite-na nontronite greigeite �omsonite-ca analcime nontronite chabazite-ca analcime figure 7. xrd patterns showing the occurrence of zeolites in amygdales and veins (see text for details) knaca-saoh, knaca silicon aluminum oxide hydrate juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts50 x-ray diffraction qualitative xrd analysis led to identifying a variety of zeolite species, including analcime, as the dominant zeolytic phase, followed by chabazite, natrolite, thomsonite, wairakite, stilbite, stilbite-ca, phillipsite and stellerite-na, thomsonite-ca, chabazite-ca and chabazite-na, with minor stellerite, laumontite, levyne, phillipsite-na, and gyrolite other secondary minerals associated with zeolites were celadonite (ferich mica), nontronite (fe-rich smectite), greigeite (iron sulfide mineral), koninckite or bobierrite (phosphates), alunite (sulphate), truscottite (silicate), calcite or dolomite, and quartz nontronite occurred in most samples figure 7 illustrates representative zeolites diffraction patterns and associated phases figure 7a reveals a typical assemblage of analcime and chabazite-ca, accompanied by nontronite the main zeolytic phases shown in figure 7b are analcime and thomsonite other minerals are nontronite and greigeite the xrd pattern in figure 7c shows chabaziteca occurrence with traces of wairakite, nontronite and koninckite a characteristic chabazite-na xrd pattern is illustrated in figure 7d figure 7e reveals the occurrence of thomsonite, chabazitena, laumontite and natrolite, along with quartz and dolomite a stilbite-ca + stelleritena assemblage is illustrated in the xrd pattern shown in figure 7f several zeolytic phases were identified in this study which could be organised into six structural groups according to meier’s classification (1968), as depicted in table 1 the heulandite group corresponding to the secondary unit c4-c4-t1 was represented by the following zeolites stellerite, stilbite, stellerite-na and stilbite-ca the chabazite group (secondary unit c6) was characterised by the presence of chabazite, levyne, chabaziteca and chabazite-na the analcime group (secondary unit c4-c4) included analcime and wairakite the natrolite group (secondary unit c4-t1) was made up of natrolite, thomsonite and thomsonite-ca the phillipsite group (secondary unit c4) was represented by phillipsite and phillipsite-na the last group consisted of secondary unit c8 with a single zeolytic phase (laumontite) the following associations of zeolytic phases were determined: laumontite + chabazite; wairakite + chabazite; chabazite + thomsonite; analcime + thomsonite; thomsonite + wairakite + chabazite; thomsonite + analcime + chabazite; natrolite + analcime + thomsonite these mineral assemblages usually showed the additional occurrence of non-zeolite phases such as nontronite and celadonite discussion on zeolite formation the isle of skye is particularly known for its abundance and variety of minerals from the zeolite group and other secondary minerals in tertiary alkali olivine basalts their distribution has been controlled by a combination of factors such as temperature, pressure, hot aqueous fluid circulation and rock’s chemical composition (walker, 1960) a study of the beinn edra group lavas by king (1977) revealed the occurrence of similar parageneses of zeolites to those of the analcime-natrolite zone defined by walker (1960) the zeolites and associated secondary minerals were sequentially deposited from the outer wall to the centre of amygdales and veins in the basalts (figure 8) chemical elements necessary for the formation of these mineral phases were released directly as a result of hydrothermal devitrification however, ferry et al , (1987), based on oxygen and hydrogen isotope evidence for widespread element exchange between rock and fluid, revealed that basalts from the skye main lava series were locally affected by contact metamorphism associated with the intrusion of tertiary cuillin complex gabbros and red hills granites and hydrothermal alteration due to the convection system induced by such intrusions zeolites were probably formed from fluids, which penetrated the vesicles and fractures, but the source of the fluid remains unknown no regional zoning of zeolite assemblages was apparent in this study figure 8 shows that the zoning sequence and textural evidence suggest the following crystallisation sequence: first k-rich clay (nontronite) followed by k-rich mica (celadonite), carbonate (calcite), cryptocrystalline silica (e g chalcedony), then thomsonite followed by chabazite, analcime, phillipsite and wairakite, and natrolite, laumontite, and stilbite-type minerals (stilbite and stellerite) zeolites’ stability fields are known from modern hydrothermal systems (kristmannsdóttir and tómasson, 1978) and the observed sequence in the samples studied here conformed to a progressive decrease in temperature favouring hydrothermal system hydration the amygdales and veins thus showed a wide range of variability regarding the zeolite minerals present the irregular zeolite and related mineral distribution in the basalts has been attributed to factors such as localised heatflow, ionic activity in pore waters and host rock permeability (pe-piper, 2000) the high initial permeability subsequently decreased due to secondary mineral deposition, leading to the formation of individual closed systems (keith and staples, 1985) although there is zoning sequence and textural evidence of open-system hydrothermal activity for the formation of the skye basalt zeolites in amygdales and veins, restricted domains may have developed as pore space became filled by precipitates of mixed-layer nontronite-celadonite, calcite, cryptocrystalline silica and zeolites the hydrothermal pathways may have been substantially different, even for quite closely spaced cavities it is probable that a variation in the nature of the exchangeable cations in zeolites occurred within a single amygdale or vein fluid composition may be important in defining the type of zeolite that occurs in a particular amygdale or vein, whereas temperature variations may influence chemical variation within such individual cavities starting material composition, especially si/al ratio, determined the type of zeolite formed (e g boles, 1977) this parameter was most important during the first stages of alteration, in which, usually, an alteration product having a si/al ratio similar to or less than that of the starting material became formed as alteration proceeded, the minerals being formed became poorer in sio2, suggesting that hydrothermal desilification took place with continued reaction the starting material’s alkali/calcium ratio may also have influenced the reaction product during early stages of alteration if the reacting solution had a low concentration of these cations table 1. comparing the zeolite phases identified in this study and the meier’s structural classification (1968) structural classification identified zeolitic phases heulandite group (c4-c4-t1) stellerite, stilbite, stellerite-na, stilbite-ca chabazite group (c6) chabazite, levyne, chabazite-ca, chabazite-na analcime group (c4-c4) analcime, wairakite, analcime-mg natrolite group (c4-t1) natrolite, thomsonite, thomsonite-ca phillipsite group (c4) phillipsite, phillipsite-na laumontite group (c8) laumontite natural zeolites filling amygdales and veins in basalts from the british tertiary igneous province on the isle of skye, scotland 51 si activity in the fluid was low by the time of zeolite crystallisation because it had been used up in chalcedony formation (chipera and apps, 2001) the secondary minerals nontronite and locally celadonite represent the first crystallised species lining the outer part of the cavities other mineral phases occurring during the early stage were calcite and cryptocrystalline silica nontronite was probably formed by basalt devitrification due to hydration, although ferromagnesian mineral phases present in the basalts might have also contributed to its formation the formation of a fe-(hydro)oxide celadonite and opaline mixture is thus suggested, which may be explained by silica glass replacement, similar to that reported by bustillo and martínez-frías (2003) high carbonate ion activity caused the early precipitation of calcite, removing most ca2+ ions and so leaving the water comparatively enriched in alkalies both celadonite precipitation (which increased the na+/k+ ratio in solution) and decreased temperature (which favoured na enrichment in the solution) may have contributed to analcime crystallisation and that of other narich zeolites (alt and honnorez, 1984) thomsonite is the most common zeolite phase formed after nontronite/celadonite chabazite, analcime, phillipsite and probably wairakite are the first zeolites which became crystallised they were followed by acicular-fibrous zeolites such as natrolite/ gonnardite, thomsonite, mesolite and scolecite analcime is usually a narich mineral phase, but may sometimes contain appreciable k (utada, 1970) however, this mineral sequence was probably affected by contact metamorphism which could explain analcime formation accompanied by decreasing si activity and increasing temperature (not reflected in figure 8) in the sio2-deficient environment defined in this study wairakite and analcime may have formed a complete solid solution na-rich zeolite precipitation on the wall of amygdales and veins reduced the solution’s alkalinity at lower temperature, na was accommodated in the structure of chabazite and other zeolites such as analcime and final crystallisation was represented by only small amounts of na-rich zeolites (natrolite group) therefore, the existence of nontronite, which is a smectite group lowtemperature mineral, along with very well-developed zeolite crystals of analcime, chabazite-na and probably phillipsite in the edges of cavities, revealed that these mineral phases were deposited prior to fibrous zeolites such as natrolite, being indicative of low-t hydrothermal alteration (<100°c) as stated by deer et al , (2004) chipera and apps (2001) have stated that phillipsite is usually caused by host basalt low-temperature hydrothermal alteration (around 90 ºc) and, referring to the fluid evolution described by cochemé et al , (1994), caand k-rich fluids changed their composition as the temperature fell depletion of these elements in the fluid during phillipsite formation promoted the appearance of narich zeolites, first revealed by the precipitation of fibrous calc-sodic zeofigure 8. xsequence of mineral phase crystallisation in cavities lites (thomsonite), followed by granular calc-sodic analcime and chabazite and na-rich zeolites as natrolite (similar to that reported by cochemé et al , 1994) si activity in the fluid was low during chabazite and analcime crystallisation due to equilibrium with silica phases and more stable phases during this stage these zeolites must have been formed at lower temperatures; otherwise, if temperature had reached 100 ºc, then stable zeolites would have been fibrous, which would have grown at the expense of previously formed zeolites (chipera and apps (2001) hay and sheppard (2001) have experimentally demonstrated that chabazite is stable up to 100 ºc, whereas analcime is stable between 100 and 200 °c in our case, the textures observed in some amygdales, showing chabazite formation after analcime, would indicate a decrease in temperature however, according to chipera and apps (2001), decreased si activity reduced the analcime stability field, promoting both natrolite formation after analcime and chabazite and thomsonite replacement by natrolite additional reduction of si in the fluid meant that thomsonite, scolecite and mesolite, along with natrolite/gonnardite, would be the most stable zeolites this suggested that zeolites formed in the edges of basalt cavities did so at higher temperature and lower si activity at the beginning of crystallisation, and that lower temperature zeolite species having more aluminous composition would form in the central part of the cavity laumontite and stilbite formation can be attributed to the nature of the hydrothermal fluids related to the emplacement of plutons, which were probably more enriched in ca, ba, and sr produced by a late event, taking into account that these zeolites usually appear filling fractures in basalts similar laumontite and stilbite association has been reported by dill et al , (2007) regarding zeolite formation in stockworks conclusions a sequential formation of zeolites and associated secondary minerals filling amygdales and veins in tertiary basaltic host rocks from the isle of skye has been described in this study from early to late formation and, from outermost to innermost portions of the cavities, nontronite, celadonite, thomsonite, phillipsite, chabazite, analcime, natrolite, mesolite, stilbite, scolecite and laumontite and stilbite were sequentially found however, individual complexly zoned amygdales and veins show evidence of multiple fluid-circulation events, showing a contrast in mineral assemblages from different cavities thomsonite, phillipsite, chabazite, analcime, natrolite, mesolite, stilbite and scolecite were associated with nontronite and celadonite in amygdales chabazite, analcime, thomsonite, laumontite and stilbite were associated with quartz and calcite in veins zeolites deposited in basalts vesicles and veins were the result of a hydrothermal process, having a variety of species and this was due to reduced si activity in the fluid reduced si activity was also revealed by the absence of sio2 polymorphs such as crystoballite, tridymite and opal in the association and because most na zeolites formed with relatively low si/al ratios compared to ca zeolites the required amounts of silica, alumina and alkali and alkaline-earth cations necessary for zeolite formation probably resulted from volcanic glass and primary igneous fenocrystal dissolution, although the plagioclase alteration reaction in the rock matrix, releasing ca, could also have been responsible for this according to the defined mineral sequence, it can be deduced that a change in fluid composition occurred as temperature dropped zeolite formation in the region studied here began with the highest si activity in fluid and low si/al ratio na zeolites and ended with the lowest si activity in fluid and high si/al ratio ca zeolites secondary minerals nontronite and celadonite represented the first crystallised species lining the outer part of the cavities other mineral phases occurring during the early stage were calcite and quartz thomsonite was the most common zeolite phase formed after nontronite/celadonite na-rich zeolites such as chabazite, analcime, phillipsite and probably wairakite were the first zeolites that became crystallised they were followed by acicular-fibrous minerals in cavities early stage late stage nontronite celadonite quartz (polymorph) thomsonite chabazite analcime phillipsite natrolite gonnardite scolecite mesolite laumontite stilbite t activity of si juan manuel triana r , javier francisco herrera r , carlos alberto ríos r , oscar mauricio castellanos a , josé antonio henao m , craig d williams and clive l roberts52 natrolite and ca-rich zeolites such as gonnardite, thomsonite, mesolite and scolecite the si activity decreased during alteration and temperature increased to 200 ºc thereby allowing analcime formation followed by that of natrolite, gonnardite, thomsonite, scolecite and mesolite laumontite and stilbite formation could be attributed to the presence of ca, ba, and sr-rich hydrothermal fluids produced by a late event, taking into account that these zeolites usually appear filling basalt fractures acknowledgments we would like to thank the university of wolverhampton, england, for supporting the research team when carrying out reconnaissance fieldwork on the isle of skye, scotland, during autumn 2008 this research forms part of jm triana and jf herrera’s universidad industrial de santander undergraduate thesis we gratefully acknowledge mrs barbara hodson for assistance with sem data acquisition at the university of wolverhampton’s school of applied sciences and to mr jose pinto in collecting xrd data at the universidad industrial de santander’s structural chemistry laboratory the authors would also like to thank the anonymous referees for their critical and insightful reading of the manuscript references alt, j c, honnorez, j , alteration of the upper oceanic crust, dsdp 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central alps, j metam geol , 28, 2010, pp 825–847 weisenberger, t , spürgin, s , zeolites in alkaline rocks of the kaiserstuhl volcanic complex, sw germany new microprobe investigation and the relationship of zeolite mineralogy to the host rock, geol belgica, 12, 2009, pp 75-91 white, r s , mckenzie, d p , magmatism at rift zones: the generation of volcanic continental margins and flood basalts, j geophys res , 94, 1989, pp 7685-7729 whitehouse, m j , bridgwater, d , geochronological constraints on paleoproterozoic crustal evolution and regional correlations of the northern outer hebridean lewisian complex, scotland, precambrian res , 105, 2001, pp 227-245 keywords: gypsum rock; dry-wet cycle; particle flow numerical simulation; water absorption; porosity. palabras clave: roca de yeso; ciclo seco-húmedo; simulación numérica de flujo de partículas; absorción de agua; porosidad. how to cite item wu, g., chen, w., & cui, k. the numerical simulation of the drying-wetting cycles' influence on the deterioration characteristics of gypsum rocks under particle flow code. earth sciences research journal, 24(1), 55-59. doi: https://doi.org/10.15446/esrj.v24n1.85486 in order to study the influence of dry-wet cycling on the deterioration characteristics of gypsum rocks and solve the problems encountered in engineering construction, in this study, gypsum rocks are taken as the research object. with the combination of laboratory test and theoretical analysis, the numerical simulation of particle flow is carried out, and the deterioration characteristics of physical and mechanical properties of gypsum rock under dry-wet cycling are studied. the results show that gypsum, quartz, zeolite and dolomite are the main components of gypsum rocks. gypsum occupies the most components in gypsum rocks, so the various characteristics of gypsum greatly affect the characteristics of gypsum rocks. the process of water absorption and loss of gypsum is similar, which shows that the rate of water absorption or loss of gypsum is faster in the early stage, and tends to be stable in the later stage. the curve of the whole process of water absorption and loss is fitted by negative exponential function, and the effect is better. the larger the porosity of gypsum rock is, the better its water absorption performance is. intergranular pore, dissolution pore and dissolution pore are the main pore types of gypsum rock. intergranular pore is the main water absorption channel of gypsum rock. the cumulative water absorption increases with the increase of wetting and drying cycles. the change of water absorption curve is mainly manifested in water absorption rate and time. the more the number of wet-dry cycles is, the higher the water absorption rate in the early stage of water absorption is, the closer the characteristic curve to the coordinate axis of water absorption is, and the shorter the water absorption time is. in contrast, the shape difference of water loss curve is very small. it can be seen from this that in the process of wetting and drying cycle, the hydrophysical and hydrochemical processes promote each other, which changes the crystal structure and pore structure of gypsum rocks, reduces the crystal strength and increases the porosity, thus leading to the deterioration of the mechanical properties of gypsum rocks. abstract the numerical simulation of the drying-wetting cycles' influence on the deterioration characteristics of gypsum rocks under particle flow code simulación numérica de la influencia de los ciclos de secado-humedecimiento en las características de deterioro de las rocas de yeso bajo el código de flujo de partículas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85486 con el fin de estudiar la influencia del ciclo seco-húmedo en el deterioro de las rocas de yeso y resolver algunos problemas de la ingeniería de construcción, las rocas de yeso son el objeto de investigación de este estudio. se realizó la simulación numérica del flujo de partículas con la combinación de pruebas de laboratorio y análisis teóricos, y se estudiaron las características de deterioro de las propiedades físicas y mecánicas de la roca de yeso expuesta al ciclo seco-húmedo. los resultados muestran que el yeso, el cuarzo, la zeolita y la dolomita son los componentes principales de las rocas de yeso. el yeso ocupa la mayoría de los componentes en las rocas de yeso, por lo que las diversas características del yeso afectan en gran medida las características de las rocas de yeso. el proceso de absorción de agua y pérdida de yeso es similar, lo que demuestra que la tasa de absorción de agua o pérdida de yeso es más rápida en la etapa inicial y tiende a ser estable en la etapa posterior. el índice de porosidad es directamente proporcional con la absorción de agua. la curva de todo el proceso de absorción y pérdida de agua se ajusta mediante una función exponencial negativa, y el efecto es mejor. los poros intergranulares y los poros de disolución son los principales tipos de poros de la roca de yeso. el poro intergranular es el principal canal de absorción de agua. la absorción acumulada de agua aumenta con el aumento de los ciclos de humectación y secado. el cambio de la curva de absorción de agua se manifiesta principalmente en la velocidad y el tiempo de absorción de agua. cuanto mayor sea el número de ciclos húmedo-seco mayor será la tasa de absorción de agua en la etapa inicial, más cercana es la curva característica al eje coordinado y más corto es el tiempo de absorción de agua. en contraste, la diferencia de forma de la curva de pérdida de agua es muy pequeña. se evidencia que en el proceso del ciclo de humectación y secado los procesos hidrofísicos e hidroquímicos se promueven entre sí, lo que cambia la estructura cristalina y la estructura de los poros de las rocas de yeso, reduce la resistencia del cristal y aumenta la porosidad, lo que conduce a la deterioro de las propiedades mecánicas de las rocas de yeso. resumen record manuscript received: 12/03/2019 accepted for publication: 03/09/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 55-59 guopeng wu1,2,3, wenwu chen1,2, kai cui1,2,3 1key laboratory of mechanics on disaster and environment in western china, department of geological engineering, lanzhou university,lanzhou 730000, china 2school of civil engineering and mechanics, lanzhou university, lanzhou 730000, china 3key laboratory of disaster prevention and mitigation in civil engineering of gansu province, department of geotechnical engineering, lanzhou university of technology, lanzhou 730050, china * corresponding author: g eo lo g ic al e ng in ee ri ng 56 guopeng wu, wenwu chen, kai cui introduction from the deep part of the earth to the surface of the earth, water-rock interaction is ubiquitous, especially the wet-dry cycle (jian & niu, 2016). it is the most closely related to human activities, and is also one of the most common water-rock interactions. for example, the precipitation and evaporation of rainwater and the rise and fall of reservoir water level are all the inducements of dry-wet circulation. the deterioration of rock mass caused by dry-wet cycling is equivalent to a kind of “fatigue effect”, and this deterioration is usually stronger than that of long-term immersion (yao et al., 2015), which has an important impact on the stability and safety of rock mass engineering (jiang & shao, 2014). therefore, it is of great engineering significance to study the deterioration characteristics of mechanical properties of rock mass and rock mass under dry-wet cycling. in recent years, scholars at home and abroad have carried out extensive experimental studies on the changing law of rock mechanical properties under dry-wet cycling, and obtained many research results (li et al., 2015). numerical simulation is one of the important means to study the mechanical characteristics of rock (gaylarde et al., 2017). in recent years, the discrete element method (dem) has been widely used in the numerical simulation of rock mechanical properties because it does not involve complex macro-constitutive models and can analyze the numerical simulation results from macro and micro levels (ion et al., 2016). particle flow code (pfc) is a kind of dem, which constructs rock materials into aggregates of particles and simulates the macro-mechanical properties of rock materials through the interaction of particles (grandclerc et al., 2016). pfc has been widely used in rock fracture analysis. in pfc, the macro mechanical properties of rock cannot be given directly, but should be matched by setting the geometric parameters and contact properties of particles and choosing reasonable meso-parameters. trial-and-error method is the most commonly used method for calibrating meso-parameters at present (sijakova-ivanova et al., 2014). by comparing the macro-mechanical parameters of rock and numerical simulation results, the meso-parameters are adjusted continuously to achieve the similarity between them. if the relationship between macro and micro parameters is not understood and the micro parameters are randomly adjusted, it may lead to a large number of numerical experiments and increase the difficulty of modeling (de weerdt & justnes, 2015). based on the above background, in this experiment, gypsum rock is taken as the research object in order to study the deterioration degree of the physical and mechanical properties of gypsum rock caused by wet-dry cycling. combined with laboratory tests, theoretical analysis and numerical simulation of particle flow, water absorption and water loss tests are carried out on gypsum rocks under different drying and wetting cycles. water absorption and water loss characteristics, pore characteristics and mechanical characteristics of gypsum rocks are obtained. the deterioration effect of gypsum rock under drywet cycling is discussed in detail, which provides a new theoretical basis for future research. method research content and technical route in this study, through data acquisition and field investigation, combined with laboratory experiments and theoretical analysis, the basic physical characteristics of gypsum rocks are described. the main components, water absorption, water loss, fracture inclination and uniaxial compressive strength of gypsum rocks are studied, and the preparation for the subsequent dry-wet cycle experiments is made in advance. the effect of dry-wet cycling on the deterioration characteristics of gypsum rocks has been studied under the pfc. gypsum rocks are subjected to dry-wet cyclic deterioration tests, and gypsum rocks with different cycles are subjected to water absorption and loss tests as well as pore characteristics experiments. then, the effects of dry-wet cycle on water absorption loss, pore structure and mechanical properties of gypsum are discussed. on this basis, the deterioration mechanism of gypsum is also analyzed. finally, the particle flow law of gypsum in different drying and wetting cycles was determined under the background of pfc. the relationship between crack inclination and uniaxial compressive strength under different drying and wetting cycles is studied through bonding clearance and meso-parameters. the mechanism of meso-deterioration of gypsum rock under drying and wetting cycles is further discussed. the technical route of this research is shown in the following figure. data collection determination of basic physical properties and selection of samples dry-wet cycle deterioration test two-dimensional particle flow simulation degradation mechanism of gypsum rock under the action of dry-wet circulating water-rock research conclusion figure 1. technical roadmap of this paper main components of gypsum minerals gypsum samples are collected from the field. in the laboratory, its components are studied experimentally. the samples are analyzed by x-ray images and the diffraction patterns are obtained. then, according to the diffraction pattern, the main components and their respective contents of gypsum rocks are calculated. according to the different contents of main minerals, the samples of experimental rock blocks are divided into four groups, numbered a, b, c and d in turn. the final results are shown in the table below. according to the table, gypsum, quartz, zeolite and dolomite are the main components of gypsum rocks. gypsum occupies the most components in gypsum rocks, so the various characteristics of gypsum greatly affect the characteristics of gypsum rocks. table 1. mineral composition and content of gypsum rocks classification of rocks gypsum (%) quartz (%) zeolite (%) dolomite (%) a > 95 1 0 < 4 b 90 2 2 6 c 83 1 0 16 d 71 1 1 27 characteristics of water absorption curve of gypsum rock in order to obtain representative standard specimens from four groups of specimens, water absorption tests are carried out at room temperature of 16°c ± 2°c. after 10 days of continuous water absorption, the water absorption of 57the numerical simulation of the drying-wetting cycles' influence on the deterioration characteristics of gypsum rocks under particle flow code of pore will change the water absorption characteristics of gypsum rocks. in order to obtain more accurate results, four test samples with different porosity are selected to study the effect of porosity on water absorption characteristics. water absorption experiments are carried out under the same experimental conditions, so water absorption curves and parameters are obtained. the experimental results show that the higher the porosity of the sample is, the higher the water absorption is and the higher the water absorption rate is. it is found that intergranular pore, dissolution pore and dissolution pore are the main pore types of gypsum rocks. intergranular pore is the main pore type. intergranular pore refers to the pore between gypsum crystals. because of the close contact between crystals, the intergranular pore becomes limited and the diameter becomes smaller, but they are the main channel of water absorption. the dissolution pore is produced by the contact between gypsum rock and groundwater. the dissolution pore of the dissolution pore is small and the regularity is not strong, and the shape and size of the dissolution pore are also different. if the connectivity is good, the dissolution pore can form a pore network together with the intergranular pore. in addition, there are some independent dissolution holes in gypsum crystal, which is caused by incomplete dissolution of gypsum crystal. the dissolution pore is relatively small in volume and low in connectivity, and has little water absorption to rocks. particle flow model of gypsum rocks when establishing the particle flow model of gypsum rock, because the macro-mechanical parameters of the model are affected by the number and size of gypsum rock particles, it is necessary to take into account the effect of res (resolution) on the macro-mechanical parameters at the minimum scale of the model. some studies believe that the number and size of gypsum particles have the least influence on the particle flow model of gypsum rocks when res ≥ 10, while others believe that the number and size of gypsum particles have the least influence on the particle flow model of gypsum rocks when res ≥ 20. therefore, in this experiment, the simulation scale and calculation speed are considered comprehensively, and the particle flow model of gypsum rock is analyzed. the calculation formula of res is as follows: res l r r r= ( ) +( ) / / /min max min1 1 (2) in the formula above, rmin represents the diameter of the smallest particle. in this study, rmin .= 0 4, and r rmax min/ .= 1 66. l is expressed as the minimum scale of the model. rmax denotes the diameter of the largest particle. uniaxial compression is achieved by imposing loading rates of the same size and opposite direction on the walls at both ends of the sample. the loading rate must be small enough to keep the sample close to static equilibrium. the results show that the loading speed of 0.2m/s is small enough to simulate the particle flow of gypsum. results and discussion change of one-dimensional water absorption curve of gypsum rocks with different cycles the following figure shows the water absorption curve of gypsum rock samples after different times of dry-wet cycles. as can be seen from the figure below, the cumulative water absorption of gypsum rock samples increases with the increase of wet and dry cycles. moreover, with the increase of wetting and drying cycles, the water absorption curve of gypsum rock samples is closer to the water absorption coordinate axis. this phenomenon appears in the characteristics of water absorption curve, which indicates that the water absorption rate increases with the increase of the number of cycles. finally, with the increase of time, the water absorption of gypsum rock samples reaches saturation, and the duration of water absorption becomes shorter. as can be seen from the above figure, the cumulative water absorption of the sample increases with the increase of the number of wet-dry cycles, which indirectly indicates that the porosity of the gypsum sample increases. the exponential function is used to fit the water absorption curve of the each sample remains basically stable. the following figure shows the variation curve of water absorption of gypsum rocks with water absorption time. during the 10-day water absorption process, the accumulative water absorption of group a rock sample is the largest, reaching 5.78g. secondly, the accumulative water uptake is 2.54g and 1.98g in group c and d, respectively. the cumulative water absorption of group b is the smallest, which is 1.46g. the cumulative water absorption of each group of samples is mainly completed in the early stage of water absorption. one day after water absorption, the accumulative water absorption of group a is 61.3%, and the accumulative water absorption of other groups is more than 36%. four days after water absorption, the accumulative water absorption of group a is over 92%, and the accumulative water absorption of other groups is over 82%. there are also obvious differences in the water absorption rate of each stage, and the water absorption rate decreases gradually with the increase of time. figure 2. water absorption characteristic curves of gypsum rocks of different groups with the increase of water absorption time, the water absorption of gypsum rocks reaches a certain degree of saturation, and the water absorption decreases slowly, gradually tends to zero. then, the water absorption characteristic curves of each group of gypsum rocks are fitted by negative exponential function. the general fitting expressions are as follows: m m aes bt=   (1) in the above formula, t represents the time when gypsum rock absorbs water. ms indicates the saturation of gypsum rock when it is saturated by water absorption. a and b represent the fitting parameters. the following figure shows the fitting parameters of water absorption curve of gypsum rock. from the following table, it can be seen that the fitting effect is good and the fitting correlation coefficient is above 0.99. table 2. fitting parameters of the whole process curve of water absorption serial number fitting parameters ms a b r2 a 5.725 5.472 0.038 0.992 b 1.503 1.412 0.019 0.995 c 2.684 2.589 0.023 0.997 d 1.975 2.015 0.015 0.994 effect of porosity of gypsum rocks on water absorption characteristics the water absorption characteristics of gypsum rocks are mainly influenced by porosity and pore structure, and the size, shape and connectivity 58 guopeng wu, wenwu chen, kai cui experimental sample. the fitting effect is good and the correlation coefficient is 0.982. the following table shows the water absorption parameters of different drying and wetting cycles. table 3. water absorption parameters of samples with different cycles of wetting and drying number of dry-wet cycles water absorption (g) 0.4 h average water absorption rate (g/h) 0 1.25 0.250 1 1.48 0.500 3 1.67 1.100 6 2.13 3.050 12 2.55 3.750 uniaxial compressive strength of complete and single crack specimens under different dry-wet cycles the following table shows the uniaxial compressive strength of complete specimens and single crack specimens under different wet and dry cycles. it can be seen from the table that the crack dip will affect the uniaxial compressive strength. the relationship between crack inclination and uni axial compressive strength under different wet-dry cycles is shown in the following figure. it can be seen from the figure that the number of dry-wet cycles and the inclination of cracks will affect the uniaxial compressive strength. the uniaxial compressive strength of single fracture specimen increases with the increase of crack inclination regardless of the number of dry-wet cycles. under the same fracture inclination, the uniaxial compressive strength of single fracture specimen decreases with the increase of the number of dry-wet cycles. with the same number of wet and dry cycles, the uniaxial compressive strength of single fracture specimen increases with the increase of fracture inclination. the uniaxial compressive strength of a single fracture specimen is the lowest when the fracture dip is equal to 30 degrees. after that, the uniaxial compressive strength of the single fracture specimen increases with the increase of the crack inclination. conclusion in this study, gypsum rock is taken as the research object for the deterioration of gypsum rock under the action of dry-wet cycle. in order to study the deterioration of the physical and mechanical properties of gypsum rocks caused by dry-wet cycling, combined with laboratory tests, theoretical analysis and numerical simulation of particle flow, gypsum rocks under different drywet cycles are tested for water absorption and water loss, and finally the water absorption and water loss characteristics, pore characteristics and mechanical characteristics of gypsum rocks are obtained. the deterioration effect of gypsum rock under dry-wet cycling is discussed in detail. experiments show that during the dry-wet cycle, the hydrophysical and hydrochemical processes promote each other, which changes the crystal structure and pore structure of gypsum rocks, reduces the crystal strength and increases the porosity, thus leading to the deterioration of the mechanical properties of gypsum rocks. due to the limited ability and time, there are still many deficiencies in the research content of this study. at present, the dry-wet cyclic deterioration tests are often aimed at intact rocks, without considering the influence of cracks on the deterioration of rock mechanical properties. in this study, the deterioration characteristics of single-fissure gypsum rocks under dry-wet cycling are preliminarily studied by means of the particle flow model of gypsum rocks. the results show that the occurrence of cracks has changed the deterioration law of rocks and has obvious influence. in practical engineering, more fractured rock mass is involved. the deterioration characteristics of fractured rock mass must be different from that of intact rock mass. therefore, in the future research, the deterioration mechanism of fractured rock mass under dry-wet cycling can be further studied. reference de weerdt, k., & justnes, h. 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(2017). epilithic and endolithic microorganisms and deterioration on stone church facades subject to urban pollution in a sub-tropical climate. biofouling, 33(2), 113-127. figure 3. one-dimensional water absorption characteristic curve of samples after different dry-wet cycles table 4. uniaxial compressive strength of complete and single crack specimens under different dry-wet cycles number of cycles crack dip complete sample 0 30 45 60 75 90 0 15.76 15.02 16.03 19.86 21.74 24.12 25.64 5 11.43 11.96 13.52 14.97 18.12 19.43 20.63 10 9.98 9.21 10.42 11.41 14.52 16.18 17.06 15 8.12 8.25 9.64 10.15 13.49 15.95 16.27 figure 4. the relationship between uniaxial compressive strength and crack inclination of single fracture specimen under different wet-dry cycles 59the numerical simulation of the drying-wetting cycles' influence on the deterioration characteristics of gypsum rocks under particle flow code grandclerc, a., gueguen-minerbe, m., nour, i., dangla, p., & chaussadent, t. 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(2015). assessment of long-term evolution of groundwater hydrochemical characteristics using multiple approaches: a case study in cangzhou, northern china. water, 7(3), 1109-1128. sijakova-ivanova, t., eric, s., & saric, k. (2014). preliminary investigations of deterioration on monument to fallen soldiers in the second world war in stip, republic of macedonia. journal of chromatography a, 8(1), 2000. yao, q., li, x., zhou, j., ju, m., chong, z., & zhao, b. (2015). experimental study of strength characteristics of coal specimens after water intrusion. arabian journal of geosciences, 8(9), 6779-6789. 1. introduction a number of previous studies (wones and eugeter, 1965; burhard, 1991; barriére and cotten, 1991; sheshtawi et al., 1993; lalonde and bernard, 1993; abdel-rahman, 1994; hecht, 1995; abdel-rahman, 1996; zhang et al., 2014) have shown that the composition of fluids associated with skarn mineralization is closely related to the physical and chemical conditions that are present during magma cooling and crystallization. previous studies also indicate that the chemical composition of biotites in granites is controlled by conditions of the wushan skarn copper deposit is genetically associated with the wushan granodiorite. in this study, we investigate the petrography and mineralogy of biotites within the wushan granodiorite. we also determine the formation conditions of these biotites and discuss the significance of these minerals in terms of petrogenesis and mineralization. electron microprobe analysis shows that biotites within the wushan granodiorite are magnesio-biotites that contain relatively high mg and ti concentrations and low fe and al concentrations. the ionic coefficient of alvi in these biotites ranges from 0.03 to 0.19, with ∑feo/(∑feo + mgo) ratios that range from 0.531–0.567 and mgo concentrations that range from 12.80–14.06 wt%. these results indicate that the wushan granodiorite is an i-type granite. the wushan biotites crystallized at temperatures (t) of 720°c–750°c, with oxygen fugacity (fo2) conditions of –11.6 to –12.5 and pressures (p) of 0.86–1.03 kb. these conditions are indicative of a crystallization depth (h) of 2.84–3.39 km. these data also indicate that the wushan granodiorite developed under conditions of high temperature and high oxygen fugacity, suggesting that the wushan granodiorite is prospective for magma-hydrothermal mineralization and that this granodiorite probably contributed to the formation of the wushan skarn copper deposit. el depósito de skarn cuprífero de wushan está asociado genéticamente con la granodiorita de wushan. en este estudio se investiga la petrografía y mineralogía de biotitas de la granodiorita de wushan. se determinan también las condiciones de formación de estas biotitas y se discute la significación de estos minerales en términos de petrogénesis y mineralización. un análisis de microsonda a electrones muestra que las biotitas de la granodiorita de wushan son biotitas de magnesio que contienen altas concentracionesrelativas de mg y ti y bajas de fe y al. el coeficiente icónico de alvi en estas biotitas oscila entre 0,03 y 0,19, con índices sfeo/(sfeo + mgo) que oscilan entre 0,531-0,567 y concentraciones de mgo que van desde 12,80 a 14,06 wt%. estos resultados indican que la granodiorita de wushan es de granito tipo i. las biotitas de wushan se cristalizaron a temperaturas (t) de 720°c–750°c, con condiciones de fugacidad del oxígeno (fo2) de -11,6 a -12,5 y presión (p) de o,86 a 1,03 kb. estas condiciones indican una profundidad de cristalización (h) de 2,84-3,39 kilómetros. los datos también indican que la granodiorita de wushan se desarrolló bajo condiciones de alta temperatura y alta fugacidad de oxigeno, lo que sugiere que la granodiorita de wushan tiene potencial para la mineralización magmática-hidrotérmica y que esta granodiorita probablemente contribuyó a la formación del depósito de skarn cuprífero de wushan. earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 39 44 abstract resumen key words: biotite, granodiorite, mineral chemistry, petrogenesis and mineralization, wushan skarn copper deposit palabras clave: biotita; granodiorita; química mineral; petrogénesis y mineralización; depósito cobrizo de wushan. record manuscript received: 13/11/2013 accepted for publication: 28/05/2014 composition of biotite within the wushan granodiorite, jiangxi province, china: petrogenetic and metallogenetic implications qian dong1, yangsong du1, zhenshan pang2, wenrui miao3, wei tu1 1. school of the earth sciences and resources, china university of geosciences, beijing 100083, china 2. development and research center of china geological survey, beijing 100037, china 3. school of electronics and information, northwestern polytechnical university, xian 710129, china correspondence should be addressed to qian dong, dongqian136@126.com magma cooling and crystallization; these conditions include oxygen fugacity, temperature, and pressure. therefore, biotite is an effective indicator that can be used to establish the physical and chemical conditions that were present during the cooling and crystallization of magma. the wushan skarn copper deposit is located in the central part of the jiujiang–ruichang district in the middle–lower yangtze river metallogenic belt (figure 1a). the deposit is genetically associated with the formation of the wushan granodiorite (figure 1b). a number of studies on the wushan granodiorite ore deposits 40 qian dong, yangsong du, zhenshan pang, wenrui miao, wei tu have determined that the magmas that formed this body contained a very high proportion of mantle component material (jiang et al., 2008). these studies also indicate that these magmas were produced by late yanshanian magmatism (145±3.9 ma), which is associated with significant crust–mantle interaction (gu, 1987; bao et al., 2002; ding et al., 2006; yang et al., 2011). these studies have also determined that the wushan granodiorite is genetically related to the wushan skarn copper deposit (kong et al., 2012; jiang et al., 2008; ji et al., 1989; huang et al., 1990; cui et al., 2002). nevertheless, the magmatic conditions of the wushan granodiorite have not been considered by these studies. in this study, we focus on the composition of biotite within the wushan granodiorite to estimate the magmatic conditions that were present and to determine the petrogenetic and metallogenic significance of this formation. this research has also led to an increase in understanding about the metallogenic processes that led to the formation of the wushan skarn copper deposit. figure 1. geological map of the wushan skarn copper deposit (after yang et al., 2011). 41composition of biotite within the wushan granodiorite, jiangxi province, china: petrogenetic and metallogenetic implications 2. geological background the wushan skarn copper deposit is located 8 km north of the city of ruichang, in jiangxi province, china (figure 1a). most of the ore bodies that make up the deposit are hosted by upper carboniferous to middle triassic carbonates, with igneous units in the area that is dominated by granodiorites, quartz diorites, quartz porphyries, and lamprophyres. faulting is widespread throughout the deposit, and the deposit is dominated by nee-striking interlaminar fractures and neand nw-striking faults. the wushan granodiorite is located within the southern wushan ore belt, and it consists of a stock that intruded into permian to carboniferous carbonates. the stick is oval-shaped in the planar view (figure 1b) and trumpet-shaped in the cross-sectional view (cui et al., 2002). figure 2. photomicrographs of biotites from the wushan granodiorite. a. euhedral tabular biotite containing magnetite inclusions; b. subhedral tabular biotite containing magnetite and apatite inclusions. bi = biotite; qz = quartz; pl = plagioclase; mt = magnetite; ap = apatite. 3. samples and analytical techniques samples were obtained from a number of underground tunnels and stopes located in the southern wushan ore belt. these tunnels and stopes were originally used for underground prospecting, mining, and other underground activities. for this study, granodiorite samples were collected from the underground in the –260 m n2 stope and from borehole zk405. these hand-collected samples have a porphyritic structure, and they contain quartz (28%–30%), plagioclase (38%–40%), potassium feldspar (18%–20%), biotite (7%–8%), and hornblende (1%–3%), with accessory minerals including titanite, apatite, zircon, and magnetite. biotites are widespread in the wushan granodiorite. they are also euhedral to subhedral, tabular, fresh, dark brown to light yellow, well-cleaved, and 1–5 mm in size. these crystals also contain earlier crystallized inclusions of magnetite, apatite, zircon, and other accessory minerals (figure 2a–b). an electron microprobe analysis (epma) was conducted at the beijing research institute of uranium geology, beijing, china employing a jxa8100 instrument that was operated using a 20 kv accelerating voltage, a 10 na beam current, and a 10 mm beam diameter. the detection limit was 0.002 wt%. for this analysis, the instrument was calibrated using albite (na), sanidine (si, al, and k), diopside (ca, mg), almandine (fe), rutile (ti), fluorapatite (p), and rhodonite (mn) standards. a calculation method developed by lin and peng (1994) was used to adjust the fe2+ and fe3+ concentrations, and we used the electrovalency balance principle to calculate the crystal formulae. the correlation calculation results are shown in table 1. table 1. representative electron microprobe analyses (wt %) and the structural formulae of biotite collected from the wushan granodiorite. no. 1 2 3 4 5 6 7 8 9 10 sio2 37.724 37.512 38.078 37.089 37.433 38.007 37.186 37.737 37.564 37.354 tio2 3.280 3.814 3.800 3.523 3.756 3.680 4.037 3.351 3.910 3.766 al2o3 13.704 13.610 13.933 13.562 13.626 13.836 13.991 13.878 13.841 13.740 feo 16.356 16.455 17.276 16.102 16.271 15.947 16.785 15.304 16.737 16.897 mno 0.048 0.066 0.109 0.072 0.034 0.029 0.105 0.095 0.057 0.042 mgo 14.055 12.821 13.180 12.802 13.637 13.925 13.359 13.521 13.598 13.822 cao 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.053 0.000 0.000 na2o 0.322 0.238 0.121 0.979 0.147 0.101 0.202 0.145 0.139 0.139 k2o 9.483 9.281 9.532 9.304 9.606 9.631 9.419 9.542 9.498 9.531 p2o5 0.027 0.034 0.000 0.000 0.000 0.000 0.000 0.082 0.055 0.000 total 94.999 93.831 96.029 93.433 94.510 95.156 95.084 93.708 95.399 95.291 structural formulae on the basis of 22 (o) 42 qian dong, yangsong du, zhenshan pang, wenrui miao, wei tu si 5.661 5.691 5.660 5.673 5.646 5.672 5.585 5.710 5.620 5.603 alⅳ 2.339 2.309 2.340 2.327 2.354 2.328 2.415 2.290 2.380 2.397 z 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 alⅵ 0.085 0.125 0.101 0.118 0.068 0.105 0.062 0.185 0.061 0.033 fe3+ 0.289 0.335 0.332 0.270 0.308 0.313 0.316 0.312 0.317 0.303 fe2+ 1.764 1.753 1.815 1.789 1.744 1.677 1.792 1.624 1.777 1.816 ti 0.370 0.435 0.425 0.405 0.426 0.413 0.456 0.381 0.440 0.425 mn 0.006 0.008 0.014 0.009 0.004 0.004 0.013 0.012 0.007 0.005 mg 3.144 2.900 2.921 2.919 3.066 3.098 2.991 3.050 3.033 3.091 y 5.658 5.556 5.607 5.511 5.617 5.610 5.631 5.565 5.635 5.673 ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.009 0.000 0.000 na 0.094 0.070 0.035 0.290 0.043 0.029 0.059 0.043 0.040 0.040 k 1.815 1.796 1.807 1.815 1.848 1.833 1.805 1.842 1.813 1.824 x 1.909 1.866 1.842 2.106 1.891 1.863 1.863 1.893 1.853 1.864 sum 15.567 15.422 15.450 15.617 15.508 15.473 15.494 15.457 15.488 15.538 σfeo/(σfeo+mgo) 0.538 0.562 0.567 0.557 0.544 0.534 0.557 0.531 0.552 0.550 fe2+/(fe2++mg) 0.359 0.377 0.383 0.380 0.363 0.351 0.375 0.347 0.369 0.370 mg/(fe2++mg) 0.641 0.623 0.617 0.620 0.637 0.649 0.625 0.653 0.631 0.630 tal 2.440 2.452 2.459 2.460 2.439 2.451 2.494 2.492 2.458 2.446 p(kb) 0.862 0.900 0.922 0.923 0.861 0.896 1.028 1.022 0.918 0.881 h(km) 2.845 2.970 3.042 3.047 2.840 2.957 3.393 3.372 3.030 2.908 p.s.: 1. see text for assignment of ferrous and ferric iron; 2. x,y,z, respectively represent the sum of the t,o,l cations in the structural formulae; 3. p= ρgh, where ρ=2700 kg/m3, g=9.8 m/s2. 4. componential characteristics the epma results listed in table 1 show that the wushan biotites contain high concentrations of mgo (12.80–14.06 wt%) and low concentrations of al2o3(13.56–13.99 wt%), with mg/(fe2+ + mg) ratios that range between 0.62 and 0.65. using a mg–(fe2+ + mn)–(fe3+ + alvi + ti) biotite classification diagram (figure 3), all of the biotites from the wushan granodiorite were classified as magnesio-biotites. the wushan biotites also contain low concentrations of feo (15.30– 17.28 wt%), with all samples falling within a narrow range of fe2+/(fe2+ + mg) ratios. all of these characteristics show that the biotites within the wushan granodiorite are primary and crystallized directly from magma (stone, 2000). figure 3. variations in mg vs. (fe2+ + mn) vs. (fe3+ + alvi + ti) in biotites collected from the wushan granodiorite; diagram after foster, 1960. 5. crystallization conditions 5.1 temperature and oxygen fugacity the data in table 1 indicate that the si within the biotite structures can be replaced by alvi, but not by alvi or ti. these results indicate that the biotites crystallized at high temperatures (deer et al., 1966), estimated to be between 720°c and 750°c (figure 4). figure 4. variations in the ti vs. mg/(mg + fe) ratio of biotite collected from the wushan granodiorite; diagram after henry et al., 2005. previous research (wones and eugeter, 1965; barriére and cotten, 1991; albuquerque, 1973; noyes et al., 1983) indicates that the atomic abundances of 43composition of biotite within the wushan granodiorite, jiangxi province, china: petrogenetic and metallogenetic implications fe3+, fe2+, and mg2+ in biotite collected from a cogenetic biotite–magnetite–kfeldspar assemblage can be used to calculate the oxygen fugacity conditions that were present during crystallization. as determined by petrographic microscopy, biotite within the wushan granodiorite is present within a hornblende + biotite + k-feldspar + magnetite + quartz assemblage indicating that the oxygen fugacity conditions of crystallization can be calculated. in the fe3+–fe2+–mg biotite diagram shown in figure 5, all of the biotites that were analyzed in this study fell between the ni–nio and fe2o3–fe3o4 buffers, indicating that they crystallized under conditions of high oxygen fugacity (wones, 1989) . figure 5. variations in fe3+–fe2+–mg in biotite collected from the wushan granodiorite; diagram after wones and eugster, 1965. in addition, figures 4 and 5, and the log fo2–t diagram for biotites (figure 6) at ph20 = 207.0 mpa (wones and eugster,1965), suggest that these biotites were crystallized at log fo2 values between –11.6 and –12.5. this result is consistent with our previous discussion. figure 6. variations in log fo2–t conditions during biotite crystallization in the wushan granodiorite; diagram after wones & eugster (1965), with the numbers 0–100 representing biotite stability as a function of (100 × fe/(fe + mg)). 5.2 pressure and depth the wushan granodiorite has been significantly altered, with mafic minerals (e.g., hornblende) undergoing variable chloritization. as a result, this mineral cannot be used as a geo-barometer (jiang et al., 2008). however, the biotites within the wushan granodiorite are unaltered (figure 2), and etsuo et al. (2007) documented a strong positive correlation between the total al (tal) content of a biotite sample and the solidification pressure (p) of the granitic host rocks, as determined using sphalerite and hornblende geo-barometers and mineral assemblages within the surrounding rocks. these results lead to the following empirical equation: p (kb) = 3.03 × tal – 6.53 (±0.33), where tal is the total al content of the biotite (calculated using 22 oxygens). therefore, this biotite geo-barometer allows us to constrain the pressures that were present during crystallization of the wushan granodiorite to 0.86–1.03 kb, equating to depths of 2.84–3.39 km. 6. petrogenesis and metallogenic significance biotite compositions enable researchers to determine the type and mineralization potential of a host granite as well as the source of magma from which these biotites formed. as previously determined, the alvi abundances of biotite permit discrimination between iand s-type granites, with whalen (1988) reporting that i-type granites are associated with biotites with low alvi abundances (0.144– 0.224), whereas s-type granites are associated with biotites with higher alvi abundances (0.353–0.561). the alvi abundances in biotites from the wushan granodiorite ranged from 0.03 to 0.19 (table 1), indicating that the wushan granodiorite is an i-type granite. abdel-rahman (1994) found that biotites that are present within i-type granites are relatively enriched in magnesium, whereas s-type granite biotites are relatively enriched in aluminum. in addition, wushan granodiorite biotites are magnesian, which also supports an i-type granite classification for this intrusion. zhou (1986) suggested that a w(∑feo)/w(∑feo + mgo) vs. w(mgo) diagram using biotite compositions could be used to discriminate between granites of differing origins. using such a diagram, biotites from the wushan granodiorite plot within the mixed mantle–crust source (mc) area (figure 7). these results are consistent with isotopic analyses showing an ehf (t) that ranged between –2.1 and –7.0 (ding et al., 2006) and an end (t) that ranged between –4.08 and –4.44 (jiang et al., 2008). figure 7. variations in the (∑feo)/ (∑feo + mgo) ratio vs. mgo in biotites from the wushan granodiorite; diagram after zhou, 1986. wyborn et al. (1994) and sun et al. (2004) determined that high oxygenfugacity environments are prospective for the precipitation and mineralization of economic metals such as cu and au. this result suggests that intrusive rocks that form in high oxygen-fugacity environments should be considered highly prospective in terms of mineralization. as described above, the crystallization temperatures of biotites from the wushan granodiorite ranged from 720°c to 44 qian dong, yangsong du, zhenshan pang, wenrui miao, wei tu 750°c, with log fo2 values ranging from –11.6 to –12.5. these results suggest that the wushan granodiorite formed at pressures of 0.86–1.03 kb, equating to depths of 2.84–3.39 km. these crystallization conditions also indicate that the wushan granodiorite formed at high temperatures, shallow depths, and under conditions of very high oxygen fugacity. in addition, these results suggest the wushan granodiorite is highly prospective for mineral exploration and that it contributed to the formation of the wushan skarn copper deposit. 7. conclusions analyses of biotites from the wushan granodiorite allowed us to reach the following conclusions. biotites within the wushan granodiorite are mgand ti-rich and fe-poor. they are classified as magnesio-biotites. the crystallization temperatures of wushan biotites ranged from 720°c to 750°c, with crystallization under log fo2 values of –11.6 to –12.5. the wushan granodiorite was formed at pressures of 0.86–1.03 kb, equating to depths of 2.84–3.39 km. these conditions indicate that the wushan granodiorite developed at high temperatures, shallow depths, and at a very high oxygen fugacity. as a result, the wushan granodiorite should be considered highly prospective, and it probably contributed to the formation of the wushan skarn copper deposit. the wushan granodiorite is an i-type granite, which was sourced from the melting of a mixed mantle–crust source. acknowledgments this study was financially supported by the china geological survey (grant no. 12120113069900 and 20089938), the national natural science foundation of china (grant no. 40672045), and the ministry of education of china (grant no. 308006). engineers guohong liu and mingjun li provided valuable assistance in the field. references a.e. lalonde and p. bernard. 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(2014). petrogenesis, geochronology, and tectonic significance of granitoids in the tongshan intrusion, anhui province, middle-lower yangtze river valley, eastern china, 79, 792-809. geofisica colombiana n°.7 pp.74-78 dlciembre de 2003 bogoli\, d.c. issn 0121 2974 application of neural networks to obtain the site response in mexico city carlos a. vargas j. universidad nacional de colombia, bogota, grupo de geofisica, cavargasj@unal.edu.co julio c. cuenca s. instituto de ingenieria, unam, mexico, d. e, mexico, jccsa@pumas.iingen.unam.mx resumen hemos implementado una red neuronal de tres capas escondidas con 40 neuronas por capa para ser usada como funciones de trasferencia suelo/roca en dos estaciones acelerornetricas en ciudad de mexico. la red fue entrenada con entrenamiento supervisado por medio de vectores de aceleracion de entrada y salida (doce registros de cinco eventos sismicos localizados en la costa de guerrero y uno al sur de puebla, 5,8 m 7,3), yprobado con tres registros no tornados en cuenta en el entrenamiento de la red. los resultados obtenidos en el dominio de la frecuencia son bastante buenos, encontrandose una amplificacion sismica entre 0,2 a 5 hz para la zona de lago (estacion rmcs). en el dominio del tiempo obtuvimos resultados que no son coincidentes. debido a los datos y a la complejidad del fenomeno, es necesario aplicar esta herramienta usando mas registros de movimientos fuertes para entrenar la red neuronal, asi el fenomeno puede ser aprendido mejor mediante una base de datos confiable. palabras clave: redes neuronales, respuesta oe sitlo, ciuoao de mexico abstract we have implemented a neural network of three hidden layers with 40 neurons each layer to be used as soil/rock transfer functions for two stations in mexico city. the net was trained with supervised learning through input and output vectors of accelerations (twelve records, from five seismic events from guerrero and puebla, 5.8 m 7.3), and tested with three records not taken in account in the training. the results in the frequency domain are good, finding a seismic amplification between 0.2 to 5 hz for the lake zone. in the time domain we obtain results that are not coincident. due to the data and the complex of the phenomena, it is necessary to apply this tool using more records for the training net, so the phenomena can be learned better through reliable database. key words: neural networks, site response, mexico city introduction the modeling of site response and specially the determination of the capacity of amplification from strong lithologic contrastshave been an aspect of relevant interest for earthquake engineering in the past three decades. a wide variety of methodologies has been proposed and applied in searching to establish transfer functions soil/rock oriented to purposes of seismic microzonation and geotechnic. some techniques were inspired in the capacity of coda waves or ambient tremors to denote changes of stiffness or attenuation of seismic energy, others were based in physical models under visco-elastic conditions in terrain with lateral homogeneity (borcherdt, 1970; su et al., 1992; field and jacobi, 1993; kato et al., 1995; su and aki, 1995; steidl et al., 1996; mucciarelli, 1998; rielpl et al., 1998; cultera et al., 1999). popular techniques such as nakamura (nakamura, 1989) and standard technique have been applied in mexican cities to study the site effect (lermo and chavez-garcia, 1994; chavez-garcia and cuenca, 1996). due to mexico city was funded over soft soils (layers of clay surrounded by basaltic material), it was known the amplified seismic waves and its long duration effect (sing and ordaz, 1993; lermo and chavez-garcia, 1994). singh manuscrito recibido para evaluacton ells de abril de 2003. articulo aceptaco para publicacion por el comrte editorial el 30 de octubre de 2003. 74 geofislca colombiana, 7, oiciembre de 2003 application of neural networks to obtain the site response in mexico city and ordaz point out that the long duration of acceleration records in the lake zone (called soft zone) is result of multipath from the source. recently, the use of artificial intelligence for purposes of site response has put of manifest the practical utility of genetic algorithm and artificial neural network for the simulation of transfer functions soil/rock and inversion of others geophysical variables (anderson, 1990; aguirre et al., 2000; moya et al., 2000; romo et al., 2000; garcia et al., 200;0;vargas et al., 2002). in this work, we try to show the importance of neural network for seismic response proposes for the lake zone in mexico city. data digital acceleration data associates to five earthquakes were selected from the instituto de ingenieria unam, the centro nacional de prevencion de desastres (cenapred) and the mexican database of strong motion. the units are gals and the series of time are 100 and 200 samples/sec depending on the local network and instrument. four earthquakes used are located at the mexican coast of guerrero state and one at south of puebla city (see figure i) covering principally three different azimuths. the accelerographic stations in mexico city taken on count due to quality of the data are rmcs (called roma "c" in surface on clay layers or lake zone) and cena (called cenapred over basaltic deposits). the station called cup4 (called c. u. patio 4 on similar basaltic deposits as in cena) was used in the case when cena not registered the events numbers i and 2 (see table 1). methodology in this work, we have tried to determine a non-analytic transfer function capable to predict seismic response under the soft formationslhard rock relation, which can consider non-linear aspects of that relation. for that reason, we have selected to train a neural network on the basis of spectral observations ofthe n-s, e-wand z components for five seismic events. taking in consideration some conclusions mentioned by vargas et al. (2002), who computed seismic amplification factor for the armenian area (colombia) using neural networks with acceleration data, pointing out that the simulations with any component must keep a similar tendency in the results. for this reason, from fifteen records (5 events with 3 components) we applied twelve records in the training phase and the others three for the evaluation phase. the utility of neural network has seen limited its use for spectral analysis. justly, the analysis on the time domain becomes a not very practical procedure due to the high computation demanding (vargas et al., 2002; garcia et al., 2000). in our case, it was proposed that inversion of the spectral acceleration for rmcs station on soil of mexico city with data of spectral acceleration from stations on rock (cup4 and cena) through the use of neural networks based on the following criteria: 1. spectra of the events coming from the seismic zones of guerrero and puebla, principal seismic sources, recorded in mexico city show components that rarely exceed the 5 hz 2. the longitudes of data accelerographic for seismic events with 5.7< mc < 7.3, generally not exceed their strong movements so far than 4 minutes. figure 2 is one example of this situation. for the 12 records of the training phase we applied fast fourier transform (fft) to spectral acceleration determination, with an analysis of 50 seconds beginning from s wave. the following step was used a neural network with supervised learning; the inputs are the spectral accelerations in the reference station (rock) and the output the spectral acceleration on other station (soil). that network must have capacity of generalization with certain detail the spectrum (training with 12 records), for that it is necessary a good number of neurons; it was selected a back-propagation topology (johansson et al., 1990) with three hidden layers of 40 neurons and activation functions [tansigmod logsigmod linear]. the first two activation functions have the capacity of learning i.inear and non-linear patrons, while that the table j. earthquake of acceleration data analyzed in this work registered in rmcs (soil) and cena (rock) i i 1 i i timag.m i dist.e) azi. location# date lat.n( 0) lon.w(o) mehh:mm:ss c 1 10 december 19-~--t:17:40.9 6.3 18.02 101.56 2.7239 nw guerrero __--i 2 14 seotember 1995 14:04:30.5 7.3 16.31 98.88 3.1 176 south coast guerrero i 3 15 june 1999 20:42:07.1 6.7 18.18 97.51 1.9 129 south puebla i i4 21 june 1999 17:43:05.5 15.8 17.99 101.72 2.7241 nw guerrero 5 29 december 1999 05:19:46 5.9 18.02 101.68 nw guerrero i geofisica colombiana, 7, oiciembre de 2003 75 third scale magnitudes of output. the learning was based in the method of conjugated gradient (hagan et ai., 1996), a powerful algorithm with rapid capacity of error convergence. finally, we tried to invert the waveform using inverse fost fourier transform (ifft) in the band 0.02 to 10 hz with intervals of 0.02 hz in the time domain. figure 3 shows the fidelity of the recorder interloper versus the true one. results and discussion it has been appreciated in several works of site response for mexico city, the amplification due to the contrast soil/rock is important in low frequencies mainly between 0.2 to 5 hz (lermo and chavez-garcia, 1994; chavez-garcia and cuenca, 1996). these observations detailed with the data of this work were simulated from the neural network. figure 4 shows an example of the capacity of reconstruction of neural network with the learned information of 12 records. we see (top of figure 4) a real amplification effect as product of soil/rock relation. however, we can obtain an simulated amplification effect similar to real one when we use as input for the net a spectra from the rock (bottom of figure 4). this procedure allows us to considerate the neural network as a well approximation for infering empirical transfer functions soil/rock by means of its generalization capability. to evaluate the inference capacity of the neural network with data unknown, we used 3 records on rock (not taken in account in the training phase) to establish what similar are the deduction of the net with respect to the true spectral accelerations on the soil. in the figure 5, we can appreciate simulated and true accelerations that follow the same tendencies, the dominants period obtained are best coincident (0.02-5 hz). thus, it is clear that the neural network has a well behavior as a transfer function soil/rock with unknown data and it can be used for inferring spectra an soil from spectra on rock. on the other hand, it looks attractive to use a net for infering the waveform on the soil from the waveform on the rock. several tests were made to obtain a transfer function in the time domain that should reach this objective. figure 6 shows real signal on the time domain (top) where we see different amplitudes as answer of the amplification effect of the soil. similarly, we see the true and simulated records on the soil with the learned data. these results satisfy in the first instance our requirements with a little change of the wave-phase. however, when we use records not taken in account in training, the waveforms were very different (see figure 7). this situation is answer of the very complex problem associated to lateral heterogeneity of soils, scattering and the structure of mexican basin. obviously, it is necessary to use more records for the training phase that can assure optima learning and permit obtain more reliable records. 76 vargaset al. conclusion we have used five triaxial accelerographic records to determine soil/rock transfer functions for two stations in mexico city with neural networks. in this case it was used a topology with three hidden layers of 40 neurons by layer. the net was trained with supervised learning through input and output vectors of accelerations (12 records), and tested with three records not taken in account in the training. in the frequency domain the results were good and the net has a valuable behavior as reliable tool for studies of site response. however, in the time domain the results were not coincident. due to the complex of the phenomena, it is necessary to use more records for the training net that can learn the nature of phenomena. we suggest to made more works on this sense where exit reliable databases that permit to show the great capacity of this technology for proposes of site response. bibliography aguirre,1., contreras,m. g. and flores,h. c. 2000. 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"using neural networks to determine site response from coda waves: application in armenia, colombia". geofisica colombiana, 5: 27-31. earthquake 1o-dec-94,16:17guerrero 10~---,....-r---r----r----..-----, -100'-------:':50:-------;-1 0::::0:-----'::5:;;-d-----;;2:';;00:;----;;-;250 time(s) powerspectrumdensity o '----jlc:.flw"'--j..m>,."-----<>-----:-----_:------:-_---;--------; o 3 5 frequency (hz) figure 3. signal in time (top) and frequency domain (bottom). note the spectrum rarely exceed the 5hz. 77 104 6 8'~':c'".,"''"'''''''''k'."''" o 2 4 6 8 frequency (hz) 10 figure 4. on the top, spectra of soil (soft line) and rock (heavy line). on the bottom, the true (soft line) and simulated (heavy line) spectral acceleration on soil, from learned data. 4 5 6 frequency(hz) 10 figure 5. true (soft line) and simulated (heavy line) spectral acceleration on soil for three different records. the simulated data obtained from spectral acceleration on rock not taken in account in the training. 78 v argas et al. true aceieratron on rock and 5011 ~:~n~ 0 u .10 -20~o 5 10 15 20 25 30 35 40 45 50 "~ 10 n~ 0 u -10 ·20 o 5 10 15 20 25 time(s) 30 45 5035 40 figure 6. on the top, true acceleration on rock (heavy line) and (soft line) soil. on the bottom, true (soft line) and simulated (heavy line) acceleration for the same time window of soil. true and simulated aceieranon::~o 5 10 15 20 25 30 35 40 45 50<:~o 5 10 15 20 25 30 35 40 45 50 .~~o 5 10 15 20 25 30 35 40 45 50 time(s) figure 7. true (soft line) and simulated (heavy line) acceleration for three different events. geofislca colombiana, 7. oiciembre oe 2003 keywords: hj-1a his; landsat-8 oli; fusion method; wetland classification. palabras clave: hj-1a his; landsat-8 oli; método de fusión; clasificación de humedales. how to cite item zhu, d., wan, l., & gao, w. (2019). fusion method evaluation and classification suitability study of wetland satellite imagery. earth sciences research journal, 23(4), 339346. doi: https://doi.org/10.15446/esrj. v23n4.84350 based on hj-1a hsi data and landsat-8 oli data, rs image fusion experiments were carried out using three fusion methods: principal component (pc) transform, gram schimdt (gs) transform and nearest neighbor diffusion (nnd) algorithm. four evaluation indexes, namely mean, standard deviation, information entropy and average gradient, were selected to evaluate the fusion results from the aspects of image brightness, clarity and information content. wetland vegetation was classified by spectral angle mapping (sam) to find a suitable fusion method for wetland vegetation information extraction. the results show that pc fusion image contains the largest amount of information, gs fusion image has certain advantages in brightness and clarity maintenance, and nnd fusion method can retain the spectral characteristics of the image to the maximum extent; among the three fusion methods, pc transform is the most suitable for wetland information extraction. it can retain more spectral information while improving spatial resolution, with classification accuracy of 89.24% and kappa coefficient of 0.86. abstract fusion methods evaluation and classification suitability study of wetland satellite imagery evaluación del método de fusión y estudio de idoneidad de clasificación de imágenes satelitales de humedales issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84350 con base en los datos hj-1a hsi y landsat-8 oli, los experimentos de fusión de imágenes rs se llevaron a cabo utilizando tres métodos de fusión: transformación de componente principal (pc), transformación de gram schimdt (gs) y algoritmo de difusión vecina más cercana (nnd). se seleccionaron cuatro índices de evaluación, desviación estándar, entropía de información y gradiente promedio, para evaluar los resultados de fusión de los aspectos de brillo de imagen, claridad y contenido de información. la vegetación de humedales se clasificó por mapeo de ángulo espectral (sam) para encontrar un método de fusión adecuado para la extracción de información de vegetación de humedales. los resultados muestran que la imagen de fusión de pc contiene la mayor cantidad de información, la imagen de fusión gs tiene ciertas ventajas en el mantenimiento del brillo y la claridad, y el método de fusión nnd puede retener las características espectrales de la imagen al máximo. entre los tres métodos de fusión, la transformación de pc es la más adecuada para la extracción de información de humedales. puede retener más información espectral al tiempo que mejora la resolución espacial, con una precisión de clasificación del 89,24% y un coeficiente kappa de 0,86. resumen record manuscript received: 20/05/2019 accepted for publication: 27/08/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 339-346 danyao zhu1,2,3, luhe wan1,2*, wei gao1,2 1heilongjiang province key laboratory of geographical environment monitoring and spatial information service in cold regions, harbin normal university, harbin 150025, china 2college of geographical science, harbin normal university, harbin 150025, china 3department of geography, mudanjiang normal university, mudanjiang 157011, china * corresponding author: wanluhe@163.com g eo ph ys ic s 340 danyao zhu, luhe wan introduction with the development of remote sensing technology, a large number of high-resolution satellites have appeared, such as irs-1c/1d, hj-1a/b, ikonos, quickbird, orbview, gf-1/2. these satellites with different band ranges, spatial resolution and spectral resolution have led to an increasing number of remote sensing data of different types in the same region (wang, li, & li, 2001) . how to integrate these remote sensing data and obtain the most applicable remote sensing image has become a difficulty in remote sensing technology. image fusion technology, by integrating the spatial and spectral advantages of remote sensing images, has continuously expanded its application field, and researchers have introduced the fusion image concept into wetland vegetation classification. wu, he and wang (2015) applied the fusion image of cbers-3 to the classification of wetlands by using four fusion methods, namely, gihs transform, wavelet transform, contourlet transform, nsct-gihs transform, and got better results. dong, lin, sun, qiu & zhang (2013) studied the applicability of wetland classification by using landsat-5 and spot-5 based on brovey transform, gram-schmidt transform and principal component (pc) transform. the results show that the fusion of 543 bands of landsat-5 and the third principal component of spot-5 are most suitable for the extraction of wetland information. wei, li, tan, & xun (2011) used the fusion image of ndvi (normalized difference vegetation index) extracted -36°chris high spectrum image and 0°chris image, adopted the method of spectral angle mapping (sam) to extract wetland vegetation type information, the experimental results show that the accuracy of vegetation classification in hyperspectral images is improved. zhu (2012) used the fusion image of hyperspectral image of hj-1a and ccd image, and extracted the typical vegetation distribution in zhalong region by using spectral angle mapping, and obtained higher classification results. the above mentioned image fusion methods are mainly focuses on the neighborhood processing of digital images. the focus of the research is how to improve the spatial resolution and spectral resolution of images through the improvement of algorithm. most scholars who do digital image fusion research tend to study in the background of computer science, but there are not many studies from the perspective of geography, especially the application of hyperspectral fusion image to wetland vegetation classification, so this paper conducts the research from this perspective. in this study, the wuyiling national nature reserve in xiaoxing’an mountains in northeast china was taken as the research area, and the hj-1a remote sensing image in july 2015 and landsat-8 remote sensing image in august 2015 were taken as the data sources. by analyzing the spatial resolution, spectral resolution, brightness, clarity, information content and the spectral features, the best fusion method applicable to the extraction of wetland information was found. the fuson image is classified by spectral angle mapping method, and the accuracy of the fusion method is verified to compare the classification accuracy of typical wetland vegetation. methods study site the images used in this study is located in wuyiling national nature reserve, geographic coordinates are 129°00′e-129°28′e,48°33′n-48°50′n, east of the xiaoxing’an mountains peak, northeast heilongjiang province. hyperspectral images can make up for the lack of distinguishing ground object information in wide-band remote sensing images (yan, 2008). however, due to the low spatial resolution of hj-1a data source (100 m), by fusion with the high-resolution landsat-8 oli full-color image (15 m), the spatial resolution of the data source was improved, so as to obtain more features of ground objects (table 1). the auxiliary data include the vector boundary map of the research area and the measured data of the field sampling points. table 1. data sources used in the study area time satellite designation sensor type 20150719 hj-1a hsi 20150815 landsat-8 oli data preprocessing hj-1a data. was preprocessed with envi5.3 software, which hj-1a /1btools tool is used to convert the data format, after calculating the signal to noise ratio of each band image, remove the first 20 bands with low signal to noise ratio. due to the existence of severe tilted stripes and bad lines in hsi data, the replace bad line function is used to repair bad lines and the global method is used to remove image stripes. radiation calibration of hsi secondary data has been carried out, and flaash atmospheric correction is still needed, and landsat8 oli images of the same period were taken as reference, select gcps: image to image tool was used for geometric precision correction, finally, the image is mosaic and cropped (fig. 2). fusion methods due to the large number of bands in hj-1a hyperspectral image, three fusion methods with unlimited band number are adopted in this paper for fusion operation. (1) principal component (pc) transform pc transform is a multidimensional orthogonal linear transformation based on statistical characteristics (ma, feng, he, & zhang, 2007). it concentrates the common information of multi-band images into the first principal component, while the unique spectral information that does not overlap with any other bands is put into other principal components, then, the panchromatic band data are stretched to have the same mean and variance with the first principal component, and which are used to replace the first principal component for principal component inverse transformation to restore to the original space and generate multi-band fusion images with high spatial resolution. (2) gram schimdt (gs) transform gs transformation is essentially a phase recovery algorithm (ma & wang, 2016). the main steps of the algorithm are as follows: (1) use the multispectral image with low spatial resolution to generate simulated panchromatic image with low spatial resolution; (2) the simulated image (as the first band) is superimposed on the multi-spectral image, and the reconstructed multi-spectral image is carried out gs positive transformation; (3) replace the first component of gs transform with high-resolution panchromatic image, then which was carried out gs inverse transformation on the multi-spectral image to get the final fusion image. figure 1. study site on xiaoxing’an mountains in heilongjiang province 341fusion method evaluation and classification suitability study of wetland satellite imagery g mn m i n j i ix y = = = +( )∑ ∑1 1 1 2 2 2   (3) where: m, n are the number of rows and columns of the image; ix , iy are the first-order difference of x direction and y direction, respectively; i, j are the line and column number of each pixel in the image of the same band. information entropy, an evaluation index of spatial detail information based on information quantity, reflects the amount of image informati on quantity. the larger the value is, the larger the information content of the fusion image be. the expression is: h l i ppi i= − =∑ 1 0 � (4) where: l represents the overall gray scale series in images; pi represents the probability of gray value of pixels in images. results and analysis the pca transform, gs transform and nnd algorithm results of hj-1a his image are shown in the figure (fig. 3.) visual interpretation it can be seen from the comparison between the fusion image and the original image that the fusion image maintains the spectral characteristics more greatly in terms of maintaining spectral information. pc fusion image and gs fusion image have moderate color contrast, and all kinds of ground objects in the fusion image show standard spectral detail features. visually speaking, there is no significant difference in spectral features between the two fusion images; the color change of nnd fusion image is serious, and spectral deformation of all kinds of ground objects has occurred, in which the construction land is green and the spectral deformation is serious. sphagnum and its surroundings appear purplish, which is quite different from the original image. it can be seen from the spatial information of the image that the fusion image is clearer and more distinguishable, with the same spatial resolution and clearer texture structure as the landsat-8 panchromatic image. in the nnd fusion image, the texture of construction land is clearer, and the edges of ground (3) nearest neighbor diffusion (nnd) algorithm nnd algorithm was proposed by sun, chen and david (2014). the main theory is that the new spectrum of high-resolution fusion images can be expressed as the weight combination of adjacent hyperpixel spectra of current pixels in low-resolution multispectral images. the fusion method based on nearest neighbor spreading can maintain accurate spectral information of multi-spectral images identified by spatial features, and is applicable to the application of accurate spectral information other than traditional visual inspection (ocalan, erdogan, tunalioglu, & durdag, 2016; gao, 2015). evaluation methods in this paper, four evaluation indexes including mean, standard deviation, information entropy and average gradient were selected to objectively evaluate the fusion image from the perspectives of image brightness, clarity and information content (fusun, saygin, mustafa, & filiz, 2017). mean, which is the gray mean value of pixels in remote sensing images, which can be expressed as: v mn m i n j f i j= = = ( )∑ ∑1 1 1 , (1) where: m, n are the width and height of the image respectively; f(i,j) is the dn value function of all pixels in the fusion image; i and j represent the row and column numbers of raster cells. standard deviation, which reflects the dispersion of dn values of all pixels and their dn means, can be used to evaluate the brightness of images. the higher the value, the more information the image contains. the specific formula is: std mn m i m j f i j v= = = ( ) −( )∑ ∑1 1 1 , (2) where: v is mean, m, n are the number of rows and columns of the image; f(i,j) is the dn value function of all pixels in the fusion image; i and j represent the row and column numbers of raster cells. average gradient, with the ability to sensitively reflect the image contrast to small details and texture transformation expression. the larger the average gradient value is, the more layers the image has and the clearer the image will be. the expression is: figure 2. preprocessed hj-1a hsi image (a. false color composite of band 111, 91, 71) and landsat-8 oli image (b. false color composite of band 5, 4, 3) 342 danyao zhu, luhe wan objects are prominent, which makes it easier to identify. however, serious fuzzy phenomena appear at the boundaries of sphagnum and marsh, the fused image is not clear enough. the color of construction land in pc fusion image and gs fusion image is relatively bright and dazzling, and the vegetation features in gs fusion image are obvious. quantitative evaluation according to the above quantitative evaluation indexes, with the support of envi/idl8.5, the eigenvalues of fusion images obtained by the three fusion algorithms used in this study are extracted, and the calculated results are shown in table 2. table 2. fusion image evaluation index calculation results mean value standard deviation average gradient information entropy original hyperspectral image 115.498 77.508 4.392 4.158 pc fusion image 111.143 92.902 12.358 4.652 gs fusion image 117.078 81.412 14.542 4.647 nnd fusion image 117.971 77.215 6.142 4.515 in terms of the brightness information of the image, the difference between the mean value of the fused image and the original image is relatively small. descending means of fusion image: nnd fusion image > gs fusion image > pc fusion image. the results show that the brightness of gs fusion image is the closest to the original image, indicating that gs fusion image has certain advantages in maintaining brightness information. from the aspect of image sharpness, average gradient can be used to evaluate the effect of improving sharpness in objective evaluation indexes. it can be seen from the table that the average gradient of gs fusion image is the largest, followed by pc fusion image, indicating that gs fusion image and pc fusion image have advantages in terms of sharpness. in terms of the information content of the image, the standard deviation and information entropy of the fused image can evaluate the improvement of the information content. the standard deviation and information entropy of pc fusion image are better than the other two fusion images. the information content of pc fusion image is the largest, followed by gs fusion image and nnd fusion image. spectral variation analysis in hyperspectral images, the radiation information of each pixel in each band can be extracted into a continuous spectral curve, reflect the spectral characteristics of ground objects, and realize the fine classification of ground objects, which is the most significant advantage of hyperspectral images (moon, choi, lee, & an, 2017; adam, mutanga, rugege, 2010). if the spectral curve of fusion image is distorted, the spectral information also changes. therefore, to evaluate which fusion method can retain the spectral information of the original image to the greatest extent, it is necessary to analyze the spectral curve variation degree of fusion images. from the fusion images and the original images, typical features of 9 ground objects samples, which concluded construction land, lakes, farmland, sphagnum, carex tato, salix brachypoda, white birch, pinus koraiensis and spruce, were selected to calculate the average spectral curve (fig. 4.). in the spectrum curve of construction land, the spectrum curve of pc, gs and nnd fusion image is similar. before 700 nm, the nnd fusion image is lower than the original image spectrum curve, and after 720 nm, the spectrum curve of the three kinds of fusion image is higher than the original image. in comparison, the spectral fidelity of nnd fusion image is better. in lake spectral curve, before 700nm, spectrum curves of three kinds of fusion image are almost the same, after 700 nm, spectrum curves of three kinds of fusion image are higher than the original image. it indicates that the spectral information of the water body of the three fusion images is seriously distorted, in which the change of the original spectral information by gs fusion is relatively small, while the change of the original spectral information by nnd fusion is relatively large; the sphagnum spectral curves of the three fusion images almost coincide with that of the original images, indicating that the fusion images have little influence on the sphagnum spectra; the spectral curve of the carex tato before and after fusion had the same trend before 700 nm, indicating that spectral fidelity was better. after 700 nm, the spectrum curve of nnd fusion image is lower than that of the original image, while the spectrum curve of pc fusion image and gs fusion image is higher than that of the original image, and keep the same trend; the spectral curves of farmland before and after fusion, after 700 nm, the spectral curves of three fusion images and the original image presented significant differences and serious spectral deformation. the reflectance of the spectral curves of the three fusion images was much higher than that of the original image and show the same trend; the spectral curves of white birch, pinus koraiensis and spruce before and after fusion, the spectral curves of the three fusion images and the original images almost overlapped before 750 nm, indicating that the fusion images retained good spectral characteristics. after 750 nm, the original spectral curve of white birch was higher than that of the three fusion images, while the original spectral curve of pinus koraiensis and spruce was lower than that of the three fusion images, indicating that there was a certain degree of spectral deformation in the fused images. classification results and accuracy evaluation in this study, the spectral angle mapper (sam) classification method with high accuracy was selected to extract the information of wetland types in wuyiling national nature reserve. sam is a hyperspectral classification algorithm designed based on the spectral shape characteristics of hyperspectral data. this algorithm has a simple design idea and does figure 3. results of different fusion methods (false color composite of band 111, 91, 71) 343fusion method evaluation and classification suitability study of wetland satellite imagery figure 4. comparison of spectral curves between fusion images and row image 344 danyao zhu, luhe wan not require data compression of hyperspectral remote sensing data. it is a relatively mature classification algorithm in hyperspectral image classification. sam classification processing is applied to fusion images of different methods (fig. 5). meanwhile, the accuracy of classification results is evaluated by using field sampling data in 2015 (tables 3-5). according to the classification results, sam was used to classify the fusion image. the accuracy of pc fusion image classification was higher and that of gs fusion image classification was lower. among them, the mapping accuracy of pinus koraiensis is 79.23% to 83.56%, and that of spruce is 94.08% to 99.15%. there are often misclassification between pinus koraiensis and spruce. in the commission errors, the values of spruce and white birch are relatively high. among the three fusion images, the commission errors of spruce and white birch in pc fusion images are relatively low. in the omission errors, the values of pinus koraiensis is larger, and the classification results of other ground objects are better because of the large difference in spatial details. conclusion and discussion in view of the low spatial resolution of hyperspectral remote sensing image of hj-1a satellite, landsat-8 panchromatic image was applied to carry out fusion experiments on three fusion methods of maintaining image’s band number, namely, pc transform, gs transform and nnd algorithm, and the fusion results was evaluated. the main conclusions are as follows: (1) compared with the original image, the fusion image is clearer and more distinguishable. the three fusion images maintain the spectral characteristics to a large extent, among which pc fusion image and gs fusion image have moderate color contrast, while nnd fusion image has serious color change. the texture of fusion image construction land is obvious, and the edges of construction land are prominent, while the edges of different vegetation types are seriously blurred. the high reflectivity of construction land in pc fusion image and gs fusion image makes it difficult to distinguish.gs image fusion significantly enhanced the vegetation characteristics. in conclusion, gs figure 5. spectral angle mapping classification results 345fusion method evaluation and classification suitability study of wetland satellite imagery fusion image and pc fusion image have advantages in sharpness. pc fusion image contains the largest amount of information, followed by gs fusion image. gs fusion image has a better performance in brightness information. (2) in the spectral variation analysis, except for the serious spectral information distortion of the lake, the spectral curves of other features are similar in the pc fusion image and the gs fusion image; the variation degree of spectrum curve in nnd fusion result is slightly less than that of pc and gs fusion results; in a variety of typical ground objects, the spectral information of lake and farmland is seriously distorted in the near infrared band, and the spectral information of salix brachypoda, spruce and sphagnum is best preserved. (3) the sam was applied to extract typical vegetation types from fusion images, and the accuracy of classification results was evaluated. the optimal overall classification accuracy reached 89.24%, and the kappa coefficient was 0.86. the classification accuracy of pc fusion image is the highest, and contains the largest amount of information; nnd fusion image have certain advantages in spectral fidelity and higher classification accuracy. the classification accuracy of gs fusion image is relatively low. table 3. pc fusion image classification accuracy confusion matrix commission errors (%) omission errors (%) mapping accuracy (%) user accuracy (%) overall accuracy (%) kappa coefficient farmland 5.29 3.11 96.89 94.71 89.24 0.86 sphagnum 2.81 2.56 97.44 97.19 carex tato 0.52 1.21 98.79 99.48 salix brachypoda 1.38 3.43 96.57 98.62 white birch 10.54 0.54 99.46 89.46 pinus koraiensis 7.15 16.44 83.56 92.85 spruce 41.92 0.85 99.15 58.08 table 4. gs fusion image classification accuracy confusion matrix commission errors (%) omission errors (%) mapping accuracy (%) user accuracy (%) overall accuracy (%) kappa coefficient farmland 7.17 3.22 96.78 92.83 87.27 0.83 sphagnum 8.54 3.85 96.15 91.46 carex tato 1.91 2.25 97.75 98.09 salix brachypoda 1.89 6.23 93.77 98.11 white birch 17.87 1.09 98.91 82.13 pinus koraiensis 5.78 20.44 79.56 94.22 spruce 43.38 5.92 94.08 56.62 table 5. nnd fusion image classification accuracy confusion matrix commission errors (%) omission errors (%) mapping accuracy (%) user accuracy (%) overall accuracy (%) kappa coefficient farmland 4.70 2.07 97.93 95.30 87.93 0.84 sphagnum 2.56 2.56 97.44 97.44 carex tato 2.87 0.52 99.48 97.13 salix brachypoda 1.22 4.70 95.30 98.78 white birch 16.86 0.54 99.46 83.14 pinus koraiensis 6.17 20.77 79.23 93.83 spruce 46.14 0.85 99.15 53.86 acknowledgements add a third grant fund: this study was supported by the national natural science foundation of china (41671100), a grant from the education department of heilongjiang province (1352msyzd003), and a grant from mudanjiang normal university (gg2017005). references adam, e., mutanga, o., rugege, d. 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(2012). zhalong wetland vegetation classification based on hyperspectral remote sensing image. harbin normal university. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.86211 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 183 190 m in in g e n g in ee r in g manuscript received: 09/04/2020 accepted for publication: 03/05/2023 analysis of the influence characteristics and correction effect of the mine direct current method in advance detection of roadway cavities haijun xie1, jingrui li2, zhiqiang li3, sheng li3 1college of geology and environment, xi’an university of science and technology 2weinan shaanxi coal qichen technology co., ltd 3china coal science and industry xi ‘an research institute (group ) co., ltd abstract keywords: mine dc electric method; advanced detection; numerical simulation; ratio method to ensure safety during coal mining, the dc electric method is frequently used for the advanced detection of concealed water-rich abnormal geological bodies located ahead of a drivage roadway. however, to some extent, the accuracy of the detection is influenced by the roadway cavity, resulting in a deviation from the actual location of the anomaly. to investigate this phenomenon, the principle of advanced detection based on a spheroidal physical model was analyzed via “comparative analyses” and “simulated” methods using cosmol multiphysics software. following the principle of numerical simulations, the influence of the roadway cavity on the accuracy of the advanced dc detection method was introduced, and the ratio of apparent resistivity calculated from actual data to that calculated only with the roadway cavity was carried out to obtain the roadway correction coefficient. this coefficient was employed to correct the apparent resistivity data for advanced 3-d detection. additionally, the response characteristics of the anomalies at different electrode layout locations in the roadway were discussed. the results show that the ratio method effectively corrects the influence of the roadway during advance detection using the dc electric method and simultaneously improves the resolution of geological anomalies and the accuracy of positioning. additionally, the influence of the roadway was related to the relative positions of the electrodes along the roadway. when the electrodes are set at the junction of the floor and the sidewall, the obtained resistivity curve is less affected. therefore, the resolutions of the anomalies are higher, and their positioning is more accurate. furthermore, a practical application of the ratio method indicated that the corrected curves reflected the geological anomalies better. palabras claves: método eléctrico de corriente continua; detección avanzada; simulación numérica; método de proporción. análisis de las características de influencia y efecto de corrección del método de corriente continua en la detección avanzada de la cavidad de la calzada en minería how to cite this item: xie, h., li, j., li, z., & li, s. (2023). analysis of the influence characteristics and correction effect of the mine direct current method in advance detection of roadway cavities. earth sciences research journal, 27(1), 183-190. https://doi. org/10.15446/esrj.v27n2.86211 resumen el método eléctrico de corriente continua (dc) se usa frecuentemente para la detección avanzada de cuerpos geológicos anormales ricos en agua y ocultos en la calzada de la mina y así asegurar la operación extractiva de carbón. sin embargo, en cierto alcance, la exactitud de la detección puede estar alterada por la cavidad de la calzada, lo que significa una desviación en la ubicación de la anomalía. para investigar este fenómeno se analizó el principio de detección avanzada, que está basado en el modelo físico esférico, a través de métodos de “análisis comparativos” y “simulados” a través del software cosmol multiphysics. de acuerdo con los principios de simulaciones numéricas, se introdujo la influencia de la cavidad de la calzada en la exactitud de la detección de corriente continua avanzada y se computó la proporción de resistividad aparente calculada con este método frente a aquella calculada solo con información de la cavidad de la calzada para así obtener el coeficiente de corrección. este coeficiente se empleó para corregir la información de resistividad aparente en la detección 3-d avanzada. adicionalmente, se discutieron las características de respuesta de las anomalías en varias posiciones con un diseño de electrodos en la calzada. los resultados muestran que el método de proporción corrige efectivamente la influencia de la calzada durante el avance de la detección a través del método eléctrico de corriente continua y simultáneamente mejora la visualización de las anomalías geológicas y la exactitud del posicionamiento de estas anomalías. además, la influencia de la calzada se ve reflejada en las posiciones relativas de los electrodos a lo largo de la calzada. cuando los electrodos están en la zona donde confluyen el suelo y la pared, la curva de resistividad obtenida se ve menos menos afectada. por lo tanto, la resolución de las anomalías es mayor y sus posiciones son más exactas. además, una aplicación práctica del método de proporción indicó que las curvas corregidas reflejaron mejor las anomalías geológicas. https://doi.org/10.15446/esrj.v27n2.86211 https://doi.org/10.15446/esrj.v27n2.86211 https://doi.org/10.15446/esrj.v27n2.86211 184 haijun xie, jingrui li, zhiqiang li, sheng li 1. introduction as an important energy resource, coal has played an irreplaceable role in the social and economic development of china (yuan et al., 2021; peng et al., 2020). during coal mining, mine water hazards, which have caused very large casualties and property loss, represent the second leading cause of coal mine accident-related deaths (cheng et al., 2014; yang et al., 2016). according to statistics, the driving face is the main place where water accidents occur. to detect water-bearing anomalies prior to excavations, advanced electrical detection technology based on the resistivity difference between groundwater and surrounding rock has become an important pre-detection method in the past 20 years (wang et al., 2021; zhang et al., 2015; yue et al., 2019). regarding mining dc advanced detection technology, many scholars have performed much research. zhou guanqun et al. (zhou et al., 2022) proposed a “triangular cone” type dc advanced detection three-dimensional observation system, which solves the problem that the traditional dc advanced detection method cannot determine the specific location of the target, avoids the existing obstacles in the current traditional tunnel dc advanced detection, and provides a new solution for advanced detection. with the help of comparative analyses, xiaolong wang and jifeng zhang (wang et al., 2011; hao et al., 2020) simulated the solutions of forward models and theoretical values using comsol multiphysics software and established 3-d geoelectric models for advanced detection in mine roadways. additionally, via forward simulations using comsol multiphysics software, they studied the shapes and positions of the extreme values on the apparent resistivity curves for the advanced detection of different abnormal mine bodies before the roadway. thus, the possibility of applying comsol multiphysics software in forward simulations for advanced dc detection in mine roadways was verified. through the combination of numerical simulations, physical simulations, and engineering applications, lu liu and wenfeng zhan (liu, 2014; zhan et al., 2018) systematically studied the impact of various influencing factors on the results of advanced detection using the dc electric method. they derived a 3-d advanced detection system and tested its efficiency in numerical simulations, physical laboratory experiments, and on-site data. thereafter, they proposed a corresponding data processing approach. based on physical experiments and numerical simulations, bingzhen ma and jifeng zhang (ma and li, 2013; zhang et al., 2016) systematically studied the effect of the size of the roadway on the accuracy of the results of advanced dc electric measurements in mines and provided a practical formula for the coefficient of the influencing roadway. advanced detection using the dc electric method in roadways has been successfully applied in some mines. however, several problems that require further investigation still exist. for example, during actual detection, the roadway cavity and its ambient environment disrupt the accuracy of the detection, resulting in differences in the electric field distribution characteristics in full space. this increases the difficulties associated with data interpretation and induces a deviation in the positioning of the abnormal bodies in front of the roadway (li et al., 2020; yang et al., 2013). if the influence of the roadway cavity is neglected (lu and wu, 2013), the advanced detection data would be inaccurate, and the resulting errors may likely lead to the distortion of the position of the anomaly to be detected (zhang, 2011). however, if it is considered, it will be very difficult to distribute the electric field in the roadway or obtain an analytical solution to the apparent resistivity of advanced detection. based on these previous studies, the numerical simulation of the resistivity method in comsol multiphysics software is emphasized in this study to investigate the response characteristics of the electric field prior to the working face in an excavation roadway. the ratio method was used to correct the influence of the roadway, and the accurate positioning of the abnormal bodies was achieved. additionally, the response results of the survey line layout at different locations along the roadway were discussed, and a theoretical basis for practical operation was proposed. 2. principle of advanced dc detection in mines 2.1. boundary value and variational problem of the stable electric field if the underground power source is the point “a” and “i”, “г”, and “ω” represent the electric current intensity, the closed surface, and the area surrounded by “γ” as shown in figure 1, respectively, the rectangular coordinate “u”, which is the boundary values that make up the 3-d electric field potential, is summarized as: (1) (2) (3) (4) (5) where “γ1”, “γ∞”, “σ”, “δ”, “n”, and “r” represent the medium interface in the area, the infinite boundary, the electrical conductivity of the underground medium, the dirac function, the coordinate variable in the normal direction of the boundary, and the vector radius from the source point to the boundary, respectively. the boundary of area “ω” represents the interface of the natural boundary conditions. it is automatically satisfied during the process of obtaining the functional extreme values; thus, its consideration is not necessary. when the finite element is used to solve the boundary value problem, it first transforms the boundary value into a variational problem, as shown in the equation below. (6) figure 1. schematic representation of the point source boundary. 185analysis of the influence characteristics and correction effect of the mine direct current method in advance detection of roadway cavities (7) substituting this equation into equation 1 provides: (8) substituting this equation into equation 4 gives: (9) after transposition is completed, the following is obtained: (10) therefore, the equivalent variational problem of the extreme boundary values of the 3-d electric field is: (11) (12) to solve the variational problem, the first step is to subdivide and discretize the model. thereafter, the tetrahedral element is used to divide the model into nonuniform grids. the division of the grids around the point power source and the abnormal body is denser. moreover, as the distance to the center of the model increases, the grids are more widely spaced. thus, both the accuracy and computation speed are enhanced. 2.2. basic principle of the advanced detection method to predict the distribution of geological anomalies in a nonexcavated and nonmined area in mines, the advanced detection dc electric method is used to obtain the characteristics of the distribution and the variation of the stable electric field by moving the measuring electrodes. in a uniform medium, the equipotential surface of the point power source is a concentric spherical surface with the source as the center. however, in a nonuniform medium, the equipotential surface is not a concentric spherical surface because the anomaly occupies the place of the equivalent spherical surface. thus, it became an equipotential curved surface with a form that is related to the nature of an abnormal body in the medium. therefore, the distortion of the spherical equipotential surface served as an anomaly indicator, i.e., the deformation of the spherical shell where the measuring electrodes are located is indicative of the presence of an anomaly. according to the principle of spherical symmetry, the anomaly can also be detected from the observation points in the roadway. thus, the advanced dc electric method detects geological anomalies in advance, as shown in figure 2. in figure 2, “a” represents the source electrodes, and “m” and “n” represent the measuring electrodes. 3. numerical simulation of 3-d advanced detection 3.1. comsol simulation process comsol multiphysics simulations can be divided into three steps as follows: preprocessing, solution, and postprocessing. the preprocessing step involves the creation of finite element models as well as the applied load, the solution step involves grid subdivision and the matrix solution, and the postprocessing step involves the visualized display of the analysis results and data output (zhao et al., 2018). in this study, by defining poisson’s equation on the basis of a mathematical module, the solution was derived using comsol multiphysics software. 3.2. comparison of the analytical and numerical solutions assuming a sphere with a radius, r0, resistivity, �2, and point a that is located outside the sphere, the analytic expression of the potential at point m in the space outside the sphere, which represents the location of the power source according to the principle of the dc detection method, would be: (13) where “�1” and “�2” represent the resistivity of the surrounding rocks and the sphere, respectively; “d” represents the distance between the point power source (a) and the center of the sphere (o); r represents the distance between the potential electrode and the center of the sphere (o); “r” represents the distance between “a” and “m”; “θ” represents the included angle between “ao” and “mo”; and pn(cosθ) represents lengendre polynomials. the above equation could be used to calculate the potential difference umn at points “m” and “n” with “amn” located along the same straight line (i.e., along the excavated roadway). thus, a sphere with a low resistivity for advanced detection in the head-on front of the roadway was simulated. the distribution of the electric field of the sphere with low resistivity in full space as well as the analytical solution of the apparent resistivity of the advanced detection could be obtained using equation 13. if the resistivity of the surrounding rocks in full space (�1, the resistivity of the sphere of low resistivity (�2), the radius (r0), and the distance (d) between the point power source (a) and the center of the sphere (o) are 100 ω·m, �2 1 ω·m, 5 m, and 15 m, respectively, the power current is 1 a. additionally, a comparison of the simulated and the analytical values of the potential is presented in table 1, which shows that closeness to the power supply point (< 2 m away) resulted in a more significant error in the measured potential. conversely, the farther the power supply point was, the smaller the error; i.e., near the boundary, the absolute error tended to 0. the relative error at 1 m away from the power supply point was 5.68%, and at 80 m away, it was 0.12%. obviously, this indicates that the computation results obtained using comsol multiphysics software for the 3-d forward modeling using the mine dc electric method are credible and have a higher computation accuracy. figure 2. schematic representation of the advanced mine dc detection principle. 186 haijun xie, jingrui li, zhiqiang li, sheng li 4. research on the advanced detection of apparent resistivity in mine roadways 4.1. analysis of the influence of the roadway cavity during actual advanced detection, the results of the detection are greatly influenced by the roadway cavity. if the influence of the roadway is neglected, the detection results will be associated with significant errors, thus directly limiting the detection accuracy. in this study, a numerical simulation for advanced detection was performed by taking into account the influence of the roadway cavity, and the model parameters were as follows: the resistivity of the roadway cavity (air) was 1010 ω·m, the cross-section of the roadway cavity was a 3 m × 3 m square, the resistivity of the surrounding rocks was 100 ω·m, the length × width × height of the abnormal body was 15 m × 5 m × 15 m, the coordinates of the central point were (10,0,0), the resistivity of the abnormal body was 1 ω·m, the coordinates of the power supply point were (-5,0,0), and the midpoint of the demarcation line of the tunnel face and the floor was the zero of the coordinates of the central point. the schematic model is illustrated in figure 4 below. the apparent resistivity curves of the advanced detection using the same model parameters in several cases are shown in the sequence in figure 5. here, ρ represents the response curve of a tabular abnormal body of low resistivity in front of the roadway. on the other hand, ρb represents the response curve of the full-space tabular abnormal body of low resistivity (assuming that there is no roadway at the rear), and ρh represents the response curve when there exists only a roadway without any tabular abnormal body at the head-on front. figure 5 reveals that the three curves showed a consistent trend. the influence of the roadway cavity resulted in a significant error difference between ρb and ρ. additionally, ρ obviously had a higher resistance, which was between those of ρh and ρb. this result demonstrates that even though the roadway cavity significantly influences the simulation results, taking this influence into account does not completely eliminate the errors associated with the advanced detection of the geological anomaly. the advanced detection curves in cases where the existence of the roadway was considered not only contained geological abnormal information regarding the front of the working face but also included errors associated with the influence of the roadway cavity. however, it was impossible to distinguish the curves using only the individual curves’ characteristics. 4.2. correction of the roadway influence during actual advanced detection using the dc electric method, the roadway possibly influences the accuracy of the detection results because it influences the distribution of the geoelectric field in the full space, making the advanced detection results more complicated. since the roadway cavity would influence the response characteristics of a geological anomaly in the front during data processing, it is necessary to eliminate the influence of the roadway cavity to avoid the detection of false anomalies. through the ratio curve, the roadway correction coefficient, f = ρ/ρh, was introduced to eliminate figure 3. spheroid in the point source current field of the full space. figure 4. schematic representation of the location of a roadway cavity and a cuboidal water-bearing structure. 187analysis of the influence characteristics and correction effect of the mine direct current method in advance detection of roadway cavities any abnormal response induced by the roadway cavity (ρ represents the existing roadway when there is a tabular abnormal body of low resistivity in front of the roadway, and ρh represents the existing roadway when there is no tabular abnormal body of low resistivity in the head-on front). the ratio method was adopted to obtain the response curves of low resistivity in the roadway space, and the results are shown in figure 6. figure 6 shows that the influence of the roadway on the curve was eliminated; thus, the resolution of the anomaly was enhanced. additionally, a minimum value appeared, and the extreme point was located 10 m away, coinciding with the location of the center of the actual anomaly. the calibration results of the roadway influence show that the ratio method can not only reduce the influence of the roadway cavity but also enhance the amplitude of the abnormal response, which is conducive to the accurate location of the abnormal body. since the roadway influences the results of dc electric detection in mines, the ratio method is more efficient in eliminating the influence of the roadway. thus, this method was preferentially adopted prior to mapping. 4.3. analysis of the influence of the electrode layout location during on-site detection using the dc electric method, the location of the electrode layout in the roadway is an important influencing factor that should also be considered. scholars have studied the relationship between the influence of the roadway cavity and the location of the electrode layout. based on their findings, the curves at the junction line of the sidewall and the floor were determined to be easy to correct. however, the corrected results were not compared. therefore, in this study, the apparent resistivity response curves at different electrode locations in the same roadway were compared. under conditions that maintain the properties of the abnormal bodies in the surrounding rocks and the roadway to be constant (the same as that in section 3.1), the results based on different electrode layout locations were simulated. regarding the same roadway, the simulation results from the different electrode layout locations with symmetric layouts coincided, i.e., the characteristics on the left and right sides of the roadway were the same. therefore, in this study, only the response results based on the left side of the roadway were considered. the location of the survey line layout is shown in figure 7. the black line, the red line and the blue line represent the response results of the electrode layout at the center of the roadway floor, on the left wall along the driving direction, and out on the included angles on the left wall and floor along the driving direction, respectively. the results of the forward modeling are shown in figure 8. figure 8 shows that as the overall trend of the three curves coincided, the resistance decreased gradually, after which a minimum value appeared. as the crossover distance increased, the influence of the electrode position weakened rapidly, and the curves tended gradually toward 1. by comparison, it was found that the first curve for the electrodes positioned along the included angle of the roadway floor and the left wall was only slightly influenced, and the resolution of the anomaly was highest. the curves for the electrodes positioned on the roadway floor as well as the included angle of the roadway floor and the left wall could facilitate the accurate determination of the location of the anomalies. however, the position of the anomalies in the curves of the electrodes positioned on the left side of the roadway showed some offset. table 1. comparison of the analytical and numerical solutions. x/m analytical/v numerical/v relative error/% 1 7.9601 7.5073 5.68 2 3.9811 4.1530 4.32 3 2.6546 2.7372 3.11 5 1.5933 16108 1.10 10 0.8856 0.8941 0.96 15 0.5315 0.5268 0.88 20 0.3987 0.3960 0.66 40 0.1993 0.1985 0.40 80 0.0996 0.0997 0.12 figure 5. comparison of the apparent resistivity response curves. 188 haijun xie, jingrui li, zhiqiang li, sheng li 5. case analysis the return airway in a mine in southern shanxi was excavated along the coal seam floor. the airway was approximately 5.6 m wide and 3.7 m high. from the head-on observation, the coal seam was perfect without obvious fractures and fragmentations. moreover, to ensure safe mining, the dc electric method was adopted to perform advanced detection. during underground operation, a three-point tripolar detection system was adopted. the distance between the power supply electrodes “a”, “b”, and “c” was 4 m. “c” was 14 m away from the head-on front, and the distance between this electrode and the measuring electrodes, “mn”, was 4 m. after data was collected, the collected data were first processed, and the results obtained are shown in figure 9. thereafter, the roadway space was corrected using the ratio method, i.e., the influence of the roadway floor and the geological bodies at the rear of the roadway was removed, and finally, mapping was performed. the results are shown in figure 10. the detection results in figure 9 show that the major anomaly of low resistivity was located at ~22–26 m in the head-on front, and a weak low resistance anomaly was observed at ~50–54 m. they were named the no. 1 anomaly and no. 2 anomaly, respectively. the apparent resistivity of the no. 2 anomaly is similar to the background field, and the anomaly is relatively weak. thus, it cannot be determined whether it is caused by a water-bearing geological structure. after correction is performed using the ratio method, it was found that the anomaly still existed at ~22–26 m, and the anomaly at ~50–54 m was evidently enhanced. it was confirmed that when the excavation reached 52 m at the head-on front, an abandoned roadway with mined-out area water was obtained, and the accumulative amount of inrush water reached approximately 180,000 m3. it is verified that the accuracy of the advanced detection results of the dc method corrected by the ratio method is higher. combined with the results of numerical simulation and engineering application research, it can be seen that using the ratio method to correct the advanced detection data of the figure 6. abnormal amplitude curve after eliminating the influence of the roadway cavity. figure 7. schematic representation of the location of the survey line in the roadway. 189analysis of the influence characteristics and correction effect of the mine direct current method in advance detection of roadway cavities figure 8. comparison of the abnormal amplitude values at different positions in the survey line layout. figure 9. results of the advanced detection using the dc electric method. fig. 10. advanced detection results after correcting the influence of the roadway. 190 haijun xie, jingrui li, zhiqiang li, sheng li direct current method can effectively reduce the influence of the roadway cavity, improve the response amplitude and detection accuracy of the water-bearing geological body in front of the roadway, and achieve the accurate location of the water-bearing geological body in front of the roadway by the dc resistivity method, which is conducive to the popularization and application of the method in the advanced detection of the roadway. 6. conclusion based on previous studies on advanced detection using the dc electric method, comsol multiphysics software was further utilized in this study to determine the influence of the roadway and the relative position of electrodes on the accuracy of the advanced detection of anomalies in mines using the dc electric method. the following conclusions were obtained. a. during advanced anomaly detection in mines using the dc electric method, the influence of the roadway is related to the location of the electrode layout. when the electrodes are set at the junction of the floor and the sidewalls, the influence of the roadway on the accuracy of the detection results is minimal, i.e., the determination of the positioning of the anomaly is more accurate, and its resolution is 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(2022). research on forward modeling of “triangular cone” type direct current method. journal of china coal society, 47(08), 3015-3023. issn 1794-6190 e-issn 2339-3459 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 39 46 g e o ph y si c s effects of vegetation height and relative submergence for rigid submerged vegetation on flow structure in open channel zhongyu wang1, huayong zhang1*, xing he1, qi jiang2, weigang xu3, wang tian1 1. research center for engineering ecology and nonlinear science, north china electric power university, beijing, china. 2. national major science and technology program management office for water pollution control and treatment, beijing, china. 3. institute of wetland research, chinese academy of forestry, beijing, china. *corresponding author: rceens@ncepu.edu.cn abstract: resumen vegetation commonly existing in natural rivers interact with water flow in a strongly nonlinear manner. in this research, vegetation height and a concept of relative submergence are adopted to experimentally investigate the effects of rigid submerged vegetation on flow structure. the results reveal that the relative submergences decrease along the submerged vegetation section. from the perspective of case-average, the linear relationship between water depth and vegetation height demonstrates a decrease trend of average relative submergence with the increase of vegetation height. the differences between the maximum and minimum water depths of submerged cases can be described by a positive linear function with vegetation height and a negative power function with relative submergence, by which, the raised water depths induced by vegetation block can be predicted under similar hydraulic conditions. the findings of the research, especially the introduction of relative submergence and its functional relationship with the water depth alternation induced by submerged vegetation, may improve the comprehensive understandings of the hydraulic behaviors in vegetated open channel and give some guidance to the practices of river ecological restoration. keywords: rigid submerged vegetation; vegetation height; relative submergence; flow structure; water depth; flow velocity distribution. https://doi.org/10.15446/esrj.v26n1.76187 efectos de la altura de la vegetación y de la inmersión relativa en vegetación rígida en la corriente de un canal abierto palabras clave: vegetación sumergida rígida; altura de la vegetación; inmersión relativa; flujo estructura; profundidad del agua; distribución de la velocidad. la vegetación en las corrientes de un río natural interactúa con el flujo de agua de una manera no lineal. en esta investigación se estudia la altura de la vegetación y un concepto de inmersión relativa para investigar experimentalmente los efectos de la vegetación rígida sumergida en una estructura de flujo. los resultados revelan que las submersiones relativas decrecen a lo largo de la sección de vegetación sumergida. desde la perspectiva del caso promedio, la relación lineal entre la profundidad del agua y la altura de la vegetación demuestra una tendencia a la baja del promedio de la inmersión relativa a medida que se incrementa la altura de la vegetación. las diferencias entre la profundidad mínima y máxima del agua en los casos de inmersión puede describirse a través de una función lineal positiva con la altura de la vegetación y una función de poder negativa con inmersión relativa, por lo que el incremento del nivel del agua ocasionado por el bloqueo de la vegetación se puede predecir con condiciones hidráulicas similares. los hallazgos de la investigación, especialmente la introducción de la inmersión relativa y su relación funcional con la alternación de la profundidad del agua inducida por la vegetación sumergida, pueden mejorar los amplios conocimientos de los comportamientos hidráulicos en un canal abierto con vegetación y pueden ofrecer información para la restauración ecológica de ríos. record manuscript received: 15/11/2018 accepted for publication: 31/03/2022 how to cite item: wang, z., zhang, h., he, x., jiang, q., xu, w., & tian, w. (2022). effects of vegetation height and relative submergence for rigid submerged vegetation on flow structure in open channel. earth sciences research journal, 26(1), 3946. https://doi.org/10.15446/esrj.v26n1.76187. https://doi.org/10.15446/esrj.v25n1.74167 40 zhongyu wang, huayong zhang, xing he, qi jiang, weigang xu, wang tian introduction submerged vegetation growing in shallow water is one of the most important components providing ecological services in natural rivers (okamoto et al., 2014; najafabadi et al., 2015; o’briain et al., 2017). for the purposes of ecological restoration and pollutant reduction, aquatic vegetation planting and management with different configurations have become common approaches in recent years (zhou et al., 2006; callow, 2012; lama et al., 2021). for the rigid vegetation, due to the additional vegetal retardance, it has been noticed that vegetation planted in rivers may raise the water depth and reduce flow velocity, and therefore increase the potential risk of flooding or sediment silting (zhang et al., 2015a; friberg et al., 2017). however, the complexity of the combinations of hydraulic and vegetation conditions hindered the development the determination of the alternations of flow state due to vegetation block (stone and shen, 2002; yagci et al., 2010). a comprehensive understanding of the effects of rigid vegetation on flow structure is therefore significant for river ecological restoration. starting from this point, numerous research works can be found in literature (fathi-maghadam and kouwen, 1997; pasquino et al., 2016). to investigate the effects of rigid vegetation on flow parameters including water depth, flow velocity, flow resistance, turbulent structure and so on, rigid cylinders made of wood, iron, plastic and some other materials were commonly utilized in previous studies to simulate rigid aquatic vegetation in river ecosystems (wu, 1999; järvelä, 2005; musleh and cruise, 2006; huai et al., 2009. temmerman et al. (2005) employed a 3-d model to investigate the flow and sedimentation patterns affected by vegetation and water level fluctuations. flume studies were carried out by järvelä (2005) to investigate the flow structure above flexible vegetation, by which the flow above submerged flexible vegetation was reported to follow the log law. a definition for the shearing velocity which can avoid complex turbulence measurements was proposed in his research. the vertical flow velocity distributions in trapezoidal open channels were found following power and logarithmic functional relationships by chanson (2004) and nezu et al. (2008). with experimental and theoretical methods, a 3-layer model was proposed by huai et al. (2009) to predict the vertical flow velocity distribution with submerged vegetation in open channel. the velocity profiles affected by configurations of cylindrical bamboo stems were investigated by yokojima et al (2015). based on the experimental and numerical results, the applicability of large-eddy simulation method was examined for the canopy modelling. water depth was selected as the hydrological controlling parameter by chauhan (2005) to investigate the vegetation structure and dynamics in a subtropical regulated river, by which, the hydrological regime was reported to dominate the developing of plant community in floodplain wetland rather than the physical connectivity. the importance of vegetation effects on turbulent structure was demonstrated by mcbride et al. (2007) through a flume experiment, in which, the flow velocity, turbulent kinetic energy, reynolds shear stress and boundary shear stress for vegetated and un-vegetated cases were found obviously different. in the studies of nepf (1999) and jack et al. (2011), the vegetation existence was found to alter the turbulent structure by changing the shear and wake productions around vegetation stems. for the research on vegetal resistance, rhee et al. (2008) found that manning’s coefficients were affected by the states of plants, whether green or dormant. from the perspective of biomechanics and streamlining of vegetation, some field experiments were also conducted (sandercock and hooke, 2010). vegetation in river can reduce the force of drag and lift acting on the boundary by increasing the effective roughness height (thorne, 1990). based on that white and nepf (2003) correctly captured the dependence of wake generation on vegetation array density and reynolds number, ben meftah and mossa (2013) established a theoretical relationship between the bulk drag coefficient cd of a random, emergent array of cylinders with vegetation density. however, flow phenomena in open channel will be more complicated with vegetation introduced, especially for that the water depth is larger than the vegetation height which means submerged vegetation (stone and shen, 2002; gualtieri et al., 2018). meanwhile, many interactive factors such as river geometry, hydraulic parameters, vegetation type, stems characteristics, planting density and so on also increase the difficulty of the research in this field (green, 2005; mcbride et al., 2007; zhang et al., 2015b). therefore, it is of great significance to find a dimensionless parameter to well describe the impact of rigid submerged vegetation. in this research, vegetation height and a dimensionless concept of relative submergence are adopted to investigate the effects of rigid submerged vegetation on flow structure. via a series of experiments, the relationship between the vegetation height and relative submergence will be determined, based on which, the variation tendencies and amplitudes of water depth and flow velocity affected by rigid submerged vegetation will be analyzed and compared regarding to both vegetation height and relative submergence. the relative submergence adopted in this study and the relevant analysis results are expected to improve the comprehensive understandings of the interactions between vegetation and water flow and provide some guidance to the engineering practices of river ecological restorations. experimental and analytical methodology experimental setup to investigate the influences of vegetation height and relative submergence of rigid vegetation on flow structure in open channel, a series of experiments are conducted in a rectangular concrete channel which dimensions where length=30 m, width=1.3 m, depth=0.8 m and bed slope i=0.0055 m/m. as shown in figure 1, from the recycling tank, water is lifted to the water flatting tower utilizing a pipeline centrifugal pump (model: 200-200(i) a, maximum discharge: 99.4 l/s). then, water flows through the water flatting tower (with an overflow launder), intake gate combined with an hqc-2 water level and discharge controller, thin-walled rectangular weir, stilling basin and energy dissipation fence in sequence to fulfil the experimental requirements of water stabilization and flow discharge control. as can be seen in figure 1, there is a 3 m long stabilizing section in front of the test section to further stabilize the water flow. within the test section (y=3-11 m), a 4 m long vegetation section (y=5-9 m) is determined. to investigate the detailed flow structure affected by the rigid submerged vegetation blocks of different vegetation heights or relative submergences, the flow discharge for this research is selected as q=90 l/s which is a higher and stable value for the utilized pump. in the experimental open channel, the water flow with free surface remains gradually varied flow states, which are also the hydraulic conditions focused on in this study. figure 1. schematic view of the experimental setup. in the experiments, woody cylinders with a uniform diameter of 7 mm are employed to simulate the rigid vegetation in rivers to avoid the influences from leaves and branches. for planting the simulated vegetation stems with uniformly staggered arrays, punched plates are used to cover the planting pool which is built concave downward the channel bed of the vegetation section (y=59 m) and filled with fine sand. on these plates, small holes of 1 cm in diameter are uniformly staggered drilled, by which, the simulated vegetation stems can be firmly planted (schematized in figure 2(a)). as demonstrated by previous studies, vegetation density alters the characteristic length and time scales of energetic motion in open channel, the standard boundary-layer flows could also be shifted to canopy flows gradually with increasing vegetation density (poggi et al., 2004), in fact, higher vegetation densities are generally used in the practices of river restorations to produce better ecological and environmental benefits (nikora et al., 2013). based on the comprehensive considerations on previous study (velasco et al., 2003, zhang et al., 2015a), field investigations and the reality of punched plates, a relatively dense vegetation density of 202 stems/m2 is used in the present research. to determine the vegetation heights above the channel bed hv which can ensure that the vegetation stems are full-course submerged, a blank test without vegetation is first conducted with flow discharge of 90 l/s. within the test section, the average water depth for the blank test is 10.03 cm. the vegetation heights hv are therefore set as 0, 1.25, 2.5, 3.75, 5, 6.25, 7.5, 8.75 and 50 cm to fully investigate the effects of relative submergence on flow structure 41effects of vegetation height and relative submergence for rigid submerged vegetation on flow structure in open channel (see figure 2(b)). for these 9 vegetation heights, 0 represent the blank test without vegetation, 50 cm means an unsubmerged test for comparisons while the other 7 tests are under submerged conditions. the experimental case with vegetation height of 50 cm is served as a control group, and the data are used for the comparison when analyzing water depth and velocity variations. this will promote our understandings on vegetation effects on water flow without affecting or changing the analysis results about vegetation heights or relative submergence. figure 2. (a) vegetation planting array and (b) vegetation heights. the flow velocities and water depths within the test section (y=3-11 m) are measured by adv (sontek 16-mhz micro acoustic doppler velocimeter) and abf2-2 water level measuring system, respectively. the distance of the probes of adv to the sampling volume is 5 cm. adv is therefore a non-contact velocimeter which has high resolution (0.01 cm/s) and accuracy (±1% of measured velocity) for the velocity measuring. with the sampling frequency of 50hz, adv was reported to get a stable average flow velocity if the measuring duration exceeds 45 s (jing et al., 2010). the sampling frequency and duration are therefore set as 50 hz and 60 s in the experiments. these 3000 instantaneous velocity samples will be used to calculate the time-average flow velocity for each measurement point. during the velocity sampling, the signal-to-noise ratios of the adv measurements maintain above 15 which can guarantee the quality of the collected velocity data (pujol et al., 2010). as can be seen in figure 3(a), flow velocities are measured at the intersections of 9 transverse-sections (y=3, 4, 5, 6, 7, 8, 9, 10, 11 m) and 5 different longitudinal-sections (x=0.05, 0.35, 0.65, 0.95, 1.25 m) within the test section. to make the velocity data more accurate, at each of these intersections, the flow velocities are measured each 0.005 m from the channel bed to the water surface (see figure 3(b)). since that the arithmetic average method is typical and critical to the calculations of cross-sectional hydraulic parameters (chow, 1959; zhang et al., 2015b), the arithmetic mean of the flow velocities collected on each transverse-section is taken as the cross-sectional average velocity, correspondingly. figure 3. schematic view of measuring sections and points: (a) plane view and (b) vertical view. with abf2-2 water level measuring system, the water depths along the measuring bridge can be automatically measured point by point with equidistance (0.1 m for the present research). its resolution and accuracy are 0.01 mm, 0.04 mm, respectively. in the experiments, the longitudinalsections for water depth measurements are same to the velocity measurements. however, along the longitudinal direction, the water depths are equidistantly measured every 0.1 m from y=3 m to y=11 m (see figure 3(a)). similar to the calculating of cross-sectional average flow velocity, the arithmetic average of these 5 values on each transvers direction is the cross-sectional water depth. analytical methodology in this research, the average reynolds number re and froude number fr are used to determine the flow state for each experimental test. as shown in table 1, the average reynolds number re is in the range of 48724-52896 for all the experimental tests which suggests that the flow in the test section is fully developed turbulent and independent of re (kouwen and fathi-moghadam, 2000). meanwhile, the average froude number fr between 0.42 and 0.78 is lower than 1 indicating the slow flow condition in the test section (table 1). the friction factor for such rough channels with vegetation covered is independent of gravitational effects (kouwen and fathi-moghadam, 2000). table 1. re and fr of different experimental cases hv (cm) 0 1.25 2.5 3.75 5 6.25 7.5 8.75 50 re 52628 52896 52858 52363 51965 51848 51255 51241 48724 fr 0.72 0.78 0.77 0.71 0.66 0.65 0.58 0.59 0.42 the relative submergence sr analyzed in the research is defined as the specific value of water depth h and the submerged vegetation height hv (equation (1)). sr = h  ⁄ hv (1) the average relative submergence sr can be got by averaging the sr values at each transverse-section corresponding to each experimental test. with the water depths experimentally measured in this research, the difference between the maximum and minimum water depths corresponding to 9 different experimental tests can be calculated with: hd = hmax hmin (2) similarly, the formula for the difference between the maximum and minimum flow velocity is: vd = vmax vdmin (3) results and discussion effects of vegetation height on relative submergence with the water depths measured in the experiments, the relative submergence at each measurement points can be calculated with equation (1) corresponding to different vegetation heights. for the case without vegetation, the relative submergence is +∞ as the vegetation height hv=0. for the other 8 tests, the variations of the relative submergences along the vegetation section are plotted in figure 4(a). as can be seen in the figure, the relative submergences present decreasing trends along the flow direction for all cases. the decreases of the water depths should be responsible to such decreases of relative submergence as the vegetation heights are constant within the vegetation section. in addition, the minimum value of the relative submergences except for the unsubmerged case (hv=50 cm) is 1.133, which appears at the exit of the vegetation block (y=9 m location) for the case of hv=8.75 cm (figure 4(a)). at this time, the water depth is only 9.9 cm (as shown in figure 5), which indicates that the vegetation stems may be submerged in the fore part of vegetation section and unsubmerged near the exit of vegetation section if another larger vegetation height hv=10 cm is introduced. such partially submerged and partially unsubmerged condition is much more complex and not considered in the research. from figure 4(a), it can also be seen that the gaps between two adjacent curves of relative submergence decrease rapidly at low vegetation heights and slowly when the vegetation heights increase to higher values. corresponding to different experimental cases except the unsubmerged one, the case-averaged water depth havg is analyzed regarding to the vegetation height hv to further realize the effects of vegetation height on average relative submergence sr . as shown in figure 4(b), the relationship between them can be described by a linear function 42 zhongyu wang, huayong zhang, xing he, qi jiang, weigang xu, wang tian havg=a+bhv (4) where a=0.102, b=0.332 are constant factors while the coefficient of determination is r2=0.928. this relationship demonstrates that the average relative submergence sr decrease with the increase of vegetation height. it is obvious that the average relative submergence of the blank test is since the vegetation height can be considered as hv=0. by extending the fitting line in the figure, a special point where both vegetation height and average water depth equals 15.22 cm is found, which makes the average relative submergence equals. that means the vegetation stems are just unsubmerged in average although it may be not rigorous. the vegetation stems are presumably partly unsubmerged while the other stems submerged because of that the variation of water depth is affected by the vegetation block planted in the channel. in this case, the average relative submergence should be higher than 1 according to its definition. the exact value of the vegetation height which can make all the stems just unsubmerged couldn’t be predicted and needs to be experimentally calibrated. but this value will undoubtedly be greater than 15.22 cm. figure 4. (a) variations of relative submergence along the vegetation section, (b) the relationship between vegetation height and average water depth. effects of vegetation height on the variation trends of water depth water depth which indicates the absolute height of water in rivers is one of the important control parameters for the designs of water courses and hydraulic structures (chow, 1959). the variations of water depth affected by rigid submerged vegetation of different vegetation heights or average relative submergences are investigated in the research. figure 5 shows the variations of measured water depths along the flow direction. as can be seen in figure 5, the variation of water depth presents a slight decreasing trend with obvious fluctuations for the experimental case of no vegetation. in this case, the irregularity of the concrete channel bed should be responsible to such fluctuations of water depth. such variations of the other 8 vegetated cases are different. in detail, the curves of water depth present increasing trends before the vegetation block (y=3-5 m) and reach maximum values near the y=5 m location which is the entrance of the vegetation block. on the whole, the maximum water depth increases with the increase of vegetation height. within the vegetation section (y=5-9 m), the water depths decrease gradually and smoothly. near the exit of the vegetation block (y=9 m), the gaps between these curves shrink rapidly to a bunch and this situation maintains after that (y=9-11 m). in summary, the curves of water depth for the vegetated cases become smoother in comparison with the blank test, but the water depths before and within the vegetation block are raised due to the vegetation blockage and retardance. and the raised amplitude of water depth increases with the increase of vegetation heights. at last, these water depths after the vegetation block resume to the level and trend of the blank test. it can be easily found that the water depths drop rapidly near the exit of vegetation block, which is well known and consistent with previous studies (zhang, 2015b). therefore, flow acceleration and fluctuation, as well as high frequency exchange of flow momentum and turbulent kinetic energy are inevitable in this zone, which may cause erosion of river bed (yagci, 2010; zhang, 2015b). enhancing the strength of river bed in this zone and regulating the flow structure by configuring the vegetation patches are undoubtedly two ways to deal with this problem. obviously, some more vegetation is needed to some distance downstream of the vegetation block in the practices of river ecological regulation. in the gap regions between the vegetation patches, the flow velocity will be reduced due to the sheltering effect, which have positive effects on the stability of river bed (zhao and huai, 2016). however, how to determine an optimal configuration of vegetation patches such as the type, height and patches number of vegetation, the distance and number of gaps, etc. needs more case studies to analyze and summarize. figure 5. variations of water depth affected by rigid vegetation of different vegetation heights. for the unsubmerged case (hv=50 cm, sr ), it can be noticed that the curve of water depth is much higher than the other cases before y=9 m location. as predicted in the research, the vegetation height making all the vegetation stems unsubmerged must be much higher than 8.75 cm. the vegetation height and average relative submergence of hv=8.75 cm, sr =1.49 are still far away from the unsubmerged case although the lowest relative submergence at y=9 m location is as low as 1.133. much higher blockage and retardance of the unsubmerged vegetation block can make the water depths before and within vegetation block higher. the effects of submerged vegetation on flow resistance are completely different to that of unsubmerged vegetation, the resistance mechanisms for the submerged and unsubmerged vegetation raising the water depths need further experimental studies (fathi-moghadam et al., 2011; zhang et al., 2014). it is obvious that vegetation blocks of different parameters show different affecting strength at different locations in the channel. for detailed analyzing the effects of rigid submerged vegetation on water depth at different locations, the average water depths at selected locations before (y=3 m), entering (y=5 m), within (y=7 m), outing (y=9 m), and after (y=11 m) corresponding to different experimental tests are plotted in figure 6. in the figure, it can be easily observed that the effects of vegetation heights or relative submergence are significant before and within the vegetation block. figure 6. variations of average water depth at different locations regarding to vegetation heights and average relative submergences. 43effects of vegetation height and relative submergence for rigid submerged vegetation on flow structure in open channel in order to determine the significance of vegetation height impacts on water depth at different locations, the relationship between them is analyzed by correlation analysis method corresponding to the integer transversesections between y=3 m to y=11 m. it should be noticed that the data from the unsubmerged case (hv=50 cm, sr =1) is removed to more accurately reflect the effects of submerged vegetation block. in shapiro-wilk normality test, the data of water depth at the transverse-section y =11 m is found not to obey the normal distribution (table 2). and spearman correlation coefficient is therefore adopted. the results of correlation analysis presented in table 2 are consistent with the conclusion directly overserved in figure 6. the effects of vegetation heights are significant before (y=3-5m) and within (y=5-9 m) the vegetation block. on the transverse-sections within these areas, the water depths increase with the increase of vegetation heights or decrease of relative submergences. for the other transverse-sections at and after the exit of vegetation block, the differences in water depth due to variable vegetation heights or relative submergences are negligible. that is why the curves for water depth in figure 5 contract to a bunch of approximatively overlapping lines at this area (y=9-11 m). although the impacts of vegetation height on the variation trends of water depth are illustrated above, the extent of such effects have not been quantified. as such, further analyses on the effects of vegetation height and relative submergence on the variation amplitude of water depth are performed. effects of vegetation height and relative submergence on the variation amplitude of water depth in recent years, vegetation planting and management have become common techniques for river ecological restoration and regulation. how the submerged vegetation blocks affect the variation amplitude of water depth or raised water depth is therefore significant to investigate. the differences between the maximum and minimum water depths hd corresponding to different vegetation heights and average relative submergence are plotted in figure 7. as can be seen in the figure, the water depth differences hd which can represent the variation amplitudes of water depths increase with the increase of vegetation height. as the water depths for 9 different cases are close near the exit of vegetation block, the variation amplitudes of water depth can represent the raised water depths in a great extent, which illustrate that the introduction of submerged vegetation can significantly alter the water surface profile in rivers. as can be seen in the figure, the water depth differences hd increase linearly with vegetation height. the variation trend can be fitted by linear function of hd = 𝓪' + b' hv (5) where a’=0.0497 and b’=0.4 are constant factors representing the intercept and the slope for the trend line, respectively. the coefficient of determination for this fitting is r2=0.999. considering the relationship between vegetation height and water depth presented in the last section, the water depth difference hd should decrease with the increase of relative submergence of vegetation. as shown in figure 7, the fitted relationship between hd and average relative submergence can be described by hd = c s d r (6) where c=0.097 and d=-0.315 are constant factors for this power function while the coefficient of determination is r2=0.945. in comparison with the linear relationship between the water depth difference hd and vegetation height hv, the power function seems more complex in form for predicting the effects of submerged vegetation on raised water depth. however, the average relative submergence sr is a more general parameter and applicable for different flow discharges in engineering practices, it just needs to calibrate the factors c and d (manes et al., 2007; wilson, 2007; fathi-moghadam et al., 2011). figure 7. variations of water depth difference regarding to vegetation heights and average relative submergences. effects of vegetation height and relative submergence on flow velocity distribution figure 8 shows the variations of flow velocity along the test section. as shown in the figure, the flow velocity of the case without vegetation changes very small along the flow direction and presents a very slight increasing trend as a whole, which is consistent with the slight decreasing trend of water depth presented in figure 5. except for these two lowest vegetation heights hv=1.25, 2.5 cm and largest average relative submergences , the flow velocities of the cases with submerged vegetation basically don’t change along the flow direction before 8 m location. when hv=1.25 and 2.5 cm, the flow velocities seem to decrease slightly in this section, which may be related to the bed roughness alternations due to the submerged vegetation with low height and large relative submergence. for table 2 statistical results of vegetation height impact on water depth location (y =) 3 m 4 m 5 m 6 m 7 m 8 m 9 m 10 m 11 m shapiro-wilk test results statistic 0.912 0.923 0.919 0.904 0.924 0.931 0.959 0.946 0.790 p-value 0.370 0.457 0.422 0.315 0.463 0.526 0.795 0.668 0.023 normal distribution p>0.05 yes yes yes yes yes yes yes yes no correlation analysis results r 1.000** 0.929** 0.905** 0.952** 0.929** 0.976** 0.714* 0.286 -0.214 p-value 0.000 0.001 0.002 0.000 0.001 0.000 0.047 0.493 0.610 note: ** represents significance level 0.01, * represents significance level 0.05. 44 zhongyu wang, huayong zhang, xing he, qi jiang, weigang xu, wang tian these two cases. the frontal vegetation area per unit bed area λ can be calculated as 0.018 and 0.035, which demonstrated bed-shear flow conditions according to the division standard (0<λ<0.04) in previous studies (nepf, 2012; zhao et al., 2019). when hv increases 3.75 cm and above, the flow regimes have changed while λ are larger than 0.04 (0.053 for the case hv=3.75 cm). however, further analysis of the bed roughness and vertical distribution of flow velocity under rigid submerged vegetation conditions are necessary to connect these two issues in the future. after y=8 m location, the curves of flow velocity for all the cases with submerged vegetation increase rapidly towards the level of the blank case. such increases near the exit of vegetation block are reasonable in considering the variations of water depth shown in figure 5. for the case with unsubmerged vegetation (hv=50 cm, sr =1), the flow velocities are obviously lower than the other cases and increase when the water just flows into the vegetation block, which presents a better consistency to the trend of water depth. these large increasing gradients of flow velocity at the interface of vegetation block agree with previous study (green, 2005; zhang, 2015b). furthermore, it can be noticed in figure 8 that the curves of flow velocity are crossed or overlapping at many locations, which makes it difficult to rank the flow velocity. anyway, the distributions of flow velocity are significantly affected by the rigid submerged vegetation of different vegetation heights or relative submergences. a problem that cannot be ignored is that high-velocity water flow may erode the river bed and banks (coon, 1998; qian et al., 2016). as reported in previous studies, vegetation patches planted properly will help to better protect against such erosion (liu et al., 2017). as pointed out by najafabadi et al. (2015), the interactions between accelerating flow velocity and vegetated banks are dominant to the flow characteristics of vegetated open channel. a further study on the special values is therefore necessary to realize the effects of vegetation heights or relative submergences on flow velocity distributions and hydraulic erosion. figure 8. variations of flow velocity along the test section corresponding to different vegetation heights and average relative submergences. as mentioned above, an analysis on the variations of the maximum, minimum flow velocities and the differences between them as well as the average flow velocities are investigated regarding to the vegetation heights or relative submergences. as shown in figure 9, the maximum flow velocities fluctuate slightly and basically don’t change with the increase of vegetation heights or decrease of average relative submergences. the fluctuation for the curve of maximum flow velocity is less than 0.135 m/s. for the curve of minimum flow velocities, the values decrease from 0.816 m/s to 0.378 m/s due to the increase of vegetation heights and therefore retardance by vegetation. the decreasing amplitude of the minimum flow velocities is up to 0.438 m/s. the variation of the average flow velocities is just higher in numerical values than the curve for the minimum flow velocities while the variation tendency is similar. it is obvious that the difference between the maximum and minimum flow velocities should increase with the vegetation heights as the maximum values change slightly and minimum values decrease continuously. such difference is as high as 0.772 m/s when the vegetation stems are unsubmerged (hv=50 cm, sr =1) and significantly higher than 0.566 m/s for the case hv=8.75 cm and sr =1.49, which is consistent to the variations of water depths. figure 9. variations of special values of flow velocities regarding to vegetation heights or average relative submergences. conclusions based on a series of experiments conducted in a laboratory channel, the vegetation height and a concept of relative submergence are adopted to analyze the effects of rigid submerged vegetation on flow structure in this research. with the experimental results of the research, the following conclusions can be drawn. (1) the relative submergences present decreasing trend along the vegetation section except for unsubmerged case of the present research. the linear regression function havg=a+bhv between vegetation height and caseaveraged water depth demonstrates a decrease trend of average relative submergence with the increase of vegetation height. (2) the differences between the maximum and minimum water depths corresponding to the submerged cases follow a linear relationship hd=𝓪'+b' hv with vegetation height and a negative power functional relationship hd=c s d r with the average relative submergence. with these relationships, the raised water depths due to rigid submerged vegetation can be predicted, especially for the relative submergence which is more general than the vegetation height. (3) for most submerged cases, the flow velocities basically don’t change before 8m location, and increase rapidly towards the level of the case without vegetation after 8m location. the differences between the maximum and minimum flow velocities increase with the vegetation heights as the maximum values change slightly and minimum values decrease continuously. some parameters in the empirical relationships obtained in this research need to be calibrated in practical applications due to the complexity of vegetation and hydraulic conditions. the findings of the research may improve the comprehensive understandings of the effects of vegetation heights and 45effects of vegetation height and relative submergence for rigid submerged vegetation on flow structure in open channel relative submergences of rigid submerged vegetation in open channel and may give some guidance to the practices of river ecological engineering design and management. acknowledgments the authors would like acknowledge with great appreciation for the support provided by the chinese national major science and technology program for water pollution control and treatment (no. 2017zx07101-002, no. 2015zx07204-007, no. 2015zx07203-011) and the chinese fundamental research funds for the central universities (no. 2021ms047). references ben meftah, m. & mossa, m. 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(2006). modeling vegetation coverage and soil erosion in the loess plateau area of china. ecological modelling, 198(1-2), 263-268. doi: 10.1016/j.ecolmodel.2006.04.019 with the development of computer vision and high-precision 3d model reconstruction, used for the measurement and 3d reconstruction of the geological landslides, acquiring a high-precision relative orientation basing multiple images is crucial and the key point to ensuring and improving the accuracy of 3d model and space position. currently, the conventional relative orientation model includes five independent parameters. for the linear relative orientation model, there are nine parameters to construct the linear space geometric relationship between the imaging and space point. to eliminate the impact of more parameterization and improve the accuracy and stability of solved parameters for the conventional direct relative orientation model, a new relative orientation model with seven constraints is proposed and validated in this paper. the additional constraints are derived from the orthogonal property of the rotation matrix of a stereo imaging pair and associated with the least squares adjustment to obtain a high-precision result of the relative orientation. through the accuracy assessment using space position, it is revealed that the new proposed model is more advantage for the conventional type of direct relative orientation, especially at 3d model reconstruction and close range photogrammetric and applications for the geological landslides measurement. con el desarrollo del entorno computacional y la alta precisión del modelo de reconstrucción tridimensional, utilizados para la medida y reconstrucción de despredimientos geológicos, es crucial la obtención de la orientación relativa de alta precisión basada en imágenes múltiples y es el punto clave para asegurar y mejorar la exactitud del modelo 3d y la posición espacial. actualmente el modelo de orientación relativa incluye cinco parámetros independientes. en el modelo linear de orientación relativa hay nueve parámetros para construir la relación geométrica espacial linear entre el sondeo y la posición espacial. para eliminar el impacto de más parametrización y mejorar la exactitud y la estabilidad de los parámetros resueltos el modelo de orientación relativa convencional, este artículo propone y valida un nuevo modelo de orientación relativa con siete restricciones. las restricciones adicionales se derivan de la propiedad ortogonal de la matriz de rotación de la imagen estéreo y se asocian con el ajuste de los cuadrados mínimos para obtener un resultado de alta precisión de la orientación relativa. al medir la exactitud con la posición espacial se revela que el nuevo modelo propuesto tiene más ventajas que aquel de orientación relativa directa, especialmente en el modelo de reconstrucción 3d y en las aplicaciones fotográmetricas de rango cercano para la evaluación de desprendimientos geológicos. abstract resumen keywords: 3d, high-precision, relative orientation, space position, additional constraints, imaging pair palabras clave: 3d, alta precisión, orientación relativa, posición espacial, restricciones adicionales, sondeo estéreo. the model of direct relative orientation with seven constraints for geological landslides measurement and 3d reconstruction record manuscript received: 19/09/2016 accepted for publication: 06/01/2017 how to cite item chen, y., xie, z., qiu, z., zhang, m., & zhong, s. (2016). the model of direct relative orientation with seven constraints for geological landslides measurement and 3d reconstruction. earth sciences research journal, 20(4), f1-f6 doi:http://dx.doi.org/10.15446/esrj.v20n4.60211 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.60211 el modelo de orientación relativa directa con siete restricciones para la medida de deslizamientos de tierra y reconstrucción tridimensional g e o t e c h n ic s earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): f1 f6 yifu chen 1, 2, zhong xie1, 2, zhenge qiu 3*, min zhang2, saishang zhong 2 1 national engineering research center for geographic information system, china university of geosciences (wuhan), wuhan 430074, p. r. china, 2 facility of information engineering, china university of geosciences (wuhan), wuhan 430074, p. r. china 3 research center for ocean mapping and applications, shanghai ocean university, shanghai 201306, p. r. china author for correspondence: zhenge qiu (e-mail: zhengeqiu@sina.com) g2 yifu chen , zhong xie, zhenge qiu , min zhang and saishang zhong introduction relative orientation is the process of recovering the position and orientation of one image with the other in an individual image space coordinate system (läbe & förstner, 2006), which is a classic topic in satellite photogrammetry, computer vision, and 3d model reconstruction. with the development of the study fields, a higher accuracy of relative orientation model is demanded and is a crucial process for ensuring and improving the accuracy of relative orientation. for the 3d measurement and space model reconstruction in the geological landslides, basing on multiple images, the accuracy of relative orientation directly determined the accuracy of the reconstructed 3d model, whose accuracy can reach the accuracy with the laser scanner in precision, theoretically. therefore, the high-precision relative orientation model is a key point for the various applications basing on space relative orientation (horn, 1990; mikhail, bethel & mcglone, 2001). many attentions are concentrated on solving relative direction elementary with the linear methods, such as eight-point algorithm, five-point algorithm, and singular value decomposition (longuet-higgins, 1987; philip, 1996; faugeras, 1993; hartley & zisserman, 2003). longuet-higgins was the first to discover that the position and orientation of one image with the other is contained in the essential matrix (longuet-higgins, 1987). in that paper, an eight-point algorithm is proposed to estimate an essential model with the limitation that it will introduce significant error because of noises in the image. to get the more definite essential matrix, nistér proposed five-point algorithm (nistér, 2004). the five-point algorithm estimates essential matrix by calculating the root of polynomial ten times. there are also methods on singular value decomposition (taylor & kriegman, 1995; ma, soatto, kosecka & sastry, 2004; hartley & zisserman). although homogeneous algebraic representation and singular value decomposition strategy can calculate relative direction elementary efficiently, it is a significant error in the result because of unmatched conjugate points. when there are outliers among the conjugate points, the least squares iterative method is one of the best choices for more accuracy results of relative orientation elementary. the least squares iterative method will decrease the adverse influence of the outliers by the iterative process. in the calculation process for relative orientation parameters, there are two solving methods. one is using iterative calculation with the least square adjustment. the method needs to give the initial parameter value. however, the initial value cannot be provided in some case, and the range of the given initial value will affect the accuracy of the relative orientation (stewenius, engels & nistér, 2006). in the worst situation, the wrong initial value will cause the equation does not converge and then have no solution, especially at close range applications (li, liu & tang, 2007; pan, 1999; duane, 1971). for large rotation angle of close range images, more strict relative orientation method is adopted to restore their relative position (tang & heipke, 1994; werner, 2003; zhang, huang, hu, wan & lin, 2011). the other method is to solve the equation of relative orientation model with one or some accurate parameter value directly. for the case, it is also difficult to acquire the few specific value which constraints its application for some fields. in this paper, a new relative orientation model, direct relative orientation with seven constraints is proposed to overcome the problems in the models described above. in the new model, the initial value is calculated by direct relative orientation with a rough bx. then, the seven constraints are used to control and adjust the solved parameter result during the bundle block adjustment. six constraints are derived from the inherent orthogonal property of rotation matrix. the seventh constraint stems from the decomposition of baseline. in the experiment, the model correction is validated, and the space position assesses the accuracy. through the experimental result, it is revealed that the proposed new model can improve and acquire a higher accuracy of relative orientation, and is suitable for large oblique angles and translation using the multiple images to measure geological landslides and perform 3d reconstruction of the geological landslides. conventional model of relative orientation 1. coplanarity condition equation relative orientation is the process of recovering the position and orientation of one image with the other in an individual image space coordinate system. given two images (left and right images) with an overlap filed from different viewpoints, a stereo model can be created and reconstruct the original epipolar geometry, represented by figure 1. assuming a ground point m is imaged at two viewpoints s1 and s2, the two image points (m1 and m2) are acquired, and the corresponding imaged ray is r1 and r2 respectively. the vector distance between the two viewpoints is called baseline b. basing on the epipolar geometry, the vectors r1, r2, and b must be in one plane, coplane, whose mathematical expression is represented by equation 1. (1)b • (r1×r2) = 0 z y x b s2 bx bz by s1 m1 m2 m (x1 y1 z1) (x2 y2 z2) μ ν r1 r2 figure 1. the diagram of the epipolar geometry relationship in the stereo images. basing on the epipolar geometry, the mathematic model of relative orientation can be expressed as follow by coplanarity equation 3 and equation 17 according to equation 1. (2) where bx, by, bz are components of baseline of a stereo pair [x1 y1 z1]=[x y -f]t and [x2 y2 z2]=r[x’ y’ -f]t coordinates of conjugate points in the image space coordinate system, (x,y) and (x’,y’) represent conjugate points in the image space coordinates system, the rotation matrix 1 2 3 1 2 3 1 2 3 a a a r b b b c c c � � � �= � � � �� � composes of the three angles φ, ω, κ, and f the focal length of image, respectively. the bx, by, bz represents the component of baseline in x, y, z directions, respectively. 1 1 1 2 2 2 f= =0 x y zb b b x y z x y z g3the model of direct relative orientation with seven constraints for geological landslides measurement and 3d reconstruction 2. direct relative orientation model in aerial photogrammetry, the values of by, bz, φ, ω, κ are set to zero as the initial value, and the accurate value of bx is given to determine the scale of the stereo model; otherwise the model scale will be any size. however, the initial value of the baseline components (by, bz) and the rotation angles is unknown in some cases, which will result in a low accuracy or an error solution for relative orientation. for this case, direct relative orientation model can be used to calculate the orientation parameters and perform the relative orientation. for the model of direct relative ordination, the value bx needs to be fixed accurately, and then the model derives to a linear mathematic construct for calculating the other parameters of relative ordination. the direct relative orientation can avoid the iteration calculation in the least square adjustment owing to the linear mathematic construct. however, two sets of parameter result for the relative ordination can be acquired, and need to determine which one is true. according to the space image coordinate of the left and right image point, the coplanarity condition equation eq.(2) can be transformed into the following linear form of eq.(3). (3) in the eq. (3), the l1, l2, l3, l4, l5, l6, l7, l8, l9 are the polynomial coefficients which are described in eq. (4), respectively. (4) eq. (5) is obtained by eq. (3) dividing l5 on both sides. (5) where 0 5/i il l l= , 0 5 1l = . eq. (5) is the basic mathematic model of direct relative orientation. this model can directly calculate the eight unknowns 0 0 0 0 0 0 0 0 1 2 3 4 6 7 8 9, , , , , , ,l l l l l l l l with eight or more conjugate points without approximate values. the baseline parameter bx has no influence on building up a stereo model; it can be assumed to be known. therefore, parameters by and bz are obtained by the following equations: (6) the following equations can get nine elements of rotation matrix r: (7) finally, three rotation angles can be decomposed by the definition of r [3, 17]. (8) according to eq. (6), l5 can be positive or negative, which does not affect the values of by and bz. however, there are two sets of possible solutions for the rotation angles φ, ω, κ. one of the two solutions only can be used to acquire the real configuration of relative orientation. therefore, the degree of the two angles φ and ω are used to determine and select a true solution, which should satisfy the following range represented in eq. (9). (9) 3. model of direct relative orientation with constraints during the conventional model of direct relative orientation, the eight parameters 0 0 0 0 0 0 0 0 1 2 3 4 6 7 8 9, , , , , , ,l l l l l l l l can be calculated when the value of bx is fixed. it is evident that the eq. (3) is linear, so the values of eight parameters can be determined uniquely without the parameters correlation. for the model, the parameters of rotation r and baseline b in the relative orientation are only constrained by the model equation, but not constrained by the relationship among the parameters, which will decrease the accuracy and stability of the parameter result and then reduce the accuracy of relative orientation. in the computer vision, it is well known that epipolar equation x’tex=0, rank-two constraint det(e)=0, and trace constraint 2eete-trace(eet)e=0 are utilized to solve essential matrix e. in the epipolar equation, the x’ and x represent the normalized image coordinates of conjugate points from the left and right images, respectively. the essential matrix e can be calculated through the matrix multiplication between the translation t and rotation r, e=tr. for the solving method, the nine components in the rotation r is constrained by orthogonal property of rotation matrix included implicitly in these equations. however, the elements in the baseline b are not limited. also, the solving method in the computer vision yields nine homogeneous polynomial equations of degree 3, and the nine similar polynomial equations are correlated. different with the conventional model of direct relative orientation and the essential matrix in computer vision that normalized image points as observations, a new relative orientation model, direct relative orientation model with seven constraints, is proposed and validated. in the new model, the original image point coordinate of conjugate points and focal lengths f are used, and the constraint includes the components relationship in rotation r and baseline b for improving and ensuring the accuracy and stability of relative orientation. the seven constraints are represented by eq. (10). (10) the proposed new model of direct relative orientation with seven constraints consisted of eq. (2), (3) and (10). the model selects the initial nine parameters of direct relative orientation as unknowns, and seven constraints are considered to improving and ensuring the accuracy of the solved parameters. 4. solution of direct relative orientation with constraints in the process of calculation of parameters in direct relative orientation with constraints, assuming eq. (2) is not equal to zero and an error v is in the equation, thus the eq. (2) is derived to eq. (11) basing on the eq. (3). (11) / 2 ; / 2p j w p� < > 1 2 3 4 5 6 7 8 9' ' ' ' ' ' ' ' ' 0v l yx l yy l yf l fx l fy l ff l xx l xy l xf= + + + + + + + + � 2 2 2 1 2 3 2 2 2 1 2 3 2 2 2 1 2 3 1 2 1 2 1 2 1 3 1 3 1 3 2 3 2 3 2 3 2 2 2 2 1 1 1 0 0 0 + + =x y z a a a b b b c c c a a b b c c a a b b c c a a b b c c b b b b � + + = � + + =� � + + =�� × + × + × =� � × + × + × = � × + × + × =� � �� 1 2 3 4 5 6 7 8 9' ' ' ' ' ' ' ' ' 0l yx l yy l yf l fx l fy l ff l xx l xy l xf+ + + + + + + + = 1 1 1 2 2 2 3 3 3 4 1 1 5 2 2 6 3 3 7 1 1 8 2 2 9 3 3 x z x z x z y x y x y x z y z y z y l b c b a l b c b a l b c b a l b a b b l b a b b l b a b b l b b b c l b b b c l b b b c = × � × = × � × = × � × = × � × = × � × = × � × = × � × = × � × = × � × 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9' ' ' ' ' ' ' ' ' 0l yx l yy l yf l fx l fy l ff l xx l xy l xf+ � + + � + + � = 2 2 2 2 2 2 2 2 22 2 0 0 0 0 0 0 0 0 0 5 1 2 3 4 5 6 7 8 9 0 5 2 1 7 2 8 3 9 2 4 7 5 8 6 9 2 / ( ) ( 1, 2, ,9) ( ) / ( ) / x i i y x z x l b l l l l l l l l l l l l i b l l l l l l b b l l l l l l b = + + + + + � � � = × = = � + + = + +  3 5 6 2 1 4 1 4 1 1 1 1 12 2 2 1 6 3 4 2 5 2 5 2 2 2 2 22 2 2 2 4 1 5 3 6 3 6 3 3 3 3 32 2 2 ; ; ; ; ; ; z y y z x y z x x z y y z x y z x x z y y z x y z x x l l l l b l b l b a l b a la b c b b b b b l l l l b l b l b a l b a la b c b b b b b l l l l b l b l b a l b a la b c b b b b b � � � + + = = = + + � � � + + = = = + + � � � + + = = = + + 3 3 2 arcsin( ) arccos cos = arccos cos b c b w j w k w = � = g4 yifu chen , zhong xie, zhenge qiu , min zhang and saishang zhong then, differential form of the eq. (11) is represented by eq. (12). the f0 is the approximate value of f, and dbx, dby … dc2, dc3 is the variation of the 12 unknown parameters for relative orientation. thus, the error equation is described as eq. (13). (12) (13) where v, x, l represent the observation vector, parameter variation value, and the free vector, respectively. to improve and ensure the accuracy of the solved parameters, the seven independent constraints described above, are considered, and its equation is expressed by eq. (14). b • x + w = 0 (14) b and w are the coefficient matrices and the observation vector of constraint condition in the bundle block adjustment, which are represented by eq. (15) and eq. (16), respectively. (15) (16) combining the error equation of observation and the equation of constraint condition, the error equation with constraint condition is expressed in eq. (17). (17) according to the least square with restrictions, the 12 unknown parameters can be computed and express by the eq. (18). (18) in the eq. (18), the u, k, and wu can be acquired by the follow eqs. (19). (19) 5. experiments and results data and experiment the indoor close range images with large oblique angles are used for experiments to validate the correctness and accuracy of the proposed new model of direct relative orientation with seven constraints. the stereo image pair was acquired by a non-calibrated sony imx135 camera with a pixel size of 1.12 μm which is shown in figure (2). the numerous conjugate points from the left and right image were detected using surf match model, and the error match points were removed to obtain the reliable relative orientation results as is shown in figure 3. from the 62 high-precision matched points, the six conjugate points were selected and used to validate the model correctness and assess the result accuracy of the relative orientation. the baseline length was measured as 0.55m. figure 2. the stereo images are taken by the same camera. the first image above is taken from the left then the other is on the right. figure 3. the result of feature point detected on the stereo pair. the experiment includes the follow procedures. 1) the 62 conjugate points were used to calculate the parameters of the relative orientation of the stereo image pair. 2) basing on the image space coordinate of the left image, the space coordinate of the object points corresponding to the selected six conjugate points were calculated using the result of the relative orientation and the forward intersection in photogrammetry. 3) the six object points were linked to 15 line segments during permutation and combination, and the distances of the 15 line segments were accurately measured as a real value. 4) to validate and assess the accuracy of the result of relative orientation, the distances of the 15 line segments were calculated using the space coordinate of the six object points, and compared with the measured true value. then, the accuracy is assessed with the standard deviation. g5the model of direct relative orientation with seven constraints for geological landslides measurement and 3d reconstruction results and analysis for validating and assessing the accuracy of the result of relative orientation, the six object points (p1, p2, p3, p4, p5, p6) are selected and used to calculate the space coordinate basing on the image space coordinate of the left picture. the selected six object points are illustrated in figure 4. figure 4. six object points are selected for validating the accuracy of relative orientation. during the forward intersection, the space coordinates of the six object points was calculated with the result of relative orientation, the rotation matrix r, and translation matrix t. the space coordinates of the six object points represent in table 1 table 1. three-dimensional coordinates of 6 object points (unit: m) to validate the correctness and assess the accuracy of the proposed new model, direct relative orientation with seven constraints, the distances of the six object points calculated with their space coordinate were compared with the corresponding measured distances, and the error rates were calculated and represented in table 2. the first column shows the line segment composed of two points, such as (p1, p2), and the calculated distance and exact distance are in the second, third column, respectively. table 2. error rate of the observed distance respect to the distance in real (unit: m) in addition, the curve of the error rates is calculated and shown in figure 5. the blue square and curve represent the error rate of each line segment and the curve of error rates, respectively. figure 5. error rate of the distance. from table 2, it is evident that the minimum error of distances reaches 0.001 m and the maximum error is 0.0057 m. in the figure 5, the range of error rates is between the -3% and +3%. the standard deviation of the distance accuracy is calculated and reaches 0.0031 m. in the least squares adjustment, a wrong initial value will result in a very low precision of relative orientation, and even cause non-convergence or convergence to an error result. to overcome this problem, a reasonable initial value of parameters (by, bz, a1, a2, a3…c2, c2, c3) is calculated and acquired by direct relative orientation using a rough bx. in the procedure, the value bx can be an arbitrary value that is greater than zero. then, the initial value of parameters in the relative orientation are used in the model of direct relative orientation with seven constraints to calculate further and adjust the parameters with the least squares adjustment, improving and ensuring the accuracy and stability of relative orientation. owing to the seven constraints, the error rate of distance used to assess the accuracy of relative orientation ranges for from -3% to +3%, and the standard deviation of the distance reaches 0.0031 m that describes the range error composed of two points. therefore, the standard deviation of the point can be the half of distance error, 0.0015 m. furthermore, the point is three dimensions; thus the standard deviation in each direction is 0.0005 m for the relative orientation. for a non-calibration and general camera sensor, the accuracy of relative orientation is excellent basing on the model of direct relative orientation with seven constraints, and the residual error may include the image distortion, match error of conjugate point, measurement error of image point, calculation error and random error. the residual error is from various factors and needed to eliminate with other methods to improve the accuracy of relative orientation further. g6 yifu chen , zhong xie, zhenge qiu , min zhang and saishang zhong conclusion in this paper, a new relative orientation model, direct relative orientation with seven constraints is proposed and validated to improve the accuracy and stability of relative ordination and overcome the problems for the large oblique angle and translation and without the initial value. the constraints are derived from the inherent orthogonal property of rotation matrix and the vectors in space translation. also, the procedure for solving direct relative orientation with seven constraints consists of the acquirement of initial value with direct relative orientation, the least square adjustment with seven constraints, and calculation of six relative orientation decompositions. the rough error is detected with data snooping techniques and used to analyze the residues of observations and the variance and covariance matrix of parameters. during the experimental result and analysis, the model of direct relative orientation with seven can obtain a high accuracy than the conventional method in some cases, such as the large oblique angle and translation and without the initial value. in the cases, some result of the conventional method has very low accuracy or wholly wrong. however, the problems can overcome during the model of direct relative orientation with seven constraints. that means the proposed model can increase the applied field of relative orientation such as close range photography, low altitude oblique photography and 3d model reconstruction and measurement of the geological landslides with multiple images. acknowledgments this research was financially supported by the projects "the research of the trajectory model with weight based on kernal function" and "the key techniques of marine surveying and mapping based on the domestic high-resolution satellite". we also thank yuan le for their support during the entire course of this study. references duane, c. b. 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(2006). recent developments on direct relative orientation. isprs journal of photogrammetry and remote sensing, 60(4), 284-294. doi: 10.1016/j.isprsjprs.2006.03.005 tang, l., & heipke, c. (1994). approach for automatic relative orientation. in: proceedings of optical 3d measurement techniques ii: applications in inspection, quality control, and robotics. international society for optics and photonics, 347-354. taylor, c. j., & kriegman, d. j. (1995). structure and motion from line segments in multiple images. ieee transactions on pattern analysis and machine intelligence, 17(11), 1021–1032. doi: 10.1109/34.473228 werner, t. (2003). constraint on five points in two images. computer vision and pattern recognition, 2. doi: 10.1109/cvpr.2003.1211471 zhang, y., huang, x., hu, x., wan, f., & lin, l. (2011). direct relative orientation with four independent constraints. isprs journal of photogrammetry and remote sensing, 66(6): 809-817. doi: http:// dx.doi.org/10.1016/j.isprsjprs.2011.09.006 palabras clave: zonificación; manejo del suelo; erosión; zona semiárida. how to cite item da silva, r. m., santos, c. a. g., araujo maranhao, k. u., medeiros silva, a., & porto de lima, v. r. (2018). geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region. earth sciences research journal, 22(3), 175-186. doi: https://doi.org/10.15446/esrj.v22n3.69904 eco-environmental change prediction models are important decision tools for policy makers, as they help to quantify environmental sensitivity and to understand the relationship between human activities and environmental quality. thus, this paper aims to analyze eco-environmental changes in monteiro county, a semi-arid zone within northeastern brazil between 1987 and 2010, by means of satellite images, geographic information system and global positioning system techniques, topographic map, climatic data and soil maps, as well as field survey. the resulting outputs were evaluated for their ability to accurately predict the most sensitive and least sensitive areas. the results showed that land cover changes have modified the environment in general and the land degradation and low soil loss processes were identified. the mean soil loss decreased from 9.67 t ha−¹ yr−¹ in 1987 to 2.67 t ha−¹ yr−¹ in 2010, as an effect of vegetation gain and particularly the conversion of thousands of square kilometers of tropophile caatinga middle slope and barren land into tropophile caatinga plain and tropophile caatinga high strand. thus, this study provides insight on territorial ordering and management of environmental services with a regional perspective. abstract keywords: zoning; soil management; erosion; semi-arid zone. geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region evaluación geoespacial de los cambios ecoambientales en el área de desertificación de la región semiárida brasileña issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.69904 los modelos de predicción del cambio ecológico ambiental son herramientas de decisión importantes para los responsables de la formulación de políticas, ya que ayudan a cuantificar la sensibilidad ambiental y comprenden la relación entre las actividades humanas y la calidad ambiental. por lo tanto, este artículo tiene como objetivo analizar los cambios ecológicos en el condado de monteiro, una zona semiárida en el noreste de brasil. este estudio utilizó medios de imágenes satelitales, sistema de información geográfica y técnicas del sistema de posición global, mapas topográficos, datos climáticos y mapas de suelos, así como datos de encuestas de campo. los resultados fueron evaluados en su capacidad para predecir con precisión las áreas más y menos sensibles. los resultados mostraron que los cambios en la cobertura terrestre han modificado el medio ambiente en general, y se han identificado dos procesos importantes de degradación ambiental: la degradación de la tierra y la baja pérdida de suelo. la pérdida media de suelo aumentó de 0.09 t ha-¹ año-¹ en 1987 a 0.18 t ha-¹ año-¹ en 2010, como un efecto del aumento de la vegetación y particularmente la conversión de miles de kilómetros cuadrados de bosque tropophile de pendiente media y tierra estéril en la llanura del bosque tropophile y el bosque alto tropophile. por lo tanto, este estudio proporciona información sobre el ordenamiento territorial y la gestión de los servicios ambientales con una perspectiva regional. resumen record manuscript received: 20/01/2018 accepted for publication: 25/05/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 175-186 richarde marques da silva1, celso a. g. santos1, karinna ugulino de araújo maranhão2, alexandro medeiros silva1 & valéria raquel porto de lima3 1 federal university of paraíba 2 federal institute of education, science and technology of paraíba 3 paraíba state university, department of geography g e o pr o c e ss in g 176 richarde marques da silva, celso a. g. santos, karinna ugulino de araújo maranhão, alexandro medeiros silva & valéria raquel porto de lima introduction land degradation is a severe environmental problem, which removes fertile top soil, and increases sedimentation in rivers and dams (shao et al., 2014). land degradation affects a large area of northeastern brazil in reasons of many factors, such as the decrease of crop productivity. these problems are even more pronounced in semi-arid regions. this region is characterized by low rainfall, high temperatures, slightly weathered soils, and low phytomass production (sousa et al., 2012). in addition, these regions have unique geo-environmental characteristics and are highly susceptible to dangers from natural and socioeconomic hazards (lal, 1986), and also receive low, erratic rainfall of up to 600 mm yr−1, with periodic droughts and various associations of vegetative cover and soils (silva et al., 2015). in this region, soil erosion process by water is a major problem that the area has suffered in the last century (santos et al., 2017). in brazil, areas prone to desertification cover 980,711 km², distributed in eight states of northeastern brazil (costa et al., 2009), and approximately 99,000 km² are in an advanced stage of desertification (sousa et al., 2012). according to anache et al. (2017), quantitative results related to erosion rates and conservation policies are not usually available for areas with erosion problems, particularly semi-arid regions which suffer from political neglect and industrial and economic stagnation (beskow et al., 2009). furthermore, the agricultural sector in the semi-arid region of brazil is dependent on natural rainfall and this high dependence on the short rainy season renders the country very vulnerable to weather hazards due to lowinvestment and family farming or traditional extensive livestock farming. therefore, proactive policies and sustainable water management strategies that are based on decision support systems are crucial in semi-arid regions, such as irrigation projects and transposition of the são francisco river (henkes, 2014). increased water scarcity, rising temperatures, and extreme climate events such as floods and droughts have grown in magnitude and frequency during the last few decades (rao et al., 2016), which directly influence the type and condition of the vegetation cover. the higher frequencies of extreme events, as droughts, represent a big threat to the economy and to the livelihoods of the population because of their many consequences to cropland, food security, water availability and health. furthermore, despite an extensive body of knowledge about the relationships among vegetative cover, soil erosion and eco-environmental changes (srinivasan & galvão 1995; marengo et al., 2016; martins et al., 2018), very little is known about such processes in the caatinga biome (schulz et al., 2017), because this biome is poorly analyzed and considered as one of the more neglected of brazilian biomes (beuchle et al., 2015; ribeiro et al., 2016). data availability of soil erosion and land changes in the caatinga is limited to thesis, few papers or technical notes published in regional journals, which makes it difficult for an international audience (silva et al., 2017). a preliminary literature review on temporal and spatial research has shown that several regional-scale studies on land change exist in brazil (portillo-quintero et al., 2013; schucknecht et al., 2013; coelho et al., 2014; salazar et al., 2015), but without integration about soil erosion and eco-environmental changes in desertification area. besides the climatic causes, the intensification and mechanization of agriculture in recent decades have been known as the main causes of the acceleration of erosion processes (cerdá et al., 2009) and consequently the acceleration of desertification. thus, changes in land cover, agricultural intensification, and increased storm intensity due to climate variability could increase erosion and eco-environmental change considerably in the coming years. environmental or erosion models are important decision tools for policy makers, as they help to quantify environmental sensitivity and to understand the relationship between human activities and environmental quality, particularly in semi-arid regions (manfré et al., 2013). the development of studies to monitor land degradation is urgently needed to preserve natural resources and increase quality of life, especially within environmentally important biodiversity hotspots such as the monteiro region of brazil. in order to study eco-environmental changes and soil loss, several approaches for vulnerability and risk assessment using geographic information systems (gis) have been developed in the last few years (braga et al., 2013; brito neto et al., 2016). these approaches allow an integrated evaluation of environmental outcomes using mathematic models, especially distributed models, which are valuable in evaluating different strategies for improvements in land use management. several physically-based models have been developed in order to estimate sediment yield, such as lisem (de roo and jetten, 1999), wepp (flanagan et al., 2012), kineros2 (woolhiser et al., 1990), and swat (arnold et al., 1998). the four models mentioned have the following disadvantages: (a) they need extensive data sets as input and have many calibration parameters; (b) they require either complex laboratory analyses or difficult and expensive field data collection, which may be infeasible in many developing countries; and (c) in spite of having some calibration parameters (zhang et al., 2013), the models (except swat) do not have an optimization method implanted in the software. some strength of these models can be emphasized: (a) they are physically or semi-physically based models, and basic processes are incorporated;(b) they take into account spatial variability of inputs and outputs, thus better representing the real world; (c) three of them are derived from the universal soil loss equation (usle). this equation was developed by wischmeier & smith (1965) and is the simplest equation for erosion estimation. it estimates mean annual soil loss for a long time and has been applied with success in several parts of the world. thus, the goal of this work is to evaluate the influence of eco-environmental changes in soil erosion in a desertification area within monteiro county, brazil. 2. material and methods 2.1 study area the study was conducted within monteiro county, inserted in the semi-arid area within the rectangle defined by the following coordinates: 37º 21′ 21″ w and 7º 42′ 55″ s, and 36º 55′ 31″ w and 8º 3′ 31″ s (figure 1). monteiro has an approximate area of 990.47 km² and is in the paraíba state, northeastern brazil. the climate is bsw type (hot semi-arid), according to köppen classification. the study area is characterized by an aridity and dry climate with historical average rainfall (1999–2011) around 700 mm (silva, 2004), absence of aquifers, low effective depth of the soils and high evapotranspiration totals. the rainfall intensity is low/moderate and generally occurs during a single rainy season causing a negative water balance (barbosa et al., 2006). as in other semi-arid regions, the seasonal variability of rainfall is high, resulting in rainfall outside of the characteristic periods, with rainfall occurring mostly during months from february and may (srinivasan & paiva, 2009). the average value for potential evapotranspiration is 1900 mm yr−1, according to santos et al. (2017). temperatures are high and quite spatially and temporally uniform, with an annual average ranging from 25 to 30 °c. monteiro has a heterogeneous landscape and lies over a crystalline basement that spreads by almost all the state of paraíba from the agreste to the sertão mesoregions. the crystalline basement is mainly composed of metamorphosed rocks and tectonically structured precambrian rocks (schist, gneiss, migmatite, quartzite) and granitic intrusions (huhn et al., 2014). quaternary alluvial deposits flank intermittent rivers and streams (de queiroz & norton 1992). this region is composed of two main units: mesoto neoproterozoic gneisses, migmatites, and meta-supracrustal rocks and neoproterozoic granitoid plutons of serra do monte and serra do bravo (souza et al., 2005). the geomorphology in general is well diversified, constituting by different forms of relief, which were worked by different processes, influenced by the semi-arid climate, and endogenous and exogenous processes. 2.2 vegetation physiognomies and land cover the study area is located in the caatinga biome, and the predominant vegetation is the semi-arid tropophile (loss of leafs in drought conditions and/or high humidity) with physiognomic aspects of caatinga vegetation (lima, 2012), with deciduous species, which lose their leaves during the dry season that occurs from june to january (silva et al., 2011). the natural distribution of trees is conditioned by the main limiting factors, such as geology, soil types, climatology and water availability. each tree species is adapted to certain conditions and is located in its niche. 177geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region the caatinga vegetation (native vegetation) has vast biodiversity, and around 50% of its original vegetation cover remains relatively intact despite the high human population density and high deforestation rates (schulz et al., 2017). northeastern brazil is composed of highly dynamic ecosystems that readily respond to climatic conditions such as precipitation. the elevation and effective root development capacity of soil thickness are the dominant factors controlling vegetation structure and distribution. this region possesses a number of species and has high importance as a biodiversity hotspot and is thus extensively studied by environmental scientists. in addition, this region is economically important to paraíba state due to bentonite mining (gopinath & lima 2011) and biodiversity (lima & artigas 2013). however, given the environmental and economic importance, there are surprisingly few studies that have examined the environmental fragility of monteiro region. caatinga vegetation is classified as steppe savannah (bush and dense), with species of genera spondias, mimosa and aspidosperma, and is a typical vegetation type in the semi-arid region of brazil. the caatinga consists of a complex mixture of deciduous woody species with an annual herbaceous understory. this biome has a large number of plant associations and floristic complexes with different physiognomies, including xerophilous, deciduous trees, thorny bushes, cactus, and bromeliads, varying from grassland to xerophytic thorn savanna, and shrub woodland (barbosa et al., 2006). in this study, three caatinga types were identified based on mansur et al. (2000) and adopted by lima & artigas (2014): (a) tropophile caatinga high strand (tchs): savannas and woody savannas as one type of caatinga dry forests with dense cover; (b) tropophile caatinga middle slope (tcms): characterized by low amounts of woody vegetation and high human-driven transformation; and (c) tropophile caatinga plain (tcp): close and open shrublands were interpreted as shrubland, i.e., xerophytic thorn scrubs or degraded caatinga dry forests with less dense and lower vegetation cover. the vegetation condition used in this study was verified by bioclimatic balance evaluation (rainfall data, soil temperature and soil texture), and such a condition was verified in the field with the objective of associating the bioclimatic condition with the vegetation physiognomy. the floristic survey technique used was that of linear transects of 50×2 m, when data of height and diameter of the vegetation were observed. 2.3 soil types the soils in the study area are morphologically diverse, including soils with reddish horizons of clay accumulation, sodium-affected soils with well-developed clay-pans, shallow undeveloped soils, alluvial soils, and cracking-clay soils (de queiroz & norton 1992). luvisols, fluvisols, lithosols, acrisols, and regosols were identified in the study area. the soil classification of fao/unesco (1988) was used in this study. luvisols are the most representative soil classes with reference to the study area and are little to moderately developed soils. these soils are developed mainly from gneisses and mica-schists, with or without the contribution of transported materials, and can be formed from other rocks such as phyllites, shales, siltstones, limestone and clayey sediments (de oliveira et al., 2009). fluvisols are young soils with weak horizon differentiation possessing ac-profiles and are predominantly brown in color, with texture ranging from coarse sand in levee oils to heavy clays in interfluves. the lithosols are characterized by young soils and normally limited to a thin a-horizon over an incipient b-horizon. acrisols are thin ochric surface horizon, dark color and lack distinct pedogenic horizons; they are shallow to moderately deep; and in paraíba state tend to be coarse-textured. regosols are very weakly developed mineral soils in unconsolidated materials that have only an ochric surface a-horizon that is not very shallow. this soil type is extensive in eroding lands, particularly in semi-arid regions (maranhão 2014). familiarity with the soil classification was gained through field observations and conversations with farmers during a detailed year-long ecological assessment of a portion of monteiro county. 2.4 land cover change and geospatial analysis of soil erosion different thematic layers were generated to spatially estimate the soil loss and eco-environmental changes between 1987 and 2010 using landsat tm imagery dataset. a multi-temporal remotely sensed imagery tm/landsat (wrs2: 215/65 and 215/66), dated may 1987 and 2010, with spatial resolution of 30 × 30 m were used. landsat satellite images were classified using supervised classification of maximum likelihood. accuracy assessment included ground truth points collected in field, high-resolution imagery from 2014, available for visualization on google earth (http:// earth.google.com), comparison to fine-resolution ikonos image, and land owners’ information, as used by garcia and ballester (2016). the areas of different land cover types and final thematic maps were produce using arcgis 10.1 (esri, 2015). then, based on these procedures, products and maps, the formal usle model was applied using a typical gis-based equation implementation (figure 2). figure 1. geographical location of monteiro county in paraíba state, brazil. figure 2. flowchart depicting steps taken to assess usle model validity for ecoenvironmental changes in desertification in the monteiro county. in this case study, estimates of soil loss from the monteiro county were estimated based on the usle to predicting the mean annual erosion and its spatial distribution during the period analyzed (1987 and 2010). the usle was selected for the current study as it yields reasonably reliable data, at least in certain conditions with moderate input required for data gathering (mohamed 2013). this equation provides acceptable estimates of average annual sheet and rill erosion from a wide range of land cover, soil, and climatic conditions. the usle is based on: y r k ls c p=     (1) 178 richarde marques da silva, celso a. g. santos, karinna ugulino de araújo maranhão, alexandro medeiros silva & valéria raquel porto de lima where y is the loss (average annual soil in t ha−¹ yr−¹), r is the rain erosivity (in mj mm ha−¹ h−¹ yr−¹), k is the soil erodibility (in t h mj−¹ mm−¹), ls is the slope-length factor, which includes slope length and slope steepness factors (booth dimensionless), c is the cover management factor (dimensionless), and p is the support practice factor (dimensionless). as there are no support and conservation practices in the monteiro, p factor values were assumed to be 1 (silva et al., 2012). thus, the cp factor (vegetation cover) was adopted. 2.5 determining usle factors 2.5.1 r factor (erosivity) in order to determinate rainfall erosivity (r factor), daily rainfall data between january 1998 and december 2010 from 12 grid points of tropical rainfall measuring mission (trmm), product 3b42 (v7), with spatial resolution of 0.25˚ × 0.25˚ were used (figure 3). the monthly rainfall erosivity of each grid point was estimated for this period as: research corporation (embrapa) in monteiro county, available at www. sisolos.cnptia.embrapa.br. the soil erodibility factor (k) was determined based on the methodology proposed by williams (1995): k f f f fsand cl si orgc hisand= × × ×− (6) where fsand is the coarse sand indices, fcl-si is the silt and clay rate, forgc is the organic carbon content and fhisand is the sand content. the parameters used to calculate the k factor were calculated using the following equations: f m m sand s silt= + × − × × −                 0 2 0 3 0 256 1 100 . . exp . (7) f m m mcl si silt c silt − = +     0 3. (8) f orgc orgc orgcorgc = − × + − ×( )         1 0 25 3 72 2 95 . exp . . (9) f m mhisand s s = − × −       −      + − 1 0 7 1 100 1 100 5 5 . exp . 11 22 9 1 100 + −                         . ms (10) where ms is the percentage of sand, msilt is the percentage of silt, mc is the percentage of clay and orgc is the organic carbon percentage contained in the a-horizon (surface soil). soil distribution in the monteiro and the k factor values are shown in table 1. table 1. soil types and soil erodibility factor values. soil types area (km²) area (%) k factor (t h mj-¹ mm-¹) luvisol 725.37 73.23 0.108031 lithosol 137.29 13.86 0.001800 fluvisol 91.53 9.24 0.184743 acrisol red yellow 34.65 3.50 0.116694 regosol 1.63 0.16 0.127641 2.5.3 ls factor (topographic) smith & wischmeier (1957) evaluated the effect of slope and length on soil erosion for several locations, and defined the dimensionless length and steepness of slope factor (ls). in the present study, the ls factor was obtained using one freely-available global digital elevation model (dem), i.e. product aster gdem2 with a spatial resolution of 30 m, which is available at https://asterweb.jpl.nasa.gov/gdem.asp. thus, in order to estimate ls factor, eq. 11 was used, as proposed by moore and burch (1986) and was also used by zhang et al. (2009): ls v=( ) ( )/ . sin / .. .22 13 0 08960 4 1 3 θ (11) where v is the product of flow accumulation and cell size and  is the slope in degrees; both are directly derived from the dem. 2.5.4 c factor (land cover) the c factor was estimated using remote sensing techniques based on vegetation indices derived from satellite images by the normalized difference vegetation index (ndvi), named ndvi-c. the ndvi measures the amount of green vegetation and is expressed as: ndvi nir ir nir ir= − +( ) / ( ) (12) figure 3. location of the trmm grid used for the erosivity factor determination. r ei n i n j m = = =∑∑ 1 1 30 100 200 6/ . (2) where r is the rainfall erosivity factor (mj mm ha−¹ hr−¹ y−¹), i is the number of years, j is the number of days per year, and ei30 is rainfall erosivity at 30 minutes per day (hundred t mm ha−1 h−1). ei ei30 144034 065 0 2695= × −. . (3) where ei1440 is erosion index per day (hundred t mm ha−1 h−1). ei pi1440 14403 856 0 0048= × −. . (4) where pi1440 is daily precipitation index (mm h−¹). pi1440 2 24=( )rainfall / (5) the daily r value obtained was converted to monthly and annual values. then, the mean annual erosivity for the 17 year period was derived. 2.5.2 k factor (erodibility) in order to obtain the values of k factor, data from analysis of eighteen soil samples were used, which were obtained by the brazilian agricultural 179geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region where nir is reflection of the near infrared portion of the electromagnetic spectrum and ir is reflection of the upper visible spectrum. in a subsequent step, ndvi-c values for each land cover were taken from each image. these sample points were used for a linear regression analysis: ndvi c ndvi ndvi= − −exp[ ( / )]2 1 (13) the c factor prediction model based on ndvi was tested here, because it was used for similar studies in the europe, asia and south america (anache et al., 2014; durigon et al., 2014; silva et al., 2013; wang et al., 2010; meusburger et al., 2010; ahmet, 2010; de asis & omasa, 2007; lin et al., 2002). a desirable feature of c factor prediction using ndvi satellite imagery is that it estimates the fractional abundance of ground cover and barren soils simultaneously, which is appropriate for soil erosion analysis. to obtain a better picture of the ndvi images, ndvi profiles were created for all land cover classes. the ndvi-c factor values obtained can be found in table 2. table 2. values of each ndvi-c factor for monteiro land cover/vegetation physiognomies ndvi-c values water 0.0 urban area 0.0 tropophile caatinga high strand 0.00004 tropophile caatinga middle slope 0.0007 tropophile caatinga plain 0.01 barren land 1.0 2.6 indictors of desertification and rates of eco-environmental degradation numerous models and indicators were developed to analyze the ecoenvironmental degradation (al-awadhi et al., 2005; garcia and ballester, 2016) and assessment of climate variability on water resources. in this study, eco-environmental degradation rate was calculated according to puyravaud modified index (puyravaud, 2003): x t t ln s s= ( ) ( )1 1002 1 2 1/ /− × × (14) where x is the landscape change rate, s1 and s2 are the land area of each land cover at time (1987 for t1 and 2010 for t2). puyravaud modified index was adopted to make the present results comparable to other studies, thus, the result of deforestation, forest degradation and land cover changes were considered as landscape loss. 3. results and discussion 3.1 analysis of eco-environmental degradation processes after the classification of satellite data, the reliability of results depends on the overall accuracies of the classified images. the result of this process indicates whether the land cover changes have been accurately identified and extracted. figure 4 shows land cover classes in monteiro for the studied years. the dominant land cover in the study area in 1987 and 2010 were tcms, tcp, tchs, barren land, water, and urban area. figure 4b shows a recovery and an increase in the tcms and tchs. table 3 shows the distribution of land cover types (change in area and percentage growth) for the study area in 1987 and 2010. the results show that there was an increase of tchs area compared to the tcms and tcp, mainly on the western edge of the municipality, whereas in the eastern portion, the areas covered by tcms were replaced by tcp. the tcp and tchs areas increased from 231.4 km² and 225.1 km² in 1987 to 282.9 km² and 275.1 km² in 2010, respectively. the results show an increase of 21.84 % of the total area of tchs, while tcms declined by 13.7 % and the barren land decreased by 64.9 % (table 3). the decrease is particularly evident in areas covered by vegetation (24.1 %) and water (15.5 %), corresponding to an average loss of 342.2 and 97.4 km² yr−1, respectively. the results also show a great growth of the urban area (> 200 %), influenced by population increase, when in 1980s the resident population was 26,852 inhabitants and in 2010s it increased to 30,852 inhabitants (210 %), with 66 % of the total population living in the urban zone (lima, 2012). from table 3, it is clear that a substantial change in land cover patterns has occurred over the studied years. as shown, urban area and tcp classes has been the main receivers of land from other classes. the increase of urbanization in the area provoked an intense land cover change during the figure 4. land cover in monteiro for (a) 1987 and (b) 2010. 180 richarde marques da silva, celso a. g. santos, karinna ugulino de araújo maranhão, alexandro medeiros silva & valéria raquel porto de lima last decades. as observed in table 3, monteiro county was mainly covered by tcms (46.1 % in 1987 and 39.8 % in 2010). in general, a substantial loss of natural vegetation (caatinga) was observed during the study period in monteiro. degraded land (barren land) presented a fluctuating pattern during the studied period, e.g. 6.7 % of the total landscape in 1987 and the minimum value in 2010 with 2.3 % of the total study area. table 3. land cover change calculated in the study area land cover classes 1987 2010 change in area (km²) percentage growtharea (km²) % area (km²) % barren land 64.8 6.6 22.7 2.3 −44.73 −66.29 tchs 225.1 22.8 275.1 27.9 50.00 21.84 tcms 231.4 23.4 282.9 28.7 −173.40 −37.58 tcp 455.6 46.1 392.9 39.8 164.70 70.17 urban area 1.5 0.2 4.7 0.5 3.21 211.18 water 8.87 0.9 8.9 0.9 0.06 0.67 the eco-environmental degradation processes in monteiro between 1987 and 2010 (figure 5) was also calculated based on puyravaud modified index. in this figure, the positive and negative bars represent land cover types that respectively expanded and decreased in extent. the results show that the environmental conversion processes occurring in monteiro can be characterized in terms of two types between 1987 and 2010: (1) barren land decreased by approximately 4.5 % and by 0.5 % for tcms; and (2) tcp and tchs increased by 0.6% and 0.7%, respectively. the results show that the decrease in the area of degraded land was more than that of reclaimed land. the urban area categories clearly increased in the period studied, followed by tcp and tchs. it was observed that, even a small increase in the land cover can cause a resignation on erosion processes. the kappa coefficients for the 1987 and 2010 maps were 0.99 and 0.97, respectively, which could be considered an almost perfect evaluation based on the reference table presented by landis & koch (1977). table 4. land cover change classification matrix for monteiro county for 1987. land use water tcp tchs tcms barren land urban area sum of rows water 3492 0 0 0 33 0 3525 tcp 0 3997 14 13 1 0 4025 tchs 0 43 3087 0 0 0 3130 tcms 0 0 0 879 3 1 883 barren land 0 0 0 6 471 11 488 urban area 0 0 0 2 25 390 417 sum of colums 3492 4040 3101 900 533 402 12468 tcp = tropophile caatinga plain, tcms = tropophile caatinga middle slope and tchs = tropophile caatinga high strand. table 5. land cover change classification matrix for monteiro county for 2010. land use water tcp tchs tcms barren land urban area sum of rows water 2703 0 7 0 0 0 2710 tcp 0 20279 105 245 11 0 20640 tchs 0 218 26044 392 12 0 26666 tcms 0 120 9 11374 139 4 11646 barren land 0 0 0 15 717 17 749 urban area 0 0 0 5 33 522 560 sum of colums 2703 20617 26165 12031 912 543 62971 tcp = tropophile caatinga plain, tcms = tropophile caatinga middle slope and tchs = tropophile caatinga high strand. figure 5. variation in eco-environmental changes among different land uses. during the analyzed period, the land cover had a spatial reconfiguration in monteiro. spatial distributions of transition among land cover classes are shown in figure 6. the pristine areas were derived from boolean operations using land use and land cover maps from 1987 and 2010. the results show that few areas remained unchanged throughout the whole landscape changing process. the areas with land cover change and without change for the studied period were 857.72 km² and 132.75 km², respectively. land cover types without changes were observed among tchs (62.53 km²), tcp (50.75 km²) and tcms (12.43 km²). main changes were observed among tcp (94.50 km²), tchs (9.12 km²), tcms (19.93 km²) and barren land (8.6 km²). tables 4 and 5 show the land cover change matrixes for monteiro county for 1987 and 2010. table 6 shows the errors of omission and commission obtained by the verification of classified images. the omission errors were small (below 2%), showing an excellent accuracy between the classes predicted and those identified in the field. commission errors showed points that were improperly included in all categories. additionally, figure 6. map of natural areas that showed no land cover or land use change in monteiro county from 1987 to 2010 (pristine). 181geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region because depends on the intensity and amount of rainfall. the main result is that the potential risk in the region of monteiro is very high due to climatic unevenness in which extreme events occur in rainy seasons. high intensity rainfall events provoke severe erosion within the area, when compared to the r factor values mapped for humid areas of brazil (anache et al., 2018; da silva et al., 2013), which were 1,000 < r factor < 5,000 mj mm ha–1 h–1 year–1 for the semi-arid region in brazil. these values showed that the region climatologically had a high erosion potential. therefore, in spite of showing higher erosivity values, it has long been recognized that the climatic conditions of these regions together with land cover, soil types, and topographic factors can increase the soil loss. figure 7b shows soil types and the spatial distribution of k factor in monteiro county. the k values varied from 0.034 to 0.052 t h−¹ mj−¹ mm−¹, with the mean value equal to 0.043 t h−¹ mj−¹ mm−¹. the major soil types in the study area are luvisols (73.23%) and lithosol (13.86 %) with 2:1 minerals that expand (swell) when wet, and shrink when dry, i.e., plastic characteristic. when studying the soil moisture in the profile corresponding to the root zone in semi-arid region of brazil, costa et al. (2013) showed that the largest runoff volumes occur in the plot with the luvisol (mean field capacity of the profile of 0.225 m3 m−3), against a value of 0.280 m3 m−3 for field capacity for plot with an occurrence a red-yellow ultisol. this characterizes the capability of a luvisol for producing of soil loss, when compared to other soils of the region. the lithosol is not more susceptible to erosion. it is noted that 9.24% of monteiro’s overall area contains fluvisols, which have the highest soil erodibility values. the soils of the region are characterized by the soil moisture variation, which provokes deep cracks during dry spells, and very plastic and sticky soil consistency, when wet (dos santos et al., 2016). the low permeability is due to the high clay content, however possesses high water adsorption, which results in high initial abstractions when dry. porosity below the inflection point corresponds to textural voids, as presented by silva et al. (2011). these factors may increase the erosion processes in the study area. the spatial distribution of ls factors is shown in figure 7c. monteiro has ls factor values which ranged from 0–0.5 to 20–40, with the highest values in the classes 0–0.5 (44 % of total area) and 0.5–1 (32 % of total area) (figure 7d). according to the spatial analysis, monteiro is characterized by its increasing elevation values from east to west, with a maximum drop of 673 m. the ls factors suggested that the largest values occur in the northwest and north portions, and at a low frequency in the central and east table 6. errors of omission and commission obtained by the verification of classified images. land use 1987 2010 omission commission omission commission water 0.94% 0.00% 0.26% 0.00% tropophile caatinga plain 0.70% 1.06% 1.75% 1.64% tropophile caatinga high strand 1.37% 0.45% 2.33% 0.46% tropophile caatinga middle slope 0.34% 2.12% 2.30% 5.42% barren land 1.26% 7.28% 2.05% 18.43% urban area 0.68% 3.99% 1.85% 10.14% it is also interesting to note that the intense anthropogenic pressures (due to removal of wood for the production of firewood and coal) in monteiro county have generated scattered occupation throughout the region, resulting in environmental degradation mainly caused by deforestation, poor soil use and careless advances on tcms areas. the erosion increase in these areas also is due to the intensification of goat breeding (agrosilvipasture and silvipasture) in the region, as a practice of economic sustainability of population to coexist with the droughts owing to the climatic conditions of the region. these areas represent zones under intense anthropogenic influence which should be considered as important regions due to their land cover changes. this can cause environmental impacts and decreased crop production in these areas in the future, especially in those related to grazing. 3.2 geospatial assessment of soil loss vulnerability after estimating all factors, a geospatial assessment of soil loss vulnerability was computed. figure 7a shows the annual rainfall erosivity calculated for the region. in this study, erosivity factor values were within a range of 3,896–4,623 mj mm ha−¹ h−¹ yr−¹. in comparison, r factor values increased from the east to west towards the north and south of the region. the results from this study show that erosivity in the region was high, figure 7. (a) map of erosivity factor, (b) map of main soil types and k factor values, (c) map of length and steepness of slope factor (ls), and (d) classes of ls values for monteiro. (continued) 182 richarde marques da silva, celso a. g. santos, karinna ugulino de araújo maranhão, alexandro medeiros silva & valéria raquel porto de lima parts, which means that there is an increase of soil loss only in a very small part of the study area that has steeper slopes, which is more susceptible to such a loss. the north and northwest areas of the region have the highest elevation. figures 8a-b show the ndvi maps for the years studied. the ndvi values varied from −1 to 0 for areas with water or barren land, while values above 0 and less than 1 represent the tcms, tcp and tchs classes. the lowest ndvi values occurred in the tcp areas characterized by small agricultural areas and in places where goat farming exist. owing to the larger amount of barren land located in the hillside edges of the valleys, higher c factor values occur in those areas as well. in 1987, the largest area with ndvi values represented the 0.25-0.50 class, while in 2010 the class with the largest area was the 0.50-0.75. in addition, many streams and reservoirs found in the images from 1987 were not present in those from 2010. this change was probably caused by land degradation or climatic conditions, even for such a short time period (i.e. 23 years). table 5 summarizes the average rate of soil loss for different land cover types. it is noted that the majority of land cover in the area (tcms, tcp and tchs) is expected to have average annual soil losses of approximately 2.83 t ha−1 yr−1 for 1987 and 1.0 t ha−1 yr−1 for 2010. these figure 7. (a) map of erosivity factor, (b) map of main soil types and k factor values, (c) map of length and steepness of slope factor (ls), and (d) classes of ls values for monteiro. figure 8. results of ndvi for the years: (a) 1987 and (b) 2010. 183geospatial assessment of eco-environmental changes in desertification area of the brazilian semi-arid region values can be considered low for these land cover types when compared to silva et al. (1999, 2012), due to the high c factor values present in a large part of the area (considering that there was no major program of natural cover restoration or erosion control) and due to climatic conditions in the study area which can be considered unusual for a semi-arid region. table 7. statistics for the estimated soil loss for each land cover type in monteiro from 1987 to 2010. land cover estimated soil loss (t ha−1 yr−1) change (%) 1987 2010 mean standard deviation mean standard deviation tcp 5.9 0.73 1.2 0.13 −80% tcms 1.1 0.60 1.5 0.23 33% tchs 1.5 1.02 0.3 0.32 −83% barren land 132.6 1.03 93.3 0.82 −30% urban area 0.6 0.02 0.01 0.01 19% figures 9a-b show annual soil loss within monteiro for the years 1987 and 2010. the results show that the soil loss values ranged from 2 to 500 t ha−1 yr−1 for two years studied, and a decrease can be observed in the areas with the highest trend to soil loss in 2010, which is a direct reflection of the ongoing reduction of vegetation cover. the current rate of soil loss from the study area was different to the mean soil loss rate obtained by silva et al. (2008), srinivasan & paiva (2009) and montenegro et al. (2013) for erosion plots of 100 m² with different surface cover types in sumé and são joão do cariri counties. however, the current rate in our study area is well above the other cover types and cultivated fields in savannah and caatinga environments (oliveira et al., 2015). the lowest rates of soil loss were observed for urban area and tchs and tcms. in this study, mean annual soil loss for barren land was 132.6 t ha-¹ yr-¹ in 1987 and decreased to 93.3 t ha−1 yr−1 in 2010, and for tcp was 5.9 t ha−1 yr−1 in 1987 and 1.2 t ha−1 yr−1 in 2010. furthermore, another important feature of this area is the undulating relief in the western edge of monteiro. areas with higher soil loss have ls factor values higher than 12.3 % and land cover classified as tcp and tcms, whereas the lower soil loss was in tchs, because it occurs in the tops of the mountains, where intact vegetation exist, which significantly reduces the soil erosion. thus, interventions should therefore be prioritized for areas with a slope steeper than 10 %. the soil erosion influenced by erosivity also is uncertain in the study area regarding rainfall magnitude and consecutive dry days (dos santos et al., 2016). human activities such as deforestation, removal of tcms for firewood and other purposes, and using arid and semi-arid lands interchangeably for cultivation and grazing significantly increase water productivity and soil loss, which in turn increases the potential for land degradation. consequently, the removal of tcms and tcp as a means to improve rangeland productivity must be accompanied by careful management. therefore, keeping suitable vegetative cover should be considered for soil and water conservation in forests and rangeland, and any cultivation or brush control should be carefully practiced to avoid retrogressive trends in these lands. the present study brings a contribution to the knowledge and discussion on historic and current status of land use and cover in the semiarid region of brazil (caatinga biome), mainly between 1987 and 2010. it provides key information to institutions concerned with land cover change assessment or environmental conservation on how much of the natural vegetation in the region has already been lost and in which rates are occurring forest and woodland losses during different historical time period, using satellite imagery and gis techniques. as example of action for the caatinga biome conservation, there is the creation of protected areas in the remaining areas of caatinga, the so-called conservation units. they are defined areas where the nature and resources could be protected by the brazilian forest code of 1965 (brazil, 1965). in addition to such measure, there is also the dissemination of sustainable technologies, reforestation, forest management, agroecology, support to the local productive groups and environmental education. such regional planning and action propositions could reduce the increase of desertification in the semi-arid region of northeastern brazil. 4. conclusions in this study, environmental and natural vulnerability maps developed for the monteiro municipality in paraíba state in the brazilian semi-arid region helped to develop a better understanding of ecosystem fragility and urban growth in desertification areas. intense anthropogenic activities take figure 9. spatial distribution of soil loss rate for monteiro: (a) 1987 and (b) 2010. 184 richarde marques da silva, celso a. g. santos, karinna ugulino de araújo maranhão, alexandro medeiros silva & valéria raquel porto de lima place at sites that are mainly related to agriculture and are aided by a lack of guidelines for local environmental management. the research shows that great changes have taken place in the eco-environment in monteiro area. soil erosion is a severe problem in semi-arid region of northeastern brazil, and study the soil loss risk and eco-environmental changes is necessary for management sustainable of land cover. the results indicate that soil losses are lower in the monteiro area, revealing the regeneration of vegetative cover and a decrease in erosion-prone areas. however, further studies are needed to confirm this tendency within other soil geomorphological units. finally, the application of the usle/gis approach resulted in a consistent pattern of soil erosion among different land cover types, slope positions and soil groups and reasonably predicted annual soil losses, locating the erosion-prone areas in monteiro county where concentrated flow created irreversible soil losses. acknowledgments the financial support provided by the national council for scientific and technological development, brazil – cnpq (grant no. 305413/2014-7, 408631/2016-3 and 304540/2017-0) and federal university of paraíba is gratefully acknowledged. this study was also financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior brasil (capes) finance code 001. references ahmet, k. 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(2009). integration of modified universal soil loss equation (musle) into a gis framework to assess soil erosion risk. land degradation and development, 20(1), 84–91.. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n1.92419 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 47 54 o r e g e o l o g y ore-controlling structures and geostatistical determination of ore-shoots in shear zone hosted lode gold type deposits, el bagre-antioquia, colombia edwin naranjo sierra1 geólogo msc. operadora minera s.a.s. clle. 5a n° 43b-25, of. 806. medellin, colombia. corresponding author: edwin.naranjo@operadoraminera.com.co how to cite item: naranjo sierra, e. (2021). ore-controlling structures and geostatistical determination of ore-shoots in shear zone hosted lode gold type deposits, el bagreantioquia, colombia. earth sciences research journal, 26(1), 47-54. https://doi.org/10.15446/ esrj.v26n1.92419 record manuscript received: 16/12/2020 accepted for publication: 12/01/2022 abstract shear zone hosted lode gold type deposits at the el bagre district, colombia, are the only known permian gold mineralization in colombia andes. multiple fluid flows are inferred in the formation of these veins, probably synchronous to several shearing episodes. nonetheless, orientation and control of high-grade gold concentrations (i.e., ore-shoots) are poorly known. in the el bagre district, three main lode systems are studied with n-nnw–s-sse orientation and variable dips (40 to 80° to the la ye and 30 to 45° to mangos and cordero). geostatistical analyses with ellipses determined by variogram modelling, reveal the anisotropy of gold mineralization and the orientation of ore-shoots. the long axes of the anisotropy ellipses trend sse, certainly related to sinistral shearing and nnw-sse faulting during d3 deformation event. this study evidences the use of a simple and effective tool for lode gold deposits exploration. keywords: el bagre; ore-shoots; geostatistics; lode gold deposits controles estructurales de mena y determinaciones geoestadísticas de zonas de alta ley en depósitos vetiformes hospedados en zonas de cizalla, el bagre-antioquia, colombia resumen los depósitos vetiformes de oro hospedados en zonas de cizalla en el bagre, al noreste del departamento de antioquia, son la única evidencia de mineralización de oro pérmica hasta la fecha en los andes colombianos. se han inferido múltiples flujos de fluidos en la formación de estas vetas, probablemente sincrónicos a varios episodios de cizallamiento. no obstante, la orientación y el control de los altos tenores (ore-shoots) de oro son pobremente conocidas. en el distrito el bagre, tres sistemas de veta principales fueron estudiados con una orientación n-nnw-s-sse y con buzamientos variables (40 a 80° para la ye y de 30 a 45° para mangos y cordero). los análisis geoestadísticos con elipses determinadas por el modelamiento variográfico, revelan la anisotropía de la mineralización de oro y la orientación de los ore-shoots. los ejes principales de los elipsoides de anisotropía tienden al sse, ciertamente relacionados con el cizallamiento sinestral y el fallamiento nnw durante el evento de deformación d3. este estudio evidencia el uso de una herramienta simple y efectiva para la exploración de depósitos de oro vetiformes. palabras clave: el bagre; altos tenores; geoestadística; depósitos vetiformes; https://doi.org/10.15446/esrj.v25n1.74167 mailto:edwin.naranjo@operadoraminera.com.co https://doi.org/10.15446/esrj.v26n1.92419 https://doi.org/10.15446/esrj.v26n1.92419 48 edwin naranjo sierra introduction orogenic lode gold type deposits are an important resource for gold (groves et al., 1998; moritz, 2000; groves et al., 2003; goldfarb et al., 2005; goldfarb and groves, 2015; groves and santosh, 2016). in the northeastern zone of antioquia department of colombia, several shear zone hosted lode gold type deposits are present. this region has long been recognized as an important historic gold district in colombia (shaw, 2000; sillitoe, 2008; londoño et al., 2009; leal-mejía, 2011). gold is frequently found in microscopic grains associated to sulphides. but at vein scale, discrete volume of rock with specific directions may contain high gold concentrations and are named “ore-shoots” (wang et al., 2019). ore-shoot occurrences are frequently related to specific structures such as faults, shear zones and folds (hulin, 1929; sibson, 2004) which can be organized in six groups (wang et al., 2019): (1) intersection of a fault or shear zone with a particular lithological unit; (2) the intersection of two syn-metallogenic faults; (3) dilation jogs, divergent bends in faults or zones of en-echelon fault segmentation; (4) fold hinge zones; (5) flexures of lode or reef surface, with axes oblique to the movement direction (commonly at a high angle), and (6) zones that plunge subparallel to the striation (wang et al., 2019). lode-gold type deposits at the el bagre, are structurally controlled and oreshoot shapes, orientation and distribution of high grades remains unresolved. based on this scenario, a geostatistical analysis of gold grade data of the main deposits was used to investigate orientation, shape and regularity of high grades. finally, the structural features of the deposits were integrated to elaborate a model for the ore shoots. geological setting regional geology in the northeastern zone of antioquia department of colombia, several shear zones hosted lode gold type deposits are reported. this region is situated in the tahamí terrane (restrepo and toussaint, 1988; cediel et al., 2003; restrepo and toussaint, 2020 and moreno-sanchez et al., 2020) and is defined by low to high grade metamorphosed metasedimentary and pre to post–tectonic granitoid intrusions that formed during a triassic metamorphism event (restrepo and toussaint, 2020). an important strike-slip ns to nnw fault system named otu-pericos is the main structure recognized in the figure 1. a). simplified map showing the location of colombia and antioquia department. b). regional map of the ne region of the antioquia department., showing the region of bajo cauca (in orange) and the main major faults. c). simplified geological map of the el bagre mining district based on gómez-tapias et al., (2015). 49ore-controlling structures and geostatistical determination of ore-shoots in shear zone hosted lode gold type deposits, el bagre-antioquia, colombia region. this fault system separates triassic metamorphic rocks associated to cajamarca complex from carboniferous intrusive rocks from el carmen stock (gómez-tapias et al., 2015). the kinematics of this regional fault are not fully understood in detail; however, right-lateral as well as left-lateral faulting and thrusting seem to have occurred (horner, 2018). to the east, along the el bagre reverse fault system, san lucas precambrian metamorphic rocks are in faulted contact with the jurassic granitic rocks of the segovia batholith. this fault system forms branches of n to nne trending palestina fault system, which has been recognized as a regional fault with right and left-lateral movement (horner, 2018). all veins at the el bagre mining district are hosted in shear zones within el carmen intrusive rocks. these shear zones have brittle-ductile deformation which produced wall rock mylonitization, boudinage, vein off-sets and reactivation of wall rock-vein contact (figure 1). the el bagre gold district the main lodes of la ye, mangos and cordero are hosted within carboniferous intrusive rocks of el carmen stock. the timing of gold mineralization at el bagre is likely to be between 284 and 276 ma according to k-ar and ar-ar sericite dating (leal-mejia, 2011 and naranjo-sierra, 2019). the gold mineralization at the el bagre mining district are the only known occurrences of permo-triassic deposits in colombia (leal-mejia, 2011; naranjo-sierra, 2019). orebodies mainly strike nnw-nne (340°-20°) and dip 30-45° to the east for mangos and cordero orebodies and at la ye dip stepper with a mean dip angle of 65° to the west. these orebodies are crosscut by enewnw and nnw-sse faults (figure 2). figure 2. local geological map showing the main lode-gold deposits. the veins consist of milky white quartz containing native gold and up to 20% vol. sulfides, dominated by pyrite with local galena and chalcopyrite (figure 3). distributions of sulphides and native gold within veins are patchy (figure 3b-d) (naranjo-sierra and alvarán-echeverri, 2020). gold mineralization only occurs in massive quartz veins associated with pyrite, chalcopyrite, galena and tellurides (calaverite (aute2), sylvanite (auagte2) and altaite (pbte) (figure 3f, g and h). close to the quartz lodes, the el carmen stock is altered by hydrothermal activity, displaying a restricted chloride halo. sericite and carbonates (figure 3e), occurring as fine-grained crystals filling fractures, are the most common alteration minerals within quartz veins (naranjo-sierra, 2019). figure 3. general view of the shear zone hosted lode gold type deposits at the el bagre. altaite (alt), calcite (cal), chalcopyrite (ccp), galena (gn), quartz (qz), pyrite (py), sericite (ser) and sylvanite (slv). mineral abbreviations according to whitney and evans (2010). a). the la ye lode gold type deposit, with coarse pyrite, massive quartz and calcite. b). high gold grade quartz vein fragment from mangos mine with semi-massive pyrite, chalcopyrite and fine-grained galena. c). local visible gold at mangos vein. d). high gold grade quartz vein fragment from the la ye mine, with semi-massive chalcopyrite and pyrite. e). fine-grained sericite. f). photomicrograph with gold, galena and chalcopyrite filling fracture in coarse pyrite crystals. g). photomicrograph with gold, sylvanite, chalcopyrite and altaite filling fracture in massive quartz. h). photomicrograph with gold, sylvanite and chalcopyrite filling fracture in coarse pyrite crystals. four deformation events have been identified and described by starling (2015); naranjo et al., (2016); naranjo-sierra and alvaran-echeverri (2018) and naranjo-sierra (2019): (i) d1 deformation event forms the ns-nnw structural corridor and is probably precoce to gold mineralization (pre-permian?). (ii) d2 deformation is mostly related to dextral shearing, generating ene/wnw lowangle thrust faults which allow percolation of fluids and significant volumes of quartz with low contents of sulphide (mainly pyrite) as well as scarce gold mineralization (naranjo-sierra and alvarán-echeverri, 2020). (iii) d3 deformation is associated to sinistral transpressional shear with nnw/sse trending riedel cross faults and deforms pre-existing veins. this deformation 50 edwin naranjo sierra stage is related to the main phase of high-grade gold mineralization as well as base-metal and telluride ore (naranjo-sierra and alvarán-echeverri, 2020). (iv) d4 deformation covers all the post-mineralization events and is frequently recognized by presence of duplication by imbrication (stacking), pinching and offsets of the vein. methods geostatistics is the study of phenomena that vary in space and/or time (matheron, 1963). this domain evaluates the spatial distribution of values and was originally developed for the purpose of ore deposit modeling and ore reserve estimation (monteiro et al., 2004). the cornerstone of geostatistics is the semi-variogram function, which geostatisticians generally refer to as a variogram. given two locations, x and (x + h), a variogram is one-half the mean square error produced by assigning the value z(x + h) to the value z(x), or the variance of the increment (matheron, g. 1963). the semi-variogram function is expressed by the following equation: ( ) = ( ) ∑ [ ( ) − ( + )]= ( ) = (1) to determine the orientation, shapes and regularity of high gold grades in lode-type deposits at the el bagre, the geostatistics method and the semivariogram function were applied to perform a gold grade analysis. the main goal is to integrate structural features of the orebodies and gold grade data set to reach the quantitative relation of mineralization and ore-controlling structures (wang et al., 2019). a gold grade data set for each lode were prepared and separated between fault blocks. a total of 35.154 samples from drill holes and channel samples (samples taken perpendicular to the vein exposures inside the mine) were used for this analysis, covering 2.2km2 (figure 4). the database was multiplied by a constant value to maintain the mining company´s confidentiality. this technique does not affect or change the sample variance and range vectors and still allows a variogram analysis (wang et al., 2019). gold grades in lode-type deposits may change depending to vein locations and directions, which induce significant anisotropies. these gold variations must be carefully evaluated in different directions (miller & wilson, 2004), helped by semi-variogram function. the backbone of semi-variogram analysis is to determine the sill or the lag distance at which the semi-variogram level off and the range or the distance at which gold grades are autocorrelative and stable. mineralization anisotropy can be described as an ellipse with a preferred direction. the ellipse that best fits the dataset of each orebody between faults was determined during variogram modeling. the longer axis of the best-fitting ellipse indicates the orientation and distance where mineralization is stable, and the shorter axis of the ellipse indicates the orientation and distance where mineralization is unstable (wang et al., 2019 and monteiro, 1996). this further indicates the orientation and size of high-grade ore shoots as ore-shoots plunge in the direction where mineralization is most stable and the range is the biggest (wang et al., 2019). based on this, several variogram direction were conducted (00°, 020°, 040°, 060°, 080°, 100°, 120°, 140°, 160° and 180°) to define the best-fit parameters of the variogram function and to determine the directional dependency of the range value. these values were used to evaluate the shape of the ellipse that best describes the mineralization of each orebody (figure 5). figure 4. plan view of vein modelling using leapfrog geo® and the sample data used in this study. 51ore-controlling structures and geostatistical determination of ore-shoots in shear zone hosted lode gold type deposits, el bagre-antioquia, colombia orebodies, ore shoots and ore-controlling structures orebodies at the el bagre mining district are generally considered as the product of ore-forming fluids derived from at least two episodes of fluid flow circulating in the shear zones (naranjo-sierra, 2019; naranjo-sierra and alvarán-echeverri, 2020), thus indicating probable multiple mineralizing stages, associated with various orebodies and ore-shoots directions. orebodies from la ye, mangos and cordero have different orientations, shapes, and sizes. according to stereonet plots, mangos orebodies tend to strike (as a whole) to the ns-nne and plunge to the ne (figure 6a), the la ye orebodies tend to strike (as a whole) to the nnw-ns and plunge to the sw (figure 6b) and cordero orebodies tend to strike (as a whole) to the nnw-ns and plunge to the ne (figure 6c). these orebodies are crosscut by late-d2 ene-wnw and d3 nnw-sse faults, divided these orebodies into several blocks (twelve) and for each, a stereonet plot was prepared to determine orebody tendency (figure 5). these tendencies were used to perform the variogram map to define the pitch direction where gold grade is more stable. the ellipses determined by variograms indicate spaces where gold grade distribution is autocorrelative, and these elliptical spaces represent individual ore-shoots (wang et al., 2019). the mineralization anisotropy diagrams indicate two preferred pitch directions; 120°-110° and 60° for mangos orebodies, 160° and 20° for the la ye orebodies, and 120° pitch direction for cordero orebodies. this anisotropy reflects the preferential direction of high grades (figure 7). shear zone hosted lode-gold type mineralization at the el bagre is characterized by an nnw/sse and ene/wnw trending major faults. on this fault planes, several linear elements were recognized (pitch/plunge). the sets 0°-5°/n and 110°-130°/se indicate early sinistral reverse movement on this orecontrolling faults during d3 deformation event. the 65°n and 90°ne sets are fairly close and defined by similar striations. these two linear sets indicate later post-mineralization normal movement related to d4 deformation. moreover, it is common to observed nw-plunging slip grooves on reactivated footwalls of the orebodies and strong fracturing with sulfides-tellurides filling due to sinistral shears and steep joints. based on this structural controls, mineralization anisotropy and thus the direction of high gold grades are related to nnw/sse trending faults during d3 deformation event. figure 5. variogram running along vein plane (dip direction/dip), showing the sill value at which the variogram level off, the nugget effect and the range. the ellipse on the right is the mineralization anisotropy (on vein plane) with the preferred direction (pitch direction) or the direction where gold grade is more stable. figure 6. stereoplots for each orebody. a) mangos orebody divided in six fault blocks (a1 to a6). b) la ye orebody divided in four fault blocks (b1 to b4). c) cordero orebody divided in two fault blocks (c1 to c2). all stereoplots are low hemisphere, equal area. 52 edwin naranjo sierra figure 7. variogram maps running along the vein plane (dip direction/ dip; based on stereoplots; i.e.; figure 6), with gold anisotropy for each orebody, showing the pitch direction (on vein plane) where gold grades are most stable and therefore the ore-shoot directions. a to f). mangos orebodies. g to j). la ye orebodies. k). cordero orebodies. 53ore-controlling structures and geostatistical determination of ore-shoots in shear zone hosted lode gold type deposits, el bagre-antioquia, colombia discussion gold mineralization at the el bagre gold district is the result of structurally controlled mineralizing fluid flow. at orebodies scale (or metric scale), significant volumes of quartz with low contents of sulphides and scarce gold mineralization were deposited during d2 deformation event. but only where d3 deformation event is present high gold grades occurred, mostly related to nnw-sse riedel shearing. individual ore-shoots plunge to the se while the orebodies plunge to the sw and ne as a whole. although hydrothermal flow is complex and poorly constrained, the fault kinematic data suggest a link between high gold grades and nnw-sse d3 faults. based on this new data and previous works presented by starling (2015); naranjo et al., (2016); naranjo-sierra and alvaran-echeverri (2018) and naranjo-sierra (2019) suggest a two stages model for the emplacement of gold-bearing veins in the el bagre district: 1) an early episode of fluid flow during nne-ssw d2 dextral shearing, with ore-forming fluids focused on dilational sites (like right stepovers/overlaps or anti-clockwise) along the trace of main veins. 2) d3 nnwsse deformation reactivates d2 structures with numerous sinistral shear zones and forming zones of fractured quartz, infilled by late fine-grained sulphides, gold and tellurides mineralization. d3 was the main phase of deformation that controlled much of the economic mineralization. moreover, d3 nnw-sse figure 8. schematic model for ore-shoots and ore-controlling structures at the el bagre gold district. a). 2d plan view of the d3 deformation event, related to sinistral shearing due to nnw/sse compression. b). ideal model ore-shoot tendency related to sinistral shearing during d3 deformation event. deformation faults show brittle-ductile strike-slip displacement of the vein in a sinistral sense that implies they formed as riedel shear components to the sinistral transpressional shearing. some of these faults display banded quartz veining that indicates that hydrothermal fluid flow was active during this event. thus, this later d3 deformation of the pre-existing massive quartz veins created zones of enhanced permeability controlling the high gold grades and therefore the ore-shoots (figure 8). conclusions gold mineralization in the shear-zone hosted lode deposits at el bagre, are structurally controlled. the ellipses of mineralization anisotropy show that ore-shoots tend to plunge to the se while orebodies plunge to the ne and sw. only where d3 deformation is present, high grades occurred, proving that later d3 nww-sse trending faults controlled the main mineralization event. combined structural analysis with geostatistics is a valuable tool for mining exploration to determine the architecture of ore-controlling structures at deposits and district scale. acknowledgment i would like to thank soma gold corp and operadora minera s.a.s for the opportunity to present this work. also, thanks to the exploration team of the company for their support and comments on the firsts editions of this document. finally, thanks to the anonymous reviewers for pointing out points of muddled thinking and for constructive comments. references cediel, f. shaw, r. p. & cáceres, c. 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interior have been studied during last century to know dimensions, properties and its reological structure. these information has been used to understand the continental and marine formation, the energy involved and the crustal dynamics in several regions in the world. on all this frame, the circum-pacific belt, and particularly, the central and south american subduction zones are constituted by a complex of structures that suggests a series of recent earthquakes and demand constant research. the earthquakes are the most important natural hazard, with considerable social and economic repercussions for latin american people. presentation american continent has been scenario of several earthquakes, some of greater energy released in the world. many ofthe most destructive earthquakes have happened throughout the central and south american subduction zones. these events have allowed to infer aspects related to processes in the crust and mantle of this region. earthquakes like colombia ecuador (mw=8.9, 1906), chile (mw=9.6, 1960), mexico (mw=8.1, 1985), chile (mw=8.9, 1995) and peru (mw=8.0, 2001), as well as an important number of destructive events related to cortical faults systems annually leave human and economic lost; this hazard scheme imposes the necessity to call the earth sciences community to explain mechanisms that govern this phenomenon. around the previous ideas, the "center of studies and research of engineering" (ceifi) of the university of quindio and the geophysical group of (geosciences department) of the national university of colombia in collaboration with the geological survey of colombia (ingeominas) and the support of many important scientific and academic institutions are preparing the first latin american congress of seismology in whose edition it tries to gather the scientific and technological experiences and accumulated developments around of "seismological aspects of central and south american subduction zones" and like departure point to form into a scientists group that work in this region and to define common fronts of work in next future. the university of quindio (armenian colombia) and the national university of colombia (bogota colombia) has taken the initiative to promote this important event to open reflection 'spaces about the earthquakes, boarded hazard from physics of phenomenon and its social and economic implications. place and dates engineering faculty, universidad del quindio. 02 09 august, 2004. topics instrument seismology and seismic signal processing macroseismic and historical seismology tectonics and seismotectonics numerical methods volcanic seismology earthquake prediction social and economic aspects of seismology education in seismology other related subjects scientific committee: armando espinosa (colombia) victor huerfano, university of puerto rico (puerto rico) giogio asteriadis, university of thessaloniki (greece) frank audemar, funvisis (venezuela) lluis pujades, universidad politecnica de catalunya (spain) hernando tavera, instituto geofisico del peru (peru) hugo yepes, escuela politecnica de quito (ecuador) ley pulliam, texas university, texas (usa) schedule pre-congress courses: 2-3 august, 2004 congress: 4-6 august, 2004 field trip: 7 august, 2004 correspondence the contributions and any question, may be sent to congress email or to congress responsible hugo monsalve or carlos a. vargas 1. key dates deadline abstracts: may 16,2004. warning of accepted works: may 30, 2004. deadline reception of full works: june 30, 2004 charges regular inscription: u$ 50.00 student: u$ 25.00 stand 2 m2 : uss 200.00 geofislca colombiana. 7, oiciembre oe 2003 79 first latin american congress of seismology contributions abstracts the abstracts are limited to a maximum of300 words in english or in spanish. it may describes methodological details, data and results of the work. roman will appear in electronic format (word) with letter time new 10, to simple space. at the end of this section will add the key words key words. full works norms the works accepted will be incorporated to a special volume of an intemationaljoumal. we recommend follow the next scheme for its presentation. abstract these instructions give you basic guidelines for preparing the papers for the first latin american congress of seismology. papers are not limited to a maximum length, but they must be submitted using this format, if you are reading a paper version of this document, please download the electronic file from the conference website so you can use it to prepare your manuscript. remember, the abstract should not exceed 300 words. include appropriate key words in your abstract, in alphabetical order, separated by commas. keyworos: fonts, formaning, margins introduction your goal is to simulate, as closely as possible, the usual appearance of typeset papers. authors' affiliations should appear immediately following their names. sub-sections are not numbered. methodology all papers must 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4----l----+---+--+-if-+4+---'-l+--++----+----+---1 i ·~·~---·-·-~···_·~i--···----i-··-······~ i i 30,0 -0.60 .l.__ ...l. __ .....l __ ----l l__ .l.__ .l.......l. __ .....l __ ----.j 0.0 120,0 figure 1. acceleration record at camacho station. 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(1999): "work of one author". journal of seismology. 19, 23, 110-120. one, 1. and 1. two (1999): "work of two authors". earth sciences research journal, 29, 213, 1110-1120. one,1., 1. two and k. three(1999):" work of two authors". journal of geophysics. 219,21, 111-120. geofisica colombiana, 7, diciembre de 2003 in soconusco, chiapas, in spite of the high availability of surface water, it is resorting to the use of groundwater. knowledge about the quality of surface or groundwater used to irrigate crops in that region is low. this paper aims to contribute to the knowledge of the quality of groundwater for agricultural use through the characterization of the spatial variability. assuming a random spatial distribution of 45 samples which were collected in situ were determined: acidity and alkalinity (ph), electrical conductivity (ec), total dissolved solids (tds), cations and anions and trace elements; in addition to the agricultural index: sodium adsorption ratio (sar), residual sodium carbonate (rsc), soluble sodium percentage (ssp), sodium percentage (% na), kelly ratio (kr), magnesium adsorption ratio (mar), permeability index (pi), effective salinity (es), salinity potential (sp) and osmotic potential (op). in general, ssp, % na, kr, pi are low, there is only one anomalous point (9) with high values at w of the study area. similarly, ps, es, cl, na and sar are low except point 16 and conversely, rsc and ph are high, except at this point located in the center of the study area. the results allow us to infer that the water in that aquifer presents no problems or sodicity toxic ions. in 27 sites sampled values above 250 µmhos/cm were found at 25°c, classified as medium to high risk of salinity, unsuitable for agricultural use. analysis of the combined effect of the presence of sodium (sar) and salinity (ec or tds) shows that 27 of analyzing sites have restricted water medium at very high for use in irrigation. abstract keywords: salinity; sodicity; osmotic pressure; sodium adsorption ratio; electrical conductivity; irrigation; aquifer. quality indices of groundwater for agricultural use in the soconusco, chiapas, mexico issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.63455 germán santacruz de león1*, josé alfredo ramos leal2, janete moran ramírez2, briseida lópez álvarez1, eugenio eliseo santacruz de león3. 1. programa "agua y sociedad" de el colegio de san luis, a.c. 2. instituto potosino de investigación científica y tecnológica. 3. universidad autónoma chapingo earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 117 127 en el soconusco, chiapas, a pesar de la alta disponibilidad de agua superficial, se recurre al uso del agua subterránea. el conocimiento de la calidad del agua superficial o subterránea utilizada para el riego de los cultivos en la región es bajo. este trabajo contribuye al conocimiento de la calidad del agua subterránea para uso agrícola a través de la caracterización de su variabilidad espacial. asumiendo una distribución espacial aleatoria, se colectaron 45 muestras, a las que se les deteminó: acidez y alcalinidad (ph), conductividad eléctrica (ce), sólidos totales disueltos (tds), cationes y aniones y oligoelementos; además se determinaron índices agrícolas: índice de adsorción de sodio (sar), carbonato de sodio residual (csr), porcentaje de sodio soluble (pss), porcentaje de sodio (% na), relación de kelly (rk), relación de absorción de magnesio (ram), índice de permeabilidad (ip), salinidad efectiva (se), salinidad potencial (sp) y potencial osmótico (op). en general, el pss, %na, rk, ip son bajas, sólo hay un punto anómalo (9) con valores altos en la parte oeste del área de estudio. del mismo modo, sp, se, cl, na y el ras son bajos, excepto en el punto 16 y en contraste, csr y ph son altos, excepto en este punto ubicado en el centro del área de estudio. los resultados nos permiten inferir que el agua en ese acuífero no presenta problemas de sodicidad y de tóxicos. en 27 sitios se encontraron valores superiores a 250 μmhos/cm a 25 ° c, clasificados como de riesgo medio a alto de salinidad, es decir inadecuados para uso agrícola. el análisis del efecto combinado de la presencia de sodio (ras) y salinidad (ce o sdt) muestra que 27 de los sitios analizados presentan restricciones de medias a muy altas para su uso en riego. resumen palabras clave: salinidad, sodicidad, presión osmótica, relación de adsorción de sodio, conductividad eléctrica, riego, acuífero. record manuscript received: 22/03/2017 accepted for publication: 21/07/2017 how to cite item santacruz, g., ramos, j. a., moran, j., lopez, b., & santacruz, e. e. (2017). quality indices of groundwater for agricultural use in the soconusco, chiapas, mexico. earth sciences research journal, 21(3), 117 127. doi:http://dx.doi.org/10.15446/esrj.v21n3.63455 h id r o g e o l o g y índices de calidad del agua subterránea para uso agrícola en el soconusco, chiapas, méxico germán santacruz de león, josé alfredo ramos leal, janete moran ramírez, briseida lópez álvarez, eugenio eliseo santacruz de león. introduction as the surface water, groundwater is part of the hydrological cycle and, a significant portion of the world population depends on it. even there are zones of the world where, for climatic or geological reasons, is the only source (price, 2003). in the past 50 years the groundwater has played a key role in agricultural production (giordano and villholth, 2007), currently, 50% of potable water and 45% of the irrigated agricultural area of the planet depend on groundwater (rifat et al., 2014). in arid and semi-arid areas of latin america, aquifers are the source of a third of the total water usage (miletto et al. 2006). in arid and semi-arid zones of mexico, groundwater is the main source of supply. in the tropics unconfined aquifers are important sources of water for agriculture and domestic use. six million hectares area irrigated, of which one third is irrigated with groundwater (marín, 2002; asad and garduño 2004; conagua, 2013). knowledge of groundwater quality in time and space, it is important to differentiate its composition and function of this, devoting it to best use. this would have generated different techniques for water quality report, include the so-called water quality index (wqi) grouping one or more physicochemical and bacteriological parameters (guzman-colis et al. 2011; kankal et al. 2012) even supported by statistical techniques (hülya and hayal, 2007; papaioannou et al. 2010; hafizan et al. 2011; mohd et al. 2011). to this, the pressures of agriculture and increasing living standards demanding better agricultural products (wijnen et al. 2012) and therefore better quality of water used for agricultural activity are added. the quality of irrigation water has repercussions on the production; it may affect plant growth and thus reduce levels of agricultural production (yesilnacar and gulluoglu, 2008; deshpande and aher, 2012). to evaluate the quality of water for agricultural use, there are considered some aspects such as salinity, effects of sodium on soil properties and toxicity of specific ions (ayers and wescot, 1985) that are included in various guidelines for interpreting quality. these guidelines have been outfitted in various indices as the sodium adsorption ratio (richards, 1954), magnesium adsorption ratio (rifat, 2014) soluble sodium percentage (todd and mays, 1980) residual sodium carbonate (eaton, 1950), among others (table 1). in mexico the knowledge about the quality of groundwater for irrigation is scarce, it is considered as a requirement of lesser importance (cardona et al., 2010). the soconusco region in the state of chiapas is in this condition (unesco, 2007 and 2010). there have been studies on the availability of water in the aquifer (diaz, 2001; conagua, 2009) and from them are considered relevant to study water quality in the soconusco to reveal current conditions and that knowledge it can be referenced for farmers, agricultural planners and for future studies. the aim of this study was to analyze from the application of various indices, the spatial variation of the quality of groundwater for agricultural irrigation in the soconusco region in the state of chiapas, mexico. materials and methods localization and characterization of the study area the aquifer of soconusco is in the region of soconusco, chiapas. this area consists of sixteen municipalities, covering an area of 5,475.5 km2 (fig. 1). agricultural activities are predominantly export-oriented crops such banana (musa paradisiaca), mango (mangifera indica l.), coffee (coffea var.) and corn (zea mays), soybean (glycine max) and beans (phaseolus vulgaris). the climate is hot and humid type with temperatures ranging from 25-34 °c in spring and summer, while for the rest of the year temperatures are average values of 18-22 °c. it presents a rainfall ranging between 1,500 mm and 4,000 mm, with a regional average of 2,450 mm (jica, 1999) due to the topographic variability. it is a region of great ecological diversity, with different strata termohídricos (flottemesch and schriker, 1993) and the presence of different types of soils (deinlein, 1993), which have their origin in volcanic ash. hydrogeological framework the basis of the stratigraphic column that emerges in the chiapas region is of proterozoic age and is composed mainly of granitoids and orthogneises (weber et al., 2006). covering discordant to the basal rocks, the rocks of the upper paleozoic are represented by the detrital series belonging to the paso hondo and grupera formations (lópez-ramos, 1980), also by metamorphic rocks, including serpentinites, schists, gneisses and quartzites (salas, 1975), which were intruded by granodiorites, diorites and granites, rocks of the batolito of chiapas. covering discordant to the sequence to the paleozoic units are detrital-calcareous sediments (conglomerates, sandstones, limes and clays) that date from the triassic and jurassic, these are the formations todos santos, grupo sierra madre and angostura (morán-zenteno, 1994). volcanic rocks cover all previous rocks and are composed of volcanic rocks whose composition ranges from acidic to basic from the chiapaneco volcanic arc in the cenozoic (mora-chaparro, et al., 2007). finally, the rocks of the pliocene-holocene are constituted by deposits of silts, sands, clays and pyroclastic deposits derived from the volcanic activities of the chichonal and tacaná; as well as by alluvial materials and residual soils. the aquifer “soconusco” is located in the physiographic province known as the highlands of chiapas, it is mainly distributed in the coastal plain, which is bounded on the south by the pacific ocean and west by the sierra of chiapas or the granite massif of chiapas (diaz, 2001; macias et al., 2010). quality indices of groundwater for agricultural use in the soconusco, chiapas, mexico. the granites and granodiorites are widely distributed in the sierra of chiapas, mainly in the northern part of the aquifer (recharge area). these rocks are one of the major sources for the formation of alluvial deposits and function as impermeable barriers to groundwater flow (diaz, 2001, macias et al., 2010). volcanic rocks of basic and intermediate composition are composed of andesites and basalts, their permeability is relatively low, as it is compact enough rocks. however, the coastal plain has been formed by the accumulation of permeable sediments flowing down from the mountains in fluvial environments; and, by the processes of coastal marine type. this group of geological formations consists mainly of unconsolidated materials as clastic, sand, gravel, boulders and thin, such as clay and silt sediments, these sediments are those that constitute the aquifer (diaz, 2001; macias et al., 2010). hydrological balance the hydrogeological balance of 2002 (conagua, 2002) indicate that the volume of natural recharge is 885.9 million m3/year and 52.2 million m3/year of induced recharge, the actual evaporation is 325.4 million m3/year natural discharge (springs) is 442.2 mm3/year. the groundwater discharge to the sea is 27.7 million m3/year and extraction wells is 1.62 mm3/year. with these figures, the conagua (2002) considers the aquifer soconusco in balance. in the 2009 update availability (conagua, 2009) consider an average annual recharge of 938.2 mm3/year. it is noteworthy that high rainfall in the region in combination with the presence of permeable materials in the plains allows high infiltration and soil washing. sampling and activities laboratory measurements in february 2013 sampling was performed on 45 samples from springs, dug wells and wells, the samples were taken during the dry season to determine major cations and anions (fig. 1). all samples were collected in bottles of high density polyethylene, washed and rinsed seven times with deionized water. the containers for collection of samples for the determination of cations and trace elements were washed with 10% hydrochloric acid. for each collected sample, was measured in situ, ph, ec, temperature, redox potential, dissolved oxygen, and alkalinity. immediately after collection, the samples for cations and trace elements were acidified with pure nitric acid to ph <2. all samples were stored at a temperature below 4 °c. the major ions and trace elements were analyzed in the laboratory of the centre for geosciences, national autonomous university of mexico. atomic emission spectroscopy was used with inductively coupled plasma (icp-oes) to determine the concentrations of cations higher calcium (ca2+), magnesium (mg 2+), sodium (na +) and potassium (k +) and higher sulphate anions (so42-) and chloride (cl-) were analyzed using liquid chromatography. alkalinity and bicarbonate (hco3 -) were determined by titration on the site. the ionic balance error (electro neutrality) in each of the 45 samples was considered less than 5%. drinking water quality analysis to determine the quality of the groundwater for drinking purposes, wqi is computed according to the following relationship (conesa, 1993): figure 1. location of the soconusco aquifer and sampling points. germán santacruz de león, josé alfredo ramos leal, janete moran ramírez, briseida lópez álvarez, eugenio eliseo santacruz de león.120 where ci is percent value function assigned to the parameters, pi weight assigned to each parameter and k constant taken from the values for the organoleptic characteristic (table 2). k value in groundwater is normally 1, except in polluted water, which uses the values in table 2. figure 2. a) piper diagram of the groundwaters of the study área, b) gibbs diagram showing the mechanisms controlling the chemistry of groundwater of major cations c) graph showing the distribution of the water quality of soconusco aquifer. figure 3. spatial distribution of indices a) ph, b) sodium adsorption ratio (sar), c) residual sodium carbonate (rsc) and d) soluble sodium percentage (ssp). classification of water in connection with agricultural use the classification of water for irrigation was made considering the conditions of acidity and alkalinity (ph), as well as sodicity rates as sodium adsorption ratio (sar), the residual sodium carbonate (rsc) is considered, the percentage sodium soluble (ssp), sodium percentage (% na), the ratio kelly (rk) and permeability index (pi) (eaton, 1950; todd and mays, 1980). to evaluate and rank the water relative to saline conditions was considered to electrical conductivity (ec), salinity effective (se), salinity potential (sp) and osmotic potential (op) which is closely related to the ec and total dissolved solids (tds) (palacios and aceves, 1970; aguilera and martinez. 1996; porta et al., 2010) the combination of these last three values allow classifying irrigation water. likewise, the water is classified considering the combined effects of salinity and sodium contents employment diagram salinity laboratory of the department of agriculture of the united states and wilcox diagram. all indices were determined in a spreadsheet, by applying standard equations. results and discussion the quality of data were analyzed based on their electroneutrality condition. the 45 groundwater samples perform that criterion. in the study área, aproximately 60% groundwater samples show distribution of mixed cations, 26 % are na+k type, 8% are ca and 6% are mg type wáter on piper trilinear diagram. among the major anions, 88% are plotted within the hco3 type, 6% are mixed type, 4% are so4 type and while only one simple has the influence of cl type. the majority of the studied samples, are plotted in the ca-hco3 type of water (60%), 18% are na-cahco3 type, 12% are na-hco3 type, 6% are ca-mg-cl type and 4% are na-cl type water on piper diagram (fig. 2a). the (gibbs, 1970) diagram was originally constructed for surface water analysis; however, it is possible to recognize some processes that occur to groundwater. the three main mechanisms that control surface water chemistry can be defined as the domain of atmospheric precipitation (rainfall), rock dominance, and control of the evaporation-crystallization process (gibbs, 1970). this diagram is constructed with the chemical data (anions and cations) of the groundwater samples. the graph of tds vs. na/(ca + na) of the gibs diagram (1970) indicates the dominance of dilution by mixing, rock alteration in the study area and in another group the evaporation process predominates (fig. 2b). the quality of water for human consumption indicates that 64% of the analyzed samples have very good quality, 20% have medium quality and 7% have poor or excellent quality and only 2% have very poor quality (fig. 2c). terms of acidity and alkalinity the quality of water used for irrigation significantly influences the productivity of crops, depending on the presence of dissolved salts in it and their concentrations. salts change the osmotic processes, and the presence of toxic substances (such as boron) can affect the metabolic processes in plants. the maximum and minimum ph values are 9.2 (point 19) and 6.1 (point 21), respectively. three (points 18, 19 and 36) of the 45 samples, have ph values above 8.5 and all are above 6 (fig. 3a). quality indices of groundwater for agricultural use in the soconusco, chiapas, mexico. 73 values above ph 8.5 and below 6 are indicative of an abnormal water quality are a signal of the presence of an ion toxic to crops. water with ph values within this range creates conditions of assimilation of different nutrients such as phosphorus and some micronutrients (ayers and wescot, 1985). terms of sodicity the presence of sodium in water at concentrations considerable permeability can affect the rate of infiltration and soil tillage (romero, 2009), so that a high proportion of sodium relative to the calcium concentration causes a decrease in infiltration its dispersing effect on soil aggregates (grattan and oster, 2003 ruda 2005). the data for assessing the quality of water relative to sodium are the sar, the rsc, the ssp, the% na, the rk and the pi. there is more possibility of sodification if the proportion of sodium is high with respect to the presence of calcium and magnesium (peinado-guevara et al., 2011). the maximum and minimum values are 10.77 sar (point 35) and 0.27 (point 22) respectively (fig. 3b). this index shows that there is no danger of sodicity in water drawn from the aquifer soconusco, only use no. 35 presents values above 10, which is the lower limit for classifying water as well. (fig. 3b). the presence of carbonates and bicarbonates have effects on the quality of water for irrigation, when their presence is greater than the concentration of calcium plus magnesium exists the possibility of forming carbonates of sodium, causing deflocculation soil (palacios and aceves, 1970; aguilera and martinez, 1996), the agricultural land irrigated with such water may become infertile (rifat et al., 2014). the rsc indicates the danger of sodium carbonate, when the cations have already reacted with carbonates and bicarbonates of calcium and magnesium, to predict the trend of these cations to precipitate in the soil when irrigated with highly carbonated water (peinado-guevara et al., 2011). four samples (items 9, 13, 29, 30) have rsc values, higher than 1.25 meq/l (fig. 3c), the rest shows values below this limit and are considered safe for use in agricultural irrigation. the maximum and minimum values, are 3.55 rsc (point 30) and -15.38 (point 16), respectively. displacement of calcium and magnesium in the cation exchange process initiated when the sodium content in solution is greater than 50% of the cations (palacios and aceves, 1970, aguilera and martinez, 1996), high percentages of sodium in water (greater than 50%) used in irrigation, prevents the growth of crops and reduce soil permeability (rifat et al., 2014), ssp values below 50 indicate good water quality and above this value indicate that the water is not suitable for irrigation (fig. 3d). the maximum value found in this sample was 80.02 (item 9) and the minimum was 15.2 (point 33), four harvesting (items 9, 12, 30, 35) present above the limit of 50 values (fig. 3d). groundwater can be grouped according to their sodium content in percent (% na), these are considered excellent for use in agricultural irrigation, and have lower values when 20%, good values 20-40%, with permissible values 40-60%, 60-80% doubtful and unsuitable when presenting values above 80% (wilcox, 1955). the maximum and minimum values found in the aquifer under study are 80.8% and 18.4%. three points (2, 7 and 33) have classified as excellent water; 24 exploitations have classified as good quality water; 14 have classified as admissible water; two points present a classification of water considered doubtful and as suitable for use in agriculture (fig. 4a). the presence of sodium with respect to the calcium plus magnesium concentration is measured by the rate or the kelly index , values greater than 1 indicate excess sodium in water, and values less than 1 indicate that water is suitable for use in agricultural irrigation (aher and deshpande, 2011; deshpande and aher, 2011, 2012). the maximum and minimum values for the aquifer soconusco were 4.01 and 0.18, respectively; four points (9, 12, 30 and 35) have values greater than 1 (fig. 4b). soil permeability is affected by the presence of sodium, calcium, magnesium, and bicarbonate in irrigation water when it is applied continually. considering this index water is classified as class i and class ii if it has more than or equal to 75% pi, this water is considered good quality for use in irrigation. in class iii, the water presents values lower than pi 25% (joshi et al., 2009; ishaku et al. 2011; obiefuna and sheriff, 2011). in this study the maximum and minimum values of pi were 161.5% and 46.5% (fig. 4c), respectively, so we can conclude that the soconusco aquifer is of good quality for irrigation. figure 4. spatial distribution of the indices a) % na, b) kelly ratio (kr), c) permeability indices (pi) and d) magnesium adsorption ratio (mar). in most of the water, calcium and magnesium remain in equilibrium, the presence of magnesium in concentrations affecting agricultural production sensitive to the chemical component (nagaraju et al., 2006) cultures. in general groundwater containing low concentrations of magnesiumcan be higher if the water is in contact with dolomite containing high concentrations of it, its content in the water is a relevant indicator for the classification of water for use in irrigation the index allows to evaluate magnesium adsorption ratio (mar), that provides percentage values greater than 50%, causes harmful effects on soil (obiefuna and sheriff, 2011; nag and ghosh, 2013; rifaf et al., 2014) values. the maximum and minimum values found in this study were 79.5% (point 5) and 20.2% (point 29), respectively. only 10 of the 45 sites sampled 50% have higher values, which are located very close to the coastline (fig. 4d). salt conditions high concentrations of salts in irrigation water can cause an increased presence of salts in the root zone and accumulation in the soil profile, causing reduction in growth, development and production of agricultural crops (grattan and oster, 2003; romero 2009). the presence of salts affects the op and reduces crop yields (palacios and aceves, 1970; aguilera and martinez, 1996). ec measures the presence of salts in the water; it is directly proportional to the concentration of salts in solution (palacios and aceves 1970; aguilera and martinez, 1996; porta et al., 2010). other criteria to evaluate the conditions of salinity in the water are the se and sp (aguilera and martinez 1996; porta et al., 2010) and the op that is closely related to the ec and with the tds, the conjunction of the latter three values allows to classify irrigation water (palacios and aceves, 1970, aguilera and martinez, 1996, porta et al., 2010). salinity is considered the most important criterion for classifying irrigation water (ghasseme et al., 1995), salinity in soils causes reduced productivity (essien and ubit, 2013). electrical conductivity is a measure of the risk of salinity, the excess of it reduces the osmotic activity and interferes with the uptake of water and nutrients from the soil to the plant, so while the higher the ec less water will be available to plants (nagaraju et al. 2006; joshi germán santacruz de león, josé alfredo ramos leal, janete moran ramírez, briseida lópez álvarez, eugenio eliseo santacruz de león. et al. 2009, ishaku et al., 2011, obiefuna and sheriff, 2011; nag and ghosh, 2013). ec values under 250 µmhos/cm at 25 °c in the irrigation water are considered excellent (c1) of 250-750 are classified as good (c2) of 750-2000 (c3) are admissible in 2000 3000 (c4) is considered to be doubtful use, so the water above 3000 µmhos/cm at 25 °c values (c5) are considered inadequate, since soil salinity tends to increase in proportion to the concentration of salts in the water that is irrigated (hamdy et al., 1993, sharma and rao 1998; perezsirvent et al., 2003), the concentration of salts in the soil can be two to six times corresponding to conductivity of irrigation water (hamdy et al., 1993; perezsirvent et al., 2003). in this study, the maximum and minimum values of ce (fig. 5a) were 3995 µmhos/cm at 25 °c and 49 µmhos/cm at 25 °c. 19 of 45 sampling points have lower values than 250 µmhos/cm at 25 °c, 18 points have values between 250-750, three point has classified as admissible water; the rest of the sampling points, 5 of 45 had higher values at 2000 µmhos/cm at 25 °c, ie, they are unsuitable for use in agriculture (fig. 5a) waters. in an area concerning tds, if these values are between 0-1000 mg/l it is considered non-salt fresh water if they are between 1000-3000 mg/l slightly saline, of 3000-10000 is considered moderately saline, values tds above 10000 mg/l are indicative of highly saline waters. the minimum and maximum values of tds for this study are 87 mg/l and 50,760 mg/l. about the 45 samples taken from the soconusco aquifer, 40 are classified as non-salt fresh water, 4 are classified as slightly saline, 1 is classified as moderately saline. it is important to remember that the presence of dissolved water in excessive amounts ions cause physical and chemical effects on the soil and plants (ahamed et al., 2013), reduces the ability of plant roots to extract water from the ground. there is a direct relationship between the presence of salts, measured from the ec, and the op. thus, as noted above, most of the near the aquifer under study olea (2013) found values ranging from 152 ec µmhos/cm, in wells near the city of huixtla, to 31,700 values µmhos/ cm wells or very close to the shoreline wells. ec values found in the soconusco aquifer, suggest that are mild problems of salinity, although the se and sp indicate that the water from this aquifer is generally of good quality. considering the above, it was deemed appropriate to classify the water according to op and the presence of tds. op values below 0.1 atm are indicators of some saline water with excellent quality, values between 0.1-0.3 are indicators of saline water with good quality, values of 0.3-0.7 indicates saline water with allowable quality water, point between 0.7-1.1 very saline water with growing problems and values greater than 1.10 atm op indicators are very saline water with significant problems. the results of this index for the soconusco aquifer show that 4 points has excellent water quality, 26 points have saline water of good quality; 9 are classified allowable salt water quality and other points (6 of 45) are classified as very saline water with major problems. figure 5. spatial distribution of the indices a) osmotic potential (op) vs electrical conductivity (ec), b) osmotic potential (op) and c) electrical conductivity (ec). samples water from wells in the soconusco aquifer are between nonsalt fresh water to considered non-salt fresh water (fig. 5c). the more realistically estimated danger of salinization, since it considers that the soluble salts of irrigation water are incorporated to soil, taking into account the precipitation of less soluble salts such as calcium and magnesium carbonates; as well as calcium sulphate (rodriguez et al., 2008, mancilla, 2012, barrios, 2014). similarly, the sp to estimate the risk that can generate the salts when the moisture content in soil is low, it is considered as one of the best estimates of the effect of salts on plants (rodriguez et al., 2008), considers the precipitation of less soluble salts such as chlorides and sulfates (nagaraju et al., 2006; mancilla, 2012; barrios, 2014), leading to increases in osmotic pressure and reduced crop production. quality indices of groundwater for agricultural use in the soconusco, chiapas, mexico. the maximum and minimum values of es obtained in this study were 30.04 meq/l (point 16) and 0.15 meq/l (point 20), respectively. of the 45 water samples taken in the soconusco aquifer, 37 es presented values lower than 3, which is the upper limit for classification as good quality water. six exploitation (items 9, 12, 14, 17, 29 and 30) have water classified as conditional (with es values above 3 but less than 15) and two harvests (points 16 and 35) are classified as not recommended water es with values greater than 15. with respect to the ps results indicate that the maximum value was 27.9 (item 16) and the minimum was 0.06 (point 34), the same limits used in the es are applied to classify water in respect to the ps; according to the above 42 of the 45 samples fall in the classification of good quality water; two (points 14 and 35) have conditioned water and one (point 16) is classified as not recommended (figs. 6a, 6b and 6c). figure 6. spatial distribution of the indices a) effective salinity (es) vs potential salinity (ps), b) potential salinity (ps) and c) effective salinity (es). classification water considering the% na, the sar and the ec the combination of sar and ec values for each of the sampling points were plotted in the diagram developed by the laboratory of the us salinity (fig. 7a), the chart allows classification of irrigation water. about the 45 samples collected in the in soconusco aquifer, 19 points are classified as c1s1 water, ie, low risk of salinity and low risk of sodification. eighteen of the 45 samples are classified as c2s1 (medium risk and low risk of salinity sodification). two samples (points 17 and 45) fall into the classification c3s1 and which are at high risk of salinity and low risk of sodification. four samples (points 18, 20, 21 and 23) are classified as c4s1, which should be only used on soils with good water permeability. the use of water from the soconusco aquifer is conditioned by the salinity levels rather than those of sodification. but also points 16 and 35 begin to show signs of high and very high risk of sodification. equal number of samples (points 18 and 16) are classified as doubtful inadequate, notably, 3 of the 45 samples taken from the soconusco aquifer are classified as inadequate (fig. 7b). it is worth remembering that there is no precedent studies on water quality to serve as reference. the % na is plotted together with the ec in the diagram wilcox and it classifies water for use in irrigation (fig. 7b). 36 of the samples collected fall into the classification of good to excellent, 2 are considered permissible to good, both fall into the category of admissible doubtful 124 germán santacruz de león, josé alfredo ramos leal, janete moran ramírez, briseida lópez álvarez, eugenio eliseo santacruz de león. the previous analysis pointed out that the problems associated with the aquifer sodicity soconusco are not considered and are presented in specific areas; however, the problems associated with salinity may increase, although they have not seen yet and there are no studies that indicate problems of soil salinization in the study region, which is due in great measure to the heavy rainfall, the soil conditions, climate and, above all, hydrogeochemical soconusco. presence of toxic ions (chloride and sodium) for plants some ions, depending on the concentration in which they are in the water, can be toxic to crops, usually they accumulate in leaves. often toxic chlorine ions, sodium and boron (ayers and wescot 1985. if the chloride concentration exceeds crop tolerance may result from the burning of leaves to tissue death, when the content of this element is less than 4 meq/l, the water is classified as good, if it is between 4-10 meq/l is classified as conditional, and if you have concentrations above 10 meq/l are classified as not recommended water. the maximum value found in this study was 21.5 meq/l (point 16) and the minimum was 0.03 meq/l, with a coefficient of variation of 11.22. of the 45 samples, 43 presented below 4 meq/l (fig. 8a) values, indicating that the soconusco aquifer presents no problems according to this chemical element and is classified as good ie may be used for irrigation. sodium often causes similar damage causing chlorides and nutritional imbalances in plants. values less than 3 meq/l of na are indicative of low toxicity hazard to plants of this; values 3-9 meq/l indicate medium risk and values of more than 9 meq/l are high risk indicators for sodium. the maximum and minimum values obtained in this sample were 16.6 meq/l and 0.07 meq/l, respectively, with a coefficient of variation of 5.6. in 38 of the 45 samples collected, sodium values obtained are less than 3 meq/l, ie it is endangered by water with low sodium. five samples (points 9, 12, 17, 29 and 30) have values between 3 and 9 meq/l, indicating water at average risk. two samples (points 16 and 35) have more than 9 meq/l values at high risk for sodium (fig. 8b). figure 7. water classification according to a) wilcox y) electrical conductivity (ec) vs percent sodium (% na), for the soconusco aquifer. conclusions geochemistry of groundwater of soconusco display the order of ionic abundance is na+>ca2+>mg2+>k+ and hco3 -> so4 2> cltrend. the type of water that predominate in the study area is ca-hco3 type. the main hydrochemical processes occurring in groundwater are dilution by mixing, water-rock interaction and only some samples show evaporation effects. wqi indicate that more than half samples are not suitable for drinking purposes (60% is very good quality). the groundwater of the soconusco aquifer has no problems of sodicity for agricultural use. the soil, climatic and physiographic conditions, as well as rainfall of around 1,500 to 4,000 mm/year in the region, favor the recharge of the aquifer and the adequate mechanisms for the natural drainage of the aquifer, avoiding the salinity of groundwater. the combination of ps/es, classifies most of the water as good for agricultural use, only some samples fall as conditioned and two samples as not recommended for this use. the combination of % na/ec, classifies the groundwater of the soconusco aquifer, as excellent, only some as admissible to doubtful and very few are classified as doubtful to inadequate. aquality indices of groundwater for agricultural use in the soconusco, chiapas, mexico. references aguilera, m. & martínez, r. 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(2014). preliminary study on irrigational quality of some ground water sources of kashmir, india. international journal of scientific & engineering research, 5(2), 318-323. weber, b.l., valencia, v.a., schaaf, p., pompa–mera, v., ruiz, j., 2008, significance of provenance ages from the chiapas massif complex (southeastern mexico): redefining the paleozoic basement of the maya block and its evolution in a peri–gondwanan realm: the journal of geology, 116, 619 639. wijnen, m., augeard, b., hiller, b., ward, c. & huntjens, p. (2012). 71742. managing the invisible: understanding and improving groundwater governance. washington, d. c.: banco mundial. informe preliminar. [en línea] retrived from: http://www-wds. worldbank.org/external/default/wdscontentserver/wdsp/ ib/2012/08/09/000333037_20120809002923/rendered/ pdf/717420wp0box370naging0the0invisible.pdf 10/07/2014 wilcox, l.v. (1955). classification and use of irrigation waters. circular 969, washington, d. c :united states deparment of agriculture. retrived from: https://ia601701.us.archive.org/23/items/ classificationus969wilc/classificationus969wilc.pdf yesilnacar, m.l. & gulluoglu, m.s. (2008). hydrochemical characteristics and the effects of irrigation on ground water qual-ity in harran plain, gap project, turkey. enviromental geology, 54(1), 183-196. 128 germán santacruz de león, josé alfredo ramos leal, janete moran ramírez, briseida lópez álvarez, eugenio eliseo santacruz de león. this work purposes a general mean velocity and a suspended sediment concentration (ssc) model to express distribution in every point of the cross section of turbulent shear flow by using a probability density function method. in order to solve turbulent flow and avoid multifarious dynamical mechanics, the probability density function method was used to describe the velocity and concentration profiles interacted on directly by fluid particles in turbulent shear flow. the velocity profile model was obtained by solving for the profile integral with the product of the laminar velocity and probability density, through adopting an exponential probability density function to express probability distribution of velocity alteration of a fluid particle in turbulent shear flow. an ssc profile model was also created following a method similar to the above and based on the schmidt diffusion equation. different velocity and ssc profiles were created while changing the parameters of the models. the models were verified by comparing the calculated results with traditional models. it was shown that the probability density function model was superior to log-law in predicting stream-wise velocity profiles in coastal currents; and the probability density function ssc profile model was superior to the rouse equation for predicting average ssc profiles in rivers and estuaries. outlooks for precision investigation are stated at the end of this article. abstract keywords: exponential probability density; mean velocity profile; concentration profile; river flow; coastal current mean velocity and suspended sediment concentration profile model of turbulent shear flow with probability density function velocidad media y modelo de perfil de concentración de sedimentos suspendidos de flujo turbulentos de cizalla turbulento con funciones de densidad de probabilidad. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.65172 este trabajo propone un modelo de velocidad media general y un modelo de concentración de sedimentos suspendidos (css) para expresar la distribución en cada punto de la sección de cruce del flujo turbulento de cizalla mediante el uso de funciones de densidad de probabilidad (pdf).. el método de funciones de densidad de probabilidad se usó para describir los perfiles velocidad y concentración que interactuaron directamente con partículas fluidas en el flujo de desprendimiento turbulento para resolver el flujo turbulento y evitar diferentes mecánicas dinámicas. el modelo del perfil de velocidad se obtuvo resolviendo el perfil integral con el producto de la velocidad laminar y la densidad de probabilidad, mediante la adopción de una función de desidad exponencial para expresar la probabilidad de distribución de la velocidad de alteración de la partícula de un fluido en un flujo de desprendimiento turbulento. también se creó un modelo de perfil css siguiendo un método similar al anterior y basado en la ecuación de difusión schmidt. se crearon diferentes perfiles de velocidad y css durante el cambio de parámetros de los modelos. los modelos se verificaron comparando los resultados calculados con los modelos tradicionales. se demostró que el pdf era superior a la ley logarítmica en la predicción de los perfiles de velocidad en corrientes costeras, y que la probabilidad del modelo del perfil de función de densidad ssc fue superior a la ecuación rouse para predecir perfiles ssc promedio en ríos y estuarios. las perspectivas para la investigación de precisión se indican al final de este artículo. resumen palabras clave: densidad de probabilidad exponencial, perfil de velocidad media, perfil de concentración, corriente de río, corriente costera. record manuscript received: 22/05/2017 accepted for publication: 23/08/2017 how to cite item wu, g., zhu, l., & li, f. (2017). mean velocity and suspended sediment concentration profile model of turbulent shear flow with probability density function. earth sciences research journal, 21(3), 129 134. doi: http://dx.doi.org/10.15446/esrj.v21n3.65172 h y d r o g e o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 129 134 guanglin wua, b, liangsheng zhua,*, fangcheng lib acollege of civil and transportation engineering, south china university of technology, guangzhou 510641, china bcollege of ocean engineering, guangdong ocean university, zhanjiang 524088, china *corresponding author. tel: +86 02087111030-3682; e-mail: lshzhu@scut.edu.cn (l. s. zhu) 130 guanglin wu, liangsheng zhu, fangcheng li introduction turbulence is much more complex than laminar flow. according to the boussinesq eddy viscosity assumption (boussinesq, 1877), the logarithmic law for velocity profile was solved by using the mixing length method (prandtl, 1925; khan et al., 2017; zawar et al., 2017). some vertical velocity distribution laws, such as the power law and the exponential law, are based on experimental considerations or semi-empirical theories (lin et al., 1953; deissler, 1954; afzal et al., 2007). other laws for turbulent velocity profile or concentration profile are based on an indirect turbulence model (bucci et al., 2008; nucci and fiorucci, 2011). logarithm law is the most popular one used to express the mean velocity in a cross section of turbulent flow for wind flow, river flow, and lake models. the logarithm law fits velocity and concentration distribution in the position far away from the interface in developed turbulent flow (wang and yu, 2007). some researchers have focused on mean velocity profiles of turbulent wall-bounded flows separately (poggi et al., 2002; buschmann and gad-el-hak, 2007; kempe et al., 2007; buschmann et al., 2009). different vertical velocity distribution laws are used to describe streamwise velocity structure in coastal currents (soulsby, 1980; anwar, 1996, 1998; de serio and mossa, 2014), in tidal channels (soulsby and dyer, 1981), and on the continental shelf (soulsby, 1983; de serio and mossa, 2010).vertical distribution of suspended sediment concentration (ssc) in rivers, estuaries, and coastal currents have been studied by many researchers through the years (rouse, 1937; taylor and dyer, 1977; li uet al., 2014).this work derives general formulas to describe the mean velocity and ssc distributions of turbulent shear flow to fit the whole cross section from the bottom boundary to the upper boundary, using a probability density function method. a brief review of classical methods for velocity profile of shear flow consider the laminar shear flow—the steady incompressible developed flow between two interfaces, with one interface moving with velocity u and the other being motionless. the flow can be solved based on 2-dimensional navier-stokes equations and expressed as (1) here h is the distance between two interfaces, μis dynamic viscosity, and d p/ d x is the pressure gradient along the x-direction. the velocity profile of shear flow can be simplified to eq. (2) when d p/ d x = 0 (2) logarithm law (prandtl, 1925), derived by the mixing length method, is the most popular equation used to express the mean velocity in a cross section of turbulent shear flow. the basic logarithm law of mean velocity profile of turbulent shear flow is (3) here, u* is the shear velocity of the fluid and y0 is hydrodynamic roughness length. but eq. (3) cannot fit select velocity profiles not full profiles because of the assumption that the mixing length is proportional to the distance away from the wall boundary. many instances and experiments have proven that the mixing length method can successfully predict velocity distributions far away from the interfaces, but it cannot predict velocity distribution near interfaces such as walls, seabed, gas-liquid surfaces, and so on. as far as we know, turbulent intensity should be dependent on the reynolds number, which is composed of the characteristic velocity, characteristic length, and kinematic viscosity. considering the mixing length method carefully, the core assumption is that the eddy viscosity is in direct proportion to the average velocity gradient and the mixing length is in direct proportion to the distance away from the wall. the method does not directly create the relationship between turbulent parameters and average velocity. exponential probability density model (epdm) for mean velocity profile of turbulent shear flow to solve for turbulent flow without closely associated defferent dynamical mechanics, a probability density function method is adopted to describe the pulsating intensity of velocity of fluid particles in this study. the pulsating intensity of every fluid particle in the profile will impact the whole profile. the effect decays at a distance from its original position and obeys a probability density function. exponential probability density function is often used to describe the decay degree distribution in many physical and chemical problems. in many situations, the exponential probability density function model is an appropriate method to approach the random decay phenomena. the primary expression of the exponential probability density function is (4) here, p(y) is the probability density of a fluid particle impacting any position, λ is the damping index, and y is the distance from the particle position yi to the impacting position y, defined by (5) as shown in fig. 1, the expression of exponential probability density function p1(y), represents the probability density that the velocity of laminar shear flow at position y1, u1, distributes to any position y due to turbulence. because the expression satisfies the boundary condition that p1(y1) = pm1 where y = y1 as shown in fig. 1, λ = pm, such as λ1= pm1, λ2 = pm2, and so on. hence, the expression pi(y) satisfies (6) figure 1. schematic diagram of exponential probability density for pulsating intensity transfer 131diffraction characteristics of small fault ahead of tunnel face in coal roadway the probability density of u, velocity of any position of y, distributes to y0, a certain position at the profile, is defined as p0 as shown in fig. 1, such as p01 and p02. we have (7) average turbulent velocity of a particular position, y0, produced by integrating the product of the local laminar velocity in eq. (7), which is solved with navier-stokes equations, and the exponential probability density function of pulsating intensity at the position, across all fluid particles in the profile, is shown as (8) here α is an adjustment. substituting u(y) with eq. (2) and p0(y) with eq. (7) in eq. (8), we get (9) eq. (9) cannot be analytically and fully solved. to solve eq. (8), we assume that (10) substituting eq. (2) and eq. (10) into eq. (8), the result is (11) integrating eq. (11), the result is (12) setting (13) and substituting y0 with y in eq.(12), the velocity profile of turbulent shear flow with this method (called pdfm) should be (14) assume that (15) eq. (14) can be calculated when the adjustable parameters, m and n, are given. the mean turbulent velocity in eq. (14) can be calculated by giving different m and n, as well as defining the dimensionless turbulent velocity and dimensionless position as (16) the calculated results are shown in fig. 2 and fig. 3. fig. 2 reveals that the mean velocity profile changes when the value of m is changed, but the mean velocity changes are weaker in the region near the wall or other interfaces than in the intermediate region while α=1. the dimensionless velocities are almost the same in the region when the value of y+ is between 0 and 0.25, but they change very evidently near the position where y+ is approximately equal to 0.8. thus, the upper mean velocity will increase along with an increasing value of m. the curve shape fits turbulent velocity profile when the value of m is lower in this case, such as m=0.25, 0.1 and 0.01. figure 2. mean velocity profiles with change of m (n= 0.25) figure 3. mean velocity profiles with change of n (m = 0.3) 132 guanglin wu, liangsheng zhu, fangcheng li de serio and mossa (2010, 2014) investigated vertical profiles of the stream-wise velocity of the coastal currents at the taranto (ta) and bari (ba) sites in italy. the comparison between the measurement data, the log law as shown in eq. (3), and the pdfm as shown in eq. (17) is plotted in fig. 4 at taranto for vertical stream-wise velocity profiles in shallow coastal currents. the calculated results show that the probability density function model (pdfm) more accurately reflects measurement data than the log-law model. ssc profiles model and its verification in river and estuary the rouse equation (rouse, 1937) was the most popular method used to calculate vertical concentration profiles of suspended load in rivers, lakes, estuarine and coastal currents, as well as on the continental shelf. the rouse formula was (18) in which ya is the thickness of the sheet-flow layer, ca is the concentration at ya which equals the bed-load concentration, z*=ω/κu* is the suspension index, and ω is the particle settling velocity. the similar expression of pdfm also can be created by the method below based on the schmidt diffusion equation (schmidt, 1925). (19a) that is (19b) in which ε is the particle diffusion coefficient, set as an assumption of the exponential probability density function (20) then (21) hence, (22) here γ is the suspension index of this model. the results of ssc profiles calculated on pdfm of eq. (22) are shown in fig. 5. the mean velocity profiles are distinct when the values of n are different, as shown in fig. 3, while α=1. the main tendency of the profile curves is shifting from the top left to the bottom right in fig. 3 along with the decreasing value of n. the tendency indicates that the parameter n has a closed relationship with turbulent intensity. verification of pdfm in coastal currents the mean velocity profile model derived in the previous section was applied to fit the investigated profiles to verify the effectiveness of the model. to use eq. (14) conveniently, the profile formula of pdfm should be transformed as (17) here β= αu/u*. figure 4. calculated and measured vertical profiles of the stream-wise velocity of coastal currents at taranto (survey data cited in de serio and mossa (2014)) 133diffraction characteristics of small fault ahead of tunnel face in coal roadway figure 5. calculated results of suspended sediment concentration profile model figure 5 shows that the slope of the profile curves is bigger near the surface than the bottom and the upper slope increases as the lower slope decrease when γ is increasing. the trend means that the particle settling velocity, particle diffusion coefficient and the water depth velocity influence γ to change the ssc distribution in the vertical cross section. the ssc is very small when γ=50, so the bed load is the dominant composition when γ≥50. the curves also show that the ssc at surfaces is greater than zero when γ≤2. on the contrary, the ssc at surfaces tends to zero when γ>2. so, this model provides an approach to avoid the surface limitation of rouse equation. zhang et al. (2007) investigated ssc of the yangtze river in detail and concluded a semi-empirical model of ssc. a comparison of predicting profiles between the pdfm (eq. 22) and the rouse equation [eq. (18)] with the measurements in the yangtze river is shown in fig. 6. as shown in the figure, the pdfm model used for this study more accurately reflects the measurement data than does the rouse equation. figure 6. calculated and measured vertical ssc profile in the yangtze river (survey data cited in zhang et al. (2007)) figure 7. calculated and measured vertical ssc profile in the yangtze estuary (survey data cited in liu, et al. (2014)) liu, yang, zhu, et al. (2014) investigated the average ssc profile in the yangtze estuary in detail. the survey data is cited here to verify the pdfm model, eq. (22). the comparison between the rouse equation, pdfm, and measurements is shown in fig. 7. the pdfm model of this study is also agreed more closely with the measurement data than does the rouse equation in the case of the estuarine area. discussions a model for solving velocity and concentration distribution of turbulent flow by using probabilistic methods is put forward in the article (khan et al., 2017; sultana et al., 2017). general models for describing the mean velocity and ssc profiles of a whole cross section in turbulent shear flow are derived based on the idea. in fact, the velocity profile formula of laminar shear flow is also the mean velocity profile solution of turbulent flow which ignores the turbulent items in reynolds equation, thus the pulsating intensity of fluid particles is apparently related to it. on the other hand, the pulsating intensity is also under the control of certain boundary conditions. the exponential probability density function may describe the pulsating intensity of fluid particles of the whole cross section correctly to study mean velocity and concentration distributions. the calculated results of the derived formulas are as diverse as the changing the damping index of exponential probability density function. an innovative approach may be found to solve complex turbulent flow by expanding on this concept. conclusions the pdfmmodel of this study possesses some advantage in the fields of predicting stream-wise velocity profiles of coastal currents and ssc profiles in river and estuaries as shown in the previous examples.further research is needed to validate the method through continued studies and carried forward to clarify and optimize the parameters of these models to apply for practical problems. for advanced research, the parameters should be studied meticulously, such as its relationship with dynamic parameters, particularly turbulent characteristic parameters. 134 guanglin wu, liangsheng zhu, fangcheng li acknowledgments this work was partially supported by the guangdong province science and technology research project (2010b030600007) and projects of enhancing school with innovation of guangdong ocean university (gdou2016050258 & gdou2017052503). references afzal, n., seena, a., & bushra, a. 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(2017). microfacies analysis and reservoir potential of sakesar limestone, nammal gorge (western salt range), upper indus basin, pakistan. pakistan journal of geology, 1(1), 12-17. zhang, r. j., xie, j. h., chen, & w. b. (2007). river dynamics (in chinese). wuhan univ. press, wuhan, hubei, p.r. china. given the difficulty of accurate online detection for massive data collecting real-timely in a strong noise environment during the complex geological mineral grade analysis process, an order self-learning arhmm (autoregressive hidden markov model) algorithm is proposed to carry out online outlier detection in the geological mineral grade analysis process. the algorithm utilizes ar model to fit the time series obtained from “online x ray fluorescent mineral analyzer” and makes use of hmm as a basic detection tool, which can avoid the deficiency of presetting the threshold in traditional detection methods. the structure of traditional bdt (brockwell-dahlhaus-trindade) algorithm is improved to be a double iterative structure in which iterative calculation from both time and order is applied respectively to update parameters of arhmm online. with the purpose of reducing the influence of outlier on parameter update of arhmm, the strategies of detection-before-update and detection-based-update are adopted, which also improve the robustness of the algorithm. subsequent simulation by model data and practical application verify the accuracy, robustness, and property of online detection of the algorithm. according to the result, it is obvious that new algorithm proposed in this paper is more suitable for outlier detection of mineral grade analysis data in geology and mineral processing. abstract keywords: arhmm; bdt; kicvc; outlier detection; online detection. online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process detección en tiempo real de valores atípicos sobre series de tiempo variable en arhmm mejorado durante el proceso de análisis de grado mineralógico issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.65215 jianjun zhaoa,b, junwu zhoubb, weixing suc*, fang liuc a school of information science & engineering, northeastern university, shenyang 110004, china; b beijing general research institute of mining & metallurgy, beijing 100160, china c school of computer science & software engineering, tianjin polytechnic university, tianjin 300387, china *email of corresponding author: 15900201597@163.com existe gran dificultad para la detección en tiempo real para series de datos masivos con altos niveles de ruido de valores atípicos. se propone un algoritmo de autoaprendizaje arhmm (modelo autoregresivo oculto de markov) para llevar a cabo la detección de dichos valores atípicos en el proceso de análisis del grado mineral geológico. el algoritmo usa un modelo ar para ajustar la serie de tiempo obtenida del “analizador de fluorescencia de rayos x” y hace uso del hmm como una herramienta básica de detección, la cual puede evitar la deficiencia de predeterminar el umbral en métodos tradicionales de detección. para actualizar los parámetros del arhmm en tiempo real, la estructura del algoritmo bdt (brockwell-dahlhaus-trindade) tradicional se mejora para ser una doble estructura iterativa en la que se aplica el cálculo iterativo en tiempo y en orden respectivamente. con el propósito de reducir la influencia de valores atípicos (o extremos) en la actualización del parámetro de arhmm, se adoptan las estrategias de detección-antes-que-actualización y la detección-basada-en-actualización, lo que también aumenta la robustez del algoritmo. la subsiguiente simulación por modelos de datos y aplicación práctica comprueba la precisión, fortaleza y capacidad de la detección en línea del algoritmo. de acuerdo con el resultado, es evidente que el nuevo algoritmo propuesto en este artículo es más apropiado para la detección de datos de valores atipicos para el análisis del grado mineral en geología y el procesamiento mineral. resumen palabras clave: modelo autoregresivo oculto de markov; detección en tiempo real; brockwelldahlhaus-trindade. record manuscript received: 21/02/2017 accepted for publication: 28/07/2017 how to cite item zhao, j., zhou, j., su, w., & liu, f. (2017). online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process. earth sciences research journal, 21(3), 135-139. doi: http://dx.doi.org/10.15446/esrj.v21n3.65215 g e o st at is t ic s earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 135 139 136 jianjun zhao, junwu zhou, weixing su, fang liu introduction mineral composition analysis is a key factor in determining whether or not to carry out mining. over the years, many scholars have proposed some new ideas and methods for accurate mineral grade assessment, many of which are based on chemical or physical test equipment to obtain the data for ingredient grade analysis (kameshwara, rao, & narayana, 2014; de'nan, naaim, & leong, 2017). therefore, the accuracy of the data used for ore composition analysis is critical to the ore grade analysis. at present, automated testing equipment is used in ore grade analysis, such as “box-a type on-stream x-ray fluorescence analyzer”, which uses spectral obtain by irradiating x-rays to the pulp to get the results of ore grade. it is worth noting that box-a type on-stream x-ray fluorescence analyzer by default is that the spectral data obtained is correct. but whether it is chemical or physical testing equipment are inevitably produced abnormal data. those outliers directly affect the analysis results of the mineral products analysiser (clarke, & levis, 1998; rivoirard, demange, & freulon, 2013). therefore, the detection and elimination of these abnormal data is the premise and key to the above ore grade analysis work. a new algorithm is proposed here to especially do outlier detection for ore inspection data which obtain from chemical or physical testing equipment. the algorithm utilizes ar model to fit the time series and makes use of hmm as a basic detection tool, which can avoid the deficiency of presetting the threshold in traditional detection methods. to update parameters of arhmm online, the structure of traditional bdt (brockwell-dahlhaus-trindade) algorithm is improved here, and a double iterative structure in which iterative calculation from both time and order is applied respectively. with the purpose of reducing the influence of outlier on parameter update of arhmm, the strategies of detection-before-update and detection-based-update are adopted, which also improve the robustness of the algorithm. subsequent simulation by model data and practical application verify the accuracy, robustness, and property of online detection of the algorithm. in this paper our innovations are shown as follow: 1. unlike other outlier detect method (such as the traditional ar model detection method), the outliers detect method proposed in this paper does not need to set the detection threshold. 2. considering the problem that the model order of chemical or physical testing equipment’s hard to be determined, the new detected method which is based on residual error has the function of model order self-learning. 3. in a view to avoid the influence of outliers on the test results, this paper proposes a detection-before-update and detection-based-update strategies. the predecessors’ achievements on outlier detection many good ideas and methods are put forward for the research of outliers detection problem, such as that barnett and lewis proposed an outliers detected method based on statistics in their word named ‘outlier in statistical data (barnett & lewis, 1994). outlier detection method based on distance is proposed by knorr and raymond (knorr & ng, 1999; edwin & raymond, 1998), an new detected method based on density is suggested by ramaswamy et al. (2000). but for ore inspection data, the detection methods based on distance, density or variance is a lack of feasibility since an online real-time detection method be needed for the ore test data. with the research of anomaly data detection technology, many new ideas and techniques are introduced, such as clustering analysis (almeida & barbosa, 2007) and neural network method (bullen, cornford, & nnbney, 2003; prakobphol, & zhan, 2008). but clustering analysis method is also not suitable for online outlier detection for extensive test data, and neural network method requires a lot of data to model learning. in 1995, ragaran and argrawal put forward the concept of "sequence anomaly" (han & micheline, 2001) and proposed the detection method based on deviation (takeuchi, & yamanishi, 2006). because this method needs to know the order of the model, it can not be directly used for the outlier detection of mineral grade analysis data. structure of double iteration in bdt to make the bdt algorithm can be calculated online, the improved bdt algorithm with double repetition structure is proposed in this paper. traditional bdt algorithm the traditional bdt algorithm is improved by levinson-durbin algorithm which is proposed by brockwell et al. (2002). for traditional bdt algorithm, using all the data to the iterative calculation of model order, in a view to obtain the order of the forward and backward ar model. xt, t =1,2,∙∙∙ is the test data waiting for detection, where xt is m -dimension vectors. so forward ar model can be express as equation 1. (1) in which, ɛk, (t) is forward residual under k order model, which obeys gauss distribution with zero means. ak (i) is the coefficient of forwarding ar model under k order model. so backward ar model can be written as equation 2. (2) with minimizing all data forward and backward residual as the target, so the generalized objective function written as equation 3 (trindade, 2003). (3) n is the number of the data. ω1 and ω2 are weighted coefficient matrix for forwarding and backward ar model, and in bdt algorithm, the values of ω1 and ω2 is 1. ἓk (t) and are estimated residual values for forward and backward ar model individually. (4) (5) in which, are m -dimension matrix. the traditional bdt algorithm can be written in full as followed: (6) (7) (8) (9) (10) (11) (12) (13) in equation 6 to equation 13, ûk and v̂k are estimated variance for forwarding and backward noise. the initial condition for traditional bdt algorithm are: (14) 137online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process (15) (16) the subscript ø express that when the initial iteration, the model order set is empty. • double iteration bdt algorithm the objective function of improved bdt algorithm also is equation 3. the dynamic performance of the algorithm is enhanced by the forgetting factor . the improved bdt algorithm has double loop structure which model order is inner loop and time is the outer loop. we set which is part of equation 6. so: (17) in equation 7, k a set maximum value for model order. considering the time-varying characteristics of the model parameters, the forgetting factor is added to the outer loop(time loop) updates. (18) (19) in equation 18, ri t is the mean of the covariance matrix for ἓi-l (t) and ἢi-l (t-i) in time . similarly, equation 13 can be rewritten as: (20) so the calculation process of double iteration algorithm is illustrated in figure 1. conditional probability. the other is expressed as observation probability matrix calculated by ar model. (21) in equation 21, n(•) is gauss function, and ʃ̂k is estimated the variance of gauss distribution. arhmm outlier detection algorithm also composed of two steps: one step-preliminary detection from equation 1, we can see that there is a deviation between estimated process data by ar model and real process data. (22) if the deviation ɛk (t) is only noise, it obeys gauss distribution. so the preliminary criteria for outlier detection are to determine the probability that the deviation follows gauss distribution. (23) in equation 23, st = 1 indicates that the real data detected is normal, st = 0 means it is the outlier. so the detection criteria can be expressed that: (24) in equation 23, the subscript p is the optimal model order calculated by kicvc criteria whose expression is: (25) in equation 25, is the mean of residual ɛk (t) under various model order (bilmes, 2006). (26) (27) two step-final detection in final detection, the result of preliminary detection is the observed value of hmm. so the final detection result obtained by viterbi algorithm (abd-krim, 2006): (28) for improved arhmm algorithm, when the data at t time is detected, the data before t time already is detected. so the traditional viterbi algorithm is request into: (29) (30) parameters updating by outlier the parameters of order self-learning arhmm algorithm need update online, and the parameters are estimated residual mean , state transition matrix a , and in improved bdt algorithm. specific update algorithm is as follows: figure 1. flow chart of double iteration algorithm implementation of order self-learning arhmm detection algorithm the traditional arhmm structure is composed of two parts (wang, & chiang, 2008): one is markov chain, which is expressed as initial state probability π and state transition matrix in which st is the state at time t, n is the total state for hmm, and is a in this research two support vector machines (svms) and a logic function were applied to identify calcareous sections automatically in wells located in the former barco concession, catatumbo basin colombia. during training stages the svms used nuclear logs, such as neutron, photoelectric factor and gamma ray in order to differentiate calcareous from clastic sections; additionally, in this stage the fractal dimension of resistive images along with mean and variance of resistivity acquired with imaging tool (of high resolution) are employed to identify textural features of the rocks. the first svm also employed in the training stage intervals manually interpreted of fossiliferous limestone, performed by a specialized geologist integrating core and logs information of a pilot well; during classification stage, this svm automatically recognized intervals with fossiliferous limestone by using only logs data of any well of the field. the second svm was also trained with nuclear logs, resistivity and fractal dimension, but in this case, with information of intervals composed of calcareous shales interbedded with limestone, recognizing automatically these rock associations during classification stage without interpretation of a geologist as input data. finally, a logic function was applied to intervals with photoelectric factor ≥ 4 and all sections not classified by the svms were grouped as laminated calcareous rocks. the svms and logic function show accuracy of 98.76 %, 94.02 % and 94.60 % respectively in five evaluated wells and can be applied to other wells in the field that have the same dataset conditions. this methodology is dependent of the data quality and all intervals affected by poor borehole conditions should be removed in order to avoid erratic interpretations. this model must to be recalibrated in case to be applied in other fields of the basin. abstract keywords: machine learning; support vector machines; borehole logs, image logs; fractal dimension; calcareous lithologies; catatumbo basin. automatic identification of calcareous lithologies using support vector machines, borehole logs and fractal dimension of borehole electrical imaging identificación automática de litologías calcáreas utilizando máquinas de vector de soporte, registros de pozo y dimensión fractal de imágenes eléctricas de pozo issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.68320 en esta investigación dos máquinas de vector de soporte (mvs) y una función lógica fueron aplicadas para identificar automáticamente secciones calcáreas en pozos ubicados en la antigua concesión barco, cuenca de catatumbo colombia. durante etapas de entrenamiento las mvs utilizaron registros nucleares, tales como neutrón, factor fotoeléctrico y rayos gamma para diferenciar secciones calcáreas de clásticas; adicionalmente, en esta etapa la dimensión fractal de las imágenes resistivas junto con la media y varianza de la resistividad adquirida con la herramienta de imágenes (de alta resolución), son empleadas para identificar rasgos texturales de las rocas. la primera mvs también empleó durante el entrenamiento intervalos manualmente interpretados de calizas fosilíferas, realizado por un geólogo especialista integrando información de núcleo y registro de un pozo piloto; posteriormente, durante la clasificación, esta mvs automáticamente reconoció intervalos con calizas fosilíferas utilizando solamente datos de registros de cualquier pozo del campo. la segunda mvs fue entrenada con registros nucleares, resistivos y dimensión fractal, pero en este caso, también con información de intervalos compuestos de lutitas calcáreas intercaladas con calizas, reconociendo automáticamente estas asociaciones de rocas durante la clasificación sin requerir interpretaciones de un geólogo como dato de entrada. finalmente, se aplicó una función lógica a intervalos con factor fotoeléctrico ≥ 4 y todas las secciones no clasificadas por las mvs fueron agrupadas como rocas calcáreas laminadas. las mvs y la función lógica mostraron precisiones de 98.76%, 94.02% y 94.60% respectivamente en cinco pozos evaluados y puede ser aplicado a otros pozos del campo que tengan igual conjunto de datos. esta metodología es dependiente de la calidad de los datos y todos los intervalos afectados por pobres condiciones del hoyo deben ser removidos para evitar interpretaciones erróneas. este modelo debe ser recalibrado en caso de ser aplicado en otros campos de la cuenca. resumen palabras clave: aprendizaje de máquinas; máquinas de vector de soporte; registros de pozo; registro de imágenes; dimensión fractal; litologías calcáreas; cuenca de catatumbo. record manuscript received:15/10/2017 accepted for publication: 16/02/2018 how to cite item leal, j. a., ochoa, l. h., contreras, c. c. (2018). automatic identification of calcareous lithologies using support vector machines, borehole logs and fractal dimension of borehole electrical imaging. earth sciences research journal, 22(2), 75-82. doi: http://dx.doi.org/10.15446/esrj.v22n2.68320 earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 75-82 jorge a. leal f.1, luis h. ochoa g.1, carmen c. contreras f.2 1 universidad nacional de colombia, departamento de geociencias, edificio manuel ancizar , bogotá d.c. colombia. jalealf@unal.edu.co, lhochoag@unal.edu.co 2principal geologist, pts-dfw. schlumberger technology, 3011 internet blvd. frisco tx 75034, u.s. ccontreras@slb.com g e o ph y si c s 76 jorge a. leal f., luis h. ochoa g., carmen c. contreras f. introduction machine learning algorithms have been broadly applied in geosciences in the latest years, most of them focus on searching natural patterns and samples classification in huge datasets. the support vector machines (svms) are part of supervised algorithms of machine learning methods with straightforward application and flexibility dealing with continuous and discrete data. in this opportunity svms have been applied to borehole logs of oil wells in the former barco concession, norte de santander department colombia (figure 1). the wells in the area were drilled in a mixed sequence, mainly of carbonates and clastic sedimentary rocks, where certain kind of limestones have been reported as hydrocarbon producer; in this research two svms were trained to identify automatically sections with fossiliferous limestones and intervals of calcareous shales interbedded with limestone; the remaining carbonates rocks, not classified by the svms, were grouped as carbonate laminated rock by applying a logic function. the objective of this model is to provide information to the well site geologist shortly after open hole logs are acquired, avoiding problems related to time consuming activities of data interpretation. the svms were developed with the free platform for data mining knime 2.7.4 (konstanz information miner, 2013) using data of a pilot well (pc-7), which has detailed image logs interpretation integrated with core-logs correlation; additionally, this well has the best coverage of the target formations in the field. the svms generated with pc-7 data were then applied to other five well of the field (named pc – 6, pc – 8, pc – 9, pc – 10 and pc 11). the set of logs were chosen because on one hand, nuclear logs provide information regard mineral composition and porosity of rocks; on the other hand, resistivity of imaging tool along with fractal dimension of the images provide information about textural features of logged formations (leal, 2014). the studied section in each well were rocks of aguardiente formation (uribante group), capacho formation (locally called cogollo formation in the area of old barco concession), and finally the data comprise the lower part of la luna formation (barrero et al., 2007). deltaic shales and sandstones that were deposited in a foreland basin. traps are wrench controlled and faulted anticlines resulted from strikeslip convergence. oil was sourced mainly from the upper cretaceous formations la luna and some intervals of capacho, as well as the lower cretaceous uribante group; oil generation began in the late eocene and continues through today (barrero et al., 2007). the sedimentary records of catatumbo basin began during syn-rift stage of cordillera oriental (branquet, et al., 2002) with normal faults controlling sedimentation. the basement consists of metamorphic rock of cambric – silurian ages according oil wells perforated in the area (sgc, 2015). from middle triassic to upper jurassic, the pangaea supercontinent starts its separation with an oceanic expansion zone between north america and gondwana, both continents presenting passive margins at each side. transform faults were produced in intra-continental zones, also producing graben structures along colombian territory (sgc, 2015). those grabens were filled with volcano-sedimentary rocks of la quinta formation, which is not cropping out in the area of old barco concession, but some tuffs have been recorded during perforation of wells. a marine transgression began at aptian times in the basin, starting sedimentation of tibu-mercedes formations and creation of the link between maracaibo and magdalena river basins (toussaint et al., 1995). this marine transgression was produced by a slow cooling lithosphere process allowing regional thermal subsidence (sgc, 2015). the catatumbo sequence studied in this research was deposited from aptian to coniaician, where succession of tectonic events along with right environmental conditions allowed sedimentation of several calcareous lithologies, resulting in a very rich petroleum system which have been producing hydrocarbon since the beginning of the last century. 2.1 aguardiente formation the aguardiente formation of lower cretaceous age was partially drilled in the six studied wells of the field (figure 2). in the area, this formation is composed of calcareous from fine to coarse-grained crossbedded glauconitic sandstone (notestein et al., 1944); in very low proportion beds and thin laminated carbonaceous-micaceous shales and thin beds of laminated limestones can be observed (ecopetrol, 2012). calcareous sandstones of this formation appear with light colors in resistive image (low conductivity), also shows low gamma rays reading and density mainly dominated by sandstone matrix. abundant glauconite and local concentrations of shell fragments indicate deposition in a marginal marine environment, where sediments rework was conducted by tidal or wave action. it is considered that this sequence may have been deposited in marginal marine environments representing an estuarine sedimentation in this part of the basin (aip, 2009). 2.2 capacho formation the capacho formation (upper cretaceous figure 2) is mainly composed of dark-gray to black shales, interbedded with fossiliferous limestones, laminated limestone and much less proportion fine-grained arenites and siltstones (ecopetrol, 2012). the upper section consists of limestone with bivalves (oysters fossiliferous limestones), while the rest of the section presents calcareous shales interbedded with limestone containing varying amounts of planktonic foraminifera, glauconite, calcium phosphate and organic matter. fossiliferous limestones appear with light color and high resistivity sections in borehole images; shales show high gamma rays reading with density matrix mainly related to clay minerals. total organic carbon reported for this formation is 2.1 % with hydrogen index of 350 mg hc/g and it is considered one of the source rocks in the basin (gonzales, et al., 2009). the rocks of lower part in this formation were deposited in less oxygenated deep marine zones, while rocks found in the top are associated with shallow marine deposits (aip, 2009). the capacho formation was totally drilled in the six study wells showing thickness from 253.59 m to 282.24 m. figure 1. catatumbo basin in northern colombia (purple line) south america and the study field (red square). 2. geological setting the catatumbo basin is limited by the serrania of perija and santander massif to west-southwest and merida andes to southeast (figure 1). the cretaceous rocks are limestones, shales and marine sandstones deposited in shallow sea environments that extended across northern venezuela and continued south through colombia to the south; cenozoic rocks are fluvialg e o ph y si c s 77automatic identification of calcareous lithologies using support vector machines, borehole logs and fractal dimension of borehole electrical imaging. 2.3 la luna formation the upper cretaceous la luna formation (figure 2) was partially logged in the wells of this research and it mainly consists of calcareous shale interbedded with limestone. the lithologies in this formation are composed of hard dark-gray abundantly foraminiferal limestones and hard black highly calcareous platy bituminous shales. bands and nodules of black chert are present in very minor amount, more numerous in the upper part. concretionary masses of dense gray limestone, ranging from few centimeters to 75 centimeters in size, are characteristic of the formation (notestein et al., 1944). this formation shows high/medium gamma rays reading with density matrix and photoelectric factor related to carbonate minerals. total organic carbon for this formation is 3.20 % with hydrogen index of 300 mg hc/g (gonzales, et al., 2009), and it is considered the main source rocks for the entire basin in colombia and venezuela (maracaibo basin). the textural features interpreted are indicating deposition in normal marine conditions, clearly offshore, but not in open marine environments (aip, 2009). there have been reported hydrocarbon production from naturally fractured intervals in la luna formation (notestein, et al., 1944), probably related to calcareous shale interbedded with limestone sections similar than observed in the six wells of this research. 3. data sets 3.1 gamma ray logs gamma ray logging is a kind of nuclear measure of natural radioactivity in formations and can be used to identify lithologies, correlating zones and determination shale (clay) volumes (track 2 figure 3). shale-free sandstones and carbonates have low concentrations of radioactive material and present low gamma ray readings. as shale content increases, the gamma ray response increases because of the concentration of radioactive material in shale. however, clean sandstone (i.e., with low shale content) might also produce a high gamma ray response if the sandstone contains radioactive minerals, or uranium-rich waters (asquith g. & krygowski, d., 2004). the gamma ray data in this research correspond to the total count of radioactive elements in the formation, without differentiating the th, u, k and other radioactive minerals content. 3.2 photoelectric factor the photoelectric factor is a continuous record of the effective photoelectric absorption cross section index or “pe” of a formation. the photoelectric absorption index is strongly dependent on the average atomic number, “z”, (i.e. atomic complexity) of the constituents of the formation, which implies the composition and by inference, the lithology. in the correct borehole environment, it can be used as quantitative indicator of lithology and certain diagenetic minerals. the use of this log is severely restricted by the fact that it is ineffective in holes with barite weighted mud, since the photoelectric absorption index for barite is nearly 150 times than most of the common minerals and when present will dominate the log response (rider, 2000); the logs of the wells in this research were acquired in free barite muds (track 3 figure 3). 3.3 neutron logs neutron logging (track 4 figure 3) is expressed in porosity units and it is based on nuclear principles that measure the hydrogen concentration in the empty space of the rocks. in clean formation (i.e., shale-free) where the porosity is filled with water or oil, the neutron log measures liquid-filled porosity. neutrons are created from a chemical source in the logging tool; when these neutrons collide with the nuclei of the formation they lose some of its energy. with enough collisions, the neutron is absorbed by a nucleus and a gamma ray is emitted. because the hydrogen atom is almost equal in mass to the neutron, maximum energy loss occurs when the neutron collide with a hydrogen atom. therefore, the energy loss is dominated by the formation hydrogen concentration (asquith g. & krygowski, d., 2004). 3.4 borehole electrical imaging the resistive image logs are acquired in wells without casing providing a two dimension image of the borehole with vertical resolution of 5.08 millimeters (track 5 figure 3). image logs allow sedimentary structures characterization, thin layers evaluation, electrofacies analysis and fractures identification. the images processing follows a standard sequence, beginning with data quality control of magnetometer and accelerometer measurements, which provide information about accurate position of all elements of the tool in space and therefore orientation of all handpicked geological features. then, corrections of velocity and acceleration that the tool had at moment of data acquisition are performed. finally, dynamic and static normalizations of resistivity data are applied; dynamically every foot in order to have a clear view of rock details and statically of whole logged interval optimizing tool operations under extreme resistivity. a color code is used to interpret resistive image logs, where light colors represent high resistivity and dark colors indicate low resistivity values. the most commonly observed geological events in image logs are well-defined planes which can be associated with bed boundaries, sedimentary structures, faults and fractures. these features are observed as sinusoids on image logs corresponding to the traces of planar events in the borehole. figure 2. stratigraphic column of catatumbo basin (compiled from barrero, et al., 2007). 78 jorge a. leal f., luis h. ochoa g., carmen c. contreras f. 3.5 mean and variance of resistivity pads the mean and variance of resistivity measured by image tool are calculated after image processing and can be seeing in the tracks 6 and 7 of the figure 3; the mean resistivity log “x̅” is generated from the arithmetic mean of resistivity measurements for each pads of the image tool in a specific depth, applying the equation (1). the figure 4 shows example with the sierpinski’s carpet and a section of resistive image, both figure with their regression and respective fractal dimension calculated with box counting method. where “x̅” represents the average resistivity log and “n” number of resistivity measurements of each pad at a specific depth “xi”. in other hand, the variance of resistivity pads “σ2” comes from the mean resistivity log using the equation (2). figure 3. typical log representation and kind of data sets used. figure 4. sierpinski carpet and section of borehole image with box counting method. 3.6 fractal dimension of borehole image in 1975 benoit mandelbrot named fractal (from latin fractus irregular), the set of forms normally generated by process of repetition characterized for having details in any observed scale (self-similarity), infinite length and fractional dimension; a dimension is fractal when the object occupies a space expressed by a fractional or decimal number (mandelbrot, 1983). the box counting method is widely used to find fractal dimension of diverse kind of images, where the studied figure is inserted in a box of side (r), then this box should be divided into four boxes with side “r/2” and the number of boxes covering any part of the figure “n(r)” must to be counted; following, resulted boxes are divided again into four boxes and the number of boxes “n(r)” containing any part of the figure must to be re-counted. this procedure is repeated, also counting the number of boxes with some part of the figure in order to plot the logarithm of the inverse size of boxes versus the logarithm of boxes with any part of the figure, or “|xj"=log (1/rj)|” and “yj = log (nj)”. finally, the slope "m" of the regression is the fractal dimension of the image obtained with the equation (3); the regression is the fractal dimension of the image obtained with the equation (3). the box counting method was applied to resistive image logs with dynamic normalization in format of bit mapped picture (bmp) with 512 pixels wide by 15378 pixels long, generating a fractal dimension curve along resistive image (track 8 figure 3), with sampling rate of 16.26 centimeters. the conversion of pixels to depth of images with vertical scale 1:5 is 512 pixels equal to 65.05 centimeters (leal, et al., 2016). 4. methods support vector machines & logic function applied a svm is a supervised classification technique that has received considerable attention during the latest years (tan, et al., 2006); this technique is based on statistical learning theory and has shown promising empirical results in many practical applications, including hand writing digit recognition, earthquake characterization (ochoa, et al, 2017), and other application related to pattern recognition. the svm also works very well with high-dimensional data and avoids the course of dimensionality problem. another unique aspect of this approach is that it represents the decision boundary using a subset of the training example, knows as the support vectors (tan, et al., 2006). the svm learning problem can be formulated as a convex optimization problem, in which efficient algorithms are available to find the global minimum of the objective function. other classification methods, such as rule-based classifier and artificial neural networks, employ a greedy-based strategy to search the hypothesis space (tan, et al., 2006), such methods in their basic approach tend to find only locally optimum solution and are not straightforward applicable to the data set used in this research. furthermore, svm performs capacity control by maximizing the margin of the decision boundary. nevertheless, the user must still provide other parameter such as the type of kernel function “k” to use and the cost function “c” for introducing each slack variable (tan, et al., 2006). the kernel function project a dataset in a space of specific characteristics and uses algorithms related to linear algebra, geometry and statistics to identify linear patterns in the dataset. any solution using kernel methods comprises two phases; first phase consists of a module that performs a mapping of the projected data; second phase contains an algorithm designed to detect linear patterns in the space where this data is projected (taylor & cristianini, 2004). the kind of kernel applied in this research was a normalized polynomial (equation 4). (1) (2) (3) 79automatic identification of calcareous lithologies using support vector machines, borehole logs and fractal dimension of borehole electrical imaging. figure 6. svm sketch for automatic identification of calcareous shale interbedded with limestone in training (a, left) and classification (b, right) stages. figure 7. flowchart of the model applied to identify calcareous lithologies in the study area. figure 5. svm sketch for automatic identification of fossiliferous limestones in training (a, left) and classification (b, right) stages. where "e" is a parameter representing the polynomial degree and "k" represents the kernel function depending on variables "x" and "y". in order to identify automatically intervals with fossiliferous limestone and calcareous shale interbedded with limestone two sub-models based on svms were trained. during training stage the first svm employs neutron, photoelectric factor and gamma rays logs; also in this stage is included mean and variance of resistivity measured for image tool, as well as fractal dimension of borehole image calculated with box counting. this svm is trained with interpreted interval composed of fossiliferous limestone of the pilot well, which has core-image log calibration. during classification stage only nuclear logs, mean and variance of resistivity tool and fractal dimension of borehole image are required to find an automatic indicator of intervals with fossiliferous limestone, which are important for their hydrocarbon production in some intervals of capacho formation. the figure 5 shows a sketch of this svm in training and classification stages. in a similar way, the second svm was trained with nuclear logs, image tool resistivity and fractal dimension of borehole images, but in this case, it was trained with interpreted intervals of calcareous shale interbedded with limestone of the pilot well (figure 6). during classification, information of calcareous shale interbedded with limestone is not required and then this svm automatically draws a flag where these lithologies are present, only using the data showed in the figure 6b. the task of this svm is important because fractured intervals of calcareous shale interbedded with limestone have been reported as hydrocarbon producer in la luna formation. the rest of calcareous rocks present in the studied sequence are easily recognize using a logic function, where all sections with photoelectric factor equal or higher than 4 are classified as laminated calcareous rocks, as long as they were not previously classified by the first svm nor the second. the figure 7 shows the model applied to identify calcareous lithologies in the study area and also relationship between the logic function and svms. 4.1 confusion matrices applied to evaluate the model the performance evaluation of a classification model is based on the counts of test records correctly classified, comparing with the interpretations of a geologist. these counts are organized in a table known as a confusion matrix. in this research each classification is considered as a two-class case with classes yes and no (“there are” or “there are not”), where each single classification has four different possible outcomes shown in the confusion matrix for a binary classification of table 1. (4) 80 jorge a. leal f., luis h. ochoa g., carmen c. contreras f. in order to evaluate the logic function and svms performance, confusion matrices were applied to the five analyzed wells. it is important to notice that data in bad hole condition was removed because of logging tools are affected along these intervals and their measures do not reflex real properties of the rocks. the sampling in confusion matrices was taken each 16.26 centimeters; therefore 1000 samples represent around 162.6 meters of length in the wells. the sampling rate of fractal dimension curves is also 16.26 centimeters (from the box counting processing), as well as it is the interpolated sampling rates of nuclear and resistivity logs data. 5. results and discussion according confusion matrix, the first svm applied to identify fossiliferous limestones presents an average accuracy of 98.8%, with best classification in the wells pc-8 and pc-9, counting 1671 samples correctly of 1688 (pc-8) and 1991 correct samples of 2011 for the well pc-9; the lower performance for this svm was in the well pc-6 with 98.5%, where 24 samples were wrongly classified of 1633, as shown in confusion matrices of table 2. in other hand, identification of interval with calcareous shale interbedded with limestone was performed by the second svm, having average accuracy of 94.0% and best execution in the well pc-6 with 96.8%, counting 1581 samples correctly of 1633; lower performance was in the well pc-10 with 90.5% where 179 samples were wrongly classified of 1889, as shown in table 3. finally, the logic function used to identify laminated calcareous rocks, has average accuracy of 94.6%, with best performance in the well pc-6 with 96.1%, counting 1569 samples correctly of 1633 and lower execution in the well pc-8 with 92.7%, where 123 samples were wrongly classified of 1688 (table 4). table 2. confusion matrix – intervals of fossiliferous limestone – 1st svm. table 3. confusion matrix – intervals of calcareous shale interbedded with limestone – 2nd svm. table 1. confusion matrix for a binary classification case. table 4. confusion matrix – intervals of laminated calcareous rocks – logic function. the true positives (tp) and true negatives (tn) are correct classifications. a false positive (fp) occurs when the outcome is incorrectly classified as yes (or positive) when it is actually no (negative). a false negative (fn) occurs when the outcome is incorrectly classified as negative when it is actually positive. the true positive rate is tp divided by the total number of positives, which is tp plus fn; the false positive rate is fp divided by the total number of negatives, (fp plus tn). the overall accuracy percentage is the number of correct classification divided by the total number of classification multiplied by 100% (equation 5), being the error 100 % minus this. figure 8. track 3 shows the model representation in a typical log template, calcareous shale interbedded with limestone flag is presented in a section of la luna formation and a contact between laminated calcareous rock and fossiliferous limestone is presented in capacho formation the final representation in a common log template is shown in track 3 of figure 8; the gamma rays and resistivity image are in track, 2 and 4 respectively. the figure 8a shows an interval composed of calcareous shale interbedded with limestone in la luna formation. the figure 8b shows example of fossiliferous limestone and laminated calcareous rocks. 5.1. fractal dimension impact on modeling performance in order to test the impact of the fractal dimension in the identification of calcareous lithologies, the same process was executed but omitting the fractal dimension curve. this test verifies the importance of the fractal dimension and the value of its generation using box counting method. the tables 5 and 6 show the model performance classifying intervals of fossiliferous limestone and calcareous shale interbedded with limestone (5) 81automatic identification of calcareous lithologies using support vector machines, borehole logs and fractal dimension of borehole electrical imaging. table 6. confusion matrix – intervals of calcareous shale interbedded with limestone – 2nd svm without fractal dimension. table 7. confusion matrix – intervals of laminated calcareous rocks – logic function from previous svms without fractal dimension. table 5. confusion matrix – intervals of fossiliferous limestone – 1st svm without fractal dimension. specifically after their processing but previous their interpretation. however, the svms of this research must to be recalibrated in case to be applied in other field of the basin. acknowledgments authors acknowledge to the national oil company of colombia (ecopetrol), for permission to use their data. authors also thank to ph.d. luis f. niño and the rest of his class of intelligent systems (2017 ii) for all advices provided, universidad nacional de colombia bogotá. references aip-asesoría en ingeniería de petróleos. (2009). caracterización de yacimientos, cuenca de catatumbo-colombia. bogotá, colombia. 6-18. asquith, g., & krygowski, d. (2004). basic well log analysis. aapg methods in exploration series 16, tulsa. the united states of america. 30-40. barrero, d., pardo, a., vargas. c, & martinez, j. (2007). colombian sedimentary basins: nomenclature, boundaries and petroleum geology, a new proposal. agencia nacional de hidrocarburos, bogotá, colombia, 32-60. http://www.anh.gov.co/informacion geologica-y-geofisica/cuencas-sedimentarias/documents/ colombian_sedimentary_basins.pdf branquet, y., cheilletz, a., cobbold, p. r., baby, p., laumonier, b., & giuliani, g. (2002). andean deformation and rift inversion, eastern edge of cordillera oriental (guateque-medina area), colombia. journal of south america earth sciences, 15(4) 391407. https://doi.org/10.1016/s0895-9811(02)00063-9 ecopetrol. (2012). informe técnico anual contractual año 2012. gerencia catatumbo orinoquia, vicepresidencia de producción, bogotá, colombia. 5-20. gonzales, m., mier, r., cruz, l., & vásquez, m. (2009). informe ejecutivo evaluación del potencial hidrocarburífero de las cuencas colombianas. contrato administrativo no. 2081941, fondo financiero de proyectos de desarrollo – universidad industrial de santander – agencia nacional de hidrocarburos, bucaramanga, colombia. 5-6. knime 2.7.4 (2013). the konstanz information miner. http://www. kanime.com (last accessed october 2017). leal, j. (2014). identificación y modelado de fracturas naturales en pozos de un yacimiento de hidrocarburos ubicado en la cuenca de catatumbo, departamento norte de santander – colombia, utilizando registros de imagénes resistivas y datos de dimensión fractal. m.sc. thesis, department of geoscience, universidad nacional de colombia, bogotá, colombia. http://www.bdigital. unal.edu.co/12849/ leal, j., ochoa, l. & garcía, j. (2016). identification of natural fractures using resistive image logs, fractal dimension and support vector machines. ingeniería e investigación, 36(3), 125-132. https://doi. org/10.15446/ing.investig.v36n3.56198 mandelbrot, b. (1983). the fractal geometry of nature. w. h. freeman and company, new york, the united states of america. 14-15. notestein, f., hubman, c., & bowler, j. (1944). geology of the barco concession, republic of colombia, south america. bulletin of the geological society of america, (55)1173-1183. ochoa, l. niño, l. & vargas, c. (2017). fast magnitude determination using a single seismological station record implementing machine learning techniques. geodesy and geodynamics, 1-8. https://doi. org/10.1016/j.geog.2017.03.010 rider, m. (2000). the geological interpretation of well logs second edition. rider – french consulting ltd, sutherland, the united kingdom. 126-128. sgc-servicio geológico colombiano. (2015). plancha 78 puerto santander. memoria explicativa. medellín, colombia. respectively. the average error increase from 1.24% to 38.88% for the 1st svm and from 5.98% to 62.42 % for the 2nd svm. the table 7 shows the confusion matrix for classification of laminated calcareous rock, applying the logic function to remaining sections not classified for the svms and photoelectric factor ≥ 4, in this case the error increased from 5.38% to 48.17%. 6. conclusions & recommendations the average accuracy of the model was 95.8% in five evaluated wells and it can be applied to recognize calcareous lithologies in this part of the basin, where the sequence is composed of mixture between clastic and carbonate; also their accuracy is higher enough to be automatically implemented during identification of intervals with capability to produce and storage hydrocarbon in the field, as in the case of fossiliferous limestones and calcareous shale interbedded with limestone, if this last is affected by natural fractures. the fractal dimension of resistive image logs along with nuclear and resistivity borehole measures can be applied to identify textural features of the formation. the average accuracy decreased from 95.8% to 50.2% when the whole model was execute without the fractal dimension curve, highlighting the contribution of this approach in textural rocks classification using borehole logs. this methodology can be applied to the rest of the wells that has the same dataset in the field, improving the sedimentological knowledge in areas without core information. for research projects in the future, the svms can be trained to identify sedimentary structures in borehole image of clastic rock, common in aguardiente and middle part of capacho formations. svms are algorithms of very simple implementation; therefore, the proposed model can be applied shortly after image logs are acquired, 82 jorge a. leal f., luis h. ochoa g., carmen c. contreras f. taylor, j., & cristianini, n. (2004). kernel methods for pattern analysis. cambridge university press, new york, the united states of america. 25-26. tan, p., steinbach, m., & kumar, v. (2006). introduction to data mining. pearson addison-wesley, boston, the united states of america. 256-276. toussaint, j. f. (1995). evolución geológica de colombia–triásico y jurásico. universidad nacional de colombia, medellín, colombia. (1) 94. keywords: combination analysis; urban agglomeration development zone; land resources; utilization potential; combination empowerment; land intensive. palabras clave: análisis de combinación; zona de desarrollo de aglomeraciones urbanas; recursos de la tierra; potencial de utilización; empoderamiento combinado; uso intensivo del suelo. how to cite item miao, j., & lu, q. (2020). an empirical study on the potential of land resources utilization in urban agglomeration development zones based on combination analysis. earth sciences research journal, 24(4), 439-447. doi: https:// doi.org/10.15446/esrj.v24n4.92069 to alleviate the contradiction between human and land caused by the limitation of land resources and the irreversibility of land use in the development zone of urban agglomeration, 24 indexes were selected from four aspects: land use status, resource security, economic feasibility, and social acceptability to establish the level evaluation index system for the sustainable use of land resources in baoding high-tech zone; also, the combination weighting method was used to determine the development level. the index weight of land resource utilization potential in the development zone was established. the subjective and objective linear combination was also established to obtain the proportion of subjective and objective. the multi-factor comprehensive evaluation model was used to evaluate the sustainable utilization degree of land in the high-tech zone. the comprehensive analysis of land-intensive use potential was made from five aspects: expansion potential, structure potential, intensity potential, management potential, and the number of years of land available; the analysis of the limiting factors and the combination of these factors of the land in the development zone was made to obtain the natural index after the renovation, and the natural quality potential of the cultivated land renovation was obtained according to the difference of the natural index before and after the renovation. the empirical results show that the high-tech zone’s land-use rationality index is relatively high during 2014-2018. during the evaluation period, the index continued to rise; the high-tech zone's resource security score was low, and there was no change during the evaluation period, which is basically at a low level. during the five years, the high-tech zone’s land-use sustainability has experienced the fluctuation process of first rising and then declining, which is in the stage of medium sustainable development. in the future, the potential for improvement is enormous. the prospect of the high-tech zone’s exploitable land structure is small, and the land structure is relatively reasonable, which is in line with the development orientation of the high-tech site. but the industrial land intensity in the zone also has a specific potential for excavation. the high-value areas of the natural quality potential of the land improvement in the high-tech site are mainly distributed in the west, northeast, and a few in the north and south. abstract an empirical study on the potential of land resources utilization in urban agglomeration development zones based on combination analysis estudio empírico sobre el potencial de utilización de los recursos de la tierra en una zona de desarrollo urbano con base en el método de análisis de combinación issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.92069 este estudio se realiza con el fin de conocer mejor el paradigma entre humanos y el uso de la tierra, mediado por la limitación de los recursos y la irreversibilidad de su uso en zonas de desarrollo de la aglomeración urbana. con este fin se seleccionaron 24 índices de cuatro aspectos: estado de uso de la tierra, seguridad de los recursos, viabilidad económica y aceptabilidad social, para establecer el sistema de índice de evaluación de nivel para el uso sostenible de los recursos de la tierra en la zona de alta tecnología de baoding; además, se utilizó el método de ponderación de combinación para determinar el nivel de desarrollo. se estableció el índice de ponderación del potencial de aprovechamiento de los recursos de la tierra en la zona de desarrollo, y se estableció la combinación lineal subjetiva y objetiva para obtener la proporción de subjetivo y objetivo. el grado de utilización sostenible de la tierra en la zona de alta tecnología se evaluó mediante el modelo de integral multifactorial. el análisis integral del potencial de uso intensivo del suelo se realizó a partir de cinco aspectos: potencial de expansión, potencial de estructura, potencial de intensidad, potencial de manejo y el número de años de tierra disponible. luego se realizó el análisis de los factores limitados y la combinación de los factores limitados del suelo en la zona de desarrollo para obtener el índice natural después de la renovación, y el potencial de calidad natural de la renovación del suelo cultivado se obtuvo según la diferencia del índice natural antes y después de la renovación. los resultados empíricos muestran que la racionalidad del estado de uso de la tierra de la zona de alta tecnología es relativamente alta durante 2014-2018. durante el período de evaluación, el índice siguió aumentando, el puntaje de seguridad de los recursos de la zona de alta tecnología fue bajo y no hubo cambios durante el período de evaluación, que básicamente se encuentra en un nivel bajo. durante los cinco años, la sostenibilidad del uso del suelo de la zona de alta tecnología experimentó el proceso de fluctuación de primero alza y luego declive, lo que se encuentra en la etapa de desarrollo sostenible medio, y en el futuro el potencial de mejora es grande; el potencial de la estructura de suelo explotable de la zona de alta tecnología es pequeño, y la estructura del suelo es relativamente razonable, lo cual está en línea con la orientación de desarrollo de la zona de alta tecnología, pero la intensidad de suelo industrial en la zona también tiene un cierto potencial de excavación. las áreas de alto valor del potencial de calidad natural del mejoramiento territorial en la zona de alta tecnología se distribuyen principalmente en el oeste, noreste y algunas en el norte y sur. resumen record manuscript received: 06/01/2019 accepted for publication: 07/07/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 439-447 jianjun miao, qian lu college of economics and management, nanjing university of aeronautics and astronautics, nanjing 210000, china *corresponding author: miaojianjun98@163.com; 18520204606@163.com en vi ro nm en ta l g eo lo g y https://doi.org/10.15446/esrj.v24n4.92069 https://doi.org/10.15446/esrj.v24n4.92069 https://doi.org/10.15446/esrj.v24n4.92069 mailto:miaojianjun98@163.com mailto:18520204606@163.com 440 jianjun miao, qian lu introduction since the establishment of the economic and technological urban agglomeration development zones in china during 1984, the cause of urban agglomeration development zone has been booming. remarkable achievements have been made in infrastructure construction, foreign capital, economic development, export expansion, talent cultivation, pilot reform, and service to the host city (wang & chen, 2016). it has also achieved good results in playing a window to the outside and radiation to the inside. however, there are too many problems in the construction of the development zone, mainly manifested in two aspects: the excessive establishment of the development zone and the rapid expansion of the development zone land, which increase the pressure of cultivated land protection. according to the regulations, the establishment of all kinds of development zones shall be subject to the twolevel examination and approval system of the state council and the people’s governments of provinces, autonomous regions, and municipalities directly under the central government. people’s governments at all levels below the provinces, autonomous territories, and municipalities directly under the central government shall not examine and approve the establishment of all kinds of development zones. some cities and counties set up all types of development zones without respecting regulations, resulting in severe waste and repeated land use (cao et al., 2016; dong et al., 2016). also, some provincial development zones set a low threshold for enterprises to enter and cannot realize the intensive use of land, which is also one reason for the rapid expansion of land use in the development zone. simultaneously, many development zones in the construction process, too much pursuit of “large and complete,” the land is going on the extensive development road of quantitative expansion. according to the ministry of land and resources, there are 6015 development zones (parks) in china, with a total planning area of 35400 square kilometers, equivalent to the entire construction land area of 660 cities and all construction towns in china. besides, a large amount of cultivated land, especially high-quality and high-yield cultivated land, is occupied in the development zone’s construction process, which causes tremendous pressure on the protection of cultivated land in china (li et al., 2017). blind expansion of development zones and extensive land use in the zones have severely damaged the development zones and urban construction, which has dramatically damaged the land resources already in short supply. therefore, evaluating the potential of land use in urban agglomeration development zone is the internal requirement of protecting cultivated land, saving urban construction investment, and strengthening land management (ding, 2019; price et al., 2019). in this paper, the index weight of land resource utilization potential in baoding high-tech development zone is determined using the combination weighting method. based on the index weight, the sustainable utilization level of land resources in the high-tech development zone is objectively evaluated by the multi-factor comprehensive evaluation model. the potential of landintensive utilization in five aspects of expansion potential, structural potential, and intensity potential are combined and analyzed. the potential of land can be changed through the development zone. the combination of unrestricted factors and restrictive factors can calculate the natural quality potential of land remediation in the high-tech zone and put forward the countermeasures for sustainable utilization of regional land, which provides an essential theoretical basis fore planning new villages in the high-tech area. basic definitions regional overview regional natural conditions: baoding’s high-tech zone is located in the ancient city of baoding, with flat terrain, an average elevation of 18.5m, and a slope of 1.0 ‰. the foundation’s bearing capacity is 18-20t per square meter, with a seismic fortification degree of 7. it belongs to a warm temperate continental semi-arid monsoon climate with four distinct seasons and mild weather. the annual average temperature is 12.3 °c; the coldest month is january, the average temperature is -4.2 °c, the warmest month is july, the average temperature is 26.7 °c, and the annual frost-free period is about 210 days. the precipitation is mainly from july to september, with an average yearly rainfall of 575mm. four distinct seasons, dry and windy in spring, hot and rainy in summer, cool in autumn, cold and little snow in winter. regional location: the central area, east area, and west area of baoding national high-tech industrial development zone are the primary areas. the central area is located in the north of baoding city, starting from torch road in the east, zhichang road in the south, xiangyang north road, luxin south road, luxin east road, luxin north road, jianshe north road in the west, and the north outer ring road in the north, with an area of 5.849km2. the eastern region is located in the east of baoding city, adjacent to expressway in the east, qiyi road in the south, baoding teachers college in the west and north, with an area of 0.185km2; the western region and the experimental site are located in baoding. in the west of the city, the west district area is 1.689km2, 800m from the flood control dike in the east to the west (li et al., 2017), 337 regiment family house in the west, jiangcheng road in the north. the experimental area is 2.890km2, from jianshe road in the east to the flood control dike in the west, jiangcheng road, baoshen railway, fuchang road in the south, kangzhuang road in the north. evaluation of land resource utilization potential based on combination weighting evaluation system the selected indicators should follow the principles of objectivity, scientifically, representativeness, comprehensiveness, dynamism, and operability to ensure the rationality of the evaluation results (arneth et al., 2018; song & deng, 2017). combined with the actual situation of land use in high-tech zone and the characteristics of sustainable land use in the county and referring to the evaluation standards of sustainable land use in china and abroad, an evaluation system of sustainable land use with the overall goal of sustainable land use in a high-tech zone is finally established, which includes four criteria levels: land-use level, resource security, economic feasibility, and social acceptability, as shown in table 1. index weight determination due to the subjective weighting method’s lack of objectivity, the weight determined by the objective weighting method will change with the environment’s change. this paper uses the subjective and objective combination method to determine the weight, the combined weighting method, to avoid the shortcomings of the subjective and objective weighting. the analytic hierarchy process (ahp) is a decision-making method that decomposes the decision-making elements into objectives, criteria, schemes, and other qualitative and quantitative analysis levels. in this paper, the ahp method is used to weight the indicators subjectively, and the results are shown in table 2. on this basis, the variation coefficient method is used to weight its indicators objectively, and the result of variation coefficient weighting is shown in table 3. the variation coefficient method directly uses the information contained in each index to calculate the weight of the index. the combination weighting is then carried out; that is, the subjective and objective linear combination is established to obtain the proportion of subjective and objective. the combination weighting value is also shown in table 3. evaluation model of land sustainable use degree factors comprehensive evaluation model and fuzzy comprehensive evaluation model are used to evaluate the sustainable use of land. because many factors affect the sustainable use of land, and applying a multi-factor comprehensive evaluation model is relatively mature, this paper uses a multifactor comprehensive evaluation model to evaluate the degree of sustainable land use in the high-tech zone (maranguit et al., 2017). suppose that there are n sub-objectives b t b t b tn1 2( ) ( ) … ( ), , , in the evaluation index system, and each sub-objective is composed of several indexes x ti ( ), where t is the year, and the constructor: a t w t b tj n j m ( ) = ( ) ( ) = ∑ 1 (1) 441an empirical study on the potential of land resources utilization in urban agglomeration development zones based on combination analysis calculation of the potential of intensive use of land in the high-tech zone (1) calculation of expansion potential the calculation formula of expansion potential is as follows: the area of available land = the land’s total area in the development zone the area of supplied state-owned construction land non-available land. (2) calculation of structural potential structural potential = area of built-up urban construction land × (ideal value of industrial land rate current value of industrial land rate). (3) calculation of strength potential strength potential = area of completed industrial and mining storage land × i i ii p i( ) / , where ii is the ideal value of comprehensive plot ratio of industrial land, building density of industrial land, input intensity of fixed assets of industrial land, output intensity of industrial land, and ip is the current value of index. b t p t x tn i i i n ( ) = ( ) ( ) = ∑ 1 (2) in formula 1, b tn ( ) is the annual evaluation index of the n sub-goal; a t( ) is the comprehensive evaluation value of annual sustainability; x ti ( ) is the standardized value of the i index, that is, the value of each index unified to dimensionless 0 ~ 1. in formula (2), p ti ( ) is the relative weight of the subgoal, and w tj ( ) is the weight of the total goal. it is necessary to set the land use potential level’s judgment standard to evaluate the degree of land use potential vividly and intuitively. according to some researches, the sustainable land use is divided into five grades, and the corresponding comprehensive evaluation values of each stage are < 0.25 (very low), 0.25-0.45 (low), 0.450.65 (medium), 0.65-0.85 (high), and > 0.85 (very high). table 1. criteria and indicators of sustainable land use evaluation system in high-tech zones criterion level land use status(b1) resource security(b2) economic feasibility(b3) social acceptability(b4) index level c11 land productivity (10,000 yuan/ha) c21 population density (person/ square kilometre) c31 per capita gdp (round) c41 per capita road length (km/ person) c12 proportion of effective irrigation area of cultivated land c22 per capita urban public green space area (square meters/person) c32 agricultural output value as a proportion of gdp c42 annual investment in education as a proportion of gdp c13 crop multiple cropping index c23 per capita arable land area (hectare/person) c33 the proportion of secondary industry output value to gdp c43 number of doctors per 10,000 people c14 the proportion of cultivated land c24 per capita food occupancy (kg/person) c34 proportion of output value of the tertiary industry in gdp c44 tap water penetration rate c12 proportion of effective irrigation area of cultivated land c25 annual reduction rate of cultivated land c35per capita net income of farmers (yuan) c45 growth rate of per capita net income of agricultural population c13 crop multiple cropping index c36 per capita gdp (10000 yuan/ha) c46 non-agricultural employment labor force ratio c14 the proportion of cultivated land c37 completion amount of fixed assets investment per unit area (10000 yuan / ha) c47 natural population growth rate c38 elasticity coefficient of land income growth table 2. subjective empowerment results index subjective weight value index subjective weight value c11 0.1560 c34 0.0074 c12 0.0593 c35 0.0108 c13 0.0928 c36 0.0324 c14 0.0288 c37 0.0228 c21 0.0166 c38 0.0442 c22 0.0237 c41 0.0190 c23 0.0971 c42 0.0316 c24 0.0641 c43 0.0939 c25 0.0368 c44 0.0470 c31 0.0156 c45 0.0686 c32 0.0035 c46 0.0097 c33 0.0050 c47 0.0134 table 3. objectively weighted and combined weighted index c11 c12 c13 c14 c21 c22 c23 c24 weighting of coefficient of variation 0.1097 0.0016 0.0033 0.0006 0.0012 0.0208 0.0012 0.0062 combined weight value 0.1328 0.0305 0.0481 0.0147 0.0089 0.0223 0.0492 0.0351 index c25 c31 c32 c33 c34 c35 c36 c37 weighting of coefficient of variation 0.1986 0.0609 0.0836 0.0347 0.0025 0.0307 0.038 0.0265 combined weight value 0.1177 0.0383 0.0435 0.0198 0.0050 0.0207 0.0352 0.0247 index c38 c41 c42 c43 c44 c45 c46 c47 weighting of coefficient of variation 0.1184 0.0541 0.0195 0.0208 0.0079 0.0673 0.0057 0.0864 combined weight value 0.0813 0.0365 0.0256 0.0573 0.0274 0.0679 0.0077 0.0499 442 jianjun miao, qian lu (4) measurement of management potential since the establishment of the high-tech development zone, the land management has been strictly controlled, and the idle land has been identified and disposed of in time (meng et al., 2018; houghton & nassikas, 2017; paulemercado et al., 2017). the land for paid use is generally supplied according to the legal maximum service life. at present, there is no idle land that has not been disposed of in the high-tech development zone, and there is no paid use of the expired land. (5) calculation of available years table 5 shows the calculation of the number of years that baoding hightech zone can provide land for expansion potential. years of land available i = expansion potential / annual average area of supply land in the previous three years; years of land available ii = expansion potential/area of supply land in the previous five years, maximum value. calculation of natural quality potential of land in the high-tech zone most of the traditional factor combination methods directly combine all factors of agricultural land classification. however, this method ignores the removability of factors, which leads to a large amount of calculation (yang et al., 2017), complex combination process, but also causes interference to the evaluation results. according to td/t1013-2013 code for acceptance of land remediation projects, adequate soil layer thickness, salinization degree, soil organic matter content, soil ph value, surface soil texture, drainage conditions, and irrigation assurance rate are the ranges for acceptance of cultivated land remediation quality. after the land remediation project, its properties can mutate, and other factors in the remediation process are all in funds. under the background of technology status and ecological environment protection, it is challenging to develop shortly; the environment is seriously damaged (chen et al., 2017) and not suitable for large-scale promotion. based on this theory, the authors could combine the land renovation factors to find the limiting factors. combination of modifiable factors according to the results of supplementary improvement of cultivated land quality in 2010 in the high-tech zone, the cultivated land quality grading factors in the zone include irrigation assurance rate, surface soil texture, soil organic matter content, and profile configuration, with 4, 3, 5 and 7 grading levels respectively (table 4), and three modifiable factors. corresponding codes express the score of modifiable factors of the agricultural land grading unit. the factor scores (10-100 points) correspond to 10 codes of 1, 2, 3, 4, 5, 6, 7, 8, 9 and + respectively. the three factors’ score codes are combined according to the order of grading factors in table 4 to be the combination type identification of modifiable factors of natural quality assessment unit. for example, if the combination type of modifiable factors in an evaluation unit is marked as “398”, it means that the irrigation assurance rate score of the unit is 30 points, corresponding to no irrigation conditions; the texture score of the surface soil is 90 points, corresponding to the surface soil is mainly clay; the quality score of the soil organic matter is 80 points, and the quality score of the corresponding organic matter is more than 2%. by analogy, all combination types of three factors of all evaluation units are obtained. combination of limiting factors because crop growth depends on the satisfaction degree of the living environment to its optimal growth environment factor value (marked by factor score in this paper), the small satisfaction degree is the main limiting factor affecting crop growth; and the main reason concerning the production capacity of agricultural land. the first limiting factor, i.e., the largest limiting factor, is the least satisfied, followed by the second limiting factor, and so on. according to this principle and grading standard, when the grading factor’s score is 50 or below, the factor will have a major limiting effect on crop growth. if the content of soil organic matter is less than 0.6% (i.e., the score of organic matter is 20), it is poor in nutrients; the texture of surface soil is gravelly soil, the salinization of soil above heavy level and the condition of no irrigation and drainage have seriously affected the cultivation of agricultural land. therefore, the factor scores between 10 and 50 (the identification code is 1, 2, 3, 4, and 5) are regarded as the limiting factors. the combination containing the limiting factors is selected from this combination of the modifiable factors to determine the type of the combination of the modifiable limiting factors, as the research object. calculation of natural quality potential according to the light and temperature production potential index and yield ratio coefficient of each designated crop, combined with the grading factor weight and factor index score of the selected crop, calculate the natural quality index before remediation following the regulations. then, combined with the study area’s actual situation, the qualitative and quantitative methods such as gis and remote sensing are comprehensively used to analyze the factors that can be transformed, and the degree of transformation. after remediation, the natural quality of cultivated land after remediation is reevaluated to obtain the index of natural quality. finally, the potential of the natural quality of cultivated land remediation is obtained according to the difference of natural index before and after remediation. the calculation formula is as follows: r a w fij tj j s k k k r j=         = = ∑ ∑ 1 1 10000/  (3) ′ = ′         = = ∑ ∑r a w fij tj j s k k k r j 1 1 10000/  (4) r r ri ij ij= ′  (5) among them, k is the number of grading factors; s is the number of designated crops; r is the number of grading factors; wk is the weight of the table 4. agricultural land grading and other factors in high-tech zones factor score factor score code irrigation guarantee rate topsoil texture soil organic matter content/% profile configuration 100 +(10) fully satisfy loam loam/clay/ soil 90 9 satisfy clay soil/sticky/ sticky, soil/ sand/soil, sand/sticky/ sticky 80 8 >2.0 stick/sand/ stick, whole body stick 70 7 general satisfaction sand sand / clay / sand 60 6 1.5~2.0 soil/sand/ sand, shallow viscous 50 5 1.0~1.5 stick/sand/ sand 40 4 0.6~1.0 sand and gravel 30 3 no irrigation conditions 20 2 <0.6 10 1 443an empirical study on the potential of land resources utilization in urban agglomeration development zones based on combination analysis k grading factors; j is the yield ratio coefficient of the j crop; atj is the light and temperature production potential index of the j crop; fk and ′fk are the index scores corresponding to the modifiable limiting factors before and after the treatment; rij, ′rij and ri respectively represent the natural quality indexes and potential of the j crops before and after treatment. results data preprocessing taking baoding national high-tech development zone as the research area, 400m×400m grid division is adopted. through the fishnet tool in arcgis software, the grid is created. after spatial connection and spatial superposition, the spatial relationship between the grid and multi-source data such as land resources, linear features is established to prepare for the grid computing of evaluation indexes. the research area is divided into 4338 grid units, as shown in figure 1. figure 1. grid unit for the quality potential evaluation of land resources in the high-tech zone evaluation results of land resource utilization potential of baoding hightech zone according to the data of baoding statistical yearbook (2014-2018) and statistics bureau of high-tech zone (2014-2018), combined with the evaluation model, the land sustainable utilization degree of the high-tech zone in 20142018 was calculated by levels, and the evaluation results are shown in table 5. on this basis, figure 2 was obtained. table 5. evaluation results of sustainable land use in high-tech zones from 2014 to 2018 criteria level evaluation particular year land use status resource security economic feasibility social acceptability comprehensive evaluation results 2014 0.48 0.21 0.34 0.42 0.43 2015 0.49 0.23 0.45 0.65 0.56 2016 0.71 0.21 0.55 0.57 0.61 2017 0.77 0.22 0.49 0.77 0.57 2018 0.81 0.21 0.46 0.61 0.51 figure 2. indicator schematic map of sustainable land use in high-tech zones from 2014 to 2018 in conclusion, the evaluation results of sustainable land use in the hightech zone are as follows: ① the rationality of land use in the area is relatively high, and the index continues to rise during the evaluation period, from 0.48 in 2014 to 0.81 in 2018, which is at a sustainable high level. ② in terms of resource security, the high-tech zone score is low, there is no change during the evaluation period, and it is basically at a low level. the main reason for the low resource security index is that the per capita urban public green space area (0.12 m2/person), per capita cultivated land area (0.034684 ha/person), per capita grain occupation (300 kg/person), and other indexes are far from the national standard value. ③ although the economic feasibility index has moved from a lower level to a medium level, it fluctuates greatly. after reaching a high of 0.55 in 2016, it shows a downward trend in the following two years. ④ the social acceptability is more reasonable. during the evaluation period, the index moved from the lower to the middle and higher levels, but its fluctuation is too large. ⑤ comprehensively, in the past five years, the sustainable degree of land use in the high-tech zone has experienced a fluctuating process of rising first and then declining, but it is basically in the stage of medium sustainable development, with great potential for future improvement. potential measurement results according to the calculation methods of expansion potential, intensity potential, structure potential, and years of land available, the current land resource potential data of baoding national high-tech development zone can be obtained as follows: 712.96 hm2 of urban construction land has been built in the high-tech development zone, and 65.91 hm2 of land has been supplied but not yet built. also, the area of non-construction land is 26.82 hm2. according to the calculation method of expansion potential, the available land in baoding high-tech development zone is 256.16 hm2. among them, according to the land use planning of high-tech development zone, 47.38 hm2 of land can be allocated, mainly for road, green space, public management, and public service; 208.78 hm2 for compensation, including 90.68 hm2 of land for industrial and mining storage; and 118.10 hm2 of another land can be used for compensation. according to the spatial distribution of the available land, it is mainly located in the third phase of the central area, the west area, and the experimental area, and all the land in the east area has been supplied. the completed industrial and mining storage land area in baoding national high-tech development zone is 427.01 hm2; the current value of the comprehensive plot ratio of industrial land is 0.60, and the ideal value is 0.65. according to the formula, the land area for improving the comprehensive plot ratio of industrial land is 32.85 hm2; the current value of industrial land construction coefficient is 41.88%, the ideal value is 45%, and the area of potential land that can be tapped by increasing the building density of industrial land is 29.61 hm2. the current value of industrial land fixed asset input intensity is 57.3557 million yuan/hm2, the ideal value is 58.5 million yuan/hm2, the area of potential land that can be tapped by increasing the input intensity of industrial land fixed assets is 8.21 hm2; the current value of industrial land output intensity is 12271. 380000 yuan/hm2, the ideal value is 123000000 yuan/hm2, and the area of potential land to increase the output 444 jianjun miao, qian lu intensity of industrial land is 0.99 hm2; the constructed urban construction land in baoding national high-tech development zone is 712.96 hm2, the current value of industrial land use rate is 59.89%, and the ideal value of industrial land use rate is 62%. according to the calculation method of expansion potential, through the adjustment of land structure, the area of industrial and mining storage land can be increased to 15.04 hm2; from 2016 to 2018, the average annual supply land of baoding high-tech zone in the first three years is 28.50 hm2, and the yearly average supply land of industrial and mining storage in the first three years is 7.60 hm2, from which it can be calculated that the number of available land years i is 8.99; and the number of available industrial and mining storage years i is 11.94. within the high-tech zone’s evaluation scope, the maximum area of land supplied in the first five years is 74.28 hm2, and the maximum area of the land provided for industrial and mining storage in the first five years is 16.19 hm2. therefore, it is calculated that the number of years of land available ii is 3.44 years, and the number of years of land available for industrial and mining storage ii is 5.60 years. combined with the above analysis, table 6 was made. table 6. estimation of the number of years available for the expansion potential of baoding national high-tech zone land-use type expansion potentialhm2 average annual supply land area in the first three years//hm2 maximum supply land area in the first five years// hm2 years of land available i//years years of land available ii//years remarks available land 256.16 28.50 74.28 8.99 3.44 including 69.74 hm2 completed rural construction land available for industrial and mining storage land 90.68 7.60 16.19 11.94 5.60 based on the current land resource potential data of baoding national high-tech development zone, the potential expansion calculation, potential strength calculation, potential structural calculation and available land year calculation, and the summary of land-intensive utilization potential were obtained, as shown in table 7. it can be seen from table 7 that the structure potential of the high-tech zone is relatively small, and the area of industrial and mining storage land can be increased by structural adjustment to 15.05hm2, which further proves that the structure of the land in the high-tech zone has been relatively reasonable and conforms to the development orientation of the high-tech site; the area of industrial and mining storage land that can be explored by improving the comprehensive plot ratio of industrial land and the building density of industrial land is 32.86 and 29.62hm2, respectively. it shows that the industrial land intensity in the zone also has an absolute mining potential; the industrial inputoutput power of intensive land use in the high-tech area has a small mining potential. by increasing the input intensity of industrial land fixed assets and industrial land output, the land volume for industrial and mining storage can be increased by 8.21hm2 and 0.99hm2, respectively. according to the calculation formula of expansion potential and baoding high-tech zone’s situation, the available land area of the evaluation scope is 256.16hm2, including 90.68hm2 of the available industrial and mining storage area. according to the calculation of the annual average supply land and the average yearly supply industrial and mining storage area of the previous three years, the actual available land number of baoding high-tech zone is 8.99 years, and the available industrial and mining storage area is determined comprehensively. according to the maximum land supply in the first five years and the maximum land supply for industrial and mining storage, it was calculated that the number of years that the high-tech zone can supply is 3.44 years, and the land supply for industrial and mining storage is 5.60 years. natural quality potential combination of modifiable limiting factors in theory, the combination type of modifiable factors of agricultural land classification unit in baoding national high-tech development zone should be 60 kinds of combination of three factors at different levels. through summary analysis, it is found that there are only 19 types of all factor combination of modifiable factors, among which eight types (3 + 6, 3 + 8, 375, 376, 378, 396, 398, 775) are qualified, and their distribution is shown in figure 3. it can be seen from figure 3 that the distribution of the combinations of the restrictions on the removability of agricultural land in the high-tech zone is relatively scattered. six of the eight combinations belong to the single restriction on the guaranteed rate of agricultural irrigation. one belongs to the single restriction on the content of soil organic matter (red area in the figure), and one belongs to the double restriction factors of “the guaranteed rate of irrigation + the content of soil organic matter.” the double restriction factors are concentrated in baizhigan town and dongcheng town. at the junction of fangzhen, the area is relatively small (purple area in the picture). therefore, it can be concluded that the natural conditions of the cultivated land in the hightech zone are mostly affected by the two limiting factors of irrigation guarantee rate and organic matter content. the irrigation guarantee rate is the leading limiting factor, and the whole area is affected by different degrees, so the land remediation project should focus on it. potential measurement and classification through the overlay analysis tool of arcmap, the overlay comparison analysis between the layers of the modifiable limiting factor unit and the non-limiting factor unit shows that 90% of the cultivated land in baizhigan town, dongchengfang town, shuangta street, yihezhuang are generally full, that is to say, the factor score code is 7. at the same time, the corresponding irrigation assurance rates of the other 11 towns are mostly basic. satisfied, that table 7. summary of land-intensive utilization potential of baoding national high-tech industrial development zone hm2 potential types potential constitution potential scale remarks expansion potential available land area 256.16 69.74 hm2 has been built into rural construction land among them: the area of warehousing land under available working conditions 90.68 structural potential adjustment of land use structure can increase the area of industrial and mining storage land 15.04 strength potential increasing the comprehensive volume ratio of industrial land and exploiting the potential land area 32.85 improving the building coefficient of industrial land and digging the potential land area 29.61 improving the intensity of investment in fixed assets of industrial land and exploiting potential land area 8.21 increasing the output intensity of industrial land and tapping potential land area 0.99 management potential land area used for compensation and expired but not disposed of 0.00 the area of idle land should be recovered 0.00 445an empirical study on the potential of land resources utilization in urban agglomeration development zones based on combination analysis is, the factor score code is 9, according to which, the score of limiting factors of irrigation assurance rate in the above four villages and towns is increased from code 3 to 7. that is, 1 level is increased, and 11 villages and towns such as songlindian are increased from 3 to 9, that is, two levels are increased. the distribution of limiting factors of organic matter is relatively centralized, the area is relatively small, and the organic matter content of the surrounding cultivated land is higher than 20%; that is, 1 level is higher. according to the calculation formula of natural quality potential, calculate the natural equivalent index and promotion potential of cultivated land in the whole area before and after the renovation. table 8 showed the combination type and natural equivalent index of some factors before and after the promotion. in table 8, the average natural equivalency index changed from 13782521 to 1851-2521. and the natural equivalency index increased by 174 on average, that is, 0.87 provincial natural classes. the national natural index increased by 211 on average, that is, 0.53 national natural grades. according to the classification method of natural discontinuities, the results are shown in figure 4. in figure 4, level 1 indicates the natural equal index’s potential range is 0-90, level 2 is 90-275, and level 3 is 275-473. the high-value areas of the natural quality potential of land remediation in the high-tech zone are mainly distributed in the west, northeast, and small in the north and south. the towns involved are dongchengfang town, wharf town, yihezhuang town, dongxianpo town, diaoweo town, linjiatun town, and haoguanzhuang town. in other areas, the promotion potential is relatively low and scattered, so it can be targeted to improve the old infrastructure such as irrigation and drainage, improve farmers’ awareness of green planting, encourage the use of organic fertilizers, and other measures. conclusions in this paper, the potential of land resource utilization in baoding hightech zone is evaluated by the method of the potential of land resource utilization based on the combination empowerment, and the potential of land intensive utilization is evaluated from the five aspects of the potential of expansion, structural potential and intensity potential of land resources in the high-tech zone. at the same time, the nature after renovation is obtained through the combination of the limited factors and the limited factors of land removability in the development zone. the analysis results show that from 2014 to 2018, the sustainable utilization index of land resources in baoding high-tech zone is fluctuating on the whole, but it is basically in the stage of medium sustainable development, with great potential for future improvement. from 0.43 in 2014 to 0.51 in 2018, the overall land utilization is in the stage of medium sustainable utilization. however, land use status has the greatest impact on the level of sustainable land use, followed by social acceptability, resource security and economic feasibility. the main reasons that restrict the improvement of the level of sustainable utilization of land resources are economic feasibility, social acceptability and resource security. these indicators are still at a medium or low level, especially the resource security is at a low level of development. therefore, in order to make a better sustainable use of land resources in the high-tech zone, the high-tech zone should start from the following aspects. (1) strengthen land consolidation and development and improve land use efficiency. although the land use situation of the high-tech zone is good, in order to make more scientific and rational use of land, we must make efforts to improve the basic conditions of agricultural production, increase the intensity of land reclamation and consolidation, increase the effective cultivated land area, and improve the efficiency of land production. (2) improve the resource security of land use and realize the transformation to the sustainable development mode. the evaluation result of land resource security of high-tech zone is still at a low level, which is not consistent with the figure 3. distribution of restrictive factor combination types table 8. combination of restrictive factors and natural index before and after renovation before rectification after renovation promotion factors combination of modifiable constraints natural equivalence index combination of modifiable constraints natural equivalence index 3+6 1577 7+6 1980 irrigation guarantee rate 3+6 1577 7+6 2190 irrigation guarantee rate 3+8 1642 7+8 2050 irrigation guarantee rate 3+8 1642 9+8 2255 irrigation guarantee rate 375 1642 775 1787 irrigation guarantee rate 375 1642 975 2040 irrigation guarantee rate 376 1443 776 1851 irrigation guarantee rate 376 1443 976 2088 irrigation guarantee rate 378 1507 778 1916 irrigation guarantee rate 378 1507 978 2120 irrigation guarantee rate 396 1779 796 2187 irrigation guarantee rate 396 1779 996 2392 irrigation guarantee rate 398 1843 798 2252 irrigation guarantee rate 398 1843 998 2456 irrigation guarantee rate 375 1642 376 1443 irrigation guarantee rate 775 1787 776 1851 irrigation guarantee rate 446 jianjun miao, qian lu level of social and economic development, which requires that in the process of economic development, high-tech zone should pay attention to improving the number of public green space and cultivated land area. (3) according to the regional characteristics, improve the level of economic development and social acceptability. formulate reasonable development strategies, change the industrial structure, increase the proportion of the second and third industries, increase the disposable income of residents, improve the land utilization rate, and promote economic development. at the same time, we should continue to improve the per capita net income of farmers, increase investment in education, improve infrastructure, increase medical and health facilities and the number of doctors, so as to improve social satisfaction. acknowledgments 1.research on measurement and reduction of undesirable output of national urban agglomeration (16bgl210) 2.empirical research on efficiency of urban land utilization base on desirable and undesirable output (15yja790046) references arneth, a., sitch, s., pongratz, j., stocker, b. d., ciais, p;., poulter, b., bayer, a. d., bondeau, a., calle, l., chini, l. p., gasser, t., fader, m., friedlingstein, p., kato, e., li., w., lindeskog, m., nabel, j. e. m. s., pugh, t. a. m., robertson, e., … zaehle, s. 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(2017). identifying separate impacts of climate and land use/cover change on hydrological processes in upper stream of heihe river, northwest china. hydrological processes, 31, 1100-1112. through a range of petrological techniques, the petrology, diagenesis, pore characteristics, and controlling factors on the regional variations of reservoir quality of the chang 7 sandstones were studied. these sandstones, mainly arkoses, lithic arkoses, and feldspathic litharenites, were deposited in a delta front and turbidites in semi-deep to deep lacustrine. the detrital constituents were controlled by the provenance and sedimentary condition, which resulted in a spatially variable composition; e.g., high biotite and feldspar contents in the northeast (ne) of the study area, and high contents of rock fragments, especially dolomite, matrix, and quartz in the southwest (sw). diagenesis includes intense mechanical compaction, cementation, and dissolution of unstable minerals. diagenetic minerals which were derived internally include quartz, ankerite, ferrous calcite, albite, illite, kaolinite, and chlorite. thus the original sandstone composition hadfirm control over the development and distribution of cement. mechanical compaction and late-stage cementations contribute to the porosity loss of sandstones of chang7 member. the dissolution porosity in major sandstone, slightly higher than primary porosity is principally dependent on the accessibility of acid fluid. the high content of plastic component facilitated the reduction of primary porosity and limited the mineral dissolution. the best reservoir sandstones are found in w, and partly from ne, m districts, with porosity are primary. the relatively high textural maturity of these sandstones reduces the impact of compaction on primary pores, and commonly existed chlorite rims limited the precipitation of pore filling quartz and carbonate cementation in late stage. abstract diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.72251 haihua zhu1,2*, guangcheng liu1,2, dakangzhong3, tingshan zhang1,2, jun lang1,2, jingliyao4 1stake key laboratory of oil and gas reservoir geology and exploitation, chengdu 610500,china 2school of geoscience and technology, southwest petroleum university, chengdu 610500, china 3school of earth science, china university of petroleum (beijing), beijing 102249, china 4exploration & development research institute of petrochina changqing oil field company, xi’an 710018, china *email of corresponding author:zhhswpu@163.com earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 129-138 a través de una gama de técnicas petrológicas se estudió la petrología, la diagénesis, las características de poro y los factores que controlan las variaciones regionales de la calidad del yacimiento de las areniscas chang 7. estas areniscas, principalmente arcosas, arcosas líticas y arenitas líticas feldespáticas, se depositaron en un frente deltáico mientras las turbiditas se depositaron en ambientes lacustres profundos y semi-profundos. los componentes detríticos dependen de la procedencia y la condición sedimentaria, lo que resultó en una composición variable en el espacio; por ejemplo, altos contenidos de biotita y feldespato en el noreste (ne) del área de estudio, y altos contenidos de fragmentos de roca, especialmente dolomita, matriz y cuarzo en el suroeste (sw). la diagénesis incluye compactación mecánica intensa, cementación y disolución de minerales inestables. los minerales diagenéticos que se derivaron internamente incluyen cuarzo, ankerita, calcita ferrosa, albita, illita, caolinita y clorita. por lo tanto, la composición original de arenisca tenía un firme control sobre el desarrollo y la distribución del cemento. la compactación mecánica y las cementaciones de la última etapa contribuyen a la pérdida de porosidad de las areniscas del miembro chang7. la porosidad de disolución en la piedra arenisca principal, ligeramente más alta que la porosidad primaria, depende principalmente de la accesibilidad del fluido ácido. el alto contenido de componentes plásticos facilitó la reducción de la porosidad primaria y limitó la disolución del mineral. las mejores areniscas de yacimiento se encuentran en los distritos occidental, y en parte de los distritos noroeste y medio, con niveles primarios de porosidad. la madurez textural relativamente alta de estas areniscas reduce el impacto de la compactación en los poros primarios, y los bordes de clorito comúnmente existentes limitaron la precipitación del cuarzo de llenado de poros y la cementación de carbonato en la etapa tardía. resumen how to cite item zhu, h., liu, g., zhong, d., zhang, t., lang, j., & yao, j. (2018). diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china. earth sciences research journal, 22(2), 129-138. doi:http://dx.doi.org/10.15446/esrj.v22n2.72251 controles diagenéticos en yacimientos de arenisca compacta: estudio de caso de las areniscas chang 7, en la formación yanchang, triásico superior, cuenca de ordos, china keywords: diagenesis; tight sandstone; reservoir quality; ordos basin palabras clave: diagénesis; arenisca apretada, calidad del yacimiento; cuenca de ordos se d im e n ta r y g e o l o g y record manuscript received: 14/11/2017 accepted for publication: 27/06/2018 130 haihua zhu; guangcheng liu, dakangzhong, tingshan zhang, jun lang, jingliyao, muhammad aqeel ashraf 1. introduction the tight oil produced from the upper triassic chang 7 lacustrine sandstone accounts for 15% of the total oil and gas resources in the ordos basin of china and is considered an essential resource for future exploration (zouet al.,2013; asghar et al., 2018).the commercial production of tight oil requires significant hydraulic fracturing, similarly to production in shale gas. however low permeability and high heterogeneity of these sandstone reservoirs due to multi-source and variable sedimentary conditions, mean that oil production varies remarkably among different plays; e.g., only 40% of wells in the changqing oilfield gain commercial oil flow rates during formation testing after conventional fracturing (duet al., 2014; nawaz et al., 2018). pore structure is critical for fluid accumulation and flows within reservoirs, and it controls the proportion of recoverable hydrocarbons as well as the results of hydraulic fracturing (shanley & cluff, 2015; indan et al., 2018). therefore, the prediction of reservoir quality and heterogeneity remains a crucial challenge for tight oil exploration and exploitation. porosity and permeability of sandstone reservoir quality is a function both of depositional conditions, which controlsfirst sediment composition, and diagenetic processes (ajdukiewiczet al., 2010; ozkanet al., 2011; harith et al., 2018). the methods of diagenesis in sedimentary basins are generally well understood (bjørlykke & jahren, 2010; bjørlykke & jahren, 2012; burley & worden, 2003; morad, 2009; worden & morad, 2009; khawaj et al., 2018). however, there is uncertainty regarding a) whether the geochemical system is open or closed during diagenesis in sedimentary basins (bjørkum & gjelsvik, 1988; bjørlykke & jahren, 2010; gluyas&coleman, 1992; sullivan et al., 1990; worden & barclay, 2000; usman et al., 2018), and b) the origin and formation process of authigenic minerals, including clay minerals, and carbonate and quartz cements in sandstone (barclay & worden, 2000; dowey, 2012;worden & morad, 2000). the chang 7 sandstone is primarily fine-grained with extremely low permeability (typically <0.3 x 10–3 um2). mass transport in the sandstone was limited during the period of deep burial. therefore the study of fine-grained sandstone helps to reveal the origins of authigenicminerals under relatively closed conditions. in this work, we address the following questions: (1) how did the original composition of the chang 7 sandstone control the species of authigenic minerals in ordos basin? (2) how did the diagenetic process influence the quality of the tight oil reservoir? 2. geological setting the research area is located in the southwest of the ordos basin, covering an area of over 50,000 km2, including the southern parts of the yishan slope and the tianhuan depression (figure 1). the triassic rocks in the study area include the liujiagou, heshanggou, zhifang, and yanchang formations, with the liujiagou formation being the lowermost unit. the upper triassic yanchang formation consists of 10 units from chang 10 at the base to chang1 at the top, which reflects the entire evolution of the basin from emergence to extinction (deng et al., 2011; naidu et al., 2018). the oil is generated mainly from the chang 73 shales (35~40m) which were deposited during the period of highest lake level and were stored in next turbiditeand delta-front sandstones of chang71~chang72 with very fine to fine grain size and low permeability (figure 2). figure 1. the study area in the southwest of ordos basin. the study area is divided into five districts with different provenances (yang et al., 2010) 131diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china figure 2. generalized stratigraphy of the 7th member (chang 7) of yanchang formation, ordos basin figure 4. qfr classification of chang 7 sandstone (after folk, 1968). the ternary diagram shows the various types of chang 7 sandstones with different sediment origins in different parts of the research area. figure 3. typical burial process of chang 7 member in the study area yang et al. (2010) split the study area into south(s), west (w) southwest (sw), northeast (ne), northwest (ne) and middle/central (m) areas with different provenances, according to the distribution of heavy and light mineral assemblages and the compositions of rock fragments. this scheme is adopted in the present study (figure 1). the sediments were derived mainly from the basic volcanic and metamorphic rocks of the yinshan upland in the northeast of the ordos basin, and from the dolomite and acid volcanic rocks of the qinlian upland in the southwest (xie, 2016; yang et al., 2011; barakat et al., 2018). the terrain slope of ne districts is gentle, and sandstones are mainly derived from braided river delta front, while in sw and m districts, sandstones deposited in meandering delta front and gravity flow are well developed(yang et al., 2010; sunny, 2018). the chang 7 section underwent rapid and continuous burial after initial deposition, with the maximum burial depth being ~4000 m in the sw of the basin and ~3000 m in the ne of the basin. subsequently, the section was uplifted to its present burial depth of 1500 m to 2500 m during the yanshan orogeny in the late cretaceous and the himalayan orogeny in the cenozoic (figure3). 3. samples and methods fifty samples collected from study area were impregnated with resin before thin sectioning for porosity identification. thin sections were stained with alizarin red s and k-ferricyanide for carbonate mineral determination. texture and composition of these sandstones were determined under a petrographic microscope. cathodoluminescence (cl) analysis of thin polished sections was carried out by a citl cold cathode luminescence 8200 mk5, excitation energy 20 kv, 400 ma. the textural studies of the cement in the sandstone samples were performed on polished blocks and fractured surfaces, coated with gold on fei quanta 200f scanning electron microscopy (sem) equipped with an energy dispersive spectrometer and operated under 15–20 kv.the analyses were performed at the state key laboratory of petroleum resource and prospecting, china university of petroleum (beijing). petrochina changqing oil field company provided additional point count data of 1493 thin sections from 361 wells, and helium porosity and permeability (6928 samples). the many thin section data provide an overall understanding of the distribution of detrital and cement minerals. 4. results 4.1 sandstone composition the chang 7 tight sandstone in the study area consists mainly of fine (0.125 mm) to very fine (0.065 mm) grained, moderate to well sorted, and poorly rounded arkoses, lithic arkoses, and feldspathic litharenites, according to the folk’s (1974) sandstone classification scheme (figure 4). point count data from 1513thin sections show that detrital quartz and feldspar are the most abundant minerals of both chang71 and chang 72 (table 1). 132 haihua zhu; guangcheng liu, dakangzhong, tingshan zhang, jun lang, jingliyao, muhammad aqeel ashraf quartz is the most abundant mineral with contents ranges from 25%~41% with an average of 36% in chang71, and 25%~40% with an average of 36% in chang 72. feldspar content of chang 71 is 19%~39%, averaging 24% and 20%~43%, averaging 25% in chang 72. lithic grains were made up mainly by eruptive rock, quartzite, phyllite, and dolomite, and to a lesser degree from slate, aphanite, schist and metasandstone, and a small amount of limestone, high grain rock. the total content of lithic grains of chang 71 ranges from 10%~23%, averaging 17%, and 12%~25%, averaging 18% of chang 72. lithic grains content of w, s, and sw districts are higher than that of ne and nw and are characterized by relatively abundant dolomite, phyllite, and quartzite. mica is familiar throughout the basin, with a content range from 5%~13%, averaging 6% of chang 71 and 3%~6%, averaging 5% of chang 72. the detrital composition shows little change vertically throughout the chang 7 unit, but spatial differences are observed. e.g., chang 71 sandstones from ne and nw districts are characterized by a relative higher content of feldspar and mica, and low quartz and lithic grains contrast with that of sandstones from s, w, and sw districts. and sandstones from chang72 show the similar distribution pattern of detrital. the matrix of the chang 7 sandstone is mainly fine-grained minerals, mostly clay minerals that fill interstitial spaces amongst the framework grains. matrix content of chang 71 ranges from 7%~11%, averaging 10%, and 5%~12%, averaging 10% of chang 72. the matrix contents in sw, s and m districts are higher than that in ne, nw and w districts (table 2). table 1. statistics of detrital components of chang 7 member sandstones table 2. statistics of matrix and cement composition of chang 7 member sandstone 4.2 diagenetic minerals the diagenetic minerals in chang 7 sandstone are mainly included fecalcite, ankerite, quartz followed by minor calcite, dolomite, albite, kaolin, and chlorite rims. total diagenetic minerals content of chang71 range from 4%~10%, averaging 7% and 4%~11%,averaging 7% of chang72. samples from ne, nw and m districts contain higher cement content than that of sandstones from w, s and sw area. 4.2.1 quartz cement quartz cement, which ranges in average abundance from 0.9% to 1% (average of 1%) in chang71 and mainly 0.8%~1.4% (average of 1.1%) in chang72, occurs as either euhedral quartz crystals that fill intergranular or dissolution pores or as syntaxial overgrowths around quartz grains. two generations of quartz cement can be distinguished. first generation quartz cement is covered by chlorite rims and filling primary pores (figure5a). second generation quartz cement grew over the chlorite rims and occurred as small euhedral or pyramidal quartz crystals filling primary and dissolution pores (figure5b). quartz cement iscommon in coarser sandstones with fewer matrix present. 133diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china figure 5. quartz cement in chang 7 sandstones. a: early quartz overgrowth been covered by and thus predate chlorite rim, well wu 62, 1827.06m. b: authigenic quartz in the intergranular pore, postdate chlorite rims in sem picture, well gao 38, 2125.46m. figure 7. fe-carbonate cement in chang 7 sandstones under thin section. (a): fe-calcite grows over quartz overgrowth, intergranular pores mostly filled by quartz and fe-calcite, well ning 31, 1654.47m. (b): ankerite occurs as overgrowth of dolomite grains and filling the intergranular residue pores after quartz cement and compaction, well li 45, 2276.02m. figure 6. calcite-ii partly or entirely replaces detrital grains and be engulfed by fe-calcite under polarizing microscope and cl figure 8. diagenetickaolin in chang 7 sandstones. a) kaolinite plates filling the primary pores, well hu 211, 2300.4m; b) kaolin with pseudo-hexagonal plates grows over and postdate by chlorite coating, well hu 211, 2300.4m. figure 9. chlorite rims on detrital (ch-1) and diagenetic quartz (ch-2) surface. the chlorite rims on authigenic quartz crystals are much thinner than that on the detrital grains. a) well gao18,1917.8m and b) well huan 54,2661.2m. 4.2.2 carbonate cement the amount of carbonate cement varies from 3% to 6%, averaging 4% in chang71 and chang72, and includes fe-calcite, ankerite, and minor calcite (~0.2%) and dolomite (generally <0.1%). calcite is stained red in thin section and bright yellow under cathodoluminescence (cl). two generations of calcite are recognized: calcite i fill large intergranular pores as poikilotopic crystals (>100um) between uncompacted detrital grains. calcite-ii partly or entirely replaces detrital grains and be engulfed by fe-calcite (figure 6). although the content of calcite cement is up to 40%, it is commonly absent and has a mean content of 0.2%. the fe-calcite is stained purple or blue-violet and dark orange under cl with content ranges from 2%~6%, averaging 2% in chang 71, and from 1%~4%, averaging 2% in chang 72. the fe-calcite is most abundant in the ne, nw districts ( > 3.5%) and decreases to < 2% in the sw, s and w districts. it usually occurs as microto coarse-crystalline (10~100μm) cement that partly or wholy fills isolated intergranular pores, engulfing quartz overgrowth, chlorite rims and calcite(figure 6, figure 7a). ankerite, stained wathlet blue under the thin section, is microto coarsecrystalline (<10~100μm) and generally occurs as overgrowths around dolomite grains, fe-calcite, and dolomite cement filling intergranular and dissolution pores (figure7b). contrary to the distribution of calcite, ankerite cement mainly occurs in samples from m, s, w, and sw districts with contents ranging from 1%~3% in chang71 and chang 72. and minor ankerite is found in ne and nw districts (0~1%). 4.2.3 clay cement clay cement identified in the studied sandstones include chlorite, kaolin, and illite. kaolin is mostly present in the samples from ne and nw districts with contents ranges from 1.9%~3.0% of chang 71 and 1.4%~3.3% of chang 72. little kaolin is found in samples from m, s and sw areas. it occurs as thin stacks of pseudo-hexagonal plates filling the primary pores and feldspars dissolution pores and partially entrapped by chlorite (figure 8). authigenic chlorite occurred in the chang 7 sandstone as two forms: grain rims and pore-filling. grain rims chlorite found in about 15% of total samples are thin (3-10μm) and discontinuous with crystals orient perpendicular to the surface of detrital and authigenic quartz crystals (figure 9). when present, it makes up <0.30% of the whole rock volume. the thickness of chlorite rims on authigenic quartz crystals is much thinner than that on the detrital grains. chlorite is often observed to wholly or partly completely replace biotite, and in places, its content can be up to ~20% of the whole rock volume. 134 haihua zhu; guangcheng liu, dakangzhong, tingshan zhang, jun lang, jingliyao, muhammad aqeel ashraf134 illite mineral occurs in every sample with three forms under sem: i) flaky and honeycomb-like crystal with spiny terminations tangentially coating detrital grains (figure 10a); ii) flaky filaments filling intergranular pores (figure 10b-e); and iii) fibrous, hair-like illiteoccurring as pore-bridging cement or filling dissolution pores (figure 10f). and most of the illite occurs as the first two forms. fibrous illite is rare in chang 7 sandstones. figure 10. the illite minerals with different crystal shapes of chang 7 sandstones. (a) flaky illites covering the detrital grains, well geng 292, 2481.01m; (b) flaky illites disorderly filling the intergranular pores, well geng 292,2481.01m; (c) flaky to hair-like illite partly filling the intergranular pores, well huan 54, 2650m; (d) honeycomb illite covering the detrital grains, well an72, 2237.43m; (e) illite with no significant crystalline morphology, well zhuang40, 1824.16m; (f) pore-bridging silkthread illite, well gao 38, 2125.46m. figure 11. plots of thin section porosity, core porosity versus depth 4.4. porosity and permeability average porosity of thin sections from different districts ranges from 1.7%~4.7% and averages 1.9% in chang 71 and from 0.7%~4.9%, averages 2.2%in chang72 (table 3). highest thin section porosities are found in samples from w, ne districts with porosity up to 4.9%. thin section and sem analyses show that there are three types of pores: intergranular pores, which make up a substantial amount of the total porosity (0.4%~3.5%, av.0.8% in chang 71 and 0.3%~3.3%, av.0.9% in chang 72); secondary pores (0.7%~1.3%, av.1.0% in chang 71 and 0.8%~1.4%, av.1.2% in chang 72) and micropores. secondary pores are mostly formed by the dissolution of feldspar and a small number of rock fragments and carbonate cement. the micropores with a pore radius of 0.5 μm or less (pittman, 1971; jefrin et al., 2018) can be of primary or secondary origin (loucks & dutton, 2007), and are associated with the clay minerals and weathered or altered rock fragments. although microporosity cannot be quantified in thin section, it is an important pore type of chang 7 sandstones. the scatter plot of thin section porosity, core plug porosity versus depths shows that the thin section porosity variation trends with depth are overall in agreement with that of core plug porosity (figure11). whereas the point count thin section porosity (mainly macropores) is significantly less than core plug porosity (0.9%~20.4%, av.9.3% in chang 71 and 01.4%~19.5%, av.9.6% in chang 72) indicating that abundant micropores in the chang 7 sandstones are not counted in thin section. 5. discussion 5.1. paragenetic sequence the main diagenetic processes affecting chang 7 sandstones include mechanical compaction, cementation of quartz, fe-calcite and ankerite and authigenesis of chlorite, kaolin, and illite. grain-coating clay formed on detrital grain surfaces in chang 7 sandstones occur as flaky texture, tangentially arranged around, and are typically originated by postdepositional infiltration (matlack et al., 1989). dominantly long and concavo-convex contacts combine with flexible grain deformation in chang 7 sandstones, suggests a moderate to intense mechanical compaction. in sandstones containing significant content of table 3. statistics of thin section porosity of chang 7 member sandstones (%) 135diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china figure 12. biotite alteration and associated products. biotite altered to chlorite and dolomite by releasing magnesium, well zhuang191, 2498.6m. figure 13. content scatterplots of (a) matrix with kaolin, (b) mica with fe-calcite, (c) feldspar with fe-calcite and (d) dolomite rock fragments with ankerite. the positive correlation of high-grade metamorphic rock fragments (hmrfs) versus chlorite contents reflects that the hmrfs could be the additional source of ions for the chlorite rims (figure13). diagenetic kaolin usually occurs in mesogenetic stage and mostly occurred in ne and nw districts where sandstones contain abundant feldspar. sem observations showed that kaolinite filling the dissolution and intergranular pores and, thus, likely formed by the alteration of detrital feldspars. the formation of kaolinite would require acidic ph condition and sufficient al and si. the acidic fluids may have been derived from adjacent shales during the maturation of organic matter (surdamet al., 1984). negative correlations between matrix content, mainly illite, versus kaolin and other cement contents suggesting that precipitation pores, which are closely related with matrix content of sandstone, are essential for diagenetic kaolin (figure 13a). ductile components, mechanical compaction is characterized by grain rearrangement and plastic grain deformation at relatively shallow depths (pittman and larese, 1991). quartz cement (>1%) are standard in chang 7 sandstones and have several sources (mcbride, 1989; worden and morad, 2000; hussin et al., 2017). the presence of syntaxial quartz overgrowth near the sites of intergranular dissolution and on tightly packed detrital quartz grains indicates a mesogenetic origin (mcbride, 1989; sibley & blatt, 1976; worden &morad, 2000; sunny et al., 2018). in chang 7 sandstones, concavo-convex contacts between detrital quartz are widely observed. it seems that in situ pressure solution provide materials for the bulk of the cement. no apparent correlations are found between average contents of detrital contents with quartz cement indicating that: 1) the silica sources, including the situ pressure solution of quartz, feldspar dissolution, and smectite illitization, exceed the demand for quartz precipitation in chang 7 sandstone; and 2) the available pore space for silica precipitate and temperature as well as ph condition are more important factors for quartz cement. diagenetic chlorite can have multi origins but typically forms at relativelyhigh temperatures (spoetlet al., 1994). chlorite may form by (i) the alteration of eogenetic, grain-coating berthierine or odinite (hillier & velde 1992; humphreys et al., 1989; kugler & mchugh, 1990), kaolinite and smectite precursor in deltaic deposits (ehrenberg, 1993; ryan, 1997; stokkendal et al., 2009); (ii) by the dissolution of feand mg-rich detrital grains (anjos et al., 2000; blackbourn & thomson, 2000; ros et al., 1994; remy, 1994; valloni et al., 1991). diagenetic chlorites in chang 7 sandstones were most probably formed by (i) massive biotite alteration, (ii) the transformation of precursor clay, and (iii) direct precipitation from pore fluids. petrographic examination showed that authigenic chlorite, especially pore filling chlorite, is often related to degradingbiotite (figure12) and, accordingly, high authigenic chlorite contents are associated with biotite-rich sandstones. the chlorite-coats grow both on detrital and diagenetic quartz surface with different thickness (figure 9), and when present on detrital grains, the chlorite coating is thicker than that of the diagenetic quartz, indicating that chlorite coating on diagenetic quartz is direct precipitates from pore water, while chlorite on detrital grain probably is firstly transformed from precursor clay coating and then continue to grow as thicker rims. the flaky and honeycomb-like illite is wide spreading in the chang 7 sandstones, indicating that illite contains smectite mixed-layer. the presence of grain coating, flaky and honeycomb-like illite crystals with sharp edges suggest a diagenetic origin (lemon & cubitt, 2003; moradet al., 2000). the extensive illitizaiton of smectite depend on high temperatures (90–130ºc) and sufficient k supply (bjørlykke, 1998; ehrenberg, 1993; moradet al., 2000; morad & de ros, 1994). the k and al ions needed for illite precipitation may have been derived from the feldspar dissolution. authigenic carbonates in chang7 sandstones occur as micro to coarse crystalline poikilotopic, pore-filling blocky and isolated pore-filling, indicating precipitation in different stages. the early calcite cement occupies an intergranular grain volume (igv) of ~40% and blocks large intergranular pores between loosely packed detrital grains indicating it occurred before intense mechanical compaction. the fe-carbonate cement typically occurs as isolated pore filling, engulfing and thus postdating quartz overgrowth, chlorite rims, and calcite cement and were mainly formed at moderate to a deeper depth and have a mesogenetic origin. homogenization temperatures of fluid inclusions reveal that the precipitation of fe-carbonate cements at temperatures of 70ºc~150ºc and mainly focused on 100ºc~110ºc(zhuet al., 2015). positive correlations between feldspar and mica versus fe-calcite (figure 13b-c) suggest that feldspar acid dissolution and mica alteration provided the bulk of fe and ca ions for late-stage fe-calcite precipitation. wang reported that light δ13c pdb values (-8.02‰ ~ -3.23‰) in fe-calcite (wang et al., 2007), indicating carbon of fe-calcite was most likely originated from the maturation of organic matter during burial process (morad, 1998). whereas evident correlation of diagenetic ankerite with dolomite rock fragments proposes dolomite grains is likely to be the primary source for ankerite cement (figure 13d). this is consistent with the heavier δ13c pdb values (-1.92‰ ~ -0.84‰) in ankerite, which indicating dolomite rock fragments may serve as the chief carbons source for later ankerite precipitation. the relative timing of diagenetic minerals of sandstones of the chang 7 member in the study area was inferred from the textural relationship as observed from thin section, sem and a paragenetic sequence is present in figure 14. 136 haihua zhu; guangcheng liu, dakangzhong, tingshan zhang, jun lang, jingliyao, muhammad aqeel ashraf figure 15. the plot of intergranular volume (%) versus cement (%) for the chang 7 member sandstones, ordos basin (after houseknecht, 1987) figure 17. scatter plots of chlorite contents (including pore filling chlorites and chlorite rims) versus primary porosity of chang 7 member sandstone, ordos basin figure 16. distribution of thin section porosity, primary porosity and dissolution porosity with the depth of chang 7member sandstones, ordos basin. authigenic minerals, especially illite, quartz and carbonate minerals occupy an average of ~6% rock volumes and also played an essential role in further reducing porosity. carbonate cement, mainly fe-calcite and ankerite, are the significant cement during the late stage, and these fill the residual intergranular and dissolution pores. the contribution of illite to the reduction in reservoir quality, by filling pore space and blocking pore throats (worden & morad, 2009), is more significant for fine sandstones than coarse sandstones (armitage et al., 2010). quartz overgrowth is avital reservoir quality reducing factor for many deep burial reservoirs (worden & morad, 2000). however, the influence of silica and albite cement on reservoir quality is limited due to their scarcity in chang7 sandstone. though the very low reservoir porosity, thin section porosity of most chang7 sandstones is still largely secondary (>50%) and thus is mainly controlled by the accessibility of acid fluid. acidic fluids generated from the maturation of organic matter within the underlie chang73 shales can directly these sandstones with well-preserved primary pore, from w, ne and m districts, are also characterized by chlorite coating and restively low quartz and carbonate cement (figure 17). it is considered that chlorite coating improves reservoir quality in most cases, especially in deeply buried sandstones (dowey, 2012; ehrenberg, 1993; tayloret al., 2010; worden & morad, 2009). the presence of chlorite coating inhibits quartz overgrowth, as well as carbonates cement by changing the grain surfaces to be oil wetting and preventing the access of water to detrital surfaces, thus stopping cementations (al-ramadan et al., 2004). figure 14. diagenetic sequences of sandstones of chang 7 members, ordos basin sandstone reservoir quality is a function both of the depositional conditions and diagenetic processes (ajdukiewicz et al., 2010). the effect of mechanical compaction on the chang 7 sandstones is evidenced by concavo-convex contacts and the deformation of micas and plastic grains. a plot of igv versus cement volume indicates that compaction contributed to the major primary porosity loss of chang 7 sandstones (figure 15). fine grain size, abundant mica, plastic grain as well as clay minerals, and rapid burial after deposition all contributed to compaction of the sandstone. migrate into the sandstones of chang 72, then chang71 generate generally upward decreasing of dissolution pores (table 3). no apparent correlation is found between feldspar content and dissolution porosity, but high content of plastic component, especially mica and clay minerals facilitate the reduction of primary porosity and permeability during compaction and make acid fluids accessing to sandstone difficult. sandstones from m districts, where thickest source rock developed, contain lower dissolution porosity than that of other areas due to the high matrix and mica contents. the best reservoir sandstones with relatively high core testing porosities (10%~20%)mainly distribute in w district and at depth ranges of 2300m–2800 m (figure 16). these sandstones porosities are primary and show relatively coarse-grained, lower matrix and well preserved primary pores. and thus reservoir quality is mainly dependent on the textural maturity of the sandstones, controlled principally by depositional conditions (tucker, 2003). 137diagenetic controls on reservoir quality of tight sandstone: a case study of the upper triassic yanchang formation chang 7 sandstones, ordos basin, china during diagenesis in sedimentary basins: constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs. aapg bulletin, 96(12), 2193-2214. blackbourn, g. a., & thomson, m. e. (2000). britannia field, uk north sea: petrographic constraints on lower cretaceous provenance, facies and the origin of slurry-flow deposits. petroleum geoscience, 6(4), 329-343. burley, s. d., & worden, r. h. (2003). sandstone diagenesis: recent and ancient. international association of sedimentologists. deng, x., fu, j., yao, j., pang, j., & sun, b. (2011). sedimentary facies of the middle-upper triassic yanchang formation in ordos basin and breakthrough in petroleum exploration. journal of palaeogeography, 13(4), 443-455.. dowey, p. (2012). prediction of clay minerals and grain-coatings in sandstone reservoirs utilising ancient examples and modern analogue studies. university of liverpool. ehrenberg, s. n. (1993). preservation of anomalously high porosity in deeply buried sandstones by grain-coating chlorite: examples from the norwegian continental shelf. aapg bulletin, 77(7), 1260-1286. gluyas, j., & coleman, m. (1992). material flux and porosity changes during sediment diagenesis. nature, 356(6364), 52-54. hillier, s., & velde, b. (1992). chlorite interstratified with a 7 å mineral: an example from offshore norway and possible implications for the interpretation of the composition of diagenetic chlorites. clay minerals, 27(4), 475-486. humphreys, b., & humphreys, s. a. s. (1989). the distribution and significance of sedimentary apatite in lower to middle devonian sediments east of plymouth sound. geoscience in south-west england, 7(2). kugler, r. l., & mchugh, a. (1990). regional diagenetic variation in norphlet sandstone: implications for reservoir quality and the origin of porosity. aapg bulletin, 74, 9(9). lemon, n. m., & cubitt, c. j. (2009). illite fluorescence microscopy: a new technique in the study of illite in the merrimelia formation, cooper basin, australia. clay mineral cements in sandstones. loucks, r. g., & dutton, s. p. (2007). importance of micropores in deeply buried tertiary sandstones along the texas gulf coast. in: american association of petroleum geologists annual convention abstracts, 84. matlack, k. s., houseknecht, d. w., applin, k. r. (1989). emplacement of clay into sand by infiltration. journal of sedimentary petrology, 59(1), 77-87. mcbride, e. f. (1989). quartz cement in sandstones: a review. earthscience reviews, 26(1-3), 69-112. morad, s. (1998). carbonate cementation in sandstones: controls by patterns of fluid flow and psycho-chemical, environmental and climatic conditions. wiley-blackwell. morad, s. (2009). carbonate cementation in sandstones: distribution patterns and geochemical evolution. blackwell publishing ltd. morad, s., & de ros, l.f. (1994). geochemistry and diagenesis of strata bound calcite cement layers within the rannoch formation of the brent group, murchison field, north viking graben (northern north sea), discussion and reply. sedimentary geology, 93, 135–147. morad, s., ketzer, j. m., & ros, l. f. d. (2000). spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. sedimentology, 47(s1), 95-120. needham, s.j., 2008. experimental production of clay rims by macrobiotic sediment ingestion and excretion processes[j]. journal of sedimentary research., 75(6), 1028-1037. ozkan, a., cumella, s. p., milliken, k. l., & laubach, s. e. (2011). prediction of lithofacies and reservoir quality using well logs, 6 conclusions (1) the upper triassic chang 7 sandstones are essential tight oil reservoirs with average porosity of 10.1% and average permeability of 0.18 × 103 μm2. they were deposited as delta front and semi-deep to deep lacustrine facies, producing a fine-grained, well-sorted, poorly rounded sandstone with a high matrix content. the detrital components were controlled by sediment provenance and are regionally distributed, with high biotite and feldspar contents in the ne and nw districts, and high contents of dolomite rock fragments and quartz in the sw, s, w districts. (2) the diagenetic evolution of the chang 7 sandstone is characterized by intense mechanical compaction and low cement content, especially low contents of quartz cement. the major authigenic minerals originated mainly from internal detrital alteration of sandstones and included quartz, ankerite, fe-calcite, albite, illite, kaolinite, and chlorite. the composition of the original sandstone was the central control on the development of cement. (3) the dissolution of feldspar is common despite intense mechanical compaction and is principally dependent on the accessibility of acid fluid. the best reservoir sandstones are found in w, and partly from ne, m districts, with porosity are primary. the relatively high textural maturity of these sandstones reduces the impact of compaction on primary pores, and commonly existed chlorite rims limited the precipitation of pore filling quartz and carbonate cementation in late stage. acknowledgments we acknowledge petrochina changqing oil field company for providing data including the results of thin section and core analyses. we are also grateful to the state key laboratory of petroleum resource and prospecting at china university of petroleum (beijing) for providing testing equipment. this work was supported by a general financial grant from the china postdoctoral science foundation (2015m580797) and by the young scholars development fund of southwest petroleum university, chengdu, china (201499010046). reference ajdukiewicz, j. m., nicholson, p. h., & esch, w.l. 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(2011). natural gas exploration domains in ordovician marine carbonates, ordos basin. acta petrolei sinica, 32(5), 733-740. zou, c. n., yang, z., tao, s. z., yuan, x. j., zhu, r. k., hou, l. h., wu, s. t., sun, l., zhang, g. s., bai, b., wang, l., gao, x. h., & pang, z. l. (2013). continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum: the ordos basin, north-central china. earth-science reviews, 126, 358-369. palabras clave: lago rift, composición química, miembros extremos, análisis multivariado, percepción remota. how to cite item noyola-medrano, c., ramos-leal, j. a., lopezalvarez, b., moran-ramirez, j., & fuentesrivas, r. m. (2019). assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical analysis, hydrogeochemical modeling and remote sensing. earth sciences research journal, 23(1), 43-55. doi: https://doi.org/10.15446/esrj.v23n1.66429 the chemical composition of a freshwater surface depends on the sources of input that can be of natural or anthropogenic origin. this study examined the spatial variability of the water quality from chapala lake and discussed the possible sources to this freshwater surface which is the largest rift lake in mexico. the methodology included multivariate statistical techniques to analyze the possible relationship between water quality and the natural and anthropogenic factors of the area. the outcome showed the existence of four groups of water mixture in the lake. each one showed two or three end members that characterized its chemical nature. the different groups showed a spatial distribution and a particular spectral behavior was detected from the analysis of a landsat 7 etm+ image. the spectral signatures extracted from the satellite image showed a high reflectivity in the range of 830 1300 nm for the water provided by the lerma-chapala (group iv) system, which represents the most polluted water in the lake. this spectral behavior is not present in the rest of the group. the western portion of the lake is fed mainly by open water. contributions from groundwater dominate the central part, and in the eastern region, the primary source is the contaminated water from the lerma river system. a high hydraulic head (hydraulic barrier) prevents the highest level of pollution from the lerma river. this hydraulic barrier constituted by a significant portion of groundwater is at the central part of the lake. furthermore, the most polluted water flows directly to the metropolitan region of guadalajara through the santiago river. the predominant role played by the groundwater in the dynamics of chapala lake is explained, in part, from its tectonic origin. the dynamics of water in the chapala lake suggest the need to implement some management plans considering the tectonic origin of the lake as an advantage for the control of pollution because of the significant contribution of groundwater in the chapala lake freshwater system. abstract keywords: rift lake, chemical composition, end members, multivariate analysis, remote sensing. assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing evaluación del origen y distribución espacial de la composición química en un lago tipo rift, utilizando modelación estadística multivariada, hidrogeoquímica y teledetección issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.66429 la composición química de una superficie de agua dulce depende de las fuentes de entrada que pueden ser de origen natural o antropogénico. este estudio examinó la variabilidad espacial de la calidad del agua del lago de chapala y discute las posibles fuentes de contribución a esta superficie de agua dulce que es el lago rift más grande de méxico. la metodología incluyó técnicas estadísticas multivariadas y análisis de imágenes satelitales, para analizar la posible relación entre la calidad del agua y los factores naturales y antropogénicos de la zona. los resultados indicaron la existencia de cuatro grupos de mezclas de agua en el lago. cada grupo muestra dos o tres miembros extremos que caracterizan la naturaleza química del agua. los diferentes grupos presentan una distribución espacial y un comportamiento espectral particular detectado a partir del análisis de una imagen landsat 7 etm+. las firmas espectrales extraídas de la imagen satelital muestran una alta reflectividad en el rango de 830-1300 nm para el agua proporcionada por el sistema lermachapala (grupo iv), que representa el agua más contaminada del lago. este comportamiento espectral no se presenta en el resto de los grupos. los datos obtenidos muestran que la parte occidental del lago es alimentada principalmente por agua superficial. los aportes de agua subterránea dominan la parte central, y en la región oriental, la principal fuente es el agua contaminada del sistema lerma-chapala. la mayor contaminación proviene del río lerma, pero ésta se rompe por una barrera hidráulica constituida por aportes considerables de agua subterránea hacia la región central. por eso, el agua más contaminada sigue directamente a la región metropolitana de guadalajara a través del río santiago. el papel predominante del agua subterránea en la dinámica del lago de chapala se explica, en parte, por el origen tectónico de este cuerpo de agua dulce. la dinámica observada en el lago de chapala sugiere que es necesario implementar planes de gestión en los cuales se considere el origen tectónico del lago con sus importantes aportes de agua subterránea como una ventaja para el control de la contaminación por la actividad humana. resumen record manuscript received: 20/07/2017 accepted for publication: 14/09/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 43-55 cristina noyola-medrano1*, josé alfredo ramos-leal2, briseida lópez-alvarez3 , janet morán-ramírez4, rosa maría fuentes-rivas5 1universidad autónoma de san luis potosí. facultad de ingeniería. area civil. geoinformática. méxico. 2instituto potosino de investigacion cientifica y tecnologica, a, c. división de geociencias. mexico. 3el colegio de san luis, a. c. programa agua y sociedad. mexico. 4centro interamericano de recursos del agua, universidad autónoma del estado de méxico 5(cira-uaemex). * corresponding author: cristina.noyola@uaslp.mx h yd ro lo g y 44 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas introduction the interaction between surface water and groundwater is a globally known phenomenon (sophocleous, 2002; rosenberry & labaugh, 2008; werner et al., 2013) of which exist a significant amount of reports on the interaction between groundwater and rivers and streams, while interaction with lakes is slightly focused (barthel & banzhaf, 2016). in the case of lakes, the interaction takes place in three primary ways: a) lakes gain water from inflow of groundwater through the lake bottom; b) they lose water to groundwater by outflow through the lake bottom, c) they do both, gaining in some areas and losing in others (winter et al., 1998). in most of the sites reported, the groundwater discharges water in a lake if the altitude of the water-table near the stream is higher than the elevation of the lake water surface. on the contrary, surface waters may infiltrate into a groundwater environment, if the altitude of the water-table near the lake is lower than the elevation of the lake water surface (winter et al., 1998; sophocleous, 2002). the chemical composition of water in lakes can be relatively simple when it has only one source of natural input; however, when it has multiple inputs, its chemistry is exceptionally complicated. thus, there are freshwater bodies whose chemical composition depends on various sources such as rivers, groundwater discharges, surface runoff, precipitations and discharges derived from anthropogenic activities (ojiambo et al., 2001; alexakis, 2011; kazi et al., 2009). particularly in the case of rift lakes, most of the reports were centered on african lakes, which showed that water in these environments present an extreme chemical variability with predominance in carbonates and sodium (tailling & tailling, 1965; chernet et al., 2001). some of these works focused their attention on the problem of eutrophication owing to human contamination (nyenje et al., 2010). other authors reported that the variability in chemistry in the rift lakes is due to changes in climate, hydrographic conditions and lithology (macintyre, 2012). in the ethiopian rift lakes, the reports indicate chemical variations related to different factors such as the dominant presence of basalt and rhyolite, contributions of meteoric water in the system and the processes of evaporation (rango et al., 2010). in contrast to a significant amount of work available on the rift lake in africa, only a few works in america have addressed the topic of chemistry variability in rift lakes. regarding the techniques used to evaluate the chemical variability of water bodies, kazi et al., (2009) reported that multivariate statistical techniques like the principal component analysis (pca) help in the interpretation of complex data matrices for a better understanding of the variability of water quality. nevertheless, the sensitivity of lakes to natural and anthropogenic variability factors depend strongly on their morphometry such that the large and shallow lakes are susceptible to natural physical drivers, whereas, in deep lakes, the biological and chemical factors prevail (tuvikene, 2018). to assess the representativeness of conventional point sampling used in water quality monitoring, anttila et al. (2008) proposed a geostatistical method where the semivariance of each observation pair is plotted against the distance of separation of these pairs. nevertheless, they showed that point source samples are representative only over small areas. in addition to the changing stationary patterns in water quality, there exist other point sources of nutrients and suspended solids from inflows or diffused sources. this kind of spatial distribution characteristic of the river mouth and shoreline areas can be detected by remote sensing (tuvikene, 2018). for these reasons, this paper deals with different statistical and geospatial techniques to analyze the origin of the chemical composition of water from chapala lake, which is the most extensive rift lake in mexico and one of the most important shallow lakes in latin america (de-anda et al., 2004). chapala lake has a total area of 1146.59 km2 and constitutes the discharge area of lerma-chapala (hernández-garcía, 2006). in addition to this condition, chapala lake receives the wastewater of the populations settled on the shoreline of the lake. this environment causes eutrophication problems in the lake as reported in several works (hansen and maya, 1997; vargas and mollard, 2005; hernández-garcía, 2006; cifuentes et al., 2011). among the main environmental problems for chapala lake is the presence of pollutants such as nitrogen and phosphorus caused by climate change and anthropogenic activity (de-anda et al., 2000; quiroz-castelán et al., 2004; de-andaand maniak, 2007; lópez-hernández et al., 2007). other studies have reported toxic pollutants such as cadmium, lead and arsenic from industrial wastewater and the geological composition of the region (hansen and maya, 1997; hansen and van afferden, 2004; záratedel valle et al., 2006; trujillo-cárdenas et al., 2010; chávez-alcántar et al., 2011), and the increase in concentration of solids as a result of the discharges from lerma river which is its main tributary (de-anda et al., 2004). de la mora-orozco et al., (2004) reported that the quality of water in chapala lake varies significantly based on seasonal factors, mainly by the revenue of water during the rainy season, allowing a recovery period that is inherent in the seasonal dynamics of the lake. the changing conditions of the lake have been analyzed with various statistical techniques and geospatial analysis (lind and dávalos-lind, 2002; de la mora-orozco et al., 2004; lópez-caloca et al., 2008). previous studies conducted on the chapala lake shows the need to characterize the predominant pollutants and their relationship with geographical space, in order to control the pollution of the lake. furthermore, it is necessary to evaluate, in first instance, the interrelatedness of the different pollutants, because some of these substances tend to form complexes with organic and inorganic elements of the water (nieboerand richardson, 1980; cruz-guzmánalcalá, 2007). therefore, the analysis of water quality must take into account this particular condition for contaminants such as manganese (mn2+), lead (pb2+), nickel (ni3+), copper (cu+), magnesium (mg2+), calcium (ca2+) and sodium (na+) among others (peña-núñez, 2006; calderón et al., 2007; díaz & colasurdo, 2008; carrión et al., 2012). putting this background into consideration, it is necessary to ask if the pollution of the lerma river and urban areas have a similar effect around the lake or if there are other factors that affect the water quality in chapala lake. the objective of this study was to identify and quantify the different contributions that give rise to chapala lake, considering its tectonic origin. to achieve this, a methodology was proposed to characterize the recent state of the quality of water in the lake, from methods of mixing water from end-members (em), statistical multivariate analysis techniques; and the geospatial distribution of the pollutant concentrations from satellite imageries visual analysis. 2.materials and methods 2.1 location of study area lake chapala is a natural reservoir of mexico, located mainly in the state of jalisco, mexico (figure 1). it has a total area of 1146.59 km2, of which jalisco occupies 86% and michoacan 14%. the full capacity of the lake is 7,897 million cubic meters (mm3). figure 1. location map of the study area. this figure shows the distribution of the surveyed sites during field work. 45assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing 2.2 geological setting the study area is a structure known as triple point junction of jalisco (tpj), and this structure is formed by the rifts of colima (n-s), tepic-zacoalco (nw-se) and chapala (w-e). in chapala, there is a significant topographic depression forming lake that extends 90 km towards the east of tpj. chapala rift is delimited by a nee-oriented step failure system (luhr et al., 1985). the chapala rift was developed at the early miocene, from a transcurrent fault of the regional system chapala tula (figure 2), with orientation e-w (rosaselguera and urrutia-fucugauchi, 1998). the north of chapala lake shows the oldest rocks (figure 2). they consist of miocene basalts aged 8.5 10 ma and they are exposed in the ne corner of the area of study (rosas-elguera et al., 1998). around the chapala basin outcrop are basaltic-andesitic rocks of late miocene-early pliocene (rosas-elguera et al., 1998). the chapala formation is visible in the western and central parts of the northern shoreline of the lake. this unit consists of alternating lacustrine sediments and pyroclastic units (ash and pumice) of late pleistocene or late pliocene (downs, 1958). the basaltic lava and basalt cones of the group crucero-ixtlan basalts were formed during the late pliocene, and their ages are between 3.3 and 2.5 ma (rosas-elguera et al., 1998). the outcrop of quaternary consists of monogenetic volcanoes, several calcoalkalinerhyolitic-dacitic domes, andesitic cones and lava flows with ages lower than 1.0 ma (rosas-elguera et al., 1998). 2.3 analytical procedures thirty-seven water samples were collected from chapala lake within three days, from march 30th to april 2nd, 2014 (figure 1). at each observation point, a wide range of physicochemical parameters was measured to ensure a good representation of a variety of potential pollutants in the dataset. water samples were collected from the whole lake, using random sampling, which was georeferenced in utm coordinates. water samples were collected at depth of 1 m and then were stored in bottles of polyethylene (hdlp) which were cleaned and rinsed seven times with deionized water. to collect the samples of cations and trace elements, the bottles were first of all washed and 10% hcl was added. for each water sample, the following parameters were determined in situ: ph, electrical conductivity, temperature, dissolved oxygen (do) and alkalinity. immediately after being collected, the samples of cations and trace elements were acidified with pure nitric acid to ph < 2. all the samples were stored in an icebox at a temperature lower than 4 °c. major ions and trace elements were analyzed in the laboratory of the institute of geology of the autonomous university of san luis potosí. the concentrations of major cations (ca2+, mg2+, na+ and k+) were determined by icpoes icp-ms “thermo x series ii”, the major anions (so24 and cl-) were analyzed using smarte 3 colorimeter, these analysis were performed in triplicates. alkalinity and hco3 were determined by titration on the site. the error ion balance (electro neutrality) was less than 5% . 2.4 multivariate statistical and end-member mixing analysis chapala lake is a large body of water comprising groundwater and surface water. the lerma river collects urban and industrial wastewater from its origin in the toluca valley to the chapala lake. meanwhile, the duero river, born in the valley of chilchota, is an urban water collection site for different towns (figure 4). other tributaries of much smaller scale, flow into the lake. this study intends to use mixing models to obtain water chemistry in lake chapala, which can be binary or ternary. the binary mixing model, can be an evolutionary process between two end members (ramos et al., 2004). finally, the water fraction type m (frm) is calculated in the mixture, for the conservative element (cl) expression (appelloand postma, 1996; generaux et al., 2002): fr m c c c c x= 2 2 1 (1) where cx, c1 and c2 represent the concentrations of the selected sample, water type c1 and c2, respectively. in this equation, the total is equal to 1. for the percentage of each em, the ternary mixture is considered for which a system of three equations with three unknown are used. starting with a general equation, where in the total concentration of the lake is cw (equation 2). c c c cw = + +1 2 3 (2) where c1, c2 and c3 are unknown members associated with different sources and cw values are equal to 1. figure 2. structural-tectonic setting and geology of chapala graben (chg); tepic-zacoalco rift (tzr); citala graben (cg) (modified of rosas-elguera & urrutia-fucugauchi, 1998). 46 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas to solve equation 1 with three unknown variables, 3 equations are needed, in which the concentrations of the conservative elements (e.g. chloride (c1-) and fluoride (f-)) from the ternary mixture model of the em are used (equations 3 and 4). c f c f c f c fw w = + +∗ ∗ ∗ 1 1 2 2 3 3 (3) c fl c cl c cl c clw w = + +∗ ∗ ∗ 1 1 2 2 3 3 (4) where c1 is the em with lower concentrations in its conservative elements, c2 is the em with high concentrations in one of the conservative elements; c3 is the em with the highest concentration in the conservative elements or a high concentration in one of them. c3 is obtained by combining equations 1 and 3 (equation 5). c c f f c f f f f w w 3 1 2 2 1 3 1 = ( ) + ( ) (5) c1 is obtained by combining equations 1 and 4 (equation 6). c c cl cl c cl cl cl cl w w 1 3 2 3 2 1 3 = ( ) + ( ) (6) the only solution for c2 when cw = 1, can be obtained by substituting c1 and c3 in equation 2 and solving c2: c c c2 1 31= (7) 2.5 geostatistical analysis the geostatistical analysis was performed based on the geochemical data obtained in the field. the analytical process includes the statistical exploration of the chemical components recorded in the thirty-seven points visited in the study area. thereafter, the structural behavior of data was explained by spherical semivariogram calculated with equation 8 (isaaks& srivastava, 1989).  h c h a h a h a( ) =     −               ≤3 2 2 3 3 (8)  h c h a( ) = >� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � where: γ(h) – variogram value at distance “h”, c – constant, h – distance, a – range. subsequently, ordinary kriging was applied to obtain the variability surface for each chemical component analyzed. this method of geostatistical estimation is widely described by wackernagel (2013). results validation was done using the cross-validation technique known as leave-one-out, which consists of removing each data location one at a time and predicting the associated data value (refaeilzadeh et al., 2009). the software arcgis 10.3 was used during the whole geostatistical procedure. 2.6. satellite images analysis in order to corroborate the observations produced from the analysis of extreme elements, a digital processing of a landsat 7 etm+ image of april 2nd, 2014 was performed. the date selected for processing of the image corresponds to the date of field work in the chapala lake when the samples were gathered for hydrogeochemical analysis. the cloud cover in this image is 0%. the software envi 5.3 was used to generate the algorithms for the processing of the satellite imagery. the treatment performed on the image consists of several steps that began with the conversion of digital number (dn) to toa reflectance. for this purpose, the criteria of chander and collaborators (2009) was used (equations 9 and 10). l grescale x qcal brescale = + (9) where: lλ =spectral radiance at the sensor’s aperture [w/(m2srμm)], qcal= quantized calibrated pixel value [dn], grescale= band-specific rescaling gain factor [(w/(m2srμm))/dn] and brescale = band-specific rescaling bias factor [w/(m2srμm)]. in order to calculate the toa planetary reflectance, equation 10 was used. ρλ= π θ λ λ * * * ( ) l d esun cos s 2 (10) where p = mathematical constant equal to ~ 3.14159 [unitless], lλ = spectral radiance at the sensor’s aperture [w/(m2srμm)], d = earth–sun distance [astronomical units], esunλ = mean exoatmospheric solar irradiance [w/(m2μm)] and θs = local solar zenith angle. the results obtained were used to fill the information gaps regarding landsat 7 etm+ image. a landsat 8 oli image of march 25th, 2014 was used to conduct this procedure. this scene presents the date closest to the landsat 7 etm+ image of april 2nd, 2014 in such a way that the landsat 8 oli image helps to cover the missing information in the landsat 7 etm+. in the landsat 8 oli image and prior to the match between images, the digital numbers of the pixels were converted to reflectance values. the algorithm used was the landsat gapfill with the subroutine of “two band gap fill (local histogram matching)” (usgs, 2004). immediately, a masking was generated to separate the land of the water from the threshold of the band 5 of landsat 7 etm+ image. the water surface isolated by the mask was used to generate a false-color image by the method of optimum index factor (oif) to determine the most informative three-band combination (chávez et al., 1982; chávez et al., 1984). the following formula was used to calculate the oif. oif max j r j i n j n = ( ) ( )   = =∑ ∑ 1 1 / (11) where oif is the optimum index factor, (i) is the standard deviation of i band, r(j) is the value of correlation coefficient between any two bands. finally, a statistical analysis was conducted to establish the most representative color for each one of the groups obtained from their hydrogeochemical characteristics.besides, the spectral signature of each group was generated from the reflectance data extracted from the different bands of the landsat 7 etm+ image. 3.-results 3.1 geochemical data and cluster analysis the dataset presented in table 1 shows the statistics data of the pollutants registered from the water samples. from this data, a cluster analysis was generated. this kind of procedure uses a series of multivariate methods to find groups of data. the hierarchical approach of cluster analysis has the advantage of not demanding any prior knowledge of the number of clusters (fraley &raftery, 1998). a dendrogram shows the distribution of quality water groups defined by the cluster analysis (figure 3). the cluster analysis identified four groups in the chapala lake (figures 3 and table 1). the group i is distributed on the west side of jocotepec. the group ii is in front of the town of chapala. the group iii is between tizapan and ocotlan. the group iv is in the eastern portion of the lake, toward the entrance of the duero river. although, this last group is classified into two subgroups of the dendrogram, it falls under the same group because they have a similar chemical behavior for chlorides, but a different concentration of fluoride. 47assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing 3.2 relationship between spectral response and distribution of groups of water. the optimal index factor analysis (oif) established bands 1, 2 and 4 as the optimal group of bands to highlight the most significant variability of quality water for chapala lake (table 2). the image obtained with combination rgb 421 is used to test the presence of 6 different colorations in the chapala lake. the predominant color for each group is shown in table 3. the color red occurs primarily on the eastern edge of the lake (figure 4). this coloration is related to the contributions of the lerma and duero rivers. the spectral response of this area corresponds to samples interpreted as group iv (ternary mixture) and characterized by a non distinct pattern in the variations of fluoride (f-) and chloride (cl-) (figures 4 and 7). the spectral signature shows the highest values of reflectance in the region situated between 830 and 1300 nm and, the shortwave infrared region present the lowest values concerning the other three groups (figure 5). towards the eastern part of the lake, a dark green coloration is presented, which can also be observed to a lesser extent in the western central part (figure 4). this color coincides with the samples belonging to groups ii and iii, considered as binary and ternary, respectively (figure 4). the group iii is more concentrated in areas of greener colorations, while group ii occurs in areas whose spectral response is purple and green. the light blue and green colorations are concentrated in the central and eastern part of the lake (figure 4). this coloration is related mainly with samples belonging to group i, corresponding to a binary group, and characterized by high values of fand cl(figures 4 and 7). the spectral signatures of groups i, ii and iii have similar patterns. the highest values of reflectance correspond to group ii; the middle values are for group i and group iii presents the lowest values of reflectance (figure 5). in figure 4 the main polygons delimiting the contribution of groundwater are marked. in figures 6 and 8, the hydrogeochemical sections show the influence on the water quality of the different sources of input to the lake of chapala. figure 3. dendrogram of the q-mode cluster analysis (the axis shown at the top indicates the relative similarity between cluster groups. the lesser the distance, the higher the similarity between water samples). table 1.-general statistics of the chemical data groups in the chapala lake water. temperature (°c) cl (mg l-1) na+ (mg l-1) mg2+ (mg l-1) ca2+ (mg l-1) tds (mg l-1) no-3 (mg l-1) fe (mg l-1) so24 (mg l-1) al3+ (mg l-1) group i minimum 20.70 37.80 136.46 22.45 47.16 699.30 3.66 0.42 104.00 0.74 maximum 23.12 60.20 144.91 23.56 49.68 714.90 10.44 0.49 140.00 0.85 mean 21.60 49.52 141.03 22.99 48.43 705.61 6.52 0.45 117.60 0.79 standard deviation 0.80 7.06 2.60 0.37 0.73 5.49 2.39 0.03 10.82 0.04 group ii minimum 20.84 5.00 136.92 22.35 47.09 685.40 1.61 0.41 42.00 0.73 maximum 24.30 10.20 146.17 23.08 50.96 703.00 11.48 0.52 66.00 0.91 mean 21.98 7.44 140.15 22.73 49.49 696.07 7.07 0.47 51.80 0.83 standard deviation 1.10 1.75 2.87 0.27 1.16 6.81 3.65 0.04 8.77 0.07 group iii minimum 21.06 24.70 137.99 22.11 48.17 684.60 3.24 0.42 68.00 0.80 maximum 22.10 52.00 146.97 22.69 50.81 694.50 10.16 0.57 90.00 0.98 mean 21.50 31.92 141.41 22.42 50.03 688.90 4.88 0.48 75.67 0.87 standard deviation 0.41 10.17 3.11 0.24 0.98 3.85 2.66 0.05 8.14 0.07 group iv minimum 21.09 23.80 145.05 22.53 50.90 690.50 5.00 0.56 90.00 0.98 maximum 24.74 52.40 160.11 23.75 59.43 755.40 17.00 1.31 114.00 2.09 mean 22.75 34.63 151.76 23.06 54.62 720.64 11.75 0.90 104.80 1.56 standard deviation 1.28 12.06 5.30 0.41 2.58 20.03 5.12 0.23 6.81 0.34 48 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas table 2. calculation of oif to establish the best combination of three bands:(i1), (i2), and (i3) are the standard deviations of three bands used in the calculation; r(j1), r(j2), and r(j3) are the correlation coefficients between the different pairs of bands generated in each combination. the combination with greater oif will show the most extensive variability of colors. combination (i1) (i2) (i3) r(j1) r(j2) r(j3) oif 157 0.006 0.007 0.006 0.827 0.835 0.919 0.007 127 0.006 0.008 0.006 0.928 0.835 0.86 0.008 257 0.008 0.007 0.006 0.855 0.86 0.919 0.008 125 0.006 0.008 0.007 0.928 0.827 0.855 0.008 137 0.006 0.007 0.006 0.627 0.835 0.614 0.009 357 0.007 0.007 0.006 0.619 0.614 0.919 0.009 135 0.006 0.007 0.007 0.627 0.827 0.619 0.010 123 0.006 0.008 0.007 0.928 0.627 0.621 0.010 237 0.008 0.007 0.006 0.621 0.86 0.614 0.010 235 0.008 0.007 0.007 0.621 0.855 0.619 0.011 347 0.007 0.009 0.006 0.683 0.614 0.309 0.014 345 0.007 0.009 0.007 0.683 0.619 0.345 0.014 457 0.009 0.007 0.006 0.345 0.309 0.919 0.014 134 0.006 0.007 0.009 0.627 0.103 0.683 0.016 147 0.006 0.009 0.006 0.103 0.835 0.309 0.017 234 0.008 0.007 0.009 0.621 0.111 0.683 0.017 145 0.006 0.009 0.007 0.103 0.827 0.345 0.017 247 0.008 0.009 0.006 0.111 0.86 0.309 0.018 245 0.008 0.009 0.007 0.111 0.855 0.345 0.018 421 0.009 0.008 0.006 0.111 0.103 0.928 0.020 table 3. statistical data about membership color. the highest percentage of points in each group regarding the color observed in the satellite image are indicated in yellow color. em (end member). group color quantity color membership (%) i light blue 7 70 i green 2 20 i purple 1 10 ii purple 5 50 ii green 4 40 ii brown 1 10 iii dark green 4 66.7 iii green 1 16.7 iii brown 1 16.7 iv red 9 90 iv brown 1 10 group em quantity main coloration i 3 light blue ii 2 purple iii 3 dark green iv 3 red 49assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing figure 4. a) lerma river basin, where is the origin of the rivers lerma (toluca) and duero (chilchota valley). b) mixes of water distributed in chapala lake. the colors result from a combination of bands 4, 2 and 1 in the channels red, green and blue, respectively. the image used was a landsat 7 etm+ from april 2nd, 2014. figure 5. spectral signature of each group of water mixes. the spectral behavior was extracted directly from the landsat 7 etm+ image from april 2nd, 2014. the influence of lerma river (rli) to the east of the lake can be observed, which is diluted by contributions from groundwater (gwi1 and gwi2) in the central part, where variations depend on ingress of polluted water from nearby towns (cwi). in the west lake, the chemical composition is altered by the entry of surface water (swi). figure 6. variability maps of a) temperature, b) ph, c) na+ (sodium), d) mg2+ (magnesium) and f) ca2+ (calcium), in the chapala lake. the graphics on the left side show the e-w hydrogeochemical sections. figure 7. diagram of clvs ffrom the samples of chapala lake. em have been marked with a black frame bordering the circle that represents the sample. 50 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas 3.3 hydrogeochemistry characterization kriging interpolation obtained the distribution of different physical and chemical components of water in chapala lake. the maps of the variability showed that the concentration of these components in the lake is not homogeneous and not gradual. towards the eastern part of the lake in the mouths of duero and lerma rivers are the highest values of chemical components. then, towards the central part, there is a decrease or increase interrupting the first trend observed in the eastern part and once again, the trend changes to the west side of the lake (figures 6 and 8). in general, all physical and chemical components present higher values in the eastern edge of the lake and the lowest values in the western part. there are some exceptions where cland mg+2 have the highest values in the western side of the lake and no3 -has the highest values toward the central part of the lake (figures 6 and 8). durov expanded diagram (figure 9) shows the relationship between different groups of water. the samples located in region 3 indicate the predominance of na-hco3. furthermore, a linear distribution was observed, that is, there is a little variation in anions and cations. based on the distribution of samples in chapala lake, it can be suggested that the primary process in the lake water is the mixture between groups of different water quality (figure 7). gibbs diagram used for the study of surface water shows three regions based on the chemistry of water: rain (precipitation domain); weathering reactions (water-rock); and evaporation-crystallization process (gibbs, 1970). the composition of the global rain dissolved solids concentrations is found in the lower right corner (figure 10). groundwater has increased due to water-rock interaction caused by weathering, thereby, increasing the proportion of dissolved solids concentrations and affecting the calcium sodium bicarbonate and chlorides content. finally, evapotranspiration in farming areas or dry surfaces receiving subsurface water runoff from the agricultural return, elevate the content of na+ and total dissolved solids (tds). similar results were reported for lakes nakuru, elementaita and naivasha in kenya (njenga, 2004). the diagram gibbs diagram was with samples of chapala lake (figure 10), which are distributed horizontally in the domain of water-rock interaction, and it can invariably be interpreted that groundwater influences the process of mixing water in the central region of the lake. in figure 9a (tds-na / (na + ca)), little variation in tds and greater variation in na / (na + ca) is due to water-rock interaction. in figure 9b (tds cl / (cl + hco3)), linear movement towards the water-rock interaction is also observed. in general, the graph of gibbs showed that the water of chapala lake has mixing processes with groundwater due to water-rock interaction. on the other hand, groundwater flows discharging into chapala lake are poorly understood and are only visible at close range. the end members of group i can be easily identified because each one of them presents color variations. for instance, the sample l-32 is located in an area that presents light blue colorations. the sample in group i (figures 7 and 8) is characterized by having the lowest values of fand cl-(figures 4, 6 and 7) and the highest values of no3 and al3+. the end member l-30 is located in an area that presents light blue and light green colorations. this sample presents intermediate values of cland the highest values of ffor this group (figures 4 and 7). the third end member, l-35 is located in an area that presents darkgreen colorations. this sample is characterized by the highest values of cl-, intermediate values of f(figure 8) and the lowest values of temperature, na+, mg2+and no3 of group i (figures 6 and 8). the end members of group ii are samples l-20 and l-22. in the case of the l-20 sample, this occurs in an area of dark blue color and is characterized by the lowest values of cland the highest values of f (figures 4 and 7). in addition, this sample contains high values of no3 and mg2+ and the highest values of temperature in the central zone of the lake (figures 5 and 7). the sample l-22 is located in an area of green and blue color, which contains the highest values of cland no3 and it has the lowest values of fand al3+for group ii (figures 4, 6, 7 and 8). for group iii, the end members are: l-1, l-13 y l-16. the sample l-1 is located in a region that presents a greyish blue color. in this sample are intermediate values of cl-, the lowest values of fand no3 -, and the highest values of al3+ (figures 4, 6, 7 and 8). the sample l-13 is located in an area of intense blue color. it is characterized as having the highest values of cl-, intermediate values of f-and no3 and high values of al3+. the third end member (l-16) is characterized by a bluish green color. this sample presents intermediate values of cl-, the highest values of no3 and fand intermediate values of al3+ (figures 4, 6, 7 and 8). all the end members of group iv show a red color. the sample l-10 presents the lowest values of f-, intermediate values of cland no3 and high values of al3+ (figures 4, 6, 7 and 8). the sample l-6 is characterized as having the highest values of f-, intermediate values of cl-and high values of no3 and al3+. the third end member (l-4) presents high values of fand cl-, the low values of no3 and intermediate values of al3+ (figures 4, 6, 7 and 8). figure 8. variability maps of a) tds (total dissolved solids), b) no3 -, c) fe (iron), d) ba2+ (barium) and f) al3+ (aluminium) in chapala lake. the graphics on the left side show the e-w hydrogeochemical sections. figure 9. durov diagram performed with samples from chapala lake. 51assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing to understand these variations and origin of groundwater, the mixture models were used, considering it as a system consisting of em, that is, those with the minimum and maximum concentrations in the system. two conservative elements (cland f-), which do not react in the system was used to identify the em. 4.-discussion the relationship between the different em; generate mixing groups of water into the lake, which when combined give rise to the chemical composition in different areas of the lake. mixtures of water in surface water bodies and groundwater have been extensively studied by other authors (barthold et al., 2011; long & valder, 2011; hooper, 2001; barth &veizer, 2004; moranramírez& ramos-leal, 2014). in addition, some of the findings regarding the sources that determine the water quality in rift lakes have been discussed for africa (rango et al., 2009; ndungu et al., 2015). however, chapala lake is not yet a standard for this type of analysis. in this paper, four groups were established based on their mix of water which has different qualities, using various statistical and geospatial analysis methods. group i, distributed to the west edge of the lake, is a ternary mixture composed of em a1 = l-32, a2 = l-30 and a3 = l-35 which has little variation of f(0.5 to 0.7 mg l-1) and a more significant difference in cl(37 60 mg l-1). the l-32 sample (west of lake and ne of the tesistan town) is influenced by surface currents flowing into the lake from sierra of tesistan. l-3 (west of lake and ne of soyatlán) shows high values of cland is related to groundwater flow. the l-30 sample, located in south of cosala, represents the em of surface water. group ii exhibits linear dispersion in me. l-20 and l-22 both have little variation in em cl(5 to 10 mg l-1) and larger variation in f (0.2 to 0.8 mg l-1); the sample 20 is located at the center of the lake and is associated with groundwater flow. the origin of this groundwater flow is associated with the fracturing generated during the development of chapala rift. this behavior is similar to that observed by rango et al., (2009) in the region of ziway-shala, where the highest concentrations of fwere related to the interaction water-rock derived from the rift environment. on the other hand, the l-22 located at the south-central portion of the lake shows low values of fand classociated with slightly contaminated water discharges in the town of tizapan. group iii represents a ternary mixture between em, l-1, l-13 and l-16. the l-1 shows relatively low values of f(0.3 to 0.5 mg l-1) and cl(24-52mg l-1), located in the eastern part of the lake, near the discharge of the lerma river and is associated with contaminated surface water at the mouth of the lermariver. the l-13 is located at the eastern part of the lake near the northern edge of the lake, and is related to groundwater flow. the l-16 sampling location is in the eastern part of the lake near the northern edge of the lake, while the water quality of this location is mainly influenced by groundwater flow. this group like the group ii have low values of f and cl-. group iv to group iii share similar characteristics, but samples of this group are located in the eastern part of the lake and are associated with the direct discharge of the duero and lerma rivers. this group has a ternary mixture with the end members samples, l-10, l-6 and l-4, with variations in the values of f(0.2-0.6 mg l-1) and cl(23-52 mg l-1). although, the sample l-10 is close to the discharge area of the lerma river, it has contributions from superficial water from the town of palma. this sample has relatively low values of f-, which helps to ascertain that the contribution of surface water have also been observed in other bodies of water. moreover, the l-6, located in the eastern part of the lake is related to the direct input of the polluted waters of the lerma river and its dynamics is of little movement. for this reason, high values of f-, clintermediate values and high values of temperature, no3 -, fe and al3+ are presented with respect to other samples from chapala. the l-4 is located in the ne part of the lake and is related to polluted waters of the lerma river. this sample shows high values of cl-, na+, mg2+ and al3+. the chemical complexity and variability of these groups are similar to that reported by chemet et al., (2001) for some lakes of ethiopia and for two estonian shallow lakes (tuvikene, 2018). considering the fractions of the mixture for each sample of group i, em a1 corresponding to contributions of surface water contributes 52%, the a2 related to the entry of groundwater in the lake participates with 38% and a2 also associated with another entry of groundwater flow in the lake contributes 10% (table 4). in group ii, the em b1 corresponding to contributions of groundwater provides 58%, the b2 related to contaminated water in the lake, near the town of tizapan, participates with 42% (table 4). in group iii, the em c1 corresponding to contributions of contaminated water provides 34%, the c2 related to the entry of groundwater into the lake participates with 20% and c3 also associated with another entry groundwater flow in the lake contributes 46% (table 4). in group iv, the em d1 corresponding to water contributions preceding contamination of the town of palma provides 45%, the d2 related to the entry of contaminated water from the rio lerma participates with 53% and d3 also associated with the entry contaminated water from the town of ocotlan contributes 2% (table 4). from the fraction of mixture, which is based on each of the ends in the groups, the chemical composition of members in each sample is obtained from the system and chemical reactions are not considered in this mixing process. the comparison between the estimated fractions from mixing and the chemical composition of the actual sample data (table 5), reveals that there are slight variations between the two results. in some individual cases, the ionic delta may be close to zero indicating the mixture as the dominant process in the system. in other cases, figure 10. gibbs diagrams for the samples of chapala: a) tds – na / (na + ca) and b) tds – cl / (cl + hco3). 52 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas table 4. proportional contributions of each em of group i, group ii, group iii and group iv. sample a1 a2 a3 at sample b1 b2 bt l-27 0.37 0.11 0.52 1.00 l-26 0.26 0.74 1.00 l-28 0.64 0.35 0.01 1.00 l-25 0.94 0.06 1.00 l-29 0.17 0.82 0.02 1.00 l-19 0.72 0.28 1.00 l-30 0.00 1.00 0.00 1.00 l-37 0.32 0.68 1.00 l-31 0.94 0.06 0.00 1.00 l-20 1.00 0.00 1.00 l-32 1.00 0.00 0.00 1.00 l-18 0.43 0.57 1.00 l-33 0.43 0.57 0.00 1.00 l-24 0.38 0.62 1.00 l-34 0.00 0.13 0.87 1.00 l-17 0.81 0.19 1.00 l-35 0.00 0.00 1.00 1.00 l-21 0.26 0.74 1.00 l-36 0.65 0.00 0.35 1.00 l-22 0.00 1.00 1.00 sample c1 c2 c3 ct sample d1 d2 d3 dt l-16 0.00 0.00 1.00 1.00 l-4 0.00 0.00 1.00 1.00 l-15 0.21 -0.19 0.98 1.00 l-8 0.49 0.43 0.08 1.00 l-23 0.33 0.08 0.59 1.00 l-10 1.00 0.00 0.00 1.00 l-14 0.50 0.12 0.39 1.00 l-6 0.00 1.00 0.00 1.00 l-1 1.00 0.00 0.00 1.00 l-5 0.59 0.41 0.00 1.00 l-13 0.00 1.00 0.00 1.00 l-9 0.18 0.93 0.00 1.00 table 5. ion difference between measured samples and calculated values for groups i, ii, iii, and iv.d: difference symbol. group i sample δclδso24 δhco3 δno-3 δca (mg l-1) δk (mg l-1) δmg2+ (mg l-1) δna (mg l-1) δf(mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1) l-27 0 -9.77 19 13.41 1.13 0.35 -0.3 1.24 0 l-28 0 -10.11 -16.57 7.53 -0.28 -1.1 -0.65 -3.16 0 l-29 0 5.46 29.32 3.84 -1.16 -1.49 -0.68 -4.74 0 l-30 0 0 0 0 0 0 0 0 0 l-31 0 -9 10.13 -8.56 -0.01 -1.1 -0.07 1.19 0 l-32 0 0 0 0 0 0 0 0 0 l-33 0 21.15 -20.16 -2 -0.42 -0.8 -0.45 -0.7 0 l-34 0 14.97 -5.71 8 0.56 0.82 -0.24 2.73 0 l-35 0 0 0 0 0 0 0 0 0 l-36 0 -1.56 -18.17 -8.54 1.51 0.95 0.27 1.15 0 group ii sample δcl δso24 δhco3 δno-3 δca (mg l-1) δk (mg l-1) δmg (mg l-1) δna (mg l-1) δf (mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1) l-12 0 5.1 -26.52 9.81 -1.83 -1.4 0.02 -4.08 0 l-13 0 -1.88 -24.04 -0.83 0.62 1.3 0.11 3.84 0 l-14 0 5.48 -59.7 1.32 -3.67 -1.14 -0.39 -6.45 0 l-15 0 0 0 0 0 0 0 0 0 l-16 0 1.96 13.03 1.28 0.12 -0.22 -0.36 -1.16 0 l-17 0 15.36 -38.68 -3.85 -0.96 -1.24 -0.11 -3.32 0 l-18 0 17.31 -31.63 1.42 0.16 -0.57 -0.39 -2.18 0 l-19 0 7.58 28.21 0.77 -0.68 -0.66 -0.06 0.69 0 l-20 0 0 0 0 0 0 0 0 0 (continued) 53assessing the source and spatial distribution of chemical composition of a rift lake, using multivariate statistical, hydrogeochemical modeling and remote sensing differences between some parameters of actual and estimated samples were observed, this behavior is related to other inputs of organic matter in the lake water as discussed in the work of trujillo-cardenas et al., (2010). moreover, discoloration observed in chapala corroborates the presence of water in different compositions. some of these compositions correspond to contaminated water, which has already been reported in chapala lake for other works (vargas and mollard, 2005; hernandez-garcia, 2006; cifuentes et al., 2011). as in other studies, a relationship between the colorimetry of water and physical-chemistry composition was observed in chapala lake, thus having different compositional areas, based on the color displayed by combining three bands of landsat 7 etm+ (kallio et al., 2008; verpoorter et al., 2012; fukushima et al., 2014). the results of variability observed with the landsat image are consistent with the data obtained from multivariate analysis and geostatistical estimations. therefore, there is a consistency in the results obtained by the different techniques applied in this study. also, in this paper, it was shown that landsat images are deemed suitable for spatial observations of shallow water quality. this observation is consistent with that reported by giordiano et al., (2014) for garda lake. finally, the hydraulic barrier related to the tectonic origin, and the interactions among the different groups of water quality of chapala lake suggest the implementation of management plans which consider the significant contributions from groundwater to the system of the lake as an advantage for the control of contaminated effluents. 5.-conclusions tectonic origin of lake chapala influences water mixing processes developed into the lake. this condition was verified by applying different techniques, such as multivariate analysis, geostatistical estimation and geospatial visualization with satellite images. the results obtained by the different methods are in agreement with each other. four mixing groups were identified, associated with the origin of the sources that feed lake chapala. group i has a strong influence on surface water. the main contribution of group ii is groundwater. group iii is powered mainly by contaminated waters of the lerma river. group iv has as main tributary the duero river. the groundwater contributions toward the central part of the lake, operate as a hydraulic barrier, which prevents the progress of pollutants from lerma river, either by its thermal gradient and by the large volumes of groundwater discharged into the middle of the lake, diluting the pollution in the central and western parts of the lake. the lerma river has significant contributions of contaminated water on chapala lake. nevertheless, the primary pollution is only perceived in the eastern portion of the lake, thus confirming that not all the contamination of the lerma river is distributed throughout the lake. the flow of the lerma river practically continues directly on the santiago river, so the most contaminated water is sent to the metropolis of guadalajara. some effects of contamination are also related to the discharge of municipal waters of some populations, chiefly on the southern border of the lake. the contributions of surface water to the west and northwestern part of the lake come from the mountainous areas close to the lake. the diagrams of gibbs and durov-expanded helped to identify the processes of interaction of water-rock into the lake. alternatively, multivariate analysis allowed the identification of four groups whose em represent changes in the chemical composition of the water due to contributions of sources of different chemical nature. the results obtained by different methods showed the mixing of water of different origins as the dominant process for chapala lake. however, in some cases, the microbial activity and the organic matter alter the chemical composition of these mixtures. the image of false color (rgb) allows the confirmation of different inputs of water on chapala lake. the colors observed are the product of the spectral behavior of the water, which can be related to the different chemical compositions found in chapala lake. this result is also corroborated by other methods explored in this study as the diagrams of durov-expanded, gibbs and the diagrams of clvs. f-. the spectral signatures of the four groups show marked differences in the values of reflectance and, group iv show a spectral behavior characterized by high values of reflectivity in the range from 830 to 1300 nm. this response is not present to the rest of the group. the results of this study suggest that the chemical variability of the lake is not only due to human activity. in the region, there are at least four factors that contribute to the chemical character of the water in the lake. in the western portion of the lake, the surface water is the primary source. in the central part of the lake, the groundwater is the dominant source, and in the east region of table 5. ion difference between measured samples and calculated values for groups i, ii, iii, and iv.d: difference symbol. group iii sample δcl δso24 δhco3 δno3 δca (mg l-1) δk (mg l-1) δmg (mg l-1) δna (mg l-1) δf (mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1) l-16 0 0 0 0 0 0 0 0 0 l-16 0 -4.2 -63.21 2.07 1.68 -0.12 0.06 -5.36 0 l-23 0 -7.23 15.3 -0.94 0.86 -0.25 0.35 -4.49 0 l-14 0 -6.84 12.45 8.09 0.76 -0.41 -0.11 -3.55 0 l-1 0 0 0 0 0 0 0 0 0 l-13 0 0 0 0 0 0 0 0 0 group iv sample δcl δso24 δhco3 δno3 δca (mg l-1) δk (mg l-1) δmg (mg l-1) δna (mg l-1) δf (mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1) l4 0 0 0 0 0 0 0 0 0 l-8 0 2.66 34.06 -8.03 0.14 -0.81 -0.13 -3.31 0 l-10 0 0 0 0 0 0 0 0 0 l-6 0 0 0 0 0 0 0 0 0 l-5 0 10.43 -31.17 -8.57 1.52 0.83 0.52 6.93 0 l-9 0 -9.31 12.14 -9.95 6.38 2.54 1.15 12.94 0 54 cristina noyola-medrano, josé alfredo ramos-leal, briseida lópez-alvarez, janet morán-ramírez, rosa maría fuentes-rivas the lake, the contaminated water of the lerma river is the main contributor. also, there are the contributions of polluted water from the urban areas along the whole shoreline of chapala lake. the dynamics observed in the chapala lake suggests the implementation of management plans which consider the tectonic origin of the lake with its substantial contributions from groundwater as an advantage for the control of pollution from human activity. this work also made it possible to differentiate between the dynamics of chapala lake regarding the rift lakes of the world. although, the tectonic scenario is similar for all places, the environment and social issues are different, which determines its significant differences in chemical variability and water quality. however, the mechanism of the interaction surface water and groundwater is similar in all cases. acknowledements the authors would like to acknowledge the program for teacher professional development in higher education (prodep) for the financial assistance granted by convention promep/103.5/13/6575. we are also grateful to the institute of geology of the autonomous university of san luis potosi (uaslp) for the services provided by performing the geochemical analyses. our sincere gratitude goes to ipicyt for the facilities and logistical support given towards the development of the field work. references alexakis, d. 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(2013). seawater intrusion processes, investigation and management: recent advances and future challenges. advances in water resources, 51, 3-26. https:// doi.org/10.1016/j.advwatres.2012.03.004 winter, t. c., harvey, j., franke, o., & alley, w. (1998). natural processes of ground-water and surface-water interaction. ground water and surface water: a single resource. us geological survey circular, 1139, 2-50. zárate-del valle, p. f., rushdi, a. i., simoneit, b. r. (2006). hydrothermal petroleum of lake chapala, citala rift, western mexico: bitumen compositions from source sediments and application of hydrous pyrolysis. applied geochemistry, 21(4), 701-712. http://dx.doi. org/10.1016/j.apgeochem.2006.01.002 ble earth sciences research journal earth sci. res. sj. vol. 17, no. 1 (june, 2013): 53 60 biogeochemical and engineering characteristics of soils and groundwater around a dumpsite odukoya am1, oresanya o2 and abimbola, a.f3 1 department of geosciences, university of lagos, nigeria. email: sesanbiiodun@yahoo.com 2 lagos state waste management authority, lagos, nigeria 3 department of geology, university of ibadan, nigeria geochemestry an active dumpsite in lagos south-western nigeria was monitored to test natural attenuation efficiency in reducing contaminants by determining soil and groundwater biogeochemical and engineering properties within and around the site. from the casagrande plasticity chart, the soil could be classified as clay or silt having intermediate to high plasticity; unsaturated zone thickness was 10-20m and permeability was low (1.96x10-5 to 41.8x10-5 m/s). appropriate microorganisms, such as bacteria, viruses and fungi which are required for natural attenuation, were naturally present. nutrients such as calcium, copper, magnesium, manganese, potassium, sodium and zinc, as well as electron donor (organic carbon) and electron acceptor (oxygen, nitrate, sulphate and iron), were also present in various amounts for effective natural attenuation. all the parameters analysed in water samples came within environmental protection agency standards, except fe, na, cl, no3, al, ba, ni, total bacteria and total fungi. phenol and total viral count were not detected in the two boreholes studied, but they showed values as high as 10,000 mpn/100ml and 230 cfu/100ml, respectively, in leachate. the concentration of contaminants in the soil was very high and inversely proportional to depth. un basurero activo en lagos, al suroccidente de nigeria, fue monitoreado para medir la eficiencia de atenuación natural en contaminantes por la determinación del suelo, las características biogeoquímicas de las aguas subterráneas y las propiedades de ingeniería adentro y afuera del vertedero de desechos. según la escala de plasticidad de casagrande, el suelo se podría clasificar como arcilloso o cienoso, con plasticidad de intermedia a alta; el espesor de la zona sin saturar fue de 10-20 m y la permeabilidad fue baja (1.96x10-5 to 41.8x10-5 m/s). están presentes los microorganismos apropiados para la atenuación natural como las bacterias, virus y hongos. nutrientes como el calcio, el cobre, magnesio, manganeso, potasio, sodio y zinc, al igual que donantes de electrones (carbón orgánico) y receptores de electrones (oxigeno, nitrato, sulfato e hierro) se encuentran presentes en diferentes cantidades para una efectiva atenuación natural. los parámetros analizados en las muestras de agua están dentro de los estándares de la agencia de protección ambiental, excepto fe, na, cl, no3, al, ba, ni, el total de las bacterias y el total de hongos. el fenol y el total viral no fueron detectados en los dos hoyos estudiados, pero estos mostraron valores de 10,000 mpn/100ml y 230 cfu/100ml en los lixiviados. la concentración de contaminantes en la tierra es muy alta e inversamente proporcional a la profundidad. key words: leachate, natural attenuation, nutrient, contaminant, electron donor. palabras claves: lixiviados, atenuación natural, nutrientes, contaminantes, donantes de electrones record manuscript received: 26/01/2013 accepted for publication: 04/04/2013 abstract resumen introduction leachate production as a by-product of organic and inorganic decomposition in landfills poses a serious threat if it becomes released to the environment (kimmel and braids, 1974; baedecker and back, 1979; arneth et al., 1989; jankowski, 1997). the waste disposal sites and landfills in the study area were neither properly designed nor constructed. after some years, a dumpsite undergoes biologically-, chemically-, geologicallyand hydro-geologically-mediated changes, resulting in weathering; consequently, it becomes a source for any aquiferous units close to them becoming polluted (altindag and yigit, 2005; adeniyi et al., 2008; arienzo, 2005; whang and zhuo, 2005; awofolu et al., 2005). many studies of urban activity-derived heavy metal contamination have been undertaken for soils, plants, stream waters and stream sediments (zvinowanda et al., 2009; kim et al., 1998; abimbola and odukoya, 2006; odukoya and abimbola, 2010). odukoya am, oresanya o and abimbola, a.f if a landfill is to be regarded as sustainable, there must be no residual risk of pollution for the environment 30 to 50 years after disposal has ceased. contamination of groundwater by landfill leachate is particularly difficult to clean up and is often associated with high costs and insoluble technical problems. similar situations, related to the anthropological impact associated with rapid development in industrial and urban regions, have been reported in other parts of the world (zhang et al., 1999; thuy et al., 2000; ansari et al., 2000; kim et al., 1998; wilcke et al., 1998 in morton-bermea et al., 2002). this has led to research into in-situ bioremediation involving naturally-occurring microorganisms which can degrade harmful groundwater contaminants; such approach is known as natural attenuation. the environmental protection agency (epa) has defined natural attenuation as, “a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater.” a variety of other terms have also been used to describe natural attenuation, including natural restoration, intrinsic remediation, intrinsic bioremediation, passive bioremediation, spontaneous bioremediation and bioattenuation (national research council, 1993; wiedemeier et al., 1996). natural attenuation focuses on the destruction rather than transfer of contaminants (national research council, 1993); however, the use of natural attenuation (i.e. chlorinated solvents) to remediate is a growing area (renner, 1998). natural attenuation is potentially applicable for some inorganic compounds, including metals and non-metals. the natural attenuation technique can also be applied to pesticides, but less effectively (federal remediation technologies roundtable, 1997). these natural processes include biodegradation, dispersion, dilution, sorption, volatilisation, radioactive decay, and chemical or biological stabilisation, transformation or destruction of contaminants [epa, 1997]. certain site parameters and characteristics are significant regarding natural attenuation; these would include groundwater flow, geological conditions, the nature and extent of contamination, microbial population, presence of carbonate minerals, nutrient supply, electron donors and acceptors. this research was aimed at determining the efficiency of natural attenuation for certain leachate compounds associated with the olusosun dumpsite and ascertaining whether this would reduce the environmental risks involved with groundwater to acceptable levels. methodology four surface soil samples were collected at depths ranging from 0-20cm using a stainless-steel hand auger within the dumpsite to determine engineering properties, in accordance with british standards (anon, 1990) which included moisture content, grain size distribution, porosity, permeability, plasticity and atterberg limits. another eight subsurface samples (0.5g) were collected vertically at 2 m intervals (2-10 m) for geochemical and microbiological analysis within the dumpsite (fig 4). subsurface samples were stored in sample bags and later air-dried in the laboratory at room temperature for two weeks. the already air-dried samples were disaggregated in a porcelain mortal and later sieved through a 2.0mm polyethylene sieve to obtain a <60µm clay fraction for geochemical analysis. the analysis took place at actlabs, ontario, canada, using an aqua regia digestion and inductively-coupled plasma mass spectrophotometer (icpms) method for trace and major elements analysis in soil. the groundwater and leachate samples collected within the dumpsite were also analysed for microbial population, nutrient supply, electron donors and acceptors, as well as other physical-chemical properties. water samples of about 125 ml were collected for multi-element analysis and acidified on site with 3 ml analytical grade hno3 to bring the water acid solution to ph~2. trace elements and cations in water were analysed by using inductively-coupled plasma-optical emission spectrometry (icpoes) while un-acidified water samples were analysed for anion concentrations using dionex dx-120 ion chromatography techniques. water samples were also analysed for microorganisms such as virus, fungi, bacteria and coliforms. results and discussion engineering properties / geological condition the geological barrier in the study area was studied to see whether it met the following conditions; (1) low permeability, (2) low (effective) porosity, (3) large thickness and (4) high natural retention capacity for hazardous substances. the casagrande plasticity chart (table 1) showed that the soils could be classified as clay or silts having intermediate to high plasticity or compressibility (mi-mh) while a few samples were classified as sand having intermediate to high compressibility, especially those at the surface within a 1 m depth interval. a high percentage of clay content in the soil in the study area could have contributed considerably to the removal of pollution by acting as an effective medium through cation exchange. unsaturated zone thickness was 10-20 m. permeability tests showed low permeability, ranging from 1.96x10-5 to 41.8x10-5 m/s (table 1). the soil could thus have provided long-term protection for the groundwater by natural attenuation as a result of unsaturated zone thickness, high clay content and low permeability. a summary of some of the soil properties is shown in table 2. moisture content ranged from 5.68% to 33.3% (table 1). these values were slightly low as maximum oxygen uptake by micro-organisms in refuse usually occurs at 40%-50% moisture content (usepa, 1998). moisture is important for micro-organism and nutrient distribution; it also flushes away degradation products, apart from being the major factor in leachate formation. water analysis results major and trace elements as well as microorganisms such as ca2+, fe2+, mg2+, k+, na+, cl-, no3 -, so4 2-, hco3 -, al, ba, cu, ni, sr, cd, cr, pb, total bacteria, faecal coliform, total fungi, total viral and phenol were ascertained in water to monitor natural attenuation effectiveness. the water samples included two boreholes, ten shallow wells and leachate samples. both leachate and monitor boreholes were located within the dumpsite while the other water samples were taken very close to the dumpsite. a summary of the results, as well as epa drinking-water standards (usepa, 2009), are presented in tables 2 and 3. ph ranged from 4.86-7.58, higher values being found in the leachate compared to the groundwater. high ph reduces metals’ solubility in leachate and increases cation sorption in soil (clay), hence inhibiting these metals’ migration. this was probably why most contaminants had a higher concentration in leachate which became reduced significantly in borehole samples. tds ranged between 26–453 mg/l in groundwater, which was within the recommended standard, but leachate had values as high as 3580 mg/l, thus being higher than the recommended value. trace elements like cd, cr and pb were only detected in leachate with the following values 1,100, 2,200 and 6,200 ug/l, respectively, and were far above the recommended values. cu, zn and sr were within the standards for groundwater but higher in leachate while al, ba, and ni ranged from 0.1-0.8, bdl-520 and bdl-1000 ug/l, respectively, most samples being higher than the recommended standard. all major elements were within usepa 2009 standard in water samples, except fe, na, cl and no3 (values as high as 0.47, 243, 406 and 250.3 mg/l, respectively) (table2). there was a general trend to become decreased in metals along the groundwater flow paths, except for some cations (fig 2). decreased concentration of metals and some anions as well as a stable or retreating plume along the groundwater flow confirmed natural attenuation, like biodegradation, sorption and dilution. microbiological analysis was carried out for the leachate and two borehole samples within the dumpsite, showing total bacterial count to be 280, 550 and 23000 cfu/ml for the two boreholes and leachate samples, respectively. neither faecal coliforms nor viruses were detected in the two boreholes while their value in leachate was as high as 10,000 cfu/100ml and 230 mpn/ml, respectively. total fungi had 21, 56 and 1,500 cfu/ ml for the two borehole samples and leachate, respectively. phenol was not detected in the boreholes but was recorded at 0.08 in leachate. however, these micro-organisms are required in certain quantities for natural attenuation but may also be hazardous for human health at certain concentrations. coliform count was above the recommended standards for the two boreholes. the absence of faecal coliforms and fungi in the two boreholes and their high concentration in the leachate could have been related to biodegradation preventing their migration to the water table (table 3). all the parameters analysed had their highest concentration in leachate and were above usepa 2009 recommended standards for drinking water. soil analysis results soil samples taken at regular depth intervals within the dumpsite were submitted to geochemical analysis. major and trace elements as well as other biological parameters like phenol and microorganisms were analysed in the soil samples to determine whether natural attenuation was occurring in the study area (table 4). the results showed that the parameters were inversely proportional to depth, which also confirmed natural attenuation (figs 5 & 6). a greater percentage of these contaminants could have been attenuated before reaching the water table (second aquifer) and this was also confirmed by the borehole samples where most contaminants came within the recommended standards. total bacterial count ranged between 14.9 and 32.5 cfu/ml105, faecal coliform ranged between 27.4 and 63.3 cfu/100ml103 while total fungi values ranged from 10.2 to 5.34 mpn/ml102 (table 4). all the values showed that the soil samples were highly polluted but became reduced as depth increased (figs 3&5), thereby confirming natural attenuation. heavy metals like as, cd, pb, mo and la were only present in soil but absent in water. the concentration of trace elements and microorganisms was highest in soil samples and this may have been due to clay’s retention ability and its high sorption ability. conclusion the dumpsite at olusosun in lagos, south-western nigeria was monitored to determine natural attenuation efficiency in reducing contaminants regarding soil and groundwater within and around the site. most parameters needed for natural attenuation to take place such as water (moisture content), microbial population (bacteria, fungi, coliforms), nutrients (nitrogen, phosphorus, other cations), electron donors (total organic carbon) and acceptors (oxygen, nitrate, iron) were present in both the soil and water within and around the dumpsite. the casagrande plasticity chart showed that the soil could be classified as clay or silt having intermediate to high plasticity or compressibility (mimh) while a few could be classified as sand having intermediate to high compressibility. clay is a geological barrier in the study area and it met the following conditions; low permeability, low (effective) porosity, large thickness and high natural retention capacity for hazardous substances. a high ph value in leachate reduced metals’ solubility and increased their sorption in the soil thus inhibiting the migration of these metals from leachate to the water table. major elements and trace elements were within usepa 2009 standards in the water samples, except for fe, na, cl, no3, al, ba and ni. the absence of phenol and total viral count in the boreholes and their presence in leachate could also have been related to natural attenuation. leachate also showed highest concentrations for all the parameters, having values above usepa 2009 standards. the soil analysis showed that all the parameters were inversely proportional to depth which also confirmed that natural attenuation was occurring. acknowledgements the author wishes to thank mr. ola oresanya, the managing director, lagos state waste management authority, lagos, nigeria, for singlehandedly sponsoring this research and for his support and encouragement. table 1: summary of soil engineering and index properties *ll liquid limit *pl plasticity limit *pi plasticity index depth (m) natural moisture content (%) atterberg limits (%) grain size distribution (%) permeability (k) coefficient x 10-5 rating ll pl pi clay silt sand gravel l1 0-1 9.17 18.69 28.13 52.92 0.28 18.6 low 1-2 9.07 22.23 34.71 42.92 0.14 8.46 low 2-3 5.68 33.27 20.99 12.28 56.03 22.05 22.93 0.00 2.61 low 3-4 10.03 13.65 23.98 59.73 2.65 41.8 low l2 0-1 17.99 30.39 15.08 15.31 56.57 20.70 22.74 0.00 2.61 low 1-2 17.07 30.59 19.83 10.76 54.56 22.11 22.73 0.61 3.35 low l3 0-1 33.33 44.29 23.36 20.93 26.57 33.40 30.68 9.36 9.09 low 1-2 23.58 46.70 25.49 21.21 32.66 26.60 31.59 6.75 5.61 low l4 0-1 11.71 37.16 22.98 14.18 44.11 32.55 23.24 0.00 7.84 low 1-2 20.01 33.65 20.79 12.86 60.72 18.91 20.21 0.17 1.96 low biogeochemical and engineering characteristics of soils and groundwater around a dumpsite table 2: summary of physical-chemical analysis result of water samples table 4: summary of soil samples’ chemical and microanalysis parameters table 3: summary of phenol and microbiological parameters in water samples nd: not detected major elements –mg/l trace elements –µg/l parameter range n=12 mean usepa (2009) leachate ph 4.86-7.58 5.21 6.5-8.5 7.58 tds 26-453 184.80 1000 3,580 al 0.1-0.6 0.29 87 0.8 ba bdl-100 61.82 1000 520 cu 4-28 16.93 1000 1,300 ni bdl-600 122.0 20 1,000 zn 30-1100 449.60 5000 2,065 sr 20-250 48 260 ca 16-128 24.55 200 407.89 fe bdl-0.47 0.13 0.3 0.79 k 0.4-194 19.05 200.28 na 4.9-243 47.88 200 254.75 mg 0.6-63.1 9.40 50 225.6 p 0.04-3.64 1.20 4.44 si 2.4-10.3 5.44 15.4 cl 11.8-406 54.08 230 657.24 no3 0.02-250.3 70.11 10 450.23 hco3 0-78 12.6 132.22 so4 2.81-187 35.59 400 56.98 cd nd 3 1,100 cr nd 50 2,200 pb nd 10 6,200 parameters borehole within dumpsite borehole near the dumpsite leachate sample who (2004) epa (2005) microbiological analysis t. bacterial (cfu/100ml) 280 550 23,000 0 0 faecal col. (cfu/100ml) nd nd 10,000 0 0 t. fungi (mpn/ml) 21 56 1,500 0 0 t viral nd nd 230 0 0 phenol nd nd 0.08 s/n parameter l1 l2 l3 l4 l5 l6 major elements 01 ca 4.36 3.52 2.14 1.58 1.61 1.96 02 fe 28.14 27.97 22.63 18.78 23.04 24.75 03 mg 4.82 3.95 2.73 2.75 2.34 1.99 04 k 6.15 3.46 4.92 2.78 3.01 2.58 05 na 5.74 4.11 5.18 3.15 2.89 3.11 06 p 19.55 20.32 67.38 20 32.98 18.47 07 sulphate 600 525 600 240 480 500 trace elements 08 cu 70 55 42 30 26 16 09 as 16 14 11 5 4 -2 10 cd 1.8 0.12 0.43 0.21 0.12 0.10 11 pb 379 223 126 162 88 50 12 cr 223.34 196 75 185.10 103.47 154.86 13 zn 887.76 758.94 836.78 1,087.11 945.84 402.54 14 mo 4 3 2 3 2 1 15 ni 32 30 26 28 21 14 16 sr 53 42 33 24 20 8 la 35 25 24 23 19 16 mn 1439 620 513 436 384 128 17 nitrogen (organic) 18.33 16.48 20.34 25.12 18.34 10.34 18 toc 6.096 10.230 10.494 14.432 11.316 12.588 19 phenol 223 196 75 185.10 103.47 154.86 microanalysis 20 total bacteria count cfu/ ml105 32.5 24.8 18.9 20.1 18.7 14.9 21 faecal coliforms cfu/100ml103 63.3 56.7 62.5 50.2 59.4 27.4 22 total fungi count mpn/ ml102 10.2 7.44 6.81 9.05 8.35 5.34 odukoya am, oresanya o and abimbola, a.f fig 1: location map showing dump site and sample points in ojota biogeochemical and engineering characteristics of soils and groundwater around a dumpsite cations -10 0 10 20 30 40 50 60 70 80 90 w-1 leachate w-2 w-3 w-4 w-5 w-6 w-7 w-8 w-9 w-10 w-11 w-12 w-13 w-14 contro l water samples co nc en tra tio n ( mg /l) ca fe k mg na p si groundwater flow path groundwater flow path anions 0 50 100 150 200 250 w-1 leach ate w-2 w-3 w-4 w-5 w-6 w-7 w-8 w-9 w-10 w-11 w-12 w-13 w-14 contro l sample locations co nc en tra tio n (m g/ l) cl no3 so4 hco3 fig 2: concentration of metals in the groundwater flow 0 5 10 15 20 25 30 c o n ce n tr at io n ( m ) 0-1m 2m 4m 6m 8m 10m depth (m) ca fe mg k na 0 10 20 30 40 50 60 70 c 0n ce nt ra tio n (p pm ) 0-1m 2m 4m 6m 8m 10m depth (m) nitrogen phosphorus 0 1 2 3 4 5 6 co nc en tra tio n (p pm ) 0-1m 2m 4m 6m 8m 10m depth (m) cd pb odukoya am, oresanya o and abimbola, a.f fig 4: sample location map 0 200 400 600 800 1000 1200 co nc en tra tio n (p pm ) 0-1m 2m 4m 6m 8m 10m depth (m) zn cu cr 0 50 100 150 200 250 co nc en tr at io n 0-1m 2m 4m 6m 8m 10m depth (m) phenol 0 500 1000 1500 2000 2500 3000 3500 t o ta l b a c te ri a l (c f u /m l1 0 3 0-1m 2m 4m 6m 8m 10m depth (m) total bactrial fig 3: concentration of cations, nutrients, trace elements, total bacteria and phenol in soil samples at different depth intervals fig 5: concentration of toc, faecal coliform/ total fungi in soil samples at various depth intervals 0 2 4 6 8 10 12 14 16 to tal o rg an ic ca rb on (% ) 0-1m 2m 4m 6m 8m 10m depth (m) toc 0 100 200 300 400 500 600 700 c on ce nt ra tio n 0-1m 2m 4m 6m 8m 10m depth (m) feacal coliform total fungi biogeochemical and engineering characteristics of soils and groundwater around a dumpsite references abimbola a.f and odukoya a.m: geochemical characterization of soils, sediments and waters around active and abandoned dumpsites in lagos 25, supplement 1 (2006) pp. 131-132, doi 10.1007/ bf02839992. adeniyi aa, okedeyi oo (2004); 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okonkwo, j. o.; shabalala, p. n.; agyei, n.m., (2009). a novel adsorbent for heavy metal remediationin aqueous environments. int. j. environ. sci. tech., 6 (3), 425-434 (11 pages). odukoya am, oresanya o and abimbola, a.f 87 earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 87 95 groundwater potential zonation by remote sensing and gis techniques and its relation to the groundwater level in the coastal part of the arani and koratalai river basin, southern india s suganthi1, l elango2 and s k subramanian3 1 department of geology, anna university, chennai 600 025, india 2 department of geology, anna university, chennai 600 025, india 3 national remote sensing centre, hyderabad, india hydrogeology groundwater is being pumped extensively from the coastal part of the arani and koratalai river basin, tamil nadu, india for irrigation and water supply to the city of chennai. the objective of this study is to delineate the groundwater potential zones of this area using remote sensing (rs) and geographic information system (gis) techniques. weighted overlay analysis was used to demarcate the ground water potential zones. various thematic layers such as geology, geomorphology, soil, lineament density, drainage density, rainfall and landuse maps were prepared. the geological map was prepared using a geological survey of india (gsi) district resource map. indian remote sensing system linear imaging self-scanning sensor iii (irs-1d liss iii) satellite imagery was used to prepare the geomorphology, soil, lineament density, drainage density, and landuse maps. the final groundwater potential map was prepared by assigning appropriate weightage to different thematic maps and adding them to the final groundwater potential map. the derived groundwater potential map was overlaid with the groundwater level and location of well fields for validation. the map prepared will help in systematic and proper development of groundwater resources in this area to meet the growing water requirements of the city of chennai. aguas subterráneas se bombean en gran cantidad desde la parte costera en las cuencas de los ríos arani, en tamil nadu, india, para el riego y el aprovisionamiento de agua a la ciudad de chennai. el objetivo de este estudio es delinear las zonas potenciales de aguas subterráneas en esta área a través de sistemas de teledeteción (rs) y de información geográfica (gis). se hizo un análisis sobrepuesto compensado para demarcar las zonas con posibilidad de tener aguas subterráneas. se prepararon mapas de uso de la tierra con varios elementos temáticos como geología, geomorfología, terreno, densidad de lineamiento, densidad de drenaje y precipitación. el mapa geológico fue preparado a partir del mapa distrital de recursos, del servicio geológico de la india (gsi). fueron utilizadas imágenes satelitales sensing system linear imaging self-scanning sensor iii (irs-1d liss iii) para definir la geomorfología, terreno, densidad de lineamientos, densidad de drenaje y mapas de uso del terreno. el mapa final de zonas potenciales de aguas subterráneas se preparó con la superposición apropiada de diferentes mapas temáticos sobre los mapas de aguas subterráneas potenciales. para la validación apropiada, el mapa derivado se solapó con los de niveles de aguas subterráneas y de zonas de pozos. el mapa preparado ayudará en el desarrollo sistemático y apropiado de las fuentes hídricas subterráneas en esta área que permita abastecer la creciente demanda de agua de la ciudad de chennai. palabras clave: atributos sísmicos, inversión sísmica, indicador de zona de flujo, índice de calidad de depósito, unidad de flujo hidráulico, redes neuronales probabilísticas. record manuscript received: 17/10/2012 accepted for publication: 26/11/2013 abstract resumen key words: irs-id liss iii satellite imagery, paleochannels, overlay analysis, potential zones, groundwater level, chennai, well fields. 88 s suganthi, l elango and s k subramanian introduction groundwater is the largest fresh water resource on the earth, which makes it an important source for human consumption and the overall development of a region. mapping groundwater potential zones is essential for planning the location of new abstraction wells to meet the increasing demand for water. the occurrence, distribution, and movement of groundwater mainly depend upon the geological and hydro-geomorphological features of the area. a detailed study of groundwater occurrences can be made by surface and subsurface investigation methods. the groundwater potential zones and paleochannels were investigated using the geological and geophysical methods reported by shirke et al. (2005) and ariyo et al. (2011). however, these methods are based on extensive and time-consuming ground measurements that can be used only for large-scale mapping. the use of remotely sensed data along with geographic information system (gis) is well suited, and it can be easily combined with the data generated from conventional and ground measurement systems. remote sensing and gis possess new possibilities for hydrogeological studies (thomas et al., 1999; lokesh et al., 2007). high-resolution satellite imageries are widely used in groundwater studies due to their high spectral and spatial resolution. they are used to identify the geology, geomorphology, soil, lineament density, drainage density, rainfall and landuse for maps that indicate the occurrence of groundwater (preeja et al., 2011). rao et al. (2004) studied the integration of groundwater resources evaluation using the remote sensing method. the groundwater prospect evaluation by highresolution satellite imageries was reported by mondal et al. (2007). the overlay analysis by gis technique was used to delineate the groundwater potential zone studied by nag (1998); sikdar et al., (2004); prasad et al., (2008); teixeira et al., (2008); girish et al., (2008); nagarajan and singh (2009); and sukumar and sankar (2010). agricultural, industry and domestic water users in the north chennai region depend upon the pumping of groundwater from the arani and koratalai river basin. in addition, groundwater from this region is also being pumped by chennai metropolitan water supply and sewerage board (cmwssb) to supply water to the city of chennai. in spite of the largescale pumping of groundwater from this area, no attempt has been made to make use of remote sensing and gis techniques to identify groundwater potential zones. hence, the present study was carried out with the objective of mapping the spatial distribution of groundwater potential zones in the arani and koratalai river basin, which will assist in the proper development of groundwater resources in this area. descriptions of the study area the study region (1792 km2) is bounded between the latitudes of 13°7’00”n and 13° 27’ 00”n and longitudes of 79°47’30”e’ and 80°20’00”e (figure 1). the study area was demarcated based on the watershed divide in the northern and southern direction. as this study area is focusing only on the coastal part of the arani and koratalai river basin where groundwater is being pumped extensively, the state boundary to the west was considered the western boundary. topographically, the study area slopes gently from east to west, with the exceptions of a few hillocks, and there is no appreciable change in the north to south direction. the highest elevation of approximately 90 m was observed in the hill areas located in the northwestern part to sea level in the east. the atmospheric temperature in this area ranges from 22 to 42 ºc. the average annual rainfall is approximately 1100 mm. approximately 40% of the annual rainfall occurs during the south west monsoon season (july to september) and remaining 60% occurs during the north east monsoon season (october to december). the rivers in this region are non-perennial and a substantial quantity of water flow only occurs during the monsoon season. groundwater is the major source of water for drinking and agriculture purposes when compared to surface water in this region. agriculture is the main occupation for livelihood in this area, where rice, groundnut, sugarcane, pulses and banana are the most important crops. dug wells are the major source for irrigation, followed by tanks and bore/tube wells. figure 1. location of the study area methodology indian remote sensing system linear imaging self-scanning sensor iii (irs-1d liss iii) satellite imagery (april 2006) was used to prepare the geomorphology, soil, lineament, drainage density and landuse maps. these maps were prepared at a 1:50,000 scale. the geological map was prepared using the geological survey of india (gsi; 2001, 2005) district resource map. the gis analysis was carried out by using arcgis 9.3 (2007) of esri inc. the drainage density and lineament density were prepared using the density analysis tool in the arc gis 9.3. the rainfall distribution map was prepared using rain gauge station data. the integration method of weighted overlay analysis was adopted to combine all thematic layers by assigning rank and weightage for each thematic layer. weighted overlay analysis is a simple method to analyze multiclass maps based on the relative importance of each thematic layer and a layers class. several studies have reported the relative importance of various thematic layers that control the groundwater potential in an area. the weight for the selected thematic maps was based on saaty’s scale (1980). suja and krishnan (2009); nag and ghosh (2012); and etishree et al. (2013) have suggested the weightages based on saaty’s (1980) scale for different thematic maps to delineate groundwater potential zones. in the present study, the weightages were assigned based on these studies. the individual layers and their classes are assigned a weightage and a rank ranging from 0.10 to 0.25 and 1 to 4, respectively. higher weightage and ranks are given to the sites that are expected to have high groundwater potential. the thematic layers were then integrated by overlay analysis in the arc gis 9.3 environment. the groundwater potential zone was prepared by delineating the study area into four classes: very high, high, moderate and low. the cumulative score index (csi) was used for this classification. csi was calculated by multiplying the rank and weightage of each thematic layer as expressed in the following equation. csi = ∑ (geology rank x weight + geomorphology rank x weight + soil rank x weight, lineament density rank x weight + drainage density rank x weight + rainfall rank x weight + landuse rank x weight) 89groundwater potential zonation by remote sensing and gis techniques and its relation to the groundwater level in the coastal part of the arani and koratalai river basin, southern india where, wj is the normalized weight of the jth factor; xi is the rank of the ith classes/features of a thematic layer; m is the total number of thematic layers; and n is the total number of classes/features in a thematic layer (deepesh et al., 2011; rose and krishnan 2009). the mean and standard deviation of csi was used to delineate the region as very high (>mean+ standard deviation), high (mean to mean+ standard deviation), moderate (meanstandard deviation to mean) and low (<mean standard deviation). results and discussion groundwater availability mainly depends on geology, geomorphology, soil, lineament density, drainage density, rainfall and landuse. hence, all the thematic maps pertaining to the study area were prepared as per the methods explained earlier. the salient aspects of these thematic maps are described below. geology geology of the study area in arani and koratalai river basin, chennai city and its surrounding is shown in (figure 2). geologically, the study area is underlined by formations ranging in age from archean to recent. the central and coastal portions of the study area are dominated by sand and sandy clay, which are partly alluvial, marine and aeolian in origin. the southern portion has purple conglomerate and laterite, which contains small amounts of sandstones. the western parts are underlined by a thick pile of gondwana shale, clays and sandstones below the recent alluvial deposits. the southeastern portion of this area is covered by small patches of clay. beyond the study area in the southern direction, charnockite, sandstone and shale are widely distributed. the unconsolidated quaternary alluvial formations up to the depth of 50 m from the ground surface consist of sand, sandy clay and clay. these alluvial deposits function as an aquifer system. due to the presence of interlaying clay lenses in this formation the lower part function as a semi-confined aquifer. the upper parts of the quaternary alluvial formation function as an unconfined aquifer. the permeability in the lower part of the sequences is high compared to upper part. the hydraulic conductivity of the aquifer varies from 30 to 260 m/d and the specific yield from 0.15 to 0.18 (undp, 1987). the area with sand and sandy clay is a groundwater potential zone due to its high porosity and high permeability. a weightage of 0.20 was assigned for this layer. geomorphology the identification of geomorphologic features is very important for demarcating groundwater potential. the hydro-geomorphological features of this area were mapped using irs-id liss iii satellite data. the major geomorphologic features are alluvial plain, paleochannels, pedifigure 2. geology of study area, chennai city and southern part 90 ments, pediplain, valley fill, coastal plain, deltaic plain and denudational hill (figure 3). the four cycles of erosion gave rise to a complex assemblage of fluvial, estuarine and marine deposits in this area. geomorphic evolution and morphostructural analysis of the area suggests neotectonic activity during the quaternary period, resulting in marine regression, large-scale changes and shifting of the course of major rivers (undp, 1987). this has resulted in burial of old river courses due to subsequent sedimentation, and these paleochannels would generally be considered potential reservoirs for groundwater (samadder et al., 2011). the paleochannels of the study area were mapped using irs liss-iii imagery based on the tonal difference. the arani and koratalai basin mostly comprises alluvial plains with paleochannels, which are expected to be zones with groundwater potential (figure 3). this thematic layer is very important due to the presence of paleochannels and deltaic plains, which are potential zones for groundwater storage. hence, a higher weightage of 0.25 was assigned to this layer. soil the soil map was prepared by using irs-id liss iii data with additional input derived from the district resource map. four types of soils, i.e., entisols, inceptisols, alfisols and vertisols, are present in the study area. according to the united states department of agriculture (usda, 1999) soil taxonomy, entisols of recent origin found in floodplains are delta deposits. inceptisols are quite young soils and are found in steep slopes and flood plains. alfisols are clay-enriched subsoils with relatively high fertility. vertisols are dark black soils that are rich in expandable clay minerals that readily swell upon wetting and shrink when dried. this type of soil is suitable for rice cultivation due to the low permeability when saturated. the areal distribution of these soils in the study area indicates that the inceptisols are widely distributed (figure 4). the inceptisols and entisols have a high infiltration rate due to the presence of sand and gravel. as vertisols and alfisols contain clays, the infiltration rate in these soils is less. hence, regions consisting of inceptisols and entisols were given high ranks, and a 0.15 weightage was assigned to this layer. figure 4. soil map of the study area figure 3. geomorphology of the study area s suganthi, l elango and s k subramanian 91 lineament density the lineaments are straight, long, linear features that are enhanced by the permeability of the terrain, which is developed by tectonic activity. the lineaments are usually associated with a linear vegetative pattern, straight drainage course, ridge topography, etc. in general, lineament controls groundwater movement and storage. the lineaments present in the study area were derived from the irs-id liss iii satellite imagery. most of the lineaments are in the central part of the study area, and most are associated with straight drainage courses as shown in figure 5. the lineament length varies from a few meters to kilometers, and lineaments trend from northeast to southwest. the lineament density was determined from the density analysis tool in the arc gis 9.3 environment. the region was classified into three categories, low (0.00-0.15 km/km2), medium (0.150.47 km/km2) and high (>0.47 km/km2). the zones with very high lineament density were classified as high groundwater potential and assigned a weightage of 0.10. figure 5. lineament density of the study area drainage density drainage density helps to assess and understand the characteristics of runoff and groundwater infiltration in this area. drainage density (total length of drainage channels per unit area) values were calculated using the natural break method (jenks, 1967). the regions were classified as low (< 0.50 km/km2), medium (0.50-1.3 km/km2) and high (> 1.3 km/km2) drainage density zones, as shown in figure 6. areas with low drainage density indicate comparatively higher infiltration and low runoff. the zones with low to moderate drainage density are considered excellent groundwater potential zones and were assigned a weightage of 0.10. rainfall rainfall is a primary source of water for groundwater recharge in this area. the theissen polygon method was used to derive the rainfall distribution map. based on the rainfall, the region was classified into eight zones, as shown in figure 7. the central and southwestern parts of the study region receive comparatively more rainfall. the northern part receives the lowest amount of rainfall. the highest mean annual rainfall of 1300 mm was observed in the southern part of the study area. receiving more rainfall is favorable for the presence of more groundwater. this layer was assigned a weightage of 0.10. landuse/land cover the landuse and land cover map was prepared from the irsid liss iii satellite data using envi 4.7 software with supervised classification, as shown in figure 8. landuse/land cover patterns provide information about infiltration and runoff, which are controlled by the nature of surface material. landuse patterns such as built-up, forest, agriculture, wasteland, wet land and salt pans were identified. agriculture is the main occupation in this area. for the landuse/land cover map, a weightage of 0.10 was assigned. groundwater potential map the relative importance of various thematic layers and their corresponding classes were used for generating the groundwater potential zone map. the rank and weightage of individual thematic layers and the mean and standard deviation of csi for arriving at the groundwater potential map are given in table 1. the groundwater potential zonation map prepared by this method is shown in figure 9. the study area was classified into very high, high, moderate and low groundwater potential zones. very high (15.70%) and high (43.15%) groundwater potential zones contain geologic units of quartz gravel, purple conglomerate, sandstone, sand and sandy clay; geomorphological units of alluvial plain, deltaic plain, paleochannels and valley fills; high lineament density; soil units figure 6. drainage density of the study area figure 7. rainfall distribution in the study area groundwater potential zonation by remote sensing and gis techniques and its relation to the groundwater level in the coastal part of the arani and koratalai river basin, southern india 92 table 1. rank and weightage of individual themes and their csi values for groundwater potential zonation themes map weight (wj) individual class/features ranks for thematic units (xi) rank x weightage of individual classes csi statistics and groundwater potential geology 0.20 sandy clay 4 0.8 sandstone 4 0.8 purple conglomerate 3 0.6 quartz gravel 4 0.8 laterite 3 0.6 clay, salt marsh and tidal flat deposits 1 0.2 geomorphology 0.25 alluvial plain, deltaic plain 4 0.8 paleo channel 4 0.8 pediplain & valley fill 3 0.60 coastal plain & pediment 1 0.20 denudational hill 1 0.20 soil 0.15 inceptisols 4 0.4 entisol 3 0.3 alfisols 2 0.2 vertisols 1 0.1 lineament density (km/km2) 0.10 low (<0.15) 1 0.25 medium (0.15-0.47) 3 0.75 high (>0.47) 4 1 drainage density (km/km2) 0.10 low(<0.50) 3 0.3 medium (0.50-1.3) 2 0.2 high (> 1.3) 1 0.1 rainfall (mm) 0.10 low 780 1 0.1 low 813 1 0.1 medium -1019 2 0.2 medium -1090 2 0.2 medium -1240 2 0.2 medium -1264 3 0.3 high -1293 4 0.4 high -1300 4 0.4 landuse/land cover 0.10 water bodies 4 0.2 agriculture 3 0.15 wasteland & wetlands 2 0.10 built up, forest & salt pan 1 0.05 csi statistics minimum:1 maximum:3.9 mean: 2.82 standard deviation:0.52 groundwater potential very high csi>=3.34 high csi>=2.82 and csi<=3.34 moderate csi>=2.30 and csi<=2.82 low csi<=2.30 93 figure 8. landuse/land cover of the study area of inceptisols and entisols; low drainage density; and high mean annual rainfall. the moderate potential zone (25.33%) is dominated by geologic units of laterite, geomorphic units such as pediments, pediplain, coastal plains, soil units of entisols, medium lineament and drainage density. a low (15.01%) potential zone is present mainly in the coastal plain, and it includes saltpan, clay patches and salt marsh, tidal flat deposits and denudational hills, built-up and forest landuse patterns, a vertisol soil type and higher drainage density. the prepared groundwater potential map was compared with a groundwater contour map from april 2006 and the locations of well fields (figure 9). it is observed that in the very high and high groundwater potential zones, the groundwater table is reasonably flat with widely spaced groundwater contours. the relatively widely spaced groundwater contours surrounded by closely spaced contours indicate that the water table is relatively flat. the well fields of minjur, panjetty, kannigaiper, flood plain, thamaripakkam and poondi are located mostly in very high and high potential zones (figure 9). as the groundwater within the study area is under threat due to seawater intrusion, the groundwater pumping in the western part of these well fields is comparatively higher than the eastern well fields. conclusion this study was carried out to map the groundwater potential zones in the region of north chennai, india, where groundwater is being over exploited to meet the demand from agriculture, industry and domestic users. weighted overlay analysis was adopted to prepare a map of groundwater potential zones using seven thematic layers: geology, geomorphology, soil, lineament density, drainage density, rainfall and landuse. the delineated zones within this region were classified as very high, high, moderate and low groundwater potential zones. a groundwater table contour map and the locations of existing well fields were overlaid with the groundwater potential map. it is observed that in the very high groundwater potential zones the groundwater table is reasonably flat. further, it was identified that the existing well fields are located in regions with high groundwater potential zones. these observations validate the map prepared in this study. the saltpans and saline water intrusion are likely due to the over-extraction of groundwater, which affects the quality of the groundwater along the coastal region. to prevent seawater intrusion, groundwater pumping from the minjur well field needs to be stopped. to meet the increasing demand and to prevent seawater intrusion, new well fields may be formed in other high groundwater potential zones, especially in the western region of the poondi well field. this study demonstrates the application of remote sensing and gis techniques for integrating surface and subsurface information in a rapid and cost-effective manner, which may assist in locating sites for the development of groundwater well fields in the future. acknowledgement the authors wish to acknowledge the research grant (nrsc/hgd/ eoam-au/2009 ) provided by the national remote sensing centre (nrsc), department of space (dos), hyderabad, india, to 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(1987). hydrogeological and artificial recharge studies, madras, india, technical report, dp/up/ind-78-029/2. united states department of agriculture (usda) (1999). a basic system of soil classification for making and interpreting soil surveys, soil taxonomy, second edition, natural resources conservation service. keywords: iron oxide; magnetic soil; mössbauer spectroscopy; x-ray diffraction; sentinel-2; palabras clave: óxidos de hierro; suelos magnéticos; espectroscopía mösbbauer; difracción de rayos x; datos sentinel-2. how to cite item hincapié, b. , cortes-soto, a., bermudez, m. a., yepez, s., trujillo-hernandez, j. s., salguero londoño, b. m., & grande, s. (2021). coffee plantation soil characterization using a multimethod approach near the volcano nevado del ruiz, colombian central cordillera. earth sciences research journal, 25(3), 263-273. doi: https://doi.org/10.15446/esrj.v25n3.79170 the presence of iron oxides may provide a sensitive indicator of the effects of cropping practices on coffee plantations. authors characterized the mineral phases present in soil a horizons at three different farms located in the department of tolima within the regions of líbano and villahermosa. our analysis includes x-ray diffraction, mössbauer spectroscopy, and remote sensing to discriminate the distribution of the different magnetic mineral phases. x-ray diffraction was used to identify the mineralogical properties of iron oxide such as hematite, goethite, and ferrihydrite (fh), as well as tectosilicate minerals such as albite and sanidine. mössbauer spectroscopy results for samples taken at room temperature indicate the presence of fe2+ and fe3+ mineral phases, which possibly correspond to ilmenite or magnetite. finally, sentinel-2a multi-spectral imager (msi) data was used to map the distribution of iron oxides and study the influence of their distribution throughout the study area. a high correlation between mössbauer spectroscopy and sentinel-2a msi data exists throughout the study area. the results suggest that farms close to the main nevado del ruíz volcano have a more significant mineralogical variability. in contrast, more distant farms are characterized by soils with more iron oxides, the product of weathering, erosion, and human activities. abstract coffee plantation soil characterization using a multi-method approach near the volcano nevado del ruiz, colombian central cordillera caracterización de suelos cercanos al volcán nevado del ruíz, cordillera central de colombia, con técnicas multiherramientas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.79170 la identificación de óxidos de hierro puede proporcionar un indicador sensible para determinar los efectos de diferentes prácticas de cultivos en plantaciones de café. por tal razón, en la presente investigación se analiza el suelo de tres fincas ubicadas en el departamento del tolima. se caracterizaron las fases minerales con hierro a lo largo de la capa productiva (horizonte a) de los suelos en las regiones de el líbano y villahermosa. se utilizó difracción de rayos x, espectroscopia de mössbauer y teledetección para discriminar la distribución de las diferentes fases minerales magnéticas. la difracción de rayos x se utilizó para identificar las propiedades mineralógicas del óxido de hierro como la hematita, la goethita y la ferrihidrita (fh), además de los minerales tectosilicatos como la albita y la sanidina. los resultados de la espectroscopia de mössbauer para muestras tomadas a temperatura ambiente indicaron la presencia de fases minerales de fe2+ y fe3+, que posiblemente corresponden a ilmenita o magnetita. finalmente, se utilizaron datos y anchos de banda de sentinel-2a msi para mapear los óxidos de hierro y estudiar la influencia de su distribución en toda el área de estudio. los datos encontrados a través de las distintas técnicas, difracción de rayos x, espectroscopia de mössbauer, y la comparación con datos sentinel-2ª, son muy consistentes a lo largo del área de estudio. nuestros resultados sugieren que las fincas cercanas al principal volcán nevado del ruíz tienen una mayor variabilidad mineralógica. en contraste, aquellas que se encuentran más distantes presentan una mayor cantidad de óxidos de hierro, producto de la erosión y las actividades humanas. resumen record manuscript received: 18/04/2019 accepted for publication: 25/11/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 263-273 brayam hincapié1, alexander cortés-soto1, mauricio a. bermúdez2*, santiago yepez3, juan sebastián trujillo-hernández1, blanca myriam salguero-londoño1, sebastián grande4, 1universidad de ibagué, colombia 2universidad pedagógica y tecnológica de colombia (uptc) 3universidad de concepción, departamento manejo de bosques y medio ambiente, chile 4departamento de geología, universidad central de venezuela *corresponding author: mauricio.bermudez@uptc.edu.co g eo lo g y https://doi.org/10.15446/esrj.v25n3.79170 https://doi.org/10.15446/esrj.v25n3.79170 mailto:mauricio.bermudez@uptc.edu.co 264 brahiam hincapie, alexander cortés-soto, mauricio a bermúdez, santiago yépez, juan sebastián trujilo-hernández, blanca myriam salguero-londoño, sebastián grande 1. introduction colombia's coffee axis, called in spanish "eje cafetero" is a famous coffee region located near the intersection of four administrative regions (departments): caldas, quindío, risaralda, and tolima, along the foothills of the los nevados national park. the coffee axis is renowned for growing and producing most colombian coffee, considered to be the best in the world. most of the population in the eje cafetero depends on coffee production. farmers in the region apply different cropping practices (organic practices and traditional cultivation). this variety in cultivation techniques is thought to bring a unique characteristic to colombian coffee. different factors influence crop growth and yield (reddy et al., 2003; kang et al., 2009; zhang et al., 2001). internal factors associated with plants (genetic) and environmental or external factors (climatic, edaphic, biotic, physiographic, socio-economic, chemical, and physical) affect crop growth and yield. inadequate nutrients result in low crop production, increased disease, and decreased plant resistance to pests. in these cases, soil intervention and the use of agrochemicals are necessary. iron plays an essential role in respiration, photosynthesis, and the production of healthy green leaves (rout & sahoo, 2015). iron can be considered a micro-nutrient because only small amounts are required. iron deficiency produces chlorosis and stunted growth. in contrast, although not common, iron in excessive quantities could produce toxicity, with symptoms including bronzing, discoloration, and stippling of leaves. soil ph influences the availability of micronutrients (boron, iron, zinc, copper, and manganese) (khadka, 2016). iron toxicity is a frequent problem in acid soils (nenova, 2006). soil ph has a dominant effect on solubility and, therefore, availability and potential phytotoxicity of ions (nutrients and toxic elements) (clark and baligar, 2000). some plants secrete acids from their roots that decrease soil phvalue and inducing iron deficiency, reported as a common problem (shenker and chen, 2005). while most of the iron in soils is ferric (fe3+), ferrous iron (fe2+) is more important from a physiological point of view (hernández and muerer, 1997). due to its low solubility, the plant roots have access to fe2+ or fe3+ ions; thus, some mechanisms are employed to reduce the fe3+ ion to fe2+ ion before reaching the root crossing the selective membranes (colombo et al., 2013). fe3+ is insoluble in soils with neutral or high ph, which means it is not available for plants in alkaline and calcareous soils. in addition, iron in this type of soil is easily incorporated in phosphates, carbonates, and hydroxide ions (lindsay, 1991). changes in soil chemical properties are the result of different input cropping practices (clark et al., 1998). the identification of iron oxides may provide a sensitive indicator of the effects of cropping practices on coffee plantations. distribution and abundance of oxides and minerals may affect the nature of the coffee crop. magnetic characterization of coffee crops soils can be studied using mössbauer spectroscopy technique to identify different compounds, hydroxides, and iron oxides. additionally, x-ray diffraction is applied identify the mineralogy of both parent material and authigenic minerals in the soil. in cropping practices management, soil oxides identification can be useful to develop effective strategies for sustainable management, especially in a scenario of increasing shortage of mineral phosphate resources (fink et al., 2016). this study allows the characterization of the soils of the farms studied using multi-tool techniques. when this information it is integrated with production rates it is possible to infer the relationship among the different cropping practices techniques used for coffee cultivation, soil type and crop management. high iron levels often cause manganese deficiency in a plant because of the competitive ionic behavior of the two micronutrients. manganese deficiency exhibits similar symptoms as iron deficiency, such as yellowing of leaves, except that manganese deficiency affects both young and old foliage, while iron deficiency affects only young foliage. iron and manganese toxicity have similar symptoms in plants as well (li & lan, 2017). iron anomalies, such as: iron ore deposits, can be detected using remote sensing techniques such as landsat etm+ (ciampalini et al., 2013). as the strong pigmenting effect of fe oxides gives most soils their color, a first mapping of iron oxides was carried out in 1984 using coarse resolution landsat multispectral scanner (mss) data. subsequently, with the advent of 30 m landsat thematic mapper (tm), different bandwidths were established to separate argillic and nonargillic materials (using near infrared 5/7 bands) with the objective to map ferric /ferrous oxides (using visible 3/1 tm bands) (van der werff and van der meer, 2016). since 2000, the 14-band, 20 m pixel resolution aster sensor on the terra satellite has been used extensively in geological application studies for surface mineralogy mapping. to provide continuity with order landsat data, a subset of aster bands overlap with those of landsat tm, as do the sentinel-2a msi bands. the sentinel-2 is an earth observation mission from the european union copernicus program, which acquires in a systematically way optical imagery at high spatial resolution often between 10 m to 60 m over land and coastal waters. in this research, we discriminate between iron oxides and overall mineralogy at different scales. at the scale of a soil pit, we use mösbauer spectroscopy and x-ray diffraction to detect different mineral species in soils from three farms, with different cropping practices, this allow us to evaluate the impact of such methodologies on soil oxidation processes. remote sensing analysis using sentinel-2a msi data, combined with detailed soil pit mineralogy from the farms, allows us to scale up observations across the landscape. in addition, results of this study provide information to determine the influence of other geological processes such as lahars, and others erosion processes. 2. study area authors chose two coffee-growing municipalities located in northern tolima department, colombia: 1) the líbano municipality, and 2) the villahermosa municipality (figure 1). average elevation of the study area is 2079 meters above sea level (m.a.s.l) with variables slopes (medium to high) where coffee is cultivated. in these municipalities, soils are poorly developed, figure 1. location of the study area and maps of soils. map of soils is based on the nrcs soils database (eswaran and reich, 2005). 265coffee plantation soil characterization using a multi-method approach near the volcano nevado del ruiz, colombian central cordillera characterized by weak horizon development, and are classified as inceptisols (eswaran and reich, 2005; figure 1). 2.1 sampling and pretreatment of samples samples were collected from three different sites and production systems: villahermosa (orange polygon, figure 1) and san carlos farm in the municipality of líbano (yellow polygon, figure 1), where different cropping practices (conventional and best agricultural practices; bap) are applied. figures 2 provides visual context for the three farms selected for this study. figure 2. sampling sites: (a) villa beatriz, (b) san carlos, and (c) la estrella farms. (d) example of the soil located to the north of the farm la estrella. (e) thin section of the soil, photo taken to 4x and polarized light (d and e graphics were modified with permission from guarin, 2004). according to procedures established by the colombian agricultural institute (ica; ccdia, 2005), thirty (30) samples were taken from the productive soil layer (a-horizon) during the summer season between october and november, 2015 from a patch of approximately 562,500 m2 (five hundred coffee plants in each line; figure 3a), these were selected and marked with a color ribbon for repeated visits. we applied the same methodology in each farm (figure 3), for a total of ninety (90) samples. we discriminated the a-horizon (between 30 to 35 cm depth) by a thin dark layer formed by organic matter, altered or partially altered, where remains of animals, leaves, and branches among others can also be observed (ccdia, 2005). this layer is characterized by an abundance of minerals because of the erosion of the surrounding mountains. 3. materials and methods the following materials were used for the sampling procedure: a clean bucket, a drill bit, a shovel, cardboard boxes, and plastic bags. we scraped off the upper centimeter of the soil surface to remove fresh residues of organic matter, road dust or other artificial contamination. a v-shaped hole was dug approximately as wide as the width of the space, a depth of 30-35 centimeters was considered, depending on the location. a 2 to 3-cm thick slice of soil was cut into the wall of the trench and a 3 to 5-centimeter-wide strip was taken from the center of that slice. this strip of soil was placed in the bucket and the operation was repeated in each location. the extracted soil samples were mixed to roughly homogenize the individual samples with a subset retained and approximately 500 g discarded back into the pits. preserved at room temperature, the samples were hygienically separated and stored in flasks to be sent to the corresponding laboratories for analyses. 3.1 chemical parameters chemical parameters in the same farms studied here, were measured successively using the same exact methods and in the same laboratories during two dry and rainy seasons. among the measured parameters are ph, cation exchange capacity (cec), soil organic matter (som), phosphorus expressed as phosphorus pentoxide (p2o5) and potassium expressed as potassium oxide (k2o). 3.2 x-ray diffraction this technique was used to identify the mineral phases present in the soil samples. x-ray diffraction measurements were made in the cetene (strategic technologies center northeast) in the state of pernambuco, brazil. we did the x-ray diffraction (xrd) analysis for the soil using a shimadzu xrd-6000 x-ray diffractometer with cu radiation κα (λ = 1.54056 å), and a 40kv. tests were carried out at a temperature of 298k and 30 ma current and the measurements were acquired at angular intervals 0o ≤ 2θ ≤ 120o, compass δ(2θ) = 0.02°δt/ and step (2θ) = 4s representing a total acquisition time of 14 h 34 min 17s. figure 3. (a) sampling strategy, (b) villa beatriz farm’s soil. (c) san carlos farm’s soil. (d) coffee plant with red ribbon. (e) stem of the coffee plant, and (f) collected soil sample. 266 brahiam hincapie, alexander cortés-soto, mauricio a bermúdez, santiago yépez, juan sebastián trujilo-hernández, blanca myriam salguero-londoño, sebastián grande 3.3 mössbauer spectrometry the samples were measured in a transmission mössbauer spectrometer using a co-57/rh source in the mössbauer spectroscopy laboratory, of the physical metallurgy and phase transitions theory group of the universidad del valle, colombia. between 512 and 1024 channels were used for spectrum storage. the spectra were fitted by using the mosfit program (varret and teillet, 1976; larson and von dreele, 2004); and pure α-fe was used for calibration. mössbauer spectroscopy has advantages over x-ray diffraction in everything related to the identification of minerals containing fe as a selective technique (byrne and kappler, 2019). 3.4 sentinel-2a data processing in this study a sentinel-2a msi l1c image was used, which was downloaded from the european space agency (esa) through the sentinel scientific data hub (https://scihub.esa.int/dhus/). the spatial resolution of sentinel-2a msi is dependent on a spectral band. there are 13 spectral bands, from the visible and the near infrared to the shortwave infrared, at different spatial resolutions ranging from 10 to 60 meters on the ground. the four bands at 10-meter resolution are normally used for basic land-cover classification. the other six bands, at 20-meter resolution, are used to obtain geophysical parameters, whereas the bands at 60-meter resolution are mainly dedicated to atmospheric corrections and cirrus-cloud screening. before using the sentinel2a msi lc1 dataset we performed a series of pre-processing tasks: 1) an atmospheric correction was implemented using sen2cor (software version 2.5.5), and 2) a combination of bands, resampled to 20 m resolution. different bandwidth of electromagnetic radiation, or ratios of bands serve as indicators for mineral sets or mineral groups. table 1 provides an overview of the landsat 5 tm and aster bandwidth suitable for use with sentinel2a msi. we focus on bands originally defined for landsat 5 tm by sabins (1999), which require sentinel-2a msi bands 2, 4, 11 and 12. additionally, the band 8a of the sentinel-2a msi was required for masking vegetation using the ndvi result. three bandwidths, defined in table 1, were selected for mapping the alteration of the mineralogy around the regions defined as study areas in the tolima department in colombia. this combination highlights hydroxyl bearing alteration (s-2a msi 11/12), discriminates iron oxides in general (s2a 4/2), and emphasizes ferrous iron oxides specifically (s-2a msi 4/11). finally, the mineral alteration map obtained from sentinel-2a msi was compared with the spatial locations of each of the three farms where the samples were collected. table 1. bandwidth applied in sentinel-2a msi as proxies for mapping mineralogy (modified after van der werff and van der meer, 2016) parameter to discriminate bandwidth landsat 5 tm ratios (compatible with sentinel 2a-msi) hydroxyl bearing alteration 11/12 all iron oxides 4/2 ferrous iron oxides 4/11 aster iron ferric iron, fe3+ 4/3 ferrous iron, fe2+ 12/8 + ¾ laterite 11/12 gossan 11/4 ferrous silicates 12/11 ferric oxides 11/8 aster silicates alteration 11/12 aster other vegetation 8/4 ndvi (8 4)/(8 + 4) 4. results 4.1 chemical parameters table 2 summarize the results found during the dry and rainy seasons, respectively. the measurements for the rainy season were carried out from may to august 2015, whereas for the dry season they were accomplished between august and december 2015. this table present the mean values, and the standard deviation (sd). during the rainy season, la estrella farm, with organic practices system, recorded the highest mean ph value (5.3); san carlos farm, with bap system, presented a ph mean value of 5.2 ± 0.3; and villa beatriz farm, with conventional system practices, obtained mean ph values of 5.1 ± 0.2, being all these values within the normal ph range for coffee cultivation (5 5.5; sadeghian, 2008). concerning the cec, villa beatriz farm gave a value of 19.7 ± 0.6 meq/100 g, san carlos and la estrella farms presented values of 18.0 ± 3.0 and 17.7 ± 3.1 meq/100 g, respectively. for the som parameter, villa beatriz farm obtained a mean value of 2.3 ± 0.1%, followed by la estrella farm with 2.1 ± 0.3% and san carlos with 2.0 ± 0.2%. all these values are less than 5%, which are considered low for coffee production (sadeghian, 2008). regarding the phosphorus oxide (p205) content, la estrella farm gives mean values of 52.7 ± 5.5 ppm, san carlos farm registers the highest mean value with 25.3 ± 12.7 ppm followed by villa beatriz with 21.7 ± 10.2 ppm. these values are all within the optimal range considered for coffee production (sadeghian, 2008). table 2. chemical parameters evaluated in three (3) farms with different coffee production systems during the rainy season (may-august 2015) and the dry season (august-december 2015). farm and/or productive system parameter meanr sdr meand sdd san carlos (bap)1 ph 5.2 0.3 5.1 0.3 cec (meq/100 g) 18.0 3.0 15.7 0.6 som (%) 2.0 0.2 1.7 0.1 p205 (ppm) 25.3 12.7 29.0 11.5 k2o (ppm) 22.7 7.4 11.3 3.1 villa beatriz (cs)2 ph 5.1 0.2 5.1 0.1 cec (meq/100 g) 19.7 0.6 19.3 0.6 som (%) 2.3 0.1 2.0 0.1 p205 (ppm) 21.7 10.2 34.7 3.8 k2o (ppm) 12.1 0.8 25.8 10.6 la estrella (op)3 ph 5.3 0.0 5.0 0.1 cec (meq/100 g) 17.7 3.1 20.7 4.0 som (%) 2.1 0.4 2.0 0.1 p205 (ppm) 52.7 5.5 43.0 3.0 k2o (ppm) 18.0 7.4 9.4 3.5 bap1: best alternative practices, cs2: conventional system, op3: organic practices. superscripts r and d correspond to rainy and dry season, respectively. concerning the potassium oxide (k2o), villa beatriz farm provides the lowest mean value with 12.1 ± 0.8 ppm, san carlos with 22.7 ± 7.4 ppm and la estrella 18.0 ± 7.4 ppm. however, all these values are within the normal range of k2o (7.8 – 27.4 ppm) according to sadeghian (2008). the ph parameter during the dry season for la estrella farm has a mean value of 5.0 ± 0.1, san carlos farm with 5.1 ± 0.3 and villa beatriz farm with 5.1 ± 0.1. for the cec, la estrella has the highest mean value with 20.7 ± 4.0 meq/100g, followed by villa beatriz farm with 19.7 ± 0.6 meq/100gr and san carlos farm with 15.7 ± 0.6 meq/100g. concerning the som parameter, the results obtained were: san carlos 1.7 ± 0.1%; villa beatriz 2.0 ± 0.1%; and la estrella 2.0 ± 0.1%, respectively. for the p2o5, san carlos farm exhibits the lowest mean value with 29.0 ± 11.5 ppm. villa beatriz farm with 34.7 ± 3.8 ppm, and la estrella farm with 43.0 ± 3.0 ppm. for the k2o, villa beatriz exhibits the highest mean values of 25.8 ± 10.6 ppm; san carlos with 11.3 ± 3.1 ppm, and la estrella with 9.4 ± 3.5 ppm. https://scihub.esa.int/dhus/ 267coffee plantation soil characterization using a multi-method approach near the volcano nevado del ruiz, colombian central cordillera 4.2 x-ray diffraction figure 4 shows x-ray diffractograms for the three samples: d1 (villa beatriz), d2 (san carlos), and d3 (la estrella), which were discriminated using the american mineralogist crystal structure database (downs and hallwallace, 2003). peaks of different intensities are associated with the presence of various minerals containing iron oxides in its structure, such as: hematite, goethite and ferrihydrite. other minerals without iron in their chemical composition were also identified, such as: illite, quartz, na-feldspar, k-feldspar, calcite, albite-oligoclase (table 3). table 3. common minerals identified by x-rays diffraction. abbreviation mineral chemical formula category q quartz sio2 mineral oxide g goethite α-feo3+(oh) mineral oxide fna na-feldspar (na,ca)(si,al)3o8 mineral tectosillicate h hematite fe2o3 mineral oxide m muscovite kal2(alsi3o10)(oh)2 mineral phylosillicate i illite (k,h3o)(al, mg, fe)2(si, al)4o10[(oh)2,(h2o)] mineral phylosillicate s sanidine k(alsi3o8) mineral tectosillicate c calcite caco3 mineral f ferrihydrite fe10 3+o14(oh)2 mineral oxide table 4. crystallographic information of the peaks of mineral phases found in sample d1 mineral name position intensity miller index (h k l) i.d (a) ferrihydrite (f) 18.0545 0.183 0 0 2 4.91 quartz (q) 20.8173 0.2812 1 0 0 4.26 goethite (g) 21.8953 0.4667 1 0 1 4.05 na-feldspar (fna) 22.8278 0.5744 1 0 0 3.84 calcite (c) 23.6601 0.6568 0 1 2 3.76 hematite (h) 24.4236 0.9736 0 1 2 3.68 quartz (q) 26.6669 0.4633 1 0 1 3.34 illite (i) 26.6693 0.3261 0 2 2 3.34 muscovite (m) 27.7562 0.4949 1 1 4 3.2 calcite (c) 29.5205 0.4351 1 0 4 3.02 illite (i) 30.2548 0.3595 -1 1 3 2.95 calcite (i) 31.4365 0.3946 0 0 6 2.84 muscovite (m) 32.8954 0.2844 -1 1 6 2.79 illite (i) 35.608 0.2927 0 0 4 2.52 ferrihydrite (f) 36.7349 0.1536 1 0 1 2.44 quartz (q) 39.267 0.2425 1 0 2 2.29 quartz (q) 42.5529 0.6747 2 0 0 2.12 illite (i) 44.9159 0.2626 -2 2 3 2.01 goethite (g) 51.4516 0.3527 4 1 1 1.67 quartz (q) 54.7409 0.1802 0 2 2 1.65 goethite (g) 57.0544 0.1686 3 1 2 1.61 calcite (c) 60.0971 0.1618 2 1 4 1.53 ferrihydrite (f) 61.3733 0.1748 1 0 5 1.51 illite (i) 62.5602 0.1748 3 3 0 1.44 quartz (q) 68.2671 0.1748 0 3 1 1.37 figure 4. x-ray diffractograms of soil a horizon samples of coffee crops soils: a) villa beatriz, b) san carlos, both farms located at the líbano municipality, and c) la estrella, villahermosa municipality, tolima department (colombia). in this figure, letters in colors correspond to: q, quartz; h, hematite; g, goethite; i ilmenite; f na na-feldspar; m muscovite; o orthoclase; mi microcline; c calcite; and f ferrihydrite. there are peaks of minerals associated with sample d1: the presence of iron oxides such as hematite, goethite and ferrihydrite, of which a peak of maximum intensity (0.9736) is observed on the diffractogram (figure 4a; table 4), this denotes the presence of hematite with an interplanar distance of 3.68 å. goethite has reflections along the diffractogram with variable intensities, ferrihydrite is present in very low amounts based on the diffractogram peaks intensities of the samples quartz and illite peaks, with intensities of 0.4633 and 0.3261, respectively, are observed with an interplanar distance of 3.34 å. quartz is abundant in all the samples, with several peaks visible in the diffractogram. the detection limit of the shimadzu xrd-6000 permits determining if the peak is present, thus no reflection in bragg diffraction (intensity of diffracted radiation) permits to discrimination of quartz, muscovite, calcite and illite. as in the case of hematite, na-feldspar was also detectable. 268 brahiam hincapie, alexander cortés-soto, mauricio a bermúdez, santiago yépez, juan sebastián trujilo-hernández, blanca myriam salguero-londoño, sebastián grande for muscovite, both with an interplanar distance of 1.87 å. muscovite and quartz are abundantly present in the sample, reflected in very well-defined peaks with relatively high intensities. the table 7 provide information on the crystalline structure and network parameters of main discriminated minerals. table 6. crystallographic information of the peaks of mineral phases found in sample d3 mineral name position intensity miller index (h k l) i.d (a) ferrihydrite (f) 18.1451 0.183 0 0 2 4.88 quartz (q) 20.9441 0.1988 1 0 0 4.24 goethite (g) 21.0190 0.3175 1 0 1 4.15 na-feldspar (f na) 22.0274 0.3175 1 0 0 3.84 muscovite (m) 23.7513 0.2327 0 2 3 3.74 orthoclase (o) 25.6915 0.2985 -1 1 2 3.46 quartz (q) 26.5976 0.2406 1 0 1 3.35 illite (i) 26.5332 0.1795 0 2 2 3.35 sanidine (s) 27.8811 0.2891 -2 0 2 3.19 orthoclase (o) 27.8814 0.2311 -2 0 2 3.19 muscovite (m) 28.5265 0.2449 -1 1 5 3.12 illite (i) 30.4259 0.2474 -1 1 3 2.93 hematite (h) 33.0678 0.2182 1 0 4 2.7 illite (i) 34.6052 0.1926 -2 0 1 2.59 ferrihydrite (f) 35.5485 0.1727 1 0 0 2.52 sanidine (s) 38.5325 0.5323 -1 1 3 2.33 orthoclase (o) 38.5242 0.4628 -1 1 3 2.33 microcline (mi) 38.5726 0.2337 -1 1 3 2.33 quartz (q) 39.7190 0.1516 1 1 1 2.26 quartz (q) 40.5794 0.1447 1 1 1 2.22 goethite (g) 41.7619 0.1635 4 0 1 2.17 muscovite (m) 44.3541 0.1586 -2 2 5 2.04 quartz (q) 45.5363 0.2251 0 2 1 1.99 calcite (ca) 48.4306 0.1484 1 1 6 1.87 muscovite (m) 48.4387 0.1065 1 3 7 1.87 quartz (q) 50.1852 0.1366 1 1 2 1.81 muscovite (m) 52.4450 0.1633 -2 2 8 1.74 quartz (q) 59.4935 0.1033 2 1 1 1.55 muscovite (m) 60.7562 0.2733 -1 3 11 1.52 ferrihydrite (f) 62.3980 0.1116 1 1 0 1.48 table 7. information on the crystalline structure and network parameters of each mineral mineral crystalline structure a (å) b (å) c (å) quartz (q) trigonal 4.9160 4.9160 5.4054 goethite (g) orthorhombic 9.9134 3.0128 4.5800 na-feldspar (f na) trigonal 3.7670 3.7670 6.1540 hematite (h) trigonal 5.0380 5.0380 13.7720 muscovite (m) monoclinic 5.1988 9.0266 20.1058 illite (i) monoclinic 5.2021 8.9797 10.2260 sanidine (s) monoclinic 8.5160 13.0230 7.2060 monoclinic (mo) monoclinic 8.5440 13.0300 7.1950 microcline (mi) triclinic 8.5784 12.9600 7.2112 calcite (ca) rhombohedral 4.9900 4.9900 17.0615 ferrihydrite (f) trigonal 2.9550 2.9550 9.3700 table 4 relates reflection angles and intensities for the minerals present in the sample. moreover, the interplanar distance (i.d.) is shown along with the miller indices (pierret, 1989) of each crystallographic plane. the diffractogram of sample d2 (figure 4b, table 5) is like d1, hematite is present but with lower intensities (0.3072, 0.2225, 0.225). goethite is present in greater quantities than d1, with a similar first peak and intensity. (d1 i = 0.4667; p = 2.8953 å, d2 i = 0.4161; p = 21.8592 å). the other goethite peaks present are well defined within the diffractogram with relevant intensities (0.3751, 0.1862, 0.2867, 01514) denoting a marked presence of this mineral. like d1, ferrihydrite, is found in small quantities. an overlap of quartz and illite can also be observed in this sample, both with an interplanar distance of 3.37 å, however a higher quartz intensity (0.7601) and a lower intensity for the illite (0.2867) is discernable. sanidine or anorthoclase with relevant intensities (0.5221, 0.3051) was found, with an interplanar distance of 3.25 å and 2.77 å, respectively, as were albite-oligoclase, muscovite, illite and calcite. table 5. crystallographic information of the peaks of mineral phases found in sample d2 mineral name position intensity miller index (h k l) i.d (a) quartz (q) 20.7783 0.2627 1 0 0 4.27 goethite (g) 21.8592 0.4161 1 0 1 4.06 na-feldspar (fna) 22.7396 0.5628 1 0 0 3.84 muscovite (m) 23.5825 0.4691 0 2 3 3.77 hematite (h) 24.3604 0.3072 0 1 2 3.65 muscovite (m) 25.4292 0.3098 -1 1 4 3.5 quartz (q) 26.4362 0.7601 0 1 1 3.37 illite (i) 26.4213 0.2867 0 2 2 3.37 orthoclase (o) 27.66 0.5221 0 4 2 3.25 muscovite (m) 28.3467 0.4533 -1 1 5 3.14 calcite (c) 29.4877 0.3388 1 0 4 3.03 orthoclase (o) 32.6397 0.3051 -1 3 2 2.77 illite (i) 35.6447 0.2955 2 0 0 2.51 quartz (q) 42.864 0.2351 2 0 0 2.12 ferrihydrite (f) 45.1995 0.1538 1 0 3 1.97 hematite (h) 49.7349 0.2225 0 2 4 1.83 goethite (g) 50.7539 0.3751 1 1 2 1.79 goethite (g) 53.2739 0.1862 2 1 2 1.71 hematite (h) 54.5495 0.225 1 1 6 1.68 quartz (q) 55.5161 0.1849 0 1 3 1.65 quartz (q) 59.8721 0.2047 1 2 1 1.54 ferrihydrite (f) 61.5306 0.1492 1 0 5 1.51 ferrihydrite (f) 62.4578 0.1543 1 1 0 1.47 goethite (g) 63.6514 0.2867 2 0 3 1.46 illite (i) 67.7404 0.187 -2 4 5 1.38 quartz (q) 68.2719 0.1778 0 2 3 1.36 goethite (g) 69.5441 0.1514 7 0 1 1.35 sample d3 (figure 4c, table 6) shows a lower presence of oxide minerals, hematite is only reflected in a peak with an interplanar distance of 2.7 å and an intensity of 0.2182. the concentration of goethite is low with interplanar distances of 4.15 å, 2.17 å at positions 21.019°, 41.7619°, respectively. as observed in the other samples, an overlap between quartz and illite is observed in this diffractogram, albeit with a lower intensity compared to the samples d1 and d2. three additional overlaps are present: sanidine and orthoclase, both minerals with an interplanar distance of 3.19 å, in which sanidine has an intensity of 0.2891 and orthoclase 0.2311; the second an overlap between sanidine, orthoclase and microcline all with an interplanar distance of 2.33 å and intensities of 0.5323, 0.4628, 0.2337, respectively; and a third overlap between calcite and muscovite with intensities of 0.1484 for calcite and 0.1065 269coffee plantation soil characterization using a multi-method approach near the volcano nevado del ruiz, colombian central cordillera 4.3 mössbauer spectrometry magnetic analysis of the homogenized soil samples by mössbauer spectroscopy, allowed a detailed and selective analysis of their mineralogical composition. this type of analysis is central to the identification of the octahedral structural sites that are occupied by the different cations of fe, considering its hyperfine parameters. results for the samples suggest two paramagnetic doublets with mineral phases containing fe+2 and fe+3. the fe+2 can be assigned to magnetite and ilmenite and 2:1 clay. in contrast, fe+3 can be associated with hematite, goethite and ferrihydrite that were identified by the x-ray diffraction technique. table 8 summarizes the hyperfine parameters obtained for the three samples. in this table, the isomeric shift is indicated by is, the line width by lw, the intensity by h1, the quadrupole splitting by sq and a is the area. the mosfit (the modular open source fitter for transients), a python 2.7/3.x package for fitting, sharing, and estimating the parameters of transients, was used. for sample d1, two paramagnetic doublets are identified, which indicate the presence of magnetic crystalline fe sites belonging to fe+2 and fe+3 and can be attributed to the presence of oxide minerals. for this sample, the clay minerals in the valence state of fe+2 is found in a greater quantity covering an area of 56.11%, with an isomeric shift of 0.409 mms-1, leaving hematite in the background, which presents a valence state of fe+3 and covers an area of 43.89% with an isomeric shift of 1.094 mms-1. for sample d2, two paramagnetic doublets are also discriminated, similar to those presented in the previous sample, ferrous clay minerals, magnetite, ilmenite (fe+2) and goethite, hematite (fe+3) are also present. the sample d3 show a single paramagnetic doublet corresponding to goethite but unlike the previous samples, them the valence state of this oxide mineral is fe+3, which has an isomeric deviation of 0.227 mms-1 and covers an area of 100% of the sample. samples d1 and d2 (figure 5) exhibit similar spectra but different intensities per area, where two paramagnetic doublets were discriminated, and associated with the presence of goethite and hematite (fe+3). such minerals are recognized as beneficial to the absorption of nutrients by plants. similarities in these spectra are due to the geographic proximity of the farms, thus soil mineral content has been altered, facilitating the accumulation of similar oxide minerals. the presence of fe+3 in these three samples suggests that this state of valence of iron is precipitated as oxide and hydroxide. then, this released iron is incorporated into lamellar silicates such as clays. subsequent mixing with soil organic matter leads to reduction of ferric iron to ferrous iron (e.g. magnetite, ilmenite or wüstite). the presence of fe+2 is attributed to magnetite and ilmenite the common soil oxides in this study’s agro-ecological conditions, however ferrous clay minerals are also present. fe+3 corresponds to goethite and hematite which are directly associated with soils in tropical areas where temperatures and ph facilitate its formation. 4.4 remote sensing high content of magnetite, ilmenite and hydroxyl bearing alteration are associated with laharic mud deposits from the last eruptions of the nevado del figure 5. mössbauer spectra of coffee crop soil a horizons: a) d1: villa beatriz farm, b) d2: san carlos, both farms located at the líbano municipality and c) d3: la estrella at the villahermosa municipality. table 8. hyperfine parameters obtained for the different samples sample doublet is (mms-1) lw (mms-1) h1 (coups) sq (mms-1) area (%) d1 (villa beatriz) magnetite 1 (fe+2) 0.403 ± 0.0391 0.230 ± 0.0514 200729219 ± 0.0333 0.650 ± 0.0671 56.11 hematite 2 (fe+3) 1.088 ± 0.0627 0.280 ± 0.0103 129102219 ± 0.0300 2.370 ± 0.0000 43.89 d2 (san carlos) magnetite 1 (fe+2) 0.403 ± 0.0380 0.205 ± 0.0490 19882527 ± 0.0364 0.622 ± 0.0663 51.47 hematite 1 (fe+3) 1.084 ± 0.0589 0.277 ± 0.0987 13874722 ± 0.0314 2.370 ± 0.0000 48.53 d3 (la estrella) goethite 1 (fe+3) 0.221 ± 0.0160 0.135 ± 0.0218 178930859 ± 0.0217 0.309 ± 0.0217 100 270 brahiam hincapie, alexander cortés-soto, mauricio a bermúdez, santiago yépez, juan sebastián trujilo-hernández, blanca myriam salguero-londoño, sebastián grande ruiz volcano on november 13, 1985. this can be observed around the volcanic cone, where a high abundance of these minerals was detected using specific bands present in the sentinel-2a satellite sensor msi which covers 13 spectral bands (443–2190 nm) (figure 6). so, fe+2 is in the form of magnetite or ilmenite that has not had enough time to decompose, because they are relatively stable; also, in pyroclastic and lahars so young, it is possible that fe+2 is in its mineral form as biotite or hornblende. large parts of these volcanic rock units fall into the image pixels that are covered by vegetation, and thus masked out for analysis (figure 6a, 7a, 7c). the san carlos and villa beatriz farms are dominated by a thick vegetation cover that masks the spectral response of the minerals present (figure 6b). despite the closeness of the farms and the small area, the synoptic analysis provided by the sentinel data, our analyses suggest that different spectral signatures related to oxide and hydroxyl bearing iron phases could be discriminated (figure 7-b). in contrast, in the la estrella farm, the abundance of iron oxiderelated minerals is much lower (figure 7-d). although the images show areas of exposed soil which are not affected by the vegetation cover, the detection of iron mineralization is almost negligible. in urban areas, our sentinel analyses show false positives, which constitute an anomaly since a large amount of infrastructure (buildings, vehicles, water pipes, etc) is concentrated in these areas. recent studies have demonstrated the sentinel’s high sensitivity to iron include depth discrimination of mines located near urban areas (mielke et al., 2014). the present investigation confirms the high sensitivity of these analyses to detect such structures. 5. discussion the soils of the three farms selected for this study are inceptisols (figure 1) derived principally from volcanic deposits and sedimentary lithic fragments. thus, the parent material is expected to be similar. our study shows significant differences across analyzed sites may linked to the proximity of the farm to the volcano. unfortunately, the number of farms considered in this work was not significant enough to relate farm agricultural practices with coffee soil production. detecting these compositional differences in the same soil type is not a simple task, and the multi-method approach used in this research allowed the detection of such differences. results indicate the presence of fe+2 and fe+3 mineral phases based on ambient temperature spectra and paramagnetic doublets for all samples. the x-ray diffraction identified iron oxide mineral phases, as well as the recognition of other minerals such as feldspars, quartz, phyllosilicates, and carbonates. for the villa beatriz (d1), a varied mineralogy (calcite, illite, quartz and muscovite) was identified, although oxide minerals such as hematite and goethite appear in small proportions. the san carlos farm (d2) has a soil rich in minerals, in addition to calcite, illite, quartz and muscovite that were present in sample d1, other minerals in d2 such as: orthoclase, hematite, goethite and ferrihydrite are present in notable quantities, suggesting a fertile soil. the soils of the la estrella farm (d3) are the most mineralogically diverse containing: feldspars such as sanidine, microcline and orthoclase, which indicate that the fertility of this soil is very high, because and together with the organic load, they favor the correct development of plants. diverse studies have shown that fe and al oxide minerals are important regulators for the stabilization of a soil's organic content (soc) in oxisols (kleber et al., 2005; zhang et al., 2010; ye et al., 2017). in the soils analyzed here, fe and na oxide minerals were found but not al; additionally, soils in our study area have a very low organic matter content (som), which is typical of inceptisols in tropical and humid areas (igwe et al., 2010). from our remote sensing analysis with sentinel-2a data, we observed a regional signal from bandwiths that indicate hydroxyl bearing iron and ferrous oxides, which could explain the high acidity of soils, and this is consistent with our ph measurements (5.0 ± 0.5). in ice sheets or glaciers, the exchange between hydroxyl-bearing minerals and water indicate effects of glacial erosion. during this process abrasion of clays and micas increases the number of broken bonds in the crystalline structure of the mineral, which promotes the development of cleavages and accelerates the rate at which these minerals exchange oxygen and hydrogen isotopes with water (souchez et al., 1990). this explains the largescale presence of hydroxyl-bearing minerals. (b) figure 6. (a) google image showing the position of the nevado del ruíz volcano with respect to the villahermosa and líbano towns. (b) a color composite (rgb: r: 4, g: 3, b:2) made with sentinel-2a msi, combination of band ratios 11/12 in red, 4/2 in green, and 4/11 in blue are showed. 271coffee plantation soil characterization using a multi-method approach near the volcano nevado del ruiz, colombian central cordillera our results demonstrate the existence of a large ice cover that could have accelerated the rate of exchange between hydroxyl-bearing minerals and water. additionally, our remote sensing analysis is sensitive enough to detect anomalies of iron oxides in large flat surfaces corresponding to cities, but also, those flat surfaces could correspond with topographic remnants that have been preserved by the different volcanic episodes. 6. conclusions mössbauer spectroscopy selectively identifies iron-bearing mineral phases in coffee-growing soils by determining the presence of fe+2 and fe+3, where the latter was more prevalent. the application of this technique allowed identification of mineral phases such as magnetite, ilmenite, hematite and goethite which were also independently identified by x-ray diffraction. the presence of fe+2 in the form of magnetite or ilmenite suggests that soils have not had time to decompose, because they are relatively stable; also, in pyroclastic and lahars so young it is possible that fe+2 is derived from rock forming minerals biotite and hornblende. in addition to the abundant presence of quartz, feldspars such as sanidine, microcline, orthoclase and sodium feldspars were found, which belong to the group of tectosilicate minerals, as well as phyllosilicate minerals such as illite and biotite. the different agricultural practices applied in the sampled productive region could have had an influence on the quality of the soils, since it is evident that those that deploy environmentally friendly practices had a greater quantity figure 7. a color composite (rgb: r: 4, g: 3, b:2) made with sentinel-2a msi (a-c) for the san carlos, villa beatriz and la estrella farms. combination of bandwidths 11/12 in red, 4/2 in green and 4/11 in blue (b-d). 272 brahiam hincapie, alexander cortés-soto, mauricio a bermúdez, santiago yépez, juan sebastián trujilo-hernández, blanca myriam salguero-londoño, sebastián grande of feldspars which are linked to the abundance of organic matter and this in turn is related to good soil quality. however, the fertility of minerals for the soil around farms is due to the proximity to the volcano, rather than to the organic matter content. the differences observed between these three farms have also been observed in other municipalities in the department of tolima, such as in ibagué and santa isabel (bustos-rodríguez et al., 2012) where the northernmost farms exhibit minor presence of fe+2. the physical-chemical analyses of the soils sampled for iron (fe) vary from low (4 to 5 mg/kg) to adequate/high (23 to 25 mg/kg). the ph data were found to be within acceptable levels for coffee cultivation. this work demonstrates the usefulness band-specific ratios of sentinel-2 msi data for the detection of areas with high surface iron content. remote sensing methodology can be applied cost-effectively in larger regions of colombia and can provide interesting insights into the possibility of detecting new areas for the cultivation of high-quality coffee. to increase the reliability of the study, it is recommended to increase the number of samples and production systems to be sure that the agricultural practices carried out contribute to the quality and sustainability of the soils. it is suggested that the measurements be made by mössbauer spectroscopy at different temperatures to be able to appreciate other oxide minerals that are not evident at room temperature. in addition, it is necessary to carry out a detailed study of inceptisols from similar parent material, in non-cultivated regions. this allow to have a frame of reference on the process of soil degradation and the influence of different agricultural practices. acknowledgements we thank to former students lina paola bermúdez-valderrama, and nicol alexa gaitán-díaz. luz patricia naranjo of the universidad de ibagué. colleagues of the group of physical metallurgy and phase transition theory of the universidad del valle for facilitate the mössbauer spectrometry analysis; the coffee producers of the villa beatriz, san carlos and la estrella farms; dairo mejía for his support and hospitality during field work. to rosa jiménez and gregory hoke for their invaluable assistance during the language review. this project was possible through funding provided by the universidad de ibagué: project 14-323-int universidad de ibagué, and project din-sgi-2666 of the universidad pedagógica and tecnológica de colombia (uptc). finally, we thanks to v. kumar, andrés torres, alexander caneva and an anonymous reviewer for their comments and suggestions. references bermúdez-valderrama, l. p., & gaitán-díaz, n. a. 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(1976). unpublished mosfit program. zhang x., wang, l., wang, l., liu, b., zheng, j., zhao, j., santana, g.p., fabris, j. d., goulart, a. t., & santana, d. p. (2001). magnetite and its transformation to hematite in a soil derived from steatite. revista brasileira de ciencia do solo, 25, 33–42. https://doi.org/10.1016/s0169-1368(99)00007-4 https://doi.org/10.1016/s0169-1368(99)00007-4 https://doi.org/10.1038/345244a0 https://doi.org/10.1016/j.rse.2014.03.022 https://doi.org/10.1016/j.rse.2014.03.022 palabras clave: depresión de la falla fangzheng; zona de falla de tan-lu; cuenca extraíble de deslizamiento; mecanismo de diferencia asimétrica de deslizamiento. how to cite item wu, j., meng, q., fu, x., ma, y., sun, m., sun, n., & tan, w. (2018). study on the relationship between multi-stage strike-slip mechanism and basin evolution in fangzheng fault depression. earth sciences research journal, 22(4), 335-339. doi: https://doi.org/10.15446/esrj.v22n4.77380 fangzheng fault depression is controlled by the northern of the tan-lu fault zone. it undergoes multi-stage strikeslip, extrusion modification, and erosion of the thermal uplift, forming a tectonic pattern of uplifts connected with sags. through the regional dynamic analysis, the study of the activity law of the western pacific plate has clarified the formation and transformation of the regional tectonic stress field. under the background of the multi-stage of the strike-slip mechanism in the northern part of the tan-lu fault, the fangzheng fault depression has a characteristic of the “left-lateral strike-slip pull-apart basin, right-lateral strike-slip extrusion transformation.” according to the difference of the strike-slip, the fangzheng fault depression has divided into two parts: the east fault depression and the west fault depression. the seismic data, seismic attribute analysis, and geological modeling techniques have applied to analyze the two fault depressions, the east fault depression has actively controlled by the strike-slip activity, and the structure is complex. the seismic data quality is poor; the structure of the west fault depression is the opposite and structural characteristics of asymmetrical difference strike-slip in the east and west fault depressions. interpretation of seismic sections through a slippery background, the strike-slip attributes of the whole fault depression from south to north are segmented, and the strike-slip mechanism from east to west is different. under the control of the multi-stage strike-slip mechanism, the fangzheng fault depression is divided into six stages of strike-slip evolution, corresponding to the six different stages of the strike-slip control basin, the formation process of the asymmetric difference strike-slip fault basin is clarified, which provides a reference for the study of the strike-slip pull-apart basin with multi-stage structure. abstract keywords: fangzheng fault depression; tan-lu fault zone; strike-slip pull-apart basin; asymmetric difference strike-slip mechanism. study on the relationship between multi-stage strike-slip mechanism and basin evolution in fangzheng fault depression estudio sobre la relación entre el mecanismo de deslizamiento por etapas y la evolución de la cuenca en la depresión por falla fangzheng issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.77380 la depresión de la falla fangzheng está controlada en el norte por la zona de falla tan-lu. esta registra un deslizamiento de varias etapas, modificación de extrusión y erosión del levantamiento térmico, lo que forma un patrón tectónico de levantamientos conectados con hundimientos. a través del análisis dinámico regional, el estudio de la ley de actividad de la placa del pacífico occidental ha aclarado la formación y transformación del campo de estrés tectónico regional. en virtud de los antecedentes de las múltiples etapas del mecanismo de deslizamiento en la parte norte de la falla tanlu, la depresión de la falla fangzheng tiene una característica de “cuenca extraíble de deslizamiento lateral izquierda, transformación por extrusión de deslizamiento lateral derecha”. según la diferencia del deslizamiento, la depresión de la falla fangzheng se ha dividido en dos partes: la depresión de la falla del este y la depresión de la falla del oeste. se aplicaron los datos sísmicos, el análisis de atributos sísmicos y las técnicas de modelado geológico para analizar las dos depresiones de fallas. la depresión de fallas del este ha sido controlada activamente por la actividad de deslizamiento y la estructura es compleja. la calidad de los datos sísmicos es pobre; la estructura de la depresión de la falla del oeste es la opuesta y las características estructurales de la diferencia asimétrica de deslizamiento en las depresiones de la falla del este y el oeste. la interpretación de las secciones sísmicas a través de un fondo resbaladizo, los atributos de deslizamiento de toda la depresión de falla del sur al norte están segmentados y el mecanismo de deslizamiento de este a oeste es diferente. bajo el control del mecanismo de deslizamiento por etapas, la depresión de falla de fangzheng se divide en seis etapas de evolución de deslizamiento, correspondientes a las seis etapas diferentes de la cuenca de control de deslizamiento. se aclara el proceso de formación de la cuenca de falla asimétrica de la cuenca de falla de deslizamiento, que proporciona una referencia para el estudio de la cuenca extraíble de deslizamiento con estructura de múltiples etapas. resumen record manuscript received: 01/08/2018 accepted for publication: 26/11/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 335-339 jingfeng wu1,2, qi’an meng2*, xiaofei fu1, yuling ma3, meifeng sun3, ning sun1, wancang tan1,2 1college of earth sciences, northeast petroleum university, daqing, 163318, china. 2exploration and development research institute of daqing oilfield company ltd, 163712, china. 3the eighth oil production plant of daqing oilfield, 163514, china. *corresponding author: qianmeng_npu@163.com g e o l o g y 336 jingfeng wu, qi’an meng, xiaofei fu1, yuling ma, meifeng sun, ning sun, wancang tan introduction fangzheng fault depression is the basin with the most significant oil and gas potential in the peripheral basin of daqing oilfield. in recent years, some progress has been made in the study of tectonic characteristics (yang et al., 2014; chen, et al., 2015), tectonic evolution (huang et al., 2003; gao & li, 197) and hydrocarbon potential. however, there are few pieces of research on the mechanism of the sliding control basin, which cannot correctly understand the formation process of the basin and restricts the progress of oil and gas exploration. in this paper, the evolution process of the fangzheng fault depression under the strike-slip mechanism is discussed. regarding the strike-slip mechanism, the predecessors have studied more on the fault zone and reached a consensus on the strike-slip zone of the tanlu fault zone (xu and zhu, 1994; mercier et al., 2007). as a significant strike-slip fault system, the formation of the tanlu fault zone, kinematics (wang, 2006; wang and zhou, 2009; tang et al., 2009; tang et al., 2010) and tectonic evolution (gilder et al., 1999; zhu et al., 2009). it has recognized for the first time since the 1950s. these research results have mostly concentrated in the southern part of the tanlu fault zone (niu et al., 2000; ge et al., 2009) and the bohai bay basin (hsiao et al., 2004; tong et al., 2008; li et al., 2013). however, the strike-slip mechanism in the northern part of the tan-lu fault zone has less studied (chen & nábelek, 1988), the sliding control basin in the northern section of the tanlu fault zone is still unclear. for the fangzheng fault depression, most of the former people analyzed the structural characteristics of the basin in the mode of stretching the basin (ren et al., 1999) and did not consider the influence of strike-slip on the basin. in the background of strike-slip (hu et al., 2010), combined with the change of regional stress field and the activity characteristics of the fault zone, the latest seismic interpretation data, seismic attribute analysis, and geological modeling techniques are used to analyze the strike-slip characteristics of fangzheng fault depression. through in-depth research, the author proposes an asymmetric difference strike-slip control mechanism. tectonic background and stress field change process fangzheng fault depression is located at the eastern end of the central asian orogenic belt and adjacent to the pacific plate in the east (fig. 1). it is the most complex area for the superposition and transformation of the paleo-asian ocean and the pacific rim. from the paleozoic to the early mesozoic, the central asian-mongolian sea experienced many times of cracking and closure, and the basement formed. at the same time, the mesozoic late triassic to early jurassic, the pacific plate subduction began and entered the pacific rim structural domain and era (hilde et al., 1977). the positive northward subduction movement of the western pacific plate since the early cretaceous caused the left lateral of the tanlu fault zone. under the mechanism of the left-lateral strike-slip pull, the fangzheng fault depression formed, and the nw direction of the late cretaceous western pacific plate subducted. fangzheng fault depression right-lateral strike-slip extrusion, leading to cretaceous strata uplift and erosion. in the early paleogene, the movement direction of the pacific plate changed to nnw direction, the tanlu fault zone began to be left-lateral strike-slip, and the basin area has further expanded. in the late paleogene, the movement direction of pacific plate has changed to nww direction, and the tan-lu fault zone was right-lateral strike-slip. the slippery feature is intensified. the post-arc expansion of the western pacific plate since the neogene has caused a lateral thrust force to the west, causing the fangzheng fault depression and tan-lu fault zone, and even the eastern part of china to be in a near-east-west squeeze environment (chen et al., 2015). fangzheng fault depression under the action of right-lateral strike-slip compression and torsion, along with the northern heat uplift, the northern part of the fault depression is uplifted and denuded, and the stress field presents a threedimensional state, forming the current tectonic pattern. fangzheng fault depression is a series of narrow and cenozoic fault basin distributed in the northeast direction in the tan-lu fault zone, which stratum develops from bottom to top:cretaceous (k), paleogene paleocene xin’ancun formation (e1x), eocene dalianhe formation (e2d), oligocene baoquanling formation (e3b: e3b1, e3b2, e3b3), neogene fujin formation (nf), and the quaternary (q) stratum, generally in the direction of the north east 40 °-50 °, the area is about 1460km2. asymmetric difference strike-slip characteristics the fangzheng fault depression has mainly controlled by tectonic activities such as strike-slip and compression of the tan-lu fault zone and the thermal uplift caused by deep geological processes. it presents the characteristics of asymmetric strike-slip pull-apart basins under the superposition of multiple tectonic activities. fangzheng fault depression from south to north seismic profile shows different fault depression structures (fig. 2). the southern structure is wast-faulting and east-overlapping (fig. 2. a-a), the middle structure is double-faulting (fig. 2. b-b, c-c), northern structure is east-faulting and west-overlapping (fig. 2. d-d). the entire fault depression structure presents an asymmetrical “warping” feature. this is entirely different from the extended fault basin. therefore, the extended basin model was used to understand the fangzheng fault depression is not comprehensive. the fault depression has controlled by three faults (fig. 2). fz1 is the western strike-slip pull-apart fault boundary; fz2 is the eastern strike-slip extrusion fault boundary; fz3 is the middle strike-slip regulation fault. fz1 and fz3 control the cretaceous basin; fz1 and fz2 control the cenozoic basin. hence, the fault depression has divided into the western and eastern fault depression by the fz3 fault control. the internal fault structure of the west fault depression and the east fault depression is entirely different. the strata in the west fault depression are stable, the cretaceous strata are developed, and the seismic data quality is good, the signal-to-noise ratio characteristic of the west fault depression is better than the east fault depression (fig. 3a). conversely, the strata continuity of the east fault depression is weak, the later transformation is strong, the cretaceous strata are not developed, and the seismic data quality is poor (fig. 3a). through the above analysis, it shows the segmentation of the strike-slip characteristics from the south to the north and the difference of the strike-slip mechanism from east to west. in particular, there are different strike-slip fault structures on both sides figure 1. fangzheng fault depression structure location 337study on the relationship between multi-stage strike-slip mechanism and basin evolution in fangzheng fault depression of the fz3 fault (fig. 3b), which have the characteristics of asymmetric difference strike-slip fault basin. evolution of multi-stage strike-slip basin because of the asymmetric difference strike-slip control basin, since the strike-slip effect through the entire basin evolution process, it cannot have simply explained by the evolution model of the conventional fault basin, and the control and influence of the strike-slip activity on the fault depression must have considered. since the formation of the cretaceous, the fangzheng fault depression has divided six evolution stages according to the different slipping periods (fig.4). including the evolution of the two stages slip-control basin in the cretaceous, the evolution of the four stages slip-control basin in the cenozoic, under the control of the six-stage strike-slip mechanism, experienced three stages of uplift and erosion, and the sedimentary subsidence center also experienced four migration process from south to north, and north to south. cretaceous slip-controlled basin evolution (1)early cretaceous strike-slip pull-apart stage early cretaceous, affected by the high-angle subduction of the western pacific plate, the leftlateral strike-slip of the jiayi fault in the north of the tan-lu fault zone, the rigid blocks on both sides of the strike-slip fault were separated, and the fangzheng fault depression began to form. an early cretaceous fault basin (western fault depression, figure 5) near the northwest-south-eastward distribution was created on the pre-mesozoic granite or metamorphic rock basement. (2)the end of early cretaceous to late cretaceous strike-slip extrusion stage as the direction of the western pacific plate changed from the north to the northwest, the northeastern part of china gradually entered the extrusion environment at the end of the early cretaceous, and the fold uplift suffered erosion of fangzheng fault depression. the cretaceous residual strata have developed in the southern part of the fault depression. the cretaceous weathering crust revealed by the northern drilling confirmed the uplift. cenozoic slip-controlled basin evolution in the early paleogene, the movement direction of the western pacific plate changed to the north-northwest, and the fangzheng fault depression was again left-lateral strike-slip. at this time, the fz2 fault began to become the eastern boundary fault of the fangzheng fault depression, and as the fault deepened, the magma and the volcanic activity promoted the extension of the lithosphere, which caused the fangzheng fault depression to enter the paleogene main basin period, and formed the east fault depression (fig. 5) between fz2 and fz3. the east fault depression and the west fault depression constitute a cenozoic fault basin (fangzheng fault depression). (1) paleocene-eocene strike-slip pull-apart stage the fangzheng fault depression began to pull-apart in the north-south direction. because the tensile stress direction is oblique to the northeast trend of the fangzheng fault depression, this oblique pull-apart has characterized by a leftlateral torsion in the fangzheng fault depression. under the action figure 2. fangzheng fault depression structure and seismic section. figure 3. 1200-2000ms signal-to-noise ratio image of the 3d seismic work area. 338 jingfeng wu, qi’an meng, xiaofei fu1, yuling ma, meifeng sun, ning sun, wancang tan of leftlateral torsional stress, the fault fz1 in the northwestern margin and the fault fz2 in the southeastern margin are strongly active, but the fz3 activity is weak, at this time, the structural of the fangzheng fault depression has a double fault, and the structural pattern of the sag-uplift fault depression is initially formed. (2) at the end of the eocene, the strike-slip extrusion stage at the end of the eocene, the fangzheng fault depression extrusion in the rightlateral background, the overall depression of the fault depression, the basin ascends as a whole, the northern part of the basin rises higher, and the south is relatively low. the xin’ancun formation (e1x) and the dalianhe formation (e2d) were strongly denuded, in particular, the dalianhe formation (e2d) has been eroded in the northern part of the fault depression.. as a result, the remnants of the xin’ancun formation (e1x) are also relatively thin. (3) oligocene-neogene differential strike-slip settlement stage during the oligocene, as the western pacific plate turned into highangle subduction in the northwest direction, fz3 began to move strongly, and the settlement of the west fault depression was small. the east fault depression received huge thick sediment under the control of fz3, and the fangzhengr fault depression began to enter the differential settlement stage. affected by the paleo-geomorphology the baoquanling formation (e3b: e3b1, e3b2, e3b3) was overtaken from the north to the south. (4) post-neogene strike-slip compression and uplift stage the fangzheng fault depression is affected by the regional rightlateral strike-slip compression stress, and the tectonic inversion occurs. the north and south sides of the fault depression are strongly uplifted and most eroded. the northern boundary forms a thrust-napped structure under the compressive stress, which is leading to the blurring of the north boundary fault. eventually, the tectonic pattern of the asymmetry difference between the east fault depression and the west fault depression was formed (fig. 5). conclusions the strike-slip effect through the evolution process of fangzheng fault depression. the formation and transformation of the basin have closely related to the strike-slip effect. therefore, it cannot have explained merely by the evolution model of the conventional fault basin. it must have applied because of evident regional stress transformation. the evolution of the slip mechanism is used to analyze the evolution process of the fangzheng fault depression. the strike-slip characteristics of the fangzheng fault depression are segmented from south to north, and the strike-slip mechanism is different from east to west, especially the different strike-slip structures on both sides of the fz3 fault, and the characteristics of the asymmetric difference strikeslip fault basin. by the research of asymmetric difference strike-slip control basin, according to the different strike-slip mechanism, there are six evolution stages in the fangzheng fault depression. early cretaceous strike-slip pullapart stage; early cretaceous to late cretaceous strike-slip extrusion stage; paleocene-eocene strike-slip pull-apart stage. at the end of the eocene the strike-slip extrusion stage, oligocene differential strike-slip settlement stage, the trend of the strike-slip compression and uplift since the neogene, corresponding to the six different strike-slip control basin backgrounds. figure 4. evolution mode of the multi-stage strike-slip basin in fangzheng fault depression figure 5. comparison of geological modeling between the east depression and the west depression 339study on the relationship between multi-stage strike-slip mechanism and basin evolution in fangzheng fault depression acknowledgments this paper is supported by the major science and technology projects fund of china national petroleum corporation (project number: 2016e0202). the authors express sincere thanks. references chen, d., zhang, f., hanlin-chen, h., yildirim, d., shufeng, y. s., qian, m. q., & yang, c. 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(2009). geochronological constraints on tectonic evolution of the tan-lu fault zone (in chinese with english abstract). chinese journal of geology, 44(4), 1327-1342. quality control can effectively improve the quality of surface meteorological observations. to ensure the stability and effectiveness of a quality control model under different terrain and climate conditions, it is necessary to structure a quality control model with strong generalization ability. algorithms such as the random forest algorithm provide such generalization ability. however, machine learning algorithms are slower than traditional mathematical models. therefore, a random forest quality control algorithm based on the principal component analysis (pca-rf) is proposed in this paper. fifteen target stations under different climatic and geomorphological conditions were selected and tested using observations collected four times daily at neighboring stations from 2005-2014. the results show that using pca to analyze the elemental composition and select elements with high correlation factors, as well as applying the random forest algorithm, can effectively reduce the run time and keep the accuracy of the model. the training sample dependence, model prediction accuracy and error detection rate of the pca-rf model are superior to those of the spatial regression method. therefore, the pca-rf method is a better quality control model for the spatial quality control of multiple elements of surface air temperature observations. abstract keywords: surface air temperature; quality control; random forest; principal component analysis; spatial quality control method for surface temperature observations based on multiple elements issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n2.65185 m e t e o r o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): xiaoling ye1,2, xing yang1*, xiong xiong1, shuai yang1, yang chen1 1school of information and control, nanjing university of information science and technology, nanjing 210044, china 2collaborative innovation center on forecast and evaluation of meteorological disasters, nanjing university of information science and technology, nanjing 210044, china *email of corresponding author: harryyangx@gmail.com el control de calidad puede mejorar efectivamente la calidad de las observaciones meteorológicas. para asegurar la estabilidad y efectividad de un modelo de control de calidad bajo condiciones diferentes de terreno y climáticas es necesario estructurar un esquema con una fuerte habilidad de generalización. algoritmos como el método de bosques aleatorios (del inglés random forest) cumplen con estas condiciones. sin embargo, los algoritmos de maquinas de aprendizaje son más lentos que los modelos matemáticos tradicionales. en este artículo se propone un algoritmo de control de calidad tipo bosques aleatorios basado en el análisis de componentes principales (pca-rf). se seleccionaron 15 estaciones objetivo bajo diferentes condiciones climáticas y geomorfológicas y se evaluaron con observaciones realizadas cuatro veces por día en estaciones vecinas desde 2005 hasta 2014. los resultados muestran que usando pca para analizar la composición elemental y seleccionar elementos con factores de correlación alta, al igual que la aplicación del algoritmo random forest, se puede reducir efectivamente el tiempo de ejecución y mantener la exactitud del modelo. la dependencia de la muestra de prueba, la exactitud del modelo de predicción y la tasa de detección de error del modelo pca-rf son superiores a aquellos del método de regresión espacial. por lo tanto, el método pca-rf es un mejor modelo para el control de calidad de elementos múltiples en las observaciones superficiales de aire y temperatura. resumen palabras clave: método de control de calidad espacial para observaciones de temperatura superficial basado en múltiples elementos record manuscript received: 23/05/2017 accepted for publication: 30/06/2017 how to cite item: ye, x., yang, x., xiong, x., yang, s., & chen, y. (2017). spatial quality control method for surface temperature observations based on multiple elements. earth sciences research journal, 21(2), 95 99. doi:http://dx.doi.org/10.15446/esrj.v21n2.65185 método de control de calidad espacial para observaciones de temperatura superficial basado en múltiples elementos 102 xiaoling ye, xing yang, xiong xiong, shuai yang and yang chen102 1. introduction meteorological observations are important for identifying and understanding variations and changes in regional and global climate (feng et al., 2004). these observations are also essential to a wide range of meteorological applications, such as climate monitoring, weather forecasting and the evaluation of numerical weather prediction (nwp) models (he et al., 2016; ingleby and lorenc, 1993). therefore, it is of particular importance to improve the accuracy of meteorological observations, especially surface observations (barnes, 1964; gandin, 1988). on the one hand, surface meteorological observations have a longterm history in china. on the contrary, surface meteorological observations can more accurately represent the atmospheric characteristics of the nearsurface compared to other meteorological observations (xu et al., 2013; hu et al., 2002; mi et al., 2014). however, surface observation stations are easily affected by the gross error and random error during the data acquisition process. thus, the primary task and purpose of quality control (qc) are to identify the gross error and large random error associated with large numbers of observations (shi-wei et al., 2009). in surface meteorological observations, air temperature is one of the most important elements because it has a considerable impact on human activity (cheng et al., 2016). hence, a variety of qc methods has been recently proposed for surface air temperature observations. the qc methods include the extreme range check (reek et al., 1992; kubecka, 2001; allen et al., 1998; forsythe et al., 1995), internal consistency check (baker, 1992), time consistency check (shafer et al., 2000; lanzante, 1996), and spatial consistency check methodologies. qc research has focused on spatial consistency check, which is used for multi-station qc. the spatial qc method predicts the value at the target station based on the values at neighboring stations, and it can evaluate the reliability of observations. the inverse distance weighting (wade, 1987) method (idw) uses the proportions of the inverse distances between multiple neighboring stations and the target stations as weights, and it requires the terrain features and distribution of neighboring stations. the bayesian qc method (lorenc and hammon, 1988) uses the probability of observations as a gross error. in 2007, hubbard et al. (2007) proposed the spatial regression test method (srt). srt calculates weights based on the standard error between the target station and neighboring stations, and it is less influenced by terrain and the distribution of neighboring stations compared to the idw method. based on the srt method, xu et al. (2014) proposed the probabilistic spatial-temporal method (srt-ps). this method considers the uncertainty in temperature observations and eliminates the associated effect on temperature fluctuations. the most widely used traditional methods discussed above are the idw method and srt method, and the srt method is considered superior to the idw method (hubbard and you, 2005). thus, the srt method is for comparison with the random forest quality control algorithm based on the principal component analysis (pca-rf) method proposed in this paper. the pca-rf method is used to reconstruct the training samples by extracting high correlation factors from meteorological elements using pca. therefore, the efficiency and accuracy of rf model are improved. the specific objectives of this paper are as follows: (1) to compare the evaluation indexes of the rf and srt methods using training samples over different periods (ridzuan et al., 2017). (2) to set different correlation factor thresholds and compare the evaluation indexes and run times of the pca-rf qc model based on these different thresholds. and (3) to compare the detection rates of the srt and pca-rf methods and evaluate the predictive performances of the qc models. 2. data china is a large country with an area of approximately 9.6 million km2. about 98% of the land area stretches between the latitudes of 20° n and 50° n. thus; the country extends from the subtropical zones in the south to the temperate zones (including warm temperate and cool temperate) in the north (yang et al., 2013; jiang et al., 2016). with recent global climate warming, the national annual average temperature increased by approximately 1.1 degrees celsius, with high temperatures in north and low temperatures in the south. additionally, the magnitude of the temperature increase in winter was larger than that in summer (asis et al., 2017) moreover, other climate factors exhibited fluctuations in different regions (zheng et al., 2010, cheng et al., 2009). therefore, it is more meaningful in spatial qc to consider different climates and geomorphological regionalization. in this paper, data were collected from january 1, 2005, to december 31, 2014, four times daily (02:00, 08:00, 14:00 and 20:00 beijing time (cst)) in different climate regions in china. the data include six types of meteorological elements: ground surface temperature (gst), pressure (pre), relative humidity (rh), temperature (tem), wind direction (wd) and wind speed (ws). these data were obtained from the china national meteorological center and had undergone basic qc; thus, the accuracy of the data is acceptable. missing data, changes in station locations and changes in precipitation measurement instruments during the observation period can influence the homogeneity of datasets (irannezhad et al., 2016). small percentages of missing data at some stations can influence an analysis, but pca can reduce the effect of missing data to a certain degree. zheng et al. (2010) suggested dividing china into 12 temperature zones, including a humid region, a semi-humid region, a semi-arid region and an arid region. li et al. (2013) discussed the specific steps and methods associated with dividing china into six major categories. in this paper, 15 stations with different climates and topographies are selected; however, stations 59948, 59985 and 59287 (in the mid-tropic, equatorial tropic and south subtropical areas) are not selected because they do not have sufficient numbers of neighboring stations (rahim et al., 2017) the target stations are shown in figure 1, and the selection of target stations accounted for the distribution of climate zones and geomorphologic regionalization. neighboring stations are selected based on the distance between the target station and neighboring stations, where the radius of the neighboring stations is not more than 100 km. the number of neighboring stations is 5-10, as shown in table 1. however, stations 55472 and 54823 have a limited number of neighboring stations or significant gaps in data. table 1. the number of neighboring stations for 15 target stations. 103spatial quality control method for surface temperature observations based on multiple elements x x xe s q= figure 1. the distribution of target stations based on geomorphological regionalization in china. to test the feasibility of the pca-rf method, artificial error is added randomly to the observations at the target station. a comparative analysis is performed using observations with artificial random error and the predicted values of the prf method. hubbard et al. (2005) presented a method of adding false error to data. a random number q with a mean of zero and a range of ± 3,5 was selected and added to the observations (hubbard et al., 2007): (1) where e and s denote the error magnitude and standard deviation, respectively, of temperature observations at the target station. additionally, x is the location of mistake value insertion. 3. method 3.1. random forest algorithm random forest (rf) is an ensemble learning method for classification and regression that constructs some randomized decision trees during the training phase and make predictions by averaging the results (scornet et al., 2015). in ensemble learning, each decision tree is a weak classifier, and some weak classifiers are combined into a strong classifier. this approach is much easier than looking for a strong classifier directly. rf generates many regression trees to improve the prediction performance of the model, and each split in the tree is determined using a randomized subset of variables/factors at each node (rahmati et al., 2016). the outcome is the average of the results of all the trees (cutler et al., 2007). 3.2. principal component analysis algorithm principal component analysis (pca) is a method for analyzing a positive semi-definite hermitian matrix. the goal of pca is to identify the most urgent basis by using a new base to filter out the noise and reveal hidden structures (shlens, 2014). notably, pca is used to analyze the covariance matrix of sampled random vectors (lloyd et al., 2014). high-dimensional data are mapped into a new coordinate system and transformed to low-dimensional data that can partly represent the original data. in this manner, pca can significantly reduce the run time of algorithms, especially in big data processing. 3.3. pca-rf algorithm combined with the above concepts, the pca-rf algorithm is proposed in this paper. the concrete steps in the algorithm are divided as follows (asis et al., 2017) step 1: the pca method is used to analyze the multiple element observations in space. assuming that n represents the number of samples, m represents the number of features, and x n,m represents the feature set, the average value of the ith feature iy is calculated using (2). (2) the standard error si of the ith feature is calculated using (3). (3) then, through standardization, (4) is obtained. (4) finally, the correlation matrix of feature sample rm,m is obtained, as shown in (5). (5) an optimal feature sample is established by extracting the matrix with high correlation factors. step 2: the rf algorithm is used to train and test the optimal feature samples, and the pca-rf model produces a predictive value esty . step 3: the predicted values esty and observed values obsy are tested for a given threshold, as shown in (6): (6) where f is the qc parameter and ∂ is the standard error between observed values and predicted values at the target station. if the conditions of (6) cannot be met, the data are considered incorrect (or associated with error). if the conditions of (6) are met, the data are considered correct values (ridzuan et al., 2017). 3.3. evaluation of model performance to analyze different forecasting models with different conditions, the root means square error (rmse) and mean absolute error (mae) were selected as evaluation metrics. rmse is the square root of the ratio of the square of the predicted value and the real value, and mae can accurately reflect the size of the prediction error. their formulas are as follows. (7) (8) 4. results and discussion the estimates using pca-rf were evaluated against the measurements at 15 target stations, and selected observations are shown in figure 1 and table 1 (you et al., 2017). in the analysis of sample dependence, observations over different periods were selected to predict observations of daily mean temperature, and the training samples were the six meteorological elements observed four times daily in the 1, 3, 5, 7 and 9 years before 2014. the testing samples were the six meteorological factors founded four times daily in 2014. in the analysis of the correlation factor, the same observations were used as the training samples and 104 xiaoling ye, xing yang, xiong xiong, shuai yang and yang chen testing samples at different correlation factor thresholds (>0, >0.5, >0.8, >0.9, >0.95). in the analysis of the detection rates of the pca-rf and srt methods, the evaluation indexes (mae and rmse) and two types of error (type i and type ii) were used to compare the performances of pca-rf and srt, and observation selection was the same as that utilized in the correlation factor analysis. observations over different periods were selected to analyze the sample dependence based on rf and srt, and the results are shown in figure 2. in general, as the number of training samples increased, the error in the testing samples decreased. as shown in figure 2, the error trends at 15 target stations exhibited this pattern. regardless of the observation period (one year, three years, five years, seven years or nine years), the mae and rmse values of the rf method are smaller than those of the srt method, and the fluctuations in training samples are smaller. hence, the rf method performed better than srt method based on the sample dependence. although the error associated with the training samples for seven years of observations is less than that associated with the training samples for nine years of observations at target station 57494, the training samples based on nine years of observations exhibited less error in general. therefore, the training sample based on nine years of observations was used in the following analysis. figure 2. evaluation indexes of the rf and srt methods over different observation periods. moreover, figure 2 shows that stations 54823, 56951, 57245 and 58606 have relatively small evaluation index values because the neighboring stations around these target stations are relatively dense (you et al., 2017). additionally, the main reason for the performance difference between the srt method and rf method is the sparse distribution of neighboring stations or low spatial correlation between values. the rf method has relatively less error at the target stations compared to the large error of the srt method, such as at stations 51477, 56684 and 57245. this result suggests that the rf method is superior to the srt method. to investigate the performance of the pca-rf method under different thresholds of the correlation factor, the evaluation index values and run time of the model obtained for seven cases (no, >0, >0.5, >0.8, 0.9, 0.95, alone) are shown in figure 3, where “no” means that the rf model does not perform pca and “alone” means that modeling is only based on temperature observations at neighboring stations. additionally, the unit of “time” (run time of the model) is 100 seconds. figure 3 shows that the evaluation index (mae and rmse) values increased slightly as the correlation factor threshold increased. moreover, the run time of the model decreased as the correlation factor threshold increased. the results show that the choice of correlation factor determines the run time of model and affects the size of the type error. a correlation factor threshold of 0.5 was chosen as the optimal value to construct the qc model. compared to the other correlation factor thresholds, the correlation factor threshold of 0.5 produced the shortest run time of the model, and the model error is relatively small. on the one hand, although the model error is reduced without pca, the run time of the model increases considerably, as pca effectively reduces the run time of the model. on the other hand, although the run time of the quality control model with only temperature elements is short, the error is larger than the error when six elements are considered in the spatial qc model. in summary, the accuracy and run time of the qc model can be improved by choosing a correlation factor threshold of 0.5. thus, the rf model performed better than the srt approach based on the evaluation indexes and error detection rate analysis. 105spatial quality control method for surface temperature observations based on multiple elements 105 figure 3. evaluation indexes and run time of the model for the 15 target stations under different thresholds of the correlation factor. the error detection rate is the ultimate criteria for a qc model. the error detection rate is related to the size of the qc parameter, and the choice of qc parameter is related to the two types of error. in type i error, the real value is treated as an incorrect value. in type ii error, an incorrect value is treated as the true value. in the realm of qc, the intersection of the two types of error is chosen as the optimal qc parameter. the optimal qc parameters for the pca-rf and srt methods obtained based on the intersection of the two types of error are listed in table 2, in which the 15 target stations are numbered 1 to 15 according to the order of their previous names. the corresponding error detection rate is shown in figure 4. table 2 and figure 4 illustrate that the error detection rate of the pca-rf method is superior to the error detection rate of the srt method. thus, the pca-rf method is more stable and efficient based on the target stations with different terrain and climate characteristics. 106 xiaoling ye, xing yang, xiong xiong, shuai yang and yang chen table 2. the optimal quality control parameters of the pca-rf and srt methods. figure 4. the detection rates of the pca-rf and srt methods at 15 target stations. 5. conclusions this paper studies a spatial qc model of six meteorological elements and temperature observations using the pca-rf model, and the results are compared to those of the srt method. the performance of qc has a considerable effect on the selection of the correlation factor threshold and the density of neighboring stations around the target station. the simulation results show that the pca-rf method is less dependent on training samples than is the srt method, and it performed better at target stations with sparse neighboring stations or low spatial correlation. the run time of the model can be reduced effectively by choosing the appropriate correlation factor at the expense of a small increase in the error. the pca-rf method is more stable and has a higher error detection rate compared to the stability and detection rate of the srt method. in the next study, time 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research on macro-mesomechanical properties of sandy pebble soil of different coarse-grained content was carried out using the method combining the indoor large-scale triaxial test of coarse-grained soil with the discrete element numerical triaxial test. the research results showed that the stress-strain curve of sandy pebble soil exhibited strain softening with the increase of coarse-grained content; when the confining pressure was the same, the stress peak increased and the strain when the peak was reached decreased gradually with the increase of coarse-grained content. it revealed the functional relationship between coarse-grained content and mechanical indexes of sandy pebble soil such as internal friction angle and cohesion, the internal friction angle and cohesion of sandy pebble soil linearly increased with the rise of coarsegrained material; it proposed the particle discrete element micro parameters of sandy pebble soil of different coarsegrained content, including contact modulus, friction coefficient, particle stiffness ratio, contact bond strength. the research results provided the theoretical support for the fine design and construction of sandy pebble stratum project. abstract keywords: sandy pebble soil; coarsegrained content; discrete element; macromicromechanical properties. macro-micromechanical properties of sandy pebble soil of different coarse-grained content issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.66105 junfu lu, di li* xiaoqiang xue, shenlin ling state key laboratory of geohazard prevention and geoenvironment protection, chengdu university of technology, chengdu610059, china *e-mail: lidi2014@cdut.edu.cn earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 65-71 la capa de arena guijosa es un manto típico de partículas inestables que consisten principalmente de arena y guijarros. sin embargo, el efecto del contenido de grano grueso en la estabilidad de la capa no es claro. el área de investigación en este estudio es la ciudad de chengdu, provincia de sichuan, en china. allí se tomaron muestras de diferente clasificación granulométrica para analizar las propiedades macro y mesomecánicas del suelo de arena guijosa a través del método combinado de la prueba triaxial a gran escala en laboratorio para suelos de grano grueso con la prueba triaxial numérica de elementos discretos. los resultados de la investigación muestran que la curva del ensayo de tracción en suelos de arena guijosa presenta un aplacamiento en la presión con el incremento del contenido de grano grueso. cuando la presión de confinamiento fue la misma, el pico de esfuerzo se incrementó y la presión empezó a caer gradualmente cuando se alcanzó el pico con el incremento del contenido de grano grueso. esto revela la relación funcional entre el contenido de grano grueso y los indicadores mecánicos para suelos de arena guijosa como el ángulo de fricción interna y la cohesión; estos factores se incrementaron con el aumento del material de grano grueso. esto sugiere los microparámetros de particulas de elementos discretos en este tipo de suelos, que incluyen el modulo de contacto, el coeficiente de fricción, la proporción de rigidez de partículas y la fuerza del vínculo de contacto. los resultados de la investigación proporcionan el apoyo teórico para el diseño y construcción de nuevos proyectos sobre la capa arenosa y guijarrosa. resumen palabras clave: suelo arenoso y guijarroso; contenido de grano grueso; elementos discretos; propiedades macro y micromecánicas. record manuscript received: 14/11/2017 accepted for publication: 23/04/2018 how to cite item lu, j., li, d., xue, x., & ling, s. (2018). macromicromechanical properties of sandy pebble soil of different coarse-grained content. earth sciences research journal, 22(1). 65-71. doi: http://dx.doi.org/10.15446/esrj.v22n1.66105 b io g e o c h e m is t r y propiedades macro y micromecánicas de suelos de arena guijosa con diferente clasificación granulométrica 66 junfu lu, di li, xiaoqiang xue, shenlin ling introduction sandy pebble soil is widely distributed in various regions of the world, and its mechanical properties are further complicated by the randomness of geometrical distribution of pebble and discreteness of particle. the sandy pebble stratum widely distributed in chengdu city, sichuan province, china has a great impact on the construction safety of tunnel and underground engineering(wei et al., 2008; omarashahestan and omarashastani, 2017), so the study on strength characteristics and stability of sandy pebble soil has been paid increasing attention (si et al., 1990; hejazi et al., 2017). guo qingguo (1998)proposed 5 mm as the dividing line of fine and coarse particles, and that takes 30% and 70% of coarse-grained contents as the two research points. at the same time, many scholars believed that the physical and mechanical properties of sandy pebble soil of different coarse-grained content such as cohesion, internal friction angle and permeability had a great impact (wei et al., 2008; yasin et al., 2017).most of them conducted studies on macromechanical properties of sandy pebble soil using the triaxial test and direct shear test but lacking micromechanical parameters of the composite structure of sand and pebble particles. discrete element method is a numerical simulation method for solving discontinuous media problems, and has received attention from many scholars and was developed since being proposed in the 1970s by cundall (1997). geng li(2011)simulated the triaxial test process using pfc3d, and analyzed the effect of particle micro parameters on the strength of coarse-grained soil, however, it researched on only a gradation of soil, so the conclusions were not representative. xu xiaofeng (2013) et al established the calculation model of discrete element simulation of four kinds of gravelly soils of different coarse-grained content, and analyzed the effect mechanism of coarse-grained content on the macro-micromechanical properties of gravelly soil, however, the micro parameters obtained were only suitable for a confining pressure, so it was not accurate enough. qi yang (2015)et al simulated the uniaxial loading test of coarse-grained soil in various main stress directions under triaxial state by means of pfc3d, and analyzed the stress-strain characteristics of coarse-grained soil in complex stress state from the macro and micro points of view, believing that the complex stress state was the inducing factor for coarse-grained soil to produce various anisotropies; in addition, many scholars at home and abroad solved the problems of sandy pebble soil encountered in the engineering by using the discrete element theory (qi et al., 2015; ahmad et al., 2017). in this paper, relying on the sandy pebble stratum in chengdu city, sichuan province, china, the influencing law of coarse-grained content on the macromechanical properties of sandy pebble soil was studied using the large-scale triaxial test of coarse-grained soil, on this basis, a particle discrete element numerical triaxial test platform was established, and the micromechanical parameters of sandy pebble soil of different coarsegrained content was studied using the discrete particle element numerical triaxial test (yasin et al., 2017). the research results provided essential reference and guidance for the fine design, construction, and maintenance of sandy pebble stratum project. research method triaxial test of coarse-grained soil test scheme in this test, a stress type large-scale triaxial test apparatus was adopted, and its specifications included a diameter of 300 mm, a height of 600 mm, a maximum axial stress of 21 mpa, a maximum vertical load of 1500 kn, maximum confining pressure of 3.0 mpa, and maximum axial stroke of 300 mm. the shear strength indexes c and φ of coarse-grained soil with the particle size of not less than 60 mm can be measured, as shown in figure 1. figure 1. large-scale triaxial apparatus of coarse-grained soil. specimens in this test, the sandy pebble soil in chengdu city, sichuan province, china was taken as the research object, for the oversized particles, the weighted average substitution was conducted using the sandy pebble soils with the maximum allowed particle size of 60 mm to 5 mm according to the content, namely equivalent substitution method (yasin, 2017; kudus et al., 2017). to obtain the shear strength index of sandy pebble soil of different coarse-grained content, a total of four groups of specimens with p5 (mass percentage of particles with particle size of greater than 5 mm) of 73%, 75%, 76.5% and 80% were adopted in this test, with density of 2.05 g/cm3, moisture content of 1.6%.each group of specimens was exerted with confining pressures of 100 kpa, 200 kpa, and 300 kpa. the particle gradation of sandy pebble soil used in the test is shown in table 1 and figure 2. table 1. composition of particle gradation of sandy pebble soil of different coarse-grained content. the triaxial specimens were divided into three layers for preparation, which allows realizing the uniform mixing of particles. the shearing with an axial strain rate of 1.5 mm/min was carried out using the test method of isotropic consolidation undrained shear until the failure of the specimen occurred (1999), and the specimens are shown in figure 3. 67macro-micromechanical properties of sandy pebble soil of different coarse-grained content figure 2. gradation curve of sandy pebble soil. figure 3. large-scale triaxial specimen of sandy pebble soil. particle discrete element numerical triaxial test establishment of numerical triaxial test platform the test silo used in the numerical triaxial test of coarse-grained soil adopted a cylinder with diameter of 300 mm and height of 600 mm, its upper and lower planes were the loading plates with stiffness of greater than particle stiffness, the wall of the test silo adopted the flexible constraint, and its normal stiffness was smaller than normal stiffness of particles (wun et al., 2017). in the effective area of the test silo, the sphere unit was adopted to replace the soil particle according to the particle gradation of sandy pebble soil tested in the test, the minimum unit diameter in the numerical triaxial test was dmin=1 mm, and the particles of large particle size were composed of small units and classified as a particle group (i.e., clump unit) (ridzuan et al., 2017). the contact bond model among particles was adopted, and the servo loading was adopted in the test to keep constant confining pressure. the model of the numerical triaxial test is shown in figure 4. figure 4. triaxial numerical model. method for determining the micro parameters of discrete element (1) determination of particle contact stiffness ec and ratio kn/ks of normal stiffness to shear stiffness. for the contact bond model, the macro elastic modulus e was mainly determined by the ratio of particle contact stiffness to stiffness. when the particle stiffness ratio was constant, the micro parameter ec and macro elastic modulus presented linear correlation (rahman et al., 2017). in the numerical triaxial test, the particle contact stiffness ec needed to be continuously adjusted until the elastic modulus of numerical model was consistent with the indoor large-scale triaxial test, and the ec at this time was the micro parameter determined. the stiffness ratio kn/ ks was mainly determined by the poisson’s ratio, the greater the stiffness ratio was, the greater the poisson’s ratio was; (2) determination of particle micro parameter friction. the friction coefficient had a great effect on the peak intensity of the model, and little impact on the elastic modulus of the material, by adjusting the friction coefficient until it was similar to the peak intensity of the indoor large-scale triaxial test, the friction coefficient can be determined; (3) determination of particle bond strengths σ and τ. the particle bond strength was mainly related to the peak intensity of the model, when the ratio σ mean/σ dev of bond strength average to unbalanced load value was constant, the peak intensity of the model was proportional to the bond strength average. table 2. microparameters of the discrete element of sandy pebble soil of different gradation. analysis of research results the stress peak of traditional the triaxial test model of particulate matter under different confining pressure was slightly lower than that of the indoor test, and potyondy and hoek et al. (1998) believed that by changing the particle micro parameters to overcome this disadvantage had little effect. in this numerical triaxial test, clump unit was introduced, which efficiently solved this problem. clump unit was a large particle composed of a plurality of balls cemented together simulating the particles of irregular shape, which can realize the irregularity of sandy pebble soil, increase the occlusion and embedding among particles, thus appropriately improving the shear strength of the numerical triaxial test. analysis of macromechanical properties of sandy pebble soil stress-strain characteristics of sandy pebble soil of different coarse-grained content from figure 5, it can be seen that the stress-strain curve of sandy pebble soil exhibited strain softening with the increase of coarse-grained content, this was because, with the increase of coarse-grained content, particles became overhead, formed large pores, and lacked fine particles to fill, resulting in the decrease of stress with the increase of strain (zhang et al., 2013; basarian and 68 junfu lu, di li, xiaoqiang xue, shenlin ling tahir, 2017). from figs. 6 and 7, it can be seen that when the confining pressure was the same, the stress maximum increased and the strain when the peak was reached decreased gradually with the increase of coarse-grained content. (a) stress-strain curve of sandy pebble soil of gradation 1. (b) stress-strain curve of sandy pebble soil of gradation 2. (c) stress-strain curve of sandy pebble soil of gradation 3. (d) stress-strain curve of sandy pebble soil of gradation 4. figure 5. stress-strain curve of sandy pebble soil of different coarsegrained content. figure 6. peak stress of sandy pebble soil of different coarse-grained content. φ =0.84p5-23.39 (1) c =4.13p5-208.8 (2) figure 7. peak strain of sandy pebble soil of different coarse-grained content. analysis of macromechanical parameters of sandy pebble soil from figure 8, it can be seen that the internal friction angle and cohesion of sandy pebble soil linearly increased with the increase of coarse-grained content, but with different increasing magnitude, by fitting, the relationship between mechanical parameters and coarsegrained content of sandy pebble soil was obtained as follows: where: φ—internal friction angle, (°); c— cohesion, (kpa) , 69macro-micromechanical properties of sandy pebble soil of different coarse-grained content (a) relationship between internal friction angle and coarsegrained content. (b) relationship between cohesion and coarse-grained content. figure 8. relationship between internal friction angle and cohesion and coarse-grained content. p5—mass percentage of particles with particle size of greater than 5 mm analysis of micromechanical properties of sandy pebble soil based on the results of the indoor large-scale triaxial test of coarse-grained soil, the optimal micro parameters of sandy pebble soil of different coarse-grained content were obtained by using the approximation method of parameters of particle numerical triaxial test. stress-strain curve of numerical triaxial test of sandy pebble soil of different coarse-grained content as can be seen from figure 9, the stress-strain curve of the discrete element numerical triaxial test was in substantial agreement with that of the indoor test, and the micro parameters of the discrete element determined by this method can well realize the macromechanical properties of sandy pebble soil. when the triaxial confining pressure was the same, the stress peak increased, but with different initial modulus, this was primarily because the basic unit of discrete element was the rigid sphere, and the occlusion of particles was weak, and the particles were arranged in single form, even if the contact bond was selected to improve the cohesion of particles (bilal et al., 2017). however, with the increase of axial load, the bond fracture led to strength characteristics of sandy pebble soil different from the actual strength characteristics of sandy pebble soil, which also needed to be improved and further optimized. (a) stress-strain curve of sandy pebble soil of gradation 1 (b) stress-strain curve of sandy pebble soil of gradation 2 (c) stress-strain curve of sandy pebble soil of gradation 3 (d) stress-strain curve of sandy pebble soil of gradation 4 figure 9. stress-strain curve of sandy pebble soil of different coarse-grained content in the real and numerical triaxial test. 70 junfu lu, di li, xiaoqiang xue, shenlin ling micromechanical parameters of discrete element of sandy pebble soil by checking calculation, the micro parameters of discrete element able to reflect the macromechanical properties of sandy pebble soil of different coarse-grained content were obtained, the determination of the micro parameters of the discrete element provided the basic data for the numerical analysis and design of sandy pebble soil. the specific micro parameters of the discrete element are shown in table 2. table 2 showed that the micromechanical parameters of sandy pebble soil were mainly determined by contact modulus, porosity, friction coefficient, bond stiffness and bond stiffness ratio. the more the coarse-grained content was, the larger the contact modulus and friction coefficient were, and the higher the internal friction angle and cohesion of sandy pebble soil were, but the other micro parameters showed little change (nema et al., 2017). therefore, the fact that the strength of sandy pebble soil increased with the increase of coarsegrained content was caused by particle stiffness and friction coefficient. analysis of displacement of sandy pebble soil particles the displacement of sandy pebble soil particles in the triaxial compression process was analyzed, to reveal the micromovement of sandy pebble soil particles in the triaxial compression process. because the coarse-grained content of sandy pebble soils of four different gradations had little difference, so their particle displacement fields showed no difference (andik and sarang, 2017). when the sandy pebble soil of gradation one was selected, the displacement was analyzed with particle displacement field of 100 kpa of confining pressure as the representative, and the microdisplacement diagram of particles under different strain is shown in figure 11. from the beginning of compression shear, due to movement of upper and lower loading plates, the particles in the upper part of the model moved downward, and the particles in the lower part of the model moved upward. as the loading progressed, the particles in the middle of the model moved outward due to pressure, resulting in the dilatancy of specimens in the middle of the model, which was the same as the failure mode (figure 10) of samples in the indoor test. figure 10. failure appearance of specimens. (a)ε=2.5% (b) ε=8% (c) ε=12% (d) ε=16.5% figure 11. displacement field of particles. conclusion in this paper, the macromechanical parameters of sandy pebble soil of different coarse-grained content were obtained using the indoor large-scale triaxial test of sandy pebble soil, and the discrete element numerical triaxial model of sandy pebble soil was established. by checking calculation, the micromechanical parameters of sandy pebble soils of four different gradations were obtained, and the effect of coarse-grained content on the mechanical properties of sandy pebble soil was analyzed from the macro and micro points of view, and the following conclusions were obtained: the stress-strain curve of sandy pebble soil exhibited strain softening with the rise of coarse-grained content; when the confining pressure was the same, the stress peak increased gradually with the increase of coarse-grained content; the shear strength of sandy pebble soil increased steadily with the increase of coarse-grained content, and the internal friction angle and cohesion of sandy pebble soil linearly increased with the rise of coarse-grained content; based on the micromechanical parameters of sandy pebble soil in the triaxial test, the micromechanical parameters of discrete element of sandy pebble soil of four different gradations were obtained using the comparative data method of numerical triaxial test, providing the theoretical support for the fine research on stability of sandy pebble soil; the increase of the coarse-grained content of sandy pebble soil resulted in increased contact stiffness and friction coefficient of micro parameters of discrete element, thus increasing the strength of sandy pebble soil; the dilatancy of specimens was caused by the particles in the upper and lower parts of the model moving to the middle and the particles in the middle of the model moving outward, and the numerical triaxial test can better reproduce the triaxial compression process. acknowledgements this work was supported by the national natural science foundation of china (51208069) and the state key laboratory of geohazard prevention and geoenvironment protection independent research project (sklgp2012z005, sklgp2016z010). 71macro-micromechanical properties of sandy pebble soil of different coarse-grained content references ahmad, n., hussain, t., awan, a.n., sattar, a., arslan, c., tusief, m.q., mariam, z. 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(2013). mesoscopic simulation of strength and deformation characteristics of coarse grained materials. rock and soil mechanics, 34(7), 2077-2083. time series models are often used in hydrology and meteorology studies to model streamflows series in order to make forecasting and generate synthetic series which are inputs for the analysis of complex water resources systems. in this paper we introduce a new modeling approach for hydrologic and meteorological time series assuming a continuous distribution for the data, where both the conditional mean and conditional variance parameters are modeled. bayesian methods using standard mcmc (markov chain monte carlo methods) are used to simulate samples for the joint posterior distribution of interest. two applications to real data sets illustrate the proposed methodology, assuming that the observations come from a normal, a gamma or a beta distribution. a first example is given by a time series of monthly averages of natural streamflows, measured in the year period ranging from 1931 to 2010 in furnas hydroelectric dam, brazil. a second example is given with a time series of 313 air humidity data measured in a weather station of rio claro, a brazilian city located in southeastern of brazil. these applications motivate us to introduce new classes of models to analyze hydrological and meteorological time series. abstract keywords: hydrology time series, meteorological time series, conditional regression models, bayesian analysis, mcmc methods. seasonal hydrological and meteorological time series issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.65577 edilberto cepeda cuervo1, jorge alberto achcar2, marinho g. andrade3. 1. statistics department science faculty universidad nacional de colombia, bogotá, colombia email: ecepedac@unal.edu.co 2. social medicine department fmrp, universidade de são paulo ribeirão preto, s.p., brazil 3. applied mathematics and statistics department icmc, universidade de são paulo sao carlos, s.p., brazil earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 83-90 los modelos de series de tiempo se usan a menudo en estudios de hidrología y meteorología para modelar series de flujos a fin de hacer pronósticos y generar series sintéticas que son insumos para el análisis de sistemas complejos de recursos hídricos. en este artículo presentamos un nuevo enfoque de modelado para series de tiempo hidrológicas y meteorológicas asumiendo una distribución continua para los datos, donde se modelan los parámetros tanto de la media condicional como de la varianza condicional. métodos bayesianos estándares que usan mcmc (markov chain monte carlo) son usados para simular muestras de la distribución a posteriori conjunta de interés. dos aplicaciones a conjuntos de datos reales ilustran la metodología propuesta, asumiendo que las observaciones provienen de una distribución normal, gamma o beta. un primer ejemplo está dado por una serie temporal de promedios mensuales de los caudales naturales, medidos en el período anual que va de 1931 a 2010 en la presa hidroeléctrica de furnas, brasil. un segundo ejemplo considera una serie temporal de 313 datos de humedad del aire medidos en una estación meteorológica de río claro, una ciudad brasileña ubicada en el sureste de brasil. estas aplicaciones nos motivan a introducir nuevas clases de modelos para analizar series de tiempo hidrológicas y meteorológicas. resumen palabras clave: : series de tiempo hidrológicas; series de tiempo meteorológicas; modelos de regresión condicional; análisis bayesiano; métodos mcmc. record manuscript received: 09/06/2017 accepted for publication: 27/04/2018 how to cite item cepeda-cuervo, e., achcar, j. a., & andrade, m. g. (2018). seasonal hydrological and meteorological time series. earth sciences research journal, 22(2), 83-90. doi: http://dx.doi.org/10.15446/esrj.v22n2.65577 series de tiempo hidrológicas y meteorológicas estacionales h y d r o m et eo r o lo g y 84 edilberto cepeda cuervo, jorge alberto achcar, marinho g. andrade 1. introduction time series models are often used in hydrology studies to model streamflow series in order to make predictions and to generate synthetic series which are inputs for the analysis of complex water resources systems (see, for example, salas et al., 1980, 1982; hosking, 1984; hipel& mcleod, 1994; montanari et al., 1997; hasebe et al., 2000). in many studies, hydrologists also use time series data to display the amount of rainfall that has fallen in a region for the past day, year or a period of 10 years (see for example, guimaraes& santos, 2011, and lee & lee, 2000). modeling hydrological variability is very important in the planning and management of water resources. many aspects of the hydrologic cycle could be described by time series data. researchers, usually use time series data to evaluate the resources of a water basin. important variables related to streamflow and watershed describes streamflow properties such as monthly flows or streamflow parameters. assuming a specified time series model, we usually could estimate the streamflow parameters using a classical or a bayesian inference approach. different time series models as arma and higher orders of ma models have been used by some authors when considering hydrologic regionalization of watersheds (see for example, chiang et al., 2002 a, b; wang et al., 2015). valipour et al.(2013) studied annual runoff time series in dahuofang reservoir, in northeast china, using autoregressive integrated moving average (arima) models coupled with ensemble empirical mode decomposition (wu, z., & huang, n. e., 2009). spectral analysis and forecasting of hydrological time series has also been considered (marques et al., 2006), including geostatistical applications (robin et al., 1993) and climate change investigations (lall and mann, 1995). considering hydrological time series, the monthly streamflow series typically have a periodic behavior in the mean and variance and in general, periodic autoregressive models are used in de analysis of the data (see, for example, modal & wasimi, 2006). in this situation, usually it is assumed that the series flow has a normal or log-normal distribution (see for example, tesfaye et al., 2006; wang et al., 2009). the behavior of meteorological time series is similar to the behavior of hydrological time series. in this case, given that the relative air humidity is a random variable with values given in the open interval (0, 1), we could assume a beta distribution to analyze the data. another possibility to analyze the data set is to consider a transformation of the data and to assume a normal distribution for the transformed data. as a special case, we could assume a logistic transformation. in this paper, a more general model is considered in the analysis of the hydrological or meteorological time series conditional to the historical available information: it is assumed that the data is generated from a normal, a gamma or a beta distribution, with conditional mean and variance, given respectively, by e [yt|yt-1] and v [yt| yt-1], where the index tis related to time. thus a general model is proposed to analyze hydrological or meteorological time series, assuming that the observations are generated from a continuous biparametric exponential family of distributions. as an illustration and motivation for the use of the proposed models, we first consider as a first example, a data set consisting of the time series of monthly averages of natural streamflows, measured in the year period ranging from 1931 to 2010, in furnas hydroelectric dam, located in southeastern brazil. this time series is shown in figure 1. from this figure, we observe that the streamflow series have a periodic behavior in the mean and variance and in this case, general periodic autoregressive models are usually assumed in the analysis of the time series data (modal & wasimi, 2006). to take into account the heteroscedasticity in the time series of streamflows showed in this first example, we propose a new model which assumes seasonal and autoregressive terms in the modeling of the mean and variance parameters. in this way, we propose a periodic and heteroscedastic model, in which the variance also presents an autoregressive structure. as a second example, we consider another time series with behavior similar to the assumed hydrological time series presented in the first example, consisting of a meteorological time series given by weekly averages of air humidity, measured in the city of rio claro, são paulo state, brazil (figure 2). in this case, given that the relative air humidity is a random variable with values in the open interval (0, 1), it is assumed a beta distribution in the analysis of the data. in this case, we propose a jointly modeling for the mean and for the variance of the data considering beta regression models, including seasonal and autoregressive terms in both regression models figure 1. time series of monthly averages of natural streamflows. figure 2. air humidity time series data. the paper is structured as follows: in section 2, it is introduced the seasonality analysis of the time series. in section 3, it is proposed seasonal autoregressive models. in section 4, it is presented the results of the analysis of the hydrological time series obtained using the proposed models assuming normal and gamma distributions. in section 5, it is presented the results of the analysis of air humidity time series. finally, in section 6 it is presented some conclusions and future research topics. 85seasonal hydrological and meteorological time series (1) (2) (6) (7) (3) (4) (5) 2. a period model in this section, we introduce a new modeling approach including seasonality terms in the model which better describes time series of monthly averages of natural stream flows, denoted by yt. in the first case, we consider the time series data related to the monthly averages of natural streamflows in furnas hydroelectric dam (figure 1). a preliminary spectral analysis is developed for this time series to determine the time periods to be considered in the mean and variance model formulations. thus, if in the spectral analysis, the number of observations is t = 2q + 1, where q is a positive integer number, the fourier time series model given by: is to be fitted by the data, where fi = i/t is the ith harmonic of the fundamental frequency 1/t and, α1,i and α2,i, i = 1,...,q, are the related coefficients and et is an error term assumed to have the first and second moments given respectively by e(et) = 0, e (et 2) = σ2 and to be uncorrelated, that is, e(et et+k) = 0 for k ≠ 0. considering the n observations of the time series, the least square estimates of the coefficients α0 and (α1,i ,α2,i), i = 1,...q, are obtained from the equations: based on these estimates, the intensity of each frequency is calculated by: observe that the highest frequency is 0.5 cycle per month (time interval) since the smallest period is 2 months. this preliminary analysis of the intensities of frequencies allows us to reduce the number of harmonics considered in the time series included in the model, considering only those frequencies that have higher intensities (marques et al., 2006). for a series consisting of 80 years of observations (that is 960 months) we would have 480 harmonics, but considering this preliminary analysis, we could reduce the number of harmonics to three or four. usually, in the case of monthly time series, these periods are given by periods of 6 and 12 months. these periods will be discussed in section 4, where the specification of the parameters related to the periods is made simultaneously with the specification of the parameters for the autoregressive models assumed for the mean and variance in the time series formulations to be introduced in section 3. if some of these parameters show to be not significant after a preliminary statistical analysis, the terms related to these parameters are deleted from the model to develop a further analysis. the final decision on which harmonics should be included in the model must be made considering some criteria for model selection or hypothesis test on the fitted coefficients α1,i and α2,i. 3. the proposed seasonal autoregressive model in hydrological or meteorological time series, we usually assume that the observations of interest are generated from a conditional continuous probability distribution function. as special cases, we could assume that the observations are generated from standard normal, gamma, beta or exponential conditional density functions, denoted by f (yt│ht-1 ), t = 1,2,...,t , where ht-1 is the available information up to time t 1 and thus yt has conditional means and variances given respectively by µt = e (yt |ht-1) and ht = var(yt |ht-1), following the models: where β = {α0, α1,1 ,..., α1,q ,α2,1,..., α2,q , ϕ1,...,ϕp ,}, is the vector of parameters for the mean model and γ = {λ0, λ1,1,..., λ1,s ,λ2,1 ,..., λ2,s, θ1,...,θr} is the vector of the parameters for the variance model, fi = i/t the ith harmonic of the fundamental frequency 1/t. in this paper, the parameters of the proposed models are estimated under a bayesian approach. in order to illustrate the proposed methodology, we also include the regression equations relating the mean and variance parameters to the assumed covariates assuming gamma and beta distributions for the data. in this way, we have, the following modeling steps: 1. if yt, t = 1,2,...,t, follows a gamma conditional distribution g(pt , qt), where g(p,q) denotes a gamma distribution with mean pq and variance pq2, the conditional mean and variance are related to the original parameters by the equations µt= pt qt and ht = µt qt. 2. if yt, t = 1,2,...,t, follows a beta distribution function b(pt,qt), we consider a reparameterization of the beta distribution density as a function of the mean and precision, ϕt = pt + qt, which results to be appropriate in order to define the joint mean and precision beta regression models as introduced by cepeda (2001). this reparameterization, where ϕ = p + q, p = µϕ and q = ϕ(1 µ), has been extensively used in the literature following the joint modeling approach for the mean and precision beta parameters introduced by cepeda (2001) and cepeda and gamerman (2005), under a bayesian approach. it is important to point out that ferrari and cribarineto (2004) also introduced a beta modeling approach for the mean but assuming constant precision parameters, under a classical approach. in all of these cases, ϕ can be interpreted as a precision parameter in the sense that, for fixed values of µ, larger values of ϕ correspond to smaller values for the variance of y. this interpretation could be not so simple. in this paper, we use the mean and variance reparameterization of the probability beta distribution in the definition of joint mean and variance beta regression models (cepeda-cuervo, 2015), taking into account that µ(1 µ) > σ2; in this way, samples of the joint posterior distribution for the regression parameters should be simulated in the subspace of parameters that satisfy this property. although this reparameterization results in a 86 edilberto cepeda cuervo, jorge alberto achcar, marinho g. andrade complex expression for the beta distribution, it leads to a best and more easily interpretation for the statistical analysis results in the applications. (8) (10) (11) (9) in this reparameterization, where the autoregressive seasonal beta regression models have the conditional mean and variance model given by the equations (6) and (7). special cases of this model could be easily obtained from this general model. a first model is given by a seasonal regression mean model, with mean given by (6) and autoregressive variance not considering the presence of seasonal terms. a second model, also a seasonal mean model, is given assuming the mean given by (6) and a seasonal variance model not considering the presence of autoregressive terms. a third model, is given by an autoregressive mean model and a variance model, not considering the presence of seasonal terms in the mean and in the variance. a fourth model, is given by an autoregressive model, with constant variance. 4. hydrological time series in this section we consider an analysis of the furnas dam hydroelectric hydrological time series dataset, introduced in section 1, assuming seasonal autoregressive conditional heteroscedastic models. the first step in the proposed analysis is to determine the period for the harmonics of higher intensity in the spectral analysis of the streamflows data. in this way, we note in figure 3 that the harmonics of higher intensity (i) corresponds to the cycle (1/fi) of 6 and 12 months, given that the periodogram shows that this time series contains two cosinesine peaks at these frequencies. there are other very small peaks in this periodogram, possibly caused by noise components. thus, the seasonal terms to be included in the mean equation model of the streamflow series are given by cos(2πt/6), sin(2πt/6), cos(2πt/12) and sin(2πt/12). the periodogram of a time series can be obtained using the r-function periodogram (x, method,..) of the library "genecycle" (ahdesmaki et al., 2012). other statistical software can be used to estimate the periodogram of a seasonal time series, for example, mathlab or the statistical software excel-xlstat. figure 3. periodogram of natural streamflows time series. many autoregressive models could be assumed to analyze this data set. as special cases, we will assume, in section 4.1, a seasonal conditional normal distribution for the data and, in section 4.2, a seasonal conditional gamma distribution for the data. in order to apply the bayesian methodology, independent normal prior distributions n(0,10k), with k = 2, are assumed for the regression parameters associated with the seasonal terms. for the other parameters of the model, we assume independent normal prior distributions n(0,10k), with k = 5. it is important, to point out that non-informative prior could be used (see, for example, geman, 2006), but in our case, the obtained results are very similar. since we are using the free available software winbugs to simulate samples for the joint posterior distribution of interest in all assumed models, which only requires the introduction of the joint distribution for the data and the prior distributions for all parameters of the model, we will not present in this paper the required full conditional posterior distributions for all parameters needed in the gibbs sampling or the metropolis-hastings algorithms. 4.1. normal seasonal time series in this section, we present the results of the bayesian analysis for the time series of monthly averages of natural streamflows, measured in the period ranging from 1931 to 2010, considering the dataset related to a hydroelectric dam introduced in section 1, assuming jointly modeling for the mean and variance, assuming a normal model for the data. from the model given by equations (6) and (7), many autoregressive models were considered in the analysis of the data, and the model with smallest dic (deviance information criterion, introduced by spiegelhalter et al, 2002) value was given by the heteroscedastic normal regression model with conditional mean and variance structures given respectively by, samples of the joint posterior distribution of interest were simulated using standard mcmc (markov chain monte carlo) methods and the free available winbugssoftware (spiegelhalter et al, 2003). in each of the cases, many samples were generated starting from different initial values. all of them showed the same behavior, after a small burn-in period consisting of 3,000 or 5,000 generated samples. convergence of the simulation algorithm was observed from trace plots of the generated gibbs samples. for the model given by equations (10) and (11), the value of the logarithm of the likelihood function evaluated at the obtained estimates for the parameters of the model was given by −2logl=10479.200 and the obtained dic value was dic=10876.500. monte carlo estimates of the posterior means for each parameter based on the generated gibbs samples and their respective standard deviations are given in tables 1 and 2, including both autoregressive and seasonal terms in the conditional mean and variance terms. table 1. normal model: bayesian estimates for the mean parameters. table 2. bayesian estimates of the variance parameters. 87seasonal hydrological and meteorological time series figure 5. normal fitted squared root of the expected volatility. figure 6. monthly averages of natural streamflow. figure 4. monthly averages of natural streamflow and normal fitted mean estimates. as an illustration of the performance of the proposed model, figure 4 shows the agreement between monthly averages of natural streamflows and the fitted mean estimate. at the same time, figure 5, depicting of the fitted squared root of the expected volatility shows periods of high volatility around of the months 200, 400, 600 and 800, as observed in the data behavior. 4.2 gamma seasonal time series in this section we present the results of the bayesian analysis for the time series for monthly averages of natural streamflows, measured in the period 1931 to 2010, in furnas hydroelectric dam, assuming joint modeling for the mean and variance autoregressive gamma models, that is, we assume that the observations of the interest are generated from a conditional gamma density function given by f (yt |ht-1), where ht-1) is the information up to time t − 1 and yt has conditional mean and conditional variance given respectively by µt = e(yt | ht-1) and σt 2 = var(yt | ht-1), and defined by (10) and (11). for this model, the logarithm of the likelihood function evaluated at the estimates for the parameters of interest is given by −2logl=10649 and the dic value is given by 10862.300. using the dic criterion to discriminate the two models (normal seasonal time series and gamma seasonal time series), we observe better fit of the data for the gamma seasonal time series model, since we have smaller dic value for this model. the bayesian estimates of the posterior means for the parameters together with the corresponding standard deviations are given in tables 3 and 4. table 3. gamma model: bayesian estimates for the mean parameters. table 4. gamma model: bayesian estimates for the variance parameters. although the mean parameter estimates given in tables 1 and 3, and variance parameter estimates given in tables 2 and 4, show some agreement between conditional normal and conditional gamma regression parameter estimates, the dic value of the conditional gamma model is smaller than that of the conditional normal heteroscedastic models, showing that the class of gamma models is better to fit the monthly averages of natural streamflow data. in order to determine the forecasting performance of the proposed gamma seasonal model, we fit this model to the first 910 observed values of the furnas dam hydrological time series, in order to predict the temporal behavior over the 50 last months. the parameter estimates agree with those reported in tables 3 and 4. figure 6 shows that this model has good performance in predicting monthly averages of natural streamflows. this figure shows the 90% prediction interval for the variable of interest in these last weeks in dashed lines, and the observed values of natural streamflows in black points. 5. beta mean and variance seasonal models applied to air humidity time series in this section it is assumed that the time series data are generated from a beta distribution, b(pi ,qi), with conditional mean and variance given, respectively, by (6) and (7). to illustrate the application of the proposed model to be fitted by the data set, we consider the time series of weekly averages air humidity, measured in rio claro city, located in southeastern of brazil, from 18/10/2002 to 08/10/2008. this time series, introduced in section 1, is shown in figure 2. as in section 4, the first step in the proposed analysis is to determine the period of harmonics of higher intensity in the spectral analysis of the humidity data. from the corresponding periodogram, figure 7, we note that the 88 edilberto cepeda cuervo, jorge alberto achcar, marinho g. andrade harmonics of higher intensity corresponds to the cycle (1/fi) of 26 and 52 days. as in the analysis of streamflow data, the periodogram shows that this time series contains two cosine-sine peaks at these frequencies, two larger peaks, and other very small peaks, possibly caused by noise components. thus, the seasonal term to be included in the mean equation model of the streamflow series are given by: cos(2πt/26), sin(2πt/26), cos(2πt/52) and sin(2πt/52). figure 8. air humidity time series data (continuous line) and fitted mean (diseased line) for double seasonal model. figure 9. fitted conditional variance time series. 5.1. seasonal mean and conditional variance models in this section, double seasonal beta repression models are proposed to analyze the air humidity time series data introduced in section 1. in this way, we conclude that the best beta seasonal mean and variance model is the one where the mean and dispersion regression structures are given by: (12) (13) assuming independent normal prior distributions n(0,100) for the regression parameters α0 and α1,i, for i = 1,2 and for λ0 and λ2,i, for i = 1,2. to fit the model 20,000 samples of the joint posterior distribution for the parameters of the model were also generated using the mcmc methods and the winbugs software (spiegelhalter et al, 2003). monte carlo estimates for the posterior means of each parameter were obtained from the final simulated gibbs sample, after an initial burn-in sample period of 2000 samples. this “burn-in sample” was discarded to eliminate the effect of the initial values in the iterative procedure. after this “burn-in sample” period we simulated another 20,000 gibbs samples choosing every 20 iteration to get approximately non-correlated samples, which gives a final sample of size 1,000 used to get the posterior summaries of interest. the posterior summaries of interest are given in table 5, for the mean regression parameters, and in table 6, for the variance regression parameters. table 5. beta model: bayesian estimates for mean parameters. table 6. beta model: bayesian estimates for the variance parameter. the logarithm of the likelihood function evaluated at the obtained estimates for the parameters of the model is given by log l = −853.082 and the dic criterion has a value equals to −825.525. in figure 8, we observe a good agreement between data and the fitted mean, showing the good performance of the proposed model to analyze this data set. figure 9, revels a good agreement for the variances, where smaller means are accompanied by smaller variance. this behavior also was observed in the original time series. that is, we conclude that this model fits by the time series data very well. to determine the forecasting performance of the proposed models, we fit the seasonal beta autoregressive model to the first 282 observed values of the air humidity time series data, in order to predict the time behavior in the 30 last weeks. figure 10 shows the 90% prediction interval for these last weeks, in dotted lines and the observed values of natural streamflows in black points. figure 7. periodogram of humidity times series. 89seasonal hydrological and meteorological time series figure 10. forecasting humidity time series. 6. concluding remarks in this paper, we introduce a new class of time series models assuming continuous random variables within the exponential family applied to hydrological and meteorological data. special cases of this proposed methodology were considered assuming, normal, gamma and beta distributions. these new models could be of great interest to analyze hydrological and meteorological data, since the original data usually could not be fitted assuming the standard modeling approach for the means of the data or assuming standard distribution assumptions for time series data. other important practical point: in practical work, usually it is required to model simultaneously the mean and the variance depending on a vector of parameters to get better predictions and to discover the real behavior of a hydrological or a meteorological time series. with this modeling approach, we could get better inferences as it was observed in the illustrations introduced in this paper. this approach usually is not considered by hydrological or meteorological researchers using standard available software for hydrology or meteorology time series. in this way, the use of bayesian methods is a good alternative to get accurate inferences and predictions, especially using mcmc methods, since these inference methods are not based on asymptotical results as its common with standard existing classical approaches based on maximum likelihood estimation procedures. other important point: under the use of a bayesian approach, we could consider informative prior distributions when is available prior opinion of experts in hydrology and meteorology. this means, better predictions. it is also important, to point out that the computational work needed in the simulation of samples of the joint posterior distribution to get the posterior summaries of interest is greatly simplified using standard existing softwares like the winbugs software.the proposed methodology was illustrated considering two brazilian data sets: a hydrological time series and a meteorological time series. a final point of relevance for the results of this paper: new statistical models for time series to model climatic variables leading to better inferences and predictions based on data from hydrology and meteorology can be of great practical interest, especially with the large climate changes that have been observed in the world in recent decades. to determine the performance of the proposed models, an extensive comparison with other models proposed in the literature should be developed. however, in order to compare the performance of the gamma seasonal models with in a model without a seasonal component, we fit the autoregressive model ar(3), for which the dic value was 11990, bigger than the dic value of the fitted gamma seasonal model. the sum of square residuals obtained from applying the ar(3) model is also bigger than that for the proposed models. for the ar(3) model, the sum of square residuals is 65679347 and for the proposed model it is 52976950. finally, we fit the ar(3) model to the humidity data, for which the dic value was -774.550, bigger than −825.525, the bic value of the fitted beta seasonal model. the sum of square residuals obtained from applying the ar(3) model to the humidity data is 17.1565, larger than the dic value (1.3786) of the proposed models. references ahdesmaki m, fokianos k, strimmer k (2012) package ‘genecycle’. r-project website. url: http://cran.r-project.org/web/packages/ genecycle/genecycle.pdf. accessed 2017 dec 5. cepeda-cuervo, e. 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(2013). comparison of the arma, arima, and the autoregressive artificial neural network models in forecasting the monthly inflow of dez dam reservoir. journal of hydrology, 476, 433-441. keywords: earthquake; prediction; prevention; risk reduction. palabras clave: benín; exponente de escalado; mfdfa; espectro de singularidad; temperatura; dinámica no lineal. how to cite item tapia-hernandez, e., reddy, e. a., & orosavilés, l., j. (2019). earthquake predictions and scientific forecast: dangers and opportunities for a technical and anthropological perspective. earth sciences research journal, 23(4), 309-315. doi: https://doi.org/10.15446/esrj.v23n4.77206 supporting earthquake risk management with clear seismic communication may necessitate encounters with various popular misapprehensions regarding earthquake prediction. drawing on technical data as well as insights from anthropology and economics, this paper addresses common and scientifically-unsupported ideas about earthquake prediction, as well as the state of science-based studies regarding statistical forecasting and physical precursors. the authors reflect on documented social and economic effects of unsubstantiated earthquake predictions, and argue that these may be dangerous but may also present certain opportunities for outreach and education in formal and informal settings. this paper is written in light of the importance that the united nations office for disaster risk reduction has placed on coordination and communication within and among diverse organizations and agencies as well as by the recent popularity of so-called earthquake prediction in mexico. abstract earthquake predictions and scientific forecast: dangers and opportunities for a technical and anthropological perspective predicciones y pronósticos científicos de sismos: peligros y oportunidades desde una perspectiva técnica y antropológica issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.77206 el apoyo de la gestión del riesgo de terremotos con una comunicación sísmica clara requiere incluir la atención de interpretaciones populares erróneas sobre la predicción de sismos. con base en datos técnicos, así como en percepciones de la antropología y la economía, este artículo discute ideas populares y sin sustento científico sobre la predicción de terremotos, así como el estado del arte de estudios científicos con bases estadísticas y con precursores físicos. se discuten los efectos sociales y económicos documentados de las predicciones de sismos sin sustento científico para argumentar su peligrosidad; pero también se hacen notar las oportunidades que representan su divulgación y educación en entornos formales e informales. el artículo discute a la luz de la importancia que la oficina de la organización de nacionales unidas para la reducción del riesgo de desastres ha otorgado a la coordinación y comunicación dentro y entre las diversas organizaciones y agencias, así como a la reciente popularidad de la llamada predicción de terremotos en méxico. resumen record manuscript received: 11/01/2019 accepted for publication: 05/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 309-315 edgar tapia-hernández1*, elizabeth a. reddy2 and laura j. oros-avilés3 1department of materials. universidad autónoma metropolitana azcapotzalco, 02200, mexico city. 2department of engineering, design and society, colorado school of mines, united states. 3department of economics. universidad autónoma metropolitana azcapotzalco, 02200, mexico city * corresponding author: etapiah@azc.uam.mx g eo lo g ic al h az ar ds 310 edgar tapia-hernández, elizabeth a. reddy and laura j. oros-avilés introduction on march 20, 2012 a strong earthquake of magnitude 7.4 struck southern mexico. over the next days, other significant earthquakes around the world were registered: an earthquake of magnitude 6.6 in new guinea on march 21; an earthquake of magnitude 7.1 in maule, chile on march 25; earthquakes of magnitude 8.6 and 5.5 in sumatra on april 11; an earthquake of magnitude 6.5 in michoacán, mexico on april 11; and another of magnitude 6.9 in the gulf of california on april 12 (usgs 2015). this cluster of activity, though far from unusual, was interpreted by at least one public as empirical confirmation of earthquake predictions (e.g. univison 2013; latinospost 2013) which drew not on peer-reviewed scientific research but rather on methods which incorporated the position of the planets, dreams and even the so-called mayan apocalypse, slated to end this age and begin a new one on december 21 of that year.1 seismicity and related hazards have killed more people than all other natural hazards combined in the past 20 years (unisdr and cred, 2016) and the danger that they continue to pose requires ongoing action. the most recent united nation sendai framework for disaster risk reduction has drawn social and physical sciences together to indicate how crucial popular knowledge about hazards can be for effective disaster prevention. this is noting that not only can clear and coherent understanding of what can sometimes be very technical issues be important for publics, but it may also be crucial to facilitate coordination between institutions and stakeholders at all levels including community-based organizations, non-governmental organizations, and governmental organizations at local, national, and international scales (unisdr 2015)2. in this context, non-expert familiarity with basic information about earthquake hazards may be essential for risk management. scientifically unsubstantiated ideas about seismicity like those that we refer to above are not necessarily opposed to good earthquake risk reduction practices. the impetus of preparation need not be a scientifically verified threat to be potentially helpful; indeed, a marked rise in discussion of earthquake hazard and interest earthquake insurance interest was documented in the wake of a spurious prediction of a 1989 earthquake on the new madrid fault (shipman et al., 1993). an understanding that a scientifically viable threat is not necessary to encourage useful activity has informed a strategy by the centers for disease control to encourage emergency preparedness through discussion of a zombie outbreak (cdc 2015).3 although preparedness activities inspired by any threat can contribute to public safety, the spread of misinformation can be dangerous. misinformation can frighten people and facilitate miscommunication or mistrust, unpredictable social trends, or demands for resources. inaccurate seismic forecasts may be detrimental to public wellbeing in ways that drawing playfully on a horrormovie hazard are not. mistaken expectations about the degree to which seismic hazard can be predicted may create confusion and has been found in some circumstances to be related to activities that might have negative impact on potentially vulnerable populations and undermine the authority of scientific experts (stallings 1995). the formal and informal outreach and education about seismicity and earthquake risk mitigation strategies can be observed in the context of popular fascination with prediction, writing editorials in newspapers, giving talks, or simply engaging in ordinary conversation with friends, family, and colleagues. in light such activities, the state of the field of earthquake forecasting is a matter of some importance for earthquake engineers and other experts. knowledge of a hazard does not necessarily spur effective disaster prevention activities (landeros-mugica et al., 2016). however, with a clear understanding of the state of research on both seismic forecasting and the dangers and opportunities provided by predictions, technically-trained experts can better consider their intervention and outreach work. such labor, whether formal or informal, paid or unpaid, may substantially support the kind of general communication and coordination that we know can support prevention, save lives, and speed recovery. 1 see, also, theories circulated by brian t. johnstone, keith hunter, and those documented in austin (2016) 2 see particularly priority one, “understanding disaster risk.” 3 jason j. morrissette (2014) contextualizes the symbolic power of the figure of the zombie in critical security studies, while julia daisy fraustino and liang ma (2015) critique the effectiveness of the social media campaign. this paper has the following goals: first, to outline current mechanisms for forecasting earthquakes, and second, to describe documented dangers and opportunities afforded by popular interest in earthquake prediction. finally, we call for further systematic research on how the formal and informal communication about earthquakes undertaken by experts may contribute to public safety as well as on the effects of erroneous predictions for both disaster prevention works. although some commentators have described “a tendency on the part of officials to see disaster planning as a product, not a process” (wenger et al. 1980), authors are of the opinion that facilitating the ongoing discussion and the circulation of useful information regarding earthquakes is part of the necessary, but sometimes frustrating, process of effective disaster preparation. on “prediction” the prediction of an earthquake is defined as future seismic event may or may not occur in a region, a period of time, and magnitude with probability p (0.0 < p< 1.0) that the earthquake occurs in order to warn the population about the risk. the most memorable prediction occurred in 1975 in haicheng, china, where the changes in ground elevation, along with levels of groundwater, the behavior of animals and short quakes allowed an evacuation, just few hours before the earthquake. however, rather than the application of a methodology to provide a certain probability of occurrence, the prediction was based on coincidences and circumstantial facts. this illustrated by the fact that local experts failed to accurately predict another similar event. indeed, thirty incorrect predictions were made just in two years (1997 to 1999). this scenario dismissed the credibility and forced the chinese government to intend to stamp out wrong warnings and mass evacuations (saegura 1999). the interest of society in predictions is supported by the uncertainty of the scientific understanding of seismic activity and the threat that represents. in fact, they might have circulation in the social media and press. some of them, that apparently account a tested methodology, are occasionally even published in scientific venues (e.g. curiel 2010; straser et al. 2019). in particular, the earthquake predictions of gabriel curiel, which suggest that gravitational forces create a “season of earthquakes” between october and december, were presented in the 2010 mexican conference of earthquake engineering and, subsequently, published on curiel’s blog and referenced in countless newspaper articles (e.g. univision 2013; latinospost 2013), despite its lack of scientific support or even any attempt to offer convincing statistical evidence (curiel 2010). despite of the fact, that it is known that the available prediction methodologies have null scientific support, forecasting approaches continue to develop. after the 2009 l’aquila earthquake, the international commission on earthquake forecasting for civil protection developed a document, which reports the state of the art of short-term predictions and indicate guidelines for identification of possible precursors in order to warn the population (jordan et al. 2011). the main recommendations are as follow: a. it is desirable to continue to track the scientific evolution of probabilistic seismic predictions and develop the expertise to utilize these data for operational intentions. b. it is necessary to establish instrumented laboratories for studying seismic activity and their generation process. c. it should develop a research project focused on seismic activity as a part of a national program to develop operational prediction methods. d. it should develop a research time-independent and time-dependent of prediction methodologies in order to improve seismic hazard maps. e. it is necessary to develop an operational capability for the prediction of aftershocks. f. the validating the forecasting models should be developed from the international efforts to develop to test earthquake prediction methods. research on precursor physical phenomena gas measurement a strong earthquake (mw= 7.1) struck the philippines on november 15 in 1994. the epicenter was located 48 kilometers from the taal volcano, where a 311earthquake predictions and scientific forecast: dangers and opportunities for a technical and anthropological perspective stake and others noted an abnormal emanation of hydrogen in bubbles of mineral springs in five locations just one month before the may 26, 1983 earthquake in atotsugawa and ushikubi, japan (mw= 7.7) and before the september 14, 1984 earthquake in nagano (mw= 6.8) (igarishi and wakita, 1995). in contrast to the above, more than 150 studies have reported that measurements of gases might not be a proper reference of chemical concentration underground at considerable depths. this is due thermo-mineral waters contacts a considerable volume of rocks at various depths, so the records are not necessarily more representative of the environment at the tectonic plates. because of this, it is desirable to study the relationship between the gas emanations and the environmental disturbances such as atmospheric pressure, tide, temperature, rainfall, barometric pressure, soil moisture, wind temperature, volcanic disturbances, etc. (segovia et al. 1999). nevertheless, the study of gases emanations has been a typical effort in lessthan-scientific earthquake predictions and, indeed, they have appeared too often in public discussions of the seismic activity. for example, the prediction made by giampaolo giuliani before the 2009 l’aquila earthquake was established form gases measurements. after the event, giuliani became recognized in social and popular media for the apparent efficacy of the measurements, despite of he had been asked to desist from frightening the society after he had made some incorrect predictions (hall 2011; cartlidge 2012). geophysical observations the seismic activity has been associated also to electromagnetic phenomena. for instance, light flashes were related with earthquakes in derr in 1973 and komogawa and others in 2005. similarly, electromagnetism effects were reported by gokhberg and others in 1982 and nagao and others in 2002. atmospheric electric fields were reported by kondo in 1968 and vershinin and others in 1999. ionospheric perturbation was discussed by nagaov and hayakawa in 1998 and liu and others in 2000 (omori et al. 2007). the chemistry and isotope components of water were studied in 9 geothermal fields located around the anatolian fault in turkey from 2002 to 2004 in order to investigate their relationship with earthquakes. the study report changes in chemical and isotopic balance, which it seemed to be related with seismic activity of moderate magnitude (süer et al., 2008). additionally, the relationship between the december 2004 earthquake in sumatra and the f-region ionospheric parameters probed remotely by a digital ionsonde over new delhi have been also studied. according to the results, perturbations in fof2 and hmf2 were noted several hours before the seismic activity (dutta et al. 2007). a correlation was established between an anomalous cloud in geostationary satellite sensor above the active iran fault by guo and wang (2008). thick clouds had spread along the tectonic plate with high temperatures, 69 days after the seismic activity. similar clouds were also noted, 64 days after an earthquake of magnitude 6.0 on december 2005. with this scenario, despite the large number of possible precursors, no definitive results have been achieved. namely, in a probability space ( p), where  is a set of events, p is a set of probabilities, m is the precursor and n is an earthquake occurrence (m, n  ). if the probability of event occurrence exists p(m) > 0 and the probability that earthquakes in that geographic region exists p(n) > 0; then, the conditional probability of n in relation of m is defined as the equation 1. p |mn p m n p n ( ) = ( ) ( )  (1) research of gas emission had been developed since june 1993. the recollected data reported, 22 days before the seismic activity, an unexpected increase in the emanation of a gas called radon. radon is a colorless, odorless and tasteless gas. it is the most studied terrestrial gas for the purpose of earthquake prediction.4 earthquake forecast research involves measuring a rays associated with the radioactive disintegration (igarashi and wakita 1995). then, the observation reported by richon et al. (2003) and other similar researches indicated the possible identification that radon gas could be associated with the increased stress in the tectonic plates which may cause an earthquake. some other researchers have reported geochemical changes before an earthquake. for instance, two weeks before the magnitude 6.8 nagano prefecture earthquake, hirotaka and others noted variations of soil-gas radon recorded 65 km from where these measurements were taken in japan in 1988 (richon et al. 2003). igarashi et al. (1995) noted similar radon anomalies as the one reported by richon et al. (2003). abnormal emanations of soil gases (radon, helium and dihydrogen) were reported at several locations following the magnitude 8.0 sichuan earthquake of may 2008 (zhou et al., 2010; zheng et al., 2012). in addition, an anomalous radon 222rn decrease was detected (fig. 1) as a different precursor starting about 2 days before a magnitude 6.0 earthquake in japan in 1991. the anomalies were reported about 200 km from the epicenter by wakita and others (igarashi and wakita 1995). figure 1. anomalous radon 222rn decrease recorded before an earthquake (adapted from igarashi and wakita 1995) with this scenario, the hypothesis was that the near-failure compression with imminent movement produces emissions of gas radon 222rn, much as if it is being squeezed from the tectonic plates. thus, an increment of emanation might anticipate a seismic activity. in controlled conditions in laboratory tests, the increment of the stress in a rock sample under uniaxial compression had been related with before an increment of the emanations of gas radon 222rn (igarashi and wakita 1995). for this reason, observations of the emanations have been investigated at various places, and different methodologies have developed to relate the radon 222rn emanation and seismic activity (e.g. segovia et al., 1999; omori et al., 2007; richon et al., 2010). however, the results are not definitive and cannot be generalized, since the seismic activity continues happening without gas emanation in all cases (richon et al., 2010). and also, earthquakes have also occurred without abnormal emanation of gas radon 222rn (peña 2003). for instance, an anomalous radon decrease was reported after the seismic activity, not before the earthquake in measurements from january and may 1987 in fukushima prefecture at the northeast of japan (fig. 2). the emanation of other gases has been also studied. abnormal measurements of helium were noted in nagano, japan from one to three months before the september 14 earthquake (mw 6.8). the emanations were reported in mineral springs and gas fumaroles between 9.0 to 95.0 kilometers from the epicenter (nagamine and sugisaki 1991). hydrogen is produced in the cortex through a complex process. for this reason, hydrogen is not studied as an indicator of earthquakes as other gasses. kato and others demonstrated in laboratory tests that samples of granite under compression load might be related to the emanation of hydrogen. furthermore, 4 there are three radon isotopes: 222rn (t1/2= 3.825 days), 220rn (t1/2= 54.5 s) and 219rn (t1/2= 3.92 s). generally, radon refers to 222rn since it has the longest half-life and, therefore, is of most geochemical interest. figure 2. observed data on radon concentration (adapted from igarashi and wakita 1995). 312 edgar tapia-hernández, elizabeth a. reddy and laura j. oros-avilés p(m|n) is interpreted whereas if n is satisfied, the conditional portion of m must be met. in other words, if the identified precursor occurs, the conditional probability of an earthquake also exists. the disadvantage of the methods discussed above is that the space where the precursor event m exists in relation to natural phenomena unrelated to seismic activity is extremely high. similarly, there is a high probability that an earthquake will occur without a precursor n. precursor events have been occurring without an earthquake and seismic activity has been reported without precursor events. in general, the studies on physical precursors have demonstrated the need for simultaneous measurement of external parameters and the creation of networked monitoring stations. nowadays, the innate randomness and frequency of earthquakes are used to sustain predictions that might produce unwarranted confidence. for the real conditions, there is still much to do to establish relevant data. also, it is necessary to study the precursors in simulations in the laboratory as a function of the variety of cases and conditions. precursors based in statistical patterns statistical correlations from catalogs of the seismic activity and other techniques developed from probabilistic models are also under investigation. these methods are based on i) diminutions of earthquakes in a region (seismic gap), ii) the tendency of the epicenters motion, iii) increments of earthquakes in the periphery of a region (donut pattern) or iv) the increase of seismic motions at low magnitudes in a particular region (swarms). statistical methods are based on data collected with stations, records of the historical seismic activity and from collaborations between historical anthropologists and geophysicists (e.g. garcía and suárez, 1996 and zuñiga et al. 1997). it is supposed, according to the elastic-rebound theory, that the tectonic plates develop stress progressively and during an earthquake, the energy is released suddenly. and after the seismic activity, it starts another energy accumulation period that will produce a new earthquake and so on. then, a seismic gap is an active fault that has not slipped in a long time when compared with other regions along the same tectonic plate. for instance, at the state of guerrero in southern mexico, an earthquake has been anticipated for many years, based on historical records. supposedly, the accumulated energy is enough to produce a strong earthquake. the information about the disastrous magnitude was spread to the society after the 8.1 earthquake of september 19, 1985 by geophysicists and the potential danger of this gap (ugm, 1986). based on this, an earthquake early warning system was expanded along the region by policymakers in mexico (espinosa-aranda et al. 2009). locally, the seismic activity has been studied using models of the physical processes and bayesian statistics in order to make adequate use of the available information since then (e.g. singh and ordaz 1994). however, the predicted strong earthquake has not struck the country. it is thought two scenarios: i) the energy continues accumulating and will lead to rupture someday, producing an even major earthquake or ii) the release movement has already happened by way of “silent” earthquakes. the release of energy through the displacement of the plates for several days is called silent slip earthquakes. nowadays, they can be distinguished by gps monitoring. for instance, figure 3 shows the location of seven sites in southwestern british columbia, canada, where red arrows show the detected displacements by continuous gps, while black arrows show the common gps motions. namely, the studied sites reversed their direction of motion. the unusual displacements were estimated about 2 centimeters in the range from 6 to 15 days; so, the slip over a 50-kilometer by 300-kilometer area was equivalent to an event of a magnitude 6.7 (dragert et al. 2001). the probability that an earthquake might occur based on the identification of historical records is uncertain. for example, in parkfield, united states was predicted an earthquake based on previous events recorded in 1857, 1881, 1901, 1922, 1934 and 1966 (bakun et al. 1987; mileti and fitzpatrick 1992) as depicted in figure 4. the identification of a seismic gap and a return period of 21.9 years were estimated with a probability of occurrence at 95%. nevertheless, the seismic activity did not occur until september 2004, deviating from the trend. thus, the search for earthquake precursors useful in a short-term has had no success yet in any sense, but their possible effects might be powerful. prevention activities and earthquake prediction disaster is a complex of social and physical effect (o’keefe et al. 1976; garcía and rojas 1994; oliver-smith 1996). disaster is not the sure outcome of a large earthquake, but instead the product of impact on people. this means that we can mitigate disaster by taking steps to prepare for earthquakes. understanding that earthquakes are likely to happen in certain areas and at certain magnitudes can support such preparation; however, when preparation is anchored to a predicted event at a given time and place, there is significant concern that this, too, can be dangerous, though in oblique ways that can be difficult to prove in the complex welter of social action. the recent un sendai framework on disaster risk reduction (unisdr 2015) addresses social and structural issues as a part of vulnerability, highlighting the ways that vulnerable people are disproportionately affected by disaster. the sendai framework defines vulnerability, in reference of hyogo framework, as “the conditions determined by physical, social, economic, and environmental factors or processes, which increase the susceptibility of a community to the impact of hazards” (unisdr 2005). this means that factors such as knowledge, income, type of work, social support network and qualities of the built environment itself such as where homes or other important structures are sited, access to services like electricity or sewage or roads and local resource depletion or contamination can be considered to contribute to vulnerability of a community to disaster. strategies suggested by the sendai framework (unisdr 2015) are wide ranging, including understanding disaster risk; strengthening disaster figure 3. description of gps sites in canada in the summer of 1999 (from dragert et al., 2001) figure 4. statistical projection of expected earthquake in parkfield, united states 313earthquake predictions and scientific forecast: dangers and opportunities for a technical and anthropological perspective risk governance to manage disaster risk, investing in disaster risk reduction for resilience, and enhancing disaster preparedness for effective response, recovery, rehabilitation and reconstruction. some of this, falls under the purview of state actors, but especially as many bureaucracies find hazards challenging in the extreme (dynes et al. 1990; perrow 2008). indeed, fostering prevention activities broadly and facilitating clear communication and integration of efforts by state and non-state actors at local, national, and international levels has been identified by the united nations office for disaster risk reduction, as a strategy to diminish the risks that potentiallydisastrous hazards pose well in advance of any danger. there is nothing controversial here, nor is there much debate regarding the understanding that a general awareness of a hazard is highly desirable for earthquake risk reduction (fema-474, 2005), especially if the members of the population are represented by the possible actions that can be taken (farley et al. 1993; mileti and fitzpatrick 1993), and can be an excellent foundation upon which to build further interventions (gratton et al. 1987; mileti and fitzpatrick 1992). this kind of awareness is certainly most helpful when it resonates with informed understandings of how earthquakes happen and are likely to affect populations and can be used in multiple and even creative ways (as barrios 2014 argues). however, it can also be troublesome. not only can emergency planning itself face significant apathy and resistance, articulating the management of action often with the management of human emotion or affect5, but also the kinds of experience with hazards and knowledge about them are also sometimes related to avoidance or unwarrantedly optimistic assessment of safety (landeros-mugica et al. 2016). despite of experts hope that they might encourage planning and preparation. prevention does not necessarily depend on scientifically informed understandings of hazard, though multi-modal and detailed communication around a threat has been found to be a key strategy for emergency hazard communication (mileti and sorensen 1990; sorensen 2000). this is a fine line to parse, but an essential one. substantial anthropological evidence indicates that particularly well-established and pervasive belief systems, whether scientifically informed or non-informed, are likely to be efficacious in the face of hazards common in the contexts in which they emerged. the adaptability and creativity based on traditions represented significant support in the past (torry 1978; oliver-smith 1994) and might offer a powerful lesson for risk mitigation (turton 1977; oliver-smith 1986). indeed, the potential utility of unscientific belief systems is supported in law; for example, the experience of l’aquila residents. in the wake of a deadly 2009 quake there, a court case was brought against the experts. they, reacting against the spurious predictions of giampaolo giuliani (noted above), assured that it was unlikely that an earthquake was forthcoming. these assurances dissuaded the population from following local custom to sleep outside in the advent earthquake swarms, and for their part in 32 l’aquila deaths, the experts were convicted of manslaughter, since overturned (alexander 2014). policy frameworks including sendai (unisdr 2015) explicitly highlight traditional informed practices potentially useful or even models for the development of effective practices. if not necessarily dangerous in and of themselves, popular expectations about the predictability of earthquakes may become so if they interfere with potentially lifesaving actions, causing them to be put off or misapplied. although research in the social sciences demonstrates that panic is often less extreme than expected (mileti and sorensen 1990, tierney 2003), anxiety involved in earthquake predictions may still have effects on institutional resources and economic activity in significant ways (haas and mileti 1976; weisbecker et al. 1977). institutional & economic impacts of earthquake predictions there are clear mechanisms by which earthquake predictions might be understood to influence expectations and affect economic activity and formal political relations. they may do so in both positive and negative ways. on the one hand, earthquake damage, often calculated as damage to a year’s expectation of economic gain, can happen here even in the absence of an actual 5 on the topic of the historical and politically-situated promises and dangers of the management of fear in particular see daston (2008) and massumi (2005). earthquake (though not in a way that insurance can be drawn upon to defray) as people evacuate, fail to visit, and change their investment plans. these kinds of impacts in advance of a hazard that may not occur can be referred to as a matter of “psychological foreshocks” (olson et al. 1989). on the other, such predictions may happen in concert with increased economic activity, as people stock up on emergency supplies (as shipman et al. suggests they did in advance of a prediction regarding the new madrid fault in 1993). they might take action, further, upon their expectations regarding possible losses, expected profits, and market depreciations. indeed, earthquake predictions based on wellestablished systems and pervasive beliefs, could modify short term spending and institutional demands with potentially wider repercussions by means of longer-term plans and market activity. in economic theory, decisions are often discussed with an inter-temporal approach: an investment decision is determined by the relationship between understood conditions of the past, of the present, and those likely in the future. this can be considered as a matter of incentives, and the scale of such incentives relate extant possibilities to potential increases in production (mankiw 2007). business owners and investors’ forecasts alike, and their confidence about forecasts, can be critical to investment decisions. their decisions do not only pertain to their own endeavors but can come to involve investors and related industries through market mechanisms, which can spread impact of investment decisions widely. the assurance with which some earthquake predictions are deployed can be very persuasive, and may not only be expressed around single events, but also with respect to longer time frames or returning seasons (“earthquake season” for example). scientific forecasting can certainly influence economic decision making. however, predictions with strong assertions of regarding when an earthquake will happen, how large it will be, and where it will be felt, may appear more concrete than the general forecasts that current state-of-the-art forecasting can offer. these, then, can motivate economic action in the same way as they motivate preparatory action: in ways that are potentially beneficial and quite troubling, but may in the end be less extreme than experts worry (olson and olson 2001). the confidence on predictions can affect economic activity of consumers, business planning, and potentially markets; they can create challenges for public facing institutions of many kinds. olson et al. (1989) document the brady-spense prediction of an earthquake in 1981 and some of the scientific and institutional conflicts which surrounded it and emerged out of it. as news of a potentially credible earthquake prediction spread and became highly politicized in the us and in peru (the territory likely to experience its impacts) people with different goals, interests, and expertise became involved in responding to the threat. many organizations were unprepared for the demands that the prediction could inspire. while olsen et al. documented how the event catalyzed new peruvian assessments of risk and disaster response capabilities, the international assistance possibilities available, and how it raised public awareness of earthquake threat, they also demonstrate the kinds of institutional pressures that were created as business as usual in state institutions. for example, peruvian civil defense was overwhelmed with requests for trainings, and could only respond to a fraction of them. predictions can create unexpected conditions, interfering with planning; and while this may open the door for new opportunities, it may also create serious trouble for midand longrange planning. conclusions in newspapers, television, and social media, predictions about earthquakes circulate. sometimes, these predictions refer to religious or mystical explanations. they may seem scientific to people unfamiliar with seismic science, referencing studies or data that readers may find difficult to parse. this paper draws on engineering, anthropological and economic scholarship to provide an overview of the current state of the field in earthquake forecasting and discuss the social and institutional implications of earthquake predictions. it pretends to give earthquake engineers the tools they need to understand current research and engage with non-expert members of the public and facilitate the kinds of coordination between institutions and stakeholders at all levels including community-based organizations, non-governmental organizations, and governmental organizations at local, national, and international scales called for by the united nations office for disaster risk reduction. 314 edgar tapia-hernández, elizabeth a. reddy and laura j. oros-avilés engineers are highly trained members of professional and nonprofessional communities who may engage with their peers on the topic of predictions formally and informally. many correct misapprehensions and answering questions, even when they are not employed to do so, providing bases by which untrained members of their communities can better understand the sometimes-confusing information they may receive about scientific research on statistically-based earthquake forecasts and seismic precursors, as well as the predictions based on ideas about earthquake seasons or ancient mystical knowledge which sometimes circulate alongside other kinds of earthquake information in the popular press. this paper provides an overview of cuttingedge scientific research into physical precursors and statistical methods of earthquake forecasting and a discussion of the potential impacts of public education regarding seismic threat can have. unlike other natural phenomena, earthquakes are not, generally speaking, predictable. statistical and precursor research is nonetheless providing key insights into seismic effects in various local environments. in correcting misapprehensions, trained experts may misestimate the negative effects that predictions can have. frustrating and troubling as they might be, they also provide opportunities that, if seized, may be turned to useful effect for 1) public education and outreach regarding earthquake safety and 2) collaboration in and between government and non-governmental institutions at local, regional, and national scales. as discussed in this paper, substantial anthropological evidence indicates that technical knowledge about seismicity may not be significantly correlated to effective public practices for earthquake risk management. nonetheless, widespread confusion and poor communication around predictions can cause interruptions to normal activity that may be both deleterious and offer opportunities for outreach about effective preparation activities. further, as responses to prediction and public concerns may necessitate the involvement of professionals without significant technical knowledge about seismicity, the understanding that a general public has of earthquake science and that of experts in the field are not the only ones of import. professionals with various responsibilities may become involved in the case of an earthquake prediction. understanding not only the geoscientific context of predictive possibility but also the social science on the risks around it may facilitate the kinds of reasonable action necessary to make the most of predictions. as erroneous earthquake predictions continue to seize popular attention without scientific support, it would be very useful indeed to critically evaluate their effects --both positive and negative-on community disaster preparation. additionally, it was noticed a great potential utility for systematic research on the effects of formal and informal expert communication about earthquakes, the state of scientific knowledge regarding forecasting, and the bases of earthquake prediction efforts. we suspect that the outreach research that we and our colleagues do can facilitate the production of shared understanding about seismic conditions and hazards and contributes to coordination that the unisdr has recognized as crucial for supporting prevention, saving lives, and speeding recovery. despite of the complex social effects of predictions that have been discussed in this paper; 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(1997). seismic hazard in latin-america and the caribbean project. international development research center. proyect 89-0190. september. ottawa, canada (in spanish). the dabashan thrust nappe structure at the southern margin of the qinling orogenic belt suffered at least two stages of evolution which are late triassic plate subduction collisional orogeny between north china block, qinling micro-plate and yangtze block followed by intracontinental orogeny since the meso-cenozoic. a prominent topography characteristic within the dabashan area is a southwestward extrusive arc (bashan arc fault) that is one of key factors to understand the geodynamic condition of the dabashan thrust nappe structure. in this work, two-dimensional plan-view models are constructed to simulate the collisional and intracontinental orogenic movements, and the factors that may control the formation of the bashan arc fault are analysed. the modelling results show that the compressive stress produced by the plates collision along both north and south boundaries is the main driving force. the dextral shearing derived from the inconsistent shape on the block margins is the main controller. rigid tectonic units such as bikou and hanan-micangshan terranes, foping and wudang domes, as well as shennongjia-huangling anticline also contribute as “anchor” effects. additionally, the rheology properties of rock material in the dabashan area affect the shape of the arc. la estructura de la falla de cabalgamiento de las montañas dabashan en el margen sur del cinturón orogénico de qinling sufrió por lo menos dos etapas de evolución, la colisión orogénica del triásico superior entre el bloque de la china del norte, la microplaca de qinling y el bloque yangtze, y la orogénesis intracontinental desde el meso-cenozoico. una característica topográfica prominente del área de dabashan es un arco extrusivo (falla arco de bashan) hacia el suroeste, que es un factor determinante para entender la condición geodinámica de la falla de cabalgamiento en las montañas dabashan. en este trabajo se construyeron modelos bidimensionales planos para simular los movimientos de colisión e intracontinental orogénicos y se analizaron los factores que podrían controlar la formación de la falla del arco de bashan. los resultados del modelado muestran que el esfuerzo de compresión producido por las placas de colisión en los límites norte y sur es la principal fuerza impulsora de la falla. la principal controladora es la fuerza de cizallamiento dextral derivada de la forma inconsistente en los margenes del bloque. las unidades tectónicas rígidas como los terrenos bikou y hananmicangshan, el domo foping y wudang, al igual que el anticlinal shennongjia-huangling tienen funciones de ancla. adicionalmente, las propiedades reológicas del material rocoso en el área dabashan afectan la forma del arco. abstract resumen keywords: numerical modelling; south qinling; bashan arc; dabashan; thrust nappe structure palabras clave: modelado numérico, qinling sur, arco bashan, montañas dabashan, falla de cabalgamiento. record manuscript received: 05/07/2013 accepted for publication: 11/11/2016 how to cite item zhang, x., & dong, y. (2016). the geological and geodynamic condition on the formation of the dabashan thrust nappe structure: based on flac numerical modelling. earth sciences research journal, 20(4), b1-b10 http://dx.doi.org/10.15446/esrj.v20n4.38666 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.38666 the geological and geodynamic condition on the formation of the dabashan thrust nappe structure: based on flac numerical modelling earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): b1 10 g e o d y n a m ic s xiaoning zhang*, yunpeng dong. state key laboratory of continental dynamics, department of geology, northwest university. china. *corresponding author: xiaoningdev@gmail.com condición geológica y geodinámica para la formación estructural de la falla de cabalgamiento en las montañas dabashan basada en el modelo numérico del software flac b2 xiaoning zhang and yunpeng dong. 1. introduction the qinling orogenic belt is located at center china, between the south china and north china blocks (fig. 1a). it is at a key tectonic position, been considered as a composite orogenic belt, formed by the collision of the south china and north china blocks (mattauer et al., 1985; sengor, 1985; zhang, 1985; ren et al., 1986; hsu et al., 1987; zhang et al., 1987, 1989; enkin et al., 1992; kröner et al., 1993; li et al., 1993; okay and sengor, 1993; ames et al., 1996; zhang et al., 1996a; hacker et al., 1998; meng and zhang, 1999; faure et al., 2001; zhang et al., 2001; ratschbacher et al., 2003, 2006; dong et al., 2011a,b,c, 2012a,b; bader et al., 2013a,b; dong et al., 2013; li et al., 2015; dong et al., 2016). qinling orogenic belt has preserved a lot of kinematic evolution records of long-term multiple stages subduction and collision between the south china and north china blocks, as well as tectonic records of overthrust thrusting, strike-slip regulation, extensional collapse, been a good region for studying orogenic tectonic process. the dabashan thrust nappe structure is one of major characteristics of qinling orogenic belt. it is located between hannan and shennongjia-huangling terranes (fig. 1b,c), as a part of the southern margin of south qinling thrust nappe structure (fig. 1a, dong et al., 2008; shi et al., 2013). the dabashan thrust nappe structure is suggested as the result of the subduction collision between north china block and yangtze block combined mesozoic-cenozoic intracontinental orogeny of qinling orogenic belt (fig. 1a, zhang et al., 2001; ratschbacher et al., 2003; wang et al., 2003; dong et al., 2008). it’s geological and geodynamic evolution has been one of research hotspots over the past decades, lots of study results have been reported in this area (e.g. cai et al., 1988; he et al., 1997, 1999; zhang et al., 2001, 2010; wang et al., 2003; dong et al., 2005, 2006, 2008, 2011; li et al., 2006; liu et al., 2006; wang et al., 2006; shi et al., 2007, 2012, 2013; tan et al., 2007; hu et al., 2009, 2012). the bashan arc is one of key factors to understand the geodynamic evolution of the dabashan thrust nappe structure. there are different evolution models about bashan arc. jiang et al (1982) emphasizes the similar shape and structure between the bashan arc and the himalayan arc and believes that the intruding of hannan and huangling terranes together with the lateral movement are the main geodynamic condition of the bashan arc. xu et al (1988) points out that the southward huge thrust and detachment shear of south qinling is the main geodynamic condition while hannan and huangling terranes play roles of baffles. zhang et al (2001) and wang et al (2003) think that the subduction from south to north of the yangtze block may also play an important role within the bashan arc. dong et al (2008) suggests that yanshanian intracontinental overthrusting from south to north developed the bashan arc. zhang et al (2010) and wang et al (2011) estimate that the pre-formed chengkou-fangxian fault is the key controller to bashan arc. he et al (1997) proposes an extrusion shear model that stresses the importance of the dextral shear in the dabashan area. in additional, durative compression model (cai et al., 1988), slip layer controlling model (li et al., 2006; liu et al 2006), bulldozer model or progressive squeeze weakened model (luo et al., 2007), two phases model (dong et al., 2006) are also suggested. despite many studies reported within this area, there are still disagreements on below issues: 1) is there dextral strike-slip around the bashan arc fault? how it formed and effected on the evolution of dabashan thrust nappe? 2) did shennongjia – huangling and bikou terranes play a role of “rigid anchor” in the formation of the dabashan thrust nappe? 3) did collision between yangtze and north china blocks cause clockwise rotation within dabashan area, and how that effect the formation of dabashan thrust nappe? this paper presents a numeric modelling using fast lagrangian analysis of continua algorithm (flac) to analyse the geodynamic evolution of the dabashan thrust nappe and the formation of the bashan arc, and mainly focuses on above issues. 2. tectonic setting the dabashan thrust nappe structure is at the northeastern of the sichuan basin and the northwestern of the central yangtze block (zhang et al., 2010, shi et al., 2013), characterized as an obvious southwestward arc and distinguished with the qinling-dabie orogenic belt (fig. 1b, zhang et al., 2001; dong et al., 2006, 2008, 2013; shi et al., 2012, 2013). the dabashan thrust nappe structure can be divided into southern and northern dabashan tectonic units which are evidently different (fig. 1c, zhang et al., 2010; shi et al., 2012, 2013), the northern dabashan tectonic unit shows nw-se direction while southern dabashan tectonic unit spreads as arc shape and displays features of two stage overprint (dong et al., 2008). from view of lithology, the southern and northern dabashan tectonic units are also distinct (e.g. middle proterozoic shennongjia group, sinian nantuo-doushantuodengying formations, cambrian sandstone-shale-carbonate for southern dabashan unit while middle-neo proterozoic wudang group and yunxi group, neoproterozoic yaolinghe group, cambrian slate-carbonate for northern dabashan unit and so on), they belong to different origin sources and were overlaid by southward thrust nappe movement (dong et al., 2008). the northern dabashan unit consists of a few thrust nappes with multiple levels and different scales that stacked southward and mainly characterized by series of nw-se faults and folds. the main structural trend is about 290°~300º paralleling to that of qinling. the southern front of the northern dabashan, close to bashan arc fault, featured by folds with southdipping axial planes, is evidently different with dominant north-dipping axial planes in both north and south dabashan units, indicating that the northern dabashan underwent at least two stages of thrust movements (zhang et al., 2001; dong et al., 2008; zhang et al., 2010). these folds with south-dipping axial planes should be formed at the early stage of collision and overprinted by following southward thrust nappes. the total shortening in the northern dabashan may reach 31%~48% (dong et al., 2008). the southern dabashan unit is located at the northeastern margin of sichuan basin, with two rigid basements (hannan-micangshan and huanglingfigure 1. schematic map of the research area. (a) the large-scale structural units sketch within china (zhang et al., 2001). (b) the bashan arc thrust nappe structure (dong, et al., 2008). (c) the regional scale structural sketch within the research area (zhang et al., 2001). b3the geological and geodynamic condition on the formation of the dabashan thrust nappe structure: based on flac numerical modelling 3 shennongjia massifs) in its east and west sides (fig. 1b,c, zhang et al., 2001; dong et al., 2006, 2008, 2011; shi et al., 2012, 2013). the unit displayed as a typical double arc-shape structure in the west flank of the huangling-shennongjia massif (yue et al., 1996; huang 2000; wu et al., 2009; shi et al., 2012, 2013). the southern dabashan is characterized by developed fold-thrust structures, can be further divided into northern foreland fold-thrust belt and southern overthrusting extended belt (fig. 1b). the former displays overprinting of two stage folds where different hinge trends indicate earlier dextral shear and later thrust superposition. the latter mainly consist of thin-skinned folds, characterized by upright and wide folds with few faults. it seems that the structural deformation in the southern dabashan is gradually weakened from northeast to southwest and the average shortening is about 36%~53% (dong et al., 2008). the bashan arc fault constitutes a boundary between the southern and northern dabashan tectonic units (zhang et al., 2001; dong et al., 2008; zhang et al., 2010). in fact, it is a thrust nappe belt consisting of a series of ne-sw or n-s thrusts. the major nappe fault was suggested to be associated within the chengkou fault that formed and been active during middle late jurassic (xu et al., 2005). the general movement of the bashan arc fault is towards southwest (240º). the vanguard portion, e.g. chengkou section, is nearly parallel with the structural trend of the north dabashan (about 290°~300º) while west and east sides cut the structural line of the north dabashan with high angles (dong et al., 2008). hannan-micangshan terrane, foping dome, shennongjia-huangling terrane, as well as wudang dome, located at the southwest, northwest, southeast, and northeast of the bashan arc fault belt respectively (fig. 2), were supposed as the “anchor” during the formation of the dabashan thrust nappe structure because of their relative rigid basements (dong et al., 2008; zhang et al., 2010; wang et al., 2011). along with the activity of the bashan arc fault, they might work together to play an important role to construct the current structural styles of the dabashan thrust nappe. bikou terrane, surrounded by qingchuan-yangpingguan fault and mianlue suture belt, located at the northwest of the dabashan thrust nappe, was supposed intruded into the south qinling and also contributed to the formation of the dabashan thrust nappe. 3. numerical method 3.1 model description a two-dimensional (2d) plan-view model (fig. 2, 3) was designed to simulate the formation of the bashan arc, and mainly focused on the condition that leads to the bashan fault bending. according to previous studies (e.g. jiang et al., 1982; xu et al., 1986; he et al., 1997, 1999; zhang et al., 2001; wang et al., 2003; dong et al., 2006; li et al., 2006; liu et al., 2006; luo et al., 2007; dong et al., 2008; zhang et al., 2010; dong et al., 2011b; wang et al., 2011, 2013), we assume that the bending conditions might include: 1) the north qinling moves towards south; 2) the yangtze block moves towards north; 3) the irregular margin of blocks produces dextral shearing and lateral extrusions in dabashan area; 4) hannanmicangshan terrane, foping dome, shennongjia-huangling anticline, as well as wudang dome act as baffles to hold down the thrust nappe (fig. 2). although bikou terrane is a little far from bashan arc fault, it is at the north of the northeast margin of sichuan basin, and very close to possible model boundary, we included it as one of experiment steps. the geometry in the 2d plan-view model is based on the reconstruction of above tectonic units, bashan arc fault, as the focus of the model, is designed as a straight line at the initial stage, while the north qinling and the sichuan basin, playing roles of parts of the boundary conditions, are moved to outside of the model boundaries (fig. 3) except for stresses or velocities applied on their south margin and northeast margin, respectively. figure 3. the geometry units and finite difference mesh for 2d plan-view model. figure 2. simplified geometry map of the research area. the map in upper-right corner indicates the location of the research area within china (dong et al., 2011b). detailed model parameters, such as rock properties, boundary and initial conditions are the key factors for numeric models and should be adjusted during model solving, they will be discussed in the numerical result session. 3.2 numerical algorithm the commonly used numerical algorithm for geology modelling can be divided into 3 groups which are: 1) continuum methods that include finite difference method (fdm) and finite element method (fem), as well as boundary element method (bem); 2) discontinuum methods that include discrete element method (dem) and discrete fracture network (dfn); 3) hybrid methods that combined continuum and/or discontinuum methods (jing, 2003). flac is one of fdm algorithm with the mixed-discretization technique (cundall and board, 1988; itasca, 2005). it has been widely used to simulate rock deformation (xia et al., 2006) and solve structural geology problems (hobbs et al., 1990; ord, 1991; zhang et al., 1996b; zhang et al., 1996c; zhang et al., 2000; strayer et al., 2001). for example, ord (1991) simulated the uniaxial compression deformation of gosford sandstone and demonstrated that the stress-strain relationships from the flac numerical models for several different confining pressures are consistent with those reported from an experimental work (edmond and paterson, 1972). similar to most mesh-based numerical modelling methods, materials (geology blocks, rocks and so on) in flac are represented by elements, or zones, which form a grid. each grid is assigned material properties, such as density and elastic moduli, and behaves according to a prescribed linear or nonlinear stress/ strain law in response to the applied forces or boundary restraints (itasca, 2005; xia et al., 2006). the material can yield and flow, and the grid can deform in large-strain mode and move with the material that is represented. using the explicit and lagrangian calculation scheme, together with the adapted mixed-discretization technique, flac ensures that material failure and large deformation are modeled accurately. the explicit method and dynamic relaxation scheme make it possible to carry out large calculations with relatively less memory requirements because of no global stiffness matrix formed during computation. however, the time-step must be small enough to ensure convergence and numerical stability (cundall and board, 1988). flac contains a powerful built-in programming language fish (short for flacish, itasca, 2005) that can be used to extend flac’s functionality, e.g. generate very complicated geology bodies, apply any kind of initial and boundary conditions, or implement special constitutive models for any materials. in this study, we select flac as the major numerical modelling platform and use fish to implement most of numerical algorithm and functionalities. b4 xiaoning zhang and yunpeng dong. 3.3 theoretical basis the elastic-viscous constitutive law (known as the maxwell substance, itasca, 2005) can be used to simulate the time-dependent behavior of materials that exhibit creep, for example, the irrecoverable deformation of rocks under small stresses and over a long period. an elastic-viscous rheology material exhibits both viscous and elastic behaviors (itasca, 2005). it is equivalent to a combination of an elastic element and a newtonian viscous element in series (turcotte and schubert, 1982; ranalli, 1987), in which the instantaneous elastic response and the viscous deformation are coupled (xia et al., 2006). the total strain ( ) is the sum of the elastic strain ( ) and viscous strain ( ), (1) the elastic strain relates to stress ( ) according to hooke’s law (2) where g, k and are shear, hand the kronecker delta, respectively, and the viscous strain rate ( ) relates linearly to stress according to the newtonian flow law (3) where is viscosity. strain rate is defined as a function of velocity gradients. the elastic–plastic constitutive law (mohr–coulomb model, itasca, 2005) is the conventional model used to represent shear failure in soils and rocks. vermeer and deborst (1984) report laboratory test results for sand and concrete that match well with the mohr-coulomb criterion. a mohr–coulomb elastic–plastic material undergoing deformation behaves initially elastically until the stress reaches a critical value known as the yield stress, at which point it begins to deform plastically, and irreversibly, to high strain (xia et al., 2006). the yield of such material may be expressed as a yield function f , given by (4) where is the maximum shear stress, is the mean stress, is the friction angle and is the cohesion. the material is in an elastic state if < 0 (the stress state is not touching the yield surface), and is in a plastic state if = 0 (the stress state is touching the yield surface) (xia et al., 2006). detailed information on using the mohr–coulomb elastic–plastic materials to simulate deformation in thrust wedge can be found in strayer et al (2001). the interface is characterized by coulomb sliding and/or tensile separation. it is designed in flac to model the geomechanics in which sliding or separation between planes is desirable, for example, joint, fault or bedding planes in a geologic medium. an interface has the properties of friction, cohesion, dilation, normal stiffness and shear stiffness, and tensile strength. the contact logic for either side of the interface is similar to that employed in the distinct element method (e.g., cundall and hart, 1992). in this study, the elastic–viscous rheology constitutive law (maxwell model) is used to simulate the tectonic transport patterns (material movement) of north and south dabashan, and the interfaces are involved to simulate the strike-slip of the bashan arc fault. the elastic–plastic constitutive law (mohr–coulomb model) is also employed for pre-calculation. as for rigid tectonic units such as bikou, hannan, foping, wudang and shennongjia, a set of rigid material properties are applied to make sure that they are hardly moved or deformed during the solving progress. 4. model and results 4.1 model design and grid generation the dabashan nappe thrusts are supposed to have suffered at least two stages of tectonic movements that are subduction collisional orogeny and subsequent intracontinental orogeny since mesozoic-cenozoic (dong et al., 2006, 2008; zhang et al., 2010). the subduction collisional orogeny results from the collisions between the north china block, qinling micro-plate and yangtze block along shangdan and mianlue sutures respectively (cai et al., 1988; he et al., 1997; zhang et al., 2001; ratschbache et al., 2003; wang et al., 2003; dong et al., 2008), suggests that the model boundaries should be put to the north of mianlue suture and to the south of the north margin of yangtze block (partly adapted according to the northeast margin of the sichuan basin). the intracontinental orogeny, as results of rock deformation under long term stress and temperature, is reflected by deformation and movement of finite difference grids inside the model and controlled by the constitutive model and material properties. the east and west boundaries of the model are put to the east of shennongjia terrane and to the west of bikou terrane to make sure that all relevant tectonic units are involved in the model. the bashan arc fault separates the north dabashan and south dabashan belts into two different tectonic units (dong et al., 2008). the tectonic orientation of the north dabashan unit is nww and almost parallel to that of the south qinling belt (about 290º– 300º, dong et al., 2008). the tectonic orientation of the south dabashan unit is more complex that may due to overprint by later tectonic actions (dong et al., 2008). in this model, we design a nww fault to represent the initial shape of the bashan arc fault. thus, the designed model skeleton consists of 8 sub-grids (fig 4), in which the null grids (spaces that divided sub-grids) 1, 2, and 3 will be replaced by interfaces to represent the dividing line between north and south dabashan, and the others (null grids) will be removed by merge and attach the related sub-grids together. the other tectonic units such as bikou terrane, hannan-micangshan terrane, foping dome, huangling-shennongjia terrane, as well as wudang dome, playing role of rigid baffles, are added to the model during the experiment process. the completed plan-view model is shown in fig. 3. 4.2 boundary and initial conditions the shape of the boundaries in the plan-view model is mainly based on a reconstructed and simplified map of the dabashan and adjacent areas (fig. 2), also consulted from a skeleton map published by he et al (1997). the north and southeast boundaries are designed as straight lines just for simplification. we tried applying declining boundaries along nee and se directions for north and southeast respectively as employed by wu et al (2009), which did not make obvious difference to the modelling results. roll boundary conditions are applied to eastern and western boundaries along which the w-e movements are limited while the n-s movements are allowed. free boundary conditions are added to the north and most of the south boundaries. to avoid unbalance failure leading by strong clockwise rotation, the corner segments of western and eastern of the south boundary are fixed. a crustal shortening rate of 1.28 mm/year (he et al., 1997) is applied to both the north and south boundaries. this rate is approximately converted to a boundary stress of 1.1×107 to 1.5×107 pa via pre-calculation with the plan-view model, which is also consistent with the boundary stress adopted in wu et al (2009). as a 2d plan-view model, the horizontal extrusion and plane displacement are mainly concerned while the initial gravity and stress applied inside finite difference mesh are set to default value of zero. figure 4. skeleton of the plan-view model, maps the study area ranging n30.5º 33.5ºand e104º-112º. it extends about 900 km in w-e direction and 350 km in n-s direction, the average length of each finite difference grid equals to about 10 km in real study area. the model is initially built as sub-grids, null grids (marked as 1, 2, 3) represent boundaries of south and north dabashan. b5the geological and geodynamic condition on the formation of the dabashan thrust nappe structure: based on flac numerical modelling 4.3 constitutive model and material properties previous field and laboratory studies (he et al., 1997; zhang et al., 2001; dong et al., 2008; wu et al., 2009; wang et al., 2011, 2013) indicate that the dabashan experienced long-term flexible deformation, therefore, large strain and viscous rheology constitutive law should be introduced for the simulation in this area (wu et al., 2009). in this study, the classical viscoelasticity model (e.g. maxwell substance) is adapted in the 2d planview model. the classical viscoelasticity model is widely used to simulate the time-dependent rock deformation, for example, intracontinental orogeny after collision. by carefully control the material properties, the classical viscoelasticity model can show behavior as either linear or nonlinear viscous rheology. as a simple time-dependent constitutive model, it has been used in lots of previous numerical simulation studies in the fields of structural geology and tectonics (dieterich and carter, 1969; treagus, 1973; williams et al., 1978; lin et al., 2004; xia et al., 2006). the material properties required for the classical viscoelasticity model are density, shear and bulk moduli (for the elastic behavior) and the viscosity (for the viscous behavior), which promise that the material flows continuously under an applied shear stress but behaves elastically under an applied isotropic stress (itasca, 2005). in this study, we use a set of homogeneous material models for different tectonic units so that the effects of material difference can be tested and compared. the basic material model is consistent with the parameters for general average crust-lithosphere materials (e.g. turcotte and schubert, 1982) that include density of 2800 kg/m3, bulk modulus of 4×1010 pa and shear modulus of 2.4×1010 pa (table. 1, xia et al., 2006). as for the value of viscosity, a set of experiments is carried out to test the affect of material rheological properties which will be discussed in section of model results. table 1. the material properties of 2d plan-view model. 4.4 model results the 2d plan-view model is calculated under larger strain and in two stages. the first stage simulates the situation of “hard” collision in which the rock deformation behaviors are mainly elasticity by set the creep time-step to zero. when the first stage finished, we checked the maximum unbalance force in the system to make sure that the model system reached a mechanics balance state. the second stage simulates the situation of “inside” deformation in which the rock deformation behaviors are mainly viscous rheology that controlled by rock creep over a long period. for creep analysis, the time-step has realistic meaning other than a virtual calculating factor employed in other static analysis (itasca, 2005). the creep time-step reflects the realistic period of rock deformation that should be determined by parameters of viscosity and shear moduli. in this study, we use an adjustable creep time-step that will be decreased whenever the maximum unbalanced force exceeds some threshold, and increased whenever it goes below some other level. by this way, we want to make sure that the model system is always under a mechanics balance state. 4.4.1 the pattern of materials movement in the dabashan area the first stage (instantaneous movement of materials under a “hard” collision) ended after 1774 static time-steps and reached a mechanics balance state. the contours plot of the displacement shows that the materials principally move towards south (fig. 5a) and the displacement in the north dabashan is larger than that in the south dabashan. there is small quantity of westward extrusion in the south dabashan (fig. 5b) and the maximum displacement in westward direction is about 20% compare to that in southward direction. the second stage (creep of materials) falls into deformation failure due to some elements within the mesh become too strongly deformed and modelling cannot be continued. fig. 6a shows the displacement pattern after 25 million years of collision. it can be clearly observed that the tectonic transport patterns (material movement) are characterized by strong ne-sw overthrust and the bashan arc fault (initialized as a straight line) shows an obvious arc shape extruded toward sw (240º) which is consistent with the present-day geomorphy in dabashan area (dong et al., 2008). the ne-sw material movement can be more clearly analysed by plotting out the contours of the displacement component in the e-w and n-s directions, respectively. fig. 6b shows the contours plot of the displacement component in the n-s direction (e.g. y-component) in which positive values indicate northward movements and negative values indicate southward movements. the maximum southward displacement of 30 km (negative contours) that distributes through the middle and the eastern part of bashan arc fault may reflect the overthrust nappe movement from north to south in tongue shape (e.g. the strong push in the middle and the weak push in the sides) that implies why the bashan fault appears as an arc shape. the displacement toward the south in the south dabashan unit is gradually smaller that may reflect that the deformation in the south dabashan is gradually weakened from north to south (dong et al., 2008). figure 5. displacement contours of the plan-view model at the end of “hard” collision. (a) contours of the y-component of displacement (the maximum y-displacement is about 0.05 km). positive values indicate northward movement and negative values indicate southward movements. (b) contours of the x-component of displacements (the maximum x-displacement is about 0.01 km). positive values indicate eastward movement and negative values indicate westward extrusion. b6 xiaoning zhang and yunpeng dong. we also notice that the areas surrounding bikou-hannan in the west and wudang-shennongjia in the east suffer minimum n-s displacement (the y-component is nearly zero) which implies an “anchor” effect of the bikouhannan and wudang-shennongjia terranes. fig. 6c shows the contours plot of the displacement component in the e-w direction (e.g. x-component) in which positive values indicate eastward movements and negative values indicate westward movements. the trend of westward movements (negative contours) runs through the dabashan area from north to south and the maximum westward displacement is at the south dabashan unit. the maximum displacement after 25 million years of collision is about 76 km (fig. 6a). to compare with the published average crustal shortening distance of 40 km in the north dabashan (dong et al., 2008) and 64 km in the south dabashan (he et al., 1997), a mark line ab is put into the model to track and calculate the average displacement in the north and south dabashan units (fig. 7), and the results are average displacement of 37 km and 41 km, maximum displacement of 39 km and 61 km for the north and south dabashan (along ab path in fig. 7), respectively. the potential underestimation of the values may be due to: 1) the 2d plan-view model could not include the slip and detachment layers beneath the dabashan area that should be also contribute to the displacement; 2) the model only calculated 25 million years of the intracontinental orogeny that should be shorter than the actual situation. the larger deviation for the south dabashan may also imply complex tectonic overprint in this area (dong et al., 2008). 4.4.2 dextral shear along the bashan arc fault to evaluate whether the tectonic movement leading strike-slip along bashan arc fault, interfaces are introduced into the model to replace the initial marker lines for bashan arc fault (fig. 4). the interfaces of 1 and 3 (in fig. 4) represent the dividing line between north and south dabashan and are limited neither to slip nor open, while the interface 2, representative of bashan arc fault, is allowed to slip or open by set relative small value to normal stiffness (kn) and shear stiffness (ks) parameters. the parameters for interface 1 are normal stiffness of 1×1010 pa/m, shear stiffness of 1×1010 pa/m and friction angle of 30º. these values are commonly used by itasca (2005) and within the data range published by kulhawy (1975), rosso (1976) and bandis et al (1983). the fault slip experiment shows an obvious right-lateral shearing along bashan arc fault (fig. 8b) compare with the initial finite difference grid (fig. 8a) where the mark line ab is put on both side of the fault to track fault offset. as the parameter values for the interface are not derived from physical tests on real joints, the value of the fault offset can not reflect the realistic fault offset. what we concerned is the direction of the fault slip, yes, right-lateral shearing slip, which may imply that the right-lateral shearing produced by incongruous margin of block may play an important role on the formation of bashan arc (he et al., 1997). 4.4.3 the effects of rigid “anchor” bikou, hannan, foping, wudang, shennongjia terranes are then removed from the initial model to estimate their contribution to bending of bashan arc, and the other conditions and parameters in the model keep consistent with that in the initial model. fig. 9a and b show the contours plot of the displacement component in the n-s and e-w directions, respectively. a strong northward extrusion appears on the y-displacement contours plot (fig. 9a) and maximum northward displacements take place in the area between bikou and hannan terranes. taking into account of the strong southward extrusion in dabashan area, the model may involve into clockwise whirl and out of balance. the x-displacement contours plot (fig. 9b) indicates that westward extrusion seems nearly zero in north dabashan unit that may be adverse to the formation of bashan arc. therefore, “anchor” effects contributed by these rigid tectonic units should exist. figure 6. results of the plan-view model. (a) distribution of displacement vectors (the maximum displacement is about 76 km). (b) contours of the y-component of displacement. positive values indicate northward movement and negative values indicate southward movements. (c) contours of the x-component of displacements. positive values indicate eastward movement and negative values indicate westward extrusion. figure 7. the marked path (ab) for calculating the average displacement in the north and south dabashan figure 8. the right-lateral shearing of bashan arc fault. (a) initial finite difference grids and the fault interface with a mark line ab. (b) deformed finite difference grids and the fault interface with a mark line ab. b7the geological and geodynamic condition on the formation of the dabashan thrust nappe structure: based on flac numerical modelling 4.4.4. the rheological properties of rocks in the dabashan area the viscosity values for general average crust-lithosphere materials is about 5×1021 pa s (turcotte and schubert, 1982; xia et al., 2006) which is converted to 1.6×1014 pa y because we use year as the unit for creep time. this value is used for both north and south dabashan tectonic units in the 2d plan-view model, while bikou, hannan, foping, wudang, as well as shennongjia terranes, a large value (more than 100 times of that for dabashan) are employed to make sure that they are rigid compare to surrounding materials during creep analysis. to estimate the relationship between viscosity values and deformation in dabashan tectonic units, we tried a set of viscosity values for experiment. the results show that: 1) if the same viscosity value is set for the north and south dabashan, the smaller the viscosity is, the larger the displacement obtained (fig. 10a); 2) if the different viscosity values are set to the north and south dabashan, enlarging the difference does not help to enlarge the displacement (fig. 10b). 4.5 model limitations this paper mainly focuses on 2d models for studying the geological and geodynamic condition of the dabashan thrust structure using flac (2d) as main modeling tool. as general limitations of using 2d models for 3d orogenic deformation simulation, the model in this paper has below limitations: 1) vertical topographical variation (li et al., 2013) is not included in the model. 2) viscosity of crustal and lithospheric rocks that depends on stress and temperature should be strongly vary with depth, is simplified as a few constant variables for different areas in this paper, 3) the mianlue suture that affects the formation of the dabashan thrust nappe, is not included in the model. these limitations will be solved in our ongoing research with cross-view and 3d models. 5. discussion the formation of the dabashan thrust nappe structure is acknowledged as results of multiple stage evolutions (he et al., 1999) that includes tectonic extension (paleozoic), tectonic inversion (middle triassic) and thrust tectonics (middle triassic to early jurassic, shi et al., 2012; dong et al., 2006; hu et al., 2009). after inter-continental collision orogeny (late triassic to early jurassic), dabashan orocline continue reshaping during late jurassic to early cretaceous, proved by the sediment sequence and strata unconformity (dong et al., 2005, 2006, 2008; meng et al., 2005; shi et al., 2007, 2012; li et al., 2007; qu et al., 2009; zhang et al., 2010; hu et al., 2012), isotopy dating (cheng and yang, 2009; hu et al., 2009; liu et al., 2011; li et al., 2010, 2011, 2012, 2013), and apatite fission-track (shen et al., 2009; xu et al., 2010; li et al., 2010, shi et al., 2012; hu et al., 2012). regarding to the key factors that affect and control the formation of the dabashan thrust nappe, there are mainly four opinions: 1) the uplift at both ends of the bashan arc fault is main mechanism (jiang et al., 1982; guo et al., 1996; wang et al., 2003; zhang et al., 2004; dong et al., 2010; xu et al., 2010; shi et al., 2012); 2) tectonic extension at the margin of paleo-contiental and irregular boundary are the decisive factors (zhang et al., 2010), the dextral shear induced by the inconsistent morphology of the continental margins also plays a key role (he et al., 1997); 3) the arc-shaped fault that has been formed in the early stage of tectonic extension controls the thrust nappe developed toward southwest (wang et al., 2011, 2013); 4) the detachment layers under dabashan unit is the main formation condition (wang et al., 2011, 2013). from our study in this paper, it seems that the margin shapes generated by early tectonic extension (he et al., 1997, 1999) and the compression stress derived by subsequent tectonic inversion are important conditions to control the mesocenozoic intracontinental orogeny, while pattern of materials movement in stage of thrust tectonics determines the formation of the dabashan thrust nappe structure. the results of the 2d plan-view models support that the shape of the north margin of yangtze block (e.g. the northeast margin of sichuan basin extrusion northeastward against qinling orogenic belt), together with the rigid bikou-hannan terrane on the northwest and wudang-shennongjia terrane on the southeast, are the main conditions of producing large-scale clockwise rotation in dabashan area. during the movement of south qinling towards south, the bikou-hannan terrane firstly obstructs locally the southward movements and compels the movement turn to eastward. the materials of eastward movement then conflict with the materials of southward movement at the north and west of the north dabashan area, and change the direction to southward. the converged southward movements are then baffled by wudang-shennongjia terrane and turn to westward. lastly, the materials of westward movement are blocked by the northward extrusion along the northeast margin of sichuan basin, some of materials turn to north (thus the clockwise whirl is shaped) and the others turn to south along the east margin of sichuan basin, as shown in fig. 5a-b and 6a-c. the existence of the clockwise whirl is confirmed by right-lateral shearing along the bashan arc fault (shown in fig. 8b). the modelling results also reveal that the predominant pattern of materials movement through the dabashan area is along an arc curve and pushes forward in a tongue-shaped in the front edge. the arc curve intersects with the bashan arc fault belt at the middle portion of the fault figure 9. the “anchor” effects of bikou, hannan, foping, wudang, and shennongjia. (a) contours of the y-component of displacement. positive values indicate northward movement and negative values indicate southward movements. (b) contours of the x-component of displacements. negative values indicate westward extrusion. figure 10. sensibility analysis of the material viscosities. (a) the same viscosity value is set to both north and south dabashan. the creep time is 10 million years after collision. (b) the different viscosity values are set to north and south dabashan. the creep time is 10 million years after collision. the broken line of “north dabashan” means an invariant viscosity (10 mpa year) for the south dabashan and gradually enlarged viscosity for the north dabashan; accordingly, the broken line of “south dabashan” means gradually enlarged viscosity for the south dabashan while the north dabashan keeping invariant. b8 xiaoning zhang and yunpeng dong. belt that leads to the southwestward displacement in the middle portion is larger than that in the eastern and western portions of the fault belt thus the fault belt displays as a southwestward extrusion arc-shape (fig. 6a). as limitation of 2d plan-view model, the slip and detachment layers beneath the dabashan area can not be simulated which may affect the estimation of the average displacement of the thrust nappe. however, the 2d plan-view model confirms that the n-s compression, dextral shearing, plus rock rheology can control the formation of the dabashan thrust nappe structure. 6. conclusion according to the modelling results and discussion above, we propose following conclusions: 1) the compressive stress produced by the plate collision along north and south boundaries of the dabashan tectonic unit is the main driving force; 2) the dextral shearing derived from the inconsistent shape on the block margins is the main controller; 3) rigid tectonic units such as bikou and hanan-micangshan terranes, foping and wudang dome, as well as shennongjia huangling anticline contribute as “anchor” effects; 4) the rheology properties of rock material in the dabashan area affect the shape of the arc. however, the difference of the rheology in the north and south dabashan may not help to enlarge the deformation. acknowledgements the authors would like to thanks reviewers for their constructive comments and suggestions. financial support for this study was jointly provided by national natural science foundation of 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(2018). empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide). earth sciences research journal, 22(3), 195-204. doi: https://doi.org/10.15446/esrj.v22n3.69491 today, geotechnical and geophysical techniques are used for landslide evaluation. geotechnical methods provide accurate data, but are time consuming and costly. geophysical techniques, however, are fast and inexpensive, yet their accuracy is lower than that of the geotechnical methods. therefore, simultaneous use of geotechnical and geophysical methods provides a suitable solution for landslide evaluation. availability of geotechnical and geophysical data makes it possible to investigate correlation between different parameters. correlating geotechnical and geophysical parameters ends up lowering field investigation costs and enhancing subsurface survey speed in a landslide zone. in the present study, in order to evaluate nargeschal landslide in iran, ambient noise measurement, ert survey, and geotechnical investigations were used. once finished with data processing, the data obtained from geotechnical and geophysical investigations were correlated. these included spt-n – electrical resistivity, soil moisture content – electrical resistivity, and spt-n – shear wave velocity correlations. the correlations were examined using two methods, namely spearman’s coefficient test and least square regression analysis. the results obtained from the two methods were in good agreement with one another. the correlations obtained in this study were of moderate to very strong strength and fell in the range of the results of previous studies. investigation of the results indicated significant influences of ground water on electrical resistivity and soil stiffness on shear wave velocity. results of this study can be used for soil classification and determination of mechanical and seismic characteristics of soil across various areas. abstract keywords: ambient noise, ert, geophysical investigation, geotechnical investigation, empirical correlation. empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide) correlación empírica entre parámetros geotécnicos y geofísicos en la zona de un deslizamiento de tierra. caso de estudio: deslizamiento de nargeschal issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.69491 actualmente, las técnicas geotécnicas y geofísicas se utilizan en la evaluación de los deslizamientos de tierra. los métodos geotécnicos proveen información exacta pero son costosos y requieren de tiempo. las técnicas geofísicas, son rápidas y económicas a pesar de que su exactitud es menor a la ofrecida por los métodos geotécnicos. el uso simultáneo de métodos geofísicos y geotécnicos provee una solución adecuada en la evaluación de estos movimientos en masa. la disponibilidad de información geotécnica y geofísica hace posible investigar la correlación entre los diferentes parámetros. esta correlación permite una reducción en los costos de la investigación y agiliza la medición subsuperficial en las zonas de deslizamiento. en este estudio, con el fin de evaluar el deslizamiento de nargeschal en irán, se utilizaron medidas de ruido de fondo, prospección eléctrica, e investigaciones geotécnicas. una vez terminado el procesamiento de los datos, se correlacionó la información obtenida de los análisis geotécnicos y geofísicos. estos incluyen correlaciones ensayo de penetración estándarprospección eléctrica, contenido de humedad del suelo-prospección eléctrica, y ensayo de penetración estándar-velocidad onda de corte. las correlaciones se examinaron a través de dos métodos, llamados la prueba de coeficiente de spearman y el análisis de regresión de mínimos cuadrados. los resultados obtenidos a través de los dos métodos coinciden entre sí. las correlaciones obtenidas en este estudio fueron de fuerza moderada a muy fuerte y se enmarcan en los resultados de estudios previos. el análisis de los resultados señalaron fuertes influencias del agua subterránea en la prospección eléctrica y la rigidez del suelo en la velocidad de la onda de corte. los resultados de este estudio se pueden utilizar para la clasificación del suelo y la determinación de características mecánicas y sísmicas del suelo a través de varias áreas. resumen record manuscript received: 18/12/2017 accepted for publication: 11/05/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 195-204 sadegh rezaei1, issa shooshpasha1 & hamed rezaei2 1 department of geotechnics, babol noshiravani university of technology, babol, iran 2 department of engineering geology, golestan university, gorgan, iran g e o t e c h n ic s 196 sadegh rezaei, issa shooshpasha, hamed rezaei introduction one of the most important steps toward landslide evaluation is to undertake subsurface surveys. using the subsurface surveys, one can evaluate geometry, layering, slip surface, lateral limits, and ground water level across a sliding zone (merrit et al., 2014). there are numerous methods for subsurface survey. one of the most common and widely applied categories of these methods is geotechnical investigations (guisheng et al., 2016). geotechnical investigations include boring boreholes or trial pits, taking disturbed and undisturbed samples, and conducting field tests (spt, cpt, vst, etc.) and laboratory experiments (determining specific gravity, atterberg limits, moisture content, particle size analysis, permeability, etc.). geotechnical investigations provide accurate data and determine characteristics of the soil across the study area. despite of the wide spectrum of their applications and reliability of their results, these methods are associated with problems which are pointed out in the following. geotechnical investigations are costly to perform. these methods are destructive and require boring the ground. moreover, the results obtained from geotechnical methods are limited to a specific point, and their operation is time-intensive (lopes et al. 2014, yilmaz and narman, 2015, szokoli et al., 2017). in addition, given that landslide areas are mostly located within remote areas of tough topography, it is very difficult to transport and deploy the required equipment for boring. another set of methods increasingly applied in subsurface surveys is geophysical investigations (rezaei and choobbasti, 2017a). nowadays, such methods as ambient noise measurement and electrical resistivity tomography (ert) are widely applied in a landslide area. geophysical methods are mostly inexpensive, fast, and non-destructive (choobbasti et al. 2013, rezaei and choobbasti 2014, rezaei and choobbasti 2017b). furthermore, one can examine a larger area of a landslide using geophysical investigations. in spite of the positive characteristics of geophysical methods, these approaches also suffer from weaknesses which are elaborated in the following. the results obtained from these methods are not as accurate as those from geotechnical methods. geological and geotechnical data are required for calibration and verification of the geophysical methods, and interpretation of geophysical data is much more complicated than those of geotechnical data. considering what was mentioned above, it is evident that a combination of geotechnical and geophysical investigations may provide an ideal approach to subsurface survey in a landslide area (ozcep et al. 2009, osman et al. 2014, sil and haloi 2017, thokchom et al. 2017, jusoh and osman 2017). accordingly, today, a great deal of study is being performed to investigate correlation between geotechnical and geophysical parameters. application of correlations can make the process of field investigation faster and less costly. numerous researches have been performed to investigate the correlation between electrical resistivity and geotechnical parameters. in their studies, researchers achieved very weak to very strong correlations between electrical resistivity and geotechnical parameters (spt-n, moisture content, coefficient of permeability, unit weight, plastic limit, liquid limit, plasticity index, etc.) (cosezna et al. 2006, oh and sun 2008, liu et al. 2008, ozcep et al. 2009, sudha et al. 2009, calamita et al. 2012, long et al. 2012, kibria and hossain 2012, siddiqui and osman 2013, fallah-safari et al. 2013, osman et al. 2014, abidin et al. 2014, hatta and osman 2015, lin et al. 2017, jusoh and osman 2017, devi et al. 2017). investigation of the correlation between spt-n and shear wave velocity has also a long history, with many correlations presented in this respect in codes and previous studies (yordkayhyun et al. 2014, lopes et al. 2014, kirar et al. 2016, anbazhagan et al. 2016, gautam 2017, salinas-jasso et al. 2017, thokchom et al. 2017, sil and haloi 2017, nejad et al. 2018, rahman et al. 2018). one of the common methods for shear wave velocity evaluation is ambient noise measurement and analysis (asten et al. 2014, büyüksaraç et al. 2014, zuccarello et al. 2016, borges et al. 2016, pischiutta et al. 2017). therefore, it is useful and important to correlate the shear wave velocity obtained from ambient noise measurement and spt-n, considering efficiency and low cost of this geophysical method. in the present research, geotechnical and geophysical investigations are used to conduct subsurface survey in a landslide zone. the geotechnical investigations include boring 6 boreholes and performing field tests and laboratory experiments. the geophysical investigations include ert survey (along two 470–m long profiles) and ambient noise measurement at 30 stations. once finished with data processing, correlations between geotechnical and geophysical parameters were investigated. these correlations included spt-n – electrical resistivity, soil moisture content – electrical resistivity, and spt-n – shear wave velocity. the correlations were examined using two test methods, namely spearman’s coefficient test and least square regression 2 study area the study area considered in the present research is nargeschal village. the village is located 18 km to the southeast of azadshahr city at an altitude of about 1080 m above mean sea level (masl) within golestan province, iran. figure 1a shows position map of the study area in iran. according to studies by geological survey and mineral explorations of iran (gsi), this area is located within very high-risk and unstable zone (figure 1b) (pourghasemi et al. 2012). figure 1. the study area (a) position map of the study area in iran, and (b) landslide risk zonation across golestan province (based on gsi). 197empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide) heavy precipitation (130 mm per 24 h) and occurrence of earthquake (with a magnitude of 3.6 richter) within a short period of time (2014/05/29 – 2014/06/04) ended up with landslide event at nargeschal on 2014/06/04. the precipitation resulted in increased ground water level, reduced matric suction, and hence weakened soil layers which together with the occurrence of a relatively moderate earthquake contributed to the occurrence of a large landslide. the landslide was roughly 750 m in length and its width was up to 80 m in the upstream and 350 m in the downstream. the slope of the area was 20° to 30° with a main slip direction at an azimuth of 80°, while the volume of slide masses was estimated to be up to 2 million m3. the landslide put the lives and properties of the residents in serious danger, so the village was evacuated and temporary accommodation was provided for the residents in a safe place (rezaei et al. 2018). geotechnical and geophysical investigations were planned across the study area to identify the landslide geometry, slip surface, layering, and strength and seismic parameters of the soil, as detailed in the following sections 3 field investigations 3-1geotechnical investigations geotechnical investigations in the present research included boring 6 boreholes and undertaking field tests and laboratory experiments. the boreholes were 9.5-24 m deep, and a total of 114.5 m soil was bored. during the boring operation, disturbed and undisturbed samples were taken for laboratory experiments. in order to characterize the soil covering the study area, such experiments as soil particle analysis, specific gravity determination, unit weight evaluation, moisture content measurement, atterberg limits, direct shear test, and permeability coefficient determination were performed. moreover, standard penetration test (spt) was conducted. the spt, which was developed around 1925, is a well-established and unsophisticated method and currently one of the most popular techniques for evaluating the geotechnical characteristics of soils to weather rock all over the world (oh and sun 2008). figure 2 shows the position map of boreholes across the study area, while figure 3 indicates logs of the boreholes. in order to stick with the main aim of the paper (investigation of the correlation between geotechnical and geophysical parameters), only results of the tests for which the obtained geotechnical parameters were acceptably correlated to geophysical parameters are presented herein. accordingly, results of soil moisture content determination and spt tests are presented. figure 4 shows variations of spt-n and moisture content (ω) in the boreholes. it is observed that spt-n shows a significant change in some boreholes. previous studies demonstrate that the boreholes drilled up to a depth below the slip surface represent such a log. similar borehole logs can be found in studies by isik et al. (2004), suryo (2013), crawford et al. (2015) and topsakal and topal (2015). 3-2geophysical investigations geophysical investigations performed in the present research included ert survey and ambient noise measurement. the aim of ert survey was to determine landslide geometry, lateral limits, slip surface, and ground water level. ambient noise measurement is performed to determine fundamental frequency, direction of site response, and shear wave velocity across the study area. ert measurements were performed along two profiles of 470 m in length using wenner-schlumberger array. figure 2 shows the position of the profiles across the study area. measurements were performed using a multielectrode geomative-gd10 device by utilizing 48 electrodes. the device is equipped with multi-core cables along with 12 electrodes spaced at maximum 10 m spacing to one another. in the present research, four cables were used. res2dinv software was utilized to calculate the values of electrical resistivity. res2dinv software is based on the least squares method and uses the finite element (fe) and finite difference (fd). it inverts the pseudo-section apparent resistivity to subsurface electrical resistivity distribution using the quasi-newton optimization technique (shemang et al. 2013; zarroca et al. 2014; akpan et al. 2015; ling et al. 2016; soto et al. 2017). in order to calculate the values of electrical resistivity, apparent resistivity and topographic characteristics of the profile path were imported into the res2dinv software. given the topography, the software employs a finite element to determine apparent resistivity. the optimization method adjusts the 2d resistivity model trying to reduce iteratively the difference between the calculated and measured apparent resistivity values. following figure 2. nargeschal landslide map. figure 3. borehole logs in a quasi 3d view. figure 4. measured spt-n and soil moisture content at boreholes. 198 sadegh rezaei, issa shooshpasha, hamed rezaei four iterations, rms values along profiles 1 and 2 were found to be 3.17% and 2.47%, respectively. figure 5 shows the distribution of electrical resistivity of soil across the study area. ambient noise measurement was performed at 30 stations via a single-station arrangement. it is worth noting that, due to steep slope of the study area (20 – 30%), the ambient noise measurement stations were not located at the same elevation level. therefore, application of arraybased methods such as spac was avoided (del gaudio et al. 2014). the equipment used in the present research included sl07 seismograph, a 7 a battery, gps, compass, and a laptop pc. the seismogram is specifically suitable for setting studies to be performed at frequencies of up to 0.2 hz (lotti 2014). least square regression analysis. linear, logarithmic, exponential and power curve fitting approximations were applied and the best approximation equation with highest r2 was selected. r2 values smaller than 0.3 indicate that there is no correlation between the considered variables. however, should the r2 value fall within the ranges of 0.3 – 0.5, 0.5 – 0.7, 0.7 – 0.9, and 0.9 – 1, the corresponding correlations are recognized as weak, moderate, strong, and very strong, respectively (jusoh and osman 2017). since most of researchers are well familiar with the least square regression analysis, no further detail on this technique is provided herein and we rather proceed to explain spearman’s coefficient test. spearman’s rank correlation coefficient (rs) is a non-parametric measure of correlation, using ranks to calculate and measure the correlation between two variables. the ordered categories can be replaced by their ranks, and the correlation coefficient (rs) calculated on these ranks, measures the strength of association between two ranked variables to indicate how closely two sets of rankings agree with each other (lin et al. 2017). spearman’s rank correlation coefficient is obtained using equation (1). r d n ns i= ( ) 1 6 1 2 2 − σ − (1) in this relationship, di is the difference between ranks, and n is the number of members of each parameter. the value of rs varies in between + 1 and – 1. positive and negative values of rs indicate positive and negative correlations, respectively. rs = 0 shows no correlation. the closer the value of rs to ± 1, the stronger is the respective correlation. the [rs] values in the ranges 0 – 0.2, 0.2 – 0.4, 0.4 – 0.6, 0.6 – 0.8, and 0.8 – 1 indicate very weak, weak, moderate, strong, and very strong correlation strength, respectively. in order to apply this method, one should begin with ranking the values of each parameter (electrical resistivity, shear wave velocity, water content of soil, spt-n, etc.). for this purpose, the smallest value of each parameter takes the rank 1 and the rest of the members are assigned subsequent ranks in the order of increasing value. sometimes, there are ties in the data. tie means that two or more values are the same, so there is no strictly increasing order (for example, the value of spt-n is 50 at more than one areas). when figure 5. distribution of electrical resistivity of soil. figure 6. ambient noise analysis results (a) h/v spectral ratio at station st22, (b) ellipticity curve at station st22, (c) shear wave velocity profile at station st22, and (d) shear wave velocity profile at st11, st14, st17, st18, st21 and st22 down to a depth of 24 m. in the course of the present research, all recommendations given by sesame team were respected. recording time and sampling rate of ambient noise measurements at each measurement station were set to 4560 min and 100 hz, respectively. in the present research, ambient noise processing and determination of h/v spectral ratio was performed via hvnr method utilizing geopsy software (nakamura 1989). the h/v is linked to the ellipticity of rayleigh wave, in situation where the high shear wave contrasts exist between soil and bedrock (bard 1999). once the h/v spectral ratio was calculated, dinver module was used to compute ellipticity curve. the module performs inversion using neighborhood algorithm. in order to perform the inversion, a set of initial parameters such as shear wave velocity, primary wave velocity, poisson’s ratio, and density is required. these initial values were determined using the borehole data. all these data can be provided to the ellipticity curve inversion at the end to get shear wave velocity profile (ullah & prado 2017). for all the models tested, the theoretical ellipticity curves of the fundamental mode rayleigh wave have been first computed (forward problem), and subsequently compared with the empirical h/v curve. the comparison provides a misfit value, which indicates the semblance between the synthetic and the observed h/v curve (mundepi et al. 2015). the model with the minimum misfit was selected as the final solution. figure 6a-6c demonstrate h/v spectral ratio, ellipticity curve, and shear wave velocity profile, respectively, at station st22. figure 6d shows shear wave velocity profile down to a depth of 24 m (maximum depth of the existing borehole) at stations in the vicinity of the boreholes, across which correlations can be established. 4 methodology in the present research, in order to study correlations between geotechnical and geophysical parameters, spearman’s coefficient test and least square regression analysis were used. typically, engineers use least square regression analysis to investigate the correlation between a pair of variables, with the quality of the obtained relationship examined using r-squared value (r2). in the present research, we begin with investigation the correlation between geotechnical and geophysical parameters using 199empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide) this happens, identical values (rank ties or value duplicates) are assigned a rank equal to the average of their positions in the ascending order of the values (lin et al. 2017). once the value of rs was calculated, one should check to ensure that the obtained value is not solely based on chance, i.e. null hypothesis shall be rejected. significance test is used for this purpose. the significance testing is run to test the significance of the relationship. for samples with more than 20 values, a t statistic can be written as, t r n rs s = − − 2 1 2 (2) the significance level  for a given hypothesis test is a value for which a p value less than or equal to  is considered statistically significant. typical values for α, are 0.1, 0.05, and 0.01. hence if p value associated with that t statistic is less than , the null hypothesis is rejected, and there is a trend in the data (lin et al. 2017). since the spearman rank coefficient method is completely different from the last square regression method, the simultaneous use of both methods is a reasonable decision. the spearman rank coefficient method does not represent a relationship but examines the quality of correlation more accurately. in contrast, the last square regression method provides relationships which can be applied by researchers and engineers in their field investigations and activities. an advantage of spearman rank coefficient is that if one or more data are far greater than other data, their correlation is not affected because just their ranks are evaluated. since the data values have great deviations and differences in this study, the use of spearman rank coefficient seems reasonable. for further detail about the spearman’s coefficient test, please refer to the studies reported by lehman (2006) and lin et al. (2017). 5 analysis of electrical resistivity – geotechnical parameters correlation in order to investigate electrical resistivity – geotechnical parameters correlation, data of the boreholes bh1, bh5, and bh7 were used. despite the closer distance of the borehole bh2 than borehole b1 to profile p1, its data was not used for investigating the correlation between electrical resistivity and geotechnical parameters. the borehole bh2 is located at upstream of the profile p1, and due to the steep slope of the ground in this area, their elevation levels differ by more than 5 m. therefore, due to the high difference in elevation level, it makes no sense to have them correlated. numerous geotechnical parameters have been evaluated for correlation investigation. among these parameters, one can refer to spt-n, moisture content, liquid limit, plastic limit, plasticity index, unit weight, specific gravity, and permeability coefficient. results of the investigations indicate very weak to weak correlation between electrical resistivity and liquid limit, plastic limit, plasticity index, specific gravity, and unit weight. the values of r2 and [rs] obtained from the correlation between the electrical resistivity and the mentioned geotechnical parameters were found to range within 0.05 – 0.2 and 0.21 – 0.41, respectively; as such, the respective results are not presented herein. giao et al. (2003), long et al. (2012), and siddiqui and osman (2013) reported similar results. they observed very weak correlations between electrical resistivity and plasticity index as well as unit weight. permeability coefficient has exhibited a strong correlation coefficient toward electrical resistivity (r2 = 0.87). however, given inadequacy of the number of available data (n < 5), the results are not presented herein. results indicate that, with increasing the permeability coefficient, electrical resistivity decreases. this is because of the fact that, an increase in permeability coefficient increases the pore space between the soil particles, and since the space is filled with water, this ends up reducing electrical resistivity considering what was mentioned above, electrical resistivity-soil moisture content and electrical resistivity – spt-n correlations are discussed in the following. 5-1electrical resistivity-soil moisture content correlation figure 7 demonstrates the electrical resistivity-soil moisture content correlation. the data presented in figures 4 and 5 are used to visualize figure 7. according to this figure, electrical resistivity-soil moisture content correlation can be obtained from equation (3). ρ ω−= 2028 2 1 496. . (3) in the above relationship,  and  are electrical resistivity and soil moisture content, respectively. the r2 = 0.68 indicates a moderate correlation. considering figure 7, it can be found that, electrical resistivity decreases with increasing the moisture content. this result resembles those of previous studies (siddiqui and osman 2013, jusoh and osman 2017, lin et al. 2017). this indicates that, variations of in electrical resistivity of the soil are affected by water, so that the larger the amount of water existing among soil particles, the lower will be the electrical resistivity of the soil. higher moisture content facilitates conduction of electrical current through movement of ions in pore water (siddiqui and osman 2013). figure 7. correlation of electrical resistivity and moisture content of soil. table 1 demonstrates the results of spearman’s coefficient test for the electrical resistivity-soil moisture content correlation. the data reported in this table are adapted from figures 4 and 5. looking at table 1, it is evident that rs = –0.6030. the negative sign indicates the inverse relationship between the two parameters considered herein (electrical resistivity and moisture content), i.e. an increase in one parameter results in a decrease in the other one. considering the criteria set in the section on methodology, [rs] = 0.6030 indicates a strong correlation. the low value of p is an indication of significance of the obtained correlation, rejecting the null hypothesis. figure 8 compares the results of the present research to those of previous research works. investigating this figure, it is clear that the results obtained in this study fall within the range of those reported in previous research works, particularly those presented by cosezna et al. (2006), fallah-safari et al. (2013), osman et al. (2014), and lin et al. (2017). the number of data used to investigate the relationship was n = 15. it is worth noting that, the number of data points used in many of the previous works has been around the same number (n ≤ 20) (cosezna et al. 2006, kibria and hossain 2012, fallah-safari et al. 2013, osman et al. 2014). 5-2electrical resistivity – spt-n correlation figure 9 shows the electrical resistivity – spt-n correlation. the data presented in figures 4 and 5 are used to visualize figure 9. according to this figure, electrical resistivity-spt-n correlation can be obtained from equation (4). =15 653 0 034. .e n (4) 200 sadegh rezaei, issa shooshpasha, hamed rezaei in the above relationship,  and n are soil electrical resistivity and spt-n, respectively. the r2 = 0.70 indicates a strong correlation. considering figure 9, it can be found that, electrical resistivity increases with increasing the spt-n. this result resembles those reported in previous studies (oh and sun 2008, liu et al. 2008, hatta and osman 2015, devi et al. 2017). this indicates that, variations of in electrical resistivity of the soil are related to soil stiffness. table 2 demonstrates the results of spearman’s coefficient test for the electrical resistivity – spt-n correlation. the data reported in this table are adapted from figures 4 and 5. looking at table 2, it is evident that rs = 0.7251. this indicates a direct, strong relationship between electrical resistivity and spt-n. the very low value of p is an indication of significance of the obtained correlation, rejecting the null hypothesis. table 3 compares the correlations obtained in the present study and those of previous research works. this table shows that, different curve fittings (linear, power, exponential, etc.) have been used in different studies. since the correlations derived from various studies have different ranges of electrical resistivity, it is not appropriate to present and compare them in form of a figure. 6 analysis of shear wave velocity – spt-n correlation in order to investigate the correlation between shear wave velocities obtained from ambient noise analysis and spt-n, the stations in the vicinity of the boreholes were used (st11, st14, st17, st18, st21 and st22). table 1. ranking orders, spearman’s rank correlation, and significance test of the electrical resistivity and moisture content of soil ert profile borehole depth (m) resistivity (ohm.m) rank ω (%) rank p1 bh1 2 30 12 16 6.5 7 28 8 15.8 5 9.5 27 7 16.4 8 11 23 4.5 21.7 13.5 12 23 4.5 21.7 13.5 14 30 12 20.5 12 15 29 9.5 16.6 9 p2 bh5 3 22 2 15 3 4.5 22 2 25.3 15 7 22 2 19.2 11 22 148 15 9.2 1 p2 bh7 4 29 9.5 16 6.5 6 25 6 17 10 8 30 12 15.5 4 20 53 14 10 2 rs -0.6030 t statistic -2.66 p (significance) 0.019 figure 8. comparison of the proposed correlation (-) with previous studies. table 2. ranking orders, spearman’s rank correlation, and significance test of the electrical resistivity and spt-n of soil ert profile borehole depth (m) resistivity (ohm.m) rank spt-n rank p1 bh1 2.5 30 10.5 17 8 7 28 7 14 3.5 9.5 27 6 14 3.5 11 23 4 11 1 13.5 30 10.5 18 9 14.5 29 8.5 16 6 p2 bh5 2.5 22 2 19 10 4 22 2 50 16.5 6 22 2 16 6 8.5 88 17 50 16.5 11 148 20 50 16.5 14.4 148 20 50 16.5 22 148 20 50 16.5 23 148 20 50 16.5 24 148 20 50 16.5 p2 bh7 3 29 8.5 16 6 6 25 5 12 2 10 38 12 50 16.5 12.5 53 14.5 50 16.5 14 53 14.5 50 16.5 16 53 14.5 50 16.5 18 53 14.5 50 16.5 rs 0.7251 t statistic 4.71 p (significance) 0.000067 figure 9. correlation of electrical resistivity and spt-n of soil. 201empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide) figure 10 shows the correlation between shear wave velocities obtained from ambient noise analysis and spt-n. the data presented in figures 4 and 6 are used to visualize figure 10. according to this figure, correlation between shear wave velocities obtained from ambient noise analysis and spt-n can be obtained from equation (5). v n= 36 592 0 6787. . (5) in the above relationship, vs and n are shear wave velocity and spt-n, respectively. the r2 = 0.75 indicates a strong correlation. considering figure 10, it can be found that, shear wave velocity increases exponentially with increasing the spt-n. this result resembles those reported in previous studies (anbazhagan et al. 2016, salinas-jasso et al. 2017). figure 10. correlation of shear wave velocity and spt-n of soil. table 3. comparison of the proposed correlation (-spt-n) with previous studies r2number of datacorrelationresearcher 0.7049n =       ρ − 6839 72 0 70 . . baraga et al. (1999) 0.4722=18 509. noh and sun (2008) 0.8213n = 2 3 2 7. .ρ+liu et al. (2008) 0.9011ρ += 20 942 281 56. .nhatta and osman (2015) 0.875ρ −=136 19 1304 83. .ndevi et al. (2017) 0.7022=15 653 0 034. .e nthis study shear wave velocity depends on soil stiffness and shear modulus (gautam 2017). therefore, spt-n, which is an indication of soil stiffness, is directly related to shear wave velocity. table 4 demonstrates the results of spearman’s coefficient test for the correlation between shear wave velocities obtained from ambient noise analysis and spt-n. the data reported in this table are adapted from figures 4 and 6. looking at table 4, it is evident that rs = 0.8230. this indicates a direct, very strong relationship between shear wave velocity and spt-n. the very low value of p is an indication of significance of the obtained correlation, rejecting the null hypothesis. in figure 11, result of this study is compared to those of previous studies. the references used in figure 11 are adapted from lopes et al. (2014), kirar et al. (2016), anbazhagan et al. (2016), gautam (2017), thokchom et al. (2017), sil and haloi (2017) and salinas-jasso et al. (2017). table 4. ranking orders, spearman’s rank correlation, and significance test of the shear wave velocity and spt-n of soil ambient noise station borehole depth (m) vs (m/s) rank spt-n rank st14 bh1 2.5 219 9.5 17 12 7 219 9.5 14 5 9.5 219 9.5 14 5 11 219 9.5 11 2 13.5 251 15.5 18 13.5 14.5 251 15.5 16 9.5 st11 bh2 3 315 19.5 50 32 5 315 19.5 20 17 11 468 29 50 32 12 216 6 20 17 15 216 6 16 9.5 17 216 6 23 19 20 496 30 50 32 st18 bh4 2 180 2.5 14 5 3.5 155 1 5 1 7 430 23.5 50 32 9 430 23.5 50 32 st21 bh5 2.5 261 17 19 15 4 334 21.5 50 32 6 334 21.5 16 9.5 8.5 444 27.5 50 32 11 444 27.5 50 32 14.4 602 33.5 50 32 22 602 33.5 50 32 23 732 43.5 50 32 24 732 43.5 50 32 st17 bh6 1.5 224 13 20 17 3 224 13 50 32 6 224 13 18 13.5 8 573 31.5 50 32 9 573 31.5 50 32 11 281 18 15 7 16 636 36 50 32 18 664 37.5 50 32 20 693 41 50 32 22 693 41 50 32 24 693 41 50 32 st22 bh7 3 195 4 16 9.5 6 180 2.5 12 3 10 432 25.5 50 32 12.5 432 25.5 50 32 14 624 35 50 32 16 664 37.5 50 32 18 686 39 50 32 rs 0.8230 t statistic 9.39 p (significance) <0.000001 202 sadegh rezaei, issa shooshpasha, hamed rezaei considering this figure, it becomes clear that, results of the present research fall within the range of the results of previous studies. relatively high shear wave velocity at high spt-n values is a result of the presence of shale, marl, and sandstone layers across the study area (figure 3). shear wave velocities of shale, marl, and sandstone were in good agreement with the results of previous studies (ahmad 1989, hiltunen 2005). process. moreover, field tests and laboratory experiments (e.g. spt, particle size analysis, specific gravity, unit weight, density, atterberg units, moisture content, and permeability) were used to compute geotechnical parameters of the soil. in order to investigate the correlation between geotechnical and geophysical parameters, two methods were used, namely spearman’s coefficient test and least square regression analysis. the results obtained from the two methods are in good agreement with one another and fell in the range of previous studies. this verifies the obtained correlations in this study. investigations indicated very weak to weak correlations between electrical resistivity and liquid limit, plastic limit, plasticity index, specific gravity, and unit weight. r2 and [rs] values obtained for the correlations between electrical resistivity and the mentioned geotechnical parameters were in the ranges of 0.05 – 0.2 and 0.21 – 0.41, respectively. permeability coefficient was found to be strongly correlated (r2 = 0.87) to electrical resistivity, so that an increase in permeability coefficient lowers electrical resistivity, due to the presence of water in pore space among the soil particles. soil moisture content exhibited moderate (r2 = 0.68) to strong (rs = –0.6030) correlation to electrical resistivity. this shows large effect of ground water on electrical resistivity. spt-n exhibited strong (r2 = 0.70), (rs = 0.7251) correlation to electrical resistivity. this shows that variations of electrical resistivity of soil are directly related to soil stiffness. furthermore, spt-n exhibited strong (r2 = 0.75) to very strong (rs = 0.8230) correlation to shear wave velocity. this is a result of the direct relationship between shear wave velocity and soil stiffness (gmax = vs 2). at areas where transporting boring equipment is not practical, the obtained correlations in this research can be used along with geophysical parameters to estimate mechanical and seismic properties of soil. these mechanical and seismic characteristics are largely applied in numerical analysis of slope stability and presentation of solutions for soil improvement and stabilization across the study area. references abidin, m. h. z., saad, r., ahmad, f., wijeyesekera, d. c., & baharuddin, m. f. t. (2014). correlation analysis between field electrical resistivity value (erv) and basic geotechnical properties (bgp). soil mechanics and foundation engineering, 51(3), 117-125. ahmed, h. (1989). application of mode-converted shear waves to rock-property estimation from vertical seismic profiling data. geophysics, 54(4), 478-485. akpan, a. e., ilori, a. o., & essien, n. u. (2015). geophysical investigation of obot ekpo landslide site, cross river state, nigeria. journal of african earth sciences, 109, 154-167. table 5. correlations between geotechnical and geophysical parameters across nargeschal landslide area rsr2correlationparameters 0.72510.70=15 653 0 034. .e n spt-n -0.60300.68ρ ω−= 2028 2 1 496. .  0.82300.75v ns = 36 592 0 6787. .vsspt-n -0.21770.06= 253 73 0 527. .ll ll -0.25750.06ρ −=100 46 0 292. .ll pl 0.22070.05=13 15 0 4865. .pl pi 0.29610.09ρ γ= 7 1182 0 8448. .e  0.41080.16= 0 00006 13 718. .gs gs -0.87ρ −=14 104 0 047. .k kfigure 11. comparison of the proposed correlation (vs-spt-n) with previous studies. 7 discussion investigation of the results obtained in this study shows correlations of different strengths between geotechnical and geophysical parameters. the correlation strength was evaluated based on r2 and rs values. it should be noted that, rs is by no means equivalent to r2. indeed, r2 is not even directly related to rs. rs is a non-parametric measure of statistical dependence between two variables. r2 is used as a measure of success of predicting the dependent variable from the independent variables. for two related samples, there is only one rs value, while there are different r2 values in different relationships (function model) (lin et al. 2017). moreover, spearman’s coefficient test investigates null hypothesis (using p value), and this enhances reliability of the solutions obtained from this method. in this research, the strengths of the obtained correlations evaluated using least square regression analyses and spearman’s coefficient test were almost the same; this verifies the obtained results. table 5 presents the results of all correlations considered in this study. as was mentioned before, details of the results of the parameters for which either very weak correlations were obtained (liquid limit, plastic limit, plasticity index, unit weight and specific gravity) or the numbers of data points were too low (coefficient of permeability) were not presented. geophysical investigations performed in this study were less costly and of better coverage than the geotechnical investigations. due to steep slope of the study area (30%), boring equipment were impossible to transport to upstream of the study area, while ambient noise measurements could be conveniently performed in those areas. using the shear wave velocity profile obtained from the ambient noise measurements and adopting the presented correlations in this study, one can extract soil properties within the study area. the obtained data have a wide spectrum of applications in soil classification, numerical analysis of slope stability, and presentation of solutions for soil improvement and stabilization across the study area. conclusions in this study, results of ambient noise measurements and ert survey along with borehole data were used to investigate the correlation between geotechnical and geophysical parameters within nargeschal landslide area. h/v spectral ratio was calculated using ambient noise measurement and analysis via hvnr method. the h/v ratio is related to ellipticity of rayleigh waves. using the ellipticity of rayleigh waves and inversion process, shear wave velocities were obtained. ert surveys were conducted using wenner-schlumberger array, and electrical resistivity of soil layers was obtained using quasi-newton optimization technique and inversion 203empirical correlation between geotechnical and geophysical parameters in a landslide zone (case study: nargeschal landslide) anbazhagan, p., bajaj, k., reddy, g. r., phanikanth, v. s., & yadav, d. n. 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(2016). shallow velocity model in the area of pozzo pitarrone, mt. etna, from single station, array methods and borehole data. annals of geophysics, 59(4), 0433. keywords: earth forest, ferruginous cement, sem, eds, clay, spherical iron compound. palabras clave: bosque de tierra, cemento ferruginoso, sem, eds, arcilla, compuesto de hierro esférico. how to cite item luo, p., zhang, s., tian, y., ding, f., & xu, z. (2019). study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china. earth sciences research journal, 23(3), 191-198. doi: https://doi.org/10.15446/esrj.v23n3.81911 the column height of earth forest is generally 8~30m in yuanmou county of yunnan province, which is far higher than the self-supporting height of general soil column. ferruginous cement is an important reason for formation of the tall and erect columnar soil. macroscopic physical and mechanical tests confirmed that the strength of the stratum containing ferruginous cement was much higher than that of the formation without ferruginous cement. the microstructure and morphology of ferruginous cement were analyzed by sem and eds. it is found that ferruginous cement is mainly composed of iron compounds and clay minerals, which only exists in certain stratum. in the process of growth, iron compounds chemically bond with clay minerals during growth to form ferruginous cements, which first form unique sphere on the surface of particles or clay minerals. it then clumps together and fills the spaces between the particles. finally, the dispersed particles are connected together to form a stable lamellar spatial structure, which greatly improves the strength of the soil. through the study of ferruginous cement, the mechanism of this natural curing agent is understood, which enriches the research content in this field. abstract study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china estudio sobre las características microscópicas del cemento ferruginoso del bosque de tierra banguo en el área de yuan mou, yunnan, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.81911 la altura de la columna del bosque de tierra es generalmente de 8 ~ 30 m en el condado de yuanmou, provincia de yunnan, que es mucho más alta que la altura autoportante de la columna de suelo general. el cemento ferruginoso es una razón importante para la formación del suelo columnar alto y erecto. las pruebas físicas y mecánicas macroscópicas confirmaron que la resistencia del estrato que contiene cemento ferruginoso fue mucho mayor que la de la formación sin cemento ferruginoso. la microestructura y la morfología del cemento ferruginoso fueron analizadas por sem y eds. se encuentra que el cemento ferruginoso se compone principalmente de compuestos de hierro y minerales arcillosos, que solo existen en ciertos estratos. en el proceso de crecimiento, los compuestos de hierro se unen químicamente con minerales de arcilla durante el crecimiento para formar cementos ferruginosos, que primero forman una esfera única en la superficie de partículas o minerales de arcilla. luego se agrupa y llena los espacios entre las partículas. finalmente, las partículas dispersas se conectan entre sí para formar una estructura espacial laminar estable, que mejora en gran medida la resistencia del suelo. a través del estudio del cemento ferruginoso, se comprende el mecanismo de este agente de curado natural, que enriquece el contenido de la investigación en este campo. resumen record manuscript received: 17/01/2019 accepted for publication: 28/05/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 191-198 penghui luo1, shitao zhang1*, yimin tian1, fei ding2, zongming xu2 1faculty of land resources engineering, kunming university of science and technology, kunming, yunnan, 650000,china 2kunming prospecting design institute of china nonferrous metal industry co., ltd, kunming, yunnan, 650000,china * corresponding author: zhangshitao9918@sina.com g eo lo g y 192 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu introduction cement refers to all kinds of authigenic minerals filling between clastic particles that are precipitated from colloids or true solutions in the sedimentary and diagenetic stages of clastic rocks. the formation of cement was affected by sedimentary environment, weathering conditions, cementitious process and other factors, whose cause of formation and composition are very complex. in recent years, with the development of technology, the research on cementing body has been developed rapidly. especially in the micro field, with the deepening of the research on the microstructure of soil, it is found that the cement, as the interparticle and interstitial material, has a great influence on the properties of soil. some scholars have studied the microcosmic nature, spatial structure, connection mode, metasomatism and other aspects of cement. cheng, kang and xu (1992) proposed that metal oxides and its hydrates deposited between clay particles play a role in cementing the soil mass, and verified the cementing role through microscopic methods and pointed out that the nature of cementing is a kind of chemical bond. jiang, zhou and chen (2013) simulated the mechanical properties of uncemented sand and cemented sand based on the rock and soil damage mechanics and the micro-damage mechanism, who proposed the constitutive model of anisotropic sand, which reflected the influence of cement on the mechanical properties of anisotropic sand from the side. li and shen (2007) studied the microscopic characteristics of structural soils in combination with the macroscopic mechanical behavior in the framework of damage mechanics, who gave the function expression of cementation and proposed the cementing crushing criterion and its mathematical expression. kong and guo (2001) further confirmed that the special properties of red clay were determined by its special cementing characteristics and microstructure, and the cementing of free iron oxide was only a part of it. zhang, kong, chen, li and liu (2017) conducted in-depth research on zhanjiang clay, who found that a large amount of iron in the water provided cementing materials for the connection of soil particles, and the acidic environment promoted the free iron oxide to produce cementing effect, thus improving the structural strength of the soil. yao, liu and cheng (2001) calculated the theoretical value of the cohesive force of guizhou red clay by using the microscopic bonding mechanism and model of synthetic gossyte bonded kaolin, which reflected the bonding effect of iron compounds from the side. gan, sheng and tian (2017). analyzed the possible chemical structure of the cementation formed by the adsorption of clay mineral cells on other cementing substances or cations in soil particles in gobi gravel soil, established the micromechanical model of the cementing force action of gobi gravel soil, and provided a beneficial reference for the analysis of the cementing characteristics of other soils. more and more scholars study the engineering application of cementation. by adding, changing and other physical or chemical actions, the cemen ting material can be used as curing agent or catalyst to improve the engineering materials, especially for the special soils such as red clay, soft soil (jian, zhang, & huang, 2013), sandy soil (li & yu, 2009), calcareous sand (zhu, zhou, & liu, 2014) and industrial raw materials (jiang, xiao, sun, & wu, 2012; feng, d. liu, f. liu, w. liu, & han, 2016; wei, yang, & gao, 2013; deng et al., 2016). author’s team conducted scientific research on earth forest (qian & ling, 1989) in banguo of yunnan province of china, who found that the height of the cylinder, generally between 8 ~ 30 m, up to 42.8 m (earth forest in langbapu of xin hua town) (fig. 1), which is far more than general soil height of self-reliance. the most important reason is that there are much ferruginous cementation body on the top and internal of the cylinder, which blocks the surface water erosion and enhances the strength of the cylinder. this paper hopes to understand the composition, microstructure, bonding characteristics and development process of the cementitious material by studying the ferric cementitious material in the cylinder of soil forest, so as to provide some theoretical basis for further research. methodology physical and mechanical test the existing earth forest in yuanmou area are mainly composed of wumao earth forest, banguo earth forest and langbapu earth forest. three representative groups of ferruginous cementitious bodies (no less than 6 samples in each group) are selected from the ferruginous cementitious layers (the strata containing ferruginous cementitious bodies) of three soil forests.:bg3①(banguo), wm1-jj(wumao), lbp1-①(langbapu). in addition, three groups of samples were taken from the strata below the ferruginous cementation layer of bangotu forest: bg3-2, bg3-3, and bg3-4 (fig. 2). firstly, ferritic cementites at different sampling points were compared, and then the most representative bg3 sampling point was selected to compare ferriferous cementitious strata with those without ferriferous cementitious strata (chapter 4 for component analysis). due to the high density, large pore ratio and poor integrity of the ferric cement of earth forest, it is easy to disintegrate when cutting. its own characteristics are irregular, so it is difficult to make standard samples for strength test. in this paper, point load meter and uniaxial press were used to conduct multiple groups of tests on a single sample, and then the relative empirical strength value was converted according to the specifications (tab.1). then the corresponding scanning electron microscope test was conducted on each group of samples. figure 1. wu mao earth forest (left) and lang ba pu earth forest (right). 193study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china granite, containing a small amount of mineral debris. the psephicity is main ly sub-circular and sub-angular, with good gradation. clay minerals are mainly kaolinite and its products in different forms, and a small amount of illite. the ferric cementite has both the lamellar structure of kaolinite and the spherical shape of ferric compound, which is generated by the chemical interaction between the two. the initial form of the ferric cementite monomer is spherical. the structure of each component can be observed under an electron microscope. we can find that the cementation type between particles are mainly basal cementation (fig. 3a) and contact cementation(fig. 3b). from figure 3, ferruginous cement with small spheroidal shape are presented obviously, some of which are growing on the surface of quartz grains (fig. 3a) and the other of which are agglomerating with clay minerals (fig. 3b, c). the clay minerals and ferruginous cement fill the void between the particles (fig. 3a, b, c), which connect and reinforce the particles, and affect the overall strength and failure mode of the soil. during the failure, the fracture surface will avoid the larger particles and rupture along the filling path. the main components of clay minerals are kaolinite and illite (fig. 3i, section 4), which are mainly in the form of cementing materials and come sem test the sem tests were carried out in yunnan key laboratory for palaeobiology and yunnan university modern analysis and testing center. fei quanta 650feg , the high-resolution field-emission scanning electron microscopy with energy dispersive spectrometer(eds) was used to get secondary electron imaging(etd), backscattered electron imaging(bsed) and the elementary composition, from which we could know the changes of microstructure and mineral composition, so as to obtain the important information such as the spatial distribution, growth process and cementation characteristics of ferruginous cement. structural features earth forest in yuanmou area were formed in the pliocene ~ holocene from semi-consolidated fluvial and lacustrine sediments cut by flowing water. its ferruginous cement layer is mainly composed of clastic rock particles, and the interstitial materials are sand, silt, clay minerals and ferruginous cement. the parent rock components of clastic rock particles are mainly sandstone and (a) (b) (c) (d) (e) (f) (g) (h) (i) figure 2. ban guo sampling points and partial formation samples (a)bg3 sampling point (b)bg3-①-1 (c)bg3-②-1 (d)bg3-③-1 (e)bg3-④-1 (f)wm1-jj-4(g)wm1-⑤-2 (h)lbp1-①-9 (i)lbp1-③-3 194 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu table 1. list of physical properties of each layer sample number natural compressive strength (mpa) dry compressive strength (mpa) saturated compressive strength (mpa) water absorption (%) porosity (%) dry density (g/cm3) wet density (g/cm3) iithology bg3-①-1 5.92 7.59 2.88 8.60 18.29 2.13 2.32 ferruginous cementation bg3-②-1 2.74 5.06 / 1.88 4.07 2.17 2.21 coarse sand bg3-③-1 3.83 9.67 / 8.04 14.47 1.8 1.95 silty clay bg3-④-1 1.49 2.57 / / / 1.59* / breccia wm1-jj-4 7.98 15.26 5.98 7.66 16.62 2.17 2.34 ferruginous cementation wm1-⑤-2 0.72 3.51 / 11.4 18.48 1.63 1.82 silt lbp1-①-9 2.29 5.49 / 10.83 21.38 1.97 2.19 ferruginous cementation lbp1-③-3 2.54 4.41 / 12.14 22.69 1.87 2.10 silt note: (1) sample number meaning: bg (sampling area) 3 (sampling point) –① (stratum) -1 (sample number); (2) * represents the physical index measured by wax seal method. by comparison, the strength of the samples with ferric cement was significantly higher than that without ferric cement. in addition to the ferruginous cement, the other samples disintegrated rapidly in water, and the saturated compressive strength could not be measured. standard samples for uniaxial compression were prepared by careful cutting of individual silt samples. with ferruginous cement. the clay minerals appear as irregular book pagelike (fig. 3d, e), lamellar (fig. 3e, i, section 4) and petal-like (fig. 3f). some microscopic processes of chemical interaction can also be observed, such as the transformation of aluminite to kaolinite (fig. 3h), the continuous growth and entanglement of clay minerals and ferruginous cement (fig. 3c). through electron microscopy scanning test, we found spherical ferruginous cement in the samples collected at three sampling points: bg3-①-1 (banguo), wm1-jj-4 (wumao), lbp1-①-9 (langbapu)(fig. 4a, b, c). however, at the same sampling point, no spheroids were found in the nonferruginous cement stratum, such as bg3-②-1, lbp1-③-3, wm1-⑤-2. only a small amount of clay minerals, quartz particles, and granular debris were observed. (fig. 4d, e, f). therefore, i ferruginous cement can greatly enhance the strength of soil. to sum up, the ferruginous cement stratum is mainly composed of ferruginous cement and a small amount of clay minerals. the ferruginous cement is formed by the chemical interaction between ferruginous compounds and clay minerals. some clay minerals react with iron oxides to form spherical ferruginous cement and some exist freely in pores. there were certain differences in the microstructure of the spherical ferruginous cement found at the three sampling points. from figure 4a (bg3-①1), the ferruginous cement was shaped like a red blood cell, with a depression in the center of the sphere and growing on the surface of quartz crystal. from figure 4b (wm1-jj-4), the spherical ferruginous cement also has depressions in the center of the spheres and is grown in lamellar kaolinit. from figure 4c (lbp1-①-9) the shape of spherical ferruginous cement is like a flower bud, and the depression in the middle is not obvious. it is more similar to many small laminated spheres, and also grows in the clay mineral dominated by lamellar kaolinite. the difference of spheroid morphology should be caused by the difference of compound composition, content and development environment. microstructure and growth process of ferruginous cement in order to further understand the characteristics of the spherical ferruginous cement, the most typical microstructure of bg3-①layer was chosen as the research object, whose ferruginous cement looked like red blood cells (fig. 5a) and had obvious grooves in the central part. through secondary electron probe and back scattering, flagella and acupuncture protrusion could clearly be seen on the surface of spheroid (fig. 5b, d). considering the angle and the penetration of the electron beam, these ferruginous cements were actually composed of many lamellar structures stacked on top of each other. they were very dense and had a high bonding strength. at the beginning, the ferruginous compounds enriched on the particle surface, who reacted with kaolinite gradually to generate small globular individuals. the number of these spheroids whose diameter was only 1 ~ 5 microns were increasing and began to agglomerate in the low depression (fig. 6a, b). at the same time, these spheroids were entangled with the free clay minerals, especially the lamellar kaolinite, forming a large layered spatial structure to fill the pores between the particles (fig. 6c). we could clearly see that the spheroids were tightly packed together, compressed and deformed into a single piece, and a flaky structure emerging from the body at the fracture. at this point, the space between the particles was filled with dense interstitial materials formed by ferruginous cement and clay minerals, which greatly improved the strength of the ferruginous cement layer. energy dispersive spectrometer (eds) eds was used to confirm the existence of iron cement and clay minerals while observing the microstructure. however, due to the influence of composition change of the compound itself and electron beam penetration and other factors, the molecular formula of the target compound could not be accurately determined, and its phase composition could only be qualitatively determined. in order to further understand the properties of ferruginous cement, energy spectrum analysis was focused on the samples of bg3-3. the four energy spectra in figure 7 were all from the bg3-3 layer. in the figure, weight% represents the mass percentage of each element at the sampling point. atomic% is the percentage of atoms. as can be seen from figure 7 (1) and (2), o, si, al, fe and k were the main cementing substances at this sampling point, with a small amount of na and mg. the selected area results show that the mass percentage contents of o, si, al, fe and k were respectively32.6%~49.7%, 21.4%~26.5%, 8.5%~15.4%, 3.7%~12.5%, 3.0%~9.3%. according to relevant works (zhang & yang, 2013) and specifications (research institute of petroleum exploration and development, 2014), it could be judged that the cementing materials here were mainly kaolinite, illite (or imonote) and iron oxides. figure 7 (3) shows the agglomeration area of ferruginous cement. the main compounds were o, fe, si and al, whose mass percentage were respectively: 28.2%~53.1%, 195study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china 17.5%~65.7%, 4.8%~32.8%, 0.91%~3.73%. the content of ferrum was obviously high. figure 7 (4) shows the spherical ferric oxide monomer. it was found that the main elements were o, fe, si, c, and a small amount of al, with the mass content of 47%, 31.1%, 15.0%, 6.53% and 0.56%. the clay minerals in the ferruginous cement were mainly kaolinite (al2si2o5(oh)4), and the structure of unit layer is 1:1 type, that is, a layer of silicon-oxygen tetrahedral sheet and an aluminum -oxygen octahedral sheet are compounded, and the two layers are connected by hydrogen bonds. the pores of the octahedron are easily filled by cations such as al, mg and fe, forming octahedral lamellar structures with common edges. meanwhile, si4+ in the tetrahedron and al3+ in the octahedron are easily replaced by other cations, such as fe3+. the excess charge generated is easy to exchange with other metal cations and adsorb other cement or soil particles with cations. therefore, it was most likely that the si4+ and al3+ between the original kaolinite layers were replaced by fe3+ to form new compounds. from figure 5 d, 6 c, the section of the spheroid was observed, whose internal structure was characterized by lamellar clay minerals, while the external morphology has the characteristic of spherical microstructure of ferric oxide (chen, 2014) (fig.7 (4)). for the ferric oxide has obvious phase, it can be judged as a new compound formed by the chemical interaction between ferric oxide and kaolinite, whose main elements were o, fe, si, c, al orderly. due to the difficulty in extracting iron cement and the lack of control materials affecting the effect of xrd, the determination of its molecular formula needs further research. conclusion (1) the main component of earth forest in yuanmou area is semi diagenetic conglomerate, whose particles with high strength comes from the mother rock clastics. its overall strength and failure mode are determined by the ferruginous cement between the particles. the formation strength with ferric cement was obviously higher than that without ferric cement. (a) (b) (c) (d) (e) (f) (g) (h) (i) figure 3. microstructure characteristics under the microscope of earth forest cementation layer (a) basal cementation (b) contact cementation (c) cement fills pores (d) book page-like and lamellar of kaolinite (e) book page-like kaolinite (f) petal-like kaolinite (g) clay mineral on the surface of quartz particles (h) conversion of alumina quartz to kaolinite (i) illite 196 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu (a) (b) (c) (d) (e) (f) figure 4. comparison of iron compounds in the cementation layer (a) bg3-①-1 (b) wm1-jj-4 (c) lbp1-①-9 (d) bg3-②-1 (e) wm1-⑤-2 (f) lbp1-③-3 (a) (b) (c) (d) figure 5. microscopic morphology of iron compounds (a) secondary electronic image (b) a larger view of fig. 5a (c) backscatter electron image (d) a larger view of fig. 5c figure 6. the growth of iron compounds (a) compound aggregation under etd probe (b) compound aggregation under cbs probe (c) gathered in layers under cbs probe 197study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china (1) clay minerals kaolinite, illite (2) clay minerals kaolinite, illite and iron oxides (3) spherical ferruginous cement (continued) 198 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu (2) the ferric cementation layer contained a large number of ferruginous cement, which were cemented with clay minerals (mainly kaolinite) to form a stable lamellar structure filling between the pores of the particles, greatly improving the strength of the ferric cementation layer. (3) the microscopic morphology of these ferruginous cement was spherical. under the microscope, it could be observed that the growth and development of the particles on the surface, agglomeration, entanglement and coiling of the clay minerals, and finally the process of connecting into a sheet. the results of eds showed that the spheroidal ferric cement was a new compound formed by ferric oxide and kaolinite, whose al3+ and si4+ between layers were replaced by fe3+. the new oxide compound had the lamellar characteristics of clay minerals in the interior and the spheroidal characteristics of iron oxides in the exterior. (4) the newly discovered spherical ferruginous cement has enriched the research content of ferruginous cement. the next step is to understand its chemical bonding process and analyze its chemical formula. based on the above results, the further possibility of using it as a natural curing agent should be carried out. reference cheng, c., kang, z., & xu, c. 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(1989). a preliminary study on the origin and types of yuanmou forest. science of china (b), 4, 412-418. research institute of petroleum exploration and development (2014). gb/t 17361-2013 microbeam analysis -scanning electron microscope and energy spectrometer method for the identification of authigenic clay minerals in sedimentary rocks. beijing: china planning press. wei, w., yang, z., & gao, q. (2013). cementing action of neotype wholetailing cementitious material. journal of building materials, 16(5), 881-887. yao, h., liu, s., & cheng, c. (2001). microscopic nature of cohesive force of a kind of natural consolidated soil. chinese journal of rock mechanics and engineering, 20(6), 871-875. zhang, r. f., & yang, z. e. (1999). scanning electron microscopy and microgeology. beijing: xueyuan press. zhang, x., kong, l., chen, c., li, k., & liu, y. (2017). effects of hydrochemistry on structural strength of zhanjiang formation clay. chinese journal of geotechnical engineering, 39(11), 1967-1975. zhu, c., zhou, b., liu, h. (2014). investigation on strength and microstracture of naturally cemented calcareous soil. rock and soil mechanics, 35(6), 1655-1663 (4) spherical ferric oxide monomer figure 7. analysis of typical clay minerals and iron compounds by energy spectrum keywords: artificial neural network; bp neural network; regional water resources; water resources carrying capacity; carrying capacity prediction; palabras clave: red neuronal artificial; red neuronal bp; recursos hídricos regionales; capacidad de carga de recursos hídricos; predicción de la capacidad de carga. how to cite item shi, c., & zhang, z. (2021). a prediction method of regional water resources carrying capacity based on artificial neural network. earth sciences research journal, 25(2), 169-177. doi: https://doi.org/10.15446/esrj. v25n2.81615 to better predict the water resources carrying capacity and guide the social and economic activities, a prediction method of regional water resources carrying capacity is proposed based on an artificial neural network. zhaozhou county is selected as the research area of water resources carrying capacity prediction, and its natural geographical characteristics, social economy, and water resources situation are explored. according to the regional water resources quantity and utilization characteristics and evaluation emphasis, the evaluation index system of water resources carrying capacity is constructed to evaluate the importance and correlation of water resource carrying capacity. the pressure degree of water resources carrying capacity is divided into five grades. according to the evaluation standard of bearing capacity, the artificial intelligence bp neural network model is constructed. based on the main impact factors of water resources carrying capacity in this area, the water resources carrying capacity grade is obtained by weight calculation and convergence iteration by using neural network model and influence factor data to realize the prediction of water resources carrying capacity. the research results show that the network model can meet the demand for precision. the prediction results have a high degree of fit with the actual data, indicating that human intelligence can obtain accurate prediction results in water resources carrying capacity prediction. abstract a prediction method of regional water resources carrying capacity based on artificial neural network método de predicción de la capacidad de carga de los recursos hídricos regionales basado en una red neuronal artificial issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.81615 para predecir la capacidad de carga de los recursos hídricos con mayor precisión y orientar mejor las actividades sociales y económicas, se propone un método de predicción de la capacidad de carga de los recursos hídricos regionales basado en una red neuronal artificial. el condado de zhaozhou se selecciona como el área de investigación de la predicción de la capacidad de carga de los recursos hídricos, y se exploran sus características geográficas naturales, la economía social y la situación de los recursos hídricos. de acuerdo con las características regionales de cantidad y utilización de los recursos hídricos y el énfasis de la evaluación, el sistema de índice de evaluación de la capacidad de carga de los recursos hídricos se construye para evaluar la importancia y el grado de correlación de la capacidad de carga de los recursos hídricos, y el grado de presión de la capacidad de carga de los recursos hídricos se divide en cinco grados. de acuerdo con el estándar de evaluación de la capacidad de carga, se construye el modelo de red neuronal bp de inteligencia artificial. con base en los principales factores de impacto de la capacidad de carga de los recursos hídricos en esta área, el grado de capacidad de carga de los recursos hídricos se obtiene mediante el cálculo del peso y la iteración de convergencia utilizando el modelo de red neuronal y los datos de factores de influencia, para realizar la predicción de la capacidad de carga de los recursos hídricos. los resultados de la investigación muestran que el modelo de red puede satisfacer la demanda de precisión y los resultados de la predicción tienen un alto grado de ajuste con los datos reales, lo que indica que la inteligencia humana puede obtener resultados de predicción precisos en el proceso de predicción de la capacidad de carga de los recursos hídricos. resumen record manuscript received: 11/08/2018 accepted for publication: 08/12/2019 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 169-177 chaoyang shi1, zhen zhang2* 1information network center, zhengzhou yellow river nursing vocational college, zhengzhou, 450066, china 2school of intelligent engineering, zhengzhou university of aeronautics, zhengzhou, china *corresponding author: pzhangza@136.com h yd ro lo g y mailto:pzhangza@136.com 170 chaoyang shi, zhen zhang introduction water resources are an important material basis to support the development of human society. it is the most widely impact factor in nature. at the same time, it is irreplaceable and limited (li et al., 2018). with the geometric expansion of the world population, the demand for water resources in people’s daily life and industrial and agricultural production has changed greatly in quality and quantity. the shortage of water resources, water pollution and low utilization rate of water resources have been paid more and more attention. people realize that the problem of water resources not only restricts the sustainable development of social economy, but also threatens the survival of human beings (menhas et al., 2019). therefore, the theory of water resources carrying capacity based on the principle of sustainable development should come into being. the research on water resources carrying capacity can be used as a tool to judge whether the regional water resources supply and social development are in a coordinated state, and also can provide certain reference for scientific management of water resources (chenini et al., 2019). zhu et al. (2019) proposed to increase the water storage capacity of the reservoir as a separate index, used a cloud model to calculate the water resources carrying capacity of hubei province, and used the dematel method to determine the importance of reservoir water storage. finally, the degree of obstacles was considered to explore the main factors affecting the water resources carrying capacity of hubei province. in liu et al. (2019) yichang city was taken as an example, and based on the comprehensive evaluation of the carrying capacity of the “water resourcessocial economyecological environment” composite system in yichang city from 2005 to 2015, the gm (1,1) grey prediction model was used to predict the water resources carrying capacity in 2020, 2025 and 2030. based on the analysis of the uncertainty of the forecast results in the future level year, the sustainability of the socioeconomic development model of yichang city under four scenarios was discussed, combining with the strictest water resources management system and river length system. jia and long (2018) used conventional trend method to evaluate water resources carrying capacity of zhongshan city, constructed sd system model of water resources carrying capacity, and compared and analyzed water resources carrying capacity under conventional development mode and different policy development scenarios. guo (2020) aimed at the problems of long time-consuming and low accuracy of prediction results of traditional prediction systems for regional water resources sustainable carrying capacity, a new prediction system of regional water resources sustainable carrying capacity was designed based on big data technology. the system hardware was composed of four parts: general control layer, subsystem function layer, subsystem function layer and system support function layer. at the same time, real-time monitoring module, information management module, decision support module and remote control module were introduced. the software display field was designed by using the display button and exit button. through system initialization, sample data reading, sample internal zero matrix definition, the proportion of each index was calculated, and the software process was composed of the output of sustainable water resources carrying capacity. the artificial neural network method has a wide range of adaptability, learning ability and mapping ability. it can approach any nonlinear function in theory, and can usually obtain satisfactory results in the modeling and prediction of multivariable nonlinear systems. because the above methods in references only consider some useful information, lack of overall understanding, and the conditions of prediction methods have certain limitations, in order to further accurately predict water resources carrying capacity, this paper proposes a new prediction method of regional water resources carrying capacity based on an artificial neural network. the main work of this paper is described as follows: (1) zhaozhou county is selected as the research area of water resources carrying capacity prediction; (2) through water resources system evaluation index, social system evaluation index, economic system evaluation index and ecological environment system evaluation index, the evaluation index system of water resources carrying capacity is constructed, which truly reflects the relationship between supply and demand of water resources, social and economic development level and regional ecological environment, and then makes a reasonable judgment on the strength of regional water resources carrying capacity; (3) through the analysis of the definition and calculation formula of each index, the classification accuracy of evaluation index grading standard is improved; (4) according to the evaluation grade standard of water resources carrying capacity, the bp neural network model is more applicable; (5) the neural network model is used to predict the water consumption. the results show that the prediction accuracy of this method is high, and has good effectiveness and practicality. overview of the study area zhaozhou county is located in the southwest of heilongjiang province, the center of songnen plain, the north bank of songhua river, close to daqing oilfield. the geographical coordinates of zhaozhou county are 124 ° 48 ′ 12 ″ 125 ° 48 ′ 03 ″ e and 45 ° 35 ′ 02 ″ 46 ° 16 ′ 08 ″ n. the county covers an area of 2445 km2, with a cultivated land area of about 2.22 million mu and a vast grassland area of 850000 mu. zhaozhou county now governs 6 towns (zhaozhou town, erjing town, fengle town, yongle town, chaoyanggou town, xingcheng town), 6 townships (yushu township, chaoyang township, xinfu township, shuangfa township, tuogu township, yongsheng township), 2 pastures (star pasture and paradise pasture), 104 administrative villages and 732 natural villages. physical and geographical characteristics topography zhaozhou county is located in the hinterland of songnen plain. there are no rivers passing through the county. there are a small number of wetlands and small marshes in some low-lying areas. the surface water resources are small, and there is no railway for the time being. however, the highway traffic is relatively developed. the higher terrain area of zhaozhou county is changwu highland in the northeast. the terrain gradually decreases from the south and west of changwu highland. the altitude of the territory is about 118-232m, the average altitude of the agricultural production area is about 167m, the elevation of the southern and western border area is about 130m, and the ground slope is generally between 1-10 degrees. the county covers an area of 2445 km2, with 2.22 million mu of cultivated land, accounting for 60% of the total area. the grassland area is 850000 mu, accounting for about 23% of the total area. the length of the north-south direction is about 77km, and the width of the east-west direction is about 72km. the terrain is flat. there has never been a flood disaster in history. climatic characteristics zhaozhou county is located in a high latitude area, belonging to the mid temperate continental monsoon climate, with short spring and less rainfall, high temperature in summer but abundant rainfall, cool and moderate temperature in autumn as well as cold and dry climate in winter. zhaozhou county belongs to the first accumulated temperate zone of heilongjiang province, with an annual sunshine duration of 2900 hours and abundant light and heat resources. the annual average active accumulated temperature is about 2800 °c and the frost free period is about 143 days. in zhaozhou county, the temperature changes greatly in spring, summer, autumn and winter. the extreme high temperature in summer reaches 38.1 °c, the extreme low temperature in winter reaches 35.3 °c, and the annual average temperature is 4.2 °c. socio-economic profile the total population of zhaozhou county is 431387, including 82970 urban population, 348417 agricultural population and 158173 households. according to the registered residence population, the urbanization rate is 19.23%. the main food crops in zhaozhou county include rice, corn, soybean, etc. the economic crops include sugar beet, garlic, oil, sunflower, melon and vegetable, hemp and flue-cured tobacco. the total sown area of crops is 150100 hm2, the effective irrigation area is 66900 hm2, and the construction of ecological high standard farmland is 10000 mu. 171a prediction method of regional water resources carrying capacity based on artificial neural network overview of water resources surface water resources there is no natural river passing through zhaozhou county, and most of the surface water resources are supplied by rainfall. the surface runoff only occurs in the rainy season of a wet year with large rainfall, and limited by many conditions, the surface water resources that can be developed and utilized are less. according to the evaluation report of surface water resources in heilongjiang province, zhaozhou county is located in the low runoff value area of heilongjiang province. the annual average annual runoff depth is 25.53mm, the average annual surface water resources is 60 million m3, and the available runoff is 40 million m3. the average annual rainfall is 456.15mm. the minimum precipitation was 236.4mm in 2001 and 696.4mm in 1981, with a frequency of 1.3% and a corresponding runoff depth of 95mm. about 82% of the annual precipitation in zhaozhou county is mainly concentrated in june, july, august and september, and the precipitation in other months is less. the total annual inflow of the north main drainage channel is 43.13 million m3, including 16.15 million m3 in zhaodong city and 22 million m3 in the west main drainage channel. groundwater resources the aquifers in zhaozhou county are mainly composed of quaternary harbin formation, lindian formation confined aquifer, qiqihar formation phreatic aquifer, tertiary daan formation and cretaceous mingshui formation confined aquifer. the characteristics of different aquifers also have certain differences. the lithology of aquifers mainly includes sand gravel and quaternary pore phreatic aquifer. due to the characteristics of thin sand layers, uneven distribution, weak water yield and great difficulty in mining, centralized mining can not be carried out. however, the tertiary and quaternary confined aquifers in the lower part of the aquifer are characterized by large sand thickness and wide distribution. it can be used as the main mining layer for people’s daily life, agricultural irrigation and industrial production water. according to the equilibrium method and pumping test method, the exploitable amount of confined water and phreatic water in zhaozhou county is 63 million m3 / year and 30.6 million m3 / year respectively. total water resources the total amount of water resources is equal to the sum of surface water resources and groundwater resources. however, under the action of natural water cycle, there is a mutual recharge relationship between surface water and groundwater, and there is a part of repeated calculation between them. therefore, the repeated calculation amount of surface water and groundwater should be subtracted when calculating the total amount of water resources. the calculation formula is as follows: w w wtotal surface water quantity underground water quantity= + −−wrepetition quantity (1) where: wtotal is the total amount of water resources; wsurface water quantity is the amount of surface water resources; wunderground water quantity is the amount of groundwater resources; wrepetition quantity represents the amount of repeated calculation between surface water and groundwater. according to daqing statistical yearbook, the statistics of water resources in zhaozhou county over the years are shown in table 1. it can be seen from table 1 that the total amount of water resources in zhaozhou county will change greatly from 134 million m3 in 2010 to 294 million m3 in 2019. among them, the surface water resources have been relatively stable, maintaining below 100 million m3, reaching the highest value of 209 million m3 between 2010 and 2019 in 2013, indicating that the surface water resources in zhaozhou county are quite scarce. construction of evaluation index system of water resources carrying capacity in zhaozhou county at present, the research on water resources carrying capacity has been continuing, and various theories, concepts and research methods have also been emerging, but a complete theoretical system has not yet been formed. evaluation index of water resources carrying capacity there are many factors that affect the carrying capacity of regional water resources. the importance and influence of indexes, regional development trends and data acquisition should be considered in the selection of specific indexes of each system. according to the establishment principle of the above index system and combined with the specific situation of zhaozhou county, the evaluation indexes are selected as follows: the available amount of water resources per unit area (10000 m3 / km2); the available amount of water resources per capita (m3); the development and utilization degree of groundwater resources (%); and the population density (person / km2); urbanization rate (%); natural growth rate (%); rural per capita water consumption (l / (day · person)); urban per capita water consumption (l / (day · person)); per capita grain occupation (kg / person); per capita gdp (10000 yuan / person); cultivated land irrigation rate (%); water consumption of 10000 yuan gdp (m3 / 10000 yuan); the proportion of added value of primary industry in gdp (%); the proportion of the added value of three industries in gdp (%); gdp growth rate (%); ecological environment water consumption rate (%); forest coverage rate (%). these indexes can reflect the relationship between supply and demand of water resources, socio-economic development level and regional ecological environment in zhaozhou county, and then make a reasonable judgment on the strength of regional water resources carrying capacity. table 1. statistics of water resources in zhaozhou county over the years particular year total water resources surface water resources ground water resources repeated calculation of surface water and groundwater annual precipitation (mm) water production coefficient calculation unit area (km2) water production scale (10000 m2 / km2) 2010 1.34 0.4 0.94 0 283.2 0.19 2445 5.5 2011 1.24 0.3 0.94 0 328.1 0.19 2445 5.07 2012 2.01 0.59 1.4 0 408.1 0.2 2445 8.21 2013 2.09 0.61 1.48 0 431 0.2 2445 8.53 2014 2.07 0.61 1.46 0 427.1 0.2 2445 8.48 2015 1.37 0.43 0.94 0.15 468.8 0.12 2445 5.6 2016 2.66 0.68 2.05 0 465.6 0.24 2445 11.18 2017 3.32 0.73 2.59 0 587.7 0.23 2445 13.38 2018 2.67 0.68 1.99 0.68 452.2 0.24 2445 10.94 2019 2.94 0.71 2.23 0 507 0.24 2445 12.03 172 chaoyang shi, zhen zhang definition and calculation formula of each index based on the comprehensive analysis of the relationship among different factors affecting water resources carrying capacity in zhaozhou county, referring to the evaluation results of water resources carrying capacity in other similar areas, according to the selection principle of evaluation index system, 17 single indexes are selected. the connotation and calculation method of each single index are described as follows. evaluation index of water resources system (1) water resources per unit area (cheng et al., 2018) (10000 m3 / km2): water resources per unit area refers to the ratio of the total water resources in the current year to the total land area of the region. the index reflects the abundance of natural water resources in this area. the larger the index value is, the richer the water resources in the region are. it has a certain reference value for measuring the water shortage in this area. water resources per unit area total water resources total a  rrea of regional land (2) (2) per capita water resources occupation (hu et al., 2019) (m3): per capita water resources occupation refers to the ratio of the total regional water resources to the total regional population. to a certain extent, the index can truly reflect the relative abundance of water resources in the region, and is an evaluation of the actual utilization effect of regional water resources. water resources per capita total water resources total popu  llation (3) (3) development and utilization degree of groundwater resources (%): the development and utilization degree of groundwater resources refers to the percentage of groundwater resources development in the total amount of groundwater resources in the region. the index is a reverse index. the more the index value is, the greater the potential of groundwater development and utilization is. development and utilization degree of groundwater resourcess development and utilization of groundwater total amount  oof groundwater resources (4) evaluation index of social system (1) population density (person / km2): population density refers to the number of people per unit land area. this index can measure the population distribution of a region and reflect the population pressure of unit land area truthfully. population density total population total area of regional = lland %×100 (5) (2) urbanization rate (xu et al., 2020) (%): urbanization rate refers to the proportion of urban population in the total population in the region. this index can reflect the level of social and economic development, the living standard of residents and the quality of population. urbanization rate urban population total population %= ×100 (6) (3) natural growth rate (‰): the natural growth rate is generally the ratio of the number of births minus the number of deaths in the current year and the average total population in the same period, expressed in thousands. the natural growth rate of population is an index reflecting the natural growth of population. the change between birth rate and mortality rate directly affects the increase or decrease of natural growth rate of the population. natural population growth rate annual number of births ann = − uual death toll annual average population %×100 (7) (4) rural per capita domestic water consumption (l / (day · person)): the rural per capita domestic water consumption refers to the ratio between the total domestic water consumption of rural residents and the total rural population. this index can reflect the living water level of rural people in the whole region. rural per capita domestic water consumption domestic water = consumption of rural residents rural population* % 365 100× (8) (5) urban per capita domestic water consumption (l / (day · person)): urban per capita domestic water consumption refers to the ratio of the total domestic water consumption of regional urban residents to the total urban population. this index can reflect the living water level of urban people in the whole region. urban per capita domestic water consumption domestic water = consumption of urban residents urban population* % 365 100× (9) (6) per capita grain occupation (kg / person): the per capita grain occupancy refers to the ratio of the total grain production and the total population in the region in the current year. this index can reflect the level of people’s life in the region as a whole. grain per capita domestic water consumption of urban resid = eents urban population* % 365 100× (10) evaluation index of economic system (1) per capita gdp (10000 yuan / person): per capita gdp refers to the ratio of the gdp of the region to the total population at the end of the year. this index can measure regional economic development. per capita gdp gdp total population at the end of the year = ×1100% (11) (2) farmland irrigation rate (%): farmland irrigation rate refers to the ratio of irrigation area to total cultivated land area, which can reflect the actual situation of regional agricultural development. irrigation rate of cultivated landirrigation rate of cultivated land irrigation area total ar = eea of cultivated land %×100 (12) (3) water consumption per 10000 yuan gdp (m3 / 10000 yuan): the water consumption per 10000 yuan gdp is the ratio of the total annual gdp to the total water consumption in 10000 yuan. the index can reflect the regional industrial structure, economic development level and water-saving measures to a certain extent. at the same time, it can also reflect the coordination of regional economic development and water resources. water consumption per 10000 yuan gdpwater consumption per yuan gdp gdp total water consumpt 10000  iion (13) (4) the proportion of the added value of the primary industry in gdp (%): the proportion of the added value of the primary industry in gdp is the ratio of the added value of the primary industry to the annual gdp in this year. the primary industry is the core and foundation of national economic development. this index is the foundation of the whole social economy, and it is related to the healthy operation of economic development. 173a prediction method of regional water resources carrying capacity based on artificial neural network proportion of added value of primary industry in gdp added = value of primary industry gdp %×100 (14) (5) the proportion of the added value of the tertiary industry in gdp (%): the proportion of the added value of the tertiary industry in gdp is the ratio of the added value of the tertiary industry to the gdp of the current year. the index can reflect the level and stage of regional economic development. the proportion of added value of tertiary industry in gdp = aadded value of primary industry gdp %×100 (15) (6) gdp growth rate (%): gdp growth is the ratio of gdp added value of this year to gdp of last year. �gdp growth rate gdp growth in this year gdp of last year = ×1000% (16) evaluation index of ecological environment system (1) ecological environment water consumption rate (%): ecological environment water consumption refers to the minimum water consumption required to ensure the benign cycle of the ecosystem. ecological environmental water use rate refers to the ratio of water consumption allocated to the ecological environment to total water consumption. water consumption rate of ecological environment water con = ssumption of ecological environment total water quantity ×1000% (17) (2) forest coverage (%): forest coverage refers to the ratio of forest area to regional land area. the index reflects the regional forest vegetation coverage and is an important index to reflect the operation of the ecosystem as a whole. forest coverage forest coverage total area of regional land = ××100% (18) determination of privacy classification criteria in order to reflect the state of regional water resources carrying capacity scientifically, reasonably, accurately and objectively, the pressure degree of water resources carrying capacity is divided into five grades according to the importance and correlation degree of the above evaluation indexes to the evaluation of regional water resources carrying capacity, and referring to the grading standards of water resources carrying capacity designed by other similar areas, the pressure degree of water resources carrying capacity is divided into five grades, respectively, v1, v2, v3, v4 and v5. from v1 to v5 means that the carrying capacity of water resources is gradually becoming fragile. where: v1 means that the regional water resources carrying capacity is strong, the regional water resources are relatively rich and the degree of development and utilization is low, which can meet the demand of regional social and economic development for water resources, and the water resources supply situation is good. v2 indicates that the carrying capacity of regional water resources is strong. the regional water resources supply and development have reached a certain degree, but there is still a large space for development and utilization. v3 indicates that the carrying capacity of water resources is medium, and the carrying capacity of water resources is in a medium level, and the water resources and social economy are in a state of coordinated development. v4 indicates that the carrying capacity of water resources in the region is light and weak, which is close to the critical value, and the potential for development and utilization is small. in the long run, problems such as water shortage and water pollution will occur. v5 indicates that the carrying capacity of water resources in the region is weak, indicating that the carrying capacity of water resources has reached the limit or is overloaded, and the development potential is extremely low. therefore, water resources will restrict the development of the social economy, and corresponding countermeasures should be taken. the classification criteria of each index are shown in table 2. analysis and calculation process of neural network model the theory of neural network systems is a frontier research field of artificial intelligence in recent years. it is based on the connection mechanism of large-scale parallel processing and distributed information storage. depending on the connection of a large number of neurons and the different excited states of neurons caused by this connection and the overall behavior of the system, it is closer to the processing mode of the human brain (pelusi et al., 2018). by using the nonlinear mapping relationship of neural networks, the complex table 2. classification standard of evaluation index evaluating index v1 v2 v3 v4 v5 water resources per unit area (10000 m2 / km2) >60 30-60 20-30 10-20 <10 available amount of water resources per capita (m2) >3000 1700-3000 1000-1700 500-1000 <500 development and utilization degree of water resources <10 10-20 20-40 40-60 >60 population density (person / km2) <40 40-80 80-140 140-300 >300 urbanization rate (%) <25 25-40 40-60 60-70 >70 natural growth rate (‰) <2 2-10 10-15 15-20 >20 rural domestic water consumption per capita (l / (day · person)) <50 50-80 80-100 100-120 >120 urban domestic water consumption per capita (l / (day · person)) <150 150-210 210-300 300-400 >400 grain per capita (kg / person) >2000 1000-2000 500-1000 300-500 <300 per capita gdp (10000 yuan / person) <0.4 0.4-0.6 0.6-2.5 2.5-8 >8 lower irrigation rate (%) <10 10-20 20-50 50-60 >60 water consumption per 10000 yuan gdp (m3 / 10000 yuan) <30 30-60 60-100 100-300 >300 proportion of positive value of primary industry in gdp <5 5-12 12-15 15-30 >30 proportion of positive value of tertiary industry in gdp >65 50-65 40-50 30-40 <30 gdp growth rate <5 5-10 10-15 15-20 >20 water consumption rate of ecological environment >5 3-5 2-3 1-2 <1 forest coverage rate (%) >50 40-50 20-40 10-20 <10 174 chaoyang shi, zhen zhang coupling system of water resources carrying capacity is abandoned to find out the inevitable relationship between them, which can avoid the difficulties encountered in finding the relationship between them by other quantitative methods. therefore, neural network theory provides an opportunity for further development of water resources carrying capacity research. the introduction of neural network theory into the field of water resources carrying capacity will promote the development of water resources carrying capacity research. artificial neurons neuron is the basic processing unit of an artificial neural network and a mathematical model of biological neurons in function and structure. it was first proposed by mcculloch-pitts in 1943, so it is also called mp model (todo et al., 2019). the mp model is a multi-input single output nonlinear element, as shown in figure 1. figure 1. mp neuron model suppose that x1, x2, … xn is the n inputs of neuron i, wji is the binding strength between the neuron and the j-th neuron from other layers, which is called the weight (par & kar, 2019); ui is the sum of inputs of neurons, i is the threshold of neurons, yi is the output of neurons, that is, the output equation of neurons is: u w xi ji i i= − = ∑i n 1 θ (19) y f ui i= ( ) (20) where: f ui( ) is a nonlinear function between input and output, which is called excitation function (wang et al., 2019). the commonly used excitation functions can be classified into three types: threshold type, s-type and linear type. the mathematical expressions are as follows: f u u ui i i ( )= > ≤    1 0 0 0 , , (21) f u ei cui ( )= + − 1 1 (22) f u ui i( )= (23) bp neural network model bp network model is the most widely used and successful artificial neural network. in 1989, robert hecht nielson proved that under certain conditions, for any given e < 0, there exists a three-layer neural network, which can approach any nonlinear function with the precision of e-mean square error. therefore, the mapping from arbitrary n-dimensional feature space to m-dimensional feature space can be completed by using a three-layer bp network. bp neural network is a multi-layer forward network with unidirectional propagation, composed of input layer ( ,..., )x xn1 , output layer ( ,..., )y ym1 and one or more hidden layers ( ,..., )h ht1 (figure 2). the input layer consists of n neurons, x i ni ( , , , )= …1 2 is the input; hidden layer consists of t neurons; the output layer consists of m neurons, y g mg ( , , , )= …1 2 is its output. there is no coupling between neurons in the same layer, and the neurons between layers are connected by connection weight w and threshold b (wang et al., 2018; kamimura, 2018). wip, b i n; p tp ( , , , , , , )= … = …1 2 1 2 denotes the connection weight and threshold between the input layer and the hidden layer respectively, and w b p t; g m, pg g ( , , , , , , )= … = …1 2 1 2 denotes the connection weight and threshold between the hidden layer and the output layer, respectively. figure 2. typical topology structure of three layer bp neural network in general, there is no threshold and activation function in the input layer of the bp network, that is, the output of neurons in the input layer is equal to the input of input neurons, while the output of neurons in the hidden layer and output layer must be processed by weight and threshold. the calculation model is as follows: the output of the hidden layer is as follows: z f w x bp h ip i p= +( )∑ (24) where i n= …1 2, , , ; p t= …1 2, , , ; xi is the input value of neurons in the input layer; zp h is the output value of neurons in the hidden layer; wip h and bp is the weight and threshold between the input layer and the hidden layer. the output of the output layer is as follows: z f w z bg y pg p h g= +( )∑ (25) where, p t= …1 2, , , ; g m= …1 2, , , ; wpg h and bg is the weight and threshold between the hidden layer and the output layer. if there is an error between the output layer actually outputs zg y and expected output yi , and the error e is greater than the specified error e0, the error signal is returned along the original path and propagates to the input layer successively by modifying the connection weights between neurons in each layer, and then goes through the forward propagation process. these two processes are repeated until e e≤ 0. at this time, the network learning process will automatically end, and the corresponding network structure and parameter setting will constitute the bp model of the problem. the commonly used error control criterion is the least square error, that is, the energy function is (li & wang, 2018): e y zi g y= −( ) = ∑1 2 1 2 i m (26) network learning is based on gradient descent, by adjusting the weight wip and wpg, the formula of weight adjustment is as follows 175a prediction method of regional water resources carrying capacity based on artificial neural network ∆ = −( ) −( ) + ∆ −( ) ∆ = −( ) = w · y z z z y ± w t w · z z pg i g y g y g y › i pg ip p h p h m 1 1 1 1i∑∑ + ∆ −( )      ´ w ± w tp pg ip 1 (27) where, wip and wpg is the weight between the input layer and the hidden layer, and the weight between the hidden layer and the output layer.  and  are called learning rate and inertia factor, respectively (wen et al., 2019; jiang & li, 2018). when  is large, the convergence is fast but unstable; when  is small, the convergence is slow;  is just opposite to  . model form and step establishment the evaluation index sequence of regional water resources carrying capacity is xij, the evaluation grade sequence is zi, i ...,n1 2, , ; j ...,m1 2, , . n and m represent the capacity of index sequence and the number of evaluation indexes respectively. there are: z f xi ij= ( ) (28) the above formula shows that the input layer of ann model has m units ( ; , ..., )x j mij 1 2 , and the input layer has a unit ( )zi . (1) according to the evaluation grade standard of water resources carrying capacity, a sufficient number of evaluation index sequences are generated by using visual basic programming technology, and the modeling sequence is composed of these index generating sequences and their evaluation grade values. the upper and lower limits of the evaluation index value in the p -th evaluation grade are respectively a j p and bj p , then the random simulation formula of evaluation index value is as follows: x b a *rnd x aij p j p j p j p= −( ) ( )+ (29) (2) standardization of evaluation index in order to eliminate the influence of different dimensions on the evaluation results, the evaluation index sequence is normalized and the following formula is used for data processing: cost indexes: u x xij ij ij / max (30) pressure type index: u x xij ij ij / min (31) (3) the number of hidden layers and units of each hidden layer is selected, and the error limit value e0 between the output and the actual output is given. according to the sequence xij and zi, the neural network is trained. when e e≤ 0, the training is over. the structure parameters, weight and threshold value of the corresponding neural network constitute the artificial neural network model for the evaluation of water resources carrying capacity. prediction of water resources carrying capacity in zhaozhou county based on neural network model the purpose of water resources carrying capacity analysis is to provide guidance and countermeasures for regional development, and the prediction of water resources demand is of great significance for the prediction of regional water resources carrying capacity. in this paper, the neural network model is used to predict the water consumption of the planning year 2025 with 2019 as the base year. selection of prediction indexes as the utilization rate of water resources, water supply modulus, per capita daily domestic water consumption, cultivated land irrigation rate, forest coverage rate, water consumption of 10000 yuan industrial output value and water consumption of 10000 yuan agricultural output value are the main impact factors affecting the carrying capacity of water resources in this area. therefore, it is used to predict the carrying capacity of water resources in the study area in this section. prediction of water resources carrying capacity of zhaozhou county based on bp neural network model preparation of e-learning samples and prediction data according to the statistical yearbook of zhaozhou county and the water resources bulletin of zhaozhou county, the original data needed to calculate the above seven impact factors are calculated, as shown in table 3. through calculation, the data of seven impact factors of water resources carrying capacity in zhaozhou county are obtained, as shown in table 4. where, the utilization rate of water resources = total annual water consumption / annual water resources, water supply modulus = 104 × water supply / area, per capita daily domestic water consumption = 107 × domestic water consumption / (population × 365), and water consumption of 10000 yuan industrial (agricultural) output value = 104 × industrial (agricultural) water consumption / industrial (agricultural) output value. combined with zhaozhou county planning outline and social development science and technology planning, based on some data in zhaozhou county statistical yearbook, and selecting 2019 as the base year, the data of seven impact factors of water resources carrying capacity in zhaozhou county in the planning level year 2025 can be obtained, as shown in table 5. table 3. original data of impact factors of water resources carrying capacity in zhaozhou county (2013-2019). particular year 2019 2018 2017 2016 2015 2014 2013 annual total water consumption (100 million m3) 97.22 94.3182 93.6993 89.9 87.164 84.17 87.67 annual water resources (100 million m3) 834.63 990.999 916.074 1117.5 972.5 1217.46 1206.2 water supply (100 million m3) 97.22 94.3182 93.6993 89.9 87.164 87.17 87.67 area (km2) 176167 176167 176167 176167 176167 176167 176167 domestic water consumption (100 million m3) 12.82 10.5603 10.4997 15.35 15.043 15.11 15.28 population (10000) 3931.12 3878.47 3869.66 3837.27 3798.51 3755.72 3605.81 effective irrigation area (10000 mu) / 1040.2864 1021.065 1007.295 989.72 980.06 965.81 common cultivated land area (10000 mu) / 2723.0128 2625.81 2654.11 2748.42 2756.22 2772.46 forest coverage rate (%) 35.9 34.9 34.9 30.83 30.83 30.83 30.83 industrial water consumption (100 million m3) 28.14 26.9346 26.3888 23.22 20.14 18.88 24.05 industrial output value (100 million yuan) 849.4621 731.7347 600.817 474.68 387.85 348.59 326.03 agricultural water consumption (100 million m3) 54.51 55.87 55.8676 51.34 51.976 50.18 48.34 agricultural output value (100 million yuan) 363.149866 329.170207 298.37 280.83 274.17 270.99 267.57 176 chaoyang shi, zhen zhang table 5. data of seven impact factors of water resources carrying capacity in zhaozhou county in 2025 impact factors base year (2019) planning year (2025) utilization rate of water resources 11.6483 18.35 water supply modulus (10000 m3 / km2) 5.5186 60.269 daily domestic water consumption per capita (l / person / d) 89.3468 200.0000 probability of cultivated land management 38.7854 55.49 forest coverage rate (%) 35.9 40.0000 water consumption of 10000 yuan industrial output value (m3 / 10000 yuan) 331.2685 173.3227 water consumption of 10000 yuan agricultural output value (m3 / 10000 yuan) 1501.0332 1298.0196 where, the utilization rate of water resources, water supply modulus, water consumption of ten thousand yuan industrial output value and ten thousand yuan agricultural output value are predicted and calculated by natural growth rate method; r1, r2, r6 and r7 represents the average annual growth rate of water resources utilization rate, annual average growth rate of water supply modulus, annual average growth rate of water consumption per 10000 yuan of industrial output value and annual growth rate of water consumption of 10000 yuan of output value, respectively; r %1 9 5217 . , r %2 1 7778 . , r %6 12 151=− . , r %7 2 864=− . ; r4 is the average annual growth rate of cultivated land from 2013 to 2019, the value is − 0.147%, and the irrigation area in 2025 is predicted to be 15 million mu. network training process and result test in this paper, seven indexes of water resources utilization rate, water supply modulus, per capita daily domestic water consumption, cultivated land irrigation rate, forest coverage rate, water consumption of ten thousand yuan industrial output value and ten thousand yuan agricultural output value are selected as the input layer of the neural network prediction model of water resources carrying capacity in zhaozhou county, and the water demand is taken as the network output; the hidden layer neuron is 7; therefore, the network topology structure is: 7 7 1  . the seven impact factor values from 2013 to 2018 in table 4 and the water consumption in corresponding years in table 3 are normalized and used as network training samples to train the network. when learning rate η= 0 1. , inertia factor α = 0 2. , error tolerance e ≤ 0 1. , that is, the accuracy is 90% (dos santos et al., 2019); when the number of iterations is 34735, the network reaches the required accuracy, e0 0999924. , that is, e e≤ 0 . table 4. data of seven impact factors of water resources carrying capacity in zhaozhou county (2013-2019) particular year current year 2019 2018 2017 2016 2015 2014 2013 utilization rate of water resources 11.6483 9.5175 10.2284 8.0447 8.9629 6.9136 7.2683 water supply modulus (10000 m3 / km2) 5.5186 5.3539 5.3188 5.1031 4.9478 4.7779 4.9765 daily domestic water consumption per capita (l / person / d) 89.3468 74.5973 74.3381 109.5956 108.4996 110.2246 116.0988 probability of cultivated land management 38.7854 38.2035 39.0000 37.9524 36.0105 35.5581 34.8358 forest coverage rate (%) 35.9 34.9 34.9 30.83 30.83 30.83 30.83 water consumption of 10000 yuan industrial output value (m3 / 10000 yuan) 331.2685 368.0924 439.2153 489.1717 519.2729 541.6105 737.6622 water consumption of 10000 yuan agricultural output value (m3 / 10000 yuan) 1501.0332 1697.2982 1872.4269 1828.1523 1895.7581 1851.7288 1806.6300 the seven impact factor values in 2018 and 2019 in table 4 and the water consumption in corresponding years in table 4 constitute test samples to test the trained network. the test results are shown in table 6. table 6. prediction test and prediction results of water resources carrying capacity neural network model in zhaozhou county particular year water consumption (100 million m3) learning outcomes actual value error 2012 84.295853 87.67 -0.03849 2013 84.240055 84.17 0.0008317 2014 87.161970 87.164 -0.0000233 2015 89.900511 89.9 0.0000057 2016 93.712300 93.6993 0.0001387 2017 94.218207 94.3182 -0.0010602 2018 94.214266 97.22 -0.0303 2019 94.277907 / / the actual value of annual water consumption in 2019 is 9.722 billion m3, and the prediction value of neural network model is 942742.66 billion m3, with the accuracy of 96.97%; then, the data of the planning level year 2025 is input into the network for prediction, and the prediction result is that the water demand in 2025 will be 94277907 million m3. the change trend chart of predicted and actual water consumption in 2013-2019 is shown in figure 3. figure 3. variation trend of water consumption predicted by neural network model and actual value in 2013-2019 conclusions as an important natural resource, water resource is the key factor to maintain the integrity and virtuous cycle of ecosystem function, and is also the basic resource and strategic economic resource to maintain human survival and 177a prediction method of regional water resources carrying capacity based on artificial neural network social and economic development. in recent years, with the increase of population and the rapid development of social economy, the water problem has become increasingly prominent. as an important part of sustainable development and water resources security strategy research, water resources carrying capacity plays an important role in realizing the harmonious development of economic, social and environmental system, and is also the focus and hot issue in the field of water resources science research. in this paper, the artificial neural network is used to predict the regional water resources carrying capacity (1) based on the analysis of the general information of the study area, the water resources of the region are determined; (2) an evaluation index system of water resources carrying capacity suitable for the study area is constructed to give full play to the role of evaluation indexes; (3) the experimental results show that: this method can effectively predict the carrying capacity of water resources, and the prediction results fit well with the actual data, which has 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(2019). evaluation of water resources carrying capacity and its obstruction factor analysis: a case study of hubei province, china. water, 11(12), 2573.1-2573.14. doi:10.3390/ w11122573 https://doi.org/10.1007/s11269-018-1957-x https://doi.org/10.1016/j.jafrearsci.2019.02.031 https://doi.org/10.1109/tr.2018.2878387 https://doi.org/10.1109/ceect50755.2020.9298636 https://doi.org/10.1016/j.neucom.2018.09.036 https://doi.org/10.1016/j.neucom.2018.09.036 https://doi.org/10.1007/s11269-018-1992-7 https://doi.org/10.1007/s11269-018-1992-7 https://doi.org/10.1007/s40815-018-0534-z https://doi.org/10.1007/s40815-018-0534-z https://doi.org/10.1016/j.eswa.2018.02.026 https://doi.org/10.1016/j.knosys.2019.03.028 https://doi.org/10.1007/s11071-018-4118-0 https://doi.org/10.1007/s11071-018-4118-0 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.68391 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 173 182 m et eo r o lo g y detecting climatic and hydrological changes in gaoyou lake, eastern china, over the last millennium using pollen, grain size, and lithology records shuheng li1,2*, wei guo3, na gao1 , jianfeng du1,2 1. school of urban and environmental sciences, northwest university, xi’an 710127, china. 2. shaanxi key laboratory of earth surface system and environmental carrying capacity, northwest university, xi’an 710127, china. 3. department of environmental sciences and technology, xi’an jiaotong university, xi’an 710049, china. *corresponding author. e-mail: lish@nwu.edu.cn abstract: numerous lakes formed in the area where the huai river and the grand canal converged during the historical period, and it played a substantial role in the evolution of the regional environment. gaoyou lake is a vital detention reservoir in the lower reaches of the huai river. variations in hydrology and basin-wide environmental changes were analyzed based on pollen assemblages, grain size data, and lithology from sediment cores collected at gaoyou lake. this study focused on variations in the hydrodynamic processes of gaoyou lake that were responses to changes in climate, regional hydrological events, and human activity. the high percentages of pollen from terrestrial herbs and results of principal component analysis suggested that the environment of eastern china was arid from ad 900 to 1300 and that this corresponded to the dry conditions of the medieval climate anomaly. the subsequent period between ad 1300 and 1650 was more humid, as revealed by an increase in aquatic pollen and a decrease in grain size. a rise in xerophytic herb taxa (particularly artemisia and chenopodioideae) indicated that the climate became arid again from ad 1650 to 1850, during the little ice age. the modern period (ad 1850 onwards) was characterized by another increase in moisture, as reflected by a decrease in herb pollen and grain sizes. the shifts in the course of the yellow river in ad 1194 and ad 1855 were recorded by abrupt changes in lithology, sediment characteristics, and the percentages of alien pollen taxa. intense human impacts were demonstrated by cereal-type pollen and an increase in pollen of other cultivated plants such as cruciferae and poaceae. this study provides a foundation for further analysis of the dynamic mechanisms between lake development, climate change, and the effects of significant river changes. keywords: paleoclimate; lake sediment; last millennium; pollen; gaoyou lake. detección de los cambios climáticos e hidrológicos en el último milenio del lago gaoyou, al oriente de china, a través del polen, tamaño del grano y registros litológicos resumen varios lagos se formaron en el área donde convergen el río huai y el gran canal durante el período histórico, lo que juega un rol importante en la evolución del entorno regional. el lago gaoyou es un importante reservorio en los segmentos bajos del río huai. en este trabajo se analizaron las variaciones en hidrología y los cambios ambientales de la cuenca amplia con base en los grupos de polen, tamaño del grano y litología de las muestras de sedimento recolectadas en el lago gaoyou. este trabajo se enfoca en las variaciones de los procesos hidrodinámicos del lago gaoyou que fueron las respuestas a los cambios del clima, los eventos hidrológicos regionales, y la actividad humana. los altos porcentajes de polen de las hierbas terrestres y los resultados de los análisis de componentes principales sugieren que el ambiente de la china oriental fue árido entre los años 900 a 1300 dc y esto se corresponde con las condiciones de sequía del período cálido medieval. el período subsecuente entre 1300 y 1650 fue más húmedo, como se revela por un incremento en el polen acuático y en el decrecimiento del tamaño del grano. un incremento en el taxón de las hierbas xerófitas (especialmente artemisias y chenopodioideaes) indica que el clima se tornó arido de nuevo entre 1650 y 1850, durante la pequeña edad de hielo. el período moderno (de 1850 en adelante) se caracterizó por otro incremento en la húmedad, como se refleja en un decrecimiento del polen de hierbas y en los tamaños del grano. los cambios en el curso del río amarillo entre 1194 y 1855 se registran por las variaciones abruptas en la litología, las características de los sedimentos y los porcentajes en los taxones de polen alienígena. los impactos humanos intensos se demuestran con la ocurrencia de polen tipo cereal y un incremento en el polen de otras plantas cultivadas como las cruciferas y las poáceas. este estudio presenta una base para análisis posteriores de los mecanismos dinámicos entre el desarrollo del lago, el cambio climático y los efectos de los mayores cambios de los ríos. palabras clave: paleoclimate; sedimentos de lagos; último milenio; polen; lago gaoyou. record manuscript received: 18/10/2020 accepted for publication: 07/06/2022 how to cite item: li, guo, gao, & du. (2022). detecting climatic and hydrological changes in gaoyou lake, eastern china, over the last millennium using pollengrain size and lithology records. earth sciences research journal, 26(2), 173-182. https://doi.org/10.15446/esrj.v26n2.68391 https://doi.org/10.15446/esrj.v26n2.68391 mailto:lish@nwu.edu.cn https://doi.org/10.15446/esrj.v26n2.68391 174 shuheng li, wei guo, na gao, jianfeng du introduction the last millennium was a critical period for understanding environmental change because of the occurrence of major changes in climate, particularly during crucial periods such as the medieval climate anomaly (mca) (approximately ad 900-1300) (soon et al., 2003; gao et al., 2015; peteet et al., 2020) and the little ice age (lia) (approximately ad 1300-1850) (mann et al., 2009; sun et al., 2020). hydroclimatic variability demonstrates significant regional differences divided by the summer monsoon boundary in china compared to increasing trends in temperature (chen et al., 2010; chen et al., 2015). most researches analyzing the variability of moisture and precipitation in eastern china over the last 1000 years have been based on historical documents (zheng and wang, 2005; zheng et al., 2006). multi-proxy records from lake sediments are main sources of evidence for paleoenvironmental changes. sedimentary lithology gives an overview about the compositional variability of the sediment records and the occurrence of unconformities. palynological investigations of lacustrine sediments are useful tools for reconstructing past vegetation and hydrologic changes in lake ecosystems that were responses to variations in climate (djamali et al., 2009). however, reconstructing climate based solely on pollen analysis could be biased due to human influence (li et al., 2014). the sizes of lacustrine sediments grains are controlled primarily by water dynamics and sediment sources, which in turn are influenced by various factors such as local climate changes, regional hydrological transitions, and human activity (lerman, 1978). therefore, the environmental significance of variations in grain size is complex. the combined study of lithology, pollen assemblages and grain size has the potential to provide different yet complementary information about the paleoenvironment (li et al., 2009). during the historical period, several lakes have developed in the area where the huai river, the yellow river, and the jing-hang grand canal converge. this vitally contributed to the water conservation, flood control, and social economy development. multi-proxy limnologic studies of these lakes using different time scales revealed that the evolution of the regional environment was frequently affected by the interaction between the large rivers, which formed the remaining lacustrine deposits, creating important archives for use in the study of the regional hydrological and environmental evolution (he et al., 2005; zhang et al., 1999; xia et al., 1995; chen et al., 2013). located in the lower reaches of the huai river and west of the jinghang grand canal, gaoyou lake is the sixth largest fresh-water lake of china (fig. 1). early studies largely based on historical records show that gaoyou lake was comprised of lagoons during the early holocene (ling, 2001). the area of gaoyou lake was diminished to a shallow bay approximately 8,000 bp, when sea transgression reached its shorelines. interfluve land was present between the gaoyou and baoying counties until 6,000 bp. in the historical period, gaoyou lake was part of the waterways of the yellow river and huai river and served as the water resource to the grand canal. as a result, the evolution of gaoyou lake was closely linked with the large river system and regional climate changes in eastern china, which made it an ideal site for the study of the regional hydrological and environmental evolution. researches during the modern period have studied the changes in the water area, biological characteristics, and water quality of gaoyou lake (wei et al., 2017; guan et al., 2019; zhang et al., 2020). however, the paleoenvironmental studies of gaoyou lake sediments have not received adequate attention. our research team combined grain size, magnetic susceptibility, and geochemical proxies in gaoyou lake sediments to infer regional climate fluctuations (li et al., 2007; li et al., 2015). yet, biological proxies such as pollen, one of the most sensitive paleoenvironmental indicators, have not been studied. this study presents the first pollen records collected from gaoyou lake, as well as grain size and lithology analyses, to provide a better understanding of the lake evolution and to deduce regional climate changes during the last millennium. the exploration of the interaction between the lake development, climate changes, and large river effects will provide scientific basis for regional ecosystem protection and construction. the aims of this paper can be summarized as: 1. reconstructing the climatic and hydrological changes of gaoyou lake during the last millennium (past 1,100 years), 2. detecting the evidence of human activities and historical events such as the course shifts of the yellow river from the core sediments of gaoyou lake. study area gaoyou lake (32°42’n–33°04’n, 119°06’e–119°25’e) spans approximately 39 km from north to south and approximately 17.3 km from west to east. it covers a 674.7 km2 area, with the lowest elevation at 3.3 m above sea level (a.s.l) and an average elevation of 4.4 m a.s.l of the lake bottom. it has an average water depth of 1.4 m and maximum depth of approximately 2.4 m (wang and dou, 1998). the average annual runoff into the lake is 2.2 × 1010 m3. water from the huai river contributes 95% of the total input for the south of the lake via the water way to the yangtze river (chen et al., 2017). several small creeks and streams–such as the tonglong river, xincun river, baita river, qinnan river, and wangqiao river–also feed the lake (fig. 1). figure 1. (a) map showing the location of gaoyou lake in china. (b) the core sampling sites of gaoyou lake the current catchment area has a northern subtropical monsoon climate, with an average annual temperature of 16.7℃ and average annual precipitation of 1,029 mm according to the meteorological records at gaoyou from 1981 to 2010 (fig. 2). precipitation during the monsoon season (june-september) accounts for 56.4% of the annual total. july (mean temperature of 27.2 ℃) and january (2.8℃) are the warmest and the coldest month, respectively. the zonal vegetation in the catchment area is a mixed forest consisting of deciduous and evergreen broad-leaved trees. the current vegetation around gaoyou lake is mostly composed of farm and economic crops. trees in the genera salix, ulmus, populus, and paulownia are commonly used in artificial cultivation (wang and yang, 1990). there are 38 genera of planktonic algae in gaoyou lake, and these are dominated by chlorophyta (40.0%), cyanophyta (24.2%), and bacillariophyta (13.64%) (wei et al., 2017). figure 2. meteorological observations of gaoyou lake (1981-2010). data collected from the national meteorological information center of china (http:// data.cma.cn) materials and methods coring, dating and grain size in november 2010, four cores (gy05, gy06, gy07, and gy08) were taken from gaoyou lake using a piston corer within a 2-m long and 80-mm diameter fitted steel coring tube. the cores were transported to the laboratory and stored at 4 °c. each core was split, and the sediments were photographed 175detecting climatic and hydrological changes in gaoyou lake, eastern china, over the last millennium using pollen, grain size, and lithology records and described in detail. one half of each core was archived and the other half was used for analysis. in this study, we focused on the pollen, particle size and lithology analysis of samples from core gy08 (158 cm long) collected from the central part of the lake (32°53’36”n, 119°21’6”e) in 1.78 m of water. the lithology of core gy07 (32°55’6”n, 119°19’39”e), collected at a water depth of 1.8 m and located approximately 12 km from gy08 was described and the core was sampled for grain size and chronological control analysis. grain size analyses were completed for 186 samples from gy08 at 0.5cm interval along the top 30 cm, and 1-cm intervals between 30 and 158 cm. h2o2 and hcl were added to each sub-sample prior to analysis to remove organic and inorganic carbon. then, the sediment was dissolved in a (napo3)6 solution for 24 h. grain size distributions were determined by a laser particle size analyzer (mastersizer 2000, malvern panalytical, united kingdom). the grain size parameters were calculated using the excel macro-program gradistat (blott and pye, 2001), based on the following equations s1 (folk and ward, 1957). the age-depth model previously developed by li et al. (2015) for the cores sampled at gaoyou lake was used for the study. in total, six calibrated accelerator mass spectrometry (ams) 14c dates were presented in this study in two batches. radiocarbon ages of four samples from core gy 07 were determined at the beta analytic radiocarbon dating laboratory in miami, usa using ams in 2013. two samples (one from core gy07 and one from core gy08) were dated using the ams system at the xi’an accelerator mass spectrometry center, china, in 2020 (table 1). the radiocarbon dates were calibrated using the software calib 6.0 and the intcal20 curve. table 1. radiocarbon chronology from gaoyou lake sediments depth /cm core dated materials δ13c (‰) calibrated 14c age 2σ /a bp (ad) lab site lab no. 7 gy07 shells -11.6 modern miami 326289* 98 gy07 shells -9.7 660-550 bp (ad 12801400) miami 326291* 150 gy07 bulk sediments -28.1 800-690 bp (ad 11601260) miami 320812* 160 gy07 bulk sediments -27.6 1060-1030 bp (ad 900-920) miami 320811* 84 gy07 shells -6.8 670-630 bp (ad 12901320) xi’an xa50651 138 gy08 bulk sediments -25.5 800-765 bp (ad 11551185) xi’an xa50652 the data of the samples with * were quoted from li et al., 2015. pollen analysis pollen samples were collected at 5-cm interval. therefore, 30 samples were obtained with approximately 150 pollen grains in each sample. the samples were processed following the standard hf technique (fægri and iversen, 1989). this involves treating 1-2 g of sediment with 10% hcl, 10% koh, and 36% hf to remove carbonates, humid acids, and silicate, respectively. lycopodium tablets were added to the samples to calculate the pollen concentrations. pollen was counted and identified using an olympus bx51 microscope at the nanjing institute of geology and paleontology, chinese academy of sciences. pollen percentage and concentration calculations were performed using the software tilia 1.5.12. only major pollen taxa with an occurrence frequency exceeding 2% in at least three samples were included in the numerical analyses and the pollen diagram. the coniss program was used to divide the pollen diagram by constrained minimum-variance cluster analysis based on percentages of pollen (grimm, 1987). pca and dca analysis principal component analysis (pca) is considered a powerful data reduction technique that produces a visual scatterplot for the qualitative assessment of the resemblances and differences among samples (ter brakk, 1988; jiang et al., 2013; geng et al., 2019). to ordinate the pollen data from core gy08 and detect patterns of vegetation development, detrended correspondence analysis (dca) and pca were conducted based on the square root transformed pollen percentage data using canoco 4.5. dca was initially performed to estimate the underlying linearity of the data. the preliminary dca yielded a gradient length of 1.176 (< 2.0) standard units, which indicates a possible linear response of pollen assemblages to environmental variables. therefore, pca was chosen to ordinate the pollen data for the gaoyou lake sediment cores. the pollen and spore data were reduced to a two-dimensional plot and the resulting data set was displayed as a plot for samples and taxa based on pca. figure 3. lithology, grain size, and ams 14c dates at gy07 and gy08. data of gy07 were extracted from li et al., 2015 figure 4. age-depth model for the core gy08 and core gy07 from gaoyou lake sediments. data of core gy07 were quoted from li et al., 2015 results core lithology and chronology the lithology of gy08 has obvious layers, which may resulted from the changing paleosedimentary environment, paleoclimate and different material components. the upper 50 cm of gy08 was composed of brown and grey mud, with plant remains at the top 0-15 cm. the 50-100 cm tier had a grey and brown mud layer, and had horizontal-wavy bedding at the 80-90 cm tier. the 100-137 cm tier was dark grey to grey clay with a lot of shells and other organic fragments at the bottom of the segment. the 137-158 cm tier was grayish black and dark brownish clay, containing rich brownish organic matter 176 shuheng li, wei guo, na gao, jianfeng du and humus. figure 3 showed the obvious stratigraphic similarities between cores gy08 and gy07. core gy07 consisted of brownish-grey mud, grey clay, and dark brownish clay. the core showed wave and horizontal bedding at 85 cm, contained plant fragments on the top segment, and contained abundant shells at 144-161 cm. both cores had the dark organic layer at the bottom (137158 cm at gy08, 144-161 cm at gy07), marked as a characteristic layer for corresponding relation between the two cores. li et al. (2015) presented an age-depth model of the sediment sequence that was based on four ams radiocarbon dates obtained for gy07 in 2013 (table 1). to establish a more accurate chronology, two more samples (one from each gy07 and gy08) were used to supplement that ams dating in 2020. here, the chronology of gy08 was developed by extrapolating the calibrated radiocarbon dates from gy07 to gy08, which was corroborated by the date at 138 cm in gy08 (fig. 4). core gy08 was formed in the 1,100 year period from ad 900 to 2010, at an average sedimentation rate of 0.15 cm/yr. the sedimentation rate was especially high during ad 1200-1400. the increase of depositional rate was assumed to be related with the result of the course change of the yellow river in ad 1194 which brought heavy sediment into gaoyou lake. the interpretation was also justified by the abrupt increasing of particle size and pollen concentration, as discussed in detail below. grain size the sediment sequence of gy08 was dominated by silt, which had a mean content of 84.5%. the remainder consisted of clay, fine sand, and medium and coarse sand–the latter two found in very small amounts. the grain size data showed a relatively stable median value and variable coarsest value. the sand fraction fluctuated greatly, varying between 0 and 40.7% at an average of 6.2%. the mean grain size of the sediment samples ranged from 4.4 to 37.2 μm, with an average of 12.5 μm. the value of sorting varied between 2.1 and 7.3, with a mean value of 3.6. the value of skewness changed from -0.5 to 0.6, averaging -0.1 while kurtosis oscillated between 2.3 and 0.8, with an average of 1.1(table 2). the results indicate that the particle sorting was poor and finer relatively. the sediment distribution was asymmetric which shows that the samples may be combined from multiple sources. according to the results of the grain size analysis, suspended loads are the main component of sediments in gaoyou lake, whereas strong inflow occurred constantly. figure 5 showed several obvious variations of most parameters at depths around 100-135 cm, 65 cm, and 20 cm indicating fluctuant sedimentary environment. table 2. the maximum (max), minimum (min) and average values of clay (%), silt (%), sand (%), mean (μm), sorting, skewness and kurtosis of gy08 value clay (%) silt (%) sand (%) mean (μm) sorting skewness kurtosis max 23.0 96.0 40.7 37.2 7.3 0.6 2.3 min 2.9 56.4 0 4.4 2.1 -0.5 0.8 average 9.2 84.5 6.2 12.5 3.6 -0.1 1.1 figure 5. mean (μm), clay (%), silt (%), sand (%), sorting, skewness and kurtosis of gy08, based on the grain-size data figure 6. percentage pollen diagram for gy08 sediment core 177detecting climatic and hydrological changes in gaoyou lake, eastern china, over the last millennium using pollen, grain size, and lithology records pollen record a total of 96 pollen and spore taxa were identified. to interpret the pollen spectra, all pollen types were classified into five eco-groups according to the life form of their parent plants. these eco-groups included tree and shrub pollen (including pinus, quercus, cyclobalanopsis, ulmus, trema, celtis, liquidambar, salix, carpinus, and pterocarya); terrestrial herb pollen (including poaceae, asteraceae, chenopodioideae, cruciferae, artemisia, polygonum, fagopyrum, plantago, sesamum indicum, and zea mays); aquatic pollen (including typha and cyperaceae); fern spores (including selaginella, hicriopteris, pteris, dicranopteris, ceratopteris); and algae (including pediastrum and concentricystes). the photographs of the main pollen, spores, and algae in core gy08 are presented in figure s1. overall, terrestrial herb and tree & shrub pollen were the dominant pollen types in the entire profile with an average percentage of 46.5% and 34.8%, respectively. the pollen data are shown in figure 6. based on the changes of pollen types, percentages and concentrations of the major taxa, and coniss results, the pollen spectra can be divided into four pollen assemblage zones. the lowest pollen zone (phase a, 158-100 cm) is characterized by the lowest percentage of tree and shrub pollen with the average value of 31.2%. out of the 27 trees and shrubs pollen taxa identified, only 14 taxa were present in phase a including pinus, quercus, cyclobalanopsis, and ulmus. terrestrial herb pollen dominated with an average percentage of 50.3% and consisted mainly of chenopodioideae (4.12%) and artemisia (3.45%). among fern spores, ceratopteris reached its highest percentage in this zone. pollen concentrations ranged from 26 to 195 grains/drop, with an average concentration of 58.3 grains/drop. phase b (100-45 cm) was notable for a clear increase in pollen contents for most taxa. the contents of aquatic pollen (12.3%) and fern spores (11.0%) significantly increased compared to the entire core. several pollen types first appeared in this zone, such as zea mays, sesamum indicum, liquidambar, salix, and pediastrum. herb decreases to 39.0%, but they were still the dominant pollen. pollen concentrations varied from 27 to 176 grains/drop, with an average concentration of 57.9 grains/drop. the remarkable characteristic of phase c (45-20 cm) was the terrestrial herbs pollen percentages increase to their highest average value of 51.3% at the expense of the fern spore percentages (6.6%). pediastrum disappeared in this zone. the percentage of cyperaceae decreased to its minimum of 2%, while the percentages of artemisia (10.4%) and chenopodioideae (6.7%) increased to their peak values over the whole core. the average pollen concentration was at the lowest value. herbs were still the dominant pollen in phase d (20-0 cm) with percentages slightly decreasing to 49.7% compared to phase c. tree and shrub pollen percentages increased to 37.3% with relatively high content of quercus (2.7%), while the percentage of ulmus dropped to 2%. the percentages of fern spores and algae both increased in comparison to phase c, with the average values being 8.1% and 0.8%, respectively. the average pollen concentration increased to the highest value. pca analysis the result showed that cyperaceae, polygonum, typha, ceratopteris and other algae adapting to wet environments tend to spread in the positive values of the first axis, while artemisia, chenopodioideae and asteraceae–which prefer an arid environment–were distributed towards the opposite direction. this indicated that the first principal component mainly reflected the changes in humidity, with the samples on axis 1 showing positive values related to wet conditions and negative values related to dry conditions (fig. 7). taxa such as dicranopteris, cruciferae, selaginella, labiatae, sesamum, and pediastrum were influenced strongly by human agricultural activity (li et al., 2008; xie et al., 2008). they were mainly distributed at positive values along the second axis, implying that the second axis may represent the intensity of human activity. the pca of the pollen assemblages of the luonan basin also showed that the first axis reflected the variation in humidity and the second axis may relate to human activity (zhang et al., 2014). figure 7. results of pca analysis in the pollen record of core gy08. liq(liquidambar); hic(hicriopteris); sal(salix); que(quercus); sesi(sesamum indicum); oth-t(other trilete spore); oth-m(other monolete spore); sel(selaginella); ped(pediastrum); lab(labiatae); cru(cruciferae); dic(dicranopteris); typ(typha); pla(plantago); zea-(zea mays); tre(trema); cel(celtis); pte(pterocarya); art(artemisia); ulm(ulmus); hum(humulus); cyc(cyclobalanopsis); che(chenopodioideae); fag(fagopyrum); com(compositae); pin(pinus); pteris(pteris); cer(ceratopteris); car(carpinus); poa(poaceae); oth-a(other algae); pol(polygonum); cyp(cyperaceae) discussion proxy interpretation and discussion the assessment of lacustrine sediment lithology may provide a clue to reconstruct lake levels and infer environmental conditions. a highly variable local environment was suggested by rapidly changing lithology. in terms of sediment transport dynamics, the presence of coarse grains in the lithology may reflect climate change or results of mass movements (kliem et al., 2013). moreover and most important, the sedimentation lithology offers the detection of flood deposits, which are marked by a high content of soil organic matter and appear as dark-brown layers that can be easily differentiated from the background (simonneau et al., 2013; zhang et al., 2015). particle size analysis has been used as a well utilized tool for reconstructing paleo-sedimentary environments based on grain size parameters such as mean diameter, sorting, skewness and kurtosis (folk and ward, 1957; folk, 1974; sun et al., 2021). grain size of the lacustrine sediment is related to hydraulic processes associated with transporting, sorting and deposition (guo et al., 2021). the particle size chiefly depends on the hydrodynamic energy of the lake water, which is strongly linked to water level, energy of the input, and the source domains (li et al., 2009). on one hand, grain size can be regarded as a lake level index that is sensitive to regional hydrological conditions. previous studies (chen et al., 2003) show that during wet periods, water levels rise and lakes expand. hence, the distance between the lake shore and coring site would increase. it is more difficult for coarse particles to reach the core site, which leads to a decrease in grain size. conversely, during dry periods, coarse sediments can more easily reach the coring site due to the diminished lake area. as a result, the mean value of particle size increases. on the other hand, transport ability of runoff was important factor that controls the grain size of sediment materials. the change of sediment source, intensity of erosion, human activities could affect the particle size by modifying the runoff transport. thus, grain size has 178 shuheng li, wei guo, na gao, jianfeng du been widely used in the studies to trace and separate material sources (ghilardi et al., 2008; jia et al., 2012; sun et al., 2015). identification of the sources of the pollen types is essential for interpretation of the pollen signal and reconstruction of palaeovegetation. the evolution of gaoyou lake is affected by the interaction of the river-lake system, thus the pollen of the lacustrine sediments may have multiple sources. the waterborne pollen by the inflowing rivers may also make a substantial contribution to the pollen influx to the lake. thus, pollen in alluvium originates from both the realm of the rivers and the lake itself. former studies of alluvial pollen on the north china plain demonstrated that most of the arboreal pollen which derived from mountain trees, such as pinus, picea, quercus, betula, tilia and castanea, were transported by rivers from the mountains to the plain. by contrast, the appearance of salix, ulmus, populus and ailanthus, which were treated as plain trees, may derived from the plain itself. most of the nonarboreal pollen were derived from the plain around the lakes. high percentages of pinus pollen can be used as an indicator of flooding because the larger and heavier pollen grains were mostly deposited by flood. artemisia and chenopodioideae pollen percentages mainly have close relations with ecological regions (xu et al., 2007). cyperaceae pollen has a strong correlation with plant cover. vegetation and climate reconstruction using pollen records are often based on the presence and percentage abundance of selected indicator taxa (herzschuh et al., 2009). artemisia spp. and chenopodioideae are widespread in temperate regions of the northern hemisphere. they are common in temperate steppes, meadows, and uplands especially in dry areas (zhao et al., 2021). thus, increases in their pollen percentages may indicate climate changes in more arid conditions. pediastrum spp. is green algae common in freshwater and lives in shallow ponds, lakes, or small rivers. when pediastrum increases, it suggests a warm and humid climate, or increased nutrients in the lake system (tong et al., 2009). pollen assemblages are the proxies commonly used in paleoclimate reconstruction to identify human influences on the environment. detecting human impacts via pollen analysis is based not only on abrupt changes in pollen concentration and richness, but also by the occurrence of cereal-type and cropland weed pollen (li et al., 2008). cereal-type pollen has been used as an ideal indicator of human activity. the occurrence and increase of cerealtype pollen such as rice, corn, and wheat in sediments is direct evidence of an agricultural area (djamali et al., 2009). poaceae pollen is ubiquitous and widely used as a paleoclimatic indicator for increased regional aridity. the duration and intensity of human occupation are also variables affecting the percentages of poaceae pollen (bush, 2002). lake evolution and environmental reconstruction a synopsis of the interpretation of grain size and pollen combined with core lithology results in the following most probable scenario of the paleoenvironmental history of gaoyou lake. during stage a (158-100 cm; ad 900-1300), the percentages of tree and shrub pollen were the lowest of all phases. terrestrial herbs such as artemisia, asteraceae, and chenopodioideae showed relatively higher percentages than the average values of the whole core, indicating a drier climate. in addition, the low humidity was implied by negative pca-1 scores. the particle size data from gaoyou lake were characterized by marked fluctuations. based on the principle of sediment sorting, the high and greatly changed coarse component suggested a low and fluctuating lake level. the synthesis of the proxies indicated low water level and unstable climate during the deposition of this layer. this relatively dry interval can be attributed to an arid situation in the southern part of eastern china during the mca period. the results were in agreement with previous studies (zheng et al., 2006; mann et al., 2009). ge et al. (2014) also found the dry period to occur during the northern song, southern song, and yuan dynasties (ad 960-1368) in eastern china. the lithology of the bottom part of this layer consisted of dark brownish clay with a large amount of organic matter and humus. previous studies demonstrated that the color of the alluvial sediments appeared brown, which is a notable characteristic thereof, and a layer with abundant biogenic debris is a signal of flood events (sirocko et al., 2013). according to historical documents, the yellow river battered its banks and began to shift southward, encroaching the huai river in ad 1128, due to human disturbance for military purposes (xu, 1993). as a result, the yellow river took over the lower channel of the huai river heading for the yellow sea in ad 1194 (fig. 8). since then, a huge sediment load from the yellow river has entered gaoyou lake. the change of material sources also led to the change of pollen concentration. consistent with the result, figure 5 and figure 6 showed the increase in sand content and pollen concentration around ad 1200. former researchers found that suspended sand increases the buoyancy of the river water. due to the high sand content of the water from the yellow river, it contains more pollen (xu et al., 1996). figure 8. sketch map showing the yellow river’s course changes since ad 900 (after xu, 1993). 1. the channel after the major course change in ad 1048; 2. the channel after the major course change in ad 1194; 3. the channel after the major course change in ad 1494; 4. the channel after the major course change in ad 1855 stage b (100-45 cm; ad 1300-1650) was separated from the previous zone by the clear increase in the proportion of aquatic pollen, accompanied by the decreasing value of mean particle size, both indicating that climate conditions were more humid. positive pca-1 denoted that the climate became wetter at this stage. it was consistent with previous studies that relatively warm and humid climate prevailed in north jiangsu before the end of 16th century (chen et al., 1998). historical documents recorded 91 heavy floods during the yuan and ming dynasties (ad 1279-1643) (yang and wang, 1995). the presence of cereal-type pollen such as zea mays (65 cm, around ad 1530) and sesamum indicum (90 cm, around ad 1340) is important as it may mark the expansion of agricultural activities during this period. productive american crops such as maize were introduced and popularized in china at the beginning of the sixteenth century, which was confirmed by the peak in maize pollen percentage at 65 cm (around ad 1530). the earliest recorded planting of maize in china can be found during ad 1511 in “ying state records”, the neighboring anhui province. the record of maize cultivation in the gaoyou lake basin first appeared in the “xinghua county annals” in ad 1559 (zhang, 1990). few pine species are found in the modern vegetation of the lake basin. consequently, pinus pollen mainly should be an alien component transported to gaoyou lake. in this study area, fluvial flow is more capable than wind to transport large quantities of exotic pollen over long distances, especially for pinus pollen because of its larger size. as a result, increase of pinus spp. content suggested that the enhanced fluvial flow or the regional climate turned wet (jiang et al., 2013). the high pinus percentage of this stage indicated a more exogenous input to the lake. since ad 1194, the yellow river has encroached the huai river basin. as a result, huge quantities of water with high sediment 179detecting climatic and hydrological changes in gaoyou lake, eastern china, over the last millennium using pollen, grain size, and lithology records load from the yellow river have poured into the lower reaches of the huai river, leading to a rapid expansion of gaoyou lake (xu, 1993). finally, the lake enlarged gradually and amalgamated to form the present gaoyou lake around ad 1600 (liao, 1992). stage c (45-20 cm; ad 1650-1850) was marked by an upsurge in terrestrial herb taxa, especially dry-tolerant plants such as artemisia, asteraceae and chenopodioideae. the negative pca-1 scores, extreme low percentages of aquatics, and absence of algae indicated further drying during this stage that was apparently connected with the lia. previous research demonstrated that the lia was characterized by a cold and dry climate throughout china from the late 16th century until the end of the 19th century, especially in the midseventeenth century (zhu, 1973; chen et al., 1998). the abrupt decrease of mean particle size and increase of sand content at 20 cm in the core (dated around ad 1860) was prominent in this stage. it coincided with the decrease in percentages of several types of tree pollen, such as quercus, ulmus, celtis, and pinus. this transition may be due to the northward shift of the yellow river in ad 1855. historical documents showed that the yellow river broke its northern bank at tongwaxiang and changed to a northeast course into the bohai sea in ad 1855 (fig. 8), ending a 700-year long occupation of the huai river (xu, 1993). thus, proportions of coarse particles dropped sharply. the reduced stream inflow may have led to the decreased percentage of exotic tree pollens. stage d (20-0 cm; ad 1850-2010) was separated from the preceding zone by the notably decreasing value of the mean particle size and the most positive pca-1 scores, suggesting a more humid period. the decreased herb pollen percentages as well as the increased proportions of fern spores and algae also confirmed this inference. the anomalous warming over china in the 20th century is well known (yang et al., 2002). previous studies also reported that moister conditions in china occurred since 1890 (peng et al., 2009). the most conspicuous feature of this stage was the appreciable increase in anthropogenic herbs, such as fagopyrum and sesamum indicum, indicating intensified cereal cultivation. the high amounts of poaceae and cruciferae were also evidence of human disturbance (li et al., 2008). the rise of pediastrum is interpreted to be related to increased nutrients in the lake. it is documented that lake reclamation in the gaoyou lake area started in 1857 (wang and yang1990). in 1988, the area of gaoyou lake shrank by 25% compared to the 1950s. the population had almost doubled during the four decades from the 1950s to the 1990s (li et al., 2011). as a result, water quality has degraded since the 1990s with accelerated urbanization. conclusion 1. this study indicated that the main component of the sediments in core gy08 was silt. terrestrial herbs were the dominant pollen in the entire profile of core gy08. 2. pollen assemblages, sediment grain size and lithology of core gy08 recorded four main phases in the climatic and hydrological changes of gaoyou lake over the last millennium. during ad 900-1300, the climate was dry, and gaoyou lake experienced a stage of low water levels. the lake began to expand under more humid conditions after ad 1300. subsequently, the water levels dropped because of the dry climate during ad 1650-1850 in conjunction with the lia. after ad 1850, a wet climate prevailed in the study area. 3. major regional events such as course changes of the yellow river during ad 1194 and ad 1855 were reflected in abrupt variations of sediment characteristics and the changing percentage of river-transported pollen from exotic plants. the first appearance of maize pollen during ad 1560 as well as the increase in pollen of plants associated with human activities in the modern era reflected a widespread human interference with the natural vegetation and an intense cultivation of the lake area. acknowledgements the authors acknowledge the national natural science foundation of china (40901101) for funding the research. we thank the reviewers for their valuable comments and suggestions. dr. yin yong, tang wei, li yanjun and li haiqing are acknowledged for their assistance during field work and experimental measurements. we are grateful to dr. mao mili for the support of pollen analysis. thanks are also extended to professor yue ming for help with plant taxonomy. the authors also would like to thank wang jiachuan and yang qi for their kind help in improving the figures. references blott, s. j., & pye, k. 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factors that influence methane production. the shallow reservoir is located in a tension zone, while the deep reservoir is located in both a transformation zone and a compression zone. although the reservoir and closure pressures increase with the burial depth, the pressures fluctuate in the deep reservoir, especially in the transformation zone. this fluctuation influences the opened degree of the fractures in the reservoir. the effective stress is lower in the deep reservoir than in the shallow reservoir, leading to higher permeability in the deep reservoir. this difference in effective stress is the key factor that influences the methane production. the combination of coal thickness and gas content also significantly influenced the methane production. influenced by the reservoir and closure pressures, the type iii coal in the shallow reservoir is more developed, while the deep reservoir contained more developed type i and type ii coal. the permeability increases exponentially with increasing thickness of type i and type ii coal, which determines the high reservoir permeability in the deep reservoir. the development of type iii coal leads to the poor reservoir hydraulic fracturing effect. however, a reservoir with thick type i and type ii coal can have a positive effect. con una profundidad de enterramiento de 1000 metros, el yacimiento de carbón del bloque yanchuan sur, en china, se divide en dos: el depósito profundo y el depósito superficial. este trabajo combina los datos de la información de producción de gas metano asociado carbón, la interpretación de registros de pozo, las simulaciones numéricas de metano asociado a carbón y las propiedades del reservorio para encontrar las características de producción a diferentes profundidades. los resultados indican que el espesor del carbón y el contenido de gas no son factores que alcancen a influir en la producción de metano. el depósito superficial se encuentra en una zona de tensión, mientras el depósito profundo está ubicado en una región tanto de transformación como de compresión. aunque el reservorio y la presión de cierre se incrementan con la profundidad de enterramiento, las presiones fluctúan en el depósito profundo, especialmente en la zona de transformación. esta fluctuación influye en el grado de apertura de las fracturas en el depósito. la tensión efectiva es más baja en el depósito profundo, lo que significa una mayor permeabilidad. la diferencia en la tensión efectiva es el factor clave que incide en la producción de metano. afectado por las presiones de cierre y del yacimiento, el carbón tipo iii en el depósito superficial está más desarrollado, mientras que el depósito profundo contiene carbón tipo i y tipo ii más desarrollado. la permeabilidad se incrementa exponencialmente con el incremento del espesor en el carbón tipo i y tipo ii, lo que determina la alta porosidad en el depósito profundo. el desarrollo de carbón tipo iii lleva a un pobre efecto de la fractura hidráulica en el depósito. sin embargo, un depósito con carbón tipo i y tipo ii espeso podría tener un efecto positivo. earth sciences research journal earth sci. res. j. vol. 20, no. 3 (september, 2016) : d1 d9 abstract resumen keywords: burial depth, cbm productivities, reservoir pressure, closure pressure, permeability palabras clave: explotación en profundidad, producción de gas metano asociado a carbón, presión de depósito, presión de cierre, permeabilidad. influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china record manuscript received: 13/06/2016 accepted for publication: 02/09/2016 how to cite item li, t., & wu, c. f. (2016). influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china. earth sciences research journal, 20(3), d1-d9. doi:http://dx.doi.org/10.15446/esrj.v20n2.58000 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n2.58000 teng li, cai-fang wu h y d r o c a r b o n s influencia de la presión, la estructura del carbón y su permeabilidad sobre la productividad de gas metano de carbón en profundidades de enterramiento del bloque yanchuan sur, china teng li, cai-fang wud2 1 introduction the production of coalbed methane from a single well is an important target to evaluate the success or failure of the coalbed methane development. however, different coalbed methane wells within the same block have different productivities. research on the factors that influence coalbed methane production show that it is a result of a coupling action, and the major factors can be restricted to three types: geological, development techniques and production management. geological factors include the reservoir burial depth, thickness, gas content, permeability, reservoir pressure, tectonic and hydrogeological conditions, all of which can radically influence the reservoir properties (kaiser et al., 1994; pashin and groshong jr., 1998; karacan et al., 2008; yao et al., 2009a; lv et al., 2012). permeability is a major factor used to evaluate the production of coalbed methane from a single well (li et al., 2011). the natural fractures in the coal are passageways for gas movement (perera et al., 2013a). the movement of gas is the result of a coupling of gasliquid-solid flows (wu et al., 2014), which has a great deal to do with the gas pressure and concentration (thararoop et al., 2012). the coal matrix swells with increasing gas pressure, especially under low gas pressure conditions (approximately 3.5 mpa), leading to the reduction in coal permeability (perera and ranjith, 2012). although the coal matrix becomes weaker with higher gas pressure, the newly created micro-fractures provide space for gas adsorption, which further enhances the coal matrix swelling (viete and ranjith, 2006). however, the increased liquid pressure may result in shrinking of the coal matrix (perera and ranjith, 2012). tectonic movements can promote coalbed methane preservation and secondary hydrocarbon generation (yao et al., 2009a), but the deformation of the coal mass structure associated with these movements should not be ignored (li, 2001; li et al., 2003). the thin-skinned structure can influence the production of coalbed methane wells (pashin and groshong jr., 1998), and the impermeable faults also affect the flow of gas and water from the coal reservoir to the well bores (karacan et al., 2008). aside from tectonic movements, gas pressure can also deform the coal mass structure, with higher gas pressure being associated with more serious deformation of the coal structure (perera et al., 2013b). the mechanical properties of the coal also contribute to the permeability, mainly as a result of the dynamic young’s modulus during the adsorption of gas, especially regarding the adsorption of co2 (perera et al., 2013c). when the gas desorbs from the surface of the coal matrix, the elastic modulus controls the coal-stress sensitivity (karacan, 2009; tao et al., 2012). research on the pore structure of 34 coal samples with various maturity from north china has shown that coal permeability is close to the coal pore fractal, especially for middleand high-ranked coal (ro,max = 1.47-4.21 %). this permeability may be influenced by the seepage pores and degree of coalification (yao et al., 2009b; meng and li, 2013). the production of coalbed methane is a coupled process, that involves the desroption and flow of methane, deformation of the coal structure, dynamic change of coal porosity and permeability (mazumder et al., 2012; tao et al., 2012; liu and harpalani, 2013; peng et al., 2014; qu et al., 2014). additionally, any external fluid could influence the reservoir permeability during fracturing (gentzis et al., 2009). essentially, dynamic permeability implies that the state of the coal reservoir varies from the initial to the final time periods (qu et al., 2014). during the production process, the effective stress of the coal reservoir and the shrinking of the coal matrix will influence the absolute permeability of the reservoir (durucan and edwards, 1986; laubach et al., 1998; chen et al., 2010; liu et al., 2010; tao et al., 2012; liu and harpalani, 2013). the permeability of the fairway wells in the san juan basin varies by approximately 100 times over the production period (palmer, 2009; mazumder et al., 2012). a study on the production of the coalbed methane wells in the south qinshui basin showed that the burial depth of the coal reservoir, the bottom bore pressure and the dewatering rate are the three main factors that control the effective stress of the coal reservoir, which further influences the absolute permeability (tao et al., 2012). when methane outputs from the bottom of the bore, the extensive underground water does not favor a reduction in pressure, and this water contributes to the groundwater flexibility energy (wu et al., 2007). this energy can keep the fractures in the coal reservoir open, thereby increasing the permeability of the reservoir, and increasing the performance of the coalbed methane wells (somerton et al., 1975; st. george and barakat, 2001; yao et al., 2009b). the decrease of bottom bore pressure promotes the desorption of coalbed methane, while the rapid decrease of pressure may lead to the gas locking in the reservoir, resulting in reduced flow rates (tao et al., 2012). the effective stress increases with increasing burial depth (mazumder et al. 2012), and permeability is more reduced in a deep reservoir is higher than that in a shallow reservoir (tao et al., 2012). conversely, lv et al. (2012) reported that methane production in a deep coal reservoir is higher than that in a shallow coal reservoir, mostly as a result of the coal thickness and gas content. however, this study did not consider the change in stress state with increasing reservoir burial depth. in the south yanchuan block, with a burial depth of 1000 m serving as the demarcation, the reservoir stress characteristics have changed from σv > σhmax > σhmnin to σhmax > σv> σhmnin (wu, 2012). this finding means that the reservoir stress state has changed from tensile to compressive, and there has been an obvious change in coalbed methane production. the methane production at various depths is differs, and this may be influenced by the change in reservoir stress state. in this paper, using the south yanchuan block as an example, the reservoir properties that influence coalbed methane production at various depths are discussed, such as pressure and the production characteristics of coalbed methane wells, characteristics and differences of coal reservoir and closure pressures, and coal structure and permeability. 2 methods 2.1 geological setting the south yanchuan block is situated in the southeast of ordos basin, china. the study area is a monoclinal structure with a northeast trend and a western dip. although the ordos basin has experienced repeated geologic history, specifically the indosinian movement, the yanshan movement and the himalayan movement, the structure in the study area is simple. the structure can be divided into five third-class structural units, the xibaigou gentle slope belt, the bai’e fault nose structure belt, the tanping gentle slope belt, the baishansi fault nose structure belt, and the wanbaoshan gentle slope belt are distributed from east to west (fig. 1) (cai et al., 2014; li et al., 2015; yan et al., 2015). the south yanchuan block was compressed during the three main tectonic movements: ns trending, nw-ne trending, and ne-sw trending movements. the ns trending stress from the indosinian period established the basement structure. the nw-ne trending stress from the yanshanian period mainly controlled the structural characteristics, including the four large-scale faults and numerous small-scale faults. the magma activity in yanshanian period was active, albeit insignificant in the south yanchuan block (li et al., 2015). the hydrodynamics in the study area are complex, including a strong runoff and recharge zone, a weak runoff zone, a stagnant zone, a strong runoff zone, a weak stagnant zone and a deep stagnant zone, situated from the east to west (cai et al., 2014). the lower permian shanxi formation (p1s) no. 2 coal seam is the target stratum for the development of coalbed methane. influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china d3 2.2 samples test and data processing there have been approximately 60 coalbed methane wells in the research area. however, most of these wells are in initial production stages, exhibiting unstable productivity. in order to ensure that the research is scientificity, coalbed methane wells that have stably produced for an excess 150 days were selected. the proximate analysis of coal samples in this study utilizes the instrumental method of proximate analysis of coal (gb/t 307322014, chinese national standard). the ro, max tests were carried out using whole-rock maceral analysis (sy/t 6414-1999, chinese petroleum and natural gas industry standard), and at least 800 points were tested. reservoir permeability can be acquired from injection/fall-off well tests, experimental test, history matching, geophysical log and constructing permeability prediction models (palmer, 2009; li et al., 2011; ma et al., 2011; thararoop et al., 2012; meng and li, 2013; perera et al., 2013b; peng et al., 2014). in this paper, the coal reservoir permeability is acquired from the injection/falloff well test, and the fitting permeability is acquired from the comet3 coalbed methane numerical simulation software. comet3 is a three-dimensional, threecomponent, two-phase, dual-porosity simulator for modeling gas and water production from a desorption-controlled coal reservoir (advanced resources international, 2005). the diffusion, desorption theory and dual-porosity/ single-permeability model are used for the coalbed methane simulation. the following equations are used to calculate the conservation of gas and water. conservation of gas: (1) conservation of water: (2) where, f indicates fracture systems, ∆ is the gradient operator, ∆• is the divergence operator, bn (n = g or w) is the gas or water shrinkage factor, γn (n = g or w) is the gas or water gradient, rsw is gas solubility in water, φ is effective fracture porosity and z is the elevation. during the simulation process, certain reservoir parameters should be inputted, including coal thickness, burial depth, reservoir pressures, porosity, permeability, gas content, sorption time, langmuir volume, langmuir pressure, and aquifers. all of these parameters can be acquired from engineering and experimental tests. if the permeability from the injection/ fall-off well test is influenced by the external fluid, then the permeability is the domain parameter that will be fitted in the simulation software. coal structure of reservoir was explained using logging data (fu et al., 2009). the coal structure is divided into three types, type i coal is primary texture coal, type ii coal is cataclastic texture coal, and type iii coal is tectonic coal. 3 results and discussion 3.1 coalbed methane well production characteristics fifteen coalbed methane production wells are studied in this paper. as shown in fig. 2, different wells have various methane and water production characteristics. according to the research of cai et al. (2014), when methane wells are located in the underground water weak-runoff and stagnant zones, the influence of underground water may be weak. the syc-1 and syc-6 wells wxhibit high methane production, with the maximum methane production of syc-6 exceeding 2000 m3/d, while the methane productions of syc-2 and syc10 are low. the output of water decreases the reservoir pressure, allowing for the methane to desorb from the coal. although the water production fluctuates, the dominant trend is decreasing. it was found that there were significant depressions in the liquid levels at syc-1 and syc-6 after the bores began to produce methane, followed by a leveling-out of the liquid levels. however, the liquid levels of syc-2 and syc-10 remained steady, indicating that the seepage of liquid is not smooth, a trait that may be related to the coal reservoir permeability. figure 2. production curves of typical coalbed methane wells in the south yanchuan block (the magenta line is the liquid level, the red line is the average methane production, and the blue line is the water production) figure 1. location of coalbed methane block 3.3 characteristics of reservoir and closure pressures influenced by depth generally, the crustal stress will increase with increasing burial depth. due to the shift in the stress state at 1000 m, the reservoir and closure pressures present typical characteristics. the reservoir in the research area is generally under pressure, with the shallow reservoir pressure is particularly low (fig. 4a). these low pressures combined with low gas saturation do not favor high production of coalbed methane. at depths of 1050 to 1150 m, the reservoir pressure fluctuates, indicating that depth is within the stress transformation zone (fig. 4a). additionally, the reservoir pressure gradient is low, and this is mainly teng li, cai-fang wud4 3.2 influence of coal properties on methane production as mentioned above, there are many factors that can influence the production of coalbed methane wells, particularly the permeability and thickness of coal and the gas content (kaiser et al., 1994; lv et al., 2012). generally, the higher the methane content and coal thickness, the higher the coalbed methane production (lv et al., 2012); however, that is not obvious in south yanchuan block. as shown in fig. 3d, the relationship between gas content and coal thickness in terms of average methane production is weak, meaning that these two factors are not the key controllers of the methane production. with increasing burial depth and coal reservoir permeability, the average methane production increases (fig. 3a, c), especially when the data from syc-3 and syc8 are removed (fig. 3b). the low methane production of these two bores will be explained later. when the burial depth exceeds 1000 m, the methane production is quiet high (fig. 3a, b). this phenomenon may be influenced by the shift of the stress state. to facilitate the discussion, the reservoir burial depth above 1000 m is referred to as the shallow reservoir, while the reservoir burial depth beyond 1000 m is referred to as the deep reservoir. the average methane production in the shallow reservoir is less than 700 m3/d, while the methane production always exceeds 500 m3/d in the deep reservoir, where it reaches 1759.37 m3/d (table 1). figure 3. relationship between burial depth, permeability, coal thickness and gas content with the average methane production types wells burial depth (m) coal thickness (m) gas content (m3/t, ad) ro, max permeability (md) mad (%) aad (%) vad (%) average methane production (m3/d) shallow reservoir syc-1 936.20 4.90 8.28 2.38 0.1735 1.48 14.58 11.40 1018.10 syc-2 898.80 5.10 7.60 2.30 0.0605 0.83 7.49 10.50 118.08 syc-4 896.00 5.10 6.98 2.19 1.09 26.04 11.16 679.96 syc-5 876.50 4.60 6.87 2.30 1.02 4.98 9.77 1253.36 syc-7 983.50 5.30 4.80 2.06 0.0490 0.64 12.86 10.20 154.36 syc-10 895.80 6.10 8.56 2.24 0.0170 0.68 14.76 10.86 267.70 syc-11 890.80 5.10 9.40 1.96 0.0281 252.95 syc-12 907.10 6.90 4.14 0.86 31.27 10.09 67.13 syc-13 928.70 5.28 8.02 2.35 0.0188 0.95 11.01 11.79 564.66 syc-14 916.40 4.28 9.50 2.46 0.98 9.30 9.46 768.21 syc-15 892.40 4.90 8.31 2.24 0.75 9.12 8.55 138.33 deep reservoir syc-3 1169.10 5.00 18.21 2.52 0.1698 0.85 11.91 8.81 544.89 syc-6 1259.40 4.80 11.60 2.57 0.1698 0.67 14.25 8.75 1759.37 syc-8 1242.40 4.30 5.10 2.02 0.89 8.81 9.67 182.75 syc-16 1064.58 5.82 14.44 2.74 0.9504 1.49 11.00 8.70 1288.86 syc-17 1498.10 3.90 10.63 3.08 0.4132 0.99 8.58 10.98 syc-18 1111.60 2.40 6.44 0.7529 0.81 7.32 8.17 syc-20 1129.31 3.94 10.74 2.65 0.0044 0.94 27.53 9.50 note: ad, the air-dry basis; -, no data. table 1. statistics of reservoir property parameters in south yanchuan, block due to the deep stagnant underground water zone (fig. 4a) (cai et al., 2014). the closure pressure also presents the same relationship with the burial depth, where the closure pressure in the transformation zone is quite fluctuant, and the maximum closure pressure of syc-20 is even higher than in deep reservoir (fig. 4b). in the deep reservoir, while the closure pressure has increased, it is not obvious (fig. 4b). where the burial depth of the reservoir exceeds 1000 m, especially in the compression zone, the crustal stress turns to compressive stress, and the closure and reservoir pressure are both high in the deep reservoir. this high reservoir pressure can maintain high stratum energy (wu et al., 2007), and the high closure pressure can keep the fracture open (perera and ranjith, 2012). with increasing the reservoir burial depth, the reservoir effective stress influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china d5 increases, and the fracture network system tends to close (laubach et al., 1998; yao et al., 2009a), which would lead to an exponential decrease in permeability (somerton et al., 1975; gray, 1987). the permeability decreases with the increase in effective stress (fig. 5a), due to the coupling of reservoir and closure pressures. the effective stress is also divided into three sections (fig. 5b). the low closure pressure gradient indicates high permeability in the deep reservoir. under the interaction between the low closure pressure gradient and the high reservoir pressure gradient, the permeability of the deep reservoir is generally more than 0.1 md. this observation has validated the conjecture, which means the coalbed methane geological condition is superior in the deep reservoir. 3.4 coal structure characteristics influenced by burial depth 3.4.1 coal structure proportion the statistical results show that in the shallow reservoir, the type i coal proportion reaches 38.18 %, the type ii coal proportion reaches 26.41 %, and the type iii coal proportion reaches 35.41 % (table 2). in the deep reservoir, the type i coal proportion reaches 25.25 %, the type ii coal proportion reaches 39.69 %, and the type iii coal proportion reaches 35.06 % (table 2). the type iii coal is mainly located in the transformation zone coal reservoir. if this section of the coal reservoir is ignored, the proportion of type i, ii and iii coal in the deep reservoir will reach 30.14 %, 50.14 % and 19.71 % respectively. high proportions of types i and ii coal is a good basis for high permeability in the deep reservoir (li et al., 2015). although the tectonics are simple in the research area, the deep reservoir (especially in the compression zone) coalbed methane wells lie in monoclinal structure. however, the shallow coalbed methane wells lie in the core of the secondary syncline or the wing figure 4. relationship between reservoir and closure pressures with the burial depth figure 5. relationship between effective stress and reservoir permeability of the secondary anticline, which would decrease permeability, and can be a disadvantage to the coalbed methane production. the development of type i coal can maintain the coal mass structure, while the development of type ii coal can increase the permeability of the coal reservoir (li et al., 2015), and this effect is the main reason for the low methane production at syc-3 and syc-8. the deep reservoir is mainly composed of the type i and ii coal structures, it has high stratum energy, and the production of coalbed methane is steady. with aslow increase of the effective stress, the fluid can discharge steadily, and the methane production effect is positive (table 3). coalbed methane production in the shallow reservoir varies, and the low reservoir pressure results in high effective stress. with the continuing production, the effective stress increases with decreasing reservoir pressure, and the fractures are rapidly closed. additionally, the development of type iii coal can make the migration of methane difficult, and can result in the methane wells’ having low to no productivity (table 3). 3.4.2 coal thickness generally, the higher the coal thickness, the larger the gas content the coal seam can maintain (yao et al., 2009a; lv et al., 2012). however, there is no marked correlation between coal thickness and coalbed methane production (fig. 3). in the shallow reservoir, the relationship between coal thickness and coalbed methane production is even negative (fig. 6a); mainly due to the development of type iii coal. the development of type iii coal can damage the reservoir permeability (li et al., 2015), and desorption of methane adsorbed at the surface of the coal matrix is difficult, resulting in low methane production. when the coal thickness is adjusted to the thickness of type i and ii coal, the methane production increases exponentially (fig. 6b). teng li, cai-fang wud6 table 2. statistics of coal structure with the logging data in south yanchuan, block types wells type i coal type ii coal type iii coal fitting permeability (md)thickness (m) ratio (%) thickness (m) ratio (%) thickness (m) ratio (%) shallow reservoir syc-1 4.90 100.00 0.00 0.00 0.00 0.00 syc-2 0.00 0.00 0.00 0.00 5.10 100.00 0.12 syc-4 0.00 0.00 0.00 0.00 5.10 100.00 syc-5 2.70 58.70 1.90 41.30 0.00 0.00 3.22 syc-7 0.00 0.00 4.10 100.00 0.00 0.00 syc-10 0.00 0.00 3.30 55.93 2.60 44.07 0.19 syc-11 5.10 100.00 0.00 0.00 0.00 0.00 1.71 syc-12 0.65 10.48 2.00 32.26 2.20 57.26 syc-13 5.30 100.00 0.00 0.00 0.00 0.00 0.81 syc-15 0.00 0.00 1.60 41.03 2.30 58.97 0.52 deep reservoir syc-3 4.50 100.00 0.00 0.00 0.00 0.00 0.29 syc-6 0.00 0.00 4.80 100.00 0.00 0.00 4.23 syc-8 0.70 17.28 1.75 43.21 1.60 39.51 0.8 syc-16 2.72 41.72 3.80 58.28 0.00 0.00 5.18 syc-17 0.00 0.00 2.10 53.85 1.80 46.15 syc-18 0.00 0.00 0.00 0.00 4.20 100.00 syc-20 0.00 0.00 0.00 0.00 3.40 100.00 0.08 note: -, no data. table 3. coalbed methane production characteristics in south yanchuan, lock types wells water drainage characteristics methane production characteristics shallow reservoir syc-1, syc2, syc-4, syc5,syc-11, syc-13, syc-14, syc-15 high maximum water production (4.02-10.13 m3/d), and water production is unstable. the water production should be regulated, then the methane production can be maintained. normally, with the decrease of the water production, the methane production gets down (118.08-1253.36 m3/d). syc-7, syc-10, syc-12 the maximum and average water produciton is low, 2.3-14.37 m3/d and 0.83-1.41 m3/d respectively. no stable methane production, even non methane produciton (67.13-267.7 m3/d). deep reservoir syc-8 high maximum and average water production, 10.80 m3/d and 5.15m3/d respectively. no stable methane production (182.75 m3/d). syc-3, syc-6, syc-16 high maximum water production (4.02-12.4 m3/d), low average water production (0.27-3.07 m3/d). methane production is stable and high (544.89-1759.37 m3/d). figure 6. relationship between coal thickness and average methane production the coal thickness also influences gas content, and the coalbed methane content unit thickness (cbmcut) can be used (lei et al., 2012). (3) where, is cbmcut, m3/(t·m); c is gas content, m3/t; h is apparent thickness, m; and α is dip angle, °. figure 7. relationship between cbmcut and average methane production cbmcut is a combined reflection of coal thickness and gas content; it increases exponentially with average methane production (fig. 7). cbmcut also has a relationship with the coal structure, and decreases with increasing coal thickness. this effect is mainly due to influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china d7 the resistance of tectonic deformation of thick coal; with thinner coal (1-2 m) more easily deformed than thick coal (> 5 m) (lei et al., 2012). 3.5 permeability in the shallow reservoir, the development of type iii coal results in the low permeability, as values are always below 0.1 md (table 1, fig. 5). the permeability of syc-11 and syc-13 coalbed methane wells with the type i coal is still low, mainly due to low reservoir pressure, and insufficient stratum energy to keep the fracture open. the deep reservoir has a single coal structure is, and the permeability is higher than that in the shallow reservoir. the development of type i and ii coal can maintain high permeability, which indicates that the type ii coal can promote the increase of permeability (fig. 8). figure 8. relationship between coal structure and reservoir permeability there is a positive exponential correlation between the fitting permeability and the thickness of type i and ii coal (fig. 9). type iii coal in the reservoirs of syc-2, syc-4, syc-7, syc-10 wells is more developed. the coal structure is complicated, and the incremental multiplier of reservoir permeability is limited after the hydrofracture, which means the fracturing effect is not obvious with the development of tectonic coal (table 2). with the development of type i coal in the reservoir of syc-11 and syc-13 wells, the permeability increased dramatically (table 2). except for the syc-3 well, the fitting permeability in the deep reservoir increased, and the development of a dirt band in the reservoir decreases the fracturing effect. this high permeability means higher methane production (fig. 2). figure 9. relationship between thickness of type i and ii coal and permeability teng li, cai-fang wud8 4 conclusions the average methane production in south yanchuan block increases with increasing coal reservoir burial depth, and coal thickness and gas content are not the key factors influencing methane production. the shift of the stress state makes the reservoir and closure pressure fluctuate, which leads to various coal structure characteristics in the shallow and deep reservoirs, and finally influences the coal reservoir permeability that controls the coalbed methane production. the reservoir in the research area is under pressure, and the shallow reservoir pressure is drastically low, which results in high effective stress. high closure pressure in the deep reservoir keeps the fracture open, and the reservoir permeability is superior. however, the reservoir and closure pressures in the transformation zone in the deep reservoir are fluctuant. the reservoir and closure pressures also influenced the coal structure. the type iii coal in the shallow reservoir is quite developed, while there is type i and ii coal in the deep reservoir. the combination of coal thickness and gas content can significantly influence the average methane content, and this also has a great deal to do with the coal structure. there is a positive exponential correlation between the fitting permeability and the thickness of type i and ii coal. the coalbed methane numerical simulation results show the development of type iii coal can decrease the fracturing effect, while it increases with the thickness of type i and ii coal. conversely, the development of type iii coal causes the methane production to be unsteady, which is a disadvantage to methane productivity. acknowledgements this work was supported by the national major special project of science and technology of china (2016zx05044001), the natural science foundation of china (41272178, 41572140), the fundamental research funds for the central universities (2014zdpy26, 2015xkzd07), the qing lan project and the research and innovation project for college graduates of jiangsu province (grant no. kylx15_1392). references advanced resources international (2005). users manual comet3 version 2.0. advanced resources international, washington, dc, usa. cai, y., liu, d., zhang, k., elsworth, d. yao, y. & tang, d. 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(2009b). fractal characterization of seepage-pores of influence of pressure, coal structure and permeability on cbm well productivity at various burial depths in south yanchuan block, china d9 keywords: local t-s model; seismic surface; nonlinear response; switching; control; damper. palabras clave: modelo local t-s; superficie sísmica; respuesta no lineal; traspuesta; control; apagador. how to cite item jiang, y., luo, j., azarkhosh, h., & wu, e. (2020). the switching control method for the nonlinear response of seismic surface based on the local t-s model. earth sciences research journal, 24(3), 327-333. doi: https://doi. org/10.15446/esrj.v24n3.90432 based on the local t-s model, the switching control method for the nonlinear response of the seismic surface is studied, and the stiffness matrix of the composite interlayer element of viscoelastic damper and herringbone support is derived. the high-order single step ß method is applied to the nonlinear seismic response analysis and the instantaneous optimal active switching control. the local t-s model and controller are designed, and the approach and control of global nonlinear responses are realized through the multi-model switching control. the results show that the proposed method can accurately reflect the reduction of seismic ground load and the switching control of the main structure when the viscoelastic damper is installed. the switching control effect of the maximum spatial displacement becomes more and more obvious with the change of time history, and the control effect of all layers applying the switching control force is the most significant. whether the controller is disturbed or not, it is always good and stable performance. abstract the switching control method for the nonlinear response of seismic surface based on the local t-s model el método de control de conmutación para la respuesta no lineal de la superficie sísmica con base en el modelo local t-s issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90432 este artículo estudia el método de control de conmutación para la respuesta no lineal de la superficie sísmica con base en el modelo local t-s, y deriva la matriz de rigidez del elemento compuesto de capa intermedia del amortiguador viscoelástico y el soporte de espiga. el método ß de un solo paso de alto orden se aplica al análisis de respuesta sísmica no lineal y al control de conmutación activo óptimo instantáneo. el modelo local t-s y el controlador están diseñados, y el enfoque y el control de la respuesta global no lineal se realizan a través del control de conmutación multimodelo. los resultados muestran que el método propuesto puede reflejar con precisión la reducción de la carga de tierra sísmica y el control de conmutación de la estructura principal cuando se instala el amortiguador viscoelástico. el efecto de control de cambio del desplazamiento espacial máximo se vuelve cada vez más obvio con el cambio del historial de tiempo, y el efecto de control de todas las capas que aplican la fuerza de control de cambio es el más significativo. ya sea que el controlador esté alterado o no, siempre es un rendimiento bueno y estable. resumen record manuscript received: 08/02/2020 accepted for publication: 11/07/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 327-333 yudong jiang1*, jinming luo2, hojatallah azarkhosh3, erjun wu3 1college of college engineering, nanjing university of aeronautics and astronautics, nanjing, 210016, china 2college of automation, shenyang institute of engineering, shenyang, 110136, china 3college of civil and transportation engineering, hohai university, nanjing, china *corresponding author: luojinming0830@163.com; hojatazarkhosh@gmail.com se is m o lo g y 328 yudong jiang, jinming luo, hojatallah azarkhosh, erjun wu introduction a large number of earthquake damage and existing records show that the earthquake will cause indelible damage to people’s lives and property. with the development of china’s national economy and the progress of science and technology, high-strength and light-weight building materials are widely used in building structures (altukhov et al., 2018), the stiffness and damping of the structures are constantly reduced, and the building structures are more sensitive to seismic excitation. in order to reduce the seismic response of the building structure, in addition to studying the seismic design structure of the bearing system of the building structure itself (zhang et al., 2017), some energy dissipation devices are set on the structure to increase the structural damping and consume the seismic energy through the nonlinear deformation of the energy dissipation materials, so as to reduce the seismic response of the main structure and effectively suppress the structural vibration (zhang et al., 2016). among these energy dissipation devices, the viscoelastic damper is the most concerned one (ding et al., 2016), because it has the following advantages compared with other energy dissipation devices: as long as the structure starts to vibrate under minor interference, it can consume energy immediately (zhang et al., 2018). therefore, even in the case of small elastic vibration, it also plays the role of earthquake control. this makes it not only suppresses the seismic response of the structure but also avoid the problem of how the initial stiffness of the damper matches the lateral stiffness of the structure existing in other energy dissipation dampers (wei et al., 2016); its “force-displacement” hysteretic curve is similar to the ellipse, so its energy dissipation is very strong (bande & choudhury, 2017). multi-model switching control is to approach the uncertainty and nonlinearity of response with multiple models. based on multiple models, a controller (viens et al., 2017) is established. multiple controllers can be selected to act on the same control object, so as to effectively control complex response (huang et al., 2018). it has nearly 40 years of development history. in recent ten years, some scholars have proposed multi-model control based on model switching. many achievements have been made in theory and practice. the t-s fuzzy system is a kind of dynamic response fuzzy identification method proposed by takagi and sugeno. its basic idea is to divide the input space into multiple fuzzy intervals, approximate each fuzzy interval with a linear model, and obtain the global model by local model interpolation. the larger the number of fuzzy rules is, the higher the precision is. however, when the number of rules of fuzzy reasoning is large, the calculation is complex and in some cases, no common positive definite matrix can be found to meet the stability conditions (acerbi et al., 2016; guo et al., 2017). in this paper, multi-model switching control of nonlinear response based on the local t-s model is proposed. the model controller is designed, and the viscoelastic damper damping device is added to the building. through the switching control, the switching control of the seismic surface response is realized. materials and methods combined stiffness matrix in the interlayer of viscoelastic damper and herringbone support stress-strain relationship of viscoelastic damping materials let and be the stress and strain of viscoelastic damping materials respectively. according to the research of tsai and lee, the stress-strain relationship satisfies the fractional derivative relationship f i t g g v i t g f i t∆ ∆ ∆( ) = +( ) ( ) + ( )0 1 1 (1) where f i t∆( ) is the original time effect of strain, and its expression is as follows: f i t i i t i v o i j v j tt t t∆ ∆( ) = ( ) + ( )    ( ) + ( )    (       1 1 11 1 )) = = ∑ j i 1 1 (2) g a t t et u fdv t t 1 0 2 0 = ( ) ( ) + ( )   ∫ ∆ γ − − θ − (3) in the equation, g0, a0 and  are all material parameters, which can be measured by experiments. according to equation (1), the stress-strain curve of viscoelastic damping material is an ellipse. stiffness matrix and switching control force vector of interlayer element if the time interval is t, t = it, according to the layer model, only one direction (x-direction) of vibration is analyzed at a time, it means the relative displacement of viscoelastic damper nodes along the x direction, then: p b p g g a h b b u g af i t be t t t{ } =   = +( )    { } + ( )  ∆ ∆0 1 1 (4) in the equation, a is the acting area of the viscoelastic damper material; h is the thickness of the viscoelastic damper material; pe{ } is the load vector of the viscoelastic damper node, and only p p pe t{ } =  × ×1 2, along the x direction is considered; u u u t { } = { }1 2, is the displacement vector of the viscoelastic damper element node. from equation 4, it can be seen that the load vector of the viscoelastic damper element is composed of two parts. the former one is similar to the load vector generated by ordinary elastic elements, and the latter one is the restoring force vector unique to viscoelastic damper elements, namely the vector of energy dissipation switching control force of the element. let pev{ } be the vector of energy dissipation switching control force of the element. according to equation (4): p g af i t b g af i tev t{ } = ( )   = ( )      1 1 1 1 ∆ ∆  (5) let the element stiffness matrix of viscoelastic damper vibrate in one direction (x direction) be ke , and it can get from equation (4): k g g a h b b a h g g ke t  = +( )     = +( )       =0 1 0 0 1 1 1 1 1 1 1 1 / /    1 1       (6) where k a h g g1 0 0 1= +( )/ is the element stiffness coefficient of the viscoelastic damper. if the coefficient of element lateral stiffness matrix is k2 and the lateral stiffness matrix of the element is ks , then: ks k  =      2 1 1 1 1   (7) where, k ea hs2 2 2 2 2 2 2 3 2 2 1 1 2= ( ) +   / / / ; as, e, 12 and h2 are the elastic modulus, cross-sectional area, span, and storey height of herringbone brace respectively. after the herringbone brace and viscoelastic damper are connected in series, the stiffness matrix of the interlayer element of the whole brace is as follows: k k k k kes  = ⋅ +      1 2 1 2 1 1 1 1   (8) high-order single step ß method for nonlinear response analysis of seismic surface considering nonlinearity, the equation of motion of shear-type structure expressed by the relative displacement between layers under the action of the seismic surface load is as follows: m x c x f m a xe s1 1 1  ′{ } +   ′{ } + { } =  { }⋅ ⋅⋅ − (9) 329the switching control method for the nonlinear response of seismic surface based on the local t-s model controller design the parallel distributed compensation algorithm (pdc) is used to design the local state feedback controller in the same region. the fuzzy control rule i in the same region is as follows: if z t is m z t m u t ki p ip i and and is then 1 1( ) … ( ) ( ) =,  ii x t( ) (18) therefore, the region controller (e.g., the q-th region rq, q  s) is expressed as: u h z t k x tq i i i r = ( )( ) ( ) = ∑ 1 (19) the global controller u t( ) is actually the output of the current regional controller selected by the multi-model switching strategy, which can be expressed as u t u t( ) = ( ) , where σ ∞t s s( )  ) → = …{ }: , , , ,0 1 2 is a piecewise constant function depending on time or state, u t( ) is generated by switching in the controller set u u us1 2, , ,…{ }, and has the form u u q st qσ ∈( ) = , , which can be further written as, u u h z t k x tt q j j r jσ −( ) = = = ( )( ) ( )∑ 1 (20) according to equation (20), the closed-loop fuzzy control system can be deduced as follows: (21) stability of nonlinear response of multi-model switching control based on local t-s model the lyapunov stability results of the multi-model switching response are all based on the following assumptions: limited switching times between local models in a limited time (wang, 2019). for the convenience of the following proof, the switching function of the multi-model switching control is described by the characteristic function q z t( )( ), namely: υ ∈ ∉ ∈q q q z t z t r z t r q s s( )( ) = ( ) ( )     = …{ } 1 0 1 , , , , , (22) it means that when the variable z(t) is in the region q, the controller q corresponding to the current region uq is selected by multi-model switching control. as a global controller u(t), controllers in other regions do not work. therefore, the global closed-loop control system and global controller of nonlinear reactive multi-model switching control based on local t-s model can be described as follows: (23) u h k xq qi qj i r q s = == ∑∑  11 (24) note: for the convenience of writing, x, q, hqi and u are used to represent the corresponding x t( ), q z t( )( ), h z tqi ( )( ) and u t( ), the same below. substituting equation (24) into equation (23), the global closed-loop fuzzy control system can be expressed as follows: (25) where, the restoring force of [m1], [c1] and {a} is calculated according to the following equation: f k x f f f k x f k xe ej ej ej i r i ef b i r b{ } =   ′{ } = = ′ + ′+1 1;   (10) p f k xpj r ej b i r b= ′ (11) where, f ej b are the current restoring force and the previous switching control restoring force generated by the i-th layer; ′xi and ′xb are the current displacement and the previous switching control displacement of the i-th layer; ppj r and ki r are the plastic load and the current stiffness of the i-th layer respectively; according to equation (9), (10) and (11), the motion equation after entering the nonlinearity can be obtained as follows: (12) where kr1    and pp r{ } represent the current stiffness matrix and plastic load matrix respectively. according to equation 12 and the assumption of the high-order singlestep method, the equation for step-by-step integration can be obtained as follows: (13) a new instantaneous optimal algorithm for active switching control the equation of motion for active switching control of structural seismic surface response is as follows: (14) where {u} represents the r1 order switching control force vector and [b] represents the nr order position matrix of switching control force action. the index function has energy meaning, and any instantaneous index function can be taken as: j x w k x x w m w t c x b t t= { }  { } + { }   +  ( ){ } +  1 2 1 2 1 2 1 2 3∆  { }( )    { }u k b u t 1 (15) where, w1, w2 and w3 are adjustable parameters of weight function. under the constraint of equation (14), let jn+ =1 0, and then it can get: b u d x d x d x d xn n n n { } =  { } +  { }   ⋅{ } +  { }⋅ ⋅+1 1 2 3 4 1 ggn+1 (16) in order to achieve closed-loop switching control, b u d x d x d x d xn n n n { } =  { } +  { }   ⋅{ } +  { }⋅ ⋅+1 1 2 3 4 1 ggn+1 is removed, so jn+1 of each step is no longer zero, that is, each step is only approximate optimization (17) from the above equation, the current applied switching control force u n{ } +1 can be obtained. 330 yudong jiang, jinming luo, hojatallah azarkhosh, erjun wu lemma 1 if at any time t, the number of activated fuzzy rules is less than or equal to r, then there are: υ − − υq qi q qi qj i ji r q s i r q s h r h h2 1111 1 1 2 0≥ ===== ∑∑∑∑∑ (26) prove according to this lemma: υ − − υ − υ −q qi q qi qj q qi qj ii r q s i j h r h h r h h2 1 2 11 1 1 2 1 1 0=     ≥ ==== ∑∑∑∑∑∑∑∑∑ ==== i r q s i r q s 1111 υ − − υ − υ −q qi q qi qj q qi qj ii r q s i j h r h h r h h2 1 2 11 1 1 2 1 1 0=     ≥ ==== ∑∑∑∑∑∑∑∑∑ ==== i r q s i r q s 1111 (27) from lemma 1, we can easily get lemma 2 as follows. lemma 2 if at any time is less than or equal to , 1 1<η ≤ , then there are: υ − − υq qi q qi qj i ji r q s i r q s h r h h2 1111 1 1 2 0≥ ===== ∑∑∑∑∑ (28) theorem it is assumed that at any time t, the number of activated fuzzy rules is less than or equal to  and . if there is a common symmetric positive definite matrix p and semi-positive definite matrix qq, the following linear matrix inequalities are obtained: g p pg qqii t qii q+ + ( )η− <1 0 (29) g g p p g g q i jqij qji t qji qji q +      + +      ≤ 2 2 0− <, (30) then the global closed-loop control system (25) based on the local t-s model is asymptotically stable. where, g a b kqij qj qi qj=  . prove let lyapunov function v x x pxt( ) = , where p is a positive definite matrix. along any trajectory of the closed-loop system (25), the derivative of v(x) with respect to time is: (31) where: g g g p p g gqij qji t qji qji= +      + +     2 2 (32) according to equation (29), (30) and lemma 2, there are: (33) from equation (29), when x t( ) ≠ 0, there is , so the global closed-loop control system (23) based on the local t-s model is asymptotically stable under the action of the global controller (24). according to the stability conditions of the above theorem, based on lmi, the design of feedback controller and local feedback gain kqi is to find a common symmetric positive definite matrix w, semi-positive definite matrix yq and matrix mqi, which makes the following linear matrix inequalities to be enabled: wa a w m b b m yqi t qi qi t qi t qi qi q+ + ( )− − η−1 0 (34) wa a w wa a w m b b m m b b m yqi t qi qj t qj qj t qi t qi qj qi t qj t qj qi q+ + +     2 0, i j wa a w wa a w m b b m m b b m yqi t qi qj t qj qj t qi t qi qj qi t qj t qj qi q+ + +     2 0, i j (35) where w p= 1, m k wqi qi= , y wq wq q= . with the help of the lmi toolbox in matlab, the feasp function is used to find the feasible solutions of w, yq and mqi which satisfy the inequality. then the local feedback gain , matrix p and qq are obtained, that is, k m wqi qi= 1, p w= 1, q py pq q= . results project overview a comprehensive building is a ten-story reinforced concrete frame structure, the height of the first to the third floor is 4.8m, the height of the fourth to the tenth floor is 4.2m, and the total height is 43.8m. the design of seismic crack resistance is 8 °, class ii site. the viscoelastic damper is placed on the top of herringbone support, beam kl1 is 550mm × 250mm, kl2 is 300mm × 250mm, column section size is 500mm × 500mm, and steel support area is 4260 mm2. in the design of viscoelastic damper support, under the principle of reducing project cost, the stiffness of viscoelastic damper support should be increased as much as possible to ensure that viscoelastic dampers can produce enough restoring control force. in this design, zn22 damping material produced by a factory is selected as the viscoelastic damping material, with material constants of g0, a0, and θ of 588, 2287, and 100.0, respectively. the unit is knm, and the area of single viscoelastic damping material is 0.7x0.7=0.49m2, the thickness is 3mm, and the working temperature is 20 ºc. calculation results and analysis in order to verify the performance of the method in this paper, the time history analysis and calculation of the nonlinear seismic response of the structure supported by the viscoelastic damper (ved), the structure only supported by the same type of steel herringbone support and the original pure frame structure are carried out respectively. in the calculation, the el-centro seismic surface acceleration record with a duration of 10.0s (its predominant period is 0.55s) is selected for the seismic surface wave. considering the rare seismic surface action, the seismic surface acceleration xg is 400gal. the calculation results of the maximum displacement of each floor of the three structures are shown in figure 1. figure 1. the horizontal displacement of each layer of the structure 331the switching control method for the nonlinear response of seismic surface based on the local t-s model table 1 shows the cracking and entering plasticity of the three structural layers. table 1. structural layer cracking and entering the plastic stage structural layer with ved cracking 1 ~ 3 layers the inelastic there is no only diagonally supported structural layers cracking 1 ~ 6 layers the inelastic 1 layer pure frame structure layer cracking 1 ~ 9 layers the inelastic 1 ~ 4 layers it can be seen from figure 1 and table 1 that the displacement of the top layer and the shear force of the bottom layer of the structure with viscoelastic damper support is reduced by about 35% compared with that of the structure with herringbone support only, and about 70% compared with that of the pure frame structure; the number of cracking layers of the structure with viscoelastic damper herringbone support is reduced, the plastic stage of the structure layer is avoided, and the collapse of the structure is prevented. the viscoelastic damper can produce a strong energy dissipation control force, significantly increase the energy dissipation capacity of the whole structure, weaken the seismic ground load transmitted to the mainframe structure, and achieve the purpose of structural vibration switching control. the results show that this method can accurately reflect the seismic response of the structure with a viscoelastic damper, and it is a more practical and accurate calculation method. comparison of maximum interlayer displacement the lateral stiffness k1 of each floor of the ten-story complex building is 3.4105, 3.26105, 2.85105, 2.71105, 2.69105, 2.43105, 2.23105, 2.07105, 1.69105, 1.37105kn/m. the layer damping coefficient ci of each floor is 490, 467, 410, 386, 375, 348, 298, 274, 243, 196 kn·s/m respectively. the ground motion acceleration lasts for 8.5 s, and the maximum acceleration is 0.1g. fig. 2 shows the switching control effect of the proposed method on the seismic surface response by comparing the maximum displacement time history under the condition of free and no switching control with the displacement time history under the switching control imposed on the top layer of the proposed method (weight function is w1= w2= w3=168). figure 2. maximum interlayer displacement response it can be seen from figure 2 that the switching control effect of the proposed method is more and more obvious with the change of time history. here, the selection of weight function is selected through comparison. for different structures, the selection of weight function is different. even for the same structure, different weight functions will lead to different switching control effects. when the weight function is not selected, it will lead to out of control. figure 3 shows the comparison of the maximum displacement time history of three different weight functions, where a is the applying switching control force on the top layer, and the weight function is w1= w2=168, w3=0; b is the applying switching control force on the top layer, and the weight function is w1= w2= w3=168; c is the simultaneously applying switching control force on the fourth, sixth, eighth and 10-th layers, and the weight function is w1= w2=1000,w3=10000. figure 3. maximum interlayer displacement response it can be seen from figure 3 that the change of w3 in the weight function has no obvious effect on the control of the maximum displacement. the effect of multi-layer simultaneous application of switching control force is more obvious than that of only the top layer. figure 4 shows the calculation results of switching control force applied to some layers and all layers. among them, c is the switching control force applied on the 4th, 6th, 8th, and 10th floor, with weight function of w1=w2=1000 and w3=10000; d is the switching control force applied on the 4th, 6th, 8th and 10th floor, with weight function of w1= w2=200 and w3=2000; e is the switching control force applied on the 10th floor, with weight function of w1= w2=200 and w3=2000. figure 4. maximum interlayer displacement response it can be seen from figure 4 that the selection of weight function has an impact on the control effect, and the control effect of switching control force applied to all layers is significant. maximum interlayer displacement response in a closed-loop state in the nominal case, that is, when there is no perturbation in the controller, the multi-model switching control method based on the local t-s model is applied to design the controller, and the reaction process is convergent 332 yudong jiang, jinming luo, hojatallah azarkhosh, erjun wu and stable; when there is a perturbation in the controller, the controller in the nominal case is still applied to realize the switching control for the reaction, and the reaction process is divergent and unstable. in both cases, the maximum interlayer displacement response in the closed-loop state is shown in figure 5. figure 5. the closed-loop maximum inter-layer displacement response of conventional switching control assuming that the controller has the same additive perturbation as in the previous case, the controller is designed by the method in this paper. the maximum interlayer displacement response of the closed-loop state is shown in figure 6, and the reaction process is convergent and stable. figure 6. in this paper, the closed-loop state maximum inter-layer displacement response of the controller is presented it can be seen from figure 5 and figure 6 that when the controller itself does not have perturbation or is disturbed by perturbation, the controller adopting the method in this paper can obtain good performance and keep stable. the simulation results show the effectiveness of the proposed method. discussions in the actual control process, the output variation range of the controller is limited (yoon et al., 2016; friederich et al., 2018), so its pole assignment is constrained when designing the controller. the multi-model switching control method proposed in this paper is used to divide the input space into several regions. considering the actual constraints, poles can be flexibly allocated in different regions (gao et al., 2016; liu et al., 2016). local controllers can be designed in each region, and then through the switching control, the approach and control of the overall nonlinear response can be realized, so that the controller can be avoided. the output saturation can also guarantee the convergence speed (cheng et al., 2016). for the reaction with a wide range of parameters, the effectiveness will be more obvious. high-order single step  method can obtain better switching control effect than other instantaneous optimization algorithms (in the past, the switching control effect of other algorithms is generally about 50% 70%) (li et al., 2016); adding damping term in the switching control index function has an impact on the switching control effect; for the value of weight function, there is the problem of optimal selection (zhan et al., 2016); there is a problem of optimal switching control layer combination in multi-level switching control. the proposed method can be applied to the design of controllers with a large range of response parameters and dynamic changes of subsystems. in addition to the application of this paper in the study of switching control of seismic surface response, it can also be applied to the air-to-air vehicle, for example. because the air-to-air vehicle has the characteristics of time-varying, uncertainty and nonlinear coupling, the parameters of system response vary greatly, and a single dynamic model has no way to accurately describe the actual changes of the system (yang & long, 2016). therefore, the method proposed in this paper can be used to approach the dynamic performance of the system, a multi-model controller is designed based on the multi-model, and multiple controllers can be selected to act on the same control object, so as to effectively control the complex system of an air space vehicle. the switching control of nonlinear response is still a problem that needs to be further studied. because the nonlinear response model is difficult to establish and control, it can use multiple linear models to approach the nonlinear model, or use multiple simple nonlinear models to approach the complex nonlinear model, and then establish the multi model controller, which will have a good control effect on some nonlinear systems. conclusions in this paper, a nonlinear response switching control method based on the local t-s model is proposed. by calculating the stress-strain relationship of viscoelastic damping materials, the stiffness matrix and switching control force vector of interlayer element, the herringbone support structure of viscoelastic damping material is applied to interlayer element, and the relative displacement of interlayer element is calculated by high-order single step  method combined with nonlinear seismic surface response analysis, which is used to represent the movement of shear structure under the seismic surface load. the controller is designed based on the local t-s model. by switching control, the nonlinear response of the whole seismic surface is controlled. in this paper, the switching control effect of the method for seismic surface response becomes more and more obvious with time. when the controller has additive perturbation, the convergence speed of the state response of the method in this paper is fast and stable, which shows the accuracy of the method in this paper for the switching control of seismic surface nonlinear response. acknowledgments this work is supported by the national nature science foundation under grant 61803271. references acerbi, e., fusco, n., julin, v., & morini, m. (2016). nonlinear stability results for the modified mullins-sekerka and the surface diffusion flow. journal of differential geometry, 113, 178-187. altukhov, v. i., sankin, a. v., sigov, a. s., sysoev, d. k., yanukyan, e. g., & filippova, s. v. (2018). schottky-barrier model nonlinear in surfacestate concentration and calculation of the i–v, characteristics of diodes based on sic and its solid solutions in the composite chargetransport model. semiconductors, 52, 348-351. bande, g. r., & choudhury, d. (2017). seismic passive earth resistance in submerged soils using modified pseudo-dynamic method with curved rupture surface. marine georesources & geotechnology, 35, 930-938. cheng, x., zhang, q., yu, x. j., du, w., liu, r., bian, q., wang, z., zhang, t., & guo, z. (2016). strike-slip fault network of the huangshi structure, sw qaidam basin: insights from surface fractures and seismic data. 333the switching control method for the nonlinear response of seismic surface based on the local t-s model journal of structural geology, 94, 1-12. ding, x. j., cheng, w., li, y., wu, j. l., li, x., cheng, q., & ding, s. (2016). an enzyme-free surface plasmon resonance biosensing strategy for detection of dna and small molecule based on nonlinear hybridization chain reaction. biosensors & bioelectronics, 87, 345-351. friederich, w., hunzinger, s., & wielandt, e. (2018). a note on the interpretation of seismic surface waves over three dimensional structures. geophysical journal international, 143, 335-339. gao, r. m., lv, t. h., & hu, j. (2016). explicit expression of limiting spectral density of high dimensional sample covariance matrices. journal of jilin university (science edition), 54, 499-505. guo, h. y., wang, l. f., ye, w. h., wu, j. f., & zhang, w. y. (2017). weakly nonlinear rayleigh-taylor instability in incompressible fluids with surface tension. chinese physics letters, 34, 60-63. huang, m., kong, l. l., cao, g. q., xiong, k., & lishen, t. (2018). analysis of short circuit failure of pouch-type lithium ion batteries. chinese journal of power sources, 42, 476-526. li, h. y., shi, p., & yao, d. y. (2016). adaptive sliding-mode control of markov jump nonlinear systems with actuator faults. ieee transactions on automatic control, 62, 1-1. liu, y., van der neut, j., arntsen, b., & wapenaar, k. (2016). combination of surface and borehole seismic data for robust target-oriented imaging. geophysical journal international, 205, 758-775. viens, l., denolle, m., miyake, h., sakai, s., & nakagawa, s. (2017). retrieving impulse response function amplitudes from the ambient seismic field. geophysical journal international, 210, 210-222. wang, c. w. 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(2017). a nonlinear method for multiparameter inversion of pre-stack seismic data based on anisotropic markov random field. geophysical prospecting, 66, 461-477. zhang, j., tang, z. h., ai, m. x., & gui, w. (2018). nonlinear modeling of the relationship between reagent dosage and flotation froth surface image by hammerstein-wiener model. minerals engineering, 120, 19-28. zhang, p., qin, z. c., & lu, z. (2016). an analytical model of space backbone network capacity for space information transmission. journal of china academy of electronics and information technology, 11, 66-72. keywords: uav; remote sensing; sea area supervision; aerial photography; image processing. palabras clave: aeronaves no tripuladas; sensores remotos; supervisión de la zona marítima; fotografía aérea; procesamiento de imágenes. how to cite item chen, r. (2021). application of uav low altitude remote sensing system in sea area supervision. earth sciences research journal, 25(1), 65-68. doi: https://doi.org/10.15446/esrj. v25n1.94162 the sea area supervision is the premise and guarantee of safeguarding national security, protecting national sovereignty, and realizing the development of marine resources, and its importance is self-evident. to carry out the national sea area work more efficiently, this study designed low altitude-unmanned aerial vehicles (uav) remote sensing system applied to the sea area supervision and analyzed the remote sensing photography technology and remote sensing image processing technology. experiments verified the effectiveness of the system. the research results show that the uav-based low altitude remote sensing system can extract high-precision sea area information through aerial images’ interpretation. it is hoped that this study can provide some reference for improving the efficiency of current sea area supervision. abstract application of uav-low altitude remote sensing system in sea area supervision aplicación del sistema de teledetección con aeronaves no tripuladas de baja altitud en la supervisión de zonas marítimas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.94162 la supervisión del área marítima es la premisa y garantía de salvaguardar la seguridad nacional, proteger la soberanía nacional y desarrollar los recursos marinos. su importancia es evidente. para llevar a cabo el trabajo de manera más eficiente, este estudio diseñó un sistema de teledetección de baja altitud con aeronaves no tripuladas (uav) aplicado a la supervisión del área marítima y analizó la tecnología de fotografía de teledetección y la tecnología de procesamiento de imágenes de teledetección. la eficacia del sistema se verificó mediante experimentos. los resultados de la investigación muestran que el sistema de teledetección de baja altitud basado en vehículos aéreos no tripulados puede extraer información del área marina de alta precisión a través de la interpretación de imágenes aéreas. se espera que este estudio pueda proporcionar alguna referencia para mejorar la eficiencia de la supervisión actual del área marítima. resumen record manuscript received: 24/06/2020 accepted for publication: 21/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 65-68 rong chen1,2* 1department of urban construction and management, xiamen city university, xiamen 361008, china 2department of urban construction and management, the open university of china (xiamen branch), xiamen 361008, china *corresponding author: crxx0708@163.com g eo ph ys ic s https://doi.org/10.15446/esrj.v25n1.94162 https://doi.org/10.15446/esrj.v25n1.94162 https://doi.org/10.15446/esrj.v25n1.94162 mailto:crxx0708@163.com 66 rong chen introduction with the rapid economic development of all countries in the world, land resources have been unable to meet the needs of national development, so the ocean with rich natural resources will become the focus of follow-up development of all countries in the world (lamchin et al., 2016; zhang et al., 2015). it is necessary to map the islands in the ocean to realize the search and development of marine resources (cao et al., 2016; gorji et al., 2017; nieuwhof et al., 2015). currently, the information acquisition method for islands and reefs in many countries is mainly naval manual measurement. however, this method is inefficient and accompanied by substantial operational risks, which can no longer meet current marine development needs (cummer & inan, 2016; liu & zhou, 2017). given this, this study proposes to use aeronaves no tripuladas (uav) instead of artificial, using uav remote sensing technology to detect islands in the sea area, hoping to provide a particular reference for the current sea area detection and supervision work. uav and gps-assisted uav remote sensing image acquisition and processing analysis of aerial triangulation the aerial triangulation method is one of the most commonly used methods of uav remote sensing measurement. this study obtains aerial image data through this method and carries out post-processing (ren et al., 2019; weixing et al., 2015). analytic aerial triangulation, also known as analytic aerial triangulation encryption, its calculation essence is to obtain the control point position through actual measurement within several air belts or in the lan composed of several air belts and carry out adjustment calculation for all image pair encryption points and external orientation elements of all images (amin et al., 2020; abboud & nofal, 2020; hoque et al., 2017). at present, three methods are mainly used in analytic aerial triangulation, which are an independent model method, flight belt method, and beam method (alfy, 2016; bi, 2020; xu, 2020). figure 1 is the schematic diagram of aerial triangulation with the beam method. in this method, the images captured by a uav remote sensing camera constitute a light beam, which is regarded as the basic adjustment calculation unit. the collinear equation of central projection is taken as the equation of adjustment calculation. after the beam’s rotation and translation, the light of the same name between the models can achieve the best intersection, and the whole region can be transformed into the ground coordinate system. then, the image’s exterior orientation elements and the ground coordinates of all the measured objects can be obtained through adjustment calculation. among the three analytical aerial triangulation methods mentioned above, the beam method has the highest accuracy and the most rigorous theory, so the beam method has become the most mainstream analytical aerial triangulation method. gps aided aerial triangulation in this study, the uav is in a low altitude flight state, its flight altitude is low, the fuselage is light, and the airflow over the ocean is large, and the airflow changes very fast. therefore, more heading and side overlap are needed in aerial measurement. at the same time, the single image area of the digital camera used by uav is small, and the field measurement workload will be doubled by using uav for aerial photography. in this case, reducing the number of ground image control points has become an urgent problem in uav remote sensing application. in order to reduce the difficulty of uav low altitude measurement, gps-aided aerial triangulation is introduced in this study. gps-assisted aerial triangulation refers to the installation of gps receiver on uav, and continuous observation of gps signal through airborne gps receiver and reference point receiver on the ground. through the gps carrier-phase differential positioning technology for data processing, the threedimensional space coordinates of uav at the time of shooting are obtained. the three-dimensional coordinate is taken as an additional observation value and introduced in the adjustment of the surveying area. the principle of converting the original ground control into air control is shown in figure 2. figure 2 is the schematic diagram of gps-aided aerial photogrammetry. when using this method, it is necessary to transform the aerial photography system and measure the eccentricity vector. the gps receiver antenna is installed on the aerial plane, and the eccentricity vector from the photographing center s to the gps receiver antenna phase center a is calculated. secondly, the gps receiver installed on the uav is used to obtain the fieldwork signal of aerial photography. according to the size of the area to be measured and the mapping scale, the ground reference stations are reasonably arranged by using differential gps positioning technology. next, the coordinates of the airborne gps station are calculated by gps observation data, and the coordinates are obtained by processing satellite ephemeris and gps observation data. finally, the photogrammetry and gps station coordinates are adjusted jointly. specifically, the spatial coordinates of the gps antenna phase center are used as photogrammetric data for regional network joint adjustment, and then the spatial coordinates and accuracy of the target object are calculated. at present, the increasingly mature uav remote sensing technology provides strong support for the sea area supervision, which can realize the rapid monitoring task of the sea area. however, the increasingly complex situation of marine supervision puts forward higher requirements for the processing efficiency of uav remote sensing photography. in this study, a variety of methods to improve the efficiency of the computer are combined to realize the rapid geometric processing of aerial photos. the image fast geometric processing process is divided into five steps. first, we need to match the image and get the corresponding connection point. under the condition of consistent images, the matching strength and speed of the algorithm are improved. secondly, combined with pos data, the rapid adjustment of the regional network is carried out, and the sparse matrix is figure 1. schematic diagram of beam method resolving aerial triangulation figure 2. schematic diagram of gps-assisted aerial photogrammetry 67application of uav-low altitude remote sensing system in sea area supervision introduced in the calculation process. after matching the connection point, the amount of adjustment calculation should be reduced as much as possible to improve the calculation efficiency. in the third step, the connection points are interpolated to obtain the dem in the survey area quickly, and the image ortho screening is carried out. next, on the basis of the face voronoi diagram image mosaic method, orthoimage mosaic is carried out to achieve less data redundancy and fast automatic mosaic. finally, automatic extraction of sea area elements from the image map of the monitoring area is carried out to obtain the location data of elements in the corresponding sea area. experimental research and analysis in order to verify the effectiveness of the uav low altitude remote sensing system, experiments were carried out to detect the coastline of islands in the sea area. it is very difficult to detect the coastline of islands by manual on-site measurement, and there are areas that are difficult to reach manually, so the fieldwork is very heavy. this study will detect the coastline of islands by uav remote sensing technology. the survey site was determined as an island bay in china. the vegetation coverage of the island bay is low, and the coastal topography is mainly hill with an altitude difference of less than 60m, which is suitable for uav low altitude aerial photography. in order to accurately grasp the location of the coastline, this study uses a three-dimensional laser to scan the coastal area of the island bay, and obtains centimeter-level fine dem. after the refined dem is obtained, gps rtk is used to measure a section of coastline in the island bay area, and the measured coastline is obtained. the coastline extracted on the basis of aerial photography and radar is compared with the field measured coastline, and the results are shown in figure 3. figure 3. coastline comparison of daowan bay figure 3 shows the comparison chart of island bay coastline results. (a), (b), (c) respectively show the comparison of general sandy shoreline, artificial shoreline, and bedrock shoreline. in figure 3(a), 15 feature points are selected for comparison; due to the difficulty of manual measurement, only 10 feature points are selected in figures 3(b) and 3(c). by comparing the coordinates of feature points obtained by different measurement methods, the measurement accuracy can be obtained. figure 4 shows the comparison results of the characteristic points of the sandy shoreline. as can be seen from figure 4, the sandy shoreline extracted from uav remote sensing images is quite different from that based on radar point cloud data. compared with the measured results, it is found that the sandy shoreline extracted from the uav remote sensing image is closer to the measured result, and the mean square deviation of both is less than 2 meters. figure 5 shows the comparison results of artificial shoreline and bedrock shoreline feature points. as can be seen from figure 5, the difference between the two methods based on uav remote sensing image and radar point cloud data is small, whether it is the coordinates of artificial shoreline feature points or bedrock shoreline feature points. through further calculation, it can be seen that the mean square deviation of the feature point coordinates is within 2.5 m in both cases. in conclusion, the uav-based low altitude remote sensing technology proposed in this study can interpret the information with high accuracy through aerial photography. this method can be combined with lidar technology in practical work, and then modify and offset the information extracted from remote sensing images to a certain extent. uav remote sensing system is expected to become an important tool for sea area monitoring. conclusion because the ocean contains rich natural resources, and it represents the national marine sovereignty, how to supervise the sea area has become the focus of attention of all countries. in this context, the study proposed to combine the sea area supervision and uav remote sensing technology, and verified the method by an example. the research results show that the sea area information obtained by the uav low altitude remote sensing system proposed in this study is closer to the measured information than lidar, and this method is expected to become an effective method to supervise the sea area. it is hoped that this study can contribute to the efficient development of marine regulatory work. in addition, there are still some problems in this study. when the uav is used to carry out aerial surveys in an area far away from the coast, the image obtained by the uav is likely to fall into the water, which will hinder the aerial triangulation to some extent, which will be improved in the future research. references abboud, i. a., & nofal, r. a. e. 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(2015). remote sensing monitoring of gullies on a regional scale: a case study of kebai region in heilongjiang province, china. chinese geographical science, 25(005), 602-611. _30j0zll _1fob9te _3znysh7 _2et92p0 keywords: ecrowd-sensing technology; geographical location, limitation; high precision; global positioning system; measurement. palabras clave: btecnología de detección de multitudes; ubicación geográfica; limitación; alta precisión; sistema de posicionamiento global; medición. how to cite item zhang, y., wang, b., & zhang, l. (2020). highprecision gps measurement method without geographical restrictions using crowdsensing technology. earth sciences research journal, 24(4), 491-497. doi: https://doi. org/10.15446/esrj.v24n4.92151 in order to improve the flexibility of gps measurement, a high-precision gps measurement method that is not restricted by the geographical location under crowd-sensing technology was proposed. the performance of the crowdsensing network was improved through a regular hexagon-based crowd-smart big data sensing network deployment mechanism. the gps /sins/dr fast and high-precision combined measurement methods were used to achieve high-precision measurement without geographical restrictions. it has been verified that the proposed method in this paper has much better stability in the deployment strategy of a regular hexagon than that of the square. the proposed method can achieve fast acquisition of satellite signals and high-precision positioning, and its measurement accuracy in the low-latitude city and high-latitude city is higher than the online measurement method based on google earth, indicating that it has significant application value. abstract high-precision gps measurement method without geographical restrictions using crowd-sensing technology método de medición gps de alta precisión sin restricciones geográficas con tecnología de detección de multitudes issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.92151 para mejorar la flexibilidad de la medición gps se propuso un método de medición gps de alta precisión que no está restringido por la ubicación geográfica y que se ejecuta bajo tecnología de detección de multitudes. el rendimiento de la red de detección de multitudes se mejoró a través de un mecanismo regular de implementación de redes de detección de macrodatos basado en hexágonos. se utilizaron los métodos de medición combinados gps/sins/dr rápidos para lograr mediciones de alta precisión sin restricciones geográficas. se ha comprobado que el método propuesto en este trabajo tiene una estabilidad mucho mejor en la estrategia de despliegue del hexágono regular que la del cuadrado. el método propuesto puede lograr una rápida adquisición de señales de satélite y un posicionamiento de alta precisión, y su precisión de medición en ciudades de baja latitud y ciudades de alta latitud es mayor que el método de medición en línea basado en google earth, lo que indica que tiene un valor de aplicación significativo. resumen record manuscript received: 03/01/2020 accepted for publication: 10/08/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 491-497 yunxiang zhang, bin wang*, lei zhang college of information science & electronic technology, jiamusi university, jiamusi 154007, china *corresponding author: jmsuwang2019@163.com g ps m ea su re m en ts https://doi.org/10.15446/esrj.v24n4.92151 https://doi.org/10.15446/esrj.v24n4.92151 https://doi.org/10.15446/esrj.v24n4.92151 mailto:jmsuwang2019@163.com 492 yunxiang zhang, bin wang, lei zhang introduction crowdsensing refers to the large-scale and complex society sensing tasks that use the mobile devices (mobile phones, tablets, etc.) of ordinary users as the basic sensing unit, through conscious or unconscious collaboration on mobile internet, to realize the distribution of sensory tasks and the collection of sensory data as well as complete those complex social sensory tasks. at present, the application scope of crowd-sensing can be divided into three categories: public facilities, environment, and society (dube et al., 2017). in terms of public facilities, crowd-sensing can detect traffic congestion, road noise or potholes, search for parking spaces, maintain public facilities such as fire hydrants and traffic lights, and have also been used in monitoring and navigation of real-time traffic (lowe et al., 2016). for example, through the sensors installed on the car and the gps, empty parking spaces can be detected and shared on the internet. through the cms system that collects passenger information on the bus, the comfort level of the bus can be evaluated and finally posted on the website. individuals collect information including pictures and names of bus stations via mobile phones, which helps to predict the arrival time of buses on mobile devices (chang et al., 2017). in terms of environment, crowd-sensing technology can be used to collect information such as co2 and pm2.5, noise, and water quality in the air. for example, trajectories corresponding to the time and place of mobile users are collected, and known models are used to calculate and collect the content of co2 and pm2.5 in the atmosphere. this information is collected through the air quality sensor of the mobile phone to realize the function of environmental monitoring. because noise is harmful to hearing, a “track-noise” relationship is collected from mobile users to form a noise map and it will be shared on the network. because people can collect water quality information such as flow, flow velocity, and amount of garbage when passing by rivers, this information collected through mobile apps can be aggregated to a background server and published online (hoang et al., 2016). in society, crowdsensing has been applied in social networking, travel, food, and tourism. for example, much social software recommends friends by mutual friends among users. mobile devices are used to collect user traveling habits and can promote environmentally friendly transportation. through building a “location-photos” library, people can use the pictures taken to query their geographic information. by personally sharing their food and beverage in the circle of friends, you can not only improve yourself but also give others a good opinion. based on gps trajectories and environment information provided by bicycle users, the best cycling route is derived (zhang et al., 2016). although crowdsensing plays a great role in clothing, food, shelter, and transportation, researchers at home and abroad have not stopped studying it in depth. in the past two years, in several well-known academic conferences, many papers have adopted more or less the basic idea of crowd-sensing. the advantage of crowd-sensing is that it has a huge mass base, which makes it outstanding in many fields. in the near future, crowd-sensing will serve society in a smarter and more convenient way (hsueh & chen, 2017). crowdsensing is a new model that utilizes ubiquitous mobile devices to efficiently collect data and is suitable for many large applications. the entire workflow of crowd-sensing is to use groups to collect data, then analyze and process these data, and finally obtain useful information. mi gensuo et al. adopted gps/rfid combined location technology and used the cp so algorithm and bp neural network combined algorithm to filter the output, which achieved high-precision measurement. foreign researchers proposed using gnss and map matching algorithms to achieve precise positioning. liu jiang et al. use a combination of gps and inertial measurement units and perform data fusion through the hcx3 robust filtering method. however, the above methods have certain constraints on geographic location. with crowd-sensing technology, the method in this paper realizes a high-precision gps measurement that is not restricted by geographical location (cai et al., 2016). method big data crowd-sensing network deployment mechanism based on a regular hexagon in this paper, vehicle positioning measurement is taken as an example to study the high-precision gps measurement method under crowdsensing technology. crowd-sensing uses the carrying smart devices or wearable devices of people as the basic sensing unit, and wireless communication within a short distance is used to collect and transmit data (cu et al., 2016). the collection of user participation data is mainly reflected in offline mobile sensing, transmitting data through the human-in-the-loop sensing mode, online social media, and contributing data through various mobile social media. however, due to the limitations of mobile node capabilities and network framework, it is difficult for a crowdsensing network to form a large scale (li et al., 2019). regular hexagonal grid deployment can seamlessly cover the target detection area. figure 1 shows the details of the regular hexagonal grid deployment mechanism of crowdsensing big data network: figure 1. regular hexagon grid deployment supposing that the side length of the regular hexagonal grid is , and  is a constant. when the mobile node is located at the positive center of the cluster, the energy consumption of transmitting data between the clusters is: e k e k dsix elec fs= × + × ×ε β (1) where k describes the length of the transceiver circuit,  fsis the parameter under the free space channel model amplifier, eelec the energy consumed to process kbit data. the energy consumption eelec is mainly related to data send distance d. d rsix sixsix = ×2 3 2 2 (2) r rsix six= 3 3 2 2 (3) where rsix is the length of the regular hexagon. from formula (2) and formula (3), it can be obtained that: d rsix = ⋅3 3 2 (4) the data transmission distance between clusters divided by a regular hexagonal grid is shown in figure 2. 493high-precision gps measurement method without geographical restrictions using crowd-sensing technology where  indicates the direction parameter. relative to the earth coordinate system, the angular velocity of the vehicle geographic coordinate system is: w w w w v r v r v r let t etx t ety t etz t yt yt xt xt yt xt =             =  tan         (7) where v is the vehicle moving acceleration of the coordinate system, r is the crowd-sensing technology communication radius of the coordinate system, l is the vehicle moving distance, the corner mark i is the inertial coordinate system, the corner mark e is the earth coordinate system, the corner mark t is the geographic coordinate system, the corner mark b is the vehicle coordinate system. the specific force information fib b obtained by the accelerometer is the specific force of each axis in the carrier coordinate system, but the specific force fit t required in this paper is the specific force of each axis in the geographic coordinate system. the two specific force can be converted by the following matrix (liu et al., 2017): f c fit t b t ib b= (8) according to the proportions, the acceleration in each direction can be calculated. the motion of the vehicle can be regarded as two-dimensional motion in the plane, so only the acceleration in the x and y directions need to be calculated (coco et al., 2016). assume that the measured attitude angles are  and , and the odometer trip is s. the equations composed of sins/dr are: v f w w v w w v v ext t itx t iez t etz t ety t iez t etz t etz t etx t = +( ) +( ) =⋅ − −2 2 ff w w v w w vitx t iez t etz t ety t iez t etz t etz t− −2 2+( ) +( ) l l r= + cosθ λ λ γ= +          r cos (9) the combined measurement results are: l t v r dt r l t v r ldt r t yt yt xt xt = + + = + =< ∫ ∫ 0 0 0 0 cos sec cos arcsin θ λ γ+ λ θ 223 13 33 γ −=                 arctan t t (10) where t describes the time. the above results were used as the update matrix for the second measurement of sins. gps satellite signal acquisition basic principles in order to track and decode the gps signal, the gps signal must be captured first, and the necessary parameters of the captured gps signal are immediately passed to the tracking process. the navigation message of the satellite can be obtained through the tracking process. signal acquisition is the process of matching the received satellite signal with the local replica signal. it is achieved by judging whether the carrier frequency and c/a code phase have the maximum correlation with the satellite signal. before the signal acquisition, the approximate doppler frequency shift and the initial position of the c/a code phase must be calculated, so the signal acquisition is a twodimensional search process (gao et al., 2018). at present, gps signal acquisition methods mainly include linear search, parallel code phase search, and parallel frequency search. parallel code phase search and parallel frequency search are realized by fourier parallel of one dimensional frequency code or phase code in the two-dimensional search, which can reduce the calculation amount of signal capture and improve the speed of two-dimensional signal search. in this paper, a faster parallel code phase search is used to further improve the acquisition speed (peyret et al., 2018). figure 2. distance between regular hexagon clusters in figure 2, r is a side length of the regular hexagon. the regular hexagonal division needs to keep the network connected, that is, d rsix ≤ . r is the communication radius. that is: d r r rsix = + ( ) =2 2 2 3 13 equation 3 is brought into the above equation: d rsix six= ⋅26 3 9 (5) to keep the network connected, d rsix ≤ is essential. gps/sins/dr-based fast and high-precision combined measurement method based on crowd-sensing technology sins/dr combined measurement algorithm the strap-down inertial navigation system (sins) has been widely used due to its comprehensive navigation information and complete autonomy in providing navigation information, but inertial navigation error will accumulate over time (kugler, 2017). for vehicle navigation applications, the dead reckoning (dr) system is as autonomous as the inertial navigation system, and the dr system error will slowly increase with the accumulation of time. in the case of long-term measurement, the measurement error of sins is much larger than that of dr (zhu et al., 2018). the advantages of the two were used to complement each other, and a sins/dr combined measurement algorithm was proposed. the algorithm in this paper introduces the strap-down inertial navigation attitude matrix into the dead reckoning state calculation matrix, so the update reckoning matrix can no longer be solved (golsorkhi et al., 2016). through this algorithm, the dead reckoning can suppress the divergence of a strap-down inertial navigation error. in this case, the error angle changes of the two are basically the same. in this method, the quaternion is calculated based on the initial attitude angle, and then the directional cosine matrix ct b is obtained. the result is: ct b = + ( ) ( ) +( )λ λ − λ −λ λ λ −λ λ λ λ − λ λ λ λ λ λ λ 0 2 1 2 2 2 3 2 2 2 0 3 2 3 0 2 1 2 0 3 02 2 2 22 1 2 2 2 3 2 2 3 0 2 1 3 0 2 2 3 0 1 0 2 1 22 2 2+ + ( ) ( ) +( )λ λ − λ λ λ − λ λ λ λ − λ λ λ λ λ λ λ − λ −−λ λ2 2 3 2+             (6) 494 yunxiang zhang, bin wang, lei zhang sins/dr-assisted satellite signal acquisition method the doppler frequency shift of a satellite signal is mainly due to the relative motion between the carrier and the satellite, the satellite clock error and the receiver clock error. its doppler frequency shift expression is: f f f fsh dy rc sc= +  (11) where fdy is the doppler frequency shift between the satellite and the motion carrier, frc is the clock error frequency of the receiver, fsc is the clock error frequency of the satellite. among them, the doppler frequency shift between the satellite and the moving carrier has the most significant influence on fsc . ignoring the receiver clock error frequency and the satellite clock error frequency, the doppler frequency between the satellite and the moving carrier can be predicted by introducing the carrier speed into the satellite navigation messages, which can be expressed as: f f fsh wdy zdy= + (12) therefore, the parallel code phase search method can be used to introduce sins/dr combined measurement data into a gps receiver, and predict the doppler frequency in advance. it improves the acquisition speed of satellite signals (gao, 2018). the implementation process is as follows: step 1: the doppler shift of satellite motion is calculated from the ephemeris: f f v r crdr r s e s = (13) where fr is the carrier frequency of the gps satellite, vs is the operating speed of the satellite in orbit, re is the radius of the earth, c is the speed of light, rs and is the average radius of the satellite orbit. step 2: calculate the doppler frequency shift of the carrier motion. from the above sins/d r combined measurement analysis, the velocity of the motion carrier can be expressed as: v v vz ets t ety t= ( ) + ( )2 2 (14) the doppler frequency shift of the motion carrier is: f f v v cz r ets t ety t = ( ) ( )2 2 (15) step 3: the doppler frequency shift can be obtained through the above calculation: f f c v r r v vsh r s e s etx t etx t= + ( ) + ( )        2 2 (16) step 4: the satellite signal was mixed with the local replica signal (with doppler frequency shift assistance) to obtain i and q signals (hoang et al., 2016). step 5: fourier transform was performed on the i and q signals. then they were multiplied with the local c/a code that completed the complex conjugate transform and fourier transform. and modulus operation was performed on the result, obtaining the last captured result (li & xu, 2016). data fusion processing algorithm when the vehicle is running at high speed, its high dynamics and complicated external environment will seriously interfere with the normal measurement work of the gps receiver. therefore, when improving the satellite signal acquisition capability, ensuring the sins/dr data connection if the satellite measurement data is interrupted is the key to improving measurement accuracy. this paper combines the least square method with the kalman filter algorithm to propose a new data fusion processing algorithm, lskf. the core idea of lskf is to determine whether the satellite measurement data is lost through the acquisition of satellite signals. in the case of data loss, the least square method is used to estimate the satellite measurement data, and the results are used as quasi observations and will be further processed by kalman filter. when the data is complete, kalman filter is directly used. this method avoids the non-linear errors in the observation equation and achieves high-precision tracking measurement of the moving vehicle. analysis of lskf algorithm model the least squares estimate is obtained from the navigation measurement data. it is assumed that the measurement data x yu u,( ) before gps satellite information loss can be measured, and u represents a constant. the route distance is regarded as an infinitely small value. for infinitely small distances, the route can be regarded as a straight line. with the least-square method, the fitting curve can be expressed as: y a bxn u= + (17) where a and b describe vectors. supposing that: j a b y a bxu u u n ,( ) = +( )  = ∑  2 1 (18) where n = 1,2,3 ..., n. the result is: nb b x y a x b x x y u u u n u n u u u u u n u n u n + = + =          == === ∑∑ ∑∑∑ 11 2 111 (19) the least squares solution can be obtained through equations (17) to (19). in order to realize the simulation calculation, the matrix is expressed as: a b x x x x x u t u       =                         1 1 1 1 1 1 1 1 1 1        xx y yu t u                       1  (20) the results obtained previously were used as quasi observations and a kalman filter calculation model was introduced. kalman filter is a recursive linear minimum variance estimation technique that uses recursive estimation to calculate continuous measurement data. the gps measurement data was processed by the least square method, and the obtained coordinates were used as quasi observations. because the processed data was approximately linear, kalman filter was used to optimize the measurement results (li et al., 2016). the kalman filter steps are as follows: (1) the initial values are calculated. when u =1 and u = 2, the train positions calculated by the least squares method are (x1, y1) and (x2, y2). and the system initial value x2 can be expressed as: x x y x x y y t 2 2 2 2 1 2 1=    (21) (2) using the initial value and state variable in step 1, the estimated value of the system when u ≥ 3 is: x k k y k k x k p k k y k k p k y q k , , , , *  1 1 1 1 1 1 1 ( ) = ( ) −( ) −( ) = −( ) −( ) + −( )     (22) where k represents the state variable, p represents the observed value, q represents the estimated value. (3) the error of the observed value to the estimated value is corrected as: 495high-precision gps measurement method without geographical restrictions using crowd-sensing technology p k p k k p r k p x k x k k p k z t( ) = −( )  + −( )      ( ) = −( ) + ( ) − − − , , 1 1 1 1 1 1 kk px k k p k p k p r kt ( ) − −( )  ( ) = ( ) −( )          − , 1 11 (23) result network connectivity in order to verify the advantages of the deployment strategy proposed by this method, matlab2016a was used to analyze the connectivity stability of the crowd-sensing network. the square grid division was compared with the regular hexagonal grid division in this paper, as shown in figure 3. figure 3. maximum euclidean distance it can be seen from figure 3 that the maximum euclidean distance gradually increases with the increase of the area. the maximum euclidean distance of the square grid division is significantly higher than that of the regular hexagonal grid division. the quality gradually becomes unstable with the increase of the maximum euclidean distance. because regular hexagonal grid division has smaller maximum euclidean distance, and its communication quality is slightly better than that of square grid division, indicating the deployment strategy of the method in this paper has advantages. gps satellite signal acquisition verification according to the “specifications for the design of high-speed railways”, railways designed for passenger trains with a travel speed of 250 to 350 km/h are called high-speed railways. therefore, in this paper, a high-speed motion carrier is used to simulate the rapid driving of the vehicle, and the debugging simulation is carried out on computer. the doppler frequency shift of the signal carrier was set as ± 5 khz and the maximum travel speed of the carrier was set as 300km/h. satellite signal acquisition simulation analysis was performed on the traditional gps receiver and the gps receiver with carrier motion doppler frequency shift. the peak values of the signals captured by the two receivers are shown in figure 4. obviously, after doppler frequency shift is introduced in this method, the correlation of acquisition has been greatly improved, and the peak value has been improved. fig. 5 is the time curve of capturing 20 signals with different intensities in both cases. without the introduction of carrier motion doppler frequency shift, the gps satellite signal acquisition time is shown in figure 5. figure 5. gps satellite signal acquisition time in figure 5, the maximum value of the acquisition time for a conventional gps receiver is 0.36s. in the case of the method in this paper, the maximum value of the acquisition time is about 0.25s. the capture speed was improved by 0.11s. suppose that 10 test positions are the coordinate positions of 10 points evenly distributed on a road. the total length of the test road is 1.5 km. positions 2, 6, and 10 are on the road section without signal blocking. signals in positions 1, 3, 4, 5, 7, 8, and 9 are blocked by trees and houses to varying degrees, and the other test environments are exactly the same. fig. 6 shows the data of satellite capture measurement data using the traditional gps receiver and the gps receiver proposed in this paper: as can be seen in fig. 6, after using a gps receiver that introduces doppler frequency shift of the motion carrier, and the method in this paper successfully captures measurements more often than an ordinary gps receiver. measurement accuracy in order to further verify the measurement accuracy of the method in this paper. the method in this paper was applied to vehicle measurement in high figure 4. verification results of gps satellite signal acquisition 496 yunxiang zhang, bin wang, lei zhang latitude city london and low latitude city lhasa. the geographic location online measurement method based on google earth was used to make a comparison. the results are shown in figure 7 and figure 8. figure 7. measurement accuracy test results of two methods in high latitude city it can be seen from fig. 7 and fig. 8 that the accuracy of the method in this paper is as high as 98.89% and 98.99% in the measurement of vehicles in low-latitude city and high-latitude city, respectively. compared with the online measurement method based on google earth, this method has higher precision. discussion as an emerging research field, a crowd-sensing network is facing many challenges, which can be summarized as the following seven aspects. (1) the common platform of a crowd-sensing network. at present, academia and industry have designed and developed a variety of crowd-sensing applications. they usually have similar or partially overlapping functions and require the same or interrelated sensing data. they are facing a series of common issues and challenges such as data collection, resource allocation, energy conservation, user motivation, security and privacy. at present, this independent development model is very inefficient, resulting in a great waste of resources. therefore, building a common platform for the crowd-sensing network is a basic problem that should be solved urgently in this field. (2) front-end processing of crowd-sensing data. the raw sensing data collected by sensors such as gps, accelerometer, microphone, and camera are usually very noisy, incomplete or redundant, and it is difficult to use them directly. therefore, effective front-end processing algorithms need to be designed, mainly including two types. one is data quality enhancement, including operations such as eliminating noise, filtering abnormal data, recovering lost data, repairing and enhancing low-quality images. the other is context inference, which includes inferring traffic patterns, sports patterns, and social occasions (such as meetings, calls, watching tv, etc.) of users and the surrounding environment (such as road bumps, noise levels, etc.). (3) efficient transmission of crowd-sensing data. many crowd-sensing applications need to continuously collect the sensing data and transmit it to the data center. the transmission method based on the mobile cellular network will consume equipment power and data traffic of the users, and put a lot of pressure on the mobile cellular network. therefore, energy-efficient data transmission methods need to be designed. for example, in the short-range wireless communication methods, the opportunity of users to contact each other or users to contact with wifi hotspots is used to forward data. (4) value mining of crowd-sensing data. crowd-sensing data comes from different users, different sensors, and has features such as multi-modality and multi-association. these massive data must be intelligently analyzed and mined to effectively exert value and form a leap from data to information and even to knowledge. the technologies involved include big data storage and processing, data quality management, and multi-modal data mining. (5) resource optimization of crowd-sensing network. overcoming the resource constraints of mobile nodes in the terms of energy, bandwidth, and computing is the key to the practical application of the crowd-sensing network. first, the number of users and the availability of sensors change dynamically over time, and it is difficult to build models and make predictions accurately based on energy and bandwidth requirements to complete specific sensing tasks. second, it is necessary to select an effective subset from a large number of users with different sensing capabilities, and reasonably schedule the sensing and communication resources under resource constraints. (6) incentive mechanism of crowd-sensing network. crowd-sensing applications rely on the participation of a large number of ordinary users, and the users will consume their device power, computing, storage, communication and other resources and bear the threat of privacy leakage when participating. therefore, a reasonable incentive mechanism must be designed as repay for the users, which ensures the quality of required data. figure 6. gps comparison of satellite acquisition and positioning figure 8. measurement accuracy test results of two methods in low latitude city 497high-precision gps measurement method without geographical restrictions using crowd-sensing technology (7) security and privacy protection of the crowd-sensing network. sensing data may leak the privacy and sensitive information of the users, so a reasonable privacy protection mechanism must be designed to ensure the privacy of the users while completing data collection as much as possible. conclusion crowd-sensing has risen in recent years and is still in its early stages. its development depends largely on the development and popularity of mobile devices. in the process of crowd-sensing, the collection and use of data is very important. based on crowd-sensing technology, this paper proposes a high-precision gps measurement method that is not restricted by geographical location. a crowd-sensing big data network deployment mechanism based on regular hexagon was designed. based on this mechanism, a rapid high-precision gps/ sins/dr combined method was used to achieve high-precision measurement without geographical restrictions. the crowd-sensing network in this paper has high network connectivity and good communication quality. and after doppler frequency shift is introduced into this method, the correlation of its acquisition has been greatly improved, and the acquisition peak has been improved. the traditional gps receiver has a maximum acquisition time of 0.36s without the carrier motion doppler frequency shift. in this method, the carrier motion doppler frequency shift is used, and the maximum acquisition time is about 0.25s, which is increased by 0.11s. and the number of acquisition measurements is higher than traditional gps receivers. in lowlatitude city and high-latitude city, the measurement accuracy of the method in this paper is as high as 98.89% and 98.99%, which illustrates that it can achieve high-precision gps measurement without geographical restrictions. in the location service application, the crowd-sensing network system should pay attention to the setting of location privacy protection and incentive mechanism. location privacy leakage is the biggest threat to such applications. in location-based services, sensitive data is primarily user location information. an attacker can infer the sensitive information of the users based on the location information sent by them, including home address, commuting route, etc. in a crowd-sensing system based on location service, each user sends data to the server. this data often contains information about user privacy, such as their location and time. in the crowd-sensing system, any individual or organization can initiate tasks and motivate users to participate in sending data, so the application server is considered untrusted. if location information of the users is leaked to an untrusted server, it will cause great harm to the user. even if the user does not send data using their real name, the server can identify the specific user or reduce the anonymity of the user according to the relevance of the data sent by the user. the goal of user location privacy protection is that the server cannot establish the association between user identity and data, so the server cannot identify a specific user based on the data sent by the user. how to protect the location privacy of the users is a major problem faced by the crowd-sensing system. reference cai, a. h., he, j. w., & wang, p. 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(2018). image feature recognition method for dangerous situation in intelligent substation. chinese journal of power sources, 42, 597-600. dynamic behavior of underground structures is controlled by the strain field imposed by wave propagation and by the interaction between rock mass and structures. shear and pressure waves propagating in the plane of the cross-section of the tunnel generate ground distortions, which tend to cause ovaling deformations of the lining. in this paper, the seismic response of a circular tunnel subjected respectively to shear waves and pressure waves will be analyzed both analytically and numerically at first, and then a complete 3d analysis will be given to show the overall effects on a tunnel induced by seismic events considering seismic inputs in three directions. abstract keywords: dynamic behavior, wave propagation, underground structures comparison between numerical and analytical analysis on the dynamic behavior of circular tunnels issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.72248 qingrui lu1,*, shijun chen1, yi chang1, chunfeng he1 1east china university of technology, school of civil and architecture engineering, nan chang, china e-mail: qingrui_lu@ecit.cn earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 119-128 el comportamiento dinámico de estructuras subterráneas se controla por el campo de deformación impuesta por la propagación de ondas y por la interacción entre la masa rocosa y las estructuras. la onda de cizallamiento y la onda de presión en el plano de sección transversal del túnel generan distorsiones del terreno que tienden a causar deformaciones ovaladas del revestimiento de la estructura. en este artículo se analiza la respuesta sísmica de un túnel circular sujeto respectivamente a ondas de cizallamiento y ondas de presión tanto analítica como numéricamente. luego se muestra un análisis tridimensional completo para mostrar los efectos generales en un túnel producidos por eventos sísmicos y donde se consideran registros en tres direcciones. resumen palabras clave: comportamiento dinámico; onda de propagación; estructuras subterráneas. record manuscript received: 14/11/2017 accepted for publication: 10/06/2018 how to cite item: lu, q., chen, s., chan, y., & he, c. (2018). comparison between numerical and analytical analysis of the dynamic behavior of circular tunnels. earth sciences research journal, 22(2), 119-128. doi: http://dx.doi.org/10.15446/esrj.v22n2.72248 comparación entre análisis numérico y método analítico del comportamiento dinámico de túneles circulares en g in ee r in g g eo lo g y 120 qingrui lu, shijun chen, yi chang, chunfeng he 1. introduction tunnel linings must be designed to withstand static overburden loads as well as seismic loads. according to dowding and rozen (1978), seismic effects on underground structures might be grouped into three categories as (1) ground shaking, (2) fault dislocation, and (3) ground failures such as liquefaction and slope instability. as referred to seismic response of underground openings, the reaction caused by ground shaking is mainly considered. seismic behavior of underground structures is controlled by the strain field imposed by wave propagation and by the interaction between rock mass and structures. according to owen & scholl (1981), three primary modes of deformation, that is, compression/extension, longitudinal bending, and ovaling of the cross section, might be undergone during seismic shaking. shear and pressure waves propagating in the plane of the cross-section of the tunnel generate ground distortions which tend to cause ovaling deformations of the lining. the resulting change of the shape of the tunnel section creates circumferential strains in the coating which can cause cracking and crushing of the concrete and reduce the carrying capacity of the lining. the transversal behavior is usually studied by analyzing the response of the cross-section to an imposed uniform strain field using the pseudo-static approach (penzien, 2000; joe et al. 2018), some analytical solutions have been developed by wang et al. (1993), penzien & wu (1998) and penzien (2000), bobet (2003) and corigliano & barla (2007) to compute internal forces in the lining due to equivalent static ovaling deformations. more recently, several numerical studies have also been carried out for circular tunnels, mainly confined to 2d plane strain analysis, to evaluate the dynamic response of underground structures subject to earthquakes (hishashi, 2001 & 2005; barla, 2008; len et al. 2018). according to the previous studies, the most critical deformation is the ovaling of tunnel cross section (wang, 1993; bobet, 2003; corigliano, 2007; rahim et al. 2018). however, the axial force in the longitudinal direction may induce tensile stresses in the lining which may be the cause of secondary effects such as water inflow, leakage, etc. thus, the longitudinal forces caused by wave motions along the structural axis cannot be completely ignored, and a 3d analysis is generally necessary to be performed to give a complete evaluation of the seismic response of underground structures. the uncoupled seismic response of a circular tunnel will be analyzed both analytically and numerically at first, and then a complete 3d analysis will be given to show the overall effects of a tunnel induced by seismic events considering seismic inputs in three directions. 2. analytical analysis of seismic response of circular tunnels based on the newmark’s approach, st. john & zahrah (1987) developed an analytical procedure for estimating the free-field longitudinal, normal, shear strain and curvature, due to p, s, and rayleigh waves, as shown in table 1, which formed the basis for the subsequent closed-form solutions for the estimation of internal forces of underground structures. 2.1 transversal response as described in the introduction, ovaling deformation of the lining, induced by the propagation of shear and pressure waves in the plane of the cross-section of the tunnel (figure 1), is generally considered as the most critical deformation pattern during a seismic event. according to penzien (2000), the response of the cross-section induced by seismic motions can be considered as the response to an imposed uniform strain field. the assumption is reasonable due to that the dimensions of a typical lining cross-section are small compared with the wavelength of the dominant ground motion producing the ovaling, and the inertial effects in both the lining and the surrounding ground are relatively small (harith et al., 2018). based on this, in most underground openings, the quasi-static conditions are usually satisfied, that is, the static far-field stresses could approximate the earthquake loading. table 1. strain and curvature due to body waves. figure 1. ovaling of circular tunnels induced by propagation of pressure wave and shear wave (esmaeili, 1998) most of the analytical solutions for the cross-section of a tunnel, in general, are developed based on the relative stiffness method, and the kinematic soil-structure interaction, which is of the foremost importance in the soilstructure interaction, is severely influenced by the relative stiffness of the ground concerning the lining (mahmood et al. 2018). the ratio, quantified by the “compressibility” and the “flexibility,” is used to represent the soil-structure interaction in the analysis. there is no unique definition of the two parameters in the available literature, and two types of descriptions are widely used in the analytical solutions. the definition, later used in the analysis of wang (1993), was proposed by hoeg (1968) and peck et al. (1972), with c=compressibility and f=flexibility, as follows: where r is the tunnel's radius; as and is are respectively are area and moment of inertia per unit length of the lining; eg, es, vg and vl correspond to young’s modulus and possion’s ratio of ground and lining, respectively. the compressibility and flexibility are measures of extensional and flexural stiffness (resistance to ovaling), respectively, of the medium relative to the lining. while another definition, expressed by einstein and schwartz (1979), was then accepted by a. bobet and corigliano & barla (2007) in their developed analytical solutions, with c*=compressibility and f*=flexibility, as follows: 121comparison between numerical and analytical analysis on the dynamic behavior of circular tunnels according to mohraz et al. (1975) and einstein et at (1979), the assumptions of full-slip or no-slip conditions which exist along the interface between the ground and the lining are very important when developing the closed-form solutions. for most tunnels, the state at the interface is between fullslip (no shear stress transmission) and no-slip (no relative shear displacement), it is suggested to investigate both cases when computing the forces and deformations in the lining, and the more critical one should be considered seriously in design (wang, 1993; abija & nwankwoala, 2018). 2.1.1 shear wave propagation in the cross-section plane when shear waves propagate transversely in the cross-section of the underground structures, the induced shear strain is equal to the state of stress shown in figure 2: figure 2. state of stress corresponding to a uniform, pure shear deformation (penzien, 1998) figure 3. state of stress corresponding to a general deformation (bobet, 2003) peck, hendron, and mohraz (1972) proposed closed form solutions regarding thrust, bending moment and displacement under external loading conditions, based on the work by burns and richard (1964) and hoeg (1968), but the solutions didn’t take the seismic loading into account. wang (1993) reformulated peck’s solutions, by replacing the in-situ overburden pressure with free-field shear stress and simulating the simple shear condition in the field, adapting to the loading caused by the seismic shear wave, and obtained the equations of internal forces for full-slip conditions. due to that the fullslip condition, according to wang, gives more conservative results concerning the maximum bending moment and lining deflections, no solution needs to be specially developed for calculating the maximum moment under no-slip condition. for both no-slip and full-slip conditions, penzien & wu (1998) and penzien (2000) developed similar solutions for evaluating the kinematic soil-lining interaction behavior of a circular tunnel produced by free-field soil response under the seismic loading as well as by overburden pressure at the soil surface and by the relaxation of the in-situ stresses near a lining. both wang’s and penzien’s solutions are based on the following assumptions: (1) linear elastic deformations of ground and structure. (2) plane strain conditions on any section perpendicular to the longitudinal axis of the tunnel. (3) ground deformations are transmitted to the structure only through shear stresses at the interface between ground and structure. according to the later validation work by hashash et al. (2005) and barla et al. (2008), the solutions obtained by two authors are identical for the condition of full-slip, but the result by penzien (2000) significantly underestimates the thrust for the circumstance of no-slip. considering the drainage conditions at the ground-line interface and the effect of groundwater pressure on ground and support response, bobet (2003) presented an analytical solution to evaluate the impact of the seismic loads or overburden loads on the support, by extending einstein & schwartz’s resolutions for the thrust and moment of the tunnel linings. but he did not mainly take into account the condition of full-slip assumption. corigliano & barla (2007) developed a simplified analytical procedure for estimating the earthquake-induced stress increment on the lining of deep tunnels. the closed-form solution addresses an unresolved issue related to the discrepancy between the closed-form solutions proposed by wang (1993) and penzien (2000) which provide different results for the case of no-slip conditions. besides, to determine the maximum shear strain in free-field conditions, which is the controlling parameter for the closed-form solutions for calculating internal forces, he estimated the displacement time histories at four points around the cross-section of the tunnel using the hisada & bielak (2003) approach through the two-point-central finite difference operators. it has been proved to be quite useful for the evaluation of the seismic response of tunnel supports. the above closed-form solutions developed respectively by wang (1993), penzien & wu (1998) and penzien (2000), bobet (2003), and corigliano & barla (2007) will be later adapted to carry out the analytical analysis for the transversal response of a tunnel, and the detailed expressions are presented in the appendix. 2.1.2 p wave propagation in the cross-section plane research has shown that transverse shear waves, generally, transmit the highest proportion of the earthquake’s energy, and amplitudes in the vertical plane have been typically estimated to be a half to two-thirds as high as those in the horizontal plane. however, in recent earthquakes such as northridge and kobe, measured vertical accelerations were equal to and sometimes larger than horizontal accelerations. the vertical component of ground motion has become an essential issue in seismic designs (hashash et al. 2001; nwankwo & nwankwoala, 2018). according to yang et al. (2002), vertical ground motion, may presumably be considered to be mainly related to p waves although in a realistic seismic environment there may exist other contributions (surface waves and converted sv waves). when pressure waves, p-waves, propagate in the direction perpendicular to the axis of underground structures, according to the analytical procedures of st. john (1987), vertical strain , could be produced, where vmax is the peak ground velocity, and vp is the ground pressure wave velocity. because the propagation of the vertical motion produced almost only compression stress (yang et al. 2002; sunny et al. 2018), the free-field vertical displacement is constant everywhere in the continuum; the horizontal strains must be zero (in the free-field any vertical plane is a plane of symmetry) (bobet, 2003; nwankwoala & ememu, 2018). accordingly, a mechanically equivalent expression regarding far-field stresses, as shown in figure 3, is: different from the solutions of wang (2003) and penzien (2002) for seismic response of tunnels, , which are mainly developed against dynamic response induced by shear wave propagation, the solutions designed by bobet (2003) and corigliano & barla (2007) however, are suitable for all far-field stress conditions based on the equivalent static assumptions. the correspondent closed-form solutions of bobet (2003) and corigliano & barla (2007) could be obtained using the above expressions of far-field stresses. 122 qingrui lu, shijun chen, yi chang, chunfeng he 2.2 longitudinal response figure 4. compression/extension induced by seismic shaking (owen & scholl, 1981) when the components of seismic waves produce motions parallel to the axis of the tunnel, alternating compression and tension will be induced, as shown in figure 4. as summarized by corigliano (2007), two approaches are available for the evaluation of the free-field response of underground openings: one approach based on local forces acting on the structure and the other based on the induced strain. however, the former method, described by st. john (1987) in detail, is primarily confined to the motion caused by sinusoidal waves. thus, this study will present the approach based on the comparison between induced strain and a limit strain which depend on the type of lining material. generally, the behavior of a tunnel in the longitudinal direction is considered to be similar to that of an elastic beam subject to deformations or strains imposed by the surrounding ground. when a por s-wave propagate at an angle ϕ to the axis of the structure, the stresses experienced by the tunnel structure can be easily calculated. according to st. john (1987), the combined longitudinal strain from axial deformation and bending deformation could be calculated using the following equations: for a p wave, for a p wave, st. john and zahrah (1987) also developed a winkler-type model to analyze the longitudinal behavior of tunnels subjected to harmonic seismic waves, which gave closed-form expressions for the maxima axial and shear forces as well as bending moments acting on a tunnel section, and in the expressions, the spring constant was defined to be a function of the wavelength, tunnel diameter, and elastic soil constants. according to corigliano (2007), the axial force, which may induce tensile stresses in the tunnel lining, can be assessed reliably through a free-field analysis, knowing the free-field axial displacement along the tunnel upon the evaluation of the axial strain : also, assuming subjected to shear wave emotion, the axial force in freefield conditions computed from the velocity time histories has been obtained from the following expression: where vff (t) is the particle velocity of the soil in free-field conditions, pg is the density of the rock mass, cs is the shear wave velocity of the soil and al is the area of the cross-section. 3. numerical analysis of a specific case because of the complexity of the seismic soil-structure interaction problem for underground structures, numerical techniques are required to be used for further analysis of such issues. for analyzing the ovaling effect of the propagated seismic wave in planes perpendicular to the tunnel axis, two-dimensional models may be sufficient to carry out plane strain analysis. considering the dynamic response in the longitudinal direction or for a large underground structure of which the three-dimensional effect cannot be neglected, it is more appropriate to utilize three-dimensional models. a large number of publications have covered the different numerical methods used to analyze wave propagation and ground/structure interaction problems. in finite difference or finite element models, the tunnel is discretized spatially, while the surrounding geologic medium is either discretized or represented by soil springs, and the use of artificial boundaries is required in the numerical model. computer codes available for these models include flac3d (itasca, 1995), sassi (lysmer et al., 1991), flush (lysmer et al., 1975), ansys-iii (oughourlian and powell, 1982), abaqus (hibbitt et al., 1999), and others. the boundary element method, which requires only a surface discretization and takes automatically into account the radiation condition at infinity in the infinite or semi-infinite soil media, may result in an exact representation of the wave scattering phenomena for media exhibiting linear material behavior, and provide all boundaries at hand (usman et al. 2018). in cases where movement along weak planes in the geologic media of shear zones, bedding planes and joints, local stress concentrations and failures in the tunnel may be caused. discrete element models may be considered for the analysis. in these models, the soil-rock mass is modeled as an assemblage of distinct blocks, which may, in turn, be shaped as either rigid or deformable materials, each behaving according to a prescribed constitutive relationship. the relative movements of the blocks along weak planes are modeled using force-displacement relationships in both regular and shear directions (power et al., 1996; veeraragavan et al. 2018). udec (itasca, 1992) and dda (shi, 1989 are two computer codes for this type of analysis. in this study, through a specific case study, the fem analysis using midas/gts was carried out due to supercomputing and mass storage capability of this program. 3.1 introduction to midas/gts on dynamic analysis for dynamic analysis using midas/gts, time history analysis is mainly carried out to seek out a solution for the dynamic equilibrium equation when the interested underground opening is subjected to dynamic loads. it calculates a series of structural responses within a given period based on the dynamic characteristics of the structure under the applied loads. before time history analysis, eigenvalue analysis is conducted to analyze the dynamic characteristics of structures. eigenvalue analysis eigenvalue analysis is also referred to as “free vibration analysis” and is used to analyze the dynamic characteristics of structures. the dynamic features obtained by an eigenvalue analysis include vibration modes (mode shapes), natural periods of vibration (natural frequencies) and modal participation factors. they are determined by the mass and stiffness of a structure. in midas gts, mode shapes and natural periods of an undamped free vibration are obtained from the characteristic equation: where k, m are respectively stiffness matrix and mass matrix, ωn 2 is n-th mode eigenvalue and ϕn is n-th eigenvector value. midas/gts adopts the subspace iteration method for the solution of an eigenvalue analysis, which is suitable for the studies of large structures. 123comparison between numerical and analytical analysis on the dynamic behavior of circular tunnels midas gts uses the modal superposition method for time history analysis, in this method, the damping matrix is composed of a linear combination of the mass and stiffness matrices, as presented below: when a time history analysis is carried out, the displacement of a structure is determined by summing up the product of each mode shape and the solution for the corresponding modal equation as expressed in eq. 17. its accuracy depends on the number of the mode used. this modal superposition method is handy and, as a result, widely used in linear dynamic analyses for large structures. boundary condition two types of boundaries are mainly adopted for the dynamical model: (1) free limit: the soil can move freely in any direction. (2) viscous damper boundary: it is based on the introduction of dampers to the edges of the mesh. loading condition for the time history analysis, dynamic loads are directly applied to the nodes or foundation of a structure, and three components of seismic motions can be simultaneously applied to the model. figure 5. acceleration time histories of the specific seismic event applied to the model. a) horizontal shear wave; b) vertical p wave; c) horizontal wave along tunnel axis. figure 6. 2d numerical model figure 7. 3d numerical model figure 8. free field shear strain time history graph table 2. characteristics of soils table 3. characteristics of lining 3.2 case parameters and seismic inputs the material properties of the soil and lining are presented in tables 2 and 3, together with the tunnel buried depth of 50 m and the radius of 5 m. seismic input waves in the three directions are shown in figure 5, and the horizontal components of the motion (longitudinal and transverse) (figure 5a and 5c) propagate in the form of shear waves, while the vertical part of the motion (figure 5b) propagates in the form of compression waves, that is, p wave. it also can be inferred that for the seismic motion adopted in this case, vertical acceleration is more significant than horizontal accelerations. 3.3 seismic analysis the fem model size for dynamic analysis depends mostly on the effect of vibration reflection from the artificial boundary, while medium frequency contests control the maximum mesh size. using midas/gts, the adopted 2-dimensional model is shown in figure 6, with the dimension of 100m*100m, and the 3-dimensional model, as shown in figure 7, has a dimension of 100m*100m*150m, and the maximal mesh size is 6m. before performing the numerical calculation of soil-structure interaction analysis, the free-field analysis is carried out to obtain strain parameters, which could be used to the available closed-form solutions to carry out the analytical study. 3.3.1 shear wave in the cross-section plane firstly, applying the seismic input shown in figure 5 a) to the freefield model in the x direction, with viscous damper boundary applied to the boundary, 2d numerical analysis was carried out to obtain the shear-straintime-history curve, as shown in figure 8, and the maximum shear strain is obtained as γmax=7.2*10-4, which is used in the closed-form solutions for shear 124 qingrui lu, shijun chen, yi chang, chunfeng he wave propagation to get the analytical results. no slip secondly, assuming no-slip conditions by extracting beam elements (to simulate tunnel lining) in 2d and plate elements in 3d model, the relevant numerical analysis considering soil-structure interaction was carried out. the thrust and bending moment contours of lining obtained by numerical 2d and a) a) a) b) b) b) figure 9. results contours of 2d numerical analysis a) thrust contour, b) bending moment contour assuming no-slip figure 13. results contours of 2d numerical analysis a) thrust contour, b) bending moment contour assuming full-slip figure 14. results contours of 3d numerical analysis a) thrust contour, b) bending moment contour assuming full-slip table 4. the maximum values of the numerical and analytical results induced by s-wave assuming no-slip figure 10. results contours of 3d numerical analysis a) thrust contour, b) bending moment contour assuming no-slip figure 11. comparison of thrust force between numerical and analytical solution assuming no-slip figure 12. comparison of bending moment between numerical and analytical solution assuming no-slip a) b) 3d analysis are respectively shown in figures 9, 10. according to equations by wang (1993), penzien & wu (1998) and penzien (2000), e bobet (2003) and equations and by corigliano (2007), the analytical solutions can be obtained with the maximum shear strain known. the obtained maximum values calculated by both analytical and numerical solutions are presented in table 4, and the comparisons of thrusts and bending moments between numerical and analytical solutions are shown in figures 11, 12. for the thrust forces, it could be concluded that the solutions by penzien result in much lower estimate compared to others, the results of corigliano and bobet are consistent very well with the numerical results; for bending moment, all the solutions seem to be in good accordance with each other. full slip considering full-slip effect, as described above, full slip represents one extreme condition that there is no shear stress transmission in the contact face, thus the full slip effect can be simulated by adding one layer with g=0 between soil and lining. by performing the numerical analysis considering full-slip assumptions, the thrust and bending moment contours of the lining are respectively shown in figures 13, 14. with the maximum shear strain γmax =7.2*10-4, the analytical results could be calculated according to analytical equations. the obtained maximum values calculated by both analytical and numerical solutions are presented in table 5, and the comparisons of thrusts and bending moments between numerical and analytical solutions are shown in figures 15,16. for the thrust forces, it could be concluded that the numerical solution results in a much higher estimate; for bending moment, all the answers are entirely identical. 125comparison between numerical and analytical analysis on the dynamic behavior of circular tunnels figure 15. comparison of thrust force between numerical and analytical solution assuming the full-slip condition figure 16. comparison of bending moment between numerical and analytical solution assuming full-slip figure 18. results contours of 2d numerical analysis a) thrust contour, b) bending moment contour figure 19. results contours of 3d numerical analysis a) thrust contour, b) bending moment contour figure 20. comparison of thrust force between numerical and analytical solution table 5. the maximum values of the numerical and analytical results assuming full-slip table 6. the maximum values of the numerical and analytical results with vertical input 3.3.2 p wave as described previously, under the vertical seismic motion, the internal forces of supporting depend majorly on the vertical strain. firstly, the vertical seismic input (shown in figure 4 b)) was applied to the free-field model and then the vertical strain time history curve could be obtained, as shown in figure figure 17. free field vertical strain time history graph 17, and the maximum vertical strain is obtained as γmax =9.02*10-4. by performing the numerical analysis with the presence of the tunnel (only no-slip condition is considered here ), the thrust and bending moment contours of lining obtained by numerical 2d and 3d analysis are respectively shown in figures 18, 19. the analytical solutions, based on the expressions by bobet (2003) and corigliano (2007), can be obtained with the maximum vertical strain known. the obtained maximum values calculated by both analytical and numerical solutions are presented in table 6, and the comparisons of thrusts and bending moments between numerical and analytical solutions are shown in figures 18,19. for the thrust forces, it could be concluded that the solutions by corigliano and bobet may result in higher results compared to those of analytical solutions; a) a) b) b) 126 qingrui lu, shijun chen, yi chang, chunfeng he figure 21. comparison of thrust force between numerical and analytical solution figure 22. free field axial displacement time history graph figure 24. free field axial strain time history graph figure 25. the axial force distribution of different time along the tunnel axis figure 23. axial displacement time history graph considering ssi interaction figure 27. the ultimate axial strain of free field and axial force distributions figure 26. the axial displacement distribution of free field for bending moment, all the answers seem to accord well with other results. 3.3.3 longitudinal response with the seismic input shown in figure 5c) applied to the model in the longitudinal direction, the numerical simulation was carried out to analyze the axial response of linings. the free-field and soil-structure interaction axial displacement time history graphs are respectively shown in figures 22, 23. as indicated from the charts, the maximum axial displacements of the free field and soil-structure interaction are respectively obtained as: dfree = 2.929x10-2, dssi = 2.617x10-2 . because of the soil-structure interaction, the axial movement is nearly decreased by 10% compared with the displacement of free-field, and the reduction is believed to depend on the relative stiffness between soil and lining structure. according to corigliano (2007), the bending effects (bending moment and shear forces) are negligible. however, the axial force may induce tensile stresses in the lining which may be the cause of secondary effects like water inflow, tunnel leakage, etc. the obtained axial strain time history graph is shown in figure 24, the maximal axial strain is εzz = 2.58x10-5 . the axial force distribution of different time along the tunnel is shown in figure 25, and the axial displacement distribution of free field is shown in figure 26. the ultimate axial strain of free field and axial force distributions are shown in figure 27. as it can be seen, the axial force is in certain proportion to the axial strain of free field. on the other hand, the axial strain could also be inferred from the axial displacement distribution in free-field condition. thus, the underground structure internal forces, especially axial forces, could be assumed through the 127comparison between numerical and analytical analysis on the dynamic behavior of circular tunnels table 7. the maximum values of the numerical and analytical results under the coaction of seismic waves figure 28. results of 2d numerical analysis a) thrust contour, b) bending moment contour of two components figure 29. results of 3d numerical analysis a) thrust contour, b) bending moment contour of three components figure 30. comparison of thrust force between numerical and analytical solution figure 31. comparison of bending moment between numerical and analytical solution deformation state of free field analysis. 3.3.4 the combination of different seismic components as described previously, an only shear wave which propagates in the cross-section plane is mainly considered in the past investigations, due to the previous cognition that shear waves transmit the highest proportion of the earthquake’s energy. however, with the increasing number of exceptions that vertical component is sometimes more significant than horizontal components, for example, according to the above case study, p wave propagation caused the most critical deformation, it is essential to take into account all the seismic components to give a more complete and accurate analysis. assuming that there is no interaction between the seismic response induced by p wave and that caused by s wave, that is, the reaction produced by p wave is uncoupled with that induced by s wave. on the other hand, the wave propagation along tunnel axis does not affect the ovaling effects in the middle cross-section of the tunnel, which could be inferred from figure 27. thus the relevant analytical solutions for could be directly expressed as the combination of equations induced by p wave and by s wave, and the correspondent closedform solutions by bobet (2003) and corigliano & barla (2007) could be obtained based on their developed theories. applying x-direction and y-direction seismic inputs to the 2d and 3d models and then applying all the three components to the 3d model, relevant numerical results could be obtained. the thrust and bending moment contours of the lining of 3d and 2d numerical simulation are respectively shown in figures 28, 29. the obtained maximum values calculated by different mathematical models are presented in table 7, and the comparisons of thrusts and bending moments among different wave combinations are shown in figures 30, 31. it could be concluded that, for thrust forces, analytical solutions result in the highest estimates, while the 2d numerical result is the lowest, for bending moment, the results of analytical solutions are in good accordance with those of 3d analysis, while 2d analysis obtains the lower estimates (sharma & yadav, 2018). on the other hand, for both bending moment and thrust forces, the seismic wave along the structure axis has little effects on the internal forces of the middle section of a) a) b) b) tunnels compared to waves propagating in the cross-section. 4. conclusion for analytical analysis of a tunnel subjected to seismic events, both the transversal and longitudinal solutions are based on the theory that the stress increment due to seismic loading can be assessed through a correct estimation of free-field deformation. through a specific case study, then, the fem analysis using midas/gts was carried out to verify the corresponding solutions. when subject to the shear wave in the cross-section plane, for no-slip conditions, penzien’s answers significantly underestimate the thrust in tunnel linings, and other solutions, including penzien’s solutions to computing bending moment, are proved to be consistent with each other in the calculation of internal forces. for full-slip conditions, a significant discrepancy exists between numerical solutions and analytical solutions for the calculation of thrust, while for bending moments, the results are rather satisfactory. when subjected to p wave in the cross-section plane, the solutions of bobet and corigliano result in a slightly higher estimate for the calculation of thrust, and the comparison of bending moment is entirely satisfactory. for longitudinal response induced by the seismic motion which propagates along the tunnel axis, the axial force can be assessed through a free-field analysis, that is, the axial force distributed along the tunnel axis is in proportion to the axial strain obtained by free-field analysis. numerical analysis and analytical analysis using the derived closedform solutions combining p and s waves were carried out to study the effects of several combinations of seismic components on the lining response. the results show that the numerical analysis also could receive satisfactory results compared with logical analysis. 128 qingrui lu, shijun chen, yi chang, chunfeng he acknowledgments this work was financially supported by the national natural science fund(51668002, 51568001), the jiangxi provincial natural science fund. 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(2002). horizontal and vertical components of earthquake ground motions at liquefiable sites. soil dynamics and earthquake engineering, 22, pp. 229-240 keywords: landform types; loess landform; karst; urban and rural areas; spatial distribution; characteristics. palabras clave: tipos de relieve; forma de relieve loess; karst; zonas urbanas y rurales; distribución espacial; características. how to cite item li, j., cai, z., & duan, l. (2020). spatial distribution characteristics and optimum layout of urban and rural areas in different geomorphological types. earth sciences research journal, 24(3), 267-275. doi: https:// doi.org/10.15446/esrj.v24n3.89450 taking jinghe river basin in the loess geomorphological area and guangnan county in the karst geomorphological area as the study area, the spatial distribution characteristics of urban and rural areas of different geomorphological types are analyzed. by using gis and related statistical analysis software, this paper summarizes three basic urban and rural types: river channel type, plateau surface type, and loess terrace horizon prototype in the loess landscape jinghe river basin. it is known that most towns in the loess plateau gully area are in the jinghe river basin. according to the spatial distribution characteristics of urban and rural areas, the optimal layout based on the main structure of five districts, nine river corridors, and four plates is proposed. using the dem module of arcgis to divide the elevation and gradient of guangnan county, we know that the density of urban and rural settlements in guangnan county is low and the spatial distribution is dispersed, and the distribution of urban and rural settlements shows a strong elevation orientation. the distribution of urban and rural settlements has a normal distribution relationship with the elevation. the largest number of urban and rural settlements is between 2.1° and 25°. according to the present situation of settlement distribution, this paper puts forward some optimization strategies, such as appropriate settlement scale, settlement space development monitoring, and so on. abstract spatial distribution characteristics and optimum layout of urban and rural areas in different geomorphological types características de distribución espacial y disposición óptima de áreas urbanas y rurales en diferentes tipos geomorfológicos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.89450 este estudio analiza las características de distribución espacial de áreas urbanas y rurales de diferentes tipos geomorfológicos al tomar como área de estudio la cuenca del río jinghe, en el área geomorfológica de loess, y el condado de guangnan, en el área geomorfológica kárstica. mediante el uso de sistemas de información geográfica y aplicaciones de análisis estadístico relacionado, este documento resume tres tipos básicos urbanos y rurales: tipo de canal de río, tipo de superficie de meseta y prototipo de horizonte de terraza de loess en la cuenca del río jesshe. en esta cuenca se encuentran asentamientos humanos principalmente ubicados sobre depósitos de loess. de acuerdo con las características de distribución espacial de las áreas urbanas y rurales, este estudio propone un diseño óptimo basado en la estructura principal de cinco distritos, nueve corredores fluviales y cuatro placas. al usar el módulo dem de arcgis para dividir la elevación y el gradiente del condado de guangnan se estableció que la densidad de los asentamientos urbanos y rurales en este condado es baja, la distribución espacial es dispersa y la distribución de los asentamientos urbanos y rurales muestra una fuerte orientación de elevación. la distribución de los asentamientos urbanos y rurales tiene una relación de distribución normal con la elevación. el mayor número de asentamientos urbanos y rurales está entre 2.1° y 25°. de acuerdo con la situación actual de la distribución de asentamientos, este documento presenta algunas estrategias de optimización, como la escala de asentamiento apropiada, el monitoreo del desarrollo del espacio de asentamiento, entre otros. resumen record manuscript received: 12/03/2019 accepted for publication: 11/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 267-275 jing li*1,2,3, zhongyuan cai4, lianru duan2 1northwest institute of historical environment and socio-economic development, shaanxi normal university, xi'an 710048, china 2school of urban planning and municipal engineering, xi’an polytechnic university, xi’an 710048, china 3state key laboratory of green building in western china, xi'an university of architecture and technology, xi’an 710048, china 4school of architecture, xi'an university of architecture and technology, xi'an 710055, china *corresponding author: lijing982426@163.com g eo m o rp h o lo g y https://doi.org/10.15446/esrj.v24n3.89450 https://doi.org/10.15446/esrj.v24n3.89450 https://doi.org/10.15446/esrj.v24n3.89450 mailto:lijing982426@163.com 268 jing li, zhongyuan cai, lianru duan introduction china has a vast territory, and there are great differences in the historical basis, natural environment, and economic development of different regions. therefore, the trend of urban-rural interaction development in different geomorphic types of regions is also different (zhang & fan, 2017). geomorphology is the undulating form of the lithospheric surface, and it is also an integral part of the natural geographical elements. in the evolution of urban and rural spatial form, on the one hand, geomorphology directly or indirectly acts on the development and expansion of urban and rural space, causing different responses and changes (aumaître et al., 2017). on the other hand, geomorphology is the main place for energy exchange and transformation between urban and rural space and other elements and becomes the basis for the development of urban and rural space. moreover, geomorphology itself has undergone profound changes under the influence of other elements, including urban and rural spatial expansion (samadzadegan et al., 2017). in a word, landform and urban-rural space interact and promote each other, making the whole living environment change constantly. correctly measuring the spatial distribution characteristics and optimizing the layout of urban-rural interactive development level is the premise of taking effective measures to coordinate urban-rural relations and narrow the gap between urban and rural areas. it is also the basis of choosing a reasonable model of urban-rural development and formulating optimal countermeasures for urban-rural interactive development in various regions. many scholars at home and abroad have carried out relevant studies on the spatial distribution characteristics of urban and rural areas in different geomorphic regions. under the background of rapid urbanization, american scholars pak and brecko analyzed the impact of land use structure change, population size, and industrial structure adjustment on the development of urban and rural settlements in suburbs. scholar liu xiantao studied the influence of topography, roads, and rivers on the spatial distribution characteristics of urban and rural settlements by using the method of cv value of the voronoi map. it was shown that topography was the dominant factor affecting the spatial distribution of urban and rural settlements, while road traffic was the second factor affecting the spatial distribution of urban and rural settlements. zhang xia, a scholar, used spatial analysis and landscape index analysis to analyze the spatial distribution pattern of urban and rural settlements in hilly areas of chongqing and its influencing factors. it was concluded that there were many urban and rural settlements in hilly areas, but the scale of urban and rural settlements was generally small. the concentration of spatial distribution was low, and with the increase of terrain gradient, the area and average scale of urban and rural settlement land showed a relatively decreasing trend. therefore, topography was the most direct factor affecting the distribution pattern of urban and rural settlements, which restricted the land use, scale, shape, and distribution characteristics of urban and rural settlements (li et al., 2017). studying the spatial distribution characteristics and optimization of urban and rural areas will help to understand the spatial distribution law of urban and rural interactive development in an all-round way and provide a decisionmaking basis for promoting urban and rural interactive development in different regions. based on the relationship between landform types and urban-rural development, this paper focuses on the spatial distribution characteristics of urban and rural areas in loess and karst landform regions, compares the spatial distribution characteristics of urban and rural areas under different landform types, and puts forward corresponding optimization measures. taking jinghe river basin as a typical area of loess landform type, this paper uses gis and related statistical analysis software to superimpose and analyze the terrain and landform, urban and rural patches of jinghe river basin, summarizes three basic urban and rural types: river channel type, plateau type, and loess terrace prototype, and quantitatively studies the spatial distribution pattern of urban and rural areas in jinghe river basin, according to the suitable index of human settlement environment and life span of jinghe river basin. from the perspective of spatial distribution characteristics of eco-spatial environment, social and economic development status, conforming to the future needs of coordinated development of environment and economy, and establishing eco-friendly “low-carbon” urban-rural system spatial structure, this paper elaborates the optimization of urban-rural spatial distribution characteristics in loess landform type regions. taking guangnan county of yunnan province as a typical karst landform, this paper uses dem module of arcgis software to analyze the basic characteristics of the spatial distribution of urban and rural settlements in guangnan county of yunnan province and explores the influence of natural factors on the spatial distribution of urban and rural settlements in karst mountain areas, so as to provide a basis for optimizing the pattern of urban and rural settlements in karst landform type regions. material and methods overview of loess geomorphology research area located in the north and west of china, the loess plateau covers a total area of 642,000 km2, of which 454,000 km2 is the most serious area of soil erosion and the most fragile ecological environment in china and even in the world. 80% of villages and towns in the loess plateau are located in small watersheds or related to small watersheds, and nearly 60% of the population live in them. the contradiction between the fragile ecological environment and the rapid development of urbanization in the loess plateau area, the contradiction between the comprehensive management of soil and water conservation ecological environment and the lagging concept of architecture discipline, gave birth to the concept of ecological living environment (lu et al., 2017). jinghe river is the largest tributary of weihe river, the first tributary of the yellow river. it originated at the eastern foot of liupan mountain in ningxia. it has successively formed radial water systems, such as namalian river, puhe river, and heihe river, and injected the weihe river near chenjiatan, gaoling county of shaanxi province. the total area of the basin is 46538 square kilometers. due to the cutting of the jinghe river and its tributaries, the jinghe river basin has formed four landforms: loess plateau-gully area, loess hilly-gully area, loess rocky mountain area, and loess terrace plain area. among them, the most representative is the loess plateau-gully landscape in the central part of the jinghe river basin and the loess hilly-gully landscape in the northern part of the jinghe river basin (yasuhara et al., 2017). therefore, the spatial distribution of urban and rural areas in the jinghe river basin on the loess plateau has unique regional characteristics. jinghe river basin is one of the most densely populated areas in the loess plateau, and the whole region contains four basic types of landforms in the loess plateau. it is also the main area for the construction of human settlements and the prevention of ecological damage and environmental pollution in the loess plateau (nematollahi et al., 2018). landform characteristics of loess the surface of the jinghe river basin decreases gradually from northwest to southeast. the ground elevation is mainly between 1000 and 1600 meters above sea level. the vast area of the jinghe river basin is the loess geomorphic area. the loess layer is deep, forming a typical loess layer profile (mori, 2018). because of the cutting of the jinghe river and its tributaries, the landform landscape of the loess with the source-gully alternation is shown in figure 1. research method of spatial distribution in this paper, based on the human settlements environment assessment index of jinghe river basin, combined with the overall goal of the development of jinghe river basin, the knowledge of ecology, geography, human settlements environment, and other related disciplines are applied, the elements of future traffic planning, industrial layout, population development prediction, etc. are added to determine the idea and overall direction of regional planning. using arcgis geographic information system software and envi remote sensing processing software, spss analysis and statistics software, and other methods and means, the development strategic planning program of the urban-rural system of jinghe river basin is formulated (yang et al., 2018). the plan is based on ecological protection, which changes the traditional planning method that takes economic benefits as the main factor and ignores ecological benefits. 269spatial distribution characteristics and optimum layout of urban and rural areas in different geomorphological types of the spatial distribution of urban and rural settlements in regional planning and hopes to provide theoretical guidance for the construction of new socialist urban and rural areas in the loess landform region (devos et al., 2018). results spatial distribution characteristics and optimal layout of urban and rural areas in loess landform area types and forms of urban and rural areas the results of the relationship between urban and rural types in different forms and topography in the jinghe river basin of the loess plateau are shown in figure 2. figure 2. relations between urban-rural types and terrain and geomorphology in different forms river channel type: the urban and rural areas are distributed on both sides or on one side of a wider river channel. due to the limitation of topography, the urban and rural land extends along the river banks or valleys, forming a zonal layout. the main characteristic of the banded layout is that its plane structure develops in the same direction with traffic lines and traffic flows, and has a strong directionality (peng et al., 2017). land tends to develop along rivers. residential, commercial services and industries are arranged on one or both sides of the main traffic lines. in the early stage of urban and rural development in chuandao, because of the small area, small scale, population concentration, low degree of regional differentiation, and less restrictive effect of terrain on urban and rural areas, the urban and rural agglomeration and compactness are high, showing a crumby structure. with the growth of urban and rural population, the expansion of urban and rural scale, and the differentiation of urban and rural functions, the spatial structure also changes. landform has a decisive impact on the expansion of urban and rural areas. urban and rural expansion generally chooses better conditions for the development of the river channel, and urban and rural areas begin to develop in a zonal manner. plateau surface type: urban and rural location is the plateau surface located in the loess plateau gully area and residual plateau gully area. generally, the large plateau surface (dongzhi plateau) is 400-500 meters higher than the valley, while the small plain (shili plateau) is 200-300 meters higher than the valley (wan mahari et al., 2018). the erosion around the plateau is active and serious, while the loess overlay on the plateau is deep, the land is flat and the soil quality is good, mainly homogeneous arable land. the cultivated land area is large, the farmland forest network is grid-like, and the patches of villages and towns are mosaic. in qingyang, dongzhi, and xiaojin towns on dong zhi plateau, changwu county on changwu plateau, majia town, and shili township on shili plateau, miaoqu town and mengba town on mengba plateau, urban and rural morphology mostly develop into blocks in the plateau survey of karst geomorphology research area research area guangnan county is located in the southeastern part of yunnan province, at the junction of yunnan, guangxi, and guizhou provinces, adjacent to guangxi zhuang autonomous region, northeastern wenshan prefecture. it is located at east longitude 104°31′105°39′, and northern latitude 23°29′-24°28′. its total land area is 7730.23 km2, including 11 townships and 166 administrative villages. it connects funing county and guangnan county to the east, xichou county, and malipo county to the south, qiubei county and inkshan county to the west, and xilin county of guangxi zhuang autonomous region to the north. it is located in the northeast of the karst plateau in southeastern yunnan, the terrain is high in the north and low in the south. the highest elevation point is barley land in the southwest, with an elevation of 2035 m. the lowest point is in the xiyang river area of banbeng township, with an elevation of 480 m. in the north, eroded low mountains, low-medium mountains, and mid-mountains are the main areas of potential rocky desertification; in the south, karst areas are serious rocky desertification, densely covered with karst rocks, mostly carbonate rocks, with thousands of karst landforms (külköylüoğlu et al., 2017). the study area is a sub-hot plateau monsoon climate with an average annual temperature of 16.6°c. at the end of 2011, the total population of guangnan county was 661.07 million, of which 660.784 million people were agricultural, accounting for 99.96% of the total population, which is a typical agricultural-dominated region. the county’s gdp is 545.5814 million yuan, per capita gdp is 824.54 yuan, and ethnic minorities are 406.39 million, accounting for 61.47% of the total population, which is a typical ethnic minority region. the per capita gdp is lower than the average level of the whole province, and the economic development is relatively backward. it is a key poverty alleviation county in yunnan province. influenced by karst landforms, topography, rivers, highways, economic development level, ethnic minorities, and other factors, there is spatial differentiation in the distribution of urban and rural settlements (zhiqiang et al., 2017). research method of spatial distribution using dem module of arcgis software, the distribution characteristics of urban and rural settlements in different areas of karst landform are analyzed by dividing the elevation and slope of guangnan county (dai et al., 2017), to explore the distribution and development trend of urban and rural settlements hidden behind the image, and to provide a research basis for further exploring the formation mechanism and historical evolution of regional settlements of karst landform type. this paper provides a scientific basis for the optimization figure 1. landscape of loess geomorphology source-gully intersection 270 jing li, zhongyuan cai, lianru duan topography. urban and rural development extends from inside to outside, mostly along traffic trunk lines. traffic trunk lines often become the guide of settlement development and evolution. the loess terrace plain type: it has a flat terrain, fertile soil, abundant water sources, convenient irrigation and drainage, convenient transportation, better soil texture, and better agricultural production conditions. the reclamation rate and cropping index are high, and agriculture and animal husbandry are relatively developed. for jingyang county, luqiao town of sanyuan county, liquan county, fenghuo town, zhao town, and so on, these common forms of urban and rural areas are centralized, compact land use, which is a compact and efficient layout form. cluster form is a common form of terrace plain in urban and rural areas. the land-use layout is compact, the road system presents a network pattern, and the development axis of urban and rural areas is not obvious. after the enlargement of urban and rural scale in the initial stage of belt development, the road system gradually evolved into a more complex network pattern, making urban and rural land more compact and saving land use. spatial characteristics of urban-rural system by the end of 2015, there were 327 urban and rural areas in jinghe river basin, including 2 prefecture-level cities, 21 county towns, and 304 townships. urban density is 0.4/10,000 square kilometers, urban and rural density (district and county level) is 4.9/10,000 square kilometers, and township density is 70.3/10,000 square kilometers. the statistical results of urban and rural quantity in the jinghe river basin are shown in table 1. table 1. quantitative statistics of urban and rural areas in the jinghe river basin grade prefecturelevel cities county town towns (townships) number of urban and rural areas 2 21 304 names of urban and rural areas qingyang city and pingliang city jingyang county, liquan county, qian county, yongshou county, chunhua county, xunyi county, bin county and changwu county in xianyang city; qingcheng county, zhengning county, ning county, heshui county, zhenyuan county, huachi county, zhenyuan county, lingtai county, jingchuan county, chongxin county and huating county in pingliang city; jingyuan county and pengyang county of guyuan city gaozhuang township, chongwen township, chonghuang township, majiawan township, arctic township, mazhai township, longgao township, etc. according to the topographic and geomorphological features, there are 170 townships in the loess plateau gully area, accounting for 55.9% of the total, while there are only 12 townships in the loess rocky mountain area, accounting for 4.6% of the total. according to the location of river channel, plateau and terrace, there are 142 townships on the surface of loess plateau, accounting for 46.7% of the total; there are only 26 villages and towns in the leung of the loess plateau, accounting for 8.6% of the total. the results of the analysis of the distribution and location of urban and rural areas according to the topography and geomorphology are shown in table 2. optimizing the spatial distribution characteristics of urban-rural system (1) according to the suitable index of human settlements environment and the spatial differentiation characteristics of the ecological space environment, the current situation of social and economic development and the need for the coordinated development of environment and economy in the future, the basic model of large dispersion and small aggregation is adopted. great decentralization maintains the institutions and functions of ecosystems in the form of zero integration. small agglomeration refers to the formation of larger and more agglomerated urban and rural areas on a large plateau and broad river roads. the main problem of the population on the loess plateau is that it is too dispersed (hua et al., 2013). the rational reorganization of urban and rural spatial structure can be achieved by guiding the population in the gully area to flow orderly to the urban and rural areas of the plateau and river channel. the urban-rural spatial system of the jinghe river basin is the main structure of five districts, nine river corridors, and four plates. five areas refer to ecological protection areas, ecological control areas, ecological coordination areas, human settlements control areas, and human settlements construction areas. the corridor of the jiujiang river system refers to the jinghe river, malian river, puhe river, heihe river, and its main tributaries, ruhe river, daxi river, milan river, malan river, and rouyuan river. the fourth plate refers to the dongzhiyuan plate dominated by qingyang city, the upper reaches of jinghe river dominated by pingliang city, the lower reaches of binchang xunjing river dominated by binchang county (changwu plateau) plate and the weihe river stepped plainly dominated by jingyang county and liquan county. the urban-rural system planning of jinghe river basin, in accordance with the principles of urban-rural coordination, perfection of functions, development of small cities and towns, and highlighting special features, promotes the functions of the core cities of pingliang city and qingyang city, optimizes the urban form, cultivates such regional sub-central cities as bin county, changwu county, jingyang county, qian county, liquan county, qingcheng county, ning county, zhenyuan county, huan county and jingchuan county, and improves the urban and rural functions of yongshou county, chunhua county, xunyi county, lingtai county, chongxin county, zhengning county, heshui county, huachi county, jingyuan county and pengyang county, so as to accelerate the key towns building, form a urban and rural system of jinghe river basin with regional characteristics of the loess plateau, and construct a five-level urban and rural system of core cities, regional sub-central cities, general cities, key towns and general towns. the scale of urban and rural classes in the jinghe river basin is shown in table 3. (2) carbon emissions in urban and rural areas can be reduced through industrial restructuring, healthy lifestyle and technological innovation, but these measures cannot change the traffic trip and its corresponding energy consumption and emissions caused by the layout of urban and rural spatial structure (jiang et al., 2013). once the urban and rural spatial structure determined by urban and rural planning is established, the structural adjustment of traffic trip will be very difficult. because of the lock-in effect of spatial structure of urban and rural systems on carbon emissions, the total amount of energy consumed and co2 and other greenhouse gases emitted by transportation increases rapidly and is very difficult to control. therefore, the establishment of eco-friendly urban-rural system spatial structure with “low-carbon” is the key to achieve the goal of ecological urban-rural construction in loess landform. table 2. distribution and location of urban and rural areas in terrain and landform (unit: number) position chuan dao tableland liang mao terrace total topographic features hilly-gullied loess region 55 0 24 0 79 loess plateau gully area 28 142 0 0 170 loess rocky mountain area 12 0 2 0 14 loess terrace plain area 0 0 0 41 41 total 95 142 26 41 304 271spatial distribution characteristics and optimum layout of urban and rural areas in different geomorphological types 1) principles of optimizing regional urban-rural spatial structure integrating the main body of urban and rural spatial planning according to loess landform characteristics, the control and guiding role of planning is strengthened; combining with the landform of jinghe river basin, a regional urban and rural spatial pattern with multi-center, network and public transport corridor-type is constructed, corridor-type development with track or regional public transport guidance is adopted, and the coordination of regional public transport network and spatial layout pattern are emphasized. functional urban areas are organized through the new functional division of labor between urban and rural areas in the region (ji & he, 2019). the adjustment of urban and rural spatial structure should be coordinated with the planning of employment and residence in order to achieve the “low-carbon” urban development goal, and the efficient public transport system should be used to effectively link urban and rural areas together to form a regional “functional balance”; the corridor-oriented spatial organization model guided by inter-city rail transit and regional public transport should replace the satellite-oriented and centripetal-oriented spatial organization model commonly used in current planning. the theoretical basis of spatial structure of urban and rural areas should shift from central geography theory to multipolar network nested theory model. the construction of largescale public facilities should be combined with a public transport hub to control the use of cars. the regional development model with weak control mainly by road network caused by the growth control mode of green belt will eventually promote the use of cars. more encouraging is the green wedge-spaced urban spatial expansion of public transport, focusing on new development of a public transport hub, to achieve a controlled compact relief. 2) strategies for optimizing regional urban-rural spatial structure according to the above theory, at the regional level, it is suggested that the urban-rural spatial structure of the loess landform type areas should follow the corridor-type development model guided by the important trunk roads such as railways and expressways, take the polarized development of urban and rural areas in key areas as the core, and combine the points and axes, form the urbanrural spatial network structure in series. in most other areas, the ecological environment is harnessed and restored to build a green base for urban and rural development (hong et al., 2018). under the restriction and influence of topographic and geomorphological conditions and water resources conditions, urban and rural construction land is mostly distributed in jinghe river. jinghe river basin is the first region to develop human settlements. apart from the military defense function of urban and rural areas, open terrain and adequate water resources are also important ecological support conditions. the main development of urban and rural areas is directly affected by the valley landform. urban and rural construction land develops along the terrain and its rivers under the stress of landform conditions, mostly on the terrace of surface runoff. among them, rural and urban reclamation, landscape, buildings and so on are distributed on the first terrace of jinghe river, while the second terrace is distributed on the low-density urban and rural construction, and the main urban and rural construction is distributed on the third terrace of jinghe river. regionally, the distribution of human settlements is uneven, and there are different distribution characteristics along with the difference of geomorphology. on the whole, the population gradually changes with the geomorphic combination characteristics, which are high in table 3. scale of urban and rural levels in the jinghe river basin scale grade level 1 urban and rural areas level 2 urban and rural areas level 3 urban and rural areas level 4 urban and rural areas level 5 urban and rural areas scale standard (person) 60 ten thousand about 10-25 ten thousand 5-10 ten thousand 3-5 ten thousand 1-3 ten thousand names of urban and rural areas central district of qingyang city and central district of pingliang city binxian county, changwu county, jingyang county, qian county, liquan county, qingcheng county, ningxian county, zhenyuan county, huating county, jingchuan county yongshou county, chunhua county, xunyi county, lingtai county, chongxin county, zhengning county, heshui county, huachi county, jingyuan county, pengyang county taicun town and malan town in xunyi county; runzhen town and fangli town in chunhua county; xinmin town, arctic town and shuikou town, binxian county; xianggong town and tingkou town, changning town, yongshou county; changning town, yongshou county; chagan town, yanxia town and caodong town in liquan county; fengyang town, qianxian county; yunyang town and wangqiao town in jingyang county; shizi town and chaona town in lingtai county; ankou town and maxia town in huating county; wangcun town and yaodian town of jingchuan county; xinyao town of chongxin county; kongtong town, baishui town and shilipu town in kongtong district; jingheyuan town and liupanshan town in jingyuan county; wang wa town and baiyang town in pengyang county; tunzi town, mengba town and pingquan town, zhenyuan county; gonghe town and yulinzi town in zhengning county; changqing bridge town, hesheng town and zaosheng town in ningxian county; old town and country in heshui county; xiaojin town and dong zhi town in xifeng district; yima town, sanlipu town and maling town, qingcheng county; huachi county yuancheng town and country; quzi town and mubo town of huanxian county other towns 272 jing li, zhongyuan cai, lianru duan the south and low in the north, and high in the east and low in the west. it also gradually decreases with the divergence of the main trunk of the jinghe river basin system to the end. spatial distribution characteristics and optimum layout of urban and rural areas in karst geomorphological area spatial distribution of urban and rural settlements in guangnan county (1) overall characteristics of spatial distribution of urban and rural settlements based on the vector distribution map of urban and rural settlements in 2015, arcgis software is used to select the comparative statistical analysis of landscape indices, such as the number of urban and rural settlements, the total area of settlements, the average area of settlements, the density of spots, the minimum area of spots and the maximum area of spots. the landscape indices of urban and rural settlements in guangnan county in 2015 are shown in table 4. table 4. landscape index of urban and rural settlements in guangnan county in 2015 scenery landscape index number of plaques (number) 14016 patch density (per /km2) 0.98% total patch area (m2) 76606672.8 minimum plaque area (m2) 399.02 maximum plaque area (m2) 266582.08 average patch area (m2) 5465.66 the density of urban and rural settlements in guangnan county is obtained by using arc info function in arcgis. generally speaking, the spatial distribution of urban and rural settlements is scattered, but not random and regular. under the influence of the unique geographical environment such as slope and elevation, the areas with moderate elevation and certain gradient are the main districts of urban and rural settlements in guangnan county. the urban and rural settlements in the areas with elevation above 1500 m and slope above 25° are sparse; the scale of urban and rural settlements along rivers is relatively large due to the influence of river system; and affected by the degree of transportation convenience and other factors, the urban and rural settlements along the highway are more intensive. (2) spatial distribution characteristics of urban and rural settlements at different elevations from 460 to 2020 m above sea level in guangnan county, the dem module of arcgis software is used to divide the altitude into five elevation zones by the equal grading method. the attribute data of urban and rural settlements are superimposed with the attribute data of elevation by overlay module, and the distribution area of urban and rural settlements in different elevation zones is obtained as shown in table 5. from table 5, it can be seen that, with the rising altitude, urban and rural settlements gradually become sparse and the climate has obvious changes, the higher the altitude is, the worse the agricultural production conditions are; the main urban and rural settlements in guangnan county live below 1500 m. adequate electricity, convenient transportation and water use are the advantages of low-altitude areas, which are convenient for villagers to live in for a long time. with a relatively large population, urban and rural settlements show a strong elevation orientation. to sum up, the distribution of urban and rural settlements and elevation shows a normal distribution relationship, as shown in figure 3. (3) spatial distribution characteristics of urban and rural settlements on different slopes using the dem module of arcgis software, the gradient is divided into five gradient zones according to the critical gradient grading method. overlay module is used to overlay the attribute data of urban and rural settlements and the related attribute data of gradient, and the distribution area of urban and rural settlements in different gradient zones is obtained, as shown in table 6. from table 6, it can be seen that the largest number of urban and rural settlement patches is between 2.1° and 25°, accounting for 89% of the total. in areas with slopes between 0° and 20°, the total proportion of urban and rural settlements is less than 10%, and the number of urban and rural settlements table 5. statistics of urban and rural settlements in different elevation zones in 2015 altitude (m) 460-955 956-1175 1176-1365 1365-1555 1556-2020 plaque (individual) 1122 2142 4922 3955 2131 number ratio (%) 7.97 15.24 35.06 26.03 15.16 plaque area (m2) 6871562.52 11636443.21 23835251.18 21549560.58 12824410.06 area ratio (%) 8.85 15.06 31.06 28.08 16.61 (a) number of patches on the map of urban and rural settlements at different elevations (b) patch area of urban and rural settlements map at different elevations figure 3. the relationship between urban and rural settlements and elevation in 2015 273spatial distribution characteristics and optimum layout of urban and rural areas in different geomorphological types above 25° is less than 5%. however, the area of urban and rural settlements in areas with slopes above 15° accounted for 76% of the total area. this is because guangnan county is a karst mountain area. limestone in the karst area is easy to be eroded by water to form groundwater. groundwater will be stored in the area with the lowest slope. soil in the rocky desertification area in the guangnan area is easy to be washed down by rainwater to the area with the lowest slope. therefore, residents will use the area with the lowest slope as cultivated land and settlements will choose the place with a certain slope. to sum up, we can get the relationship between the distribution of urban and rural settlements and the size of slope, as shown in figure 4. (a) number of patches on urban and rural settlements maps with different slopes (b) patch area of urban and rural settlement maps with different slopes figure 4. the relationship between urban and rural settlements and slope in 2015 optimizing the spatial distribution characteristics of urban and rural settlements from the above, it can be seen that karst landform determines the landscape index, spatial distribution characteristics and spatial distribution characteristics of urban and rural settlements at different elevations and slopes in guangnan county (soto et al., 2017). therefore, the planning of socialist urban and rural construction in karst geomorphic regions, the optimization of spatial structure of urban and rural settlements and the development of small and medium-sized urban and rural areas should be based on the full investigation of spatial structure and distribution of karst geomorphic settlements. on the other hand, karst environment is a very fragile ecological environment. the premise of every work is to harness, protect and improve the ecological environment, so we must optimize the settlement space structure based on the ecological concept, respect and follow the natural process. (1) respect the spatial self-organization model of existing urban and rural settlements the present settlement is a long-term adaptation to the fragile karst ecological environment and the result of the continuous harmony and unification of the contradictions between human and land. therefore, when optimizing the living space, we must respect the spatial self-organizing mode of the existing settlement, and consider the natural conditions, social and economic development and residential culture of the karst geomorphological type area (martín-díaz et al., 2018). (2) moderate settlement scale karst is a very fragile ecological environment with many mountains and few flat lands. therefore, the environmental capacity of urban and rural settlements development is very limited, and the appropriate scale of settlements must be considered. appropriate settlement scale in karst urban and rural areas should be based on population, land use of settlements, the relationship between them and regional carrying capacity of cultivated land resources, land use structure, water resources, rocky desertification and soil erosion, labor employment, population migration, social and economic development and other factors. regardless of the population of settlements or the scale of residential land, there are huge spatial differences in karst-plateau-basin area, plateau-mountain area and plateau-canyon area. it is necessary to evaluate comprehensively from various aspects and complete the investigation and prediction of the current situation with different methods in order to formulate an appropriate and reasonable scale of urban and rural settlements and optimize the spatial structure of urban and rural settlements in karst landform type areas. (3) monitoring of settlement space development in recent years, urban and rural settlements in karst areas have many new characteristics in spatial extension, such as the empty and waste of old settlements, the strip extension of new buildings along highways, etc. (yan et al., 2018). in addition, the changes of settlement land use, regional land use type and structure, especially the changes of rocky desertification and soil erosion, have an important impact on the development of settlement space, and directly penetrate into residential culture. therefore, it is necessary to use the dem module of arcgis software to monitor real-time in order to investigate the problems in settlement development, predict the trend of settlement evolution, and make a good spatial planning of settlement development. (4) optimizing the residential space of settlements on the basis of the existing settlement spatial pattern, we should optimize the spatial pattern and focus on the control and restoration of the ecological environment, especially the control of rocky desertification and soil erosion. on this basis, we should realize the rational utilization of land and the optimization of spatial structure, strive for more and more reasonable land for urban and rural table 6. statistics of urban and rural settlements in different slope zones in 2015 slope (°) 0-2 2.1-6 6.1-15 15.1-25 >25 plaque (individual) 111002 418703 605243 458222 81206 number ratio (%) 6.62 24.89 36.03 27.25 4.76 pplaque area (m2) 576393931.50 275426853.04 984562766.30 2477144677.65 3416321408.14 area ratio (%) 7.35 3.45 12.63 32.04 44.09 274 jing li, zhongyuan cai, lianru duan settlement spatial development, and develop and protect the cultivated land, forest land and garden land on which we depend for survival. emphasis should be placed on supporting facilities construction, environmental improvement and transportation facilities construction (hong, 2017). discussion the regional topography and physiognomy of loess landform types play a decisive role in the spatial distribution of urban and rural areas. this paper uses gis and related statistical analysis software to analyze the overlap and correlation of the topography and physiognomy of jinghe river basin, urban and rural patches, and summarizes three basic urban and rural types, namely, river channel type, plateau surface type and terrace type. at the same time, the spatial distribution characteristics of urban and rural systems are analyzed in detail. according to the suitable index of human settlements environment and the spatial differentiation characteristics of ecological space environment in jinghe river basin, the present situation of social and economic development and the need of coordinated development of environment and economy in the future, the main structure of urban and rural space system, corridor of five districts, nine rivers system and four plates is put forward under the background of the new round of urbanization in the jinghe river basin, in order to solve the contradiction between urban and rural development and ecological environment deterioration in jinghe river basin, provide guidance for the coordinated development of regional natural environmental protection and social economy, and promote the scientific and orderly development of urban and rural areas. arcgis is used to extract the urban and rural settlement patches and generate the density map of urban and rural settlement in guangnan county. the total area of the patches, the average area of the patches, the density of the patches and other landscape indices are obtained for comprehensive analysis. elevation map and slope map are generated by dem module, and then overlapped with urban and rural settlement patches by overlay module. through the above analysis, the following conclusions can be drawn: (1) due to the high environmental vulnerability and sensitivity of karst mountainous areas and the constraints of human activities, rocky desertification in guangnan county is serious, and the area of rocky mountains is wide and dispersed, which results in the extremely dispersed distribution of settlement patches. because of the difference of natural conditions (elevation, elevation, slope) in guangnan county, there are obvious spatial differences in the distribution of urban and rural settlements. (2) with the change of different elevation zones, the pattern of urban and rural settlements also changes. the pattern of urban and rural settlements at 1100-1500 m elevation is the most, which indicates that this elevation zone is most suitable for the location of urban and rural settlements. (3) from the view of urban and rural settlement patches in different slope zones, there are obvious differences and uniqueness between slope and plain. the number of settlement patches is normal distribution with slope, but the area of urban and rural settlement patches is proportional to slope. conclusions natural environment is the material basis for human survival and social and economic development in urban and rural areas. different geographical units limit the spatial environment for urban and rural development. distribution characteristics and trends of urban and rural space in different geomorphic regions are not only influenced by socio-economic development and national cultural customs, but also by the surrounding natural environment conditions, which have a far-reaching impact on them, and even affect the appearance, size, layout and trend patterns of urban and rural settlements. therefore, in view of the spatial distribution characteristics of urban and rural areas of loess and karst landforms, this paper puts forward the optimal layout strategy, which provides the basis for choosing the reasonable model of urban and rural development and formulating the optimal countermeasures of urban and rural interactive development. there are some shortcomings in this study. because of the different data sources, the results of spatial feature analysis may be different. with the continuous updating and improvement of data sources, this study will be more accurate. in addition, this paper only analyses the natural environmental factors, but fails to carry out relevant research based on the regional socio-economic data of different geomorphological types. therefore, it fails to mine deeply into the characteristics, structure and laws of urban and rural settlements, which will become the direction of further research in the future. acknowledgments this work is supported by national natural science foundation of china, study on the green construction of rural settlement units in northwest china (51678481); shaanxi provincial education department special research project (17jk0460); xi’an university of architecture and technology, 2019 special research fund for new urbanization, (general items); xi’an polytechnic university, the project of general project of philosophy and social science (2019zxsk02). references aumaître, g., bourlès, d. l., & keddadouche, k. 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(2017). tilt license plate image edge fuzzy feature recognition method research. computer simulation, 34, 372-375. zhiqiang, l., pang, r., zhu, j., deng, r., & shen, l. (2017). the correlation analysis of surface thermal environment and settlement characteristics in the hilly city. journal of chongqing university, 40, 60-71. http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201802011 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201802011 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201802011 about:blank http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201701080 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201701080 about:blank investigating the effects of groundwater level changes on gnss observations in the konya closed basin erdoğan, h.,1 oktar, o.,1 gezgin, c.,1* poyraz, f., 2 arslan, n.,3 yılmaztürk, f.1 1 aksaray university, engineering faculty, department of geomatics, aksaray, turkey 2 cumhuriyet university, engineering faculty, department of geomatic engineering, sivas, turkey 3 çukurova university, engineering faculty, department of geomatic engineering, adana, turkey *corresponding author: cemilgezgin@aksaray.edu.tr keywords: konya closed basin; gnss; time series analysis; fast fourier transform; groundwater level. palabras clave: cuenca konya; sistemas globales de navegación por satélite; análisis de series temporales; transformada rápida de fourier; nivel de aguas subterráneas. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.92490 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 405-414 h yd ro lo g y record manuscript received: 23/12/2020 accepted for publication: 29/09/2021 abstract: the konya closed basin (kcb), inhabited by approximately 4 million people, has a semi-arid climate and has been an important agricultural and economic production area with its fertile alluvial soils. in recent years, groundwater use as a primary source in the intense need for irrigation due to climate change, drought, and wrong agricultural policies triggered land subsidence in the basin by causing the aquifer system compaction. in this study, the effects of groundwater level changes obtained from groundwater monitoring stations on the positions of continuously operating gnss stations located in kcb were investigated by time series analysis. as a result of trend component analyses of time series processed based on the itrf08 fixed frame, it was determined that the horizontal positions of stations in this region were moving in the northeast direction (18.74 mm/year). for the up coordinates, while stations anrk, kluu, and nigd had movements 0.56 mm/year in the up direction, the other stations moved in the down direction. the vertical movement of the kny1 station based on 560-day data was in the down (-) direction, with an annual movement of 70.96 mm, and this was the maximum movement or velocity compared to the other stations in this region. the trend analysis of 19 wells revealed linear groundwater level changes in the down direction with a mean value of -39.22 cm/year. the decrease in the water level of the wells in the konya closed basin was statistically significant for the majority of the wells and at meter level in some wells. it was determined that the linear decrease was generally caused by the reduced groundwater level in this region. the periodic component analysis showed that the movements of the gnss stations changed from a day to a year due to several factors, such as atmospheric and hydrological loading and climatic effects. investigación de los efectos del cambio en el nivel de aguas subterráneas a partir de observaciones de sistemas globales de navegación por satélite en la cuenca de konya, turquía resumen la cuenca de konya, habitada por cerca de cuatro millones de personas, tiene un clima semiárido y ha sido una importante área de producción agrícola y económica con sus fértiles suelos aluviales. en los años recientes, el uso de las aguas subterráneas para satisfacer la demanda de irrigación, en un contexto de cambio climático, sequía y políticas agriculturales erróneas, ha ocasionado la subsidencia de los suelos en la cuenca a través de la compactación del sistema acuífero. en este estudio se investigan a través del análisis de series temporales los efectos en los cambios de los niveles de aguas subterráneas obtenidos de las estaciones de monitoreo en la cuenca que operan continuamente con sistemas globales de navegación por satélite (gnss, global navigation satellite system). como resultado del análisis del componente de tendencia en las series temporales con base en el marco ajustado itrf08 se determinó que las posiciones horizontales de las estaciones en esta área se movieron en la dirección noreste (18.74 mm/year). en las coordenadas superiores las estaciones anrk, kluu, y nigd tienen movimientos de 0.56 mm por año hacia arriba, mientras que las otras estaciones se mueven hacia abajo. el movimiento vertical de la estación kny1 de acuerdo con la información de 560 días fue hacia abajo, con un movimiento anual de 70,96 mm. este fue el máximo movimiento o velocidad comparado con las otras estaciones. el análisis de tendencia de 19 pozos revela los cambios lineales de aguas subterráneas hacia abajo con una media de -39.22 centímetros por año. el descenso en el nivel del agua de los pozos de la cuenca konya fue estadísticamente significante para la mayoría de los pozos y en el nivel métrico de algunos pozos. se determinó que el descenso lineal fue causado generalmente por la reducción del nivel de aguas subterráneas en la región. el análisis del componente relacionado a la frecuencia muestra que los movimientos de las estaciones gnss cambian desde un día hasta un año debido a varios factores como la carga atmosférica e hidrológica y los efectos climáticos. how to cite item: erdogan, h., oktar, o., gezgin, c., poyraz, f., arslan, n., & yilmazturk, f. (2021). investigating the effects of groundwater level changes on gnss observations in the konya closed basin. earth sciences research journal, 25(4), 404-414. https://doi.org/10.15446/esrj. v25n4.92490 mailto:cemilgezgin@aksaray.edu.tr https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.92490 https://doi.org/10.15446/esrj.v25n4.92490 406 erdoğan, h., oktar, o., gezgin, c., poyraz, f., arslan, n., yılmaztürk, f. introduction with the help of global navigation satellite system (gnss) stations that continuously collect data, it is possible to examine changes using positional information as a function of time. these data are generally used to observe the slip movements of the earth’s crust and to estimate the slip velocity of the ground (langbein & johnson, 1997; williams et al., 2004; uzel et al., 2013; tiryakioğlu et al., 2017; gezgin et al., 2020; yalvaç, 2020). it is assumed that gnss signals fit a model consisting of components, such as linear slip, semi-annual and annual oscillations, post-earthquake logarithmic oscillation, a starting point, and a noise component. the correct estimation of the model parameters depends on the appropriate modeling of the data (baykut, 2012; gülal et al., 2015; oktar & erdoğan, 2018; özkaymak et al., 2019; tiryakioğlu et al., 2019). there are also other parameters that affect the position accuracy of gnss stations. some of these parameters, such as device selection, measurement method, measurement time, software selection, and process of measurements can be eliminated in stages. the selection of station locations is another important factor affecting position accuracy. since certain areas where gnss stations are used require very high accuracy, stations should be located in areas that are tectonically stable, they should be built on solid rock or hard ground, and the ground point should be monument (calais, 1999). due to withdrawal of groundwater, the compression in aquifer systems can create a vertical deformation on the ground of these systems (üstün et al., 2015; calo et al., 2017; özkaymak et al., 2017; orhan et al., 2020; tiryakioğlu & bozkuş, 2020). some partial land subsidence events in the earth’s crust are being monitored and some are studied with geodetic methods. üstün et al. (2007) observed the gnss stations in the konya closed basin (kcb) located in the central anatolia region in turkey to determine groundwater withdrawal rates and identified changes in the height component between -8 and 18 mm in five months. üstün et al. (2010) carried out a study in the kbc and detected the vertical displacements between -12 and −52 mm/year based on the gnss observations processed between 2006 and 2009. üstün et al. (2015) performed a deformation analysis in the same basin using the gnss stations between 20062012 and insar observations from 2002 to 2009 and obtain land subsidence resulting in the rate of 1–4 cm/year in kcb. another land subsidence study was conducted in konya city center and its vicinity by orhan (2021). it was revealed that there were land subsidence rates reaching −75 mm/yr in the period between 2014 and 2018 using the sentinel-1a sar data. in this study, the daily position information (north, east and up) of 16 continuously operating reference stations-turkey (cors-tr) stations in and around the konya closed basin were used. for 15 stations, the data covered between 2008 and 2014 and for kny1 station 560-day data (2014-2015) were used. the time series of all stations were created to demonstrate the behavior of the stations under various effects at global, regional, environmental and seasonal level, different environmental and climatic conditions, such as crustal movement, movement of groundwater resources, precipitation, and sunspot change, and other factors related to reference system errors (e.g., linear and periodic movements, signal exchange and drifts). in the last decade, it has been seen that there is a decrease in groundwater due to the drought resulting from global climate change and excessive water use (orhan et al., 2014, calo et al., 2018). in turkey, kcb has been affected by land subsidence and sinkhole formation (figure 1). this site is an important agricultural basin of the country. over the years, agricultural activity in the basin was conducted by non-irrigated farming. however, since the 2000s, irrigated farming areas have increasingly become dependent on groundwater usage in kcb (orhan et al., 2021). the increased usage of groundwater in the basin, increased land subsidence and sinkhole formations in the region over the past decade (ahmed et al., 2020; orhan, 2021). this negative variation is considered to affect the movement of gnss stations in the vertical direction. therefore, in this study, the changes in groundwater levels were also examined. study area the konya closed basin is located in the central and southern part of the central anatolia region in turkey. the field is surrounded by bozdağ and obruk plateau in the north and northwest; the inner slopes of toros mountains arc from sultan mountains to the south of the karaman province in the south; ereğli plain in the east; and takkeli mountain, gevele mountain, and loras mountain in the west. the boundaries of the konya closed basin include konya, niğde, isparta, aksaray, ankara, karaman and nevşehir (fig. 1). figure 1. map of the konya closed basin (adapted from orhan et al., 2020). konya closed basin covers 800 km² of hotaş plain (including marsh), 700 km² of karapınar plain, 500 km² of karaman plain, and 2500 km² of ereğli plain (including ayrancı plain) (i̇nan et al. 2006). the formation of sinkholes has increased in recent years in and around the konya closed basin. this is presumed to be due to the decrease in groundwater levels, which has been reported in akgöl (ereğli), acıgöl (karapınar) and timraş lakes, and meke salt pan (karapınar) (göçmez et al., 2004). climate: according to the data analysis of the meteorological stations selected from the konya closed basin the average annual temperature varied between 11 °c and 11.8 °c, and the average annual precipitation ranged from 281.4 mm to 329.2 mm. the difference in the precipitation values is related to the geographical formations and the effects of air masses. at 310.3 mm, the average annual precipitation in the konya closed basin is the lowest in turkey. methodology time series analysis the behaviors of continuously operating gnss stations under different effects described above can be explained by the analysis of time series. excluding artificial or co-seismic and post-seismic slip, the time series y(ti) obtained from the gnss measurements at ti (i=1, 2, 3, …, n) can be generally divided into the following three components (gülal et al., 2013): (1) in the analysis of time series, firstly, the time-axis graph of the series is drawn and the unusual measurements (e.g., gross errors) are excluded to obtain a general interpretation of the series. a suitable low-pass filtering operation (e.g., five-order floating weighted average) is applied to the series to eliminate the noise effect in the series and reveal trend and periodic components more clearly (erdoğan et al., 2007; gülal et al., 2013). then, it is determined whether the series has a trend component, which represents long-term changes that can modeled by the polynomial function in the time domain as: (2) where ck (k=1,2,…,m) are the parameters depending on the order of the polynomial function. linear changes (velocities) in the increasing or decreasing direction of gnss stations are defined by taking m=2. however, the magnitude of linear variation in gnss stations is significantly affected by periodic movements. the effects of atmospheric, tidal and earth rotation 407investigating the effects of groundwater level changes on gnss observations in the konya closed basin parameters are modeled in the processing step of gnss observations using academic software. however, behaviors resulting from periodic effects that are not modeled at gnss stations can also be determined by time series analysis. for this reason, the periodic component must be absolutely determined in the time series analysis of gnss stations. the periodic component in the series can be modeled by a trigonometric function with equation 3 based on fs (s=1,2,...,p) frequencies and p(ti) measurements. (3) where unknown parameters ck,, as, bs and standard deviations of these parameters mck, mas and mbs are estimated by the least square method (lsm). the test size determined for each parameter is compared to ±tf,1-α/2 confidence limits of t-distribution for 1-α confidence limit and f degrees of freedom. the parameters that are found insignificant in the test are removed from the function, and the process is continued until only significant parameters are left. in equation 3, the frequencies of movements must be known to calculate the periodic movement. for this purpose, firstly, it is necessary to determine the trend component(s) of the series using equation 2 and remove them if they are significant. otherwise, these components affect the results of spectral analysis on the series and produce a peak at zero frequency. after the elimination of the trend components from the series, transformation from the time domain to the frequency domain of the series is performed using the fast fourier transform (fft), which is defined by equation 4. (4) where f is the frequency, and e2πjt is the complex and exponential function. since the values obtained using fft are complex numbers, they are still not sufficient to determine the frequency of the series. therefore, the module square of x(f) is calculated and called the fourier spectrum pxx(f). (5) thus, the frequencies of the periodic movements in the series are determined by detecting at which frequencies there is more energy from the signal. the parameters of frequencies were tested by substituting these frequencies in equation 3. research findings and discussion this study investigated the linear movements (velocities) and periodic movements of continuously operating gnss stations in the konya closed basin and the changes in groundwater level. the stochastic components of time series analysis were not analyzed in this study. gnss stations the daily position data (north, east and up) of 16 gnss (cors-tr) stations in and around the konya closed basin were used. the data from 15 stations belonged to 2008 to 2014 and that of kny1 station was obtained from 2014 to 2015 (560 days in total). due to the slip seen in the knya station time series, which was previously installed in the region, kny1 station was later established instead of this station in 2014. therefore, kny1 station has 560-day data. the time series of the 16 stations were created (fig. 2). the data obtained from the general command of mapping and the itrf08-fixed frame were used in the data analysis process. the time series in the north and east directions of the kamn, kapn, knya and kny1 stations from 48 (16.3 = 48) time series obtained for 16 stations are given in figures 3 and 4. moreover, figure 3 shows the linear movements and equations of the series. figure 2. positions of cors-tr stations and wells. figure 4 also shows the up time series, linear changes, function of linear change, and frequency spectrum of the same stations. due to the high number of stations, the time series graphs of other stations are not given in this paper. however, linear functions and station velocities (annual) are given in tables 1 and 2. the time series plots in figures 3 and 4 were obtained by subtracting the first coordinate value of the time series of each station from the other coordinate values. in this case, the signal change at any time point in the time series is at the start time of the signal. it can be seen that the time series in the north and east directions of the stations show a linear increase in the same directions (fig. 3). the same results were obtained in the time series of the other stations, but these are not presented in a figure. in addition, pulse movement was seen at marked points in the north and east time series and the last part of the series in knya. this was even more prominent for the up coordinate values (fig. 4). for this reason, before and after the first pulse were taken into account separately in the calculation of the velocities at the knya station for north, east and up coordinate time series. due to the insufficient amount of data after the second pulse, the remaining days were not taken into account. this situation is shown in the figure 4. the linear movements (velocities) and periodic movements of the stations given in figures 3 and 4, as well as those that are not presented in this paper were determined by performing time series analysis. trend component analysis it was determined whether the time series in the north, east and up directions of gnss stations indicated any linear movement using the proposed function in equation 2. the c1 and c2 parameters of the linear function generated by taking m=2 and standard deviations of these parameters were estimated by lsm. the significance of the deviations of the calculated parameters from the “0” value was determined based on α=0.05 error possibility according to t-distribution (for f>2000, =2 and for f>500, ). as a result of the trend component analysis of gnss stations, the time series of the north and east coordinates of all stations showed a statistically significant linear movement in the positive (increasing) direction. the linear movements and linear functions are given in figure 3. table 1 shows statistically significant linear movements (linear models) and annual linear changes (velocities) for all stations. the “c2” parameter in the trend function indicates the daily (+) increasing or (-) decreasing direction of the linear change. using this parameter, it is possible to identify linear changes or movements of the stations any time. 408 erdoğan, h., oktar, o., gezgin, c., poyraz, f., arslan, n., yılmaztürk, f. figure 3. coordinate time series, linear movements, and linear functions of kamn, kapn, knya, and kny1 stations in the north and east directions. 409investigating the effects of groundwater level changes on gnss observations in the konya closed basin figure 4. coordinate time series, linear movements, and frequency spectrums of stations in the up direction. 410 erdoğan, h., oktar, o., gezgin, c., poyraz, f., arslan, n., yılmaztürk, f. as shown in table 1, gnss stations in the study area had linear movements in the northeast direction (itrf08 fixed frame). after the removal of the stations knya, knya* and knya**, annual mean movement or velocities of the stations calculated as 18.74 mm/yr. as it can be seen in figure 3 and table 1, the pre-slip (knya*) velocity of the knya station is 15.20 mm/yr in the northeast direction, and the post-slip (knya**) velocity is 17.61 mm/yr in the same direction. the velocity calculated for all data belonging to the knya station is also obtained as 21.02 mm/yr in the northeast direction. the results obtained show that the slips change the velocity value and therefore slips should be taken into account before time series analyzes. the stations having an annual velocity greater than 2 cm were; aksr, kamn, kapn, nevs, nigd, kny1. the maximum velocity was 26.55 mm/year, which was obtained from the kny1 station. the minimum velocity was found in akhr at 9.84 mm/year. the linear movements and functions of the up coordinate time series of the gnss stations (kamn, kapn, knya and kny1) are shown in figure 4. in addition, table 2 presents the statistically significant linear movements (linear models) and annual linear changes (velocities) of these stations. table 2. linear functions and annual velocities of gnss stations (up). station up linear functions (y=c2t+c1) linear changes (velocities) (mm/year) akhr y(trend)=-0.0143t+14.3868 -5.22 aksi y(trend)=-0.0063t+22.6557 -2.30 aksr y(trend)=-0.0102t-1.5357 -3.72 anrk y(trend)=0.0023t-8.4197 0.84 beys y(trend)=-0.0025t+5.2138 -0.91 ciha y(trend)=-0.0035t+3.5352 -1.28 hymn y(trend)=-0.0037t-0.1510 -1.35 kamn y(trend)=-0.0400t-9.8436 -14.60 kapn y(trend)=-0.0200t+5.8245 -7.30 kirs y(trend)=-0.0023t-8.6258 -0.84 kluu y(trend)=0.0018t-5.7144 0.66 nevs y(trend)=-0.0007t+5.2945 -0.26 nigd y(trend)=0.0005t+10.1856 0.18 yunk y(trend)=-0.0026t+2.009 -0.95 table 1. linear functions and annual velocities of gnss stations (north and east). station north linear functions (y=c2t+c1) linear changes (mm/year) east linear functions (y=c2t+c1) linear changes (mm/year) resultant changes (mm/ yıl)/ directions akhr| y(trend)=0.0245t+1.7226 8.94 y(trend)=0.0113t+1.6671 4.12 9.84/northeast aksi y(trend)=0.0302t+14.0774 11.02 y(trend)=0.0406t+0.0612 14.82 18.47/northeast aksr y(trend)=0.0513t-0.7534 18.72 y(trend)=0.0229t+1.3993 8.36 20.50/northeast anrk y(trend)=0.0427t+1.2507 15.59 y(trend)=0.0115t+1.6134 4.20 16.15/northeast beys y(trend)=0.0313t+1.5188 11.42 y(trend)=0.0283t+3.0769 10.33 15.40/northeast ciha y(trend)=0.0418t-2.7231 15.26 y(trend)=0.0191t-2.8808 6.97 16.78/northeast hymn y(trend)=0.0361t+4.5038 13.18 y(trend)=0.0148t-0.9256 5.40 14.24/northeast kamn y(trend)=0.0511t+2.5236 18.65 y(trend)=0.0390t+4.7394 14.24 23.46/northeast kapn y(trend)=0.0566t+0.6830 20.66 y(trend)=0.0406t+4.0371 14.82 25.43/northeast kirs y(trend)=0.0490t+7.9869 17.89 y(trend)=0.0192t-1.0199 7.01 19.21/northeast kluu y(trend)=0.0435t+5.2655 15.88 y(trend)=0.0196t+1.2331 7.15 17.42/northeast nevs y(trend)=0.0535t+1.2779 19.53 y(trend)=0.0256t+1.7963 9.34 21.65/northeast nigd y(trend)=0.0550t-0.6508 20.08 y(trend)=0.0334t+1.1063 12.19 23.49/northeast yunk y(trend)=0.0314t+0.6469 11.46 y(trend)=0.0135t-2.7270 4.93 12.48/northeast kny1 y(trend)=0.0723t-1.3919 26.39 y(trend)=0.0079t-0.2729 2.88 26.55/northeast knya y(trend)=0.0484t+2.7589 17.67 y(trend)=0.0312t+3.3720 11.39 21.02/northeast knya* y(trend)=0.0365t+5.952 13.33 y(trend)=0.0200t+6.6192 7.30 15.20/northeast knya** y(trend)=0.0409t-1.5028 14.92 y(trend)=0.0256t-1.0583 9.35 17.61/northeast * before slip ** after slip mean = 18.74mm/year station up linear functions (y=c2t+c1) linear changes (velocities) (mm/year) kny1 y(trend)=-0.1944t-13.4857 -70.96 knya y(trend)=-0.0106t-6.9657 -3.87 knya* y(trend)=0.00026t-9.3247 0.095 knya** y(trend)=0.00084t-0.6248 0.31 the up-coordinate time series of all stations had statistically significant linear movements in the positive and negative directions. as with the north and east components, up values of the knya station were analyzed in two parts as pre-slip (knya*) and post-slip (knya**) (fig. 4 and table 2). velocities were found to be 0.095 mm/yr for knya* and 0.31 mm/yr for knya**, respectively, and both velocities were in the positive direction. when the time series of the station is examined as a whole, it is seen that the velocity is negative and greater. accordingly, the slip must be considered as it affects the direction and magnitude of motion at this station. if it is not considered, it causes miscalculation and misinterpretation of the movement. including the kny1 station velocity, the mean value of linear movements in the negative direction for the up coordinates was -9.14 mm/year except for knya, knya* and knya** (table 2). the annual vertical movement of the kny1 station with 560-day data was 70.96 mm in the down (-) direction, which was also the highest value of the vertical movements of all stations in the study area. kny1 is the new gnss station set up in 2014 since there was a peak (fig. 3 and fig. 4) in the knya station time series. regarding the significant increases in both agricultural and residential areas in konya city and its surroundings, it can be concluded that the rise in the groundwater depletion in the district was inevitable (orhan, 2021). therefore, the reason for the maximum land subsidence rate in kny1 station was the acceleration in the land subsidence rate in recent years related to the increasing extraction of groundwater in the konya plain (özdemir, 2016; calo et al., 2018). excluding the annual velocity of the kny1 station, the mean value of vertical changes in the negative direction was -3.52 mm/year. the vertical movements of the anrk, kluu, and nigd stations were in the up direction, and their mean annual movement was 0.56 mm (table 2). except for kny1 station, the annual velocities of the stations for the up coordinates were relatively small values. at -1.5 cm/year velocity, kamn had the second highest velocity among all the stations investigated in the study area (table 2). 9.84/northeast 18.47/northeast 20.50/northeast 16.15/northeast 15.40/northeast 16.78/northeast 14.24/northeast 23.46/northeast 25.43/northeast 19.21/northeast 17.42/northeast 21.65/northeast 23.49/northeast 12.48/northeast 26.55/northeast 21.02/northeast 15.20/northeast 17.61/northeast 18.74mm/year 411investigating the effects of groundwater level changes on gnss observations in the konya closed basin periodic component analysis the periodic component analysis, which was the second component of the time series, was performed after removing the trend components from the time series of the gnss stations. with the help of the function given in equation 3, significant periodic movements existing in the series were detected. however, in equation 3, the f frequencies had to be known. in order to determine the frequencies in the equation, fft was applied to the series without the trend components. thus, the series were transformed from the time domain to the frequency domain, and all existing frequencies in the time series of the gnss stations were determined. the frequency spectrums of kamn, kapn and kny1 are given in figures 4 and 5. these gnss stations had daily, monthly, seasonal, semi-annual, annual, and even biennial periodic movements. the results for the other stations were similar. moreover, periodic movements obtained for this region supported those reported for other gnss stations around the world. many researchers have previously shown that gnss stations have annual periodic movements (herring & springer, 2002; mao et al., 1999; wdowinski et al., 1997). it was considered that this annual periodic effect in the time series was due to atmospheric and hydrological loading. calais (1999) reported a long-term periodic signal with a large amplitude over approximately 450 days. it was deduced that the periodic movement not being seasonal and the signal being larger in the east and west components resulted from the faults in the reference system or satellite orbit or ground routing parameters. furthermore, concerning the signal, snow and ice accumulation on the antenna and antenna protectors, as well as climatic factors had an effect on annual, half-annual, four-monthly and threemonthly periodical movements found by scherneck et al. (2002) and scherneck et al. (1998). groundwater level changes in this study, 19 wells were measured monthly in order to determine the groundwater level changes in the konya closed basin. for the wells in this area, minimum 11-year and maximum 49-year data were available (from 19 stations) (fig. 2). the time-dependent changes (monthly) of the two wells used to determine the groundwater level changes are given in figure 6. figure 5. fourier spectrums of the kamn, kapn and kny1 stations for the north and east coordinate time series. 412 erdoğan, h., oktar, o., gezgin, c., poyraz, f., arslan, n., yılmaztürk, f. figure 6. time series of doğanbey and bulduk stations (wells). as in the trend analysis of gnss stations, a trend analysis was performed for 19 wells. the linear functions and annual changes are given in figure 7 and table 4. the results showed linear changes in the decreasing direction with a mean of -39.22 cm/year for 19 wells; for example, -15.56 cm/year for three wells in konya-cihanbeyli, -63.48 cm/year for four wells in konya-selçuklu, -68 cm/year for six wells in karaman, and -19.50 cm/year for two wells in niğde. the annual change in the konya-selçuklu-meydan, karaman-ayranci–huyukburun, and karaman-merkez–sudurağı wells was over 1 m (table 4). however, the linear function of the konya-kulu and konya-seydişehir wells showed an insignificant linear change at 95% confidence level. in addition, a linear increase (69.72 cm/ year) was detected in the konya-beyşehir-doğanbey well. the majority of the water level of the wells in the study area had a statistically significant decrease, and this change was at meter level for some wells. from past to present, climatic, geological and geomorphological factors have influenced the changes in the level of groundwater in the konya closed basin. changes in groundwater level occur both within a year and between years. there is a decrease in the june-september period, which characterizes the dry season within the year, and an increase in the rainy season in januarymay. it is observed that there is a significant decrease of up to 1 m in groundwater levels in 30-40-year water wells distributed in the region (göçmez & i̇şçioğlu, 2004; doğdu et al., 2007; üstün et al., 2007). at some stations, the monthly water level data have been recorded since 1960. üstün et al. (2007) reported a decrease in the groundwater level of 24 wells from the same area. as a result of examining the time series of the wells, it was determined that the decrease in most of these wells was not linear; the groundwater level of the wells around karapınar was found as 16.9 m in september 1970 and 31.2 m in september 2003. göçmez et al. (2004) reported a decrease of 14.3 m in the level of groundwater in a 33-year period. as a result, these reduced levels in the aquifers of the konya closed basin and surrounding areas are caused by the table 4. linear functions and annual changes of 19 wells in the study area. name of well well number linear functions y(trend)=c2t+c1 linear changes (cm/year) results konya-beyşehi̇r-doğanbey 52770 y(trend)=0.0581t-17.0946 69.72 significant konya ci̇hanbeyli̇ bulduk 53705 y(trend)=-0.0109t-11.2754 -13.08 significant konya-ci̇hanbeyli̇-kartal 53706 y(trend)=0.0046t-24.4201 5.52 significant konya-ci̇hanbeyli̇-sigircik 53704 y(trend)=-0.0326t-22.3711 -39.12 significant konya-karapinar-gulfet-yayla 52258 y(trend)=-0.0520t-10.6263 -62.40 significant konya-kulu 53707 y(trend)=-0.0039t-11.3281 -4.68 insignificant konya-selçuklu 37325 y(trend)=-0.0363t-5.4342 -43.56 significant konya-selçuklu-meydan 46392 y(trend)=-0.1098t-7.4969 -131.76 significant konya selçuklu-saricalar 10472 y(trend)=-0.0257t+0.1759 -30.84 significant konya selçuklu-tutup 62564 y(trend)=-0.0398t+2.1481 -47.76 significant konya-seydi̇şehi̇r 49340 y(trend)=-0.0003t-2.2111 0.36 insignificant karamanayranci -huyukburun 30270 y(trend)=-0.0933t-66.2993 -111.96 significant karamankazimkarabeki̇r-merkez 1167 y(trend)=0.0024t-7.8545 2.88 significant karaman -center -akcasehir 13314 y(trend)=-0.0585t-33.6459 -70.20 significant karaman -center -emi̇nler 17171 y(trend)=-0.0379t-2.9531 -45.48 significant karaman -center -kocahoyuk 52260 y(trend)=-0.0610t-0.9697 -73.20 significant karamancenter -suduraği 20122 y(trend)=-0.0922t-23.1213 -110.64 significant nigdealtinhisarcenter 52263 y(trend)=-0.0256t-7.9697 -30.72 significant nigde -bor –bahceli̇ 52261 y(trend)=-0.0069t-1.8532 -8.28 significant use of water in the existing reservoir in the basin. in addition, it was observed that the decrease in water levels compared to previous years was caused by the unconscious consumption of water (göçmez et al., 2004). according to the comparison of well measurements between 2003 and 2008, the level of meram-alakova was 12.5 m lower, meram-hataf 18.5 m lower, çumra-fethiye lower, and çumra-batum 18.9 m lower in 2008. narrowing/constriction (press) in the aquifer systems due to groundwater drawdown can create a vertical deformation (downfall) in the floor over these systems. given similar examples elsewhere in the world, it is likely to encounter such a situation for the konya closed basin. some examples of partial downfall events of the earth’s crust and more detailed information on the geodetic monitoring of these components can be found in the following studies: phienwej et al. (2006), abidin et al. (2007), galloway & hoffman (2007) and üstün et al. (2007). similarly, in the current study, the -70.96 mm/year linear decrease observed in the up coordinate of the kny1 station calculated from the 560-day time series can be attributed to the decrease in the groundwater level in this region. figure 7. linear functions of well data in konya and karaman. the linear analysis of well data in the konya closed basin revealed that in the last decade, there was a decrease in the groundwater level, especially due to the drought that resulted from the global climate change and excess water use. conclusion in this study, the following results were obtained from the analysis of time series of the north, east and up coordinates and groundwater levels of stationary gnss stations in the konya closed basin. 413investigating the effects of groundwater level changes on gnss observations in the konya closed basin • with the time series analysis, important results were obtained concerning the changes in the behavior of gnss stations, the reasons for these changes, the effect of groundwater level (linear, periodical, etc.), mathematical models and the state of recently installed stations. • the time series of the konya station was analyzed in two parts, before and after the slip and data after the second slip was not analyzed because it was scarce. as a result of the analysis, it was observed that the direction and magnitude of the velocities changed if the slip was not taken into account in the time series. • horizontally, the gnss stations in the konya closed basin had linear movements in the northeast direction (itrf08 fixed) with an annual mean movement or velocity of 18.74 mm/yr except knya, knya* and knya** stations. moreover, while the vertical movements of anrk, kluu, and nigd stations were in the positive direction with a mean annual movement of 0.56 mm, other stations moved in the negative direction except knya, knya* and knya** stations. • the annual vertical movement of the kny1 station with 560day data was 70.96 mm in the negative (-) direction, which was the highest value of vertical movements among all stations in this region. • in the periodic component analysis, the signals of the stations contained periodic movements with time varying from day to year. the reason for these movements was considered to be atmospheric and hydrological loading, and climatic effects similar to previous reports on global gnss stations. • the trend analysis performed for 19 wells showed that they had linear movements in the down direction with a mean of -39.22 cm/ year. • the annual change in the konya-selçuklu-square, karamanayranci–huyukburun, and karaman-center–sudurağı wells was over 1 m. however, the linear function of the konya-kulu and konya-seydişehir wells showed an insignificant linear change at 95% confidence level. • a linear increase (69.72 cm/year) was detected in the konyabeysehir-doğanbey well. • the water level of the majority of the wells in the region statistically significantly decreased, and this was at meter level for some wells. the linear decrease of -70.96 mm/year in the up coordinates of the kny1 gnss station with 560-day data was considered to be mostly caused by this decrease in the groundwater level. • in the last decade, there has been a decrease in the groundwater level in the konya closed basin due to the drought resulting from global climate change and excess water use. acknowledgments this study was financially supported by aksaray university scientific research projects coordination unit (project number: 2015-064). authors would like to thank aksaray university for their support, and the general command of mapping of turkey for gnss data and the general directorate of state hydraulic works of turkey for the observations of groundwater level in the konya closed basin. references abidin, h. z., andreas, h., djaja, r., darmawan, d. & gamal, m. 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(2020). estimation of gps l2 signal observables using multilayer perceptron artificial neural network for positional accuracy improvement. earth sciences research journal, 24(1), 97-103. doi: https://doi.org/10.15446/esrj.v24n1.78880 in recent decades, due to the increasing mobility of people and goods, the rapid growth of users of mobile devices with location-based services has increased the need for geospatial information. in this context, positioning using data collected by the global navigation satellite systems (multi-gnss) has gained more importance in the field of geomatics. the quality of the solutions is related, among other factors, to the receiver’s type used in the work. to improve the positioning with low-cost devices and to avoid additional user expenses, this work aims to propose the implementation of an artificial neural network (ann) to estimate the gps l2 carrier observables. for this, a network model was selected through the cross-validation (cv) technique, the observations were estimated, and the accuracy of the solutions was analyzed. the cv technique demonstrated that a multilayer perceptron with four intermediate layers and one with one intermediate layer are the most appropriate configurations for this problem. the dual-frequency rinex processing (with artificial data) revealed significant improvements. for some tests, it was possible to comply with the rural property georeferencing regulations of the brazilian national institute of colonization and agrarian reform (incra). the results indicate, therefore, that the methodological proposal of the present investigation is very promising for approximating the quality of positioning reachable using a dual-frequency receiver. abstract estimation of gps l2 signal observables using multilayer perceptron artificial neural network for positional accuracy improvement estimación de los observables de señal gps l2 utilizando redes neuronales artificiales de perceptrón multicapa para mejorar la precisión posicional issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.78880 en las últimas décadas, debido a la creciente movilidad de personas y bienes, el rápido crecimiento de los usuarios de dispositivos móviles con servicios basados en la ubicación ha aumentado la necesidad de información geoespacial. en este contexto, el posicionamiento utilizando los datos recopilados por los sistemas globales de satélite de navegación (multi-gnss) ha ganado más importancia en el campo de la geomática. la calidad de las soluciones está relacionada, entre otros factores, con el tipo de receptor utilizado en el trabajo. para mejorar el posicionamiento con dispositivos de bajo costo y evitar gastos adicionales del usuario, este trabajo tiene como objetivo proponer la implementación de una red neural artificial (ann) para estimar los observables del operador gps l2. para esto, se seleccionó un modelo de red a través de la técnica de validación cruzada (cv), se estimaron las observaciones y se analizó la precisión de las soluciones. la técnica cv demostró que un perceptrón multicapa con cuatro capas intermedias y uno con una capa intermedia son las configuraciones más apropiadas para este problema. el procesamiento rinex de doble frecuencia (con datos artificiales) reveló mejoras significativas. para algunas pruebas, fue posible cumplir con las regulaciones de georreferenciación de propiedad rural del instituto nacional de colonización y reforma agraria (incra). los resultados indican, por lo tanto, que la propuesta metodológica de la presente investigación es muy prometedora para aproximar la calidad de posicionamiento accesible utilizando un receptor de doble frecuencia. resumen record manuscript received: 04/04/2019 accepted for publication: 15/11/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 97-103 cassio vinícius carletti negri, paulo cesar lima segantine university of são paulo (usp) * corresponding author: cassiovcn@usp.br g ps m ea su re m en ts 98 cassio vinícius carletti negri, paulo cesar lima segantine introduction the increasing mobility of people and goods, as well as the rapid growth of users of mobile devices with location-based services, significantly expands the need for geospatial information gathered from artificial earth satellites. to address this challenge, the integrity of the multi-gnss (us american gps, russian glonass, european galileo, and chinese beidou) propagated into national or international positioning services is of great importance. the gps (global positioning system) can be highlighted here due to the stability of the system over a long period of time and the robustness of its signals. initially, its satellites emitted the signals in two frequencies (l1 and l2) only. over the years, on the basis of its modernization program, the outdated satellites of the space segment of the gps were replenished by satellites of new generations emissions as well new navigation signals that were added to the existing signals (leick, rapoport, and tatarnikov, 2015; segantine, 2005). however, one of the advantages of dual or triple-frequency receivers is the possibility of making linear combinations between the observables of different carriers to improve the positioning accuracy by reducing the ionospheric effect and by also eliminating the errors of receiver and satellite clocks. in recent years, to avoid the high costs associated with the use of double or multifrequency receivers, some studies have been conducted to assess the possibility of using superficial artificial neural networks (anns) to generate l2 carrier observations from data gathered by a single-frequency receiver within a continuous monitoring network (mazzetto, 2017; c. a. u. da silva, 2003). although some unsatisfactory results were obtained in certain tests during the learning process of the networks, in general, the relative positioning solutions estimated using the generated l2 observables were more accurate than those using the original l1 data only. this suggests that it is possible to model the behavior of gps observations for up to 30 minutes to generate the l2 carrier observables, improving the positioning accuracy of low-cost single-frequency receivers. according to hornik, stinchcombe and white (1989), a shallow multilayer perceptron (mlp) can approximate any function at a particular accuracy level. however, bengio and lecun (2007) argue that although networks with one hidden layer have advantages, deeper models can represent some systems more efficiently because they can learn lower-level abstractions (characteristics of the characteristics). in addition, goodfellow, bengio and courville (2016) have shown from empirical tests that anns with two or more hidden layers have a better generalization capacity than the simpler models. thus, this paper aims to implement anns with more than one hidden layer, through the tensorflow library in the python computer language, to estimate the gps l2 carrier observables. in addition, the present investigation seeks to validate a network topology through the cross-validation (cv) technique by random sampling and compare the results with those obtained in other research. literature review this research requires the union of two areas of knowledge: geodesy, with a gps problem in geomatics, and computer science, with anns being a possible solution. due to this association, it became necessary to detail the state of the art of anns and their applications in the area of geomatics and similar areas. starting with the ann theme, mcculloch and pitts (1943) developed the concept of the first artificial model of a biological neuron, exhibiting its computational capacities. posteriorly, rosenblatt (1958) proposed a new model named perceptron. in the following years, widrow and hoff (1960) created the delta learning rule to minimize the errors of an ann based on gradient descent. in 1982, hopfield’s proposition of a recurrent ann gave the area quite prominence, and in the following years, the backpropagation algorithm creation (rumelhart, hinton, and williams, 1986) and computational advances have consecrated the ann studies. several practical applications have since been performed with anns. among the research developed, there is a current that relates them to the satellite positioning segment with the gnss. in mosavi (2004), anns were used in the estimation of observations corrections for differential global positioning system (dgps). furthermore, kubik and zhang (1997) and indriyatmoko et al. (2008) also applied the ann tool to predict the dgps corrections. jwo and chin (2002) used a superficial ann to determine the precision geometric dilution (gdop) parameter, and jwo and lai (2007) conducted tests with several architectures and topologies, trying to select the ideal configuration to estimate gdop values. the results showed that all candidate networks met expectations regarding the accuracy of their outputs, but those trained by the backpropagation algorithm presented a longer learning process. in addition, a recurrent wavelet neural network (rwnn) was implemented in mosavi (2007) to reduce gps receiver time data noise and to model the errors of this source. anns were also applied to improve positioning during gps signal blocking. semeniuk and noureldin (2006), for instance, created an alternative method based on an ann that estimates the speed and position errors of an inertial navigation system to correct and improve positioning in locations where the gps system is not available. more recently, a hybrid between a wavelet neural network (wnn) and the strong tracking kalman-filter (stkf) was proposed in an attempt to improve the positioning by satellites during this type of situation (chen et al., 2013). the combination of these two tools in the work of chen et al. (2013) was based on the wnn outstanding performance in data prediction and on the capacity of the stkf to estimate inertial navigation system (ins) errors. thus, it has been proposed to represent a sequential process because when there is no blockage in the gps signal, the wnn is trained using the predicted prior positioning errors, which were provided by the stkf, and it generates, as an output, the current positioning error. approaching the topic of this paper, silva (2003) sought to improve the positioning with single-frequency receivers, creating a method to estimate the gps l2 carrier observables using an ann. for estimating the l2 carrier phase, an mlp with one hidden layer and 6 artificial neurons was chosen; for estimating the l2 carrier code, the author adopted the same architecture but with 30 artificial neurons in one hidden layer. both configurations were implemented in matlab 6.1 and trained by the levenberg-marquardt algorithm, reaching up to 5,000 iterations. the results of silva (2003) indicate that the tool can successfully model the observable behavior. in another investigation, leandro, silva and santos (2007) evaluated the best method for feeding an ann with gps observations. in addition, mazzetto (2017), adopting the same topologies indicated by silva (2003), proposed a method in which only the signals with higher strength obtained in four stations near the receiver, where it is desired to estimate the observables, should be used because, theoretically, they experience the same atmospheric effects. in this research, ann training processes required up to 8 minutes and 15,000 iterations to complete. although so much time has been expended, some validation mse values were quite high and not converging as expected. despite these factors, he was able to increase the prediction time of the gps observables and improve the positioning accuracy in certain tests performed. the literature review showed that the studies used only an intermediate layer ann to estimate the gps l2 carrier observables. in addition, there is no ann topology validated through the cv technique to estimate the gps l2 carrier observables. therefore, the use of an appropriate ann model to estimate the l2 carrier data, capable of extracting the characteristics of the process more efficiently than a shallow mlp and being tolerant to the noises present in the input signals, represents an important step towards the enhancement of postprocessed solutions with single-frequency devices. materials and methods in this investigation, it was decided to implement the network in the tensorflow library through the python language. designed initially for machine learning applications, the low-level structure tool is currently being used in different fields and by several large companies (tensorflow, 2018). to select the appropriate ann model, the cv technique was applied, bypassing the training and validation samples, which were obtained in the dual-frequency rinex files generated in the inconfidentes/mg (mgin), piracicaba/sp (sppi), lins/sp (spli) and jaboticabal/sp (spja) stations. then, the ann was tested in the l1 carrier observables generated in the receiver fixed in são carlos/sp (eesc). figure 1 shows this configuration, chosen to compare the results obtained with those obtained in previous works, 99 estimation of gps l2 signal observables using multilayer perceptron artificial neural network for positional accuracy improvement in the receiver. in addition, the positioning solutions originated based on data postprocessing. for the extraction of the gps observables from the rbmc station rinex files, the rinex adapter software was used. this program was developed in the research of mazzetto (2017). in the creation of new rinex files with the artificial observables, the software deletes the original l2 carrier data and multiplies the l1 carrier phase values in the first epoch (from each satellite) by a factor k, or any other desired value, to determine the initial number of the l2 carrier phase since the ambiguity term is unknown. the generation of observations occurred differently for each type of observable, as these do not behave equally over time. thus, two ann models were implemented: one that used the phase differences of the l1 carrier as the input signal and the phase differences of the l2 carrier as the output. the second model used the c/a code observations as input signal and the p2 code data as the output, using the reverse truncation process. a similar method for treating observable gps for anns was used in leandro, silva and santos (2007). the topology for each ann, that is, the optimal number of hidden layers and neurons, the optimization algorithm and other parameters, was determined with tests that used the random subsampling cross-validation technique, where the available dataset was randomly subdivided into training and validation subsets five times (cv = 5), seeking to analyze the performance of each candidate towards data drawn differently for each test (haykin, 2008; i. n. silva, spatti, and flauzino, 2010). the new rinex files produced with the artificial observables were processed in geo office software v. 5, from leica, available at the department of transportation engineering, at são carlos school of engineering (university of são paulo), by the relative method to the eesc station with the other four stations as references. the positioning solutions were evaluated for the accuracy of the coordinates and compared with those produced by previous research. table 2 shows the official coordinates of the eesc rbmc station used to determine the accuracy of the positioning solutions. to evaluate the performance of the new model using exactly the same data used by mazzetto (2017). next, table 1 shows the test dates and times. the variation in the ionospheric refraction mainly depends on the latitude of the station, the angle of elevation, and the time of observation. near the magnetic equator, the ionospheric refraction varies more rapidly in time. hence, in our example, we use data of satellites that were received on different days between 01:00 and 03:30 pm due to strong solar activities. the mse values were calculated according to equation 1. mse n d yi i i n = ⋅ − = ∑1 1 2( ) (1) where: n: number of samples; di: observed output i (true value); yi: predicted output i. it is important to highlight that this proposed method for generating the l2 carrier observables was applied after the l1 carrier data were collected; that is, the estimation of these values was not performed in real-time with the ann figure 1. arrangement of rbmc stations used adapted from mazzetto (2017) table 1. test dates and times adapted from mazzetto (2017) test date gps time 1 09/15/2014 01:00 – 01:30 pm 2 01/10/2015 02:30 – 03:00 pm 3 06/10/2015 01:30 – 02:00 pm 4 01/20/2016 03:00 – 03:30 pm 100 cassio vinícius carletti negri, paulo cesar lima segantine the basic processing settings adopted in the geo office 5.0 software were l1 and l1 + l2 frequencies; hopfield tropospheric model; leica geo office standard (standard) v. 5 ionospheric model; broadcast ephemeris; elevation mask: 15º; and gps system data only. the gnss device models used in this work are the following: 1) eesc à receiver: leica gr10; antenna: leica ar10 (773758); 2) spja à receiver: trimble netr8; antenna: gnss choke ring (trm59800.00); 3) spli à receiver: trimble netr8; antenna: gnss choke ring (trm59800.00); 4) sppi à receiver: trimble netr8; antenna: gnss choke ring (trm59800.00); 5) mgin à receiver: trimble netr5; antenna: zephyr gnss geodetic model 2 (trm55971.00). the spatial deviations (sd) were calculated according to equation 2. sd n n e e a ao p o p o p= − + − − −( )² ( )² ( )² (2) where n0 = the official north coordinate, in meters; e0 = the official east coordinate, in meters; a0= the official geometric altitude coordinate, in meters; np = the processed north coordinate, in meters (relative positioning method); ep = the processed east coordinate, in meters (relative positioning method). ap = the processed geometric height coordinate, in meters (relative positioning method). results and discussions in this section, we first present the tests and results regarding the choice of the topologies of each ann model, validated by the cv technique by random sampling. then, the training times and the validation mse of each test are shown. finally, the spatial deviations of each positioning solution using the artificial observations were compared with those obtained in other works and with the single-frequency files. ann implementation the initial step was to develop the generic algorithm of a network, from the input data loading and normalization to the determination of the number of hidden layers, neurons and mathematical models. it is known that the optimal topology selection of an ann occurs through attempts and errors. each problem requires a different model. with the generic mlp, the tests were started with the observations obtained in the spja, spli, sppi and mgin rbmc stations. the evaluation of the best model was due to the search for lower validation mse values in the four experiments. carrier code ann model first, the intervals in which the data should be normalized was tested, choosing the limits between 0 and 1. this step was performed through the class minmaxscaler, provided by the package scikit-learn (scikit-learn, 2018). the restriction of the network inputs in relation to signal strength (0, 5 and 7), as successfully proposed in mazzetto (2017), was applied in the present investigation. however, inserting all the observations of the code without eliminating those of lower quality proved to be the best option, unlike his conclusion. this situation can be explained by the noise tolerance in the input signals that the newly implemented ann has developed. regarding the number of hidden layers and the number of neurons, several topologies were investigated. figure 2 shows the average validation mse values for each tested configuration. although the results are from one date, the other learning processes and the results obtained on other days were similar to those shown in the image below. 0 0.02 0.04 0.06 0.08 0.1 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs 25 30 35 15-3 23-7 25-10 25-15 18-10-5 25-12-7 14-8-4-3 18-8-7-5 20-10-7-5 figure 2. comparison between models with different numbers of hidden layers and artificial neurons day 01/20/2016 – fourth test day (03:00 03:30 pm) in black (30), the topology chosen in silva (2003), applied in mazzetto (2017), and in red (18-8-7-5), the configuration chosen in this investigation are shown. in relation to the tests performed to choose the activation function, figure 3 shows a test that analyzed the rectified linear unit (relu) and logistics (sigmoid) functions. it is observed that the relu produced lower validation mse values, although in some evaluations, both functions performed satisfactorily, but with relu converging faster. géron (2017) states that in practice, this function generally produces excellent outputs, is fast in processing, and works well in deep networks. 0.00 0.05 0.10 0.15 0.20 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs relu sigmoid figure 3. comparison of activation functions 01/10/2015 – second test day (02:30 03:00 pm) to select the optimization algorithms available in the tensorflow library, only the two most popular algorithms were used: adam and the stochastic gradient descent (sgd). based on figure 4 (mse values reached for table 2. official coordinates of the são carlos rbmc station (eesc) – ibge (2018a) station geographic coordinates (sirgas 2000) precision (m) eesc latitude 22 ° 00 ‘ 17.81599 “ s 0.001 longitude 47 ° 53 ‘ 57.04968 “ w 0.001 geometric height (m) 824.587 0.005 101 estimation of gps l2 signal observables using multilayer perceptron artificial neural network for positional accuracy improvement 09/15/2014), it is possible to verify that the former presented higher efficiency, converging rapidly. this result was expected because kingma and ba (2014) already demonstrated the effectiveness of the algorithm in training mlps, comparing it with four others. 0.00 0.05 0.10 0.15 0.20 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs adam sgd figure 4. comparison between optimization algorithms 09/15/2014 first test day (01:00 01:30 pm) the method for initializing the synaptic weights in each layer was also tested by choosing the variance scaling type, whose objective is to keep constant the variance scale of the input signals to the final layer of the network. figure 5 presents the average validation mse values for tests performed on 06/10/2015. 0.00 0.20 0.40 0.60 0 25 50 75 100 125 150a ve ra ge v al id at io n m se (c v = 5 ) training epochs glorot uniform random normal random uniform variance scaling figure 5. comparison between the initialization methods of synaptic weights day 06/10/2015 – third test day (01:30 02:00 pm) carrier phase ann model for the carrier phase, again using the minmaxscaler class from the scikit-learn library (scikit-learn, 2018), the data were normalized between the intervals -1 and 1, reducing the propagation of the error, as the normalization factor is smaller (leandro et al., 2007). in addition, at this stage of the research, it was chosen to restrict signal strength (intensity equal to 5), training the ann only with the highest quality observations. the tests on the number of hidden layers and neurons were limited to evaluating only one or two intermediate layer topologies since, above that, overfitting of the samples was verified. figure 6 shows the results obtained for the fourth test day (01/20/2016). in red (8), the topology selected in this work is highlighted, and in black (6), the configuration adopted by silva (2003) and used in mazzetto (2017) is shown. regarding the activation functions, two types were investigated: linear and hyperbolic tangent (tanh). figure 7 shows the results. based on all tests, it was found that the tanh function converged faster and produced the best positioning solutions and, for these reasons, was adopted for the ann of the carrier phase. regarding the optimization algorithms, adam and sgd were tested again, and the first algorithm was chosen because it provided the best results. figure 8 shows one of the tests carried out that corroborates this conclusion. 0.00 0.10 0.20 0.30 0.40 0.50 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs adam sgd figure 8. comparison between optimization algorithms 06/10/2015 – third test day (01:30 02:00 pm). finally, the last analysis verified which initialization methods of the synaptic weights generate the lowest validation mse values. according to 0.00 0.02 0.04 0.06 0.08 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs 5 6 8 10 11 3-2 5-2 5-3 6-2 6-4 figure 6. comparison between models with different numbers of hidden layers and artificial neurons day 01/20/2016 – fourth test day (03:00 03:30 pm) 0.00 0.04 0.08 0.12 0.16 0.20 0 25 50 75 100 125 150 a ve ra ge v al id at io n m se (c v = 5 ) training epochs linear function tanh function figure 7. comparison of activation functions day 09/15/2014 – first test day (01:00 01:30 pm). 102 cassio vinícius carletti negri, paulo cesar lima segantine figure 9, it is observed that starting weights randomly and with a uniform distribution generated good results. 0.00 0.03 0.05 0.08 0.10 0 25 50 75 100 125 150a ve ra ge v al id at io n m se (c v = 5 ) training epochs glorot uniform random normal random uniform variance scaling figure 9. comparison between the initialization methods of synaptic weights 01/10/2015 – second test day (2:30 3:00 pm). ann training and validation with the network configurations and all parameters selected, the l2 carrier phase and code observations of the eesc station were generated for the four days. table 3 shows the total sum of training epochs of the anns, the learning time and the validation mse for each observable.. table 3. total sum of the number of training epochs and time for generating the l2 carrier observables test date validation mse (p2 code) validation mse (l2 phase) training epochs duration (s) 1 09/15/2014 0.002882 0.003958 369 36.77 2 01/10/2015 0.004782 0.007323 219 29.28 3 06/10/2015 0.001514 0.034828 88 91.77 4 01/20/2016 0.000095 0.210098 23 4.95 it was verified that the mse performance parameter of the code presented very low values, close to zero, unlike the ones exposed in previous works (mazzetto, 2017), where these numbers were not even below the unit. in general, the number of training periods of anns, considering both models, represents only 1.9% of the number of iterations that were performed in past research for the same sample data, while the total process time equals only 8.1%. these numbers point to the optimization of the process of generating observations with anns, being considerably faster, even when adopting models with more than one hidden layer. gps data processing as the main objective of this paper was to evaluate the possibility of improving positioning solutions using anns, this section shows the results regarding the processing of gps data with artificial observables. parallels were made between the deviations obtained with the single-frequency solutions; the l1 + l2modif data, obtained in mazzetto (2017); and the l1 + l2a observables generated in the present investigation, aiming to prove the validity of the proposed method. figure 10 shows the spatial deviations (sd) obtained in processing for the first test day. for the second test (01/10/2015), spatial deviations are shown in figure 11. next, figure 12 shows the spatial deviations obtained on 06/10/2015 (test 3). finally, the spatial deviations of the processing performed on 01/20/2016 (test 4) are shown in figure 13. as the above figures demonstrated, applying the l1 carrier observations together with those of the l2 carrier, which were generated in this work, it was possible to reduce the positioning errors in several baselines (on average, a 40.2% reduction) when compared to the results obtained with only the original l1 data. therefore, it can be stated that the process of gps l2 carrier observables estimation using anns can improve relative positioning solutions with data obtained in single-frequency receivers. 1.348 2.016 0.936 1.0831.386 1.355 1.030 0.955 0.349 0.543 0.762 0.541 0.00 0.50 1.00 1.50 2.00 2.50 mgin-eesc spja-eesc spli-eesc sppi-eesc sp at ia l d ev ia tio n (m ) baseline l1 l1/l2modif mazzetto (2017) l1/l2a figure 10. spatial deviations from relative processing for 09/15/2014 (test 1) 1.655 0.664 1.782 0.7820.637 0.498 0.463 0.492 0.717 0.609 0.954 1.086 0.00 0.50 1.00 1.50 2.00 2.50 mgin-eesc spja-eesc spli-eesc sppi-eesc sp at ia l d ev ia tio n (m ) baseline l1 l1/l2modif mazzetto (2017) l1/l2a figure 11. spatial deviations from relative processing for 01/10/2015 (test 2) 2.141 2.182 1.136 1.911 0.969 1.026 0.938 0.699 1.060 0.275 0.497 0.91 0.00 0.50 1.00 1.50 2.00 2.50 mgin-eesc spja-eesc spli-eesc sppi-eesc sp at ia l d ev ia tio n (m ) baseline l1 l1/l2modif mazzetto (2017) l1/l2a figure 12. spatial deviations from relative processing for 06/10/2015 (test 3) 2.399 1.230 2.226 1.123 0.451 0.506 0.448 0.401 2.427 0.611 0.589 1.523 0.00 0.50 1.00 1.50 2.00 2.50 mgin-eesc spja-eesc spli-eesc sppi-eesc sp at ia l d ev ia tio n (m ) baseline l1 l1/l2modif mazzetto (2017) l1/l2a figure 13. spatial deviations from relative processing for 01/20/2016 (test 4) 103 estimation of gps l2 signal observables using multilayer perceptron artificial neural network for positional accuracy improvement conclusion dual-frequency receivers help to improve the positioning accuracy by enabling the formation of linear combinations between the observables of different carriers. this allows us to use differencing techniques between the phase or code measurements in data processing to reduce the ionospheric effect and to also eliminate the errors of receiver and satellite clocks. those so-called geodetic receivers are associated with high costs and, therefore, are waived by the service provider in many cases, especially in large-scale networks where a high number of these types of instruments are needed. in recent years, some studies have been carried out to check the achievable accuracy using the generated l2 carrier observations based on data gathered by a single-frequency receiver. this study aims to determine the quality of the gps l2 carrier observables estimated by applying anns to answer the question of whether a dual-frequency receiver can be replaced by a cost-efficient single-frequency receiver approaching its achievable accuracy. for this purpose, according to the literature review, for the first time, two ann topologies were validated and selected through the cv technique to estimate the gps l2 data, among a wide universe of possible configurations. concisely, the mlps selected in this research contain up to 4 hidden layers and 38 processing units. despite this, the training and validation process of the models was extremely fast, only a few seconds, and represented less than 10% of the time spent in other research. regarding the validation mse values, all numbers were very close to zero. it was verified that by applying the observations estimated by the anns, together with the first carrier originals, it was possible to achieve significant reductions of the spatial deviations in several baselines when compared to those obtained with only the l1 observable. in addition, some vectors presented smaller errors than those presented in mazzetto (2017), and others have complied with the incra rural georeferencing standards, which determine a plane deviation of less than 50 cm. however, it should be noted that it was not possible to apply the l3 (iono-free) combination in the relative processing of the dual-frequency rinex files, similar to what has been evidenced in previous research. for future research, the use of genetic algorithms in the search for an even better ann configuration for this problem is indicated, as they can analyze a vast number of candidates in a much shorter period of time than a human would need. additionally, as future research, it is proposed that an analysis be performed to identify which factors influence the final results of the gps data processing because, in some tests, the ann could not successfully estimate the observations of the l2, resulting in positioning accuracy degradation when using the artificial observables. acknowledgments this study was financed in part by the coordenação de aperfeiçoamento de pessoal de nível superior brasil (capes) finance code 001 (no. 1680400). references bengio, y., & lecun, y. 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(1960). adaptive switching circuits. neurocomputing: foundations of research, 123–134. keywords: jadeite; color-causing; standard light source; color; quality grading. palabras clave: jadeíta; causante de color; fuente de luz estándar; color; clasificación de calidad. how to cite item pan, x., guo, y., liu, z., zhang, z., shi, y. (2019). impact of different standard lighting sources on red jadeite and color quality grading. earth sciences research journal, 23(4), 371-378. doi: https://doi.org/10.15446/esrj.v23n4.84113 the purpose of this paper is to investigate the standard light source for grading and displaying the color of red jadeite and to classify the color. with raman spectrometer, ultraviolet-visible spectrophotometer and x-ray fluorescence spectrometer, the results show that, the fe 3+ is the main chromogenic mineral of red jadeite, which negatively correlates with the tonal angle, while the color of red jadeite has a positive correlation with the hematite content. the color of 120 red jadeite samples was examined by collecting the reflective signaled from the sample surface using an integrating sphere with the portable x-rite sp62 spectrophotometer based on cie 1976 l*a*b* uniform color space. the color parameters of jadeite samples under d65, a and cwf standard light sources were analyzed. the light spectrum of d65 light source is continuous, relatively smoothed with high color temperature, which makes the sample color close to that under the natural light and can be used as the best evaluation light source. a light source contributes to improve the red tone of jadeite, which is the best light source for commercial display of red jadeite. cwf light source can be used as the auxiliary lighting for color evaluation. the color of red jadeite is divided into five levels from best to worst using k-means cluster analysis and fisher discriminant analysis under d65 light source: fancy vivid, fancy deep, fancy intense, fancy dark and fancy. abstract impact of different standard lighting sources on red jadeite and color quality grading impacto por diferentes fuentes de iluminación estándar en la clasificación de jadeíta roja y la calidad del color issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84113 el propósito de este trabajo es investigar la fuente de luz estándar para calificar y mostrar el color de jadeíta roja y clasificarlo. con el espectrómetro raman, el espectrofotómetro ultravioleta-visible y el espectrómetro de fluorescencia de rayos x, los resultados muestran que el fe 3+ es el principal mineral cromogénico de la jadeíta roja, que se correlaciona negativamente con el ángulo tonal, mientras que el color de la jadeíta roja tiene una correlación positiva con el contenido de hematita. el color de 120 muestras de jadeíta roja se examinó recogiendo el reflectante señalado desde la superficie de la muestra utilizando una esfera de integración con el espectrofotómetro portátil x-rite sp62 basado en espacio de color uniforme cie 1976 l*a*b*. se analizaron los parámetros de color de las muestras de jadeíta bajo las fuentes de luz estándar d65, a y cwf. el espectro de luz de la fuente de luz d65 es continuo, relativamente suavizado con alta temperatura de color, lo que hace que el color de la muestra sea similar al de la luz natural y puede usarse como la mejor fuente de luz de evaluación. una fuente de luz contribuye a mejorar el tono rojo de jadeíta, que es la mejor fuente de luz para la exhibición comercial de jadeíta roja. la fuente de luz cwf se puede utilizar como iluminación auxiliar para la evaluación del color. el color de la jadeíta roja se divide en cinco niveles, de mejor a peor, utilizando el análisis de conglomerados k-means y el análisis discriminante de fisher bajo la fuente de luz d65: fancy vivid, fancy deep, fancy intense, fancy dark y fancy. resumen record manuscript received: 12/07/2019 accepted for publication: 07/10/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 371-378 xin pan, ying guo*, ziyuan liu, zikai zhang, yuxiang shi school of gemology, china university of geosciences (beijing), beijing, 100083 china * corresponding author: guoying@cugb.edu.cn m in er al o g y 372 xin pan, ying guo*, ziyuan liu, zikai zhang, yuxiang shi introduction jadeite is generally considered to be a high-pressure mineral (yang, yang & long, 2015), which is called the “king of jade” in china. it generally occurs as block, and is a rare rock type formed in subduction zones with two origins: the wholesale metasomatic replacement and the vein precipitation (yui & fukuyama, 2015; mevel & kiénast, 1986). based on possible formation models, here we propose to classify jadeite into two types: fluid precipitates (p-type) and metasomatic replacement (r-type) (tsujimori & harlow, 2012). geologically, jadeite is a fibrous mineral aggregate composed of pyroxene minerals with jadeite and omphacite as the main minerals. it typically has different percentages of diopside (camgsi2o6)/ kosmochlor (nacrsi2o6 ), iron-rich pyroxene (cafe2 + si2o6 ) and aegirine (nafe3 + si2o6 ), which coexist with jadeite as solid solutions (harlow & olds, 1987). to petrologically define the jadeite, the jadeite pyroxene content in rock must reach 90-95%, or it may be too rigidity to be defined as jadeite-rich rock. beside basic mineral composition, the composition of jadeite varies with place of origin, e.g., jadeite from the burma contains soda amphibole (including ekman rutile, blue amphibole, sodium tremolite and amphibole), albite, zeolite, tremolite, rutile, clinochlore, barium albite and chromite, etc (htein & naing, 1994, 1995). jadeite, a polycrystalline heterochromatic jadeite, is transparent and colorless when purified, which usually white due to the scattering of cracks, tiny grain boundary edges and fluid inclusions. the color of jadeite can be divided into primary one and secondary one. the crystal field theory shows that the color mechanism of jadeite is mainly composed of the following two forms: (1) metamorphic differentiation: the color-causing trace elements such as cr3+ and fe3+ migrated and aggregated, which then replaced al3+ in the crystal of jadeite naalsi2o6, (2) ductile deformation: jadeite could be ductile deformed due to the tectonic compression under high pressure and with cr3+and fe3+rich fluid that flows into jadeite crystal lattice (ying, zong, qi, zhang & wang, 2018). the jadeite is mostly blue-green or lividity and blue-black when ferrous (fe2+) and iron (fe3+) are mixed into it. the darkest black-green is caused by a small amount of iron oxides and omphacite (rossman, 1974). furthermore, charge transferring (fe2+-o-fe3+) between ferrous and trivalent iron can also form purple in nearly pure jadeite (ponahlo, 1999; hughes et al., 2000). the secondary color of jadeite refers to the color formed by the exogenous geological process after being exposed to the surface, which is generally produced by the micro-mineral impregnation on the fine boundary, e.g., jadeite contaminated by water-bearing iron oxides is commonly red to orange-brown, iron-rich clay minerals often bring dark green stripes to jadeite, while graphite makes jadeite gray to black (guo, wang & du, 2016a). color plays a significant role in jadeite quality evaluation, typically determining the quality and value of jadeite. the color grading of jadeite was first summarized and described by professionals using subjective words. china launched the national standard of gb/ t 238852009 jade grading in 2009 to classify the jadeite color using the colorimetric method under standard environment (guo et al., 2016a). the main parameters include color hue, lightness and chroma (guo, 2017a). in this standard, the color is measured based on spectrophotometry with standard a or d light sources being recommended. however, standard a light source (color temperature is about 2856k) and standard d light source (color temperature is about 5000-7500k) are two completely different ones (china national bureau of technical supervision, 2009). also, purple and red jadeite are still graded according to the standard of green jadeite with no separate color grading standard for red jadeite. currently, all kinds of gemstones in the world have their own color grading methods, e.g., ruby, sapphire, emerald, etc., while most of the color grading systems refer to the color grading system of gia color diamonds (china national bureau of technical supervision, 2010; 2016a, 2016b). diamond color is classified and described into nine levels via gia according to their chroma and color hue, including fancy vivid, fancy deep, fancy intense, fancy dark, fancy, fancy light, light, very light, faint. materials and methods one hundred and twenty natural red jadeite with good texture, well purity and polished surface were selected and prepared in size of 5mm *6mm to 7mm *8mm, which varies in color from orange red to brownish red with continuous change of color hue and lightness. some typical samples are shown in figure 1. raman spectroscopy was carried out using hre volution laser raman spectrometer from japan horiba company. excitation light source wavelength: 532 nm; resolution: 1 cm-1; laser power: 30-40 mw; scanning times: 3; single integration time: 10 seconds; testing range: 100-2000 cm-1. ultraviolet-visible spectrums were analyzed using uv-3600 uv-vis spectrophotometer. measurement conditions were listed as followings: detector conversion wavelength: 200-900 nm; grating conversion wavelength: 830.00 nm; s/r shift: normal mode; light source: automatic; detector lock: automatic; slit program: normal; beam mode: double beam; scanning speed: high; sampling interval: 0.5s; scan mode: single. all-element analysis was performed on ourstex100fa portable energy dispersive x-ray fluorescence spectrometer. measuring mode: monochrome or continuous x-ray with pd target; tube voltage: 40kv; tube current: -0.051ma; detector: sdd (siliocon drift detector); detector refrigeration mode: peltier refrigeration mode (-28oc); sample environment: low vacuum (103 pa); diameter and height (expandable) of sample chamber: 20 cm and 10-30 m; measuring time: 100-300 s. x-rite sp62 was employed to collect reflective signals from forsterite surface through integrating sphere based on cie1976 l*a*b* uniform color space system (figure 2). test conditions were described as following: reflection, not include the specular reflection; light source: d65 standard one; observer view: 2°; measuring range: 400-700nm; measuring time:<2.5s, wavelength interval:10nm; voltage: 220v; current: 50-60hz. figure 2. schematic diagram showing experimental device quantitative color analysis is based on l*a*b* uniform color space (cie, 1976), which consists of plane color a*, b* axis and l* vertical axis. the positive and negative ends of a* axis represent red and green, respectively, the positive and negative ends of b* axis represent yellow and blue, respectively, and the l* axis represents brightness. the chroma cab* and color hue hab * can be calculated by color a* and b*, respectively. c a b* * *= ( ) ( )      2 2 1 2  (1) figure 1. images showing typical red jadeite samples 373impact of different standard lighting sources on red jadeite and color quality grading fe3+ is typical 3d5 configuration, whose compounds are usually brown, while the charge transfer band of fe3+ usually occurs at 200 nm in the far ultraviolet region, indicating that the peak at the former region (200-320 nm) is resulted from the charge transfer between fe2+ and fe3+. therefore, it can be confirmed that fe3+ is the main color-causing mineral of red jadeite, while the gradation varies as a function of iron content, which can be expressed by the absorption intensity in the spectra. figure 5. impact of fe content on hue angle of typical samples x-ray fluorescence spectrometer of four samples with gradient color shows that the main elements of red jadeite are na, al and si, which is h b a ° = arctan * * (2) cie de2000 (e00) color difference formula was employed to calculate the color difference of jadeite under different background (perez et al., 2011), which is the latest color difference formula recommended by cie. the formula includes not only weighting function of lightness difference, color hue difference and chroma difference, but also the cross term of color hue difference and chroma difference as well as the term of improving the prediction ability of neutral grey color difference. compared with cie lab (e*ab) color difference formula lacking of visual uniformity (guo, 2017b), cie de2000 is an improved version that can better express fine chromatic aberration in green area. cie de2000 (δe00) color difference formula is as following: ∆ ∆ ∆ ∆ ∆e l k s c k s h k s r c k st t c c k k t c 00 2 2 2 = ′    ′    ′    ′    cc k k h k s     ′    ∆ where δl’, δc’ and δh’ are lightness difference, color hue difference and chroma difference for certain group of color data; rt is the conversion function for resolving the interaction between color hue and chroma in blue region; sl, sc and sh are position functions to correct the visual uniformity in cie lab; kl, kc and kh are calibration parameters under experimental environment, while two groups of parameter combinations are widely used:  cie de2000 (1:1:1): kl=1ˎ kc=1ˎ kh=1(cie technical report: improvement to industrial colordifference evaluation, 2001; luo et al, 2001; ghinea et al, 2010);  cie de2000 (2:1:1): kl=2ˎ kc=1ˎ kh=1(mangine et al, 2005; perez et al, 2011) (guo, wang, li & dong, 2016b). this paper employs sensitive parameters to evaluate color difference: cie de2000 (1:1:1). the standard light source box is selected as experimental environment. the inner wall of the box is composed of standard neutral gray matte material. the standard light source tube is put on top to simulate the natural light source, which can be switched freely. three lighting sources were used based on the national standard gb/t 20146-2006 requirements for standard lighting sources, as well as the requirements for the detection and quality evaluation of jewelry and jade, and the uniformity of lighting sources (guo, zhang, li & zhang, 2018a). the d65 light source (related color temperature: 6504 k) was used for simulating sunlight in cloudy days. the a light source (related color temperature: 2856 k), a typical incandescent lamp, was selected for lighting in households or stores, and finally, the cwf light source of the cold white fluorescent (related color temperature: 4150 k) for the business and office in the united states was used for reference. results and discussion color-causing analysis of red jadeite raman spectroscopy was carried out on the sections with uniform color that can represent the average color of each sample. the raman spectroscopy measurement shows that all samples have raman peaks of jadeite, a collective mineral phase, indicating that all samples are natural jadeite. raman spectra of three samples with obvious color gradient were analyzed to identify their peaks (figure 3). these peaks represent lepidocrocite (γ-feo(oh)) and hematite (α-fe2o3), which work together to express natural red-yellow. the lepidocrocite is the dominated mineral with relatively complete crystals, while hematite is of secondary importance. the sample color indicated by the peak intensity shows a positive correlation with the hematite content. ultraviolet-visible absorption spectra (figure 4) of three red jadeites with large color hue differences (hue angle: 35.11°, 43.52° and 50.46°, respectively) shows that the absorption spectra basically follows the same trend with slight shift of certain absorption peaks and minor difference in the intensity. it can be inferred that the lower the hue angle is, the lower the absorption of ultraviolet and visible light spectrum is, the darker the color is. generally, the weak absorption at 430-470 nm and absorption at 730-800 nm are associated with occurrence of fe3+. figure 3. raman spectra of red jadeite figure 4. uv-vis absorption spectra of red jadeite 374 xin pan, ying guo*, ziyuan liu, zikai zhang, yuxiang shi consistent with other jadeite colors. however, the fe, ca and mg contents are higher than other jadeite colors, while the fe content, one of main elements affecting the jadeite color, varies greatly among them. table 1 shows the fe content in the tested samples increases successively. it can be inferred that the fe content in red jadeite will increase with color concentration in the same color system under the same conditions. color variation of red jadeite under three light sources a portable x-rite sp62 spectrophotometer was used to measure the color parameters of red jadeite samples under standard light source d65, cwf light source and a light source, respectively. the color parameters are l* (lightness), a* (red-green coordinates), b* (yellow-blue coordinates), cab* (chroma) and h0 (hue angle). two sets of 2880 data from 120 jadeite samples were obtained, including tristimulus values (x, y and z) and 24 variables. each sample was measured 3 times to ensure the accuracy (guo, zong & qi, 2018b). range of color parameters firstly, the color data of jadeite samples measured under three different light sources were sorted out (table 1). the results show that, under the standard d65 light source, l* ranges from 37.06 to 52.82, a* ranges from 13.05 to 28.18), b* ranges from 8.19 to 30.02, ho ranges from 22.64° to 57.66°, c* ranges from 15.95 to 38.33. under the cwf light source, l* ranges from 37.94 to 54.89, a* ranges from 9.62 to 21.33, b* ranges from 9.56 to 34.04), ho ranges from 27.59° to 68.19°, c* ranges from 13.97 to 47.25. under the a light source, l* ranges from 39.27 to 58.68, a* ranges from 15.56 to 30.23, b* ranges from 10.75 to 34.90), ho ranges from 36.09° to 57.21°, c* ranges from 19.83 to 44.67. obviously, the color is concentrated in the reddish area with a yellowish hue with different brilliance and medium and low brightness. the maximum and minimum values of measured data under each light source were selected, and the average and variance were calculated. the comparison shows that, c* value under a light source is obviously higher than that under other two light sources, which means, chroma of jadeite samples under a light source is higher. the b* value under cwf light source is higher than a*, and their difference is the largest, indicating more yellow tone. however, the result under d65 light source is different from that, showing more red tone. the measurement matches well with the observation. to visualize the color distribution under three light sources, the measured color data is projected onto l*a*b* uniform color space system and a*b* chromaticity diagram, as shown in figure 6-2. the horizontal axis and the vertical axis represent a*axis (red-green direction) and b*axis (yellow-blue direction), respectively (shams-nateri, 2009). compared with d65 and cwf light sources, the chroma of sample is higher under a light source (figure 6). meanwhile, sample under d65 light source is generally red, and sample under cwf light source is commonly orange, which agrees with the color perception under naked eyes. the influence of spectrum on the jadeite color comparison of the theoretical values of spectrum of d65, cwf and a light sources (figure 7) suggests that the curve under a light source is a generally inclined straight line. the spectral radiation of red zone is the strongest in the visible light region with color-rendering index (ra) up to 92.39. importantly, the input of orange-red color makes it suitable for illuminating red jadeite (guo, zong, qi, zhang & wang, 2018c). the spectrum curve under d65 light source is uniform in green, yellow and red zone with the highest peak in the blue-green zone at 480 nm. the ra (80.02) is lower than that of a light source, and the spectrum curve is uneven, showing that its illuminance on red jadeite is worse than a light source. generally, both standard a light source and standard d65 light source are high quality color-rendering light source. the spectrum curve table 1. color parameters of jadeite samples under different light sources light source l* a* b* c* h0 d65 range average value 37.06~52.82 44.68 13.05~28.18 20.63 8.79~30.02 19.83 15.95~38.33 28.69 22.64~57.66 43.13 cwf range 37.94~54.89 9.62~21.33 9.56~34.04 13.97~47.25 27.59~68.19 average value 46.07 15.40 22.36 27.43 54.56 a range 39.27~58.68 15.56~30.23 10.75~34.90 19.83~44.67 36.09~57.21 average value 47.84 23.40 24.77 34.24 46.41 figure 6. l*a*b* uniform color space system and a*b* chromaticity diagram showing red jadeite under d65, cwf and a light sources. 375impact of different standard lighting sources on red jadeite and color quality grading of cwf light source is generally sharp with peaks at 440 nm and 540-580 nm, which is unsuitable to illuminate jadeite red. figure 7. spectrum of d65 light source, cwf light source and a light source the color temperature of a light source (2856k) is lower than that of d65 light source (6504k), and its energy in the long-wave red zone is obviously higher than that in the short-wave blue-purple zone with overall light color of yellowish as a whole. the energy distribution of d65 light source in the range of 360 nm to 780 nm is more uniform than that of a light source, and the relative spectrum of d65 light source is close to that of sunlight (guo, 2016a, 2016b). therefore, the light color of d65 light source is closest to that of natural sunlight with no other obvious hues, which can better express the object color. ten red jadeite samples were randomly selected to be observed by naked eye and be simulated by computer, and the color difference was calculated using formula (3). the result confirms that l* and c* of the samples are the highest under a light source, a good indicator of brighter color. the samples under cwf light source are generally dark yellow-green tone, and color under d65 light source is closer to the red of the jadeite observed under sunlight (table 2). comprehensive analysis two-way analysis of variance since the light source and sample are two independent factors determining the jadeite appearance, the influence of light source and sample on their color index is determined through two-way analysis of variance via spss software (table 3). table 3. two-way analysis of variance of different light sources and samples on red jadeite difference source samples light source l* c* ho l* c* ho p 0.000 0.000 0.000 0.000 0.000 0.000 f 100.174 39.036 15.578 768.864 920.394 714.955 the results show that the l*, c* and h0 are significantly governed by both light source and sample itself, because p << 0.01 suggests a significant level. f value can be used to represent the significance of difference in the variance analysis, e.g., f≥p is a good indicator of significant difference. the f values in the table 3 is higher than p values, verifying the conclusion that color index varies greatly with sample itself and light source (wang, li, you & zhao, 2011). f-statistic from the variance analysis can also be used to determine the difference of the impact of the sample/ light source on l*, c* and ho values, specifically, the impact of sample can be summarized as: l*> c* >ho, while the impact of light source can be described as following: c* > l* >ho. one-way anova it is essential to conduct the one-way anova to specify the light source transformation that has a great influence on color index (china national bureau of technical supervision, 2001, 2004; cha & lee, 2009). the p > 0.05 was used to confirm homogeneous variance in the homogeneity test of variance, where the lsd post-comparison was employed table 2. red jadeite samples under three standard light sources and color block simulated by computer no. 1 6 12 18 24 30 36 42 48 54 color parameters l* 37.41 40.55 42.65 44.25 51.37 49.18 40.26 41.90 43.09 48.20 a* 13.05 17.89 19.68 20.69 22.21 16.65 18.39 20.52 22.18 24.45 b* 9.17 14.31 14.33 19.64 26.91 21.54 15.72 17.71 19.39 24.67 samples d65 cwf a color difference δe00 d65 4.89 6.65 9.20 16.58 13.27 5.35 7.27 8.73 13.69 cwf 3.41 5.91 7.07 11.17 19.49 16.37 6.68 8.62 10.06 15.91 a 2.88 829. 10.10 13.01 21.02 16.76 8.75 10.82 12.52 18.14 simulated color block d65 cwf a 376 xin pan, ying guo*, ziyuan liu, zikai zhang, yuxiang shi because of its highest sensitivity. the p < 0.05 is an indicator of inhomogeneous variance, where tamhane method was used to compare the data (nayatani & sakai, 2007; shams-nateri, 2009). table 4. test of variance homogeneity of red jadeite with different light sources l* c* h0 p 0.630 0.568 0.007 the pl*>0.05 and pc*>0.05 in the homogeneity test indicate that lsd post-comparison method should be used to analyze the influence of different light sources on brightness and chroma, while ph0<0.05 suggests that tamhane method is supposed to be used to analyze the influence of different light sources on the red tone. table 5. one-way anova of light source on red jadeite calculation method (i) light source (j) light source averaged difference (i-j) p value l lsd d652 f22 -1.36100 0.005 f22 a2 -1.84292 0.000 a2 d652 3.20392 0.000 c lsd d652 f22 1.26300 0.044 f22 a2 -6.81683 0.000 a2 d652 5.55383 0.000 h0 tamhane d652 f22 -11.43317 0.000 f22 a2 8.15092 0.000 a2 d652 3.28225 0.000 the variance analysis (table 5) shows that the brightness, color hue and chroma of red jadeite are significantly affected by changing light source. the p values of these parameters after changing light sources are <0.05, which illustrates that the light source can exert significant impact on brightness, color hue and chroma. impact of standard light source on brightness, color hue and chroma of red jadeite figure 8. impact of three light sources on the l* of red jadeite three light sources have weak influence on the brightness (figure 8), while the brightness value under a light source is higher than that under d65 light source and cwf light source. also, a light source can increase the brightness value, making samples brighter, since its light color tends to be orange-red because of higher energy at long wave (figure 3). three light sources have great influence on the color hue that increases in an order: cwf light source, d65 light source and a light source. this can be explained by that, cwf light source, a cold and white light source in the united states, has a higher energy at wavelength of 550-620 with yellow and green tone. the red color is disturbed and the chroma value is reduced when the yellow-green light is superimposed on the red jadeite. d65 light source represents the average daylight with light distribution more uniform in all color light, so the red color of jadeite under d65 light source is more similar to that under sunlight, and the color displayed is more objective. as a color light, a light source has higher energy in a wide wavelength range with orange-red tone. the orange-red light can increase color value of the jadeite, making the red color more saturated. jadeite red has the largest tone angle under cwf light source, while d65 and a light sources can bring larger biases to samples with lower tone angle and smaller biases to samples with lower tone angle. thus, color hue of red jadeite varies greatly with light sources. this is because the cwf light source has higher energy in the purple band, enabling the purple light to neutralize the yellow tone of the sample and make the tone angle larger. orange-red light derived from a light source can make the color of the red jadeite more orange-red. summarily, d65 light source is suitable for the jadeite evaluation, a light source is suitable for exhibiting the red color of jadeite, which can be used to sale and display jadeite. the cwf light source can add more red tone to the orange-yellow jadeite, which can be used in special cases. color grading of red jadeite the light source has an important influence on the color grading of gemstones. as discussed above, the energy of d65 light source in the range of 360-780 nm is much more uniform than that of a light source and cwf light source, which is close to the relative light spectrum of the sunlight. thus, light color of d65 light source is the closest to the natural sunlight with no obvious other hues, which can better reflect the object color. therefore, we recommend d65 light source to be used for the color grading of red jadeite. three independent color parameters, e.g., l*, a*, b*, are used for cluster analysis based on the international grading scheme of gem color (guo, 2017). acceptably, the clustering performs best for the grading number 5 with uniform distribution of each category. also, the sig value of the clustering result is < 0.001 (table 1), suggesting that a valid grading. therefore, 120 red jadeite samples were classified into 5 categories. fisher discriminant function was used to exam the clustering, corresponding discriminant functions: f1 = 22.417 l* + 10.690 a* 9.536 b* (12) f2 = 23.384 l* + 12.586 a* -7.723 b* (13) f3 = 23.033 l* + 11.648 a* -8.605 b* (14) f4 = 24.546 l* + 11.623 a* -6.833 b* (15) f5 = 23.864 l* +10.854 a* -7.966 b* (16) the significance (sig) of the five grading is < 0.001 with the accuracy of color data backdating of 100% (table 6), confirming that the scheme is valid and feasible (tang, guo & xu, 2018). table 6. fisher’s discriminant analysis of jadeite-jade red color. (%) 1 2 3 4 5 total 1 100.0 0.0 0.0 0.0 0.0 100.0 2 0.0 100.0 0.0 0.0 0.0 100.0 3 0.0 0.0 100.0 0.0 0.0 100.0 4 0.0 0.0 0.0 100.0 0.0 100.0 5 0.0 0.0 0.0 0.0 100.0 100.0 377impact of different standard lighting sources on red jadeite and color quality grading currently, high-quality red jadeite is characterized by pure color, bright and uniform distribution, thus, c* and l* were used to evaluate color. as we know, high lightness makes the jadeite bright. thus, with the same chroma, jadeite with middle lightness is regarded as high quality. consequently, jadeite with hue range of 22.64-57.66°is divided into five levels, including fancy vivid, fancy deep, fancy intense, fancy dark and fancy, respectively. a 3d model of red jadeite is established accordingly (figure 4), specifically, the l* of fancy vivid ranges from 35 to 47, c* ranges from 30 to 40; the l* of fancy deep ranges from 47 to 53, the c* ranges from 30 to 40; the l* of fancy intense ranges from 35 to 44, the c* ranges from 24 to 30; the l* of fancy intense ranges from 44 to 53, the c* ranges from 24 to 30; and the l* of fancy intense ranges from 35 to 53, the c*chroma ranges from 15 to 24 (pan et al., 2019). figure 9. color solid of red jadeite conclusion conclusions were given based on comprehensive analysis: (1) fe 3+ is the main chromogenic mineral of red jadeite. the fe content has negative correlation with the tonal angle, and the color of red jadeite positively correlates with the hematite content. (2) d65 light source, a light source and cwf light source can exert great influence on the brightness, color hue and chroma of red jadeite. the light color of d65 light source distributes more uniformly, allowing red color of jadeite more objective and more similar to that under sunlight, which is suitable for the jadeite evaluation. a light source is suitable for exhibiting the red color of jadeite, which can be used to sale and display jadeite. the cwf light source can add more red tone to the orange-yellow jadeite samples, which can be used in special cases. (3) k-means clustering analysis and fisher discriminant analysis were used to determine the color from best to worst under the d65 standard light source (the most suitable light source for jadeite color grading): fancy vivid, fancy deep, fancy intense, fancy dark, fancy. the l* of fancy vivid ranges from 35 to 47, c* ranges from 30 to 40; l* of fancy deep ranges from 47 to 53, the c* ranges from 30 to 40; l* of fancy intense ranges from 35 to 44, c* ranges from 24 to 30; l* of fancy intense ranges from 44 to 53, c* ranges from 24 to 30; and l* of fancy intense ranges from 35 to 53, c*chroma ranges from 15 to 24. acknowledgments the authors gratefully acknowledge the financial supports by the innovation and entrepreneurship training program for undergraduates of china university of geosciences (beijing) under project numbers x201911415404. it also thanks dr.guo and xue-ding wang for their suggestions and help in colorimetric research. references cha, h. s. & lee, y. k. 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(2015). a revisit to the yorii jadeite–quartz rock, the kanto mountains, central japan: implications for petrogenesis. journal of asian earth sciences, 108, 58-67. doi: 10.1016/j. jseaes.2015.04.022. structured soils, such as residual laterites or volcanic ashes, abound in tropical regions. the hydromechanical behavior of these soils is clearly related to their structure. new techniques based on microstructural studies constitute a powerful tool for investigating these soils. this work presents an experimental investigation that documents the relationships between the microstructure and mechanical behavior of these soils. this study focuses on the microstructural behavior of brasília clay, a structured, highly porous tropical soil (n>50%) with a bimodal pore size distribution (psd). the microstructure was investigated using psd analyses of different structural states and void ratios. the mercury intrusion porosimetry (mip) technique was used to explore the psd of samples under various conditions, i.e., natural, compacted, slurry or consolidated states. modeling of the psd curves was performed using the bimodal van genuchten curve, which permits the linking of psd curves to the water retention curve. we observed that loading and compacting did not affect the micropores of this soil and concluded that the changes occurred entirely within the macropores. abstract keywords: hydromechanical behavior, microstructural behavior, pore size distribution. study of the relationship between the hydromechanical soil behavior and microstructure of a structured soil issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.65640 manoel porfirio cordão neto1, oisy hernández2, raydel lorenzo reinaldo3, camila borges4, bernardo caicedo5 1. department of civil and environmental engineering, university of brasilia, brazil 2. department of civil engineering, federal university of west bahia, brazil 3. department of civil engineering, federal university of tocantins, brazil 4. department of civil engineering, federal institute of goias, brazil 5. geotechnical post-graduation program, university of andes, colombia earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 91-101 los suelos estructurados, tales como las lateritas residuales o las cenizas volcánicas, abundan en las regiones tropicales. el comportamiento hidromecánico de estos suelos está claramente relacionado con su estructura. las nuevas técnicas basadas en estudios microestructurales constituyen una poderosa herramienta para investigar estos suelos. este trabajo presenta una investigación experimental que documenta las relaciones entre la microestructura y el comportamiento mecánico de estos suelos. este estudio se centra en el comportamiento microestructural de la arcilla de brasília, un suelo tropical estructurado y altamente poroso (n> 50%) con una distribución de tamaño de poros (psd) bimodal. fueron utilizados los análisis de psd para investigación de la microestrutura del suelo en diferentes estados y con diferentes índices de vacíos. se utilizó la técnica de porosimetría por intrusión de mercurio (mip) para explorar la psd de muestras en diferentes condiciones, es decir, natural, compactado, lodo o estados consolidados. se realizó la modelación de las curvas psd utilizando la curva bimodal de van genuchten, que permite el enlace de curvas psd a la curva de retención de agua. observamos que la aplicación de carga sobre las muestras de suelo y la compactación de las mimas no afectaron los microporos y se concluyó que los cambios que ocurrieron en este suelo fueron completamente dentro de los macroporos. resumen palabras clave: comportamiento hidromecánico; comportamiento microestructural; distribución del tamaño de los poros. record manuscript received: 13/06/2017 accepted for publication: 16/03/2018 how to cite item cordão neto, m., p., hernandez, o., lorenzo, r., borges, c., & caicedo, b. (2018). study of the relationship between hydromechanical soil behavior and microstructure of a structured soil. earth sciences research journal, 22(2), 91-101. doi: http://dx.doi.org/10.15446/esrj.v22n2.65640 estudio de la relación entre el comportamiento hidromecánico y la microestructura de un suelo estructurado g e o t e c h n ic s 92 manoel porfirio cordão neto, oisy hernández, raydel lorenzo reinaldo, camila borges, bernardo caicedo 1. introduction since the initial description of effective stress for unsaturated soils, significant progress has been made in defining constitutive models. most of these models are overviews of cam clay or modified cam clay (alonso et al., 1990, 1999, 2013; wheeler et al., 2003; sheng et al., 2008). currently, most studies attempt to link the soil microstructure to the overall soil behavior (gens and alonso, 1992; alonso et al., 1999, 2010, 2013; sanchez et al., 2005; koliji et al., 2008; pinyol et al., 2012; della vecchia et al., 2013). this trend is justified by the fact that according to mitchell and soga (2005), the mechanical properties of soil, including stressdeformation behavior, strength, compressibility and permeability, depend on its structure. for this reason, microstructural experimental studies have been an important tool for improving the research community’s understanding of the relationship between the microand macroscopic behaviors of soils (romero, 2013). several studies (e.g., alonso et al., 1999, 2010, 2013; loret and khalili, 2000; della vecchia et al., 2013; romero, 2013) have followed this line of research. based on constitutive laws and modeling, microstructural analysis distinguishes between variables that depend on the state of the soil, e.g., slurry, undisturbed or compacted. an example of the methodology used to distinguish between state-dependent and intrinsic soil properties is the concept of the intrinsic compression line, first presented by burland (1990). figure 1 shows the same soil in three different states: intact, slurry and remolded. the three curves, which represent the different soil states, tend to converge. a simple conclusion is that the initial structural state is destroyed in the presence of plastic strain and that at a certain point, no memory of the initial state remains, and the sample response is either equal or very similar. figure 1. structure states (vaughan and leroueil, 1990). figure 2. grain size distribution. (a) laser granulometric test; (b) hydrometer test. microstructural studies have been found to be useful in studying soils with limited bonding between particles. these studies are very useful for understanding the behavior of structured tropical soils, such as brasília porous clay (camapum de carvalho and mortari, 1994), colombian volcanic ash (lizcano et al., 2006) and san juan alluvium (zhang and rothfuchs, 2004). these soils have a very complex structure; therefore, defining a constitutive model that extracts all the features of these materials is difficult. in this work, we investigate the relationship between the mechanical behavior and microstructural features of brasília clay via mip tests. the mip samples were in different states (slurry, undisturbed (natural) and compacted) with different void ratios following the consolidation curve. all mip tests were modeled using the modified van genuchten equation (van genutchen, 1980). modeling analyses were conducted to better understand how mechanical loading affects the microstructural behavior. the results show that the mechanical loading erases the initial structure and leads to a convergent behavior. materials and methods the characteristics of brasília porous clay are highlighted here. this clay contains aluminum and iron oxides, as well as bonding particles, which form clumps of particles (figures 2 and 3), i.e., soil aggregates. these aggregates produce a bimodal soil structure that is evident in the pore size distribution (psd) curve (figure 4) and the soil-water retention curve (figure 5). figure 2 also shows how the aggregates (package) are subsequently destroyed by the dispersant and the ultrasound. furthermore, this clay contains two dominant pore sizes (microand macropores), and the distance between pores is very large (approximately 3 orders of magnitude) compared to other soils. finally, the micropores are not affected by loading or compaction processes (figures 4 and 5). (a) (b) all tests presented in this study were performed on soil samples obtained from the experimental field of the university of brasília (unb) (from a depth of 2 m) in brasília city, df, brazil. the characteristics of this site have been investigated in detail by many researchers of the unb geotechnical group. the first ~8 m of this site are characterized by a lateritic porous clay known as “brasília porous clay” (camapum de carvalho and mortari, 1994). this soil’s high porosity and weak cementation between particles originate from two specific physicochemical geological processes: lixiviation and laterization (camapum de carvalho and mortari, 1994). in general, the macrostructure of lateritic soils depends on the laterization and weathering processes (miguel and bonder, 2012). this soil is characterized by a high percentage of clay aggregates, which are often described as “packages” of clay (figure 3). despite the clayey matrix, the soil’s hydraulic behavior is similar to that of granular soil (kw=10-5 m/s), and the air entrance value (aev) of the water retention curve is approximately 10 kpa. evidence of a granular nature can be seen 93study of the relationship between the hydromechanical soil behavior and microstructure of a structured soil in the grain size distribution curves measured using a laser granulometer, with and without ultrasound (figure 2a), and in the hydrometer test results, with and without dispersant (figure 2b). furthermore, two other important structural components of the brasília soil are its weak cementation (aluminum and iron oxides) and high porosity values (greater than 50% in the natural state), which give the soil a very high potential for volumetric collapse (figure 6). figure 3. photomicrograph of an intact sample of brasília porous clay (guimarães, 2002). figure 6. consolidation curve for natural water content (wnat) and saturated (sr=100%) conditions. depth=2.0 m (guimarães, 2002). figure 4. pore size distribution and its evolution with loading for the brasília porous clay (silva, 2009). figure 5. water retention curve of brasília soil in different states. another important characteristic of this tropical soil is the bimodal distribution of the psd curve. figure 4 illustrates a typical psd curve for compacted brasília porous clay and shows how it changes along a loading path. the same feature (i.e., bimodal distribution) is observed in the water retention curve presented in figure 5. despite the different states and void ratios, the micropores do not change (suction greater than 10.000 kpa), as observed in the psd curves. the soil samples present a liquid limit wl=36%, plastic limit wp=26% and specific solid weight gs=2.74. table 1 summarizes other geotechnical properties of the samples. furthermore, the samples presented in table 1 had different fabric consolidation states. the following nomenclature is used to describe the initial structure and consolidated states: natural soil (n) denotes an intact sample, compacted soil (c) represents soil compacted using the standard proctor procedure on the dry side (figure 7), and slurry (s) denotes samples with water contents at the liquid limit (wl). the procedure used to produce slurry samples (s) is similar to the one used to obtain the consistency index of soils, i.e., the liquid limit and plastic limit. table 1 also lists the effective vertical stresses applied during the consolidation tests; for instance, n-50 indicates a natural sample consolidated using a vertical effective stress equal to 50 kpa. 94 manoel porfirio cordão neto, oisy hernández, raydel lorenzo reinaldo, camila borges, bernardo caicedo table 1. geotechnical properties of the samples. figure 7. compacted curve – standard proctor. field data from (silva, 2009). figure 8. comparison between mip samples and the respective consolidation load points. (a) natural sample; (b) slurry and compacted samples. results the mercury intrusion porosimetry (mip) technique has been used to easily and rapidly study structural psds in soils. although mip has certain limitations regarding the minimum and maximum measurable pore radii, it is one of the most commonly used techniques for describing the structure of soils and can provide insights into the link between the microstructure and the geotechnical properties of a soil (romero, 2013). prior to mip testing, all samples were freeze-dried via the lyophilization technique with liquid nitrogen. the drying process helps avoid strains on the samples. next, to investigate how the consolidation process affects the psd, the samples were subjected to consolidation. a sample was placed in the equipment and saturated, and then a target load was applied and maintained for 24 hours. then, the sample was removed, and an mip sample was prepared. a different sample was used for each vertical effective stress. figure 8 presents a comparison between the mip results obtained by the above procedure, as well as their consolidation curves. in general, the results obtained from this procedure follow the consolidation curve. the natural sample was very heterogeneous; consequently, differences exist between the mip samples and the consolidation curves. (a) 95study of the relationship between the hydromechanical soil behavior and microstructure of a structured soil figure 9. brasília porous clay in different states. (a) water relation; (b) pore size distribution. figure 10. brasília porous clay in the natural state. (a) water relation; (b) pore size distribution. (a) (a) (1) (b) (b) in general, the results verify the influences of the initial structure and changes during loading on the microscopic behavior of brasília clay. starting with the initial structural state, figure 9 shows the psds for three different structures (natural, compacted and slurry). the vertical axis, enw, represents the ratio between the volume of the voids filled with mercury and the volume of the solids. the psd is defined as follows: where d is the apparent pore diameter. the range of diameters shown in the psd is approximately 0.01 μm to 200 μm. bimodal behavior is observed in this range, and the distance between dominantly microand macropores is nearly 3 orders of magnitude. this type of behavior has been observed in other studies, such as almeida and futai (2005) and miguel and bonder (2012), which present psd curves for similar tropical soils. due to the limitations of mip associated with pores smaller than 0.1 μm, otálvaro (2013) performed both mip and n2 adsorption tests on the same soil and concluded that the differences between the micropores determined via mip and the n2 tests were not significant. the micropores are probably associated with aggregates (packages), as shown and discussed above (figure 2). interestingly, the samples with different initial structures (natural, compacted and slurry) featured similar pore distributions for diameters less than 1 μm. furthermore, the extrusion curves were also very similar. therefore, we conclude that the compaction and slurry processes are not able to affect the micropores inside the aggregates. consequently, these micropores inside the aggregates influence the liquid and plastic limits. the natural and compacted samples have very similar psds, and their main distinction is the final void ratio. according to their curves, these samples are similar for pore diameters smaller than 1 μm, and their distributions of micro and macropores are similar. for >1μm pore diameters, certain differences exist, with the main difference being that between the inflection points in the slopes of the curves. based on the comparison of these two specific curves with that of the slurry sample, the dominant macropore diameter in the slurry is obviously smaller than that in the other two. however, verifying the macropore range uniformity is not possible. figures 10, 11 and 12 illustrate the changes in the psd during loading for different initial structures. the consolidation process similarly affects the psd of natural, compacted and slurry samples. overall, no noteworthy variations are present in the curves for pore diameters smaller than 0.5μm (micropores). additionally, the extrusion curves are similar. the majority of differences are observed among the macropores. 96 manoel porfirio cordão neto, oisy hernández, raydel lorenzo reinaldo, camila borges, bernardo caicedo for the natural sample, the macropores are quite uniform, and the dominant diameter is approximately 50 μm. the macropore distribution was less uniform during loading, indicated by the change in slope of the curves (figure 10a) and the pore size density (figure 10b). in addition, the dominant pore size peak is also reduced. importantly, if the effective vertical stress was increased, the dominant diameter peak for macropores would disappear, i.e., the psd would assume a unimodal behavior. the mip results for the slurry sample are presented in figure 11. most of the above observations also apply to the slurry sample. however, the distribution of >1 μm pore diameters is less uniform than that of the natural and compacted samples, and the distribution is even less uniform after the consolidation process (figure 11b). the modeling results presented in the next section enable the evaluation of the uniformity of the curves using a model fitting parameter. figure 11b shows that the pore size density of macropores decreases more slowly in the slurry than in the natural samples. (a) (a) (b) (b) figure 11. brasília porous clay in the slurry state. (a) water relation; (b) pore size distribution. figure 12. brasília porous clay in the saturated compacted state. (a) water relation; (b) pore size distribution. the behavior of the compacted sample (figure 12) is similar to that of the other two states. however, the compacting process slowed the pore size changes, i.e., curves with loads less than 800 kpa are similar. this behavior agrees with the fact that a compacted sample is less deformable; consequently, it will experience fewer changes in structure. however, after the sample point c-800, the changes are noteworthy. importantly, the position of c-400 is unexpected because it is located near sample c-200 and not directly in between c-800 and c-200 (figure 12a). finally, as in other samples, unimodal behavior is expected if larger loads are applied. the results presented in figures 11 and 12 agree with the water retention curves presented in figure 5. the water retention curves for natural (guimarães, 2002; silva, 2007), compacted ((silva, 2009) e=1.16), and compacted and loaded states ((silva, 2009), e=0.98 and e=0.77, respectively) coincide at suction values larger than 1000 kpa because the water content links the micropores. compaction and loading also affect the curves at values less than 100 kpa, a range associated with macropores. figure 13 presents the water retention curves obtained via conventional methods (prapaharan et al., 1991) and the mip results. the water retention curve obtained via the mip test was calculated using the procedure presented by otálvaro et al. (2016). although the curves do not coincide, the mip results reflect the aevs of the macroand micropores. 97study of the relationship between the hydromechanical soil behavior and microstructure of a structured soil figure 13. comparison between the water retention curves obtained via conventional techniques and mip. table 2. pore size distribution parameters for full-fit procedure. modeling and discussion because a visual analysis of the effects of the consolidation process would be difficult, the results were modeled, and an analysis of fitting parameters was performed to better understand the effects of structural and consolidation state changes on the psds. the cumulative pore size curves were fitted using the equation proposed by durner (1994). this equation is a modification of the van genutchen (1980) equation and is able to separate and identify the microand macrocomponents of the soil structure. thus, the cumulative pore size curve was adjusted using equation 2: (2) where the parameters with the subscript m are associated with micropores and the parameters with the subscript m are associated with macropores. the parameters em and em are the void ratios; αm and αm represent the dominant pore sizes; and nm and nm are the fitting parameters associated with pore size uniformity. the fitting was done in two steps. first, all six parameters of equation 2 were calculated using a standard procedure, which means that all possible variables were varied until the best fit was obtained. this step is called full-fit; the results are presented in table 2. in the next step, only the fit parameters related to the macropores (em, αm and nm) were adjusted; the micropore parameters (em, αm and nm) remained constant and equal to the average of values obtained in the first step (full-fit) considering all the states and consolidation loads. additionally, for this procedure, em was computed by imposing the total void ratio. this step is referred to as macro-fit; the results are presented in table 3. according to tables 2 and 3, the r2 coefficient is greater than 97%, which means that the modeling process was able to mimic the main features of the data. the micropore parameter αm, which is associated with the dominant pore size, exhibits little variation. this agrees with the observations in figures 10, 11 and 12. additionally, the experimental results lead to the conclusion that all micropore ranges are small. consequently, the macro-fit results can be used to identify the main features of the data (r2>97%), demonstrating that the micropore parameters had little influence on the final results. figure 14 presents the fit and experimental data for the smaller r2 value. even this result exhibits a good agreement and allows us to conclude that the fit parameters are representative of the data. the macropore parameters evolved as expected. for example, the macropore void ratio (em) decreased during loading. however, the fit parameters highlight another point. table 4 presents the void ratio of the micropore curve (em). the suggestions of alonso et al. (2013) and delage and lefebvre (1984) were used to determine the values of em. in general, the values obtained by modeling were similar to but less than the extrusion curve values. the fit parameter em is the value around the inflection point of the micropores, and the data show that the fit parameter is an alternative to determining the microand macropore void ratios. 98 manoel porfirio cordão neto, oisy hernández, raydel lorenzo reinaldo, camila borges, bernardo caicedo table 3. pore size distribution parameters for macro-fit procedure. (b) (a) (c) (d) figure 14. water relation curves and adjustment with model validation. (a) n, (b) c, (c) c-50, and (d) s. 99study of the relationship between the hydromechanical soil behavior and microstructure of a structured soil (b) (b) (a) (b) (a) (a) figure 15. variations in the fit parameters of the natural sample. (a) dominant pore size (αm); (b) uniformity coefficient (nm). figure 17. variations in the fit parameters of the compacted sample. (a) dominant pore size (αm); (b) uniformity coefficient (nm). figure 16. variations in the fit parameters of the slurry sample. (a) dominant pore size (αm); (b) uniformity coefficient (nm). table 4. microstructural void ratio values from the extrusion curve. the behavior of the psd curves can be better analyzed by using the parameters presented in table 2 and table 3. figures 15, 16 and 17 present variations in the parameters related to the dominant macropores (αm) and macropore uniformity (nm). 100 manoel porfirio cordão neto, oisy hernández, raydel lorenzo reinaldo, camila borges, bernardo caicedo for all samples, we observed that αm decreases during loading in the full-fit and macro-fit results (figures 15a, 16a, and 17a). the slope of the αm decrease is steeper for the natural samples than for the compacted and slurry samples. this behavior can be attributed to the presence of weak cement bonding between aggregates that is not present in the slurry and compacted samples. after the bonds are destroyed, the natural samples behave similar to the other samples. furthermore, despite the differences in initial structures, the values of αm approach zero, meaning that all samples exhibit unimodal behavior. the macropore uniformity parameter (nm) did not change after the consolidation process. for the macro-fit procedure, the curves presented in figures 15b, 16b, and 17b can be considered constant at approximately 1.5 (except for the highest void ratio). the compacted sample results of nm indicate that the samples become more uniform during the full-fit procedure. however, according to these results, the variation is small and can be regarded as a statistical phenomenon. the natural samples from n (e=1.69) to n-50 (e=1.55) change significantly (see figure 18), indicating that the consolidation process does not affect the distribution of pores beyond a certain point. this result agrees with the above αm results, in which the natural sample begins to behave like the other samples when the weak bonds are destroyed. across all the samples with different structures and consolidation states, the value of nm was approximately 1.5. therefore, this parameter is not related to the structure and is instead an intrinsic soil property associated with grain size, grain shape, interactions between mineral-clays and water, etc. the number of tests in this study is not sufficient to make our results conclusive, but our results can be considered the first insight into how mip results can be used to identify non-structural properties. final discussion and conclusions here, we present the test results and discuss our conclusions: 1. for brasília clay, the micropores (<0.1 μm in diameter) are not affected by consolidation, compaction, or processes used to produce a slurry. 2. the macropores are destroyed by the consolidation process, which can shift the sample from bimodal to unimodal distributions. 3. the initial structure is destroyed by the consolidation process, and after a certain degree of consolidation, the natural, compacted and slurry samples achieve very similar structures. the third point can be linked with the macroscopic behavior of the soil. as discussed at the beginning of this paper, the mechanical properties of soils depend on the soil structure. however, if the consolidation process destroys the initial structure, samples with different initial structures tend to exhibit similar behaviors after the initial structure is destroyed. this conclusion agrees with the concept of the intrinsic consolidation line presented by burland (1990) and can be observed in figures 1 and 18 for two different soils. figure 18 presents the consolidation curve for brasília clay with different initial states (natural, compacted and slurry). despite the different initial behaviors (which are related to the formation processes), all consolidated samples tend to exhibit similar behaviors, and the consolidation curves converge. the conclusions presented in this study need to be verified for other soils because brasília clay has very specific features, such as aggregations, a bimodal structure and the conservation of the micropore structure. however, the methodology used to investigate the effects of the consolidation process on psd proved to be efficient. additionally, the modeling process aided our understanding of the microstructural behavior and proved to be a very efficient tool for analyzing the psd. figure 18. results of oedometric compression tests on brasília clay in different initial states. references almeida, m. s. s. & futai, m. m. 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(2004). experimental study of the hydromechanical behaviour of the callovo-oxfordian argillite. applied clay science, 26(october), 325–336. the reservoir structural fractures have excellent fractal characteristics, as well as self-similarities. based on the fractal theory, the surface fractal characteristics of faults and the fractal characteristic of fractures in the core of the xujiahe formation in the fault-fold belt of the central yuanba area were studied, and a quantitative relationship was set up between them. based on the fractal characteristics of faults, predictions were made of the favorable fracture zone, which provides a new idea for the research of fracture, as well as offers theoretical references for exploring the fracture development zone during oil-gas exploration. the results show the following: the seismic value of reflection fault fractal dimension of the xujiahe formation is 1.5284; the correlation coefficient r2 is bigger than 0.9901; the capacity dimension linear regression correlation coefficient of the fracture in core of the xujiahe formation is bigger than 0.98; the fractal dimension d can well reflect the fault and fracture development degree, as well as the complexity of the fracture system; it can quantitatively calculate the density of the fracture of the reservoir in the area; the areas of capacity dimension bigger than 1.45 are the fracture development zones in the fault-fold belt of the central yuanba area; the oil and gas enrichment degree is high; the areas with the fractal dimension value between 0.95 and 1.45 are the fracture relatively-developed zones; the fractal dimension with values smaller than 0.95 are the lack of fracture areas. abstract keywords: fractal characteristics, structural fracture, xujiahe, formation, central yuanba fractal characteristics of reservoir structural fracture: a case study of xujiahe formation in central yuanba area, sichuan basin issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.72250 cunhui fan1,2, qirong qin1,2*, dongfeng hu4, xiaolei wang1,2, mengyue zhu1,2, wei huang1,2, yuxi li1,2, muhammad aqeel ashraf3 1school of geoscience and technology; southwest petroleum university; chengdu 610500; china 2state key laboratory of oil and gas reservoir geology and exploitation, southwest petroleum university, chengdu, 610050, china 3international water, air & soil conservation society iwascon 59200 kuala lumpur, malaysia 4exploration company sinopec chengdu 610041, china corresponding author: 986336692@qq.com earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 113-118 las fracturas estructurales de yacimientos tienes excelentes características fractales y autosimilares. basados en la teoría fractal, se estudiaron las características fractales superficiales de las fallas y las características fractales de las fracturas en el núcleo del cinturón falla-pliegue de la formación xujiahe, área central de yuanba (china). luego se estableció una relación cuantitativa para estas relaciones. según las características fractales de las fallas, se predijo la zona favorable de fractura, lo que ofrece una nueva idea para el conocimiento de esta y a la vez brinda referencias teóricas para la exploración de la zona de desarrollo de la fractura durante la exploración de petróleo y gas. los resultados muestran lo siguiente: el valor sísmico de la dimensión fractual de la falla en la formación xujiahe es de 1.5284; el coeficiente de correlación r2 es mayor a 0.99901; la dimensión del coeficiente de correlación de regresión lineal en el núcleo de la fractura de la formación xujiahe es mayor a 0.98; la dimensión fractal d puede reflejar la falla y el grado de desarrollo de la fractura, al igual que la complejidad del sistema de fractura; también puede calcular cuantitativamente la densidad de la fractura del yacimiento en el área; las áreas con capacidad mayor a 1.45 son las zonas de desarrollo de la fractura en el cinturón falla-pliegue del área central de yuanba; el grado de enriquecimiento de petróleo y gas es alta; las áreas con valor de dimensión fractal entre 0.95 y 1.45 son las zonas de fractura de desarrollo relativo; la dimensión fractal con valores menos a 0.95 son las áreas que carecen de fracturas. resumen palabras clave: características fractales; fracturas estructurales; formación xujiahe, yuanba central. record manuscript received: 14/11/2017 accepted for publication: 27/06/2018 how to cite item fan, c. h., qin, q. r., hu, d. f., wang, x. l., zhu, m. y., huang, w., li, y. x., & ashraf, m. a.(2018). fractal characteristics of reservoir structural fracture: a case study of xujiahe formation in central yuanba area, sichuan basin. earth sciences research journal, 22(2), 113-118. doi: http://dx.doi.org/10.15446/esrj.v22n2.72250 características fractales de fracturas estructurales de yacimientos: caso de estudio de la formación xujiahe en el área central de yuanba, cuenca de sichuan st r u c t u r a l g e o l o g y 114 cunhui fan, qirong qin, dongfeng hu, xiaolei wang, mengyue zhu, wei huang, yuxi li, muhammad aqeel ashraf 1. introduction oil and gas explorations concentrate on fracture-developed zones. tight sandstone reservoirs were regarded as their target areas. this type of reservoir comprises fractures with tectonic origins as their main seepage path and reservoir space (bahrami et al., 2012; abija & nwankwoala, 2018). the formation, development, and evolution of structural fractures are the critical issues and difficulties in oil and gas exploration and development, especially in the study of the characteristics and division of the formation stages of reservoir fractures. this research area has received considerable attention from domestic and foreign experts and scholars (larsen et al., 2010; nelson, 2001; ali et al., 2018). mandelbrot, a french mathematician, proposed the fractal theory to study a complicated phenomenon with similar features and suggested making a quantitative description of fractal complexity with fractal dimension (mandelbrot, 1982; roslee, 2018). scholz & aviles (1985) employed the fractal theory in the fracture study for the first time, providing a new mean for quantitative characterization of spatial distribution and the geometry of fracture structure. barton & hsieh (1989) made systematic research on the statistical law of rupture and cracks on field outcrops, in which he found fractal features of fracture distribution. zhang et al. (1996) reported that fractal dimension of tectonic fissures in an oil and gas reservoir reflects fracture development degrees of different lithological reservoirs. both fractures and cracks are a geological phenomenon and that rocks generate cracks or displacement under stresses. a self-similar structure is formed in a particular scale range (deng et al., 2006; rahim et al., 2018). the fractal dimension of fault and core cracks could contribute a quantitative description of the spatial development of cracks in reservoirs. recently, the fraction theory is widely applied in the geological field. it provides a new means for the quantitative characterization of spatial distribution and the geometry of fault structure, as well as a new idea for studying crack distribution in reservoirs (takayuki et al., 1987; usman et al., 2018). the yuanba area is located in the northeastern part of the sichuan basin. yuanba spans several administrative divisions of bazhong city, as well as the counties of cangxi, tongjiang, and nanjiang, among others. remarkable breakthroughs in oil and gas exploration have been achieved through perennial geological research in the continental and marine strata. commercial oil and gas were found in the wells of several series of gas-bearing formations, which includes: the lei-3 section of the leikoupo formation; second, third, and fourth sections of the xujiahe formation; zhenzhuchong and daanzhai sections of the ziliujing formation; and the middle jurassic qianfoya formation (zeng et al., 2010; nwankwoala & oborie, 2018). the productive natural gas flow was obtained in multiple wells. among these, the yl7 well in the third section of the xujiahe formation was tested and produced a gas flow of approximately 120.3×104 m3/d. these discoveries indicate good prospects for gas exploration. however, gas productivity varies dramatically from well to well. the daily gas output of a single well ranges from 0.1×104 to 120×104 m3/d. in the study area, the reservoir has low porosity and low permeability with fracturedominated productivity (zeng & li, 2005; zeng et al., 2008; jiang et al., 2004; veeraragavan et al., 2018). structural fractures have relative development due to the multi-stage tectonic deformation they undergo. fractures in different stages dominate and regulate hydrocarbon migration and accumulation. the presence of these fractures is likewise an indispensable condition for high gas production (zeng, 2008; raj & prabhakaran, 2018). therefore, the author used the fractal theory to study fractal features of cracks in reservoirs of the xujiahe formation, in the fault-fold belt of the central yuanba area. the capacity dimension on the t3x fracture and fractal dimension (d) of cracks on 12 cores were also calculated. fractal features of cracks were explored based on the relationship between core crack density of a cored well and the capacity dimension of cracks. the fractal characteristics of fractures and the threshold of the fractal dimension (d) of cracks in the study area were determined by combining the verification of oil and gas statistics of the study area. on this basis, crack distribution in the study area was predicted. 2 geological characteristic figure 1. map showing the general structural belts in the study area (large map) in regional tectonics, the yuanba area is located in the transition region between the frontal zone of micangshan and the gentle fold belt of the central sichuan basin, which connects the micangshan-dabashan foreland fold-thrust belt on the north and the longmenshan orogenic belt in the northwest (guo, 2011; lin et al., 2011; len et al., 2018) (fig.1). central yuanba lies close to the subsurface of yuanba, which connects the jiulongshan structure in the north and the tongnanba structure in the northwest. proximity to the subsurface causes the structural features of the study area to have a close genetic relation to its nearby regional structure. the surrounding structural belts of the longmenshan, dabashan, and micangshan exert varying degrees of influence on tectonic formation and evolution. ever since the indosinian movement, these surrounding structural belts have been reformed and superposed by multi-stage tectonic shifts. the study area and southern qinling in the yanshanian and himalayan movement stages were intensively compressed from the north to the south. the structural movement patterns of the himalayan scene inherit the characteristics of the yanshanian stage. the study area was thrust continually from the north to the south. thus, the structure was ultimately formed in the late-himalayan stage. the principal structures of the study area can be divided into two secondorder units: namely, low-flattish fold-thrust belts and syncline structural belts. the degree of the folds’ development is weak, and the structural height is not extremely obvious when compared with that of the nearby jiulongshan and tongnanba structures (roslee & tongkul, 2018). the major faults developed are mostly of three sets: namely, the nearly sn thrust faults and the nw and ne compresso-shear faults, which are mainly middle-shallow and middle-small faults. most faults can only break through the xujiahe formation because of the small scale and range. finally, the faults disappear upward in the zhenzhuchong formation and downward in the leikoupo formation. the faults that appeared on the seismic reflection profiles are mainly high-angle reverse faults. the reservoir lithology of the xujiahe formation in the central yuanba area mainly consists of medium-grained lithic sandstone, quartz sandstone, and siltstone. primarily, the sedimentary environment is a lake-deltaic sedimentary system, and the reservoir thickness is generally from 270 to 700m. the pore space of the reservoir is mainly composed of residual intergranular pore, intergranular dissolved pore, repitation, and fractures. the distribution of the reservoir porosity ranges from 0.61% to 6.88%, and the average value is 2.46%. permeability distribution ranges from 0.0012×10-3 to 26.0086×10-3 µm2 with an average value of 0.2944×10-3. the reservoir has the typical low porosity and low permeability characteristics of a tight sandstone reservoir (nwankwo & nwankwoala, 2018). the distribution of the middle and shallow hydrocarbon 115fractal characteristics of reservoir structural fracture: a case study of xujiahe formation in central yuanba area, sichuan basin figure 2. faults systems diagram of xujiahe formation in fault-fold belt of central yuanba area figure 3. characteristic of fractures of xujiahe formation in fault-fold belt of central yuanba area, a) low angle shear fracture yl2, t3x and b) low angle shear fracture yl2, t3x reservoir is controlled by a large area of overlapping, deltaic sand bodies. the development of fractures and faults provide favorable transportation conditions for hydrocarbon migration. some sand bodies that are blocked by a reverse-fault offer a favorable trap condition for hydrocarbon enrichment and accumulation (wali et al., 2018). located in the gently low porosity and low permeability hydrocarbon reservoir, the multiphase petroleum system with a complicated oil-water-air relationship finally forms. 3 self-similarity of middle xujiahe formation in yuanba reservoirs of the xujiahe formation in the intermediate zone of the fault-fold belt of the central yuanba area are common pore-crack reservoirs. crack development mainly determines oil-gas enrichment and productivity. as viewed from the causes, fault, and fracture are different fractural structures of rocks with different cracking degrees and displacement under the effect of a uniform, tectonic stress field. they have self-similarity and meet the fractal theory. 3.1 self-similarity of fault and fracture azimuth t3x reflects faults in the xujiahe formation of fault-fold belt of the central yuanba area, which mainly suffers tectonic extrusion of the longmenshan tectonic zone, dabashan fault zone, and micangshan tectonic zone. it is dominated by the nearly sn-oriented and nnw-oriented fracture system, followed by the ne-oriented faults (fig. 1). in the northeast area of the study area, considering the effect of the dabashan arcuate tectonic belt, the advantageous fault azimuth is in the nw direction. the advantageous azimuths of the yl3 and yb4 cracks in this region are in the nw direction and the nnw direction. the nw direction is likely to be the advantageous azimuth. influenced by the micangshan tectonic zone, the fault azimuth at the middle area is close to the sn and nnw direction. the advantageous azimuth is close to the sn direction, while the advantageous azimuths of the yl1 and yl5 cracks that are located in the middle zone are close to the sn direction (fig. 2). to sum up, the central crack azimuth of each well conforms suitably to the fault strike surrounding the borehole, which indicates the apparent self-similarity between fault and fracture layout. 3.2 self-similarity between fault and fracture combinations according to the rock sound emission test and analysis on homogenization temperature and the tectonic structure of inclusion, the tectonic system in this area was mainly developed during the late yanshan movement-early himalayan movement. during the late yanshan movement-early himalayan tectonic movement, this area, together with the south qinling mountains, suffered sturdy extrusion from the north to the south and formed the nw-oriented and ne-oriented fracture system under the nearly sn-oriented tectonic extrusion. during the middle and late himalayan tectonic movement, the roughly snoriented fault system was developed under the ne-oriented tectonic extrusion of the dabashan arcuate tectonic belt, which reconstructed and superposed the previous nw-oriented and ne-oriented fault system. as a result, fault often represents the convergence of the ne-oriented fault tail and the sn-oriented fault, or the convergence of the nw-oriented fault tail and the sn-oriented fault. the result of the faults shows a distinctive zigzag structure. this result reflects the recombination and superposition of two tectonic movements. moreover, the apparent x-shaped combination of faults could be seen along different fault extension directions. tectonic rupture and fault are material reflections at different stages of evolution from the same tectonic stress field. tectonic fracture has evident direction. it often extends along the fault zone or forms a sharp angle with the fault zone. the discovery of the fracture azimuth of the xujiahe formation was introduced based on imaging logging data of yl5 and yl2. fractures still extend mainly toward sn, nnw and ne directions, conforming to the fracture azimuth and height on the t3x reflection tectonic structure. core observation found that there are mostly two types of fractures. one is shear cracks. they often have two groups, and most are perpendicular to bedding. they extend stably with apparent striations and have broad distributions, presenting an “x-shaped” conjugate (fig. 3a). the other type of fracture is tension fissures. they are often indentation, manifested in tracing and reconstruction of the early two groups of shear cracks (fig. 3b). the self-similarity between fault and core fracture demonstrates that fault combinations are the product of the typical stress field. 4. fractal features of fault and core fracture 4.1 principle and process of fractal dimension according to the definition of mandelbrot (1982), the fractal has a similar composition to the whole in some way. he applied fractal dimension (d) to make a quantitative description of fractal complexity. for quantitative research on the fractal problem, a fractal set could be defined as the following: (1) (2) where nn is the number of objects with appointed features, rn is the radius of the particular object, d is the fractal dimension, and c is the proportional constant. calculate the natural logarithm to both sides of the equation (1), then, it can be seen from equation (2) that lnnn and lnrn are in a linear relationship, and d is the slope of the straight line. a physical model was designed according to the definition of fractal dimension and equation (1), and thereby d of each entrapment was calculated. there are a lot of methods to calculate the distribution fractal of reservoir fault and fractures, such as the box covering method, the cantors’ dust method, 116 cunhui fan, qirong qin, dongfeng hu, xiaolei wang, mengyue zhu, wei huang, yuxi li, muhammad aqeel ashraf the box-counting method. in this paper, the fractal dimension of fault and fractures was calculated by using the box covering method, which is easy to be implemented on the computer. elementary steps of this method include the following: use a square grid (side length=r) to cover the study area to make statistics on the number of squares covering fault zones and fractures (nn). next, change r gradually and make a statistic on corresponding nn. establish a rectangular coordinate system taking lnrn as the x-axis and lnnn as the y-axis, in which a regression analysis of statistical data is implemented by using the least square method. if fault and fracture distribution in the study area have fractal features, lnrn and lnnn shall meet the linear relationship of d=lnnn/δlnrn. at this moment, the slope of this regression line is d of each fault distribution. d of the fault and fracture is calculated similarly except for the different standard of r. figure 4. calculation of information dimension of faults table 1. fractal statistics of fractures of the reservoir in the study area figure 5. contour map of the distribution of faults capacity dimension of t3x 4.2 fractal features of fault this paper carried out different-scale gridding and calculated corresponding fractal dimensions to the t3x fault tectonic map (fig.3) of the fault-fold belt of the central yuanba area using the box covering method. in the t3x fault tectonic map, faults were vectorized with coredrawx6 and divided into 200 sub-regions (grids with an actual side length of 1.5km). the capacity dimension of each sub-region was calculated. subsequently, the plane distribution map was drawn by using sufer8.0. when calculating the fractal dimension of each sub-region, the subregion was covered by same-scale (r=1, 0.8, 0.6, 0.4, 0.25 and 0.2km) gridding. d of this tectonic map was calculated as 1.5284, and the correlation coefficient was r2=0.9901. when the minimum size of the box reaches the length of the smallest fault and the correlation coefficient is higher at 0.92, it can be viewed meeting requirements of statistical self-similarity. therefore, t3x faults in the study area have fractal features. figure 4 shows calculated results of the subregion in which the yb3 well lies. the absolute value of the straight slope is 1.5213, which is the capacity dimension of faults in this sub-region. after the information dimension of all sub-regions is calculated, the horizontal distribution of the information dimension of t3x faults in the study area could be drawn (fig. 5). 4.3 fractal features of fractures fracture pictures were collected from drum scanning of 18 rock samples of the t3x in the study area. the side length of the square gridding was set as r=2, 3, 4, 5, 8, 10, 14, 16, 18, 20, 25 and 30mm. the physical number of squares containing fractures was counted. the other calculation process is same as the calculation of the fractal dimension of fault. finally, the capacity dimension of fractures on each rock sample dh was calculated (table 1). according to the calculated results, all correlation coefficients are higher than 0.98, indicating the desirable fractal feature of the general fracture distribution of the core samples. therefore, distribution law of fractures could be predicted by using the fractal theory. due to different lengths of cores and different contributions of each section to the overall core density of the studied core segment, as well as the lack of a representative core section, the weighted average method was applied to calculate fracture density of the overall coring section of each well. in other words, the contribution of each rock sample to the fracture density of the whole well-section being studied was taken into account. this could be expressed as: where pi=li/l; pi is the weight of each rock sample; ρ1 is the coring density of the whole well-section being studied; ρ1i is the density of each rock sample; l is the total coring length of the whole study well-section, and li is the length of each coring. coring fracture densities of yl2 and yl3 were calculated from the equation, 2.01 fractures/dm2, and 3.43 fractures/dm2, respectively. 4.4 quantitative relationship between fault and fracture fractal features since fault and fracture have the same causes and self-similarity in term of fractal significance, fracture distribution could be predicted from fault distribution, as long as some internal quantitative relationship between them was discovered. the capacity dimension of the fault on the sub-region and the capacity dimension of fractures on the coring section were connected through fracture density on the core. finally, capacity dimension distribution of fault on the t3x tectonic map was transformed into the capacity dimension distribution of fractures. through statistical fitting on coring data (fig.7), it was discovered that there is some relationship between the fracture density of different wellsections on the core and the capacity dimension of corresponding fractures: where k is the fracture density on the core cross-section (fracture/dm2). dh = 0.0012k2 + 0.086k + 1.1422 the fracture density of the coring well and the capacity dimension of the fault (3) (4) 117fractal characteristics of reservoir structural fracture: a case study of xujiahe formation in central yuanba area, sichuan basin figure 6. the fitting relation between fracture face density and fracture capacity dimension figure 8. prediction graph of fracture favorable zones of t3x figure 7. the fitting relation between faults capacity dimension and fracture face density k = 8.6596d‘ – 11.014 on the sub-region of this coring well have the following fitting relationship: where d‘ is the capacity dimension of the fracture on the sub-region. the capacity dimension of the macro fault can be transformed into the capacity dimension of the micro-fracture through equation (1) and (2). fig. 8 shows the capacity dimension of the fracture transformed from that of fault. comparing fig. 6 and fig. 8, the region with higher d of fracture has higher d of fracture, indicating the higher development of fractures; the vice versa. 5 fractal dimension prediction and oil & gas productivity according to the distribution of discovered commercial hydrocarbon flow and oil & gas wells, regions with higher than 1.45 fractal dimension of fault often have better oil & gas flows. the fractal dimension of regions predicted favorable for fracture development from the above two formulas shall be higher than 1.4. hence, regions with d > 1.4 are defined as favorable regions for fracture development. regions with 0.95 < d < 1.4 are defined relatively favorable regions for fracture development, and regions with d < 0.95 are unfavorable regions for fracture development. in fig.8, regions with fracture development in the xujiahe formation of yuanba mainly concentrate within the isoline region (d > 0.95) and surrounding areas. for example, well-areas like yb3, yl3 and yl4 show a higher than 1.5 fractal dimension of fracture and enjoy high productivity of commercial hydrocarbon flow. this agrees well with fracture prediction. wells in the isoline region (d < 0.95), that is, predicted secondary fracture development regions, show no oil and gas. this confirms that the fractal theory is a useful technology to predict fracture development law. 6 conclusions 1.fault distribution and fracture distribution controlled by fault have remarkable self-similarity and fractal features. therefore, studying fracture development law with the fractal theory is a reliable, technological idea and an effective approach. 2.the capacity dimension of the fault and fracture distributions (d) is calculated based on the fractal theory by using the box covering method, which could reflect the development degree of fault and fracture, as well as the complexity of the fracture system. as a result, d could be used as one quantitative parameter to study spatial development and the distribution characteristics of fractures. 3.according to the prediction results, fault distribution and fracture distribution have evident fractal features. their correlation coefficients are all higher than 0.99. higher d represents dense fracture development. regions with d > 1.45 are favorable regions for fracture development and have reservoirs with good physical 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(1996). a fractal analysis on structure fractures of reservoirs (in chinese). petroleum exploration and development, 23(4), 65-68. this paper explores the applicability of a modified q classification system and its component parameters for analysis and conclusion of site investigation data to estimate rock slope stability. based on the literature, q classification system has high applicable potential for evaluation of rock mass quality. therefore, in this study, it was used with rmr and smr rock mass classification systems to assess stability or instability of different rock slopes along the hamedan-ganjnameh-tuyserkan road, hamedan province west of iran. furthermore, a modified rock mass classification system namely slope quality rating (sqr) was proposed based on the correction of the q classification parameters and calculating some new parameters such as dip and strike of discontinuities and the method of rock excavation or blasting. for this purpose, the smr and rmr rock mass classifications were also needed. by measuring sqr for different rock slopes, it will be possible to measure slope mass rating (smr). este artículo explora la aplicabilidad del sistema de clasificación q modificado y sus parámetros para analizar y determinar la información estimada de estabilidad de pendiente de roca en el sitio determinado de estudio. según la literatura, el sistema de clasificación q tiene un alto potencial de aplicabilidad para la evaluación de la calidad del macizo rocoso. en este estudio además se utilizó el sistema q junto con los sistemas índice de masa de pendiente (smr) y clasificación geomecánica de bienawski (rmr) para evaluar la estabilidad e inestabilidad de diferentes pendientes rocosas en la carretera hamedan-ganjnameh-tuyserkan, de la provincia de hamedan, en el oeste de irán. además, se propone el índice de calidad de pendiente (sqr), un sistema de clasificación de macizo rocoso modificado, a partir de la corrección de los parámetros de clasificación q y el cálculo de nuevos parámetros como pendiente y caída de las discontinuidades y el método de excavación o explosión de la roca. para esta propuesta también se utilizaron las clasificaciones smr y rmr. la medición sqr en diferentes pendientes hizo posible el cálculo del sistema smr. abstract resumen keywords: rock mass classification, rock slope, q, rmr, smr, sqr. palabras clave: clasificación del macizo rocoso, pendiente rocosa, sistema q, clasificación de bienawski, índice de masa de pendiente, índice de calidad de pendiente. record manuscript received:12/02/2015 accepted for publication: 14/06/2015 how to cite item fereidooni, d., reza, g. and heidari, m. (2015). assessment of a modified rock mass classification system for rock slope stability analysis in the q-system . earth sciences research journal, 19(2) 14 7 152. http://dx.doi.org/10.15446/esrj.v19n2.49127 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n2.49127 evaluación del sistema q modificado de clasificación del macizo rocoso para el análisis de estabilidad de pendiente de roca earth sciences research journal earth sci. res. j. vol. 19, no. 2 (december, 2015): 147 152 g e o t e c h n ic s assessment of a modified rock mass classification system for rock slope stability analysis in the q-system davood fereidooni*1, gholam reza khanlari2, mojtaba heidari2 1 school of earth sciences, damghan university, damghan, iran d.fereidooni@du.ac.ir (*corresponding author) 2 department of geology, bu-ali sina university, hamedan, iran 148 davood fereidooni, gholam reza khanlari and mojtaba heidari 1. introduction rock slope failure along roads, highways, railways and in urban and industrial areas is one of the geologic hazards that will destroy engineering projects such as dams, buildings, roads and railroads. this natural hazard shows the importance of the engineering geology and rock mechanics studies when engineering constructions are performed. in the past decades, the rock mass classification concept has been extensively applied in engineering design of different projects such as tunnels, slopes and foundations. many researchers have proposed or used rock mass classification systems as an experimental method to assess rock mass quality or rock slope stability (e.g. bieniawski 1973; barton 1974; selby 1980; haines and terbrugge 1991; hack et al. 2003; liu and chen 2007; pantelidis 2010; hajiazizi and khatami 2013). the primary objective of rock mass classification is to provide quantitative data and guidelines for engineering purposes that can improve originally abstract descriptions of geological formation (liu and chen 2007). over the last several years, different rock mass classification systems have been proposed for rock cuttings with high risk to identify their failure potential to present preventive and improvement methods. also, rock mass classification is mean for the evaluation of the performance of rock cut slopes based on the most important inherent and structural parameters (pantelidis 2009). in other words, rock mass classification systems try to consider the most important aspects affecting the rock mass, to rate its quality (tzamos and sofianos 2007). based on bieniawaski (1989), the main aim of rock mass classification systems is dividing particular rock masses into groups of similar behavior and also providing a basis for understanding the characteristics of each cluster to yield quantitative data for engineering purposes, and at least providing a common basis for communications between the researchers. most of the multi-parameter classification schemes are developed from civil engineering case histories in which all of the components of the engineering geological character of the rock mass are included. different classification systems place different emphases on the various parameters (hoek 2007). these statements show that the study of rock mass classifications must be done before performing engineering constructions. some of rock mass classifications proposed by different researchers need to be modified to improve their applications. for example, romana (1985) changed the rmr rock mass classification system based on four different parameters for rock slope stability analysis. in this research, the rock masses in the margin of hamedan-ganjnameh-tuyserkan road in hamedan province, west of iran, have been studied based on rmr, smr, and q rock mass classification systems. the q classification system was modified using the parameters proposed by romana (1985) and also some other parameters. also, based on the data analysis from the seven rock slopes chose in the study area, a new rock mass classification system (slope quality rating, sqr) is applied and proposed. 2. geological setting the study area, longitude from 48° 10' e to 48° 35' e and latitude from 34° 30' n to 34° 52' n, is located on the sanandaj-sirjan structural zone, west, and south west of iran. one of the most important and interesting plutonic rocks of iran named alvand batholith is situated in this area. alvand granitic rock mass is limited from the north to hamedan, from southeast to tuyserkan and from northwest to assad-abad. it covers an area of about 400 km2 which makes it the largest plutonic rock mass in iran. according to sepahi gero (1999), the major part of alvand plutonic rock mass consists of porphyritic granites (monzo granite-granodiorites), mezocrate granites and holocrate granites. the metamorphic rocks occurred around alvand granitic rock masses are pelitic hornfels and schist. various discontinuities such as tectonic joints, schistosity and cleavage planes were developed in these rocks and can cause rock slope instability, especially on the margin of the ganjnameh-shahrestaneh road. the geology deposits close to the north and east side of the study area are the quaternary alluvial fans. fig. 1 shows the location of the studied rock slopes on the geological map of the region. fig. 2 shows a general view of the two of the seven studied rock slopes in the study area. physical and lithological properties of the seven studied rock slopes have been listed in table 1.properties of the seven studied rock slopes have been listed in table 1. fig. 1 geological map of the study area. fig. 2 general view of the rock slopes in the study area. 149assessment of a modified rock mass classification system for rock slope stability analysis in the q-system table 1 physical and lithological properties of the studied rock slopes 3. most important existing rock mass classifications rock mass classifications are a universal communication system for their users. some of rock mass classifications such as q, rmr and mrmr systems have been applied successfully in tunneling and underground excavations. some of these systems have been used for slopes (e.g. q and rmr system) or have been modified for slopes (e.g. the rms, smr, srmr and csmr systems comprise modifications of the rmr system). although, the classification schemes are appropriate for their original application (tunneling), especially when used within the bounds of the case histories from which they were developed, considerable caution must be exercised in applying rock mass classifications to other rock engineering problems (pantelidis 2009). a detailed list of the most important empirical rock mass classification methods developed world-wide is presented in table 2. table 2 most important existing rock mass classification systems 654321 rrrrrrrmr +++++= the rmr and q classification systems, two of the most important systems, were proposed by bieniawski (1973) and barton et al. (1974), respectively. bieniawski (1989) developed his rock mass classification scheme (rmr) using data obtained mainly from civil engineering excavations in sedimentary rocks in south africa. the classification system based on rock mass quality (q) was originally developed by barton et al. (1974) to assist in the empirical design of tunnel and cavern reinforcement and support, but it has been used for several other tasks in rock engineering in recent years by barton et al. (2002). common parameters in these classifications are those concerning rock structure and joint surface conditions. the rmr-value is obtained from the following equation (bieniawski 1989): (1) where, r1 is uniaxial compressive strength of rock material , r2 is rock quality designation (rqd), r3 is spacing of discontinuities, r4 is condition of discontinuities, r5 is groundwater conditions and r6 is orientation of discontinuities. the q-value is estimated from the following expression (barton et al. 2002): (2) where, rqd is the percent of competent drill-core sticks >100 mm in length in a selected domain, jn is the rating for the number of joint sets (9 for 3 sets, 4 for 2 sets, etc.) in the same domain, jr is the rating for the roughness of the least favorable of these joint sets or filled discontinuities, ja is the rating for the degree of alteration or clay filling of the least favorable joint set discontinuity, jw is the rating for the water inflow and pressure effects, which may cause outwash of discontinuity infillings, and srf is the rating for faulting, for strength/stress ratios in hard massive rocks, for squeezing or for swelling. with attention to the equations of the q and rmr systems, it is noticed that the rmr value is obtained from summation of five parameters whereas the q value is obtained by products and divisions of the six parameters. so, the number of the parameters participated in the q system is more than that in the rmr system. the rmr parameters are uniaxial compressive strength (σc), rock quality designation (rqd), spacing of the joint sets, discontinuities conditions, and ground water condition. in the q system, in addition to the above parameters, some new parameters such as jn (factor of joint sets number), jr (factor related to the joint roughness), ja (factor related to the joint alteration), and srf (stress reduction factor) are also employed. in the q equation, the ratio of (rqd/jn) clearly indicates the effect of block size on the instability of the rock masses. other ratios are (jr/ja) and (jw/srf) related to the shear strength and stress conditions, respectively, and they have very important roles in the behavior of the rock masses. one of the most important problems in the stability of rock slopes is shear strength of discontinuities and joint surfaces which are affected by the block size and the roughness of discontinuities. in addition, the weathering of discontinuity surfaces will have a negative effect on the shear strength of rock masses. another factor which has a negative effect on the shear strength of rock masses is the presence of the water on the discontinuities which will increase the pore pressure and decrease the shear strength. these conditions will lead to the instability of rock masses and all of them are contributed in the q classification. therefore, the presented contributed parameters in the q equation show that this classification system has a high potential in comparison with the rmr system to evaluate the rock mass quality. as a result, it is possible to say that the q system can present a good quantitative description of rock mass quality. 4. relationship between rmr and q the application details of the rmr and q classifications for the seven studied rock slopes are shown in tables 3 and 4. in an overall view, the results of the classifications show that the rock slopes in the study area are placed in the good rock class. to make a correlation between rmr and q, the values of these parameters related to the seven studied rock slopes have been plotted in fig. 3. 150 davood fereidooni, gholam reza khanlari and mojtaba heidari y = 12.64 ln(x) + 41.63 r² = 0.55 50 55 60 65 70 75 80 85 90 4 6 8 10 12 14 rm r q table 3 rmr classification and the values of its parameters for the studied rock slopes table 4 q classification and the values of its parameters for the studied rock slopes 0 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 rm r/ q /s m r no. of rock slopes rmr q smr fig. 3 relationship between rmr and q for the rock slopes in the study area according to the fig. 3, it can be concluded that the relationship between the two mentioned parameters can be estimated as follow: rmr = 13 lnq + 42 (3) the equations similar to equation 3 have been proposed by some researchers such as bieniawski (1989) which is as follow: rmr = 9 lnq + 44 (4) singh and goel (2000) mentioned that due to the low correlation coefficient of such equations, they have low validity. 5. modification of q classification the rock mass rating system (rmr) was modified by romana (1985) for rock slope stability analysis purposes. romana (1985) proposed some new factors such as discontinuity orientations, pattern of discontinuities and the methods of blasting for modification of the rmr system. these factors depend on the existing geometrical relationship between discontinuities affecting the rock mass and the slope excavation method. he proposed slope mass rating (smr) which can be measured from the following equation: smr = rmrbasic (f1 × f2 × |f3|) + f4 (5) where, smr is slope mass rating, rmrbasic is basic rock mass rating, f 1 is difference between slope face and critical discontinuity strike, f 2 is discontinuity dip angle, f 3 is difference between the slope face angle and the dip of critical discontinuity and f 4 is method of excavation. among all available rock mass classifications, smr is the most extended one and is applied for rock slopes due to its ease and exhaustive, well established, quantitative definition of correction factors. fig. 4 shows a comparison between the values of rmr, q and smr for the seven rock slopes. the values of these parameters have a good coordination. fig. 4 comparison between the values of rmr, q and smr for the rock slopes in this research, presented parameters in equation 5 were used to modify of the q system. because of some difficulties derived from using the q system, now it is used for rock slope stability and open pit mining in some cases. due to the stress condition at the surface and depths, distinguishing between these two conditions is necessary. for this reason, it seems that there is necessary to make some changes to the value of srf parameter. ajoodani-namin (1999) proposed that we can use (jcs / h ) factor of srf in order to incorporate the effect of stress differences in different conditions (depth and surface of the rock masses). based on a try and error method, ajoodani-namin (1999) suggested following conditions: if; (jcs / h) < 160 →srf = 0.35 if; (jcs / h) ≥ 160 →srf = 0.11 where, jcs is joint compressive strength (mpa), is unit weight of rock slope materials (g/cm3) and h, is the height of rock slope (m). based on this condition, the value of srf for the study area was chosen 0.11 and, therefore, the value of q was corrected for the slope stability applications. details of modified q (qm), rmr, jcs, srf and other parameters are listed in table 5. table 5 the values of different parameters to calculate qm 151assessment of a modified rock mass classification system for rock slope stability analysis in the q-system y = 12.69 ln(x) + 1.87 r² = 0.55 50 55 60 65 70 75 80 85 90 100 150 200 250 300 rm r qm in order to make a correlation between rmr and qm, the values of these parameters are plotted in x-y system (fig. 5). according to fig. 5, the relationship between the two mentioned parameters is as follow: rmr = 13 lnqm + 2 (6) 0 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 sm r no. of rock slopes from equ. 3 from equ. 7 fig. 5 relationship between rmr and qm for the rock slopes in the study area 6. calculation of sqr with combining equations 5 and 6, it is possible to achieve the value of sqr based on the following processes: (7) as it is clear from equation 7, the value within bracket is rating of sqr for the rock masses of the study area which can be calculated from the below equation: (8) as a result, the relationship between smr and sqr will be as follow: (9) based on the table proposed by romana (1985), the values of f1, f2, f3 and f4 for the studied rock slopes are listed in table 6. table 6 the values of sqr and smr for the studied rock slopes fig. 6 shows a comparison between calculated values of smr from equations 5 and 9 for the seven rock slopes. the values of these parameters have a good coordination. fig. 6 comparison between the calculated values of smr 7. conclusions the modified q classification system (sqr) is one of the rock mass classifications that can be applied for rock slope stability analysis. this classification has been concluded from three rock mass classifications means rmr, q, and smr. application of sqr classification system for the rock slopes in the study area has presented good results and shows that this classification is a very useful method to analyze the rock slope stability. furthermore, with the application of this method, we can calculate smr values. the estimated values of rmr in the seven studied rock slopes show that the minimum value of rmr (62) is related to the rock slopes no. 3 and the maximum value of rmr (77) is linked to the rock slopes no. 2. by the results, the studied rock slopes are placed in the good rock class (class ii). the measured smr for the seven studied rock slopes shows that the minimum value of smr (66) is related to the rock slopes no. 3 and the maximum value of smr (84) is linked to the rock slopes no. 2. the results show that the studied rock slopes have good quality and stable condition (class ii). the calculated values of q for the seven studied rock slopes show that the minimum value (5.8) is related to the rock slopes no. 3 and the maximum value (12.8) is linked to the rock slopes no. 5. the results show that the rock qualities of the rock slopes are fair and good, respectively. the measured sqr for the seven rock slopes shows that the minimum value of sqr (177) is related to the rock slopes no. 3 and the maximum value of sqr (497) is linked to the rock slopes no. 2. the results show that the studied rock slopes have good quality. the values of smr from the equation proposed in this research (equation 9) have proper coordination with the values of smr from the equation suggested by romana (1985) (equation 5). in other words, the values of smr, from two mentioned methods, for the rock slope no. 3 and the rock slope no. 2 are minimum and maximum, respectively (fig. 6). in this study, similar results were obtained from rmr, smr, q and sqr rock mass classification systems for all seven studied rock slopes. this means that these classifications are in coordination with each other in assessing rock slope stability analysis in the study area. 152 davood fereidooni, gholam reza khanlari and mojtaba heidari references ajoodani-namin s. (1999). proposed a new method for rock slope stability analysis with modification of q system, msc thesis, tarbiat modarres university, tehran, iran, (in persian). barton n. r., lien, r. and lunde j. (1974). engineering classification of rock masses for the design of tunnel support, rock mechanics and rock engineering. 6, no. 4, 189–239. barton, n. (2002). some new q-value correlations to assist in site characterization and tunnel design, international journal of rock mechanics and mining sciences. 39, 185–216. bieniawski, z. t. (1973). engineering classification of rock masses, trans south african institute of civil engineering. 15, 335–44. bieniawski, z. t. (1976). rock mass classification in rock engineering, proceedings of the symposium on explorer for rock engineering, johannesburg, 97–106. bieniawski, z. t. (1979). the geomechanics classification in rock engineering applications, proceedings of the 4th international congress on rock mechanics, montreux, 41–48. bieniawski, z. t. (1989). engineering rock mass classifications, new york: wiley chen, z. (1995). recent developments in slope stability analysis, proceedings of the 8th international congress isrm, tokyo, 1041–8. deere, d. u. (1963). technical description of rock cores for engineering purposes, rock mech. eng. geol. 1, no. 1, 17–22. haines, a. and terbrugge, p. j. (1991). preliminary estimation of rock slope stability using rock mass classification systems, in: wittke, w. (ed.), proc 7th congr on rock mechanics 2, isrm, aachen, germany. balkema, rotterdam, 887–892. hajiazizi, m. and khatami r. s. (2013). seismic analysis of the rock mass classification in the q-system, international journal of rock mechanics and mining sciences. 62, 123–130. hack, r., price, d. and rengers n. (2003). a new approach to rock slope stability-a probability classification (sspc), bulletin of engineering geology and the environment. 62(2), 167–184. hoek, e., kaiser p.k. and bawden, w. f. (1995). support of underground excavations in hard rock. rotterdam: balkema. hoek, e. (2007). practical rock engineering, http://rockeng.utoronto.ca/ rok/hoek/hoeknote2007.htm. 2008-11-04. laubscher, d. h. (1997). geomechanics classification of jointed rock masses-mining applications, trans inst. min. metall. 86, 1–8. lauffer, h. (1985). gebirgs klassifizierung fur den stollenbau, geol bauwesen. 24, no. 1, 46–51. liu, y. c,. and chen, c. s. (2007). a new approach for application of rock mass classification on rock slope stability assessment, engineering geology. 89, 129–143. pantelidis, l. (2009). rock slope stability assessment through rock mass classification systems, international journal of rock mechanics & mining sciences. 46, no. 2, 315–325. pantelidis, l. (2010). an alternative rock mass classification system for rock slopes, bulletin of engineering geology and the environment. 69, 29–39. robertson, a. m. (1988). estimating weak rock strength, proceedings of the sme annual meeting, phoenix, 1–5. romana, m. seron, j. b. and montalar, e. (2003). smr geomechanics classification: application, experience and validation, proceedings of the international symposium on role of rock mechanics, south african institute of mining and metallurgy, 1–4. romana, m. (1985). new adjustment ratings for application of bieniawski classification to slopes, proceedings of int. symp. role of rock mech., isrm, zacatecas, mexico, 49–53. selby, m. j. (1980). a rock mass strength classification for geomorphic purposes: with tests from antarctica and new zealand, zeitschrift fur geomorphologie, 23, 31–51. sepahi-gero, a. a. (1999). petrology of alvand plutonic with emphasis on granitoieds, phd thesis on petrology, kharazmi university, tehran, iran, (in persian). singh, b. and goel, r. k. (2000). rock mass classification (a practical approach for civil engineering), elsevier ltd. terzaghi, k. (1946). rock defects and loads on tunnel supports, rock tunneling with steel supports. vol. 1, youngstown, oh: commercial shearing and stamping company, 17–99. tzamos, s. and sofianos, a. i. (2007). a correlation of four rock mass classification systems through their fabric indices, international journal of rock mechanics & mining sciences. 44, 477–495. wickham, g. e., tiedemann, h. and skinner, e. h. (1972). support determination based on geologic predictions, proceedings of the 1st north american rapid excavation tunneling conference, new york: aime, 43–64. keywords: small strain shears modulus; standard penetration test (spt); seismic piezocone penetration test (scptu); regression relation; palabras clave: modulo cortante a pequeñas deformaciones; ensayo de penetración estándar; ensayo de penetración de cono sísmico; relación de regresión; how to cite item chu, y., liu, s., & cai, g. (2021). characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china. earth sciences research journal, 25(2), 225-235. doi: https://doi. org/10.15446/esrj.v25n2.62287 small strain shear modulus plays a fundamental role in the evaluation of site response parameters. the standard penetration test (spt) and seismic piezocone penetration test (scptu), which can quickly get the density and shear wave velocity (vs), were rarely used in small strain shear modulus estimating. in this study, an attempt has been made to develop the regression relationship between standard penetration test (spt) n values and the small strain shear modulus (gmax). for this purpose, field investigations spt and seismic piezocone penetration test (scptu) data from locations in su-xin expressway of china have been used, which were also used for ground improvement project. the in-situ density of soil layer was estimated using undisturbed soil samples from the boreholes. the vs profiles with depth were obtained for the locations close to the boreholes. the values for small strain shear modulus have been calculated by measured vs and in situ soil density. about 50 pairs of spt-n and gmax values were used for regression analysis. the differences between measured and corrected values which were used in fitted regression relations were analyzed. most of the existing correlations were developed based on the studies carried out in japan and in india, where n values are measured with hammer energy of 78%, which may not be directly applicable for other regions because of the variation in spt hammer energy which in china is about 55%. a new correlation has been generated using the measured values in silts of china. from this study, it is found that uncorrected values of n and modulus give the best fit regression relations when compared to corrected n and corrected modulus values. with most equation was used for sand and clay, the regression relations between corrected values of n and modulu gives the equation of silts in china. abstract characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china caracterización de la correlación entre los valores n del ensayo de penetración estándar y el módulo cortante a pequeñas deformaciones gmax en sedimentos de jiangsu, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.62287 el modulo cortante a pequeñas deformaciones juega un rol fundamental en la evaluación de parámetros de respuesta en sitio. el ensayo de penetración estándar (spt) y el ensayo de penetración de cono sísmico (scptu), los cuales pueden rápidamente obtener la densidad y la velocidad de la onda de corte (vs), se han utilizado poco en la estimación del modulo cortante a pequeñas deformaciones. en este estudio se realiza un acercamiento para desarrollar la relación de regresión entre los valores n del ensayo de penetración estándar y el módulo cortante a pequeñas deformaciones (gmax). con este objetivo se utilizó la información de las investigaciones de campo del spt y del scptu en las ubicaciones de la autovía su-xin de china, que también se usaron en el proyecto de mejoramiento del terreno. la densidad in-situ de la capa de suelo se estimó a través de muestras inalteradas tomadas por perforación. los perfiles vs con profundidad se obtuvieron de ubicaciones cercanas a las perforaciones. los valores del módulo cortante a pequeñas deformaciones fueron calculados por la medición de los valores vs y la densidad del suelo in-situ. cerca de 50 acoples de valores spt-n y gmax se utilizaron en el análisis de regresión. además, se analizaron las diferencias entre los valores medidos y los valores corregidos que fueron usados en las relaciones de regresión ajustadas. gran parte de las correlaciones se desarrollaron con base en estudios realizados en japón e india, donde los valores n se midieron con martillo eléctrico de 78 %, que no sería directamente aplicable para otras regiones debido a que la variación en el ensayo de penetración estándar del martillo eléctrico en china es del 55 %. una nueva correlación se ha generado a través de valores medidos en sedimentos en china. en este estudio se encontró que los valores n no corregidos y del módulo proporcionan las relaciones de regresión más adaptables cuando se comparan con los valores n corregidos y del módulo. con las ecuaciones usadas para la arena y la arcilla, las relaciones de regresión entre los valores n corregidos y del módulo proporcionan la ecuación de sedimentos en china. resumen record manuscript received: 29/01/2019 accepted for publication: 15/01/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 225-235 ya chu*, songyu liu, guojun cai department of geological resources and engineering, nanjing tech university, nanjing, jiangsu, china 211816 *corresponding author: 136235507@qq.com so il s https://doi.org/10.15446/esrj.v25n2.62287 https://doi.org/10.15446/esrj.v25n2.62287 https://doi.org/10.15446/esrj.v25n2.62287 mailto:136235507@qq.com 226 ya chu, songyu liu, guojun cai introduction the site specific ground response study requires the soil parameters of thickness, density, and shear modulus of each layer as inputs. the soil type and thickness of each layer are generally obtained by drilling boreholes and logging the borehole information. the in-situ soil densities of each layer are usually obtained from the undisturbed soil samples collected in boreholes. in most cases, the gmax for site response analysis is evaluated using relationships based on the standard penetration test (spt) n values. there are many available regression equations between n values and small strain shear modulus in literatures for different soils by different researchers. the existing correlations were developed by imai and yoshimura (1970), ohba and toriumi (1970), ohta et al. (1972), ohsaki and iwasaki (1973), hara et al. (1974), imai and tonouchi (1982), seed et al. (1983, 1986) and anbazhagan and sitharam (2010). most of the old correlations are listed in the popular text book of kovacs (1981) and kramer (1996). kovacs (1981) has presented the summary of spt-n values and gmax correlations based on the above first five research works. kramer (1996) has modified the correlation developed by imai and tonouchi (1982) for a sandy soil by replacing the measured n values with energy corrected n values (n60). seed et al. (1983) presented the correlation based on their previous studies. seed et al. (1986) have presented gmax correlation based on the ohta and goto (1976) data. correlation proposed by seed et al. (1983, 1986) and kramer (1996) is being used in shake2000 site response software to estimate the shear modulus from the spt-n values. a correlation by anbazhagan and sitharam (2010) is a recently developed one, after 27 years of gap. this paper presents the summary of the above correlations and comparisons. a new correlation has been developed considering the measured old and new data from japan and india, where n values are measured with hammer energy of 78%. the modification factor for old and new correlations is suggested for other regions, where the spt-n values are measured with different hammer energies. these relationships are region specific, which depends on the type and characteristics of the soil in the respective region. it is not reasonable to use existing correlations to obtain shear modulus for ground response studies without considering local soil condition. hence, in this paper an attempt has been made to develop a relationship between spt-n value and gmax considering spt and scptu data from su-xin expressway, china, where is rich in the content of silt. in this paper the small strain shear modulus is measured using the shear wave velocity obtained from scptu and in situ density from undisturbed soil samples obtained at the same depth in the corresponding boreholes. these values are used to generate a correlation between spt measured and corrected n values and gmax. the developed relationships are compared with a similar relationship available in the literature. small strain shear modulus the stiffness of a body (or structure) is defined as the resistance of that body to deformation under applied force (abbiss, 1979). it is derived from: (a) the shape of the body; (b) boundary conditions, such as fixities and load positions; (c) the stiffness properties of the constituent materials (shear modulus, etc.) (anderson, 1978). thus, deformation depends upon stiffness, which in turn depends on the stiffness properties. then the subject of this paper is how to estimate stiffness (small strain shear modulus). in geotechnical engineering practice stiffness is normally defined within the context of the mathematical theory of elasticity, although this is not strictly necessary. tests on reconstituted materials have shown the important influence that “state” (void ratio, current effective stresses) and stress history can have on stiffness (ballard, 1975; burland, 1989; cai et al., 2014). the shear modulus of a granular material at very small strain levels is affected fundamentally by three factors: (a) the void ratio of the specimen; (b) inter particle contact stiffness, which will depend upon particle mineralogy, angularity and roughness, and effective stress; (c) deformation and flexing within individual particles, which will depend on particle mineralogy and shape. the shear modulus at low strain level for soil layers has been determined using shear wave velocity from scptu and density from undistributed soil samples using the following equation (abbiss, 1979) g vsmax = 2 (1) where ρ is density measured from the undisturbed sample and, vs is shear wave velocity measured using the scptu. gmax has been evaluated for corresponding depth of n values in the respective locations. a great many techniques exist from which stiffness parameters can be derived, ranging from the “simple” spt to the sophisticated self-boring pressuremeter. this paper considers a limited selection of more unusual techniques, based on the author’s experience and belief that they will have value in many situations (clayton, 2011; matthews, 2000; gordon, 1997). in particular, two classes of test are reviewed: the first one is traditional seismic testing consisting three tests, (a) continuous surface wave testing, (b) downhole geophysics, (c) cross-hole geophysics; another one is advanced seismic testing including seismic piezocone penetration test (scptu) and multichannel analysis of surface wave (masw) in this paper, the shear wave velocity (vs) is measured by seismic piezocone penetration test (scptu). figure 1. the location of su-xin expressway 227characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china site description in this paper the su-xin expressway (jiangsu) is as a basic project. the expressway from suqian to xinyi is an important part of expressway network planning in jiangsu province, where the location of su-xin expressway is shown in figure 1. this construction has a very vital significance to improve the road network layout structure of the transport corridor in jiangsu province and share responsibility for the beijing-shanghai expressway traffic flow. test section is located at the first section in su-xin expressway: k7+630~ k7+730. the handled length in k7+630~ k7+710 is 80 m, while the width is 40m and the gross area is 3200 m2; the handled length in k7+710~ k7+730 is 20 m, while the width is 40 m and the gross area is 800 m2. the testing site is nearby luoma lake with a distribution of many rivers and flat terrain. the main geological structure is tancheng-lujiang fault zone which is still active. the seismic intensity is about 8 in such intra-regional. the geomorphic types of testing sites are principally the alluvial plain of the waste of the yellow river, rolling plain and scattered denudation monadnock. the soil of this location is almost unconsolidated quaternary sediment. the ground surface is recent artificial miscellaneous fill and cultivated soil, below which is silt and silt sand. the groundwater level in winter is 3.8m. based on the research content in this paper the supplementary investigation was been made in the testing site to get more valid data. according to the supplementary investigation, the soil layer of this location roughly divided into four levels from top to bottom, which can be obtained by figure 2. table 1 gives the main physico-mechanical indexes of soils figure 2. engineering geological profile of site geotechnical data the shear wave velocity of hole 1 with depth is shown in figure 4. typical recorded wave arrivals for a source to first receiver distance of each 1 m and the processed data are shown in table 2. shear wave velocity of 5 holes are presented the trend of the overall increase with depth (the locations of 5 holes were shown in figure 3). the shear wave velocity values of sandy silt layer range from 70 to 100 m/s, and the shear wave velocity values of silt layer range from 100 to 200 m/s. shear wave velocity and cone tip resistance have a similar trends, shear wave velocity obviously increased with the suddenly increase of the cone tip resistance, the reaction of shear wave velocity data have a consistency with cone tip resistance and side friction by revealing the changes in soil. the equivalent soil shear wave velocity varies from 50 to 200 m/s. most of the area, having an average shear wave velocity of 100-150 m/s, can be classified as medium to dense soil. figure 3. the number and locations of site figure 4. shear wave velocity variety with depth the spt is one of the oldest, most popular, and most common in situ tests used for soil exploration in soil mechanics and foundation engineering. this test is used for many geotechnical projects because of the simplicity of the equipment and test procedure. in particular spt tests are used for seismic site characterization, site response, and liquefaction studies towards seismic microzonation (bolton seed, 1985; kayen, 1992; cetin, 2004). this test is quite crude and depends on many factors due to the variations of applications carried out in the test and some equipment used in the test. those factors includes the drilling methods, drill rods, borehole sizes and stabilization, sampler, blow count rate, hammer configuration, energy corrections, fines content, and test procedure (schmertmann and palacios, 1979; kovacs et al., 1981; farrar et al., 1998; sivrikaya and togrol, 2006). the combined effect of all these factors can be accounted by applying the correction factors separately or together. the spt-n values may vary even for identical soil conditions because of sensitivity table 1. main physico-mechanical indexes of soils layer name depth/m specific gravity(gs) clay content/% moisture content/% wl/% ip spt-n63.5 value ① silt 3.8 2.70 3.7 24.1 28.1 5.8 4 ② silty sand 9.4 2.69 5.4 33.6 28.7 5.7 10 228 ya chu, songyu liu, guojun cai to operator techniques, equipment, malfunctions, and poor boring practice. so the spt based correlations may be used for projects in preliminary stage or where there is a financial limitation, but for important projects, it is preferable to measure dynamic properties directly by using suitable field tests (anbazhagan and sitharam, 2008). boreholes with a diameter of 110 mm were drilled using hydraulic rotary drilling rigs up to the hard stratum. spt tests were conducted at a regular sampling interval of 1.5 m in each borehole, and additional disturbed soil samples were also collected. most of the penetration resistances (spt-n values) in the boreholes were measured using donut hammer. the undisturbed soil samples are collected according to chinese standard gb 50021 (2009). the undisturbed soil sample is procured by driving the thin walled sampler of diameter of 75 mm and 700 mm length tube into the borehole at desired depth/ change of strata. to avoid densification of soil samples due to hammering, the following precautionary steps are followed in the field: (a) the length of tube is marked on the driving rod and the driving is carried out carefully up to tube length (up to mark) by adjusting hammer height of fall, and (b) usually, the height of the hammer fall is limited to 76 cm. after ensuring complete penetration due to hammering, the tube is turned at least for two revolutions to shear the sample off at the bottom. the loose/disturbed soil in the upper end is removed and waxed on either end and taken to the laboratory. these samples are used to evaluate in situ densities of the soil layers. in most of the locations, the boreholes are drilled up to weathered rock and in few locations boreholes reached up to hard rock. spt-n values of hole 1 with depth are shown in figure 5. these boreholes were also used for ground improvement project of su-xin expressway and other studies. n and vs corrections shear wave velocities are corrected for overburden stress using traditionally following equation (sykora, 1987; robertson et al., 1992; andrus and stokoe, 2000; youd et al., 2001; juang et al., 2002; andrus et al., 2004). v v cs s v1= (2) c pv a v= / ' . σ 0 0 25( ) (3) where vs1 is overburden stress corrected shear wave velocity, cv is factor to correct measured shear wave velocity for overburden pressure, σ’v0 is effective overburden pressure, pa is 100 kpa. a maximum cv value of 1.4 is generally applied to vs at shallow depth (andrus and stokoe, 2000). the overburden stress corrected shear wave velocity vs1 is evaluated for each layer, which is used to estimate the overburden stress corrected shear modulus gmax1. the n values from field boreholes have been corrected for various corrections. one of the important factors that affect the spt-n value is the energy transferred from the falling hammer to the spt sampler. the energy ratio (er) delivered to the sampler depends on the type of hammer, anvil, lifting mechanism, and the method of hammer release. where energy measurements cannot be made, careful observation and notation of the equipment and procedures are necessary to estimate the ce value. the use of good-quality testing equipment and carefully controlled testing procedures will generally yield more consistent ers. for liquefaction calculation, yilmaz and bagci (2006) took the ce value as 0.7 for the spt hammer donut type, delivering 60% energy. in this study the delivered hammer energy is not measured, but the hammer used is donut hammer. from the table 3 of summary of rod energy ratios, it can be seen that the delivered energy ratio (er) is 55% and ce is taken as 0.9 (skempton, 1986). the different spt hammer was shown in figure 6. since the energy ratio for spt is known in this article, the energy-ratio-corrected n-values n60 can be obtained by using the following equation: n n ce60= × (4) where n60 is the energy-ratio-corrected n-values, ce is the hammer energy parameter. table 2. shear wave velocity of 5 holes with depth depth (m) shear wave velocity (m/s) hole1 hole2 hole3 hole4 hole5 1 74.16 69.41 70.07 64.48 64.48 2 98.82 105.76 95.19 104.95 104.95 3 113.7 120.99 109.28 125.04 125.04 4 123.8 130.28 99.54 129.63 129.63 5 128.44 144.48 105.52 125.50 125.50 6 143.34 142.68 135.04 135.11 135.11 7 148.86 152.53 144.67 139.63 139.63 8 151.69 156.50 142.29 155.40 155.40 9 163.34 167.35 137.70 149.24 149.24 10 170.52 165.71 149.28 151.12 151.12 11 165.82 174.85 159.51 160.44 160.44 12 172.24 177.75 150.86 170.01 170.01 13 180.73 180.85 159.35 179.71 179.71 14 190.92 185.30 165.07 185.82 185.82 figure 5. spt-n values variety with depth 229characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china table 3. the summary of rod energy ratios country hammer release er (%) ce (er/60%) japan donut tombi 78 1.3 donut 2 turns of rope 65 1.1 china pilcon type trip 60 1.0 donut manual 55 0.9 usa safety 2 turns of rope 55 0.9 donut 2 turns of rope 45 0.75 uk pilcon, dando trip 60 1.0 old standard 2 turns of rope 50 0.8 the n values from field boreholes have been corrected for various corrections, such as (a) overburden pressure cn, (b) hammer energy ce, (c) borehole diameter cb, (d) presence or absence of liner cs, (e) rod length cr, and (f) fines content cfine (seed et al., 1983, 1985; skempton, 1986; youd et al., 2001; cetin et al., 2004; pearce and baldwin, 2005). corrected n60 value (n1)60 are obtained by using the following equation: n n c c c cn b s r1 60( ) × × × ×( )60 = (5) where cn is the overburden pressure parameter and not exceeding a value of 1.7. the spt-n values recorded in the field increase with increasing effective overburden stress; hence overburden stress correction factor is applied (seed and idriss, 1982). this factor is commonly calculated from the equation developed by liao and whitman (1986). however kayen et al. (1992) has suggested the following equation, which limits the maximum cn value to 1.7 and provides a better fit to the original curve specified by seed and idriss (1982): c pn v a=2 2 1 2 0. / . /'+( )σ (6) where σ’v0 is effective overburden pressure, pa is 100 kpa, and cn should not exceed a value of 1.7. this empirical overburden correction factor is also recommended by youd et al. (2001). the other correction for borehole diameter, rod length, and sampling methods are modified from skempton (1986). the correction factors are listed by robertson and wride (1998). the borehole diameter correction factor of 1.00 for 110 mm borehole diameter is used. the rod length correction factor cr is applied based on the length of the rod. sampler correction factor cs for the presence or absence of liner is taken as 1.0 for standard sampler. the corrected n value (n1)60 is further corrected for fines content based on the revised boundary curves derived by idriss and boulanger (2004) for cohesionless soils as described below: n n n1 1 1( ) ( ) + ∆ ( )60cs 60 60 = (7) ∆ ( ) + + − +             n1 2 1 63 9 7 0 001 15 7 0 00160 = fc fc exp . . . . . (8) table 4. spt-n correction values of hole 1 borehole water table = 1.7 m depth (m) field n value density (kn/m3) ts (kn/m2) es (kn/m2) cn correction factors for n60 (n1)60 δ(n1)60 corrected n value, (n1)60cs hammer effect (ce) bore hole diameter (cb) rod length (cr) sample method (cs) 1.30 7.00 19.60 25.48 25.48 1.51 0.90 1.00 0.75 1.00 6.3 5.55 5.52 11.07 2.80 5.00 20.10 55.63 44.65 1.34 0.90 1.00 0.75 1.00 4.5 3.52 5.61 9.13 4.30 7.00 20.10 85.78 60.10 1.22 0.90 1.00 0.85 1.00 6.3 5.08 5.61 10.69 5.80 14.00 19.70 115.33 74.95 1.13 0.90 1.00 0.85 1.00 12.6 9.41 5.61 15.02 7.30 10.00 19.70 144.88 89.80 1.05 0.90 1.00 0.95 1.00 9 6.98 5.61 12.59 8.80 14.00 19.60 174.28 104.50 0.98 0.90 1.00 0.95 1.00 12.6 9.12 5.61 14.73 10.30 11.00 19.60 203.68 119.20 0.92 0.90 1.00 1.00 1.00 9.9 7.08 5.57 12.65 11.80 25.00 20.20 233.98 134.80 0.86 0.90 1.00 1.00 1.00 22.5 15.05 5.57 20.62 13.30 24.00 20.20 264.28 150.40 0.81 0.90 1.00 1.00 1.00 21.6 13.61 5.60 19.21 14.80 34.00 20.40 294.88 166.30 0.77 0.90 1.00 1.00 1.00 30.6 18.33 5.60 23.93 16.30 36.00 20.40 325.48 181.60 0.73 0.90 1.00 1.00 1.00 32.4 18.40 5.60 24.00 figure 6. spt hammers: (a) old standard; (b) donut; (c) trip (skempton, 1986) 230 ya chu, songyu liu, guojun cai where fc is percent fines content (percent dry weight finer than 0.075 mm). typical corrected n values for hole 1 is shown in table 4. similarly spt-n corrected values with depth have been determined for all the boreholes. relation between gmax and spt-n values the correlation between measured gmax (calculated from measured shear wave velocity and density of each layer) and the measured spt-n values is attempted. from the 6 sets of scptu and spt testing points, about 50 pairs of n and gmax values have been used for the regression analysis. to obtain the practical relationship between shear modulus and n values, also to understand data matching, different combinations of corrected and uncorrected values were attempted, which are discussed in the following. before starting the discussion, we first analyzed the relationship between the cone tip resistance qt and shear wave velocity vs. the figure 7 was the regression relation between cone tip resistance qt and shear wave velocity vs. in test site. from this figure we could see that the shear wave velocity was mainly increased with the cone tip resistance. the relationship was shown with by index function and the coefficient of determination was good (r2=0.94). the regression equation between qt and vs. is given below v qs t=102.344 0 234. (9) figure 7. the relationship between cone tip resistance and shear wave velocity according to related research the spt-n values had a positive correlation with the cone tip resistance (anbazhagan p and sitharam tg, 2010). on the other hand, there was also a positive correlation between shear wave velocity and small strain shear modulus. thus we could infer that there was also a positive correlation between spt-n values and the small strain shear modulus which was speculated by the shear wave velocity. relation between corrected n and uncorrected gmax values according to the research experience of the previous scholars, many kinds of equations were used to compare with which is the best equation for the regression relation between spt-n values and small strain shear modulus (ohta, 1972; imai, 1982; anbazhagan, 2010). studies showed that when using exponential function, the related equation had a highest coefficient of determination and there was a better fitting. so the predicted small strain shear modulus in this paper could be used for the engineering. there is an exponential relationship between measured values of spt-n and shear modulus gmax. correlation between measured values of spt-n and shear modulus gmax is presented in figure 8(a). the regression equation between gmax and n is given below: g nmax .=12.05 0 53 (10) where gmax is low strain measured shear modulus in unit of mn/m2, n is measured spt-n value. (a) (b) (c) figure 8. shear modulus versus corrected spt-n values 231characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china figure 8(a) also shows the actual data and fitted equation with ±10 % error bars. it gives a visual sense of how well the data defines the best fit curve (motulsky, 2004). the best fit equation has the r squared value of 0.918. to study the difference between corrected and uncorrected spt-n values in the regression equation, two combinations of plots were generated. figure 8 shows correlations that are (b) corrected n values without fines content correction and measured modulus and (c) corrected n values with fines content correction and measured shear modulus. in the first case corrected n values are estimated excluding fines content correction factor according to equation 5, and in the second case the corrected n values are estimated including fines content correction factor according to equation 7. the both are giving a similar r2 (0.859 and 0.843). the data range in the first case (n1)60 is distributed from 2 to about 21, but in the second case, the (n1)60cs is distributed from 8 to about 28. for the purpose of developing the regression equation, any one of the corrected n value without or with fines content correction ((n1)60 or (n1)60cs) may be used. this is so because the corrected n values without or with fines content correction yield similar best fit equations and r2 values. the developed regression equations for the corrected n values without or with considering fines content correction are given below: without fines content correction g nmax . =12.66 1 60 0 36( )  (11) with fines content correction g nmax . =4.68 cs1 60 0 68( )  (12) relation between corrected n and corrected gmax values the overburden stress corrected shear modulus has been evaluated using traditional vs correction factor given in equation 3. figures 9a and 9b show the corrected shear modulus gmax1 versus corrected n values of (n1)60 and (n1)60cs. it was found that the regression fit is poor and gives lower r2 values when compared to the relationship between corrected n and measured gmax values. also similar to the above results, the corrected n values without or with fines content correction gives similar fitted equation, but the first one gives slightly higher r2 value (0.716) when compared to the second one (r2=0.69). the regression equation for the corrected shear modulus (gmax1) and corrected n values without or with considering fines content correction is given below: without fines content correction (a) (b) figure 9. corrected shear modulus (gmax1) versus corrected spt-n values g nmax . 1 1 60 0 22 =20.83 ( )  (13) with fines content correction g nmax . 1 1 60 0 45 =9.19 cs( )  (14) to trace out the problems, an attempt is made to use similar overburden stress correction factor applied to n values. the overburden stress corrected shear modulus of each layer has been evaluated using the correction factor given in equation 6. figures 10a and 10b show the corrected shear modulus (gmax2) versus corrected n values of (n1)60 and (n1)60cs. the regression fit is very poor and gives low r2 values when compared to corrected n and corrected (gmax1) values. the regression equation for the corrected shear modulus (gmax2) and corrected n values without or with fines content correction is given below: without fines content correction g nmax 2 1 60 =11.2 0.44( )  (15) with fines content correction g nmax . 2 1 60 0 85 =3.36 cs( )  (16) from this study, it is clear that the correlation between the normalized shear modulus and n does not give a better regression equation even though the data are same. it is necessary to review the traditionally used overburden stress correction factor for vs. the traditional shear wave velocity correction factors may be revised in future based on these data or by including more data if available. results and discussions to validate n versus gmax relation presented in this study, it has been compared with existing relations available in the literature. many regression equation between n, (n1)60, and (n1)60cs with vs and vs1 are available in the literature for different soils by different researchers, but limited regression equation are available for n versus gmax. the popularly used correlations are developed by ohta and goto (1976) and imai and tonouchi (1982), but in the relationship presented by imai and yoshimura and the relationship presented by ohba and toriumi (1970) were developed by assuming uniform density. 232 ya chu, songyu liu, guojun cai recently, anbazhagan and sitharam (2010) developed a correlation between measured spt-n and shear modulus values using data measured for seismic microzonation study of bangalore, india. the original equation proposed by anbazhagan and sitharam (2010) is g nmax ..=24 28 0 55 (17) where the unit of gmax is mn/m2. the equations were reproduced in anbazhagan and sitharam (2010) for the corrected n values (corrected n values without or with fines content) for silts and with less percentage of clay, which is as follows: g nmax . .=29 17 1 60 0 57( )  (18) g n csmax . .=17 12 1 60 0 69( )  (19) figure 11 shows the equation 17, equation 18, and equation 19 using uncorrected values of n. in order to compare equations developed in our study, it is assumed that equation 18 and equation 19 based corrected n values (n1)60 and (n1)60cs as highlighted in the work of anbazhagan and sitharam (2010). figure 11. comparison of shear modulus equations (anbazhagan and sitharam, 2010) in figure 11, horizontal line gives uncorrected or corrected spt-n values based on the equation. if spt-n value of x is uncorrected for equation 10, the same x is corrected n value without fines content correction for equation 11 and corrected n value with fines content correction for equation 12. this similar explanation also applies for figures 13 and 15. from figures 11 and 12, it is clear that corrected n values without fines content corrections yield a higher shear modulus (gmax) against measured n values. from figure 12, gmax equations developed using corrected n considering fines content correction (eq. 12) matches with gmax equation using measured n values (eq. 10) up to the n values of 15 and gmax equation using corrected n without considering fines content correction for n values about 30. for the n values between 15 and 30, equation 12 lies between equation 10 and equation 11. figure 12. comparison of shear modulus versus n values developed in this study figure 13 shows the comparison of equations developed in this study with equation 17 presented by anbazhagan and sitharam (2010). from figure 13, it is clear that corrected n values with fines content corrections yield a higher shear modulus (gmax) against equation 17, but equation 17 has a similar trend with the equation 12; this may be attributed to soil type and number of data. in anbazhagan and sitharam (2010) most of the data are above the n value of 50 (for clay and sand) and the average shear wave velocity is about 250-500 m/s while the average shear wave velocity in this study is about 100-150 m/s. hence, the gmax may have some distinction with different soil type. figure 15 shows a comparison of equations developed in this study for corrected values, with equations 18 and 19 presented by anbazhagan and sitharam (2010). shear modulus values have been evaluated considering (n1)60 and (n1)60cs (a) (b) figure 10. corrected shear modulus (gmax2) versus corrected spt-n values 233characterization on the correlation between spt-n and small strain shear modulus gmax of jiangsu silts of china using equation 18 and equation 19. from figure 14, regression relations developed in this study between corrected n values ((n1)60 and (n1)60cs) and measured shear modulus (eq. 11 and eq. 12) have similar trends with those of anbazhagan and sitharam (2010). the correlations developed in this study used the measured spt-n values of up to 20 (refusal), whereas anbazhagan and sitharam (2010) had used a measured n value of up to 50. the corrected average shear wave velocity in anbazhagan and sitharam (2010) is about 300500 m/s while the average shear wave velocity in this study is about 80-140 m/s. so, the gmax may have some distinction with different soil type. also, the proposed correlation is unique, independent of the lithology, soil grading, age, cementation, etc. the developed equations in this study can be directly used for soil type “jiangsu silts of china” however for the important structures; the best way is to measure gmax directly using in situ seismic tests. comparison of measured and calculated gmax values the shear wave velocities measured in the seismic piezocone penetration test (scptu) can be compared with those estimated using empirical models which are developed in this study above. the spt-n and gmax correlations of jiangsu silts of china with respect to the aforementioned relationships are presented with the small strain shear modulus determined from the scptu results. moreover, the small strain shear modulus determined from the corrected n values (without or with considering fines content correction) is compared in figure 15. although a continuous small strain shear modulus profile down to a depth of 16 m could be obtained from both scptu vs measurements (a) (b) (c) figure 15. comparison of small strain shear modulus gmax determined from scptu and empirically-calculated: (a) calculated gmax by n; (b) calculated gmax by n60; (c) calculated gmax by n60cs figure 13. comparison of relations developed in this study with anbazhagan and sitharam (2010) figure 14. comparison of relations developed in this study based on corrected n values with anbazhagan and sitharam (2010) 234 ya chu, songyu liu, guojun cai and spt-n and gmax correlations in this study. the correlation of small strain shear modulus profiles in both scptu measurements and n60 or (n1)60 and gmax correlations is similar for the available data as seen in figure 15. but the data calculated using (n1)60cs and gmax correlations has a larger error with measurements. conclusions regression relation between spt-n and gmax values have been developed using 50 pairs of spt-n and gmax from geotechnical boreholes and scptu data. the regression equation using measured values gives best fit and r2 values when compared to the corrected n and corrected gmax. the regression relation between corrected n values without considering fines content correction (n1)60 or with considering fines content correction (n1)60cs and gmax are also giving similar r2 values. in the relation between corrected n values and measured shear modulus, any one of the corrected n values (without or with considering fines content correction) can be used for regression analysis. the relation between corrected n and gmax shows poor regression relation for the same data. the traditional overburden stress correction factors applied for shear wave velocity need a relook. the traditional shear wave velocity correction factors may be revised in the future based on these data or including more data if available. the proposed relations are comparable with the existing relations similar to this study. existing relations were developed with many assumptions during the developing stage of geotechnical earthquake engineering (gee). these relations have to be reviewed with present knowledge of gee and may be updated and reproduced in the future. the developed equation between n and gmax is more suitable for jiangsu silts of china. acknowledgements majority of the work presented in this paper was funded by the key project of natural science foundation of china (grant no. 41330641), the “twelfth five-year” national science and technology support plan (grant no. 2012baj01b02), and the research innovation project of ordinary university graduate student of jiangsu 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(2001). liquefaction resistance of soils: summary report from the 1996 nceer and 1998 nceer/nsf workshops on evaluation of liquefaction resistance of soils. journal of geotechnical and geoenvironmental engineering, 127(4), 297-313. doi:10.1061/ (asce)1090-0241(2001)127:10(817) https://doi.org/10.3208/sandf1972.13.4_61 https://doi.org/10.3208/sandf1972.13.4_61 https://doi.org/10.1139/t92-075 https://doi.org/10.1139/t92-075 http://dx.doi.org/10.1139/t98-017 https://doi.org/10.1061/(asce)0733-9410(1983)109:3(458) https://doi.org/10.1061/(asce)0733-9410(1983)109:3(458) https://doi.org/10.1061/(asce)0733-9410(1986)112:11(1016) https://doi.org/10.1061/(asce)0733-9410(1986)112:11(1016) https://doi.org/10.1016/j.enggeo.2006.05.002 https://doi.org/10.1016/j.enggeo.2006.05.002 https://doi.org/10.1680/geot.1986.36.3.425 https://doi.org/10.1680/geot.1986.36.3.425 https://doi.org/10.1680/geot.1995.45.2.267 https://doi.org/10.1680/geot.1995.45.2.267 https://doi.org/10.1007/s00254-005-0112-1 ole_link15 ole_link22 ole_link11 ole_link12 ole_link35 ole_link36 ole_link13 ole_link14 ole_link72 ole_link73 ole_link23 ole_link29 ole_link38 ole_link37 ole_link39 ole_link9 ole_link10 ole_link25 ole_link46 ole_link70 _hlk59803026 ole_link1 ole_link2 ole_link3 ole_link4 ole_link5 ole_link6 ole_link30 ole_link40 ole_link43 ole_link44 ole_link45 ole_link49 ole_link50 ole_link51 ole_link52 ole_link53 ole_link60 ole_link61 ole_link31 ole_link32 ole_link54 ole_link55 ole_link56 ole_link57 ole_link58 ole_link59 ole_link62 ole_link63 ole_link64 ole_link65 ole_link68 ole_link66 ole_link67 ole_link76 ole_link77 ole_link78 ole_link79 ole_link80 ole_link81 ole_link82 ole_link83 ole_link84 ole_link85 ole_link86 keywords: earthquake; geological hazards; longitudinal; land subsidence; deformation monitoring; simulator; sensing optical cable. palabras clave: terremoto; peligros geológicos; longitudinal; hundimiento de la tierra; monitoreo de la deformación; simulador; sensor de cable óptico. how to cite item xu, a., azarkhosh, h., & wu, e. (2020). monitoring method of longitudinal land subsidence and deformation in seismic geological disasters. earth sciences research journal, 24(3), 259-266. doi: https://doi. org/10.15446/esrj.v24n3.90290 monitoring of longitudinal land subsidence and deformation in seismic and geological hazards plays an important role in preventing and curing land collapse, land subsidence, and ground cracks. in this paper, a distributed monitoring model experiment on vertical land subsidence and deformation of seismic and geological hazards is carried out by brillouin optical frequency-domain analysis technology (botda). by using the self-made indoor longitudinal land subsidence and deformation simulation box, different intensity seismic ground is simulated by air bag. distributed optical fibers are used to monitor the longitudinal land subsidence and deformation during different intensity seismic and geological disasters. according to different intensity seismic and geological disasters, distributed sensing optical fibers cooperate with the ground to compress or stretch longitudinally and obtain the data of longitudinal land subsidence and deformation. the correction coefficient is introduced to modify the monitoring data of confining pressure-sensing optical fiber and complete the precise monitoring of vertical land subsidence and deformation in seismic and geological hazards. the experimental results show that this method can monitor the vertical ground subsidence and deformation of seismic and geological hazards under different conditions, and the monitoring efficiency and cost are superior to gps and inertial monitoring methods, and the practical application value is high. abstract monitoring method of longitudinal land subsidence and deformation in seismic geological disasters método de monitoreo de hundimiento y deformación longitudinal de la tierra en desastres sísmicos geológicos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90290 el monitoreo del hundimiento longitudinal de la tierra y la deformación en los riesgos sísmicos y geológicos juega un papel importante en la prevención y corrección del colapso, el hundimiento de la tierra y las grietas del suelo. en este documento, la tecnología de análisis de dominio de frecuencia óptica de brillouin (botda) sirve para llevar a cabo un experimento de modelo de monitoreo distribuido sobre hundimiento vertical de la tierra y deformación de riesgos sísmicos y geológicos. mediante el uso de la caja de simulación de deformación del terreno longitudinal interior de fabricación propia se simula un suelo sísmico de diferente intensidad mediante una bolsa de aire. las fibras ópticas distribuidas se utilizan para controlar el hundimiento y la deformación del terreno longitudinal durante desastres sísmicos y geológicos de diferente intensidad. de acuerdo con estos movimientos, las fibras ópticas de detección distribuida cooperan con el suelo para comprimir o estirar longitudinalmente y obtener los datos de hundimiento y deformación del terreno longitudinal. el coeficiente de corrección se introduce para modificar los datos de monitoreo de la fibra óptica de detección de presión de confinamiento, y completar el monitoreo preciso del hundimiento y deformación vertical de la tierra. los resultados experimentales muestran que este método puede monitorear el hundimiento vertical del suelo y la deformación de los riesgos sísmicos y geológicos en diferentes condiciones. la la eficiencia y el costo del monitoreo son superiores a los métodos de monitoreo inercial y gps, y el valor práctico de la aplicación es alto. resumen record manuscript received: 20/05/2019 accepted for publication: 03/02/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 259-266 aimei xu, hojatallah azarkhosh2, erjun wu2 1school of civil engineering and architecture, xinyu university, xinyu, 338000, china 2college of civil and transportation engineering, hohai university, nanjing, china *corresponding author: yanyy1209@126.com g eo lo g ic al h az ar ds mailto:yanyy1209@126.com 260 aimei xu, hojatallah azarkhosh, erjun wu introduction land subsidence and deformation is a geological phenomenon in which the surface rock and soil subsides downward under the action of natural or manmade factors and forms a subsidence pit on the ground. according to the causes of subsidence deformation, land subsidence and deformation can be divided into natural subsidence deformation and artificial subsidence deformation. natural subsidence and deformation occur under the influence of natural environmental factors, and the main inducements include earthquakes, rainfall, and so on. artificial subsidence deformation refers to the surface subsidence deformation caused by overexploitation of groundwater and drainage of mine pits. with the rapid development of the city, the underground space of the city has been continuously developed. the underground of most cities is densely covered with various pipelines, tunnels, and other underground “holes” (tan et al., 2016). these underground “voids” are prone to collapse and then cause land subsidence and deformation under the action of external factors such as ground load and disturbance of underground engineering construction. in view of the increasingly serious development trend of land subsidence and deformation, monitoring of vertical land subsidence and deformation has become a new research hotspot, and many scholars at home and abroad have done a lot of research on it. common monitoring technologies include insar, gps, ground-penetrating radar, and transient electromagnetic methods. however, land subsidence and deformation are a geological hazard phenomenon of vertical compression and horizontal deformation after the coordinated deformation of soil structure. it is difficult for traditional point monitoring technology to fully understand the deformation characteristics of the whole and local layers of the soil-structure system, the compatibility of deformation among different layers, and the law of development. it is also difficult to predict the deformation process of the soil in the ground subsidence. distributed fiber optic sensing (dfos), as a new monitoring technology, has the advantages of high spatial resolution, high precision, and high repeatability. it can effectively solve the problems of traditional monitoring technology (diao et al., 2016). at present, this technology has been widely used in the monitoring of slope, embankment, ground subsidence, and ground fissures. distributed optical fiber sensing technology can sense and measure one-dimensional deformations along the direction of optical fibers. in practical monitoring, various conversion methods or optical fiber layouts are often needed to achieve the purpose of monitoring the required deformations of the measured objects (bornyakov et al., 2016). after more than ten years of research and accumulation, the author’s research group has developed a variety of technology-based deformation monitoring methods (sayle et al., 2016). the following mainly introduces the distributed optical fiber sensing monitoring method of soil longitudinal subsidence and deformation. distributed optical fiber monitoring technology is based on botda (brillouin optical time-domain analysis) monitoring technology. botda is a time-domain analysis technology of brillouin scattering light based on stimulated brillouin scattering. its sensing principle is that a certain frequency of pulsed light is injected into one end of the fiber, and the pulsed light interacts with the acoustic phonon in the fiber, resulting in brillouin scattering. the light-receiving unit and signal acquisition unit of botda mainly analyze and process the back brillouin scattering light which returns to the injecting end of the pulsed light along the original optical fiber path. the optical signal is converted into an electrical signal by the photodiode, and the electrical signal is amplified by a broadband amplifier into the heterodyne receiver. the digital signal is averaged and the scattering spectrum of each sampling point along the optical fiber is obtained. brillouin scattering light is affected by strain and temperature, that is, when the fiber is stretched (compressed) along the axis or the temperature along with the fiber changes, the corresponding changes in the stretching (compressed) or temperature range of the fiber have a good correlation with brillouin frequency shift (xu et al., 2018). therefore, the brillouin frequency drift can accurately reflect the temperature and strain changes of optical fibers. in this paper, bofda technology is used to conduct a distributed monitoring model test of vertical land subsidence and deformation in seismic and geological hazards. using the self-made physical model of land subsidence and deformation (tehrani et al., 2016), the longitudinal deformation of different types and thicknesses of land in the process of subsidence and deformation is monitored by distributed optical fiber. the experimental results are compared and analyzed to verify the feasibility of distributed optical fiber sensing technology in land subsidence monitoring (kim et al., 2016). it also provides a reference for further research and application of distributed optical fiber sensing technology. materials and methods test material (1) sand the sand used in this experiment is taken from the jiangning area (song et al., 2016), nanjing. the grain size distribution curve of sand is shown in figure 1, and its physical and mechanical parameters are shown in table 1. figure 1. diameter distribution curve of sand table 1. the physical parameters of the sand proportion maximum dry density/ (g/cm3) minimum dry density/ (g/cm3) maximum porosity minimum porosity cohesion/ kpa internal friction angle/(º) 2.64 1.69 1.35 0.97 0.14 0 30.12 (2) clay the clay used in this experiment is xiashu soil (li et al., 2016), which is widely distributed in the nanjing area. the natural state is yellowish-brown, mainly plastic to rigid plastic, with compact structure (chen et al., 2017). its physical and mechanical parameters are shown in table 2. table 2. the physical parameters of the caly relative density bulk density liquid limit plastic limit plasticity index natural moisture content cohesion/ kpa internal friction angle/(º) 2.72 19.8 30.3 17.5 12.8 20-28 15.8 6.3 (3) gasbag in this experiment, gasbags are used to simulate seismic and geological hazards. the gasbag is made of black rubber, ellipsoid, with a long axis of 20 cm, a short axis of 15 cm, and a height of 15 cm. it is connected with an air pump and a pressure valve. this kind of gasbag has the characteristics of good air tightness, strong elasticity, high wear resistance, high acid resistance, and aging resistance (lu et al., 2018). it is flattened after natural ventilation, which meets the test requirements. 261monitoring method of longitudinal land subsidence and deformation in seismic geological disasters effectively and comprehensively study the mechanism and impact of ground subsidence. in the experiment, two main parameters are changed: the thickness of the upper soil layer and the content of clay. filling thickness (h) refers to the thickness of the soil on the top of the optical fiber. the specific parameters are 5 cm and 10 cm. clay content refers to the content of clay in the upper layer of optical fiber. the specific parameters are 0% and 5%. the monitoring variables of land subsidence deformation are vertical deformation of soil, including longitudinal compression and rebound. by laying distributed sensing optical fibers vertically in ground subsidence monitoring, appropriate backfilling is selected according to stratum lithology. with the increase of laying depth, the lateral earth pressure will increase continuously, and the deformation coupling between distributed sensing fiber and soil will be stronger (jiang & zhang, 2017). therefore, when the soil is compressed or rebounded, the distributed sensing fiber will co-occur axial compression or tension. the distributed sensing fiber data will be collected regularly to obtain the strain increment of distributed sensing fiber (shi et al., 2017), and the product will be used. the deformations of distributed sensing optical fibers can be obtained separately, as shown in formula 1. longitudinal land subsidence and deformation information can be obtained according to the integral results. ∆l l dl i i d i l l d l l = ( ) = ( ) + +( ) = ∑∫ ε ε − 1 2 1 2 1 1 2 (1) where l is the longitudinal tension or contraction /m between the sensing cable l1 and l2; (l) is the longitudinal strain at the sensing cable lm; and d is the step length /m of data acquisition by sensing cable. test steps (1) laying cushion: dry sand with a thickness of 5 cm is laid on the bottom of the model box with sand rain normal direction and leveling the surface (li et al., 2018). (2) installation of gasbags: the three full-air gasbags in the model box are placed. from left to right, they are no. 1, no. 2 and no. 3, and the spacing of the gasbags is 40 cm. the air duct connecting the gasbag is led out of the model box to control the air pressure inside the gasbag during the test. (3) sand layer landfill: dry sand is laid around the gasbag by sand rain method until it stops 5 cm above the top of the gasbag and the surface is leveled. (4) embedding of optical fibers: the optical fibers are passed through the reserved holes and welded with jumpers. the fused optical fiber is connected with bofda optical fiber monitoring instrument. two heat sources are set at both ends of the model box, and the monitoring software is run to determine the specific location of the optical fiber test section. after the fixed point is completed, the optical fibers are fixed and pre-stretched on the outside of both ends of the model box (zhang et al., 2017). (5) test soil layer landfill: the upper test soil is filled into the model box. after the fill reaches the predetermined height, the soil surface is leveled and placed for 12 hours. (6) land subsidence simulation test: before the beginning of the test, the pre-stress applied on the optical fiber is removed, the initial data is recorded, the no. 2, no. 1, and no. 3 gasbags are filled in turn, and the filling interval is 30 minutes. the data before each filling is measured, and the last measurement is 30 minutes after the completion of the no. 3 gasbag filling. in the process of aeration and measurement, the change of soil surface is photographed, and the change of soil surface during the test is recorded accurately. (7) subsidence information record: after data acquisition, excavates along with the optical fiber until the bottom of the gasbag, obtain the longitudinal profile of the soil after deflation, and observe the shape of the cavern and record and describe it. test conditions tests are carried out under the following four working conditions, and the soil conditions under different working conditions are as shown in table 3. condition 1: gasbag 2 deflates naturally and collects data after 30 minutes. (4) in this experiment, two kinds of sensing optical cables are used: a. polyurethane strain sensing optical cable with a variable diameter (2mm cd), is a polyurethane optical cable with 2mm diameter, and 25 mm length quartz glass tube at 20 cm intervals. a concave-convex variable diameter sensing optical cable (dyshlyuk et al., 2017) is fabricated, as shown in figure 2; b. polyurethane strain sensing optical cable (2mm cd for short) is a conventional optical cable with a diameter of 2 mm, that is, a sensing optical cable without a quartz glass tube. figure 2. polyurethane sensing optical fiber with a changed diameter monitoring program the model used in this experiment is a self-made indoor simulation box for longitudinal land subsidence and deformation (foumelis et al., 2016), which length is 200 cm, width is 50 cm and height is 50 cm. three air bags are buried in the set position of the box (deverel et al., 2016), as shown in figure 3: figure 3. indoor longitudinal land subsidence deformation simulator according to the test requirements, layered backfill soil samples are buried with distributed sensing optical fibers in the upper soil of the gasbag during the backfill process. after backfilling is completed, the air pressure inside the gasbag is increased at a certain speed through the external air pressure switch, so that the soil around the gasbag will be deformed due to the excessive support at the lower part, and then land subsidence will occur on the surface of the gasbag (zhang et al., 2017). by changing different test variables, this experiment can simulate different seismic and geological engineering conditions, and then 262 aimei xu, hojatallah azarkhosh, erjun wu condition 2: gasbag 1 deflates naturally and collects data 30 minutes later. condition 3: gasbag 3 deflates naturally and collects data after 30 minutes. conditions 4: gasbag 1-3 deflates naturally and collects data after 60 minutes. table 3. detailed contents of different working conditions working condition thickness sand content/% clay content/% 1 5 100 0 2 10 100 0 3 5 95 5 4 10 95 5 data correction method for sensing optical fiber according to the indoor test results, when the sensing fiber is embedded in a certain depth of soil, the confining pressure of the soil will affect the testing effect of the sensing fiber. therefore, when analyzing the sensing optical fiber data in deep soil, it is necessary to introduce correction coefficient to modify the monitoring data of sensing optical fiber under different confining pressures (tao et al., 2019), as shown in formula 2 and 3: ∆ ∆l k ls s= (2) where, ls is the longitudinal tension or contraction of sensing optical fibers in soil/m, and ks is the correction factor of coupling between sensing optical fibers and soil deformation. k k k i s is i i = + +( ) + 2 1 1 ≤ < (3) ki is the drawing efficiency of sensing fiber and soil when confining pressure is impa. the modified coefficient ks is introduced to modify the monitoring data of sensing optical fiber, and the corrected monitoring data are accurate monitoring results of vertical land subsidence and deformation. results in the course of the test, the changes in the soil surface in the model box at each stage are fully recorded. it is found that the upper soil has a great influence on the surface morphology of the ground subsidence, and different test phenomena will appear. the vertical subsidence and deformation of the soil surface under four working conditions are described by figure 4 (a), figure 4(b), figure 4(c) and figure 4(d), respectively. it can be seen that different seismic actions have different effects on the vertical subsidence and deformation of the soil surface. the longitudinal subsidence and deformation of soils under different working conditions will be monitored by the proposed method as shown in table 4 (yang et al., 2020). table 4 shows the changes in soil longitudinal subsidence and deformation under different working conditions monitored by the proposed method. among them, the results of soil subsidence and deformation measured under working conditions 2 and 3 have less valid data. in order to verify the correctness of the results of the proposed method in monitoring the vertical land subsidence and deformation in seismic and geological hazards, the results of soil subsidence and deformation monitored by the proposed method under working conditions 1 and 4 are compared with the actually measured soil subsidence. the results of deformation are compared and described in figure 5 and figure 6 respectively. the average vertical subsidence of soil under working conditions 1 and 2 measured by two optical fibers in this paper is described in table 5. from the data in table 5, it can be seen that the method can accurately monitor the (a)longitudinal land subsidence deformation result of working condition 1 (b)longitudinal land subsidence deformation result of working condition 2 (c)longitudinal land subsidence deformation result of working condition 3 (d)longitudinal land subsidence deformation result of working condition 4 figure 4. longitudinal subsidence and deformation of soil surface under four working conditions 263monitoring method of longitudinal land subsidence and deformation in seismic geological disasters vertical subsidence and deformation of soil structure at different locations under different seismic and geological hazards (wu et al., 2020). table 5. average longitudinal subsidence deformation of soil layer measured by this method/mm working condition working condition 1 working condition 4 0 cm -20cm 4.5 5.05 21 cm -40 cm 5.6 25.5 41 cm -60 cm 4.1 8.3 61 cm -80 cm 13 6.75 81 cm -100 cm 17.7 33.7 101 cm -120 cm 12 9.45 121 cm -140 cm 6.6 6.25 141 cm -160 cm 3.85 22.8 161 cm -180 cm 0 12.55 181 cm -200 cm 0 6.25 from the curves of figure 5 and figure 6, it can be seen that the results of longitudinal land subsidence and deformation monitored by the proposed method are almost identical with those measured in practice, which indicates that the method has high accuracy in actual subsidence and deformation monitoring, and can be widely used in areas with a high incidence of seismic and geological hazards, so as to warn the displacement of vertical land subsidence and to take timely measures to reduce personnel and property losses (gao et al., 2018). based on the accuracy of monitoring the vertical land subsidence deformation in seismic geological hazards by the proposed method, the monitoring efficiency and economy need to be verified. a qualified monitoring method for vertical land subsidence deformation in seismic geological hazards needs both high precision and high-efficiency deformation monitoring results and low monitoring cost. the experiment is based on gps technology to monitor the vertical ground subsidence deformation in seismic geological hazards (cheng et al., 2020). as an experimental reference, the methods of vertical land subsidence and deformation monitoring based on inertial technology for seismic and geological hazards are compared in terms of time and cost of precise vertical land subsidence and deformation monitoring for seismic and geological hazards under work conditions 1 and 4. the results are shown in table 6. from the data in table 6, it can be seen that the time and cost of the proposed method for monitoring vertical land subsidence and deformation of seismic and geological hazards at different distances are lower than those of the other two methods. the time and cost of the proposed method for monitoring subsidence and deformation at different distances of working conditions 1 and 4 are respectively 0.91 million yuan and 0.41 s and 0.75 million yuan and 0.41 s. the time and cost of gps technology for monitoring subsidence and deformation at different distances of working conditions 1 and 4 are 0.91 million yuan and 0.41 s, respectively. the maximum time and cost of inertial technology monitoring for subsidence and deformation in table 4. longitudinal subsidence and deformation of soil under different conditions working condition 1 2 3 4 l1 optical fiber mean subsidence deformation/mm l2 optical fiber mean subsidence deformation/mm l1 optical fibermean subsidence deformation/mm l2 optical fibermean subsidence deformation/mm l1 optical fibermean subsidence deformation/mm l2 optical fibermean subsidence deformation/mm l1 optical fibermean subsidence deformation/mm l2 optical fibermean subsidence deformation/mm 0 cm -20cm 5.6 3.4 0 0 0 0 4.5 5.6 21 cm -40 cm 6.6 4.6 0 0 0 0 23.4 27.6 41 cm -60 cm 4.8 3.4 18.8 6.7 0 0 8.9 7.7 61 cm -80 cm 15.7 10.3 0 0 0 0 7.8 5.7 81 cm -100 cm 20.7 14.7 0 0 8.8 6.5 32.7 34.7 101 cm -120 cm 11.5 12.5 0 0 11.7 12.9 10.2 8.7 121 cm -140 cm 8.7 4.5 0 0 0 0 5.8 6.7 141 cm -160 cm 4.3 3.4 0 0 0 0 21.8 23.8 161 cm -180 cm 0 0 0 0 0 0 11.3 13.8 181 cm -200 cm 0 0 0 0 0 0 6.7 5.8 figure 5. the method presented in this paper and the actual measured subsidence and deformation results of the soil under working condition 1 264 aimei xu, hojatallah azarkhosh, erjun wu working conditions 1 and 4 are 12.3 million yuan and 15.7 million and 15.4 million yuan and 1.34 s, respectively. the maximum time and cost of inertial technology monitoring for subsidence and deformation in working conditions 1 and 4 are 15.4 million yuan and 1.52 s and 22.4 million yuan and 0.95 s, respectively. the time and cost of three methods for monitoring subsidence and deformation in working conditions 1 and 4 are summarized as: the proposed method<inertial technology monitoring method < gps technology monitoring method. it has strong economic benefits and monitoring speed, high practical value, and can be widely used in seismic and geological hazard monitoring, with a strong economy. figure 6. the method presented in this paper and the actual measured subsidence and deformation results of the soil under working condition 4 table 6. monitoring results of three methods method working condition 1 working condition 2 the method in this paper monitoring method of gps technology inertial technology monitoring method the method in this paper monitoring method of gps technology inertial technology monitoring method cost/ten thousand yuan time/s cost/ ten thousand yuan time/s cost/ ten thousand yuan time/s cost/ ten thousand yuan time/s cost/ ten thousand yuan time/s cost/ ten thousand yuan time/s 0 cm -20cm 0.65 0.35 1.55 1.26 1.23 0.59 0.73 0.26 1.28 1.02 2.26 0.65 21 cm -40 cm 0.84 0.25 1.26 1.35 1.5 0.68 0.62 0.32 1.54 1.13 2.15 0.54 41 cm -60 cm 0.91 0.31 1.54 1.45 1.02 0.78 0.55 0.36 1.26 1.24 1.59 0.95 61 cm -80 cm 0.65 0.41 1.58 1.35 1.35 1.52 0.64 0.35 1.14 1.16 2.24 0.85 81 cm -100 cm 0.65 0.26 0.64 1.21 1.36 1.05 0.57 0.24 1.05 1.15 1.68 0.86 101 cm -120 cm 0.68 0.34 1.59 1.57 1.24 0.65 0.65 0.26 0.62 1.25 1.54 0.87 121 cm -140 cm 0.64 0.35 1.88 1.24 1.52 0.54 0.59 0.35 0.54 1.34 2.14 0.83 141 cm -160 cm 0.67 0.31 1.54 1.23 1.36 0.64 0.64 0.34 0.69 1.06 2.15 0.87 161 cm -180 cm 0.68 0.26 2.01 1.45 1.54 0.85 0.75 0.37 0.85 1.04 1.64 0.64 181 cm -200 cm 0.64 0.28 2.23 1.33 1.36 1.03 0.62 0.41 1.32 1.26 1.82 0.87 total 7.01 3.12 15.82 13.44 13.48 8.33 6.36 3.26 10.29 11.65 19.21 7.93 discussion in this paper, botda is used to monitor the longitudinal ground subsidence and deformation of the simulated seismic and geological hazards. the soil model box is used to simulate the surface environment. three airfilled gasbags are installed in the soil of the model box to simulate the seismic and geological hazards with different intensities. distributed sensing optical fibers are installed in the soil to obtain the longitudinal ground subsidence and deformation. the results of longitudinal land subsidence and deformation under four different working conditions show that the proposed method can monitor the subsidence and deformation at different locations of monitoring objects. the results are compared with the measured values. from figure 5 and figure 6, it can be seen that the average longitudinal land subsidence and deformation of soil at different distances from working condition 1 and 4 are almost identical with the measured values, which shows that the proposed method can monitor the subsidence and deformation of soil at different distances from working condition 1 and 4. in addition, by comparing the proposed method with inertial technology monitoring method and gps technology monitoring method in terms of the monitoring efficiency and cost, it can be seen that the proposed method can not only obtain high-precision subsidence deformation results but also improve monitoring efficiency, reduce the cost of the monitoring process and enhance economic benefits. the results of subsidence and deformation monitoring of indoor model box simulation tests show that reasonable monitoring methods can accurately detect soil deformation by sensing optical cable, and the monitoring accuracy is better than the existing conventional monitoring technology. how to apply distributed optical fiber sensing technology to the monitoring of geological hazards such as land subsidence and ground fissures more reasonably and efficiently and develop it into a mature and popular monitoring method is the main content of the next research. according to the monitoring situation in this paper, the development direction of optical fiber sensing monitoring technology for vertical land subsidence and ground fissures is summarized as follows: (1) introducing fiber bragg grating sensing technology fiber bragg grating (fbg) sensing technology has very high accuracy, but also has the unique advantages of anti-interference and corrosion resistance. 265monitoring method of longitudinal land subsidence and deformation in seismic geological disasters in the monitoring of geological hazards such as land subsidence, the distributed optical fiber sensing technology, and fiber bragg grating sensing technology are combined to achieve the complementary advantages of the two technologies and achieve the purpose of efficient monitoring. (2) developing vertical buried monitoring methods the vertical buried monitoring method can realize the distributed monitoring of soil compression and deformation. how to solve the problem of shallow surface temperature compensation is the focus of the next step of the proposed method. if the target monitoring layer is deeply buried, the temperature compensation can be avoided. the construction technology of this monitoring method is basically mature. the next step is to select the location with subsidence monitoring significance for drilling construction, lay the sensing cable through the main subsidence compression layer, and realize the vertically distributed monitoring of soil compression deformation. at the same time, the monitoring data are combined with the geological environment information to carry out the research on the mechanism and development law of land subsidence based on the vertical deformation distributed measured data. (3) improving and optimizing the fixed-point monitoring party the fixed-point monitoring method can realize the monitoring of soil micro-deformation in-ground fissure area. in terms of the effective monitoring length of monitoring points, the dislocation deformation of soil mass on both sides of the crack area can be accurately detected, and the monitoring accuracy can reach the fixed point distance. the monitoring effect is better than the direct buried and pipe-type monitoring methods used at the same time. therefore, in the next step of distributed optical fiber ground fissure monitoring, we should focus on the fixed-point monitoring method, and further optimize and improve the monitoring method. the research on the proposed method can be carried out from indoor tests and field monitoring. the main work can be carried out around the following points: a. the monitoring effect of this method under different modes of dislocation deformation of ground fissures is studied through laboratory tests, and the design and laying methods of anchorage components are improved to realize directional and quantitative monitoring of soil movement. b. the monitoring accuracy of this method is further improved by testing various sensing optical cables and fixing modes. c. suitable monitoring sites are chosen to conduct a wide range of distributed monitoring of ground fissures with different spacing. on the basis of monitoring existing fissures, pre-warning, and monitoring of safety in fractured areas are carried out. conclusions ground, as the main transportation place, plays an important role in the development of the national economy and society. ground safety is more directly related to people’s lives and property security. the ground is usually used for a long time, often affected by various loads and external seismic and geological disasters. the structure has different degrees of deformation. the vertical land subsidence deformation in the early stage of seismic and geological disasters is very weak, and the location is relatively hidden. the traditional detection methods are difficult to achieve the task of precise monitoring of the vertical land subsidence deformation under seismic and geological disasters. distributed optical fiber sensing technology based on brillouin optical frequency domain analysis can make up for the shortcomings of traditional methods, and can accurately measure the average vertical subsidence deformation in different distances of the ground to be monitored. it has good application prospects in the monitoring of vertical land subsidence deformation and improves the accuracy of vertical land subsidence deformation. compared with gps technology and inertial technology, the proposed method is more effective in monitoring vertical land subsidence deformation. the rate is higher and the cost of monitoring is lower. reference bornyakov, s. a., miroshnichenko, a. i., & salko, d. v. 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(2017). a queue management mechanism to improve 3d video transmission in wlan. journal of china academy of electronics and information technology, 12(3), 246-250. 10.3390/math 10.1016/j.aquaculture 10.1016/j.aquaculture this work purposes a general mean velocity and a suspended sediment concentration (ssc) model to express distribution in every point of the cross section of turbulent shear flow by using a probability density function method. in order to solve turbulent flow and avoid multifarious dynamical mechanics, the probability density function method was used to describe the velocity and concentration profiles interacted on directly by fluid particles in turbulent shear flow. the velocity profile model was obtained by solving for the profile integral with the product of the laminar velocity and probability density, through adopting an exponential probability density function to express probability distribution of velocity alteration of a fluid particle in turbulent shear flow. an ssc profile model was also created following a method similar to the above and based on the schmidt diffusion equation. different velocity and ssc profiles were created while changing the parameters of the models. the models were verified by comparing the calculated results with traditional models. it was shown that the probability density function model was superior to log-law in predicting stream-wise velocity profiles in coastal currents; and the probability density function ssc profile model was superior to the rouse equation for predicting average ssc profiles in rivers and estuaries. outlooks for precision investigation are stated at the end of this article. abstract keywords: exponential probability density; mean velocity profile; concentration profile; river flow; coastal current mean velocity and suspended sediment concentration profile model of turbulent shear flow with probability density function velocidad media y modelo de perfil de concentración de sedimentos suspendidos de flujo turbulentos de cizalla turbulento con funciones de densidad de probabilidad. issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.65172 este trabajo propone un modelo de velocidad media general y un modelo de concentración de sedimentos suspendidos (css) para expresar la distribución en cada punto de la sección de cruce del flujo turbulento de cizalla mediante el uso de funciones de densidad de probabilidad (pdf).. el método de funciones de densidad de probabilidad se usó para describir los perfiles velocidad y concentración que interactuaron directamente con partículas fluidas en el flujo de desprendimiento turbulento para resolver el flujo turbulento y evitar diferentes mecánicas dinámicas. el modelo del perfil de velocidad se obtuvo resolviendo el perfil integral con el producto de la velocidad laminar y la densidad de probabilidad, mediante la adopción de una función de desidad exponencial para expresar la probabilidad de distribución de la velocidad de alteración de la partícula de un fluido en un flujo de desprendimiento turbulento. también se creó un modelo de perfil css siguiendo un método similar al anterior y basado en la ecuación de difusión schmidt. se crearon diferentes perfiles de velocidad y css durante el cambio de parámetros de los modelos. los modelos se verificaron comparando los resultados calculados con los modelos tradicionales. se demostró que el pdf era superior a la ley logarítmica en la predicción de los perfiles de velocidad en corrientes costeras, y que la probabilidad del modelo del perfil de función de densidad ssc fue superior a la ecuación rouse para predecir perfiles ssc promedio en ríos y estuarios. las perspectivas para la investigación de precisión se indican al final de este artículo. resumen palabras clave: densidad de probabilidad exponencial, perfil de velocidad media, perfil de concentración, corriente de río, corriente costera. record manuscript received: 22/05/2017 accepted for publication: 23/08/2017 how to cite item wu, g., zhu, l., & li, f. (2017). mean velocity and suspended sediment concentration profile model of turbulent shear flow with probability density function. earth sciences research journal, 21(3), 129 134. doi: http://dx.doi.org/10.15446/esrj.v21n3.65172 h y d r o g e o l o g y earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 129 134 guanglin wua, b, liangsheng zhua,*, fangcheng lib acollege of civil and transportation engineering, south china university of technology, guangzhou 510641, china bcollege of ocean engineering, guangdong ocean university, zhanjiang 524088, china *corresponding author. tel: +86 02087111030-3682; e-mail: lshzhu@scut.edu.cn (l. s. zhu) 130 guanglin wu, liangsheng zhu, fangcheng li introduction turbulence is much more complex than laminar flow. according to the boussinesq eddy viscosity assumption (boussinesq, 1877), the logarithmic law for velocity profile was solved by using the mixing length method (prandtl, 1925; khan et al., 2017; zawar et al., 2017). some vertical velocity distribution laws, such as the power law and the exponential law, are based on experimental considerations or semi-empirical theories (lin et al., 1953; deissler, 1954; afzal et al., 2007). other laws for turbulent velocity profile or concentration profile are based on an indirect turbulence model (bucci et al., 2008; nucci and fiorucci, 2011). logarithm law is the most popular one used to express the mean velocity in a cross section of turbulent flow for wind flow, river flow, and lake models. the logarithm law fits velocity and concentration distribution in the position far away from the interface in developed turbulent flow (wang and yu, 2007). some researchers have focused on mean velocity profiles of turbulent wall-bounded flows separately (poggi et al., 2002; buschmann and gad-el-hak, 2007; kempe et al., 2007; buschmann et al., 2009). different vertical velocity distribution laws are used to describe streamwise velocity structure in coastal currents (soulsby, 1980; anwar, 1996, 1998; de serio and mossa, 2014), in tidal channels (soulsby and dyer, 1981), and on the continental shelf (soulsby, 1983; de serio and mossa, 2010).vertical distribution of suspended sediment concentration (ssc) in rivers, estuaries, and coastal currents have been studied by many researchers through the years (rouse, 1937; taylor and dyer, 1977; li uet al., 2014).this work derives general formulas to describe the mean velocity and ssc distributions of turbulent shear flow to fit the whole cross section from the bottom boundary to the upper boundary, using a probability density function method. a brief review of classical methods for velocity profile of shear flow consider the laminar shear flow—the steady incompressible developed flow between two interfaces, with one interface moving with velocity u and the other being motionless. the flow can be solved based on 2-dimensional navier-stokes equations and expressed as (1) here h is the distance between two interfaces, μis dynamic viscosity, and d p/ d x is the pressure gradient along the x-direction. the velocity profile of shear flow can be simplified to eq. (2) when d p/ d x = 0 (2) logarithm law (prandtl, 1925), derived by the mixing length method, is the most popular equation used to express the mean velocity in a cross section of turbulent shear flow. the basic logarithm law of mean velocity profile of turbulent shear flow is (3) here, u* is the shear velocity of the fluid and y0 is hydrodynamic roughness length. but eq. (3) cannot fit select velocity profiles not full profiles because of the assumption that the mixing length is proportional to the distance away from the wall boundary. many instances and experiments have proven that the mixing length method can successfully predict velocity distributions far away from the interfaces, but it cannot predict velocity distribution near interfaces such as walls, seabed, gas-liquid surfaces, and so on. as far as we know, turbulent intensity should be dependent on the reynolds number, which is composed of the characteristic velocity, characteristic length, and kinematic viscosity. considering the mixing length method carefully, the core assumption is that the eddy viscosity is in direct proportion to the average velocity gradient and the mixing length is in direct proportion to the distance away from the wall. the method does not directly create the relationship between turbulent parameters and average velocity. exponential probability density model (epdm) for mean velocity profile of turbulent shear flow to solve for turbulent flow without closely associated defferent dynamical mechanics, a probability density function method is adopted to describe the pulsating intensity of velocity of fluid particles in this study. the pulsating intensity of every fluid particle in the profile will impact the whole profile. the effect decays at a distance from its original position and obeys a probability density function. exponential probability density function is often used to describe the decay degree distribution in many physical and chemical problems. in many situations, the exponential probability density function model is an appropriate method to approach the random decay phenomena. the primary expression of the exponential probability density function is (4) here, p(y) is the probability density of a fluid particle impacting any position, λ is the damping index, and y is the distance from the particle position yi to the impacting position y, defined by (5) as shown in fig. 1, the expression of exponential probability density function p1(y), represents the probability density that the velocity of laminar shear flow at position y1, u1, distributes to any position y due to turbulence. because the expression satisfies the boundary condition that p1(y1) = pm1 where y = y1 as shown in fig. 1, λ = pm, such as λ1= pm1, λ2 = pm2, and so on. hence, the expression pi(y) satisfies (6) figure 1. schematic diagram of exponential probability density for pulsating intensity transfer 131diffraction characteristics of small fault ahead of tunnel face in coal roadway the probability density of u, velocity of any position of y, distributes to y0, a certain position at the profile, is defined as p0 as shown in fig. 1, such as p01 and p02. we have (7) average turbulent velocity of a particular position, y0, produced by integrating the product of the local laminar velocity in eq. (7), which is solved with navier-stokes equations, and the exponential probability density function of pulsating intensity at the position, across all fluid particles in the profile, is shown as (8) here α is an adjustment. substituting u(y) with eq. (2) and p0(y) with eq. (7) in eq. (8), we get (9) eq. (9) cannot be analytically and fully solved. to solve eq. (8), we assume that (10) substituting eq. (2) and eq. (10) into eq. (8), the result is (11) integrating eq. (11), the result is (12) setting (13) and substituting y0 with y in eq.(12), the velocity profile of turbulent shear flow with this method (called pdfm) should be (14) assume that (15) eq. (14) can be calculated when the adjustable parameters, m and n, are given. the mean turbulent velocity in eq. (14) can be calculated by giving different m and n, as well as defining the dimensionless turbulent velocity and dimensionless position as (16) the calculated results are shown in fig. 2 and fig. 3. fig. 2 reveals that the mean velocity profile changes when the value of m is changed, but the mean velocity changes are weaker in the region near the wall or other interfaces than in the intermediate region while α=1. the dimensionless velocities are almost the same in the region when the value of y+ is between 0 and 0.25, but they change very evidently near the position where y+ is approximately equal to 0.8. thus, the upper mean velocity will increase along with an increasing value of m. the curve shape fits turbulent velocity profile when the value of m is lower in this case, such as m=0.25, 0.1 and 0.01. figure 2. mean velocity profiles with change of m (n= 0.25) figure 3. mean velocity profiles with change of n (m = 0.3) 132 guanglin wu, liangsheng zhu, fangcheng li de serio and mossa (2010, 2014) investigated vertical profiles of the stream-wise velocity of the coastal currents at the taranto (ta) and bari (ba) sites in italy. the comparison between the measurement data, the log law as shown in eq. (3), and the pdfm as shown in eq. (17) is plotted in fig. 4 at taranto for vertical stream-wise velocity profiles in shallow coastal currents. the calculated results show that the probability density function model (pdfm) more accurately reflects measurement data than the log-law model. ssc profiles model and its verification in river and estuary the rouse equation (rouse, 1937) was the most popular method used to calculate vertical concentration profiles of suspended load in rivers, lakes, estuarine and coastal currents, as well as on the continental shelf. the rouse formula was (18) in which ya is the thickness of the sheet-flow layer, ca is the concentration at ya which equals the bed-load concentration, z*=ω/κu* is the suspension index, and ω is the particle settling velocity. the similar expression of pdfm also can be created by the method below based on the schmidt diffusion equation (schmidt, 1925). (19a) that is (19b) in which ε is the particle diffusion coefficient, set as an assumption of the exponential probability density function (20) then (21) hence, (22) here γ is the suspension index of this model. the results of ssc profiles calculated on pdfm of eq. (22) are shown in fig. 5. the mean velocity profiles are distinct when the values of n are different, as shown in fig. 3, while α=1. the main tendency of the profile curves is shifting from the top left to the bottom right in fig. 3 along with the decreasing value of n. the tendency indicates that the parameter n has a closed relationship with turbulent intensity. verification of pdfm in coastal currents the mean velocity profile model derived in the previous section was applied to fit the investigated profiles to verify the effectiveness of the model. to use eq. (14) conveniently, the profile formula of pdfm should be transformed as (17) here β= αu/u*. figure 4. calculated and measured vertical profiles of the stream-wise velocity of coastal currents at taranto (survey data cited in de serio and mossa (2014)) 133diffraction characteristics of small fault ahead of tunnel face in coal roadway figure 5. calculated results of suspended sediment concentration profile model figure 5 shows that the slope of the profile curves is bigger near the surface than the bottom and the upper slope increases as the lower slope decrease when γ is increasing. the trend means that the particle settling velocity, particle diffusion coefficient and the water depth velocity influence γ to change the ssc distribution in the vertical cross section. the ssc is very small when γ=50, so the bed load is the dominant composition when γ≥50. the curves also show that the ssc at surfaces is greater than zero when γ≤2. on the contrary, the ssc at surfaces tends to zero when γ>2. so, this model provides an approach to avoid the surface limitation of rouse equation. zhang et al. (2007) investigated ssc of the yangtze river in detail and concluded a semi-empirical model of ssc. a comparison of predicting profiles between the pdfm (eq. 22) and the rouse equation [eq. (18)] with the measurements in the yangtze river is shown in fig. 6. as shown in the figure, the pdfm model used for this study more accurately reflects the measurement data than does the rouse equation. figure 6. calculated and measured vertical ssc profile in the yangtze river (survey data cited in zhang et al. (2007)) figure 7. calculated and measured vertical ssc profile in the yangtze estuary (survey data cited in liu, et al. (2014)) liu, yang, zhu, et al. (2014) investigated the average ssc profile in the yangtze estuary in detail. the survey data is cited here to verify the pdfm model, eq. (22). the comparison between the rouse equation, pdfm, and measurements is shown in fig. 7. the pdfm model of this study is also agreed more closely with the measurement data than does the rouse equation in the case of the estuarine area. discussions a model for solving velocity and concentration distribution of turbulent flow by using probabilistic methods is put forward in the article (khan et al., 2017; sultana et al., 2017). general models for describing the mean velocity and ssc profiles of a whole cross section in turbulent shear flow are derived based on the idea. in fact, the velocity profile formula of laminar shear flow is also the mean velocity profile solution of turbulent flow which ignores the turbulent items in reynolds equation, thus the pulsating intensity of fluid particles is apparently related to it. on the other hand, the pulsating intensity is also under the control of certain boundary conditions. the exponential probability density function may describe the pulsating intensity of fluid particles of the whole cross section correctly to study mean velocity and concentration distributions. the calculated results of the derived formulas are as diverse as the changing the damping index of exponential probability density function. an innovative approach may be found to solve complex turbulent flow by expanding on this concept. conclusions the pdfmmodel of this study possesses some advantage in the fields of predicting stream-wise velocity profiles of coastal currents and ssc profiles in river and estuaries as shown in the previous examples.further research is needed to validate the method through continued studies and carried forward to clarify and optimize the parameters of these models to apply for practical problems. for advanced research, the parameters should be studied meticulously, such as its relationship with dynamic parameters, particularly turbulent characteristic parameters. 134 guanglin wu, liangsheng zhu, fangcheng li acknowledgments this work was partially supported by the guangdong province science and technology research project (2010b030600007) and projects of enhancing school with innovation of guangdong ocean university (gdou2016050258 & gdou2017052503). references afzal, n., seena, a., & bushra, a. 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(2007). river dynamics (in chinese). wuhan univ. press, wuhan, hubei, p.r. china. this issue of earth sciences research journal we at the earth sciences research journal hope all our readers have a happy and productive 2018. our last issue of esrj in 2017 presents a number of papers on different topics within the earth sciences. in alvarez-ysabel et al. a 150+ year database of tropical cyclone activity is analyzed for the dominican republic (dr). a total of 112 events have tracks at most 300 km from the dr are studied in terms of their seasonal distribution and long-term distribution, their frequency and landfall probability. the authors also present a review of trocial cyclone activity in the region. ogly gahramanov uses a combination of geological and geochemical information to model the formation of hydrocarbon deposits in the south caspian basin, in particular for the south caspian depression. the authors argue that large pressures and tectonic history have a large effect on the appropriate selection of potential drilling sites. using high-resolution aeromagnetic survey data in combination with other geological-geophysical thematic maps, batista takes advantage of gis tools for identifying different tectonic regimes in the sabinas basin, northeastern mexico. regions where the basin is deepest and the delineation of faults allows for the determination of block boundaries as well. he et al. use a modified direct shear test apparatus to determine shear strength parameters of unsaturated soils under various conditions. the paper proposes that the direct shear test analysis is an efficient way to analysis soil samples. zhuo and chen explore the improved water retention capacity of sandy soils by adding nanocarbon made from coconut shells. different amounts and thicknesses of nanocarbon were added to the soil, increasing water content with increased nanocarbon. loess deposits can have very large thicknesses, and for engineering purposes it is important to better understand the geotechnical response of the loess deposits. wang et al. investigate the collapsibility of loess deposits in the xixian new region using compressions tests under different pressure and water content conditions. results presented may have important application in future geotechnical studies in the region. germán a. prieto editor-in-chief th is is su e o f ea rt h s ci en ce s r es ea r c h jo u r n a l earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017) keywords: nested decomposition; gpu parallel; linear solution; cpr. palabras clave: descomposición anidada; gpu paralela; solución lineal; cpr. how to cite item how to cite item: shi, x. & di, y. (2019). gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation. earth sciences research journal, 23(3), 249-257. doi: https://doi.org/10.15446/esrj.v23n3.81669 this paper designs a highly parallel nested factorization (nf) to solve large linear equations generated in reservoir numerical simulation problems. the nf method is a traditional linear solution preprocessing method for reservoir numerical simulation problems and has regained attention in recent years due to its potential to extend to parallel architectures such as gpus (graphics processor units). the parallel algorithm of this paper is based on the mpnf (massively parallel nested factorization) framework proposed by appleya. the mpnf algorithm designed in this paper focuses on its efficient implementation on the gpu parallel architecture. its features include: using a custom matrix structure to achieve merge access, improving access bottlenecks, and improving the efficiency of the spmv algorithm. it is also applicable to the two-stage preprocessing method cpr (constrain pressure residual). cpr pressures solution and global preprocessing stage; the mpnf method is extended to the solution of the 2.5-dimensional unstructured grid problem. the parallel algorithm in this paper has been integrated into the reservoir numerical simulator. for the spe10 (million grid, highly heterogeneous) standard example, the gpu-based parallel nf algorithm is in the structured grid model and the equivalent 2.5-dimensional non-on the structured grid model. compared with the serial version of the nf method, the acceleration ratios of 19.8 and 17.0 times were obtained, respectively; compared with the mainstream serial solution method; the efficiency was also improved by 2 to 3 times. abstract gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation método de descomposición anidada a través de paralelización con unidades de procesamiento gráfico para resolver sistemas lineales grandes en la simulación numérica de yacimientos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.81669 este artículo diseña una factorización anidada (nf) altamente paralela para resolver grandes ecuaciones lineales generadas en problemas de simulación numérica de yacimientos. el método nf es un método tradicional de preprocesamiento de solución lineal para problemas de simulación numérica de yacimientos, y ha recuperado atención en los últimos años debido a su potencial para extenderse a arquitecturas paralelas como las gpu (unidades de procesador de gráficos). el algoritmo paralelo de este artículo se basa en el marco de mpnf (factorización anidada masivamente paralela) propuesto por appleya. el algoritmo mpnf diseñado en esta contribución se enfoca en su implementación eficiente en la arquitectura paralela de gpu. sus características incluyen: usar una estructura de matriz personalizada para lograr un acceso combinado, mejorar los accesos de cuellos de botella y mejorar la eficiencia del algoritmo spmv. también es aplicable al método de preprocesamiento en dos etapas cpr (solución de presión residual de restricción) presión solución y etapa de preprocesamiento global; el método mpnf se extiende a la solución del problema de cuadrícula no estructurada de 2.5 dimensiones. el algoritmo paralelo en este artículo se ha integrado en el simulador numérico de yacimiento. para el ejemplo estándar de spe10 (millones de cuadrículas, altamente heterogéneo), el algoritmo nf paralelo basado en gpu está en el modelo de cuadrícula estructurada y el equivalente de 2.5-no dimensionalen el modelo de cuadrícula estructurada, en comparación con la versión en serie del método nf, los índices de aceleración de 19.8 y 17.0 veces se obtuvieron respectivamente; en comparación con el método de solución serial convencional, la eficiencia también se mejoró de 2 a 3 veces. resumen record manuscript received: 20/03/2019 accepted for publication: 15/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 249-257 xin shi1, yuan di1* 1college of engineering, peking university, beijing, 100871, china co m pu te r sc ie nc e ap pl ie d to g eo lo g y 250 xin shi, yuan di gpu parallel architecture gpu multi-core parallel architecture has received more and more attention in the field of high-performance computing in recent years. gpu was originally used for graphics processing acceleration tasks. with the development of general-purpose gpu (general-purpose computing on gpu, gpgpu-related) software and hardware platforms, gpus are gradually applied to parallel acceleration in the field of scientific computing. one of them is nvidia. launched the compute unified device architecture (cuda) platform (appleyard, appleyard, wakefield, & desitter, 2011; nvidia, 2009). the algorithm in this paper is based on cuda implementation. the gpu achieves higher computing performance by integrating more computing units on the same chip area. take nvidia’s kepler architecture gk110 chip as an example. the most basic computing unit is the cuda core. a complete gk110 chip contains 2,880 cuda cores. the k10c gpu with gk110 chip has 5gb gddr5 (graphics double data rate, version 5) memory, and its single-precision and double-precision floating-point operation peak speeds are 3.52 and 1.17 tflops respectively (float operation per second, floating-point operations per second) ). the peak bandwidth has reached 249.6 gb/s. these metrics far exceed the peak data of the same level of cpu. gpu parallelism is often memory-constrained, so it must be welldesigned to accommodate the gpu’s multi-level memory structure to maximize gpu computing performance (nvidia, 2012a; nvidia, 2012b). linear system solution in reservoir numerical simulation control equation in reservoir numerical simulation, the basic governing equation is the mass mass conservation equation. for any discrete grid system, the mass conservation equation for a certain mesh i and component c can be written as: ∆ ∆ φ ρ ρ − − −γ − t v s x x q t i j p p j p p i p p g d j p p cp p cp w p p ∑ ∑( )    + ( ) , dd i pj neighbor                   =∑∑ 0 ∈ (1) which represents volume, represents porosity, represents phase saturation, represents phase density, represents component mass fraction, represents source term (well term), represents all meshes connected to i, represents a phase, between a connected mesh conductivity, which represents pressure, represents the mass density of the phase, represents the gravitational acceleration constant, and represents the mesh depth. additional constraints are required to solve the above system. for the most common case, in the constraint equation v represents volume,  indicates porosity, indicates phase saturation s p p ∑ = 1 (2) xcp c ∑ = 1, p (3) f p x t f p x tcp p cp cq p cq, , , ,( ) = ( ) (4) equation (4) represents the fugacity balance of the components between phases and . the above systems are usually solved using the newton iteration method. at each newton iteration step, the corresponding jacobian matrix and the rightend term are obtained, and the linear system is solved, and the newtonian update is performed until the nonlinear equation converges, and then the simulation of the next time step is performed. in the above process, the linear system solution takes up the most time. in the reservoir numerical simulation, the krylov subspace-based iterative solver is generally used to solve such large sparse linear systems. generalized minimum residual (saad, 2003; saad & schultz, 1986) (generalized minimum residual, gmres) is one of the most commonly used solvers. however, the performance of such solvers depends to a large extent on the pre-processing methods used with them. cpr two-stage pretreatment method the equation system of the reservoir problem has mixed characteristics, which are close to the part of the elliptic equation (pressure equation) and the part close to the hyperbolic equation (convection equation). in order to solve this mixed equation problem, wallis et al. (wallis, 1983; wallis, kendall & little, 1985) proposed a two-step pretreatment method, constrained pressure residual (cpr). in this method, the first step extracts the pressure matrix from the complete jacobi matrix, solves the pressure matrix to eliminate the low frequency error existing in the pressure variable, and the second step preprocesses the complete matrix to eliminate the high frequency error (or local error). ). the mathematical characterization and algorithm steps of the cpr method are as follows. the preprocessing matrix defining the cpr method is m1,2 m m i am m1,2 1 1,2 1 1 -1 1 -1=     + (5) where m1 and m2 are the preprocessing matrix for the first and second steps, respectively. it is the original matrix, which is the unit matrix. call cpr method to get x m v= 1,2 -1 . it can be broken down into two separate pre-processing processes to calculate: 1) call the first step of the preprocessing algorithm, calculate x m v= 1 -1 ; 2) calculate v v ax2 1=  ; 3) call the second step preprocessing method, calculate x m v2 2 1 2= − ; 4) calculate x x x= +1 2; reasonable selection of two separate preprocessing methods as cpr components, the cpr method proved to be very efficient in dealing with reservoir simulation problems, on a series of complex models including standard wells, multi-segment wells and unstructured grid models. can have very good performance (jiang, 2007; jiang & tchelepi, 2009). the pressure solution process in cpr generally uses the multigrid method (stüben, 1983), and the global preprocessing stage generally uses the ilu0 method. this paper retains the cpr solution framework and uses the mpnf method as a two-stage preprocessing method. nested decomposition traditional nf method the nf method (stüben, 1983) is a classical linear solution preprocessing algorithm for reservoir numerical simulation problems for structured grids. its basic idea is to use the nested three-diagonal matrix structure corresponding to the structured mesh to solve the problem quickly. for structured grid systems that are small (within hundreds of thousands of grids), the nf method has high efficiency. mpnf method nf is a completely serial algorithm, both in the decomposition phase and in the solution phase. in order to make it parallel on the gpu, the algorithm needs to have a large number of parallelizable units. mpnf is such a variant. mpnf achieves good fine-grained parallelism through dyeing and rearrangement. for example, a set of meshes extending in the z direction (other directions are also possible) is called a kernel; then coloring is performed at the core level (that is, the xy plane), and the dyeing operation refers to each unit is assigned a color, so that adjacent units always belong to different colors; finally, the grid system is rearranged by number, first in color order, and the same color is sorted by core (the order between cores can be arbitrary) ), the core internals are sorted from small to large in the z direction. 251gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation pre-processed gmres for the cpr first-stage solution, and introduced the cuda-based gpu algorithm implementation in detail, and this method applied to multi-gpu parallelism. this paper implements a complete gpu parallel linear algorithm based on cuda. the algorithm uses cpr as the preprocessor and gmres as the accelerator solution framework. the preprocessor selection of cpr two stages is mpnf method. consolidated access the global memory is the memory outside the gpu chip. it has the largest storage space and the largest access latency. when the algorithm works, it will store all the data that needs to be read and written from the host thus, the inner mesh of the core is a one-dimensional connection, and the numbers are adjacent, resulting in a three-diagonal structure. kernel d u l d u l u l d i i i i i i i nc i nc i nc i = − − 1 1 1 2 2 2 1 1 0 0 0 0 0 0 l o m o o m o o o l                     (6) where nc represents the number of grids in each core, representing the serial number of the core. it can be a single element or a dense matrix block, depending on the current matrix type (pressure matrix or main matrix) and the number of implicit variables in each grid block. the cores in the same color are not connected to each other, forming a block diagonal structure. color kernel kernel kernel i i i nk i =     1 2 0 0 0 0 0 0 0 0 0 0 0 l o o m o o m o o o l               (7) where nk represents the number of cores in a color (the number of cores in different colors is generally different), representing the serial number of the color. think of a grid within a color as a whole, as long as the dyeing algorithm ensures that the different colors are also one-dimensional connections, then the colors also form a block-like three-diagonal structure. a color u l color u l u l color = − − 1 1 1 2 2 2 1 1 0 0 0 0 0 0 l o m o o m o o o l nco nco nco ii                 (8) where nco representative dye number, l,u represents the non-diagonal elements produced by the connection between colors. compared with nf mehtod, mpnf through dyeing and rearrangement, a matrix of two-level three-diagonal nested structure is formed, and a similar factorization can be performed corresponding to the pre-processing matrix. since at the color level, the cores inside the same color are always not adjacent, the parallelism is obtained between the cores. in the decomposition and solution phase of the preprocessing, each core can be executed in parallel when it involves operations within each color. the simplest method of dyeing is the checkerboard dyeing of two colors, alternating the two colors to complete the staining on the plane. as shown in figure 1. in the figure, each core contains 4 grids, which are arranged in the longitudinal direction. after dyeing, the two colors each contain 10 cores, and the cores in the same color are not connected to each other. fewer staining results in higher parallelism, but at the same time the approximation of the preprocessing matrix is degraded, and convergence requires more iterations. appropriate increase in the number of stains may contribute to the parallel effect as a whole. a four-color oscillatory format staining method is shown in figure 2. the dyeing starts from one corner and is sequentially dyed according to the strip parallel to the diagonal, and the color is circulated in the order of 1→2→3→4→3→2→1. obviously, this dyeing method can still ensure that the cores in the same color are not connected, and the color is a one-dimensional connection. figure 3 is the corresponding matrix structure. efficient implementation and expansion of mpnf methods appleyard (2011) describes the principle of the mpnf method, but there is no specific implementation details; zhou (2012) used the mpnf figure 1. 5×4×4 regular sorting of the grid (left) and checkerboard sorting (right) figure 2. four-color oscillating format dye sorting 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 nz = 448 figure 3. matrix structure diagram corresponding to four-color oscillating format dyeing sorting 252 xin shi, yuan di rows is the same as the number of rows of the original matrix, and the number of columns is the same as the maximum number of non-zero elements in each row of the original matrix, and the position where there is a gap after compression by the row is filled with 0 elements. val is stored in column priority order. the col_idx storage format is the same as val, recording the original column index of each corresponding position element. when using cuda for spmv operations, the operation of each row in the ellpack format is handled by one thread, and the padded 0 element ensures that access to the matrix elements is a merged access. this mode allows it to perform better than the csr format when the number of non-zero elements in each row is not much different. in the reservoir numerical simulation problem, the maximum number of non-zero elements per row of the matrix depends on the number of connections of the grid (a grid is connected to at most 6 grids), which is relatively fixed, and after the grid number is determined, the maximum the number of non-zero elements can be calculated, and it is suitable to use ellpack format storage. combined with the characteristics of the matrix, this format can be further improved. one of the characteristics of such a matrix is that the first layer of the block structure is constructed in a core organization. in the matrix shown in fig. 3, the upper non-diagonal element representing the connection between color 1 and color 2 is taken as an example (see fig. 5), and the red box marked portion represents the connection relationship between the cores of the two colors, and constitutes a connection relationship. a layered structure, in which each small matrix block is a diagonal matrix, which is hereinafter referred to as a core block. another feature is that each diagonal element in the core block (ie, the black patch in the figure) is also a block structure. with these two layers of block structure, a more compact b-ellpack format can be constructed for spmv operations. figure 5. two-layer block structure diagram figure 6 is a schematic diagram of the storage of figure 5 as a b-ellpack. in this format, only the column subscript of the block block of the core block is stored in col_idx, and the column subscript of each non-zero element is not required to be recorded, and the required storage space and the access frequency thereof are greatly reduced, thereby improving the correlation. the efficiency of the operation. when performing spmv operation, each thread processes a block operation of a core block. the storage order of specific elements in the val array should be: firstly stored in the order of the block block of the core block; sorted by layer in the block column of the same core block (assume that the core is distributed along the z direction); the same layer is arranged in the order of the columns of the small matrix blocks; finally, the core blocks are sorted by the block rows. the matrix representing the connection between colors can be stored in the b-ellpack format. the spmv operation needs to be performed when solving the block-shaped three-diagonal matrix composed of colors, and the obtained result vector is used for the right-end term of the inner three-diagonal solution of the next color. therefore, in addition to the higher efficiency, this format can also make the order of the result vectors obtained by the matrix after the spmv operation coincide with the order of the right end items required for the inner tridiagonal solution of the color (ie, the order). for p-gmres and b-gmres, the spmv operation of the pressure matrix and the main matrix is required in the algorithm. a simple modification to the above format is applicable. this variant is hereinafter referred to as the b-diag-ellpack format. (host) memory. copy to the device (device, also known as gpu), stored in the global memory for thread access. access to global memory needs to satisfy coalesced access: if an instruction needs to access specific data in global memory, and the access locations of all threads (within the same thread bundle) are consecutive in memory, then the merge access is satisfied. . if the merge access condition is satisfied, only one read operation is required, and all threads in the warp can obtain their respective data; otherwise, each thread must perform a separate read operation to obtain the respective data, which greatly reduces the effective bandwidth. for the mpnf algorithm, the initial matrix data of the host does not meet the merge access requirements, and needs to be rearranged before being passed to the device for parallel algorithms. the data that the mpnf algorithm needs to use includes: matrix data, unknown vectors, and right-end vectors. the structure is similar and is illustrated by an example. after the mesh is dyed, the initial structure of the diagonal elements is first sorted by color number (that is, the elements whose subordinate color numbers are small are always in front of the elements with large numbers), the same color is internally sorted according to the core number, and the same core is internally meshed according to the grid. the number is sorted. finally, if the middle element is a matrix block (for b-mpnf), the matrix blocks are arranged in column priority order. since the parallelism of the mpnf algorithm exists between the cores in the same color, the thread responsible for processing different cores should access the contiguous memory under the same operation to satisfy the merge access, so it should be rearranged in the following order: firstly, the order is the same according to the color number. inside the same color, first arranged in layer order (assuming the core is in the z direction); if the middle element is a matrix block in the same layer, it is arranged in the order of the columns in the matrix block; the positions in the same matrix block are in the order of the core number arrangement. if the above arrangement order in the same color is denoted as in_ ker_order, in_blk_order, ker_order, the arrangement before and after rearrangement is changed from “ker_order” “in_ker_ order” “in_blk_order” to “in_ker_order” “in_blk_order” “ker_order”. in short, parallelism exists between cores, and only needs to be sorted by core number in the end, and the other order is unchanged to ensure merge access. in addition to using the same arrangement, the incoming right-end vectors should be rearranged in the same way, as is the calculated solution vectors. the intermediate variables calculated in this way also have the same order. block-based b-ellpack format the ellpack format is a sparse matrix storage format, and its storage mode is similar to the csr format. the difference is that after the matrix is compressed by row, the ellpack format fills 0 elements in rows with fewer nonzero elements so that all rows are compressed to the same length. see figure 4 for an example of its storage format. where val is a dense matrix, the number of figure 4. ellpack storage format example. 0 represents the filled 0 element, * represents the subscript corresponding to the filled 0 element, can be any value, will not be accessed when used 253gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation take the main matrix as an example. the main matrix differs from that of figure 5 in that the block diagonal elements are internally trigonal rather than diagonal. the diagonal block always exists, so only the diagonal block needs to be stored in the top position of each block row, and col_idx does not need to store the column index of the diagonal block, and the correlation operation is performed. the block column can be specially treated. the order of the special block columns can be various, and it is only necessary to ensure that the order of the last two steps is arranged in the order of in_blk_order ker_order. fig. 7 is a behavior example corresponding to the first color in fig. 3, showing the corresponding b-diag-ellpack storage format. correspondingly, the spmv results of the main matrix are also stored in the same order. when performing other vector parallel operations, they must first be converted back to the normal order. mpnf method for 2.5d unstructured grid the simulation of fractured reservoirs uses the discrete fracture model (dfm). the dfm mesh model can be a completely unstructured tetrahedral model or a semi-structured layered model (ie 2.5d). a layered model is a grid system that is still divided into layers in the longitudinal direction and is formed by triangulation (or other splitting format) on the plane. compared with the tetrahedral model, the layered model has limitations in applicability (requires cracks to be high-angle seams), but it has significant advantages in terms of sectioning difficulty, grid quality, formation information, and speed of solution. an important unstructured grid model. kuznetsova et al. (2007) introduced a method of applying the nf method to a layered model: dyeing on a plane to construct a two-layer one-dimensional joint structure (the grid in the same color is a one-dimensional connection, and the color is one between dimensional links, plus one-dimensional connections in the vertical direction, finally construct a three-dimensional one-dimensional structure in the unstructured grid. similarly, the mpnf method itself does not require the distribution of the mesh on the plane: as long as one direction can be selected as the direction of the core extension (requires the same number of core meshes, and is a onedimensional connection), and in the direction of the core extension a dyeing method can be given on a vertical plane to ensure a one-dimensional connection between colors, and the mpnf method can be applied. appleyard (2016) presented a method for staining unstructured meshes on a plane. first select the color number and the stimuli format of the dye sequence; select the starting grid, assign it the starting color; find the current unstained, but at least one of the grids connected to the dyed grid to form a candidate list; statistical candidate list the number of different colors connected to the grid is arranged in descending order of different color numbers; the next color in the color sequence is taken, and the grids satisfying the requirements (the same color cannot be connected) are sequentially given in the order of the candidate list until the traversal is completed. finally, look for a new candidate list and repeat the above process until all the grids have been dyed. this is a general dyeing method that works equally well for structured grids. when applied to a structured grid, the number of colors is at least 2; however, when applied to an unstructured grid, the number of colors required is generally large (eg, 6). figure 8 is a schematic diagram showing the results of staining on a plane using a dyeing method for the above dyeing method. the unstructured mesh is generated by triangle (shewchuk, 2002) (using the -q command to control the triangle element as close to the regular triangle as possible). it can be seen that the dye spreads outward from the layer at the initial grid, and the shock stimuli sequence is still well maintained. figure 9 is a layered model (divided into 41 triangular grids on the plane, divided into 3 layers in the longitudinal direction) and rearranged by 6 colors according to the above method, corresponding to the matrix structure diagram. the part marked by the red frame is a three-diagonal structure composed of connections between colors. it can be seen that for non-structural meshes, this method of dyeing does not completely guarantee a one-dimensional connection between colors, so there will be a small amount of elements that fall into the tri-diagonal off-band. when using mpnf preprocessing, these off-band elements can be ignored. although there is some loss in accuracy, a strict three-diagonal structure is guaranteed, so that the algorithm can be performed normally. but whether it is the gmres algorithm in the pressure solution process or the gmres algorithm used in conjunction with cpr, the spmv part still uses the exact matrix. figure 6. b-ellpack format diagram figure 7. b-diag-ellpack format diagram figure 8. 2.5d grid 6-color turbulence format staining result (staining start point at the center) 254 xin shi, yuan di study case test algorithm convergence performance and time-consuming analysis this section selects the first 10 layers of the spe10 (van der vorst, 1992) example, and performs performance test and time analysis on the cpr twostage mpnf algorithm. the grid size is 60×220×10, a total of 132,000 grids, and the fluid is oil-water two-phase. the mpnf method in the pressure phase is combined with gmres, that is, the one pressure matrix solution may need to call multiple mpnf solving processes, and the convergence error is set to 0.1; the second phase mpnf is used alone, that is, the second phase mpnf solving process is performed only once per cpr call. . the first is the test of convergence speed. ilu0 and amg were selected for comparison with p-mpnf, and bilu0 was selected for comparison with b-mpnf. the mpnf method also selects the number of stains as 3, 4, and 5 for comparison. if the mpnf method is not used in both stages of cpr, the grid number is in natural order. if mpnf is used in any stage, the grid number is the order after dyeing. when ilu0 is used in the pressure solution phase, it is also used in combination with gmres. the pressure solution convergence error is 0.1, and amg is used alone. the second phase of bilu0 is also used separately. the test platform hardware is configured for one xeon x5670 cpu and one k20c gpu. table 1 shows the test results. first, the number of newton iterations is almost the same in all the examples. this is because the selection of the specific components of the linear solution does not affect the newton convergence speed under the same linear solution strategy and convergence error. the number of linear iterations is the number of iterations of the outer gmres. cpr preprocessing is called once per iteration. the size reflects the validity of the cpr combination. the cpr combination of the example 1 achieves the best effect, which is consistent with the usual understanding. amg is best suited as the choice for the stress solution phase in cpr. in the comparison between the case 2 and the case 3, the average number of iterations required for each pressure solution is used as a standard, and the ilu0 is used for the pressure solution to be slightly weaker than the mpnf. considering that the calculation example 3 is based on the dyeing rearrangement, the rearrangement will make the quality of the matrix worse in the sense of solution, and thus the number of linear iterations is more than that of the second example. comparing example 3 with example 4, the number of linear iterations is very close, reflecting that mpnf as a second-stage preprocessing is similar to bilu0. a comparison of the examples 3, 4, and 5 shows that the increase in the number of stains is beneficial to the matrix solution, whether it is a pressure matrix or an outer iteration. but the difference between the three in this test case is not obvious enough. first, the mpnf decomposition phase time consumption is negligible compared to the solution phase. this is because for p-mpnf and b-mpnf, the decomposition phase is only executed once for each newton iteration; and the solution phase for b-mpnf is every time. the cpr call is executed once. for p-mpnf, it is executed once per pressure iteration in each cpr call; the number of times of pressure solution iteration is very large, and the number of executions in the decomposition phase and the solution phase are very different (for p-mpnf and bmpnf is 1:174 and 1:6) respectively. blas operation has good parallelism, but the computational density is very low (the ratio of the number of memory reads to the number of calculations), which is limited by the gpu bandwidth cannot achieve high performance; the reduction operation decreases rapidly with the parallelism of the algorithm. also can’t have a good performance. the number of these two operations is affected by the number of gmres iterations. as the number of iterations increases, the number of blas and reduction operations performed per iteration increases linearly. therefore, the two occupy a large proportion in the algorithm time composition. in order to reduce the general blas and reduction operations for parallel performance, the bicgstab (stabilized double conjugate gradient method) solver can be used instead of gmres, which is used in conjunction with the pressure matrix solution. bicgstab is also a subspace iterative method for general matrices. experience has shown that gmres is more stable because it guarantees that the norm of the residual is monotonically decreasing after each iteration. however, bicgstab is different from gmres in that the number of blas and reduction operations does not change with the number of iterations in each iteration. for the problem that the average number of iterations is close to 30, the ratio of these two operations is greatly reduced. on the other hand, the stress solving phase in cpr does not require high accuracy. convergence criteria for relaxation stress solving can be considered to reduce the number of iterations of excessive pressure system solutions. 0 20 40 60 80 100 120 0 20 40 60 80 100 120 nz = 611 figure 9. 2.5d grid 6 color oscillating format dyeing corresponding matrix structure table 1. comparison of iteration speeds of different cpr combinations (3c, 4c, 5c represent the number of dyes, respectively, 3, 4, 5) study number cpr combination newton iterations linear iterations pressure solution iteration number average number of iterations per iteration 1 amg+bilu0 147 667 667 1.0 2 ilu0+bilu0 145 786 25073 31.9 3 p-mpnf+bilu0(4c) 147 871 25770 29.6 4 p-mpnf+b-mpnf(4c) 147 869 25635 29.5 5 p-mpnf+b-mpnf(3c) 147 941 28412 30.2 6 p-mpnf+b-mpnf(5c) 147 840 24501 29.2 255gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation the pressure solution was changed to bicgstab and the pressure convergence criterion was relaxed to 0.5 and tested again. the comparison results are shown in table 2. the number of newton iterations is constant, the number of linear solutions is increased by 40%, but the average number of iterations of the solution is reduced to 11 times, and the total time consumption is reduced by more than 40%. the increase of the number of linear solutions is caused by the lower precision pressure solution, which increases the time consumption of the outer gmres and b-mpnf, but also reduces the average pressure solution iteration number, and finally reduces the total pressure solution iterations. . in addition, it should be noted that the total time consumption of mpnf has not decreased, but has increased. on the one hand, bicgstab will call p-mpnf solution twice in one iteration, so that the actual number of p-mpnf calls in two tests is close; the number of b-mpnf calls increases as the number of linear iterations increases, and the time spent on a single call is also higher than that of p-mpnf. thanks to the use of bicgstab, the proportion of time for reduction and blas operations decreased from 59% to 21%. parallel performance the general proportion of calculated components is reduced, which is the reason for the significant reduction in total time consumption. under the new setting, the mpnf solution stage time consumption is 44.7% (mainly p-mpnf solution stage, more than 90%), which occupies the main part. at the same time, the proportion of the remaining serial part in the linear solution algorithm is also more significant (close to 20 %). although this ratio is very high (resulting in an acceleration ratio of up to 5), the main time composition of the serial part is derived from the pressure matrix decomposition process. for more complex problems, a pressure matrix decomposition will correspond to more linear iterations, corresponding strings. the proportion of the line portion will also decrease. spe10 study test this section uses the complete spe10 model for parallel algorithm testing. the complete spe10 model consists of 60 x 220 x 85 with a total of 1.1 million grids. the model hole permeation field height is heterogeneous and is a standard example designed to evaluate the performance of linear solvers. the fluid uses two phases of oil and water. the model consists of four production wells located at the four corners of the model and one injection well located in the center of the model. all wells are shot through all formations. the test platform is the same as before. this paper implements a serial version of the two-stage mpnf algorithm as a reference to measure the acceleration of parallel implementation. at the same time, the gmres method preprocessed by cpr (amg+bilu0) under the serial algorithm is also added to compare the advantages of the parallel algorithm compared with the current mainstream serial algorithm. the comparison is classified according to the total time taken by the linear solution, the time-consuming of the pressure matrix, the second-stage pre-processing and the outer gmres time-consuming. the parallel version of mpnf achieved an acceleration ratio of 19.8 over the entire linear solution phase, and an acceleration ratio of 23.9 and 27.0 was obtained in the pressure solution phase and the second phase pretreatment and the outer gmres, respectively. since the total speedup includes additional overhead and other parts that cannot be paralleled, it is lower than the other two parts; the second stage preprocessing and outer gmres are similar in composition to the stress solution stage (average gmres iterations are 12 times) ), but thanks to the use of the block matrix, the access limit is smaller, so the acceleration ratio is higher than the pressure solution phase. the serial version of mpnf is significantly slower than the serial version of the mainstream solution combination. among them, the velocity matrix solution speed is very different, which verifies that amg is much better than mpnf in the serial hardware. the pressure matrix solution occupies most of the time (more than 70%) of the linear solution, so although the time consumption of the second stage preprocessing is not much different (about 2 times, the number of cpr calls caused by inaccurate pressure solution increases) cause), but the total linear solution time is still much higher. by comparing the latter two, it can be seen that the shortcomings of the mpnf preprocessing method itself can be compensated for on parallel hardware due to better parallelism. the parallel version of p-mpnf is more than twice as fast as amg, and the serial version of b-mpnf is more than 10 times faster than bilu0. the total linear solution speed is nearly three times faster. this proves the practicability of this parallel solution algorithm, and it can also bring about a real speed increase compared with the optimal strategy of the serial version. unstructured equivalent spe10 study test this section performs a crack-free layered model mpnf method test. the unstructured mesh (see figure 10) model is generated based on the spe10 example, keeping the number of layers unchanged, and the horizontal direction is re-divided into 12,680 meshes, and the attribute field is mapped according to the closest distance of the center point of the mesh. the unstructured model has 1,077,800 grids. in parallel mpnf vs. serial mpnf acceleration ratio, the structural model has decreased. this is because this study uses 6-color shading, and the number of parallel tasks drops, failing to take full advantage of hardware performance. the average number of iterations for the pressure solution is 20, which is more than the structured model, reflecting the consequences of ignoring elements outside the strip. but the parallel version of mpnf still has an advantage over the amg+bilu0 combination. table 2 . comparison of iterative speeds between different pressure solver selections and convergence errors study setting newton iterations linear iterations pressure solution iteration number average number of iterations per iteration total time consumption / s mpnf total time consumption / s p-gmres convp<0.1 147 869 25635 29.5 48.7 11.2 bicgstab convp<0.5 147 1233 13551 11.0 27.5 12.4 figure 10. layered model generated by spe10 study and dyeing results 256 xin shi, yuan di 2.5d non-structural example test with cracks next is a layered model test with cracks. the model is divided into 10 layers vertically, with a total of 26,418 grids in the horizontal direction, including 417 crack grids. the fracture permeability was 1000 md and the matrix permeability was 1.0 md on average. there is a production well in the two corners of the injection well. the fluid uses two phases of oil and water. the model is shown in figure 11. the model uses 6 color stains. the convergence speed information of the mpnf method is shown in table 3. although the elements outside the tridiagonal strip are rounded off, the overall convergence performance of the cracked example is still relatively stable. the number of linear iterations is not much higher; the average pressure matrix solution iterations is slightly higher than the regular grid, but it is also within the acceptance range. the speedup results are similar to the previous few examples. it is worth noting that the advantage of mpnf compared to amg+bilu0 combination is more obvious in this example, mainly because the total number of linear iterations is not much different between the two examples. conclusion the linear solution part is the most time-consuming part of reservoir numerical simulation. this paper mainly discusses how to use the gpu as a multi-core parallel architecture to parallelize the linear solution algorithm. parallel algorithm in the cpr two-step preprocessing solution framework, the mpnf method developed by the classical nf method is selected as the two-step preprocessor selection. the algorithm proposes a data rearrangement method for the two-stage mpnf preprocessing algorithm to ensure the merge access; the block-based b-ellpack structure is used to accelerate the spmv operation in the global gmres algorithm; bicgstab is used as the pressure stage solver. the weaker pressure is used to solve the convergence criterion to reduce the proportion of the less conservative reduction operation in the algorithm. through these optimization methods, the parallel solution algorithm achieves a good parallel effect. compared with the serial mpnf method, the complete spe10 algorithm obtains a speed ratio of more than ten times. compared with the mainstream linear algorithm, it also obtains 2-3. the speedup ratio is doubled. in addition, this paper uses the general dyeing algorithm to extend the mpnf method applicable to cpr two-stage preprocessing to 2.5 unstructured grid model, which expands the practicability of the mpnf method. in the 2.5d unstructured grid, the dyeing process requires more dyeing numbers, and there are a small number of elements located outside the three diagonal strips. these elements are ignored in mpnf preprocessing, and the parallel effect may be inferior to the structured net. grid. the algorithm was tested on a custom example containing discrete cracks and an example derived from spe10 mapping (both 2.5d unstructured grid studies). the results were shown on a 2.5d unstructured grid. the mpnf algorithm still has obvious advantages over the serial algorithm. references appleyard, j., appleyard, j., wakefield, m., & desitter, a. 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(1983). algebraic multigrid (amg): experiences and comparisons. applied mathematics and computation, 13(3): 419-451. table 3. comparison of iterative speeds of layered models with cracks solution strategy newton iterations linear iterations average number of iterations per iteration amg+bilu0 573 9166 1 p-mpnf+b-mpnf 571 9953 14.8 a) matrix part b) crack section figure 11. layered model with crack 257gpu parallelization nested decomposition method for solving large linear systems in reservoir numerical simulation van der vorst, h. a. (1992). bi-cgstab: a fast and smoothly converging variant of bi-cg for the solution of nonsymmetric linear systems. siam journal on scientific and statistical computing, 13, 631-644. wallis, j. r., kendall, r. p., & little, t. e. (1985). constrained residual acceleration of conjugate residual methods. spe reservoir simulation symposium. society of petroleum engineers. wallis, j. r. (1983). incomplete gaussian elimination as a preconditioning for generalized conjugate gradient acceleration. spe reservoir simulation symposium. society of petroleum engineers. zhou, y. (2012). parallel general-purpose reservoir simulation with coupled reservoir models and multisegment wells. stanford university. an inverse mechanics model has been built under work face mining conditions, according to the law of working surface pressure distribution, to explore the mechanical characteristics and stability of the reverse fault under the influence of mining. as a result, a theoretical calculation equation of the normal and shear stresses in the fault zone has been deduced to obtain the stress variation rule between the working surface and the fault layer, under distance conditions of 10, 30, 50, and 70m. with distance conditions of 10 and 30m, the working surface mining stress had a noticeable effect on the reverse fault, resulting in a changing trend of firstly increasing, then decreasing, and increasing again in the normal and shear stresses of the fault zone as a whole. with distance conditions of 50 and 70m, the working face mining stress had little effect on the reverse fault; furthermore, the normal and shear stresses exhibited a changing trend of gradually increasing. at a later stage, a simulation of the above distance plans was conducted using the flac3d numerical simulation software. the results demonstrated that the influence range of the mining stress on the working face under the spacing distances of 10 and 30m included the fault zone, while under the distance conditions of 50 and 70m, the fault zone was excluded. on this basis, the fault zone stability was analyzed under four types of spacing conditions using the mohr-coulomb theory rule and fault activation determination. it is concluded that the fault zone stability was high, with increasing distances between the working face and fault zone. the least sufficient stability was located near the working face, where the fault zone stability was so weak that it is likely to result in impact fracture. abstract keywords: reverse fault; normal stress; shear stress; fault instability; rock burst; analysis of mechanical characteristics and instability law of fault under the influence of mining issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.72249 yuanhui li1,rui zhou1,2* 1key laboratory of ministry of education on safe mining of deep metal mines, northeastern university, shenyang 110819, p.r. china. 2china coal technology engineering group, shenyang research institute, shenyang 110016, p.r. china. * email of corresponding author: 3416197979@qq.com earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 139-144 se construyó un modelo mecánico inverso bajo las condiciones de un frente de trabajo minero, de acuerdo con las condiciones de distribución de presión en la superficie de operación, para explorar las características mecánicas y la estabilidad de la falla inversa en estas áreas de explotación minera. como resultado, se dedujo una ecuación teórica para calcular el esfuerzo normal y el esfuerzo cortante en la zona de falla y con el fin de obtener el régimen de variación de esfuerzos entre la superficie de trabajo y la capa de la falla a distancias de 10, 30, 50 y 70 metros. a 10 y 30 metros de distancia, el esfuerzo en la zona de trabajo tiene un efecto notorio en la falla inversa, lo que significa un cambio en la tendencia de los esfuerzos normal y cortante, que primero se incrementan, decrecen y nuevamente aumentan en toda la zona de la falla. a 50 y 70 metros, el esfuerzo en el frente de trabajo tiene poco efecto en la falla inversa; además, los esfuerzos normal y cortante mostraron una tendencia cambiante de crecimiento gradual. por último, se realizó una simulación en cada una de las distancias con el software de simulación numérica flac3d. los resultados demuestran que el rango de influencia del esfuerzo en el frente de trabajo a distancia de 10 y 30 metros incluyen la zona de falla, mientras que a 50 y 70 metros la zona de falla está excluida. de acuerdo con estos resultados, se analizó la estabilidad de la zona de falla en cuatro condiciones de espacio con la teoría de mohr-coulomb y la determinación de activación de falla. se concluye que la estabilidad en la zona de falla es mayor con el incremento de las distancias entre el frente de trabajo y la zona de falla. el menor punto de estabilidad suficiente se ubicó cerca del frente de trabajo, donde la estabilidad de la zona de falla es tan baja que podría derivar en una fractura de impacto. resumen palabras clave: falla inversa; esfuerzo normal; esfuerzo cortante; inestabilidad de falla; explosión rocosa. record manuscript received: 14/11/2017 accepted for publication: 11/06/2018 how to cite item li, y., & zhou, r. (2018). analysis of mechanical characteristics and instability law of fault under the influence of mining. earth sciences research journal, 22(2), 139-144. doi: http://dx.doi.org/10.15446/esrj.v22n2.72249 análisis de las características mecánicas y las condiciones de inestabilidad de fallas en áreas de explotación minera m in in g e n g in e e r in g 140 yuanhui li, rui zhou 1 introduction coal reserves of 1.48 trillion tons have been identified in china, ranking third in the world. however, coal seams are buried deeply, with many folds, faults, and other geological structures, particularly in the tectonic region of fault. the damaged continuity of coal and rock seams poses a challenge to coal mining (tan et al., 2011; xiao et al., 2016; sufiyan et al., 2018). moreover, fault instability generated by the coal mining influence in areas affected by the fault structure may result in disasters such as rock bursts (li et al., 2014; rahim et al., 2018; mahmood et al., 2018), water inrush through the fault (lamoreaux, wu, & zhou, 2014; khanchoul et al., 2018), and coal and gas outbursts (díaz-aguado, & gonzález-nicieza, 2007). the changing law of mining mechanics under the influence of fault has always been the focus of rock burst preve, particularly in the prevention and control of rock burst. gibowicz (1984) and mcgarr (1984) determined that an ore quake is similar to a natural earthquake source mechanism, and shear slip in the fault zone mainly causes large-energy mine earthquakes. the majority of theories regarding the seismic source mechanism can be used for mine earthquakes. joughin and jager (1984), potgieter and roering (1984) discovered that large-energy mine earthquakes are closely related to shear slip in their study of a mine earthquake that occurred in a particular south african gold mine. chinese scholars jiang and liu (2010) proposed a correlation between different fault types and rock burst occurrences. these are more likely to occur in the pressure bump when the working surface is overtaken by the reverse fault, according to data prepared by jiang et al. (2014). it is concluded that a rock burst induced by the reverse fault is more likely to accumulate a significant amount of elastic energy with strong destruction, upon conducting a statistical analysis of a rock burst disaster induced by the reverse fault. through micro-seismic, electromagnetic radiation and other equipment, it is possible to monitor the fact that a mechanical indication in some form occurs before the rock burst induced by the reverse fault, and such that evidence serves as identification of fault instability (veeraragavan et al., 2018; nwankwoala & ememu, 2018). although various scholars have studied the phenomenon, the majority of studies have focused mainly on field monitoring data analysis. less analysis has carried out on the theoretical causes of this mechanical indication, and the changing law of the standard and shear stresses causing the evidence in the fault zone. therefore, according to the actual geological characteristics of the mining face under the influence of a mining area’s reverse fault, the distribution law of the supporting pressure in the coal seam direction is analyzed in this paper (sudhakaran et al., 2018; guan leong et al., 2018). moreover, a mechanics model of the stope has been established under the reverse fault to explore the pressure changing laws and instability of the reverse fault under the mining influence, providing a reference for the prevention and control of the pressure bump induced by the reverse mistake (wali et al., 2018). 2. materials and methods 2.1. mechanics model of the working surface tending to the reverse fault the tendency of the mining working face of the coal seam in a mining area to be affected by the f16 reverse fault was evaluated. the thickness of the coal seam is 5m; the upper roof of the coal seam is a conglomerate of 190m in diameter, and the fault angle is 60°. after the mining face, the overlying strata are collapsed. assuming that the stratigraphic coal and rock mass are an isotropic elastic body, the top plate plane would be taken above the working surface. the pressure distribution of the overlying strata trending along the working surface is shown in fig. 1 (qian, 2003; thiruchelvam et al., 2018; tair & dell, 2018). the pressure of the -∞ to a and i to + ∞ zones indicate the stress of the original rock γh. ai is the stress affecting the working face zone, where ab, bc, fg, and gi are the stress increasing zones (simplified as a linear increase). furthermore, cd, de, and ef are the stress reduction zones (simplified as a linear decrease). the intersection of the fault and the roof plane is the origin o, while the angle with the y-axis is the dip angle θ of the fault (asghar et al., 2018). the working surface is at point c, the working face stress concentration coefficient is k1, and the working face stress reduction coefficient is k2. according to elastic mechanics theory, the stress of a point m (y, x) on the fault can be obtained as follows: figure 1. pressure distribution along working face where σx is the tension of the x-direction in the fault zone, mpa; σy is the stress of the y-direction in the fault zone; τxy is the shear stress of the fault zone, mpa; γ is the rock mass density, kg/m3; h is the buried depth of the working face roof, m; ξ is a variable; g is the gravity acceleration, n/kg; and k1 and k2 are the stress concentration and reduction coefficient, respectively. the specific meanings of the length units a, b, c, d, e, f, g, and i are shown in fig. 1. the usual and shear stresses of the fault zone can be calculated as follows: (4) (5) 141analysis of mechanical characteristics and instability law of fault under the influence of mining table 1. parameter selection figure 2. normal stress distribution of the fault zone under different spacing distances figure 4. numerical model table 2. rock mechanical parameters figure 3. shear stress distribution of the fault zone under different spacing distances where σn is the general stress of the fault zone; τn is the shear stress of the fault zone, mpa; and θ is the dip angle, °. because it is difficult to find an analytical solution with the above calculation equations, relevant parameters are assigned according to the actual coal mine site conditions, to analyze the stress distribution in the fault zone. the specific parameters selected are shown in table 1. the parameters in table 1 are substituted into equations (4) and (5) to obtain the stress distribution of the normal and shear stresses of the fault zone under spacing distances of 10, 30, 50, and 70m between the working surface and fault, as shown in figure 2 and figure 3. it can be observed from figure 2 that the maximum normal stress value in the fault zone appears at a distance of 100m between the fault zone and origin o; that is, the closest position to the working face. here, the normal stress of the fault zone is relatively large at most places under the condition that the working surface is 10m away from the fault; however, the stress is relatively small when the working surface is 70m away from the fault. at a distance of 30 to 40m, the normal stress is relatively large under the condition that the working surface is 70m away from the fault, while it is relatively small under the distance of 30m. it happens because the mining working surface is causing additional normal stress in the fault zone, and the closer the distance to the fault, the more significant the influence will be. it can be seen from figure 3 that the maximum shear stress value in the fault zone is at 100m from the origin o under different distances between the working surface and fault; that is, the closest position to the working face. here, the shear stress of the fault zone is relatively large at most places under the condition that the working surface is 10m away from the fault; however, the stress is relatively small where the working surface is 70m away from the fault. likewise, the shear stress is relatively large under the condition that the working surface is 70m away from the fault, while it is relatively small at 30m, as a result of additional working surface shear stress. combining figure 2 with figure 3, it can be seen that the working face mining stress has a significant effect on the fault zone, causing the changing trend of the normal and shear stresses of the fault zone first increasing and then decreasing, under the conditions that the working surface is 10 and 30m from the fault. the working face mining stress has a small effect on the fault zone, resulting in the changing trend of the normal and shear stresses of the fault zone increasing gradually, under distance conditions of 50 and 70m. 2.2. numerical model analysis a working face-mining-reverse-fault-simulation model was established by utilizing the numerical simulation software flac3d, to better analyze the variation law of the fault zone under the influence of mining, as well as to verify the fault mechanics model analysis. the mining range of the model is 100m in length, 540m in trend length, and 100m in height, as illustrated in figure 4. the working surface excavation process is simulated using dummy cells, using the mohr-coulomb model for simulating the mechanical changes in the coal and rock mass. based on the geological survey and related rock mechanics test results, the rock mass size effect is taken into account. the parameters of each rock formation adopted in the simulating calculations are displayed in table 2. based on the theoretical calculation scheme, a simulation scheme with distances of 10, 30, 50, and 70m between the working face and fault is established when the fault dip angle is 60°. the cross-section of the longitudinal section of the working face mining position and the roof plate horizontal position are intercepted to analyze the effects of the distance between the mining working surface and fault. the simulation results are displayed in figure 5. 142 yuanhui li, rui zhou (a) stress distribution with a distance of 10m between working face and fault (a) variation law of fault instability coefficient (b) stress distribution with a distance of 30m between working face and fault (b) variation law of shear stress with normal stress (c) stress distribution with a distance of 50m between working face and fault (d) stress distribution with a distance of 70m between working face and fault figure 5. stress distribution with different distances between working face and fault figure 6. variation law of fault with different distances it can be seen from the longitudinal section in figure 5 that the stress concentration lies in the roof plate position on the front face of the working surface under four distance conditions between the working surface and fault. the stress in this position should be the maximum; the closer the distance to the fault, the larger the additional stress generated in the fault will be. this point is the same as the maximum position of the fault zone in the theoretical analysis. it can be seen from the horizontal cross-section in figure 5 that when the working surface is 10 and 30m, respectively, from the fault, the roof plate stress range should cover the position of the fault zone, apparently affecting the fault zone stress. under the distance conditions of 50 and 70m, the working surface stress range excludes the fault zone position. according to the numerical simulation results, the influence of the mining stress on the fault zone stress is gradually reduced with an increase in the working distance between the working face and fault, which is also the case in the theoretical calculation. therefore, when combining the elastic mechanic’s theory and numerical simulation results, it can be concluded that the working surface trending mechanics model of the working fault in the working face can be effectively reflected by the variation law of the fault zone stress. 2.3 stability analysis of reverse fault zone two methods for the mohr-coulomb strength criterion and fault activation determination were selected to analyze the fault stability. in determining the strength criterion, the fault friction angle is selected as 10°, and the cohesion is 0.1mpa. since the friction angle and cohesive force of the fault are determined according to the empirical value, and the characteristics of each fault in the fault zone differ, the qualitative analysis could only be conducted on the possibility of fault instability, rather than determining whether the fault was stabilized. the fault instability coefficient is set as α, according to the mohr-coulomb theory strength criterion equation, to facilitate analysis, as is showed in formula 6. the larger the value of α, the higher the probability of fault instability; the smaller the value of α, the more stable the fault will be. the fault activation determination method is conducted according to the ratio between the shear and normal stress of the fault zone; the more significant the ratio, the higher the possibility of instability. (6) where cf is the fault cohesion, mpa, and ɸ is the internal friction angle, °. employing this calculation, when the distances between the working surface and fault are 10, 30, 50, and 70m, respectively, the ratio variation law among the instability coefficient of the reverse fault, the shear stress, and the normal stress can be obtained, as indicated in fig. 6. it can be observed from figure 6 that the fault stability results obtained by the two methods are almost the same. the position with poor stability in the fault zone is located at a distance of 100m between the fault zone and origin o; that is, the closest position to the working face. here, the poorest fault stability is under the distance condition of 10m between the working surface and fault, 143analysis of mechanical characteristics and instability law of fault under the influence of mining figure 7. layout of 25011 working face while the highest fault stability can be obtained at a distance of 70m. under distance conditions of 50 and 70m between the working face and fault, the working face mining stress has no apparent effect on the fault zone; thus, the fault zone stability is gradually increased. under distance conditions of 10 and 30m between the working face and fault, the working face mining stress results in apparent additional pressure on the fault zone, causing its overall stability to exhibit a trend of first increasing, then decreasing and growing again. moreover, negative values exist in the calculation of the ratio of the instability coefficient, shear stress, and normal stress in the fault zone. the physical meaning of the fault zone is that the stress leading position of the fault is different from other areas. in this case, the possibility of fault fracture is also intensified, in opposition to fault stability. therefore, such a stress state should be avoided in actual production work. in summary, it is clear that the working face mining pressure significantly influences the fault stability. moreover, the closer the fault, the higher this influence will be. therefore, the minimum spacing between the working face and fault should firstly be determined in field work, to avoid the risk of fault instability caused by different stress directions in the fault zone. then, comprehensive consideration should be given to actual situations, such as roof plate support on site and economic benefits, to reasonably determine the width of the coal pillar between the working face and reverse fault. 2.4 analysis of actual situations on site the 25110 working face of a particular mine consists of fully mechanized caving faces, located at the level of -445m of the mine 25 mining area. the 2-1 coal seam was evacuated on the working surface. the coal ranges from 7.4 to 13.8m in thickness, moving towards 112 to 27° with a tendency of 202 to 217° and a dip angle of 12°, and belongs to the slowly inclined thick coal seam. the lower lane of the working surface of 25011 is near the f16 reverse fault, as indicated in figure 7. according to the actual situation on site, a total of eight rock burst events occurred in the 25110 working face during the mining process, all within the lower lane of 25011. the first to seventh events occurred during the tunnelling process, while the eighth occurred during the mining process. therefore, we can conclude that the 25011 lower lane approaching the reverse fault is a major area for rockburst occurrence. furthermore, as the mining distance during the early stage approached the reverse fault, one rockburst event was also caused during the mining process; thereafter, no such event occurred. thus, it can be determined that the closer the dip distance of the working face to the fault, the more easily the rock burst will occur, which is consistent with previous analysis results. 3 conclusions (1) based on the law of the working face dip pressure distribution, a reverse fault mechanics analysis model was established to deduce the calculation equation of the normal and shear stresses of the fault zone under the influence of the mining working face. it could be observed that the normal and shear stresses of the fault zone exhibited a changing trend of first increasing and then decreasing under the conditions in which the working surface was 10 and 30m from the fault. the normal and shear stresses of the fault zone were exhibited a changing trend of increasing gradually under distance conditions of 50 and 70m between the working surface and fault. the stress distribution of the longitudinal and transverse horizontal sections under spacing conditions of 10, 30, 50, and 70m between the working and fault distance were obtained by means of the flac3d numerical simulation method. the results indicate that the influence range of the mining stress on the working face at spacing distances of 10 and 30m included the fault zone, while at spacing distances of 50 and 70m, the fault zone was excluded. consequently, it can be concluded that the mining stress not only affects the fault zone, but also changes the variation trend of its normal and shear stresses under spacing conditions of 10 and 30m; however, there was less influence under spacing conditions of 50 and 70m. (2) two mohr-coulomb theory rule methods and fault activation determination were applied to analyse the fault zone stability. when the working surface was 10 and 30m away from the fault, negative values existed in the calculation of the instability coefficient, shear stress, and normal stress ratio in the fault zone. the physical meaning of the fault zone is that the stress leading position of the fault is different from other positions. in such a case, the possibility of fault fracture is also intensified, resulting in a possibility of rock burst. when the spacing was 50 and 70m, the fault zone stability exhibited an overall changing trend of gradually increasing. moreover, the most dangerous fault zone positions under four spacing conditions were located at the closest position to the fault. (3) based on actual site conditions, rock burst was more likely to occur in the lanes approaching the fault zone, indicating that the spacing between the working surface and fault zone was an important factor affecting the occurrence of rock burst. therefore, determining a reasonable distance between the mining working face and inverse fault zone was a foundation for preventing rock burst. acknowledgments this research was financially supported by the yong foundation of china coal technology engineering group (no. 2014qn003); the state key research development program of china (no. 2016yfc0801605); the national natural science foundation of china (no. 51674061); and the state key research development program of china (no. 2016yfc0600706). references asghar, z., ali, w., nasir, a., & arshad, a. 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(2016). rock mass failure mechanisms during the evolution process of rockbursts in tunnels. international journal of rock mechanics and mining sciences, 83,174-181. keywords: soil losses; erosivity; usle; slopes; pineapple; palabras clave: pérdidas del suelo; erosividad; usle; laderas; piña; how to cite item martínez-lópez, c., rivera-paja, a. o., & menjivar j. c. (2021). susceptibility to erosion risks in soils dedicated to pineapple cultivation in the department of valle del cauca, colombia. earth sciences research journal, 25(2), 201-206. doi: https://doi.org/10.15446/esrj. v25n2.84601 in colombia, almost half of the soils are prone to erosion, where valle del cauca is one of the most affected departments by its severity. in order to identify the susceptibility to erosion risks in terms of the rainfall erosivity and the incidence of the topographic factor in the main pineapple producing municipalities of the department, a study was carried out that considered the historical climatological information of more than 50 years, for which all available weather stations in the area were selected. the erosivity index (r – ei30), the modified fournier index (mfi), and the topographic factor (ls) were estimated. the results indicate that the average mfi values ranged between (91.31 and 582.79) which correspond to the moderate, high and very high categories, the results of r-ei30 presented moderate, high, very high and extremely severe values (5076.91 mj.mm.ha-1 22718.83 mj.mm.ha-1), while the highest values of the topographic factor (with slopes up to 81°) coincide with the upper part of the river basin dagua, predominantly in the municipality of dagua. these values indicate that the soils in the area are susceptible to erosion risks depending on the rainfall erosivity and the topographic factor at a moderate, high and very high level, which can generate significant soil losses, and therefore they should be considered when establishing a pineapple crop. abstract susceptibility to erosion risks in soils dedicated to pineapple cultivation in the department of valle del cauca, colombia susceptibilidad a riesgos de erosión en suelos dedicados al cultivo de piña en el departamento del valle del cauca, colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.84601 en colombia casi la mitad de los suelos son propensos a la erosión. el valle del cauca es uno de los departamentos más afectados respecto a la severidad de la misma. con el objetivo de identificar la susceptibilidad a riesgos de erosión en términos de la erosividad de la lluvia y la incidencia del factor topográfico en los principales municipios productores de piña del departamento se realizó este estudio que contempló la información histórica climatológica de más de 50 años, para lo cual se seleccionaron todas las estaciones meteorológicas disponibles en la zona. se estimó el índice de erosividad (r–ei30), el índice modificado de fournier (imf), y el factor topográfico (ls). los resultados indican que los valores promedio del imf oscilaron entre 91,31 y 582,79, los cuales corresponden a las categorías moderada, alta y muy alta; los resultados de r-ei30 presentaron valores moderados, altos, muy altos y extremadamente severos (5076,91 mj.mm.ha-1 22718,83 mj.mm.ha-1), mientras que los mayores valores del factor topográfico (con pendientes de hasta 81°) coinciden con la parte alta de la cuenca del río dagua, predominando fundamentalmente en el municipio de dagua. estos valores indican que los suelos de la zona son susceptibles a riesgos por erosión en función de la erosividad de la lluvia y del factor topográfico en un nivel moderado, alto y muy alto, lo que puede generar importantes pérdidas de suelos, y por lo tanto deberían ser consideradas a la hora de establecer un cultivo de piña. resumen record manuscript received: 16/01/2020 accepted for publication: 11/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 201-206 carolina martínez lópez, albin osvaldo rivera paja, juan carlos menjivar flores universidad nacional de colombia *corresponding author: camartinezlo@unal.edu.co so il s https://doi.org/10.15446/esrj.v25n2.90210 https://doi.org/10.15446/esrj.v25n2.90210 https://doi.org/10.15446/esrj.v25n2.90210 mailto:camartinezlo@unal.edu.co 202 carolina martínez lópez, albin osvaldo rivera paja, juan carlos menjivar flores introduction land degradation defined by fao-unesco as the loss of its current and / or potential capacity to provide goods and services, is one of the major international concern. that is how the united nations points out that it is one of the biggest challenges for the sustainability of food production and water supply in the 21st century (gabriel, 2017), where agricultural practices and inappropriate land use have been the main causes of accelerated erosion in many developing countries (iaea, 2015). the tropics are one of the most affected areas where water and wind erosion are the most contributing factors, with reported values of 915 x 106 and 474 x 106 degraded hectares, respectively (lal, 1994; velasquez el at., 2007). erosion is understood as the process that involves the detachment, transport and deposition of soil particles (blanco-canqui et al., 2016). in this regard, it is estimated that between 25.000 million and 40 billion kilograms of soil surface layer are eroded each year, and if this trend continues, it could represent losses of 1.5 million square kilometres of land suitable for agriculture by 2050 (fao, 2019). colombia is no exception, since almost half of the soils are prone to erosion (ideam & udca, 2015), but the problem is aggravated if one considers that, of the 53.6 million hectares intervened in the country, 32,7 million have an inappropriate use (igac, 2019), which puts their productive capacity at risk. in colombia, the high vulnerability to erosion is not only favored by its geomorphological and climatic characteristics (idiger, 2019) (rivera et al., 2010), but also by processes such as deforestation, soil cover removal, excessive tillage, poorly designed irrigation or drainage channels, and roads without adequate works (ideam & udca, 2015). likewise, it has been pointed out that the highest degradation rates appear with tillage practices in favor of the slope (sagredo, 2005), and when intense rainfall impacts bare soils with low structural stability (castillo, 2004). according to the national study of soil degradation in colombia, conducted by ideam in 2015, valle del cauca is one of the departments most affected by the severity of erosion, whose figure corresponds to 5,4%, and this is mostly explained by the broad extension of the hillside area that it has (970,702 ha), with soils mainly corresponding to inceptisols, andisols, entisols and alfisols (cvc, 2013), the vast majority of which are susceptible to erosion (tafur et al., 2006). in this regard, in the dagua river basin, one of the main agricultural activities in hillside areas is pine growing (loaiza et al., 2012), so most of its soils have been used for this crop, which is generally established on any type of soil and slope, without considering how vulnerable the soils are to erosion due to the topography of the land and how aggressive the rains are. in addition to this, farmers, in order to avoid the occurrence of certain diseases related to high humidity, sow pineapple as a clean crop (absence of plant cover), establish it in favor of the slope and worse still do not use conservation practices, which favors erosive processes and the decrease of productive potential (gómez et al., 2006) (montiel, 2015) (rivera et al., n.d). thus, erosion losses have been reported in pineapple soils of 49 to 142 t/ha/ year, data corresponding to severe erosion (álvarez & peña, 2013). likewise, it has been found that in low fertility soils, the decrease in nutrients n, fe and zn, as well as organic matter, has been considerable (garcía, 2008), so that the productive potential of these soils in areas of high precipitation could fall in less than 20 years (quijandria et al., 1997). thus, in pineapple growing municipalities such as dagua with slopes greater than 20% (chaparro et al., 2012), heavy rains have induced landslides for many years (lozano, 2000), and the appearance of gullies have been reported (rivera et al., n.d). currently, the largest producer of pineapple in the department is the municipality of dagua, since for the year 2018 a production of 46,680 t, was reported. then there are the municipalities of restrepo with 35,700 t, vijes with 14,400 t, and la cumbre with 14,179 t (agronet, 2019). therefore, the objective of this study is to identify the susceptibility to erosion risks in soils of the main pineapple producing municipalities, located in hillside areas of the valle del cauca department, based on climatic and geomorphological characteristics (rainfall erosivity and the slopes). this work is a tool to determine in which areas to sow and what type of management should be given to soils and crops to avoid degradation. materials and methods the present work was carried out on an area located in the valle del cauca, whose geographic coordinates of the centroid are 3°40’34.324” n and 76°39’20.266” w, which covers the municipalities of dagua, restrepo, la cumbre and vijes, as the main pineapple producers of the department. the study area has a surface area of 138,800,885 ha, of which 61.38% corresponds to the municipality of dagua, 14,45% to the municipality of restrepo, 16% to the municipality of la cumbre and 8,17% to the municipality of vijes, being mostly in a mountain area, according to the basic secondary information of the thematic cartography (geomorphology) of the regional autonomous corporation of valle del cauca (cvc). politically, the territory is divided into 52 districts, in which agricultural exploitation activities are mainly carried out (loaiza et al., 2012) (gonzalez et al., 2016). the geographic information was provided by the cvc (2013), and is available in the corporate geographic advanced viewer, while the base mapping was downloaded from open data of the agustín codazzi geographic institute (igac). the precipitation of the study area was characterized by historical information available in thirteen (13) meteorological stations belonging to the cvc, which have started operating in several years ranging between 1966 and 1986, currently remaining active (table 1). rainfall erosivity the dagua river basin, according to a study carried out by the university of valle (2016), has been morphogenetically cataloged as a “hydrological system with rapid response and torrential characteristics”, which presents table 1. location of the cvc weather stations. name municipality basin coordinate start date x y aguacatal-montebello cali cali 1058549.999 877405.6117 jun-30-1969 buenos aires yotoco yotoco 1071854.129 916464.6178 may-31-1971 costa rica ginebra sabaletas 1094047.494 907024.5997 feb-28-1975 dagua dagua dagua 1043265.57 895668.4842 oct-31-1983 dapa yumbo arroyohondo 1058101.811 885462.6364 may-31-1971 diguita dagua anchicaya 1033515.712 882755.42 mar-31-1971 guachazambolo palmira guachal 1070380.2995 878515.6218 mar-31-1973 matapalo palmira guachal 1071509.964 887218.9652 sep-30-1972 riobravo calima calima 1058104.345 930664.9313 mar-31-1966 san pablo cali cali 1050759.763 880677.3158 nov-30-1969 san vicente jamundí jamundí 1050137.334 853301.2787 apr-30-1971 triana buenaventura dagua 1030107.961 918238.2662 may-31-1982 yumbillo yumbo yumbo 1058618.72 887682.2518 mar-31-1986 203susceptibility to erosion risks in soils dedicated to pineapple cultivation in the department of valle del cauca, colombia precipitation levels that range from 900 to 8.000 mm / year, an aspect that contrasts with the high soil losses, because of the steep slopes, typical of the region (aguirre et al., 2017). on the other hand, the area presents a monomodal rainfall regime, with a period of intense rainfall between the months of april and december, and a period where rainfall decreases, between the months of january-february-march, where the lowest value of average precipitation occurs in the month of march, while the maximum value is recorded in october (cvc, 2017). the modified fournier index (mfi) was used to calculate the rainfall erosivity, which corresponds to the monthly average proposed by arnoldus (1977) (equation 1), and which has been widely used in the colombian coffee zone (ramirez et al, 2007; echeverri & obando, 2010; lince & castro, 2015). once the mfi values were obtained, they were replaced as an independent variable to get the ei30 values, as suggested by ramirez et al (2007) (equation 2). for equation 2 the average mfi values were used. imf p p i i t = = ∑ 1 12 2 (1) where: mfi = modified fournier index (mm) pi = average monthly rainfall (mm) pt = average annual rainfall (mm) ei imf30 12 1 38 4 28 3= ( ) +∑ , , (2) where: mfi = modified fournier index in mm ei30 = erosivity in mj.mm.ha-1 the mfi and r classification (ie30) are presented in tables 2 and 3. table 2. mfi classification. source of consultation: apaydin et al, 2006. mfi class rank classification 1 <60 very low 2 60-90 low 3 90-120 moderate 4 120-160 high 5 >160 very high table 3. the classification of r (ie30). source of consultation: rivera and gomez, 1991. class erosivity r (mj.mm.ha-1) classification 1 <1.000 natural 2 1.000-2.500 very low 3 2.500-5.000 low 4 5.000-7.500 moderate 5 7.500-10.000 high 6 10.000-15.000 very high 7 15.000-20.000 severe 8 >20.000 extremely severe the method of spatial autocorrelation taken into account for the spatialization of the r factor was the regularized spline, using erosivity records. the precipitation data are not found with a normal distribution, due to the limited existence of monitoring stations, therefore, it is necessary to use trend analysis such as the statistical method and non-parametric regression “regularized spline”. this method allows spatially visualizing the spatial distribution of precipitation in the study area in a smoothed way, achieving a better understanding of the distribution of precipitation in space for the given temporality, since it adjusts functions by portions to groups of data, producing a surface with minimal curvature. for this purpose, the arcgis software and the geostatistical analyst extension were used. topographic factor for the preparation of the slope map, a digital elevation model (dem) taken from alos palsar was used, with a resolution of 12.5 m and radiometric correction. the geoprocesses were carried out to determine the slope map. the topographic factor calculation (l and s) was made based on the expressions proposed by renard et al (1997) and desmet and govers (1996). for this purpose, a reclassification was carried out on the slope map, and specifically for the length of the slope (l), a dem fill was used, from which it was possible to obtain a slope raster (degrees), used to create the factor with equations 3, 4 and 5: β θ θ =     ( ) + sin sin 0 0896 3 0 0 560 8 . . .. (3) where: θ = it is the angle of the slope expressed in degrees. subsequently, the ß factor was obtained, using the raster calculator tool, and with these results it was possible to find the m factor, from equation 4: m = +( ) β β 1 (4) the l factor was calculated by equation 5: l a d a m x d i j i j m i j m m m, , , * * .( ) ( ) + ( ) + = +( ) − + ( ) 2 1 2 1 22 13 (5) where: a (i, j) [m] = is the unit input area to the input of a pixel (cell). d = is the size of the pixel. x = is the form correction factor. on the other hand, for the calculation of factor s, equation 6 was used: s i j i j i j i j , , , , . sin . tan . . sin( ) ( ) ( ) ( ) = + < − 10 8 0 03 0 09 16 8 β β β 00 5 0 09. tan ., β i j( ) ≥     (6) where: s (i, j) = slope of the coordinate factor (i, j) ß (i, j) = slope (degrees) with the coordinates (i, j) when obtaining the sub-factor raster of (s), the slope of the terrain will be found. after calculating l and s separately, the ls factor is found. areas susceptible to erosion risks once the maps of the rainfall erosivity and the topographic factor were obtained, these were used as input to establish the areas susceptible to erosion risks. to this end, the categories of very high and severe rainfall erosivity were unified, and were grouped into a single class (very high), the above with the purpose of generating five classification criteria that are comparable with those 204 carolina martínez lópez, albin osvaldo rivera paja, juan carlos menjivar flores obtained for the topographic factor. based on these criteria, vector analysis was carried out, and an overlay of maps to obtain the map of susceptibility to erosion risks based on rainfall erosivity and topographic factor, classifying them through a decision matrix into five variables which are: very low (1), low (2), moderate (3), high (4) and very high (5) (table 4). table 4. decision matrix ls r very low low moderate high very high very low 1 2 2 3 3 low 2 2 3 3 3 moderate 3 3 3 4 4 high 4 4 4 4 4 very high 5 5 5 5 5 results and discussion modified fournier index (mfi) the modified fournier index presented average values ranging between 87,79 and 582,79, which according to the classification proposed by apaydin et al (2006) correspond to a low and very high mfi classification. likewise, it is evident that the high erosivity values extend covering the entire northwestern to southwestern area of the study area, corresponding mainly to the municipality of dagua. from the historical analysis of the monthly precipitation data for the different weather stations, it was found that only matapalo showed low mfi values (87, 79), the rest of the stations presented values that range between (91,31 and 582,79), which are part of the moderate, high and very high categories (table 5). these values indicate that soils can be affected by intense climatic aggressiveness with rain as an erosive agent. although, there are no studies reported in this type of analysis for the valle del cauca region, there are investigations carried out in areas of neighbouring departments such as caldas and quindio, in which data with similar categories to those found (ramirez et al., 2009) (echeverri & obando, 2010) (castro et al., 2017). rainfall erosivity values between 3710.8 mj.mm.ha-1 and 22718.83 mj.mm.ha-1 were found which, according to the classification of (rivera & gómez, 1991), correspond to erosion levels between low and too severe. from the total of the study area, it was found that the municipalities of restrepo, la cumbre and vijes showed, depending on the area, a rainfall erosivity mainly of low category. in contrast, dagua presented severe, high and very high r values (table 3). the graph presents a trend that shows an increase in r values as it approaches the western flank. an analysis based on the information available at the stations indicates that, of the thirteen evaluated, five presented low rainfall erosivity values (3710.8 mj.mm.ha-14815.9 mj.mm. ha-1), and the rest were among moderate, high, very high and extremely severe values (5076.91 mj.mm.ha-1 – 22718.83 mj.mm.ha-1). the results found are similar to those reported for some departments of the andean area (ramírez et al., 2009) (castro et al., 2017) who primarily indicate erosivity classified as high and very high. these data are also consistent with the ranges indicated by pacheco et al (2019) who reported erosivity values between 3162.12 and 12683.72 mj.mm.ha-1 for an ecuadorian tropical region and liu et al (2020), who obtained average erosivity values for south america of 14378.64 mj.mm.ha-1 between the years 1980-1999 and of 10273.67 mj.mm.ha-1, between 2000-2017. figure 1 shows the distribution of the r factor in the area. topographic factor figure 2 demonstrates that the highest values of the topographic index in the basin of the study area extend again from the northwestern zone to the southwestern zone, mainly covering the municipality of dagua. this coincides with the upper part of the basin, that is, where the slopes that can reach 81° are located. these results are consistent with that indicated by cocuyame and salazar (2015), who point out that the highest values of the topographic index are found on the steepest slopes of the dagua river basin. map of susceptibility to erosion risks based on erosivity and topographic factor the map with the areas of susceptibility to erosion risks based on rainfall erosivity (r) and topographic factor (ls) is presented in figure 3. the results indicate that of the four municipalities evaluated, dagua has the highest susceptibility, the 74.46% of the area is under the high and very high categories, which corresponds to 63419.86 ha, followed by restrepo, where 77.38% of the area is under the moderate to high categories, which is equivalent to 15513.9 ha, while most of the municipalities of vijes and la cumbre show moderate to low degrees of susceptibility to erosion risks, taking into account that it is a region with high slopes and intense rainfall with short return periods (rivera et al., n.d) (alarcón & reyes, 2013). table 5. distribution of the mfi and the r factor. name municipality mfi total classification (mj.mm.ha-1) classification aguacatal-montebello cali 116.57 moderate 4815.94 low buenos aires yotoco 141.70 high 5780.81 moderate costa rica ginebra 174.65 very high 7046.01 high dagua dagua 103.03 moderate 4296.07 low dapa yumbo 123.37 high 5076.91 moderate diguita dagua 260.65 very high 10348.45 very high guachazambolo palmira 91.31 moderate 3845.74 low matapalo palmira 87.79 low 3710.80 low riobravo calima 241.73 very high 9622.12 high san pablo cali 143.11 high 5834.89 moderate san vicente jamundí 259.01 very high 10285.62 very high triana buenaventura 582.79 very high 22718.83 extremely severe yumbillo yumbo 109.78 moderate 4555.30 low 205susceptibility to erosion risks in soils dedicated to pineapple cultivation in the department of valle del cauca, colombia figure 3. susceptibility to erosion risks based on rainfall erosivity and ls factor. conclusions the rains in the region that covers the main pineapple producing municipalities in the department of valle del cauca have high erosive potential, being more relevant in the area of influence of the climatic seasons of the municipality of dagua. regarding the topographic factor, it is possible to notice that this index is higher towards the western flank of the study area, which corresponds mainly to the municipality of dagua, which aggravates the processes of water erosion, and which in turn explains the behaviour of the high and very high susceptibility to erosion risks presented by this municipality. it is important to ensure the sustainability of productive systems; new practices must be implemented by farmers, leading to promoting soil conservation and thus reducing erosive processes. it is relevant to promote practices such as maintaining a continuous vegetation cover, planting through the slope, the use of living barriers and ditches. likewise, it is recommended to carry out studies that allow the determination of erosivity with the support of geographic information systems and the influence of the topographic factor at the department level, which will allow a better comparison and validation of the information regarding future researches. acknowledgement to the national university of colombia, palmira campus. to the soil and water research group with emphasis on soil degradation. to the project “increasing the competitiveness of hillside agriculture in the entire department of valle del cauca, west”. to the regional autonomous corporation of valle del cauca, from which the cartographic information of the study area was taken. references aguirre, a., lópez, l. i., bolaños, f. v., gonzález, d. f. & buitrago, o. 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(2020). global rainfall erosivity changes between 1980 and 2017 based on an erosivity model using daily precipitation data. catena, 194, 1-12. https://doi.org/10.1016/j. catena.2020.104768 https://www.agronet.gov.co/estadistica/paginas/home.aspx?cod=4 https://doi.org/https://doi.org/10.22458/rc.v16i1.59 https://doi.org/10.1016/j.jhydrol.2006.01.019 https://doi.org/10.1016/j.jhydrol.2006.01.019 https://doi.org/10.2136/sssaj2015.07.0254 https://doi.org/10.22490/21456453.1828 http://www.fao.org/news/story/es/item/357165/icode/ https://repository.agrosavia.co/bitstream/handle/20.500.12324/2207/44194_56407.pdf?sequence=1&isallowed=y https://repository.agrosavia.co/bitstream/handle/20.500.12324/2207/44194_56407.pdf?sequence=1&isallowed=y https://repository.agrosavia.co/bitstream/handle/20.500.12324/2207/44194_56407.pdf?sequence=1&isallowed=y https://doi.org/10.14483/udistrital.jour.tecnura.2016.2.a07 https://doi.org/10.14483/udistrital.jour.tecnura.2016.2.a07 https://www.iaea.org/sites/default/files/documents/tc/water-spanish.pdf https://www.iaea.org/sites/default/files/documents/tc/water-spanish.pdf http://documentacion.ideam.gov.co/openbiblio/bvirtual/023647/protocolo-erosion.pdf http://documentacion.ideam.gov.co/openbiblio/bvirtual/023647/protocolo-erosion.pdf https://www.igac.gov.co/es/noticias/de-las-53-millones-de-hectareas-intervenidas-en-colombia-el-61-presenta-un-uso-inadectado https://www.igac.gov.co/es/noticias/de-las-53-millones-de-hectareas-intervenidas-en-colombia-el-61-presenta-un-uso-inadectado https://www.igac.gov.co/es/noticias/de-las-53-millones-de-hectareas-intervenidas-en-colombia-el-61-presenta-un-uso-inadectado https://doi.org/10.13140/rg.2.1.1024.0082 https://doi.org/10.13140/rg.2.1.1024.0082 http://www.asohofrucol.com.co/archivos/biblioteca/biblioteca_117_pfnvalledelcauca.pdf http://www.asohofrucol.com.co/archivos/biblioteca/biblioteca_117_pfnvalledelcauca.pdf https://doi.org/10.1016/j.catena.2020.104768 https://doi.org/10.1016/j.catena.2020.104768 _hlk29894229 _hlk29912642 _hlk29913036 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n4.82748 earth sciences research journal earth sci. res. j. vol. 26, no. 4 (december, 2022): 321 333 o r e g eo lo g y investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration narges habibkhah1*, hossein hasani1, abbas maghsoudi1, mehdi honarmand2 amirkabir university of technology, iran university of advanced technology, iran *corresponding author: n.habibkhah@aut.ac.ir manuscript received: 09/10/2019 accepted for publication: 11/01/2023 how to cite this item: habibkhah, n., hasani, h., maghsoudi, a., & honarmand, m. (2022). investigation of spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration. earth sciences research journal, 26(4), 321-333. https://doi.org/10.15446/esrj. v26n4.82748 keywords: structural controls; porphyry cu deposits; fry; fractal; distance-distribution analysis; urmia-dokhtar magmatic belt; iran. palabras clave: controles estructurales; depósitos de pórfido cuprífero; método de fry; fractal; análisis de distribución de distancias; cinturón de urmia-dokhtar; irán abstract known to be triggered and controlled by tectonic systems, magmatism plays a significant role in the deposition and emplacement of hydrothermal mineral systems. it is, therefore, of paramount importance to recognize the tectonic processes that are genetically associated with hydrothermal mineral systems. this study seeks to address this research gap by recognizing the main tectonic processes that have controlled the distribution of porphyry copper (cu) deposits in dehaj area on the urmia-dokhtar magmatic belt, central iran. for this purpose, spatial associations of 31 known porphyry copper deposits, faults, and fractures were evaluated by multiple numerical techniques including fry, fractal, (box-counting and radial-density methods), and distance-distribution analyses to investigate structural controls of the porphyry copper mineralization. results of the fry analysis revealed three mineralization trends, namely ns, ne, and nw, clearly following the existing fault systems in the area. application of the fractal method demonstrated that structural controls on mineralization have operated on two different scales, regional and local scales. distance-distribution analysis was further used to spatially correlate known porphyry copper deposits to fault traces, supplementing the results of fry and fractal analyses by quantitative measurements. compiling the results of these three methods, it was shown that nw-trending faults have plausibly controlled the magmatism on a regional scale. on a local scale, however, neand n-trending faults have probably acted to control the channelling and emplacement of mineral-bearing fluids. investigación de la distribución espacial de depósitos de pórfido cuprífero a través del método de fry y métodos fractales en el área de dehaj, kerman: implicaciones para la exploración resumen el magmatismo, conocido por ser desencadenado y controlado por sistemas tectónicos, juega un rol importante en la deposición y emplazamiento de sistemas minerales hidrotermales. es fundamental, entonces, reconocer los procesos tectónicos que están genéticamente asociados con los sistemas minerales hidrotermales. este trabajo está enfocado en esta falta de datos al establecer los principales procesos tectónicos que han controlado la distribución de depósitos de pórfido cuprífero en el área de dehaj, en el cinturón magmático de urmia-dokhtar, en el centro de irán. con este propósito se evaluaron las asociaciones espaciales, las fallas y las fracturas de 31 depósitos conocidos de pórfido cuprífero a través de técnicas numéricas que incluyen el método de fry (extrapolación), fractal (método de conteo de cajas y de densidad radial), y análisis de distribución de distancia para investigar los controles estructurales de la mineralización del pórfido cuprífero. los resultados de los análisis de extrapolación revelaron tres tendencias en la mineralización, ns, ne, y nw, lo que claramente coincide con los sistemas de fallas en el área. la aplicación del método fractal demostró que los controles estructurales en la mineralización han operado a escala regional y a escala local. el análisis de distribución de distancias fue usado luego para relacionar los depósitos de pórfido cuprífero con las trazas de las fallas y complementar así los resultados de las medidas cuantitativas del método de fry y de los análisis fractales. la compilación de los resultados de estos tres métodos muestra que las fallas con tendencia nw problablemente han controlado el magmatismo a escala regional. a escala local, sin embargo, las fallas con tendencia ne y n han controlado la canalización y el emplazamiento de fluidos con minerales. https://doi.org/10.15446/esrj.v26n4.82748 mailto:n.habibkhah@aut.ac.ir https://doi.org/10.15446/esrj.v26n4.82748 https://doi.org/10.15446/esrj.v26n4.82748 322 narges habibkhah, hossein hasani, abbas maghsoudi, mehdi honarmand 1. introduction an array of hydrothermal mineral deposits, including porphyry cu deposits (sillitoe, 1972; sillitoe, 2010) are conventionally thought to be controlled structurally (pirajno, 2012). that is, structural features, ranging from faults to fractures, act as major corridors for migration of mineral-bearing fluids (pirajno, 2012; parsa and maghsoudi, 2018). although distribution and emplacement of hydrothermal mineral systems are inextricably linked to fault systems, not all fault systems are associated with this type of mineralization (cf. faulkner et al., 2010). it is, therefore, crucial to answer the question of how is the distribution of hydrothermal mineral deposits associated with faults? the answer to this question is important in that the fault systems that exhibit spatial and genetic link with mineral deposits can serve as exploration criteria for the delineation of exploration targets (e.g., carranza, 2009; carranza and sadeghi, 2014; adib et al., 2017a; adib et al., 2017b; parsa et al., 2018a; parsa and maghsoudi, 2018). located in the southern part of urmia-dokhtar magmatic belt (fig. 1), dahej area is dominated by magmatic sequences. in this area, the magmatism is known to have been commenced by the deposition of acidic-to-intermediate extrusions in eocene, and then lasted till quaternary when basaltic outcrops have been emplaced. the acidic-to-intermediate plutonic sequences have been then intruded by shallow oligocene-miocene dioritic and granitic intrusions. not only did these intrusions trigger the development of extensive alteration zones, but they hosted a number of metallic deposits (feizi et al., 2017a) such as porphyry cu mineralization in the area (feizi et al., 2017b); that is, these intrusions are spatially and temporally associated with mineralization (zarasvandi et al., 2016). although many studies have been conducted on these deposits (e.g., khoie et al., 1999; bazin and hubner, 1969; dehghani, 2000; zarasvandi, 2004; mehrabi et al., 2005; zarasvandi et al., 2016), their spatial link with structural features remains unanswered. accordingly, the present research aimed at tackling the foregoing caveat gap through spatial and numerical techniques. faults, fractures, shear zones, and various spatial stresses tend to concentrate the magma and trigger its movement through shallow zones of the crust, indicating their crucial role in the distribution of magmatic fluids (ryan and dewey, 1990). therefore, investigation of the geometry of the fault mechanisms associated with porphyry cu orebodies can lead to very significant insights into the choice of proper locations for exploring porphyry bodies (walker, 2006). different analyses have been conducted to assess spatial distribution of mineral deposits; however, the most profound methods of analysis are seemingly the fractal (mandelbrot, 1983) and fry analyses (fry, 1979), which we were used in this study. in addition, there are myriad numerical techniques available for assessing spatial association of faults systems with mineralization. these include, for example, the t-student spatial statistics (zuo et al., 2009; xiao et al., 2012; zuo and wang, 2015; parsa et al., 2017a; parsa et al., 2017b; parsa et al., 2017c), success-rate curves (agterberg and bonham-carter, 2005; mirzababaei, 2016; parsa et al., 2016a; parsa et al., 2016b), distance-distribution analysis, and receiver operating characteristics (roc) curves (berman, 1977; berman, 1986; haddad-martim et al., 2017; parsa et al., 2018b). among others, roc curves provide for superior robustness and visual assessment of the relationship between structures and mineral deposits (cf. carranza, 2009; agterberg, 2013; lisitsin, 2015; haddad-martim et al., 2017; parsa et al., 2018a; ramezanali et al., 2019). in iran, roc analysis has been used to investigate spatial distribution of chromite (yaghoubpour and hassan nezhad., 2006), pb-zn (hassannejad et al., 2001), iron (najafi et al., 2010) and cu deposits (tanhaei et al., 2010, zarasvandi et al., 2016). in this paper, two principal objectives were followed: (1) to investigate structural elements and (2) to identify relationship between these structures and mineralization. in this regard, fry (fry, 1979), fractal (mandelbrot, 1983) and distance-distribution (berman, 1977, 1978) analyses were utilized to investigate structural controllers. as a result of these analyses, we could develop a suitable method for determining the main directions of dispersion of deposits with respect to linear structures. a combination of these methods has rarely been devised to investigate the spatial pattern of porphyry cu deposits, with the results contributing to delineation of prospective areas for exploration activities. 2. geological setting study area is located in the southwest of central iran, covering parts of the sanandaj – sirjan zone and urmia-dokhtar magmatic belt (dimitrijevic, 1973). 2.1. stratigraphy on the surface, the study area is covered by sedimentary, igneous, and metamorphic rock units that have been dated back to palaeozoicto the present age, generally trending along a nw – se direction. dated back to palaeozoic, the oldest deposits constitute the bedrock and are, in most cases, metamorphized. exposed in parts of the sanandaj – sirjan zone and central iran, these rocks are separated from the sahebabad – marvast desert embayment by the marvast – herat fault which strikes along an nnw – sse direction. this fault has remained active during the quaternary, leading to a drop down of the eastern wall with respect to the western wall. metamorphized and non-metamorphized permian rocks represent the youngest units of palaeozoicage in this area. the rock units in this area have been subjected to two phases of metamorphism; one during the triassic and another one during the upper jurassic. various zones are separated from one another by faults in the nw-se direction, some of which are further inclined toward an e-w direction (soheili, 1981; khakzad and jaafari, 2002). an upper cretaceous colouredmélange zone is developed in the form of a narrow stripe with a width of about 3 -17 km and a length of about 125 km along the general trend of the region. this zone is mainly composed of basalt, split rock, green tuff, tuffy sandstone, ultra-basaltic rocks, limestone, and radiolaria, which are exposed in the tectonic zone. adjacent to this zone, multiple faults are developed along a nw – se direction, which together comprise a part of the naein – baft main fault. in between this zone and the marvast – herat fault, there is an extensive embayment in the form of a graben, which includes the sahebabad – marvast desert, that is itself a part of the greater embayment of gavkhooni – sirjan and hosts quaternary deposits. besides the colouredmélange unit, there are eocene volcanic rocks, indicating that the mentioned tectonic zone has been partly active in eocene as well (soheili, 1981). the urmia – dokhtar zone includes the highlands in the west, centre, and north of the mapped area. in this zone, limestones comprise the oldest rock unit, while the upper cretaceous densely folded flysches are ended up located at the centreof an anticline in the east. due to the continental drift in eocene, large amounts of lava and volcanoclastic rocks with basic and alkaline composition were emitted through deep paleo-fractures, with the eocene sedimentary deposits formed at the same time. following the eocene, the compressive force melted the continental crust, developing intrusive rocks with the general composition of granodioritic and monzonitic quartz that cut through the eocene rocks. during the neogene, another round of extensive volcanic activities occurred with a moderate to acidic composition, and associated lava and volcanoclastic rocks covered the older rock units. an example of such volcanic activities occurred at mozahem volcano in the ne of shahr-e-babak (soheili, 1981). in quaternary, a semi-deep mass of porphyry and dacitic microgranodioritic rocks cut through the older strata, forming some light-coloured uplifts in mountainous areas. intrusion of these semi-deep rock units and associated metamorphism have contributed to the formation of porphyry cu orebodies and veins and minor accumulations of lead and zinc in this area. in the nw corner of the mapped area, the upper cretaceous is represented by highly folded and partly metamorphized flysches, and a nw-se-trending syncline is seen in this area. the syncline is followed by kerman conglomerates and hosts some middle eocene lavas in the middle. the anar embayment is seen as a graben in between the badbakhtkooh and mountain ranges in the central part of the mapped area, encapsulating the quaternary deposits. rock bodies of granite, andesite, granodiorite, and diorite are dispersed as intrusions into the space between the main rock units of region (fig. 2) (soheili, 1981; mirzababaei, 2016). a total of 130 metallic and non-metallic mineralization occurrences are reported in this study area, including metallic reserves of cu, cugold, iron, and lead-zinc and non-metallic reserves of bentonite, stone, gypsumanhydrite, industrial, phosphate, kaolin, soils, perlith, silica and pozzolan. 323investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration 2.2. structural geology in addition to the marvast – herat fault, there are three other faults in the mapped area, namely anar, shahr-e-babak, and rafsanjan faults. a major part of anar fault is extended beyond this area, but its southern end passes through the west of anar city. shahr-e-babak is a dextral strike-slip fault with a length of 235 km that is developed along a nw-se trend. seemingly, anar and rafsanjan faults intersect on the west of the area (soheili, 1981; zarasvandi et al., 2016). figure 1. location map of the study area along the urmia-dokhtar magmatic belt (modified from alavi, 1994) figure 2. simplified geologic map of the study area which is classified by age (modified from soheili, 1981) 324 narges habibkhah, hossein hasani, abbas maghsoudi, mehdi honarmand 3. materials and methods there are a total of 31 porphyry cu deposits distributed in the area (table 1). here, we applied fry, fractal, and distance-distribution analyses in a row to achieve the objectives of our study. 3.1. fry analysis as a visual tool for understanding the distribution of mineral deposits, fry analysis considers mineral deposits as points (vearncombe and vearncombe, 1999; hassan nezhad et al., 2001; najafi et al., 2010; zuo, 2015; parsa et al., 2016b) to represent their spatial distribution (e.g., kreuzer et al., 2007; carranza, 2009; parsa et al., 2018a). the method has been thoroughly described by carranza (2009). fry analysis is a geometrical method for evaluating conformity of a geological point structure. it has been used by many geologists to analyse geological controllers of deposits (kreuzer, et al., 2007 – zuo, et al., 2009 – carranza, 2009). it works by a transparent paper with horizontal and vertical axes intersecting at the centre of the paper. the centre point, marked by a sign, is superimposed on the corresponding point to a particular deposit and locations of other points are marked on the paper. the process is iterated for all points. overall, the method leads to a total of n2-n fry points, or the so-called translated points, from the analysis of n mineral deposit locations. the translated points are then demonstrated on rose diagrams manifesting the directions through which deposits are distributed. the rose diagrams are developed using the length and trend of the obtained points (carranza, 2009). dominant trends on the rose diagram demonstrate the trend of probable mineralization controllers, which needs to be confirmed with the help of local structural geology of the area, deposit type, and geological conditions of the deposits (haddadmartim et al., 2017). the rose diagrams can be produced in two different ways, namely using all points or a given set of them. if the former is the case, the rose diagram will serve as a visual aid for interpreting regional-scale distribution of mineral deposits. in contrast, the latter case shows the distribution of mineral deposits on a larger scale, depending on the selected intervals (carranza, 2009; haddadmartim et al., 2017). interested readers are referred to carranza (2009) for further discussion. the processes can be easily implemented using relevant software tools such as geo fry plot, given probable linear trends as output. 3.2. fractal analysis fractal measures have been frequently applied in different branches of earth sciences (e.g., afzal et al., 2011; mansouri et al., 2015; yousefi et al., 2015; yousefi et al., 2016; parsa et al., 2017a; parsa et al., 2017d), especially for understanding the subtle patterns embedded in the spatial distribution of mineral deposits (e.g., carlson, 1991; raines, 2008; carranza, 2008; carranza, 2009; carranza and sadeghi, 2014). among a whole host of fractal measures proposed, the s-called box-counting (pruess, 1995) and radial-density (mandelbort, 1983) approaches can be used to assess spatial distribution of deposits, on which we will elaborate here. 3.2.1. box-counting (b-c) measure fractal dimension of mineral deposits can be determined by the boxcounting (b-c) method. in this method, a sheet is divided into a mesh of square cells of the size ε, and the cells with known mineralization points n (ε) are counted. in order for the distribution pattern of the counted points to follow the fractal geometry, the conditions of the below power-law function have to be satisfied (mandelbort, 1983; feder, 1988; pruess, 1995; carranza, 2008; carranza, 2009; carranza and sadeghi, 2014). ( ) ( ) n βε ∞ ε − (1) in the above equation, ∞ means proportion and β is the fractal dimension, which either ranges between 0 and 2, or is equal to 2. the latter case shows a complete spatial randomness. in a log-log plot, the equation will be a linear relationship, or (carranza, 2008): log (n(ε)) = α β . log(ε) (2) where a is the intercept and β is the slope of the linear log–log plots of n (ε) versus ε. 3.2.2. radial-density (r-d) measure the radialdensity (r-d) measure of fractality is expressed by the below equation (mandelbrot, 1983): d = c * r (d 2) (3) in which d is the point density within a given location, c is a constant, r is the circle radius, and d is the fractal dimension. the foregoing function means that spatial distribution of points around a reference point is fractal should the point density decreases with distance from the reference point (haddad-martim et al., 2017). actually, this method calculates the deposit density (i.e., the number of deposits / unit circle area) from a reference point. interested readers are referred to agterberg (2013) for further discussion. 3.3. distance-distribution analysis this method is perhaps the most popular method for measuring the spatial association of mineral deposits with geological structures (berman, 1977, berman, 1978 berman, 1986) in a quantitative way (carranza, 2008). details about this method have been extensively explained by carranza (2009), precluding us from further explanation. it works on the basis of three curves, namely, d1, d2, and the contrast curve (i.e., c = d2 – d1) measures the distance from each point to deposit locations, manifesting a complete spatial randomness. d2, however, measures the distance of each structure from different deposit locations. positive and negative values of c show positive and negative spatial associations of structures with mineralization, respectively (carranza, 2008). 4. results since eocene, joints of the region have been among the oldest geological features and significantly contributed to forming regional faults. joints play a crucial role in providing dikes with the required space for intrusion. formation and concertation of joints into regional intrusive bodies have been affected by shearing mechanisms, geothermal solution pressure, and intrusive process of rock mass emplacement. in order to determine the relationship between cu mineralization and geological structures, data of faults and deposits across the study area was drawn (fig. 3) and subjected to independent fractal analysis. 325investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration ta bl e1 . l oc at io ns o f i nd ic es an d cu d ep os its in th e s tu dy ar ea (t o be co nt in ue d) . m ai n or e m in er al s a lte ra tio n ty pe s h os t r oc ks y (d d ) x (d d ) n am e c ha lc op yr ite , c ha lc oc ite c ov el lit e, p yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry 54 .9 9 30 .5 a ba di a ba gh 1 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 55 .3 0 30 .3 2 a bd ar 2 2 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry g ra no di or ite po rp hy ry ,a nd es ite , 55 .3 2 30 .3 0 a bd ar 1 3 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry g ra no di or ite po rp hy ry ,a nd es ite , 54 .9 4 30 .5 2 a bd ar c ho za m 4 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry , g ra no di or ite po rp hy ry ,a nd es ite , 55 .2 3 30 .3 1 a di ba gh 5 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry , g ra no di or ite po rp hy ry , a nd es ite 55 .1 3 30 .4 9 b id o 6 py rit e, c ha lc op yr ite , c ha lc oc ite ph yl lic , a rg ill ic , p ot as si c, p ro py lit ic , ja ro si tiz at io n d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry 55 .1 7 30 .4 1 c ha h m es i 1 7 py rit e, c ha lc op yr ite , c ha lc oc ite ph yl lic , a rg ill ic , p ot as si c pr op yl iti c, ja ro si tiz at io n d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry 55 .0 2 30 .4 c ha h fir oo ze h 8 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry , a nd es ite 55 .2 7 30 .4 1 e e m ey do k 9 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .9 2 30 .5 1 ei jo 1 10 py rit e, c ha lc op yr ite , c ha lc oc ite , m al ac hi te ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .9 6 30 .5 6 ei jo 2 11 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , p ro py lit ic , p ot as si c g ra no di or ite p or ph yr y 55 .0 5 30 .5 0 g od k ol v ar i 1 12 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry , a nd es ite 54 .9 7 30 .6 1 g od k ol v ar i 2 13 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry ,a nd es ite 55 .0 1 30 .6 1 g od k ol v ar i 3 14 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .9 9 30 .5 9 g od k ol v ar i4 15 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , a rg ill ic , p ot as si c, p ro py lit ic , ja ro si tiz at io n, s ili ci fic at io n d io rit e po rp hy ry , m ic ro d io rit e po rp hy ry 54 .9 5 30 .5 5 ei jo 3 16 py rit e, c ha lc op yr ite c ha lc oc ite , m al ac hi te , ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .7 4 30 .6 1 k oh e m od av va r 18 py rit e, c ha lc op yr ite , g al en a ph yl lic , a rg ill ic , p ro py lit ic g ra no di or ite p or ph yr y, d ac ite , a nd es ite , d io rit e 55 .1 9 30 .3 8 k oh e m oz ah em 19 (c on tin ue d) 326 narges habibkhah, hossein hasani, abbas maghsoudi, mehdi honarmand m ai n or e m in er al s a lte ra tio n ty pe s h os t r oc ks y (d d ) x (d d ) n am e py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , g ra no di or ite p or ph yr y, d ac ite , a nd es ite , d io rit e 55 .1 6 30 .4 0 la ch ah 1 20 c ha lc op yr ite , m al ac hi te , a zu rit e c ar bo na tiz at io n, s ili ci fic at io n, p ro py lit ic , c hl or iti za tio n, s er ic iti za tio n, a nd es ite , p yr oc la st ite s 55 .1 6 30 .4 1 m ey do k 21 c ha lc op yr ite , c ha lc oc ite , s ph al er ite , m al ac hi te , a zu rit e, g al en a pr op yl iti c, p ot as si c a nd es ite , d io rit e po rp hy ry 54 .7 4 30 .5 4 n ah ro 22 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .9 0 30 .2 7 n am ak za r 1 23 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 54 .8 1 30 .3 7 n am ak za r 2 24 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry ,a nd es ite 55 .1 3 30 .4 4 pa rk am 25 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic d io rit e po rp hy ry 55 .1 3 30 .4 4 sa ra 26 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic d io rit e po rp hy ry 54 .9 9 30 .4 8 se re nu 27 m al ac hi te , a zu rit e, c hr ys oc ol la , p yr ite , c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , p ot as si c d io rit e po rp hy ry 55 .0 6 30 .5 7 ta za rj 28 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic , s ili ci fic at io n d io rit e po rp hy ry , g ra no di or ite po rp hy ry , a nd es ite 55 .0 8 30 .4 9 ti r k oh 29 c ha lc op yr ite , m al ac hi te , a zu rit e c ar bo na tiz at io n, s ili ci fic at io n, p ro py lit ic , c hl or iti za tio n, s er ic iti za tio n, a nd es ite , p yr oc la st ite s 55 .0 9 30 .4 0 w e m ey do k 30 py rit e, c ha lc op yr ite ph yl lic , a rg ill ic , p ro py lit ic d io rit e po rp hy ry , q ua rtz d io rit e po rp hy ry ,a nd es ite 54 .9 8 30 .4 7 za vi e ol ya 31 ta bl e1 . l oc at io ns o f i nd ic es an d cu d ep os its in th e s tu dy ar ea 327investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration figure 3. location map of faults and deposits across the study area 4.1. fractal analysis of faults figure 4. the rose diagram showing the distribution of faults in terms of length and direction acting as controlling factors to orebodies, patterns of local and regional fractures and geology of the area are of paramount importance. satellite images enable geologists to identify associations between orebodies and regional lineaments. in a mineralization zone, the frequency of fractures and lineaments can lead exploration activities as they serve as channels for fluid flow. lineaments and fractures in a mineral zone may not be easily identifiable from satellite images. accordingly, filtering is used to detect these features on the satellite images. the most popular kernels for feature detection are 3 * 3, 5 * 5, and 7 * 7 kernels. knowing that the band 8 of the etm+ sensor provides for a higher spatial resolution than the other bands of the same sensor, we herein used band-8 data to detect lineaments, followed by applying three oriented filters at 00, 450, and 315° with a 3 * 3 kernel to identify regional fractures along n-s, ne-sw, and nw-se directions, respectively. once finished with extracting the faults and fractures by means of the mentioned methodologies and correlating them to the fractures indicated on 1:100,000 map of the region, a new map of fractures was prepared. afterwards rose diagrams of fractures and faults were plotted. the map of spatial distribution of faults, determined by the analysis of remotely sensed data, shows three major ne-sw, ns, and nw-setrending fault systems (fig. 4). while nw-trending faults run parallel to magmatism in the area, ne-trending faults, that are younger than the first group (mirzababaei, 2016; zarasvandi et al., 2016), intersect with older geological units. the ns-trending faults, however, are apparently the youngest reportedly active faults on a local scale (meshkani et al., 2013). to determine fractal dimension of the faults, first, fracture maps of the area were divided into 4 squares grids of 48 km in side length (grids a, b, c, and d), with each grid sub divided into four identical squares. this process was iterated 6 times to come up with different numbers of squares and square sizes r at each iteration. then, each square was analysed separately and a fractal dimension was calculated for each square grid (a, b, c, and d) (fig. 5). figure 5 shows a stepwise presentation of the box-counting method for measuring the fault fractal dimension (d). at each stage, the cells with the size r containing faults were counted and considered as n (r) in equations 1 and 2 (table 2). on the basis of the logarithmic graphs plotted (fig. 6), four fractal dimensions were calculated for the four grids a, b, c, and d (fig. 6). the calculated values are da= 1.4339, db= 1.4971, dc=1.5528, and dd= 1.2777. table 2. square dimension (r) and the number of cells containing faults (n) for 4 square grids a, b, c and d. level r (km) box count (na) box count (nb) box count (nc) box count (nd) 1 48 1 1 1 1 2 24 3 4 4 3 3 12 8 12 13 7 4 6 23 31 35 19 5 3 61 88 110 38 6 1.5 150 205 260 84 according to the results, the square grid c exhibits the largest fractal dimension, implying that it hosts the largest number of intersecting faults and hence, making it the most favourable part of the study area for prospecting further porphyry cu deposits (cf. carranza, 2009). interestingly, deposits are more frequent in this region, showing the efficacy of the fractal method adopted. in other regions, however, the fractal dimension declines as they host fewer faults. 328 narges habibkhah, hossein hasani, abbas maghsoudi, mehdi honarmand step 1 step 2 step 3 step 4 step 5 step 6 figure 5. stepwise presentation of the box-counting method. figure 6. log log plot of the number of cells containing faults n(r) versus inverse of side length for square grids a, b, c, and d. 329investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration 4.2. fractal analysis for deposits in order to implement the box-counting procedure, the study area was divided into a grid of cells with different dimensions in several steps and those that were superimposed by the deposits were counted. the log-log plot of the number of cells containing deposits n (ε) versus the square length was prepared (fig. 7). the b-c analysis exhibits that the pattern of the cu deposits has two fractal dimensions, i.e., β = (d1 = 0.74, and d2 = 0.85) and determined by one breakpoint on the plot (≈ 6 km). figure 7. log-log plot of the box–counting method showing n (ε) versus ε for the deposits authors also used the r-d fractal modelling (fig. 8) to further investigate spatial distribution of the porphyry cu deposits in dehaj area. for this purpose, the r-radius circles hosting cu occurrences were counted. the result was, then divided by the area to obtain d in equation 3 (table 3). figure 9 shows the loglog plot of radius (r) versus the density (d). as it can be observed in this figure, two fractal dimensions were observed, which were separated at around 6 km. table 3. radius (r) and density (d = number of cu occurrences in the circle/circle area). number of cu occurrencesr (km)level 31481 23242 17123 664 135 01.56 results of the two fractal methods (b-c, r-d) are almost identical, showing that two structural processes at different scales (< 6 km and > 6 km from the deposit locations) have controlled the emplacement of porphyry cu deposits in dehaj area (figs. 7 and 9). figure 8. stepwise presentation of the r–d method figure 9. log-log plot of the r–d method demonstrating radius (r) versus density (d) for deposits 330 narges habibkhah, hossein hasani, abbas maghsoudi, mehdi honarmand 4.3. fry analysis for deposits the fry analysis was applied on the 31 known porphyry cu deposits in dehaj area, resulting in 930 fry points (fig. 10). initially, we used a rose diagram to show the trends of all fry points (fig. 11a). this method sheds lights on the structural controls on mineralization that have acted on a regional scale (e.g., parsa et al., 2018a). according to fig. 10a, the regional controls on mineralization have plausibly operated in a major nw-se and a minor e-w direction. furthermore, with reference to the breakpoints separating fractal dimensions in figs. 6 and 9, which are 6 km apart, we further developed a rose diagram for fry points that are within 6 km of one another (fig. 11b). this figure shows a general n-s inclination, manifesting that local-scale controls on mineralization have probably contributed to the development of mineralization in an n-s direction. figure 10. points corresponding to local indices and fry figure 11. the rose diagram of (a) all fry analytical points, (b) points within 6 km of one another local investigation of cu indices shows that, for the most part, cu mineralization has occurred within intersection of faults with the mentioned trends. the fry pattern of the region and rose diagram of cu index orientations indicate a general nw-se trend, which is in good agreement with the trend of dominant fractures in the region. 4.4. distance-distribution analysis various structures, including faults with diverse directions, are resulted from different tectonic processes; however, only a limited number of tectonic processes are associated with mineralization (faulkner et al., 2010). therefore, mineralization-related tectonic processes should be identified. one way for recognizing mineralization-related fault systems is through measuring their spatial association with mineral deposits by the distance-distribution analysis; a positive relationship manifests a mineralization-related fault system while a negative relationship shows fault systems that are not associated with mineralization (carranza, 2008). in this study, three distances -distribution graphs were plotted for three fault systems in the area, nw-, ne-, and n-trending faults (fig. 12). according to this figure, all of these three fault systems are associated with porphyry cu mineralization, as indicated by their positive association with mineral deposit locations. the highest positive spatial association between the ne-trending faults and mineralization was seen for the deposits within 6 km of the faults (fig. 12a). likewise, the highest positive spatial association was observed within a distance of about 10 km from the nw-trending faults (fig. 12b). however, the most significant spatial correlation between mineralization and the n-trending faults is observed within a distance of merely 4 km from these faults (fig. 12c). this shows that the nwand netrending fault systems have controlled the distribution of faults on a regional scale while the n-trending faults have controlled the distribution of faults on a local scale. 5. discussion porphyry cu deposits are structurally controlled (sillitoe, 1972, 2010; padilla et al., 2001). therefore, recognition of structural controls of this type of deposits is very important in exploration surveys. analyses of spatial distribution of mineral deposits and spatial association of mineral deposits with fault systems can pave the way for addressing this challenge (vearncombe and vearncombe, 1999; kreuzer et al., 2007; lisitsin, 2015). in order to investigate the regional fractures, an analysis was performed on the faults (including continuous and discontinuous faults) and the joints systems. regional faults are dominantly strike-slip in nature (accompanied with small thrust or graben components in some cases) and follow different trends. investigations showed that the regional faults have been formed as a result of the shearing effect applied to the urmia-dokhtar magmatic belt. upon continuing the shearing mechanism and the resultant progressive deformation, alterations occurred to the faults and their geometrical associations. regional joints systems are also developed as a result of the shearing mechanism and/ or folding process. the progressive deformation within the shear zone has further affected the joints systems. a large number of jointed tuff strata have been rotated, with the other intrusive tuff bodies further experiencing shearinginduced changes. another factor contributing to deformation of the region is the small shearing zones developed between the regional faults. such shearing zones tend to further complicate the joints systems, mix up the strata, etc. since the study area is located on the urmia – dokhtar igneous belt within the dextral shearing zone delimited by rafsanjan fault in the north and shahr-e-bakak fault in the south (both of which are dextral strike-slip faults with some dip-slip components), then reginal fault analysis was performed on the basis of shearing mechanism. existing literature have shown that nw-se faults are the oldest in the region. for most part, these faults are longer than the other faults in the region, being observed along the longitudinal axis of intrusive bodies. due to erosion and lack of visible evidence of the fault surface to ensure reactivation 331investigation of the spatial distribution of porphyry copper deposits using fractal and fry analysis methods in dehaj area, kerman: implications for exploration of these faults, they seem to be primary faults in the region, which are formed sometime after the eocene. if the amount of fault gouge along the fault surface is zero or negligible, the fault is said to be a periodic fault. this feature is seen for most of the faults in the region. another characteristic of the faults in the region is the absence of fractures on the fault surface, which is another indication of faulted joints. dominant trend of joints in the region is well in agreement with the trend of adjacent faults. last but not the least, in-fills of chlorite and calcite can be seen on the fault surfaces for most of the faults in the region. the fact that the lithology of these in-fills is identical to that of near-fault periods further confirms that the mentioned faults are faulted joints. the fractal analysis of the fault systems (e.g., zhao et al., 2011), showed that the faults are significantly associated with mineralization (figs. 5 and 6). in other words, an abundance of mineral deposits and fault systems is observable in the se part of the area (grid c in fig. 6) in which the fractal dimension is at its peak (1.748). these results are significant as fault systems are corridors for raising magmatic and hydrothermal fluids that develop mineralization (pirajno, 2012). results of fry analysis (fig. 11), fractal analysis (figs. 7-9), and distancedistribution analysis (fig. 12) were in agreement with one another showing that the nwand netrending faults were responsible for regional-scale processes that have contributed to the development of mineralization. furthermore, these analyses demonstrated that the n-trending faults are the local-scale controls on mineralization. these results have been achieved by numerical techniques in this paper and comply with the previous results achieved by other studies in dehaj area (meshkani et al., 2013; zarasvandi et al., 2016). in these studies, it has been reported that the nw-trending dehshir-baft strike-slip fault is the major structural control on the magmatism and the development of porphyry cu deposits in the study area. 6. conclusion in this study, a range of numerical techniques were applied, including fry, fractal, and distance–distribution analyses, for the recognition of structural controls on porphyry cu mineralization in dehaj area. by combining these methods, useful results were obtained to help understand the processes contributing to the formation of mineral deposits. indeed, porphyry cu masses were found to be more probable to intrude and accumulate along the margins of huge faults and shear zones. it was further indicated that evaluation of mineralization direction with respect to fault direction can provide insights into the role of faults in emplacement of cu mineralization. based on the results of spatial analysis, mineralization direction was conformed to faults directions in most parts of the study area. the fry analysis demonstrated that the study area had three intersecting fault mechanisms. fractal analysis of the fault systems revealed that the fractal dimension reaches its peak in the se part of the area where there is an abundance of faults and mineral deposits, showing that faults are inextricably linked with porphyry cu mineralization. in addition, the results of fry, fractal, and distance–distribution analyses of mineral deposit locations were in agreement with findings of previous geological studies of the area. these techniques demonstrated that whilst the nwand ne-trending faults are responsible for the regional-scale deposition of mineralization in the area, the n-trending faults have probably acted as the local-scale controllers of mineralization in the area. figure 12. distance-distribution analysis results to investigate 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(2022). fuzzy approach for the safety risk assessment in dimension stone mining. earth sciences research journal, 27(1), 37-45 https://doi.org/10.15446/esrj.v27n1.85482 record manuscript received: 02/03/2020 accepted for publication: 02/03/2023 keywords: dimension stone mines; safety risk assessment; risk priority number; fuzzy inference process abstract mining activities are liable to injuries and different types of diseases. the occurrence of an accident threatens safety in dimension stone mines. therefore, the safety risk assessment in such mines is an important issue that needs special consideration. in this paper, the safety risk of incidents in dimension stone mines in iran is evaluated using the fuzzy inference system. the fuzzy analytical hierarchy process is used to identify the importance degree of each incidence and then, the overall risk priority number is calculated based on the fuzzy inference process. the results of this study show that vehicle traffic and wire rupture are the most hazardous incidents. resumen las actividades mineras son responsables de lesiones y diferentes tipos de enfermedades. la ocurrencia de un accidente amenaza la seguridad de la minería de rocas de dimensión. además, la evaluación de riesgos de seguridad en estas minas es un tema importante que necesita consideración especial. este artículo evalúa el riesgo de seguridad en incidentes de minería de rocas de dimensión en irán. el proceso analítico jerárquico difuso se usa en este trabajo para identificar el grado de importancia de cada incidente y, luego, se calculó el valor de prioridad de riesgo general con base en la inferencia difusa del proceso. los resultados de este estudio muestran que el tráfico de vehículos y la ruptura de cables son los incidentes más peligrosos. https://doi.org/10.15446/esrj.v27n1.85482 mailto:rahimdel@birjand.ac.ir https://doi.org/10.15446/esrj.v27n1.85482 38 mohammad javad rahimdel 1. introduction risk assessment, as a primary part of occupational health and safety management, is a critical state in industrial activities. dimension stone mining using diamond wire cutting is one of the most hazardous mining activities. accidents in this type of rock exploitation cause serious injuries, even fatalities, and have a considerable effect on financial losses. regarding the national statistical center of iran (nsci) in 2020, more than 12% of total mines in iran are in the category of dimension stone mines (nsci, 2020). there are more than 10,000 workers in the dimension stone mines of iran, and therefore, the safety risk assessment in this type of mine is necessary. over the last few years, numerous studies have been done in the field of safety risk analysis in quarries. sanmiquel et al. (2014) studied occupational safety management and its effect on the incidence rate of occupational accidents in dimension stone mines in spain from 2004 to 2008. in this research to evaluate the quality of occupational safety management in each mine, a questionnaire with four main categories was used. the main categories included preventive organization, treatment plants, workshops, storages, dimension stone mine-related items, and other general questions. results of the mentioned study showed that, in the dimension stone mine-related items, the signs for entries and tracks, electrical facilities, and operating slopes had the worst condition. sanmiquel et al. (2015) studied the accidents in spanish mines from 2003 to 2012. they expressed that the leading causes of accidents were collisions with a moving object and physical effort. they concluded that most accidents were because of the previous causes before the accidents which were the electric problem, collapse, loss of the machinery control, fall of a person, body movement with or without physical effort, prevention organization, experience, and age of the injured worker. khalilabad et al. (2018) provided a model to analyze the safety risk of dimension stone mines. in the mentioned study, fault tree analysis under the fuzzy environment was used to analyze hazards related to the wire-cutting machine in a quarry mine in iran. results of the reviewed study showed that the dangers of the rupture of wire, diversion of wire, and the existence of mud inside the stone blocks were basic events that had the highest occurrence probability. marras and careddu (2018) studied occupational injuries in the dimension stone mines of italy industry. in the mentioned study, the work-related injuries and fatal accidents caused for quarrying of stone, sand, and clay mines were analyzed from 2012 to 2019. regarding the results of the reviewed study, the role of human behavioral factors on safety is decisive and the competence of safety measures and identification of unambiguous regulations are necessary to prevent quarry accidents. melodi et al. (2020) studied the risk management analysis for labor and equipment in quarry mines in three states of nigeria. in this study, the level of risks and likelihood of occurrence of potential hazards were identified and analyzed. results of the mentioned study showed that slips and trips, noise pollution, and dust impact were the most potential safety hazards. bogoly and fuzesi (2021) studied the slope stability of a dolomite quarry in hungary by using deterministic and probabilistic methods. in this study, the factor of safety was obtained from the deterministic approaches and then, compared to the results of probabilistic methods. results of the mentioned study showed that probabilistic methods are more flexible to design the slope properties. hazard identification skills of workers in dimension stone mines were studied by bae et al. (2021). in the mentioned study safety behaviors of workers in a quarry mine in the midatlantic region of the united states were explored by using interviews and field notes collection. results of the reviewed study indicate that the quarry workers identified hazards and improved their safety knowledge from their interaction with other workers, hands-on experience, and sharing their responsibilities among the team members. esmaeilzadeh et al. (2022) used the failure modes and effect analysis (fmea) method for the safety risk assessment of quarry mines. in this research, the main causes of risks in the west-azerbaijan quarry mines of iran were identified and studied. results of the reviewed study showed that the diamond cutting wire breaking, rock-fall, and car accidents had the highest risk priority number. moreover, some preventive activities such as the planned cutting wire replacement, application of an intelligent system to control cutting tools, and training workers to mitigate the safety hazards. reviewing the papers, mentioned above, shows that the safety risk of quarry mines has been studied by researchers. some of these studies have usually focused on the frequency of accidents without considering their hazardous consequences. these studies didn’t take into account some factors such as weather-related or delivering services of the health, safety, and environment (hse) unit, which might be the leading cause of some serious incidents. moreover, all safety incidents have no same importance degree and then, considering the importance measure of them during is essential. furthermore, obtaining the risk evaluation factors using the crisp values may bring a different level of uncertainty and ambiguity in the form of doubt and hesitancy. in such cases, using the fuzzy set theory could reduce these uncertainties. in this paper, to overcome these scarcities, the importance degree of the most frequent incidents in iran’s dimension stone mining is considered and then the safety risk is evaluated under the fuzzy environment. to achieve this, in the first step, the fuzzy analytical hierarchy process (fahp) is used to find the importance degree of each incidence. there have been many successful applications of this method for vague and uncertain decision-making problems (karimnia and bagloo, 2015; alizadeh et al., 2016; modak et al., 2017). then the risk priority numbers (rpns) for each incidence is computed under the fuzzy environment by applying the fuzzy inference system (fis). finally, the overall rpn is calculated for each safety incident. the result of this study defines the most hazardous incidents during dimension stone mining in iran. these results are helpful for the mine engineers and directors to predict the most hazardous incidents during dimension stone mining, create a safe working place for the employees and accordingly prevent the occupational incidents. this paper is organized as follows. in section 2, the research methodology is introduced. in this section, first, the fuzzy ahp is presented, and then and fuzzy inference process is introduced. in section 3, the safety risk of the most frequent incidents during the dimension stone mining is evaluated and discussed. 2. theoretical foundation in this section, first, the fuzzy ahp is presented. then, the risk level assessment by using the fuzzy inference system is explained. 2.1. fuzzy analytical hierarchy process the ahp is based on the innate human ability to make judgments about small problems. ahp is one of the well-known multi-criteria decision-making methods which was first presented by saaty (1980). this method is a simple, flexible, and practical approach that is widely used to obtain the importance degree of the decision criteria. in the application of ahp, both quantifiable and nonquantifiable information can be evaluated, any level of detail about the main goal of the problem can be structured, and scientific judgment can be combined with personal judgment in the evaluation process (hakan and kanik, 2012; qureshi and harrison, 2017). the methodology of conventional ahp is based on comparisons of objectives and alternatives in a natural and pairwise manner to evaluate the customer’s needs using the point scales. in conventional ahp, the decision-makers are only able to focus on a limited number of items at the same time. zadeh (1965) proposed a new method as the theory of fuzzy sets based on the generalization of the classical methods. the crisp set allows full membership or no membership at all for an element, while a fuzzy set is an extension of a crisp set that allows partial membership. the range of values of membership from zero to one was suggested by zadeh to show the object’s membership in a fuzzy set. the complete membership and non-membership are represented by one and zero, respectively and values between one and zero indicate the intermediate membership degrees. the fuzzy set is characterized by a membership (or characteristic) function which assigns to each object a grade of membership between zero and one (rahimdel and ghodrati, 2021). in this paper, triangular fuzzy numbers (tfns) are used because tfns have the characteristics of being constructive and easy to calculate in comparison to trapezoid and gaussian fuzzy numbers (rahim, 2017; tsai et al., 2022). tfns expressed with m̃ = (l,m,u), where l ≤ m ≤ u, in which the parameter l indicates the smallest possible value and m and u, respectively, represent the most promising and the largest possible value with the membership function as (rahimdel and ghodrati, 2021): = 0 < − − ≤ ≤ − − ≤ ≤ 0 > m (1) 39fuzzy approach for the safety risk assessment in dimension stone mining in using real crisp numbers, the ratio of the pairwise comparison is linguistic and vague and the human assessments cannot reflect the human thinking style. therefore, the ahp method seems inadequate to determine the customers’ importance measures, accurately. to overcome these kinds of shortcomings, the fuzzy sets were used with the pairwise comparison to extend conventional ahp, named fuzzy ahp (fahp), to handle the linguistic variables. on the other hand, reflecting the uncertain preferences of the decision-making group using crisp values is impossible. therefore, the ahp under the fuzzy environment is used. in the last decades, fuzzy ahp has been successfully applied in various mining and mineral-related fields such as risk assessment in coal mines (li et al., 2020; rahimdel et al., 2022), carrying capacity prediction of the water resource (chi et al., 2019), selection of the process mining technology (dogan, 2021), mining method selection (bajić et al., 2020), mine reclamation (yu et al., 2020), and mining equipment selection (patyk and bodziony, 2022). different approaches have been proposed to drive the priorities in fahp from the fuzzy pairwise comparison matrices that the logarithmic least squares method (van laarhoven and pedrycz, 1983), the geometric mean method (buckley, 1985), the fuzzy modification of the logarithmic least squares method (boender et al., 1989), synthetic extent analysis (chang, 1996), the fuzzy least square method (xu, 2000), lambda-max method (csutora and buckley, 2001), fuzzy preference programming and two-stage logarithmic programming (wang et al., 2005) are some of them. among these approaches, the extent analysis method was found to be the most commonly applied method because of its computational simplicity. this approach was proposed to handle the pair-wise comparison scale using the tfns for the synthetic extent value of the pairwise comparison. in this method, first, the tfns are used for pairwise comparison, and then, the extent analysis method is applied to obtain the weight vector by using synthetic extent values. the steps of the fuzzy ahp method, introduced by chang (1996), are summarized as follows: step 1: constructing the triangular fuzzy judgment matrix the pair-wise comparison matrix is constructed by using tfns via pairwise comparison as follows: 1~~ 1 ~1~ ~~1 21 221 112 ~ nn n n aa aa aa a= ··· ··· ··· · · · · · · · · · (2) where, ã is an n×n fuzzy matrix containing the tfns ãij in which the ãji equals 1⁄ãji. the fuzzy numbers in the corresponding of each linguistic variable are defined regarding the uncertain linguistic variables of the human judgments as table 1. table 1. linguistic variable terms and their corresponding tfn linguistic scale tfn ãij reciprocal tfn (1⁄ ãij) just equal (1,1,1) (1/1,1/1,1/1) equal preferred (1,1,3) (1/3,1/1,1/1) weak preferred (1,3,5) (1/5,1/3,1/1) moderately preferred (3,5,7) (1/7,1/5,1/3) strongly preferred (5,7,9) (1/9,1/7,1/5) very strongly preferred (7,9,10) (1/10,1/9,1/7) extremely preferred (9,10,10) (1/10,1/10,1/9) let the object set x = {x1, x2, x3... xn} and the goal set g = {g1, g2, g3... gn}. each object is considered and the extent analysis for each goal is performed. therefore, m extent analysis values for each object can be obtained as: m m m gi gi gi m1 2 , , ..., i = 1,2,...,n where all of the m m m gi gi gi m1 2 , , ...,, (j = 1,2,...,n) are tfns representing the performance of object xi with regard to each goal uj. step 2: calculating the fuzzy synthetic extent values the value of fuzzy synthetic extent with respect to ith criterion can be obtained as: s m m l m u i gi i j m gi j j m i n j j j j m j m j m �� �� �� �� �� �� �� �� �� �� ���� �� ������ �� ���� ���� 1 11 1 111 , ,���� �� �� �� �� �� ���� �� �� ������ �� �� �� ���� �� �� �� ���� �� �� �� 1 1 1 1 1 1 u m l jj m jj m jj m , , (3) , where si is the fuzzy synthetic extent and ⊗⊗ is the fuzzy multiplication operator. step 3: calculating the possibility degree of decision matrices in this step, synthetic extent is compared to other synthetic extent values of decision matrices. the degree of possibility is assumed to be m1 ≥m2, where m1=(l1,m1,u1) and m2=(l2,m2,u2). the degree of possibility is obtained by using the following equation: ( 2 ≥ 1) = 1, 0 l₁ u₂ (m₂ u₂) (m₁ u₁) if m₂ > u₂ if l₁ < u₂ otherwise (4) , where v (m1 ≥ m2) is degree of possibility for m1 and m2, d is the highest intersection point between µm1 and µm2 as shown in figure 1. to compare m1 and m2, both values v (m1 ≥ m2) and v (m2 ≥ m1) are needed. it is worth noting that the degree of possibility is an index to compare two tfns and cannot be used to represent their relative importance. figure 1. intersection point d between two fuzzy number m1 and m2 step 4: calculating the weight vector the possibility degree for a convex fuzzy number to be greater than k convex fuzzy numbers mi=(i =1,2,...,k) is given by the following equation: v (m ≥ m1, m2, … , mk) = min v (m ≥ mi), i= 1, 2,…, k (5) letting d′(ai) = min v (si ≥ sk) for k = 1, 2, …, n; k ≠ i, then the vector of weight (w′) is obtained as: w′ = (d′(a1), d′(a2), …, d′(an))t (6) , where ai = (i = 1, 2, …, n) are n elements. the normalized weight vector is calculated as follows: w = (d(a1), d(a2), …, d(an)) t (7) , where w is not a fuzzy weight number. step 5: checking the degree of inconsistency of the judgements to assure a certain quality level of a decision, the consistency of evaluation is analyzed. according to saaty (1980), the consistency ratio is used to verify the consistency of the comparison matrix. the consistency ratio is computed using the consistency index and random index as follows. cr ci ri == (8) 40 mohammad javad rahimdel ci n n �� �� �� �� max 1 (9) , where cr is the consistency ratio, ci is the consistency index, ri is the random index, λmax is the maximum eigenvalue of the pair-wise comparison matrix, and n is the size of the comparison matrix. the random index represents the consistency index of a randomly generated reciprocal matrix, is also calculated as follows: ri = 1.98 × [(n 2)/n], (10) if the consistency ratio is high, it means that the input judgments are not consistent and consequently are not reliable. therefore, it requires that the consistency ratio smaller than 0.10 to avoid too much inconsistency. on the other hand, if the consistency ratio smaller than 0.1 it means the pair-wise comparisons are supposed consistent within acceptable random variations. if the consistency ratio is higher than 0.1, the judgments are not reliable, and a new comparison matrix needs to be solicited. 2.2. risk level assessment based on the fuzzy logic theory risk is defined as the uncertainty and lack of awareness about the consequences which can lead to a loss or benefit of action or incident. the risk priority number has been the most widely used technique for analyzing the risk level of potential incidents. the risk priority number guides ranking the potential incidents and is calculated as (zhang and chu, 2011): rpn = s × o × d (11) where rpn is the risk priority number, s is the severity, which means the level of damage effects that occur, o is the occurrence, which represents the frequency of incident, and d is detectability, which indicates the ability to detect the potential incident. in the traditional rpn, the s, o, and d values are defined by crisp point scales and some problems are made in interpreting the results of the conventional quantitative rpn method. because in this way, the rpns are calculated by only multiplying three crisp numbers. in some cases, the different level of risk evaluation factors leads to the same rpn. on the other hand, the real crisp numbers are linguistic, and vague and therefore analyzing the rpn in this way seems inadequate. in calculating the final rpn in the traditional approach, it is assumed that all incidents have equal importance. therefore, fuzzy logic is applied to work with the linguistic terms directly. to prioritize the risk in the fuzzy environment, in comparison with the traditional method, fuzzy linguistic terms are used. in this approach, the linguistic term and corresponding fuzzy membership function of each incident are used as table 2 (zhang and chu, 2011). table 2. linguistic terms and the corresponding triangular fuzzy numbers (zhang and chu, 2011) fuzzy number linguistic terms fuzzy membership function 1 equally important (1,1,2) 2 intermediate value (1,2,3) 3 weakly important (2,3,4) 4 intermediate value (3,4,5) 5 fairly important (4,5,6) 6 intermediate value (5,6,7) 7 strongly important (6,7,8) 8 intermediate value (7,8,9) 9 very strong importance (8,9,10) concerning the scores of rpns, the incidents can be ranked, and accordingly, suitable actions can be taken on the incidents with high-risk priority numbers. the safety risk analysis in this paper is based on expert judgments. however, the evaluation of the same event by different experts is usually subjective and ambiguous. in such conditions dealing with imprecise and partial data information, a fuzzy inference system (fis) is applied. mamdani-type (mamdani and assilian, 1975), and sugeno-type (sugeno and kang, 1988), are well-known fuzzy inference systems. the mamdani fuzzy inference method, which is known as the mamdani method, is the most common fis method. the mamdani-type method employs the defuzzification of a fuzzy output, while the sugeno computes the crisp output. mamdani’s approach is not dependent on a data set, involves sufficient expertise in the system, and is a generalized model that can be applied for effective future predictions (khalifa et al., 2015). moreover, it has a more interpretable rule base and is well-suited to human input (bobzin et al., 2022). the rest of this section is devoted to explaining the mamdani fuzzy inference method. the mamdani method has three main steps including fuzzification, rule processing, and defuzzification as shown in figure 2. step 1 step 2 step 3 crisp values crisp values fuzzy input fuzzy output fuzzier if-then rules interface defuzzification figure 2. the fuzzy logic system in the fuzzification step, all risk factors and risk priority levels are converted to fuzzy numbers regarding the expert’s judgment. in this research, the triangular fuzzy numbers described in table 3 are used to interpret the linguistic terms of experts. table 3. linguistic variable and corresponding membership function for all risk factors and risk level severity (s) occurrence probability (o) probability of detection (d) risk level (rpn) membership function none rarely very high none (1,1,2) very minor remote high very low (1,2,3) minor slight moderately high low (2,3,4) very low low moderate high low (3,4,5) low moderately low low low moderate (4,5,6) moderate moderate moderately low moderate (5,6,7) high moderately high very low moderate high (6,7,8) very high high remote low high (7,8,9) serious very high very remote high (8,9,10) hazardous almost certain almost impossible very high (9,10,10) in the rule processing step, the relationships between all o, s, d, and risk levels were created and characterized by fuzzy “if–then” rules obtained from experts’ knowledge and opinions. table 3 is used to achieve this. the “if-then” rules, by using the mamdani algorithm, can be presented in the following form (kalogirou, 2009): “if xi is aii and xr is air then y is bi” for i = 1, 2, . . ., k, where xi and xr are the input variables, aii, air, bi are the linguistic terms (fuzzy sets), y is the output variable, and k is the number of rules. after creating rules, the rule consequences are obtained by combining the results based on the system’s input values. in this paper, the “min-max” composition, as the most commonly used 41fuzzy approach for the safety risk assessment in dimension stone mining technique, is used. in this method, the truth value of the rule was defined as the minimum value. if the output of fuzzy sets were more than one rule, this subset was defined as the maximum real value. the outputs of the fuzzy inference system are the fuzzy values. in the third step, the defuzzification process, all fuzzy conclusions were defuzzified. there are different algorithms such as the center of the area (coa), the bisector of area (boa), the mean of maximum (mom), smallest of maximum (som), and the largest of maximum (lom) methods for defuzzification (rahimdel et al., 2022). in this paper, the center of the area method, due to simplicity and ease of computation, is used. in the coa method the crisp value is obtained from the following equation: x x x dx x dx aa b aa b �� ���� �� �� �� ( ) ( ) (12) where, x* is the defizzified value for x output and µa(x) is the aggregated output membership function for the interval a to b. 2.3. the overall risk priority numbers after calculating rpn for each hazard, by using the above-mentioned procedure, importance degree of each incidence (derived from the fuzzy ahp) is integrated with the rpn values (derived from the fuzzy inference process) to obtain the overall risk priority numbers as follows: rpn w rpn overall i i i �� �� (13) , where, rpn w rpn overall i i i �� �� is the overall rpn for hazard i, wi is the non-fuzzy weight for hazard i, and rpn is the risk priority number. 3. results and discussion in this section, first, the safety risk of dimension stone mining in iran is evaluated using the procedure mentioned in the previous sections. the hazardous incidents in the dimension stone mines of some other countries are also surveyed and discussed. 3.1. safety risk assessment of iran dimension stone mines according to the reports of nsci (2020) and also the experts’ opinions on the dimension stone mines in iran, the serious incidents were identified. crucial incidents in the dimension stone mines of iran are summarized in table 4. the importance degree of the incidents is obtained using the fuzzy ahp, and then, considering the risk-evaluating values in the fuzzy environment, the overall rpn of each incident is calculated. in the first step of the fuzzy ahp method, the linguistic variables, mentioned in table 1, were used by the group of decision-makers to obtain the fuzzy judgment matrix. this matrix is given in table 5. in the next step, the fuzzy synthetic extent values (si) are calculated from the equation 2 for each incident. accordingly, the degree of possibility for each pairwise incident was calculated by using equation 4 and given in table 6. table 4. the crucial incidents in the dimension stone mines of iran (nsci, 2020) safety incident code description rupture of the diamond cutting wire i1 the diamond wire cutting rupture because of the wire exhaustion in incurrent or overtime usage. fall from the bench i2 falling the workers/equipment/machinery from the height is one the most frequency incidents in iran’s quarries due to the slimy place or lack of caution. vehicles traffic i3 vehicles accidents with other vehicles or employees. rockfall i4 falling the rocks because of loose blocks or poor scaling. electrical shocks i5 rupturing the worn-out cables and exposing people to the conductive materials. machine-related incidents i6 the incidents because of vehicles failures e.g., vehicle brake damage, poor maintenance, or working with more than the design capacity. operator-related incidents i7 the incidents due to the poor skill level of workers or tiredness and sleepiness because of the working overtime. weather-related incidents i8 the incidents because of adverse weather conditions such as raining, snowfall, high temperature. poor delivering the hse unit services i9 the incidents due to the poor service of hse unit in the installation of awareness signs in the needed places, persuading worker to consider the safety issue, weak training schedule. table 5. the fuzzy judgment matrix i1 i2 ... i4 i9 i1 (1,1,1) (1,3.67,7) ... (5,8.5,9) (3,6.333,9) i2 (0.143,0.227,1) (1,1,1) (3,5.5,7) (1,2.333,5) i3 (0.333,0.040,1) (1,1.67,3) (5,8.5,9) (0.3,0.57,1) i7 (0.333,1,1) (1,1,1) ... (5,8.5,10) (7,9.5,10) i8 (0.111,0.118,0.2) (0.143,0.182,0.333) (1,1,1) (1,3.68,5) i9 (0.111,0.158,0.3) (0.2,0.429,1) (0.2,0.27,1) (1,1,1) 42 mohammad javad rahimdel table 6. the degrees of possibility for the fuzzy judgment matrix v (i1 ≥ i2) = 1 v (i2 ≥ i1) = 0.684 v (i3 ≥ i1) = 0.884 v (i4 ≥ i1) = 0.346 v (i5 ≥ i1) = 0.404 v (i1 ≥ i3) = 1 v (i2 ≥ i3) = 0.779 v (i3 ≥ i2) = 1 v (i4 ≥ i2) = 0.702 v (i5 ≥ i2) = 0.790 v (i1 ≥ i4) = 1 v (i2 ≥ i4) = 1 v (i3 ≥ i4) = 1 v (i4 ≥ i3) = 0.417 v (i5 ≥ i3) = 0.488 v (i1 ≥ i5) = 1 v (i2 ≥ i5) = 1 v (i3 ≥ i5) = 1 v (i4 ≥ i5) = 0.898 v (i5 ≥ i4) = 1 v (i1 ≥ i6) = 1 v (i2 ≥ i6) = 1 v (i3 ≥ i6) = 1 v (i4 ≥ i6) = 0.972 v (i5 ≥ i5) = 1 v (i1 ≥ i7) = 1 v (i2 ≥ i7) = 1 v (i3 ≥ i7) = 1 v (i4 ≥ i7) = 0.441 v (i5 ≥ i6) = 0.515 v (i1 ≥ i8) = 1 v (i2 ≥ i8) = 1 v (i3 ≥ i8) = 1 v (i4 ≥ i8) = 1 v (i5 ≥ i7) = 1 v (i1 ≥ i9) = 1 v (i2 ≥ i9) = 1 v (i3 ≥ i9) = 1 v (i4 ≥ i9) = 1 v (i5 ≥ i8) = 1 v (i6 ≥ i1) = 0.286 v (i7 ≥ i1) = 0.846 v (i8 ≥ i1) = 0.019 v (i9 ≥ i1) = 0.088 v (i6 ≥ i2) = 0.670 v (i7 ≥ i2) = 1 v (i8 ≥ i2) = 0.298 v (i9 ≥ i2) = 0.355 v (i6 ≥ i3) = 0.350 v (i7 ≥ i3) = 0.697 v (i8 ≥ i3) = 0.010 v (i9 ≥ i3) = 1.000 v (i6 ≥ i4) = 1 v (i7 ≥ i4) = 1 v (i8 ≥ i4) = 0.667 v (i9 ≥ i4) = 0.724 v (i6 ≥ i5) = 1 v (i7 ≥ i5) = 1 v (i8 ≥ i5) = 0.534 v (i9 ≥ i5) = 0.605 v (i6 ≥ i7) = 0.375 v (i7 ≥ i6) = 1 v (i8 ≥ i6) = 0.652 v (i9 ≥ i6) = 0.706 v (i6 ≥ i8) = 1 v (i7 ≥ i8) = 1 v (i8 ≥ i7) = 0.016 v (i9 ≥ i7) = 0.112 v (i6 ≥ i9) = 1 v (i7 ≥ i9) = 1 v (i8 ≥ i9) = 1.006 v (i9 ≥ i8) = 1.000 regarding table 6, the normalized weight of each incident was calculated using equations 6 and 7: wi1=0.220, wi2=0.150, wi3=0.194, wi4=0.076, wi5=0.089, wi6=0.063, wi7=0.186, wi8=0.002, wi9=0.019. to calculate the consistency index, first, the fuzzy judgment matrix was defuzzified to a crisp number by using the center of gravity (cog) method (wang and luoh, 2000). based on our conclusions, the eigenvalue of the pair wise comparison matrix (λmax) is calculated as 10.192. the dimension of the matrix is nine, and then the random index is 1.540 (=1.98 × ((9 2) / 9)). the consistency index is 0.149 (= 10.192 – 9) / (9 – 1)). therefore, the consistency ratio of the matrix is calculated as 0.096 (=0.149/1.540)) < 0.1 which means the judgment in the comparison matrices is acceptable, and the judgment errors are tolerable. in the rest of this section, the overall risk priority number is calculated for each incident by using the fuzzy inference system (fis), as described in subsection 2.2. the membership functions corresponding to linguistic terms are considered for each safety incident. to define the severity, occurrence, and probability of detection for each incident, some questionnaires were prepared. these questionnaires were sent out to the experts and directors in the field of dimension stone mining to fill them out. in this way, the severity, occurrence, and detectability of each incident were defined by using fuzzy linguistic scales as described in table 3. in this paper, the mamdani fuzzy inference process was performed in matlab software (johanyák et al., 2006). the structure of the applied fuzzy inference model with attribute inputs and the corresponding output is shown in figure 3. it notes that all risk factors were considered as inputs and the rpn was considered as output variables. then, the fuzzy inference model was created by matching inputs to output variables against the “if-then” rules. these rules were defined with the aims of the experts with enough experience in the field of dimension stone mining. considering ten states for all three risk factors, a total (103=) 1000 rules were created. some of these rules are given in table 7. fuzzy inference system (mamdni) severity rpn ocurrence not-detection figure 3. structure of the fuzzy inference model in the matlab software table 7. creating rules in the fuzzy inference process no. of rule if (inputs) then (output) severity (s) occurrence (o) probability of detection (d) rpn 1 none rarely very high none 2 none rarely high none 3 none rarely moderately high very low 4 none rarely moderate very low 5 none rarely low low 6 none rarely moderately low low 7 none rarely very low high low 8 none rarely remote moderately low 1000 hazardous almost certain almost impossible very high in the next step of the fuzzy inference process, the rpn values were defuzzied from the fuzzy results set. to achieve this, the center of the area method was applied. then, the overall rpns are obtained from equation 13 by multiplying the incidents’ importance weight by the risk priority number obtained from the fuzzy inference process. the results are shown in figure 4. in this way, the incidents can be prioritized. o ve ra ll r pn 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 l3 l1 l7 l2 l4 l5 l6 l9 l8 figure 4. the overall rpns for each safety incident 43fuzzy approach for the safety risk assessment in dimension stone mining according to figure 4, vehicle traffic (i3), and rupture of the diamond cutting wire (i1) are the most hazardous incidents during the dimension stone mining. therefore, effective risk adjustment approaches need to be considered for occurrence preventing of them. 4.2. hazardous incidents in the dimension stone mines of some other countries incident comparison of the other dimension stone-rich countries can provide helpful information to suggest suitable strategies experienced by more advanced countries in safety issues. incidence rate in spanish mining sector is much higher compared with other countries. in 2007, the incidence rate was 8.9 times higher than in the united states and 19.6 times higher than in the australian state of queensland. occupational health and safety studies of quarries in spain from 2007 to 2008 show that 2,452 accidents caused 60,194 days away from work. more than 200 non-fatal accidents caused at least 60 days of lost work. the incidence rate of quarries in spain was also higher in treatment plants, workshops, and storage than the other workplaces. physical over-exertion on the muscular-skeletal system and being hit by falling objects were the leading causes of accidents (sanmiquel et al., 2014). studying the occupational accidents in marble quarries in the diyarbakir province of turkey showed that about 42.9% of incidents occurred because of the cutting wires rupture and 17.8% and 3.6% were because of the blasting and falling from the bench, respectively. it is worth noting that 10.7% of these incidents resulted in death (gumus and akkyun, 2006). the main accidents which occurred in the marble quarries of turkey were respectively because of the broken wires, slipping, and falling. more than 4% of accidents caused death or permanent disability, and more than 60% of accidents caused a harmful impact. moreover, there was an extremely negative exponential relationship between the safety index and the accident index, which means the importance of safety measures in reducing accidents. it should be noted that 40% of incidents in these mines were so dangerous for workers (ersoy, 2013). in the dimension stone mines of australia, about 2600 deaths were reported from 2003 to 2012 (swa, 2013). the quarries safety reports show that the vehicle accident was the cause of more than 50 present of incidents in quarries from 2013 to 2016 (dnrm, 2016). regarding the incident analysis of australian quarries, the accident rate (accident per one million hours worked) from 2015 to 2016 was about 2.7 while, in the other surface and underground mineral types was 0.2 and 0.7, respectively. moreover, vehicle collision was the most dangerous hazard in more than one-third of high-potential incidents (dnrm, 2017). the hazardous area of italy’s quarries is the overburden areas where the construction machines move the overburden. while the most hazardous area of turkey’s mine was the benches. in this area, falling pieces could injure the people who work in front of the bench, because of the weight of construction machines or vibrations. regarding the mentioned study, the area of italy’s quarries is safer than turkey’s quarries both in terms of working fields and observed issues (ersoy and yesilkaya, 2016). regarding the occupation and safety condition of different dimension stone-rich countries, mentioned above, the vehicle collision is the most hazardous incident of iranian, australian and italian quarries. while, the rupture of diamond cutting wire is the leading cause of the incidents in dimension stone mines of turkey. it is also worth noting that, the most frequent incident in the dimension stone mines of spain caused by falling objects. teaching the operators and persuading and encouraging them to consider the safety issues would have a considerable effect on reducing the operator-related dangers. different actions such as proper maintenance of the installations in workplaces with unprotected moving parts, walkways and railings, materials in poor conditions, defective electrical installations and dust emissions or noises should be considered to avoid vehicle accidents. in terms of observed issues, using the diamond cutting wires, correctly, checking and inspecting the wires, periodically, avoiding to use the worn-out wires and also selecting the optimum length of diamond wires should be taken to account decrease the safety risk level. 5. conclusions this paper analyzed the safety risk of the most frequent incidents in dimension stone mines of iran which exploited by using the diamond wire cutting method. the importance degree of each incident was obtained using the fuzzy ahp. then, the overall risk priority numbers were calculated using the fuzzy inference system. the results of this study showed that the vehicle traffic, rupture of the diamond wires, falling from the bench, and human imprudence are the most critical incidents. while the weather-related incidents and the insufficient delivering services of the hse unit are the lowest portion of unsafety conditions. the incidents dealing with traffic, rupturing the diamond wires, and falling from the bench have the highest level of severity and occurrence, while the low detectability level of them is the main reason for the high-risk priority. therefore, considering the effective measures such as encouraging and persuading the vehicles’ operators to observe the safety issues, trying to improve the skill levels of operators, regular checking and inspecting the wires, scaling and cleaning the benches from the loosed rocks and slimy materials are recommended. the results of this study are helpful for the dimension stone mine directors and managers to improve the safety level. although this study considered the most common incidents in dimension stone mines in iran, there were some restrictions during the research. dimension stone mines have different maintenance and inspection plans for equipment and vehicles. therefore, the frequency of the vehicle-related incident can be different from one mine to others. economic aspects of the safety measure were another limitation that should be taken into account in future studies. studying effects of the risk adjustment issues for reducing the safety risk level, ranking different types of dimension stone mines based on the safety risk levels, and economic analysis of the measures taken to ensure the safety mining in comparison with the production lost costs are also recommended for the future studies. real-time safety risk assessment and applying artificial intelligence approaches to perform safety risk assessment are recommended, as well. acknowledgement the author would like to thank all the members of staff at department of mining engineering, university of birjand, for their assistance during this research. references alizadeh, s., rad, m. m. s., & bazzazi, a. a. 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(2011). risk prioritization in failure mode and effects analysis under uncertainty. expert systems with applications, 38(1), 206-214. https://doi.org/10.1016/j.eswa.2010.06.046 https://doi.org/10.1016/j.fss.2004.10.020 https://doi.org/10.1016/j.fss.2004.10.020 https://doi.org/10.1016/s0165-0114(97)00376-x https://doi.org/10.1016/s0165-0114(97)00376-x https://doi.org/10.3390/su12052015 https://doi.org/10.1016/s0019-9958(65)90241-x https://doi.org/10.1016/s0019-9958(65)90241-x https://doi.org/10.1016/j.eswa.2010.06.046 _hlk122385311 _hlk123034168 _hlk122809990 table1 _hlk122898949 _hlk122962891 _hlk122986023 _hlk129238588 _hlk129239689 _hlk129239077 _hlk129238960 _hlk129239243 _hlk129238691 _goback keywords: manabí province; git; soil erosion risk. palabras clave: erosión; mdt; ndvi; usle; ndvi; imágenes de satélite. how to cite item pacheco, h. a., mendez, w., & moro, a. (2019). soil erosion risk zoning in the ecuadorian coastal region using geotechnological tools. earth sciences research journal, 23(4), 293-302. doi: https://doi. org/10.15446/esrj.v23n4.71706 the activation of erosive processes on the slopes of the drainage catchments in the ecuadorian coastal region represents a serious environmental problem that results in a loss of soil resource with the consequent reduction in agricultural productivity, increasing lack of areas with vegetation, and decreased water quality. these issues increase, among other things, the probability of flooding. the purpose of this research is to zone soil erosion risk in the manabí province, which is located on the ecuadorian coast. the methodology is supported by the employment of geographic information technologies (git) in order to access and process information of interest, such as: precipitation data with which to calculate the r factor of the universal soil loss equation (usle); topography, which was employed to create a slope map from the digital elevation model (dem) aster; vegetation cover, by applying the normalized difference vegetation index (ndvi); land use, through the interpretation of orthophotos and a field survey; and infiltration capacity, obtained by considering the texture of the soil. the factors were combined by means a weighted relationship (map algebra) and exploiting the potential of software for the design of geographic information systems (gis). the results indicate the presence of soils with little or limited vegetal protection, in areas with steep slopes, moderate rainfall and extensive farming activities. these characteristics determine conditions of high soil erodibility, and propitiate the triggering of erosive processes. indeed about 19% of the surface of the manabí province classifies with soil erosion risk between high and very high. these areas should be prioritized by regional government agencies to implement soil restoration and protection policies. surfaces with medium soil erosion risk represent 67% of the territory, which are also areas that require certain levels of attention. the surfaces with low soil erosion risk represent only 14% of the province, corresponding to areas of low slope. abstract soil erosion risk zoning in the ecuadorian coastal region using geo-technological tools zonificación de riesgo erosivo en la costa ecuatoriana usando herramientas geotecnológicas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.71706 la activación de fuertes procesos erosivos en las vertientes de las cuencas hidrográficas de la provincia de manabí, representa una seria problemática ambiental que se traduce en la pérdida del valiosísimo recurso suelo, con la consecuente disminución en el rendimiento de la productividad agrícola, el incremento de áreas desprovistas de vegetación, la disminución de la calidad del agua, la intensificación de las inundaciones, entre otras cosas. el objetivo de ésta investigación consiste en zonificar el riesgo erosivo en la provincia de manabí; para ello se hace uso de las tecnologías de la información geográfica en el acceso y procesamiento de información de interés como es el caso de las precipitaciones con el cálculo del factor r de la usle, topografía, elaborando un mapa de pendientes a partir del mde aster, cobertura vegetal, usando el índice normalizado de diferencia de vegetación (indv), uso del suelo, a través de interpretación de ortofotos y levantamiento de campo, y capacidad de infiltración, considerando la textura del suelo. la combinación de los factores se realizó a través de una relación ponderada, aprovechando las potencialidades de softwares para trabajo en ambiente gis, específicamente las relacionadas con el álgebra de mapas. los resultados indican la presencia de suelos con escasa o nula protección vegetal, en áreas de pendientes elevadas, pluviosidad considerable y actividades agropecuarias extensivas, lo cual condiciona la fuerte acción de los procesos erosivos, con las consecuencias de bajo rendimiento en la producción agropecuaria, azolve y contaminación de los cuerpos de agua con la consecuente disminución de la calidad de vida en la población, evidenciando la necesidad de implementar políticas, estrategias y acciones, por parte de los gobiernos autónomos descentralizados, orientadas a impulsar el fortalecimiento de capacidades institucionales y comunitarias que promuevan la sostenibilidad ambiental. resumen record manuscript received: 14/04/2018 accepted for publication: 16/08/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 293-302 henry antonio pacheco1*, williams méndez1, armando moro1 1universidad técnica de manabí * corresponding author: henrypacheco@gmail.com g eo lo g ic al e ng in ee ri ng 294 henry antonio pacheco, williams méndez, armando moro introduction one of the main environmental problems treated in recent years has been to solve soil erosion, which has increased worldwide, due to the need to expand the agricultural frontier to produce food and provide habitat for a growing population. situation that has led to the analysis, in many countries, looking to establish as anthropogenic conditionings, interact in the activation of erosive processes, in order to evaluate and zone the risk levels, and to design measures responding to each environmental reality. in the case of ecuadorian coastal region the socio-environmental conflicts, are specifically because more than 60% of the population, is engaged in agricultural activities not productive, in addition to the fact that they employ few mechanisms that are sustainable for the environment, which leads to pollution and the degradation of soils, forests and waters, thus decreasing the population’s quality of life (instituto nacional de estadística y censos [inec], 2010). in the drainage catchments of the manabí province, the sensitivity to the activation of erosive processes, represents a serious environmental problem, that results in the loss of soil resource, with the consequent reduction in agricultural productivity, increase in areas without vegetation, decrease in water quality, and the intensification of floods, among other scenarios (desinventar, 1994). so the qualification and quantification of soil erosion, is important as regards, assessing land units because it has effects such as the loss of the nutrients required for crop growth, the accumulation of sediments in river channels, and a reduction in water storage capacity owing to soil loss, thus reducing the natural flow of catchments (fonseca et al., 2014). it is as these in recent years, there have witnessed a significant increase in the use of geotechnological tools (remote sensing [rs] and gis) to study soil erosive processes. the principle objectives of many of these contributions are: (a) the multi-temporal and spatial modeling of erosive processes (zeng et al., 2013); (b) the analysis of the effects of the changes in vegetal cover and land use (lech-hab et al., 2015); (c) the quantification of soil loss (parveen and kumar, 2012; farhan et al., 2013; amsalu and mengaw, 2014; lahlaoi et al., 2015; ganasri and ramesh, 2016; tahiri et al., 2016); (d) the zoning of the hazard and risk control (prasannakumar et al., 2012; gaatib and larabi, 2014; uddin et al., 2014; chirala et al., 2015); and (e) soil mapping and classification (alves et al., 2015; rizzo et al., 2015; moura-bueno et al., 2016). likewise, in most of the contributions mentioned above, emphasis is placed on the use of the r factor of the revised universal soil loss equation (rusle) as an indicator of the soil erosion risk. however, little has been done about it in ecuador. in this sense, quantitative studies conducted in the ecuadorian sierra by de noni and trujillo (1986) reveal that the land lost due to erosion are considerable. for example, the results fluctuate among 200 to 500 t.ha-1.y-1 for 1982. the experiments clearly demonstrated the close relationships between runoff and rainfall intensity, in particular the maximum rainfall intensity in mm h-1 during 30 min and of medium frequency (im30), which fluctuate in the ecuadorian coastal region between 40 and 70 mm h-1. in this order of ideas and taking into account, the relevance of the manabí province, for the ecuadorian productive sector, in terms of agricultural potential and the propensity of the physical environment, to activate erosive processes, the purpose of this paper, was to use geotechnological tools, to zone the soil erosion risk, in order to spatiality analyse the problem, identify the most critical areas in the province, and propose possible control measures. study area the manabí province is divided into 22 cantons that cover an area of 1,942,764.33 ha located on the north-western pacific coast of ecuador and whose geoastronomical coordinates are latitude of -1.39° and 0.55° s and n, respectively, longitude of 79.38° 80.93° w. the area has an altitudinal range from 0 to 851 m.a.s.l. (figure 1). the rainfall regime is unimodal with more than 9 dry months on the coast with annual amounts of less than 200 mm, and progressive increase in a w-e direction until reaching an annual rainfall of 3,000 mm in the north-eastern region (instituto nacional de meteorología e hidrología [inamhi], 2016). large coastal plains and fluvial valleys, in addition to some mountainous systems with maximum elevations of 851 m.a.s.l, mainly represent the relief. the major fluvial systems are formed of the drainage catchments of the chone and portoviejo rivers (winckell et al., 1997). the lithology consist of more than 90% poorly consolidated sedimentary rocks. this is represented by rock outscrops belonging to the following lithological units: piñón formation (cretaceous; basaltic lavas, tuff and breccias), cayo formation (cretaceous; grauwackes, shales and basaltic mantles), san mateo formation (eocene; sandstones and conglomerates), zapallo formation (eocene; conglomerates, sandstones and shales), dos bocas formation (oligo-miocene; shales and mudstones), villingota formation (miocene; shales and sandstones), angostura formation (miocene; coquinas, sandstones and mudstones), onzole formation (miocene; shales and siltstones), borbón formation (pliocene; tobaceous sandstones), balzar formation (pliocene; sands, conglomerates and clays), san tadeo formation (pliocene; volcanic fan and lahars), tablazo formation (pleistocene; bioclastic marine terraces) and marine terrace (holocene; estuarine marine clays) (ministerio de minería [mm], instituto nacional de investigación geológico minero metalúrgico [inigmm], 2017; enríquez, 2018). figure 1. location of the study area, manabí province, ecuadorian coastal region. residual soils are composed primarily of very plastic and expansive clays. there are also large areas that are covered with silt, which has a high degree of collapsibility and scattering materials. these materials are easily mobilized and lose their strength as soon as they are excavated, thus making them easily erodible by runoff action (aguirre and chávez, 2009). the coverage of the native forest in the ecuadorian coastal region has been reduced and continues to decrease significantly in habitat extension and quality. this has caused an irreparable loss of its biodiversity, as a consequence of intensive deforestation and other human activities (cornejo, 2011). the land use and cover is primarily agricultural and pasture and it is the first-ranked province in the country as regards total agricultural land with 1,643,428 ha (inec, 2015). this represents 21.16% of the country’s productive activities in the economic sector, which accounts for 14% of provincial gross added value (inec, 2016), and which represents 37.1% of the economically active population (inec, 2010). this productivity is mainly supported by family farming occupying average areas of 4 ha per farmer (monteros and salvador, 2015). material and methods soil erosion risk the soil erosion risk was estimated by employing a combination of the precipitation, the r factor of the usle, topography with a slope map from 295soil erosion risk zoning in the ecuadorian coastal region using geo-technological tools ri = mean rainfall of month i (mm) rt = annual mean rainfall (mm) the information from each station was then employed to carry out a spatial interpolation in the gis using the ordinary kriging method according to the criteria of plouffe et al. (2015) and panagos et al. (2016). the values of the r were subsequently employed to reclassify the map of rainfall erosivity into 8 classes by adapting criteria of rivera and gómez (1991), páez (1999) and olivares et al. (2013). slope (s) the slope is one of the main dynamic factors to activate mass movements because it determines the kinetic and potential energy of an unstable mass. this increases susceptibility to the production of gravitational movements as the ground tilt increases. this instability is determined by the characteristics of relief, which have a significant influence on many variables involved in ecosystems dynamics and equilibrium (sánchez et al., 2002). the slope is, overall, one of the main conditioning factors of erosive processes, sediment transport and runoff (as a result of both surface flow and subsurface flow). the ground slope was calculated using the gis spatial analysis tools, taking the aster dem of 30 meters of resolution as a basis. the software slope tool has predefined algorithms with which to calculate the change in the maximum rate of elevation value of each cell with regard to its eight neighbours. the slope map was generated and reclassified according to the criteria suggested by pacheco (2012), which is based on a detailed analysis of the ground slope distribution using topographic profiles that allowed the determination of clinometric ranges. normalized difference vegetation index (ndvi) the variable related to vegetation was obtained from calculations of the ndvi using landsat le70110612003128edc00.tif satellite image. the image was processed with atmospheric and geometric corrections (chávez, 1989; chávez and kwarteng, 1989) in order to apply a cloud mask that allowed us to obtain the ndvi. this index was calculated by considering only two of the original bands provided by the sensor according to the expression (chuvieco, 1996): ndvi = −( ) +( )pir vis pir vis/ (4) where: ndvi = normalized difference vegetation index pir = near infrared band vis = visible red band the application of this equation enables an image with normalized values between 1 and -1 to be generated (karaburun, 2010; pacheco, 2012). the values close to 1 represent very dense vigorous vegetation, while negative and positive values near zero indicate regions devoid of vegetation. the method resulted in a regression model between the vegetation cover values measured in the field or obtained from the guide tables and ndvi values derived from the satellite image. this information was then used to reclassify the standard deviation of the ndvi values (intervals = 1/3 of the deviation). the results show 6 classes, of which less than 4 correspond to water bodies, and whose variations are possibly owing to sedimentation, depth and vegetation. the highest values correspond to vegetation in different states of health, density and size. land use (lu) the land use factor was evaluated and weighted by employing the map of coverage and land use of ecuador elaborated by ministerio del ambiente de ecuador (mae) – ministerio de agricultura, ganadería, acuacultura y pesca (magap) (2015) at scale of 1:100,000 as input. land use information from the map in question was obtained from field work and the analysis and interpretation of satellite imagery and high resolution orthophotographs. the spatial units of use include forests, agricultural lands, shrub and herbaceous vegetation, water bodies, anthropic areas (populated areas and infrastructure) and other types of land. relative weights were assigned to each type of use depending on its influence on the activation of erosive processes. the criteria utilized for the allocation of weights and reclassification of land use map were the dem aster, vegetation cover using ndvi, land use and infiltration capacity variables from ministerio de agricultura, ganadería, acuacultura y pesca [magap] (2016). the variables were reclassified into five classes of soil erosion risk, as follows: 1 (very low), 2 (low), 3 (moderate), 4 (high) and 5 (very high). the variables were combined using a weighted ratio, obtained from expert judgment by means of a multi-criteria evaluation (starkloff and stolte, 2014; vulević et al., 2015), and using software for the analysis and interpretation of satellite images and gis, specifically those related to map algebra. the weighting process of the factors determined the following results: 20% (0.2) for the usle r factor, 30% (0.3) for the topographic slope, 30% (0.3) for the ndvi, 10% (0.1) for land use, and 10% (0.1) for soil texture, expressed as follows: soil erosion risk r* s* ndvi* lu* st*= ( ) + ( ) + ( ) + ( ) +0 2 0 3 0 3 0 1 0. . . . ..1( )(1) where: r = usle r factor (rainfall) (mj mm ha-1 h-1 y-1) s = slope (°) ndvi = normalized difference vegetation index (dimensionless; the ndvi values range between -1 and 1, where the negative values correspond to areas without vegetal coverage and the positive values at different levels of vegetal coverage, with 1 being the highest density of vegetation) lu = land use (dimensionless; the types of land uses were reclassified into 5 classes according to the following criteria: current land use, land use change and land protection policies) st = soil texture (dimensionless; the types of soil textures were reclassified into 5 classes according to the following criteria: coarse, moderately coarse, medium, fine and very fine) the erosion risk map was reclassified into four classes according to the author’s criteria. for the validation and calibration of the results obtained for this map, 50 control points were georeferenced in the field, which were superimposed onto the map in question. usle r factor the usle r factor was determined from processing rainfall records weather stations camposano, chone, el carmen, jama, julcuy, olmedo, pedernales, portoviejo and rocafuerte, provided by the instituto nacional de meteorología e hidrología (2016) for the period 1984-2011. any missing rainfall data were estimated by using rational and average substitution methods, while homogenization and consistency were estimated by means of the “rachas” or round method (soares et al., 2012). the influence of rainfall on erosive processes was estimated by considering the equation proposed by echeverri and obando (2010). this was generated and validated by carrying out a linear regression analysis of the modified fournier index (mfi) and the monthly average i30 reported for 15 year records, with a determination index of 0.84 and a significance level of 5%. a spearman correlation coefficient matrix was produced after considering the cases of significant and highly significant correlations. the simple linear regressions were determined to a significance level of 5%. the models with a determination index of ≥ 0.75 and a low variation coefficient (≤ 12%) were selected according to the expression: r = ( ) +∑ 12 1 30 4 28 3. .mfi (2) where: r = erosivity (mj mm ha-1 h-1 y-1) mfi = modified fournier index the mfi was estimated according to the equation proposed by arnoldus (1977) to represent climate aggression: mfi = ∑ 12 1 2r ri t/ (3) where: 296 henry antonio pacheco, williams méndez, armando moro based on those proposed by paroissien et al. (2015), who consider current land use, land use change and land protection policies. soil texture (st) the soil texture map developed by magap (2016) at scale of 1:1,250,000 was used as the basis for the evaluation and weighting of this factor. the information presented on the map was obtained from field work and an analysis of soil samples, discriminated by lithological and geomorphological units. the textural groups presented on the map are coarse, moderately coarse, medium, fine and very fine (ditzler et al., 2017). the assignment of relative weights to textural classes was based on expert judgment. the map was reclassified on the basis of the criteria proposed by de lannoy et al. (2014) and cisty et al. (2015), who considered hydric models with high precision algorithms and techniques. results and discussion usle r factor the rainfall amounts (annual means) and their spatial distribution in the manabí province can be observed in table 1 and figure 2. it is characterized by a progressive increase in rainfall from the coast towards the mountains in a westeast direction, with annual amounts of less than 400 mm on the coast, which is distinctive from the arid areas in the region located between montecristi, manta, jaramijó and sucre. amounts significantly increase to annual values of 1,600 mm towards the north and south of the province, while maximum amounts close to 3,000 mm have been recorded in the north-eastern part, as registered in el carmen canton. the erosivity shows the same spatial distribution of the rainfall with the presence of low, moderate, high and very high classes. according to rivera and gómez (1991) páez (1999) and olivares et al. (2013), the territory is mostly covered by class 4, which is categorized as moderate erosivity (table 1), and extends from north to south across the province as a central strip. this is followed by the high class to the center and east of the territory in a north-south direction. the low class is toward the coastal area in the jama, rocafuerte, portoviejo and manta axis. finally, the very high class that covers the smallest area is concentrated to northeast of the manabí province. the usle r factor is a measure of the erosive power of the rain that reflects its kinetic energy, it is directly correlated with the loss of soil and the amount of sediment carried by runoff and can, therefore, be considered as a good indicator of the erosion risk. actions for the practice of sustainable agricultural activities should, therefore, be considered in sectors with high and very high erosivity. however, in areas with low erosivity levels, it is possible to intensify erosion processes triggered by extraordinary rainfalls. in this sense, according to data from portoaguas (2017), with a total rain sheet of 70 mm for a duration of 24 hours, a turbidity in the portoviejo river of 25,000 ntu was measured, which reflects a high erosion rate associated with that particular event, as well as a high amount of sediment carried. slope (s) the low slopes are in the coastal areas and intermountain valleys (table 2 and figure 3), which serve as settlements for the largest cities in the province, of which manta, portoviejo, chone, calceta, junín and tosagua stand out. the erosive processes in these locations are minimal while, conversely, floods and sediment accumulation predominate. these areas account for over 74.65% of the territory and are categorized as having a low and very low influence on the activation of erosive processes. the high and very high slopes cover 6% and the moderate slopes 19% of the province (table 2) and are mainly located in the central mountain range region (flavio alfaro, chone, bolívar, junín and santa ana cantons) (fig. 3) and, to a lesser extent, in the pedernales and jama northern and jipijapa and puerto lópez southern cantons. it is in these cantons where the slope conditions greatly influence the genesis of intense erosive processes. table 1. annual mean values of rainfall and erosivity in the manabí province, ecuadorian coastal region. station annual rainfall (mm) erosivity r (mj mm ha-1 h-1 y-1) area (km2) area (%) erosivity class description rocafuerte 471.04 3,162.12 4,149.97 21.36 3 lowporto viejo 603.85 3,797.41 jama 687.52 3,990.93 pedernales 1,049.18 5,767.14 8,067.56 41.53 4 moderatejulcuy (jipijapa canton) 869.16 6,024.03 chone 1,144.15 6,407.31 camposano (paján canton) 1,364.27 7,815.62 6,187.25 31.85 5 high olmedo 1,625.53 9,001.34 el carmen 2,945.62 12,683.72 1,022.86 5.26 6 very high total 19,427.64 100.00 figure 2. spatial distribution of the annual rainfall in the province of manabí, ecuadorian coastal region. 297soil erosion risk zoning in the ecuadorian coastal region using geo-technological tools table 2. area and percentage distribution of the slope classes in the manabí province, ecuadorian coastal region. class slope (°) description area (km2) area (%) 1 ≤ 5 very low 5672.87 29.20 2 4 – 15 low 8829.86 45.45 3 15 – 25 moderate 3772.85 19.42 4 25 – 35 high 1023.84 5.27 5 ≥ 35 very high 128.22 0.66 total 19,427.64 100.00 figure 3. spatial distribution of the topographic slopes in the province of manabí, ecuadorian coastal region. normalized difference vegetation index (ndvi) the ndvi has values ranging between -0.26 and 0.64 (table 3 and figure 4), thus indicating the presence of uncovered or few vegetation covered areas with below zero values. the majority of these areas are located in the coastal cantons, mainly in the center and south of the province. the presence of the densest vegetation cover with values above 0.5, although not completely vigorous, is observed with increased abundance in the cantons located in the eastern sector of the province. li et al. (2015) reported the recovery of vegetal cover in regions which ecological engineering projects were been carried out. this could be assessed by changes observed in the ndvi on satellite images. their findings have shown that vegetation is effective in protecting soil from erosive processes. land use (lu) according to the inec (2015), manabí is located between the ecuadorian provinces with the largest areas of agricultural activity (permanent, temporary and fallow crops, natural and cultivated pastures). the cultivated and natural pastures comprise 886,633 ha, which allows them to provide habitats for the largest livestock production in the country, with 17.76% of the national total. land use is a dynamic process that spans several decades and has a significant influence on the activation of erosive processes. this is understandable when considering the soil disturbance prompted by local people on the mountainous hillsides, and that adversely affects the ecosystem functions. the annual and seasonal crops are those most prone to being affected by vulnerability, resulting in serious problems such as the indiscriminate felling of native trees and the pollution of water resources. these activities take place in order to obtain greater extensions of land devoted to the planting of agricultural products or livestock breeding (rodríguez and sandino, 2010). the values shown in the table 4 and figure 5 indicate that more than 50% of the territory is predominately covered by different crops and pastures, located in the northern center of the province in the portoviejo, santa ana, chone, flavio alfaro, jama and pedernales cantons. these uses condition the activation of erosive processes. on a small area of the province the forest is preserved, in which the effects of erosion are decreased by the protection that this biome offers to the soil (garcía-ruiz et al., 2008; nadal-romero et al., 2012). another class includes the combined coverage of pastures and crops where there is bare soil during the dry season. this condition is highly prone to the activation of erosive processes with the first rains in the winter season, which move sediments from eroded hillsides toward the water bodies and urban areas, thus increasing the possibility of overflows and floods in addition to table 3. area and percentage distribution of erosion risk according to the ndvi, manabí province, ecuadorian coastal region. class ndvi description area (km2) area ( % ) 1 > 0.60 very low 835.39 4.30 2 0.40 – 0.60 low 3122.02 16.07 3 0.30 – 0.40 medium 4509.16 23.21 4 0.20 – 0.30 high 6469.40 33.30 5 < 0.20 very high 4491.67 23.12 total 19,427.64 100.00 figure 4. spatial distribution of ndvi in the province of manabí, ecuadorian coastal region. 298 henry antonio pacheco, williams méndez, armando moro decreasing the quality and quantity of water intended for human consumption and the practice of agricultural activities. areas of greater disturbance resulting from uses associated with anthropic activities condition more severe soil erosion responses (high runoff coefficients) owing to their greater sensitivity to the impact of high intensity rainfall events (regüés et al., 2014). soil texture (st) the distribution percent in the textural composition of soils (table 5 and figure 6) reflects the fact that fine (58.21%) and very fine (6.55%) material predominates almost 65%, which reduces the infiltration capacity and favors both the accelerated increase in runoff and the erosion rates. the hydraulic parameters of the soil are strongly linked to its texture (de lannoy et al., 2014). for example, sandy soils drain more easily than clay soils and have, given their moisture content, a much higher hydraulic conductivity that maximizes when soil physicochemical characteristics are altered by the occurrence of forest fires (inbar et al., 2014). these fires occur frequently in the manabí province, these fires are generated frequently, as is reported by some residents of rural parishes. table 4. area and percentage distribution of the land use classes, manabí province, ecuadorian coastal region. land use class description area (km2) area ( % ) 100 % artificial water body 1 very low 2,273.03 11.70 100 % natural water body 100 % urban area water 100 % sand banks 2 low 423.52 2.18100 % shrimp farms permanent crops forest 3 medium 10,710.46 55.13 forest + cultivated forest forest + crops forest + grass forest + shrubby vegetation parks protection shrubby vegetation cultivated forest 4 high 2,109.84 10.86 cultivated forest + grass crops + shrubby vegetation grass + shrubby vegetation transients transients + grass crops + grass 5 very high 3,910.78 20.13 grass total 19,427.64 100.00 figure 5. spatial distribution of the land uses in the province of manabí, ecuadorian coastal region. table 5. area and percentage distribution of the soil texture, manabí province, ecuadorian coastal region. textures class description area (km2) area (%) urban area 1 very low 64.11 0.33 water bodies 25.26 0.13 coarse texture 48.57 0.25 beaches, barrier islands, sand bank 1.94 0.01 shrimp farms 2 low 184.56 0.95 moderately coarse texture 1,466.69 7.55 medium texture 3 medium 5,055.07 26.02 fine texture 4 high 11,308.83 58.21 very fine texture 5 very high 1,272.51 6.55 total 19,427.64 100.00 spatially, there is a fine texture from north to south throughout the province with a compact presence in the western sector, while the very fine texture is concentrated in the chone and tosagua cantons. the mean texture is most predominant in the upper part of the chone and portoviejo river basins, and in the vicinity of the floodplain of the latter. the percentage of coarse and moderately coarse textures is concentrated in the el carmen canton of the north-eastern sector of the province, and in some lots isolated from the upper and middle parts of the portoviejo river basin. erosion risk map as part of the validation and calibration process of the results obtained for the soil erosion risk map, 50 control points were georeferenced in the field in 299soil erosion risk zoning in the ecuadorian coastal region using geo-technological tools which evidence of erosive processes was observed, such as gullies and erosion grooves. these control points were superimposed onto this map, thus allowing us to discover a correspondence of more than 90% with the high and very high categories. this map (figure 7) allows us to visualize the spatial distribution of soil erosion risk in the province of manabí, and also enables us to relate the different risk classes with the factors that have the greatest weight as regards their determination by interest areas (compared by the superposition of the final erosion map onto the map of each of the factors). this has allowed us to identify that the factors with the greatest influence on soil erosion risk are the slope and the soil coverage, thus corroborating the point made by vulević et al. (2015). in manabí, the cantons (flavio alfaro, chone, bolívar and pichincha) with highest erosion risk are located in the central-eastern sector of the province, concurring with the highest slopes, deforested areas and agricultural activity with inappropriate intensive use practices, such as the felling of native forests and the systematic burning of crop waste. the main cantons with medium erosion risk (portoviejo, santa ana, 24 de mayo, junín and olmedo) are located in the southeast part of the province, coinciding with low to moderate slopes and moderate precipitation amounts. the cantons with the least erosion risk (manta, montecristi and jaramijó) correspond to those located in coastal areas of center of the province, where gentle slopes prevail, and there are low amounts of rainfall and urban land use, protected by asphaltic and concrete infrastructures. in the area distribution of erosion risk shown in table 6, it observed that a low percentage of the territory shown a very high erosion risk. however, when considering the real extent that this percentage represents, government agencies consider that the erosion risk may have a high impact, and this is reflected in their planned protection activities. as an example, the project for provincial forestation and reforestation developed by the provincial government of manabí, which must invest for more than 7 years in order to cover solely this portion of manabí territory, fulfills the goal of 10,000 ha reforested per year. for this program to be successful it is necessary to propose alternative technological changes in agricultural practices so as to avoid and reduce, for example, the felling of forests and fires in crop areas, which cause changes in the characteristics of the soil surface (stoof et al., 2015). this modifies the roughness, thus increasing the erosion risk by surface flow, particularly when considering the projections of routschek et al. (2014), who claim that one of the climate change scenarios reflects the increased intensity of rainfall, with a severe impact on soil loss, until at least 2050. additionally, paroissien et al. (2015) reported that, in order to ensure soil resource sustainability, changes in their use must be taken into account. they proved that the soil erosion rate and its life expectancy are more sensitive to land use changes than to variations in rainfall. they also found that for simulations of the a1b scenario proposed by the intergovernmental panel on climate change (ipcc), the erosion rate was quantified as being 4.2 t.ha-1.y-1 with a soil life expectancy of 249 years. this reflects a decrease of 24 years when compared to current conditions. the manabí province should be of particular interest in this scenario, changes in land use associated with agricultural activities conditioned by the effects of el niño are reported, which are attributed to substantial losses of economic resources by farmers of permanent crops such as coffee and cocoa. these farmers have seen the need to migrate to short-cycle crops and pastures in order to achieve livelihoods. according to ministerio del ambiente de ecuador [mae] (2012), deforestation maps for 1990-2008 show an accelerated process of this type of anthropic intervention, with a rate over 10,000 ha.y-1 in the coastal region, conditioning the existence of soils with scarce or no vegetal protection in areas of steep slopes, moderate rainfalls and extensive agricultural activities. these features determine the strong action of erosive processes and loss of soil resources, with the consequences of a low yield in the agricultural production, figure 6. spatial distribution of the soil textures in the province of manabí, ecuadorian coastal region. figure 7. spatial distribution of soil erosive risk in the province of manabí, ecuadorian coastal region. table 6. area and percentage distribution of the soil erosion risk in the manabí province, ecuadorian coastal region. class value description area (km2) area (%) 1 ≤ 2 low 2,737.35 14.09 2 2 – 3 medium 12,925.21 66.53 3 3 – 4 high 2,989.91 15.39 4 ≥ 4 very high 775.16 3.99 total 19,427.64 100.00 300 henry antonio pacheco, williams méndez, armando moro siltation and the pollution of water bodies, and an increased hazard of floods and waterborne diseases. the combination of the erosion risk with the massively high rate of deforestation in the province has caused an increase in flood risks, not only because of the material volumes carried but also owing to the silting of riverbeds. the impacts are thus manifested in the flooded areas of the manabí province, located in the lower parts of the bolívar, chone, tosagua and portoviejo cantons and as an exceptional case in manta. the erosion risk combined with anthropic factors such as deforestation, indiscriminate land use, road construction, excavation and overloads generated by the development of structures on steep ground, similarly have significant influences on the genesis of mass removal processes. the cantons located in the central mountain range of the coast, are in this respect, those with an increased susceptibility to this morphodynamic. a direct consequence of erosion is the loss of good quality soils, thus decreasing agricultural production and signifying that farmers must use agrochemicals to improve yields or decide to fell additional extensions of forest, making the problem a vicious circle. the sediments generated by erosion are incorporated into the discharge of the rivers, creating operational problems in hydraulic projects, such as the chone-carrizal multipurpose (hydraulic infrastructure for irrigation, human consumption, hydroelectric energy and flood control, in the bolívar, chone and tosagua cantons), which is less effective owing to sedimentation, and prolonging the problem associated with the lack of water in the province. the increased sediment load in the rivers affects biodiversity because it prevents the natural reproduction of some species of fish and plants, thus causing an imbalance in the ecology of natural systems such as mangroves and tidelands, among others, and generating important losses as regards fisheries and aquaculture. finally, environmental degradation affects the socio-economic activities of the inhabitants of this territory, inducing demographic processes such as migration, which can lead to what the organización de las naciones unidas [onu] (2015) has called “environmental refugees” (human groups that move driven by environmental degradation) as the result of deforestation, rising sea levels, expanding deserts and catastrophic climatological events. all these environmental problems exist in the manabí province, where the number of environmental refugees could exceed the number of refugees caused by wars (federación internacional de la cruz roja [ficr] y la sociedad de la media luna roja [smlr], 2012). the environmental problems dealt with in this paper are of current and global importance, as was understood in the post-2015 schedule of the onu (2015), with the proposal of sustainable development goals (sdg), which emphasizes environmental concerns and economic growth. these objectives are accompanied by goals that are operationalized by means of indicators focused on measurable results whose aim is to promote to global and universal actions in nature that are applicable to all countries, considering the particular characteristics that distinguish them. conclusions the git allow the processing of useful, timely, current, accurate and abundant information regarding geospatial features. in this particular case, they have allowed us to zone the soil erosion risk in the manabí province. the rainfall erosive factor was determined by using the mfi. the ndvi proved to be a good indicator of the vegetation cover spatial distribution, and it correlates very well with the risk of soil erosion. a total of 365,872.95 ha that represent 19.38% of manabí territory are in the categories of high and very high erosion risk. the cantons of manabí with the highest soil erosion risk are located in the central-eastern sector of the province, such as flavio alfaro, chone, bolívar and pichincha, where the highest slope values with deforested areas and agricultural use with inappropriate intensive practices are. the cantons with medium erosion risk are located in the southeast part of the province, standing out among them portoviejo, santa ana, 24 de mayo, junín and olmedo. the level of risk in these areas is determined mainly by the predominance of low to moderate slopes and moderate rainfall amounts. the lower erosion risk corresponds to the coastal areas in the center of the province that are highly represented in the manta, montecristi and jaramijó cantons, with predominantly gentle slopes, low amounts of rainfall and urban land use protected by an asphaltic and concrete infrastructure. strong inter-institutional support is required in order to promote erosion risk mitigation actions. it is recommended that the decentralized autonomous governments (dag) and productive sectors make use of this information to plan the optimal use and occupation of geographical space in view of their agroecological conditions. in this respect, it is important to promote the development of programs for forestation and reforestation, such as those put forward by the provincial government of manabí. acknowledgements the authors are very grateful to the secretaría nacional de educación superior, ciencia y tecnología (senescyt) of ecuador for the financial support of the research project from which this paper has been derived. the authors would also like to thank the gobierno provincial de manabí (gpm) and the universidad técnica de manabí (utm) for the logistic support of the project as host institutions. the authors are also grateful to professor yuly asención (phd) from the university of arizona and mrs. sally newton for the reviewing the english in the manuscript of this paper. references aguirre, m. & chávez, m. 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(2013). dynamic monitoring of plant cover and soil erosion using remote sensing, mathematical modeling, computer simulation and gis techniques. american journal of plant sciences, 4(7), 1466-1493. http://dx.doi.org/10.4236/ajps.2013.47180 keywords: complex geological environment; gear and bearing; fault diagnosis; palabras clave: entorno geológico complejo; engranaje y cojinete; diagnóstico de fallas. how to cite item wang, s., liu, s. y., & meng, f. (2021). fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment. earth sciences research journal, 25(1), 115-122. doi: https://doi.org/10.15446/ esrj.v25n1.93184 the traditional research method of fault diagnosis mechanism has poor stability, which leads to the difference of fault diagnosis and location results. therefore, under the complex geological environment, a new research method of fault diagnosis mechanism of gear and bearing for coal mining equipment is proposed. this method calculates gears and bearings’ yield strength by analyzing coal mining equipment’s bearing capacity elasticity. according to the fitting degree, the equipment sample’s projection space is confirmed, the fault features of gear and bearing are extracted by segmentation algorithm, the optimal fitness is set by positioning algorithm, the location of fault center is obtained, and the fault mechanism diagnosis is studied. experimental results show that compared with the traditional method, the proposed method is more stable, and the difference in fault diagnosis results is minimal. it can be seen that this method is more suitable for fault diagnosis of coal mining equipment. abstract fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment diagnóstico de fallas de engranajes y cojinetes para equipos de minería de carbón en entornos geológicos complejos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93184 el método de investigación tradicional del mecanismo de diagnóstico de fallas tiene poca estabilidad, y puede conllevar a diferencias entre el diagnóstico de fallas y los resultados de ubicación. por lo tanto, en un entorno geológico complejo se propone un nuevo método de investigación del mecanismo de diagnóstico de fallas de engranajes y cojinetes para equipos de minería de carbón. este método calcula el límite elástico de engranajes y cojinetes al analizar la elasticidad de la capacidad de carga de los equipos de minería del carbón. de acuerdo con el grado de ajuste, se confirma el espacio de proyección de la muestra del equipo, las características de falla del engranaje y el cojinete se extraen mediante un algoritmo de segmentación, la aptitud óptima se establece mediante el algoritmo de posicionamiento, se obtiene la ubicación del centro de falla y se estudia el mecanismo de diagnóstico de falla. los resultados experimentales muestran que, en comparación con el método tradicional, el método propuesto es más estable y la diferencia entre los resultados del diagnóstico de fallas es muy pequeña. se puede ver que este método es más adecuado para el diagnóstico de fallas de equipos de minería de carbón. resumen record manuscript received: 16/03/2020 accepted for publication: 28/11/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 115-122 shuilin wang1,2*, songyong liu1, fanping meng3 1school of mechatronic engineering, china university of mining and technology, xuzhou, 221116, china 2xuhai college, china university of mining and technology, xuzhou, 221008, china 3yongcheng coal & electricity holding group co. ltd, shangqiu, 476600, china *corresponding author: wslxhxy2008@163.com m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v25n1.93184 https://doi.org/10.15446/esrj.v25n1.93184 https://doi.org/10.15446/esrj.v25n1.93184 116 shuilin wang, songyong liu, fanping meng introduction with the rapid progress of science and technology and the continuous improvement of modernization, coal mining equipment is increasingly developing towards the direction of large-scale, integrated, high-speed, precise and intelligent. with the development trend of mechanical equipment, it is the complexity of equipment. the relationship between different equipment and different parts of the same equipment is more complex, forming a unified whole. when any one of the components fails, not only the operation of the equipment itself, but also the operation of other equipment associated with the equipment will be affected, which will cause a series of chain reactions, resulting in the interruption of the operation of the whole equipment, affecting normal production and product quality. on the other hand, affected by the geological conditions and hydrological environment of coal mining, under the complex natural environment, the mining difficulty of coal mining equipment is greatly increased. excessive operation makes the gears and bearings in the equipment prone to failure. therefore, through real-time monitoring of the working conditions of gears and bearings, the traditional research methods, obtain the operation parameters of parts, and statistics the fault location through the difference method, to find the nodes prone to failure and the time of failure. it is found that the signal of the fault location is unstable due to the poor ability to analyze the fault characteristics, which makes the accuracy of the fault mechanism decline. therefore, a new fault diagnosis mechanism research method is proposed. according to the relationship between gears and bearings, the method resets more stable fault feature extraction and location according to the acquisition method of cyclostationary signal and projection decomposition operation method to ensure the reliability of fault diagnosis mechanism (morris et al., 2018). this method not only solves the existing problems of the traditional methods, but also provides scientific and technical solutions for the fault diagnosis mechanism research of gears and bearings in other fields, and provides reasonable research methods for the development of national equipment and technological innovation. research method of fault diagnosis mechanism of gears and bearings for coal mining equipment in complex geological environment calculation of yield strength of gears and bearings in complex geological environment on the basis of defining the basic parameters of a complex geological environment, the yield strength of gears and bearings is calculated. as all gears and bearings will not be perfect, under the influence of a complex mining geological environment, the top of gears may buckle within a reasonable range due to the influence of bearing force during operation (balestro et al., 2018). therefore, to determine the directional elasticity of the stability of gears and bearings, the method of angular displacement can be used for directional analysis. under the complicated geological conditions, the equation of angular displacement of gears after bearing the axial force is as follows: β µ µ λ β µ µ λ a a b b b a f a b a b d f b a a b d = + − +( )          = + − +( )               (1) in the formula, f ei d= / represents the linear stiffness of the bearing, f ei d= / , f ei d= / represents the size of the gear, a and b represent the left and right ends of the gear respectively, a and b represent the angular displacement of the two ends of the gear, a and b represent the deformation angle of the two ends of the gear respectively,  represents the lateral displacement value of the bearing, a and b represent the stability function and the bending stiffness coefficient. figure 1 is a schematic diagram of the gear tooth deformation curve under the axial force. in the figure above, it is the displacement of a single gear tooth after deformation, and the critical load can be calculated only according to the rai sed section recessed section force ba force amplification gear tooth top cross section figure 1. schematic diagram of the elastic load capacity of the gear 117fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment balance equation. however, the fault situation shown in the figure is relatively simple. under the condition of bearing and gear tooth deformation, the tooth slot will also be deformed. therefore, the shear balance equation should be established to solve the elastic buckling load. figure 3 is the structural diagram of the gear after the deformation of the teeth and grooves. assuming that there is vertical load on the axis at the upper and lower ends of the structure, it is represented by p1 and p2 (wang et al., 2018). reverse rotation forward rotation support force contact force a b f2 f1 r1r2 figure 2. schematic diagram of the load structure of the gear under the load according to the above figure, in order to facilitate the analysis of the load elasticity of the gear, it is assumed that the a and b ends of gear mi buckle at the same time with the other gear mj in contact with each other, and that aa' and bb' are the contact ends. at this time, the end angles of the cross section in the same layer of gear are the same size and opposite direction, that is, a a a a =− ' , a a b b =− ' . it is known that the k value between the gears is the same, and the bending stiffness coefficients are both a b= and a b= . by default, the gear in the same layer buckle at the same time when losing stability. then, according to formula 1 and the balance condition p p 1 2 = − , the elastic value of the bearing capacity of the gear slot under the tooth deformation is calculated by using the angle displacement method. f ei k a b a b f ei k a b a b aa m a b bb m b a i j ′ ′ = + − +( )    = + − +( ) 1 2 β β η β β η           (2) in the above formula, k1 represents the elastic value of bearing capacity of contact point aa', k2 represents the elastic value of bearing capacity of contact point bb', eim i represents the transfer force of target gear mi, eim j represents the bearing force of gear mj adjacent to mi,  represents the force balance coefficient, and  represents the force control coefficient. it is known that the sum of equilibrium conditions of node a and node b is 0, and there is f f faa bb' '=− +∆ , that is, f f faa bb' '+ −( )+∆ = 0 , where f is the force of other values. according to the equilibrium condition, let k k k= +( )1 2 2 , thus, the instability condition of gear under high strength operation is as follows: sk f f mhaa bb′ ′+      −( ) = 2 02 (3) in the formula, s represents the linear stiffness of the gear under the action of external force, m represents a non-zero natural number, and h represents the constraint condition. the trigonometric function is substituted to the above formula, and j= f ei mh/ is made to obtain the buckling equation of gear teeth and bearings: sk f f k aa bb′ ′+ −       + = 2 02j j jtan (4) where  is the buckling evaluation parameter. the above equation is the elastic analysis equation of bearing capacity of gear and bearing. it should be noted that the bearing will also be affected by the horizontal load force, so under the condition of meeting the above formula, the elastic calculation expression of gear shear balance is: ′ = × +( ) − +( ) + k ei d a b a b ab b c (5) in the formula, c represents the constant of change under the influence of bending stiffness coefficient a and b, and ei force modulus. when k' is greater than the specified elastic value, it means that the gear is distorted under the influence of load. under the influence of formula 5, the plastic value of the bearing capacity of the false gear is set as f ei d= /, and when the value of f ei d= / exceeds the maximum limit of the gear, the yield strength of the gear is calculated. according to the yield strength value, the critical value of functional failure of gear bearing capacity is confirmed. the basic material composition of the gear is known, and the amplification coefficient of the compressive strength of the material is calculated. the formula is: cdif p p e e = =         1 2 1 2 j (6) in the formula, e1 represents the strain rate and e1 ≥ 30s-1, e2 also represents the strain rate and e2 = 30  10-6 s-1; p1 represents the dynamic compressive strength of the material under e1, p2 represents the static compressive strength of the material under e2; j= +      −3 4 3 1p u represents the force angle under impact, wherein p3 represents the compressive strength under the total volume of the material (jeong et al., 2018). the increase coefficient of tensile strength is: tdif q q = =         1 2 1 2 τ τ σ (7) in the formula,  1 represents the strain rate under the influence of the force,  1 ≤ 1s-1,  2 represents the strain rate under the influence of the force, and 2 = 10-6 s-1; q q1 2= represents the dynamic tensile strength of the material composed of gears under  1 , q q1 2= represents the static tensile strength of the material under  2 ;  = 10-6 n-2 represents the force angle, where 118 shuilin wang, songyong liu, fanping meng n q q = +      − 1 8 13 0 1 , q q3 0= represents the standard static tensile strength that the material can bear, q q3 0= represents the static tensile strength of a variable material. by combining the formulas 6 and 7, the yield strength calculation formula is obtained z f m z ej j eff= +                     +( )1 1 0τ ω α ω (8) in the formula, z0 represents the initial yield stress,  represents the strain rate, weff represents the equivalent plastic strain, ej represents the plastic hardening modulus,  represents the hardening parameter, when  = 0 represents the follow-up strengthening, m and j represent the constants respectively. when z < 0.7, it means that the frame gear can bear the current working strength, the gear is in good condition, and the bearing capacity function has not been lost; when 0.7 ≤ z ≤ 0.8, it means that the gear can bear the current working strength, but affected by the geological conditions, at this time, the gear has the situation of tooth section depression or external convex, and part of the bearing capacity function; when e > 0.8, it means that the gear is seriously deformed or the gear teeth are directly broken, unable to withstand the current operation, and the bearing capacity function of the gear is completely lost (walker et al., 2018). extraction of gear and bearing fault features the real-time running state of the bearing is obtained by using the monitoring system. it is known that the monitoring data will be distorted due to salt and pepper noise, gaussian noise and reflected half point noise in the process of recording and transmission. therefore, the bearing monitoring data is estimated by using the geometric shape analysis method and digital analysis method of mathematical concept. through the analysis of the relationship between the elements, the bearing monitoring data is estimated, and the linear least square method is used to measure the impact of noise on the data, that is, according to the calculated minimum average variance, the monitoring background that can be fitted is determined. therefore, the calculation formula of the fitting degree is: f x k x k x k x k x( )= + + +1 2 3 4 (9) in the formula, k1, k2, k3 and k4 are respectively represented as the influence of background information on noise in each direction, and x is the mutation value of noise (spartalis et al., 2018). the average variance is calculated according to the formula 9: s z x k xi i i n 2 2 1 = − ( )    = ∑ (10) in the formula, xi represents the gray value of the i-th column of the monitored data. at this time, the calculation result is the noise factor of background information in four positive directions. due to the inaccurate fault location and calculation deviation, the information interference in other directions is corrected by information extraction, and the median filter is weighted and added. at this time, the corrected background noise estimate is: v u u w= +1 2 2g (11) in the formula, according to the 2 value obtained by the above fitting, w represents the median filter, u1 and u2 represent the weighting coefficient, and u1 + u2 = 1, so as to obtain a more accurate noise background. according to this value, the interference noise is proposed, and the complete bearing monitoring data is obtained. in order to detect the defects of the pre-processing monitoring data, it is necessary to determine the fault area and segment it. according to the monitoring data, roi is the region of interest. the bearing in this region is judged and segmented according to the result. according to the imaging principle, known bearings can be divided into three areas: the upper background area, the middle imaging area and the lower turntable area (ganson, 2018). according to the principle of uniform direction and gray projection algorithm, the distribution rule of bearing projection is analyzed to determine the scope of fault analysis. according to the above calculation results, the gray projection on the calculation axis is: g y v h x y i v ( )= ⋅ ( ) = ∑1 1 , (12) in the formula, v is the background value calculated by formula 11, h(x, y is the projection normal distribution function. according to the calculation results, the curve characteristics are analyzed, to determine the bearing monitoring data on the y-axis of the imaging boundary. figure 3 is the sample space schematic diagram of the projection curve. boundary conditions projection range figure 3. sample curve projection space the two curves in the image are the constraint conditions for generating projection curves for non damaged samples, scraped samples, bumped samples and over sharpened samples. it is known that no matter what type and specification of the bearing for coal mining equipment, its projection shape is similar, the upper background area has no detection objects, and there is no effective monitoring data; the bottom area has a certain degree of fluctuation because of the problem of reflection; the middle position is the main data monitoring area of the bearing, because the bearing itself is smooth, there will be reflection phenomenon, thus, the curve of this projection will fluctuate violently. according to the above analysis results, the region roi calculation algorithm is set up: the gray projection curve is filtered by means of mean value, and the curve is derived by smooth value, so that it passes through the zero point, and then the maximum value point and secondary value point of the curve are found out from the zero point, the abscissa is set as x1 and x2, and (0,c1 and (0,c2 are taken as control points to establish a rectangular region, which is roi (liu & zhu, 2018). because the image boundary of the bearing in the roi area at this time is relatively fuzzy, which will affect the classification of defect categories, the wave value transformation segmentation algorithm is adopted to accurately segment the image area of the bearing, as shown in figure 4. 119fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment background area transition area target area figure 4. schematic diagram of the segmentation process it can be seen from the figure that there may be errors in the boundary of the segmented region. one is to divide the background into the target region, and the other is to divide the target pixels into the background region. in order to avoid such a situation, morphological processing is carried out. morphological treatment is to expand and corrode. expansion is to merge all the contact points of the object’s contact background into the object, so that the boundary expands outwards, while corrosion is to eliminate the fictitious contact points, so that the boundary contracts inwards. the calculation process is as follows: u u s w w s u w s 2 1 1 1 2 2 1 1 3 − = − = − =      (13) in the formula, u1 and u2 represent the area obtained by two expansions, w1 and w2 represent the area obtained by two corrosions, s1 represents the part around the boundary that does not belong to the target range, s2 represents the area around the boundary that belongs to the target range, s3 represents the area that cannot be determined, so the area is divided into bundles. on the basis of region segmentation, the feature statistics and extraction of these data are carried out, and the rule subsets are established according to these features, so as to reduce the deposition data and improve the accuracy of fault diagnosis. the shape feature of gears and bearings in coal mining equipment is one of the important features of fault identification, so the common fault shape feature shown in table 1 below is listed (ali et al., 2018). table 1. basic shape characteristics when gears and bearings fail serial number characteristic shape feature description 1 area amount of data in the split area 2 perimeter amount of data in adjacent divided regions 3 diameter the horizontal distance between the two furthest data points on the boundary 4 center of gravity horizontal center of gravity, vertical center of gravity 5 aspect ratio aspect ratio 6 number of cracks easy to describe and quickly calculated values in the segmented area 7 thin length ratio of perimeter to area, ratio of diameter to area 8 compactness data to measure the complexity of the boundary 9 similarity ratio of vectors 10 symmetry symmetric distance after region rotation according to the data in table 1, the bearing deformation features are extracted. it is known that the color distribution of bearings and gears is divergent under the condition of light source, light and object reflection in machine vision. assuming that the hue is m, the saturation is d, and the brightness is r, the color distribution is calculated: µ= +      + − +       + = ∑e m m d d d d i n max min max min ln 1 1−− −      r 0 5 0 5 . . (14) in the formula, e represents the parameters of the segmented area, and m represents the hue resolution. through the color resolution model, the color features of gear and bearing images are obtained, and then the texture features are counted. because the change of texture image has certain regularity, which is embodied in the attributes of density, thickness, uniformity, regularity, linearity, frequency, etc., according to these attributes, the image area is set as w(x,y, the size is i×j, and the calculation formula is as follows: i x f x j x i f x i i i n i i i n = ( ) = −  ( )      = = ∑ ∑ 1 1 (15) in the formula, xi represents the gray level series, f xi( ) represents the probability function of gray level series, which is the statistical result of texture features. in the process of feature extraction, it is necessary to make the selected features meet the separability constraints, which is to be representative, so that they can not only reflect the characteristics of all aspects, but also reflect the personality of the object; secondly, the graphic features should have very strong anti-interference, so as to ensure that the influence of different lighting, light source, shape and other factors is small and stable, the feature values of image content in the segmentation region are similar, and the selected features are independent of each other. in this paper, the scale feature extraction method is used to extract data based on the statistical results of features. the image is transformed to another space by function or matrix. the transformation methods include frequency domain transformation and geometric projection transformation. the data with different functional features in feature grouping are extracted and a new feature set is established. according to this feature, the calculation results are as follows: ε µζ κ κ= ⋅ ⋅ +( )n i j2 2 (16) in the formula, n represents n shape values, , i and j are the result statistics values of formula (2-7), so as to realize the extraction of gear and bearing fault feature data in coal mining equipment in a complex geological environment (sayers et al., 2019). fault location parameters setting of the location algorithm on the basis of defining the fault characteristics of gears and bearings, the fault location algorithm is developed according to the basic operation mode of coal mining equipment. generally speaking, the first step is to determine the fault range of gears and bearings. using cs technology to set up a search program, the target range of fault location is defined. the step factor is calculated, which is the key parameter to control the breadth of the search area. when the step factor is too large, the search program is controlled to start a full regional search; when the step factor is too small, the parameter can control the search program to start a local search in a local area. in order to control the search balance of the search program in the whole region and the local region, a balance adaptive adjustment parameter is added to make the search program distributed and synchronous search. the calculation formula of the step factor is: 120 shuilin wang, songyong liu, fanping meng r r k r r n f f fs = − −( ) × − max max min t 1 2 1 (17) in the formula, rs represents the search step of the s-th time, where s represents the current number of times; k represents the control parameter; rmax represents the maximum upper limit of step factor; rmin represents the minimum lower limit of step factor;  represents the adjustment parameter introduced; n represents the maximum number of iterations; f f f fi ri1 2= = = represents the optimal function under the initial fitness; f f f fi ri1 2= = = represents the optimal function suitable for this iteration. at the same time, under the search control of this parameter, the discovery probability of the approximate location of the equipment fault is calculated. in cs technology, the discovery probability is generally between 0.2-0.3, and the probability under this parameter is set to p. when the probability is always in a large value, the convergence speed of the fault search can be accelerated; when the value of the discovery probability p is small, the convergence rate is reduced. therefore, the optimal fitness is set by the ratio of the optimal fitness and the previous generation solution. f f f f f i ri i ri = > ≤     1 0 (18) in the above formula: f f f fi ri1 2= = = represents the optimal fitness of the previous generation; f f f fi ri1 2= = = represents the fitness of the i-th solution under the action of rs. according to the above calculation formula, the detection probability calculation results obtained by adjusting the adaptability of the detection probability are as follows: ′ = = + −( )⋅ ′      = ∑p d f p p p p p i i n / min max min 1 (19) in the formula, p' represents the high-quality probability of the solution after iteration; di represents the optimal fitness for the i-th solution; n represents the total number of solutions; pmin and pmax represent the upper and lower limits of the discovery probability respectively. combining the above two sets of formulas, the search parameters are obtained, and the capture function is set by the parameters to extract the fault nodes of gears and bearings within the range of the target. according to the carrier frequency of the data, the capture function can get the code phase of the defect data and realize the acquisition of data nodes in any area. the capture times of the capture function are calculated as follows: n j d j b= × × +      1 0 2 (20) in the formula, n represents the number of acquisition times, ji represents the number of times of code phase, j0 represents the total step of wave frequency, d represents the step of wave frequency of defect data, and b represents a fixed constant, usually 1. the capture function is set according to the capture times, and the capture parameter f x( ) is introduced, as follows: dft f x q dx w s g t q dx a st t n f a st t = ( )    = ( ) ( )    −∫ ∫ 2 0 2 0 p p = ( ) ( )w s w sn f * (21) in the above calculation expressions, dft represents the fourier calculation result of the acquisition function under the condition of discrete transformation; q represents the degree of acquisition; x represents the independent variable of the acquisition function; q represents the exponential function; a represents the frequency of acquisition; s represents the conjugate value; t represents the target range space; g tf ( ) represents the real-time fault signal; wn(s represents the fixed value of the acquisition strength of the fault signal under n times of acquisition; w sf * ( ) indicates the actual acquisition strength of fault data when the acquisition strength is f ei d= /, * indicates the transformation flag. in the obtained defect nodes, two adjacent defect nodes are randomly selected as samples, to calculate the defect center point by distance scaling method, and the fault sample curve is selected, as shown in figure 5 (heffner et al., 2018). change time sub-peak maxim num peak d yn am ic p ea k ch an ge in di ca to r figure 5. fault peak curve the selected fault samples need to meet the basic requirements of two peaks. the fault data of gears and bearings are “flat”. assuming that the abscissa of high peak value and sub peak value in “flat area” is x1 and x2 , the center position c0 and peak width l of the area are calculated. the calculation results are shown in the following formula: c x x sh l x x 0 1 2 1 2 = − = −      h (22) in the formula, s is the number of peaks, and s ≥ 2; h is the error correction coefficient;  is the width measurement parameter. according to the above calculation results, the distance between the fault node and the fault center position is determined. according to the real-time changing fault intensity indicator, combining formulas 21 and 22, the random dynamic change parameter  is calculated. the formula is: γ ε ε α = + + × ⋅ d c l dft 0 0 1 (23) in the above formula,  is the adjustment control factor of defect peak; d0 is the adjustment factor of dynamic parameter;  is the peak active direction of the fault with different intensity; dft is the discrete transformation parameter. the linear regression model with activation function is as follows: 121fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment y z q z h zi i i n ( )= ( ) + ( ) = ∑ w 1 (24) in the formula,n is the number of multi-peak defect nodes between gear and bearing; y(z is the expected output value; qi(z is the calculation regression factor in the i-th fault section of gear or bearing; i is the connection weight between fault nodes; h(z is the calculation residual. according to the model and the variation parameter , the defect center is obtained c y z y z = ( ) ( ) ∑ ∑ g 2 2 (25) according to the obtained central coordinate position, the hardware fault edge is determined, and the specific range of fault position is obtained (barrero et al., 2019). the boundary coordinates of the fault location are obtained by the least square method. according to the above calculation, the defect center c is obtained. supposing there are n anchor nodes from the center. within the determined fault target range, each coordinate can be represented by a1,a2,...,an. then, for each anchor node, there is an= {(x,y) | (x1,y1),(x2 ,y2),...,(xn ,yn)}, an unknown node is set as aj , the node coordinate is (xj , yj), and the node is assumed to be the true coordinate of the fault. the relationship between the unknown node and the anchor node is: x x y y d x x y y d x x y y j j j j j n j n −( ) + −( ) = −( ) + −( ) = −( ) + −( 1 2 1 2 1 2 2 2 2 2 2 2 2  )) =      2 2dn (26) d1,d2, ...,dn in the above formula represents the estimated distance between the unknown node and the anchor node. the coordinate boundary obtained by the formula is shown in figure 6. failure node tracking function lossless node figure 6. obtained coordinates of fault boundary it can be seen from the above figure that the gray area in the figure represents the true range of the fault, and the white area is the location of other nodes of the equipment affected by the fault. the boundary coordinates are obtained by the least square method. the formula (3-10) is rewritten into the form of ax=b, and the objective function min ||ax=b ||2 is obtained by using the least square method x x y =         0 0 (27) in the formula, x0 and y0 represent the abscissa and ordinate of the fault boundary respectively. through the calculation process, the specific boundary coordinates of the fault in figure 6 above are obtained, the influence scope of the fault in the mining equipment is determined, and the diagnosis mechanism of gear and bearing fault for mining equipment is studied under complex geological conditions (vaziri et al., 2018). experimental study in this paper, a comparative simulation experiment is proposed to simulate the complex geological environment in the process of coal mining (yang et al., 2020). opening the simulation test software, the bearing fault effect in the coal mining equipment is simulated. table 2 below is the fault value to be simulated and the fault frequency parameters to be input (wu et al., 2020). table 2. simulated bearing failure frequencies item number fault frequency name failure frequency a1 rolling bearing outer ring failure frequency 162.178 a2 rolling bearing inner ring failure frequency 227.822 a3 rolling bearing rolling element failure frequency 86.593 a4 failure frequency of cage touching inner ring 12.475 a5 failure frequency of cage touching outer ring 17.525 different bearing speeds are set. when the bearing speed is too high, the gear working frequency will be too high. at this time, the bearing signal indicators that cause bearing failure and gear failure are shown in table 3 below. table 3. indicator values of bearing vibration signals at different speeds different speeds and signals segment indicators rotating speed signal m1 m2 m3 m4 m5 800r/min normal 1 0.6055 0.4791 0.3451 0.2477 0.0342 normal 2 0.6012 0.4792 0.3487 0.2921 0.0371 internal loss 1 0.5805 0.4788 0.3561 0.2858 0.0475 internal loss 2 0.5865 0.4874 0.3563 0.2687 0.0411 external loss 1 0.6345 0.4851 0.3522 0.2751 0.0480 external loss 2 0.6327 0.4792 0.3566 0.2741 0.0382 1200r/min normal 1 0.6281 0.4853 0.3485 0.2736 0.0567 normal 2 0.6204 0.4722 0.3669 0.2819 0.0552 internal loss 1 0.5977 0.4621 0.3851 0.2654 0.0523 internal loss 2 0.5947 0.4673 0.3554 0.2647 0.0604 external loss 1 0.6381 0.4615 0.3582 0.2708 0.0478 external loss 2 0.6296 0.4628 0.3479 0.2432 0.0455 according to the data in the table above, the fault parameters of gears and bearings for coal mining equipment is simulated in a complex geological environment under high-intensity work. two methods are used to analyze 122 shuilin wang, songyong liu, fanping meng the fault diagnosis mechanism. the proposed research method is used as the experimental group, and the traditional research method is used as the control group. the test results are shown in figure 7 (cheng et al., 2020). stationary curve a m pl itu de (v ) a m pl itu de (v ) difference curve testing time(s) (a) experimental group test result testing time(s) (b) control group test result expectation curve stationary curve difference curve expectation curve figure 7. experimental test comparison results according to the curve trend in the figure above, it can be seen that the stability curve of the proposed research method is similar to the expected curve, while the stability curve of the traditional research method fluctuates violently, which is quite different from the expected curve. it can be seen that the stability difference value of the proposed research method is far lower than the 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(2020). effects of water temperature on tissue depletion of florfenicol and its metabolite florfenicol amine in crucian carp (carassius auratus gibelio) following multiple oral doses. aquaculture, 515, 9. doi:10.1016/j. aquaculture.2019.734542 climate teleconnection signals are one of the main factors influencing the earth's climate oscillations in global and regional scales. in the present research, the effect of these signals on precipitation and discharge of madarsoo watershed at the upstream of golestan dam was investigated. for this purpose, three raingauges and hydrometric stations with respectively 40 and 38 years of daily rainfall and discharge data were selected. pearson-correlation coefficient was used to consider the correlation between climate signals and monthly mean and extreme precipitation and discharge. the results showed strong correlation between monthly total precipitation of tangrah and tamer with sea level pressure (slp) of caspian sea with 7 and 9 months of lag, respectively, and galikesh with sea surface temperature (sst) of caspian sea with 9 months of lag. for monthly mean discharge of tangrah and galikesh, the maximum correlation was calculated for slp of caspian sea with a lag of 9 months and tamer with sst of greenland with a lag of 7 months. for the extreme monthly data, very strong correlation was detected between the precipitations of all raingauges in june with greenland sst with 8 months of lag. in contrast, for maximum monthly discharge of tangrah and galikesh hydrometric stations, the maximum correlation coefficients were calculated for sst of black sea with 4 months of lag in august and for tamer with slp of caspian sea with 4 months of lag in august. with regard to the results, the utilization of these signals, especially slp and sst, is strongly suggested to predict the maximum and mean monthly precipitation and discharge over this region. las señales de teleconexión atmosférica son uno de los factores principales que influencian las oscilaciones climáticas de la tierra a escala global y regional. en el presente estudio se investigan los efectos de estas señales en la precipitación y caudal de la cuenca hidrográfica madarsoo, río arriba de la presa golestan. con este propósito se seleccionaron tres estaciones pluviométricas e hidrométricas con registros diarios de lluvia y caudal durante unas cuatro décadas. se utilizó el coeficiente pearson para considerar la correlación entre las señales climáticas y los índices medio y extremo mensual de precipitación y caudal. los resultados muestran una fuerte correlación entre la precipitación total mensual de las estaciones tangrah y tamer con la presión barométrica a nivel del mar (slp, inglés) en el mar caspio, en un período de siete y nueve meses, respectivamente, y de la estación galikesh con la temperatura a nivel del mar (sst, inglés) en el caspio durante nueve meses. para la media mensual del caudal en tangrah y galikesh, la correlación máxima se calculó a través de la slp del mar caspio con un período de nueve meses, y la de tamer con la sst del mar de groenlandia en un lapso de siete meses. para el índice extremo mensual se detectó una fuerte correlación entre las precipitaciones medidas en todos los pluviómetros en junio con la sst de groenlandia en un período de ocho meses. en contraste, para el índice máximo mensual de caudal en las estaciones hidrométricas tangrah y galikesh se calcularon los coeficientes de correlación máxima para la sst del mar negro en un lapso de cuatro meses en agosto y para la estación tamer con la slp del mar caspio durante cuatro meses, también en agosto. con respecto a los resultados, se recomienda la utilización de estas señales, especialmente la slp y la sst, para predecir los índices de precipitación y caudal medio y máximo en esta región. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 65 72 abstract resumen key words: climate signals, sea surface temperature, sea level pressure, precipitation, discharge correlation palabras clave: señales climáticas, temperatura a nivel del mar, presión barométrica a nivel del mar, precipitación, caudal, correlación. assessing the correlation between climate signals and monthly mean and extreme precipitation and discharge of golestan dam watershed hossein ruigar¹, saeed golian² 1m.sc. student, civil engineering department, university of shahrood, email: hossein_ruigar@yahoo.com 2assistant professor, civil engineering department, university of shahrood, email: s.golian@shahroodut.ac.ir meteorology record manuscript received: 27/11/2013 accepted for publication: 14/04/2015 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.40996 66 hossein ruigar, saeed golian introduction in recent decades, the water demand has increased significantly in all domestic, agricultural and industrial sections, especially in arid and semi-arid areas where supplying water for different sectors is facing with serious problems. prediction of rainfall and surface water runoff can help in managing water-related plans. in recent years, the interaction of climate factors on land, ocean and atmosphere has been investigated by many hydro-climatological scientists. climate teleconnection signals in global and regional scales are one of the main factors influencing climate oscillations of earth. the most important widely-used signals are el-nino southern oscillation (enso), north atlantic oscillation (nao), arctic oscillation (ao), pacific decadal oscillation (pdo) and southern oscillation index (soi). periodic changes in sea level pressure (slp) and sea surface temperature (sst) are other climate signals which have presented the opportunity to test the scientific community’s skill in applying climate prediction models and estimating their effects on global climate variables, e.g. precipitation and temperature. many studies have shown the significant effects of all aforementioned climate signals on precipitation and discharge of large basins, worldwide. angel (1981) compared sst variations in the equatorial eastern pacific (0–10°s, 180–90°w) with variations in atmospheric temperature, circulation, rainfall and trace-constituent amount. a significant negative correlation was acquired between the values of sst in the east pacific and queensland of australia rainy season. nichols (1989) showed that the surface temperature can be part of the indian ocean and the arabian sea. two patterns were correlated to sea surface temperatures in the indian and pacific oceans. the first rainfall pattern was best related to the difference in sea temperatures between the indonesian region and the central indian ocean. the second rainfall pattern was related to equatorial pacific sea surface temperatures. this relationship reflected the influence of the southern oscillation on both sea surface temperatures and australian rainfall but the relationship between the first rainfall pattern and the difference between indonesian and central indian ocean sea surface temperatures was largely independent of the southern oscillation. kahaya and dracup (1993) studied the relationship between the el niño/southern oscillation (enso) and unimpaired streamflow over the contiguous united states. coherent and significant streamflow responses to hypothesized enso forcing were found in four regions of the united states: the gulf of mexico, the northeast, the north central, and the pacific northwest. once an enso event set in, a long-range forecasting utility may be available for these regions. roucou et al. (1996) studied the atmospheric structure over north-east brazil during anomalous rainfall years in the 11 levels of the outputs of the laboratoire de météorologie dynamique atmospheric general circulation model (lmd agcm). the simulated march-april rainfall was in good agreement with observations. correlation of simulated rainfall and three sst indices was relative to the equatorial pacific and northern and southern parts of the atlantic ocean exhibited stronger relationships in the simulation than in the observations. hughes and saunders (2001) had considered monthly temperature and rainfall for a period of 95-year at the area of europe and the southern oscillation index (soi) and nino3,4 index were used to evaluate the correlation with rainfall. in their study it was shown that the correlation between seasonal rainfall (during march, april and may) with seasonal nino 3.4 (during december, january and february) was more than monthly and yearly correlation. moron et al. (2001) have done three experiments started from different initial conditions with the echam-4 atmospheric general circulation model (gcm) integrated at t30 resolution forced with the observed sea-surface temperature (sst) over the period 1960–1994. they showed that 35 to 55 percent of annual rainfall oscillations in tropical america in the amazon area are explained with oscillations in tropical pacific waters. smolenski (2004) examined the mechanisms involved in the interactions among large-scale atmospheric circulation patterns and how they are related to surface air temperature anomalies in the north american arctic. he believed that the negative phase of the arctic oscillation pattern, at low temperatures in north america and europe, can cause different large-scale atmospheric circulation regimes. gachrel (2004) found a good correlation (0.47, 5-month time delay) between the enso activity and the discharge of the main rivers in french guiana. shrestha and kostaschuk (2005) used harmonic analysis to examine the impact of the el nino/southern oscillation (enso) on mean-monthly streamflow variability in nepal. el nino caused below normal streamflows in two core regions: the western region and the eastern region. there was a stronger el nino influence on streamflows compared to la nina. jochen and wolfgang (2006) presented retrospective forecasts of the maximum water level in central amazonia by models based on the soi and sst anomalies of the el niño 3.4 region in february, four months before its appearance. the forecast of the floodpulse allowed also predicting parameters correlated with the flood-pulse (e.g., tree growth, biogeochemical cycles) and increases the efficiency in planning and executing of economic activities by the human population (e.g., fishery, timber extraction, agriculture).kim et al. (2007) analyzed the effects of large-scale climate signals such as nao and soi on the seasonal rainfall of colorado river basin in us. their results showed that modeling the dynamic systems of the climate can be useful in developing a long-term prediction model which will be valuable for the management of water resources. in another study, revadekar and kulkarni (2008) had considered the influence of enso on extreme winter rainfall in india. the relationship of el nino-southern oscillation index with these extremes showed that this index can be used to predict frequency and intensity of extreme precipitation events, 4–6 months in advance. some research have been conducted in various parts of iran to investigate the relationship between large-scale climate signals with precipitation and discharge. nazemosadat and cordery (2000) showed that the enso phenomenon has a significant effect on winter and particularly autumn precipitation in iran. their investigations suggested that, due to the intense warm (or el-niño) and cold (or la niña) phases of enso, the intensity and predictability of dry and wet conditions in iran are substantially altered. their studies indicated that winter precipitation (january to march) in these regions is in inverse proportion to sst of the persian gulf. ghayour and khosravi (2001) studied enso signals and impacts on precipitation and other related elements during summer and autumn. the results suggested that precipitation significantly correlated with enso in autumn and its amount in enso warm phases (el nino) was greater than cold and natural phases. yarahmadi and azizi (2007) studied on the relationship between rainfall in autumn, winter and spring with climate indices through regression analysis of the 25 synoptic stations of iran. their results indicated that from climate indices associated with enso, niño 3.4, have the highest relationships with iran autumn and winter rainfall. motamedi et al. (2007) used annual and seasonal correlations between soi and rainfall data and also between soi and temperature data. they concluded that when soi amounts are increased the temperature in khorasan province will be decreased. fallah-qhalhry et al. (2009) evaluated the role of climate change in signals on spring rainfall oscillations in the khorasan province. results showed a strong relation between sea level pressure (slp) and sea level pressure difference (δslp) changes with the rainfall of the studied areas zarehabyaneh and bayat varkeshi (2012) studied the number of rainy days and effect of enso phenomenon at the country level over iran. the results of applying the pearson correlation analysis, expressed the relationship between enso climatic indexes and the number of annual rainfall days. the present research aims to assess the correlation between large scale climate signals, i.e. enso indices (including nino1+2, nino3.4, nino3, nino4, mei), ao, nao and pdo and also sst and slp signals with monthly rainfall and discharge data (average and extreme values) of some raingauges and hydrometric stations over madarsoo watershed at upstream of golestan dam, iran. 2case study: the study area is madarsoo watershed at the upstream of golestan dam, north of iran. madarsoo watershed is located between 53° 51' 32'' e to 56° 19' 21'' e longitude and 36° 30' 43'' n to 38° 08'13'' n latitude. it 67assessing the correlation between climate signals and monthly mean and extreme precipitation and discharge of golestan dam watershed covers a 5155 km2 area and stretches between three provinces, northern khorasan, semnan, and golestan. the average annual temperature and precipitation are 17.8 °c and 450 mm, respectively. data for three raingauges, namely tangrah, tamer, and galikesh, as well as three hydrometric stations with the same names were used to determine the correlation between climate signals and rainfall and discharge of this basin. figure 1 shows the location of the watershed as well as the raingauge and hydrometric stations used in this study. table (1) shows some statistical properties for total monthly precipitation and mean monthly discharge in the raingauges and hydrometric stations used in this study. table 1some statistical properties of precipitation and discharge data figure 1 location of raingauges and hydrometric stations in madarsoo watershed for this purpose, the average monthly rainfall and discharge of all the stations as well as the maximum monthly values were calculated for different months. 3methodology 3-1introducing climatic indices el niño/southern oscillation (enso) event is one of the most important and prominent events that leads to the emergence of major climate anomalies in many parts of the world (alexander et al., 2005, nazemosadat et al., 2003). walker and bliss (1932), for the first time, observed global scale fluctuations at sea level pressure and named it southern oscillation (so). el–nino is the ocean component of enso and is related to large changes in the sea surface temperatures in the tropical region of pacific ocean. el-nino phenomenon is always associated with some changes in atmospheric pressure at sea level; that is why, meteorologists have never considered it as an isolated phenomenon; but, they are usually referring to el-nino southern oscillation or enso. enso resembles a pendulum with oscillation between el-nino conditions (hot water) and la-nina events (cooling water). there are many indices related to enso, which include nino1+2, nino3, nino3. 4, nino4, mei (multivariate enso index), and tni (trans niño index). enso occurs when the difference between sea level pressure (slp) is high at the equator and the sea surface temperature (sst) is low (neelin et al. 1998). mei index (multivariate index enso) is one of the most important enso indices, which considers many parameters as follows: mei= f(p+v+u+s+a+c) where u is zonal wind component surface, p is pressure at sea level, s is sea surface temperature, v is meridian wind component level, c is the cloud grade in the sky, and a is the surface temperature (wolter, 1987). north atlantic oscillation (nao) index is defined as the tendency of pressure to be low near iceland in winter where pressure is high near the azores and southwest europe. nao is calculated based on the normalized sea level pressure difference (p) between the tropical azores (a) and nearpolar low pressure area in iceland (i) as follows (marshall et al., 2001): nao= p(a) p(i) another teleconnection climate index is arctic oscillation (ao) pattern. this index as one of the north hemisphere teleconnection patterns which is defined as the anomalies of atmospheric sea level pressure in the north arctic and mid-latitude areas (45° north latitude). iran is located in a region where various pressure systems, namely mediterranean, atlantic, and siberian, are affecting the country. the formation of these pressure centers, under the governance of arctic pressure patterns, can introduce a remote driver for atmospheric pressure centers in the northern arctic latitudes and temperature oscillations in iran (regior et al., 2000). pacific decadal oscillation (pdo) is a climate phenomenon related to anomalous circulation oscillations which occurs in a decadal time scale. pdo is associated with the stability of two-phase climate patterns in the north pacific with 50-year time period variations (bond and harrison, 2000). pdo is determined using the patterns of sst in the north pacific ocean. southern oscillation index (soi) is used as the primary measure of the state of the enso. it is computed as the normalized difference in standardized slp anomalies between tahiti (as a high pressure center located at 17° south and 150° west) and darwin (as a low pressure center located at 12°south and 130° east) relative to its root mean square (barton and ramirez, 2004): soi= ptahiti –pdarwin 3-2climate signals data for the study area, 40 years of rainfall and 38 years of discharge data were provided for three raingauges and hydrometric stations across madarsoo watershed. data filling, where required, was performed using a multivariate regression method between the adjacent stations. next, statistical tests were conducted for all the stations in the studied area to check the quality of data, e.g. randomness and consistency. the monthly data of climate signals and sst and slp were downloaded from (www.esrl.noaa.gov) and (www.cpc.ncep.noaa.gov) from 1972 to 2011 on 15 august, 2014. the spatial resolution for sst and slp data was 2 degrees. in order to investigate the relationship between the variations of sst and slp with precipitation and discharge data, some points which were proved to have more influence on iran were studied as the indicators of each sea (karamouz, 2005; fallah-ghalhary, 2009 and 2011). location of these indicator points including the name of sea/ocean in which they are located and their coordinates are included in table 1 (fallah-qalhry, 2009). figure (2) shows the location of the indicator points used for sst and slp data. 68 hossein ruigar, saeed golian table 2name and coordinates of the points used in the analysis of relationship between sst, slp, and rainfall and runoff data (fallah-ghalhary, 2009 and 2011) figure 2 schematic view of the indicator points used for sst and slp analyses (after ruigar and golian, 2015) 3-3-correlation analysis in this study, to assess the relationship between rainfall and discharge data and large-scaled climate signals, pearson correlation test was utilized. to compare precipitation and discharge data with data of climate signals, all the data were standardized. the following equation was used for data standardization: (4) where x is the initial (unchanged) data value and x and s are average and standard deviation of data, respectively. pearson correlation coefficient is also called a zero-order correlation coefficient or pearson product-moment correlation. the value of pearson coefficients varies from 1 to 1. if the two variables are in perfect linear relationship, the correlation coefficient will be either 1 or –1. the sign depends on whether the variables are positively (directly) or negatively (inversely) related. the correlation coefficient is 0 if there is no linear relationship between the variables. also, the p-values, with 0.05 significance level, were calculated to accept (reject) the statistical significance of the correlation between these two variables. the null hypothesis was that the value of r in the population was zero. if the calculated p-value was less than 0.05, the null hypothesis ho (no correlation) would be rejected and vice versa; if p-value> 0.05, it means the correlation would be statistically significant. correlation coefficients and p-values were calculated using openstat v.2013 software. 4results and discussion in this section, the amount of correlation between the observed data, e.g. average monthly and maximum monthly values of precipitation and discharge data and climate signals including nino1+2, nino3, nino4, nino3. 4, mei, tni, ao, nao, soi, pdo, sst, and slp is evaluated. 4-1correlation of monthly average data with climate indices after standardizing discharge and precipitation data, values of the correlation coefficient and p-value for different lags from 1 to 24 months were calculated. maximum values of correlation coefficient and the relevant p-values are presented in tables (2) and (3). it can be observed from the tables that the maximum values of correlation coefficient for all the stations was derived for nino1+2 climate index with 11 and 13 months of lag for precipitation and discharge data, respectively. nino 1+2 indicated oscillations in the sea surface temperature in the eastern part of pacific ocean. the correlation between precipitation and discharge with other signals was all less than that of nino1+2 for the study region. 69assessing the correlation between climate signals and monthly mean and extreme precipitation and discharge of golestan dam watershed table 3maximum correlations for total monthly precipitation and climate indices table 4maximum correlations for average monthly discharge and climate indices 4-2correlation of average monthly data and sst p-values and correlation coefficients between total monthly precipitation and average monthly discharge and sst with 1 to 24 months of lag were calculated. the maximum values of the correlation between sst data of different locations presented in table 1 and precipitation and discharge data are shown in figures (3) and (4), respectively. it can be seen that the maximum value of correlation for tangrah and tamer raingauges was derived for sst data of the south of red sea with 6 and 3 months of lag, respectively. the maximum value of correlation coefficient for galikesh raingauge was related to sst of caspian sea with 9 months of lag. figure 3maximum calculated correlations for total monthly precipitation and sst data for hydrometric stations, the maximum amount of correlation coefficients for tangrah was calculated for soran network sst with 2 months of lag. for tamer and galikesh hydrometric stations, this value was derived for the sst of greenland and soran network with 7 and 15 months of lag, respectively. figure 4 maximum calculated correlations for average monthly discharge and sst data 4-3correlation of average monthly data and slp figure (5) shows that the correlation between tangrah and tamer raingauges was maximum with the slp of caspian sea with 7 and 9 months of lag, respectively, while for galikesh, the maximum correlation was calculated for the slp of the southern persian gulf with 9 months of lag. also, figure (6) demonstrates that the maximum correlation for tangrah, tamer, and galikesh hydrometric stations were calculated with the slp of caspian sea with 9 months of lag. figure (5) maximum correlations for total monthly precipitation and slp data figure (6) maximum correlations for average monthly discharge and slp data 4-4correlation of monthly extreme data with climate signals: p-value and correlation coefficients between maximum monthly rainfall and discharge and climate signals for different lags were calculated for every month, separately. the results for maximum correlation values for rainfall and discharge data are presented in figures (7) and (8) as well as tables (4) and (5), respectively. 70 hossein ruigar, saeed golian it can be seen that, for tangrah raingauge, the highest correlation coefficient (equal to 0.641) was calculated between maximum monthly precipitation of may and sst of eastern mediterranean sea with a lag of 7 months. for tamer, the maximum correlation coefficient of 0.537 was calculated for monthly maximum precipitation of october and slp of west mediterranean sea with 11 months of lag; for galikesh, this value was 0.459 in august with a lag of 4 months with slp of atlantic ocean. the next highest correlation coefficients calculated for tangrah was 0.543 for the rainfall of december with a lag of 5 months with atlantic slp; for tamer, it was 0.519 in may with the lag of 11 months with sst of eastern mediterranean sea; also, for galikesh, it was 0.456 with a lag of 4 months in august with the slp of west mediterranean sea. the highest correlation coefficients were calculated as 0.937, 0.782, and 0.926 for tangrah, tamer, and galikesh hydrometric stations, respectively, with a lag of 8 months between maximum monthly discharges of april and slp of aden gulf. the next highest correlation value was calculated for the maximum monthly discharge of tangrah was 0.544 with a lag of 8 months in march with black sea sst; for tamer, it was equal to 0.764 with a lag of 7 months in march with slp of aden gulf; also, for galikesh, it was 0.515 with a lag of 4 months in august with slp of black sea. according to these results, a significant relationship between maximum monthly precipitation and discharge data and sst/slp signals was detected across the study region. figure (7) correlations for maximum monthly precipitation with climate signals figure (8) correlations for maximum monthly discharge with climate signals table 5p-values and lags of maximum monthly precipitation and climate signals 71assessing the correlation between climate signals and monthly mean and extreme precipitation and discharge of golestan dam watershed table 6p-values and lags of maximum monthly discharge and climate signals 4-5discussion in summary, the correlation between hydroclimatologic variables, e.g. average monthly rainfall, and runoff large scale climate signals, e.g. enso indices, was investigated at the watershed in iran. for instance, in galikesh raingauge, it was demonstrated that the correlation between average monthly rainfall and nino1+2 index was maximum with the lag of 11 months. it means that, when nino1+2 index increased (decreased), the value of precipitation in galikesh would increase (decrease) with 11 months of lag. for galikesh hydrometric station, the maximum correlation was observed for nino1+2 index with a lag of 13 months. from table 6, it can be deduced that, for average monthly data, the correlation coefficients derived for sst/slp signals were higher than those of climate indices, e.g. nino1+2. in other words, sst and slp had the most influence on precipitation and discharge of the studied area. slp of caspian sea had the most control on the total monthly precipitation of tangrah and tamer, while sst of caspian sea had more influence in galikesh. for average monthly discharges in tangrah and galikesh, the slp of caspian sea and, for tamer, the sst of greenland, had maximum influence. results of the correlation analysis for extreme precipitation values also showed a very good agreement between precipitation data and slp of caspian sea and black sea and slp of the west and east of mediterranean sea. for maximum monthly discharge, the most correlations were observed for sst of atlantic sea and greenland sea and slp of black sea and caspian sea with different lags. the highest value of correlation for maximum discharge was calculated in august with 4 months of lag with sst of black sea. 5conclusion: in the present research, the effect of large-scale climate signals including nino1+2, nino3, nino4, nino 3.4, mei, tni, ao, nao, soi, pdo, sst, and slp on precipitation and discharge of madarsoo watershed at the upstream of golestan dam was investigated. for this purpose, three raingauges and hydrometric stations with 40 and 38 years of daily rainfall and discharge data were selected. the correlation coefficients were calculated for total and maximum monthly precipitation and also average and maximum monthly discharge data. in monthly scale, the maximum correlation between total precipitation and mean discharge was calculated for nino1+2 climate index, sst of greenland, north and south of the persian gulf, southern red sea, slp of caspian sea and sst of soran network. it means that these indices had maximum impact on monthly precipitation and discharge of the watershed. for galikesh raingauge and hydrometric station, the maximum correlation coefficients between monthly rainfall and discharge data with nino1+2 index was calculated to be 0.4 and 0.51 with 11 and 13 months of lag, respectively. by analyzing sst data, maximum correlation coefficients for precipitation data of tangrah, tamer, and galikesh raingauges was equaled to 0.455, 0.478, and 0.44 with 6, 3, and 9 months of lag, respectively. for discharge data of tangrah, tamer, and galikesh hydrometric stations, the correlation values were 0.442, 0.576, and 0.552 with the lags of 2, 7, and 15 months, respectively. analysis of slp data showed that the maximum correlation coefficients for total precipitation data of tangrah, tamer, and galikesh raingauges equaled to 0.487, 0.487, and 0.425 with 7, 9, and 9 months of lag, respectively. for discharge data of tangrah, tamer, and galikesh hydrometric stations, the correlation values were 0.594, 0.433, and 0.61, respectively, all with 9 months of lag. for all the investigated large-scale climate signals, the maximum correlation coefficients between precipitation data of tangrah and tamer raingauges were derived with slp of caspian sea while sst of caspian sea, has more influence in galikesh station. for discharge data, the maximum correlation for tangrah and galikesh hydrometric stations was calculated for slp of caspian sea. for tamer, the correlation coefficient was obtained with greenland sst. the highest correlation coefficient (equal to 0.641) was calculated between maximum monthly precipitation of may and sst of eastern mediterranean sea with a lag of 7 months. for tamer, the maximum correlation coefficient of 0.537 was calculated for the monthly maximum precipitation of october and slp of west mediterranean sea with 11 months of lag; for galikesh, this value was 0.459 in august with a lag of 4 months with slp of atlantic ocean. the highest correlation coefficients were calculated as 0.937, 0.782, and 0.926 72 hossein ruigar, saeed golian for tangrah, tamer, and galikesh stations, respectively, with a lag of 8 months between maximum monthly 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(in persian) keywords: cesar-rancheria basin; cesar sub-basin; los cuervos formation; source rock; geochemical modelling; porosity-depth relation. palabras clave: algoritmo de localización 3d; interferencia masiva y no masiva; geoestratigrafía; roca circundante; ree; rastro. how to cite item avendaño-sanchez, g. m., garcia-gonzalez, m., cruz-guevara, l. e., & cruz-ceballos, l. f. (2021). thermal evolution of los cuervos formation in the southern area of the cesar subbasin (colombia), based on geochemical and petrophysical data. earth sciences research journal, 25(2), 179-192. doi: https://doi. org/10.15446/esrj.v25n2.86025 the tectonic complexity that occurred during the cenozoic at the cesar-ranchería basin (colombia) has generated changes in the evolution of its petroleum system. proof of this is the lack of stratigraphic records in the cesar subbasin during both the early eocene and in the late eocene-early miocene interval. in this study, authors analyzed information already published regarding the sub-basin and new geochemical data obtained from rock-eval pyrolysis analyses and vitrinite reflectance (ro%). the core samples used are from the anh-la loma-2 and anh-crlos cerezos-1x wells. based on this information, los cuervos formation was classified as a source rock for its geochemical characteristics. additionally, the geochemical and petrophysical results indicate continuous deposition of sediments from the paleocene to the late eocene. these results were obtained using samples from two wells (calenturitas and la jagua) at los cuervos formation and the paleo-geothermometer data of paleocene formations. based on density and compaction conditions, it is clear that it corresponds to a sequence of quartz sandstones. therefore, it is expected the eocene sedimentites unit has reached a thickness between 2.5 to 3.5 km. according to recent thermo-chronological studies, this unit began to be eroded from the late eocene to the early miocene. the evidence obtained helped improve the knowledge in the thermal evolution model of the petroleum system, establish the time when the organic transformation ratios (tr) occurred, and estimate the current state of the cretaceous source rocks of the cesar sub-basin. abstract thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data evolución termal de la formación los cuervos, en el área sur de la subcuenca cesar, con base en datos geoquímicos y petrofísicos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.86025 la complejidad tectónica a la cual fue sometida la cuenca cesar-ranchería durante el cenozoico generó cambios en la evolución de su sistema petrolífero. prueba de ello es la falta de registro estratigráfico en la subcuenca cesar durante el eoceno temprano y el intervalo eoceno-mioceno temprano. en este estudio los autores analizan la información publicada y nuevos datos de pirolisis rock-eval y de reflectancia de vitrinita (ro%) con muestras de núcleos de perforación de dos nuevos pozos (anh-la loma-2 y anh-cr-los cerezos-1x). basados en esta información fue posible clarificar las características geoquímicas de la formación los cuervos como roca fuente de hidrocarburos. adicionalmente, los resultados geoquímicos y petrofísicos, obtenidos con muestras de dos pozos (calenturitas y la jagua) de la formación los cuervos y datos de paleogeotermómetros de formaciones del paleoceno, indicaron la existencia de una depositación continua de sedimentos desde el paleoceno hasta el eoceno tardío. con base en las condiciones de densidad y compactación observadas es claro que esto corresponde a una secuencia de areniscas de cuarzo. por lo tanto, se espera que la unidad de sedimentitas del eoceno haya alcanzado un espesor de 2.5 – 3.5 km. esta unidad de roca, de acuerdo con estudios recientes de termocronología, comenzó a ser erosionada en el eoceno tardío y hasta el mioceno temprano. la evidencia obtenida permitió mejorar el modelo de evolución termal del sistema petrolífero, establecer el tiempo en el cual ocurrieron las transformaciones orgánicas y estimar cómo están actualmente las rocas generadoras del cretáceo de la subcuenca cesar. resumen record manuscript received: 03/04/2020 accepted for publication: 16/10/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 179-192 gladys marcela avendaño-sánchez¹,², mario garcía-gonzález¹,³, luis enrique cruz-guevara¹,³, luis felipe cruz-ceballos¹,². 1. universidad industrial de santander 2. grupo de investigación en geología básica y aplicada (gigba) 3. energy and other non-renewable resources research group (eonr3g) g eo lo g y https://doi.org/10.15446/esrj.v25n2.86025 https://doi.org/10.15446/esrj.v25n2.86025 https://doi.org/10.15446/esrj.v25n2.86025 180 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos introduction numerous studies have been carried out in the cesar sub-basin. this sub-basin is part of the cesar-ranchería basin, northern side of colombia (figure 1). these studies have determined the source rock formations and their hydrocarbon potential, and have also indicated the generation potential, the organic matter contents, and the potential of accumulation as shown in the study from garcía et al. (2008). other authors such as mesa and rengifo (2011) pointed out that about 50 wells were drilled in the cesar sub-basin and it was determined a petroleum system in the sub-basin consisting of source rocks: lagunitas, aguas blancas, la luna, molino, and los cuervos formations; reservoir rocks: with fractured reservoirs of lagunitas and aguas blancas formations; and seal rocks: represented by a thick mudstone sequence of the molino formation. the hydrocarbon traps are pop-up structures, and fault propagation folds. the overburden sequence is integrated by lagunillas to cuesta-san antonio formations, and the regional seal on top is the miocene cuesta-san antonio formation. in the cesar basin, the barco formation consist of quartz sandstone and claystone, and los cuervos formations consists of claystone, siltstone, and several coal seams, according to guo et al. (2018). barco and los cuervos formations were deposited during the paleocene in a continental environment as a product of a slow regression (hernández, 2003). los cuervos formation corresponds to a sequence of carbonaceous shales, siltstones, sandstones, and coal beds (mesa and rengifo, 2011). studies carried out by barrero et al. (2007) and mesa and rengifo (2011) show the importance of the exhumation of the perijá range (pr) for the cesar sub-basin petroleum system. this is because this exhumation generates the formation of traps and marks the change in the existing conditions in terms of accommodation space. consequently, it also controls the increase of the lithostatic charge, thus increasing the maturation of the source rocks. evidence of the time of occurrence of this event was provided by bayona et al. (2007), who stated that the northern sector of the pr was at the early paleocene-eocene period; contributions were already being made to both the ranchería sub-basin and the maracaibo basin (bermúdez et al., 2017). other authors, like shagam et al. (1984), established that this uplift started from the cretaceous-paleogene limit and had successive pulses of uplift throughout the cenozoic. this study uses published information of the cesar sub-basin, 31 data from the maximum pyrolysis temperature (tmax) and 13 data from vitrinite reflectance (ro%) from los cuervos formation in the calenturitas and la jagua wells; and new geochemical data, 24 samples from tmax and 14 samples from ro% from the anh-la loma-2 and anh-cr-los cerezos-1x wells. this study aims to establish the current maturity state and the potential for hydrocarbon generation (quality and quantity) of the rocks in the los cuervos formation. furthermore, the goal is to recreate the burial history scenario through numerical modelling with the petromod software (version 2012). the modelling is based on geochemical data, the stratigraphic/structural architecture as interpreted by 2d seismic reflection data, and the several uplift events of the perijá range that affected the burial history. it also provides insights that figure 1. cesar sub-basin, wells, seismic cross lines, surface samples location and additional geochemical tmax and ro% and aft data. capital letters a, b, c and d represent seismic cross sections (figure 2). 181thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data would help understand the current state of maturity, as well as the organic transformation ratios (tr) of the cretaceous formations underneath los cuervos and barco formations. this is fundamental for any understanding of the organic geochemical characteristics of los cuervos formation, as well as how the basin was filled with sediments. this process has direct implications regarding the maturation of organic matter of the units as well as the generation of hydrocarbons. the interpreted 2-d seismic lines with structural reconstruction, main faults, truncated seismic refractor and on lap-strata terminations of the cretaceous, paleogene, and neogene formations are presented in figure 2. data and methods geochemical analysis was used to establish the type and quality of organic carbon and other geochemical properties of rocks, which contributed to the evaluation of the potential production of conventional or unconventional hydrocarbons in a prospective play. geochemical data were obtained using core samples from los cuervos formation of two wells, anh-la loma-2 and figure 2. interpreted 2-d seismic lines in two-way time (twt) with location of the wells; a. cv-1979-03 seismic line; b. cv-1979-11 seismic line; c. cv-1979-04 seismic line; d. cv-1979-08 seismic line. anh-cr-los cerezos-1x wells. vertical sampling was not performed at equal intervals (figure 3). the selection criterion was based on the visible presence of organic matter using an ultraviolet lamp. the geochemical analysis was carried out at uis petroleum geochemistry laboratory (pgl). geochemical analysis of rock-eval pyrolysis (hi, oi, pi, s1, s2, s3, tmax, py), total organic carbon (toc), and total sulphur content (ts) (table 1), ro% (table 2). additional existing geochemical data (tmax and ro%) used were obtained from hernández and jaramillo (2008); mora et al. (2007); ayala (2009) and guo et al. (2018). thermal maturation data of the cesar a-1x, compae-1, and el paso-3 wells was obtained from aguilera et al. (2010), ayala (2009), mora et al. (2007) and guo et al. (2018) including ro% and tmax data of surface samples. petrophysical analysis including permeability and porosity of sandstones and mudstones (514 samples) were performed at the uis petrophysical analysis laboratory (supplementary materials table s1). the colombian agencia nacional de hidrocarburos (anh) and the universidad industrial de santander (uis) have granted access and approved the use of all data for this study. 182 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos the toc analysis was carried out in 38 samples to quantify the amount of organic matter present in the sample. in the first instance, following the usual methodology reported in (schmacher, 2002), the content of carbonates present in the sample was eliminated by using hcl to dissolve inorganic caco3. after that, the total carbon was determined using the leco sc-144dr automated analyzer. it also allowed us to calculate the ts. likewise, to evaluate the quality of this organic matter, a rock–eval pyrolysis analysis was performed in each of the 24 samples with rock–eval 6 technology equipment. this technique presented by barker (1974) and behar et al. (2001) uses programmed heating to a small amount of rock (70 mg) or coal (30–50 mg) in an inert atmosphere (helium or nitrogen) in order to determine the quantity of free hydrocarbons present in the sample (s1 peak), as well as those that potentially can be released after maturation (s2 peak). this procedure permitted us to know the maximum pyrolysis temperature (tmax) and the current state of thermal maturation. the toc had to be determined along with the mineral carbon content (minc), the volume of free hydrocarbon generated before pyrolysis (s1), the amount of table 2. measurements of maceral materials of los cuervos formation from anh-la loma-2 and anh-cr-los cerezos-1x wells. ro% (minimum, maximum and mean), toc and ts. well name coordinates wgs 84 depth (m) ro% min. ro% max. measurements (number) standard deviation ro% mean toc % ts % anh-la loma-2 longitude -73.5527 latitude 9.6142 322 0.51 0.54 14 0.01 0.53 3.62 5.23 anhla loma -2 344 0.51 0.54 15 0.01 0.53 5.26 3.44 anhla loma -2 364 0.53 0.61 40 0.02 0.57 4.17 1.06 anhla loma -2 509 0.53 0.55 10 0.01 0.54 0.20 3.56 anhla loma -2 549 0.49 0.58 35 0.02 0.55 4.71 5.27 anhla loma -2 590 0.52 0.57 22 0.01 0.55 0.04 1.29 anhla loma -2 657 0.51 0.55 23 0.01 0.53 2.13 1.77 anhla loma -2 661 0.52 0.56 26 0.01 0.54 4.76 3.78 anhla loma -2 669 0.51 0.57 43 0.02 0.54 52.68 8.64 anh-cr-los cerezos-1x longitude -73.5826 latitude 9.4569 604 0.44 0.47 30 0.01 0.45 7.17 1.41 anh-cr-los cerezos-1x 640 0.49 0.56 35 0.02 0.51 2.04 1.55 anh-cr-los cerezos-1x 678 0.47 0.52 50 0.01 0.49 15.68 1.42 anh-cr-los cerezos-1x 751 0.45 0.47 15 0.01 0.46 1.65 12.12 anh-cr-los cerezos-1x 752 0.44 0.49 40 0.01 0.46 7.09 2.91 table 1. geochemical analysis results of los cuervos formation from core samples in anh-la loma-2 and anh-cr-los cerezos-1x wells. toc and ts (%); hi (mg hc/g toc); oi (mg co2/g toc); pi (toc%); s1, s2 and py (mg hc/g rock); s3 (mg co2/g rock) and tmax (ºc). well name coordinates wgs 84 depth (m) toc % ts % hi oi pi s1 s2 s3 tmax ºc s2/s3 py=s1+s2 anhla loma -2 longitude -73.5527 latitude 9.6142 292.7 0.50 0.22 59.42 79.23 0.26 0.14 0.40 0.30 416.00 1.33 0.54 anhla loma -2 309.3 0.81 0.09 62.84 85.02 0.15 0.12 0.69 0.51 422.00 1.35 0.81 anhla loma -2 310.8 2.75 0.19 82.67 60.46 0.08 0.14 1.66 2.27 424.00 0.73 1.80 anhla loma -2 364.1 4.17 1.06 21.56 296.81 0.03 0.44 12.39 0.90 423.00 13.77 12.83 anhla loma -2 364.3 4.28 0.66 49.75 104.63 0.09 0.46 4.48 2.13 419.00 2.10 4.94 anhla loma -2 392.7 0.65 0.09 76.51 68.86 0.39 0.29 0.45 0.50 428.00 0.90 0.74 anhla loma -2 394.1 0.44 0.11 72.02 56.27 0.39 0.16 0.25 0.32 426.00 0.78 0.41 anhla loma -2 546.0 0.31 0.02 94.69 71.83 0.58 0.31 0.22 0.29 424.00 0.76 0.53 anhla loma -2 590.1 2.99 2.17 66.97 44.54 0.13 0.20 1.33 2.00 419.00 0.67 1.53 anhla loma -2 619.6 1.06 0.18 33.90 115.84 0.07 0.09 1.23 0.36 431.00 3.42 1.32 anhla loma -2 650.7 1.97 0.19 23.37 259.57 0.02 0.10 5.11 0.46 429.00 11.11 5.21 anhla loma -2 657.4 2.13 1.77 49.34 95.39 0.03 0.06 2.03 1.05 426.00 1.93 2.09 anh-crlos cerezos -1x longitude -73.5826 latitude 9.4569 513.74 2.04 0.406 72.00 50.94 0.08 0.13 1.47 1.04 417.00 1.41 1.60 anh-crlos cerezos -1x 521.76 0.63 0.166 65.00 85.61 0.23 0.12 0.41 0.54 420.00 0.76 0.53 anh-crlos cerezos -1x 566.53 0.82 0.065 60.74 100.83 0.19 0.12 0.50 0.83 422.00 0.60 0.62 anh-crlos cerezos -1x 640.23 2.04 1.552 48.16 67.81 0.06 0.06 0.98 1.38 427.00 0.71 1.04 anh-crlos cerezos -1x 678.18 15.68 1.416 271.81 48.58 0.06 2.74 42.63 7.62 426.00 5.59 45.37 anh-crlos cerezos -1x 751.91 7.09 2.913 66.00 103.52 0.03 0.15 4.68 7.34 419.00 0.64 4.83 hydrocarbon generated during pyrolysis (s2), and the amount of emitted carbon dioxide (s3). derivations of these parameters such as hydrogen index (hi = s2/ toc × 100), the oxygen index (oi = s3/toc × 100), the production yield (py = s1 + s2), and the production index (pi = s1/(s1 + s2)) are calculated. the vitrinite reflectance (ro%) that indicated organic matter content, was measured in 14 samples using zeiss imager microscopy. vitrinite reflectance was measured according to the method proposed by the astm d7708-14 (2014). sample chips or sidewall core samples were cleaned to remove drilling mud or mud cake and then crushed using a mortar and pestle to a grain-size of less than 3 mm. these samples were mounted in cold-setting resin and polished “as received”, so that whole-rock samples were examined, excluding concentrates of organic matter. the core samples were mounted and sectioned perpendicularly to the bedding. vitrinite reflectance measurements were made using immersion oil of refractive index 1.518 at 546 nm at 23ºc and at spinel and garnet standards of 0.42, 0.917, as well as 1.726% reflectance for calibration. after calibrating the equipment an average of 28.4 ro% readings were taken for each polished 183thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data section. during optical analysis the vitrinite macerals were identified on the polished surface, taking into account that the standard deviation of the 28.4 readings of ro% per polished section must be less than 0.003. a correlation of ts concentration vs ro% was performed to determine the reflectance of suppressed vitrinite according to mukhopadhyay (1994), who indicated that the suppression occurs between values of 0.2 to 1.4, this suppression takes place in vitrinite due to excess sulphur of more than 1% [w/w] according to carr (2000). this last author established that in some situations, vitrinite reflectance values may be either ‘suppressed’ or ‘enhanced’ and said that low vitrinite reflectance or vitrinite suppression may be caused by perhydrous-vitrinite, impregnation of vitrinite with hydrogen rich amorphous material, and in coals with high sulphur content. fluorescence-mode observations were made on all samples, providing supplementary evidence concerning organic matter type, exinite abundance, and maturity. for fluorescence-mode, a 3-mm bg-3 excitation filter was used with a tk400 dichroic mirror and a k490 barrier filter. the data correlation of the average ro% versus tmax is presented in figure 4 to show the change in thermal maturity of los cuervos formation in the la loma-2 and anh-crlos cerezos-1x wells. petrophysical analyses were performed in addition to the organic geochemistry analysis to give greater certainty of the thermal evolution of the rancheria sub-basin. petrophysical analysis, porosity and permeability in anh-la loma-2 well were determined for 262 and 55 samples of sandstone and mudstone, respectively; and in anh-cr-los cerezos-1x well were determined for 141 and 56 samples of sandstone and mudstone, respectively. porosity and permeability were measured at uis petrophysical analysis laboratory, using conventional core analysis practices from american petroleum institute (1998). petrophysical sample analysis consists of two stages. first: sample preparation that comprises cutting, profiling, cleansing and drying plugs; and second: determination of petrophysical properties (porosity and permeability). petrophysical information was plotted to visualize its distribution on profiles using free software panplot2 as described by sieger and grobe (2013).petrophysical data allowed establishing and characterizing the thickness of the sequence deposited, during the early eocene, in the interval from the late eocene to the early miocene. this sedimentary sequence was lost in the geological record due to regional tectonic events (see figure 2). this lost sequence managed to exert a lithostatic charge that increased the maturation of the source rocks and decreased the porosity of reservoir rocks. the 2d seismic lines in two-way time (twt) used in this research were cv-1979-03, cv-1979-11, cv-1979-04 and cv-1979-08; the interpretation of the lines was used for the purpose of defining the cutting relationships between lithological units present in the basin. additionally, the seismic lines had the geographic and stratigraphic positions of the wells used for this study. one-dimensional modelling was generated using the petromod software in order to reconstruct the burial history and thermal maturation of the geological units present in the wells. this modelling used the vitrinite thermal maturation model (easy ro%) proposed by sweeney and burnham (1990), based on the arrhenius first-order parallel-reaction approach with a distribution of activation energies. easy ro% were calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. the ages and minimum and maximum thicknesses of the rio negro, lagunitas, aguas blancas and lower part of the molino formations reported for the sub-basin by mesa and rengifo (2011) were used. for barco, los cuervos and upper molino formations, we used the data obtained in anh-la loma-2, anh-cr-los cerezos-1x, la jagua and calenturitas wells. the eroded thickness in the sequence was estimated considering the maximum thickness reported and the thickness present in the wells. the heat flow was established according to mesa and rengifo (2011): the syn-rift stage goes from 200 to 116 million years ago (mya) and the postrift stage from 116 to 20 mya (figure 5a). the paleo water depth (pwd) for la luna and molino formations was established considering patarroyo et al. (2017) see table 3 and figure 5b. sediment water interface temperature (swit) was calculated based on wygrala (1989), assuming a current position in the northern hemisphere on the continent of south america at a latitude of 10° (figure 5c) as obtained by montes et al., (2019). multiple kinetics based on lithology were used: lagunitas, aguas blancas, la luna and lower molino formations, the kinetics ies-tii_brown_limestone, for the middle molino formation the kinetics ies-tii_toarcian_shale, for los cuervos formation the kinetics handil_mahakamdelta-tiii and ies-tiii_tertiary_coal_2c according to diprimio and horsfield (2006). figure 3. lithologies, samples site (+) with ro%, pyrolysis rock-eval and petrophysical analysis for anh-la loma-2 and anh-cr-los cerezos-1x wells. 184 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos table 3. paleo-batimetry conditions (pwd) vs time used to generate the geochemical modelling. pwd for la luna and molino formations (patarroyo et al., 2017). 0 m deep during the early rift stage (120 mya) according to mesa and rengifo (2011). age(ma) pwd(m) 0 0 50 0 70 15 79 100 92 200 103 100 116 100 120 0 the burial history and exhumation event of the cesar sub-basin was obtained from hernández and jaramillo (2008). the burial depth of los cuervos formation and the thickness of eocene age formation, were obtained using the relationship between porosity and depth, known as ‘compaction trend’ (lee et al., 2019), and the one-dimensional geochemical model obtained. results and discussion in this section we summarize the results obtained, corresponding to geochemical and petrophysical analysis for anh-la loma-2 and anhcr-los cerezos-1x wells. the figure 3 shows a stratigraphic column with samples collected for vitrinite reflectance (ro%), pyrolysis rock-eval and petrophysical analysis. the figure 4 shows a correlation between tmax versus ro% for the samples analyzed from anh-la loma-2 well (depths of 95.2 to 668.6 m) and anh-cr-los cerezos-1x well (depths of 513.7 to 751.9 m), respectively. figure 4 shows a good correlation between data to illustrate the change in thermal maturity of los cuervos formation in the anh-la loma2 and anh-cr-los cerezos-1x wells. figure 5 shows the basin modelling boundary conditions (heat flow, paleo water depth, swit). multiple scenarios cases for erosions events allow us to discriminate the best calibration for each well (figure 6 and 7). the burial history diagrams for anh-la loma-2 and anh-cr-los cerezos-1x wells, and for calenturitas mine and la jagua mine wells, respectively (figure 8 and 9). the transformation ratios (tr) are shown at the figure 10. the transformation ratio (tr), as defined by tissot and welte (1984), is the ratio of the petroleum (oil and gas) actually formed by the kerogen to the total amount of petroleum that the kerogen is capable of generating. it depends on the origin of the organic material (kerogen type) and the maturation history associated to the geological history of a basin. figure 11 and 12 shows the variations in depth of permeability and porosity for anh-la loma-2 and anh-cr-los cerezos-1x wells, respectively. figure 13 shows permeability vs porosity correlation in anh-la loma-2 and anh-cr-los cerezos-1x wells, respectively. these results of porosity and permeability were used to determine the depth to which the lithologies were buried (figure 14 and 15). in the anh-la loma-2 well (figure 3) the following results were obtained for 18 mudstone samples with toc values with a min. of 0.306%, max. of 5.257%, a mean of 2.595% and a standard deviation (sd) of 1.709%. the sulphur content with a min. of 0.016% [w/w] (were [w/w] means percentage concentration by weight), max. of 5.274% [w/w], mean of 1.516% [w/w] and sd of 1.778% [w/w]. a sample of coal with 52.68 toc and sulphur content of 8.64% [w/w]. the 2 samples of sandstone with toc values with a min. of 0.04%, max. of 0.20%, a mean of 0.120%. the sulphur content with a min. of 1.286% [w/w], max. of 3.564% [w/w], mean of 2.425% [w/w]. in the anh-cr-los cerezos-1x well (figure 3) the following results were obtained for 10 mudstone samples with toc values with a min. of 0.631%, max. of 15.684%, a mean of 6.112% and a sd of 5.638%. the ts with a min. of 0.166% [w/w], max. of 12.125% [w/w], mean of 2.587% [w/w] and sd of 3.465% [w/w]. a sample of sandstone with 0.82 toc and sulphur content of 0.065% [w/w]. figure 4. data correlation of maximum pyrolysis temperature (tmax) versus vitrinite reflectance (ro%), to indicate source rock thermal maturity in los cuervos formation of the anh-la loma-2 and anh-cr-los cerezos-1x wells. multiple modellings were made using tmax and ro% data and with different thicknesses of the eocene sequence (e.g. from 2000 to 4700 in 300 m intervals) were made (figure 6b and 6d). the modelling that best fit for anh-la loma-2 and anh-cr-los cerezos-1x wells was 3200m and 3500m respectively (figure 6a and 6c). figure 7 shows a calibration with ro% and tmax for la jagua and calenturitas mines using data of guo et al. (2018). figure 5. boundary conditions used for 1d modelling of burial history of lithological units in the anh-la loma-2 and anh-cr-los cerezos-1x wells: (a) heat flow, (b) paleo water depth, and swit. 185thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data figure 6. best calibration and multiple models for calibration of vitrinite reflectance (ro%) and tmax with depth: (a) and (b) for the anh-la loma-2 well, and (c) and (d) for anh-cr-los cerezos-1x well. figure 7. best calibration and multiple models for calibration of vitrinite reflectance (ro%) and tmax with depth: (a) and (b) for the calenturitas well, and (c) and (d) for la jagua well. 186 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos the burial diagrams obtained for the anh-la loma-2, anh-cr-los cerezos-1x, calenturitas and la jagua wells (figures 8 and 9) are consistent with the two major deformational phases affecting the cesar-ranchería basin, first to a middle eocene convergent phase related to the accretion of the great arc of the caribbean (sánchez and mann, 2015; cardona et al., 2011; villagomez et al., 2011); and a second phase in the miocene age characterized by large-displacement thrusting in the eastern cesar ranchería basin driven by late miocene collision of the panama arc with south america plate (sánchez and mann, 2015; taboada et al., 2000; vargas and mann, 2013). the geochemical modelling (figures 8 and 9), assumes a continuous deposition during the middle eocene. since middle eocene the accumulated thickness cretaceous-middle eocene begins their erosion, without significant loss of sedimentary record. this erosion event is prior to the deposition of the cuesta formation and is more pronounced closer to the perijá range (pr). the minimum thickness values of each formation in the cesar sub-basin used to generate model (figure 8) was obtained from the seismic interpretation (figure 2b) and mesa and rengifo (2011). the model of burial history (figure 8 and 9), with overlapping values and the different hydrocarbon generation windows were coloured as proposed by sweeney and burnham (1990). these models projected to the economic basement (río negro formation), the lagunitas, aguas blancas and luna formations are expected to enter the main oil generation window, while the molino formation is expected to enter the early oil window in the middle eocene just before the loss of sedimentary record. the geochemical modelling of the calenturitas and la jagua wells (figures 9a and 9b) was achieved using data of guo et al. (2018). the modelling permit simulate the evolution in thermal maturation and transformation ratios over time. it was only possible to determine how the sedimentary sequence behaved for the calenturitas well using seismic (figure 2a). but a fault was found just below the synchronization line where los cuervos formation is completed. the fault repeats the cretaceous sequence and cause errors in the determining the thickness units to be modelled. los cuervos formation in the calenturitas well, is expected to be immature and the molino formation enters the early oil window during the middle eocene (figure 9a). the molino formation in la jagua well, is expected that reached the early oil window during the same period (figure 9b). the diagrams of tr variation in time from source rocks in anh-la loma-2 and anh-cr-los cerezos-1x wells, are presented in figure 10a and 10b). these subsidence model implies that the tr of source rocks do not increase after the eocene. the units of the lower cretaceous (lagunillas and aguas blancas formations) reached the highest tr of the modeled source rocks, obtaining its maximum advance during the paleocene. the formations of the upper cretaceous (la luna and los cuervos formations) reached its maximum advance during the eocene. los cuervos formation is not expected to have such high tr but around 66%, while la luna formation is expected to be hovering around 95% of tr in this area. likewise, it is evident that los cuervos formation did not advance in the transformation of its organic matter in hydrocarbons. based on the models, the tr are larger in anh-cr-los cerezos-1x well followed by anh-la loma-2 well. figure 8. burial history using modelled temperature variation: (a) for anh-la loma-2 well and (b) for anh-cr-los cerezos-1x well. 187thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data the petrophysical analysis results of the porosity and permeability of the sandstone and mudstone lithologies of the anh-la loma-2 well, show a decrease in their values with increasing depth (figure 11). sandstones beds with high permeability values are also highlighted. however, these are local and are distributed especially towards the top of the well, while the permeability values in the mudstone remain very low. for anh-cr-los cerezos-1x well, the porosity and permeability values (figure 12) also decrease with increasing depth. however, higher values in mudstone permeability are evident, which can be up to 3 times higher in some cases, while sandstones are maintained with permeabilities that do not exceed 2500 md, unlike those observed for anh-la loma-2 that can reach 6000 md. the porosity and permeability values show a positive correlation (figure 13), in anh-la loma-2 well (figure 13a), a significant number of samples of sandstones are dominated by porosity from 13.16% to 29.62%, with mean of 21.37% and a standard deviation (sd) of 5.52%. for mudstones, a porosity values predominates between 4.86% and 14.60%, with mean of 10.75% and sd of 4.61%. permeability values range from 0.1 to 560 md for sandstones, with mean of 461.27md and sd of 930.54md. for mudstones permeability values are between 0.01 and 1.27md, with mean 0.95md and sd of 1.62md. in anh-cr-los cerezos-1x well (figure 13b), a significant number of samples of sandstones are dominated by a porosity between 15.25% and 31.02%, with mean of 24.78% and sd of 4.72%. for mudstones, a porosity predominate value is between 8.98% and 23.96%, with mean of 17.64% and sd of 6.68%. permeability values range from 0.39 to 250 md for sandstones, with mean of 322.54 md and sd of 565.12 md. for mudstones are between 0.37 and 6.47md, with mean 3.89 md and sd of 6.55 md. bjørlykke (2014) describes two different types of compaction processes (mechanical and chemical). compaction generates a loss of porosity in siliceous sediments during burial in sedimentary basins. being the mechanical compaction caused by the effective stress more relevant before exceeding temperatures greater than 100 °c. chemical compaction begins after 100 ºc to dominate the process of loss of porosity, it is based on the chemical reactions that occur according to the lithology (mineralogy). in cesar sub-basin, through modelling with petromod (avendaño-sánchez et al., 2019), shows that the analyzed lithologies did not exceed this temperature limit, therefore it is considered that the loss of porosity was due to the effect of mechanical compaction. accordingly, and using the models of kim et al. (2018) who performed numerical modelling of the decrease in porosity with respect to depth in sandstone, mudstone, and limestone, the following low-end, mean and highend curves were obtained. the porosities obtained by petrophysical analysis for the anh-la loma-2 well (figure 14) in both sandstones and mudstones show that these lithologies were at depths between 2.5 and 3 km. while for the anh-cr-los cerezos-1x well (figure 15) is expected these lithologies were at depths less than 2.5 km. the stratigraphic record deposited between the eocene and the neogene in the cesar sub-basin is very thin, hernández (2003) reported 10 meters of slightly conglomerate quartz sandstone in la jagua de ibirico syncline, these would be the 10 meters that mesa and rengifo (2011) describe in its generalized column of the basin as the eocene sedimentitas unit. in ayala (2009), a paraconformity is proposed between the eocene sedimentitas unit and los cuervos formation. both are considered the product of continuous succession without the presence of erosive contact. in the ranchería sub-basin, figure 9. burial history using modelled temperature variation: (a) for calenturitas well and (b) for la jagua mine well. 188 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos figure 10. hydrocarbon transformation ratios (tr) for the cretaceous and paleocene formations: (a) in the anh-la loma-2 well, and (b) in anh-cr-los cerezos-1x well. ronderos (1957) reported that eocene aguas nuevas formation is made up of reddish conglomeratic sandstones, glauconite-containing sandstones, reddish sandstones, and shales. the lack of eocene age sedimentary record in the cesarrancheria basin that kellogg (1984) describes as the result of compressional and transpressional tectonics associated with tectonic convergence along the southern margin of the caribbean and the northern sector of the andes during the cenozoic. authors such as mesa and rengifo (2011) propose subsidence models for the cesar sub-basin, in which a significant cenozoic sedimentation occurred during the paleocene (about 2.1 km) corresponding to the barco formation. the formations deposited during the eocene have been little investigated, with a reported thickness of 300 meters, which is presumed to have been completely eroded. the model also presents a large amount of sedimentation in less than 5 million years (my) time, while sedimentation was scarce for the 20 my of the eocene. however, authors such as garcía et al. (2008), and sánchez and mann (2015) give more relevance in their models to the formation deposited in the eocene, assigning a thickness of 1 km and 2 km, respectively. garcía et al. (2008), and sánchez and mann (2015) establishes that its sequence was eroded during the oligocene-miocene and oligocene-pliocene, respectively. but it is important to highlight that the three models mentioned do not show the calibration with paleo-geo-thermometer data and do not show the boundary conditions used. garcía et al. (2008) used a variation of the heat flow that reached 100 mw m-2 between 110 and 90 mya. these values differ with those used in our modelling, which do not exceed 60 mw m-2. these values are in the phase with crustal thinning and increased accommodation space that occurred in the cretaceous. patiño et al. (2019) with new and by compiling thermochrometry data of apatite fission track aft (villagómez et al., 2011) and new apatite (ahe) and zircon (zhe) (u-th)/he, established that the highest peak of exhumation rates of 0.9 km/my. this event affected the sierra nevada de santa marta (snsm) during the late eocene to early miocene and resulted in their vertical exhumation of 15-19 km, based on geobarometry of granitoids (cardona et al, 2011). bermúdez (2009) from aft analysis on synorogenic sediments supports the eocene-oligocene origin of the andes range in venezuela. this exhumation rates coincides with the deformation front of the caribbean began a more eastern migration with respect to south america in the late eocene oligocene as proposed by montes et al. (2019). it can be concluded that late eocene to early miocene tilting event that occurred in the maracaibo block may have been the cause of the total or partial loss of the deposited sedimentary units between paleocene to the middle eocene in some areas of the cesar sub-basin. the correlation between tmax and ro% data which are conditioned by the increase in temperatures, and the porosity data that are conditioned by the compaction of the sedimentary units, excluding the possible effect of cementation, allow to corroborate that it is not necessary to assign higher values in the heat flow within the sub-basin in order to reach the same point of thermal maturation. the results obtained in the burial modelling carried out in this study contrast with the work proposed by martínez et al. (2012), where the methodology used to replicate the effect of the perijá range (pr) thrusting on the cesar sub-basin. here, the maturation of the cesar subbasin rocks is a consequence of the effects of continuous sedimentation until the middle eocene, at which time the regional uplift event of the maracaibo block 189thermal evolution of los cuervos formation in the southern area of the cesar sub-basin (colombia), based on geochemical and petrophysical data figure 11. variations in depth of permeability and porosity from different lithologies for anh-la loma-2 well. figure 12. variations in depth of permeability and porosity from different lithologies for anh-cr-los cerezos-1x well. interrupted de sedimentation. this model coincides with that proposed by sánchez and mann (2015), which also proposes that the thickness of the units deposited in the eocene is greater in the direction of the maracaibo basin. conclusions it can be concluded that late eocene to early miocene tilting event that occurred in the maracaibo block may have been the cause of the total or partial loss of the deposited sedimentary units between paleocene to the middle eocene in some areas of the cesar sub-basin. the maturation of the cesar subbasin rocks is a consequence of the effects of continuous sedimentation until the middle eocene, at which time the regional uplift event of the maracaibo block interrupted de sedimentation. the correlation between tmax and ro% data which are conditioned by the increase in temperatures, and the porosity data that are conditioned by the compaction of the sedimentary units, excluding the possible effect of cementation, allow to corroborate that it is not necessary to assign higher values in the heat flow within the sub-basin in order to reach the same point of thermal maturation. using the data geochemical obtained for los cuervos formation (416 431 ºc of tmax, and 0.46-0.57 of ro%), it can be established that this formation in the southern area of the cesar sub-basin is thermally immature, near the oil generation window, but this formation presents higher values reported in the la jagua well towards the perijá range. based on the porosity values, it is expected that the sandstones and mudstones of los cuervos formation, have reached depths from 2 to 3 km in the study area. furthermore, according to the values of tmax and ro%, it is expected that los cuervos formation has been at a depth from 2600 to 3800m for anh-la loma-2, from 2900 to 4100m for anh-cr-los cerezos-1x, from 2600 to 4400m for calenturitas well, and from 3500 to 5300 for la jagua well, taking into account that this thickness was considered with the properties of quartz sandstones. 190 gladys marcela avendaño-sánchez, mario garcía-gonzález, luis enrique cruz-guevara, luis felipe cruz-ceballos according to the modelling carried out for this study area, the late cretaceous formations (aguas blancas and lagunitas) have had their greatest advance in hydrocarbon transformation ratios during the paleocene, while the early cretaceous formations (la luna and molino) peaked in hydrocarbon transformation ratios from the early eocene to the middle eocene. similarly, through the modelling of the lagunitas, aguas blancas and la luna formations, it is concluded that they had transformation ratios greater than 80% while the molino formation has transformation ratios around 50%. figure 14. petrophysical analysis results of porosity and permeability for anh-la loma-2 well. figure 15. petrophysical analysis results of porosity and permeability for anhcr-los cerezos-1x well. acknowledgments special thanks to the agencia nacional de hidrocarburos (anh) from colombia and the universidad industrial de santander (uis) for providing the resources and facilities for the development of this research. anh give permission to show the seismic lines. thanks to the petroleum geochemistry figure 13. permeability vs porosity for (a) anh-la loma-2 and (b) anh-cr-los cerezos-1x wells. laboratory (lgp) of the universidad industrial de santander (uis) for performing the geochemical analysis for this work and to the uis petrophysical analysis laboratory for performing the petrophysical analysis. thanks to the style-writer corrector, german morales. we thank the anonymous reviewers for their valuable contributions, allowing the document to improve substantially. references aguilera, r., sotelo, v., burgos, c., arce, c., gómez, c., mojica, j. and osorno, j. 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(1989). integrated study of an oil field in the southern po basin, northern italy. ber. kernforschungsanlage jülich, 2313: 1-217. http:// juser.fz-juelich.de/record/153416/files/fzj-2014-03033.pdf https://doi.org/10.1785/0120120328 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010tc002835 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010tc002835 http://juser.fz-juelich.de/record/153416/files/fzj-2014-03033.pdf http://juser.fz-juelich.de/record/153416/files/fzj-2014-03033.pdf _hlk46235819 _hlk47027899 _hlk45546900 _hlk46413529 _hlk49164585 _hlk49165014 _hlk49165353 _hlk46497191 _hlk49193137 _hlk46418243 _hlk46416674 _hlk46419489 _hlk46574981 _hlk46590815 _hlk47947098 _hlk47947674 _hlk46585413 keywords: soil investigation; masw; remi; hvsr technique; site classification. palabras clave: investigación del suelo; masw; remi; técnica de relación espectral horizontal a vertical; clasificación de sitio. how to cite item sertcelik, i., kurtulus, c., sertcelik, f., livaoglu, & sas c. (2020). investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor. earth sciences research journal, 24(4), 473-484. doi: https://doi.org/10.15446/esrj. v24n4.79123 in this study, shallow seismic surveys, including seismic refraction, multichannel analysis of surface waves (masw), refraction microtremor (remi), and microtremor measurements were conducted to estimate site characterization at 26 strong-motion stations of afad (disaster and emergency management presidency) in the province of hatay, situated in one of the most seismically active regions in southern turkey. the horizontal to vertical spectral ratio (hvsr) technique was applied, using smoothed fourier spectra derived from a long duration series to determine dominant frequency values at different amplification levels. shear wave velocity up to 30 m of the ground was detected with masw analysis. in the remi analysis, up to 80 m was reached with a corresponding average of 650 m/s shear wave velocity. the shear wave velocities estimated by the masw method up to 30 m were compared with those found by the remi method, and they were observed to be very compatible. the province of hatay was classified according to vs30 based nehrp provisions, eurocode-8, the turkish building earthquake regulation (tbdy-2018), and rodriguez-marek et al. (2001). the shear-wave velocity (vs30), horizontal to vertical ratio’s (h/v) peak amplitude, dominant period, and site class of each site were determined. the h/v peak amplitudes range between 1.9 and 7.6 while the predominant periods vary from 0.23 sec to 2.94sec in the study area. these results are investigated in order to explain the consistency of site classification schemes. abstract investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor caracterización del suelo en la provincia de hatay, turquía, a través de los métodos refracción sísmica, análisis multicanal de ondas superficiales y microtemblores issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.79123 en este trabajo se realizaron sondeos sísmicos superficiales como la refracción sísmica, el análisis multicanal de ondas superficiales (masw, del inglés multichannel analysis of surface waves), la refracción de microtemblores (remi, del inglés refraction microtremor), y la medición de microtemblores para una caracterización en 26 estaciones de registros sísmicos de la afad (autoridad para la administración de desastres y emergencias) en la provincia de hatay, que está situada en una de las regiones sísmicamente más activas del sur de turquía. se aplicó la técnica de relación espectral horizontal a vertical con un espectro fourier regular derivado de una serie de larga duración para determinar los valores de frecuencia dominante a diferentes niveles de amplificación. la velocidad de la onda de corte por encima de los 30 metros de profundidad se detectó a través de la técnica masw. el análisis remi por encima de los 80 metros se logró con una media correspondiente de 650 m/s en la velocidad de la onda de corte. las velocidades en las ondas de corte alcanzadas por el método masw se compararon con aquellas de la técnica remi, y se observó que son muy compatibles. la provincia de hatay fue clasificada de acuerdo con la norma nehrp con base en vs30, eurocode-8, la entidad de regulación sísmica de turquía (tbdy-2018), y rodríguez-marek et al. (2001). se determinó la velocidad de la onda de corte (vs30), la amplitud del pico de la relación horizontal a vertical (h/v), el período dominante, y el tipo de cada sitio analizado. las amplitudes del pico en las relaciones h/v están entre 1.9 y 7.6, mientras que los períodos predominantes varían entre 0.23 segundos y 2.94 segundos en el área de estudio. estos resultados se investigaron para explicar la consistencia de los esquemas de clasificación de sitios. resumen record manuscript received: 15/04/2019 accepted for publication: 07/08/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 473-484 cengiz kurtuluş, i̇brahim sertçelik*, fadime sertçelik, hamdullah livaoğlu, cuneyt saş kocaeli university, department of geophysics, kocaeli, 41100, turkey *corresponding author: isert@kocaeli.edu.tr se is m o lo g y 474 i̇brahim sertçelik, cengiz kurtuluş, fadime sertçelik, hamdullah livaoğlu, cuneyt saş introduction the hatay area is located in a seismically active region at the western end of the 1000-km long border formed by arabic and turkish plates. the tectonic activity of the region shows a complex tectonic behavior under the influence of faults extending to the dead sea, eastern anatolia, and cyprus (beyen et al., 2003). more than ten of the most disastrous earthquakes during the past century (1937–2015) occurred in this area, indicating that local site conditions have a major effect on ground shaking. it was reported that the tsunamis that occurred during the1822 and 1872 earthquakes in hatay caused the death of 20 thousand people (tarı et al., 2013). these devastating earthquakes from the past have shown that local ground condition is a major influence on ground amplification which can be calculated by a s-wave velocity structure and microtremor studies. there have been several site effect studies in the forms of a microtremor survey, earthquake observations, a surface-wave survey, and borehole data for various regions in turkey such as bayülke (1982), erdik (1984), ambraseys et al. (1993), inan et al. (1996), aydan & hasgör (1997), durukal et al. (1998), zaré & bard (2002), ozalaybey et al. (2002), rathje et al. (2003), ergin et al. (2004), kalafat et al. (2007), sandıkkaya (2008), sandıkkaya et al. (2010), kalkan & gürkan (2004), ulusay et al. (2004), and yılmaz et al. (2008). zaré & bard (2002) used a (h/v) spectral ratio to estimate turkey’s strong motion site conditions, and rathje et al. (2003) and gülkan et al. (2007) used geophysical and geotechnical studies to determine the site classification of turkish strong-motion sites. many local site investigations have been conducted by many researchers in turkey (kurtulus et al., 2016; ongun, 2010; alçık et al., 1995; dikmen & mirzaoglu, 2005). however, only a few studies about site effect were performed in the hatay region. büyüksaraç et al. (2014) conducted a site response analysis, compiling data such as density, seismic wave velocity, and soil cross-sections from previous seismic microzonation studies and acceleration data from four free-field strong-motion stations of the seramar project. they determined three different layers for the ranges 0-5 m, 5-15 m, and 15-25 m. the seismic velocities of these layers vary between 380 and 470 m/s for 0-5 m, 320 and 480 m/s for 5-15m, and 470 and 750m/s for 15-25m. over et al. (2011) analyzed the microtremor data for antakya and proposed a preliminary microzonation map for the province of antakya based on dominant periods ranging from 0.2 to 0.8 sec and shear wave velocities of the sediments covering the area. korkmaz (2006) studied the relationship between ground conditions and earthquake effect in antakya. horoz (2008) studied the seismicity of the region between adana-antakya-kahramanmaraş with the parameter “b” and risk analysis. ceken & polat, (2014) tried to investigate local site effects with matnet installed near the hatay-kahramanmaraş areas, consisting of 55 triaxial force-balance accelerometers and capable of recording explosions and evaluating them as a part of an early warning and preliminary damage estimate system. ozmen et al., (2017) conducted a microtremor array exploration of shallow s-wave velocity profiles at 28 sites in the provinces of hatay and kahramanmaraş. they determined fundamental frequencies from 0.8 to 10 hz in hatay and peak amplitude values of more than 5 in kahramanmaraş. there is also site predominant based seismic site classifications regarding sediment thickness carried out by researchers which promising reliable preliminary results (livaoglu h., & irmak t.s., 2017; livaoğlu, h., & f. sertçelik, 2017). they have used the strong ground motion data and evaluated the site predominant periods of the installed stations via response spectra of h/v. in this study of seismic refraction, multichannel analysis of surface waves (masw), a refraction microtremor (remi) and single station microtremor studies were conducted at 26 strong-motion stations of afad located in the province of hatay to investigate s-wave velocity distribution in shallow soil which is important in the design of seismic hazards. geology and tectonics of the region hatay and its surrounding are located to the south of the antakyakahramanmaraş graben, which developed under the influence of the dead sea and eastern anatolian faults and the cyprus spring. the south of the graben is shaped by the dead sea fault. however, arabian and african plates move northward under the influence of convective currents in the mantle over the weakly resistant (fluid) upper asthenosphere. the slip vectors of earthquakes in the region, the fault systems, the global kinematic models based on the oceanic spread, indicate that the arabian plate moves north-northwest to eurasia at an average speed of 25 mm per year. the african plate is moving northward at about 10 mm per year in relation to eurasia (reilinger, et al.1997). although many destructive depressions have been exposed in the historical period, there has not been an earthquake in the area for 135 years. this situation increases the risk of destructive earthquakes every day (fig. 1). the otokton arabian platform rocks with the main lines of the geology of the amanos mountains covering the vast majority of the region; ophiolite nappes; neo-autoclaved sediments covering nappes and autochthonous units are defined. the amanos mountains located at the southern tip of the eastern taurus mountains and the kızıldağ ophiolite massif in the vicinity of them also have an important place in the geology of the region (fig. 2). data acquisition and analysis seismic refraction, masw, remi, and single station microtremor data were collected at afad’s 26 strong motion stations in the province of hatay in southern turkey. the seismic refraction data were recorded for 2.0 sec with a sampling rate of 1.0 msec using a 50 kg weight-dropping energy source with a hydraulic weight-lifting unit operated by a battery powered electric motor. number of 48 pwave geophones with 4.5 hz were fixed in line form at 2.0 m intervals (fig. 3). data were collected from forward, midpoint, and reverse shootings with two m offsets; transferred to 48 channels geode recording units; and converted from analog to digital form. the midpoint of the geophone spread was placed as close as possible to the strong motion station on a flat area so that the static corrections of the shot and geophone points were neglected. an example of the shot records obtained at station 3112 is shown in figure 4a. the first arrival times of the refracted p-waves were picked from the field record, and the corresponding travel time-distance curves of the profile were determined (figure 4b). the initial p-wave velocity-depth model was then obtained from the first arrival times. then, this initial model was processed iteratively by nonlinear travel time tomography (zhang & toksöz, 1998) to obtain the final p-wave velocity-depth model. iteration was stopped when the differences between the first arrival times and the modeled were reduced to an acceptable minimum value as 8. the p-wave velocity–depth cross-section of the site was determined laterally averaging the p-wave velocity-depth model (fig. 4c). shot records on both sides were used in the analysis of multichannel analysis of surface waves (masw) to estimate the vs profiles of the top 30 m. it was observed that layers varying in thickness from 5m to 18m up to 30m. the shot record was isolated from the refracted by muting before and after. low and high frequency noises were filtered through a bandpass filter with frequencies (2, 4, 36, 48) hz (figure 5a). thereafter, plane wave decomposition was applied to convert the data from the offset time to the phase velocity versus the frequency domain (park et al., 1999; xia et al., 1999). the dispersive curve related to a fundamental mode of surface waves was picked from this domain. the s-wave velocity was determined as a function of depth by inverting the dispersion curve. it has been observed that each frequency component of rayleigh waves propagates at different phase velocities; in other words, these waves have scattering properties on the ground. the greatest share of the energy of the rayleigh waves belongs to the fundamental mode, although not for every case. for this mode, dispersion curve representing the change of the phase velocity relative to the frequency is determined. this dispersion curve was used to determine the s-wave velocity-depth profile of the stationary region by inversion method (fig. 5b). the dispersion curve for the basic mode showing the change of the frequency and the phase velocity is shown in figure 5c. a remi study was performed to check the velocity-depth profile obtained by the masw method at a depth of up to 30 m as well as to find the velocity-depth profile at the lower half of 30m. the remi data were recorded for 32 sec with 2.0 msec sampling rate in the same field layout using ambient noise from wind and traffic as a seismic source. 24-25 records were stacked in each remi installation to remove the cultured noise from the microtremor data at each site. rayleigh waves are distinguished from other waves by using two-dimensional slowness frequencies (p-f) transformations of noise records spreading in both directions. the rayleigh wave dispersion is selected along the minimum velocity envelope of energy in the spectral image of the slowness frequency. 475investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor figure 1. tectonics of the study area, location of stations and epicenter of the earthquakes that have taken place in the last 10 years. thick black dash arrows shows the plate’s movements. red triangles shows the stations. the colored circle points shows the earthquakes according to their magnitudes and depths figure 2. the geology map of the study area (emre et al., 2013; akbaş et al.,2002a). 476 i̇brahim sertçelik, cengiz kurtuluş, fadime sertçelik, hamdullah livaoğlu, cuneyt saş figure 3. masw acquisition data scheme with a 48-channel linear receiving array figure 4. sample seismic refraction record taken at the station coded 3112. (a) seismic refraction records with picked first arrival times showing as a red line, (b) observed and calculated travel time curves, (c) p-wave velocity-depth model along the profile a b c 477investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor the spectrum is normalized as the ratio of the power spectrum at certain frequency slowness over the mean slowness values of that frequency. the best way to obtain accurate phase velocities is to choose the fundamental mode phase rate of the rayleigh wave dispersion along the minimum edge of the vertical steep slopes of the dispersion curves (louie, 2001). these rayleigh wave dispersion picks are modeled using the experiment and error settings of the velocity-depth model to determine the shear wave velocity-depth profile. the time-distance data are shown in fig. 6a. the dispersive curve for the fundamental mode of surface waves are transformed to frequencyphase velocity domain to pick fundamental dispersion curve (fig 6b) and is inverted to obtain the s-wave velocity depth profile (fig. 6c). a velocity based broad-band sensor (flat response between 0.03-50 hz) was used for ambient noise recording. whole data were acquired with the same sensor. (figs. 7a and 7b). the estimated spectra of the three components microtremor data are given in fig. 8c. the h/v amplitude spectrum of dominant frequency and amplification of the 3112 coded station are given in fig. 7c. as seen from fig.7c, the dominant frequency of the ground is 5 hz (period is 0.2 sec.) and h/v ratio or peak amplitude at this frequency is 6.6. the theoretical h/v amplitude spectra of the stations were also calculated to check the validity of the observed values. the theoretical amplification functions of stratified soil layers (table 1) were calculated according to the recursive algorithm proposed by tsai (1970) and modified for calculations (see also tsai and housner, 1970, herak, 2008). the theoretical amplification functions of stratified soil layers (table 1) were calculated according to the recursive algorithm proposed by tsai (1970) and modified for code (see also tsai & housner, 1970, herak, 2008). the h/v transfer functions were synthetically derived for the station 3112 as shown in fig. 8. observed h/v curve were also estimated from the microtremor measurements for the 3112 coded station. the values from the table 1 are in-situ measurements and results from the field near the stations. it can be clearly seen from the figure that the fundamental peak frequency corresponding to the h/v amplitudes are in step with each other which indicate 2.55 and 3.17 hz respectively which are in the similar site class for predominant based site classifications. the significant difference between the fundamental frequencies and h/v amplitudes of the site could be originated from lateral irregularities. besides, the fundamental peak frequency corresponding to the h/v amplitudes are compatible. the shear wave velocities, vs30, soil classes, h/v peak amplitudes, and dominant soil periods are given in table 2. where vp and vs are respectively the p and swave velocities, qp and qs are the quality factors related with p-and s-velocities, ƍ is density, and h indicates the layer thickness. sites utilizing the information obtained from shallow field tests are classified according to bssc (2003), cen (2003), tbdy (2018), and rodriguez-marek et al. (2001), all of which recommend criteria based on the average shear wave velocity vs30 of the top 30 m soil/rock profile. the mean shear wave velocity of the uppermost 30 m at a site is calculated by where di and vs are respctively the thickness, shear wave velocity of the ith layer , and n is the number of layers in the top 30 m. the vs profile obtained from the masw measurements is used to compute the vs30 for each strong motion station. velocity-depth values obtained by masw up to 30m depth are in good agreement with the values found by the remi method. figure 5. (a) the shot record separated from the refraction and reflected waves at the station coded 3112, (b) phase velocity-frequency and picking dispersion curve , (c) vs-depth profile and modeled dispersion curve. the s-wave-depth profile (blue) and the modeled dispersion curve (black). a b c 478 i̇brahim sertçelik, cengiz kurtuluş, fadime sertçelik, hamdullah livaoğlu, cuneyt saş site classification according to nehrp provisions the nehrp recommends six field classes (a, b, c, d, e, and f) for identifying site-specific design spectra, shown in table 3. based on vs30, eight of the 26 sites are classified as class d, 16 as class c and two as class b. in other words, 30.7% of the total places correspond to class d, 61.5% to class c, and 7.69% to class b. site classification according to eurocode-8 the field classification scheme proposed by eurocode-8 is quite similar to the nehrp provisions (table 4). the difference between the two is that the a and b classes of the nehrp provisions are included in eurocode-8 as class a. in addition, the limit of 760 m/s between the a and b classes in the nehrp provisions is set to 800 m/s in eurocode-8. according to eurocode-8, eight sites correspond to class c (30.77%), 17 to class b (65.38%), and one to class a (3.84%). as can be seen, classes d, c, and b in in the nehrp provisions correspond to classes c, b, and a in eurocode-8. site classification according to tbdy (2018) the tbdy (2018) involves six site classes (za, z b, z c, zd, ze, and zf) to identify the site-specific design spectra shown in table 5. as can be seen in table 2, eight sites are classified as class zd (30.77%), 16 as zc (61.54%), and two as classified as zb (7.69%), according to the vs30 criterion. site classification according to rodriguez-marek et al. (2001) rodriguez-marek et al. (2001) proposed a geotechnical classification scheme based on results from the ground motion data analysis shown in tables 5 and 6. as seen in table 2, 11 sites are classified as class d1 (42.3%), three as d-2 (11.54%), eight as d-3 (30.77%), one as c-1(3.85), one as c-2 (3.85), and two as c-3 (7.69%), according to the site period. site amplification of hatay province experimental amplification ratios (h/v) and predominant periods were obtained at 26 locations using the güralp cmg 6td with a three-component broadband velocity meter and a 12-volt power supply. the sensitivity of the seismometer sensor to ground motion varied in a wide frequency band from 0.033 to 50 hz. microtremor data were obtained in gcf (guralp compressed file) format. the three recorded components were viewed and edited with noise data scream (figure 9). the records were evaluated according to the single station method (h/v) by using geopsy (sesame europe project, 2004) software. firstly, a trend analysis was applied to data for removing base-line figure 6. (a) remi data, x-t domain at the station coded 3112, (b) the dispersion spectrum calculated by separating the plane-wave components of the surface waves present in the seismic record, (c) remi s-wave velocity-depth profile (blue) and modeled dispersion curve (black). the vertical axis indicates the s-wave velocity and the horizontal axis shows the frequency of dispersion curve. a b c 479investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor figure 7. (a) filtered time signal series, (b) the amplitude spectra of the three components microtremor data, the peak curves (average curve, black line), illustrate the corresponding resonant frequencies and (c) the h/v amplitude spectra. the dashed lines show the standard deviation of amplitude, and the two vertical gray areas show the peak frequency standard deviation domains. trend and then a 5% rounded cosine window was applied on the edges of the raw data to simulate sudden changes at both ends of the timeline. the amplitude of the side-oscillations that can be seen on the resultant spectra was reduced, and the data was filtered with a band-pass butterworth filter between 1-20 hz. thus, the noises of very low and high frequencies are removed from the signals, and the frequency content of the microtremors is left behind. in evaluation of microtremor data, the number of windows is taken into consideration such that the number of distinct cycles in the entire record is greater than 200, the case where the fo (dominant frequency) value is greater than or less than 0.5 hz, (table 2). in the evaluation of microtremor data; criteria such as number of windows, and the standard deviation values of the h / v curves were considered according to sesame project (site. effects assessment using ambient excitations, contract no. evg1-ct-2000-00026) the h/v peak amplitudes and predominant periods were interpreted according to sesame (2004) criteria. the top 30 m site conditions were characterized in accordance with the nehrp provisions (bssc 2003), eurocode-8 (cen2003), tbdy (2018) and a b c 480 i̇brahim sertçelik, cengiz kurtuluş, fadime sertçelik, hamdullah livaoğlu, cuneyt saş rodriguez-marek et al. (2001), as shown in table 2. the peak amplitudes of the stations (h/v) vary between 1.3-7.6 (fig. 9a) and their dominant periods range from 0.23 to 3.3 s (fig.9b). discussion this article uses site responses, site amplification variations using microtremor data of 26 strong motion stations of afad in hatay region. the shear wave velocities, vs30, up to 30 m of depth vary between 221 and 597m/s at the 3112, 3113, 3114, 3115, 3116, 3117, 3119, 3120, and 3121coded stations. these velocity values are almost the same as those determined by ozmen et al. (2017) at the same stations. the difference between these two sets of velocity ranges between 0.07% and 2.95%. büyüksaraç et al. (2014) have determined the velocity model of the underground in four stations. they determined the seismic velocities ranging between 380 and 470 m/s at a depth of 0-5m, between 320-480 m/s at a depth of 5-15 m, and between 470-750 m at a depth of 15-25 m. this data are in good agreement with the velocity data obtained in this study (figs. 5c and 6c). in addition, the soil amplification was calculated as 1.5 in periods of 0.4sec and 0.5sec and 2.0 in the other two stations in 0.25sec and 0.4sec periods, which approximately correspond to the values table 1. observed soil model of the 3112 station for theoretical h/v calculation compressional wave velocity, vp (m/s) shearwave velocity, vs (m/s) quality factors p-velocity qp quality factors reletad with s-velocity qs density ƍ(kg/m3) thickness h(m) 431 431 431 431 431 431 440 440 515 ∞ 217 219 216 207 203 217 243 270 304 ∞ 5 15 20 25 30 100 1 5 10 20 30 70 1.8 1.9 2.1 2.2 2.5 2.6 0-1.43 1.433.21 3.215.44 5.44-8.23 8.2311.72 11.72-16.07 16.07-21.52 21.52-28.33 28.33-36.83 figure 8. synthetic and observed h/v curves at the 3112 station. figure 9. (a) h/v peak amplitudes, (b) pre-dominant periods a b 481investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor table 2. combined data set of study in hatay province no station code lat. lon. vs30 (m/s) h/v peak amplitudes t0 (s) nehrp e-code-8 tbdy rm-2001 lithology 1 3112 36.59 36.15 233 1.3 0.7 d c zd c-3 unidentifıed quaternary alluvium 2 3113 36.58 36.16 221 4.8 1.85 d c zd d-3 basalt 3 3114 36.57 36.15 215 4.1 2.5 d c zd d-3 unidentifıed quaternary alluvium 4 3115 36.55 36.16 424 2.5 1.1 c b zc d-1 unidentifıed quaternary alluvium 5 3116 36.62 36.21 781 1.6 0.23 b b zb d-1 unidentifıed quaternary alluvium 6 3117 36.56 36.17 597 2.67 0.24 c b zc c-1 clastic and carbonates rocks 7 3119 36.58 36.17 374 3.9 1.51 c b zc d-1 clastic and carbonates rocks 8 3120 36.59 36.21 455 2.87 1.9 c b zc d-1 clastic and carbonates rocks 9 3121 36.66 36.22 271 5.5 0.55 d c zd c-3 clastic and carbonates rocks 10 3122 36.03 36.11 1011 3.1 1.45 b a zb c-2 clastic and carbonates rocks 11 3124 36.24 36.17 283 4.1 0.81 d c zd d-1 clastic and carbonates rocks 12 3125 36.24 36.13 448 7.6 1.38 c b zc d-1 clastic and carbonates rocks 13 3126 36.22 36.14 350 3.1 2.8 d c zd d-3 clastic and carbonates rocks 14 3127 36.21 36.14 404 2.9 1.96 c b zc d-3 clastic and carbonates rocks 15 3128 36.21 36.15 329 2.36 1.96 d c zd d-3 clastic and carbonates rocks 16 3129 36.19 36.13 447 2.7 3.3 c b zc d-3 clastic and carbonates rocks 17 3130 36.18 36.15 447 4.2 1.4 c b zc d-1 clastic and carbonates rocks 18 3131 36.19 36.16 567 4.2 1.3 c b zc d-1 clastic and carbonates rocks 19 3132 36.21 36.17 377 3.2 2.94 c b zc d-3 clastic and carbonates rocks 20 3133 36.24 36.57 471 4.6 1.19 c b zc d-2 clastic and carbonates rocks 21 3134 36.83 36.20 374 4.1 0.87 c b zc d-2 clastic and carbonates rocks 22 3135 36.41 35.88 460 2.5 2.9 c b zc d-3 clastic and carbonates rocks 23 3142 36.50 36.37 191 1.6 1.06 d c zd d-2 clastic and carbonates rocks 24 3143 36.85 36.56 444 2.2 1.42 c b zc d-1 alluvium fan 25 3144 36.76 36.49 485 2.2 1.1 c b zc d-1 basalt 26 3145 36.65 36.41 533 2.2 1.66 c b zc d-1 alluvium fan that we determined. over et al. (2001) found the average vs velocities from the remi studies 413m/s at 0-13m of depth, 420m/s at 13-33m, 560m/s at 33-62m, and 537m/s at about 100 m. these values are very close to the values that we determined by applying the masw and remi methods. they also calculated the dominant soil periods between 0.2sec and 0.8sec and site amplification values between 1.1 and 1.8, which correspond to what we found. the sites were classified in accordance with nehrp provisions (bssc, 2003), eurocode 8 (cen. 2003), tbdy (turkish building earthquake regulations, 2018), and rodriguez-marek et al. (2001), and tabulated in table 2. the site classes were found at the stations located to the west of the amanos mountains as c and d 482 i̇brahim sertçelik, cengiz kurtuluş, fadime sertçelik, hamdullah livaoğlu, cuneyt saş according to nehrp and b and c according to eurocade-8. in both systems, the ground class was calculated as b at station 3116. soil classes were the same in the nehrp and tbdy-2018 systems. soil classifications obtained in nehrp and tbdy systems were similar. h/v peak amplitude values were calculated as 2.2 at stations 3143, 3144, and 3145. the h/v values of the stations coded 3123 and 3125 are 3.9 and 4.6, respectively. the h/v value was calculated between 3.9 and 4.6 for stations 3113, 3114, 3119, 3124, 3130, 3131, 3133, and 3134 and between 1.3 and 3.3 for other stations. the dominant periods (t0) of the stations were between 0.23sec (at station 3112) and 3.3sec (at station 3129). conclusion in this study, the subsurface soil condition and experimental site amplification ratios at the strong motion stations of afad in the province of hatay were investigated through geophysical methods including seismic refraction, masw, remi, and microtremor surveying. the studies were carried out at 26 strong motion stations to evaluate the site effects and seismic hazard of the hatay region. the shear-wave velocity profiles of the upper 30m of the sites were determined with masw and remi techniques. transfer functions (maximum h/v amplitude ratios) and predominant periods were obtained at the same locations by applying microtremor studies. the shear-wave velocities determined with the masw technique were compared with those obtained employing the remi technique. the site class and soil amplification ratio as well as the dominant period of each site are tabulated in table 2. the peak frequency of the h / v ratio in a field can be used to create a standard h / v response spectrum, which is a good diagnosis of the field condition. the peak amplitude of the h/v function varies between 1.3 and 5.5. the shape of the standard h / v ratio curve is very similar across regions. vs30 values (191-1011m/s) corresponding to these peaks show scattering because the studied environments are in a very complex structure. two types of regions are recognized based on their dominant period, to, characteristics: (1) to value, respectively, in hatay region is generally higher than 0.87sec and (2) in i̇skenderun region generally lower than 2.5sec. the database supported by the precise descriptions of site conditions presented here will provide valuable information and will contribute to further table 3. site class definitions in nehrp provisions (bssc 2003) site class vs,30 m (m/s) a vs,30 m > 1,500 b 760 < vs,30 m ≤ 1,500 c 360 < vs,30 m ≤ 760 d 180 ≤ vs,30 m ≤ 360 e vs,30 m < 180 any profile with more than 3 m of soft clay defined as soil with pi > 20, w ≥ 40% and su < 25 kpa table 4. site class definitions in eurocode-8 (cen 2003) site class vs,30 m (m/s) a vs, 30 m > 800 b 360 < vs,30 m ≤ 800 c 180 ≤ vs,30 m ≤ 360 d vs,30 m < 180 e a soil profile consisting of a surface alluvium layer with vs in the range given for sites of types c or d, with thickness varying between 5 and 20 m and which is underlain by stiffer material with vs > 800 m/s s1 deposits containing a layer of soft clays/silts with a high plasticity index (pi > 40) and high water content with vs < 100 m/s or 10 < su < 20 kpa and at least 10 m thick s2 deposits of liquefiable soils, of sensitive clays, or any other soil profile not included in types a-e or s1 table 5. turkey earthquake building regulations (tbdy-2018) local soil classes soil type vs30 [m/s] za strong, hard rocks > 1500 zb weak altered, medium strong rocks 760-1500 zc very stiff sand, gravel and hard clay layers or latered, very cracked weak rocks 360-760 zd medium-hard sand, gravel or very hard clay layers 180-360 ze soft sand, gravel or soft-hard clay layers or profiles containing a soft clay layer (cu < 25 kpa) of a total thickness of 3 meters providing conditions of pi > 20 and w > 40% < 180 table 6. site classes proposed by rodriguez-marek et al. (2001) site description site period comments a hard rock <0.1 s hard, strong, intact rock (vs ≥ 1500 m/s) b rock <0.2 s most “unweathered” california rock cases (vs ≥ 760 m/s or <6 m of soil) c-1 weathered/soft rock <0.4 s weathered zone >6 m and <30 m (vs > 360 m/s increasing to > 700 m/s) c-2 shallow stiff soil <0.5 s soil depth >6 m and <30 m c-3 intermediate depth stiff soil <0.8 s soil depth >30 m and < 60 m d-1 deep stiff holocene soil, either s (sand) or c (clay) <1.4 s soil depth >60 m and < 200 m. sand has low fines content (< 15%) or nonplastic fines (pi <5). clay has high fines content (> 15%) and plastic fines (pi > 5). d-2 deep stiff pleistocene soil, s (sand) or c (clay) <1.4 s soil depth >60 m and < 200 m. see d1 for s or c subcategorization. d-3 very deep stiff soil <2.0 s soil depth >200 m e-1 medium depth soft clay <0.7 s thickness of soft clay layer 3 m to 12 m e-2 deep soft clay level <1.4 s thickness of soft clay layer >12 m f special, e.g. potentially liquefiable sand or peat = 1.0 s holocene loose sand with high water table (zw ≤ 6 m) or organic peat 483investigation of soil characterization in hatay province in turkey by using seismic refraction, multichannel analysis of surface waves and microtremor development of methods to assess the relationship between site conditions and strong motion parameters. acknowledgment this study was funded and supported by the afad (disaster and emergency management presidency) national earthquake research program (udap-g-15-04) within the scope of the “determination of soil parameters of turkish strong motion recording stations project.” references akbaş, b., akdeniz, n., aksay, a., altun, i., balcı, v., bilginer, e., bilgiç, t., duru, m., ercan, t., gedik, i., günay, y., güven, i.h., hakyemez, h.y., konak, n., papak, i., pehlivan, s., sevin, m., şenel, m., tarhan, n., ... yurtsever, a. 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(2002). strong motion dataset of turkey: data processing and site classification. soil dynamics and earthquake engineering, 22(8), 703-718. zhang, j. & toksöz, m. n. (1998). nonlinear refraction traveltime tomography. geophysics, 6, 1726 1737. palabras clave: función de base radial en redes neuronales; modelo bursa-wolf; validación cruzada de k-iteracciones; transformación coordinada; remuestreo estadístico. how to cite item ziggah, y. y., youjian, h., tierra, a. r., & laari, p. b. (2019). coordinate transformation between global and local data based on artificial neural network with k-fold crossvalidation in ghana. earth sciences research journal, 23(1), 67-77. doi: https://doi.org/10.15446/esrj.v23n1.63860 the popularity of artificial neural network (ann) methodology has been growing in a wide variety of areas in geodesy and geospatial sciences. its ability to perform coordinate transformation between different data has been well documented in literature. in the application of the ann methods for the coordinate transformation, only the train-test (hold-out cross-validation) approach has usually been used to evaluate their performance. here, the data set is divided into two disjoint subsets thus, training (model building) and testing (model validation) respectively. however, one major drawback in the hold-out cross-validation procedure is inappropriate data partitioning. improper split of the data could lead to a high variance and bias in the results generated. besides, in a sparse dataset situation, the hold-out cross-validation is not suitable. for these reasons, the k-fold cross-validation approach has been recommended. consequently, this study, for the first time, explored the potential of using k-fold cross-validation method in the performance assessment of radial basis function neural network and bursa-wolf model under datainsufficient situation in ghana geodetic reference network. the statistical analysis of the results revealed that incorrect data partition could lead to a false reportage on the predictive performance of the transformation model. the findings revealed that the rbfnn and bursa-wolf model produced a root mean square horizontal positional error of 0.797 m and 1.182 m, respectively. the rbfnn model results per the cadastral surveying and plan production requirement set by the ghana survey and mapping division are applicable. this study will contribute to the usage of k-fold crossvalidation approach in developing countries having the same sparse dataset situation like ghana and in the geodetic sciences where ann users seldom apply the statistical resampling technique. abstract keywords: radial basis function neural network, bursa-wolf model, k-fold cross-validation, coordinate transformation, statistical resampling. coordinate transformation between global and local data based on artificial neural network with k-fold cross-validation in ghana transformación coordinada entre información global y local basada en redes neuronales artificiales con validación cruzada de k-iteraciones en ghana issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.63860 la popularidad de la metodología de redes neuronales artificiales está en crecimiento en varias áreas en geodesia y en las ciencias geoespaciales. su capacidad de realizar una transformación coordinada entre diferente información ha sido bien documentada en la literatura. en la aplicación de métodos de redes neuronales artificiales para la transformación coordinada solo se ha evaluado el desempeño del enfoque de prueba de adiestramiento (validación cruzada por método de retención). en este punto, la información se divide en dos subconjuntos diferentes: adiestramiento (modelo de construcción) y verificación (modelo de validación). sin embargo, una desventaja en el procedimiento de validación cruzada por método de retención es inapropiada durante la división de información. una partición no adecuada en la información podría llevar a una gran diferencia o a un sesgo en los resultados generados. además, ante una situación de un conjunto de datos disperso la validación cruzada por método de retención no es adecuada. por estas razones se recomienda la validación cruzada de k-iteraciones. por consiguiente, este estudio, por primera vez, explora el potencial de usar el método por validación cruzada de k-iteraciones en la evaluación de ejecución de la función de base radial en redes neuronales y el modelo bursa-wolf en una situación de información insuficiente en la red de referencia geodética de ghana. el análisis estadístico de los resultados muestra que una partición incorrecta de información puede llevar a un registro falso en la ejecución predictiva del modelo de transformación. los resultados demuestran que la función radial y el modelo bursa-wolf producen un error posicional de media cuadrática horizontal de 0.797 m y 1.182 m, respectivamente. los resultados del modelo radial por la medición cadastral concuerdan con los requerimientos del plan de producción instaurados por la divisón de mapeo del servicio geológico de ghana. este estudio contribuirá en la usabilidad del método de validación cruzada de k-iteracciones en países en desarrollo que tienen conjuntos de datos dispersos, como ghana, y en las ciencias geodésicas donde los usuarios de redes neuronales casi nunca aplican la técnica estadística de remuestreo resumen record manuscript received: 03/04/2017 accepted for publication: 09/11/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 67-77 yao yevenyo ziggah1, hu youjian1, alfonso rodrigo tierra2, prosper basommi laari3 1china university of geosciences (wuhan), china. 2universidad de las fuerzas armadas espe, ecuador. 3university for development studies, ghana. * corresponding author: ziggah78@yahoo.com; hyj_06@163.com g eo de sy 68 yao yevenyo ziggah, hu youjian, alfonso rodrigo tierra, prosper basommi laari introduction positional information about natural and man-made features is shown on maps as coordinates. hence, coordinates has become an indispensable representative means for accurately mapping out natural resources. for example, the geologist needs coordinate to carry out geological mapping, while the drilling engineer require the position to be drilled as well as the azimuth the drilling should be done. in view of the foregoing discussion, it is clear that accurate positional information should be provided for proper planning, management and decision making. in view of the above, global navigation satellite systems (gnss), particularly, global positioning system (gps) have been widely adopted in geospatial sciences and its related earth science disciplines for geodetic purposes. since its arrival, gps has become an essential technology that has revolutionized data collection and surveying practices at large. however, for the gps data to be used locally so that compatibility could be created between maps and other geospatial data produced in the national datum, there is a need to perform coordinate transformation (featherstone, 1996; yang, 2009). in doing this, earth scientist can comfortably apply the gps measurement locally with minimal degree of errors that are usually created due to different datum size, shape and origin between the national data (non-geocentric) of countries and the global datum (geocentric) of the gps. as a means to accomplish such task, empirical based coordinate transformation equations have widely been used. notable among them is the three-dimension (3d) conformal transformation models like bursa-wolf (bursa, 1962; wolf, 1963), molodensky-badekas (molodensky et al., 1962; badekas, 1969), abridged molodensky (molodensky et al., 1962) and veis (veis, 1960). conversely, in recent times, artificial neural network (ann) techniques have also been applied for coordinate transformation (gullu, 2010; gullu et al., 2011; lin and wang, 2006; mihalache, 2012; tierra et al., 2008, 2009; tierra and romero, 2014; turgut, 2010; yilmaz and gullu, 2012; zaletnyik, 2004; konakoğlu and gökalp, 2016; konakoğlu et al., 2016; elsayed and ali, 2016; ziggah et al., 2016; ziggah et al., 2017a). it was, however, evident in literature that, the statistical test called the hold-out cross-validation has been the most commonly used technique to evaluate coordinate transformation models performance. in this hold-out cross-validation method, it was noticed that, most authors are primarily concerned with randomly partitioning the available data into two disjunct samples: a training set and a testing set. here, the training set is used to determine transformation parameters in the case of the empirical models and for ann model construction. the testing data, on the other hand, is used to measure the predictive accuracy and stability of the chosen trained ann model, and the empirical model developed with respect to the transformation parameters computed. however, many studies have reported some significant limitations for using the hold-out cross-validation procedure. one of the main issues is related to how appropriate the available data set is divided (reitermanová, 2010). it has been indicated that partitioning the data into training and testing sets reduces the number of samples for calibrating the model, and the results produced is based on the unsystematic chosen split (train, test) sets. moreover, improper data separation could lead to an unreasonably high inconsistency of the model performance. by virtue of this, any bias resulting from the improper split of the data set could have an adverse effect on the model performance (kohavi, 1995). besides, the hold-out cross-validation method heavily depends on having large datasets making it unsuitable to be applied in data-insufficient situations (sparse dataset). the reason being that in sparse dataset, a single train-and-test experiment performed could yield unstable estimates that do not provide enough convincing evidence of the models generalization ability. this could be attributed to: (i) the inadequate amount of observational data available; (ii) unsuitable train-test split selected; and (ii) high variance of the model performance (bengio and grandvalet, 2004). in continuance of this, there is often not enough data for holding some of it for testing purposes in sparse data situation. to combat these defects, the use of k-fold cross-validation (kcv) technique has been recommended (burman, 1989; reitermanová, 2010; kohavi, 1995). it is for these reasons that this study applied for the first time, the kcv technique to evaluate the coordinate transformation performance of the widely used ann (radial basis function neural network) as well as the similarity model (bursa-wolf). the present authors were also motivated to apply the kcv approach on ghana geodetic reference network data for several reasons. first, the ann approach has only been tested for transforming coordinates between two local geodetic data, namely accra and leigon data in ghana. this can be found in ziggah et al. (2016). in their study, only 27 local common points were available. although the data is quite small, the hold-out cross-validation procedure was implemented where 20 co-located points were used to form the model and 7 evenly distributed points were used to test the model. in addition, ziggah et al. (2017a) developed an error compensation model based on ann capable of improving the performance of the geocentric translation model. here, 19 co-located points were divided into 11 training points and 8 testing points, respectively. due to data limitation, it is suggested by the present authors that the kcv technique may have been more appropriate to represent the true reflection of the ann performance. it is important to note that no such kind of study has also been carried out in ghana using the ann approach to directly transform coordinates between the global datum and ghana’s local geodetic datum. in continuance of that, the widely adopted gnss for geodetic purposes has necessitated the transformation of its data to the local datum to make it applicable. in line with that, the present study applied the ann approach which usage in ghana for global to local datum transformation has been hampered due to limited data availability. in addition, the similarity, affine and projective transformation models (ayer, 2008; ayer and fosu, 2008; dzidefo, 2011; ziggah et al., 2013a, b; kumi-boateng and ziggah, 2016; anann et al., 2016; laari et al., 2016; ziggah et al., 2017b) which have been utilised in ghana’s geodetic reference network could be compared with the ann via kcv approach. the study will further serve as the foundation for exploiting kcv as an alternate way of assessing transformation models performance in ghana geodetic reference network. study area and data this study was carried out in ghana, located in the western part of africa (fig. 1). it lies between latitudes 4°30′ n and 11o n, and between longitudes 3o w and 10 e. it has a land area of 238, 540 km2 (fosu et al., 2006) with the highest elevation range not exceeding 880 m above mean sea level located at mountain afadjato in the volta region (berry, 1995). the land has general characteristics of low grassland and savanna with a divided plateau at the south-central part of the country (baabereyir, 2009). ghana shares border with ivory coast to the west, togo to the east, burkina faso to the north and gulf of guinea to the south. the local ellipsoid used in ghana for its mapping and surveying undertakings is the war office 1926 suggested by the british war office. the war office 1926 ellipsoid has a semi-major axis a = 6378299.99899832 m, semi-minor axis b = 6356751.68824042 m, and flattening f = 1/296 (ayer, 2008; poku-gyamfi, 2009; mugnier, 2000). the ghana coordinate system is a projected grid coordinates based on the transverse mercator. the origin of the transverse mercator is longitude 01o 00’ w (central meridian) and latitude 04o 40’ n with 274319.736 m as the false easting added to all y coordinates to avoid negative coordinates and the false northing set to zero. a scale factor of 0.99975 is used at the central meridian so that the scale distortion exceeds the projection values only at the extreme ends of the country (mugnier, 2000). therefore, positions of features of all survey maps in ghana are the projected grid coordinates of easting and northing derived from the transverse mercator 10 nw. for ghana to capitalise on the potential of gnss technology, the ghana survey and mapping division of lands commission with funding from the world bank and other stakeholders embarked on the land administrative project (lap). one of the major aims of the lap is to establish a nationwide gnss reference network for ghana. the lap has been divided into three phases namely the golden triangle, northern triangle and the kintampo link, and the nationwide coverage (poku-gyamfi, 2009). currently, it is only the golden triangle phase covering five out of the ten administrative regions in ghana that has been completed (fig. 1). the golden triangle (fig. 1) was strategically established in these five regions due to their massive contributions to the country’s economic growth. this is because almost all the natural resources like gold, oil, timber, cocoa, diamond, manganese, bauxite, limestone to mention but a few are located in these regions (poku-gyamfi, 2009). the golden triangle comprises of three permanent reference stations fixed at the apex of the three largest cities situated at the western, ashanti and greater accra regions. the reference stations form a triangle of sides 200 km and radii 69coordinate transformation between global and local data based on artificial neural network with k-fold cross-validation in ghana coverage of 100 km with area coverage of 79857 km2 representing 33.5 % of the total land area (238,540 km2) for ghana (fosu et al., 2006). in the establishment of the gnss reference network, a continuous twelvehour observation was made on 19 historical triangulation stations located in the golden triangle by dual frequency gps receivers. the coordinates provided by the gps receivers were then differentially processed with the international gnss permanent stations to obtain the respective common points on the wgs84 ellipsoid. these 19 satellite coordinates are defined in the international terrestrial reference frame 2005 specified at epoch 2007.39 (kotzev, 2013). data used for the study is from the lap comprising of 19 golden triangle common points on the war office 1926 ϕ, λ,h war( ) and wgs84 ϕ, λ,h wgs( ) 84 reference frames. here,  is the geodetic latitude,  is the geodetic longitude and h is the ellipsoid height. it is imperative to note that the local geodetic network of ghana involves data in geodetic latitude, geodetic longitude and orthometric height (h) without the existence of ellipsoidal height. this, however, impedes the direct conversion of geodetic coordinates to cartesian coordinates. in view of this, the abridged molodensky transformation model (molodensky et al., 1962) was applied in the lap to get the war office ellipsoid heights. this was done by applying the abridged molodensky model to estimate the ellipsoid correction factor (∆h) between the wgs84 and war office 1926. the war office 1926 ellipsoid height (hwar) was then calculated using the relation h h hwar wsg= 84−∆ . methods conversion of geodetic coordinate to cartesian coordinate the common points geodetic coordinates, ϕ, λ,h wgs( ) 84 and ϕ, λ,h war( ) on wgs84 and war office 1926 was converted to cartesian coordinates (x, y, z) using the standard forward equations from heiskanen and moritz (1967). the rectangular coordinates derived from the conversion is designated in this study as (x, y, z)wgs84 and (x, y, z)war, respectively. similarity transformation model this study applied the 3d similarity model of bursa-wolf (bursa, 1962; wolf, 1963) to transform coordinates from wgs84 to war office 1926. the bursa-wolf model considers the misalignment of the x, y, z axes between two reference systems. the bursa-wolf model is made up of three rotations, three translations and a scale factor which in totality form a seven parameter transformation approach. one important characteristic feature of this model is that the shape of the geodetic network containing the coordinates to be transformed is preserved, hence angles are not altered after transformation, but the distance between the transformed coordinates and their original positions could be changed (constantin-octavian, 2006; ghilani, 2010). mathematically, the bursa-wolf model integrates two sets of threedimensional (3d) rectangular coordinates defined in two different coordinate systems using equation 1. x y z t t t r x y z war war war x x z wgs wgs wg           =           ⋅+η 84 84 ss84           (1) where (x, y, z)wgs84 and (x, y, z)war are the respective rectangular coordinates for wgs84 and war office 1926 ellipsoid. the (tx, ty, tz) are the translations along x, y, z-axes of the two reference systems, η is the scale factor and r is the total rotation matrix, respectively. expansion and simplification of eq. (1) into least squares form can be found in (deakin, 2006). in this study, the total least squares (tls) approach was used to determine the unknown transformation parameters. the solution of the tls was done using singular value decomposition (svd) method. here, the svd was first applied on the augmented matrix [a:f]. here, a is the design matrix and f is the observation matrix. the svd on augmented matrix [a:f] could be defined by equation 2 (golub and reinsch, 1970; van huffel and vandewalle, 1991; markovsky and van huffel, 2007) as a f usvt:[ ] = (2) where u u u u u u u u u r u u iu n m i m t mn = [ ] =   = …[ ] ∈ =… +1 2 1 1 2 1: , , , , , ,, ; v v v v v v v v r v v i d n n d n d i n d t=       = …[ ] ∈ =+ +11 12 21 22 1 , , , nn d+ ; s diag r t m n dn m n d= ∑ ∑       = …( ) ∈ = { }+ +( )1 2 1 1 0 0 σ σ −×, , , min , ; ∑ = …( ) ∈ ∑ = …( ) ∈+ + ( ) 1 1 2 1diag r diag rn n m n n t m n dσ σ σ σ× − ×, , , , , , and σ ≥…≥σ ≥1 1 0n+ the i are the singular values of a and [a:f], and the vectors ui and vi are the ith left and right singular vector of a and [a:f], respectively. a tls solution exists if and only if v22 is non-singular and the solution is unique if and only if σ ≠σn n+1. the tls solution for the unknown parameters was determined using equation 3. (3) the corresponding tls correction matrix ∆ctls( ) is given by equation 4 as ∆ ∆ ∆c a l udiag vtls tls tls t= [ ] = ∑( ): , 0 2 (4) figure 1. study area: gnss reference network coverage 70 yao yevenyo ziggah, hu youjian, alfonso rodrigo tierra, prosper basommi laari radial basis function neural network the basic radial basis function neural network (rbfnn) structure consist of highly interconnected neurons arranged into three layers: input, hidden and output layer respectively, as shown in figure 2 where (x1, x2, …, xi) is the input layer data and (y1, … ,yk) is the output layer target. the input layer accepts the input variables of the problem to be solved and deliver straightforwardly to the hidden layer without weighting. the obtained input data in the hidden layer are then transformed by weight multiplication and the results moved into a nonlinear system by means of a radial basis function (rbf). the most generic rbf widely used in rbfnn known as the gaussian function was applied in this study. this type of rbf is highly characterized by a centre position and a width parameter which regulates the amount of decrease of the function during training. the gaussian function (jain et al., 2011) is defined in equation 5 as , (5) where m xi v( ) denote the hidden layer output value of the ith unit, h is the number of rbf units, x jv is the jth variable of the input data v, is the centric position of ith rbf unit for input variable j and i is the spread parameter of ith rbf unit. it should be noted that, in the course of rbfnn training, the rbf parameters (eq. (5)) are optimized and determined in a threestep procedure. first, the euclidean based clustering technique is applied to determine the rbfs centres. the spread parameters are then calculated by the nearest neighbour method. to this end, the weights (w) linking the hidden layer neurons (rbf units) and the output layer are estimated using a linear regressor. the estimated output layer results for the rbfnn could be given by equation 6 as y w m x wi i v o u h = ( ) + = ∑ 1 , (6) where h is the number of radial basis function, wi is the output weight that matches to the association between a hidden node and an output node, while wo is the bias. m xi v( ) is defined in equation 5. the mean squared error (mse) for the vth data is then estimated using equation 7. mse y v y va t= ( ) ( )( )1 2 2  , (7) where y va ( ) and y vt ( ) are the actual output and target for the vth data, respectively. repeat the training process until the rbfnn reach the desired error value. the error function ek( ) (jain et al., 2011) could be estimated using equation 8. e mse y v y vk v v a t v v = = ( ) ( )( ) = = ∑ ∑1 2 1 2 1 max max  , (8) where vmax represents the total number of training data. the optimum trained rbfnn model was then tested using a test data (untrained). it important to note that, in order for us to carry out the rbfnn training described, both the input and output data were first scaled into the interval [-1, 1] using equation 9 (mueller and hemond, 2013). scaling the data set into a bounded interval gave constant variation in the data and thus improved training speed of the network. q q q q b b b bi i= + ( ) × ( ) ( )min max min min max min    , (9) where qi represents the normalized data, bi is the measured coordinate values, while bmin and bmax represents the minimum and maximum value of the measured coordinates with qmax and qmin values set at 1 and -1, respectively. statistical resampling technique generalisation is a generic operational procedure use to assess the efficiency of developed mathematical and statistical models. in common terms, generalisation is the ability of a model to correctly learn the significant parameters of a prediction function (training set) and predict well on yet unseen data (testing data) (urolagin, 2011). the general way adopted in the majority of research works relating to coordinate transformation for assessing the generalization capability of a model is by using the hold-out cross-validation technique (gullu, 2010; lin and wang, 2006; mihalache, 2012; tierra et al., 2008, 2009; tierra and romero, 2014; turgut, 2010; gullu et al., 2011; yilmaz and gullu, 2012; zaletnyik, 2004; konakoğlu and gökalp, 2016; konakoğlu et al., 2016; elsayed and ali, 2016; ziggah et al., 2016; ziggah et al., 2017a). however, as stated earlier (introductory section), the hold-out cross-validation method has some deficiencies to be surmounted. therefore, a solution to the enumerated problems has led to the usage of k-fold cross-validation (kcv) as an alternative way to estimate out-of-sample accuracy. the kcv is a procedure that employs the entire dataset as training and testing sets (bengio and grandvalet, 2004). by virtue of using a combination of more tests, a stable estimate of the model error could be achieved (reitermanová, 2010). this is because each point in the entire dataset will be in a test set just once, and will be in the training set the number of folds. hence, for kcv approach how the entire dataset is divided is less of a concern as compared with the hold-out approach. in the kcv method, a k-fold partition of the entire dataset is first carried out. here, the dataset m is separated into k disjoint blocks of approximately equal size as the target data such that m m i k i= =1  (konaté et al., 2015). subsequently, for each k trials of cross validation the union of k-1 folds is used as training data for model development while the remaining part is used as the testing data for the resulting model validation (stone, 1974). in this study, the k-1 disjoint set was used to train the rbfnn as well as to determine the transformation parameters for the bursa-wolf model. the remaining subset acting as the test data was used to validate the results produced by rbfnn and bursa-wolf model, respectively. this k-fold methodology was applied to the 19 co-located points (table 1) located in the golden triangle (fig. 1) from the land administration project. in practice, the 19 common points was divided into four disjoint subsets (four-fold) as shown in table 1 as subset 1, 2, 3 and 4 respectively. it can be seen from table 1 that subsets 1 to 3 comprised of five common points, while subset 4 consisted of four data points. after dividing the entire dataset into fourfolds (table 1), four models were then built using each subset. in this process, input layer hidden layer output layer x1 x2 y1 x3 yk x i 1 k 1 2 3 i 1 2 3 i figure 2. rbfnn representation 71coordinate transformation between global and local data based on artificial neural network with k-fold cross-validation in ghana table 1. co-located point in the ghana geodetic reference network showing k-fold partition (subset 1, 2, 3 and 4). the wgs84 coordinates for longitude, latitude and ellipsoid height are represented as wgs long, wgs lat in decimal degree unit and wgs hght in meters. war long, war lat, war hght are the corresponding coordinates on the war office 1926 ellipsoid. point id subset wgs long (degree) wgs lat (degree) wgs hght (metre) war long (degree) war lat (degree) war hght (metre) pt1 1 -0.423560 5.460090 78.2744 -0.423846 5.457304 82.0662 pt2 -0.559317 5.625798 304.9379 -0.559597 5.623015 307.9119 pt3 -1.501102 5.455087 275.1437 -1.501354 5.452274 279.5252 pt4 -0.734406 5.282744 83.4515 -0.734678 5.279958 88.3693 pt5 -0.121995 5.939034 524.5492 -0.122291 5.936247 525.5954 pt6 2 -1.033307 6.372118 492.5083 -1.033574 6.369342 491.7309 pt7 -1.286211 6.053792 437.6990 -1.286464 6.051007 438.7549 pt8 -0.729705 5.943107 311.0926 -0.729978 5.940331 312.4534 pt9 -0.765572 6.575797 782.2084 -0.765843 6.573035 780.2024 pt10 -0.748904 6.128197 327.0218 -0.749175 6.125418 327.4004 pt11 3 -1.164663 6.571357 615.7568 -1.164932 6.568592 613.9824 pt12 -1.925156 6.484775 643.5756 -1.925406 6.482004 642.6334 pt13 -1.630465 7.235861 536.0062 -1.630717 7.233129 530.9264 pt14 -2.016758 6.912480 560.8285 -2.017006 6.909735 557.6481 pt15 -1.445338 6.992209 620.9316 -1.445591 6.989461 617.0558 pt16 4 -1.743418 6.846344 417.0231 -1.743673 6.843595 414.0673 pt17 -1.411897 6.556916 503.7124 -1.412161 6.554147 502.1391 pt18 -1.694831 6.471558 472.1430 -1.695085 6.468791 471.1630 pt19 -1.966153 5.849618 399.3477 -1.966403 5.846824 401.8272 table 2 four-fold cross-validation technique structure number of common points per subset first trial second trial third trial fourth trial subset 1 subset 2 subset 3 subset 4 5 testing training training training 5 training testing training training 5 training training testing training 4 training training training testing for each trial one part of the data is kept for testing and the remaining parts for training as shown in table 2. that is, in the present study, there are three parts for training and one part for testing at each step. this is evident from table 2 where it can be seen that the first row in subset 1 has been singled out for testing, while the second to fourth rows data are used for the training. in subset 2, the second row was set for testing while the first, third and fourth rows were used for training. this same procedure was continued for subsets 3 and 4 as indicated in table 2, respectively. a critical study of table 2 shows that there is no overlap between the subsets and no overlap between the testing data. however, there is significant overlap of the training parts. this implies that in the k-fold cross-validation system each data subset is passed exactly once as a test sample. in the present context, rbfnn and bursa-wolf model was trained and tested in four trials with four coordinate transformation models being developed simultaneously. therefore, the average accuracy produced over all the four disjoint subsets served as a better indicator for the out-of-sample rotation performance of the rbfnn and the bursa-wolf model in the study area. it should be known here that the same subset data applied in the rbfnn was used for the bursa-wolf model. statistical performance metrics the usefulness of any applied mathematical or statistical model depends on how close their predicted outcomes fit well with the observed data. by applying statistical quantitative methods, an objective evaluation of the quality of the results produced by the model could be done. in lieu of this, the following statistical performance metric (pm) tools were employed. (i) root mean square error the root mean square error (rmse) is a type of dimensioned error statistic that is always non-negative and incorporates the concept of bias and standard 72 yao yevenyo ziggah, hu youjian, alfonso rodrigo tierra, prosper basommi laari deviation. it is used to quantify the degree of dispersion of model predictions to observed data in a system. ideally, an optimum model performance should have a rmse value of zero. however, in practice, the rmse could vary from zero to infinity subject to the units of the forecasted variable. equation 10 was applied in the rmse estimation expressed as rmse n o pi i i n = ( ) = ∑ 1 2 1 . (10) (ii) rmse horizontal positional residual the rmse horizontal positional residual (rmsehe) technique was used to determine the total uncertainties in the integrated data set used. the rmsehe (paredes-hernández, 2013) is given by equation 11 as follows: rmse rmse rmsehe easting northing= ( ) + ( )2 2 , (11) where rmse easting n e ei obs i pred i = ( )∑ 1 2 and rmse northing n n ni obs i pred i = ( )∑ 1 2 . the rmseeasting and rmsenorthing is the rmse in easting and northing coordinates. ei obs, ei pred , ni obs and ni pred signify the observed and predicted easting and northing coordinates. (iii) standard deviation the standard deviation (sd) was computed to assess the precision of the transformed coordinates produced by the model. it is calculated in equation 12 as . (12) here, e represent the deviations between the observed and predicted data and is the average of the deviations. (iv) horizontal positional residual the horizontal positional residual (he) was used to ascertain the horizontal accuracy associated by integrating the transformed horizontal coordinates for each position. the he could be represented by equation 13 as he e e n no p o pi i i i = ( ) + ( )  2 2 , (13) where (eo, no) are the observed coordinates and (ep, np) are the transformed coordinates given by the model. application transformation parameters determined and rbfnn model developed the application of the similarity transformation model of bursa-wolf requires the determination of seven unknown coordinate transformation parameters. this study deduced these parameters from a sequence of 19 common points in wgs84 and war office 1926 reference frames. it was however observed that the number of available common points will generate more equations than the seven unknown transformation parameters needed. this will as a result, create an over-determined system of linear equations that require a more pragmatic technique to find solutions to them. to overcome this problem, the total least squares adjustment technique described in sect. 3.2 was applied to the bursa-wolf model for the calculation of the transformation parameters and their standard deviations. it must be known that the training and testing data for each subset (table 2) was used to determine the transformation parameters and checking model performance. these determined parameters involved three translation vectors (tx, ty, tz), three rotational parameters (rx, ry, rz), and one scale factor (sf) as shown in table 3. for the purpose of this study and to provide adequate objective comparison of the bursa-wolf model to the rbfnn, the transformation parameters were determined for each subset. table 3 presents the transformation parameters, with the addition of their standard deviations for each subset. the proposed rbfnn model developed for transforming coordinates from wgs84 to war office 1926 reference frame comprise of three layers: input layer, hidden layer and output layer. in the rbfnn model development process, the same k-fold split data (tables 1 and 2) set used for the bursawolf was employed. the supervised training technique was adopted in the rbfnn model formulation. the data was first normalised into the range of [-1, 1] using equation 9. this was necessary in order to bring the entire data onto a common scale interval thereby achieving equal variations among the datasets. in this study, the rbfnn structure consists of two inputs, one hidden layer using gaussian function as the non-linear transfer function, and an output layer having linear transfer function. the data for each subset (see tables 1 and 2) was trained and tested by the rbfnn. to do that, several input and output data scenarios were tested with the objective of selecting the one that produced better transformed coordinate values. here, (x, y, z)wgs84 and (x, y, z)war was first applied as the input and output data, respectively. secondly, ϕ, λ,h( )wgs84 and ϕ, λ,h( )war was used as input and output. finally, ϕ, λ( )wgs84 was used as input with ϕ, λ( )war as the output. the coordinates used here are defined in table 1. the input-output scenario was made possible due to the non-parametric capability of the rbfnn. thus, the rbfnn does not require a mathematical function to describe the input-output relationship among the variables. it was observed that the third scenario yielded more satisfactory results. in deciding the optimal rbfnn for each subset (see tables 1 and 2), the mse (equation 7) of all the trained models were examined at each stage of training and testing. here, the mse was serving as the optimality criterion to facilitate in choosing the best rbfnn structure for each trained subset table 3. computed transformation parameters for the bursa-wolf transformation model for on each partition parameter symbol and unit subset 1 subset 2 subset 3 subset 4 tx (m) -117.636±19.796 -163.811±22.733 -182.277±18.359 -181.962±33.147 ty (m) 88.337±28.057 178.050±40.437 57.558±26.115 287.919±57.782 tz (m) 165.156±31.213 187.775±38.282 109.870±30.099 300.8797±56.300 rx (rad) -2.71e-07±3.11e-06 2.75e-07±3.60e-06 4.84e-06±2.88e-06 -2.0282e-06±5.24e-06 ry (rad) 2.59e-05±4.91e-06 2.15e-05±6.02e-06 3.33e-05±4.73e-06 3.5482e-06±8.85e-06 rz (rad) 8.85e-06±4.38e-06 2.29e-05±6.31e-06 4.32e-06±4.08e-06 3.9979e-05±9.02e-06 sf (ppm) -9.523±3.08e-06 2.321±3.54e-06 1.470±2.86e-06 -1.1354±5.16e-06 73coordinate transformation between global and local data based on artificial neural network with k-fold cross-validation in ghana suitable for coordinate transformation from wgs84 to ghana war office 1926. therefore, the trained network that produced the lowest mse from the testing dataset was chosen as the best rbfnn scheme. after several trials, the optimum rbfnn scheme for subsets 1, 2 and 3 (tables 1 and 2) was [2-14-2], while subset 4 had [2-15-2], respectively. the interpretation here is that, two inputs (φ, λ)wgs84, one hidden layer of 14 neurons and two outputs (φ, λ)war, was achieved for subsets 1, 2 and 3 whereas, subset 4 differed only by the number of hidden neurons (15) as its optimal number. the results produced by these optimum rbfnn models were then projected onto the transverse mercator 10 nw to get the projected grid coordinates. this was done by using equations from ayer (2008). for the case of the bursawolf model, the transformed rectangular coordinates was first converted into geodetic coordinates using bowring inverse equation (bowring, 1976) and then projected onto the transverse mercator. these projections were necessary because ghana utilises the 2d projected grid coordinate system (easting, northing) for its surveying and mapping related activities. k-fold cross-validation analysis the k-fold cross-validation technique enables the modeller and the user to have an honest assessment of the models predictive performance. the objective here is to test the potential of rbfnn via kcv for the first time in coordinate transformation under data-insufficient situation in ghana geodetic reference network. in order to evaluate the extent at which the rbfnn and bursawolf model transformed coordinates deviate from the measured, horizontal positional residuals using eq. (13) were estimated and evaluated. the essence is to know the practical application of the bursa-wolf and rbfnn transformed coordinates. the various positional errors determined at the training and testing stages for all the four-folds are shown in figures 3 to 10, respectively. with reference to figures 3, 4, 5 and 6, it is known that the respective transformed training coordinates from the rbfnn model demonstrated low bias in the training data than the bursa-wolf model. this means that the rbfnn transformed outputs do not differ greatly from the measured (target) training data and thus was able to learn the training data in a more effective manner due to its adaptive computational capabilities compared with the parametric method of the bursa-wolf model. this phenomenon was observed in all the four-folds of the training data as shown in figures 3, 4, 5 and 6, respectively. figures 7, 8, 9 and 10 display how well the bursa-wolf and rbfnn models generalised on the testing data for the four-folds. it is acceptable that the strength of a model lies in its ability to give a least prediction error when unseen data is introduced into the model. a visual inspection in figures 7, 8, 9 and 10 clearly exposed the strength of rbfnn generalisation over the bursawolf model in terms of horizontal accuracy. moreover, figures 7, 8, 9 and 10 suggest that encouraging horizontal coordinates were produced by the rbfnn model. furthermore, these assertions are buttressed by the interpretation of the quantitative estimations of the horizontal positional accuracy assessment for the training and testing set as presented in table 4. it is well understood that to evaluate the overall performance of a model via kcv technique, there is the need to find the average across the k-folds ‘optimal’ models error estimates as quantified by the performance metrics (jung and hu, 2015). that is, find the average of each performance metric results presented in table 4 across the entire four-fold. hence, the four-fold cross validation technique average performance based on the statistical analytical tools applied to the bursa-wolf and rbfnn transformed coordinates is given in table 5. as can be seen from the descriptive statistics in table 5, it is noticeable that the rbfnn model performed better than the bursa-wolf model. the rmsehe test results (table 5) indicate that the rbfnn model had the least total horizontal dispersion of 0.797 m, while 1.1820 m was achieved by the bursawolf model. these rmsehe calculated values describe the total uncertainties exhibited in the entire integrated horizontal coordinates. on account of the mean he test results (table 5), it could be stated that when the rbfnn model is applied in the study area, a horizontal positional error in average of 0.704 m would be produced as compared with 1.149 m by the bursa-wolf model. these mean he values denote the achievable average horizontal positional error for bursa-wolf and rbfnn models, respectively. the maximum and minimum hes (table 5) suggest the interval at which the error produced by figure 3. horizontal positional residuals for subset 1 (training) figure 4. horizontal positional residuals for subset 2 (training) figure 5. horizontal positional residuals for subset 3 (training) 74 yao yevenyo ziggah, hu youjian, alfonso rodrigo tierra, prosper basommi laari bursa-wolf and rbfnn varies in the study area. observation from table 5 shows that the rbfnn achieved the best minimum (0.241 m) and maximum (1.351 m) he test results. thus, the rbfnn has the capability of producing satisfactory transformation results in the ghana geodetic reference network (golden triangle) than the bursa-wolf model. from the sd computed values (table 5), it could be seen that similar transformation precision was produced across the four-folds by rbfnn and bursa-wolf model, respectively. consistent with the results in table 5, it is clear that for the study area the rbfnn approach is superior to the bursa-wolf model. the strength of the rbfnn could be attributed to its non-parametric properties whereby it has the capability to adapt to the dataset without a priori knowledge of the underlying functional relationship describing the input and output dataset. from these results, it can be stated that the application of rbfnn via kcv approach for coordinate transformation in ghana geodetic reference network has been duly demonstrated. furthermore, the transformation errors produced in this study could possibly be attributed to the coordinates related to the ghana local geodetic reference system (accra datum) than the global wgs84 data. this is because with all the attendant problems of local geodetic networks as indicated by several authors (see e.g. poku-gyamfi, 2009; varga et al., 2017 and references therein), the coordinates of the accra datum lacks homogeneity. it is therefore logical to state that these distortions inherent in the network might have contributed to the level of accuracy achieved in this study. nonetheless, it is suggested here that the obtained results produced could still be used in ghana for low-order survey works such as data collection for geographic information figure 6. horizontal positional residuals for subset 4 (training) figure 7. horizontal positional residuals for subset 1 (testing data) figure 8. horizontal positional residuals for subset 2 (testing data) figure 9. horizontal positional residuals for subset 3 (testing data) figure 10. horizontal positional residuals for subset 4 (testing data) 75coordinate transformation between global and local data based on artificial neural network with k-fold cross-validation in ghana table 4. statistics of the total horizontal residual assessment for the training and testing data across the 4-folds (unit: metre) pm subset 1 subset 2 training testing training testing bw rbfnn bw rbfnn bw rbfnn bw rbfnn rmsehe 0.686 0.509 1.085 1.479 1.098 0.379 0.963 0.624 mean he 0.637 0.353 1.003 1.275 0.969 0.353 0.940 0.484 min he 0.144 0.099 0.641 0.442 0.199 0.160 0.626 0.123 max he 1.015 0.792 1.777 2.587 2.131 0.587 1.280 1.165 sd 0.265 0.145 0.463 0.839 0.537 0.145 0.209 0.394 pm subset 3 subset 4 training testing training testing bw rbfnn bw rbfnn bw rbfnn bw rbfnn rmsehe 1.090 0.583 1.434 0.514 1.120 0.796 1.246 0.570 mean he 0.979 0.539 1.320 0.469 1.039 0.694 1.335 0.587 min he 0.381 0.161 0.575 0.231 0.429 0.054 0.869 0.168 max he 1.905 0.881 2.030 0.841 2.088 1.691 1.911 0.809 sd 0.497 0.233 0.626 0.236 0.506 0.439 0.457 0.285 table 5. statistics of the overall average horizontal positional errors for bursa-wolf (bw) and rbfnn across the 4-folds (unit: metre) pm training testing bw rbfnn bw rbfnn rmsehe 0.999 0.567 1.182 0.797 mean he 0.906 0.485 1.149 0.704 min he 0.288 0.118 0.678 0.241 max he 1.784 0.988 1.750 1.351 sd 0.451 0.240 0.439 0.439 system geodatabase generation, reconnaissance survey, small scale topographic surveys and for land information system works. this assertion was based on the maximum allowable horizontal positional error tolerance of ± 0.9144 m set by the ghana survey and mapping of lands commission for cadastral applications and plan production in ghana (yakubu and kumi-boateng, 2015). concluding remarks the hold-out cross-validation procedure has been widely adopted to assess the performance of the coordinate transformation methods. however, studies have shown that the kcv offers some advantages over the holdout approach. therefore, the main contribution of this study is to apply and demonstrate the potential and applicability of the rbfnn via kcv technique on the lap sparse dataset in ghana geodetic reference network. the obtained rbfnn results were compared with the bursa-wolf model. the findings revealed significantly that improper split of the sparse dataset for the ghana geodetic reference network into single train-test experimentation could produce misleading results that is totally dependent on how the data was partitioned and the split set selected. these are evident from the four-fold results produced by bursa-wolf and rbfnn where if a particular split fold is chosen for the case of hold-out cross-validation would give a misrepresentation of the models capability. hence, it has been demonstrated in this study that for proper assessment of model predictive performance especially in sparse dataset situation, the kcv offers a better solution. the conclusion made from the overall statistical analyses was that, for ghana geodetic reference network the rbfnn has the potential and strength to account for the uncertainties in the data related to the different data (war office 1926 and wgs84) more effectively than the bursa-wolf model. the result suggests that the rbfnn will be more applicable to low-order accuracy surveys. this is because the rbfnn achieved maximum horizontal positional error was more than the ± 0.9114 m tolerance set by the ghana survey and mapping division of lands commission for cadastral applications. acknowledgement the authors are grateful to the ghana survey and mapping division of the lands commission for given us the land administration project data for this work. our sincere gratitude also goes to the anonymous reviewers for their helpful comments. references annan, r. f., ziggah, y. y., ayer, j., & odutola, c.a. 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(2017b). coordinate transformation using featherstone and vaníček proposed approach a case study of ghana geodetic reference network. geoplanning: journal of geomatics and planning, 4, 1, 19-26. keywords: batch equilibrium test; freundlich isotherm; langmuir isotherm; potentially toxic elements; sorption palabras clave: prueba de equilibrio por lotes; isoterma de freundlich; isoterma de langmuir; elementos potencialmente tóxicos; adsorción how to cite item raimondi, i. m., silvestre-rodrigues, v. g., zanin-lima, j., pelinson-marques, j., artimonte-vaz, l. a., & vieira, e. m. (2021). assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study. earth sciences research journal, 25(1), pending pages. doi: https://doi.org/10.15446/esrj.v25n1.84276 peat is an organic material that has been widely used as an efficient and low-cost adsorbent. as many studies tend to focus on temperate peats, there is a lack of knowledge about the adsorption mechanism of tropical peats. this paper investigates the use of two brazilian peats (cravinhos c and luis antônio la) from the mogi-guaçu river basin for the adsorption of lead (pb), zinc (zn), and cadmium (cd), in order to contribute to the use of local and easy access materials to remediate contaminated sites. the peats adsorbed a high percentage of cations, especially pb cations (100.0-46.3%), with commercial peat c showing higher adsorption than peat la. the removal order was pb2+ > cd2+ ≥ zn2+ for c and pb2+ > zn2+ > cd2+ for la. the batch data for both peats and for all metals were better fit by the langmuir isotherm, with adsorption capacities (qm) for pb, zn, and cd of 37.3134, 29.0674 and 21.2890 mmol kg-1 in peat c and 21.7391, 14.2550 and 3.6460 mmol kg-1 in la, respectively, values comparable to those of other peats and biosorbents. the studied peats are considered efficient, alternative and low-cost adsorptive materials for these metals. the proximity of peatlands to areas with high potential for contamination necessitates the use of local materials to reduce remediation costs. abstract assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study evaluación del uso de turbas tropicales como materiales alternativos locales para la adsorción de pb, zn y cd: un estudio de equilibrio issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.84276 la turba es un material orgánico que se ha utilizado ampliamente como adsorbente eficiente y de bajo costo. dado que muchos estudios tienden a centrarse en las turbas templadas, hay una falta de conocimiento sobre el mecanismo de adsorción de las turbas tropicales. este trabajo investiga el uso de dos turbas brasileñas (cravinhos c y luis antônio la) de la cuenca del río mogi-guaçu para la adsorción de plomo (pb), zinc (zn) y cadmio (cd), con el fin de contribuir al uso de materiales locales y de fácil acceso para remediar sitios contaminados. las turbas adsorbieron un alto porcentaje de cationes, especialmente cationes pb (100,0-46,3%), mostrando la turba comercial c una mayor adsorción que la turba la. la orden de eliminación fue pb2+ > cd2+ ≥ zn2+ para c y pb2+ > zn2+ > cd2+ para la. los datos de equilibrio por lotes para turbas y para todos los metales se ajustaron mejor a la isoterma de langmuir, con capacidades de adsorción (qm) para pb, zn y cd de 37,3134, 29,0674 y 21,2890 mmol kg-1 en turba c y 21,7391, 14,2550 y 3,6460. mmol kg-1 en la, respectivamente, valores comparables a los de otras turbas y biosorbentes. las turbas estudiadas se consideran materiales adsorbentes eficientes, alternativos y de bajo costo para estos metales. la proximidad de las turberas a áreas con alto potencial de contaminación requiere el uso de materiales locales para reducir los costos de remediación. resumen record manuscript received: 20/12/2019 accepted for publication: 28/08/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 29-40 isabela monici raimondi1, valéria guimarães silvestre rodrigues1*, jacqueline zanin lima1, jéssica pelinsom marques1, luiz augusto artimonte vaz2, eny maria vieira3 1department of geotechnical engineering, são carlos school of engineering, university of são paulo 400 trabalhador são carlense ave. são carlos, brazil 13566-590 2cronos consultoria 911 josé vaz de camargo rd. matão, brazil 15993-028 3são carlos institute of chemistry, university of são paulo 400 trabalhador são carlense ave. são carlos, brazil 13566-590 * corresponding author: valguima@usp.br g eo ch em is tr y https://doi.org/10.15446/esrj.v25n1.84276 https://doi.org/10.15446/esrj.v25n1.84276 30 isabela monici raimondi, valéria guimarães silvestre rodrigues, jacqueline zanin lima, jéssica pelinsom marques, luiz augusto artimonte vaz, eny maria vieira introduction peat is formed by the decomposition of several plant materials in the waterlogged environments of marshes, bogs and swamps (spedding, 1988; zulkifley et al., 2016) and is present on all continents, from sea level to high altitudes (joosten and clarke, 2002). peatlands cover an estimated area of 400 million hectares. however, peats located in tropical and subtropical regions correspond to a small portion of the total: 35.80 million hectares (andriesse, 1988; ips, 2008). the typical characteristics of a tropical climate (high precipitation, high evapotranspiration and high average annual temperatures) directly affect the type of peat formed in this region. climate has both direct (for example, via hydrology) and indirect (via the vegetation species; trees, containing woody material, are typically present in tropical regions, whereas sedges and sphagnum moss are found in temperate regions) effects on peatlands. the temperature also influences the rate of oxidation of the material, and since tropical soils frequently have larger sesquioxide contents than soils in temperate regions, their iron and aluminum contents may be higher. the tropical climate region is defined as that located between the tropics of cancer and capricorn. however, the typical characteristics of tropical peat do not necessarily coincide with the geographical limits, especially at high altitudes. thus, tropical peatlands can be found on the entire african continent and in most of south america (notably brazil and uruguay), southern florida, north carolina and southeast asia (which contains the largest extension, approximately 57%, of tropical peat deposits) (andriesse, 1988). peat is a material with a high organic matter content, consisting of mainly lignin, cellulose and humic substances (which include humic acids, fulvic acids and humin). the presence of functional groups (such as carboxylic acids, phenolic hydroxides and alcohols) associated with these organic compounds imparts a polar character; therefore, peat is a material with high potential for adsorbing metals and polar organic molecules (couillard, 1994). due to its characteristics, peat has been studied over the past decades as a low-cost, efficient and environmentally friendly adsorbent for potentially toxic elements (ptes) (babel and kurniawan, 2003; bolan et al., 2014; zehra et al., 2014; leiviskä et al., 2018; lima et al., 2018). contamination by ptes contributes to environmental degradation. several anthropogenic sources, such as fertilizers, mines, landfills and industrial sites, may result in potential hotspots of ptes pollution (orgiazzi et al., 2016; mohamed et al., 2017). the presence of ptes as contaminants can create significant environmental risks over time, as these metals are nondegradable and can become mobile and available to biological receptors. these elements may bioaccumulate and biomagnify to critical levels in the food chain, generating grave effects on human health as well as on flora and fauna (siegel, 2002; yong and mulligan, 2003). pb, zn and cd are ptes frequently present in areas contaminated by industrial and mining activities (kasemodel et al., 2016; marques et al., 2019). pb is a toxic element that is nonessential and has the potential to accumulate in humans, affecting fundamental biochemical processes; it has the ability to imitate ca and consequently inhibit ca-protein interactions (atsdr, 2007). cd is another environmental pollutant. it is toxic to plants and organisms at smaller concentrations than zn, pb and copper (cu) are (adriano, 1986). studies have provided evidence that cd is a carcinogenic element and that acute exposure can lead to death in humans and animals (atsdr, 2012; santos et al., 2016). zn is an essential nutrient for plants and animals and it is important for the biological functions of living tissues. however, when present in high concentrations in the environment, zn becomes toxic to organisms (kabata-pendias, 2011). chronic ingestion of zn is also dangerous and may have negative effects on the gastrointestinal and hematological systems (who, 2001; atsdr 2005). the treatment of pte-contaminated water and soil includes several processes, including adsorption. to date, knowledge is still limited, and a complete understanding of the mechanisms involved in adsorption has not been achieved. although several studies have explored adsorption by peat from temperate and boreal climate zones (e.g., coupal and lalancette, 1976; gosset et al., 1986; chen et al., 1990; brown et al., 2000, qin et al., 2006; kalmykova et al., 2008; koivula et al., 2009; bartczak et al., 2018; egene et al., 2018; hammi et al., 2019), there are little works using tropical peat deposits (petroni, 2004; balasubramanian et al., 2009; abat et al., 2012; lima, 2017; santos et al., 2018; marques et al., 2020). nonetheless, interest in pte adsorption by tropical peat is increasing due to its low cost and high availability. in brazil alone, there are approximately 24,000 km2 of peatlands (wer, 2013), therefore, there are many areas still available as a natural resource. according to franchi et al. (2006), the use of peat in countries with tropical climate is recent, increasing the interest in these areas. the mogi-guaçu river basin, located in the northeast of the state of são paulo (brazil), and drainage area of approximately 15,000 km², is an example of an area with extensive peatland that can be easily exploited (perh, 2005). the mogi-guaçu peat deposits are found in the extensive flood plains associated with the meandering river system (franchi et al., 2006). these peatlands originate from predominantly the remnants of grasses or, alternatively, woody plants (franchi, 2004). brazil, like many other developing countries, struggles with pte contaminations (reis et al., 2019; trevizani et al., 2019; fernandes et al., 2020). as regards mogi-guaçu river basin, some studies have shown the presence of pb, cd, and zn (among others pte) in high concentrations in water, sediments, and living organisms; wherein, their concentrations can vary according to the period of floods (tomazelli and martinelli, 1999; tomazelli, 2003; silva, 2005; la serra, 2015). in addition, the mogi-guaçu river basin committee (sigrh, 2020) establishes that the basin is classified as industrial, with emphasis on agribusiness (including sugarcane and orange), pulp and paper industry, slaughterhouses, among others. the agriculture is considered an important source of pte for soil and water bodies, which can occur through fertilizers, pesticides, and soil amendments (silva et al., 2016; shanshan and yanqing, 2020); however, the effluents of the pulp and paper industry stand out due to the presence of pb, cd, zn and cu in their composition (frizzo et al., 1996). perh (2005) confirmed the high demand of water for industrial activity in the mogi-guaçu river system. these pointed factors could increase the risk of contamination of the region by pte in a short or long-term. some studies have shown good results regarding the adsorption of metals to brazilian peats from various river basins in the states of sergipe, bahia, são paulo, rio de janeiro and santa catarina (santos, 1998; lamim et al., 2001; franchi, 2004; petroni, 2004; crescêncio junior, 2008; batista et al., 2009; cerqueira et al., 2012; oliveira et al., 2015; carvalho, 2015; marques et al., 2020). however, studies are necessary to evaluate the adsorption capacity of peat from the mogi-guaçu river basin, since this is a strategically located area (industrial area with high potential for pb, zn and cd contamination) containing peatlands with easy access, which would facilitate the use of and reduce the acquisition costs associated with this reactive material. because of this lack of knowledge, the aim of this study was to comparatively assess the pb, zn and cd adsorption capacities of two brazilian peats collected in two cities from the mogi-guaçu river basin: cravinhos (already commercialized in the country) and luis antônio. this study also intends to verify whether the adsorption results are compatible with those of other peats (including tropical and temperate ones), as well as those of other biosorbents. studies such as this might encourage the use of locally available adsorptive materials and might also provide a promising direction for the development of low-cost and ecofriendly technologies for soil and water remediation, especially in tropical contaminated areas. materials and methods field sampling peat samples for this study were sourced from the mogi-guaçu river basin (são paulo state, brazil) in two different cities: cravinhos (peat c) and luis antônio (peat la) (figure 1). peat c is exploited for commercial use since its modified form (as liquid humic acid) is widely used as a fertilizer for local crops. the peat c is extracted and placed in piles in the field (figure 2) to subsequently proceed to air drying (in covered area) and to modification (marques et al., 2020). peat c collection area by the company covers a few square kilometers in the municipality. for this study, the company provided samples without any treatment or chemical modification, as research intention. peat la was collected in november of 2017 at kilometer 40 of highway sp-255 in luis antônio city (21° 35’53.2 “s and 47° 57’03.3” w). 31assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study figure 1. locations of the peat collection sites in the mogi-guaçu river basin: cravinhos city (peat c) and luis antônio city (peat la). adapted from shimada et al. (1981). peat sample preparation peat c is a commercialized peat, and after receiving it from the company, the samples were sieved through 2 mm mesh, dried at 50°c for 48 h and then homogenized. peat la was first dried at room temperature, homogenized in a porcelain mortar, sieved through 2 mm mesh, and subsequently dried at 50°c for 48 h. both peats were stored at room temperature prior to analysis. some physicochemical properties of both peats were published by raimondi et al. (2019) and are summarized in table 1. table 1 also shows additional characteristics to those presented by raimondi, such as specific surface area and pore volume, which are important properties for adsorption studies. according to raimondi et al. (2019), both peats have similar characteristics, and the results indicate that these materials are favorable for pte ion retention. most of the characteristics of the two peats were similar, but peat c had a higher organic matter content, and specific surface area, besides a lower cation exchange capacity (cec). table 1. physicochemical properties of peat c and peat la parameter peat c peat la reference organic matter 520.43 ± 2.08 g kg-1 510.06 ± 8.26 g kg-1 raimondi et al. (2019) ash content 47.9 ± 0.2% 48.9 ± 0.8% cation exchange capacity (cec) 91.0 cmolc kg-1 116.0 cmolc kg-1 ph h2o 5.1 ± 0.1 5.9 ± 0.1 specific surface area 3.45 m2 g-1 2.04 m2 g-1 this study pore volume 0.025 cm3 g-1 0.014 cm3 g-1 metal adsorption experiments batch adsorption experiments were carried out to evaluate the adsorption capacities of the peats. these procedures were based on and adapted from the figure 2. peatland areas cravinhos (peat c). methodology used by roy et al. (1992), soares and casagrande (2000) and astm d4646 (2016). one gram of the tropical peat samples was added to a falcon tube containing 50 ml of a single metal solution. this peat/solution ratio (1/50) was used because it is the most favorable condition according to the preliminary step presented in lima (2017) and raimondi et al. (2019), studies that characterized both tropical peats from the mogi-guaçu river basin. in the batch adsorption assays, eight initial concentrations of the three ions (pb2+, zn2+, and cd2+), which ranged from 0.10 to 2.36 mmol l-1, were used. the most concentrated solutions of each metal (1.05, 2.36 and 1.45 mmol l-1 for pb2+, zn2+ and cd2+, respectively) were prepared by dissolving the corresponding chloride salt (pbcl2, zncl2 and cdcl2·h2o) in deionized water. the remaining concentrations were obtained by diluting the concentration solutions in deionized water. the ph of the initial suspensions was not adjusted, and the ph of the solutions ranged from 4.0 to 6.0 depending on the peat and the pte analyzed. the suspensions were agitated on a horizontal shaking table (novatécnica, nt 155) at an agitation rate of approximately 120 rpm. after being agitated for 24 h (contact time determined according to astm d4646 (2016), the samples were centrifuged and filtered through unifil filter paper (weight of 80 g m-2 and particle retention of 4-12 μm). the pb2+, cd2+ and zn2+ concentrations in the filtrates were determined by an atomic absorption spectrometer (perkinelmer pinaacle 900f). the instrument standard curves had three points at different concentrations (dilution of the respective perkinelmer standards iso 9001 purity certification in deionized water), and correlation coefficients higher than 0.995 were utilized. the solutions were diluted according to the instrument’s quantification limits (0.448 mg l-1 pb2+, 0.006 mg l-1 zn2+, and 0.018 mg l-1 cd2+), and the maximum value indicated by the manufacturer for a linear fit zone of the equipment (10.00 mg l-1 pb2+, 0.75 mg l-1 zn2+, and 1.00 mg l-1 cd2). batch equilibrium tests were performed in triplicate at room temperature (approximately 25°c). the efficiency of metal removal by both peats was calculated as a percentage, as shown in equation 1: metal removal c c x c e %( )= −( )0 0 100 (1) the ions adsorbed by a mass of material at equilibrium (qe) in mmol kg-1 was calculated by equation 2: q c c v me e = −( )0 � (2) in equations 1 and 2, c0 is the initial concentration of ions in solution (mmol l-1), ce is the ion concentration after adsorption equilibrium (mmol l-1), v is the solution volume (l), and m is the mass of peat (kg). the langmuir (types i and ii) and freundlich (linearized forms) isotherm models were chosen to investigate the mechanism of target 32 isabela monici raimondi, valéria guimarães silvestre rodrigues, jacqueline zanin lima, jéssica pelinsom marques, luiz augusto artimonte vaz, eny maria vieira metal adsorption. according to febrianto et al. (2009), both langmuir and freundlich isotherms are widely used to describe ptes adsorption by biosorbents (which include peat). the freundlich isotherm has been used to interpret adsorption onto heterogeneous surfaces or surface sites with different affinities (bulgariu et al. 2011, freitas et al. 2018). the freundlich isotherm is expressed in equation 3: q k ce f e n . (3) the linearized form of the freundlich isotherm is expressed in equation 4: log log logq k n ce f e= + (4) the langmuir isotherm is based on monolayer adsorption onto homogeneous surfaces and can be used to estimate the maximum adsorption capacity of the surface (langmuir, 1997). this isotherm is expressed in equation 5: q q k c k ce m l e l e = +1 (5) the linearized forms of the langmuir isotherm are expressed in equation 6 for langmuir i and equation 7 for langmuir ii: c q k q c q e e l m e m = + 1 (6) 1 1 1 q k q c qe l m e m = + (7) in equations 3 to 7, kf is the freundlich equilibrium constant (l kg-1), n is the freundlich exponential coefficient (dimensionless), qm is the adsorption capacity or maximum amount of ions or molecules to be absorbed (mmol kg-1), and kl is the langmuir adsorption constant related to the binding energy (l mmol-1). the separation factor (rl) is a dimensionless constant calculated to predict the feasibility of the langmuir isotherm (foo and hameed, 2010) and is defined in equation 8: r k cl l = + 1 1 0 (8) the adsorption process can be described as a function of rl as follows: rl>1: unfavorable; rl=1: linear; 0<rl<1: favorable; rl=0: irreversible additionally, the change in free energy (∆g) was evaluated from the langmuir constant. thermodynamic parameters are often used in environmental engineering to determine whether the process will occur spontaneously (ho, 2006). the ∆g calculation is presented in equation 9. ∆ =−g rt klln (9) in equation 9, r is the universal gas constant (8.314 j mol-1 k-1), and t is the absolute temperature (k). the temperature in the experiments was 25°c. results and discussion in the batch adsorption experiments, the adsorption percentage of both peats was calculated as a function of the initial concentration, and the results are shown in figure 3. the ion adsorption on peat occurred in single-solution systems. the removal percentage of the three ions was dependent on the initial concentration, and higher removal occurred at lower concentrations, with maximum removal reached at the lowest concentration (for example, 100% pb, 81.4% zn and 90.8% cd for peat c). peat c had a higher efficiency than peat la, and the difference in metal ion removal can be seen in figure 3. both peats adsorbed more pb than cd and zn, showing the preference of peat for this cation. in peat c (figure 3a), the pb adsorption was above 70% at all concentrations (from 100.0 to 71.9%), while in la (figure 3b), the pb removal ranged from 98.6 to 46.3%. for peat c, the removal order was pb2+ > cd2+ ≥ zn2+. the removal efficiency was very similar for zn and cd (81.4 to 27.8% zn and 90.8 to 34.4% cd) but was slightly higher for cd. for peat la, the removal efficiency followed an order of pb2+ > zn2+ > cd2+, with zn removal ranging from 39.6 to 12.5% and cd removal from 52.6 to 9.8%. figure 3b shows that cd removal achieved higher percentage (52.6%) over zn (39.6%) at the lowest initial concentration, however, its removal was generally lower. therefore, following the order of pb2+ > zn2+ > cd2+. according to qin et al. (2006), adsorption affinity is frequently correlated with electronegativity and softness. the preference of peat for pb2+ can be explained, for example, by the theory of hard and soft acids and bases (hsab) (pearson, 1968). according to this theory, “c class cations”, such as pb2+, form stronger complexes with fulvic and humic acids in peat than do “b class ions” (zn2+ and cd2+) (kalmykova et al., 2008). when considering electronegativity, the more electronegative the element is, the more strongly it is attached to the surface (mckay and porter, 1997). pb has the maximum adsorption capacity and the greatest electronegativity (2.33) (table 2). nonetheless, the order of cd (1.69) and zn (1.65) held true for only peat c. however, it is important that the electronegativity of cd and zn are similar, as are the removal efficiency results for both metals in the peats. regarding the order zn2+ > cd2+ for peat la, liu et al. (2009) reported that the greater ionic radius of cd may interfere with its access to the adsorbent pores, even though it has a higher affinity for binding sites. in fact, the larger pore volume of peat c (0.025 cm3 g-1) could allow greater access to cd ions than the la peat pore volume (0.014 cm3 g-1) could allow. table 2. properties of the metal ions metal ions electronegativity1 pearson’s hsab2 pb2+ 2.33 between hard and soft zn2+ 1.65 soft cd2+ 1.69 soft 1 alleoni et al. (2009) 2 hsab = hard and soft acids and bases (pearson, 1968) the superior efficiency and higher maximum adsorption capacity of peat c in the removal of all three tested ions from solution could be explained by its specific surface area (3.45 m2 g-1 for c and 2.04 m2 g-1 for la – table 1) as well as its higher organic matter content (520.4 g kg-1 for peat c and 510.06 g kg-1 for peat la – table 1). these characteristics may favor the performance of specific adsorption mechanisms. the higher efficiency of c may also be associated with the functional groups present on the surface of this peat. the ph variation during the batch equilibrium test was also verified (figure 4). in figure 4, the “solution ph” was considered the ph of the metallic ion solution (ph at each initial concentration), “initial ph” was the ph after contact between the peat and metallic ion solution, and “final ph” was the ph after 24 h of the experiment. batch equilibrium tests were performed without buffering the ph because the resulting initial ph of all samples (between 4 and figure 3 percentage of metal ion removal versus the initial concentration of pb2+, cd2+ and zn2+ for two natural brazilian peats: (a) peat c, (b) peat la 33assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study 6 – figure 4) was within the range considered ideal for the adsorption of metal ions onto peat and soils, between approximately 4 and 8 (couillard, 1994; petroni et al., 2000; qin et al., 2006; lim and lee, 2015). peats generally have a ph of approximately 4.0 due to the humic acids present, and their structures degrade at ph values higher than 9.0 and below 3.0, decreasing their chelation capacity. in addition, at low ph values, the high concentration of h+ in solution competes with metallic ions in ion exchange reactions, and at high ph values, the formation of metal oxide precipitates is the main active retention mechanism (couillard, 1994). as shown in figure 4, in general, the “initial ph” tended to decrease in relation to the “solution ph”. the “initial ph” resulted from an equilibrium between the “solution ph” and the ph of the material. this decrease compared to the “solution ph” is mainly due to the acidic properties of the carboxylic and phenolic functional groups present in peat (bloom and mcbride, 1979; sharma et al., 2011). the “initial ph” of peat c was lower than the “initial ph” of la (comparing the same metal – figure 4). as an example, the “initial ph” of pb for c ranged from 4.4 to 3.9 as the initial concentration increased, while in la, this variation was from 4.7 to 4.5. this slight difference in the “initial ph” of the peats may have resulted from the ph of each peat, because the ph of phh2o (measured in water) for c was 5.1 and that for la was 5.9 (according to raimondi et al., 2019). the difference in “initial ph” may have contributed to the higher efficiency of peat c. ph is one of the main parameters that influences adsorption processes, and at lower ph values, pb, zn and cd tend to be ionized and, therefore, more soluble and available for adsorption. therefore, the higher efficiency of peat c than of la may be a result of their properties (such as the specific surface area, organic matter content and surface functional groups, as previously mentioned) and the lower ph values under the initial test conditions. for both peats and for the three metals analyzed, the “final ph” of the samples (ph after 24 h of the experiment) was between 4 and 6. koivula et al. (2009) emphasized that the final ph is governed by both the initial ph and the ions present in solution. their research verified that the final ph decreases as the initial concentration increases and suggested that this occurs due to the ion exchange principle. according to the ion exchange reactions, with the increase in the number of adsorbed ions on peat, a greater number of h+ ions are released in solution, decreasing the ph (lima et al., 2018). a decrease in the final ph as a function of the increase in the initial concentration can be observed in figure 4 for both peats, except for cd adsorption by peat la, where the ph remained constant throughout the test as a function of the characteristics of this material, with stable ion exchange. the experimental data were also fitted to isotherm models to understand the adsorption behavior of the pb2+, cd2+ and zn2+ ions onto two tropical peats. adsorption isotherms describe the distribution of metal ions between the phases of the solid peat and the aqueous solution and are important elements for obtaining valuable information about the mechanism and nature of the adsorption process (sposito, 2004; balan et al., 2008). the isotherm models fitted to the experimental data were freundlich and langmuir (linearized form i and ii), and table 3 provides the parameters of the adsorption models for the three metal ions and their correlation coefficients (r2). the equilibrium adsorption isotherm and fitting of the models to the experimental data are shown in figures 6, 7 and 8 for pb2+, zn2+, and cd2+, respectively. the highest values of the correlation coefficient r2 (above 0.9000) are highlighted in gray in table 3. as seen from the r2 data, the langmuir i model fits the data on pb and cd for peat c (values of 0.9940 and 0.9366, respectively) and pb, zn and cd for peat la (0.9369, 0.9772 and 0.9264, respectively), and langmuir ii was a good fit of the data on cd for peat c (0.9607) and zn for peat la (0.9938). the freundlich isotherm model also fit the data for most metallic ions, except for zn on peat c (0.6378) and cd on peat la (0.6574). the correlation coefficient of the freundlich model was 0.9891 for pb and 0.9387 for cd with peat c and 0.9357 for pb and 0.9674 for zn with peat la. the obtained r2 results (from table 3) suggest that the langmuir isotherm model is a better fit for pb, cd and zn adsorption on both tropical peats, which is in agreement with other studies of peats, including those from tropical and temperate areas (bencheikh-lehocine, 1989; mckay and porter, 1997; franchi, 2004; qin et al., 2006; batista et al., 2009; sõukand et al., 2010; carvalho et al., 2017). brown et al. (2000) found that langmuir isotherms were frequently effective in studies modeling the binding of a metal to peat. for the case of peat c and zn, the good fit was not obtained for any of the models (freundlich or langmuir) (r2 < 0.7000). as can be seen in figures 3a and 7, the adsorbed zn increased rapidly with increasing zn equilibrium concentrations in the isotherm initial stage and then increased slowly, tending to reach saturation condition. according to the system of isotherm classification (giles et al., 1974), this behavior suggests that data experimental fit in the “l”type curve (i.e., langmuir). this finding is similar to the other metals studied. however, the batch equilibrium results for peat c and zn revealed two points of adsorption capacity associated with the highest initial concentrations that appear to be above this saturation plateau. this may indicate a change in the adsorbate retention mode evidenced by the completion of the first adsorption monolayer and the start of a second adsorption layer. this may have been the cause of the lack of adjustment to the adsorption models. the langmuir isotherm model is assumed to represent monolayer surface coverage without interaction between the adsorbed ions, and the existence of a figure 4. the ph variation variation during the batch adsorption tests of pb, cd and zn ions onto two tropical peats (c – cravinhos; la – luis antonio) 34 isabela monici raimondi, valéria guimarães silvestre rodrigues, jacqueline zanin lima, jéssica pelinsom marques, luiz augusto artimonte vaz, eny maria vieira value for qm (maximum adsorption) means the presence of a finite supply of sorption sites (langmuir, 1997). table 3 shows that peat c had higher maximum adsorption capacities (37.3134, 29.0697 and 21.2890 mmol kg-1 for pb, zn and cd, respectively) than peat la (21.7391, 14.2550 and 3.6460 mmol kg-1 for pb, zn and cd, respectively). these qm values and all parameters obtained from the langmuir model were considered from the best fit of the langmuir model: langmuir ii for the adsorption of cd onto peat c and zn onto peat la and langmuir i for the others. these considered values are highlighted with an asterisk in table 3. table 4 provides the monolayer adsorption capacities of some materials tested as low-cost adsorbents for metallic ions; the peats tested in the present work presented lower values than those in other studies but were still comparable. from table 4, it is also verified that even among brazilian tropical peats, the parameter qm varies widely, presenting values ranging from 752.86 mmol kg-1 (lattuada et al., 2014) to 150.36 mmol kg-1 (franchi, 2004) for pb, for example. following the langmuir model, the adsorption capacity increased in the order pb2+ > zn2+ ≥ cd2+ for peat c and pb2+ > zn2+ > cd2+ for peat la, showing the higher affinity of the peat functional groups for pb2+ and confirming the preference of peat for this cation, as previously determined. these orders are due to the characteristics of the analyzed organic materials. a similar order was obtained for the removal efficiency of peat la (figure 3b). for peat c, the qm values were very similar for zn and cd but larger for zn. the removal efficiency was also similar for cd and zn but was slightly higher for cd. for this sequence, the ion electronegativity may explain the selectivity for the metals (table 2). another parameter obtained from the langmuir isotherm model is kl, which is related to the binding energies of the adsorption process. the high kl parameters for pb and cd suggest stronger interactions with surface groups than those for zn binding. the kl parameters for pb, cd and zn with peat c were 89.3333, 63.7354 and 2.9913 l mmol-1, respectively, while for peat la, the values were 20.9091, 11.9781 and 3.0077 l mmol-1, respectively. these values were considered from the best fit of the langmuir model (langmuir i or langmuir ii) and are highlighted with an asterisk in table 3. other parameters, such as the separation parameter (rl) and the change in free energy (∆g), were calculated from the best fit of the langmuir model. the separation parameter (rl) values are shown in figure 5; all values were in the range 0-1, suggesting that pb, zn and cd adsorption in both tropical peats are favorable. in general, for both peats, the rl values indicated more favorable sorption at high initial metal ion concentrations than at a lower initial concentration. according to poots et al. (1978) and ho et al. (2002), values closer to zero indicate more favorable adsorption. on the basis of the rl parameter, peat c was very favorable for pb and cd adsorption but less favorable for zn adsorption. in contrast, peat la was more favorable for pb and zn adsorption than for cd adsorption, following the order of preference obtained from the langmuir model (pb2+> zn2+ > cd2+). overall, peat c is more favorable than peat la for the adsorption of metallic ions, except for zn. the negative values for the change in free energy (∆g) (table 3) ranged between -28.2588 and -19.8527 kj mol−1) and indicate that metal ion adsorption is spontaneous and feasible at 25°c, while the absolute magnitudes indicate a physisorption process, in accordance with the results determined from the ph data that suggested an ion exchange process. adsorption with ∆g between -20 and 0 kj mol−1 implies a physical process, while values from -80 to -400 kj mol−1 correspond to chemisorption (fernandes et al., 2010). in comparison, ho (2006) obtained a value of -11.1 kj mol−1 for the adsorption of pb, while balan et al. (2008) obtained a value of -26.054 kj mol−1 for cd adsorption. both experiments were carried out in sphagnum moss peat with temperatures close to 30°c, and the parameters were obtained from a langmuir i linearization. these two values are close to those obtained for the peats in this study and show that the process of metallic ion adsorption in peat is generally achievable and spontaneous and is characterized by physical processes. figure 5. plot of rl versus the initial concentrations (co) of pb2+, cd2+ and zn2+ for the two brazilian peats table 3. isotherm parameters for pb2+, cd2+ and zn2+ adsorption by two natural tropical peats (peat from cravinhos – peat c – and peat from luis antonio – peat la) isotherm parameter peat c peat la pb2+ zn2+ cd2+ pb2+ zn2+ cd2+ freundlich r2 0.9891 0.6378 0.9387 0.9357 0.9674 0.6574 kf (l kg-1) 45.9934 20.9218 29.9778 24.3276 10.1438 3.3527 n 0.1750 0.2575 0.2278 0.2577 0.3993 0.1586 langmuir i r2 0.9940 0.6837 0.9366 0.9369 0.9772 0.9264 qm (mmol kg-1) 37.3134* 29.0697* 25.9067 21.7391* 15.2905 3.6460* kl (l mmol-1) 89.3333* 2.9913* 19.3000 20.9091* 2.3956 11.9781* langmuir ii r2 0.8040 0.6701 0.9607 0.8276 0.9938 0.5037 qm (mmol kg-1) 28.8150 20.3250 21.2890* 15.0140 14.2550* 2.9610 kl (l mmol-1) 1428.7505 20.5000 63.7354* 348.8515 3.0077* 54.9888 ∆g (kj mol-1) 28.2588 -19.8388 -27.4219 -24.6591 -19.8527 -23.2781 r2 – correlation coefficients; kf – freundlich equilibrium constant (l kg-1); n – freundlich exponential coefficient (dimensionless); qm – adsorption capacity (mmol kg-1); kl – langmuir adsorption constant (l mmol-1); ∆g – change in free energy (kj mol-1); * represents values considered from the best fit of the langmuir model; highlighted in gray means r2 above 0.9000 35assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study the freundlich isotherm of sorption is also one of the most commonly used mathematical descriptions due to its wide adjustment range for experimental concentrations (mobasherpour et al. 2012). the isotherm theory assumes that the adsorption energy decreases logarithmically as the surface becomes covered by the adsorbate. the freundlich isotherm is better adjusted to heterogeneous surfaces (soares and casagrande, 2000). as occurred for the langmuir model, most metallic ions were well adjusted to the freundlich model, with the exception of cd on peat la (r2 of 0.6574) and zn on peat c (0.6378) (table 3). for the adjustment of cd on peat c and pb on peat la, the correlation coefficients of the freundlich isotherm (0.9387 and 0.9357, respectively) were practically the same as those of langmuir i (0.9366 for cd and 0.9369 for pb). lattuada et al. (2014) and koivula et al. (2009) also found freundlich to be the most suitable isotherm for pb adsorption to tropical peat (from brazil) and temperate peat (from finland); lattuada et al. (2014) and kalmykova et al. (2008) found the same for zn, and qin et al. (2006), balan et al. (2008) and kalmykova et al. (2008) found the same for cd. the freundlich constant n for all ions was less than 1 and ranged from 0.1586 to 0.3993 (table 3), indicating favorable adsorption, strong interaction and surface heterogeneity. the closer the value n is to zero, the more heterogeneous the surface is (soares and casagrande, 2000; bulgariu et al., 2011). zhang et al. (2010) described n values of humic acid from sediment ranging from 0.735-0.902 and suggested unevenly distributed sorption sites on this material. kf is considered a freundlich parameter related to the adsorption capacity (oliveira et al. 2015), and the determined values indicate the great ability of peat c and peat la to retain pb (kf of 45.9934 and 24.3276 l kg1, respectively). for zn, the parameters were 20.9218 l kg-1 for peat c and 10.1438 l kg-1 for peat la, while for cd, the values were 29.9778 and 3.3527 l kg-1, respectively. the kf values from this study are in agreement with values obtained for adsorption in soils, such as zn and cd adsorption in 11 soils (spodosols and other types); the kf ranged from 2.1 to 776.3 l kg-1 and 5.4 to 755.09 l kg-1, respectively (buchter et al., 1989). in addition to the adsorption efficiency of the material, the cost is also an important parameter and is even considered essential in the selection of any treatment process. as this study classifies tropical peat as a low-cost adsorbent, price analysis was also conducted. in the brazilian market, local peats have been sold on a large scale in 2019 for an average of u$0.05 per kilo (approximately r$0.22 in national currency). on the other hand, in the same market, activated carbon is traded at between u$1.20 and u$2.10 per kilo (approximately r$5.00 – r$9.00), average prices that are 33 times higher than the price estimated for peats. activated carbon is the most common and efficient adsorbent but has disadvantages due to its cost (especially in developing countries) (silva et al., 2020). the financial advantage of peat as an alternative and lowcost adsorptive material confirms the importance of the study of local and efficient materials. in general, both tested tropical peats are efficient adsorptive materials, especially for the removal of pb. the peat deposits are located near industrial regions and are already close to areas with great potential for contamination. both peats are therefore easily accessible materials for the remediation of these contaminated areas and can be used in adsorption technologies. these materials can maintain high efficiencies for contaminant removal but reduce the associated treatment costs. thus, these findings provide direction for the use of local materials in a tropical area for soil and water treatment. according to crini (2006) and sharma et al. (2011), an adsorption process can be an attractive technology if the low-cost adsorbent is easy to acquire and ready for use. conclusion the batch adsorption experiments showed a high percentage of pb, zn and cd removal by both tropical peats, with the removal efficiency being a function of the initial concentration of the cation. the removal order was pb2+ > cd2+ ≥ zn2+ for peat c and pb2+ > zn2+ > cd2+ for peat la, showing the higher affinity of both peats for pb. pb removal by both materials ranged from approximately 46.0 to 100.0%. regarding zn and cd, the efficiency of removal by peat c remained between 90.8 and 27.8%, while that by peat la was between 52.6 and 9.8%. table 4. comparison of adsorption capacities (qm) for pb2+, zn2+ and cd2+ of some adsorbents metal adsorbed adsorbent qm (mmol kg-1) reference pb2+ peat (brazil) 752.86 lattuada et al. (2014) peat (denmark) 730.00 qin et al. (2006) peat (romania), alkaline treated 660.30 bulgariu et al. (2011) peat (estonia) 571.00 sõukand et al. (2010) peat (romania) 562.40 bulgariu et al. (2011) peat (china) 428.00 qin et al. (2006) peat (poland) 397.23 bartczak et al. (2018) peat (brazil) treated with hcl 155.69 franchi (2004) peat (brazil) 150.36 franchi (2004) sugar cane bagasse 419.67 abdelhafez and li (2016) sugar beet pulp 356.00 reddad et al. (2002) orange peel 134.45 abdelhafez and li (2016) sugar cane bagasse 61.78 mahmood-ul-hassan et al. (2015) peat c (brazil) 37.31 this study peat la (brazil) 21.74 this study zn2+ peat (brazil) treated with hcl 247.78 franchi (2004) peat (brazil) 183.54 lattuada et al. (2014) peat 179.00 mckay and porter (1997) peat (brazil) 102.78 franchi (2004) sugar beet pulp 272.00 reddad et al. (2002) commercial activated carbon 160.60 lattuada et al. (2014) rice husk 35.64 lattuada et al. (2014) peat c (brazil) 29.07 this study peat la (brazil) 14.25 this study cd2+ peat (denmark) 447.00 qin et al. (2006) peat (estonia) 374.00 sõukand et al. (2010) peat (china) 285.00 qin et al. (2006) peat 188.00 mckay and porter (1997) peat (brazil) treated with hcl 149.76 franchi (2004) peat (romania) 92.27 balan et al. (2008) peat (brazil) 59.36 franchi (2004) sugar beet pulp 217.00 reddad et al. (2002) compost of organic waste 14.28 simantiraki and gidarakos (2015) sugar cane bagasse 6.98 mahmood-ul-hassan et al. (2015) peat c (brazil) 21.29 this study peat la (brazil) 3.65 this study 36 isabela monici raimondi, valéria guimarães silvestre rodrigues, jacqueline zanin lima, jéssica pelinsom marques, luiz augusto artimonte vaz, eny maria vieira figure 6. adsorption isotherms of pb ions onto peat c (cravinhos) and peat la (luis antonio), using freundlich, langmuir i and langmuir ii model fitting linearization figure 7. adsorption isotherms of zn ions onto peat c (cravinhos) and peat la (luis antonio) using freundlich, langmuir i and langmuir i model fitting linearization peat c was slightly more efficient than peat la, which may have been due to the characteristics of peat c that were more favorable to adsorption than those of peat la, such as the higher surface area, higher organic matter content, and lower ph of peat c, which resulted in a lower ph in the initial test condition (“initial ph”). peat c is already commercialized in brazil, which would further facilitate its use as an adsorptive material. during the batch assays, the increase in solution ph after adsorption as the initial concentration increased suggested an ion exchange mechanism. this finding is in accordance with the parameter ∆g, which implied a physisorption process. according to the parameters ∆g and rl, adsorption occurred in favorable, spontaneous and feasible conditions. among the isotherm models used to describe the metal adsorption, langmuir had the best correlation, which included the best fit for both the langmuir i and langmuir ii models. these best fits assume a monolayer sorption capacity (qm), decreasing in the order of 37.3134, 29.0697 and 21.2890 mmol kg-1 for peat c and 21.7391, 14.2550 and 3.6460 mmol kg-1 for peat la for pb, zn, and cd, respectively. such adsorption capacities are comparable to the values for other peats (including temperate and tropical 37assessment of the use of tropical peats as local alternative materials for the adsorption of pb, zn and cd: an equilibrium study peats), as well as for other biosorbents, proving the potential use of the tested peats. in some cases, the freundlich model also achieved a high adjustment, comparable to that of the langmuir model. the removal of pb and cd by peat c (r2 of 0.9891 and 0.9387, respectively) and pb and zn on peat la (r2 of 0.9357 and 0.9674, respectively) indicated favorable adsorption, strong interaction and surface heterogeneity. regarding costs, the price of peat in the brazilian market was found to be 33 times lower than that of activated carbon (an efficient adsorbent with regular use). in general, both tropical peats are efficient, alternative and low-cost adsorption materials, especially with respect to the removal of pb. sources of both peats are located near large industrial areas in brazil, and the presented findings contribute to the use of local materials to remediate contaminated sites, thus lowering costs and maintaining efficiency. acknowledgments to the national council for scientific and technological development (cnpq) for the phd scholarship (process number 154134/2016-3) and for productivity in the research fellowship (process number 305096/2015-0) and to the são paulo research foundation (fapesp) for the master’s scholarships (numbers 2015/02529-4 and 2016/17894-2) and financial support (process number 2014/07180-7). references abat, m., mclaughlin, m. j., jirby, j. k. & stacey, s. p. 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(2020). optimal allocation algorithm of geological and ecological high-resolution remote sensing monitoring sampling points. earth sciences research journal, 24(1), 105-110. doi: https://doi.org/10.15446/esrj. v24n1.85531 the purpose of this study is to solve the problem of an unsatisfactory image representation of monitoring sampling points in high-resolution remote sensing due to the complexity of geological ecology. firstly, three algorithms used in remote sensing technology were introduced, that is, extraction algorithm of monitoring sampling point (selective search algorithm), discriminant algorithm (support vector machine), and bing algorithm. then, the bing algorithm was improved. finally, the superiority of the improved bing algorithm was verified through the experimental data set. the results showed that the selective search algorithm could generate more candidate windows in remote sensing images and had better adaptability. the improved algorithm had a higher quality of candidate windows extracted from remote sensing images. although the ibing algorithm could significantly improve the extraction speed of remote sensing, the detection time of each image became larger. such testing times were still acceptable. therefore, in this research, the allocation algorithm of geological and ecological high-resolution remote sensing monitoring sampling points was optimized, which had a good guiding significance for the application of remote sensing technology in environmental and geological research. abstract optimal allocation algorithm of geological and ecological high-resolution remote sensing monitoring sampling points algoritmo de asignación óptima de puntos de muestreo de monitoreo de detección remota geológica y ecológica de alta resolución issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85531 el propósito de este estudio es resolver el problema de la representación de imagen insatisfactoria de los puntos de muestreo de monitoreo en la teledetección de alta resolución debido a la complejidad de la ecología geológica. en primer lugar, se introdujeron tres algoritmos utilizados en la tecnología de detección remota, a saber, el algoritmo de extracción del punto de muestreo de monitoreo (algoritmo de búsqueda selectiva), el algoritmo discriminante (máquina de vectores de soporte) y el algoritmo bing. luego de esto el algoritmo bing fue mejorado. finalmente, la superioridad del algoritmo bing mejorado se verificó con base en un conjunto de datos experimentales. los resultados mostraron que el algoritmo de búsqueda selectiva podía generar un mayor número de ventanas candidatas en la imagen de teledetección y tenía una mejor adaptabilidad. el algoritmo mejorado tenía mayor calidad de ventanas candidatas extraídas de imágenes de teledetección. aunque el algoritmo ibing podría mejorar en gran medida la velocidad de extracción de la teledetección, el tiempo de detección de cada imagen se hizo mayor. estos tiempos de prueba aún eran aceptables. por lo tanto, en esta investigación se optimizó el algoritmo de asignación de puntos de muestreo de monitoreo de detección remota geológica y ecológica de alta resolución, que tenía una alta importancia orientadora para la aplicación de la tecnología de detección remota en la investigación geológica y ecológica. resumen record manuscript received: 06/01/2019 accepted for publication: 26/07/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 105-110 taifu bi 1key laboratory of regional environment and eco-restoration, shenyang university, shenyang liaoning 110044, china * corresponding author: co m pu te r s ci en ce ap pl ie d t o ge ol og y 106 taifu bi introduction the existing remote sensing technology has many advantages, such as wide coverage, fast updating speed and large amount of information. the above advantages enable remote sensing technology to acquire large range of data in a relatively short time, so remote sensing technology is widely used. at present, it is most widely used in environmental and geological fields, which can monitor soil erosion, environmental protection, vegetation cover and land use (gorji et al., 217). remote sensing technology can be divided into broad sense and narrow sense. in the broad sense, it refers to the use of remote non-contact means to capture the information of monitoring research objects. in a narrow sense, the specific meaning refers to the technology that uses existing space technologies (satellites or aircraft) as a carrier platform and carries relevant sensors to perceive or observe some characteristics of the research target and analyze its characteristics (gao et al., 2017). in remote sensing image technology, target detection sampling point is one of the difficulties in remote sensing monitoring for geological ecological monitoring system. with the development of science and technology, the operating time and spatial resolution of remote sensing technology have been continuously improved, and the information of research objects collected by remote sensing images has become richer (lengyel et al., 2017). at present, the application of target detection technology based on remote sensing image is more and more extensive. high-resolution optical remote sensing images can obtain more shape and texture information of ground objects, have stronger discrimination ability to ground objects, and can more intuitively display the surface conditions. however, due to the complexity of geology and ecology, it is difficult to extract images and express effects when monitoring sampling points. therefore, these problems of target detection based on high resolution optical remote sensing image need to be solved. most of the target detection technologies of remote sensing images are infrared remote sensing, synthetic aperture radar or medium-low resolution remote sensing images. these traditional detection technologies can’t effectively adapt to the complexity of high-resolution optical remote sensing images and the diversity of targets (silva et al., 2017). and as the spatial resolution of remote sensing gets higher and higher, the image gets closer and closer to the natural image. however, the target detection technology of natural image (such as artificial intelligence algorithm) has developed rapidly in recent years. the combination of artificial intelligence algorithm and image target detection will further improve remote sensing technology. therefore, it is of practical significance to explore more effective target detection algorithms based on high-resolution optical remote sensing images. to sum up, in this research, the configuration algorithm of high-resolution remote sensing detection technology was optimized by studying highresolution remote sensing target detection technology. it is hoped that the effect of high-resolution remote sensing target detection can be improved. methods monitoring sampling point extraction algorithm selective search algorithm firstly divides the pixels of the same object with similar features (such as color, texture and scale) in the image into super pixels, and obtains the initial small area set. then, the similarity definition and merge strategy definition of pixels are carried out to merge super pixels and form a hierarchical structure, which can generate candidate windows. in order to obtain better robustness, selective search algorithm adopts multiple color spaces and multiple similarity calculation strategies for better robustness (sürme et al., 2017). bing algorithm is simple and efficient in the extraction of candidate windows, which is very suitable for the extraction of candidate windows. bing algorithm is a classifier with two stages. first, the first stage is to scan the images waiting to be detected according to different scales of sliding. then the subgraph of the candidate window is unified to 8*8, and finally the linear scoring model needs to be used for scoring. the linear scoring model can be expressed as equations (1), (2), and (3). w r 64 (1) s w gl l= ( ), (2) l i x y= ( ), , (3) among them, l represents the window position; i represents the window scale; (x, y) indicates the coordinates of the upper left corner; sl represents filter score; and gl represents the window feature vector. in the bing algorithm, the w linear model is trained by svm in the first stage, the normal pattern gradient features of the real target window are taken as positive samples, and the paradigm gradient feature of the randomly sampled background window is taken as a negative sample. the second stage is to solve the problem that different scales contain different possibilities of targets. it needs to train a scoring calibrator for each scale window. the candidate window score extracted in the first stage is corrected, as shown in equation (4). l i l iv s t= • + (4) among them, vi and sl represent the calibration coefficients of i learning at different scales. in order to speed up the process of image extraction and scoring, the bing algorithm uses a binary approximation method. the two-valued basis vector is used to approximate the linear model, as shown in equation (5). w j jj nw ≈ β α =∑ 1 (5) among them, nw represents the number of base vectors; j represents a base vector; and j indicates the coefficient. j can be represented by another binary vector and its complement form, as shown in equation (6). α α −αj j j= + + (6) among them, α ∈j + { }0 1 64, . equation (6) can perform the fast bitwise and bit count operation of the score of the binarized feature b, which can be expressed by equation (7). w b b bj jj nw , ,( ) ( )( )+ =∑≈ β α −2 1 (7) bing algorithm expresses ng eigenvalue through equation (8): g bl b k k lk ng ≈ −2 1 ,=∑ (8) through the above changes, the score of whether the final image window contains the target can be approximated as equation 9: s w g b bl l j b k k n j k lj n gw = ( ) ( )( )= + = ∑∑, , ,≈ β α −−2 2 21 (9) among them, 2 2b k j k lb b− α −+( )( ), , can perform cpu atomic operations, which improves the speed of bing algorithm. discriminant algorithm support vector machine (svm) is adopted in the general discriminant algorithms of remote sensing image technology. the basic idea of support vector machine is shown in figure 1. the algorithm derivation process of svr is as follows (ellis & coppins, 2017). first, the training set sample pairs were given:t x y i li i= ( ) = …{ }, , , , ,1 2 , x ri d , y ri  . in the high-dimensional eigenspace, the linear regression function is established. 107optimal allocation algorithm of geological and ecological high-resolution remote sensing monitoring sampling points f x a a x x b a a k x x b i i i l i j i i i j i l (16) the principle diagram of support vector regression machine is shown in figure 2. k(x1,x) k(x2,x) k(xn,x) ∑ the hidden layer the input layer x=[x1,x2,…,xn] y bias x2 x1 xn figure 2. schematic graph of svr improved bing algorithm because the bing algorithm was easy to miss the fuzzy monitoring samples, in this research, the candidate region extraction algorithm was improved. firstly, different weak feature channels were extracted from each training sample, and a scoring model was developed for each weak feature channel. then the score of a scoring model was trained by each weak feature channel as the score of candidate window. because the integration part adopted a simple linear weighting method, the accuracy of each model determined the weight of the model. thus, the corresponding scoring model for each window was obtained, as shown in equation (17) and (18). s w gkl k kl= ( ), (17) l i x y= ( ), , (18) among them, wk is the parameter of scoring model k; skl represents the score of window l by the scoring model; and gkl indicates the feature value k at window 1. therefore, the final window score in the first stage is expressed by equation (19). g sk kln = ∑  (19) among them, k is the weight of the eigenvalue of the scoring model. after correction according to the scoring model of the second stage, the window score is shown in equation (20). o a s v tl k kl i in = ∗ +∑ (20) in bing algorithm, ng feature graph is calculated for test sample points, and then binary approximation window ng feature and scoring model f x w x b( ) = ( ) + (10) among them, w is weight vector; (x) represents the nonlinear mapping function; and representative bias. the c-linear insensitive loss function is defined as equation (11). l f x y y f x y f x y f x ( )( ) = ( ) ( ) ( )     , , , , ε − ≤ ε − −ε − >ε 0 (11) among them, f(x) is the predicted value, y is the corresponding real value, and  is the insensitive coefficient to control the fitting accuracy. secondly, relaxation variables, that is, i and j, are introduced. according to srm criterion, the minimization objective function is constructed, as shown in equation 12. min *1 2 2w c i i i l l + +( ) = ∑   (12) s t y w x b y w x b i l i i i i i i i . . , , , , , * − φ − ≤ε ξ − φ ≤ε ξ ξ ≥ ξ ≥ ( ) + + ( ) + + = …   1 2 0 0     (13) among them, c > 0 is the penalty factor, and represents the punishment intensity for samples with an error greater than 1 during training. lagrange equation is introduced into equation (13) to obtain dual optimization problem, as shown in equation (14) and (15): min * * * * , 1 2 111 a a k a a a a a a yi i j j i i i i i i l i l i j − − − − ε −( ) ( ) ( ) + ( ) === ∑∑ ll ∑           (14) s t a a a a c i i i l i i . . , , *− ∈ ( ) =         = ∑ 0 0 1 (15) among them, k x x x xi j i j,( ) = ( ) ( )  is a kernel function. the commonly used kernel functions are polynomial kernel function, radial basis kernel function and sigmoid kernel function. and the radial basis kernel function was selected in this research. fourthly, ai and ai * can be solved from equation (15), and then w* and b * can be obtained, the regression function is shown in equation (16). f(x) support vector optimal classification line * * * * * * * * * * * · · ·· ·· · · · figure 1. the basic idea of svr 108 taifu bi parameters are used. such a process is to only use cpu atomic operation. ng feature graph calculates the gradient amplitude of each pixel by collecting the edge intensity information in the original graph. however, because the logic of the ng feature map is relatively simple, the influence of noise on it is particularly prominent. based on the shortcomings of bing algorithm mentioned above, sobel feature graph with better edge information capture effect was adopted in this research to replace ng feature graph with weak edge information. moreover, the local binary pattern (lbp) feature graph and difference of gaussian (dog) feature graph were introduced into the sobel diagram. the meaning expressions of sobel feature graphs are shown in equations (21)~(23): g ax = + + +           ⊗    1 0 1 2 0 2 1 0 1 (21) g ay = + + + +           ⊗ 1 2 1 0 0 0 1 2 1    (22) g g gx y= +2 2 (23) among them, a represents the original graph matrix; gx is the gradient of the horizontal direction of the image; gy is the gradient for the vertical direction of the image; and g is the gradient magnitude of the feature graph matrix. as can be concluded from equation (21), sobel feature diagram adopts the grayscale diagram of the original image of 3*3 sobel operator convolution. the approximate gradients of the horizontal and vertical directions are calculated respectively, and then the sum of the squares of the approximate gradients of the horizontal and vertical directions is carried out to obtain the corresponding gradient amplitude of each pixel. lbp feature map is a texture feature proposed by yavad and congalton (2017) to describe images. with the development of technology, it is widely used in human classification and pedestrian detection. the calculation process of lbp code is shown in figure 3. dog feature map is obtained by subtracting two fuzzy images of different degrees from the original image. the two-dimensional gaussian kernel function is defined as follows: g x y x y σ πσ − σ1 1 2 21 2 2 2 1 2, exp( ) = +      (24) g x y g x y f x y1 1 , , ,( ) = ( )∗ ( ) (25) g x y g x y f x y2 2 , , ,( ) = ( )∗ ( ) (26) the dog feature map can be obtained by subtracting the two images, as shown in equation (27). g x y g x y g x y f x y g x y f x y1 2 1 2 , , , , , ,( ) ( ) = ( )∗ ( ) ( )∗ ( )− −σ σ = ( )∗ ( )g g f x yσ σ− 1 2 , = ∗ ( )dog f x y, (27) that is, dog can be expressed by equation (28): dog g g e e x y x y ≈ − π σ − σσ σ − σ − σ 1 2 2 2 1 2 2 2 2 21 2 1 1 1 2 2 2 =       +( ) +( )/ / (28) among them,  1 and 2 represent gaussian kernel parameters. when dog images are applied in different scenarios, the proportions of two gaussian kernel parameters are also different. for example, when dog is used for image enhancement,  1 2 1 4: := . in this study, the purpose of dog was image enhancement, so 1 0 5= . , and 2 2 0= . were adopted. results and discussion analysis of the results of the selective search algorithm in this research, the applicability of selective search algorithm in remote sensing image to target extraction of monitoring sampling points and the superiority of improved bing algorithm were mainly verified. the data sample set was divided into training set and test set, and the ratio of training set to test set was 2:8, this experiment was tested in fast mode, and only two different color spaces and two similarity functions were used. in remote sensing images, the size of target collection points was determined by the spatial resolution of images. therefore, in this research, unreasonable candidate windows were filtered when selective search algorithm results were obtained, as shown in figure 4. according to figure 4, with the increase of pixel size of remote sensing image, the number of candidate windows generated by selective search algorithm was also increasing. in addition, when there was no constraint, the average number of candidate windows generated by selective search algorithm under different pixel sizes was greater than the average number of candi date windows generated by selective search algorithm after filtering unreasonable windows. moreover, when there was no constraint, the maximum number of candidate windows generated by selective search algorithm under different pixel sizes was larger than that generated by selective search algorithm after filtering unreasonable windows. studies showed that selective search algorithm could generate about 2000 candidate windows on average in natural images (fan et al., 2017). in this experiment, the adopted remote sensing images could generate 3000 candidate windows on average. by comparison, selective search algorithm could generate more candidate windows in remote sensing image. table 1 shows the running time of selective search algorithm in figure 3. calculation process of lbp code 109optimal allocation algorithm of geological and ecological high-resolution remote sensing monitoring sampling points remote sensing images of different sizes. as can be observed from table 1, with the increase of pixel of remote sensing image, the running time of the algorithm had been increasing. when the pixel size was 1500*1500, it took 30s to get a candidate window of image in remote sensing image, which indicated that this algorithm did not occupy an advantage in remote sensing pixel size. figure 4. the average number of candidates extracted by selective algorithm for remote sensing images of different scales table 1. the average running time of selective algorithm for candidate extraction of remote sensing images in different scales remote sensing image scale / pixel 500*500 800*800 1000*1000 1500*1500 average processing time / s 2.7 7.6 14.3 29.8 comparison and analysis of the effects of bing algorithm and ibing algorithm figure 5 is the candidate effect diagram of extraction on high-resolution remote sensing images by bing algorithm and ibing algorithm. as can be concluded from figure 5, when the number of candidate windows was the same, the dr/mabo value of ibing algorithm was 97.37%, while the dr/ mabo value of bing algorithm was 95.13%. therefore, the extraction effect of ibing algorithm was better than that of bing algorithm. similarly, the mabo value of ibing algorithm was 65.36%, while the dr/mabo value of bing algorithm was 63.21%, indicating that the quality of candidate boxes extracted from remote sensing images by the improved algorithm was higher. figure 6 shows the extraction effect diagram when the number of candidate areas is 1000~10000. it can be observed that as the number of candidate regions increased, both the dr and mabo indicators tended to rise and then stabilize. when the number of candidate areas was less than 5000, the index was rising, and when the number was greater than 5000, it did not change. this indicated that the number of targets in large-format remote sensing images was relatively large, and the range of possible positions of targets increased significantly, so more candidate windows were needed to achieve a certain detection rate. comparison and analysis of average detection time between bing algorithm and ibing algorithm figure 7 shows the average detection time of bing algorithm and ibng algorithm on different data sets. as can be observed from the figure, under the bing algorithm, the detection time increased with the increase of size, but the maximum time did not exceed 1s. however, the increase of ibing algorithm was obvious, which increased from 0.18s to 0.64s. this showed that although ibing algorithm could greatly improve the extraction speed of remote sensing, the detection time of each image was increased, and such detection time was acceptable. figure 7. average detection time of bing algorithm and ibng algorithm on different data sets figure 5. candidate renderings extracted from high-resolution remote sensing images by bing algorithm and ibing algorithm figure 6. extraction effect in 1000~10000 candidate areas 110 taifu bi conclusion in this research, the algorithm was optimized for high-resolution remote sensing detection sampling points under geological ecology. the three algorithms involved in traditional remote sensing technology are selective search algorithm, svm algorithm and bing algorithm respectively. in this paper, the bing algorithm was improved to obtain the ibing algorithm. according to different data sets, the selective search algorithm in remote sensing image was verified for the applicability of target extraction of monitoring sampling points, and the improved bing algorithm was also verified. the results showed that selective search algorithm could generate more candidate windows in remote sensing image and had better adaptability. the improved algorithm had higher quality of candidate boxes extracted from remote sensing images. although the ibing algorithm could greatly improve the extraction speed of remote sensing, the detection time for each image became larger. and such detection time was also acceptable. the algorithm optimization of high-resolution remote sensing monitoring sampling points in geological ecology was carried out, and the superiority of the algorithm was verified through experiments, which promoted the application of high-resolution remote sensing technology in geological ecology. reference ellis, c. j., & coppins, b. j. 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(2017). issues with large area thematic accuracy assessment for mapping cropland extent: a tale of three continents. remote sensing, 10(2), 53-57. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n1.62063 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 83-89 g eo m o r ph o lo g y record manuscript received: 19/01/2019 accepted for publication: 21/01/2022 how to cite item: qian, f., wang, w., zhang, x., zhang, j., & bian, z. (2022). comprehensive landscape characteristics of the arable land red line in donggang, china. earth sciences research journal, 26(1), 83-89 https://doi. org/10.15446/esrj.v26n1.62063 comprehensive landscape characteristics of the arable land red line in donggang, china fengkui qian1*, weiwen wang 2, xuefeng zhang3, jingye zhang4, zhenxing bian1 1. college of land and environment, shenyang agriculture university, shenyang, china, 2 college of geography and environmental science, henan university, kaifeng, china 3 dalian tianshi real estate consultant co., ltd, dalian, china 4 liaoning longfeng land real estate and assets appraisal co., ltd, shenyang, china. *corresponding author: fkqian@syau.edu.cn abstract: in the face of endless idle and even abandoned farmland, it is trying to protect farmland resources and ensure food supply by determining the bottom line (red line) of arable land that cannot be changed in use. the red line has landscape characteristics and functions, but there remain limited understandings of how to judge the dominant landscape characteristics of cultivated land. therefore, this study divides landscape characteristics into aesthetic value and stability according to landscape pattern. a comprehensive evaluation system was developed to explore the dominant landscape features of the cultivated resources based on a multi-factor comprehensive evaluation. on this basis, this study conducts an empirical study in donggang, a coastal city in china. the results show that cultivated land resources with high aesthetic value are distributed near cities, major roads, and important natural resources. the cultivated land resources with greater continuity and compatibility with adjacent plots are relatively stable, mainly paddy fields. the red line of cultivated land should focus on the characteristics of cultivated land stability. at the same time, the aesthetic value of cultivated land resources is also an important factor (related degree is 0.852). according to this evaluation system, the dominant area of arable land protection in donggang city is 88,027.21hm2, which is the preferred area for the red line of arable land. keywords: cultivated land; red line; aesthetic landscape; landscape pattern; donggang análisis amplio de las características de la “línea roja” cultivable en donggang, china resumen: con el fin de resolver el alto índice de vacancia o incluso el abandono de tierras de cultivo, en este trabajo se busca proteger los recursos de tierras cultivables y asegurar el suministro de comida al determinar el margen inferior (línea roja) del terreno apto para sembrar al cual no se le debe cambiar el uso. para efectos de este estudio, se estableción la línea roja en 1800 millones de mu (cada mu corresponde a 1/15 de hectárea). la línea roja tiene funciones y características paisajísticas pero deben estar definidos los criterios de cómo juzgar las características dominantes paisajísticas de las tierras cultivadas. además, en este estudio se dividen las características paisajísticas entre valores estéticos y de estabilidad de acuerdo con los patrones del paisaje. se desarrolló un sistema amplio de evaluación para explorar las características dominantes del paisaje en los terrenos cultivados con base en la evaluación amplia multifactorial. con base en este enunciado, este trabajo consiste en un estudio empírico en la ciudad costera de donggang, china. los resultados empíricos muestran que las tierras cultivadas cerca de las ciudades, carreteras principales y reservas naturales tienen un valor mayor estético, cerca de 85440 hm2, y que corresponden a un 81.80 % de las tierras cultivadas. las tierras adyacentes cultivadas y las tierras altamente compatibles para el cultivo en las planicies tienen una alta estabilidad, y el tipo de paisaje dominante es el arrozal, con una área de unos 91682 hm2, que corresponde al 87.77 %. los análisis de las características del paisaje con las ventajas compuestas de las tierras cultivadas muestra que el área de los recursos cultivados tanto con las ventajas estéticas y de estabilidad es de cerca de 82.653hm2, que corresponde al 79,13%. este es el límite sugerido para establecer la línea roja de protección a la tierra cultivable. palabras clave: terreno cultivado; línea roja; paisaje no estético; patrón de paisaje; donggang https://doi.org/10.15446/esrj.v26n1.62063 https://doi.org/10.15446/esrj.v26n1.62063 https://doi.org/10.15446/esrj.v26n1.62063 mailto:fkqian@syau.edu.cn 84 fengkui qian, weiwen wang, xuefeng zhang, jingye zhang, zhenxing bian introduction arable land is a significant resource for the livelihood of communities (wang et al., 2021). for agriculture-based civilizations, arable land supplied basic human needs (hanioka et al., 2018, yang et al., 2021). during industrialization, arable land was a solid foundation for economic and social development as the carrier of production materials (lageras and fredh, 2020; zhou and lv, 2020). during ecology-centric construction, arable land can provide landscape value beyond basic resources (liang et al., 2020; schirpke et al., 2021). therefore, arable land as a complex system has been involved in an aesthetic landscape function and played an important role in providing social security. the term of arable land “red line” proposed by the chinese government, should be strictly used as the baseline for arable land protection (zhou and lv, 2020; qian et al., 2021). from an ecological landscape perspective, arable land can also contribute to an ornamental landscape (shifaw et al., 2019). arable land is an important factor in the rural landscape and can be used to enhance diversity with mountains and lakes (jiang et al., 2021). furthermore, improvements in arable land patterns can strengthen ecological protection, improve the connectivity of green spaces, and enhance the beautification of the rural landscape (liang et al., 2020). the concept of landscape ecology originated in the 1930s, was defined as the science and art of studying and improving the relationship between spatial patterns and ecological processes at multiple scales and organizational levels (gao and liu, 2021; mansourian, 2021). since landscape ecology was introduced in china regarded as the unity of ecology and geography, domestic scholars have achieved a lot of farmland landscape designs, aesthetics, and ecological planning incorporating chinese characteristics (mirghaed et al., 2020; isik and vessel, 2021). along with increased research into landscape patterns, the ornamental value of the farmland landscape has become a research focus (mirghaed et al., 2020). most foreign scholars have focused on landscape quality and on the aesthetic value of the landscape (schirpke et al., 2021; yazici and asur, 2021). traditional research on landscape quality has involved investigation and analysis of the visual characteristics of a target area or the visual landscape of a geographic area (gao et al, 2022; isik and vessel, 2021). keywords for this research have included visual landscape quality, landscape visual quality, scenic beauty, and so on, with research content focusing on the evaluation of visual aesthetic quality (krupowicz et al., 2020; isik and vessel, 2021; yazici and asur, 2021). compared with foreign research, environmental impact assessments of the visual landscape in china are mainly focused on aspects including landscape design for urban planning and road aesthetics, and research methods have prioritized theoretical analysis over quantitative or spatial evaluation (jiang et al., 2020a; gao and liu, 2021; qian et al., 2021). the visual landscape is an important index for assessing aesthetic value, which will be quantified in this paper. figure 1. research theoretical framework in china, arable land protection has been improved from quantitative protection to comprehensive protection of quality and landscape (yang et al., 2020; wang et al., 2021). the arable land red line represents the cordon of arable land protection, has ensured both stable patterns of food production and great aesthetic values for environmental protection (sakellariou et al., 2021). therefore, by using an evaluation index system, this study tentatively divided the landscape characteristics of the arable land red line into two categories: the aesthetic landscape characteristics and landscape patterns static characteristics (figure 1). the main objectives of this research are to (1) evaluate the aesthetic landscape characteristics of arable land; (2) quantify the landscape pattern characteristics of arable land. and (3) establish a framework to explore comprehensive features of the arable land using an empirical model and providing a basis for the determination of the arable land red line. materials and methods study region descriptions donggang city is located between 123°22′30″ and 124°22′30″ e and 39°45′ and 40°15′ n near the liaodong peninsula and the mountains of eastern liaoning in china (figure 2). its geographical conditions include the yellow sea in the south and the yalu river in the east. the terrain of the study area drops in elevation from north to south. the countryside is hilly in the north, low hilly areas are found in the central region, and coastal plains with flat topography and fertile soil ideal for planting high-quality rice are distributed in the south. donggang city is influenced by a humid monsoon climate with annual mean precipitation of 899 mm. the climate is suitable for agricultural production. donggang city is a major rice-producing area within liaoning province. according to the overall plan of land utilization of donggang city (2006–2020), arable land and primary arable land shall be less than 88,500 hm2 76,370 hm2 in 2020. the arable land resources are rich, which can not only ensure food security but also prompt a focus on ecological protection of the aesthetically pleasing scenic line in the construction and development of urban areas. data sources and methods the main data sources included: (1) land survey data including land use type (e.g. ecological land, traffic routes, the urban boundary); (2) digital evaluate map (dem) of donggang city, which provided slope attribute data, and (3) farmland classification data, from which the arable land data were extracted. this study analyzed the asymmetry of comprehensive landscape features, which can reveal the frequency distribution rule of each value (xiang and li, 1981), as listed in equation 1. 3 1 ][ )2)(1( ∑ = -= n i i s xx nn ncso (1) where cso is the deviation coefficient, n is the number of samples, xi is the value of sample i, x is the median of the sample, and s is the standard deviation of the sample. this study analyzed the influence of aesthetic landscape characteristics and landscape patterns characteristics on comprehensive characteristics using the pearson correlation coefficient (zhang et al., 2005), as listed in equation 2. y i n i x i s yy s xx n r )()(1 1 -= σ = (2) where r is the pearson correlation coefficient, n is the number of samples, xi and yi are the values of x and y variables of sample i respectively, `x and`y are the mean values of x and y respectively, and sx and sy are the standard deviations of x and y respectively. evaluation processes the evaluation processes included five steps: (1) construction of the evaluation unit; (2) index selection for the aesthetic landscape characteristics; (3) index selection for landscape patterns characteristics; (4) calculation of indices score; and (5) building the evaluation model. d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); 85comprehensive landscape characteristics of the arable land red line in donggang, china construction of the evaluation unit the evaluation unit, which is the most basic evaluation object, has internal attribute uniformity. evaluation units have the same soil properties (e.g. organic content and surface soil texture), and the same land-use type and topography. in this study, the evaluation unit was formed by overlaying soil maps, topographic maps, and land use maps with the same spatial scale. according to this principle, all arable land plots were divided into 30,656 evaluation units using the second national land survey data of donggang city in 2012. aesthetic landscape characteristics of arable land landscape aesthetics, closely related to the landscape pattern, not only contains social and cultural values but also reflects ecological values. it should be paid attention to when carrying out resource protection and planning to give play to the aesthetic value of landscape utilization(kerebel et al., 2019; liu and schroth, 2019). landscape aesthetics can be studied quantitatively, and the commonly used methods are psychophysical (schirpke et al., 2019). the main idea is to understand the relationship between landscape and aesthetics as a stimulus-reflective relationship (tribot et al., 2018; ha and yang, 2019). therefore, aesthetic landscape characteristics were further divided into the number of views and ornamental value. the number of views is very important to the quality of life for current residents (a crucial factor in attracting future growth and development to the valley) (ha and yang, 2019). this factor was evaluated considering various viewpoints to provide a scientific basis for the aesthetic protection of the farmland landscape. this study used the access gallery and city boundary as principal viewing positions and for calculating land plot numbers. that is the number of points that can be seen from arable land by human activity sites including urban boundaries and roads, regardless of any angle. larger numbers indicated greater visual landscape features and a significant landscape aesthetic function. scores for the number of views were calculated using the cut-off value method. the ornamental value of arable land is mainly based on the minimum distance from the arable land patches to regions of important natural resources. reservoirs, seashores, and nature reserves are crucial natural resource regions and serve as important ecological barriers to land use. it is also important to protect the ecological function of arable land resources. this study calculated ornamental value scores using the cut-off value method. landscape patterns characteristics of arable land landscape pattern generally refers to its spatial pattern, that is, the spatial arrangement and combination of landscape elements of different sizes and shapes, including the type, number, and spatial distribution, and configuration of landscape components (jiang et al., 2018, jiang et al., 2019, jiang et al., 2020b). therefore, the quantitative indicators of landscape pattern characteristics in this study include compatibility with adjacent plots, degree of continuity, fractional dimension (fd), and shannon diversity index (shdi). compatibility with adjacent plots is an important index for assessing the stability of arable land resources. greater compatibility means a lower risk of development, greater stability, and more sustainable utilization of arable land resources. compatible plots included arable and ecological land (such as garden plots, woodlands, grassland, water areas, and beaches). compatibility value was based on a ratio of compatible length to the total length of every arable land plot. this study calculated the score of compatibility with adjacent plots using the cut-off value method. figure 2. location and land use status of the study area 86 fengkui qian, weiwen wang, xuefeng zhang, jingye zhang, zhenxing bian the degree of continuity is related to the scale of land use and aid in characterize agricultural landscape patterns. thus, the continuity of arable land should be a conservation priority. in this study, the degree of continuity is taken to be the covered area of intersecting arable land resources within a 10-m buffer area. the degree of continuity was calculated using the cut-off value method. fd is a landscape pattern index measuring the shape of landscape patches, and the fd values of each arable land patch were calculated according to equation 3. fd values theoretically range from 1.0 to 2.0. values close to 1.0 indicate stronger self-similarity and geometric rules, as well as greater effects of human disturbance (frohn, 1997). fd scores were calculated using the cutoff value method. csln fd += = )(ln)4/(l 2 k k (3) where fd is the fractional dimension, l is the perimeter, s is the area, k is the slope of the regression system, and c is the offset. the shdi is based on information theory and reveals landscape heterogeneity. it is especially sensitive to the unbalanced distribution of various types of patches. the shdi values of each arable land patch were calculated according to equation 4. the shdi emphasizes the contribution of rare patches. furthermore, shdi is a biochemical indicator for the comparison and analysis of changes in diversity and heterogeneity over different periods. when shdi equals 0, the whole landscape consists only of one plot. a larger shdi value indicates balance and diversity in the distribution of patch types (shannon and weaver, 1998). in this study, scores were calculated using the cut-off value method. )(ln i m i ppshdi σ-= (4) where shdi is the shannon diversity index, m is the number of patch types, and pi is the area of land type i. calculation of indices score this study divided the indices into ranges and calculated their scores using the cut-off value method (table 1). evaluation model construction the assessment model was based on a multi-factor comprehensive evaluation method (saaty, 1988), as summarized in equation 5. ∑ = = n j ijiji fwc 1 (5) where ci is the comprehensive score of evaluation unit i, fij is the score of evaluation unit i for evaluation factor j, and wij is the weight of evaluation unit i for evaluation factor j. the evaluation methods for index weight primarily included the analytical hierarchy process (ahp) and expert scoring method (table 2). table 2. index weight evaluation factors destination layer index weight aesthetic landscape characteristics number of views 0.25 ornamental value 0.25 landscape patterns characteristics compatibility with adjacent plots 0.15 degree of continuity 0.15 fd 0.1 shdi 0.1 results and discussion evaluation analysis of aesthetic landscape characteristics the aesthetic landscape is a significant perspective in landscape protection that can reflect the aesthetic value of arable land resources. in this study, aesthetic landscape characteristics were analyzed using the number of views and ornamental value. this study synthetically analyzed the aesthetic landscape characteristics (figure 3a) and further analyzed features of the number of views (figure 3b) and ornamental value (figure 3c). from figure 3a, aesthetic landscape characteristics scores were mainly distributed within the range of 76-88. the greatest-scoring areas were located around the city center, along both sides of arterial traffic, and adjacent to important natural resources. arable land in those regions had a greater number of views and ornamental value. areas with dynamic characteristic scores greater than 80 covered approximately 22,659 hm2, primarily distributed along both sides of china national highway 201, and near-coastal benched and other water areas located in the jurisdictions of haiyanghong farm, huangtukan farm, donggang city economic zone, and xinglong farm. the arable land in these regions was considered to have the best visual landscape with an aesthetic scenic line. this feature can also be incorporated into the red line to control the boundary of urban development. the visual landscape is beneficial to the groupand series type urban development, as it helps prevent a ‘street city’ condition in which all cities are connected directly without a buffer zone (e.g. farmland or forest). the number of views ranged from 0 to 606. in the north, the area with a view score equal to 100 covered approximately 3,980 hm2 and was distributed primarily around the danda highway. in the south, the area scoring 100, covered approximately 14,216 hm2 and was distributed in peri-urban areas, and along both sides of the roadway. scores in the north were significantly lower than those in the south owing to the more complex terrain in the northeast. in the north, low mountains, and hilly land block the line of sight. the south has an open field of vision, along with the main traffic corridors. the city center is also located in the southeast and relates to these roadways. thus, arable land resources in the south had greater view scores. the area of these high-scoring regions, distributed along the perimeter of the center and on both sides of arterial traffic, covered approximately 9,210 hm2. these regions also exhibited higher-quality natural conditions and location advantages. at the policy level, table 1. evaluation index calculation destination destination layer index score summary aesthetic landscape characteristics number of views 0 20 40 60 80 100 land plot numbers can be viewed in principle position0 [1-3] [4-8] [5-21] [22-53] >54 ornamental value >2000 [1001-2000] [701-1000] [401-700] [201-400] [0-200] distance from patches of arable land to important natural land landscape pattern characteristics compatibility with adjacent plots 0 (0-0.66] (0.66-0.82] (0.82-0.95] (0.95-0.99] >0.99 side proportion of compatible plots degree of continuit [0-0.32] [0.32-0.70] [0.70-1.37] [1.37-2.66] [2.66-5.54] >5.54 contiguous area fd 1 (1-1.06] (1.06-1.08] (1.08-1.11] (1.11-1.14] >1.14 self-similarity shdi —— 1.1194 1.5790 1.7039 1.8013 1.8846 balance degree of land utilization d:/youdao/dict/6.3.69.8341/resultui/frame/javascript:void(0); 87comprehensive landscape characteristics of the arable land red line in donggang, china the recommended arable land red line in this region satisfied the construction needs for permanent capital farmland. in terms of the visual landscape, arable land in this area is indispensable. thus, the aesthetic value and red-line function of arable land resources are consistent. the ornamental value of arable land was mainly based on the distance from patches of arable land to important natural resources. these distances ranged from 0 to 7,474 m, with a mean distance of approximately 247 m. the important natural resources of donggang city are mainly distributed in the north and along the southern coast. the areas with ornamental scores of 100 cover approximately 24,709 hm2. these regions should be classified as ecological conservation areas. arable land in this region, reliant upon the natural terrain, maintains the continuity of the natural landscape and, in conjunction with the adjacent landscape, can provide valuable ecological services. figure 3. aesthetic landscape characteristics(a) and explanatory variables: number of views (b) and ornamental value(c) of arable land in donggang city (source: farmland classification map of donggang city in 2012) evaluation analysis of landscape pattern characteristics the landscape pattern characteristics mainly reflect the diversity and disturbance degree of the landscape, which in turn indicates the stability and degree of development and utilization of the land. in this study, landscape pattern characteristics were analyzed by their compatibility with adjacent plots, degree of continuity, fd, and shdi. a higher score meant greater permanence and stability and prominent landscape characteristics (figure 4a), and further analyzed the features of compatibility with adjacent plots (figure 4b), fd (figure 4c) degree of continuity (figure 4d), and shdi (figure 4e). landscape pattern characteristics scores were mainly distributed between 70 and 82. the area with scores greater than 75 covered approximately 12,931 hm2, mostly distributed in the jurisdictions of wusi farm, xiaomianzi town, beijingzi town, and changshan town. arable land in the high-scoring region exhibited less conflict with adjacent plots, greater compatibility and continuity, a more regular shape, and a more stable ecosystem. the arable land of these regions should be well-maintained to reduce the impacts of human activity and should be incorporated into the permanent prime farmland area. in terms of compatibility with adjacent plots, proportions ranged from 0 to 1. there were approximately 14,186 plots with a value of 1. the arable lands in these areas had greater stability, continuity with the adjacent ecological land, protection of biodiversity, and overall environmental quality. the best-scoring jurisdiction was shifan farm, in which 97.26% of arable lands had a value of 1, followed by the jurisdictions of haiyanghong farm, xiaomianzi town, changshan town, and heigou town. the contiguous total area of arable land which the buffer intersects ranged between 0.06 and 242.94 hm2. in the northwest arid areas, arable land had low contiguity, and the mean contiguous area was approximately 2.22 hm2. in the southeastern coastal areas, the mean contiguous area was approximately 5.74 hm2. this area hosts abundant water resources, and the arable land comprises mostly paddy fields. the most contiguous jurisdiction was the developed zone, where the mean contiguous area was 6.79 hm2, followed by the jurisdictions of wusi farm, dadong office, beijingzi town, and changshan town. arable land with a greater degree of continuity has prominent landscape characteristics and greater stability and makes greater use of scale. fd values ranged from 1 to 1.39. lower scores indicated that arable land had greater self-similarity and stability and more geometric rules. regular patches of arable land also have greater aesthetic value than do irregular patches at large scales. the region with fd scores of 100 was mostly distributed around the city center and along both sides of arterial traffic in the southeast of donggang city and included the jurisdictions of xinglong farm, wusi farm, beijingzi town, and changshan town. shdi values ranged from 1.1194 to 1.8846. the number of patches with scores of 100 was approximately 97.26%. greater shdi values indicate balanced land utilization, a stable ecological system, and a highquality ecological environment. the areas with high shdi values included the jurisdictions of xincheng office, dadong office, the developed zone, huangtukan farm, shifan farm, wusi farm, xinxing office, and xinglong farm. these regions should be priorities in the protection of landscape pattern characteristics. figure 4. landscape pattern characteristics(a) and explanatory variables: compatibility with adjacent plots (b), fd (c), contiguous total area (d), and shdi (d) of arable land in donggang city (source: farmland classification map of donggang in 2012) 88 fengkui qian, weiwen wang, xuefeng zhang, jingye zhang, zhenxing bian evaluation analysis of comprehensive landscape features in terms of the comprehensive landscape features, uniform distribution ranged from 0 to 606, with a standard deviation of 16.835, and asymmetry was -0.073 (equation 1). the correlation with the aesthetic landscape characteristics was 0.852 (equation 2), and the correlation with landscape pattern characteristics was 0.609. thus, the effect of aesthetic landscape characteristics was relatively prominent, indicating that aesthetic features of arable land resources are important in determining the arable land red line. this research divides the dominant zones and non-dominant zones according to the arable land protection objectives issued by the higher-level government. the node is a comprehensive value of 42, which can be used as the best choice for permanent basic farmland. as shown in figure 5, the areas with comprehensive landscape features scores greater than 42 covered approximately 88,027.21 hm2. in the south, this area covered approximately 49,570.56 hm2. in the north, this area covered approximately 38,456.62 hm2. this region was located along the perimeter of the central city and on both sides of arterial traffic and adjacent to important natural resources. these areas, which can be classified as permanent basic farmland not only have a low-relief surface, broad visibility, and greater ornamental value, but also regular shape, greater continuity, greater stability, and greater use of scale. aesthetic landscape characteristics are prominent in the dominant landscape area, of which the value of the number of views ranged from 41 to 606, and the ornamental value ranged from 0 to 638.61. meanwhile, the index of the compatibility with adjacent plots and shdi are advantages in the landscape pattern characteristics, with values ranging from 0.87 to 0 and 1.81 to 1.88, respectively. therefore, in the recommendations for the arable land red line, not only the resource value of arable land should be protected but also the landscape value. conclusion the result showed that the arable land red line is not only a quantitative line for ensuring food security, but also has a landscape value. this study discussed the aesthetic value of arable land resources through assessing the aesthetic landscape characteristics and landscape pattern characteristics. finally, the comprehensive landscape features of arable land resources wereb discussed, and the distribution of dominant landscape features was confirmed. the result confirms that arable land with greater aesthetic landscape characteristics is distributed around the city center, along the sides of arterial traffic, and adjacent to important natural resources. this area covered approximately 81.80% of arable land in donggang city. arable land with stable landscape patterns is distributed on plain terrain, which covered approximately 87.77% of arable land in donggang city. the arable land in this region comprises mostly paddy fields and has greater continuity and compatibility with adjacent plots. the evaluation of dominant landscape features found that areas with better visual features and stable landscape patterns covered approximately 79.13% of arable land in donggang city. from these findings, the paper recommends that the arable land red line should consider the aesthetic value and landscape patterns of arable land. in this study, we evaluated the landscape value of the arable land red line, using limited indices of aesthetic and landscape patterns features in a typical region of china. further study will include a more comprehensive index that considers a larger scale. in addition, index differences will also be considered for different geomorphological regions. figure 5. the distribution map of comprehensive landscape features of arable land resources in donggang city (source: farmland classification map of donggang in 2012) 89comprehensive landscape characteristics of the arable land red line in donggang, china acknowledgments the authors sincerely thank all the students and staff who provided their input into this study. this research was financially supported by the national key r&d program of china (grant no.2021yfd1500200) and the national natural science foundation of china (grant no.42077149). our acknowledgments also extended to the anonymous reviewers for their constructive reviews of the manuscript. references gao, j., gong, j., yang, j. x., li, j. y. & li, s.c., (2022). measuring spatial 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(2020). understanding the dynamics of farmland loss in a rapidly urbanizing region: a problem-driven, diagnostic approach to landscape sustainability. landscape ecology, 35, 2471-2486. https://doi. org/10.1007/s10980-020-01074-w https://doi.org/10.1016/j.landurbplan.2021.104260 keywords: virtual reality; monocular vision; volcanic rock; cycle; eruption; period. palabras clave: realidad virtual; visión monocular; roca volcánica; ciclo; erupción; período. how to cite item yang, j., & wang, x. (2020). eruption cycle of volcanic rocks based on virtual reality technology. earth sciences research journal, 24(3), 277-284. doi: https://doi.org/10.15446/ esrj.v24n3.89468 the eruption cycle of volcanic rocks based on virtual reality technology is studied to reveal the spatial distribution of volcanic rocks and the genetic relationship between different lithologic assemblages, and effectively guide volcanic exploration. taking xujiaweizi fault depression and yingshan depression in songliao basin as the research area, the monocular vision method of virtual reality technology is used to collect data from the study area. then, the threedimensional dynamic scene of the study area was constructed. based on this, combined with the research steps and division basis of the volcanic eruption cycle, the eruption cycle of the volcanic region in the study area is studied. according to the characteristics of volcanic activity, volcanic facies sequence, and rock rhythm combination in xujiaweizi fault depression, the first and three sections of yingcheng formation are divided into three eruption periods according to the contact relationship between sedimentary strata and volcanic rocks in the eruptive intermittent period. y1c1, y1c2, y1c3, y3c1, y3c2, and y3c3 are respectively from bottom to top. yingshan volcanic rocks in yingshan depression are divided into three cycles and six periods. each stage has significant stages and differences, and the eruption intensity is different. the overall performance is gradually enhanced, and the internal cycle is characterized by the first strong and then weak. abstract eruption cycle of volcanic rocks based on virtual reality technology ciclo de erupción de rocas volcánicas con base en tecnología de realidad virtual issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.89468 este estudio analiza el ciclo de erupción de las rocas volcánicas basado en tecnología de realidad virtual con el fin de revelar la distribución espacial de las rocas volcánicas y la relación genética entre diferentes conjuntos litológicos y, además, guiar efectivamente la exploración volcánica. se definió la depresión de falla xujiaweizi y la depresión de yingshan en la cuenca songliao como el área de investigación. el método de visión monocular de la tecnología de realidad virtual se utilizó para recopilar datos del área de estudio, y se construyó la escena dinámica tridimensional del área de estudio. con base a esto, además de los pasos de investigación y la base de división del ciclo de erupción volcánica, se estudia el ciclo de erupción volcánica del área. de acuerdo con las características de la actividad volcánica, la secuencia de facies volcánicas y la combinación de ritmo de roca en la depresión de la falla xujiaweizi, las tres secciones de la formación yingcheng se dividen en tres períodos de erupción de acuerdo con la relación de contacto entre los estratos sedimentarios y las rocas volcánicas en el período eruptivo intermitente. y1c1, y1c2, y1c3, y3c1, y3c2 y y3c3 están, respectivamente, de abajo hacia arriba. las rocas volcánicas de yingshan, en la depresión de yingshan, se dividen en tres ciclos y seis períodos. cada etapa tiene diferencias significativas y la intensidad de la erupción es diferente. el rendimiento general se mejora gradualmente y el ciclo interno se caracteriza por primero fuerte y luego débil. resumen record manuscript received: 01/06/2019 accepted for publication: 11/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 277-284 jiqiang yang1*, xing wang2 1college of creative culture and communication, zhejiang normal university, jinhua 321000, china 2school of physics and physical engineering, qufu normal university, qufu 273165, china *corresponding author: yangjiqiang_zjnu@126.com co m pu te r sc ie nc e ap pl ie d to g eo lo g y https://doi.org/10.15446/esrj.v24n3.89468 https://doi.org/10.15446/esrj.v24n3.89468 https://doi.org/10.15446/esrj.v24n3.89468 mailto:yangjiqiang_zjnu@126.com 278 jiqiang yang, xing wang introduction with the development of oil and gas exploration, volcanic oil and gas reservoirs have become an essential part of oil and gas exploration. volcanic rocks are complex in rock type, uneven in regional distribution, and fast in lateral lithology. volcanic oil and gas reservoir exploration technology is a difficult point in petroleum technology research today (zhu et al., 2017). the volcanic gas reservoirs in the songliao basin are controlled by space-time matching relationship between source rocks, volcanic traps, and fault transport channels, which provides a basis for finding desirable exploration areas for volcanic rocks. stages and differences characterize the volcanic eruption, lithology, and structure are complex and diverse, and physical properties are different. each facies belt has different lithological combinations, forming processes, diagenetic modes and structural structures, and has different pore structure characteristics. volcanic eruption cycle study can reveal the spatial distribution of volcanic rocks and the genetic relationship among diverse lithologic assemblages, which provides useful guidance for volcanic exploration (tian et al., 2017). chen jianwen et al. found that oil and gas accumulation is not only related to the lithology of volcanic rocks, but also related to the general characteristics of volcanic rocks, and natural gas is usually enriched in the central part of volcanic structures. by planke and others, the favorable reservoir is mainly distributed in the favorable lithofacies area of the volcanic crater near fire pass facies belt. in recent years, favorable lithologic and lithofacies zone drilling productivity has been found to be closely related to volcanic type. tang jianren et al. have discovered that volcanic reservoirs are controlled by volcanic structures and lithofacies. different types of volcanic structures have different lithofacies distribution characteristics and hydrocarbon bearing properties. therefore, studying the eruption cycle of volcanic eruption and defining the distribution characteristics and evolution rules of volcanic apparatus have important guiding and practical significance for volcanic oil and gas exploration. volcanic rocks are formed by magma rising from the weakest zone of the strata along the cut through basin basement and the deep fault of the crust. compared with sedimentary rocks, volcanic rock deposits are not controlled by the accommodation of water bodies. the formation characteristics of this “water and fire” symbiosis make volcanic magma form superposed volcanic bodies in multi-stage and intermittent eruptions (skelin et al., 2018). the change of volcanic rocks in material composition and lithology is the basis for dividing cycles and periods. in different volcanic eruption cycles or different periods of the same cycle, the material composition is different, resulting in the regularity of volcanic lithology in longitudinal direction, and the corresponding logging curve has the characteristics of prosodic variation (yang et al., 2018). the seismic reflection characteristics of different lithological characteristics are different in continuity, stratification and amplitude intensity. songliao basin is a deep structural depression with large scale and controlled by tensional and torsional faults. yingshan depression and xujiaweizi fault depression are the main areas of deep natural gas exploration. the volcanic reservoirs are of strong heterogeneity and lateral variation. the separate volcanic reservoirs can form an independent reservoir forming system, which may appear in a plane with overlapping distribution. favorable reservoirs are controlled by facies belts (wendt et al., 2017), the exploration of volcanic gas reservoirs is rather difficult. in order to identify the macroscopic rules of the distribution of volcanic reservoirs in different stages of the study area, in order to guide the exploration of volcanic rocks, this paper divides the eruption characteristics of volcanic rocks in songliao basin into cycles (geurden et al., 2017). due to the less drilling and core data in the study area, the virtual reality technology was used to collect the three-dimensional scene data of songliao basin, and the characteristics of the distribution and evolution of each sub volcano were obtained. based on the study of the step and division of volcanic cycles, the eruption cycle of songliao basin was studied in detail, and the subsequent reservoir prediction and meticulous research were carried out. it has important guiding significance for the following reservoir prediction and fine research work. material and method survey of research areas the study area is located in songliao basin. the deep layer of songliao basin consists of three first-order tectonic units: eastern fault depression zone, central paleo-uplift zone and western fault depression zone. take the xujiaweizi fault depression located in the northern songliao basin eastern fault depression and the main part of yingshan depression zone in the north of the songliao basin as the key research area, the volcanic eruption cycle study is carried out (chen, 2017). xujiaweizi fault depression is close to the central paleo-uplift belt and is a deep and large-scale fault depression. it extends nearly north-south with a length of 90 km. the widest part in the middle is 5 km and its area is 4300 km2. the fault depression is a single fault graben fault depression in the west to the east. it can be divided into many structural units, such as the western fault depression belt, the sag depression structural belt, the eastern slope zone, the funyue low uplift and the low uplift. according to the results of the third resource evaluation, the total deep resources of xujiaweizi fault depression are 6772×108 m3. as of the end of 2007, 105 wells were drilled in the deep part of the xujiaweizi fault depression. a total of 5058.8 km2 of pre-stack time migration processing has been completed in the deep layer, with a total proved natural gas reserve of 2457×108 m3 handed in, with a proven rate of 36.2% (bracquart et al., 2017). yingshan depression covers an area of 400 km2, and its syncline and xujiaweizi fault depression are divided by chaoyang valley anticline belt, which is an asymmetric graben type double fault depression under the control of four station fault and linjiang fault. before the deposition of the doutlouku formation, the structure is controlled by volcanic action, forming two ancient landforms with four concave uplifts, namely, the paleo uplift, the qingshan uplift and the yingshan depression. the north is a low-lying area. before the deposition of the group, the north south differential subsidence further strengthened the southern structure, and the amplitude of the western paleo long structure decreased. up to now, strong tectonic inversion has occurred in the north, slight inversion in the south, and stable subsidence in the middle, resulting in the formation of today’s surrounding high, middle low tectonic form. yingcheng formation is located in the fault depression period of the basin. at present, there are five exploration wells in yingshan depression. there are few logging and drilling data, and the overall exploration and development degree is weak (pavey et al., 2017). research method for eruption cycle of volcanic rocks virtual reality technology is used to reconstruct the volcanic terrain in the study area, and the monocular vision method is applied to collect the data collected in the study area. the image machine is used as the image receiving collector to carry out the data measurement and acquisition in the research area needed for comprehensive three-dimensional modeling, such as image processing, data acquisition and visual capture. according to the three dimensional reconstruction data of dynamic scenes in the study area, combined with the research steps of volcanic eruption cycle and volcanic eruption cycle stages, the lithology, transport characteristics and lithofacies periodic changes of volcanic rocks in the study area were analyzed, and the results of the eruption cycle of volcanic rocks in the study area were obtained. three-dimensional reconstruction of dynamic scene in research area based on virtual reality technology by using monocular vision method, the collected image can be either a single image of a single viewpoint or multiple images of several different viewpoints (lozano-bilbao et al., 2017). this method can recognize the degree of light and shade, texture lines, distance points, outlines and so on. this method is simple in equipment, simple in structure, and can reconstruct the three-dimensional model of the object using only a single or a few images. these matching points are used to describe and obtain the coordinate information points of the three-dimensional object in space, so as to realize the three-dimensional reconstruction of the research area (sun, 2017). (1) design of photometric stereo vision virtual reality although conventional brightness methods can reconstruct a threedimensional model from a single image, there is less data information available from a single image. therefore, the sfs method was improved in woodham, and the photometric stereo vision method (englberger & dörnbrack, 2018) was introduced. the method is based on the brightness equation, as shown in formula (1): 279eruption cycle of volcanic rocks based on virtual reality technology x x x f a x x b y y c z z a x x b y y c − ∆ − − − − − −1 1 1 1 1 1 1 3 2 2 2 3 ( ) + = ( ) + ( ) + ( ) ( ) + ( ) + zz z y y x f a x x b y y c z z a x x − − ∆ − − − − − 2 0 2 1 2 1 2 1 3 2 ( )       ( ) + = ( ) + ( ) + ( ) ( )) + ( ) + ( )      b y y c z z2 2 3 2− − (4) in the formula: x x 1( ) is the differential prime of one-dimensional coordinates of the model; y y 0( ) is the data validation level variable of twodimensional coordinates of the model. after the first display, the collected data will be allocated to a certain area. after the display, the collected data can be calibrated by photometric stereo yyue. the process is shown in formula (5): x s a a s a l y l b b s b l = + + + = + + +    0 1 2 0 1 2 (5) in the formula, a0, a1 and a2 are the three-dimensional coordinate valid values of points in the photometric stereo yyue calibration process, and s is the optimum photometric stereo yyue calibration parameter value. the point coordinates ml(x, y) and f(x, y) can be obtained intuitively, which greatly improves the occurrence of visual disturbance and ensures the threedimensional dynamic rendering of the dynamic scene in the study area (rocholl et al., 2017). the effect of three-dimensional reconstruction of the scene in the study area is shown in figure 2. figure 2. scene 3-d reconstruction effect map of research area d o g g d d o d di i j j i i i j j i j n ij n i 1 2 1 2 −α − − ψ ψ − ′ ′ ∈∈ + ( ) + ( ) + ( )    ( )( ) ∑∑       = 0 (1) in the formula: 1oi( ) denotes the intensity coefficient of the light source reflected from the surface of the object; g j is the normal vector of the kinetic energy of the object; gi is the refraction vector of the light source, ψ′i is the image pixel; d dj i( ) denotes the two-dimensional parametric transformation value, which is the value of the response acquisition data to ensure the integrity of the data in the process of data analysis; j n i ( ) denotes the value range of the data acquisition point. photometric stereo vision uses multiple sources of light to obtain multiple images of the same object. by combining the conjugate equations of different images, the normal vector g j of the kinetic energy of the object and the refraction vector gi of the light source can be solved. finally, the three-dimensional shape of the object can be restored. so, the current methods basically use multiple (4-6) light sources for 3d reconstruction, using matrix representation as shown in formula 2. gh =                             σ σ σ σ σ σ α α α 1 2 1 2 2 2 2 2 2 0 0  (2) in the formula, gh matrix represents the mode of data storage calculation and 1 2 represents the number of convertible and simplified conditions of light source. pixels simplified by transformation can ensure the clarity of the picture in the process of data conversion, and it is more suitable for multi-object acquisition data conversion under different conditions (xiang et al., 2018). the transformed data need to be distinguished before they can be used, as shown in formula (3): x x i i x x i i 1 0 2 0 2 4 0 2 4 0 = ( ) + ( )      = ( ) + ( ) − − β − β − − β − β exp exp exp exp       = ( ) + ( )       = ( ) + x x i i x x i 3 0 4 0 2 4 0 2 4 − − β − β − − β exp exp exp expp − βi( )                                   0 (3) in the formula, exp − βi( ) represents the high-order attribute distortion vectors after data transformation, which can reflect the validity of the transformation process; x0 represents the variable parameters that data can effectively retain in the transformation process; x1, x2, x3 and x4 represent the conversion success rates. through the above process, the design data of photometric stereo vision virtual reality is obtained, and the image of information particle operator distribution is shown in figure 1. (2) three-dimensional reconstruction the principle of 3d reconstruction is mainly to get images from the rgb camera and the far-infrared camera in the same direction, so that the direction of the two cameras can be as parallel as possible. this will help reduce the amount of computation when the camera is corrected, and get the rgb and depth images (zhang et al., 2019) respectively. in order to solve the problem of image center deviation caused by different positions of two cameras, rgb images are first transformed into coordinates to align with the depth image coordinates. then the coordinate positions of space points x and y are calculated. finally, the three-dimensional coordinate point cloud data (x, y, z, r, g, b)is displayed through lod hierarchical model, as shown in formula (4): figure 1. information particle operator distribution 280 jiqiang yang, xing wang study on volcanic cycles study on the step of volcanic cycle according to the 3d reconstruction data obtained from virtual reality technology in the study area, the same volcanic cycle in the volcanic strata in the study area is represented by a set of transitional lithological assemblages, and there are obvious changes in the occurrence of different volcanic cycles. therefore, the interface of the cycle is manifested as an integrated surface from an unconformity to a sedimentary area in the volcanic area. on the top of the interface, volcanic rocks are obviously overlying. the thickness of a volcanic cycle is generally more than 80m, which can be used for lateral correlation of volcanic cycles with seismic profiles. in the cycle, we can use single well phase contrast (zhou et al., 2017) according to the change of lithofacies. the steps of the study of volcanic cycles and phases are: (1) comprehensively using coring and thin slices to identify various data, determine the rock types and the lithofacies types of coring segments in the area, and identify all kinds of geological interfaces (weathering crust, volcanic ash layer and sedimentary interlayer); (2) make full use of oil field drilling, logging and logging data to establish single well volcanic lithologic columnar section, according to rock assemblage. (3) based on the classical volcanic activity facies sequence model, the primary division of volcanic cycles and stages of single well is carried out flexibly; (4) based on the results of the zircon dating and the division and correlation of volcanic cycles and periods, we should set up separate wells and intervals and push them out according to the principle of “proximity” in order to clarify the eruption cycle and eruption period of the whole area. classification basis the division of volcanic eruption periods should be based on the principle of isochronous comparison and hierarchical control, that is, in the same eruption cycle, the lithology, lithofacies, electricity, eruption sedimentary cycle and seismic reflection characteristics can be divided into two parts (bolgar et al., 2018). the recognition of volcanic eruption stage should pay attention to unconformity, weathering crust, volcanic rock composition, color, structure, sedimentary rock interlayer, and so on, and combine seismology, geology and logging. in addition, the periodic changes of volcanic facies and the cyclical changes of volcanic components, such as volcanic channel facies, explosive phases, intrusive facies, overflow facies, volcanic sedimentary facies, periodic variations of basic, neutral and acidic volcanic rocks, and alkaline and subalkaline periodic changes are also the basis for dividing volcanic eruption stages (liu et al., 2017). results the eruption period of volcanic rocks in the xujiaweizi fault depression. lithology and prosody characteristics of volcanic rocks in different stages of volcanic activity, volcanic matter composition, different eruption modes, energy differences, and later crystallization and differentiation make each stage of volcanic rock have different lithological types, showing a certain prosody feature in lithological combination and logging response characteristics. the periodic variation of volcanic lithology and the characteristics of rhythmic combination are the reflection of the intensity of volcanic eruption, so it can be used to divide volcanic eruption times. photographs of volcanic cores at different eruption stages are shown in figure 3. the lithology and logging response characteristics of different types of volcanic rocks are shown in figure 4. the same period of secondary volcanic rocks in xujiaweizi fault depression usually has the following lithologic variations: (1) mainly volcaniclastic. usually the central eruption or central fissure eruption, its eruption energy is strong, the eruption product is mainly volcaniclastic, and has positive rhythmic lithological characteristics. in general, volcanic eruption material is composed of caking, volcanic breccia and tuff at the top. figure 3 (a) is the third stage volcanic rock of the sheng shen 2 well with this lithological feature. the electrical characteristics of this kind of combination, such as xushen 8 well, are shown in figure 4 (a). from the electrical characteristics of xushen 8 well, the block rock has high resistivity, medium and low natural gamma, while the top tuff has low resistivity and high natural gamma. the top and bottom of this type of volcanic lithologic assemblage are usually volcanic sedimentary rocks or tuff, and the electrical characteristics are the decrease of resistivity and the increase of natural gamma, indicating the beginning and end of a volcanic eruption. (2) lava is dominant. the energy of fissure eruptive volcanic rocks is weak, and the eruptive products are mainly volcanic lavas, which generally contain only a small amount of pyroclastic rocks. this lithologic assemblage usually occurs in the shengping-anda area and the southern fung lok area in the northern part of the xujiaweizi fault depression. the volcanic lavas are mainly dominated by acid rhyolite, while the northern part of the andesite is dominated by basic basalt and neutral andesite (zong et al., 2018). figure 3 (b) core image of the shenshengeng 203 well shows that the top of the lava is usually very porous, reflecting the end of the eruption of the volcanic eruption, and the lava exposing the surface; at the end of the eruption, due to weathering and surface water communication, the pores on the top of lava are very developed. these large numbers of primary stomata are signs of the end of the eruption period. figure 4 (b) shows the lithology and electrical characteristics of the basic lava in the northern part of the study area. it can be seen that the third stage volcanic rocks in dashen 6 well are mainly basalts with a small amount of andesite. it has the characteristics of high resistivity and low natural gamma. figure 4 (c) shows the lithological and electrical characteristics of acid lava in the central and southern part of the study area. it can be seen that the second stage of shengshen 4 well is mainly fissure eruption, and the rock type is mainly rhyolite. if the lithologic assemblage of these volcanic rocks is thicker, the middle part tends to show higher natural gamma and high resistivity characteristics. the top and bottom are usually volcanic sedimentary rocks or tuff, and the electrical characteristics are the decrease of resistivity and the increase of natural gamma, indicating the beginning and end of the eruption. (a) sheng shen geng 2 well, 2902.67 m (b) sheng shen geng 203 well, 3006.5 m figure 3. core photographs of volcanic rocks at different eruption stages 281eruption cycle of volcanic rocks based on virtual reality technology (3) the interbedding of lava and pyroclastic. the energy of volcanic eruption in the center fissure type is slightly weaker than that in the center. volcanic eruption products are usually dominated by pyroclastic and lava, indicating that the energy of volcanic eruption from strong to weak or weak to strong. as shown in figure 4 (d), the deep well of wangshen 903, with low energy at the beginning of the eruption, is dominated by rhyolite, and then gradually increases, the volcanic ejecta is breccia, and then the energy gradually weakens. overall, the process of weakening one strong and one weak indicates the ending of a volcanic activity. variation based on the volcanic lithofacies belt cycle in the same period, the cyclical changes of volcanic rock composition, structure and structure make volcanic rocks exhibit certain prosody features. these prosody features overlap or appear repeatedly, reflecting the pattern and output pattern of spatial distribution of volcanic eruption in the same period, and the appearance characteristics of these products, which is the “facies” of volcanic rocks (cheng et al., 2017). each stage of volcanic rock is usually composed of one or more phase sequences, showing a continuous or quasi continuous phase sequence. a complete volcanic facies sequence assemblage in the longitudinal direction is volcanic channel facies explosive phase overflow facies intrusion facies volcanic sedimentary facies. in fact, only 2-3 lithofacies types or single lithofacies types can be found in the facies sequence assemblages, and the phenomenon of duplication or inversion may also occur. the change of phase sequence is actually the manifestation and result of the process of volcanic activity. therefore, the periodic variation of phase sequence can be used as a basis for dividing volcanic eruption periods. volcanic facies in xujiaweizi fault depression are usually divided into 5 subfacies, 15 facies subfacies, including volcanic channel facies, outburst facies, overflow facies, intrusive facies and volcanic deposits. each facies zone is subdivided into 3 sub facies. if the eruption energy of the volcano is strong in the early stage and weakened in the later stage, and the eruption time is long, there may be a complete phase sequence combination near the crater. if the energy of the volcanic eruption is weak in the initial stage and strong in the late stage, the combination of the phase sequence will be reversed (peng et al., 2016). the phase sequence changes of volcanic rocks in yingcheng formation of xujiaweizi fault depression mainly begin with explosive facies or overflow facies, and seldom start by volcanic channel facies. moreover, the volcanic sequence assemblages are usually composed of 2-3 facies belts, and the complete facies assemblages are rare. the characteristics of different eruptive subvolcanic facies in xujiaweizi fault depression are shown in table 1. (a) xushen 8 well (b) dashen 6 well (c) shengshen 4 well (d) wangshen 903 well figure 4. lithology and logging response characteristics of different types of volcanic rocks 282 jiqiang yang, xing wang division result from the analysis of lithologic combination and electrical characteristics of these volcanic rocks, we can see that at the beginning and end of the first eruption, the lithology changes are volcanic sedimentary rocks (or tuff) pyroclastic rocks (or lavas) volcanic sedimentary rocks (or tuff). the characteristics of electricity are the decrease of density and resistivity, and the increase of natural gamma (wang et al., 2020). these characteristics are the signs of volcanic intermission. on the basis of lithology and facies combination, according to the regularity of volcanic activity and the particularity of volcanic strata, and combining the characteristics of volcanic lithology, lithologic assemblage, volcanic facies belt, sedimentary rock intercalation and electrical characteristics of the core and volcanic eruption, the first and three sections of yingcheng formation in xujiaweizi fault depression are divided into 3 eruption periods, followed by y1c1, y1c2 and y. 1c3, y3c1, y3c2, y3c3. the results are shown in table 2. eruption stages and characteristics of volcanic rocks in yingshan depression the volcanic eruption cycle of yingshan depression in the study area experienced a quiet period of intense volcanic eruption and quiet period of volcanic activity, while sedimentary intercalation is the representative lithology of the quiescence period and the most obvious indicator of the interface of the cycle. the eruption characteristics of volcanic rocks in yingshan depression are shown in table 3. cycle 1 is divided into stages 1 and 2 from bottom to top. the period 1 is mainly neutral or moderately acidic volcanic rocks. it shows layered like reflection in seismic sections. sedimentary facies and sedimentary tuff are the main sedimentary facies in stage 2, and the thickness of sedimentary interbeds is about 150m. it can be used as volcanic eruption times alone, indicating volcanic eruption into a long quiet period. the activity of fire mountain is from strong to weak, and it shows good stratification and continuity in seismic profile, and is easy to track and compare horizontally. at the top of the cycle 2, there is a thin sedimentary rock with a thickness of about 50m, showing a continuous strong reflection coaxial axis on the seismic profile, which is the interface between cycle ii and cycle iii, indicating that the two cycles experienced only a relatively short eruption period. on the whole, the cycle 2 is mainly volcanic rock, and from bottom to top, it is divided into stage 3 and stage 4. three of them were mainly volcanic lava or volcanic breccia, locally developed fluffy tuff and granularity. phase 4 was mainly composed of fused tuff and rhyolite tuff, with smaller granularity. it showed that the eruption of multi crater gradually evolved into the eruption of large scale volcanic eruptions. the obvious alteration of granularity and lithology of these volcanic rocks indicates the vertical migration eruption of volcanic rocks at gr. the curves show rhythmic changes, and the reflection of the upper part relative to the lower part of the layer is disorderly on the seismic section. cycle 3 is divided into stages 5 and 6 from bottom to top. the stage5 is dominated by rhyolite and rhyolite breccia lava, and the granularity is large. stage 6 is mainly tuff, with smaller granularity, indicating vertical migration and eruption of volcanic rocks. volcanic activity gradually ends, and the upper part of the seismic section shows relatively good stratification, while the lower table 1. lithofacies association and variation characteristics of subvolcanic rocks at different eruption stages in xujiaweizi fault depression representative well well da shen 1 well wang shen 1 well xu shen 8 xu shen 9 well eruption period phase i of yingsan section phase ii of yingsan section phase iii of yingsan section phase i of yingsan section phase ii of yingsan section phase iii of yingsan section phase i of ying phase i phase ii of ying phase i phase iii of ying phase i phase i of ying phase i phase ii of ying phase i phase iii of ying phase i volcanic activity intensity strong weak strong strong weak strongweak weak strongweak strong weak strongweak strong intermittence strong weak strong intermittence strong weak strong weak weakstrongintermittent types of phase sequence combination explosiveoverflow facies spillwayexplosive facies overflowexplosive phase superposition overflowexplosive phase superposition explosiveoverflow facies explosiveoverflow facies explosiveoverflow facies explosiveoverflow facies volcanic channel faciesexplosive facies explosiveoverflow facies eruptive faciesvolcanic sedimentary facies eruptiveoverflowvolcanic sedimentary facies eruption mode fissure type fissure type central type fissure type table 2. volcanic eruption stages in xujiaweizi fault depression, songliao basin stratum ying one section ying two section ying three section lithology rhyolitic debris tuff, rhyolitic breccia, tuff agglomeration, fused breccia tuffaceous sandstone, mudstone, conglomerate and silty conglomerate andesite, andesite basalt, diorite porphyrite and tuffaceous mudstone phase sequence combination spillover facies dominates eruptive phase eruptive phase lacustrine fan delta facies overflow phase volcanic sedimentary facies overflow phase overflow phase eruption period y1c1 y1c2 y1c3 volcanic-sedimentary cycle y3c1 y3c2 y3c3 cycle eruption cycles i depositional cycle eruption cycles ii representative well xu shen 1 well well xu shen 6 xudong area well da 2 well 283eruption cycle of volcanic rocks based on virtual reality technology part reflects chaos and vermicular reflection, and the stratification is poor. on the whole, the first stage is controlled by the four station fault, the sub top interfaces are overlapped to the four station fault, and the area is above the first bottom interface. discussions volcanic eruption period is a relatively continuous volcanic activity. its material composition and eruption intensity also have regular changes in the longitudinal direction. a volcanic eruption cycle consists of multiple eruption periods. the lithology and seismic profile characteristics of the same cycle differ from each other, such as volcanic lithology change, volcanic particle size change and seismic reflection characteristics change, etc., which is the basis of the internal division period of the cycle (bergroth et al., 2018). in this paper, the virtual reality technology is used to study the eruption cycle of volcanic eruption in the xujiaweizi fault depression and yingshan depression. the data of the study area are collected by monocular vision based on the 3d reconstruction of the virtual reality technology, which ensures data blocking, avoids the transition of the connected data, and ensures the maximum degree of clarity. the dynamic state of data such as texture and transparency can achieve the best imaging state and realize the dynamic rendering of 3d scene in the study area. on this basis, the eruption cycle times of volcanic rocks in the study area are divided according to the classification steps and division basis of the cycle stages. during the mesozoic, the study area experienced the regional tectonic evolution of “north-south strike, east-west subduction, mantle crust, and opening and closing alternation”, and 3 major volcanic eruptions occurred in succession, namely, jurassic flint rock formation and cretaceous yingcheng formation. each volcanic eruption cycle has erupted intermittently. compared with the eruption cycle, the eruption period is much shorter than that of the first cycle. a volcanic eruption cycle is usually composed of multiple eruption periods (zhou & zhang, 2017). the same sub volcanic rocks in xujiaweizi fault depression usually have three lithologic variation assemblages, mainly volcaniclastic, mainly lava and interbedding of lava and pyroclastic rocks. they reflect the rhythmic characteristics of lithological assemblage and logging response, reflecting the strong and weak eruption of volcanoes. in the same period, volcanic eruption materials are spatially distributed volcanic rocks. finally, the first and three sections of yingcheng formation in the xujiaweizi fault depression are divided into 3 eruption periods, followed by y1c1, y1c2, y1c3, y3c1, y3c2 and y3c3. the yingshan section of yingshan depression table 3. characteristics of each eruption period of volcanic rocks in yingshan depression cycle cycle i cycle ii cycle iii period period 1 period 2 period 3 period 4 period 5 period 6 lithologic characteristics neutral or intermediate-acid volcanic rocks predominate sedimentary rock or tuff fused tuff is locally developed mainly from volcanic lava or volcanic breccia with large grain size. fused tuff and rhyolitic tuff are the main types. rhyolite and rhyolitic breccia are dominant with larger grain size. tuff is the main type earthquake reflection characteristics layer-like reflection with thinner formation thickness good stratification, continuous and clear reflection inphase axis intermittent and disorderly reflection, poor stratification, large bottom thickness intermittent reflection is better than continuous reflection and stratification is better. the reflection is chaotic and wormlike, with poor stratification and large formation thickness. the formation thickness is thin and the stratification is relatively good. stratigraphic contact relation the stratum overlaps the sizhan fault and partially exceeds t41. the stratum overlaps the sizhan fault and partially exceeds t41. the stratum overlaps the sizhan fault and partially exceeds t41. the stratum overlaps the sizhan fault and partially exceeds t41. load cutting, local strata overlap the fourth station fault the stratum overlaps the sizhan fault interface marker cyclic interface intermittent depositional interlayer with larger thickness intermittent depositional interlayer with smaller thickness periodic interface 1. volcanic metamorphic sedimentary rocks 2. better stratification and continuity of seismic reflection characteristics 1. the size of volcanic rocks decreases 2. better seismic reflection characteristics 1. the size of volcanic rocks decreases 2. better seismic reflection characteristics volcanic apparatus total area (km2)/number (individual)/ maximum amplitude/(m) 25.36/8/71 31.78/10/37 149.19/25/91 82.14/20/100 151.71/15/121 73.31/18/161 distribution location sporadically distributed near well-4 sporadic distribution at the edge large area is distributed in the middle and south, along the fault small area in central and northern china widespread distribution in the west small areas are distributed in the west and south 284 jiqiang yang, xing wang in the study area is divided into three cycles, i. e., cycle 1, ii and iii, which can be subdivided into six stages. among them, cycle i develops large sets of sedimentary rock intercalations, showing obvious stratification, high continuity and stable equilibrium and strong amplitude energy characteristics in seismic profiles. cycle ii only develops a thin sedimentary interlayer on the top, showing only a continuous strong reflection axis on the seismic profile, but it is obviously different from the chaotic reflection characteristics of the volcanic rocks. the lateral correlation of volcanic eruption cycles can be achieved. the eruption of volcanic rocks has obvious stages and differences. the intensity of each eruption is different, and the overall performance is gradually enhanced. conclusions volcanic activity is characterized by polycyclic and multi-stage activities. the magmatic products of a volcanic activity cycle and their volcanic structural traces have certain unity and coordination in time and space distribution. volcanic rocks of different stages of the same cycle, or volcanic rocks of different stages of the same period, are different in volcanic lithofacies zones due to different energy and eruption modes. different volcanic lithofacies zones control the development of reservoirs and the enrichment and high production of oil and gas. therefore, by using virtual reality technology to build three-dimensional dynamic scenes in the study area, it is of great significance to study volcanic eruptions cycles times and effectively divide volcanic eruption stages to identify favorable facies zones of volcanic rocks, analyze the inducing factors of reservoir development, and hydrocarbon distribution and accumulation rules. references bergroth, j. d., koskinen, h. m. k., & laarni, j. o. 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(2018). energy security in group of seven (g7): a quantitative approach for renewable energy policy. energy sources part b-economics planning and policy, 13(3), 173-175. http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dianyjs201711044 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201706028 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201805073 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201805073 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201805073 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201901011 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201901011 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201901011 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201701020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201701020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201701020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201702005 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201702005 palabras clave: modelos análogos; fracking; acuíferos. how to cite item martínez-sánchez, d. a., & jimenez, g. (2019). hydraulic fracturing considerations: insights from analogue models, and its viability in colombia. earth sciences research journal, 23(1), 5-15. doi: https://doi.org/10.15446/esrj.v23n1.69760 hydraulic fracturing, known like fracking, is being considered around the world as a potential method in the hydrocarbons extraction given the increase of production in usa and canada during the last years with its implementation. the most criticized feature of the technique of hydraulic fracturing is the contamination of underground waters by fluids connected through the generated and/or pre-existing fractures. this work evaluates the viability of hydraulic fracturing in colombian territory based in the study of the analogue models, considering the elastic properties of the reservoir rocks using gelatin, sand and clay that apply scaled pressure (lithostatic pressure to scale). the gelatin is used to simulate the source and reservoir zone, the clay will simulate one seal and the sand will simulate reservoir rocks of underground water. as a result seven different models were generated. there were simulated as 1) the anisotropy presence, 2) the rock seal presence, and 3) the injection groove of fluids in the pipeline. the completed models show that the required distance between the unconventional reservoir and underground waters to avoid contamination by fluids is ~200 m, also between the hydraulic fracturing point and the faults that connect with the shallows area is ~350 m, and it is suggested to study in detail the permeability of both: nearby faults and the rock seal. two colombian basins (middle magdalena valley and llanos) were taken as the main base to analyze the assessment of hydraulic fracturing according to the previously mentioned results, concluding that it is viable particularly in areas like llanos basin and with some precautions in similar zones to middle magdalena valley basin. abstract keywords: analogue models, hydraulic fracturing, aquifer. hydraulic fracturing considerations: insights from analogue models, and its viability in colombia consideraciones del fracturamiento hidráulico: a partir de modelos análogos y su viabilidad en colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.69760 la técnica del fracking a nivel mundial está siendo considerada como un método efectivo en la extracción de hidrocarburos teniendo en cuenta que paises como estados unidos y canada incrementaron la producción de hidrocarburos al implementar esta técnica. la principal crítica a la técnica del fracking es la contaminación de acuíferos por fluidos conectados a través de las fracturas generadas y/o pre existentes. en el presente trabajo se evalúa la viabilidad del fracking en el territorio colombiano con base en el estudio de modelos análogos, considerando las propiedades elásticas de la roca usando gelatina, arena y arcilla, simulando presiones o cargas escaladas (litostática, a escala). la gelatina es utilizada para simular la zona fuente y reservorio, la arcilla para simular sellos y la arena para simular zonas de acuíferos. en total se generaron 7 modelos diferentes, donde se simuló 1) la presencia de anisotropías pre existentes, 2) la presencia de roca sello y 3) ranuras de inyección de fluido en la tubería. los modelos realizados muestran que la distancia requerida entre el reservorio no convencional y los acuíferos para evitar la contaminación por fluidos es de ~200 m, entre el punto de fracturamiento hidráulico y las fallas que conectan de una manera u otra con la superficie es de ~350 m, y se sugiere estudiar de manera detallada la permeabilidad tanto de las fallas cercanas como de la roca sello. dos cuencas colombianas (valle medio del magdalena y llanos) se tomaron como base para analizar la viabilidad de la técnica del fracking según los resultados mencionados anteriormente, concluyendo que es viable especialmente en áreas como las de los llanos y tomando algunas precauciones en zonas similares a las del valle medio del magdalena. resumen record manuscript received: 12/01/2018 accepted for publication: 03/10/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 5-15 martínez-sánchez, dilan a¹,³; jiménez, giovanny¹,² ¹universidad industrial de santander. ²grupo de investigación en geología básica y aplicada (gigba) ³energy and other non renewable resources research group (eonr3g) g eo lo g y 6 martínez-sánchez, dilan a, jiménez, giovanny introduction hydraulic fracturing is being used extensively as a new source of hydrocarbon. some countries around the world are taking serious considerations into developing this technique in order to provide security for the energy’s future. these ideas have generated various debates due to the potential negative environmental impact (goodman, 2016). the more relevant considerations are associated to different settings during the hydraulic fracturing planning (figure 1); related to faults, fractures and the well stability, which could favor the fluids connection of fracturing with shallows aquifers (scotchman, 2016; yoxtheimer, 2014). colombia has created a series of regulations for exploration and exploitation of unconventional hydrocarbon (anh, 2014a). in addition, forums of debate about the environment factor were carried out on the agencia nacional de hidrocarburos web (anh), due to the fall of the production of hydrocarbon in the world during recent years (mayer, 2016; bp, 2016). while planning the production assembly to carry out the fracturing, it is considered the well orientation; since if the well is located parallel or perpendicular to the main horizontal stress, it will generate longitudinal or transversal fractures respectively. (fossen, 2016; brady et al., 1992; hubbert y willis, 1957). when applying the technique is ideal to carry out transversal fractures to improve the vertical flow during production, considering that it will complement the well orientation. however, a transversal fracture will be only connected with the well in one point, whereby is important to plan the distribution of multiple transversal fractures along the well (yew and weng, 2015). the fracturing will be different according to the fluid viscosity (figure 2) (spe, 2016); considering: 1) the spatial variation of the fracture (figure 2a), 2) the change of pressure in the well while pumping (figure 2b), and 3) the fracture’s width (figure 2c). in summary, a high viscous fluid generates great fractures, high pressure in the well, and a slow range in the rotation of the fracture (yew and weng, 2015). taking into account the examples of aquifer contamination in the usa that happened based on the wrong regulation on the beginnings of the hydraulic fracturing process (osborn et al., 2011). the investigation’s objective is the evaluation in a laboratory scale about the ratio between aquifers and unconventional deposit zones when using the hydraulic fracturing technique; evaluating the following variables: 1) the presence of pre-existent anisotropies, 2) the presence of seal rock and 3) the configuration of pipeline to change the direction of the fractures system in regard of the well. method considering that the colombian deposits have an average depth near to two kms (tissot and welte, 1984). seven models were made in a tank of acrylic with dimensions of 20*30*40 cm, varying the disposition of materials with particular rheological properties. the materials used in the experiments are gelatin, sand and clay (figure 3), to simulate a viscous-elastic and clearly brittle mechanical stratigraphy. some of these models have planes of anisotropies which are generated by cuts made with a thin sheet of aluminum. scaling the density of the upper crustal can vary between 2.4 and 2.6 g/ml depending on its composition (nikkilä et al., 2015; wu et al., 2015; rosas et al., 2014; bonini, 2007), an average value of 2.5 g/ml was used in this work. the gelatin in the models is used with a concentration of 7% in ratio mass-volume, to simulate the rheological properties of the source and reservoir rocks, such as elasticity and toughness (otterloo and cruden, 2016; di giuseppe, 2009). hence, the ratio of the density for the gelatin is: rg = =1 02 2 5 0 408. . . (1) where rg is the density ratio of the gelatin, the density ratios of the sand and clay were also obtained: ra = =1 2 2 5 0 48. . . (2) rs = =1 7 2 5 0 68. . . (3) where, ra is the density ratio of the sand and rg is the density ratio of the clay. figure 1. possible risk settings of planning during the development of hydraulic fracturing; where fractures, faults and the well stability can favor the connection with shallow zones. modifies of scotchman (2016). figure 2. graphics of the behavior of fracturing fluid depending on its viscosity. a) variation of the fracture rotation in two dimensions considering the vertical axis (z) and the horizontal axis (x), assuming one stress ratio of 2.5 between the maximum horizontal stress and the minimum horizontal stress. b) variation of the net pressure exerted in the center of the well as the pumping time advances. c) variation in the width of the center of the fracture generated in relation to the pumping time. modified from yew and weng (2015). 7hydraulic fracturing considerations: insights from analogue models, and its viability in colombia the longitudinal scale is 5*105 assuming that the acceptable ranges are 106 lr 1 (twiss and moores, 2006; yagrupsky, 2009; pollard and fletcher, 2010), therefore in the model 5 cm are equivalent to 1 km of nature, then the volumetric scale is vr = 1.25*1013 for the injected fluid. nevertheless, just two dimensions are considered for the generated fractures because to the width is negligible to this scale, so the areal scale is ar = 2.5*1011. the stress scale (r) is defined from longitudinal scale, this was calculated considering that the gravity ratio is 1 (yagupsky, 2009: dooley and achreurs, 2012; pollard and fletcher, 2010), and the mass ratio is equal to the multiple from the volumetric ratio and the density ratio. so, we obtained the force ratio. σ ρ ρr r r r r r r r f a v a l= = =* * (4) therefore, it is possible to consider that the stress ratio is equal to the density ratio per the longitudinal ratio. this value is used to calculate the elasticity properties of the gelatin, young´s module and the poisson ratio, which were measured of uniaxial way. then, knowing that the stress scale for the gelatin is 2.04*105, the young´s module measured is ~40 kpa and the poisson ratio is 0.35, additionally, the compression toughness is ~2.7 kpa . these values in natural prototype scale are equivalent to rocks like shales and sandstone (acevedo, 2009; fossen 2010; gercek, 2007). but also, we know the equivalent value in the nature of the stress applied by the sand and/or clay depending on the model, knowing that the stress is 637 pa in the models with only sand and 638 pa in the models with sand and clay. in each model is represented one sedimentary basin of 3 km taking into account the defined scale; where the first upper kilometer is only composed of sand or 4 cm of sand above 1 cm of clay, and the two kilometers below are composed of one gelatin bed, the pipeline is located in the central zone of this bed, that is to say about 2 km of depth. finally, the fracturing fluid have a key role in the dimensions of the generated fractures depending on its viscosity knowing that the width, the vertical and horizontal extension are major when the fluid is more viscous (figure 2) (yew and weng, 2015). then, the density fluid is ~ 5000 cp, the fluid is highly viscous to increase the influence zone contemplating the damage caused by the small anisotropies that the rocks present, that can´t be represented in these models for the scale. in each model will be injected 2 ml, which it is equivalent to one hundred times the fluid used in perforation, 2-20 million of gallons of water (meng, 2016; jackson et al., 2014; epa, 2016; iea,2012), to look at the dimension of the fractures. clarifying that the generated fractures will be 10 times bigger in each axis (horizontal and vertical), if we consider that its width is negligible. results six models were generated with the configuration used to shape parallel fractures to the pipe to consider areas where the pipeline is located parallel to the figure 3. schematic models of the tank of acrylic with pipelines; the horizontal pipeline will be interchangeable in order to have diverse options of grooves to inject the fluid. a) stable model without seal (clay) in the mechanical brittle zone; b) model with pre-existent anisotropies without seal presence; c) stable model with seal presence in the mechanical brittle zone; d) model with pre-existent anisotropies in the deposit zone and seal presence in the mechanical brittle zone; e) model with abundant pre-existent anisotropies. 8 martínez-sánchez, dilan a, jiménez, giovanny horizontal main stress. the seventh model has a different configuration in the injection groove to generate transverse fractures to the pipe. the first three models were built with two beds; the first bed is composed of gelatin and the second bed is composed of sand (figures 4, 5 and 6). model 1 (figure 4), has the setting of the assemblage a) specified in the methodology, which does not have anisotropies and the propagation of fractures is free in the gelatin bed. in the figure 4 can be seen both the initial and final phase, further a real-scale digital model (this sequence of images is used for each model). model 2 (figure 5) has the setting of the assemblage b) specified in the methodology, in which were created planes in the hydrocarbon zone to see the response of the fluid when intersecting these planes in order to establish some considerations on the areas with this geological setting in the hydrocarbon zone. these anisotropies are considered as fault zones and were distributed in the following form inside the gelatin bed: three in the upper zone and other two in the lower zone (figure 5a). while the fluid is injected it can be seen that the fluid migrates when the generated and the initial fractures connect consequently and the fractures propagation stops. for model 3 (figure 6), we have the setting of the assemblage e) specified in the methodology where several randomly fractures are generated. (figure 6a.) to able to see the behavior of the injected fluid when connecting the generated fractures with these planes. this model during the final phase shows that the fluid migrates through the interconnected planes affecting the zone of sand. in the model 4 (figure 7) is not generated anisotropies on the hydrocarbon zone and one new bed of clay is added between the gelatin bed and sand bed, this model has the setting of the assemblage c) specified in the methodology. in this model the volume of the injected fluid is exaggeratedly greater than the volume used in the production fields in order to see the behavior of the fluid when it reaches a good seal. as result it can be seen that the injected fluid stops its ascend and the fracture grow laterally when the fracture reach to the seal bed. further, the fluid escapes between the clay bed and the gelatin bed at the moment that the injection of fluid is stopped. followed, the fluid ascends through the space between the tank and beds until it reaches the sand bed (figure 7) now, in model 5 (figure 8) one clay bed is put in the same way as in model 4. but, in this case were generated anisotropies, this model has the setting of the assemblage d) specified in the methodology to be able to see the fluid migration when it reaches the clay bed through the anisotropies. in the final phase was made a cut to see better the fluid migration because in the front picture it cannot be seen clearly. the flow is different in each anisotropy, counting from left to right, in the third anisotropy the fluid escapes ascending as in previous models, but in the first anisotropy the fluid descended (figure 8b;8c). finally, model 6 (figure 9) is the last with the setting of grooves used to generate longitudinal fractures to the pipeline, this model has bubbles series considered as small anisotropies not connected in the hydrocarbon zone. in the results of this model is very important to highlight the irregular shape of the fractures because the fluid connects the nearest bubbles. in model 7 (figure 10) the setting of grooves in the pipe is changed by injection points in circle about on the pipeline several zones to perform perpendicular fractures. nevertheless, during this stage of laboratory was impossible to generate transversal fractures for this model because in this assembly are not applied the horizontal stresses. the final phase (figure 10) shows cuts of the generated fractures where it can be seen that the dip of each fracture is not vertical as in the first models possibly associated with the change of the grooves, but the fractures keep longitudinal to the pipe considered as the main cause of the inexistence of the different horizontal stresses. discussion the results obtained in the models show that the fractures generated by hydraulic fracturing will not reach aquifer zones in the areas where there are not anisotropies and the exploitation zones have an average depth of 660 ft (~2 km), considering the first kilometer as the aquifers zone. knowing that, how was mentioned in the scaling, the injected fluid is greater than the amount used figure 4. model 1.a) initial phase, the image shows a homogeneous bed of gelatin below of one sand bed. b) final phase, this image shows three generated fractures in the injection area which are highlighted with black arrows. c) digital model scaled to the nature which shows the generated fractures with red color and each division of the beds with planes. 9hydraulic fracturing considerations: insights from analogue models, and its viability in colombia figure 5. model 2, a) the image shows the configuration of the model with anisotropies in the gelatin bed which are highlighted with black arrows. b) initial phase, shows the previous gelatin bed below one sand bed. c) final phase, the picture shows the three generated fractures highlighted with black arrows, these fractures stop growing when doing contact with the initial planes of anisotropies. d) digital model scaled to nature is showing the generated fractures with green color and the initial anisotropies with red color. figure 6. model 3, a) assembly configuration, this picture has a gelatin bed with anisotropies which are highlighted with red lines. b) initial phase, this picture shows the previous gelatin bed below of the sand bed. c) final phase, this picture shows two generated fractures which are marked with black arrows, these fractures stop to grow when connecting with the anisotropy planes. further, the migrated fluid is indicated with blue arrows. d) digital model scaled to nature, shows the generated fractures with green and the anisotropy planes with red. in addition, the orientation of fluid migration is indicated with black arrows. 10 martínez-sánchez, dilan a, jiménez, giovanny figure 7. model 4, in images a), b) and c) are showing the initial phase, final phase and the digital model respectively. further, in pictures d), e) and f) it can be seen the scape of the fluid. d) the picture shows the fracture when coming into contact with the clay bed. e) in the picture can be seen the lateral growth of the fracture and fluid´s flow between the two beds considering that the fracture cannot grow vertically. f) this picture is a top view where can be seen the fluid mixed with the sand bed after ascending through the space between the tank and the clay bed. figure 8. model 5, a) initial phase, shows the anisotropies in the gelatin beds indicated with red arrows below one clay bed and one sand bed. b) final phase, shows the growth of two small fractures in the central zone and mainly the fluid migration through the created anisotropies which is marked with black arrows. c) digital model scaled to nature, evidences clearly the generated fractures with green color and the initial anisotropies with red color besides highlighting the migration of fluid with black arrows. 11hydraulic fracturing considerations: insights from analogue models, and its viability in colombia figure 9. model 6, a) initial phase, shows the gelatin bed with small bubbles and one anisotropy which is indicated with a red arrow below a sand bed. b) final phase, shows two generated fractures and the growth of the initial anisotropy, further on one of the fractures generated is made a cut to show the irregular shape. c) digital model scaled to nature, shows the generated fractures with green color. figure 10. model 7, a) initial phase, shows a homogeneous gelatin bed below a sand bed. b) final phase, shows the growth of small fractures, in each fracture is made a cut to evidence the dip of the fractures. c) digital model scaled to nature, shows the generated fractures in green. 12 martínez-sánchez, dilan a, jiménez, giovanny vertical transmission and lateral seal (vice versa), and intermittent transmission faults (aydin, 2000). further, the tectonic activity could stimulate the migration through the fault planes (moretti, 1988). in models 5 and 3 it can be seen these dynamics in the migration of the fluid (figure 13), due to some sections of the planes of anisotropies were occasionally closed in these models by the sand which produce a vertical compression distributed aleatory, probably associated with the dip of the planes. with respect to the models must be added that the models 4 and 5 show that a good seal can be an ideal insulating to avoid migration of the fluid. although, the clay bed has to be studied and delimited due to that any anisotropy can enable the contamination of shallow zone. ¿colombia? according to barrero et al. (2007), the fifteen sedimentary basins have the potential for the exploitation of hydrocarbon in colombia, of which seven basins (middle magdalena valley, upper magdalena valley, eastern llanos, sinú-san jacinto, cauca, eastern cordillera, and catatumbo) have been identified with potential to shale oil and shale gas. in addition, colombia is classified as the third country in south america with great potential of unconventional deposits, after brazil and argentina, stablishing that the prospectivity of unconventional gas and oil for colombia will have contributions of hydrocarbon between the 32-60% and 11-26% respectively (anh, 2014b). based on the results of the 7 realized models, the viability of hydraulic fracturing in colombia is evaluated considering two basins with different strain levels, the basin of the middle magdalena valley and the llanos basin . (figure 14) (moretti etal., 2009; vayssaire et al., 2014; gómez-alba et al., 2015). therefore, if it is thought to use the hydraulic fracturing in the source rocks of the cretaceous could be made successfully due to its depths and the few faults existent that can connect the aquifers with the deposit zones. the models in the middle magdalena valley basin are characterized by great reverse faults (figure 15) (moretti et al., 2010; spickert, 2014). for that reason, implementing the hydraulic fracturing might be possible but before is necessary to do detailed studies of the seal rock in shallow unconventional deposits and faults with potential risks. in real cases, we take the length obtained for each fracture axis on our models and thus a possible damage zone is considered. according to king (2012), the hydraulic fracturing has to be made to greater depth than 2000 ft; but, he considers an aquifer zone shallower than the used in our models in which the aquifers zone is defined in the first kilometer, for that reason we consider a depth greater than 3800 ft (~1300m) to practice this fracturing technique, taking into account that the generated fractures in the models has a vertical length in the range of 150 and 200 m (figure 11).. the international energy agency, iea (2012), proposes in “the gold rules for the exploitation of unconventional hydrocarbon” that the geology must be known in detail, mainly associated to the great faults of the interest zone. since, as scotchman (2016) mentions, these planes serve as migration routes. however, in the studies previously mentioned is not propose the required distance between the injection groove and the faults to avoid the escape of fluids. so, in the results of the models 1 and 2 (figure 12), is showed that the ideal distance is approximately 400m. although, we have to consider that the length of the fractures is larger because the amount of fluid is greater to assume the damage area. also, we have to consider the permeability properties and the tectonic activity of the faults nearest to production area because the properties of the fault rocks in the damage zone are fundamentals in the fluid migration, knowing that the faults can be classified in: transmission faults, seal faults, faults of figure 11. digital image of the model 1 where is indicated the vertical length of the generated fractures. figure 12. digital images of the model 1 and 2 from left to right, in these ones is indicated the horizontal length since the injection groove until the boundary of the fracture. l 13hydraulic fracturing considerations: insights from analogue models, and its viability in colombia figure 13. the fluid migration through the generated anisotropies planes in model 5 depending on its properties. figure 14. simplified section of the llanos basin and puerto gaitan field. modified of gómez-alba et al. (2015). 14 martínez-sánchez, dilan a, jiménez, giovanny conclusions the experimental assemblage made is an appropriate tool to study the generation of hydraulic fractures, due to the good scaling process of the materials. the main parameters and considerations in the hydraulic fracturing from an experimental view are: a) the vertical distance between the production zone and the aquifer zone must be greater than 750 ft. b) the horizontal distance between the injection points and the faults that connect the production zone with the aquifer zone must be greater than 400m. c) the properties of permeability and the tectonic activity of the nearest faults to the deposit zones must be known. also, the seal rock qualities have to be mapped with detail. according to the considerations in colombia may be possible stablishing several areas to use the hydraulic fracturing with success for the unconventional hydrocarbon production; mainly highlighting structurally similar areas to the llanos basin. further, if it is had a strict control to the nearest structures to the production area and the seal rock should be considered other basins or zones with a greater structural complexity. acknowledements we acknowledge to the corrigan fund conformed by the asociación colombiana de geólogos y geofísicos del petroleo (acggp), and the ares geological corporation for the economic support to carry out this investigation. additionally, to the reviewer of the investigation proposal and the evaluators of the thesis for your commentaries and contributions. references acevedo a. o. d. 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(2014). shale energy fluids management practices. forum on hydraulic fracturing, agencia nacional de hidrocarburos, bogotá, colombia. keywords: large dip angle; thick coal seam; support instability risk; mechanical model; control measures palabras clave: ángulo grande de inmersión; filón de carbón espeso; riesgo de inestabilidad de apoyo; modelo mecánico; medidas de control. how to cite item zhang, w., wang, t., zhang, d., zhang, y., xu, p., & duan, x. (2021). a mechanical analysis of support instability risk along the strike of coalface in thick coal seam with large dip angle: a case study. earth sciences research journal, 25(1), 101-108. doi: https://doi.org/10.15446/ esrj.v25n1.74167 studying the support stability of a coalface with a large dip angle (lda) and large mining height (lmh) under the corresponding conditions is essential to ensure safe and high-efficiency mining. this study is based on the #3up509 coalface of the gaozhuang coal mine (gcm) affiliated with the zaozhuang mining area. the authors analyzed the mechanical characteristics of support in the coalface with lda and lmh. on this basis, the mechanical models for support tilting and sliding in the coalface were developed. then, support stability along the coalface’s strike during the normal mining period (nmp) and special mining period (smp) was analyzed. the results show that the critical support tilting resistance during the nmp is 52.2 kn, and the critical support sliding resistance is 183.75 kn. and for the smp, the values are 229.7 kn and 4425 kn. the use of two-leg shield support, known as zy6600-25.5/55 (its rated working resistance is 6600 kn), is investigated, which is proved reasonable for the coalface. finally, some technical measures, such as installing an interlock set to fasten support and adjustable lifting jack, increasing the setting load of the support, and optimizing the support displacement method, are taken to improve the coalface’s overall support stability. the initial aim for safe and high-efficiency mining of the #3up509 coalface has been achieved through these measures. abstract a mechanical analysis of support instability risk along the strike of coalface in thick coal seam with large dip angle: a case study análisis mecánico del riesgo de inestabilidad en los apoyos de un frente de trabajo en un filón de carbón espeso con ángulo de inmersión grande: estudio de caso issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.74167 estudiar la estabilidad del soporte de un frente de explotación carbonífero bajo las condiciones de un ángulo de inmersión grande (lda) y una explotación minera alta (lmh) es esencial para garantizar la seguridad y la alta eficiencia minera. este estudio se basa en el frente de trabajo número #3up509 de la mina de carbón gaozhuang (gcm) afiliada al área minera de zaozhuang. los autores analizaron las características mecánicas de apoyo en un frente de trabajo que cumple con las condiciones lda y lmh. con esta base se desarrollaron los modelos mecánicos que permitan al apoyo deslizarse e inclinarse en el área del frente de trabajo. luego, se analizó la estabilidad del apoyo en el rumbo del frente de trabajo durante un periodo normal de explotación (nmp) y en un período especial de explotación (smp). los resultados muestran que la resistencia crítica de la inclinación del apoyo durante el período nmp es de 52.2 kn, y la resistencia crítica del deslizamiento es de 183.75 kn. y para el período smp, los valores son 229,7 kn y 4425 kn. además, se investigó el uso de un apoyo de escudo con dos pies, conocido como zy6600-25.5/55 (su capacidad de resistencia está calculada en 6600 kn), que funciona bien en los frentes de trabajo. finalmente se tomaron algunas medidas técnicas como instalar un enclavamiento para amarrar un apoyo y un gato de elevación, lo que incrementa la capacidad de carga del apoyo y optimiza el método de desplazamiento del apoyo, para mejorar la estabilidad general del apoyo en el frente de trabajo carbonífero. el objetivo inicial de seguridad y alta eficiencia minera para el frente de trabajo carbonífero #3up509 se alcanzó con estas medidas. resumen record manuscript received: 09/08/2019 accepted for publication: 02/10/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 101-108 wei zhang1,2, tianyi wang2, dongsheng zhang1,2*, yandong zhang2, peng xu2, xu duan2 1state key laboratory of coal resources and safe mining, china university of mining and technology, xuzhou 221116, china 2school of mines, china university of mining and technology, xuzhou 221116, china * corresponding author: dshzhang123@cumt.edu.cn m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n1.74167 102 wei zhang, tianyi wang, dongsheng zhang, yandong zhang, peng xu, xu duan introduction coal is still the major energy source in china, occupying more than 60% of the total energy consumption (huang et al., 2017; zhang et al., 2019). along with the improvement of china’s planning on strategic coal reserves and the exhaustion of high-quality coal resources with good occurrence conditions, the need for coal mining under complex geological conditions has been growing. in china, mineable reserves of steeply inclined coal seams account for approximately 17 % of the total coal reserves (chi et al, 2019; tu et al., 2015; wu et al., 2000; wu et al, 2014). the large dip angle coal seams (ldacss) are mainly located in sichuan, xinjiang, ningxia, gansu, inner mongolia, and heilongjiang provinces. due to a series of technical difficulties such as violent underground pressure behavior, complex rule of overburden movement, and difficult control of coalface roof (tu et al., 2015; wang et al., 2011; yun et al., 2017; yin et al., 2001; zhang et al., 2019), the mining technique for ldacss is much less developed compared with the mining technique for the gently inclined coal seams. the commonly used mining methods for thick coal seams in china are slice mining, top coal caving mining, and large cutting height mining (tu et al., 2009). slice mining is a conventional mining method, in which multiple mining operations will cause coalface roof broken, making it difficult to control, while top coal caving mining is an advanced method that greatly increases the mining efficiency of thick coal seams (deng et al., 2015; klishin & klishin, 2010; singh et al., 2002; wu et al., 2008; wang et al., 2019). however, top coal caving mining has some inherent drawbacks, such as low recovery rate of coal resources, susceptibility to spontaneous combustion of coal seams, and a considerable amount of coal dust in the working space. recent years have witnessed a constant advancement in the research and development of coal mining machinery, accelerating the application of large cutting height mining while improving the level of fully-mechanized mining of thick coal seams (rakesh et al., 2015; wang, 2014). for thick ldacss, large cutting height mining can greatly increase the coal recovery rate, reduce refuse content and dust production rate of the coalface (yang et al., 2015). but it should be pointed out that, support tilting and sliding are usually the major factors that restrict the use of large cutting height mining (lin et al., 2004; wang & jiao, 2016; yuan et al., 2015; yang et al., 2018; zhang et al., 2019). to ensure safe and highefficiency production of the mine, it is necessary to assess the support stability in the coalface under the corresponding conditions. compared with the gently inclined coal seams, the mining of ldacss causes a reduction in the normal component and an increase in the tangential component of support’s gravity (singh & singh, 2009; tu et al., 2008). the reduction in the normal component leads to a reduction in the weight that the support can bear and hence a smaller working resistance of the support. a high tangential component will cause an increase in the sliding power acting on the support along the inclination direction of the coalface. as a result, support is likely to tilt and slide. at present, many studies have been conducted on support stability along with the dip of the large dip angle coalface (ldac). however, support stability in ldac is not only affected by the dip angle of the coal seams, but also by the strike angle of the coal seams (i.e. underhand or overhand mining). moreover, during the smp (e.g. passing through faults and other tectonic zones) (li & zhou, 2018), the stress condition of the support is also different from that during the nmp (hosseini et al., 2017; hua & wang, 2008; ma et al., 2015; ostayen et al., 2004; wang et al., 2015; wu et al., 2014). in particular, the support in the coalface is subjected to large lateral stress imposed by the roof, which will greatly reduce the support stability (li et al., 2017). in this study, based on the specific mining conditions of the #3up509 coalface with lda and lmh of the gcm, zaozhuang mining area, china, the stress characteristics of support in the coalface are analyzed, and the mechanical models for support tilting and sliding are respectively developed. using these two models, the support stability along the strike of the coalface is assessed during the nmp and the smp. furthermore, the influencing factors of support stability along the strike of the coalface are discussed. finally, some countermeasures for preventing support tilting and sliding are proposed, to ensure the safe and high-efficiency mining of the #3up509 coalface, and achieve good technical and economic benefits. overview of mine and mining conditions the gcm, one of the main mines of zaozhuang mining group, is located in the southwest of tengnan coalfield. the coal mine under investigation is located in weishan county, shandong province, china. the geographic location of the gcm is shown in figure 1. the north-to-south distance of the gcm is 8.50 km and the west-to-east distance is 2.50-5.50 km, with an area of 32 km2. the gcm is situated in a monocline, with a northeast-southwest strike and a dip angle of 5°-18°, the maximum being 25°. the dip angle gradually decreases from the southeast to the northwest. the coal-bearing strata are permocarboniferous systems consisting of five exploitable coal seams (#3up, #3down, #12up, #16, and #17 coal seam). the #3up and #3down coal seams are the main mineable coal seams. multi-level vertical shaft development is implemented, with centralized appose ventilation. the production levels of the gcm are -370 m and -430 m, and #3up and #3down coal seams are mainly exploited. the #3up509 coalface of gcm is located in the western fifth mining district at the level of -430 m. on the south is the #3up507 coalface in which extraction has been completed; on the north is the level of -600 m which is to be extended; the east side is close to the shaoji fault, and the west side is the down-dip entries of the western fifth mining district. the length of the #3up509 coalface along the strike is 1180 m, and 220 m along with the dip. the mining area is 259600 m2, and the corresponding ground elevation is +32.76-+33.70 m; while the underground elevation is -244.6--414.0 m. the coalface is located in the #3up coal seam of the permian shanxi series in a monocline. the density of the coal seam is about 1.35 t/m3, and the thickness is 3.75-6.20 m, with an average of 5.20 m. a gangue layer about 0.30 m thick is observed locally. the dip angle is 12°-31° with an average of 19°, indicating that the coal seam is a typical ldacs. the roof is mainly composed of mudstone and sandstone, and the floor is predominantly of siltstone. the tectonic development of coalface revealed a total of nineteen faults. among them, nine faults have a throw over 2.0 m, where the maximum throw is 12 m. inclined longwall mining (underhand mining) is implemented (average mining height 5.0±0.1 m), and the maximum angle of the underhand mining is 10°. goaf roof is managed by the all-collapse method. two-leg shield support (zy6600-25.5/55) is used for the coalface, with a maximum height of 5.50 m and a minimum height of 2.55 m, with a center distance of 1.5 m, as well as with the rate-setting load of 5067 kn and rated working resistance of 6600 kn. figure 1. the geographic location of the gcm. stress characteristics of support in the coalface with lda and lmh large mining height supports usually have poor stability. moreover, due to the different movement states of the roof, support, and floor in the ldac and 103a mechanical analysis of support instability risk along the strike of coalface in thick coal seam with large dip angle: a case study underhand mining operation (zhang & cao, 2015), support stability usually decreases dramatically. the components of self-gravity associated with large mining height support in the ldac using an underhand mining method are illustrated in figure 2. it can be seen from figure 2 that the self-gravity (g) of large mining height support has component g1 (along with the dip of the coal seam) and g3 (along the strike of the coal seam), positioned perpendicular to each other. the presence of these two components is not conducive to the stability of the large mining height support. in contrast, component g2 along the normal direction of the coal seam can promote support stability. the support stability is jointly affected by the dip and strike of the coal seam. if only component g1 is considered in calculating the support stability along the strike, then the influence of component g3 on the support stability will be neglected. to make the analysis of support stability more accurate, both the strike and dip of the coal seam are considered. here, the resultant force of components g1 and g3 is newly introduced as g3 into the equation for support stability along the strike. this can ensure a certain safety factor. from the geometric relationship in figure 2 and the aforementioned mechanical simplification, the following equations are derived. g g2= ⋅ ⋅cos cosα β (1) g g3 2 21= × − ⋅cos cosα β (2) where α is the dip angle of the coal seam; and β is the angle of the underhand mining. figure 2. schematic of components of self-gravity of large mining height support. mechanical analysis of support stability along the strike during the nmp the nmp refers to that there is no severe situation affecting the safety production of coalface in advance. for example, the accident of local roof caving does not occur in the coalface. during the nmp, the roof of the coalface with lda and lmh is usually intact, and it will produce a frictional force on the large height support beam that prevents the support from tilting and sliding. under the influence of the friction force, the instability possibility of the large height support in ldac will be greatly reduced. in this section, the support stability of ldac along the strike during the nmp has been analyzed by the mechanical models of support tilting and sliding. mechanical model of support tilting the mechanical model of support tilting along the strike of the coalface during the nmp (underhand mining) is shown in figure 3. figure 3. mechanical model of support tilting along the strike during the nmp. from figure 3 the following stress state equations of support are derived: f f f g f r r g r fh f h r l m g l g h 21 22 3 22 22 21 2 22 22 22 2 32 3 2 + + = = = + + + − + − = µ ( ) 00         (3) solving the above equations will yield the equation for the critical support tilting angle: β α = + − arccos( cos ) 24 3 36lm h n m n (4) where m r m lr fh r h= − − −22 22 22 µ ; n gl gh= +16 92 2 ; g is the self-gravity of the support, taken as 260 kn; f is the support force imposed by the coal wall on the support; μ is the coefficient of friction between the support and the stratum, taken as 0.30; d is the length of the support beam, taken as 4.11 m; m is the distance between the tail of the support beam and the tail of the support base, taken as 1.43 m; l is the length of the support base, taken as 3.19 m; x is the distance from the center of gravity of the support to the tail of the support base, taken as l/3; r22 is the support resistance of the support (setting load or working resistance). in equation 3, underhand mining status during the nmp, the critical support tilting angle (β) is inversely proportional to the self-gravity of support (g), mining height (h) of the coalface, and dip angle (α) of the coal seam; it is directly proportional to the coefficient of friction (μ), length of the support base (l), support resistance (r22) of the support, and the support force (f) imposed by the coal wall to the support. introducing the support-related parameters of the #3up509 coalface into equation 3 and letting f=0 (i.e. the coal wall provides a small support force to the support, so its influence on support stability is neglected), it is calculated that the minimum r22 is 52.20 kn (i.e. the minimum 104 wei zhang, tianyi wang, dongsheng zhang, yandong zhang, peng xu, xu duan initial support force or the force of support displacement with pressure is not below 52.20 kn). that means the support will not undergo tilting along the strike. mechanical model of support sliding the mechanical model of support sliding during the nmp (underhand mining) is shown in figure 4. figure 4. mechanical model of support sliding along the strike during the nmp. from figure 4 the following stress state equations of support are derived: f f f g f r f r r g r fh f h r l m d 21 22 3 21 21 22 22 21 2 22 22 22 2 + + = = = = + + + − − + µ µ ( ) 22 3 2 2 02 3 21g l g h r l − − =            (5) solving the above equations will yield the critical support sliding angle: β µ µ µ α = + − − + arccos[ ( ) ( )cos ] g m m g 2 2 2 2 1 1 (6) where m f r= +2 22µ ; g is the self-gravity of the support, taken as 260 kn; f is the support force imposed by the coal wall on the support; μ is the coefficient of friction between the support and the stratum, taken as 0.3. in equation 6, underhand mining status during the nmp, the critical support sliding angle (β) is inversely proportional to the self-gravity (g) of support and dip angle (α) of the coal seam; it is directly proportional to the coefficient of friction (μ), support resistance of support (r22) and the support force (f) imposed by coal wall onto the support. introducing the supportrelated parameters of the #3up509 coalface into equation 6 and letting f=0, it is calculated that the minimum r22 is 183.75 kn (i.e. the minimum initial support force or the force of support displacement with pressure is not below 183.75 kn). that means the support will not undergo sliding along the strike. mechanical analysis of support stability along the strike during the smp the smp refers to that there are some severe situations affecting the safety production of coalface in advance, e.g. failure of the main roof caused by its weight, and coalface cutting through a fault, the coalface roof is likely to break up. due to the dip angle influence of the coalface, the breaking roof will have a downward trend. in this case, the roof will produce considerable lateral stress (f1) on the large height support beam (see figure 5), which has a severe adverse impact on support stability. moreover, due to the rib spalling influence of the large height coalface, this lateral stress (f1) will aggravate the deterioration of the relationship between the support and surrounding rock, and seriously affect the normal production of ldac. in this section, the mechanical models of support tilting and sliding have been established separately, and the support stability of ldac along the strike during the smp will be analyzed. figure 5. schematic of lateral stress acting on the support beam. the lateral stress imposed by the roof on the support should be processed as follows when performing mechanical analysis of the support stability along the strike. the overlying strata above the caving zone can form a structure. so the pressure borne by the support mainly comes from the weight of the overlying strata within the caving zone (klishin et al., 2013; miao et al., 2011; zhang et al., 2018). based on the mining conditions of the #3up509 coalface, the pressure imposed by the roof onto simple support (fr) is calculated as fr=γ×hi ×li×b=25.0×10.4×6.0×1.5=2340 kn (where γ is volume force of overburden, taken as 25 kn/m3; hi is the thickness of immediate roof, taken as 10.4 m; li is the hanging length of the immediate roof, taken as 6.0 m; and b is the support width, taken as 1.5 m). that is, the total pressure imposed by the roof to single support is 2340 kn. using the method for decomposing the support’s selfgravity in section 3, the pressure imposed by the roof is decomposed in three directions, namely, in parallel with the strike of the coalface, in parallel with the dip of the coalface, and perpendicular to the floor of the coalface. the same method of mechanical simplification is implemented to obtain the lateral stress imposed by the roof to a single support. f1=2340× 1 2 2− ⋅cos cosα β =1254 kn (7) mechanical model of support tilting the mechanical model of support tilting during the smp (underhand mining) is constructed as in figure 6. 105a mechanical analysis of support instability risk along the strike of coalface in thick coal seam with large dip angle: a case study figure 6. mechanical model of support tilting during the smp. from the perspective of the support mechanism, the tilt hydraulic support cannot normally provide support resistance to the coalface. it is inappropriate to refer to r22 as the working resistance, but the normal component of fr. then from figure 6 the following stress state equations of support are derived: f f f g f f r g r f r r l m f 21 1 3 21 2 22 21 11 22 − + = ⋅ ⋅ ⋅ = + = − + − 1 r= cos sinµ α β µ ( ) ( ff h g l g h 1 2 32 3 2 0) + − =            (8) solving the above equations will yield the critical support tilting angle: β α = arccos( cos ) 3 36 242h n m lm n (9) where m lr fh f h r m= + − −22 1 22 ; n g l h= +( )16 92 2 ; g is the self-gravity of the support, taken as 260 kn; f1 is the lateral stress imposed by the roof onto the support beam; f is the support force imposed by the coal wall on the support; μ is the coefficient of friction between the support and the stratum, taken as 0.30; d is the length of the support beam, taken as 4.11 m; m is the distance between the tail of the support beam and the tail of the support base, taken as 1.43 m; l is the length of the support base, taken as 3.19 m; x is the distance from the center of gravity of the support to the tail of the support base, taken as l/3; r22 is the normal component of fr. according to equation 9, underhand mining during the smp, the relations between the critical support tilting angle (β) and other factors are consistent with those during the nmp. however, it is directly proportional to the lateral stress (f1) imposed by the roof onto the support beam. introducing f1=1254 kn and the support-related parameters of the #3up509 coalface into equation 9 and letting f=0, it is calculated that the minimum r22 is 229.7 kn (i.e. the minimum initial support force or the force of support displacement with pressure is not below 229.7 kn). that means the support will not undergo tilting along the strike. mechanical model of support sliding the mechanical model of support sliding during the smp (underhand mining) is developed, as shown in figure 7. figure 7. mechanical model of support sliding along the strike during the smp. from figure 7 the following stress state equations of support are derived: f f f g f r r g r r l m d f h fh g l g h 21 1 3 21 21 21 2 22 22 1 2 3 2 2 3 − + = = = + − − − + + − + µ ( ) rr l21 2 0=         (10) solving the above equations will yield the critical support sliding angle: β µ µ µ α = + + − + arccos[ ( ) ( ) cos ] m g m g 2 2 2 2 1 1 (11) where m f f r= − −1 22µ ; g is the self-gravity of the support, taken as 260 kn; f1 is the lateral stress imposed by the roof onto the support beam; f is the support force imposed by the coal wall on the support; μ is the coefficient of friction between the support and rock stratum, taken as 0.30; r22 is the support resistance of the support (setting load or working resistance). according to equation 11, underhand mining during the smp, the relations between the critical support tilting angle (β) and other factors are consistent with those during the nmp. however, it is inversely proportional to the lateral stress (f1) imposed by the roof onto the support beam. introducing f1=1254 kn and the support-related parameters of the #3up509 coalface into equation 11 and letting f=0, it is calculated that the minimum r22 is 4425 kn (i.e. the minimum initial support force or the force of support displacement with pressure is not below 4425 kn). that means the support will not undergo sliding along the strike. 106 wei zhang, tianyi wang, dongsheng zhang, yandong zhang, peng xu, xu duan discussions influencing factors of support stability as analyzed above, the values of minimum support resistance (i.e. critical support resistance) necessary for maintaining support stability along the strike (no tilting or sliding) during the nmp and the smp are listed in table 1. table 1. calculation results of support stability in the #3up509 coalface mining status critical support tilting resistance /kn critical support sliding resistance /kn nmp 52.2 183.75 smp 229.7 4425 as shown by the mechanical analysis above, support stability along the strike is mainly affected by the following factors: (1) technical parameters of the support a. self-gravity of support: as the self-gravity of the support increases, the support stability in the coalface decreases. that is why large-tonnage supports are not suitable for the coalface with lda and lmh. b. height and width of the support: the height of the support is generally determined by the geological conditions of the coal seam, and the width is restricted by the underground transport conditions. therefore, if the height and width of the support are fixed, the mining height of the coalface can be decreased to enhance support stability in specific situations. c. length of support beam and length of support base: the length of the support beam and length of the support base are restricted by the equipment matching and underground mining space, so it is difficult, if not impossible, to control support stability by changing these two parameters. d. setting load and working resistance of the support: for coalface with lda and lmh, it must be ensured that the support resistance of the support has reached the required setting load and working resistance. this is crucial for controlling the strata movement of the roof and for increasing the overall stability of the support. (2) geological conditions of the coalface the geological conditions of the coalface mainly consist of the dip angle of the coal seam, the strike angle of the coalface (the angle of underhand or overhand mining), lithology of the floor and roof (which determines the coefficient of friction), and support provided by the coal wall to the holding rack. (3) special situation during the mining process the aforementioned mechanical calculation of support stability along the strike is based on the assumption that the roof is static. but for coalface with lda and lmh, roof, support, and floor may undergo different movement due to the influence from the dip angle of coal seam and strike of the coalface. such special situations, including the failure of the main roof caused by its weight, rib spalling, and coalface cutting through a fault, will produce large lateral stress on the support, thus posing a great threat to support stability. therefore, some corresponding control measures should be taken to prevent the supports tilting and sliding in field application. control measures for support stability (1) installing interlock set to fasten the support when the dip angle of the coal seam and the strike angle of the coalface increased to a certain level, the support is very likely to tilt during the support movement (singh & singh, 2012). the maximum dip angle of the #3up509 coalface is 31° with lmh (the maximum being 5.50 m), to prevent support tilting, an interlock set to fasten support is usually installed (see figure 8). to ensure sufficient support force during movement (i.e. support movement with pressure), the original method of support movement was changed. that is, the support is moved from the tail to the head of the scraper conveyor. th orientation of the support is adjusted timely using the side rack so that the verticality of the support is maintained within ±50°. the above control measures can effectively prevent support tilting and improve overall support stability. (2) installing adjustable lifting jack to the support base when the support is in normal conditions, it will not slide due to the presence of support force. when the support is displaced or the roof is suspended, one or several supports in the coalface may slide (liu et al., 2016). to prevent support sliding, an adjustable lifting jack can be installed to the support base (see figure 9). when moving the support, the support base is adjusted using the adjustable lifting jack to prevent support sliding and to increase the overall support stability. by adopting the above control measures, there is no instability accident such as tilting and sliding of the supports in #3up509 coalface (see figure 10). that is to say, the support stability has been effectively controlled, which has greatly guaranteed the safe and high-efficiency mining of the coalface, achieved good technical and economic benefits. (a) overall status (b) amplified status figure 8. interlock set to fasten support. (a) extension status (b) retraction status figure 9. adjustable lifting jack to the support base. 107a mechanical analysis of support instability risk along the strike of coalface in thick coal seam with large dip angle: a case study (a) real-time monitoring of the support state (b) viewing angle of upper end (c) viewing angle of lower end figure 10. the actual working state of the supports in #3up509 coalface. conclusions (1) mechanical calculation of support stability along the strike indicates that the critical support tilting resistance is 52.2 kn and the critical support sliding resistance is 183.75 kn during the nmp. for the smp, the critical support tilting resistance is 229.7 kn, and the critical support sliding resistance is 4425 kn. multiplying the above maximum value (4425 kn) by a safety factor of 1.3 will yield the rated working resistance of support in the #3up509 coalface, which is 6600 kn. this indicates that the support stability along the strike can meet the requirement during the mining process. (2) the influencing factors of support stability along the strike of the coalface with lda and lmh include the following: technical parameters of the support itself (self-gravity, width and height, length of support beam and length of support base, setting load and working resistance of support), geological conditions of the coalface (the dip angle of the coal seam, the strike angle of the coalface, lithology of roof and floor, and support force provided by the coal wall to the side rack), as well as special situations occurring during mining (e.g. coalface cutting across a fault). (3) during the mining of the #3up509 coalface, interlock set to fasten support is installed to the beam and the adjustable lifting jack to the support base to increase the support force and change the method of support movement. these effectively reduce the risk of support tilting and sliding in the coalface and increase the overall support stability, as well as ensure the safe and high-efficiency mining of the #3up509 coalface, and achieve good technical and economic benefits. research achievements can provide theoretical guidance and technical support to risk assessment and control of support instability in other ldacss of zaozhuang mining area. acknowledgments this research was financially supported by the fundamental research funds for the central universities (no. 2017xkqy022). we wish to thank the gcm of the zaozhuang mining area in shandong province, china, for supporting to conduct this important study. special thanks are given to mapletrans company in wuhan, china, for its professional english editing service. the authors are also grateful to the anonymous reviewers for their constructive comments and helpful suggestions. references chi, x. l., yang, k., & fu, q. 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(2018). new method and experiment for detecting relative position and posture of the hydraulic support. ieee access, 7(1), 181842-181854. keywords: deformation features; destabilized mining slope; fault-controlled conditions. palabras clave: características de deformación; pendiente minera desestabilizada; condiciones controladas por falla. how to cite item lv, y. (2020). numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions. earth sciences research journal, 24(1), 61-69. doi: https://doi.org/10.15446/esrj.v24n1.85290 this paper took the fengmaoding slope, typically found in the mountainous areas of central shanxi province, as its research area. the stratum structure of the fengmaoding slope is a typical coal measures stratum covered by a layer of loess. there are communication towers on the slope mass and top of the slope; railways, roads, and rivers at the foot of the slope. there are many villages across from the slope, therefore, the buildings and structures nearby, even the lives of the villagers are threatened by the damages caused by the deformation of the slope. tectonic development and mining activities have very profound effects on the stability of the slope. considering the above, this paper analyzed the effects of the fault in controlling the deformation features of the destabilized slope during underground mining by adopting the method of numerical simulation from these perspectives including displacement, stress, strain, among others. under different circumstances (the faulted and unfaulted) so as to provide guidance for future studies about landslides of the same type and give preventive proposals for reference. abstract numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions simulación numérica y modelo de características de deformación de la pendiente minera desestabilizada en condiciones controladas por fallas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85290 este documento tomó la pendiente de fengmaoding, que generalmente se encuentra en las áreas montañosas de la provincia central de shanxi, como su área de investigación. la estructura del estrato de la pendiente de fengmaoding es un estrato típico de medidas de carbón cubierto por una capa de loess. hay torres de comunicación en la masa de la pendiente y en la parte superior de la pendiente; ferrocarriles, carreteras y ríos al pie de la pendiente. hay muchos pueblos al otro lado de la pendiente, por lo tanto, los edificios y estructuras cercanas, e incluso la vida de los aldeanos se ve amenazada por los daños causados por la deformación de la pendiente. el desarrollo tectónico y las actividades mineras tienen efectos muy profundos en la estabilidad de la pendiente. teniendo en cuenta lo anterior, este artículo analizó los efectos de la falla en el control de las características de deformación de la pendiente desestabilizada durante la minería subterránea mediante la adopción del método de simulación numérica desde estas perspectivas, incluido el desplazamiento, el esfuerzo, la tensión, etc., en diferentes circunstancias (con fallas y sin fallas) para proporcionar orientación para los futuros estudios sobre deslizamientos de tierra del mismo tipo y dar propuestas preventivas de referencia. resumen record manuscript received: 24/07/2019 accepted for publication: 13/11/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 61-69 yiqing lv* college of mining engineering, taiyuan university of technology, taiyuan, 030024, china * corresponding author: gdlvjujian@yeah.net g eo lo g ic al e ng in ee ri ng 62 yiqing lv introduction shanxi province is known for its abundance in coal resources. with the continuous development of coal mining, the mined-out area resulting from coal mining now accounts for 98.6% of the total mined-out area in shanxi and this helps the formation of mining landslide. therefore, more and more researchers attach importance to the study of deformation features of the destabilized slope during underground mining and the relevant preventive measures so as to minimize the losses incurred by similar problems. to achieve this goal, we need to have a more precise and comprehensive understanding of the slope during underground mining. researchers have tried in many different perspectives to do an in-depth study on it, such as inducing factors, formation mechanism, deformation mechanism, stability analysis, etc. yang & huang (1999) used elastic-plastic finite element analysis to discuss the changing patterns of stress field and displacement field induced by underground coal extraction, and he put forward the geological pattern for the deformation mechanism of the slope during extraction; song & nie (2003) discussed the changing pattern of the slope by analyzing different combinations of the directions of underground mining and the structural types of the rock mass of the slope; fan (2006) made an in-depth analysis on the process of deformation damage at the mined-out side-slope and summarized the evaluation steps, key points, etc. of the loess side-slope. chen et al. (2008) used the software geo-slope in their study of the relationship between engineering geological condition and stability of the mined-out side-slope, and then put forward some measures to deal with a landslide. gao et al. (2009) used the software flac3d to evaluate the damage to side-slope and the stability of the management results to counteract it. shangguan & wang (2009) applied limit equilibrium theory and finite difference numerical simulation method to the analysis of mined-out landslide and concluded the intrinsic mechanism of it. taking the fengmaoding slope typically found in the mountainous areas of central shanxi province as its studying object, this paper adopted the method of numerical simulation to analyze the effects of the fault in controlling the deformation features of the destabilized slope during underground mining by adopting the method of numerical simulation from these perspectives including displacement, stress, strain, etc. under different circumstances (the faulted and unfaulted) so as to provide guidance for the future studies about landslides of the same type and give preventive proposals for reference. the slope deformation development characteristics after underground coal mining are given. the lithology of fengmaoding slope consists of soil and bedrock. the slope is about 140m long and 210m wide. the area is 29,400m2 and its volume is 58,8000m3. the principal axis direction of the slope is 318°, and the average slope is 50°. the front elevation of the slope is 1036m and the rear elevation is 1146m. the slope appears irregular semicircle in the plane, and a convex on the section approximately, as shown in figure 1. the field investigation suggests that the slope deformation-failure is presented as a ground fissure. 25 fissures are found, most of them are distributed on the mountaintop. usually, they are tensing fissures and strike from north-east to south-west, which is presented as a linear distribution along the mined-out area. the width of these fissures is different, which is usually about 2cm. and some fissures are buried by diluvium (the fengmaoding unstable geological body of survey report in luyukou, 2009; lv et al., 2011). engineering geological conditions in the research area geographic and geomorphic conditions located on the east side of the central part of luliang ridge, the research area is a mid-low mountainous area with loess on its top and bedrock exposed on both sides of the river valley at its foot. the north-west part of the slope is standing next to the lion riverbed. the elevation in the highest point of the fengmaoding is 1145m, while the beach of the western fenhe river is only 1030m in elevation. the maximum relative relief is about 115m. as to the maximum slope steepness, the bedrock region is 48°, the loess region is 60°and the average steepness is 35°. the erosion gullies divide the north-east part and the south-west part of the slope. the railway special line, county road and lion riverbed lie in the western part of the research area. formation lithology conditions the research area is situated in the north-west corner of xishan coalfield. the exposure strata are lower permian shanxi formation, lower permian lower shihezi formation, higher permian higher shihezi formation in permian system; neogene upper pliocene baode formation, quaternary middle pleistocene lishi formation, upper epipleistocene malan formation, and holocene stratum. the former three strata mainly exist on both sides of the branches of the lion river, while the rest mainly spread at the mountain ridges and riverbed regions (mese & tutuncu, 1997). geological structure condition located at the north-west edge of taiyuan xishan coalfield, the area is a part of graben formed by lion river-malan syncline fold belt of taiyuan xishan syncline subsidiary fold, subsidiary fold in haojiagou north syncline, jiulongta fault (f17) and hongyazi fault (f49). the anticline is located in western research area, whose west wing is the strata of shanxi formation and lower shihezi formation in the lower permian system. the stratigraphic dip generally ranges from 5°-9°, and some can reach 14°. the surface of the east wing of the anticline is largely covered by neogene and quaternary strata, underlying is the strata of shanxi formation and lower shihezi formation in the lower permian system. both wings are substantially symmetric. the axial direction of north-north-east extends close to a line, and the extended length is more than 1500m pitching toward the south. according to the surface investigation and analysis of the relevant geological data, three normal faults (f17, f19, and f20) are found in the research area whose strike is almost parallel with the lion river. these faults lean south-east with a larger dip. the fault displacements range from 5m to 40m combined into steps. f17 normal fault: the fault strikes to north-east of 30°, and leans to the south-east with an angle of inclination of 65°. the fault displacement is about 40m. f19 normal fault: the fault is about 130 m from the west side of f17 normal fault. it strikes to the north-east of 25° and leans to the south-east with an angle of inclination of 65°. its fault displacement is about 6m, and the extended length is about 900m. f20 normal fault: the fault is about 20 m from the west of f17. it strikes to the north-west of 31° and leans to the south-east with 60°. its fault throw is about 20m, and the extended length is about 350m. coal mining activities the main coal-bearing stratum in the research area distributes at the taiyuan formation in the upper carboniferous system and shanxi formation of the lower permian system. the research area mines the no.2+3 coal seam, and the working face is arranged in a longwall face along strike when mining. the strike length is 630m, and the inclined length is 120m. the caving method and high-grade conventional mining are used, and the former is applied for managing the roof. the mined-out area is right below the slope top and its area is 24,000 m2. calculation principle and the establishment of model principle when the finite element technique used by midas gts nx is applied for stability analysis, the initial stress field of the mechanical model is calculated, and then the strength reduction method is used to reduce the strength of rocksoil mass continuously in order to reach the non-convergence state. at last, the corresponding fracture point and fracture plane will be found according to the stress field distribution of the research object. the model should comply with the mohr-coulomb criterion. the formula is (irwin, 1977): f cs = + δ −δ ϕ − ϕ −2 − ϕ − ϕ1 3 1 1 1 1 sin sin sin sin 1: the maximum principal stress; 3: the minimum principal stress; c: cohesive force of the rock-soil mass; : internal friction angle. 63numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions third, considering the limits for processing capability and the memory size of computers, the unit size should fit for the calculation program to process. this model has more than 70 thousand units. whereby, the mesh division of key areas including loess on the earth surface, the f19, f20, and f17 faults, no.2-4 coal seam, mined-out area and no.8, no.9 coal seam, is shown in figure 2. analysis of numerical simulation results comparative analysis of the whole displacement vector field faulted condition: the x directions of the foot in front of the slope and the f20 fault footwall (railway and road included) are mainly negative, manifested as the surface is extruding by displacing towards free face and the slope is deforming outward (see figure 3); the x directions of f20 fault hanging wall and f17 fault footwall are mainly positive, showing that rocksoil mass is displacing towards mined-out areas and the slope is deforming inward as a result of the mining of coal seam (see figure 3-a). the complex establishing a finite element model establishing the model with a width of 400m (in x direction), a length of 470m (in y direction), a base elevation of 850m at the lower boundary, and the maximum elevation of about 1170m from upper boundary to the surface by using midas/gts numerical simulation software based on the actual slope topography. and two kinds of geologic models (the faulted and unfaulted) are established to analyze the effect of the fault on the slope body, as shown in figure 1. the division of mesh and the establishment of a three-dimensional finite element mesh calculation model for the unstable slope of fengmaoding are based on the 3 principles. first, it should be able to reflect the geological features of unstable slopes and the underlying mined-out area of fengmaoding. the unit block in the fault area, the mined-out area, and the surface, etc. should be subdivided to ensure the calculation model’s ability in showing the true geometrical features of the rock mass. then ensure the consistency of the calculation results for stress, displacement, during the numerical calculation. figure 1 engineering geological model of unstable slope in fengmaoding figure 2. mesh plot of unstable slope in fengmaoding (a) unfaulted unstable slope (b) faulted unstable slope (a) unfaulted unstable slope (b) faulted unstable slope 64 yiqing lv y displacement vector field is controlled by the subsidence of mountain mass towards mined-out areas, showing that the rock-soil mass on both sides of the mined-out areas is displacing towards mined-out areas (see figure 3-b). the displacement vector field in the z direction is relatively simple, mainly showing a feature of subsidence, and the largest subsidence displacement value of the mountaintop is in the z direction as a result of the mining of coal seam. note that there are red arrows on some parts of the mountainside over the railway, which indicate that the displacement field is uplifting. and it is consistent with the actual deformation features of displacement (see figure 3-c). unfaulted condition: the surface of the mountain slope is displacing horizontally in the positive x direction which is affected by mining subsidence, and the magnitude of displacement value in the direction deviating from the railway at top of the slope is about 0.03m, which is a large value. while the magnitude of displacement value at the mountainside and at the foot of the mountain is about 0.01m. and the magnitude of displacement value at the foot of the mountain is about 0.006m, which is smaller than the magnitude of displacement value at the mountainside. the soil mass of the steep local part of the slope shoulder is displacing in a negative direction in response to its own gravity, i.e. the soil mass is displacing toward the free face of the slope mass, and the magnitude of displacement value is about -0.01m. from the simulation calculation result, we can see that the existence of fault has a significant impact on the displacement direction in the x direction of surface and the magnitude of displacement value (see figure 4-a). the whole magnitude of displacement value in the x direction of the surface is relatively small. the magnitude of displacement value at road and railway is about 0.001˜0.005m, the magnitude of displacement value at the mountaintop is about 0.01˜0.03m. the positive displacement in the y direction is predominant; however, the displacement in some local areas is negative under the control of the topographic condition. the existence of fault has little impact on the displacement direction in the y direction of the surface and the magnitude of displacement value based on the simulation calculation result (see figure 4-b). the displacement in the z direction of the surface is mainly subsided. the magnitude of subsidence displacement value at the top of the slope is about -0.2m, which is large. the vertical subsidence displacement value at mountainside and foot is relatively small, whose magnitude of subsidence displacement value is about -0.1m. and the magnitude of subsiden ce displacement value at road and railway is about -0.001m˜-0.002m, which is small. the existence of fault under the control of the fault plane has a significant effect on the magnitude of subsidence displacement value in the z direction of the surface judging from the calculation result (see figure 4-c). figure 3. faulted surface displacement (unit: m) figure 4. displacement of unfaulted surface (unit: m) (a) x direction (b)y direction (c) z direction (a) x direction (b)y direction (c) z direction 65numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions analysis of deformation of different sections in the y direction existence of fault: cutting the section in different positions in y direction (y=10m, 50m, 100m, 150m, 200m, 250m, 300m, 350m, 400m, 450m, 500m, 550m), as shown in figure 5. the x displacements of different sections are shown in figure 6 and the z displacements are shown in figure 7. as shown in figure 7, the displacement varies in different positions because of the mined-out area resulting from the excavation of coal seam. as shown in section y = 10m, the relatively uniform x displacement map indicates the displacement changes little. with the excavation of coal seam, as shown in section y = 100m, y = 150m, the f20 fault hanging wall in the position near the mined-out area begins to make a large positive displacement in x direction (the yellow); the f17 fault footwall is making a large negative displacement in x direction toward mined-out area (the light green). in the maximum coal-mining position, as shown in section y = 350m, y = 300m, the largest displacements are the positive displacement in the x direction along the f20 fault in the f20 fault hanging wall (the red), and the negative displacement toward the mined-out area in f17 fault footwall (the blue). meanwhile, the rock mass in the f20 fault footwall is making a relatively large negative displacement, shown as the displacement toward railway and road. (j) y = 450m figure 6. x displacement of different sections in y direction figure 5. different section positions in the y direction (a) y = 10m (b) y = 50m (c) y = 100m (d) y = 150m (e) y = 200m (f) y = 250m (g) y = 300m (h) y = 350m (i) y = 400m 66 yiqing lv though coal seam will not exist in the north of the mined-out area, there is still displacement in x direction affected by the mined-out area, such as the section y=450m. the displacement maps of different sections in x-direction indicate that big displacements in x direction basically occur on both sides of the f20 fault, with the hanging wall of f20 having displacement in positive direction (red) and the footwall having displacement in negative direction (green), showing that f20 is an important controlling factor to prevent the mountain from extruding to the free face. as shown in figure 7, mined-out areas are formed due to the coal mining, and the displacements are different due to different locations of the mined-out areas. the displacement maps of the sections y=10m and y=450m on z direction are uniform, which means the changes of their displacements are small. for the parts near the mined-out area, such as section y=100m and section y=150m, the roof of the mined void has negative-direction displacement in the z direction (green), with a manifestation of sinking, the base of the minedout area has positive-direction displacement in the z direction (red), with the manifestation of swelling. for the biggest coal mining area, such as the sections y=350m and y=300m, the roof and the base of the mined void have the biggest displacement in the z direction, with the roof sinking up to 1.44m and the base swelling up to 0.44m. it indicates that all the big displacement changes occur in the hanging wall of f20, and the displacement of the rock mass is small in the footwall of the f20 fault (yellow), so f20 is an important controlling factor to prevent the mountain body from having subsidence displacement. unfaulted condition: cutting the section in different positions in y direction(y=10m, 50m, 100m, 150m, 200m, 250m, 300m, 350m, 400m, 450m, 500m, 550m), as shown in figure 8. displacements of different sections are generated in x direction (see figure 9) and z direction (see figure 10) respectively. (j)y = 450m figure 7. z direction displacement of different sections along y-direction (unit: m) (a) y = 10m (c)y = 100m(b)y=50m (d)y = 150m (e)y = 200m (f)y = 250m (g)y = 300m (h)y = 350m (i)y = 400m 67numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions figure 8. different sections positions in the y direction. as shown in figure 9, y=10m in the x direction is uniform, which means the displacement change is small. the part near the mined-out area forms the mined-out area, y=100m, and y=150m, the positive direction displacement happens. comparing to the faulted condition, the displacement happens only affected by the minedout area. at the center of the mined-out area, such as the sections y=350m and y=300m, the positive direction displacement in the x direction is the biggest. as section y=450m, the farther the distance to the center of the mined-out area, the smaller the displacement in the x-direction. it’s showing that the fault is an important controlling factor to prevent the mountain from extruding to the free face. as shown in figure 10, in the z direction, the roof of the mined void has negative-direction displacement, with the manifestation of sinking, the base of the mined void has positive-direction displacement, with the manifestation of swelling. y=350m and y=300m, comparing to the faulted condition, the fault controls the displacement border of the east side and the west side. (j)y = 450m figure 9. x displacement of different sections in y direction (unit: m) (a) y = 10m (c)y = 100m(b)y=50m (d)y = 150m (e)y = 200m (f)y = 250m (g)y = 300m (h)y = 350m (i)y = 400m 68 yiqing lv conclusion from the numerical simulation results, the characteristics of earth surface displacement in the mountaintop are mainly sinking, in the mountainside and at the foot of the mountain are mainly extruding outside dislocation and swelling deformation, in accordance with deformation characteristics on the actual earth surface. at the mountainside and the foot of the mountain to the west of the f20 fault, including the railway and road, the slope deformation is mainly the displacement in the x direction, with the manifestation of extruding outwards the free face and outward deformation of the slop, i.e. the horizontal deformation – swelling belt. at the mountainside and the mountaintop to the east of the f20 fault, the deformation of the slope is mainly downward displacement in z direction, and the displacement of the rock-soil mass on the mountaintop is towards the mined-out area, with strong sinking deformation; the horizontal displacement characteristic is displacement in reverse direction to the free face and inward deformation of the slope, i.e. the horizontal deformation – sinking belt. regarding the fengmaoding slope, geological and numerical models of underground mining condition are established, and the destruction and deformation processes of the slope with and without fault are given through simulation, by which we can draw the opinion that: under the condition of underground mining, the slope destruction deformation stress and strain distribution range and degree without fault are smaller than that with fault, i.e. underground mined-out area of a certain size is the main factor affecting slope stability, if fault exists in the slope, the destruction deformation of that unstable slope will further increase. acknowledgment basic research plan projects of shanxi science and technology department (no.201701d121015). references chen, s., yang, t., & zhang, h. (2008). ping shuo open-pit slope stability under the condition of underground mining. journal of coal, 11, 148-152. fan, s. (2006). the stability problem of slope on the goaf. journal of resources and environment and engineering, s1, 617-627. gao, l., kou, j., & jiawei, f. (2009). discrete element numerical simulation application in highway tunnel surrounding rock deformation and failure mechanism study. journal of xiangfan college, 11, 41 46. (j)y = 450m figure 10. displacement of different sections in z direction along y-direction (unit: m) (a) y = 10m (c)y = 100m(b)y=50m (d)y = 150m (e)y = 200m (f)y = 250m (g)y = 300m (h)y = 350m (i)y = 400m (d)y = 150m (e)y = 200m (f)y = 250m 69numerical simulation and model of deformation features of destabilized mining slope under fault-controlled conditions irwin, r. w. (1977). subsidence of cultivated organic soil in ontario. asce journal of the irrigation and drainage division, 103(2), 197-205. lv, y., liu, h., & yang, j. (2011). study of failure mechanism of anti-dip rocks materials slope undermining subsidence. advanced materials research, v143-144, 1097-1101. mese, a., & tutuncu, a. n. (1997). an investigation of mechanical, acoustic and failure properties in unconsolidated sands. international journal of rock and mining sciences, 314, 467. shangguan, k. & wang, g. (2009). kiln street mine mining influence mechanism of landslides study. journal of coal science and technology, 42 -45. song, y., & nie, d. (2003). mining slope deformation failure mode analysis and prediction. journal of disaster science, 11, 34-39. the fengmaoding unstable geological body of survey report in luyukou. (2009.) yang, z, & huang, k. (1999). mechanism of slope deformation induced by underground mining. journal of xi ‘an institute of mining, 11, 10-14. http://www.engineeringvillage.com/controller/servlet/controller?cid=quicksearchcitationformat&searchword1=%7byang%2c+jianjun%7d&section1=au&database=1&yearselect=yearrange&sort=yr keywords: gps; lava area; geology; exploration; remote; monitoring. palabras clave: gps; área de lava; geología; exploración; teledetección; sondeo. how to cite item xu, a. (2020). remote monitoring system of geological exploration in lava area based on gps. earth sciences research journal, 24(1), 207-214. doi: https://doi.org/10.15446/esrj. v24n2.87956 a remote monitoring system for geological exploration in a lava area based on gps is designed. the system mainly collects geological exploration data in a lava area using multi-s integration through the data acquisition module of geological exploration. the data collected is then transmitted for analysis to the data analysis and management module of geological exploration data in a lava area using a gps network. the monitoring data processing method based on the unascertained filtering method is used to remove gross errors in geological exploration, identify abnormal values, and transmit the abnormal values to the gps-based geological anomaly location module. the gps-based geological anomaly location module locates the geological anomaly according to the abnormal values, thus completing the remote monitoring of the lava area's exploration. the test results show that when the system is applied to remote monitoring of geological exploration in a lava area, the positioning error of small soil caves is low, and the reliability of monitoring data is high, which meets the needs of remote monitoring of geological exploration in a lava area. abstract remote monitoring system of geological exploration in lava area based on gps sistema de monitoreo remoto de exploración geológica en el área de lava basado en gps issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87956 en este artículo se diseña un sistema de monitoreo remoto para exploración geológica basado en gps en una área de lava. el sistema recopila principalmente datos mediante la integración multi-s a través del módulo de adquisición de datos de exploración geológica. luego, los datos recopilados se transmiten al módulo de análisis y gestión de datos de exploración geológica en el área de lava a través de una red gps. el método de procesamiento de datos de monitoreo basado en un método de filtrado no determinado se usa para eliminar errores graves en los datos de exploración geológica del área de lava, identificar valores anormales y transmitir los valores anormales al módulo de ubicación de anomalías geológicas basado en gps. el módulo de localización de anomalías geológicas basado en gps ubica la anomalía geológica de acuerdo con los valores anormales, completando así el monitoreo remoto de la exploración del área de lava. los resultados de la prueba muestran que cuando el sistema se aplica al monitoreo remoto de la exploración geológica en un área de lava, el error de posicionamiento de las pequeñas cuevas del suelo es pequeño y la confiabilidad de los datos de monitoreo es alta, lo que satisface las necesidades del monitoreo remoto de la exploración geológica en una zona de lava. resumen record manuscript received: 12/03/2019 accepted for publication: 17/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 207-214 aimei xu school of civil engineering and architecture, xinyu university, xinyu, china gp s m ea su re m en ts a pp li ed to g eo lo gy https://doi.org/10.15446/esrj.v24n2.87956 https://doi.org/10.15446/esrj.v24n2.87956 https://doi.org/10.15446/esrj.v24n2.87956 208 aimei xu introduction lava landform is also called karst landform. karst geomorphology refers to all kinds of landforms formed by dissolution, transportation, and sedimentation of soluble rocks (limestone) by water, such as stone buds, stone ditches, stone forests, peak forests, falling water caves, funnels, karst depressions, karst caves, underground rivers, etc. (masiokas et al., 2017). in the karst geomorphological development area, the ground is often crowded with odd peaks, the surface water system is relatively scarce, but the groundwater system is relatively developed (jian et al., 2017). guangxi, guizhou, yunnan, and other places in china widely distribute karst landforms, which is one of the most typical karst landforms in the world (zhu, 2018). engineering geological survey, geophysical prospecting, and drilling are mainly used in the geological exploration of lava area, in order to obtain the relevant information of lava landform, development degree, development law, hazard nature and geological activity law of highway section (gu et al., 2017). the first is the lava survey of the highway subgrade section. when the lava geomorphology is located on both sides of the highway subgrade, through the investigation and information collection of the direction of surface water runoff, the estimation of water volume, the development degree of surface karst groove and the scale of karst depression in a certain area on both sides of the subgrade, the development degree of lava in this area is determined, and then targeted drilling is arranged to reveal the buried depth of karst cave through drilling. the safety thickness of the lava roof can be obtained by the roof and floor thickness and filling species, which can provide corresponding excavation and filling parameters for highway subgrade construction. next is the lava survey in the bridge area (zhao et al., 2017). when the bridge crosses the lava area, through the geological survey in a certain range of the bridge site area, the data of surface water runoff direction, water flow, the development degree of surface dissolution ditches, the scale of dissolution depression, the development degree of bank slope dissolution on both sides and underground rivers are obtained. according to the characteristics, the related data such as drilling hole encounter rate, the vertical height of karst cave and depth of roof and floor are further obtained to determine the development degree of lava in the bridge site area and to provide the basis for the elevation design of bearing stratum of pile foundation (wu et al., 2018). finally, it is the tunnel lava survey. generally speaking, it is more difficult to survey tunnels in the lava area, and the safety problems in later construction and operation are more prominent. therefore, the investigation of tunnels in the lava area, especially long tunnels, is the focus of tunnel lava exploration (liu, 2017). firstly, engineering geology survey and hydrogeology survey are used to make a detailed investigation of surface water runoff direction, spring outcrop, surface dissolution depression and falling tunnel in a certain range of tunnel site area to draw engineering geology map and hydrogeology map and to preliminarily determine the relationship between surface water and groundwater, runoff model and fault structure (kugler, 2017). then geophysical exploration and drilling methods are used to further determine the relationship among the underground lava pipeline, underground river, karst cave, and the design cavern, and the connectivity and water flow of the underground river of lava pipeline are determined by means of connection test analysis. the surrounding rock of the cavern and the water inflow after excavation are accurately classified to provide reliable geological parameters for the design (wang et al., 2018). monitoring is widely used in a safe city, railway monitoring, bank monitoring, warehouse monitoring, construction site, and other occasions because of its intuitive, accurate, timely, and rich information content (khan et al., 2017). remote monitoring and control of geological exploration refer to the exploration of the geology of lava area by various means and methods, to determine appropriate bearing strata, identify geological details and the technical conditions for mining and utilization, provide the provision of geological information needed for highway construction and design, and investigate and study geological conditions such as the rocks, structures, minerals and strata in a certain area, so as to provide favorable data support for geological research in lava areas (zhao & miao, 2018). this paper designs a remote monitoring system for geological exploration in the lava area based on gps in order to contribute to construction data and exploration data in the lava area. materials and methods remote monitoring system of geological exploration in lava area based on gps the architecture of system hardware geological exploration in lava areas requires high equipment requirements. all remote monitoring equipment selection has been certified by the security department (riordan et al., 2017). the three core function modules of a remote monitoring system for geological exploration in lava area based on gps are the data acquisition module of geological exploration in lava area, the data analysis and management module of geological exploration in lava area, and the location module of geological anomaly based on gps (he & yan, 2017). the data acquisition module of geological exploration in the lava area is the monitoring end of the system. it transmits the collected geological data of the lava area to the data analysis and management module of geological exploration in the lava area using a gps network. after filtering and analyzing the acquired data, if there are abnormal values in exploration data, it transmits positioning instructions to the geology based on gps. the anomaly location module carries out positioning feedback to users (liu et al., 2017). the overall structure of the remote monitoring system for geological exploration in the lava area based on gps is shown in figure 1. (1) data acquisition module for geological exploration in the lava area the main task of the data acquisition module of geological exploration in lava area is to combine the old three pieces (geological hammer, compass, magnifier) and the new three pieces (notebook, digital camera, gps) to achieve multi-s integration and to collect formation information, rock information and structural information of lava area (geng et al., 2017). the structure of the data acquisition module for geological exploration in the lava area is shown in figure 2. (2) data analysis and management module of geological exploration in the lava area the data analysis and management module of geological exploration in the lava area is responsible for receiving, analyzing, and managing the geological data of the lava area collected by the data acquisition module of geological exploration in the lava area. after receiving the geological data collected by the data acquisition module of geological exploration in the lava area, the module filters the geological data in the lava area using the method of monitoring data processing based on unascertained filtering. according to the data, it judges whether there are abnormal values in geological exploration in the lava area. if there are abnormal values, it transmits positioning instructions figure 1. structural diagram of the remote monitoring system for geological exploration in the lava area based on gps 209remote monitoring system of geological exploration in lava area based on gps a. spatial part the space part of gps is composed of 24 gps working satellites, which together constitute the satellite constellation. among them, 21 are localizable satellites and 3 are standby microsatellites. the 24 satellites are evenly distributed in six orbital planes about 20183 km from the ground. the angle of intersection between each orbital plane and the equatorial plane is 55°, and the distance between the orbital planes is 60° (i.e. the ascending point of the orbits is 60° different from each other). there are four satellites in each orbit. each gps working satellite sends out signals for locating abnormal materials in the geology of lava areas. b. ground control part the ground control part is composed of a monitoring system consisting of several tracking stations distributed around the world. according to their different functions, these tracking stations are divided into main control stations, monitoring stations, and injection stations. c. user equipment section the user equipment is composed of gps receiver, data processing software, and corresponding user equipment. its function is to receive gps satellite signals to obtain necessary positioning information. monitoring data processing method based on unascertained filtering during the operation of the remote monitoring system for geological exploration in the lava area based on gps, there are more or fewer errors in monitoring data due to the influence of monitoring instrument accuracy and observation conditions. when analyzing the monitoring data, it will be found that there will be jumps in the monitoring data and local jumps in the timedisplacement curve drawn. on this basis, the analysis and survey of geological stability in the lava area will encounter difficulties in data screening. therefore, before analyzing the monitoring information, it is better to do filtering to minimize the systematic error, random error, and gross error in the monitoring data, so that the monitoring data can better reflect the geological conditions of lava areas. in the monitoring of geological exploration in the lava area, abnormal data processing is very important. there are generally two reasons for abnormal monitoring data: errors caused by monitoring, called gross errors; abnormal monitoring data caused by geological abnormalities, called abnormal values. in data processing, gross errors should be eliminated, abnormal values should be processed and analyzed, and effective measures should be taken to deal with the geological abnormal part of the lava area (hu et al., 2017). to the geological abnormal situation positioning module based on gps (kostopoulou et al., 2017). the structure of the data analysis and management module for geological exploration in the lava area is shown in figure 3. figure 3. structure diagram of data analysis and management module for geological exploration in the lava area (3) geological anomaly location module based on gps a geological anomaly location module based on gps consists of a space part, a ground control part, and a user part (alison et al., 2017). details are shown in figure 4. figure 2. structure diagram of geological data acquisition module in the lava area figure 4. gps based geological anomaly location module structure 210 aimei xu based on the modern error theory, this paper uses the unascertained filtering method to judge the gross errors and outliers of monitoring data, to effectively eliminate gross errors and identify abnormal values (song et al., 2017). there are essential differences between gross errors and abnormal values. the greatest difference is that gross errors are contingent, and the frequency of occurrence is not high. compared with the monitoring data of this group, gross errors are usually single and have a certain contingency. the abnormal values are continuous and accumulative after the catastrophic data are generated, which is essentially due to the geological anomalies in lava areas. in this paper, we use unknown rational numbers to distinguish gross errors from abnormal values (amiri-simkooei et al., 2017). taking the monitoring data sequence v v v vn1 2 3, , , ,… as an example, an unascertained rational number is defined. a v v f x=   ( ){ }min ,max ,1 1 (1) where, 1 1≤ ≤ ( )v n f x, , , f (x) is the distribution density function of the reliability of remote monitoring data for geological exploration in the lava area. the main purpose of processing monitoring data is to identify and screen anomalous data, eliminate the gross errors caused by various factors, and reduce their impact on data analysis. through the analysis of gross error and abnormal value of monitoring data, it is found that if the value of gross error appears in the monitoring sequence, the data must exist in isolation, and there is only one [a, b] in the remote monitoring data interval of geological exploration in lava area; if the value is abnormal value, the adjacent monitoring sequence has continuity. over time, this data appears more and more frequently. therefore, the combination of monitoring data and its reliability density function f (x) can effectively eliminate gross errors, identify outliers, and reduce its impact on monitoring data analysis. if the trend of monitoring data sequence changes too much, it will affect li number of neighborhoods of monitoring sequence, thus affecting the judgment of gross errors. in order to reduce the impact of sequence trend change on vi , n detection values of monitoring data can be divided into several data segments, each data segment has k + 1 detection value, k< n, then: when x = vi , f x i i i n( ) = ∑   (2) when x vi≠ , f x( ) = 0 (3) where, x represents the monitoring value in remote monitoring data interval [a, b] of geological exploration in the lava area, and i represents the number of (i  j) in vi field. the size of the credibility density function determines the probability of vi as the likelihood of gross error, the greater the reliability of vi is, the smaller the probability of gross error is; and the smaller the reliability of vi is, the greater the probability of gross error is. then the gross error is determined according to the value of f (x). after determining the gross error in the monitoring sequence, the gross error needs to be eliminated. in order to eliminate the gross error, the unascertained expectation of vi and its monitored values in the neighborhood is used as the final result of the monitoring sequence. it is not known that the expected e (a) calculation method is: e a l f v v i i l l k i l l k( ) = ( ) + + ∑ ∑ (4) where, l means that in the original monitoring data, the value of monitoring data starts from l, and the length of the data segment is k+1. l value varies with the location of vi in the selected monitoring data. let m be a definite even number. the selection of monitoring data is as follows: when i = 1, the first and second monitoring data are taken as the search area of i in the data sequence segment where vi is located. when formula (2) and formula (3) are used to calculate, k of confidence density function f (x) is taken as 1, and l = 1 of unascertained expectation is calculated by the formula (4). when 1 2  i m≤ , the values of i m  2 to i m+ 2 in the monitoring data are taken as the sequence search area, and then k = m, l i m=  2 . when n m i n− < 2 ≤ , the values from 2i ‒ n to n-th in the monitoring data are taken as the sequence search area, where k = 2 (n ‒ i), l l m=  2 . when i = n, the n-1 and n values in the monitoring data are taken as the search area of the sequence, and then k = 1 and l = n ‒ 1 are taken. gross error recognition depends on the reliability of monitoring data. the reliability depends on the number of monitoring data in the vi neighborhood, and the number of monitoring data depends on the size of  in the neighborhood radius. the value of  is determined according to the actual situation in the field. when the average difference between two adjacent monitoring values is large, the value of  is larger. when the average difference between two adjacent monitoring values is small, the value of  is smaller. in practical engineering, if the value of  is too large, the ability to identify gross errors will decrease; if the value of  is too small, the normal data may be misidentified as gross errors, resulting in increased workload and inaccurate calculation. therefore, the reasonableness of  value will directly affect the effectiveness of data processing by filtering method (yu & lu, 2017). for the value of , it is usually determined by the method of calculating the difference variance of the monitoring data sequence. with the help of the criterion of double or triple median error in gross error identification, the difference variance of double or triple is obtained. considering that  is bigger in the monitoring data sequence with gross errors,  is taken as doubledifference variance in the processing of remote monitoring data of geological exploration in the lava area. λ − − − = ( )+∑ 2 1 1 2 1 1 v v n i i n (5) the upper and lower limits of the neighborhood of the selected monitoring sequence are determined according to the neighborhood radius calculated by the formula. neighborhood upper limit = monitoring value + r (6) neighborhood lower limit = monitoringvalue – r (7) where  is the threshold parameter of gross error recognition. in the process of gross error identification, gross errors need to be eliminated and the remaining monitoring data need to re-calculate  until there is no gross error (li et al., 2017). because the value of k depends on the selection of m, m generally chooses all even numbers from 2 to n/2 and calculates the value of r iteratively. the value that makes  converge sufficiently and can identify gross errors is taken as the selection value of m. in concrete calculation, if m is too small and there are continuous gross errors, there will be zero gross errors, and the total reliability of the data segment is zero. at this time, e (a) cannot replace gross errors. we should select a larger data segment to obtain the unascertained expected value, or select more data to expand the neighborhood radius. for the monitoring data sequence with a small change range, m usually chooses a larger value to get a better result. if the data segment is wider, the ability of gross error recognition is stronger; for the monitoring data sequence with the large change, if m is selected, it is difficult to obtain satisfactory results. the reason is that the detection data sequence has volatility, which will result in the occlusion of gross error recognition, thus affecting the accuracy of data processing results (wang et al., 2017). 211remote monitoring system of geological exploration in lava area based on gps test settings the strata in the test area are mainly limestone of the tianziling formation in the devonian. in the detailed exploration stage of engineering geology in this area, besides ascertaining the stratum structure and geophysical and mechanical properties (gao et al., 2013; liu et al., 2014), the emphasis is on ascertaining the distribution, shape, size, scale of lava and the associated soil caves and ground subsidence in the overlying strata. this system is used to monitor the process of engineering geological exploration in this area. (1) location function test in the lava monitoring area used in the experiment, 12 small caves are set up. the system is used for remote monitoring of exploration, and the positioning function of the system is tested (hendrikx & gestel 2017). (2) reliability testing of monitoring data combined with the distribution of small soil caves in the experimental area, the monitoring data of 23 m and 3.2 m geological survey points in the lava area are extracted as the analysis objects. 23 m: a total of 22 monitoring data from may 2019 to july 2019 are selected for analysis. when the serial number of the monitoring data is greater than 3, the monitoring data is taken as the center and three values are taken forward and backward as their neighborhoods to calculate the reliability of the monitoring data in their neighborhoods (liu et al., 2018). 3.2 m: through collating and analyzing the monitoring data of this system, 22 monitoring data from august 29, 2019, to august 12, 2019, are selected for analysis. results test results of location function in this paper, the location function test results of 12 small soil caves in the geology of lava area are shown in table 1. the statistical results of the positioning errors of the system in this paper are shown in table 2. according to table 1 and table 2, we can see that the maximum error is 0.3 when the system locates 12 small soil caves, and the error is small. the reliability test results of monitoring data of the systemin this paper (1) reliability test results of monitoring data at 23m survey point the calculation process is shown in table 3, where i represents the number of monitoring data in the neighborhood. in this paper, the difference variance of r is  times. the reliability of the monitoring data of the system is tested several times by setting different monitoring dates. according to table 3, the reliability of monitoring data before may 28, 2019 is 1, and the reliability of monitoring data is high. during the period from may 28, 2019 to june 2, 2019, the reliability of monitoring data fluctuates to a certain extent, but it remains above 0.8, which can be considered that the monitoring data is basically in the normal range. from june 8, 2019 to june 23, the reliability of monitoring data is maintained above 0.8, and the reliability is basically stable. the reliability of monitoring data from june 28, 2019 is 0.81, and that of monitoring data from june 30, 2019 to july 2, 2019 is 0.83, which basically meet the requirements of the project. after july 2, 2019, the reliability of monitoring data has remained basically stable. according to the reliability analysis of the monitoring data of the system in this paper, we can see that most of the monitoring data have high reliability, which is basically maintained above 0.8. the reliability of the monitoring data in june-4, 2019 and june-28, 2019 are both 0.81, which indicates that the monitoring data in these two periods are encouraging, and the credibility is the lowest compared with other monitoring data. on the whole, the reliability of the monitoring data in this system is better, which meets the remote monitoring standard of geological exploration in lava area. (2) reliability test results of monitoring data at 3.2m survey point combined with the distribution of small soil caves in the experimental area, the monitoring data of 3.2m geological survey points in the lava area are table 1. the test results of the system positioning function are presented in this paper order number actual point coordinates the results of system monitoring in this paper x / cm y / cm h / cm x y h x y h 1 3824861.671 636956.611 286.178 3824861.681 636956.621 286.183 0.1 -0.2 -0.1 2 3824879.657 637304.244 331.949 3824879.649 637304.238 331.956 0.1 0.1 -0.1 3 3824940.057 637351.918 330.205 3824940.051 637351.912 330.214 0.1 0.1 -0.2 4 3824936.230 637412.885 335.384 3824936.225 637412.879 335.378 0.1 0.1 0.2 5 3824844.611 637524.535 324.801 3824844.618 637524.541 324.811 -0.1 -0.2 -0.2 6 3824926.865 637556.532 330.699 3824926.885 637556.542 330.685 -0.2 -0.1 0.4 7 3825026.065 637637.839 333.185 3825026.043 637637.821 333.171 0.2 0.1 0.4 8 3825145.265 636400.321 239.529 3825145.274 636400.328 239.533 -0.1 -0.1 -0.1 9 3825221.939 636475.104 248.448 3825221.945 636475.111 248.454 -0.6 -0.2 -0.1 10 3824771.613 638169.588 333.643 3824771.633 638169.598 333.625 -0.3 -0.2 0.2 11 3824874.477 637735.863 335.710 3824874.492 637735.882 335.723 -0.5 -0.1 -0.3 12 3824770.723 637906.581 336.103 3824770.729 637906.591 336.113 -0.2 -0.5 -0.2 table 2. systematic positioning errors in this paper roll call 1 2 3 4 5 6 7 8 9 10 11 12 positioning error 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.1 0.3 0.2 0.3 0.3 note: x and y are coordinate values of soil cave and h is height value. x, y and h are positioning differences. 212 aimei xu extracted as the analysis object. through collating and analyzing the monitoring data of this system, 22 monitoring data are selected from august 29, 2019 to july 2, 2019. details are shown in table 4. according to table 4, the reliability curve of monitoring data of 3.2m survey point is drawn, as shown in figure 5. among them, the abscissa represents the monitoring day, the ordinate represents the credibility data, and the maximum value is 1. during the period of august 29, 2018 to dec. 29, 2018, the credibility of monitoring data is 1, and the credibility of monitoring data is high in this period; during the period of january 1, 2019 to february 25, 2019, the credibility of monitoring data continues to decrease, to a minimum of 0.81; during the period of march 14, 2019 to may 7, 2019, the credibility of monitoring data continues to rise, to a maximum of 1; after may 7, 2019, the credibility of monitoring data remains stable, at this time, the credibility of monitoring data is high. discussions in this paper, the application of gps in remote monitoring of geological exploration in lava area is discussed. gps has the following advantages: (1) high accuracy in the past engineering surveying and mapping work in china, the relevant workers usually use traditional surveying and mapping technology, but the traditional surveying and mapping technology is gradually replaced by gps engineering surveying and mapping technology because of its shortcomings table 3. reliability calculation table of monitoring data at 23m survey point monitoring time displacement value neighborhood lower limit neighborhood upper bound i reliability 2019-5-21 1222.4 1218.0 1226.8 3 1 2019-5-25 1220.6 1216.2 1225.0 4 1 2019-5-28 1218.9 1214.5 1223.3 4 0.86 2019-5-30 1216.2 1211.8 1220.6 3 0.86 2019-6-2 1217.1 1212.7 1221.5 4 0.87 2019-6-4 1199.8 1195.4 1204.2 0 0.81 2019-6-8 1216.2 1211.8 1220.6 4 0.87 2019-6-10 1217.1 1212.7 1221.5 5 0.83 2019-6-13 1218.0 1213.6 1222.4 5 0.83 2019-6-16 1221.1 1216.7 1225.5 5 0.83 2019-6-20 1218.2 1213.8 1222.6 5 0.83 2019-6-23 1217.1 1212.7 1221.5 5 0.83 2019-6-28 1213.1 1208.7 1217.5 2 0.81 2019-6-30 1218.3 1213.9 1222.7 5 0.83 2019-7-2 1218.0 1213.6 1222.4 5 0.83 2019-7-4 1217.4 1213.0 1221.8 6 1 2019-7-6 1218.8 1214.4 1223.2 6 1 2019-7-9 1217.1 1212.7 1221.5 6 1 2019-7-13 1216.2 1211.8 1220.6 6 1 2019-7-23 1215.3 1210.9 1219.7 5 1 2019-7-26 1217.1 1212.7 1221.5 4 1 2019-7-31 1216.2 1211.8 1220.6 3 1 table 4. reliability calculation table of monitoring data at 3.2 m survey point monitoring time displacement value neighborhood lower limit neighborhood upper bound i reliability 2018-8-29 37.0 31.4 42.6 3 1 2018-9-3 35.7 30.1 41.3 4 1 2018-9-18 36.5 30.9 42.1 5 1 2018-9-21 36.4 30.8 42.0 6 1 2018-9-29 36.5 30.9 42.1 6 1 2018-10-29 36.7 31.1 42.3 6 1 2018-11-29 38.5 32.9 44.1 6 1 2018-12-15 38.4 32.8 44.0 6 1 2018-12-29 40.9 35.3 46.5 6 1 2019-1-6 40.2 34.6 45.8 5 0.83 2019-1-23 40.9 35.3 46.5 4 0.87 2019-2-25 40.1 34.5 45.7 3 0.81 2019-3-14 51.9 46.3 57.5 3 0.83 2019-4-10 52.1 46.5 57.7 4 0.83 2019-5-1 53.0 47.4 58.6 5 0.83 2019-5-7 53.3 47.7 58.9 6 0.83 2019-5-14 53.6 48.0 59.2 6 1 2019-5-21 55.8 50.2 61.4 6 1 2019-6-12 56.0 50.4 61.6 6 1 2019-7-12 56.4 50.8 62.0 5 1 2019-8-12 57.7 52.1 63.3 4 1 2019-9-17 59.3 53.7 64.9 3 1 figure 5. reliability curve of 3.2m monitoring data at survey point 213remote monitoring system of geological exploration in lava area based on gps in surveying accuracy and material consumption. for gps engineering survey technology, it has the perfect advantage of high accuracy. gps engineering surveying and mapping technology itself has the function of static surveying, and this function of static surveying can make the accuracy of surveying and mapping results to millimeter units, which is incomparable with traditional surveying and mapping technology in china. therefore, in this respect, gps engineering survey technology shows its unique advantages. (2) the measurement time is short and the efficiency is high. for gps engineering surveying technology, it not only has the characteristics of high accuracy and stronger adaptability, but also has the characteristics of short surveying time and high efficiency. in the traditional engineering surveying and mapping technology of china, due to the lack of relevant application software, it usually consumes a lot of manpower to complete the surveying and mapping work. however, gps engineering survey technology can solve this problem very well. gps engineering surveying and mapping technology is mainly the application of gps system, which can work uninterruptedly without the influence of time, geographical environment and weather, thus greatly saving working time and improving the overall surveying efficiency. (3) more adaptable gps engineering surveying technology adopts the grid method in the process of surveying, and the grid structure is very simple. the density of each point and the length of edge distance can be freely selected according to the actual situation of surveying and mapping, even if the known point is far away from the measuring point, it can also be measured. in addition, for gps engineering survey technology, this technology effectively solves the problem that points cannot be seen each other, and can select known points without elevation. for gps engineering survey technology, when surveying a large grid, gps engineering survey technology can show the advantages of high accuracy and uniform error of gps grid. therefore, for gps engineering survey technology, it has certain adaptability. conclusions at present, although china has achieved certain success in the development of geological survey and research work, the investigation of lava area is still the key and difficult problem that puzzles project researchers. how to carry out a series of economic, reasonable, technically feasible, effective and scientific exploration work and realization of remote and high-precision monitoring on the basis of fully understanding the actual geological background of local lava area and the limitation of existing resources of the project is a subject worthy of study. this paper designs a remote monitoring system for geological exploration in lava area based on gps. the system consists of three functional modules: data acquisition module for geological exploration in lava area, data analysis and management module for geological exploration in lava area, and positioning module for geological anomaly based on gps, so that the three major functions of geological exploration in lava area: data acquisition, analysis and management and anomaly positioning can be realized. the system is applied to the remote monitoring task of geological exploration in a lava area. several conclusions are drawn: (1) when the system locates 12 small soil caves, the maximum error is 0.3, and the error is small. (2) when the geological survey point in the lava area is 23m, according to the reliability of the monitoring data of this system, it can be seen that most of the monitoring data have high reliability, which basically maintains above 0.8. (3) when the geological survey point in the lava area is 3.2 m, the minimum and maximum reliability of the system monitoring is 0.81 and 1, which is high. with its unique and powerful functions and advantages, gps technology fully demonstrates its superiority in the field of development, as well as greater and broader development space. however, some shortcomings have also been exposed in the actual construction process and in the construction and monitoring of follow-up projects in this field. (1) the key of precise positioning of gps system lies in the accurate calculation of the distance between satellite and receiver. according to the fixed mode: distance = speed× time, after the time is determined, the speed is determined by the propagation speed of the electromagnetic wave. it is well known that electromagnetic waves propagate rapidly in vacuum, but the atmosphere is not a vacuum state, and the signals are subject to heavy interference from the ionosphere and troposphere. the gps system can only calculate the signal averagely and there must be some errors in some specific areas. because of the influence of high-rise buildings and trees on the signal in big cities or mountain areas, the non-linear propagation of the signal will also be caused, and some errors will be introduced in the calculation. 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(2018). simulation of target location signal detection in satellite gps geological exploration. computer simulation, 35, 445-448. keywords: cyanobacterium, extracellular polymeric substances (eps), microbially-induced sedimentary structures (miss), microbial mat features, biosignature, geobiology. palabras clave: cianobacteria, sustancias poliméricas extracelulares (eps), estructuras sedimentarias inducidas por microorganismos (miss), características de tapetes microbianos, biofirma, geobiología. how to cite item osorio-rodriguez, d., & sanchez-quinonez, c. a. (2018). biological and geological characterization of modern biofilms and microbial mats and comparison with similar lithified structures in colombian cretaceous formations. earth sciences research journal, 22(3), 159-168. doi:https://doi.org/10.15446/esrj.v22n3.68839 microorganisms may play an important role in the binding of sediments and the formation of sedimentary structures, by means of the formation of biofilms and microbial mats. in this work, filamentous cyanobacteria from three different environments were compared: a biofilm from a lacustrine environment with intervals of flooding/desiccation, a biofilm from a continental saline environment, and a microbial mat from a thermal spring. the optical identification of the cyanobacteria allowed to establish that in the biofilms there is a dominance of the order oscillatoriales, while in the microbial mat both cyanobacteria of the orders nostocales and oscillatoriales dominated. two rock samples isolated from the thermal spring which genesis was possibly influenced by the activity of cyanobacteria are described and classified. one of them is a travertine/microbial framestone with stromatolitic and thrombolytic texture. the second one is classified as a mudstone/microbial boundstone. finally, a comparison between the sedimentary structures identified in those rocks with similar structures in the formations la luna, paja and tetuán, deposited during regressive phases of the colombian cretaceous epyric sea, and microbial mat features previously described is performed. based on morphological resemblance, fibrillar networks identified locally in those formations are interpreted as possible biolaminations originated from the activity of cyanobacteria. abstract biological and geological characterization of modern biofilms and microbial mats and comparison with similar lithified structures in colombian cretaceous formations caracterización biológica y geológica de biopelículas y tapetes microbianos actuales y comparación con estructuras litificadas similares en formaciones cretácicas de colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.68839 los microorganismos pueden jugar un papel importante en la cohesión de los sedimentos y la formación de estructuras sedimentarias, a través de la formación de biopelículas y tapetes microbianos. en el presente trabajo se compararon las cianobacterias filamentosas de tres ambientes diferentes: una biopelícula de un ambiente lacustre con intervalos de inundación/desecación, una biopelícula de un ambiente continental salino, y un tapete microbiano aislado de una fuente termal. la identificación óptica de las cianobacterias permitió establecer que en las biopelículas predomina el orden oscillatoriales, mientras que en el tapete microbiano predominan a la vez cianobacterias de los ordenes nostocales y oscillatoriales. se describen y clasifican dos muestras de roca tomadas de la fuente termal cuya génesis posiblemente estuvo influenciada por la actividad de cianobacterias. una de ellas es un travertino/ “framestone microbiana con textura estromatolítica y trombolítica. la segunda se clasificó como una lodolita/ “boundstone” microbiana. finalmente, se realiza una comparación entre las estructuras sedimentarias identificadas en dichas rocas con estructuras similares en las formaciones la luna, tetuán y paja, depositadas durante fases regresivas del mar epicontinental de colombia durante el cretácico, y las características de tapetes microbianos descritas previamente. con base en las similitudes morfológicas, las redes fibrilares identificadas de manera localizada en dichas formaciones se interpretan como posibles bioláminas originadas por la actividad de cianobacterias. resumen record manuscript received: 15/11/2017 accepted for publication: 01/06/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 159-168 daniela osorio-rodriguez & carlos alberto sanchez-quiñónez universidad nacional de colombia departamento de geociencias daosorioro@unal.edu.co, casanchezq@unal.edu.co g e o l o g y 160 daniela osorio-rodriguez & carlos alberto sanchez-quiñónez introduction the surface adhesion of microorganisms by means of extracellular polymeric substances (eps) favors the formation of thin organic laminae known as biofilms (marshall, 1992; costerton et al., 1995). these structures may grow to form thick stratified layers called microbial mats (castenholz, 1994; gerdes, 2010), fibrillar networks that often irreversibly bind filaments of cyanobacteria and sediments. they are characterized by an inner stratification with functional groups of microorganisms that coexist, generate symbiotic relationships and potentially modify the characteristics of sediments and sedimentary rocks (walter, 1976 in golubic et al., 2000), particularly in extreme environments. microbial mats have been regarded as advanced stages of a biofilm that build laminae on the top of layered surfaces and reflect gaps in sedimentation (gerdes, 2010). the transition from a biofilm to a microbial mat involves several weeks of non-burial and the formation of fibrillar condensed networks (schieber et al., 2007), as well as the incorporation and stabilization of sediments (stolz, 2000). the interaction between eps and sediments, either calcareous or siliciclastic, allows the preservation of characteristics that provide evidence of the presence of microorganisms. those microbially-induced sedimentary structures (miss) have been categorized into 17 different types and can be found not only in current environments but also in the lithological record (noffke, 2009). in addition, schieber et al. (2007) describes four main categories of microbial mat features in the rock record, from which the biolaminites are the most abundant. both miss and microbial mats are considered biosignatures, chemical or physical evidence of biological activity (farmer and des marais, 1999; summons et al., 2011; westall and cavalazzi, 2011). the study of miss and microbial mat features is highly relevant to understand the paleobiological implications of the associations between microorganisms and sediments, the paleoecology of microorganisms and their contribution to the sedimentary record through the formation of structures and biominerals. some other applications include: basin analysis and the detection of sea-level oscillations in sequence stratigraphy, oceanographic and paleoclimatic research, geochemical and paleoenvironmental reconstructions of the early earth (noffke, 2010), and the potential for the search of extraterrestrial life (foster and mobberley, 2010). studies on geobiology and astrobiology in colombia are scarce and their development has not been extensively considered in the main geosciences research schools and centers that traditionally have given more relevance to other areas such as geochemistry, petrology, geophysics, and oil and mineral deposits. this is one of the pioneering works in the geobiology and astrobiology lines of study at the geosciences department at the national university of colombia. the aim of this study was to identify the filamentous cyanobacteria isolated from biofilms and microbial mats collected in three sampling sites, considered representative of present-day extreme environments, in order to assess their morphological variability and find correlations between the morphotypes and the environment. in addition, we analyzed the role of the cyanobacteria in the formation of the rocks in which they were growing, as an analog for the identification of potential microbial mat features on several cretaceous formations in colombia. 2. data the samples analyzed in this study were microbial mats and the rocks that they were growing on top of, collected from the thermal spring “agua caliente” at el rosal (cundinamarca), and biofilms from a channel with organic sludges adjacent to the salt mine at nemocón (cundinamarca) and a flooding land surface nearby the colombian agricultural institute (ica) inside the campus of the national university of colombia, bogotá (figure 1). table 1 indicates the sampling coordinates and figure 2 shows the location of the sampling sites. the rock samples collected with the microbial mat growing on top were macroscopically identified as a travertine (r-1) and a mudstone (r-2). the rocks, microbial mats and biofilms from each location are illustrated in figure 3. 3. methodology 1 cm2 of the biofilm (less than 1 mm thickness), or 1 cm3 of the microbial mat (1cm thickness) collected at each location was separately dispersed on a glass slide with the aid of forceps. nanopure water was added as necessary to dilute each sample until it was possible to distinguish cellular structures with transmitted light microscopy. the observation was made on a nikon eclipse e100 microscope at the laboratory of microbial ecology of the national university of colombia, bogotá. photographs at different scales were taken (figures 4-6) and compared to the revised classification castenholz (2002) and considering the taxonomic revisions of komárek & anagnostidis (2005), and komárek et al. (2014). table 1. location and characteristics of the sampling site of biofilms and microbial mats analyzed in this study. sampling site coordinates (bogotá origin) height (m) water temperature (°c) at the sampling time annual ranges of air temperature (°c) average air temperature (°c) thermal spring “agua caliente”, el rosal (a) 4°55’44,09’’n/ 74°15’39,16’’ w 2258 42 6.9-19.3 13.3 salt mine, nemocón (b) 5°3’55,10’’n/ 73°52’41,08’’w 2595 11 6.7-20.7 13.8 flooding plain, unal (c) 4°38’6,72’’ n/ 74°5’11,43’’w 2600 14 7.2-19.4 13.5 figure 1. images of the sampling sites of biofilms and microbial mats analyzed in this study. (a). thermal spring “agua caliente”, el rosal (cundinamarca). (b). runoff channel with organic sludge adjacent to the nemocón salt mine. (c). flooding land surface inside the campus of the national university of colombia, bogotá. 161biological and geological characterization of modern biofilms and microbial mats and comparison with similar lithified structures in colombian cretaceous formations from quebrada la sorda, santander, where they are characterized by the dominance of foraminifera biomicrites with partial silicification, pseudospar and siliciclastic sediments of silt and sand sizes, mainly towards the top of the unit (sarmiento et al. 2015). tetuán formation (albian) samples were taken from quebrada motilona, paicol, huila, where it is composed of marls and biomicrites deposited during the second marine cretaceous flooding (gaona, 2015, hay & floegel, 2012, keller, 2008 in gaviria, 2016) and simultaneously with two different global anoxic events, which probably produced the high organic matter accumulations that characterize this unit (villamil et al., 1999 in gaviria, 2016). finally, samples from paja formation (hauterivian-late aptian) were taken from the uppermost part of the intermediate level “lodolitas abigarradas” near villa de leyva, boyacá, characterized by fissile black mudstones interlayered with micrites, calcareous concretions and gypsum (patarroyo, 2000). to identify potential microbial mat features in samples r-1 and r-2 collected at the thermal spring “agua caliente”, as well as in the thin sections of rocks from the la luna, paja and tetuán formations, the petrographical definition and characterization at gerdes (2007), schieber (1986), schieber (2007) and ulmer-scholle et al. (2014) was used. for the characterization of sample r1 (travertine), neither the classifications of folk (1959), dunham (1962) or embry & klovan (1971), traditionally used for calcareous rocks, nor mount’s (1985) classification for mixed rocks were considered. these classifications do not take into account fossils of microorganisms, so burne & moore (1987) classification for microbialites was used instead. 4. results 4.1 identification of cyanobacteria the microscopical observation of cyanobacteria from the three sampling locations led to the morphological identification of organisms to order, and to genera in some cases. none unicellular cyanobacteria were recognized (orders chroococales and pleurocapsales). in the samples from the thermal spring “agua caliente”, five morphotypes of filamentous cyanobacteria were identified: narrow trichomes (2.5 μm thickness), 5-8 μm length cells, abundant gas vesicles bound to the walls of the filaments. order oscillatoriales, unknown genus (figure 4 (a), morphotype *). narrow trichomes (< 2 thickness), 3.3-6.6 μm length cells. order oscillatoriales, genus leptolyngbya sp. (figure 4 (a), morphotype **). trichomes with helicoidal coiling in a close to partially unwinded helix, with a width of 1.4 to 2.9 μm. crossed wills with nearly invisible cells. order spirulinales, genus spirulina sp. (figure 4 (b)). cyanobacteria with heterocysts, predominantly intercalar (inside the filaments). vegetative cells spherical to ovoid. the trichomes have a slight coiling. order nostocales. possible genera: nostoc sp./ anabaena sp. (figure 4 (c)). trichomes composed of vegetative cells with discoid shape. presence of heterocysts and possible akinetes. order nostocales, genus nodularia sp. (figure 4 (d)). at the biofilm from the runoff channel nearby the salt mine of nemocón it was possible to recognize two morphotypes of filamentous cyanobacteria similar to those found at the thermal spring “agua caliente”: order oscillatoriales, unknown genus (figure 5 (a), morphotype *); order oscillatoriales. genus leptolyngbya sp. (figure 5 (a), morphotype **). trichomes of 5-6 μm width, with discoid cells (wider than long). order oscillatoriales, genus oscillatoria sp. (figure 5 (b)). finally, at the flooding surface inside the national university of colombia, bogotá, one of the same morphotypes of filamentous cyanobacteria found at the channel nearby the salt mine of nemocón was identified, assigned to the order oscillatoriales, genus oscillatoria sp. (figure 6). figure 2. satellite image illustrating the location of the sampling sites at the department of cundinamarca. (a). thermal spring “agua caliente”, el rosal. (b). runoff channel with organic sludge near the salt mine of nemocón. (c). flooding plain, universidad nacional de colombia. (qgis, 2018). figure 3. (a). travertine from the thermal spring “agua caliente”, el rosal. (b). mudstone from the thermal spring “agua caliente”, el rosal. (c). microbial mat growing on top of the rock samples shown in a. and b. (d). biofilm from a runoff cannel with organic sludge adjacent to the nemocón salt mine. (e). biofilm from a flooding land surface nearby the colombian agricultural institute (ica) inside the campus of the national university of colombia, bogotá. for assessing the presence of microbial mat features in the colombian cretaceous sedimentary record, we performed a petrographic analysis on rock samples from la luna, paja and tetuán formations. these units were selected considering that they were formed in a shallow marine setting, the one where microbial mats have been mostly studied and described, and the previous observation of organic matter laminae in rocks from these formations (e.g. sarmiento et al. 2015, gaviria, 2016). although the rocks analyzed in the present study are continental, we consider the comparison valid since the microbial mat features preserved in rocks are usually consistent over a wide range of environments (schieber et al., 2007). the samples from la luna formation (turonian-coniacian) were taken 162 daniela osorio-rodriguez & carlos alberto sanchez-quiñónez 4.2 lithologic characterization of microbialites related to the microbial mat isolated from the thermal spring “agua caliente”, el rosal (cundinamarca) the macroscopic observation of the samples r-1 and r-2 and the related microbial mat collected at the thermal spring “agua caliente” allowed the identification of processes of carbonate biomineralization, binding of quartz grains and a well-defined layering (figures 7, 8 and 9, respectively). sample r-1 was classified as a travertine according to the definition given by pentecost (2005), and as a microbial framestone (burne & moore, 1987). it was possible to recognize brown lenses with a possible microbial origin, similar to the ones that form the microbial mat found on top of the rock. layering is evident in some areas, while in some others the rock has a clotted texture (figures 7 (a) and 8). therefore, according to kalkowski (1908) the rock has a stromatolitic texture in the layered areas, and it also has a dendritic texture in the zones with macroscopic patches (aitken, 1967). besides, a brownish tone can be observed in some layers, which could be ascribed to the presence of iron from pore fluids, that allowed the precipitation of hematite and sideritization of the carbonates nearby. the observation of the sample r-2 allowed to establish the presence of terrigenous sediments and a very small fraction of calcareous material. only at the uppermost part of the rock, it was possible to determine an association between the microorganisms from the mat and some crystals of carbonate and quartz. however, at the middle part of the sample layered structures with iron oxides were distinguished, similar to biolaminations (figure 7 (b)). the observation of thin sections under the petrographic microscope allowed to determine that the sample is predominantly made of quartz grains (> 90%; 15% fine sand, 25% very fine sand, 60% mud). besides, it contains iron oxides (5%), opaque minerals (1-3%), calcium carbonate (1-3%) and glauconite (<1%) (figure 9 (a) and (b)). this sample was classified as a mudstone (folk, 1959), a sandy mudrock (mount, 1985), and a microbial boundstone (burne & moore, 1987). the microbial mat that was found growing on the surface of samples r-1 and r-2 contains laminae with a thickness of less than 1 mm, and a green internal coloring, related to the photosynthetic pigments figure 4. light microscope photographs of the cyanobacteria found in the microbial mat collected at the thermal spring “agua caliente”, el rosal (cundinamarca), that allowed the morphological identification. (a). it is possible to differentiate two morphotypes of filamentous cyanobacteria: *, order oscillatoriales, unknown genus; **, order oscillatoriales, genus leptolyngbya sp. (b). morphotype of filamentous cyanobacterium with narrow trichomes and closed to slightly unwinded helicoidal coiling. order spirulinales, genus spirulina. (c). filamentous cyanobacterium with recognizable heterocysts (light rounded cells, arrows) and possible akinetes (thick, strongly pigmented cells, asterisks). order nostocales, genus nostoc sp./anabaena sp. (d). filamentous cyanobacterium with discoid to ovoid cells. an apical heterocyst (arrow) and a possible akinete (asterisk) can be observed. order nostocales, genus nodularia sp. each field of view was cut and fitted to show the more relevant characteristics. figure 5. light microscope photographs from the cyanobacteria found at the biofilm from the runoff channel with organic sludge near the salt mine of nemocón (cundinamarca). (a). filamentous cyanobacteria; two morphotypes were identified: *, order oscillatoriales, unknown genus; **, order oscillatoriales, genus leptolyngbya sp. (b). order oscillatoriales, genus oscillatoria sp. abundant oscillatoriales filaments from the morphotypes identified in a. can be observed in the background. figure 6. light microscope photographs of cyanobacteria found in the biofilm retrieved from the flooding plain near ica at national university of colombia, bogotá. (a) and (b). order oscillatoriales, genus oscillatoria sp. figure 7. mesoscale photographs of r-1 and r-2 samples, and the microbial mat growing on top of them, isolated from the thermal spring “agua caliente”, el rosal (cundinamarca). (a). r-1 sample. the stromatolitic (above the green contour) and thrombolytic (below the green contour) textures can be visualized inside the same rock body, as well as the microbial mat growing on the surface of the sample. (b). r-2 sample. it is possible to observe that this is a fine grain siliciclastic rock, with iron oxides lenses. the microbial mat growing at the surface of the sample can also be seen. (c). transversal cut of the microbial mat growing on top of the samples r-1 and r-2. the orange tones suggest cyanobacterial pigment production and the presence of iron oxides, while the green layers suggest the presence of chlorophyll, also from cyanobacteria. 163biological and geological characterization of modern biofilms and microbial mats and comparison with similar lithified structures in colombian cretaceous formations of cyanobacteria and with a brownish coloring at the borders (figure 7 (e) and (f)). the microscopic observation of the mat allowed to establish the presence of iron oxides in the brown-reddish layers (figure 10 (a)) and of calcium carbonates between the filaments formed by the cyanobacteria (figure 10 (b)). 4.3 identification of possible biolaminations and microbial mats in cretaceous formations in colombia potential lithified microbial mats were found in thin sections from the la luna (figure 11), tetuán (figure 12) and paja (figure 13) formations, that were interpreted based on the images from the figures 8-10 and those presented in schieber (1986), schieber (2007), gerdes (2007) and ulmerscholle et al. (2014). the most similar structure to a microbial mat from all the observed sections was the one from from the la luna formation. it consists of a network of light grey fibers, inside which it was possible to recognize carbonate crystals (figure 11 (a) and (b)). reddish brown continuous and discontinuous laminae, interpreted as biolaminations, were also identified. these laminae may contain microbial kerogen and carbonate aggregates inside the filament sheaths and networks (figure 11 (c)-(g) and (h)). light grey localized lenses with elongated surfaces, distinguishable from other structures similar to fibrillar networks, were observed in some sections (figure 11 (i) and (j)). figure 8. microphotographs of thin sections of the r-1 sample, a travertine with a dominance of stromatolitic textures and some thrombolitic patches collected at the thermal spring “agua caliente”, el rosal. (a). inner section of the sample. carbonate crystals were deposited between the laminae of organic matter, which is interpreted as in situ biomineralization, possibly aided by the presence of cyanobacteria. (b). crossed section from the surface of the sample, over which the microbial mat was found growing. brown laminae are thought to be lithified microbial mats, inside which carbonate crystals can be observed, as well as sideritization due to a potential iron enrichment. figure 9. microphotographs of thin sections of the r-2 sample, mudstone (folk 1959) with iron oxides laminae and organic matter, or microbial boundstone. (a). and (b). laminae and lenses rich in organic matter with iron oxides. figure 10. microphotographs of thin sections of the microbial mat related to the samples r-1 and r-2 isolated from the thermal spring “agua caliente”, el rosal (cundinamarca). (a). transversal cuts in which an internal lamination can be observed. (b). precipitation of carbonate crystals between the laminae of microorganisms, that suggests the possible coexistence of processes of biomineralization and inorganic precipitation of the dissolved carbonate. xpl. figure 11. microphotographs of thin sections of foraminifera biomicrites from the la luna formation in which structures that resemble microbial mats can be observed. (continue) 164 daniela osorio-rodriguez & carlos alberto sanchez-quiñónez on the other hand, the observation of possible biolaminations and biomineralizations associated to the presence of microorganisms in thin sections of the tetuán formation allowed the identification of laminar, discontinuous and linear dark-brown colored structures, that signal the presence of organic matter (figure 12 (a) and (b)). some of these form aggregates and very small and localized fibrillar networks (figure 12 (e) and (f)). in one of the sections, the simultaneous presence of phosphatic lenses, mature kerogen, and possible biolaminations was observed, and apparent stylolites were identified (figure 12 (c) and (d)). figure 11. microphotographs of thin sections of foraminifera biomicrites from the la luna formation in which structures that resemble microbial mats can be observed. (a). potential microbial mat (center) in which a braided structure and a grey coloring distinguish it from the adjacent kerogen accumulations. (b). zoom of the square region in (a)., where carbonate crystals can be observed in the inner part. (c). possible biolaminations with a microbial origin, pointed out with red arrows. the purple stars show similar structures, that may be interpreted as organic matter that follows stylolites. these can be differenced because they are curved, with serrated, and are truncated against the minerals. (d). accumulations of kerogen that acquire a texture of clear biolaminations braided towards the bottom, inside which interlaced fibers and foraminifera aggregates can be observed. (e), (f), (g) and (h). laminae and parallel lenses of organic matter, similar to biolaminations and fragments of microbial mats. the continuity, parallelism, and zoning inside the sample are criteria that reinforce the hypothesis that these structures are lithified microbial mats. (i) and (j). possible biolaminations surrounding a lighter central structure, that could evidence reworking inside the basin according to schieber et al. (2010). (k) and (l). laminar surfaces with accumulations of organic matter. the binding of grains, the presence of an inner network and the localized distribution suggest that these laminae have a biogenic origin, however, the curved and serrated shape of these surfaces does not allow to preclude a genesis related to stylolites. figure 12. microphotographs of thin sections of foraminifera biomicrites and pseudosparites from the tetuán formation. (a) and (b). surfaces of organic matter accumulation relatively linear and continuous, that are interpreted as possible biolaminations. (c) and (d). phosphatic laminae (red asterisks), laminae of accumulation of mature kerogen (orange arrows) interspersed with potential biolaminations (pink arrows). (e) and (f). predominantly discontinuous lamination that might correspond to fragments of microbial mats. these laminae are less linear that those in (a) and (b) and sometimes are found surrounding foraminifera carapaces. observation of thin sections from the paja formation allowed to establish that these are rocks with flat parallel lamination and high fissionability, with a variable content of organic matter. microphotographs show that its accumulation occurs in the form of sinuous fibrillar networks that produce dense to disperse laminae, concentrated in some regions of the samples (figure 13 (a)) or disseminated, without any specific distribution (figure 11 (b)). the laminae, that potentially have a microbial origin due to their high similarity to the structures reported in the literature, border the clay grains and the high interference tones in polarized light can be attributed to a high concentration of micas (figure 13 (c)). 165biological and geological characterization of modern biofilms and microbial mats and comparison with similar lithified structures in colombian cretaceous formations figure 13. microphotographs of thin sections of mudstones from the paja formation. (a) and (b). laminar and braided structures with organic matter, with a non-homogeneous distribution. (c), (d), (e) and (f). detail of the structures observed in (a). in (c), the crystals do not have a defined shape and are entrapped inside the net formed by those structures, similar to the microbial mats described by schieber (1986). we interpret the high interference tones as localized gypsum replacements. xpl. 5. discussion 5.1. lithologic characterization of microbialites associated with the microbial mat isolated from the thermal spring “agua caliente”, el rosal (cundinamarca) sample r-1 was classified as a travertine (pentecost, 2005). in travertines, carbonate precipitation is mainly produced by the transference (evasion or invasion) of carbon dioxide from or to a source of groundwater that produces a calcium carbonate supersaturation, with nucleation and growth of crystals over an immersed surface. they can be stratified and the stratification is frequently irregular (pentecost, 2005). it is produced because of compositional changes of the material dissolved in the water during their deposition -which is slow and often discontinuoussuch as iron, organic matter, and other substances that precipitate together with the carbonate and may produce its characteristic band pattern (ujueta, 1960). some travertine deposits have been identified in colombia, mainly in the departments of caldas and boyacá, this last one being the one that registers the most travertines in the villages of villa de leyva (ujueta, 1960), firavitoba and tibasosa. the geologic characterization of a travertine from pesca was performed recently (vargas, 2013). because of their localization and distribution, travertine deposits in colombia have been associated with the volcanic activity of domes and the ascent of hydrothermal fluids through fractures (vargas, 2013). for the area of el rosal-san franciscosubachoque, no travertines have been reported. the calcareous supply for the formation of these bodies possibly comes from the seca formation (maastrichtian-paleocene; de porta, 1966), over which it is located. in the microbial mat growing on the surfaces of the samples r-1 (travertine) and r-2 (mudstone), structures related to the microbial growth together with the binding of sediments, such as biolaminations and biovarves, were identified (schieber, 2004). the light laminae in biovarves have been related to the growth of coccoid cyanobacteria, which are the main eps produces and may favor the precipitation of gypsum and carbonates (gerdes, 2010). carbonates and iron oxides in microbial mats have been considered a product of possible simultaneous processes of inorganic precipitation and biomineralization, classified as structures produced by metabolic effects (schieber, 2004). photosynthesis may shift carbonate solubility enough to promote the precipitation of these minerals, which induces the formation of irregular ooids, disseminated grains, cementation in specific laminae, early diagenetic dolomite, etc. we also identified siderite, that was probably produced by a diagenetic substitution of calcium by iron in the carbonate structure. the layers with iron enrichment may be explained by the activity of sulfate-reducing bacteria. the binding of iron to polysaccharide sheaths promotes the fixation of carbon dioxide and limits photorespiration, and the iron hydroxides below the cyanobacteria layer protect them from an excess of sulfur and keep oxygen from reaching the anaerobic community of microorganisms (schieber et al., 2007). the identification of microbial mats with similar textural characteristics in the samples r-1 (travertine) and r-2 (mudstone) provides an evidence of the ability of filamentous cyanobacteria to promote carbonate biomineralization and bind siliciclastic sediments. therefore, the composition of the grains, the sedimentary textures, and the microbial mat features in any environment might be mainly influenced by its physicochemical properties and the input of sediment. we thus hypothesize that the species of cyanobacteria that colonize microbial mats could be very variable, since they could be interchangeable as long as their ecological role within the microbial community is fulfilled. 5.2 cyanobacteria identification several groups of cyanobacteria have been found growing on travertine surfaces. this can be explained by the fact that many of these grow well in environments with high temperatures enriched in sulfur, variable light intensities, high carbon dioxide concentrations, and are resistant to desiccation (castenholz, 2002). the cyanobacteria genera identified in the present study match those who have been recognized as the most predominant in fresh to brackish water environments with mild temperatures (castenholz, 2002; komárek and anagnostidis, 2005; komárek et al., 2014). these cyanobacteria have also been recognized as some of the main contributors to the formation of miss and modern microbial mats (e.g. schieber et al., 2014) for the observation and morphological characterization of cyanobacteria, more advanced microscopy techniques had been used, such as confocal, phase-contrast, fluorescence and electronic microscopy (castenholz, 2002). getting a precise identification of cyanobacteria in different environments would be critical to know the diversity of these microorganisms, track the appearance of their characteristics and adaptations, and determine the conditions that drove the evolution of oxygenic photosynthesis, one of the most relevant events studied by geobiologists. the present is an exploratory, morphological study, and further work considering the use of molecular identification techniques is strongly encouraged to achieve a more precise discrimination and resolution of the cyanobacteria present in each location, given that the morphological identification may be subject to errors and does not allow to achieve the resolution needed to differentiate species, and in some cases, genera (komárek et al., 2014). 5.3 identification of possible biolaminations and microbial mats in cretaceous formations in colombia most of the structures with a possible microbial origin identified in la luna, paja, and tetuán formations are related to growth and binding of particles, such as biolaminations and biovarves, and with metabolic effects (biomineralizations) in the case of the la luna and tetuán formations. it is possible that the differences between the lithified and modern structures identified here and those that have been previously reported obey to two main factors: first, most reported miss and microbial mat features have 166 daniela osorio-rodriguez & carlos alberto sanchez-quiñónez been observed and classified on coastal and hypersaline environments (schieber, 1986; gerdes, 2007; schieber, 2007; ulmer-scholle et al., 2014), which might have contrasting conditions to the paleoenvironments in which the la luna, paja, and tetuán formations were deposited. in addition, there are diagenetic phenomena that can alter the appearance of the structures observed in the rock record with respect to modern microbial mats and biofilms, from which is worth mentioning burial, compressive forces (that can produce faulting, fracturing and pressure dissolution), mineral replacement and weathering. these diagenetic processes can alter the integrity of lithified microbial mats, which makes it possible to find fragmented and transported fossil fragments inside of them. in the analyzed thin sections from la luna formation, light lenticular structures were found, that have been interpreted as intraclasts, produced by erosion and reworking from deep currents inside the basin (schieber et al., 2010). conversely, in modern settings, deformed microbial mats with folds, bulges and roll-ups have been reported to form as a consequence of tidal currents (cuadrado et al., 2015), while sand layers interspersed within the microbial mats have been considered a record of highly energetic storm events (cuadrado et al., 2013). none of these processes can be discarded as a potential factor involved in the formation of the structures that we described. it is also important to note that in some of the studied sections, the possible lithified microbial mats can be mistaken with surfaces in which dissolution of carbonates had occurred due to compressive forces (stylolites). some criteria that can be used to differentiate them are the tendency of stylolites to be continuous, have curved and serrated edges and truncate carbonate crystals. however, the simultaneous presence of both structures cannot be discarded (figure 11 (c), (k), and (l)). on the other hand, although the presence of phosphates and dissolution surfaces at the tetuán formation had been previously acknowledged (gaviria, 2016), the present study is the first to question the nature of stylolites and some kerogen laminae in this unit and to propose a possible microbial origin for those structures. finally, on the thin sections from paja formation highly continuous and abundant biolaminations were recognized (figure 13 (d)(f)), together with diagenetic gypsum at outcrop scale. as previously stated, the la luna, paja and tetuán formations are characterized by low-energy marine facies, with variations in depth and fossil content and with intermittency in the rates of sediment and continental matter supply. these characteristics have been previously regarded as ideal for microbial mat proliferation (schieber, 1986). the finding of biolaminations with high similarity to those reported by gerdes (2007), schieber (1986), schieber (2007) and ulmer-scholle et al. (2014), mainly in the la luna and paja formations is consistent with the depositional environment and constitutes the first morphological evidence of fossilization of microbial mats or their remnants in the rock record in colombia. it is very important to remark that observation and identification of miss and microbial mat features should, whenever possible, be complemented with additional tests to provide confidence and robustness to their interpretation. the fact that morphology is an interpretative biogenicity criterium makes it susceptible to confusion with similar structures such as stylolites, intraclasts, and non-microbial kerogen. some of the most used complementary analytical techniques are advanced microscopy techniques and geochemical analyses like chromatography and mass spectrometry to detect chemical biomarkers such as associated heavy elements and rare earth elements, as well as carbon, oxygen, sulfur and nitrogen isotopes in terrestrial samples studies (westall, 2008). 6. conclusions the detailed observation of organic matter laminae in sedimentary geologic units, mainly in marine settings, has allowed the reinterpretation of some of these as potential microbial mat features. most of these structures manifest themselves as continuous biolaminations, inside which a fibrillar mesh produced by the filaments of cyanobacteria can be differentiated, related to biomineralization and sediment binding. diagenetic effects can alter the integrity of lithified microbial mats, which, together with the characteristics of the depositional environment, could account for the differences between the structures observed in the modern and the cretaceous microbial mat features. the observation of filamentous cyanobacteria isolated from the microbial mat from the thermal spring “agua caliente”, el rosal (cundinamarca) and from the biofilms collected from a runoff channel nearby the salt mine of nemocón and a flooding plain at the national university of colombia, bogotá, allowed their identification until genus. most of them belong to the order oscillatoriales, that has a worldwide distribution in fresh, brackish and marine waters, with representatives that require or tolerate extremes of salinity and temperature. two rock samples from the thermal spring “agua caliente”, el rosal (cundinamarca), whose genesis was influenced by the growth of a microbial mat, were characterized. one of these, a travertine and microbial framestone, had stromatolitic and thrombolitic textures. in the second one, a mudrock and microbial boundstone, biolaminations and biomineralizations were identified. the comparison between the textures observed in a modern microbial mat and its associated rocks, and those reported in the literature, allowed the identification of potential microbial mat features in the la luna, paja and tetuán formations. this finding is highly relevant given that it makes up the first report of biolaminations and biomineralizations produced by microoganisms in cretaceous rocks in colombia, and may be complemented with additional studies to perform paleoclimatic and sedimentological inferences about its depositional environment. despite the fact that the observation of microbial mat features is a strictly morphological criterium to assign a microbial origin to a sedimentary structure, it is a critical tool for the identification of biosignatures inside the frame of the search of life in extraterrestrial settings. however, we encourage performing additional tests in future studies to provide more reliability to the biogenicity interpretation. acknowledgments we would like to thank the department of geosciences of national university of colombia, bogotá, for the use of the laboratories, supplies and equipment that allowed the development of this work. we also want to thank professor jimena sánchez, from the microbial ecology laboratory (ecomic) of the department of biology of national university of colombia, bogotá, for the kind supply of microscopes and materials for the biological characterization of cyanobacteria. furthermore, the borrowing of thin sections made by jonatan bustos, néstor gaviria and professor gustavo sarmiento, respectively, was fundamental for the development of this work. we would also like to thank armando sánchez from the laboratory of thin sections at the geosciences department of national university of colombia, bogotá, for his help with the elaboration of thin sections. miguel gómez and david valentín helped in the elaboration of the map. yael natalia méndez, a student of m.sc. in geosciences at national university of colombia, provided important ideas for the development of this research. finally, we want to acknowledge the comments and suggestions from two anonymous reviewers, which greatly contributed to the improvement of the manuscript. references aitken, j. d. 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(2011). biosignatures in rocks. in v. thiel & j. reitner (eds.), encyclopedia of geobiology (189-201). springer, berlin.. keywords: small baseline subset (sbas); timeseries insar; ningdong coalfield; loess plateau; deformation monitoring. palabras clave: subconjunto de referencia pequeño (sbas); series de tiempo insar; campo de carbón de ningdong; meseta de loess; monitoreo de deformación. how to cite item ma, f., & sui, l. (2020). investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china. earth sciences research journal, 24(3), 373-386. doi: https:// doi.org/10.15446/esrj.v24n3.90123 ground deformation characterization was difficult to obtain over large spatial areas before the invention of the satellite radar interferometry (insar) technique. especially underground mining in the loess plateau of china, it causes large-scale ground damage within a short period of time. a small baseline subset (sbas) algorithm can overcome some limitations of insar technology, such as temporal decorrelation, spatial decorrelation, and atmospheric delay. in this study, sbas-insar technology was applied to process 19 scenes of sentinel-1a data in ningdong coalfield, china. we investigated and analyzed the mining subsidence status from march 2015 to june 2016. there are 6 ground deformation areas in the cumulative subsidence maps, and the maximum cumulative subsidence value is -178cm distributed in the renjiazhuang mining area during this period. the deformation rate map shows that the maximum deformation rate was -117cm/year. gps data above the working tunnel was collected in six mining areas in shigouyi. the subsidence value of sbas data is consistent with gps observation station data. the results reveal the evolution process of subsidence in mining subsidence and are helpful to the early warning of the mine disaster. abstract investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china investigación en subsidencia minera con datos de sentinel-1a por tecnología sbas-insar: estudio de caso de ningdong coalfield, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90123 antes de la invención de la técnica de interferometría de radar satelital (insar) la caracterización de la deformación del suelo era difícil de obtener en grandes áreas. la minería subterránea en la meseta de loess de china causa daños a gran escala en un corto período. el algoritmo de subconjunto de línea de base pequeña (sbas) puede superar algunas limitaciones de la tecnología insar, como la decorrelación temporal, la decorrelación espacial y el retraso atmosférico. en este estudio se aplicó la tecnología sbas-insar para procesar 19 escenas de datos sentinel-1a en la mina de carbón de ningdong, china. además, se investigó y se analizó el estado de subsidencia minera desde marzo de 2015 hasta junio de 2016. hay seis áreas de deformación del suelo en los mapas de subsidencia acumulada y el valor de subsidencia acumulativo máximo es -178cm distribuido en el área minera de renjiazhuang durante este período. el mapa de la tasa de deformación muestra que la tasa de deformación máxima fue de -117 cm/año. se visitaron estas seis áreas mineras y se recopilaron datos de gps sobre el túnel de trabajo en el área minera de shigouyi. el valor de subsidencia de los datos sbas es consistente con los datos de la estación de observación gps. los resultados revelan el proceso de evolución en el hundimiento minero y son útiles para la alerta temprana del desastre de la mina. resumen record manuscript received: 25/11/2019 accepted for publication: 07/05/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 373-386 fei ma1, lichun sui1,2,* 1school of geological engineering and geomatics, chang’an university, xi’an, china. 2national administration of surveying, mapping, and geoinformation engineering research center of national geographic conditions monitoring, xi’an, china *corresponding author: sui1011@chd.edu.cn m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v24n3.90123 https://doi.org/10.15446/esrj.v24n3.90123 https://doi.org/10.15446/esrj.v24n3.90123 mailto:sui1011@chd.edu.cn 374 fei ma, lichun sui introduction ningdong coalfield is one of the largest coal production bases in china. the large-scale ground damage in ningdong coalfield will profoundly influence the ecological environment of the loess plateau of china. therefore, investigating the status of ground damage and analyzing the trend of ground deformation in ningdong coalfield are important for revealing the evolution process and helpful to the early warning of the mine disaster. experience in previous work, mining engineers always analyze ground deformation through monitoring points using level measurements, global navigation satellite system measurements, or geodesy. the following shortcomings were exhibited in these approaches. (1) they are low spatial resolution, they can only monitor the value of control points but not in other unknown regions; (2) they are long work cycle and costly but low efficiency, the survey crew have to spend a long time to monitor the deformation points, and it is difficult to monitor the deformation in bad condition zones. (3) limited to the spatial resolution, they cannot monitor the unknown subsidence in the mining area; they are inapplicable to monitor the deformation in large-scale coalfield fleetly and accurately. (4)because of the limited theoretical analysis, it is difficult to identify the mining subsidence regularity using within-data observation of discrete points, and also it cannot satisfy the actual requirements of disaster prevention and prediction in the mining area (chen et al., 2010; wang et al., 2014). interferometric synthetic aperture radar (insar) offers an approach to observation earth using active electromagnetic signals. graham’s findings indicated that insar can measure ground topography; insar has the advantages of all-weather, all-day, and cloud influence free monitoring (mukul et al., 2016). graber et al. discovered differential insar (d-insar) technology, and it can monitor centimeter-level or smaller ground deformation along the line of sight (los) of a radar satellite (graber et al., 1996). after that, scholars research landslides (huang et al., 2017; cigna et al., 2014), mining subsidence (mroz & perski, 2003; fan et al., 2015), urban water-loss settlement (bai et al., 2016; liu et al., 2016) volcanic activities (rosen et al., 1996; schaefer et al., 2016), glacial shift (muskett et al., 2008; necsoiu et al., 2016), earthquake deformation (massonnet et al., 1993; chao & xinjian, 2006) using this approach. however, d-insar technology’s monitoring gradients are limited (wang et al., 2014), mining subsidence with immense deformation in a short time will cause decorrelation in the interferogram. moreover, it is difficult to remove phase errors, including such as atmospheric delay and orbit residue error for the traditional d-insar. nonetheless, many scholars reported that traditional d-insar technology has been experimented in many areas successfully. wherein, some scholars have acquired a lot of research findings in selected mining areas using d-insar technology, and some of them proposed some new ideas. wegmuller et al. surveyed surface subsidence caused by underground mining using d-insar successfully (wegmuller et al., 1999; normand & heggy, 2015; huang et al., 2019; raucoules et al., 2018; ng et al., 2009; yufeng et al., 2014). daqing ge et al. have outlined deals largely with mining subsidence monitoring experiments using d-insar, and they confirmed the feasibility of using d-insar in large-scale mining subsidence monitoring (wu et al., 2005; daqing et al., 2007; jingfa et al., 2008; dong & tang, 2018; yang et al., 2018; yang et al., 2010; zhu et al., 2011; wang et al., 2015). therefore, the significant features of the traditional d-insar technology are low costs and observation efficiency, a couple of sar images with an appropriate interferometric strategy can recognize surface deformation associated with past-mining activities (temporim et al., 2018). usai et al. present a least-squares (ls) database approach for sar interferometric data, they computed the deformation velocity of highly coherent target points via ls estimation, and successfully overcame the shortcomings of d-insar and acquired accurate subsidence data (usai, 2003). after that, the small baseline subset (sbas) insar technology developed from the traditional d-insar technology. berardino et al. proposed the small baseline subset (sbas) algorithm based on the ls model, and then bateson et al. applied the sbasinsar method to the south wales coalfield successfully. they selected insar data pairs with small spatial baselines and short temporal baselines; put forward the singular value decomposition (svd) method to combine multiple small baseline subsets (berardino et al., 2002; samsonov et al., 2013; bateson et al., 2015). therefore, sbas-insar technology does not only have the advantages of all-weather, all-day, and cloud influence free monitoring but can also enlarge monitoring gradients effectively, solve the spatial and temporal decorrelation among sar data sets, acquire deformation values in high coherence targets during different time periods. bateson et al. developed the three-dimensional phase unwrapping algorithms; yin et al. put forward extraction algorithms of coherent target points and error elimination algorithms, and lanari et al. improved the time-series deformation model, these algorithms and improvement optimized sbas-insar technology and improved monitoring accuracy (yin et al., 2011; zhou et al., 2013; zhao et al., 2013; lanari et al., 2006; lanari et al., 2007). coal is one of the most important energy resources, it represents the largest proportion of chinese petrochemical energy, and therefore mining area subsidence hazard caused by coal mining has attracted much more attention. mining industries are mainly distributed in northern, and north-western china. many chinese scholars have used sbas-insar technology to monitor surface subsidence in coalfields. for instance, fan et al. monitored the huaibei mining area and the yulin mining area using sbas-insar technology, found the deformation areas (fan et al., 2014; fan et al., 2017). zhao et al. (2018) combined the probability integral method and time-series insar technology to predict the deformation area in the fengfeng mining area (diao et al., 2016). but there are few pieces of research on the loess plateau in the west of china. there are several challenges to monitoring mining deformation in the loess plateau of china using sbas-insar technology: (1) there is loose soil, plateau beam broken, gully aspect, it is difficult to find high coherent target points. (2) due to the loose soil on the surface, mining activities underground will trigger landslides which are difficult to monitor. (3) mining subsidence in plateau beam broken reveals unique characteristics, such as non-linearity and sudden collapse, which results in phase unwrapping errors. in this paper, ningdong coalfield is selected as the research area, deformations in the mining region are presented with c-band data, which were gathered in descending paths of sentinel-1a images, and sbas-insar method is introduced to study the ground subsidence caused by the exploitation of mineral resources in this area. the time-series deformation information is obtained, the spatial and temporal development of the subsidence centers in the mining area is analyzed. sbas-insar technology provides an opportunity to determine the temporal evolution of land subsidence phenomenon with the insar time series in the loess plateau of china. materials and methods site selection ningdong coalfield is located in the east of yinchuan, the capital city of ningxia province, as shown in figure 1. its geographical coordinates are 37°07’15’’-38°17’17’’ n and 106°18’49’’-106°57’44’’ e. the mine area consists of a series of folds (nnw or nearly sn) and faults, and it extends eastward to the erdos syncline and stretches westward to tectonic zone in liupan mount. its north-south length is approximately 210 km and its east-west width is approximately 23-35 km, thereby covering an area of approximately 5900 km2. the proven reserves are 273.14gt. ningdong coalfield covers a cluster of the mines of 1-million-tons annual production that comprise more than 10 underground coal mines, including hongshiwan(hsw) mining area, renjiazhuang(rjz) mining area, yangchangwan(ycw) mining area, meihua(mh) mining area, qingshuiying(qsy) mining area, lingxin(lx) mining area, shigouyi(sgy) mining area, jinfeng(jf) mining area and zaoquan(zq) mining area, etc. among them, the rjz mining area, ycw mining area, qsy mining area, sgy mining area, and jf mining area are being exploited and more than 5-million-tons annual production. ningdong coalfield is suffering from a large-scale, centralized distribution of mining subsidence and ground damage because it is located on the loess plateau. the geological structure above mineable coal beds is fragile. data selection in this paper, 19 sar images(c-band) for a fifteen months period between 09 march 2015 and 07 june 2016 were processed, which were acquired by the ascending orbits on-board esa’s sentinel-1a satellite. the image stack covers an area of 250 km by 250 km, and the slant range and azimuth pixel spacing 375investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china assuming that a set of n + 1 sar images acquired at ordered times (t0, · · · , tn) covering ningdong coalfield, m interference fringe images are generated based on the baseline threshold. the number of interference fringe images satisfies the following inequality: n m n n+ ≤ ≤ +    1 2 1 2 (1) considering two interference fringe images acquisition at times ta and tb (tb > ta), any interferogram j is generated between time ta and tb. the unwrapped phase can be expressed as follows : δφ φ −φ ≈ φ φj b a def j topo jx r t x r t x r x r x r, , , , , , ,, ,( ) = ( ) ( ) ( ) + ( ) + φ φatmj noise jx r x r, ,,( ) + ( ) (2) in formula (2), j x r,( ) is the j image and the interference phase in pixels of azimuth and range coordinates (x,r).  t x rb , ,( ) and  t x ra, ,( ) are phase values of sar images at two different times. def j x r, ,( ), topo j x r, ,( ), atmj x r,( ), noise j x r, ,( ) represent deformation phase, residual phase due to inaccuracies in reference dem, depicts atmospheric phase error random noise phases between times tb and ta respectively. to achieve deformation information def j x r, ,( ), topo j x r, ,( ), atmj x r,( ), and noise j x r, ,( ) should be separated from j x r,( ). there are m equations in equation (2) after removing the topo, atmj, and noise, written in matrix forms: of the scenes are 5m and 20m, the repeat cycle of sentinel-1a sar sensors is 24 days. ningdong coalfield is contained in the image stack, considering the computational efficiency, the sentinel-1a sar image stack covered ningdong coalfield was extracted to estimate vertical average surface subsidence velocity and time-series deformation maps. specific parameters of sentinel-1a data are illustrated in table 1. orbit state vectors for the 19 single look complex scenes were released by the european space agency (esa) who improved and minimized orbital errors firstly. the orbit state vectors were named as precise orbit ephemerides (pod) data, which were available for the orbital refinement and phase re-flattening. the three arc-second shuttle radar topography mission (srtm) dem provided by the national aeronautics and space administration (nasa) was adopted to remove topographic phases. fundamental principle of sbas-insar technique the sbas-insar technique processing is divided into two stages: d-insar processing and time series processing. d-insar processing generates differential interferograms produced by sar image pairs based on baseline threshold (including flat-earth phase removal, topographic phase removal, interferogram filtering, phase unwrapping, and baseline refinement). time series processing estimates deformation information of every single differential interferogram and regards them as measured values. the deformation rate and time series can be solved by the least square (ls) method; the measured values are solved by singular value decomposition (svd) [34,37,40]. processing steps are as follows: china map 0 300 600 1.200 1.800 2.400 67’0’0’’ 72’0’0’’ 77’0’0’’ 82’0’0’’ 87’0’0’’ 92’0’0’’ 97’0’0’’ 102’0’0’’ 108’0’0’’ 114’0’0’’119’0’0’’ 125’0’0’’ 131’0’0’’ 137’0’0’’ 143’0’0’’ 57 ’0 ’0 ’’ 52 ’0 ’0 ’’ 47 ’0 ’0 ’’ 42 ’0 ’0 ’’ 37 ’0 ’0 ’’ 32 ’0 ’0 ’’ 27 ’0 ’0 ’’ 22 ’0 ’0 ’’ 17 ’0 ’0 ’’ 12 ’0 ’0 ’’ 7’ 0’ 0’ ’ 3’ 0’ 0’ ’ km 107’0’0’’106’0’0’’105’0’0’’104’0’0’’ 36 ’0 ’0 ’’ 37 ’0 ’0 ’’ 38 ’0 ’0 ’’ 1:7,868,095   figure 1. the geographical location of ningdong coalfield. the yellow lines are boundaries of mining areas. the background is the optical imagery of ningdong coalfield acquired in 2016. 376 fei ma, lichun sui aφ δφ= (3) in formula (3), a is an m × n coefficient matrix, m is the number of interference phase images, and n represents the numbers of sar images.   t nt t= ( ) … ( ) 1 , , represents the vector of the phase of the highcoherence pixel. δφ δφ φt n= … 1, , are unwrapped phase values of differential interference images. the deformation rates of high-coherence pixels were integrated into the time interval between master and slave images. the matrix form can be written as follows: bυ δφ= (4) in formula (4), b represents an m  n coefficient matrix, it can be solved by using the least-squares (ls) or singular value decomposition (svd) algorithm (lanari et al., 2004), and t can be represented as follows: υ υ φ − υ φ −φ − −1 −1 t n n n n nt t t t = = … =      1 1 1 0 , , (5) then the minimum norm solution of the data vector can be acquired. finally, deformation rate and deformation time series can be achieved and derived according to the time interval between sar images. data processing in this study, there are two steps for data processing as the abovementioned: generating differential interferogram, retrieval deformation rate, and time series. the specific steps are as follows: generate differential interferogram the proper sar pairs are selected within the thresholds of the perpendicular and temporal baseline; only small baseline interferometric images will be generated. then the sar data pair connection diagram is generated in figure 2. in this experiment, considering the c-band sentinel-1a data with small perpendicular baselines are coherent. the maximum spatial baseline was set at 300 m. the maximum temporal baseline was set as 200 days. the system selected the super master image automatically which was acquired on 2 april 2015. all the other 18 scenes sar images were selected as slave images which were co-registered and resampled to the super master image. interferometric pairs with high coherence and fine unwrapping were screened, 78 differential interference images remained. remove topographic phases and flat-earth phases. the topographic phase was eliminated using the srtm dem which was provided by nasa. and the flat-earth phase was removed using pod data released by the esa. table 1. sentinel-1a data covering ningdong coalfield used in this work no. acquisition date (yyyymmdd) track product type mode polarization orbit direction 1 20150309 157 slc iw vv ascending 2 20150402 157 slc iw vv ascending 3 20150426 157 slc iw vv ascending 4 20150520 157 slc iw vv ascending 5 20150613 157 slc iw vv ascending 6 20150707 157 slc iw vv ascending 7 20150731 157 slc iw vv ascending 8 20150824 157 slc iw vv ascending 9 20150917 157 slc iw vv ascending 10 20151011 157 slc iw vv ascending 11 20151104 157 slc iw vv ascending 12 20151222 157 slc iw vv ascending 13 20160115 157 slc iw vv ascending 14 20160208 157 slc iw vv ascending 15 20160303 157 slc iw vv ascending 16 20160327 157 slc iw vv ascending 17 20160420 157 slc iw vv ascending 18 20160514 157 slc iw vv ascending 19 20160607 157 slc iw vv ascending (a) (b) figure 2. (a) time–position of sentinel-1a image interferometric pairs; (b) the time–baseline of sentinel-1a image interferometric pairs. the yellow diamond denotes the super master image. blue lines represent interferometric pairs. green diamonds denote slave images about:blank 377investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china subsidence rate and time series retrieval subsidence rates can be estimated by the ls method because the combination of differential interferogram was integrated into the time interval between master and slave images (goldstein & werner, 1998). the estimated linear subsidence was subtracted first from raw subsidence time series. then eliminated atmospheric phase, random noise phases, and only nonlinear subsidence remained, it can be separated through temporal and spatial bandpass filtering (hooper, 2008). finally, adding up nonlinear subsidence and linear subsidence we could retrieve subsidence time series. results subsidence rate map figure 3 shows the subsidence rate map derived using sbas insar; the map is superimposed on the optical imagery of ningdong coalfield acquired in 2016. positive values of rate map indicate that the surface is uplifting in the vertical direction, whereas negative values denote surface subsidence in the vertical direction. sbas insar-derived results in figure 3 reveal that annual average subsidence rates in ningdong coalfield from -117 cm/year to 18 cm/ year, and the largest subsidence rate was detected in rjz mining area. subsidence time series figure 4 shows the surface cumulative subsidence time series in ningdong coalfield from 09 march 2015 to 07 june 2016. the sentinel-1a tops sar image acquired on 09 march 2015 was considered a reference time of the time series in the sbas insar method. the maximum cumulative subsidence in this period is -178 cm, and it was located in rjz. southern parts of ningdong coalfield are relatively stable, and most of the cumulative subsidence in these areas range from −10 cm to 10 cm. the magnitude of subsidence in major subsidence areas (i.e., rjz mining area, qsy mining area, sgy mining area) gradually increased with time. size of these subsidence areas also gradually expanded. discussion mining subsidence status in ningdong coalfield as shown in figure 4, several deformation areas were identified in ningdong coalfield whose characteristics were prominent uneven subsidence patterns. the major subsidence areas include rjz mining area, qsy mining area, ycw mining area, sgy mining area, jf mining area. we detected six deformation centers which subsidence rates greater than -48cm/year (table 2). among the above-mentioned six regions, rjz, qsy, sgy, and jf mining areas feature subsidence bowls, and most of the serious subsidence bowl is distributed in the rjz mining area. jf mining area features the smallest subsidence bowls, but its subsidence range is gradually expanding. excessive deformation gradient changes the ground surface echoing characteristics in the center of the ycw mining area, resulting in decorrelation in the center area as well as the missing of large gradient deformation information. additionally, these subsidence areas are mainly distributed in the 5-million-tons annual production mining area. in addition to the above-mentioned regions, other areas present small surface subsidence, and most subsidence rates are less than -20 cm/year. after inquiring about the construction history data, it was found that other mining areas were under construction in 2016, the intensity of underground activities is small. from figure 4, it can be seen that the sbas technique can well monitor the range of influence and the trend in the development of mining subsidence. figure 4(r) represents that the center areas of coal mining subsidence distribute in rjz mining area, ycw mining area, qsy mining area, sgy mining area and jf mining area. because of the past mining subsidence and lack of field data, the reliability of interferometric measurements needs to be verified and validated. we visited these six mining areas, and only collected global position system (gps) observation stations’ data in sgy mining area. according to the geographic coordinates of deformation pits, we found six deformation pits using hand-held gps receivers in rjz mining area, ycw mining area, qsy mining area, sgy mining area, and jf mining area. there are different deformation features in different mining areas. for instance, underground mining activities caused a large-scale basin on the ground in the rjz mining area and formed a bowl with a diameter of 12 meters in the center of the mining area. coal mining caused large-scale ground damage in the ycw mining area; it formed ground fissures with 3 meters width in the south and 28 meters north of the ycw mining area. and little-mode landslides occurred continually during mining in the qsy mining area. a long deformation basin formed in the sgy mining area, a collapse pit generated in the center of the deformation basin. the narrow ground fissure is the main deformation feature in the jf mining area. comparative analysis of sbas and gps data in sgy mining area to verify the accuracy and reliability of the experiment results, we collected the gps data and sbas data in sgy mining area for analysis and comparison. in figure 6, they are a time series of deformation maps from march 2015 to june 2016 in sgy mining area, the green circles are gps observation stations, the red rectangles are the working panels. it can be known that the main deformation basin appeared in the research area for the continuous mine exploitation since march 2015, the maximum deformation in sgy mining area reached -94cm. this movement is a representative class of the settlement basin commonly involving digging and mining in the mining area. especially, the observed displacements at the top of the mining area are consistent with the downward movement usually figure 3. vertical deformation rate derived by sbas insar for the whole study area during the period from 09 march 2015 to 07 june 2016. the background is the optical imagery of ningdong coalfield acquired in 2016. 378 fei ma, lichun sui (a) (b) (c) (d) figure 4. cumulative subsidence maps time series from 09 mar. 2015 to 07 jun. 2016. the background is optical imagery of ningdong coalfield acquired in 2016. (a)-(r) are cumulative subsidence maps of 15 months in ningdong coalfield. (a) 09 mar. 2015 to 02 apr. 2015; (b) 09 mar. 2015 to 26 apr. 2015; (c) 09 mar. 2015 to 20 may 2015; (d) 09 mar. 2015 to 13 jun. 2015; (e) 09 mar. 2015 to 07 jul. 2015; (f) 09 mar. 2015 to 31 jul. 2015; (g) 09 mar. 2015 to 24 aug. 2015; (h) 09 mar. 2015 to 17 sep. 2015; (i) 09 mar. 2015 to 11 oct. 2015; (j) 09 mar. 2015 to 04 nov. 2015; (k) 09 mar. 2015 to 22 dec. 2015; (l) 09 mar. 2015 to 15 jan. 2016; (m) 09 mar. 2015 to 08 feb. 2016; (n) 09 mar. 2015 to 03 mar. 2016; (o) 09 mar. 2015 to 27 mar. 2016; (p) 09 mar. 2015 to 20 apr. 2016; (q) 09 mar. 2015 to 14 may 2016; (r) 09 mar. 2015 to 07 jun. 2016. 379investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china (e) (f) (g) (h) figure 4. cumulative subsidence maps time series from 09 mar. 2015 to 07 jun. 2016. the background is optical imagery of ningdong coalfield acquired in 2016. (a)-(r) are cumulative subsidence maps of 15 months in ningdong coalfield. (a) 09 mar. 2015 to 02 apr. 2015; (b) 09 mar. 2015 to 26 apr. 2015; (c) 09 mar. 2015 to 20 may 2015; (d) 09 mar. 2015 to 13 jun. 2015; (e) 09 mar. 2015 to 07 jul. 2015; (f) 09 mar. 2015 to 31 jul. 2015; (g) 09 mar. 2015 to 24 aug. 2015; (h) 09 mar. 2015 to 17 sep. 2015; (i) 09 mar. 2015 to 11 oct. 2015; (j) 09 mar. 2015 to 04 nov. 2015; (k) 09 mar. 2015 to 22 dec. 2015; (l) 09 mar. 2015 to 15 jan. 2016; (m) 09 mar. 2015 to 08 feb. 2016; (n) 09 mar. 2015 to 03 mar. 2016; (o) 09 mar. 2015 to 27 mar. 2016; (p) 09 mar. 2015 to 20 apr. 2016; (q) 09 mar. 2015 to 14 may 2016; (r) 09 mar. 2015 to 07 jun. 2016. 380 fei ma, lichun sui (i) (j) (k) (l) figure 4. cumulative subsidence maps time series from 09 mar. 2015 to 07 jun. 2016. the background is optical imagery of ningdong coalfield acquired in 2016. (a)-(r) are cumulative subsidence maps of 15 months in ningdong coalfield. (a) 09 mar. 2015 to 02 apr. 2015; (b) 09 mar. 2015 to 26 apr. 2015; (c) 09 mar. 2015 to 20 may 2015; (d) 09 mar. 2015 to 13 jun. 2015; (e) 09 mar. 2015 to 07 jul. 2015; (f) 09 mar. 2015 to 31 jul. 2015; (g) 09 mar. 2015 to 24 aug. 2015; (h) 09 mar. 2015 to 17 sep. 2015; (i) 09 mar. 2015 to 11 oct. 2015; (j) 09 mar. 2015 to 04 nov. 2015; (k) 09 mar. 2015 to 22 dec. 2015; (l) 09 mar. 2015 to 15 jan. 2016; (m) 09 mar. 2015 to 08 feb. 2016; (n) 09 mar. 2015 to 03 mar. 2016; (o) 09 mar. 2015 to 27 mar. 2016; (p) 09 mar. 2015 to 20 apr. 2016; (q) 09 mar. 2015 to 14 may 2016; (r) 09 mar. 2015 to 07 jun. 2016. 381investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china (m) (n) (o) (p) figure 4. cumulative subsidence maps time series from 09 mar. 2015 to 07 jun. 2016. the background is optical imagery of ningdong coalfield acquired in 2016. (a)-(r) are cumulative subsidence maps of 15 months in ningdong coalfield. (a) 09 mar. 2015 to 02 apr. 2015; (b) 09 mar. 2015 to 26 apr. 2015; (c) 09 mar. 2015 to 20 may 2015; (d) 09 mar. 2015 to 13 jun. 2015; (e) 09 mar. 2015 to 07 jul. 2015; (f) 09 mar. 2015 to 31 jul. 2015; (g) 09 mar. 2015 to 24 aug. 2015; (h) 09 mar. 2015 to 17 sep. 2015; (i) 09 mar. 2015 to 11 oct. 2015; (j) 09 mar. 2015 to 04 nov. 2015; (k) 09 mar. 2015 to 22 dec. 2015; (l) 09 mar. 2015 to 15 jan. 2016; (m) 09 mar. 2015 to 08 feb. 2016; (n) 09 mar. 2015 to 03 mar. 2016; (o) 09 mar. 2015 to 27 mar. 2016; (p) 09 mar. 2015 to 20 apr. 2016; (q) 09 mar. 2015 to 14 may 2016; (r) 09 mar. 2015 to 07 jun. 2016. 382 fei ma, lichun sui observed in rotary sliding: as continuous digging and mining in the mining area, the subsidence center moves from northwest to southeast, and the settlement area is obviously enlarged, and the settlement range extends southward. there are twelve automatic real-time gps observation stations above the tunnel in the sgy mining area. thirteen gps receivers were used simultaneously to carry out surveys in triangulation mode (keeping one at the base station). gps data processing was done by gamit/globk software in triangulation mode with respect to a reference station which is located in a relatively unaffected place in the neighborhood. the coordinate of the reference point was computed with respect to an existing control point of survey. along (q) (r) figure 4. cumulative subsidence maps time series from 09 mar. 2015 to 07 jun. 2016. the background is optical imagery of ningdong coalfield acquired in 2016. (a)-(r) are cumulative subsidence maps of 15 months in ningdong coalfield. (a) 09 mar. 2015 to 02 apr. 2015; (b) 09 mar. 2015 to 26 apr. 2015; (c) 09 mar. 2015 to 20 may 2015; (d) 09 mar. 2015 to 13 jun. 2015; (e) 09 mar. 2015 to 07 jul. 2015; (f) 09 mar. 2015 to 31 jul. 2015; (g) 09 mar. 2015 to 24 aug. 2015; (h) 09 mar. 2015 to 17 sep. 2015; (i) 09 mar. 2015 to 11 oct. 2015; (j) 09 mar. 2015 to 04 nov. 2015; (k) 09 mar. 2015 to 22 dec. 2015; (l) 09 mar. 2015 to 15 jan. 2016; (m) 09 mar. 2015 to 08 feb. 2016; (n) 09 mar. 2015 to 03 mar. 2016; (o) 09 mar. 2015 to 27 mar. 2016; (p) 09 mar. 2015 to 20 apr. 2016; (q) 09 mar. 2015 to 14 may 2016; (r) 09 mar. 2015 to 07 jun. 2016. table 2. characterization of deformation pits in ningdong coalfield. no. geographic coordinates (wgs84) mining area location subsidence rates (cm/year) characterization 1 longitude:106.479502 latitude:38.185305 renjiazhuang -117.66 formed a “bowl” with a diameter of 12 meters in the farmland 2 longitude:106.739867 latitude:38.136653 qingshuiying -53.05 formed landslide in farmland 3 longitude:106.622403 latitude:38.027403 yangchangwan -80.67 formed ground fissure with 3 meters width and collapse in farmland 4 longitude:106.633408 latitude:37.966727 yangchangwan -73.97 formed ground fissure with 28 meters width and collapse in foul land 5 longitude:106.469058 latitude:37.721456 shigouyi -82. 88 formed deformation basin with 18 meters width in the wasteland 6 longitude:106.860975 latitude:37.642009 jinfeng -48.04 formed ground fissure with 0.5 meters width in farmland about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank 383investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china figure 6. interferogram of sbas-insar in sgy mining area 384 fei ma, lichun sui figure 7. comparison curves between sbas-measured and gps-measured deformation, the black curves show time-series deformation in gps monitoring points using sbas-insar technology, and the blue curves show time-series deformation in gps monitoring points using gps technology, curves(a)-(l) denote gps monitoring points from gps1 to gps12. 385investigation on mining subsidence based on sentinel-1a data by sbas-insar technology—case study of ningdong coalfield, china northwest to southeast direction, gps observation stations were named gps1 to gps12 in sequence. the time series of sar data and gps data covered the ranges from march 2015 to june 2016. comparison results are seen in figure 7. the x-axis represents the date of data acquisition, ranging from 09 mar. 2015 to 07 jun. 2016, and the y-axis expresses the deformation values in the vertical direction. gps data are the high precision real-time observation data; we considered them to be the truth values to verify the accuracy of sbas data. from figure 7, it can be seen that the subsidence curve of sbas data is consistent with the subsidence curve of gps observation station data. in particular, sbas data agree well with gps data in gps1-gps6, gps8, and gps9. the gps data have the same trend as sbas data in gps7 and gps1012. nonetheless, it was found that some uplifts occur in gps7 and gps10-12 observation stations which may be caused systematically by orbit error, or by an inappropriate selection of the reference points, we will try to understand this phenomenon and deal with it in our future work. conclusions in this study, sbas-insar techniques allow us to understands the annual growing characteristics of subsidence in ningdong coalfield. surface deformation in the sgy mining area extracted from sar images is discussed, and validation was conducted between sbas data and gps data. 1. from time-series image analysis results, the subsidence basins in the coal mine are found, the subsidence development trend in space and time are analyzed, and the mining subsidence rate is revealed, and the analysis of sbas data indicates that the maximum rate is about -117cm/year in ningdong coalfield. 2. the time-series analysis of sbas-insar achieves more reliable results and more accurately reflects the mining subsidence characteristics of ningdong coalfield. five deformation centers that were distributed in the 5-million-tons annual production mining area are found. 3. the comparisons between the deformation obtained by the sbas techniques and gps results in the sgy mining area have been performed. the experimental results indicate that the settlement variation obtained by two different types of technology is consistent. 4. in addition, some factors, such as the inaccuracy of phase estimation error and dem accuracy, will also affect the measurement results of sbas; 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(2011). monitoring of ground surface deformation in mining area with insar technique. chinese journal of nonferrous metals, 21, 2564-2576. 10.1109/tgrs 10.1109/tgrs 10.3390/rs 10.1016/j.epsl 10.1016/j.epsl 10.1016/j.rse 10.1186/bf 10.1109/tgrs 10.1016/j.jag 10.1016/j.jag 10.3390/rs 10.1109/tgrs 10.1109/igarss https://doi.org/10.1016/s1674 10.1109/jstars 10.4304/jcp 10.3390/rs 10.3390/rs letter from the editor it's time to present our second volume of 2017. the result of a great effort to put into your hands some works of the best quality. with this selection of articles, an invitation to the colombian geology congress. as every two years, in a city strategically picked up to show the best of the colombian and latinamerican geology, the sociedad colombiana de geologia entity (colombian geology society) presents the xvi congress of geology and the third explorers symposium. the venue is santa marta, a touristic city in the caribbean. register for technical courses and field visits are still open. the event is scheduled august 28 until september 1, 2017. recent months, it has been confirmed in colombia a gas reservoir with at least 520 meters of netgross at the kronos well, which suggests a substantial increase in the gas reserves of the country. sure, a subject to be analyzed during the congress. the discovery, which left the first results in 2015, is proved in a depth of 4,795 meters, including a 1,835-meter water table. the state-managed company ecopetrol holds a 50% share in this block, according to news releases by them, as the 50% remaining is of anadarko, an operator company. these figures are on the same way that the energy reserves around the world, where new techniques and exploration have reached peaks to cover the current demand for oil and related products for decades. this growth of reserves in the world goes hand in hand with the investment. ecopetrol, for example, announced the increase of its exploration budget this year to us$ 650 million with the final purpose of intensifying the search for new hydrocarbon reserves. but mining in the earth would be not the unique perspective to analyze. goldman sachs wrote a recent note explaining that “space mining could be more realistic than perceived,” according to an interesting article published by the washington post on 28 april 2017. the report analyzes the possibilities and bets the mining out of the earth is a decade away. for the next year, the nasa plans to vacuum materials from a 2,000-foot-wide asteroid called bennu; sure it would be the cornerstone. thanks for your attention to this number of the earth sciences research journal and the little reflections proposed in this letter. with the best regards, carlos alberto vargas jimenez editor in chief le tt er f r o m t h e ed it o r earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017) keywords: choiyoi group; magmatism; petrography; geochemistry; gondwana; argentina. palabras clave: grupo choiyoi; magmatismo; petrografia; geoquimica; gondwana; argentina. how to cite item martínez, a., gallardo, a., giambiagi, l., & tobares, l. (2020). the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry. earth sciences research journal, 24(2), 121-132. doi: https://doi.org/10.15446/esrj.v24n2.79515 the choiyoi group from the permo-triassic, is one of the most conspicuous volcano-sedimentary suites of southern south america, considered critical to understand the geological evolution of the western margins of gondwana. in this regard, petrography, geochemistry, and structural data were examined to better elucidate the physical character and emplacement conditions of the unit in the cordón del plata range, within the frontal cordillera of mendoza, argentina. the site is representative of the magmatism and deformation through different andean cycles. results of the study indicate three facies of increasing felsic composition upwards. mafic units consist of basalts, andesite and andesitic breccias at the base of the sequence. felsic rocks such as rhyodacites, granites and welded tuffs are predominant above. the fault zone of la polcura – la manga is the most prominent structural feature in the region, which presumably controlled the emplacement of breccias and ignimbrites within the middle and upper members. these compositional variations suggest a magma evolution from subduction to a rifting environment after the san rafael orogeny in the late palaeozoic. in this line, the lower choiyoi was observed to overlie the san rafael structures indicating thus, that compression ceased before the volcanic extrusion. geochemistry data indicate that mafic rocks are mostly high-potassium, calc-alkaline volcanics derived from the mantle wedge above the subduction zone. in contrast, the upper choiyoi corresponds to high-potassium rhyolites to shoshonites, typically depleted in eu. this indicates partial melting of a lithospheric mantle in an average to thin crust. abstract the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry el grupo choiyoi en el cordon del plata, oeste argentino: estratigrafía, petrografía y geoquímica issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.79515 el grupo choiyoi del permo-triásico es una de las suites volcánicas y sedimentarias más conspicuas del sur de sudamérica, considerada crítica para comprender la evolución geológica de los márgenes occidentales de gondwana. en este sentido, se examinaron datos petrográficos, geoquímicos y estructurales para dilucidar mejor el carácter físico y las condiciones de emplazamiento de la unidad en el área del cordón del plata, dentro de la cordillera frontal de mendoza, argentina. el sitio es representativo del magmatismo y la deformación a través de diferentes ciclos andinos. los resultados del estudio indican tres facies litológicas de aumento de la acidez hacia arriba. las unidades máficas consisten en basaltos, brechas andesíticas y andesítas en la base de la secuencia. predominan las rocas félsicas, como las riodacitas, los granitos y las tobas soldadas. la zona de falla de la polcura la manga es la característica estructural más prominente de la región, que presumiblemente controla el emplazamiento de brechas e ignimbritas dentro de los miembros medios y superiores. estas variaciones en la composición sugieren una evolución magmática de la subducción a un entorno de ruptura después de la orogenia de san rafael en el paleozoico tardío. en esta línea, se observó que el choiyoi inferior cubría las estructuras de san rafael, lo que indica que la compresión cesó antes de la extrusión volcánica. los datos de geoquímica indican que las rocas máficas son en su mayoría vulcanitas calcoalcalinos de alto potasio derivados de la cuña del manto por encima de la zona de subducción. en contraste, las rocas felsicas van desde las riolitas con alto contenido de potasio hasta las shoshonitas, típicamente pobres en eu. esto indica la fusión parcial de un manto litosférico en una corteza media a delgada. resumen record manuscript received: 06/05/2019 accepted for publication: 28/02/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 121-132 amancay martinez1, adrian gallardo1,2, laura giambiagi3, laura tobares1 1san luis national university, fcfmyn, department of geology, san luis, argentina 2conicet (argentina national scientific and technical research council), san luis, argentina. 3ianigla-conicet cct mendoza. adrián ruiz leal s/n, parque san martín. (5500). mendoza, argentina. * corresponding author: adgallardo@geowater.com.au g eo lo g y https://doi.org/10.15446/esrj.v24n2.79515 https://doi.org/10.15446/esrj.v24n2.79515 mailto:adgallardo@geowater.com.au 122 amancay martinez, adrian gallardo, laura giambiagi, laura tobares introduction the basement of the central andes of argentina is largely constituted by an assemblage of rhyolites, andesite, mesosilicic igneous complexes and interbedded volcanic breccias and tuffs of late permian – middle triassic age. these rocks have been included within the choiyoi group (choiyoi), which covers an area of at least 200,000 km2 and may be up to 500,000 km2 in central western argentina and chile (llambias et al. 2003). in general, the base of the unit is characterised by rocks of mafic to intermediate composition whilst, the upper members are dominated by felsic volcanites, including rhyolites. in argentina, the sequence is exposed across several geologic provinces, from the neuquén basin in the south (~ 39º s) to the andes of mendoza and san juan (i.e. principal and frontal cordillera), at an approximate latitude of 30º south. due to its widespread extension, the choiyoi has been related to the magmatism that occurred during the breakdown of the western margins of gondwana in the late palaeozoic (zeil, 1981; ramos. 1999; rocha-campos et al. 2011). more specifically, the volcanism appears related to large-scale faults associated to anisotropy in the upper crust (giambiagi and martinez, 2008). as the backbone of the andes of southern south america and being one of the most important geological units of argentina, the choiyoi has been extensively studied over the years (e.g. gonzalez diaz, 1957; stipanicic et al. 1968; ramos et al. 1988; gregori and benedini, 2013; sato et al. 2015; spalletti and limarino, 2017; and many others.). despite the large amount of work that has been done, the large geographical extent, lithological diversity and complex structural setting, means that a clear understanding of the origin and evolution of the choiyoi is still elusive. furthermore, most papers about the unit deal with its facies distribution and lithological features, but only a few examine the geochemical, chronological and tectonic aspects of the unit (strazzere et al. 2006). in this regard, the present work integrates petrography, geochemistry and structural data from an area where the magmatism is well represented, in order to better recognise the geologic features of the choiyoi in central argentina and to help constrain its tectonic evolution along the southwestern margins of gondwana. the study is focussed on the northeast of the cordón del plata range (cordón del plata), in the frontal cordillera of mendoza, argentina. in here, the basement is represented by a variety of rocks and structures that reflect the evolution of the unit through different andean cycles. even though the paper provides an overview of the geology in this area, the broader aspects of the rock characteristics as well as the geodynamic processes are expected to be common to other regions and ultimately, have implications for the entire formation. location and regional setting the study area is located west of the town of potrerillos in the cordón del plata range, within the margins of the frontal cordillera in the central andes of argentina. in particular, the site locates between about 32º 45´ s and 33º 10´ s, on the north-northeast section of a major mountain chain that extends 100 km by 50 km between the mendoza and las tunas rivers (figure 1). morphologically, the ridge includes numerous valleys and peaks with an elevation close to 6,000 m. the oldest rocks recognised in the area correspond to the palaeozoic basement, which consists mostly of sedimentary and metamorphic deposits of variable characteristics. the lower member is represented by conglomerates with volcanic clasts, stratified quartzite and litharenites assigned to the vallecitos beds, of pre-carboniferous (devonian?) age (figure 2). near las tunas river, the unit is known as the guarguaraz complex, which consists of metasedimentary and mafic and ultramafic rocks deformed in the middle devonian and later in the early carboniferous (lopez de azarevich et al. 2009; willner et al. 2011). further east, in the precordillera belt, these rocks are partially overlapped by a sequence of silurian-devonian turbiditic sandstones intruded by granitoids, commonly referred as the villavicencio group (harrington, 1957). the tectono–sedimentary environment of the unit has been interpreted as a passive continental margin or the continental side of a back-arc system (kury, 1993). a hiatus separates the vallecitos beds from the upper carboniferous strata. in some places this hiatus would correspond to an angular unconformity attributed to the chanic diastrophism (furque and cuerda, 1979). in the frontal cordillera of mendoza, the upper carboniferous is largely composed of marine clastic deposits assigned to the el plata formation (fm.) and loma de los morteritos fm. (caminos, 1965; rolleri and criado roqué, 1969). in particular, these units have been described as a succession of feldspathic figure 1. location of the cordón del plata range, mendoza province, argentina. 123the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry determine the main geochemical characteristics of the sequence. the samples were analysed for major and trace elements by x–ray fluorescence spectroscopy (xrf) and inductively coupled plasma-mass spectrometry (icp–ms) at the laboratories of the university of cornell, new york and at actlabs, ontario, canada. the icp–ms is widely used as a conventional method to determine ultra trace compositions using solutions prepared by digestion of rocks. for more details about the methodology, the reader is referred to tamura et al. (2015). similarly, xrf is a well–established analytical technique for geochemical investigations (young et al. 2016). as described by potts (1992), samples are prepared as compressed powder pellets or fused glass discs and excited with x-ray radiation. interaction between this primary radiation and atoms of the sample causes ionization of discrete orbital electrons. during the subsequent electronic rearrangement by which the atom de-excites back to the ground state, fluorescence x-rays of energy characteristic of that element are emitted. the emission intensity of this characteristic radiation is ultimately measured with a suitable x-ray spectrometer and compared with that from a standard sample. several other specimens were made into polish thin sections and selected for petrographic analyses at the san luis university, argentina. the slides were examined by a polarizing–light microscope and the observations ultimately captured in higher–magnification images. finally, calculations and plotting of the rock’s geochemical data were done using the gcdkit software package from janousek et al. (2006). litharenites with subordinate litharenites and lithic arkoses, which reflect terrigenous contributions (spalletti et al. 2011). simultaneously, the deposition of proximal alluvial fans and glacial debris would be represented by the rio blanco conglomerates, an array of sedimentary and volcanic pebbles from the upper carboniferous – lower permian. the palaeozoic basement is unconformably overlain by volcanics from the choiyoi group (groeber, 1946) and intruded by plutons of permian – middle triassic age (figure 3). more recently, llambias et al. (2003) and strazzere et al. (2016) included the rio blanco conglomerates in the choiyoi group, as the sedimentary unit was considered transitional to the volcanic rocks above. the choiyoi outcrops extensively along the cordón del plata and neighbour localities such as cordón del portillo, cerro cantera, and quebrada de santa elena. in general, the lower half of the unit comprises andesitic lavas and breccias, whilst the upper half consists of rhyodacitic and lithic pyroclasts and rhyolite lava-flows (martinez, 2005). it is widely accepted that there were at least four major deformation events in the frontal cordillera: a) compression related to the famatinian and chanic orogeny in the pre – early carboniferous; b) widespread contraction and crustal thickening known as the san rafael orogeny in the early permian; c) extensional tectonics from the andean cycle in the late permian – middle triassic and; d) compression and uplift of the cordón del plata in the cainozoic. in this regard, the choiyoi in the study area is affected by north – south trending thrusts and strike–slip faults related to stress propagation during the pangea breakup. additionally, the elongated geometry of the triassic granitoids with the same trend of the normal faults indicates that their intrusion could be structurally controlled by these faults (heredia et al., 2012). methods and materials a field survey was undertaken in the northeast section of the cordón del plata to enable a systematic characterisation of the stratigraphy and to identify the most prominent features of the bedrock. geological mapping, lithological logging and structural measurements were carried out simultaneously to assess stratigraphic relationships and mineralogical variations of the deposits. fifteen samples of fresh vulcanites and plutonic rocks were additionally collected to figure 2. general stratigraphy of the cordón del plata and surroundings (modified from giambiagi et al. 2011). figure 3. geological map and sampling of the cordón del plata range and southern precordillera of mendoza (modified from martinez, 2005; strazzere et al. 2016). 124 amancay martinez, adrian gallardo, laura giambiagi, laura tobares results historically, the composition of the choiyoi would be increasingly silicic upwards, from mafics near the base to rhyolites and felsic rocks at the top (polanski, 1958; llambias et al. 1993; strazzere et al. 2016). we defined the lower choiyoi in the cordón del plata as typically basaltic and andesitic unlike the upper member, largely composed of dacites, rhyolites and granitic rocks (table 1). the mafic facies correspond to basalts, andesite and andesitic breccias outcropping in the vicinities of the cerro negro ridge. the sequence is separated from the palaeozoic bedrock by a high angular unconformity. the basalts are typically fractured and cut by subvertical red, rhyolitic dikes up to 350 meters (m) thick, possibly related to the major granitic bodies that intrude all units (caminos, 1965; martinez, 2005). the texture of the basalts is predominantly porphyritic, with phenocrysts of plagioclase in a pilotaxitic groundmass (figure 4). bands of andesitic breccias of northeast orientation are also present. with a thickness of about 500 m, these greenishblack breccias are highly consolidated, with much of the material consisting of euhedral plagioclase (an50) and hornblende randomly distributed in a consolidated fine-grained paste. the felsic volcanic facies of the choiyoi correspond to a maximum of 800 m of rhyodacitic lavas with numerous outcrops showing columnar disjunction, whilst others are characterised by some flow banding defined by the parallel arrangement of planar minerals. the thickness of the bands seldom exceeds a few centimetres. they are commonly folded and moderately foliated, which indicates a ductile deformation of the lava front during its advance. generally, the rhyodacitic lavas exhibit a porphyritic texture with euhedral plagioclase (an30) and biotite phenocrysts in a dark brown paste (figure 5). mineralogically, the dikes are characterised by a porphyry texture with phenocrysts of plagioclase and biotite embedded in a felsic paste and isolated spherulites of similar composition to the phenocrysts. a suite of rhyolitic tuffs is interlayered within the sequence. usually, these tuffs have phenoclasts of quartz, sanidine and plagioclase (an20) into a spherulitic and eutaxitic matrix. similarities in colour, structure and rock coherence difficult the recognition of facies in the field. nonetheless, the mineralogical examination reveals at least two distinctive types of tuffs: a) lithic tuffs containing abundant fragments of accidental pelite and phyllites, commonly predominant in the north of the cordón del plata; b) multi-coloured pumicitic tuffs enriched in quartz, alkali feldspar and biotite embedded in a glassy paste. these rocks are often amalgamated with light-colour pumice clasts in a highly vesicular matrix. additionally, some moderately-welded ignimbrites as described by marshall (1935), outcrop in the region. the last stage of the choiyoi magmatism in the region is represented by granitoids whose largest manifestation occurs in the peaks of the cerro médanos, south of the andean precordillera of mendoza. these rocks, known as the guido stock, have an ellipsoid pattern that cover an area of approximate 60 km2 and forms a mountainous terrane mostly over 2,000 m above sea level (orme et al. 1996). the granitoids are pink to white, typically composed of coarse–grained quartz, k–feldspar and plagioclase (an16), with minor table 1. modal composition of igneous rocks in the choiyoi group (in percentage). component essential primary accessories secondary pyroclastic unit pl ag io cl as e k fe ld sp ar q ua rtz py ro xe ne a m ph ib ol e b io tit e a pa tit e zi rc on o pa qu es ti ta ni te c hl or ite c ar bo na te s se ric ite ep id ot e q ua rtz pa st e li th ic s pu m ic e lavas & andesitic breccia 45 5 15 × × × 30 basaltic lavas 35 × × × × × × × × 60 lithic ignimbrites 10 20 15 × × × 30 20 vitreous ignimbrites 5 15 15 5 × × × 15 10 30 rhyodacitic lavas 15 10 10 15 × × × × 45 felsic dikes 20 10 10 10 × × × × 45 granitic intrusives 20 35 30 5 5 × × × × × mafic dikes 35 15 25 20 × × × × × figure 4. phenocrysts of plagioclase and pyroxene embedded in a groundmass of plagioclases in the basaltic lava unit. 125the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry hornblende and flakes of biotite (figure 6). titanite, zircon and some opaques are the main accessory minerals. near its top, the stock shows a conspicuous dike of basaltic horizons, together with the local development of quartz–biotite hornfels product of the reaction with the host rock during emplacement. the basalts are black, aphanitic, and show an excellent development of columnar disjunction up to 10 m thick. furthermore, the stock is dissected by numerous reddish hypabyssal rocks predominantly porphyritic, mostly consisting of phenocrysts of quartz, feldspar and plagioclase in a microgranular groundmass. from the tectonic point of view, convergence of the oceanic crust against the pacific edge of gondwana resulted in several episodes of deformation during palaeozoic to caenozoic times (ramos, 1988; giambiagi and martinez, 2008). in general, the region is characterised by fractures and faults trending west–northwest to north–northeast, associated to a network of minor and discontinuous structures that cross cut the basement in multiple directions. in particular, the frontal cordillera can be divided into the structural western and eastern domains, which are separated by the la carrera fault system, an array of north–south reverse faults active until recent times. the western domain, where the cordón del plata locates, is distinguished by east-vergent reverse faults crossed by sinistral strike–slip faults of northwest orientation (giambiagi et al. 2014). chemical composition, and different diagrams can be used to discriminate the origin of the volcanic rocks in the choiyoi group (appendix i). a detailed interpretation of the results is presented in the following sections. discussion lithological facies it is clear that the choiyoi magmatism resulted in a complex array of volcanic and igneous rocks that partly intercalates with pyroclastic beds, conglomerates and sandstones deposited after tectonic reactivation at a more local scale. ignimbrites and rhyolitic tuffs are also present in the uppermost part of the sequence (tobares et al. 2015). in this context, the lowermost part of the choiyoi group would represent an intermediate to mafic subduction– related magmatism (gregori and benedini, 2013) whilst, the breccias could be the end product of the fracturing and opening of vents at the initial stages of the figure 5. typical stratigraphic column of the choiyoi group. a-b) rhyolitic vitreous tuff with quartz and feldspar with preserved shreds; c-d) rhyolitic lithic tuff with phyllite and sandstone from the pre-volcanic basement; e) rhyolitic vitreous tuff with folding eutaxitic texture; f) phenocrysts of euhedral quartz with deformed vitroclastics; g-h) rhyodacitic lava flows with phenocrysts of biotite and plagioclase in felsitic paste; i) andesitic breccia with block of andesitic lava flows; j) phenocrysts of euhedral honblende in paste with plagioclase. 126 amancay martinez, adrian gallardo, laura giambiagi, laura tobares volcanic eruption. based on field relationships and lithological similarities, the mafic facies has been correlated to the portezuelo del cenizo fm., outcropping in region of quebrada del tigre, north of the study area (folguera et al. 2001). in addition, k/ar measurements in an andesite from quebrada del tigre suggest that the unit is from the lower permian, with an estimated age of 281 ± 10 ma (rapalini and vilas, 1991). higher in the sequence, the rhyodacites of the cordón del plata appear closely related to the dacites of pampa de la canota, which outcrop in the precordillera region of mendoza to the north. the dacites registered a k/ar age of 263 ± 15 ma and therefore, are attributed to the middle permian (folguera et al. 2001). these deposits, which can be considered part of the vitreous tuffs group, are characterised by pink to orange tonalities, ill-defined bedding and a distinctive eutaxitic texture product of the compaction of glass shards and pumice fragments during the mobilisation of the pyroclastic flow. poor particle sorting and the absence of clear sedimentary structures suggest that part of the ignimbrites would have been deposited as pyroclastic flows. thus, the stratified ignimbrites are consistent with an explosive eruption that fragmented the volcanic conduits and incorporated lithic fragments into a density current flow along valleys and topographical depressions. simultaneously, part of the pumice materials remained in suspension, to be dispersed by the wind and be deposited at lower rates throughout the region. in this energetic regime, the preferential down-current transport of selected clast types produces a decrease in clasts density with distance, from dense bombs in the proximal breccia to low–density pumices and ash in distal ignimbrites (branney and kokelaar, 1997). the ignimbrites of the cordón del plata were intruded by felsic dikes with a maximum thickness of 5 m. their texture is mostly porphyritic, with phenocrysts of plagioclase and biotite in a fine-grained mass. considering that the dikes present compositional similarities to the host rock, it is argued that the intrusion occurred as part of a residual magma soon after the volcanic eruption. cortés et al. (1999) presented k/ar results from biotite minerals in the dikes, which resulted in an age of 241 ± 7 ma, equivalent to the olenekian – anisian in the triassic. finally, the felsic bodies at the top of the sequence are likely related to late pulses of the felsic magma that originated the bulk of the granites in the area. the absence of internal deformation suggests that the intrusions would be post– tectonic, possibly emplaced contemporaneously with the stock solidification (caminos, 1979; cingolani et al. 2012). thus, the stocks’ emplacement could be linked to the initial phases of rifting and extensional relaxation along the western margins of pangea, after the peak in volcanic activity during the permian – early triassic. in this line, isotopic dating also indicated that the guido stock could be ascribed to early – middle triassic times. in effect, k/ar analyses in samples from cerro médanos yielded and age of 244 ± 10 ma (early triassic) whilst, rocks at the neighbour cerro arenal constrained the magmatism to 237 ± 10 ma, in the middle triassic (caminos et al. 1979). tectonic structures and deformation as previously mentioned, a regional west–northwest fault zone develops in the area of la polcura – la manga. this is also indicated by additional sinistral strike–slip and dip–slip faults longer than 5 km (figure 7). the la polcura and la manga faults are parallel but dip into one another. thus, they represent a graben–like zone that separates westerly–oriented structures in the precordillera from north–south faults in the frontal cordillera (giambiagi and martínez, 2008). the la carrera fault would have originated during the uplift of the range in the late cainozoic (caminos, 1965; polanski, 1972). pre–choiyoi structures these structures include thrusts and folds of variable wavelength and amplitude, ranging from a few cm to several km in length. the deformation especially affects the marine deposits from the upper –carboniferous to the early permian but has no influence over the choiyoi rocks indicating thus, an early permian age (giambiagi et al. 2011). rocks of the early palaeozoic are characterised by numerous brittle structures somewhat difficult to differentiate from the main andean synorogenic ones formed after the miocene. an exception occurs when the structures are unconformably overlain by the permo – triassic vulcanites, which act as a diagnostic marker in the tectonic record. to the west, palaeozoic faults mostly strike north–south to north–northwest, unlike the eastern domain where the prevailing faults’ direction is northeast to north– northeast. similarly, the folds generally trend north–northwest to northeast, with their axial planes dipping either east or west. as recognised by von gosen (1995), deposits of the late palaeozoic are crosscut by dextral north–northwest structures and sinistral north–northeast strike–slip faults, which cannot be traced into the permo – triassic sequence. mutual crosscutting relationships between opposite dipping faults suggest that these structures would have developed as a conjugate set. some of the strike– slip faults cut the thrust faults and some do not, but they all predate the permo – triassic vulcanites. therefore, the strike–slip faults seem to be coeval with the late palaeozoic thrust faults, while others are likely to be genetically related to the permo – triassic extension (giambiagi et al. 2011). kinematic analyses of the palaeozoic structures suggest a contractional regime with a west– northwest trending λ3 in the western domain, shifting to a northwest trending λ3 in the east. although the permian structures result primarily from orogen– normal contraction, the strike–slip fault systems affecting the belt indicate a left-lateral strike–slip regime associated with a north–south trending λ3 and an east–west trending λ1 (giambiagi et al. 2011). middle permian – late triassic deformation a widespread contractional episode known as the san rafael orogeny generated a crustal thickening in the early permian (azcuy and caminos, 1987). figure 6. microphotograpy of the granites showing coarse-grained quartz, k-feldspar and plagioclase with subordinated biotite. 127the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry the lower choiyoi unconformably covers the san rafael structures indicating thus, that compression ceased before the extrusion of the volcanic rocks. the middle permian to late triassic represented an extensional period, concomitant with an intense magmatism responsible for the bulk of the vulcanites in the choiyoi. the peak in volcanic activity took place in the permian, followed by the development of a continental rift basin during the triassic. the choiyoi rocks, both vulcanites and volcaniclastics, are dissected by normal and oblique–slip normal faults that trend west–northwest to northwest (figure 8). the great thickness and facies variations of the rocks near these structures, and the relationship between the structures and the intrusion of plutonic rocks suggest that they are syn-magmatic permo – triassic (giambiagi and martinez, 2008). as previously mentioned, the la polcura and la manga faults are two of the most prominent structures from the permo – triassic in the region. in particular, the la polcura fault extends for over 30 km and shows a vertical displacement of at least 500 m. slickensides on the exposed surface indicate two sets of striae. the older set corresponds to a sinistral oblique–slip normal movement attributed to the reactivation of a strike–slip fault during the permo – triassic extension in the andean orogeny. in contrast, the younger fault system exhibits a sinistral strike–slip movement ascribed to reactivation during the andean shortening. the la manga fault runs approximately in the same direction than la polcura, to finally merge into a single structure southward. nonetheless, the tilted blocks of both la polcura and la manga dip toward each other to form a regional graben that presumably enabled the emplacement of andesitic breccias and ignimbrites of the middle and upper sections of the choiyoi group, respectively. fault–slip data from major and mesoscale faults in the permo – triassic volcanics (50 m > displacement < 50 m, respectively) indicates a homogeneous orientation of north–northeast maximum stretching direction (giambiagi and martínez, 2008). this deformation episode contributed to the evolution of two groups of faults: a) structures of west–northwest to northwest strike, with normal offset and a minor to negligible strike–slip component; b) normal, oblique–slip faults of sinistral offset, typically oriented north–northwest such as the la manga fault. even though the major north–northwest, oblique–slip faults could be attributed to the reactivation of palaeozoic structures during the andean orogeny, the mesoscale oblique faults were identified exclusively in the permo – triassic volcanics. therefore, it is postulated that both sets of faults are newly created, broadly formed during the complex crust extension that extended from the permian to the cretaceous. andean deformation the formation of the frontal cordillera can be related to uplift and exhumation of the palaeozoic and mesozoic basement during the andean orogeny. the orogeny initiated in the principal cordillera in the early miocene, migrating towards the cordón del plata in the late miocene – early pliocene (giambiagi et al. 2003). as a result of the compression, the palaeozoic structures were reactivated as reverse faults oriented with respect to the maximum stress propagation. on the other hand, reactivated faults from the permo – triassic exhibit a sinistral displacement (terrizano et al. 2010). in this regard, the la figure 8. normal faults affecting the andesitic lavas and breccias of the choiyoi group. figure 7. schematic cross-section a-a´ (see figure 3 for location) showing the graben-like structure delimited by the la manga and la polcura faults. 128 amancay martinez, adrian gallardo, laura giambiagi, laura tobares carrera fault system has traditionally been considered to be part of the process that gave rise to the cordón del plata (caminos, 1965). geochemical considerations mafic volcanic rocks different types of mafic rocks were recognised in the succession, as indicated by the tracers zr/tio2 – nb/y. in the diagram of winchester and floyd (1977), samples plot in the field of an andesite (figure 9a). as for the dikes, the total alkali versus silica (tas) relationship (middlemost, 1994) indicates a gabbroic diorite of subalkalic signature (figure 9b). in turn, the k2o vs. sio2 discrimination (pecerillo and taylor, 1976), shows that the rocks are predominantly high–k, calc–alkaline volcanics (figure 9c). this is consistent with strazzere et al. (2016), who also distinguished three types of volcanic rocks, all k–rich, in the portezuelo del cenizo fm, between the cordón del plata and the frontal cordillera of mendoza. using the more mobile elements th and co (hastie et al. 2007), mafic samples mostly fall in the basalt–andesite field. in contrast, the andesitic calc–alkaline rocks plot in the high–k, shoshonitic category (figure 9d). in this regard, the whole rock geochemistry of the choiyoi volcanics suggest a transition from calc–alkaline magmatism at ~266 ma to more alkaline/shoshonitic compositions at ~248 ma (kimbrough et al. 2015). the tectonic discrimination diagram of schandl and gorton (2002) shows little scattering for the mafic rocks, with the bulk of the samples reflecting the homogeneous provenance of the rocks, which plots in the active continental–margin field (figure 9e). within the generally mobile lile group, th is a relatively stable and reliable indicator, whose enrichment relative to other incompatible elements (especially nb and ta) is taken to represent the subduction zone component (wood et al. 1979). in this context, the calc–alkaline basalts of the choiyoi are part of the destructive plate margins described by wood (1980) (figure 9f). the th/yb – ta/yb binary plot is useful to assess the role of source heterogeneities, metasomatism, and crustal contamination in the generation of magmas (pearce, 1983). the plot shows that samples are displaced toward higher th/yb ratios, which is consistent with rocks from the central andes subduction zone (figure 9g). low ta/yb ratios point to the involvement of a depleted mantle (ta/yb< 1), whilst the high th/ yb ratio may be attributed to the enrichment of this mantle by a subduction component carrying th, but not ta or yb (pearce et al. 1990). the mafic rocks are moderately enriched in light rare earth elements (lree) and have a nearly flat signature in heavy elements (hree) normalised to chondrite (figure 9h). as interpreted by sun et al. (2019), this signature would imply a mantle source in a volcanic arc environment and a convergent plate margin. thus, the melt would have been generated in the mantle wedge above the subduction zone, possibly representing the initial stages of basaltic volcanism. felsic rocks the felsic rocks are mainly rhyolites and granites, with sio2 close to 80 % and na2o + k2o of about 6%. additionally, the general composition of the unit ranges from high–k rhyolites to shoshonites. all samples exhibit a somewhat flat pattern in the chondrite–normalized ree, with negative eu anomalies. the eu anomaly can therefore be used as an indication of the amount of fractionation that magma has undergone (tobares et al. 2015). a slight eu depletion was observed in rhyodacite lava flows and rhyolitic tuffs (figure 10a), unlike the granites that showed a pronounced reduction in the eu content (figure 10b). thus, the overall patterns of the felsic rocks are considered to be the end result of arc magmatism in an average crustal column, with plagioclase fractionation occurring at low pressures (< 1kbar). concluding remarks the present work examined the petrography, geochemistry, and structural style of the choiyoi group in the cordón del plata range of argentina, to better understand the rocks’ physical character and the evolution of the western margins of gondwana since the palaeozoic. three distinct lithologic facies figure 9. variation diagrams for geochemical discrimination of the choiyoi rocks. a) zr/tio2 vs. nb/y diagram of winchester and floyd (1977); b) tas diagram of middlemost (1994); c) sio2 vs. k2o plot of peccerillo and taylor (1976); d) th-co diagram of hastie et al (2007); e) diagram of schandl and gorton (2002) for discrimination of felsic to intermediate volcanic rocks; f) th-hf-nb tectonic discrimination of wood (1980); g) morb normalized diagram of pearce (1983); h) chondrite-normalized ree diagram of nakamura (1974) for mafic rocks. were recognised in the sequence, which becomes increasingly felsic upwards. in this regard, the base of the choiyoi consists of mafic rocks, mostly basalts, andesite, and andesitic breccias that correlate with the portezuelo del cenizo formation, from the lower permian. the second facies includes rhyodacites and minor rhyolitic tuffs attributed to the middle permian. these rocks are commonly foliated and folded, likely as a result of continuous compression and surface cooling at the advancing lava. in particular, the tuffs can be subdivided into two main groups: lithic tuffs prevailing in the north, and vitreous tuffs and pumices to the south. the poor stratification, eutaxitic texture and intensity of the welding suggest deposition from a pyroclastic density current, followed by the downfall of lighter pumices and ash. following the volcanic eruption, the magma triggered many dike injections in the triassic. the third lithofacies is represented by the granites of the guido stock, correlated to the final stages of magmatism in the early to middle triassic. in addition, the pluton displays a wide range of dike swarms and metamorphic contacts either syntectonic or partly brought during subsequent stages of the tectonism. the san rafael orogeny has been recognised to be responsible for the deformation in the 129the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry carboniferous to middle permian. nevertheless, the choiyoi basement in the cordón del plata unconformably overlies the san rafael structures indicating thus, that volcanism postdates the orogeny. in this regard, the mafic rocks at the bottom of the studied sequence would derive from a primary magma at the base of the continental crust, in a convergent plate margin. the magma ascent would have contributed to thermal maturation and crustal melting and the subsequent generation of felsic tuffs and lava flows. migration of the magma might have preferentially occurred through reactivated high–angle, oblique–normal faults such as the la manga – la polcura system. the magmatic cycle culminated in the triassic with the emplacement of large granite bodies controlled by extensional structures associated to the breakup of gondwana. acknowledgments this research was supported by grants from the agencia nacional de promoción científica y tecnológica of argentina (pict 99-6729) and conicet (pip 5843 and pip 638). many thanks to dr. suzanne kay, cornell university, for her assistance with the geochemistry. the authors are especially grateful to dr. victor ramos, katja deckart, vanesa litvak, alicia folguera, mariela etcheverría, florencia bechis, josé mescua, maisa tunik and silvia barredo, for the discussions and comments to improve the manuscript. special thanks also to diego di carlo, ezequiel garcía morabito, natalia marchese, guillermo peralta, cristian guerra and diego márquez, for the collaboration during field work. the authors also thank dr. lucy jacksonville, ahs consulting, for her review and assistance in improving the grammar and writing style of the manuscript. last but not least, we are also deeply grateful for the review, comments and suggestions made by the anonymous reviewers. references azcuy, c. l. & caminos, r. 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(1981). volcanism and geodynamics at the turn of the paleozoic to the mesozoic in the central and southern andes (chile-argentina). zentralblatt für geologie und palaontologie, 1, 298-318. https://doi.org/10.1007/978-1-4615-3270-5 https://doi.org/10.1007/978-1-4615-3270-5 10.1016/j.tecto 131the choiyoi group in the cordón del plata range, western argentina: structure, petrography and geochemistry appendix i andesitic breccia basalts and andesite lavas mafic dikes rhyodacite lava vitreous ignimbrites felsic dikes granites sample id: 217 218 312 308 301 318 225 230 223 224 241 320 305 311 313 primary (%) sio2 56.7 58.9 60.8 51.6 75.6 76.2 65.7 77.3 al2o3 17.5 15.2 18.1 16.0 11.7 11.2 15.2 11.8 tio2 1.0 1.1 0.6 1.4 0.1 0.1 0.6 0.1 fe2o3 5.9 6.73 7.8 5.6 5.2 8.7 1.5 2.4 2.2 3.2 1.8 1.5 1.8 2.3 1.3 mno 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 mgo 2.3 1.1 0.7 5.0 0.1 0.1 0.9 0.2 cao 6.5 6.9 2.3 7.4 1.2 0.7 1.9 0.6 na2o 8.08 5.42 3.3 3.7 7.9 3.4 3.2 6.1 5.1 8.1 5.0 0.3 1.7 5.4 3.7 k2o 1.6 3.5 2.6 1.7 5.5 6.7 4.1 4.3 p2o5 0.3 0.4 0.3 0.4 0.0 0.0 0.1 0.0 loi 2.4 4.6 1.6 3.3 3.7 1.8 2.5 1.0 total 99.6 101.1 100.3 99.1 99.8 100.3 98.8 100.4 trace (ppm) ba 1230 1696 949 1090 1110 421 588 899 1200 922 972 101 94 934 139 cs 1.9 1.8 2.1 3.5 0.8 0.9 3.4 1.5 1.6 1.6 0.8 2.9 4.5 1.8 5.6 sr 881 789 545 480 791 372 186 243 185 175 148 22 38 250 29 rb 45 138 48 44 197 202 97 368 cr 14 7 -20 -20 -20 82 1 1 0 0 1 -20 -20 -20 -20 sc 11.8 14.6 16 15 6 24 3.7 5.5 8.3 8.7 7.4 9 7 7 2 ni 7 4 -20 26 -20 41 3 1 2 4 4 -20 -20 -20 -20 co 14 18 28 22 20 35 3 3 1 2 1 12 15 16 19 v 123 161 87 195 8 29 29 6 cu -10 27 -10 29 -10 -10 21 -10 zn 76 165 87 101 103 60 40 35 ta 0.37 0.34 0.4 0.7 0.3 0.5 0.46 0.8 0.88 1.14 0.68 1.1 1 0.7 3.1 u 1.3 1.1 1 4.8 1.2 1.7 1.7 3.7 3 3.5 3 3.3 2 3 12.1 th 3.2 3.2 4.5 10.9 3.5 6.5 9 15.7 13.3 16.3 8.9 14.1 11.9 13.5 43.3 hf 6.2 5.8 5.6 9.3 6.2 5 5 9 8.5 12.1 7.9 7.3 8.2 8.8 7.1 y 27 31 21 27 54 40 25 73 zr 233 338 259 213 208 248 345 199 nb 8 11 7 8 15 14 10 24 w 27 48 67 43 72 87 51 131 tl 0.4 0.3 0.2 0.2 1.2 0.8 0.5 2.2 pb 14 42 46 56 21 18 10 21 (continued) 132 amancay martinez, adrian gallardo, laura giambiagi, laura tobares andesitic breccia basalts and andesite lavas mafic dikes rhyodacite lava vitreous ignimbrites felsic dikes granites trace (ppm) bi -0.4 -0.4 -0.4 -0.4 -0.4 -0.4 -0.4 -0.4 ga 23 24 20 20 23 20 20 25 ge 1 1 2 1 1 2 -1 2 as 8 -5 -5 -5 -5 30 -5 -5 mo -2 2 -2 -2 -2 -2 -2 -2 ag -0.5 -0.5 2.1 -0.5 -0.5 -0.5 -0.5 -0.5 in -0.2 -0.2 -0.2 -0.2 -0.2 -0.2 -0.2 -0.2 sn -1 14 1 2 2 -1 -1 4 sb 1.2 1.8 1.9 2.8 -0.5 0.9 -0.5 -0.5 be 2 2 3 2 3 4 3 7 rre (ppm) la 32.5 28.5 32.8 38.6 29.4 29.4 21.6 32.6 40.6 50.3 36.0 37.3 60.4 32.7 42.9 ce 68.6 64.8 74.7 81.1 68.6 67.7 50.9 77.4 90.3 82.8 93.4 90.6 132.0 77.2 90.9 pr 8.8 10.7 8.1 8.0 11.1 14.6 8.9 11.3 nd 36.3 34.3 31.1 42.4 28.4 29.9 19.2 31.5 45.7 32.7 38.5 38.6 57.8 30.7 41.1 sm 6.4 6.5 5.5 7.0 5.0 6.0 3.8 6.4 8.0 8.1 7.3 7.7 11.4 5.6 7.7 eu 1.7 1.9 1.7 1.5 1.6 2.0 0.6 0.9 1.2 1.4 1.3 0.2 0.1 1.2 0.3 gd 5.4 6.9 4.5 5.5 8.7 9.3 4.8 7.9 tb 0.7 0.8 0.8 1.1 0.6 0.8 0.4 0.8 1.1 1.0 0.9 1.4 1.5 0.7 1.5 dy 4.5 5.5 3.5 4.8 8.6 7.7 4.2 9.8 ho 0.9 1.0 0.7 1.0 1.9 1.4 0.8 2.1 er 2.7 3.4 2.0 2.6 5.6 3.7 2.4 7.3 tm 0.4 0.5 0.3 0.4 0.9 0.5 0.4 1.2 yb 2.5 2.4 2.4 3.0 2.0 2.5 1.3 2.6 4.3 4.1 3.6 5.2 3.4 2.4 8.4 lu 0.3 0.3 0.4 0.4 0.3 0.4 0.2 0.4 0.6 0.5 0.5 0.8 0.5 0.4 1.2 chemical composition of the choiyoi group rocks. small fault ahead of the tunnel face in coal roadway is the important hidden hazard factor of coal and gas outburst accidents. the study of small fault prediction has important practical significance, which is the urgent demand of coal mine safety production. the diffraction of breakpoint can be used to identify the fault. however, unlike surface seismic exploration, the diffraction is with approximately horizontal incidence when the advanced detection is carried out in the roadway. the common advanced detection system is mainly as the reference of traffic tunnel, without considering the influence of low-velocity coal seam. considering the influence of an acoustic wave of the roadway cavity and channel wave of the coal seam, the advanced detection model of small fault ahead of tunnel face is established. diffraction advanced observation system in which sources located in front of tunnel face is constructed, and the numerical calculation of the high-order staggered-grid finite difference is carried out. the simulation results show that: compared with the data collected by reflection observation system, in seismic records acquired by diffraction observation system, the suppression effect of acoustic wave is appeared. the diffracted p-wave of the breakpoint of component x is clear with strong energy and short-wave group. multiple diffractions of the breakpoint are not found, but the multiple diffraction of tunnel face endpoint is obvious. the difference between breakpoint diffraction and multiple diffractions of the endpoint is clear, and diffracted p-wave of the breakpoint is easy to identify. the multiple reflected channel wave between the fault and the tunnel face is very obvious, and the reflected channel wave of small fault is so hard to identify. migration results show that the imaging resolution of diffracted p-wave of small fault is higher than the reflected channel wave, and breakpoint location of imaging is consistent with the actual model. abstract keywords: coal roadway; small fault; diffraction; reflected channel wave diffraction characteristics of small fault ahead of tunnel face in coal roadway issn 1794-6190 e-issn 2339-3459 earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): 95 99 bo wang1, shengdong liu1*, fubao zhou2, jun zhang1, fangkun zheng1 1 state key laboratory of deep geomechanics & underground engineering and school of resource and earth science, china university of mining and technology, xuzhou 221116, china 2 school of safety engineering, china university of mining and technology, xuzhou 221116, china *email of corresponding author: wbsyes@126.com la falla pequeña en la calzada de carbón de un túnel es un factor importante no visible en los accidentes por explosión en minas de gas y carbón. el estudio de la predicción de falla pequeña tiene un importante sentido práctico: la demanda urgente de seguridad en la producción carbonífera. el punto de quiebre de la difracción puede utilizarse para identificar la falla. sin embargo, al contrario que la exploración sísmica superficial, la difracción se acerca a la prevalencia horizontal cuando se realiza la detección avanzada en la calzada. el sistema común de detección avanzada se usa principalmente para referenciar el tráfico del túnel, sin considerar la influencia de la baja velocidad en la veta de carbón. al valorar la respuesta de la onda acústica en la cavidad de la calzada y la onda de canal de la veta de carbón se establece el modelo de detección avanzada de pequeña falla para la calzada del túnel. se construyó el sistema de observación avanzada de difracción en el cual las fuentes se localizan en frente de la cara del túnel y se realizó el cálculo de diferencia finita en una red escalonada de orden alto. los resultados del modelo muestran que a diferencia de la información recolectada con el sistema de observación de reflexión, en los registros sísmicos adquiridos con el sistema de observación de difracción se puede ver el efecto de supresión de la onda acústica. el punto de quiebre de la onda p difractada para el componente x es claro, con energía fuerte y en el grupo de onda corta. no se encontró el punto de quiebre para difracciones múltiples pero es evidente la difracción múltiple para el punto final de la cara del túnel. es clara la diferencia entre el punto de ruptura de la difracción y las difracciones múltiples del punto final, mientras el punto de ruptura de la onda p difractada es fácil de identificar. los resultados de migración muestran que la resolución de imágenes de la onda p difractada de falla pequeña es mayor que la onda de canal reflejada y la ubicación del punto de quiebre de la imagen es consistente con el modelo actual. resumen palabras clave: calzada de carbón; pequeña falla; difracción; onda de canal reflejada. record manuscript received: 15/05/2017 accepted for publication: 30/06/2017 how to cite item: wang, b., liu, s., zhou, f., zhang, j., & zheng, f. (2017). diffraction characteristics of small fault ahead of tunnel face in coal roadways. earth sciences research journal, 21(2), 95 99. doi:http://dx.doi.org/10.15446/esrj.v21n2. 64938 características de difracción de falla pequeña en la calzada de carbón de un túnel http://dx.doi.org/10.15446/esrj.v21n2.64938 s e is m ic p r o c e ss in g 96 wbo wang, shengdong liu, fubao zhou, jun zhang, fangkun zheng 1 introduction in china, the deaths from 2012 to 2016 was 451 because of coal mine gas accidents (liu et al., 2016), so coal mine gas prevention and control are very grim, and gas control is the most important work of coal mine safety production. most accidents of coal and gas outburst in tunnel face of coal roadway occur near the geological structure (shepherd et al., 1981; lama et al., 1998), especially small fault (gao et al., 2015). this paper focuses on the research of small fault ahead of the tunnel face, which is the important hidden factor of coal and gas outburst accident. it is of great practical significance to the prevention and control of gas accidents. at present, there are a lot of advanced geophysical exploration methods adopted in the roadway, which can be divided into three classes: seismic class, electromagnetic class, and other class (roslee et al., 2017). at present, the seismic wave method is less affected by the detection environment, which is most suitable for the prediction of geological structure (lüth et al., 2006; jetschny et al., 2011). the seismic wave method of advanced detection mainly includes the reflected wave method, surface wave method, scattered wave method, channel wave method, diffraction method and so on (wang et al., 2016). the advanced detection by using reflected wave method is started early in the field of traffic tunnel. according to the launching order, seismic advanced detection technology includes: hsp, tvsp, tsp, trt, isis, tgp, tsd, usp, tswd, etc. (inazaki et al., 1999; otto et al., 2002; lüth, et al., 2005; jetschny et al., 2010). compared with the advanced tunnel prediction, the research on reflected wave in coal roadway is few, including rtsp, msp, etc. (wang et al., 2015). in the aspect of advanced detection of surface wave and scattered wave in underground coal mine, the advanced detection technique of rayleigh surface wave was carried out in fangezhuang mine (cheng et al., 2014). zhao et al. (2006) obtained higher image positioning accuracy based on inverse scattered imaging theory. cheng et al. (2013) studied the seismic scattered wave imaging in coal roadway by using numerical simulation method. in the aspect of reflected channel wave, essen et al. (2007) analyzed the response characteristics of rayleigh channel wave of split coal through numerical simulation; yang et al. (2012) carried out numerical simulation research with seismic wave field of small structure, and then pointed out that the rayleigh surface wave generated from the reflection of rayleigh channel wave can be used as the characteristic wave of the advanced detection of small structure. lu et al. (2013) obtained the reflected channel wave by using tunnel boring machine as the source to carry out the advanced detection survey with a vertical fault with fault throw of 8m. in the aspect of the diffraction, yang et al. (2010) analyzed the wavefield of the advanced detection model of the geological interface, and then pointed out that the diffraction was the powerful wave to identify the interface. deng (2012) pointed out that the diffraction of breakpoint due to the large-scale fault was the effective wave to detect the position of the coal seam. however, the studies on advanced detection in the underground coal roadway do not include the study of diffraction characteristics of small fault; therefore, the characteristics of diffraction wavefield of small fault are studied in this paper through the numerical simulation, so as to solve the fine imaging of small fault ahead of tunnel face. 2. principle when a seismic wave meets coal seam breakpoints, these discontinuity points can be regarded as the new sources, which can produce a kind of new disturbance to propagate around the elastic medium (lindang et al., 2017; kamsani et al., 2017). this wave of disturbance is called diffraction. in essence, the physical premise of diffraction generation is the wave impedance difference and minor geological structure. the diffraction follows a concept of full wavelength, and its propagation is in agreement with the “huygens fresnel” principle. the relation between the incident wave and the diffracted wave is one-to-many, while the relation between the incident wave and the reflected wave is that one-to-one, as shown in figure 1. the reflected wave is generated when the seismic wave encounters the medium of sudden change in the propagation process, which is the change of travel time and amplitude caused by large-scale heterogeneity lai et al., (2017). because the fault throw in coal roadway is less than 10m, it is hard to form the reflection of the fault. however, due to the small scale of the fault and the appear wave impedance difference between coal seam and surrounding rock (the average density and p-wave velocity of coal seam and surrounding rock are respectively as: ρ1=1400kg/m3, v1=2000m/s; ρ2=2800kg/m3, v2=3500m/s), the obvious diffraction could be formed in the process of seismic advanced detection in coal roadway, as shown in figure 2. figure 1. diagram of reflection and diffraction. figure 2. diffraction of advanced detection. 3. high-order staggered-grid finite difference in general, the structure of coal seam is layered. coal seam, layered roof, and floor can be regarded as horizontal isotropic media (lai et al., 2017). the equation of the acoustic wave propagating in two-dimensional isotropic media is as follows: (1) where p = p(x,z,t) represents acoustic wave field, v = v(x,z,t) represents velocity field. the difference scheme of time second-order and space 2nth-order difference accuracy is as follows: (2) the source is the spike pulse, and ricker wavelet is used as spike pulse source and time domain formula of ricker wavelet is as follows: (3) where a is the amplitude, f0 is the dominant frequency of wavelet, t0 is the beginning time? 97diffraction characteristics of small fault ahead of tunnel face in coal roadway when the forward modeling of 2nth-order difference accuracy in the condition of the regular grid is carried out, the stability condition is as follows: (4) taking into account the requirements of computational efficiency and simulation accuracy, time two-order and space tenth-order are adopted in this paper and c(l) is 0.541. the pml absorbing boundary is adopted as a model boundary to reduce the boundary disturbance caused by the artificial boundary. 4. wavefield characteristics analysis figure 3 shows the advanced detection model of small fault ahead of tunnel face with the fault throw of 10m. the dip angle of fault is 45 degrees, and the model size is 300 m × 300 m. the grid spacing is set as 0.25 m × 0.25 m, and sampling interval is δt=0.1 ms. taking into account the source features of field survey, the seismic frequency is set as 400 hz. the coordinate range of the tunnel face is x=130 m and z=149 ~ 152 m, while receivers are respectively located in the roadway roof and floor. the number of receivers is 40 in the roof and floor. (a) x-component receiver in roof (b) x-component receiver in floor figure 3. model of advanced detection. the receivers are located in the scope of x=91 ~ 169 m with trace intervals of 2 m. the depth of the receivers is 2 m in roof and floor. the two components x and z are used for geophone receiving, and x direction denotes the tunneling direction, and z direction denotes the direction being perpendicular to the floor (roslee et al., 2017). the coordinate of the source is (134, 149), and specific medium parameters of the model are shown in table 2. table 1. model parameter. (a) z-component receiver in roof (b) z-component receiver in floor figure 4. seismic data of diffraction advanced observation system. (a) x-component (b) z-component figure 5. seismic wavefield snapshots. 98 wbo wang, shengdong liu, fubao zhou, jun zhang, fangkun zheng figure 4 shows the seismic data of component x and component z with 0~80 ms, while figure 5 shows the wavefield snapshots of components x and z with 16 ms, 40 ms, and 57 ms. according to the seismic records and the wavefield snapshots of the model, with reference to kinematics and dynamics characteristics of various types of seismic wave, all kinds of waves can be recognized from the complex seismic wavefield, and they are as follows: a direct p-wave, this direct p-wave is with circular propagation from the source to the surrounding rock. b diffracted p-wave, is formed by the diffraction of the direct p-wave when propagating to breakpoint of the fault. c 1# reflected channel wave, is formed by the reflection of the direct channel wave when encountering fault. d 2# reflected channel wave, is formed by reflection of 1# reflection channel wave when encountering the tunnel face; e 3# reflected channel wave, is formed by the reflection of 2# reflection channel wave when encountering fault; f the roadway acoustic wave, is formed by the direct p-wave when propagating into the cavity of the roadway; g the diffracted p-waves at the endpoint of the tunnel face are the multiple diffractions when direct p-wave encounters the endpoint (kamsani et al., 2017). the fundamental difference b between g diffracted p-wave and diffracted p-wave is as follows: b diffracted p-wave can also be observed from the receivers ahead of tunnel face and behind tunnel face, while g diffraction can only be observed from the receivers behind tunnel face. the diffraction b can be clearly identified based on the above difference. it can be seen from figure 4 that the resolution of breakpoint diffraction b is highest in seismic data of component x, and the diffraction is recorded by the receivers in the floor. the waveform of the diffraction b is clear with strong energy and short-wave group. in the figure, the reflected channel wave of the fault is also obvious, but the wave group of the channel wave is wider than diffraction, which causes the low resolution when using the channel wave. due to the existence of the channel wave in the coal roadway and the roadway axial being parallel with the horizontal coal seam, the common observation system of the tunnel reflected wave should be with poor applicability when detecting an advanced fault. concerning to reflection observation system commonly used in traffic tunnel (liu et al., 2016), the sources are arranged in the rear of tunnel face, which results in figure 6 (kamsani et al., 2017) . from the figure, it is found that the acoustic wave in seismic data is specially developed and the diffraction of the breakpoint is not identified. it can be seen from figure 7, the lateral variation range of the fault location of reflected channel wave is about 20 m, and the distribution range of the channel wave migration imaging is large, and it is difficult to determine the position of small fault. the lateral variation range of breakpoint of diffraction is about 2 m, and the wave group of diffraction is short and high resolution, and imaging position of breakpoint of small fault is in accordance with the actual model, and the direction of breakpoint in full space is easily computed by polarization migration (wang et al., 2016). (b) diffracted p-wave figure 7. migration profile. 6. conclusion in this paper, considering the influence of the roadway cavity and coal seam with low velocity, the advanced detection model of small fault in front of heading face is established. the numerical simulation calculation is carried out by diffraction advanced observation system. the through comparative analysis between the diffraction and reflection observation system, and there are not acoustic wave and obvious diffracted p-wave with short wave group by diffraction observation system in seismic data of component x. compared with multiple diffractions of the endpoint, the diffracted p-wave is clear and easy to recognize and extract, and the reason is that: diffraction of small fault can also be observed from the receivers ahead of tunnel face, but diffraction of the breakpoint should not be observed from the receivers ahead of tunnel face. the reflected channel wave of small fault is difficult to identify because there are multiple reflected channel waves between the fault and the tunnel face. therefore, the diffraction of coal seam breakpoint is the naturalwave of small fault, and the results of the migration show that the imaging resolution of diffraction is high and breakpoint location of imaging is consistent with the actual model. acknowledgements this research has been performed by the national natural science foundation project (no. 41604082, 51323004, 41474122) and joint funding project of national natural science foundation and shenhua group corporation ltd (no.u1261202), and the fundamental research funds for the central universities (2014xt02). a project funded by the priority academic program development of jiangsu higher education institutions. (a) x-component receiver in roof (b) x-component receiver in floor figure 6. seismic data of reflection observation system. 5. advanced imaging of small fault based on the above analysis, the diffracted p-wave and the reflected channel wave are both the effective waves of advanced detection of a small fault in coal roadway. based on the principle of prestack diffraction migration, the contrastive study is carried out with diffraction of the breakpoint and the channel wave (roslee et al., 2017; lindang et al., 2017; kamsani et al., 2017; lai et al., 2017) . figure 7 shows the imaging results of b diffracted p-wave and c 1# reflected channel wave of component x which is received on the floor. the starting position of x-axis is 91m. 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aperture condition.” acta geodynamica et geomaterialia, 13 (2016): 47-58. yang, s. t., cheng, j. l., “numerical simulation of fore detecting with seismic in coal roadway and study of wave field characteristics.” journal of china coal society, 35 (2010): 1633-1637. yang, s. t., cheng, j. l., “the method of small structure prediction ahead with rayleigh channel wave in coal roadway and seismic wave field numerical simulation,” chinese journal of geophysics, 55 (2012): 655-662. zhao, y. g., jiang, h., zhao, x. p., “tunnel seismic tomography method for geological prediction and its application.” applied geophysics, 3 (2006): 69-74. given the difficulty of accurate online detection for massive data collecting real-timely in a strong noise environment during the complex geological mineral grade analysis process, an order self-learning arhmm (autoregressive hidden markov model) algorithm is proposed to carry out online outlier detection in the geological mineral grade analysis process. the algorithm utilizes ar model to fit the time series obtained from “online x ray fluorescent mineral analyzer” and makes use of hmm as a basic detection tool, which can avoid the deficiency of presetting the threshold in traditional detection methods. the structure of traditional bdt (brockwell-dahlhaus-trindade) algorithm is improved to be a double iterative structure in which iterative calculation from both time and order is applied respectively to update parameters of arhmm online. with the purpose of reducing the influence of outlier on parameter update of arhmm, the strategies of detection-before-update and detection-based-update are adopted, which also improve the robustness of the algorithm. subsequent simulation by model data and practical application verify the accuracy, robustness, and property of online detection of the algorithm. according to the result, it is obvious that new algorithm proposed in this paper is more suitable for outlier detection of mineral grade analysis data in geology and mineral processing. abstract keywords: arhmm; bdt; kicvc; outlier detection; online detection. online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process detección en tiempo real de valores atípicos sobre series de tiempo variable en arhmm mejorado durante el proceso de análisis de grado mineralógico issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.65215 jianjun zhaoa,b, junwu zhoubb, weixing suc*, fang liuc a school of information science & engineering, northeastern university, shenyang 110004, china; b beijing general research institute of mining & metallurgy, beijing 100160, china c school of computer science & software engineering, tianjin polytechnic university, tianjin 300387, china *email of corresponding author: 15900201597@163.com existe gran dificultad para la detección en tiempo real para series de datos masivos con altos niveles de ruido de valores atípicos. se propone un algoritmo de autoaprendizaje arhmm (modelo autoregresivo oculto de markov) para llevar a cabo la detección de dichos valores atípicos en el proceso de análisis del grado mineral geológico. el algoritmo usa un modelo ar para ajustar la serie de tiempo obtenida del “analizador de fluorescencia de rayos x” y hace uso del hmm como una herramienta básica de detección, la cual puede evitar la deficiencia de predeterminar el umbral en métodos tradicionales de detección. para actualizar los parámetros del arhmm en tiempo real, la estructura del algoritmo bdt (brockwell-dahlhaus-trindade) tradicional se mejora para ser una doble estructura iterativa en la que se aplica el cálculo iterativo en tiempo y en orden respectivamente. con el propósito de reducir la influencia de valores atípicos (o extremos) en la actualización del parámetro de arhmm, se adoptan las estrategias de detección-antes-que-actualización y la detección-basada-en-actualización, lo que también aumenta la robustez del algoritmo. la subsiguiente simulación por modelos de datos y aplicación práctica comprueba la precisión, fortaleza y capacidad de la detección en línea del algoritmo. de acuerdo con el resultado, es evidente que el nuevo algoritmo propuesto en este artículo es más apropiado para la detección de datos de valores atipicos para el análisis del grado mineral en geología y el procesamiento mineral. resumen palabras clave: modelo autoregresivo oculto de markov; detección en tiempo real; brockwelldahlhaus-trindade. record manuscript received: 21/02/2017 accepted for publication: 28/07/2017 how to cite item zhao, j., zhou, j., su, w., & liu, f. (2017). online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process. earth sciences research journal, 21(3), 135-139. doi: http://dx.doi.org/10.15446/esrj.v21n3.65215 g e o st at is t ic s earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 135 139 136 jianjun zhao, junwu zhou, weixing su, fang liu introduction mineral composition analysis is a key factor in determining whether or not to carry out mining. over the years, many scholars have proposed some new ideas and methods for accurate mineral grade assessment, many of which are based on chemical or physical test equipment to obtain the data for ingredient grade analysis (kameshwara, rao, & narayana, 2014; de'nan, naaim, & leong, 2017). therefore, the accuracy of the data used for ore composition analysis is critical to the ore grade analysis. at present, automated testing equipment is used in ore grade analysis, such as “box-a type on-stream x-ray fluorescence analyzer”, which uses spectral obtain by irradiating x-rays to the pulp to get the results of ore grade. it is worth noting that box-a type on-stream x-ray fluorescence analyzer by default is that the spectral data obtained is correct. but whether it is chemical or physical testing equipment are inevitably produced abnormal data. those outliers directly affect the analysis results of the mineral products analysiser (clarke, & levis, 1998; rivoirard, demange, & freulon, 2013). therefore, the detection and elimination of these abnormal data is the premise and key to the above ore grade analysis work. a new algorithm is proposed here to especially do outlier detection for ore inspection data which obtain from chemical or physical testing equipment. the algorithm utilizes ar model to fit the time series and makes use of hmm as a basic detection tool, which can avoid the deficiency of presetting the threshold in traditional detection methods. to update parameters of arhmm online, the structure of traditional bdt (brockwell-dahlhaus-trindade) algorithm is improved here, and a double iterative structure in which iterative calculation from both time and order is applied respectively. with the purpose of reducing the influence of outlier on parameter update of arhmm, the strategies of detection-before-update and detection-based-update are adopted, which also improve the robustness of the algorithm. subsequent simulation by model data and practical application verify the accuracy, robustness, and property of online detection of the algorithm. in this paper our innovations are shown as follow: 1. unlike other outlier detect method (such as the traditional ar model detection method), the outliers detect method proposed in this paper does not need to set the detection threshold. 2. considering the problem that the model order of chemical or physical testing equipment’s hard to be determined, the new detected method which is based on residual error has the function of model order self-learning. 3. in a view to avoid the influence of outliers on the test results, this paper proposes a detection-before-update and detection-based-update strategies. the predecessors’ achievements on outlier detection many good ideas and methods are put forward for the research of outliers detection problem, such as that barnett and lewis proposed an outliers detected method based on statistics in their word named ‘outlier in statistical data (barnett & lewis, 1994). outlier detection method based on distance is proposed by knorr and raymond (knorr & ng, 1999; edwin & raymond, 1998), an new detected method based on density is suggested by ramaswamy et al. (2000). but for ore inspection data, the detection methods based on distance, density or variance is a lack of feasibility since an online real-time detection method be needed for the ore test data. with the research of anomaly data detection technology, many new ideas and techniques are introduced, such as clustering analysis (almeida & barbosa, 2007) and neural network method (bullen, cornford, & nnbney, 2003; prakobphol, & zhan, 2008). but clustering analysis method is also not suitable for online outlier detection for extensive test data, and neural network method requires a lot of data to model learning. in 1995, ragaran and argrawal put forward the concept of "sequence anomaly" (han & micheline, 2001) and proposed the detection method based on deviation (takeuchi, & yamanishi, 2006). because this method needs to know the order of the model, it can not be directly used for the outlier detection of mineral grade analysis data. structure of double iteration in bdt to make the bdt algorithm can be calculated online, the improved bdt algorithm with double repetition structure is proposed in this paper. traditional bdt algorithm the traditional bdt algorithm is improved by levinson-durbin algorithm which is proposed by brockwell et al. (2002). for traditional bdt algorithm, using all the data to the iterative calculation of model order, in a view to obtain the order of the forward and backward ar model. xt, t =1,2,∙∙∙ is the test data waiting for detection, where xt is m -dimension vectors. so forward ar model can be express as equation 1. (1) in which, ɛk, (t) is forward residual under k order model, which obeys gauss distribution with zero means. ak (i) is the coefficient of forwarding ar model under k order model. so backward ar model can be written as equation 2. (2) with minimizing all data forward and backward residual as the target, so the generalized objective function written as equation 3 (trindade, 2003). (3) n is the number of the data. ω1 and ω2 are weighted coefficient matrix for forwarding and backward ar model, and in bdt algorithm, the values of ω1 and ω2 is 1. ἓk (t) and are estimated residual values for forward and backward ar model individually. (4) (5) in which, are m -dimension matrix. the traditional bdt algorithm can be written in full as followed: (6) (7) (8) (9) (10) (11) (12) (13) in equation 6 to equation 13, ûk and v̂k are estimated variance for forwarding and backward noise. the initial condition for traditional bdt algorithm are: (14) 137online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process (15) (16) the subscript ø express that when the initial iteration, the model order set is empty. • double iteration bdt algorithm the objective function of improved bdt algorithm also is equation 3. the dynamic performance of the algorithm is enhanced by the forgetting factor . the improved bdt algorithm has double loop structure which model order is inner loop and time is the outer loop. we set which is part of equation 6. so: (17) in equation 7, k a set maximum value for model order. considering the time-varying characteristics of the model parameters, the forgetting factor is added to the outer loop(time loop) updates. (18) (19) in equation 18, ri t is the mean of the covariance matrix for ἓi-l (t) and ἢi-l (t-i) in time . similarly, equation 13 can be rewritten as: (20) so the calculation process of double iteration algorithm is illustrated in figure 1. conditional probability. the other is expressed as observation probability matrix calculated by ar model. (21) in equation 21, n(•) is gauss function, and ʃ̂k is estimated the variance of gauss distribution. arhmm outlier detection algorithm also composed of two steps: one step-preliminary detection from equation 1, we can see that there is a deviation between estimated process data by ar model and real process data. (22) if the deviation ɛk (t) is only noise, it obeys gauss distribution. so the preliminary criteria for outlier detection are to determine the probability that the deviation follows gauss distribution. (23) in equation 23, st = 1 indicates that the real data detected is normal, st = 0 means it is the outlier. so the detection criteria can be expressed that: (24) in equation 23, the subscript p is the optimal model order calculated by kicvc criteria whose expression is: (25) in equation 25, is the mean of residual ɛk (t) under various model order (bilmes, 2006). (26) (27) two step-final detection in final detection, the result of preliminary detection is the observed value of hmm. so the final detection result obtained by viterbi algorithm (abd-krim, 2006): (28) for improved arhmm algorithm, when the data at t time is detected, the data before t time already is detected. so the traditional viterbi algorithm is request into: (29) (30) parameters updating by outlier the parameters of order self-learning arhmm algorithm need update online, and the parameters are estimated residual mean , state transition matrix a , and in improved bdt algorithm. specific update algorithm is as follows: figure 1. flow chart of double iteration algorithm implementation of order self-learning arhmm detection algorithm the traditional arhmm structure is composed of two parts (wang, & chiang, 2008): one is markov chain, which is expressed as initial state probability π and state transition matrix in which st is the state at time t, n is the total state for hmm, and is a 138 jianjun zhao, junwu zhou, weixing su, fang liu (a) : if is normal data, then maintained by equations 30, 31, otherwise, not updated at time t. (b) : since there are two states in arhmm algorithm. so the updated algorithm is: (31) in equation 31, n (ay) indicates the times of the situation that st-1 = i, st = j, appears (lou, 195). (c) , : is normal data, then , calculated by equation(14), otherwise, using data means to replace xt. (d) is normal data, then calculated by equation 19; otherwise, it calculated by equation (32). (32) (e) is normal data, then calculated by equation (18); otherwise, it calculated by equation 33. (33) results and discussion model-based validations to verify the accuracy of the algorithm to detect the order of the model, the three order model which reacting ore detection interference process are used to generate the data. the data is shown in figure 2-(a), and the order detection results are shown in figure 2-(b). in which: (35) as can be seen from (34), there are time vary parameters on the denominator. to verify the robustness of the algorithm, we add 10% of the white noise and eight anomalies. the data is shown in figure 3-(a), and the order detection results and outliers detected results are shown in figure 3-(b) and figure 3-(c). figure 2. data and results of order detection as can be seen from the order detection results, the proposed algorithm can accurately detect the model order through the short-term adjustment process. with a view to verify that the proposed algorithm not only can identify the optimal model order but also can detect the abnormal data accurately. we modified the open-loop model mentioned in the alex alexandridis paper to get the second set of data (alexandridis, sarimveis, & bafas, 2003). rabiner 1989). the modified model is as follows: (34) figure 3. data and detection results for model data fig.3-(a) is the data waiting for detecting. fig.3-(b) is the order estimated result, and fig.3-(c) is the outlier detection result. as can be seen from the result, for the third-order nonlinear time-varying system, the model-order self-learning algorithm proposed in this paper can find its optimal model order and accurately detect all the anomaly data. application to further verify the practicality of the improved arhmm outlier identified method, it is applied to the on-stream x-ray fluorescence analyzer, which, first of all, in the last century 70's by the finotec outotec company successfully developed and implemented to mineral processing practice. so far, finland outotec company still has more than 80% market share. in china, the beijing institute of mining and metallurgy following its analytical principles, developed in 2014 with the same function grade analyzer boxa type on-stream x-ray fluorescence analyzer (hekimoglu, eernoglu, & kalina, 2009). according to foreign reports, the measurement accuracy of the analyzer increased by 1%, will effectively improve the metal recovery rate of 0.2 or more, and now from the hardware to improve measurement accuracy has been very difficult, or input-output serious disproportionate, so more scholars turn to the analysis of the modeling technology to improve research. based on this situation, the accuracy of the data for the model of learning is essential our comparative tests are as follows: the first set of data is obtained as follows: first of all, we use the improved arhmm outlier detection algorithm proposed in this paper to pre-process the spectral signal which as the input of the analyzer, and get the ore grade results as the first set of data. 139online outlier detection for time-varying time series on improved arhmm in geological mineral grade analysis process the second set of data is obtained as follows: we use the traditional ar outlier detection algorithm proposed in paper by northey, mohr, & mudd (2014) to pre-process the spectral signal which as the input of the analyzer, and get the ore grade results as the second set of data. finally, the two sets of data are compared with the results of the ore grade laboratory test, the error of the two groups of test results as shown in table 1. table 1. the result of comparison between with outlier detection process and without outlier detection process it can be seen from the table that there is higher accuracy when using the improved arhmm outlier detection algorithm proposed in this paper to do the pre-processing of the spectrum compared to tradition ar outlier detected method, which since the improved arhmm outlier detected algorithm have more robustness and more suitable for non-linear systems. conclusions taking into account the lack of arhmm algorithm for ore grade analysis process, an order self-learning arhmm algorithm is proposed in this paper, whose innovation points are summarized as: first, unlike other outlier detection method (such as the traditional ar model detection method), the outliers detection method proposed in this paper does not need to set the detection threshold. second, considering the problem that the model order of control system‘s hard to be determined, the new detected method which is based on residual error has the function of model order self-learning. and third, to avoid the influence of outliers on the test results, this paper proposes a detectionbefore-update and detection-based-update strategies. so under above improving, arhmm algorithm can more accurately use to analysis the data in the geological mineral grade analysis process. in other words, the application field of arhmm algorithm has been expanded. subsequent simulation by model data and practical application verify the accuracy, robustness, and property of online detection of the algorithm. according to the result, it is evident that new algorithm proposed in this paper is more suitable for outlier detection in the geological mineral grade analysis process. acknowledgments this research is partially supported by national natural science foundation of china under grant 51607122, 61602343. references abd-krim, s. (2006). vector autoregressive model-order selection from finite samples using kullback’s symmetric divergence. ieee transactions on circuits and systems i: regular papers, 53(10), 2327. alexandridis, a., sarimveis, h., & bafas, g. (2003). a new algorithm for online structure and parameter adaptation of rbf networks. neural networks, 16(7), 1003-1017. almeida, j. a. s., & barbosa l. m. s. (2007). a new method with outlier detection and automatic clustering. chemometrics and intelligent laboratory systems, 87(2), 208. barnet, v., & lewis, t. (1994). outlier in statistical data. john wiley & sons, chichester. bilmes, a. j. (2006). what hmms can do. ieice transactions on information and systems, e89-d(3), 1. brockwell, p. j., dahlhaus, r., & trindade, a. a. (2002). modified burg algorithms for multivariate subset autoregression. technical report 2002-015, department of statistics, university of florida. bullen, r. j., cornford, d., & nnbney, i. t. (2003). outlier detection in scatterometer data: neural network approaches. neural networks, 16(3-4), 419. clarke, b. r., & levis, t. (1998). an outlier problem in the determination of ore grade. journal of applied statistics, 25(6), 751-662. de'nan, f., naaim, n., & leong, l. c. (2017). behaviour of flush end-plate connection for perforated section. engineering heritage journal, 1, 11-20. edwin, m. k. & raymond, t. n. (1998). algorithms for mining distancebased outliers. proceedings of the twenty-fourth international conference on very large data bases, c. 392. han, j. w., & micheline, k. (2001). data mining concepts and techniques. machinery industry press, china. hekimoglu, s., eernoglu, r. c., & kalina, j. (2009). outlier detection by means of robust regression estimators for use in engineering science. journal of zhejiang university-science a, 10(6), 909. kameshwara, r., rao, c. r., & narayana, a. c. (2014). assessing grade domain of iron ore deposit using geostatistical modelling: a case study. journal of the geological society of india. 83(5), 549-554. knorr, e. m. & ng, r. t. (1999). finding intentional knowledge of distance-based outliers. proceedings of the twenty-fifth international conference on very large data bases, c. 211. lou, h. l. (1995). implementing the viterbi algorithm. ieee signal processing magazine, 1053-5888, 42. northey, s., mohr, s., & mudd, g. m. (2014). modeling future copper ore grade decline based on a detailed assessment of copper resources and mining. resources conservation and recycling, 83, 190-201. prakobphol, k., & zhan j. t. (2008). a novel outlier detection scheme for network intrusion detection systems. proceedings of the second international conference on information security and assurance, c. 555. rabiner, r. l. (1989). a tutorial on hidden markov models and selected applications in speech recognition. proceedings of the ieee. 77(2), 257. ramaswamy, s., rastogi, r., & kyuseok, s. (2000). efficient algorithms for mining outliers from large data sets. proceedings of the acm sigmod international conference on management of data dallas, c. 427. rivoirard, j., demange, c., & freulon, x. (2013). a top-cut model for deposits with heavy-tailed grade distribution. mathematical geosciences, 45(8), 967-982. takeuchi, j., & yamanishi, k. (2006). a unifying framework for detecting outliers and change points from time series. ieee transactions on knowledge and data engineering, 18(4), 482. trindade, a. a. (2003). implementing modified burg algorithms in multivariate subset auto-regressions modeling. department of statistics, university of florida. wang, j. s. & chiang, j. c. (2008). a cluster validity measure with outlier detection for support vector clustering. ieee transactions on systems, man, and cybernetics, part b (cybernetics), 38(1), 78. palabras clave: experimento de columna predicción del transporte de solutos; columna de suelo no perturbado. how to cite item godoy, v. a., napa-garcía, g. f., & zuquette, l. v. (2018). determination of the sodium retardation factor using different methods: analysis of their characteristics and impact on the solute movement prediction in a structured brazilian soil. earth sciences research journal, 22(4), 281-289. doi: https://doi.org/10.15446/esrj.v22n4.68100 the retardation factor (rd) is one of the main important solute transport parameters. its value can vary significantly depending on the method used for its determination. in this paper, the sodium rd is experimentally determined using undisturbed sandy soil columns to compare four methods of rd determination and assess the impact of the chosen method on the prediction of sodium movement. column experiments in undisturbed soil samples and analytical analysis were performed. the results showed that the soil has dual-porosity and preferential pathways. the breakthrough curves were in accordance with the soil’s physical characteristics. the rd values ranged from 1.7 to 7.77 depending on the initial concentration and on the method used. these differences arise from the conceptual model of each rd determination method. the experimental and analytical analysis indicated that the higher the rd, the slower the movement. the methods that best reproduced the laboratory sodium movement were ogata and banks’ (1961), langmuir and freundlich’s isotherms. the prediction models presented smaller errors with the increase of the initial concentration. in these cases, the predicted concentrations can be overestimated up to 22.5 % when using a not suitable method. these results suggest that the rd determination method can strongly affect the prediction of the sodium movement. because of that, it is of vital importance to evaluate each method and how they can be adequate for the soil under investigation when determining rd. abstract keywords: column experiment; solute transport prediction; undisturbed soil column. determination of the sodium retardation factor using different methods: analysis of their characteristics and impact on the solute movement prediction in a structured brazilian soil determinación del factor de retardo del sodio mediante métodos diferentes: análisis de sus características e impacto en la predicción del movimiento del soluto en un suelo brasileño estructurado issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.68100 el factor de retardo (rd) es uno de los principales parámetros para el transporte de solutos. su valor puede variar significativamente dependiendo del método utilizado para su determinación. en este artículo, el rd del sodio se determina experimentalmente utilizando columnas de suelo arenoso no perturbado, para comparar cuatro métodos de determinación de rd y evaluar el impacto del método elegido en la predicción del movimiento de sodio. se realizaron experimentos en muestras de suelo no perturbado y análisis analítico. los resultados mostraron que el suelo tiene doble porosidad y caminos preferenciales. las curvas de llegada estaban de acuerdo con las características físicas del suelo. los valores de rd variaron de 1.7 a 7.77 dependiendo de la concentración inicial y del método utilizado. estas diferencias surgen del modelo conceptual de cada método de determinación de rd. el análisis experimental y analítico indicó que cuanto mayor es la rd, más lento es el movimiento. los métodos que mejor reprodujeron el movimiento de sodio obtenido en el laboratorio fueron las isotermas de ogata y banks (1961), langmuir y freundlich. los modelos de predicción presentaron errores más pequeños con el aumento de la concentración inicial. en estos casos, las concentraciones pronosticadas pueden sobreestimarse hasta un 22,5% cuando se utiliza un método no adecuado. estos resultados sugieren que el método de determinación de rd puede afectar fuertemente la predicción del movimiento de sodio. debido a eso, al determinar rd, es de vital importancia evaluar cada método y cómo pueden ser adecuados para el suelo bajo investigación. resumen record manuscript received:03/10/2017 accepted for publication: 29/06/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 281-289 vanessa a. godoy1,3, gian f. napa-garcía2, lázaro valentin zuquette3 1institute of water and environmental engineering, universitat politècnica de valència 2vale institute of technology 3department of geotechnical engineering, university of são paulo. godoyalmeida@gmail.com en vi ro nm en ta l g eo te ch ni cs 282 vanessa a. godoy, gian f. napa-garcía, lázaro valentin zuquette introduction maintenance of groundwater resource quality depends on the knowledge of how solutes move in the aquifer to estimate the groundwater contamination and therefore to manage the health risks. as the processes associated with the groundwater flow and solute transport are complexes, numerical modeling is a powerful tool to perform predictions and base the decision-making processes. numerical models require input parameters that must be determined reliably to guarantee the quality of the prediction. one of the main input parameters in the modeling of reactive solute transport is the retardation factor (rd) (fetter, 1999). because of the physical, biological and chemical mechanisms involved between a liquid solution and particle surfaces of the soil, the velocity of water percolation, v, differs from the velocity of the solute, vs (freeze and cherry, 1979). the relationship between these two velocities is called retardation factor. when the solutes do not interact with the solid medium (are nonreactive), rd = 1, the percolation velocity of the solute and the water are equal. solutes with rd > 1 are called reactive solutes (freeze and cherry, 1979; shackelford, 1994). according to jury, gardner and gardner (1991), retardation factors exceeding 3 indicate a high degree of solid particles-solute interaction and a very reactive solute. the determination of the rd can be performed based on the direct evaluation of the experimental breakthrough curves (btcs) from column experiments (freeze and cherry, 1979; van genuchten and parker, 1984; godoy, gómez-hernández and zuquette, 2018). the rd can also be determined using inverse modeling by fitting the advection-dispersion equation (ade) to the experimental data (ogata and banks, 1961; markhali and ehteshami, 2016). solute transport experiments can be performed using undisturbed soil cores (vanderborght, timmerman, and feyen, 2000; logsdon keller and moorman, 2002; jarvis, 2007; jellali et al., 2010), however, in a more general way, disturbed soils columns are used (dousset et al., 2007; eberemu, amadi, and edeh, 2013; delwiche, lehmann and walter, 2014; liu et al., 2014; internò, lenti and fidelibus, 2015; silva et al., 2016). batch tests, despite their limitations, are also used to determine sorption isotherms and then rd (brigham, reed and dew, 1961; fagundes and zuquette, 2011; fonseca et al., 2011; barbieri et al., 2012; liu et al., 2014; arthur et al., 2017). sorption isotherms are efficient in the rd determination but their determination must consider the soil heterogeneity. the characteristics of the rd determination methods may affect its quantification. the use of an inadequate method to determine rd values may cause under or overestimation of the fate and solute transport, impairing the design of clean-up techniques and health risk analyses. this is especially important by considering that those who work with numerical modeling usually use the provided parameters with no concerns about the method used in its determination. studies which aim to compare rd determination methods are rare and this paper intends to contribute to reduce this lack (shackelford, 1995; porfiri et al., 2015) (3. additionally, the use of undisturbed soil samples to perform solute transport test and determine sorption isotherm have not been identified in the literature, and are also presented in this work. the objective of this study is twofold: (i) to compare four methods of rd determination and (ii) to assess the impact of the chosen method in the prediction of sodium movement. in order to achieve that, first, the sodium rd is experimentally determined using undisturbed sandy columns and then the rd were determined based on the methods proposed by freeze and cherry (1979), by van genuchten and parker (1984), by the fit of the sorption isotherms models to the experimental data obtained from column experiments, and by fitting the analytical solution of the ade to the experimental data. second, the rd obtained with all these methods were used as input parameter to reproduce the experimental btc using the analytical solution of the ade to evaluate the impact of a different rd in the prediction of the sodium movement. data and methods soil sampling and characterization the study was carried out in an experimental site located at the city of cabrália paulista (22°27′37″ s, 49°20′16″ w), state of são paulo, brazil. this area is constituted by the residual soil of sandstones from the adamantina formation, bauru group (k). the area was chosen because its geological materials are spread out at about 117,000 km² in the state of são paulo (paula e silva, kiang, and caetano-chang, 2003) and incorporate the guaraní aquifer system and some of its aquifer unities such as the lower and medium bauru unities. the soil samples were taken from an area quite close to one where salty wastewater from domestic sewage has been used for irrigation purpose. because of the high sodium (na+) concentration in the wastewater, this was the solute used in this study. we cautiously collected ten undisturbed samples from hand-excavated trenches using rigid pvc cylinders (150 mm length and 97.2 mm inner diameter). undisturbed samples were used to conduct water flow and solute transport experiments in the laboratory and to analyze the soil porosity. porosity analysis by mercury intrusion (mip) was performed in three samples (washburn, 1921). mip is based on the penetration of mercury into the soil pores, which occurs only under pressure since the mercury is a nonwetting fluid. the mercury penetration pressure should be higher the smaller the pore diameter, according to p t d s= −4 cos (1) where ts is the surface tension; α is the contact angle between the liquid and the surface of the solid; p is the applied pressure; and, d is the pore diameter. during the test, as the pressure is increased, the volumes of mercury inserted into the sample are monitored. using equation 1 the dv/dd differentials were obtained, which allowed obtaining the pore distribution of the sample. the apparatus used in these tests was the “pore size 9320 micromeritics instrument corporation”, with a maximum pressure of 30 000 psi. disturbed samples were taken to characterize, in three replicates, the physical, chemical, mineralogical and physicochemical properties of the soil (godoy and zuqutte, 2013). the mineralogical analysis was performed using x-ray diffraction(xrd) (azaroff and buerger, 1953), particle size analysis was performed according to astm d422-63. the density of the solids was determined according to astm d854-10. we also determined in three replicates the ph in h2o and in kcl, reduction potential (eh), electrical conductivity (ec) (donagema and campos, 2011), delta ph (phkcl – phh2o) (mekaru and uehara, 1972), point of zero charge (pzc) (2phkcl – phh2o) (keng and uehara, 1974), and organic matter content according to astm d2974 – 00. the arithmetic mean of the three replicates was used as the value of the measured parameter. the methylene blue adsorption test using the filter paper method described by pejon (1992) was used to determine cec in one replicate from each soil sample. in order to visually analyze the soil fabric characteristics as well as its capacity to retain solute, a solution of methylene blue (adsorbing dye) was percolated through the soil, a traditional method used to visualize flow pathways (zehe and flühler, 2001; dousset et al., 2007). first, the filter paper was placed across the base of an unsaturated and undisturbed soil column, so that the preferential paths would be marked in the cross-sectional area of the column. second, a volume of 1.5e-03 m³ of a methylene blue solution at a concentration of 3 kgm-3 was percolated through the soil. the soil was set aside to allow the excess solution to drain. third, the soil sample was removed from the pvc cylinder and longitudinally cut to observe the methylene blue pathways and visually analyze the retention and the flow through the pores. column experiments the pvc cylinders used for collecting the undisturbed soil samples were used as rigid-wall permeameters, in which column experiments were 283determination of the sodium retardation factor using different methods: analysis of their characteristics and impact on the solute movement prediction in a structured brazilian soil conducted. the soil columns were first slowly saturated from the bottom with distilled water to remove entrapped air. to ensure complete saturation, the distilled water was allowed to overflow 1 cm on the soil surface for two days. after column saturation, the flow was reversed, and the tests were performed under a constant hydraulic gradient of 1.5. once the steady-state condition was verified, that is, when flux (q) variations were below 5% in a week, the following water flow parameters were obtained from each column: the saturated hydraulic conductivity, k (darcy, 1856); darcy velocity, q (darcy, 1856)and average linear velocity, v (freeze and cherry, 1979). the mean effective porosity, ne, which is used in the calculation of v, was adopted as the percentage of pores larger than 10 µm based on the mip results (ahuja et al., 1984). three undisturbed soil columns were percolated with nacl and the effluent was collected at each pore volume (godoy, zuquette and napa garcía, 2015). the concentrations of the initial solution inserted in the columns 1, 2 and 3 were, respectively, 50, 90 and 117 mg.l-1 based on previous studies (patterson, 1997; medeiros et al., 2005; silva et al., 2012). solute displacement tests were carried out under isothermal (20 °c) conditions. the ph and electrical conductivity, both of the input solution and of the percolated effluent solution, were monitored throughout the test. the na+ concentrations were determined in three replicates using a micronal b26 flame photometer at a 1:21 dilution ratio. the column experiments design was also described by godoy, zuquette and napa garcia (2015). determination of retardation factors the rd values were determined by four methods. first, rd was determined inversely by fitting the analytical solution of the ade (ogata and banks, 1961) to the experimental btc, r c t d c x v c xd h       = −² ² (2) where c is the concentration [ml-1], dh is the longitudinal dispersion coefficient [l2t-1], v is the linear velocity of the fluid through a column [lt-1], and rd is the retardation factor. the analytical solution of the equation 2 was proposed by ogata and banks (1961) and lapidus and amudson (1952) and was used to solve the ade when the initial condition is c=0 for the entire sample, and the boundary conditions are c=c0 at the inlet and c=0 at an infinite distance from the inlet. c c r l vt d r t vl d r l vt d r d h d h d h0 1 2 2 1 2 2 = −      +     +erfc erfcexp ddt               (3) second, rd was determined using the method proposed by van genuchten and parker (1984) where the rd is the area above the btc; third, the rd determination was performed by using the method proposed by freeze and cherry (1979) where the rd is equal to the pore volume when the relative concentration (c/c0) is 0.5; and fourth, the rd was found by fitting the sorption isotherms, obtained through column test, to the linear, freundlich and langmuir isotherm models (freeze and cherry, 1979; garga and o’shaughnessy, 1994; fetter, 1999; asgari et al., 2014). the ade and the isotherms models’ fits were performed using mathematica 9.0. table 1 shows the equations that were used to determine the rd from different sorption isotherms. lastly, the rd obtained with the aforementioned methods were directly used to predict the experimental btc using the analytical solution of the ade. the coefficient of determination r2 was computed and used to assess the goodness of the rd to explain the experimental data. the simulated btcs were used to assess the impact of the rd determination method in the sodium movement prediction. results and discussion soil characterization and flow parameters texturally, the soil is clayey silty sand and consists of 79.4% sand, 10.5% clay and 10.1% silt particles. the specific weight of the solids was consistent with sandy soil values (26.4 knm-³). the main minerals are quartz, kaolinite, and gibbsite and the results of the xrd are showed in figure 1. table 1. equations used to determine rd from sorption isotherms eq. 3 eq. 4 eq. 5 r n n kd = ( )1 1 + − r n n b cd s e b= ( )1 1 1 12+ − ρ − r n n k k cd s e = ( )1 1 1 1 2 + − ρ + where n represents the porosity [ ]; k is the henry coefficient for linear isotherm [ ]; b1 is the freundlich coefficient [ml-3(-b2)-1]; b2 is the freundlich exponent [ ]; k1 is the langmuir sorption coefficient[]; k2 is an exponential coefficient [l3m-1]; ce is the equilibrium concentration [ml-3]. 0 200 400 600 800 1000 5 15 25 35 45 55 in te ns ity (c ps ) angle of bragg 2θ normal condition 3.34 (quartz) 3.54 (kaolinite) 4.26 (gibbsite) 7.25 (kaolinite) 0 200 400 600 800 1000 5 15 25 35 45 55 in te ns ity (c ps ) angle of bragg 2θ ethylene glycol 3.34 (quartz) 4.26 (gibbsite) 10.00 (haloisite) 0 200 400 600 800 1000 5 15 25 35 45 55 in te ns ity (c ps ) angle of bragg 2θ 550 °c 3.34 (quartz) 4.26 (gibbsite) 7.25 (kaolinite) 3.54 (kaolinite) a c b figure 1. xrd diffractograms of the studied soil soil: a normal condition; b ethylene glycol; c 550 ֯c 284 vanessa a. godoy, gian f. napa-garcía, lázaro valentin zuquette table 2 summarizes the soil physicochemical parameters. the soil presented values of 5.23 and 4.20 for ph in h2o and in kcl, respectively, and contains a small amount of organic matter (1.18%). with these results, it is possible to note that there was an indication of the electrostatic adsorption due to the negative delta ph (-1.03) and pzc lower than the phh2o (4.18). according to the mip, the distribution of pore sizes is: 5% macropores, 50% mesopores and 45% micropores (koorevaar, menelik and dirksen, 1983). according to godoy (2015), the soil also contains macropores larger than 100 µm that cannot be identified in the porosimetry analysis due to characteristics of the test. these pores are up to 4 mm in diameter and were presumably important in determining the movement of the sodium in the soil. table 3 shows the flow parameters obtained from column experiments. the average k coefficient is in the typical range of sandy soils (zuquette and palma, 2006). the mean v can be considered high, most probably due to the presence of preferential pathways caused by macropores (mcmahon and thomas, 1974; dousset et al., 2007; delwiche, lehmann and walter, 2014; silva et al., 2016). analysis of breakthrough curves figure 3 shows the btcs of the na+ plotted using the relative concentration measured in the leachate (c/c0) vs. the number of eluted pore volumes and table 4 summarizes the results obtained from column experiments. with the increase of the initial concentration there is a displacement of the btc to the left, indicating that the higher the concentration of the initial solution, the faster the solute is identified in the effluent solution. this can be related to the limited sorption sites in the soil as well as to the soil physical heterogeneities. the btcs have the appearance of a continuous flow of initial solution. the tails show that in the beginning, the solute speedily left the soil column and after that, there was a slow transport and a significant tailing occurred. this can indicates physical nonequilibrium and the presence of heterogeneous adsorption sites in the soil, caused by smallscale heterogeneity (gerritse, 1996; vanderborght, timmerman and feyen, 2000; dousset et al., 2007; jarvis, 2007). the small-scale heterogeneity may be due to the dual-porosity characteristic of the soil. the dual-porosity affects the timing and shape of the btc as mentioned by rezanezhad et al. (2012). that occurs because part of the solutes can move fast due to advection in large pores and part of them can be trapped in immobile regions due to percolation through micropores and non-interconnected pores, behavior also stated by others (van genuchten and wierenga, 1976; jarvis, 2007; silva et al., 2016). visual analysis of the dye transport the visual analysis of the dye transport pathways indicated that, although the methylene blue solution was equally distributed in the upper part of the column, there was a greater flow in one location than in the rest of the column and it is evident in figure 4 which shows the location where preferential flow occurred (labeled in the photo). as only water was observed in the effluent, the methylene blue is believed to have been subject to physical retardation in macropores, which do not connect to the bottom of the soil sample (figure 5), and chemical retardation due to the soil and solute chemical characteristics. table 2. physico-chemical parameters of the soil used in the column experiments ph δph pcz eh c.e ctc m.o ph(h20) ph(kcl) (mv) (ms) meq/100g (%) 5.23 4.20 -1.03 4.18 264 55.70 0.152 1.18 to ta l i nt ro du ce d vo lu m en (% ) pore diameter (1e-6 m) 0.001 0.01 0.1 1 10 100 1000 100 80 60 40 20 0 sample 1 sample 2 sample 3 figure 2. porosimetry tests results total volume of mercury versus pore diameter table 3. flow parameters obtained from column experiments column1 column2 column3 ksat(m/s)a 3.060 x 10-05 3.50 x 10-05 4.43 x 10-05 ib 1.5 1.5 1.5 q (m/s)c 4.59 x 10-05 5.25 x10-05 6.65 x 10-05 v(m/s)d 2.18 x 10-04 2.50 x10-04 3.16 x 10-04 a: saturated hydraulic conductivity; b: hydraulic gradient; c: darcy velocity; d: average linear velocity figure 3. breakthrough curves of the na+ table 4. results obtained from column experiments column 1 colum 2 colum 3 pore volume c/c0 c/c0 c/c0 0 0.00 0.00 0.00 0.395 0.00 0.11 0.09 1.185 0.10 0.36 0.38 1.975 0.40 0.56 0.66 2.765 0.50 0.72 0.77 3.555 0.60 0.78 0.83 5.135 0.70 0.83 0.91 6.715 0.80 0.86 0.94 9.085 0.85 0.91 0.96 11.455 0.95 0.96 0.98 285determination of the sodium retardation factor using different methods: analysis of their characteristics and impact on the solute movement prediction in a structured brazilian soil a similar result was found by dousset et al. (2007) who verified physical retardation of a dye tracer in non-connected pores. even after introducing three pore volumes of the solution, the methylene blue remained in the surface portion of the column. the retention of methylene blue indicates that the soil has very significant cation sorption, contrary to what is expected for sandy soils since the mineralogy of these soils is not composed by reactive clay minerals. sodium sorption isotherm the sodium sorption isotherm is shown in figure 6. there is an increase in sorption over time and saturation is reached when no additional sorption of na+ occurs. different results were obtained by müller (2005) and humenick and mattox (1978). the maximum na+ sorption value (0.1 gg-1) did not exceed the maximum soil sorption (0.5 gg-1) obtained by the cec analysis. thus, the sorption values probably are primarily related to physical mechanisms such as retention in micropores which represent 45% of the total porosity. with this result, it is possible to suppose that chemical and electrical mechanisms were probably of minor importance in the na+ retardation. it is important to mention that four points may not be sufficient to define the isotherm accurately but can show the trend of the sorption behavior as obtained by önal et al. (2007). table 5 presents the fit of the experimental data to the mathematical models of the langmuir, freundlich, and linear isotherms. the langmuir and freundlich models fitted well to the experimental data (r2 = 0.99). according to önal et al. (2007), the langmuir model can be successfully applied to many real sorption processes. the linear model had the worst fit (r2 = 0.84) indicating the nonlinearity of the experimental isotherm. retardation factor and its impact on the sodium movement prediction in this section, the rd values obtained with different methods are shown and discussed. table 6 shows the retardation factors obtained using the methods proposed by freeze and cherry (1979), van genuchten and parker (1984), ogata and banks (1691) and by the fit of the sorption isotherms. figure 4. a water spot on the filter paper indicating the location where preferential flow occurred figure 5. visual analysis of the dye transport pathways: retardation in connected macropores that are not connected to the bottom of the soil sample figure 6. sorption isotherm of na+ table 5. parameters obtained by fitting mathematical sorption models to experimental data model parameters fitted langmuir r2 0.99 k1 12.42 k2 112.1 freundlich r2 0.99 b1 0.14 b2 0.14 linear r2 0.84 k 3.17 table 6. retardation factors obtained with different methods method retardation factor column 1 column 2 column 3 (50 mg.l-1) (90 mg.l-1) (117 mg.l-1) freeze and cherry (1979) 2.75 1.75 1.51 ogata and banks (1691) 7.8 4.18 3.84 van genuchten and parker (1984) 4.42 3 2.17 linear isotherm 4.2 4.2 4.2 freundlich isotherm 7.78 4.5 3.63 langmuir isotherm 7.69 4.51 3.6 286 vanessa a. godoy, gian f. napa-garcía, lázaro valentin zuquette the rd values can be considered very high, except for the one obtained with the freeze and cherry (1979) method (jury, gardner, and gardner, 1991). rd values were higher even when compared to results obtained in compacted kaolin at low flow rates, which has a high retardation factor since they are used as liners (shackelford and redmond, 1995). using the linear isotherm model, garga and o’shaughnessy (1994) found sodium retardation factor ranging from 2.22 to 3.6 in clay soil columns with high cec. the high values of rd obtained are in accordance with the results obtained with the sorption isotherm and with the dye transport. the sorption isotherm yields values of rd smaller than those obtained by the cec from the blue methylene adsorption test suggesting a physical dependence originated by e.g. preferential pathways which were also evidenced in the dye transport pathway. given that high average linear velocity values were observed during the experiments (table 3), small values of rd can be expected because of the reduction of the contact time between the solute and soil particles, as observed by hoag and price (1997). the soil sorption and physical-chemical characteristics did not indicate that rd values would be high. however, the physical characteristics played a key role in the rd quantification, as was also noticed by rezanezhad et al. (2012). there is a trend to decrease rd with increasing initial concentration. this behavior is in accordance with the limited sorption sites in the soil but could also reflects soil heterogeneities between the columns, since they are natural soil cores and do not have the same pore configuration, as observed by dousset et al. (2007). the coefficients of determination (r2) of the prediction of the btc using the analytical solution of the ade are shown in table 7. figure 7 shows the fit of the experimental btc to the analytical equation using rd obtained with the methods used in this study. in figure 7, it can be observed that the higher the rd, the slower the movement and the btc displace to the left, as mentioned by other authors (freeze and cherry, 1979; shackelford, 1991; malusis and shackelford, 2002). the rd values obtained with the method of freeze and cherry (1979) were the smallest. this method can be suitable for soils with no preferential pathways, but when there is a dual-porosity, part of the solute will appear faster in the effluent solution, and part will be retained in micropores (rezanezhad et al., 2012). thereby, half of the initial concentration (c/c0) = 0.5, can be detected too fast in the effluent solution, not representing the solute that still is retained and that is also very important in the determination of the correct rd. according to shakelford (1994), this method can be adequate when pure advection occurs, i.e., without dispersion/diffusion. it is suitable when high ( > 50) péclet numbers are achieved (only advection) but is not appropriate when there is dispersion. moreover, this method is based on the sigmoidal btc of homogeneous soils, which was not observed in the results. therefore, this method is not adequate to determine the retardation factor of the sodium in the studied soil. the method of freeze and cherry (1979) also overestimated na+ movement prediction once the fitted btc is displaced to the left in comparison to the experimental curve (figure 7). the r2 obtained with this method were the smallest but increased with increasing concentration of the initial solution. the method proposed by ogata and banks (1961) resulted in higher rd values for all columns. this method is suitable for homogeneous soils, with linear sorption, characteristics not suitable for the studied soil. however, the r2 was 0.99 for all initial concentrations, indicating that the ade model explained the experimental btc. therefore, it is possible that the ade can be adequate to reproduce btc in heterogeneous materials for small-scale studies, in which the macrodispersion has no place. the method proposed by van genuchten and parker (1984), besides being based on the ogata and banks (1691) transport equation, considers the mass balance of the solute in the soil and can so be capable of capturing the mass of solute retained in the soil regardless of whether there is equilibrium or nonequilibrium transport. the relatively good suitability of this method was verified with r2 ranging from 0.78 to 0.91. however, in figure 7 it is noticeable that the sodium movement is overestimated. it is important to mention that the correct use of this method depends on whether the solute had reached inflow concentration, which can sometimes take a long time. in this research, the inflow concentration was not reached in any column test and to use this method, an extrapolation was made, which can explain the errors in the na+ movement prediction. column 1 50 mg/l column 2 90 mg/l column 3 117 mg/l times [minutes] times [minutes] times [minutes] c /c 0 c /c 0 c /c 0 experimental btc langmuir isotherm freundlich isotherm linear isotherm van gen. and parker ogata and banks freeze and cherry experimental btc langmuir isotherm freundlich isotherm linear isotherm van gen. and parker ogata and banks freeze and cherry experimental btc langmuir isotherm freundlich isotherm linear isotherm van gen. and parker ogata and banks freeze and cherry 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 60 120 180 240 300 360 0 60 120 180 240 300 360 0 60 120 180 240 300 360 figure 7. fit of the experimental btc to the analytical solution of the ade using rd obtained with all the methods used in this study work 287determination of the sodium retardation factor using different methods: analysis of their characteristics and impact on the solute movement prediction in a structured brazilian soil conclusion aiming to compare four methods of sodium rd determination and assess the impact of the chosen method in the prediction of sodium movement, we did laboratory tests and used the analytical solution of the ade to perform solute transport predictions. with the presented study, it was possible to highlight some conclusions. in columns containing natural and completely weathered soils, with preferential paths and dual-porosity, the solute transport parameters obtained using some of the most common methods found in the literature may result in non-representative rd values mainly due to the natural heterogeneity of the soil column. the suitability of the method used for determining rd depended on the concentration of the initial solution. the higher the initial concentration, the smaller the errors in the prediction of sodium movement. for the conditions studied, the method proposed by ogata and banks (1961) and by the fit of the freundlich and langmuir isotherms models resulted in the best reproduction of the experimental breakthrough curve. differently, the method proposed by freeze and cherry (1979) resulted in the worst result. the fit of the experimental btc to the ade analytical models showed that the predicted concentration can be overestimated up to 22.5% when using a not suitable method. with this study, we conclude that the rd determination method can significantly affect the prediction of the sodium movement. because of that, it is important to evaluate carefully when a method can be adequate to determine the rd for the soil under investigation and when cannot be adequate. acknowledgements the authors would like to thank the teachers and staff of the state technical school “astor de mattos carvalho” for allowing collecting soil samples on the school’s area. this research was made possible by the involvement of financial support from the coordination for the improvement of higher education personnel (capes). references ahuja, l. r. (1984). macroporosity to characterize spatial variability of hydraulic conductivity and effects of land management. soil science society of america journal, 48(4), 699. url: https:// www.soils.org/publications/sssaj/abstracts/48/4/ss0480040699. for column 1, the use of the rd obtained with the linear isotherm model is not adequate for the prediction of the na+, since the movement is overestimated and r2 was small (0.29). these results are adequate with the fact that the soil isotherm is nonlinear and that the soil has a limited sorption capacity, which is not represented by the linear model. however, for columns 2 and 3, this model seems to be suitable with r2 equal to 0.99. this method should be used with caution since its suitability depends on the concentration of the initial solution. the results obtained with the other two isotherm models, langmuir and freundlich, were quite similar and, in comparison to the experimental btc, the simulated curves using those methods reproduced very well the experimental btc with r2 equal to 0.99 for both isotherms. these results show the efficiency of the use of isotherms obtained from undisturbed soil columns to determine rd. the comparison of the simulated btc evidenced the impact of the method used to calculate de rd in the sodium movement prediction and this impact is more pronounced with the decrease in the initial concentration of the inlet solution. the r2 increased with the increase of the initial concentration for almost all conditions studied. it is possible that, for a low concentration of the initial solution, the equilibrium adsorption has not yet been reached and the rd determination could be more difficult to achieve. by increasing the concentration of the initial solution, the rd determination is more stable and almost all methods work relatively well. table 8 shows the comparison between na+ concentrations obtained experimentally and after 180 minutes using different methods of rd determination in the analytical solution of the ade. from table 8 it is noticeable that for column 1, the predicted concentration using the rd calculated with the method of ogata and banks (1961) was 3.75 % smaller compared to the experimental data; with the langmuir and freundlich isotherms, it was 2.5 % smaller. the linear isotherm, freeze and cherry’s and van genuchten and parker’s methods overestimated the concentration in 16.25%, 22.5 %, and 15%, respectively. for column 2 and 3, all concentrations were overestimated, but this effect was smaller when compared to the results of column 1. for column 2, the overestimation of the concentrations was 4.5 % when using the method of ogata and banks, 13.80% for the method proposed by freeze and cherry, 10.34% for the methods of van genuchten and parker and the langmuir and freundlich isotherms, and 6.8 % for the linear isotherm. for column 3, the increase in the prediction of the concentration ranged from 1 % to 6%, and the worst method was the one proposed by freeze and cherry. for all columns, the highest overestimated percentage was obtained using the method proposed by freeze and cherry, as expected since this method resulted in the smallest rd due to its characteristics as discussed above. table 7. the coefficient of determination (r2) of the prediction of the btc using the analytical solution of the ade method column 1 column 2 column 3 (50 mg.l-1) (90 mg.l-1) (117 mg.l-1) r2 r2 r2 freeze and cherry (1979) 0.18 0.66 0.72 ogata and banks (1691) 0.99 0.99 0.99 van genuchten and parker (1984) 0.28 0.93 0.88 linear isotherm 0.29 0.99 0.99 freundlich isotherm 0.99 0.98 0.99 langmuir isotherm 0.99 0.98 0.99 table 8. comparison between sodium concentrations obtained experimentally and after 180 minutes using a different method of rd determination in the analytical solution of the ade method concentration (mg/l) after 180 minutes column 1 column 2 column 3 (50 mg.l-1) (90 mg.l-1) (117 mg.l-1) experimental 40 78.3 108.81 freeze and cherry (1979) 49 89.1 115.83 ogata and banks (1691) 38.5 81.9 111.15 van genuchten and parker (1984) 46 86.4 112.32 linear isotherm 46.5 83.7 109.98 freundlich isotherm 39 86.4 112.32 langmuir isotherm 39 86.4 112.32 288 vanessa a. godoy, gian f. napa-garcía, lázaro valentin zuquette arthur, j. d. 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(2006). avaliação da condutividade hidráulica em área de recarga do aqüífero botucatu. rem: revista escola de minas, 59(1), 81–87. url: http://www.scielo.br/scielo.php?script=sci_arttext&pid=s0370-44672006000100011&lng=pt&tlng=pt. keywords: stable isotopes; epithermal; intermediate sulfidation; gold mineralization; fluids mixture; miocene. palabras clave: isotopos estables, epitermal, sulfuración intermedia, mineralización aurífera, mezcla de fluidos, mioceno. how to cite item rojas barbosa, s., molano mendoza, j. c., & cramer, t. (2020). petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia). earth sciences research journal, 24(1), 5-18. doi: https://doi.org/10.15446/esrj.v24n1.63443 the gold mineralization located in vetas, santander, consists of auriferous quartz veins hosted in bucaramanga gneiss rocks, intrusive jurassic rocks, and intrusive to porphyritic miocene rocks. this study identified four mineralizing events: (1). sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz, sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite. the age for this stage is 10.78 ±0.23ma (ar/ar on sericite). (2). molybdenite, magnetite with exsolution of ilmenite, as-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant and microcrystalline textures and, tourmaline and sericite. (3). gold and tennantite associated with sphalerite, fineand coarse-grained pyrite, as-pyrite, chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and “comb” textures, and tourmaline. stage 2 and 3 happened from 7.58 ±0.15 ma to 6,89±0,41ma (ar/ar on sericite). (4). thick, thin, and pyrite with arsenic, hematite and microcrystalline quartz (forming breccia texture), and sericite. the age for this stage is 5.24 ±0.10 (ar/ar on sericite). post-mineral: quartz comb, alunite, halloysite, kaolinite, and ferrum hydroxides. the stable isotopes, ∂18o, ∂d, and ∂34s and fluid inclusions analysis infer that fluids were producing a mixture of meteoric and magmatic fluids with low salinity and minimum trapping temperatures between 200°c to 390°c. the mineralogy association, and fluid inclusions, in the first event show characteristic of low sulfidation epithermal. the second stage was hottest and with more magmatic signature over printed an intermediate sulfidation system; show a little more salinity on the fluids and more mineralogical diversity, the third and four events, could show an evolution of this fluid, where it was cooling and impoverishing on metals. two initials stages are contemporaneous with two magmatic miocene pulses on the area: the first one of granodiorite composition 10, 9± 0.2 ma (u/pb zircon), and the other one rhyodacite with 8.4 ±0.2 y 9.0 ± 0.2 ma. abstract petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) petrografía, microtermometría e isotopía de las vetas auríferas de vetas, santander (colombia) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.63443 la mineralización de oro ubicada en vetas, santander, consiste en vetas auríferas de cuarzo alojadas en rocas del gneis de bucaramanga, rocas jurásicas intrusivas y rocas intrusivas porfiríticas miocenicas. este estudio identificó cuatro eventos de mineralización: (1). sericita, carbonato (ankerita y calcita?), cuarzo masivo y microcristalino, esfalerita, adularia, albita, galena, pirita delgada, pirrotita, calcopirita. la edad para esta etapa es 10.78 ± 0.23ma (ar / ar en sericita). (2). molibdenita, magnetita con exsolución de ilmenita, arsenopirita, esfalerita, pirita de grano fino un poco de calcopirita y cuarzo con textura plumosa, mosaico fino, flamboyante y microcristalino, turmalina y sericita. (3). oro y tenantita asociados con esfalerita, pirita de grano fino y grueso, pirita de arsenopirita, calcopirita como inclusiones, y cuarzo con texturas flamboyante, mosaico, masiva y en "peine", y turmalina. los eventos 2 y 3 ocurrieron entre los 7,58 ± 0,15 ma a 6,89 ± 0,41ma (ar / ar en sericita). (4). pirita en cristales finos y gruesos con arsénico, hematita y cuarzo microcristalino (formando una textura brechosa), y sericita. la edad para este evento es de 5.24 ± 0.10 (ar / ar en sericita). el evento post-mineral: peine de cuarzo, alunita, haloysita, caolinita e hidróxidos de hierro. los isótopos estables, ∂18 o, ∂d y ∂34 s y el análisis de inclusiones de fluidos infieren que los fluidos fueron producto de una mezcla de fluidos meteóricos y magmáticos con baja salinidad y temperaturas de atrapamiento mínimas entre 200° c y 390° c. la asociación mineralogica y las inclusiones fluidas, en el primer evento, muestran características de un ambiente epitermal de baja sulfuración; el segundo evento más caliente presenta una firma magmática más prominente sobreimponiendo un sistema epitermal de intermedia sulfuración sobre el de baja sulfuración, con un poco más de salinidad en los fluidos y más diversidad mineralógica. para el tercer y cuarto eventos muestran una evolución de este fluido, donde se enfría y se empobrece en metales. de acuerdo a las edades reportadas, los dos eventos iniciales son contemporáneos con dos pulsos magmáticos del mioceno que fueron identificados en sectores circundantes del área, el primero de composición de granodiorita 10, 9 ± 0.2 ma (circón u / pb, mantilla, et al. 2011), y el otro de riodoritas con 8.4 ± 0.2 y 9.0 ± 0.2 ma. resumen record manuscript received: 22/03/2018 accepted for publication: 24/04/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 5-18 sonia rojas barbosa1, juan carlos molano mendoza2, thomas cramer2 1servicio geológico colombiano 2universidad nacional de colombia * corresponding author: srojasb@unal.edu.co; jcmolanom@unal.edu.co g eo lo g y 6 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer introduction this work shows the physicochemical characteristics of the mineralizing fluids, and determine the possible metallogenic environment in which it was formed in addition to its age, based on petrographic, fluid inclusions, and isotopic studies. the vetas gold deposit is located in the area of vetas, santander, 50 km far from bucaramanga toward the ne. tectonically, the area is placed at the northwest of south american plate, limited by the bucaramanga fault (principal regional structure on the area) and the oca fault. those faults are strike–slip types, and the movement is a product of the subduction of nazca and caribbean plate (figure 1). mineralization is untaken as a series of veins embedded in metamorphic rocks of bucaramanga gneiss and jurassic-tertiary igneous rocks that intrude them. geological setting the study area is located in santander massif, which consists of precambrian, metamorphic rocks, jurassic intrusive igneous rocks, and tertiary subvolcanic rocks (miocene), also with colluvial deposits (figure 1). bucaramanga gneiss rocks in this area are outcrops paragneisses (quartz-feldspathic, quartz-biotitic, quartz feldspathic, and biotitic and silimanithic), quartzites and amphibolites. these rocks present retrograde metamorphism for biotite, evidenced by the presence of chlorite (urueña & zuluaga, 2011). these rocks have a strong hydrothermal alteration, as evidenced by the presence of chlorite, epidote, feldspar, sericite, carbonates, and silicification associated with plagioclase and aluminosilicates, and veins. jurassic intrusive igneous rocks petrographically, these rocks are holocrystalline, fine-to-medium grained with a very light pink and gray color; their composition varies from granite, diorite, and tonalitic igneous rocks, and correspond to different bodies but contemporaneous (ward et al., 1973); which lie in the north and east of the study area (figure 1). these rocks, close to mineralization zone have a moderate and zoned hydrothermal alteration, characterized by silicification and pyritization on the periphery and argilic alteration near the mineralized veins. the argilic alteration is strong and pervasive masking of the rock´s original texture. these rocks are strongly fractured, helping the formation of stockwork mineralization. miocene intrusive rocks these rocks occur as intrusives and dikes outcropping in the northern part of the study area, in the road to reina de oro mine and within a few tunnels. the rocks are porphyritic quartzmonzonites, light pinkish to gray and composed of quartz, plagioclase (oligoclase) and biotite, zircon as accessories. in some tunnels some dikes of this unit were observed cutting veins. in the la reina de oro tunnel, a sample of dacite was taken, where fine disseminated pyrite, arsenious pyrite, chalcopyrite and magnetite (figure 2) was identified. figure 1. location of the study area and tectonic setting (modified from rojas, 2013; ward et al, 1973). 7petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) 1. quartz with massive texture (characterized by the presence of large, subhedrales to euedral. it was identified in early mineralizing stages (first, second and third stages). 2. quartz with flamboyant and feathery texture on the second stage. 3. quartz with mosaic texture (figure 4b), occurs as irregular, finegrained to micro-crystalline and aggregate. in the late events of mineralization (second and third stages). 4. cryptocrystalline quartz (figure 4b), latest event and acts as a cement in places where brecciated texture is presented (fourth event) 5. quartz with “comb” texture (figure 4 a), appears as prismatic medium to coarse-grained crystals, this texture was formed on the third stage mineralization and a post-mineral event. structural control setting the vetas mineralizations have a structural control related to the bucaramanga (nw direction), cucutilla (ne direction) faults, and their associated structures, which create a distensive tectonically originating spaces that served as conduits where the bodies were emplaced (mantilla et al, 2009 and mantilla et al, 2011). structurally reefs are related to fractures set with n55°w and n30°60° strikes, with veins strikes of n30° to 50°e, n60°to 88ºw and ew, and with dips between 30° to 90°. dip variations are observed in places that vein thickness change (mantilla and mogollón, 1991), that situation is caused for little faults that cut the reefs. the el volcán fault has a sinistral (left-lateral) movement, moving veins from 10 to 15 meters. other smaller faults with a ne strike late to mineralization are significant because they cause truncations and wedge veins (figure 2 and 3). ore deposit mineralogy the mineralization occurs mainly in veins with thicknesses ranging from 30 cm to 1 meter. the most of the veins are banded, where the central strip correspond to recrystallized quartz and sulphides, occasionally tourmaline and sericite, this area contains higher gold concentrations. on both sides of these bands of quarts are presented primary and recrystallized quartz, and sulfides, also a significant increase in the intensity of sericite alteration. the most outer part of the vein corresponds to an argillic and mineralized zone, which is characterized for a bluish green color, and also presents significant gold concentrations. gangue minerals quartz is the most abundant mineral, and was identified in all mineralizing stages (4 mineralizing stages and the post mineral event). quartz occur with different textures (dong et al., 1995) that were used as a guide for recognizing the paragenetic sequence. these textures are: figure 2. argillic dacite dike cutting by some mineralized veinlets, and the same time the dike cut other veins too. reina de oro mine figure 3. vein wedged by faulting, la peter mine 8 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer ore minerals the paragenesis includes gold, sulfides like pirrotite, pyrite, tennantite, pyrrotite, galena sphalerite and traces of molybdenite and oxides like, magnetiteilmenite, the oxides being the most abundant. these minerals occur in veinlets or disseminated in intergrowth textures, replacement and filling spaces in the veins and host rocks. gold: it was identified in the third mineralizing stage, included in tennantite or fractures of low iron sphalerite (figure 5), and massive quartz associated with tourmaline and quartz with flamboyant and massive texture. gold is irregularly shaped, its size varies between 26 and 41 µm. other authors reported free gold associated with quartz and euhedral pyrite (cortés, 2002 and garcia y uribe, 2003). silver: it has been reported in other works (cortés 2002, garcia y uribe, 2003), and others) associated with pyrite and galena. within the electron probe micro-analyzer (empa) analysis show, between 10% and 15%. ag in the tennantite. pyrite: three generations were identified: 1. fine pyrite, 2. coarse-grained (>100 microns) pyrite associated with aspyrite (lighter and anisotropic) and 3. fine-grained pyrite (±50 microns, figures 6 and 7). some crystals of pyrite are product of recrystallization of pyrrhotite (figure 6). galena: it has coarse particle size (≥100 microns) and is associated with quartz with flamboyant and massive texture; in some cases, it is being replaced by sphalerite, so this indicates that was formed during first mineralizing event (figure 5 and 6). sphalerite: two species were identified, one clear and the other dark related to variations in the iron content possibly (table 1). the first one crystallized during the first stage associated with carbonate when is disseminate in the host rock close to the veins and sericite, also appears as islands in pyrite, sometimes like inclusions and other times filling pores or associated .with tennantite (figure 5 and 6). the clear (visible with transmitted light) sphalerite is found mainly in the central part of the veins, as coarse-grained, euhedral crystals. it can be found inter-grown with tennantite, replacing galena and pyrite and filling spaces in pyrite and as-pyrite. tennantite (cu12agzn3assb4s16-cu5ag2znassb2s8): occurs in coarsegrained aggregate, with clear sphalerite. its composition was determined by empa analysis (figure 5 and 6). molybdenite: is pinkish dirty pale gray color, with a fibrous and pseudohexagonal habit, presents a strong anisotropy and pleochroism. it occurs in the second stage of mineralization (figure 6). chalcopyrite: occurs as pyrite islands in “atoll” textures or in contact (figure 6) with fibrous cristal of molybdenite, indicating that they were precipitated on first to early second mineralization stages, even though some crystal of chalcopyrite occur in the three and four mineralizing event. according to some empa analysis, they contains arsenic in their composition (figure 6). pyrrhotite: occurs as fine-grained, pink crystals, they are presented in the first mineralization event associated with chalcopyrite and dark sphalerite (figure 6). magnetite: the magnetite have exsolutions of ilmenite, this indicate ti in the structure of magnetite. this mineral appear in the second mineralization stage (figure 6). alteration minerals and hydrothermal alterations sericite and illite (figure 7): main minerals alteration has good crystallinity and its size varies from fine to very coarse-grained (> 100 microns, possibly muscovite). it is presented as alteration in sillimanite and plagioclase alteration from the host rock, and is associated with sulfide and quartz veinlet in the first and second mineral stage, when the granulometry of sericite is fine-grained is presented with illite (identified by drx). adularia: is the second alteration mineral in abundance, is found as coarse-grained (100 to 500 microns), euhedral crystals in veinlets, sometimes associated with albite and sericite. it represents in the first stage of mineralization. figure 4. a. quartz (qz) textures. a. quartz with “comb” texture (center of the picture), quartz with mosaic texture and tourmaline (left) of the third event, and microcrystalline quartz (right), b. fragment of massive quartz with tourmaline (to) in a matrix of cryptocrystalline quartz (brecciated texture). a b qz 100 microns 100 microns to to figure 5. gold (au) include in tennantite (tnt), in paragenetic association with sphalerite and galena early. 100 microns tnt sp au qz 9petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) table 1. sphalerite composition (epma analysis) sample as% s% mn% fe% cu% zn% pb% ag% au% cd% sb% total% formule 1019 0,097 33,829 0,106 0,004 0,755 66,109 0,255 0,257 0,000 0,391 0,201 102,002 cuzns 1019 0,161 33,376 0,112 0,000 0,580 66,170 0,209 0,401 0,015 0,482 0,301 101,806 cuzns 1019 0,000 33,712 0,022 0,613 0,171 67,130 0,086 0,022 0,091 0,597 0,023 102,467 fezns 1019 0,000 34,001 0,042 0,977 0,233 66,600 0,106 0,015 0,000 0,398 0,013 102,384 fezns 1019 0,047 33,650 0,110 0,000 0,383 66,819 0,105 0,368 0,000 0,546 0,161 102,190 zns 1019 0,115 33,580 0,099 0,014 0,453 66,311 0,226 0,094 0,000 0,677 0,141 101,710 cdzns 1019 0,094 33,551 0,073 0,000 0,288 66,771 0,091 0,181 0,000 0,498 0,149 101,695 zns 1019 0,000 33,761 0,106 0,000 0,166 67,069 0,099 0,067 0,000 0,498 0,017 101,781 zns 1019 0,439 32,604 0,123 0,000 1,148 64,029 0,248 0,968 0,083 0,461 0,397 100,500 cuzns 1019 0,000 34,060 0,100 0,000 0,264 68,080 0,152 0,000 0,000 0,460 0,016 103,131 zns 1019 0,000 34,184 0,092 0,007 0,371 67,619 0,102 0,006 0,000 0,512 0,014 102,907 zns 1031 0,051 33,311 0,036 0,987 0,094 65,353 0,009 0,019 0,041 0,281 0,044 100,232 fezns 1031 0,000 33,228 0,039 2,199 0,201 65,827 0,056 0,000 0,021 0,248 0,000 101,818 fe0,1zns 1031 0,000 33,311 0,043 2,925 0,062 64,896 0,120 0,033 0,082 0,189 0,009 101,670 fe0,1zns 1031 0,000 33,190 0,000 1,051 0,001 67,632 0,134 0,020 0,000 0,348 0,000 102,375 zns 1031 0,000 33,276 0,039 1,992 0,150 66,101 0,175 0,028 0,000 0,232 0,044 102,038 fe0,1zns 1031 0,193 32,883 0,048 0,696 0,792 66,247 0,086 0,213 0,007 0,248 0,165 101,578 fecuzns 1035 0,000 32,994 0,073 0,649 0,208 66,544 0,018 0,028 0,131 0,260 0,107 101,011 fezns 1035 0,000 32,945 0,035 0,004 0,044 68,203 0,040 0,000 0,138 0,241 0,000 101,649 cuagzns 1035 0,000 33,196 0,008 0,007 0,025 68,747 0,095 0,000 0,297 0,245 0,002 102,622 zns 1035 0,000 32,905 0,014 0,000 0,130 68,493 0,231 0,033 0,090 0,213 0,000 102,109 zns 1035 0,000 32,818 0,028 0,016 0,257 67,990 0,000 0,128 0,000 0,340 0,049 101,657 zns 1035 0,046 33,095 0,070 0,008 0,384 67,346 0,134 0,293 0,000 0,272 0,135 101,783 zns 1035 0,010 32,756 0,037 0,572 0,153 67,342 0,102 0,217 0,097 0,290 0,088 101,672 zns 1035 0,077 32,997 0,035 0,026 0,386 67,484 0,105 0,138 0,000 0,364 0,174 101,783 zns 1035 0,000 32,978 0,049 0,020 0,055 68,332 0,201 0,035 0,103 0,284 0,000 102,057 zns 1035 0,072 32,817 0,056 0,016 0,524 67,130 0,173 0,340 0,041 0,328 0,236 101,732 zns 1035 0,025 33,067 0,023 0,233 0,212 67,703 0,156 0,128 0,000 0,278 0,119 101,943 zns 1035 0,000 42,300 0,076 0,028 0,319 67,880 0,153 0,039 0,341 0,000 0,000 111,174 zns 1019a 0,002 42,178 0,134 0,003 0,668 66,201 0,171 0,000 0,508 0,000 0,159 110,070 aucuzns ankerite (identified by sem) is restricted to the areas close to veins, where the rock alteration is not strong (figure 8). tourmaline: it is characterized by having prismatic habit, thin and elongated crystals with brownish green color. it is associated with the ore-rich area of the veins, accompanied by quartz with flamboyant and massive texture between second and third stage. other identified alteration minerals include chlorite and epidote, in remote areas of the mineralization; kaolin, affecting feldspars from the host 10 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer rock and alunite (identified by sem and drx), possibly product of supergene alteration as well as iron oxides and hydroxides are also recognized. paragenetic sequence four mineralizing events and one post-mineral stages separated by fracturing, brecciation and mineralogical changes were identified. such associations are mentioned below and are represented in figure 9: stage 1: association of sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz (possibly recrystallized demoustier, castroviejo, & charlet, 1998), sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite. mineralogical change is produced by a temperature increase and enrichment of fluids. stage 2: molybdenite, magnetite with exsolution of ilmenite, as-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant (product of recrystallization of massive quartz first event or from chalcedony demoustier, castroviejo, & charlet, (1998)) and microcrystalline textures quartz and, tourmaline and sericite. brittle deformation, which produced conduits where new minerals were mobilized and precipitated. stage 3: gold and tennantite associated with sphalerite, fine-grained pyrite, coarse-grained pyrite and as-pyrite, sometimes with chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and “comb” textures, and tourmaline. stage 4: fine-grained pyrite microcrystalline quartz and sericite. a generation of fine-grained pyrite with microcrystalline quartz that produces brecciated textures. post-mineral stage: this event happens in open spaces result of brittle deformation or remaining space of crystallization, which where was formation of small quartz geodes with “comb” texture, ferrum hydroxide and alunite, halloysite and kaolinite. fluid inclusions mycrothermometry the study of fluid inclusions provides information related to changes in the temperature and composition of the mineralizing fluids. the measured fluid inclusions correspond to 1, 2, 3 mineralizing stage and the post-mineral event (figure 10). the research was carried out in five polished thin sections chosen because of their mineralogical characteristics and characterized by their textures and abundance of fluid inclusions that distinguish the different mineralizing pulses. the measured fluid inclusions correspond to 1, 2, 3 mineralizing events and the paragenetic post-mineral sequence (figure 9). during the analysis, the following was discovered: biphasic inclusions (the most abundant), very small monophasic and three phasic (only two examples, with crystals of opaque minerals). biphasic inclusions have sizes ranging from less than 5 µm (the most abundant), 10µm – 30µm and the biggest range from 80µm to over 100µm. a lot of varieties of shapes are observed: regular, irregular, ovoid and tabular. the primary inclusions in quartz with massive texture are the biggest, usually they appear lonely or as isolated groups and in the center of the crystals. some pseudo-secondary inclusions are found as smaller aligned sets along microfractures that do not touch the edges of the crystals secondary inclusions were observed in the recrystallized edges of the quartz with flamboyant and feathery textures or in fractures that cross a lot of crystals (figure 11). figure 6. ore minerals. a. chalcopyrite crystals in the contact with molybdenite, b. magnetite intergwith ilmenite exsolution, c. tennantite in paragenesis with sphalerite and chalcopyrite, pyrite later or in paragenesis with sphalerite, d. pyrrhotite and pyrite, before sphalerite , e. asspyrite intergrown with pyrite (porous maybe product of pyrrhotite recrystallization) f. chalcopyrite in paragenesis with pyrrhotite before as pyrite. 11petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) figure 7. a. veinlet of adularia in gneiss quartz feldspathic and sericitization on plagioclase, b. plagioclase with sericitic (coarse sericite) alteration. c. veilent of adularia. d. ankerite and calcite of alteration in quartzmonzonite. figure 8. analysis of ankerite with electronic microscopy was identified calcium, iron and magnesium. ad qz pg ser ser qz ad qz qz ank a b c d 12 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer fluid inclusions that have an evidence of strangulation or fluid leakage was not measured. four temperature changes were visualized: first melting temperature. hydro-halite melting temperature. final melting temperature of ice. homogenization temperature. these temperature changes allowed the authors to not only determine the chemical system but also the minimum trapping temperature, which allowed to determine important variations in temperature during deposit formation. first melting temperatures are quite low, indicating the presence of salts in the water, and can be divided into three groups of temperature: -40°c to -35°c, -30°c to -25°c and -23°c to -20°c. this range of temperatures show that the composition of the mineralizing fluids was quite complicate, where possibly a saline mixture of nacl, cacl2 and mgcl2 is presented (table1). figure 9. paragenetic sequence 13petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) in some of the fluid inclusions changes at temperatures between -25°c and -20°c corresponding to the final hydro-halite melting point (table 2). low temperatures of the first melting and final melting hydro-halite, in most of the measurements in events 1, 2 and 3 indicate that the chemical system is complex, with at least three different kinds of solutes presented: h2o – nacl, h2o – nacl – cacl2 y h2o – nacl – mgcl2, of which nacl is the predominant, while the mgcl2 is the lowest in abundance. to determine the presence of these salt systems the following programs were used: aqso1e, aqso2e and aqso3e, software package fluids, version 1, for aqueous fluid inclusions developed by ronald bakker of the university of leoben in austria (table 2). first melting temperatures are also variable, and generally low, being samples between -6°c to 0°c and others between -12°c to -8 ° c, that in terms of salt concentration correspond to approximate salinities of 16 wt%eqnacl to 11 wt%eqnacl and 10 wt% eqnacl to 0 wt%eqnacl respectively. inclusions related to the early mineralizing events are a little more saline, for example, for the first and second stage the salinity may be above 10 wt%eq nacl, for the case of second stage can be up 18 wt %eqnacl (for the first stage 12 wt %eqnacl), meanwhile for the third and post mineral stage only achieve to 10 %eqnacl (figure 12). fluid inclusions of the first mineralizing stage show that temperatures are between 210°c and 250°c, with an average of 220°c and minimum trapping pressure (calculated using the software package fluids, version 1. of zhang and frantz, 1987), varies from 1 to 2 mpa and depths calculated in general under 1000 m, showing that the conditions were propitious for the formation of epithermal deposits (camprubi & albinson, 2006). the second mineralizing stage show a significant increase in the homogenization temperatures, reaching values above 350°c (the range the temperatures is from 227 to 387°c), with an average of 279°c. these high temperatures are consistent with the presence of minerals such as tourmaline, and molybdenite in this event. values calculated show a minimum pressure range from 3 to 9 mpa, these being higher values compared to the first mineralizing event. this change in conditions could be explained as an increase in pressure associated with the emplacement of igneous bodies that heated the system contemporaneous with the second mineralizing stage like was observed in the reina de oro tunnel. the fluid inclusions in the third mineralizing event measurements show a decrease in temperature, ranging between 200°c and 240°c, this change is also evident in the mineral assemblages presented in this event, where hightemperature minerals were not identified. the calculated pressures are from 1 to 2,8mpa, which are similar to the first mineralizing event. for the fourth mineralizing event it was not possible to make any microthermometry measurement, because the quartz type present in this event is very fine-grained, cryptocrystalline quartz. however according to the mineralogical scarcity could be inferred that fluids were becoming more impoverished, and the fluid continuous with the cooling process. in the post-mineral mineralizing event few measurements were made because most inclusions were found liquid-rich, biphasic or with a very small size. of the few that could be measured, low homogenization temperatures were obtained from 125°c to 190°c (figure 13). figure 13. histogram showing the homogenization temperature. stable isotopes around 25 sulfur isotope analysis (δ34s) in pyrite and galena, 7 isotopes of o (δ18o) and 6 isotopes h (δ2h) were performed. these analyzes were carried out in the laboratory of the united states geological survey usgs for stable isotopes in denver, colorado (δ34s, δ 18o and δ 2h) and university of salamanca in spain (d18o and δ 2h). the results of isotopic analysis δ34s are reported, based on the standard values of troilite from diablo´s canyon; for δ 2h and δ 18o isotopes values are given according to the isotopic signature of seawater. deuterium and oxygen analyzes were performed into sericite and quartz. sericite was separated from samples of mineralized veins and backups, which were crumpled and pulverized, and quartz was obtained from mineralized veins. figure 10. fluid inclusions in quartz with massive texture of the second mineralizing stage. figure 11. secondary fluid inclusions in quartz with flamboyant texture of the third mineralizing event. figure 12. histogram showing the salinity of fluid for the stage measured. the salinity is similar for the all satage, but for the second event is showing some pikes of salinity, and the most dates are between 6 and 10 %wt nacl eq. 14 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer table 2. measurements and calculated parameters from fluid inclusions mycrothermometry. the analyses of mycrothermometry was doing with the bakker – fluid, software aqso1e aqso2e y aqso3e y loner 38, for the watery inclusions. (t ff: first fusion, thh: final fusion of hydrohalite, tfi: final fusion of ice, th: homogenization temperature, l: liquid, v: vapor). 15petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) sulfur isotopes isotope values for δ34s show a bimodal trend with values between 1 ‰ to -6 ‰ and -17.6 ‰ to -20 ‰ (figure 14), indicating two different sources of sulfur. the closer to zero values are associated with felsic igneous rocks (campbell & larson, 1998), sulfur was possibly leached from the sulfides presented in igneous rocks (rye & ohmoto, 1974). the most negative δ34s values are the result of leached of metamorphic rocks (from sedimentary) for reactive fluids during hydrothermal process, and imposed their isotopic signature to mineralization (tang et al., 1998 from y. zhang et al., 2017) table 3. isotope (𝛿34s (‰)) results for mineral separates sample mineral 34s (‰) 1003 py 0,48 1009 gn 0,0 1009 py -1,2 1010 py -0,44 1012 py -5,7 1014a py 0,9 1018 py -0,6 1019a py -2,0 1022 py -3,4 1028 py 0,7 1031 py -16,7 1031 py -17,8 1033 py 1,4 1034 py 0,8 1035 py -3,4 1043 py 2,5 1019 py -0,6 1050 py 1,0 1050 ga -19,0 1050 ga -20,0 figure 14. sulfur isotope analysis (𝛿34s) deuterium and oxygen isotopes analyses were performed on samples obtained from quartz veinlets and sericite from the alteration zone of the host rocks in the vein zone. the isotopic values of 𝛿18osmow‰ are close to 10 indicating that fluid have strong magmatic affinity (campbel & larson, 1998), in the case of 𝛿dsmow ‰ the situation is the same the most values have magmatic signature (campbel & larson, 1998). when deuterium and oxygen results are compared it (figure 15), a strong magmatic affinity is observed. this affinity is illustrated by the fact that most of the points fall very close to the field of magmatic waters and even two of the values are within the field. the points that are outside of this field tend to move toward the line of meteoric water, indicating that they are also related. according to the previous facts it can be interpreted that the mineralizing fluids were generated due to a mixture of meteoric and magmatic waters, being magmatic waters dominant. table 4. stable isotopes of deuterium and oxygen results muestra mineral 18osmow‰ dsmow ‰ isotopic fractionation factor of o (1000 lnαa-b) h20% 1002 sericite 11,2 -77,1 5,95 2,3 1014 quartz 11,4 4,86 0,3 1018 quartz 15,7 -77,6 9,16 0,3 1022 quartz 11,8 -88 2,08 0,3 1035 quartz 13,2 -80,7 3,40 0,5 1035 quartz 13,1 -80,7 3,30 0,5 1043 quartz 13,6 -93,8 5,23 0,4 1035 sericite 12,9 8,50 -when deuterium and oxygen results are compared it (figure 15), a strong magmatic affinity is observed. this affinity is illustrated by the fact that most of the points fall very close to the field of magmatic waters and even two of the values fall within the field. the points that are outside of this field tend to move toward the line of meteoric water, indicating that they are also related. according to the previous facts it can be interpreted that the mineralizing fluids were generated due to a mixture of meteoric and magmatic waters, being magmatic waters dominant. figure 15. diagram of 𝛿18o vs 𝛿d. modified from taylor, 1979 in barnes, 1997 ore deposit age four rock samples were taken from three different veinlets; these were chosen according with their high content of sericite. the rock samples were pulverized, sieved and clays were separated by decanting. in the case of thicker sericite these were separated by hand picking method under stereomicroscope. the samples were analyzed in the laboratories of the university of oregon. 16 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer as shown in the table 5 and figure 16, mineralization originated from miocene to pliocene, where 3 different hydrothermal pulses were identified, the oldest at 10.78±0,23 ma, an intermediate between 7,68±0,15 ma and 6,89±0,41 ma and the last one of 5,24±0,10ma. the first pulse could be contemporaneous with intrusion bodies, one of them a granodiorite with age is (u/pb on zircon) 10.9±0.2 ma, located in the meseton sector (mantilla et al, 2011) 1 to 3 km at nw of mineralizations visited on vetas, indicating that body like that could be source of fluids and temperature for the first stage of the mineralization. the others ages 7,68±0,15ma and 6,89±0,41ma could be with the second and possibly third stage, because the petrography in that samples corresponding with that stages, and the 5,24±0,10ma corresponding to fourth stage. also, there are some intrusive rhyodacitics bodies younger in the area with age from 8.4 ±0.2 y 9.0 ± 0.2 ma (mantilla et al, 2009), that indicate that the magmatism was continue for a long time, and contribute to warm and elements to mineralizing fluids. it’s therefore possible there are intrusive bodies younger no date in the area table 5. ar/ar age of the analyzed samples ages obtained for mineralizations sample number mineral for analyzing rock type age plateau (my) age inverse isochron (my) 1002 sericite vein (loscas north) 10.78 ±0.23 11,53±0.84 1012 sericite backup vein, la gloria mine 5.24 ±0.10 5.26±0.13 1018 sericita backup vein, salvación mine 7.68±17 6.75±0.1 1018d sericite backup vein, salvación mine 7.58 ±0.15 7.19±0.54 1022 sericite vein, la locura mine 6.89 ±0.41 6.83±059 discussion the vetas gold deposit was formed by four mineralizing events and one post-mineral event. the first stage shows typical neutral characteristics of low sulfudation epithermal environments according to their mineralogical association, fluids composition, low saline and temperature of deposit formation (camprubi et al., 2003). for the second event, conditions change, due to temperature increase associated with the possible injection of hot and mineralrich fluids to the system, increasing salinity slightly in fluids. during the second stage, the magmatic influence is more evident than the first stage, such as the stable isotopes with a magmatic signature, seems greater relationship with marginal porphyritic conditions, overlap intermediate epithermal environment on the initial conditions. the fluid which interacted with the host rocks, was cooled, developed, neutralized and mixed with meteoric water, precipitating minerals with intermediate sulfidation like sulfosalts (tennantite) silver-rich and zinc-rich, plus gold, low iron sphalerite, pyrite and chalcopyrite (yilmaz, et al., 2010; wang et al, 2019). in the third mineralizing event, fluid and had lower salinity. possibly due to an increase in pressure, a hydraulic micro-brecciation, whose matrix is composed of cryptocrystalline quartz and pyrite is generated. the little mineralogy could be showing that the fluid was getting colder and low in dissolved elements when the four stage happened. finally single crystals of quartz with “comb” texture were precipitated in the open spaces, at temperatures below 190°c. such magmatic influence on the ore deposit becomes more apparent during the second hydrothermal event where a drastic increase in temperature (˃300°c) found by measurement of fluid inclusions and represented by the mineralogy presented during this event is observed. this mineralogy is characterized by high-temperature minerals such as molybdenite, magnetite and tourmaline, showing almost porphyritic features, possibly caused for the overprinted of different hydrothermal systems, product of continue magmatism for 2.5 ma during mioceno (mantilla et al., 2011), the first one, previous to contemporaneous with the first hydrothermal stage with of 10, 9± 0.2 ma (u / pb zircon, mantilla, et al 2011), and the other one dated with 8.4 ±0.2 y 9.0 ± 0.2 ma (mantilla et al, 2009), before second stage (7,58±0,15ma and 6,89±0,41ma), but is necessary dated others porphyritic bodies identified in the area to check their real relation with different stages of the mineralization, being that the magmatic affinity is important. the mineralogical association and salinity of the mineralizing fluids, strong magmatic affinity evidenced with isotopic results, in addition to the pressure and temperature conditions allow the classification of the ore deposit as intermediate sulfidation epithermal deposit (hedenquist et al, 2000, camprubi & abilson, 2006, yilmaz, et al., 2010 and wang et al, 2019). those characteristic could be originated for entry of fluids magmatic and warm to the system during the second stage, that magmatic influence could be over print marginal porphyritic conditions, but is necessary check cores to verify the presence porphyry on depth. solutions neuters to alkaline (illite –sericite association) and low salinity at temperatures near 330°c, allow the conclusion that gold could be transported by bisulphides complexes to shallow levels where the ambient is most oxidant and possibly most acid allowed more precipitation. (hedenquist et al. 2000, camprubi et al. 2006, williams-jones, et al., 2009). metallogenic model the low to intermediate sulfidation epithermal ore deposit located in vetas, santander, originated in a tectonic framework associated with the bucaramanga fault (main structure) and crossings structures with strikes of ne and nw, which helped as a duct for the magmas ascending, product of caribbean plate depth subduction under the south american plate (mantilla et al. 2011) . these magmas with ryodacitic to quartz-monzonites composition were important in the ore deposit formation; because they were initially the main source of mineralizing fluids. the first one of 10, 9± 0.2 ma (mantilla et al, 2011), was the source of first stage possibly, and 8.4 ±0.2 and 9.0 ± 0.2 ma (mantilla et al, 2009) of second stage, even though other bodies more recent no dated. between the miocene pliocene, magmatic fluids, reacted with bucaramanga gneiss rocks and intrusive igneous rocks (jurassic and tertiary) altering it. during the first stage (10.78 ±0.23) these fluids were mixed with meteoric water; they were neutralized and the metals were transported by bi sulphides complexes because according with the alteration mineralogy (illite – sericite carbonates), and temperature (200 to 240°c in the first stage) was the most appropriated mechanism, on shallower depth the enviroments is most oxidant and the metals deposited on open spaces in an low sulfidation environment. a magmatic pulse between first and second stage of 8.4 ±0.2 and 9.0 ± 0.2 ma was identified to 8 km nw approx. this pulse could be related to the second stage (7,58±0,15ma and 6,89±0,41ma ). the difference between ages is almost 1 ma, but is possible the presence of others bodies no dated but identified in the area (figure 1). the mechanism of transported to the second stage its considered similar to the first stage, but the mineralogy, the fluid inclusions show more temperature and stable isotopes more magmatic affinity, this aspects allow suggest that overprinted a system of intermediate sulfidation to almost marginal porphyritic conditions on low sulfidation environment. the third stage is considered like a progression of second stage, but with the coolest and poorest (in metals) fluid, was separated of first event because the minerals were deposited in new spaces opened, product of fracturing event. the four stage dated with (5.24 ±0.10 ma), could be representing a progression on the fluid evolution, with more interaction of meteoric fluids, produce a hydraulic brecciating (mciver, 1997). the post mineral stage is the continuation of evolution of fluid colder and poor in metals, possibly on areas close to surface with a strong oxidation activity and an acid ph (alunite, kaolinite and halloysite). conclusions vetas gold deposit, santander, is a low to intermediate sulfidation epithermal ore deposit in which this study has identified four mineralizing events and one post-mineral event. 17petrography, microthermometry, and isotopy of the gold veins from vetas, santander (colombia) figure 16. age plateau and inverse isochron. the graphics show satisfactory results because the plateaus are almost perfect, with exception to the samples 1002 and 1018 (duplicate), where the 10% of the released gas is below the average age and the sample and the sample 1022 where 10% of the last released gas is above of the average age. the ages calculate with the inverse isochron are very similar to the plateau values, even though in some case there are small variations but those values are into the margin of error, only in the case of sample 1018 the values are more scattered, for this reason the assay was repeated. 11.53±0.84 ma 5.26±0.13 ma 6.75±0.1 ma 7.19±0.54 ma 6.83±0.59 ma 18 sonia rojas barbosa, juan carlos molano mendoza, thomas cramer fluids salinity is high and complicated because there are at least three types of chemical systems: naclh2o, cacl2 – nacl h2o y mgcl2, – nacl h2o, with concentrations reaching up to 16% wteqnacl for the first and third mineralizing events, 18 % wteqnacl for the second event and 8 to 10% wteqnacl for the post-mineral event. these values of salinity are similar of low and intermediate epithermal gold deposits in mexico (camprubi & albinson, 2006). the ore deposit was formed from miocene to pliocene (between 10,7ma – 5,6 ma), this time period is contemporaneous during the first 2.5 ma with magmatic bodies dated on the area by mantilla et al, (2009; 2011). this magmatism had a strong relationship with the actual configuration of deposit, evidenced on stable isotopes result and the temperature pikes (second stage mainly). them this magmatism could over printed marginal porphyritic features to the deposit in the second stage, evidenced on the temperature (390°c), high temperature mineralogy, and caused environments with intermediate sulfidation characteristics. stable isotopes of δ18o, δ 2h, allow to infer that the mineralizing fluids were a mixture of meteoric and magmatic waters where the latter are dominant. stable isotopes of 𝛿34s, show a magmatic reservoir to the mineralization principally, and other possible reservoir is from sedimentary metamorphic rocks of bucaramanga neis, one of the host rocks in the area. the alteration minerals like sericite, carbonates and illite shows that mineralizing fluids were neutral, except on the postmineral stage that be shallower and environment more oxidant the ph was more acid (alunite, kaolinite and halloysite). the identified gold is associated to tennantite and low-iron sphalerite, this was possibly carried by bisulphides complexes, according to neutral ph, homogenization temperature and the ore deposit formation depth. although free phase silver not was identified, this chemical element is part of the tennantite composition, where it is found in high proportions. recommendations make a petrographic, geochemical and geochronology study of the intrusive bodies outcropping in the ne of vetas town, in order to differen tiate accurately miocene (pliocene?) igneous pulses there, clearly determine their chemical characteristics and their associated tectonic environment. date directly the metallic mineralization with methods like re/os in molybdenite. produce a hydrothermal alteration map and make a review of drilling hunches in order to establish their possible association with porphyry ore deposit. acknowledgments the authors would like to thank the united states geological survey (usgc), universidad nacional de colombia and the igac geographic institute for their collaboration in the development of analytical process. in addition, the authors are very grateful to the cb gold company for logistical support and all artisanal miners who allowed us to work on their projects and be in their tunnels. references barnes, h. l. i. 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(1991). evaluación geológica y determinación de las reservas de oro y plata de la mina “la botella”. municipio de vetas, santander. universidad industrial de santander. proyecto de grado. mantilla, l. c., valencia, v., barra, f., pinto, j., & colegial, j. (2009). geocronología u-pb de los cuerpos porfiríticos del distrito aurífero de vetas-california (dpto. de santander, colombia). boletín de geología, 31, 1, 31-43. mantilla, l. c., mendoza, h., bissig, t & craig, h. (2011). nuevas evidencias sobre el magmatismo miocénico en el distrito minero de vetas – california (macizo de santander, cordillera oriental de colombia). boletín de geología, 33, 1, 43-58. mciver, d. a. (1997). epithermal precious metal deposits: physicochemical constraints, classification characteristics, and exploration guidelines rhode university. master tesis. 141 pags. rojas, s. 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(1973). geología de los cuadrángulos h12 bucaramanga y h13 pamplona, departamentos de santander y norte de santander. u.s. geological survey, ingeominas. boletín geológico, xxi, 1-3, 1p. williams-jones, a., bowell, r. & migdisov, a. (2009). gold in solution. elements, 5, 281–287. yilmaz, h., oyman, t., sonmez, n., arehart, g., & billor, z. (2010). intermediate sulfidation epithermal gold-base metal deposits in tertiary subaerial volcanic rocks, sahinli/tespih dere (lapseki/ western turkey). ore geology reviews, 37, 3-4, 236-258. zhang, y. g. & frantz, j. d. (1987). determination of the homogenization temperatures and densities of supercritical fluids in the system naclkclcacl2h2o using synthetic fluid inclusions. chemical geology, 64, 335-350. zhang, y., shao, y., chen, h., liu, z. & li, d. (2017). a hydrothermal origin for the large xinqiao cu-s-fe deposit, eastern china: evidence from sulfide geochemistry and sulfur isotopes. ore geology reviews, 88, 534-549. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.90532 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (septiembre, 2023): 313 319 so il s abstract the enhanced electrokinetic (ek) technique is an innovative approach and increasingly used in remediation of the fine-grained texture soils contaminated with different types of contaminant. in this study, the ek is enhanced by adding an intermediate compartment and using purging solutions at the electrode’s compartments. the natural clayey soil samples were obtained from al-ahadab oil field and artificially contaminated with two ratios of copper sulfate (66666.67 and 26666.67 mg/kg). the feasibility of using ek technique was evaluated by the efficiency of extracting copper from soil and the magnitude of recovery in the geotechnical properties of soil. the duration of remediation was continued for about 240-250 hours depending either on the continuity of electroosmosis flow or reach a constant rate of electrical current. the removal efficiency of copper from contaminated soil samples was ranged from 98.4% to 99.6%. the ek remediation technique proved a marginal impact on the specific gravity and atterberg’s limits, but it has a significant effect on the shear strength parameters and compressibility of soil. furthermore, the ek technique can be considered an efficient method for removing the low and high concentrations of copper sulfate from clayey soil of low permeability. influence of remediation by electrokinetic on the chemical and geotechnical properties of clayey soil contaminated by copper sulfate mahdi o. karkush1*, shahad d. ali1, junied a. bakr2 1. department of civil engineering, university of baghdad, baghdad, iraq. 2. department of civil engineering, university of anbar, ramadi, iraq. * corresponding author: mahdi_karkush@coeng.uobaghdad.edu.iq orcid number: 0000-0003-1304-0303 manuscript received: 14/09/2020 accepted for publication: 13/09/2023 how to cite item: karkush, m. o., ali, s. d., & bakr, j. a. (2023). influence of remediation by electrokinetic on the chemical and geotechnical properties of clayey soil contaminated by copper sulfate. earth sciences research journal, 27(3), 313-319. https://doi. org/10.15446/esrj.v27n3.90532 keywords: remediation; copper sulfate; electrokinetic; heavy metals; geotechnical properties; clayey soil. resumen la técnica de electrocinética mejorada (ek) es una acercamiento innovador que ha incrementado su uso en la remediación de suelos de textura de grano fino contaminados con diferentes fuentes. en este estudio, la técnica ek se mejoró al adicionar una sección intermedia y usar soluciones de purificación en las secciones de electrodos. las muestras naturales de suelos arcillosos se obtuvieron del campo petrolífero al-ahadab y se contaminaron artificialmente con dos adiciones de sulfato de cobre (66666.67 y 26666.67 mg/kg). la viabilidad de usar la técnica de electrocinética mejorada se evaluó por la eficiencia al extraer el cobre de la muestra y la magnitud de la recuperación de las propiedades geotécnicas del suelo. la duración de la remediacion fue continuada entre 240 y 250 horas dependiendo en la continuidad del flujo de electroósmosis o en alcanzar una corriente eléctrica estable. la eficiencia en la remoción de cobre en muestras de suelo contaminado osciló entre 98.4 % y 99.6 %. la técnica de remediación por electrocinética mejorada presentó un impacto marginal en la gravedad específica y en los límites de atterberg pero tuvo un efecto significativo en los parámetros de resistencia a la cizalladura y compresibilidad del suelo. además, la técnica ek puede considerarse un método eficiente para la remoción de pequeñas y altas concentraciones de sulfato de cobre en suelos arciollos de baja permeabilidad. palabras clave: remediación de suelos; sulfato de cobre; electrocinética; metales pesados; propiedades geotécnicas; suelos arcillosos. influencia de la remediación electrocinética en las propiedades químicas y geotécnicas de suelos arcillosos contaminados con sulfato de cobre https://doi.org/10.15446/esrj.v27n3.90532 mailto:mahdi_karkush@coeng.uobaghdad.edu.iq https://doi.org/10.15446/esrj.v27n3.90532 https://doi.org/10.15446/esrj.v27n3.90532 314 mahdi o. karkush, shahad d. ali, junied a. bakr list of notations c1 is the symbol of soil sample contaminated with 66666.67 mg/kg of copper sulfate c1r is the symbol of soil sample c1 after remediation by ek method c2 is the symbol of soil sample contaminated with 26666.67 mg/kg of copper sulfate c2r is the symbol of soil sample c2 after remediation by ek method cao is calcium oxide cc is the compression index cd is the cadmium cf is the final concentration ci is the initial concentration cl-1 is chloride content cr is the recompression index cu is the copper in soil cu is the undrained shear strength cv is the consolidation coefficient eo is the initial void ratio er is the removal efficiency gl is the ground level gs is the specific gravity k is the coefficient of permeability ll is the liquid limit mv is the coefficient of volume compressibility omc is organic matter content pc is the preconsolidation pressure pb is the lead ph is the power of hydrogen pl is the plastic limit sio2 is silicon dioxide so3 is sulfur trioxide tss is the total soluble salts usepa is the united states environment protection agency ε is the axial strain 1. introduction the urban development and industrial expansion in the world have produced a remarkable increase in the sources and quantity of contaminants, one of the common problems is soil contamination with heavy metals (hms). the excessive use of pesticides and fertilizers, leakage from storage tanks and pipes, municipal wastes disposal without treatment, and industrial wastes are the main sources of contaminants in soil (wuana and okieimen, 2011; shehzad et al., 2015). the demand for soil remediation has recently increased to control contamination and exploit the contaminated sites especially in the countries that have a limited area of land. however, many methods can be used in the remediation of contaminated sites, where the selection of suitable methods for remediation depends on the type of soil, type of contaminants, the concentration of contaminants, and the change and continuity in the source of contamination. the ek method is one of the most reliable methods that can be used to remove the hms from fine-grained textured soils. these soils have low permeability, complex structure, and heterogeneous state (e.g., clay lenses inside the sand structure). sometimes, the soils may be contaminated by several contaminants (e.g., organic compounds integrated with heavy metals), and therefore combined techniques can be used for remediation of the contaminated soils (reddy, 2010). faisal and hussein (2013) carried out numerical modelling for the 1-d mass transport equation to simulate the transport copper in the soil during the remediation process by the ek method. the influence of several factors such as ph, adsorption, aqueous phase reaction, and precipitation on the transport phenomena was investigated by the proposed numerical model. further, the constant electrical current was assumed with time and the effects of temperature and dissolution of soil constituents during the remediation process were ignored during the simulation of the problem. the ph profile and concentration of copper in the sandy soil predicted by the numerical model were in good agreement with those measured experimentally (faisal and hussein, 2013). the relationship between the geochemical properties of soil and the ability of soil for retention of heavy metals (pb and cd) depend on the type of soil and heavy metal, where the retention ratio ranges from 10 to 100% (defo et al., 2016). also, this study demonstrated that the organic matter content has a major influence on the retention of pb and cd in the ntem watershed soil while the clay content, ph, and cation exchange capacity have a slight effect. however, chinade et al. (2017) studied the impacts of the chemical solution that contains different types and concentrations of heavy metals on the shear strength of the soil. the unconfined compression strength tests were conducted on soil samples exposed to chemical solutions infiltration for several periods of 7, 21, 42, 84, and 120 days. the results of tests proved that the shear strength was reduced when the period of permeating by the infiltration of municipal solid wastes was increased. chen et al. (2019) investigated the impacts of the electrokineticspersulfate process on the remediation of soil artificially contaminated with decabromodiphenyl ether (bde-209) and copper. the results show that there was significant removal of bde-209 and cu from the soil. mcd assisted process gave the highest bde-209 removal (88.6%) and the third largest cu removal (54.3%) from the soil. comparatively, the highest cu removal (92.5%) and the second largest bde-209 removal (85.6%) were achieved by the joint application of mcd and citric acid in anolyte during the ek-persulfate treatment. shen et al. (2020) investigated the effect of electric potentials on the efficiency of ek remediation on the removal of cu and zn from soil. the results of study showed increasing the removal efficiency of cu and zn with increasing the electric potential of the cathode from 0 to 10 v. also, the ion migration to maintain electrical neutrality played an additional role in the variations in cu and zn removal-efficiencies and energy consumption. mostly, increasing the electric potentials causes raising the temperature of soil which causes evaporation of soil moist and may causes reverse osmosis. sharma and kumar (2020) reviewed using utilization of industrial wastebased geopolymers in the stabilization of soft and weak soil. in addition to the improvement of geotechnical properties of soil, the study focused on the cost and the amount of co2 gas emission and it is responsibility of global warming. effects of different parameters of geopolymers such ratios of mixing (%), molarity, temperature, curing time, alkaline activator ratio, and water/cement ratio on the geotechnical properties of soil have been discussed. ghobadi et al. (2021) used electrokinetic remediation technology integrated with compost as recyclable reactive filter media for removal of copper from kaolinite soil the compost placed near the cathode served as an adsorbent to bind copper ions while buffering the advancement of the alkaline front in soil. the total copper removal rate increased from 1.03% in ek to 45.65% in ek with 100% of compost under an electric gradient of 10 v. in the present study, the natural fine-grained textured soil sample was contaminated with two ratios of copper sulfate (66666.67 and 26666.67 mg/ kg) for 30 days and treated by enhanced ek method for 240-250 hours. the ek method is enhanced by using several purging solutions and mid-compartment of activated carbon to increase the efficiency of this method for removing copper sulfate from the soil. the geotechnical and chemical characteristics of soil are measured before and after remediation to evaluate the efficiency of the proposed ek method in the remediation of clayey soil samples contaminated with copper sulfate. the efficiency of the remediation technique can be evaluated either by the removal efficiency of contaminant from soil or by the magnitude of recovery of the key geotechnical properties such as shear strength and compressibility of soil. 2. effects of heavy metals on geotechnical properties of soil heavy metals have been produced from different sources and they have a severe influence on the geotechnical properties of soil. table 1 shows the maximum concentration of heavy metals in soil (saleem et al., 2011). the heavy metals adhere to the particles of soil and they are immobile. this process is called sorption and describes the heavy metal distribution between the solid and the soil solution. sorption can be divided into adsorption and absorption, where adsorption is the critical mechanism of sorption. adsorption can be defined as binding the contaminants on the surface of the soil while absorption can be described as the binding of these contaminants in solid components of the soil. the primary mineral components of soil such as clay minerals, organic matter, silica, aluminium, iron, manganese oxides, carbonates, and phosphates sorb heavy metals (nystrøm, 2001). 315influence of remediation by electrokinetic on the chemical and geotechnical properties of clayey soil contaminated by copper sulfate table 1. maximum concentration levels of hms in soil (saleem et al., 2011). maximum level in soil, mg/kg heavy metal dutch standardsusepa 0.30.5mercury (hg) 0.80.6cadmium (cd) 0.71.7selenium (se) 2914arsenic (as) 920cobalt (co) 3532nickel (ni) 8560lead (pb) 36100copper (cu) 100120chromium (cr) 140220zinc (zn) up to date, very limited studies have been studied the effect of heavy metals on the geotechnical properties of soil and these studies addressed that the presence of heavy metals in the soil mass causes a reduction in the shear strength parameters of soil, cohesion (c) and angle of internal friction (φ). however, it was also observed that the presence of heavy metals in the soil raises compressibility, increases the average dry density, and reduces the optimal soil moisture content and permeability. studies on the impact of heavy metals on soil geotechnical properties are however quite limited (karkush et al., 2013). 3. electrokinetic remediation technique the ek is one of the most well-known recent technique developed to remove the heavy metals and/or organic compounds from the contaminated soils which have low permeability (karim, 2014). this remediation method can extract contamination (radionuclides, certain organic compounds, and heavy metals) from the soil or groundwater by applying an electrical current between two electrodes. during the application of dc electrical current, three phenomena can occur electroosmosis, electromigration, and electrophoresis. several previous studies have shown that the use of water as an electrolyte is an ineffective way to give a high removal efficiency of contaminant, especially when the soil is contaminated with two or more contaminants. therefore, several techniques can be combined with the ek method to improve the removal efficiency of contaminants from the soil as listed below: a. increasing the period required for treatment, but it is limited by the flow of electrical current and osmotic flow. b. increasing the electric gradient, but is limited by the temperature of the soil sample. c. using the improvement of chemical solutions (solvents or surfactants). d. using washing by chemical solutions. the use of purging solutions in the electrodes compartment is the most promising strategy for obtaining high removal efficiency of contaminant from soil or groundwater (reddy, 2013). according to the type of contaminant and soil, it can be identified the type and concentration of purging solution. also, soil washing is one of the remediation techniques that can integrated with the ek method (dermont et al., 2008). soil washing is ordinarily utilized for isolating the contaminants from soil particles and the most contaminated fraction of the soil will be disposed outside of the site, where several extracting fluids such as acids/bases, salts, chelating agents, surfactants, or redox agents can be used to move the metals from the soils into the hydrous solution. 4. experimental work the experimental work consists of two main stages: the fieldwork stage and laboratory work stage. the fieldwork involves soil sampling and measuring the field total density and moisture content. however, the laboratory work includes investigating the influence of ek method remediation on the geotechnical and chemical properties of soil samples contaminated with copper sulfate by conducting a wide range of traditional laboratory tests (karkush and ali, 2019a). soil sampling and field tests the natural soil samples are obtained from al-ahdab oil field in the east of iraq by digging a trial pit to a depth of 3 m under the ground level (gl) and the groundwater table encountered at depth of 2.65 m from the gl. the field density of soil was 1984 kg/m3 measured according to astm (d2937) and the water content was 28.6% measured according to astm (d2216) (karkush and ali, 2019a). contamination of soil samples two soil samples are contaminated artificially in the laboratory with two ratios of copper sulfate (66666.67 and 26666.67 mg/kg ) to investigate the influence of contaminants on the geotechnical and chemical properties of clayey soil samples. the soil samples are soaked by contaminant solutions for thirty days in two separate plastic containers. the duration of saturation, 30 days, is assumed enough to complete the chemical reactions between the contaminant and mineral composition of the soil (karkush et al., 2013)). further, this duration should be enough for contaminant to reach the farthest points in the soil and ensure homogenous distribution for the contaminant in the soil samples. it can be confirmed this state visually by using transparent plastic containers. the contaminant solution consists of copper sulfate (100 and 400) g and 10 l of distilled water. the quantity of water used in this process must be enough to cover the soil sample and the saturation of soil samples (zhou et al., 2004; karkush et al., 2013). the designation of soil specimens, the concentrations of copper sulfate (cuso4.5h2o), and copper (cu) used in the current study are given in table 2. table 2. definition of tested soil samples. soil sample definition concentration of copper sulfate concentration of copper c1 remolded soil sample contaminated with 100 g of copper sulfate (cuso4.5h2o) 66666.67 mg/kg 1696.732 mg/ kg c1r decontaminated soil sample c1 with enhanced ek method c2 remolded soil sample contaminated with 400 g of copper sulfate (cuso4.5h2o) 26666.67 mg/kg 6786.933 mg/ kg c2r decontaminated soil sample c2 with enhanced ek method remediation of contaminated soil samples the ek setup utilized for remediation of contaminated soil samples consists of two main compartments (cathode and anode) in addition to the suggested mid compartment of activated carbon. the ek cell has a length of 50 cm, and a height of 10 cm of high with 10 cm of width, and it was supplied with a power supply and multimeter as shown in figure 1. the dimensions of carbon electrodes used in this study are 10 cm of width, 10.5 cm of height and 2 cm of thickness and the used voltage gradient is 1.3 vdc/cm. the ek method setup is enhanced in two ways: using a purging solution in the three compartments instead of distilled water and using an activated carbon barrier. the purging solutions used in this study are 0.001 m of nitric acid (hno3) at the anode, mid, and cathode compartments at the beginning of the test. when the ph value at the anode compartment reaches to a value about 2, the purging solution is replaced by 0.1 m of sodium hydroxide (naoh) to keep the value of ph higher than 2 to prevent the reverse electroosmotic flow from occurring (karkush and ali, 2019a). also, the anolyte was circulated with controlled constant flow rate during the test as shown in figure 1. the remediation period is continued for 316 mahdi o. karkush, shahad d. ali, junied a. bakr 240-250 hours and it is stopped when the electrical current becomes constant or there is no osmotic flow. figure 1. the ek setup used in the remediation of soil samples. chemical and geotechnical properties of soil a specific laboratory-testing program is carried out on contaminated and decontaminated soil samples in the laboratory of soil mechanics at the university of baghdad. the chemical and geotechnical properties of soil samples are measured according to the astm standards. understanding the effects of contamination by copper sulfate and remediation by ek method requires detailed testing program to measure the mineral composition of soil samples and the variation in the physical and mechanical properties of these soil samples. the copper sulfate has a significant influence on the chemical and geotechnical properties of clayey soil. therefore, the use of such soils in the construction or as the foundation for the structures makes researchers think seriously in the remediation of such soils before they decide to use them (karkush and ali, 2019a). 5. results and discussion the removal efficiency of copper from the soil samples after remediation by ek method can be calculated by: er, cu = (ci cf)/ ci × 100 (1) where er, cu: is the is the removal efficiency of copper from the soil. ci: is the initial concentration of copper in the soil (mg/kg). cf: is the final concentration of copper in the soil (mg/kg). the initial and final concentrations of copper in the soil samples can be computed by: atomic mass of used quantitycu (%) atomic mass of 4 2 100 5 cu cuso h o × ∗  = (2) according to equation 2, the initial and final concentrations of copper in the tested soil samples and the removal efficiency of copper from contaminated soil samples calculated by using equation 1 are given in table 3 (karkush and ali, 2019a). the use of the enhanced ek method in the remediation of clayey soil contaminated with copper sulfate is very efficient regardless of the initial concentration of copper sulfate in the soil samples. table 3. removal efficiency of copper from tested soil samples. initial concentration of copper mg/kg final concentration of copper mg/kg removal efficiency % 1696.732 soil sample c1 27.56 soil sample c1r 98.4 6786.933 soil sample c2 28.46 soil sample c2r 99.6 impacts of copper on the chemical properties of soil the chemical composition of clayey soils plays an important role in determining the geotechnical properties of soils, especially when the soil samples are contaminated. the chemical tests conducted on the soil samples before and after remediation to measure the contents of so3, cl-1, sio2, cao, omc, gypsum, ph, and tss to evaluate the impacts of remediation process on the chemical composition of the soil samples. these tests conducted according to the astm specifications. the results of the chemical tests are presented in table 4. it can be noted from table 4 that the remediation by ek method causes a reduction in the contents of so3, cl-1, sio2, omc, gypsum, and tss because of adsorption of these compounds by the surrounded soil particles. however, the ek method causes increasing the contents of cao and ph value of soil. a strong alkaline used as a purging solution in the ek method causes a slight rise in the ph value of soil. based on this, it can be generally said that the ek method has slight impacts on the contents of so3, sio2, cao, and ph value while it has significant influences on the contents of cl-1, omc, gypsum, and tss. table 4. results of chemical tests conducted on soil samples. soil sample so3 % cl-1 % quartz % cao % omc % gypsum % ph value tss % c1 0.620 0.836 32.58 17.93 0.610 0.043 7.5 3.53 c1r 0.572 0.416 32.48 18.28 0.551 0.031 7.6 2.59 c2 0.293 1.187 31.64 18.11 0.656 3.56 7.3 6.92 c2r 0.291 0.819 33.41 18.12 0.637 3.23 7.4 2.33 impacts of copper on physical properties of soil the results of particle-size distribution, atterberg limits, specific gravity, and hydraulic conductivity of tested soil samples are presented in table 5 and the particle-size distribution curves are shown in figure 2. the physical properties of soil samples are measured according to the astm standards. it can be observed that the contamination by copper sulfate causes increasing of the sizes of the soil particles and increasing the concentration of copper sulfate in the soil leads to a reduction in the percentage of fine particles in the soil samples. accordingly, the stability of aggregates in the soil is noted to be increased because the copper metal (cu+2) can adsorb on the surfaces of clay particles and create a cation bridge between the clay particles. however, after the remediation process, the percentage of fine particles is observed to be increased while the percentage of sand and silt tends to be decreased. it can be confirmed based on the previous observations that although the contaminants are removed from soil by using the ek method, the percentage of sand and silt is still less than that of natural soil samples because the salts of copper sulfate are not completely extracted from the soil after the remediation process. the specific gravity of soil samples is increased with increasing the content of copper sulfate in the soil. this result may be attributed to the precipitation of copper sulfate in the pores of soil and the specific gravity of copper sulfate ishigher than that of natural soil (karkush et al., 2013). after the remediation process, it can be observed that there was insignificant decrease in the values of specific gravity. a possible explanation for these results may be the migration of dissolved salts and colloids in leaching water during the ek method. besides, the increase of the content of copper sulfate in the soil samples causes a reduction in liquid and plastic limits (karkush and ali, 2019a), but the remediation by the ek technique leads to increasing the liquid limit by 8 and 8.7% in soil samples cr1 and cr2, respectively. the liquid limit increased because the content of fine particles in the soil samples after remediation is increased and the removal of copper sulfate from 317influence of remediation by electrokinetic on the chemical and geotechnical properties of clayey soil contaminated by copper sulfate the soil is increased the reactions between charged clay particles with water. the pl of soil sample cr1 does not change after remediation, but in soil sample (cr2) is increased by 8.7%. the hydraulic conductivity of soil samples tested by falling head tests (fht) is reduced by increasing the content of copper sulfate in the soil specimen, where the hydraulic conductivity of soil sample c2 is less than of soil sample c1 by 33%. table 5. physical properties of tested soil samples. soil samples sand % silt % clay % gs ll % pl % k×10-8 cm/sec c1 0.104 34.896 65.0 2.77 50 27 1.63 c1r 0.103 28.891 71.0 2.76 54 27 c2 0.285 47.432 52.0 2.82 46 23 1.06 c2r 0.188 36.812 63.0 2.79 50 25 figure 2. particle size distribution curves of tested soil samples. impacts of copper on mechanical properties of soil the stability of structures foundations depend on the compressibility and shear strength of soils beneath them, and therefore the structures can be damaged or collapsed if the soil fails in shear strength and/or the compressibility exceeds the allowable limits. well understanding of the shear strength and compressibility of the soil is necessary to design the foundations and earth structures. the copper sulfate contamination causes a significant decrease in the shear strength parameters and the compressibility parameters of soil (karkush and ali, 2019b). the remediation by the ek method is used to regain the strength of soil lost by contamination. the consolidation parameters are obtained in the current study by conducting a 1-d consolidation test while the shear strength parameters are obtained by performing the unconfined compressive strength test (ucs) and unconsolidated undrained triaxial test (uut). the results obtained from 1-d consolidation tests are the initial void ratio (eo), compression index (cc), swelling index (cr), preconsolidation pressure (pc), coefficient of volume compressibility (mv), and coefficient of consolidation (cv) and they are presented in table 6. further, the variation of void ratio with pressure obtained from 1-d consolidation test is shown in figure 3. the coefficient of consolidation (cv) is increased after remediation by the ek method. a possible explanation for these results may be the increase of the hydraulic conductivity of the contaminated soil samples. however, the compression and swelling indices are noted to be increased for the tested soil samples, and there are several reasons caused this action: (1) the presence of copper as a lubricant agent in the voids of soil could cause the sliding of soil particles; (2) the nature of the fluid and the charge of the soil particles could influence the adsorbed cations; and (3) perhaps, this increase caused by dissolving of salts (grim, 1968; karkush and altaher, 2017). it can be also noted that the contamination causes increasing the coefficient of volume change and this could be because of the re-arrangement of the recently bonded soil particles into the macro voids generated from contamination of soil samples with copper sulfate. in the soil matrix, this reaction causes the generation of a large space between the particles of soil (ota, 2013). after remediation, it can be noted that the initial void ratio is decreased, but the compression index, swelling index, the coefficient of volume change, and the coefficient of consolidation are increased relative to the corresponding values of contaminated soil specimens. table 6. compressibility and shear strength parameters of soil samples. so il sa m pl e 1-d consolidation test ucs uut eo cc cr pc kpa mv m2/kn cv cm2/sec ɛ % cu kpa cu kpa c1 0.881 0.123 0.033 45 0.00027 0.00018 9.276 65.52 94.145 c1r 0.829 0.199 0.046 110 0.00044 0.00032 12.223 68.09 105.99 c2 0.893 0.125 0.034 33 0.00046 0.0001 17.039 18.82 23.28 c2r 0.866 0.205 0.045 33 0.00089 0.00042 16.710 26.65 74.35 figure 3. void ratio versus pressure of tested soil samples by 1-d consolidation tests. the shear strength of soil samples has been measured before and after remediation by ek method using (ucs) and (uut) tests. the results recorded from ucs are given in table 6 and the stress-strain relation is shown in figure 4. the contamination of soil by heavy metals (hms) has a negative impact on the undrained shear strength of the soil. the contaminated soil samples (c1 and c2) are classified according to the undrained shear strength as firm and very soft soil respectively (craig, 2004). the lower value of shear strength parameters of contaminated soil samples is obtained because of the presence of the dissolution of salts that connect the particles of soil altogether and consequently, the contact area between the soil particles will be reduced, where the percentage of fines is decreased by contamination and this ratio is increased after remediation (karkush and altaher, 2017). however, it can be observed that the undrained shear strength of soil samples after remediation is increased by 3.92 and 29.55% for the soil samples c1r and c2r respectively. the undrained triaxial tests (uut) are conducted on soil samples before and after remediation under different cell pressure (100, 200, and 300) kpa. in uut test, the angle of internal friction (φ) is equal to zero if the soil is completely cohesive and fully saturated, so if the degree of saturation is not equal to 100%, the failure envelope of mohr-coulomb circles is usually a curve and that the angle of internal friction (φ) is not equal to zero (ota, 2013). the results obtained from uut test are given in table 6 and the variation of deviatoric stress with the axial strain is shown in figure 5. the results obtained from uut are demonstrated that the copper has a high impact on the undrained shear strength of the soil and the ek remediation method remarkably influences the recovery of the undrained shear strength of contaminated soil samples. it is observed that the undrained shear strength is recovered by 13 and 220% for the soil samples c1r and c2r respectively. generally, both ucs and uut showed the same trend of stress-strain relation, but the results of the uut test are still considered more reliable than ucs test due to neglecting the effect of confining pressure in ucs. 318 mahdi o. karkush, shahad d. ali, junied a. bakr figure 4. stress-strain relationships for soil samples obtained from unconfined compressive strength tests. figure 5. deviator stress versus axial strain for soil samples obtained from unconsolidated undrained triaxial tests. 6. conclusions the present study investigates the impacts of remediation of clayey soil samples contaminated with two percentages of copper sulfate using enhanced ek technique on the chemical and geotechnical properties of soil samples. the main conclusions from this work are: • the copper sulfate has slight effects on the particle-size distribution curve, but the remediation of contaminated soil specimens has contributed to an increase of the percentage of fines particles having a size less than 0.005 mm and a slight impacts on the atterberg’s limits and the specific gravity of clayey soil samples. • the compressibility parameters such compression index (cc) and the coefficient of consolidation (cv) are adversely increased after remediation. • the undrained shear strength of the soil is recovered after remediation by enhanced ek technique. the magnitude of recovery increased significantly with increasing the initial concentration of copper in the soil 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(2004). electrokinetic remediation of a cu contaminated red soil by conditioning catholyte ph with different enhancing chemical reagents. chemosphere, 56(3), 265-273. https://doi.org/10.1016/j.chemosphere.2004.02.033 https://doi.org/10.1080/01496395.2020.1825967 https://doi.org/10.1080/01496395.2020.1825967 https://doi.org/10.5402/2011/402647 https://doi.org/10.5402/2011/402647 https://doi.org/10.1016/j.chemosphere.2004.02.033 https://doi.org/10.1016/j.chemosphere.2004.02.033 _hlk44029663 la estructura elástica bajo australia es mostrada por medio de mapas de velocidad de onda s, para profundidades que varían desde cero a 400 km, determinada por regionalización e inversión de dispersión de ondas rayleigh. las trazas de 233 terremotos, ocurridos desde 1990 hasta 2010, han sido usadas para obtener datos de dispersión de ondas rayleigh. estos terremotos fueron registrados por 65 estaciones sísmicas localizadas en australia y el área circundante. las curvas de dispersión fueron obtenidas para periodos entre 5 y 250 s, por filtrado digital con una combinación de las técnicas de filtrado: mft (técnica del filtro múltiple) y tvf (filtrado en tiempo variable). luego, todos los eventos sísmicos (y algunas estaciones) fueron agrupados para obtener una curva de dispersión para cada trayecto fuente-estación. estas curvas de dispersión fueron regionalizadas e invertidas de acuerdo con la teoría de la inversión generalizada, para obtener modelos de velocidad de onda de cizalla para una rejilla regular de tamaño de celda de 2.5º×2.5º. la estructura de velocidad de cizalla obtenida a través de este procedimiento es mostrada en los mapas de velocidad de onda s representados para diversas profundidades. estos resultados concuerdan bien con la geología y otros resultados geológicos y geofísicos previamente obtenidos. los modelos de velocidad de onda s obtenidos sugieren la existencia de heterogeneidad lateral y vertical. las zonas con estructuras antiguas y bien consolidadas presentan mayores valores de velocidad de onda s que los correspondientes a otras zonas, aunque esta diferencia puede ser muy pequeña o despreciable en algún caso. no obstante, en el rango de profundidad de 15 a 50 km, las diferentes profundidades del moho presentes en el área de estudio generan la principal variación de velocidad de onda s. un comportamiento similar es encontrado par el rango de profundidad desde 65 a 180 km, en el cual la frontera litosfera-astenosfera genera la principal variación de velocidad de onda s. finalmente, debería ser notado que una nueva e interesante característica fue obtenida en este estudio: la definición de la base de la astenosfera, para profundidades que varían desde 155 a 280 km, en australia y el área circundante. esta característica está también presente en los continentes: américa del sur, antártida y áfrica, los cuales fueron parte del mismo súper-continente: gondwana, en el mesozoico temprano antes de fragmentarse. earth sciences research journal eart sci. res. j. vol. 18, no. 2 (december, 2014): 87 98 abstract resumen key words: rayleigh wave, shear wave, crust, upper mantle, australia. palabras clave: estructura de velocidad de onda s de australia a partir del análisis de ondas rayleigh record manuscript received: 05/12/2013 accepted for publication: 21/09/2014 seismology the elastic structure beneath australia is shown by means of s-velocity maps for depths ranging from zero to 400 km, determined by the regionalization and inversion of rayleigh-wave dispersion. the traces of 233 earthquakes, occurred from 1990 to 2010, have been used to obtain rayleigh-wave dispersion data. these earthquakes were registered by 65 seismic station located in australia and the surrounding area. the dispersion curves were obtained for periods between 5 and 250 s, by digital filtering with a combination of mft (multiple filter technique) and tvf (time variable filtering), filtering techniques. later, all seismic events (and some stations) were grouped to obtain a dispersion curve for each source-station path. these dispersion curves were regionalized and inverted according to the generalized inversion theory, to obtain shear-wave velocity models for a rectangular grid of 2.5º×2.5º mesh size. the shear-velocity structure obtained through this procedure is shown in the s-velocity maps plotted for several depths. these results agree well with the geology and other geophysical results previously obtained. the obtained s-velocity models suggest the existence of lateral and vertical heterogeneity. the zones with consolidated and old structures present greater s-velocity values than those in the other zones, although this difference can be very little or negligible in some case. nevertheless, in the depth range of 15 to 50 km, the different moho depths present in the study area generate the principal variation of s-velocity. a similar behaviour is found for the depth range from 65 to 180 km, in which the lithosphere-asthenosphere boundary generates the principal variations of s-velocity. finally, it should be highlighted a new and interesting feature was obtained in this study: the definition of the base of the asthenosphere, for depths ranging from 155 to 280 km, in australia and the surrounding area. this feature is also present in the continents: south america, antarctica and africa, which were part of the same super-continent gondwanaland, in the early mesozoic before fragmentation. shear-wave velocity structure of australia from rayleigh-wave analysis v. corchete higher polytechnic school, university of almeria. email: corchete@ual.es issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v18n2.41078 http://dx.doi.org/10.15446/esrj.v18n2.41078 88 v. corchete 1. introduction the great ability of the rayleigh-wave dispersion analysis to delineate the earth’s deep structure is well known (corchete, 2012). this methodology allows the features of the earth’s structure to be determined, because it is well known that the surface-wave dispersion (rayleigh-wave dispersion) is related to the earth’s structure crossed by the waves (represented by the s-velocity structure). thus, the features of the elastic structure for any region of earth can be studied through the analysis of rayleigh-wave dispersion. in this paper, the features of the elastic structure beneath australia (and the surrounding area) will be revealed through this analysis. australia and its surrounding area are ones of the regions for which there are relatively few seismic studies concerning deep mantle structure (e.g. zielhuis and van der hilst, 1996; simons et al. 1999; fichtner et al., 2009; fishwick and rawlinson, 2012). since the geologic setting of this area is very complex, knowledge of detailed seismic structure of this area would be of great importance. nevertheless, the elastic structure beneath australia is among the least studied structures on earth, due to the particular conditions of this continent (a large part is covered by deserts), which have made difficult the development and maintenance of high-quality seismic networks. fortunately, in the recent years, some international agencies (as gsn, iris geofon and geoscope) joint to local agencies (as geoscience australia) have been able to develop and support new and permanent broadband digital seismic stations (and portable deployments within australia). as a result, in the last twenty years high-quality seismic records have been made available from seismographic databases for australia and its surrounding area. in pioneer studies, ellis and denham (1985) and denham and ellis (1986) investigated the crust and upper mantle structure using surface waves. they used periods of the fundamental modes ranged from 8 to 60 s, obtaining a few shear-velocity profiles from 0 to 140 km of depth. later, zielhuis and van der hilst (1996) performed the first rayleigh-wave tomography for australia and the surrounding area. in this study, they constructed a 3-d model of shear velocity for the upper mantle (and transition zone beneath eastern australia and the adjacent oceanic regions), using both body and rayleigh waves (fundamental and higher modes). they obtained shear-velocity maps from 80 to 400 km of depth (although the resolution decreases drastically with depth from 200 km). few years later, simons et al. (1999) performed a more complete rayleigh-wave tomography for australia and the surrounding area. in this study, they obtained shear velocity maps from 80 to 400 km of depth (although the resolution decreases drastically with depth from 210 km), using periods of fundamental modes ranged from 40 to 200 s and periods of higher modes ranged from 20 to 125 s. more recently, fishwick et al. (2005), fishwick and reading (2008) and fishwick and rawlinson (2012) obtained the upper-mantle s-velocity structure beneath australia, from surface-wave tomography, for depths ranging from 75 to 300 km (although the resolution decreases drastically with depth from 225 km). fishwick and rawlinson (2012) compared their surface-wave inversion results with those calculated by fishwick et al. (2008), finding very similar patterns of s-velocity in both studies (also in both studies the resolution decreases drastically with depth from 225 km). the results obtained by fishwick and rawlinson (2012) are also very similar to those calculated by fichtner et al. (2009), who obtained the upper-mantle s-velocity structure in the australasian region, from a full seismic waveform tomography, for depths ranging from 80 to 400 km (although the resolution of their 3d s-velocity model decreases drastically with depth from 250 km). finally, saygin and kennett (2012) obtained the first 3-d s-velocity structure for the australian crust (from 0 to 30 km of depth), using surface-wave group velocity (from 1 to 32 s of period) extracted from ambient noise. this s-velocity model for the crust was combined with all the information retrieved from the abovementioned studies, by kennett and salmon (2012), to perform their ausrem (australian seismological reference model). this recent model is displayed from 0 to 400 km of depth (although its resolution decreases drastically with depth from 225 km). in the crust, ausrem represents the australian crust via a 5-layered structure (i. e. only 5 layers are considered for the whole australian crust). these five layers are: sediments, basement, upper crust, middle crust and lower crust. this simple form to give a model for the crust, provides considerable flexibility to be used in seismic-receiver function studies, but it provides considerable lesser information than the s-velocity model performed by saygin and kennett (2012), which is a true 3d model (although it is only displayed from 0 to 30 km of depth). it should be noted that no 3d s-velocity structure has been obtained for australia and its surrounding area, from surface-wave tomography, for depths ranging from 30 to 75 km. furthermore, the s-velocity structure, obtained in the above-mentioned previous studies, shows a poor resolution for the depth range from 200 to 400 km. for it, the goal of the present study is the determination of the elastic structure (s-velocity structure) beneath australia and the surrounding area, from rayleigh-wave analysis, for the whole depth range from 0 to 400 km, because no 3d s-velocity structure has been obtained, until now, for this study area in this complete depth range. this rayleigh-wave analysis will consist of filtering of rayleigh waves, to obtain dispersion curves from 5 to 250 s. this dispersion analysis will be performed in several steps. first, all seismic events will be grouped in source zones to get an average dispersion curve for each source-station path. then, the dispersion curves will be obtained by digital filtering with a combination of the multiple filter technique (dziewonski et al., 1969) and time variable filtering (cara, 1973). thus, a set of source-station averaged dispersion curves will be calculated. this set of dispersion curves will be regionalized and then inverted according to the generalized inversion theory, to get s-wave velocity models for a rectangular grid defined in the study area. finally, these models will be plotted to obtain a 2d mapping of the 3d s-wave velocity structure, for the study area. the period range to be obtained in the present study, by digital filtering, will be wider than the period range measured in the above-mentioned previous studies. thus, the determination of the 3d s-velocity structure, in the whole depth range from 0 to 400 km, will be more accurate in the present study than the other above-mentioned previous studies, in which no surface-wave dispersion was measured in the period ranges from 5 to 40 s and from 200 to 250 s. this lack of information in the above-mentioned previous studies has caused the lack of a 3d s-velocity structure, for the depth range from 0 to 80 km, and the bad resolution of the 3d s-velocity structure for the depth range from 200 to 400 km. 2. data set in this study, a list of 233 earthquakes, occurred from 1990 to 2010 in the neighbouring of australia, was considered (supplement 1). these earthquakes were registered by 65 seismic stations located at this continent and the surrounding area (supplement 2). from these stations digital data are available, recorded with a different instrument for each available channel (supplement 6). from all these channels, only the records for the channels: bhz, lhz, blz, llz, bh1, lh1, bh2, lh2; have been considered as the most suitable data for this study, because the period interval of the best registration for these channels is between 1 or 5 to 200 or 250 s (period interval in which the frequency response is almost flat). this period interval (from 1 to 250 s) is more suitable for exploring the elastic structure of the earth, for a depth range of 0 to 400 km of depth. for this reason, the above mentioned channels haven been selected from the all available channels. also, to ensure the reliability of the results, only the earthquake traces in which a well-developed rayleigh wave train is present, with a very clear dispersion, have been considered. thus, many records have been discarded. logically, the instrumental response must be taken into account to avoid the time lag introduced by the seismograph system (and all distortions produced by the instrument). this correction recovers the true amplitude and phase of the ground motion, allowing the analysis of the true dispersion of rayleigh waves. for this reason, all the traces considered in this study were corrected for instrument response. 3. pre-processing and filtering for the pre-processing and calculation of dispersion curves, the computation method detailed by corchete et al. (2007) was followed. in this paper, only a brief review of the principal concepts of this methodology will be presented. 89shear-wave velocity structure of australia from rayleigh-wave analysis grouping of stations and seismic events. it is well known that generally the rayleigh waves propagated along very near epicentre-station paths show similar dispersion curves, because they cross the same earth structure and sample the same elastic properties of the medium. as a result, it is possible to group all seismic events listed in supplement 1 in source zones, as listed in supplement 3, to get source-station paths from epicentre-station paths. these source zones are defined as a location at which seismic events with similar epicentre coordinates have occurred. the coordinate differences for a group of events must be less than or equal to 1 degree in latitude and longitude, to be able to group them in the same source zone. then, an average dispersion curve for each source-station path can be obtained, when the dispersion curves calculated for the traces of the events of this source zone (recorded at the same station) are averaged. in this way, any small deviations obtained are considered errors, which can be described by the standard deviation. it should be noted that some stations considered in this study also show very similar coordinates. thus, they have been grouped using the same criteria described above for the events, defining new station codes to denote these average stations. these new stations are listed in supplement 4. figure 1 shows the path coverage obtained for the study area given by the source-station paths (the source coordinates are listed in supplement 3 and the station coordinates are listed in supplement 2 and 4). dispersion analysis. the rayleigh-wave group velocity for the trace of each event registered has been measured by means of the combination of digital filtering techniques: multiple filter technique (mft) and time variable filtering (tvf), as shown in the flow chart displayed in figure 2. as an example of this filtering process, the application of this combination of filtering techniques to the trace of the event 31 (recorded at station can) will be detailed. the mft has been applied firstly to this trace as it is shown in figure 3. this trace and the dispersion curve obtained are used to compute the digital filtered signal using the tvf. figure 4 shows the time-variable filtered signal resulting from the tvf. finally, figure 5 shows the final dispersion curve obtained after application of the mft to the filtered signal. a comparison between figures 3 and 5 shows the benefits of the proposed filtering process. it should be noted that a combination of mft and tvf works better than the application of the mft only, because the signal/noise ratio is highly increased. figure 2. the scheme followed in the filtering process of each event-station seismogram to obtain its dispersion curve. the circles are used to denote the application of a digital filtering technique and the rectangles are used to denote the results obtained. the enhanced rectangles are used to show the initial data and the final result. figure 3. (a) observed seismogram corresponding to the event 31 (supplement 1) recorded at station can (supplement 2), instrument corrected. (b) contour map of relative energy normalized to 99 decibels, as a function of the period and the group time (white points denote the group times inferred from the energy map). (c) group time curve inferred from the energy map. (d) group velocities calculated from the group times and the epicentral distance (group velocity is the epicentral distance divided by the group time for each period). figure 4. (a) observed seismogram corresponding to the event shown in figure 3a. (b) group velocity dispersion curve obtained after application of the mft (as shown in figure 3). (c) time-variable filtered seismogram. figure 5. (a) time-variable filtered seismogram obtained after application of the tvf, as shown in figure 4. (b) contour map of relative energy normalized to 99 decibels as a function of the period and the group time (white points denote the group times inferred from the energy map). (c) group time curve inferred from the energy map. (d) group velocities calculated from the group times and the epicentral distance. average dispersion curves and estimation of the error. when the dispersion curves of the traces for each event involved in a source-station path have been calculated, by means of the above-described filtering process, the average group velocity for this path can be obtained as a mean of the groupvelocity values calculated for each period. an estimation of the error for this media is calculated by the standard deviation (1-s error). for paths with only one event involved, a mean of the standard deviations obtained for the other near paths (which have more than one event involved), can be considered as a good estimation of the error. the average group velocities obtained for the 412 source-station paths considered in this study are listed in supplement 5 for several periods. the periods for these dispersion curves ranges from 5 to 250 s. it should be noted that the period range obtained in the present study largely exceeds the period range obtained in previous studies (simons et al., 1999; fishwick et al., 2005; fishwick and reading, 2008; fichtner et al., 2009). this fact points to a better determination of the elastic structure beneath australia for large depths, if the present dispersion data are used. 4. regionalization of group velocities the above-described group velocities constitute the dispersion curves to be considered for regionalization. the period range of these curves goes from 5 to 250 s (supplement 5) and there are 412 dispersion curves corresponding to the source-station paths shown in figure 1, a dispersion curve for each sourcestation path. thus, for each above-mentioned path, the forward problem in the regionalization of the surface-wave velocities is expressed as (keilis-borok, 1989) (1) where ues(t) is the group velocity (dispersion curve) observed between the source and the station along a great-circle path connecting those two points (the source-station path), des is the source-station distance (des = òds) and u(t, p) is the local group velocity at a period t and a point p of the earth surface (on the great-circle path). the inverse problem is posed to calculate the local group velocity u(t, p), at each period t and at each point p, from the average group velocity ues(t) previously measured for a number m of source-station tes (t ) = δes ues (t ) = ds u(t,p)e s ∫ 0 25 50 75 100 125 150 175 200 225 250 2.0 2.5 3.0 3.5 4.0 4.5 0 25 50 75 100 125 150 175 200 225 250 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 2400 0 25 50 75 100 125 150 175 200 225 250 900 1200 1500 1800 2100 period (s) g ro up t im e ( s) 89 90 91 92 93 94 95 96 97 98 99 db (d) g ro up v elo cit y (k m /s) period (s) (c)g ro up t im e ( s) period (s) (b) time (s) (a) 0 25 50 75 100 125 150 175 200 225 250 2.0 2.5 3.0 3.5 4.0 4.5 0 25 50 75 100 125 150 175 200 225 250 900 1200 1500 1800 2100 300 600 900 1200 1500 1800 2100 2400 0 25 50 75 100 125 150 175 200 225 250 900 1200 1500 1800 2100 period (s) g ro up t im e (s ) 89 90 91 92 93 94 95 96 97 98 99 db (d) g ro up v el oc ity (k m /s ) period (s) (c) g ro up t im e (s ) period (s) (b) time (s) (a) 0 50 100 150 200 250 2.0 2.5 3.0 3.5 4.0 4.5 300 600 900 1200 1500 1800 2100 2400 300 600 900 1200 1500 1800 2100 2400 g ro up v el oc ity (k m /s) period (s) (b) time (s) (a) time (s) (c) tvf 90 v. corchete paths. the inverse problem defined in these terms is known as regionalization of the surface-wave velocities. this inverse problem can be simplified, if the reciprocal of the local group velocity u(t, p), or equivalently the slowness s(l, f), is expanded into fourier series by the formula (2) where x is l l 0 and y is f f 0 , (l 0 , f 0 ) are the geographical longitude and latitude of the point at the bottom left corner of the study area (figure 1), x is lmax l 0 and y is fmax f 0 , (lmax, fmax) are the geographical longitude and latitude of the point at the top right corner of the study area (figure 1) and (anm, bnm, cnm, dnm, ny and nx) are constants. it should be noted that u(x, y) takes the velocity values of u(t, p), i. e., u(x, y) = u(l, f) and s(x, y) = s(l, f), for each period t, because each point p (on the earth surface considered as a sphere) has coordinates (x, y) and (l, f). if formula (2) is introduced in the relationship (1) the group time can be expressed as (3) figure 1. the path coverage of the rayleigh waves (412 paths). where ti is the group time of the ith source-station path (with i =1, 2, …, m), ci is the great-circle that connects the source and station points of the ith source-station path. m is the number of source-stations paths measured for the period t (the maximum is 412 for this study). equations (3) also can be written in the form where bnm is (anm; bnm; cnm; dnm) and δx,δxt δx,δxt = b δy,δyt bt δx δy,δyt then, equations (3) can be expressed as a system of linear equations, for each period t, in the form y = ax (4) where formula (4) is a set of m linear equations, for a given period t, with n unknowns xj (with j =1, 2, …, n), where n = 4(nx+1)(ny+1). there is a forward problem posed for each period t, for which the equations (4) can be written. in each forward problem, the number of equations m can take different values depending on the considered period t, because the path coverage can be different for each period (supplement 5). as a consequence of this, the number of unknowns n that can be properly determined may be also different in each problem, because always n must be less than or equals to m. otherwise, formula (4) will be a set of equations with more unknowns n than equations m, being very difficult or impossible to determine of the unknowns xj or equivalently the bnm coefficients of formula (2). once the forward problem has been defined by formula (4), for each period t, the regionalization of the surface-wave velocities is performed by the inversion of formula (4), or equivalently inverting the matrix a. the inverse matrix of a can be obtained using the inverse method detailed by corchete and chourak (2010). this inverse matrix is given by b = vpλp(λp 2 + αi)-1up t (5) the matrix b defined by formula (5) is called the damped generalized inverse. then, the linear equations given by formula (4) are inverted to obtain x by means of x = by (6) where baxx = the product of matrixes b and a is called resolution matrix and it is obtained through ba = vplp 2 (lp 2 + αi)-1vp t (7) if the resolution matrix given by formula (7) is the identity matrix, the obtained solution x is unique and the resolution is perfect. nevertheless, the addition of the factor α in the main diagonal makes that the equality ba = i is only reached when the damping factor α is zero. the introduction of the damping parameter degrades resolution, but stabilizes the obtained solution by means of the reduction of the variance (corchete and chourak, 2010). the error δx in the model parameters (or equivalently the error in the solution x ) due to the error dy in the data (i. e. the error in the group times), is given by the corresponding covariance matrix as (aki and richards, 1980) (8) where indicates averaging, b is given by (5) and the covariance matrix for is calculated from the covariance matrix of the error in data δy. assuming that all the components of the data vector y are statistically independent (or assuming an independent error for each individual measurement), i. e. assuming that the fluctuation in any data is entirely independent of the fluctuation of other data, the off diagonal elements are made zero. then, the main diagonal elements of the data covariance matrix are the variances of the data. if a similar behaviour is assumed for the model parameters x (i. e. assuming that all the components of the vector x are statistically independent), yi = ti (t ) aij = anm i x j = bnms(x, y) = anm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+ bnm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+ ⎡ ⎣ ⎢ m=0 ny ∑ n=0 nx ∑ + cnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+ dnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ ti (t ) = s(x, y) ci∫ ds = anm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+ bnm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+ ⎡ ⎣ ⎢ m=0 ny ∑ n=0 nx ∑ci∫ ti (t ) = anm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ + bnm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ + ⎡ ⎣ ⎢ m=0 ny ∑ n=0 nx ∑ + cnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ + dnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ ⎤ ⎦ ⎥ ti (t ) = bnmanm i m=0 nx ∑ n=0 ny ∑ anm i == cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ ; cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ ⎛ ⎝ ⎜ ; sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds; sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ds ci∫ci∫ ⎞ ⎠ ⎟ 91shear-wave velocity structure of australia from rayleigh-wave analysis the off diagonal elements are made zero. then, the main diagonal elements of the model covariance matrix are the variances of the model parameters. the obtained solution x (or equivalently the set of coefficients bnm) is calculated by introducing the relation (5) in the equations (6). then, the slowness s(x, y), or equivalently the reciprocal u(x, y), is computed from these coefficients bnm using formula (2). the errors in the model parameters (or equivalently the errors in the coefficients bnm) are obtained by means of the relation (8). later, the error in the slowness s(x, y) is computed from the errors in the coefficients bnm using the partial derivative of formula (2) given by finally, the error in the local group velocity u(x, y) is computed from the error in the slowness s(x, y), using also the partial derivative of the formula then (9) figure 6 shows the regionalized group velocity u(l, f) obtained following the above-described method for the periods 15, 75 and 175 s. figure 7 shows the errors of the regionalized group velocity du(l, f) for the same periods. the geographical distribution of group velocities shown in figure 6, presents the highest values for the consolidated and old structures. the group velocity presents the highest values at the western part of the continent, while the group velocity values at the eastern part are the lowest ones. this group velocity pattern agrees well with that shown in the group velocity maps performed by saygin and kennet (2012). nevertheless, the group velocity maps shown in figure 6, present a better resolution than that achieved in the group velocity mapping of saygin and kennet (2012). the identification of the principal geological units of australia, in terms of group velocity, is clearer in the present study. figure 6. rayleigh-wave group velocity u(l, f) computed by means of formula (2) at the periods 15, 75 and 175 s. the isolines interval is 0.1 km/s. u(x, y) = 1 s(x, y) δu(x, y) = ∂u ∂s δs(x, y) = 1 s2 (x, y) δs(x, y) =u 2 (x, y)δs(x, y) figure 7. error in rayleigh-wave group velocity du(l, f) calculated by means of formula (9) at the periods 15, 75 and 175 s. the isolines interval is 0.01 km/s. 5. inversion of regionalized dispersion curves the above-described group velocity distributions with period must be inverted to obtain the shear-wave velocity distribution with depth (the 3d s-velocity structure for australia and its surrounding area). for it, these group velocity surfaces u(l, f) calculated for each period, from 5 to 250 s, are sampled in a rectangular grid with 2.5º×2.5º mesh size, from 110 to 160ºe and from 0 to 45ºs. these grid data can be inverted to obtain a shear velocity model (a shear velocity distribution with depth) for each grid point of the study area, achieving thus a 3d s-velocity model that represents the 3d elastic structure beneath australia and its surrounding area, obtained from 110 to 160ºe and from 0 to 45ºs. the inversion of the group velocity, for each grid point of the study area, will be performed using the computation method described in detail by corchete et al. (2007). here only a brief review of the principal concepts of this methodology is presented. selection of an initial earth model. the selection of an initial earth model is a preceding step before the inversion process. the initial model must be prepared considering all information available for the study area, with respect to the s-wave, p-wave and density distributions with depth. for the present study, this geophysical and geological information has been obtained from previous studies developed in the study area, which are quoted in the introduction section of this paper. thus, an initial model has been prepared for each grid point for depths ranging from 0 to 400 km. for depths deeper than 400 km, the prem model developed by dziewonski and anderson (1981) has been used. these models must provide a theoretical dispersion curve as close as possible to the observed curve, because the inversion scheme to be used only considers small perturbations for the wave velocities involved. as an example, the initial model considered for the inversion of the dispersion curve (group velocity), associated with the grid point with geographical coordinates (28.75ºs, 118.75ºw), is listed in table 1 and its shear-wave velocity distribution with depth is plotted in figure 8a (dashed line). the theoretical group velocity shown in figure 8c (dashed line) is calculated by forward modelling (corchete et al., 2007), from the starting model in table 1. δs(x, y) = ∂s ∂bnm δbnm m=0 nx ∑ n=0 ny ∑ = δanm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ m=0 ny ∑ n=0 nx ∑ + +δbnm cos 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+δcnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟cos 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟+δdnm sin 2nπ x x ⎛ ⎝ ⎜ ⎞ ⎠ ⎟sin 2mπ y y ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ 92 v. corchete table 1. initial earth model (a: p-wave velocity, b: s-wave velocity and r: density) considered for the inversion of the group velocities (obtained by regionalization), for the grid point with geographical coordinates (28.75ºs, 118.75ºw). thickness (km) a (km/s) b (km/s) r (g/cm3) 5 4.33 2.50 2.30 5 5.20 3.00 2.77 10 5.70 3.30 2.77 10 6.60 3.80 2.95 5 6.60 3.80 3.20 15 7.80 4.50 3.20 30 7.80 4.50 3.35 100 7.80 4.50 3.40 100 7.50 4.30 3.53 120 9.41 5.09 3.60 100 9.41 5.09 3.64 100 9.72 5.26 3.84 40 9.97 5.26 4.08 30 10.54 5.70 4.16 130 10.68 5.85 4.22 200 11.10 6.26 4.43 200 11.48 6.44 4.61 200 11.78 6.54 4.74 200 12.06 6.64 4.83 ∞ 12.32 6.75 4.92 computation of the inversion. when the starting model is ready, the group velocity can be inverted to obtain the corresponding s-velocity distribution with depth. figure 8 show the results of an example of such inversion. in this example, the dispersion curve corresponding to the grid point with geographical coordinates (28.75ºs, 118.75ºw), has been inverted. the s-velocity model shown in figure 8a (continuous line) is the final model obtained for this grid point. the model improvement iteratively obtained, is checked by the comparison between the observed group velocity (considered as observed data) and the theoretical group velocity (calculated from the actual model by forward modelling and considered as theoretical data). when the theoretical curve falls within the vertical error bars of the observed curve, as shown in figure 8c, the inversion process is finished, because the obtained s-velocity model describes the observed curve within its experimental error. figure 8. (a) shear-wave velocity (final model) obtained after the inversion process for the grid point (28.75 ºs, 118.75 ºw), plotted with continuous line only from 0 to 800 km of depth. the horizontal bars show standard deviation for each layer considered in this inversion process. the shear velocity distribution of the initial model listed in table 1 is plotted with dashed line, only from 0 to 800 km of depth. (b) resolving kernels of the inversion problem posed (plotted only from 0 to 800 km of depth). the reference depths are marked by vertical bars for the media depth of each layer considered. (c) the theoretical group velocity obtained from the final model plotted in figure 8a, is shown with continuous line. the theoretical group velocity obtained from the initial model listed in table 1 (and plotted also in figure 8a), by means of forward modelling, is plotted with dashed line. the dots line denotes the average group velocity, calculated by regionalization, for the above-mentioned grid point (considered as observed data). the vertical bars show the standard deviation in group velocities at each period (1-s errors in the observed data). resolution of the final model. the resolving kernels shown in figure 8b indicate the reliability of the solution obtained for the inverse problem posed. a good engagement between the calculated solution and the true solution (which implies the reliability of the estimated solution), is obtained when the absolute maxima of these functions fall over the reference depths. this reliability is also related to the width of these absolute maxima. the solution for the inversion problem is more reliable when the maxima of these resolving kernels are narrower. the s-velocity models (figure 8a) and the resolving kernels (figure 8b), are plotted only for depths above 800 km. this fact is due to the bad resolution obtained for depths deeper than 400 km. figure 8b shows that the deeper layers (below 400 km) have a poor resolution. this is shown in the lack of coincidence between the resolving-kernel absolute maxima and their reference depths, for depths down to 400 km. therefore, the results obtained for depths below 400 km have been left out, because the resolution at these deeper depths is poor. for depths of 0 to 400 km, the s-velocity models must be considered as valid models. thus, the s-velocity final model shown in figure 8a (continuous line), is the s-velocity distribution with depth obtained for the grid point with geographical coordinates (28.75ºs, 118.75ºw). shear velocity mapping. the above-described s-velocity distributions with depth, obtained for each grid point of the rectangular grid defined in the study area from 110 to 160ºe and from 0 to 45 ºs (with 2.5º×2.5º mesh size), are plotted with contour maps. this 3d s-velocity structure is mapped with contours by depth intervals, obtaining 2-d images of shear velocity for the study area, as shown in figure 9. such 2-d images are suitable for an easy interpretation and correlation, with other geophysical and geological information available for the study area. the 1-sigma errors in the s-wave velocities are also represented in 2-d images. these errors are plotted in figure 10, for some depth intervals, to check the error spatial distribution of the s-wave velocities mapped in figure 9. 800 700 600 500 400 300 200 100 0 2 3 4 5 6 0 25 50 75 100 125 150 175 200 225 250 2.0 2.5 3.0 3.5 4.0 4.5 starting final (a) shear velocity (km/s) d ep th (k m ) observed starting final (c)g ro up v el oc ity (k m /s) period (s) 0 100 200 300 400 500 600 700 800 (b) depth (km) resolving kernels 93shear-wave velocity structure of australia from rayleigh-wave analysis figure 9. geographical distribution of the s-velocity as a function of depth. the interval between isolines is 0.1 km/s. white lines are used to plot contours associated with s-velocities greater than or equals to 4 km/s. . 94 v. corchete figure 10. geographical distribution of the 1-sigma errors arisen in computation of the s-velocities, for the depth ranges: 30-35 km, 105-130 km and 230-280 km. the interval between isolines is 0.01 km/s. 6. results and discussion in this section, the results obtained in this study and shown in the s-velocity mapping of figure 9, will be presented and discussed. the structural features shown by these images will be also correlated with other geophysical studies (developed in the study area) and correlated with the geology (figure 11). figure 11. geologic map of australia with definitions of the units (simons et al., 1999). the dashed line is the tasman line, which divides the precambrian western australia from phanerozoic eastern australia. depth range: 0 – 5 km. the s-velocity map shows clearly the distribution of sedimentary basins located in the study area (figure 11). the s-velocity presents the lowest values for the basins and the highest values for the consolidated and old structures. the results obtained, for this depth range, agree well with those obtained by ellis and denham (1985) and denham and ellis (1986), who have determined s-velocity distributions with depth along a few paths on the australian continent. also, it is found a good coincidence with the s-velocity mapping performed by saygin and kennett (2012), for the depths: 1, 3 and 5 km. nevertheless, the resolution of the present study is clearly better than that achieved in previous studies. the sedimentary basins present in the study area, can be more clearly identified by lower s-velocities in figure 9, for this depth range. these s-velocity values are also influenced by the thickness of the sedimentary basins. the lowest s-velocities can be correlated with the locations of the thicker sedimentary basins, which are shown in the sedimentary basin thickness map performed by kennett and salmon (2012). depth range: 5 – 15 km. the geographical distribution of the s-velocity values shows an s-velocity pattern different from the s-velocity pattern obtained for the previous depth range. for this depth range, the lowest s-velocity values are not correlated with the location of the sedimentary basins. the s-velocity presents the lowest values for eastern part of australia. again, the s-velocity presents the highest values for the consolidated and old structures, but the s-velocity presents highest values at the west of the tasman line (figure 11), while the s-velocity values at the east of the tasman line are the lowest ones. these results agree well with those obtained by ellis and denham (1985), who obtained s-velocities, for the seismic paths at the west of the tasman line, higher than those for the paths at east of this line. the s-velocities calculated in the present study, for the eastern part of australia, also agree well with those obtained by denham and ellis (1986). nevertheless, the better resolution of the present study allows the clear identification of the principal geological units of australia, in terms of s-velocity. this identification is more difficult in the previous studies performed by ellis and denham (1985) and denham and ellis (1986), because they only analysed a few seismic paths. also, it is more difficult in the s-velocity mapping performed by saygin and kennett (2012), in which even there is no evidence of lower s-velocity values at the east of the tasman line, as it could be expected. in general, the new 3d s-velocity model presented in this paper, from which an ensemble of 2d plots is shown in figure 9, presents the lower s-velocities for the eastern part of australia, at the east of the tasman line, in which there is a higher heat flow (cull, 1982). these low-velocity values are particularly low for the eastern margin of australia, from 5 to 130 km of depth, coinciding with the sites of cenozoic volcanism (johnson, 1989). this coincidence and the existence of a high heat flow in this region (cull, 1982), suggest to zielhuis and van der hilst (1996) that these low velocities, found for the eastern margin of australia, could have thermal origin. depth range: 15 – 50 km. the geographical distribution of the s-velocity values for this depth range shows higher values (4 km/s and higher) in zones of the study area in which the moho depth is overcome (i. e. zones which the depth range considered is located below the moho discontinuity). for each depth range shown in figure 9 (between 15 and 50 km), the moho depth is overcome in different areas. the moho depth is firstly overcome in off-shore areas and lastly in on-shore and continental areas. in general, the s-velocity increases with depth for the whole study area, as shown by figure 9, for each depth range between 15 and 50 km. again, the s-velocity presents the lowest values at the east of the tasman line, while the highest ones are present at the west of this line. this feature doesn’t appear in the s-velocity mapping performed by saygin and kennett (2012). for this depth range, there is no s-velocity change associated with the tasman line, in their mapping. the s-velocity presents higher values for the consolidated and old structures. nevertheless, the s-velocity difference associated with this structural difference is minor than the s-velocity difference due to the moho depth overcoming, because the s-velocities associated with the crust are much minor than those associated with the upper mantle. the results obtained for this depth range (from 15 to 50 km) agree well with those obtained by ellis and denham (1985) and denham and ellis (1986). although, the few seismic paths considered by them only allow the determination of media values for the moho depth, which agree well with the moho map obtained in the present study (figure 12). in the present study, an s-velocity value greater than 4 km/s is considered associated with the uppermantle structure. thus, if the s-velocity value of 4 km/s is used to find the moho depth in the whole study area, considering that an s-velocity value greater than 4 km/s is associated with upper-mantle structure, then it is possible to plot a map of the moho discontinuity, such as the map shown in figure 12 (top), from the s-velocity mapping shown in figure 9 (which is in fact an ensemble of 2d plots for a 3d s-velocity distribution). this moho map agrees well with other 95shear-wave velocity structure of australia from rayleigh-wave analysis moho maps previously calculated for australia, as the moho maps obtained by aitken (2010), kennett et al. (2011), kennett and salmon (2012) and saygin and kennett (2012). it is very noticeable that the most recent maps calculated for australia (kennett and salmon, 2012; saygin and kennett, 2012) are very similar to ausmoho (kennett et al., 2011), as consequence of the use of similar databases for their calculation. thus, a comparison of the moho map shown in figure 12 (top) with anyone of them gives the same. figure 12. the map of the moho depth (top), the lithospheric thickness (middle) and the base of the asthenosphere (bottom); obtained from the 3d s-velocity structure beneath australia and its surrounding area, shown in the 2d s-velocity mapping of figure 9. the moho discontinuity is located, for any point of the study area, at the depth in which the s-velocity value of 4 km/s is overcome by first time. the lithospheric thickness is defined, for any point of the study area, as the depth in which the boundary lithosphere-asthenosphere (the top of the asthenosphere) is located. the base of the asthenosphere is defined, for any point of the study area, as the depth in which the low s-velocity of the asthenosphere increases to higher values. the interval between isolines is 5 km. depth range: 50 – 65 km. again, the s-velocity presents the highest values at the west of the tasman line (figure 11), while the s-velocity values at the east of the tasman line are the lowest ones. moreover, the s-velocity presents higher values for the consolidated and old structures, which allows the identification, in terms of s-velocity, of the principal units that compose australia. on the other hand, the s-velocity values calculated for the northern part of the study area (the south of indonesia and the surrounding seas) agree well with those obtained by okabe et al. (2004). nevertheless, a better resolution is achieved in the present study, because they divide the study area in blocks with 5º×5º in size, while this area is gridded with 2.5º×2.5º mesh size in the present study. a comparison with previous studies developed in the australian continent is not possible, because no 3d s-velocity structure was obtained for depths ranging from 30 to 75 km. the results obtained in this study, for this depth range, are a new and very interesting feature never presented before. depth range: 65 – 80 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the south-eastern and north-eastern parts of australia, in which the s-velocity shows lower values. fishwick and reading (2008) confirmed this pattern of low velocities for south-eastern and northeastern parts of australia. also, fishwick and rawlinson (2012) confirmed with their results that the s-velocity values, at the east of the tasman line, are the lowest obtained for australia. moreover, the results obtained by them, for the whole study area (australia and its surrounding area), agree well with those obtained in the present study, although the resolution achieved in the present study is the best. on the other hand, the s-velocity shows low values in the south-eastern and north-eastern parts of australia, because the asthenosphere is reached at this depth range (from 65 to 80 km) and for these areas, as shown in figure 12 (middle). the lithosphere-asthenosphere boundary (or equivalently the lithosphere thickness) shown in figure 12 (middle), has been computed from the 3d s-velocity structure, calculated in the present study and represented by the 2d s-velocity mapping shown in figure 9, considering for each point of the study area the depth in which the s-velocity starts to decrease with depth. the lithosphere thickness map shown in figure 12 (middle) agrees well with the lithosphere-asthenosphere boundary (lab), obtained by ford et al. (2010) from sp wave imaging. nevertheless, a better resolution is achieved in the present study because they interpret a strong coherent negative sp phase in eastern australia as the lab, but no tomography is performed in their study. also, a better resolution is achieved for the lab in figure 12 (middle) comparing with that performed by kennett and salmon (2012). depth range: 80 – 105 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the north-eastern part of australia, in which the zone of lower s-velocity is increased respect to the previous depth range, because the asthenosphere is reached, at this depth range, for a wider zone of the north-east of australia, as shown in figure 12 (middle). this feature is confirmed by the lab obtained by ford et al. (2010) and is also confirmed by previous studies (zielhuis and van der hilst, 1996; simons et al., 1999; fishwick et al., 2005; fishwick and reading, 2008; fichtner et al., 2009; fishwick and rawlinson, 2012). the results obtained in these previous studies also agree well with those obtained in the present study, for the whole study area. concretely, all s-velocity maps calculated in the studies performed until now (including the present study), show high values of s-velocity at the west of the tasman line (figure 11), while the s-velocity values at the east of the tasman line are the lowest ones. moreover, the s-velocity presents higher values for the consolidated and old structures. nevertheless, the identification of the principal units that compose australia, in terms of s-velocity, is clearer in the s-velocity maps of the present study than in those of any previous study. on the other hand, the s-velocity values calculated for the northern part of the study area (the south of indonesia and the surrounding seas), in the present study, are higher in the northwest than in the northeast. although, in the s-velocity map of present depth range (from 80 to 105 km), appears a zone of high velocity in the north-eastern part of the study area. this high s-velocity also appears in the same zone of the s-velocity maps obtained by fichtner et al. (2009), for the depths of 80 and 110 km. these s-velocity values agree well with those obtained by okabe et al. (2004). nevertheless, a low resolution of their maps (they divide the study area in blocks with 5º×5º in size) makes difficult the comparison with the results of the present study. depth range: 105 – 130 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the eastern part of australia, in which the s-velocity shows the lowest values, because the asthenosphere is reached, at this depth range and for this area, as shown in figure 12 (middle). again, this feature is confirmed by the lab obtained by ford et al. (2010) and is also confirmed by other studies (zielhuis and van der hilst, 1996; simons 96 v. corchete et al., 1999; fishwick et al., 2005; fishwick and reading, 2008; fichtner et al., 2009; fishwick and rawlinson, 2012; kennett and salmon, 2012). nevertheless, the identification of the principal units that compose australia, in terms of s-velocity, is again clearer in the s-velocity maps of the present study than in those of any previous study. also, for the present depth range, the s-velocity values calculated for the northern part of the study area agree well with those obtained by okabe et al. (2004), although, the low resolution of their maps makes difficult the performing of a precise comparison. depth range: 130 – 155 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the central-eastern part of australia (from the tasman line toward east, approximately), in which the s-velocity shows the lowest values, because the asthenosphere is reached, at this depth range and for this area, as shown in figure 12 (middle). again, this feature is confirmed by the lab obtained by ford et al. (2010), which is also in agreement with the s-velocity distributions with depth shown in figure 13, for perpendicular cross-sections along the parallels of 25 and 30ºs. these crosssections have been obtained from the 3d s-velocity structure calculated in the present study, by vertical interpolation from the 2d s-velocity mapping shown in figure 9. it should be noted that the cross-sections obtained in the present study show a better resolution than those obtained by fichtner et al. (2009), for depths greater than 250 km. this improvement in the resolution at the deeper depths achieved in this study is related to the wide period range, in which the rayleigh-wave group velocities have been measured. in the present study, dispersion curves (group velocities) until 250 s of period have been determined, providing observed data to fit the deep s-velocity structure with more accuracy than any previous studies. figure 13. s-velocity cross sections along the longitudes of 25 and 30ºs. the interval between isolines is 0.1 km/s. depth range: 155 – 180 km. the s-velocity pattern shown is similar to that in the previous depth range, except for the north-eastern and south-eastern parts of australia, in which the s-velocity shows higher values comparing with those in the previous depth range, because the asthenosphere is overcome, at this depth range and for these areas, as shown in figure 12 (bottom). the boundary of the asthenosphere base (or equivalently the lithosphere-asthenosphere system thickness) shown in figure 12 (bottom), has been computed from the 3d s-velocity structure, calculated in the present study and represented by the 2d s-velocity mapping shown in figure 9, considering for each point of the study area the depth in which the s-velocity starts to increase with depth below the lab. it was found that the map of asthenosphere base, shown in figure 12 (bottom), is a new and very interesting feature never obtained before the present study. also, it is very noticeable that the mantle low-velocity zone (i. e. the asthenosphere), identified in australia from the lab (figure 12 middle) to the asthenosphere base (figure 12 bottom), is shallower for the eastern part of australia, at the east of the tasman line, coinciding with the australian region of higher heat flow (cull, 1982). this increment in the heat 115 120 125 130 135 140 145 150 400 300 200 100 115 120 125 130 135 140 145 150 400 300 200 100 east degrees d ep th (k m ) 4.0 4.5 5.0 5.5  (km/s) 25ºs 30ºs east degrees d ep th (k m ) flow, found at the east of the tasman line, could be originated precisely by an asthenosphere nearer to earth surface, for the eastern part of australia (i. e. a thinness of the lithosphere towards the east from the tasman line). fishwick et al. (2008) have presented evidence for the progressive eastward thinning of the lithosphere across australia. this behaviour is also confirmed by kennet and salmon (2012), who have observed that the dominant control on the seismic velocities in the mantle is from temperature, with a decreasing in velocity associated with the increasing in temperature. they suggest that the variations in the seismic velocity with depth can be interpreted in terms of changes in the lithospheric thickness (i. e. variations on the asthenosphere depth). thus, the regions, in which are present consolidated and old structures characterized by higher s-velocities (located at the west of the tasman line), are underlain by a thick mantle lithosphere, while the lithosphere is generally thinner beneath the regions at the east of the tasman line, characterized by lower s-velocities. this shallower part of the asthenosphere is particularly the shallowest part for the eastern margin of australia (i. e. the thinnest lithosphere), coinciding with the sites of cenozoic volcanism (johnson, 1989). on the other hand, a low-velocity zone is also present, for a similar depth range, in the mantle of the continents: south america (corchete, 2012), antarctica (corchete, 2013a) and africa (corchete, 2013b), which were part of the same super-continent gondwanaland (supplement 7), in the early mesozoic before fragmentation. this new and very interesting feature proves that it is possible the precise reconstruction of gondwanaland, comparing the high and low velocity areas found in south america, antarctica, africa and australia. depth range: 180 – 230 km. the s-velocity pattern shown is very different from that in the previous depth range, because the asthenosphere is reached for the major part of the study area, as shown in figure 12 (middle), except for the north-eastern and south-eastern parts of australia, in which the asthenosphere was overcome for the previous depth range. opposite to this, the asthenosphere is overcome for the eastern part of australia, as shown in figure 12 (bottom). for this reason, the s-velocity shows higher values (for this area and for the present depth range) comparing with those shown for the previous depth range, but in general, the s-velocity (in the present depth range) presents values lower than those shown the previous depth range, for the major part of the study area, as a consequence of the asthenosphere presence in the major part of the study area. this feature is also shown in figure 13 and confirmed by the lab obtained by ford et al. (2010). on the other hand, they have obtained that the lab is defined by a sharp discontinuity in the eastern part of australia, while this boundary is gradual in the western part of this continent. this feature is confirmed by kennet and salmon (2012), who have found that the lithosphere beneath the older parts of the continent can be readily recognised by fast s-velocities, but the transition to the asthenosphere is not marked by any sharp transition. this behaviour is also shown in figure 13, because the s-velocity decreases gradually with depth (from 180 to 280 km) in the western part of australia, while the s-velocity distribution with depth shows a sharp lab in the eastern part of australia. this lab is located at different depths for eastern part of australia, as shown in figure 12 (middle). depth range: 230 – 280 km. the s-velocity pattern shown is similar to that in the previous depth range, because the asthenosphere was reached for the major part of the study area in the previous depth range. opposite to this, the asthenosphere is overcome for the central-eastern part of australia (from the tasman line toward east, approximately), as shown in figure 12 (bottom). for this reason, the s-velocity shows higher values (for this area and for the present depth range) comparing with those shown for the previous depth range. nevertheless, the s-velocity in the depth range from 230 to 280 presents the lowest values calculated for the study area, comparing with those calculated for the depth range from 50 to 400 km (figure 9). this feature can be more clearly observed in figure 13, in which the depth range from 230 to 280 presents, in general, the lowest values of s-velocity. previous studies (simons et al., 1999; fishwick et al., 2005; fishwick and reading, 2008; fichtner et al., 2009; fishwick and rawlinson, 2012; kennet and salmon, 2012) also obtained the lowest s-velocity values for this depth range, but the resolution of these previous studies is poorer than that from the present study, for depths greater than 250 km. the improvement in the resolution at the deeper depths achieved 97shear-wave velocity structure of australia from rayleigh-wave analysis in this study is related to the wide period range, in which the rayleigh-wave group velocities have been measured. in the present study, dispersion curves (group velocities) until 250 s of period have been determined, providing observed data to fit the deep s-velocity structure with more accuracy than the above-mentioned previous studies. depth range: 280 – 400 km. the s-velocity values shown in figure 9 jump respect to those shown in the previous depth range, from low values (4.2 – 4.4 km/s) to high values (4.8 – 5.1 km/s). this jump of the s-velocity, at this depth range, can be more easily observed in both cross-sections of figure 13. obviously, the low-velocity channel associated with the asthenosphere, shown in the previous depth range, has been overcome for the whole study area in the present depth range. again, the s-velocity presents the highest values at the west of the tasman line (figure 11), while the s-velocity values at the east of the tasman line are the lowest. moreover, the s-velocity presents higher values for the consolidated and old structures, but the identification, in terms of s-velocity, of the principal units that compose australia is more difficult for this deeper depth range, because the s-velocity differences are very little for this depth range. the s-velocity contrast decreases with depth while the s-velocity values increase with depth, obtaining an s-velocity pattern of similar high s-velocity values, for the whole study area. thus, at the opposite of the results obtained in the previous depth ranges, the s-velocity remains almost the same with depth, for the whole study area. this pattern of high velocities also have been obtained beneath australia, for this depth range, in other previous studies (simons et al., 1999; fishwick et al., 2005; fishwick and reading, 2008; fichtner et al., 2009; fishwick and rawlinson, 2012; kennet and salmon, 2012) but their resolution is very low at these deeper depths, which makes it difficult to compare the results of these previous studies, with those obtained in the present study. on the other hand, a similar smooth correlation between older (or more consolidated) structures and high s-velocity values has been found (at this depth range) for the continents: south america (corchete, 2012), antarctica (corchete, 2013a) and africa (corchete, 2013b), which were part of the same super-continent gondwanaland. this new and very interesting feature proves again that it is possible the precise reconstruction of old super-continents (now fragmented), comparing the high and low velocity areas present in the modern continents. 7. conclusions the results presented in this paper show that the techniques used here are a powerful tool for investigating the elastic structure of earth. this methodology that is a good tool to investigate the physical properties of the earth structure, in any region of earth with a variety of tectonic features, has been improved in the present paper performing a new method for the regionalization of surface-wave dispersion, based on fourier series, never published before in any previous paper. by means of this analysis, the principal structural features beneath australia and its surrounding area have been revealed. such features include the existence of lateral and vertical heterogeneity in the whole study area, which is concluded in figures 9, 12 and 13. the zones with consolidated and old structures present higher s-velocity values than the other zones (which allows the identification of the principal geological units of australia, in terms of s-velocity), although this difference can be negligible in some cases (e. g. for depths greater than 280 km). nevertheless, in the depth range of 15 to 50 km, the different moho depths present in the study area generate the principal variation of s-velocity, because the s-velocities associated with the crust are less than those associated with the upper mantle. a similar behaviour is found for the lab, which generates the principal variations of s-velocities in the depth range from 65 to 180 km, because the s-velocities associated with the lithosphere are much major than those associated with the asthenosphere, as is well known. on the other hand, a new and interesting feature, obtained in this study, is the definition of the base of the asthenosphere, for the whole study area, for depths ranging from 155 to 280 km. this boundary joint to the lab define a low-velocity zone in the australian mantle, which is also present for a similar depth range in the continents (south america, antarctica and africa) that were part of the same super-continent gondwanaland. acknowledgements. the author is grateful to the incorporated research institutions for seismology (iris), for providing data used in this study. the seismograms and the instrumental response have been provided by iris, through the internet utility wilber ii and the database: iris dmc metadata aggregator (available at www.iris.edu), respectively. the author is also grateful to the international and local agencies (and persons), who have been able to develop and support the seismic stations (portable deployments within australia and permanent installations), which data has been used in this study. references aitken a., 2011. moho geometry gravity inversion experiment (moggie): a refined model of the australian moho, and its tectonic and isostatic implications. earth and planetary science letters, 297, 71-83. aki k. and richards p. g., 1980. quantitative seismology. theory and methods. w. h. freeman and company, san francisco. cara m., 1973. filtering dispersed wavetrains. geophysical journal of the royal astronomic society, 33, 65-80. corchete v., chourak m. and hussein h. m., 2007. shear wave velocity structure of the 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(ed.), 1989. intraplate volcanism in eastern australia and new zealand. cambridge university press, melbourne. keilis-borok v. i., 1989. seismic surface waves in a laterally inhomogeneus earth. kluwer academic publishers, london, uk. kennett b. l. n., salmon m., saygin e. and ausmoho working group, 2011. ausmoho: the variation of moho depth in australia. geophysical journal international, doi: 10.1111/j.1365-246x.2011.05194.x. kennett b. l. n. and salmon m., 2012. ausrem: australian seismological reference model. australian journal of earth sciences, accepted. okabe a., kaneshima s., kanjo k., ohtaki t. and purwana i., 2004. surface wave tomography for southeastern asia using iris-farm and jisnet data. physics of the earth and planetary interiors, 146, 101-112. roult g. and rouland d., 1994. antarctica i: deep structure investigations inferred from seismology; a review. physics of the earth and planetary interiors, 84, 15-32. saygin e. and kennett b. l. n., 2012. crustal structure of australia from ambient seismic noise tomography. journal of geophysical research, 117, b01304, doi: 10.1029/2011jb008403. simons f. j., zielhuis a. and van der hilst r. d., 1999. the deep structure of the australian continent from surface wave tomography. lithos, 48, 17-43. zielhuis a. and van der hilst, r. d., 1996. upper-mantle shear velocity beneath eastern australia from inversion of waveforms from skippy portable arrays. geophysical journal international, 127, 1-16. letter from the editor on september 2017, two large earthquakes struck méxico. numerous casualties and at least 40 buildings collapsed in mexico city. the earthquakes are explained by the tectonic setting of central america, the cocos plate subducts underneath the north american plate at about 7-8 cm a year, making mexico a seismically active region. but the two earthquakes with hypocenter depths between 50 and 60 km – did not occur in the contact between the two tectonic plates as is usually expected, but rather within the cocos plate as it bends downward within the mantle. both events showed a normal faulting mechanism, and although they were widely felt, their major impact was in densely populated mexico city, where wave amplification is expected due to the geological features of the city's soil structure. we know that we cannot predict earthquakes, and although this is the holy-grail in seismology, it seems like we are not close to accurately predict them. nevertheless, early warning systems have been developed in various places, including mexico, and they were successful in issuing an alarm, although with a few seconds before the strong shaking. these developments can save lives and continue to be relevant and are likely to be implemented in other regions, including the pacific coast of the us. another aspect that can save lives is people’s awareness. the m7.1 earthquake occurred in the afternoon of the anniversary of the 1985 michoacan earthquake, just a couple of hours after an earthquake drill in mexico city, so people had just recently been remained of what to do. this most likely saved lives. we should ask ourselves, are we prepared? is our city prepared? germán prieto editor in chief carlos a. vargas former editor le tt er f r o m t h e ed it o r earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017) keywords: earthquake early warning; rapid response; earthquake arrival azimuth; seismic event; bogota–colombia; support vector machine (svm); seismology; earthquakes. palabras clave: alerta temprana de terremoto; respuesta rápida; azimuth de llegada; evento sísmico; bogotá-colombia; máquina de soporte vectorial (msv); sismología; terremotos. how to cite item ochoa-gutierrez, l. h., vargas-jimenez, c. a., & niño-vásquez, l. f. (2019). fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques. earth sciences research journal, 23(2), 103-109. doi: https://doi.org/10.15446/esrj.v23n2.70581 the objective of this research is to develop a new approach to estimate earthquake arrival azimuth using seismological records of the “el rosal” station, near to the city of bogota – colombia, by applying support vector machines (svms). the algorithm was trained with time series descriptors of 863 events recorded from january 1998 to october 2008, considering only events with magnitude ≥ 2 ml. the earthquake signals were filtered in order to remove diverse kind of low and high-frequency noise not related to typical seismic activity in the area. during training stages of svms, several combinations of kernel exponent and complexity factor were applied to time series of 5, 10 and 15 seconds along with earthquake magnitudes of 2.0, 2.5, 3.0 and 3.5 ml. the best classification of svms was obtained using time windows of 5 seconds and earthquake magnitudes greater than 3.0 ml with kernel exponent of 10 and complexity factor of 2, showing an accuracy of 45.4 degrees. this research is an improvement of previous works related to earthquake arrival azimuth determination from single station data employing machine learning techniques. abstract fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques estimación rápida del azimut de llegada de un terremoto utilizando registros de una sola estación sismológica y técnicas de aprendizaje de máquinas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.70581 el propósito de esta investigación es desarrollar un nuevo enfoque para estimar el azimut de llegada de terremotos utilizando registros sismológicos de la estación el rosal, cercana a la ciudad de bogotá – colombia, mediante la aplicación de máquinas de vectores de soporte (mvs). el algoritmo fue entrenado con descriptores de series de tiempo de 863 eventos adquiridos desde enero 1998 hasta octubre de 2008, considerando solamente eventos con magnitudes ≥ 2 ml. las señales de los terremotos fueron filtradas para remover diversos tipos de ruidos de alta y baja frecuencia no relacionados con la actividad sísmica típica en el área. durante las etapas de entrenamiento de la mvs fueron aplicadas varias combinaciones del exponente kernel y factor de complejidad, a series de tiempo de 5, 10 y 15 segundos junto con terremotos de magnitudes mayores a 2.0, 2.5, 3.0 y 3.5 ml. la mejor clasificación de la mvs fue obtenida utilizando ventanas de tiempo de 5 segundos y terremotos de magnitud mayor a 3.0 ml con exponente kernel de 10 y factor de complejidad de 2, mostrando una precisión de 45.4 grados. esta investigación es una mejora a trabajos previos relacionados con determinación del azimut de llegada de un terremoto a partir de datos de una única estación sismológica empleando técnicas de aprendizaje de máquinas. resumen record manuscript received: 22/02/2018 accepted for publication: 15/03/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 103-109 luis hernán ochoa gutiérrez, carlos alberto vargas jiménez, luis fernando niño vásquez universidad nacional de colombia * corresponding author: lhochoag@unal.edu.co se is m o lo g y 104 luis hernán ochoa gutiérrez, carlos alberto vargas jiménez, luis fernando niño vásquez introduction this study is part of a research line which proposes calculation of earthquake hypocentral parameters applying artificial intelligence methods in order to develop an early warning system for the city of bogota. in case of a destructive seismic event in this area, the entire country would face many harmful social and economic effects; this is why a seismic early warning system around bogota is important and the earthquake arrival azimuth estimation is one of the main parameters in this system. nearly a third of colombia’s population lives in bogota’s savannah and surrounding which is the country’s main economic center with around 40% of the gross domestic product (ojeda et al., 2002). the density of seismological stations around the city is not high enough, this makes the required time for seismic events localization to be longer than the travel time to areas where the early warning is required. an alternative solution to this problem is employing seismological data from previous events recorded at one single station to estimate the earthquake hypocentral parameters (ochoa et al., 2014). automatic computation algorithms in a single broadband three-component station have been mainly developed for p and s waves onsets detection, allowing estimation of source location using the back-azimuth and the apparent surface speed measurements (magotra et al., 1987; roberts et al., 1989; saita & nakamura, 2003), or seismic moment estimation (talandier et al., 1987; reymond et al., 1991; odaka et al., 2003; horiuchi et al., 2005; wu et al., 1998; espinosa, 1995). supervised machine learning techniques based on kernel methods have become a very powerful tool for mathematicians, scientist, and engineers, providing solution in areas like signal processing and pattern recognition; its implementation is quite simple and can be performed by applying mathematical functions that combine input variables as a combination of themselves, obtaining an enhanced new space with more dimensions where separation of classes can be achieved. there are several methods to detect seismic wave and its arrival azimuth in a single three-component station (magotra et al., 1987; anant & dowla, 1997); these authors employed algorithms that measure the level of linear polarization in the p wave’s arrival. the methodology proposed in this research consists of applying svms along with a kernel function in order to estimate the arrival azimuth with minimal processing of data acquired at the station, similar to methodology applied to a fast determination of earthquake magnitude and epicenter distance using a single seismological station (ochoa et al., 2017; ochoa et al., 2018). data sets and methods the dataset used in this research belongs to the “el rosal” seismological station, located toward north-west bogota as shows figure 1. this station is part of the colombian seismic network operated by the “servicio geológico colombiano sgc” (colombian geological service). the “el rosal” station employs a guralp cmg t3e007 sensor in three components and a nanometrics rd3-hrd24 digitizer, which provides simultaneous sampling of three channels with 24-bit of sampling rate (bermudez & rengifo, 2002). the data correspond to the three component raw waveforms recorded directly in this station and a seismic catalog with 2164 characterized events, selected between january 1st 1998 and october 27th 2008; all of them located less than 120 kilometers from the station. the colombian seismic network consists of 42 stations, with an average distance of 162 kilometers between them, which record and transmit seismic data in real time for the entire country as shown in figure 2. before starting the processing related to svms, waveform files from “el rosal” station were converted to the american standard code for information interchange (ascii) format, using a seisan package tool; earthquakes with 1200000 1150000 1100000 1050000 1000000 9500000 9000000 800000 850000 900000 950000 1000000 1050000 1100000 1150000 1200000 villavicencio bogota d.c. tunja el rosal staton ibague depth 0-30 km 30-70 km 70-120 km 120-180 km figure 1. location of the “el rosal” seismological station and earthquake distribution around bogota. 105fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques magnitudes lower than 2.0 ml were ignored; therefore the followed processes were applied on the remaining 1011 events. since the selected seismic records present variable levels of noise, it was necessary to filter them out with both high and low-frequency filters. low frequencies correspond to instrumental noise that can be easily eliminated through implementation of a high-pass filter with cut-off frequency of 0.075 hz (wu & zhao, 2006), while high frequencies were removed with a low-pass filter with cut-off frequency of 150 hz. the statistical distribution of azimuth values is presented in figure 3, where the main distribution of the whole dataset is observed. this histogram shows a high frequency in 240 degrees (azimuth), indicating an active seismogenic zone in that direction. although this is not a homogeneous distribution, it represents the regular seismic behavior of the area. descriptors – input data set of the svm in the first stage, parameters that have been previously used by other authors to earthquake magnitude estimation were calculated and employed as input variables or descriptors for the svm of this research. in this sense, the relationship between maximum p wave amplitudes and local earthquake magnitudes was considered (wu & kanamori, 2005), where a linear regression was performed for each one of the three components of the station. three parameters were taken from this linear regression which corresponds to the slope (m), an independent term (b) and correlation coefficient (r). the maximum amplitude values (mx) obtained for each component’s time window were used as descriptors as well. therefore, each event had 12 descriptors associated with this concept. in the second place, 9 descriptors used for epicenter distance estimation were added adjusting a linear regression of an exponential function in time (t) by applying the expression “bt exp (-at)”; this expression belongs to the envelope of the seismic record in logarithmic scale (odaka et al., 2003) determined also by a linear regression and its respective correlation coefficient (r), for each of component in the seismic station. the correlation coefficient (r) along with the parameters (a) and (b) were calculated for each component; where (b) represents the slope of initial part of p waves and (a) is a parameter related to amplitude variations in time. finally, parameters for back-azimuth determination were used to include information about sources location of the seismic events into the model. maximum eigenvalues of two-dimensional covariance matrix were employed as input, calculated as described in magotra et al., 1987, and magotra et al., 1989. a windowing scheme with one second time windows was performed to obtain consecutive values for which a linear regression was calculated, also determining the slope (m), the independent term (b), the regression correlation factor (r), and this time with addition of the arithmetic mean of the eigenvalues (p). this last processing works with all components of the station at the same time, thus 4 descriptors were added as input related to this process. in summary, the svm of this study employs 25-time signal descriptors as input (table 1); 12 of them related to works on magnitude calculation, 9 were associated with epicenter distance estimations and the last 4 were used in the back-azimuth determination. these descriptors were calculated for 5, 10 and 15 seconds signal of the 863 selected events. the svm model a svm is a supervised classification technique that has its roots in statistical learning theory and has shown promising empirical results in many practical applications, from handwritten digit recognition to text categorization. svm also works very well with high-dimensional data and avoids the curse of dimensionality problem (tan et al., 2006). in geosciences e.g., it have been applied in earthquake characterization (ochoa et al., 2017), automatic recognition of natural fractures (leal et al., 2016), automatic indicator of lithologies in open hole logs (leal et al., 2018), among other applications related to pattern recognition. the svm model of this research was trained with the refined data set for each time window using the waikato environment for knowledge analysis weka 3.6 (frank et al., 2016) and the 25 descriptors explained before. this algorithm has strong statistical support and can be easily implemented on the station by electronic processing cards. 1800000 1600000 1400000 1200000 1000000 800000 600000 400000 200000 0 400000 600000 800000 1000000 1200000 1400000 1600000 1800000 colombia venezuela ecuador brasil panama figure 2. colombian seismic network. backazimuth magnitude ml ≥ 2.0 fr eq ue nc y 400 350 300 250 200 150 100 50 0 0 30 60 90 120 150 180 210 240 270 300 330 360 figure 3. statistical distribution of earthquake azimuth recorded at “el rosal” station. table 1. summary of descriptors employ as input data related to earthquake magnitude estimation (wu & kanamori, 2005) related to epicenter distance estimation (odaka et al., 2003) related to back-azimuth estimation (magotra et al., 1987; magotra et al., 1989) slope (m) independent term (b) correlation coefficient (r) maximum amplitude (mx) 12 descriptors (4 in each component of the station). parameter (a) parameter (b) correlation coefficient (r) 9 descriptors (3 in each component of the station). slope (m) independent term (b) correlation coefficient (r) maximum eigenvalues (p) 4 descriptors (all components work at the same time). 106 luis hernán ochoa gutiérrez, carlos alberto vargas jiménez, luis fernando niño vásquez after performing several processing tests, a standard normalized polynomial kernel was selected. in order to choose the kernel exponent and the complexity factor, correlation factors and minimum absolute error obtained by a 10 fold cross-validation process were compared. these processes were carried out testing multiple combinations of exponents and complexity factor for selected earthquake magnitudes and time signals. the correlation coefficient calculated for each partition corresponds to the pearson’s coefficient, which measure the linear relationship between two variables independently of their scales. this coefficient takes values between 1 and – 1; a value of zero means that a linear relationship between two variables could not be found. a positive value of this relation means that two variables change in the same way, i.e. high values of one variable correspond to high values of the other and vice versa. the closer this value is to one, the greater certainty that two variables have a linear relation. the pearson’s coefficient in this research is as high as 0.6, showing the relevance of svm in the estimation of earthquake arrival azimuth; the lower value of this coefficient was 0.096 as shown in table 4. results and discussion using the 25 descriptors and real magnitudes for each seismic event, a group of 12 datasets was evaluated (table 2). each dataset corresponds to combinations of 4 minimum magnitude filters (2.0, 2.5, 3.0 and 3.5 ml) and 3 signal length filters (5, 10 and 15 seconds), evaluating combinations of 7 values for kernel exponent (e = 1.5, 2, 4, 5, 10, 20 and 50) and 6 values for complexity factor (c = 1, 3, 5, 10, 20 and 50); testing 504 models of svms in order to find the combination of parameters with the best correlation factor in arrival azimuth determination. table 2 shows values of correlation coefficients in each combination of cut-off magnitude and time signals where kernel exponents and complexity factors were calculated. according to table 2, the best correlation coefficient is 0.6 for a time signal of 15 seconds (magnitude ≥ 2 ml). table 3 shows statistical summary for the best model of earthquake arrival azimuth in each combination of time signal (15, 10 and 5 seconds) and magnitude (2, 2.5, 3 and 3.5 ml). according to skewness values, all distributions present similar shape except samples from 5 seconds and 3.5ml (skewness = -4.86), theses samples show a negative skew related to low number of samples for this set (count = 33). the mean arithmetic can be affected by extremely low and high values; therefore, any interpretation from this parameter might produce a wrong perception of the data. high kurtosis values are related to over-fitting in some combinations and lower kurtosis are related to time signals of 10 and 15 seconds with cut-off of 3.5 ml, which can be interpreted as the arrival azimuth estimation may be reliable for earthquake greater than 3.5 ml and time signal of 15 seconds; however, the objective of this research is to develop the best model with the lowest base magnitude; in consequence and according to table 3 the best time signal should be 5 seconds and cut-off magnitude of 3.0 ml. the choice of svm final parameters is shown in table 4, where pearson’s correlation coefficient and mean absolute error are presented for each combination of kernel exponent “e” and complexity factor “c”, all of them for the time signal and the cut-off magnitude previously selected. these parameters were calculated using svm algorithms in weka 3.6 (frank, et al. 2016) with a standard normalized polynomial kernel and 10 fold cross-validations. according table to 4, the earthquake arrival azimuth can be performed at “el rosal” station using the svm based model with a normalized polykernel of exponent 2 and complexity factor of 10 for a time signal of 5 seconds and 3.0 ml of earthquake cut-off magnitude, showing a standard deviation of 45.4 degrees. figure 4 shows the cross-plot with a relationship between the real arrival azimuth (x-axis) and the azimuth calculated by the model (y-axis), where a normal statistical pattern can be observed in the distribution of residual values (histogram). the dashed blue line represents the linear behavior of predicted data, corresponding to the locus where prediction is equal to real values. this model tends to place the prediction further south to the real location, because of seismogenic zones toward east and west of the station producing more data in those directions and lower among of information from north and south; this is a normal operational condition of “el rosal” station and therefore, it is a behavior implicit into the model. 360 315 270 225 180 135 90 45 0 c al cu la te d a zi m ut h fr eq ue nc y real azimuth residual azimuth 0 45 90 135 180 225 270 315 360 azimuth svn t05 s mg > =3.0 mean typical error standard deviation kurtosis asymmetric coefficient 2.81 4.05 45.43 2.60 0.48 40 35 30 25 20 15 10 5 0 -90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 figure 4. correlation between real and calculated arrival azimuth with svm. 107fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques table 2. cut-off magnitude and time length combination pearson’s coefficient for each combination of magnitude and time signal magnitude ≥ 2.0 ml – “e” magnitude ≥ 2.5 ml – “e” ti m e si gn al = 1 5 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.423 0.442 0.523 0.538 0.58 0.607 0.561 1 0.397 0.415 0.483 0.507 0.559 0.545 0.489 3 0.45 0.489 0.548 0.561 0.6 0.594 0.51 3 0.447 0.466 0.523 0.543 0.578 0.518 0.443 5 0.47 0.511 0.557 0.574 0.608 0.575 0.479 5 0.462 0.498 0.536 0.569 0.552 0.507 0.412 10 0.496 0.525 0.575 0.588 0.603 0.54 0.437 10 0.485 0.502 0.572 0.581 0.508 0.464 0.302 20 0.516 0.537 0.587 0.59 0.574 0.495 0.403 20 0.496 0.5 0.575 0.57 0.476 0.437 0.387 50 0.535 0.555 0.582 0.582 0.513 0.419 0.373 50 0.49 0.531 0.553 0.525 0.439 0.378 0.387 ti m e si gn al = 1 0 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.429 0.454 0.506 0.521 0.553 0.578 0.565 1 0.427 0.449 0.494 0.519 0.535 0.522 0.474 3 0.457 0.483 0.534 0.544 0.57 0.587 0.512 3 0.456 0.47 0.432 0.539 0.525 0.489 0.419 5 0.488 0.498 0.542 0.552 0.579 0.575 0.484 5 0.467 0.488 0.534 0.538 0.514 0.466 0.411 10 0.485 0.521 0.554 0.56 0.587 0.546 0.459 10 0.481 0.491 0.536 0.535 0.489 0.425 0.408 20 0.51 0.536 0.561 0.569 0.577 0.5 0.45 20 0.477 0.499 0.529 0.504 0.456 0.382 0.408 50 0.535 0.556 0.566 0.573 0.536 0.439 0.442 50 0.479 0.513 0.481 0.471 0.394 0.351 0.408 ti m e si gn al = 0 5 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.379 0.399 0.442 0.449 0.467 0.471 0.412 1 0.312 0.354 0.419 0.442 0.484 0.467 0.416 3 0.428 0.439 0.457 0.463 0.473 0.469 0.365 3 0.41 0.419 0.458 0.475 0.491 0.469 0.373 5 0.431 0.439 0.465 0.471 0.485 0.455 0.342 5 0.406 0.422 0.467 0.479 0.49 0.447 0.351 10 0.434 0.447 0.466 0.464 0.474 0.726 0.328 10 0.414 0.432 0.473 0.481 0.481 0.416 0.33 20 0.445 0.462 0.449 0.45 0.45 0.397 0.305 20 0.422 0.451 0.467 0.47 0.46 0.38 0.305 50 0.462 0.444 0.424 0.435 0.419 0.329 0.285 50 0.45 0.44 0.44 0.456 0.41 0.334 0.265 magnitude ≥ 3.0 ml – “e” magnitude ≥ 3.5 ml – “e” ti m e si gn al = 1 5 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.431 0.43 0.422 0.421 0.386 0.37 0.341 1 0.283 0.327 0.366 0.361 0.295 0.237 0.135 3 0.433 0.412 0.405 0.411 0.426 0.362 0.34 3 0.364 0.419 0.475 0.37 0.348 0.241 0.135 5 0.416 0.407 0.417 0.427 0.405 0.368 0.348 5 0.401 0.454 0.392 0.383 0.351 0.241 0.135 10 0.415 0.413 0.441 0.457 0.382 0.388 0.34 10 0.456 0.404 0.357 0.341 0.351 0.241 0.135 20 0.426 0.42 0.461 0.437 0.387 0.368 0.34 20 0.412 0.397 0.325 0.34 0.351 0.241 0.135 50 0.419 0.431 0.431 0.398 0.388 0.368 0.34 50 0.379 0.323 0.325 0.34 0.351 0.241 0.135 ti m e si gn al = 1 0 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.388 0.412 0.41 0.428 0.458 0.491 0.334 1 0.305 0.339 0.375 0.37 0.313 0.318 0.121 3 0.411 0.408 0.427 0.442 0.532 0.41 0.333 3 0.324 0.33 0.317 0.334 0.326 0.32 0.121 5 0.409 0.422 0.45 0.487 0.509 0.382 0.333 5 0.299 0.29 0.346 0.366 0.327 0.32 0.121 10 0.42 0.409 0.486 0.513 0.435 0.382 0.333 10 0.247 0.274 0.382 0.346 0.327 0.32 0.121 20 0.403 0.411 0.494 0.496 0.374 0.382 0.333 20 0.245 0.329 0.357 0.347 0.327 0.32 0.121 50 0.393 0.433 0.467 0.425 0.352 0.382 0.333 50 0.318 0.357 0.357 0.347 0.327 0.32 0.121 ti m e si gn al = 0 5 s – “ c ” 1.5 2 4 5 10 20 50 1.5 2 4 5 10 20 50 1 0.336 0.407 0.47 0.46 0.456 0.343 0.182 1 0.369 0.427 0.52 0.534 0.518 0.267 -0.34 3 0.549 0.542 0.524 0.496 0.387 0.312 0.183 3 0.523 0.551 0.586 0.599 0.468 0.168 -0.34 5 0.571 0.561 0.494 0.462 0.346 0.305 0.183 5 0.556 0.563 0.609 0.609 0.413 0.168 -0.34 10 0.572 0.588 0.447 0.406 0.326 0.29 0.183 10 0.556 0.572 0.6 0.546 0.418 0.168 -0.34 20 0.501 0.531 0.369 0.308 0.321 0.298 0.183 20 0.552 0.567 0.532 0.525 0.418 0.168 -0.34 50 0.538 0.436 0.249 0.251 0.325 0.298 0.183 50 0.552 0.552 0.536 0.525 0.418 0.168 -0.34 108 luis hernán ochoa gutiérrez, carlos alberto vargas jiménez, luis fernando niño vásquez table 3. summary for the best model of earthquake arrival azimuth in each combination summary in determination of earthquake arrival azimuth residual signal time = 15 s signal time = 10 s signal time = 05 s minimum magnitude 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5 skewness 0.64 1.11 1.16 0.07 1.27 1.10 1.62 0.43 0.78 1.29 0.48 -4.86 kurtosis 4.14 4.89 7.68 1.58 6.83 3.26 16.24 2.39 1.93 4.09 2.60 27.14 mean 2.62 2.69 -0.14 4.71 3.96 5.74 -0.09 -3.10 6.04 5.79 2.81 -1.94 standard deviation 46.3 40.7 31.8 35.5 37.8 52.0 27.6 34.8 53.1 46.4 45.4 17.6 standard error 1.58 2.09 2.84 6.19 1.29 2.67 2.46 6.69 1.81 2.39 4.05 3.06 count 863 379 126 33 863 379 126 33 864 377 126 33 level of confidence (95%) 3.093 4.109 5.614 12.605 2.528 5.252 4.863 13.628 3.545 4.702 8.010 6.231 table 4. pearson’s coefficient and mean absolute error used to determent “e” and “c” pearson’s coefficient exponent “e” 0.5 0.8 1.1 1.5 2 4 5 10 20 50 c om pl ex ity f ac to r “c ” 0.1 0.096 0.12 0.125 0.137 0.159 0.209 0.23 0.272 0.28 0.187 0.5 0.165 0.192 0.226 0.264 0.303 0.387 0.401 0.414 0.386 0.195 0.8 0.192 0.234 0.267 0.307 0.366 0.45 0.451 0.452 0.355 0.182 1 0.207 0.25 0.276 0.339 0.409 0.47 0.459 0.458 0.347 0.182 2 0.225 0.287 0.365 0.472 0.522 0.51 0.506 0.417 0.327 0.172 4 0.285 0.406 0.529 0.57 0.55 0.51 0.475 0.361 0.31 0.183 5 0.324 0.454 0.557 0.571 0.56 0.494 0.462 0.346 0.304 0.183 10 0.409 0.498 0.555 0.573 0.588 0.447 0.406 0.326 0.29 0.183 20 0.438 0.507 0.553 0.6 0.532 0.368 0.308 0.321 0.298 0.183 50 0.38 0.398 0.583 0.538 0.436 0.25 0.252 0.352 0.298 0.183 mean absolute error exponent “e” 0.5 0.8 1.1 1.5 2 4 5 10 20 50 c om pl ex ity f ac to r “c ” 0.1 53.1 52.7 52.5 52.4 52.2 51.9 51.8 51.3 51.1 52.3 0.5 52.3 52.6 52.6 52.4 51.9 50.5 50.2 50.4 51.7 56.4 0.8 52.6 52.7 52.5 52 51.1 49.2 49.4 49.9 54 57.7 1 52.7 52.6 52.5 51.6 50.3 48.8 49.3 49.7 55.2 57.9 2 53.1 52.9 51.5 48.8 47.5 48.3 48.3 53.2 58.2 58.5 4 53.7 51.6 47.8 46.2 47.4 48.5 50.3 60.4 60.3 58.2 5 53.9 50.6 46.8 46.4 47.3 49.6 51.9 63 60.9 58.2 10 54.6 50.3 47 46.4 45.6 55.9 59.9 67.4 62.7 58.2 20 56.7 52.5 47.1 45 49.9 65.8 73.2 69 62.8 58.2 50 81.3 65.4 45.1 51 61.7 85.2 85 73.1 62.8 58.2 109fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques conclusions & recommendations this model is proposed and evaluated for fast earthquake arrival azimuth determination, based on support vector machine regression through pattern recognition and characterization of earthquake signals recorded on a three components seismic station in only 5 seconds, anticipating the arrival of earthquakes in the city of bogota. additionally, this model can be implemented directly in the electronic devices of a seismological station, where the main mathematical process corresponds to a simple matrix product, a kernel function of exponent 2 and complexity factor of 10 with an earthquake of 3.0 ml. the accuracy reported in this research is lower compared with results reported by lockman & allen, 2005 and eisermann et al., 2015; however it must to be consider that these authors works with data from several seismological station and not with a single station as this research does. nevertheless, the result of 45.4 degrees in earthquake arrival azimuth estimation showed in this research is an improvement on that of noda et al., 2012, who had standard deviation between 49.0 and 67.9 degrees working also with a single station. the implementation of additional input variables such as predominant period, fourier and wavelet frequency spectra should be considered in order to obtain higher correlation factors. furthermore, the use of an updated dataset is recommended, adding information from october 27th 2008 to present; this new data along with additional input variables might improve the model performance and reaching better estimation of earthquake arrival azimuth. it is important to find ways to improve the prediction accuracy based on further research, supported by computational intelligence and geophysics research groups as well as the seismological network in bogota’s savannah and its surroundings managed by the universidad nacional de colombia. acknowledgments the authors are grateful to the servicio geológico colombiano (sgc) for providing the dataset used in this study and to universidad nacional de colombia for supporting our efforts to 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(2006). magnitude estimation using the first three seconds p-wave amplitude in earthquake early warning. geophysics research letter, 33(16), l16312. https://doi.org/10.1029/2006gl026871 wu, y. m. & kanamori, h. (2005). rapid assessment of damage potential of earthquakes in taiwan from beginning of p waves. bulletin of the seismological society of america, 93(1), 526-532. https://doi. org/10.1785/0120040193 wu, y. m., shin, t. c. & tsai, y. b. (1998). quick and reliable determination of magnitude for seismic early warning. bulletin of the seismological society of america, 88(5), 1254-1259. http://citeseerx.ist.psu.edu/ viewdoc/download?doi=10.1.1.462.985&rep=rep1&type=pdf keywords: tight sandstone; pore size structure; sem; porosimetry; diagenetic effects. palabras clave: areniscas compactas; estructura del tamaño de poros; microscopio electrónico de barrido; porosimetría; efectos diagenéticos. how to cite item wang, w., yu, c., zhao, l., xu, s., & gao, l. (2020). combing sem and mercury intrusion capillary pressure in characterization of porethroat distribution in tight sandstone and its modification by diagenesis: a case study in the yanchang formation, ordos basin, china. earth sciences research journal, 24(1), 19-28. doi: https://doi.org/10.15446/esrj.v24n1.84838 determining the characteristics of pore-throat structures, including the space types present and the pore size distribution, is important for the evaluation of reservoir quality in tight sandstones. in this study, the results of various testing methods, including scanning electron microscopy (sem), pressure-controlled porosimetry (pcp) and rate-controlled porosimetry (rcp), were compared and integrated to characterize the pore size distribution and the effects of diagenesis upon it in tight sandstones from the ordos basin, china. the results showed that reservoir spaces in tight sandstones can be classified into those with three types of origins (compaction, dissolution, and clay-related) and that the sizes and shapes of pore space differ depending on origin. considering the data obtained by mercury injection porosimetry and the overestimation of pore radii by pressure-controlled porosimetry, the full-range pore size distribution of tight sandstones can be determined by combining data from pcp with corrected rcp data. the pore-throat radii in tight sandstone vary from 36 nm to 200 μm, and the distribution curve is characterized by three peaks. the right peak remains similar across the sample set and corresponds to residual intergranular pores and dissolution pores. the middle and left peaks show variation between samples due to the heterogeneity and complexity of nano-scale throat bodies. the average micro-scale pore content is 33.49%, and nano-scale throats make up 66.54%. the nano-scale throat spaces thus dominate the reservoir space of the tight sandstones. compaction, dissolution, carbonate cementation and clay cementation have various effects on pore-throats. compaction and carbonate cementation decrease the pore body content, pore-bridging clay cementation decreases the throat space content, and simultaneously, pore-lining clay cementation preserves pore space. abstract combining sem and mercury intrusion capillary pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis: a case study in the yanchang formation, ordos basin, china combinación de los métodos de microscopio electrónico de barrido e intrusión capilar de mercurio a presión en la caracterización de la distribución de gargantas de poro en areniscas compactas y su relación con la diagénesis: caso de estudio en la formación yanchang, cuenca ordos, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.84838 para evaluar la calidad de yacimiento en areniscas compactas es importante determinar las características de las gargantas de poros, entre ellas las clases de espacio presentes y la distribución de los tamaños de poros. en este trabajo se comparan e integran los resultados de varios métodos de prueba, entre ellos el microscopio electrónico de barrido (sem), porosimetría de presión controlada (pcp), y porosimetría de velocidad controlada (rcp), para caracterizar la distribución de los tamaños de poros y los efectos de la diagénesis en las areniscas compactas de la cuenca ordos, en china. los resultados muestran que los espacios de yacimiento en areniscas compactas se pueden clasificar de acuerdo con tres tipos de origen (compactación, disolución y arcilloso) y que los tamaños y formas del espacio de poros difiere de acuerdo con el origen. de acuerdo con la información obtenida por la porosimetría con inyección de mercurio y la sobreestimación de los radios de poros por la porosimetría de presión controlada, se puede determinar el rango completo de la distribución del tamaño de poros en areniscas compactas al combinar la información del método pcp con la información corregida del método rcp. el radio de la garganta de poro en areniscas compactas varía de 36 nm a 200 μm; la curva de distribución se caracteriza por tres picos. el pico de la derecha permanece similar a lo largo de todas las muestras y corresponde a los poros intergranulares residuales y a los poros de disolución. el pico central y el de la izquierda muestran la variación entre las muestras debido a la heterogeneidad y la complejidad de los cuerpos de garganta a nano-escala. el promedio del contenido poroso a micro-escala es de 33.49 %, y las gargantas a nano-escala ascienden al 66.54 %. esto significa que los espacios de gargantas a nano-escala dominan el espacio del yacimiento en areniscas compactas. la compactación, disolución, cementacion de carbonato y cementación arcillosa tienen varios efectos en las gargantas de poros. la compactación y la cementación de carbonato decrecen el contenido del cuerpo del poro. el puente arcilloso de poros reduce el contenido del espacio de las gargantas. mientras que la cementación arcillosa de revestimiento de poros preserva el espacio entre los poros. resumen record manuscript received: 30/01/2019 accepted for publication: 12/06/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 19-28 wei wang1,*, caili yu2, le zhao2, shuang xu2, lei gao2 1college of chemistry and chemical engineering, yulin university, yulin, pr china 2petrochina changqing oilfield company, xi’an, pr china * corresponding author: wangwei_1988@yulinu.edu.cn g eo lo g y 20 wei wang, caili yu, le zhao, shuang xu, lei gao introduction with the gradual decline of hydrocarbon reserves in conventional reservoirs and the increasing global energy demand, the great potential of unconventional reservoirs has made them the focus of global oil production (ghanizadeh et al., 2015). tight sandstones are characterized by extremely low porosity and permeability, the presence of nano-scale throat space, and having undergone strong diagenesis (rezaee et al., 2012; zou et al., 2013), all of which influence the characteristics of seepage from tight reservoirs (jarvie et al., 2007; nelson, 2009; xi et al., 2016). therefore, a comprehensive understanding of the pore-throat distribution in tight sandstone and how it is affected by diagenesis is necessary for quality-evaluation and successful production. the pore size distribution (psd) in tight sandstones can be characterized by many techniques, such as thin section analysis, scanning electron microscopy (sem), x-ray computed tomography (ct), pressure-controlled porosimetry (pcp), rate-controlled porosimetry (rcp), nuclear magnetic resonance (nmr) and n2 adsorption, but these techniques all have limitations that hinder determination of the characteristics of pore-throats of a wide range of sizes. sem (fe-sem) and thin sections provide direct information on the morphological characteristics of reservoir space and can be used to determine the genetic origins of pore-throats, but they fail to provide quantitative data (klaver et al., 2012; loucks and ruppel, 2007). pcp can be used to obtain quantitative data on pore-throat radius from samples and effectively identify nanopores but fails to measure large pores due to the shielding effect (gane et al., 2004). rcp can examine pore spaces and throats spaces separately according to the oscillation of intrusion pressures (yuan and swanson, 1989) but cannot reveal the distribution of tiny throats due to the limited maximum intrusion pressure (6.22 mpa) (wang et al., 2018; zhao et al., 2015). ct can be used to reconstruct 3d images of pore-throats, but its application is restricted by its high cost and the inevitable error that arises in calculating pore-throat radius (mayo et al., 2015). nmr can be employed to obtain the pore size distribution through a transformation of the value of transverse relaxation time (daigle and johnson, 2016; gao et al., 2019). however, the test results need to be corrected, and the test range is limited by surface relaxivity (xiao et al., 2017a). nitrogen adsorption can only obtain information on nanopores in tight sandstones (yao and liu, 2012) stated thus, one single technique can only partially reveal the pore-throat distribution of a tight sandstone, so integration of several techniques appears to be necessary for characterizing their overall pore-throat distribution. therefore, scholars have combined pcp, rcp, nitrogen adsorption, and nmr to study the pore-throat distribution of tight sands by considering the limitations of the different tests (xiao et al., 2017b; yan et al., 2017; yao et al., 2010; zhang et al., 2017; zhao et al., 2015) this approach has expanded the range of pore sizes that can be investigated and has improved the accuracy of pore size characterization. rcp and pcp are based on the same principle, so the results from these two methods can be well-integrated to reveal the pore size distribution of tight sandstones. rcp overestimates the size of pore bodies (xiao et al., 2017b), however, so the data derived by rcp must be corrected to obtain accurate pore distribution data. the evolution of pore-throats is influenced by several factors such as the original rock composition, sedimentary environment and diagenesis. sedimentation forms the basis of the pore-throats, and diagenesis modifies them (waples, 2002; zhu et al., 2014). many scholars have studied the evolution of pore-throats during diagenesis. some of the conclusions that have been reached are as follows: (1) compaction and cementation reduce the space between gains and are the main diagenetic processes that reduce the pore-throat space in tight sandstone (hillier, 2000). (2) chemical dissolution increases the relative content of secondary pores and improves the physical properties of the reservoir (jia et al., 2018). however, less work has been done separating the effects of compaction and cementation on pores and throats because data are not available on the distribution of pore-throats of a wide range of sizes, which is essential for the study of pore-throat characteristics. in this study, we first analyze pore-throat structures based on thin sections and sem observations. pore and throat types are divided into those with three origins: compaction, dissolution and clay-dominated. pore-throats with different origins are shown to have different morphological characteristics. secondly, the full-range psd is established through a combination of pcp and corrected-rcp results. finally, the influence of various diagenetic processes on the pore-throat distribution of tight sandstones is discussed. samples and experiments seven cylindrical core plug samples with a diameter of 2.48 cm and a length of 5.00 cm were prepared. oil was removed from the samples with a mixed solution of benzene and alcohol, and they were then dried in a bake oven at 115°c for 12 h. the samples were analyzed via porosity and permeability tests and were then were divided into parts for further experiments. samples all of the samples were chang 7 tight sandstones, which from part of the upper triassic yanchang formation in the ordos basin (fig. 1). the chang 7 sandstones were deposited in a semi-deep and deep lake sedimentary environment and are dominantly fine-grained sandstone. chang 7 developed in the period when lakes were at their deepest, leading to the formation of highquality source rocks within it. the crude oil generated from the source rocks in chang 7 migrated immediately to the adjacent chang 7 tight sandstones as the rocks were maturing (zhang et al., 2009), making the chang 7 tight sandstone a tight oil reservoir. experimental methods the morphological characteristics of pores and throats were measured with an fei quanta 400 feg sem at 30 kv and 1.2 nm. each sample was first polished and then gold-coated to better capture the morphological characteristics of the pore-throats. an autopore 9420 mercury porosimeter was used for pcp. the maximum injection pressure was 200 mpa, corresponding to the ability to detect a pore radius of 36 nm. rcp was carried out with an aspe730 mercury porosimeter, and the rate of mercury intrusion was kept constant at a value of 0.001 μl/s. the maximum intrusion pressure was 6.8 mpa (1000 psi), corresponding to a pore radius of 0.1 μm. results lithological characteristics of tight sandstone the mineralogical compositions of the seven samples are shown in table 1. the dominant minerals are quartz, feldspar, clay and carbonate. the quartz content ranges from 31% to 43%, with an average value of 37%. the feldspar content varies from 40% to 51%, with an average value of 46%. the carbonate content is from 2% to 18%, with an average value of 8.7%. the clay content ranges from 5% to 15%, with an average value of 8.2%. the porosity values of the samples vary from 9.54% to 12.26%, with the average value being 10.79%. their permeability varies from 0.06×10-3 μm2 to 0.32×10-3 μm2, with an average value of 0.19×10-3 μm2. the tight sandstone thus has extremely poor petrophysical properties. the pores in the samples can generally be classified into three types on the basis of the sem and thin section images: residual intergranular pores, mineral dissolution pores and intercrystalline pores. residual intergranular pores are the major pore type in the tight sandstone and formed through compaction, cementation and dissolution (fig. 2a and b). mineral dissolution pores are generated by dissolution and were usually found in feldspar and debris (fig. 2c and d), though dominantly in feldspar. intercrystalline pores were mainly found in illite and chlorite (fig. 2e and f). the size and shape of the three types of pores vary significantly. the residual intergranular pores take the form of triangles and regular polygons, and their edges are generally clear and straight. the radii of residual intergranular pores are relatively large, ranging from 50 μm to 200 μm, and the connectivity between pores is good. the feldspar dissolution pores are irregularly shaped, and their sizes are generally <100 μm. the connectivity between feldspar dissolution pores is worse than between residual intergranular pores. intercrystalline pores range in size from 1.5 μm to 5 μm, and their connectivity is the worst of the three types. the throats in all of the samples were classified into three main types on the basis of the sem images and thin section images, namely compacted throats, dissolution throats and intercrystalline throats. with intense compaction and cementation, the primary pore space in the samples is gradually squeezed, and compacted, cement-infilled throats are formed. soluble mineral particles eroded by acidic fluids form dissolution throats, which are connected with dissolution pores. the intercrystalline pores in chlorite or illite form the intercrystalline throats. compacted throats are necking and range in radius from 0.8 μm to 4 μm (fig. 2g), while dissolution 21combining sem and mercury intrusion capillary pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis the mercury injection curves for the tight sandstone samples can be divided into two types. j12, j13, j17 and j20 belong to type i. these mercury intrusion curves contain one platform. when the mercury injection pressure is < 7 mpa, the corresponding mercury saturation increases at a low rate with the increase in pressure, forming an obvious platform in the curve. when the mercury injection pressure is > 7 mpa, the corresponding mercury saturation increases at a higher rate. the threshold pressure of this type is the higher of the two types, with a mean value of 2.35 mpa. this indicates that the maximum pore space that allows the fluid to enter the sample is small. the curves for j5, j6 and j10 belong to type ii. there are two platforms in mercury intrusion curves of this type. when the mercury intrusion pressure is < 3mpa, a platform appears due to a slow increase in the mercury intrusion saturation. when the mercury intrusion pressure is between 3 mpa and 10 mpa, the mercury intrusion saturation increases rapidly. when the mercury intrusion pressure is > 10 mpa, the mercury saturation increases more slowly, forming throats are flake-like and are in the range 0.3–0.8 μm (fig. 2h). intercrystalline throats are tube bundles, and their radii are mainly <0.2 μm (fig. 2i). pore-throat structures mercury injection is an effective way to obtain quantitative data on micro–nano-scale pores. the intrusion pressure pc was converted to pore or throat radius by applying the washburn equation (washburn, 1921), adopting an interfacial tension value of 485 dyn/cm and a wetting angle of 140°. pcp curves variation in the slopes of the mercury intrusion curves was attributed to variability in the pore size distribution (fig. 3); the corresponding parameters are given in table 2. figure 1. geological and lithological features. (a) location and tectonic units of the research area (modified from yang et al. (2005)); (b) comprehensive stratigraphic column of tight sandstone in the ordos basin (data on lithological features from wu et al., 2018) table 1. characteristics of seven tight sandstone samples. samples depth lithology porosity perm content of xrd(%) relative content of clay(%) (m) (%) (md) quartz feldspar carbonate clay kaolinite chlorite illite i/s mixed layer j5 2620 sandstone 9.54 0.19 43 47 4 6 27 22 11 40 j6 2404 sandstone 11.99 0.28 38 49 8 5 10 33 4 53 j10 2394 sandstone 11.92 0.14 37 48 2 13 10 33 4 53 j12 2270 sandstone 12.26 0.32 39 51 5 5 25 24 11 40 j13 2554 sandstone 10.58 0.11 31 46 8 15 30 26 8 36 j17 2367 sandstone 10.11 0.06 35 43 14 8 25 10 18 47 j20 2306 sandstone 9.16 0.22 34 40 20 6 27 8 16 49 22 wei wang, caili yu, le zhao, shuang xu, lei gao the second platform. the threshold pressure of this type of curve is low, with a mean value of 1.01 mpa. there is some discrepancy in the medium saturation pressures of the seven samples: the largest, 17.56 mpa, is for sample j17, and the smallest, 4.19 mpa, is for sample j6. this variation indicates that the pore-throat distribution of the samples is inhomogeneous. the maximum mercury intrusion saturations and residual mercury saturations of the samples differ greatly, with the average values being 88.78% and 57.13%, respectively. the difference between the mercury intrusion saturation and the mercury extrusion saturation indicates that a large amount of mercury residue remained in the pore-throat space of the samples. rcp curves rcp can provide information on the pore–throat structure because the data derived by this method reflects the pore space and throat space separately. figure 2. primary pore and throat types in sandstone samples in thin section and sem images. (a) residual intergranular pores preserved by chlorite (j6), (b) residual intergranular pores that are triangular and regularly polygonal (j20), (c) dissolution pores in feldspar (j6) (d) dissolution pores in debris (j10), (e) intercrystalline pores in illite (j12), (f) intercrystalline pores in chlorite (j13), (g) compacted throats (j5), (h) dissolution throats (j10) and (i) intercrystalline throats formed by clay-space necking (j17). figure 3. capillary pressure curves from pressure-controlled porosimetry (pcp) the samples were differentiated into two types based on the characteristics of the intrusion curve (fig. 4); the full rcp results are given in table 2. for sample j6, the total mercury saturation curve shows a similar trend with increasing pressure as the throat injection and pore injection curves, and the total mercury intrusion can be seen to be controlled by both throats and pores. for sample j20, the mercury intrusion saturation was controlled by pores at the beginning of mercury intrusion, but, with increasing pressure, throats became the controlling factor. the threshold pressures of the two types of samples are similar. the average value for type i is 1.04 mpa, and that for type ii is 0.97 mpa, indicating that the maximum throat radius controlling the intrusion of fluids into pores is similar in type i than in type ii. for type i, the final intrusion saturation ranges from 39.55% to 85.41%, with an average value of 62.26%, and for type ii, the final intrusion saturation varies from 45.13% to 73.07%, with an average value of 60.07%. the final mercury saturation of type i is similar to that of type ii, indicating that the macro-scale throat content of the samples is similar. the similar intrusion satruration and threshold indicates that micro-scale pore spaces is similar in two types i and ii. discussion features of the pcp results the pore-throat radius distribution can be obtained from the saturation values corresponding to different pressures along the mercury intrusion curve (fig. 5). the psd obtained by pcp shows that the pore radius ranges from 36 nm to 1.01 μm, and is mainly within the range of 0.01–0.4 μm. the pore-throat distribution curve for type ⅰ forms a single peak at a pore radius of 0.1 μm. the pore-throat distribution of type ⅱ is bimodal, indicating that there are two pore-throat development regions. the smaller pore-throats are 0.04 μm and the larger pore-throats are 0.8 μm. the pcp mercury intrusion curve shows a smaller pore radius than the thin section and sem observations and cannot identify residual intergranular pores with a larger radius. features of the rcp results when calculating the reservoir space by rcp, the pore space is considered to be spherical. however, because of compaction and cementation in tight 23combining sem and mercury intrusion capillary pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis table 2. pore-throat structure parameters of seven samples on the basis of pcp and rcp samples pcp rcp threshold pressure (mpa) medium saturation pressure (mpa) maximum mercury intrusion saturation (%) residual mercury saturation (%) threshold pressure (mpa) final mercury intrusion saturation (%) pore mercury saturation (%) throat mercury saturation (%) j5 1.16 10.30 91.91 55.89 1.48 45.13 26.33 18.80 j6 0.71 4.19 90.53 60.46 0.88 62.00 41.43 20.57 j10 1.16 12.63 90.91 52.76 0.56 73.07 39.88 33.19 j12 1.81 10.55 86.17 59.12 0.79 85.41 49.66 35.75 j13 2.91 10.90 90.14 56.04 0.56 42.52 16.09 26.43 j17 2.91 17.54 87.18 53.71 1.98 39.55 24.09 15.47 j20 1.81 11.14 84.51 61.95 0.84 81.57 47.69 33.88 figure. 4 mercury intrusion curves for samples j6 and j20 (rcp) figure 5. pore radius distribution derived by pcp (a) type i and (b) type ii 24 wei wang, caili yu, le zhao, shuang xu, lei gao sandstones, the shape of pore space is dominantly tubular (xiao et al., 2017b). thus, the final pore distribution is obtained by first multiplying the initial pore radius given by rcp by a shape factor (2/3) to account for this tubular shape. the radii of the pores range from 50 μm to 200 μm, and the psd is characterized by two peaks (fig. 6a). the right peak ranges from 100 μm to 200 μm, which mainly corresponds with residual intergranular pores filled with cements, which have a large radius. the left peak ranges from 50 μm to 100μm, which is mainly attributable to feldspar dissolution pores, which have small radii. the throat sizes are widely distributed, ranging from 0.2 μm to 1.4 μm (fig. 6b), implying that the nano-scale reservoir spaces have been strongly influenced by diagenesis. the psd can be calculated by consideration of the pore distribution and throat distribution data from rcp (fig. 7). the pore-throat distributions of the seven samples are characterized by having two peaks. the distribution of the right peak is consistent with the information obtained via thin section and sem examination, ranging from 60 μm to 150 μm, while the distribution of the left peak is limited by injection pressure and ranges from 0.2 μm to 1.2 μm. psd calculated by pcp and rcp differences between the pcp and rcp results as discussed above, both pcp and rcp have limitations. it is impossible to obtain the full-range pore-throat distribution using just one of them. rcp can distinguish pores and throats, but the maximum mercury intrusion pressure is low, and throats with a radius of under 0.12 μm cannot be characterized. pcp can provide information on smaller pore-throats because of its high mercury intrusion pressure, but it cannot distinguish pores and throats, so it has poor ability to recognize pores with a large radius. therefore, it is best to combine the two test datasets to obtain the overall psd, taking advantage of the strengths of both pcp and rcp. since pcp and rcp are calculated under the same test principle, samples should have the same mercury intrusion curves under the same mercury intrusion pressure (fig. 8). in reality, the mercury intrusion curves obtained by pcp and rcp are not exactly the same due to the differences in contact angle and interfacial tension in pcp and rcp. figure 6. rcp-derived distributions of (a) pores and (b) throat bodies in seven tight sandstone samples figure 7. corrected-rcp-based pore size distribution in seven tight sandstone samples. the mercury saturation reached in the pcp tests varied from 84.51% to 91.91%, with an average value of 88.76%, which is larger than that arrived at through rcp (averaging 63.98%), indicating that there is a lot of nano-scale pore space in tight sandstone that cannot be probed by rcp. this is caused by the limitation on the mercury intrusion pressure that can be applied. full-range psd calculated by pcp and rcp the measurement range of rcp means that it can characterize pores but cannot identify nano-scale throat bodies, while pcp can characterize the overall distribution of nano-scale throats. thus, the combination of the throat distribution derived by rcp and the pore distribution derived by rcp is an efficacious method for revealing the pore-throat distribution. the mercury intrusion curve of pcp overlaps with that of rcp when the intrusion saturation is low. this portion of the data reflects the pore distribution of the sample, which can be obtained from the psd given by rcp. when the mercury intrusion saturation in rcp reaches its maximum, the distribution of the remaining throats in the sample is derived from the pcp mercury intrusion curve. thus, the distribution of micro-scale spaces is derived from rcp, and the distribution of nano-scale spaces is derived from pcp. the pore-throat sizes detected in the tight sandstone range from 36 nm to 200 μm in radius (fig. 9). the full-range psd curves are dominated by three peaks. the right semi-peak ranges from 50 μm to 200 μm, with a peak value of 90 μm; the corresponding pore types are residual intergranular pores and 25combining sem and mercury intrusion capillary pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis dissolution pores. the middle semi-peak ranges from 0.2 μm to 1.5 μm; throats in this size range are compacted throats. the left peak is between 36 nm and 0.2 μm and reaches a maximum at different values for the two sandstone types: 0.1 μm for type ⅰ and 0.04 μm for type ⅱ. throats in this size range are dissolution throats and intercrystalline throats. the peak values of the middle peak and right peak to not differ significantly for the two types, indicating there is no obvious discrepancy between the content of pores and compacted throats in the two types. the higher peak value for the left peak for type ⅰ than for type ⅱ, meanwhile, implies that dissolution throats control more mercury intrusion space in type ⅰ samples than in type ⅱ samples. when pore radii are <1.5 μm, their pore-throats are dominantly nanothroats, including compacted throats, dissolution throats and intercrystalline throats. the sizes of compacted throats are between 0.2 μm and 1.5 μm, the sizes of intercrystalline throats are <0.05 μm, and dissolution throats usually range from 0.05 μm to 0.2 μm. the results show that the content of the different throat types differs significantly between the tight sandstone samples. the saturation of micro-scale pore bodies ranges from 15.87% to 49.66%, with an average value of 33.49%, and the average saturation of nanoscale throats is 66.54%, indicating that nano-scale throat bodies, rather than micro-scale pore bodies, dominate the pore spaces of the tight sandstones. with a decrease in the permeability of the samples, the nano-scale throat content increases, while the proportion of micro-scale pores decrease. the influences of diagenesis on the full-range pore-throat distribution diagenesis after sedimentation has an important influence on pore throat formation, giving rise to the complexity and heterogeneity of pore-throat space in tight sandstones. the effects of diagenesis on pores and throats can be inferred from the full-range pore-throat distribution. (1) compaction: samples j5, j6, j10, and j12 have similar contents of mineral particles, as well as a relatively low cement content (<15%) (table 1). with increasing sample depth from 2270 m to 2620 m, the pore distribution range remains stable and the pore-space saturation decreases gradually from 49.66% (sample j12, 2270 m) to 26.33% (sample j5, 2620 m) (fig. 10a, b and c). there is no obvious variation in the peak value of throats with a radius of under 1 μm in the four samples. with increasing depth, the porosity decreases from 12.26% to 9.54%, though the magnitude of the decline is relatively small for samples j12, j10 figure 8. comparison of pcp and rcp intrusion curves for samples j6 and j20 figure 9. pore size distribution derived by combining pcp and rcp results. (a) type i and (b) type ii 26 wei wang, caili yu, le zhao, shuang xu, lei gao and j6. this decrease may have been caused by feldspar dissolution and the formation of clay minerals during early cementation (figs. 2c and 10e, f), which can expand or preserve the pore space from compaction (berger et al., 2009). this implies that compaction has a considerable pore-space and porosity-reducing effect. (2) cementation the cements in the tight sandstones are mostly carbonate, while clay cements are less dominant. carbonates generally take a double-crystal shape, and clay appears both in the form of a coating on mineral particles and as crystals bridging spaces between grains. these cements have an important effect on reservoir quality. (i) carbonate. samples j10, j17, and j20 have similar burial depths and mineral particle content, and the cements in the three samples are dominantly carbonate and clay. with increasing carbonate content from 2% (j10) to 20% (j20) (table 2), the content of pore bodies becomes significantly lower, dropping from 39.88% to 15.88% (figs. 11a, b and c). porosity also shows corresponding changes. that may be caused by carbonate infilling of pore spaces during the later stages of cementation (fig. 11d), implying that carbonate plays an important role in reducing pore size. (ii) clay. with increasing clay content from 6% (j20) to 13% (j10), the distribution of the throat space becomes wider and the content of throat bodies becomes lower. that may be caused for throat space being filled and segregated by pore-bridging clay (figs. 2e and 11e). correspondingly, the permeability decreases. the clay minerals in samples j17 and j20 are mainly kaolinite and illite, while those in sample j10 are dominated by chlorite (table 2). with a decreasing relative chlorite content, the content of pore space becomes lower. the higher pore space content in the chlorite-rich sample may be attributable to porelining-type authigenic chlorite forming during the early stages of cementation (fig. 11a) preventing compaction and pore-bridging clay (e.g. kaolinite) from decreasing pore spaces (huang et al., 2004). this indicates that pore-bridging clay cements can reduce the throat size by filling the throat, while pore-lining clay cements can preserve pore space by coating the grains. figure 10. graphs shown the effect of compaction on the pore size distribution of (a) j12, (b) j10, (c) j6 and (d) j5, which have different burial depths but similar mineral compositions. (e-f) thin-section images showing (e) the residual intergranular pores in sample j12 and (f) isolated feldspar-dissolution pores in sample j5. note that there is evidence of a decrease in the pore space and porosity as compaction increases. figure 11. graphs showing the effect of clay and carbonate cementation on the pore size distributions of (a) j20, (b) j17 and (c) j10, three tight sandstone samples that have similar burial depths but various contents and types of cements. (d) thin-section image showing that sample j20 contains carbonate with a double-crystal shape. (e) sem image demonstrating that sample j10 contains abundant illite. with an increasing content of pore-bridging cement, the content of throat bodies and permeability decrease appreciably, while the content of pore bodies does not. with an increase in carbonate content, the pore body content and porosity decrease substantially. 27combining sem and mercury intrusion capillary pressure in the characterization of pore-throat distribution in tight sandstone and its modification by diagenesis conclusions sem, thin section, pcp and rcp analyses were performed on seven samples from the upper triassic yanchang tight sandstone from the ordos basin so as to bring the combined strengths of a range of methods to bear on determining the full-range distribution of pore bodies and throats and the effects of diagenesis upon them. the conclusions are as follows: (1) the sem and thin-section experimental results indicate that pores and throats formed in the tight sandstones by various diagenetic processes. these spaces can be distinguished between those with a compaction origin, a dissolution origin or a clay-dominated origin; these mainly formed by compaction, dissolution and cementation, respectively. pore-throats with different origins have different sizes and shapes. (2) the full-range pore size distribution of tight sandstones can be determined by combining the experimental data from pcp and rcp. the detected pore sizes of these tight sandstones range from 36 nm to 200 μm, and the psd curves of the samples are polymodal. the right peak remains stable across the sample set and corresponds to residual intergranular pores and dissolution pores. the middle and left peaks show variation between samples due to the complexity and heterogeneity of nano-scale throat bodies. the average content of micro-scale pores is 33.46%, and that of nano-scale throats is 66.54%. nano-scale throat spaces thus dominate the pore spaces in the tight sandstones. 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(2014). microscopic characteristics and formation mechanism of upper triassic chang 7 tight oil reservoir in the southwest ordos basin. journal of china university of mining and technology, 43, 853-863. zou, c., yang, z., tao, s. z., yuan, x. j., zhu, r. k., hou, l. h., wu, s. t. ... pang, z. l. (2013). continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum: the ordos basin, north-central china. earth-science reviews, 126, 358-369. keywords: low frequency; rock petrophysics; lithology; coalbed methane; enrichment. palabras clave: baja frecuencia; petrofísica de roca; litología; carbón metano; enriquecimiento. how to cite item he, y., wang, x., sun, h., xing, z., chong, s., xu, d., & feng, f. (2019). coal seam roof: lithology and influence on the enrichment of coalbed methane. earth sciences research journal, 23(4), 359-364. doi: https://doi. org/10.15446/esrj.v23n4.84394 to identify the lithology of coal seam roof and explore the influence of these roofs on the enrichment of coalbed methane, low-frequency rock petrophysics experiments, seismic analyses and gas-bearing trend analyses were performed. the results show that the sound wave propagation speed in rock at seismic frequencies was lower than that at ultrasound frequencies. additionally, the p-wave velocities of gritstone, fine sandstone, argillaceous siltstone and mudstone were 1,651 m/s, 2,840 m/s, 3,191 m/s and 4,214 m/s, respectively. the surface properties of the coal seam roofs were extracted through 3d seismic wave impedance inversion. the theoretical p-wave impedance was calculated after the tested p-wave velocity was determined. by matching the theoretical p-wave impedance of the four types of rocks with that of the coal seam roofs, we identified the lithology of the roofs. by analyzing known borehole data, we found that the identified lithology was consistent with that revealed by the data. by comparing and analyzing the coal seam roof lithology and the gas-bearing trends in the study area, we discovered that the coal seam roof lithology was related to the enrichment of coalbed methane. in the study area, areas with high gas contents mainly coincided with roof zones composed of mudstone and argillaceous siltstone, and those with low gas contents were mainly associated with fine sandstone roof areas. thus, highly compact areas of coal seam roof are favorable for the formation and preservation of coalbed methane. abstract coal seam roof: lithology and influence on the enrichment of coalbed methane cimas con gas de filones de carbón: litología e influencia en el enriquecimiento de metano en capas de carbón issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84394 para identificar la litología de una cima con gas de filones de carbón y explorar la influencia de estas cimas en el enriquecimiento de metano en capas de carbón, se realizaron experimentos de petrofísica en rocas de baja frecuencia, análisis sísmicos y análisis de tendencias con gas. los resultados muestran que la velocidad de propagación de la onda de sonido en roca a frecuencias sísmicas fue menor que la de las frecuencias de ultrasonido. además, las velocidades de la onda p de arenisca, arenisca fina, limolita arcillosa y lutita fueron 1.651 m/s, 2.840 m/s, 3.191 m/s y 4.214 m/s, respectivamente. las propiedades de la superficie de los techos con gas de filones de carbón se extrajeron mediante inversión de impedancia de onda sísmica 3d. la impedancia teórica de la onda p se calculó después de determinar la velocidad de la onda p probada. al hacer coincidir la impedancia teórica de la onda p de los cuatro tipos de rocas con la de las cimas con gas de filones de carbón, se identificó la litología de las cimas. al analizar los datos conocidos del pozo, se encontró que la litología identificada era consistente con la revelada por los datos. al comparar y analizar la litología de la cima con gas de filones de carbón y las tendencias con gas en el área de estudio, descubrimos que la litología de esta cima estaba relacionada con el enriquecimiento del metano de las capas de carbón. en el área de estudio, las áreas con alto contenido de gas coincidieron principalmente con zonas de techo compuestas de lutita y limolita arcillosa, y aquellas con bajo contenido de gas se asociaron principalmente con áreas de techo de arenisca fina. por lo tanto, las áreas altamente compactas del techo con vetas de carbón son favorables para la formación y preservación del metano de las capas de carbón. resumen record manuscript received: 27/04/2019 accepted for publication: 11/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 359-364 yunlan he1, 2*, xikai wang3, hongjie sun3, zhenguo xing3, shan chong1, dongjing xu2, feisheng feng3 1state key laboratory of coal resources and safety mining, china university of mining & technology, beijing 100083, china 2shandong provincial key laboratory of depositional mineralization & sedimentary minerals, college of earth science & engineering, shandong university of science and technology, qingdao 266590, china 3college of geoscience and surveying engineering, china university of mining & technology, beijing 100083, china * corresponding author: 55069008@qq.com h yd ro ca rb o ns 360 yunlan he, xikai wang, hongjie sun, zhenguo xing, shan chong, dongjing xu, feisheng feng introduction coalbed methane exists in coal rock, and coalbed methane enrichment remains a difficult research topic. in addition to the coal seam, which influences methane enrichment (groshon, pashin, & mcintyre, 2009; kędzior, 2009; hemza, sivek, & jirásek, 2009; gao et al., 2012; moore, 2012), the coal seam roof also affects the enrichment of coalbed methane. thus, the role of the roof has attracted increased research attention (chen, cui, liu, & lang., 2006; zhou, 2013; song et al., 2013). the properties of coal seam roofs are difficult to determine based on sparse borehole data; however, 3d seismic data collected at low frequencies (5-100 hz) can greatly improve the density of control points and identification accuracy. the theories regarding the propagation velocity of sound waves in rock at seismic frequencies have not been supported by experimental data, although some have described the velocity and attenuation of sound waves in fluid-saturated rock at seismic frequencies (ba, 2010; white, 1975; biot, 1956). these theoretical models are mainly based on experimental data at ultrasonic frequencies (mhz). tutuncu, gregory, sharma, and podio (1998) measured the p-wave and s-wave velocities of a saline-saturated sandstone at frequencies of 10 hz-1 mhz, and the results showed that both the p-wave and s-wave velocities displayed upward-increasing trends. batzle, han, and hofmann (2006) tested and compared the propagation velocity of sound waves at seismic and ultrasonic frequencies using a low-frequency stress-strain method and an ultrasonic test. wang, zhao, harris, and quan (2012) measured the bulk modulus of different types of rocks at 600 hz using resonance spectroscopy and compared the results with those at ultrasonic frequencies. using a low-frequency stress-strain method, wei, wang, zhao, tang, and deng (2015a; 2015b) discussed the factors that influence the propagation velocity of sound waves in sandstone and studied the propagation velocity of sound waves and their dispersion characteristics under different conditions. in this paper, the propagation velocity of sound waves in four types of rocks was measured and studied using a low-frequency stress-strain method. by matching the tested velocity with 3d seismic data, we identified the coal seam roof lithology to explore the influence of the roofs on the enrichment of coalbed methane. we hope that this study will improve studies of the propagation velocity of sound waves at low frequencies and the enrichment trends of coalbed methane (hungerford, 2013; li & wu, 2016; liu, 2018). research methods low-frequency rock petrophysics experiments the samples used in the low-frequency rock petrophysics experiments included four representative types of rocks: gritstone, fine sandstone, argillaceous siltstone, and mudstone. the rocks were collected from the top of the #3 coal seam of the permian shanxi formation at the sihe coal mine in the southern qinshui basin, china. the gritstone was coarse-grained quartz sandstone with no primary pore development, some micro-fissures, and a porosity of less than 3%. the fine sandstone was fine quartz sandstone with primary pore development and a porosity of approximately 20%. the argillaceous siltstone was composed of fine particles and miscellaneous bases with poor separation, no primary pore development, and porosity of almost 0%. the mudstone was collected from the immediate coal seam roof, was composed of argillaceous rocks, and had a porosity of 0%. in this experiment, the physical system used to test the rock samples at low frequencies (fig. 1) was developed by the colorado school of mines, and a low-frequency stress-strain method was used as the experimental method. the tested rock samples were cylindrical, with a diameter of 38 mm and a length of 50-80 mm. each sample had cylindrical aluminum block bases and pand s-wave probes at both ends. additionally, longitudinal and transverse strain gauges were fixed on the aluminum block bases and rock samples, respectively. the physical testing system can produce acoustic waves with frequencies of 3-2,000 hz, and the vertical and transverse strains can be tested with fixed strain gauges on the aluminum block and rock samples. the young’s modulus of the aluminum block was 69.8 gpa. each rock sample and the aluminum block bases comprised a complete unit, so their stresses were the same during the test. notably, the stress on the aluminum block al( ) was equal to that on the rock sample s( ) (formula 1). σ =σal s (1) the stress on the aluminum block al( ) was as follows. σ ε −al al al ve= ⋅ (2) the young’s modulus of the rock sample (es) can be deduced from formulas 1 and 2. e e gpa s s s v al al v s v al v s v = = ⋅ = ⋅σ ε ε ε ε ε− − − − − 69 8. (3) the poisson ratio of a rock sample (v) can be obtained by formula 4. v s v s h = ε ε − − (4) because the young’s modulus and poisson ratio of a rock sample were given, the other parameters of the rock sample at different frequencies and conditions could be obtained by the formulas listed in table 1. figure 1. physical testing system of the rock at low frequencies 361coal seam roof: lithology and influence on the enrichment of coalbed methane results results of the low-frequency experiments the propagation velocity of sound waves in the four types of rocks was measured at 16 frequencies from 3-100 hz. the p-wave and s-wave velocities of the rocks at the 16 frequencies are shown in fig. 2. fig. 2a shows the relationship between the p-wave velocity and frequency. the p-wave velocity of the gritstone varies little at different frequencies, with an average of 1,651 m/s. the p-wave velocities of the fine sandstone, argillaceous siltstone and mudstone exhibited little variability, with average values of 2,840 m/s, 3,191 m/s and 4,214 m/s, respectively. fig. 2b shows the relationship between the s-wave velocity and frequency, and the resulting trends are similar to those for the p-wave velocity, but with smaller variations and average values of 1,233 m/s, 1,492 m/s, 1,778 m/s and 3,514 m/s for the four materials. by comparing the propagation velocity of sound waves in the four types of rocks, we found that both the p-wave and s-wave velocities successively increased at different frequencies. wave impedance properties of the coal seam roof because wave impedance inversion is important for identifying rock properties, it is a common method used to identify sandstone and mudstone based on the sound wave velocity and density log curves. figure 3 shows the p-wave impedance surface properties of the #3 coal seam roof in the study area. the depth of the roof was 10 m higher than the bottom boundary of the #3 coal seam, and the p-wave impedance varies between the roof and the boundary. according to the characteristics of the p-wave impedance of the roof, the impedance was divided into three types: yellow-red p-wave impedance (6,400 m/s·g/cc to 7,800 m/s·g/cc), blue p-wave impedance (7,800 m/s·g/cc-9,400 m/s·g/cc) and purple p-wave impedance (> 9,400 m/s·g/cc). the p-wave impedance, with a range of approximately 6,400 m/s·g/cc-10,000 m/s·g/cc, greatly differed and displayed obvious partitioning and zoning. gas content of the study area the gas properties in the study area were analyzed with a kriging interpolation method based on the gas contents of the boreholes in the #3 coal seam (fig. 4). the overall gas content in the area was high but considerably table 1. formulas for calculating the elastic parameters of the rock samples elastic parameter formula shear modulus g e v = ( )2 1 bulk modulus k e v = ( )3 1 2 longitudinal wave velocity v k g p = + ρ 4 3 transverse wave velocity v g s =  3d seismic data and gas content of the study area the 3d seismic data had a cmp mesh density of 5 m×10 m, with 16 stacked layers. first, the data underwent a series of processes, including static correction, amplitude preservation, denoising, deconvolution, and velocity analysis. then, post-stack wave impedance inversion was performed using the processed data. in terms of the stratum of the coal seam roof, the depth of the coal seam roof can be determined by observing the strong reflection interface formed by the coal seam and the surrounding rock. from the post-stack wave impedance inversion data, the p-wave impedance surface properties of the coal seam roof strata were extracted. to identify the coal seam roof lithology, the properties of the coal seam roof were correlated and matched with the product of the propagation velocity of the sound waves and the density of rock measured in the low-frequency experiments. using the gas contents measured in boreholes, the content difference and distribution were analyzed. the study area was 3.57 km long and 0.87 km wide, with an area of approximately 3.10 km2. the gas contents were collected from 20 wells, and measurements were obtained from cores via a direct analytic method for the #3 coal seam. when analyzing the gas content, a content distribution map was drawn using the kriging interpolation method according to the gas contents of the boreholes. figure 2. relationships between the propagation velocity of sound waves in the four types of rocks and the frequency 362 yunlan he, xikai wang, hongjie sun, zhenguo xing, shan chong, dongjing xu, feisheng feng varied. the study area (3.10 km2) exhibited a gas content ranging from approximately 19 m3/t to nearly 29 m3/t, a difference of nearly 10 m3/t. three sub-areas with gas contents greater than 27 m3/t were observed and are colored in red in figure 5. four sub-areas with gas contents lower than 20 m3/t were observed and are colored in yellow in figure 5. discussion wave impedance inversion can be used to analyze the properties of coal seam roof strata. wave impedance is the product of the propagation velocity of a sound wave in rock and the density of the rock, and it can be used to identify the lithology. however, sandstone and mudstone cannot be distinguished simply by wave impedance and can be influenced by many factors. moreover, the logging velocity was obtained at high frequencies in the process of wave impedance inversion. after well logging calibration was performed for the velocity and density log curves and 3d seismic data, wave impedance inversion was performed for data interpolation. low-frequency experiments were performed to obtain the propagation velocity of sound waves in four types of rock, and the velocity was used to calculate the theoretical wave impedance. the results did not match the wave impedance values obtained by 3d seismic inversion at higher frequencies. for example, murphy (1984) measured the propagation velocity of sound waves in sandstone at different frequencies using the resonance bar technique, and the results showed that the propagation velocity of sound waves in sandstone at 500 khz was 10-25% higher than that at 5 khz. tutuncu et al. (1998) measured the propagation velocity of sound waves in saline-saturated sandstone, and the p-wave velocity and s-wave velocity increased by 33% and 20%, respectively, as the frequency increased from 10 hz to 1 mhz. batzle et al. (2006) measured the p-wave velocity of high porosity and high permeability sandstone (with a porosity of 35% and a permeability of 8.7×10-12 m2) using a stress-strain method and ultrasonic measurement technique. the p-wave velocity of the 90% saturated sandstone at 0.8 mhz was 32% higher than that at 5-10 hz (batzle et al., 2006). these experiments confirmed that the propagation velocity of sound waves at seismic frequencies is lower than that at ultrasonic frequencies. according to the collected borehole data, the coal seam roof was mainly composed of gritstone, medium sandstone, fine sandstone, sandy mudstone, siltstone, and mudstone. in this study, the propagation velocity of sound waves was monitored at both low and ultrasonic frequencies (1 mhz). the p-wave velocity at low frequencies was lower than that at ultrasonic frequencies, but the p-wave velocities of rocks with different lithologies considerably differed. to better match the theoretical wave impedance calculated according to the experimental results, the p-wave velocity was increased by 20% to compensate for the difference in velocity due to the difference in frequency. the specific data are shown in table 2. figure 3. p-wave impedance properties of the #3 coal seam roof figure 4. gas content distribution in the study area table 2. the p-wave velocity and p-wave impedance of the rocks lithology density (g/cm3) vp-lf (m/s) vp-1mhz (m/s) vp-lf×(1+20%) (m/s) vp-lf×density (m/s·g/cc) (1.2×vp-lf)×density (m/s·g/cc) gritstone 2.30 1651 1805 1981 3797 4557 fine sandstone 1.95 2840 3854 3408 5538 6646 argillaceous siltstone 2.41 3191 3588 3829 7690 9228 mudstone 2.20 4214 5070 5057 9270 11125 *vp-lf is the vp value of the low-frequency condition, and vp-1mhz is the vp value of the 1 mhz frequency condition. 363coal seam roof: lithology and influence on the enrichment of coalbed methane the difference in wave impedance for different lithologies was essential for matching the theoretical wave impedance with the wave impedance of 3d seismic inversion. the p-wave velocity of the gritstone was low, possibly due to the existence of micro-fissures. the p-wave impedance did not match that based on 3d seismic inversion, and the weighted p-wave impedance of the gritstone was 4,557 m/s·g/cc. the p-wave impedance of fine sandstone was 6,646 m/s·g/cc, which fell within the scope of the yellow-red wave impedance (6,400 m/s·g/ cc-7,800 m/s·g/cc) when matching the wave impedance obtained by 3d seismic inversion. the p-wave impedance of argillaceous siltstone was 9,228 m/s·g/ cc, which is within the scope of blue wave impedance (7,800 m/s·g/cc-9,400 m/s*g/cc). additionally, the p-wave impedance of the mudstone exceeded 10,000 m/s·g/cc, which is within the scope of purple p-wave impedance (> 9,400 m/s·g/cc). the lithology of the identified coal seam roof was tested according to the known borehole data. the depth of the coal seam roof was 10 m higher than the bottom boundary of the coal seam, and 2000 m/s was selected as the average propagation velocity of the sound wave. fig. 5 shows the coal seam of the selected test well and the top and bottom lithology. the identified coal seam roof lithology was consistent with that revealed by the borehole data. in terms of lithological identification, sandstone could not be distinguished from fine sandstone, and sandy mudstone could not be distinguished from mudstone. however, the fine sandstone, siltstone and mudstone could be distinguished. in addition to coal seams, coal seam roof also influence the enrichment of coalbed methane. during the diagenetic evolution of coal, a ton of coal, from lignite to anthracite, can produce 268-393 m3 of gas (zhang & li, 1988), but only a fraction of this coalbed methane is eventually preserved. the coal seam roof directly contacts the coal seam and undergoes diagenetic evolution together with the coal seam, and the compactness of the coal seam roof has a direct influence on the residual coalbed methane content (li et al., 2003; liu, li, yang, liu, & yan, 2012). by comparing and analyzing the coal seam roof lithology and gas content, we found that the coal seam roof lithology is related to the enrichment of coalbed methane. the coal seam roof in the three areas with high gas contents was mainly mudstone and argillaceous siltstone, and the coal seam roof in the four areas with low gas contents was mainly fine sandstone. the mudstone and argillaceous siltstone, which are highly compact, were more conducive to the preservation of coalbed methane, and the fine sandstone, which is poorly compact, was not. conclusion in this paper, the variable propagation velocity of sound waves in four types of rock was measured at 3-100 hz using a low-frequency stress-strain method. the p-wave and s-wave velocities of the gritstone, fine sandstone, argillaceous siltstone, and mudstone successively increased at the same frequency. the propagation velocity of sound waves in the four types of rock at seismic frequencies was lower than that at higher frequencies. after the p-wave velocities obtained via testing were considered, the theoretical p-wave impedance of the four types of rocks was calculated, and the values were then matched with the wave impedance values of seismic inversion for the coal seam roof. the roof was mainly composed of mudstone, argillaceous siltstone, and fine sandstone, and the boreholes verified that the lithology. the lithology of the roof is related to the enrichment of coalbed methane. the roof in areas with high gas contents was mainly mudstone and argillaceous siltstone, and that in areas with low gas contents was mainly fine sandstone. compact areas of the coal seam roof were more conducive to the formation and preservation of coalbed methane. acknowledgments the project was financially supported by the subject of open fund of state key laboratory of coal resources and safe mining, china university of mining & technology, beijing (sklcrsm19zz01), the fundamental research funds for the central universities (2019qmo1) and the national natural science foundation of china (41602168). references ba, j. (2010). wave propagation theory in double-porosity medium and experimental analysis on seismic responses. scientia sinica (physica, mechanica & astronomica), 40(11), 1398-1409. batzle, m. l., han, d. h., & hofmann, r. (2006). fluid mobility and frequencydependent seismic velocity-direct measurements. geophysics, 71(1), n1-n9. biot, m. a. (1956). theory of propagation of elastic waves in a fluid-saturated porous solid. i. low-frequency range. the journal of the acoustical society of america, 28(2), 168-178. chen, t. j., cui, r. f., liu, e. r., & lang, y. q. (2006). prediction of coal seam methane enriched areas using seismic data. journal of china university of mining & technology (english edition), 16(4), 421-424. gao, l. j., tang, d. z., xu, h., meng, s. z., zhang, w. z., meng, y. j., & wang, j. j. (2012). geologically controlling factors on coal bed methane (cbm) productivity in liulin. journal of coal science and engineering (china), 18(4), 362-367. figure 5. the lithological column sections from the known wells 364 yunlan he, xikai wang, hongjie sun, zhenguo xing, shan chong, dongjing xu, feisheng feng groshong jr, r. h., pashin, j. c., & mcintyre, m. r. (2009). structural controls on fractured coal reservoirs in the southern appalachian black warrior foreland basin. journal of structural geology, 31, 874-886. hemza, p., sivek, m., & jirásek, j. (2009). factors influencing the methane content of coal beds of the czech part of the upper silesian coal basin, czech republic. international journal of coal geology, 79, 29-39. hungerford, f. (2013). in-seam directional drilling for improving safety and productivity in underground coal mines. journal of mines, metals and fuels, 61(7-8), 213-218. kędzior, s. 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(2012). differential acoustic resonance spectroscopy for the acoustic measurement of small and irregular samples in the low frequency range. journal of geophysical research atmospheres, 117(b6), 6203. wei, x., wang, s. x., zhao, j. g., & deng, j. x. (2015a). laboratory investigation of influence factors on vp and vs in tight sandstone. geophysical prospecting for petroleum, 54(1), 9-16. wei, x., wang, s. x., zhao, j. g., tang, g.y., & deng, j. x. (2015b). laboratory study of velocity of the seismic wave in fluid-saturated sandstones. chinese journal of geophysics, 58(9), 330-3388. white. j. e. (1975). computed seismic speeds and attenuation in rocks with partial gas saturation. geophysics, 40(2), 224-232. zhang, w & li, x. (1988). analysis of the hydrocarbon-producing potential of main coal measures source rocks in china. coal geology & exploration, 16(3), 31-37. zhou, q. q. (2013). geological control law of coalbed methane accumulation of heshun block in qinshui basin. china coalbed methane, 10(2), 12-15. keywords: overhead mining; floor roadway; mining-induced stress; roadway stability. palabras clave: minería aérea; superficie de calzada; esfuerzo inducido por la minería; estabilidad de calzada. how to cite item wang, p., jiang, l., ma, c., & yuan, a. (2020). evolution laws of floor stress and stability of floor roadway affected by overhead mining. earth sciences research journal, 24(1), 45-54. doi: https://doi.org/10.15446/esrj. v24n1.67925 based on the geological conditions of a drainage roadway in the xinglongzhuang coalmine of shandong province (china), a mechanical model of a working face for overhead mining over a roadway was established, and the laws influencing the mining-induced stress were obtained. the evolution of mining-induced stress in the floor with different horizontal distances between working face and floor roadway was examined using universal distinct element code (udec) numerical simulation; the stability of the floor roadway was analyzed. the results of the numerical simulation were verified via on-site tests of the deformation of surrounding rocks and bolt pull-out readings from the drainage roadway. the results indicated that the floor roadway was significantly affected within a certain depth and then the effect weakened with the depth increasing. in view of the roadway stability, the roadway was observed with worse stability of surrounding rocks during the period of the working face mining near the roadway or just passing the roadway; moreover, this exploitation was not safe and needed to strengthen the support for the roadway. the results may provide a scientific basis for determining reasonable locations and support for roadways in xinglongzhuang coalmine and other coal mines with similar coal measures strata. abstract evolution laws of floor stress and stability of floor roadway affected by overhead mining leyes de evolución en el esfuerzo del suelo y de estabilidad de la superficie de una calzada afectada por minería aérea issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.67925 este artículo estableció un modelo mecánico de un frente de trabajo aéreo sobre una calzada con base en las condiciones geológicas de la calzada de drenaje de la mina de carbón xinglongzhuang y se obtuvieron los factores determinantes del esfuerzo inducido por la minería. parte del estudio examinó la evolución del esfuerzo inducido por minería en el suelo con diferentes distancias horizontales entre el frente de trabajo y la superficie de la calzada a través de una simulación numérica basada en el código de elemento distintivo universal (udec, del inglés universal distinct element code). la estabilidad del suelo también fue analizada. los resultados de la simulación numérica se verificaron a través de pruebas de campo de la deformación de las rocas circundantes y de las muestras de la calzada de drenaje. los resultados indican que la superficie de la calzada fue afectada significativamente hasta cierta profundiad y luego el efecto se debilitó a medida que la profundidad se incrementa. los resultados muestran que la estabilidad de la calzada registró el peor índice de las rocas circundantes durante el tiempo en que el frente de trabajo estuvo cerca de la calzada o tuvo que pasar por la calzada; lo que significó que esta explotación no es segura y necesitó el fortalecimiento de los soportes de la calzada. los resultados podrían proveer una base científica para determinar las ubicaciones razonables y los soportes adecuados para calzadas en la mina xinglongzhuan y en otras minas de carbón con medidas de capas similares. resumen record manuscript received: 24/08/2018 accepted for publication: 25/10/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 45-54 pu wang1, lishuai jiang1*, changqing ma1, anying yuan2 1department of resources and civil engineering, shandong university of science and technology, tai’an, 271019, china 2state key laboratory of mining disaster prevention and control co founded by shandong province nd the ministry of science and technology, shandong university of science and technology, qingdao 266590, china * corresponding author: skwp0701@hotmail.com g eo lo g ic al e ng in ee ri ng 46 pu wang1, lishuai jiang, changqing ma, anying yuan introduction given the increase in the demand for coal, there is an increase in the depth and intensity of mining activities. in order to improve the recovery rate of coal resources and increase their economic benefits, overhead mining technology is widely used. it includes several advantages, such as a low drivage ratio, rational exploitation of resources, and stress-relaxed state for the roadway, and is especially applied to short-distance coal seams and thick coal seams (kang et al., 2011; wang et al., 2017). coal mining induces a stress readjustment and stress transfer to the deeper part of the floor. overhead mining significantly affects the floor roadway, and high mining stress concentrates in the surrounding rocks of the roadway to a certain depth, which induces serious deformation such as roof falls, floor heaves, and rib spalling (wang et al., 2015; wang et al., 2017; liu et al., 2017). several on-site photos of roadway instability are shown in figure 1. the stress and deformation of the floor roadway induced by mining activities differ with the position during overhead mining. hence, it is important to examine the evolution laws of the mining stress on the floor, and the stress distribution and failure of the floor roadway to select the location and roadway parameters. several studies have used different methods to focus on the distribution of the mining stress on the floor and the deformation-failure of floor roadway. the stress distribution and its propagation laws on the floor were studied by means of elastic mechanics theory, and the deformation characteristics of surrounding rocks of a floor roadway were obtained (yin et al., 1992; yuan et al., 2016). the distribution laws of the mining stress in floor and its additional stress were analyzed by utilizing numerical simulations and physical simulations, and reasonable parameters for a floor roadway layout were obtained (gao et al., 2015; ma et al., 2015; liu et al., 2016; feng et al., 2016). in terms of the deformation and stability of a roadway, the deformation characteristics of the surrounding rocks, such as the floor heave and roof fall of the roadway, were examined using on-site observations, physical simulations, and numerical simulations (islam and shinjo, 2009; li et al., 2016; jiang et al., 2016, 2017a). given the instability of roadways with a soft interlayer, the physical and chemical properties, along with the engineering mechanical properties of the soft interlayer were studied using rock mechanical tests, physical simulations, numerical simulations, and theoretical analysis. moreover, the instability characteristics and failure mechanism of a roadway with a soft interlayer were analyzed (li, 2011). because of the heterogeneity and anisotropy of the coal measures strata in different coal mines, as well as discrete factors such as interface joints, the commonly used method (i.e., theoretical analysis or empirical formula) can not accurately reflect the engineering problems that we intend to study. hence, based on the geological conditions of a drainage roadway in xinglongzhuang coalmine of shandong province (china), a mechanical model of a working face for overhead mining over the roadway is established. the influence of mining stress on the surrounding rocks of the roadway for different spatial positions of the working face and floor roadway are studied in detail by utilizing a universal distinct element code (udec) numerical simulation, and the danger area of the roadway layout under this coal measures strata condition is determined. this may serve as a scientific basis for floor roadway selection, which is affected by overhead mining in xinglongzhuang coalmine or its surrounding coal mines. it may also provide some roadway support experience and supporting parameters for similar coal measures strata. stress analysis of floor roadway affected by overhead mining geological conditions of overhead mining the drainage roadway under coal seam no. 3 of the shanxi group in the 10th district of the xinglongzhuang coalmine has a depth of approximately 606 m, which is close to coal seam no. 3. the surrounding rocks are primarily carbonaceous mudstone, and the internal fissures are extremely well developed. serious damage, including cracking and failure of the surrounding rocks of the roadway, had been induced by the overhead mining of panel 10302, which was in need of repair and support prior to reuse. mechanical analysis of floor stress affected by overhead mining given the relationship between the advancing direction of working face and floor roadway, the overhead mining was divided into mining with the advancing direction of the working face parallel to the roadway and mining in which the advancing direction was perpendicular to the roadway. hence, the effects of mining on the floor roadway differed. on-site observations indicated that the mining stress in the surrounding rocks of the floor roadway was affected by overhead mining when the advancing direction perpendicular to the roadway exceeded that of the other type of overhead mining, and the mining influence of the former was more evident (dai et al., 1999; yuan et al., 2016). hence, a mechanical model of overhead mining when the advancing direction was perpendicular to the roadway was established, and the laws influencing the mining stress of a floor roadway with different layers were analyzed. front abutment stress can be formed ahead of the coal wall because of the mining activities. in order to solve the problem conveniently, the front abutment stress could be simplified and deemed as a uniform load according to the reference written by lu et al. (2017). the distribution zone of the front abutment stress approximately corresponded to a rectangle with a length l and width w. the overlying load was viewed as a uniform load q, as illustrated in figure 2a. when the peak value of the abutment stress was just above the middle axis of the roadway, the mechanical model was simplified as a plane stress model, as shown in figure 2b (dai et al., 1999), and the vertical stress component σh of the surrounding rocks of the roadway was obtained as follows: σ πh q l h lh l h = − + +    2 4 4 2 2arctan (1) where q denoted the average value of front abutment stress (mpa); l denoted the length of the working face (m), according to the geological report of the working face, l is 160 m; and σh denoted the vertical stress component of surrounding rocks of the roadway (mpa); h denoted the vertical distance between the working face and the floor roadway. figure 1. the deformation of the floor roadway is affected by mining activities (li, 2011). http://fanyi.baidu.com/#auto/auto/drivage ratio http://fanyi.baidu.com/#auto/auto/drivage ratio http://fanyi.baidu.com/#auto/auto/rectangle 47evolution laws of floor stress and stability of floor roadway affected by overhead mining maintained at about 23mpa. moreover, the curve slope that could reflect the attenuation rate of the stress in this range was very small, so it was in the low attenuation rate state and the roadway was affected by mining activities notably due to the high stress. with an increased in h, the σ0 h linearly increased, while the σh decreased when 40 m < h ≤ 160 m. moreover, the attenuation rate of the vertical stress decreased notably due to the curve slope obviously increasing. hence, the difference between σh and σ0 h decreased. these indicated that the effect of mining activities on the floor roadway weakened. the relationship between the vertical stress σh and vertical distance h between the working face and the floor roadway was obtained by curve fitting, as shown in eq. (3), and a high fitting accuracy was observed. the expression was as follows: h h h r= − × − × + =− −1 58 10 1 28 10 23 20 0 9774 2 3 2. . . . (3) evolution of mining stress in floors affected by overhead mining establishment of udec numerical model the numerical simulation methods commonly used in engineering and theoretical analysis can effectively solve complex engineering problems. the udec is a two-dimensional discrete element calculation program for based on the geological report of the working face, the mining depth h was approximately 566 m. thus, the in situ stress σ corresponding to level 566 m was calculated by eq. (2) as follows: σ γ= = × =h 25 5 566 14 43. .kn/m m mpa3 (2) where γ denoted the unit weight of the rock mass. in the present study, the unit weight of the overlying rock mass was assumed to be 25.5 kn/m3 (sainoki and mitri, 2015; wang et al., 2018). hence, the average value of the front abutment stress q was 23.09 mpa because the average value of the abutment stress concentration factor was 1.6, as indicated by the geological report of the working face. thus, the vertical stress σh of the surrounding rocks of the roadway with different positions was calculated by eq. (1), and the values were listed in table 1. table 1. vertical stress σh of surrounding rocks of the roadway with different positions. h(m) σh (mpa) σ0 h (mpa) 10 23.09 14.69 20 23.08 14.94 40 22.96 15.45 60 22.66 15.96 80 22.16 16.47 100 21.49 16.98 120 20.69 17.49 140 19.81 18.00 160 18.90 18.51 180 17.99 19.02 as shown in figure 3 and table 1, when the vertical distance between the working face and floor roadway was less than 160 m (i.e., 0 m < h ≤ 160 m), the vertical stress σh of the surrounding rocks for the roadway always exceeded the corresponding in situ stress σ0 h (i.e. the in situ stress of level which corresponds to different positions of the floor roadway). this indicated that the mining activities in this range affected the floor roadway and the propagation depth of the mining stress was about 160 m. when 0 m < h ≤ 40, the value of σh was greatly larger than its corresponding in situ stress σ0 h and figure 2. a mechanical model for overhead mining: (a) spatial model and (b) planar model. (a) (b) figure 3. vertical stress σh of surrounding rocks of the roadway with different positions. 48 pu wang1, lishuai jiang, changqing ma, anying yuan discontinuous media that allows large deformation sliding, rotation, and detachment along the joint surface of a discrete rock mass. the structure of the overlying strata and distribution of the mining stress in the roof and floor are clearly reproduced. thus, it satisfied the needs of the present study (shnorhokian et al., 2014). based on the geological conditions of panel 10302 in xinglongzhuang coalmine, a udec numerical model was established with respect to the overhead mining and floor roadway. the thicknesses of the roof and floor of panel 10302 were simplified and adjusted to satisfy the research requirements. given the boundary effect, a 340 m (length) × 150 m (high) calculation model was established, as shown in figure 4. the mining depth and thickness of the coal seam in the model were 500 m and 10 m, respectively. the vertical distance between the coal seam and floor roadway was 5 m, and the horizontal distance from the left boundary of the model was 150 m. in the mine-wide model, meshes were more densely discretized around the coal from which the stopes were extracted compared to those in areas near the model boundary. thus, the discretization method made it possible to simulate the stress state in an active mining area as accurately as possible using the available computation capability (hofmann and scheepers, 2015; sainoki and mitri, 2015). figure 4. a numerical model for calculation. the mechanical properties assigned to the blocks and joints of the model were conventionally derived from laboratory testing programs (yu et al., 2016; jiang et al., 2017b). the physical and mechanical parameters of the rock mass derived from the geological report and rock mechanical test report of the working face employed in the model were listed in table 2. the mohrcoulomb model was used to simulate the continuous part of the rock because of the elastic and plastic deformation of the overlying strata, and the joint surface contact–coulomb slip model was used to simulate the joint calculation (itasca, 2005; sainoki and mitri, 2014). the full displacement constraint boundary was used at the bottom of the model, and the horizontal displacement constraint and vertical displacement free conditions were used at the left and right sides, respectively. the top corresponded to a free boundary. the simulated depth of the coal seam was 500 m, and the roof height of the model was 30 m. thus, the simulated failure of the overlying strata with a height of 470 m was assumed to be a result of the vertical stress that acted on the top of the numerical model. hence, the compensated load σv could be calculated by eq. (2) as follows: σ γv vh= = × =25 5 470 11 99. .kn/m m mpa3 where hv denoted the height of the failed simulated strata. in the present study, the height of the failed simulation was set at 470 m. according to the geological report from the xinglongzhuan coalmine, the value of the horizontal stress applied to the two sides of model with the trapezoidal distribution was half of the value of vertical stress. evolution of mining stress in floors affected by overhead mining the floor roadway was significantly influenced by overhead mining within the range of 0 m < h ≤ 40 m, as discussed in section 2, but the effect decreased with an increase in h. hence, in this study, based on the condition of the floor roadway located below a working face of 5 m, the laws influencing the mining stress in floor of the roadway (as affected by the overhead mining) were examined. the horizontal distance between the working face and floor roadway was defined as ld, corresponding to 60 m, 40 m, 20 m, 0 m, -20 m and -60 m (where the negative value indicates the working face had passed the roadway) as shown in figures 5 – 10. abutment stress was not only concentrated in front of the coal wall, but also would transfer to the bottom of coal floor affected by mining activities. as shown in figures 5 and 6, when the horizontal distance between the working face and floor roadway were 60 m and 40 m (i.e., ld = 60 m, 40m), the abutment stress was formed ahead of coal wall with 56 m and 47 m, respectively. however, the floor roadway was beyond this range when ld = 60 m which indicated that it was not affected by overhead mining; thus the surrounding rocks of floor roadway did not fail. when ld =40 m, the floor roadway was in the margin of the abutment stress-affected zone with the small stress concentration factor which reflected the degree of stress concentration (stress concentration factor was the ratio of concentration stress to in situ stress); thus, only the roadway roof failed. hence, it indicated that the effect of overhead mining on the floor roadway was slight and the roadway support did not need to be strengthened when ld≥40 m. from figures 7 and 8, as the working face gradually approaching the roadway, the effect of overhead mining on the roadway enhanced and the floor roadway was located in the stress significant influence area with a high-stress concentration factor of 1.6; thereby the surrounding rocks of roadways seriously destroyed, and plastic failure distributed around the roadway. hence, when 40m>ld≥0 m, the effect of overhead mining on the floor roadway enhanced notably by the high concentration stress and the roadway support should be paid more attention to. from figure 9, when 0 m>ld≥-20 m (the negative value indicated that the working face had passed the floor roadway), the working face was passing through the roadway and the roadway was beneath the goaf of the working face; thereby locating in a stress-relaxed zone with a small stress concentration factor of 0.2 when ld=-20 m. it meant that the effect of overhead mining on the floor roadway decreased gradually due to the small vertical stress. however, during this progress of mining operation, the roadway had experienced the effect of table 2. mechanical properties of the rock mass. lithology thickness (m) density (kg.m-3) bulk (gpa) shear (gpa) poisson ratio tensile strength (mpa) friction angle (º) siltstone 18 2530 1.34 0.76 0.2 3.08 37 medium sand 10 2520 1.83 1.15 0.18 7.00 41 mudstone 2 2450 0.98 0.71 0.24 1.80 31 coal seam 10 1350 1.05 0.65 0.28 1.50 20 siltstone 15 2530 1.34 0.76 0.28 3.08 37 medium sand 20 2520 1.83 1.15 0.18 7.00 41 mudstone 75 2340 0.98 0.71 0.24 1.80 31 49evolution laws of floor stress and stability of floor roadway affected by overhead mining figure 5. floor stress affected by overhead mining when ld = 60 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. figure 6. floor stress affected by overhead mining when ld = 40 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. figure 7. floor stress affected by overhead mining when ld = 20 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. figure 8. floor stress affected by overhead mining when ld = 0 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. (a) (b) (a) (b) (a) (b) (a) (b) 50 pu wang1, lishuai jiang, changqing ma, anying yuan the largest stress and then the effect decreased with the stress dropping; thereby inducing serious failure of surrounding rocks of roadway. hence, in this stage, even though the roadway located in the goaf-affected zone gradually, it had experienced the notable effect of the largest stress, hence the serious failure was observed and the range of plastic failure increased as shown in figure 9(b). as shown in figure 10, when -20 m>ld≥-60 m, as the goaf zone increased, the vertical stress in the goaf recovered and increased with constant compaction of migrated overlying strata. the effect of the stress on the floor roadway increased gradually and the stress concentration factor increased to 1.2 which indicated the floor roadway re-located in the stress increasing zone when ld=-60 m. hence, the vertical stress in the goaf re-affected the roadway. this effect by the vertical stress in recovery zone of the goaf was often neglected although it might influence the roadway significantly, the failure of surrounding rocks of roadway aggravated again as shown in figure 10(b). hence, when -20 m>ld≥-60 m, the roadway was affected by the recovered stress in the goaf and the failure of roadway aggravated further which indicated that the effect of the recovered stress on the roadway should not be ignored and necessary support should be considered with respect to the roadway. hence, according to the aforementioned analysis, the evolution laws of the vertical stress and the effect on the floor roadway were obtained, as shown in table 3 and figure 11. the entire process of overhead mining with respect to the effect on the floor roadway was divided into four stages. stage i: 40 m ≤ ld ≤ 60 m. the stress concentration factor of the rocks surrounding the roadway exceeded 1 and rose slightly with a decrease in ld. hence, the effect of overhead mining on the floor roadway was slight and the roadway did not need strengthened support. stage ii: 0 m ≤ ld < 40 m. the stress concentration factor of the rocks surrounding the roadway significantly increased and tended to be stable with a decreased in ld, and the maximum value increased to 1.6. hence, the effect of overhead mining on the floor roadway enhanced notably affected by high concentration stress and the roadway support should be paid more attention to. stage iii: -20 m ≤ ld < 0 m. the roadway was in the stress-relaxed state and the stress concentration factor significantly decreased with the working face passing through the roadway. however, it had experienced the notable effect of the largest stress during this stage, hence the serious failure of roadway was observed and the range of plastic failure increased. thus the floor roadway support should be more strengthened. stage iv: -60 m ≤ ld < -20 m. the abutment stress in the goaf corresponded to recovery and increased with the constant compaction of the migrated overlying strata. the floor roadway re-located in the stress increasing zone. hence, the roadway was affected by the recovered stress in the goaf and the failure of roadway aggravated further which indicated that the effect of the recovered stress on the roadway should not be ignored and necessary support should be considered with respect to the roadway. figure 9. floor stress affected by overhead mining when ld = -20 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. (a) (b) figure 10. floor stress affected by overhead mining when ld = -60 m: (a) distribution of floor stress; (b) plastic zone of coal roof and floor. (a) (b) table 3. stress concentration factor of rocks surrounding of floor roadway. ld (m) stress concentration factor 60 1 40 1.2 20 1.6 0 1.6 -20 0.2 -40 0.3 -60 1.2 51evolution laws of floor stress and stability of floor roadway affected by overhead mining engineering case a significant effect of overhead mining on the floor roadway was observed in the above analysis of the theory and numerical simulation. hence, onsite observations of the deformation and failure of a drainage roadway in xinglongzhuang coalmine were used to verify the previously mentioned results. deformation monitoring of roadway. in this study, the deformation monitoring of the roadway mainly included the relative displacement of roof to floor, which reflected the vertical deformation, and the relative displacement of two ribs, which represented the horizontal deformation. hence, seven sections of the roadway (marked as 1–7), which corresponding positions were shown in figure 12, were surveyed at 30 m intervals along the roadway with the mining range of 300 m. make a sectional plane at the midpoint of working face, and obtain the three states between the working face and the floor roadway, including ahead of floor roadway, right above floor roadway and rear of floor roadway, as shown in figure 13. figure 14a shows the the monitoring points of surrounding rock deformation for the roadway section. table 4 and figure 14 showed the final results of deformation survey for the floor roadway within the mining range of 300 m when the working face had passed through the roadway and the roadway was no longer affected by mining operation. hence, it can be seen the vertical and horizontal deformations of the drainage roadway in sections 1–4 were large, with both values approximately 1100–1300 mm. this was because the roadway in this testing range was notably affected by the highly vertical stress of panel 10302 mining, and a large-scale plastic failure zone was formed in the rocks surrounding the figure 11. relationship between ld and stress concentration factor. figure 12. plan of panel 10302 and the layout of roadway sections. (a) ahead of roadway (b) right above roadway (c) rear of roadway figure 13. three states related the working face and the floor roadway. table 4. results of roadway deformation. sequence of sections relative displacement of roof to floor convergence (mm) relative displacement of two ribs convergence (mm) no.1 1156 1203 no.2 1321 1078 no.3 1210 1135 no.4 1098 1254 average value 1196 1167 no.5 701 743 no.6 621 703 no.7 598 664 average value 640 703 52 pu wang1, lishuai jiang, changqing ma, anying yuan roadway. thus, the strength of the surrounding rocks decreased, and large deformation was observed in the roadway. moreover, many fractures formed in the floor strata because of the mining activities. this resulted in seepage from the goaf penetrating the floor, which further reduced the strength of the surrounding rocks. however, the deformations of sections 5–7 were relatively small, with values of 600–700 mm. according to the geological report for the xinglongzhuang coal mine, this testing range (sections 5–7) was affected by mining activities slightly compared to that of sections 1–4, and the deformation was mainly contributed by the weakening effect of the seepage from the goaf. thus, the deformations in sections 5–7 were much smaller than those of sections 1–4. hence, it can be concluded that the effect of overhead mining on the floor roadway was significant, and the deformation of the surrounding rocks increased notably. so this exploitation is not safe and the support for the roadway should be strengthened. in order to study the relationship between the roadway deformation and the mining activities in detail, section 2 which was notably affected by mining were selected; and then the deformation of roof-floor and two ribs with the mining range of 300 m was surveyed. figure 15 showed the variation curves of deformation for the roadway with mining activities. as can be seen from figure 15, the variations of roof-floor convergence and two ribs convergence of section 2 were similar and showed obvious stage characteristics during the face progress. when the horizontal distance between working face and floor roadway exceeds 47 m, the roadway was affected by mining activities slightly, so both the roof-floor convergence and two ribs convergence were small and had no much change, as shown in figure 15 with stage (a), which the pressure appearance from the filed observation was similar to the simulation results with 40 m ≤ ld ≤ 60 m in section 3.2. when the horizontal distance between working face and floor roadway was in range of -34 m ~ 47m (the negative value indicated that the working face had passed the floor roadway), the deformation of roadway occurred significantly due to the violet effect of mining activation, resulting in both the displacement of rib-floor and two ribs convergence increasing sharply, as shown in figure 15 with stage (b). hence, compared with the fore-mention numerical simulation, the pressure appearance of filed observation was similar to the simulation results with -20 m ≤ ld < 40 m in section 3.2. when the working face passed through the floor roadway exceeds 34 m, the floor roadway was under the goaf of panel 10302, and the effect of mining operations on the roadway reduced gradually where roadway failure was no longer aggravated. hence, both the roof-floor convergence and two ribs convergence rose slightly and tended to be stable, as shown in figure 12 with stage (c). however, the both displacements were large and peak values in survey range were 1321 mm and 1078 mm (shown in table 4) respectively. at this stage, the pressure appearance of filed observation was similar to the simulation results with -60 m ≤ ld < -20 m in section 3.2. pull-out test analysis of bolts. based on the stress characteristics of the bolts, it was possible to assess the stability of the surrounding rock and the rationality of the bolt support of the roadway. hence, pull-out tests of the existing bolts used for the drainage roadway were performed using a pull-out device (zy-30), in which the maximum pull-out force was 300 kn. three bolts (distributed in the roof and two ribs) were arranged in each section for the test, as shown in figure 16a. the results of the pull-out tests are shown in table 5 and figure 16b. as shown in table 5 and figure 16, the pull-out force for most of the bolts was unable to satisfy the minimum requirement of 50 kn as indicated in the geological report of xinglongzhuan coalmine. the qualified rates for the bolts in the roof and two ribs corresponded to 42.9%, 28.6%, and 42.9%, respectively, with an average value of only 38.1%. this indicated that the existing support of the roadway had a poor performance during the overhead mining of panel 10302. based on the study of lu et al. (1998), an important reason for a low bolt pull-out force was an inability to fully utilize the anchoring capacity of the bolt system (i.e., in fractured surrounding rock, the existence of more cracks lead to bolt failure) or the existence of serious damage to the surrounding (a) layout of monitoring points (b) deformation of roadway figure 14. results of roadway deformation. figure 15. deformations of section 2 of roadway with the working face mining. 53evolution laws of floor stress and stability of floor roadway affected by overhead mining rock, which prevented the bolt from reaching its anchorage capacity. however, according to the results of the roadway deformation monitoring, the overhead mining of panel 10302 significantly affected the floor roadway, many fractures formed in floor strata, and the deformation of the surrounding rocks increased notably. the strength of the surrounding rocks decreased. hence, the bolts might not have fully reached their anchoring capacity as a result of the overhead mining, which produced a low pull-out force for the bolts. thus, the on-site results for the surrounding rock deformation and the pull-out tests of the existing bolts of the drainage roadway indicated that the floor roadway was affected by overhead mining significantly and its deformation-failure were serious; thus more attention should be paid on the similar coal measures strata. additionally, the observation results and the numerical simulation results showed a good agreement which can further explain rationality of numerical model. conclusions in this study, the laws influencing the mining-induced stress in different position of a roadway and the evolution of the mining-induced stress in floor with different horizontal distances between the working face and the floor roadway were examined to investigate the influence of overhead mining on a floor roadway. a mechanical model of a working face for overhead mining over a roadway was established, and a numerical model was generated using udec software. it was found that the depth influencing the mining stress on the floor corresponded to 160 m. thus, the mining stress was high and slowly decreased within a range of less than 40 m, through which the floor roadway was significantly affected, while the stress significantly decreased within a range exceeding 40 m, through which the effect of the stress on the roadway evidently decreased. the mining stress on the floor gradually increased, which led to an increase in the failure of the rocks surrounding the floor roadway within 0 m ≤ ld ≤ 60 m, and the effect was particularly evident in a range of 0 m ≤ ld ≤ 40 m. although the floor roadway was in a stress-relaxed state, the worse stability of the surrounding rock was observed in a range of -20 m ≤ ld < 0 m. the floor roadway was affected by the recovery of the abutment stress in the goaf when -60 m ≤ ld < 20 m, and thus attention should be focused on strengthening the support. finally, the surrounding rock deformation and bolts pull-out tests of a drainage roadway in xinglongzhuang coalmine were used to verify the results of the numerical simulation. the results could be utilized to serve as a scientific basis for floor roadway selection, which was affected by overhead mining in the xinglongzhuang coalmine or its surrounding coal mines and might also provide some roadway support experience and supporting parameters for similar coal measures strata. acknowledgments: this study was funded by the national natural science foundation of china (no. 51574155, no. 51504145, no. 51704182 and no. 51704185), the natural science foundation of shandong province (no. zr2017bee050 and no. zr2017bee045), the shandong provincial key r&d plan (public welfare special program) of china (no. 2017ggx20125), scientific research foundation of shandong university of science and technology for recruited talents (no. 2015rcjj057), tai’an science and technology development plan of shandong province (no. 2015zc1058), science and technology innovation fund of the college of the mining and safety engineering, shandong university of science and technology (no. kykc17008). references dai, j., li, h., li, f. h., & liu, t. x. 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(2016). across mechanical analysis and control technology of surrounding rock of roadway under the influence of dynamic pressure. metal mine, 45(2), 47–50. http://www.sciencedirect.com/science/article/pii/s1631072117301249 http://www.sciencedirect.com/science/article/pii/s1631072117301249 http://www.sciencedirect.com/science/article/pii/s1631072117301249 keywords: soft-sediment deformation structure, sand body architecture, chang 6 oil member, yanchang formation, ordos basin. palabras clave: estructuras de deformación de sedimentos blandos; arquitectura de sedimentos blandos; miembro petrolífero chang 6; formación yanchang; cuenca ordos. how to cite item liang, q., tian, j., wang, f., & zhang, x. (2019). soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic yanchang formation, southwestern ordos basin, china. earth sciences research journal, 23(2), 119-126. doi: https://doi.org/10.15446/esrj.v23n1.69760 soft-sediment deformation (ssd) structures of the upper triassic yanchang formation are laterally widespread in the ordos basin. in the huachi-qingyang (h-q) area of the ordos basin, the chang 6 oil member of the upper triassic yanchang formation is among the most significant mesozoic oil-bearing strata. it is characterized by the development of reservoir sand bodies. during the depositional evolution of the chang 6 oil member, ssd structures induced by paleo-seismic events developed in the h-q area in the middle of the basin. the ssd structures developed in the sand bodies of the chang 6 oil member are mainly ball-and-pillow structures, fold structures, sand dikes, irregular convolute stratifications, and synsedimentary faults. the architecture of the sand bodies resulted from paleo-seismic events and gravity slumping and mainly include two types of structures: i) ssd structures driven by paleo-seismic events with delta front sand body (sd-ssd) and ii) ssd structures driven by paleo-seismic events with turbidites (formed by delta-front slumping and re-distribution due to seismic action) (st-ssd). as a consequence, genetic models of the sand bodies formed by different sedimentation processes are established. abstract soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic yanchang formation, southwestern ordos basin, china estructuras de deformación de sedimentos blandos y arquitectura de cuerpos de arena en el miembro de petróleo chang 6 de la formación yanchang del triásico superior, al suroeste de la cuenca ordos de china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.69760 las estructuras de deformación de sedimentos blandos de la formación yanchang del triásico superior están extendidas lateralmente en la cuenca ordos. en el área huachi-qingyang de esta cuenca, el miembro de petróleo chang 6 está entre los estratos petrolíferos más significativos del mesozoico. este se caracteriza por el desarrollo de cuerpos de arena en el yacimiento. durante la evolución deposicional del miembro chang 6, las estructuras de deformación de sedimentos blandos se desarrollaron en el área huachi-qingyang, en el medio de la cuenca, inducidas por eventos paleosísmicos. las estructuras de deformación de sedimentos blandos que se desarrollaron en los cuerpos de arenas del miembro de petróleo chang 6 son principalmente estructuras esféricas y acojinadas, estructuras plegadas, canales de arena, estratificaciones irregulares enrolladas y fallas sinsedimentarias. la arquitectura de los cuerpos arenosos son el resultado de eventos paleosísmicos y la caída por gravedad; se reconocen principalmente dos tipos de estructuras: 1. estructuras de deformación de sedimentos blandos por eventos paleosísmicos con cuerpos arenosos del frente del delta. 2. estructuras de deformación de sedimentos blandos traídas por eventos paleosísmicos con turbiditas (formadas por la caída del frente del delta y redistribuidas debido a la acción sísmica). como consecuencia, se establecieron los modelos genéticos de los cuerpos arenosos formados por diferentes procesos de sedimentación. resumen record manuscript received: 25/05/2018 accepted for publication: 08/03/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 119-126 qingshao liang1, 2, jingchun tian1, 2*, feng wang1, 2, xiang zhang1, 2 1institute of sedimentary geology, chengdu university of technology, chengdu 610059, china; 2state key laboratory of oil and gas reservoir geology and exploitation, chengdu university of technology, chengdu 610059, china; * corresponding author: tjc@cdut.edu.cn pe tr o le um g eo lo g y 120 qingshao liang, jingchun tian, feng wang, xiang zhang introduction soft-sediment deformation (ssd) structures and the architecture of sand bodies can provide information on depositional history and reveal aspects of basin evolution. ssd structures are related to liquefaction and fluidization. liquefaction and fluidization can be induced by many natural processes, including earthquakes, overloading, unequal loading, and impulsive stresses (allen,1982; owen, 1987). whatever the trigger mechanism, the cause of the ssd structures can be attributed to a decrease in shear resistance in watersaturated and unconsolidated sediments (moretti and sabato, 2007). the intensity of the trigger mechanism, the physical properties of the sediments, and the sedimentary environment determine the types and scales of ssd structures. because of the diversity of trigger mechanisms, it is particularly important to determine their origin. thus, is it possible to distinguish ssd structures produced by seismic and non-seismic causes? the distinguishing criteria, proposed and elucidated by sims (1975), owen and moretti (2011) and liu et al. (2016) used to identify paleo-earthquakes are as follows: (i) ssd structures are deformed in a tectonically active basin near an active fault, (ii) ssd structures occur in the vertical direction, separated by undeformed beds, and are developed continuously in the lateral direction, (iii) the morphologies of ssd structures are comparable to ssd structures confirmed from active earthquake belts, (iv) ssd structure types occur in complex combinations, (v) both deformed and undeformed beds develop in similar lithologies and depositional environments (montenat and sabato, 2007; owen and moretti, 2011; liu et al., 2016). ssd structures related to seismically induced liquefaction or fluidization are seismites (seilacher, 1969, 1984). layers that record paleo-earthquakes have been analyzed by many researchers and reported from many areas in various sedimentary environments, particularly lacustrine (scott and price, 1988; rodríguez-pascua et al., 2000; becker et al., 2002). the presence of coarse sediments can be indicative of rapid deposition in a slumping environment. coarse sediments mainly develop via processes of mass transport, e.g., slumping, sandy debris flows, and turbidites, or with oversteepening of depositional slopes (owen and moretti, 2011; oliveira et al., 2011; van loon and pisarska-jamrozy, 2014). rapid sedimentation can only be preserved where the overlying sediments are thick and massive and have a uniform grain size. the theory of seismogenesis has been frequently quoted as the most appropriate for the interpretation of numerous ssd structures (papadopoulos and lefkopoulos g, 1993; tuttle and schweig, 1996; alfaro et al., 2002; bachmann and aref, 2005; montenat et al., 2007; zhang et al., 2007; obermeier, 2009; ghosh et al., 2010; moretti and ronchi, 2011; brandes and winsemann, 2013; van loon and pisarska-ja-mrozy, 2014; liu et al., 2016 ; onorato et al., 2016). this study aims to identify typical ssd structures in the chang 6 oil member of the yanchang formation of the southwestern ordos basin, based on a large number of research results on the characteristics of seismites, include their identification marks and classification, and analysis of the types and characteristics of the ssd structures and their sedimentary environments (montenat, 2007). moreover, the difference between ssd structures formed by a paleo-seismic event and a gravity slump is analyzed. the architecture of sand bodies from different genesis are studied to build a genetic model of sand bodies occurring from different sedimentation processes in the study area. these methods could be beneficial in further analysis of the southwestern ordos basin and oil-gas exploration in the area. geological setting the mesozoic-cenozoic ordos basin is the second-largest sedimentary basin in china, developed on the paleozoic north china craton atop a proterozoic crystalline basement (qiu et al., 2014). tectonically, it can be subdivided into the following eight major structural units: the west margin thrust belt; tianhuan depression; yishan slope; jinxi fault-fold belt; lvliangshan fault-fold belt; yimeng uplift; weibei uplift; and weihe graben (figure 1). the study area is on the southwestern yishan slope, which is mainly in the central ordos basin. to better establish the stratigraphic relationships, the petrochina changqing oilfield company subdivided the yanchang formation into 10 informal oil members, named chang 10 to chang 1 (from bottom to top) based on marker beds, sedimentary cycles or lithological associations (qiu et al., 2015) (figure 2). the upper triassic yanchang formation is dominated by alluvial fan, alluvial plain, deltaic, fluvial and lacustrine complexes that are figure 1. regional location of study area. (a) location of the ordos basin, china; (b) geological setting of the study area and periphery; (c) well location of the case studies in the study area illustrated the present paper. 121soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic yanchang … composed of sandstones, siltstones, mudstones and tuff intercalations, with a thickness of approximately 1000-1300 m (figure 2) (qiu et al., 2014). the sedimentary environment of the chang 6 oil member in the huachiqingyang (h-q) area is a delta front and shallow lake. it is among the most significant mesozoic oil-bearing strata, with oil reserves reaching more than 70% of the proved geological oil reserves and crude oil output in the mesozoic ordos basin (qiu et al., 2015). ssd structure types various ssd structures, related to paleo-seismic events and gravity slumps, are developed in the chang 6 oil member of the upper triassic yanchang formation in the h-q area of ordos basin. these ssd structures can be divided into five categories based on morphologic and deformational type: ball-and-pillow structures, fold structures, sand dikes, irregular convolute stratifications, and synsedimentary faults. there are described and interpreted as follows. ball-and-pillow structures ball-and-pillow structures are the most numerous ssd structures in the chang 6 oil member in the study area. under the action of earthquakes and gravity, balls or pillows composed of fine-grained sandstone fall into underlying muddy layers, and ball-and-pillow structures are formed. both the overlying and the underlying layers exhibit liquefaction phenomena (figure 3-a). portions of dense overlying liquefied sediments can become completely separated balls and pillows from the parent rock, and the diameter of individual balls and pillows range from 1 to 5 cm, although they can remain interconnected each other floating in the underlying less dense sediment. the ball-and-pillow structures are associated with seismic action, and they can be interpreted to be driven by downslope slumping, but most results from loading processes related to variations in gravitational potential energy and are driven by variations in bulk density (calvo et al., 1998). factors that favor the development of sand ball-and-pillow structures, in contrast to load casts that affect the base of a laterally continuous bed, include a driving force of large magnitude, a combination of driving forces, a very weak lower layer, or a long duration of deformation. fold structures fold structures are common in the upper part of the yanchang formation. these ssd structures are mainly represented by obvious folds occurring in layers of siltstone and mudstone strips and form a series of different forms of small folds that fluctuate in a wave and groove shape, equivalent to “seismic corrugated rock,” as termed by qiao et al. (1994). fold structures are more typically shown in the zhen-80 well (figure 3-b) and ta-17 well (figure 3-e). the fold structures can be interpreted as ductile deformation; however, asymmetric geometry and overturned folds in the study area suggest that the deformation occurred in a hydroplastic manner rather than a ductile deformational process (martín-chivelet et al., 2011). the sedimentary environment of the chang 6 strata is lacustrine, and hydroplastic deformation can occur via downslope movement of sediments (slump fold). the fold structures are characterized by laterally continuous layers, which are underlain and overlain by undeformed beds with an irregular and transitional boundary. according to the well cores, most of the folded layers are 3-6 cm thick (up to 10 cm in thickness) and consist of thick sandstone and relatively thin mudstone layers. internally, individual folded beds or laminae have an asymmetric geometry with different vergences in a single folded layer. sand dikes sand dikes are composed of homogeneous liquefied sandstone injected into existing cracks. during the deformational process, small-sized breccias might dislodge from the surrounding rock and float within the sand sediment. sand dikes are more typically observed in the shan-111 well (figure 3-c). sand dikes refer to sandy veins or dikes developed in the interbedded sediments of sand and mudstone. they are stretched in an irregular shape and cut the horizontal bedding of the adjacent rock. the sand dikes are vertical or oblique to the bedding, a few of which are nearly parallel to the bedding. sand dikes differ in size, with widths generally ranging from 0.2 to 5 cm and lengths from 1 cm to dozens of centimeters. the shapes are also varied, and are often represented by irregular vein, plate or worm shapes. some are larger in the middle part, thinning towards the two ends before wedging out, and are bifurcated. there is no uniform shape on the surface, while some are larger in the upper part instead of the lower part. the dike body is bent when it cuts through the surrounding rock, and is particularly convex in the upper part and concave in the lower part. u ni t su bun it th ic kn es s lithology sedimentary facies legend c h1 c h2 c h3 c h6 c h7 c h8 c h9 c h1 0 c h4 +5 facies subfacies d el ta d el ta d el ta fr on t d el ta fr on t d el ta r iv er la ke la ke d el ta p la in sandstone silstone sandy mudstone silty mudstone mudstone oil shale prodelta prodelta prodelta prodelta shallow lake shallow lake deep lake delta front delta front delta front meandering river (m) up pe r t ria ss ic ya nc ha ng f or m ati on 100 200 300 400 500 600 700 800 900 1000 1100 1200 figure 2. stratigraphic column of the upper triassic yanchang formation in the ordos basin (ch=chang, modified from qiu et al., 2015). 122 qingshao liang, jingchun tian, feng wang, xiang zhang irregular convolute stratifications irregular convolute stratifications are structures induced by the liquefaction of sediments (liu et al., 2016). irregular convolute stratifications are generally present in fine-to-medium-grained sandstone. the morphology they exhibited is highly distorted stratification, and they more typically occur in the zhen-279 well (figure 3-d). the irregular convolute stratifications can be interpreted as liquefaction of sediments, and the morphologies of irregular convolute stratifications are similar to synsedimentary faults, but the liquefied degree represented by the two structure types is very different. irregular convolute stratifications always found in fine-medium-grained sandstone, sometimes mixed with coarse sandstone. synsedimentary faults small-scale synsedimentary faults are mainly observed in the chang 6 oil member, and typically occur in the gu-9 well (figure 3-f). these faults, which show a few centimeters of displacement along their lengths of <6 cm, truncate at both the overlying and underlying coarse-grained sandstone beds. these faults display pairs of conjugate geometry. synsedimentary faults generally represent brittle deformation; however, they occur in consolidated, partially consolidated, and even non-consolidated sediments. in particular, the dragged layer and dis clasts in the reverse fault zone indicate that they were incompletely lithified at the time of faulting and truncate only the restricted layer. thus, synsedimentary faults may result from different figure 3. types and features of ssd structures in the study area. (a) ball-and-pillow structure from the gao-40 well, chang 6 oil member, 1927.00-1927.35 m. (b) fold structures from the zhen-80 well, chang 6 oil member, 2021.49-2021.78 m. (c) sand dikes from the shan-111 well, chang 6 oil member, 1951.87-1951.96 m. (d) irregular convolute stratifications from the zhen-279 well, chang 6 oil member, 2154.30-2154.72 m. (e) fold structures from the ta-17 well, chang 6 oil member, 1433.95-1434.10 m. (f) synsedimentary faults developing in two opposite directions from the gu-9 well, chang 6 oil member, 1679.00-1679.28 m. 123soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic yanchang … physical properties during plastic deformation. in addition, the coexistence of both normal and thrust faults can be interpreted as a result of local extension and compression. differences between the ssd structures induced by earthquakes and gravity slumps the ssd structures developed in the chang 6 oil member of yanchang formation in the study area are limited to a single layer, of which the upper and lower parts are non-deformational layers, showing synsedimentary deformation controlled by geological events. generally, there are only two driving mechanisms in a lacustrine environment, namely earthquakes and underwater gravity slumps. the main differences between the two mechanisms are shown as follows: i) because an underwater landslide is controlled by the basin slope, its ssd structures have a dominant orientation. ii) underwater gravitational slidings are allogenic deposits with a base sliding surface, different from the non-deformational layers, and the ssd structure caused by seismic activity occurs in the in situ formation. iii) seismic activities may form a uniform liquefaction layer, while a slump cannot. as recognized as a sign of ssd structures induced by earthquakes, uniform liquefaction layers are under the water and the sedimentary surface. they are liquefied during the shake of an earthquake, and the primary beddings are destroyed. following the seismic activity, the normal sedimentation of the water body is recovered, and the primary stratification appears again. the uniform layer and overlying nonearthquake layer in the ta-17 well are well preserved (figure 3-e). in addition, the movement direction of the landslide is consistent with the direction of the paleoslope, while there is no clear correspondence between the movement direction of deformation driven by an earthquake and slope direction (zhang et al., 2007). the ssd structure stratum developed in the chang 6 oil member of yanchang formation in the study area is consistent with the upper and lower non-deformational strata in terms of lithology, rock association and sedimentary environment, which contradicts the ssd structures formed by a landslide. moreover, the two sets of synsedimentary faults found in well gu 9 break in two opposite directions (figure 3-f), which indicates that the formation of the fault is not controlled by the paleoslope. if the fault direction is related to the paleoslope, the faults break towards the direction of slope. the fault should be formed under the seismic tensile stress of two seismic events that occur within a short time interval, and the focus of the two seismic events should also be different. based on the aforementioned principles, and from all the systematic research on the relationship between ssd structures and depositional environment and paleoslope, it was determined that a seismic event was the driving mechanism for the formation of the ssd structures in the chang 6 oil member. different types of sand body architecture under the influences of two driving mechanisms in a lacustrine environment, two types of sand body architecture can form as follows: i) ssd structures driven by a seismic event with delta front sand body (sd-ssd) and ii) ssd structures driven by seismic event with turbidites (st-ssd). architecture of sd-ssds for the architecture of the sand bodies in the study area, the layer with the ssd structures is interbedded with the normal delta front sedimentary sand body in a vertical direction (figure 4-a, 4-b). this is because the material source of the sand bodies with ssd structures are the delta front sand bodies, which formed by liquefaction during seismic activities. during the period between two seismic events, ssd structures overlaid the delta front sand body formed by normal deposition, resulting in a superimposed composite feature of ssd structures formed by the seismic event and a normal delta deposit sand body during the sedimentary evolutionary phase (figs. 4-a, 4-b). architecture of st-ssds during the sedimentary evolution of the chang 6 oil member in the h-q area, not only were ssd structures developed related to paleo-seismic events, but turbidite sand body also developed which widely formed via the deposition of turbidity currents (especially in qingyang, guchengchuan, heshui, and ningxian). these two aforementioned deposited sand bodies are subject to vertical superimposition and horizontal close intergrowth, forming a superimposed composite sand body of the sedimentary body of seismic event genesis and a sand body resulting from a turbidity current (figure 5-a, 5-b, 5-c). core coreorigin originsedimentary structure sedimentary structure flat bedding flat bedding sliding deformation sliding deformation parallel bedding parallel bedding parallel bedding wavy bedding lenticular bedding microfault slumping deformation seismite ssd structures driver by earthquake (seismite) ssd structures driver by earthquake (seismite) ssd structures driver by earthquake (seismite) normal sand body (distributary channel of delta front) normal sedimentation (distributary bay) normal sedimentation (delta front) normal sedimentation (delta front) a b figure 4. architecture of a delta front sand body and ssd structural layer in the h-q area, ordos basin.(a) gao-59 well, chang 6 oil member, 1903.49-1903.61 m; (b) gao-41 well, chang 6 oil member, 9-33-36/56. 124 qingshao liang, jingchun tian, feng wang, xiang zhang core coreorigin originsedimentary structure sedimentary structure cyclothem turbidite cyclothem cuclothem turbidite slumping deformation slumping deformation slumping deformation seismite seismite seismite microfold sand pillow turbidite a b collapsed structure and ball-and-pillow structure depth (m) 2457.8 2463.8 thick bedded turbidite thick bedded turbidite medium bedded turbidite medium bedded turbidite thick massive turbidite thick massive turbidite thick massive turbidite thick massive turbidite thick massive turbidite medium bedded turbidite thin bedded turbidite seismite c figure 5. architecture of turbidite sand body and ssd structural layer in the h-q area, ordos basin. (a) mu-11 well, chang 6-3 oil member, 9-6/67; (b) zhuang-42 well, chang 6-3 oil member, 10-61/106; (c) huan-62 well, chang 6-2 oil member. discussion mechanism of deformation there are three sand body types were developed during the sedimentary evolution of the chang 6 oil member in the h-q area in the central ordos basin. including i) sand body include ssds related to paleoseismic events, ii) turbidite sand bodies formed by turbidity events, and iii) delta front sand bodies. these three sand body types are superposed vertically and closely coexist on the plane, which constitute the superposition of paleoseismic event sediments, turbidity event sediments and delta front sediments. although they are different in characteristics and genetic mechanism, the causes of their formation are closely related. there is general agreement that ssd structures as described maybe caused by liquefaction of sediments induced by seismic shock. according to the criterion of the seismites summarized by sim (1975), compared with the characteristics of chang 6 ssd structures: the chang 6 ssd structures meet these seven criteria, and they are most likely due to seismic shock. in addition, turbidites are widely developed in the chang 6 oil member in the southern ordos basin. the turbidites also develop ssd structures such as ball-and-pillow structures, fold structures, sand dikes and irregular convolute 125soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic yanchang … stratifications, during the deposition process. some former researchers have seen typical phenomena of turbidity deposition in the ssd layer or in the adjacent layer of the study area, such as grain-order bedding or bog sequenced, indicating that ssd structures will also occur during turbidity deposition (montenat et al., 2007). this is not inconsistent with the statement that the seismic caused the ssd structures in the study area, because the seismic is an important factor in triggering gravity flow. sedimentary model it can be observed from the regional depositional setting that during the chang 6 sedimentary period, the h-q area was exactly at the superimposition part of the northeastern delta front and the southwestern delta front (fu et al., 2005). the ordos basin began to contract and the delta kept expanding towards the lake basin via progradation, carrying a large number of terrigenous clasts to the lake basin. because of the vibrations caused by seismic action, the unstable irregular lobate body of the delta front may have collapsed and formed seismite first, and the top of the seismite could have transformed into a turbidite body due to gravity. the aforementioned process can recur when new seismic events occur, which results in a vertical superimposition and horizontal intersection of the ssd structural layer related to the paleo-seismic event and the sand body formed by a normal delta front and turbidity current deposition (figure 6). table 1. comparison between criteria of seismites (sim et al., 1975) and characteristics of chang 6 ssd structures. no. criteria of seismites (sim et al., 1975) characteristics of chang 6 ssd structures references 1 proximity to the active seismic zone there are synsedimentary activity faults in the study area. zhang et al., 1998; qiu et al., 2014; gao et al., 2016 2 sediment has potential liquefaction capacity the sediments of the yanchang formation are mainly fine sandstone, siltstone and mudstone. the better sorting of siltstone and fine sandstone are more likely to occur liquefaction. yan et al., 2009; yang et al., 2016, 2017; pang et al., 2012; xia et al., 2007; li et al., 2010; tian et al., 2015 3 similar structures can be seen in the experiment the morphological characteristics of the ssd structures are basically consistent with those observed by yan et al. (2009) in the experiment. 4 traces of sediment liquefaction in the deformed layer as shown in figure 3, the liquefaction traces are clearly visible. 5 ssd is limited to a single layer the ssd structures observed in the core are all intra-layer defoemations. 6 the ssd layer is visible in a wide range the deformed layer can be compared in a large range, and the precise contrast is difficult, but the deformation layer can be observed extend in the field outcrop. 7 slope environment during the chang 6 period, the study area was a steep slope topography. li et al., 2009; zheng et al., 2009 figure 6. sketch showing formation process of the overlay between the seismite and delta front sand body or turbidite sand body. 126 qingshao liang, jingchun tian, feng wang, xiang zhang conclusions (1) various earthquake-related ssd structures developed in the chang 6 oil member sand body in the h-q area in the central lake basin during the depositional stage of the chang 6 oil member of the triassic yanchang formation in the ordos basin. these mainly included ball-and-pillow structures, fold structures, sand dikes, irregular convolute stratification and synsedimentary faults. (2) the obvious distinctions between the ssd structures formed by paleoseismic events and ssd structures resulting from gravity slumps mainly are apparent in three aspects, including dominant orientation of the ssd structures, the existence of a glide plane and the existence of a uniform liquefaction layer. (3) the sand body architecture formed between the ssd structural layer resulted from paleo-seismic events and those of other genetic types, which mainly includes two types: i) ssd structures driven by seismic events with delta front sand body and ii) ssd structures driven by seismic events with turbidite sand bodies formed by a delta-front slumping and redistribution due to seismic action. acknowledgements this study was jointly supported by the national science and technology major project of the ministry of science and technology of china (nos. 2016zx05046005-001, 2016zx05007004-002). references alfaro, p., delgado, j., estévez, a., molina, j. m., moretti, m. & soria, j. m. 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(1998). extension in the graben systems around the ordos (china), and its contribution to the extrusion tectonics of south china with respect to gobi-mongolia. tectonophysics, 285, 41-75. zheng, r. c., wang, c. y., wang, h. h, wang, c. y. & wen, h. g. (2009). sequence-based lithofacies and paleogeographic characteristics of 6th oil-bearing member of the yanchang formation of upper trissic in the shijiawan area, ordos basin (in chinese with english abstract). journal of mineralogy and petrology, 29(4), 16-24. keywords: remote sensing image; gis technology; small watershed topography; hydrological characteristics. palabras clave: imagen de teledetección; tecnología sig; topografía de cuencas pequeñas; características hidrológicas. how to cite item pan, h., & khabazi, m. (2020). study on the evolution monitoring of topographic and hydrological characteristics of small watershed based on remote sensing and gis. earth sciences research journal, 24(3), 297-303. doi: https://doi.org/10.15446/esrj. v24n3.90340 soil and water conservation is one of the key measures to improve the ecological environment of small watersheds and maintain the healthy life of the region. with the current method used to monitor the evolution of topographic and hydrological characteristics of a small watershed, the edge keeping index of the image and the signal-to-noise ratio of the image are low, the monitoring results are inaccurate, and there are some problems such as low edge keeping ability, poor denoising effect, and low monitoring accuracy. a monitoring method based on remote sensing and gis for the evolution of topographic and hydrological characteristics of small watersheds is proposed. the hyperspectral data was transformed by the principal component analysis (pca). group principal component images, use sparse representation method based on adaptive dictionary and dual-tree complex wavelet transform method to denoise principal component images with a small amount of information were the objectives of the paper; also the use the multi-scale wavelet transform to detect image edge; build a binary model of a pixel, extract vegetation index, and terrain factor based on a binary model of the pixel, and realize terrain and hydrological characteristics of the small watershed. the experimental results show that the proposed method has the high edge-preserving ability, good denoising effect, and high monitoring accuracy. abstract study on the evolution monitoring of topographic and hydrological characteristics of small watershed based on remote sensing and gis estudio de la evolución de las características topográficas e hidrológicas en pequeñas cuencas hidrográficas con base en teledetección y en sistemas de información geográfica issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90340 la conservación del suelo y el agua es una de las medidas clave para mejorar el entorno ecológico de las pequeñas cuencas y mantener la vida de la región sana. con el método actual para monitorear la evolución de las características topográficas e hidrológicas de las cuencas hidrográficas pequeñas, el índice de mantenimiento de bordes de la imagen y la relación señal/ruido de la imagen son bajos, los resultados del monitoreo son inexactos y hay algunos problemas como la capacidad baja del mantenimiento de bordes, un efecto de eliminación de ruido deficiente y una precisión de monitoreo baja. en este trabajo se propone un método de monitoreo basado en teledetección y en sistemas de información geográfica para determinar la evolución de las características topográficas e hidrológicas de pequeñas cuencas hidrográficas. los datos hiperespectrales fueron transformados por el método de análisis de componentes principales (pca). agrupar las imágenes de componentes principales, utilizar un método de representación disperso basado en un diccionario adaptativo y un método de transformación de wavelet complejo de árbol dual para reemplazar las imágenes de componentes principales con una pequeña cantidad de información fueron los principales objetivos de este trabajo; también usar la transformación de wavelet multiescala para detectar el borde de la imagen; construir un modelo binario de píxeles, extraer el índice de vegetación y el factor de terreno basado en el modelo binario de píxeles, y realizar las características del terreno y las características hidrológicas de las pequeñas cuencas. los resultados experimentales muestran que el método propuesto tiene una alta capacidad de preservación de bordes, buen efecto de eliminación de ruido y alta precisión de monitoreo. resumen record manuscript received: 13/11/2019 accepted for publication: 20/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 297-303 hong pan1*, mostafa khabazi2 1institute of history and cultural cartography, sanjiang university, nanjing 210012, china 2assistant professor of geomorphology, shahid bahonar university kerman, kerman, iran *corresponding author: h yd ro g eo g ra ph y https://doi.org/10.15446/esrj.v24n3.90340 https://doi.org/10.15446/esrj.v24n3.90340 https://doi.org/10.15446/esrj.v24n3.90340 298 hong pan, mostafa khabazi introduction with the continuous development of human society and the economy, various environmental problems have been paid attention to all over the world. in particular, soil erosion has become one of the most serious environmental problems in the world (cheke et al., 2016). since the soil erosion was recognized by people, it has been widely concerned by all walks of life, and it has seriously affected the environment on which people live (li et al., 2016). however, due to its complex formation mechanism and process, people can hardly find a single research method to reveal its process. this makes it necessary for people to seek a fast, scientific and economic investigation and research method, to dynamically reflect the current situation and development trend of soil and water loss, and to develop into a cutting-edge topic in the research of soil erosion (archer et al., 2016). in recent years, many ecological projects have been implemented, such as natural forest protection projects, conversion of farmland to forest, conversion of farmland to grass, etc., many of which are related to soil and water conservation. the serious situation of soil and water loss in the water source area of the middle line project has begun to get better. however, due to the development of the regional economy and the development of the resettlement projects in the reservoir area, the situation of soil and water loss has gradually changed, mainly reflected in two aspects. there are two aspects: first, the area and intensity of soil and water loss have been gradually reduced since the comprehensive treatment of soil and water loss was carried out on a large scale in more than ten years, and remarkable benefits have been achieved in ecological, economic and social aspects; second, with the continuous construction of the middle route project of south-to-north water transfer, the land cover has been formed in the construction process of the urban and other infrastructure of the reservoir resettlement project. the structure and industrial structure have been adjusted to create man-made soil erosion (shevchenko et al., 2018). therefore, it is of great theoretical and practical significance to monitor the evolution of topographic and hydrological characteristics of small watersheds. at present, there are some problems in the monitoring method of the evolution of topographic and hydrological characteristics of small watersheds, such as low edge retention ability, poor denoising effect, and low monitoring accuracy. therefore, it is necessary to study the monitoring method of the evolution of topographic and hydrological characteristics of small watersheds (faiz et al., 2018). zhu et al. (2017) used the image analysis method to segment the terrain of small watersheds, taking the branch point of the valley as seed point and the curvature characteristic of the surface profile as growth criterion, and realized the automatic segmentation of the area between the ditches, the ditch slope, and the ditch bottom. on the basis of the automatic segmentation of the ditch slope terrain, the object-oriented target classification and recognition idea was used, combined with the object. in order to monitor the evolution of topographic and hydrological characteristics of small watersheds, the edge-preserving index of this method is low, and there is a problem of low image edge-preserving ability (zhu et al., 2017). cao et al. (2018) spliced the dem of each sample area standard map into a data unit, and extracted the slope length with the standard map of the medium basin and buffer zone as the unit, and compared and analyzed the graph, statistical characteristics, and work efficiency of the extraction results, so as to realize the monitoring of the evolution of topographic and hydrological characteristics of the small watershed. this method cannot remove the remote sensing image, there is a problem of poor denoising effect (cao et al., 2018). xiao (2018) analyzed the topographic and hydrological characteristics of the small watershed based on the hydrological and sediment measurement and remote sensing image data. different coastal sediment transport formulas were used to compare and calculate the coastal sediment transport volume of waves. the evolution law of hydrological characteristics of the small watershed was studied by using the measured topographic data for many years. the monitoring results obtained by this method were inconsistent with the actual situation, and the monitoring accuracy was low (xiao, 2018). zhao et al. (2016) and others summarized and classified the common simulation methods of middle-class runoff in hydrological cycle simulation according to the principle of middleclass runoff in hydrological cycle simulation. on this basis, the main methods of precipitation runoff correlation coefficient method, full storage runoff, and super permeability runoff in runoff generation simulation were analyzed. in the simulation method, the parameterization methods of topography, land use cover, and soil type are analyzed and discussed to realize the monitoring of the evolution of topographic and hydrological characteristics of small watersheds. the remote sensing image obtained by this method has a low signal-to-noise ratio and poor denoising effect (zhao et al., 2016). in order to solve the problems in the above-mentioned methods, a monitoring method based on remote sensing and gis for the evolution of topographic and hydrological characteristics of small watersheds is proposed. (1) denoising and edge detection of remote sensing data. (2) based on the preprocessed remote sensing image, a binary pixel model is constructed. (3) vegetation index and terrain factors are extracted on the basis of pixel binary models to detect the evolution of topographic and hydrological characteristics of small watersheds. (4) experiments and discussions verify the overall effectiveness of the monitoring method based on remote sensing and gis in three aspects: edge detection ability, denoising ability, and monitoring accuracy. (5) conclusion. materials and methods image denoising based on the advantages of spectral dimension transformation and dictionary learning, the evolution monitoring method of topographic and hydrological characteristics of small watersheds based on remote sensing and gis solves the problem of remote sensing image denoising. combined with the classic principal component analysis and the image denoising model based on the k-svd algorithm, a new remote sensing image denoising method is proposed. the specific process is as follows: (1) spectral dimension pca transformation. assuming that the spatial dimension of the noisy remote sensing image is mn and the number of wavebands is p, it is expressed as the matrix xx of p row mn column, where x x x xp t = …   1 2, , , , p p=1 2, , is the column vector of mn1 dimension that rearranges the image of the i waveband in the order of column first and row second; the transformed data y is obtained by pca transformation of x. (2) spatial dimension denoising in the pca domain. the first k principal components are retained, and the remaining principal components are denoised as follows: the principal components are transformed into a 2d image with mn size; the standard deviation of the noise is estimated by robust median estimation; the noise in the principal component image is removed by k-svd denoising model (su et al., 2016). (3) pca domain spectral dimension denoising. in the pca domain, each pixel after transformation is denoised in the following way: the first k components of the transformed spectral dimension remain unchanged, and the remaining components of pk dimensions are denoised. (4) data reconstruction. all the principal components processed by the above method are inverted by pca, and the obtained data are rearranged into a matrix of mm size, i.e. the remote sensing data after denoising (zhao et al., 2017). the principal component analysis is one of the most basic data dimensionality reduction methods, mainly through the eigenvalue decomposition of the covariance matrix of the data. after the eigenvalue decomposition, the eigenvector corresponds to the principal component of the data, and the eigenvalue corresponds to the weight of the data on each principal component (lv et al., 2016). on the basis of principal component analysis, the k-svd model is used to denoise remote sensing images. the denoising principle is as follows: suppose f = u+v, where the size of noise component image f is m row n column, u is the image to be restored, v is the gaussian white noise with a mean value of 0 and standard deviation of . the corresponding energy form of image denoising model based on the k-svd dictionary learning algorithm is as follows: (1) among them, riju is the binary operator that extracts n n× size image block from the top left corner of the image u with the j ij ij, , , ,= …1 2 299study on the evolution monitoring of topographic and hydrological characteristics of small watershed based on remote sensing and gis where nr represents the number of pixels in the neighborhood and t is the threshold. it can be seen from the above formula that 01. the meaning of the parameter  is: when  = 1, because the length of the edge chain exceeds t, it indicates that (i, j) is the edge point; when (i, j) is in the smooth region,  is very small. with the increase of  value, the possibility that point (i, j) is the edge increases. edge detection of images is carried out by a multi-scale wavelet transform. (1) if the gray level of any point (x, y) in the image is f (x , y), its wavelet transform can be simplified as follows: wf x y f x y x y, , * ,( ) = ( ) ( ) (5) where (x, y) is the wavelet generating function if (x, y) is the first partial derivative of the smoothing function  (x, y). (2) firstly, two wavelets are defined, which are the partial derivatives of two-dimensional continuous smoothing function  (x, y) along the direction of x and y. φ ∂θ ∂ 1 x y x y x , ,( ) = ( ) (6) φ ∂θ ∂ 2 x y x y x , ,( ) = ( ) (7) then the binary wavelet decomposition algorithm of the two-dimensional image is used to obtain the wavelet transform in x direction and y direction. w f x y f x y x y w f x y f x y x y j j j j 2 1 2 1 2 2 2 2 , , * , , , * , ( ) = ( ) ( ) ( ) = ( ) ( )       (8) according to the above formula: w f x y w f x y f b x y j j j j2 1 2 2 2 32 , , * , ( ) ( )           = ( )( ) �� (9) (3) calculate the local maxima of wavelet in x and y directions. the edge pixel is defined as the local maxima in x and y directions. according to the canny edge detection method, if the modulus of the gradient vector at (x0, y0) point along the gradient vector direction f (x, y) is local maximum, then (x0, y0) point is called edge point (yuan et al., 2019). therefore, the multiscale edge of a two-dimensional image signal can be characterized by the horizontal and vertical components of the wavelet transform. if the modulus and amplitude of the two-dimensional wavelet transform are respectively: m f x y w f x y w f x yj j j2 2 1 2 2 2 2 , , ,( ) = ( ) + ( ) (10) a f x y w f x y w f x y j j j 2 2 1 2 2 , arctan , , ( ) = ( ) ( ) (11) the modulus of the gradient vector  �� f b x yj* , 2 3( )( ) is proportional to the modulus m f x yj2 ,( ) of the wavelet transform, and the modulus maximum of the wavelet transform corresponds to the edge point. when the image is translated, the modulus maximum of the wavelet transform also changes with the translation (yang et al., 2020). (4) in order to overcome the sensitivity to noise and avoid detecting too many small edges or non-edge points, according to the multi-scale idea of wavelet and considering the singularity of edge points, the multi-scale wavelet generating function is selected.  1 1 2 1s x y x s y s s , ,( ) =     (12) element arranged in the order of column first and row second and converts it into n1 size column vector, ij m n n n= +( ) ⋅ +( ) 1 1 is the number of image blocks; d is the dictionary, n is the number of pixel points in the image block, k is the number of dictionary atoms, is the sparse coefficient vector of r uij on the dictionary d. , and respectively represent the sparse representation coefficient vector group, the dictionary to be learned and the denoised image corresponding to all image blocks;  is the weight of fidelity between the estimated image and the noisy image, which is related to the noise intensity. when the noise is large,  should take a smaller value; ij is the weight of sparse representation coefficient ij; ij 0 is the number of non-zero elements in vector . after proper initialization, the variables to be solved in the above optimization problem can be solved by the method of alternating iteration as follows: (1) sparse coding: fixed , solution: ; (2) dictionary update: fixed , solution: . after proper conversion, sparse coding can be solved by orthogonal match tracking algorithms (naik et al., 2016). according to the property r u d y daij ij fij − α − 2 2 2=∑ of matrix norm, where a ij= …   , , ,2 . the dictionary atoms are updated one by one. the energy functional corresponding to step (2) can be converted into: d a y d a d ak k t d a k k t k k t k k f d i i ki ki t j j i ki , argmin , argmin , { } = =∑− − ≠ 2 ,,a k k ki t f ki t i i e d a− 2 (2) where ak t i represents line ki of matrix a. in the above formula, since dki and ak t i are column vectors and row vectors respectively, eki can be regarded as the product of matrix dki and ak t i of two ranks 1. according to the singular value decomposition e u vk t i = ∆ of the error matrix, the new dictionary atom d uki = and corresponding coefficient a vk t i =   ×∆ 1 1 1, are obtained by rank 1 approximation of eki. step 1 and step 2 iterate alternately until the stop condition is met. after the sparse coding and dictionary are obtained according to the above steps, the terms related to u in formula (1) are taken and the derivative of u is obtained easily. (3) where i is the unit matrix. the above formula actually indicates that the value at each pixel point of is obtained by the weighted average of the pixel values of the corresponding pixel points of all image blocks including the pixel point in the noisy image and the denoised image (cutter et al., 2016). edge detection firstly, the gradient operator is used to filter the image, extract the edge pixels, and form the edge image (yan et al., 2016). take the edge pixel as the candidate edge pixel point. the length of the edge chain is obtained by linear connection of the edge points. using the canny operator to detect edge points, let the neighborhood of pixel (i, j) be r, let lij and gij represent the edge chain length and gradient amplitude passing through (i, j) respectively, and gij represent the maximum gradient amplitude in the neighborhood, then define a measurable parameter of an effective edge in the neighborhood as : µ < ∈ = ≥            ( ) ∑1 1 n g m l t l tr ij i j r ij ij , (4) 300 hong pan, mostafa khabazi  2 2 2 1s x y x s y s s , ,( ) =     (13) where s is the scale coefficient, then: w f x y f x y x y w f x y f x y x y s s s s 1 1 2 2 , , * , , , * , ( ) = ( ) ( ) ( ) = ( ) ( )       (14) if the right scale is chosen, the location of the edge can be accurately determined. results (i) binary model of pixel it is assumed that in remote sensing images, the spectral information of each pixel is linearly combined by the spectral information of twocomponent factors, namely, the surface factor with vegetation coverage and the surface factor without vegetation coverage. the proportion of each component in the corresponding ground area of each pixel is the information weight of the component factor. the vegetation coverage of this pixel is the proportion of the vegetation coverage in the image (wu et al., 2020). from the above model, it can be concluded that the sum of the information ss contributed by the soil component and the information sv contributed by vegetation component is the information s observed by remote sensing sensor, namely: s s sv s= + (15) if the vegetation coverage of a pixel is fc, the ratio of bare soil coverage is 1  fc. when the surface is completely covered by vegetation, assuming that the remote sensing information is sveg, the product of sveg and fc is the vegetation part of the mixed pixel, as shown in the following formula: s f sv c veg= × (16) similarly, it can be concluded that when the ground is completely composed of soil, the covered pixel is a pure pixel, and the remote sensing information obtained is ssoil (huang et al. 2017), then the information ss contributed by the soil part of the mixed pixel can be expressed as the product of ssoil and 1  fc: s f ss c soil= ( ) ×1 (17) bring formula (16) and formula (17) into formula (15): s f s f sc veg c soil= × + ( ) ×1 (18) the above formula is transformed to obtain the calculation formula of vegetation coverage: f s s s sc soil veg soil =   (19) using remote sensing information to estimate vegetation coverage only needs to know the parameters of the binary model of ssoil and sveg, which can be obtained according to formula (19). (ii) vegetation index vegetation index refers to the multispectral data obtained by remote sensing sensors, which is composed of linear and non-linear combinations and has a certain significance for vegetation. it is an indirect index that is calculated from the spectral characteristics of vegetation reflection and reflects the growth, coverage, biomass, and characteristics of vegetation on the surface. the results show that the vegetation index has a good correlation with vegetation coverage, and it is suitable to calculate vegetation coverage (cheng et al., 2020). in order to monitor vegetation coverage, ratio vegetation index (rvi) was first proposed. the ratio vegetation index (rvi) is very sensitive to the atmospheric conditions, and the atmospheric effect reduces its sensitivity to vegetation detection. in order to eliminate the influence of the atmosphere on the different nonlinear attenuation of the two bands, it is often calculated after the atmospheric correction data or the gray value is converted to the reflectivity. when the vegetation coverage is high, rvi is very sensitive to vegetation, when the vegetation coverage is less than 50%, this sensitivity is significantly reduced (gao et al., 2018). the normalized vegetation index ndvi is the result of the non-linear normalization of rvi. the calculation formula of ndvi is: ndvi nir r nir r = +  (20) among them, nir is the reflectivity of the near-infrared band, r is the reflectivity of the red band. ndvi is a comprehensive reflection of vegetation information in unit pixel, and its value is determined by vegetation coverage fc and leaf area index, so it can be used to estimate both. ndvi is widely used because of its wide range of vegetation coverage detection, good temporal and spatial adaptability. according to the binary model theory, the ndvi value of a pixel can be divided into two parts: the green vegetation part ndviveg and the bare soil part ndvisoil. bring ndvi into formula (19): f ndvi ndvi ndvi ndvic soil veg soil =   (21) where ndvisoil is the ndvi value of the area completely covered by bare soil or vegetation. ndviveg represents the ndvi value of the pixel when it is completely covered by vegetation, that is, the ndvi value of the pure vegetation pixel. ndvi is the best indicator of vegetation growth and vegetation coverage. the ratio in the formula is used to partially eliminate the influence of changes in irradiance conditions related to satellite observation angle, topography, solar altitude angle, cloud/shadow, and atmospheric conditions. ndvi is more sensitive to the change of soil background. because the reflection of cloud, water and snow is higher in the visible band than in the near-infrared band, its ndvi value is negative; while the reflection of rock and bare soil is close in the two bands, so its ndvi value is close to 0; while the reflection of vegetation in the near-infrared band is greater than that in the visible band, its value is positive, and it has a positive correlation with the vegetation coverage (gao et al., 2019). (iii) terrain factor the topographic factors affecting soil erosion include slope, slope direction, and gully density. topographic factors have a great influence on soil erosion, and the same cultivation conditions have great differences in soil erosion intensity due to different topography. there are two ways to express the slope, degree, and percentage. the degree value refers to the angle between the slope and the horizontal plane, and the percentage refers to the ratio of the vertical height to the horizontal length. in arcgis, window differential analysis is used to calculate the slope. the slope is calculated in 33 dem grid windows. the calculation equation for determining the slope value i of the central pixel is as follows: θ −− i i jh h d = max tan 1 (22) in the formula, i is the slope of the pixel to be calculated, expressed in degrees; hi is the elevation of the pixel; hj is the elevation value of the eight pixels (j) around it; d is the center distance of the two adjacent pixels, when j = 2,3,6,8 d is the side length of the pixel, when j = 1,3,5,7, d is the diagonal length of the pixel. slope direction is defined as the angle between the projection of slope normal on the horizontal plane and the true north direction, that is, from the true north direction, clockwise rotation to the angle between the normal on the horizontal projection line. gully density refers to the total length of a gully in a specific area, and its mathematical expression is as follows: 301study on the evolution monitoring of topographic and hydrological characteristics of small watershed based on remote sensing and gis d l a =  (23) in the formula, d refers to gully density; l refers to the total length of gully within a specific area of a; a refers to a specific area within the study area. gully density is a term used to describe the degree to which the ground is cut and broken. on the other hand, it is easy to form surface runoff. the higher the gully density, the faster the surface runoff and soil erosion. gully density is an important characteristic value of terrain development stage, precipitation or elevation difference, soil permeability, and surface erosion resistance. therefore, gully density is a basic index reflecting the local climate, geology, and landform, which is of great significance to the monitoring of soil and water conservation ecological construction. (iv) feature evolution monitoring because of the influence of many factors such as surface humidity, roughness, soil type, and color, the ndvisoil value of most bare land surfaces is kept between 0.1 and 0.2. ndviveg represents the maximum value of vegetation coverage pixel, and its value will change with different space and time. ndvi stands for vegetation index, expressed by fraction, which can be calculated by original digital data, atmospheric reflection, radiation or surface reflection. the size of ndviveg will be affected by the type of data used. even for the same image, the ndvi value is not fixed when calculating vegetation coverage. for any two pixels a1 and a2 in an area a, the known vegetation coverage is fc1 and fc2, and the application of these two pixels has the following formula: f ndvi ndvi ndvi ndvic soil veg soil 1 1=   (24) f ndvi ndvi ndvi ndvic soil veg soil 2 2=   (25) the following formula can be obtained by combining the above two formulas: ndvi f ndvi f ndvi f fsoil c ci c c = ×2 1 2 2 1   (26) ndvi f ndvi f ndvi f fveg c c c c = ( ) × ( ) ×1 12 2 1 2 2 1     (27) in principle, the pixels a1 and a2 can be any pixel, but it can be seen from formula (26) that if the value of fc2fc1 is larger, the error between ndvisoil and ndviveg will be smaller. when fc1=0 and fc2=1, then the pixel a1 is vegetationfree and the pixel a2 is completely vegetation covered. take ndvi1 as ndvimin, ndvi2 as ndvimax; fc1 as fcmin, fc2 as fcmax, and substitute into formula (26) and formula (27) to get: ndvi f ndvi f ndvi f fsoil c c c c = × ×max min min max max min   (28) ndvi f ndvi f ndvi f fveg c c c c = ( ) × ( ) × 1 1    min max max min max min (29) discussion in order to verify the overall effectiveness of the monitoring method based on remote sensing and gis, it is necessary to test the monitoring method based on remote sensing and gis. this test is completed in the simulink platform, and the operating system is windows. let ies represent the edge keeping index. the higher the value of edge keeping index is, the higher the image edge keeping ability is. the calculation formula is as follows: i dn dn dn dn es r r i m r r i m= = = ∑ ∑ 1 2 1 1 2 1 ′ ′ ′ ′ − − (30) in the formula, m represents the number of image pixels; dnr1  and dnr2  represent the gray value of left and right or upper and lower adjacent pixels at the edge junction after filtering; dnr1  and dnr2  represent the gray value of left and right or upper and lower adjacent pixels at the edge junction of the filtering front. based on remote sensing and gis, the evolution monitoring method of topographic and hydrological characteristics of the small watershed, zhu et al. (2017), cao et al. (2018), and xiao (2018) were used to test, and the edge retention index of four different methods were compared. the test results are as follows: (a) edge retention index of the proposed method and zhu et al. (2017) method (b) edge retention index of cao et al. (2018) and xiao (2018) methods figure 1. edge preserving index of four methods according to the analysis of figure 1, in many iterations, the edge keeping index of the monitoring method based on remote sensing and gis is higher than that of zhu et al. (2017), cao et al. (2018) and xiao (2018) , the higher the edge keeping index is, the higher the image edge keeping ability is, 302 hong pan, mostafa khabazi and the image edge of the monitoring method based on remote sensing and gis is verified. it has a high conservation capacity, which provides a basis for monitoring the evolution of soil and water characteristics in small watersheds. an important parameter to measure image quality is the signal-to-noise ratio (snr), which is defined as follows: snr p p x n =10 10log (31) where, px is the energy of pure signal xi j k , ; pn is the energy corresponding to the noise in signal containing noise. based on remote sensing and gis, the methods of evolution monitoring, zhu et al. (2017), cao et al. (2018) and xiao (2018) are used to test the signalto-noise ratio of four different methods. the test results are as follows: (a) signal to noise ratio of the proposed method and zhu et al. (2017) method (b) signal to noise ratio of cao et al. (2018) and xiao (2018) methods figure 2. signal to noise ratio of four different methods it can be seen from the analysis of figure 2 that the signal-to-noise ratio of the proposed method in multiple iterations is higher than that of zhu et al. (2017), cao et al. (2018), and xiao (2018), and the higher the signal-to-noise ratio is, the better the denoising effect of the method is. combining the classic principal component analysis and k-svd algorithm to denoise the image, it improves the signal-to-noise ratio of the image and verifies the denoising effect of the method based on remote sensing and gis. taking the monitoring accuracy as the test index, the validity of the monitoring method, zhu et al. (2017), cao et al. (2018), and xiao (2018) methods based on remote sensing and gis is further verified. the test results are as follows: (a) monitoring accuracy of the proposed method and zhu et al. (2017) method (b) monitoring accuracy of cao et al. (2018) and xiao (2018) methods figure 3. monitoring accuracy of four different methods it can be seen from figure 3 that the monitoring accuracy of the evolution monitoring method of topographic and hydrological characteristics of small watershed based on remote sensing and gis in multiple iterations is higher than that of cao et al. (2018) and xiao (2018), because the method extracts vegetation index and terrain factor based on the binary model of pixel, improves the accuracy of monitoring results, and verifies the topography of small watershed based on remote sensing and gis. the monitoring accuracy of the hydrological characteristics evolution method is high. conclusions soil and water loss is the main ecological environment problem in china. with the development of society and economy, the regional land cover situation changes, this has an impact on the generation and development of soil and water loss. it is necessary to monitor the evolution of the topography and hydrological characteristics of small watersheds. at present, there are some problems in the monitoring method of the evolution of topographic and hydrological characteristics of the small watersheds, such as the low ability of edge keeping, the poor effect of noise removal, and the low accuracy of monitoring. based on remote sensing and gis, this paper puts forward a monitoring method for the evolution of topographic and hydrological characteristics of small watershed, which can accurately monitor the evolution of topographic and hydrological characteristics of the small watershed, solve the existing problems in the current method, and provide a basis for promoting the good development of ecological environment. 303study on the evolution monitoring of topographic and hydrological characteristics of small watershed based on remote sensing and gis references archer, n. a. l., otten, w., schmidt, s., bengough, a. g., shah, n., & bonell, m. 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(2017). the loess terrain automatic segmentation and gully head recognition method based on dem image analysis. geography and geo-information science, 33, 74. http://c.old.wanfangdata.com.cn/periodical-zgstbckx.aspx 10.1016/j.matlet 10.1109/access 10.1109/access 10.3390/math http://c.old.wanfangdata.com.cn/periodical-sygc.aspx 10.1016/j.aquaculture 10.1016/j.aquaculture about:blank about:blank under freeze-thaw cycles, the relationship between rock microstructure deterioration and its macroscopic mechanical characteristics has drawn extensive attention from engineers. with the objective to incorporate freeze-thaw cycle experiment into headrace tunnel engineering, in the present study two groups of andesite rock samples in different states are tested under the conditions of the lowest freezing temperature of –40 ºc and the thawing temperature of 20 ºc. damage detection was performed by magnetic resonance imaging for the interior microstructure of rock samples subject to different freeze-thaw cycles, and the relationship between the sample mechanical properties and gradual deterioration of rock microstructures was discussed. the results demonstrate evident influence of freeze-thaw cycle on the damage and deterioration of internal pore structure in andesite, and the rock uniaxial compressive strength and elasticity modulus exhibit a decreasing trend with the increase of freezethaw cycles. after 40 cycles, the strength of naturally saturated rock samples decreases by 39.4% (equivalent to 69.4 mpa) and the elasticity modulus drops by 47.46% (equivalent to 3.27 gpa). for rock samples saturated by vacuum, 40 freeze-thaw cycles lead to a decrease of 36.86% (equivalent to 58.2 mpa) in rock strength and a drop of 44.85% (equivalent to 2.83 gpa) in elasticity modulus. therefore, the test results quantitatively elucidate the substantial influence of freeze-thaw cycle on the damage and deterioration of internal structure in andesite. abstract keywords: freeze-thaw cycle; microstructure; progressive deterioration; strength damage the relationship between mechanical properties and gradual deterioration of microstructures of rock mass subject to freeze-thaw cycles issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.66108 haibo jiang* college of hydraulic and architectural engineering, shihezi university, xinjiang 832000, china. earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 53-57 en el ciclo de congelación y descongelación, la relación entre el deterioro de la microestructura de la roca y sus características mecánicas macroscópicas ha atraido un amplio interés de los investigadores. con el objetivo de incorporar las pruebas del ciclo congelación y descongelación del canal de un túnel, en este estudio se evaluaron dos grupos de muestras de andesitas en diferentes estados bajo las condiciones de congelación a -40 ºc y de descongelación a 20 ºc. la detección del daño se realizó por el escaneo de resonancia magnética en el interior de la microestructura de las muestras de roca sujetas a diferentes ciclos de congelación y descongelación, y se analizó la relación entre las propiedades mecánicas de las muestras y el deterioro gradual de las microestructuras de la roca. los resultados muestran una influencia evidente del ciclo de congelación y descongelación en el daño y deterioro de la estructura interna de los poros de la andesita y la fuerza compresiva uniaxial de la roca, mientras el módulo de elasticidad presenta una tendencia decreciente con el incremento de los ciclos aplicados. después de 40 ciclos, la fuerza de las muestras de roca saturadas naturalmente cae en un 39,4 % (el equivalente a 69,4 mpa) y el modulo de elasticidad baja en un 47,46 % (equivalente al 3,27 gpa). para las muestras saturadas por aspiradora, los cuarenta ciclos de congelación y descongelación llevan a un decrecimiento del 36,86 % (equivalente a 58,2 mpa) en la fuerza de la roca y una bajada del 44,85 % (equivalente a 2.83 gpa) en el módulo de elasticidad. además, los resultados de la evaluación cuantitativa esclarecen la influencia sustancial de los ciclos de congelación y descongelación en el daño y deterioro de la estructura interna de la andesita. resumen palabras clave: ciclo congelacióndescongelación; microestructura; deterioro progresivo; daño a la resistencia de la roca. record manuscript received: 05/07/2017 accepted for publication: 21/03/2018 how to cite item jiang, h. (2018). the relationship between mechanical properties and gradual deterioration of microstructures of rock mass subject to freeze-thaw cycles. earth sciences research journal, 22(1), 53-57 doi: http://dx.doi.org/10.15446/esrj.v22n1.66108 g e o ph y si c s 54 haibo jiang introduction the expansion of water during freezing process and its shrinking during thawing process inside rock mass is the primary factor for freezingthawing damage in underground engineering projects in cold area (wen, 2013). the temperature differences caused by the alternation of seasons induce repeating freeze-thaw cycles of water in rock mass, lead to enormous volume expansion force and result in the damage of rock mass (javier, 2013). the macroscopic brittle failure and strength damage of rock under freeze-thaw cycles are caused by the germination, expansion, growth and connection of many micro pore structures in the rock (rostasy, 1980; li, 2013). many laboratory experiments and detection results show that the damage of rock is caused by the continuous development of microscopic pore structure in rock mass, and rock mass experiences damages during the process in which the large pore volume increases and small pore volume decreases (mutlutürk et al, 2004). therefore, the detection and analysis of gradual deterioration progress of rock microstructures are of great significance to both the construction of correct macroscopic constitutive relation and the accurate measurement of rock strength characteristics (yan, 2015). the research on the relationship between the macroscopic mechanical characteristics and the gradual deterioration progress of rock mass microstructures under freeze-thaw cycles can greatly benefit and facilitate underground engineering projects. over the years many researchers developed the understanding through engineering practice that the effects of freeze-thaw cycle of water in rock pores change the microscopic pore structure of rock mass and cause variations in physical and mechanical properties, e.g. elastic modulus, strength and poisson’s ratio of rock mass. through a thermal expansion straining test under single freeze-thaw cycle, yamabe (2001) studied the relationship between the mechanical characteristics of rock and freeze-thaw cycle. the test proved that a freeze-thaw cycle causes the sample to deform elastically in axial direction and the saturated sample to deform plastically. they also found that there existed strong effect of freeze-thaw cycles on the damage of saturated rock. de argandona (1999) detected the damage of dolomite using x-ray, found serious weathering of rocks in a few short cycles, and assessed the damage according to the experimental results. nicholson (2000) performed the freeze-thaw cycle test for ten types of rock, and studied the impacts of lithology on the deterioration degree of rock during freeze-thaw cycles. zhang (2008) summarized the factors that cause the damage and deterioration of rock during freeze-thaw cycles, and studied the corresponding mechanism and mechanical properties of rock mass during freeze-thaw cycles. liu (2014) held the view that the elastic modulus of intact rocks decreases with the increase of freeze-thaw cycles, and through laboratory test they comparatively analyzed the freeze-thaw damage change rule of joint rock mass. the strength loss of limestone in cold regions subject to freeze-thaw recycles was studied by fatih (2012). from the studies above, the indoor freeze-thaw test demonstrates significant effects of freeze-thaw cycles on the macroscopic mechanical characteristic of the rock. the macroscopic strength decrease of rock mass has a close relation with the gradual deterioration of micro pore structure of rocks. there are many crisscross microscopic pore structures of various sizes within the rocks. the temperature differences caused by the alternation of seasons induce repeated freeze-thaw cycles of water in rock mass, and then lead to the germination, expansion, growth and connection of many micro pore structures in rocks, and ultimately lead to macroscopic brittle failure and even strength damage of rocks. under different hydration conditions, the relationships between damage and deterioration of microstructure and macroscopic mechanical characteristics of rock have drawn great attention from engineers. the recent research on rock freeze-thaw mainly concentrates on the laboratory test, but the combination of laboratory findings to practical engineering projects is still scarce. the freezethaw cycle test and the detection of micro pore structure of rock samples after the recycle were closely linked to headrace tunneling practice. in the present work the freeze-thaw tests were carried out for two groups of andesite sample rocks in different states under the conditions of the lowest freezing temperature of 40 ºc and the thawing temperature of 20 ºc. damage detection was performed by magnetic resonance imaging for the interior microstructure of rock samples subject to different freeze-thaw circles, and the relationship between the mechanical properties and the gradual deterioration of microstructures of rock mass was also discussed. the damage and deterioration mechanism of micro pore structure and macroscopic mechanical characteristic of rocks under different hydration conditions were analyzed. the results provide theoretical reference for the construction and engineering safety in cold regions. test of rock microstructure gradual deterioration subject to freeze-thaw cycles selection of samples table 1. physical mechanical property of rock samples. andesite samples were taken from a headrace tunnel project of a hydropower station in xinjiang and these sample lumps had dark gray surfaces and regular structures. most rock fracture had closed and fresh surfaces with a few with slight color-fade. occurrence was 270° ~ 285° ne <70° ~ 75° and < 275° ~ 285° ne <25° ~ 30°, smooth straight sliding surface, filled quartz vein and calcium. the physical mechanical properties of rock samples are shown in table 1. the headrace tunnel of the hydropower station is deeply buried, which allowed most of the tunnel surrounding rocks embedded in fresh rock mass. the onsite engineers tested the in-situ deformation of rock mass and obtained the field data of its mechanical characteristics, which provided references for the excavation of surrounding rocks and the design of lining structure. the deformation and elastic modulus of the tunnel rock in different states of stress were measured as shown in table 2. table 2. in-situ deformation of tunnel rock in the headrace hydropower station. note: the elastic modulus surface is located at a depth of 9.0 m in tunnel pd6. the rock was andesite with a smooth surface, and it was saturated for 7 days. the force on the elastic modulus surface was perpendicular to the bottom of tunnel. experiment program to understand the gradual deterioration progress of micro pore structure of rock mass under freeze-thaw cycles, the damage caused to rock samples was detected by magnetic resonance imaging. the results allowed us to understand how the micro pore structure of rock mass changes and to analyze the gradual deterioration of microstructure of rock mass under freezing-thawing cycles. the test procedure is stated as follows. the rock samples were generated by water drilling which drilled the cores along the direction perpendicular to the bedding plane of rock mass. after cutting and further processing, ten standard cylindrical rock mass samples were made which had a height-to-diameter ratio of 2 : 1, a diameter of 50 ± 2 mm and a height of 100 mm, according to the rock mechanics test regulations. in 55the relationship between mechanical properties and gradual deterioration of microstructures of rock mass subject to freeze-thaw cycles addition, the mass and size of the specimens were measured and each rock sample was visually inspected, as shown in figure 1. figure 1. the first group of andesite samples. (b) the samples were divided into two groups with five samples in each group. the first group (a1, a2, a3, a4, a5) were in natural saturation state, while the second group (b1, b2, b3, b4, b5) reached the saturation state with the aid of vacuum saturating device at 0.1 mpa vacuum pressure for 4 h of pumping. then both groups of samples were soaked in distilled water for 24 h. (c) based on the climate conditions at the construction site and the operating procedure of the freeze-thaw cycle test of rock mass, the freezing temperature was set at – 40 ºc and the freezing time was 4 hours. then the samples were thawed in water at temperature of 20 ºc for 4 hours. this constituted a freeze-thaw cycle, and the cycle was repeated. the two groups of rock samples were subjected to repeating freeze-thaw cycles, and data were acquired at the cycle number of 10, 20, 30 and 40. tds-300 freeze-thaw cycle machine was used for the repeating cycle test. (d) upon the completion of 10, 20, 30 and 40 cycles, the rock samples were measured by nuclear magnetic resonance relaxometry and nuclear magnetic resonance (nmr) system. the distribution curves of t2 spectrum of rock samples were obtained for two states and various freezethaw cycles. according to the area difference of sample t2 spectrum before and after the designated cycles, the rock porosity and its change rate were analyzed for different freeze-thaw cycles. (e) the freeze-thaw deterioration process of rock samples was observed and recorded, and the test results were analyzed. analysis of test results during the freezing process, the moisture in rock mass was frozen to ice. the accompanying volume expansion pushed and pressed the pore surface, which led to the extension of pore structure in rock and the generation of new pores (matsuoka, 2001). under saturation condition, pore structures in rock were filled with moisture. therefore the change of pore structure in rock mass can be characterized by the signal strength of water indicated by the nuclear magnetic resonance (nmr) system, and the relationship between nmr t2 spectrum distribution and pore size was established. nmr t2 spectrum also reflected the distribution of pores. in the t2 spectrum distribution, the peak position related to poresize, and the peak area related to the quantity of pore with corresponding aperture (li, 2012). the samples were imaged by nuclear magnetic resonance, and the transversal relaxation time of t2 spectrum distributions was obtained. the relaxation time of t2 is proportional to the pore diameter, while the signal strength reflects the quantity of pore with different apertures (zhou, 2014). by observing the changes in t2 spectrum distribution, the change of pore structure in rock can be analyzed. the t2 spectrum distributions of rock samples subject to different freeze-thaw cycles are shown in figures 2 and 3. as shown in figures. 2 and 3, after the freeze-thaw cycles, the t2 spectrum distribution of the rock samples exhibited two primary peaks. the morphological distribution of t2 spectrum shifted to the upper-left with the increase of freeze-thaw cycles, which indicates the enhancement in the t2 spectrum signal of small pores and the generation of new micro-pore structures in rock under the freeze-thaw cycles. therefore, the freeze-thaw cycles gradually deteriorated the rock microstructure. by analyzing the t2 spectrum distribution in figure 2 and 3, it was found that the first peak in the t2 spectrum distribution is significantly higher than the second for all the samples tested. the trend shows that new micropore structures were generated in rock samples. during freeze-thaw cycles, the moisture migrated to new pore structure, and the t2 spectrum signal strength was enhanced. the results show that under the freeze-thaw cycles, pore structures of all sizes expanded inside rock mass. not only were new pore structures generated, but also the original micro-pore structures extended, with a growth in the size of pore structure. this caused further damage to the rock microstructure. 0 50 100 150 200 250 300 350 400 450 500 0.01 0.1 1 10 100 1000 10000 relaxation time (t2) after freezing and thawing cycles/ms si gn al a m pl itu de . without freezing-thawing cycles after 10 freezing-thawing cycles after 20 freezing-thawing cycles after 30 freezing-thawing cycles after 40 freezing-thawing cycles figure 2. t2 spectrum distribution of naturally saturated rock samples. 0 50 100 150 200 250 300 350 400 450 0.01 0.1 1 10 100 1000 10000 relaxation time (t2) after freezing and thawing cycles/ms sig na l a mp lit ud e. without freezing-thawing cycles after 10 freezing-thawing cycles after 20 freezing-thawing cycles after 30 freezing-thawing cycles after 40 freezing-thawing cycles figure 3. t2 spectrum distribution of rock samples saturated by vacuum. the peak of t2 spectrum distribution for naturally saturated samples was obviously higher than that of the samples saturated by vacuum. this indicates that naturally saturated rock samples had many micro-pore structures generated after the freeze-thaw cycles, and had faster generation and expansion speeds of fracture structure. these results explain the further deterioration of rock microstructure. table 3. the rock porosity and its change rate under different freezethaw cycles. 56 haibo jiang table 3 shows the rock sample porosity and its change rate after 10, 20, 30 and 40 freeze-thaw cycles. as shown in table 3, the porosity of the rock increased with the freeze-thaw cycles and there was small variation of amplitude among the samples of each group. for naturally saturated samples, their porosity increased by 42.56% ~ 72.72% after 20 freeze-thaw cycles, 145.45% ~ 103.81% after 30 cycles and 146.71% ~ 180.24% after 40 cycles. for samples saturated by vacuum, the porosity increased by 38.46% ~ 67.74% after 20 freeze-thaw cycles, 99.67% ~ 142.12% after 30 cycles and 149.0% ~ 188.97% after 40 cycles. it is noted that the number of freeze-thaw cycles has essential impact on the porosity of rock sample. the increase of freeze-thaw cycles resulted in faster generation and expansion speeds of fracture structure, as well as more evident change of porosity and gradual deterioration of rock microstructure. macroscopic mechanical characteristics of the rock mass subject to freeze-thaw cycles test method and scheme to establish the relationship between macroscopic mechanical characteristics and gradual deterioration of rock mass microstructure, the quality of rock samples subject to freeze-thaw cycles was measured by nuclear magnetic resonance (nmr) technique in order to investigate the rapid non-destructive detection characteristics. the samples subject to 0, 10, 20, 30, 40 cycles were tested for compression strength. the relationship between microscopic gradual deterioration and macroscopic mechanical characteristics of rock samples was analyzed for different freeze-thaw cycles. after the rock samples were detected by nuclear magnetic resonance (nmr), compression strength test and elastic modulus measurement were immediately performed using a universal qw-w300 microcomputercontrolled low-temperature electron test equipment. the data in each group were recorded and the results were analyzed systematically. the analysis of uniaxial compression characteristics table 4. strength and elastic modulus of andesite under different freezethaw cycles. due to the difference in lithology, mineral composition, porosity and water content, etc., the damage and degradation patterns in rocks subject to freeze-thaw cycles are also different. during the freeze-thaw cycles, the respective strength damage laws of rock also varied. table 4 lists the uniaxial compression strength and elastic modulus of andesite subject to different freeze-thaw cycles. figure 4 and figure 5 show the strength damage curves and elastic modulus curves of two groups of andesite under different freezethaw cycles. as shown in figure 4, with fewer freeze-thaw cycles, the compression strength of andesite is stronger. the increase of freezethaw cycles results in decreasing compression strength. the detection of microscopic pore structure of the rock samples uncovered that new microscopic pore structures were generated with the increase of freezethaw cycles. during freeze-thaw cycles, moistures migrated to new pore structures. continuous freeze-thaw cycles resulted in gradual deterioration of microstructure and larger compression strength of the rock samples. 0 50 100 150 200 0 10 20 30 40 cycles of freezing-thawingun ia xi al c om pr es si on s tr en gt h/ m p a naturally saturated saturated by vacuum 0 2 4 6 8 0 10 20 30 40 cycles of freezing-thawing e la st ic m o d u lu s/ g p a naturally saturated saturated by vacuum figure 4. uniaxial compression strength of andesite. figure 5. elastic modulus of andesite. during the test, no pore structure change was observed after small number of freeze-thaw cycles. after 10 ~ 20 cycles, the original micropore structures began to extend. in the process of freeze-thaw cycles, the newly generated micro-pore structures were observed through nuclear magnetic resonance (nmr) detection. during freeze-thaw cycles, the damage kept deepening and the cracks extended with the increase of cycles. the test data and figure 4 show that, the strength of andesite rock samples decreased by different amount under various freeze-thaw cycles. the strength of naturally saturated rock samples decreased by 7.66% (equivalent to 13.5 mpa) after 10 cycles, 16.5% (equivalent to 29.1 mpa) after 20 cycles, 30.17% (equivalent to 53.2 mpa) after 30 cycles and 39.4% (equivalent to 69.4 mpa) after 40 cycles, respectively. for rock samples saturated by vacuum, the strength decreased by 7.03% (equivalent to 11.1 mpa) after 10 cycles, 14.88% (equivalent to 23.5 mpa) after 20 cycles, 25.84% (equivalent to 40.8 mpa) after 30 cycles and 36.86% (equivalent to 58.2 mpa) after 40 cycles. based on the test data and figure 5, the elastic modulus of naturally saturated rock samples decreased by 21.77% (equivalent to 1.5 gpa) after 10 cycles. the reduction continued for more cycles, which is 31.35% (equivalent to 2.16 gpa) after 20 cycles, 41.51% (equivalent to 2.86 gpa) after 30 cycles and 47.46% (equivalent to 3.27 gpa) after 40 cycles. for rock samples saturated by vacuum, the reductions of elastic modulus are 18.54% (equivalent to 1.17 gpa) after 10 cycles, 29.95% (equivalent to 1.89 gpa) after 20 cycles, 38.67% (equivalent to 2.44 gpa) after 30 cycles and 44.85% (equivalent to 2.83 gp) after 40 cycles, respectively. conclusions the macroscopic mechanical properties of andesite deteriorate gradually with the increase of freeze-thaw cycles. the deterioration mainly shows that, both the strength and the elastic modulus decrease gradually with the increase of freeze-thaw cycles. the macroscopic strength damage of rock mass is closely related to the gradual deterioration of its microstructure. the microscopic pore structures in rocks continuously develop and expand with the increase of freeze-thaw cycles, accompanied by the migration of moisture. continuous freeze-thaw cycles cause the gradual deterioration of microstructure and larger strength damage of the rock samples. (2) the gradual deterioration test of micro-structure of rock samples under freeze-thaw cycles suggests that the development and expansion speed of pores vary in different freeze-thaw stages. the nuclear magnetic resonance area of each rock sample increases with the freeze-thaw cycles. there are evident and distinct changes of nuclear magnetic resonance areas of all rock samples after 10, 20 and 30 cycles, which demonstrates different rates of freeze-thaw damage in different stages. (3) the influence of freeze-thaw cycles on the mechanical properties of andesite rock samples under different conditions is substantial. with the increase of freeze-thaw cycles, the uniaxial compression strength and elasticity modulus of rock present a clear reduction trend. the strength of 57the relationship between mechanical properties and gradual deterioration of microstructures of rock mass subject to freeze-thaw cycles naturally saturated rock samples decreased by 39.4% (equivalent to 69.4 mpa) and the elasticity modulus decreased by 47.46% (equivalent to 3.27 gpa) after 40 cycles. in contrast, the strength of rock samples saturated by vacuum decreased by 36.86% (equivalent to 58.2 mpa) and the elasticity modulus decreased by 44.85% (equivalent to 2.83 gpa) after 40 cycles. the results demonstrate evident influence of freeze-thaw cycle on the damage and deterioration of internal structure in andesite. it should be noted that the strength damage and even deterioration caused by freeze-thaw cycles in cold regions is a complicated process in rock engineering, and this is a complex thm multi-physics problem at low temperature. the phase change of pore water and ice in the rock mass is the key factor of rock freeze-thaw damage. by microscopic detection, the relationship between the macroscopic mechanical characteristics and the gradual deterioration of microstructure of rock mass under freezethaw cycles was investigated in this work. the study was based on the microscopic dimensions of the gradual degradation of rock microstructure under freeze-thaw cycles, and then extended to the effect of freeze-thaw cycles on macroscopic mechanical characteristics of rock mass. the present work contributed to discovering the damage mechanism of frozen rocks. the present work suggests that further study on rock freeze-thaw cycle damage should be based on the microscopic dimensions, extend to the effect of freeze-thaw cycles on the development of rock mass fracture network, and focus on the macro mechanical characteristics of rock. acknowledgements this research was financially supported by the national natural science foundation of china. 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(2004). study on the damage propagation of surrounding rock from a cold–region tunnel under freeze-thaw cycle condition. tunneling and underground space technology, 19, 295-302. zhou, k. p., hu, z. x. & li, j. l. (2014). study of marble damage evolution laws under unloading conditions based on nuclear magnetic resonance technique. chinese journal of rock mechanics and engineering, 33, 3523-3530. the poor water retention capacity of sandy soils commonly aggregate soil erosion and ecological environment on the chinese loess plateau. due to its strong capacity for absorption and large specific surface area, the use of nanocarbon made of coconut shell as a soil amendment that could improve water retention was investigated. soil column experiments were conducted in which a layer of nanocarbon mixed well with the soil was formed at a depth of 20 cm below the soil surface. four different nanocarbon contents by weight (0%, 0.1%, 0.5%, and 1%) and five thicknesses of the nanocarbonsoil mixture layer ranging from 1 to 5 cm were considered. cumulative infiltration and soil water content distributions were determined when water was added to soil columns. soil water characteristic curves (swcc) were obtained using the centrifuge method. the principal results showed that the infiltration rate and cumulative infiltration increased with the increases of nanocarbon contents, to the thicknesses of the nano carbon-soil mixture layer. soil water contents that below the soil-nano carbon layer decreased sharply. both the brooks-corey and van genuchten models could describe well the swcc of the disturbed sandy soil with various nano carbon contents. both the saturated water content (θs), residual water content (θr) and empirical parameter (α) increased with increasing nano carbon content, while the pore-size distribution parameter (n) decreased. the available soil water contents were efficiently increased with the increase in nanocarbon contents. abstract keywords:soil-nano carbon mixture layer; infiltration process; soil water characteristic curves; available water content effect of nano-carbon on water holding capacity in a sandy soil of the loess plateau issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n4.66104 earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017): 189-195. beibei zhou* and xiaopeng chen state key laboratory base of eco-hydraulic engineering in arid area, xi’an university of technology, xi’an 710048, china la poca capacidad de retención de agua en suelos arenosos es un factor determinante en la erosión del terreno y en el entorno ecológico de la meseta de loes, en china. debido a su capacidad de absorción y a su amplia superficie específica, se investigó el uso de materiales nanocarbonados hechos de cáscara de coco de forma que con la estabilización del suelo se pueda mejorar la retención de agua. se realizaron experimentos con muestras tomadas en el área de estudio en las cuales se mezcló una capa de materiales nanocarbonados a una profundidad de 20 cms. los análisis se realizaron a partir de cuatro parámetros establecidos por el contenido en peso de los materiales nanocarbonados (0 %, 0.1 %, 0.5 % y 1 %) y de cinco parámetros basados en el espesor de la mezcla de nanocarbonados con el suelo, que van de uno a cinco centímetros. las distribuciones de infiltración acumulada y contenido de agua frente a material sólido se determinaron al añadir agua a las muestras de suelo. a través del método de centrifugación se obtuvieron las relaciones agua-suelo (swcc, del inglés soil water characteristic curves). los principales resultados muestran que el índice de infiltración y la infiltración acumulada aumenta con proporción directa a los contenidos nanocarbonados, al igual que crece el espesor de la capa mezclada del suelo con materiales nanocarbonados. los contenidos de agua y suelo en la capa de suelo nanocarbonada se redujeron pronunciadamente. los modelos de brooks-carey y van genuchten describen las relaciones de agua-suelo en suelos arenosos irregulares con varios contenidos de nanocarbón. tanto el contenido de saturación de agua (θs), como el contenido de agua residual (θr) y el parámetro empírico (α) aumentaron proporcionalmente con el contenido de nanocarbón, mientras que el índice de distribución de los poros (n) bajó. los contenidos disponibles de agua y suelo se incrementaron eficientemente con el aumento de los contenidos nanocarbonados. resumen palabras clave: mezcla de suelo nanocarbonado; proceso de infiltración; relación agua-suelo; contenido disponible de agua. record manuscript received: 05/07/2017 accepted for publication: 16/11/2017 how to cite item: zhou, b. & chen, x. (2017). effect of nanocarbon on water holding capacity in a sandy soil of the loess. earth sciences research journal, 21(4). 189-195. doi: http://dx.doi.org/10.15446/esrj.v21n4.66104 efecto de materiales nanocarbonados en la capacidad de retención de agua en suelos arenosos de la meseta de loes g e o c h e m is t r y 190 beibei zhou and xiaopeng chen introduction the loess plateau, located in northwest china, is a key agricultural area for the country. however, it is also susceptible to some of the most severe soil and water losses on the slope in the world (bo et al., 2011; chen et al., 2007; li et al., 2015; mi et al., 2014). as a result, numerous studies have investigated the soil and water losses on the hillslope from the plateau (she et al., 2010; el-jakee et al., 2016). these studies have mainly focused on ecosystem management to reduce the soil and water losses by optimizing land use structure, which could involve steep terracing slopes, converting cultivated steep slopes to forests and grasslands, banning grazing, constructing reservoirs and check dams, etc. therefore, there is an urgent need for developing novel and effective measures to improve soil structure at the loess plateau. the implementation of these measures facilitated vegetation restoration and ecological conservation. however, even though the soil layer on the plateau is very thick, its weak structure and/or coarse texture often leads to a low soil water holding capacity. furthermore, surface runoff generated during rainstorms, which can be intense, can still reduce the effectiveness of local soil erosion and water controls. in the past decade, there has been a rapid growth in the manufacture and use of soil amendment in helping with the soil water capacity, such as biochar and charcoal. but high rates of this amendment, always 100 and 200 t/ha, which is not practically feasible at farmer level (zhou, 2006; liu and liu, 2010). nanomaterials have been used in a wide range of industrial and agricultural fields (kammann et al., 2013; nayak et al., 2013). nanocarbon has been found to improve plant growth. khodakovskaya et al. (2009) found that nanocarbon was able to penetrate the thick coat of tomato seeds and enhance water uptake into the seed, which improved seed germination and the growth of the tomato seedlings in water culture experiments. subsequently, kammann et al. (2013) reported that tomatoes produced two times more flowers and fruit when grown in soil containing carbon nanotubes than when grown in soil without carbon nanotubes. fan et al. (2012) concluded that the application of different levels of nitrogen fertilizer and nanocarbon significantly affected nitrogen use in a rice paddy field with a saline-alkali soil, and that yield increased when nitrogen was added with nanocarbon. when wu (2013) applied nanocarbon contained within two slowreleased fertilizers, rice yields and nitrogen use efficiency both increased while nitrogen losses in the surface water runoff from the paddy soil for both slowreleased fertilizers decreased when compared to the fertilizers alone. the capacity of nanocarbon to adsorb other elements is also of importance. liu and cheng (2012) found that cu2+ and cd2+ were better adsorbed onto a modified oxidized nano-carbon black sorbent. wang et al. (2009) applied nanocarbon near the roots of ryegrass and found that there were reductions in the cu and zn concentrations in the ryegrass and that the ryegrass biomass increased. khaydarov et al. (2012) reported that nanocarbon-polymer compositions were effective in removing metal ions from contaminated groundwater and soil. there have been many studies about uses of nanocarbon in agriculture. however, most of them focused on the ability of nanocarbon to adsorb heavy metals and improve crop yields. there is a scarcity of published studies on the effects of nanocarbon on soil water holding capacity, especially in sandy soils. soil water holding capacity is a major factor that can affect soil erosion by water, as well as plant growth (heathman et al., 2003; she et al., 2014).tan et al. (2014) found that nano carbon could increase the water contents effectively in columns of the disturbed soil column. these findings are important since they imply that nanocarbon would also reduce water losses due to percolation to deeper soil layers and that soil layers containing nanocarbon can hold more water for a longer time, for example, in soil near to plant roots. one of the advantages of nanocarbon made of coconut shell, when compared to metallic nanomaterials, is that carbon naturally exists which is also good for controlling carbon emission (nair, 2010). consequently, the use of nano-carbon materials in agriculture is more acceptable and has been widely propagated. therefore, in this paper, the effects of different nano-carbon contents of layers containing nanocarbon-soil mixtures on soil water infiltration and soil water characteristic curves (swcc), as well as on soil water content distributions in the soil profile after infiltration ceases were examined. since nanocarbons are expensive, we also investigated the effect of reducing the thickness of the soil-nanocarbon layers, which would, in turn, reduce the costs, on these soil water properties. figure 1. columns of sandy soil with soil-nanocarbon mixture at different nanocarbon contents and thicknesses. materials and methods soil and nano carbon the soil used in the study was an aeolian sandy soil, typical of soils on the loess plateau. it developed from loessial parent material and was characterized by a loose structure and high erodibility. the soil was collected from the shenmu soil water conservation field station, the chinese academy of sciences (110°11′-110°23e, 109°19′23″e, 38°46′-38°50e), yulin county, shaanxi province. the collection site has a semiarid area climate with a mean annual rainfall of 437 mm, of which about 70% falls in intense storms between june and september that can cause high soil and water losses. to reduce the influence of organic matter found in the topsoil, the upper 10 cm soil layer was removed first, and only soil from the 10~25 cm layer of the profile was collected. debris, which included dead leaves and animals, was removed, and the soil was airdried and passed through a 2-mm sieve. particle size analysis was determined by sieving and the pipette method (lv, 1992).the particle size distribution is as follows, sand content, 0.482 kg kg−1; silt content, 0.255 kg kg−1, which belong to the aeolian sandy soil according to the usda soil system (hillel, 1982). the nanocarbon used in this study was supplied by the hainuo nano material company (shanghai), china and the mean specifications of the particles were: 40 nm diameter; 0.99 g cm-3 density; purity 99%. infiltration experiment in this part, the effect of nanocarbon contents and thicknesses of soilnanocarbon mixture layer on infiltration was studied. the nanocarbon was mixed well with the air-dried soil samples to give four different gravimetric contents (0%, 0.1%, 0.5%, and 1.0%). then the mixtures with different thickness (1 cm, 2 cm, 3 cm, 4 cm and 5 cm) were applied to the packed soil columns (10 cm in diameter, 50 cm in length) 20 cm below the surface where is near most of the plant roots (shown in figure 1). 191effect of nano-carbon on water holding capacity in a sandy soil of the loess plateau the bases of all of the columns were perforated and were covered by a coarse filter paper, to permit water to drain and prevent air entrapment while retaining the soil. the surfaces of the soil mixtures were also covered with a circle of filter paper to reduce disturbance from the inflowing water. infiltration experiments were carried out by rapidly establishing and then maintaining a constant 2 cm head of deionized water on the surface of the soil using a mariotte bottle. the cumulative infiltration rate was initially determined from the volume of water supplied from the mariotte bottle measured at timed intervals. the infiltration experiments were carried out in three replications in a laboratory where the mean temperature was 28±3 °c. relative humidity was not controlled but remained at 40%±10%. the top 40cm of the soil columns at every 3cm depth increment was sampled just after the infiltration process, and the gravimetric soil water content was determined by mass loss during ovendrying at 105 °c. soil water characteristic curve (swcc) to understand the effect of nanocarbon on swcc, the air-dried sandy soil was thoroughly mixed well with the nanocarbon with five different gravimetric contents (0%, 0.1%, 0.5%, 0.7% and 1%). all the mixtures were packed into a cutting ring (100 cm3 in volume) to give a bulk density of 1.4 g cm-3. the rings with the combinations were placed in distilled water to become slowly saturated within about 12 hours. each saturated mixture was placed in a centrifuge tube. the tubes were centrifuged in a state make and model of centrifuge. to measure the water contents of the mixtures at different pressure heads, centrifugation was carried out at specific rotation speeds corresponding to known pressure heads. after attaining equilibrium at each given pressure head, the tubes were removed from the centrifuge to determine the gravimetric water content of the mixture. excess water was poured off, and the mass of the mixture was measured using an electronic balance (precision of 0.01 g). the procedure was repeated sequentially as the pressure head was increased to the highest level of 10200 cm. centrifugation was carried out at 20. at the end of the experiment, the gravimetric water content of the mixture samples was put into the ovendrying at 105°c for 24 hours. subsequently, the volumetric water contents of the different mixtures at each pressure head were determined from the dry bulk density and gravimetric water content. the swcc was then obtained by plotting the pressure head (cm) against the volumetric water content (cm•cm-3). water repellency of the tested nano carbon-soil mixture samples the soil samples were mixed well with nanocarbon contents at five different gravimetric contents (0%, 0.1%, 0.5%, 0.7% and 1%). a wdpt test (water droplet penetration test) was also used to measure water repellency (king, 19981 in this study to understand the effect of nanocarbon contents on water repellency. the penetrate time was recorded. each treatment has ten replicates. and the results are shown as follows with the mean values (in table 1). table 1. effect of nanocarbon contents on water repellency. models brooks-corey model (bc) according to brooks and corey (1964), swcc functions are given by: (1) where se is the degree of saturation; q is the volumetric water content (cm3 cm-3); qs is the saturated volumetric water content (cm3 cm-3); qr is the residual volumetric water content (cm3 cm-3); a is an empirical parameter (cm1) whose inverse is often referred to as the air entry value or bubbling pressure; h is the soil water pressure head (cm); and n is a pore-size distribution parameter affecting the slope of the retention curve. van genuchten model (vg) van genuchten (1980) found relatively simple functions could describe the swcc well: (2) where se, q, qs, qr, a, h, n, and ks are the same as in equation (1), and m is an empirical parameter. according to the statistical pore-size distribution model of mualem (1976), the parameter m is expressed as m=1-1/n (n>1). burdine’s model burdine (1953) proposed a model that described the hydraulic conductivity (k). when the bc is used in burdine’s model, the hydraulic function is given by the expression: where, l is a pore-connectivity parameter, which was assumed to be 2.0 in the original study of brooks and corey (1964); se and n are the same as in equation (1), and ks is the same as in equation (3). mualem’s model mualem (1976) described a new model for predicting the hydraulic conductivity. combined with vg, the hydraulic conductivity is given by: 1/2 ++= ln es skk (3) (4) (5) where ks, l, and se are the same as in equation (4), and m is the same as in equation (2). the pore-connectivity parameter, l, in the hydraulic conductivity function was estimated to be about 0.5 and was considered as a mean value that could be valid for many soils (mualem et al., 1976). to analyze the cumulative infiltration data, we fitted the experimental data with philip’s infiltration equation (equation (5)), the parameters of which are presented in table 2: (5)where i is the cumulative infiltration (l), t is the time (t), and s is the sorptivity (l t-1/2), a is the steady-state infiltration rate (l t-1). 192 beibei zhou and xiaopeng chen table 2. experimental treatments. results and discussion effect of soil-nano carbon mixture layers on soil water infiltration processes effect of different nano carbon contents on cumulative infiltration figure 2. effect of nanocarbon contents on soil water infiltration for 5 soil-nano carbon layer thicknesses. figure 2 presents the effect of nanocarbon contents on the soil water infiltration process. smooth cumulative infiltration curves were obtained for all nanocarbon contents and followed a similar pattern to those typical for homogeneous soils without nanocarbon. this indicated that the fine soil matrix was continuous in all cases. during the early stage of the infiltration process, there was no noticeable effect of nanocarbon content on cumulative infiltration since the wetting front had not yet entered the soil-nanocarbon mixture. when the wetting front reached the soil-nanocarbon mixtures, 20 cm below the soil surface, the nanocarbon content had significant effects on the infiltration rates of all four soilnanocarbon mixtures in each of the five-layer thicknesses. the cumulative infiltration increased with increasing nanocarbon content. this was the opposite effect of that reported by tan et al. (2014) for various mixture compositions in different soil columns that were each filled by a particular homogenous soil-nanocarbon mixture. cumulative infiltration increased at significantly faster rates when nanocarbon was present than when it was not present. this indicated that the presence of a soil-nanocarbon layer at a depth of 20 cm within the soil profile could accelerate infiltration. there was no noticeable difference between the effects of the 1% and 0.5% nanocarbon contents on cumulative infiltration. soil water infiltration could be affected greatly by hydrophobic materials which are either intermixed with the soil or coat the mineral soil particles. the infiltration curves for wettable and water repellent soils presented earlier reflect this effect (figure 2). the practical implications of water relations in these hard-to-wet soils (such as the effects on plants, runoff, and erosion) have led to numerous studies on water movement. the effect of hydrophobic substances on water movement in soils provides a basis for characterizing water repellency. effect of different thicknesses of nano carbon-soil layers on soil water infiltration incorporating nanocarbon into the soil profile at depth incurs high costs and requires more labor. therefore, it is desirable to apply the most costeffective amount of nanocarbon possible to reduce the expense of the treatment. this raises the important question as to how much nanocarbon is needed in the sandy soil. therefore, we investigated the effect of changing the thicknesses of the soil-nano carbon layers on soil water infiltration (figure 3). smooth cumulative infiltration curves were obtained for all soil-nanocarbon layer thicknesses, which followed a pattern similar to those typical for homogeneous soils without nanocarbon. this indicated that the fine soil matrix was continuous in all cases. cumulative infiltration increased at a significantly faster rate when the thickness of the soil-nanocarbon mixture increased. this also indicated that the presence of soil-nanocarbon at 20 cm depth within the soil column could accelerate infiltration of all the layer thicknesses investigated. figure 3. effect of soil-nanocarbon layer thickness on soil water infiltration for three nanocarbon contents. the effect of the three thicker soil-nano carbon layers (3, 4, and 5 cm) on cumulative infiltration was considerably greater than that of the two thinner layers (1 and 2 cm). statistically, there were no significant 193effect of nano-carbon on water holding capacity in a sandy soil of the loess plateau differences for cumulative infiltration among the mixture layers with thicknesses of 3, 4 and 5 cm. this would suggest that a layer of the soil-nanocarbon mixture that was 3 cm thick would be sufficient to improve cumulative infiltration and that increasing the thickness of the layer would not result in further improvement. to analyze the cumulative infiltration data, we fitted the experimental data with philip’s infiltration equation, the values of the fitted parameters for each of the 20 treatments are presented in table 3. philip’s infiltration equation considers both the downward movement of water, due to gravity, and advection due to matric forces. the coefficient of determination, r2, values were all above 0.99, which indicated that the equation effectively described the relation between cumulative infiltration and time during the course of our experiment. both the sorptivity and the steady state infiltration were affected similar by either increasing the nanocarbon contents or the thicknesses of the soil-nanocarbon mixture layer (table 3). figure 4. effect of nanocarbon layer content and thickness on soil water contents at ten 3-cm depth increments in the soil profile. table 3. parameters for the philip infiltration equation determined for columns with soil-nanocarbon mixture layers having various nanocarbon contents and layer thicknesses. note: r2 is the coefficient of determination for the fit of i = st 1/2 to the experimental data, where i is the cumulative infiltration, t is the time, s is the sorptivity, and a is the steady-state infiltration rate, with the experimental data the main effect of the presence of the nanocarbon is to act as a strong absorptive for water transmission. the values of sorptivity determined by philip’s equation were higher for nanocontents of 0.5% and 1% than for the soil alone and may be an indication of this increased soil water infiltration ability. during the initial stage of the infiltration process, water movement into the dry soil was faster due to the presence of the matrix potential that existed at the wetting front between the wet and dry soils. the presence of the soil-nanocarbon layer might be expected to represent a discontinuity in the soil pore system. however, the matrix potential might also be expected to increase with increasing nanocarbon contents when the wetting front reached the soil-nanocarbon layer. however, from our data, it appears that there was a critical nanocarbon content of about 0.5%, for the 3-cm thick soil-nanocarbon layer. we attributed this to a greater degree of special characteristic of nanocarbon in the soil profile. effect of different thicknesses of soil-nano carbon layers and nano carbon contents on soil water contents in the soil profile the findings of this study presented above have shown that the soilnanocarbon could increase soil water infiltration rates. this might help reduce the amount of runoff considerably from the slopes on the loess plateau. this suggestion should be investigated further in rainfall simulation experiments. however, it is not clear that the nanocarbon layer could hold sufficient soil water for crop growth on the semi-arid loess plateau. after the infiltration experiments ended, the soil in the column was sampled at every 3-cm depth increment to find out where the infiltration water was stored. the measured soil water contents of these soil samples are shown in figure 5. the soil water content in the control soil column varied little with depth, and the soil water content was relatively high even at the lower end of the soil column; this distribution of soil water contents was similar to that of the column containing the layer with a nanocarbon content of 0.1%. with the increases in the nanocarbon content and in the thickness of the soil-nanocarbon mixture layer, the soil water contents below 20 cm were considerably reduced. this was especially the case when the thickness of the soil-nanocarbon mixture was 3-5 cm. these results showed that the sandy soil with nanocarbon could hold more available water for plant growth as well as reducing runoff. a positive correlation between applied nanocarbon contents and mixture thickness was obtained. therefore, nanocarbon has the potential to serve as a soil amendment that could increase the plant available soil water reservoir. effect of nano carbon contents on swcc in the disturbed sandy soil the swcc represents the relationship between the matric potential and the water content or the degree of saturation. it is related to pore-size distribution, structure, and the nature and extent of the soil surfaces. figure 5. soil water characteristic curves for a disturbed sandy soil with different contents of nanocarbon. 194 beibei zhou and xiaopeng chen194 as the matric potential is less than zero, the soil energy state was better expressed by the pressure head (the negative value of the metric potential) to give a more readily interpreted illustration of the swcc. figure 5 presents volumetric water content-pressure head equilibrium data points. smooth swccs were obtained for the five nanocarbon contents, which followed similar patterns to that of the homogenous sandy soil without nanocarbon. at the lower pressure heads (0-102 cm), since most of the drainage water was due to gravitational forces, the differences in volumetric water content changes for soil containing the various nanocarbon contents were not evident. changes began to exhibit differences in the high-pressure head section (>102 cm) of the swcc. the volumetric water content for all treatments presented a sudden reduction, but the ranges were different. in this study, the slope dh/dq at 102 cm pressure head was used to describe the intervals. the slopes for the five nanocarbon contents between 0% and 1% were -2.23×10-3, -1.73×10-3, -1.49×10-3, -1.40×10-3, and -1.34×10-3, respectively. similarly, with increasing pressure head, reductions in volumetric water content for soils containing nanocarbon were less than those for the disturbed sandy soil alone. the phenomenon manifested itself in that the slope increased with increasing nanocarbon content and when the soil contained nanocarbon the slope became steeper. furthermore, these data points in figure 5 indicated that the range of pore sizes was greater for the soil samples that contained nanocarbon than for the soil without nanocarbon and increased with the nanocarbon content. due to its large specific surface area and ultrafine size, nanocarbon tended to increase the fine pores between soil particles by partially filling those spaces. this caused a shift in the pore-size distribution by decreasing the number of larger pores and increasing the number of smaller pores in the system. at low-pressure heads, water tended to pass through larger pores. as the pressure head increased (>100 cm), water in the macropores was emptied and only remained in the smaller pores. there were larger suctions in the finer pores. thus, water discharge for soil with nanocarbon was less than for the homogeneous soil without nanocarbon. consequently, nano carbon could improve soil water retention capability to some extent. table 4. parameters of the soil water characteristic curve for a disturbed sandy soil with various nanocarbon contents fitted by the brooks–corey (bc) and van genuchten (vg) models. note: r2 is the coefficient of determination for the fit of the vg or bc model to the experimental data; θs, saturated water content; θr, residual water content; α, empirical parameter; n, pore-size distribution parameter the minimum data requirement for swcc generation is at least five q – h pairs (nimmo and akstin, 1988). thus, to determine the effect of nanocarbon on the swcc shape and related physical properties of the disturbed sandy soil, this study used the brooks-corey (bc) (equation (2)) and van genuchten model (vg) (equation (4)) to describe the dehydration process of the swcc occurring under centrifugation. the fitted parameters of these two models are given in table 3. the r2 values for the vg and bc models relating the predicted values to the measured data values were above 0.99 and 0.98, respectively. this indicated that, as with the homogenous sandy soil alone, the swccs for the sandy soil with different nanocarbon contents could be described well by both models although the vg was slightly better fitted. from table 4, it was clear that both qs and qr were affected in a similarly increasing manner by the increases in nanocarbon contents. moreover, the increased available water contents due to nanocarbon in the soil could be calculated by qsqr, which further indicated that nanocarbon could improve the water capacity. table 5. the relationship between nano carbon content and parameters of the brooks–corey (bc) and van genuchten (vg) models. note: * p<0.05; ** p<0.01; α, empirical parameter; n, pore-size distribution parameter comparing the two models, a and n in vg were about 1.5 and 3 times larger than in bc, respectively. in both the vg and bc models, the value of a increased, except for the 0.7% nanocarbon content, and n decreased with growing nanocarbon contents. the value of a expresses the inverse of the air entry value. the increasing values of a suggested that air entry values decreased and soil water capacity increased with the increases in nanocarbon content. this further confirmed that sandy soil containing nanocarbon had more available water for plant growth. the parameter n is related to pore-size distribution. the values of n decreased with increasing nanocarbon content which indicated that the fine pores increased (table 4). due to the special characteristics of nanocarbon (i.e., the large specific surface area and high adsorption capacity), adding nanocarbon to the soil could reduce the proportion of larger pores while increasing that of the small pores. consequently, the saturated water volumetric content increased with the increases in nanocarbon content. also, it was clear that the parameters of the two models were linearly related to the nanocarbon content. conclusions this study investigated the effects of infiltration and the soil water holding capacity of adding nanocarbon to a sandy soil. the addition of nanocarbon to the sandy soil had a significant effect on soil water holding capacity, as well as on infiltration. based on the analysis of swccs, the presence of nanocarbon enhanced water retention and increased the proportion of fine pores. both the brooks-corey and van genuchten models could well describe the swccs for the disturbed sandy soil with different nanocarbon contents. in general, qs, qr and a increased with increases in nanocarbon content, while n decreased. nano carbon content was linearly related to the parameters of the vg and bc models. infiltration experiments investigated four different nanocarbon contents of a soil layer with five different thicknesses within a soil profile. cumulative infiltration increased with increasing nanocarbon contents and with the thickness of the soil-nanocarbon mixture layer. this was especially evident when the nanocarbon content was higher than 0.5%, and the thickness of the mixture layer was between 3 and 5 cm. storage of infiltrated water increased, which indicated that nanocarbon could be beneficial for plant growth and could reduce losses of water due to percolation to deep soil layers. this study provides some basic information about the effects of nanocarbon on the water holding capacity of a sandy soil on the loess plateau. it suggests that nanocarbon may provide a means by which to improve the poor structure of loess plateau soils. however, more detailed and systematic research is required to determine optimal levels of nanocarbon and in what manner it is added to the soil, as well as the effectiveness ofnano carbon over time. acknowledgements the work in this paper is supported by the national natural science foundation of china (51239009; 41371239), the science and technology planning project of shaanxi province (2013kjxx-38), the shaanxi water conservancy project (2013slt-04), and the doctoral fund of xi’an university of technology (106-211301), which are all gratefully acknowledged. 195effect of nano-carbon on water holding capacity in a sandy soil of the loess plateau 195195 references abbasi, n., & yasin, m. 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(2006). modeling vegetation coverage and soil erosion in the loess plateau area of china. ecological modelling 198, 263-268. keywords: expansion intensity; land use; hotspot analysis; the wanjiang city belt along the yangtze river. palabras clave: intensidad de expansión; uso del suelo; análisis de puntos críticos; el cinturón de la ciudad de wanjiang a lo largo del río yangtze. how to cite item cao, y., liu, m., cao, y., chen, c., & dapeng, z. (2020). change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example. earth sciences research journal, 24(2), 215-223. doi: https://doi. org/10.15446/esrj.v24n2.87710 the construction land includes urban land, rural residential areas, and other construction lands. the wanjiang city belt along the yangtze river is a vital demonstration area for undertaking industrial transfer in china. with the accumulation of factors relative to economic development, the construction land has increased sharply, and the regional ecological security pattern is facing new challenges. after collecting the image interpretation data of multi-period land use of the wanjiang city belt, this work studies the characteristics of construction land change patterns since 1995. the driving mechanism was also analyzed based on the gis platform, land use transfer matrix, expansion intensity index, hotspot analysis, and mathematical statistics. the results showed that: (1) from 1995 to 2015, the urban land and other construction lands in the wanjiang city belt have increased, but the rural residential areas decreased in 20102015. the three types of lands had the most significant changes in 2005-2010, and the other construction land was particularly prominent. (2) the hotspots for construction land expansion are mainly in urban areas with rapid economic development such as hefei, wuhu, maanshan, and tongling, where the land-use changes most severely. (3) the driving factors for the shift in construction land area include natural and social factors. among social and economic factors, the gdp, industrial added value, secondary output value, and urbanization rate are the main driving forces for changes. in the past 20 years, the construction of china’s undertaking industrial transfer demonstration area has changed the optimal land allocation and intensive use mode in the region, providing the basis for resource development and utilization, economic development, and industrial structure adjustment. abstract change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example patrón de cambio y mecanismo de conducción de terrenos de construcción en el área de empresas de transferencia industrial chinas: ejemplo del distrito de wanjiang, en la ribera del río yangtze issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87710 el terreno de construcción incluye el terreno urbano, las áreas residenciales rurales y otros terrenos de construcción. el distrito industrial de wanjiang, en la ribera del río yangtze, es un área importante para llevar a cabo la transferencia industrial en china. con la acumulación de factores relacionados con el desarrollo económico, el terreno de construcción ha aumentado considerablemente y el patrón de seguridad ecológica regional enfrenta nuevos desafíos. después de recopilar los datos de interpretación de imágenes del uso de la tierra por períodos múltiples del cinturón de la ciudad de wanjiang, este trabajo estudia las características del patrón de cambio de tierra de construcción desde 1995 y su mecanismo de conducción basado en la plataforma sig, matriz de transferencia de uso de la tierra, índice de intensidad de expansión, análisis de puntos críticos y estadísticas matemáticas. los resultados mostraron que: (1) de 1995 a 2015, el suelo urbano y otros terrenos de construcción en el cinturón de la ciudad de wanjiang han aumentado, pero las áreas residenciales rurales disminuyeron en 2010-2015. los tres tipos de terreno tuvieron los mayores cambios en 2005-2010 y el cambio en la otra zona de construcción fue particularmente prominente. (2) los puntos críticos para la expansión de la zona de construcción se encuentran principalmente en áreas urbanas con un rápido desarrollo económico como hefei, wuhu, ma’anshan y tongling, donde el uso de la tierra cambia más severamente. (3) los factores que impulsan el cambio del área de tierra de construcción incluyen factores naturales y sociales. entre los factores sociales y económicos, el pib, el valor agregado industrial, el valor secundario de producción y la tasa de urbanización son las principales fuerzas impulsoras de los cambios. en los últimos 20 años, la construcción del área de empresas de transferencia industrial chinas ha cambiado la distribución óptima de la tierra y el modo de uso intensivo en la región, proporcionando la base para el desarrollo y la utilización de recursos, el desarrollo económico y el ajuste de la estructura industrial. resumen record manuscript received: 20/05/2019 accepted for publication: 16/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 215-223 yuhong cao1*, meiyun liu1, yuandan cao1, chen chen1, dapeng zhang2 1school of environmental science and engineering, anhui normal university, wuhu 241002, china 2school of geography and tourism, anhui normal university, wuhu 241002, china * corresponding author: hong923@ahnu.edu.cn la nd u se https://doi.org/10.15446/esrj.v24n2.87710 https://doi.org/10.15446/esrj.v24n2.87710 https://doi.org/10.15446/esrj.v24n2.87710 mailto:hong923@ahnu.edu.cn 216 yuhong cao, meiyun liu, yuandan cao, chen chen, dapeng zhang introduction a country and region will undergo a process of rapid industrialization and urbanization on the way to modernization. the expansion of construction land is always accompanied by this process as an important feature of industrialization and urbanization. with industrialization and urbanization, the current social economy is in a critical period of rapid transformation and development. in addition, the disorderly expansion of construction land in recent years has restricted the intensification of agricultural land management and neglected the benefits of environmental economy. the expansion of construction land has attracted high attention from scholars at home and abroad (ye et al., 2013; liu et al., 2016). there are few studies on the expansion of construction land in foreign, and the related research mainly focuses on urban expansion. the expansion control theory of urban construction land originated in the early 20th century, including the unified planning, strict policy and laws and regulations. the urban land is divided into areas according to the land development plan, specifying the use of construction land in each area, building volume ratio, building density, building height, etc. (seto et al., 2012). the concept and identification method of urban spatial form were proposed respectively (boyce & clark, 1964; lee & sallee, 1970). arlinghaus (1985) first proved the central theory of urban expansion through fractal theory. frankhauser (1990) used the fractal theory to analyze the characteristics of urban expansion patterns and urban expansion. batty et al. used the fractal theory to analyze the shape of urban expansion (batty & xie, 1994). and in terms of urban expansion mode, forman (1995) proposed the filling, edge and enclave. sdss technology was utilized to construct the spatial decision support system, which is used to evaluate the driving factors of urban and rural construction land use (xuan et al., 1996). sharifi and van keulen (1994) used the gid and linear programming models to design and analyze the driving force of factors of urban and rural construction land use in decision support systems. american scholar ventura et al. (1988) used the multi-purpose planning to analyze the driving force of rural residential areas, and obtained the driving influence on rural residential areas, which provides the scientific support for subsequent decision-making. the domestic research on the construction land mainly focuses on the spatial characteristics of urban expansion at different scales, the driving mechanism of urban expansion and the impact of urban expansion on the environment. it scientifically guides the expansion of construction land and maintains urban ecological security. wherein, the cities with the fastest expansion of construction land, such as beijing, shanghai, guangzhou and shenzhen, are studied in depth, analyzing the overall situation of urban land expansion at the national or regional scale (chen & wu, 2014; liu, liu, kuang & ning, 2016; qu & qiu, 2013; xu et al., 2012). chi and hu (2011) used the markov model to simulate the future changes of land resources in chongqing, and played the leading role in the land use planning of chongqing in the future. the sleuth model was utilized with gis and rs technology, the simulation and prediction of the spatial expansion of jiuquan city and jiayuguan city were carried out, showing that the urban expansion under different scenarios is different (jiang et al., 2017). lv and chen (2009) analyzed the relationship between manufacturing location selection and urban spatial reconstruction. chen et al. (2014) taking nanjing (the central city of the yangtze river delta) as an example, used the remote sensing and land using survey data and other data to study the expansion process, pattern and mechanism of construction land in metropolitan areas (chen et al., 2014). yao et al. (2018) used the dynamic change measurement methods of land use and land use multi-functional methods for the cities in the middle reaches of the yangtze river from the perspective of socio-economic systems. besides, they measured the spatial structure and internal function transformation of construction land use (yao et al., 2018). with the advancement of science and technology, technical means such as aviation and remote sensing have been applied, with the trend, mode and driving force of construction land expansion analyzed. in recent years, with the emergence of environmental issues, domestic scholars pay attention to the relationship between land use change and environmental benefits. by integrating the time scales of social and environmental changes, the china’s cultivated land transformation and land consolidation have been studied, proposing the theoretical assumptions of china’s cultivated land transformation (li et al., 2018; qu et al., 2014). therefore, the work measured the change of construction land in the wanjiang city belt by the remote sensing images of 1995, 2000, 2005, 2010 and 2015, the detailed investigation and change data of land use from 1995 to 2015, and the gis grid analysis method. in the process of expansion intensity, hotspot area and spatial transformation process, the work analyzed the evolution trend and reasons of construction land in the wanjiang city belt from 1995 to 2015. data sources and research methods the land-use data comes from the resource and environmental science data center of the chinese academy of sciences (http://www.resdc.cn), and the socioeconomic data from the anhui statistical yearbook. the main research methods are as follows: land use change rate it is used to describe the change in a type of land use within a certain period, reflecting that the type of land use is affected by human activities. its formula is: k s s s t t t 1 2 1 1 100= =  % (1) where ki is the land use change rate of the land use of type i in the period from t1 to t2; st1 and st2 are the areas of the land use of type i in the initial and final stages of the study. spreading strength index (sprawl intensity index) the expansion strength index can reflect the expansion speed of a certain type of land within a region. by standardizing the land area of a certain type, the expansion strength index in the regional space can be obtained. the standardized value can be used for different research periods. the comparison between the growth rates of a certain type of land area in a region during the period can be expressed as: sii a a t sii= × ×10000 (2) where asii is the growth area of a certain type of land in different time periods; a the total area; t the year of the study period. the study area is divided into a total of 3,325 cell grids of 5 km*5 km, with the expansion strength index of each cell grid calculated. we can obtain the expansion strength index of the cell grids of the wanjiang city belt in 1995-2000, 2000-2005, 2005-2010 and 2010-2015. hotspot analysis (getis-ord gi*) hotspot analysis is generally used to identify the clustering of hot spots and cold spots in an area. by looking at the value of each element in the area and the value of its neighborhood, the z-score and p-value represent the clustering of hotspots and cold spots, and the results are often statistically significant. its formula is as follows: g w x x w n w w i i j j i j j n j n i j i j j n j n * , , , , =           == == ∑∑ ∑∑   11 2 1 2 1           n s 1 (3) x x n j j n = = ∑ 1 1 (4) http://www.resdc.cn 217change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example county, feixi county, feidong county, hefei city, jin’an district, shucheng county, he county and dangtu county; the area has been reduced by more than 100 hectares. due to the gradual development of the wanjiang city belt, the process of urbanization is also accelerating, and the rural population in most areas’ flocks to the towns, which reduces the area of rural settlements. during the period 1995-2010, the area of construction land in most areas of the wanjiang city belt has decreased in small range, while in 2005-2015, the area of other construction land has increased significantly. changfeng county, feidong county, wuhu city and other places have increased by more than 2,000 hectares, and the increase in most areas is more than 1,000 hectares. since 2006, the wanjiang city belt has been included in the key development areas of the country’s central improvement strategy. some large industrial areas have been gradually developed, including the construction of traffic roads and airports, which has increased the area of other construction land. analysis of the expansion strength of construction land in wanjiang city belt the superposition analysis function in arcgis software was used to obtain the average expansion intensity of each city in the above 4 different time periods. the results are shown in the following table 2. generally speaking, the construction land of the wanjiang city belt from 1995 to 2015 has always shown the continuous expansion. the overall expansion intensity index from 2000 to 2005 was the lowest in the study period, and its value dropped from 10.78 in the previous period to 4.78, with the urban expansion rate greatly delayed. however, the overall expansion intensity index from 2005 to 2010 increased rapidly to 28.92, which was about six times the growth rate from 2000 to 2005 (it is the highest in the study period). by 20102015, it has dropped to a level similar to that from 1995 to 2000, dropping to 10.69, and the rate of urban expansion has once again fallen sharply. from different cities, during the period of 1995-2000, maanshan city had the highest urban expansion intensity, and its expansion intensity was as high as 25.5, about 15 times that of the lowest value of chizhou city. it was followed by chuzhou city and hefei city (18.43 and 18.3), respectively. the areas with the highest expansion intensity index in 2000-2005 were hefei city and tongling city, with values of 10.67 and 9.2, respectively (wang, 2013). the expansion strength index of all prefecture-level cities showed a significant decrease (below 5) except for tongling city during this period. it was indicated that the expansion speed of construction land has been controlled during this period, and the degree of human activities tended to slow down. in 2010-2015, the expansion intensity index of each region has dropped significantly from the previous period, and it was similar to the initial period, showing a relatively gradual growth trend. (1) this trend is often attributed to two reasons. first, in the process of rapid and rapid development of the city, the city will encounter restrictions and reduce its own expansion speed to adapt to the local economic development. secondly, with the development of the economy, people’s protection and management of the ecology has also been improved. the expansion of construction land has gradually been suppressed. these reasons lead to a sharp decline in the urban expansion intensity index of various cities after experiencing rapid growth. table 2. various prefecture-level cities expansion strength analysis of wanjiang city belt 1995-2000 year 2000-2005 year 2005-2010 year 2010-2015 year anqing 2.06 4.01 17.21 5.79 chizhou 1.69 1.05 16.72 7.64 chuzhou 18.43 2.02 16.85 4.11 hefei 18.3 10.67 45.71 16.25 maanshan 25.5 5.69 52.78 19.91 tongling 4.52 9.2 62.47 36.83 wuhu 7.87 3.68 35.33 15.43 xuancheng 3.17 1.19 13.88 2.56 luan (part) 3.32 4.55 17.28 11.51 overall 10.78 4.78 28.92 10.69 s x n x j j n = ( )= ∑ 2 1 2  (5) where xj is the expansion strength index of the j-th unit; wi j the spatial weight of the i-th unit and the j-th unit; n the total number of cells (baruah & pathak, 2011). the calculated g* i represents the z-score. if it is statistically significant, the larger value means the higher value around the cell and the closer cluster, belonging to the hot spot region. the smaller value means the lower the value around the cell, and the more dispersed cluster, belonging to the cold spot region. construction land change pattern in the process of industrial transfer analysis of the characteristics of construction land use the construction land includes urban land, rural residential areas, and other construction land. table 1 shows the variation range of various construction land. the maximum change of all types of construction land is in c period (2005-2010), and the absolute value of area change is higher than those of other periods. with the same time of change, the land use is different, and the minimum value of urban land appears in period a (19952000). the rural residential area is in the d period, and other construction land appears in the b period (2000-2005). judging from the change rate of construction land area, other construction land is higher than the other two categories from 1995 to 2015. the size is as follows: other construction land> urban land> rural residential area (hao & chunjing, 2013). the maximum change rate of urban land and other construction land area appears in the c period, while the rural residential area is in the a period. the time when the minimum rate of change occurs is also different for each land use, and the urban land used in the a period is 8.8%. the rural residential area is in the d period, and other construction land appears in the b period. table 1. changes in construction land in the study area type change amount (hm2) rate of change (%) δsa δsb δsc δsd ka kb kc kd urban land 3859.17 13088.47 45032.27 28308.98 8.80 27.44 74.08 26.75 rural residential area 32036.17 4351.57 33448.80 -4005.90 8.73 1.09 8.30 -0.92 other construction land 1634.29 1257.08 24064.21 12886.86 42.15 22.81 355.50 41.80 note: δs in the figure is the area of the node at the next time minus the area of the node at the previous time; k is the rate of change; a is 1995-2000; b is 2000-2005; c is 2005-2010; d is 2010-2015; t is the total change from 1995-2015. using the spatial analysis function of arcgis, the area change of urban land, rural residential areas and other construction land in various areas of the wanjiang city belt is made into a spatial variation distribution map according to different variation ranges (see figures 1 and 2). the analysis of the following figures shows: (1) the area of urban land used has a small change in 1995-2005, and most of the areas have increased, but the increase is less than 100 hectares. during the period of 2005-2015, the urban land area of feixi county has increased by more than 4,000 hectares, and the increase in other areas is less than 2,000 hectares. this is related to the construction of hefei high-tech zone in feixi county and the main carrier of hefei’s industrial development, which enables feixi county to increase a large amount of construction land. during the period of 1995-2010, the most areas of rural residential showed a small increase trend. some areas such as changfeng county, dingyuan county, feixi county and feidong county increased by more than 2,000 hectares. however, in 2010-2015, the area of rural residential reduction has increased a lot compared with the previous periods, especially for changfeng 218 yuhong cao, meiyun liu, yuandan cao, chen chen, dapeng zhang figure 1. changes in urban land in the four periods hot spot analysis of construction land expansion in research area the above analysis only counts the average index of urban expansion intensity of different regions in the wanjiang city belt from the perspective of the city. it is impossible to judge which specific areas of the wanjiang city belt belong to the hotspots or cold spots of construction land expansion from the regional space. through the hotspot analysis tool in arcgis, the expansion intensity index of each cell in the study area was calculated, and the result of expansion intensity hotspot analysis was obtained (see figure 4). we take the yangtze river as the axis and hefei as the core, with the advancement of urbanization; the location of the hotspots is also complex and changeable. however, the central urban area has always been the main hotspot 219change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example figure 2. changes in rural residential area during the four periods distribution and expansion area. in addition, the cold spot area rarely appears, different from the trend of spreading development of hotspots. in 2010-2015, only a few cold spot areas appeared in the northern districts and counties of hefei. during the period of 1995-2000, the hotspots for construction land expansion in the wanjiang city belt were still in the northern part of the region. with hefei as the center, the northward development was connected to chuzhou, and the southeast was extended to connect maanshan and wuhu city, eventually forming a large u-shape agglomeration hotspot with high expansion strength. the hotspots with large expansion intensity mainly included hefei city, fengyang, dingyuan, quanjiao, lai’an and other counties in chuzhou, 220 yuhong cao, meiyun liu, yuandan cao, chen chen, dapeng zhang figure 3. changes of other construction land in the four periods including hanshan and hexian in maanshan, wuwei county and wujiang district in wuhu, and daguan district of anqing. except for the districts of hefei, jiujiang district and daguan district, others are all county-level units of cities. the areas with weak development foundations such as county towns were still the main development direction in the early urban expansion of the wanjiang city belt. from 2000 to 2005, the hotspots of expansion strength agglomeration gradually dispersed, and hefei city was still the main development of the core hot area. in addition, small hot spots appeared in langya district and tianchang city of chuzhou, huashan district, yushan district of maanshan, yijiang district of wuhu, tongling county and its suburbs and tongling city, yixiu district, daguan district, tongcheng city and huaining county of 221change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example anqing, and jin’an district and shucheng county of luan. the hotspots with county-level units as the expansion direction were gradually transferred to the district units within the city, indicating that the spread and development of the city to the periphery was faster than the development speed of the county and surrounding towns during this period. construction land around the city had been utilized. in 2005-2010, while hefei was still the core hotspot, there were also non-aggregating non-significant areas in the city center, and the surrounding areas such as changfeng county and feidong county also showed an agglomeration hotspots of expansion strength. maanshan and wuhu spread to the surrounding area. the hotspots in tongling continued to agglomerate and expand. in addition, compared with the previous period, new hotspot clusters figure 4. expansion intensity hotspot analysis of wanjiang city belt from 1995 to 2015 have appeared in fengyang county, handan district, xuanzhou district, and guichi district, opposite in jin’an district and shucheng county, yixiu district, daguan district and tianchang. during 2010-2015, the hotspots in various regions have a narrow development trend. most of the hotspots were located in the surrounding areas of cities, such as the urban areas of hefei, chaohu city, yushan district of maanshan, jiujiang district of wuhu, and shizishan district of tongling , guichi district of chizhou, daguan district of anqing and yixiu district, and xuanzhou district of xuancheng. other areas were basically insignificant areas, affected by economic restructuring and ecological protection policies. construction land no longer blindly expanded, and the speed of urban expansion tended to be stable. in addition, for the first time at the junction of hefei and a.1995-2000y b.2000-2005y c.2005-2010y d.2010-2015y 222 yuhong cao, meiyun liu, yuandan cao, chen chen, dapeng zhang luan and the junction of hefei and chuzhou, there was a cold spot area with an expansion strength index, affected by the introduction of ecological protection policies. the unreasonable construction land had changed into ecological land during the construction process, with the improved ecological environment. the relationship between man and land had moved toward a friendly development direction. driving mechanism of construction land change in wanjiang cit belt there are many factors driving the change of construction land, including natural and socio-economic factors, such as the mentioned elevation, slope and climatic conditions. the socio-economic factors mainly refer to the macroeconomic conditions that affect the changes of the social environment, gdp and industrial added value, the output value of the second production, the output value of the three productions, the investment in fixed assets, and the total retail sales of social consumer goods. the polarization region of wanjiang city belt is in the subtropical zone—the terrain is dominated by plains, and the topography of the mountains and low hills intersect. the correlation measure is performed by selecting the correlation between the main socio-economic development indicators and the construction land area of the region. table 3. construction land and major economic development indicators of wanjiang city belt y x1 x2 x3 x4 x5 x6 x7 1995 414544 864.19 178.9 306.4 412.48 252.01 286.46 22.57 2000 452074 1763.18 201.2 355.2 803.07 627.40 645.36 21.64 2005 470771 3562.46 1142.6 1080.2 1600.58 1416.39 1127.43 23.45 2010 573354 8904.99 6717.1 3756.3 4929.11 3040.25 2699.51 48.20 2015 611931 15650.31 16204.4 6845.4 8255.08 6105.28 5778.11 55.74 table 3 shows a simple pearson correlation analysis of construction land and other economic and social development indicators in the above five groups of data. table 4. pearson correlation between construction land and other economic factors y x1 x2 x3 x4 x5 x6 x7 y 1 x1 0.967** 1 x2 0.923* 0.989** 1 x3 0.960** 0.998** 0.993** 1 x4 0.973** 0.999** 0.986** 0.999** 1 x5 0.949* 0.997** 0.992** 0.994** 0.992** 1 x6 0.937* 0.994** 0.996** 0.993** 0.989** 0.998** 1 x7 0.973** 0.963** 0.940* 0.969** 0.975** 0.940* 0.935* 1 note: y is the construction land area; x1 the gdp; x2 the fixed asset investment; x3 the industrial added value; x4 the second production value; x5 the third production value; x6 the social consumer goods retail sales; x7 the urbanization rate (%). **: significantly correlated at the 0.01 level (both sides); *: significantly correlated at the 0.05 level (both sides). table 4 shows that the construction land area has strong correlation with the above-mentioned types of economic development indicators, and the construction land use are expanded by the gdp, fixed asset investment, industry, secondary or tertiary services. wherein, the gdp, industrial added value, secondary output value and urbanization rate are all significantly correlated at the level of 0.01 (on two sides). the main factors causing land change in the development of the undertaking industrial transfer area as wanjiang city belt are industries and cities, consistent with the industrialization and urbanization in the past 20 years. these two factors promote the region. the urbanization driven by industrialization and industrialization is the core driving force for the increase of construction land in the demonstration area for undertaking industrial transfer. conclusions from 1995 to 2015, the changes in construction land in various areas of the wanjiang city belt are complex and diverse. by analyzing the variation characteristics of its spatial quantity and its factors, the following conclusions can be obtained. from the area and extent of time and space changes in construction land, the area changes of urban land in the wanjiang city belt from 1995 to 2015 was higher than the other two categories. the order of its size was as follows: urban land > rural residential area > other construction land. among them, the urban land and other construction land area showed an increasing state at each time period, while the area of rural residential areas showed an increase state in the first three periods, and the last period of time d (2010-2015) decreased. the urban land area of the wanjiang city belt increased in small range in most areas from 1995 to 2005, and was less than 100 hectares. the change ranged from 2005 to 2015 was larger than 2,000 hectares. rural settlements increased in most areas during 1995-2010, but continued to decrease during 2010-2015. other construction land area decreased in most areas in 19952005, and increased in 2005-2015. in 2005, the state officially implemented the construction of new countryside. the wanjiang city belt was included in the key development areas of the national central rise strategy. policy support and economic development were the main factors for its change. from the perspective of the expansion of construction land, it has always shown a growing trend of outward expansion for nearly 20 years. the expansion strength index shows a trend of decrease, rise and decrease. the expansion between 1995 and 2005 was relatively flat. the overall expansion intensity index from 2000 to 2005 was the lowest in the study period, and its value dropped from 10.78 in the previous period to 4.78. the rate of urban expansion was greatly delayed. the overall expansion intensity index between 2005 and 2010 increased rapidly to 28.92, which was the highest in the study period. the urban expansion rate in 2010-2015 was greatly reduced, and gradually became flat. hotspots in various regions had a narrow development trend, and most were located in the surrounding areas of the city, such as the urban area of hefei, chaohu city, yushan district of maanshan, and jiujiang district of wuhu. other areas were not significant. under the influence of economic restructuring and ecological protection policies, construction land no longer expanded blindly, and the rate of urban expansion stabilized. in addition, due to the impacted of the ecological protection policy, unreasonable construction land had been changed into ecological land during the construction process. for the first time, at the junction of hefei and luan and the junction of hefei and chuzhou, there was a cold spot area with an expansion intensity index. the ecological environment gradually improved, and the relationship between man and land tended to be friendly. the two important driving factors for the change of construction land area include natural and social. among them, gdp, industrial added value, secondary output value and urbanization rate are the main driving forces for changes in social and economic factors. as a strategic fulcrum for regional coordinated development, the wanjiang city belt is an important part of the yangtze river delta urban agglomerations. it is not only an important link for the coordinated development of east, central and west, but also an important growth pole for the implementation of the strategy of the rise of the central region. the undertaking industrial transfer policies and economic development in china have led to changes in construction land in the region. acknowledgements this work was funded by the national natural science foundation of china (41571124). 223change pattern and driving mechanism of construction land in china’s undertaking industrial transfer demonstration area: taking the wanjiang city belt along the yangtze river as an example references arlinghaus, s. l. 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(2013). research on the influence of site factors on the expansion of construction land in the pearl river delta, china: by using gis and remote sensing. international journal of applied earth observation & geoinformation, 21(4), 366373. keywords: the yellow river estuary; sediment transport; numerical simulation; summary and prospect. palabras clave: estuario del río amarillo; transporte de sedimentos; simulación numérica; resumen y perspectiva. how to cite item jian, j., & chen, h. (2019). advance of research on the numerical simulation of sediment transport in the yellow river estuary. earth sciences research journal, 23(4), 379-383 doi: https:// doi.org/10.15446/esrj.v23n4.84100 the numerical simulation of sediment transport in the yellow river estuary is an important method to quantitative analyze the problems by water and sediment movement, including sediment deposition and river bed evolution, beach deposition and development, the formation and development of the delta, and so on. making an intensive study of the problems is useful to study the principles of sediment movement and the evolution law of the estuary bed, and it is also important significance to scientific demonstrate of the estuary sediment deposition, sea water intrusion prevention treatment scheme and solve the problems of engineering. based on the mathematical modeling of sediment transport and our research work, this paper analyzes the research statement, the unsolved issues and the developed trend of this kind problem, to provide the theoretical reference on more accurate numerical simulation of sediment transport in the yellow river estuary and the scientific reference on the demonstration and decision of estuarine regulation scheme. abstract advance of research on the numerical simulation of sediment transport in the yellow river estuary avance de la investigación sobre la simulación numérica del transporte de sedimentos en el estuario del río amarillo issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84100 la simulación numérica del transporte de sedimentos en el estuario del río amarillo es un método importante para analizar cuantitativamente los problemas por el movimiento del agua y los sedimentos, incluida la deposición de sedimentos y la evolución del lecho del río, la deposición y el desarrollo de las playas, la formación y el desarrollo del delta, y demás. hacer un estudio intensivo de los problemas es útil para estudiar los principios del movimiento de sedimentos y la ley de evolución del lecho del estuario, y también es importante demostrar científicamente la deposición de sedimentos del estuario, el esquema de tratamiento de prevención de intrusiones de agua de mar y resolver los problemas de ingeniería. basado en el modelo matemático del transporte de sedimentos y en nuestro trabajo de investigación, este estudio analiza la declaración de la investigación, los problemas no resueltos y la tendencia desarrollada de este tipo de problemas, para proporcionar la referencia teórica sobre una simulación numérica más precisa del transporte de sedimentos en el estuario del río amarillo y la referencia científica sobre la demostración y decisión del esquema de regulación estuarina. resumen record manuscript received: 27/04/2019 accepted for publication: 25/09/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 379-383 jinfeng jian*, huanzhen chen 1school of mathematics and statistics, shandong normal university, jinan 250014, china * corresponding author: jjf_sdnu@163.com h yd ro lo g y 380 jinfeng jian, huanzhen chen introduction the mathematical model of sediment transport of estuary is a branch of river simulation. it is an important method to quantitatively predict the problems caused by water and sediment movement, including river bed evolution, formation and development of deltas, expansion of siltation areas, transport and deposition of silt and sediment, and so on. the yellow river estuary is a strong accumulative estuary with frequent fluctuation of weak tides and much sediment. its evolution is influenced not only by the little water and much sediment of upper reaches, but also by the main factors such as tidals and ocean dynamic characteristics in coastal areas, including tides, currents and waves. at the same time, the influence of interaction between cohesive sediment and turbulence, the flocculation of cohesive sediment and the ocean chemistry should be considered. the problem is complex, the simulation is difficult and the cost is high. with the rapid development of modern computing technology, especially parallel computing technology, compared with traditional simulation methods such as measured model and physical similari ty model, the numerical simulation of sediment transport by using mathematical model has the advantages of low investment, short cycle, repeatability, and easy improvement, which has become an important method of estuary simulation. the thorough study of this kind of problem can help people to more clearly grasp the mechanism and form of the sediment transport in the estuary. it will also be helpful for the scientific demonstration and engineering solution of the estuary control schemes such as governing estuarine siltation, slowing down estuarine extension velocity, preventing seawater intrusion, which has a positive and important significance for the effective implementation of the national strategies of “development plan of high efficiency ecological economic zone in yellow river delta” and “development plan of shandong peninsula blue economic zone”. there are many mathematical models describing the sediment transport problem of the yellow river. different hydrodynamic conditions, different demands and different boundary conditions can lead to subtle differences in the models. the one-dimensional model can simulate the siltation and deformation of riverbed sediment in long-term and non-uniform sand, and it is often used in the study of river channel simulation in the long reach. the two-dimensional model can describe the horizontal or vertical distribution of velocity field and sediment field in water area and it is often used in the study of riverbed deformation in short-term river sections such as estuaries and ports. however, the sediment content in the practical problem is generally non-uniformly distributed along the water depth, and the sediment transport presents a typical three-dimensional property. at this time, the three-dimensional mathematical model can simulate the sediment transport and sedimentation law more accurately and completely. however, the development of three-dimensional sediment mathematical model is slow, the main reasons are: (1) the study of the three-dimensional sediment movement law is still not perfect, the sediment transport rate, resistance and other sediment parameters still adopt onedimensional sediment transport calculation formula. (2) the existing numerical calculation format has a complex structure, large calculation amount, and it is not easy to be visualized and softwareized. (3) the boundary conditions are not easy to be obtained. because of the complexity of riverbed terrain and the complex and variable dynamic conditions of rivers and oceans, the real initial boundary conditions are not easy to be obtained. in recent years, hydraulic engineers and computational mathematicians at home and abroad have proposed one-dimensional and two-dimensional mathematical models of sediment transport in estuaries under certain special simplifying assumptions, and realized the numerical simulation of finite difference method, finite element method and finite volume method, which have been applied to solve large-scale engineering sediment problems. however, in the general sense, the mathematical model and numerical simulation of sediment transport in the yellow river estuary, especially the three-dimensional problem, are still rare, and the systematic and rigorous theory of numerical analysis has not been reported. on the basis of referring to relevant literature and combining with our research work, this paper attempts to make a brief analysis and comment on the current situation of numerical simulation of sediment transport problem in estuaries, and put forward its own views and opinions on the future development, with a view to benefiting. two-dimensional mathematical model of sediment transport in the water era of the yellow river estuary, the sediment movement is mainly suspended load, and the planar flow velocity is significantly higher than the vertical flow velocity. considering the influence of turbulence, tidal current, wave and the assumption of hydrostatic pressure distribution, by performing reynolds average, wave period average and water depth average on the threedimensional sediment transport model, a planar two-dimensional sediment transport model composed of hydrodynamic model (water flow continuous equation, water flow movement equation) and sediment transport model (sediment continuous equation, riverbed deformation equation) can be obtained (yang, 1993; dou, dong, dou & li, 1995; cao & wang, 1993). water flow continuous equation van rijin, tan (1) in the equation, h represents the water depth, (u, v) represents the component of the vertical average velocity along the direction of x and y, and it is understood as different composite velocity fields in different water areas (for example, estuary water area is understood as a composite field of wave, runoff and tidal current velocity ). water flow movement equation ∂ ∂ + ∂ ∂ + ∂ ∂ + ∂η ∂ + τ ρ − τ ρ − + ρ ∂ ∂ + ∂ hu t huu x hvu y gh x fv s x s bx m wx m m xx ( ) ( ) 1 xxy by m wx m y hu hu t huu x hvv y gh y ∂ ε ∂ ∂ + ∂ ∂ + ∂ ∂ + ∂η ∂ + τ ρ − τ ρ     = ( ) ( ) v −− + ρ ∂ ∂ + ∂ ∂ ε fu s x s y hv m yx xy 1     = v (2) in this equation,  represents the water level,  = h + zb, zbrepresents the riverbed elevation, m represents the fluid density, hx, by represents the component of the shear stress on the bottom of the riverbed in the direction of x and y, wx, wy represents the component of the surface wind shear stress in the direction of x and y, f = 2 sin  represents the coriolis force coefficient reflecting the effect of the earth’s rotation,  represents the angular velocity of the earth’s rotation,  represents the local latitude, sxx, sxy, syx, syy represents the wave radiation stress term, i.e. the residual momentum flux caused by fluctuations, and  is the turbulent viscous coefficient. sediment continuous equation ∂ ∂ +υ ∂ ∂ + ∂ ∂ − s t s x v s y d s f h sv = (3) in this equation, s represents the sediment concentration, fs represents the function of sediment scouring and silting on the riverbed, generally taken as fs = (s* − s),  represents the sediment settling velocity,  represents the recovery saturation coefficient, s* represents the sediment transport capacity, and d represents the sediment diffusion coefficient. this is a uniform sediment model. if the non-uniform sediment is taken into account, the sedi ment concentration of each group of sediment gradation satisfies this equation. suspended load riverbed deformation equation ′ =γ ∂ ∂ − z t fb s (4) in this equation, ´is the dry bulk density of sediment. 381advance of research on the numerical simulation of sediment transport in the yellow river estuary the windward finite element, mass concentrated windward finite element and other methods are produced. these methods are effective, but due to the introduction of excessive artificial viscosity, the discrete accuracy is reduced and the discontinuous screeding phenomenon is caused. petrov-galerkin and streamline diffusion methods can better avoid numerical dispersion and non-physical numerical oscillations, but they still have problems when dealing with interrupted flow. high-resolution format finite element method can better avoid the high frequency oscillation of the interrupted flow and the false oscillation caused by the convection dominant by introducing minimum and sufficient consumption in the stiffness matrix, but no strict mathematical proof has been given. at present, the models used in the engineering are mainly tabs-2 developed by the us army corps of engineers waterway experimental station, which can be used for the simulation of rivers, reservoirs and estuaries, and the cche2d established by jia,wang ,which can simulate the secondary flow effect formed in the curve. finite volume method (guo, han & he, 1996; pan, lu & yu, 2009; hu & tan, 1995; wu & wang, 2012; benkhaldoun, elmahi & seaid, 2010) the finite volume method is more suitable for complex regions than the finite difference method, and its calculation amount is lower than that of the finite element, so it has been widely used. there are two key problems to be solved in the finite volume method, one is the arrangement of variable nodes in the integral unit, the other is the calculation method of boundary flux of the integral unit. the traditional finite volume method can not deal with shock wave problem, but it has the ability to deal with discontinuous problem when using riemann solution operator, which can accurately capture the shock wave. the finite volume method can use unstructured grid at the same time. in the volume method of second-order and higher accuracy, the limiter function should be introduced to eliminate numerical oscillation in the process of flux reconstruction. at present, the finite volume method is the mainstream method to solve the problem of sediment transport. it mainly uses the body-fitted grid to establish the fast2d model in the curvilinear coordinate system to simulate the sediment transport and topographic dynamics the alluvial channels; the sutrench-2d model established by vanrijin,tan is used to simulate the sediment transport at the bottom of the sedimentary river bed and the change of the riverbed elevation under the effect of quasi-steady-state water flow and wind-induced waves, and there is also no strict mathematical proof. characteristic finite element method (li & chen, 2011; wang & liu, 2003; liu, li & chen, 2013; zhou & chen, 2007; luo, zhu, zeng & xie, 2007; dawson & martinez, 2000) since the convection terms of the first three equations of the sediment transport model can lead to numerical dispersion and non-physical oscillation, and have hyperbolic characteristics, numerical dispersion along the characteristic line can improve the stability of the format. at the same time, the truncation error about time is smaller, so the larger time step can be adopted. the characteristic line method is suitable for the strong estuary areas where the hydrodynamic changes are severe. firstly, the characteristic finite element scheme of shallow water equation is proposed by, dawson,monica, and the suboptimal error estimation is obtained. wang jiwen and liu ruxun get the cn-galerkin method based on the characteristic direction of the shallow water wave equation, and obtain the suboptimal error estimation; zhou zhaojie and chen huanzhen apply it to the planar two-dimensional water and sediment model, and a suboptimal error estimation is obtained. mixed finite element method (luo, zhu, zeng & xie, 2004; lu, 2007; xia, 2013) the mixed finite element can approximate the function and its gradient with high accuracy at the same time, which overcomes the shortcomings of the standard finite element method in calculating the gradient by solving itself, and the requirement for the solution of space smoothness becomes lower. luo zhendong and others give a mixed finite element method for two-dimensional sediment transport model, give the existence and uniqueness of generalized solution and the mixed finite element solution, and obtain the optimal order error estimation by increasing the number of interpolations in the test function space of flow velocity field and sediment field. lu xiuying and xia kaifeng the conditions of initial boundary values. initial water level (water depth) usually starts from downstream water level, and the initial value of the water level at each point is given by linear interpolation according to the average river bed gradient. the initial flow velocity and sediment concentration are given if there are measured values. otherwise, they can be taken as 0. with the process of numerical calculation, the influence of initial values will gradually disappear. the initial siltation elevation is usually taken as zero. on this basis, the calculated positive value is the siltation thickness, while the negative value is the scouring thickness. the inflow boundary generally gives the flow or water level, and for the suspended load, the sediment concentration should also be given. at the outflow boundary, the sediment flux is generally considered to be zero. for the boundary conditions of the fixed shore, the condition of no slip of flow is generally adopted, and the velocity field is taken as 0. when studying the sea area, due to the periodicity of the tide level, the edge beach changes with the water level, which leads to the change of boundary. at this time, the boundary “freezing” technology can be used to process the complex moving boundary into the fixed boundary. the determination of parameters. in the two-dimensional sediment transport model, the determination of the parameters describing sediment characteristics and hydrodynamic forces, such as shear stress at the bottom of the riverbed, shear stress of surface wind, wave radiation stress, sediment transport capacity of current, recovery saturation coefficient, sedimentation velocity and dry bulk density of sediment, have not been effectively expressed in the twodimensional motion theory. most of them directly use the corresponding data or regular formulas of one-dimensional sediment transport. numerical simulation method the numerical simulation mainly includes the discrete format construction, the mathematical demonstration of the format (the uniqueness, stability, and convergence of the existence of the solution), the program implementation of the format, and the practical application of the format. in terms of format construction methods, there are mainly finite difference, finite volume element, finite element, mixed finite element, characteristic finite element, discontinuous finite element and other methods. at the same time, in order to better deal with the problem of discontinuous (shock) flow, sometimes the technique of windward and adding artificial dissipative items is combined. finite difference method (lu & peng, 1985; zheng, 2003; spasojevic & holly, 1990; chang, 1998) the finite difference method which is widely used in the planar twodimensional sediment transport model is the alternating direction implicit format method (adi). the traditional difference method has the advantages of simple structure and easy implementation. however, it is inflexible and the accuracy is difficult to improve when dealing with irregular boundary and complex boundary conditions. although it can basically adapt to irregular boundary after using the curve coordinate technology, there are still some shortcomings. in order to solve the numerical oscillation that occurs when the shock current is generated, the windward adi method is formed in combination with the windward. at present, the main model used in engineering is mobed2 model established by spasojevic and holly in curvilinear coordinate system, which can simulate sediment deposition in waterways, estuaries and coastal areas; the fluvial12 model established by chang in rectangular coordinate system, which can simulate the changes of riverbed section and sediment gradation caused by river bending; the delft-2d model used to simulate wave and current movement can describe the influence of wave motion, but these models do not give strict mathematical proof. finite difference method (li & zhang, 1999; zhang & yin, 2002; chippada, dawson, martinez & wheeler, 1998; chippada, dawson, martinez & wheeler, 1998; dawson & martinez, 2000) the finite element method can deal with irregular boundaries and complex boundary conditions better, and it can improve the calculation accuracy more conveniently, but the storage amount and calculation amount are larger than the finite difference method. both the finite difference method and the finite element method can generate numerical dispersion due to the existence of convection terms. combined with the windward technology, 382 jinfeng jian, huanzhen chen propose a characteristic mixed finite element method for shallow water equation and sediment transport equation, and obtain suboptimal error estimation. discontinuous finite element method (shi, 2013; dawson & proft, 2004) discontinuous finite element not only retains the advantages of the finite element which can be applied to irregular boundary and easy to improve accuracy, but also has the advantages of easy processing discontinuous problems, easy carrying out parallel computation, good stability and entropy compatibility. dawson,proft use the methods of discontinuous finite element and finite element coupling to study the shallow water equations, gives the calculation format, and obtains the suboptimal error estimation of the l2 model. shi baohai proposes a semi-discrete format for the two-dimensional sediment transport model by using discontinuous finite element method, estimates the errors, and obtains suboptimal error estimation of the l2 model. further research and outlook at present, although the research on the establishment of mathematical models and the technology of numerical simulation for sediment transport problems have made great progress, some major problems such as the construction and simulation of mathematical models in three dimensions need to be further studied and explored. (1) research on the basic laws of sediment movement. the factors that strongly influence the law of sediment diffusion and transport include riverbed resistance, sedimentation, sedimentation velocity and recovery saturation coefficient in the sediment transport model, and flocculation caused by seawater temperature and salinity changes. these laws do not necessarily conform to the classical gauss distribution, and may conform to some algebraic distribution. this needs to be further explored in mathematical theory and physical experiments to form a constitutive equation that reflects the mathematical physical nature of practical problems, which can more accurately describe the process of sediment transport. (2) research on the basic laws of hydrodynamics. sediment deposition or seawater erosion in the waters of the estuary coast is affected by a variety of hydrodynamic factors. therefore, we should consider the wavecurrent coupling model formed by runoff, tidal current and wind, and pay attention to the shape, function and development law of shear peak and pinnacle peak, which are caused by runoff and ocean dynamics, and the mechanism of sediment accumulation and capture, so as to form an accurate hydrodynamic system. (3) research on numerical simulation technology. based on the hydrodynamics and laws of sediment movement in the estuary, we should consider factors such as strong convection, nonlinearity, strong coupling, and data roughness in practical problems to establish computable models and mathematical theories, which provide a solid theoretical basis for a more accurate simulation of sediment transport in the yellow river estuary, and provide a reliable scientific basis for the demonstration and decisionmaking of the control scheme of the yellow river estuary. acknowledgement the paper is supported by the national natural science foundation of china (no. 11471196, 11301311, 10971254) and the natural science foundation of shandong province (no. 2014zrb01849). references benkhaldoun, f., elmahi, i., & seaid, m. 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(2013). a characteristic-mixed finite element method for the simulation of two-dimensional water and sediment transport model. jinan: school of mathematical science, shandong normal university. yang, g. (1993). river mathematical model. beijing: ocean press. zhang, x. b. & yin, r. l. (2002). planar 2-d flow and sediment mathematical modeling. advances in water science, 12 (6): 665-669. zheng, j. (2003). development and application of numerical models for flow motion and sediment transport in an orthogonal body-fitted coordinate system. marine science bulletin, 22(1), 1-8. zhou, z. & chen, h. (2007). a characteristic finite element simulation for current and silt sedimentation model in two space dimensions. numerical mathematics a journal of chinese universities, 29(3), 245-256. keywords: artificial intelligence; low permeability reservoir; petroleum logging; lithology identification palabras clave: inteligencia artificial; embalse de baja permeabilidad; explotación de petróleo; identificación de litología how to cite item shang, f., cao, m., & wan, c. (2021). application of artificial intelligence in lithology recognition of petroleum logging in low permeability reservoirs. earth sciences research journal, 25(2), 255-262. doi: https://doi.org/10.15446/ esrj.v25n2.80895 in low permeability reservoirs, the conversion accuracy of the existing petroleum logging lithology identification method to small pore capillary pressure curve is not high, resulting in a low rock mass identification accuracy. therefore, artificial intelligence technology is considered in this study to enhance the accuracy of lithology identification in low permeability reservoirs. firstly, the radar mapping program is used to predict the position of reservoir oil logging, and then the small pore capillary pressure curve is converted by using the conversion method of piecewise power function scale to obtain the pore characteristics of low-permeability reservoir rocks. on this basis, the crossplot method is used to gather the pore characteristic data in well logging and form a plan, and the response parameters of well logging rock mass are obtained to realize the identification and analysis of lithology. the experimental results show that, compared with the existing identification methods, the accuracy of lithology identification in low-permeability reservoir logging is significantly increased after the application of artificial intelligence technology, and the identification process takes less time, which fully proves that the application of artificial intelligence technology is conducive to improving the performance of lithology identification. abstract application of artificial intelligence in lithology recognition of petroleum logging in low permeability reservoirs aplicación de la inteligencia artificial en reconocimiento litológico de extracción de petróleo en yacimientos de baja permeabilidad issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.80895 en los yacimientos de baja permeabilidad, la precisión de conversión del método de identificación de litología de extracción de petróleo existente a la curva de presión capilar de poros pequeños no es alta, lo que da como resultado una baja precisión de identificación del macizo rocoso. por lo tanto, en este estudio se considera la tecnología de inteligencia artificial para mejorar la precisión de la identificación litológica en yacimientos de baja permeabilidad. en primer lugar, el programa de mapeo de radar se usa para predecir la posición del registro de petróleo del yacimiento, y luego la curva de presión capilar de poros pequeños se convierte utilizando el método de conversión de la escala de función de potencia por partes para obtener las características de los poros de las rocas del yacimiento de baja permeabilidad. sobre esta base, se utiliza el método de parcelas cruzadas para recopilar los datos característicos de los poros en el registro de pozos y formar un plan, y se obtienen los parámetros de respuesta del macizo rocoso del registro de pozos para realizar la identificación y análisis de la litología. los resultados experimentales muestran que, en comparación con los métodos de identificación existentes, la precisión de la identificación litológica en el registro de yacimientos de baja permeabilidad aumenta significativamente después de la aplicación de tecnología de inteligencia artificial, y el proceso de identificación lleva menos tiempo, lo que demuestra plenamente que la aplicación de la tecnología de inteligencia artificial es propicia para mejorar el rendimiento de la identificación litológica. resumen record manuscript received: 05/07/2019 accepted for publication: 02/10/2019 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 255-262 fuhua shang1, maojun cao1*, caizhi wang2 1school of computer & information technology, northeast petroleum university, daqing, 163318, china 2department of well logging & remote sensing technology, research institute of petroleum exploration and development, beijing, 100083, china *corresponding author: caomj007@126.com g eo lo g y mailto:caomj007@126.com 256 fuhua shang, maojun cao1, caizhi wang introduction with the development and progress of the society, the demand for energy in various fields is constantly increasing. therefore, petroleum geological exploration technology is facing more severe challenges. multi channel, multi means, multi discipline cross fusion has become the development trend of exploration and development technology (deng et al., 2017). the early geological exploration objects in china are mainly sand and mudstone reservoirs, and lithology identification is relatively simple. with the continuous development of geological exploration and development, the requirement of well logging technology is gradually improved, and in order to form a more reasonable well logging interpretation method, lithology identification technology is also improved. lithology identification belongs to the category of logging engineering and is an important research content in reservoir description, formation evaluation and drilling monitoring. the commonly used methods to obtain formation lithologic information include cuttings logging, coring section analysis and comprehensive treatment of logging data (wei et al., 2017). among them, the quality of cuttings logging depends on the operation of technicians, which is greatly influenced by human factors. the process of core slice analysis is time-consuming and laborious, and it is difficult to describe the complete well formation profile. in terms of comprehensive processing of logging data, crossplot method can intuitively reflect the boundary interface and lithological distribution area of different lithologies, and has the advantage of intuitive reading. however, its operation process is complex, and it cannot identify the whole well or interpret lithological information of the well. the mathematical statistical method is suitable for well logging data with good physical characteristics, and has a good application effect when there are few core data and more logging data, but it is very difficult to obtain the empirical formula (xie et al., 2018; jia et al., 2018; jiang et al., 2018). artificial intelligence is a branch of computer science. it is a technology used to simulate, extend and expand human intelligence. it can simulate human consciousness, thinking and information formation process. research in this field includes robotics, language recognition, image recognition, natural language processing and expert systems. it can be said that artificial intelligence is a very challenging science, which requires not only computer knowledge, but also knowledge in fields such as psychology and philosophy (deng et al., 2018). artificial intelligence research, on the whole, one of the main goal is to make the machine to be able to do some human intelligence is needed to complete a complex work, namely through the use of computers to simulate human thinking process and intelligent behavior of the discipline, can say it is almost covered all subjects of natural science and social science, its scope has been far beyond the scope of computer science (guzman & aoyama, 2018). in recent years, artificial intelligence has become one of the main methods of pattern recognition in various fields. for example, bp neural network and fuzzy cluster analysis method. although the bp neural network method has some disadvantages such as local minimization, slow convergence rate and different structure selection, it has the ability of distributed processing, self-learning, self-organization, highly nonlinear mapping and fault tolerance, which greatly makes up for the deficiency of self-recognition performance. therefore, in order to improve the ability of identifying lithology of oil logging in low permeability reservoirs, artificial intelligence technology is introduced in this study. application research on reservoir oil logging location prediction when predicting the location of reservoir oil logging, first of all, do not collect logging data. in this process, various interference factors inevitably exist, which affect the resolution of the signal. linear combination of sensitive indexes with high correlation is an effective method to improve the accuracy of logging interpretation. therefore, on the basis of selecting the logging data of the core well, the logging data is compared with the core data by the multi-index radar chart comparison method. the execution flow of the radar chart drawing program is shown in figure 1. according to the implementation process of the radar chart drawing program as shown in figure 1, sensitive indexes that can reflect the lithologic change characteristics are selected, and then the correlation coefficient of the selected sensitive indexes is obtained, and the indexes with good correlation are combined linearly. although the noise is random in the radar map drawing process, the linear combination process can enhance the formation characteristic information, and highlight the useful information by superposition signal, thus reducing the signal interference (han et al., 2019). the dimensions of different logging indicators are often different. in order to combine the indicators of different dimensions, it is necessary to normalize the indicators and then combine the dimensionless data to form new analysis indicators. firstly, the radar map is used to analyze the logging data, and the log phase identification curve set is established through the standard well. then, the logging location of the reservoir of the unknown well segment is identified by using the curve difference comparison, and the logging data of the core well is obtained after sorting out and correcting the analysis data, and the clustering normalization is completed. after calculation, the following results can be obtained: x x x x x = −∗ ∗ ∗ ∗ min max min (1) among them, x represents the normalized log curve, x* represents the original log data, x* min and x* max represent the maximum and minimum values of the log curve respectively. it is easy to realize the linear normalization method with the powerful matrix operation of matlab, but in the transverse comparative analysis of multiple wells, the difference of normalization results will be caused by the different selection of maximum and minimum values, thus affecting the analysis results. therefore, data expansion or shift processing is carried out in this paper according to the value situation (zhou et al., 2019; jia & deng, 2018). for example, after analyzing logging and core data, it can be seen that a certain reservoir is located in the second section, which is dominated by grayish-white medium-grain lithic sandstone, and its density (den) value is mostly around 2~3 g/cc. at this point, if we can extract the fractional part, magnify the difference between maximum and minimum, we will get a better comparison effect. on this basis, all the above density measurements were summarized and five parameters of ac, den, cnl, gr and rd were selected to depict the logging phase position radar map of the traveling gas layer, gas bearing layer and gas layer, as shown in figure 2. figure 1. radar chart drawing program execution flow diagram 257application of artificial intelligence in lithology recognition of petroleum logging in low permeability reservoirs figure 2. logging locations corresponding to different density values table 1. rock mass characteristic data table serial number of rock mass porosity/ % t2cutoff/ ms nmr bound water saturation/ % maximum throat radius /μm mean throat radius/μm sorting coefficient expulsion pressure/ mpa t2lm/ ms permeability/ 10-3μm-2 12-1 6.71 22.68 64.26 11.92 0.44 0.14 0. 26 1.70 0.07 61-1 6.77 21.99 65.77 11.41 0.60 0.16 0.31 1. 26 0.13 64-1 7.89 11.01 73.22 575 0.55 0.05 -0.31 1.36 0.11 62r 13.50 16.26 43.01 21.25 1.00 0.05 -0.43 0.75 0. 56 18.00 6.90 7.90 42.21 11.06 1.22 0.04 -0.66 0.61 0. 28 51b 10.60 18.00 40.60 26.24 1.15 0.12 -0.18 0.65 0.42 9-35 6.99 2.66 52.40 3.96 1.19 0.02 -0.94 0.63 0.68 70-1 5.62 1.55 57.21 1.93 0.33 0.00 -0.86 2.28 0.08 19-1 9.73 16.79 41.10 28.16 1.17 0.06 -0.31 0.64 0.23 34-4 12.77 5.85 46.97 9.78 2.60 0.21 -0.20 0.29 3.59 52-1 11.37 10.54 83.37 3.01 0.34 0.05 -0.42 2.21 0.15 64-1 8.12 7.41 64.15 6.47 0.89 0.07 -0.46 0. 84 0.41 93-1 16.65 8. 26 42.68 17.19 20.06 0.17 -0.53 0. 20 42.30 115-1 14.92 10.39 52.38 10.46 3.81 0.03 -0.74 0.56 4. 85 9-1 11.57 6. 80 61.11 5.54 1.35 0.07 -0.32 0.17 0.91 20-1 11.76 6.67 46.52 9.32 4.29 0.91 0.41 0.98 17.27 68-1 6.38 2.75 85.97 1.09 0.76 0.02 -0. 94 1.39 157 137.00 10.45 7.77 79.32 2. 98 0.54 0.03 -0.75 1.40 0.79 152-1 8.35 40.30 83.07 7.01 0.54 0.04 -0.48 0.98 0.87 3-1 6.26 27.35 67.35 9.17 0.76 0.30 0.81 2.01 0.63 30-1 5.52 1.68 91.38 0. 87 0.37 0.04 -0.40 0.22 0.52 91-1 10.56 13.81 45.54 15.45 3.35 0.19 -0.20 1. 26 3.38 144-1 8.21 61.52 67.08 35.12 0.60 0.10 -0.02 1.09 0.70 216-1 9.47 52.66 59.96 41.56 0.69 0.17 0.23 1.23 0.97 236-1 8.17 36.92 62.34 27.80 0.61 0.16 0.34 1. 20 0.85 257-1 6.90 46.74 56.18 42.06 0.63 0.21 0.62 0. 20 0.42 according to the different density ranges shown in figure 2, it is obvious that cnl can reflect different density differences to a certain extent, which corresponds to the location of oil logging in low-permeability reservoirs. samples of rock sections are randomly selected within the determined location range to determine the pore characteristics of rocks in low-permeability reservoirs. pore characteristics of low permeability reservoir rock when judging non-uniform properties of low permeability reservoirs, pressure pump experiments should be carried out in low permeability reservoirs according to thin sections obtained by well logging, and rock characteristic data information should be obtained by nuclear magnetic test. the characteristic data are shown in table 1. referring to the data information in table 1, the “three porosity component percentage method” was used to process the above 26 rock samples, to qualitatively classify the pore types of the rock samples, and to draw the corresponding cross plot of pore structure parameters (wang & siau, 2019), as shown in figure 3. by referring to the cross plot of pore structure parameters shown in figure 3, it can be seen that the t2 spectrum measured by nmr consists of three kinds of transverse relaxation time, that is, the t2 spectrum observed is: 1 1 1 1 t t t tb d s2 2 2 2 = + + (2) 258 fuhua shang, maojun cao1, caizhi wang where, t2 denotes transverse relaxation time, t2b denotes volumetric (free) relaxation time of the fluid, t2d denotes diffusive relaxation time, and denotes surface relaxation time. t2s and t2d were characterized by specific surface area, diffusion coefficient, magnetic field gradient, echo interval and other parameters, and then: 1 1 122t t s v d gt b e 2 2 2 '= + + ( ) ρ γ      (3) among them,  2 represents the transverse surface relaxation strength of the rock, s represents the surface area of pores, v represents the volume of pores, v represents the specific surface area of characterized rock samples, d represents the diffusion coefficient, g represents the magnetic field strength, te represents the echo interval,  represents the magnetic rotation ratio, and the remaining parameters remain unchanged. for the uniform magnetic field with a small echo interval, equation (2) can be transformed into: ρ ρ 2 21s v t2      = =' f rs c (4) where, fs represents the geometric shape factor, and for spherical pores, the value of fs is 3; for columnar pores, fs has a value of 2. rc represents the pore radius. generally speaking, the relaxation time of pore fluid is related to the size and shape of pore space (grzonka et al., 2018; gobashy et al., 2020). since there should be a corresponding transverse relaxation time for a particular rc , t2 and rc should have a one-to-one correspondence. however, due to the constant change of fs , the corresponding relationship between t2 and rc also changes. for medium high porosity and permeability reservoirs, reservoir heterogeneity is weak, and the corresponding relationship between t2 and rc has little change. however, for tight sandstone reservoirs with complex pore structure and pore type, the corresponding relationship between t2 and rc changes greatly due to their strong reservoir heterogeneity. according to fluid mechanics, there is a certain relationship between capillary pressure and pore radius: p rc c = 2σ θcos (5) where, pc represents capillary pressure,  represents fluid interfacial tension, and  represents wetting contact angle. for mercury-air system, equation (4) can be rewritten as: p rc c ' .  0 735 (6) the comprehensive calculation formulas (3) and (5) can be obtained as follows: c pc= ×t2 (7) where, c represents the conversion coefficient, and. therefore, if the conversion coefficient can be accurately calculated by using the well log, the capillary pressure curve at different depths of the whole well can be evaluated by using the nmr log, and the variation of various pore structure parameters with depth can also be evaluated (zibret, 2019). generally, the pseudo-capillary pressure curve and the capillary pressure curve of the pore segment cannot be superimposed together, because of the accumulation of thin film bound water with large pores in t2 distribution to small pores (zhang, 2019). based on this idea, a piecewise power function calibration method is proposed to solve the problem of low precision of pressure curve conversion of small pore capillary. in theory, the specific surface area has a linear relationship with the pore size, but for the actual formation, the pore structure is very complex, and the specific surface and pore size often have a nonlinear relationship, which can be expressed as follows: f rc( )= ρ2 1 t2 (8) where, f(rc ) represents the function of rc . according to equation (5) and equation (7), the function of t2 spectrum can be calculated as follows: g pc 1 t2      = (9) according to the above formula, the pore characteristics of lowpermeability reservoir rocks can be divided into large pores and small pores, and the pore characteristics can be described as follows: p a p a cb b cs b =       =               1 2 2 1 1 1 t t 2 2 (10) where, a1 and b1 represent the corresponding parameters of small pores, and a2 and b2 represent the corresponding parameters of large pores. referring to the pore characteristics obtained, crossplot method can be used to obtain logging response characteristics that can represent lithology. obtain logging rock mass response parameters the cross-plot method is used to summarize the pore data obtained above and form a plan. according to the coordinates of the intersection point, the numerical value and aggregation trend of the parameters can be seen. in the process of using the crossplot method to identify lithology, two groups of appropriate logging data are first selected, and coordinate points are drawn in the crossplot coordinate system according to the changes of logging data. the aggregation trend of coordinate points is expressed in the form of intuitive data, and the distribution range and boundary of different lithology can be reflected visually. the crossplot drawn should be compared with the coring lithology data for better accuracy of lithology identification. cross-graphs such as gr-pb and gr-ac can be made according to the core analysis data. according to these cross-graphs, the positions of different measured lithologic points, the lower limit of lithologic logging parameters and the lithologic distribution trend can be determined. referring to logging rock masses of different densities and using the ratio of spectral intensity of elements, structural logging rock characteristic parameters are as follows: figure 3. cross plot of pore structure parameters 259application of artificial intelligence in lithology recognition of petroleum logging in low permeability reservoirs z r sp = ×φ0 5421 1 0458 0 3181 . . . (11) among them, z represents logging rock characteristic parameters,  represents porosity calculated above, r represents average pore throat radius, sp represents sorting coefficient, and two constants represent permeability measured. logging rock characteristic parameters were defined to be around 80-110, and the acoustic wave presented high value, which was generally above 67, with the development of porosity structure. when the characteristic parameters of logging rocks are above 120 and the acoustic values are generally above 70, the fused tuff appears in the logging interior. when the characteristic parameters of logging rocks are distributed around 40-120 and the acoustic values are generally below 65, the characteristic values cannot be determined, and the rock mass is also a fused tuff. according to the calculated logging rock characteristic parameters, the cross map of gr-ρb within the logging location was obtained, as shown in figure 4. as can be seen from the logging characteristic parameters shown in figure 4, different rock assemblages have great differences in layers, and their electrical properties are also different. therefore, before obtaining the response characteristics in the logging, linear transformation should be adopted to normalize the electrical properties in the logging characteristic parameters, and the standardization process is as follows: x x x x n x n x x t n i n i i i i n n n i i n i n n i = − = = − −           = = ∑ ∑ σ σ 1 1 1 1 2 1 ( )  (12) where, where, 111 represents the n the standardized value of xi , x̄i represents the mean value of the electrical variable, i represents the standard deviation of the electrical variable, and n represents the number of logging characteristic parameters. the response values of acoustic time difference (ac), natural gamma (gr) and resistivity (rt) obtained by using the above standardized electrical property values combined with the curve overlap method are comprehensively judged. the difference, acoustic time difference (ac), natural gamma (gr) and resistivity (rt) of porosity d and n extracted by the overlapping method are shown in table 2. the data shown in table 2 are integrated into the calculation formula of logging response characteristic parameters, as shown below: n m t t nf nma ma f f ma ma f = − − = − −        φ φ ρ ρ ρ ρ ∆ ∆ (13) among them, tf represents acoustic time difference of pore fluid, tma represents acoustic time difference of rock skeleton, ma represents pore fluid density, ma represents neutron porosity of rock skeleton, and nf represents neutron porosity of pore fluid. by referring to the calculation results of different characteristic parameters, the response values of corresponding logging lithology were calculated, as shown in table 3. table 3. response values of logging lithology logging lithology ac (us/ft) den (g/cm3) cnl (p.u.) gr (api) cloud mass limestone 45.25~47 2.17~2.79 -1~0 10 gray dolomite 43.5~45.25 2.79~2.81 0~1 10 limestone 48 2.71 -1 10 dolomite 43.50 2.87 1 10 argillaceous limestone 59 2.12 21.50 57.50 anhydrite 49.30~51.80 2.91~3.02 -2 1.50~6 paste dolomite 43.50~46.75 2.87~2.91 -1~0.50 5.80~10 cream quality limestone 49 2.71~2.915 -1.50~1 5.80~10 corresponding to the response values of different characteristics shown in table 3, the properties of different logging rocks can be determined. thus, the application of artificial intelligence in lithology identification of low permeability reservoirs is completed. simulation experiment and result analysis in order to verify the practical application performance of the low permeability reservoir logging lithology identification method designed in this paper by using artificial intelligence, the following simulation experiment is designed for verification.figure 4. gr-ρb cross map of oil logging in low permeability reservoir table 2. parameter set matrix table overlapping layer d − n the mean ac(us/ft) the mean gr(api) the mean rt(·m) 1 -0.179 53.094 49.902 230.441 2 -0.143 53. 512 54.729 235.899 3 -0.135 53. 539 52.784 238.887 4 -0.132 53. 539 47.642 242.040 5 -0.089 52. 870 42.829 253.606 6 -0.011 51.491 39.953 263.617 7 -0.056 50.794 41.874 268.003 8 -0.081 50.525 45.082 285.795 9 -0.105 50.318 47.851 321.116 10 -0.108 50.347 45.627 337.462 11 -0.110 50.453 45.927 339.846 12 -0.109 50.542 43.827 294.277 13 -0.121 50.374 42.100 263.586 14 -0.085 53.412 49.902 213.427 260 fuhua shang, maojun cao1, caizhi wang the experiment to prepare firstly, a basic libs detection system is set up, which mainly includes light source, light splitting system, photoelectric detector, timing control module, collimation, focusing light path, light collection lens group, sample table and so on. imaging of laser-induced plasma is also required during the experiment, so imaging modules should be set up in the libs detection system. the experimental platform structure is shown in figure 5. figure 5. experimental platform structure diagram as shown in figure 5, the main performance parameters of the me5000 medium-step grating spectrometer in the experimental platform are shown in table 4. table 4. main performance parameters of me5000 medium step grating spectrometer parameter data escher spectrometer focal length (mm) 195 aperture f7 wavelength range (nm) 230~950 wavelength accuracy (nm) ±0.05 focal plane size (mm) 13.3×13.3 stray light 1.5×10-4 iccd active pixel 1024×1024 effective pixel size (nm) 13 effective area (mm) 13.3×13.3 peak quantum efficiency up to 18% total insertion delay (ns) ~35 ddg insert delay (ns) ~19 gate delay 0~25s delayed resolution 25ps width of the gate 1ns~25s width resolution 25ps under the control of the performance parameters shown in table 4, 8 logging blocks as shown in figure 6 are prepared. use as shown in figure 6 to mudstone, shale, sandstone, dolomite, asphalt, volcanic ash, conglomerate and paste rock, in the experimental environment, were used in this paper, design of using artificial intelligence low permeability oil reservoir logging lithology identification method and the traditional lithology recognition method based on laser induced breakdown spectroscopy, logging lithology recognition method based on boosting tree algorithm comparing experiment, from lithology recognition accuracy and recognition process takes two angle compared three methods of recognition performance. experimental results and analysis taking mudstone samples as an example, based on the above experimental preparation, the standard reference spectrum of mudstone samples was first obtained by using me5000 medium-step grating spectrometer, as shown in figure 7. figure 7. standard reference spectra of mudstone samples after obtaining the standard spectrum of mudstone samples, under the same experimental conditions, used respectively in this paper, the design of the use of artificial intelligence in the low permeability oil reservoir logging lithological identification method and the traditional lithology recognition method based on laser induced breakdown spectroscopy, logging lithology recognition method based on boosting tree algorithm for mudstone samples of spectral information. in order to avoid the spectral line position drift caused by temperature change, the experimental room temperature should be controlled within the range of 25-30 °c. on this basis, the similarity between the spectral information of mudstone samples obtained by three different identification methods and the standard spectra was compared. when the position and relative strength of spectral lines between the three different identification methods were consistent, the recognition was deemed as a success. the recognition results obtained by the three recognition methods are shown in table 5. according to the experimental results shown in table 5, the three identification methods can be used to identify the same number of different types of logging rock mass information, and the three identification methods figure 6. experiment to prepare rock samples 261application of artificial intelligence in lithology recognition of petroleum logging in low permeability reservoirs show different identification capabilities. after applying the logging lithology identification method based boosting tree algorithm, the number of rock masses identified is too small and the identification accuracy is low. however, the identification accuracy of lithology based on laser-induced breakdown spectroscopy has been improved, but the information of other rock masses has not been fully identified except for the gypsum rock. after the application of this method, except dolomite and shale, the information of other rock mass is correctly identified, and the correct identification rate of this method is the largest among the three methods. based on the above experimental results, it can be seen that the rock mass identification method designed in this paper with artificial intelligence technology can avoid the deficiency of identifying too few rock masses, and the identification accuracy is higher, which is suitable for the identification of lithology of oil logging in low-permeability reservoirs. on this basis, the identification performance of the three methods is verified by taking the identification process time as the index. the results are shown in figure 8. figure 8. time consuming comparison diagram of different methods to identify the process according to the experimental results shown in figure 8, the three identification methods can be used to identify the same number of different types of logging rock mass information, and the time of the three identification methods is greatly different. the well logging lithology identification method based on boosting tree algorithm has a identification process of 8-12min. the lithology identification method based on laser induced breakdown spectroscopy technology is higher and keeps between 12-15min. however, the identification process of the method in this paper takes the least time among the three methods, which is always kept below 7min. based on the above experimental results, it can be seen that the rock mass identification method designed in this paper with artificial intelligence technology can quickly identify the lithologic characteristics of oil logging in low-permeability reservoirs. conclusion automatic identification of lithology is based on the combination of mathematical theory and computer software. this process fully embodies the interdisciplinary application, which can be used as a supplement to the conventional lithology identification method and promote the development of the lithology identification technology with multiple approaches, methods and angles. the appearance of artificial intelligence lithology identification software plays a powerful role in promoting the technology of complex formation lithology classification. in this paper, the application of artificial intelligence technology to low permeability reservoir oil logging lithology identification is proposed. it has been proved by practice that this method can effectively improve the accuracy of lithology identification in low permeability reservoir, and the identification process is time-consuming. acknowledgments this paper is supported by national science and technology major project “well logging interactive fine fusion processing platform” (no. 2017zx05019-005), heilongjiang natural science foundation “quantum intelligent optimization and application in real-time adjustment of logging interpretation model while drilling” (no. lh2019f004). reference deng, l. (2018). artificial intelligence in the rising wave of deep learning: the historical path and future outlook [perspectives]. ieee signal processing magazine, 35(1), 180-177. doi: 10.1109/ msp.2017.2762725 deng, c. x., pan, h. p., & luo, m. (2017). joint inversion of geochemical data and geophysical logs for lithology identification in ccsd main hole. pure and applied geophysics, 174(12), 4407-4420. https://doi. org/10.1007/s00024-017-1650-7 gobashy, m., abdelazeem, m., abdrabou, m., & khalil, m. h. (2020). estimating model parameters from self-potential anomaly of 2d inclined sheet using whale optimization algorithm: applications to mineral exploration and tracing shear zones. natural resources research, 29 (1), 499-519. https://doi.org/10.1007/s11053-019-095260 grzonka, d., jakóbik, a., koodziej, j., & pllana, s. (2018). using a multi-agent system and artificial intelligence for monitoring and improving the cloud performance and security. future generation computer systems, 86(sep.), 1106-1117. doi:10.1016/j.future.2017.05.046 guzman, a., & aoyama, a. (2018). pipeline risk assessment using artificial intelligence: a case from the colombian oil network. process safety progress, 37(1), 110-116. doi:10.1002/prs.11890 table 5. statistical table of identification results of different methods rock mass classification number of test samples /number logging lithology identification method based on boosting tree algorithm method of this paper lithology identification method based on laser induced breakdown spectroscopy correctly identify quantity/number recognition accuracy/% correctly identify quantity/number recognition accuracy/% correctly identify quantity/number recognition accuracy/% mudstone 8 4 50 8 100 6 75 sandstone 8 3 37.5 8 100 5 62.5 shale 8 4 50 7 87.5 6 75 volcanic ash 8 3 37.5 8 100 7 87.5 dolomite 7 3 42.8 6 85.7 5 71.4 cream rock 2 1 50 2 100 2 100 conglomerate 7 5 71.4 7 100 6 85.7 asphalt 4 2 50 4 100 3 75 http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(chengxiang+deng) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(chengxiang+deng) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(chengxiang+deng) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(miao+luo) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(miao+luo) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(mohamed+gobashy) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(mohamed+gobashy) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(mohamed+gobashy) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(maha+abdelazeem) http://scholar.cnki.net/result.aspx?q=%e4%bd%9c%e8%80%85:(mohamed+abdrabou) 262 fuhua shang, maojun cao1, caizhi wang han, f. l., zhang, h. b., guo, q., & rui, j. (2019). lithological identification with probabilistic distribution by the modified compositional kriging. arabian journal of geosciences, 12(18), 1-14. doi:10.1007/s12517019-4775-4. jia, h., & deng, l. h. (2018). water flooding flowing area identification for oil reservoirs based on the method of streamline clustering artificial intelligence. petroleum exploration and development, 45(02), 328335. https://doi.org/10.1016/s1876-3804(18)30036-3 jia, j. w., fu, h. b., & wang, h. d. (2018). lithology identification methods based on laser-induced breakdown spectroscopy technology. chinese journal of quantum electronics, 35(03), 264-270. jiang, k., wang, s. d., & hu, y. j. (2018). lithology identification model by well logging based on boosting tree algorithm. well logging technology, 42(04), 395-400. wang, w. y., & siau, k. (2019). artificial intelligence, machine learning, automation, robotics, future of work and future of humanity: a review and research agenda. journal of database management, 30(1), 61-79. doi:10.4018/jdm.2019010104 wei, j. l., liu, x. n., ding, c., liu, m., jin, m., & li, d. (2017). developing a thermal characteristic index for lithology identification using thermal infrared remote sensing data. advances in space research, 59(1), 7487. https://doi.org/10.1016/j.asr.2016.09.005 xie, y. x., zhu, c. y., zhou, w., li, z., liu, x., & tu, m. (2018). evaluation of machine learning methods for formation lithology identification: a comparison of tuning processes and model performances. journal of petroleum science and engineering, 16(03), 182-193. https://doi. org/10.1016/j.petrol.2017.10.028 zhang, l., liang, y. c., & niyato, d. (2019). 6g visions: mobile ultrabroadband, super internet-of-things, and artificial intelligence. china communications, 16(8), 1-14. doi: 10.23919/jcc.2019.08.001 zhou, y., zhang g. z., gao g., zhao, w., yi, y., & wei, h. (2019). application of kernel principal component analysis in lithologic identification of well logging turbidite. oil geophysical prospecting, 54(03), 667-675, 490. zibret, g. (2019). influences of coal mines, metallurgical plants, urbanization and lithology on the elemental composition of street dust. environmental geochemistry and health, 41(3), 1489-1505. doi: 10.1007/s10653018-0228-3 about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank https://doi.org/10.1016/s1876-3804(18)30036-3 http://www.wanfangdata.com.cn/details/javascript:void(0) http://www.wanfangdata.com.cn/details/javascript:void(0) http://www.wanfangdata.com.cn/details/javascript:void(0) http://www.wanfangdata.com.cn/details/javascript:void(0) http://dx.doi.org/10.4018/jdm.2019010104 https://doi.org/10.1016/j.asr.2016.09.005 https://doi.org/10.1016/j.petrol.2017.10.028 https://doi.org/10.1016/j.petrol.2017.10.028 in this study, experimental investigations were carried out on polypropylene fiber (pf), steel mesh (sm) and pf+sm reinforced shotcrete (rs) panels to evaluate performance characteristics of energy absorption and load capacity. in addition to this, material cost evaluation of shotcrete mixtures for unit energy absorption and load capacity has been given. the panel tests, along with the european specification for sprayed concrete (efnarc), were done on 18 prismatic specimens having the same mix designs and were cured for 28 days but reinforced at varying fiber dosages. test results indicate that in terms of unit cost performance for pf, the best dosages were 2 kg/m³ and 5 kg/m³ respectively. when compared with sm, it was determined that pf is more expensive within the range of 37 53%. en este estudio se realizaron investigaciones experimentales en fibras de polipropileno, en mallas de acero y en paneles de hormigón proyectado reforzados con fibra de polipropileno y con mallas de acero para evaluar las características de desempeño en absorción de energía y en capacidad de carga. adicionalmente, se provee una evaluación del costo de materiales en mezlas de hormigón proyectado por unidad de absorción de energía y por capacidad de carga. los exámenes en paneles, con las especificaciones europeas para hormigón proyectado (efnarc, inglés), se realizaron en 18 muestras prismáticas con los mismos diseños de mezcla y se curaron por 28 días con refuerzos de diferentes dosis de fibras. los resultados de la evaluación indican que, en términos de desempeño por unidad de costo para fibras de polipropileno, las mejores dosis fueron 2 kg/m3 y 5 kg/m3, respectivamente. cuando se comparó con las mallas de acero, se determinó que las fibras de polipropileno son entre el 37 % y el 53 % más caras. earth sciences research journal earth sci. res. j. vol. 19, no. 1 (june, 2015): 81 84 abstract resumen key words: shotcrete, polypropylene fiber, energy absorption, panel test. palabras clave: hormigón proyectado; fibras de polipropileno; absorción de energía; evaluación de paneles. determining optimal polypropylene fiber dosages in sprayed concrete for mining and civil engineering applications bayram kahraman dokuz eylül university, department of mining engineering, turkey bayram.kahraman@deu.edu.tr geotechnics record manuscript received: 27/11/2013 accepted for publication: 30/09/2014 issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v19n1.40995 82 bayram kahraman82 introduction sprayed concrete is a mixture of cement, aggregate, and water projected pneumatically from a nozzle into place to produce a dense homogeneous mass. sprayed concrete normally incorporates admixtures and may also include additions or fibers or a combination of these (efnarc, 1996). shotcre te (sprayed concrete) is widely used as rock support in mines and civil engineering projects. it is applied through a process by which concrete or mortar is sprayed onto a surface to produce a compacted self-supporting and load-bearing layer. the main design principle for shotcrete as well as for other rock support elements is to help the rock to carry its inherent loads (malmgren, 2005). the use of shotcrete for the support of underground excavations was pioneered by the civil engineering industry. reviews of the development of shotcrete technology have been presented by morgan (morgan et al., 1989). rabcewicz was largely responsible for the introduction of the use of shotcrete for tunnel support in the 1930s, and for the development of the new austrian tunneling method for excavating in weak ground. in recent years, the mining industry has become a major user of shotcrete for underground support (hoek, 2007). in mining, flexible linings are preferred because large displacements of underground openings are allowed. it often results in extensive cracking of the shotcrete. therefore, the toughness of shotcrete is very important which is enabled by the use of reinforcement (morgan et al., 1989). toughness is the amount of energy that is absorbed before and after fracture. ductility and high-fracture strains are also important characteristics of fiber-reinforced shotcretes (frs) because the main reason for incorporating fibers in concrete and shotcrete is to impart ductility to an otherwise brittle material. in addition, fiber reinforcement improves the energy absorption and crack resistance of shotcrete (morgan et al., 1995). with an increasing load on the composite, fibers will tend to transfer the additional stress to the matrix through bond stresses until they fail or pull out (mehta and monteiro, 1993). in this way, they enable shotcrete to continue to carry the load after cracking, the so-called post cracking behavior (vandewalle, 1997). in this study, rs panels were made with pf, sm and pf+sm. these rs panels, moreover, examined for load capacity and energy absorption tests. this study shows that the use of pf in rs, with sufficient fiber content can greatly improve energy absorption and ultimate load capacity. pf, sm and pf+sm concrete mixture estimation costs, furthermore, were compared in terms of unit load and energy absorption cost. 2.materials and experimental work in previous studies, malmgren discussed the comparison of steel fibers and pf and its effects on shotcrete. results of malmgren’s study show us the shotcrete reinforced with steel fibers and the pf had almost the same energy absorption. in this study, to investigate the load capacity and the energy absorption of reinforced shotcrete, efnarc panel test was used. firstly energy absorption and peak load of pf at different dosages and sm is examined. then the pf content was evaluated in terms of unit cost. for this aim, 27 shotcrete mixtures are prepared and then the energy absorption of mixtures is tested. 2.1.materials in the design of test panels, turkish portland cement was used for the production of concrete mixtures (pc 42.5 r, astm type i). crushed rock having a particle size between 0-3 mm and 3-7 mm were equally used as aggregates. the sieve analysis of aggregate is given in table 1. as a high range water reducing admixture, rheobuild® t90, was used in shotcrete mixtures. the specifications of pfs are given in table 2. the steel mesh (diameter: 5.5 mm; intervals: 100 mm) was used as a reinforcement. table 1. sieve analysis of aggregates in shotcrete table 2. specification of pf (rheobuild® t90) 2.2. shotcrete mixtures the shotcrete mix proportions used in this experimental study are given in table 3. shotcrete mixes having 0.47 water/cement ratio were prepared and put into molds. 83determining optimal polypropylene fiber dosages in sprayed concrete for mining and civil engineering applications materials (kg/m3) pf-0* pf-2 pf-3 pf-4 pf-5 pf-6 pf-8 pf-4+ sm** sm cement 450 450 450 450 450 450 450 450 450 coarse aggregate 825 825 825 825 825 825 825 825 825 fine aggregate 825 825 825 825 825 825 825 825 825 water 210 210 210 210 210 210 210 210 210 admixture (wra) 4 4 4 4 4 4 4 4 4 pf 0 2 3 4 5 6 8 4 +sm sm *:pf dosages **:steel mesh table 3.base concrete mix proportions 2.3. experimental work the efnarc panel test was used to compare the toughness of shotcrete panels with different reinforcement and amount of reinforcement. the toughness was calculated as the absorbed energy corresponding to the area under the load-displacement curve between 0 and 25 mm (efnarc, 1996). the test panel 600 mm x 600 mm x 100 mm was supported on its four edges rigidly (figure 1-b). the free face of the panel is 500 mm x 500 mm because of the thickness of the support (figure 1-a). the load was applied through a contact surface of 100x100 mm at the center of the panel by 100 kn testing machine at 1.5 mm/min deformation rate. during the test load and displacement at the center are recorded up to of 25 mm deflection has occurred at the center of the panel. fig. 1. a. efnarc panel test (efnarc, 1996), b. testing machine (100 kn) used for panel test (a) (b) 2.4. shotcrete cost the required amount of shotcrete, to cover 10 cm thick, and 1 m long shotcrete of a decline of 5x5 m cross-section was taken as basis for calculating the shotcrete cost. in these calculations, rebound was assumed to be % 20. accordingly the amount of shotcrete was found to be 1.93 m3 for 1 m advancement. the cost of 1.93 m3 shotcrete was found for every mixture using the current prices given in table 4, these values were divided by load capacity and energy absorption of each mixture, and thus the unit load and energy cost performance of the mixture were calculated (table 4). table 4.shotcrete costs concrete mix materials unit unit cost aggregate (0-7 mm) $/t 9,90 cement, cem 1 42,5 $/t 56,81 rheobuild® t90 $/kg 0,57 fiber meyco® fib sp 540 $/kg 6,01 steel mesh (5x2 m) $/unit 31,03 3. results and discussion the load–deformation curves (figure 2) were obtained as an average of the results of the tests performed on nine groups having three panels each. pf-0, pf-2, pf-3, pf-4, pf-5, pf-6, pf-8, pf-4+sm, sm panels. fig. 2. load deformation curves then the energy absorption of shotcrete lining is calculated by integrating the area under the load-displacement curve. the load– deformation curves of tests are given in figure 2. the area under the load displacement curve between 0 and 25 mm calculated by the aim of cad based program and the results are given in table 5. also unit costs per peak load and unit costs per absorbed energy are calculated and given in table 5. 84 bayram kahraman table 5. results of panel tests the test results show us the dosage of pf has a moderate effect on the peak load of panel samples. on the other hand, energy absorptions of samples have considerable differences by the changes of fiber dosages. the results from panel tests are summarized in figure 3. as inferred from the table 4 and figure 3, use of both pf and sm has the maximum energy absorption but it should also be evaluated in terms of costs. however, in any case both pf and sm will be used for high-stress areas like as intersections of underground spaces. 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0 10 20 30 40 50 60 70 pe ak lo ad (k n ) peak load (kn) energy absorption (kj) en er gy a bs or bt io n (k j) fig. 3. peak load and energy absorption of test samples when the effect of pf in a mixture is discussed, the tests show us increasing of pf dosage have a positive effect on energy absorption of shotcrete lining. another issue that must not be forgotten is the effect of mixture ratios on shotcrete. the results are discussed in terms of cost per load and cost per energy absorption. as seen from the table when the fiber dosage increased from 2 kg/m3 to 8 kg/m3 the peak load increases 19% whereas the unit cost per peak load increases 26%. on the other hand, the energy absorption increases 29% but the cost per energy absorption increases only 15%. similarly when the sm and sm + pf are discussed the cost per peak load and the cost per energy absorption decrease up to 50%. 4. conclusion the experimental investigation was performed on polypropylene fiber, steel mesh, and polypropylene fiber + steel mesh panels to evaluate performance characteristics of energy absorption and load capacity. in addition, material cost evaluation of shotcrete mixture for unit energy absorption and load capacity were discussed. the obtained results are summarized as follows; 1. the dosage of pf has not considerable effect on peak load of shotcrete linings, but the use of sm or sm+pf together has a positive effect on peak load. 2. according to the achieved results, by the increasing of pf dosage a linear rise of energy absorption of shotcrete linings is observed. especially compared with zero dosage of fiber, the existence of pf has a favorable effect on shotcrete lining. 3. if the cost of pf discussed in terms of energy absorption the study shows when the energy absorption increases 29% the cost per energy absorption increases 15%. however, if the peak load discussed the cost per peak load increases with the increase of fiber dosages, because the fiber dosages have no remarkable positive effect on peak load 4. the uses of pf and sm together have the maximum energy absorption and minimum cost per energy absorption. although the sm and sm + pf has a few disadvantages such as time-consuming installation, difficult installation, etc., especially in the unstable formations the sm and sm + pf linings should be preferred 5. it should be kept in mind that the change of energy absorption is not related to only the fiber dosages. it is also the function of mixture component ratios. references [1] european federation of producers and applications of specialist products for structures, european specification for sprayed concrete, efnarc (1996). loughborough university, pp. 8 – 10. [2] malmgren, l. (2005). interaction between shotcrete and rock experimental and numerical study, doctoral thesis, luleå university of technology, department of civil and environmental engineering, division of mining and geotechnical engineering rock mechanics and rock engineering, sweden. [3] morgan, d.r., mcaskill n., richardson b.w., zellers r.c. (1989). a comparative evaluation of plain, polypropylene fiber, steel fiber and wire mesh reinforced shotcrete, transp. res. rec. 1226, washington, d.c., 78 – 87. [4] hoek, e. (2007). practical rock engineering. chap. 12, rock-support interaction analysis for tunnels in weak rock masses, pp 1-19, www.rockscience.com. [5] morgan d.r., chen l., beaupre d. (1995). toughness of fibre reinforced shotcrete, asce shotcrete underground support vii, austria, 66– 87. [6] mehta p.k., monteiro p.j.m. (1993). concrete, microstructure, properties, and materials, 2nd ed., the mcgraw-ill companies, inc., p. 405. [7] vandewalle m. (1997). tunneling the world, bekaert s.a., belgium. the current study represents an evaluation of the petrophysical properties in the well shaikhan-8 for the garagu, sarmord and qamchuqa formations in shaikhan oilfield, duhok basin, northern iraq. the petrophysical evaluation is based on well logs data to delineate the reservoir characteristics. the environmental corrections and petrophysical parameters such as porosity, water saturation, and hydrocarbon saturation are computed and interpreted using interactive petrophysics (ip) program. neutron-density crossplot is used to identify lithological properties. the qamchuqa formation in the shaikhan oilfield consists mainly of dolomite with dolomitic limestone, and the average clay volume is about 13%; while sarmord formation composed of limestone and dolomitic limestone, the average clay volume in this formation is about 19%; also the garagu formation consists mainly of limestone and dolomitic limestone in addition to sandstone and claystone, the volume of clay in the garagu formation is about 20%. pickett plot method is used to calculate formation water resistivity (rw), saturation exponent (n) and cementation exponent (m) the values are 0.065ohm, 2, and 2.06 respectively. the porosity ratio (ø) of the qamchuqa formation ranges between 7-15%; this indicates that the lower part of the formation has a poor-fair porosity (7%), while the upper part of the formation has a good porosity (15%). the porosity value decrease toward sarmord formation especially in the lower part of the formation, it has a poor porosity (5%), whereas this value reaches to 13% in the upper part of the formation, indicates for fair porosity. garagu formation has good porosity, reaches 20% in the lower part, but in the upper part of the formation, this value decreases to 3%. water saturation (sw) value which is calculated by archie equation ranges between 14-33%, while saturation in the flushed zone (sxo) ranges between 52-73%, these indicate for good movable hydrocarbons are present in the studied interval (840-1320m), and from the total 480m the early cretaceous formations in well shaikhan-8 have 178m pay. abstract keywords: qamchuqa, sarmord, garagu, shale volume, porosity, saturation; petrophysical properties of the early cretaceous formations in the shaikhan oilfield/northern iraq issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.66088 wrya j. mamaseni (1), srood f. naqshabandi (1), falah kh. al-jaboury (2) 1 salahaddin university/college of science/ geology department 2 knowledge private university / petroleum engineering department wriajihad@yahoo.com; sroodfn@gmail.com; falhkh@yahoo.com earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 45-52 este estudio presenta una evaluación de las propiedades petrofísicas en el pozo shaikhan-8 para las formaciones garagu, sarmord y qamchuga, en el campo petrolero de shikhan, cuenca duhok, al norte de irak. la evaluación petrofísica se basó en la información de registro de pozos, lo que permitió delinear las características del depósito. las correcciones ambientales y los parámetros petrofísicos como porosidad, saturación del agua, y saturación de hidrocarburo se calcularon e interpretaron a través del programa interactive petrophysics (ip). la gráfica de interrelación de densidad de neutrones se utilizó para identificar propiedades litológicas. la formación qamchuqa consiste principalmente de dolomita y caliza dolomítica, y el promedio de volumen arcilloso es cercano al 13 %; mientras que la formación sarmord se compone de caliza y caliza dolomítica, con un promedio de volumen arcilloso de 19 %; la formación garagu está formada por caliza y caliza dolomítica con adición de arenisca y arcillolitas, y su volumen arcilloso es de 20 %. la gráfica de pickett se utilizó para calcular en las formaciones la resistividad del agua (rw), el exponente de saturación (n) y el exponente de cementación (m); los resultados en las tres formaciones de estudio fueron de 0.065ohm, 2, y 2.06. la proporción de porosidad (ø) de la formación qamchuqa oscila entre 7 y 15 %; esto indica que la parte más baja de la formación tiene una porosidad entre regular y baja (7 %), mientras que la parte más alta tiene un alto índice de porosidad (15 %). el valor de porosidad cae hace la formación sarmord, especialmente en su parte más baja donde tiene un índice bajo (5 %), mientras que en la parte alta alcanza el 13 %, lo que indica un nivel regular. la formación garagu tiene un buen índice de porosidad que alcanza el 20 % en la parte más baja, pero que en la parte alta cae al 3 %. el valor de saturación de agua (sw), que se calcula por la ecuación de archie, oscila entre el 14 y el 33 %, mientras que la saturación en la zona lavada (sxo) oscila entre el 52 y el 73 %; esto demuestra que los hidrocarburos móviles están presentes en el intervalo de estudio (840-1320m), y que del total de 480 metros de las formaciones del cretácico temprano en el pozo shaikhan-8, aproximadamente 178 metros de zona productora. resumen palabras clave: qamchuqa; sarmord; garagu; volumen de esquistos; porosidad; saturación; record manuscript received: 04/07/2017 accepted for publication: 17/11/2017 how to cite item mamaseni, w. j., naqshabandi, s. f., al-jaboury, f. k. (2018). petrophysical properties of the early cretaceous formations in the shaikhan oilfield/northern iraq. earth sciences research journal, 22(1). 45-52. doi: http://dx.doi.org/10.15446/esrj.v22n1.66088 propiedades petrofísicas de las formaciones del cretácico temprano en el campo petrolero de shaikhan, en el norte de irak pe t r o ph y si c s 46 wrya j. mamaseni, srood f. naqshabandi, falah kh. al-jaboury introduction the mesozoic era in northern iraq are considered one of the most suitable locations for deposition carbonate rocks, due to ecumenical temperature rise the best source and reservoir rocks in the jurassic and cretaceous period are made (jassim and goff, 2006). the cretaceous succession has been extensively studied because it contains abundant reservoir intervals (al-sakini, 1992). the major of discovered hydrocarbon reserves in iraq are in cretaceous reservoirs, although numerous oil and gas fields have been discovered recently in deeper jurassic and cretaceous reservoirs in the kurdistan region, including the shaikhan, atrush, swara-tika, bakrman, bina-bawi, and mirawa oilfields (verma et al., 2004). most of the significant hydrocarbon discoveries in the kurdistan region occur in compressional structures formed during the zagros orogeny. the subsidence in the developing zagros foredeep induced a major phase of source rock evaluation and hydrocarbon expulsion, and it is probable that hydrocarbon charging of the zagros anticlines occurred via relatively short-distance migration from the adjacent synclinal kitchens (joseph et al., 2015). reservoir rocks, where petroleum accumulations have been found, are mainly siliciclastic rock (sandstone), or carbonate rocks (limestone and dolomite). fluid flow through heterogeneous carbonate reservoirs (limestone and dolomite) is a substantially different process from the flow through the homogeneous sandstone reservoir, this variation is largely cause to the fact that carbonate rocks tend to have a more complex pore system (i.e the interrelationships among depositional lithologies, the geometries of depositional facies, and diagenesis) than sandstone (amin, et. al., 1987). three carbonate formations were selected in well shaikhan-8 for this study, which is located 60 km southeast of duhok city at the high folded zone (figure 1). the shaikhan structure is an anticline at the northwestern terminus of the zagros fold belt, and affected by several faults (gulf keystone 2013). the studied formations that contain qamchuqa, sarmord and garagu are belonging to the late tithonian early turonian period; and defined by wetzel, 1950 in northern of iraq (bellen et al., 1959). lithologically the formations are composed of grey to greenish grey dolomite with greyish yellowish limestone and dolomitic limestone in addition to grey shale and sandstone. the thickness and intervals for three formations qamchuqa, sarmord and garagu are 186m from the interval 840-1026m, 113m from the interval 1026-1139m and 181m, from the interval 1139-1320m respectively. the main objective of this study is to determine petrophysical evaluations (porosity, water saturation, and hydrocarbon saturation) for the early cretaceous formations at duhok basin. figure 1. location and tectonic map of the studied area (jassim and goff, 2006) material and method this research involves the analysis of petrophysical properties using data from the available logs such as caliper, gamma ray, density, neutron, sonic, and resistivity logs, the available open hole logs data imported into the appropriate interactive petrophysics software (ipversion 3.5) for analysis and interpretation. one reading per 0.5m depth is selected for recording the input data measurements. log data are adversely influenced by the borehole environment such as borehole size, pressure, salinity, drilling mud, filter cake. the effects of these environment conditions on the log data may be eliminated through environment correction. after the well log curves were checked and calibrated to the depth by using gamma ray log as a reference curve, all environment corrections (gamma ray, density, neutron, and resistivity logs) have been done using schlumberger chart (slb, 2005), which is supplied to this software program as the environmental correction module (figure 2); then shale volume (vsh) calculations were made, and the lithology and mineralogy discriminations were interpreted through appropriate crossplots, and porosity (ф) estimated from neutron and density logs. the resistivity logs were utilized in order to obtain water saturation (sw). figure 2. environment corrections for the studied formations in well shaikhan-8 47petrophysical properties of the early cretaceous formations in the shaikhan oilfield/northern iraq results and discussion 1. shale volume estimation (vsh) in order to calculate the volume of shale, gamma ray log is the most common indicator. the maximum gamma ray response is taken as the shale point and minimum response as the clean point. the lower shale content usually reveals a better reservoir whereas increase in volume of shale decreases the effective reservoir capacity. the volume of shale must be correct, if it is not corrected, gives erroneous during calculation volume of hydrocarbon, porosity and water saturation. to calculate the volume of shale for the early cretaceous formations at first gamma ray index was calculated by using the following equation: and then this gamma ray index is used to calculate the volume of shale by using the larionov old rock equation: where: igr= gamma ray index grlog= gamma ray log reading of formation grmin= minimum gamma ray reading in clean zone, (clean sand or carbonate) grmax= maximum gamma ray reading in shale zone vsh =volume of shale there are a lot of methods used to calculate volume of shale according to age of the formation; larionov method is used for mesozoic formations (figure 3), regarding to this method the average volume of shale ranges between 1320% (figure 4). qamchuqa formation contains minor volume of shale in well shaikhan-8 (13%), while this value increase in the two other formations ranges between 19-20%, for sarmord and garagu formations respectively (table 1). table 1. minimum and maximum value of caly for the studied formations in well shaikhan-8 (1) (2) (3) figure 3. different methods used to calculate volume of shale figure 4. volume of clay for the studied formations in well shaikhan-8 2. formation temperature estimation the temperature gradient module in the software was used to create a continuous temperature curve for the well shaikhan-8 by entering temperatures at fixed points (table 2). the software extrapolates between the points. the following equation was used for formation temperature determination: where: t2= formation temperature d = log depth bht = bottom hole temperature t1= surface temperature td = total depth table 2. temperature and total depth in well shaikhan-8 48 wrya j. mamaseni, srood f. naqshabandi, falah kh. al-jaboury48 3. determination of lithology and mineralogy determination of the lithology and mineralogy composition of the rock formation is important for the evaluation of a reservoir. lithology often describes the solid (matrix) portion of a rock. the neutron-density crossplot is one of the oldest techniques used for determining the formation lithology. this method consists of plotting neutron porosity (øn) versus bulk density (ρb) on a standard plot (hughes, 1992). the neutron log is used to measure the amount of hydrogen in the formations, which is assumed to be related to porosity. the density log is used to measure electron density and from that the formation bulk density, when the two logs are used together, lithology can be determined (asquith and krygowski, 2004). crossplot illustrates the separation between the sandstone, limestone and dolomite lines, the lithology lines are marked with porosity values (wyllie 1963). according to the neutron-density crossplot, the qamchuqa formation in well shaikhan-8 composed mainly of dolomite with some dolomitic limestone and the porosity ranges between 7-15%. the lower part of the formation has fair porosity, but it has good porosity in the upper part of the formation. the sarmord formation consists mainly of limestone, dolomitic limestone with some claystone, the porosity ranges between 5-13%, which indicates poor porosity in the lower part of the formation while fair porosity for the upper part of the formation. the garagu formation is mainly composed of limestone and minor dolomitic limestone, in addition to claystone and sandstone. the porosity range between 3-20% the upper part of the formation has very poor porosity (3%), whereas the lower part of the formation has enough porosity (20%) for hydrocarbon accumulation. there are some points that are scattered due to the influence of borehole enlargement and gas effective (figure 5). another relationship is m-n crossplot. this technique is used when lithology of a formation is more complex. this crossplot requires the sonic log along with the neutron and density logs. the sonic log is a porosity log that measures interval transit time (asquith and gibson, 1982). the terms m and n are defined as follows (schlumberger 1972): (4) (5) where: ∆tf: transit time for the fluid in the formation (189 for fresh and 185 for salt water) ∆t: transit time in the formation (from log) ρb: bulk density of formation (from log) ρf: density of fluid (1.0 for fresh water, and 1.1 for salt water) ønf: neutron porosity of fluid formation (use 1.0) øn: neutron porosity of formation (from log) based on the m-n crossplot, the main constituent of the qamchuqa formation is dolomite, calcite and little anhydrite, while sarmord formation consist of calcite with little dolomite, whereas garagu formation contain calcite with little dolomite and anhydrite (figure 6). figure 5. neutron-density crossplot for the studied formations in well shaikhan-8 figure 6. m-n crossplot for the studied formations in well shaikhan-8 4. porosity the porosity of a rock is the volume of the non-solid portion of the rock that is filled with fluids divided by the total volume of the rock (crain, 1986). in general porosities tend to be lower in deeper and older rocks due to cementation and overburden pressure stress on the rock (rider, 1996). generally, porosity has two main types: total porosity and effective porosity. total porosity is defined as the ratio of the entire pore space in a rock to its bulk volume (nnaemeka 2010), or the porosity derived directly from the 49petrophysical properties of the early cretaceous formations in the shaikhan oilfield/northern iraq logs without correction for the clay content. effective porosity represents the pore space that contains hydrocarbon and bound water (nnaemeka 2010). the resultant porosity in effective porosity can be determined after removal of the effect of clay. in an interval without shale, the total porosity equals the effective porosity. the porosity from logs can be estimated from a single porosity log (density, neutron, sonic) or a combination of porosity logs. the relationship between the total porosity (øt) and the effective porosity (øe) can be represented by: (6) (7) (8) (11) (9) (10) in the current study the total porosity ranges between (8%-14%), while these values decrease to (7%-12%) when calculate the effective porosity, due to influence of shale volume on the porosity (table 3). the porosity derived from the density log (фd) is defined by the following relationship (dresser, 1979): where: фdcorr. = corrected density for shale effect ρma = matrix density (s.st = 2.65, l.st=2.71, dolomite=2.88) ρb= log density vsh = volume of shale ρsh = density value of adjacent shale formation ρf= formation fluid density for: fresh water mud, ρf = 1.0 g/cc and salt water mud, ρf = 1.1 g/cc, gas mud, ρf = 0.70 g/cc in a shaly formation, the neutron log will appear to be more porous. the effect of shale on the neutron log can be corrected by the following equation: where: фncorr. = corrected neutron porosity фnlog= neutron log reading of the interval vsh = volume of shale фnsh = neutron log of the adjacent shale formation the total porosity can be determined from density and neutron porosities by the following equation: the velocity of sound through sedimentary rocks depend on lithology and porosity, shale has the lowest sonic velocity, while limestone and dolomite have higher velocities; the velocity of sound is slower and need long travel time when travel through liquids and gas, while at compact zone this velocity is fast and need short travel time to reach receivers. the following formula can be used for calculating the sonic porosity after remove the shale effect (dresser, 1979): where: ∆tlog= is the reading on the sonic log in /ft ∆tma =is the transit time of the matrix material (s.st= 55.5, l.st= 47, dol. = 43 µs/ft) ∆tf =is the transit time of the saturating fluid (fresh = 189, salt = 185 µs/ft) δtsh = interval transit time for adjacent shale vsh = volume of shale table 3 . average total and effective porosity in the studied formations in well shaikhan-8 5. formation water resistivity (rw) formation water resistivity represents the resistivity value of the water which is uncontaminated by the drilling mud that saturates the porous formation. pickett’s (1966) method is a graphical solution to archie’s equation so that plotting deep resistivity (lld) against effective porosity (phie) (both on logarithmic scales) will produce a linear equation: in water bearing zone, sw = 1 in the plot of the true formation resistivity versus effective porosity, the archie’s parameters were calculated (a=1, m=2.06 and n=2). the red line that passed through points have lowest deep resistivity and highest porosity values which represent the water zone at which water saturation equals one. the slope of the line represents the porosity exponent (m) and the intercept represents the formation water resistivity (rw) which is equal 0.065 in this well (figure 7). shaikhan-8 lld / phie interval : 840. : 1320. 1. 10. 100. 1000. lld 0.01 0.02 0.05 0.1 0.2 0.5 1. ph ie 0. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. sw 0.2 0.3 0.5 853 points plotted out of 961 parameter : rw : 0.0653 parameter : rw form temp : 0.0653 parameter : m exponent : 2.05 parameter : n exponent : 2. parameter : a factor : 1. well depths (-1) shaikhan-8 840.m 1320.m figure 7. archie parameters and rw by pickett’s plot in well shaikhan-8 6. reservoir fluids and formation analysis the reservoir rock fluid saturation is referred to as the percentage of the pore space filled with a particular fluid and the sum of all the fluids in the pore spaces equals one hundred percent. the saturation of water is always part of the fluids that occupy the pore spaces of reservoir rocks (dresser, 1975). connate water usually is in the range of 10 % to 40 % in most rocks. this means that the connate water occupies about 10 to 40 percent of the pore volume of 50 wrya j. mamaseni, srood f. naqshabandi, falah kh. al-jaboury the reservoir rock and other fluids (hydrocarbon) occupy the rest of the pore volume. free water is the water that resides within the macro-porosity region of the interconnected pore space of a reservoir and is able to flow under an applied pressure gradient (newman, 1987). a. water saturation (sw) it is the amount of pore volume in a rock that is occupied by formation water. water saturation is represented as decimal fraction or as percentage (asquith and krygowski 2004). sw of the uninvaded zone of a reservoir is calculated by the archie’s (1942) formula (12) (13) (17) (14) (15) (16) where: sw = water saturation, fraction rw = formation water resistivity, ohm-m rt = true formation resistivity for uninvaded zone, ohm-m ф = effective porosity, and, a, n, and m: archie’s parameters, dimensionless water saturation of the formation’s flushed zone (sxo) is based on the archie’s equation, which is expressed by: water saturation of the flushed zone (sxo) can be used to indicate the movable hydrocarbon. if the value of sxo is much higher than sw, then hydrocarbons in the flushed zone have probably been moved or flushed out of the zone nearest the borehole by the invading drilling fluids (asquith, and krygowski, 2004). in the present study the values of the sxo are greater than sw; this indicates that the formations under study contain a good movable hydrocarbon (table 4). b. bulk volume analysis (bvw) bulk volume analysis represents the amount of the bulk water volume that preserve in sediments during deposition and lithification before the hydrocarbon migrating to the reservoir rocks, which is known as connate water (tiab and donaldson, 1996) the product of the formation water saturation (sw) and the effective porosity gives the bulk volume of water (bvw) in the uninvaded zone, and the product of the formation water saturation (sxo) and the effective porosity give the bulk volume of water (bvsxo) in the flushed zone (spain, 1992). comparison of sw and sxo in the hydrocarbon zone gives the movable hydrocarbon (sxo sw), which is equal to the fraction of the movable hydrocarbons in the formation. the volume percentage in terms of the reservoir is given by multiplying in term of the porosity, i.e. percentage of volume of reservoir with movable hydrocarbons equal to (rider, 1996). the area between bvwsxo and bvw represents the movable hydrocarbon, whereas the area between bvwsxo and øe represents the residual hydrocarbon (figure 8). c. hydrocarbon saturation (shc) the hydrocarbon saturation can be deduced from the water saturation by the following relationship: hydrocarbon saturation is normally differentiated into the nonexploitable or residual hydrocarbon (shr) and the exploitable or movable hydrocarbon (shm), as follow: movable hydrocarbon represented by yellow color on the computer processing interpretation, (figure 8) while residual hydrocarbon represented by green color. table 4. saturation fluid value within the studied formations in well shaikhan-8 7. reservoir cutoff cutoff values such as shale volume, porosity, and water saturation are very important parameters in calculation of the original oil in place (ooip) and the reservoir net pay. the results show that the lower cretaceous formations in the well shaikhan-8 have more than 178m pay from a total 480m thick (table 5). table 5. reservoir cutoff for the studied formations in well shaikhan-8 51petrophysical properties of the early cretaceous formations in the shaikhan oilfield/northern iraq figure 8. computer processing interpretation for the studied formations in well shaikhan-8 discussion this study has shown the petrophysical evaluation of the early cretaceous reservoirs along shaikhan block in the duhok basin. even some previous studies have been conducted around that area; there is not detailed published petrophysical evaluation. this study would be the first petrophysical properties calculation in that area. therefore, it is hoped that this report would help for future explorations in the area. the early cretaceous formations in the studied area lithologically composed of dolomite, limestone to dolomitic limestone with some claystone and sandstone. the carbonate formations have fair to good ratio of porosity, especially in the upper part of the qamchuqa and sarmord formations, in addition to the lower part of the garagu formation that indicate to classify this period as best medium for hydrocarbon accumulation. one of the most important procedures in petrophysical evaluation is calculating the amount of shale in the formations, any formation which contains a high ratio of shale could not be considered as a good reservoir. the amount of shale is very rare in the early cretaceous formations, particularly in qamchuqa formation. 52 wrya j. mamaseni, srood f. naqshabandi, falah kh. al-jaboury conclusions gamma ray log detected very little shale volume in the qamchuqa formation (13%), whereas, the shale volume increases in the sarmord and garagu formations (19-20%) respectively. neutron-density crossplot show that the qamchuqa reservoir consists mainly of dolomite, and some dolomitic limestone, but the sarmord formation consists of limestone with some dolomitic limestone, whereas the garagu formation is limestone, dolomitic limestone in addition to claystone and sandstone. m-n crossplot claimed that the main constituent mineral of the qamchuqa formation is dolomite while calcite is the main constituent for the other formations. porosity logs show that the qamchuqa formation has fair porosity in the lower part (7%) and good porosity in the upper part (15%). the sarmord formation has poor to fair porosity except some beds in the upper part of the formation that have good porosity, especially in the intervals 1030-1050m,10751080m and1093-1100m respectively, but the garagu formation has good porosity in the lower part of the formation (20%) and very poor porosity in the upper part of the formation (3%). according to the saturation models, good movable hydrocarbon occurs in this interval (840-1320m), qamchuqa formation has 49m pay zone from 186m, the sarmord formation has a 36m pay zone from 113m, whereas 93m pay zone from 181m in garagu formation. acknowledgements our gratitude and thanks are extended to the staff of the ministry of natural resource especially dr. muhammadamen atrushi, for providing most of the data and information related to this work. we would like to extend our appreciation to staff of gulf-keystone company especially to mr. hemn sabah tahir, for helping us to provide data that required accomplishing this study. references al-sakini, j.a. (1992). summary of the petroleum geology of iraq and the middle east. in northern oil company press (naft-al shamal co.) kirkuk, iraq (in arabic), 179 p. amin, a.t., watfa, m. and awad, m.a. (1987). accurate estimation of water saturation in complex carbonate reservoir, middle east oil show, 7-10 march, bahrain, spe-15714-ms, 16 p. archie, g.e. (1942) the electrical resistivity log as an aid in determining some reservoir characteristics. trans. am. inst. mech. eng., v.146, 54–62 p. asquith, g.b. and gibson, c. (1982). basic well log analysis. second edition aapg, tulsa, oklahoma, 239 p. asquith, g. and krygowski, d. (2004). “basic well log analysis”, 2nd.ed, 244 p. bellen, r.c. dunnington, h.v., wetzel, r. and morton, d. (1959). lexique stratigraphic international. asie, fasc. 10a, iraq, paris, 333 p. crain, e.r. (1986). the log analysis handbook; penn-well publishing company, tulsa, oklahoma, usa. 700 p. dresser a. (1975). log interpretation fundamentals. dresser atlas division, dresser industries, houston, tx 125 p. dresser a. (1979). log interpretation charts; houston, dresser industries inc., 107 p. gulf keystone company, (2013). internal report, the final well report of shaikhan-8 well, (unpublished report) 22 p. hughes, b. (1992). advanced wierline and mwd procedure manual huston, 192 p. jassim, s.z. and goff, j.c. (2006). geology of iraq. published by dolin, prague and moravian museum, brno, czech republic, 341 p. joseph, m.e., grenville. l., luke, f. and george, y. (2015). geologic evolution of the iraqi zagros, and its influence on the distribution of hydrocarbons in the kurdistan region 231-272 p.. newman, a.c.d. (1987). the interaction of water with clay mineral surfaces. in chemistry of clay and clay minerals, harlow, england. longman scientific and technical, 237-274 p. nnaemeka, e. (2010). petroleum reservoir engineering practice: porosity of reservoir rocks, 816 p. pickett, g.r. (1966). a review of current technique for determination of water saturation from logs. spe 1446 p. rider, m. (1996). “the geological interpretation of well logs”, 2nd ed., petroleum exploration consultant rider french consulting ltd. aberdeen and sutherland, 278 p. schlumberger (1972). log interpretation manual principles. vol. i, schlumberger well services inc., houston, 113 p. spain, d.r. (1992). petrophysical evaluation of a slope fan / basinfoor fan complex, cherry canyon formation, ward county, texas, aapg bulletin, vol.76, no.6, 805-827 p. tiab, d., and donaldson, e.c (1996). “petrophysics, theory and practice of measuring reservoir rock and fluid transport properties”: huston, texas, 706 p. verma, m.k., ahlbrandt, t.s. and al-gailani, m. (2004). petroleum reserves and undiscovered resources in the total petroleum systems of iraq: reserve growth and production implications. geoarabia, 9 (3), 51-75 p. wyllie m.r.j., (1963). the fundamentals of well log interpretation. new york academic press, 238 p. keywords: seismic amplification; sediment thickness; soil; gis. palabras clave: amplificación sísmica; espesor de sedimentos; suelo; sig. how to cite item moreno-ceballo, r., gonzalez-herrera, r. g., paz-tenorio, j. a., aguilar-carboney, j. a., & del carpio-penagos, c. u. (2019). effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo, chiapas, mexico. earth sciences research journal, 23(2), 111-117. doi: https://doi.org/10.15446/ esrj.v23n2.72623 local variations in soil type and (to a lesser extent) topography determine seismic amplification, thereby establishing spatial patterns in the damages caused by large earthquakes, such as the two that occurred in chiapas, mexico on 6 october 1975 and 7 september 2017. the latter, of mw 8.2, was the region’s strongest during the last 100 years. based upon damage patterns, an analysis was made to identify the effects of sediment thickness and type upon seismic amplification in the urban zone of chiapa de corzo, chiapas. salgado et al., (2004) had shown that the fundamental periods of soil vibration in the city vary from0.14 s to0.39 s. from those periods, and using the model presented in newmark and rosenbleuth (1976), with an average shear-wave velocity of 150 m/s, the variation of the sediment thicknesses that cause site effects in the area was determined. the thicknesses ranged between 7.12 and 14.6 m. finally, through the use of a map prepared with gis software, a spatial correlation was made with the historical earthquake damage recorded within the city. abstract effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo, chiapas, mexico efectos del espesor de sedimentos en la amplificación sísmica de la zona urbana de chiapa de corzo, chiapas, méxico issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.72623 las variaciones locales del tipo de suelo y la topografía en menor medida, determinan la amplificación sísmica y han establecido un patrón en los daños observados durante grandes terremotos, como el que se produjo el 06 de octubre de 1975 y el 07 de septiembre de 2017mw 8.2, siendo el más importante en la historia sísmica del último siglo en la región. con estas bases se realiza un análisis para identificar la participación del espesor de sedimentos en la amplificación sísmica en la zona urbana de chiapa de corzo, chiapas. para obtener el espesor en el área de estudio se utilizaron los períodos fundamentales de vibración del suelo obtenidos por salgado et al. (2004), que varían entre 0.14 s y 0.39 s. además, a través del modelo utilizado por newmark y rosenbleuth (1976), se determinó la variación de los espesores de sedimentos que conducen el efecto de sitio en la zona, para lo cual se considera una velocidad media de onda de corte de 150 m/s. finalmente, mediante el uso de software tipo gis, se elaboró un mapa que representa los espesores de sedimentos en la ciudad de chiapa de corzo, que oscilan entre 7.12 m y 14.62 m y se hizo una correlación espacial con los daños históricos por sismo en la ciudad. resumen record manuscript received: 01/06/2018 accepted for publication: 21/12/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 111-117 roberto moreno ceballo1, raúl gonzález herrera1, jorge antonio paz tenorio1, jorge alfredo aguilar carboney2, carlos uriel del carpio penagos1 1universidad de ciencias y artes de chiapas 2universidad autónoma de chiapas * corresponding author: roberto moreno geo9006@gmail.com se is m o lo g y 6 roberto moreno ceballo, raúl gonzález herrera, jorge antonio paz tenorio, jorge alfredo aguilar carboney, carlos uriel del carpio penagos introduction of all natural phenomena that can present serious threats to property and human life, earthquakes probably claim the most victims and cause the greatest material losses per unit of time (perepérez, 2014).therefore, researchers attempt to find meaningful temporal patterns in the occurrence of earthquakes along specific faults (dowrick, 2003; işık and kutanis, 2015), as well as spatial patterns in the associated damages to structures. it is notably accepted among the earthquake engineering that local geology especially sediment thickness has a crucial effect on seismic motion (livaoğlu et al., 2015). soft sediments make an important component of the subsurface lithology and carry a special nuisance in seismic hazards, as they serve as a source of seismic amplification (khan & khan, 2016). among the factors that determine those spatial patterns are topography, magnitudes of regional earthquakes, and distances of population centers from epicenters. in addition, it has been noted repeatedly that adjacent areas underlain by different soil types suffer different degrees of damage during earthquakes (meslem et al., 2012, vucetic, 1992). such is the case in the state of chiapas, mexico, where local soil conditions play a fundamental role in seismic responses of structures. dynamic responses of soils to earthquakes vary significantly, and include such phenomena as soil failure and liquefaction. for that reason, municipal planners must consider the seismic response of each locality´s subsoil, in order to define the specific parameters of structural design seismic resistance appropriate to the seismic history of the region (ipcmird, 2010).a detailed spatial analysis of the history of damages caused by earthquakes in a specific locality is essential for identifying that locality´s amplification zones. studies such as the present, which consist basically of defining micro-zoning of seismic hazards, must look for evidence of varied soil responses within the area of interest, then communicate that information to builders and urban planners by means of maps in a cartographic database (tupak, 2009). using geo-referenced information from such databases as inputs, software for modeling seismic responses of structures can estimate the level of damage that will result to buildings at specific sites, during earthquakes of magnitudes expected within the region (işık and kutanis, 2015). the area of study in the present article is chiapa de corzo, in the center of the state of chiapas (fig 1). the city has suffered frequent earthquakes during the last century because of the subduction of the cocos tectonic plate under that of north america (garcía and suárez, 1996). for example, the traumatic earthquake of 1975changed the city radically by destroying traditional adobe houses that dated to the city´s foundation (nandayapa, 2011).infrastructure systems failed, and most of the city was affected. table 1 lists dates, magnitudes, and epicenter locations for 18 of the most significant earthquakes that have affected chiapa de corzo during the last 120 years. in the present article, which seeks a correlation between sediment thickness underlying chiapa de corzo and the city’s historic patterns of seismic damage, we use historical records from two events:  the earthquake of magnitude 4.8 on monday, 6 october 1975, at 12:55 a.m. local time. it caused major damage to chiapa de corzo. the epicenter was located in ribera de cupía, amatal and américa libre rural localities approximately 4 km southeast of chiapa de corzo. this earthquake was produced by cortical failure, induced by filling the reservoir of the “la angostura” dam (nandayapa, 2011).  the magnitude-8.2 earthquake of 7 september 2017, whose epicenter was in the gulf of tehuantepec, 133 km southwest of pijijiapan, chiapas. this earthquake caused heavy damage in metropolitan areas of chiapas (ssn, 2017). figure 1. location of the study area. 7effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo, chiapas, mexico zoning studies are needed urgently in chiapa de corzo because of the city’s many historically significant buildings, high level of marginalization, and high degree of deterioration of the housing stock. geological characteristics of the study area chiapa de corzo lies along the banks of the grijalva river, and is contained within the river’s flood plain. the entire urban area is built upon alluvium (schmudde, 1968).in general, seven types of rock are exposed within metropolitan regions of chiapas. soils in most locations are quaternary deposits. three types of bedrock predominate: limestone (34.46% of the surface), siltstone-sandstone (29.01%), and shale-sandstone (16.55%). cretaceous deposits are also present: siltstones (8.99%) and alluvial (7.22%). the remaining 5.40% of chiapas’s surface area is covered by soils of varied compositions (inegi, 2004; inegi, 2008; pdm, 2017). the study area’s shales are fine-grained rocks that fracture in laminated slabs more or less parallel to the stratification. particles within these rocks are so small that their mineral compositions are difficult to determine precisely. it is important to note that the shales contain clay minerals, as well as silt, quartz, feldspar, calcite, and dolomite particles (don leet and judson, 1980; blanquer, 2011). in contrast, sandstone is formed by the consolidation of individual grains of the size of the sand (1/16 mm to 2 mm diameter). sandstone occupies, then, an intermediate position between conglomerate, which is coarse-grained, and the lodolite, which is fine-grained. grain size varies from one sandstone to another. sandstones and shales constitute about 99% of all sedimentary rocks, the shales being the most abundant (don leet and judson, 1980). methods this research seeks to propose the relationship between flood zones associated with areas of greater sediment thickness and site-effect zones as a preliminary analysis to identify these areas in regions where studies have not been conducted. the methodology followed in this work consists of three fundamental stages (fig 2). in the first stage, a mathematical model was defined to calculate thicknesses of sediment layers in chiapa de corzo. next, the fundamental vibrational periods of soils in each zone of the city were obtained from salgado et al. (2004). finally, historical records of earthquake damage to the city were reviewed in order to correlate soil thicknesses with local seismic amplification. model newmark and rosenbleuth(1976) derived the following equation for the fundamental vibrational period (t) of a terrain that is underlain by n sediment strata: t h ² i n i i = ∑4 , (1) where hi and βi are, respectively, the thickness of the ith stratum and the propagation rate of shear-waves within it. to estimate the total thickness of sediment at each of the 20 locations for which salgado et al. (2004) determined the terrain’s fundamental period, we assumed, based upon (narcía et al., 2006), that β = 150 m/s within all sediment layers throughout chiapa de corzo´s urban area. therefore, our estimate of the thickness h present at a location with period t seconds was h tx² i i= ( ) 4 .(2) fundamental periods of soil vibration as noted in the previous section (“model”), fundamental periods for 20 locations within chiapa de corzo’s urban area were obtained from salgado et al., (2004). periods ranged from 0.14 to 0.39 s; with most near 0.3 s. historical damage the last step in the methodology was to develop a database, in excel (fig 3), of levels of damage caused to structures in chiapa de corzo by the earthquakes of 6 october 1975 and 7 september 2018. through our own on-site examinations and from previous research, we also investigated whether the inherent seismic vulnerability of structures (for example, that due to design, materials, and construction methods) varied significantly among the twenty sites at which salgado et al. (2004) measured fundamental periods. table 1. historical and recent local seismicity of chiapa de corzo (salgado et al., 2004; ssn, 2017) date latitude longitude magnitude 05/06/1897 16.30 -95.40 7.4 19/04/1902 14.90 -91.50 7.5 23/09/1902 16.60 -92.60 7.7 14/01/1903 15.00 -93.00 7.6 09/12/1912 15.50 -93.00 7.0 30/03/1914 17.00 -92.00 7.2 10/12/1925 15.50 -92.50 7.0 28/06/1944 15.00 -92.50 7.1 26/09/1955 15.50 -92.50 6.9 09/11/1956 17.45 -94.08 6.3 29/04/1970 14.52 -92.60 7.3 05/10/1975 16.74 -92.92 4.8 10/09/1993 14.20 -92.80 7.2 14/03/1994 15.98 -92.43 6.8 21/10/1995 16.81 -93.47 7.1 18/11/2001 15.45 -93.60 6.3 16/01/2002 15.58 -93.60 6.3 07/09/2017 15.76 -93.70 8.2 model fundamental periods thickness comparison with historical damage figure 2. sequence of steps for determining effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo. 8 roberto moreno ceballo, raúl gonzález herrera, jorge antonio paz tenorio, jorge alfredo aguilar carboney, carlos uriel del carpio penagos results fig. 4, prepared by arcmap via linear interpolation of fundamental-period data from salgado et al., (2004), shows the spatial variation of fundamental vibration period within chiapa de corzo’s urban area. sediment thicknesses, as estimated via eq. (2) using data from salgado et al., (2004), ranged from 7.12 m to 14.6 m. fig.5 contrasts surface topography with estimated thickness of underlying sediment in the central zone of chiapa de corzo.comparing the interpolated spatial distribution of estimated thicknesses (fig. 6) to historical patterns of damage (fig. 7), we find that damage has been greater where sediments are estimated to be thicker. importantly, the inherent seismic vulnerability of structures did not vary significantly among the twenty sites. (for example, see cardona, 2008). in addition to the graph shown in fig.8, a correlation between the period of vibration and the thickness of sediments is observed. conclusions for the purposes of the work reported here, sediments underlying each site in chiapa de corzo could be modeled as homogeneous deposits with β = 150 m/s. the resulting estimates of thicknesses correlate with degrees of structural damage that occurred during the earthquakes of 6 october 1975 and 7 september 2017: where the estimated thickness is greater, so was the degree of damage. thus, in the case of chiapa de corzo, estimates of sediment thickness calculated from measurements of the terrain’s fundamental vibrational period can be a fundamental input for defining micro-zones based upon dynamic response of soils. 80 70 60 50 40 30 20 10 0 36 10 6 1 5 g1 g2 g3 g4 g5 total % grado de daño figure 3. degrees (grades) of damage caused to buildings in the urban area of chiapa de corzo bythe earthquakes of 6 october 1975 and 7 september 2017. grades ranged fromg1 (negligible) to g5(total collapse of the building). figure 4. micro-zoning of the city of chiapa de corzo according to the fundamental period of the underlying sediment. circles show locations of salgado et al. (2004)’s measuring stations. 9effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo, chiapas, mexico figure 5. comparison between surface topography and estimated thickness of underlying sediment in the urban area of chiapa de corzo. the green arrow points toward north. figure 6. spatial variation of estimated sediment thickness in the urban area of chiapa de corzo. figure 7. comparison of sediment thicknesses and earthquake damage suffered by buildings in the urban area of chiapa de corzo. degrees of damage range from 1 (negligible) to complete collapse (5). 10 roberto moreno ceballo, raúl gonzález herrera, jorge antonio paz tenorio, jorge alfredo aguilar carboney, carlos uriel del carpio penagos figure 8. correlation between period of vibration (from salgado et al. (2004)),thickness of sediments (as estimated in the present article), and historical patterns of eartthquake damage to buildings in chiapa de corzo. this conclusion would not be tenable if the inherent seismic vulnerability of structures varied significantly among 20 sites at which salgado et al. (2004) measured fundamental periods (padrón et al., 2011). that caution is an important one which future research in chiapa de corzo or elsewhere should consider when choosing sites at which to estimate sediment thicknesses. in summary, dynamic response of subsoil is an important phenomenon that is sometimes given insufficient attention during the structural design of buildings and other engineering works. (see, for example, gonzález-herrera et al. (2013) regarding subsoil conditions in tuxtla gutiérrez, chiapas.) useful correlations for micro-zoning based upon this phenomenon can be developed by carefully analyzing and cross-referencing spatial patterns of earthquake damage and shear-wave velocities. the results obtained in this work should be supplemented by more-detailed studies that include current measurements located throughout chiapas de corzo’s urban area, because population growth has been considerable during recent years. acknowledgements we want to thank dr. james smith for his valuable involvement in translating this article and in the contribution he made with his wise remarks. references arbeláez, a. c., posada, l., & vélez, m. v. (2002). usos de suelo en la zona inundable del río san carlos, colombia. xv seminario nacional de hidráulica e hidrología. blanquer, j. m., ibáñez, s., & moreno, h. (2011). vertisoles. universidad politécnica de valencia. cardona, o. (2008). (transcript of personal interview). en padrón, c. (comp). lineamientos para la consideración de riesgo sísmico en la planificación urbana del municipio chacao, estado miranda. informe de pasantía (véase padrón, 2009), universidad simón bolívar, caracas, 254-271. don leet, l., & judson, s. (1980). fundamentos de geología física. méxico, d.f. limusa. dowrick, d. (2003). earthquake risk reduction. wiley, england. gonzález-herrera, r., mora-chaparro, j. c., aguirre-gonzález, j., & novelocasanova, d. a. (2013). la estructura de velocidades y su relación con el peligro sísmico en tuxtla gutiérrez, chiapas. revista mexicana de ciencias geológicas, 121-134. inegi. (2004). guía para la interpretación de cartografía: edafología. inegi. (2008). prontuario de información geográfica municipal de los estados unidos mexicanos. ipcmird. (2010). sistema estatal de protección civil del estado de chiapas. plan operativo de protección civil por riesgo sísmico, tuxtla gutiérrez, chiapas, 2010. isik, e. & kutanis, m. (2015). determination of local site-specific spectra using probabilistic seismic hazard analysis for bitlis province, turkey. earth sciences research journal, 19(2), 129-134. khan, s., & khan, m. a. (2016). mapping sediment thickness of islamabad city using empirical relationships: implications for seismic hazard assessment. journal of earth system science, 3, 623-644. livaoğlu, h., irmak, t., güven, t., & ozer, f. (2015). an empiric relationship between sediment thickness of different data and resonance frequency which calculated by using the h/v ratio method of seismic noise for gölcük-değirmendere area (turkey). european geosciences union general assembly 2015, 17. vienna. meslem, a., yamazaki, f., maruyama, y., benouar, d., kibboua, a., & mehani, y. (2012). the effects of building characteristics and site conditions on the damage distribution in boumerde`s after the 2003 algeria earthquake. earthquake spectra, 28 (1), 185-216. nandayapa, a. (2011). temblores de 1975 en la ciudad de chiapa de corzo. tuxtla gutiérrez. narcía lópez, c., cruz díaz, r., aguilar carboney, j., ramírez centeno, m., & gonzález herrera, r. (2006). el período natural de vibración del suelo en la ciudad de tuxtla gutiérrez, chiapas. quehacer científico en chiapas, 1(1), 22-36. newmark, n. & rosenbleuth, e. (1976). fundamentos de ingeniería sísmica. 1a ed., diana, méxico, d.f. padrón, c., mendes, k. c., schmitz, m., & hernández, j. j. (2011). la microzonificación sísmica en el proceso de planificación urbana. caso de estudio: municipio chacao. revista de la facultad de ingeniería u.c.v, 26(2), 89-101. perepérez, b. (2014). la peligrosidad sísmica y el factor de riesgo. informes de la construcción, 66(534): e018. doi: http://dx.doi.org/10.3989/ ic.13.018. 11effects of sediment thickness upon seismic amplification in the urban area of chiapa de corzo, chiapas, mexico salgado, a., escamirosa, f., & calvo, a. (2004). zonificación sísmica de tres centros históricos del sureste mexicano. sociedad mexicana de ingeniería estructural, 11-15. schmudde, t. h. (1968). floodplain. in: r.w. fairbridge. the encyclopedia of geomorphology. new york: reinhold, 359-362. secretaría de hacienda. (2017). secretaría de hacienda. retrieved 15 may 2017, from http://www.haciendachiapas.gob.mx/planeacion/informacion/ desarrollo-regional/prog-regionales/metropolitana.pdf ssn. (2017). reporte especial: sismo de tehuantepec (2017-09-07 23:49 m8.2). suárez, g. & singh, k. (1986). tectonic interpretation of the transmexican volcanic belt discussion. tectonophysics, 127, 155-160. tiryakioglu, i., yavasoglu, h., ugur, m. a., özkaymak, c., yilmaz, m., kocaoglu, h., & turgut, b. (2017). analysis of october 23 (mw 7.2) and november 9 (mw 5.6), 2011 van earthquakes using long-term gnss time series. earth sciences research journal, 21(3), 147-156. doi: https://dx.doi.org/10.15446/esrj.v21n3.62812 tupak, o. (2009). microzonificación sísmica. universidad internacional de andalucía, unía huelva, españa. vucetic, m. (1992). soil properties and seismic response. in proceedings 10th world conference on earthquake engineering, pp. 1199–1204. keywords: waste; management; construction and demolition waste; life cycle assessment; impact category. palabras clave: residuos; gestión; residuos de construcción y demolición; evaluación del ciclo de vida; categoría de impacto how to cite item suarez silgado, s. s., calderon valdiviezo, l. j., & mahecha vanegas, l. f. (2021). application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study. earth sciences research journal, 25(3), 341351. https://doi.org/10.15446/esrj.v25n3.82815 the construction industry not only consumes more raw materials and energy than any other economic activity, but also generates the largest fraction of waste, known as construction and demolition waste (cdw). this waste has major environmental implications, most notably in south american countries such as colombia, where it is handled inappropriately. in this study, the management processes that are currently used for fractions of construction and demolition waste (cdw) generated in ibagué (colombia) were evaluated and the environmental impacts of the management of 1 kg of cdw were calculated. other cdw management alternatives were evaluated, in which the percentage of the fraction of the waste and/or the treatment or management process that is used was modified to determine its environmental and economic viability. the information was obtained through telephone interviews and visits to recycling plants, construction companies, quarries, government entities, and inert landfills in the country. it was completed with secondary sources and the ecoinvent v.2.2 database. life cycle assessment (lca) methodology and simapro 8 software were used to calculate the environmental impacts. an economic study of each management process and each alternative was also carried out. a comparison of the alternatives revealed the current alternative contributes most to the environmental impacts in all categories. the results of this study indicate that the most beneficial alternative in environmental and economic terms in ibagué (colombia) is that in which 100% of the metals are recovered, 100% of excavated earth is reused, and 100% of the stone waste is recycled (alternative 3). when a sensitivity analysis was carried out with different distances (30 km and 50 km), alternative 3 continued to be the most favorable. abstract application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study aplicación de la metodología de análisis de ciclo de vida (acv) y evaluación económica para los residuos de construcción y demolición: un caso de estudio colombiano issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.82815 la industria de la construcción no solo consume más materias primas y energía que cualquier otra actividad económica sino que también genera la mayor fracción de residuos, conocidos como residuos de construcción y demolición (rcd). estos residuos presentan importantes implicaciones ambientales, especialmente en países de américa del sur como colombia, donde se manejan de manera inapropiada. en este estudio se evaluó el sistema de gestión actual de los residuos de construcción y demolición (rcd) en ibagué (colombia) y se calcularon los impactos ambientales de la gestión de 1 kg de rcd. se evaluaron otras alternativas de gestión de rcd para determinar su viabilidad ambiental y económica, en las cuales se modificó el porcentaje de la fracción de los residuos y el proceso de tratamiento o de gestión. para la consecución de la información se realizaron entrevistas telefónicas y visitas a plantas de reciclaje, empresas constructoras, canteras, entidades gubernamentales y vertederos inertes en el país. dicha información se completó con fuentes secundarias y con la base de datos ecoinvent v.2.2. para calcular los impactos ambientales se empleó la metodología de análisis de ciclo de vida (acv) y el software simapro 8. también se realizó un estudio económico de cada proceso de gestión y de cada alternativa. una comparación de las alternativas reveló que la alternativa de gestión actual contribuye en mayor medida a los impactos ambientales en todas las categorías. los resultados de este estudio indican que la alternativa más beneficiosa en términos ambientales y económicos en ibagué es aquella en la que se recupera el 100% de los metales, se reutiliza el 100% de la tierra excavada y se recicla el 100% de los residuos de piedra (alternativa 3). cuando se realizó un análisis de sensibilidad con diferentes distancias (30 km y 50 km), esta alternativa continuó siendo la más favorable. resumen record manuscript received: 12/10/2019 accepted for publication: 16/04/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 341-351 sindy sofía suárez silgado1*, lucrecia janneth calderon valdiviezo2, leandro fernando mahecha vanegas1 1universidad antonio nariño, ibagué, colombia. 2escuela técnica de arquitectura, barcelona, españa. *corresponding author: sindysofia@uan.edu.co g eo te ch ni ca l en g in ee ri ng https://doi.org/10.15446/esrj.v25n3.79170 https://doi.org/10.15446/esrj.v25n3.79170 mailto:sindysofia@uan.edu.co 342 sindy sofía suárez silgado, lucrecia janneth calderon valdiviezo, leandro fernando mahecha vanegas 1. introduction construction is one of the essential industrial activities for the development and progress of cities. however, it is also one of the sectors that contributes most to environmental impacts, due to the extraction of raw materials, energy use, and waste generation. it is considered that the construction sector consumes more raw material and energy than any other economic activity, and generates the largest fraction of waste. in europe alone, around 900 million t of construction and demolition waste are produced every year (bravo et al., 2015). according to ramesh et al. (2010), the term construction and demolition waste (cdw) refers to solid waste produced in the construction sector. more specifically, the term is defined as the waste that arises from construction, renovation and demolition activities. cdw includes a range of materials such as ceramic products, concrete waste and asphalt material, and to a lesser extent other components such as wood, glass and plastics (yuan and shen, 2011). the main components of this waste depend on the materials used, the construction practices, and the technological development of the sector. there are various management options for this waste, whose hierarchy depends on the environmental impacts they generate. the five levels of low to high environmental impact are: reduction, reuse, recycling, incineration and final disposal. some authors (yuan and shen, 2011) have grouped these six levels into four: waste reduction, reuse, recycling and disposal. however, the impacts of management systems vary. for example, inadequate disposal of construction and demolition waste (cdw) can generate negative environmental impacts such as soil degradation and erosion, destruction of vegetation, and loss of environmental services (mejía et al., 2015). the impacts of waste disposal in landfill are associated with the extraction or obtaining of raw material, and land occupation. if waste is sent to landfill, then new materials or products must be manufactured from original raw material (suárez silgado, 2017). faced with this situation, european countries have tried to find innovative alternatives for the recovery of waste from construction and demolition. yilmaz et al, (2018) have used construction and demolition waste as cemented paste backfill material for underground mine openings. also, numerous eu regulations have been drawn up on this topic. one is the european waste directive, which foresees that by 2020, 70% of cdw should be properly valued. the objective is to achieve much higher levels of recycling by minimizing the extraction of additional natural resources. thus, prevention and recycling are key elements of the new waste policy in europe (suárez-silgado, 2016). in this same line, several studies have used life cycle assessment for effective municipal waste management, because it helps in environmental evaluations of alternative waste management systems (koci and trecakova, 2011). according to iso 14040 (2006), lca is composed of an inventory of the relevant inputs and outputs of the system, the definition of the goal and the scope, an assessment of the potential environmental impacts associated with these inputs and outputs and, finally, an interpretation of the results of the inventory and impact phases in terms of the study objectives. this methodology has been used by several prominent authors in the field, including zabalza et al., 2011; monahan and powell, 2011 and tošić et al., 2015. mercante et al. (2012), coelho and de brito (2012), yeheyis et al. (2013), carpenter et al. (2013), dahlbo et al. (2015), guignot et al. (2015) and wang et al. (2018) have used the lca for the environmental assessment of cdw management systems in europe, north america and asia. the situation has led to the development and implementation of technologies in the international area. the cdw in china are usually randomly dumped or disposed in landfills and the average recycling rate of cdw is only about 5% (huang et al., 2018). in countries such as denmark, the netherlands, and belgium, the recycling of cdw for uses other than landfill is promoted, and recycling percentages of over 75% are achieved. this high level of recycling is mainly due to the shortage of natural aggregates and space for landfill sites (zabalza bribián et al., 2011). one factor that has increased recycling rates has been the increase in the cost of landfilling, or its prohibition in some cases, such as in denmark or the netherlands. however, in most south american countries this is not the situation. the management of cdw waste is carried out inadequately, and there is clearly a large gap between south american and other countries in terms of management and technology. in colombia, cdw is sometimes managed using a controlled discharge system, but usually its disposal is uncontrolled. the authorized sites for waste disposal are disseminated widely, and there are few alternatives for recovery, recycling or reuse. for this reason, only 5% of cdw is recycled in colombia (castaño et al., 2013). the generation of cdw has great environmental implications in this country, as it is sometimes disposed of in illegal landfills or thrown on public roads contributing to changes in the landscape and urban areas (aguilar et al, 2010; pinzón, 2014). the situation is particularly palpable in some municipalities in the country, such as ibagué, where around 488.000 t of cdw are generated per year (ibagué limpia, 2017), without counting the waste that is generated and disposed of clandestinely. the problem has increased in recent years, due to population growth, increased construction activity, and remodeling of buildings. however, an attempt has been made to advance in this area, and for this reason regulations and programs have been promulgated at district level that encourage the adequate disposal of cdw. examples are resolution nº1115/2012, which technically regulates the treatment and/or use of cdw in the capital district; and the municipal development plan “bogotá humana” (2012-2016) with the “zero waste-rubble zero” program. in ibagué, there have been new initiatives for cdw management, expressed in agreement no. 19 of 2013. the agreement implements environmental compare as a tool for citizens related to the proper management of solid waste in ibagué, and the integral solid waste management plan of ibagué (pgirs, 2015), which aims to promote the integral management of cdw, broaden the characterization of this waste, and design programs to take advantage of cdw through feasibility studies. in the same field, studies have been carried out at national level on the perspectives and limitations of cdw management (castaño et al., 2013; pinzón, 2014), and on the current waste situation in some municipalities (jiménez, 2013). technical diagnoses have been made of the use of cdw in the capital district (escandón, 2011; chávez et al., 2014), along with studies of quantification and characterization of cdw (sda, 2012; pinzón, 2014). finally, in some cases, pilot proposals for recycling plants have been made (chávez et al., 2014). however, despite these advances at national level, no studies have been undertaken to date on the environmental assessment of managing fractions of this waste. therefore, to help solve the problem of cdw generation in colombia, the objective of this study was to evaluate the current treatment or management of each fraction of cdw. subsequently, other alternatives were evaluated in which either the treatment that is used for fractions of waste or the percentage of waste in the treatments was modified. hence, the objective was to determine which is the most beneficial management alternative at environmental and economic level. the area chosen for the study was the department of tolima, specifically, its capital ibagué. this area was selected for two main reasons: the current problems associated with the generation of cdw because of population growth and an increasing number of buildings, and the new waste management initiatives described in agreement no. 19 of 2013 and the pgirs (2015), mentioned above. the research is novel because the environmental impacts of waste management systems in colombia were evaluated using life cycle assessment (lca), to obtain a more realistic view of the impacts in this country, specifically in ibagué. as life cycle analysis (lca) is a decision tool, the results could help to encourage or give greater impetus to the development of projects and programs that contribute to cdw recycling or reuse. the management scenarios were also evaluated from an economic perspective. in addition to applying lca methodology, a sensitivity analysis was carried out to determine how the final results are affected. the results obtained may lead to the creation of more demanding regulations in the field of waste management in colombia. 2. data and methods 2.1 study area this study was carried out in ibagué, a city located in the center-west of colombia on the central mountain range of the andes, at an altitude of 1.285 m.a.s.l. this city has an extension area of 4.605 hectares, according to the territorial ordinance plan (pot, 2015), and a population of 553.524 inhabitants (dane, 2015). the average annual generation of construction and 343application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study demolition waste is 488.000 t, according to data provided by ibagué limpia (2017). for this study, it is considered that waste is generated in the expansion zone, where the largest amount of waste is produced. 2.2 description of the construction and demolition waste in the study area in accordance with the regulations that apply in this country (resolution 1115/2012), construction and demolition waste are generated during the development of a construction project, and include excavation products, leveling and leftovers from site preparation; products used for foundations and pilings; stone waste (concrete, sand, gravel, pieces of bricks and blocks, ceramics, leftovers of mortar and concrete mix); and non-stone waste (glass, wood, plastics, metals, cardboard and gypsum). construction companies in the city were visited to gather information about the current management of cdw in the city of ibagué. direct contact was also established through telephone calls and email. according to the information obtained from the companies, over 80% of them are engaged in constructing new dwellings. therefore, the largest amount of waste in this sector comes from construction processes. of all the construction activity, 92.8% corresponds to dwellings (icer, 2016). figure 1 shows the composition of cdw in the study area. the highest percentage corresponds to excavated earth. the amount of excavated land that is generated is high compared to other countries. the reason is that in colombia construction companies (in which the earth movement phases are notable) predominate over demolition companies. in another similar study, a high percentage of this waste was found in ibagué (suárez-silgado et al, 2018). figure 1. composition of cdw in ibagué the management system currently applied for cdw in the city of ibagué (according to the information provided directly by construction companies in the city in 2017) consists basically of the generation of waste in construction, and its subsequent transport to the landfill or final disposal site, without any treatment or recovery. in some cases, around 50% of construction companies separate the metal and excavated earth from the rest of the waste. the metals that are separated are transported by managers to a metal classification or preparation plant for subsequent recycling in a smelting plant in another city. excavated earth is separated to be reused in the same construction or on a nearby site. in some construction projects, waste is not separated and is simply taken to landfill. this management system is due partly to the fact that ibagué does not have a recycling plant for cdw, and therefore the waste is mostly taken to the final disposal site. however, in the absence of sufficient control and monitoring in the management of these sites, waste is also transported to non-formal disposal sites in some cases, causing great damage to public roads, riverbeds, and vacant lots. exact data on this form of disposal are not known. 2.3 life cycle assessment (lca) methodology 2.3.1 goal and scope definition considering the current management of cdw in ibagué and the problems associated with waste generation in this city, the objective was to identify the potential environmental impacts associated with the management of construction and demolition waste fractions and compare them with new alternatives. the functional unit of the study was 1 kg of waste. based on the characterization of waste in ibagué (figure 1) and the information obtained from construction and other companies that were consulted directly, the three waste fraction considered were: stones (19.5%) (concrete, brick and other inerts), excavated earth (80%) (essentially sands and clays; the organic material content within the ground was considered negligible) and metals (0.5%) (steel). 2.3.2 alternatives three alternatives were evaluated in the life cycle assessment (table 1). they were chosen according to the quantity and characterization of the waste and the current waste management system in the study area; information that was provided by ibagué limpia (2017). the first alternative (a1) corresponds to the current management of waste in the city of ibagué. in this alternative, all stone waste is taken to landfill, metals are taken to the sorting and compaction plant and 50% of excavated earth is reused on a nearby construction site (5 km), while the other 50% is taken to landfill. in two alternatives (a2 and a3), the waste treatment and percentages were modified, to determine viability. in alternative a2, the stone waste management was modified. in this case, 50% of stone is recycled and the other 50% is taken to landfill. in alternative a3, 100% of all waste is recovered. all the stone is recycled and all the excavated earth is reused. figure 2. alternatives evaluated figure 3 shows the limits of the study systems. the generation and separation of waste fractions were not considered, as waste is separated on site. only the stages of transportation and waste management were included. once the waste has been generated and separated, it is transported for treatment or final disposal. thus, the type of management (recycling/recovery, reuse and disposal) varied depending on the waste fraction. finally, the loads of processes that are avoided were subtracted. figure 3. limits of study systems. 344 sindy sofía suárez silgado, lucrecia janneth calderon valdiviezo, leandro fernando mahecha vanegas 2.3.3 life cycle inventory the aim was to obtain all data directly through company visits and incorporate them into the inventory. data that could not be obtained in this way (above all, emissions data) were complemented with the ecoinvent database. in all cases, data were contextualised for the colombian context. information was obtained from primary sources through visits, telephone calls and emails to construction companies, cdw and metal recycling plants, inert landfill sites and quarries. government organizations such as ibagué limpia and cortolima were contacted. fifty-six construction companies registered in ibagué, the existing legal inert landfill, 4 metal recycling plants, and 6 quarries near the study area were also contacted. as the city of ibagué does not have a cdw recycling plant, data had to be obtained by visiting waste recycling plants that are currently in operation in colombia. data were also collected from other facilities at national level and from 2 landfills to complement and compare with data from landfills in the study area. the data correspond to different years (2015-2017) with a similar production capacity. the information was supplemented with secondary sources such as articles, journals, projects on the same subject, and the ecoinvent v3 database. to evaluate the environmental impacts of each of the management processes applied to the cdw fractions in ibagué, new processes were created in ecoinvent v3, taking as a starting point the information collected from companies, recycling plants and landfill, completed with the inventory of the ecoinvent v3 database. the electricity mix of colombia was used for the calculations. each of the treatment or management processes was evaluated per kg of waste to obtain the environmental impacts. to determine the environmental impacts of managing 1 kg of cdw generated in ibagué, a new process was created in which each treatment was incorporated, considering the percentage that it occupies in the total construction and demolition waste (figure 2). in this way, each of the alternatives was compared. next, each type of treatment or management was defined: earth reuse, metal recovery (classification and compaction), stone recycling, stone and earth landfill. • excavated earth reuse the process of reusing excavated earth was evaluated in another nearby construction site. it was assumed that the excavated earth was not contaminated and could therefore be reused without any other type of treatment. the external transportation to the other site is considered. according to the information obtained from the companies, 5 km was considered the maximum distance between the extraction site and the reuse site. the compaction of earth on the new construction site and the emissions (leached) due to its use (100 years) were also considered. solid waste was considered as output, which is transported to a sanitary landfill located 11 km from the site on which the waste is generated. the reuse of excavated earth on a nearby construction site avoids disposal of this material in landfill. therefore, the costs of transporting excavated earth to the landfill and its disposal were subtracted in this case. table 1 shows the input and output of the reuse of excavated earth and the process that is avoided. table 1. input and output of the reuse of excavation lands excavated earth reuse cant unit source input transport 5 km companies consulted, 2017 diesel (compaction) 0.001 mj companies consulted, 2017 output emissions to air table 6 ecoinvent v3 emissions to water (leached) table 5 lópez and lobo (2014) waste solid (transport-11 km) 1.00e02 kg companies consulted, 2017 avoided products cant unit source earth landfilling/ transport 1 kg ecoinvent v3; companies consulted, 2017 • metal (steel) recovery for the recovery of metal, the stage considered was transport to the treatment plant, where the waste is prepared (classified and compacted) before it is cast, since there is currently no foundry for this material in ibagué. it was assumed that waste comprised of steel is separated on the construction site and transported separately to the treatment site. the process in these plants basically consists of classifying the material and compacting it, for subsequent export and smelting. according to the companies consulted the steel waste is transported from the expansion area of ibagué to the classification and compaction plants located at an average distance of 9 km. the production capacity of the metal treatment plant was taken to be approximately 3500 t/year of waste, with an infrastructure of 1500 m2. as the recovery of this waste and its preparation for use as secondary raw material (scrap) replaces the production of iron ore, this production is classified as an avoided product, so the impacts due to this process can be subtracted. table 2 shows the inputs and outputs of the recovery of metal and the process that is avoided. table 2. inputs and outputs of the recovery of the metal metal (steel) recovery cant unit source input transport 9 km companies consulted, 2017 diesel 0.03 mj companies consulted, 2017 electricity 5.00e-03 kwh companies consulted, 2017 output emissions to air table 6 ecoinvent v3 waste solid (transport-26 km) 1.00e-02 kg companies consulted, 2017 avoided products cant unit source extraction of iron ore 1 kg ecoinvent v3 • stone recycling taking into account the information collected and based on the existing unit processes in the ecoinvent v.3 database, a process called stone recycling was identified. from the amount of cdw generated in the city of ibagué (2017), it was determined that stone waste made up of 45% brick and 55% concrete. for stone waste recycling, crushing of the aggregate was considered in a fixed recycling plant. as there is currently no recycling plant in the city of ibagué, a possible location was chosen, considering the map of the area and the territorial ordinance plan (pot, 2015). a viable site would be suburban land in this city, which would allow the location and development of an industry of this type, since it is inside the industrial zone. the site chosen for a possible location of the recycling plant is 7.5 km from where the waste is generated (ibagué expansion zone). this distance is well below the limit proposed in ulubeyli et al. (2017), which states that the distance between facilities should be no more than 50 km for them to be viable environmentally and economically. based on these data, the t-kilometers to be entered in the ecoinvent v3 database were calculated. it was found that 0.008 tkm is used to transport 1.02 kg of stone waste. it was assumed that ceramics and concrete waste had been separated from the rest of cdw in work. the output of the recycling process was considered to be air and water emissions, the product of the crushing process, and the steel recovered from the reinforced concrete that is included in concrete and brick stone waste. to calculate the amount of steel generated by mixed aggregate, it was assumed that 1 kg of concrete generates 0.02 kg of steel (nsr, 2010). as the mixed aggregate was composed of 55% concrete, according to data from ibagué limpia (2017), then 0.55 kg of concrete generated 0.01 kg of steel. this value was taken as the maximum amount of steel generated from the mixed aggregate. it was assumed that the generated steel waste could be transported to the treatment plants that are currently situated in ibagué, which classify and 345application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study compact the metal. therefore, in this study, the steel management system included only transport to these plants. in this case, an average distance of 16 km was selected, according to the maps (pot, 2015). the transport of aggregate extracted from the quarry was considered a process that was avoided. it was assumed that the recycling plant had a maximum production capacity of 100000 t/year. table 3 shows the inputs and outputs of stone recycling and the process that is avoided. table 3. inputs and outputs of stone recycling stone recycling cant unit source input transport 8 km companies consulted, 2017 diesel 0.05 mj companies consulted, 2017 electricity 8.00e-05 kwh companies consulted, 2017 water 6.00e-03 kg ecoinvent v3 output particulates <2.5 um 1.66e-05 kg ecoinvent v3 particulates >10 um 8.35e-05 kg ecoinvent v3 particulates >2.5 um and <10 um 6.34e-05 kg ecoinvent v3 water 1.91e-04 m3 ecoinvent v3 waste reinforcement steel (transport16 km) 1.00e-02 kg nsr (2010); ibagué limpia (2017) avoided products cant unit source extraction of aggregates/transport 1 kg ecoinvent; companies consulted, 2017 • excavated earth and stone landfilling the stages of landfill disposal are similar for stone and earth. this process considers landfill infrastructure, land occupation, fuel consumption for compaction and grading of the material, and the effect of landfilled waste (leachate). in this process, it was considered that waste from construction works is transported from the expansion zone to the inert waste landfill located at 13 km. factors that were considered included the landfill infrastructure (14 hectares), the use of land, and fuel (diesel) consumed in the compaction and grading of the material. table 4. inputs and outputs of earth and stone landfilling excavated earth and stone landfilling cant unit source input occupation, construction site 4.44e-05 m2a ecoinvent v3 occupation, dump site 0.000444 m2a ecoinvent v3 transformation, from dump site 4.44e-05 m2 ecoinvent v3 transformation, to dump site 4.44e-05 m2 ecoinvent v3 diesel 1.00e-03 mj companies consulted, 2017 electricity 2.70e-06 kwh companies consulted, 2017 transport 13 km companies consulted, 2017 output emissions to air table 6 ecoinvent v3 emissions to water (leached) table 5 lópez and lobo (2014) a life cycle inventory (lci) for inert landfill rarely includes leachate, since in general the waste material in this kind of landfill has a low pollutant content, and is chemically inert to a large extent. however, according to bovea et al. (2016), models for inert material landfills should take these emissions into account, since a small percentage of biodegradable materials may be disposed of at these sites. considering studies by butera et al. (2015) and colomer et al. (2017), the leachate emissions framework was considered to be 100 years. the leachate composition from cdw landfill according to lópez and lobo (2014), bovea et al. (2016), and ecoinvent v3 was taken into account. table 5 shows the composition of the leachates in the inert waste landfill. the landfill gas collection system was excluded, since this was not relevant in a landfill for cdw (butera, 2015; bovea et al., 2016). the table 4 shows the inputs and outputs of this process. table 5. primary data for leachate composition from inert landfill (lópez and lobo, 2014) composition quantity cod (mg/l) 1571 bod5 (mg/l) 227 nh4-n (mg/l) 401 sulfates (mg/l) 405 ca (mg/l) 150 na (mg/l) 495 cr (µg/l) 105 cd (µg/l) 27 cu (µg/l) 28 zn (µg/l) 276 pb (µg/l) 987 ni (µg/l) 59 as(µg/l) 233 hg (µg/l) 1.4 table 6. emissions to air (ecoinvent v3) emmissions to air cant unit acetaldehyde 1.50e-07 kg acrolein 5.83e-08 kg ammonia 5.18e-07 kg arsenic 3.66e-12 kg benzaldehyde 4.51e-08 kg benzene 2.30e-09 kg butane 4.94e-09 kg cadmium 3.18e-10 kg carbon dioxide 0.1147 kg carbon monoxide 0.00011 kg chromium 1.10e-09 kg chromium vi 2.20e-12 kg copper 7.76e-10 kg dinitrogen monoxide 6.14e-06 kg ethane 9.88e-10 kg formaldehyde 2.76e-07 kg heptane 9.88e-09 kg lead 1.90e-09 kg mercury 1.94e-10 kg methane 8.10e-08 kg m-xylene 3.23e-08 kg nickel 3.22e-10 kg (continued) 346 sindy sofía suárez silgado, lucrecia janneth calderon valdiviezo, leandro fernando mahecha vanegas emmissions to air cant unit nitrogen oxides 5.22e-05 kg nmvoc 2.67e-06 kg o-xylene 1.31e-08 kg pah 2.86e-09 kg particulates <2.5 4.94e-07 kg pentane 1.97e-09 kg propane 3.30e-09 kg selenium 3.66e-12 kg styrene 1.84e-08 kg sulfur dioxide 5.66e-07 kg toluene 3.29e-10 kg zinc 6.36e-08 kg 2.3.4 life cycle assessment for the study of the lca, the impact categories were chosen based on several studies in this field. these studies are shown in table 7. the impact assessment method selected for the study is impact 2002+, which has been used by several authors (hossain, 2016; hossain, 2017; and suárez et al., 2016). this method is one of the most feasible and widely used environmental impact assessment methodologies. it combines a midpoint and damage approach, and links all types of life cycle inventory results via several midpoint categories to several damage categories (hossain, 2017). the software used to perform the calculations was simapro 8. simapro is one of the most widely used and accepted lca tools. it helps to model various products and processes, and analyses the results to achieve sustainability goals. simapro includes many lci datasets, including the renowned ecoinvent v3 database, the new industry-specific agri-footprint database, and the elcd database (pré consultants, 2017). table 7. impact categories evaluated references c ri old po lo a e gw nre me bueno et. al. (2015) x x x x x x hoxha et. al. (2017) x x liu et. al. (2017) x x x x x x x ripa et al. (2017) x x x x x x x heinonen et al. (2016) x x x x x x x x x hossain (2016) x x x x x x x x x x bovea et al. (2016) x x x x x x x x butera et al. (2015) x x x x x x x x hossain (2017) x x jullien (2014) x suárez et al. (2016) x x x x x x x x x x milutinovic´ (2017) x x x x x x yay (2015) x x x x x x x x c= carcinogenic; ri= inorganic respiratory; old= depletion of the ozone layer; po= photochemical oxidation; lo= land occupation; a= acidification; e= eutrophication; gw= global warming; nre= non-renewable energy; me= mineral extraction. 2.3.5 interpretation the interpretation includes the presentation and evaluation of results, and a sensitivity analysis to check the reliability and robustness of the results by varying assumptions and methods. a sensitivity analysis with different distances was performed. 2.4 economic evaluation to determine the economic viability of each management process and, subsequently, the feasibility of each alternative, a study was carried out in which the cost of each treatment per kg of treated waste and the cost of each alternative was calculated, considering the type of treatment and the percentage of each fraction of waste within the total generated cdw. the economic data were obtained from the construction companies, recycling plants and entities visited in ibagué. 3. results and discussion 3.1 results of the management processes the results of the management processes that are currently applied in ibagué for each waste fraction are presented below. these results are given per kg of waste. 3.1.1 stone recycling on a relative scale of 0%–100%, where positive percentages represent impacts and negative percentages represent savings, figure 4 (a) shows that the avoided process of gravel extraction contributed to the greatest savings in all the categories that were evaluated (-100%). the other processes contributed to environmental impacts. diesel consumption had the greatest environmental impacts in all categories, apart from photochemical oxidation. the crushing process had major impacts on the category of respiratory effects (52%) due to the emission of particles. the transport process affected the soil occupation category (28%) as did the infrastructure of the recycling plant (14%). the environmental burdens of the rest of the processes were minimal. 3.1.2 excavated earth reuse figure 4 (b) shows that the avoided process of earth landfilling contributed to great environmental savings in all the categories (-100%). the other processes contributed to environmental impacts, but in a smaller proportion (less than 20%). transportation was the process that contributed most to the impacts on the categories, particularly global warming (17%), mineral extraction (13%) and photochemical oxidation (11%). diesel consumption and leachate emissions during land compaction did not have significant impacts in the process of earth reuse. 3.1.3 excavated earth and stone landfilling the process of earth and/or stone disposal does not avoid any processes, which is why it contributed to positive environmental loads. the process that contributed most to environmental impacts was the infrastructure of the landfill, especially in the categories carcinogenic (81%), acidification (77%), nonrenewable energy (77%), depletion of the ozone layer (77%) and eutrophication (74%). another process that contributed to environmental impacts was transport, particularly in the global warming category (44%). transformation and use of earth for landfill had an impact on the land occupation category (41%) (figure 4 [c]). leachate emissions are not relevant in the process of earth and stone disposal, as they are inert materials. 3.1.4 metal (steel) recovery metal recovery plays an important role as it avoids the process of extracting iron ore. this had environmental benefits in all the impact categories. the greatest savings were found in the inorganic respiratory (-100%), photochemical oxidation (-100%) and mineral extraction (-100%) categories. significant savings were also generated in the following categories: acidification (-80%), reduction of the ozone layer (-37%), non-renewable energy (-29%) and global warming (-27%). the infrastructure of the classification plant contributes to greater impacts in the evaluated categories. the greatest impacts were identified in the categories of eutrophication (98%), land occupation (97%) and carcinogenic effects (91%) (figure 4 [d]). 347application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study 3.2 comparison of the evaluated processes in figure 5, the processes that were evaluated are compared to determine the environmental impacts of each one per kg of waste. figure 5 shows that the metal recovery process contributed to impacts on the carcinogenic effects (100%), ozone depletion (67%), land occupation (100%), acidification (64%), eutrophication (100%), global warming (100%) and non-renewable energy categories (100%). however, there were also savings in the inorganic respiratory (-100%), photochemical oxidation (-63%) and mineral extraction (-100%) categories. these savings were due to avoidance of the processes of extracting and producing raw material. the results agree with those found by mercante et al. (2012). the stone recycling process led to savings in all categories. the biggest savings were in photochemical oxidation and acidification (-100%). this is mainly due to the avoidance of natural aggregate extraction loads. the lowest savings in the inorganic respiratory category (-1%) were due to the emission of particulate material during the stone crushing process. as in mercante et al. (2012), it was found that stone recycling led to savings in all categories. the results agree with those of mercante et al. (2012), coelho and de brito (2012), kucukvar et al. (2014) ,guignot et al. (2015) and wang et al. (2018), who also found that recycling cdw led to environmental savings. the process of earth reuse resulted in environmental savings in all categories, as the high loads in the earth landfilling process are avoided. the biggest savings were in the categories of ozone depletion (-89%) and non-renewable energy (-86%). significant savings were also identified in photochemical oxidation (-46%), acidification (-38%) and soil occupation (-38%) categories. the disposal of earth and/or stones had an impact on all categories. the greatest impacts were found in the category of ozone depletion (100%) and non-renewable energy (97%) as a result of energy consumption during the process. there were also impacts on land occupation due to the transformation and use of the earth. the impacts of leachate were not relevant in this process; therefore, eutrophication effects were not very high (4%). the results coincide with butera (2015), who found impacts due to waste disposal and no savings from this process. figure 5. comparison of the processes evaluated for each type of waste per kg 3.3 analysis of the different alternatives finally, the environmental impacts of the current management system were evaluated, considering the waste fraction and the management process applied to each type of waste that make up 1 kg of the cdw generated in ibagué (colombia). this management alternative (a1) was compared with two other alternatives in which the percentage of the waste fraction and/or the management process was modified (a2 and a3). the results in figure 6 show that the current management alternative (a1) for 1 kg of cdw generated in ibagué had an impact on all categories, except mineral extraction, in which environmental savings were found (-21%) due to the metal recovery process. the greatest impacts were in the eutrophication category (47%). this may be due to the fact that in this alternative, 100% of the stone and 50% of the excavated earth were dumped, which has a greater impact on this category due to the infrastructure of the landfill and transport of waste to the landfill. other impacts associated with this alternative were in the categories of global warming (34%), non-renewable energy (30%), land occupation (30%) and ozone depletion (29%). these impacts are due to fuel consumption and the use and transformation of the earth in the waste disposal process. in addition, borghi et al. (2018) evaluated the environmental performance of the regional management of construction and demolition waste in italy. the results showed that the induced environmental impacts are generally greater than the benefits arising from recycling activities. figure 6. comparison of the scenarios per kg of cdw alternative 2 had lower environmental impacts than a1 in all categories. in the categories of photochemical oxidation and acidification, it led to environmental savings, unlike a1, because it includes the process of recycling 50% of the stone waste, thus reducing the disposal of this material. a) b) c) d) figure 4. environmental impacts. a) stone recycling; b) excavated earth reuse; c) excavated earth and stone landfilling; d) metal recovery. 348 sindy sofía suárez silgado, lucrecia janneth calderon valdiviezo, leandro fernando mahecha vanegas alternative 3 led to environmental savings in all the categories (-100%). this is due to the fact that disposal of waste is eliminated and the percentage of stone waste recycling and reuse of earth increased. in other words, in a3 all generated cdw is recovered. since the impacts that are avoided when waste materials are recovered and used instead of virgin material are much greater than the impacts that are generated, the net result is that recovery or recycling contributes to savings. these results are consistent with those obtained by coelho and de brito (2012) who found better results for management alternatives in which 96% recycling was carried out, resulting in lower environmental impacts in the categories of climate change, acidification, summer smog, nitrification and heavy metals than the alternatives in which recycling was null or scarce. in addition, rosado et al. (2019) evaluated the environmental performance of construction and demolition waste management in brazil. the results highlighted the importance of the impacts that are avoided by recovering materials, particularly those related to steel, glass and plastics recycling. table 8 shows the environmental impacts of the alternatives evaluated per kg of cdw. these results coincide with those found by coelho and de brito (2013a) who in their study concluded that the impacts prevented through material recycling can be up to one order of magnitude greater than those generated. according to the world bank (2013) ibagué is classified as the second city with the most growth and strongest development perspective in colombia. this is why it is believed that this region could have good demand for this type of materials for infrastructure construction. table 8. environmental impacts of the different alternatives evaluated per kg of cdw impact category unit a1 a2 a3 c kgc2h3cl eq 1.83 e-5 4.82e-6 -6.36e-5 ri kg pm2.5 eq 8.57e-7 1.05e-7 -5.17e-6 old kg cfc-11 eq 4.15 e-10 1.53e-10 -1.41e-9 po kg c2h4 eq 8.79e-7 -1.11e-7 -3.73e-6 lo m2org.arable 0.000209 0.00011 -0.000709 a kg so2 eq 6.67e-6 -1.97e-6 -3.08e-5 e kg po4 p-lim 3.47e-7 1.75e-7 -7.33e-7 gw kg co2 eq 0.00136 0.00037 -0.00402 nre mj primary 0.0381 0.0146 -0.128 me mj surplus -3.65e-5 -7.19e-5 -0.000177 3.4 sensitivity analysis it was considered necessary to carry out a sensitivity analysis to determine how the final results varied. it was assessed how the results changed when the distance to the recycling plant was altered. previous studies were taken into account to select the distances. rodríguez et al. (2015) indicated that the maximum viable distance between the sources of residues and recycling facilities is 30 km. ulubeyli et al. (2017) proposed that the maximum limit for viability is 50 km between facilities. therefore, a sensitivity analysis was required, for which scenario 3 was taken as a basis and the environmental impacts were evaluated according to the distances of 30 km (a4) and 50 km (a5). figure 7 reveals that a3 continued to be the most favorable scenario, due to the greater savings in the evaluated categories. also when coelho and de brito (2013b) performed a sensitivity analysis, they found that in spite of the variations in overall primary energy and co2eq balances, the prevented impacts are always higher than the generated impacts. when the transport distance to the recycling plant was increased, savings were still made in the alternatives. the savings decreased as the distance to the recycling plant increased. these results agree with those found by butera (2015), who considered that the process of transporting cdw to the recycling plant contributes to environmental impacts. butera (2015) concluded that transportation contributed 30 per cent of carcinogenic human toxicity impacts, but only played a minor role with respect to non-carcinogenic human toxicity. blengini and garbarino (2010) reached a similar conclusion. in their case, it was concluded that the transportation distance of recycled aggregate should increase two to three times before the induced impacts outweighs the avoided impacts. figure 7. sensitivity analysis for different distances 3.5 economic evaluation the economic factor was also relevant when different alternatives were assessed. in this study, the economic evaluation was undertaken on the basis of the cost of each management process per ton, and considering the cost avoided by the management process (table 9). the net cost of each alternative (table 10) was obtained from: the values of the net cost of table 9 and the percentages of each process within the alternative (figure 2). table 9 shows that a3 was the most beneficial scenario, from the economic perspective, and that increasing the distance to which the recycling plant would be located (50 km, a5) would continue to lead to economic savings, compared to the current management alternative (a1). 3.6 study limitations the limitations of the study mainly refer to the fact that not all the inventory analysis data could be obtained through visits to companies. consequently, they were completed with the ecoinvent v.2.2 database, which was contextualized for the colombian context. table 9. economic cost of each process per kg waste management process process cost ($/kg) avoided cost ($/kg) net cost ($/kg) source stone recycling cra1+tc2=16+27=43 cna4+tc2=23+136=159 -116 1 recycling plants consulted 2017; 2 ibagué limpia 2017; 3 metal recovery plants 2017; 4 quarries consulted, 2017; 5 companies consulted, 2017 landfilling ct2+clr2=44+5=49 ----- 49 earth reuse ex situ tc2 + c= 17+20=37 tc2+cg2=36+49=85 -48 landfilling ct2+clr2=44+5=49 ----- 49 metals recovery tc2+ccc3=30+22=52 ciron=167 -115 cra= cost of recycled aggregate; tc= transport cost; clr= cost of landfill rate; lc= labour cost (separation); ccc= classification and compaction cost; cna= cost of natural aggregate; cg=cost of earth; sc= scrap cost 349application of life cycle assessment (lca) methodology and economic evaluation for construction and demolition waste: a colombian case study table 10. net economic cost of each alternative evaluated process a1 a2 a3 a4 a5 stone recycling -----11.3 -22.6 -7.99 5.07 metal recovery -0.58 -0.58 -0.58 -0.58 -0.58 earth reuse -19.2 -19.2 -38.4 -38.4 -38.4 stone landfilling 9.55 4.77 ------------earth landfilling 19.6 19.6 ------------net cost by scenarios ($) 9.38 -6.70 -61.6 -46.97 -33.91 4. conclusion in this study, management processes for construction and demolition waste fractions in ibagué (colombia) were evaluated. the environmental impacts of the management of 1 kg of generated cdw were calculated, considering the current management of each fraction of waste and the percentage of the fraction within the total cdw that is generated (a1). new management alternatives were evaluated (a2 and a3) in which the percentage of the waste fraction or the type of treatment applied to each one was varied, to determine viability. the environmental evaluation was performed using lca methodology. an economic evaluation of alternatives was also carried out to determine viability. the comparison of processes evaluated per kg of waste fraction revealed that metal recovery contributed to impacts in some categories and savings in others. likewise, the process of earth reuse resulted in environmental savings in all categories, due to the high loads of the process that was avoided (disposal of earth in landfill). the last process to be evaluated corresponded to earth and stone landfilling, which had an impact on all categories. the comparison of management alternatives evaluated per kg of cdw revealed that a1, which corresponds to the current management of cdw in ibagué, contributed to environmental impacts in all categories, except mineral extraction. a2 led to savings in photochemical oxidation, acidification and mineral extraction. a3 brought about great savings in all categories. thus, the results of this study allow us to verify that the most beneficial cdw management scenario from the environmental and economic point of view in ibagué is that in which 100% of the metals are recovered, 100% of the earth is reused, and 100% of the stone waste is recycled. the results could help to raise awareness among all the agents involved in the construction sector, who can clearly see the vital role they play in the proper management of cdw and, as a result, promote the implementation of mechanisms and infrastructure for waste recovery and recycling. likewise, the results could contribute to enhancing environmental awareness in the educational and professional field, leading to more sustainable production and consumption habits in colombia. finally, it is hoped to help solve current problems related to the inadequate management of cdw, and contribute to progress in this area, as has occurred at international level. 5. acknowledgments the authors thank antonio nariño university for the financial support. we also appreciate all the support provided by construction companies and organizations in ibagué, as well as recycling plants, governmental entities and inert landfills in colombia that provided information for the project. references aguilar, m., & ardila, f. 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(2022). the effects of water acidity and engineering properties on rock durability. earth sciences research journal, 26(1). 6982. https://doi.org/10.15446/esrj.v26n1.76468 record manuscript received: 28/11/2018 accepted for publication: 01/02/2022 fifteen sedimentary rock samples from four rock types, including sandstone, limestone, travertine, and conglomerate, were collected from northern iran’s damghan area. mineralogical studies and laboratory experiments were performed to assess the rocks’ physical properties, schmidt hardness, p-wave velocity, slake-durability index, uniaxial compressive, point load, brazilian tensile, and block punch strengths. the studied rock samples are mainly composed of quartz, calcite, and dolomite with different textures. the slake-durability test was carried out up to ten cycles in fluids with different phs. the utilized test fluids were natural water with a ph of 7 and sulfuric acid solutions with ph of 5.5 and 4. based on the results, the slake-durability index is affected by the ph of the test fluids. also, the different rock types had distinctive slaking behaviors. the decreasing rate of the slake-durability index at initial cycles is higher than the end cycles. regression analyses showed that the evaluated parameters are correlated to the slake-durability index. in other words, the slake-durability index of the studied rock samples is greatly affected by their mineral composition, texture, physical properties such as porosity, and it is closely related to the strength parameters of the rocks. these preliminary results led to the extraction of empirical equations for determining essential characteristics of the rocks from the slake-durability index. keywords: rock; physical properties; mechanical properties; slake-durability; ph; efectos de la acidez del agua y las propiedades ingenieriles en la durabilidad de la roca resumen para este trabajo se recolectaron quince muestras tipo sedimentarias de cuatro clases de rocas, donde se incluyen areniscas, calizas, travertinos y conglomerados, en el área damghan, en el norte de irán. estudios mineralógicos y experimentos de laboratorio se realizaron para medir las propiedades físicas de las rocas, el índice de dureza de schmidt, la onda de presión, el índice de durabilidad, el de compresión uniaxial, el punto de carga, la prueba brasileña de tracción y la resistencia de golpe de bloque. las muestras de roca estudiadas se componen principalmente de cuarzo, calcita y dolomita con diferentes texturas. el índice de durabilidad se realizó en diez ciclos con fluidos de diferente alcalinidad. los fluidos utilizados para el índice fueron agua natural con ph de siete y soluciones de ácido sulfúrico con ph 5.5 y 4. con base en los resultados, el índice de durabilidad se afecta por el ph de los fluidos del ensayo. también los tipos de las rocas tienen diferente comportamiento en la durabilidad. la caida en el índice de la durabilidad de los ciclos iniciales es mayor que en los ciclos finales. los análisis de regresión muestran que los parámetros evaluados están correlacionados con el índice de durabilidad. en otras palabras, el índice de durabilidad de las muestras de rocas estudiadas está ampliamente afectado por su composición mineral, textura, y propiedades físicas como la porosidad, y está relacionado de cerca con los parámetros de resistencia de las rocas. estos resultados preliminares condujeron al diseño de ecuaciones empíricas para determinar las características esenciales de las rocas en el índice de durabilidad. palabras clave: rocas; propiedades físicas; propiedades mecánicas; índice de durabilidad; https://doi.org/10.15446/esrj.v26n1.76468 https://doi.org/10.15446/esrj.v26n1.76468 70 reza khajevand & davood fereidooni70 introduction the slaking and durability behaviors of rocks is generally evaluated by using the slake-durability test. the test method is described in isrm (1979) and as an international standard in astm (1990). the test index namely slakedurability index (id) is an important parameter to investigate the engineering behavior of rock-materials and rock masses, especially for weak and soluble rocks including clay-bearing, carbonate and evaporate rocks (franklin and chandra, 1972; onodera et al., 1974; crosta, 1998; koncagul and santi, 1999; gokceoglu et al., 2000; dhakal et al., 2002; singh et al., 2005; gupta and ahmed, 2007; yagiz, 2011). slaking and durability behaviors of various rocks were studied by many scholars using the slake-durability test (e.g. bell, 1992; moon and beattie, 1995; gokceoglu, 1997; sadisun et al., 2005; fuenkajorn, 2011; sebastian brysona et al., 2012; yavuz, 2012; keaton, 2013; heidari et al., 2015; yavuz et al., 2017). in this regard, gokceoglu et al. (2000) assessed the effect of mineralogy and strength on the durability of different types selected from turkey. singh et al. (2006) reported that the strength characteristics of marble are highest at ph=7 and the strength reduction is greater in the acidic conditions than in the alkaline. dhakal et al. (2002) indicated that the durability of rocks is affected by their mineralogy. the durability of some carbonate rocks in acidic environments was studied by gupta and ahmed (2007). kolay and kayabali (2006) identified the influence of aggregate shape and angularity on the slake-durability index for the same rocks. the effect of water ph on slaking process of different rocks has been analyzed by kayabali et al. (2006), gupta and ahmed (2007) and zhou et al. (2005). ghobadi and fereidooni (2015) studied the influence of mineralogy on the durability and strength of hornfelsic rocks under acidic rainfall in urban polluted areas. many other researchers have used different methods including regression analysis, fuzzy logic system and grey system modeling prediction to estimate the slake-durability of rocks (lu et al., 2005; singh et al., 2005; moradian et al., 2010; tasdemir et al., 2013; fereidooni, 2016). singh et al. (2005) investigated the effect of acidity of water on the slake-durability index of shales and used the artificial neural network and neuro-fuzzy logic systems for predicting the parameter for shales. sharma and singh (2008) tested various rocks including sandstone, basalt, mica schist, coal, and shale, to assess the relationships between the slake-durability index and uniaxial compressive strength (ucs), impact strength, schmidt hardness and p-wave velocity. yagiz (2011) carried out a comprehensive study to determine relationships between the slake-durability index and physical and mechanical properties of some carbonate rocks from turkey. in this study, dry density, saturated density and schmidt rebound hardness are related to the first cycle slake-durability index. but uniaxial compressive strength is related to the fourth cycle slake-durability index. momeni et al. (2017) have investigated the effect of weathering on durability and deformability properties of three different types of granitoid rocks. in their research, to assess the weathering effect on the durability behavior of these rocks, a slake durability test was performed up to 40 cycles. the results showed that the slake durability index can be used for determining the weathering grade and the long-term durability assessment of the rocks. kahraman et al. (2017) found the correlation between the slakedurability index and uniaxial compressive strength of pyroclastic rocks. monticelli et al. (2020) assessed the relationship between durability index and uniaxial compressive strength of a gneissic rock and found that the slake -durability test is a suitable test for predicting susceptibility of the studied rock to weathering, loss of strength, and durability. many other researchers have the slake -durability test for assessing behaviors of various rocks that a summarized history of their researches is presented in table 1. table 1. a summarized history of application of the slake-durability test by previous researchers no. reference studied rock types description number of test cycles applied solution 1 gokceoglu et al. (2000) clay-bearing rocks study the effect of the number of wetting-drying cycles on the durability of weak rocks four cycles (id4) natural water at 20 °c 2 dhakal et al. (2002) pyroclastic rocks, sandstones and mudstones investigated the relationship between mineralogy and durability in different solutions ten cycles (id10) distilled water, nacl and cacl2 solutions 3 kayabali et al. (2006) gypsum the influence of ph of testing liquid on the slake durability index (id) two cycles (id2) hcl and naoh solutions with ph values of 2, 4, 6, 8, 10, and 12 4 gupta and ahmed (2007) limestone, shale and siltstone evaluating the effect of mineralogical characteristics and ph of water including acidic and alkaline environment on the durability of rocks five cycles (id2) ph of the liquid is 2, 4, 6, 8, 10 and 12 at about 20 °c 5 sharma and singh (2008) sandstone, basalt, phyllite, mica schist, coal, shale use of regression analysis for correlation between ultrasonic wave velocity (vp) and slake durability index one cycle (id) natural water at 25 °c 6 kolay et al. (2010) sandstone, limestone, granite, tuff, marl present empirical relationships between durability index and point load strength by some soft computing approaches two cycles (id2) natural water 7 ghobadi and momeni (2011) granitic rocks assessing the effect of ph on the degradability of rocks by slake durability test fifteen cycles (id15) sulfuric acid (h2so4) with ph 2.5, 4, and 5.5 8 yagiz et al. (2012) carbonate rocks effect of the slake durability cycles on the modulus of elasticity (e) and uniaxial compressive (ucs) strength of rocks ten cycles (id10) natural water https://pubs.geoscienceworld.org/eeg/search-results?f_authors=r+g.+bell https://pubs.geoscienceworld.org/eeg/search-results?f_authors=jeffrey+r.+keaton 7171the effects of water acidity and engineering properties on rock durability 9 tasdemir et al. (2013) sandstone, marl, ignimbrite, limestone, tuff, granite use of artificial neural network (ann) approaches for estimating slake durability index two cycles (id2) t natural water 10 ghobadi and mousavi (2014) sandstones evaluating the influence of ph and salty solutions on the durability of rocks fifteen cycles (id15) acidic (h2so4) and alkaline (naoh) watery environments and saline solutions (nacl and na2so4) 11 gautam and shakoor (2016) claystone, mudstone, siltstone and shale performing slake durability test for laboratory comparing the slaking behavior of common clay-bearing rocks five cycles (id5) natural climatic conditions 12 kahraman et al. (2017) pyroclastic rocks prediction of uniaxial compressive strength from slake durability index by using regression analysis four cycles (id4) tap water at about 20 °c 13 fereidooni and khajevand (2018) travertine establish relationship between number of cycles of slake durability test and geotechnical properties of rocks such as physical properties, uniaxial compressive strength, brazilian tensile strength, point load strength, block punch strength and schmidt rebound hardness ten cycles (id10) natural water with ph = 7 and sulfuric acid solution (h2so4) with ph of 5.5 and 4 14 yagiz (2018) travertine and limestone assessing the effect of ph of testing solution on degradability of rocks by performing slake durability test five cycles (id5) various acidic (hcl) and alkaline (naoh) watery environments with ph from 2 to 10 15 fereidooni and khajevand (2019) different sedimentary rocks purposed new weathering method for evaluating the durability of rocks under different ph conditions ten cycles (id10) of slake durability test and ten cycles (awi10) accelerated weathering test (awt) natural water and sulfuric acid solution with different ph conditions 16 liu et al. (2020) sandstone evaluating the effect of various chemical additives in the slaking fluid on the durability, by slake durability test and dual rotation test two cycles (id2) distilled water and dual rotation test in sds, nacl, mgcl2, na2co3, k2co3, na2so4, al2(so4)3 solutions 17 selen et al. (2020) flysch and serpentinite study the slaking and disintegration behavior of the weak rock mass consisting of clay and clay-like minerals for hydropower project using modified slake durability test two cycles (id2) tap water at 20-25 °c 18 khajevand (2021) carbonate rocks study the effect of mineralogy, petrographic and textural properties on slake durability index three cycles (id3) natural water of the region 19 arman (2021) evaporate rocks correlation between uniaxial compressive strength and second cycle of slake durability index and present empirical equations two cycles (id2) natural water 20 momeni et al. (2017) granitoid rocks the effect of weathering on durability and deformability properties of granitoid rocks up to 40 cycles (id40) natural water 21 monticelli et al. (2020) gneiss relationship between durability index and uniaxial compressive strength of a gneissic rock at different weathering grades two cycles (id2) natural water 72 reza khajevand & davood fereidooni72 in the current research, the effect of number of wetting-drying cycles and ph of water on the values of the slake-durability index was investigated for fifteen different rock samples including sandstone, limestone, travertine, and conglomerate collected from northwestern parts of damghan, north of iran. also, the relationships between slake-durability index (id10) from one side and physical properties (dry and saturated unit weights, porosity, water absorption), index properties (schmidt rebound hardness, p-wave velocity), and mechanical properties (uniaxial compressive strength, point load index, brazilian tensile strength and block punch strength) from the other side have been investigated for the rocks. these rocks outcropped on the area and are used as construction materials and building stones in nearby cities. in previous researches, different rocks were subjected to two cycles of the slake-durability test, this doesn’t exactly simulate the natural conditions to evaluate rock durability. in the present study, the test is carried out up to ten cycles and the relationship between slakedurability indexes of all cycles and engineering properties of the rocks were investigated. it can be considered as an innovation for the research. sampling locations the northwestern parts of damghan, northern iran, were selected as sampling locations. the area is dominantly composed of various sedimentary rocks with a longitude of 54° 08’ e to 54° 15’ e and a latitude of 36° 14’ n to 36° 17’ n. it is elevated between 1380 and 1445 m above the mean sea level with a typical continental climate. fifteen rock samples from four rock types including three sandstones, three limestones, six travertines, and three conglomerates were collected from this region for laboratory testing. the sandstone samples were collected from the devonian padeha formation from a ballast mine on the chesmeh-ali road. the limestone samples were selected from the bahram formation of the middle devonian outcropping in northern parts of the ballast mine. the travertine samples were collected from the chemeh-hafez quarry located near the astaneh village. these travertines occur along the minor branch of the astaneh fault from spring waters passing within dolomites and vermicular limestones of the elika formation. they were deposited between the pishsarkoh and anbehkoh mountains. the conglomerate samples are obtained from the fajan formation of the paleocene-eocene outcropping on a new road cutting in the margin of chesmeh-ali road. methodology to do the current study, the block samples of sandstones (ss), limestones (ls), travertines (ts) and conglomerates (cs) were selected from outcrops available in mines, quarries and road cuttings in the study area. the rock samples were obtained from the depth of excavated locations and they were completely fresh. then they were transferred to the laboratory. in the laboratory, core specimens were prepared from the rock blocks using the core-drilling machine. the laboratory specimens were prepared in three shapes; irregular or lump, disc, and cylinder. the irregular or lump specimens were used for the slake-durability test. the disc specimens were used for the brazilian and punch tests. the cylindrical specimens were used for measuring physical properties, ultrasonic wave velocity, point load, and uniaxial compressive strength. mineral composition and textural properties of the samples were determined using a polarizing microscope and x-ray diffraction (xrd) based on astm (2009) and isrm (2007) standard procedures. for assessing the effect of mineralogy on rock durability, microscope investigation of thin section and xrd analyses were performed on the samples after the slake-durability test in acidic conditions in 2θ ranges from 4° to 70°. physical properties including specific gravity (gs), dry density (γdry), saturated density (γsat), water absorption (wa), and porosity (n) were determined based on isrm (2007). these tests involved five sets of experiments carried out on each rock types. index and mechanical properties including schmidt rebound hardness (hs), ultrasonic p-wave velocity (vp), uniaxial compressive strength (ucs), point load index (is(50)), brazilian tensile strength (bts) and block punch strength (bps) were obtained in accordance to related international standard methods. hereof, the schmidt rebound test was performed according to isrm (1981) and astm (2001c) by a schmidt hammer with 2.207 n/m energy on rock blocks of the studied samples. according to the standards, it must do 20 impacts on rock surface and record them. then, half of the less valuable data (10 impacts) are omitted and averaged over the rest. the p-wave velocity test was applied based on isrm (2007) and astm (1996). p-wave velocity of the tested rocks was determined from five set of experiments for each rock types performed directly on cylindrical shaped specimens in the laboratory by using a peroceq© digital ultrasonic tester (model: nd 180; trade name; pundit lab+; transit time range: 0.1-9999 µs; energizing pulse: 125, 250, 350, 500 v; frequency range: 24-500 khz). the uniaxial compressive strength was performed based on the methods suggested by isrm (1979; 2007) and astm (1995). to determine ucs, five cylindrical specimens with the length to diameter ratio of 2-2.5 were tested of each rock sample. the point load tests were undertaken for the rock specimens with unit length to diameter ratio according to isrm (1985) and astm (2001a). the brazilian tensile test was performed on rock specimens. the length to diameter ratio of the samples was from 0.5 to 0.75 in accordance with the isrm (1978) and astm (2001b). ten sets of experiments on each rock types were performed for determining the point load index and the brazilian tensile strength of the studied rocks in the laboratory. the block punch test was used to assess the shear strength of the studied rocks. for this purpose, the cores are cut into thin discs of various raw thicknesses ranging between 5 and 15 mm using a diamond saw (ulusay et al., 2001). the slake-durability tests were applied on the fifteen rock samples based on isrm (2007) and astm (1990). 450 rock lumps (ten pieces of about 40–60 g of each sample) were prepared for performing the test. the rock lumps were rotated for 10 min in a test drum made of a standard sieve mesh so that slaking products would be finer than 2 mm and pass through the drum. the drum was half immersed in water. the rock pieces retained in the drum were dried at 105 °c for a full day in the oven. then, they were cooled and weighed. the slake-durability index (id) corresponding to each cycle of the test was calculated as the percentage ratio of final to initial dry weights of rock lumps in the drum after the cycle. also, in order to understand the effect of wetting and drying cycles and acid solutions on the durability of the rock samples, the slake-durability test was performed up to ten cycles in different ph conditions. the used slaking fluid were natural water with ph=7 of the region and sulfuric acid solutions with ph of 5.5 and 4 prepared in the laboratory. in fact, there are many polluting gases in the air that one of their most important is sulfur dioxide (so2). this gas combines with rainfall waters in the air to produce sulfuric acid. we select this acid due to two reasons; 1) it is abundant gas in polluted cities of iran, 2) if the other acids were selected, the research became very long. so, the authors decided to perform the research with only one acid namely sulfuric acid. finally, correlations between the slake-durability index and physical, dynamic and mechanical properties were assessed by regression analyses in different ph conditions. regression analyses led to new empirical equations for predicting physical and mechanical properties of the studied rocks from the slake-durability index. results and discussions mineralogical and petrographic properties the studied rocks are commonly composed of quartz, calcite, aragonite and dolomite. the sandstone samples are dominantly composed of quartz, and calcite with minor dolomite and hematite. mono crystalline and fine grain quartz (smaller than 0.125 mm) and rhombohedra dolomite are the important textural characteristic of these samples. the limestone samples are dolomitic limestone composed of calcite, dolomite, and quartz. breccia structure and dolomitic vein with fossil fragments are the textural characteristics of these samples. mineral composition of the travertines is aragonite, calcite, quartz, and admixture of fluorite and bornite. these samples have a layered texture that is due to the succession of crystalized aragonite and calcite and micritic layers. the travertine samples are porous and dominantly have mixture cement composed of calcite and aragonite. the conglomerate samples are composed of micritic limestone, crystalline calcitic limestone, sandstone and siltstone fragments with calcitic cement. rock fragments larger than 2 mm with low textural maturity and poor sorting are the most important textural characteristics of the conglomerate samples. rock types and the modal abundance of the minerals in the rock samples are given in table 2. 7373the effects of water acidity and engineering properties on rock durability table 2. type and mineral composition of the rocks mineral content (%) rock typesample other mineralsbor.hem.dol.arg.cal.qtz. 3-213-2161sandstoness1 2-310-2560sandstoness2 1-39-2859sandstoness3 11221-5520dolomitic limestonels1 21220-5520dolomitic limestonels2 21419-5519dolomitic limestonels3 1-1-55358travertinets1 5---454010travertinets2 2---45458travertinets3 11--484010travertinets4 11--50408travertinets5 11--49409travertinets6 2-65-4542conglomeratecs1 7-53-4540conglomeratecs2 7-45-4539conglomeratecs3 qtz.: quartz (sio2); cal.: calcite (caco3); ara.: aragonite (caco3); dol. dolomite ca.mg(co3)2; hem.: hematite (fe2o3); bor.: bornite (cu5fes4) physical properties the average values of specific gravity (gs), dry density (γdry), saturated density (γsat), water absorption (wa), and porosity (n) are presented in table 3. based on the results, the values of dry density increase in the samples of travertine, sandstone, conglomerate, and limestone, respectively. the values of porosity increase in the samples of limestone, travertine, conglomerate, and sandstones, respectively. in this regard, the samples of ts3 and ls1 have the lowest and highest values of dry density and the ts1 and ss3 samples have the lowest and highest values of porosity, respectively. according to the rock classification suggested by iaeg (1979), most of the samples have moderate dry density and low to medium porosity. table 3. values of physical properties of the rocks description of n (iaeg 1979) description of γd (iaeg 1979) physical properties sample n (%) wa (%) γs (g/ cm3) γd (g/ cm3) gs mediummoderate7.583.692.532.442.49ss1 mediummoderate8.413.422.522.452.49ss2 mediummoderate10.324.822.462.352.35ss3 mediumhigh5.802.322.672.612.66ls1 mediumhigh7.833.332.602.522.56ls2 lowhigh4.411.792.702.612.65ls3 lowmoderate2.801.222.352.322.37ts1 mediummoderate5.602.412.262.202.25ts2 mediummoderate4.821.332.332.282.32ts3 mediummoderate5.041.842.302.262.30ts4 lowmoderate4.711.152.392.372.32ts5 mediumlow6.692.312.142.092.13ts6 mediummoderate5.442.252.572.512.56cs1 mediummoderate8.223.112.502.432.48cs2 mediummoderate6.983.012.572.492.54cs3 table 4. values of the dynamic and mechanical properties of the rocks bps (mpa) bts (mpa) is(50) (mpa) ucs (mpa) vp (m/s)hssample 8.156.678.8546.29385429ss1 6.835.386.5432.46351726ss2 5.903.815.8016.09309324ss3 9.367.9310.3250.13517136ls1 7.834.768.2025.19467228ls2 10.279.2211.8651.51528436ls3 6.157.749.1531.96598430ts1 4.255.086.2819.37419820ts2 5.136.857.0025.34540225ts3 6.467.188.6128.35478929ts4 5.506.197.5425.89470726ts5 4.885.166.4018.52342720ts6 6.164.668.8626.15373527cs1 4.992.975.1614.97293118cs2 5.733.887.6723.88378928cs3 dynamic and mechanical properties the average values of the dynamic and mechanical properties for the rocks are outlined in table 4. based on the results, the values of these properties increase in the samples of limestone, sandstone, travertine, and conglomerate, respectively. in this regard, the sample of cs2 has the lowest value and the samples of ls1 and ts1 have the highest value of schmidt rebound hardness. the values of p-wave velocity range from 2931 to 5984 m/s. the cs2 and ts1 samples have the lowest and highest values of the parameters, respectively. based on the iaeg (1979) classification, these samples are classified as the rocks with low to very high p-wave velocity. the samples of cs2 and ls3 74 reza khajevand & davood fereidooni74 have the lowest and highest ucs values, equal to 14.97 and 51.51 mpa, respectively. based isrm (2007), the tested samples are classified as weak to strong rocks. the cs2 and ls3 samples have the lowest and highest point load index values, equal to 5.16 and 11.86 mpa, respectively. based on the broch and franklin (1972) classification, the tested samples are classified as very strong to extremely strong rocks. based on the results, cs2 and ls3 samples have the minimum and maximum values of bts, equal to 2.97 and 9.22 mpa, respectively. the samples of ts2 and ls3 have the lowest and highest values of point load index, equal to 4.46 and 10.27 mpa, respectively. based on ulusay et al. (2001), the samples of ts3 and cs2 are weak rocks and the other samples are classified as moderate rocks. slake-durability index the values of the slake-durability index for the samples of sandstone, limestone, travertine, and conglomerate are outlined in tables 5, 6, 7 and 8, respectively. based on the results, the values of the slake-durability index increase in the samples of conglomerate, sandstone, travertine, and limestone, respectively. in this regard, the samples of ts1 and cs2 with slake-durability index equal to 96.72% in ph=7 and 85.85% in ph=4, have the maximum and minimum values of the parameter after ten cycles of the test, respectively. according to the gamble (1971) classification, the tested samples are classified as durable and very durable rocks after the first cycle of the test (fig. 1). table 5. values of the slake-durability index of sandstone samples in different ph conditions ph sample slake-durability index (id) (%) id1 id2 id3 id4 id5 id6 id7 id8 id9 id10 ph=7 ss1 98.10 97.25 96.56 95.90 95.31 94.74 94.21 93.70 93.47 93.03 ss2 98.19 97.26 96.42 95.56 94.64 94.17 93.54 92.97 92.34 91.85 ss3 98.04 97.04 95.54 95.23 94.52 93.79 93.13 92.47 92.12 91.62 ph=5.5 ss1 97.98 97.06 96.21 95.47 94.76 94.17 93.61 93.04 92.25 92.05 ss2 98.13 97.23 96.42 95.68 92.23 94.06 93.44 92.88 92.23 91.61 ss3 97.50 96.31 95.31 94.35 93.06 92.29 91.57 90.91 90.17 89.79 ph=4 ss1 97.90 97.02 96.43 95.67 95.06 94.44 93.58 93.06 92.77 92.36 ss2 97.84 96.81 95.86 95.03 94.29 93.55 92.89 92.25 91.58 90.98 ss3 97.36 95.99 94.88 93.79 92.78 91.70 90.87 90.09 89.40 88.67 table 6. values of the slake-durability index of limestone samples in different ph conditions ph sample slake-durability index (id) (%) id1 id2 id3 id4 id5 id6 id7 id8 id9 id10 ph=7 ls1 98.71 98.28 97.98 97.66 97.49 97.14 96.89 96.63 96.39 96.19 ls2 98.76 98.30 97.95 97.58 97.36 96.97 96.67 96.38 96.12 95.88 ls3 98.60 98.28 98.01 97.75 97.54 97.30 97.07 96.83 96.64 96.46 ph=5.5 ls1 98.58 98.11 97.60 97.11 96.80 96.51 96.21 96.06 95.84 95.67 ls2 98.82 98.42 97.83 97.48 97.17 96.87 96.46 96.29 96.05 95.87 ls3 98.83 98.58 98.29 97.73 97.52 97.30 97.13 96.99 96.88 96.95 ph=4 ls1 98.84 98.44 97.98 97.59 97.28 96.97 96.73 96.47 96.26 96.16 ls2 98.62 98.19 97.59 97.16 96.76 96.40 96.08 95.75 95.51 95.39 ls3 98.58 98.22 97.89 97.58 97.33 97.09 96.86 96.65 96.46 96.34 table 7. values of the slake-durability index of travertine samples in different ph conditions ph sample slake-durability index (id) (%) id1 id2 id3 id4 id5 id6 id7 id8 id9 id10 ph=7 ts1 99.44 98.97 98.54 98.21 97.95 97.66 97.43 97.16 96.92 96.72 ts2 98.34 97.85 97.49 97.15 96.83 96.58 96.30 96.04 95.77 95.31 ts3 99.01 98.53 98.14 97.81 97.51 97.20 96.93 96.68 96.40 96.12 ts4 99.04 98.55 98.16 97.81 97.51 97.19 96.92 96.64 96.38 96.13 ts5 99.11 98.68 98.30 97.99 97.69 97.43 97.13 96.87 96.25 96.25 ts6 98.53 97.90 97.39 96.94 96.54 96.16 95.77 95.37 95.02 94.45 ph=5.5 ts1 99.05 98.50 98.17 97.88 97.60 97.38 97.11 96.79 96.59 96.36 ts2 98.48 98.05 97.73 97.44 97.15 96.91 96.66 96.44 96.23 96.01 ts3 98.90 98.52 98.21 97.91 97.46 97.22 96.97 96.76 96.52 96.31 ts4 98.75 98.35 97.95 97.62 97.30 97.02 96.75 96.47 96.24 95.99 ts5 99.01 98.72 98.41 98.15 97.65 97.42 97.22 96.99 96.60 96.60 ts6 98.70 98.11 97.57 97.12 96.71 96.31 95.96 95.43 95.06 94.76 7575the effects of water acidity and engineering properties on rock durability ph sample slake-durability index (id) (%) id1 id2 id3 id4 id5 id6 id7 id8 id9 id10 ph=4 ts1 99.07 98.68 98.26 97.96 97.65 97.40 97.14 96.91 96.68 96.46 ts2 98.52 98.17 97.71 97.33 96.99 96.67 96.34 96.02 95.73 95.43 ts3 99.09 98.57 98.16 97.81 97.49 97.10 96.27 96.01 95.74 95.52 ts4 98.29 97.76 97.23 96.78 96.08 95.66 95.26 94.89 94.54 94.21 ts5 99.13 98.71 98.30 97.97 97.65 97.34 97.04 96.65 96.11 96.11 ts6 98.66 97.50 96.82 96.33 95.71 96.34 94.94 94.57 94.22 93.85 table 8. values of the slake-durability index of conglomerate samples in different ph conditions ph sample slake-durability index (id) (%) id1 id2 id3 id4 id5 id6 id7 id8 id9 id10 ph=7 cs1 98.11 97.35 96.64 96.06 95.73 95.20 94.78 94.40 94.06 93.76 cs2 96.68 95.45 94.53 93.72 93.17 92.39 91.81 91.19 90.66 90.12 cs3 97.22 96.21 95.43 94.70 94.18 93.45 92.88 92.31 91.83 91.39 ph=5.5 cs1 97.74 97.09 96.31 95.65 95.14 94.63 94.21 93.78 93.46 93.23 cs2 96.60 95.12 93.64 92.96 92.35 91.76 91.21 90.74 90.38 90.02 cs3 97.57 96.77 95.80 94.79 94.25 93.76 93.31 92.85 92.48 92.11 ph=4 cs1 98.05 97.39 96.55 95.33 93.86 91.70 91.18 90.80 90.52 90.21 cs2 95.74 93.77 92.35 91.10 89.94 89.04 88.28 87.21 86.53 85.85 cs3 97.24 95.71 94.40 93.58 92.63 91.68 91.04 90.35 89.87 89.61 figure 1. classification of the studied rocks based on the gamble (1971) classification the effect of wetting-drying cycles on the slake-durability index of the studied rock samples in different ph conditions are presented in figure 2. also, the results of the 10th cycle of the slake-durability test as comparative diagram are shown in figure 3. according to these figures, as the number of cycles increased, the values of id decreased (specimen weight loss increased) for all the rocks. this is similar to the obtained results presented in all researches summarized in table 1. the decreasing rate of id in the initial cycles is higher than the end cycles. also, in more samples, a constant pattern exists between the slake-durability index and ph of the testing solutions so that the id decreases with decreasing ph from 7 to 4. this is coordinated with the results presented by kayabali et al. (2006), ghobadi and momeni (2011), ghobadi and mousavi (2014), fereidooni and khajevand (2018), and fereidooni and khajevand (2019). this occurs because all the samples have calcite (caco3) which is a highly susceptible mineral when to be attacked by acidic agents. it has also been observed that porous travertine ts6 is more susceptible to degradation as compared to the other rock samples. the samples of conglomerate exhibited an entirely different pattern so that the minimum values of the slake-durability index were obtained for these samples in comparison to the other samples which possess a middle range of the slake-durability index. as a matter of fact, the higher values of id, especially in acid solutions, must represent the higher resistance of a sample to degrade under a given set of conditions. but this is not true for the samples of the conglomerate. because these samples have weak cement and their rock fragments simply cut out from the rock lumps due to collision and abrasion to each other during the slake-durability test. this lead to obtain low values of id being obtained in the samples of conglomerate. figure 2. influences of number of wetting-drying cycles and ph on the slakedurability index of the rocks 76 reza khajevand & davood fereidooni76 figure 3. comparative diagram of the 10th cycle slake-durability index in different ph conditions xrd analyses the obtained results from xrd analyses, as shown in figure 4, indicate that quartz and calcite are the most important minerals in the rock samples. the crystallinity degree of these minerals has been evaluated in the 2θ ranges of 20° and 50°. the mineral composition of the rock samples contains a large amount of quartz, calcite and a small amount of other minerals such as hematite and bornite. therefore, the thin section mineralogical studies have been confirmed by xrd analyses. mineral composition of the fresh samples and the samples under slakedurability test was approximately similar to each other. the diffractograms for the two states are different only in the intensity of the mineral peaks indicating a decreasing trend of calcite peaks. this means that acid solutions have a specific effect on chemical weathering of the rock’s minerals. the sulfuric acid solutions with ph=4 could react with calcite and break it down. this is confirmed by ghobadi and momeni (2011), ghobadi and mousavi (2014), and fereidooni and khajevand (2019). the decrease in peaks of calcite in the diffractograms of the durable rock samples is evidence for this occurrence. figure 4. diffractograms of the fresh samples (left) and the samples under slakedurability test at ph=4 (right) regression analyses one of the commonly accepted methods for investigating empirical relationships between different rock properties is regression analysis (khanlari et al., 2014 a, b; fereidooni et al., 2015; ghobadi and babazadeh, 2015; fereidooni et al., 2016). simple regression analyses were performed to develop relationships between slake-durability and physical, index, and mechanical properties of the studied rocks. in addition, the relationships between experimental and calculated results are developed based on equations obtained for the rocks. although in momeni et al. (2017) and monticelli et al. (2020), id2 is selected to be correlated to the other rock properties, in the present research, the values of id10 were correlated to the values of physical, index and mechanical properties of the studied rocks. because the effect of water ph changing from 7 to 4 on id is more visible after 10 cycles of the slake durability test. correlations between the slake-durability index and physical properties correlations between id10 in natural water with ph=7 and physical properties are shown in figure 5, and the extracted empirical equations between the parameters are presented in table 9. in this research, the fitting lines are drawn separately for each type of rock sample. because in this case, a better relationship is obtained between the two variables with a higher determination coefficient (r2). based on the results, obtained correlation coefficients range from 0.22 to 0.99. the samples of limestone have better correlations than the other samples. the least correlation belongs to the samples of sandstone. slake-durability index decreases with increasing porosity and water absorption. therefore, correlations between the parameter and porosity and water absorption are inverse linear. correlations between experimental and calculated values of dry and saturated density, porosity and water absorption based on the equations for all tested samples are shown in figures 6 and 7, respectively. for evaluating the validity of the results, the 1:1 relationship (y=x) has been plotted in these figures. it is clear that the lines closely match the line of y=x, which confirms the validity of the equations. table 9. empirical equations between the slake-durability index with porosity and water absorption of the rocks sample r2 equation x – y ss r² = 0.31 γd = 0.040 id10 1.294 id10 – γd r² = 0.52 γs = 0.036 id10 0.833 id10 – γs r² = 0.69 n = -1.538 id10 + 150.560 id10 – n r² = 0.22 wa = -0.465 id10 + 46.839 id10 – wa ls r² = 0.78 γd = 0.159 id10 -12.663 id10 – γd r² = 0.97 γs = 0.174 id10 – 14.052 id10 – γs r² = 0.99 n = -5.913 id10 + 574.660 id10 – n r² = 0.98 wa = -2.670 id10 + 259.270 id10 – wa ts r² = 0.86 γd = 0.112 id10 8.470 id10 – γd r² = 0.87 γs = 0.100 id10 7.313 id10 – γs r² = 0.84 n = -1.436 id10 + 142.570 id10 – n r² = 0.72 wa = -0.583 id10 + 57.592 id10 – wa cs r² = 0.81 γd = 0.020 id10 + 0.618 id10 – γd r² = 0.59 γs = 0.017 id10 + 1.007 id10 – γs r² = 0.98 n = -0.749 id10 + 75.627 id10 – n r² = 0.94 wa = -0.247 id10 + 25.455 id10 – wa 7777the effects of water acidity and engineering properties on rock durability figure 5. correlations between the slake-durability index and, a) dry density, b) saturated density, c) porosity and d) water absorption for the rocks figure 6. correlations between experimental and calculated values of dry and saturated densities for the rocks figure 7. correlations between experimental and calculated values of porosity and water absorption for the rocks correlations between the slake-durability index and index properties relationships between the slake-durability index and schmidt rebound hardness and p-wave velocity are shown in figure 8. as presented in table 10, correlation coefficients obtained from the regression analyses range from 0.50 to 0.97. the samples of sandstone have better correlations than the other samples. the least correlation belongs to the samples of the conglomerate. relations between id10 in natural water with ph=7, hs and vp are direct linear in all of the samples. the validity of the obtained equations is characterized by correlations between experimental and calculated values of hs and vp (fig. 9). for comparison of the results, a 1:1 relationship (y=x) has been plotted in these figures. it is clear that correlation lines match the line of y=x, which confirms the validity of the equations. figure 8. correlations between the slake-durability index and a) schmidt hardness rebound, b) p-wave velocity for the rocks table 10. empirical equations between the slake-durability index, schmidt rebound hardness and p-wave velocity of the rocks sample r2 equation x – y ss r² = 0.97 hs = 3.978 id10 339.490 id10 – hs r² = 0.82 vp = 456.750 id10 – 38609.000 id10 – vp ls r² = 0.78 hs = 14.088 id10 1321.600 id10 – hs r² = 0.91 vp = 1068.800 id10 97749.000 id10 – vp ts r² = 0.73 hs = 4.277 id10 384.740 id10 – hs r² = 0.87 vp = 1023.000 id10 – 93285.000 id10 – vp cs r² = 0.50 hs = 2.104 id10 168.730 id10 – hs r² = 0.68 vp = 189.860 id10 – 13936.000 id10 – vp figure 9. correlations between experimental and calculated values of a) schmidt rebound hardness and b) p-wave velocity for the rocks 78 reza khajevand & davood fereidooni78 correlations between the slake-durability index and mechanical properties relationships between the slake-durability index and mechanical properties including uniaxial compressive strength, point load index, brazilian tensile strength, and block punch strength were performed by regression analyses. figure 10 shows relationships between id10 in natural water with ph=7 and the properties. the equations extracted from these relations are direct linear and outlined in table 11. this means that slake-durability index increases with increasing rock strength in all samples. moreover, strong correlation coefficients were found between id10 and the mechanical properties in the range of 0.46 to 1.00. the samples of limestone have better correlations than the other samples. the best and least correlations belong to the samples of limestone and travertine, respectively. the relationships between experimental results and calculated ucs values based on id10 for all tested rock samples are shown in figure 11. for comparison of the results, a 1:1 relationship (y=x) has been plotted in this figure. the correlation line shows that the equations are reliable. also, the relationships between experimental results and calculated values of is(50), bts and bps based on id10 are shown in figure 12. it is clear that the three lines match the line of x=y, which confirms the validity of these equations. figure 10. correlations between the slake-durability index and a) uniaxial compressive strength, b) point load index, c) brazilian tensile strength, d) block punch test for the rocks 7979the effects of water acidity and engineering properties on rock durability table 11. empirical equations between the slake-durability index and mechanical properties of the rocks sample r2 equation x – y ss r² = 0.83 ucs = 18.252 id10 1650.600 id10 – ucs r² = 0.99 is(50) = 1.870 id10 165.360 id10 – is(50) r² = 0.83 bts = 1.723 id10 153.520 id10 – bts r² = 0.93 bps = 1.435 id10 125.310 id10 – bps ls r² = 0.82 ucs = 46.24 id10 4404.800 id10 – ucs r² = 0.99 is(50) = 6.323 id10 598.030 id10 – is(50) r² = 0.96 bts = 7.7520 id10 738.250 id10 – bts r² = 1.00 bps = 5.052 id10 476.600 id10 – bps ts r² = 0.86 ucs = 5.898 id10 539.600 id10 – ucs r² = 0.64 is(50) = 1.149 id10 102.620 id10 – is(50) r² = 0.76 bts = 1.166 id10 105.330 id10 – bts r² = 0.46 bps = 0.679 id10 59.663 id10 – bps cs r² = 0.77 ucs = 2.802 id10 235.470 id10 – ucs r² = 0.87 is(50) = 0.952 id10 80.084 id10 – is(50) r² = 0.95 bts = 0.447 id10 37.186 id10 – bts r² = 0.90 bps = 0.304 id10 22.231 id10 – bps figure 11. correlations between experimental and calculated values of uniaxial compressive strength (ucs) for the rocks 80 reza khajevand & davood fereidooni80 figure 12. correlations between experimental and calculated values of point load index (is), brazilian tensile strength (bts) and block punch strength (bps) for the rocks conclusion in this study, an attempt has been made to investigate the durability properties and geotechnical characteristics of the selected rock samples and correlated slake-durability index to physical, dynamic, and mechanical properties of the rocks. these rocks are dominantly composed of calcite and quartz and they are classified as weak to strong rocks based on ucs. slakedurability test was carried out up to ten cycles in natural water with ph=7 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(2019). measurement of radon in soils of lima city peru during the period 20162017. earth sciences research journal, 23(3), 171-183. doi: https://doi.org/10.15446/esrj. v23n3.74108 lima city is situated on alluvial fan deposits of rivers flowing through geological formations that contain different levels of uranium. in this paper, a study is made on the average spatial and temporal behavior of radon gas in soils of lima city. radon concentration was determined using the lr-115 type 2 track detector during 36 periods, of 14 days each, in twenty holes distributed in the fifteen districts of lima city. radon concentration in soil pores ranged from 0.1 to 64.3 kbq/m3 with an average value of 5.6 kbq/m3. the average radon concentration in soil gas was about two times lower in winter than in the other seasons. high radon values during october/november 2017 were related to the earthquakes perceived in lima city in that period. the highest radon concentrations were found in areas of alluvial deposits whose parental material has been removed from the quilmaná and huarangal volcanics by the chillón and huaycoloro rivers. soil gas radon concentrations were even higher in areas closer to volcanic and less distant from rivers. during the period of maximum flooding of the chillón, rímac and lurín rivers, due to the natural phenomenon “el niño costero”, anomalous high soil radon concentrations were observed in most of the measurement sites located near rivers. these high radon values were associated with ground vibrations caused by rock and debris avalanches in rivers and creeks. abstract measurement of radon in soils of lima city peru during the period 2016-2017 medición del radon 222 en suelos de la provincia de lima perú durante el período 2016-2017 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.74108 la ciudad de lima está situada sobre depósitos de abanicos aluviales de ríos que fluyen a través de formaciones geológicas que contienen diferentes niveles de uranio. en este trabajo se realiza un estudio sobre el comportamiento espacial y temporal promedio del gas radón en suelos de la ciudad de lima. la concentración de radón se determinó utilizando el detector de trazas lr-115 tipo 2 durante 36 períodos, de 14 días cada uno, en veinte hoyos distribuidos en los quince distritos de la ciudad de lima. la concentración de radón en los poros del suelo estuvo comprendida entre 0.1 y 64.3 kbq/m3 con un valor promedio de 5.6 kbq/m3. la concentración promedio de radón en el gas del suelo fue aproximadamente dos veces menor en invierno que en las otras estaciones. los altos valores de radón durante octubre/noviembre de 2017 se relacionaron con los terremotos percibidos en la ciudad de lima en ese período. las concentraciones más altas de radón se encontraron en áreas de depósitos aluviales cuyo material parental ha sido removido de los volcánicos quilmaná y huarangal por los ríos chillón y huaycoloro. las concentraciones de radón en el gas del suelo fueron incluso mayores en áreas más cercanas a los volcánicos y menos distantes de los ríos. durante el período de máxima inundación de los ríos chillón, rímac y lurín, debido al fenómeno natural "el niño costero", se observaron altas concentraciones anómalas de radón en el suelo en la mayoría de los sitios de medición ubicados cerca de los ríos. esos altos valores de radón se asociaron con las vibraciones del suelo causadas por avalanchas de rocas y escombros en ríos y quebradas. resumen record manuscript received: 08/08/2018 accepted for publication: 21/06/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 171-183 lázaro luís vilcapoma1, maría elena lópez herrera1, patrizia pereyra1, daniel francisco palacios1, bertin pérez1, jhonny rojas1, laszlo sajo-bohus2 1pontificia universidad católica del perú, lima, peru 2universidad simón bolívar, caracas, venezuela * corresponding author: dpalaciosf@pucp.edu.pe en vi ro nm en ta l g eo lo g y 172 lázaro luís vilcapoma, maría elena lópez herrera, patrizia pereyra, daniel francisco palacios, bertin pérez, jhonny rojas, laszlo sajo-bohus introduction radon gas is produced by the radioactive decay of 226ra, which in turn is derived from the 238u series. uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. since its discovery, at the beginning of the 20th century, the radon gas 222rn (indicated as rn or radon in the text), has awakened considerable interest due to its harmful effects on human health and its applications in geosciences. in the present study two areas of interest related to radon are combined: as a health hazard and as a natural tracer. the connection of radon with uranium makes it useful as a pathfinder element when searching for uranium rich minerals and indirectly, to identify radon-prone areas that may have the potential to generate high indoor radon levels. radon generated within the mineral grains of rocks and soils can escape from them by recoil into air or fluid filled cracks and into the contiguous pore space, respectively. this can be quantified by the emanation coefficient, which depends on the availability of 226ra, porosity, moisture content and temperature (nazaroff, 1992). subsequent transport from the place of escape into the gas/fluid phase towards the earth surface occurs by diffusion and convection/advection. radon transport is influenced not only by local geology as rock types, but by other geophysical and geochemical parameters as joints, fractures, hydrology, porosity, permeability and presence of other gas carriers. these characteristics are, in turn, determined by the character of the bedrock, glacial deposits, or transported sediments from which the soil was derived (gundersen et al., 1992; alonso et al., 2019). soil-gas radon concentrations can vary in response to climatic and weather changes on hourly, diurnal, or seasonal time scales (sundal et al., 2008; zafrir et al., 2012). the most important meteorological factors appear to be barometric pressure, wind, relative humidity, rainfall, and temperature. geology is the most important factor controlling the source and distribution of radon (appleton & miles, 2010). the release of radon from rocks and soils is controlled largely by the types of minerals in which uranium and radium occur. granite, metamorphic rocks, black shales, and some volcanic rocks may be enriched in uranium; these rocks, and the soils derived from them, are the most common sources of radon gas (sprinkel & solomon, 1990). uranium concentrations in igneous rocks are usually higher compared to sedimentary ones, excepting shale, deep-sea sediments and phosphate rocks, while metamorphic rocks have concentrations similar to those of the rocks from which they were derived. the content of uranium in igneous rocks depends on the origin and composition of the rock, for example, high silica igneous rocks contain relatively higher u concentrations compared to such basic low silica igneous rocks as basalt (iaea, 2014). because of its inert gas geochemistry and its radioactivity, radon has been recognized as a potential tracer of fault systems (al-tamimi & abumurad, 2001; walia et al., 2008), as an important tool to forecast earthquakes and volcanic eruptions (zmazek et al., 2005; neri et al., 2006; giammanco et al., 2007; pérez et al., 2007), as a good indicator of hydrocarbon leakage from reservoirs (matolin & stráník, 2006; palacios et al., 2016), to wide range of applications as a tracer in marine and hydrological settings. several studies have demonstrated that radon concentrations in soil and groundwater are controlled not only by types of rocks and geohydrological characteristics of rock mass but also by tectonic processes (choubey et al., 1999; choubey et al., 2010). landslides, which result due to geodynamic processes, have also been considered as a source of radon (purtschller et al., 1995). recent studies revealed that spatial distribution of indoor radon concentration in lima city can be represented by two different areas: one with concentrations <100 bq/m3 and the other in the range (100 300) bq/m3 (pereyra et al., 2018). several factors may have influenced on this distribution, however the two major factors determining indoor radon concentration are the entry rate and the ventilation rate. the entry rate is the most important factor in determining houses with high radon levels (nazaroff & nero, 1988). it depends on soil gas radon concentration, on soil physical characteristics such as permeability, on meteorological circumstances such as temperature and pressure, and on building-related factors. geogenic radon potential (grp) provides information about the source of indoor radon concentration and depends on the equilibrium activity concentration (rn concentration in soil gas at about 1 m depth), and soil permeability. criteria have also been established for radon hazard assessment considering only the soil radon as the source of high indoor radon concentration (åkerblom et al., 1984; snih, 1992). most of the area of lima city is built on alluvial deposits originating by the chillón and rímac rivers, which eroded, transported and deposited materials from the geological formations present in their beds. throughout their courses, these rivers interact with rock formations of different geological origin, and different chemical (e.g., uranium content) and mineralogical composition. it is to be expected, then, that one of the main source of variation of radon concentration in soil pores beneath and around dwellings in lima city could be related to the type of rock originating the alluvial deposits. thus, some association could be found between radon indoors and main parent material of the alluvial soil on which dwellings are located. indoor radon measurements in lima city shows the highest concentration in houses located at northeast of the rímac district (pereyra et al., 2018). in this area, landslides and avalanches of rocks, debris and sediments caused by the floods of the rímac and huaycoloro rivers have historically occurred. although the alluvial parent material in the lima city area is related to the occurrence of two lithologies, with rocks containing different amounts of uranium, the relationships between uranium content in parent material and soil gas radon in a given area is not obvious, since many factors and processes may contribute to influence the latter. in this paper a study is made on the average spatial and temporal behavior of radon in alluvial soils of lima city in order to evaluate the possible relationship between soil radon concentration and the origin of deposited parent material. in addition, to evaluate if temporal variations of soil gas radon were related to the seismic events and to the natural phenomenon “el niño costero” occurred during measurements. material and methods characteristics of the study area lima city is located at the west central peru, in a desert stripe between the pacific ocean and the andes, mostly on flat terrain of the peruvian coastal plain within the valleys of the chillón, rímac and lurín rivers (fig. 1). the chillón river is to the north, the rímac river in the center and the lurín river is to the south. these rivers have contributed significantly to the erosion and transport of loose materials, resulting from the erosion of hills present along their channels. the geology of the lima region reflects the subduction of the nazca plate under the south american plate and consequent uplift and volcanic activity. this subduction zone is constantly generating seismic activity. the geomorphology of lima is a tectonic and plutonic result superimposed by the processes of geodynamics. according to the lithology, in the northern area of lima there are volcanic rocks along the chillón river resulting from the presence of the chain of huarangal and quilmará volcanoes, formed at the end of the mesozoic and beginning of the cenozoic age (80 million years ago) and currently all are inactive (hakl et al., 1992). the alluvial cones were the result of the accumulation of debris formed by blocks, clasts and gravels of volcanicsedimentary origin with silty sandy matrix. in the central part of lima, where the rímac river flows, we find intrusive and metamorphic rocks, with little presence of plutonic rocks, predominating granodiorites that are found along the course of the huaycoloro river (tributary of the rímac river). from the upper part to the dejection cone, the rímac river and its tributaries have eroded shale sedimentary rocks, paleogene-neogene volcanics, and middle cretaceous to upper and triassic-jurassic limestones. at lower elevations and farther to the west, intrusives emerge at the edges of the dejection cone of the rímac river, mostly of upper cretaceous intermediate composition. in the area of lima city, volcanic rocks of the middle cretaceous casma formation, compact sedimentary rocks of sandstones, shales, and lower cretaceous limestones emerge (palacios et al., 1992). unlike the main parent material deposited by the chillón river, most of the material deposited in the alluvial fan of the rímac river generally has low uranium content. in the south, the lurín river, with similar characteristics to the rímac river, transports meteorite material from the hills to the lurín valley (farmland) until it reaches the sea (see fig. 1). lima city has an arid climate, with deficiency of rainfall throughout the year, maintains a semi-warm temperature and a humidity of 64% to 75%. because it usually does not rain and the atmospheric pressure practically does not change, the observed seasonal changes are possibly due to changes in temperature. 173measurement of radon in soils of lima city peru during the period 2016-2017 radon measurement method a hole of about 1.8 m deep was drilled at each site where radon will be monitored. this depth ensures the measurement of the equilibrium activity concentration, a determining factor of the grp. the hole was sheathed with a pvc tube approximately 2 m long. the tube was inserted inside the hole and dropped to its bottom. the space left between the tube and the subsoil is filled with part of the extracted soil material. an integrating track detector, cellulose nitrate lr-115 type 2, was fixed at a distance of 20 cm from the bottom of the hole staying its sensitive side facing the soil radon emanating surface. in this configuration, the lr-115 detector was exposed just to the volume of gas within 20 cm under it, registering only the alpha particles emitted from the effective volumes of each airborne radionuclide. the effective volume of 222rn is located approximately between 1 3 cm from detector surface, but approximately between 3 6 cm and 6 8 cm for the most energetic radon and thoron progenies, respectively (rojas et al., 2018). fig. 2 shows details of the installation of pvc tube in the hole and the sketch of the monitoring system. during the exposure period of detector, a cap screwed tightly on top of the tube housing isolates it from the outside. bore-holes closed from environmental air exchange do not allow the soil gas to flow freely into the atmosphere, therefore, the rate of diffusion controls the radon migration process (shweikani & hushari, 2005; papp et al., 2008). the detector assembly enclosed in a pvc pipe housing reduces the influence of environmental factors such as temperature, air pressure, wind and moisture. using similar procedures, other authors have considered the method as stable since the atmospheric parameters have little influence on the radon concentration inside the hole (ciotoli et al., 1998; king & minissale, 1994; papp et al., 2008). the distance of the lr-115 detector to the bottom of the hole causes negligible contribution of thoron gas to the track density because it decays before reaching the effective volume. the distribution of 220rn atoms exhaling uranium occurrences in peru are distributed according to different geological environments (magmatic-sedimentary) that do not obey exclusively to a geological time. in lima region, five prospects of uranium deposits of relatively low average grade (0.060) were identified, totalling 10,556 metric tons of u3o8 (rivera, 2010). they are located in the area of volcanic rocks northeast of lima city (fig. 1), particularly in andesitic lavas of quilmaná volcanic (upper cretaceous) that emerge in the chillón river (palacios et al., 1992; perales, 1994). in this area, which has the casma group as the main host rock, deposits of massive volcanogenic sulphides of pb-zn-cu were formed (chira et al., 2018), confirming the behavior of these elements as pathfinders associated with uranium (kurt et al., 2009). the quilmaná and huarangal formations present in the chillón and huaycoloro river basins are part of the casma group, which consists mainly of volcanic materials including rocky fragments of shales and sandstones, and andesitic lavas (villacorta et al., 2015). previous analysis indicates that erosion materials deposited in the dejection cone of chillón and rímac rivers are associated to rock formations with different uranium contents. due to the natural phenomenon “el niño costero”, intense anomalous rainfall caused multiple phenomena of mass movements (debris and sludge flows, landslides, rock falls, river erosion of hillsides, etc.) and floods (huaycos), which affected a large part of lima city in summer of 2017. the erosion of slopes in the outcrops of volcanic and intrusive rocks, very fractured and weathered, provided material for transport and subsequent deposition. the most important forces and phenomena related to the physical processes of huaycos might be gravitational acceleration down slopes, the resistance experienced by the flow and the processes of erosion and deposition of particles, by which the flow exchanges mass with the underlying boundary. however, fundamental understanding of the mechanisms of particle erosion from an underlying substrate by a huayco is lacking (hogg, 2018). figure 1. map of lima city quadrangle showing its lithology (palacios et al., 1992), the main rivers that formed the alluvial fan deposits on which lima city is situated, and spatial distribution of the 20 measurement points locations (holes) in the 15 districts. 174 lázaro luís vilcapoma, maría elena lópez herrera, patrizia pereyra, daniel francisco palacios, bertin pérez, jhonny rojas, laszlo sajo-bohus spatial distribution of measurement sites radon measurements in soil pores were carried out in places near and far to chillón, rímac and lurín rivers. the selection of the observation sites was conceived in order to cover all districts of lima city. to get an idea of the potential contribution of soil to indoor radon, 20 measurement sites were distributed in the 15 districts of lima city. detectors located inside the pvctubes were coded from s1 to s20. table 1 shows the coding of each one and the distances of each hole to the nearest riverbank. the minimum and maximum distances between a hole and the riverbank were 0.4 km and 9.4 km, respectively. the holes were drilled in ground of residential houses to ensure the ease and safety of access for the recovery and periodic change of detectors. figure 1 also shows the locations of the measurement sites (tip of the arrows). previsions were taken so that measurement sites were not affected by rains. exposure time and detector processing all detectors were exposed simultaneously during periods of 14 days in the pvc tubes installed permanently in soil. data collection was conducted over 36 consecutive periods in the 20 holes from april 2016 to september 2017 (table 2). after exposure, detectors were chemically etched in 2.5 n naoh solution at 60°c during 90 minutes (fleischer et al., 1975; salama et al., 2006; mehta et al., 2014; pereyra et al., 2015). etched detectors were analyzed in an automatic reading system (polytrack) to determine the net track density. the calibration factor that converts the track density rate to time-averaged radon concentration was determined in a radon chamber. environmental conditions within the chamber were similar to those existing in measurement holes. reference radon concentration in the chamber was continuously measured by means of the alphaguard df 2000 monitor, traceable to the primary rn standard in physikalischtechnische bundesanstalt (ptb), braunchweig, germany. from soil surface decreases exponentially with the distance to the bottom of the hole, in a manner analogous to what happens in front of a wall (godoya et al., 2002; hadler & paulo, 1994; chauhan et al., 2017) or inside a diffusion chamber (urosevic et al., 2008; nikezic & stevanovic, 2007). since the main mechanism of gas transport within the pvc tube is diffusion, very few thoron atoms can reach 20 cm from the source because their diffusion length is approximately 2-3 cm for the commonly used air diffusion coefficient values. for example, assuming a diffusion coefficient in air of 1.2·10-5 m2/s, the diffusion lengths (l d=  ) for 220rn and 222rn would be 3.1 cm and 239.2 cm, respectively (huxol et al., 2012). thus, at a distance (d) of 20 cm from a surface only 0.16% of the exhaled 220rn atoms would arrive ( n n d l0 = −    exp ), while 92% of the exhaled 222rn atoms could reach the same distance (bonotto & andrews, 1999). the first 220rn progeny (216po) behaves as its progenitor due to its very short half-life. the rest of the 220rn progeny, and practically all 222rn daughters, are almost completely deposited on all internal surfaces within the pvc tube due to the plate-out effect (hadler & paulo, 1994; paulo et al., 2001; nikezic & stevanovic, 2007) such as the bottom hole, inner surfaces of the pvc wall and lid, and surfaces of detector support. in short, due to very short half-life (t1/2=55.6 s), 220rn atoms cannot be homogenously distributed into the pvc tube and decreases exponentially with the distance from the bottom of the hole in opposite to the 222rn atoms (t1/2=3.82 d) (urosevic et al., 2008). previous considerations seem to indicate that radon is the main contributor to track density in the lr-115 detector, so in this study no filtering membrane was used at the entrance of the radon cup. kumar et al. (2017) used the same principle for soil radon measurement and discrimination of radon and thoron concentrations in soil gases. furthermore, some negative aspects of the filtering membranes, such as large variation of their gas permeability with temperature and reduction of detector sensitivity to radon (fleischer et al., 2000; tommasino, 2016), supported the decision of not using them. in summary, using the described setup the measurement of 222rn is practically not influenced by the 220rn soil gas concentration. figure 2. installation of a measuring point in san juan de lurigancho district (left), and schematic view of the hole with closed pvc tube and the radon measurement system (right). 175measurement of radon in soils of lima city peru during the period 2016-2017 results and discussion comments on results of soil gas radon concentrations and on the measurement methodology the minimum and maximum values of soil gas radon concentration in the study area were 0.1 kbq/m3 and 64.3 kbq/m3, respectively, which show great variability of the data. the average radon concentration ranges from 1.5 kbq/m3 in winter to 11.9 kbq/m3 in spring. the average concentration of rn in the 20 holes during the entire observation time was 5.6 kbq/m3. there is no history of rn measurements in soil gases in lima city. the obtained results are compatible with those reported in similar studies, both using active (choubey et al., 2005; szabó et al., 2013; duggal et al., 2014; cinelli et al., 2015) and passive methods (kareem, 2016). they are also similar to those found in zones of fluvial deposits and poorly developed soils whose associated lithologies correspond to plutonic rocks, volcanic magmatic rocks and differentiated magmatic rocks (alonso et al., 2019). nevertheless, the average radon concentration of 5.6 kbq/m3 is quite low compared to that reported by cothern & smith (1987). the relatively low value could be table 1. codes of measurement sites and distances of each detector hole to the nearest riverbank. site code district latitude longitude nearest river distance (km) s1 chorrillos -12.17 -77.02 surco 0.73 s2 huaycan -12.02 -76.82 rímac 3.00 s3 santa eulalia -11.90 -76.66 santa eulalia 0.17 s4 cuspanca (chosica) -11.89 -76.65 santa eulalia 0.06 s5 villa el salvador -12.24 -76.92 lurín 2.81 s6 villa maría del triunfo (1) -12.19 -76.94 lurín 7.48 s7 villa maría del triunfo (2) -12.15 -76.93 lurín 8.72 s8 san juan de lurigancho (3) -11.94 -76.99 rímac 9.40 s9 san juan de lurigancho (1) -11.97 -77.00 rímac 6.13 s10 san juan de lurigancho (2) -12.01 -77.01 rímac 1.90 s11 chaclacayo -11.98 -76.80 rímac 0.40 s121 ate -12.04 -76.93 rímac 2.06 s13 manchay -12.10 -76.88 lurín 7.06 s14 independencia -11.98 -77.04 chillón 6.84 s15 carabayllo -11.86 -77.06 chillón 2.67 s16 puente piedra -11.93 -77.10 chillón 1.56 s172 san martín de porres -11.99 -77.09 rímac 4.45 s18 pueblo libre -12.07 -77.07 rímac 4.47 s19 jesús maría -12.09 -77.06 pacific coast 2.44 s203 miraflores -12.11 -77.04 surco 4.74 1also at 3.9 km of confluence of rivers huaycoloro and rímac 2also at 5.34 km from chillón river 3also at 1.00 km from pacific ocean coast table 2. codes of measurement periods. code from/to/year code from/to/year code from/to/year p1 ap. 17/may 01/2016 p13 oct. 2/oct. 16/2016 p25 mar. 19/ap. 2/2017 p2 may 1/may 15/2016 p14 oct. 16/oct. 30/2016 p26 ap. 2/ap.16/2017 p3 may 15/may 29/2016 p15 oct. 30/nov. 13/2016 p27 ap. 16/ap. 30/2017 p4 may 29/jun. 12/2016 p16 nov. 13/nov. 27/2016 p28 ap. 30/may 14/2017 p5 jun. 12/jun. 26/2016 p17 nov. 27/dec. 11/2016 p29 may 14/may 28/2017 p6 jun. 26/jul. 10/2016 p18 dec. 11/dec. 25/2016 p30 may 28/jun. 11/2017 p7 jul. 10/jul. 24/2016 p19 dec. 25/2016/jan. 8/2017 p31 jun. 11/jun. 25/2017 p8 jul. 24/aug. 7/2016 p20 jan. 8/jan. 22/2017 p32 jun. 25/jul. 9/2017 p9 aug. 7/aug. 21/2016 p21 jan. 22/feb. 3/2017 p33 jul. 9/jul. 23/2017 p10 aug. 21/sep. 4/2016 p22 feb. 3/feb. 19/2017 p34 jul. 23/aug. 6/2017 p11 sep. 4/sep. 18/2016 p23 feb. 19/mar. 5/2017 p35 aug. 6/aug. 20/2017 p12 sep. 18/oct. 2/2016 p24 mar. 5/mar. 19/2017 p36 aug. 20/sep. 3/2017 176 lázaro luís vilcapoma, maría elena lópez herrera, patrizia pereyra, daniel francisco palacios, bertin pérez, jhonny rojas, laszlo sajo-bohus explained considering that most measurement sites meet one or more of the following points: 1. disequilibrium between radium and uranium due to leaching action (valid for all alluvial surfaces), 2. although the permeability in alluvial soils is fairly high, low emissions of radon can occur in deposits of parent material of volcanic origin due to the low specific surface area of the uranium minerals, 3. generally fairly low uranium concentration in the parent material of plutonic origin, 4. very low bedrock, 5. low moisture content of the soil most of the time. one might also think that the relatively low value is because measurements made with passive nuclear track detectors do not represent the exact radon concentration in soil gas. however, szabó et al. (2013) found even lower values of average radon concentrations in soils with similar characteristics to the alluvial soils of lima city, and measurements were made by an active method. furthermore, the relative low average radon concentration in soil pores is in agreement with the average radon concentration in dwellings of lima city (83 ± 11 bq/ m3), also relatively low (pereyra et al., 2018). the minimum value, (11 ± 6) bq/m3, was found in coastal districts away from rivers, while the maximum, (407 ± 55) bq/m3, was in the rímac district located to the northeast of lima province, at the right margin of the rímac river, in an area formed predominantly by andesitic volcanic rocks interbedded with sandstones and siltstones. the obtained maximum value (64.3 kbq/m3) is much higher than the median of made ground, typical of urban areas, even higher than the median of undisturbed soil according to values reported by wiegand (2001). elío et al., (2015) found lower values of radon by passive etched track detectors than those obtained using an active monitor, but, unlike the conditions in our study area, radon measurements were carried out for monitoring geological storage of co2, i.e., detection of co2 leakages, where co2 acts as carrier gas. other authors have used active and passive systems to measure soil gas radon and radon emanation factor, and results were in general agreement with each other (giammanco et al., 2009, lópez-coto et al., 2009). the values measured by different methods depend, among others, on the physical characteristics of soils. for example, ruckerbauer & winkler (2001) found no significant differences for active and passive procedures in a porous soil. nevertheless, passive procedures yield soil radon concentrations significantly lower than obtained with the active procedure in an impermeable clay soil. it is important to note that soils of the study area in lima city are mostly porous and permeable. a more detailed discussion on the use of active and passive methods for radon measurements in soil gas can be found in palacios et al., (2016). time series of soil gas radon concentrations relationships whit seasons and some meteorological parameters time series of the average soil radon concentrations obtained biweekly from the twenty positions are shown in fig. 3. medium-term (the measurement periods) and seasonal changes in rn activity concentration in borehole air were observed. apparently, seasonal changes in radon activity concentration in borehole air were associated with atmospheric temperature changes. winter is characterized by approximately twice less average soil gas radon concentration than the average for the other seasons. during the total period of measurements the atmospheric pressure remained almost constant, and rainfall occurred only during the “el niño costero” event. descriptive statistics of radon concentration and some meteorological parameters is presented in table 3. meteorological parameters represent average values of data reported by nine meteorological stations distributed nearly uniformly in the study area. radon levels in soil gas and atmospheric pressure were the variables that showed the highest and lowest variability, respectively. variation of meteorological parameters (normalized to their maximum values) is shown in fig. 4. table 3. descriptive statistics of radon concentration and meteorological variables. variable mean sd min max cov(%) rn concentration (kbq/m3) 5.6 2.9 1.5 11.9 51.8 humidity (%) 76.4 5.1 66.6 83.1 6.7 temperature (°c) 19.8 3.3 15.9 25.9 16.7 pressure (mb) 985.4 1.3 983.1 987.0 0.1 dispersion graphs and the moderate correlation coefficients show the low relation between radon and meteorological variables (fig. 5). from fig. 5 it is clear that the measured radon concentration shows low negative, positive, and negative correlations with relative humidity, temperature, and atmospheric pressure, respectively. these results support 13 12 11 10 9 8 7 6 5 4 3 2 1 0 r n (k b q/ m 3 ) observacion period p1 p2 p3 p4 p5 p6 p7 p8 p9 p1 0 p1 1 p1 2 p1 3 p1 4 p1 5 p1 6 p1 7 p1 8 p1 9 p2 0 p2 1 p2 2 p2 3 p2 4 p2 5 p2 6 p2 7 p2 8 p2 9 p3 0 p3 1 p3 2 p3 3 p3 4 p3 5 p3 6 figure 3. average rn concentration per observation period in the 20 wells. the horizontal solid line represents the radon concentration value averaged over the entire measurement period and the dotted lines represent the deviation (s) from the average value. 177measurement of radon in soils of lima city peru during the period 2016-2017 that the variation of soil gas radon concentration in time is only weakly influenced by the meteorological conditions at the surface. however, the level of significance (p) was less than 0.05 in all cases, indicating statistically significant correlations. pca analysis of the data was performed to investigate the variability, both seasonal and throughout the study period, of radon concentration, relative humidity, temperature and barometric pressure. the results are summarized in table 4. in most cases the variability of the four parameters showed differences between different seasons of the same year and between the same seasons of different years. from the higher variability of radon concentration compared to the variability of humidity, temperature, and barometric pressure it can be concluded that there should be other agents governing the radon concentration variation besides the analyzed meteorological variables. 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5n or m al iz ed t em pe ra tu re , h um id ity a nd p re ss ur e observation period p1 p2 p3 p4 p5 p6 p7 p8 p9 p1 0 p1 1 p1 2 p1 3 p1 4 p1 5 p1 6 p1 7 p1 8 p1 9 p2 0 p2 1 p2 2 p2 3 p2 4 p2 5 p2 6 p2 7 p2 8 p2 9 p3 0 p3 1 p3 2 p3 3 p3 4 p3 5 p3 6 figure 4. plot showing the temporal variation of average values of pressure, humidity and temperature. table 4. results of the principal component analysis. time interval season percentage of variance (%) radon concentration humidity temperature barometric pressure p1 p5 autumn 87.3 11.3 1.4 0.1 p6 p12 winter 42.6 28.1 20.6 8.7 p13 p18 spring 73.9 20.1 5.9 0.1 p19 p24 summer 66.2 21.9 10.6 1.4 p25 p31 autumn 87.5 7.9 4.1 0.5 p32 p36 winter 58.7 33.8 7.2 0.3 p1 p36 all periods 76.0 20.0 2.8 1.2 14 12 10 8 6 4 2 0 -2 -4 14 12 10 8 6 4 2 0 -2 -4 10 15 20 25 3060 66 72 78 84 90 r = 0.35 p = 0.031 r = -0.36 p = 0.027 temperature (˚c) r n (k b k/ m 3 ) r n (k b k/ m 3 ) humidity (%) 14 12 10 8 6 4 2 0 -2 -4 982 984 986 988 990 r n (k b k/ m 3 ) pressure (mb) r = -0.38 p = 0.023 figure 5. dispersion graphs, pearson correlations (r) and p-values (p) for soil gas radon and meteorological variables. confidence level for ellipses is 95%. 178 lázaro luís vilcapoma, maría elena lópez herrera, patrizia pereyra, daniel francisco palacios, bertin pérez, jhonny rojas, laszlo sajo-bohus other factors having influenced the temporal variation of radon concentration in soil pores closeness to volcanic formations as can be seen in fig. 3, the average radon concentration in the 20 holes has high values in the first two measurement periods. the greatest contribution to these high values came from the hole located in carabayllo district (s15). in those two periods, two of the highest concentrations measured throughout the study (53.1 kbq/m3 and 21.1 kbq/m3) were found in that hole. the s15 hole is near the chillón river and is also the closest measurement site to the uraniumrich volcanic rocks of quilmaná and huarangal formations. no seismic activity or particular meteorological conditions were observed that could explain the high average values found during those periods. occurrence of earthquakes the high values observed in october and november 2016 (p13 – p16 in fig. 3) could be associated with the earthquakes occurred in lima during those months. in addition to the large number of reports on radon anomalies prior to earthquakes, anomalies have also been registered during their occurrence (petraki et al., 2015). a possible cause of the apparent simultaneity could be related to the time resolution of the radon measurement system. the time resolution of active systems, such as continuous radon monitors, can be as small as 10 minutes. however, passive systems, such as nuclear track detectors, integrate radon concentrations over long time intervals (at least > 1 4 weeks), i.e., they provide coarse time-series estimations. for example, as an illustration, suppose that the empirical equation of sultankhodzhayev et al. (1980), which relates precursor time with earthquake magnitude and epicentral distance, is fulfilled as a first approximation. thereby, for earthquakes of magnitudes 4 and 5, occurring at epicentral distances between 50 and 300 km, the maximum precursor times would be approximately between 1 and 28 days. most of the seismic events perceived in lima city during the study had magnitudes between four and five, and their epicenters were at distances from approximately 50 to 300 km from lima city. since soil gas radon concentrations are integrated in periods of about two weeks, anomalies could be recorded in periods prior to the earthquake or in the period in which the event occurred. as can be seen in fig. 6a, the number of earthquakes perceived in lima city is greater during two consecutive periods in october-november 2016 (seismic bulletins, 2017), and in the period from the end of june to the beginning of july 2017. however, the magnitudes of the earthquakes of the last period were smaller, being much greater the energy released during the october earthquakes (fig. 6b). the two earthquakes perceived most intensely in lima city during the period of measurements (according to the modified mercalli scale) occurred precisely in 2016, on october 19 and 28 (fig. 7). a more detailed study on the association of seismic events with radon in soil gases is planned to be carried out in the near future. rock and debris avalanches due to flooding of rivers during the “el niño costero” phenomenon a significant peak in radon concentration was also observed during the exposure period of detectors from march 5 to 19, 2017 (p24 in fig. 3). this could be associated with the natural phenomenon “el niño costero” which greatly affected several areas in lima city. heavy rains during this period triggered flows of detritus, rock falls and landslides. they also caused overflows and floods of the chillón, rímac and lurín rivers, where large avalanches of rocks, debris and mud were generated. the increased flow in these rivers caused the dragging of rocks and debris. rock motions, debris flows, rock and debris avalanches, rocks impacting on the ground, and rocks impacting or rolling on the channel bed generate ground vibrations, which could cause the increase of radon emanation in holes located near rivers and creeks. our assumption is supported by studies on the influence of vibrations on emanation and exhalation of soil gas radon, and on ground vibrations produced by rock motions and debris flows. moharram (2000) found increased radon concentration in soil pores and enhanced radon exhalation in areas nearby railway and motor vehicle traffic tracks. the author considers the increased delivery of radon from the subsoil due to the enhanced upward diffusion. cai et al. (2018) and cai et al. (2019) studied the influence of low-frequency vibrations on the radon exhalation from uranium samples. the results showed that the radon exhalation increased with the rising vibration frequency until frequencies of about 50 to 70 hz, but fell slowly after reaching the peak. studying ground vibrations produced by rock motions and debris flows, huang et al. (2007) found that ground vibrations caused by individual rocks were 10-150 hz and that larger stones generate ground vibrations with lower peak frequencies. the frequency of the debris flows is lower than 50 hz, but after the front passed, the frequency is between 50 and 100 hz. in these frequency ranges there is an increase in radon emanation and exhalation according to cai et al. (2018) and cai et al. (2019). these authors confirmed that one of the main sources of ground vibration caused by debris flows is the interaction of rocks or boulders with the channel bed. the following facts also support our assumption about the cause of the increased radon concentration in soil gas during the “el niño costero” event: 1all the holes had the same depth; 2detectors were placed at the same distance from the bottom of the hole; 3in no hole there was water infiltration or water vapor condensation on detector surface that could affect the measurements; 4in each measurement period all stations experienced approximately the same meteorological conditions since the measurements were carried out simultaneously; 5overflowing of natural channels of rivers during floods did not affect any measurement site; 6the lowermost pvc tube was never submerged because of rising groundwater levels; the groundwater level in the measurement sites is very deep (inrena, 2004) and its possible fluctuations should have affected little the soil radon emanation; 7if below the bottom of the hole the soil water content would have been very high, contrary to what happened, the radon concentration should have decreased 6 5 4 3 2 1 0 n um be r o f e ar th qu ak es en er gy re le as ed (t on s o f tn t) p1 p2 p3 p4 p5 p6 p7 p8 p9 p1 0 p1 1 p1 2 p1 3 p1 4 p1 5 p1 6 p1 7 p1 8 p1 9 p2 0 p2 1 p2 2 p2 3 p2 4 p2 5 p2 6 p2 7 p2 8 p2 9 p3 0 p3 1 p3 2 p3 3 p3 4 p3 5 p3 6 period of observation period of observation p1 p2 p3 p4 p5 p6 p7 p8 p9 p1 0 p1 1 p1 2 p1 3 p1 4 p1 5 p1 6 p1 7 p1 8 p1 9 p2 0 p2 1 p2 2 p2 3 p2 4 p2 5 p2 6 p2 7 p2 8 p2 9 p3 0 p3 1 p3 2 p3 3 p3 4 p3 5 p3 6 30.000 25.000 20.000 15.000 10.000 5.000 0 figure 6. number of earthquakes felt in the lima city (left) and energy released by earthquakes (right) during the period of measurements. 179measurement of radon in soils of lima city peru during the period 2016-2017 considerably; 8during the period p24 there was only one earthquake on 03/13/2017 at local time 10:46:00, latitude -12.63 and longitude -76.97, and 53 km depth (seismic bulletins, 2017). although its magnitude was 4.5, it was perceived in lima city with the lowest intensity an earthquake could be reported (ii level) according to the modified mercalli scale. combined spatial and temporal behavior of soil gas radon concentrations radon concentration in different holes during the period of maximum flow of rives due to phenomenon “el niño costero” to help interpret the results, in particular regarding the effect of floods on the increase of radon emanation from the base of the tube holes, the average radon concentration in each of the 20 holes is shown in fig. 7 for three cases: 1. during the entire period of measurements (epm), 2. during the entire period of measurements except periods of the “el niño costero” event (epm-pec), and 3. during the period of maximum flood of chillón, rímac and surco rivers (mfr). as described, lima city is situated on alluvial fan deposits of the chillón and rímac rivers. alluvial soils of the study area are mainly differentiated by deposited parent material, which is associated with the fluvial erosion of geological formations that contain different uranium content. consequently, figure 7 should provide information on the spatial distribution of deposited parent material, given by the spatial radon distribution, and on the temporal variation of radon in the most important considered periods. radon concentrations were generally higher than the total average concentration in most holes near rivers (s1, s10, s11, s12, s15, s16) regardless of the period of time in which they were averaged. in contrast, radon concentrations were below the total average concentration in most holes located far from rivers and they were approximately the same for the different averaging periods. table 5 and the scatter plot in fig. 8 show a significant negative correlation between soil radon concentration and distance from the hole to the nearest river bank for the different averaging periods, however, the correlation and gradient were higher for the period of maximum flow of rivers. for analysis, the 2-tailed test of significance was used. as can be seen in figure 7, radon concentrations averaged during epm and during epm-pec were very similar in most of the holes. there figure 7. isoseismal maps of the two most intense earthquakes perceived in lima city during soil gas radon measurements (hernando et al., 2016). 30 25 20 15 10 5 0 r n (k b q/ m 3 ) observation well s1 s2 s3 s4 s5 s6 s7 s8 s9 s1 0 s1 1 s1 2 s1 3 s1 4 s1 5 s1 6 s1 7 s1 8 s1 9 s2 0 1 2 3 figure 8. average radon concentration in each hole during the entire period of measurements. (1). during the entire period of measurements except periods of “el niño costero” event. (2). and during the period of maximum flow of chillón and rímac rivers (3) (bars with red inclined lines correspond to holes where very large increases in radon concentration occurred). horizontal and dotted lines represent the same as in figure 3. table 5. pearson and spearman correlation coefficients between soil gas radon concentration and distance from the measurement site (hole) to the nearest river bank for the different averaging periods. pearson corr. p-value spearman corr. p-value mfr -0,67* 0,0049 -0,76* 0.0007 epm -0.60* 0.010 -0.61 0.0089 epm-pec -0.55* 0.0213 -0.61 0.0089 *correlation is significant at the 0.05 level 180 lázaro luís vilcapoma, maría elena lópez herrera, patrizia pereyra, daniel francisco palacios, bertin pérez, jhonny rojas, laszlo sajo-bohus was a large increase in radon concentration in the mfr period in most holes where radon concentrations averaged during the epm were similar or greater than the total average concentration; those holes were located relatively close to rivers. hence, the largest source of temporal variation of radon in holes located relatively close to chillón and rímac rivers was associated with the mfr period, where large floods occurred with avalanches of rocks, debris and mud. the hole located near the surco river in chorrillos district (s1), the holes located along the channel of the rímac river in san juan de lurigancho (s10), chaclacayo (s11) and ate (s12) districts, and the holes situated near the chillón river in puente piedra (s16) and san martin de porres (s17) districts, showed large increase in radon concentration during the period of maximum flow (higher flood) of these rivers. although the s17 hole was relatively far from rivers, it is located in an area historically affected by the overflow and flooding of the chillón river. rocks and debris transported during the period of maximum flow of the chillón river reached an area close to this measurement site. the hole located in independencia district (s14) showed relatively high radon concentration during the epm, but the increase in radon concentration was not as great as in other holes during the mfr period. this hole was located far from rivers, however, the nearby steep and short distance creeks were activated during the period of higher intensity of rainfall, producing debris flows accumulated on their slopes and beds. possibly, the intensity and frequency of resulting vibrations were different from those produced by the large avalanches of rocks and debris occurred during the floods of chillón and rímac rivers; this could be the cause of the lower increase in radon concentration in the hole. radon concentration in each hole averaged over all measurement periods results of radon concentration averaged over the entire study period could be related to the parent material of alluvial soils in the monitoring sites. fig. 7 shows that the highest average radon concentration was registered in ate hole. this may be related to the origin of alluvial deposits in the area around the hole, which are most likely associated with the material transported during the floods of the huaycoloro river in the pleistocene era (about 2.5 million years ago). this river has eroded in its course volcanic rocks rich in 238u (ingemmet, 1999; sebrier & macharé, 1980). the high average values of radon emanation found in carabayllo (s15) and san martín de porres (s17) holes, are due to the presence of alluvial deposits of pleistocene and recent (less than 0.13 million years ago), corresponding to the floods of chillón river that eroded the chain of volcanic hills located on its slopes. soils around these holes are shallow and they have developed from hard, coherent and consolidated rock. the lithological composition of these soils is variable such as quartzites, sandstones, limestones, shales and volcanic rock; some of them commonly present high 226ra content. differently, the floods of rímac river have contributed less in the deposition of materials with relatively high content of radon parents, since the predominant geological formation along its channel and slopes has lower uranium content compared to the casma group (where the chillón and huaycoloro rivers flow). the inferred permeability of the medium confirms the direct association between the measured radon levels and the origin of the alluvial deposits. most of the area of lima city is covered by an alluvial gravel deposit which is composed of a medium to dense coarse gravel with a sandy matrix. these materials are mostly from the alluvial fan of the rímac river. in such permeable deposits radon moves readily and could reach the measurement site from relatively large distances. in these circumstances, high radon levels would be generated at the measurement site if the emanation power of the medium is high. since the measured radon concentrations were relatively low, the low emanation power of materials deposited in the alluvial fan of the rímac river is confirmed. however, in the northern districts of puente piedra, comas and carabayllo, and in the eastern districts of la molina and ate vitarte, there 40 30 20 10 0 -10 -20 40 30 20 10 0 -10 -20 r n (k b k/ m 3 ) -8 -4 -2 0 2 4 6 8 10 12 14-8 -4 -2 0 2 4 6 8 10 12 14 distance to the river (km)distance to the river (km) r n (k b k/ m 3 ) 40 30 20 10 0 -10 -20 r n (k b k/ m 3 ) -8 -4 -2 0 2 4 6 8 10 12 14 distance to the river (km) a b c figure 9. scatter plot of soil gas radon concentration averaged during the epm (a), epm-pec (b), and mfr (c) periods vs. distance from the hole to the nearest riverbank. the confidence level for ellipse was 95%. 181measurement of radon in soils of lima city peru during the period 2016-2017 are silty sand deposits. in these less permeable deposits, the measured radon comes from shorter distances, thus providing more local information about the emanation power of the medium. since the highest levels of radon were found in these sites, the high emanation power of materials deposited by the chillón and huaycoloro rivers is confirmed. semkov (1990) and sutherland (1994) have also suggested correlation between specific surface area and emanating powers of radon. thus, particle-size fractions due to fracturing and crushing have possibly played a major role in increasing the radon emanation at the measurement sites of districts indicated above. a significant contribution to the radon concentration was found when volcanic materials were present at the measurement sites. these materials have been transported and deposited by the chillón and rímac rivers and come from the disaggregation by meteorization and erosion of the casma group’s volcanic rocks. however, measurements made directly on slope of hills of volcanic (s2) or plutonic origin (s8, s9, s13), yielded the lowest values of radon concentration. in these places there is no means of transport and thus erosion is slower compared to sites located in the riverbeds. according to ennemoser et al. (1993), the emanation rate of radon from soil depends not only on the concentration of uranium and radium content but to some extent on emanating power and the diffusion coefficient of radon in soil. the alluvial deposits, compared to the rocky areas on the hill slopes, has an increased emanating power and diffusion coefficient due to intense fracturing, and crushing down to the grain and sub-grain size, resulting in an increased surface area and porosity (purtschller et al., 1995). radon measurements in the studied area have shown that the highest levels of radon are in accordance with the type of lithological material present in riverbeds and with its disintegration, erosion, transport and distribution by the action and flooding of rivers. the grain-size reduction due to fracturing and further weathering of transported parent materials expose the uraniumrich mineral thus giving rise to high values of radon concentration in the zones of higher accumulation. floods of rivers associated with “el niño costero” have been repeated over thousands of years and have great responsibility in the geomorphology of the current lima. thus, the results of this study provide information not only on the effects of the event on soil gas radon during the period of measurements, but also help in understanding the causes of differentiated geogenic radon potential in the study area. conclusions 1. total average soil gas radon concentration (spatial and temporal) was 5.6 kbq/m3, while the minimum and maximum values for the whole set of measurements were 0.1 kbq/m3 and 64.3 kbq/m3, respectively. these results are included within values reported in similar studies. 2. total average value of radon concentration in alluvial soil of lima city is in correspondence with the relatively low radon values reported in dwellings. however, more studies are required to obtain statistical evaluation of spatially geogenic radon potential with indoor radon. 3. the average radon concentration in soil gas is about two times lower in winter than in the other seasons. although there is a moderate relation between soil radon concentration and meteorological variables, it was shown that other agents controlled the radon concentration variation. 4. high values of soil gas radon concentration during october/november 2017 apparently were related to the earthquakes perceived in lima city in that period. 5. the highest radon concentrations were found in areas of alluvial deposits whose parental material has been removed from the volcanic quilmaná and huarangal by the chillón and huaycoloro rivers flowing through them. soil gas radon concentrations were even higher in areas closer to volcanic and less distant from rivers. 6. in general, the lowest average concentrations of radon in soil gas were found in areas far from rivers, on hills of volcanic origin, and in geological formations where intrusive rocks predominate or in alluvial fans of rivers flowing through them (rímac and lurín rivers). 7. anomalous high soil gas radon concentrations were found during the period of maximum flooding of rivers due to “el niño costero” phenomenon. they were associated with ground vibrations caused by rock and debris avalanches in rivers and creeks. 8. the study of radon emanation in pores of alluvial soils allowed to identify the sources of deposited materials. thus, the application of this methodology could in principle help in the characterization of geodynamic events in geological and geomorphological contexts similar to that of our study area. 9. the identification and distribution of the parent material in alluvial deposits of lima city, and the influence on them of the historical occurrence of “el niño costero”, help to interpret the geogenic radon hazard variation defined by radon indoors studies. acknowledgements this study was carried out as part of the “measurement of radon-222 in soils, using nuclear track detectors” project (cap 339-2016-pucp). references åkerblom, g., andersson, p. & clevensjö, b. 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(2012). differentiation between the effect of temperature and pressure on radon within the subsurface geological media. radiation measurements, 49, 39-56. zmazek, b., zivcic, m., todorovski, l., dzeroski, s., vaupotic, j. & kobal i. (2005). radon in soil gas: how to identify anomalies caused by earthquakes, applied geochemistry, 20, 1106-1119. http://www.ingemmet.gob.pe/documents/73138/202784/p11_expo_uranio.pdf http://www.ingemmet.gob.pe/documents/73138/202784/p11_expo_uranio.pdf keywords: three-dimensional modelling; key algorithms; geological bodies; virtual display. palabras clave: modelado tridimensional; algoritmos clave; cuerpos geológicos; pantalla virtual. how to cite item feng, w. (2019). virtual display system of geological body based on key algorithms of three-dimensional modelling. earth sciences research journal, 23(3), 185-189. doi: https://doi.org/10.15446/esrj.v23n3.81809 to study the virtual display system of geological body, the key algorithm of three-dimensional modelling is used to analyze it, and the virtual display scheme of geological body is realized. three-dimensional modelling of geological bodies is a frontier field of current geoscience research, and also a hot issue in computer science. its research can complement and improve the theoretical basis of virtual reality technology and expand its application fields. geological body three-dimensional modelling software can meet the needs of geoscientists to study geological problems more effectively from three-dimensional space, and has considerable practical value. in the network environment, users' information needs are diversified and individualized, and special attention is paid to the breadth, novelty and timeliness of information. faced with massive information resources, the information users need is a drop in the ocean in the huge and disorderly network information space. users need to spend a lot of time and energy to find valuable information in all kinds of uniform systems. traditional system information services can no longer help users effectively get rid of information overload and information lost. therefore, the concept of personalized service system has become a hot spot of people' s attention as soon as it is put forward. in recent years, personalized systems have emerged in endlessly. in the early stage of formulation and implementation of personalized service strategy, the existing personalized service generally lacks the attention to context information such as user' s environment. there are weak links in personalization and intellectualization of service, which damages user experience. therefore, shepard algorithm and kriging algorithm of three-dimensional modelling are used to study the virtual display system of geological bodies. the results show that these two key algorithms play a very important role in the research process of geological virtual display system, and also provide a new idea for further research on the popularization of geological knowledge. abstract virtual display system of geological body based on key algorithms of three-dimensional modelling sistema de visualización virtual del cuerpo geológico basado en algoritmos clave de modelado tridimensional issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.81809 para estudiar el sistema de visualización virtual de un cuerpo geológico se utiliza el algoritmo clave del modelado tridimensional, y se realiza el esquema de visualización virtual del cuerpo geológico. el modelado tridimensional de cuerpos geológicos es un campo fronterizo de la investigación actual en geociencias, y también un tema importante actualmente en la informática. su investigación puede complementar y mejorar las bases teóricas de la tecnología de realidad virtual y ampliar sus campos de aplicación. el software de modelado tridimensional del cuerpo geológico puede satisfacer las necesidades de los geocientíficos para estudiar problemas geológicos de manera más efectiva desde el espacio tridimensional, y tiene un valor práctico considerable. en el entorno de red, las necesidades de información de los usuarios están diversificadas e individualizadas, y se presta especial atención a la amplitud, novedad y puntualidad de la información. frente a los recursos de información masivos, la información que los usuarios necesitan es una gota en el océano, en el enorme y desordenado espacio de información de la red. los usuarios necesitan gastar mucho tiempo y energía para encontrar información valiosa en todo tipo de sistemas uniformes. los servicios tradicionales de información del sistema ya no pueden ayudar a los usuarios a deshacerse efectivamente de la sobrecarga de información y la información perdida. por lo tanto, el concepto de sistema de servicio personalizado se ha convertido en un punto de atención de las personas tan pronto como se presenta. en los últimos años, los sistemas personalizados han surgido en forma interminable. en la etapa inicial de formulación e implementación de la estrategia de servicio personalizado, el existente generalmente no presta atención a la información de contexto, como el entorno del usuario. existen vínculos débiles en la personalización e intelectualización del servicio, que dañan la experiencia del usuario. por lo tanto, el algoritmo de shepard y el algoritmo de kriging del modelado tridimensional se utilizan para estudiar el sistema de visualización virtual de cuerpos geológicos. los resultados muestran que estos dos algoritmos clave desempeñan un papel muy importante en el proceso de investigación del sistema de visualización virtual geológico, y también proporcionan una nueva idea para futuras investigaciones sobre la divulgación del conocimiento geológico. resumen record manuscript received: 20/05/2019 accepted for publication: 15/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 185-189 wei feng institute of mud logging technology and engineering, yangtze university, hubei jingzhou, 434023, china g eo lo g y 186 wei feng introduction china is a country with large geological resources. generally speaking, china has a well-developed stratum, diverse sedimentary types, complex geological structures, frequent coexistence of magmatic activities in active zones and stable areas, and diverse types of long-term geohistory metamorphism with different metamorphic degrees (sinitski, thompson, godsell, honey, & besemann, 2018). china is one of the regions with important geological characteristics in the world, and also one of the important areas to study the evolution of continental geological structures, especially the mesozoic and cenozoic crustal structures (naqvi, ali hassan, pervaiz, & ni, 2018). therefore, in order to better and more comprehensively understand the current situation of geological resources in china, many researchers use three-dimensional modelling algorithm and virtual display technology to study geology, which can not only help people to have more knowledge of geography, but also help researchers to study geological strata (handong, tuo, & changhong, 2006). this is better than simple data for researchers to have a deeper understanding and experience of geological formations. visualization of scientific computing emerged in the late 1980s (li, cai, & lu, 2018). it uses computer technology to convert the data of scientific or engineering calculation process and calculation results into graphics or images, and display these graphics or images on the screen for interactive processing theory, methods and techniques (wu, zhu, lin, zhuang, & ma, 2018). three-dimensional geological modelling technology is a branch of scientific computing visualization technology. it is the development and application of scientific computing visualization technology in the field of geosciences. therefore, in order to exploit and utilize the underground resources according to the specific national conditions of china, it is necessary to clarify the shape, attributes and spatial relationship of the underground geological structure, understand the basic structural and material characteristics of the geology, so as not to blindly exploit and utilize the underground resources and avoid unreasonable development and loss (liu et al., 2018). however, due to the late start of three-dimensional geological modelling technology in china, the lack of geological data, and the complexity, fuzziness and uncertainty of geological spatial distribution and structure, it is difficult to study threedimensional geological modelling and visualization (zhao, wang, & martinez, 2018). however, with the development of science and technology, some new methods and techniques have emerged in the exploration and acquisition of underground resources. people can obtain more detailed, comprehensive and abundant geological data through high-tech, which also provides material and data basis for the establishment of three-dimensional geological visualization model (buttussi & chittaro, 2018). on this basis, an intuitive, clear and complete three-dimensional geological visualization model is established, and the structure and characteristics of geological bodies are presented concisely and concisely by means of virtual images (zhu, lu, & rätsch, 2018). this model can not only describe the geological structure and characteristics comprehensively, but also relieve the pressure of staff members. at the same time, it can also provide certain theoretical basis for relevant staff to make reasonable exploration and development decisions. on the other hand, it is also conducive to the popularization of geological knowledge. methodology key algorithms for 3d geological modelling geological body is a term often used in geological work. it usually refers to the product of geological processes that occupy certain space in the earth’ s crust, especially the inherent components and can be distinguished from the surrounding materials. no matter how large or small the geological body is, whether it reaches a rock mass or an inclusion, it can be called a geological body. in geophysical prospecting, it refers to the object to be looked for when applying geophysical methods to study geological problems. therefore, geological body is often the geological factor causing anomalies. sometimes, it is common with anomalies. geological bodies occupy a certain space in the crust and have certain composition, which is a natural entity different from its surroundings. the study of geological bodies is of great significance in searching for various useful deposits and seismotectonic active belts. the data format and data structure of geological data play an important role in the three-dimensional geological body modelling. its organization mode determines the methods used in the three-dimensional geological modelling. therefore, understanding the data organization commonly used in the threedimensional geological modelling is of great value to the three-dimensional geological modelling. the geological data which are studied more at present is chosen as the research object, that is, borehole data. it provides data support for the algorithm research of three-dimensional geological modelling. diamond data is the most common type of data in three-dimensional geological modelling. borehole data is obtained by engineering drilling method. it can directly obtain detailed rock and soil distribution by drilling core. it is necessary to obtain borehole data, distribution data of boreholes and strata, and the description of the global bottom, which lays a theoretical foundation for the follow-up study of geological virtual display system. borehole data is very important in three-dimensional geological modelling because of its intuitive, accurate and detailed characteristics. the flow chart of three-dimensional geological modelling of borehole data is shown in figure 1. stratigraphic division is to determine how many strata there are in a geological body and the interface data of each stratum. assuming that there are five drilling points in a drilling area and obtain data from them, each drilling point has coordinates of x and y directions. geological information obtained at different heights may be different. stratigraphic division is based on the numerical information of each drilling hole in different z coordinates. in this study, shepard processing algorithm and kriging algorithm are used to process it. the corresponding interpolation models are as follows: ξ µ µ x y z r x y r x y z r ii n i i i i n, , , ( ) = ( )  ( )  =       = = ∑ ∑ 1 1 1 0       = ri 0 (1) r x y x x y yi i i,( ) = ( ) ( )− + − 2 2 represents the distance from the interpolated point (x, y) to the sampling point (xi, yi ). the value of  is taken as 2. the influence of the function value on the weight function is inversely proportional to the distance from the interpolated point to the sampling point. the larger the distance is, the smaller the impact is. the smaller the distance is, the greater the impact is. the two points are close to each other and have similar relationship in geological data, that is, the properties of the adjacent points are similar. kriging method is a spatial local interpolation method. it evaluates regionalized variables in the interpolation region by using covariance function, variation function and other models. the formula is as follows: z x z xi i i n * 0 1 ( ) = ( ) = ∑  (2) where z x* 0( ) represents the value of the point to be interpolated, z xi( ) represents the value of the first sampling point, and i is the effect of the i-th sampling point on the interpolation point. in kriging algorithm, the ordinary kriging method has been widely studied in the field of geology and has shown good results. therefore, due to the uncertainty of drilling location and geological structure, some variables in the address data show random and structural characteristics and regionalized variables in spatial distribution. the difference between regionalized random variables can be expressed by spatial covariance. cov z x z x h e z x z x h e z x e z x h( ) ( )  = ( ) ( )  ( )  ( ) , + + − + (3) the variation function of z x( ) at points x and x+h is denoted as  x h,( ), and its expression is as follows: 187virtual display system of geological body based on key algorithms of three-dimensional modelling environment through three-dimensional equipment. moreover, people can freely control the three-dimensional graphics environment to achieve specific purposes. virtual reality technology has the characteristics of immersion, interaction, multi-perception and sense of autonomy. its greatest feature is that people have natural functions and perception, and can interact with computergenerated virtual systems in real time. virtual reality system provides us with a convenient and realistic way to communicate with computers. it is a powerful tool for us to understand and transform the world. it has attracted the attention and research of many wellknown scholars all over the world. through the efforts of various researchers, it has made some brilliant achievements. it can be foreseen that with the rapid development of computer hardware and software technology, virtual reality technology will further develop and change our lives. as a result, the national geological information library successfully launches the virtual exhibition hall. on the one hand, it can give full play to the role of scientific popularization of geological data and documents, and popularize geological knowledge. on the other hand, it also provides scientific and technological means for researchers to study geological conditions and disaster situations more comprehensively. in the specific design of the display system, many perceptrons, reasoners, ontologies and service providers are involved in the structural model. next, these key points are elaborated in detail. by constructing a series of generic ontologies, a bridge for context-aware intelligent mobile services can be built to communicate and express with each other. in this three-dimensional virtual display platform, the ontology structure is divided into several sub-ontologies, which are object ontology, service ontology, computing ontology, perception ontology and storage ontology. object ontology includes user ontology and product ontology. service ontolo gy includes services. when the system decides that users need some information, γ − + − + x h var z x z x h e z x z x h,( ) = ( ) ( )  = ( ) ( )  1 2 1 2 2 − − + 1 2 2 e z x z x h( ) ( ) { } (4) on this basis, unbiased estimates are made, and the estimated variance is as follows: σ − − λe i i i n e z x z x e z x z x2 2 1 2 = ( ) ( )    = ( ) ( )         = ∑* (5) σ − λ + µe i i i n c x x c x x2 1 = ( ) ( ) = ∑, , (6) by analyzing the variance of kriging estimation, whether the estimated value is the best one can be judged. virtual display system as one of the emerging technologies, spatial information has become the main source of information for human beings to understand the real world around them and explore the unknown world. establishing realistic largescale virtual scene can help us understand spatial information. therefore, in recent years, researchers pay more attention to virtual simulation technology. moreover, it can realize human-computer interaction. computer generates three-dimensional graphics and makes people feel three-dimensional start select drilling hole and extract drilling data standardized processing of drilling data underlying partition algorithm generate low level data ground surface reconstruction algorithm texture mapping algorithm for true feeling of plane end 3 d geological modeling algorithm for zone faults fast slicing algorithm construct 3d geological body model satisfied ？ three dimensional analysis of the model y n figure. 1 flow chart of 3-d geological modelling based on borehole data 188 wei feng it feeds back the information to users and service system to recommend products to users in time. computing ontology contains three reasoners, which is the core part of the system. there are four sensors in perceptual ontology, and storage ontology is fused in the above ontology in practical application. to deal with the huge and strict data information, it is necessary to use data storage and computing tools in colleges and universities, as shown in figure 2. results and discussion this study collects borehole data of a certain area, as shown in table 1. table 1 borehole data of a certain area geological condition well drilling p[1] well drilling p[2] well drilling p[3] well drilling p[4] below depth (1) 1 1 2 1 below depth (2) 3 2 3 2 below depth (3) 5 3 5 4 below depth (4) 6 5 6 5 below depth (5) 7 7 7 6 stratigraphic division of geological conditions is carried out by the algorithm, and the results are obtained, as shown in figure 3. the three-dimensional geological model thus established is shown in figure 4. the result of virtual display is shown in figure 5. in addition, through the analysis of the calculation results of the algorithm, it can be seen that the parallel algorithm of shepard method and kigring method is more efficient than that of the single one in the case of two cores. moreover, in the case of four cores, the efficiency of the parallel algorithm is much higher than that of the two cores, which shows that the parallel algorithm can reduce the events of program execution and improve the efficiency of modelling. it is also very helpful to the operation of virtual display system. in summary, on the basis of collecting relevant data of geological body, using the key algorithm of three-dimensional modelling not only improves the operation efficiency, but also provides a good data support for the establishment of virtual display system, so that the final image can display the specific effect well, which is conducive to people’ s further understanding of geological conditions and provides a good model for researchers to study. user ontology product ontology reasoning machine i reasoning machine ii reasoning machine iii sensor i sensor ii sensor iii sensor iv storage of ontology service i service ii figure. 2 five ontology structures and their relationships figure. 3 stratigraphic division result map figure. 4 main interface of 3d geological simulation system 189virtual display system of geological body based on key algorithms of three-dimensional modelling conclusion with the continuous development of virtual display technology, researchers have a better understanding of the relevant information of geological bodies, and use the three-dimensional modelling algorithm to analyze, and ultimately complete the design of the display system. based on the three-dimensional geological modelling process of borehole data, the three-dimensional geological modelling modules are divided. the algorithm of stratum division and surface reconstruction are studied and realized. in the algorithm of surface reconstruction, shepard and kriging are used to implement the two methods respectively, and the two algorithms are analyzed and compared in detail. the design and research of the two algorithms in the geological virtual display system are realized. however, in the design of virtual display system, less consideration is given to the data that may affect the model, so more relevant data will be studied in the future. this study also provides ideas for other aspects of three-dimensional modelling, and has a reference significance for the wider application of virtual display technology. references buttussi, f., &chittaro, l. (2018). effects of different types of virtual reality display on presence and learning in a safety training scenario. ieee transactions on visualization and computer graphics 24.2,1063-1076. handong, t., tuo, t., & changhong, l. (2006). the parallel 3d magnetotelluric forward modelling algorithm. applied geophysics 3.4,197-202. liu, m., zhu, c., cui, t., zhang, h., zheng, w., & you, s an alternative algorithm of tunnel piston effect by replacing three-dimensional model with two-dimensional model. building and environment 128,55-67. naqvi, s. a. r., ali hassan, s., pervaiz, h., & ni, q. (2018). drone-aided communication as a key enabler for 5g and resilient public safety networks. ieee communications magazine 56.1,36-42. sinitski, e. h.., thompson, a. a., godsell, p. c., honey, j., & besemann, m. (2018). postural stability and simulator sickness after walking on a treadmill in a virtual environment with a curved display. displays 52,17. li, w. d., cai, y. l., & lu, w. f. (2007). a 3d simplification algorithm for distributed visualization. computers in industry 58.3,211-226. wu, w., zhu, h., lin, j., zhuang, x., & ma, g. (2018). tunnel stability assessment by 3d dda-key block analysis. tunnelling and underground space technology 71,210-214. zhao, r., wang, y., & martinez, a. (2018). a simple, fast and highlyaccurate algorithm to recover 3d shape from 2d landmarks on a single image. ieee transactions on pattern analysis and machine intelligence 40.12,3059-3066. zhu, x., lu, h., & rätsch, m. (2018). an interactive clothing design and personalized virtual display system. multimedia tools and applications 77.20,27163-27179. figure. 5 virtual display result image palabras clave: fase estacionaria; no consolidado; falla; sellado vertical; método de evaluación. how to cite item sun, n., fu, g., liu, i., & wu, j. (2019). unconsolidated fault vertical sealing ability evaluation method and its application during stationary phase. earth sciences research journal, 23(1), 35-42. doi: https://doi. org/10.15446/esrj.v23n1.78831 to study the vertical distribution of oil, gas in the fracture zone of the oil and gas bearing basin, based on the mechanism and influence factors of vertical sealing oil and gas, this paper has studied the necessary conditions of unconsolidated fault vertical sealing oil, gas, determination method for compaction and diagenetic of the fault filling. concluded that the static period of fault vertical sealing oil and gas required that fault should be cut by mudstone, through comparing the relative period of the fault rock starting consolidate and the fault stopping activity to determine whether the fracture fillings are compacted into rock. if the time fault rocks began compaction into rock is earlier than the time fault stopped activity, fault filling has been consolidated; otherwise not compaction. by determining the displacement pressure of the fracture filling material and the rock displacement pressure of the underlying reservoir, a method is established to evaluate static unconsolidated fault vertical sealing oil and gas ability by comparing the relative size of the two. applied it in the evaluation of the vertical sealing capacity f1 in the one to three sections of the dongying formation and 5 sections of the formation in the southern bohai bay basin. the results show that the fault f1 in one to three of the dongying formation have the greater filler displacement pressure than the displacement pressure of underlying reservoir rock, as the vertical sealed is favorable for oil and gas accumulation and preservation. it’s accordant with oil and gas distribution has been found in dongying formation at present in the formation of the south fort 5 structure, which indicates that the method is feasible to evaluate the vertical sealing ability of the fault in the stationary phase. abstract keywords: stationary phase; unconsolidated; fault; vertical sealing; evaluation method. unconsolidated fault vertical sealing ability evaluation method and its application during stationary phase método de evaluación de la capacidad de sellado vertical de fallas no consolidadas y su aplicación durante la fase estacionaria issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.78831 con el fin de estudiar la distribución vertical de petróleo y gas en la zona de fractura de la cuenca que los contiene, según el mecanismo y los factores de influencia del sellado vertical del petróleo y el gas, en este artículo se estudian las condiciones necesarias de la falla no consolidada de sellado vertical de petróleo y gas, el método de determinación por compactación y la diagenética del llenado de fallas. se concluye que el período estático de la falla de sellado vertical de petróleo y gas requería que la falla fuera cortada por piedra fangosa, mediante la comparación del período relativo en que la falla de roca se comienza a consolidar y la actividad de detención de fallas para determinar si los rellenos de fractura están compactados en la roca. si el tiempo en que las rocas comenzaron a compactarse es anterior a que se detuvieran las fallas, el relleno de fallas se ha consolidado; de lo contrario, no hubo compactación. al determinar la presión de desplazamiento del material de relleno de fractura y la presión de desplazamiento de roca del reservorio subyacente, se estableció un método para evaluar la capacidad de gas y petróleo de sellado vertical de falla estática no consolidada comparando el tamaño relativo de los dos. se aplicó en la evaluación de la capacidad de sellado vertical f1 en una de las tres secciones de la formación dongying y en cinco secciones de la formación en la cuenca sur de la bahía de bohai. los resultados muestran que la falla f1 en uno a tres de la formación de dongying tiene la mayor presión de desplazamiento del relleno que la presión de desplazamiento de la roca del yacimiento subyacente. el sellado vertical es favorable para la acumulación y conservación de petróleo y gas. se ha encontrado que concuerda con la distribución de gas y petróleo en la formación dongying en la actualidad en la formación de la estructura south fort 5, lo que indica que el método es factible para evaluar la capacidad de sellado vertical de la falla en la fase estacionaria. resumen record manuscript received: 01/09/2018 accepted for publication: 13/02/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 35-42 ning sun1, guang fu1, lili liu1, jingfeng wu1,2* 1northeast petroleum university, daqing, 163318; 2exploration and development research institute of daqing oilfield company ltd, 163712, china. * corresponding author: jingfengwu_nepu@163.com pe tr o le um g eo lo g y 36 ning sun, guang fu, lili liu, jingfeng wu introduction in oil and gas bearing basins, due to the different stopping time of different fractures, the fracture fillings of the compaction and diagnosis degree of are different, and the vertical sealing ability of oil and gas will also be different, thus different vertical distribution characteristics of oil and gas are resulted. it can correctly understand that the vertical sealing mechanism and ability of fracture fillings with different compaction, the diagenetic degrees is very important for oil and gas exploration in fault development areas of the bearing basins. there are a lot of researches and discussions on the vertical sealing ability of oil and gas in fractures. there are mainly two methods concluded. one is to study whether the oil and gas in fracture are vertically sealed or not based on relative magnitude between the positive pressure on the fault surface and the plastic deformation limit of mudstone. if the positive pressure on the fracture is greater than the limit of plastic deformation of mudstone, then the mudstone in the fault zone plastically flows and blocks the leakage space left after the fracture is closed, and a vertical sealing is formed, otherwise, vertical seals cannot be formed. another method is to treat the fracture as a layer of rock that is dumped in a surrounding rock stratum. by measuring the depth of solid buried rock and mud content in fault rock, this paper uses the empirical relationship between measured displacement pressure of surrounding rock and compacted burial depth and mud content to determine the displacement pressure and calculate the displacement pressure. this paper compares the relative magnitude of the displacement pressure of fault rock and the displacement pressure of the rock in the bottom reservoir, and studies the vertical closed oil and gas in the fault. however, these two methods are both aimed at the study of vertical sealing oil and gas of the fault rock type fractures in the stationary phase faults. in the meanwhile, relatively few studies have conducted on the vertical sealing oil and gas of the non-rock type fault in the stationary phase, and some are only qualitative, lacking quantitative research, which is undoubtedly unfavorable to further exploration of oil and gas in the fault development area of the oil and gas bearing basin. therefore, it is of great significance to study the method of evaluating the vertical sealing ability of unconsolidated faults in stationary phase for correct understanding of the distribution of oil and gas in fault development zones of the oil and gas bearing basins and guiding oil and gas exploration. the necessary conditions for the vertical sealing of oil and gas of unconsolidated faults in the stationary phase the fracture just stopped its activities. due to its relatively shallow burial, the pressure of the overlying sedimentary load is relatively small, and the fault filling has not been compacted and diagenetic. now, whether or not the oil and gas can have sealed in fractures in the upright vertical direction depends on the relative sizes of the fracture filling displacement pressure and the rock replacement pressure in the underlying reservoir. if the fracture filling displacement pressure is greater than or equal to the underlying replacement pressure of reservoir rock, then the fracture is closed vertically, as shown in figure 1a. on the contrary, if the fracture filling displacement pressure is less than the underlying displacement pressure of reservoir rock, then the fracture is not closed vertically, as shown in figure 1b. from this, it can be seen that the conditions required for vertical sealing of oil and gas in the unconsolidated fault in stationary phase should be that the fracture filling is dominated by argillaceous matter, while the fracture of the argillaceous fracture filling should be faulted in the mudstone seal. determination methods of fracture filling compaction and diagenesis in order to determine whether the fracture filling is compacted and diagenetic, it is assumed that the fault rock and surrounding rock should have the same compaction and diagenesis history. thus, the initial porosity and compaction diagenesis coefficient of the fault rock should be the same as that of surrounding rock. the inverse relationship (equation 1) between the compaction diagenesis degree of the fault rock and the surrounding rock with the same burial depth, which can be considered as its diagenetic pressure and action time, and the ratio of the porosity can be used to derive the fault rock compaction diagenetic burial depth zf, which is shown in equation 2. ρ ρ φ φ − w f w s o cz o cz zt zt e f = equation 1.(guang & dan, 2005) in the equation: z—fault rock burial depth, m; zf—fault block compaction diagenetic burial depth, m; tf—fault rock compaction diagenetic time, ma; ts—compaction diagenetic time of surrounding rock with the same burial depth as the fault rock, ma; ρw—density of formation water, g/cm3; φo—initial porosity of surrounding rock, decimal; c—surrounding rock compaction diagenetic coefficient. z z c t tf f s =  1 in equation 2.(guang, yuxi & yunfeng, 1997) figure 1. schematic diagram of unconsolidated fault in static period vertical seal oil and gas condition 37unconsolidated fault vertical sealing ability evaluation method and its application during stationary phase t t ef s cz=  equation 3.(xiaofei, xue & lixu, 2012) t t tf o fo=  equation 4.(tong, xiaofei & haixue, 2016) from equation 2, taking zf=0 (indicating that the fracture filling is not compacted into rock), the time for the fracture to start compaction and diagenesis is shown in equation 3. if the tfo is earlier than the fracture stopping time to, then the fracture filling is not compacted diagenetic, on the contrary, if the time tfo for starting compaction and diagenesis of the fracture filling is later than the fracture stopping activity time to, then the fracture filling has started compaction and diagenesis. the compaction diagenetic time can have calculated by equation 4. evaluation method of vertical sealing ability of oil and gas of unconsolidated fractures of fault in stationary phase it can be seen from the above that it is necessary to determine the displacement pressure of the fracture filling and the displacement pressure of the underlying reservoir rock to judge whether the unconsolidated fractures in the stationary phase seal off oil and gas. determination of displacement pressure of fracture filling due to the limitations of drilling and coring, it is impossible to test the displacement pressure of the fracture filling directly at present. it can only be determined indirectly through the experimental results of the physical model. in order to obtain an empirical relationship between the fracture filling displacement pressure and the pressure and mud content, clay and silt are mixed to simulate fracture filling with different mud content according to the ratio of 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100 respectively, poured into a stirrer and stirred evenly, and moistened with an atomizer. a manual pressure pump has used to apply pressures of 1 mpa, 5 mpa, 10 mpa, and 15 mpa to simulate fracture filling under conditions of 100 m, 500 m, 1000 m, and 1500 m underground respectively. then, 20 pieces of fracture filling samples with a diameter of 2.5 cm will be taken out, compressed, dried, vacuumized, and treated with saturated kerosene. finally, the samples have placed in the core holder and the displacement pressure has tested using high-pressure solid gas. the results are shown in table 1. by mathematical regression between the test results of displacement pressure of the fracture filling and the pressure and mud content in table 1, the formula for calculating the displacement pressure of the fracture filling by using the pressure and the mud content of the fracture filling is available as shown in equation 5. the pressure of the fracture filling nf in equation 5 is calculated by the equation 6 according to the burial depth, rf in equation 5 can be calculated from equation 7 based on the fault displacement and the thickness and mud content of the formation that is faulted by it. p ef n rf f= 0 1197 3.  equation 5 (yanfang, xuan & xiaofei, 2007) in the equation: pf—fracture filling actual measured displacement pressure, mpa; nf—pressure on fracture filling, mpa; rf—fracture filling mud content, decimal. n zf w=  equation 6 (huiting, long & yan, 2014) r h r lf i i i n = = ∑ 1 equation 7 (guang, haoran & j., 2014) in the equation: rf—fracture filling mud content, decimal; hi—thickness of layer i stratum that faulted by faults, m; ri—mud content of layer i stratum that faulted by faults, decimal; n—number of strata that have been faulted by faults; l—fault displacement, m. determination of displacement pressure of underlying reservoir rock the displacement pressure of the underlying reservoir rock can be obtained by direct coring test. if there is no core, it can also be calculated by the table 1. the measured displacement pressure and the pressure and mud content data of the fracture filling material sample no. depth(m) displacement pressure mud content(%) 1 1500 2.11 100 2 1500 1.10 80 3 1500 0.54 60 4 1500 0.32 40 5 1500 0.19 20 6 1000 0.86 100 7 1000 0.50 80 8 1000 0.31 60 9 1000 0.18 40 10 1000 0.12 20 11 500 0.41 100 12 500 0.28 80 13 500 0.18 60 14 500 0.12 40 15 500 0.08 20 16 100 0.26 100 17 100 0.16 80 18 100 0.12 60 19 100 0.08 40 20 100 0.06 20 38 ning sun, guang fu, lili liu, jingfeng wu empirical correlation (equation 9) between the measured displacement pressure of the reservoir rock in the study area, the compacted diagenetic burial depth, and mud content by using its compacted diagenetic burial depth. if there are no obvious overlying strata of the uplift and denudation, and it can be replaced by the current burial depth and the mud content, which can be calculated by equation 8 using its natural gamma data. vsh g i g cur gr cur = ⋅2 1 2 1   equation 8. (jijian, lee & guang, 2012) thereinto i gr gr gr grgr =  max min in the equation: gr—natural gamma-ray logging value of target formation; grmax—natural gamma-ray logging value of mudstone formation; grmin—natural gamma-ray logging value of sandstone formation; igr—natural gamma relative value; gcur—the empirical coefficient related to formation, 3.7 for new formation, 2.0 for old formation. p aer br zf f= (9) in the equation: pr—measured displacement pressure of reservoir rock, mpa; zr—reservoir rock compaction diagenesis burial depth, m; rr—reservoir rock mud content, decimal; a,b—constants related to area. evaluation of vertical oil and gas sealing capacity of unconsolidated faults in stationary phase by comparing the relative size of the above-identified fracture filling displacement pressure and the displacement pressure of underlying reservoir rock, the vertical oil and gas sealing ability of the unconsolidated fracture in the stationary phase can be evaluated. if the fracture filling displacement pressure is greater than or equal to the displacement pressure of underlying reservoir rock, then the fracture has closed vertically, conversely, if the fracture filling displacement pressure is less than the underlying displacement pressure of reservoir rock, then the fracture is not closed vertically. example application selecting the f1 unconsolidated fault of the nanpu no.5 structure in the nanpu sag of the bohai bay basin as an example, using the above method, this paper studies the vertical sealing ability of the stationary phase in the first, second and third sections of the dongying formation. through the analysis of the relationship between the research method and distributions of oil and gas found in the first, second and third sections of the dongying formation of nanpu no. 5 structure at present, the feasibility of the proposed method in evaluating the vertical sealing ability of the unconsolidated faults in the stationary phase is verified. the nanpu no. 5 structure is located in the northwest of the nanpu sag. it is an important oil and gas bearing structure in the nanpu sag. the structure feature is generally a draping structure developed on the background of buried hill, and its tectonic direction is northeast to east. it is complicated by near-eastwest fracture cutting, as shown in figure 2. the formations developed from the bottom to the top are the kongdian formation, the shahejie formation, the dongying formation of the paleogene, and the guantao formation, the minghuazhen formation of the neogene, and the quaternary. up to now, this structure has verified a large number of oil and gas shows in shahejie formation and dongying formation. among them, there are relatively more showing in the first, second, and third sections of dongying formation, as shown in figure 3. the f1 fault is located in the core section of nanpu no. 5 structure, of which the trend is northeast or east, with a relatively large scale of extension, as shown in figure 2. the displacement of the lower part of f1 fault is larger than the upper part, and the average displacement is 70-145m. the dip varies greatly from 49o-72o. from sha-1 member of f1 fault to the ground surface, the fracture filling has not compacted and consolidated so far. whether it can form vertical sealing for oil and gas in the first, second and third sections of the dongying formation should be the key to oil and gas exploration in the dongying formation of nanbao no. 5 structure. the burial depth of f1 fracture in the first, second and third sections of the dongying formation can be obtained from figure 3, and the compacted diagenetic time (deposition time since its inception) of the surrounding rock with the same burial depth as the f1 fracture can be obtained from the geological time scale. the diagenesis coefficient of the surrounding rock and the initial porosity can be determined from fig. 4. calculated from the equation 3, the time of compaction and diagenesis of the f1 fracture is 6.58ma, which is obviously earlier than the f1 fault stopping activity time (0ma), indicating that the f1 fracture has not been compacted and consolidated although the activity has stopped so far. from the burial depth of the fault f1 in the first, second, and third sections of the dongying formation, and the pressure received by the fracture filling can be calculated from equation 6, and then from fault displacement of the fault f1 in the first, second and third section of the dongying formation. the thickness of the stratum faulted and mud content have calculated by equation 7, the mud contents of the fracture filling are 0.27, 0.29, and 0.31 respectively. finally, the pressure received by fracture filling of f1 fault in the first, second, and third segments of the dongying formation and mud content are substituted into equation 5, and the displacement pressures of the f1 fault filling in the first, second, and third sections of the dongying formation are calculated as 0.41 mpa, 0.59 mpa, and 0.75 mp, respectively. from the compaction diagenetic burial depth of reservoir rocks in the first, second and third sections of the dongying formation, where the fault f1 is located, because there is no significant uplift and erosion, and it can have replaced by the current burial depth. the natural gamma logging has calculated by equation 8, the mud contents of reservoir rocks in the first, second and third sections of dongying formation are 0.15, 0.17, and 0.10 respectively. the compaction diagenetic burial depth and mud content of reservoir rocks in the first, second and third sections of the dongying formation have substituted into the relationship between the nanpu depression reservoir rock measured displacement pressure and its compaction diagenetic burial depth and mud content, where we can see in table 2 and equation 10. the displacement pressures of reservoir rocks in the first, second and third sections of the dongying formation are 0.12 mpa and 0.15 mpa and 0.10 mp respectively. comparing the relative size among the displacement pressures of the fracture f1 filling in the first, second and third sections of the dongying formation and the displacement pressure of the underlying reservoir rock. it can be seen that the displacement pressures of the fault fracture f1 filing in the first, second and third sections of the dongying formation are greater than them of the underlying reservoir rocks. in addition, the fault is close vertically, which is conducive to the accumulation and preservation of oil and gas in the first, second and third sections of the dongying formation. p er z rf f= 0 0593 0 1662, . (10) table 2. the measured displacement pressure of the reservoir rocks in the south of the depression and its compaction and the data of the depth of the rock and the mud content well no. depth/m mud content/% displacement pressure/mpa g49 2448.6 5.64 0.89 np1-37 3045.5 8.79 0.62 m28×1深 3268.6 24.48 1.34 g62 4054.6 22.28 1.31 np5-10 3320 32.4 2.82 np4-51深 2449.79 54.6 3.18 b7 3597.58 38.7 4.12 m5 2768.36 51.21 3.77 g3106 3899.5 36.52 2.06 b5 4219.4 35.89 4.25 m108×1 3345.3 46.16 3.11 39unconsolidated fault vertical sealing ability evaluation method and its application during stationary phase figure 2. the relationship map of f1 fault and hydrocarbon distribution in nanpu 5 figure 3. reservoir profile of f1 fault in nanpu no. 5 40 ning sun, guang fu, lili liu, jingfeng wu in the equation, pr, zr, and rr have the same meaning as the equation 8. seeing from fig. 3, there is oil and gas showing in both ed-1 and ed-3 of the f1 fault in nanpu 5 structure. although the f1 fault is not compacted as diagenetic, the displacement pressure of fracture filling is greater than that of the underlying reservoir rock, which is conducive to the accumulation and preservation of oil and gas. moreover, during the f1 fault activity, the breaking thickness of the guan-3 member volcanic cap rock and the ed-3 mudstone cap rock (173m and 177m, respectively) is greater than the minimum breaking thickness required for sealing oil and gas (figure 5). oil and gas cannot transport and dissipate upward along f1 fault. it is also conducive to hydrocarbon accumulation and preservation. there is no oil and gas showing in ed-2 because the breaking thickness of the mudstone cap rock in ed-2 (110m) is smaller than the minimum breaking thickness required for sealing oil and gas (figure 5). the f1 fault activity is connected up and down in ed-2 mudstone cap rock. thus the underlying oil and gas dissipate along the fracture of f1. even though the displacement pressure of the fracture filling is greater than the displacement pressure of the underlying reservoir rock after the gas stops moving, and it is vertically sealed, there will be no accumulation of oil and gas. conclusions the required condition for the vertical sealing of oil and gas of unconsolidated fault in the stationary phase is that the fault being fractured should be mudstone cap rock. based on the predicted time for the initiation of compaction and diagenesis of the fault filling and the time when the fracture ceases activity, compaction and diagenesis can be determined. if the time that the fault filling begins to form earlier than that the fracture stops activity, then the fault rock is not compacted and consolidated, otherwise it is compacted and consolidated. in this paper, the displacement pressure of the fault filling and the rock mass of the bottom reservoir is determined, and the both relative sizes have compared. in this paper, a set of evaluation methods for the vertical sealing ability of the loose phase and the stationary phase has established. it has figure 4. relation of porosity and burial depth of sandstone and mudstone formation in nanpu sag a．ed-3 mudstone cap rock 41unconsolidated fault vertical sealing ability evaluation method and its application during stationary phase b．ed-2 mudstone cap rock c．guan-3 mudsone cap rock figure 5. determination map of the minimum breaking thickness of cover to seal hydrocarbon in nanpu depression 42 ning sun, guang fu, lili liu, jingfeng wu applied to the vertical oil and gas seal of the first, second and third sections of the dongying formation by the fixed-phase structural loose fault f1 in the blocking ability evaluation of nanpu sag, bohai bay basin. the results show that the displacement pressure of fault filling of f1 fault in the first, second and third sections of the dongying formation is greater than the displacement pressure of the underlying reservoir rock, and it is vertically closed, which is conducive to oil and gas accumulation and preservation. it matches the existing discovered oil and gas distributions in the first, second, and third sections of the dongying formation near the f1 fault of the nanpu no.5 structure. it shows that the method is feasible to evaluate the vertical oil and gas sealing ability of the unconsolidated faults in the stationary phase. references guang, f., & dan, y. (2005). shurennouer fault vertical sealing distribution and its relationship with hydrocarbon accumulation. acta petrolei sinica, 06, 40-45. guang, f., haoran, w., & j, h. (2014) fault vertical sealing off the pressure difference method for the pressure difference method and its application. journal of petroleum, 04, 685-691. guang, f., yuxi, l., & yunfeng, z. (1997) research methods of fault vertical sealing oil gas and its application. natural gas industry, 06, 31-34+5. huiting, h., long, w., & yan, l. (2014) research methods and application of vertical sealing ability of the overpressure mudstone cap rock in the vertical sealing ability. oil and gas geology, 03, 359-364. jijian, s., lee, l., & guang, f. (2012) caprock fault vertical sealing evaluation method and application of quantitative. journal of jilin university (earth science edition), s2, 162-170. tong, w., xiaofei, f., & haixue. (2016). the quantitative evaluation of the brittle plastic deformation characteristics and sealing ability of gypsum rock salt. geological theory, 01, 127-137. xiaofei, d., xue, g., & lixu, z. (2012) the formation and evolution of mudstone smear and oil and gas migration and sealing. china university of mining and technology, 01, :52-63. yanfang, l., xuan. s., & xiaofei, f. (2007). the vertical sealing of fault quantitative evaluation method and its application. acta petrolei sinica, 05, 34-38. this study estimates the regional environmental consequences of the impactor extraterrestrial body that could produce the probable vichada impact crater structure on the vichada plain, in colombia, south america. this paper details the parameter assumptions upon which the estimation is made. it describes an approach to quantifying the principal impact processes that could have affected the landscape in the vicinity of the probable vichada impact event in the past. the key parameters are impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, and the distance from the impact at which the environmental effects are to be calculated, and the target type of sedimentary rock or crystalline rock. these parameters were chosen with support from the vichada structure dimensions obtained from remote sensing data interpretation, regional geologic mapping and interpreted satellite data and ground-based gravity and magnetic anomalies. the calculations are based on compiled novel algorithms for estimating the thermal radiation emitted by the impact-generated vapor plume or fireball, and the intensity of seismic shaking. model validation is performed by obtaining the approximates various dimensions of the vichada impact crater and ejecta deposit, as well as estimating the severity of the air blastin both crater-forming and air burst impacts. we illustrate the utility of the calculations by examining the predicted environmental consequences in seven localities of the colombian territory, through hypothetical impact scenarios occurring in cumaribo and puerto carreño (vichada), puerto inirida (guainía), puerto gaitán and villavicencio (meta), mitú (vaupes) and bogotá, d.c. it is concluded that the most wide-reaching environmental consequence is seismic shaking. both ejecta deposit thickness and air-blast pressure decay much more rapidly with distance than with seismic ground motion. close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon. these results would guide further detailed fieldwork hunting for direct impact crater evidences and/or interpreting high resolution geophysical studies and borehole that could be carried out in the probable vichada impact crater area in the near future. abstract keywords: vichada structure; impact asteroid crater; environmental effects vichada asteroid impact effects from simulation of regional environmental consequences of a meteoroid impact on earth efectos del impacto de asteroide de vichada a partir de la simulación de las consecuencias ambientales regionales de un impacto de meteorito en la tierra issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.65459 este estudio simula las consecuencias ambientales regionales del cuerpo impacto de un asteroide que pudo producir la probable estructura de cráter de impacto de vichada, en la llanura de vichada, colombia, américa del sur. este artículo proporciona los parámetros a partir de los cuales se hace la estimación. describe un enfoque para la cuantificación de los procesos principales del impacto que podrían haber afectado el paisaje en las cercanías del evento de probable impacto de vichada en el pasado geológico. los parámetros clave son diámetro del asteroide, densidad, velocidad de impacto antes de la entrada atmosférica, ángulo del impacto y la distancia entre el sitio impacto y el lugar en el cual se calculan los efectos ambientales y el tipo de roca sobre la que pudo ocurrir el impacto. estos parámetros fueron seleccionados a partir del análisis de las dimensiones de la estructura de vichada obtenidos a partir de la interpretación de imágenes de sensores remotos, cartografía geológica regional y la interpretación de anomalías gravimétricas y magnéticas satelitales, aerotransportadas y terrestres. los cálculos se basan en algoritmos novedosos compilados para la estimación de la radiación térmica emitida por la pluma de vapor generado por el impacto o bola de fuego y la intensidad de la vibración sísmica. la validación del modelo se realiza mediante la reconstrucción de las dimensiones del posible cráter de impacto de vichada y depósitos de eyecta, así como la severidad de los efectos explosivos del aire y partículas del material de eyecta. ilustramos la utilidad de los cálculos mediante el examen de las consecuencias ambientales pronosticadas en siete localidades del territorio colombiano, a través de escenarios de impacto hipotético en cumaribo y puerto carreño (vichada), puerto inírida ( guainía), mitú (vaupés), puerto gaitán y villavicencio (meta) y bogotá, d.c. se concluye que la consecuencia ambiental de más regional es la asociada a eventos sísmicos. tanto el espesor del material de eyecta y la presión atmosférica de las ondas de choque disminuyen más rápidamente al aumentar la distancia con respecto al sitio de impacto que los eventos sísmicos. cerca del sitio del impacto, el efecto más devastador está asociado a la radiación térmica; sin embargo, la curvatura de la tierra implica que localidades distantes están protegidos contra la radiación térmica directa porque la bola de fuego está por debajo del horizonte. estos resultados se constituyen en una guía para los posteriores trabajos de geología de campo, en donde se buscarán evidencias directas en la zona del cráter de impacto e interpretación de estudios geofísicos de alta resolución y datos e perforaciones que se realizaran en la zona del probable cráter de impacto de vichada en un futuro cercano. resumen palabras clave: estructura de vichada; cráter de impacto de asteroide; efectos ambientales. record manuscript received: 05/06/2017 accepted for publication: 01/12/2017 how to cite item hernández, o., & alexander, g. c. (2018). vichada asteroid impact effects from simulation of regional environmental consequences of a meteoroid impact on earth. earth sciences research journal, 22(1), 7-12. doi: http://dx.doi.org/10.15446/esrj.v22n1.65459 earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 7-12 hernández, o.1, alexander, g.c.1 1 universidad nacional de colombia, departamento de geociencias, edificio manuel ancizar of 335, bogotá d.c. colombia ohernandezp@unal.edu.co, gcalexandere@unal.edu.co g e o ph y si c s 8 hernández, o.and alexander, g.c. introduction asteroid and comet impacts have played a major role in the geological history of the earth. it is widely proposed that the chicxulub impact event, 65 million years ago (alvarez, 1980) and the wilkes lands impact event in antarctica, 320 million years ago (von freseet al., 2009), could have perturbed the global environment so catastrophically that major life extinction could occurr by these comic impacts. therefore, both the scientific community and the general public are increasingly interested in both the threat to civilization and the potential environmental consequences of impacts. this study is based on model simulation offered by the earth impact effect program (collins et al., 2005) to estimate the severity of several of these environmental effects, which focuses on the consequences of an impact event for the regional environment; that is, from the impact location or ground cero to a few hundred kilometers away. the purpose of this paper is to model the asteroid impact effects that could be registered in the geological record of the vichada structure and surrounds to guide a fieldwork searching for evidences of thermal radiation, seismic shaking, and air blast and ejecta effects that could confirm the probable asteroid impact origin of the vichada structure and to classify it as a proven asteroid crater. previous papers have examined, in detail, the geomorphological and satellite, airborne and ground gravity and magnetic evidences of the probable vichada impact crater (hernandez et al., 2009, 2011). the vichada impact crater structure the vichada structure is a probable asteroid impact structure along the vichada river in colombia (4°30′n69°15′w, vichada department, colombia, south america; figure 1). it is most likely the largest probable impact structure in south america. the structure was discovered by rocca (2004) by examining tm landsat imagery. the structure consists of a central flat depression surrounded by two concentric rings of hills of approximately 30 and 50 km diameter, respectively. the vichada river anomalously flows around the outer ring of small hills. the structure has the typical multi-ring shape configuration of large impact structures: a central peak ring impact structure. the central core consists of a ring about 30 km. in diameter which surrounds a central depression of circular shape and 20 km. in diameter. in this innermost region, there is a basin, the relief is quite smooth and it is the deepest part of the structure. the central basin is covered by jungle and it is surrounded by 2 concentric rings of low hills. the outer most ring is 50 km in diameter and in the vichada river flows around it in a perfect semi-circle following the external limits of this outer ring of hills. this flow around feature of the river is very interesting and anomalous. the geology of the area is not known in very detail. rocks exposed in the rio vichada structure include precambrian metasedimentary and granitic rocks with an extensive sedimentary cover. the sedimentary cover is composed by a heterogeneous sequence of conglomerates, sandstones and claystones. the oligocene to pliocene covering strata lie on top of precambrian crystalline basement rocks. this circular structure is probably older than 30 million years. a combined satellite and airborne gravity and magnetic study were carried out over the 50 km wide structure (hernandez et al., 2009).as a result, a prominent positive free-air gravity anomaly mapped over a roughly 50-km diameter basin is consistent with a mascon centered on (4o30`n, -69o15`w) in the vichada department. follow up, ground-based gravity and magnetic anomalies were modeled confirming the regional free air gravity anomalies (hernandez et al., 2011). these potential field anomalies infer a hidden complex impact basin structure filled with tertiary sedimentary rocks and recent quaternary deposits. negative bouguer anomalies of -8 mgals to -15 mgals are associated with a concentric sedimentary basin with a varying thickness from 100m to 500 m in the outer rings to 700m to 1000m at the center of the impact crater basin. strong positive magnetic anomalies from 100nt to 300nt infer the presence of a hidden local precambrian crystalline basement that could be affected by intensive faulting producing tectonic blocks, that are dipping to the center of the structure, showing a typical “domino structure” of impact craters such as that of sudbury, ontario, canada (evans, 1993). basic to intermediate mineralized veins and dikes with contrasting density and magnetic susceptibility properties could be emplaced along these faulting zones, as inferred from local gravity and magnetic highs. the geologic mapping of the area is limited by the flat topography and absence of outcrops/ geomorphologic units. nevertheless, observed local normal faults along the inner ring together with radially sparse irregular blocks over flat terrains can be associated with terraced rimsor collapse of the inner crater structure and ejecta blanket, respectively (hernandez et al., 2011). figure 1. geographic location of the vichada structure in eastern colombia, northwestern south america showing the inner and outer rings of the vichada structure and the semicircular path of the vichada river that flows from west to east, over a landsat tm image. asteroid impact model simulation the formation of an impact crater is an extremely complicated and dynamic process (melosh 1989). the abrupt deceleration of a comet or asteroid as it collides with the earth transfers an immense amount of kinetic energy from the impacting body to the target. as a result, the target and impactor are rapidly compressed to very high pressures and heated to enormous temperatures. between the compressed and uncompressed material, a shock wave is created that propagates away from the point of impact. in the wake of the expanding shock wave, the target is comprehensively fractured, shock-heated, shaken, and set in motion, leading to the excavation of a cavity many times larger than the impactor itself. this temporary cavity, termed as the transient crater (dence et al., 1977) subsequently collapses under the influence of gravity to produce the final crater form. as the crater grows and 9vichada asteroid impact effects from simulation of regional environmental consequences of a meteoroid impact on earth collapses, large volumes of rock debris are ejected onto the surface of the earth surrounding the crater. close to the crater rim, this ejecta deposit forms a continuous blanket smothering the underlying terrain; further out, the ejecta lands as a scattered assortment of fine grained dust and larger bombs that may themselves form small secondary craters. in addition to cratering the surface of the earth, an impact event initiates several other processes that may have severe environmental consequences. during an impact, the kinetic energy of the impactor is ultimately converted into thermal energy in the impactor and target, seismic energy, and kinetic energy of the target and atmosphere. the increase in thermal energy melts and vaporizes the entire impactor and some of the target rocks. the hot plume of impact-generated vapor that expands away from the impact site, referred to as the “fireball, radiates thermal energy that may ignite fires in combustible materials along a ‘line of sight’ with of the fireball. as the impact generated shock wave propagates through the target, it eventually decays into elastic waves that travel great distances and cause violent ground shaking several crater radii away. in addition, the atmosphere is disturbed in a similar manner to the target rocks; a shock wave propagates away from the impact site compressing the air to high pressures that can pulverize animals and vegetation. immediately behind the high-pressure front, violent winds ensue that may flatten forests and scatter debris. all of these impact-related processes are extremely important as they could be preserved in the geological record in order to be discovered by detail fieldwork geological and geophysical surveys, allowing to confirm or to reject the probable asteroid impact origin of the vichada structure. due to the lack of direct geological evidences in the field by the limited number of outcrops for geological observation, the modeling of the environmental effects is obtained from of theoretical parameters to estimate the impact energy and average time interval between impacts of the same energy somewhere on earth; the resulting crater size and volume of the melt produced; the thermal radiation damage from the fireball; the impact-induced seismic shaking; the extent and nature of the ejecta deposit; and the damage caused by the blast wave (collins et al., 2005). the simulation requires six parameters (figure 2): (1) the diameter of the impactor l0, (2) the impactor density ρi, (3) the impact velocity v0, (4) the angle that the trajectory of the impactor subtends with the surface of the earth at the impact point ɵ, (5) the target rock type and (6) the distance away from the impact at which the environmental consequences are calculated r (collins et al., 2005). figure 2. diagram illustrating the input parameters for the earth impact effects program: l0 is the impactor diameter at the top of the atmosphere, v0 is the velocity of the impactor at the top of theatmosphere, ρiis the impactor density, ρt is the target density, and θ is the angle subtended between the impactor’s trajectory and the tangent plane to the surface of the earth at the impact point. the distance r from the impact site at which the environmental consequences are determined is measured along the surface of the earth; the epicentral angle ∆between the impact point and this distance r is given by ∆= r/re, where re is the radius of the earth (collins et al., 2005). imput parameters for the simulation of environmental consequences the following input parameters were selected with support from local and regional geological and geophysical studies of the probable vichada impact structure (de la espriella et al., 1990; hernandez et al., 2009; 2011; khurama, 2007; rocca, 2004), complemented with information from theoretical asteroid impact crater effects impactor density (ρi): asteroids are made of rock (ri ~2000–3000 kg/m3; hilton, 2002) or iron (ri ~8000 kg/m3). about eight out of ten meteorites seen to fall are chondrites (bevan and laeter, 2002). therefore, a density value of 3.0 kg/m3 has been assigned to the extraterrestrial impact body to represent a chondritic meteorite. impact velocity (v0): asteroids typically collide with the earth’s atmosphere at velocities of 12–20 km/s (bottkeet al., 1994). taking into account the diameter of 3220m and density of 3.0 kg/m3 of the impactor, a high impact velocity is expected because the earth´s atmosphere is not capable of retarding such a large mass (heide and wlotzka, 1995). in other words, the probable asteroid that could produce the vichada structure could strike the surface of the earth after having loss only a small fraction of its cosmic velocity; therefore, an impact velocity of 20 km/s has been selected (heide and wlotzka, 1995). similar simulated environmental effects have used an impact dunite impactor, impacting at 20 km/s and found none of the impactor remained solid (pierazzo & melosh 2000) impacting angle (ɵ): the impacting angle is that angle at which the trajectory of the impactor subtends with the surface of the earth at the impact point (shoemaker, 1962). an impacting angle of 900 has been selected based on the perfect symmetric spherical shape of the vichada impact structure and taking into account the simplified case of vertical impact of an airless planetary body without a surficial water layer or target heterogeneity (gareth et al., 2012). most impacts occur at an oblique angle, the percentage of impactors that strike at impact angles less than 60o and less than 30o to the target surface is 75% and 25%, respectively (shoemaker, 1962). therefore, the vertical impact case is the best studied and serves as a useful exemplar of many of the processes common to all impacts (gareth et al., 2012). target rock: a sedimentary cover, composed by a heterogeneous sequence of conglomerates, sandstones and claystones has been selected as representative of the target rock hit by the asteroid. the specific density of the target rock is 2.4kg/ m3. this value was also assigned to the sedimentary cover in the inverse modeling of ground complete bouguer gravity anomalies by hernandez et al., (2011). diameter of the impactor (l0): taking into account the density of the target and impactor, the impact velocity and the angle of impact, several calculation were made by applying the utilities of the impact calculator (lowe et al., 2017) to obtain a final crater dimension of 50 km of the vichada structure obtaining an impactor diameter of 3220m, being consistent with the measured impact crater structure from the tm-landsat imagery, geographical features and fieldwork reconnaissance. distance away from the impact (r): seven different distances away from the impact crater central point were chosen at which the environmental consequenceswere calculated, including cumaribo, vichada (70km away), puerto inirida (150 km away), puerto carreño (260 km away), puerto gaitán (320 km away), mitú (370 km away) villavicencio (494km away) and bogotá d.c. (545km away). the point data results obtained at each location have been linearly interpolated to map regional the environmental consequences. based on these results, a window of the fist 100km away from the vichada structure has been selected to carry out fieldwork activities among with seismic lines and borehole data compilation and interpretation. 10 hernández, o.and alexander, g.c. results the environmental consequences are estimates based on limited understanding of the impact process and come with large uncertainties; they should be used with caution, particularly in the case of peculiar input parameters with a weak geological support. all values are given to seven different locations away from the center of the impact crater structure (tables 1 and 2). data processing of the point data solutions has been applied to obtain the regional environmental effects (figures 3 to 5). uncertainty associated with these calculations is discussed in collins et al., 2005. the main environmental effects are: time for maximum radiation: 2.19 seconds after impact thermal exposure: 1.01 x 109 joules/m2 duration of irradiation: 9.47 minutes radiant flux (relative to the sun): 1770 transient crater diameter: 31.7 km transient crater depth: 11.2 km final crater diameter: 50 km final crater depth: 961 meters the crater formed is a complex crater. the volume of the target melted or vaporized is 93.1 km3 roughly half the melt remains in the crater, where its average thickness is 118 meters. table 1. table 2. 11vichada asteroid impact effects from simulation of regional environmental consequences of a meteoroid impact on earth figure 3. air blast maximum wind velocity, in meters per second , with maximum velocities above 800 m/s in the vichada structure figure 5. ejecta mean fragment diameter, in centimeters, with fragments of about 70cm in mean fragment diameter in the surround of the vichada impact structure that could be preserved in the geological record. figure 4. average ejecta thickness, in meters, showing more than 25m of ejecta thickness in the surround of the vichada structure that could be preserved in the geological record. conclusions the hypothetical regional environmental consequences of the collision of the probable vichada impactor have been calculated providing a catastrophic scenario for the first 100km away from the impact center. the seismic shaking of 8.9 intensity in the richer scale could be the most significant effect at short and large distances from the impact site; effects of ejecta fallout and the air blast decrease much more rapidly with distance away from the impact site, being significant at cumaribo (70 km away), almost undetectable at puerto inirida (150km away) and neglectable and impossible to recognize at distances greater than 150km away, such as puerto carreño (220km away) and the other four points of observation in puerto gaitán, mitú, villavicencio and bogotá. the timing analysis of the environmental consequences shows that the seismic shaking could take only up to two minutes from the impact center to the larger distance at bogotá d.c, while the ejecta arrival could take near six minutes (three times longer) and the air blast takes about 28 minutes (14 times longer) to arrive at the larger observation point. this difference of the arrival time at each location is extremely important, due to the fact that local deformation and disruption of lithological units and ejecta material could be preserved near the impact crater, in the first 100km from the impact site. this is a significant result as it allows reducing the studied area to search and to hunt for direct geological evidence of the vichada impact crater. estimates of the average ejecta thickness and ejecta mean fragment diameter are also important to conduct detailed geological studies, and borehole and seismic data compilation and interpretation in the inner ring, outer ring and the first 50km away from the probable impact crater, where we expect to find ejecta thickness greater than 20m with mean fragment diameter greater than 1m. estimates of maximum fireball radius, air blast wind 12 hernández, o.and alexander, g.c. velocity and air blast sound intensity are almost impossible to validate from the geological and geophysical record. recommendations the calculated environmental effects of the probable vichada impact crater has applied a modeling in which it was necessary to make general assumptions in the input parameters, limiting the accuracy of the results. therefore, the obtained results must be use with care. until now, the vichada impact structure is not a proven asteroid impact crater. we expect to provide a hypothetical scenario waiting to be proven or disproven by future detailed geological and geophysical studies in the area. acknowledgements this work has been partially supported by a grant from tep ltda. we also thank the geosciences department of the national university of colombia for 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academicpress. pp. 283–359. vonfrese, r. r. b., l. v. potts, s. b. wells, t. e. leftwich, h. r. kim, j. w. kim, a. v. golynsky, o. hernandez, and l. r.gaya-pique, 2009, grace gravity evidence for an impact basin in wilkes land, antarctica, geochem. geophys. geosyst.,10, q02014, doi:10.1029/2008gc002149. a mathematical approach for assessing landslide vulnerability hernán eduardo martínez carvajal1,2*, maria tâmara de moraes guimarães silva3, edwin fabián garcía aristizábal4, edier v. aristizábal-giraldo2, mayra alejandra larios benavides2 1university of brasilia. campus “darcy ribeiro” sg-12. ppg-geotecnia, enc/ft. brasilia df, brazil. 2universidad nacional de colombia, sede medellin, colombia. medellin, colombia 3goiás federal institute, av. pedro ludovico, s/n, residencial reny cury. anápolis-go, brazil. 4universidad de antioquia. udea. medellin, colombia *corresponding author: carvajal@unb.br palabras clave: vulnerabilidad por deslizamientos; cuantificación de la vulnerabilidad; modelo t de vulnerabilidad. how to cite item martínez-carvajal, h. e., guimaraes-silva, m. t. m., garcía-aristizábal, e. f., aristizábalgiraldo, e. v., & larios-benavides, m. a. (2018). a mathematical approach for assessing landslide vulnerability. earth sciences research journal, 22(4), 251-273. doi: https://doi.org/10.15446/esrj.v22n4.68553 a natural phenomenon (hazard) may be characterized in terms of temporal, spatial and magnitude probabilities. the effects of the interaction between the hazard and the exposed element depend on the intensity of the hazard and on the resistance, sometimes called susceptibility, of the element at risk, which describes the propensity of a building or other infrastructure to suffer damage from a specific hazard impact. consequently, a modern concept of vulnerability must consider the intensity of the hazard as well as the structural resistance of the exposed infrastructure. this concept is referred to as physical vulnerability, and the most accepted definition is a representation of the expected degree of loss quantified on a scale of 0 (no damage) to 1 (total destruction). thus, this work presents a mathematical model for landslide physical vulnerability assessment, here named the t-model, based on the “principle of natural proportionality” and calibrated with field observations of the massive rainfall-triggered landslide event that occurred in nova friburgo, brazil in november of 2011. the model was also calibrated for a flow-type movement that is based on field observations of the failure of a tailing dam that affected the district of bento rodrigues, brazil in november of 2015. the results showed a good agreement between predictions and the observed level of damages. thus, it is possible to conclude that from a mathematical point of view, the model may be qualified as universal. it is recognized that a real universal objective model for vulnerability to landslides is not practical at present. more important than the model itself is the methodology that is presented here, which leads the user to take qualitative damage information from the field and develop it into a quantitative mathematical framework. potential users of the t-model must be cautious regarding the values of parameters that are presented in this paper. the t-model is just a modest proposal that requires further calibration and deep expert criticisms. abstract keywords: landslide vulnerability; quantitative vulnerability; vulnerability assessment; landslide risk analysis; vulnerability t-model. un modelo matemático para la evaluación de la vulnerabilidad por deslizamientos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.68553 un fenómeno natural (peligro) puede caracterizarse en términos de probabilidades temporales, espaciales y de magnitud. los efectos de la interacción entre el peligro y el elemento expuesto dependen de la intensidad del peligro y de la resistencia, a veces llamada susceptibilidad, del elemento en riesgo, que describe la propensión de un edificio u otra infraestructura a sufrir daños de un peligro específico. en consecuencia, un concepto moderno de vulnerabilidad debe considerar la intensidad del peligro así como la resistencia estructural de la infraestructura expuesta. este concepto se conoce como vulnerabilidad física, y la definición más aceptada es una representación del grado de pérdida esperado cuantificado en una escala de 0 (sin daño) a 1 (destrucción total). por lo tanto, este trabajo presenta un modelo matemático para la evaluación de la vulnerabilidad física por deslizamientos, aquí denominado el modelo t, basado en el "principio de proporcionalidad natural" y calibrado con observaciones de campo del evento de deslizamientos provocado por la lluvia intensa que tuvo lugar en nova friburgo, brasil en noviembre de 2011. el modelo también fue calibrado para un movimiento de flujo a partir de observaciones de campo de la falla de una presa de relaves que afectó al distrito de bento rodrigues, brasil en noviembre de 2015. los resultados mostraron un buen ajuste entre las predicciones y el nivel observado de daños. por lo tanto, es posible concluir que desde un punto de vista matemático, el modelo puede calificarse como universal. se reconoce que un modelo objetivo universal real para la vulnerabilidad a los deslizamientos de tierra no es práctico en la actualidad. más importante que el modelo en sí es la metodología que se presenta aquí, que conduce al usuario a tomar información de daños cualitativos del campo y desarrollarla en un marco matemático cuantitativo. los usuarios potenciales del modelo t deben ser cautelosos con respecto a los valores de los parámetros que se presentan en este documento. el modelo t es solo una propuesta modesta que requiere una mayor calibración y profundas críticas de expertos. resumen record manuscript received:28/10/2017 accepted for publication: 16/08/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 251-273 g eo te c h n ic a l en g in ee r in g 252 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides introduction landslide risk analysis is inherently complex. the greater difficulties in achieving reliable results for landslides in comparison with other natural threats, such as earthquakes and floods, have been highlighted in the literature. such difficulties are due to the complexity of modeling landslide hazard, intensity and the vulnerability of the built environment to landslides (uzielli et al., 2008). the concept of vulnerability is often related to the degree of loss of an element suffering damage during the interaction with an external force of a given intensity (hollenstein, 2005; uzielli et al., 2008; fuchs, heiss, and hübl, 2007; unesco, 1979). some other definitions and related concepts have been studied in several technical publications. thus, the terminology is sometimes frustrating and even confusing as visible in the paper by nicu (2016) where the concept of vulnerability is associated to a semi-quantitative index that represents a measure of the exposure of a population to some hazard; the index is a composite of five quantitative indicators properly combined via ahp (analytic hierarchic process) to obtain a numerical result. in this case, it is clear that exposure is proposed as synonymous of vulnerability which differs from some classical papers (li et al, 2010; fell et al., 2005) where the exposure (e) is a measure of the probability of physical interaction between the hazard and the vulnerable (v) element (people, buildings, environment, infrastructure). once the interaction is materialized, a consequence (c=vxe) came to appear in the form of economic/environmental/societal loss. the external force represents a natural phenomenon (hazard) that may be characterized in terms of temporal, spatial and magnitude probabilities. the effects of the interaction between the hazard and the exposed element depend on the intensity of the hazard, which may be defined as “a set of spatially distributed parameters describing the destructiveness” of the hazard process (hungr, 1995) and, alternatively, on the resistance, which sometimes is called susceptibility, of the element at risk, which describes the propensity of a building or other infrastructure to suffer damage from the impact of a specific hazard (li et al., 2010) vulnerability is widely accepted to be defined as the degree of loss (or damage. consequently, a modern concept of vulnerability must consider not only the intensity of the hazard but also the structural resistance of the exposed infrastructure. this concept is referred to as physical vulnerability, and the most accepted definition, per (fell et al., 2005), is a representation of the expected degree of loss quantified on a scale of 0 (no damage) to 1 (total destruction). thus, a vulnerability assessment requires an understanding of the interaction between the hazard and the exposed element. this interaction can be expressed using empirical vulnerability curves (quan luna et al., 2011), mathematical – decisional models (pascale, sdao, and sole, 2010), heuristic rules (birkmann et al., 2013), physical vulnerability curves (kang and kim, 2015), quantitative vulnerability functions (fuchs, heiss, and hübl, 2007), expert knowledge systems (bell and glade, 2004; zêzere et al., 2008), numerical modeling (llano-serna, farias, and martínez-carvajal, 2015) and quantitative estimation of vulnerability that are based on scenarios that consider different kinematic intensity models (li et al., 2010). from the natural science perspective, vulnerability assessment can be split into two main procedures, which require different methods and assumptions: estimation of the vulnerability of life and vulnerability of property. despite its importance, defining the potential fatalities has not been intensely considered in landslide risk management, perhaps due to the intrinsic difficulty of its objective definition (bell and glade, 2004). only recently, some authors have approached the problem, largely relying upon considerations on the host structures and infrastructure, population census data such as density, education level or average age (liu and lei, 2003) or consequence analysis (uzielli et al., 2008). previous studies about landslides and debris flows have mainly focused on landslide hazard assessment and the corresponding triggering mechanisms (jaiswal, van westen, and jetten, 2010; van westen, rengers, and soeters, 2003). however, there is an increasing interest in research on risk assessment (totschnig and fuchs, 2013; nicu, 2018). for a risk assessment, vulnerability assessment is required for the elements at risk during landslide/flow. as stated in (mergili et al., 2012), the term debris flow does not always refer to exactly the same process. some authors consider it a landslide with a fluid-like motion (corominas et al., 2003; mergili et al., 2012), others consider it runoff with very high sediment concentration (o’brien et al., 2007). however, these two approaches are not necessarily contradictory— they depend on whether the observer has a geotechnical or a hydrological background. as presented in figure 1, there are a number of similarities between landslide and debris flow related to the vulnerability assessment. thus, it is possible to postulate that a unique model, when properly defined, may consider the quantification of the physical vulnerability for both processes. the main objective of this paper is to present a physical vulnerability model for diverse types of structures to allow a quantitative assessment of landslide/debris flow risks. additionally, the vulnerability function is characterized according to the structural resistance of the buildings and their spatial location in relation to the hazard position. the proposed model has potential applications in regional quantitative assessment of the physical vulnerability of buildings to landslide and debris flow events. landslide debris flow hazard: landslide. effect on buildings: major/total damage depending on the structural resistance and intensity of sliding mass. persons in buildings: injuries depend on the velocity of the landslide and the structural resistance of the building. hazard assessment: volume, water content, type of soil, triggering factors, temporal/spatial probability. vulnerability assessment: distance to element at risk, local topography along possible track, speed of sliding mass. hazard: debris flow. effect on buildings: filled with mud. partial/major damage. persons in buildings: minor injuries depend on the velocity of the debris and the structural resistance of the building. hazard assessment: volume, slope steepness, water content, type of soil, triggering factors, temporal/spatial probability, size of blocks transported. vulnerability assessment: distance to element at risk, local topography along possible track, speed of sliding mass, travel distance prediction. figure 1. schematic representation of landslide and debris flow damages related to the location of the elements at risk in relation to the hazard. modified from (van westen, van asch, and soeters, 2006). 253a mathematical approach for assessing landslide vulnerability vulnerability framework according to (li et al., 2010), vulnerability depends on both the characteristics of the element at risk and the landslide intensity. the abovementioned authors proposed a model in which the vulnerability (v) is analyzed on average as a function of the hazard intensity (i) associated with exposed elements at risk and the resistance ability (r) of the elements to withstand a threat, as presented in equation 1, where both intensity (i) and resistance (r) is expressed in non-dimensional terms. the intensity may be expressed using any of the next variables or their combinations: velocity of sliding mass, energy, volume and depth of mobilized detritus. as depicted in figure 1, the quantification of the intensity may be assessed using the velocity, as presented in equation 2. v f i r i r i r r i r = ( ) = ≤ − −( ), ; . ; 2 0 5 1 2 2 2 2 2 0 i r i r . ; 5 1 1 1 < ≤ >            (1) i i idyn dpt= * (2) where idyn is the dynamic intensity factor (eq. 3) and idpt is the debrisdepth factor (eq. 4). i c dyn = ≤0 00 5. ; xx mm s log c mm s c x mm s 10 1 36 6 3 5 10 7 10 2 7 − − ( ) +( ) ( ) > ( )     . (3) i *ddpt dpt= 0 1. (4) in the previous equations, c represents the averaged velocity (in mm/s) of sliding mass, and ddpt is the debris depth (in meters) at the building location. taking into consideration that the methodology must be applied as a planning tool prior to the occurrence of the landslide/debris event, the estimation of the parameters c and ddpt must be performed using empirical, simplified analytical or numerical simulation methods. considering that the simplified and empirical methods do not always guarantee the necessary accuracy for most of the applications and that the numerical simulations demand high computational costs, the proposed vulnerability model is difficult to incorporate, at least as a regional tool, in a gis (geographic information system) environment for planning purposes. using the creativity of li´s model, a new variable was defined, named t, which is presented in equation 5 and which was originally proposed by (guimaraes-silva 2015). t z r = (5) where r represents the structural resistance of the building and z represents a modified gradient defined in equation 6. z h d = (6) the extreme values of v are zero and 1. according to equation 5, the independent variable z represents the intensity of the landslide process; consequently, it strongly controls the value of the vulnerability. the introduction of variable t is necessary because it considers not only the intensity, but also the structural resistance of the buildings, r. the lower the resistance of the building, the higher the vulnerability. figure 2 shows a schematic representation of the topographic variables h and d. it is noticed that d must be measured along the possible track of the sliding mass from the hazard location until the center of the plan area of the building. the measurement of d is straightforward when using the appropriate tools of most of the common gis software already available. agreeing with iverson, logan, and denlinger (2004), debris flow can be divided into hillslope debris flow (landslides) and channelized debris flow. hillslope debris flow has a smaller size, short travel distance and faster moving speed compared with channelized debris flow. to understand the type of debris flow, the abovementioned author studied the mobility index of debris as the ratio of the horizontal distance l between the source area and distal limit of the deposit to the difference in height h, which corresponds to the mobility of gravity-driven mass flows. the origin of such approximation is the fahrboschung, tanα=h/l, which is defined by the ratio between the start–stop point elevation difference of the debris (h) and the corresponding horizontal travelled distance (l). the fahrboschung angles range from 21° to 24° for the coarse portion of the debris and from 11° to 14° for the respective fluid portions (nocentini et al., 2015). in this study, both the mobility index and the fahrboschung were tested as descriptors of the landslide/debris intensity without satisfactory results. the higher the sinuosity of the track of the sliding mass, the poorer the capacity of these variables to properly approximate the landslide intensity. finally, the resistance (r) is easily estimated by li´s model, and consequently, it is not necessary to introduce modifications to the original formulation. thus, equation 7 may be used directly. r sty sht smn sfd= ( )   . . . 1 4 (7) where sty, sht, smn, and sfd are the resistance factors of the structure type, height, maintenance state and foundation depth, respectively. these factors are comprehensively enlightened, and their values are presented in li et al. (2010). figure 2. definition of the topographic variables h and d. modified from glade, anderson, and crozier, 2005. 254 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides theoretical background for the t-model origin according to juarez-badillo (1985), all natural phenomena have a quality of beauty, which means that they are ordered and simple. the quality of order means that these phenomena may be described using mathematical equations. on the other hand, the quality of simplicity means that the referred equations are harmonic. in this context, harmony means that the equations must be complete and symmetrical. appropriate variables are those that are capable of describing the phenomena in the simplest way possible. for example, for a defined change in volume in a soil mass, the volume (v) together with the variable (1+e) are the appropriate variables, whereas the void ratio (e) wouldn’t be by itself. consider a phenomenon that is described by the appropriate variables x and y in such a way that y=y(x) with x being in the real domain from zero (0) to infinite (∞). if for a set of extreme values of y, y0=y(0) and y∞=y(∞), then the real domain of y is not complete. then, consider ζ=ζ(y) the appropriate function of y with a complete real domain, and this is ζ(y0)=0 and ζ(y∞)=∞. there are two complementary basic principles: • the equation that relates y and x may exist uniquely through a nondimensional parameter and must satisfy the boundary conditions y0=y(0) and y∞=y(∞) independently of critical points. • the relationship between y and x may exist merely through the appropriate function ζ, and it must possess a satisfactory non-linear proportionality, as shown in equation 8. d dx x ζ ζ α= (8) where α is the proportionality dimensionless parameter. equation 8 defines an appropriate non-linear proportionality between y and x through the function ζ = ζ (y), which is presented in equation 9.  = − − −∞ ∞ 1 1 0 y y y y (9) where y y∞ < 0. note that in equation 9, ζ(y0)=0 and ζ(y∞)=∞. additional details about the philosophical conception of this procedure are presented in juarez-badillo (1985), where the author uses the code, named “principle of natural proportionality”, to obtain general equations for describing stressvolume-time and stress-strain behavior of geomaterials, explain common geotechnical problems as settlements, consolidate clayey soils and cyclic behavior of granular soils, among others. returning to the previously presented vulnerability model, it is noticeable that the phenomena that describe the interaction between the sliding mass (landslide/debris) and the building are represented using two simple variables: v and t. the real domain of t (eq. 5) is from 0 to ∞, and consequently, the vulnerability function assumes values in the domain from 0 to 1. in other words, when t is minimum (equal to zero), the vulnerability (v) is zero; on the other hand, when t reaches its maximum theoretical value (∞), the vulnerability is 1. the relationship between these variables, according to the “principle of natural proportionality,” must be stated thorough an “appropriate” function, with a complete real domain between 0 and ∞. therefore, the functions  = −1 1 v and t are the simplest with real complete domains, and consequently, it is possible to verify that when t varies from 0 to ∞, ζ varies from ∞ to 0. the relationship between these appropriate functions must be of the type expressed in equation 10. d dt t ζ ζ −δ= (10) where δ is the proportionality coefficient. figure 3 presents a schematic representation of v=f(t) in terms of appropriate variables with a complete real domain. when t varies from 0 to ∞, the variable 1/ζ also varies from 0 to ∞, and thus, equation 11 can be presented as d dt t 1 1 ζ ζ δ= (11) the left term of equation 11 can be rewritten to obtain d d d 1 1 1 2       = − = − − (12) by combining equation 11 and equation 12, the initial equation 10 is obtained, which must be integrated to obtain in inζ ζ ζ −δ] =  1 1 t t t (13) where (t1 1, ) is a known point that leads us to ζ ζ δ 1 1 =     − t t (14) from figure 3, the expression for the variable ζ is introduced in equation 14 to obtain equation 15 to equation 17. figure 3. schematic representation of v=f(t) using an appropriate variable ζ. 255a mathematical approach for assessing landslide vulnerability 1 1 1 1 1 1 v v t t − − =     − (15) 1 1 1 1 1 1v v t t = −         + − (16) v v t t = −         + − 1 1 1 1 1 1  (17) the known point ( , )t v1 1 must be encountered from field observation. here, it defines t1 as the “characteristic value” of t when v1=0.5, and then, equation 17 becomes v t t =     + − 1 1 1  (18) v t t =     +         − − 1 1 1  (19) finally, equation 19 is the theoretical vulnerability model, where t is the variable previously defined in equation 5 and δ is a coefficient that governs the average relationship between the sliding mass and the building. the calibration process leads to the determination of the best values for both t1 and δ. calibration procedure for the t-model to obtain the “characteristic value” t1, a methodology was adopted that is based on the observation of the consequences of several landslides, which occurred in nova friburgo, brazil during the extreme rainfall event on the 11th and 12th of november, 2011. the catastrophic rainfall produced 1620 landslides in the urban area of nova friburgo and more than 7000 landslides in the rural area of the “serrana region” that includes the municipalities of nova friburgo, petrópolis, conselheiro paulino, cachoeiras do macacu and bom jardim. because of the high number of fatalities (more than 1000) and extreme economic losses, this event is currently considered the worst meteorological disaster in brazil (entralgo, 2013). further details on the event itself are presented in margottini, canuti, and sassa, 2013, and entralgo, 2013. the calibration procedure involves a three steps methodology where the first step is the analysis of the landslide inventory, then it comes the determination of the parameters t1 and δ; and finally the analysis of the effect of the modified gradient on the theoretical vulnerability. every one of these steps is detailed presented in next sections. landslide inventory according to entralgo (2013), the number of landslides inventoried in “serrana region” was 7 268 in an area of 1 217.67 km2 (figure 4). because the majority of landslides occurred in rural areas, it was necessary to include the urban area of nova friburgo in a polygon where the consequences on buildings and civil infrastructure were more severe (figure 5). figure 6 was prepared to show a zoomed area of the selected polygon with the intention of showing an example of the type of landslide and its effect on buildings. the next step was to identify all structures inside the polygon that were affected by landslides and using historical imageries, to make a before-after comparison of damages, as shown in figure 7. the coordinates datum used in this study was utd/datum sirgas 2000. using the gis tools, it was possible to identify the position of the buildings before landslide occurrence, as seen in figure 8. a total of 166 structures were identified, and a database was created to store landslide information (table 1) and building information (table 2 ). with the purpose of evaluating the resistance parameters for damaged buildings, according to li´s model (eq. 7), the google street view tool was used, which allows for the observation of buildings from the ground, as shown in figure 9 and figure 10. the observed vulnerability was initially defined in a qualitative way by discerning the degree of damage on a scale of: total (vobs=1), intense (vobs=0.7-0.9), medium (vobs=0.4-0.6), and slight (vobs<0.3). figure 4. map showing the landslide inventory of the extreme rainfall event on november 11-12, 2011 in “serrana region”, rio de janeiro, brazil. (entralgo, 2013). 256 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides figure 5. aerial view of urban landslides in the municipality of nova friburgo. figure 6. typical appearance of landslides in the urban area of nova friburgo and effects on buildings. graphical scale in figure. 257a mathematical approach for assessing landslide vulnerability resistance parameters from li´s model were properly identified, and a database was created, as shown in table 3. determination of parameters t1 and δ. the structures that have values of observed vulnerability between 0.4 and 0.6 (medium damage) were separated from the original database, and variables z and t were calculated. in total, only 17 structures satisfied this condition, and their calculations are presented in table 4. the adopted strategy to calibrate the model was to use a characteristic value of the vulnerability equal to 0.5. because the buildings affected by the nova friburgo landslide events presented a wide range of vulnerabilities and taking into consideration that estimation of the damage was made by direct field observation, it was decided to use those buildings that presented observed vulnerabilities in the range 0.4 to 0.6 with the purpose of having a mean value as close as possible to 0.5. figure 7. comparison (before/after) of google images that show damage to the building produced by the landslide. figure 8. aerial view of the landslide number 4 479 and 13 damaged buildings. table 1. example of the database for the landslide 4479. landslide id area (m2) ctmax (m) ctmin (m) 4 479 4 312 927 872 notes: id – identification number of landslide; ctmax – maximum height above the sea level; ctmin – minimum height above the sea level. table 2. example of the database for the buildings affected by the landslide 4479. building id area (m2) e coordinate n coordinate d(m) as defined in figure 2 4479-1 95.0 753946.7 7538609.0 128 4479-2 159.2 753960.5 7538611.9 139 4479-3 49.6 753966.2 7538606.5 140 4479-4 57.2 753956.6 7538601.3 131 4479-5 87.7 753948.3 7538592.5 124 4479-6 34.9 753973 7538596.9 142 4479-7 83.2 753964.9 7538595.3 134 4479-8 58.7 753954.6 7538590.8 122 4479-9 150.3 753985.7 7538610.9 157 4479-10 147.5 753989.6 7538622.7 167 4479-11 70.7 754015.3 7538639.9 129 4479-12 50.0 754009.6 7538638.8 119 4479-13 86.5 753999.7 7538636.4 109 notes: id –identification number of the building; d – track distance of debris from the highest point of the landslide to the center of the building. 258 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides figure 9. floor view of buildings 1, 3, 4, 5, 11, 12 and 13 damaged by the landslide 4 479, acquired using google street view. table 3. example of the database for the buildings damaged by the landslide 4 479. building id vobs observed damage resistance parameters r ξsfd ξsty ξsmn ξsht 4479-1 1.0 total 0.60 0.8 0.4 0.1 0.37 4479-2 1.0 total 0.60 0.8 0.4 0.1 0.37 4479-3 1.0 total 0.60 0.8 0.4 0.1 0.37 4479-4 1.0 total 1.00 0.8 0.4 0.4 0.60 4479-5 1.0 total 1.00 0.8 0.4 0.4 0.60 4479-6 1.0 total 0.65 0.8 0.8 0.4 0.64 4479-7 1.0 total 0.80 0.8 0.8 0.4 0.67 4479-8 1.0 total 0.90 0.8 0.8 0.4 0.69 4479-9 1.0 total 1.00 0.8 0.4 0.4 0.60 447910 0.3 slight 1.00 0.3 0.4 0.4 0.60 447911 0.6 medium 1.00 0.6 0.4 0.4 0.60 447912 0.7 intense 1.00 0.8 0.4 0.4 0.60 447913 0.6 medium 1.00 0.4 0.4 0.4 0.60 notes: id – identification number of the building; vobs – observed vulnerability according to image analysis; ξsfd; ξsty; ξsmn and ξsht – resistance factors according to li´s model; r – building resistance according to equation 7 figure 10. floor view of the building 10 that was damaged by the landslide 4 479, acquired using google street view. as previously mentioned in the formulation of equation 19, the known point (t z1 1, ) must be found from field observation in such a way that t1 is the “characteristic value” of t when v=v1=0.5. as seen in table 4, the mean value of observed vulnerability is 0.46. thus, it can be approximated to 0.5. on the other hand, the mean value of variable t is 0.55. thus, it is possible to conclude that those values precisely table 4. values of the variable t for the structures with observed vulnerability between 0.4 and 0.6. building id h(m) building area (m2) d (m) as defined in figure 2 observed vulnerability r z=h/d t 3149-2 14.0 57.8 31.0 0.5 0.79 0.45 0.57 3438-5 11.0 60.5 51.7 0.5 0.60 0.21 0.36 3530-3 27.0 61.6 62.6 0.4 0.60 0.43 0.72 3814-2 18.0 54.7 54.9 0.5 0.75 0.33 0.44 3817-2 13.0 119.7 32.3 0.5 0.79 0.40 0.51 3834-2 24.0 78.6 70.9 0.5 0.60 0.34 0.57 3834-3 24.0 138.1 72.7 0.5 0.50 0.33 0.66 4089-2 42.0 166.8 108.5 0.4 0.79 0.39 0.49 4224-6 20.0 173.0 42.4 0.4 0.60 0.47 0.79 4479-11 55.0 70.7 129.1 0.6 0.60 0.43 0.71 4479-13 50.0 86.5 109.3 0.4 0.60 0.46 0.76 4635-11 60.0 56.8 170.6 0.5 0.50 0.35 0.70 4639-1 38.0 110.7 77.9 0.4 0.87 0.49 0.56 4639-2 38.0 114.1 170.8 0.5 0.87 0.22 0.26 4639-4 38.0 130.2 168.7 0.5 0.87 0.23 0.26 4782-1 18.0 8326.9 149.0 0.4 0.75 0.12 0.16 5016-13 62.0 56.2 92.9 0.4 0.75 0.67 0.89 mean values 0.46 0.69 0.37 0.55 notes: id – identification number of the building; h – height difference between the landslide initiation point and structure; d – track distance of debris from the highest point of landslide to the center of the building; r – building resistance (eq. 7); z – modified gradient (eq. 6); t – t-model´s main variable (eq. 5). 259a mathematical approach for assessing landslide vulnerability effect of the modified gradient (z) landslides as previously mentioned, the modified gradient (eq. 6) is directly proportional to the height difference between the hazard location and the building and inversely proportional to the distance measured along the possible track of the sliding mass. in a qualitative sense, z may be intended as a measure of landslide potential intensity when interacting with the structure. the histogram of z (figure 12) was prepared with all values measured in the nova friburgo massive landslide event, and it indicates that 70% of the cases are included in the interval (0.15<z<0.55). in a complementary way, a histogram for the resistance was prepared, and it is presented in figure 12. a summary of main statistics is presented in table 5. considering the mean values of the variables involved and their standard deviations, curves of vulnerability as a function of structural resistance were prepared for three different values of the modified gradient: zmean=0.5; zmean+1std.dev. =0.8; and zmean -1std.dev =0.2. these curves are presented in figure 13 together with a schematic representation of the effect of z in a structure with a definite structural resistance. match the known point (t z1 1, ), which has been previously defined as the “characteristic values” of the t-model. the introduction of “characteristic values” in eq. 19, is the way to obtain eq. 20, which is straightforward. v t=     +         − − 0 55 1 1 .  (20) the next step is the calibration of the coefficient δ, which governs, on average, the relationship between the variables v and t. in the authors’ opinion, δ depends mainly on the type of mass movement: debris or landslide, as shown in figure 1. thus, the value must be unique for all landslides that are studied in nova friburgo. by testing different values of δ, theoretical curves of v vs. z were compared with the observed values. the best fit for the complete set of structures in the database was reached for δ=3.5. thus, equation 20 becomes v t=     +         − − 0 55 1 3 5 1 . . (21) to improve visualization, figure 11 shows the curves v vs. z for three theoretical values of structural resistance (r=0.2; 0.5 and 0.8). the points represent observed values for all structures in the database, which are grouped into three categories: low resistance (r<0.4) – represented by an r=0.2 theoretical curve; medium resistance (0.4<=r<0.7) represented by an r=0.5 theoretical curve; and high resistance (r>=0.7) represented by an r=0.8 theoretical curve. figure 11. theoretical curves of v vs. z and comparison against the observed field values for δ=3.5 and three different structural resistance values: r=0.2; 0.5 and 0.8 (from left to right, respectively). figure 12. frequency histogram for the modified gradient z (left) and structural resistance r (right). table 5. summary of basic statistics for the modified gradient and structural resistance. variable minimum maximum mean value std. deviation modified gradient, z 0.04 2.2 0.45 0.30 structural resistance, r 0.22 1.5 0.56 0.21 260 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides theoretical curves presented in figure 13 show that for a specific value of resistance, the higher the modified gradient is, the higher the vulnerability. on the other hand, it is clearly shown that for low values of the modified gradient (see the z=0.2 curve), the vulnerability significantly decreases when resistance increases. this observation may be of remarkable importance in risk management planning processes when reinforcement of existing structures is considered as an acceptable measure for risk reduction and control. flows when compared to landslides, the flow-type movements are characterized by bigger sizes and long travel distances. therefore, their interaction with buildings must be fundamentally different and characterized using a different δ coefficient in equation 20. to find a proper value for δ and considering the lack of available quantitative information about damages produced by these type of processes, field observations were used, which were made in the district of “bento rodrigues”, minas gerais, brazil, which was destroyed by an earth flow that was produced by the failure of a tailings dam in november of 2015 (figure 14)1. figure 15 shows a zoomed area where field observations were carried out three months after the tragedy. five points were selected for the assessment of the vulnerability, as indicated in the same figure. selected points (letters a to e) are also illustrated in the photographs of figure 16. from the direct observation of buildings, which correspond to photos a, b and c in figure 16, it was deduced that they presented an average vulnerability of 0.5. thus, they can be used to calibrate the model and obtain the δ coefficient value. the first step is the determination of the “characteristic value” t1, which is related to v=v1=0.5. from the digital terrain model and ground gps data, the difference in height between buildings and the dam breaking point was determined (see values on level lines in figure 15). table 6 presents the corresponding values. thus, it can be concluded that t1=0.05. the next step is to test different values of δ to find the one that reproduces the observed vulnerability of all buildings. the best value is δ=1, and thus, equation 22 is the expression used to estimate the vulnerability of buildings due to this flow-type mass movement. v t=     +         − − 0 05 1 1 1 , (22) 1. http://g1.globo.com/minas-gerais/videos/v/imagens-por-satelite-mostram-como-bento-rodrigues-ficou-apos-rompimento-de-barragens/4600744/ table 7 presents the vulnerability data collected for the five selected houses in “bento rodrigues” and two additional structures included with the intention of verifying the consistency of the model. the vulnerability values of houses e and d (0.82 and 0.78 respectively) agree with the field-observed behavior (see photos in figure 16). the additional structures are the church of “paracatu de baixo” district and a theoretical typical house of the region characterized, on average, as having a very low structural resistance. “paracatu de baixo” is a small village located approximately 38 km downstream from the tailings dam. the village´s church was partially covered by the earthflow without suffering appreciable structural damage (figure 17). according to newspapers, most houses of the village were destroyed. however, reliable technical information about damage quantification is not available. the estimated average vulnerability using the t-model is 0.59 (medium vulnerability), which represents significant structural damage. considering the mean values of the variables involved, curves of vulnerability as a function of structural resistance were prepared for three different values of modified gradient z: 0.05; 0.01 and 0.005 (see figure 18). these curves are presented together with a schematic representation of the effect of z in a structure with a definite structural resistance. application of the t-model to landslides in medellín, colombia medellin is the capital city of the province of antioquia, which is located in the central part of the nw colombian andes. the city occupies the bottom and partially hillslopes of a deep valley that is locally named the “aburrá valley” with an extension of 1152 km2. due to the complex geomorphological and geological framework of the valley, several catastrophic landslides have occurred in the last century. the uncontrolled occupation of the hillslopes without adequate building figure 13. effect of z in the landslide vulnerability of buildings and curves v vs. r (structural resistance) on different values of z. the schematic representation was adapted from glade, anderson, and crozier, 2005. table 6. geometrical and resistance parameters of three selected buildings with v=0.5 in bento rodrigues, mg, brazil. building geometrical parameters for eq. 6 structural resistance parameters of li´s model (li et al. 2010) r t from eq. 5 h (m) d (m) z ξsty ξsmn ξsht ξsfd a 50 2500 0.020 0.4 0.8 0.1 0.42 0.34 0.059 b 56 2300 0.024 0.8 0.4 0.4 0.64 0.53 0.046 c 56 2900 0.019 0.4 0.8 0.1 0.42 0.34 0.057 characteristic value of t (named t1 in eq. 19) 0.054 table 7. vulnerability data for five buildings in “bento rodrigues”, the church of “paracatú de baixo” district and a typical very low resistance house. building track distance, d (m) modified gradient, z structural resistance, r t from eq. 5 vulnerability a 2500 0.020 0.34 0.059 0.60 b 2300 0.024 0.53 0.046 0.50 c 2900 0.019 0.34 0.057 0.50 d 2300 0.028 0.20 0.139 0.82 e 2600 0.024 0.20 0.118 0.78 paracatú de baixo church 38000 0.006 0.86 0.01 0.12 very low resistance buildings in paracatú de baixo 38000 0.006 0.08 0.07 0.59 261a mathematical approach for assessing landslide vulnerability figure 14. aerial view (from google earth) before and after the failure of the tailings dam in bento rodrigues, minas gerais, brazil. zoomed rectangle can be seen in figure 15. 262 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides figure 15. detailed view of the central part of bento rodrigues district and five selected points for analyzing vulnerability. the values indicated on level lines correspond to the difference in height between buildings and dam breaking point. figure 16. photographs of selected structures located at the positions indicated in figure 15. photos were taken in february of 2016 during a field trip, three months after the tragedy. 263a mathematical approach for assessing landslide vulnerability practices produces a high vulnerability condition, and consequently, a very high number of lives have been claimed by massive landslides, which has also resulted in considerable economic losses (aristizábal et al., 2011). figure 19 presents an inventory of the most catastrophic landslides registered in the aburrá valley since 1920. the last three events (la gabriela, poblado-alto verde, and el socorro) where chosen for creating vulnerability estimations according to the t-model to verify the robustness of the model. el socorro landslide according to aristizábal-giraldo (2008), this tragic event claimed the lives of 27 people and left 16 seriously injured at the el socorro settlement. this event, together with rosellón (1927), media luna (1954), santo domingo (1974), villatina (1987) and el barro (2005), represents one of the most tragic events in the aburrá valley history. the mass movement was classified as a simple rotational landslide in the ne direction. due to high saturation levels, the sliding mass moved 140 m ahead in the se direction, and finally, the mass turned to the ne. the triggering causes are associated with antecedent rainfall from the previous 15 days and a very intense rainfall event during may 30th, when 83.3 mm fell in just 2 hours and 30 minutes along the central part of the valley. in order to illustrate the reader about the precipitation regime of medellin, it is mentioned that the mean historical precipitation for the month of may is 200 mm. together with october (mean historical precipitation 207 mm), may is the rainiest month of the year. the driest month is january with mean historical precipitation of 52 mm. figure 20 shows an aerial view of the landslide. the structural resistance of the structures was determined in a generalized way from the descriptions presented in aristizábal-giraldo (2008) and complemented with newspaper photographs. a total of 21 buildings were analyzed, and their vulnerability was calculated, as presented in table 8. the location of the analyzed buildings is presented in figure 21, where contour lines of equal z values are shown. in comparison with the aerial view in figure 20, it is observed that the actual path of the landslide debris is enclosed by the z=0.3 line. figure 17. photos of the church of “paracatú de baixo”, mg, brazil.2 figure 18. effect of z in the debris-flow vulnerability response of buildings and curves v vs. r for different values of the modified gradient, z. the schematic representation was adapted from glade, anderson, and crozier (2005). 2. http://g1.globo.com/minas-gerais/desastre-ambiental-em-mariana/noticia/2015/11/quase-90pessoas-seguem-vivendo-perto-de-area-atingida-por-lama.html table 8. modified gradient (z), structural strength (r) and physical vulnerability (v) of the buildings affected by the el socorro landslide. building id z (modified gradient) ξsfd ξsty ξsmn ξsht r t v 1 0.31 0.42 0.3 0.6 0.1 0.29 1.04 0.90 2 0.32 0.42 0.3 0.6 0.1 0.29 1.09 0.92 3 0.32 0.42 0.3 0.6 0.1 0.29 1.07 0.91 4 0.33 0.42 0.3 0.6 0.1 0.29 1.11 0.92 5 0.33 0.42 0.3 0.6 0.1 0.29 1.11 0.92 6 0.33 0.42 0.3 0.6 0.1 0.29 1.11 0.92 7 0.32 0.42 0.3 0.6 0.1 0.29 1.09 0.92 middle 0.31 0.42 0.8 0.7 0.7 0.64 0.48 0.39 9 0.33 0.42 0.3 0.6 0.1 0.29 1.12 0.92 10 0.32 0.42 0.3 0.6 0.1 0.29 1.09 0.92 11 0.33 0.42 0.3 0.6 0.1 0.29 1.12 0.92 12 0.34 0.42 0.3 0.6 0.1 0.29 1.17 0.93 13 0.36 0.42 0.3 0.6 0.1 0.29 1.22 0.94 14 0.37 0.42 0.3 0.6 0.1 0.29 1.25 0.95 15 0.38 0.42 0.3 0.6 0.1 0.29 1.27 0.95 south 0.29 0.42 1.0 0.6 0.9 0.69 0.43 0.29 north 0.29 0.42 0.3 0.4 0.4 0.38 0.76 0.76 18 0.32 0.42 0.3 0.6 0.1 0.29 1.09 0,92 19 0.32 0.42 0.3 0.6 0.1 0.29 1.10 0.92 20 0.33 0.42 0.3 0.6 0.1 0.29 1.11 0.92 21 0.33 0.42 0.3 0.6 0.1 0.29 1.12 0.92 264 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides figure 19. catastrophic landslides in the last century in the aburrá valley, colombia. adapted from claghorn et al. (2016). figure 20. aerial view of the el socorro landslide depicting the path of the sliding mass (aristizábal-giraldo, 2008). 265a mathematical approach for assessing landslide vulnerability because of the intensity of the landslide and the low structural strength of buildings, all houses were entirely destroyed but three, which were located at the border of contour line z=0.3 suffered partial to intense damage only. the mean vulnerability theoretical value of destroyed buildings is 0.92, which agrees with the catastrophic damages observed. the buildings that figure 21. location of the buildings affected by the el socorro landslide. points represent the buildings. contour lines correspond to equal values of the modified gradient z. figure 23. photographs of the landslide pathway and three buildings that were partially destroyed by the el socorro landslide. figure 22. location of three buildings that suffered partial damage due to the el socorro landslide. the values indicate the vulnerability index of the buildings. suffered partial damage, named north, middle and south, presented theoretical vulnerability values of 0.76, 0.39 and 0.28, respectively (figure 22). the pictures presented in figure 23 show the level of damage suffered by the buildings named north (v=0.8), middle (v=0.4) and south (v=0.3). the vulnerability values agree with the observed damage. 266 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides alto verde landslide a massive landslide occurred at the alto verde residential complex at the end of 2008 in the city of medellin, colombia, claiming the lives of twelve people and destroying six houses (llano-serna, farias, and martínezcarvajal, 2015). the poor geotechnical properties of dunite residual soil and heavy antecedent rainfall were the main causes of the landslide. images in figure 24 show the situation of the alto verde neighbourhood before and after the occurrence of the catastrophic landslide. in those images, it is possible to see the position of the six houses that were destroyed and house 1 that remained undamaged. the cross-section aa’ was used by (llano-serna, farias, and martínez-carvajal 2015) for numerical analysis of the propagation of the sliding mass using the mpm methods (material point method). detailed geotechnical information can be found in the abovementioned reference. the vulnerability calculations require the definition of the points that represent the position of the six destroyed houses, which can be seen in figure 25 together with the contour lines of the modified gradient. because of the existence of natural channels, it is necessary to limit the analysis of the modified gradient behavior to the area where the movement of the sliding mass can naturally occur. those limits are also represented in figure 25, which indicates that vulnerability analysis makes sense only in the area between them. according to the literature review, the sliding mass was completely saturated, which makes the soil behave as a dense fluid. thus, it was characterized as a debris-flow, and consequently, the parameters used in the t-model must be t1=0.05 and δ=1. figure 26 shows the contour map of the modified gradient (z), which can be interpreted as a measure of the spatial evolution of the landslide intensity. as observed in the el socorro landslide, here, the contour z=0.3 also enclosed the limits of actual sliding mass taking into consideration the topographic limitations imposed by the axis of the natural channels mentioned before. the structures that were destroyed are numbered from av1 to av6, and their structural characteristics were obtained from the published literature, which is also presented in (llano-serna, farias, and martínez-carvajal 2015). vulnerability calculations are presented in table 9, where the high vulnerability indexes of the houses are evident, which agrees with field observations. despite the high structural strength of the buildings, their proximity to the sliding source and the high velocity of the sliding mass due to its flow behavior produced catastrophic vulnerability values (v>0.9). photos in figure 27 show the degree of destruction of houses av1 and av3. la gabriela landslide this landslide occurred on the 5th of december in 2010 during a sunny day, affected the la gabriela neighborhood and claimed the lives of 85 people. it is unknown why a formal investigation was never carried out. thus, technical information about the geological and geotechnical aspects is not available. the landslide main scarp was close to the medellin bogotá highway in a place used by local inhabitants for placing demolition waste. regardless of the fact that the landslide was initiated in waste material that is different from the natural soil, the decision of using the vulnerability model was justified because the theoretical background used to support the model is independent of the type of material in the sliding mass. because the waste was not in a saturated situation, the parameters used for the t-model were t1=0.55 and δ=3.5. figure 28 is an aerial view of the la gabriela landslide including marks that indicate the position of damaged buildings and the topographic limit, which corresponds to a local channel that is strongly affected by gravel mining. figure 24. aerial view of the alto verde landslide in 2008 (before) and in 2011 (after). house 1 remains undamaged. table 9. modified gradient (z), structural strength (r) and physical vulnerability (v) of the buildings affected by the alto verde landslide. house id z (modified gradient) xsfd xsty xsmn xsht r t v av1 0.40 0.51 0.4 0.8 0.3 0.47 0.60 0.92 av2 0.37 0.51 0.4 0.8 0.3 0.47 0.56 0.92 av3 0.38 0.51 0.4 0.8 0.3 0.47 0.56 0.92 av4 0.35 0.51 0.4 0.8 0.3 0.47 0.53 0.91 av5 0.34 0.51 0.4 0.8 0.3 0.47 0.52 0.91 av6 0.34 0.51 0.4 0.8 0.3 0.47 0.52 0.91 267a mathematical approach for assessing landslide vulnerability figure 25. location of center points representing the positions of destroyed houses during the alto verde landslide. figure 26. contour map of the modified gradient for the alto verde landslide. points av1 to av6 represent the position of the destroyed houses. 268 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides the photo presented in figure 29 helps the reader to understand the general context of the site. the physical condition of the buildings was comparable to those described for the el socorro landslide. in this case, 44 buildings were analyzed, and all of them had very low structural strength (rmean=0.29). the mean value of the modified gradient was found to be z=0.33. thus, the vulnerability index was 0.9 for all buildings, which agrees with the field observation of the destructiveness of the landslide. a visual comparison between the landslide pathway and contour lines in figure 30 allows us to conclude that the contour z=0.3 enclosed the limits of the actual sliding mass, taking into consideration the topographic limitations imposed by the abovementioned channel. discussion as presented before, the vulnerability model, which was explored here, is a mathematical function that depends on two main variables: t and figure 27. photographs showing the damage to houses av1 and av3 during the alto verde landslide. figure 28. aerial view of the la gabriela landslide site. 269a mathematical approach for assessing landslide vulnerability figure 29. panoramic view of the la gabriela landslide. note the topographic limit of the sliding mass. figure 30. contour map of the modified gradient z for the la gabriela landslide. 270 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides δ (eq. 23). the first variable takes into consideration the relationship between a measure of the potential intensity of the landslide (here named modified gradient, z) and the structural resistance of the buildings. the coefficient δ is responsible for including the effect of different types of mass movements in the model. the characteristic value t1 must be calibrated with field observations and correspond to the value of t that produces v=0.5. v t t =     +         − − 1 1 1  (23) thus, the vulnerability is a continuous surface, as presented in figure 31, which includes all values into the real domain [0,1] depending on the combination of the type of movement and variable t. the general expression presented in equation 23 may be labelled universal because of its capacity for representing, in theory, the structural damage due to the interaction of any type of mass movement and any building. according to van westen, van asch, and soeters (2006), the vulnerability of elements at risk, which are related to landsliding, is extremely difficult to establish for most landslide types because very limited damage data are available. table 10 gives a schematic overview of different types of damage that may be caused by different landslide types and different elements at risk, including notes on the applicability of t-model for assessing landslide physical vulnerability of the structures. unlike other types of hazards, such as earthquakes, flooding or windstorms, relatively little work has been done on the quantification of the physical vulnerability due to landslides (ciurean and schröter, 2013). the problem with landslide vulnerability assessment is that there are many types of landslides, and each one should be evaluated separately (table 10). such vulnerability information should come from historical studies in the first place but can be combined with modelling approaches, empirical approaches and theoretical approaches. assessment of the level of risk using mathematical approaches, as the t-model demonstrated, is sufficient for use at medium to regional scales to decide which areas should be avoided for new developments or to identify areas of relatively high risk for more detailed investigation. at medium scales, the most important input data are event-based landslide inventory maps, which are made directly after any major disaster (earthquake, rainstorm, hurricane, etc.). such an inventory by geomorphologists should emphasize landslide characteristics (types and volumes) as well as damage caused to the different elements at risk (klose, maurischat, and damm, 2015). in addition to the capability of the t-model to represent, in theory, any type of landslide vulnerability due to the impact interaction between a mass and a building, there is a potential for easy and straightforward application in gis environments for spatial analysis. conclusions a theoretical model was presented to assess the physical vulnerability of structures that are impacted by landslides and debris flows. the vulnerability is directly proportional to the value of a new topographic figure 31. graphic representation of the vulnerability surface according to the t-model. 271a mathematical approach for assessing landslide vulnerability table 10. applicability of the t-model for physical vulnerability assessment depending on the type of landslide. modified from van westen, van asch, and soeters (2006). type before after notes on the applicability of the t-model impact by a large rock mass. fully applicable since the movement is massive. coefficient δ must adopt high value. z values would probably be from z=2 to 0.5. characteristic value t1 is difficult to calibrate due to the lack of field observations. impact by single blocks not applicable because individual falling blocks do not slide on terrain. the movement is not massive. impact by a landslide mass. already calibrated in this work. t1=0.55 and δ =3.5. modified gradient values from z=0.2 to 0.8. loss of support due to undercutting. not applicable. the t-model is not intended for erosional effects due to undercutting. differential settlement / tilting due to slow movement. not applicable. the t-model is not intended for tilting effects of buildings on top of slow mass movements. impact by debris flow on a slope. already calibrated in this work. t1=0.05 and δ =1.0. z values are from z=0.005 to 0.05. also applicable to supersaturated landslides with z values as high as 0.4. flooding by debris flow on an alluvial fan. most likely applicable depending on the magnitude of the sliding mass in relation with the size of the channel and its water discharge. if debris is much larger than the river, the model is likely to be used. if not, the model cannot be used because the phenomenon would be closer to flooding. impact by sturzstrom fully applicable since the movement is massive. coefficient δ must adopt a high value. modified gradient values would probably be from z=0.1 to 0.05. characteristic value t1 is difficult to calibrate due to the lack of field observations. liquefaction not applicable because the model is intended for vulnerability due to impact and not due to a building that sinks deep-seated creep movement not applicable. the t-model is not intended for the tilting effects of buildings that are settled on creeping hillslopes. 272 hernán eduardo martínez carvajal, maria tâmara de moraes guimarães silva, edwin fábian garcía aristizábal, edier v. aristizábal-giraldo, mayra alejandra larios benavides variable named the “modified gradient” (eq. 6) and inversely proportional to the structural resistance of buildings, which is measured as proposed by li et al. (2010). the proposed theoretical model is based on a philosophical analysis of nature’s behavior, which has been originally proposed in juarezbadillo (1985) and named the “principle of natural proportionality”, which establishes that all natural phenomena can be described in an ordered and simple manner using “appropriate” variables. the model was initially calibrated using the database collected from a catastrophic rainfall-triggered landslide event that occurred in nova friburgo, brazil, in november of 2011. the proportionality between the variables involved in the proposed model is made using a coefficient, δ, which holds the nature of the sliding mass in a broad way, that is involving the physical characteristics of the process that governs its destructive capacity. the results of the nova friburgo landslide vulnerability simulations agree with the observed damages. it was deduced that for the soil-slip landslides, such as those that occurred in nova friburgo, the coefficient of proportionality adopts the value of δ=3.5. in contrast, when the sliding mass behaves as a flow, the coefficient of proportionality acquires a lower value, δ=1. it was revealed that to calibrate the model, the user must look for field evidence of buildings that suffered 50% of structural damage after being impacted by landslides. depending on the type of landslide, this damage level may appear as close as 50 m from the hazard source or as distant as a few kilometers (as in the case of flow-type movements). according to the authors’ opinion, once calibrated for a certain type of mass movement, the model may be used in a general way regardless of the site or the geographical situation. thus, the general expression for the model (eq. 19) may be considered universal. in addition to the capability of the t-model to represent, in theory, any type of landslide vulnerability due to the impact of an interaction between a mass and a building, there is a potential for its easy and straightforward application in gis environments for spatial analysis, which represents one of the most important advantages over other vulnerability models that are already available. on the other hand, the introduction of the modified gradient concept and its physical interpretation leads to the direct application of the model in risk reduction processes. values of z=0.3 show good agreement with the actual deposition paths of the analyzed landslides. finally, the authors must state that the universality of the t-model must be assumed as a simple mathematical expression. it is recognized that a real universal objective model for vulnerability to landslides is not currently practical. more important than the model itself is the methodology that is presented here to take field qualitative damage information and turn it into quantitative information in a mathematical framework. as other vulnerability models, the t-model is not able to fully exploit its potential until a more systematic analysis of data of the damage induced by landslides on the built environment is performed. of course, the potential users of the t-model must be critics in relation to the values of the parameters that are presented in this paper. the t-model is just a modest proposal that requires further calibration and deep expert criticism. acknowledgements the work presented in this paper was supported by the university of brasilia, brazil, through the graduate program on geotechnical engineering, the cnpq (brazilian national council for scientific and technological development) and the faculty of engineering of the university of antioquia at medellin.). the authors gratefully acknowledge the support. references aristizábal-giraldo, e. v. 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(2008). probabilistic landslide risk analysis considering direct costs in the area north of lisbon (portugal). geomorphology, 94, 467–495. https:// doi.org/10.1016/j.geomorph.2006.10.040. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n4.94720 earth sciences research journal earth sci. res. j. vol. 26, no. 4 (december, 2022): 345 358 g eo te c h n ic s sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india vijendra kumar pandey1*, milap chand sharma2 1department of geography, kirori mal college, university of delhi, delhi, india 2centre for the study of regional development, jawaharlal nehru university, new delhi, india *corresponding author’s email: vijendrapandey@kmc.du.ac.in manuscript received: 26/03/2021 accepted for publication: 11/01/2023 how to cite this item: pandey, v. k., & sharma, m. c. (2022). sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india. earth sciences research journal, 26(4), 345-358. https://doi. org/10.15446/esrj.v26n4.94720 abstract every year during the indian summer monsoon, large number of landslides occur in the lesser and the greater himalayan rock formations, triggered by intense rainfall episodes coupled with physiography and anthropogenic activities. the present study investigates the slope failure mechanism’s relationship with slope material compositions. hence, sediment samples of 25 landslides were collected along the road corridors. these samples were collected from the lesser and greater himalayan ranges and rock formations. the sediment was collected from the active landslides to understand particle size, clay content, moisture content, mineral composition, crystallographic structures, and the influence of geomorphic processes on the landslide failure processes. the samples were analyzed using a sieve, x-ray diffractometry (xrd), and scanning electron microscopy (sem) to accomplish the study’s objectives. the analysis indicates that the lesser himalayan meta-sedimentary rock formations have a high composition of fine and medium-size particles, lesser quartz mineral compositions with calcite, and a highly crushed and fractured presence, conchoidal fractures types of morphological features. micrographs obtained from the schist and phyllite rock of the lesser himalayan origin shows highly sheared and crushing, crystal overgrowth; and, in turn, have a higher susceptibility to landslides. the relationship between slope materials and instability has shown a definite pattern in the study area. the debris flow and slump have a comparatively higher percentage of clay and silt compared to debris fall, debris slide, and rockfall. the particle size composition of sediment collected from the slip zone is significantly related to the types of landslides. the present study is helpful in understanding the sediment composition and slope failure mechanism. keywords: landslide; grain size distribution; clay mineralogy; x-ray diffraction; slope instability; garhwal himalaya. características sedimentológicas y su relación con los corrimientos de tierra en la cuenca de bhilangana, garhwal himalaya, india resumen cada año durante el verano monsónico en la india ocurren varios corrimientos de tierra en las formaciones rocosas de lesser himalayan y greater himalayan desencadenados por las intensas lluvias y afectados por actividades fisiográficas y antropogénicas. este artículo investiga el mecanismo de falla de taludes y su relación con los materiales que componen el terreno. con este fin se recolectaron muestras en 25 escenarios de corrimiento a lo largo de los corredores viales. estas muestras se tomaron en las cordilleras y en las formaciones rocosas del lesser himalayan y del greater himalayan. el sedimento fue recolectado en corrimientos activos para analizar el tamaño de las partículas, el contenido de arcillas, la proporción de humedad, la composición mineral, las estructuras cristalográficas y la influencia de procesos geomórficos en los procesos de falla de los corrimientos. las muestras fueron analizadas con cedazo, con difracción de rayos x y con microscopio electrónico de barrido para conseguir el objetivo del estudio. los análisis indican que las rocas metasedimentarias de la formación del lesser himalayan tienen una composición alta de partículas de tamaño fino y medio, composiciones minerales bajas en cuarzo con calcita y una alta presencia de tipos de fracturas concoides, molidas y fracturadas, con características morfológicas. las micrografías obtenidas de los esquistos y rocas de filita del lesser himalayan muestran cristales sobrecrecidos, cortados y molidos. por ende, estos tienen una mayor susceptibilidad a corrimientos. la relación entre los materiales del terreno y la inestabilidad muestran un patrón definido en el área de estudio. el flujo de detritos y de materiales poco consolidados tienen un porcentaje comparativamente mayor de arcillas y cieno en comparación con los detritos caidos, detritos deslizados y rocas caidas. la composición del tamaño de la partícula del sedimento recolectado en la zona de caida está significativamente relacionado a los tipos de corrimientos. el presente estudio es útil para entender la composición del sedimento y el mecanismo de falla de taludes. palabras clave: deslizamiento de tierra; distribución del tamaño del grano; mineralogía de arcillas; difracción de rayos x; inestabilidad de pendiente; garhwal himalaya https://doi.org/10.15446/esrj.v26n4.94720 https://doi.org/10.15446/esrj.v26n4.94720 https://doi.org/10.15446/esrj.v26n4.94720 346 vijendra kumar pandey, milap chand sharma 1. introduction landsliding is the dominant mass-wasting process in the mountainous area (burbank et al., 1996; hovius et al., 2000; meunier et al., 2008; brunetti et al., 2014), where young streams continuously undercut the slopes. a significant challenge in landslide investigation is to identify the slope failure mechanism under different physiography and its relationship with slope material compositions. a critical relationship was found between the characteristics of slope materials and the composition of clay-silt sediment on the sliding surface; and the slope failure mechanism (summa et al., 2010; azañón et al., 2010; brunet et al., 2016; summa et al., 2018). geomorphological investigation of slope instability is very crucial to understanding the causes of a landslide in geo-dynamically active terrain like the garhwal himalaya. the factors which induce landslides in the area include; seismo-tectonic movements (brunsden et al., 1981; owen, 1991; owen et al., 1995, 1996; owen and sharma, 1998; barnard et al., 2001), heavy and prolonged spells of rainfall leading to saturation and increasing the incumbent load on the slope that substantially increase shear stress (bartarya and sah, 1995; paul et al., 2000; srivastava et al., 2013; owen et al., 1996; shroder and bishop, 1998); and anthropogenic activity (barnard et al., 2001; begueria 2006; glade, 2003; remondo et al., 2005). the reduction of shear strength is the principal cause of slope instability and often triggers landslides. therefore, it is essential to analyze the material characteristics of the landslide slip zone to understand the fundamental mechanisms of slope instability, and to delineate susceptible areas (baoping and haiyang, 2007). hence, analysis of the sedimentary depositional environment can provide crucial insight into the displacement mechanism of materials (dufresne et al., 2016; xiaomin et al., 2019). shear strength reduction at the slope slip zone could be a result of mineral composition, grain size, and the processes that are operating over the slope. therefore, a comprehensive investigation of slope material is required to analyse the mineral composition, lithology, and grain morphology, under which landslide occurs (wiemer et al., 2015). the mineral and chemical composition of a landslide-slip zone material, generally those of slide bodies and bedrocks, could provide insight into the landslide characteristics. it is considered to be one of the effective ways to examine the causes of instability in the landslide-slip zone, and the mechanism of slope failures (dill, 1998). although, the x-ray diffraction analysis of the mineralogical composition of slope material does not have a direct relationship with landslide yet; it was used in another way, such as site-specific sediment mineralogy for understanding the mechanism of landslide slip zone (cafaro and cotecchia, 2001; bogaard et al., 2007; summa et al., 2010; xie et al., 2022). the abundance of fe2o3 minerals within the slip zone of landslide materials is saturated due to rain-water infiltration and groundwater, leading to the hydrolysis of quartz, and the formation of clay minerals, particularly kaolinite (summa et al., 2010). hence, the increase of clay minerals concentration in the figure 1. geological map of the garhwal himalaya (modified after srivastava and ahmad, 1979). 347sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india slope material, particularly at the slip-zone, acts as a lubricant and reduces shear strength; and activates landslide (summa et al., 2010; wang and sassa, 2003). ascertaining links between slope failure mechanism and micro-surface features present on the slope materials can provide insight into slope failure. systematic studies of quartz grains offer comprehensive information on sedimentary histories such as erosion, deposition, and the tectonic environment (krinsley and doornkamp, 1973; mahaney et al., 2001). the presence of micro-surface texture digenetic changes can be analyzed to understand the alteration, and cementation of original sediments, and the nature of pore-fluid that moved through the sedimentary sequence (tucker, 1988; mahaney, 2002; moral cardona et al., 2005; pandey, 2011). sedimentological properties have been found to reflect the interaction between source material, energy, and climatic regimes in high mountainous areas (sharma, 1996). therefore, particle size, micro-fabric, and quartz grain texture analysis are very significant for the identification of processes and intensity of slope instability (pandey, 2011; vos et al., 2014). the term slope failures, which includes all types of the mass movement, has been used as a synonym for landslide in the present study. to understand the surface processes; that cause slope instability, sediment samples were collected from landslides and processed using sieve analysis, x-ray diffraction (xrd) powder, and scanning electron microscopy (sem) techniques. 2. geological setting of the area the bhilangana basin covers both the lesser and greater himalayan ranges, separated by the main central thrust (mct). the area is seismically very active, and a large number of earthquakes occur along mct ((https://earthquake.usgs. gov/earthquakes/map/?), inferring continual neo-tectonic activity along the thrust plane (fig. 1). lithological formations of the area include the higher himalayan crystalline (hhc) that formed of granite and gneiss of vakirta and munisiari groups to the north of mct. the lesser himalayan rock group includes two types of formations, i.e., the lesser himalayan crystalline (lhc) of ramgarh group and the lesser himalayan sedimentaries (lhs) including subathu, tejam, damtha and chandpur formations (valdiya, 1980; pandey, 2011). the geotectonic setup and lithology of the garhwal himalaya represent quaternary to protozoic formations (fig. 1) with three distinct geotectonic units. two of which are located in the central himalayan crystalline group of rocks; and the third is deoban/garhwal tectonic unit. the crystalline and sedimentary rocks are separated from each other by thayeli thrust (rao and pati, 1982; valdiya, 1980). it is a low-angle thrust separating these two tectonic units and is given the site-specific name of thayeli. a major north-south direction fault, known as the balganga fault, offsets the thayeli thrust from a distance of 50 km in the balganga valley (sakliani, 1989). the central himalayan crystalline rocks are subdivided into two distinct geological units separated from each other by a tectonic plane (table 1). the lower bhilangana formation comprises low to medium-grade metamorphoses such as porphyroblastic biotite streaky genesis, quartzite, quartz-schist, quartz-felspathic schist, chlorite schist persistent limestone bands, epidiorite, and amphibolite. medium represents the higher unit, the gangi-kedarnath formation to high-grade metamorphics like garnetiferous quartz schist, biotite schist, graphite schist, carbonaceous phyllite, porphyroblastic gneiss, granulites, marble, and amphibolite. the upper proterozoic rocks include bhilangana, ghansali formation, and garhwal/deoban basic volcanic rocks. the proterozoic rock groups constitute gneisses, migmatites, and intercalated metasediments known as the vakirta group (naqvi, 2005). table 1. tectonic sequence and lithological unit in bhilangana basin (modified after; rao and pati, 1980). tectonic unit lithology central himalayan crystalline group gangi kedarnath formation rambara member marble, quartzite, garniferous schist, granulite, gneiss and amphibolite. trijuginarayan member alternate schist and quartzite, gneiss and amphibolite with lenticular bands of marble kinkolakhal member marble, quartzite graphite schist and other schist with aplitic granitic rocks panwali thrust bhilangana formation pokhar member streaky porphyroblastic gneiss quartzite, schist, marble, limestone, amphobolite ghuttu member quartzite, schist, carbonaceous phyllite, crystalline limestone main central thrust/ thayeli thrust lesser himalayan sedimentaries deoban/ garhwal group ghansali formation quartzite and slate sequence with gritty quartzite and limestone lenses in the upper horizons and basic and granite intrusive rocks agastmuni formation schist, quartzite with dolomite and limestone bands and intrusive chandrapuri gneiss https://earthquake.usgs.gov/earthquakes/map/ https://earthquake.usgs.gov/earthquakes/map/ 348 vijendra kumar pandey, milap chand sharma sample failure type rock type location elevation (m) slope angle (degrees) slope direction quartz (%) rock strength (mpa) volume (m3) area (m2) dfl84 debris fall dolomite n30 22.736 e78 47.052 1689 42 sw 84.8 20.1 108 162 dfl123 debris fall quartzite n30 29.396 e78 42.684 1332 32 e 99.1 35.8 990 1485 dfl127 debris fall quartzite n30 27.211 e78 41.702 1190 65 se 56.9 28.5 196 294 dfl132 debris fall quartzite n30 27.186 e78 41.512 1207 47 ne 74.3 19.9 2333.3 3500 dfl145 debris fall quartzite n30 33.898 e78 38.288 1316 69 se 83.4 23.7 2392 3588 dfl162 debris fall dolomite n30 27.132 e78 38.207 935 67 nw 99.4 38.2 1206 1809 dfl167 debris fall dolomite n30 26.205 e78 38.495 927 62 sw 99.1 18.9 3578.7 5368 dfl172 debris fall dolomite n30 25.720 e78 38.785 908 62 sw 99.5 47 172.7 259 dfl187 debris fall quartzite n30 27.565 e78 43.922 1766 42 w 62 15.6 2172.3 3258.5 dfl298 debris fall conglomerate n30 22.570 e78 31.404 895 28 ne 63.6 17.6 345.3 518 dfl299 debris fall conglomerate n30 22.674 e78 31.378 889 36 n 80.8 24.6 132 198 dfl304 debris fall quartzite n30 22.656 e78 30.369 894 49 ne 73 23.3 298.7 448 dfw96 debris flow dolomite n30 24.289 e78 42.233 1058 42 e 95 26.3 277.3 416 dfw119 debris flow quartzite n30 29.700 e78 43.165 1353 44 se 80.2 29.1 174 261 dfw120 debris flow quartzite n30 29.873 e78 42.979 1342 37 se 83.3 47.4 1057.3 1586 dfw154 debris flow conglomerate n30 30.928 e78 38.547 1103 45 w 48.8 16.2 408 612 dfw229 debris flow quartzite n30 23.195 e78 34.289 1187 43 s 83.5 56.6 232 348 dfw306 debris flow quartzite n30 22.752 e78 29.891 915 42 n 49.6 14.3 1916.7 2875 rfl80 rockfall quartzite n30 23.460 e78 49.138 2034 36 sw 97.7 38.5 203 304.5 rfl118 rockfall quartzite n30 29.792 e78 43.335 1390 71 se 80.5 42.6 468 702 rfl151 rockfall quartzite n30 32.089 e78 38.921 1220 63 se 94.9 48.6 288 432 rfl308 rockfall slate n30 22.696 e78 29.402 947 65 n 74 17.6 823.3 1235 slm82 slump quartzite n30 23.438 e78 48.449 1951 45 sw 99.5 46.3 186 279 slm153 slump quartzite n30 31.256 e78 38.654 1155 55 sw 87.8 42.3 627 940.5 slm301 slump slate n30 22.606 e78 30.949 889 42 w 74.2 19.6 381.3 572 table 2. sediment sample collected from the slope failures in bhilangana basin, during sept.-oct. 2009. 349sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india 3. materials and methods during the fieldwork of september 2009, we surveyed 135 landsides, out of which 25 sites were selected for the sedimentological analysis to understand the failure mechanism (table 2). sediment samples collected from each landslide occurred in the different lithological formations and were well distributed in the study area (fig. 2). as sediment samples were collected from the failed slope materials, it necessitates collecting the mixture of material across the landslide body to understand its instability process. the samples were taken from three positions for each site including slide head, intact materials, and dislodged material. in addition to the measurement of landslide geometry, in-situ rock strength was also assessed using the proceq digi-schmidt 2000 (concrete test hammer model nd/ld); also termed an original hammer. schmidt hammer uses the energy of impact released by a spring-controlled plunger. we have taken ten impacts point in each slope material across the slide body, and the mean value of these measurements was used as rock mass strength (mpa). figure 2. location map of selected landslides in the bhilangana basin (uttarakhand, india). grain-size distribution analyses of sediment were carried out using a motorized sieve shaker with sieve mesh sizes varying from 63 µm to 2 mm, capable of differentiating grain size intervals from 0.2 to 200 μm. sediment samples collected from the sites typically contain moisture, and therefore, are air-dried at room temperature for three weeks. on average, 500 gm of the sample was used for grain size distribution. since these samples have coarse grains, the optimum weight of 500-1000 grams of sediment is usually preferred for this analysis. the duration of the mechanical shaking of samples was kept 15 minutes for each sample. the standard time used for processing in most laboratories is 15-20 minutes but it is recommended to be 10-15 minutes for samples consisting of coarse grains (pye, 2007). the mineralogical analysis was carried out by x-ray diffraction (xrd), using pan analytical pw3050 xpert-pro powder diffractometer (cukα radiation and secondary monochromatic sample spinner). the procedure was applied to obtain mineralogical data of bulk rock using the barahona fernandez (1974) method. this technique depends on the integrated intensities of measurements of all the crystalline phases. however, the results obtained were cross-checked, comparing mineralogical library data. the scan axis position (°2th) was taken to be 5.03 to 89.99 with continuous scan type and logging interval of one second. the sediment size was < 2µm crystal powdered mounted in a substantial sample holder. the analyses of mixed-layer clay properties were performed using moore and reynolds (1989) and semi-quantitative estimates succeeding biscaye (1965); with small amendments. further, sem analysis and powder diffraction (xrd) techniques were used in this study to analyze the crystallographic structure, crystallite size (grain size), and preferred orientation in polycrystalline powdered bedrock sediments. diffraction peaks of unknown substances were compared with the diffraction database maintained by the international centre for diffraction data to identify the mineralogical composition present in the sediments. also, powder diffraction is a standard method used for determining lattice strains in crystalline materials. sem analyses of quartz grains have been done to identify the microsurface texture, morphological features, and processes. the quartz grains were separated from sediment samples and dipped for 20 minutes in hydrochloric acid (hcl) to remove impurities. though quartz grains are a highly conductive element, the gold coating (au) of these grains made for better conductivity. carl zeiss evo 40 instrument, on 20 kv was used for sem analysis. quartz sand grains were mounted on 10mm diameter aluminium stubs using doublesided carbon adhesive discs. it was considered sufficient to photomicrograph and measurement of grains into two orthogonal orientations. the grains are mounted in a linear array, as on the left-hand stub. the measurements and photomicrographs were acquired into three orientations of grains mounted on a triangular array to study elongation, flatness, and sphericity. the proposition is that surface textural features and assemblage records provide information about the origin, transport history, and subsequent weathering processes or diagenetic processes of materials investigated. presence or absence of different types of features recorded, either regarding presence/ absence or relative abundance; such as absent or rare (<5% of grains), scant (5 to 25%), common (25 to 75%), and abundant (>75%). however, the analysis shows that surface textural features present on the grains investigated do not relate to unique environmental processes or conditions. it is very significant to understand the procedures used to identify and classify surface features in different environments (tickell, 1965). analyses of the internal surface textural features of grains, combined with mineralogical characteristics such as precipitates, coatings, adhering particles, and surface contaminants, will be very significant in analyses of the micro-environmental process (pye, 2007). in the present study, 104 grains were analysed from 25 landslides sediment using sem to understand the process of slope instability and its relationship with mass movement, types of failure, and lithology. 4. results 4.1 grain-size analysis the grain size distribution analysis of the landslide sediments mainly, rock slide, debris slide, and debris fall occurred in the quartzite rocks have a high proportion of gravel, coarse sand, and less percentage of silt. the % of clay concentration was almost insignificant in these sediments. the grain size distribution composition is significant to analyses of slope material which usually determines infiltration capacity, soil moisture condition, water pressure, and cohesive strength. the composition and texture of the sediment show a direct relationship between the proportion of gravel, sand, and silt with the mechanism of slope failures. it is observed that a higher percentage of gravels and coarse sand are present mainly in debris fall and slump types of the landslides, which are rotational slides (fig. 3). however, rockfall and debris fall a vertical collapse occurred in the quartzite and slate have also a high proportion of gravels. the grain size composition of the debris fall shows a more or less similar pattern to the rockfall with a moderately high concentration of gravel. the investigated sediment samples, except dfw120, have a higher concentration of sand and silt. on further analyses with lithology, conglomerate rock shows a similar grain size composition. the compositions of landslide sediments of quartzite lithology have a similar composition to debris falls. the grain size composition of all the gravity-driven failures, such as debris fall, slump, and rockfall, have similarities in the slope material. these have almost moderate to high concentrations of gravel and a lesser amount of fine particles. the sediment composition of debris fall consists of medium to large gravels and coarse-grained sand. these sediments have the least amount of silt, i.e., less than 9% of the material. however, it has also been observed that a few debris fall sites have a more significant concentration of sand grains and a lesser proportion of gravel and silt. http://en.wikipedia.org/wiki/powder_diffraction http://en.wikipedia.org/wiki/crystallite http://en.wikipedia.org/wiki/international_centre_for_diffraction_data http://en.wikipedia.org/wiki/international_centre_for_diffraction_data http://en.wikipedia.org/wiki/strain_scanning 350 vijendra kumar pandey, milap chand sharma on examination of the rockfall sediments, having a large part of gravel, it was found that in-situ weathering of slope material dominates the process of failures, having a little run-out distance. a large proportion of gavels and the least concentration of fine sediments suggest that there is little run-out but are gravity-controlled vertical failures. similarly, debris falls along the road corridors had a different pattern with lithology. the average concentration of gravel in the sediments of the slump is 41%, debris fall is 40.9%, rockfall is 38.5%, and 25.3% in a debris flow. thus, indicating a high concentration of finegrain sediment in the debris flow. conglomerate rocks have a high proportion of sand and silt concentration in comparison to quartzites. the sediments of debris flow have an average percentage of sand, and silt, but a comparatively higher concentration of clay. the detailed analyses show consistency in grain size composition of slope materials with the type of failures. except for debris fall and debris flow other failed base rocks, have a similar pattern of particle size composition. the sediment composition of rockfalls investigated in the area has a predictable pattern with similar grain size composition. gravel and sand composition of sediments collected from quartzite, slate, and schist-phyllite bedrock have a similar composition as in rockfall and slump. each type of landslide has a varying composition of grain size composition and texture, hence these results are. analysis of the failed slope material shows a definite pattern of grain size composition associated with the type of landslide as well as lithology. rock falls, debris falls, and slumps occurred in similar lithologies, such as quartzite have similar grain size compositions with few exceptions. therefore, gravity-driven failures is reveals lithological controls on the sliding mechanism. the analysis concludes that the volume of debris displaced due to landslides has a significant relationship with grain size and lithology in the region. 4,2 mineralogical composition the mineral composition and crystallographic structure of slope material in powder form were examined using the x-ray diffraction technique. as given in supplementary table 1, quartz mineral is abundant in all the sediment samples collected from landslide sites. however, the concentration of quartz is lesser, mainly, in the conglomerate’s rocks. silicates, carbonates (calcite>>dolomite), and feldspars minerals are also found in a lesser amount in a few samples, whereas traces of gypsum and hematite (fe2o3) are present in the failed slope materials. it is notable that hematite concentration found in slope material representative of active slide surfaces and discontinuities probably have reactivated landslides. the higher silica (sio2) concentrations of minerals are found in quartzite rocks of the lhc rock formations, particularly the partapnagar brown quartzite which has a 93% sio2 concentration. the diffraction data reveals that high and thin sio2 peaks, particularly in quartzite rocks, show a well-crystalline structure and lesser lattice strain. the width of peaks in a particular phase figure 3. cumulative particle size % of the slope material collected from landslides: (a) sediment of debris fall, it has lesser fine particles and high % coarse material, (b) sediment of debris flow, it has lesser % coarse material moderate concentration of fine particles, (c) rockfall sediment has high proportions of coarse particles; and (d) slump sediment composition is similar to debris fall, moderate to high % of coarse particles. 351sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india pattern indicates average crystalline size. large crystalline size gives rise to sharp peaks, while peak width increases as crystalline size decrease. the peak broadening also occurs as a result of variation in’ spacing caused by microstrain in the rock. a broad base of peaks shows significant’ spacing of molecules. it is often called void space between facies. peak-to-peak comparison of mineral reflection provides details of the structural deformation process and lattice strain in the diffraction data. particularly, broader peaks exist in the dolomite rock while quartzite rocks have a narrow and high-rise peak, possibly showing the vast extent of lattice strain. the difference in these peaks appears due to a single phase because the relative intensities vary considerably among different slope materials. the rock mass strength measured of the landslide sites compared with mineral composition shows that highly fractured and sheared bedrock has weak strength and vice-versa. figure 4b illustrates the regression value (r2) is 0.34, which indicates a reasonable relationship with mineral composition. however, the sio2 composition of slope materials and rock strength has a strong correlation; that varies in different lithologies. quartzite rocks, in particular, have high strength in comparison to dolomite, slate, and schist-phyllite. comparatively, high rock strength is found in the rockfall and debris fall slope materials. the bedrocks have large cracks, and fractures show weak strength. the slope materials are having alternate bands of quartzite and dolomite strata, are highly weathered, and have poor (14.3mpa) rock strength. it shows a direct relationship between types of lithology and weathering of material related to mineral influences slope failures. the volume of displaced material varies according to the strength of the rock and slope inclination. slope lithology in failed sites has relatively low rock mass strength, which represents debris flow and debris fall, while sites with high rock mass strength are prone to rockfalls and rock slides. the relative proportions of quartz minerals were subsequently assessed using lithology and sem analysis. the peak of each sediment sample shows higher variations in the concentration of minerals and phases (fig. 4). those rocks are having a more significant proportion of quartz minerals show thin and high peaks in these sediments. some sediment sample has more than 2000 counts of phases in the samples. rockfall sediment shows varying proportions of quartz minerals as high as 91 to 94%. lithologies, with a high concentration of quartz minerals, have greater rock strength in comparison to a lesser concentration of sio2. figure 5a illustrates the sediment analysis of schist-phyllite rocks of debris fall (dfl132) and the composition of minerals. it has 74.3% quartz (sio2), magnetite (fe2o3) 14.4%, and dilanthanum tris (molybdate) 10.4% minerals; figure 5b presents the composition of debris flow (dfw120) in quartzite lithology. figure 4. x-ray diffraction of the quartzite rock specimen, showing a sharp and high peak of sio2 show well defined crystalline structure and lattice strain from the lesser himalayan crystalline rocks, using cu kα radiation anode 4.3 sem analysis meticulous examination of quartz grain micro-surface features done using sem showed that quartz grains are ambiguous and also challenging to distinguish between morphological processes. the majority of the grains’ outlines are angular and sub-angular and may be of glacial and glacial-fluvial origin. these slope material samples were collected from altitudes between 1600-2700m above sea level. grain outline shows the roundness or angularity of the features of the grain subdivided into angular grains with sharp edges subangular with slightly blunt edges, and rounded grains with smooth edges (fig. 6). grain outline mainly relates to the mode of transportation process, distance, time, and up to a certain extent with particle size. though, it is equally a function of the original grain shape and size from the source rock (goudie and watson, 1981; kleesment, 2009; costa et al., 2013; vos et al., 2014). sub-angular to rounded grains are produced in upper flow regimes as it requires severe abrasion to round the particle edges (mahaney, 2002). it indicates that flow regimes at this altitude were many times higher in the bhilangana river as compared to the present. the angular grains’ outline occurs in glacial-dominated landscapes where they are crushed, abraded, and plucked in high-energetic subaqueous environments. these areas have limited transport distances of sediment, causing grain breakage without rounding edges (helland and holmes, 1997). it indicates that glaciers extended downstream to much lower altitudes in the past to have left these signatures on the quartz grains. the glacial process is mainly dominated by grinding and abrasion during ice transport and glacio-fluvial activity. the digenetic/ alteration in the environment is very widely defined, comprising all processes, mostly chemical, altering the grain surface after deposition and before metamorphism of the source rock (vos et al., 2014). figure 5. x-ray diffractograms of clay minerals: (a) diffraction peaks of dolomite rocks showing order reflection and broad base, with less sio2 mineral composition, and (b) well-defined peaks of red quartzite rock, a high concentration of quartz with a mixture of trace minerals. 352 vijendra kumar pandey, milap chand sharma figure 6. micrographs showing the surface texture of quartzite rock specimen collected from debris fall, lesser himalaya: (a) grain with conchoidal shape and fractures partly smoothened by abrasion. probably it is a grain of glacial origin, deposited into glacio-fluvial environment, (b) abundance of arcuate steps and quartz crystal overgrowth, (c) solutions pits calcite overgrowth, (d) fracture plane and mica adhering particles, (e) highly fractured angular facieses of glacio-fluvial sediments with carbonate evaporates, and (f) grooves and conchoidal flacks of schist-phyllite angular flakes and medium cracks. morphological features, which occupy a large proportion of grain surface, include conchoidal fractures, multi-stepped striations, grooves, dishshaped depressions, large pits, ridges, and crystalline overgrowths with or without planar crystalline facies (supplementary table 2). small-scale features that have been identified and measured are small pits and various types of grooves, and mineral precipitates. usually, the grain surface is characterized by the presence of a vast number of closely spaced pits; and dominant small projections are often defined as consuming a frosted advent, whereas surfaces are having a smaller number of irregularities often denoted to pitted structures. grain outline is possibly altered by chemical interactions, such as solution and precipitates of minerals. conchoidal fractures are typically curved and shelllike breakage patterns, of the grains lacking clear cleavage directions. these micro-surface features are the result of powerful impact or pressure on the grain surfaces. crystal lattice generates a ribbed appearance, cracks, and fractures due to the dominating impact of pressure waves into the grains. small-scale pits (<1 μm) have often been observed on these fracture planes as mineral insertions, which are weakening the crystal lattice. hence fractures and cracks developed on the grains (vos et al., 2014). conchoidal fracture planes are occupied by the arcuate and straight steps (cardona et al., 1997). the depths of these steps are usually varied to several micrometres, with an irregular spacing of successive steps up to 5μm. the steps were formed due to the imminence pressures of impacts when the conchoidal fracture plane intersects with the cleavage planes of the quartz crystal. however, these features are inherently associated with conchoidal fracture planes. meandering ridges are also noticed on the grain surface as the cleavage planes intersect on slightly curved conchoidal fractures. however, the sediment collected from the landslides is highly crushed, deformed, and metamorphosed in the phase of the orogenetic process of the himalayas. the small surface features are varying with lithological formations. the grains collected from quartzite rocks are highly weathered due to the mechanical weathering process. the micrograph shows the presence of fractures, crystal overgrowths, adhering particles, arcuate steps, and quartz grains collected from rockfall (fig. 6d) in the pratapnagar brown quartzite. the presence of large cracks and fractures, crystal overgrowth, and minor stratification with conchoidal shape and grooves marked on the grain surfaces. therefore, it indicates that slope failures have occurred due to weak rock mass strength, highly disintegrated minerals, cracks, and fractures (fig. 6). micrographs of quartz grains studied from slump sites in quartzite rock of the ghansali formations show angular facies, crystal overgrowth, medium size groves with minor fractures, disintegrated granules and foliated structures (fig. 7). dolomite rock sites where debris fall has occurred show less disintegrated and angular flakes, but the presence of pits and mediumsized grooves on the surface, indicating active chemical weathering (fig. 8). sediments of dolomite and limestone rocks have small to medium pits, grooves, and carbonate precipitate in grain texture. angular, sub-angular facies, disintegrated flakes, striation, and crystal overgrowth suggest glaciofluvial transportation of sediment. micrographs of sediment collected from slate and schist-phyllite rocks show disintegrated angular flakes, fractured rocks, irregular grain shape, and texture. debris fall and rockfall types of failure occur due to weak lithology and highly weathered surfaces in such rock groups (fig. 9). the presence of large cracks, medium relief, and crystal overgrowth might suggest sedimentation strain due to seismicity in the region as well. these grains have rounded to a sub-rounded shape, which shows the transportation of unconsolidated materials and deposition below the elevation of 1400 meters in the lesser himalayan meta-sedimentaries rock formations. the results of the analysis have compared frequency histograms (fig. 10). 353sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india figure 7. micrograph of debris flow: (a) sub-angular shape of quartz grain with the abundance of arcuate steps and smooth surface due to abrasion, (b) small irregular pits, medium conchoidal fractures, (c) fine edge fractured amphibole textures with minor cracks, (d) angular outline of grain specimen of rockfall, it has fine edge fractured amphibole textures with minor cracks; fine edge fractured amphibole textures, (e) large conchoidal fractures with minor cracks schist-phyllite platy structures, crystal overgrowth, minor fractures and rounded grooves, and (f) fractured and crystal overgrowth, small pits of carbonate evaporates shows strong chemical weathering environment. it has straight steps with high relief. figure 8. micrograph of slump sediment: (a) meandering ridges with curved scratches, high relief, (b) fractures plates and siliceous precipitates angular foliated structures of glacio-fluvial origin, (c) presence of large size grooves, sharp edges and highly weathered crystalline structures, disintegrated material of quartzite rocks, and (d) large size cracks and grooves in the amphiboles and small crystal overgrowths. the minerals show characteristics of the physical weathering process with disintegrated minerals of mica-schist, minor fractures, and small crystal growth are persistent. 354 vijendra kumar pandey, milap chand sharma a. b c d. e figure 9. frequency distribution of quartz grain micro-surface texture presence on the sediment of landslide in the basin: (a) micro-surface features of debris fall sediment dfl298 of conglomerate rock, having both rounded and angular facies, cracks and conchoidal fracture and small solutions pits, presence of adhering particles; (b) debris fall occurred in quartzite rock dfl304, showing conchoidal fractures and solution crevasses, disintegrated surface; (c) debris flow in quartzite rock dfw120, moderately high cracks and fractures, small grooves and irregular pits, rounded outline; (d) rock fall in mixed slate/phyllite rocks rfl308, medium irregular pits, small conchoidal fractures, angular outline, meandering ridges and adhering particles; and (e) slump in quartzite/conglomerate lithology, occurred due to seepage in cracks and joints, showing medium irregular pits, arcuate steps, solution crevasses, cracks and joints (slm153). 355sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india a b c d e figure 10. debris fall (dfl132) occurred in bhilangana basin: (a) photographed of debris fall showing active slip zone and slide head in the disposed of materials, (b) highly fractured brown quartzite rock of pratapnagar formation, exposed on the slide head where the slip zone initiated, (c) diffraction peak of clay mineralogy of the sediment collected from the site, showing a sharp and thin peak of sio2. the mineral composition of the slope material is 74.3% quartz, magnetite 14.4%, dilanthanum tris (molybdate) 10.4% and sodium cobalt oxide hydrate 0.9%. due to the presence of fe2o3, chemical dissolution in slope material by rainfall infiltration induced active slip zone at slide head, (d) micrographs showing the fractured presence on the quartz grains as seen in the (b), small pits, adhering particles and sharp flacks, and (e) frequency histograms showing the micro-surface features analyzed from the sediment. 356 vijendra kumar pandey, milap chand sharma 5. discussion and conclusion the grain size distribution, mineral composition, and micro-surface texture analysis indicate a strong relationship with slope failure mechanisms in diverse lithologies. however, the search for a link between slope instability and compositional parameters, such as textural, mineralogical, and micro-surface features, is still a developing field of research (summa et al., 2013). in the present study, compositional variables and the heterogenetic nature of the data makes the analyses rather complicated. the grain size distribution of landslide sites shows a definite pattern of material composition and its relationship with the types of landslides. sediments are having a large proportion of granules, coarse sand, and lesser silt mainly found in rockfall, debris fall, and slump. the medium to fine size silt and sand, with a sufficient proportion of clay composition, influences debris flow, mudflow, and soil creep and are associated with slumps and debris fall in the area. the mineralogical composition of fine sediment fraction analyses shows that variations in the concentration are a cause of landslides (fig. 11). the high sio2 composition is significantly related to rock mass strength (mpa). the sediment sample, which has > 80% of quartz minerals, has rock strength above 30 mpa (r2 0.43). further, the rock strength was correlated with the size and volume of landslides and found it was necessary for slope failures that occurred in the lower himalayan meta-sedimentary rock formations. the fine sediment composition of slope materials and the chemical, mineralogical, and textural compositions of sediment, as well as the study of the process interaction of material involved, are very crucial in influencing the physical-mechanical characteristics of the landslide. the fe2o3 and calcite develop a suitable orientation of particles, thus providing some degree of lubrication, whereas mixed layers are prone to the aggregation of particles in the sub-humid environment where the materials are deposited. diffraction data of clay mineralogy provided clear foresight of the mineral composition of sediments. the analysis shows that minerals having large’ spacing between molecule arrangements, are observed from the weak rock strength and reach the critical limit of tensile strength, causing failures. the results of mineralogical analysis and rock mass strength have a strong relationship and to a certain extent, are supportive in understanding slope failure processes. the quartzite rocks exposed to the weathering process are highly disintegrated, thus reducing rock strength, and causing rockfalls and slumps in the area. the textural changes induced by water chemistry depend on mineralogical composition to a large extent. the analysis of quartz grain surface textures shows that precipitates of silica and carbonates indicate the influence of regional climate. these areas have undergone various phases of climatic change and tectonic activity in recent geologic history. hence, the glacial and deglacial processes have played a significant role in the supply of sediments deposited in the river valleys. the debris flows sediment samples collected near ghuttu village show angular pattern, stairation surface, and grooves indicative of a glacial environment. the analyses conclude that slope material is deposited during a glacial advance. during the monsoon season, debris flow, debris fall, and debris avalanches frequently occur in these unconsolidated lithofacies due to lubrication. grain textures in these areas have undergone a lot of geodynamic processes during recent geological history through neo-tectonics. sem analysis has added many advantages to geomorphology, including the determination and origin of depositional landforms, the source of sediments, the energy of the environment and processes of diagenesis, weathering, and development through time. it provides details of the origin of fine silt and clay particles in the geological column, fracture-abrasion mechanisms in the field and laboratory, and analysis of grain modifications under different weathering regimes. the microscopic study of quartz grain is used to understand texture, pattern, internal morphology, and weathering pattern prevalent in the area, and its correlations with slope instability. it provides a close relationship with rock strength and the volume of displaced material. the study of microtextures is, therefore, a useful technique to reconstruct the sedimentary history of the displaced materials, and should be judiciously used more often for studies of clastic sediments. the present study is an experiment that analyses the significance of sediment characteristics in the slope instability processes. it also provides new stimuli to further site-specific investigations such as grain size distribution, mineralogical and micro-surface texture, and their relationships. the mineral composition is closely related to producing sliding surfaces and discontinuities in the slope materials. the present study finds that the slope instability process is controlled by site-specific material composition, the presence of minerals and the depositional environment. the geotechnical parameters such as liquid limit, plastic limit, and angle of shearing resistance on the sliding surface may require such analysis to determine the relationship with the failure mechanism. however, it is distinguishing between the cause and effect even though some risk factors were identified, which are inducing slope failures. the present study will be very useful for the site-specific slope instability analysis and selection of appropriate stabilization measures. the in-depth analyses of these parameters will contribute to the knowledge of the geotechnical characteristics of landslide investigations. a. b. figure 11. analysis of landslide pattern and mineral composition (a) relationship between the composition of sio2 and rock mass strength, and (b) rock mass strength has significantly influenced area and volume of failures. 357sedimentological characteristics and their relationship with landsliding in the bhilangana basin, garhwal himalaya, india acknowledgement the author would like to thank advanced instrumentation research facility (airf), jnu, new delhi; for providing laboratory instruments and technical support. we acknowledge the unconditional support given by dr manoj pratap singh (system analyst) and dr ruchita pal (senior technical assistant) for xrd and sem analyses. this research is a part of v k pandey’s phd thesis. pandey is also thankful 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(2020). consolidation and deformation characteristics of soft rock foundation in a hydrological wetland environment. earth sciences research journal, 24(2), 183-190. doi: https://doi.org/10.15446/ esrj.v24n2.87920 the widespread distribution of soft rock and soft soil in a hydrological wetland environment is a common geotechnical engineering problem encountered in coastal engineering construction. a study method for consolidation and deformation characteristics of soft rock and soft soil foundation in a hydrological wetland environment is proposed to solve this problem. the study area was limited to the k9+280-k11+120 section along the fu-nehe section of national highway 111. the consolidation and deformation characteristics and loading conditions of soft soil foundation under embankment filling load, treatment methods of the soft rock foundation, stratum conditions, temperature changes, and time effects are analyzed. the results show that although the wetland soft rock and soil layer is not thick, the settlement of soft rock and soil accounts for more than 80% of the total settlement. the negative temperature has a certain influence on the consolidation settlement of soft rock foundation, which is mainly manifested in the difference between the settlement process of the central separation zone and the roadbed soft soil foundation. the pore water pressure of soft rock foundation dissipates to varying degrees. according to the monitoring results of settlement and pore water pressure, bagged sand wells are more suitable for soft rock foundation engineering treatment in a hydrological wetland. the research results can provide a reference for the study, calculation, and design of the consolidation and deformation of soft rock foundation in a hydrological wetland. abstract consolidation and deformation characteristics of soft rock foundation in a hydrological wetland environment características de consolidación y deformación de una fundación de roca blanda en el entorno de humedales hidrológicos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87920 la distribución generalizada de roca blanda y suelo blando en el entorno de humedales hidrológicos es un problema común de ingeniería geotécnica en las zonas costeras de construcción. para resolver este problema se propone un método de estudio para las características de consolidación y deformación de rocas blandas y cimientos de suelos blandos en ambientes de humedales hidrológicos. como área de estudio se definió la sección k9+280-k11+120 a lo largo de la sección fu-nehe de la carretera nacional 111. se analizaron las características de consolidación y deformación y las condiciones de carga de los cimientos del suelo blando bajo la carga de relleno del terraplén, los métodos de tratamiento de los cimientos de rocas blandas, las condiciones del estrato, los cambios de temperatura y los efectos del tiempo. los resultados muestran que aunque la capa de suelo y roca blanda del humedal no es gruesa, el asentamiento de roca blanda y suelo representa más del 80 % del asentamiento total. la temperatura negativa tiene cierta influencia en el asentamiento de consolidación de los cimientos de rocas blandas, que se manifiesta principalmente en la diferencia entre el proceso de asentamiento de la zona de separación central y los cimientos de suelos blandos de la calzada; la presión de poro del agua de la base de roca blanda se disipa en diversos grados. de acuerdo con los resultados de monitoreo del asentamiento y la presión del agua de los poros, los sacos de arena son más adecuados para el tratamiento de ingeniería de cimientos de rocas blandas en humedales hidrológicos. los resultados de la investigación pueden proporcionar una referencia para el estudio, el cálculo y el diseño de la consolidación y la deformación de los cimientos de rocas blandas en humedales hidrológicos. resumen record manuscript received: 18/12/2018 accepted for publication: 11/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 183-190 bin liu1, xiugen jiang2* 1water conservancy project & civil engineering college, tibet agriculture & animal husbandry university, nyingchi 860000, china 2college of water resources & civil engineering, china agricultural university, peking 100083, china * corresponding author: xyfdgzc@sina.com g eo te ch ni ca l en g in ee ri ng https://doi.org/10.15446/esrj.v24n2.87920 https://doi.org/10.15446/esrj.v24n2.87920 https://doi.org/10.15446/esrj.v24n2.87920 mailto:xyfdgzc@sina.com 184 bin liu, xiugen jiang introduction the shrinkage of soil under pressure is called the compressibility of soil (baral et al., 2018). the process of soil compression increasing with time and dissipation of excess pore water pressure is called consolidation of soil, which is generally divided into primary consolidation and secondary consolidation (huang et al., 2017). main consolidation refers to the compression process caused by the gradual dissipation of excess pore water pressure and the increasing effective force in soil (zhang, 2017), while secondary consolidation refers to the slow deformation caused mainly by creep of soil skeleton (tao et al., 2018) after the basic dissipation of excess pore water pressure. the characteristics of soft rock and soil deformation with time under load easily lead to uneven settlement and excessive post-construction settlement of roadbed built on soft rock foundation (zhang et al., 2019; zhang & wang, 2018). due to the different origins of soft rock and soil, their respective engineering properties also show some differences. hydrological wetland ecosystem is located in the transitional area between water ecosystem and terrestrial ecosystem (yang et al., 2017). specific hydrological conditions are the driving force of wetland formation and maintenance. small changes in hydrological characteristics will lead to changes in wetland ecosystem. therefore, hydrological wetland ecosystems are vulnerable to climate change and other human activities. the response and feedback mechanism of hydrological wetland ecosystem under the influence of global change and human activities is the focus and frontier issue of common concern in the international community (guo, 2018). consolidation and deformation characteristics of soft rock foundation in hydrological wetland environment are affected by many factors, such as physical and mechanical properties of soft rock and soil, sludge thickness of hydrological wetland, loading capacity, loading mode and field environment (xu et al., 2017; kotani et al., 2017). in the hydrological wetland area, in order to carry out the construction of soft rock foundation smoothly, cofferdams are built on both sides of most sections before foundation treatment, and counter-pressure barriers are built between cofferdams and roads. due to the influence of hydrological tide, cofferdam and back pressure guard, the consolidation degree of silt foundation in hydrological wetland area is difficult to meet the design requirements during the design preloading period. in order to study the consolidation and deformation of silt in this kind of area, a lot of field tests have been carried out. however, the parameters of soil layer, initial conditions and boundary conditions of field prototype tests are difficult to adjust, and the influence of each factor on the deformation characteristics cannot be accurately analyzed. with the development of computer technology, numerical calculation greatly strengthens the ability to solve complex geotechnical engineering problems. the numerical method is more and more used in soft foundation treatment projects, but the numerical analysis of consolidation and deformati on characteristics of silt roadbed in hydrological wetland area is rare. the overload preloading of bagged sand well is a common method in soft foundation treatment (zhu & wen, 2017). for the consolidation analysis of sand well foundation, it is a three-dimensional problem with too much calculation. generally, it is simplified to the plane strain problem of sand wall foundation (yan et al., 2017). in order to better reflect the actual situation of the project, in recent years, in the construction of high-grade highway, on-site monitoring has been carried out in many soft rock base sections. although the cost of onsite monitoring is high, which brings some inconvenience to the construction of the project, the monitoring results can be used to improve the calculation results and predict the post-construction settlement and final settlement of soft rock foundation. based on this, the research method for consolidation and deformation characteristics of soft rock foundation under hydrological wetland environment is put forward, which provides the main analysis basis for judging whether the safety and preloading of embankment construction progress in soft rock foundation of hydrological wetland have reached stability. materials and methods project survey the geological conditions along the fuyu-nehe section of national highway 111 are relatively complex. the test section is k9+280-k11+120, which is about 20 km away from the hometown of red-crowned crane zhalong nature reserve. the two layers of soft soil encountered are deposited by hydrological wetlands (ma et al., 2018). the first layer of soft rock is with the thickness of 0.3-0.5 m and is silty clay (guang-ze et al., 2018); the second layer of soft rock is with the thickness of 2-6 m and is silty clay, which has poor engineering properties. two layers of undisturbed soil samples of soft rock are tested in laboratory. the main physical and mechanical indexes are listed in table 1. table 1. basic physical properties of soft rock and soil determination index silty clay silty clay sampling depth/m 1.9—2.5 4.6—5.2 moisture content/% 60.01 53.03 density/g·cm-3 1.57 1.7 relative density 2.16 2.37 void ratio 1.21 1.13 saturation/% 100 100 liquid limit/% 58 50.27 plastic limit/% 30.57 26.78 liquid index 1.11 1.12 plastic index 27.43 23.49 in the test section, 8 sections are selected for field study, and the monitoring results of one of the typical sections are analyzed (k9 + 280 and the treatment method is to lay geogrids on bagged sand wells) (zhou et al., 2017). the geological conditions of the engineering in monitoring section and the buried position of the instruments and equipment are detailed as shown in figure 1. determination of permeability coefficient sand wells have changed the drainage boundary conditions of the original hydrological wetland foundation (hao et al., 2017), shortened the drainage distance, and the drainage conditions have been greatly improved compared with the natural foundation. radial drainage consolidation is the main factor in sand well foundation, and the vertical permeability coefficient can be obtained from the geotechnical test value. because of the heterogeneity of the sludge in the hydrological wetland area, the permeability coefficient obtained from indoor and outdoor tests is discrete, and the measured settlement curve can reflect the consolidation condition of the foundation comprehensively. the horizontal permeability coefficient (sun et al., 2017) can be calculated by using settlement data. the theoretical solution of average consolidation degree of soil under various drainage conditions can be summed up as a general expression: u e t= 1−α −β (1) where, t is time,  and  are parameters to be determined. according to the measured settlement-time curve, any three time t1, t2 and t3 after the load stops are selected and t3 ‒ t2 = t2 ‒ t1. it can be obtained that: β − − − = 1 2 1 2 1 3 2t t s s s s ln (2) in the formula, the settlement values at time t1, t2 and t3 are s1, s2 and s3 respectively. for the consolidation of shaft foundation (ideal shaft), there are: c d f n c c d h h e v h e = ( ) + β 2 2 2 2 8 4 π (3) 185consolidation and deformation characteristics of soft rock foundation in a hydrological wetland environment results analysis of layered settlement monitoring results figure 2 shows the change process of embankment filling height of test section. the starting time of abscissa is november 5. before the installation of the observation equipment, the test section has been filled with gravel drainage cushion and part of embankment fill, and geogrid has been laid. because of the influence of climate, the embankment has not been filled and the soft rock foundation has not been loaded during the whole winter (liu, 2018). the main loading time is concentrated in the following five, six and seven months, which lasts more than 90 days. the thickness of the embankment is 1.8 m and the height of the embankment is 4.0 m. figure 2. change process of embankment fill height in test section where, de is the diameter of equivalent circle, d l le = =2 3 1 05 π . , l is the distance between sand wells, h is the thickness of unilateral drainage soil layer, f n n n n n n ( ) = ( ) 2 2 2 21 3 1 4  ln , n is the diameter ratio, cv and ch are the vertical and horizontal consolidation coefficients, respectively. transforming the permeability coefficient of sand well foundation into that of sand wall foundation starting from baron’s theory (wei et al., 2018), and considering the effects of smear, lateral deformation and vertical seepage, the equivalent method between plane and axisymmetric problems of sand well foundation can be obtained. this method can only adjust the permeability coefficient (zhang et al., 2017), and the spacing of sand wall can still be determined according to the need of meshing (yang et al., 2018). the transformation formula is as follows: k d k k d khp h h vp v v= =; (4) where, kh and kv are horizontal and vertical permeability coefficients of sand well foundation, khp and kvp are equivalent horizontal and vertical permeability coefficients of sand wall foundation, and dh and dv are adjustment coefficients. the vertical permeability coefficient kv of silt roadbed is 2.82×10-7 cm/s in laboratory test, and the horizontal permeability coefficient kh=2.60×10-7 cm/s is calculated according to settlement data (wang et al., 2018). the horizontal permeability coefficient and vertical permeability coefficient are khp=8.92×10-7 cm/s and kvp=2.54×10-7 cm/s respectively when the sand well treatment area is converted into sand wall foundation. figure 1. geological bubble surface of test section, i.e. instrument burial map 186 bin liu, xiugen jiang figure 3 and figure 4 show the settlement curves of the magnetic rings on the subsidence tube of the test section. the buried position of each subsidence pipe and each magnetic ring on each subsidence pipe is shown in figure 1. figure 3. settlement curves of magnetic rings in no. 1 settling pipe of test section figure 4. settlement curves of magnetic rings in no. 2 settling pipe of test section figure 3 shows the settling process curves of the magnetic rings of no. 1 settlement pipe in the test section. the layered settlement pipe is buried on both sides of the embankment fill in the test section, i.e. at the shoulders of both sides of the embankment. the characteristics of the settlement of the magnetic rings are as follows: from november 19 to january 9, there is no obvious settlement process; from january 9 to february 17, the settlement of the magnetic rings has increased, and then until may 1, that is, before the embankment filling load, the settlement of the magnetic rings of the settlement pipe at the shoulder has not increased significantly. after may 1, the embankment filling began to load, and the settlement of the magnetic rings of the settlement pipe at the shoulder increased obviously. the main reason for this settlement process is that the temperature decreases from november 19 to january 9, and the embankment fill gradually freezes in depth. from january 9 to february 17, it is the coldest season in the region (issaka & ashraf, 2017; liu & baghban, 2017). the freezing depth reaches the maximum. the embankment fill forms a shell-like freezing layer with the thickness of 2.5 m, which has good integrity and high strength. it can transfer the load from the middle to the edge of the embankment, resulting in the edge soil. the pressure increases, which is consistent with the observation results of the increase of the earth pressure in the process of embankment freezing. figure 4 shows the settling process curves of the magnetic rings of the no. 2 settlement pipe in the test section. the layered settlement pipe is located in the central partition zone of the embankment fill. from november 19 to february 17, the following year, during the freezing process of embankment fill in winter, there is no obvious settlement of soft soil foundation at the central separation zone of the two test sections. from february 17 to may 1, it is the season when the temperature of embankment filling gradually rises. the shelllike freezing layer of embankment filling gradually melts. the pressure on the soft soil foundation at the central separation zone increases gradually. the settlement of the magnetic rings of the subsidence tube increases, especially the settlement of the magnetic rings near the surface of the soft rock foundation. every year from may to september is the construction season of highway construction in the northern region. the embankment filling speed is faster, especially concentrated in may, june and july. the settlement of soft rock foundation has an obvious acceleration process. the more the closer to the surface settlement of soft rock foundation is, the greater the settlement of soft soil is, and the settlement of soft soil accounts for more than 80% of the total settlement. in addition, the treatment method of soft soil foundation for bagged sand well is adopted in the test section. from the settlement data of the test section, it can be seen that the slope foundation of the settlement curve of the soft soil foundation treated by the bag sand well is small at the later stage, which indicates that the treatment of the shallow soft rock foundation of hydrological wetland is more conducive to drainage consolidation of the soft rock foundation and reduce the post-construction settlement. analysis of pore water pressure monitoring results tables 2 and 3 describe the variation curve of pore water pressure in the test section. tables 2 and 3 show that pore water pressure dissipates in varying degrees during the freezing process of embankment fill. during the whole winter, from feb. 17 to pre-loading, the observed pore water pressure increases gradually, but the rising amplitude and rate are not obvious. the main reasons are: (1) the temperature gradually rises, snow melts and rainfall influences, and the groundwater level gradually rises. (2) due to the melting of embankment fill, the frozen crust layer begins to melt. except near the slope angle, the soft rock foundation is subjected to the increase of the base pressure caused by embankment fill. since the beginning of may, embankment filling begind to be constructed. the additional stress of soft rock foundation increased under the action of filling loading, resulting in a significant increase in excess pore water pressure in the foundation soil. the change process of excess pore water pressure is very synchronous with the loading process of filling. that is, the excess pore water pressure increases obviously during loading. the faster the loading rate is, the larger the loading is, the faster the excess pore water pressure increases. the excess pore water pressure dissipates obviously after stopping loading. from the curve of pore water pressure change of two test sections, it can be concluded that the drainage consolidation process of soft rock foundation is greatly influenced by geological conditions. normally, the shallower the pore water pressure probe is buried, the larger the excess pore water pressure is generated in the initial stage of loading, because the excess pore water pressure is caused by additional stress. the more the nearer to the surface of soft rock foundation is, the greater the additional stress caused by embankment filling load is. from tables 2 and 3, it is easy to find that the excess pore water pressure dissipation of the deep buried probes no. 5 and no. 1 is very fast, basically after loading, the excess pore water pressure dissipation has been completed, because the two probes are buried in the well permeable gravel layer. through the curve of pore water pressure change in the test section, it can be concluded that the excess pore water pressure dissipates quickly in 187consolidation and deformation characteristics of soft rock foundation in a hydrological wetland environment soft soil foundation treated by bagged sand wells. this shows once again that the use of bagged sand wells to treat the soft rock foundation of hydrological wetland is more conducive to the drainage consolidation of soft rock and soil, and increases the stability of soft rock foundation. effect of water level variation on consolidation and deformation characteristics of soft rock and soil consolidation and deformation characteristics of water level with the same change range figure 5 is the settlement time history curve, and figure 5 (a) is the whole settlement time curve of surcharge preloading. figure 5 shows that settlement deformation mainly occurs during loading and decreases with the increase of depth. sediment curve on top of silt takes eight data points per day, while other curves only take one data point per day. settlement value changes periodically every day, and the change range is basically unchanged. from figure 5 (b), it can be clearly seen that the periodic variation of settlement caused by hydrological tides is the same as that caused by tidal frequencies, and its influence decreases rapidly with the increase of depth. the amplitudes of settlement cycle change are 35 mm and 5 mm at the top and 16 m below the silt surface, respectively. figure 6 is a pore pressure-time history curve, from which the effect of tide on pore pressure is found. figure 6 (a) is the pore pressure-time history curve in whole process. at the initial stage of loading, because of the small load, the hydrological tide has a great influence on pore pressure. the pore pressure increases with the depth. however, with the development of loading process, the pore pressure decreases with the depth increasing, indicating that the pore pressure caused by loading is greater in the shallow part than in the deep part, but when the drainage consolidation process is almost finished, pore pressure increases again with the increase of depth. it can be seen from figure 6 (b) that hydrological tide causes periodic variation of pore pressure. the frequency of variation is the same as that of hydrological tide, and the law of variation of pore pressure is opposite to that of hydrological tide. this is because pore pressure is influenced by both hydrological tides and the upper load caused by hydrological tides, and the variation of pore pressure with depth is small. effect of water level variation on drainage consolidation the actual situation of consolidation of silt roadbed in hydrological wetland area is very complicated. even if the external hydrological tide changes the same, because of the difference of the elevation of the silt top surface itself, the change of water level in the roadbed will be different, and most sections are filled with cofferdams, whose permeability varies greatly. therefore, the influence of hydrological tide on pore pressure and settlement in the measured values of each section is also quite different. the actual effect of hydrological tide in engineering is to change the range of water level change at the top of silt, and the range of influence is different at different sections. from figure 7, it can be seen that the larger the variation range of water level on the top of silt is, the larger the variation range of settlement is. this effect is a linear relationship. from figure 7, it can also be found that when the hydrological tide falls below table 2. pore water pressure variation curve of no. 2 hole in test section time no. 8 void hydraulic probe (initial buried depth 3.6 m)/kpa 7 void hydraulic probe (initial buried depth 4.7 m)/kpa no. 2 void hydraulic probe (initial buried depth 5.6 m)/kpa no. 5 void hydraulic probe (initial buried depth 7.4 m)/kpa november 5th 5.89 17.43 31.78 36.54 december 15th 2.76 15.04 30.65 33.99 january 25th -2.31 13.03 28.89 30.94 february 27th -2.24 12.4 30.23 30.23 april 7th 1.95 13.86 32.06 33.15 may 17th 3.6 17.66 33.74 35.42 june 26th 11.22 23.79 42.73 46.91 august 5th 7.05 17.68 33.15 35.06 september 15th 6.97 19.07 29.66 35 table 3. pore water pressure change curve of test section 3 time no. 6 void hydraulic probe (initial buried depth 3.6 m)/kpa no. 3 void hydraulic probe (initial buried depth 4.3 m)/kpa no. 9 void hydraulic probe (initial buried depth 4.9 m)/kpa no. 1 void hydraulic probe (initial buried depth 6.0 m)/kpa november 5th 3.98 11.03 21.18 32 december 15th 3.99 8.57 18.26 30.1 january 25th 4.02 6.02 10.05 27.95 february 27th 4.1 5.11 7.6 16.24 april 7th 5.17 7.58 7.93 29.17 may 17th 6.46 9.74 12.15 31.03 june 26th 15.03 15.37 19.34 38.86 august 5th 11.2 14.19 16.82 28.62 september 15th 9.85 14.3 16.91 27.44 188 bin liu, xiugen jiang the top of the silt, the settlement varies with the variation of water level. the larger the variation of water level is, the smaller the settlement is. this indicates that the hydrological tide will reduce the consolidation settlement. the larger the variation range of silt top water level is, the greater the influence on pore pressure variation range is. the influence relationship is also a linear relationship, as shown in figures 8, 9 and 10. when the water level is the same as the top of the silt, the variation range of water level has little effect on pore pressure, but from the whole pore pressure-time history curve in front, it can be seen that the variation range of pore pressure curve caused by each variation range of water level increases with the development of consolidation and drainage process. therefore, attention should be paid to the influence of hydrological tide on settlement and pore pressure in site measurement during construction of hydrological wetland areas. discussion considering the role of hydrological tide in the silt roadbed in hydrological wetland, the horizontal permeability coefficient of sand well silt roadbed is calculated based on the field measured data, and the three-dimensional sand well roadbed is converted into the plane strain sand wall roadbed, and the equivalent permeability coefficient is determined. the consolidation and deformation characteristics of soft rock foundation under hydrological wetland environment are calculated as follows: (1) at the shoulder position on both sides, the settlement of embankment fills is not obvious at the initial stage of freezing. in the coldest season, the settlement increases. then, until the embankment fills are loaded, the settlement near the slope does not increase significantly; at the central partition zone of (a) settlement-time curve of preloading with surcharge (b) settlement-time curve 4 days before construction commencement figure 5. settlement-time curve (a) the whole process (one calculation per day) (b) former 4d figure 6. pore pressure-time curve 189consolidation and deformation characteristics of soft rock foundation in a hydrological wetland environment embankment fills, there is no obvious settlement in the whole winter. as the temperature rises gradually, the settlement increases, and the settlement near the surface of the soft rock foundation increases obviously. during the construction season, the settlement of the soft rock foundation increases obviously, and the closer to the surface the settlement is, the larger the settlement of the soft rock soil accounts for more than 80% of the total settlement. the slope of the settlement curve of soft rock foundation treated by bagged sand well is smaller in later stage, which indicates that bagged sand well treatment method is beneficial to drainage and consolidation of shallow soft rock foundation of wetland in cold region, and reduces post-construction settlement of soft rock foundation. (2) during the whole winter, pore water pressure dissipates in varying degrees; the change of excess pore water pressure is very synchronous with the loading process of filling soil, and the excess pore water pressure increases obviously during loading. the faster the loading rate is, the larger the loading is, and the faster the excess pore water pressure increases. the excess pore water pressure dissipates obviously after stopping loading. the excess pore water pressure of soft rock foundation treated by bagged sand wells dissipates quickly, which is more conducive to drainage consolidation of soft rock and soil, and increases the stability of soft rock foundation. (3) during the freezing process of embankment fill, the earth pressure near the central separation zone decreases gradually, while the earth pressure near the embankment slope increases gradually. during the melting process of embankment fill, the earth pressure near the central separation zone increases gradually, while the earth pressure near the embankment slope decreases gradually. (4) hydrological tides and back pressure guards play an important role in drainage and consolidation of silt roadbed. the regularity of settlement and figure 7. settlement-time relationship curve under different water level variation ranges figure 8. pore pressure-time relationship curve under different water level variation ranges figure 9. settlement variation mud top water level variation figure 10. pore pressure variation range mud top water level variation range 190 bin liu, xiugen jiang deformation caused by hydrological tide changes. the settlement-time history curve changes periodically with the increase of hydrological tide, and its influence decreases with the increase of silt depth. the pore pressure dissipation curve is also affected by hydrological tides. with its periodic variation, the frequency of variation is the same as that of hydrological tides. the law of variation of pore pressure is contrary to that of hydrological tides, and its influence decreases with the increase of depth. (5) when the variation of silt top water level caused by tide is different, the periodic variation range of settlement and pore pressure is linearly related to the variation range of silt top water level. when the hydrological tide is lower than the silt top, the variation range of silt top water level has no effect on pore pressure, but it has obvious effect on settlement. the larger the variation range of water level is, the smaller the settlement is, indicating that the hydrological tide will decrease the settlement at consolidation stage. conclusions the engineering geological problems faced by the construction of highgrade highway on soft rock foundation are mainly the settlement and stability of soft rock foundation. the engineering properties of soft rock and soil are different because of different causes. in this paper, the research method of consolidation and deformation characteristics of soft rock foundation in hydrological wetland environment is proposed. the correlation of engineering properties, treatment methods and time effect of soft rock foundation in hydrological wetland is studied by combining on-site monitoring with numerical calculation. in the future study and work, we should carry out further work in this area, especially the experimental study on the structure of soft rock and soil and the compression and strength characteristics of soft rock and soil at different depths, as well as on-site monitoring of deformation of soft rock foundation in hydrological wetland during embankment filling. by evaluation of post-construction consolidation settlement and stability, construction loading rate is controlled by using monitoring data in further study, to ensure that the stability of soft soil foundation can guide construction more effectively. acknowledgements the study was supported by “the national natural science foundation of china, china (no. 11672362)”, and “natural science foundation of tibet autonomous region, china (no. xz 2018 zr g-32(z))”. references baral, p., rujikiatkamjorn, c.,, indraratna, b., & kelly, r. 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(2017). intelligent power monitoring system of building equipment based on internet of things. chinese journal of power sources, 41, 1775-1777. http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201805020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201805020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201805020 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201804015 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201804015 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201804015 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201802097 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201802097 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201704002 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201704002 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201704002 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dianyjs201712032 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dianyjs201712032 anomalous geoelectric potential variations observed along a gas pipeline section in argentine, possible intensification with variations of the earth’s magnetic field patricia larocca1*, m.a. arecco1,2, a.c. macrino3 universidad de buenos aires, facultad de ingeniería, instituto de geodesia y geofísica aplicadas (university of buenos aires, school of engineering, institute of applied geodesy and geophysics), buenos aires, argentina. universidad de la defensa nacional, instituto universitario naval, escuela de ciencias del mar (university of national defense, naval university institute, marine sciences school), buenos aires, argentina. universidad nacional de la plata, facultad de cs. naturales y museo, departamento de geología (university of la plata, school of natural sciences and museum, geology department). *corresponding author: plarocc@fi.uba.ar keywords: generalized regression neural network (grnn); radial basis function neural network (rbfnn); multiple linear regression (mlr); interpolation methods; geoid determination palabras clave: gasoducto; potenciales caño/suelo; tormentas geomagnéticas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.91059 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 363-369 g eo ph ys ic s record manuscript received: 19/10/2020 accepted for publication: 19/11/2021 abstract significant anomalous geoelectric potential variations have been observed in a section of the neuba ii gas pipeline along its route in the district of saavedra, near the area of goyena, province of buenos aires (argentine), where it goes through major lithological, edaphological and hydrological variations. detailed research was conducted, showing that these disturbances may be intensified with variations of the earth’s magnetic field, during a magnetic storm, as the pipe-to-soil potential (psp) values remained constant for weeks and then fluctuations from 0.1 v to 0.15 v were recorded in various parts of the pipeline. on the other hand, to provide another analysis of these variations, models based on the distributed source transmission line (dstl) theory were used, proposing a uniform geoelectric field along the pipeline route. a design was proposed that would allow modeling the response of the pipeline to variations of induced geoelectric fields, taking into account their intensification based on points of discontinuity due to subsoil characteristics or differences in its structure. good consistency was achieved between the observed and modeled psps. the analysis and monitoring of these psps is a useful tool to identify the potential risks caused by geomagnetically induced currents in the pipes that would increase the effects due to the structure or the environment in which it is buried. variaciones anómalas en potenciales geoeléctricos observadas a lo largo de un gasoducto en argentina y su posible intensificación debido a variaciones del campo magnético terrestre resumen se han observado importantes variaciones anómalas en el potencial geoeléctrico sobre un tramo del gasoducto neuba ii a lo largo de su recorrido en el distrito de saavedra, cercano a la zona de goyena, provincia de buenos aires (argentina), donde atraviesa suelos de características litológicas, edafológicas e hidrológicas distintas. se realizó una investigación detallada que mostró que estas perturbaciones pueden intensificarse con variaciones del campo magnético de la tierra, durante una tormenta magnética, ya que los valores de la diferencia del potencial tubería-suelo (psp) permanecían constantes durante semanas y luego se registraron fluctuaciones de 0.1 v a 0.15 v en varias partes del gasoducto. por otro lado, para brindar otro análisis de estas variaciones, se utilizaron modelos basados en la teoría de la línea de transmisión de fuente distribuida (dstl), proponiendo un campo geoeléctrico uniforme a lo largo de la ruta del ducto. se propuso un diseño que permite modelar la respuesta del gasoducto a variaciones de campos geoeléctricos inducidos, teniendo en cuenta su intensificación en base a puntos de discontinuidad por características del subsuelo o diferencias en su estructura. se logró una buena consistencia entre los psp observados y modelados. el análisis y seguimiento de estos psps es una herramienta útil para identificar los riesgos potenciales provocados por corrientes inducidas geomagnéticamente en las tuberías que incrementan estos efectos debido a la estructura o el entorno en el que está enterrado. how to cite item: larocca, p., arecco, m. a., & macrino, a. c. (2021). anomalous geoelectric potential variations observed along a gas pipeline section in argentine, possible intensification with variations of the earth’s magnetic field. earth sciences research journal, 25(4), 363-369. https://doi.org/10.15446/esrj.v25n4.91059 mailto:plarocc@fi.uba.ar https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.91059 364 patricia larocca, m.a. arecco, a.c. macrino introduction the geomagnetic field protects the earth from cosmic rays (high-energy particles coming from the galaxy) and the solar wind (ionized gas or plasma flowing out of the sun), just like a barrier (owens et al., 2013). without this magnetic field barrier, the solar wind could blow away the atmosphere of the earth over geological time, and the high-energy particles might damage the health of living beings and the proper functioning of technological systems (fernberg et al., 2007). the study of geomagnetism is, therefore, directly linked to the survival of humans and other species on the earth. the sun is emitting solar wind all the time. at the earth’s orbit, it has a density of about 5 particles cm-3, and speeds of 400 km/s on average. both density and velocities are variable, and the speeds can exceed 1500 km/s (owens et al., 2013). the interaction of the solar wind with the geomagnetic field leads to the formation of the magnetosphere. due to this interaction, the magnetic field on the dayside is compressed whereas on the nightside it is stretched in the form of a long tail. periodically the sun becomes highly active, causing on its surface eruptions of clouds of electrified magnetic gas, referred to as coronal mass ejections (cme), which are hurled into space from the solar corona, travelling at speeds of up to 1500 km/s, and reaching the earth’s magnetosphere. the solar wind-magnetosphere coupling is enhanced when these cmes are accompanied by long intervals of southward interplanetary magnetic field. this situation, in general, leads to the occurrence of a geomagnetic storm characterized by a main phase during which the horizontal component of the earth’s magnetic field is significantly depressed over a time span of one to a few hours. this is followed by a recovery phase, which may extend for ~ 10 hours or more (gonzález and tsurutani, 1987; gonzález et al., 1989; tsurutani and gonzález, 1997; tsurutani et al., 1999). the intensity of a geomagnetic storm is measured in terms of the disturbance storm time index (dst). magnetic storms with a dst < -200 nt are considered intense and those with a dst < -350 nt are super intense (tsurutani et al., 1999). during a magnetic storm, a large amount of energy is dissipated in the polar regions, resulting in profound changes in the global morphology of the upper atmosphere (prölss, 2011). the storm is generally accompanied by great auroral brightness; it occasionally moves equator-ward from its usual location (known as expansion of the auroral oval). the intensity of the equatorial magnetic field, measured by the dst index, is directly related to the total kinetic energy of the ring current particles (rostoker et al., 1997), thus, the dst index is also often used to determine whether or not a storm happened, to define the duration of a storm, and to distinguish between quiet and disturbed geomagnetic conditions. however, dst does not identify all storms because, dst is incomplete in describing storm evolution. the ae or the am indices may be much more appropriate for estimating storm intensity and for statistical studies (borovsky and shprits, 2017). on the other hand, the auroral electrojet (ae) index is designed to provide a quantitative measure of auroral zone magnetic activity produced by ionospheric currents flowing below and within the auroral oval. defined and developed by davis and sugiura (1966) and improved by ahn and collaborators (2000), the ae index has been usefully employed both qualitatively and quantitatively in studies of the morphology of geomagnetic storms (newell and gjerloev, 2011), and their influence on the coupling between the interplanetary magnetic field and the earth’s magnetosphere (akasofu, 1981 a, b; akasofu, 1989). the impact of the space weather on the earth’s infrastructures is caused by the combined effects of large variations in the geomagnetic field, the ground conductivity structure and the topology of the affected network (yu et al., 2019). these impacts range from slow, cumulative and excessive corrosion of pipeline systems (ingham and rodgers, 2018) to a rapid collapse (in minutes) of electricity grids (trichtchenko and boteler, 2001). buried pipelines are coated with low-conductivity materials and are kept with a slightly negative potential difference with respect to the ground of 0.850 v (cathodic protection system) in order to reduce the flow of currents along them and to mitigate corrosion effects. however, corrosion may increase due to the electric currents spread through the soil during magnetic disturbances (trichtchenko and boteler, 2001). during periods of intense magnetic storms, telluric currents caused by these geomagnetic disturbances in geomagnetic storm periods may be large enough to keep a pipeline unprotected for a significantly long time, which may reduce its useful life increasing the possibility of corrosion. the influence of geomagnetic activity on psp has been studied in many cases, e.g. campbell (1979, 1980), boteler (2000), pulkkinen et al. (2001), and hejda and bochn´ıˇcek (2005), osella and favetto (2000), in particular where a pipeline crosses a highly resistive intrusive rock. it is a common practice to check pipes once a year to measure the voltage at test posts so as to track that pipe-to-soil potential (psp) variations are within the safety range (from -0.850 to -1.350 volts) provided by cathodic protection systems (yu et al., 2019). psp readings, however, are usually irregular and fall outside the recommended range. their interpretation is difficult because several factors may influence the variation of psps measured locally at a given time: • geomagnetic activity (magnitude and frequency of magnetic variations); • profile of the soil conductivity below the pipe; and • piping structure (presence of curves, rims and finishings, division or combination of one or two pipes, etc.) and geographic orientation. in the framework of a series of control observations made by the gas pipeline integrity management of the transportadora de gas del sur (tgs) company, between 2013 and 2018 psp values outside the safety range were recorded, in a series of checkpoints along the neuba ii gas pipeline, concentrated in a particular area of the province of buenos aires. in this work, based on the data provided by the company, the possible causes of these anomalous values in the potential readings at the checkpoints were analyzed, focusing the study on the aforementioned factors. in particular, periods of geomagnetic activity were analyzed, when these periods coincided with the dates of captured data. the influence of the pipeline section topology was also studied from the application of the distributed source theory model, dstl (boteler, 2000; 2013) and the soil conductivity profile. area of study this paper analyzed the records of variations in the pipe-soil potential (from a cu / cus04 reference electrode placed at the control posts) of the neuba ii gas pipeline, in the district of saavedra in the area of goyena, province of buenos aires (argentine), on a 200 km-long section, between 2013 and 2018 (figure 1). the section of the pipe analyzed is located in the southwest of the province of buenos aires, going through the districts of puan and saavedra. it runs west of the sierras de ventania system, more specifically west of the stratigraphic group la lola, which is the most extended group in the curamalal series (harrington, 1947). this examined section of the gas pipeline goes through quaternary unconsolidated loessoid sand-clay silt sediments with levels of relatively thick and discontinuous hardpan of an epigenic origin, where the buried section from kilometer 699 to 730 runs largely through silts and calcareous clayey silts and sandy limolites, gravels and subordinate sands. in particular, the soils near the town of goyena are identified as loessic plain soils, quartzites, quartzitic sandstones and scattered outcrops of granites and porphyries that are characterized by their low to zero conductivity (pereyra et al., 2001). methodology this paper uses periodic psp records provided by the company that maintains and controls the proper functioning of the cathodic protection of the neuba ii gas pipeline. psp measurements were carried out by tgs, recording data at each of the test points, sequentially along the pipeline. we have the specific information of psp (on-off), time and date of registration of each test point along the gas pipeline, between the years 2013 to 2018. surveys on the gas pipeline were carried out annually, in 2013 (8/26 to 9/2), in 2014 (8 /8 to 16), in 2015 (9/14 to 17), in 2016 (11/30 to 12/3), in 2017 (11/28 to 30) and 2018 (9/11 to 14) (figure 1). in those days when bad weather did not allow measurements to be made, they were done the next day. the data were recorded in different campaigns, which explains the difference in their amplitude but the same trends, since they were certainly taken by different measuring equipment from one campaign to another. in all cases the measurement precision was 1 mv. https://www.ngdc.noaa.gov/stp/iono/ionohome.html 365anomalous geoelectric potential variations observed along a gas pipeline section in argentine, possible intensification with variations of the earth’s magnetic field usually, the pipelines are protected by arrays of batteries with the negative terminal connected to the pipeline and the positive terminal to a supplementary anode (cathodic protection) with the purpose of diminishing the amount of current that circulates on the pipe. during the routine control surveys by personnel, the records were collected with the protection system both disconnected and connected (off/on), (osella and favetto, 2000). on figure 2, it is possible to see that, along the 200 km of the gas pipeline analyzed, there are off-range records in the various measurements repeated from 2013 to 2018, accounting for anomalies relative to the geological contrast of the soil or changes in the pipeline structure. earth conductivity contrasts can create amplitude variations of surface geoelectric fields resulting in important psp variations (osella and favetto, 2000), principally where a pipe crosses a highly resistive intrusive rock. this is mainly observed in the measurements of the psp off that become more negative in the range between 680-720 km, a result observed in all the records in figure 2. in parallel, the periods of geomagnetic storms occurring during that length of time were analyzed based on the records of the dst and ae indices (campbell, 1979); since the study area is in mid-latitudes, a comparison was made with both indices to get an idea of the global geomagnetic activity. in addition, data was collected from two geomagnetic observatories located in the vicinity of the study area. these observatories of the intermagnet network are trelew and pilar, not more than 300 km from the study area. as the data recorded in these observatories are not continuous, it was alternated between one and the other to observe variations in the components of the magnetic field parallel (by) and perpendicular (bx) to the pipeline on the days chosen as disturbed and that there was a psp record.the geomagnetic indices data used in this paper were provided by the wdc, kyoto (http://wdc.kugi.kyoto-u.ac.jp/ wdc/sec3.html). to identify periods of intense geomagnetic storms, figure 3 shows the values of geomagnetic indices dst and ae for the years from 2013 to 2018. we examined geomagnetic activity during the time the measurements were recorded, and selected disturbed periods, as highlighted in figure 3 and listed in table 1. the storm times were identified according to loewe & prolss (1997) a weak storm has −30 nt > dst > −50 nt, a moderate storm has −50 nt > dst > −100 nt, a strong storm has −100 nt > dst > −200 nt, a severe storm has −200 nt > dst > −350 nt, and a great storm has dst < -350 nt, and confirmed with ae index up to 500 nt. the storm periods are defined when dst index significantly depressed and then it is followed by a recovery phase to return to initial level (see figure 3, gray shaded rectangle). figure 2 shows psp voltages on that go outside the safe range in periods coinciding with geomagnetic storms (disturbed days). table 1 remarks the test point measured during the storm periods to see both when data do and don’t go over the safety range to discuss the relationship between psp and geomagnetic storms. figure 1. map of the province of buenos aires showing the location of the neuba ii gas pipeline (green line) (source: https://www.enargas.gob.ar) and a detail showing a satelital image of saavedra district in the area of goyena and control posts (red dots) (source: pipeline integrity/management, tgs). figure 2. on (red dots) and off (blue dots) potentials measured along the gas pipeline during the 2013 2018 period. dates of disturbed days (in 2013 (8/27 to 9/2), in 2014 (8/18 to 8/22), in 2016 (11/22 to 11/27) and 2018 (9/11 to 9/14) and -0.85 v and -1.35 v protection limit (grey and black horizontal lines). they are spot measurements taken over several days. the grey boxes highlight the section between 680-720 and also highlight out of safe zone points. figure 3. time data of the ae (in red line) and dst (in blue line) indices for the 2013-2018 period coinciding with the days of psp observation. geomagnetic storms periods where identified by asterisks and gray shaded rectangle. record.the http://wdc.kugi.kyoto-u.ac.jp/wdc/sec3.html http://wdc.kugi.kyoto-u.ac.jp/wdc/sec3.html https://www.enargas.gob.ar 366 patricia larocca, m.a. arecco, a.c. macrino to observe the local incidence of storm periods, the geomagnetic field values were recorded in geomagnetic observatories belonging to the intermagnet network near the study area. these observatories are trelew (colatitude 133.3 °, longitude 294.7 °) and pilar (colatitude 121.4 °, longitude 296.12 °) both within 300km of the pipeline route. we used the horizontal components of the field (bx and by) and their time derivative (dbx/dt and dby/dt), at a 1 minute sampling rate. as the induced psp voltage depends not only on the magnitude of the geomagnetic field but also on its direction with respect to the pipeline, we computed both directions to observe the important correspondance with the table 1. selected periods by their geomagnetic activity. storm type according to loewe & prolss (1997) a weak storm has −30 nt > dst > −50 nt, a moderate storm has −50 nt > dst > −100 nt, a strong storm has −100 nt > dst > −200 nt, a severe storm has −200 nt > dst > −350 nt, and a great storm has dst < -350 nt. date 27 to 30-ago-13 31-aug to 2-sep-13 20 to 22-aug-14 22 to 27-nov-16 11 to 14-sep-18 dst index (nt) -60 -40 -30 -30 -60 storm type moderate (*1) weak (*2) weak (*1) weak (*1) moderate (*1) ae index (nt) 1000 700 800 1300 1000 number of psp test points meassured 740-800 600-730 710-800 620-690 780-800, 600-640 psp voltage (trichtchenko and boteler, 2001). figure 4 shows psp voltages on that go outside the safe range in periods coinciding with geomagnetic storms for example, it is noted that the peak in db/dt in fig 4a coincides with the psps going out of range. but there are other times when the psps go out of range when the db/dt is quiet, for example on 08/27/13, and 08/21 in 4b. in 2016 and 2018 the db/dt is very small and gappy so its really hard to actually see any activity. finally,the pipeline was modeled using the dstl theory representing the pipeline as an electrical transmission line, modeled by proposing multiple segments with various lengths and orientations to represent the pipeline path along its route (boteler, 2000; 2013). the electric field is represented by voltage sources distributed along the transmission line (northward), see figure 5. the pipeline electric properties are series impedance per unit of length, given by the resistivity of the pipeline steel and the pipeline cross section, and parallel admittance per unit of length, given by the conductance through its coating. these parameters determine the propagation constant. figure 5. distributed source transmission line model of a pipe section, schematic representation. (modified boteler, 2000) along most of the route, there are two parallel buried pipelines. for individual pipelines with impedance z and parallel admittance y, the impedance of both parallel pipelines is z/2 and their effective parallel admittance is 2y, according to boteler (2000). this results in a decreased characteristic impedance; however, the propagation constant remains unchanged. the following were considered as dielectric constants: the magnetic permittivity of the surrounding medium and the coating coinciding with those of a vacuum and that of steel 1000 times that of a vacuum and the same permittivity in all the mediums coinciding with that of a vacuum (favetto and osella,1999). results in figure 2 we can see that in the different years where the psp was registered in the area comprised by km 680-720, the psp off values fall outside the safety range, becoming more negative than -1.35 v. this is corrected with cathodic protection system on. since this effect is repeated in all the series of measurements carried out, it can be assumed that it would be linked to the geological characteristics of the site. in addition, psp on voltages that go out of safe range at non-fixed checkpoints based on the time they are measured. therefore, the anomalous potentials were compared with the values of the dst and ae indices to see if they coincided with periods of geomagnetic storms. figure 4. north, bx, and east, by, components of the geomagnetic field (blue lines) registered using data from the observatories trelew (2013) and pilar (2014, 2016, 2018) and corresponding time derivatives, dbx/dt and dby/dt, of the geomagnetic field (orange lines). the sampling rate is 1 min. scatter graphs of the psp voltage (v) computed for the whole periods selected. 367anomalous geoelectric potential variations observed along a gas pipeline section in argentine, possible intensification with variations of the earth’s magnetic field contrasting figures 2a and 3a, the large potential variations may be associated with the geomagnetic storm period from 8/27/2013 to 8/30/2013. on this period, there were moderate geomagnetic storms. there is an interesting correspondance at 8/27 (fig. 2a) that is still replicated in the on potential going outside the safety range. there’s some correspondance between the magnetic activity and the psp on voltages, particularly when the dst index does become strongly negative ( fig.3a 1*). when comparing figures 2b and 3b, a coincidence can be found again with the storms recorded from 8/19/2014 (fig. 3b, *1) at km 710. the period evaluated in 2014 only includes one moderate storm (8/19), during which some of the psp on measurements went out of range. the comparisons between figures 2c and 3c and figures 2e and 3e show that those observations of potentials were made in calm periods, with very weak storms. we can see some correspondence between disturbed days and abnormal record on 11/22/16. finally, the effect is seen on figures 2f and 3f for the 2018 period, when, again, a moderate geomagnetic storm occurred in the observation period (*1), increasing the potential levels around the area of kilometers 620640 and 780, although not exceeding a lot the protection levels. figure 4 represents the north and east components of the magnetic field and their temporal derivatives recorded in the vicinity of the study area. it can be seen in figure 4a that the record of maximum variation of the dby / dt component coincides with the period in which the psp records go outside the security range at checkpoints 740 to 790. the same is observed for the dbx / dt component in figure 4b at positions 710 to 720. this is repeated in figure 4c (km 790) and figure 4 d (km 620-640). but there are other times when the psps go out of range when the db/dt is quiet, for example on 08/27/13, and 08/21 in 4b. in 2016 and 2018 the db/dt is very small and gappy so its really hard to actually see any activity. focusing on the soil study at control points 680 and 720, as can be seen on the maps of figure 6, point 700 is located in alluvial plain and terrace soils, formed by unconsolidated sediments of clayey silts, silty clays and sands, and sandy gravels, with moderate water erosion and high risk of flooding; while control post 730 is located in loessic plain soils with hardpan, formed by sediments similar to those of post 700, with moderate water erosion but a low to null risk of flooding (pereyra et al., 2001). regarding the electrical conductivity of the study area, it has been observed, during the campaigns of the argentine mining geological service (segemar), that it increases in the direction of the flow network from the mountain recharge area to the discharge areas (pereyra et al., 2001). however at checkpoint 712, where there are the changes in the psp voltages, the continuity is uniform. the relative size of the psp variations was compared to determine whether there was lateral psp variability along the oil pipeline route and whether this is correlated with a characteristic of the pipeline. to this end, the records of 2013 were taken as a case study because it is the year where the greatest amplitudes in potential differences were recorded. we know that the year also coincides with the most geomagnetically disturbed year when measurements were made; however, the measurements made during the storm are outside of the main section of pipeline that shows the negative values of interest here. figure 7 shows the psp relative amplitude coefficients as a function of the distance along the pipeline, observing that it changes in the section around control post 712, going from a dual to a single buried pipeline, and subsequently becoming a dual one again (figure 8). they allow us to know how sensitive the psp variable is to change with respect to position variable (the rate of change of psp(mv) per kilometer. figure 6. lithological (left), floodability (centre), and edaphological (right) maps. control points from 700 to 730 (red dots). 368 patricia larocca, m.a. arecco, a.c. macrino figure 7. relative size of the psp variations recorded for 2013 along the pipeline route. detail of control points from 700 to 730 (red oval). figure 8. detail image of the gas pipeline (green lines) that crosses provincial route 67 and gral. roca railway. it’s on the right a zoom of the characteristics of the pipeline structure along its route. (image source: google earth); checkpoints on the gas pipeline (red dots); control point numbering (white rectangle). for this study, a dstl model was defined, considering a northward electric field of 10 mv/km. by the pipeline route there are two pipes in parallel. but, the pipeline is reduced to a single buried pipe for sections immediately before a compressor station. designs were made for a steel pipeline with a 762 mm outer diameter, and a 50 km propagation constant, for the 200 km of the pipeline between general cerri (compression plant at km 592) and saturno (compression plant at km 794). figure 9 shows the psp amplitude variations along the pipeline route, accounting for an anomalous area of great amplitude at the testing post extending from km 670 to km 720 of the distance along the pipeline. the maximum amplitude relationships have been recorded at the testing post located at km 700 of the pipeline route, and they decrease in both directions from this location, where the pipeline structure changes from dual to single. a change from 2 to 1 pipes also represents a change in the pipeline characteristics seen by telluric currents. figure 9. pipe-soil potential modeled considering variations in the pipeline sections. above the model is shown as double-single-double (single pipe between km 712 and km 713) (see figure 8). conclusions we analyse a series of annual measurements from control points along a pipeline in argentina over 6 years to analyse the local influence from geomagnetically induced currents from space weather activity and local influence from geology or soils. we were able to observe that anomalies increase in periods of greater geomagnetic activity, as most notably seen in the 2013 observations coinciding with the period of maximum solar activity (activity peak of solar cycle 24). the psp values showed fluctuations in various parts of the pipeline, from 0.1 v to 0.15 v, compared with the records of the same location in calm periods. the results obtained can be compared with those shown by yu et al. (2019) and ingham et al. (2018) who also accounted for the effects of geomagnetic storms in pipelines buried in mid-latitudes. in ingham’s study focused more on observing variations in cathodic protection potential on a natural gas pipeline in new zealand where that variations occur in the electric field across the pipeline affecting it more than those parallel to the pipeline obtaining variations between -0.5 mv and about +0.8 mv, they concluded that observed variations in the cp potential arise because of changes in the local potential of the ground. what’s more, yu et al. (2019) observed that during the geomagnetic storm, the psp positive offset of the pipelines can exceed the limit of the technical standard for currents interference mitigation of buried pipeline, even when the effects of geomagnetic storms are less in low and mid latitudes, they observed the psp values of the pipeline in changyi station. the anomalies recorded in the pipeline section from km 699 to 730 observed repeatedly from 2013 to 2018 could be related to the peri-hill gravel, sand and silt deposits, with piedmont levels, as the remaining route runs through relatively thick and discontinuous loessial sand-clayey silts with hardpan levels of an epigenic origin (segemar, geological and mining map of the province of buenos aires), accounting for a geological boundary in the area of the anomalies observed. the great psp variations observed along the pipeline in the area under study could be related to the local geology traversed by the gas pipeline and to discontinuities of the pipeline structure along its route. the psp modelled using the dstl method showed a good consistency between the observed and modeled psps mainly in the increases of potential due to changes in the topology of the pipeline. it would be interesting to perform some surface impedance calculations based on magnetotelluric data in the area to corroborate this assumption and to determine the magnitude of this contribution. on the other hand, the present results did not enable any deeper insight into the problem. this is why we hope to obtain more data from a greater number of measuring stations and apply quantitative models of geomagnetically induced currents and voltages in the pipeline networks. 369anomalous geoelectric potential variations observed along a gas pipeline section in argentine, possible intensification with variations of the earth’s magnetic field acknowledgements: we thank the referees for their valuable comments. the p/s voltage data are in the ownership of the joint-stock company tgs gas pipeline. the authors appreciate the data contributed by e.f. lara, c.a. deloso, d.e. molina, d.r. falabella, g. avila, and s.n. río, from tgs gas pipeline integrity/management. this work was partly funded by projects no. 279/2018 (undefi) of the national defense university and 200201601000088ba (ubacyt) of the university of buenos aires. references akasofu, s. i. 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(2019). monitoring experiment of electromagnetic interference effects caused by geomagnetic storms on buried pipelines in china. ieee access, 7, 14603-14610. doi:10.1109/access.2019.2893963. https://doi.org/10.1002 https://doi.org/10.1093/gji/ggs113 10.1029/ja 10.1029/ja https://doi.org/10.12942/lrsp-2013-5 https://doi.org/10.12942/lrsp-2013-5 https://repositorio.segemar.gov.ar/handle/308849217/2574 https://repositorio.segemar.gov.ar/handle/308849217/2574 https://doi.org/10.1007/s10712 10.1109/access _hlk48586445 keywords: contaminated soil; total petroleum hydrocarbons; atterberg limits; density; clay; sand. palabras clave: suelo contaminado; hidrocarburos totales de petróleo; límites de atterberg; densidad; arcilla; arena. how to cite item zahermand, s., vafaeian, m., & bazyar, m. h. (2020). analysis of the physical and chemical properties of soil contaminated with oil (petroleum) hydrocarbons. earth sciences research journal, 24(2), 163-168. doi: https:// doi.org/10.15446/esrj.v24n2.76217 this paper is focused on analyzing the engineering properties and geotechnical changes of the soil contaminated by petroleum hydrocarbons, compared to non-contaminated soil. also, identifying the physical and chemical behaviors and the interaction mechanisms between soil and contaminants at the time of contamination is far-reaching. this study aims to investigate the effects of petroleum contaminants on atterberg limits, cation-exchange capacity, electrical conductivity and acidity, maximum dry density, and optimum moisture content of clay soil compared to non-contaminated samples with similar geological conditions in gachsaran petroleum refinery region. the significant effects of contaminants on clay soil include an increase in cation-exchange capacity and electrical conductivity, but a decrease in acidity compared to non-contaminated samples. the results of the atterberg limit as a physical index of the soil showed an increase in the petroleum hydrocarbon concentration, which enhanced the plastic and liquid limits, but decreased the plasticity index. the results of compaction tests using water and oil fluids indicated that the concentration of petroleum hydrocarbons in sandy soil increased maximum dry density and decreased optimum moisture content. moreover, in a soil sample with stable pollution levels, maximum dry density and optimum moisture content were higher and lower in the compaction test using water fluid compared to those in compaction tests using oil fluid, respectively. finally, increasing the soil pollution level reduced the difference between maximum dry density and the optimum moisture content in the compaction test using water fluid compared to the same test using oil fluid. abstract analysis of the physical and chemical properties of soil contaminated with oil (petroleum) hydrocarbons análisis de las propiedades físicas y químicas de suelos contaminados con hidrocarburos de petróleo issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.76217 el presente trabajo se enfoca en analizar las propiedades de ingeniería y los cambios geotécnicos de los suelos contaminados por hidrocarburos del petróleo comparados con suelos no contaminados. tambien en identificar los comportamientos físicos y químicos y los mecanismos de interacción entre el suelo, los contaminantes y el alcance de estos. este estudio busca investigar los efectos de los contaminantes de petróleo en los límites de atterberg, en la capacidad de intercambio de cationes, la conductividad eléctrica y la acidez, la densidad seca máxima, el contenido óptimo de humedad en suelos arcillosos frente a muestras de suelo con condiciones geológicas similares en la refinería de petróleo de gachsaran, irán. los efectos determinantes de los contaminantes en suelos arcillosos incluyen un incremento en la capacidad de intercambio de cationes y en la conductividad eléctrica, pero un decrecimiento en la acidez frente a las muestras no contaminadas. los resultados de los límites de atterberg como un índice físico del suelo muestran un incremento en la concentración de hidrocarburos de petróleo, lo que intensifica los límites plásticos y líquidos, pero reduce el índice de plasticidad. los resultados de las evaluaciones de compactación al añadir agua y fluidos de crudo indican que la concentración de hidrocarbonos de petróleo en suelos arenosos incrementan la densidad seca máxima y reducen el contenido de humedad óptimo. además, en una muestra de suelo con niveles de contaminación estables, la densidad seca máxima y el contenido óptimo de humedad registraron el mayor y el menor índice en la evaluación de compactación con agua comparado con las pruebas donde se utilizó crudo. finalmente, al incrementar el nivel de contaminación del suelo se reduce la diferencia entre la densidad seca máxima y el contenido óptimo de humedad en la prueba de compactación cuando se usa agua comparado con la prueba donde se usa crudo. resumen record manuscript received: 17/11/2018 accepted for publication: 12/02/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 163-168 samad zahermand1, mahmood vafaeian2*, mohammad hosein bazyar3 1department of civil engineering, isfahan (khorasgan) branch, islamic azad university, isfahan, iran 2department of civil engineering, isfahan university of technology, isfahan, iran 3department of civil engineering, yasouj university, yasouj, iran * corresponding author: mahmood@cc.iut.ac.ir so il s https://doi.org/10.15446/esrj.v24n2.76217 https://doi.org/10.15446/esrj.v24n2.76217 https://doi.org/10.15446/esrj.v24n2.76217 mailto:mahmood@cc.iut.ac.ir 164 samad zahermand, mahmood vafaeian, mohammad hosein bazyar introduction the pollution of soil and underground water causes many environmental problems. among all the environmental pollutants, oil and petroleum hydrocarbons have been attracting more attention as global concerns. soil pollution with petroleum compounds is one of the most prevalent ecological problems. the leakage of petroleum compounds leads to soil contamination and physical and chemical transformations in the soil. this could happen due to the leakage of oil-transferring pipes, worn-out and old petroleum reservoirs, underground and sea oil transmission systems, natural leakages, as well as oil well fires. the amount of pollution depends on soil characteristics and the chemical compounds of pollutants, making it complicated to get removed from the soil. there are different methods for the removal of the contaminants from soil. in general, these methods are divided into three different categories of physical (such as soil vapor extraction), chemical (such as soil washing), and biological (such as phytoremediation). sometimes, a combination of these methods can be applied (nathanail & bardos, 2004; sharma & reddy, 2004; aiban, 1998). in some cases, since soil washing is not cost-effective, petroleumcontaminated soils are used for civil engineering applications like constructing roads. to take the most advantage of the soil, it is essential to precisely study the features and behavior of such soils after contamination. there is no doubt that such actions and features cannot be realized without considering the soil’s geotechnical parameters, including atterberg limits, density, optimum moisture, compaction, cohesion, friction angle, etc. it is not possible to understand the behavior of the structures and their proper designing on the contaminated soil unless all the geotechnical calculations and behavioral parameters of the soil have been explored. therefore, to achieve an accurate and adequate design, it is necessary to examine different soils and oil pollutants to minimize the probable expenses and damages caused by the geotechnical risks of petroleum contaminated soils. few studies have been conducted to investigate the physical and chemical characteristics of oil-contaminated soils under natural conditions. cook et al. (1992) conducted an analysis to specify the compaction and strength of sands contaminated with oil in the laboratory environment. they reported that, while oil contamination enhanced the compatibility of sandy soils, friction angle decreased. shin et al. (1997) and meegoda & ratnaweera (1994) obtained similar results for sandy and clay soils, respectively. stewart et al. (2003) discovered that the plasticity features of soil induced changes in clay soil permeability; for example, with a decrease in the plasticity index, the permeability of soil increased. yong (2000) and ouhadi & yong (2006) argued that the liquid limit and plasticity index of fine-grained soils were important in using these soils for clay coverage. khamehchiyan et al. (2007) reported a relationship between decreasing strength, permeability, maximum possible dry unit weight, and atterberg limit and increasing oil contamination in sandy and clay soils. al-sanad et al. (1995) found a decrease in the strength and permeability, but an increase in the compatibility of oil-contaminated sands in kuwait. al-sanad & ismael (1997) investigated the long-time effect of reducing oil concentration in which the stability of oil-contaminated sands increased over time. according to jia et al. (2011), increasing the contamination led to a rise in the plastic limit, liquid limit, and coefficient of compressibility in the soil. kermani & ebadi (2012) stated that intensifying oil contamination increased the friction angle, maximum possible dry unit weight, the compression index, and atterberg limit, but decreased optimal moisture and cohesion. also, the long-time effect of contaminants caused more reduction in cohesion but did not affect friction angle. the studies mentioned above showed that the penetration of petroleum contaminants in the soil led to changes in soil properties in different ways. there is an obvious need for more comprehensive studies under natural conditions on the oil compounds released into the environment to obtain their real effects on the soil. this article aims to investigate the effects of crude oil contaminants on the physical and chemical characteristics of the soil under natural conditions, compared to those in non-contaminated soil samples in the nearby region. material and methods the region under observation is located around gachsaran city in kohgiluyeh and boyer-ahmad province at the farthest end of oil-producing zones in iran, which is about 220 km to the southeast of ahvaz (figure 1). gachsaran oil field was first studied during 1923 and 1924. oil production in this field was started in 1937, after drilling some wells and more specifically producing from the well number 3 with an amount equal to 26000 barrels per day. gachsaran oil field is today one of the five most significant oil-producing areas in iran, producing 20% of the country’s oil. it consists of 10 exploration units, ten gas injection units, three gas refineries, five salt removal units, with 6700 km long oil-transferring pipes. there are 13 oil-producing reservoirs in this corporation, with the total oil production, equal to 470,000, 105,000, and 65,000 barrels per day from gachsaran, bibi hakimeh, and rag safid2 reservoirs, respectively. the reservoirs of bink, golkhari, nargesi, chelinger, pazenan2, siahmekan, gorangan, solabdar, rodak, and khoez rank 4th to 13th, concerning the production volume, respectively. also, the reservoirs of kilvar kerim, zageh, chaharbisheh, mansorabad, and koohkaki have a total production capacity of 18,000 barrels per day. these reservoirs have been excavated in kohgiluyeh and boyer-ahmad, bushehr, fars, and khuzestan provinces. gachsaran oil reservoir is globally the third-largest oil reservoir after saudi arabia and kuwait oil reservoirs. geologically, the study region consists of asmari and gachsaran formations. the most massive oil production is provided by asmari formation, which is made up of carbonate stones and is the wealthiest reservoir of oil in iran and the middle east (figure 2). in this study, eight oil-contaminated samples (samples 1, 2, 3, 4, and 5 with cl classification and samples 7, 8, and 9 with sm classification), containing similar oil combinations with different degrees of contamination and two non-contaminated samples (sample 6 with cl classification and sample 10 with sm classification) in a neighboring area with similar geological conditions were selected as the study samples. also, four regions of 0-500, 500-1000, 1000-1500, and 1500-2000 meters length were selected and categorized as a, b, c, and d. in each region, four samples of soil within 125 meters distance from each other were collected and then were separately mixed. samples a, b, and d had sm classifications, while sample c had cl classification. the samples were then dried and sifted with a 2mm sifter. some of the chemical and physical properties of the soil were examined via the following methods: electrical conduction (ec) using the ec instrument (sparks, 1996), acidity (ph) using standard astm e 7097, total petroleum hydrocarbons (tph) via aglient 7890a gas chromatograph with flame ionization detector (hutchinson et al., 2001), cation exchange capacity (cec) (bower et al., 1952), soil classification via the astm d 422-63 standard test method, atterberg limits according to astm d 4318, and finally the compaction according to the astm d-698 standard. according to the unified classification system, two types of soils in the study area, including silty sand (sm) and clay of low plasticity low-plasticity clay (cl), were detected. the results of gas chromatography (gc), including the percentage of particles in soil samples of cl and sm, along with the physical and chemical properties of cl soil samples are represented in tables 1, 2, and 3. table 1. results of the gas chromatography test and the percentage of particles in cl soil samples clay (%)silt (%)sand (%)tph(g/kg)sample 16.8038.2045.00126.361 18.3034.6047.1081.882 17.4035.5047.1076.503 22.0038.6039.4057.324 19.2040.3040.5044.245 21.0030.2048.8024.20c 25.4028.4046.20-6 table 2. physical and chemical properties of cl soil samples cec (cmol/kg)ec (ms/cm)phsample 15.8910.506.431 16.738.906.602 15.529.106.623 12.566.606.954 15.954.907.055 13.262.107.30c 7.292.007.906 165analysis of the physical and chemical properties of soil contaminated with oil (petroleum) hydrocarbons in contaminated soil samples compared to non-contaminated ones increased, which could have been due to higher crude oil salts. as shown in figure 3, the plastic limit has increased in line with an increase in the contamination. it is higher in contaminated soils compared to non-contaminated ones. the liquid limit of the contaminated soil was found out to be higher than that of the non-contaminated soil. still, it was less noticeable than differences in the plastic limit. increasing the contamination level hasn’t made a significant effect on the amount of liquid limit. however, though the liquid limit had a slight decrease in the concentration of 126.36 grams of oil per kilogram soil (g/kg), it was still higher than that in non-contaminated soil. figure 3. the relationship between atterberg limits and total oil hydrocarbons table 3. results of the gas chromatography test and the percentage of particles in sm soil samples clay (%)silt (%)sand (%)tph (g/kg)sample 10.0034.5055.50146.827 16.3032.8050.90111.268 12.5035.5052.0068.129 13.4021.2065.4030.30a 11.0029.0060.0025.56b 10.5020.9068.6018.76d 12.1026.3061.20-10 results and discussion cl soil comparing the chemical and physical properties of oil-contaminated soil with those in non-contaminated soil at the refinery revealed excessive hydrocarbon concentrations in oil-contaminated soil, indicating noticeable crude oil contamination in the soil area. influenced by the amount of contamination in the soil, soil ph decreases with an increase in the number of pollutants, which can be due to the presence of organic compounds containing hydrogen that can be separated (laurent et al., 2012). the cation exchange capacity (cec) in the non-contaminated soil was less than that of the contaminated soil. also, electrical conductivity figure 1. study region figure 2. geological map of the study region 166 samad zahermand, mahmood vafaeian, mohammad hosein bazyar over the production years, the refinery has accumulated considerable amounts of oil-contaminated soils. yet, the duration has been long enough for creating equilibrium between crude oil and the soil. thus, there can be four conditions influencing the presence of oil in the soil. some volatile matter in the oil has evaporated due to the sustainability of heat in soil particles and the low thermal conductivity coefficient of clay soil. some parts of the oil have undergone chemical reactions with the soil, while some others have covered the soil particle surfaces, and the rest settled down in pores of the soil. since some parts of soil particles are covered by oil, they do not let water molecules reach double layers; thus, more water is needed for the soil to get to the plasticity. however, the portions of water which are unable to access the double layer settle down in the free water layer and increase the soil’s liquid limit. this could be the underlying reason for higher liquid and plastic limits in oilcontaminated soils compared to the non-contaminated ones (estabragh et al., 2016; kermani & ebadi, 2012). according to some researchers, higher liquid and plastic limits in oilcontaminated soils compared to the non-contaminated ones can be due to an increase in the presence of oil contamination in the clay, where the oily material has been absorbed in the clay mineral interlayer space, thus creating enough volume for the water penetration thereby causing an increase in the double layer thickness in clay minerals (sivapulliah, 1988; shah et al., 2003). as shown in figure 3, the plastic limit of the non-contaminated soil is twice less than that of the contaminated soil. in contrast, the percentage of clay in the non-contaminated soil is 51.2% higher than in the most contaminated soil of the refinery. according to some researchers, the plastic limit of the soil is influenced by the cation exchange capacity, the amount of clay, and the remaining moisture in the soil (seybold et al., 2008). therefore, in this study, it is evident that the cation exchange capacity has been more effective than the clay percentage in determining the plastic limit. the highest contaminated soil with low clay percentage has shown more plastic limit than non-contaminated soil with a high clay percentage. in contrast to the results of the present study, the studies by khomehchiyan et al. (2007) and lashkaripoor et al. (2010) showed that an increase in the contamination decreased the plastic limit. however, khoshneshin langroodi et al. (2010) showed a positive relationship between soil contamination and the plastic limit, which supports the results of the present study. in contaminated soils, with an increase in the amount of oil contamination initially, the liquid limit increases, but then it decreases. this can be explained according to the double layer theory. as oil is a non-polar solvent, when clay particles are surrounded by it, no double layer of oil and clay is formed, so the moisture increases. however, due to the lubricating property of the oil, water molecules flow smoothly, and less moisture gets to the liquid limit at the end. meegada et al. (1994) and sridharan et al. (2000) mentioned that the soil liquid limit depends on the percentage of clay in the soil. thus, there could be two reasons for explaining the decrease in the liquid limit, when the level of oil contamination increases. first, the clay percentage of studied soils in the present research decreases with an increase in the contamination. second, since crude oil is non-polar and its dielectric constant is lower than that of water, it reduces the repulsion force and increases the gravity force between clay particles and flocculates them (kaya & fang, 2000). therefore, since a significant part of the liquid limit is dependent on the clay soil, due to the flocculation, the amount of clay soil is reduced. in consequence, the liquid limit decreases. olgun & yildiz (2012) argued that the existence of oil in soil forces small particles to move towards larger ones. thus the soil is transformed into grain structure. in another study, khamehchiyan et al. (2007) and rahman et al. (2010) reasoned that the presence of oil around clay particles in the soil reduces the moisture of the soil, as oil gets in contact with the loaded surfaces of clay faster than water, settles down in interlayer space and repulses water. with an increase in oil contamination and clay particles surrounded by the oil, clay particles slide over each other more quickly, and smaller particles get settled among bigger ones to fill soil pores. thus the soil gets to the liquid limit with less water. the plasticity index in non-contaminated soil is higher than that in contaminated soil. with an increase in the contamination level of crude oil in contaminated soil, the plasticity index decreases. the plasticity index depends highly on the amount of clay in the soil, and non-contaminated soils contain more clay than the contaminated soils. therefore, the high clay amount directly affects the plasticity limit and, in consequence, makes the soil change from plastic limit to liquid limit. another factor in increasing the plasticity index is the percentage of silt in the soil. in sum, based on the findings, it can be concluded that crude oil-contaminated soil should not be used for construction purposes in the refinery zone. plus, it is suggested that for using crude oilcontaminated soils, appropriate and economic strategies be applied to minimize the contamination. sm soil to determine the amount of maximum dry density and optimum moisture content in the silty-sand soils, the proctor compaction test was conducted according to the astm d-698 standard. the results are presented in table 4. table 4. the amount of maximum dry density and optimum moisture content based on the proctor compaction test optimum moisture content (%)dry concentration (g/cm3)sample 8.001.727 8.501.828 9.501.859 12.801.81a 13.001.79b 13.81.76d 14.21.7310 as represented in table 4, an increase in the crude oil concentration in the soil up to 68.12 g/kg, increased the maximum dry density with less optimum moisture than the non-contaminated soil. this can be due to the lubricity properties of oil hydrocarbons, making sand and silt particles more slippery. in consequence, more compaction of particles together with a decrease in the friction between sand grains and silt, as well as the closer positioning of grains with each other, cause the number of particles per unit volume to increase. for the samples 7 and 8, due to a higher level of contamination and lower optimum moisture than other samples, their maximum dry density has decreased. the contaminated soil sample 7 with the maximum crude oil concentration of 146.82 g/kg has the least optimum moisture and maximum dry density. this could be because, with an increase in the crude oil contamination in the soil, the empty spaces between particles are filled with oil, so less soil content is added into the test mold. thus the maximum dry density decreases. also, the high viscosity of crude oil doesn’t allow the oil to move easily between the empty spaces, resulting in a decrease in slippery surfaces, which lead to a reduction in the density. in general, the results of this study show that an increase in oil contamination is correlated with an improvement in the compressibility of grain soils. the present findings are consistent with those obtained by al-sanad et al. (1995), according to which an increase in oil contamination in sandy soil up to 4% of the soil weight, led to a decrease in optimum moisture and an increase in the density. however, contrary to the current study, khamehchyan et al. (2007) indicated a relation between higher oil contamination and less maximum dry density. given the percentage of optimum moisture in the non-contaminated soil at the refinery, with an increase in the crude oil contamination up to 30.30 g/ kg, more oil is absorbed by soil particles, with a small part being flown in the space between particles. thus, the added water increases slipperiness, and in turn, more compressibility is achieved. at 68.12 g/kg, compared to the noncontaminated soil, the optimum moisture is reduced by 33.10%, which shows that after the particles have absorbed the oil, empty spaces between particles are filled with oil. then, less water is required to reach maximum dry density. at 111.26 g/kg and higher levels, a smaller decrease occurs in optimum moisture, and maximum dry thickness is reduced. the clay percentage has been effective in the compression of soil samples. the physicochemical behavior of clay in maximum dry density increase in sm soils can be explained as follows. clay particles are surrounded by oily materials, causing double layers to decrease. after that, an increase in the gravity force between soil particles makes them more compact and flocculated, 167analysis of the physical and chemical properties of soil contaminated with oil (petroleum) hydrocarbons so the clay’s behavior becomes similar to silt and sand. thus, since clay absorbs more water compared to the silt and sand, optimum moisture is decreased. table 5 represents non-contaminated soil sample 10 as well as contaminated soils in which oil is used as the fluid of the proctor compaction test (samples a and d). table 5. the amount of optimum moisture and maximum dry density obtained from the proctor compaction test using oil fluid optimum moisture content (%)dry concentration (g/cm3)sample 13.21.77a 15.11.56d 15.71.5310 as shown in table 5, with an increase in the crude oil concentration in non-contaminated soil samples, there is a decrease in optimum moisture, but maximum dry density enhances. the underlying reason is that, due to more lubricating properties between soil particles and less friction force in highly oilcontaminated soil, the particles settle closer to each other, thereby increasing the density. the same results were obtained for samples 10, d and a in the proctor compaction test using liquid water. however, higher maximum dry density and lower optimum moisture were observed compared to proctor compaction test using oil fluid. in fact, as a result of the presence of oil instead of water in the compaction test, the soil becomes more contaminated, so oil is positioned between the soil particles more easily and fills the empty spaces. consequently, less soil is added to the test mold. the reason for lower optimum moisture in the compaction test using water fluid compared to the same test using oil fluid in a soil sample with fixed crude oil concentration can be due to lower viscosity of water compared to the oil, where water molecules move easily between soil pores and thus, produce more slippery surfaces. as shown in figure 4 and figure 5, with an increase in the contamination level, the difference in maximum dry densities between the compaction test using water fluid compared to the same test with oil fluid decreased. therefore, in sample a with a higher level of contamination than sample d, maximum dry density in the compaction test using water fluid was more than that in the same experiment using oil fluid by 2.26%. also, in sample d with a lower level of contamination than sample a, maximum dry density in the compaction test using water fluid was more than that in the same test using oil fluid by 12.82%. furthermore, in non-contaminated soil (sample 10), maximum dry density in the compaction test using water fluid was more than that in the same test using oil fluid by 13.07%. also, with an increase in the crude oil contamination concentration, the difference in optimum moisture between the compaction test using water fluid and the same test with oil fluid decreased. for sample a, the optimum moisture content in the compaction test using water fluid was by 3.03% less than that in the same test with oil fluid. for sample d, the optimum moisture content in the compaction test using water fluid was by 8.61% less than that in the same test with oil fluid. besides, in the non-contaminated soil, the optimum moisture content in the compaction test using water fluid was by 9.55% less than that in the same test with oil fluid. one justification for this could be that in soils with higher contamination, particles move more easily due to the lubrication property, and less water and oil fluids are needed to get compacted. therefore, particles in the contaminated soils get maximum dry density with less water and oil contents. conclusions this research was conducted to analyze the concentration of total petroleum hydrocarbons (tph) on the atterberg limit of clay soil (cl), as well as its effect on the maximum dry density and optimum moisture in the compaction test using water and oil fluids for sandy soil (sm). therefore, contaminated soils, together with non-contaminated soil samples with similar geological conditions, were collected from different locations in the gachsaran refinery. according to the results, soil behavior is varied and dependent on the amount of soil contamination, i.e., the existence of oil contaminants in clay soils causes an increase in the plastic and liquid limits and plasticity index compared to non-contaminated soils. a significant relationship was found between the plastic limit and cation exchange capacity (cec). in addition, the liquid limit was found to be highly affected by the clay percentage in the soil. also, increasing crude oil contaminants in clay soil increased cation exchange capacity and electrical conductivity, but decreased the ph of soil. standard compaction tests of contaminated sandy soils (sm) revealed a decrease in optimum moisture and an increase in maximum dry density, compared to non-contaminated soils. with an increase in the oil concentration in the contaminated soils, the soil gets to the maximum dry density with less optimum moisture. with an increase in the contamination concentration by more than 68.12 g/kg, the maximum dry density decreases. when the oil percentage gets to a certain amount, due to the filling of pores and spaces between particles by oil, less water is needed to get to the maximum density, and thus less soil is added into the test mold. in the proctor compaction test, using oil fluid, optimum moisture, and maximum dry density for contaminated and non-contaminated soils were less than those in the compaction test using water fluid. comparing the results obtained from the compaction test using oil fluid versus using water fluid in different samples showed that optimum moisture and maximum dry density were lower in the compaction test using oil. increasing the crude oil concentration in the non-contaminated soil decreased the 9.55% optimum moisture difference between the compaction test using oil fluid versus compaction test using water fluid to 3.03% in sample a. moreover, increasing the crude oil concentration in the no-contaminated soil decreased the 13.07% maximum dry density difference between compaction test using oil fluid versus compaction test using water fluid to 2.26% in sample a. when the percentage figure 4. the relationship between maximum dry density in proctor compaction tests using water and oil fluids, and total petroleum hydrocarbons figure 5. the relationship between optimum moisture content in proctor compaction tests using water and oil fluids and total petroleum hydrocarbons 168 samad zahermand, mahmood vafaeian, mohammad hosein bazyar of the oil concentration is high due to lubricating properties, the particles move on the surface of each other more easily; thus, less water or oil is needed to get to the maximum dry density. references aiban, s. a. 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(2000). geoenvironmental engineering, contaminated soils, pollutant fate and mitigation. crc press, boca raton, 320 pp. https://www.tandfonline.com/bssc20 https://www.tandfonline.com/bssc20 https://link.springer.com/journal/13369 https://link.springer.com/journal/10706 https://link.springer.com/journal/10706 https://www.onepetro.org/search?q=dc_creator%3a%28%22shin%2c+e.c.%22%29 https://www.onepetro.org/search?q=dc_creator%3a%28%22lee%2c+j.b.%22%29 palabras clave: guangdong coast; maritime cyclones; statistical characteristics; temporalspatial characteristics; path analysis. how to cite item chen, c., wang, l., chen, r., xing, f., & chen, j. (2018). temporal-spatial characteristics and path analysis of maritime cyclones in guangdong coastal areas in the south china sea. earth sciences research journal, 22(4), 319-325. doi: https://doi.org/10.15446/esrj.v22n4.77361 this study was conducted to analyze the maritime cyclone characteristics in guangdong coast in the years of 1949 to 2016, including inter-annual variation, the intensity of tropical cyclones, generating location and time, and path direction. also, the temporal-spatial characteristics were also studied. results show there were 183 tropical cyclones landed in guangdong coast in the past 68 years, with an average of 2.7 each year, which more than 60 percent were a typhoon. most of the tropical cyclones were generated in the northwest pacific, spanning from april to december. the path directions were mainly north, northwest, and west. the strengths of the tropical cyclones were reduced from central guangdong coast to the east and the west sides, and the section of zhanjiang city to shenzhen city was the most vulnerable to tropical cyclones. tropical cyclones that generated in the south china sea tend to attack the west of the guangdong coast, while the ones that produced in the northwest pacific tend to attack the east of the guangdong coast. in the study area, the tropical cyclones frequently occurred from july to september and became strongest in september. there are a most common landing section and path direction for each month. finally, based on the statistical data and research results, the tropical cyclone paths in guangdong coast were preliminarily analyzed. abstract keywords: guangdong coast; maritime cyclones; statistical characteristics; temporal-spatial characteristics; path analysis. temporal-spatial characteristics and path analysis of maritime cyclones in guangdong coastal areas in the south china sea características temporales y espaciales y análisis de recorrido de ciclones marítimos en las areas costeras de guangdong, en el mar de la china meridional issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.77361 este estudio se realizó para analizar las características de los ciclones marítimos en la costa de guangdong entre los años 1949 y 2016, además de su variación interanual, la intensidad de los ciclones tropicales, el lugar y el momento de creación, y su recorrido. las características temporales y espaciales también fueron estudiadas. los resultados muestran que 183 ciclones tropicales tocaron tierra en la costa de guangdong en estos 68 años, con un promedio de 2.7 por año, de los cuales el 60 por ciento se consideran tifones. muchos de los ciclones tropicales se generaron en el noroeste del pacífico, y van de abril a diciembre. las direcciones de recorrido son principalmente norte, noroeste, y oeste. la fuerza de los ciclones tropicales se redujeron desde la costa central de guangdong hacia el este y el oeste, mientras que el sector entre las ciudades de zhanjiang y shenzhen es el más vulnerable a los ciclones. los ciclones tropicales generados en el mar del sur de china tienden a tocar tierra en el oeste de la costa de guangdong, mientras que aquellos que se producen en el noroeste del pacífico suelen recorrer hacia la costa este. en el área de estudio, los ciclones tropicales frecuentemente ocurren de julio a septiembre y son más fuertes en septiembre. se evidenciaron recorridos y direcciones de los ciclones propias a cada mes. finalmente, de acuerdo con la información estadística y los resultados de investigación, el recorrido de los ciclones tropicales en la costa de guangdong fueron analizados preliminarmente. resumen record manuscript received: : 24/07/2018 accepted for publication: 01/12/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 319-325 cheng chen1*, lei wang2, ruozhou chen2, fangliang xing2, jun chen2 1college of civil engineering, fuzhou university, fuzhou, 350116, china 2the pearl river hydraulic research institute, guangzhou, 510611, china *corresponding author: chencheng_1117@163.com h y d r o m e t e o r o l o g y 320 cheng chen, lei wang, ruozhou chen, fangliang xing, jun chen 1. introduction a tropical cyclone is an intense natural phenomenon, and the destructiveness of tropical cyclones over the past years is increasing (emanuel, 2005). it is generated in the ocean, which may cause substantial wind waves and storm surge. therefore, devastating damages will be induced in the coastal areas due to a tropical cyclone (yuan et al., 2010). china is one of the most vulnerable countries that suffered from the wind waves and storm surge disasters (jia et al., 2010), and according to the official records, there were thirteen major maritime disasters possibly caused by the tropical cyclones (liu et al., 2017). consequently, more and more researchers are focusing on this topic (li et al., 2004; yuan et al., 2011; zhang & cui, 2013). guangdong province is located on the west bank of the pacific ocean, and it is on the brink of the south china sea. it is frequently subjected to the invasion of tropical cyclones that generated in the northwest pacific or the south china sea (wen, 2002; tang et al., 2003). the cyclone weather conditions and huge waves generated by tropical cyclones will affect the fishing vessels (rezaee et al., 2016). also, huge waves are a severe threat to the structural safety of coastal, offshore, and marine projects, as well as the daily life of coastal residents (xu et al., 2008; zhao et al., 2010). three rings of the coastal planning record cyclones when they attack the coast (tucker et al., 2018). also, coastal planning such as mangrove creeks provides the service of protection (williams et al., 2007). it is essential to study the characteristics of the tropical cyclones that generated in the ocean to mitigate the disasters in coastal areas (zheng et al., 2014). also, observations of the maritime cyclones help forecasts of the maritime disasters (liou and elsberry, 1987). in this study, the landing tropical cyclones characteristics in guangdong coast in the years of 1949 to 2016 were analyzed, including inter-annual variation, the intensity of tropical cyclones, generating source and time, and path direction. based on the statistical data and research results, the tropical cyclone paths in guangdong coast were preliminarily analyzed, which may provide some useful information for wind wave simulations and disaster prevention in coastal engineering. 2. material and methods 2.1. tropical cyclone classification on the sea in the south china sea, most of the tropical cyclones that affect the guangdong coastal areas are generated in the south china sea and northwestern pacific. the intensity of the tropical cyclone is based on the maximum wind speed near the center of the tropical cyclone. generally, the intensity of the tropical cyclone is divided as follows: 1) super strong typhoon, the maximum average wind speed near the bottom centre is greater than 51.0 m/s (16 stages and above); 2) strong typhoon, the maximum average wind speed near the bottom center is 41.5-50.9 m/s (14-15 stage); 3) typhoon, the maximum average wind speed near the bottom centre is 32.7 41.4 m/s (12-13 stage); 4) strong tropical storm, the maximum average wind speed near the bottom center is 24.5-32.6 m/s (10-11 stage); 5) tropical storm, the maximum average wind speed near the bottom center is 17.2-24.4 m/s (8-9 stage); 6) tropical depression, the maximum average wind speed near the bottom center is 10.8-17.1 m/s (6-7 stage). the typhoon, strong typhoon and super strong typhoon are classified as typhoons in this study because the cyclones stronger than typhoon will demonstrate destructive power to coastal engineering. 2.2. data collection in this study, the data of the cyclones were collected from the fujian water conservancy information network in china. the cyclone data include all the previous tropical cyclone numbers, chinese and english names, hourly path (in coordinates), wind stage, maximum wind speed, central pressure, shift speed and direction of the wind center, radius of 7 and 10 stages wind, etc. the cyclones that weaker than 7 stage were not analyzed in this study because they hardly cause huge damages. the cyclones are chosen from the years of 1949 to 2016, with the period of 68 years. the statistical space area is a square area (17-29 °n, 108-120 °e), including guangdong province and its coastal area. in data selection, 4 data points (i.e. 0:00, 6:00, 12:00, 18:00) are selected at a daily interval. as the wind power is weaker than stage 7, it was assumed the cyclone is disappeared, and the data collection is stopped. the coastal line of guangdong is divided into six sections, to distinguish the landing area of the tropical cyclone in guangdong province, starting from the xuwen county in the west, to the raoping county in the east. figure 1 shows the classification of the sections: a xuwen county; b zhanjiang city; c yangjiang city; d shenzhen city; e shanwei city; f shantou city; g raoping county. 3. results 3.1. inter-annual variation figure 2 presents the number of tropical cyclones landing in guangdong province every year (from 1949 to 2016). results show that the frequency distribution of tropical cyclones which landed in guangdong province every year is not uniform, and the tendency is irregular. during the 68 years, 183 tropical cyclones landed in guangdong province, with an average of 2.7 per year. to be more specific, the largest number of landing time is in the year 1999, which exceeded seven times. the second largest amount of the landing time is the years of 1961, 1993, and 2008, which exceeded six times. the smallest number of the landing time is the years of 1950, 1956, 1959, and 2000, which is 0. that means no tropical cyclone was observed in the study area in these years. for most of the years, the landing time of the tropical cyclones is 2 4, which also accords with the landing time in recent eight years. 3.2. tropical cyclone intensity in the coastal area in this study, the strength of the tropical cyclone that lands in guangdong coast is divided into four types: 1) tropical depression, 2) tropical storm, 3) strong tropical storm, and 4) typhoon. figure 3 shows the intensity distribution of tropical cyclones. results show that the landing times increases as increasing cyclone intensity. to be more specific, typhoon occurs for the most of the landing times, which exceeded to 114 times (accounting for more than 60% of the total), and the averaged landing time in each year is 1.7. for the strong tropical cyclone, it is the second most of the landing times, which exceeded to 40 times (accounting for about 20% in total), and the averaged landing time in each year is 0.6. however, the tropical depression that landing guangdong coast occurred for only seven times during the 68 years (only accounting for 4% of the total), which means it lands guangdong coast once every ten years. according to the statistical results, the strongest typhoon during these 68 years is no. 16 "sally" in 1946. the highest stage of typhoon "sally" was 17 stage, and the maximum wind speed near center during the moving exceeded to 87 m/s. the second strongest cyclones during these 68 years are no. 27 in 1954 and no. 14 in 1968. the intensities of both of them also exceed to the 17 stage, and their maximum wind speeds near center during the moving exceeded to 77 m/s. 3.3. generation location and time the generation sea areas of the tropical cyclones that land in guangdong province are the south china sea and the northwest pacific. one hundred eighty-three tropical cyclones fell in guangdong province during these 68 years, in which 134 cyclones were generated in the northwest pacific, and only 49 cyclones were generated in the south china sea. the first and the last cyclones that landed in guangdong coastal area each year were selected and analyzed in this study. for the first cyclones every year during the 68 years, the earliest time is april 19, which occurred in no. 1 cyclone in 2008, while the latest time is october 6, which is the no. 13 321temporal-spatial characteristics and path analysis of maritime cyclones in guangdong coastal areas in the south china sea figure 1. study area and section classification. figure 2. times of tropical cyclones that landing guangdong each yea figure 3. intensity distributions of tropical cyclones 322 cheng chen, lei wang, ruozhou chen, fangliang xing, jun chen cyclone in 1975. in general, the first cyclones that landed in guangdong coastal area every year occurred in june or july. for the last cyclones every year during the 68 years, the earliest time is june 6, which occurred in no. 5 cyclone in 1955, while the latest time is december 2, which is the no. 27 cyclone in 1974. in general, the last cyclones that landed in guangdong coastal area every year occurred in september, october or november. 3.4. moving directions when the tropical cyclone is moving into the guangdong coastal area, it presents a general moving direction. table 1 listed the moving direction distributions of the tropical cyclones. from the table, it can be seen that in the past 68 years there were mainly three moving directions when the cyclones entered the study area, north, northwest and west, and the northwest direction was the most common direction, which took up half of the total. it is relatively rare for the moving directions of southwest and south because their total number was only four. the four of them are as follows: (1) no. 18 tropical cyclone in 1952, moving from fujian province to meizhou city via guangzhou city, guangdong province. after that, it was heading south and finally entered the south china sea. (2) no. 14 tropical cyclone in 1966, moving from fujian province to shantou city. after that, it was heading south and finally entered the south china sea. (3) no. 3 tropical cyclone in 1999, landing between shantou city and shanwei city, heading south along the coast of guangdong province, turning to the southwest near yangjiang city, and then entering to guangxi province. (4) no. 7 tropical cyclone in 2009, landing near yangjiang city, heading southwest to zhanjiang city, and then moving towards southwest to hainan province. the four cyclones above were unique. also, there was only one cyclone that ran towards the south for 68 years, which only account for 0.5 %. it should be noted that there were no cyclones that moved towards east or southeast during the 68 years. table 1. moving directions of tropical cyclones in guangdong coastal areas moving direction frequency of occurrence percentage (%) north 38 20.8 northeast 8 4.4 south 1 0.5 southwest 3 1.6 west 49 26.8 northwest 84 45.9 3.5. spatial characteristics of landing sections the coastal line of guangdong province that was attacked by the tropical cyclones is divided into six sections (shown as figure 1). table 2 lists the spatial characteristics of landing locations. results show that the tropical cyclones most frequently attacked the section b-c (from zhanjiang city to yangjiang city), followed by c-d (from yangjiang city to shenzhen city), and these two sections are subjected to 43% cyclones in total. therefore, the coastal line from zhanjiang city to shenzhen city, which is the most developed area in guangdong province, is most vulnerable to the tropical cyclones. to be more specific, during 68 years 30 tropical cyclones were landing from xuwen county to zhanjiang city, with an annual average of 0.44. forty-three tropical cyclones fell from zhanjiang city to yangjiang city, with a yearly average of 0.63. forty-one tropical cyclones landed from yangjiang city to shenzhen city, with an annual average of 0.60. thirty-four tropical cyclones fell from shenzhen city to shanwei city, with a yearly average of 0.50. there were 27 tropical cyclones landing from shanwei city to shantou city, with an annual average of 0.40. and there were eight tropical cyclones landing from shantou city to raoping county, with a yearly average of 0.12. for the first landing month in each year, the most common month is may; the earliest month is april, and the latest month is june. for the last landing month in each year, the most common months are october and november; the earliest month is september, and the latest month is december. the period of the first landing month to the last landing month decreases from the center of the guangdong coastal line to the east and west sides (decreasing from 9 months to 5 months). for the moving direction, most of them are west and northwest. notably, the moving direction is north for the sections c-d and f-g. for the intensity of the tropical cyclones, most of them are typhoons. specifically, the strongest intensity of the tropical cyclones occurred between the yangjiang city and shenzhen city (the occurrence frequency of the typhoon is 25 with the annual average of 0.37). the occurrence frequency of typhoon decreases from the coastal center line to east and west sides (dropping from 25 to 5). the generation locations of the typhoon landing in guangdong sea areas are also studied. the tropical cyclones affecting the guangdong coastal area mainly generated from northwest pacific. the cyclones that produced from the south china sea tend to land in the west of the guangdong coastal line, i.e., xuwen county to shenzhen city (section a–d), while the cyclones that generated from northwest pacific tend to land in the east of the guangdong coastal line, i.e., shenzhen city to raoping county (section d–g). it can be concluded that the landing location of the cyclones is likely influenced by its generation location. also, according to the data of the cyclones from 1949 to 2016, the first cyclones every year is likely to attack yangjiang city to shenzheng city (section c–d). however, the last cyclones every year is expected to attack zhanjiang city to yangjiang city (section b – c). table 2. spatial distribution of landing sections for tropical cyclones landing section times first landing month latest landing month direction intensity generation location td ts sts ty south china sea northwest pacific a b 30 june october west / northwest 1 4 5 20 9 21 b c 43 june november west / northwest 3 6 11 23 17 26 c d 41 april december north / northwest 2 5 9 25 10 31 d e 34 may november west / northwest 1 4 5 24 7 27 e f 27 may october west / northwest 0 2 8 17 4 23 f g 8 may september north 0 2 1 5 2 6 note: location a, b, c, d, e, f, g are shown in figure 1; td tropical depression; ts tropical storm; sts strong tropical storm; ty typhoon 323temporal-spatial characteristics and path analysis of maritime cyclones in guangdong coastal areas in the south china sea 3.6. month distribution of cyclones in the coastal area although the occurrence time of tropical cyclones attacking guangdong province is not evenly distributed among each year, it demonstrates certain regularity in the distribution of the months. table 3 lists the monthly distribution of tropical cyclone landings guangdong coastal area. results show that the generation time of the tropical cyclones is from april to december. most cyclones are generated in july, and there are more than 70% of cyclones generated in three months (july, august, and september), which indicates the concentricity of the tropical cyclones in the generation time. to be more specific, during 68 years, there were 49 tropical cyclones striking guangdong coastal area in july, with an annual average of 0.72; there were 40 tropical cyclones attacking guangdong coastal area in august, with a yearly average of 0.59; and there were 42 tropical cyclones attacking guangdong coastal area in august, with an annual average of 0.62. variations of the landing locations are also observed for each generation month. specifically, the most frequent landing location for july is in between shanwei city to shantou city (in section e-f). the most frequent landing locations for june and august are in between zhanjiang city to yangjiang city (in part b-c). and the most frequent landing locations for the rest of the months are in between yangjiang city to shenzhen city (in section c-d). the results above suggest that the landing locations may be influenced by the generation month. also, the main moving directions for each month are north, northwest, and west. for the intensity, the strongest tropical cyclones tend to occur in september (29 typhoons, with an annual average of 0.43), followed by july and august. for the generation location, the tropical cyclones that generated in april and may are likely produced in the south china sea, while the tropical cyclones that generated in other months are probably caused in the northwest pacific. it was found that in the earlier time each year, guangdong coastal areas are vulnerable to the tropical cyclones generated in the south china sea; while in the latter time each year, guangdong coastal areas are vulnerable to the tropical cyclones generated in the northwest pacific. 4. discussion 4.1. analysis procedure of moving characteristics this study collected the data of the tropical cyclones in guangdong coastal areas during 68 years (from 1949 to 2016) and selected all on the tropical cyclone path of them in 6 hours interval (in latitude and longitude). therefore, a total of 2040 path points were analyzed. also, to analyze the tropical cyclones that occurred in summer (the peak period), 511 records, 467 records, and 502 records for july, august and september were also analyzed, respectively. the moving path characteristics on the sea were analyzed according to the statistical results. the analysis procedure is as follows: (1) the most frequent landing location was marked as the constraint condition for a certain month, and the middle point of the section in guangdong coast was taken as the landing location of the specific path. (2) the trend line for a particular month was fitted from the path points of the tropical cyclone that in the study area. (3) the moving direction for the path was used as the check condition of the path characteristics analysis. 4.2. equations for moving path the center coordinates of the tropical cyclones are used in the trend line fitting. the latitudes are used as independent variables, and the longitudes are used as dependent variables. the least squares approximation method is used to propose the functions for the tropical cyclone paths in this study area, so the moving path characteristics are described. the tropical cyclone paths in the peak period months (july, august, and september) are shown in figure 4(a), figure 4(b), and figure 4(c), respectively. the equations are described as follows: the traditional path for july is: 0 3843 87 879 5000 0 115 1202. . ,x x y x− + + = ( )   (1) the traditional path for august is: 0 2561 50 0 110 8 120. , .x y x+ − = ( )   (2) the traditional path for september is: 1 1294 150 0 110 8 120. , .x y x+ − = ( )   (3) in which, is the longitude (in °e), y is the latitude (in °n). based on the frequency analysis of the path of all tropical cyclones, a dense area of path points can be obtained, which falls between the upper boundary and the lower boundary, as shown in figure 4 (d). the upper boundary is: 0 55 85 0 112 120. ,x y x+ − = ( )   (4) the lower boundary is: 0 27 50 0 109 2 120. , .x y x+ − = ( )   (5) in which, x is the longitude (in °e), y is the latitude (in °n). 5. conclusions this study analyzes the tropical cyclones that affect the guangdong coastal areas in the south china sea. the tropical cyclone data from 1949 table 3. temporal distribution of landing month for tropical cyclones month times moving path intensity generation location most common landing location main direction td ts sts ty south china sea northwest pacific april 1 c d west 0 0 0 1 1 0 may 6 c d west 0 0 1 5 5 1 june 25 b c northwest 2 6 4 13 10 15 july 49 e f northwest 4 4 14 27 8 41 august 40 b c northwest 0 6 12 22 14 26 september 42 c d west 1 6 6 29 9 33 october 15 c d north 0 1 1 13 2 13 november 4 c d northwest 0 0 1 3 0 4 december 1 c d north 0 0 0 1 0 1 note: location a, b, c, d, e, f, g are shown in figure 1; td tropical depression; ts tropical storm; sts strong tropical storm; ty typhoon 324 cheng chen, lei wang, ruozhou chen, fangliang xing, jun chen to 2016 were collected. key characteristics of the tropical cyclones on the sea were investigated, such as the inter-annual variation, the intensity of tropical cyclones, the generating source and time, and the path directions. the results are summarized as follows: 1) there were 183 tropical cyclones landed in guangdong province in the past 68 years, with an average of 2.7 each year, which more than 60 percent was a typhoon. 2) most of the tropical cyclones were generated in the northwest pacific, spanning from april to december. the path directions were mainly north, northwest, and west. 3) the strengths of the tropical cyclones were reduced from central guangdong coast to both sides, and the section of zhanjiang to shenzhen was the most vulnerable to tropical cyclones. 4) tropical cyclones that generated in the south china sea and the pacific northwest tend to land in the west and east of the guangdong coastline, respectively. 5) cyclones frequently occurred from july to september and became strongest in september. 6) there are a most common landing section and path direction for each month. finally, the temporal-spatial characteristics were investigated, and the functions for describing moving path characteristics are proposed based on the data. acknowledgments the authors gratefully acknowledge the support of the national natural science foundation of china (grant no. 51809047) and the youth teacher education scientific research project in the education department of fujian province, china (grant no. jt180049). references emanuel, k. 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(2008). numerical simulation of typhoondriven-waves from the east china sea to yangtze estuary. chinese journal of hydrodynamics, 23(6), 604-611. yuan, f. c., chen, m. n., & chen, d. w. (2010). changes of tropical cyclones in china and fujian in recent 60 years. marine forecasts, 27(5), 3439. yuan, j. j., ding, z. y., & wang, l. (2011). a statistical study and composite analysis on the characteristics of the extratropical transition of landfall typhoons during 1949-2007. journal of tropical meteorology, 27(4), 529-541. zhang, w. l., & cui, x. p. (2013). review of the studies on tropical cyclone genesis. journal of tropical meteorology, 29(2), 337-346. zhao, h. j., song, z. y., xu, f. m., & cheng, c. (2010). numerical simulation of typhoon waves in the south china sea – a case study of typhoon chanchu. the ocean engineering, 28(3), 128-134. zheng, c. w., zhou, l., song, s., & pan, j. (2014). simulation of the wave field caused by no. 1307 typhoon “soulik.” journal of xiamen university (natural science), 53(2), 257-262. palabras clave: imágenes sísmicas; cálculo del tiempo de viaje; método de marcha rápida; construcción de frente de onda; procesamiento sísmico. how to cite item sun, h., meng, f., zhang, z., gao, c., & liu, m. (2018). high-precision joint 2d traveltime calculation for seismic processing. earth sciences research journal, 22(4), 327-334. doi: https://doi.org/10.15446/esrj.v22n4.77362 fast marching method (fmm) boasts high calculation efficiency and strong adaptability and stability while being applied to seismic traveltime. however, when it is applied to the largescale model, the calculation precision of fmm is insufficient. fmm has poor calculation precision near the source, which is an essential reason for the low accuracy of the whole algorithm. this paper puts forward a common traveltime calculation method to address the problem. wavefront construction (wfc) with a relatively high calculation accuracy rather than fmm is adopted for calculation of the grid nodes’ traveltime near the source. after that, fmm is used to calculate the seismic traveltime in the remaining area. joint traveltime calculation method greatly improves the calculation accuracy of the source’s surrounding area and the calculation accuracy of fmm. according to the new method, fmm is still adopted for the calculation of most grid nodes in the model, so the high calculation efficiency of fmm is maintained. multiple numerical models are utilized to verify the above conclusions in the paper. abstract keywords: seismic imaging; traveltime calculation; fast marching method; wavefront construction; seismic processing. high-precision joint 2d traveltime calculation for seismic processing cálculo de alta precisión del tiempo de viaje integrado bidimensional en procesamiento sísmico issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.77362 se presupone que el método de la marcha rápida (fmm, del inglés fast march method) tiene una alta capacidad de calcular y una fuerte adaptabilidad y estabilidad cuando se aplica en el tiempo de viaje sísmico. sin embargo, cuando se aplica a un modelo de gran escala su precisión de cálculo es insuficiente. el fmm tiene una baja precisión de cálculo cerca de la fuente, lo que explica su poca exactitud en todo el algoritmo. en este artículo se presenta un método de cálculo común de tiempo de viaje para acercarse al problema. la construcción de frente de onda (wfc, del inglés wavefront construction), con una mejor precisión que el fmm, se utilizó para el cálculo de la cuadrícula de nodos del tiempo de viaje cerca de la fuente. después de esto, el fmm se utilizó para calcular el tiempo de viaje sísmico en el área restante. el método conjunto aumentó considerablemente la exactitud en el cálculo del área alrededor de la fuente y la exactitud en el cálculo del mecanismo fmm. de acuerdo con este nuevo método, el fmm se utiliza en el cálculo de la cuadrícula de nodos, con lo que se mantiene su alta eficiencia de cálculo. varios modelos numéricos se utilizaron para verificar las conclusiones de este trabajo. resumen record manuscript received: 24/07/2018 accepted for publication: 29/11/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 327-334 hui sun1,2,3, fanchang meng4*, cheng gao5, zhihou zhang1,3, mingchen liu6 1moe key laboratory of high-speed railway engineering, southwest jiaotong university, chengdu, 610031, china 2shandong provincial key laboratory of depositional mineralization and sedimentary minerals, college of geological science and engineering, shandong university of science and technology, qingdao, 266590, china 3faculty of geosciences and environmental engineering, southwest jiaotong university, chengdu, 611756, china 4institute of geology and geophysics, the chinese academy of sciences, beijing, 100029, china 5school of mines, inner mongolia university of technology, huhhot, 010051, china 6college for geoexploration science technology, jilin university, changchun, 130026, china *corresponding author: mfc@mail.iggcas.ac.cn se is m o l o g y 328 hui sun, fanchang meng, cheng gao, zhihou zhang, mingchen liu introduction as the difficulty of finding oil and gas resources in shallow seismic exploration increases, in-depth seismic exploration is gaining more and more attention (liang et al., 2016). however, deep seismic exploration imposes higher requirements on seismic exploration methods because of the larger area of calculation and the complexity of the structure. seismic traveltime has a broad application in seismic exploration methods, including migration, demigration, tomography, and forward modeling (wang et al., 2018; sun et al., 2017; wan, wei & wu, 2017; yang & zhu, 2017; wang, li & chen, 2017; yang et al., 2018; huang & sun, 2018; wang et al., 2016; sun et al, 2018). it is of great significance to study seismic traveltime calculation methods for in-depth seismic exploration. currently, traveltime calculation methods can mainly be divided into three kinds, including ray-based methods, finite difference methods, and new ray tracing methods based on graph processing techniques. regarding ray-based methods, they can be divided into two kinds. the first kind is traditional ray tracing methods (mainly including the trial ray method and the shooting method); the second kind is wavefront construction (wfc). the trial ray method and the shooting method (julian, 1977) were widely applied to seismic exploration techniques in the past, but these two methods face with prominent problems, such as low calculation efficiency and low adaptability to complex models. wfc was first put forward by vinje (1993) and improved later by various scholars (vinje, 1997; coman, r., & gajewski, 2005). chambers and kendall (2008) presented a practical wfc implementation method in 3d isotropic media. hauser et al. (2008) obtained the multiarrival traveltime by the wfc method. han et al. (2009) proposed a new positioning method for grid points in wfc. bai et al. (2011) studied the seismic wavefront evolution of multiply reflected, transmitted, and converted phases in 2d/3d triangular cell model. the method does not break new ground regarding principles. it is still a new technique to realize traditional ray tracing methods in essence, but the technique succeeds in playing an important role in popularizing the application of ray theories to seismic exploration. wfc boasts a good calculation precision, but its calculation efficiency is low because lots of rays are required to insert into the calculation process to ensure the ray density of the model space. combining graph processing techniques and traveltime calculation, modern ray tracing methods mainly include: interpolation method (asakawa & kawanaka, 1993; cardarelli & cerreto, 2002; nie & yang, 2003); shortest ray path tracing method (moser, 1991; bai, greenhalgh & zhou, 2007; sun z., sun j., & han, 2009). these methods can help obtain both traveltime and ray paths, but their calculation process calls for the screening of authentic ray paths from numerous useless ones. thus, methods of the kind are low in calculation efficiency. a finite difference method for traveltime calculation was first put forward by vidale (1988). methods of the kind obtain the viscosity solution of the eikonal equation through numerical analysis. their calculation efficiency is high, but their defect is striking, in that, their calculation accuracy is low especially in the area near the source. fast marching method (fmm) is one of the finite difference methods, which was raised by sethian (1996). the technique was originated from the level set method (sussman, smereka & osher, 1994). it integrates the narrow-band technology and the heapsort technique into its calculation process to achieve high efficiency, stability, and adaptability. due to salient advantages, fmm was favored by many scholars (sun & fomel, 1998; alkhalifah & fomel, 2001; rawlinson & sambridge, 2004) not long after its emergence, but it still failed to solve inadequate calculation accuracy of traveltime near the source. for the strong adaptability of fmm, it was often used for calculating models under rugged topography (sun, z., sun, j. & han, 2010; sun, z., sun, j. & han, 2011; sun, j. & han, 2012). liu et al. (2014) proposed a tti traveltime computation method based on a dynamic programming approach. ma and zhang (2014) obtained the ray information from the first arrival traveltime by solving the topography dependent eikonal equation. sun et al. (2016) analyzed the effects of seawater velocity variation on deep-water seismic migration and found that the traveltime difference was a significant factor. huang et al. (2016) solved the complex eikonal equation to obtain the traveltime by a nonuniform grid-based finite-difference method. sun et al. (2016) traced the ray in the random media with complex topography by employing the gmm-ulti method. sun et al. (2017) calculated the seismic traveltime under complex conditions using the group marching upwind hybrid method. huang and greenhalgh (2018) further proposed an approximate solution for the seismic complex traveltime. lan and chen (2018) presented an upwind fast sweeping scheme for seismic traveltime calculation under the irregular free surface condition. based on the advantages and disadvantages of fmm and wfc, this paper puts forward a joint traveltime calculation method for deep-sea geology. according to this method, wfc is adopted for traveltime calculation in the surrounding of the source; and fmm is for the traveltime calculation of the remaining area. by improving the traveltime calculation accuracy near the source, the joint traveltime calculation method achieves the goal of enhancing the overall calculation accuracy while maintaining the high calculation efficiency of fmm. the above conclusions are verified through the error analysis of the homogeneous model and the trial of the gradient model, the layered model, the marmousi model, and sigsbee model. methods fmm is an efficient and stable method to calculate seismic traveltime. its calculation of grid nodes’ traveltime relies on the information of the surrounding nodes already derived. since there is little node information near the source, the calculation accuracy of the area is relatively weak. inadequate calculation accuracy in the area also influences the overall calculation accuracy. to improve the efficiency of fmm near the source is of vital importance to the improvement of the method. wfc is also an effective method for traveltime calculation. however, with the increase of the calculation area, the blind calculation zone will appear, and the calculation efficiency is not high, but its calculation accuracy near the source is relatively high. based on the characteristics of the two traveltime calculation methods, this paper puts forward a joint traveltime calculation method based on the two calculation methods. as is shown in figure 1, the new method employs wfc for traveltime calculation in the surrounding of the source, and fmm for traveltime calculation of the remaining area. before introducing the joint traveltime calculation method, this part first gives a brief introduction of basic principles of fmm and wfc; then puts forward strategies to realize the combination of the two ways, and finally provides the schematic flow diagram. principles of fmm by solving the eikonal equation through the upwind method, fmm obtains the seismic traveltime. before the introduction of the method, the 2d eikonal function is first introduced (bai, greenhalgh, zhou, 2007). figure 1. fmm and wfc calculation regional diagram in the joint calculation method. wfc calculates the gray region, fmm calculates the white part. 329high-precision joint 2d traveltime calculation for seismic processing ∇ =t s (1) where, t is the seismic traveltime; s is the slowness;  is the gradient symbol. in (1), the upwind difference of the gradient item is approximate to (sethian, j. a., & popovici, 1999): ∇ = + + + − + − t d t d t d t d i j x i j x i j z i max( , ) min( , ) max( , ) min( , , , , 0 0 0 2 2 2 jj zt+          , ) / 0 2 1 2 (2) where, d ti j x , − , d ti j x , + , d ti j z , − and d ti j z , + are the forward and backward difference operator in the direction of x and z. below are the specific forms: d t t t x d t t t x d t t t z d i j x i i i j x i i i j z i i i j , , , , , , − − − − = − ∆ = − ∆ = − ∆ 1 1 1 + + +xx i it t t z = − ∆ +1 (3) (2) could be reduced to a simpler form: ∇ = −( ) + −( )   − + − +t d t d t d t d ti j x i j x i j z i j zmax , , , max , ,, , , ,0 0 0 2 2   1 2/ (4) during the realization process, fmm adopts the narrow-band technique as the wavefront expansion method. the first step of the narrowband procedure is to establish the first narrow-band. usually, the grid points near the source were selected as the elements of the first narrowband, which is shown in figure 2. during the narrow-band initialization, traveltime of the source (is, js) is zero. the traveltime of the points near the source can be expressed as t i j h v t i j h v t i j h v s s i j s s i j s s s s s s ±( ) = ±( ) = ± ±( ) = ± ± 1 1 1 1 1 1 , / , / , / , , ii js s± ±1 1, (5) where, t stands traveltime;v for velocity; h for grid space. when the traveltime calculation of the grid nodes is complete, they will be inserted to the narrow band to establish the initial narrow band. figure 2. narrow-band initialization. the red star is the source; gray points are the initial elements of the narrow-band. after the narrow-band initialization is finished, the wavefront will be expanded as is shown in figure 3. in this step, the minimum traveltime grid node within the narrow band need to be found, and the heapsort technique is employed to improve the calculation efficiency. figure 3. narrow band extension schematic diagram of fmm. a is the minimum traveltime grid node within the narrow-band. principles of wfc the theoretical basis of wfc for traveltime calculation is to solve the kinetic radial tracing (krt) equations. the krt equations are shown below: dx d v p dp d v v x i i i i i   = = − ∂ ∂       = − 2 1 1 2 , (6) where, xi stands coordinates in space; v for velocity; pi for slowness component product;  for the value of traveltime. figure 4. conditions for inserting a new ray. by solving the above equations, traveltime of discrete nodes along the ray paths can be obtained. for the reason that wfc is only employed near the source, just the distance factor is considered to insert a new ray to ensure the ray coverage. as is shown in figure 4, new rays are added only when the distance is higher than the critical value. in the joint traveltime calculation method, traveltime of regular grid nodes is needed. below is a brief introduction of calculating traveltime of regular grid nodes based on the information along the ray paths. figure 5. wfc traveltime calculation schematic diagram. 330 hui sun, fanchang meng, cheng gao, zhihou zhang, mingchen liu the adjacent rays and the wavefronts form irregular quadrangles. (see fig. 5) concerning any irregular quadrangle, abcd, regular grid nodes within the quadrangle are first chosen out, the process of which is discussed in detail by han et al. (2009). then, based on the information of four vertexes, namely a, b, c and d, the traveltime of regular grid nodes within them can be interpolated (see fig. 6). figure 6. sketch map of the vector product method. once the position of the target point is determined, its traveltime could be calculated by the information of the quadrilateral vertices. as is the shown in figure 7, r is the target point; a, b, c and d are the quadrilateral vertices. oa, ob, oc and od are the virtual sources, which are determined the information of quadrilateral vertices. the traveltime of r can be expressed as: t t t t t t v t v t v t r ar br cr dr ar r br r cr r = + + + = = = 0 25. *( ) / , / / , ro ro ro a b c ddr rv= rod / (7) where, vr stands the velocity at point r. figure 7. sketch map of the vector product method. principles and realization strategies of joint traveltime calculation based on fmm and wfc the joint traveltime calculation method is proposed to combine the advantages of fmm and wfc. the new method calculates the grid nodes’ traveltime with wfc in a small area near the source. the maximum single ray tracing distance is chosen between 5% and 10% of the model side length. three aspects were taken into consideration when the scope was selected. firstly, wfc takes relatively more time, so the calculation area will influence the computation efficiency of the new algorithm. secondly, when the wfc calculation area becomes larger, it’s easy to appear a blind area in the model and may need to insert new rays to satisfy the precision of the algorithm, which will further reduce the efficiency of the new method. finally, it can be seen from the accuracy of the analysis below is that the calculation accuracy of the technique has been greatly improved when wfc is used within that scope. though the combination of the two traveltime calculation methods can make the best of -both in different model areas, fmm and wfc are independent of each other and have little association, so the key to realizing the joint traveltime method lies in how to combine the two ways into a whole. this process could be realized through the narrow-band technique of fmm. narrow-band technique plays an essential role in fmm. as the wavefront expansion mode of fmm, it endows fmm with robust stability and flexibility. before the use of the narrow-band technique, grid nodes in the whole space are divided into three kinds, namely accepted nodes, narrowband nodes, and far-away nodes. accepted nodes refer to grid nodes which have been calculated and confirmed. narrow-band nodes refer to grid nodes which have been calculated but have not yet been finally confirmed. faraway nodes refer to grid nodes which have not been calculated. narrowband refers to the collection of all narrow-band nodes. in fmm, it is used to stay close to the wavefront. the above process of forming the narrow band is called narrow-band initialization. in the joint traveltime calculation method based on fmm and wfc, the grid nodes’ traveltime calculation sequence is: first, wfc is used to finish the traveltime calculation of the area near the source; then, fmm is used to calculate traveltime of the remaining area. therefore, the narrow-band initialization in the joint method is a step after wfc finishing the traveltime calculation. as is shown in figure 8, the imaginary line stands for the length of the single ray; the nodes within the enclosed full line graph are grid nodes already calculated by wfc. as to the narrow-band initialization in the joint method, the grid nodes outside the enclosed full line graph are defined as far-away nodes. (see white nodes in fig. 8) nodes within the enclosed full line graph are a collection of accepted nodes and narrow-band nodes. therefore, they should be further divided. as to grid nodes already calculated by wfc, if a grid node’s surrounding (back, front, left and right) nodes are all calculated by wfc, it is defined as the accepted node (see black nodes in fig. 8); otherwise, it is the narrow-band node (see gray nodes in fig. 8). move all narrow-band nodes into the narrow band to finish the step of narrow-band initialization in the joint traveltime method. the completion of the above step means that wfc and fmm finish the connection. below is an introduction of how to use fmm to achieve traveltime calculation of the remaining grid nodes. the process is defined as the narrow-band expansion, which can generally be divided into the following three steps: (1) choose the minimum traveltime node in the narrow band which have been initialized through wfc, and change the attribute of the node from the narrow-band node to the accepted node; (2) judge the characteristic of the surrounding nodes of the newly-confirmed accepted node. if the node is a far-away node, the finite difference method is employed to work out its traveltime and change its attribute to the narrowband node. if the node is a narrow-band node, its traveltime value should be updated, but its characteristic should be maintained; if the node is an accepted node, the attribute and the traveltime value of the node should both be maintained; (3) repeat step 1 and step 2 until the narrow band is empty. through the above analysis, it can be seen that the key to the joint traveltime calculation method lies in the flexible application of the narrowband technique. the technique is a bridge to connect fmm and wfc. below is a realization flow diagram of the joint traveltime calculation method put forward in this paper (see fig. 9) 331high-precision joint 2d traveltime calculation for seismic processing figure 8. graphical description of narrow band’s initialization in the new method. experiments and analysis this part uses the homogeneous model to analyze the accuracy of the joint traveltime calculation method based on fmm and wfc, and multiple complex numerical models to verify the stability and adaptability of the new approach. the size of the homogeneous model is 601×601; the grid spacing is 10m×10m; the velocity is 1,000m/s. in the joint traveltime calculation method, the maximum length of the single ray is 500m. figures 10a, 10b and 10c show the relative error of the first-order fmm, the second-order fmm, and the fmm and wfc joint traveltime calculation method on the model. (the fmm used in the joint traveltime calculation method is the first order.) it can be seen that the calculation accuracy of the fmm and wfc joint traveltime calculation method has greatly improved compared with the first-order fmm, and is even higher than that of the second-order fmm. (a) (b) (c) figure 10. homogeneous medium relative error schematic diagram: first-order fmm relative error (a); second-order fmm relative error (b); fmm and wfc joint traveltime calculation method relative error (c).figure 9. flow diagram of joint traveltime calculation program.. 332 hui sun, fanchang meng, cheng gao, zhihou zhang, mingchen liu table 1 shows the calculation accuracy of the joint traveltime calculation method put forward in this paper in different areas. the traveltime calculated by the joint method can be divided into two parts: one part is traveltime obtained through wfc, and the collection of the regular grid nodes in the area is called a; the other part is traveltime obtained through fmm, and the group of nodes of this part is called b. table 1 shows that the average relative error of the three methods in part a, part b, and all grid nodes. regarding velocity models of different sizes, the ratio of the maximum ray tracing length to the model’s unilateral length is 1:12. in this way, the number of grid nodes in part a steadily accounts for around 1.1% of the total. from table 1, it can be seen that the accuracy of the joint traveltime calculation method based on the first-order fmm and wfc registers an obvious improvement compared with the first-order fmm. the joint way improves the calculation accuracy not only in the area of part a through wfc, but also in the area of part b. though part b is calculated through the first-order fmm, it can be seen that, due to the improvement of the traveltime calculation accuracy in part a, the calculation accuracy of the first-order fmm in part b is higher than that of the second-order fmm. in the former part, the calculation of the joint traveltime calculation method is analyzed. the following section is an analysis of the new method’s calculation efficiency. table 2 shows the time consumed by the calculation of homogenous models of different sizes through the first-order fmm, the second-order fmm, and the joint traveltime calculation method. it can be found the calculation efficiency of the new approach is a little lower than that of the first-order fmm but is far higher than that of the second-order fmm. generally speaking, the fmm and wfc joint traveltime method has good accuracy and efficiency. complex models are adapted to verify the stability and adaptability of the fmm and wfc joint traveltime calculation method. figure 11a is the isochronal graph of the gradient model, whose model size is 401 x 401; the grid spacing is 10m x 10m ; the velocity distribution is v = 1000m + 0.4z. figure 11b is a layered medium, whose model size is 801 x 601; grid spacing is 10m x 10m; the up-down layer velocity is 1500m/s, 2000m/s and 2500m/s, respectively. figure 11c is the isochronal graph of marmousi model, whose model size is 2085 x 603; grid spacing is 5m x 5m. figure 11d is the isochronal graph of sigsbee model, whose model size is 3210 x 1201; grid spacing is 25m x 25m. based on the calculation results of figure 11, it can be found that the new method can obtain the traveltime results met the laws of seismic wave propagation and boast good stability and adaptability to complex models. (a) (b) table 1. the average relative error comparison between fast marching methods and the method combining the fast marching method with the wavefront construction method. model size average relative error(%) region a region b entire model 1st order fmm 2nd order fmm new method 1st order fmm 2nd order fmm new method 1st order fmm 2nd order fmm new method 401×401 2.411 0.659 0.002 0.471 0.078 0.054 0.493 0.085 0.040 501×501 2.110 0.554 0.002 0.396 0.062 0.043 0.415 0.068 0.038 601×601 1.899 0.481 0.002 0.344 0.053 0.036 0.360 0.057 0.036 701×701 1.717 0.421 0.001 0.304 0.045 0.031 0.320 0.049 0.033 801×801 1.576 0.376 0.001 0.273 0.040 0.027 0.288 0.043 0.031 table 2. the computational cost(s) of different methods model size computational cost(s) 1st order fmm 2nd order fmm new method 1001×1001 0.718 2.187 0.824 2001×2001 2.649 9.962 3.033 3001×3001 6.168 21.006 7.057 4001×4001 9.812 36.763 11.217 333high-precision joint 2d traveltime calculation for seismic processing (c) (d) figure 11. traveltime results of different models: isochronal graph of the gradient model (a); isochronal graph of the layered model (b); isochronal graph of the marmousi model (c); isochronal graph of the sigsbee model (d). conclusions fmm boasts high stability and efficiency while calculating the seismic traveltime, but the calculation accuracy near the source is low, which might influence the accuracy of the whole calculation method. in response to the problem, this paper puts forward a joint traveltime calculation method based on fmm and wfc. the core idea of the new way is to calculate the small area near the source with wfc and calculate the major remaining area with fmm. the calculation accuracy and calculation efficiency of the joint calculation method are analyzed, and the complex geological models are adapted to verify that the joint traveltime calculation method put forward in this paper is a seismic traveltime calculation method accommodating to both calculation efficiency and accuracy and boasting strong stability and adaptability. the new approach can serve as a powerful traveltime calculation tool for seismic exploration techniques, such as migration, demigration, and tomography. acknowledgments this work was supported in part by the natural science foundation of china under grant 41804100, in part by the fundamental research funds for the central universities under grant 2682018cx36, in part by the china postdoctoral science foundation under grant 2018m640910, in part by the science research project of inner mongolia university of technology under grant zd201622, in part by the science research project of institutions of higher learning in inner mongolia under grant njzz18079, and in part by the china geological survey project under grant 121201011000150013-05. the authors would like to thank prof. sun jianguo, prof. sun zhangqing, prof. han fuxing and prof. gong xiangbo in jilin university for their kindness help. references alkhalifah, t., & fomel, s. 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(2018). time-domain least-squares migration using the gaussian beam summation method. geophysical journal international, 214(1), 548-572. doi: https://doi.org/10.1093/ gji/ggy142 keywords: groundwater; sensitivity analysis; pilotpoints technique; pest; inverse parameterization. palabras clave: agua subterránea, análisis de sensibilidad técnica de puntos piloto, parametrización inversa how to cite item arenas, m. c., pescador, j. p., donado, l. d., saavedra, e. y., & arboleda obando, p. f. (2020). hydrogeological modeling in tropical regions via feflow. earth sciences research journal, 24(3), 285-295. doi: https://doi. org/10.15446/esrj.v24n3.80116 this paper presents the analysis of a regional groundwater model via feflow in tropical regions using two techniques: pilot points (pp) and constant zones (cz). these methodologies allow the proper identification of biased parameters and heterogeneities of hydraulic properties. for this purpose, we developed a numerical density-variable model that is limited to reinterpreted data from real measurements. for the cz, the initial parameters are considered constant; in contrast, in the pp technique, the initial parameters are assigned according to interpolations using in-situ point measurements. the developed model was applied in an area under the influence of the inter-tropical convergence zone, located in the middle valley of magdalena (mmv). this area is important in the development of colombia due to its contribution to gross domestic product, and it has been subject to significant changes in land use, as a result of intense economic activities. the established model shows a link between the observed state variable (hydraulic head) and hydraulic conductivity (k) proving the importance of spatial heterogeneity in k. the model is calibrated in order to establish k, the porosity and the specific storage capacity, reducing the mean square error of the state variable dependable on the observation points. the results show that the pp system approach provides a better heterogeneity representation and shows that each parameter is sensitive, and does not depend on other parameters. this research compiles the first breakthrough toward a methodology to assertively restrict a highly parameterized inverse regional model in a tropical basin. abstract hydrogeological modeling in tropical regions via feflow modelación hidrogeológica en regiones tropicales a través de feflow issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.80116 la modelación hidrogeológica se realiza comunmente por la solución de problemas inversos y la estimación de técnicas de parámetros no lineales. a pesar de este escenario común, el uso de estas directrices se limita al muestreo adecuado de los datos de campo. este muestreo implica una variedad de datos que generalmente tienen poca disponibilidad, especialmente en regiones donde la variabilidad geográfica y climática no permite una medición constante. en este artículo presentamos el análisis de un modelo de flujo subterráneo regional con base en dos técnicas: puntos piloto (pp) y zonas constantes (cz). estas metodologías permiten identificar correctamente si hay parámetros sesgados y heterogeneidad de las propiedades hidráulicas. para este propósito desarrollamos un modelo numérico de densidad variable que está limitado con datos reinterpretados de mediciones reales. para la técnica cz, los parámetros iniciales se asignan de acuerdo con su capa, y cada capa se considera constante para los valores de los parámetros; en contraste con la técnica de pp, los parámetros iniciales se asignan de acuerdo con las interpolaciones de mediciones de puntos in situ. el modelo desarrollado se aplicó en un área bajo la influencia de la zona de convergencia intertropical (zcit) ubicada en el valle medio de magdalena (mmv). esta área es importante para el desarrollo del país debido a su contribución al pib y ha estado sujeta a cambios significativos en el uso de la tierra, como resultado de intensas actividades económicas como la agricultura, la energía hidroeléctrica y la producción de petróleo y gas. el modelo establecido muestra un vínculo escaso con la variable de estado observada (cabeza hidráulica -k); esto demuestra la importancia de la heterogeneidad espacial en k. el modelo se calibra para establecer k (como una variable anisotrópica que varía espacialmente), la porosidad (η) y la capacidad de almacenamiento específica (ss) en el pp y cz, y que reduce un error de "cuadrado medio" de la variable de estado dependiente de los puntos de observación. los resultados muestran que el enfoque del sistema pp proporciona una mejor representación de heterogeneidad y muestra que cada parámetro es sensible y no depende de otros parámetros, lo que da a los resultados de la evaluación independencia de los hechos y autenticidad. esta investigación compila una metodología para restringir asertivamente un modelo inverso altamente parametrizado con datos de campo para estimar parámetros de acuíferos que varían espacialmente a escala regional. resumen record manuscript received: 05/06/2019 accepted for publication: 17/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 285-295 maria cristina arenas1*, juan pablo pescador1, leonardo david donado1, edwin yesid saavedra1, pedro felipe arboleda obando1 1universidad nacional de colombia sede bogotá *corresponding author: mcarenasb@unal.edu.co h yd ro g eo lo g y https://doi.org/10.15446/esrj.v24n3.80116 https://doi.org/10.15446/esrj.v24n3.80116 https://doi.org/10.15446/esrj.v24n3.80116 mailto:mcarenasb@unal.edu.co 286 maria cristina arenas, juan pablo pescador, leonardo david donado, edwin yesid saavedra, pedro felipe arboleda obando introduction 2d and 3d representations in hydrological modeling allow to exemplify static and dynamic conditions of hydrogeological systems in natural or hypothetical scenarios, and their relationships with surface water bodies and atmospheric inputs (alberti, colombo, & formentin, 2018; gogu, carabin, hallet, peters, & dassargues, 2001). this type of modeling can be used to analyze the effects of groundwater extraction, to evaluate irrigation strategies in order to establish an appropriate correspondence with aquifers, or to simulate different water management scenarios (white, 2018). these models are classified in three types (hughes, 2016; sieber, yates, huber-lee, & purkey, 2005): i.) physical models that follow the laws of physical and chemical processes, and can be determined and described by differential equations, ii.) analogs, which are simulations of electrical order and iii.) mathematics: which are simplified representations of physical processes that are expressed in mathematical terms to simulate complex processes taken from a specific key component of a determined system. however, hydrogeological modeling implies a variety of data that generally has little availability, especially in regions where geographical and climatic changes do not allow constant measurement (comunian & renard, 2009). this is why the calibration of hydrogeological models is important (klaas, imteaz, sudiayem, klaas, & klaas, 2017). the calibration process must be used to acquire reliable modeling results (wu, liu, cai, li, & jiang, 2017). a good calibration of a model ensures that the vestigial data between the observed and the simulated data are minimized and the parameters uncertainties are lower (gan et al., 2018; simmons et al., 2017). this procedure was developed using the inverse method (m. usman et al., 2018; x. wang, jardani, & jourde, 2017). the inverse method application and the non-linear parameter estimation techniques are common in hydrogeological modeling (carrera, hidalgo, slooten, & vázquez-suñé, 2010; x. wang et al., 2017; white, fienen, & doherty, 2016). this given the swiftness that this technique shows to determine the best adjustment parameters, applying low subjectivity in the calibration procedure and also the availability of hydrogeological modeling software to easily integrate the hydrological results (n.-z. sun & sun, 2015; m. usman et al., 2018; zhou, gómez-hernández, & li, 2014). despite this advantage, the method is limited to the satisfactory sampling of field data (llopis-albert, merigó, & palaciosmarqués, 2015; pool, carrera, alcolea, & bocanegra, 2015; h. zhang, li, saifullah, li, & li, 2016). additionally, hydrogeological models have a high spatial-temporal variability rate that causes non-linearity which also increases the correspondence within different model input parameters (gaganis & smith, 2006; marchant & bloomfield, 2018). an unwanted effect in this situation can be the appearance of unrealistic parameter distributions, or the calibration falling into a local minimum without exploring other declines (hu & jiao, 2015; klaas et al., 2017). bearing in mind that the calibration process does not guarantee the total model reliability and that the results obtained are as real as the veracity in the assumptions used in the conceptual model (betancur, mejia, & palacio, 2009; kpegli, alassane, van der zee, boukari, & mama, 2018; linde, renard, mukerji, & caers, 2015), it is appropriate to analyze the sensitivity and uncertainty associated with the parameters. the uncertainty in the hydrogeological models is related to: i.) inaccurate measurement of input data, ii.) inadequate simplification of the represented system and iii.) calibration process (carrera, mousavi, usunoff, sánchez-vila, & galarza, 1993; chen, izady, abdalla, & amerjeed, 2018; meeks, moeck, brunner, & hunkeler, 2017). in particular, the calibration process of any hydrogeological model is based on some method of spatial characterization or zone focused parameters (b. dewandel et al., 2012; benoît dewandel, jeanpert, ladouche, join, & maréchal, 2017). the model of this research was divided into different hydrogeological units based mainly on geological properties (custodio et al., 2016; islam et al., 2017; t. wang et al., 2014). this subdivision leads to suppose hydraulic uniformity properties in each zone (cherry et al., 2004). meaning that the hydraulic parameters values at any location are weighted averages of the actual values measured at different points within an area (hassane maina, delay, & ackerer, 2017; irsa & zhang, 2012; m. zhang, burbey, nunes, & borggaard, 2014). the sensitivity analysis allows to address uncertainties identifying the most important model input parameters (karay & hajnal, 2015). additionally, this analysis provides an overall system understanding, reduces uncertainties and improves the calibration and validation processes (carrera et al., 1993; ehtiat, mousavi, & ghaheri, 2015; sanchez-león, leven, haslauer, & cirpka, 2016). the use of those cz for the values assignation to hydraulic parameters can lead to unnecessary uncertainties in the modeling and, therefore, to produce erroneous parameter values and current geological homogeneity (linde, ginsbourger, irving, nobile, & doucet, 2017; tiedeman & green, 2013; woodward, wöhling, & stenger, 2016). having this into consideration, the pp technique has been proposed to help improve the spatial variability interpretation of parameters, in cases of groundwater flow modeling that cannot be obtained by working with cz (christensen & doherty, 2008; jiménez, mariethoz, brauchler, & bayer, 2016; le ravalec-dupin, 2010; m. usman et al., 2018). this technique interpolates the spatially correlated hydraulic properties values of a set of points distributed along the model domain (ma & jafarpour, 2017); generating a uniformed distribution of parameter values with less uncertainty. however, the use of a larger number of pps or the area subdivision could result in extensive modeling times (christensen & doherty, 2008; white et al., 2016). the application of this technique has been widely used throughout many related issues as shown (a. alcolea, carrera, & medina, 2006, 2008; christensen & doherty, 2008; hernandez, neuman, guadagnini, & carrera, 2003; jung, ranjithan, & mahinthakumar, 2011; le ravalec-dupin, 2010; le ravalec & mouche, 2012; ma & jafarpour, 2018; panzeri, guadagnini, & riva, 2012). nonetheless, the use of this technique can generate over-parameterization, situation that would lead to the optimization problem instability (amini, johnson, abbaspour, & mueller, 2009). the instability can imply unlimited or very large ranges, where the parameters variations are established, large variations in the parameters value in small distances, non-existent correlations due the small amount of data, and non-uniform distribution in large areas, and lastly very large second derivatives in the hydraulic properties (monica riva, panzeri, guadagnini, & neuman, 2011; tóth et al., 2016; yeh et al., 2015). the use of variograms to correlate the initial value of hydraulic properties and limit their search ranges, can help to improve their stability (friedel & iwashita, 2013; jardani et al., 2012; kashyap & vakkalagadda, 2009; sheikholeslami, yassin, lindenschmidt, & razavi, 2017). although the biggest issue with application of pp, is associated with the number of parameters to be calibrated, which is actually the definition of pp parameters itself, where each of those parameters is another constraint. determination of pp number within the model can be defined in a fixed way associating a spatial location or a random variation during the optimization process (christensen & doherty, 2008; jiménez et al., 2016; d. sun, zhao, wei, & peng, 2011). the usage of a reduced pp value can improve model stability, although this approach could generate a larger homogeneity of hydraulic properties, approaching it to cz technique and making the problem sensitive to the point locations (ma & jafarpour, 2017). however, andrés alcolea, carrera and medina, (2006) established that when implementing pp and regularization processes, it is recommended to implement as many number of pps as computational viability is allowed. traditionally, surface water has been the main source of supply in colombia. however, in the last decade, groundwater has become more important than surface water due to easy access, the relatively low cost of handling and its influence on the industrial and economic development of the country (ideam, 2014). colombia has possibilities of using underground water in 74% of its extension. however, 56% of this area corresponds to geographic regions with high surface water yields and low percentage of the settled population (vargas martínez, campillo pérez, garcía herrán, & jaramillo rodríguez, 2013). the mmv basin is located in the andean region, the most densely populated in the country, with 106,131 km2 of surface area with resources and groundwater reserves. these reserves are equivalent to 12.5% of the total area covered by hydrogeological basins in the country. this potential has led to the hydrogeological study of the area, through the development of conceptual and mathematical models to understand its functioning (ideam, 2014). this project focuses on the groundwater behavior analysis in the mmv area, through the numerical simulations, and whose objectives are: i) to identify the properties of aquifers present in the area, and ii) to analyze the flow dynamics of underground water. this document shows the theoretical framework in section 2, in section 3 we present the study area. sections 4 and 5 show data sources and the groundwater model. section 6 presents the results and finally, the conclusions are indicated in section 7 theoretical framework feflow software version 7.1 was used for the numerical model development (trefry & muffels, 2007) to compare the underground regional 287hydrogeological modeling in tropical regions via feflow to evaluate the model performance, statistical measures were taken to quantify the differences in observed and simulated groundwater heads. in the present study, were used the coefficient of determination (r2), the mean square error (rmse) and mean absolute error (mae) (carrera et al., 1993; nash & barsi, 1983; sahoo & jha, 2017; muhammad usman, liedl, & kavousi, 2015). study area the results are explored within a regional flow model that presents high hydraulic conductivity and is under the effects of the inter-tropical convergence zone, which characterize meteorological and hydrologic features. a domain of 17,000 km2 was defined in the central area of mmv, colombia (figure 1). the average amount of rainfall overpasses 2000 mm/year, and the main river has an annual average flow of 2361 m3/s. the average annual temperature is higher than 24°c throughout the territory, and the terrain elevation varies between 16 and 3110 m.a.s.l (ideam, 2014). the study area presents a high geological complexity, with variable depths in all the hydrogeological units. its formation took place during the tertiary, at the time of high volcanic activity of the central cordillera (mojica & franco, 1990). result of this geological background, the area is tilted towards the east, having a monoclinal structure, disturbed by some folds and faults (servicio geológico colombiano, 2014). it occupies a structural depression that is considered a half graben tilted towards the east, where it limits with inverse faults such as salina (ingrain, 2012). in general, rocks lean monoclinally from the cordillera central towards the eastern cordillera, but they are interrupted in-situ by faults and bends. at this basin, recent clastic sediments of alluvial type and sedimentary rocks of quaternary and tertiary age developed. the main characteristic of this material is the low consolidation and sediments predominance such as sand and gravel, interspersed with finegrained materials. in the mmv basin, groundwater is extracted from units that function as an aquifer. these units are recent alluvial and terrace deposits that emerge in the magdalena river proximity with an average productive thickness of approximately 150 m (gallego, jaramillo, & patiño, 2015) whose origin is associated with this body of water, and with the mesa and real formations unconsolidated detrital sediments (sandstones, conglomerates). the underlying rock represented at the east limit is used as a vertical and lateral no-slip boundary condition for every aquifer formation, which is not continuous through the whole domain. the following aspects were taken into account for the conceptual model integration: i) the relationship between each formation material, ii) homogeneity of hydraulic properties for regional scales, iii) an electrical resistivity analysis for each formation material, and iv) common permeability values. at a regional scale, the geological reinterpretation of the study area led to a simpler model of the hydrogeological basin. it included the depth of sedimentary formations, the salina-buitima fault, and every main fold. in addition, interpolation algorithms were used including secondary information from seismic and magnetotellurics studies, superficial geological interpretations, and stratigraphic from drilling well. in the end, seven layers resulted from this exercise, each one separated from the other by lithological contacts or erosive surfaces. near-surface formations are considered the most important given the current water resource exploitation from those aquifers. data sources to describe a three-dimensional groundwater flow model geometry, it required a digital elevation model (dem). it was obtained from srtm satellite data derivation with a 30 m resolution between the coordinates 6° 7’ 46’’ n and 74° 39’ 47’’ w and an average vertical altitude error of 6.2 m (90% accuracy level) and a geolocation error of 9 meters for south america (sharma, tiwari, & bhadoria, 2011). the recharge was obtained through a prior semidistributed hydrological model consolidation: topmodel, whose information may be reviewed in (arenas-bautista et al., 2017). the piezometric levels and wells were obtained from the national underground water forms (funias), compiled specifically for the national inventory of groundwater led by the regional autonomous corporations (car). from that point on, geographical location information and type of points were collected. water well pumping tests were obtained from the hydrogeological studies developed by ideam and flow with cz and pp. for this model, we used the configuration of a saturated model whose solution is represented by darcy’s law (equations 1 y 2). q s b s h t q s y = +( ) ∂ ∂ + ∇ ⋅ ′ * (1) ′ = − ⋅∇q kbf hu (2) here, ′q is flow (m/s), q is the volumetric flow (m3/s), ss is the specific storage, b is the thickness in the saturated phreatic aquifer (m), sy is the specific performance, k is the hydraulic conductivity tensor (m/s), fu is the function of viscosity and h is the hydraulic head. a triangular two-dimensional mesh algorithm was used and mesh refinements in the river and well lines were carried out. then, this mesh was projected in-depth to seven layers defined in the conceptual model, leading to six-nodes finite elements. the first layer was configured as a free aquifer. the code used for this mesh was developed by jonathan shewchuk at uc berkeley, ee.uu (shewchuk, 1997) and was used due to its high speed, and its ability to accommodate complex configurations of polygons, lines, and points. the model was run in steady and transient states, adopting an adamsbashforth scheme for the latter. the adams-bashforth outline permits to control automatically the time-step scheme, providing an easier convergence and stability in model executions (trefry & muffels, 2007). in the transient model, the steady state model groundwater hydraulic loads were used as initial conditions. the system of symmetric equations (flow simulation) in stable state was solved with the algebraic multigrid samg method (by k. stüben, fhg-scai). this method is sharper for stable state models and simulations with a variety of element sizes. samg internally analyzes the best method and applies a cg (conjugate gradient method) or samg (algebraic multigrid) type solution. for the transient state, pcg (preconditioned conjugate gradient) method was used. although the direct method assures a better model stability, the demand for memory and execution time for models with more than 100 thousand nodes, is not applicable. the objective function  is defined to iteratively optimize the model parameters (carrera, alcolea, medina, hidalgo, & slooten, 2005; medina, galarza, carrera, jódar, & alcolea, 2001). this optimization is expressed by measuring the objective function (equation 3).  = = −( ) −( )− == ∑∑m h ti ti h ti ti t n i op w h h v h h1 11 (3) here, i is the number of observation points (op), pp or cz, t is the observed time interval, h* is the hydraulic head vector at the observation points, h is the simulated hydraulic head vector, vh the covariance matrix, wh is the observations weights which has a value of one in our problem. for m estimation, pest was applied. nonlinear pest software (modelindepend parameter estimation) developed by john doherty (1994), uses an inverse code. pest executes the simulation software iteratively by adjusting the parameters values to be estimated until the errors between the observed and simulated data reach the finishing criteria (maheswaran et al., 2016; woodward et al., 2016). the parameter search is carried out using the glm algorithm (gauss-levenberg-marquardt). this method linearizes the state variables dependence on the model parameters, while imposing a change on the parameter pk for each iteration (equation 4) (finsterle & kowalsky, 2011; yao & guo, 2014). h i p gk k k k+( ) = − ∆ (4) here, hk represents an approximation of hessian matrix of . gk is the gradient in the model parameter vector in k iteration i, is the identity matrix and k is the marquardt parameter. after each iteration, the model parameter vector is updated. before consolidating this step, it is checked if the vector parameters components by iteration meet the assigned upper limit. this process is iterative until: 1.) if i ≤0.0001, where  is the of value at the end of the optimization iteration ith, 2.) when 30 is the maximum number of optimization iterations, 3.) if    i min i − ≤ 0 1. , where min it is the lowest of reached so far. for the development of this model, was established as convergence control numeral 1. ee.uu 288 maria cristina arenas, juan pablo pescador, leonardo david donado, edwin yesid saavedra, pedro felipe arboleda obando the national university of colombia (arismendy, salazar, vélez, & caballero, 2004; betancur et al., 2009), and the production blocks known as guane (ingenieria geotec, 2016) and vmm-39 (ecopetrol, 2016). the rivers levels for both steady and transient conditions were taken from the ideam, limnimetric stations available in the area. additionally, it became necessary to obtain the bathymetry and bed river level to consolidate the model input data. the geostatistical model was defined by the transmissivity variogram in the model domain. the soil texture details for different depth profiles were available from well logs that were used to establish the initial values of different hydraulic parameters in each zone (freeze & cherry, 1979). the methodology used to generate the geological profiles and the consolidation of hydrogeological units is described in (pescador-arévalo, arenas-bautista, m. c., donado, guadagnini, & riva, 2018). in the model development, eight constant zones were defined which correspond to the geological zones defined in the study area (figure 2a). as opposed to the cz technique, the pp method allows to estimate fields of continuous parameter distribution. the fundamental principle of this constraint is to prefer homogenous distributions of parameters whose values show as small a deviation as possible from those expected. in a geological context, a pp should preferably have a similar value to its neighbors within a certain distance. this distance and the strength of correlation are defined through a semi-variogram. in this research, due to the study area does not have sufficient and reliable data throughout the domain, and the available data are pooled in small areas of the study area, was decided to generate the pps with uniform distribution. the model ran 50 times, and in each run the pps were distributed so that their locations were different (x, y, z), but the space between them was constant. it means that a rectangular pattern was used for every run, but the pp locations changed after every run, always keeping the same pattern (figure 2b). the random location of pps with a uniform distribution allowed to represent without bias the locations that had no field measurements in the calibration process and the post-calibration analysis identified the parameter contributions to the uncertainty. the variogram that defined the spatial behavior of pp’s was an exponential model, with a range of 2000 m and a variance of 2 without nugget effect. the k values obtained vary from 10-5 to 35 m/day. we defined 167 observation points based on the piezometric levels (figure 1), because all piezometric data levels registered in funias were taken on different dates, and it was not possible to find records of continuous time series monitoring for these measurements. an average data level was used as reference in the numerical model in steady state. for transient state, a measurement grouping by month was made. this grouping assumes that levels in the same months of different years, exhibit a similar behavior due to the results of the hydrological figure 1. location of influence area in the model (red). in addition to this the figure shows the locations of piezometric levels, limnimetric stations, and wells figure 2. definition of geological units in the model. 289hydrogeological modeling in tropical regions via feflow processes analysis. finally, a series of monthly data was consolidated for an average year. the extraction data comes from 78 independent pumping tests in the wells where information was found within the model domain (figure 1). pumping rates range between 10-2 m3/day and 250 m3/day. groundwater model conceptual model figure 3a represents the conceptual application model area. the zone is limited to the east and west by the limits of savmm aquifer system that acts as an impermeable barrier due to its igneous-metamorphic nature (ideam, 2014). the south and north limits of model act as an inflow and outflow, respectively, in the different zone sections. the groundwater flow is estimated by darcy’s law (karay & hajnal, 2015; şen, 2015) using piezometric data, and is generated in south-east to north-west direction. the elevation in the focus zone descends smoothly in south-north direction, conserving the properties of an alluvial valley and, therefore, provoking a regional movement of underground water throughout this depression (ingrain, 2012). additionally, there is a register of low scale domestic and industrial use of groundwater collection obtained with pumping equipment (gonzalez, saldarriaga, & jaramillo, 2010). most recharge of groundwater occurs at the mountain ranges, from rainfall infiltration, and at the low-lands, from channel drainage losses. the region weather is mainly semiarid with high temperatures, especially during dry season, which causes a loss of water in the form of evapotranspiration (asociacion colombiana del petroleo, 2008). the hydraulic properties of the aquifer system make possible to identify areas with primary and secondary porosity and characterize them as units able to store and transmit water with relative ease. the horizontal (x axis) permeability is an order of magnitude higher than the vertical (han & cao, 2018; xie, hu, jiang, & liu, 2006). the system total porosity ranges between 25% and 3%, with an average specific yield of around 14%. the test holes pumping results and the lithological analysis allow us to conclude that the hydraulic conductivity varies from 20 m/day to 0.1 m/day numerical model the recharge was estimated with a one-year time scale, taking proportions from average monthly data ranging from 12 years, between 2000 and 2012. the information was consolidated on a spatial scale of 6 km × 6 km resulting from hydrological modeling (arenas-bautista et al., 2017). the recharge is assigned as a material associated property, which becomes variable in time for transient model (figure 4). the loading boundary conditions (figure 3b) were assigned as a first type variable dirichlet condition in south and north edges from the model domain, according to water level historical records. for the seven consolidated rivers in the model domain a cauchy boundary condition was applied. this requires the hydraulic conductance of the riverbed material and its geometry procurement, which were obtained establishing the initial conductivity from the pumping tests and the cross-section geometry, supplied by the limnmetric stations in the area. the east and west model limits were conditioned by the application of neumann boundary conditions; given they are considered as a watershed. the inlet and outlet flow sections and the model domain flow were identified using piezometric water level contours, and applying darcy’s law. finally, pumping wells were assigned as a well edge condition where a defined extraction was applied to a node. for cz application a pattern on materials disposition according to the hydrogeological units was defined. for the pp, it was observed that it was not possible to obtain a defined pattern for the same materials, so the information was spatialized. for this, the pp spatial distribution model of the hydraulic properties used kriging (tsai & yeh, 2011). this technique was used given its additional use for regularization processes, which are based on the same variograms used for regionalization (janetti, riva, straface, & guadagnini, 2010; kashyap & vakkalagadda, 2009; m. riva, guadagnini, neuman, janetti, & malama, 2009; tsai & yeh, 2011). for the development of this model, each layer was treated as a single zone and tested with a variable number of pp to evaluate the response model. the principle of parsimony establishes that the numbers of parameters considered in an estimation process must be kept to a minimum (golmohammadi, khaninezhad, & jafarpour, 2015; merritt, croke, & jakeman, 2005; tonkin, doherty, & moore, 2007). however, when pp are used in conjunction with regularization, this number tends to increase (panzeri et al., 2012). in order to check the results from the calibration process, an uncertainty analysis was performed. in this analysis, some basic metrics are reported such as rmse, pearson correlation and nash efficiency coefficients. results and discussion the two calibration processes (cz and pp) are compared briefly in their value parameters, model fit and predictions. this comparison focuses on the model ability to simulate the real system and to show conceptual differences. the model residuals respect to the observation points allows us to infer that model calibration allows to adequately represent the assumptions presented in the conceptual model, both in the cz and in the pp technique (figure 5a). the figure 3. hydrogeological conceptual model. 3a-represent the conceptual model of influence area. 2b-show the boundary conditions used in the model. 290 maria cristina arenas, juan pablo pescador, leonardo david donado, edwin yesid saavedra, pedro felipe arboleda obando average residual differences in pp results is lower than 1 m. the cz technic exhibits oddment points between 1 and 3 m. the possible reason for these mayor differences would be associated with the large number of wells in this region, which pumps extensively the groundwater. therefore, the possibility of an occasional partial inflow across the study area boundaries, especially on the eastern and western boundaries, could not be well represented by the input/output flow limits conditions. the error obtained in the observation points is summarized in table 1. these results include r2, mae and rmse. the value of r2 for the calibration model is 0.99; likewise, mae, which is an efficiency of calibration indicator, is 0.25. figure 5b allows to evidence optimal simulation results, as the observation values are adequately represented under the numerical model assumptions, and the figure 6 presents the results of transient-state calibration. the hydrographs of simulated versus observed heads over time show a relatively good match between observed and simulated heads, even if the model tends to overestimate the piezometric levels. the general behavior of pp technique offers average values of 142 m in the of. however, the results obtained in cz show a variation of 6 m above the results with respect to values obtained from pp. these results are in agreement with the findings of (andrés alcolea et al., 2006) where it is concluded that the pp technique offers better results for the analysis of heterogeneity. the hydraulic conductivity analysis (figure 7) in the model shows that, in the quaternary, the average kx values in cz is different from the average pp values. this occurs because of the lack of knowledge of hydraulic properties of outcrop geological units in the zone. the most information and monitoring compiled for this model has an 80 m depth (for mesa and real formations); because of this, the model representation fits with higher certainty. in the seven (7) remaining model units, the kx values assignment does not represent a direct link either with cz nor different pp. this may be due to the ignorance of depths of hydraulic properties higher than 600 m. additionally, the scarce available information is focused on specific influence areas, which gives another struggle for its representation. finally, results show that in the deepest model units equifinality issues can arise when the same model performance with a different parameter for the same node is generated. additionally, is evident a strong separation of the quaternary aquifer and mesa formation with values of 10 m/day and 2.5 m/day respectively. it is evident in figure 7, that the pp calibration process presents a k three times higher than the cz model. in general, the k values for all hydrogeological units are within the reported range in literature. comparing the two methods evaluated through the cz and pp, they show similar model characteristics: i.) the east and west regions of the model, corresponding to jurassic and early cretaceous units, have a uniformly low conductivity, which corroborates the conceptual model assumption establishing those zones as water divide, ii.) the model presents small areas of high conductivity in the north and central part and, iii.) the highest conductivity is found in quaternary and mesa formation. an interesting fact is that the estimated field begins to reflect some large known scale geological structures also perceived on the cz model. the storage does not significantly influence the hydraulic head during the one-day simulation period. therefore, it can be concluded that k is the only definitive parameter in hydraulic head modeling and that each parameter is sensitive, and does not depend on other parameters. in summary, compared to the previous cz technique, pp approach is not only able to provide satisfactory matches for all measured heads and reproduce large-scale and local features of the measured groundwater level, but it can also extract more information of heterogeneity at several scales using the same amount of measured information. all the results shown above indicate that the pp approach provides a more realistic model to simulate river-groundwater interactions based on parameters. taking into account the parameters calibrated in both techniques, the ratio between the horizontal and vertical hydraulic figure 4. recharge data used in the transient model. this series was built for an average year, based on result of the hydrological model (arenas-bautista et al., 2017). figure 5. result of the conceptual model from the residual values of the numerical model. 5a-shows the model residual respect to the observation points in the cz and pp techniques 5b-shows the result of graphical fitting of observed and simulated heads at the observation points. 291hydrogeological modeling in tropical regions via feflow conductivity is at the center of its range with a value of around 0.1. therefore, the vertical flow in the aquifer is ten times slower than the horizontal flow that coincides with the regional dips (sub-horizontal layers) conclusions this article has been focused to identify if there are biased parameters in the model solution and determine the heterogeneity of the hydraulic properties by applying a calibration scheme based on cz and pp techniques in tropical basins. subsequently, the optimized model has been used to quantify the groundwater flow through topographic boundaries. the analysis of the pps used for estimation of hydraulic parameters offers a guide to estimate the heterogeneity in hydraulic anisotropic conductivity in unconfined aquifers. the regional groundwater model provided information within regional flow patterns. the flows through all internal limits constitute 3 to 16% of their discharge and are significant in relation to pumping; therefore, it is important to consider these flows in hydrological models, since they could have a great impact on the water currents simulation. additionally, this highlights the need for flow models (surface and underground) coupled on a regional scale due to most surface water models assume zero flow between topographic divisions. the use of real data as initial aquifer parameters reduces the interactions necessary to minimize ϕ, improving potential estimates and estimating the possible convergence. table 1. experiment description and result of the statistical metrics figura 6. results of observed and simulated levels in the model for trasient state. the results are shown for dour wells (the most representatives in the area): felicidad, coyote, sam 1.1 and m-5237 figure 7. result of k in the proposed hydrogeological model 7a. the model developed with pp. 7b. the model developed with cz. 292 maria cristina arenas, juan pablo pescador, leonardo david donado, edwin yesid saavedra, pedro felipe arboleda obando the inverse model results in the calibration process demonstrated a better response in the real representation of the system. the hydraulic head error was reduced by 3% compared to the conceptual model developed. although the minimum square error of the state variable was minimized at the observation points, the model validation showed consistent results at the control points. this idea supports the model solution, showing that the model calibration through the pps is more robust and flexible in contrast to the cz parameterization given its lower subjectivity and lower of the heterogeneity representation of the hydraulic properties. in conclusion, the research results indicate a less significant pattern for the cz method compared to the pp-based model. therefore, to identify the spatial value of the observations there must be a certain degree of flexibility and variability in the parameterization of the model. although the pp method reduces the homogeneity produced by cz, it has some limitations, e.i., to overlook other sources of model error and result in an over-parameterization or equifinality. an important uncertainty of the pp method is the choice of the number and location of the pps, as well as the effect it has on reducing the variability of the parameters in the model during the investment problem. although there is no defined guideline for generating these points, the results of this article indicate that better results can be obtained using a uniform pattern, reducing the spaces between them which is useful to correlate the hydraulic properties as a guide for the separation distance. in the same way, the interpolated fields of k resulting from the application of the pp method work properly for the estimation of the flow at the regional scale; however, it can lead to significant limitations for other applications, such as solute transport 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(2019). paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan. earth sciences research journal, 23(4), 347357. doi: https://doi.org/10.15446/esrj. v23n4.84502 by observing a large number of conjugate shear joint data in the field and underground of the study area, the conjugate shear joint data is analyzed by the stereographic projection method, and the paleo-tectonic stress field experienced in the mining area since the mesozoic is inferred. based on the above research, the tectonic evolution process of the eastern mining area of pingdingshan is discussed from the aspect of dynamic mechanism. the results show that: (1) the eastern mining area of pingdingshan experienced the indosinian, yanshan early-middle, yanshanian, and xishanian tectonic stress fields, with the directions of ns, nw, ne, and nne, respectively. (2) the tectonic evolution of the mining area can be divided into four stages: (a) the indosinian tectonic stress field has the weakest effect on the study area, the coal-bearing strata are gradually uplifted by the stress field in this period, and some small structures are formed in the coal-bearing strata; (b) the early-middle yanshanian tectonic stress field has a certain transformation effect on the coal-bearing strata in the eastern part of pingdingshan. it mainly forms the secondary structure of the mining area such as huoyan normal fault and small fault-fracture structure; (c) the late yanshanian tectonic stress field has the strongest effect, and the control complex fold structure with the axis near nw direction is formed under the tectonic stress field of this period, such as likou syncline, baishishan anticline, lingwushan syncline, guozhuang anticline, niuzhuang syncline, etc. the near-nw direction of the control fault structure is also formed under the action of the late yanshanian tectonic stress field, such as the niuzhuang reverse fault, the original no.11 mine reverse fault, etc.; (d) the himalayan tectonic stress field mainly forms some secondary and small structures in the mining area and has a certain transformation effect on the structure formed by the early tectonic stress field. the tectonic pattern of the eastern mining area of pingdingshan is the result of the above-mentioned tectonic stress field acting in sequence with the coal-bearing strata, causing structural deformation of the coal seam. abstract paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan campo de estrés paleotectónico y evolución tectónica desde el mesozoico en el área minera oriental de pingdingshan issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84502 tras recopilar una gran cantidad de datos de cizallamiento en campo y bajo tierra del área de estudio, se analizó esta información con el método de proyección estereográfica, y el campo de esfuerzo paleo-tectónico experimentado en el área minera desde que se infiere el mesozoico. luego de este análisis de información se discutió el proceso de evolución tectónica de la zona minera oriental de pingdingshan con enfoque en el mecanismo dinámico. los resultados muestran que: (1) el área minera oriental de pingdingshan experimentó los campos de estrés tectónico indosiniano, yanshan temprano-medio, yanshaniano y xishaniano, con las direcciones de ns, nw, ne y nne, respectivamente. (2) la evolución tectónica del área minera se puede dividir en cuatro etapas: (a) el campo de tensión tectónica indosiniana tiene el efecto más débil en el área de estudio, los estratos portadores de carbón se elevan gradualmente por el campo de tensión en este período, y se forman algunas estructuras pequeñas en los estratos portadores de carbón; (b) el campo de tensi ón tectónica yanshaniana de principios y medios tiene un cierto efecto de transformación en los estratos portadores de carbón en la parte oriental de pingdingshan. esto forma principalmente la estructura secundaria del área minera, como la falla normal de huoyan y la estructura de fractura de fallas pequeñas; (c) el campo de estrés tectónico de yanshanian tardío tiene el efecto más fuerte, y la compleja estructura de control de plegado con el eje cerca de la dirección nw se forma bajo el campo de estrés tectónico de este período, como la sinclinal de likou, la anticlinal de baishishan, la sinclinal de lingwushan, la anticlinal de guozhuang, la línea de sincronización de niuzhuang, etc. la dirección cercanw de la estructura de falla de control también se forma bajo la acción del campo de tensión tectónica de yanshanian tardío, como la falla inversa de niuzhuang, la falla inversa original de mina no.11, etc.; (d) el campo de tensión tectónica del himalaya forma principalmente algunas estructuras secundarias y pequeñas en el área minera, y tiene un cierto efecto de transformación en la estructura formada por el campo de tensión tectónica temprana. el patrón tectónico del área minera oriental de pingdingshan es el resultado del campo de tensión tectónica mencionado anteriormente que actúa en secuencia con los estratos portadores de carbón, causando deformación estructural de la capa de carbón. resumen record manuscript received: 28/04/2019 accepted for publication: 07/11/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 347-357 weidong gong* school of safety engineering, henan university of engineering, zhengzhou 451191, china * corresponding author: gwd1202@126.com te ct o ni cs 348 weidong gong introduction china is the world’s largest coal-producing country with abundant coal resources. the country’s proven coal geological reserves are 114 billion tons, and its associated coalbed methane (mainly gas) is rich in resources, reaching 36.8 trillion m3, ranking third in the world. although coal’s share of energy consumption has been declining in recent years, coal accounts for nearly 60% of china’s primary energy consumption in 2018. pingdingshan mining area is one of the 14 large coal bases planned by china (wang, 2017). however, the mining area, especially in the eastern mining area, is generally deep, with a depth of up to 1200 m. the risk of geological disasters such as coal and gas outbursts and rock outbursts is high. how to prevent and control the above geological disasters has been a hot topic. nearly 80% of coal and gas outburst accidents in the eastern mining area of pingdingshan occur in geological structure areas (gong & guo, 2018). therefore, studying the geological structure characteristics and its formation and evolution process can provide a geological guarantee for safe and efficient mining of coal mines. due to the lack of corresponding magmatic activity in the mining area, it is difficult to determine the tectonic deformation period by the traditional isotope dating method, which makes the formation order and evolution law of the fault-fold structure always unclear. in this case, it is a more effective way to demonstrate the tectonic evolution process through the inversion of the tectonic stress field. the eastern mining area of pingdingshan is located in the southern margin of the north china plate and in the central part of henan province. the coal accumulation period of the mining area is mainly in the carboniferous and permian. the tectonic stress field evolution of the coalfield in this mining area is mainly studied after the coal formation, that is, the tectonic stress field after the mesozoic. there are many pieces of research on the regional paleo-tectonic stress field in the qinling-dabie tectonic belt of the southern margin of the north china plate and the henan province. however, there are few studies on the evolution of the paleo-tectonic stress field in the eastern part of the pingdingshan, and the predecessors have carried out the inferential analysis, which is difficult to provide an effective basis for the tectonic evolution law of the mining area. li (1987) believes that the evolution of the north china plate can be divided into six stages: craton, hyperplasia, initial cleavage, shear translation, fault depression, and equilibrium. zhou and wang (1999), and jia, he and lu (2004) believed that the neogene proterozoic in the southern margin of the north china plate can be divided into four stages: volcanic activity, depression filling, differential lifting, and continental proliferation. nie (1985) conducted a detailed study on the yanshanian tectonic stress field in the north china plate, and clarified the role of the five-curve tectonic movement in the yanshanian period, and discussed the tectonic activity laws and mechanisms of the north china plate in the yanshanian period. zhang, dong, zhao, and zhang (2007) divided the tectonic evolution of the north china plate during the jurassic period into three stages. in the early jurassic, the plate stratigraphy was uplifted, and the early middle jurassic plate was in a weakly extended state. the middle and late jurassic plate was at squeeze the deformation state. wu, zhang, and zhao (2000) studied the layered landforms of the plateau mountains in north china plate and concluded that the first scene of the xishan movement occurred in the eocene to the oligocene, and the second scene occurred in the miocene to the pliocene, the third scene began in the quaternary and continues to this day. liu and zhang (2008) analyzed the formation of the basin-mountain system in the eastern qinling-dabie structural belt and considered that it was controlled by the tectonic stress field generated by the collision between different plates. wang (2009) studied the tectonic evolution of the henan region and believed that the henan strata experienced the tectonic stress field of the indosinian, the early yanshanian, the late yanshanian, the early xishan, and the late xishan in the late triassic to the quaternary. the five-stage tectonic stress field determines the state and occurrence of coal seams and the distribution of structural coal in the coal-producing area. pei, ba, and wang (2007)believed that the pingdingshan area experienced the three tectonic stress fields of the indosinian, yanshanian and himalayan periods during the mesozoic era, and the strong squeezing action of the yanshanian tectonic stress field formed a large number of fault-reducing structures in the area, which basically laid the structural pattern. zhang and zhang (2003) believed that the compressive stress field from the southwestern side of the yanshanian period caused the thrusting nappe action in the pingdingshan mining area and formed the main structure of the mining area. yan, zhang, and wang (2015) inferred from the regional tectonic evolution that the pingdingshan mining area mainly experienced the three tectonic stress fields of the indosinian, yanshanian and himalayan periods; cui and cai (2000) based on the interaction and movement process between the continental plate and the surrounding plate, it is believed that the pingdingshan mining area experienced the indosinian, yanshan early-middle, yanshanian, and xishanian tectonic stress fields. in short, regarding the evolution of the tectonic stress field in the eastern mining area of pingdingshan, the research that has been carried out is very limited, and the understanding of its evolution law is very limited. it is necessary to carry out systematic research work and provide key information for the tectonic evolution of the area. based on the above situation, this paper carried out extensive field and underground observations on the joint structure of the eastern pingdingshan mining area, obtained a large number of conjugate shear joint data. through the stereonet stress field inversion software operation, the 4th paleo-tectonic stress field in the mining area since the mesozoic was studied. on this basis, the tectonic evolution of the study mining area is analyzed, which provides important information for the tectonic evolution process and laws of the mining area, and provides a geological guarantee for safe mining in geological structure areas. regional geological background the eastern mining area is located in the eastern part of the pingdingshan coalfield, which includes the no. 8 mine, no. 10 mine, no. 12 mine, and shoushan mine. the main geologic structure of the mining area is the likou syncline; the axial direction is roughly nw50°, and the axial surface is nearly upright. the northwest limb is relatively gently dipping, the southeast limb is relatively tight, and the two limbs are basically symmetrical; the stratigraphic inclination of this structure is generally 5-15°. there are many secondary folds that were developed by the two limbs of the likou syncline, which are mostly nw-oriented, including the niuzhuang syncline, the guozhuang anticline, the baishishan anticline, and the lingwushan syncline. the fault structure is very developed in the mining area and mainly includes normal faults and strike-slip faults. the fault directions are divided into two groups: one is nw, including the baishigou fault, the huoyan fault, the f1 fault, and the niuzhuang reverse fault; the other is ne, including the goulifeng normal faults and some small faults (figs. 1-3). figure 1. geological structure outline and geostress measuring point distribution map of the pingdingshan eastern mining area study on paleo-tectonic stress field in the eastern mining area of pingdingshan principle of inversion of paleotectonic stress field by conjugate shear joint joints are microfractures which are formed by the tectonic stress field acting on the rock stratum (eyal, gross, engelder, & becker, 2001; bai & pollard, 2000; de guidi, caputo, & scudero, 2013). according to the type of tectonic stress field formed by joints, they are divided into tension joints and shear joints (de guidi et al., 2013; mandl, 2005). the relationship between shear joint and principal stress is similar to that of fault, but the accuracy of 349paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan the stereographic projection method for azimuth correction (eyal et al., 2001; rawnsley et al., 1998; arlegui & simon, 2001). the projection tool of the stereographic projection is a sphere. the planar structure such as joint and linear structure can be expressed by the surface or line passing through the center of the sphere. the intersection of the planar, linear structure and the projection sphere is a circle and a point, respectively. all intersections between the planar structure and the lower hemisphere are connected to the pole (divided into upper and lower poles) by straight lines. these lines form part of the conical surface, and the intersection of this cone and the horizontal plane passing through the center of the sphere is an arc. this arc is the stereographic projection of the planar structure (fig. 5), the projection principle of the linear structure is similar to the planar structure, and its red projection is a little (fig. 6) (fosse, 2010). the above pole is the connection point, called the lower hemisphere projection, otherwise, it is called the upper hemisphere projection. in the joint analysis of this paper, the lower hemisphere projection is used in this paper. the correction of the joint occurrence of the inclined rock layer is to project the observed joint onto the red plane through the stereographic projection theory. around the strike line, the joint rotation is at the same angle as the inclination angle, and the rotation direction is opposite to the joint tendency, that is, the corrected shear joint data is obtained. the corrected joint occurrence data can be solved according to the relationship between the conjugate shear joint and the three principal stresses. in this paper, the conjugate shear joint occurrence is corrected by the stereo projection analysis software stereonet 10.0, and the a shear joint inversion tectonic stress field is insufficient, and the reliability of conjugate shear joint inversion is greatly improved. therefore, conjugate shear joints are commonly used in geological research to invert paleo-tectonic stress fields (negrete-aranda et al., 2010; federico, crispini, & capponi, 2010; veloso et al., 2015; mazzoli, santini, macchiavelli, & ascione, 2015; naimi-ghassabian, khatib, nazari, & heyhat, 2015). the distribution of joints produced by the same tectonic stress field has a certain regularity, usually a large area is developed in the rock mass to form a specific structural pattern. the research area often experiences the multi-stage tectonic stress field. each tectonic stress field forms a joint with a specific direction and mechanical properties. the joints formed in the early stage may be reconstructed by the late tectonic stress places, even superimposed with the joints formed by the late tectonic stress fields. therefore, how to accurately match and stage the joint data is crucial for determining the direction and timing of the tectonic stress field. the staging of joints is to classify the joints produced by the tectonic stress field in different periods and divide the joints generated by the same tectonic stress field into a group. the matching of the joints is to determine whether the joints of different directions and properties are the products of the same tectonic stress field, that is, whether it is possible to form a conjugate shear joint. the shear joints of conjugate shear joints are formed in the same tectonic stress field, which should have similar spacing and density, and the joint fillings should be similar (tang, yang, lv, tang, wang, 2018; jiang et al., 2016; fang, sang, wang, shiqi, & ju, 2017). conjugate shear joints generally have microdislocations, and one set on the left, the other group on the right. the two shear joints with the above characteristics can be judged as conjugate shear joints, and conversely, they are not conjugate shear joints. if the observed joints are at the horizontal level or approximately at the horizontal level, the three principal stress directions can be directly inverted (eyal et al., 2001; rawnsley, peacock, rives, & petit, 1998; arlegui & simon, 2001). the relationship between the spatial orientation of the three principal stresses and the conjugate shear joint is: the direction of the bisector of the acute angle between the two sets of shear joints is the action direction of σ1, and the direction of the blunt angle of the obtuse angle is the action direction of the minimum principal stress σ3. the intersection line direction of the two sets of shear joints face is the action direction of the intermediate principal stress σ2 (fig. 4). the rock formation usually has a certain angle with the horizontal plane, especially the structural areas such as faults and folds that may intersect the horizontal plane at a large angle. when using the shear joints in the inclined rock to carry out the stress field inversion, the joint production needs to use figure 2. likou syncline-baishishan anticline-lingwushan syncline profile map (a-a) figure 3. niuzhuang syncline-guozhuang anticline-likou syncline profile map (b-b) figure 4. relationship between plane conjugate shear joint and principal stress axis 350 weidong gong direction of the tectonic stress field is analyzed by the software. conjugate shear joint inversion of paleo-tectonic stress fields has been successfully used in several studies (federico et al., 2010; mazzoli et al., 2015; foeken, bertotti, & dunai, 2006). joint features of the eastern mining area of pingdingshan in this paper, in the study of the tectonic stress field in coal mining areas, the underground conjugate shear joint is innovatively combined with the field conjugate shear joint to invert the tectonic stress field, which improves the accuracy and has significant significance for studying the tectonic stress field evolution in the mining area. considering the topography, the field geological structure, and the underground geological structure factors, 32 conjugate shear joint points are finally selected. among them, there are 26 field measurement points, at least 40 joints are measured at each measurement point, and a total of 1483 joints are measured. the stratum is mainly the triassic lower purple-red layered medium-grain sandstone and the permian shiqianfeng group coarsegrained sandstone; there are 6 downhole measuring points, and at least 40 joints are tested at each measuring point. a total of 333 underground joints are tested. the stratum is mainly composed of permian lower ash fine sandstone. the distribution of field and underground conjugate shear joints measurement points is shown in figure 7. in this section, according to the conjugate shear joint data of the test, the rosette diagram of the joints occurrence of each test point is drawn, the dominant orientation and representative occurrence of the conjugate shear joint are determined, and the extrusion direction of the paleotectonic stress field in each stage of the mining area is finally analyzed. the joint features of different measuring points have common features and differences. the common features of joints are: most of the measuring points develop 2-3 stable shear joints, and the rock is often cut by the shears joint in a rhombic shape or a checkerboard shape, and the two shear joints of the conjugate shear joint are almost isometric arrangement; the shear joint is generally smooth, and the shear joint is usually closed when it is not filled, and the scratches left by the shear stress field are occasionally observed on the joint surface; the conjugate shear joints are larger and the density is smaller when the rock layer is thicker, on the contrary, the scale is smaller and the development density is larger. (figs. 8-9). there are differences in the shear joints of different measuring points. some shearing joints have small scales, and the conjugate shear joints have a diamond-shaped side length of only about 10 cm, but the conjugate shear joints have a higher density. while some of the measuring points have larger scales, the diamond or chess side is up to 50 cm long and its density is relatively small (figs. 8-9). figure 7. measuring point and joint strike rose distribution diagram in the eastern mining area of pingdingshan the conjugate shear joints in the eastern part of pingdingshan are mostly high-angle joints, and the rose diagram of joint strike and the dip distribution histogram are drawn for the 1816 joints measured (figs. 10-11). the results show that the joint trend is most developed with nee, followed by ew and nnw, which reflects the role of the multi-period tectonic stress field in the study area. the joint inclination angle is most developed at 80-85°, followed by 75-80° and 85-90°. the high-angle joints belong to the early plane conjugate shear joints, after the slope of the stratum in the later period, the joint occurrence changes. therefore the joint occurrence is corrected first, and then the direction of the tectonic stress field can be analyzed. figure 5. the principle of plane stereographic projection figure 6. the principle of a stereographic projection of points 351paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan measuring point 1 measuring point 2 measuring point 4 measuring point 5 measuring point 7 measuring point 8 measuring point 20 measuring point 21 figure 8. joint development characteristics of field measurement points 352 weidong gong figure 10. the rose diagram of shear joints strike inversion results of the paleo-tectonic stress field the shear joints were corrected by stereonet 10.0 software, and the stress field inversion was finally performed. the inversion results are shown in figure 12 and table 1. the maximum principal stress direction of the inversion results is mainly concentrated in four directions. the direction of the 12 measuring points is near the ne direction, the specific orientation is 212.6239.5°, and the average is 222.7°; the maximum principal stress direction of 8 measuring points is nearly nw. the specific orientation is 305.8-336.9°, with an average of 317.9°; the maximum principal stress direction of 8 measuring points is near nne, the specific azimuth range is 194.6-200.9°, and the average measuring point 27 measuring point 29 measuring point 30 measuring point 31 figure 9. joint development characteristics of underground measuring points figure 11. the distribution histogram of shear joints inclination 353paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan is 198°; the maximum principal stress direction of 4 measuring points is near ns direction, the specific orientation is 354.1-6.3°, with an average of 359.1° (table 1). according to the above results, the eastern mining area of pingdingshan has experienced at least four tectonic stress fields. the first stage tectonic stress field is in the direction of ns, the conjugate shear joints preserved in this tectonic stress field are the least, indicating that the tectonic movement in this period is relatively early, combined with the evolution process of the paleo-tectonic stress field of the north china plate, the tectonic stress field should be the indosinian tectonic stress field; the direction of the second-stage tectonic stress field is near nw direction. the conjugate shear joints produced by the tectonic stress field in this period are more preserved, indicating that the stress field in this period is more intense. combined with the table 1 inversion results of principal stress directions of joint points in the eastern mining area of pingdingshan measuring point number representatives occurrence of joint set 1 representatives occurrence of joint set 2 principal stress axes period of tectonic stress field action σ1 σ2 σ3 1 86°∠78° 355°∠84° 311°∠4.2° 58.9°∠76.6° 220∠12.7° the indosinian period 2 115°∠78° 344°∠80° 229.1°∠25.1° 51.9°∠64.9° 319.6°∠1.1° the late yanshanian period 3 60°∠83° 118°∠80° 359.4°∠3.1° 106.9°∠79.8° 268.9°∠9.7° the indosinian period 4 95°∠80° 344°∠82° 219.3°∠15.6° 43.9°∠74.3° 309.6°∠1.2° the late yanshanian period 5 174°∠85° 85°∠78° 219°∠5° 106.5°∠77.1° 310°∠11.8° the late yanshanian period 6 182°∠83° 89°∠70° 317.2°∠10.2° 110.3°∠68.7° 224.0°∠8.9° the early-middle yanshanian period 7 146°∠69° 70°∠85° 20.4°∠12.8° 123.2°∠73.9° 339.5°∠13.1° the himalayan period 8 254°∠89° 143°∠80° 19.1°∠9.6° 169.1°∠78.9° 288.2°∠5.5° the himalayan period 9 75°∠82° 168°∠80° 214.4°∠3.7° 112.5°∠72.7° 305.5°∠16.9° the late yanshanian period 10 84°∠80° 191°∠82° 317.7°∠14.9° 132.7°∠75° 227.4°∠1.2° the early-middle yanshanian period 11 328°∠82° 241°∠84° 194.6°∠1.5° 293.1°∠80.3° 104.4°∠9.6° the himalayan period 12 165°∠70° 93°∠75° 40.1°∠4.2° 140.8°∠68.2° 308.4°∠21.3° the late yanshanian period 13 260°∠85° 182°∠69° 308.7°∠12.6° 183.2°∠69° 42.5°∠16.5° the early-middle yanshanian period 14 45°∠89° 123°∠86° 354.1°∠2.4° 120.6°∠86° 264.9°∠3.2° the indosinian period 15 332°∠75° 248°∠83° 200.9°∠6° 312.4°∠74.1° 109.3°∠14.7° the himalayan period 16 186°∠72° 103°∠75° 55°∠2.3° 150.7°∠68.3° 324.1°∠21.6° the late yanshanian period 17 92°∠82° 20°∠79° 145.7°∠2.6° 43.5°∠78° 236.2°∠11.7° the early-middle yanshanian period 18 255°∠80° 25°∠83° 140.5°∠19.5° 315.25°∠70.4° 49.9°∠1.7° the early-middle yanshanian period 19 76°∠79° 324°∠85° 199.4°∠14.1° 34.3°∠75.4° 290.3°∠3.6° the himalayan period 20 310°∠87° 82°∠81° 195.3°∠14.5° 28.6°∠75.1° 286.1°∠10.3° the himalayan period 21 270°∠80° 340°∠82° 34.8°∠1.7° 295.8°∠78.9° 125.1°∠10.9° the late yanshanian period 22 175°∠83° 92°∠78° 223°∠3.8° 116.7°∠76.8° 313.9°∠12.6° the late yanshanian period 23 215°∠88° 97°∠79° 336.9°∠12.5° 133.3°∠76.4° 245.7°∠5.2° the early-middle yanshanian period 24 240°∠76° 116°∠82° 356.9°∠22.5° 186.4°∠67.2° 88.3°∠3.4° the indosinian period 25 167°∠80° 136°∠75° 239.5°∠9.3° 114.9°∠74° 331.7°∠13° the late yanshanian period 26 152°∠81° 62°∠86° 17.3°∠3.5° 128.2°∠80.2° 286.7°∠9.2° the himalayan period 27 77°∠88° 175°∠84° 305.8°∠6.1° 149.5°∠83.4° 36.1°∠2.6° the early-middle yanshanian period 28 342°∠85° 263°∠88° 212.6°∠2.4° 330°∠84.9° 122.4°∠4.5° the late yanshanian period 29 309°∠84° 244°∠82° 6.3°∠1.9° 263.7°∠81.5° 96.6∠8.3° the indosinian period 30 323°∠84° 73°∠78° 197.3°∠15.4° 31.3°∠74.1° 288.3°∠3.7° the himalayan period 31 122°∠76° 161°∠81° 230.1°∠7.4° 109.3°∠75.7° 321.7°∠12.2° the late yanshanian period 32 90°∠76° 343°∠82° 215.7°∠18.1° 48.2°∠71.5° 306.9,°∠3.7° the late yanshanian period 354 weidong gong fi gu re 1 2. s ch em at ic d ia gr am o f t he ac tio n di re ct io n of p al eo -te ct on ic st re ss fi el d at ea ch m ea su rin g po in t i n th e e as te rn m in in g ar ea o f p in gd in gs ha n 355paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan tectonic movement evolution process of the north china plate and previous research data, it is believed that the period of tectonic stress field in this period should be yanshan early-middle; the direction of the tectonic stress field in the third stage is near ne direction. the shear joints generated by the tectonic stress field in this period are the most preserved in the study area, indicating that the tectonic stress field has a strong effect in this period, and a large number of tectonic deformations are formed in the mining area. combined with the regional tectonic evolution process, the direction of the tectonic stress field in this period is consistent with the late yanshanian, and it should be the late yanshanian tectonic stress field; the direction of the tectonic stress field in the fourth stage is near nne. the shear joints generated in this period of tectonic stress field are preserved in the study area, but the intensity of the action is weak and the late yanshan tectonic stress field, combined with the regional tectonic evolution history, the stress field should be himalayan tectonic stress field. in summary, according to the measured shear joint data, the strata of the eastern pingdingshan mining area have experienced at least the indosinian, yanshanian, yanshanian and himalayan period four tectonic stress fields. tectonic evolution since the mesozoic in the eastern mining area of pingdingshan the stratum of the eastern mining area of pingdingshan was uplifted in the caledonian period, causing the surface to be denuded and the mining area to develop into a quasi-plain. this is an important factor affecting the stable deposition of the carboniferous and permian coal seams in the pingdingshan mining area. the pingdingshan mining area finally formed a carboniferouspermian coal-bearing stratum with a thickness of 800 m, of which the thickness of the coal seam is nearly 30 m, and the coal quality is mainly coal, fat coal, and coking coal. after the formation of the coal-bearing strata, the eastern mining area of pingdingshan mainly experienced the indosinian period, the yanshan early-intermediate period, the late yanshanian period, the himalayan period tectonic stress field. (1) indosinian tectonic stress field the indosinian tectonic movement occurred in the early mesozoic era and was of great significance to the entire chinese region. yin and nie believe that the splicing of the north china plate and the yangtze plate in the indosinian period is completed from east to west, and the splicing line is the yanlu faultdabieshan-qinling structural belt. the large number of tectonic traces generated by the tectonic stress field in the north china plate has been identified by many scholars. these studies all indicate the existence of the indosinian tectonic stress field. the study area is located in the southern part of the north china plate, which was the ancient qinling ocean during the early indosinian period, and the ancient qinling ocean disappeared in the late indosinian period, then the area was elevated to land. the conjugate shear joint data observed in the study area shows that the direction of the indosinian tectonic stress field in this area is close to the n-s direction, and the intensity is not strong. the indosinian tectonic stress field has little effect on the tectonic pattern of the pingdingshan mining area, and there is almost no ew structure in this area which can support this view very well (fig. 13). the indosinian tectonic stress field is also weak for the coal seam transformation, mainly manifested by the gradual uplift of the coal-bearing strata. the coal seam reaches the maximum depth (more than 2000 m) at the end of the early triassic, and the coal body undergoes deep metamorphism during the ascending process, the effect is accompanied by a large amount of gas generation. (2) the early-middle yanshanian tectonic stress field the early-middle tectonic stress field of yanshan movement is mainly caused by the subduction of the western pacific plate to the north china plate, which causes the north china plate to be subjected to the near nw direction of the compressive stress field. the tectonic stress field has a strong effect on the eastern part of the north china plate, and a series of nne thrust structures are formed in the eastern part. however, the energy loss in the eastern part is large, and the energy transfer to the southwestern part of the north china plate is weak. the eastern mining area of pingdingshan is located at the southern margin of the north china plate. therefore, the mining area is weakly affected by the near nw-oriented compressive tectonic stress field in this period, and does not form a controlled structure of the mining area. only a small number of secondary and small fold structures with a near-ne direction are formed, such as the jiaozan syncline and some small folds in the 12 minefields. moreover, a small amount of near-nw secondary strike-slip faults and normal faults are also formed under the stress field of this period, such as the huoyan normal fault (fig. 14). a large number of small normal faults with nw or nnw strike are developed in the coal-bearing strata of the mining area. these faults are mainly formed by the tectonic stress field in this period. though the coal-bearing strata in the eastern part of pingdingshan are in the gradual uplifting stage in the early middle yanshan, the buried depth of the coal seam is still very deep, and the gas storage conditions in the coal seam are good, so it is still an important period for the massive formation of coalbed methane and the deep metamorphism of coal. figure 14. the early-middle yanshanian tectonic stress field in pingdingshan eastern mining area (3) the late yanshanian tectonic stress field at the end of the eocene, the pacific plate swooped directly into the east asian continent, moving in the direction of nww, and acting strongly on the east asian continent in the form of strong convection, leading to the rifting climax of the period during this period; at the same time, from the late jurassic to the early cretaceous, the qinling-dabieshan tectonic belt experienced a strong thrust-overlifting movement in the near-ns direction. the above tectonic movement caused the late yanshanian north china plate to be subjected to the near-ne thrusting tectonic stress field. according to the inversion result of the conjugate shearing joint in the previous section, the study figure 13. indosinian tectonic stress field in the eastern mining area of pingdingshan 356 weidong gong area was also subjected to a strong compressive stress field near ne in this period. the near ne direction of the compressive stress field causes the regional strata to undergo more intense fold orogeny on the basis of the pre-structural movement. a series of controlled fold structures and reverse faults in the mining area approaching the nw direction were formed during this period, such as likou xiangshi, baishishan anticline, lingwushan syncline, guozhuang anticline, niuzhuang syncline, niuzhuang inverse. fault, baishigou reverse fault, the original eleventh reverse fault (figs. 15-17). in this period, the tectonic stress field exhibits a stretching effect in the nw direction. some near-ne positive faults are formed in the study area, such as the goulifeng normal fault, but the ne-trending tectonic development is far less than the nw-trending structure. the nw and nww structure are often cut off by the ne and nne structures, such as the likou syncline, the lingwushan syncline, and the baishishan anticline, which are cut by the goulifeng normal fault, indicating that the ne and nne structures were formed later than nw and nww structures. likou syncline and lingwushan syncline are the two longest fold structures in the mining area. likou syncline ne wing inclination angle is 8-23°, the sw wing inclination angle is 10-25°, and the sw wing is steeper than the ne wing. lingwushan is asymmetric with two wings, the sw wing is steep, the dip angle is 10-25°, the ne wing is slow, and the dip angle is 10-18°. both of the synclines show that the sw wing is steep, reflecting that the active force direction of the compressive stress field should be on the southwest side. in addition, a large number of small normal faults with ne strike and a few of small reverse faults with nw strike are developed in the coal seam by the late yanshanian tectonic stress field, and tectonic coals in these tectonic zones are widely developed. a large amount of magmatic activity occurred in the eastern and northern parts of the north china plate during this period. however, almost no magmatic intrusion activity was found in each mine in the eastern mining area of pingdingshan. (4) himalayan tectonic stress field in the late eocene, the north china plate entered the himalayan tectonic movement, and the northward movement of the indian plate collided with the eurasian plate. the northeastern part of the indian plate wedged northward into the tibet-yunnan foreland basin, and the stress field appeared as an extruded type. in the western part of the chinese mainland, the himalayas and the gangdese mountains were formed. in this period, the tectonic stress field is mainly characterized by large-scale sedimentation and stratigraphic depression in the east, and the north china plate was dominated by regional sedimentation that accompanied the uplift of the surrounding mountainous areas. according to the results of the conjugate shearing inversion in the previous section, the eastern mining area of pingdingshan is subject to the near nne-oriented squeeze tectonic stress field during the himalayan period, but the stratigraphic deformation in this tectonic movement is weaker than that in the late yanshanian tectonic movement, some secondary or small folds and faults were formed during this period. the structure formed in the early stage is reconstructed by the tectonic stress field in this period. for example, the strike of the zhangwan normal fault is deflected from the near nee to the near ew direction (fig. 18). conclusion (1) through the field and underground conjugate shearing joint observation, the direction of paleo-tectonic stress field in the eastern mining area of pingdingshan was determined by using stereographic projection method. the results show that the eastern mining area of pingdingshan has experienced the indosinian, yanshan early-middle, yanshanian and xishanian tectonic stress fields, with the directions of ns, nw, ne, and nne, respectively. the late yanshanian tectonic stress field has the strongest intensity and the indosinian period has the lowest intensity. figure 16. the formation of the partition style folded belt figure 17. the formation of horst structure figure 15. the late yanshanian tectonic stress field in pingdingshan eastern mining area 357paleo-tectonic stress field and tectonic evolution since the mesozoic in the eastern mining area of pingdingshan (2) indosinian tectonic stress field is mainly characterized by the gradual uplift of coal-bearing strata, and a small number of near-ew-oriented small structures are formed in this period; the early-middle yanshanian tectonic stress field is stronger than the indosinian period. a small number of secondary and small fold structures with a near-ne direction are formed, such as the jiaozan syncline. a small amount of near-nw secondary strike-slip faults and normal faults are also formed, such as the huoyan normal fault. moreover, a large number of small normal faults with nw or nnw strike are developed in the coal-bearing strata of the mining area; the late yanshanian tectonic stress field has the strongest effect. a series of controlled fold structures and reverse faults in the mining area approaching the nw direction were formed during this period, such as likou xiangshi, baishishan anticline, lingwushan syncline, niuzhuang reverse fault, baishigou reverse fault, et al. in addition some nearne positive faults are formed in this period, such as the goulifeng normal fault; some secondary or small folds and faults were formed by the himalayan tectonic stress field, and the structure formed in the early stage is reconstructed by the tectonic stress field. references arlegui, l. & simon, j. l. 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(1999). feature and tectono-paleogeography evolution of the southern margin of the north china continent in mesoproterozoic and neoproterozoic ear. geoscience, 3, 261-267. figure 18. himalayan tectonic stress field in eastern pingdingshan mining area keywords: heavy metal pollution; soil; ecological compensation; internet of things. palabras clave: contaminación por metales pesados; suelo; compensación ecológica; internet de las cosas. how to cite item rojas barbosa, s., molano mendoza, j. shanshan, j., & yanqing, z. (2020). ecological compensation method for soil polluted by heavy metals based on internet of things. earth sciences research journal, 24(2), 153-161. doi: https://doi.org/10.15446/esrj. v24n2.87441 the traditional ecological compensation method for soil pollution by heavy metals is complicated. it consumes a lot of human and material resources and cannot quickly meet real-life needs. this study takes an area where heavy metals pollute the soil. increasing soil retention in this area is the target for ecological compensation, and the method is based on the internet of things. users' willingness to participate in the land heavy metal pollution control project was studied, and the pollution degree was calculated. the area was linked to the internet tools according to the agreement, and the information exchange was carried out through information-sensing equipment of radio frequency identification. this process provided the basis for the ecological compensation standard of soil heavy metal pollution area. according to the content, changing trend, and characteristics of pollutants in the study area, the best forest species were selected for this area. the soil retention was increased to the maximum extent to realize the ecological compensation to the area polluted by heavy metals. the final results show that the ecological compensation method for soil contaminated with heavy metals based on the internet of things is cost-effective and has excellent feasibility, which can be the essential means for sustainable development of ecological construction. abstract ecological compensation method for soil polluted by heavy metals based on internet of things método de compensación ecológica para suelos contaminados por metales pesados con base en el internet de las cosas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87441 el método tradicional de compensación ecológica para el suelo contaminado por metales pesados es complicado, consume mucha mano de obra y recursos materiales, y no puede satisfacer las necesidades de la vida real. para este trabajo se tomó como ejemplo un área donde el suelo está contaminado por metales pesados. el aumento de la retención del suelo en esta área es el objetivo específico de la compensación ecológica, y este aumento se busca alcanzar a través de métodos relacionados con el internet de las cosas. se estudió la disposición de los usuarios a participar en el proyecto de control de la contaminación por metales pesados y se calculó el grado de contaminación. a través del equipo de detección de información de identificación por radiofrecuencia, el área de contaminación de metales pesados del suelo se conectó a herramientas de internet según el acuerdo, y el intercambio de información se llevó a cabo, a fin de proporcionar la base para el estándar de compensación ecológica del área de contaminación de metales pesados del suelo. según el contenido, la tendencia cambiante y las características de los contaminantes en el área de estudio, se seleccionaron las mejores especies forestales para esta área, y la retención del suelo se incrementó al máximo para obtener la compensación ecológica al área contaminada por metales pesados. los resultados finales muestran que el método de compensación ecológica para el suelo contaminado con metales pesados basado en el internet de las cosas es rentable y tiene una gran viabilidad, que puede ser el medio importante para el desarrollo sostenible de la construcción ecológica. resumen record manuscript received: 13/11/2019 accepted for publication: 27/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 153-161 jia shanshan1*, zhou yanqing2 1school of inner mongolia vocational college of chemical engineering, department of computer and information engineering, hohhot, 010010, china; 2inner mongolia agricultural university, college of computer and information engineering, hohhot, 010018, china * corresponding author: jiashanzi@163.com; hellozhouyanqing@126.com so il s https://doi.org/10.15446/esrj.v24n2.87441 https://doi.org/10.15446/esrj.v24n2.87441 https://doi.org/10.15446/esrj.v24n2.87441 mailto:jiashanzi@163.com mailto:hellozhouyanqing@126.com 154 jia shanshan, zhou yanqing introduction the main sources of heavy metal pollution (gasiorek et al., 2017) include industrial pollution, metal mining, sewage irrigation, solid waste disposal, etc., and have received extensive attention from researchers in recent years. the accumulation of pollutants (diepens & koelmans, 2018) in the soil often presents a high degree of spatial variability due to the enhancement of human activities, which makes the soil heavy metal pollution more complicated, and often forms one or more trends in space. since heavy metals are not easily mobile and dissolved, they accumulate after entering the organism and have an adverse effect on the organism. for example, when the low concentration of hg is present in the soil, it can promote in the early stage of wheat germination. however, as the time increases, the respiration will decrease and eventually manifest as inhibition. it can be seen that the latent time of heavy metal pollution is long. heavy metals that are not easily degraded have adverse effects on the quality and yield of crops, and can even affect human health through the food chain. the construction of ecological compensation mechanism is an important means to improve the quality of the ecological environment and coordinate the contradiction between environmental protection and economic development. many experts and scholars at home and abroad have done a lot of research on this and achieved good results. various methods of ecological compensation standards and their application cases are analyzed to determine the ecological compensation method based on the internet of things (kokkonis et al., 2017), which provides the scientific basis for the determination of ecological compensation standards and the construction of mechanisms. (1) in order to solve the ecological environment loss caused by industrial pollution and determine reasonable ecological compensation standards, energy theory is used to calculate energy loss. taking the measured ecological environment loss as the ecological compensation standard, the research draws the isw pollution tax to balance the pollution control effect (liu et al., 2018). (2) the concept of theoretical deficit. based on the theoretical deficit, the calculation method of compensation cost between contaminated areas is improved. at the same time, compensation costs are predicted by combining population size, wealth and technology. finally, the compensation indicators for each contaminated area are determined (yang et al., 2019). (3) first-hand information obtained through public statistics and field surveys as the source of data. conditional valuation method (cvm) is used to estimate the source data and contaminated receptor data associated with the compensation area. according to the survey results, the compensation standard is determined, and the compensation amount is estimated, then the ecological compensation is finally realized (shu, 2018). (4) an area-based ecological compensation method is proposed. based on the traditional ellipsoidal area and 103 regions as reference, the biomass and ecological compensation are determined by surface area method. in this way, we will balance the interests of different regions and make reference for ecological sustainable development (he et al., 2018). due to the complexity of the process, it will consume a lot of human and material resources. therefore, an ecological compensation method based on the area where the soil is contaminated by heavy metals is proposed. by analyzing the living and production of the contaminated area, the cost of pollution compensation is calculated to establish a sustainable ecological compensation plan. the method has short calculation time, easy operation and high accuracy, and has excellent application promotion value. materials and methods the overall structure of the internet of things first, the concept, necessity, and positioning of the internet of things operating system are briefly analyzed. the basic functions of the internet of things operating system (gao et al., 2019) are similar to those of the mobile operating system in the field of mobile internet. first of all, the role of the mobile operating system is to achieve the separation of terminal hardware and software. the developer of any application does not have to consider the physical hardware configuration of the terminal. it can be run on all mobile operating system-based terminals simply by writing an application based on the programming interface provided by the phone. the function of the hardware is limited. if the software and hardware are tightly bundled and not separated, the functions of the entire system are limited. however, by stripping out the hardware functions, the functionality of the entire system can be improved by software changes. secondly, for the new internet of things, to achieve a benign development, the effect of software and hardware separation must be achieved. the internet of things has limited hardware capabilities. by writing the corresponding application by the specific developer, you can extend the capabilities of the hardware and make full use of the hardware. in addition, the internet of things needs to achieve another separation: the separation of hardware drivers from the operating system kernel. when there is the problem with the hardware of the internet of things used, only the problem hardware needs to be replaced, and the operating system automatically configures the corresponding driver. the installation process is transparent to the programs running on the operating system. the following diagram illustrates the internet of things ecosystem built on these two separations (application software (millard et al., 2018) is separated from the hardware; device drivers are separated from the operating system kernel),the work flow diagram of the internet of things is shown in figure 1: figure 1. work flow chart of the internet of things in order to obtain accurate spatial distribution information of pollutants in the soil by using the internet of things, the investigation of heavy metal pollution in the soil usually includes multiple stages such as preliminary investigation and detailed investigation of pollution. the main purpose of the preliminary investigation is to identify major pollutants and potentially contaminated areas of the soil (monaco et al., 2017), usually with a small sample size. the detailed investigation of pollution is based on the preliminary investigation, and the sample points in the potential pollution area are increased to determine the extent of the pollution area and the degree of pollution. the errors in the soil pollution survey program mainly include the underestimation of the contaminated area and the overestimation of the clean area. among them, the former will lead to the pollution control of the pollution area is not effectively 155ecological compensation method for soil polluted by heavy metals based on internet of things application for this hardware, while third party enterprises can use the java interface to develop applications that can be widely ported to rfid information sensing equipment to exchange information. (3) the framework of the driver of the internet of things operating system should be flexible enough and compatible enough to dynamically load and unload device drivers (myhre et al., 2017); (4) the version of the internet of things operating system has a variety of branches or variants. for each industry, even each type of hardware (such as cars, refrigerators, etc.) will have a corresponding version, which requires good support for compiling development tools. based on the “the internet of things + ecology” concept, the internet, gis and the principles and methods of image graphics, the ecological compensation of soil contaminated by heavy metals is discussed. based on the connotation and principles and theoretical basis of ecological planning, the basic procedures, contents and methods are elaborated, and the role of the internet of things is emphasized (li et al., 2017). from the perspectives of the diversity, comprehensiveness, systemicity and logic of ecological factors, the ecological compensation ideas are analyzed. the internet of things operating system kernel, peripheral modules, application development environment, etc., are software system platforms that support the upper-level industry application software. the industry application software is the software that finally releases the hardware function of the iot terminal. such the iot operating system is the basis for the rapid industrialization of industrial applications. software and hardware vendors and users can only benefit from the standard and secure internet of things operating system. the internet of things can also develop rapidly, laying a good foundation for ecological compensation. the ecological compensation structure based on the internet of things is shown in figure 3. ecological compensation assume that the factors affecting farmers’ compensation (kan et al., 2017) include age (fage), gender (fsex), cultural level (fedu), labor population controlled, and the latter will lead to unnecessary investment. in order to obtain accurate information on contaminated areas, it is often necessary to increase the sample size, but this will result in an increase in the cost of sampling analysis. the efficient sampling scheme minimizes the total cost of economic losses due to sampling survey costs and survey uncertainty. the goal of sampling scheme optimization is to seek the best sample information to reduce the uncertainty of pollution repair. the spatial distribution of soil pollutants is affected by factors such as pollution sources, environmental conditions, and pollutant properties. it exhibits varying degrees of spatial correlation and variability in space. the description of the spatial variability of soil contaminants is the key to the impact of the survey results. therefore, combined with the specific needs of soil pollution compensation, an encryption method based on pollution probability and local spatial variability of pollutants is proposed to improve the estimation accuracy of ecological compensation methods in terms of pollution range and pollution degree. it also provides methodological support for ecological compensation in soil contaminated areas. based on the architecture of this ecological environment (lin et al., 2018), the architecture of the internet of things operating system is shown in figure 2: hardware design (1) the overall image size of the internet of things operating system must be highly scalable to accommodate applications with limited hardware resources. it can not only adapt to the application scenarios where the hardwa re resources such as the wristband are relatively rich, but also adapt to applications such as environmental monitors that are not very intelligent. specifically, the size of the operating system should be scalable from 10k to 10 megabytes; (2) in addition to providing a java interface, the internet of things operating system should also provide a standard c language interface to deal with high efficiency and real-time applications. for example, the manufacturer of the internet of things terminal can use c interface to develop efficient figure 2. the general mode of ecological compensation based on the internet of things 156 jia shanshan, zhou yanqing figure 3. structure diagram of ecological compensation based on the internet of things (fwrf), family cultivated area (fsize), whether to waste (fabd), degree of farmland pollution (fdgr) and bid amount (fbid). these eight factors are used as independent variables to establish multiple linear regression equations: inw f f f f f f age sex edu wrf size a = + + + + + + α β β β β β β β 1 2 1 3 4 5 6 bbd dgr bidf f+ +β β7 8 (1) based on the internet of things, the detailed analysis is carried out to determine a set of relatively fixed methods for analyzing the preference data (chen et al., 2017). the utility level (u) of the farmer is as follows u j sy s j= ( ) +, ,  (2) in the above formula, v is an observable variable; ε is an unobservable variable. j = 0 indicates that the sample is willing to stop farming and participate in the project, j = 1 indicates that the sample continues to farm and refuses to participate in the project. y is the income level of the sample under different decision conditions; s is a series of potential influencing factors (including age, gender, etc.) of the sample’s willingness to pay. the basic information of the respondents is shown in table 1. assuming the given compensation criterion is a (he et al., 2017), when the given compensation criteria increase the utility level, the survey sample is likely to choose to participate in the project. at this time there are  0, , ,y a s( )+ ≥ (3) assume that the utility difference (δu) caused by farmers’ participation/ non-participation in the project is: u u u= 0 1 (4) table 1. basic information of respondents variable distribution ratio /% variable distribution ratio /% age cultural level 20~29 2.5 no 4.6 30~39 15.8 primary school 29.2 40~49 33.4 junior high school 51.7 50~59 33.3 high school 14.5 ≥ 60 15.0 university and above 0 gender cultivated land area/hm2 male 62.1 ≤ 0.33 22.9 female 37.9 > 0.33~0.67 48.4 working population > 0.67~1.00 19.5 0 2.1 > 1.00 9.2 1 7.1 whether to waste 2 56.3 yes 50.4 3 12.1 no 49.6 4 16.7 degree of pollution ≥ 5 5.7 1 12.0 2 47.8 3 40.2 157ecological compensation method for soil polluted by heavy metals based on internet of things when expressed as an observable variable, the utility difference is u = +α β (5) in the above formula, is a constant term;  is the coefficient of influence of the given compensation criterion on the willingness to participate. when multiple variables are involved, they can be written as u n= + + +…+α β β β1 2 (6) at the same time, let η ε −ε= 0 1 (7) it is concluded that the possibility of local farmers participating in the project p0 is p p v y a s v y s0 0 10 1= +( ) + ≥ ( ) + , , , ,  (8) it can also be written as following. p f u0 = ( )η ∆ (9) p is the probability value; f uη ∆( ) is the probability distribution function of u with respect to . according to the logit model, there is: f u e uη −∆ ∆( ) = + 1 1 (10) for the model integral, the upper limit of the score is the upper limit of the bidding interval, and the estimated value of the average lower limit of the willingness of compensation wmean can be obtained: w f u dmean = ( )∫ η ∆ 0 max (11) where a is expressed as: a e dau a = +∫ 1 10 max (12) cost of ecological compensation the use of market theory to calculate the conditions of ecological compensation is very strict, and the establishment of the ecosystem of the ecosystem (kaiserbunbury et al., 2017) is very difficult. semi-market theory can determine the criteria for ecological compensation if the market fails to function. the theory is to use the market supply (zabihollah, 2017) and demand to determine the compensation standard. according to the market theory, there is a corresponding relationship between the quantity and price of goods, and the supply curve and the demand curve are given from the supplier and the demander respectively. according to the theoretical amendments proposed by mankiw in the principles of economics, the formula is as follows: p f q qs d= ( ), (13) q f i p p a nd o o r d= ( ), , , , (14) q f i t a ns p s= ( ), , , (15) in the above formulas,p is the price; qs is the supply curve; qd is the demand curve; io is the income; po is the price of the relevant product; pr is a preference; a is expected; nd is the number of demanders; ip is the input; t is technology; a is expected; ns is the number of sellers. similar to the market, the determination of ecological compensation standards is also affected by various factors, summed up as formula: s f q qs d= ( ), (16) s f q f mac mics s s s= ( ) = ( ), (17) s f q f mac micd d d d= ( ) = ( ), (18) where,s is the compensation standard; ss is the compensated standard; sd is the compensator standard; mac is the macro factor; mic is the micro factor;  is the noise (other factors). due to the uncertainty and complexity of the macro factors (zhang et al., 2018) in the study area, there is currently little research on the impact of changes in macro factors on ecological compensation standards. most of the relevant research is based on microscopic factors (sun & ma, 2017). for providers of compensation, the micro-factors (micd) that affect their compensation criteria include income, preferences, and expectations, which can be reflected by the provider’s willingness. for the recipients of compensation, the main influencing factors are income, direct costs, expectations and opportunity costs (lomas et al., 2018). the first three can be reflected in a willing manner, and the opportunity cost can be used as a basis for determining the criteria. in summary, semi-market theory is based on market theory, and in fact it evaluates the unilateral criteria of providers and recipients of ecological compensation separately. in the end, find the influencing factors and determine the criteria for ecological compensation. in the investigation of pollution sources, it is known that there are large copper pipe plants, metal material plants and electroplating plants in the research area. many factories have heavy metals in the waste residue. the survey and analysis of the willingness to abandon contaminated land is shown in table 2. table 2. investigation and analysis of the willingness to abandon farming of contaminated land willingness to abandon farming due to serious soil pollution number percentage % proportion of the total number of interviewees the proportion of people who are unwilling to give up the total willing 223 58.37 unwilling 159 41.62 land pollution is not serious 9 2.35 5.66 replanting other crops 13 3.41 8.18 losing living security 78 20.41 49.06 insufficient compensation 54 14.14 33.96 polluted, but has no effect on your health 5 1.31 3.14 based on the above, the following research priorities are determined: first: the type of ecological compensation is indeed divided. for different types of ecological compensation, specific methods for determining ecological compensation standards were established. different ecological compensation objects are also classified. according to the different functions of ecosystem services, it can be divided into four types of ecological compensation for biodiversity, carbon emissions, water resources and landscape protection. there are also many projects that combine these ecosystem services to compensate. according to the scope of compensation, it can be divided into global scale, national scale, regional scale and watershed scale. there is no method for determining the corresponding ecological compensation standard for each type. second: based on the three theories (value theory, market theory and semi-market theory), the basic research on the method of determining ecological 158 jia shanshan, zhou yanqing compensation standards is further strengthened. among them, the breakthrough point of value theory is to find the appropriate functional relationship to transform the value of ecosystem service function into ecological compensation standard. the difficulty of market theory is how to effectively establish the market and make the standards of ecological compensation self-regulating. third, exploring and clarifying the key factors affecting the determination of ecological compensation standards. it is an inevitable trend of ecological compensation standards to determine the standard of compensation by finding one or several specific parameters. fourth, with the rapid development of information technology, the study of ecological compensation standards should further strengthen the connection with computers. the wta distribution feature of the interview area is shown in figure 4. figure 4. wta distribution characteristics in the interviewed area sampling in order to improve the accuracy of the ecological compensation method, the reliability of the survey sampling information increases, as shown in table 3. table 3. sources of data information data attribute date data sources the date of internet of things date of land use chinese academy of sciences resource and environmental data center date of soil socioeconomic data statistics and survey data digital elevation model (dem) spatial cloud of geographic data pollution erosive factor calculated basic data and others soil erodibility factor calculated vegetation cover and management factors based on models and related research results maximum parameter drawing on and adopting relevant research results analysis of ecological compensation data based on the internet of things calculation of new soil retention soil retention mainly includes erosion control and sediment retention (kim et al., 2018). the soil retention is calculated by the soil retention module of the invest model, which consists of two main components. the first is the amount of sediment retained by the plot itself, expressed as the product of the uphill sand content and the sediment interception rate. the second is the amount of soil erosion reduced by vegetation cover and soil and water conservation measures (cunha et al., 2017), expressed as the difference between the potential soil erosion and the actual soil erosion. sedret rklsx uslex sedrxxd = + (19) sedrx sex usley sez z y x y x = ( ) = += ∏∑ 1 1 1 1 1   (20) where, sedretxd is the amount of soil retention intercepted by the grid x itself´ is the amount of sediment retention intercepted by the grid x itself. rklsx is the amount of potential soil erosion, uslex is the actual amount of soul erosion. sex is the interception rate, usley is the actual soil erosion amount of the uphill grid y, sez is the sediment interception amount of the uphill grid z. the new soil holding amount is obtained by the internet of things data to know the unit soil holding amount after the land use type is changed. in the process of calculation, different forest species (deciduous broad-leaved forest, sparse shrub forest, coniferous and broad-leaved mixed forest, evergreen coniferous forest and deciduous broad-leaved shrub forest) will be produced during the conversion of cultivated land to forest land. e e ei fi ci=  (21) s s si fi ci=  (22) where, si is the unit new soil holding capacity, expressed as t/hm; sfi is the soil holding capacity of different forest species per unit area, expressed as t/ hm; i represents the soil holding capacity per unit area of cultivated land, t/hm. the income item sets the planting area, production and selling price of the main crop; the expenditure item takes into account the cost of inputs such as seeds, fertilizers and pesticides. the difference between the income and the expenditure is divided by the planting area, which is the opportunity cost per unit area of land. as shown in equations (23)-(25). c c soppi i i = (23) c c ci ti si=  (24) c c c c c csi z f n h y= + + + + (25) where coppi is the opportunity cost amount; si is a good planting area; ci is the amount of net income of farmers’ cultivated land; cti is the total income of the crop industry; csi is the total amount spent on agriculture; cz is the amount of seed purchase fee; cf is the amount of fertilizer purchase; cn is the amount of pesticide purchase; ch is the total harvesting amount of the main crop; cy is the total amount spent on refueling agricultural machinery. data processing according to the recommended range and value of heavy metals in gb15618-1995 china soil environmental quality standard, the pollution index of each heavy metal is calculated (oliveira et al., 2017). due to the different dimensions of the raw data for each indicator, the magnitude difference is also very different. to eliminate the original data dimension, merge the order of magnitude to make it comparable. the raw data is first preprocessed. assuming the sequence x x x x ni i i i= ( ) ( ) … ( )( )1 2, , , , the sequence after processing is ′ = ′ ( ) ′ ( ) … ′( )( )x x x x ni i i i1 2, , , . when analyzing the system, different data can be selected for pre-processing of the original data according to different situations. the main methods of raw data preprocessing are as follows. initial value transformation: 159ecological compensation method for soil polluted by heavy metals based on internet of things x k x xi i k i ( ) = ( ) ( ) 1 (26) mean transformation: x k x k x x x ki i k n ( ) = ( ) = ( ) = ∑, 1 (27) percentage transformation: x k x k k x k i i i ( ) = ( ) ( )max (28) multiple transformation: x k x k k x k i i i ( ) = ( ) ( )min (29) range transformation x k x k k x k k x k i i i i ( ) = ( ) ( ) ( )  min max (30) interval value transformation x k x k k x k k x k k x k i i i i i ( ) = ( ) ( ) ( ) ( )   min max min (31) the preprocessed data sequence is x x x x ni i i i= ( ) ( ) … ( )( )1 2, , , (32) where i = 0, 1, 2, …, m. assume that the compensation indicator based on the internet of things is: x x x x n0 0 0 01 2= ( ) ( ) … ( )( ), , , (33) then ∆ −0 0i ik x k x k( ) = ( ) ( ) (34) where, k = 0, 1, 2, …, n, m is the number of samples; n is the number of pollutions. then the difference matrix is ∆ ∆ ∆ ∆ = ( )… ( ) ⋅ ⋅ ⋅ ( )…                 01 01 1 0 m n m (35) find the maximum (major difference) and minimum (very small difference) in the difference matrix, expressed by max and min. the correlation coefficient is δ ∆ ε∆ ∆ ε∆0 0 i i k k ( ) = + ( ) + min max max (36)  in the formula (36) is the resolution coefficient, δ∈ 0 1, . the degree of association is  0 0 1 i i n k= ( )∑ (37) based on the internet of things, the calculation of ecological compensation related data has lower requirements on data, and the calculation amount is small, which is convenient for popularization and application. accurate measurement of new soil holdings is a key issue in the design and evaluation of ecological compensation projects. compared with previous empirical models, it more accurately reflects the amount of soil retention in different spatial contexts. the internet of things data (liu et al., 2017) provides quick and timely access to soil and land cover information for the study area. results the ecological compensation method based on the internet of things is to change the regional land use type through ecological compensation. it changes the cultivated land to forest land, thereby changing the supply of various ecosystem services such as soil conservation, water supply and carbon sequestration, and achieving the goal of improving the ecological environment. the working schematic diagram of the ecological compensation system based on the internet of things is shown in figure 5. the regional background value of heavy metal contaminated soil and the statistical value of the study area are shown in table 4. the mass ratio of cd in soil sample bt-34 was 0169μg/g, and the single factor evaluation reached 2123, which exceeded the moderate pollution index in the national standard. due to the low mass ratio of the other six heavy metals such as as, the average value of the soil sample is low, and the nemero index is evaluated as mild pollution. the mass ratio of zn in soil sample bt-87 was as high as 572μg/g, and the single factor was evaluated as 3136, far exceeding the figure 5. working diagram of ecological compensation system based on the internet of things 160 jia shanshan, zhou yanqing table 4. regional background values of soil contaminated by heavy metals and statistical values of study areas background values minimum sampling value maximum sampling value average sample value intermediate sampling value as 15 5 76 7.34 6.00 cd 0.2 0.051 1.76 0.19 0.14 cr 90 41 180 63.68. 60.00 cu 35 11 104 20.63 19.00 hg 0.15 0.011 1.114 0.08 0.05 ni 40 5 41.00 25.46 25.00 pb 35 16 654 38.24 22.00 zn 100 35 3075 112.89 56.50 table 5. soil contamination results of association analysis number clean mild pollution moderately pollution severe pollution results bt-34 0.898 0.693 0.693 0.484 mild pollutio bt-37 0.758 0.660 0.714 0.552 severe pollution bt-111 0.949 0.69 0.619 0.441 clean bt-177 0.944 0.635 0.598 0.443 clean bt-178 0.942 0.640 0.592 0.434 clean bt-179 0.940 0.635 0.593 0.439 clean table 6. compensation standards for soils in the study area and the amount of soil added compensation area (a) (b) (c) (d) area / 10,000 hm2 0.11 0.14 0.66 0.27 compensation standard / 10,000 yuan 0.46 0.49 1.12 1.05 capital demand / 10,000 yuan 506.67 686.70 745.01 2803.60 new soil holding capacity / 10,000 144.00 63.04 574.20 245.50 forest soil retention / 10,000 1152.00 504.32 4593.60 1964.00 suitable forest plants evergreen coniferous forest evergreen coniferous forest deciduous broad-leaved forest deciduous broad-leaved forest figure 6. compensation results of the study area national standard for heavy pollution. however, the nemero evaluation index of the soil sample was 21,528, which was evaluated as moderate pollution. in the soil sample bt-111, the mass ratio of cd was at the critical point of light pollution, and the mass ratios of the other seven heavy metals were all within the clean range, which was evaluated as appropriate for cleaning. however, the nemero evaluation index at this point was 01857, which was evaluated as mild pollution. the sampling points of soil samples bt-177, bt-178, and bt-179 are less than 3km apart, and the soil pollution levels in the vicinity of more than ten kilometers are clean. however, since the cd in the soil sample bt-178 exceeded the light pollution threshold of 0104μg/g, it was evaluated as mild pollution, which resulted in a small lightly contaminated area in the spatial interpolation analysis. the degree of pollution is magnified, which affects the correct judgment of local pollution. through comparative analysis, it can be seen that the gray correlation analysis method (the corresponding soil sample analysis results are shown in table 5) can more appropriately assess the heavy metal pollution of the samples. the diversity and complexity of pollution factors in the soil determine that it is difficult to directly analyze the relationship between heavy metal pollution indicators and soil environmental quality. the standard of soil compensation and the amount of soil added in the study area are shown in table 6. as shown in figure 6, the four regions (a), (b), (c), and (d) are used as research areas, and the information of the internet of things is processed and extracted based on other information such as the internet of things, land use map, economic data, and basic data. the invest model was used to simulate the changes in soil conservation of different forest species before and after returning farmland to forest. the invest model was used to simulate the changes in soil conservation of different forest species before and after returning farmland to forest. the spatial distribution of the new soil retention curve for the best forest species was derived using the minimum data method. the relationship between the new soil conservation and the compensation price of the best forest species under different land use conversion ratios was calculated, and the impact of ecological compensation on regional soil conservation was analyzed. with the main goal of increasing soil conservation, the soil in the areas contaminated by heavy metals is all retreated. the best planting species in the research area (b) is evergreen coniferous forest, which needs to be compensated for 0.49 million yuan/ (hm2•a), and the soil holding capacity can be increased by 630,400 t/a. in 8 years, the soil holding capacity was increased by 5,043,200 t/a. the best forest species in the study area (c) is deciduous broad-leaved forest, which needs to be compensated for 112,000 yuan/ (hm2•a), and the newly added soil holding capacity is 5,742,200 t/a. the polluted area (d) needs to be compensated for 10,500 yuan/ (hm2•a), and the newly added soil holding capacity is 2,455,000 t/a, and then the soil holding capacity is increased by 9.64 million t/a in 8 years. in the research area (a), the best forest species is coniferous and broad-leaved mixed forest, which needs to be compensated by 4.6 thousands yuan/ (hm2•a), and the soil holding capacity can be increased by 7.151 million t/a. in general, the total compensation for the above polluted areas was 178.8319 million yuan / a, and the total new soil holding capacity was 17.4084 million tons / a. 161ecological compensation method for soil polluted by heavy metals based on internet of things conclusions the proposed method realizes the spatialization and dynamism of quantitative evaluation of ecosystem service functions, and can visually display the spatial distribution and difference of soil conservation in the study area, which is a good choice for ecological compensation research. after years of development, ecological compensation has become an important means of ecological environment protection and heavy metal pollution control in china, but the current ecological compensation model is still too extensive. the internet of things data, the invest model and the minimum data method are flexibly utilized. the reasonable ecological compensation project can effectively improve the efficiency of ecological compensation and enhance the sustainability of the project, which helps to achieve visual and refined ecological compensation. reference chen, i. a., markowitz, v. m., chu, k., palaniappan, k., szeto, e., pillay, m., ratner, a., huang, j., andersen, e., huntemann, m., varghese, n., hadjithomas, m., tennessen, k., nielsen, t., ivanova, n. n., & kyrpides, n. c. 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(2018). spatial downscaling of trmm precipitation data considering the impacts of macro-geographical factors and local elevation in the three-river headwaters region. remote sensing of environment, 215, 109-127. http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jsjfz201710058 about:blank keywords: flac; neural network; landslide; deformable body; jm tunnel. palabras clave: flac; red neuronal; deslizamiento de tierra; cuerpo deformable; túnel jm. how to cite item zhou, h., wang, y., shen, t., & feng, q. (2019). the comprehensive treatment of the ancient landslide deformable body in the giant deep rock bedding based on the flac analysis model. earth sciences research journal, 23(4), 303-308. doi: https://doi.org/10.15446/esrj. v23n4.84008 in previous studies, most of the studies are based on the failure mechanism of the landslide deformable body on tunnel and the safety monitoring and early warning after the completion of the tunnel, while there is less research on tunnel construction. in order to study the comprehensive treatment of the deformable body of the ancient landslide in the giant deep rock bedding better, the treatment of jm tunnel landslide deformable body was selected to carry on the empirical study. after a detailed analysis of the geological characteristics of jm tunnel engineering landslide, the stability analysis of the landslide and the deformation body and the calculation of the thrust value based on the flac model were carried out on the basis of the monitoring data. after the construction of the model, the intensity parameters of the slip mass deformation and the sliding zone were obtained from the back analysis. on the basis of the above, the comprehensive treatment scheme for the rear side of the old landslide deformation body was determined, including earthwork cleanup, grading supporting, and the back pressure by abandoning the cleaned earthwork at the leading edge. the study is of great significance to the design of the construction of undercrossing tunnel of landslide under the complicated geological conditions. abstract the comprehensive treatment of the ancient landslide deformable body in the giant deep rock bedding based on the flac analysis model tratamiento integral del deslizamiento de un cuerpo deformable antiguo en un gigante lecho rocoso profundo con base en el modelo flac issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84008 estudios anteriores suelen basarse en el mecanismo de falla de un cuerpo deformable por deslizamiento de tierra en un túnel y en el monitoreo de seguridad y alerta temprana después de la finalización del túnel, mientras que hay menos investigación sobre la construcción del túnel. para estudiar mejor el tratamiento integral del cuerpo deformable de un antiguo deslizamiento de tierra en un lecho de roca profunda gigante, se seleccionó el tratamiento del cuerpo deformable de deslizamiento de túnel del túnel jm para llevar a cabo el estudio empírico. después de un análisis detallado de las consideraciones de deslizamiento por las características geológicas del túnel jm, se realizaron el análisis de estabilidad del deslizamiento de tierra y el cuerpo de deformación y el cálculo del valor de empuje basado en el modelo flac y en el monitoreo de información. después de la construcción del modelo, los parámetros de intensidad de la deformación de la masa de deslizamiento y la zona de deslizamiento se obtuvieron del análisis posterior. después de esto, se determinó el esquema de tratamiento integral para la parte posterior del antiguo cuerpo de deformación por deslizamiento de tierra, incluida la limpieza del movimiento de tierras, el soporte de nivelación y la contrapresión al abandonar el movimiento de tierra limpio en el borde delantero. el estudio es de gran importancia para el diseño de la construcción del túnel de deslizamiento de tierra bajo condiciones geológicas complicadas. resumen record manuscript received: 27/04/2019 accepted for publication: 25/09/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (january, 2020): 303-308 hongsheng zhou1,2,*, yunsheng wang1,2, tong shen1,2, qianqian feng1,2 1state key laboratory of geohazard prevention and geoenvironment protection, chengdu university of technology, chengdu, 610000, china; 2college of environment and civil engineering, chengdu university of technology, chengdu, 610000, china * corresponding author: geophysics_chow@sina.cn g eo lo g ic al e ng in ee ri ng 304 hongsheng zhou, yunsheng wang, tong shen, qianqian feng introduction the excavation of the tunnel may lead to the deformation of the original mountain, coupled with the original landslide geological conditions, the comprehensive consideration of all kinds of complex factors is necessary in the corresponding reinforcement construction (chen, xiao, sun, & fang, 2014). there are many studies on the investigation and support design of slope landslide in china, while there are few studies on the existing landslide with complicated construction conditions (liu & qian, 2014). the traditional analysis methods of slope stability are mainly qualitative analysis and quantitative analysis of the two, in which, the qualitative analysis includes the geological analysis method, the engineering analogy method and the graphic method, while the quantitative analysis method includes the limit equilibrium method, the numerical analysis method and so on (shuan-cheng, wang, wang, & he, 2015). these methods can be used to analyze and predict the process of slope landslide, but the parameters needed are many, and the calculation is large (ming-zhu, kong, & zheng 2011). to this end, the empirical study on the comprehensive treatment of the jm tunnel landslide deformable body was carries out, the relevant parameters of the landslide deformable body were calculated on the basis of the construction of the flac two-dimensional numerical analysis model and the uniform design test method (sun, wong, shang, shen, & lü, 2010). the study has certain reference significance to the construction of the modern expressway in china and the reinforcement supporting engineering of slope under the complicated geological conditions, which is helpful to promote the application of neural network model in engineering design. description of the problem jm tunnel from xy to gl expressway is divided into two tunnels, on the left and right; the tunnel passes through the landslide two times, the left hole body is 103m, the entrance section is 347m and the right hole section is 204m. the jm landslide belongs to the old landslide of the giant bedding rock, therefore, it has the characteristics of multi-sliding, which is influenced by tunnel excavation; the large sliding deformation of landslide deformable body causes significant surface cracks and tunnel cracks, as shown in figure 1. the main treatment difficulties include: (1) the geological condition is very complex, the jm landslide belongs to the multi-level sliding deformation, the landslide areas are many, and the stress change is complex; (2) the duration of construction is tight, the entire expressway from xy to gl must be opened to traffic in 2016; (3) the project cost is needed to control. therefore, it is necessary to make a comprehensive analysis of the jm expressway landslide deformable body, so as to provide the basis for the selection of the treatment plan. for the supporting scheme of landslide deformation, it is necessary to analyze the sliding surface and the sliding bed characteristics of the landslide and deformable body, and calculate the stability and the thrust value of the landslide and the deformation body, so as to provide data basis for the choice of treatment plan. china is vast in territory, and the geological conditions are complicated (xiao & huang, 2013). affected by natural factors, the main threat to road safety in mountainous areas is the landslide (zhou, zeng, & zhu, 2012). in the engineering construction of several decades, affected by the landslide, there have been many traffic disruption and engineering accidents caused by landslide disaster in china, which has caused serious economic losses to our country; this is not conducive to the construction of infrastructure in our country (zhenyu, chen, & wan, 2014). but whether it is expressway construction or railway construction will face the situation through the landslide mountain areas (song-yue, cao, hong, & zhou, 2009). in the process of slope excavation and tunnel construction, there are serious security problems because of the landslide (deng, 2014). especially in the southwest, heavy rain may further lead to instability of the slope in the rainy season, and exacerbate landslide deformation (jiang, 2010). the use of tunnel construction is to avoid the disturbance caused by the slope excavation; however, affected by natural factors and human impact of construction, it is difficult to carry on the supporting work of the slope (lin, 2011). figure 1. deformation and cracks of tunnel landslide in jimei 305the comprehensive treatment of the ancient landslide deformable body in the giant deep rock bedding based on the flac analysis model furthermore, assuming that the velocity component of node was ui, then the constant strain triangular element was obtained on the basis of the gauss’s theorem: ∂υ ∂x a u n s i i i s = ∑1 ∆ (2) the nodal velocity was used to calculate the strain increment of a time step unit: ∆ ∆e u x u x tij i j j i = 1 2 ∂ ∂ + ∂ ∂ (3) the total stress was calculated by the constitutive relation function. assuming that the stress tensor was ij, so there was: ∆ ∆ij ijf e= ( )……, (4) assuming that the unbalanced component of node in i direction was f ti ( ) at t time, so that the motion equation of node was expressed as:   u t f t m i i= ( ) (5) based on the principle of virtual work and the formula (5), formula (6) was obtained: u u f t mi t t i t t i t+ − + ∆ ∆ ∆2 2         ( ) = ∑ (6) firstly, the inverse analysis of parameters of slip soil was carried out under the natural working condition, the 6-6’ section plane in the zoneⅰ was used as the main sliding profile, and the flac two-bit numerical analysis model was established by means of the generalized engineering geological profile. at the same time, the homogeneous design test method was used to establish the computational scheme table of numerical analysis experiment homogeneous design of sliding deformation parameters and sliding zone intensive parameters (yan, guan, & xie, 2006). according to the homogeneous design test plan, the experimental parameters were substituted into the flac model, thus, the displacement values of the same position of the model and field monitoring were calculated (wang, kang, & wang, 2011). the coefficient reduction was used to obtain the parameters of the new rock and soil to carry out the cycle computing, so as to make the stability judgment. assuming that the reduction coefficient was f, the shear strength indexes were ct t, after the reduction, so there was: c c ft = / (7)  t f= ( )arctan tan / (8) the stability limit of the slope was determined by the mohr kulun yield criterion. the ratio of the maximum shear strength of slope and the actual shear stress of slope was expressed as: f c dt dll t n t t = ( )∫ ∫ +σ ϕ τ tan (9) combined with the strength factor reduction method, there was: 1 =     = ( )∫ ∫ ∫ ∫ c f f dll c dll dll n t n t+σ σ τ +σ ϕ τ tan tan (10) based on the analysis of the geological conditions of each partition, it can be seen that the whole jm landslide initially slid from the soft stratum of bedrock, and gradually disintegrated under the influence of external stress after a long period of time, which was characterized by multi-zone, multi-layer and multi-period sliding (shi, 2012). among them, there was an extrusion affected zone at the junction between the southeast of ⅰ3 and the zoneⅱ, the bedrock occurrence of the landslide was 14-55°∠33-15°, while the bedrock occurrence of slide bed in zoneⅰ3 was 14-31°∠33-35°, and the angle between the deformation direction of deformable body was 19-36°. the sliding surface slope of bedding landslide and the direction of the maximum sliding force were bedding tendency, namely 14-31°. in which, the direction of the maximum slip force was the northwest direction. the traction, extrusion and rub occurred in the east side of zoneⅰbecause of the influence of sliding body, thus forming a lateral impact area, as shown in figure 2. as a result, the surface topography in the affected area was messy, and the secondary landslide was developed. figure 2. deformation force and deformation direction of deformable body methodology in order to put forward the feasible measures for the treatment of jm tunnel landslide, it is necessary to analyze and calculate the displacement deformation value of the deformable body and tunnel, so as to ensure the safety of the whole project, and provide data support for the optimization of the scheme (xueyong, cai, zhao, & zhu, 2013). taking into account the actual situation of the project, the calculation results of special numerical analysis which are widely used in the analysis and calculation of slope and landslide of hydropower system at present were used as the main basis, at the same time, the monitoring data of the deformable body and tunnel deformation was adjusted and perfected to realize informatization construction and dynamic design (mei, peng, & xiao-hong, 2008). based on this, the flac model was used to carry on the specific analysis and research: flac finite difference mesh was used to carry on the calculation, which was divided into quadrilateral elements, each quadrilateral was made up of 2 triangular elements with constant strain. the weighted average values of all variables in a triangular element were taken as the element variable value of the quadrilateral. assuming that the integrals of the closed surface boundary and surface area of were s , a. the unit normal vector of the surface s is ni, and the increment ar length was ds. the lagrange’s method and the gauss’s theorem were used for the calculation:   f x da fn dsi sa = ∫∫ (1) 306 hongsheng zhou, yunsheng wang, tong shen, qianqian feng then the deformation displacement was used as the input variable, and the sliding deformation parameters and sliding zone intensive parameters were taken as output variables, so that the neural network rbf model was established (shuanping, 2010). the calculation model of 6-6’ profile is shown in figure 3. the homogeneous design test analytical data sample was used to train the grid, so that a neural network model was established to accurately reflect the nonlinear relationship among the deformation parameters of the sliding body, the sliding zone intensive parameters and the displacement value of the monitoring points, then the intensive parameters of slip deformation and slip zone were obtained by substituting the measured displacement values into the model. figure 3. calculation model of section 6-6 ‘section the anti-slide pile was used in the treatment scheme, and the pile element was used to simulate the anti-slide pile in the latter part of the calculation, so that the solid element was used to simulate the soil of sliding zone (liu, 2012). the shear force was generated between the constitutional unit of anti-slide pile and the surrounding rock mesh because of the slippage. assuming that the axial displacement of the pile was up, and the axial displacement of the rock and soil surface was um, and the length of the unit was l , so there was: f l cs u us sstiff p m= ( ) (11) the maximum shear force of unit length pile was: f l cs cs perimeters scoh c t sfric max tan= × ( ) ×+σ (12) the calculated results of the back analysis of parameters are shown in table 1. table 1. back analysis of parameters elastic modulus of sliding body e/ (pa) poisson’s ratio μ elastic model of slip zone e/(pa) poisson’s ratio μ slip band cohesion c/(kpa) internal friction angle of sliding belt phi(°) 7.00e+07 0.33 4.00e+07 0.35 33.2 18.6 secondly, the numerical analysis of the deformable body and tunnel was carried out, so that the rationality of the subsequent program was verified to guide the later supporting design. in this paper, the 6-6’ profile was selected to carry out the stress-deformation analysis of typical profile, and four kinds of working conditions were fully taken into account in the calculation: (1) natural + tunnel excavation + earthwork non-cleanup; (2) natural + tunnel excavation + earthwork cleanup; (3) rainstorm + tunnel excavation + earthwork cleanup; (4) rainstorm + tunnel excavation + earthwork cleanup + supporting structure. among them, for the third working condition, the strength of slip soil was reduced by 1.15 under the condition of rainstorm; for the fourth working condition, a row of anti-slide piles was arranged at the right side of the left and right tunnel hole of 10m, respectively. the calculated parameters of the model are shown in table 2: table 2. model parameter table lithology elastic modulus e/pa poisson ratio μ cohesive force c/ pa internal friction angle /° bulk density kn/m3 natural saturated slip zone 4.00e+07 0.35 30 19 2350 2400 sliding body 7.00e+07 0.33 36 28 2350 2400 bedrock 5.00e+09 0.26 6000 42 2640 primary support 9.00e+08 0.24 500 38 2500 two support 9.00e+09 0.2 6000 40 2500 results analysis through the analysis of the different working conditions of the 6-6’ profile after the tunnel excavation, the deformation of the landslide was understood, and the equivalent nephogram of landslide deformation was obtained, as shown in figure 4. as can be seen from the figure, under the condition of the natural state of tunnel excavation, the maximum deformation of slope was 57cm, the deformation mainly was concentrated in the upper part of the deformable body, the lower part would not happen (yang, peng, gong, & wu, 2012). under the condition of rainstorm + earthwork cleanup, the deformation of the slope was only about 2.3cm, and the deformation around the tunnel was very small. figure 4. equivalent nephogram of landslide deformation the maximum shear stress equivalent nephogram of the landslide is shown in figure 5. as can be seen from the figure, the maximum shear stress was mainly concentrated around the tunnel; the maximum shear stress value 307the comprehensive treatment of the ancient landslide deformable body in the giant deep rock bedding based on the flac analysis model was about 5.4mpa under the working condition of the natural + earthwork noncleanup, while the maximum shear stress value was about 2.5mpa under the working condition of rainstorm + earthwork cleanup. figure 5. the maximum shear stress equivalent nephogram of landslide through further analysis, the plastic zone distribution map of the deformation was obtained, as shown in figure 6. as can be seen from the figure, there was a shear stress strain increment zone in the upper part of landslide under natural conditions, but there was no in the lower part. it can be found that the whole slope was stable and there might be local sliding. under the working condition of rainstorm + earthwork cleanup, the strength of slip soil was reduced by 1.15 times, and the shear stress strain increment zone mainly appeared in the lower part of the landslide. thirdly, as can be seen from the analysis of the deformation and stress distribution law of tunnel, under the condition of rainstorm, the maximum shear stress around the tunnel was about 2.45mpa after the earthwork cleanup of deformable body, and the displacement value was about 5.7mm. when the anti-slide pile was applied to the periphery, the stress and displacement value of the tunnel were not changed greatly. in the previous calculations, under the condition of rainstorm, soil cohesion was reduced by 26.09kpa according to the natural condition, and the friction angle was also reduced by 16.6°. according to the previous calculation method, the 6-6’ profile was calculated. under the condition of rainstorm + earthwork cleanup, the deformation range of the landslide increased with the decrease of the cohesion of the sliding body, the corresponding displacement values of the sliding body also increased, the deformation of the left line tunnel was obvious, while the deformation of the right line tunnel was relatively small. when the soil cohesion values of sliding zone were 26.09kpa, 22.0kpa and 18.0kpa, respectively, the maximum displacement values of the corresponding slope were 2.13cm, 2.55cm and 5.38cm. the maximum shear stress increment of the landslide concentration gradually increased with the decrease of sliding soil cohesion, and enhanced with smooth consistency. it can be concluded that under the working condition of rainstorm + earthwork cleanup, the slope safety coefficient of different sliding soil cohesion of the landslide was taken, as shown in table 3. as can be seen from the table, the slope safety coefficient decreased with the decrease of sliding soil cohesion, but which was still greater than the requirements of the safety factor. table 3. the slope safety coefficient of different sliding soil cohesion of the landslide sliding soil cohesion (kpa) 26.09 22 18 safety factor 1.38 1.35 1.3 the results were calculated by using the neural network rbf model, and the inverse analysis was used to obtain the sliding soil cohesion value and the internal friction angle value under the natural working condition. according to the actual situation of the landslide, the slip soil cohesion value of zoneⅰwas 30.0kpa under the natural working condition, and the internal friction angle was 19°. under the working condition of natural + tunnel excavation, each section of the landslide body underwent significant deformation; and the deformation was concentrated in the middle and upper part of the deformable body, while the deformation of the lower part was not obvious. on the whole, jm landslide was in a relatively stable state. based on the comparison and analysis of the scheme of earthwork cleanup, the measures of earthwork cleanup for the rear part of the deformed body were determined; at the same time, taking into account the rainstorm condition, the sliding soil strength was reduced by 1.15 times, so that the overall sliding surface was not formed, which shows that the deformation induced by jm landslide and tunnel excavation was in a stable state. a row of anti-slide piles was arranged at the right side of the left and right tunnel hole of 10m, which had no effect on the deformation of the tunnel, therefore, the implementation of the anti-slide pile supporting engineering of two holes was not considered. through the comprehensive analysis, the in-situ treatment was determined, and the anti-slide pile was given priority. because the protective range of anti-slide piles on the tunnel and the top was greater than 5m, the pile top and above sliding body could be transferred to the leading edge slide to offset the sliding thrust, so that the sliding thrust of the tunnel range was considered. the residual sliding thrust values of the right side of left and right hole were calculated as 13674kn/m, 11653kn/m. therefore, multi rows of anti-slide pile needed to be set up in the upper deformation. figure 6. distribution of plastic zone of landslide body 308 hongsheng zhou, yunsheng wang, tong shen, qianqian feng conclusions because the excavation construction of jm tunnel may cause the old landslide which originally belongs to the giant bedding rock have further large sliding deformation, the cracks appeared in the tunnel and the ground surface need to be treated. in order to put forward the feasible measures for the treatment of jm landslide, it is necessary to analyze and calculate the displacement deformation value of the deformable body and tunnel, so as to ensure the security implementation of the whole treatment project and provide data support for the optimization of the scheme. the 6-6’ profile was taken as the research object, and the flac two-dimensional numerical analysis model was constructed; combined with the analytic results of uniform design test and actual monitoring displacement value, the intensity parameters of the slip mass deformation and the sliding zone were obtained by forward and reverse analysis. at the same time, under four different working conditions, the maximum displacement value of the slope obtained from different sliding soil cohesion was considered, it can be seen that the safety coefficient of slope decreased with the decrease of sliding soil cohesion, but which was still greater than the requirements of the safety coefficient. on the basis of the above analysis, the comprehensive treatment scheme of the jm tunnel landslide deformation body was proposed, including earthwork cleanup, grading supporting, and the back pressure by abandoning the cleaned earthwork at the leading edge. this study is based on the reinforcement supporting of landslide deformation as the main research object, although the mechanism of the tunnel and landslide deformation has been studied, the reinforcement supporting of the tunnel has not been studied in depth. acknowledgments this study has been supported by the funds for creative research groups of china (grant no. 41521002), the national key research and development program of china (grant no. 2017yfc1501000), the national natural science foundation of china (grant no. 41877235), and the independent fund of state key laboratory of geohazard prevention and geoenvironment protection (grant no. sklgp2015z001). the authors would like to express their gratitude for these financial supports. references chen, c., xiao, b., sun, x., & fang, g. 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(2008). analysis of reviviscence characteristic and optimal selection of treatment projects of ancient landslide body located on the exit of zhangping tunnel. communications standardization, 7. sun, h. y., wong, l. n. y., shang, y. q., shen, y. j., & lü, q. (2010). evaluation of drainage tunnel effectiveness in landslide control. landslides, 7(4), 445-454. wang, k., kang, h. g., & wang, h. t. (2011). recovery execution in landslide at the portal of highway tunnel. advanced materials research, 250253, 1823-1826. xiao, z. w. & huang, l. (2013). analysis and treatment of landslide at the tunnel portal in nanjing road. applied mechanics & materials, 353356, 686-691. xueyong, y. u., cai, q., zhao, d., & zhu, d. (2013). analysis on genesis of landslide at the exit of wusha tunnel and the treatment design. subgrade engineering, 4, 167-171. yang, x. g., peng, j. g., gong, x. w., & wu, s. (2012). comprehensive treatment on landslide at the entrance of shejiapo tunnel on xuyong-gulin railway. subgrade engineering, 2012-05. yan, l. i., guan, z. h., xie, z. w. (2006). analysis on the causes of landslide in beihai road tunnel and the plans of emergency repair. west-china exploration engineering, 18, 164-166. zhenyu, y. i., chen, h., & wan, p. (2014). discussion of treatment techhique on great collapse of hanlinjie tunnel. highway engineering, 1, 177179. zhou, k., zeng, y., & zhu, w. (2012). slope treatment of highway tunnel with water-softening surrounding rock under complex geological conditions. journal of water resources & water engineering, 23(6). keywords: fuzzy statistics; rock slope; stability; finite element analysis palabras clave: estadísticas difusas; pendiente de roca; análisis de elementos finitos. how to cite item wang, z., & lin, m. (2021). finite element analysis method of slope stability based on fuzzy statistics. earth sciences research journal, 25(1), 123-130. doi: https://doi. org/10.15446/esrj.v25n1.93320 in order to reduce the risk of slope stability evaluation due to the fuzziness of calculation parameters, a finite element analysis method of slope stability based on fuzzy statistics is proposed. based on the principle of quasi-static method and with the help of the finite element software comsol multiphysics, this paper studies the stability of the gravel soil accumulation rock slope under the action of different seismic acceleration. by analyzing the displacement, plastic zone and safety factor of the rock soil slope, the stability of the rock soil slope is analyzed. the research results show that the fuzziness of mechanical parameters of rock and soil slope will lead to the fuzziness of position displacement and stress analysis results of rock and soil slope, and the analysis of rock and soil slope with the method of fuzzy finite element analysis can strengthen the comprehensive understanding of position displacement, stress and safety of rock and soil slope by engineers and technicians, and reduce the stability of rock and soil slope due to the fuzziness of calculation parameters to a certain extent evaluate the risk qualitatively. abstract finite element analysis method of slope stability based on fuzzy statistics método de análisis de los elementos finitos para estabilidad de taludes con base en estadísticas difusas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93320 con el fin de reducir el riesgo de evaluación en la estabilidad de taludes debido a la falta de claridad de los parámetros de cálculo, en este estudio se propone un método de análisis de elementos finitos con base en estadísticas difusas. basado en el principio del método cuasi-estático y con la ayuda del software multifísico de elementos finitos comsol, este trabajo estudia la estabilidad del talud de roca con acumulación de grava en suelo bajo la acción de diferentes aceleraciones sísmicas. al analizar el desplazamiento, la zona plástica y el factor de seguridad, se analiza la estabilidad del talud del suelo rocoso. los resultados de la investigación muestran que la falta de claridad de los parámetros mecánicos de la pendiente de la roca y del suelo conducirá a la falta de claridad del desplazamiento de posición y los resultados del análisis de tensión de la pendiente de la roca y el suelo. el análisis de la pendiente de la roca y el suelo con el método de análisis difuso de elementos finitos puede fortalecer la comprensión integral del desplazamiento de posición, el estrés y la seguridad de la pendiente de la roca y el suelo por parte de ingenieros y técnicos, y reducir la estabilidad de la pendiente de la roca y el suelo debido a la falta de claridad de los parámetros de cálculo y, hasta cierto punto, evaluar el riesgo cualitativamente. resumen record manuscript received: 13/11/2019 accepted for publication: 01/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 123-130 zhongjie wang1,*, min lin2 1department of architectural engineering, yantai vocational college, yantai, 264670, china 2yantai architectural design and research co. ltd, yantai, 264000, china *corresponding author: jzgcxjyk@163.com; yt_yt@126.com; g eo te ch ni ca l en g in ee ri ng https://doi.org/10.15446/esrj.v25n1.93320 https://doi.org/10.15446/esrj.v25n1.93320 https://doi.org/10.15446/esrj.v25n1.93320 mailto:jzgcxjyk@163.com mailto:yt_yt@126.com 124 zhongjie wang, min lin introduction small and medium-sized landslides often occur in rock and soil slopes, most of which occur in a short time after heavy rain (canli et al., 2018). landslides undoubtedly pose a huge hidden danger to traffic safety. due to the lack of understanding of the characteristics of the special regional soil and the rock soil slope composed of it, the design and construction personnel are often unable to reasonably evaluate the stability of the rock soil slope and take reasonable protection and reinforcement measures (norouzi et al., 2019). therefore, it is necessary to systematically study the deformation mechanism and law, stability evaluation method, protection and reinforcement of rock and soil slope. because the parameters used in geotechnical analysis have a certain type of fuzzy uncertainty, the uncertainty or quantity is scattered. this is due to many reasons, such as test methods, etc., people can not accurately obtain the calculation mechanical parameters needed for calculation and analysis, and the value of mechanical parameters at a certain point in the calculation model can only be given a general range, for example, the measured value of friction angle or cohesive force in the rock and soil slope can never be a fixed value (tamate & hori, 2018). each experiment will lead to some data that is different from the previous values, thus showing a certain distribution in a certain range. in the past, the uncertainty (fuzziness or randomness) of each calculation parameter is eliminated according to the statistical analysis method before calculation, and a certain value is selected to evaluate the stability of rock and soil slope. the rigid body limit equilibrium method is widely used in the stability analysis of rock and soil slope. however, due to the limitations of the method itself and the uncertainty of many factors affecting the stability of the rock slope, the results of the solution are difficult to be directly used by the engineers, and its accuracy often depends on the experience of the users (karray et al., 2018). this results in a lot of phenomena that are judged to be stable in the calculation conditions, but in the actual situation, instability and failure occur. in view of this kind of problems, this paper studies the finite element analysis method of rock and soil slope stability. with the wide application of the finite element technology in the engineering field, the research on the application of the finite element method to evaluate the stability of the rock and soil slope has gradually increased. in these studies, all kinds of parameters are analyzed after being treated as deterministic parameters. the influence of the fuzzy uncertainty of the calculation mechanical parameters on the stress and displacement is also very significant, and the impact on the project cannot be ignored (naderiboldaji et al., 2018; xiao et al., 2018). due to the strong water permeability, wide particle size distribution and large pores of the accumulated gravelly soil, it is very easy to lose stability and induce landslide disaster under the action of rainfall or earthquake (chinkulkijniwat et al., 2019; provost et al., 2018). therefore, it is very important to deepen the understanding of the failure mode of the rock and soil slope and analyze the stability of the rock and soil slope for the prevention and control of the rock and soil slope disaster, which has certain research value and engineering practical significance (cao & wen, 2019). in this paper, the fuzzy finite element method is used to study the stability of rock slope. design of the finite element analysis method for the stability of rock and soil slope based on fuzzy statistics establishment of numerical model of rock and soil slope the finite element model is established based on the geotechnical slope data. the specific geometric dimensions of the geotechnical slope model are shown in figure 1. in the simulation, it is covered with gravelly soil and phyllite. model parameters: the slope height is 56m, the slope is about 19 °, the calculation area is 130m long, and the total height of the model is 75m. the model boundary is a fixed constraint, i.e. the displacement in x and y directions on the bottom boundary is 0; the left and right boundaries are roller supports, i.e. the displacement in the direction perpendicular to the boundary (normal) is zero, but the boundary can move freely in the tangent direction; the slope surface is a free boundary, and the specific boundary conditions are shown in figure 2. the parameters of rock and soil used for the stability calculation of rock and soil slope are determined according to the physical and mechanical test values provided in the rock and soil test report, as shown in table 1. table 1. rock and soil parameters material science gravels phyllite natural severe / (kn/m3) 18.4 23.5 modulus of elasticity /(mpa) 80 840 poisson’s ratio 0.35 0.24 cohesive force / (kpa) 17.6 45.00 internal friction angle / (°) 24.00 30.00 according to the statistics, the strength parameters c and  of gravelly soil on the slope, and the change interval of fuzzy statistical coefficient is 0.05, and the statistical results are shown in table 2. table 2. fuzzy statistics of strength parameters of gravelly soil fuzzy statistical coefficient cohesionc/ kpa internal friction angleψ/ (°) 1.00 17.60 24.00 1.05 16.68 22.86 1.10 16.00 21.82 1.15 15.30 20.87 1.20 14.67 20.00 1.25 14.08 19.20 1.30 13.54 18.64 1.35 13.03 17.78 1.40 12.58 17.14 1.45 12.13 16.55 1.50 11.73 16.00 figure 1. numerical model of geotechnical slope figure 2. boundary conditions of geotechnical slope 125fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment in the finite element method, mesh generation has a great influence on convergence and solution speed. the free triangulation grid provided by comsol multiphysics software (kwan et al., 2019; bouida et al., 2018) is adopted for the geotechnical slope. the whole grid consists of 11086 triangle cells, 431 edge cells, 41 top cells, 0.5314 minimum cell mass, 0.9844 average cell mass, 0.05057 cell area ratio, 6703m2 grid area, 2.247 maximum growth rate and 1.095 average growth rate. calculation of the load on the slope it is necessary to calculate the safety degree of the fuzzy point before calculating the load on the slope. the fuzzy point safety factor ks is an index used to evaluate the safety degree of each point in the slope land. to some extent, it can reflect the local stability of the slope. the fuzzy limit equilibrium method can be used to study the fuzzy evaluation of the overall stability of geotechnical slope (sajjad et al., 2018). the safety factor ks of fuzzy point is a ratio of fuzzy stress and shear strength of rock and soil. according to the mohr coulomb criterion, the calculation formula of point safety factor is: k c f s = + + + −      − σ σ σ σ α σ σ α 1 2 1 2 1 2 2 2 2 2 2 cos sin (1) α σ σ σ σ = − −      + +      −1 2 2 2 1 1 2 1 2 cos f c f (2) in the calculation of anti slope stability, because the structure of rock and soil slope has no bottom, the soil in the slope cannot participate in the anti slope work. according to the research results on the structure of rock and soil slope, 70% of the weight of soil in the slope can be taken as the effective weight of anti slope stability, then the expression of gravity g is: g g h blt i i= + ( )∑0 7. γ (3) where: gt is the weight of the slope, i and hi are the weight and depth of the i layer soil in the slope, b is the width of the simplified structure, and l is the distance between the centerline of the rock soil slope structure and the centerline of the subgrid bin. it is assumed that the earth pressure on both sides of the slope will reach the limit value when the immersed rock soil slope structure reaches the ultimate failure state. according to the analysis in 1.1, the active earth pressure on the landside of the slope is multiplied by the rankine active earth pressure resultant force on the imaginary vertical plane by the fuzzy statistical coefficient of 0.75, and the passive earth pressure on the sea side of the slope is multiplied by the rankine passive earth pressure resultant force on the imaginary vertical plane by the coefficient of 1.2. the specific expression is: f l e dya a h t = + ∫0 75 0 0 . (4) e h k c k qka i i a a a=( ) − +γ 2 (5) f l e dyp p t = ∫1 2 0 0 . (6) e h k c kp i i p p=( ) +γ 2 (7) in the formula, h is the height of the non embedded part of the slope, t0 is the depth of the slope, ea and ep are the active earth pressure and passive earth pressure values at a certain depth, fa and fp are the active and passive earth pressure resultant values of the wall, i and hi are the weight and depth of the soil in the i layer, l is the distance between the centerline of the geotechnical slope ground structure and the centerline of the sub compartment. according to the american api code, the friction force f ei d= / of soft clay to the unit area of rock soil slope is not greater than the undrained shear strength cu of soft clay; the maximum end resistance stress (soil reaction at the bottom of slope) is 9cu. assuming that when the rock soil slope structure reaches the ultimate failure state, the side friction and bottom reaction of the slope body reach the limit value, then: f l c dyu t  = ∫0 0 (8) f l c dyu h t  = + ∫0 0 (9) f c l d xr u= +( )9 β (10) among them, the value of cu is derived from the shear strength formula and c and . at a point where the depth from the ground is y, the undrained shear strength of soft clay is: c k u h cu t i i= ( ) +∑0 γ ϕtan (11) where: γ i ih( )∑ is the weight of all soil layers from above y to the ground, and the meaning of other symbols is the same as before. at the depth of slope bottom, the undrained shear strength of soft clay is: c u h cu t i i= ( ) +∑ γ ϕtan (12) where: γ i ih( )∑ is the weight of all soil layers above the slope bottom, and other symbols have the same meaning. in the formula, c is the cohesion of rock mass; f ei d= / is the friction coefficient  1 of rock;  2 is a positive value at compressive stress. finite element analysis of the stability of rock soil slope the key factor to determine the stability of rock soil slope is the embedment of foundation soil. in the limit state, it is considered that the actual shear stress produced by the external load of rock soil slope is equal to the minimum shear strength that the soil can play to resist the external load, that is, when the actual strength index of soil is reduced continuously until reaching the ultimate failure state, the shear strength of soil is equal to the external load the strength of shear stress produced by loading is equal. the ultimate failure state of rock soil slope can be simulated by weakening the strength of soil. especially for the geotechnical slope structure, the external load it bears is complex, including the wave force, the earth pressure generated by the filler behind the wall, the earth pressure generated by the variable load on the wharf surface, the residual water pressure caused by the water level difference, the stacking load, the flow mechanical load and other ground service loads (saneie et al., 2018). at the same time, in the stability analysis of geotechnical slope structure, 126 zhongjie wang, min lin the load combination under various adverse conditions must be considered. therefore, by using the fuzzy statistical method, the stability and safety factor of the structure can be obtained simply by weakening the strength parameters of the soil and simulating the instability and failure of the rock soil slope structure. in this paper, the fuzzy statistical method is used, and the large-scale finite element analysis software is used abaqus, a three-dimensional elasticplastic overall analysis model of interaction between rock soil slope structure and soil mass is established. the basic contents include: selection of model calculation domain, model boundary conditions, selection of model calculation unit, constitutive model of structure and soil mass, model of contact surface between soil mass and structure, simulation of initial stress field of foundation, etc. (kundu et al., 2018). taking the structure of rock and soil slope as an example, the typical plan and section of the structure are shown in figure 3 and figure 4 respectively. figure 3. plan of geotechnical slope structure (unit: mm) figure 4. cross section of geotechnical slope structure the structure of rock and soil slope is formed by a series of continuous arrangement of slopes, with arcade walls and obvious spatial bearing characteristics. when the action direction of wave load is perpendicular to the wall axis of the structure, the structure is symmetrical about the plane parallel to the action direction of wave force. therefore, for the convenience of analysis, it is assumed that the action direction of wave load is perpendicular to the wall axis of rock soil slope structure. using the spatial symmetry of load and structure, half of a slope and a certain range of soil before and after the wall axis are taken as the area to establish the finite element model, and symmetrical boundary conditions are set on the symmetrical boundary. among them, the calculation domain of soil mass is: in the direction perpendicular to the wall axis, take the area within 5 times of the diameter of the geotechnical slope on both sides; in the direction of the structural depth of the geotechnical slope, take the area within 3 times of the depth of the slope below the slope (chiorescu et al., 2019). the calculation domain plan of the overall model is shown in figure 5. front side wave direction right side structure of rock soil slope slope top left side rear fill 14m 26m 130m 130m calculation area of finite element model rear side figure 5. plan of finite element analysis calculation area in the setting of boundary conditions in the finite element calculation domain, the influence of boundary conditions on the finite element analysis domain should be reduced as much as possible. therefore, in the finite element analysis model of geotechnical slope structure, the basement surface is fixed boundary, the surface is free boundary, the left and right sides are symmetrical boundary, the front and rear sides are side boundary (cifuentes et al., 2019). generally, the structure of rock soil slope is made of steel. the strength and rigidity of the structure are far greater than that of the soil. the displacement and instability of the whole model system mainly depend on the deformation and bearing capacity of the foundation soil. therefore, in the overall analysis model, the slope structure is simulated by elastic model, while the constitutive model of the soil is simulated by abaqus mohr coulomb model in finite element analysis software is used for simulation (kalateh, 2019). for the three-dimensional elastic-plastic finite element model, due to the existence of soil with incompressible plastic deformation, which to some extent limits the choice of model element type. in order to avoid over constraint and reduce calculation cost as much as possible, eight node three-dimensional reduced integral solid element is used for soil and rock slope structure elements in this paper. in order to simulate the phenomenon of adhesion, slippage and separation between soil and rock slope structure under the action of wave force, residual water pressure, stacking load and other external loads, a master-slave contact surface is set up in the area where the structure contacts the soil. considering that the elastic modulus of the structure is far greater than the elastic modulus of the soil, the contact surface on the structure is designated as the main contact surface, and the contact surface on the soil is from the contact surface. the normal direction of the contact constitutive model is hard contact, and the tangential direction is coulomb friction constitutive model. according to the american api code, it is set that the friction between cohesive soil and slope wall per unit area f ei d= / is not greater than the undrained shear strength cu of clay (talesnick, 2013). according to the principle of fuzzy statistics, the stability analysis steps of rock soil slope structure under various external load combinations are as follows: step 1: establish an elasto-plastic three-dimensional finite element analysis model of interaction between rock soil slope structure and soil mass; step 2: according to the actual situation of the project, calculate the external load of the geotechnical slope structure, and according to its load distribution form, set the corresponding load distribution in the finite element analysis model to solve the stress and displacement of the soil and structure system under the design load; 127fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment step 3: the strength parameters (including cohesion c and tangent value tan  of internal friction angle) of all soils in the overall finite element analysis model are calculated step by step to solve the stress and displacement of soil and structure system under the corresponding soil strength until the finite element calculation does not converge; step 4: draw the relationship curve between the fuzzy statistical coefficient of soil strength parameters and the structural displacement; step 5: determine the criteria of structural instability; step 6: according to the criteria of structural instability, the strength reduction coefficient corresponding to structural instability is determined, which is defined as the safety coefficient of structural stability of geotechnical slope (chen et al., 2019; feng et al., 2019). comparative analysis of experiments construction of experimental model the stratum of the slope is from triassic to jurassic, in which triassic is a set of thick to very thick sandstone, mainly siltstone and fine sandstone, with weak argillaceous rock and soft hard inter bedded rock group of coal line; jurassic is a set of soft rock with hard rock group of siltstone and mudstone as main inter bedded sandstone. the artificial high rock and soil slope is 1300.0m long, with the maximum excavation elevation of 425.00m and the maximum slope height of 150.0m. in order to study the artificial high rock slope of the ship lift section on the left bank of xiangjiaba, the uncertainty analysis of plane stress and plane deformation of typical section 4 (maximum section height) is carried out by using the fuzzy finite element theory. the finite element mesh of rock slope is shown in figure 6. figure 6 finite element mesh division of rock slope the physical and mechanical parameters of rock mass are shown in table 3. the values outside the brackets in the table are calculated parameter values based on the statistical mean, while the values inside the brackets are the range values of each physical quantity. boundary conditions: the left and right boundaries of the experimental model are respectively constrained by the horizontal x-direction hinge displacement, and the lower boundary of the model is constrained by the vertical y-direction hinge displacement, forming the displacement boundary conditions. quadrilateral 4-node element is used for calculation. table 3. mechanical parameters of rock mass in slope land formation lithology t3 2-6 、 t3 4 sandstone t3 3 siltstone, siltstone with argillaceous rock argillaceous rock with sandstone strong weathering rock mass type iv iv iv f  0.75(0.62-0.81) 0.65(0.52-0.73) 0.55(0.42-0.65) c' /mpa 0.60(0.52-0.73) 0.50(0.45-0.64) 0.30(0.23-0.41) rock density /(g·cm-3) 2.50(2.44-2.61) 2.46(2.4-2.55) 2.40(2.35-2.52) modulus of elasticity / gpa 5.0(4.68-5.72) 4.2(3.78-5.1) 3.3(2.7-4.0) poisson’s ratio 0.25(0.23-0.26) 0.28(0.24-0.31) 0.30(0.25-0.32) moderate weathering rock mass type iii iii iii f  1.05(0.94-1.13) 0.95(0.89-1.02) 0.8(0.68-0.96) c' /mpa 1.0(0.88-1.11) 0.9(0.84-0.97) 0.7(0.66-0.79) rock density /(g·cm-3) 2.57(2.51-2.62) 2.58(2.40-2.67) 2.56(2.39-2.61) modulus of elasticity / gpa 12.0(11.3-13.1) 9.0(8.3-10.2) 5.0(4.6-5.3) poisson’s ratio 0.23(0.21-0.245) 0.25(0.24-0.263) 0.28(0.265-0.3) breeze rock mass type ii ii ii f  1.2(1.01-1.3) 1.0(0.85-1.17) 0.9(0.85-1.1) c' /mpaz 1.4(1.13-1.56) 1.0(0.87-1.18) 0.8(0.73-0.89) rock density /(g·cm-3) 2.60(2.55-2.75) 2.62(2.54-2.73) 2.58(2.5-2.7) modulus of elasticity / gpa 19.0(17.4-21.7) 12.0(10.2-13.7) 8.0(7.3-9.1) poisson’s ratio 0.22(0.2-0.24) 0.23(0.21-0.24) 0.25(0.23-0.27) 128 zhongjie wang, min lin load condition: self weight, excavation unloading. in order to simulate the excavation and unloading of rock and soil slope, loading is carried out in two steps. firstly, the natural rock and soil slope before excavation is discretized, and the whole model is discretized into 1372 nodes and 1294 units. after the calculation, the node forces on the excavation boundary are reversely applied on the corresponding nodes of the calculation grid after excavation to simulate the unloading process in the excavation area of rock and soil slope. after excavation, there are 1255 nodes and 1178 elements in the finite element model. during the experiment, it is considered that the displacement under the action of self weight has been completed, so the displacement caused by excavation is only the displacement caused by unloading of the slope, that is to say, when calculating the deformation caused by excavation of the slope, the displacement under the condition of self weight is deducted, and the stress state caused by self weight is retained. test steps the following simplified calculation method is established for the structure of rock and soil slope, and the solution process is realized by fortran programming language: step 1: set the stability safety factor value k required by the geotechnical slope structure; step 2: according to the basic principle of the finite element strength reduction method, the strength parameters c and  of all soils are reduced to the soil parameters c' and ' under the condition of the stability safety factor, and are substituted into the load calculation formula; step 3: combine the equation and the calculation formula of reaction fr at the bottom of slope to find the depth of rock and soil slope that meets the set safety factor: a. let x0 = 0, solve the equation, and get the initial value t0 of the depth of the slope. b. from (1) the t0 value is used to solve all the load values, and the equation is used to get the reaction value of rock soil slope; c. the fr value obtained from (2) is used to calculate the distance x between the turning point and the centerline by the following formula: f c l d xr n= +( )9 2β β (13) d. according to the relationship between x and x0 , the x0 value under the condition of meeting x obtained in (3) is obtained, which is: x d d x d x 0 2 2 = − +( ) = − β β β (14) if |x'0−x0| <  and  are given accuracy requirements, calculation will be stopped; otherwise, continue to the next step. e. let x x x 0 0 0 2 = − , solve the equation, and obtain the value t0 of the depth of the slope under this condition. continue (2) and the following steps until the end of the calculation. at this time, the calculated t0 is the depth of the rock slope to meet the set stability safety factor value, and x is the distance between the turning point of the rock slope and the central axis. analysis of experimental results a. analysis of the characteristic values of fuzzy stress and fuzzy displacement at the measuring points of geotechnical slope figure 7 is a schematic diagram of the location of geotechnical survey points. see table 4 for the characteristic value of stress and displacement at the geotechnical survey point. figure 7. location of geotechnical survey points table 4. characteristic values of stress and displacement at measuring points of geotechnical slope point number most probable value minimum value maximum value p/l h/p 1 horizontal displacement /mm -0.55 -0.44 -0.78 1.26 1.42 vertical displacement /mm -0.42 -0.34 -0.76 1.21 1.84 shear stress / kpa -2.34 0.97 -6.33 -2.41 2.71 maximum principal stress /kpa 14.71 12.66 66.73 1.16 4.53 minimum principal stress / kpa -25.7 -11.42 -84.89 2.26 3.29 2 horizontal displacement /mm 0.93 0.91 1.92 1.02 2.05 vertical displacement /mm -2.39 -2.19 -4.32 1.09 1.18 shear stress / kpa 2.20 1.44 3.84 1.53 1.74 maximum principal stress /kpa 13.30 8.30 42.99 1.60 3.23 minimum principal stress / kpa -19.55 -11.03 -64.57 1.77 3.30 3 horizontal displacement /mm 0.85 0.85 1.80 1.00 2.11 vertical displacement /mm -2.77 -2.55 -4.94 1.09 1.78 shear stress / kpa -1.79 -1.00 -3.96 1.79 2.21 maximum principal stress /kpa 10.63 7.06 32.42 1.50 3.05 minimum principal stress / kpa -20.49 -11.30 -71.76 1.81 3.50 4 horizontal displacement /mm 1.89 1.85 3.94 1.02 2.08 vertical displacement /mm -5.21 -4.68 -9.42 1.07 1.77 shear stress / kpa -16.28 -14.14 -37.89 1.15 2.33 maximum principal stress /kpa -11.41 -6.24 -129.76 1.83 11.37 minimum principal stress / kpa -58.68 -51.73 -336.01 1.13 5.73 5 horizontal displacement /mm -0.78 -0.57 -1.17 1.37 1.50 vertical displacement /mm -2.36 -2.19 -4.14 1.08 1.75 shear stress / kpa -8.18 -6.46 -19.27 1.27 2.36 maximum principal stress /kpa -28.34 -12.92 -146.71 2.19 5.18 minimum principal stress / kpa -52.18 -30.43 -240.683 1.71 4.61 129fault diagnosis mechanism of gears and bearings for coal mining equipment in a complex geological environment it can be seen from table 2 that due to the complexity of the geometric boundary of the experimental section and the fuzziness of the material, the distribution state of the earth stress and displacement of the rock slope may change greatly. when the fuzziness of materials is small, that is, the value of mechanical parameters of materials is close to the most possible value (mean value), and the deviation of calculation results is small. when the fuzziness of materials is large, that is, the value of mechanical parameters of materials is far from the most possible value (mean value), and the deviation of calculation results is large. the ratio of possible value to minimum value (p / l) and the ratio of maximum value to possible value (h / p) in table 2 show this one o’clock. table 2 shows that the deviation of the maximum principal stress can reach 11.37 times. the calculation also shows that the change of the position shift of the rock slope caused by the fuzziness of the rock slope material is smaller than the change of the earth stress of the rock slope. the ratio of the maximum to the possible value (h / p) is less than 2.0, and the ratio of the maximum to the possible value (h / p) is more than 2.0. it can be seen that the fuzziness of rock and soil slope material has a great influence on the stress estimation of rock and soil slope. b. fuzzy displacement analysis of excavated slope figure 8-11 shows the contour map for calculating the displacement of rock slope. due to the high level of membership (0.8, 0.9, 1.0), it can be found that the distribution of horizontal displacement and vertical displacement are basically the same under the condition of excavation and unloading. the horizontal displacement of the slope shows an inward movement above 320m and an outward movement below 320m. the displacement model of rock slope is suitable for the material composition of rock mass and the geometric properties of material interface. due to the formation of the free surface at the foot of the slope, the downward vertical displacement occurs after unloading. the displacement of rock and soil slope caused by excavation and unloading is small, and its magnitude is millimeter. the maximum value of vertical displacement under different membership levels appears at the top of the slope, and the value of vertical displacement gradually decreases from top to bottom. the maximum value of horizontal displacement under different levels of subordination degree generally appears on the rock slope excavated at the elevation of 450 m. conclusions in this paper, based on fuzzy statistics, the finite element analysis method of rock and soil slope stability is proposed. the two-dimensional finite element analysis method of rock slope stability is established by introducing the fuzzy statistical method. based on the principle of quasi-static method and with the help of the finite element software comsol multiphysics, the stability of rock and soil slope of gravel soil accumulation under different seismic acceleration is studied. by analyzing the displacement, plastic zone and safety factor of the rock soil slope, the stability of the rock soil slope is analyzed. the research results show that the fuzziness of mechanical parameters of rock and soil slope will lead to the fuzziness of position displacement and stress analysis results of rock and soil slope, and the analysis of rock and soil slope with the method of fuzzy finite element analysis can increase the comprehensive understanding of position displacement, stress and safety of rock and soil slope by engineers and technicians, and reduce the stability evaluation of rock and soil slope caused by the fuzziness of calculation parameters price risk. figure 8. upper limit of horizontal displacement of geotechnical slope profile 4 (μ = 0.8) figure 9. lower limit of horizontal displacement of geotechnical slope profile 4 (μ = 0.8) figure 10. lower limit of vertical displacement of geotechnical slope profile 4 (μ = 0.8) figure 11. lower limit of vertical displacement of geotechnical slope profile4 (μ = 0.8) 130 zhongjie wang, min lin references bouida, n., bouchikhi, a. s., megueni, a., & gouasmi, s. 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(2018). monitoring shear strain in shallow subsurface using mini pipe strain meter for detecting potential threat of slope failure. geotechnical testing journal, 41(2), 413-424. xiao, s., guo, w. d., & zeng, j. (2018). factor of safety of slope stability from deformation energy. canadian geotechnical journal, 55(2), 296-302. _gjdgxs the band-e narges deposit is located about 70 km northeast of the city of badrud, northern isfahan province. band-e narges ore deposit is mining for magnetite. to release valuable minerals, crushing and grinding implemented for separation ore from the gangue. magnetic separation and flotation methods for upgrading magnetite iron ore were carried out in different experimental conditions with varied parameters. the particle size of the initial content was 74 microns for flotation, and 150 microns for magnetic separation. the initial samples, with the iron grade of 43.4% and sulfur of 1.9%, are individually subjected to upgrading by floatation and magnetic separation during which the affecting parameters for each method were optimized. the improvement in the optimal condition for magnetic separation culminated in 60.85% for the iron grade, 85.21% iron for recovery and 1.08% for sulfur content. the upgrading by floatation in the optimal mode produced 60.02% iron grade, 80.41% iron recovery and 0.95% sulfur content. to determine the best method for the pre-concentration stage of ore, the content gained from each technique passed reclining for grad improvement. the final content obtained from the magnetic separation of was undergone the floatation test yielded to a content with 64.3% iron grade, 77.15% iron recovery and 0.7% sulfur content. the use of magnetic separation as a pre-concentration stage for floatation method is proposed as an economical method for improving the grade of the iron and reduce the sulfur content and to avoid the high cost of grinding, which is costly part of processing procedures. abstract keywords: magnetite iron ore; floatation; magnetic separation; reduction of processing costs. characterization of band-e narges magnetite iron ore for mineral processing issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.73438 amir pazoki1*, reza rashidi-khabir1, reza jahanian2, ali pourbahaadini2 1assistant professor, mine engineering department, lorestan university, khorramabad, iran *e-mail: pazoki.a@lu.ac.ir 2ms student, mine engineering department, lorestan university, khorramabad, iran earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 145-148 el yacimiento de hierro band-e narges se ubica a 70 kilómetros al noroeste de la ciudad de bad rud, en la norteña provincia de isfahan, irán. en este yacimiento se extrae magnetita. para la extracción de minerales valiosos, se utiliza la separación por molienda y triturado desde la ganga. se utilizaron los métodos de separación magnética y flotación con diferentes parámetros para la recuperación de la magnetita. el tamaño de la partícula inicial fue de 74 micrones en flotación y de 150 micrones en la separación magnética. las muestras iniciales, con un 43.4 % de hierro y 1.9 % de súlfuro, fueron sujetas individualmente al mejoramiento por flotación y separación magnética, proceso donde se optimizaron los parámetros afectados por cada método. el mejoramiento por la separación magnetica registró un 60.85 % de hierro, mientras que por recuperación fue 85.21 % de hierro y 1.08 % de contenido sulfúrico. el mejoramiento por flotación en condiciones óptimas produjo un 60.02 % de hierro, con un 80.41 % de hierro recuperado y un 0.95 % de contenido sulfúrico. para determinar el mejor método en la etapa de preconcentración de la mena, la ganancia de cada técnica estuvo reclinada para mejorar el contenido. el contenido final obtenido de la separación magnética se sometió a la prueba de flotación con un 64,3 % de hierro, con el 77.15 % de recuperación de hierro, y un 0.7 % de contenido sulfúrico. el uso de la separación magnética como una etapa de pre-concentración para el método de flotación se propone como un medio económico para mejorar el grado del hierro y reducir el contenido sulfúrico además de evitar el alto costo de la molienda, que es la parte más costosa de los procedimientos. resumen palabras clave: yacimiento de hierro; flotación; separación magnética; reducción de los costos de producción. record manuscript received: 18/01/2018 accepted for publication: 13/06/2018 how to cite item pazoki, a., rashidi-khabir, r., jahanian, r., & pourbahaadini, a. (2018). characterization of band-e narges magnetite iron ore for mineral processing. earth sciences research journal, 22(2), 145-148. doi: http://dx.doi.org/10.15446/esrj.v22n2.73438 o r e g e o l o g y caracterización del yacimiento de hierro band-e narges (irán) para el procesamiento mineral 146 amir pazoki, reza rashidi-khabir, reza jahanian, ali pourbahaadini introduction methods available for upgrading iron or aree physical (magnetic), physicochemical (floatation) or chemical methods, amongst which the physical methods are the most cost effective (rezaei, 1999). when the particle size of the impurities is large, upgrading is possible in larger dimensions and the cost will decrease. rock upgradation ability and upgrading costs are important factors in the evaluation of iron ore (cohen, 1986). the type of processing and its stages depend on the ore and its complexity. in general, magnetite iron ore upgrades initially include two or three stages of crushing and dry magnetic separation (svoboda and fujita, 2003). for mechanical floatation to function effectively the energy should be sufficient to meet the requirements of floatation; therefore, optimal energy use is critical. some of the energy consumed is transmitted across the cell as kinetic energy available in the vortices of various sizes, but most of it is lost through micro-interactions between the water, air and solid phases (schubert, 1999; shahbazi et al., 2015; schubert and bischofberger, 1978; ahmed and jameson, 1985; deglon et al., 1998; pyke et al., 2003; newell and grano, 2006; amini et al., 2013). after grinding to release the minerals, the valuable minerals are separated from the gangue. floatation is a commonly used separation process (chau, 2009; vaqar, 1997). magnetic separation is a physical and low-cost upgradation method for minerals having magnetic properties. the use of flotation depends on the surface characteristics of the minerals. because rock is composed of different minerals, different methods must be employed to process them according to their economic viability and in terms of the percentage of recovery and improvement in the grade. the current study was undertaken to determine the optimal state of the parameters affecting floatation and magnetic separation and the sequence of use of each method is investigated. research method the study area is the band-e narges iron ore mine located in northern isfahan province about 70 km northeast of the city of badrud. for analysis of the use of floatation and magnetic separation, the samples were crushed by jaw, cylindrical and conical crushers into dimensions of less than 2 mm. the product was then sieved with 18, 30, 50, 80, 100, 120, 150, 170, 200 and 230 mesh. representative samples were collected from each sieve class to make a smooth section. analysis of the smooth sections allowed determination of the degrees of freedom (dof) and the dispersion of mineral particles. testing was then conducted for different sizes and intensities to determine the parameters affecting magnetic and floatation separation. after determining the optimum parameters and obtaining concentrates using each method, the floatation concentrate was further processed by magnetic separation and the primary magnetic separation concentrate was further processed by floatation and the results were compared in terms of grade, recovery and sulfur content. discussion degree of freedom economic recovery of valuable minerals depends on several factors. grind ability is the most important characteristics determining the grinding mechanism, which includes crushing and smelting. one goal of these processes is the release of valuable minerals from waste material in the largest possible particle size. for present study, the samples were crushed using jaw and conical crushers to dimensions of less than 2 mm and were then run through mesh of different sizes (18, 30, 50, 80, 100, 120, 150, 170, 200, and 230). from each size of sieve, samples were collected to determine the dof and prepare smooth sections. analysis of the smooth sections showed that the particles from the 120-mesh sieve class (125-150 micron) featured the required dof of d80. analysis of the smooth sections showed that the magnetite from the 100 mesh at 150 micron had the highest amount of release (d80); therefore, this range of granulation was used for processing. the mineral ore must be upgraded after grinding in accordance with its mineralogical characteristics. however, in smooth sections at this range of granulation, there is contact between the magnet and disturbing minerals; thus, the processing orbit appropriate to the type and characteristics of minerals should be carefully selected. if the processing orbit is designed precisely, the cost and time of the process will be minimized at the operational stages. because the samples tested in this study were identified to be magnetite, a magnetic separation technique was used to upgrade it as the most cost-and time-efficient method. the magnetic properties of different materials generally depend on the amount of electrical charge and the magnetic torque (spin) of the electrons. the motion of an electron cloud around an atomic nucleus in the form of opposing orbitals creates a magnetic dipole torque. the sum of the magnetic torque generated by the spins when moving around the nucleus and the magnetic dipole torque caused by an external magnetic field gives the induction dipole torque (skoog et al., 2013). davis tube testing was used to determine the optimal dof with the highest grade at the best field intensity. it was concluded that the optimal dof for this ore occurred at a mesh size of +150, although better grades of fractions were produced at +200 and +230. these latter sizes required more grinding and more expense although the grades obtained at +150, +200 and +230 did not differ significantly. to reduce the cost of grinding and in accordance with the results of davis tube testing, the optimum dof for the ore was chosen to be that for the +150 mesh. dry magnetic separation tests particles with the dimensions of 0 to 150 micron produced the highest dof and particles with 1000 to 2000 micron in size with an intensity of 2000 gauss had the highest percentage of recovery. the difference in recovery was not very high for these two granulation ranges. in terms of grade, the grade of the particles 0 to 150 micron in size was significantly higher for those 1000 to 2000 micron in size; thus, the optimal size and field intensity were selected to be 0 to 150 micron and 2000 gauss, respectively. the grade of magnetite considered suitable for the industry is more than 60%. after one stage of magnetic separation, a grade of 54.74% was obtained with particles of 0 to 150 micron in size; thus, more than one stage of magnetic separation was required to reach the appropriate grade for the industry. because the optimal dimensions were 0 to 150 micron and optimal magnetic field intensity was 2000 gauss, further other magnetic separation was carried out for these dimensions and field intensity. in order to increase the grade, the concentrate obtained from the magnetic separation of particles 0 to 150 micron in size with a magnetic field intensity of 2000 gauss was run through two more magnetic separation stages. the results are listed in table 1. table 1. results of stages 2 and 3 of magnetic separation of 0 to 150 μm particles after three stages of magnetic separation at a 2000-gauss field intensity on particles of 0-150 micron, a product with a grade of 60.85% and a recovery of 85.21% was obtained that is usable in the industry. the sulfur content of this product was 1.08%, so chemical methods such as floatation had to be used to reduce the sulfur content. use of floatation on magnetic separation concentrate since after three stages of magnetic separation, the sulfur content was 1.08%, the concentrate was subjecting to a next grounding with 147characterization of band-e narges magnetite iron ore for mineral processing table 2. results of floatation on magnetic separation concentrate table 3. steps of magnetic separation on floatation concentrate table 4. final specifications after magnetic separation on floatation concentratea mill to reach to gain size of less than 74 micron. the flotation was carried out in three stages to reach a concentrate with an iron grade of 67.9%, recovery of 76.39% and sulfur content of 0.48%. the results are shown in table 2. as seen, after floatation on the magnetic separation concentrate, the amount of sulfur decreased to 0.48% while the iron content increased. use of magnetic separation on floatation concentrate when the floatation parameters were optimized, five stages of floatation were performed and a product with a grade of 60.02%, a recovery of 80.41% and a sulfur content of 0.95% was obtained. the concentrate was next subjected to magnetic separation. because floatation is performed only on particles of less than 74 micron, magnetic separation could only be performed on particles of that size. it was possible, however, to examine the intensity of the field at 1000, 1500 and 2000 gauss. it was determined that, for a floatation concentrate of less than 74 micron in size, the highest grade was produced at 1000 gauss; thus, three stages of magnetic separation were performed on the concentrate at 1000 gauss. the results are shown in table 3. after three stages of magnetic separation a product was attained with a grade of 64.3% and a final recovery of 77.15% iron and a sulfur content of 0.74% (table 4). conclusion the type and complexity of ore determines the type of processing used. in addition, factors such as the simplicity, cost-effectiveness, speed and level of environmental pollution of the various processes must be taken into account. the results of this study showed that the use of the magnetic separation method as a pre-concentration method for floatation is more suitable than using the flotation method as a pre-concentration method for magnetic separation. the following conclusions were drawn. 1. the most appropriate dimensions for magnetic separation are equivalent to a dof of 150 micron, while the proper dimensions for floatation are 74 micron. this indicates that the use of magnetic separation before floatation will reduce the cost of grinding. 2. the optimal magnetic field intensity for magnetic separation when it is used for pre-concentration was 2000 gauss for a particle size of up to 150 micron. when flotation was used as a pre-concentration before magnetic separation, the optimal field intensity was 1000 gauss because the particle size was a maximum of 74 micron. this occurred because, when small particles are subjected to high field intensities, they create magnetic masses from the soft particles created. these masses trap waste and reduce the grade at high intensity; thus, the intensity of the field is reduced. 3. the use of floatation method as pre-concentration for magnetic separation required a total of five stages. this increases the cost of chemicals consumed, the difficulty of the process and amount of environmental pollution produced. 4. the use of magnetic separation before floatation for preconcentration creates a product with a higher grade, recovery and sulfur content. the high sulfur content should be reduced by chemical methods such as floatation. 5. when magnetic separation is used as pre-concentration, its product has a higher grade and a lower sulfur content, which is a drawback. although the recovery was less, its high grade and low sulfur content justifies this approach. references ahmed, n., jameson, g. j. 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(2006). hydrodynamics and scale up in rushton turbine flotation cells: part 2. flotation scale-up for laboratory and pilot cells. international journal of mineral processing, ‏.65-78 ,(2)81 pyke, b., fornasiero, d., ralston, j. (2003). bubble particle 148 amir pazoki, reza rashidi-khabir, reza jahanian, ali pourbahaadini heterocoagulation under turbulent conditions. journal of colloid and interface science, 265(1), 141-151.‏ rezaei, b. (1999). mineral processing technology (magnetic upgradation). amir kabir university of technology publishing center. first edition. schubert, h. (1999). on the turbulence-controlled microprocesses in flotation machines. international journal of mineral processing, ‏.257-276 ,(1-4)56 schubert, h., bischofberger, c. (1978). on the hydrodynamics of flotation machines. international journal of mineral processing, 5(2), 131‏.142 shahbazi, b., rezai, b., koleini, s. m. j., noaparast, m. (2015). estimation of gas holdup and input power in froth flotation using artificial neural network. iranian journal of materials science and engineering, 12, 12-20.‏ skoog, d. a., west, d. m., holler, f. j., crouch, s. (2013). fundamentals of analytical chemistry. nelson education.‏ svoboda, j., fujita, t. (2003). recent developments in magnetic methods of material separation. minerals engineering, 16(9), 785-792.‏ vaqar, r. (1997). preparation of the loads of iron and steel furnaces, tehran university press. keywords: gis remote sensing technology; tourist attractions; heavy metal pollution; pollution tracing; coefficient of variation; correlation analysis; palabras clave: tecnología de teledetección sig; atracciones turísticas; contaminación por metales pesados; rastreo de contaminación; coeficiente de variación; análisis de correlación; how to cite item mo, j., tian, x., & shen, w. (2021). tracing the source of heavy metal pollution in water sources of tourist attractions based on gis remote sensing. earth sciences research journal, 25(2), 207-214. doi: https://doi.org/10.15446/ esrj.v25n2.84631 to effectively prevent heavy metal pollution in water sources in tourist attractions, clarify the degree of control of heavy metal pollution sources, and improve the accuracy of tracing results, a gis-based remote sensing method of heavy metal pollution in tourist attractions is proposed. using gis spatial analysis method, the dem elevation data monitored by remote sensing is obtained, the watershed geographic information is compiled, and the gps obtains the longitude and latitude coordinates to locate the source of heavy metal pollution. the plug-in application framework is designed, and the watershed geographic information and plug-in application framework are integrated to build the pollution tracing platform. according to the mixing direction of pollutants after entering the water source, the migration and diffusion coordinate system of heavy metal pollution in the water source is established. the spatial-temporal distribution function model of heavy metal pollutants in water sources is constructed through the migration, transformation, and concentration of heavy metal pollutants in water sources. the tracing results of heavy metal pollution in water sources of scenic spots are obtained. the results showed that the order of variation coefficient of heavy metal pollution elements was cr > cd > cu > ni > zn > pb. the spatial distribution of heavy metal pollution elements was extremely uneven. there was a certain positive correlation between ni and cr, and the correlation coefficient of cu and zn was 0.78. the positive correlation was very significant, and the homology was very strong. moreover, the identification result of the proposed method is very close to the real value, which can accurately trace the source of heavy metal pollution in the water source of tourist attractions, with small tracing error and high accuracy of tracing result evaluation. abstract tracing the source of heavy metal pollution in water sources of tourist attractions based on gis remote sensing trazabilidad de la contaminación por metales pesados en fuentes de agua de atracciones turísticas a través del sistema de información geográfica issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.84631 para prevenir eficazmente la contaminación por metales pesados en las fuentes de agua en las atracciones turísticas, aclarar el grado de control de las fuentes de contaminación por metales pesados y mejorar la precisión de los resultados de rastreo, se propone un método de detección remota basado en sig de la contaminación por metales pesados en las atracciones turísticas. utilizando el método de análisis espacial sig, se obtienen los datos de elevación dem monitoreados por teledetección, se compila la información geográfica de la cuenca y se obtienen las coordenadas de longitud y latitud mediante gps para ubicar la fuente de contaminación por metales pesados. el marco de la aplicación de complemento está diseñado, y la información geográfica de la cuenca hidrográfica y el marco de la aplicación de complemento están integrados para construir la plataforma de rastreo de la contaminación. de acuerdo con la dirección de mezcla de los contaminantes después de ingresar a la fuente de agua, se establece el sistema de coordenadas de migración y difusión de la contaminación por metales pesados en la fuente de agua. a través de la migración, transformación y concentración de contaminantes de metales pesados en la fuente de agua, se construye el modelo de función de distribución espaciotemporal de los contaminantes de metales pesados en la fuente de agua y se obtienen los resultados de rastreo de la contaminación por metales pesados en la fuente de agua de los lugares escénicos. los resultados mostraron que el orden del coeficiente de variación de los elementos contaminantes por metales pesados fue cr> cd> cu> ni> zn> pb. la distribución espacial de los elementos contaminantes por metales pesados fue extremadamente desigual. hubo cierta correlación positiva entre ni y cr, y el coeficiente de correlación de cu y zn fue de 0,78. la correlación positiva fue muy significativa y la homología fue muy fuerte. además, el resultado de la identificación del método propuesto está muy cerca del valor real, que puede rastrear con precisión la fuente de contaminación por metales pesados en la fuente de agua de las atracciones turísticas, con un pequeño error de rastreo y una alta precisión en la evaluación del resultado del rastreo. resumen record manuscript received: 10/09/2019 accepted for publication: 19/01/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 207-214 jianghong mo1, xinling tian2, wei shen2,3,* 1. school of tourism management, guilin tourism university, guilin 541006 china 2. college of tourism management, enshi polytechnic, enshi 445000 china; 3. school of business administration, zhongnan university of economics and low, wuhan 430074 china; *corresponding author: 201701080025@stu.zuel.edu.cn re m o te s en si ng https://doi.org/10.15446/esrj.v25n2.84631 https://doi.org/10.15446/esrj.v25n2.84631 https://doi.org/10.15446/esrj.v25n2.84631 mailto:201701080025@stu.zuel.edu.cn 208 jianghong mo, xinling tian, wei shen introduction at present, with the rapid development of the industrial economy and the continuous development of tourist attractions, the point pollution and surface pollution in the river basin and around the scenic spot are increasing day by day. the water source of the scenic spot is affected mainly by the precipitation and runoff process, water and soil loss in the upper reaches, and the pollution brought by the surrounding areas of the scenic spot, which makes the pollution load of the scenic spot increase sharply and the water quality of the scenic spot deteriorate (smith et al., 2018). due to the prominent problem of eutrophication of water sources in tourist attractions, people pay more attention to the study of nutrients in surface runoff, while ignoring the threat of other heavy metal substances. the potential harmfulness of heavy metal pollution has brought serious threat to the water quality of water sources in tourist attractions. the accumulation of heavy metals in water sources not only directly affect the physical and chemical properties of water sources, reduce the biological activity of water sources, hinder the effective supply of nutrients, but also enter the food chain and groundwater through plant absorption and leaching, posing a serious threat to the ecological environment and human health (liu et al., 2020). there is not only a problem of heavy metal pollution in surface water sources. there are prominent water quality problems in urban groundwater to varying degrees, and urban domestic pollution, industrial pollution, irrigation of farmland with untreated sewage and deterioration of geological conditions are the main causes of pollution (wang et al., 2018). therefore, the problem of heavy metal pollution in water sources of tourist attractions needs to be solved. in view of the prevention and control of heavy metal pollution in water sources, it is of great significance to accurately trace the source of heavy metal pollution in water sources of tourist attractions by using reasonable scientific research methods. tracing the source of heavy metal pollution in water sources of tourist attractions is to trace the sources of heavy metal pollutants in water sources of tourist attractions. it is the key technology and prerequisite for scientifically and accurately predicting the pollution of any water area. at present, great progress has been made in the study of pollution tracing. liu et al. (2019) proposed a method for tracing thallium pollution in sediments of the northern pearl river basin in south china. an improved irmm sequential extraction procedure for selected depth profiles showed that the enrichment of thallium in geochemical flow components was extremely high. the near-range quantitative source analysis using lead isotope fingerprint technology showed that along the depth profile 80%~90% of the thallium pollution is from lead-zinc smelting waste. the results emphasize the key role of smelting activities in the discharge of large amounts of geochemical flow thallium into the sediments. wang et al. (2018) proposed a method for tracing the source of heavy metal pollution in reservoirs in hilly areas of southern china, and studied the pollution characteristics, sources and health risks of soluble heavy metals in 19 representative reservoirs in hilly areas of southern china. the results show that due to the interference of human activities, the concentration of heavy metals in the studied reservoirs has great spatial heterogeneity. however, the pollution tracing error of the above methods is relatively large, and the evaluation accuracy of the tracing results is low. in view of the above problems, a method based on gis remote sensing is proposed to trace the source of heavy metal pollution in water sources of tourist attractions. using gis spatial analysis method, compiling remote sensing monitoring basin geographic information, using gps to obtain longitude and latitude coordinates, positioning source of heavy metal pollution, designing plug-in application framework, integrating watershed geographic information and plug-in application framework, and building pollution tracing platform. based on the analysis of the migration and transformation mechanism of heavy metal pollution in the water source of tourist attractions, the migration and diffusion coordinate system of heavy metal pollution in water sources is established. through the migration, transformation and concentration of heavy metal pollutants in water source, the spatial-temporal distribution function model of heavy metal pollution in water source is constructed, and the error of tracing results of heavy metal pollution in water source of tourist attractions is obtained. the identification results of the proposed method are very close to the real value, and can accurately trace the source of heavy metal pollution in the water source of scenic spots. the tracing error is small, and the evaluation accuracy of the tracing results is high. establishment of pollution tracing platform based on gis remote sensing technology gis remote sensing technology geographic information system gis is a computer technology system which takes the spatial data with geographical location attribute as the research object, takes the spatial database as the core, adopts the method of spatial analysis and spatial modeling, and provides various spatial and dynamic resources and environmental information timely. it is the product of the combination of traditional disciplines and modern science and technology, and a general technology for analyzing and processing massive spatial data (kong et al., 2018). it can be used for dynamic description of spatial information, simulate and demonstrate the development and change process of things through space-time modeling, so as to provide services for scientific research, management and decision-making. its biggest characteristic is that it can organically combine all kinds of information in social life with the graphic information reflecting geographical position, thus making it possible to solve complex spatial problems scientifically. global positioning system gps is a system that uses gps positioning satellites to locate and navigate the world in real time. it is called the global satellite positioning system. gps is a kind of satellite navigation system with omni-directional, all-weather, alltime and high-precision developed by the u.s. department of defense. it can provide global users with low-cost, high-precision three-dimensional position, speed and precise timing navigation information. it is an application model of satellite communication technology in the field of navigation. it greatly improves the information level of the earth society and effectively promotes the development of the digital economy. remote sensing technology remote sensing is a comprehensive technology of earth observation. in a broad sense, it refers to all non-contact remote detection, including the detection of electromagnetic field, force field, mechanical wave, acoustic wave, seismic wave, etc. in practical work, the detection of gravity, magnetic force, acoustic wave and seismic wave is classified as geophysical exploration, so only electromagnetic wave detection belongs to the category of remote sensing (ahmed et al., 2018). in the narrow sense, it refers to the comprehensive detection technology that the electromagnetic wave characteristics of the target are recorded from a distance without contact with the detection target by using the detection instrument, and the characteristics and changes of the object can be revealed through analysis. remote sensing technology can provide geometric and physical information of a large area and its surrounding environment in real time and quickly. the massive spectral information of ground observation provides accurate qualitative and quantitative data for target recognition and scientific law detection. platform design based on gis plug-in and application framework the construction of this platform adopts the design concept of application framework and is based on arc gis engine series products of arc gis company. the application framework has five main features: modularity, reusability, extensibility, simplicity and maintainability. the application framework can be logically divided into multiple levels and modules, so as to improve the aggregation of applications and reduce the coupling of applications. each independent module communicates and interacts through a unified pipeline or protocol. when a module changes significantly, as long as its exit and entrance remain unchanged, the effectiveness and reliability of the system will not be affected. extensibility is one of the significant characteristics of application framework, which means that the function of application framework has the ability to grow. the extensibility of framework enables us to realize different functions based on one platform and meet different functional needs. 209tracing the source of heavy metal pollution in water sources of tourist attractions based on gis remote sensing compiling watershed geographic information in order to improve the effect of trace analysis of heavy metal pollution in water sources of tourist attractions, it is necessary to use gis spatial analysis method to analyze water source direction through dem elevation data monitored by remote sensing, so as to obtain complete and detailed water source data. taking the water source basin of a tourist attraction as an example, the digital water source drainage network is extracted. the data source comes from the remote sensing elevation data obtained by the us space shuttle columbia when it takes off. the data source covers more than 80% of the world and the whole territory of china (xu et al., 2019). this article uses geographic information software for the arc gis series software released by esri, uses arc gis desktop version to process it, and stitches the two areas together to obtain a digital elevation map of a tourist attraction. use the hydrology hydrology analysis tool in spatial analyst tools in the arc gis desktop system toolbox to analyze, obtain the water source basin information of the entire tourist attraction, and complete the compilation of the basin geographic information. locate the source of heavy metal pollution in water sources in order to trace the sources of heavy metal pollution in water sources of scenic spots more quickly and conveniently, it is necessary to calibrate the data of heavy metal pollution sources in water sources of scenic spots on the basis of the compilation of watershed geographic information. the longitude and latitude coordinates obtained by gps are used to locate the source of heavy metal pollution in water sources, which provides theoretical support for the construction of water source heavy metal pollution tracing platforms. through the actual investigation, combined with the census data of pollution sources, the names and coordinates of pollution sources were investigated. by using the add xy data tool in arc gis tools, the coordinates in the map coordinate system were automatically captured, the information of pollution sources was displayed, and each pollution source was marked according to the name. the gis map built by the platform was obtained, and the location of heavy metal pollution sources in water sources was completed. design plug-in application framework because the whole pollution source tracing platform takes into account information query, pollution source tracing algorithm, geographic information positioning, decision support and other functions, its platform structure is complex and its requirements are changeable. therefore, application development is a highly targeted work. the module design, hierarchical division, language selection and system deployment of the program all depend on the specific needs of the end user. in this paper, the plug-in programming is based on the interface. the framework program is established. the attributes and methods of each excuse are analyzed one by one. after analyzing all the interfaces, the plug-in and the whole framework are linked to complete the whole framework. integrating gis plug-ins and frameworks in order to further maintain the extensibility of the plug-in framework design, the plug-in set is encapsulated. after the plug-ins are written, they are compiled into the form of dynamic link library. enter the dynamic link library file into the folder specified in the framework compilation. after the framework runs, it automatically traverses the dynamic link library plug-ins and interprets them on the framework to realize the corresponding functions. through the use of geographic information arc gis series software, the digital elevation map of a tourist attraction is obtained, and the hydrological analysis tool in arc gis is used to analyze the water source basin information of the whole scenic spot, and the geographic information of the river basin is compiled. based on the longitude and latitude coordinates obtained by gps, the source of heavy metal pollution in water source is located, and the plug-in application framework is designed. the plug-in set is encapsulated and compiled into the form of dynamic link library. the plug-in application framework is integrated to realize the construction of pollution tracing platforms. basic theory of heavy metal pollution in water sources pollution characteristics and toxicity of heavy metals in water sources heavy metals in aquatic ecosystems are distributed in various components of the system, which have an impact on the components of aquatic ecosystems. when the accumulation of heavy metals in organisms reaches a certain degree, the symptoms of injury will appear, physiological obstruction, development stagnation, and even death. at the same time, the structure and function of the entire aquatic ecosystem will be damaged or even collapsed (kang et al., 2019). the pollution characteristics are as follows: (1) trace heavy metals in natural water can cause biological poisoning. for example, the heavy metals cadmium and mercury can produce toxic effects only in the concentration range of 0.001-0.01mg/l. (2) heavy metals are enriched in organisms. the content of heavy metals in the food chain can be increased to tens of thousands of times in the last level of the food chain through the biological amplification of the food chain. therefore, heavy metals may cause greater harm to people and the environment. for example, when the concentration of mercury in seawater is 0.0001mg/l, the mercury content in plankton reaches 0.01~0.002 mg/l, that in small fish is 0.2~0.5 mg/l, and that in large fish is 1~5 mg/l. the mercury content in large fish is 10000-60000 times higher than that in seawater. (3) not only can heavy metals not be degraded by microorganisms, on the contrary, certain heavy metal elements can be converted into metal organic compounds under the action of microorganisms. for example, the methylation process of mercury is to convert inorganic mercury into organic mercury through bacterial strains. (4) there are various ways for heavy metals to enter the human body. for example, they can be ingested into the human body through breathing, drinking water, food, etc., which have toxic effects on human health. due to the cumulative effect of heavy metals in the human body, some heavy metals often take decades to realize their harmful effects on the human body (merinomascorro et al., 2018). in a word, heavy metals are potentially hazardous pollutants. the water source is a dynamic, open and complex multi-phase system. the water environment medium, water quality physical and chemical factors are easy to affect the water source. the solid-phase material contains a large number of clay minerals, organic matter, metal oxides and their hydrogen oxygenates that can adsorb and invade its interior. the harm of heavy metals to the water environment will not show in a certain period of time, but when the environmental conditions of the water source change or the accumulated amount of toxic substances exceeds its bearing capacity, all kinds of ligands in the water source will suddenly activate to form chelates and chelates, thus causing serious ecological harm. the harm of heavy metals in water can be divided into three categories: (1) it is harmful to aquatic organisms and organs. (2) algae, which are harmful to aquatic plants, play a very important role in the food chain of aquatic ecosystems. once algae absorb heavy metals, they will cause disorder of their physiological function, growth and metabolism. the decrease of cytochrome and inhibition of photosynthesis lead to cell distortion, tissue necrosis and even poisoning death, which changes the composition and species of algae in the natural environment. therefore, the sensitivity of aquatic organisms and algae is often used as a heavy metal pollution detector. (3) harmful to human health, heavy metals entering the human body are not easy to excrete and accumulate gradually, causing various and multi-level hazards to human health. its toxicological effects are mainly manifested in the influence on the normal development of the fetus, reproductive disorders, and the reduction of human quality. migration and transformation mechanism of heavy metal pollution in water source in order to effectively trace the source of heavy metal pollution in the water source of tourist attractions, it is necessary to study and analyze the migration and transformation mechanism of heavy metal pollutants in the water source of tourist attractions, master some laws of migration and diffusion 210 jianghong mo, xinling tian, wei shen of heavy metal pollutants in water sources of tourist attractions, and then, according to these laws, study and operate relevant mathematical tracing methods to realize the tracing and tracing of heavy metal pollution in water sources of tourist attractions. establishment of migration and diffusion coordinate system of heavy metal pollution in water source after a heavy metal pollution event occurs, the heavy metal pollutants in the water source need to move for a period of time before they can be completely mixed in the water source. that is, any type of pollution must go through the following three mixing stages in sequence during this period to achieve complete mixing with the water source body: (1) the mixing stage along the water depth in the water source, that is, the vertical mixing stage, has a range of influence called the near zone. the movement of pollutants in this area is particularly complex. the location of the pollution source, the form of discharge, and the hydraulics of the water body are the parameters. the main influencing factor of the mixing distance x1 in the direction of the water flow in the stage. (2) from the completion of the first stage to the fully mixed stage of the water source, that is, the mixing stage, the intensity of the pollution source, water area conditions, and hydrological factors are the main factors affecting the mixing distance x2 and concentration distribution of the water flow direction in this stage. (3) the area where the pollutants are fully mixed in the water source is the complete mixing stage, also known as the vertical mixing stage. the type and nature of the pollutants, the flow rate of the water source, and microorganisms are the main factors affecting the concentration changes in the area. it can be seen that after heavy metal pollutants enter the water source, they show vertical, horizontal and vertical mixing zones as figure 1. figure 1. various mixing stages after heavy metal pollutants enter the water source in figure 1, the migration and diffusion patterns of pollutants in each mixing region are different. in order to accurately describe the migration and diffusion of pollutants in water source, the coordinate system of pollutant migration and diffusion should be constructed according to the mixing direction of pollutants after entering the water source. for example, the direction of migration and diffusion of pollutants entering the water source in the lateral mixing zone is the x direction, that is, the x direction is defined as the direction of water flow along the water source (liu et al., 2018). the direction of migration and diffusion of pollutants in the longitudinal mixing zone is the y direction, that is, the y direction is defined as the direction perpendicular to the water surface. the direction of migration and diffusion of pollutants in the vertical mixing zone is the z direction, that is, the z direction is defined as the direction of gravity. through analysis, it is found that the pollutants are along the direction of the water flow (x direction). because the thrust of the water flow is dominant, the migration of the pollutants in this direction is mainly carried out with the water flow. in the horizontal plane perpendicular to the direction of water flow (y direction), since the concentration of pollutants has a gradient, in this direction pollutants mainly migrate from areas with high concentrations to areas with low concentrations. along the direction of gravity (z direction), due to the exchange of momentum, heat and mass between the pollutant and the water source, the migration and diffusion in this direction is more complicated, and the pollutant migration speed is affected by the size of the contact area between the pollutant and the water source. migration and transformation of heavy metal pollution in water source colloidal substances existing in natural water sources are deposited in suspended particulate matter of water sources. the main colloidal substances include organic and inorganic, including inorganic polymer compounds composed of hydrated oxides and soil minerals, and organic polymer compounds composed of organic matter, humus and protein (jia et al., 2020). colloid has a huge specific surface area. because of the surface energy and charge, a large number of ions can be adsorbed on the surface of various colloidal particles, such as pesticides and heavy metal elements in the water source, and a variety of physical and chemical reactions occur at the solid-liquid interface, which makes heavy metals transfer from aqueous phase to solid phase. therefore, the migration and transformation of heavy metal ions are closely related to the existence of colloidal substances. the migration and transformation of heavy metal ions transferred from liquid phase to solid phase water source are closely related to organic carbon, sulfide, ph, redox potential, carbonate, particle size, and primary minerals in sediment. under the influence of these conditions, it has adsorption, coordination, precipitation and dissolution with the water source and the substances in the water source to realize the transformation of heavy metal ions in the water source. therefore, heavy metal ions in the water source have complicated physical and chemical reactions. therefore, the pollutants in the water source have the characteristics of position change and attenuation reaction in various forms under the joint action of water flow, gravity and various resistances, including push flow migration, dispersion dilution and transformation and migration: (1) push flow migration: also known as flow migration, refers to the behavior of pollutants moving in the x, y, z direction under the action of the water body after entering the water body. however, this migration behavior of pollutants only changes its spatial position in the water body, and does not reduce its concentration value. in general, if the pollutants in the water source only migrate with the flow, the product of the concentration of pollutants in the water body and the flow velocity component in the x, y, z direction is equal to the pollutant migration flux in the x, y, z direction, namely: p u c p u c p u cx x y y z z  , , (1) in formula (1), px , py , pz represents the flux of pollutants in the x, y, z direction, ux , uy , uz represents the water velocity component in the x, y, z direction, and c represents the concentration of pollutants. (2) dispersion and dilution: due to the uneven flow in all directions of the water source and the large gradient of pollutants entering the water source, the pollutants entering the water source will not only migrate in the flow direction, but also have diffusion behaviors such as molecular diffusion, turbulent diffusion or shear flow dispersion (liu et al., 2019). therefore, any type of pollutant entering the water source exists dispersion dilution, that is, dispersion dilution is a basic movement form of pollutant in water source. molecular diffusion: diffusion refers to the phenomenon that pollutants migrate to homogenization due to the existence of gradient, which is a common phenomenon after pollutants enter the water body. among them, the molecular diffusion of pollutants refers to the phenomenon of random dispersion of pollutants caused by the irregular movement of pollutant molecules, that is, when the concentration of pollutants in water is not completely mixed, the pollutant molecules will move from the high concentration area to the low concentration area. therefore, the molecular diffusion process of pollutants in water source obeys fick’s first law: i e c x i e c y i e c zx m y m z m=− ∂ ∂ =− ∂ ∂ =− ∂ ∂ , , (2) 211tracing the source of heavy metal pollution in water sources of tourist attractions based on gis remote sensing in the formula (2), ix , iy , iz represents the molecular diffusion flux in the x, y, z direction, c̄ represents the concentration of heavy metal pollutants in the water source, em represents the molecular diffusion coefficient of the pollutants, and the partial derivative term on the right side of the equation represents the concentration gradient along the x, y, z direction. the negative sign indicates that the diffusion of pollutant molecules is consistent with the negative concentration gradient. turbulent diffusion: turbulence refers to the random pulsating diffusion of various physical quantities contained in any pollutant point in the water flow field with time variation. turbulent diffusion, also known as turbulent diffusion, refers to the phenomenon of pollutant migration from high concentration area to low concentration area caused by turbulent field of water body. its size is closely related to the whirling motion degree of water flow. therefore, the turbulent diffusion law of pollutants in water source can be expressed by fick’s first law: u e c x u e c y u e c zx x y y z z=− ∂ ∂ =− ∂ ∂ =− ∂ ∂ , , (3) in formula (3), ux , uy , uz represents the turbulent diffusion flux of pollutants along the x, y, z direction, and ex , ey , ez represents the turbulent diffusion coefficient of pollutants along the x, y, z direction. shear flow dispersion: the pollutants entering the flowing water body are usually transported along with the water flow, and the dispersion coefficient is closely related to the average cross-section velocity and pollutant concentration (ren et al., 2020). however, due to the influence of perimeter and water viscosity, the concentration and velocity distribution of pollutants on the same section are often non-uniform, that is, there will be shear stress and velocity gradient in the water source. similarly, the first law can be used to characterize the mass flux of pollutants produced by dispersion: o d c x o d c y o d c zx x y y z z=− ∂ ∂ =− ∂ ∂ =− ∂ ∂ , , (4) in formula (4), ox , oy , oz respectively represents the pollutant diffusion flux generated by dispersion in the x, y, z direction, and dx , dy , dz represents the dispersion coefficient along the x, y, z direction. the difference between molecular diffusion and turbulent diffusion is that only after pollutants enter the water source does the shear flow dispersion occur, and the coefficient magnitude is generally 10~104m2/s. (3) transformation and transportation: generally speaking, the pollutants that enter the water body are usually adsorbed by suspended substances such as sediment and aquatic organisms in the water body. the role of the water body makes the pollutant concentration in the water body change in time and space by means of aggregation, precipitation and resuspension, that is, the pollutant has a transformation and transportation effect in the water body type. generally, different types of pollutants will be transformed and transported in different ways after entering the water source. for example, particulate heavy metal pollutants generally move with water in the form of ions, molecules or colloids dissolved in water. dissolved heavy metal pollutants are transformed and transported through food chain. the insoluble pollutants such as na, k, ci and f can only migrate with the flow because of no transformation and attenuation. speciation and bioavailability of heavy metal pollution in water source usually, the toxicity and bioavailability of heavy metals in water sources are studied together. the toxicity of heavy metals is closely related to the speciation of heavy metals in water sources. therefore, the bioavailability of heavy metals in water sources also depends on their speciation. due to the variety of heavy metal forms, some of the heavy metals in the water source are easy to form insoluble hydroxides, carbonates and sulfides, and some of them can be transferred into the bottom water source through adsorption and polymerization with coarse particles. therefore, the existing forms of heavy metals in water sources mainly include the following two aspects: (1) the form of heavy metals in natural water and the form of heavy metals in water sources. water source water source can not only provide nutrients for aquatic organisms, but also store toxic substances. as an important part of the water environment, it is not only a carrier of water source pollutants, but also an indicator of water source pollution. (2) the environmental behavior and toxic effects of heavy metals are not only related to the content of heavy metals, but also depend on the existing forms, migration and transformation characteristics and bioavailability of heavy metals in environmental media. therefore, in the process of studying the pollution of heavy metals in water sources, simply analyzing the content of heavy metals can not truly reflect the pollution degree of heavy metals. combined with its morphological analysis, the pollution level is comprehensively considered (sun et al., 2018). through the analysis of the form of heavy metals to study its geochemical process and toxic effect has a great role in promoting, based on this to evaluate its potential migration law and environmental hazards, so that the toxicity, bioavailability, chemical activity and re migration of heavy metals can be better revealed. there are different forms of heavy metals in water sources, and their migration ability and toxic effects are quite different. exchangeable state is easily dissolved in water or exchanged by other polar ions when environmental conditions change. because of the instability of its existing form, it is an important part of water quality. carbonate bound state is especially easy to re-enter the water source when the water source environment changes. the strong oxidation of iron will be partly released into the water source due to environmental changes. the organic state is relatively stable and is not easy to be absorbed and utilized by organisms. the residual form is the most stable of all forms, and it is not easy to release into the water environment for a long time. for organisms, the toxicity of metal elements in dissolved state is the largest, while that of particulate metal is relatively small. if a kind of compound which is completely insoluble in water can be formed and firmly bound in the water source, it will be safer and more ideal. tracing method of heavy metal pollution in water source taking the heavy metal pollution incident of water sources in tourist attractions as an example, a mathematical model that can grasp the relevant information of pollution sources is established, that is, tracing the source of heavy metal pollution in water sources of tourist attractions as figure 2. figure 2. source tracing of heavy metal pollution in water sources of scenic spots if the geometry of the water source canal remains unchanged, a number of observation points qj are arranged in the calculation area [0, l], and q(t) represents the pollutant concentration distribution function observed during the t period. therefore, the tracking and tracing of such sudden water pollution is how to determine the model parameters, the location of the pollution source, the time of discharge and the intensity based on the known information. the migration and transformation equation of pollutants in sudden water pollution incidents can be approximated to: ∂ ∂ =− ∂ ∂ + ∂ ∂ ∂ ∂      + + c t u c x x c x s cλ δ (5) in formula (5), u represents the average water flow velocity of the water source,  represents the longitudinal dispersion coefficient of the water source,  represents the attenuation coefficient of the pollutant in the water source, and s represents the source term of the pollutant in the water source. 212 jianghong mo, xinling tian, wei shen if f  0 is used to represent the location of the pollution event, and the water source area [0, l] is taken as the research object, the pollutants entering the water source section will move with the flow, longitudinal dispersion and attenuation under the combined action of water flow and shear force. if it is assumed that the pollutant concentration of the upstream inflow and downstream outflow of the research object are both 0, then: c x x l c t s t t t c l t t t , , , 0 0 0 0 0 0 0 ( )= < < ( )= ( ) < < ( )= < <         (6) in formula (6), s(t) represents the intensity of pollutants entering the canal. to construct a water source heavy metal pollution tracing model based on the above formula, that is, the time and space distribution function model of heavy metal pollutants in water sources is: c x t s t x t x u t t t t , exp( )= ( ) −( ) − − −( )  −( ) − −( )   ∫0 32 1 1 4 1 1 πλ λ δ   dt (7) in addition, due to the accident site, monitoring equipment, sampling, etc., there is an observation error in the pollutant concentration observation sequence value q(t), and this error plays a decisive role in the accuracy of the tracing result. therefore, the pollutant concentration time series value of the observation point can be expressed as: q q q j mj noisy j exact j exact, , , , ,...,= + =εω 1 2 (8) in the formula (8), qj,noisy ,qj,exact represents the observation value with error and the accurate observation value,  represents the error level,  represents a set of random numbers conforming to the standard distribution, and m represents the number of observations. the error of the tracing result of heavy metal pollution in this type of water source can be expressed as: e n s ss e k t k k n = ( ) −( )  = ∑1 2 1 (9) in formula (9), es represents the error value of water source heavy metal pollution tracing results, st represents the actual value of water source heavy metal pollution tracing, se represents the tracing result of water source heavy metal pollution, and n represents the output number of water source heavy metal pollutant emission intensity and water source heavy metal pollutant concentration spatial and temporal distribution. through the above steps, according to the mixing direction of pollutants entering the water source, the migration and diffusion coordinate system of heavy metal pollution in the water source is established. through the migration, transformation and concentration of heavy metal pollutants in the water source, the spatial-temporal distribution function model of heavy metal pollutants in the water source is constructed, and the tracing error of heavy metal pollution in the water source of tourist attractions is obtained. experimental analysis overview of the study area the research area is a tourist attraction with a total water source of about 6km. the terrain belongs to the typical low mountain and hilly area. the terrain in the study area fluctuates greatly. the general trend of the terrain is high in the southeast and low in the northwest. the basic landform is erosion erosion landform, denudation landform and accumulation landform. the average altitude of the scenic spot is 1000~1500m, and the total area of the scenic spot is 16,400 km2. the climate of the tourist attraction is a typical warm temperate zone and semi-humid continental monsoon climate with four distinct seasons. summer is hot and rainy, winter is cold and dry, and spring and autumn are short. the annual average temperature is 13.6 °c, the annual frost-free period is 180~200 days, and the annual average rainfall is 624.8mm. data acquisition and processing in this study, spss13.0 was used for basic statistical analysis, gs7.0 was used to analyze the spatial variability of heavy metal content data. spatial interpolation module of arc gis desktop 9.2 software was used to make and output the distribution of heavy metal index sample points in water source, as shown in figure 3. figure 3. distribution of sample points of heavy metal index content in water sources the samples were dried in a vacuum freeze dryer. the dried samples were filtered with a 4-mesh standard screen to remove the large stones, garbage and other debris, and then sealed in a dry and clean self sealing bag. put the appropriate amount of sample into agate mortar and grind it through 100 mesh sieve. mix the screened samples evenly, put them into self sealed bags and label them. after that, the samples were placed in a refrigerator at 4 °c and kept away from light to avoid the degradation of light and microorganisms and ensure the reliability of the samples. take a proper amount of treated sample, continue to grind and make it pass through 200 mesh screen. these samples are used to determine the speciation of heavy metals. analysis of human disturbance on heavy metal elements coefficient of variation is a variable that describes the spatial distribution. through the study of the coefficient of variation, we can understand the volatility of heavy metal content changes, and then analyze the degree of heavy metal pollution by human beings. the larger the coefficient of variation is, the greater the degree of heavy metal pollution is affected by human beings. that is to say, heavy metals with obvious external pollution can be identified and analyzed. its calculation formula is as follows: c s xv d (10) in formula (10), sd represents the standard deviation of the sampled sample, and x represents the sample mean. the classification of the degree of variation is shown in table 1. 213tracing the source of heavy metal pollution in water sources of tourist attractions based on gis remote sensing table 1. classification of variation degree coefficient of variation degree of variation <15% low variation 15%~36% moderate variation >36% high variation the coefficient of variation of heavy metal elements in water sources of tourist attractions is as table 2. table 2. variation coefficient of heavy metal elements in water sources of tourist attractions heavy metal elements coefficient of variation degree of variation zn 56.24% high variation cd 96.29% high variation cu 66.54% high variation cr 16.87% moderate variation ni 43.09% high variation pb 58.35% high variation according to the data in table 2, the order of variation coefficient of heavy metal elements in water sources of tourist attractions is cd > cu > pb > zn > ni > cr. except for cr, the other metal elements are highly variable, especially cd, with a coefficient of variation of 96.29%, indicating that its spatial distribution is extremely uneven and highly interfered by human beings. the variation coefficient of heavy metal pollution elements in water sources of tourist attractions is as table 3. the order of variation coefficient of heavy metal pollution elements in water sources of tourist attractions is cr > cd > cu > ni > zn > pb. except for pb, the variation of other elements is high. the variation degree of ni and cu is 56%, while that of cr and cd is about 70.5%. therefore, ni-cu and cr-cd have certain similarities in spatial distribution. compared with the data in table 2 and table 3, the results of coefficient of variation of cr and pb are quite different. in the analysis of the coefficient of variation of heavy metal elements in the water source of tourist attractions, cr belongs to moderate variation, while in the variation coefficient table of heavy metal pollution elements in tourist attractions, it is highly variable. the coefficient of variation of heavy metals in water sources of tourist attractions is 58.35%, but the variation coefficient of heavy metals pollution in water sources of tourist attractions is the lowest, only 32.24%. the results show that the concentration of pb in the sediment of water source polluted by heavy metals in scenic spots is relatively close, and the concentration of pb does not exceed the national secondary standard and remains at a low level. it can be concluded that the discharge of heavy metal pollution in the water source of tourist attractions is not the main source of pb pollution. the reasons for the difference are: based on the principle of average distribution, the sampling results reflect the overall pollution status of water sources in a long period of time, while for the heavy metal pollution of water sources in tourist attractions, the samples are taken after field investigation. the nature of heavy metal pollution elements in water sources of tourist attractions depends on the types of pollution sources. therefore, the results of the coefficient of variation are quite different. table 3. variation coefficient of heavy metal pollution elements in water sources of tourist attractions heavy metal elements coefficient of variation degree of variation pb 32.24% moderate variation ni 56.48% high variation cu 56.87% high variation zn 46.09% high variation cr 73.62% high variation cd 68.35% high variation correlation analysis of heavy metal elements generally, there is a significant correlation between heavy metals as one of the sources of heavy metals. therefore, in order to find homologous heavy metals, it is necessary to carry out correlation analysis. the specific steps of correlation analysis are as follows: (1) perform z-score standardization on all data first to prevent interference with the analysis results due to the difference in concentration of different types of heavy metals. (2) the correlation analysis of heavy metals and organic matter content (loi) was carried out. (3) in order to ensure the accuracy of the correlation analysis results, the bilateral significance level test was carried out on the correlation analysis results. the results of correlation analysis of heavy metal elements are as table 4. the results of the significance level test are as table 5. table 4. correlation analysis results of heavy metal elements z-score (ni) z-score (cu) z-score (zn) z-score (cr) z-score(cd) z-score(loi) z-score(ni) 1.000 z-score(cu) -.045 1.000 z-score(zn) -.045 .768 1.000 z-score(cr) .517 -.287 -.405 1.000 z-score(cd) .058 .187 -.108 .385 1.000 z-score(loi) .479 .139 .167 -.175 .195 1.000 table 5. significance level test results z-score(ni) z-score(cu) z-score(zn) z-score(cr) z-score(cd) z-score(ni) z-score(cu) .872 z-score(zn) .839 .002 z-score(cr) .047 .294 .135 z-score(cd) .848 .503 .713 .165 z-score(loi) .093 .634 .578 .548 .487 214 jianghong mo, xinling tian, wei shen according to the results in table 4 and table 5, there is a certain positive correlation between ni and cr, the correlation coefficient is 0.517, and the p value in the test results is 0.048 (< 0.05), so the two are significantly positive correlation. the correlation coefficient of cu and zn is 0.78, and their significance level is 0.01 (< 0.05), which means that the positive correlation between cu and zn is extremely significant and the homology is very strong. there is a positive correlation between ni and organic matter content, but their significance level is 0.095 (> 0.05), which indicates that organic matter content has a certain influence on ni, but it is not the main influencing factor. trace error analysis of heavy metal pollution in order to verify the effectiveness of the tracing method of heavy metal pollution in tourist attractions based on gis remote sensing, it is assumed that the error follows the gaussian distribution   0.01, and the pollution source parameter g(x1, x2, s1, s2) is uniformly distributed and independent of each other. among them, x1, x2 obeys uniform distribution h(0,100), s1, s2 obeys uniform distribution h(0,10). using the proposed method, the liu et al. (2019) method and the wang et al. (2018) method for iterative calculation, the tracing errors of different methods are obtained as table 6. according to the data in table 6, the absolute error, relative error and standard deviation of the proposed method are less than 0.2%, 0.4% and 0.15 respectively, and the absolute error, relative error and standard deviation of pollution source intensity are less than 0.02%, 0.4% and 0.06 respectively, and the tracing value is in good agreement with the real value. it can be seen that compared with the liu et al. (2019) method and the wang et al. (2018) method, the tracing identification result of the proposed method is very close to the true value, and the accuracy of the prior distribution of the unknown parameter is higher under the same random sampling frequency. the tracing error is smaller, and the tracing assessment accuracy is higher. conclusion this paper proposes a method based on gis remote sensing to trace the source of heavy metal pollution in the water source of tourist attractions, which can accurately trace the source of heavy metal pollution in the water source of tourist attractions, with small tracing error and high accuracy of tracing results evaluation. however, it has not formed a clear definition of the trace source of heavy metal pollution in water sources, and lacks a set of systematic and complete tracing theory. therefore, a clear and complete tracing theory is proposed in the next research to provide more accurate theoretical and methodological support for the tracing and tracing of heavy metal pollution in water sources. references ahmed, w., lobos, a., senkbeil, j., peraud, j., gallard, j., & harwood, v.j. 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regression analysis; gnss error sources; cors; gnss palabras clave: diseño factorial; análisis de regresión; fuentes de error gnss; estaciones de referencia en funcionamiento continuo; sistemas globales de navegación satelital issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.95060 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 391-396 g eo de sy record manuscript received: 14/04/2021 accepted for publication: 03/11/2021 abstract although researchers have widely studied the analysis and modeling of error sources on global navigation satellite systems positioning, some of these errors have not been eliminated significantly. only some of the global navigation satellite system’s data are modeled. the present work was undertaken to determine the effect of different variables: season, the number of visible satellites, and dilution of precision on the efficiency of horizontal and vertical cors (continuously operating reference stations) positioning. the cors data was collected at 14 different test points during 600 epochs with 1-second intervals for this aim. factorial designs supply an efficient solution to understand the impact of several factors on a response variable. a full factorial design with three factors at two levels was applied for these purposes. the main and the interaction effects of factors were analyzed on the cors horizontal and vertical positioning. according to the full factorial design results, while all main and interaction effects of factors significantly affected the cors horizontal positioning error, some elements did not affect the cors vertical positioning error. also, the regression equations were obtained for all situations to investigate the other level of selected factors in the response variables. estudio de modelado por diseño factorial en posicionamiento de sistemas globales de navegación satelital resumen a pesar de que los investigadores han analizado y modelado ampliamente las fuentes de error de posicionamiento en los sistemas globales de navegación satelital (gnss, global navigation satellite system), algunos de estos errores no se han eliminado significativamente. y solo parte de esta información de los sistemas satelitales de navegación global ha sido modelada. este trabajo se realiza con el fin de determinar los efectos de diferentes variables: temporada, número de satélites visibles, e imprecisión en la eficiencia de posicionamiento horizontal y vertical cors (estaciones de referencia en funcionamiento continuo). la información cors se recolectó en 14 escenarios de prueba durante 600 períodos y con intervalos de un segundo para este objetivo. los diseños factoriales proveen una solución eficiente para entender el impacto de varios factores ante una respuesta variable. en este trabajo se aplicó un diseño factorial completo con tres factores en dos niveles. los efectos principales y de interacción de factores se analizaron con el posicionamiento cors horizontal y vertical. de acuerdo con los resultados del diseño factorial completo, mientras que los efectos principales y de interacción de factores afectaron significativamente el error de posicionamiento horizontal, algunos efectos principales y de interacción de factores no afectaron el posicionamiento vertical cors. también las ecuaciones de regresión se obtuvieron para todas las situaciones con el fin de investigar los otros niveles de factores seleccionados en la respuesta de variables. how to cite item ilci, v., & sisman, y. (2021). a modelling study by factorial design on gnss positioning. earth sciences research journal, 25(4), 391-396. https://doi. org/10.15446/esrj.v25n4.95060 mailto:veli.ilci@omu.edu.tr https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.95060 https://doi.org/10.15446/esrj.v25n4.95060 392 veli ilci, yasemin sisman introduction global navigation satellite systems (gnss) have been commonly used for positioning for decades. however, the positioning accuracy obtained by the gnss is prone to error sources, which can be primarily categorized as clock-related errors, signal propagation errors, system errors, and intentional errors (selective availability and signal jamming etc.) (karaim et al., 2018). the quality of the obtained positioning accuracy is expressed with two metrics. the first metric is the user equivalent range error (uere) that defines the total errors on the pseudorange. the range between satellites and the receiver is called the pseudorange due to the noise and errors. the second quality metric is the dilution of precision (dop) that specifies the effect of geometry on the relationship between positioning and measurement (chen et al., 2013; li et al., 2018; teng et al., 2015; teng & wang, 2016). accurate positioning can be obtained with the good spatial spread of the visible satellites that provide lower dop values (banerjee & bose, 1996; verma et al., 2019). dop is utilized for the optimum selection of visible satellites (teng & wang, 2014) and is categorized as the horizontal dilution of precision (hdop), vertical dilution of precision (vdop), and time dilution of precision (tdop). the combination of hdop and vdop is named position dilution of precision (pdop), and the variety of pdop and tdop is called the geometric dilution of precision (gdop). global positioning system (gps) is still the most reliable satellite-based positioning system designed to have at least six satellites at any part of the earth (busznyák et al., 2019; wang et al., 2011). however, in some environments such as urban areas, heavy tree cover, open-pit mines, etc., the desirable number of gps signals cannot reach the receiver due to the signal blockage by obstacles (alkan et al., 2015). in this case, a weak or no positioning solution can be obtained. in the last decades, multi-gnss solutions have been investigated to exceed this limitation and acquire more accurate and reliable positioning results (alkan et al., 2017). in the last years, multi-constellation solutions have been worked with gps/glonass, gps/beidou, gps/galileo, gps/ glonass/beidou, gps/glonass/galileo, and gps/glonass/ galileo/beidou in many studies. the multi-constellation positioning ensures improved positioning solutions using more visible satellites with better geometric distribution and more satellite availability (wang et al., 2019). the tropospheric and ionospheric errors are the two primary error sources related to signal propagation. the seasonal changes of atmospheric variables like temperature, water vapour, hydrostatic pressure, and humidity significantly affect these errors. the effect of seasonal variation on the gnss accuracy was analyzed by dogan et al. (2014). they concluded that the gnss positioning accuracy in summer is better than that in winter. additionally, saracoglu & sanli (2020) studied the effect of seasonal changes on gnss positioning in different world regions. they concluded that the seasonal impact on positioning accuracy changes according to climate zones. in zheng et al. (2018), zenith tropospheric delay accuracy was investigated on the gnss data. the results showed that the accuracy in winter is better than in the other seasons. many seasonal atmospheric factors affect gnss signals quality and positioning. therefore, the results of the seasonal effects on gnss positioning accuracy differ from the others. the analysis of the gnss measurements is critical in geodesy due to working conditions in the field. some experiments can be performed, and the results can be investigated to shorten the working time in the field. in the process of obtaining measurements, a lot of factors affect the gnss measurements. the experimental design methods can be used frequently to determine the significant factors on the response variable. the primary and interactive effects of dependent variables (x) can be determined using the experimental design process on the response variable (y). the statistical analysis and graphical presentation make the interpretation of the results easier. also, a suitable experimental design model will reduce the required data and spend time on the field (çoruh et al., 2012; seltman, 2018). there are different experimental designs such as full factorial, plackettburman, tagucci box–behnken design, and central composite design (gündoğdu et al., 2016). full factorial design (ffd) is a widely used procedure to determine dependent variables’ primary and interactive effect on the response variable in different levels, such as 2p, 2p-k, 3p (george et al., 2005; navidi, 2008; sisman, 2014a). the experimental design has been used in many engineering applications. geomatics engineering uses many types of application data from different sources for positioning. the size of application data has increased a lot in recent years, and therefore the analysis of application data has become more critical. experimental design is one of the analyzing methods of application data, but the experimental design is limited in geomatics. the main aim of this study is to investigate the factorial effects of the season, the number of satellites, and dop on the cors horizontal and vertical positioning errors using ffd. although there are some studies in geodesy that have investigated the factor effects on the gnss positioning (abad & suárez, 2004; ahmad, 2015; brenneman et al., 2010; cai & gao, 2007; catania et al., 2020; pirti, 2008; raghunath et al., 2011; stone & powell, 1998; svabensky & weigel, 2004; wielgosz et al., 2019; wing et al., 2008; yoshimura & hasegawa, 2003), there is no study in literature for network-real-time-kinematic (nrtk) using ffd. the factorial effects at two levels were studied using a 23 ffd in this study. the regression equation as y = f(x) was obtained from the nrtk horizontal and vertical positioning error results. the rest of this paper is organized as follows. in section 2, gnss positioning and the factorial design are explained in detail. the results and discussions are presented in section 3. finally, the conclusions are given in section 4. material and methods gnss positioning this study was conducted along a 280 km long route from samsun to the kırıkkale provinces of turkey. fourteen geodetic points were established with 20 km intervals through the course. the locations of geodetic points and the stations of the continuously operating reference stations network of turkey (called cors-tr or tusaga-aktif) around the geodetic points are shown in figure 1. figure 1. study area. tusaga-aktif delivers real-time gnss correction data to the receivers of nrtk systems 24/7 like the other countries. tusaga-aktif system was established with 146 reference stations using baselines ranging 70-100 km across turkey and northern cyprus (aykut et al., 2015; gülal et al., 2013). this system transmits the correction data with virtual reference stations (vrs), flachen korrectur parameter (fkp), and master auxiliary concept (mac) techniques in gsm, networked transport of rtcm via internet protocol (ntrip), and radio connections by radio technical commission for maritime services (rtcm) 3.0 and higher protocol (bakici et al., 2017). the obtained 3d coordinates are in the international terrestrial reference frame 1996 (itrf96) datum and 2005.0 epoch with cm-level accuracy (i̇lçi, 2019). a trimble r10 gnss receiver for gnss observations was used. the site surveys were conducted in two different periods (winter and summer) to determine the seasonal effect on gnss measurement accuracies. in winter, the site surveys were conducted on two consecutive days (december 1st and 2nd, 2017), and site surveys in summer were executed on june 2nd and 3rd, 2018. the gnss receiver was mounted on the geodetic points to determine the 393a modelling study by factorial design on gnss positioning reference position of the sites in the winter and summer periods. (soler et al., 2006) revealed that 2-hour static sessions are sufficient to obtain the desired accuracy for 280 km baseline length between the base and the rover stations. (firuzabadì & king, 2012) obtained mm level horizontal and vertical precisions where the baseline lengths were less than 200 km in 2 hours. in this study, taking into account the baseline lengths between 1 to 100 kilometres and the open sky area conditions, we determined the gnss static observation duration as 100 minutes. simultaneously, the receiver was connected to the tusagaaktif service and the cors data were collected for 10 minutes with 1-second intervals, and the elevation mask was set to 10 degrees. although the tusagaaktif service only transmits the correction data related to gps and glonass constellations, we have observed all gps, glonass, galileo, beidou, and qzss constellations at all geodetic points for further analyses. 2.2 factorial design the factorial design investigates the effect of two or more factor levels on the response variable using designed experiments. thus, the mathematical model can be obtained between the main and interactive effects of factors and response variables; also, the time, effort, and operational cost can be reduced (george et al., 2005; ismail et al., 2008; montgomery, 2001; navidi, 2008). if the factors have a main or interactive effect on the response variable in the experimental design, their results can be determined as significant. although there are several factorial design methods, the 2p ffd is the most preferred method. 2 and p show different levels and the number of factors, respectively (box et al., 2005; gygi et al., 2006; ismail et al., 2008; wu & hamada, 2009). the two aims were realized by ffd. one of them is mathematical model development between selected factors and response variables. this mathematical model is a regression equation, including main and interactive effects, given in equation (1). (1) here; b0, bi, and bij are the main and interactive effects’ coefficients; and are the factors; ε is the error of the mathematical model. the second aim of ffd is to test significance of factors on the obtained mathematical model. in this stage, the null and alternative hypotheses are established. the main and interactive effects of factors are tested according to the selected significance level using the analysis of variance (anova). more details of ffd can be found in (george et al., 2005; gygi et al., 2006; montgomery, 2001; navidi, 2008). results and discussion in this study, the season, number of satellites, and dop data were taken as factors; the cors’s horizontal and vertical positioning errors were taken as a response variable, and 23 ffd was established. the level of factors was taken as high (+1) and low (-1). the levels of the season, satellite number and dop were selected for summer as (high) and for winter as (low); >15 (high) and <15 (low); <1.4 (high) and >1.4 (low), respectively. the high (+1) and low (-1) levels of factors are shown in table 1. table 1. the levels of factors. factor low level (-1) high level (+1) season (x1) winter summer satellite number (x2) <15 >15 dop (x3) >1.4 <1.4 the design matrix of application data was taken from the data obtained from cors-tr, as explained in section 2.1 (table 2). minitab 16 statistical software was used for all the analyses of the experimental process (minitab, 2021). table 2. the design matrix of application data. run no. factor rms x1 x2 x3 horizontal errors (mm) vertical errors (mm) 1st trial 2nd trial 1st trial 2nd trial 1 winter <15 >1.4 15.2 15.3 20.6 20.8 2 summer <15 >1.4 10.1 10.2 16.5 16.7 3 winter >15 >1.4 9.11 9.6 14.3 14.6 4 summer >15 >1.4 10.4 11.0 17.0 17.9 5 winter <15 <1.4 10.1 10.3 15.3 15.7 6 summer <15 <1.4 10.3 10.5 15.8 17.5 7 winter >15 <1.4 10.5 11.1 16. 3 17.8 8 summer >15 <1.4 9.2 9.9 15.0 16.0 the descriptive statistic is a supply to understand the data by users using central tendency (mean, median, mode, quartiles) or variability (range variance, skewness, etc.) (sharma, 2019). there are several graphical methods to represent the descriptive statistic parameters (potter, 2006). the box plot analysis of application data was realized (table 3 and fig. 2). table 3. results of the descriptive statistical analysis of application data. application data mean (mm) st.dev. (mm) minimum (mm) median (mm) maximum (mm) horizontal errors 10.8 1.8 9.1 10.3 15.3 vertical errors 16.7 1.9 14.3 16.4 20.8 figure 2. the box plot of application data. the box plot represents that the min, max, q1 (%25), q2 (median), q3 (75%) and mean values of application data. firstly, the mathematical model was obtained for the application data (table 4). the significance of factors was determined using a hypothesis test. if the p-value of the factor is bigger than the significance value (selected as %5 in this study), it is decided that the factor is insignificant in the regression model. in table 4, coef. and p-values, s and r-sq represent the regression equation coefficient, the student-test value of factors, standard deviation and the ratio of explained variation to total variation, respectively. it was seen that all main and interactive factors except x1 and x1*x3 on the vertical positioning had a significant effect on the response variable. in this case, the insignificant term should be removed, and the regression coefficient determined with substantial factors. the regression equation of the response variable, for horizontal and vertical positioning, is given according to equations 2 and 3, respectively. st.dev 394 veli ilci, yasemin sisman (2) (3) the response variable can be increased or decreased according to the multiplication of coefficients and the levels of factors. since the corshor. and corsver. are supposed to decrease; this multiplication is desired to have a negative sign. in this study, the main effects should have negative signs in equation 2 and equation 3. therefore, the level factors can be taken as (+1) for equations 2 and 3 for the main effects. on the other hand, the interactive effects of factors should be considered. according to the magnitude of coefficients in equation 2, since the coefficients of x1*x2 and x2*x3 are bigger than x1 and x3 coefficients and the coefficient of x1*x2*x3 are the maxima the x1, x2, and x3 should be taken as (-1), (+1) and (-1). also, according to the magnitude of coefficients in equation 3, since the coefficients of x2*x3 is bigger than x2 and x3 coefficients and the coefficient of x1*x2*x3 is the maximum, the x1, x2, and x3 should be taken as (-1), (+1) and (-1). the r2 is the rate of explained variability and total variability and describes the goodness of fit for the model (mason et al., 2003; seltman, 2018; sisman, 2014b). in this study, r2 was equal to 98.56% and 92.91% for the cors horizontal and vertical positioning errors, respectively. this means that 98.56% and 92.91% of the application data can be explained with the obtained mathematical model. moreover, the anova test is realized in the ffd. the anova results of horizontal and vertical cors positioning errors were obtained for application data (table 5). it is decided that the effect has a significant impact on the response variable if the p-value is more powerful than the selected significance value (5%). in table 5, df, adj ss, adj ms, p-value represent the degrees of freedom, adjusted sums of squares, adjusted mean squares, the fisher-test value of factors, respectively. some graphical representations such as the normal plot of the standardized effects can also obtain the effects of factors on the response variable. the normal probability plots are used to estimate the significance of interaction effects in a factorial design (kavuri et al., 2009). the magnitude, direction, and importance of the effects can be determined using the normal probability plot of the effects. figure 3 illustrates the normal plot for application data. figure 3. normal plot for horizontal (left) and vertical (right) cors positioning. if the effects are close to the distribution fit line, they have no significant impact on the response variable. it was also seen that although all main and interactive results were significant on the cors horizontal positioning error, the x1 and x1*x3 effects were not significant on the cors vertical positioning error. the change effect of the factor level can be seen with the main effect plots. the main effect plots are given in figure 4. figure 4. main effects plot for cors horizontal (left) and vertical (right) positioning error. it was determined that the (+1) level of all factors had reduced the horizontal and vertical cors positioning errors. also, it was seen that the x1 (i.e. the seasonal effect) was not a significant effect on the response variable of the vertical component. since the effects have a high slope, it is decided that selecting factor levels is suitable for this study. the interactive effect of the factor level can be obtained from interaction plot. for this study, the interaction plots were given in figure 5. table 4. estimated effects and coefficients for cors horizontal (left) and vertical (right) positioning errors. term effect coef. p-value term effect coef. p-value constant 10.8110 0.000 constant 16.740 0.000 x1 -1.2030 -0.6015 0.000 x1 -0.392 -0.196 0.288 x2 -1.4069 -0.7035 0.000 x2 -1.259 -0.629 0.006 x3 -1.1158 -0.5579 0.000 x3 -1.140 -0.570 0.011 x1*x2 1.2480 0.6240 0.000 x1*x2 1.097 0.549 0.013 x1*x3 0.7005 0.3502 0.002 x1*x3 0.175 0.087 0.625 x2*x3 1.2840 0.6420 0.000 x2*x3 1.483 0.742 0.003 x1*x2*x3 -1.9609 -0.9804 0.000 x1*x2*x3 -2.436 -1.218 0.000 s=0.299 r-sq=98.56% s=0.688 r-sq=92.91% table 5. anova for horizontal (left) and vertical (right) cors positioning. source df adj ss adj ms p-value source df adj ss adj ms p-value x1 1 5.7893 5.7893 0.000 x1 1 0.6132 0.6132 0.288 x2 1 7.9177 7.9177 0.000 x2 1 6.3372 6.3372 0.006 x3 1 4.9800 4.9800 0.000 x3 1 5.1951 5.1951 0.011 x1*x2 1 6.2300 6.2300 0.000 x1*x2 1 4.8155 4.8155 0.013 x1*x3 1 1.9627 1.9627 0.002 x1*x3 1 0.1225 0.1225 0.625 x2*x3 1 6.5947 6.5947 0.000 x2*x3 1 8.7977 8.7977 0.003 x1*x2*x3 1 15.3799 15.3799 0.000 x1*x2*x3 1 23.7319 23.7319 0.000 395a modelling study by factorial design on gnss positioning it was seen that while only the x1*x3 interaction does not have a significant effect on the cors vertical positioning error, the other interaction effects were substantial in both the cors horizontal and vertical positioning errors. if the 2-effect factor levels have the same slope, it will not be meaningful on the result variable. the main results of the study can be summurazed as follows; • r2 values were satisfactory (98.56% for cors horizontal, 92.91% for cors vertical positioning errors). • all main and interactive effects of factors were significant on cors horizontal positioning errors at the 5% level. • season (x1) and season*dop (x1*x3) effects of factors were insignificant on cors vertical positioning errors at the 5% level. • the 3-way interaction effect season*satellite number*dop(x1*x2*x3) had the most significant coefficient on the regression models of cors horizontal and vertical positioning errors. conclusions this study aimed to investigate the effects of factors on cors positioning using statistical design experiments. for this, a 23 full-factorial design (three factors at two levels) was established to evaluate the main and interaction effects of season, the number of satellites, and dop on the cors positioning. the main conclusions for the study are shown below: • the minimum values of cors horizontal and vertical positioning errors were 9.1 mm and 14.3 mm in the case of the winter season, bigger than 15 satellite numbers, and bigger than 1.4 dop, respectively. • the maximum values of cors horizontal and vertical positioning errors were 15.3 mm and 20.8 mm, in the case of the winter season, lower than 15 satellite numbers, and bigger than 1.4 dop, respectively. • the factors levels should be taken as (+1) levels (summer, >15, and < 1.4) considering the main effects of factors. but it is seen that when the interactive effects were added to the regression model, the factors should be taken as (-1) levels for season and dop, (+1) level for the number of satellites. • the value of corshor. was calculated 9.6 mm with (+1) levels (summer, >15 and <1.4) of factors. but the value of corshor. was calculated 9.4 mm with (-1) levels (winter and >1.4) of seasons and dop and (+1) level (>15) of the number of satellites because of the interactive effects of factors on the response variable. • while the value of corsver. is calculated 15.6 mm (+1) levels (summer, >15 and <1.4) of factors. but the value of corshor. was calculated 14.1 mm with (-1) levels (winter and >1.4) of seasons and dop and (+1) level (>15) of the number of satellites. • in the light of these explanations, it can be said that the main and interactive effect of the number of satellites was always (+1) levels (>15); the other factor levels (seasons and dop) can be changed for application data. many different application data sources must be analyzed attentively in applied sciences such as geomatics engineering. it is seen that the experimental design is quite helpful and practical for positioning applications, as it demonstrates the factor effects on positioning error. the statistical analysis and graphical presentation of the experimental setup are easier to understand the application data. also, the regression equations derived from the experimental design allow investigating different levels of selected factors on the corshor. and corsver. easily. it is suggested that the experimental design studies must be carried out in other areas of geomatic engineering such as remote sensing, photogrammetry and surveying. references abad, p., & suárez, j. p. 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(2019). characteristics and dolomitization of the lower qiulitage group reservoir in bachu uplift, tarim basin, china. earth sciences research journal, 23(3), 199-208. doi: https://doi.org/10.15446/esrj.v23n3.81914 the characteristics of the lower qiulitage group in bachu uplift, which originated in the upper cambrian period, are not clear. based on four core observations, identification of 40 thin sections, and geochemical analysis of samples, this study concludes that crystal dolomite reservoirs developed in the lower qiulitage group in bachu uplift. intercrystalline pores and dissolved pores formed the main reservoir space, while dissolution pores and fractures were developed in the middle and bottom part of the formation. the reservoir features low porosity and low permeability, and the finemedium crystalline dolomite reservoir has the best pore structure. according to a petrochemical analysis, the lower qiulitage group reservoir experienced intense dolomitization, with slightly higher content of trace elements mn and sr and negative oxygen isotropy. tectonic hydrothermal dolomitization developed locally under the influence of base faults, and vug pores were filled with saddle dolomites and siliceous filling. the presence of rare earth elements ce and eu presented positive anomaly characteristics. it suggested that the co2 acid dissolution caused by hydrothermal fluids may be a new mechanism for the increase in pores in deeply buried reservoirs, which is worth further study. abstract characteristics and dolomitization of the lower qiulitage group reservoir in bachu uplift, tarim basin, china características y dolomitización del reservorio del grupo qiulitage inferior en el levantamiento de bachu, cuenca de tarim, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.81914 las características del grupo qiulitage inferior en el levantamiento de bachu, que se originó en el período cámbrico superior, no están claras. con base en cuatro observaciones centrales, la identificación de 40 secciones delgadas y el análisis geoquímico de muestras, este estudio concluye que los reservorios de dolomita de cristal se desarrollaron en el levantamiento del grupo qiulitage inferior en bachu. los poros intercristalinos y los poros disueltos formaron el espacio del depósito principal, mientras que los poros de disolución y las fracturas se desarrollaron en la parte media e inferior de la formación. el reservorio presenta baja porosidad y baja permeabilidad, y el reservorio de dolomita cristalina fina-media tiene la mejor estructura de poros. según un análisis petroquímico, el reservorio del grupo qiulitage inferior experimentó una intensa dolomitización, con un contenido ligeramente mayor de oligoelementos mn y sr e isotropía de oxígeno negativa. la dolomitización hidrotermal tectónica se desarrolló localmente bajo la influencia de fallas de base, y los poros vug se llenaron con dolomitas de silla y relleno silíceo. la presencia de elementos de tierras raras ce y eu presentó características de anomalías positivas. esto sugirió que la disolución de ácido de co2 causada por los fluidos hidrotermales puede ser un nuevo mecanismo para el aumento de poros en depósitos profundamente enterrados, lo que merece un estudio más profundo. resumen record manuscript received: 10/04/2019 accepted for publication: 24/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 199-208 bin li1,2,*, qingsong xia1,2, jun peng1,2, suju yang3, qingqi xu3, lei zeng1 1school of geoscience and technology, southwest petroleum university, chengdu 610500, china. 2state key laboratory of oil and gas reservoir geology and exploration, southwest petroleum university, chengdu 610500, china. 3design and planning institute of sinopec northwest company, urumqi, xinjiang 830011, china. * corresponding author: lbin@swpu.edu.cn g eo lo g y 200 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu introduction with the development of oil and gas exploration in the tarim basin, researchers have increasingly focused on the prospect of dolostone in the upper cambrian salt in the bachu uplift (he, 2013; yang, liu, tian, wanbin & zhu, 2011; jiao et al., 2011; cai, li, quian & zhang, 2010; chen, wang, meng & zheng, 2002) . many gas logging anomalies appeared in the ht1, bt5, mb1, and t1 wells drilled by the lower qiulitage group in the upper cambrian system (fig. 1), indicating that the lower qiulitage group reservoir could be well developed (huang, jiang & ma, 2014; zhu, meng, jin, liu & hu, 2015; cai, mao, qian, chen & you, 2009). but, due to the limitation of burial depth and exploration conditions, and to the lack of drilling data, the reservoir characteristics and genesis of the lower qiulitage group were doubtful, which restricts further explorations of the upper cambrian. in this paper, the reservoir characteristics and genetic model of the lower qiulitage group are studied through core observation and petrochemical analysis of the upper cambrian samples, to provide theoretical support for further risk exploration in the bachu uplift. figure 1. distribution of tarim basin well locations in bachu area. reservoir properties petrologic features based on core observation of bt5, h1, mb1, and t1 in the bachu uplift, combined with identification of 40 thin rock sections (table 1), it was found that the crystalline dolomite of the lower qiulitage group was developed with an average dolomite content of 72.3-82.2%, followed by micrite dolomite. residual granular dolostone developed relatively less in the formation, with content between 1.8% and 13.5%. the particles’ residual structure was clearly observed under the microscope due to the incomplete dolomitization and recrystallization in the bachu uplift (fig. 2a). some intergranular and vug pores were filled with anhydrite, secondary quartz mineral, and pyrites; the content of anhydrite was between 0.5% and 0.9%, the secondary quartz was 0.2%, and pyrite was between 0.67% and 2.6%. crystal dolomite the crystalline dolomites were mainly formed by metasomatism or recrystallization. because the original rocks have undergone thorough dolomitization, it was difficult to recognize their original structure (liu et al., 2010). the cambrian crystalline dolomites in bachu uplift are characterized by silty crystalline, fine crystalline, and a med-coarse crystalline structure (fig. 2), as the distribution of silty dolomite in bachu is relatively low, mainly in the eastern area. in the mb1 well and h1 well, dolomite crystal is in its anhedral-subhedral form, with a muddy crystal surface. the crystal dolomites are in mosaic contact, and the intercrystalline pores and dissolved pores are not developed. the surface porosity is low (fig. 3a), and a number of fractures and stylolites were developed. the fracture was mostly filled with dolomite and organic matter (fig. 3b), and needle pores were formed by later dissolution on the core. figure 2. distribution of dolomite in different crystal types of lower qiulitage group. fine dolomites are widely distributed in the bachu uplift, mostly in anhedral-subhedral form. the crystal planes are turbid, and the crystals often appear with a concave-convex contact. some of them can be seen in the foggy center with a bright edge or dolomite ring structure, and occasionally with a residual grain phantom structure. the original intercrystalline pores and fractures were filled with organic matter (fig. 3c). the fine dolomite with higher euhedral was distributed in the dissolution pores or at the edge of the fracture, and the intercrystalline pores and dissolution pores are relati vely developed (fig. 3d). the medium-coarse crystalline dolomites were mainly developed in well t1 and bt5 in the west bachu uplift, mostly with anhedral-subhedral form. the crystal surface is turbid, and the intercrystalline pores and intercrystalline dissolved pores are not developed (fig. 3e). the medium-coarse dolomites were greatly affected by the later dissolution, and plenty of large dissolved pores were formed (fig. 3f). the fine dolomite with higher euhedral was distributed in the dissolution pores, or at the edge of the fracture, and the intercrystalline pores and dissolved pores are relatively developed. where the dissolution was intense, it can be seen that the dolomite crystals are partially dissolved to form intragranular dissolved pores. on the whole, the dolostone has obvious differences in distribution. in the west bachu uplift, the fine and med-coarse dolomite are dominant, while the silty and fine dolomite are developed in the east. residual granular dolomite the granulated dolomites in bachu are mainly composed of sand debris, gravel, and oolitic grains (figs. 3g, 3h). some particles are mainly composed of mud microcrystalline dolomite, which was rich in organic matter. some particles recrystallized to form silty dolomite. the crystal grains are larger, the outline of the particles is not clear, and the remaining particles retain the original structure. micrite dolomite the micrite dolomite was relatively developed, and the lamellar structure and birds-eye structure are both visible. a small amount of fractures and stylolite was developed, with dissolved vugs mostly filled with dolomite, calcite, and anhydrite. the dissolved pores were developed, but most of them were filled or semi-filled later. the crystal dolomites had mosaic contact, and the porosity and permeability are very low. porosity and permeability the reservoir property analysis of 26 samples from the lower qiulitage group showed that the porosity ranged from 0.14% to 6.7% with an average value of 2.28% (fig. 4a). the permeability ranged from (0.01-4.5)10-3m2, with an average value of 1.1210-3m2, so it was defined as a low porosity and low 201study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china table 1. statistical table of reservoir composition of lower qiulitage group in bachu uplift well bt5 ht1 mb1 t1 depth (m) 5043.39 5788.30 6257.75 6437.95 6158.87 6184.21 3173.01 main constituents av. max. av. max. av. max. extrabasinal constituents 1.8 4.3 2.2 5.7 13.5 47.0 8.7 intrabasinal constituents 8.4 71.0 37.4 77.7 37.2 65.0 0.0 primary carbonate constituents 8.1 71.0 37.3 77.7 37.2 65.0 0.0 anhydrite 0.5 4.3 9.3 61.7 0.8 2.3 0.0 dolomite replacing primary constituents 80.2 94.7 73.7 97.3 66.4 88.7 80.0 radial dolomite replacing dolomite 0.3 8.0 0.0 0.0 0.0 0.0 2.3 saddle dolomite replacing dolomite 10.8 62.0 6.8 29.0 4.9 9.7 58.0 dolomite pore-filling 1.4 12.0 0.9 9.3 4.4 22.0 0.0 total dolomite 82.2 97.3 74.6 97.3 72.3 88.7 82.3 total quartz 0.5 4.7 0.2 1.7 1.1 5.3 0.0 total pyrite 1.2 4.3 1.5 5 2.6 7.3 0.67 intragranular pore 0.6 2.0 0.6 7.0 0.0 0.0 7.7 intercrystalline pores 4.8 14.3 1.8 12.7 6.1 16.1 0.0 fracture pores 0.6 3.7 2.8 13.0 1.0 3.0 0.3 vug pores 1.3 9.7 1.0 3.0 1.8 4.3 0.0 total porosity 7.3 21.0 6.2 14.3 8.9 21.7 8.0 dolomite crystal size mode(mm) 0.18 0.40 0.05 0.10 0.07 0.20 0.60 sample numbers 19 13 7 1 figure 3. microscopic characteristics of crystalline dolostone of lower qiulitage group a. residual granular dolostone, mb1, 5632m, 3ql, plane polarized light, ×50; b. silty dolostone, bt5, 4743.37m, 3ql, plane polarized light, ×100; c. fine dolostone, dolomite was with euhedral, intergranular pores filled by organic matter, mb1, 530m, 3ql, plane polarized light, ×100; d. fine-middle dolomite, dissolved in beaded, bt5, 4812.9m, 3ql, blue cast, plane polarized light, ×50; e. med-coarse dolostone, vug pore, and intergranular pore developed, bt5, 3ql, 4812.9m, blue cast, plane polarized light; f. med-coarse dolostone, a small amount of unfilled intragranular and intergranular pore developed, bt5, 3ql, 4812.9m, blue cast, plane polarized light, ×50; g. residual debris dolostone, showing a large amount of residual particle structure, intergranular pores and fractures filled with organic matter, ht1, 3ql, 5816.8m, plane polarized light, ×50; h. residual debris dolomite with a large amount of the residual particle structure, bt5, 4739.32m, 3ql, plane polarized light, ×50; i. micrite dolostone and fracture filled with dolomites, mb1, 5658m, 3ql, plane polarized light, ×50) 202 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu permeability reservoir. reservoir heterogeneity was strong and the correlation between porosity and permeability is not obvious. the fine dolomite reservoir had higher porosity and a better pore-permeability relationship according to the crystal type (fig. 4b). reservoir space microscopic analysis demonstrated that the reservoir space was of various types, and that the intercrystalline pores and the intercrystalline dissolved pores were relatively developed (figs. 5a, 5b, 5c). the morphology of the intercrystalline dissolved pores are irregular, with many harbors, and some of the pores and vugs were filled with quartz and dolomite (figs. 5d, 5e, 5f). the scale of intracrystalline dissolved pores was small, and the contribution to the reservoir space was limited. a large number of dissolution pores were developed in the middle and lower sections of the lower qiulitage group and showed in beaded and honeycomb form on the core. the pore sizes were different, mostly 0.1-2mm, in various states such as unfilled, semi-filled, and fully filled. unfilled dissolution holes are the main reservoir space (figs. 5d, 5f). the statistics of the dissolved pores of the lower qiulitage group showed that the distribution of reservoir space was heterogeneous (fig. 6), mainly composed of dissolved pores and fractures. some of the pores visible on the core were filled with dolomites and calcite, mainly semi-filled vugs. the wells of fang1 and ht1 are characterized by dissolved pores and mainly dominated by small vugs. discussion dolomitization the dolomite genesis is an important part of the study of dolomite reservoirs (zhu, zhang, xie, huang & he, 2011; han, li & shen, 2017; liu, huang, wang & shi, 2016). the reservoirs of the lower qiulitage group in the bachu uplift were greatly modified by the dolomitization, with complex causes (peng, cao, li, xia & liu, 2018). according to the petrological characteristics and geochemical test results (huang, liu, ye & li, 2013; zhang et al., 2009; chen, zhou & yang, 2009), the characteristics of buried and tectonic hydrothermal dolomitization of the lower qiulitage group dolomite are more obvious. figure 4. diagram of porosity-permeability (a) and surface porosity (b) of the lower qiulitage group figure 5. reservoir types of the lower qiulitage group of the bachu uplift a. shallow gray silty dolostone with dissolved pore and fracture, bt5, 4811.61 -4811.71m,3ql; b. gray fine dolostone with three very thin horizontal fractures. t1, 3174.2m, 3ql. c. gravel-bearing dolostone with small dissolved pores along the thin fracture. ht1, 6163.85, 3ql. d. med-coarse dolostone with dissolved pore semi-filled with pyrite. bt5, 4739.88m, 3ql, plane polarized light, ×10. e. med-coarse dolostone with intragranular and intercrystalline pore filled with quartz. bt5, 3ql, 3173.01m, cross-polarized light, ×5. f. intragranular and intercrystalline pore developed, semi-filled with late dolomites. bt5, 6162.45m, 3ql,×10.) 203study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china buried dolomitization the crystal structure of the lower qiulitage group and the residual granular dolomite are relatively developed. the dolomite grains mostly appeared as anhedral-subhedral with a foggy heart and bright edge structure, and the grain phantoms are more obvious (fig. 7a). under cathode luminescence, the matrix dolomite shows a dark red light, while the dolomite ring and spray cement shows a bright red color (figs. 7b, 7c) with high luminescence intensity, indicating that there is a certain difference between late diagenetic fluid and the early stage. trace element analysis showed that the mn content of silty and finemedium dolomites was slightly higher than that of micrite dolomite, the sr content was significantly lower (fig. 8a), and the fe and mn contents were increased (fig. 8b). this indicates that the dolomite was formed in the burial environment, proving that they were formed in a shallow burial diagenetic stage. carbon and oxygen isotope measurements show that the carbon-oxygen intragranular intercrystalline fracture vug total porosity 10 9 8 7 6 5 4 3 2 1 0 su rf ac e po po si ty (% ) bt5 ht1 mb1 t1 figure 6. histogram of the pore distribution of lower qiulitage group cores figure 7. cathode luminescence characteristics of bottom qiulitage group (a, d. med-coarse dolostone with developed intergranular pores, vugs filled with dolomites and quartz, and dolomite with purple light, bt5, 3ql, 4809.09m, cathode luminescence, ×40. b, e. fine dolostone with intergranular pores and dolomite with dark red light, bt5, 3ql, 4806.85m, cathode luminescence, × 40. c, f. vug pore filled with medium dolomites with bright red light, with obvious ring texture, bt5, 3ql, 4806.85m, cathode luminescence, × 40.) 204 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu isotopes of various crystalline dolomites have no distinct differentiation (fig. 9a), and the whole has a negative δ13c value bias and a δ18o value greater than -9‰, close to that of paleo-seawater (zhou et al., 2012). this is combined with the empirical formula proposed by keith and weber (keith & weber, 1964): z a c b o= × +( ) + × +( ) 13 1850 50 (13c, δ18o belong to the pdb standard; a=2.048, b=0.489; z120 is marine facies, z<120 is freshwater facies) the z values of the residual dolostone and fine-med dolomite in the study area are both greater than 120, indicating that the dolomite of the lower qiulitage group was mainly influenced by marine fluid in the early diagenesis. strontium isotope analysis shows that the 87sr/86sr value of mud microcrystalline dolomite and residual granular dolomite is similar to that of cambrian seawater (fig. 9b). due to the influence of the diagenetic modification intensity, the 87sr/86sr value of silty dolomite is obviously higher than that of cambrian sea water. it is indicated that the dolomitization fluid was residual seawater during the shallow burial period. hydrothermal dolomitization in recent years, hydrothermal dolomitization has attracted more and more attention, and its impact on reservoirs has been increasingly valued (boni, parente, bechstadt, de vivo & iannace, 2000; ma et al, 2009; he, liu & qin, 2010). it is generally believed that saddle dolomite can be used as the main indicator of structural hydrothermal dolomite (li, jiao, wu, rong & wang, 2008). petrological observation of the lower qiulitage group in bachu uplift shows that saddle dolomite develops in reservoir rocks (table 1). the average content is between 4.9% and 10.8%, most of which is fine-medium crystalline and close mosaic contact. under cathodoluminescence, the dolomites are bright red with dark fringes (fig. 7c) and the ring-like structure is obvious. the intergranular pores are filled with quartz and illite (fig. 7a), and the calcination and gypsum are generally developed, which indicates that the formation has been subjected to obvious hydrothermal dolomitization. trace element analysis showed that the filling dolomite had a higher mn content and a lower sr content, indicating that it was formed in a deep burial reduction environment. carbon and oxygen isotopes show that the δ18 opdb values of the lower qiulitage group crystal dolomite and granular dolostone have an obvious negative bias (fig. 9a). likewise, a large amount of siliceous filling observed under the microscope indicates that it was more likely to be modified by hydrothermal fluid. the distribution pattern of rare earth elements shows that there are obvious positive anomalies of ce and eu in different crystalline dolomites (fig.10), which confirms they were modified by hydrothermal fluids. the homogenization temperature of the two-phase inclusions of the distributed saddle dolomites is 100 °c [22]. the buried history map of bt5 in bachu shows that the depth of the reservoir buried in the himalayan period is more than 7000 meters, and the maximum temperature of buried depth reaches 180-200 °c. it is shown that the formation of the hydrothermal transport system is easily affected by basement faults (li et al., 2016). according to the comprehensive petrological characteristics and geochemical tests, it can be seen that the hydrothermal action characteristics of the lower qiulitage group are obvious. the magnesia-rich hydrothermal fluid mainly invades the well-permeability surrounding rock, along with the fault system, and formed hydrothermal dolomite (cai, mei, li & zeng, 1997). the effect of the hydrothermal was controlled by the deep fault system, and the distribution was relatively limited. figure 8. element content of fe, mn, and sr in the dolomite of the lower qiulitage group (a. characteristics of trace elements in sr and mn of dolomite; b. characteristics of trace elements in fe and mn of dolomite.) figure 9. isotope data of carbon, oxygen, and strontium of the lower qiulitage group (a. carbon and oxygen isotope data; b. strontium isotope data) 205study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china dissolution of reservoir freshwater dissolution the diagenetic evolution of carbonate rocks suggests that the atmospheric freshwater dissolution occurs mainly in the eodiagenesis and mostly through selective dissolution. it was observed that some unstable minerals or cement dissolved into intragranular dissolved pores and molded pores, developed in bt5 and ht1 in the bachu uplift (figs. 4d, 4f). these early dissolution pores provided the main seepage channels for the diagenetic fluids entering the rocks during the burial period, but they were mostly filled with the later diagenetic cement (fig. 5f). so, they are less effective and have little significance for the formation of reservoirs. burial dissolution it can be seen that the lower qiulitage group is in the stage of continuous subsidence from the regional burial history chart (fig. 11), there is no obvious exposure and denudation in the burial stage, and the formation of dissolution holes should be relatively late. the existing studies show that the pore development of carbonate reservoirs in the deep burial stage is mostly related to acidic fluids, and the deep acidic fluids in the deeply buried stage are mainly organic acids and co2 (jiang et al., 2008). some scholars believe that the dissolution of organic acid fluids to reservoirs was limited in a closed environment (ehrenberg, walderhaug & bjφrlykke, 2012; ehrenberg, & bjφrlykke, 2016). at the same time, according to regional data, the cambrian source rocks in the bachu uplift are not developed (yang et al., 2017). organic acids should not be the cause of vug pores in the lower qiulitage group. in recent years, many scholars have paid attention to the influence of co2 in hydrothermal fluid on carbonate reservoirs (biehl et al., 2016). zhu dongya studied the fluids in the permian igneous rocks in the central and northern tarim basin and showed that co2 was predominant in the gases released from igneous rocks, up to 91.47% (zhu, meng, xie & jin, 2011). liu wei collected fluorite and calcite samples in the xikeer outcrop area of bachu at the western margin of the tarim basin and analyzed the fluid inclusion composition by laser raman microprobe, finding that a large amount of co2 existed in fluids during hydrothermal activities (liu et al., 2016). according to the statistics of the reservoir surface porosity (figs. 12a, 12b), in that range of less than 4% of porosity, the content of the saddle dolomites is slightly positively related to the vug pore, indicating that the corrosion effect has a significant transformation effect on the reservoir. and large pores are easily filled with authigenic minerals, such as area b in the figure 12b. the content of saddle dolomite formed by hydrothermal interaction is still positively correlated with porosity and fractures, which indicates that the early hydrothermal dissolution has a significant effect on the increase of reservoir porosity. in this paper, it is considered that the co2 acid dissolution caused by hydrothermal fluids in the deeply buried environment may be an important reason for the increase of the buried porosity of reservoirs. genesis of crystal dolomite reservoir the reservoir of the lower qiulitage group in bachu is dominated by crystal dolomite. due to the lack of drilling and coring, the formation evolution of the reservoir is unclear. some scholars believe that the reservoir of the lower qiulitage group in this area is affected by sea level exposure and that the karst caves developed belong to the karst dolomite reservoir (zhu et al., 2011; huang et al., 2016), while other scholars believe that the reservoir was greatly altered by diagenesis (yang et al., 2011). under the constraints of buried history, the sequence of reservoir genesis and evolution of the lower qiulitage group has been established by combining reservoir void characteristics and surface porosity identification (fig. 11). the restricted platform environment developed in the late cambrian in bachu (ni, chen, zhu, yang & xiong, 2015; he, peng, shao, gao & shi, 2002). the sediments are dominated by micrite. gypsum and other evaporative minerals are common in gray sediments. during the eodiagenesis period, the formation of silty dolomite was promoted by the shallow burial dolomitization, while the crystals were mainly anhedral-subhedral and intergranular pores developed (figs. 2e, 2f). in this stage, gypsum nodules are easily dissolved by atmospheric fresh water to form a small number of gypsum molds (zheng, shen & liu, 2013). with the increase of the buried depth, the reservoir enters the mesodiagenesis stage, and the compaction strengthens to form the suture structure. the late tectonic uplift of the middle diagenesis results in the generation of fractures, promoting the burial dolomitization and formation of fine-med dolomites. the cements are mainly siliceous and sporadic pyrite. the reservoir rocks entered the hydrothermal transformation stage during the himalayan period, which was affected by the basement fracture. the co2 acid gas was brought by the hydrothermal solution under high temperature and highpressure conditions (wang et al., 2016), the residual pores were dissolved along the seepage channels such as fractures and intergranular solution pores, and the residual pores were corroded and altered along the seepage channels such as fractures and intergranular solution pores. the formation of a large number of secondary pores is the key event for the formation of high-quality reservoirs. hydrothermal dolomitization promotes the development of saddle dolomite. in the late hydrothermal stage, the filling of quartz and anhydrite produced locally reduced the reservoir space to a certain extent and formed the presentday grain dolomite reservoir. based on the above research, it is believed that the lower qiulitage group develops crystal dolomite reservoirs under the limited platform. but it is greatly affected by burial and hydrothermal transformation. the reservoir quality of dolomite reservoirs in the basement fault area is better. figure 10. normalized pattern (npdw) of rare elements of dolostone in the lower qiulitage group (part of the data from the northwest bureau of sinopec) 206 penghui luo, shitao zhang, yimin tian, fei ding, zongming xu figure 12. crossing plot of saddle dolomites and surface porosity. (a. fracture pore and saddle dolomites; b. total porosity and saddle dolomites.) figure 11. paragenesis of the lower qiulitage reservoir of cambrian in tarim basin. 207study on the microscopic characteristics of ferruginous cement of banguo earth forest in yuan mou area, yunnan, china conclusion (1) the lower qiulitage group mainly developed crystal dolomite reservoirs in the bachu uplift, with an average dolomite content of 72.3-82.2%. the common authigenic minerals are quartz and anhydrite. there are obvious differences in distribution, with fine and medium-coarse dolomites in the west, and silty and fine dolomites in the east. micrite dolomite and (residual) grain dolomite were less developed in this area. (2) there are various types of reservoir space in the lower qiulitage group. the intercrystalline and intercrystalline pores are relatively developed. the middle and lower reservoirs are mainly composed of dissolved pores and fractures. the pore types are mainly small pores and reservoir fractures, and are mostly corrosion fractures. the reservoir porosity ranges from 0.14% to 6.7%, the permeability range is (0.01-4.5)×10-3 m2, and the average value is 1.12× 10-3 m2. the reservoir properties include low porosity and low permeability characteristics. the fine-medium dolomite has the best property of the pore structure. (3) the lower qiulitage group in bachu is mainly in the shallow burial stage. it is affected by the residual seawater in the stratum, the crystal structure is relatively developed, the trace elements mn and sr are slightly higher, and the oxygen is in a negative position. in the deep burial stage, saddle dolomite and siliceous filling are developed due to the increase of fluid temperature and local fracture. the δ26 mg isotope value of the filling dolomite is low and the rare earth element ce and eu shows a positive anomaly, indicating that hydrothermal transformation is obvious. (4) the lower qiulitage group dolomite reservoir was greatly influenced by buried dolomitization and hydrothermal dolomitization. the pores and fractures formed by dolomite are important channels for diagenetic fluid reconstruction reservoirs. the dolomite reservoir has better quality in the fracture area. 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(2020). wheat physiological traits response in standard and ecological agricultural systems. earth sciences research journal, 25(1), 131-136. doi: https://doi.org/10.15446/esrj.v25n1.94403 this research was conducted on a farm of payam-e-noor university in piranshahr, in west azerbaijan province of iran, during 2014-2015 and 2015-2016 crop sessions to investigate the wheat physiological traits response to standard and ecological agricultural systems. the experiment was conducted as a split-plot design with two factors and three replications. the first factor, which was considered the main plot, included high, low-input, and ecological agricultural systems. subplots were seed pre-treatment, including control (without priming), hydro priming, and food priming with ascorbate. the results showed that the least photosynthesis rate and mesophilic conductance were related to lowinput agricultural system; however, in the ecological system, the transpiration, substomatal carbon dioxide, and water use efficiency of photosynthesis were the lowest. also, seed priming increased the photosynthesis rate, mesophilic conductivity, and water use efficiency of photosynthesis. abstract wheat physiological traits response in standard and ecological agricultural systems respuesta de los rasgos fisiológicos del trigo en sistemas agrícolas estándar y ecológico issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.94403 este estudio se realizó en la granja de pruebas de la universidad de payam-e-noor, en la ciudad de piranshahr, provincia de west azerbaijan, irán en la cosecha de los años 2014-2015 y 2015-2016, para analizar la respuesta de las características morfofisiológicas del trigo en sistemas de agricultura común y de agricultura ecológica. la fase experimental se diseñó en una área dividida, con dos factores y tres réplicas. el primer factor, que se consideró como el del lote principal, se dividió en los niveles de alta, baja contribución y sistemas de agricultura ecológica. los sublotes fueron sembrados antes de los tratamientos que incluyen control (sin preparación), preparación hidráulica y preparación de nutrientes con ascorbato. los resultados muestran que el menor índice de fotosíntesis y de conductividad mesófila estaba relacionado con los sistemas agrícolas de baja aportación. sin embargo, la transpiración, el dióxido de carbono subestomático y el uso eficiente del agua durante la fotosíntesis en el sistema ecológico fueron los más bajos. además, la preparación de la siembra incrementó el índice de fotosíntesis, la conductividad mesófila y el uso eficiente del agua durante la fotosíntesis. resumen record manuscript received: 05/02/2020 accepted for publication: 13/06/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 131-136 hamid fateh, ahmad toube*, abdul qayum gholipuri the university of mohaghegh ardebili, ardabil, iran. *corresponding author: ahmadtobeh1340@gmail.com en vi ro nm en ta l g eo lo g y https://doi.org/10.15446/esrj.v25n1.94403 https://doi.org/10.15446/esrj.v25n1.94403 mailto:ahmadtobeh1340@gmail.com 132 hamid fateh, ahmad toube, abdul qayum gholipuri introduction agricultural operations are based on the traditional knowledge and experience of human beings. an appropriate solution to increase agricultural products is to increase yields per unit area. now, in addition to increasing production, stability is also essential. therefore, various farming systems and their production potential should be investigated and revised (fateh, 2001). fateh et al. (2013) explained that the biggest challenge in the next 50 years is to double food in ways that do not harm the environment and consumer health. also, studies on agricultural systems are essential for understanding the developing factors and limiting factors. today, the global approach is towards ecological agriculture, which is a sustainable agricultural branch. this farming system tries to avoid chemical origin supplies (choghmaqi & moradi, 2007). the conventional agricultural system in the use and management of resources has not achieved acceptable success because pesticides and fertilizers have led to the emergence of unsustainable farming ecosystems, environmental pollution, and quantitative and qualitative reduction of the product and energy efficiency. lotter et al. (2003), while studying the standard agricultural and biological systems, reported that absorption and storage of water of rainfall in plots with the biological system are 100% higher than in the standard method, which could improve gas exchanges and consequently performance. they also reported that the performance of the ecological system is 38 percent higher than the standard system. jahan et al. (2009) reported that more nutritional elements, especially nitrogen, increase the water requirement proportionally to the plant’s growth in the high-input system. they also stated that moisture is more available to the plant for transpiration in a low-input ecological agricultural system than in a high-input system. and the plant can maintain a higher temperature difference with ambient temperature through more transpiration. higher soil capacity to retain moisture in ecological agricultural systems, especially biological systems, has been discussed in several sources (lotter et al., 2003; hole et al., 2005). meanwhile, the existence of animal manure in the ecological agricultural system has exacerbated this issue. jahan et al. (2009) stated that a high-input system is superior in terms of traits such as seed yield, number of maize, and soil respiration. in terms of plant height, the difference between systems is not significant. the canopy temperature in the ecological system was approximately 10% lower than the high-input system. the differences between ecological and low-input systems are shallow, and at the same time, they are in the middle between input and moderate-input systems. in other words, a nearly 10% reduction in the performance of ecological and low-input systems is compensated not only by lower supplies in terms of costs but also for the stability of these systems in the long term. oehl et al. (2004) reported that the consumption of fertilizers and energy in the biological farms decreases by 34-53 percent compared to the common ones. in contrast, the performance decrease of the biological method is only 20%. what is essential in ecological systems is forming mutual effects for increasing the efficiency of material and energy consumption, self-sufficiency of reversibility, and sustainability over time. economic performance in these systems among the high ecological values is not much attended (gosling et al., 2006; pimentel et al., 2005). materials and methods this research was conducted on a farm of payam-e-noor university, in piranshahr, west azerbaijan province, during 2014-2015 and 2015-2016 crop sessions. the experiment was conducted as a split-plot design with two factors and three replications. the first factor, which was considered the main plot, consisted of high-input, low-input, and ecological agricultural systems. subplots were seed pre-treatment, including control (without priming), hydro priming (seeding in distilled water for 8 hours), and food priming with ascorbate (100 ppm ascorbic acid solution for 8 hours) (harris et al., 2004). the wheat used is azar 2 certified, which a semi-water cultivar was prepared from oshnavieh gold cluster company. all crop operations, including plowing, pest, and weed control, were applied as registered in table 1. the traits related to gas exchange, including photosynthesis, transpiration, sub-stomatal carbon dioxide, and mesophilic conduction, the activity of antioxidant enzymes, leaf chlorophyll content, ion leakage percentage, the relative water content of leaves, and protein content of the seed, were measured. the infrared gas analyzer (irga) model lca4, manufactured by adc company of england, was used to measure photosynthesis per unit area of a leaf (µmol co2 m-2s-1), transpiration rate (mmol m-2s-1), and co2 sub-stomata concentration (µmol mol-1). all measurements were carried out at 10 to 12 hours in the light intensity of 1200-1400 micromoles photon per square meter per second. before the measurement began, the device was turned on for 20 minutes. in each treatment, the middle part of the leaf under the flag of the main stem was placed in the glass chamber in eight stages, from flowering to the hard seed; and after one minute, the device data was recorded. water use efficiency of photosynthesis (µmol co2 mol-1) was also determined by dividing the photosynthesis rate per unit area of the leaf to transpiration (siosemardeh et al., 2014). results and discussion the variance analysis results showed that the effect of year on photosynthesis was significant at 5% level, and the impact of agricultural system and seed priming system was significant at 1% level (table 2). as shown in figure 1, the photosynthesis rate in the first year was approximately 12 μmol co2 s-1, while in the second year, it decreased to 6.10. it should be noted that the precipitation in the second year was lower than in the first year. also, comparing the agricultural system’s average interactions showed no significant difference between the high-input and ecological agricultural systems regarding the photosynthesis of the wheat plant’s flag leaf. in contrast, the lowest photosynthesis rate was associated with the low-input agricultural system (fig. 2). the highest photosynthesis rate in the treatments was related to seed treatment with ascorbic acid. however, there was no statistically significant difference with hydro priming treatment, but there was a substantial difference with the control (fig. 3). this study’s results are consistent with fateh et al. (2013) on the positive effects of ascorbic acid priming on photosynthesis. plants from primed seeds have expanded their root system compared to plants from untreated seeds during a shorter time. they reach the autotrophic stage with the better absorption of water and food and the production of photosynthetic green sectors. the realization of biological and ecological conditions gives a particular position to plants primed with seeds. these conditions allow the plant to use environmental elements, such as water and light, more appropriately. these cases’ results can eventually lead to an increase in the photosynthetic rate and area in these plants. consequently, carbon dioxide stabilization and, naturally, the production of assimilates and the storage of non-structural hydrocarbons in different parts of the plant have increased. and as a result, higher biomass will be produced (duman, 2006). the main effects of seed priming include seed germination, initial seedling establishment, increasing grain yield and biomass through increased photosynthesis due to faster plant establishment (farooq et al., 2006; harris et al., 2001; duman, 2006). table 1. consumption of supplies and crop operations in different agricultural systems agricultural system high-inputlow-inputecologicalinput consumed 1--plowing times 11-cultivator times 400-200-150200-150-50biofertilizerfertilizer(n-p-k) kg/ ha --25manure (ton/ha) 21chemical struggle with pests and diseases(times) 21by handweed combat (times) 133tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm figure 1. comparison of the average effect of year: y1 (first year), y2 (second year) on the amount of photosynthesis of wheat. figure 2. comparison of the average effect of agronomic system: s1 (high-input), s2 (low-input) and s3 (ecological) on the amount of photosynthesis of wheat. figure 3. comparison of the average effect of seed pre-treatment: p1 (control), p2 (hydro priming) and p3 (ascorbic acid) on the photosynthesis of wheat. table 2. analysis of variance of studied traits (as=agricultural system. p=priming, y=year) mean of square degree of freedom (df) source of change grain yield water use efficiency mesophyll conductance sub stomata co2 transpiration photosynthesis 115/0 023/0 173/13 407/1625 018/0 336/0 replication 003/2 037/0 71/204** 074/1968 592/2* 582/27* y 543/0 014/0 796/0 630/135 119/0 286/0 main error 645/5** 929/2** 778/99** 352/5756** 343/4** 838/15** as 812/0** 021/0 992/3 574/668 069/0 247/0 p 562/0** 465/0** 386/14* 963/48 197/0 423/2** y× as 015/0 041/0 930/2 963/44 037/0 286/0 p ×y 062/0 085/0 506/5 713/599 050/0 261/0 as×p 020/0 036/0 429/1 602/99 059/0 132/0 y×p× as 052/0 053/0 327/4 060/875 131/0 227/0 total error 03/4 74/8 07/8 70/6 28/8 21/4 coefficient of variation (c.v %) * and ** there is a significant difference between them at the probability level of 5% and 1%, respectively. effect of year and agricultural system on transpiration rate was significant at 5% and 1% respectively (table 2). as shown in figure 4, the photosynthesis and transpiration rates were also higher in the first year than in the second year and, in the second year, the transpiration rate decreased by about 10% compared to the first year. however, unlike photosynthesis, transpiration rate was the lowest in ecological agricultural system (fig. 5). the main factor in reducing the photosynthesis and transpiration rates under stress conditions is due to this strategy that, the plant uses the limited amount of water that it has to avoid drought and, it tries to close its stomata to prevent the water loss. with the onset of the dry period, the plant maintains its transpiration at maximum level for a while but, with the continuation of the dry period, it tries to narrow its stomata and eventually close those (moradi et al., 2005). figure 4. comparison of the average effect of year, y1 (first year), y2 (second year) on the amount of transpiration rate of wheat. figure 5. comparison of the average effect of agronomic system: s1 (high-input), s2 (low-input) and s3 (ecological) on the amount of transpiration rate of wheat. 134 hamid fateh, ahmad toube, abdul qayum gholipuri the effect of high-input, low-input and ecological systems at the level of 1% on the sub-stomatal carbon dioxide was significant (table 2). the results showed that, under ecological conditions, the carbon dioxide content in the sub-stomatal cavity is less than that of the high-input system. application of chemical fertilizers and chemical pesticides seems to cause stress in the plant and has increased carbon dioxide levels in the sub-stomatal cavity in the highinput system (fig. 6). fateh et al. (2011) reported that the sub-stomatal co2 concentration under stress conditions was about 13% higher than the control condition. under stress conditions, co2 injected into leaves is not well used in photosynthesis. this suggests that any kind of stress action on the plant, in addition to closing the stomata and preventing the entry of co2, also prevents the leaves from being processed by co2 through the internal mechanism (siosemarde et al., 2003). the reason for this is the effect of metabolic factors during stress on the reduction of carboxylation and the efficiency of carbon dioxide emissions into chloroplast and mesophilic cells. under these conditions, it seems that non-stomatal limiting factors play an important role in reducing photosynthesis in drought stress. (nazemosadat & kazemini, 2008; siosemarde et al., 2003). figure 6. comparison of the average effect of agronomic system: s1 (high-input), s2 (low-input) and s3 (ecological) on sub-stomatal conductance rate of wheat. effects of crop year and system mesophilic conductivity was significant at 1% level and seed priming effect at 5% level on (table 2). as with the photosynthesis and transpiration rates, mesophilic conductivity was reduced in the second year. the decrease in mesophilic conductivity in the second year was about 14% compared to the first year, which was slightly higher than the decrease in transpiration (fig. 7). figure 7. comparison of the average effect of year: y1 (first year), y2 (second year) on the mesophilic conductance of wheat. figure 8. comparison of the average effect of agronomic system: s1 (high-input), s2 (low-input) and s3 (ecological) on mesophilic conductance rate of wheat also, the mesophilic conductivity in the ecological agricultural system was higher than the low-input system however, there was no statistically significant difference with the high-input system (fig. 8). in a study on durum wheat, in severe drought stress treatments, an increase in the concentration of sub-stomatal co2 was associated with a significant decrease in mesophilic conduction and thus reduction of photosynthetic capacity of chloroplast. in this case carbon dioxide injected into the leaves has not been properly used in the photosynthesis process (nazemosadat, & kazemini, 2008). in addition, with the mean comparison between priming treatments in this study it was found that, as with other traits studied so far, applying seed food priming with ascorbic acid or vitamin c has led to increased mesophilic conductivity in wheat plants due to faster growth and better growth and more efficient use of resources than control treatments (fig. 9). figure 9. comparison of the average effect of seed pre-treatment: p1 (control), p2 (hydro priming) and p3 (ascorbic acid) on mesophilic conductance of wheat the effect of agricultural system and also seed priming on the photosynthetic water use efficiency was significant at the level of 1%. comparison of mean of treatments showed that in ecological agricultural system, photosynthetic water use efficiency was higher in comparison with low-input and high-input systems and, this difference is significant. the photosynthetic water use efficiency in the ecological system was 2.79 mmol/ mol/sec which was reduced by 12 and 7 percent, respectively, in high-input and low-input systems (fig. 10). it was also determined that photosynthetic water use efficiency was higher in priming treatments (fig. 11). jahan et al. (2009) stated in their research that in general, in low-input and ecological agricultural systems, moisture available to the plant was higher than the highinput system for transpiration and the plant has been able to maintain a higher temperature difference with more transpiration. several sources have referred to the greater soil capacity to maintain moisture in ecological agricultural systems, especially biological systems (hole et al., 2005; lotter et al., 2003), while the existence of animal manure in the ecological agricultural system has exacerbated this issue. figure 10. comparison of the average effect of agronomic system: s1 (highinput), s2 (low-input) and s3 (ecological) on water use efficiency of photosynthesis of wheat. 135tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm figure 11. comparison of the average effect of seed pre-treatment: p1 (control), p2 (hydro priming) and p3 (ascorbic acid) on water use efficiency of photosynthesis of wheat. the interaction between the crop year and the agricultural system and also the effect of seed pre-treatment on wheat grain yield was significant at the level of 1% (table 2). comparison of the average of treatments using duncan test at 5% probability level showed that in both the first and second year, the grain yield in the high-input system was higher than the other treatments. however, there was no significant difference between the low-input and ecological agricultural systems in both years (fig. 12). the results also showed that seed priming with ascorbic acid in comparison with control increased the grain yield, and this increase was statistically significant (fig. 13). figure 12. comparison of the interaction effect of year: y1 (first year), y2 (second year) and agronomic system, s1 (high-input), s2 (low-input) and s3 (ecological) on grain yield of wheat figure 13. comparison of the average effect of seed pre-treatment: p1 (control), p2 (hydro priming) and p3 (ascorbic acid) on grain yield of wheat the photosynthesis rate did not differ in ecological and high-input farming systems. the highest photosynthesis rate and mesophilic conduction among treatments were seed priming treatment with ascorbate and hydro priming. seed priming leads to accelerated germination and establishment of the green surface, and can eventually lead to an increase in the photosynthesis time and area (duman, 2006). under ecological system conditions, the intercellular co2 concentration (ci) was less than that of the high-input system conditions. it seems that the application of chemical fertilizers and chemical pesticides caused stress in the plant and increased the intercellular co2 concentration (ci) in the high-input system. under stress conditions, co2 injected into leaves is not well used in photosynthesis. in ecological farming system, the efficiency of photosynthetic water use was higher than the high and low input system. it was also determined that the relative water content of the leaf in the high-input system relative to the ecological system decreased by about 12%. it seems that in a low-input ecological farming system compared to high-input system, higher moisture has been available to the plant for transpiration and the plant could maintain higher temperature difference through more transpiration. higher photosynthetic water use efficiency is due to more soil capacity to maintain moisture and also the presence of manure in ecological farming system. however, there was no significant difference between the two years in the low-input and ecological farming system. common agriculture is based on maximum production. agricultural sources such as water, soil and genetic diversity have been exploited excessively and are being destroyed. the only remaining choice is to preserve productivity of agricultural land for a long time along with the change in consumption pattern of inputs. maintaining the long-term yield of agricultural land is a requirement for sustainable food production. this sustainability can be achieved through ecological processes on farms (kohnaward et al., 2011). the ecological farming system, while protecting soil fertility, increases the production of the product without relying on the use of chemicals and achieving a stable production. lower average yield in the long-term ecological system is possible through rising prices for these products. but the fact is that removal of chemicals is not enough to move from the common agricultural toward ecological farming (kohnaward et al., 2011). during the transition period of the common systems to low-input systems, problems such as nutrient deficiencies and weed and pest problems are among the major factors affecting yield loss (behdani et al., 2017). increasing yield in the common agricultural system is due to increased consumption of foreign inputs such as fertilizer, pesticide and herbicide. generally, the yield 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(2008). the effects of water stress and photosynthesis active radiation on photosynthesis rate of wheat. journal of agricultural sciences and natural resources, 14(6), 31-39. oehl, f., sieverding, e., mader, p., dubois, d., ineichen, k., boller, t., & wiemken, a. (2004). impact of long-term conventional and organic farming on the diversity of arbuscular mycorrhizal fungi. oecologia, 138, 574-583. pimentel, d., & burgess, m. (1980). energy inputs in corn production. in: pimentel, d. (ed.). handbook of energy. utilization in agriculture, 6784. crc press, inc. siosemardeh, a., ahmadi, a., poustini, k., & ebrahimzadeh, h. (2003). stomatal and nonstomatal limitations to photosynthesis and their relationship with drought resistance in wheat cultivars. iranian journal of agricultural science, 34(4), 93-106 siosemardeh, a., fateh, h., & badakhshan, h. (2014). responses of photosynthesis, cell membrane stability and antioxidative enzymes to drought stress and nitrogen fertilizer in two barley (hordeum vulgare) cultivars under controlled condition. iranian journal of field crops research, 12(2), 215-228. keywords: regularization; iterative continuation; anomaly separation; physical parameters. palabras clave: regularización; continuación iterativa; separación de anomalías; parámetros físicos; inversión. how to cite item wang, y., li, j., wang, x., & jian, x. (2019). inversion technique of physical parameters based on regularization extension depth constraint. earth sciences research journal, 23(4), 331-338. doi: https://doi.org/10.15446/ esrj.v23n4.84340 through the regularization downward continuation of gravity and magnetic anomalies, the depth of the field source can be solved. however, due to the gibbs effect, the horizontal resolving power of the field source is poor. in view of this, based on the depth of field source established by regularization downward continuation, this paper proposes a physical property parameter inversion method based on iterative continuation and anomaly separation, which can effectively improve the inversion accuracy of superimposed anomaly physical parameters, and provide a new idea for solving the physical parameters of superposition gravity and magnetic anomalies. abstract inversion technique of physical parameters based on regularization extension depth constraint técnica de inversión de parámetros físicos basada en la regularización de restricción de extensión de profundidad issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.84340 a través de la regularización de la gravedad y las anomalías magnéticas, se puede resolver la profundidad de la fuente del campo. sin embargo, debido al efecto gibbs, el poder de resolución horizontal de la fuente de campo es poco. en vista de esto, basado en la profundidad de la fuente de campo establecida por la continuación de la regularización hacia abajo, este artículo propone un método de inversión de parámetros de propiedades físicas basado en la continuación iterativa y la separación de anomalías, que puede mejorar efectivamente la precisión de inversión de los parámetros físicos de anomalías superpuestas, y proporcionar una nueva idea para resolver los parámetros físicos de la superposición de gravedad y anomalías magnéticas. resumen record manuscript received: 01/04/2019 accepted for publication: 09/08/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 331-338 yang wang1, 2, jun li1, 3*, xuben wang1, 3, xingxiang jian1, 3 1college of geophysics, chengdu university of technology, chengdu 610059, china 2public class teaching department, sichuan vocational and technical college of communications, chengdu 611130, china 3ministry of education key laboratory of earth exploration and information technology, chengdu university of technology, chengdu 610059, china * corresponding author: lijun3@cdut.edu.cn g eo ph ys ic s 332 yang wang, jun li, xuben wang, xingxiang jian introduction the downward continuation of the potential field can provide abundant information on the spatial distribution of the potential field and is an important part of the interpretation of gravity and magnetic anomalies (zhu, cao, & lu, 2014; deng, liu, song, & chen, 2014). luan wengui adopted another regularization functional and deduced another response. according to the same regularization functional, another response formula was derived by luan wengui, which is suitable for understanding the distribution characteristics of the source potential field in the near field (zhang, dai, & liu, 2014). c. w. groetsch, p mauriello, d patella, xu deshu, zeng hualin, wang xuben, gao yongcai and other scholars have also studied the extension method. through research and comparison, it shows that the regularization method has the best downward extension effect. but due to the influence of the gibbs effect, on both sides of the main anomaly zone, there is often a “positive and negative alternating “ ” shaped weak anomaly zone with the main source distribution area as the center, which may be a separate anomaly or composite anomalies formed by the superposition with other anomalies, thus increasing the difficulty to explain the field source. to solve this problem, based on the gravity and magnetic data regularization downward extension, this paper puts forward downward gravity and magnetic property parameters inversion method of downward continuation and anomaly separation based on the iterative method, providing a new way of thinking for the comprehensive interpretation of the field source from the material properties and geometric parameters (al farajat et al., 2015; li & zhang, 2017; mazgaj, szular, & szczurek, 2017). basic principle regularization extension technique according to the laplace equation and the boundary condition of the potential field, there is: ∂ / ∂ +∂ / ∂ φ 2 2 2 2 0 0 u x z x u x z z u x z u x z f x z x l z x , , , , , ( ) ( ) = ( ) = ( ) = ( )  → = ( )       (1) u x z,( ) is the potential field at the measuring point x z,( ), and f x z,( ) is the known field value on the measuring point;  x( ) is the height of the measuring point. by using the method of separating variables and the boundary condition, the general formula of the series decomposition of the field potential can be obtained (boehm & ulbrich, 2015). f x z a x ei i z i n , cos ,( ) = ( ) = ∑   0 (2) set m as the number of points at the measuring point, and x is the distance between the measuring points, there are x i x= ( )⋅1 ∆ λ π / −i i m x= ⋅ ( )⋅1 ∆ this result is the series decomposition method of the traditional downward continuation (raknes & arntsen, 2014). in order to overcome the oscillatory effect, the series is still taken as the basic form of the field value solution. in solving the continuation function, a decomposition function with a relatively smooth variation is selected and the error of fitting it to the original field is minimized on the original observation profile, so that the analytical function of regularized downward continuation series after correction can be obtained (zhang et al., 2015; fontchastagner, lubin, mezani, & takorabet, 2018). f x z a a x e vi i z i i n i, cos( ) = ( ) = ∑1 2 0 1 + λ λ (3) vi is called the regularization factor, which is 1 1 2 2/ +αλ λ i ze i( ), a function related to the frequency i , continuation depth z and coefficient . it corrects the frequency of the series term of the continuation field, and it is proved by practice that as long as the regularization function is properly selected, it cannot only guarantee the non-singularity of the potential field in the process of continuation, so as to obtain the complete field value distribution in the lower half space, but also the gibbs effect cannot be very significant. iterative continuation technique it is often necessary to extend downwards to the top of different depth layers to separate the anomalies produced by different depth layer sources on the ground. the stability and depth of continuation directly affect the result of continuation. to deal with the instability of potential field downward continuation and the limited depth of downward continuation by the classical fft method, xu shizhe put forward a downward continuation method in spatial domain, which is implemented based on stable fft upward continuation. the principle is simple and the downward continuation is stable. the basic idea of the downward continuation of the iterative method is to assume that there are two planes, a and b, representing the ground surface and the plane of a certain depth under the ground, respectively. between the two, there is no field source distribution, that is, there is a passive space, so that the downward continuation iterative solution of the known depth based on upward continuation can be realized according to the following diagram (chen, 2015). figure 1. implementation diagram of downward extension through the iterative method inversion of physical parameter of gravity and magnetic anomaly assuming the thickness of a certain depth of the underground layer as h, the layer is divided into mn mn m n= ×( ) small vertical prisms of the same size with a finite extension, and the coordinate of the center point of the small prisms is x yj j0 0,( ) and residual density is  j j mn=( )1 2 3, , , , ; its side length is a and b respectively; the top depth of the prism is h and the height is dh. thus, the formula for the frequency spectrum g u vj ,( ) of the gravity anomaly g x yj ,( ) generated by the ground x y,( ) of each prism in the frequency domain is as follows: g u v g uv au bv e r ej j hr dhr, sin sin( ) = ⋅         ⋅ ⋅ ( )2 4 2 2 1π ρ − − − ⋅⋅ ( )e i ux vyj oj− +0 (4) 333inversion technique of physical parameters based on regularization extension depth constraint thus, by fourier inverse transformation, the residual density values of each point x y, �( ) on the top surface of the depth layer can be obtained: ρ π − − − x y f g u v g uv au bv e r dhr, , sin sin ( ) = ( ) ⋅         ⋅  1 2 4 2 2 1               (9) formula 9 is the inversion formula for calculating the relative physical parameters of the gravity anomaly of the known field source. the apparent density corresponding to the gravity anomaly can be calculated by the formula. for the magnetic source anomaly, the gravity anomaly of the magnetic source can be obtained first, and the magnetic source density or magnetization intensity can be inversed by the same method. in view of the space, its calculation formula is not deduced in this paper. gravity anomaly regularized continuation depth constraint inversion regularized continuation depth constrained inversion of single model gravity and magnetic anomalies in this paper, a 101 × 101 mesh is designed, the distance between points and that between lines is all 1 (unit: meter), that is, the measuring area is 100 × 100m2. the specific parameters of the sphere model are shown in table 1. table 1. specific parameters of a single sphere model spherical center coordinate (m) sphere radius (m) residual density (g/cm3) magnetization intensity (a/m) magnetization inclination magnetization deviation angle (50,50,20) 5 0.5 100 45° 0° the corresponding gravity anomalies (δ g) and magnetic anomalies (δ t) are derived from the forward calculation of the above model, as shown in figure 2: figure 3 is a section of the regularization result of the above magnetic anomalies, which shows that the center of the anomaly is consistent with the depth of the given model. however, due to the influence of the gibbs effect, in the upper formula, u, v represent the wave number of x, yrespectively; g is the gravitational constant; and r u v= +2 2 . the frequency spectrum g u v,( ) is made fourier inversion to obtain the gravity anomaly g x y,( ) in the space domain. according to the superposition property, the frequency spectrum g u v,( ) of gravity anomaly g x y,( ) produced by the whole depth layer at the ground x y, �( ) is as follows: g u v g uv au bv e r e dhr dhr j , sin sin( ) = ⋅         ⋅ ⋅ ( )2 4 2 2 1π − ρ − − ⋅⋅ ( ) = ∑ e i ux vy j mn oj− +0 1 (5) formula 5 is the forward formula of field source gravity anomaly with known physical parameters. if the gravity anomaly g x y,( ) generated by the depth layer in the top surface is known, h = 0 (if h ≠ 0, that is, the buried depth is not zero, and it can be extended to the top interface first, and then: ρ ρ − + u v ej i ux vy j mn oj,( ) = ⋅ ( ) = ∑ 0 1 (6) thus:  g u v g uv au bv e r u v dhr , sin sin ,( ) = ⋅         ⋅ ( ) ⋅ ( )2 4 2 2 1 π − ρ − (7) the following formula can be deduced from the above:   ρ π − − u v g u v g uv au bv e r dhr, , sin sin ( ) = ( ) ⋅         ⋅2 4 2 2 1 (8) figure 2. gravity anomaly (left) and magnetic anomaly (right) of the sphere model 334 yang wang, jun li, xuben wang, xingxiang jian on both sides of the main anomaly zone, there is often a “positive and negative alternating “øè” shaped weak anomaly zone with the main source distribution area as the center, which may be a separate anomaly or form composite anomalies by the superposition with other anomalies, thus increasing the difficulty to explain the field source. figure 3. magnetic source gravity anomaly of sphere model figure 4. the principal curve of magnetic source gravity anomaly and its regularization downward extension fracture surface regularization continuation depth constrained inversion of superimposed gravity and magnetic anomalies in order to carry out theoretical calculation and verification, a 201 × 201 mesh is designed in this paper. the distance between points and that between lines is all 5 (unit: meter, that is, the range of measuring area is 1000 × 1000m2. the model consists of three vertical prisms, in which the column centers of model 1 and model 3 are on the same vertical line, and the specific parameters of each column model are shown in table 2: according to the relevant parameters of the above model, the gravity anomaly and magnetic anomaly (unit: nt) of the model can be obtained by forward modeling (unit: mgal). the result is shown in figure 6. in order to extend this method to the processing of magnetic anomalies effectively, the magnetic anomalies’ magnetic source gravity is solved first, and the magnetic anomalies are also solved according to the gravity anomaly process. figure 7 shows the magnetic source gravity anomaly of the magnetic source model. the anomaly of the main section of the model is selected for regularized downward continuation, so that the depth of the field source is obtained respectively. it can be seen from the diagram that the depth of the regularization extension has a high accuracy. the regularization is used to establish the depth of the field source for iterative continuation. considering that the basic requirement of iterative continuation is that the two depth interfaces are passive field spaces, the idea of this paper is to carry out the continuation of shallow source depth first; then, the shallow-source anomaly is separated by cutting method, and the cutting radius is established according to the anomaly range of shallow-source anomaly. (the idea of this paper is to use laplace operator to establish abnormal boundary and then the radius is taken as (l+d)/2) according to the anomaly boundary.) 335inversion technique of physical parameters based on regularization extension depth constraint figure 5. magnetic source gravity anomaly (left) and apparent density inversion results (right) table 2. specific parameters of the combined model parameters cylinder cylindrical center coordinate (m) cylinder length (m) cylinder width (m) cylinder height (m) density (g/cm3) magnetization intensity (a/m) declination magnetic dip 1 (650,650,500) 300 300 100 0.4 80 0° 45° 2 (200,200,50) 50 50 50 0.5 100 0° 45° 3 (650,650,50) 50 50 50 0.5 100 0° 45° (a) spatial position of the model (b) model plane position and parameters figure 6. model and its forward modelling anomaly 336 yang wang, jun li, xuben wang, xingxiang jian (c) gravity anomaly g of model forward modelling (d) magentic anomaly ta of model forward modelling figure 6. model and its forward modelling anomaly figure 7. magnetic source gravity of the model figure 8. anomaly curve of diagonal section of magnetic source gravity anomaly and its regularized extension 337inversion technique of physical parameters based on regularization extension depth constraint figure 9. magnetic source gravity’s anomaly value after the downward extension of 50 m figure 10. shallow anomaly (left) and deep anomaly (right) obtained by cutting 50 m depth anomaly figure 11. the density value obtained by the shallow anomaly inversion 338 yang wang, jun li, xuben wang, xingxiang jian conclusion based on the potential field theory and regularization theory, a method for inversion of gravity and magnetic physical property parameters of downward continuation and anomaly separation based on iterative method is proposed. the following points should be pointed out: 1) the inversion effect based on single model depth constraint is better; 2) the lateral superposition model can obtain higher lateral resolution anomalies in the process of iterative downward continuation. therefore, the inversion of physical parameters has a higher accuracy, while the vertical superposition anomaly and physical properties are related to the separation accuracy of anomalies. 3) the study of high accuracy anomaly separation is the key to the inversion of depth constrained physical parameters of complex geological models. acknowledgement we acknowledge the support sullied by the national key research and development program (granted by 2017yfc0601504) and the valuable review comments proposed by the reviewers. references al farajat, m., schaefers, b., al hassanat, h., al atteyat, n., al jahed, n., & khataibeh, j. (2015). using gis and geophysics in selecting suitable basins with freshwater aquifers for an efficient exploration strategy a case study from petra-region, jordan. earth sciences research journal, 19(1): 39-50. boehm, c. & ulbrich, m. (2015). a semismooth newton-cg method for constrained parameter identification in seismic tomography. siam journal on scientific computing, 37(5), s334-s364. chen, y. (2015). iterative deblending with multiple constraints based on shaping regularization. ieee geoscience & remote sensing letters, 12(11), 2247-2251. deng, c. l., liu, h. j., song, b. h., & chen, j. (2014). density parameters inversion for structures based on regularization method. journal of guilin university of technology. fontchastagner, j., lubin, t., mezani, s., & takorabet, n. (2018). design optimization of an axial-field eddy-current magnetic coupling based on magneto-thermal analytical model. open physics, 16(1), 21-26. li, c. & zhang, f. (2017). ava inversion based on the l1-norm-based likelihood function and the total variation regularization constraint. geophysics, 82(3), 1-54. mazgaj, w., szular, z., & szczurek, p. (2017). calculations of magnetic field in dynamo sheets taking into account their texture. open physics, 15(1), 1034-1038. raknes, e. b. & arntsen, b. (2014). time-lapse full-waveform inversion of limited-offset seismic data using a local migration regularization. geophysics, 79(3), wa117-wa128. zhang, f., dai, r., & liu, h. (2014). seismic inversion based on l1-norm misfit function and total variation regularization. journal of applied geophysics, 109, 111-118. zhang, y,. yan, j., li, f., chen, c., mei, b., jin, s., & dohm, j. h. (2015). a new bound constraints method for 3-d potential field data inversion using lagrangian multipliers. geophysical journal international, 201(1), 267-275. zhu, z. q., cao, s. j., lu, g. y. (2014). 3d gravity inversion with bound constraint based on hyper-parameter regularization. the chinese journal of nonferrous metals, 24(10), 2601-2608. figure 12. abnormal value of the extension of 450 m of depth anomaly (left) and its inversion density (right) keywords: river erosion; plucking; basalts; fractures. palabras clave: erosión fluvial; lecho rocoso; basalto; fracturas. how to cite item lima., a., g., pelegrina, m. a., & pontarolo, m. (2021). fracture variability in basalts and its effect on river erosion: a case study in the paraná volcanic province. earth sciences research journal, 25(1), 13-20. doi: https:// doi.org/10.15446/esrj.v25n1.74167 the variation in the structural characteristics (cooling joints and tectonic fractures) of basaltic flows implies potential variability in the intensity of erosion by plucking. the erosive behavior of the rivers that sculpt these areas depends on their interaction with the diverse fracture systems. in view of this, we analyzed the effect of fracture variability (tipology, density and direction) in basalts on erosion in a bedrock river reach located in the continental volcanic province of the paraná basin, southern brazil. the fracture density and fracture direction were evaluated through field photogrammetry in seven sample areas distributed along a reach of 120 m. the fracture direction and main erosion axes were also surveyed by remote piloted aircraft (rpa) aerial imaging. the main erosion axes coincide with the principal fracture directions (tectonic fractures), which are disposed obliquely to the channel flow direction, making an average angle of 50°. the small, more abundant, and multidirectional cooling joints control the plucking process, but do not determine the erosion direction. the fracture density systematically decreases upstream from 9.62 to 3.73 m/m², probably related to distance from a structural lineament which river crosses downstream. the higher fracture density favors more intense plucking due to decrease in the size of the rock blocks. the lower fracture density limits the plucking and favors the macroabrasion, mainly if associated with vesicular-amygdaloidal basalt. abstract fracture variability in basalts and its effect on river erosion: a case study in the paraná volcanic province variabilidad de fractura en basaltos y su efecto en la erosión fluvial: un estudio de caso en la provincia volcánica de paraná issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.74167 la variedad de características estructurales (juntas de enfriamiento y fracturas tectónicas) de los derrames basálticos implica una variabilidad potencial en la intensidad del proceso de erosión fluvial por extracción de bloques (plucking). el comportamiento erosivo de los ríos que esculpen estas áreas depende de la interacción con la diversidad de los sistemas de fractura. en vista de esto, buscamos analizar el efecto de la variabilidad de las fracturas en los basaltos sobre la erosión en un tramo fluvial de lecho rocoso, ubicado en la provincia volcánica continental del paraná, en el sur de brasil. el tramo, de 120 m de largo, está parcialmente sujeto a la influencia de una posible falla que lo atraviesa cerca de uno de sus extremos. evaluamos la densidad y dirección de fracturas, mediante fotogrametría de campo, en siete áreas de muestra distribuidas a lo largo del tramo y la dirección de las fracturas en el tramo integral mediante imagen aérea de remote piloted aircraft (rpa). con la imagen de rpa, también se identificaron los principales ejes de erosión. fracturas tectónicas visto en el campo no siempre aparecen en la inspección de las áreas de muestra, pero son evidentes en la imagen de rpa. los ejes principales de erosión coinciden con las direcciones principales de fractura (tectónicas), que son oblicuas a la dirección del flujo del canal, formando un ángulo promedio de 50°. las diaclasas, más abundantes y multidireccionales, funcionan como controles sobre el proceso de plucking y no como determinantes de la dirección de la erosión. la densidad de las fracturas disminuye progresivamente con la distancia desde el punto de cruce de la falla (de 9.62 a 3.73 m/m²), aunque el valor más bajo está influenciado por una zona de basalto amigdaloide. la mayor densidad de fracturas favorece la intensificación de la erosión por extracción de bloques. resumen record manuscript received: 13/02/2020 accepted for publication: 28/08/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 13-19 adalto gonçalves lima, marcos aurelio pelegrina, murilo pontarolo universidade estadual do centro-oeste, guarapuava, paraná, brazil *corresponding author: adalto@unicentro.br g eo lo g y https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n1.74167 14 adalto gonçalves lima, marcos aurelio pelegrina, murilo pontarolo introduction complexity of fracture systems may vary broadly in basalts. these rocks have different fracture patterns depending on the morphology and thickness of the lava flow (degraff & aydin, 1993; thordarson & self, 1998; bondre et al., 2004; vye-brown et al., 2013). in addition, a significant number of fractures of tectonic origin may be present in these rocks (scheidegger, 1978; puffer & student, 1992; fernandes et al., 2016). abrasion is the predominant erosion process in rocks with few fractures, while rocks with submetric spacing of fractures favor plucking. (hancock et al., 1998; whipple et al., 2000a). abrasion is produced by the removal of particles from the rock surface by the impact of sediment carried by the flow, while plucking is a process of extracting blocks bounded by joints, only by hydraulic forces. thus, cooling joints in basaltic riverbeds typically provide an erosive response involving plucking (whipple et al., 2000b; lima & binda, 2013; lima & flores, 2017). although differences in fracture style are not significant for long-term river erosion through plucking, these differences can influence bed morphology on the scale of reaches. two features of fracture systems are critical in determining differences in erosive intensity (scott & wohl, 2019). the first is fracture density, which determines the size of rock blocks susceptible to plucking and macroabrasion (whipple et al., 2000b). the second is fracture direction, which can favor the plucking process depending on its relationship with the channel flow direction (dubinski & wohl, 2013). the fracture density has been recognized as a determining variable of the type of erosion (hancock et al., 1998; whipple et al., 2000a). however, the fracture density has not been sufficiently studied as a factor in the variation in erosion intensity within the plucking domain. this gap in the literature is even greater for basalts, partly because of a lack of detailed knowledge of the plucking erosion mechanics in complex fracture systems. the effect of the fracture direction on river erosion has been further studied in terms of the preferential direction imposed on drainage channels (e.g., pelletier et al., 2009). regarding the effect on the erosion intensity, some studies (e.g., miller, 1991; dubinski & wohl, 2013) have shown that the fracture direction determines the plucking efficiency, and greater exposure of a fracture face to the flow facilitates block sliding. in this study, the effect of variability in fracture style of basalts on plucking erosion in a bedrock river reach was analyzed in detail. the studied reach presents fracture variations due to the internal zones of common basaltic flow from the paraná volcanic province. moreover, the area is influenced by tectonic fractures imprinted over cooling joints. thus, it was possible to evaluate the effect of density variation and fracture direction on plucking behavior in a relatively small area. these two factors that in part control the relationship between river erosion processes and volcanic rocks, will be important to understand other areas because continental flood basalt provinces worldwide are similar (jerran & widdowson, 2005). study area the studied river is in the paraná volcanic province (south america), which is one of the largest igneous provinces of continental basalts on earth (frank et al., 2009). this volcanism was dated between 137 and 127 ma (turner et al., 1994) and about 90% of the rocks are basalts and andesites (piccirillo et al., 1988; lopes, 2008). the area is a bedrock reach of the bananas river, which is a tributary of the jordão river (figure 1), which in turn is a tributary of the right branch of the iguaçu river in southern brazil. this bedrock reach is a knickzone, that is, a reach with a higher slope situated between reaches with lower relative slopes comprising a sequence of small knickpoints and rapids (figure 2). the bedrock reach has a variable width of 45 to 90 m and a length of 400 m. this area was selected because of the excellent exposure of bedrock, especially during the winter low flows. the studied reach is approximately 80 m upstream from an intersection with a structural lineament (n45e), which is approximately 18 km long (figure 1c). another lineament (n70e) is situated about 500 m downstream of the study area. considering that these lineaments may represent fault lines, fracturing of the bananas river bedrock may have been affected by the stress associated with these faults or other regional structures (figure 1b). one possible effect of this stress is an increase in the fracture density of the basalts near these lineaments. the basaltic flows in the region surrounding the study area are of the simple type (walker, 1971), and are characterized by large lateral extensions and large thicknesses (30 m on average; nardy, 1995). the flows have thick massive basalt cores with relatively thin zones at the top (0.3–3.0 m) consisted of amygdaloidal basalt filled with quartz, chalcedony, and zeolites (arioli et al., 2008; serviço geológico do paraná, 2013). locally, in the bananas reach, the outcrops share these regional morphological characteristics. the upper part is a vesicular-amygdaloidal zone with approximately 3 m thick. the lower part consists of massive basalt with an outcrop thickness of approximately 6 m (figure 2). the flow structure is similar to that described by thordarson & self (1998) for what they called proximal sections in the roza member of the columbia river basalt group. unlike the pattern widely described in the literature, in which hexagonal columns are well developed, the basalts in this area are not columnar with well-defined geometry. in the core of the lava flow, the main joints have well-developed curvilinear faces that produce columns of variable horizontal dimensions, with maximum value less than 1 m. the curved figure 1. (a) location of the study area in the regional context and (b) in relation to the main drainage and main structural lineaments, according to the serviço geológico do paraná (2013). (c) shaded relief map with some lineaments (red lines) and the study area (yellow box). figure 2. longitudinal profile of the studied reach and selected field views. jointing pattern are not to scale. note that the positions of the sample areas are approximate because the profile was outlined more to the right side of the channel while the sample areas were located near the left side. 15fracture variability in basalts and its effect on river erosion: a case study in the paraná volcanic province joints tend to be tapered at the base. the main joints intersect with other joints, many of which are short, while others are longer and straighter. according to piccirillo et al. (1988), the central region of the paraná volcanic province (and study area) occupies a transitional zone between southern (basalts tio2 < 0.2%) and northern regions (basalts tio2 > 0.2%) based on geochemical and petrographical criteria. the rocks exposed in the riverbed and surrounding area are dark gray with mesoscopic fine-grained phaneritic to aphanitic texture. the low density of vesicles viewed in the exposed riverbed (figure 2) is related to the base of that basaltic flow zone. methods and materials to evaluate the influence of fracture characteristics on river erosion in the studied area, the survey was designed to measure the density and direction of fractures along the bedrock reach. these two objectives required two approaches to data collection, one based on field observations and the other on remote sensing. in the field, seven sample areas (figure 3) were photographed vertically with a 14.3 megapixel digital camera at an average height of 1.6 m. consequently, the average diameter of the circular areas was 1.34 m. the photographs were digitally analyzed to enhance the fractures and measure the directions and densities. although the bedrock reach is approximately 400 m long, the sample areas were only 120 m long (figure 3). despite the low flow, river water and sediments covered the lower portions of the bed, so field photographs could not be made in this part of the reach. figure 3. locations of riverbed sample areas (first box) and field photograph with digitally enhanced fractures (white lines), the circle (yellow) was used as a scanline for the calculations (see text for details). the scale card on the riverbed is 8 cm long. fracture density was calculated using the circular scanline technique (mauldon et al., 2001), which was adapted for use in field photography. in each photograph, a circle was drawn with an average diameter of 1.34 m, along which the number of intersections with fractures was counted (figure 3). the density (d) was obtained using: d = n/4r (1) where n is the number of fractures intersected by the circle, and r is the radius of the circle. data from the field sampling areas were first used to determine the fracture directions. many long fractures with significant erosion control were observed and could not be sampled, because the spacing between them, in many cases, exceeded the size of the circular sample areas. thus, the bedrock reach was imaged with a remote piloted aircraft (rpa, phantom 4 advanced) carrying a camera with a 20-megapixel cmos sensor. images were taken at an altitude of 99.8 m and a resolution of 2.46 cm/pixel. the 188 photographs obtained were digitally processed using agisoft photoscan software to generate an overall orthophoto of the area (figure 3). the orthophoto was used to map the main fractures in the bed. this procedure avoids the inclusion of data on small fractures, i.e., cooling joints or even tectonic fractures, which were surveyed with circular scanlines. some tectonic fractures could be identified in the field because they are relatively long, straight and intersect the cooling joints and sometimes cause displacement. in general, it is difficult to differentiate among tectonic and nontectonic fractures in the field, particularly small fractures, and the irregular pattern of cooling joints in the lava flow type present in the study area. in this study, differentiating the nature of minor fractures was not considered because their effect on erosion was assessed indirectly from fracture density. instead indications of tectonic participation in fracturing of volcanic rocks can be better evaluated from the preferential directions expressed in the fracture system, which we do following the methods of other authors (scheidegger,1978; rogers et al., 1996). the survey data were analyzed to evaluate the relationships between (1) fracture direction and direction of the main erosion axes, (2) channel flow direction and direction of main erosion axes, (3) variability in fracture density and possible controls, and (4) fracture density and erosional characteristics (typology and intensity). to analyze the first relationship, it was assumed, based on field observations and aerial image examination, that erosion produced step-pool features in which the main axes are located where the bed is deeper. many pools have an elongated shape, which served to define the erosion axes. the general directions of these erosion axes were traced using images from rpa. to analyze the relationship between fracture density and erosional characteristics, we qualitatively evaluated the spatial distribution of the type and intensity of erosion processes (abrasion and plucking) based on field observations and rpa imaging. results fracture directions the main fracture directions differed among sample area (figure 4) and a more detailed analysis revealed some peculiarities. first, the lineament directions (n45e and n70e) did not correspond to the main direction of fractures in any of the sample areas, except in area 0435 where the direction n50-60e is remarkable. another peculiarity involved fractures with main directions close to north (~20°), which appeared in four areas. directions close to east-west occurred in three of the seven areas. in addition to the presence of preferential maxima in most areas the fractures exhibit a broad range of directions, except for the upstream area (2784), in which the number of fractures was smaller and concentrated in two distinct directions. unlike field sampling, the high-resolution rpa images allowed us to observe the major fractures (i.e., the longest ones) in the bed (figure 5). the directions of these major fractures have four maxima, one oriented nw (50– 70) and three oriented ne (10–20, 50–70, and 80–90). if only the main and secondary maxima of the circular samples (figure 3) are observed, there is some level of correspondence with the rpa survey. all circular sample areas had some maxima that corresponded to the rpa survey. areas 2758 and 2764 presented the highest correspondences with three maxima each one. 16 adalto gonçalves lima, marcos aurelio pelegrina, murilo pontarolo segment has a different direction distribution. in the upper segment, erosion axes with a direction of n60–70e are predominant. in the middle segment, the main direction is n80-90e, whereas in the lower segment, the main direction is n10–20e. the lower segment is distinguished from the others by the almost absolute predominance of a single erosion axis direction. each of these major directions of the erosion axes coincides with the major directions of bed fracture (figure 5). in the upper segment, even the minor directions of the erosion axes correspond to major fracture directions. the overall channel flow direction changes in each segment of the reach. it ranges from 50 nw in the upper segment to 40 nw in the middle segment and finally 30 nw in the lower segment. the smallest angle between the general direction of flow in the channel and the main direction of the erosion axes in each segment is 40–60°. in the upper segment, where three erosion axes dominate, the angle with respect to the main direction is 60°, that with respect to the secondary direction is 40°, and that with respect to the third direction is 50°. although there are differences in the angular relationship between erosion axes and channel flow direction, there is a consistency in the values around 50°. as noted earlier, some fracture directions surveyed in the circular areas match the maxima obtained from the rpa (figures 4 and 5), while others do not. unmatched directions are related to minor and multidirectional fractures, which may be tectonic or cooling joints. these fractures are not related to the directions of erosion axes but contribute to overall erosion by plucking. an example of these distinct roles is evidenced in sample area 3684 (figure 7). in this area, a conjugate system of tectonic fractures is well-defined. the n85w main fracture act as a control on the erosion direction. the river flow direction at this site is approximately n40w, which result in an angle of 45° to the main fracture. minor and long joints are crossed by the main fracture, producing small blocks ready for plucking. in fact, erosion by plucking produced a small step in the bed. here and in other places in the study area the main fracture is opening and facilitates hydraulic wedging (hancock et al., 1998), which occur as clasts are trapped in fractures, forcing displacement of blocks. fracture density table 1 presents the fracture density calculated from measurements made in the seven sample areas. the areas appear in the order of their location along the analyzed reach, that is, from downstream to upstream (figure 3). data analysis reveals that fracture density tends to decrease in the upstream direction. the more upstream area (2784) is on the amygdaloidal basalt. at the other end of the analyzed transect, by contrast, the fracture density of area 0435 is three times higher than area 2784. sample area 0435 is situated on a massive basalt and is closest (~80 m) to the lineament n45e which river crosses. figure 4. fracture directions in the sample areas of the studied reach. the fourdigit numbers identify the sample areas (see figure 3); letters l, m and u indicate the lower, middle and upper segments of the reach, respectively, according the river flow direction (see figure 6); and (n) is the number of fractures. the diagrams are arranged sequentially from downstream to upstream. figure 5. fractures in the bedrock reach, according to the rpa image survey and the respective rose diagram. figure 6. (a) main erosion axes in the studied river reach, (b) general distribution of directions, and (c) distribution by channel segments (l: lower, m: middle, u: upper) according to the change in flow direction, indicated by an arrow in a dashed-line. erosion axes versus fracture direction the main erosion axes of the studied reach have two well-marked maxima at n00–10e and n80–90e (figure 6). the n80–90e direction coincides with the major maximum of the bed fracture directions (figure 5). by contrast, the directional axes at n00–10e are close to another directional maximum of fractures oriented n10-20e. the number of erosion axes with a nw direction is lower than the number with ne directions; however, there is a small concentration around the n50-70w direction, which coincides with one of the main fracture directions (figure 5). in the studied reach, the river channel develops a wide curve, changing the flow direction, which in turn can influence which fracture directions facilitate erosion. to verify this hypothesis, the direction of the erosion axes was evaluated separately in three channel segments (figure 6). in fact, each 17fracture variability in basalts and its effect on river erosion: a case study in the paraná volcanic province discussion tectonic fractures and cooling joints basaltic rocks have many joints owing to the cooling process of lava. these joints assume various directions depending on their location in the lava flow body or even in a limited area. as shown in figures 3 and 4, there is a wide distribution of fracture directions in the study area. however, tectonic fracturing is possibly indicated by the presence of some main directions, that agree with some authors (e.g., rogers et al., 1996). the presence of tectonic fractures in the study area is directly confirmed by some field observations (figures 3 and 7); however, considering that the joint directions are distributed among several classes, fractures of tectonic origin may overprint other types of fractures because pre-existing joints may accommodate subsequent tectonic stress (patrick muffler et al., 1994; grant & kattenhorn, 2004; moir et al., 2010). main fractures and erosion the importance of main fractures to erosion in the studied river reach is highlighted by the strong relationship between the erosion axis directions and the main fracture directions (figures 5 and 6). the tectonic nature of some fractures with greater pervasiveness and directional continuity makes them preferable for erosion. this also may be true for some long (3-4 m) cooling joints. the more abundant and multidirectional small joints act to control the plucking process because they limit the polyhedral blocks, but not to determine the main erosion direction (figure 7). most of the main erosion axes, as defined in this study, are related to step-pool morphology zones. the erosive evolution of these zones from long fractures can be understood primarily in terms of the higher density of fractures along one direction. over a long fracture, the number of intersections with other natural basalt joints becomes larger. in addition, tectonic fractures will often have subsidiary (pinned) joints, which increase the density of rock discontinuities along the main direction (peacock, 2001). as a result, the blocks available for simple plucking and breakage by impacts from sediments in transit (macroabrasion, whipple et al., 2000a) are potentially smaller than those in other bed areas and erosion is concentrated along this line (figure 7). these preferential linear erosion zones may eventually be interrupted owing to local variations in the channel flow direction and/or the presence of basalt joints. especially at the intermediate levels of the thick basaltic flows of the paraná basin province (serviço geológico do paraná, 2013) and of the studied area, columns with metric dimensions (height and width) delimited by joints with curvilinear planes are common. the river flow can create alternative paths along these joints. flow and erosion direction as noted earlier (results section), the overall flow direction makes an angle of 40–60° with respect to the main direction of the erosion axes in each segment into which the reach has been divided. furthermore, these axes directions correspond to major fracture directions. the slope of the channel has no discernible effect on this angular relationship. in the lower segment, which has a higher slope, the angle is 40°. in the middle segment, which has a smaller slope, the angle is 50°. the slope of the upper segment is between those of the other two segments; however, the main angular relationship is 60°, although the 40° and 50° relationships also occur in secondary directions. the angular relationship between the flow and fracture directions (erosion axes) within the observed range apparently depends on the variability of the channel flow hydrodynamics, the variability of the interaction between tectonic fractures and cooling joints, and the lithological characteristics. considering the large number of variables involved, the only possible conclusion is that the average relationship between the channel flow direction and erosion axis direction is on the order of 50°. satellite observations of other parts of the bananas river and other rivers in the region, which run over basalts, confirm the average angular relationship of 50° between the direction of flow and the erosion axes, especially the knickpoint face directions. miller (1991), who studied rivers in siliciclastic and carbonate rocks, also frequently observed oblique arrangements of knickpoints generated by fractures. fracturing is much more complex in basalts of the study area than in that sedimentary rocks. the presence of more pervasive and continuous tectonic fractures seems to be essential for establishing the angular relationship with the flow direction, similar to the case in simpler fracture systems. why the river flow tends to have this specific angular relationship with the major fractures remains an open question. although a definitive answer requires further investigation, it is possible to deduce that angles much smaller than 40° between the flow direction and the direction of these pervasive fractures favor the adjustment of the channel along the structures. as several surveys have shown, rivers tend to exploit directions represented by vertical fractures (e.g., eyles et al., 1997; pelletier et al., 2009). flume experiments (dubinski & wohl, 2013; wilkinson et al., 2018) with regular fracturing suggest that an orthogonal arrangement of fractures with respect to the flow direction favors the removal of bed blocks. although the experimental results presented by george et al. (2015) for blocks with complex geometries indicate that the erodibility is greater when the angle with the flow is 75°, it is worth noting that this is the angle with respect to the central axis and not that with respect to the face of the block, owing to its tetragonal geometry. correcting the angle to that with respect to the face of the block gives an approximate value of 55°, which is consistent with the average angle obtained for the bananas river. fracture density variation lima & binda (2013) found that the average fracture density in the basalts of the das pedras river, close to the bananas river (figure 1), in the reaches associated with structural lineaments, is 6.05 m/m². the average obtained for the bananas river study (6.7 m/m²) is similar (table 1). overlap between tectonic fractures and basalt joints occurs effectively, as confirmed by the presence of preferential directions (figure 5) and field observations (figure 7). this implies that the fracture density is increased by tectonic effects. as previously described, two major lineaments are close enough to the area to eventually contribute to the generation of fractures. however, the upstream decrease in fracture density (table 1) is depending on the distance from the structural lineament (n45e) that the channel crosses? the maximum fracture density (9.2) appears in the zone closest to that lineament (table 1). sample area 2784 is farthest from the lineament and has the lowest fracture density (3.73). contrary to what was expected if the study area is associated with a damage zone, there is no maximum direction parallel to the lineament (n45e), although it is possible for joints to form at high angles to the strike of a fault (kattenhorn et al., 2000). figure 7. example of differential role of fractures in erosion of riverbed in the study area. (a) vertical view of sample area 3684. heavy lines highlight the conjugate system of tectonic fractures, and the arrow indicate river flow direction. (b) upstream oblique view of the area. table 1. fracture density in bananas river sampling areas. sample area n r d 0435 25 0,65 9,62 2758 20 0,67 7,46 0397 19 0,62 7,66 2764 18 0,77 5,84 3684 14 0,64 5,47 3674 19 0,67 7,10 2784 10 0,67 3,73 n: number of fractures that intersect the circumference of the sample area; r: sample area radius (meters); d: fracture density according mauldon et al. (2001), d = n/4r. 18 adalto gonçalves lima, marcos aurelio pelegrina, murilo pontarolo fracture density in basalts varies depending on the internal lava flow structure. in fact, area 2784 is on an amygdaloidal zone, whereas the others are on massive basalt. for amygdaloidal zones, fracture densities equal to or greater than those of massive zones have been reported (faust, 1977; thordarson & self, 1998), although lower densities are also possible (lima & binda, 2013). one aspect that favors the damage zone hypothesis is the gradual decrease in fracture densities upstream. transitions in the pattern of fractures between internal zones of the lava flows are usually abrupt. moreover, the absence of fractures parallel to the lineament could be masked by the cooling joints which served to accommodate subsequent tectonic stress, as described in the first section of this discussion. further and detailed analysis will be necessary to describe the nature of the lineament and to define if there is a real relationship with the pattern of fracture density detected in the study area. fracture density and erosion regardless of the causative factor, could the variation in the fracture density affect the erosion processes? theoretically, variation in fracture density can affect the type of erosion processes that occur and their intensity. low fracture density, as found upstream in the study area, favors abrasion (whipple et al., 2000b). the formation of potholes indicates abrasive processes. however, in the studied reach, these abrasive features appear in both upper and lower segments, regardless of the different fracture densities (9.6 and 3.7, respectively). despite these signs of abrasion, plucking and macroabrasion are the main processes acting along the entire reach. the increase in fracture density (table 1), which coincides with massive basalt zone, favors plucking because it decreases the size of the rock blocks. by contrast, the decrease in fractures densities culminates in an amygdaloidal basalt zone (sample area 2784) that is less resistant and more susceptible to weathering (lima & binda, 2013). low fracture density limits the plucking of blocks because they are larger. however, the high susceptibility to weathering and low rock resistance favor macroabrasion (small et al., 2015). this is observed in the increase of the chipping features of the bed surface (sample areas 3674 and 2784, figure 3) and in the smoothing of the reach morphology (figure 2). while erodibility may be the same throughout the reach, bed morphology and slopes may differ. conclusions the study of a bedrock fluvial reach that developed on basalts allowed us to verify the effects of spatial variability in density, typology, and direction of fractures on erosion behavior. although it does not change the type of dominant process, which in this case is plucking, this variability modifies the intensity and the distribution of erosion in the channel, with implications for bed morphology. the presence of preferential fracture directions attributed to tectonic activity was verified. owing to their pervasiveness and length, some of these tectonic fractures control the main axes where bed erosion is concentrated. the small, abundant, and multi-directional basalt cooling joints control the plucking process, but do not the erosion direction. the general direction of river flow is at an average angle of 50° with the main erosion axes, that is, with the direction of the main tectonic fractures. although further investigation is required, available field findings and experimental data of the others authors suggest that this behavior is probably related to shear force optimization necessary to perform the plucking process. in the studied reach, fracture density decreases progressively upstream from 9.62 to 3.73 m/m², related to distance from a structural lineament transverse to the channel. the decrease in fracture density does not affect the dominant erosion process, i.e., plucking. however, owing to the textural change in the basalt, specifically, a decrease in rock resistance, macroabrasion is favored. acknowledgements this work was partially supported by funding of the universidade estadual do centro-oeste, guarapuava, pr. we thank prof. ms. paulo nobukuni, laboratório de planejamento e gestão territorial (unicentro), for providing access to the rpa used in the field survey. we are grateful to four anonymous reviewers for their careful reading and comments that enabled us to improve our original manuscript. references arioli, e. e., licht, o. a. b., vasconcelos, e. m. g., bonnet, k. l. & santos, e. m. 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(2018). plucking phenomena in nonuniform flow. geosphere, 14(5), 2157-2170. issn 1794-6190 e-issn 2339-3459 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 275-284 g eo lo g y keywords: cesar sub-basin, well log analysis, reservoir rock, petrophysical properties. palabras clave: subcuenca cesar; análisis de registros de pozo; roca reservorio; propiedades petrofísicas; how to cite item alarcon olave, h. f., & herrera otero, e. h. (2021). petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar subbasin, colombia. earth sciences research journal, 25(3), 275-284. doi: https://doi. org/10.15446/esrj.v25n3.89293 the cesar-ranchería basin has all the necessary elements for the generation, expulsion, and migration of hydrocarbons and considerable potential for coal bed methane (cbm) in colombia. previous studies in the cesar basin focused on understanding the tectonic evolution, stratigraphy, hydrocarbon generation potential, and evaluation of reservoir potential in cretaceous calcareous units and quartzose sandstones from the paleocene barco formation. these studies had confirmed the existence of an effective petroleum system, with several episodes of oil expulsion and re-emigration in the miocene period, turning the cenozoic clastic succession (barco, los cuervos, la loma, and cuesta formations) into an element of significant exploratory interest to clarify the potentiality of the basin in terms of hydrocarbon accumulation. the petrophysical parameters of cenozoic units (shale volume, porosity, water, and oil saturation) were determined by integrating wells log and core samples analyses from three stratigraphic wells. the integration of these results synthesizes the petrophysical behavior of the units. it defines intervals with clay volumes of less than 30%, effective porosity around 20%, which means favorable characteristics as reservoir rocks that need to be considered in future exploratory projects. abstract petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar sub-basin, colombia determinación de propiedades petrofísicas a través del análisis de registros de pozo para conocer las primeras ideas en la evaluación de reservorios clásticos cenozoicos en la subcuenca cesar, colombia https://doi.org/10.15446/esrj.v25n3.89293 la cuenca cesar ranchería contiene todos los elementos necesarios para la generación, expulsión y migración de hidrocarburos y el potencial de gas asociado a carbón (cbm) en colombia. aunque algunos investigadores se han centrado en el entendimiento de la evolución tectónica de la cuenca, estratigrafía y potencial de generación de hidrocarburos, los trabajos en busca de la caracterización de rocas reservorio solo se han llevado a cabo en formaciones cretácicas, consideradas el principal objetivo reservorio, sin hallazgos importantes. investigaciones recientes que confirman la existencia de un sistema petrolífero efectivo, episodios de expulsión y remigración de hidrocarburos en el mioceno, convierten la sección clástica cenozoica en un elemento de significante interés exploratorio para aclarar el potencial de acumulación de la cuenca. el presente estudió se realizó en la sección superior de la sucesión barco-cuervos, las formaciones la loma y cuesta, en la parte sur de la subcuenca cesar, una área prospectiva para la industria de los hidrocarburos, la cual tiene la presencia de estructuras que preservan el mayor espesor de unidades cenozoicas. a través del análisis de registros de pozo y la integración con información de núcleos se determinaron los parámetros petrofísicos (volumen de arcilla, porosidad, saturación de agua y aceite), se generó un modelo de litotipos y se identificaron unidades hidráulicas de flujo. la integración de estos resultados sintetiza el comportamiento petrofísico de las unidades y propone intervalos con características favorables como roca reservorio para las formaciones clásticas cenozoicas presentes en los pozos de estudio, lo que proporciona nuevos datos que deben ser considerados en futuros proyectos exploratorios y contribuye al conocimiento geológico de la subcuenca cesar. resumen record manuscript received: 20/07/2020 accepted for publication: 25/06/2021 helmer fernando alarcón olave*, edwar hernando herrera otero universidad industrial de santander * corresponding author: helmer2188218@correo.uis.edu.co https://doi.org/10.15446/esrj.v25n3.89293 https://doi.org/10.15446/esrj.v25n3.89293 https://doi.org/10.15446/esrj.v25n3.89293 mailto:helmer2188218@correo.uis.edu.co 276 helmer fernando alarcón olave, edwar hernando herrera otero introduction the cesar-ranchería basin (figure 1) fulfills all the requirements for hydrocarbon expulsion, generation, and migration (barrero et al., 2007; hernández & jaramillo, 2009; mesa & rengifo, 2011), and it is also considered to hold the most extensive coal bed methane (cbm) potential in colombia (garcía-gonzaléz, 2010). although the mid-cretaceous calcareous succession and the barco formation has been considered the primary target reservoir in the area (garcía et al., 2007; mesa & rengifo, 2011), new core data and episodes of expulsion and probable remigrations in the miocene time (ayala, 2009; sánchez & mann, 2015) turns the cenozoic clastic rocks into an attractive exploratory section to evaluate its potential as a hydrocarbon reservoir the southern part of the cesar sub-basin is considered a prospective area for the hydrocarbon industry. it preserves several tens of meters of reservoir units, the most significant thickness of the cenozoic clastic section compared to the other sub-basin sectors (martinez et al., 2012; sánchez & mann, 2015), offering an excellent opportunity to apply well log interpretation techniques. this allows the estimation of petrophysical parameters, such as total porosity, effective porosity, permeability, water, and oil saturation, acquiring a quantitative perspective and providing accurate and essential information in reservoir evaluation (rider & kennedy, 2011; wu & grana, 2017). in this context, by examining the relationship among the petrophysical rock properties, petrophysical rock types, and hydraulic flow units, this research aims to provide data to understand the petrophysical behavior while contributing to the knowledge of hydrocarbon accumulation potential in bypassed reservoir units within cuesta, la loma, and los cuervos formations. such new data should be considered when studying the feasibility of future exploration projects in the area. geological framework the hinterland cesar-ranchería basin, located at the northern part of colombia, is bounded by the santa marta massif (smm) to the west, the perijá range (pr) to the east and the northern termination of the santander massif to the south (figure 1a). the geological evolution of these three mountain systems controlled the geodynamics of sedimentation and subsidence of the cesarranchería basin (ayala et al., 2009, bayona et al., 2011, sánchez & mann, 2015). presently, the basin is divided into two sub-basins by the valledupar high the ranchería sub-basin to the north and the cesar sub-basin to the south (figure 1a). the analyzed anh-1 and anh-2 wells are located in the southeastern sector of the cesar sub-basin in both flanks of la loma syncline. which is a structural low with 5 km of structural relief that preserves the major thickness of cretaceous and cenozoic units, limited at the western by la loma fault, a nw-se striking thrust, and at the eastern by the tigre fault an eastwarddipping, nw-se striking thrust with ca 1–1.5 km slip (sánchez y mann., 2015) (figure 1b,1c). whereas anh-3 well is located farther south at the foothills figure 1. a). regional tectonics of the caribbean-south american plate boundary and geological setting of the cesar-ranchería basin (crb). b). location map of the study wells in the southern part of the cesar sub-basin. modified from bayona et al. (2009) and sgc (2015). c) seismic line gm-81-07 from mora & garcía (2006), showing the geological setting of the anh1, anh-2 wells, located in la loma syncline, d) schematic diagram based on surface and well’s information of anh-3 well, located next to the perija range foothills. 277petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar sub-basin, colombia of the junction of the northern santander massif and the southern part perija range. this area constitutes another tectonic block in the hanging wall of the tigre fault where most of the late cretaceous and upper sections are truncated by eocene unconformity and shows a complex faults pattern with a ne-sw main fault trend, and a secondary fault trend composed of ew and nw-se impacting fault populations (sánchez & mann., 2015) (figure 1b,1d). cesar sub-basin volcano-sedimentary rocks from the lower jurassic la quinta formation are the oldest sedimentary rocks drilled in the cesar sub-basin (mesa & rengifo., 2011), studies report the existence of an extensional domain, accompanied by intense volcanism and the consequent development of volcano-sedimentary units (la quinta formation) (maze, 1984, zuluaga et al., 2015). this configuration prevailed until the early cretaceous, allowing the deposition of post-rift sediments (rio negro formation) (cáceres et al., 1980, ayala, 2009). with the start of a passive-margin regime in the aptianmid campanian the depositional environment changed to marine environments from internal to external carbonate platforms, giving way to the development of limestones, mudstones, and calcareous shales which are occasionally rich in organic matter (cogollo group and la luna formations) (cáceres et al., 1980, gómez, 2001, ayala, 2009). there is evidence of high tectonic subsidence in the maastrichtian as there is a major unconformity separating la luna from molino formation (martínez & hernandez, 1992). the anoxic-sulfidic settings of la luna formation were transitionally replaced by post-oxic conditions in molino formation (or colon formation) when aerobic environments were established in the upper maastrichtian (martínez & hernandez, 1992). the molino formation is divided into tres esquinas and socuy members at the base, marked by two major paleobathymetric fluctuations. the accumulation in the lower tres esquinas member is mainly chemical with precipitation of glauconita. the upper socuy member records a definitive regression with increasing oxygenation and the input of synorogenic (metamorphic and plutonic) and siliciclastic detritus mixing with calcareous shales (martinez 1985; martinez & hernandez, 1992). the marine sequence is followed by the presence of transitional to continental cenozoic deposits, whose sediments mainly came from the southeastern flank of the smm (bayona et al., 2007, ayala et al., 2009), represented in the cesar sub-basin by paleogene sandy catatumbo and barco formations that changed up-section to sandy-muddy lithology of the los cuervos formation with the presence of associated coal seams (ayala et al., 2009). coarse-grained fluvial and alluvial fan deposits of lower eocene age have been reported by mora et al. (2006) and ayala et al. (2009) as la loma unit. sandy continental deposits of miocene age (cuesta formation) are found between two angular unconformities present in the basin of eocene middleoligocene and upper miocene-pliocene age (sánchez and mann, 2015) (figure 2). material and methods the available data for this study were provided by the agencia nacional de hidrocarburos (anh) of colombia. these new data include stratigraphic core information and digital well logs from three stratigraphic wells (anh-1, anh-2, and anh-3). the logs set includes all the basic tools, such as caliper (cali), spontaneous potential (sp), gamma ray (gr), shallow and deep resistivity (lld, ils), density (rhob), neutron (nphi) and photoelectric factor (pef) logs. petrophysical software was used to integrate and conduct the qualitative and quantitative analysis of the information. environment corrections and depth calibrations were done prior to the well log analysis using core information and gamma-ray log as a reference curve. the shale content (v-sh) was calculated for each operative interval from gamma-ray gr log (equation 1) (dresser atlas, 1979). estimations of porosity (ø) were made from density (rhob) log (equations 2,3). v-sh parameter and resistivity logs (lld) in the indonesian equation (equation 4) were used to obtain water saturation (sw). gamma-ray log has a direct relation with shale volume: a low response of gamma ray indicates clean intervals. in order to reveal intervals with better reservoir probability, the volume of shale in the cenozoic clastic section was estimated from a gamma-ray log, using the equation (1).    vshgr gr gr gr gr log max max min (1) where: vshgr = shale volume from gamma ray grlog = gamma-ray reading of formation grmax = maximum gamma-ray reading in the shale zone grmin = minimum gamma-ray reading in the clean zone. total porosity (øt) can be derived from density, neutron, or sonic logs. however, the effective porosity (øe) requires a correction of the clay content, as shown in equation (3). in this study, the porosity estimated through density log (phid) by equation (2) (dresser, 1979), after shale effect correction, provides the best results when compared against laboratory data.    phid rhob rhobma rhobf rhobma (2) where: phid = porosity derived from density log rhob = log density rhob ma = matrix density (ss = 2.65 g/cc, ls = 2.71 g/cc, dolomite = 2.87 g/cc) rhob f = fluid density (fresh water mud = 1.0 g/cc, salt water mud =1.1g/cc). figure 2. generalized stratigraphic column of the cesar sub-basin. modified from mora & garcía, 2006. 278 helmer fernando alarcón olave, edwar hernando herrera otero     e t vsh* 1 (3) where: øe = effective porosity øt = total porosity vsh = shale volume. an important step in petrophysical evaluation is to determine the percentage of rocks filled with specific fluids, such as water, oil or gas (tiab et al., 2012). water saturation is always part of the fluids that occupy the pore spaces of reservoir rocks (dresser, 1979). water saturation was estimated using the indonesian equation (4), due to the presence of clay content on the sandy facies in the studied section.                                           sw vsh rsh rt rt a rw* vsh m n 1 *0.5 0.5 0.5 2/ (4) where: sw = water saturation rsh = resistivity measure in 100% shale rt = true resistivity rw = formation water resistivity a = tortuosity exponent m = cementation exponent n = saturation exponent. the pickett graphical method (pickett, 1966), with theoretical values of m = 2, n = 2, and a = 1, was used to determine the formation water resistivity. based on the fluid saturation property, expressed as the fraction or percentage of the total pore volume occupied by the oil, gas or water (tiab et al., 2012), the hydrocarbon saturation can be deduced by the relation (5).  sw sw1 (5) relations between permeability and porosity were used to identify points with similar flow conditions and to clarify the presence of hydraulic flow units (lee et al., 2002; perez et al., 2005; mahjour et al., 2016). hydraulic flow units (hfu) were calculated using the winland r35 method (windland, 1972; gunter et al., 2014) that refers the radius of the pore throat, in which the poral networks are interconnected and form a pattern of continuous flow (r35 factor), providing good results in to correlate porosity and permeability laboratory data (ross-coss et al., 2016). the use of hydraulic flow units allows the identification of pathways with similar hydraulic flow conditions widely related to the reservoir quality properties (lee et al., 2002; perez et al., 2005). the identification and classification of hfus were based on winland’s methodology (gunter et al., 2014), using a log linear cross plot of permeability (md) vs porosity (v/v) and r35 value through equation (6).    log r kc c( 35) 0.732 0.588 0.864 (6) where: r35 = radius of the poral throat when the core samples are 35% saturated kc = average air permeability øc = measured porosity of the core. petrophysical rock types were calibrated with core information and defined from logs through an unsupervised model by hierarchical cluster method in the ipsom tool of the petrophysical software. results lithological units identified in the three wells the miocene cuesta, the lower to middle eocene la loma formations, and the upper paleocene los cuervos formation were identified based on sedimentological core description and well log responses, preserving their stratigraphic sense and in accordance with seismic interpretations of the southern part of the cesar sub-basin (mora & garcía, 2006; mesa & rengifo., 2011; sánchez & mann, 2015), along with the characteristics described for la loma formation in the area (mora et al., 2006; ayala, 2009). next to perija range, basal cretaceous sandstones (rio negro formation?) were identified in anh-3 well (figure 3). a brief description of each formation is presented below from the stratigraphic columns provided by the anh. la loma syncline area (anh 1 and anh-2 wells): los cuervos formation shows in the lower segments of the cores bioturbated carbonaceous mudstones with plane parallel lamination and sheets of coal intercalated with thin bedded, glauconitic, and fine-grained sandstones. at the middle and upper parts of the cores increase the presence of laminated, fineto medium-grained litharenites. the presence of organic matter and sulfur decrease at the top (figure 3). la loma formation is composed by medium-to coarse-grained sandstones (mainly sub-lithoarenites and quartz sandstones), locally conglomeratic, with cross beds and oxidation marks. these sandstones are intercalated with clast and matrix supported conglomerates. gray mudstones and siltstones interbeds are present in the middle segments of this unit. according to mora & garcía. (2006), the contact with the cuervos formation is probably a paraconformity. the cuesta formation is stratigraphically in discordant contact over la loma unit. this is a heterogeneous unit characterized by thin intercalations of poorly sorted litharenites, arkosic sandstones with polymictic conglomerates, and mudstones. perijá foothills area, anh-3 well. the formations exhibit important lithological and thickness variations at east of the tigre fault where most of the late cretaceous and upper sections are truncated by eocene unconformity (sánchez & mann., 2015) and the rocks have major influence from the santander massif and perijá range as sediment sources (ayala et al, 2012; ojeda & sánchez., 2013). in anh-3 well, upper cretaceous and paleocene sections are absent, hence la loma formation it’s in discordant contact with cretaceous basal sandstones. cretaceous basal undifferentiated, rio negro formation? composed of sub-lithoarenites and quartz-sandstones, fine to coarsegrained well cemented occasionally conglomeratic with cross-stratification and siliceous conglomerates with imbricated intraclasts. there are sporadic intercalations with sandy mudstones which contain calcareous layers and lenses. sulfur, organic matter, carbon drapes, and chip drapes, dispersed pyrite, carbonate-filled fractures, and oxidation marks are common throughout the range. la loma formation the presence of accommodation space and sediment supply nearby source areas allow the accumulation on foothills areas of the basin (ayala, 2009) favoring the deposition of lower eocene rocks closer to the santander-perija ranges. la loma formation, in anh-3 well, is in contact discordant with basal cretaceous sandstones and consist of conglomerates with sub-angular muddy intraclasts and very fine to coarse-grained lithoarenites-quartz sandstones which contain muddy, lithic, and chert intraclasts. at the top decreases, the grain size of the sandstones and the conglomerates are less common. cuesta formation the cuesta formation is stratigraphically in discordant contact over la loma unit. this is a heterogeneous unit characterized by thin intercalations of poorly sorted litharenites, sub-litharenitas sandstones occasionally conglomeratic with polymictic conglomerates, and mudstones. the data analysis of the anh-3 well will be carried out separately due to the important variations of lithology, and tectonic story in the cenozoic between la loma syncline area (anh-1 and anh-2 wells) and the hanging wall of the tigre fault, near the foothills of the santander-perija junction area (anh-3 well). 279petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar sub-basin, colombia shale volume estimation (vsh) la loma formation has the lowest vsh values in the interest formations, ranging from 21% to 25%, with 23% on average. the cuesta formation shows shale volume trends from 28% to 37%, with 33% on average in anh-1 and anh-2 wells and 30% in anh-3 well. the uppermost 70 m of los cuervos formation has the highest shale volume compared to the other formations with values from 28%-39%, and 34.5% on average. the basal cretaceous sandstones only, present in anh-3 well, has 22.5% of shale volume on average (table 1). porosity (ø) histograms of effective porosity model on anh-1 and anh-2 wells show that the cuesta formation has the highest data frequency, from 20% to 27%, with 23.5% on average. similarly, the data from la loma formation ranges from 15% to 23%, with an average value of 19%, and the los cuervos formation data, from 13% to 18%, with 15.5% on average (figure 4). the anh-3 well presents ranges of porosity from 0,6% to 3% on average, in all formations (figure 4). combination with other porosity estimations through neutron-density and sonic logs were included in a cross plot to determine the type of porosity on each well, as proposed by hakimi, et al. (2012). these porosities are predominantly primary or intergranular more than the secondary porosity type found on anh-1 and anh-2 wells. conversely, there is an absence of primary porosity on the anh-3 well, corresponding mainly to the presence of secondary porosity (figure 5). figure 4. porosity values and frequencys histogram of effective porosity model in the formations of the study a) cuesta formation, b) la loma formation, c) los cuervos formation, and d) anh-3 well. table 1. minimum, maximum, and average values of shale volume in the studied formations. anh-1, anh-2 wells anh-3 well formations min vsh (%) max vsh (%) average vsh (%) min vsh (%) max vsh (%) average vsh (%) cuesta 28 37 33 26 33 30 la loma 21 25 23,5 20.5 25 23 cuervos 28 39 34,5 rio negro? 20 25 22,5 figure 3. stratigraphic correlation, datum in 150 m for the three wells of the study, showing gamma-ray patterns of each formation and their lateral variations. the blue squares represent oil impregnations; however, they do not represent the thickness or level of impregnation. 280 helmer fernando alarcón olave, edwar hernando herrera otero water saturation (sw) the estimated water saturation values show some intervals with less than 50% sw in the three studied formations, reflecting interest intervals according to cut offs values standard (table 2). hydrocarbon saturation (sh) this estimation allows finding notable hydrocarbon saturations on some intervals having a good correlation with oil impregnations reported in cores (see figure 8 for data of anh-2 well) (table 2). petrophysical rock types well logs reflect direct and indirect measures of rock properties, providing the possibility to determine groups of rocks with particular characteristics that distinguish them from other rock types (gunter et al., 1997b). these rock groups are called lithotypes or petrophysical rock types. relations between electrical log responses were studied to define how many rock types can be determined and what specific characteristics they have particular to them. this analysis defined four petrophysical rock types (figure 6). subsequently, these petrophysical rock types were included in a neuronal network to generate a lithotype model unsupervised by the hierarchical cluster method. the results in the three wells were compared to a discrete log from core descriptions, showing a correlation of more than 80% (figure 8). this first model allows the prediction of lithologies in the studied area using well log measurements. the summary line contains the same parameter for the four lithotypes in the same histogram and shows that each lithotype can be characterized by a unique petrophysical measurement, if the vertical axis differences are considered. hydraulic flow units (hfu) six hfus were defined in the study wells (figure 7). hfu 6 represents the best hydraulic flow conditions with clusters showing the interactions between high values of porosity and permeability, and hfu 1 reflects the least remarkable values of porosity and permeability. a bimodal trend is expressed in hfu´s graph (figure 7). one trend illustrating values of porosity less than 0.1 (v/v), widely related to anh-3 well data, and the other, porosity values from 0.2 (v/v) to 0.35(v/v), corresponding to anh-1 and anh-2 wells data. table 2. intervals classified as best conventional reservoirs in the studied wells and cutoffs. thickness (m) vsh (%) øe fluids wells depth (m) gross (m) net reservoir (m) net pay (m) vsh (%) phie (%) sw (%) sh (%) anh-1 cuesta 133,4 3,7 3,1 1.6 22 23 47 53 anh-1-2 la loma 301,3 26,9 24 13,8 24 23 47 53 426,8 13,5 13,1 12,2 22 23 45 55 464,3 5,4 4,8 2,5 26 21 49 51 494,5 4,5 1,5 1 13 15 46 54 517,2 17,6 11,6 4,1 16 13 45 55 633 9.9 8,6 5,5 19 20 46 54 anh-1-2 cuervos 661 2,5 2,4 1,9 21 19 46 54 748,3 5,12 2,3 1 36 18 47 53 503,9 5,4 5 1 27 15 46 54 578,3 12,5 9,4 1,6 24,5 18 45 55 vsh: shale volume, phie: effective porosity, sw: water saturation, sh: hydrocarbon saturation, cutoffs for net pay: vsh <30%; phie > 15%; sw <50%. figure 5. cross plots of porosity (neutron-density) vs. porosity (sonic) depicting the type of porosity in the cenozoic clastic rocks per well from the area of the study. 281petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar sub-basin, colombia figure 7. identified hydraulic flow units and their correlation factor using porosity vs permeability plot using the winland (1972) method. the relationship among petrophysical parameters, rock types, hydrocarbon occurrence reports, and hydraulic flow units are represented in templates, depicting a direct relation between hydraulic units 4-5-6, petrophysical rocks types 3,4 to hydrocarbon presence data reports, and the lowest values of water saturations (figure 8). discussion the characteristics of these cenozoic formations in the cesar sub-basin as reservoir rock have only been studied from macroscopic and qualitative points of view (mesa & rengifo, 2011). cretaceous fractured carbonate rocks and paleocene sandstones of barco formation sandstones have been cataloged as the main target reservoirs in the basin (mora & garcía, 2006; garcía et al., 2007); these target reservoirs have been the object of petrophysical evaluations and the purpose of the development of exploratory wells, without favorable results in terms of findings commercial hydrocarbon accumulations (mesa and rengifo, 2011). however, in prolific hydrocarbon neighboring basins that were connected to the cesar-ranchería basin until eocene (maracaibo & catatumbo basin) and miocene time (middle magdalena basin) (gómez et al., 2005; mesa & rengifo, 2011; sánchez & mann, 2015). cenozoic clastic reservoirs are the main hydrocarbon hosts in equivalent formations to barco, los cuervos and la loma (escalona & mann, 2006; sarmiento, 2011; pedraza y ramírez., 2011). la loma syncline area: in this study we made the first quantitative perception as a conventional hydrocarbon reservoir quality of clastic cenozoic rocks (los cuervos, la loma y la cuesta formations), in terms of their petrophysical properties in the cesar sub-basin. our study demonstrates for the wells in the la loma syncline anh-1 and anh-2, the existence of intervals thicker than 1,5 m with favorable characteristics to accumulate hydrocarbons, represented by clay volumes < 30%, water saturations < 50%, ranges of porosity from 15% to 28% and permeabilities in a wide range from 5 to 300 md (table 2, figure 8). these properties are catalogued by shogenov et al. (2015) as average to good quality hydrocarbon reservoirs (ø (%)19-25.5; k(md) 113-295.5). our study documents some intervals, presenting the most common porosity and permeability values in productive reservoirs (ø (%) 15-25), according to dikkers (1985). the generation of the lithotype model and the identification of hydraulic analysis flow units enhance the understanding of the petrophysical behavior of the study units in the la loma syncline area (anh-1 and anh-2 wells). these intervals of interest have a direct relationship with the confluent of these factors corresponding to: 52% of occurrence with the lithotype (4) sandstone plus the hydraulic flow units 5 or 6 and 25% when lithotype (4) is linked with hfu 4. the conglomerate’s lithotype (1) with the hydraulic flow units 4 and 3 have a correspondence of 16% with the interest intervals defined. in less proportion, 7%, the lithotype (3) heterolitic sandstone with hfus (3,4 or 5) is associated with outstanding reservoir intervals (figure 8). the porosity values, measured from sandstones and in the porosity model trend, progressively decrease from the cuesta formation to the deeper cored unit los cuervos formation in anh-1 and anh-2 wells. this behavior could suggest the influence of the depth and lithostatic pressure (rider, 1996), and reflects the typical shale composition of los cuervos and the consequent shale effect that reduces the effective porosity values around 3%, in respect to the total porosity estimations (figure 4). figure 6. frequency histograms of gamma ray, density, porosity and resistivity values for the four lithotypes: 1. conglomerate 2. mudstone + claystone 3. heterolithic sandstones 4. sandstones. 282 helmer fernando alarcón olave, edwar hernando herrera otero the santander-perija junction foothill area low porosity (< 5%) and permeability (< 10 md) values in anh-3 well could be associated with the local tectonic evolution of the southeastern sector of this segment of the cesar sub-basin, which is adjacent foothills of the santander massif and perija range. this sector underwent high subsidence rates, as it is exposed by its burial histories as proposed by sanchez and mann (2015), and probably, compaction slightly higher than in the loma syncline area to the west of tigre fault (figure 1 a and c), expressed by the absence of primary porosity (figure 4). another factor is the possible existence of no differentiated diagenetic processes, as this well is closer to perija-santander junction foothills, and the direct influence exerted by this kind of processes in the modification of the petrophysical properties of the rocks (rahman & mccann., 2012; rahman & worden., 2016). next steps for exploration different relations between petrophysical characteristics and other parameters and techniques have been used to enhance exploration and production processes in well-known oil fields (baouche & netjari, 2006; hakimi, et al., 2012; amigun et al., 2012). however, in unexplored and emergent basins, like the cesar sub-basin and the foothills of the southern perija range and northern santander massif, these parameters, need to be related to the geological setting. this relation helps to understand (1) the anh-3 well behavior and (2) identify that in la loma syncline area at the top of los cuervos formation when the organic matter decrease there are sandy intervals with hydrocarbon reservoir potential in a unit typically considered a regional seal. from the petrophysical properties point of view, cuesta, la loma and the upper part of los cuervos formations have intervals with favorable characteristics as reservoir rocks in la loma syncline area; however, it is necessary to identify other elements for the hydrocarbon accumulation which cannot be achieved in this research. based on studies indicating intervals in the oil and gas generation window (martinez et al, 2012), the generation and migration episodes documented in miocene time in the southern part of cesar sub-basin (ayala, 2009; sánchez and mann, 2015), without considering migration paths or trapping oil processes while taking into account the results here obtained, there are favorable arguments to propose that la loma formation and the sandy intervals at the top of los cuervos formation, in la loma syncline area have exploratory interest. furthermore, la loma syncline area presents the main accumulations of source rock in the cretaceous section, where short and vertical migration routes could take place along vertical faults, as proposed by ayala (2009); sánchez and mann (2015). in contrast in perijá range foothills area, the source rock was eroded and cenozoic rocks rest upon lower cretaceous rocks suggesting that there is not figure 8. petrophysical 1d model of well anh-2, shale volume, effective porosity and permeability models, fixed with laboratory data, water saturation model determined by the indonesian model and hydraulic flow units. 283petrophysical properties of bypassed cenozoic clastic reservoirs in the cesar sub-basin, colombia petroleum system in this area, or at least not with the same mechanism of the timing and migration paths proposed for the la loma syncline. in addition, lows porosities (< 5%) and permeabilities (< 10 md) values on the rocks in this sector, evidence of diagenetic processes, higher compaction, and more deformation expected by the fact that is into a tectonic complex zone closer to northern foothills of the santander massif turns this area in no attractive prospect for hydrocarbon accumulation. conclusions based on the petrophysical characterization, carried out on cenozoic clastic units recovered in anh-1 and anh-2 wells, there are intervals that can be interpreted as good quality reservoirs in la loma and los cuervos formations, with clay volumes of less than 30%, effective porosity around 20% and water saturations of less than 50%. la loma syncline area presents the best reservoir rocks characteristics in the sw-ne trend (ø: 15-30%; k: 10-800 md), while the least prospective area, in terms of reservoir rock, corresponds to the southeast of the studied area in the anh-3 well (ø < 10% and k: < 10md). the adjustment of the electrical logs to the lithological properties obtained from the cores establishes the relationship between direct observations and the different associations of indirect measures of the particular characteristics of the rocks taken from the logs. this relation provided elements of judgment for the generated models and allowed the identification and discretization of the total of lithofacies in 4 petrophysical rock types (or lithotypes): 1. conglomerates 2. claystones 3. heterolytic sandstones 4. sandstones. the successful use of the lithotype model, by hierarchical group’s method, includes the previous understanding between indirect measures and their relationship with direct observations. the input parameters used in this work provide models that offer a good correlation with the core information and increase the probability of predicting lithologies in other wells in the cesar sub-basin. the integration among the geological setting, petrophysical parameters, lithotypes and hydraulic flow units contributes to the synthesis of the petrophysical behavior of the studied formations, restricting the intervals with the best petrophysical properties (øe (%) 15-23; k(md) 100-300; vsh(%) <30) to accumulate hydrocarbon to the lithotypes 1 and 4, and the flow units 4, 5 and 6. this result is a strong input to examine interest intervals as a conventional reservoir rock in the area. the limited exploratory activity, the restricted availability of information and the complex tectonic evolution; continue to constitute a great risk for the exploration of hydrocarbons in the cesar sub-basin. however, the integration of results presented in this study expose a strong input that contributes to the development of future exploratory and/or academic studies and to the geological knowledge of the cesar sub-basin. acknowledgments this research was financially supported by the fp4842-454-2017 agreement signed among colciencias, universidad industrial de santander and the agencia nacional de hidrocarburos (anh; entity that was represented by fiduciaria la previsora s.a.). this agreement included the project “certificación de estratigrafía física y de la calidad de los núcleos de perforación recuperados por la anh en las cuencas valle inferior del magdalena, cesar y ranchería”. in addition, we thank héctor hugo pérez and luis enrique cruz for the observations and comments that helped to improve the presentation of data and ideas. we appreciate the master students and professors from universidad industrial de santander for the formidable work environment and discussion comments that enhance the development of the project. references amigun, j. o., olisa, b. & fadeyi, o. 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(2015). jurassic silicic volcanism and associated continental-arc basin in northwestern colombia (southern boundary of the caribbean plate). aapg especial volumes, memoir, 1081, 37-159. https://doi.org/10.1306/13531934m1083640 keywords: linear regression model; mann kendall; sen’s slope; climatic variables; borneo. palabras clave: modelo de regresión lineal; técnica de estimación de inclinación; variables climáticas; isla de borneo. how to cite item ninu krishnan, m. v., prasanna, m. v., & vijith, h. (2020). trend characteristics of rainy days and evaporation at a tropical rainforest region in east malaysia, borneo. earth sciences research journal, 24(3), 305-315. doi: https:// doi.org/10.15446/esrj.v24n3.77670 impact of climate change over the hydrological system in a region can be identified through statistical characterization of hydrometeorological parameters such as rainfall, temperature, humidity and evaporation. in order to understand the influence of climate change, statistical trend characteristics of rainy days, non-rainy days and evaporation rate in the limbang river basin (lrb) in sarawak, malaysia, northern borneo was assessed in the present research. annual rainfall and monthly evaporation data, over a period of 46 years, (1970 2015) corresponding to three rain gauging stations, the limbang did, ukong and long napir were used in the research. linear regression model, mann kendall and sen’s slope estimator techniques were applied to detect the statistical trends in rainy days and evaporation. a statistically significant increasing trend in annual rainy days was found at all the stations. non-rainy days showed a statistically significant decreasing trend at limbang did and ukong. monthly evaporation rates showed an overall increasing trend and the greatest increasing trend in evaporation was observed in september (2.55 mm/year) for the limbang did and in december (2.61 mm/year) for ukong. evaporation measured at the ukong station also showed a non-significant decrease during june and september. a comparison of the evaporation controlling meteorological variables such as rainfall, temperature and relative humidity indicates inter-influence at various strengths. along with local precipitation characteristics, wind and fluctuation of atmospheric temperature over the region plays a vital role in increased rate of evaporation from the region. overall, the analysis identified a statistically significant increasing trend in rainy days and evaporation in the lrb. the results of the present research can be used as critical planning data for micro and macro hydroelectric projects in the river basin. abstract trend characteristics of rainy days and evaporation at a tropical rainforest region in east malaysia, borneo características de tendencia de días lluviosos y evaporación en una región tropical boscosa del este de malasia, en la isla de borneo issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.77670 el impacto del cambio climático en los sistemas hidrológicos de una región pueden identificarse a través de la caracterización estadística de parámetros hidrometeorológicos como la lluvia, la temperatura, la húmedad y la evaporación. para entender la influencia del cambio climático este estudio evalúa las tendencias estadísticas en los días lluviosos, en los días no lluviosos y en la tasa de evaporación en la cuenca del río limbang, en sarawak, malasia, al norte de la isla de borneo. para la investigación se utlizó la pluviosidad anual y la información mensual de evaporación en un período de 46 años (1970-2015) para tres estaciones pluviométricas: limbang did, ukong y long napir. el modelo de regresión lineal y las técnicas de estimación de inclinación de mann kendall y sen se aplicaron para detectar las tendencias estadísticas en días lluviosos y de evaporación. para todas las estaciones se encontró un incremento en la tendencia anual de días lluviosos. los días no lluviosos mostraron una baja estadística significativa en las estaciones de limbang did y de ukong. los índices mensuales de evaporación mostraron mostraron un incremento generalizado, mientras que la mayor tendencia de incremento se observó en los meses de septiembre (2.55 mm/año) para la estación limbang did, y para ukong en los meses de diciembre (2.61 mm/año). la evaporación medida en la estación de ukong también muestra una baja no significativa para junio y septiembre. una comparación de la evaporación y del control de variables meteorológicas como la pluviosidad, la temperatura y la húmedad relativa indica una influencia relacionada en varias intensidades. las características locales de precipitación, de viento y de fluctuación de temperatura atmosférica sobre la región juega un rol importante en el incremento de la evaporación. el análisis identificó una tendencia de incremento estadística significativa en los días lluviosos y de evaporación en la cuenca del río limbang. los resultados del presente estudio pueden usarse como información de planeación determinante para micro y macroproyectos hidroeléctrios en la cuenca del río. resumen record manuscript received: 05/02/2019 accepted for publication: 28/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 305-315 ninu krishnan mv*, prasanna mv, vijith h department of applied geology, faculty of engineering and science, curtin university malaysia, malaysia *corresponding author: mvninukrishna@gmail.com m et eo ro lo g y 306 ninu krishnan mv, prasanna mv, vijith h introduction efforts to understand the impacts of climate change at a regional or even local scale, in this case a river basin, has acquired greater importance during the last few decades due to its increased adverse effects on hydrological, agricultural, industrial as well as economic sectors. the effects of the earth’s changing climate are demonstrated through varying hydrometeorological characteristics at a spatial and temporal level (frich et al., 2002). spatial and temporal variability of the impacts of climate change are not only limited to the amount of rain falland an increase in temperature but also include increased rates of rainfall in some regions, frequent flash floods, continuous droughts (meteorological as well as agriculture), decreasing groundwater levels, cyclones, melting of glaciers and so on (raziei et al., 2005; ipcc, 2007; sharad & vijay 2012; zafor et al., 2016a). all these phenomena have been well studied during the last three decades in an effort to reduce the crumbling effects of climate change (crisci et al., 2002; walther et al., 2002; benestad, 2013; paparrizos & matzarakis, 2017; sobral et al., 2019; parry et al., 2020). this data collection has been improved due to the development of sophisticated climatic parameter measuring and monitoring instruments as well as the increased availability of satellite-based observations and measurements (schloss et al., 1996; rodell et al., 2004; crétauxet et al., 2011; rhee & im, 2017; asfaw et al., 2018). along with this increased availability of higher resolution gridded climate data sets increased the ease of regional scale statistical analysis of climatic parameters (abatzoglou, 2013; cecil et al., 2014; dumitrescu & birsan, 2015; akinsanola et al., 2017; serrano-notivoli et al., 2018; simpson & mccarthy, 2019; singh & xiaosheng, 2019; hassan et al., 2020; xu et al., 2020; yao et al., 2020). among the observations and research carried out to understand the effects of climate change, statistical analysis of rainfall trends has attained wider importance at a global as well as local scale (yue & wang, 2004; shadmani et al., 2012; ambuan et al.,2013; nyatuame et al., 2014; ahmad et al.,2015; zhang et al., 2015; teodoro et al., 2016; gavrilov et al., 2018; sayyad et al., 2019; sobral et al., 2020; wang et al., 2020). along with rainfall trend analysis, the climatic parameters such as air temperature, evaporation and humidity have also been analyzed at various scales through statistical measurements (santhe et al.,2012; mikkonen, et al., 2015; gebremicael et al., 2017; livada et al., 2019; behzadi et al., 2020). global scale studies on the effect of climate change in rainfall indicate an increase of 0.5 – 1% in decadal rainfall and a 2% increase in annual average regional rainfall during the 20thcentury (mosmann et al., 2004; xu et al., 2003; 2005; yu et al., 2006; ipcc, 2007). global-scale studies will help to improve a broader understanding of the effects of climate change for the planet in general but to better understand and develop management strategies, more detailed local as well as regional studies are necessary. in order to estimate the effects of climate change, most researchers have studied trends in monthly, seasonal and annual rainfall using historical datasets and by applying a range of scientific methods irrespective of climatic conditions (lau & wu, 2007; allan et al., 2010; uduak et al., 2012; zafor et al., 2016b; asfaw et al., 2018; sa’adi et al., 2019; wang et al., 2020). most recent studies have revealed increasing, decreasing or no trend in rainfall amounts indicating the influence of climate change over rainfall (in the case of regions showing increasing or decreasing trends) or no influence (regions showing no trends). a few studies have also reported statistical trends in rainy days, evaporation rates, air temperature and extreme rainfall events correlated with climate change scenarios (xu et al., 2006; burn & hesch, 2007; bandyopadhya et al., 2009; fu et al., 2009; kumar & jain, 2010, 2011; abtew et al., 2011; jhajharia et al., 2012; soltani et al., 2012; kumar et al., 2012; madhu et al., 2015;soro et al., 2016; djaman et al., 2017; oliveira et al., 2017; toride et al., 2018; yang et al., 2019; behzadi et al., 2020). the statistical trend results provide evidence for climate change effects on parameters analyzed and its contribution to regional climatic changes. numerous studies have been reported from malaysia related to determining statistical rainfall trends using mann kendall, spearman’s rho, sen’s slope and linear regression techniques and have been concentrated in river basins in west malaysia (suhaila et al., 2008, 2010; deni et al., 2010; zin et al., 2010; suhaila & jemain, 2012; palizdan et al., 2014, 2015, 2017; cheros et al., 2016; suhaila & yusop, 2018;yatim et al., 2019; pour et al., 2020). when looking at the east malaysian region, especially sarawak, very few local or regional reports are available regarding the statistical trend determination of rainfall (manton et al., 2001; hua et al., 2013; krishnan et al., 2018, 2019a,b; sa’adi et al., 2017a, 2019). further regional scale climatic model for the southeast asian region (including the study area) indicates an increasing trend in rainfall along with temperature and also suggesting extreme rainfall in very localized nature and increase in dry areas in upper river basins in east malaysia (christensen et al., 2007; chotamonsak et al., 2011; sa’adi et al., 2017b; tahir et al., 2018). though these studies reported a significant increase in rainfall in the region, no attempts have been made to assess the characteristics of distribution and trend patterns of annual rainy or non-rainy days and evaporation from the region. therefore, the present study has been framed with the objective of analyzing statistical trends in annual rainy days, non-rainy days and monthly evaporation rates at a pre-determined region in northern borneo. the limbang river basin (lrb), which possesses the unique characteristics of an equatorial tropical river basin, was selected for a pilot study to determine trends in rainy as well as non-rainy days and evaporation by applying welldefined and widely used statistical techniques. the findings of the present study will help researchers to detect the impacts of climate change by applying various hydro-meteorological parameters, especially those from tropical regions. this is why these regions are significantly influenced by regional as well as local climate conditions and which often result in low agricultural productivity, forest deterioration and loss of biodiversity (thomas et al., 2004; thompson et al., 2009; piao et al., 2010; bálint et al., 2011). currently, most researchers have concentrated on determining trends in rainfall and temperature (krishnan et al., 2019a, b; sa’adi et al., 2019). however, information related to trends in evaporation rates as well as rainy and non-rainy days is a basic pre-requisite for the planning and development of major hydroelectric projects and large-scale agricultural schemes (jana et al., 2016; eymen & köylü, 2019; liu et al., 2019; mandale et al., 2019; ruiz-alvarez et al., 2019; ghumman et al., 2020). study area borneo, the third largest island in the world with a tropical equatorial rainfall regime is located in south east asia. the limbang river basin (lrb) is one of the largest river basins in the state of sarawak, malaysia and covers a total area of 3,687 km2 , located between n latitudes 030 47’ 7” to 040 54’ and e longitudes 1140 46’ 7” to 1150 31’ 30” (figure 1). the limbang river flows a distance of 196 km before it discharges into the south china sea, at brunei bay. the lrb exhibits varied terrain characteristics and a highly undulating nature with an elevation ranging from sea level to 2,400 m above mean sea level. the majority of the lrb is covered with primary and secondary forests (more than 90%) but also includes oil palm plantations and mixed agriculture. sarawak receives rainfall of various amounts throughout the year in two monsoon seasons, which determine the climatic conditions of the study area. the monsoon seasons in this region last from may to september (southwest monsoon) and from november to march (northeast monsoon) but these are separated with two short inter-monsoon months i.e. april and october. the northeast monsoon is the wettest period and the southwest monsoon comparatively drier (did, 2015). the study area (lrb) receives an annual average rainfall of 3,851 mm with monthly mean rainfall exceeding 250 mm. due to the high amount of rainfall and the peculiar relief characteristics of the research site, lower areas in the lrb experience repeated severe flash flooding during the peak monsoon seasons. the average daily temperature ranges from 25˚c to 35˚c during the day and 18˚c to 21˚c at night with the humidity varying from 70 to 90 percent (tuh et al., 2013).the major human development activity in the study region is timber harvesting, which alters the terrain conditions and induces severe soil erosion during heavy rainfall as well as affecting the climatic conditions of this region (vijith et al., 2018). materials and methods before collecting the necessary hydrometeorological information, the available data were obtained from the department of irrigation and drainage (did), sarawak, malaysia. it was noted that, though a number of rain gauging stations had been installed in the lrb, only three rain gauge stations, the limbang did, ukong and long napir possessed long-term measurements, which is essential for statistical trend determination. the did measured rainfall using manual as well as automated weather stations and the rate of evaporation was measured from a pan by assessing the change in the level of its free water 307trend characteristics of rainy days and evaporation at a tropical rainforest region in east malaysia, borneo methods were implemented to determine the trends in rainy and non-rainy days and evaporation in the lrb. statistical techniques used in the present study are discussed in the following sections. linear regression model linear regression model is one of the simplest method to calculate the trend of time series data. linear regression model considers normally distributed data and analyzes the linear relationship between time and variables, which are temporally and spatially independent (helsel & hirsch, 1992; wang & zhou, 2005; soro et al., 2016). the linear regression equation is: y a bx�� �= + (1) where, y is the dependent variable, x is the explanatory variable, b is the slope and a is the intercept. the slope of regression can describe the positive and negative nature of the time series data. an assumption of normal distribution is necessary for linear regression model. the null hypothesis is that the slope of the line is zero or no trend in the data. the probability value shows the significance of the slope. a positive value indicates an increasing linear trend and a negative value a decreasing linear trend (brito et al., 2017). mann kendall test the mann kendall test is the non-parametric test used to statistically assess the trend in a series of monotonic variables such as hydrological, climatic and environmental data which may be serially independent (mann, 1945; kendall, 1948). mann kendall test statistics are widely used to detect the trend characteristics of stream flow, rainfall and temperature in connection with the assessment for the impact of climate change on these variables (partal & kahya, 2006; dindang et al., 2013; diop et al.,2016). mann kendall statistics for time series data of x1, x2, x3, . . .xn can be determined as: s = −( ) = − = + ∑∑ i n j i n sgn xj xi 1 1 1 (2) where, xi and xj are ith and jth value of data, n is the number of observations and sgn xj xi−( ) is the sign function calculated as : sgn xj xi −( ) = + −      − >1 0 1 0if xj xi if xj xi if xj xi − = − < 0 0 ��� (3) if the number of observation n> 10, the mean (µ(s)) and the variance (σ2 (s)) are calculated using the following equation: µ s( ) = 0 (4) 2 1 2 5 1 2 5 18 1= −( ) +( ) − −( ) +( ) =∑n n n t t t i m i i i (5) where, ti is the number of observations in the ith tied group and m is the number tied groups. if the number of ties are absent in the observation then the variance is calculated as: from the evaporation pan with a height of 254 mm and a diameter of 1,206 mm (did, 2017). these stations represent different terrain (physiographic) and conditions (coastal, midland and highland). daily rainfall and evaporation data were collected from the did and processed. the general characteristics of the rain gauge stations and the data sets collected are given in table 1 and the locations of the rain gauges as well as the evaporation measuring stations are shown in figure 1. the time span of the meteorological data collected ranged from 35 to 46 years (1970 to 2015) and the percentage of the missing data ranged from 4 to 17%. the elevation of the rain gauge stations varied from 7 m (limbang did), 22 m (ukong) and 147 m (long napir). among the rain gauge stations, only limbang did and ukong had evaporation measurements (figure 1 and table 2). before proceeding to the calculation of the missing data and the statistical trend determination, a lag-1 autocorrelation test was performed to determine the method of preprocessing and to test for homogeneity and correlation (tabari et al., 2012; zhaoet al., 2015). this test classified data as homogenous but non-correlated and suggests the suitability of the collected data for statistical analysis without applying any pre-whitening technique to determine the trend. then the missing data in the daily evaporation were filled through a normal ratio method. three different statistical methods, the linear regression model (parametric), mann kendall and sen’s slope (non-parametric) figure 1. location map showing the limbang river basin with rain gauges and evaporation stations. table 1. details of rainfall, rainy days and non-rainy days at the rain gauging stations considered in the present study. stations data availability elevation (m) data missing (%) accumulated rainfall (mm) rainy days (mean) non rainy days (mean)from to minimum maximum mean limbang did 1980 2015 7 4.67 2033.00 4554.20 3093.63 203 153 ukong 1970 2015 22 17.83 3152.00 5233.00 3885.54 183 165 long napir 1977 2015 147 13.51 1855.00 3582.00 2480.71 195 123 308 ninu krishnan mv, prasanna mv, vijith h 2 1 2 5 18 = −( ) +( )n n n (6) the normalised standard test statistics is calculated as: z s s if s ifs = − +         > 1 0 1 0  ss if s = < 0 0 (7) the zs value will determine whether the observed data possess a statistically significant trend. if the zs value is greater than zero this indicates a positive trend and a value, less than zero signifies a negative trend. to test the monotonic trend, either positive or negative, at α level of significance, the null hypothesis h0 (absence of monotonic trend/ identical distribution) is rejected if the absolute value of zsis greater than zs (1-α/2), and the alternative hypothesis h1 (follows a monotonic trend) is accepted (turgay & ercan, 2006; karpouzos et al., 2010; tabari et al., 2015). sen’s slope the sen’s slope method helps to predict the magnitude of the trend of the variable under consideration by estimating the slope of the trend (yue et al., 2002). the sen’s slope estimator is considered an alternative to the parametric least-squares regression line, which estimates the slope using a weighted mean, whereas the sen’s method uses median values to estimate the true slope of the existing trend (sen, 1968; gilbert, 1987). the slope ti can be calculated as: t x x j ki j k= − − � (8) where, xi and xk are the data values at the time j and k (j>k) and with i varies 1, 2, 3, …,n. the sen’s estimator of slope is the median of n values of ti and can be calculated as: q t n isodd t t n is even i n n n = +             + + 1 2 2 2 2 1 2 ��     (9) a two-sided non-parametric test at 100 (1α) % confidence interval is used to compute the value of qmed. a positive value if qi represents an upward trend, a negative value demonstrates a downward trend and no trend is represented by zero. the annual distribution of rainy days and non-rainy days were used to identify trends in rainy and non-rainy days whereas monthly evaporation rates were used to calculate evaporation trends. results of the trend patterns of rainy, non-rainy days and evaporation are given in tables 3 and 4 and are discussed in detail below. results and discussion general characteristics of rain and evaporation gauging stations the rainfall analyzed at three rain gauge stations in the lrb showed a range of characteristics (table 1). the monthly mean evaporation rate measured by the two stations also showed fluctuation during each month and varied widely between the stations (table 2). the rain gauge station limbang did, located in the lower alluvial plains and peat land of the lrb showed an annual rainfall in the range of 2,033 mm to 4,554 mm with a mean of 3,093 mm. in the case of rainy and non-rainy days, the limbang did station recorded a mean total of 203 and 153 rainy and non-rainy days, respectively. the mean monthly evaporation rate measured varied between 125 mm (february) to 149 mm (august). regarding the ukong station, which is located in the center of the lrb, the annual rainfall ranged from 3,152 mm to 5,233 mm with a mean rainfall of 3,885 mm. this station recorded 183 mean rainy days and 165 mean non-rainy days and the mean monthly evaporation varied between 94 mm (february) to 115 mm (august) during the period of analysis. at the long napir station, the annual rainfall amount ranged from 1,833 mm to 3,582 mm with a mean rainfall of 2,480 mm and indicated 195 mean rainy and 123 non-rainy days. evaporation measurements were not present at this station. comparing the three rain gauge stations, the ukong station exhibited the longest rainfall record (45 years) and recorded the highest annual rainfall and mean rainfall of all three stations. however, when analyzing the mean rainy days, the limbang did station recorded the maximum rainy days followed by the long napir station, whereas the ukong station logged more non-rainy days. the mean monthly evaporation rate was comparatively higher at the limbang did station than at the ukong station. these results indicate the ideal location of the ukong station (located in the central catchment region of the limbang river basin) which received the maximum rainfall recorded during the least number of days and where less water evaporated despite the daily temperature range being more or less similar throughout the region. statistical trend characteristics of rainy and nonrainy days and evaporation rainy day and non-rainy day trend trend of rainy days and non-rainy days recorded at gauging stations in the lrb indicate an increasing trend in rainy days and decreasing trend in nonrainy days with varying statistical significance.the results of the trend analysis are given in table 3 and the linear relationship of rainy and non-rainy days during assessed periods are shown in figure 2. results show an increasing trend in rainy days at 99% confidence at all three stations. the linear regression model shows an increase of 2.08 rainy days/year at the limbang did station (figure 2a) whereas the mann kendall and the sen’s slope estimates show the increase of rainy days as 4.16 and 2.27 days/year, respectively. regarding the rain gauge station at ukong, the data showed the rainy days increased by 2.03 days/year using linear regression (figure 2b), 3.53 days/year by means of mann kendall and 2.75 via sen’s slope methods of trend estimation. among the stations, long napir showed the maximum increase in rainy days i.e. 4.91 days /year employing the mann kendall test, 3.8 utilizing the sen’s slope estimation and 2.89 days/year applying linear regression (figure 2c) methods. the data for non-rainy days' showed a direct correlation with the rainy days data as seen in the linear trend graphs. in the present analysis, the representative rain gauge stations selected in the lrb showed significant and a non-significant decreasing trend in non-rainy days (table 3 and figure 2df). non-rainy days recorded at the limbang did station showed a statistically significant (99% confidence) decreasing trend in all three methods (figure 2d), used at -1.48 days/year in linear regression, mann kendall (-2.92 days/year) and sen’s slope (-1.7 days/year). the ukong station showed the maximum decreasing trend of -3.49 days/year using the mann kendall followed by sen’s slope with -2.76 and -2.03 days/year in the linear regression test (figure 2e). the long napir station showed a non-significant decreasing trend in non-rainy table 2. mean monthly evaporation measured at evaporation gauging stations in lrb. stations data availability mean monthly evaporation (mm) from to jan feb mar apr may jun jul aug sep oct nov dec limbang did 1983 2015 129.29 125.65 148.98 139.54 138.62 138.72 144.67 149.23 139.20 143.15 133.25 134.59 ukong 1973 2004 106.40 94.14 119.47 111.80 109.78 111.22 113.02 115.69 110.99 108.90 101.92 104.92 309trend characteristics of rainy days and evaporation at a tropical rainforest region in east malaysia, borneo days except for the linear regression model which showed a decreasing trend with a value of -0.65 days/year (figure 2f), whereas the mann kendall and sen’s slope methods showed -0.69 days/year and -0.15 days/year, respectively. overall, the results point towards increasing trend characteristics of rainy days. at the same time, regional spatial differences in the trend patterns were also found in the lrb. this is clearly demonstrated by the trend characteristics of the long napir and ukong stations, which show a comparatively higher increasing trend in rainy days and a decreasing trend in non-rainy days compared to the limbang did station. these results may be influenced by the geographical location of the rain gauging stations. the long napir and ukong stations are located in comparatively higher elevations and are surrounded by mountains of varying elevation, whereas the limbang did station is situated near the coastal area and will experience the direct influence of wind blowing from the south china sea. the increasing trend characteristics of rainy days can be correlated with the amount of annual rainfall received in the region during the last few decades. studies on rainfall trend characteristics carried out in sarawak suggest an increasing trend in rainfall in the region, which supports the results of the present study (tuh et al., 2013, sa’adi et al.,2017a). moreover, a recent study conducted by krishnan et al., (2019 a,b) on the monthly and annual rainfall trend characteristics of the limbang river basin identified a statistically significant increase in monthly rainfall in the river basin. monthly evaporation trends the results of monthly evaporation trend characteristics analyzed through linear regression, mann kendall and sen’s slope statistics are given in table 4 and the monthly linear trend characteristics are shown in figures 3 and 4. during the month of january, the limbang did station showed a nonsignificant increasing trend in evaporation (0.55 mm/year, 1.02 mm/year and 0.75mm/year, respectively for linear regression model, mann kendall and sen’s slope methods). the ukong station also showed an increasing trend in evaporation with a 95% confidence level in the mann kendall test (1.75 mm/year) compared to the other two methods, which showed a non-significant increase (0.53 mm/year and 1.02 mm/year, respectively for linear regression and sen’s slope methods). in the month of february the limbang did station showed a statistically significant increasing trend (95% confidence) in the linear regression (0.74 mm/year) mann kendall' (2.04 mm/year) and sen’s slope (0.98) tests. the ukong station continued to show non-significant increasing trends in the linear regression and sen’s slope methods, whereas in the mann kendall test, it showed a statistically significant (90% confidence) increasing trend of 1.8mm/year. in the months of march and april, the limbang did and ukong stations showed a non-significant increasing trend in evaporation. the limbang did and ukong stations continued to show a nonsignificant increasing trend in evaporation during the month of may while a maximum increasing trend was shown using the mann kendall test at 0.94 mm/ year and 0.85 mm/year for the limbang did and ukong stations, respectively. in june, the limbang did station continued to show a nonsignificant increasing trend in evaporation with a maximum of 1.25 mm/year. a change in trend characteristics was observed at the ukong station, which showed a non-significant decreasing trend in the monthly evaporation rate in the mann kendall and sen’s slope tests (-0.40 mm/year and -0.07 mm/year, respectively) whereas in linear regression tests it showed a slight increasing trend (0.21 mm/year). in the month of july, evaporation measured at both stations showed significant as well as non-significant increasing trends. the limbang did station showed an increase of 1.75 mm/year in the mann kendall test and 0.79 mm/year in the linear regression with 90% confidence. the sen’s slope method also showed an increasing trend (0.77). during the same month, the ukong station showed a maximum increase (non-significant) of 1.37 mm/ year by means of the mann kendall test, whereas the linear regression method showed a statistically significant (90% confidence) increase of 0.63 mm/year. the sen’s slope method also showed an increasing trend (0.34) during this month. in august, both stations showed a non-significant increasing trend. the limbang did station showed a maximum increase of 0.99 mm/year using a linear regression, whereas the ukong station showed an increase of 0.45 mm/ year (mann kendall test). during september, both stations showed distinct characteristics of a trend in evaporation rates. the limbang did station showed a statistically significant trend whereas, the ukong station showed a non-significant decreasing trend as well as no trend. the limbang did station data using the mann kendall and linear regression tests, revealed an increase in evaporation rates of 2.55 mm/year (95% confidence) and 0.79 mm/year (99% confidence), respectively. the sen’s slope trend method also showed an increasing nature (0.53). while the ukong station under the linear regression method showed a non-significant decreasing trend (-0.10 mm/year), the mann kendall test showed no trend (0) and the sen’s slope showed a slightly increasing trend (0.20). in the month of october, both stations showed increasing trends. the limbang did station showed an increasing trend in the evaporation rate by means of the linear regression (1.00 mm/year at 99% confidence), the mann kendall test (2.55 mm/year at 95%) and a comparatively higher increase of 1.12 for the sen’s slope test. the ukong station showed a maximum increase of 1.83 mm/year (90% confidence) using the mann kendall test and a non-significant increase of 0.28 mm/year via a linear regression and 0.82 mm/year via a sen’s slope table 3. statistical trend characteristics of rainy days and non-rainy days in lrb. variables analysis methods rain gauging stations limbang did ukong long napir rainy days linear regression model 2.08*** 2.03*** 2.89*** mannkendall 4.16*** 3.53*** 4.91*** sen’s slope 2.27 2.75 3.8 non-rainy days linear regression model -1.48*** -2.03*** -0.65 mannkendall -2.92*** -3.49*** -0.69 sen’s slope -1.7 -2.76 -0.15 note: bold values indicate statistically significant trends; *** 99% confidence level. figure 2. linear trends in rainy days (a) limbang did (b) ukong and (c) long napir and non-rainy days (d) limbang did (e) ukong and (f) long napir. 310 ninu krishnan mv, prasanna mv, vijith h figure 3. linear trend of monthly evaporation in station limbang did. table 4 statistical trend characteristics of monthly evaporation in lrb. station name analysis methods months jan feb mar apr may jun jul aug sep oct nov dec limbang did linear regression model 0.55 0.74** 0.06 0.12 0.49 0.40 0.79* 0.99 0.79*** 1.00*** 0.49 0.789** mannkendall 1.02 2.04** 0.12 0.21 0.94 1.25 1.75* 0.58 2.55** 2.47** 0.75 1.54 sen’s slope 0.75 0.98 0.39 0.2 0.81 0.34 0.77 0.61 0.53 1.12 0.33 1.01 ukong linear regression model 0.53 0.58 -0.05 0.30 0.23 0.21 0.63* 0.17 -0.10 0.28 0.05 0.65*** mannkendall 1.75* 1.8* 0.45 1.07 0.85 -0.40 1.37 0.45 0 1.83* 0.6 2.61*** sen’s slope 1.02 0.9 0.22 0.58 0.33 -0.07 0.34 0.30 0.20 0.82 0.49 0.86 note: bold values: statistically significant trends; italics – decreasing trends; *** confidence level 99%; ** confidence level 95%; * confidence level 90%. test. during the months of november and december, both stations showed an increasing trend in monthly evaporation rates, but a statistically significant increase was noted during the month of december only. during november, the limbang did station showed a maximum increase of 0.75 mm/year in the mann kendall test, whereas the ukong station showed a maximum of 0.60 mm/year. during the month of december, the limbang did station showed a statistically significant (95% confidence) increasing trend in linear regression (0.78 mm/year) and a non-significant increasing trend in mann kendall (1.54 mm/year) test. the ukong station showed a statistically significant (99% confidence) increasing trend in the linear regression (0.65 mm/year) and mann kendall (2.61 mm/year) tests. the sen’s slope method also showed increasing trends for the limbang did and ukong stations (1.01 and 0.86, respectively). 311trend characteristics of rainy days and evaporation at a tropical rainforest region in east malaysia, borneo overall, the monthly evaporation rate in selected rain gauge stations in the lrb showed an increasing trend with varying statistical significance. in the limbang did station, statistically significant increasing trends were associated with the months of february, july, september, october and december. at the ukong station a significant increase in the evaporation rate was observed during the months of january, february, july, october and december and a nonsignificant decreasing trend during june and september. being located in an equatorial tropical rainforest region with very dynamic climatic characteristics and varying terrain characteristics likely contributed to comparatively higher evaporation rates. daily variation in temperature, wind speed, relative humidity and rainfall is the major factors controlling the rate of evaporation for this region. further, it was also noted that all the regional scale climate models suggest the presence of extreme rainfall with increasing trend in temperature in these regions (christensen et al., 2007; chotamonsak et al., 2011). comparing the results of evaporation trends (present research) with monthly rainfall trend pattern in the region carried out by krishnan et al., (2019a), both shows very correlating characteristics. it was noted that monthly rainfall shows increasing trend characteristics in most months similar to the evaporation trends particularly for limbang did and ukong station. both the stations record maximum monthly rainfall higher than 750mm and average annual rainfall > 3000mm. this suggests the increased rate of evaporation associated with heavy rainfall in the region. if the rate of rainfall is lower, then the evaporation rate less and vice versa and also indicate the close relationship between local rainfall and evaporation rate (yang et al., 2016).this might be due to the increase in difference of maximum and minimum temperatures over these regions and also due to sunshine duration, wind speed and saturation demand of the atmosphere (mondal et al., 2013; madhu et al., 2015; hounguè et al., 2019). in order to make a comprehensive analysis of the relationship between mean evaporation rates with rainfall, temperature and relative humidity, linear regression and correlation analyses were performed (figure 5). it was noted that, there was a negative correlation between the monthly mean rainfall and the monthly mean evaporation (r = -0.43 and r2 = 0.18), whereas a positive correlation was observed between the monthly mean temperature and the mean evaporation (r = 0.46 and r2 = 0.21). though the monthly mean rainfall and evaporation shown a negative correlation, as mentioned earlier, the evaporation rate is highly depends on the local rainfall characteristics. however, relative humidity and evaporation showed a comparatively higher negative correlation (r = -0.57 and r2 = 0.32). though the correlation and linear regression was moderate, the combined effects of these parameters likely influenced the higher rate of evaporation in the lrb. in order to establish and understand the exact co-existence of hydro-meteorological variables more detailed measurements of rainfall, daily temperature, humidity and wind speed along with evaporation is necessary. conclusion statistical trend analysis for annual rainy days, non-rainy days and evaporation data from the limbang river basin analyzed through linear figure 4. linear trend of monthly evaporation in station ukong. 312 ninu krishnan mv, prasanna mv, vijith h regression model, mann kendall and sen’s slope methods indicates significant increasing trends in annual rainy days and evaporation rates and a decreasing trend in non-rainy days. all the three stations showed an increasing trend in annual rainy days with highest increase in long napir (4.91 days/year). similarly in the case of non-rainy days trend, the ukong station showed the maximum decrease (-3.49 days/year). increase in the number of rainy days in the study area suggests a positive impact on the rainfall amount in the region. monthly evaporation trends showed an increasing trend throughout the year at the limbang did station whereas ukong station showed varying trend characteristics with a maximum increase in december and a decreasing trend in june and september. correlation analysis between the mean evaporation rate with the monthly mean rainfall, monthly mean temperature and relative humidity suggests a combined influence on increased evaporation rates. further, the increased evaporation rate in the region is linked to the local precipitation characteristics, diurnal variation in temperature, wind characteristics and humidity. moreover, the lrb suffers from a lack of detailed observation records of evaporation from different locations, which are critical for deriving more comprehensive evaporation characteristics of the region. overall, the limbang river basin is now experiencing a greater number of rainy days with an increasing evaporation rate. the present research provided a new applied theory in the field of statistical trend determination of hydro-meteorological parameters, especially for those who are working on hydroelectric projects and the impacts of climate change at a local/regional level. acknowledgment references abatzoglou, j. t. 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(2015). evaluating the suitability of trmm satellite rainfall data for hydrological simulation using a distributed hydrological model in the weihe river catchment in china. journal of geographical science, 25(2), 177-195. statistical analyses, a piper diagram, the saturation index and the correlations of chemical parameters were used to reveal the hydrogeochemistry and hydrogeochemical evolution of shallow groundwater in the southern part of the zhongwei section of the yellow river alluvial plain. the water quality for agricultural and domestic uses was also assessed in the study. the results suggest that the shallow groundwater in the study area is fresh to moderately mineralized water. higher ca2+ and hco3 are observed in the less mineralized water, whereas na+ and so4 2are common ions in the highly mineralized water. the major hydrochemical facies for groundwater with total dissolved solids (tds) <1 g/l are hco3-ca·mg and hco3-ca·na·mg, and for groundwater with tds > 1 g/l, so4·cl-na and so4·cl-na·mg·ca are the predominant hydrochemical types. the main reactions in the groundwater system are the dissolution/precipitation of gypsum, fluorite, halite, calcite and dolomite. cation exchange is also important in controlling the groundwater chemistry. the water samples assessed in the paper are of acceptable quality for agricultural use, but most of them are not fit for direct human consumption (drinking). tds, total hardness (th), cland so4 2are the main indices that result in the poor drinking water quality. análisis estadísticos, un diagrama de piper, el índice de saturación y la correlación de los parámetros químicos fueron utilizados para revelar la hidrogeoquímica y la evolución hidrogeoquímica de las aguas subterráneas poco profundas en la parte sur de la sección zhongwei en la planicie aluvial del río amarillo. la calidad del agua para el uso doméstico y agrícola también fue evaluada en este estudio. los resultados sugieren que las aguas subterráneas poco profundas en el área de estudio son entre frescas y moderadamente mineralizadas. un índice mayor de ca2+ y hco3 se observó en las aguas menos mineralizadas, mientras que na+ y so4 2son iones comunes en las aguas altamente mineralizadas. los perfiles hidroquímicos predominantes para las aguas subterráneas con total de sólidos disueltos (tds) <1 g/l son hco3-ca·mg y hco3-ca·na·mg, y para las aguas subterráneas con tds >1 g/l, so4·cl-na y so4·cl-na·mg·ca. las mayores reacciones en el sistema de aguas subterráneas son la disolución/ precipitación de yeso, fluorita, halita, calcita y dolomita. el intercambio de cationes también es importante en el control de la química de las aguas subterráneas. las muestras de agua evaluadas en este manuscrito son de calidad aceptable para el uso agrícola, pero la mayoría no son aptas para el consumo humano. el índice tds, la dureza total del agua (th), cly so4 2son las razones principales que influyen en la baja calidad de esta agua. hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china peiyue li 1, 2*, jianhua wu 1, 2, hui qian 1, 2 1. school of environmental science and engineering, chang’an university, no. 126 yanta road, xi’an, 710054, china *email: peiyueli@chd.edu.cn, fax: 86-29-82339952, tel: 86-29-82339327 2. key laboratory of subsurface hydrology and ecological effect in arid region of the ministry of education, chang’an university, no. 126 yanta road, xi’an, 710054, china earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 27 38 abstract resumen key words: hydrogeochemical characteristics; groundwater protection; hydrochemistry; water-rock interaction; groundwater. palabras clave: características hidrogeoquímicas; protección de aguas subterráneas; hidroquímica; interacción agua-roca; aguas subterráneas. hydrogeology record manuscript received: 18/10/2012 accepted for publication: 29/04/2014 28 peiyue li, jianhua wu, hui qian introduction groundwater is an important source of fresh water for domestic, agricultural and industrial uses in china, especially in north china and northwest china, where surface water is scarce (wang et al. 2007). compared to deep groundwater, shallow groundwater is much easier to exploit, but it is vulnerable to pollutants, and its quality is easily influenced by natural water-rock interactions and anthropogenic activities. thus, shallow groundwater requires more attention and protection. shallow groundwater underneath the alluvial plain is usually of good quality because it usually receives large amounts of recharge from the rivers. however, the aquifers underlying the alluvial plain are usually composed of coarse sands and gravels, which not only enables the groundwater to flow quickly through the pores but also allows pollutants to travel easily through the aquifers. a small disturbance from natural processes or human activities can easily change the chemical constituents and quality of shallow groundwater. therefore, the understanding of the hydrogeochemical evolution and water quality status is a key component in addressing effective protection measures and implementing sustainable management of shallow groundwater beneath alluvial plains. the investigation and assessment of groundwater chemistry and quality is a hot topic worldwide, triggering international attention from both researchers and policy makers due to its relevant link with environmental objectives and regional development. groundwater chemistry has been extensively studied, and a great deal of literature can be found in various scientific journals and databases. these previous investigations have involved many different hydrogeological and hydrogeochemical methods and techniques, including multivariate statistics, major solute analyses, stable oxygen and hydrogen isotope analyses and groundwater geochemical modeling. for example, reghunath et al. (2002) studied the interaction of river water with adjacent groundwater and the mixing/ non-mixing of different types of groundwater in the nethravathi catchment (india) using multivariate statistical techniques. similarly, menció and maspla (2008) used principal component analysis (pca) and cluster analysis (ca) to determine surface water origins and interactions with groundwater in mediterranean streams. guo and wang (2005) researched the geochemical characteristics of shallow groundwater in the datong basin. xu et al. (2010) measured major ion concentrations in various waters around the lake qinghai catchment and identified the major factors influencing their chemical characteristics. isotopic and geochemical modeling techniques are rapidly becoming widely used in the study of groundwater chemistry and groundwater cycling. li et al. (2010a) investigated the geochemical processes of groundwater in the southern plain area of pengyang county using a geochemical modeling approach. qian et al. (2007) used hydrogen and oxygen isotopes to study the dynamics of groundwater and surface water. the evaporation fraction was also calculated in their study by hydrogen and oxygen isotopes. kanduč et al. (2012) investigated the spring water chemistry and carbon cycling of alpine springs in slovenia using stable isotopes. all these existing studies have provided useful instructions for groundwater protection. the shallow groundwater in the yellow river alluvial plain is a regional strategic resource for sustainable economic development. for better management of the groundwater resources, it is necessary and important to study the groundwater characteristics and hydrogeochemical processes. however, the shallow groundwater in the yellow river alluvial plain (zhongwei section) is rarely investigated because there are no large cities, only small towns and villages. the struggling economy has resulted in few government investments, and therefore no funds have been arranged to support environmental studies. in recent years, with the increased awareness of groundwater protection, the local government has been trying to change this situation and provide some economic support to perform various studies related to the water environment. for example, wu et al. (2014) performed an investigation of soil salinization in this area, and they related secondary soil salinization to irrigation and groundwater quality. the purposes of the present study are (1) to investigate the hydrogeochemistry and geochemical evolution of shallow groundwater and (2) to assess the groundwater quality for drinking and agricultural uses. the study will be extremely important and useful in regional groundwater protection and management. study area location and climate the study area is a part of the yellow river alluvial plain. it is located to the south of the yellow river, between longitudes 105°09′05″ to 105°29′55″e and latitudes 37°22′22″ to 37°30′06″n, covering an area of 79.28 km2. the location of the study area is shown in fig. 1. the yellow river, the main surface water body primarily used for irrigation, runs through the area along the northern boundary. zhongwei city, to the north of the area, is the economic and cultural center of the area. nanshantaizi, composed of quaternary alluvial and pluvial deposits, lies to the south of the study area. located in a semiarid area, the study area is characterized by scant precipitation, strong winds in the winter, and high temperatures and intense evaporation in the summer. according to the data from the zhongwei weather station, the multi-year average temperature is 9.37°c in the area. the highest and lowest temperatures are usually observed in january and july, respectively. the multi-year mean precipitation is 185.18 mm, with the majority (72%) falling between june and september (fig. 2). the mean evaporation of the area is 1774.25 mm, and over 73% of the annual evaporation occurs between april and september. fig. 1 study area and sampling locations. fig. 2 monthly average meteorological elements. 0.00 50.00 100.00 150.00 200.00 250.00 300.00 1 2 3 4 5 6 7 8 9 10 11 12 month mm -10.00 -5.00 0.00 5.00 10.00 15.00 20.00 25.00 ℃ precipitation(mm) evaporation(mm) temperature(℃) 29hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china geology the study area lies within a fault-formed basin. the occurrence and distribution of groundwater in the area are controlled by geologic structures. the basin belongs to the cenozoic zhongwei-chenmajing depression zone. pluvial and piedmont deposits formed the red tertiary clasolite underlying the quaternary deposits. according to borehole data, the quaternary deposits can be over 130 m in thickness, comprising sands and gravels interbedded with loess-like clayey sands. the upper part of the quaternary deposits forms a phreatic aquifer or semi-confined aquifer. the neotectonic movement in the area has been intense. the nanshantaizi is rising relative to the alluvial plain. the elevation of the front zone of nanshantaizi is 100 m higher than that of the alluvial plain. the underlying tertiary formations contain fissure water, but the groundwater is poor in quality and not suitable for use. the alluvial plain is composed of gravels and sands, which are good groundwater-bearing formations. calcite, dolomite, fluorite, gypsum and halite are common minerals in the aquifer media of the plain. hydrogeology since the quaternary, a thick pile of loose sediments has been deposited in the area, forming good water-bearing formations. from the top down, holocene and pleistocene deposits are observed as per the borehole data, and they constitute the phreatic aquifer and confined aquifer. the phreatic aquifer in the alluvial plain was formed by holocene (q4 al) and late pleistocene (q3 al) alluvial deposits. the thickness of the phreatic aquifer can reach over 50 m (fig. 3). this aquifer is the most important for water supply in the area and is characterized by a broad and stable aquifer distribution, shallow groundwater table, good water quality and abundant water yield. according to field investigations, the groundwater depth in general is 0.11-4.99 m, and the tds content in most areas is upper than 1 g/l. highsalinity water is observed along the front zone of nanshantaizi. fig. 3 cross section of a-a′. the confined aquifer in the alluvial plain is composed of alluvial sands and gravels from the middle pleistocene (q2 al). this aquifer is rarely used because its distribution is not stable and its depth is too great. the thickness of the confined aquifer is within the range of 18.85-67.38 m. the shallow groundwater flows in general from south to north and from west to east, influenced by topography and artificial irrigation over the nanshantaizi area. the yellow river water is diverted by pipes and channels to nanshantaizi for agricultural uses, after which it infiltrates into the aquifer and flows back to the alluvial plain. during the diversion, a large amount of water percolates into the phreatic aquifer through channel leakage. a water budget of the area (including the nanshantaizi) during the period of september 2009 to august 2010 was calculated. the total recharge of the period is 0.5322×108 m3, and the channel leakage and irrigation infiltration account for over 75% of the total recharge (table 1). precipitation is also an important recharge of the aquifer, although the annual precipitation is less than 200 mm in the area. groundwater is mainly discharged by evaporation and as boundary outflow during the period; artificial extraction only represents a small proportion (less than 4%). this small amount of extracted groundwater is used mostly for drinking and irrigating vegetables in the inhabitants’ gardens. table 1 the shallow-groundwater budget of the study area (including the nanshantaizi area). recharge amount ×108 m3/a percentage % discharge amount ×108 m3/a percentage % irrigation infiltration 0.1607 30.20 evaporation 0.4186 82.69 precipitation 0.0870 16.35 boundary outflow 0.0694 13.71 flood river 0.0155 2.91 artificial extraction 0.0182 3.60 boundary inflow 0.0104 1.95 channel leakage 0.2586 48.59 total 0.5322 100 total 0.5062 100 materials and methods data collection and analysis for this work, a total of 27 shallow groundwater samples (nw1, nw2, and nw4 to nw28) and 1 yellow river water sample (nw3) were collected in august 2010. all groundwater samples were collected from hand-pumped wells at depths of 8-12 m. the sampling locations are shown in fig. 1. most of the samples are located in the eastern portion of the study area, and only two samples were collected from the western portion because most villages are located in the east. samples were collected in 5 l white plastic bottles. prior to sampling, the containers were washed three times with the water to be sampled. after the collection, the containers were labeled for identification and brought to the laboratory. during sample collection, handling, and preservation, standard procedures recommended by the chinese ministry of water resources were followed to ensure data quality and consistency. the groundwater samples were analyzed in the laboratory of the ningxia geo-environmental monitoring station. all samples were analyzed for total dissolved solids (tds), ph, total hardness (th), and major ions, including sodium (na+), potassium (k+), calcium (ca2+), magnesium (mg2+), sulfate (so4 2-), chloride (cl-), carbonate (co3 2-) and bicarbonate (hco3 -). certain trace constituents, such as free carbon dioxide (co2), total soluble iron (tfe), nitrate-nitrogen (no3-n), ammonia-nitrogen (nh4-n), nitrite-nitrogen (no2-n), fluoride (f-), and metasilicic acid (h2sio3), were also analyzed. among the analyzed parameters, na+ and k+ were determined by using a flame photometer. ca2+, mg2+, hco3 -, and clwere analyzed with the titrimetric method, and fe was analyzed with an atomic absorption spectrophotometer. the ph and electrical conductivity (ec) were measured in situ. after physiochemical analysis, the accuracy of the analysis results was checked via charge balance. generally speaking, the relative error should be within ±5%, i.e., ( cation anion) / ( cation + anion)= 5% the results suggest that the maximum relative error is 4.56%, which is within the acceptable error limit. the analysis results are accurate and can be used for the study. 30 peiyue li, jianhua wu, hui qian saturation index the saturation index (si) is a widely used indicator in hydrogeochemical studies. the geochemical program phreeqc (parkhurst and appelo 1999) was used to compute the saturation index (si) of certain major minerals, including calcite, dolomite, fluorite and gypsum. when si = 0, the minerals in the aqueous solution are in equilibrium. when si < 0, the minerals in the aqueous solution are unsaturated and follow a dissolution trend, and when si > 0, the supersaturated status of minerals in the aqueous solution causes deposition to occur (li et al. 2010a). the saturation index (si) is defined as lg iapsi k = (1) where iap is the relevant ion activity product in a mineral dissolution reaction, which can be obtained by multiplying the ion activity coefficient γi and composition concentration mi; and k is the equilibrium constant of mineral dissolution at a certain temperature. irrigation water quality assessment there are several methods for assessing irrigation water quality (li et al. 2013a). the sodium adsorption ratio (sar) is likely the most popular metric used around the world. it is calculated from the ratio of sodium to calcium and magnesium (fipps 2003). the equation is expressed as follows (hakim et al. 2009): (2) where all the ions are expressed in meq/l. the soluble sodium percentage (%na) is also a useful indicator in determining the suitability of groundwater for agricultural uses. the parameter %na is defined as the ratio of sodium to the total cations in meq/l (li et al. 2010b). water with a %na value greater than 60% may result in sodium accumulations that will cause a breakdown in the soil’s physical properties (khodapanah et al. 2009). the index %na is expressed as follows: + + + 2+ 2+ na%na 100 na +k +ca +mg = � (3) where all the ionic concentrations are expressed in meq/l. drinking water quality assessment for drinking water quality assessment, the most common method used in china is the comprehensive water quality index method (cwqi), which is recommended by the quality standard for groundwater of china (bureau of quality and technical supervision of china 1994). as per the standard, the groundwater is classified into five grades: excellent (grade i), good (grade ii), fair (grade iii), poor (grade iv), and very poor (grade v). the cwqi can be calculated using formulas (4) and (5). groundwater with grades i and ii is of excellent and good quality and is suitable for all uses. grade iii is fairquality water, which is generally suitable for drinking, irrigation, and industrial production. grade iv is poor-quality water, which is fit only for irrigation and industrial production and may be used for drinking after proper treatment. grade v groundwater is very poor-quality water that cannot be used for any purpose. 2 2 mean max( ) ( ) 2 ij ij j f f f + = (i=1, 2, 3, …, n) (4) (i=1, 2, 3, …, n) (5) where fj is the comprehensive evaluation value of sample j, fij is the evaluation value of the ith index of the jth water sample, which can be determined as per the quality standard for groundwater (bureau of quality and technical supervision of china 1994). the term (fij)mean represents the average value of fij for n indices of the jth water sample, (fij)max is the maximum value of fij, and n is the numbers of index for sample j. based on the calculated comprehensive evaluation value fj, the groundwater quality is classified according to table 2. table 2 standards of groundwater quality classifications. grade excellent good fair poor very poor fj fj<0.8 0.8<fj≤2.5 2.5<fj≤4.25 4.25<fj≤7.2 fj>7.2 results and discussion general hydrochemical characteristics the analytical results of the physiochemical indices are shown in table 3. table 3 also lists the statistical analysis results. the tds in the groundwater ranges from 539.75 to 2248.98 mg/l, indicating fresh to moderately mineralized water (table 3). the ec of the groundwater is higher than that of the surface water, suggesting mineral dissolution in the groundwater. among the mean ion concentrations, the main anion is hco3 -, ranging from 213.69 to 609.65 mg/l, followed by so4 2and cl-, with averages of 267.92 and 207.50 mg/l, respectively. the main cations are na+ and ca2+, with averages of 146.26 and 129.07 mg/l, respectively. the ph of the groundwater is within the range of 8.01 to 8.15, indicating an alkalescent tendency. the contents of tfe and fare low, and both are within the permissible limits for drinking purposes. th ranges from 363.10 to 1108.69 mg/l and is therefore categorized as hard or very hard water. no ammonia nitrogen and nitrate nitrogen pollution was detected, but local nitrite nitrogen pollution was observed. the coefficients of variation can reveal the spatial variability of ions. the statistical analysis results listed in table 3 show that the ec possesses the highest coefficient of variation, suggesting a higher spatial variability than the other indices. the spatial variabilities of tds, th and hco3 are also very high. the physiochemical analysis of the yellow river water (nw3) was also included in table 3 for comparison. the comparison reveals that most major indices of the surface water have lower values than those of the groundwater. this suggests that the water quality of the yellow river water is better than that of the groundwate. tri-linear diagrams are perhaps the most commonly used technique for finding hydrochemical patterns in major ion data. in the present study, a piper diagram (piper 1953) was used to understand the major hydrochemical facies (fig. 4). these figures were drawn in aq•qa® (rockware, inc. 2004). fig. 4 reveals that ca2+ is the predominant cation in groundwater with tds < 1 g/l, followed by na+ and mg2+. for groundwater with tds > 1 g/l, na+ is the major cation, followed by mg2+ and ca2+. in groundwater with tds > 1 g/l, the na+ content is higher, but the ca2+ content is lower compared to groundwater with tds < 1 g/l. with respect to anions, hco3 is predominant in groundwater with tds < 1 g/l, but in groundwater with tds > 1 g/l, so4 2and clare the most prevalent anions. the main hydrochemical facies for groundwater with tds <1 g/l are hco3-ca·mg and hco3-ca·na·mg, and for groundwater with tds > 1 g/l, so4·cl-na and so4·cl-na·mg·ca are the predominant hydrochemical types. the ph values of groundwater with tds <1 g/l, in general, are lower than those of groundwater with tds > 1 g/l. the yellow river water possesses the lowest tds among all collected water samples. 2+ 2+ nasar ca +mg 2 + = x 31hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china table 3 results of the physiochemical analyses and statistical analyses. sample no. k+ na+ ca2+ mg2+ clso4 2hco3 tfe fnh4-n no3-n no2-n co2 h2sio3 tds th ph ec nw1 8 200 205.44 164.12 286.52 916.85 282.83 0.04 0.4 0 5.42 0.16 8.59 17.9 1946.22 1108.69 8.01 2728.8 nw2 5 215 122.88 143.17 252 519.86 534.23 0.08 0.4 0 16.95 0.14 17.18 18.2 1560.71 798.16 8.05 2256.6 nw4 7 121 101.76 32.59 100.11 121.45 452.52 0.03 0.2 0 2.26 0 21.12 15.3 727.93 403.06 8.15 1127.8 nw5 6 145 103.68 59.36 186.41 175.26 433.67 0.05 0.1 0 1.13 0.06 12.89 15.8 909.63 489.13 8.09 1417 nw6 7 400 224.64 111.74 676.59 700.23 213.69 0.05 0.4 0 0.23 0 8.59 21.3 2248.98 1009.15 8.05 3168.7 nw7 4 89 80.64 44.23 96.66 79.3 427.38 0.03 0.2 0 2.26 0.05 12.89 15.6 625.63 374.62 8.09 999.14 nw8 6 480 132.48 75.66 569.58 625.24 226.26 0.03 0.5 0 1.8 0 4.3 19.8 2024.19 624.42 8.04 2848.8 nw9 7 288 186.24 111.74 503.99 568.23 238.83 0.03 0.5 0 2.71 0 8.59 18.5 1806.33 893.95 8.05 2633.7 nw10 7 113 138.24 46.56 151.89 178.23 458.81 0.05 0.3 0 2.94 0.12 12.89 16.9 884.58 554.4 8.08 1366.5 nw11 5 72 130.56 66.35 179.5 185.23 358.25 0.02 0.3 0 4.52 0.1 8.59 15.8 838.48 590.72 8.09 1344.2 nw12 5 57 161.28 65.18 227.83 221.35 276.54 0.02 0.2 0 4.97 0.08 8.59 15.9 897.07 679.39 8.08 1447.7 nw13 5 64 86.4 53.34 72.49 124.35 402.24 0.03 0.2 0 1.81 0.05 8.59 14.9 623.87 419.83 8.07 999.43 nw14 4 135 115.2 46.56 117.37 180.23 496.52 0.04 0.2 0 3.62 0.05 12.89 12.6 866.09 485.28 8.08 1321.2 nw15 5 68 165.12 71 248.54 245.23 270.26 0.02 0.3 0 7.91 0.06 8.59 16.3 962.6 708.37 8.07 1541 nw16 7 64 99.84 57.04 131.18 135.23 345.68 0.02 0.2 0 8.48 0.12 8.59 14.9 690.82 470.63 8.06 1119.3 nw17 4 86 88.32 50.05 96.66 52.31 509.09 0.02 0.2 0 0 0 4.3 14.9 646.98 415.12 8.09 1034.7 nw18 3 65 88.32 40.74 82.85 105.23 358.25 0.02 0.2 0 0.9 0.02 12.89 14.8 580.18 387.18 8.09 937.42 nw19 3 62 76.8 44.23 72.49 60.21 408.53 0.02 0.3 0 1.24 0.01 8.59 15.2 539.75 363.1 8.1 877.33 nw20 4 72 94.08 36.08 89.75 72.15 408.53 0.05 0.2 0.02 0 0 12.89 15.1 587.65 390.49 8.1 945.53 nw21 4 74 96 47.72 107.01 121.3 383.39 0.02 0.2 0 3.62 0 8.59 15.2 660.75 431.17 8.09 1059.1 nw22 5 137 105.6 68.68 151.89 241.23 446.24 0.02 0.4 0 7.91 0 4.3 17.6 958.42 522.83 8.09 1463.6 nw23 4 102 157.44 43.07 144.98 166.35 496.52 0.03 0.2 0 11.3 0.01 12.89 17.5 895.11 601.52 8.09 1374.4 nw24 5 137 207.36 80.32 276.16 240.12 609.65 0.02 0.2 0 2.26 0 12.89 18.2 1271.44 863.03 8.06 1920.7 nw25 5 348 190.08 81.48 386.62 653.89 389.67 0.02 0.3 0 1.13 0 12.89 18.1 1879.44 814.68 8.04 2633.1 nw26 5 84 80.64 43.07 89.75 99.23 383.39 0.02 0.1 0 0.34 0 4.3 16.3 610.12 371.12 8.1 977.44 nw27 4 115 132.48 44.23 144.98 205.23 389.67 0.02 0.3 0 3.62 0.07 8.59 16.3 861.05 530.14 8.07 1334.8 nw28 7 156 113.28 76.82 158.79 240.23 546.8 0.04 0.2 0 0.23 0 12.89 16.9 1042.85 570.31 8.08 1578.4 max 8 480 224.64 164.12 676.59 916.85 609.65 0.08 0.5 0.02 16.95 0.16 21.12 21.3 2248.98 1108.69 8.15 3168.7 min 3 57 76.8 32.59 72.49 52.31 213.69 0.02 0.1 0 0 0 4.3 12.6 539.75 363.1 8.01 877.33 mean 5.22 146.26 129.07 66.86 207.5 267.92 398.05 0.03 0.27 0 3.69 0.04 10.33 16.51 1042.48 587.8 8.08 1572.46 sd 1.37 110.52 43.63 32.34 157.34 231.07 101.57 0.01 0.11 0 3.93 0.05 3.96 1.81 510.64 207.57 0.03 676.73 cv 26.2 75.57 33.81 48.37 75.83 86.25 25.52 47.45 40.28 519.62 106.6 123.47 38.33 10.99 48.98 35.31 0.34 43.04 nw3 3 43 53.76 27.94 55.23 83.56 207.41 0.05 0.2 0 1.36 0.01 4.3 14.2 385.96 245.09 8.09 643.47 note: units for all indices are mg/l, except ph and ec (μs/cm). max=maximum value, min=minimum value, sd=standard deviation, and cv=coefficient of variation. 32 peiyue li, jianhua wu, hui qian ▲ yellow river water ● groundwater with tds <1 g/l ■ groundwater with tds >1 g/l fig. 4 piper diagram of the groundwater and yellow river water. saturation index the saturation indices for gypsum, fluorite, dolomite and calcite were calculated with phreeqc (parkhurst and appelo 1999). the calculation results are listed in table 4. the si values of calcite and dolomite for all groundwater samples are greater than zero (table 4), indicating oversaturation and potential precipitation of these carbonate minerals. in contrast, the si values of gypsum and fluorite for all groundwater samples are less than zero (table 4), indicating that gypsum and fluorite could feasibly be dissolved by the groundwater along the flow path. generally speaking, groundwater in the recharge zones or in the upgradient areas of a regional flow system is unsaturated with regard to gypsum, fluorite, dolomite and calcite because of short contact time with the aquifer media and/ or insufficient mineral sources. with the groundwater flowing to the runoff area and then to the discharge area, many water-rock interactions will take place, and groundwater may reach equilibrium with these minerals. accordingly, the tds in groundwater will also increase along the groundwater flow path due to progressive mineral dissolution. therefore, the plot of si versus tds (fig. 5) is very useful in analyzing groundwater evolution along the flow path. fig. 5 plot of the saturation indices of the relevant minerals versus tds. the si values of calcite and dolomite range from 0.645 to 1.456 and from 1.909 to 3.166, respectively. the correlations between them and the tds are not significant (fig. 5), which indicates that calcite and dolomite do not table 4 calculated saturation indices of calcite, fluorite, dolomite and gypsum. sample no. sicalcite sifluorite sidolomite sigypsum sample no. sicalcite sifluorite sidolomite sigypsum nw1 1.064 -1.203 2.451 -0.299 nw16 0.979 -1.942 2.081 -1.205 nw2 1.163 -1.394 2.900 -0.724 nw17 1.339 -2.001 2.984 -1.679 nw4 1.071 -1.975 2.538 -1.298 nw18 0.952 -1.982 2.086 -1.350 nw5 1.070 -2.595 2.607 -1.167 nw19 1.008 -1.678 2.237 -1.636 nw6 0.871 -1.229 2.333 -0.464 nw20 1.031 -1.961 2.262 -1.495 nw7 0.775 -2.039 1.909 -1.536 nw21 1.034 -1.963 2.272 -1.274 nw8 0.645 -1.266 2.190 -0.745 nw22 1.149 -1.384 2.734 -1.022 nw9 0.969 -1.064 2.462 -0.565 nw23 1.339 -1.800 2.811 -0.993 nw10 1.257 -1.506 2.748 -1.022 nw24 1.456 -1.733 3.059 -0.784 nw11 1.018 -1.502 2.097 -0.990 nw25 1.142 -1.522 2.884 -0.527 nw12 1.034 -1.766 1.936 -0.827 nw26 0.861 -2.637 2.056 -1.433 nw13 1.000 -1.995 2.185 -1.291 nw27 0.868 -1.523 1.997 -0.977 nw14 1.388 -1.943 3.166 -1.103 nw28 0.994 -1.973 2.452 -1.016 nw15 0.994 -1.417 1.924 -0.789 33hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china continue to dissolve along the flow path. in contrast, the si values of fluorite and gypsum are less than zero, and they exhibit good positive correlations with tds (fig. 5), which suggests that gypsum and fluorite dissolve into the groundwater along the flow path and increase the concentrations of tds. the dissolution of gypsum and fluorite will increase the ca2+ in groundwater, which will in turn increase the si values of calcite and dolomite and may make them precipitate from the groundwater. the slight increase in the si values of calcite and dolomite supports this inference (fig. 5). correlation of major chemical indices the relationships among tds and the major ions are useful to interpret the major hydrogeochemical evolution processes within an aquifer and can also be used to deduce the sources of ions and the origin of the groundwater (kumar 2009; wu et al. 2013). for cations, there is a good correlation between na+ and tds (r2=0.828, fig. 6a), and the correlations of ca2+ and mg2+ with tds are also good, with r2=0.626 and 0.650, respectively (figs. 7b and 7c). for anions, so4 2and tds exhibit the highest correlation (r2= 0.922, fig. 6e), followed by the correlation between cland tds (r2=0.859, fig. 6d). considering the slopes of the fitted lines, na+ increases with tds more rapidly than other cations, and the rate of increase of so4 2is much larger than that of cl-. the concentration of hco3 decreases with tds (fig. 6f). the correlations between these major ions and tds and the saturation indices of minerals listed in table 4 suggest the following reactions along the flow path: 2+ 24 4caso =ca +so (r1) + -nacl=na +cl (r2) 2+ 2caf =ca +2f (r3) when there is excessive ca2+, calcite may become oversaturated and precipitate from the groundwater (r4), decreasing the concentration of hco3 -. this may be the reason why hco3 decreases with increasing tds. 2+ + 3 3ca +hco =caco +h� (r4) fig. 6 plots of the major ions versus tds. 34 peiyue li, jianhua wu, hui qian however, many reactions are involved in the evolution of groundwater, and a great deal of work is required to explain the ionic characteristics in the groundwater system. generally, the ratios of different physiochemical parameters are useful in explaining the groundwater evolution. in the present study, certain parameters are used to interpret the possible processes controlling the groundwater chemistry. the ratio of cland ca2+ in meq/l is a parameter charactering the hydrodynamics (li et al. 2011). usually, ca2+ is a dominant cation in a lowmineralized environment. with the tds increasing, mg2+ will gradually increase, and with further tds increasing, na+ will become the dominant cation in the groundwater. when the tds value is high, clis difficult to migrate; therefore, the concentration of clwill increase with tds (li et al. 2011). as a result, the cl-/ca2+ will increase with tds, suggesting a worsening of the hydrodynamic conditions. with respect to the results in the present study, cl-/ ca2+ exhibits a linear increase with tds (fig. 7a), which indicates the presence of the aforementioned processes. theoretically, if na+ and care derived solely from the dissolution of halite, the ratio of na+ and clin mmol/l should be equal to 1 (li et al. 2013b). in the present study, most samples are plotted below the 1:1 line (fig. 7b), which suggests that the clcontent is higher than na+. in other words, the dissolution of halite is not the only source of na+ and cl-, and other reactions influencing the contents of na+ and clmay have occurred as well. cation exchange (r5 and r6) may explain the deficiency of na+. the cation exchange makes na+ in groundwater adsorbed onto the surface of the aquifer media, resulting in the decrease of na+ in the water and forcing the samples to be plotted below the 1:1 line (fig. 7b). 2na++cax2=ca2++2nax (r5) 2na++mgx2=mg2++2nax (r6) fig. 7 plots of the correlations between different ions. 35hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china additionally, the ratio of na+/clless than 1:1 may also be explained by the infiltration of domestic sewage and animal excrement, which contain excessive chloride. there are no large cities or chemical plants in the area, and there is no waste-water treatment system either. the residents living in small villages raise many animals, such as pigs, sheep and chickens. the domestic sewage and animal excrement are indiscriminately discharged without any proper supervision or management. the pollutants contained in the domestic sewage and animal excrement may infiltrate into the aquifer with the precipitation, causing the increase of clin the groundwater. similarly, if the dissolution of gypsum is the only explanation to the origin of ca2+ and so4 2-, the ratio of ca2+ and so4 2in mmol/l should be 1:1. as shown in fig. 7c, ca2+ exhibits a significant positive correlation with so4 2 (r2=0.563), but the ratio of ca2+ and so4 2is only 0.142. the calcite is saturated; therefore, calcite precipitation (reaction r4) is most likely the most appropriate explanation for the ca2+ deficiency versus so4 2-. if the origins of ca2+ and hco3 can be exclusively explained by the dissolution of calcite, the slope for ca2+/hco3 in mmol/l should range from 1:1 to 1:2 due to the following: + 2+ 3 3caco +h =ca +hco � (r7) 2+ 3 2 2 3caco +co +h o=ca +2hco � (r8) the concentration of ca2+ exhibits a slight negative trend with hco3 (fig. 7d). because the concentration of hco3 decreases with tds (fig. 6f), it can be inferred that the concentration of ca2+ increases with tds along the flow path, which is verified by fig. 6b. in fig. 7d, most samples are plotted below the line of 1:2, which indicates that the dissolution of calcite is not the only source of ca2+. furthermore, groundwater is oversaturated with respect to calcite within the study area, which is supported by the sicalcite value listed in table 4. therefore, calcite dissolution is not likely to take place within the study area. likewise, the sidolomite is greater than 0, and the concentration of ca2++mg2+ does not increase with hco3 (fig. 7e), indicating that the dissolution of dolomite is not likely to occur in the area either. considering the dissolution of gypsum and fluorite and the cation exchange dynamics discussed above, it is reasonable to conclude that the concentration of ca2+ is mainly controlled by the dissolution of gypsum and fluorite, and cation exchange between ca2+ and na+ occurred within the study area. however, the dissolution of carbonate minerals no doubt contributed greatly to the regional groundwater evolution, especially with respect to the origins of ca2+, mg2+, and hco3 -, because most samples are plotted between the 1:1 and 1:2 lines in fig. 7e. the dissolution of the two minerals may have taken place in the upper reaches of the study area, and the groundwater may become oversaturated with the two minerals before the groundwater flows into the study area, which can be verified by the results of saturation index. the plots of so4 2+hco3 versus ca2++mg2+ in meq/l (fig. 7f) also indicate the contribution of the dissolution of carbonate minerals to the relevant ions. in summary, the dissolution of carbonate minerals (calcite and dolomite) may make important contributions to the ca2+, mg2+ and hco3 in groundwater, but their dissolution may have occurred in the upper reaches rather than taking place within the study area. the groundwater has already become oversaturated with calcite and dolomite before it flows into the study area. in contrast, gypsum and fluorite are undersaturated in the groundwater, and their dissolution can be a significant contribution to the increased ca2+ in the groundwater. the increase of mg2+ in the groundwater may be explained by cation exchange. for na+, the dissolution of halite contributes greatly to its concentration, but cation exchange makes the na+ and clvary differently. furthermore, the potential artificial pollution in the area may also contribute to the different patterns of na+ and cl-. the reactions in a groundwater system are complex but interesting, and therefore require much more work. irrigation water quality assessment the suitability of groundwater for agricultural purposes depends on the effect of the water’s mineral constituents on both plants and the soil (tyagi et al. 2009). wilcox (1948) and the us salinity laboratory staff (ussl 1954) proposed irrigation specifications for evaluating the suitability of water for irrigation use. the ussl diagram relates sar (representing the alkalinity hazard) to ec (representing the salinity hazard). the ec of irrigation water can be classified into low (c1), medium (c2), high (c3) and very high (c4) salinity zones. in terms of the classification of irrigation water, the alkalinity hazard is classified as low (s1, <10), medium (s2, 10-18), high (s3, 18–26) and very high (s4, >26). the irrigation water quality was assessed with the methods introduced above. the sar value varies from 0.95 to 8.21, with an average of 2.51, indicating a low-alkalinity hazard according to the ussl classification. in the present study, the values of sar and ec were plotted on a us salinity diagram (fig. 8). the results show that most groundwater samples belong to the category c3s1, which is generally suitable for agricultural purposes. nw1 and nw2 belong to c4s1, and nw6, nw8, nw9 and nw25 belong to c4s2, which indicates that although these samples represent a low alkalinity hazard, they are not suitable for irrigation under ordinary conditions because they represent a very high-salinity hazard. however, these waters may be used for salt-tolerant plants on permeable soils with special management practices. in the wilcox diagram, the ec is related to %na (meq/l). generally speaking, a %na value lower than 60% is acceptable. in the present study, most samples in the wilcox diagram are plotted in the good to permissible zone, accounting for 77.78% of the total samples. nw1, nw2, nw8, nw9 and nw25 belong to doubtful to unsuitable category, which requires special practices for hazard control due to the high-salinity hazard. nw6 is unsuitable for irrigation (fig. 9). fig. 8 plot of the sar versus ec (ussl 1954). 36 peiyue li, jianhua wu, hui qian fig. 9 plot of %na (meq/l) versus ec (wilcox 1948). there is no high-alkalinity hazard in groundwater with respect to agricultural uses, and the only potential agricultural hazard may be the salinity hazard. therefore, the spatial variation of ec can reflect the distribution of irrigation water quality. fig. 10a shows the ec distribution, and it is clear that the groundwater is acceptable for agricultural use in most areas. however, in the western area and in local areas of the middle and eastern areas, the groundwater has ec values of more than 2000 μs/cm, indicating doubtful utility for irrigation. fig. 10 spatial distribution of (a) ec (μs/cm), (b) tds and (c) overall water quality. drinking water quality assessment tds and th are two important parameters in assessing drinking water quality. tds represents the total weight of dissolved solids in a solution and expresses the degree of salinity of a medium (mitra et al. 2007). according to the level of tds, groundwater can be classified as fresh groundwater (tds<1000 mg/l), brackish water (1000<tds<10000 mg/l) and saline water (tds>10000 mg/l) (wanda et al. 2011). th is a measure of dissolved ca2+ and mg2+ in water and is expressed as caco3. as per the grading standards of th (as caco3), groundwater can be classified as soft water (th<150 mg/l), moderately hard water (150<th<300 mg/l), hard water (300<th<450 mg/l) and very hard water (th>450 mg/l) (li et al. 2011). three major zones in the study area can be identified according to the level of tds (fig. 10b). fresh water is mainly located in the middle and east, and groundwater with tds values of 1-2 g/l is observed in the western and southeastern areas. groundwater with tds values higher than 3 g/l is found only locally in the east. it appears that the groundwater around xuanhe town is good with respect to tds. figs. 10a with 10b appear similar. this is because the two parameters both reflect the groundwater salinity, and they can be approximately converted into each other using the relationship ec=γtds, where γ is in the range of 0.55-0.75 (qian and ma 2005). the plot of tds versus th (fig. 11) suggests that the groundwater samples lie in the zones z3, z4 and z8, indicating different quality levels among these samples. samples that belong to zone z3 are suitable for human consumption because they are fresh water with acceptable degrees of hardness. however, the samples that belong to zones z4 and z8 are not fit for direct consumption because they are either too hard or too salty. fig. 11 plot of tds versus th expressed in mg/l as caco3. although tds and th are two very important parameters for indicating the suitability of water for drinking, they do not comprehensively reflect the overall water quality. other indices may also have large influences on the overall water quality. in the present study, the comprehensive water quality index (cwqi), recommended by the quality standard for groundwater of china (bureau of quality and technical supervision of china 1994), was selected to comprehensively assess the drinking water quality, and the comprehensive assessment results are illustrated in fig. 10c. 37hydrogeochemistry and quality assessment of shallow groundwater in the southern part of the yellow river alluvial plain (zhongwei section), northwest china fig. 10c reveals that poor-quality water, with respect to drinking purposes, is widespread in the study area, and goodand fair-quality water is only observed locally near xuanhe town and yongkang town. in the front zone of nanshantaizi, very poor quality water is observed. the results suggest that the groundwater in most regions of the study area is not fit for direct consumption, and special pretreatments should be performed if it is used for drinking. the poor quality of the groundwater is caused by excessive levels of tds, th, cland so4 2-, which result from the dissolution of minerals in the aquifer media. no serious anthropogenic pollution was detected in this study. however, agricultural activities in nanshantaizi, such as irrigation by diverting water from the yellow river, may cause water quality deterioration along the front zone of nanshantaizi, and this requires further specific investigation. conclusions this research demonstrated the hydrogeochemical evolution of shallow groundwater via multiple approaches. the groundwater quality with respect to agricultural and drinking purposes was assessed. the conclusions can be summarized as follows. • there are higher concentrations of major ions and higher values of chemical parameters in the groundwater than in the yellow river water. the groundwater in the study area is fresh to moderately mineralized, with tds values ranging from 539.75 to 2248.98 mg/l. no ammonia nitrogen pollution or nitrate nitrogen pollution was detected, but local nitrite nitrogen pollution was observed. the groundwater samples with different concentrations of tds possess different hydrochemical characteristics. for groundwater samples with tds < 1 g/l, ca2+ is the predominant cation, followed by na+ and mg2+, whereas for groundwater samples with tds > 1 g/l, na+ is the major cation, followed by mg2+ and ca2+. for anions, hco3 is predominant in the groundwater samples with tds< 1 g/l. in contrast, for groundwater samples with tds > 1 g/l, so4 2and clare the most prevalent anions. the main hydrochemical facies for groundwater samples with tds < 1 g/l are hco3-ca·mg and hco3ca·na·mg, but for groundwater samples with tds > 1 g/l, so4·cl-na and so4·cl-na·mg·ca are the predominant hydrochemical types. the ph values of groundwater samples with tds < 1 g/l are lower than those of groundwater with tds > 1 g/l. • most major ions, with the exception of hco3 -, increase with tds. the increase of ca2+ and so4 2can be attributed to the dissolution of gypsum and fluorite. the deficiency of ca2+ against so4 2may be attributed to the precipitation of calcite and dolomite along the flow path. the increase of mg2+ may be explained by the cation exchange. the dissolution of halite contributes greatly to the increase of na+, but cation exchange and potential artificial pollution make the na+ and clvary with different patterns. the reactions in the groundwater system are very complex. • most water samples are acceptable for agricultural use. the suitability of groundwater with regard to agricultural use is controlled by salinity. groundwater with acceptable quality is mostly distributed in the middle and east, whereas in the west and in local areas of the middle and east, the groundwater is of doubtful quality for irrigation. necessary management practices are required when doubtful-quality water is used for irrigation. • groundwater in most areas is not fit for direct human consumption. the poor drinking water quality is caused by excessive levels of tds, th, cl and so4 2-, which are mainly caused by the dissolution of minerals in the aquifer media. the groundwater pollution along the front zone of nanshantaizi can also be attributed to the irregular irrigation patterns in this area. acknowledgement the research was supported by the doctor postgraduate technical project of chang’an university (2013g5290002 and 2014g5290001), the national natural science foundation of china (41172212), the special fund for ningxia environmental protection (2011hb001) and the technical program of environmental protection of ningxia environmental protection department (201108011). we thank wei yani, song baode and zhang xuedi for their help with the field investigation and sampling. the editor and anonymous reviewers are also acknowledged for their useful comments on the paper. references bureau of quality and technical supervision of china. 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(2010). major ion chemistry of waters in lake qinghai catchments, ne qinghai-tibet plateau, china. quaternary international, 212, 35-43. doi: 10.1016/j.quaint.2008.11.001 keywords: scenic spot; remote sensing technology; natural resources; dynamic monitoring; modis remote sensing data; forest resources. palabras clave: lugar escénico; teledetección; recursos naturales; monitoreo dinámico; datos de teledetección modis; recursos forestales. how to cite item li, m. (2021). dynamic monitoring algorithm of natural resources in scenic spots based on modis remote sensing technology. earth sciences research journal, 25(1), 57-64. doi: https://doi.org/10.15446/esrj.v25n1.93869 a dynamic monitoring algorithm of natural resources in scenic spots based on modis remote sensing technology is proposed to improve natural resources monitoring accuracy in scenic spots. the remote sensing images of scenic spots obtained by modis were preprocessed by tm image processing, atmospheric correction, and other technologies to get high-precision remote sensing images. the remote sensing images of scenic spots were segmented by the multiscale segmentation method, and then the hierarchical supervision classification method was used. the change points of natural resources were extracted. the resource changes and independent variables of scenic spots were analyzed based on the least square method to realize the dynamic monitoring of natural resources in scenic locations. the experimental results show that the technique can accurately monitor the dynamic changes of forest resources and water resources in scenic spots, and the monitoring results have high accuracy. abstract dynamic monitoring algorithm of natural resources in scenic spots based on modis remote sensing technology algoritmo de monitoreo dinámico de recursos naturales en lugares escénicos con base en la tecnología de detección remota modis issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93869 con el fin de mejorar la precisión del monitoreo de recursos naturales en lugares escénicos, se propone un algoritmo de monitoreo dinámico de recursos naturales con base en la tecnología de teledetección modis. las imágenes de teledetección de lugares escénicos obtenidas por modis fueron preprocesadas con el método de imágenes tm, corrección atmosférica y otras tecnologías para obtener imágenes de teledetección de alta precisión. las imágenes de teledetección se dividieron mediante el método de segmentación de múltiples escalas y se utilizó el método de clasificación de supervisión jerárquica. se extrajeron los puntos de cambio de los recursos naturales y se analizaron los cambios de recursos y las variables independientes de los lugares escénicos con base en el método de mínimos cuadrados, para realizar el seguimiento dinámico de los recursos naturales en los lugares escénicos. los resultados experimentales muestran que el método puede monitorear con precisión los cambios dinámicos de los recursos forestales y los recursos hídricos en lugares escénicos. los resultados del monitoreo tienen una alta precisión. resumen record manuscript received: 03/02/2020 accepted for publication: 21/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 57-64 maolin li* school of tourism management, guilin tourism university, guilin 541006, china *email of the corresponding author: limaolin2245@163.com g eo ph ys ic s https://doi.org/10.15446/esrj.v25n1.93869 https://doi.org/10.15446/esrj.v25n1.93869 mailto:limaolin2245@163.com 58 yan lv, laijun lu, mengxue cao introduction remote sensing technology (rs) is a high-tech applied to agriculture in the late 1970s. according to the theory of electromagnetic waves, it is a comprehensive technology (chen, 2017) to collect, process, and image the electromagnetic wave information radiated and reflected by a longdistance target using various sensing instruments, which allows detection and identification of multiple scenes on the ground. the popularization of rs technology has driven the development of some related technologies, and its application has been continuously improved. now it is widely used in the dynamic monitoring of land use. it can quickly and accurately analyze the dynamic changes in the interpretation of landforms, textures, colors, or other features in towns, rice fields, gardens, woodlands, and specific locations. (li et al., 2016). for the research of remote sensing of natural resources, the coupling research of natural resources and ecological environment system supported by remote sensing technology is paid more attention in the world at present, which is very important for mastering the spatial and temporal distribution law of the quantity and quality of natural resources (du et al., 2018; hu, 2018; lisowski, 2018; zeng et al., 2018). typical research examples include nasa, engineers, and soldiers. they use the method of combining remote sensing and natural resource models to study the integrated natural resource simulation and management in the united states (mcroberts et al., 2018). the environmental simulation and resource management of four rural natural resources in the netherlands conducted by the netherlands soil research institute (bi, 2020; pan, 2018; pei et al., 2018). the main contents of these studies are the migration of natural resources and their behavior process in the ecosystem, the driving and restricting factors of the behavior process of natural resources. with the advent of multi-angle remote sensing, it is possible to determine the structure parameters of the ground and then the spectral parameters of the components through the inversion of the bidirectional reflection model, which makes the expectation of quantitative remote sensing higher and higher. in the 1990s, verstrateet, pinty, and myneni published their research results, trying to theoretically and conceptually define the potential and limitations of remote sensing inversion to extract surface information. all three scholars are experts who have made great achievements in modeling bidirectional reflection and multi-angle remote sensing. they have a profound understanding and incisive opinions on the essence and difficulties of remote sensing inversion. they have incisive discussions on many aspects, such as the limitations of empirical models, the necessity of quantitative remote sensing, the information fusion of multi-angle remote sensing and multispectral remote sensing, and put forward the “model for (specific data)”. at present, experts engaged in remote sensing model inversion in china have done a lot of work in the inversion of soil moisture, vegetation biomass, and other surface parameters. scholars and professors represented by li xiaowen have summed up the “data-based inversion target decision” from practice, and the staged segmentation of data set and parameter set based on the “uncertainty and sensitivity matrix of parameters”. the development of hyperspectral resolution remote sensing or imaging spectral remote sensing technology is one of the major technological breakthroughs made by human beings in earth observation in the future and is the leading technology of remote sensing at present. after a long time of development, a series of hyperspectral imaging systems have been successfully developed in the world and have been widely used in aviation platforms (dong et al., 2018; kundakci & nas, 2018; li et al., 2018). after a series of important technical problems in the application of hyperspectral remote sensings, such as the calibration and quantification of hyperspectral imaging information, the visualization and multi-dimensional expression of imaging spectral image information, image spectral transformation, and extraction of spectral information, a large amount of data processing, spectral matching, spectral recognition, and classification, they have been basically solved. on the one hand, hyperspectral remote sensing is in the experimental research stage gradually turns to the practical application stage (cai et al., 2017). the remote sensing instrument used in this paper is modis, which is used to monitor the natural resources of the libo scenic area. after image processing, the image is segmented and classified in multi-scale. on this basis, the stepwise regression analysis method based on the least square estimation is used to build the model, which can improve the water resources and forest resources in libo scenic spot action monitoring (xie et al., 2018). materials and methods research object libo scenic spot is located in libo county in the south of guizhou province, belonging to qiannan buyi and miao autonomous prefecture. it is the abbreviation of zhangjiang national key scenic spot (national 5a level) in libo, which is composed of eight natural scenic spots and five red scenic spots. it belongs to the subtropical monsoon humid climate area. the general trend of temperature distribution is higher in the south and lower in the north. when the terrain is increased by 100 m, the temperature drops by 0.55°c; the valley area is higher than the mountain area of the same height, the east-west valley is higher than the north-south valley, the south slope is higher than the north slope, and the closed valley basin is higher than the same high slope. the annual average temperature of the county is 18.3°c. the hottest month is july and the coldest month is january. the average temperature in july is below 27.0°c, and the extreme maximum temperature is not more than 40.0°c; the average temperature in january is above 5.5°c, and the extreme minimum temperature is above -10.0°c. the terrain gathers the comprehensive advantages of the guizhou plateau and guangxi hills. there are both mountains and hills, which belong to the karst landform. the rivers in libo belong to the longjiang river system and pearl river basin. the larger river systems in the county include the dagou river system (including the mainstream of zhangjiang river and the tributaries of fangcun river), the sancha river system, and the jiacai river system. libo scenic spot contains typical cone-shaped karst landform and related surface and underground karst morphology; special karst primitive forest ecosystem known as “emerald” and related natural elements such as rock, soil, hydrology, animal and plant communities, which show the ongoing biological ecological process; many endangered, endemic and protected animals and plants and their dependence habitat for survival; it also includes various landforms and ecological landscapes showing the natural beauty of this area. libo scenic area has rich natural resources and beautiful scenery. remote sensing data processing the flow chart of remote sensing image processing with modis is shown in figure 1. radiation calibration modis data geometric correction band combination radiation values extraction of related parameters remote sensing light temperature geographic information vegetation coverage emissivity image enhancement information reflection valuesimage information enhancement loading geographic information atmospheric correction tm image processing technology correction of solar altitude angle figure 1. data processing flow chart the omdis detector is a passive swing scanning detector with a transverse scanning angle of ±55’. each scan line consists of 13 541 km pixels or 2 708 500 m pixels or 5 461 250 m pixels. due to the curvature of the earth, the actual span of the scan line is approximately 2 340 km (reiche et al., 2018). for each scan, the modis detector travels 10 km along the orbit, forming a scan strip. after obtaining modis remote sensing data, tm image processing technology is used for image processing, and the solar altitude angle, geometry, 59tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm and atmosphere are corrected. after obtaining the reflection value and radiation value, relevant parameters are extracted. the applied parameters include remote sensing brightness temperature, vegetation coverage, image enhancement information, geographic information, and specific radiation rate. tm remote sensing image processing technology the main purpose of remote sensing image processing is to grasp the situation of forest resources in a wide range, to provide information quickly and macroscopically, to output high-quality images, and to provide a basis for visual interpretation and classification. (1) geometric correction according to the characteristics of the tm remote sensing image, we choose the geometric correction method of a quadratic polynomial. when using this method for correction, we choose the same name ground feature points that are evenly distributed and easy to determine on the image map and topographic map, such as the intersection of farmland forest network; the intersection of roads, rivers, canals, etc. (wei et al., 2016). in general, as long as the control points are selected properly and used accurately, the error is usually less than one pixel. (2) band combination the information of tm channels is related to the spectral characteristics of vegetation reflection, so it is very important to optimize tm band combination for the application of tm image in forest resource monitoring. according to the characteristics of plant reflection spectrum and vegetation tm spectrum, the combination of tm4.5.3 band is a better pseudo color synthesis scheme. (3) image information enhancement for the pseudo color image of tm4.5.3 after synthesis, in order to highlight the information of remote sensing image and distinguish different land type features, the multi-stage gray stretch transform is adopted, which can stretch all the ground objects linearly in the range of 0-255 of national radiance level, or stretch different targets in a certain gray range, and compress other non-target ground objects processing to achieve the purpose of extracting and enhancing the target features (pahlevan et al., 2017). the general expression of the multi-segmental gray-scale twist to stretch transformation is as follows: y l l a a x a li n i i i i i i= + + = − + −( )+1 1 1 (1) where: x and y are the luminance values x y, 0 255∈ −( ) after tm image stretching transformation; ai+1 and ai, li+1 and li are the stretching range required by the original image determined by the user and the corresponding luminance value range after stretching; n is the number of segments. (4) loading geographic information in order to facilitate the use of images, loading the geographic grid information can make the satellite image better applied. combined with the fourth reexamination of forest resources in liaoning province, 2kin×2kin network and 1/50000 international topographic map frame line were loaded. use a topographic map for a sampling survey to verify whether the grid point is correct. after processing the modis remote sensing image, in order to obtain high-precision remote sensing images, it is necessary to correct and remove the interference data in the image. the specific methods are as follows. atmospheric correction from the perspective of radiation data processing, the radiation distortion components entering the sensor include extinction (absorption and scattering) of the atmosphere, sky-light (atmospheric scattering) irradiation = path radiation. it is difficult to correct the influence of the atmosphere on radiation transmission because of the inaccuracy of atmospheric environmental parameters (liang et al., 2016). there are two kinds of common correction methods. one is to solve the radiative transfer equation (approximately). another method is to use the data of a ground survey. set a marker with known radiation characteristics, and the sensing instrument outputs its radiation data. at the same time, the measurement does not include the radiation output data of the atmospheric path. by comparing the two, we can get the effect of the atmosphere. it is obvious that this method is limited by the correction factor to eliminate the influence of the atmosphere. using american modtran4, 6s, lowtran7 and other atmospheric correction software to carry out an atmospheric correction for modisbi data flaash is an extension module of envi for rapid atmospheric correction analysis of spectral data. the purpose of atmospheric correction is to eliminate the influence of atmosphere and light on the reflection of surface objects and to obtain the real physical model parameters of the surface, such as the reflectance, emissivity, and surface temperature. flaash can not only eliminate the influence of water vapor, oxygen, carbon dioxide, methane, and ozone on the surface reflection but also eliminate the influence of atmospheric molecules and aerosol scattering. flaash can process any hyperspectral data, satellite data, and aviation data (860lnn/1135lnn). acceptable sensor types include htmap, vairis, casi, hydice, orbview-4, nemo, and more (yang et al., 2016). at the same time, flaash can also correct the vertical imaging data and the observation imaging data. this module is initiated by airforceresearchlabs (afr)l, which studies dtran atmospheric transport models. it is developed by spectarlseiences, inc. (551), a leader in atmospheric correction algorithm research. research systems, inc. (rsi) is responsible for the gui design and module integration of the flaash module. solar height angle correction of reflectivity (1) modislbi data product is the radiation and reflection of the top layer of the atmosphere (to) a, which has been positioned and calibrated. the reflectivity conversion formula is: reflectante = refc tan ce − scale[b](si − reflectance − offsets[b]) (2) where reflectante is the reflectivity, refc tan ce − scale[b] is the reflection calibration coefficient, reflectance − offsets[b] is the reflection offset, and si is the standard quantity. (2) the reflectivity is related to the solar altitude angle and changes with the change of the solar altitude angle. the relationship between reflectivity and solar altitude angle:  = cos, where  is the top reflectivity;  is the solar altitude angle. (3) because the influence of the atmosphere on modis 1, 2, and 6 band data is relatively small, the influence of the atmosphere can be ignored. data bad value and noise removal due to the bad band value of the data caused by the sensor’s test sale, the sensor drift causes noise in the data. the interpolation and the fourier transform are used to eliminate the noise as much as possible. the elimination of the bad value of the strip is to use the method of linear mashing to fill the bad points with the linear combination of the values of the adjacent points (li, 2016). the data obtained from the remote sensing sensor is the radiation value of the atmosphere surface (toa). in the process of data processing, it is converted into the reflection value (*=cos) according to the following formula: ρ θ π cos = d r e toa 2 0 (3) where, rtoa is the radiation value of the surface layer of the atmosphere; cos is the reflectivity; e0 is the solar radiation in outer space; (wm-2um-1)d is the distance from the sun to the earth. in practical application, we need the  value of solar vertical reflectance, so we need to divide the existing reflectance data by the reflectance of vertical solar radiation cos . in the high-resolution remote sensing image, the influence of solar height angle is very small because of the small scanning width. however, for modis data with a wide scanning range, the correction of solar 60 yan lv, laijun lu, mengxue cao height angle is very necessary. if the data describing the solar altitude angle cannot be obtained in the calculation process, for example, the resolution data of 250 m and 500 m only contains coordinate data, and the satellite and solar position data need to be obtained from 100 m data. in this case, the solar high angle of production can be calculated directly through the longitude and latitude information. because the terra satellite belongs to a sun-synchronous satellite, the transit time is 10:30 a.m. local time. if the terra satellite is an earra satellite, the transit time is 2:30 p.m., which can be converted into greenwich time: 2:30 and 6:30 respectively, so that the value of solar altitude angle at any position on the earth can be determined. the dynamic monitoring method of natural resources after removing the interference data in the image and obtaining highprecision remote sensing images, the following methods are used to carry out dynamic monitoring of natural resources in libo scenic areas. multiscale segmentation of remote sensing image because the multi-scale segmentation considers the multi-level of the field pattern of the earth’s surface and overcomes the fixed size of the data source, it can make full use of all kinds of information and select the most appropriate segmentation scale to segment-specific terrain types, so this segmentation adopts the multi-scale segmentation method. firstly, the small spot layer and remote sensing image are overlapped, and the image is initially segmented by using the chessboard segmentation algorithm. the boundary of the segmented object is the small spot boundary, and each polygon corresponds to a small spot object. using this method to segment, the boundary information of the original small spot layer can be preserved completely, which is conducive to updating the small spot data after the completion of classification. if the image is directly segmented for image segmentation, the boundary generated by the segmentation object will be inconsistent with the original speckle boundary, resulting in a larger work amount of updating the speckle after classification (sun, 2016). then the segmentation is carried out again, and the segmentation scale is selected as 50, 150, and 250. the comparison of the segmentation effect of the test sample area is shown in figure 2. after a comparative analysis of the segmentation results of the three corresponding regions in the figure, it is found that when the segmentation scale is set to 150 (figure 2(b)), the land class can be segmented better, and there are fewer over-segmentation phenomena. after re-segmentation, a new class hierarchy is generated, and the original land class information is integrated, and then each object generated after segmentation is given. at this time, the generated object is the fine segmentation object inside the original speckle object. after classification, the changing area can be quickly extracted. hierarchical supervision classification the method of hierarchical supervised classification can make full use of the inner land classes of each layer. when selecting the training sample area, aiming at the phenomenon of the same object and different spectrum, the method of type subdivision can be used to improve the separation degree between land classes and improve the classification efficiency and accuracy to a certain extent. therefore, this classification adopts the method of hierarchical supervision and classification. in this classification, all land types are divided into forest land and non-forest land. after the forest land classification system is determined, the classification is directly carried out in ecognition. for eight different bands, different combinations of different bands or image bands can be selected. the most significant difference between different categories or a certain number of training samples can be selected to supervise the classification and gradually extract the objects of different categories. finally, the objects of different categories can be obtained. taking no forest land (200 > dl > 1500) as an example, this paper introduces the process of changing area extraction in no forest land thematic layer. in view of the small spot objects that have become forestland, the normalized vegetation index (ndvi) is selected for discrimination extraction. by selecting a certain number of training samples, it is found that the threshold value is set to 0.25, then the small spot objects (200 > dl > 150, ndvi > 0.250) (a) points case scale 50 (b) points case scale 150 (c) points case scale 250 figure 2. results in different segmentation scales that have become forestland in the original forestland free area can be extracted, and the small spot objects (mainly including water) that have become non-forestland for the forestland free area can be extracted. in the false-color image, the water body is dark black, and the color of cultivated land and construction land without crops is gray or white, which is obviously different from other types of objects. the training samples can be selected, the nearest classifier can be used to supervise and classify, and these three types of objects can be extracted; for sparse forest land, unforested forest land, nursery land, and cultivated land with crops. because the area of these four kinds of ground objects is not large, they can be modified manually later. 61tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm based on the natural resource change map obtained by classification monitoring, a model is established to monitor the dynamic changes of natural resources. model building with the help of remote sensing (rs), geographic information system (gis), and global positioning system (gps), the remote sensing monitoring of various variables of natural resources in libo scenic spot has been studied by many scholars at home and abroad. the basic idea is generally, the gray value, gray ratio, and gis information of the rs image corresponding to the ground sample plot are used as independent variables, and the sample plot volume is used as the dependent variable. through the quantitative method, the monitoring model between the resource change and the independent variable is established. the variable monitoring model of natural resources is generally expressed as formula (4): v i x e e e cov e i n n = + + ( )= ( )= γ σ γ0 0 2 (4) in the formula, 0 is the parameter of the undetermined constant term, e is the vector of n×1 which represents the random error of observed value of resource change, cov(e) is the covariance matrix of the observed value of canopy density or resource change of n sample plots, 2 is the observed variance of canopy density or resource change of sample plots, in is the unit matrix of n order. in the data obtained in this study, there are few independent variables that can be used as a resource change monitoring model, which is suitable for least square regression modeling. in the limited independent variables, in order to ensure the significance of the variables in the model, stepwise regression analysis is needed. therefore, in this study, the stepwise regression analysis method based on the least square estimation is used to establish the model. the independent variable ldlx is land type, pd is slope,x1 is tm1, x2 is tm (5+7−2) / (5+7+2), x3 is tm (4+5−2) / (4+5+2), x4 is tm5/4, x5 is ndvi, y is unit volume. through the stepwise regression analysis of the least square, the optimal monitoring model is finally established as follows: y ldlx x x x x x = − + + −   80 341 8205 978 14536 277 3130 191 2 427 1 2 3 4 5 . . . . .         / ( / )300 180cos pdπ (5) where cos(pd/180) is the cosine of the slope (radian). from formula (5), it can be seen that the variable ldlx (land type) is the slope of the model, which plays an important role in monitoring the change of natural resources; the land type is not a fixed value, and different land types have different values, that is to say, the change of resources is monitored in layers according to the value of land type. the complex correlation coefficient of the regression model is 0.650, which can better reflect the relationship between resource change and remote sensing variables. the significance test of the regression model uses statistics as an f test. when the test reaches the significant or extremely significant level, it can be considered that the regression model has a significant linear regression relationship. f test the regression model. the f (6, 53) value is 6.466. according to the f distribution table, the f value is 2.93 at the level of significance 0.05, less than 6.466, and p = 0.000. the f test reaches an extremely significant level. therefore, it is considered that there is a significant linear regression relationship between resource changes and remote sensing variables. results libo scenic area is located in the south of guizhou province. the charming karst landform breeds unique scenery and rich natural resources. among them, laya waterfall, 68 level waterfall group, primitive forest on the back of tortoise, and water forest are famous in china and abroad. aiming at the characteristic forest resources and water resources in the libo area, this paper uses the method to carry out dynamic monitoring. dynamic monitoring results of forest resources the distribution map and current situation map of forest resources change in libo scenic area monitored by this method are shown in figure 3. (a) change of forest resources (b) status of forest resources figure 3. changes and status of forest resources in libo scenic spot it can be seen from figure 3 that the area of positive change (change of increase of forest coverage) and reverse change (change of decrease of forest coverage) of forest resources in libo scenic spot reaches 5205.186 hm2 and 546.524 hm2 respectively, the area without change is 42159.603 hm2, the area with original land type empty is 895.514 hm2, and the area with positive change is obviously large in the area of reverse change. in the early years, in order to develop the economy in the libo area, the forest burning and logging in large areas greatly reduced the forest area and destroyed the ecological balance of the original forest. in recent years, the tourism industry has been vigorously developed. under the policy of returning 62 yan lv, laijun lu, mengxue cao farmland to the forest to maintain the original forest landform in the libo scenic area, the large-scale forest building activities in the libo area have achieved positive results, overcoming the impact of large-scale economic construction on the reduction of forest area. at the same time, the forest coverage area has been greatly increased, providing an ecological guarantee for the long-term development of the libo scenic area. this classification accuracy evaluation is mainly based on the data collected by remote sensing technology and field investigation. it mainly focuses on the inspection and verification of the changed objects, especially the reverse change objects of forest resources. at the same time, 342 objects whose image characteristics are not obvious are selected from 3814 changed objects for field verification, and interpretation marks are established. then visual inspection and inspection are carried out for each object see table 1 for the results. table 1. accuracy verification table category change question pattern spot change no question pattern spot total change pattern spots no change pattern spot all figure spot the number of totals 425 3639 4064 2577 6641 interpretation right 385 3372 3757 2526 6283 interpretation errors 810 267 1077 49 1126 interpretation accuracy/% 91% 93% 92% 98% 95% from table 1, it can be seen that 385 out of 425 change question spots are interpreted correctly, with a correct rate of 91%; 3757 out of 4064 change spots are interpreted correctly, with a correct rate of 92%; for all 6641 spots, with a correct rate of 95%. the results show that the dynamic monitoring results of forest resources in libo scenic spots are good. see figure 4 and table 2 for the dynamic changes of natural resources in libo scenic spots in recent ten years. according to the observation of figure 4 and table 2, the change of forest resources in libo scenic area in recent ten years is as follows: in recent ten years, the largest change of land type area of forest resources is the mixed deciduous broad-leaved forest, followed by the evergreen broad-leaved forest, and the smallest change of area volume of the mixed evergreen broad-leaved forest. from figure 4, it can be seen that there was a significant increase in the area of three land types in 2013. before 2013, there was almost no change in the area of the three land types. this is because the large-scale development of agriculture in the last century did not maintain the forest resources after burning mountains and logging, and let it grow naturally. since 2013, the government has carried out the policy of returning farmland to forests and actively developed the tourism industry. therefore, since 2013, the government has carried out the policy of returning farmland to forest, the area change of forest resources has increased year by year. the area of evergreen broad-leaved forest has increased from 420.64 km2 in 2009 to 516.78 km2 in 2019, an increase of 3.94%; the area of evergreen broad-leaved mixed forest has increased from 365.14 km2 in 2009 to 491.23 km2 in 2019, an increase of 3.47%; the area of deciduous broadleaved mixed forest has increased from 466.70 km2 in 2009 to 569.24 km2 in 2019, an increase of 9.14%. dynamic monitoring results of water resources libo scenic area is rich in water resources, and its famous water features include yuanyang lake, qinglongtan, dongshan shuibi, zhangjiang scenery belt, among others. the experiment uses this method to monitor the dynamic changes of the above waters. taking yuanyang lake as an example, the change of water level at different times is shown in figure 5. it can be seen from figure 5 that the annual water level of yuanyang lake in libo scenic spot monitored by this method has less precipitation in winter figure 4. libo scenic area in the last 10 years table 2. libo scenic area in the last 10 years plant type evergreen broadleaf forest mixed evergreen broad-leaved forest deciduous broadleaf mixed forest combined 2009 area/km2 420.64 365.14 465.70 1251.48 percentage/% 19.65 16.35 18.89 54.89 2010 area/km2 420.64 365.24 465.90 1251.54 percentage/% 19.67 16.37 18.94 54.98 2011 area/km2 420.74 365.24 466.87 1252.85 percentage/% 19.78 16.47 18.96 55.21 2012 area/km2 420.89 365.41 467.55 1253.85 percentage/% 19.89 16.57 19.34 55.98 2013 area/km2 489.42 469.59 473.51 1432.52 percentage/% 22.52 18.87 26.10 67.49 2014 area/km2 489.65 470.65 482.67 1442.97 percentage/% 22.57 18.96 26.16 67.69 2015 area/km2 497.65 473.58 487.41 1458.64 percentage/% 22.89 19.04 27.01 68.94 2016 area/km2 499.65 479.65 490.22 1469.52 percentage/% 22.97 19.21 27.40 69.58 2017 area/km2 502.36 485.59 494.58 1482.53 percentage/% 23.25 19.75 27.34 70.64 2018 area/km2 506.28 489.22 503.14 1498.64 percentage/% 23.27 19.75 27.46 70.78 2019 area/km2 516.78 491.23 569.24 1477.25 percentage/% 23.59 19.82 28.03 71.44 63tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm and spring, and it is in the dry period. the water level of yuanyang lake is low and the water resources are not rich. in summer and autumn, the precipitation increases and the water level rises. especially from june, the precipitation suddenly increases and the water level rapidly increases, until august, when the water in libo scenic spot reaches its peak abundant in natural resources and beautiful scenery. see table 3 for the growth of water resource areas in the libo scenic area at different times. it can be seen from table 3 that the change of water resource area is monitored by the method in this paper. due to the drought in winter and spring, the area of water resources has negative growth, and the area of water area is decreasing. after entering the rainy season, the area of water area has an upward trend and abundant water resources. it shows that this method can effectively monitor the dynamic changes of natural resources in the libo scenic spot. discussions the dynamic monitoring of natural resources based on remote sensing technology has the characteristics of real-time, long-term, comprehensive, and periodic. the survey results must have high accuracy and accuracy. it is feasible to use remote sensing images to monitor the change of natural resources. the remote sensing information has great potential in the monitoring of the change of natural resources. it has great potential in the practical application of production above the county-level broad prospects (low et al., 2016). the remote sensing image is the ideal data for the spectral remote sensing monitoring of natural resources change. its spatial resolution can basically meet the requirements; the spectrum information is very rich, including visible light, near-infrared light, and medium infrared light; the economic cost is low, and part of the data can be downloaded free of charge on the internet. in the quantitative monitoring of multi-scene or multi-time image remote sensing image of natural resource change, image segmentation is necessary to establish a unified monitoring model of the gray value of different scene or time phase image and natural resource change, improve the stability of the model, and then improve the monitoring accuracy (benacchio et al., 2016). the correlation between land type and monitoring image is very high, which is an indispensable factor for remote sensing monitoring of natural resources change, and has a significant impact on the monitoring accuracy of natural resources change. without land type, the monitoring result of natural resource change is only a gray-scale image the change of unit area (pixel area) (gray value), which is a semi-finished product with both natural resource change and non-daily resource change. only through the control of land type can the final monitoring of natural resource change be completed. moreover, land information can be extracted directly from remote sensing images. the remote sensing images used in the extraction of land information and the monitoring of change amount should be different. when extracting the land type information, the greater the difference between the natural resource information and the non-natural resource information in the remote sensing image is, the better, the middle of march or the middle of october is suitable; when monitoring the change of natural resources, the monitoring model is established with the remote sensing image band combination (equivalent to vegetation index) as the variable, and the image phase is suitable for the natural normal growing season in nature (liu et al., 2017). the establishment of monitoring models is the key to the success or failure of natural resource change monitoring. there are nearly 20 independent variables of remote sensing factors. first, the correlation between these variables and the change of natural resources is very different, and some variables with low correlation need to be eliminated through correlation analysis; second, there is a correlation between variables, and the correlation between variables must be eliminated through principal component analysis; third, there are many variables involved in modeling, and the least square stepwise regression method is used, the variables are filtered one by one to determine the modeling variables. through the above three links, the fitting and stability of the monitoring model are improved. when the discontinuities of vegetation structure and soil moisture coexist, the model error is the largest, the sub-pixel composition proportion is closely related to the scale error, and the change of wind speed has an obvious impact on the scale error. according to the basic rule of scale expansion, the parameter scale expansion formula of the double-layer model is derived, and the zero error flux result can be obtained by using this expansion method no matter what the non-uniformity is. the method of reducing the resolution and aster images is used to test the actual flux scale error. the results show that the error is obvious at the junction of different coverage types, and the rule of this error is difficult to determine due to the discontinuity of the algorithm itself. in addition, the useless information of roads and buildings for flux calculation is one of the reasons for the large pixel scale error. in the monitoring of forest resources and water resources, remote sensing technology overcomes the limitation of single point observation and is the best means of large-scale energy balance and water dynamic monitoring. with the gradual improvement of the quantification degree of remote sensing parameters, it is possible to accurately estimate the evaporation rate of the surface, and further realize the remote sensing quantitative measurement of regional water resources. in the selection of ground model, remote sensing can be directly used sensing data and other auxiliary data-driven models are still the first choice for water volume calculation, while the surface temperature provided by the thermal infrared band is one of the most important sources of evaporation information, and it also includes vegetation leaf index and ground relative humidity (stepenuck & genskow, 2017). in this paper, based on modis remote sensing data, the multi-scale segmentation of remote sensing images is carried out, and the stepwise regression analysis method based on the least square estimation is used to establish the model to monitor the forest resources and water resources in libo scenic area dynamically. in the research, because the model refinement and the lack of parameter information are always a pair of irreconcilable contradictions, a large part of the energy of this paper is to try to solve this problem, and also has some progress, but under the existing research level and daily operation ability of surface parameter remote sensing, some figure 5. water level changes at different times table 3. variation of the water resource area month yuanyang lake /km2 qinglong pool/ km2 dong shan shui bi/km2 zhang jiang /km2 1 -5 -2 -4 -10 2 -8 -4 -6 -15 3 -2 -1 -1 0 4 1 3 1 2 5 2 1 2 4 6 8 5 7 9 7 10 9 11 16 8 12 11 13 15 9 9 8 6 9 10 4 6 4 7 11 2 3 2 1 12 0 0 0 0 64 yan lv, laijun lu, mengxue cao models still have some difficulties, more simple and efficient models and more parameters. the method of data extraction is two aspects that need to be solved. because this paper involves many fields and aspects, the research on the remote sensing quantitative measurement of the dynamic change of water resources in the basin is only preliminary, and the new and effective methods need to be further studied. in addition, combining the flux results of remote sensing calculation with the research in other fields is also a work to be strengthened in the future to serve more application needs; from the perspective of methods and models, future research needs more innovation and breakthrough in thinking, and more full use of various types of remote sensing information, while the effective verification of large-area water resources dynamic change methods and remote sensing simulation results is. in order to promote the development of new models and methods, remote sensing experiments and simulation studies should be strengthened. conclusions in this paper, after the remote sensing image is preprocessed by tm image processing, atmospheric correction, correction of solar altitude angle of reflectivity, data bad value, and noise removal, a dynamic monitoring model of natural resources is established by using the stepwise regression analysis method based on the least square estimation. the dynamic changes of forest resources and water resources in the libo area are tested. as an example, the effectiveness of this method is verified. the results show that this method can realize the dynamic monitoring of natural resources in the libo scenic spot, and the monitoring accuracy is high, which can provide references for the application of related industries. acknowledgment the national social science fund of china (no. 17bmz094). references benacchio, v., piégay, h., buffin-belanger, t., & vaudor, l. 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(2018). a co-occurrence region based bayesian network stepwise remote sensing image retrieval algorithm. earth sciences research journal, 22, 29-35. http://www.wanfangdata.com.cn/details/javascript:void(0) http://www.wanfangdata.com.cn/details/javascript:void(0) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.97735 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 91 106 g eo ph y si c s detection and study of a high magnitude seismic event from gps data: case study of the 2011 tohoku-oki earthquake ángel cibeira urtiaga1, manuel berrocoso1, belén rosado1, antonio pazos2 1. laboratorio de astronomía, geodesia y cartografía, departamento de matemáticas, facultad de ciencias, campus de puerto real, universidad de cádiz, 11510 puerto real, spain 2. san fernando naval observatory, plaza tres marinas s/n, 11110 sanfernando, spain abstract the advent of gps provided a new way of measuring surface displacements due to earthquakes by deploying gps networks within active seismic areas. japan is located in the confluence of several tectonic plates, hence its seismicity. in order to surveille this activity, one of wider gps network in the world was deployed, i.e., geonet. by processing data from 93 geonet reference stations, we analyze the 2011 tohoku-oki earthquake using ppp strategy. we studied the time series during the event setting up a threshold value at we consider the time series are being altered by the earthquake. we also identified the time after the occurrence when the maximum displacements happen. with the study of these two parameters, we aim to show their different behavior as the main shock propagates along the japan islands, with a focus on a better understanding of the earthquake and its propagation. to achieving this, a least square adjustment method was used to relate epicentral distance to topocentric displacements and the time of detection to epicentral distance. the results show an exponential behavior of the distance-displacement regression versus a linear behavior of the distance-time regression. besides, we use the former linear regression to calculate and approximation of the velocity of the shock waves. keywords: seismogeodesy; gps; tohoku-oki; geodesy. detección y estudio de un evento sísmico de gran magnitud a partir de datos gps: caso de estudio del terremoto tohoku-oki de 2011 resumen la aparición del gps proporcionó una nueva forma de medir los desplazamientos en superficie debidos a los terremotos mediante el despliegue de redes de gps en zonas sísmicas activas. japón está situado en la confluencia de varias placas tectónicas, de ahí su sismicidad. para vigilar esta actividad, se desplegó una de las redes de gps más amplias del mundo, i.e., geonet. al procesar los datos de 93 estaciones de referencia de geonet, analizamos el terremoto de tohoku-oki de 2011 utilizando la estrategia ppp. estudiamos las series temporales generadas durante el evento estableciendo un valor umbral en el que consideramos que las series temporales están siendo alteradas por el terremoto. también identificamos el momento posterior al suceso en el que se producen los máximos desplazamientos. con el estudio de estos dos parámetros, pretendemos mostrar su diferente comportamiento a medida que la sacudida principal se propaga a lo largo de las islas de japón, con el objetivo de comprender mejor el terremoto y su propagación. para ello, se utilizó un método de ajuste de mínimos cuadrados para relacionar la distancia epicentral con los desplazamientos topocéntricos y el tiempo de detección con la distancia epicentral. los resultados muestran un comportamiento exponencial de la regresión distancia-desplazamiento frente a un comportamiento lineal de la regresión distancia-tiempo. además, utilizamos la primera regresión lineal para calcular y aproximar la velocidad de las ondas de choque palabras clave: sismogeodesia; gps; terremoto; tohoku-oki. record manuscript received: 20/08/2021 accepted for publication: 19/05/2022 how to cite item: cibeira urtiaga, a., berrocoso, m., rosado, b., & pazos, a. (2022). detection and study of a high magnitude seismic event from gps data: case study of the 2011 tohoku-oki earthquake. earth sciences research journal, 26(2), 91-106 https://doi.org/10.15446/esrj.v26n2.97735 92 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos 1. introduction initially, gnss systems and permanent station networks were used to create time series, from which the velocities of the stations that comprises the geodetic networks were extracted, thus allowing the creation of a deformation model that enables the study of the geodynamics of the area covered (feigl et al., 1993; freymueller et al., 1993). for these kinds of studies, a 30 or even 300 second decimate is sufficient, since the time series are created from several years of observations (members bifrost project, 1996). the implementation of geomatic techniques were also expanded to other fields like hydrology with successful results (arab amiri & gocic, 2021; arab amiri & gocic, 2021; gocic & arab amiri, 2021). the development of gnss receivers allowed large sampling frequencies (1 hz or higher). on the other hand, it also improved the quality of accurate products (bock et al., 2009; altamimi et al., 2007) (i.e. corrections of satellite clock states, orbits, etc...). this improvement in the precise products is materialized into the evolution of the processing techniques, allowing better estimates that are close to the processing in near real time (colosimo et al., 2011). such development allows the use of conventional models for the analysis of gnss data generated by the stations during seismic events (ge, 1999; ge et al., 2000; hirahara et al., 1994; larson, 2009), where large displacements are obtained in short intervals of time, typical circumstance during a great magnitude earthquake. the increasing number of geodetic networks with sampling frequencies of 1 hz or higher, is a very useful tool for the study of seismic events (zheng et al., 2012), complementing the traditional methods (li et al., 2013). they allow, in addition to the study of the cortical deformation in time (bock, 1991), before and after the rupture, to record the displacements experienced by the stations during the event (michel et al., 2017), with increasing accuracy. gnss systems have great advantages over traditional seismic equipment. for instance, they do not get saturated in case of big accelerations and offer a direct measurement of the cosmic displacement, being the seismogeodetic networks a useful tool for the study of seismic events. the area that encompasses the islands of japan is one of the most tectonically active areas, hence the great importance of cortical deformation monitoring. it is a region characterized by the interactions of 4 tectonic plates (yoshioka et al., 1994) (the euro-asian plate, the pacific plate, the philippine sea plate and the okhotsk plate) (bird, 2003), being the central part of japan the most active (figure 1). the tectonics of the region originates a high seismic and volcanic activity. subduction zones are created in the convergence zone of two plates, when the oceanic plate slides under the continental one. in japan, there are several of them, like the convergence of the pacific and euro-asian plates (hashimoto et al., 2004) or in the southern zone, with the euro-asian and philippine sea plates (furukawa, 1999). the landslide that occurs in the subduction zone (brizzi et al., 2018; métois et al., 2012; mazzotti et al., 2000) is the result of a combination of displacements generated by both earthquakes and the displacement of the plates (chlieh et al., 2008). the accumulated movement during the inter-seismic period generates the energy that is later released with a seismic event. the measurement of the cortical deformation during this period is key to understand the recurrence of earthquakes (wallace et al., 2004), especially those of great magnitude. the central area of the main island of japan is affected by the interaction of the plates formerly mentioned that generates subduction zones due to their collides (loveless, j. p., and b. j. meade, 2010). the pacific and euro-asian move opposite each other subducting beneath the kanto region, close to the epicenter of the earthquake. this fact is one of the reasons the high number of figure 1. representation of the limits of the four plates that converge in the zone of japan represented by the red line, the centroids of the earthquakes of magnitude greater than 4 occurred in the year before the earthquake, the volcanoes and the epicenter of the earthquake represented by a star. 93detection and study of a high magnitude seismic event from gps data seismic events (nishimura t., sagiya, t., stein, r. 2007) that have struck the surrounding areas. there are several of these events recorded, like the 1923 kanto earthquake whose rupture stroke the area along the confluence of the euro-asian and philipine plates (shishikura, 2003; ishibashi, 2004). another interaction that affects the central region of japan is the okhotsk plate moving south while collides with euro-asian plate on its way to east (zhao et al., 2018). to the south, the philippine sea plate is subducting beneath the euro-asian plate. besides, the pacific plate subducts beneath the philippine sea plate due to their different velocities. the former interactions imply the central region of japan is surrounded by a complex tectonic activity that generates several subduction areas (katsumata, k., n. wada, and m. kasahara, 2003) and faults, source of the high seismicity in the region (hindle, d., and k. mackey, 2011) and its recurrency (shelly, r. et al., 2006; kano, m. et al., 2018). as a consequence, a great number of high magnitude events were recorded in the area. some examples are the 1993 kushiro-oki earthquake (suzuki and kasahara, 1996; ozel and moriya, 1999) and the 1994 hokkaido-toho-oki earthquake (kikuchi and kanamori, 1995; katsumata et al., 1995) both with a magnitude greater than 7.5. the cortical deformation (fujita et al., 2019; kumar et al., 2002) generated by the tectonic plates in the area reaches values over 50 mm/ year in several points (sagiya et al., 2000; sagiya et al., 2002; simons et al., 2011). the deformation occurs along the different collide zones of the plates, generating several subduction zones (ozawa et al., 2004; kumar et al., 2002). the geodynamic complexity of the zone is materialized in an important seismic activity (ozawa et al., 2005), with events of great magnitude along history (e.g., ozawa et al., 2004; watanabe et al., 2006). the register of past events is a tool that allows to characterize the seismicity of the zone (tajima et al., 2013; ide & aochi, 2013). another tool that has allowed the study of the seismic of the zone is the satellite positioning systems, which allow to measure the surface deformation (li et al., 2020; el-fiky & kato, 2006) giving an idea of the displacement of the plate on which the station is located as well as the numerical simulation of earthquakes (sagiya et al., 2000). on march 11, 2011, a strong earthquake occurred off the northeast coast of honshu island, shaking the islands of japan. the event was originated in the subduction zone located at the confluence of the pacific and north american tectonic plates (i.e., okhotsk plate), which have a relative motion velocity of about 8 cm/year. although the epicenter is located 130 km from japanese territory, its effects on the main island were devastating. this fact make very complicated the measurement of near field offsets (p. k., gautam et al., 2019). this earthquake is one of the largest recorded with seismic instrumentation (m 9.0) and gps. its ground motion effects have been widely studied using seismic equipment (goto, h & morikawa, h., 2012; hirose, f., 2011; ishii, m., 2011; suzuki, w. et al., 2011), since there are also huge seismic networks all over the islands (e.g., k-net). the processing of the gnss data allows to obtain the field of coseismic displacements on the ground, obtaining values without the need of integrating acceleration values coming from seismometers. while traditional methods use seismic data, gnss data or a combination of both for studying seismic events (melbourne, t. et al., 2002; hsu,y. et al., 2006; ohta, y. et al., 2012; larson, k. m., p. bodin, and j. gomberg 2003; ohta, y. et al., 2008), we propose to focus on the combination of temporal and displacement values derived only from gps data to evaluate the propagation of the earthquake instead of the mechanism process (yakota, y., et al., 2009). after constructing the temporal series of each station, we managed to figure out a threshold value to assume the station is by fact being affected by the seismic waves. from there, we analyzed the trend of this time variation versus the epicentral distance, what also let us calculate an approximation of the wave velocity based only on the time series of the gps stations, not on seismic data (stein, s., and e. a. okal 2005). previous aproximations of the velocity of the wave were made only by using a few stations (zheng, y., et al., 2012), but with higher rates of data (5 hz). furthermore, we calculated the coseismic displacements again based only on gps data, with independence of seismic data inversion. 2. data and processing japan has a dense geodetic network in its territory, known as geonet, whose deployment began in 1993 by the geographical survey institute (gsi), motivated by the number of seismic events that were recorded in the area. in 1996, the network had more than 400 stations, growing to more than 1300. this network makes japan one of the countries with the greatest amount of gnssbased information for geodynamic studies in the world. stations maintain a homogeneous pattern, being the typical configuration a monument on which the antenna is installed, a dual-frequency gps receiver and a modem connected to a telephone line for data transmission. the receivers are trimble or topcon brand, and all the stations are equipped with chokering type antennas, model trm29659.00. the average distance between stations is 20 km, with a maximum distance of 40 km (sagiya, 2004), which provides an enormous amount of data. initially the network was configured to work at a decimate of 30 seconds, although technological evolution allowed the gsi authority to increase the sampling frequency of stations to 1 hz. on the other hand, the increase in bandwidth allows observation data to be transmitted in real time to the control center for processing. for this study, data from 93 stations belonging to the geonet network have been used, located in the area where the largest cosmic displacements were recorded (grapenthin & freymueller, 2011) during the 2011 tohoku-oki earthquake (between latitude 37 and 40 degrees north, 136 and 142 degrees east, figure 2). with this work we aim to a better understanding of the earthquake focusing on the study of different parameters. by defining a time of detection and comparing it with the time when the maximum displacement is reached, we were able to see the evolution of the shock wave by comparing the different results obtained by each station. besides, by calculating the displacements and obtaining a least square regression, we manage to model the real effect of the earthquake on each site, with the possibility of giving an approximate value of displacement and time of arrival of the seismic wave. this information could be very valuable to reinforce earthquake early warning systems, when using igs ultra-rapid ephemerides. once the earthquake is detected, according to the methodology, in the closest gps station to the epicenter, a prediction of the displacements and time of arrival could be calculated. 3. analysis the analysis of the time series is based on the study of different parameters related to time and displacement (normal distribution is plotted on figure 5), obtaining also, from the local geodesic coordinates, the 2-d topocentric displacements (i.e., the horizontal displacement). the studied temporal parameters aim to detect the influence that the earthquake exerts on the time series in local geodetic coordinates. therefore, what is sought are the moments in which it is appreciated in an unequivocal way that the variation on the time series generated by each station is due to the earthquake (i.e., the main tremor causes an alteration in the series not induced by the errors of the processing strategy), as well as the time values in which the maximum variation in the time series is registered. the latter, when compared with the distance to the epicenter, allows us to see the behavior of the event as the seismic waves evolve throughout the japanese territory. the displacement parameters characterize the magnitude of the event and are defined from the averages of the time series in the instants previous to the earthquake (from the second 19000). when these averages exceed the threshold value of 3 times the average (value defined for this study), it is considered that the effect of the earthquake is recorded by the station. likewise, to characterize the evolution of the event, the maximum values of the local geodetic coordinates e and n are used. daily (24 hours) rinex files were processed from 93 stations located in the central and northern regions of japan, corresponding to the 70th day of the year 2011 (march 11), with a sampling frequency of 1 hz, covering the entire period of the earthquake (starting at 05:46:23 utc). gipsy-oasis software (lichten & border, 1987; zumberge et al., 1997), version 6.1, is used for processing, using the precise products (héroux et al., 2001) generated by nasa’s jet propulsion laboratory. specifically, the final ephemeris, calculated for the igb08 reference frame, were used, refining the models of the phase centers of the receiver and satellite antennas (schmid et al., 2007). these ephemerides have a latency of 14 days and provide 3-d rms accuracies of 2.5 centimeters in the determination of the position of the satellites of the constellation. the gmf function (boehm et al., 2007) is used to reduce errors related to the tropospheric delay. the oceanic tidal load model used is fes2014b, whose values are obtained from the website http://holt.oso.chalmers.se/loading/. http://holt.oso.chalmers.se/loading/ 94 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos figure 2. disposition of the stations belonging to the geonet network that were used in this study, as well as the epicenter of the tohoku-oki earthquake 95detection and study of a high magnitude seismic event from gps data figure 3. e and n time series corresponding to stations 049, 175, 206, 546, 550, 551, 795, 933, 936, 937 939, 940. the vertical red line marks the beginning of the earthquake. the period represented goes from 500 seconds before the earthquake to 500 seconds after it. stations distributed along the study area are chosen the data are processed in kinematic mode using the ppp strategy (ge et al., 2008; gao & shen, 2002; martín et al., 2012). kinematic processing is chosen to avoid the effects of precursor events occurring in the hours prior to the earthquake. after the processing, the series corresponding to the geocentric cartesian coordinates in the igb08 reference frame are calculated. these series of coordinates are transformed into local geodetic coordinates e, n, u using figure 4. e and n time series corresponding to stations 049, 175, 206, 546, 550, 551, 795, 933, 936, 937 939, 940. the series begin in the instant of the beginning of the earthquake (second 20783 of the day in utc time), and extends an interval of 300 seconds. the image is represented as a zoom to better appreciate the evolution of the earthquake matlab software, using first the function “ecef2geodetic” that converts the ecef (earth centered earth fixed) coordinates of each station into a reference value in geodetic coordinates (latitude, longitude and height). the time series are then generated in local geodetic coordinates using the “ecef2enu” function (figure 3). in addition, matlab will also be used to study the processing results, programming the least-squares regression algorithms in this software. 96 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos different parameters related to time and displacement will be studied on the time series, obtained also, from the local geodesic coordinates, the 2-d topocentric displacements (i.e. the horizontal displacement). the magnitude of the event is represented with horizontal displacements that in several cases reach meters (figure 4), showing its attenuation as the stations move away from the epicenter (e.g., there is a large difference between the magnitude of the displacement between stations 206 and 546, whose distances to the epicenter vary by 74 km). the representation of the series corresponding to the different stations also gives an idea of the arrival time of the main shock. the time differences are quantifiable and are parameter to analyze the evolution of the earthquake along the territory. however, this only reflects the propagation of the wave, without allowing to calculate its characteristics or a detailed observation of the rupture process (ozawa et al., 2011), serving as an initial approximation focused on a quick determination of parameters of interest related to the earthquake, which allows to feed, for example, early warning systems, being, for instance, a verifying element that an earthquake is happening. to obtain rupture models or waveforms from gps data, more information from other measuring equipment (i.e., seismic equipment) is needed (zheng et al., 2012). the positions of the stations represented in figures 3 and 4, as well as the distances (measured in kilometers) to the earthquake epicenter are shown in table 1. table 2 shows the values of the e and n displacements, as well as the displacement modulus. these 12 stations have been chosen as a representation of the study carried out following a criterion of distance to the epicenter. stations with a wide range of distances ranging from a minimum of 87 kilometers to a maximum of 248 kilometers have been taken, the minimum distances used in the study being 87 kilometers and the maximum 654 kilometers. the horizontal displacements are obtained from the modulus of the vector resulting from the composition of the displacement vectors e and n in local geodetic coordinates and centered at every station. the horizontal displacement vectors are the result of the difference between the mean values of the time series between the beginning of the day and the second 20783 utc (instant at which the earthquake occurs) and between the second 21283 utc and the end of the day (or the end of the records, since some stations stop working due to structural or electrical network damage after the main shock). it is during the 500 seconds after the main shock that peak displacement values are recorded (simons et al., 2011). the results show a maximum displacement of 5.02 meters and a minimum value of 0.04 meters in the e coordinate, and a mean value of 1.48 meters and a standard deviation of 0.98 m, while the maximum and minimum displacement values (in absolute value) for the n coordinate are 1.82 meters and 0 meters respectively, with a mean value of 0.58 meters and a standard deviation of 0.50 m. on the other hand, we obtained a maximum 2d topocentric displacement of 5.24 meters and a minimum value of 0.04 meters in the farthest station studied, with a mean value of displacement of 1.62 meters and standard deviation of 1.05 m. table 1. location and distance to the epicenter of stations represented in figures 3 y 4 station latitude longitude distance (km.) 49 139.5096 e 38.2307 n 250.4 175 141.4494 e 38.6827 n 89.65 206 139.3643 e 37.3028 n 287.73 546 141.5755 e 39.1431 n 114.32 550 141.5007 e 38.3012 n 75.98 551 139.8489 e 39.8911 n 279.01 795 141.4526 e 39.8490 n 187.15 933 140.2205 e 38.6284 n 190.54 936 140.4468 e 37.8335 n 177.15 937 139.9727 e 37.6693 n 222.60 939 139.8355 e 37.4613 n 242.45 940 140.4640 e 37.4449 n 193.80 table 2. e and n coordinates displacement, additionally module of the topocentric 2-d displacement of the stations represented in figures 3 and 4. station e (m.) n (m.) displacement (m.) 49 0.95 -0.17 0.97 175 4..4 -1.54 4.66 206 0.70 -0.01 0.70 546 2.90 -1.68 3.35 550 5.02 -1.51 5.24 551 0.55 -0.33 0.64 795 1.01 0.89 1.35 933 1.50 -0.44 1.56 936 1.84 -0.19 1.85 937 1.21 -0.09 1.21 939 1.03 -0.05 1.03 940 1.62 -0.08 1.62 table 3. maximum, minimum, mean and typical deviation of the results of the temporal series e and n and 2-d displacement, according to d=√(e2+n2 ) of the geonet stations used in the study fixed to a normal distribution. values are represented in meters. maximum value minimum value mean typical deviation e coordinate 5.02 0.04 1.48 0.98 n coordinate 1.82 0.00 0.58 0.50 2-d displacement 5.24 0.04 1.62 1.05 figure 5. normal distribution with frequency histograms of the 2-d displacement values, local geodesic coordinate e and n, of the stations comprised in geonet used in the research. the resulting vectors are used to generate the field of horizontal seismic velocities. in the areas closest to the epicenter, values up to 5 meters of horizontal displacement are obtained, showing the magnitude of the earthquake. the effects of the mainshock were recorded in gnss equipments in regions far away from the epicenter (wang et al., 2011; feng & jónsson, 2012). the co-seismic displacements are an earthquake-specific parameter (melgar et al., 2013), and decrease as the stations move away from the earthquake epicenter. the decreasing can be studied as a function of time, measured in seconds from the event, as well as distance (zhan et al., 2013). the attenuation of the earthquake effects is due to the decrease in the amplitude of the surface waves generated by the earthquake. in this way it is possible to quantify the evolution of the event along the japanese territory. 97detection and study of a high magnitude seismic event from gps data figure 6. horizontal velocity field caused by the tohoku-oki earthquake, obtained from e and n displacements in local geodetic coordinates. figure 7. co-seismic displacement plotted versus distance to epicenter. data fitted to a least-squares exponential regression: y=aebx. the results obtained are a= 5,8112 and b =-0.3278. 98 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos the surface waves generated at the epicenter of the earthquake dissipate as they travel through the study area. this behavior causes the displacements recorded at the stations (figure 6) to decrease as the distance from the epicenter increases. plotting the 2d displacement versus distance from the epicenter, data show an exponential decay trend (figure 7). since the measurements made have the usual errors associated with satellite positioning techniques (e.g., errors due to atmospheric refraction), a least square fit (deng et al., 2019) is used, using an exponential regression. the goal is to find a curve that minimizes the discrepancy between the real data and the approximation, in order to obtain a model of the attenuation of the co-seismic displacement as a function of the epicentral distance. on the other hand, the time value at which the station detects the event is calculated. due to the precision of the ppp strategy (szołucha et al., 2018), it is necessary to define a value from which it is considered that the station is detecting a displacement, so that the errors intrinsic to the positioning strategy do not cause the appearance of false positives. for this purpose, the average of the positions of the e, n, u coordinates is calculated from the second 19000 within the day in utc time, up to the second 20783, instant in which the earthquake occurs. it is assumed that for detection to occur, the values of the local geodetic coordinates must be greater than 3 times the calculated mean, thus ensuring that all positions are already being affected by the displacement resulting from the event. unlike the exponential behavior shown by the 2d seismic displacement versus distance to the earthquake epicenter, the representation of the results obtained show a linear trend. analogously to the previous case, a model that fits the observed data with the smallest possible error is sought. therefore, a least squared regression is performed to adjust the results to a straight line, obtaining the results shown in figure 8. based on the results obtained in the previous processing, it can be observed that the trend of the seismic displacement, caused by the surface waves on their way through the main island, is an exponential decay. however, the time it takes for the station to start recording the effects of the displacement from the beginning of the event, with the rule that the disturbance recorded in the time series of the e and n coordinates is three times higher than the average of the values prior to the beginning of the event, shows a linear character. figure 8. representation of the epicentral distance of each station versus detection time in seconds from the onset of the earthquake together with the line obtained in the least squares regression. the model fits a straight line according to: y = a x + b, obtaining values of a=2.7179 and b= -33.8587. in addition to the co-seismic displacement, another determining factor is the maximum displacement generated by the earthquake. this value allows us to appreciate the magnitude of the surface waves in the area studied and given that the surface waves that generate the seismic displacement are the same, it is to be expected that the behavior of the maximum displacement will be similar in its evolution as the distance to the epicenter increases. to check the results obtained, the evolution of this magnitude is studied as the distance to the epicenter increases, obtained from the series of e and n coordinates of the network stations. in order to obtain a better characterization of the event and its evolution, a new time parameter is defined. once selected the instant in which the station is sensitive to the earthquake, the time value is calculated for all station as well as its maximum displacement values. again, since these are the same waves, as the distance to the epicenter increases, a similar behavior is expected. the time value in which each station records the maximum displacement is calculated, measured from the moment in which it has been defined that the earthquake is detected (i.e., 3 times the mean value of the local geodetic coordinates) until the instant in which the maximum displacement value is reached. figure 8. representation of the epicentral distance of each station versus detection time in seconds from the onset of the earthquake together with the line obtained in the least squares regression. the model fits a straight line according to: y = a x + b, obtaining values of a=2.7179 and b= -33.8587. following the previous methodology, it is again observed that the displacement undergoes a decay as the station moves away from the earthquake epicenter, following a decreasing exponential pattern similar to when the co-seismic displacement was studied. in the same way, the evolution of the calculated time from detection to the maximum value shows an increase with the epicentral distance with a linear behavior as shown in figure 9, although this trend is less marked than in the case of the time in which the event is recorded at the station. the least squared regression is calculated for the case of the maximum 2d displacement, fitting to an exponential, whilst time values are fitted to a straight (figure 9). 99detection and study of a high magnitude seismic event from gps data 4. discussion in this research a methodology has been designed for the tohokuoki earthquake using only observations from gps stations of the geonet network. the ppp strategy is used as it is considered the most appropriate, since the magnitude of the earthquake makes the use of other strategies, such as double differences, implies using reference stations located at a great distance from the geodetic network. in addition, given the magnitude of the earthquake and the displacements obtained, the ppp strategy offers enough precision to carry out the research. due to the precisions of the ppp strategy in kinematic mode, it is necessary to establish a minimum value, so that the station can be suffering the effects of surface waves generated by an earthquake. for this reason, a value 3 times higher than the mean of the records obtained from the second 19000 to the second 20783 is used as a threshold. as the effects of the earthquake are detected on the series of each station, the evolution of the surface waves can be seen. this wave is responsible for the horizontal displacement in each station, getting lower as the epicentral distance is bigger (figures 10 and 11). when comparing it to the distance to the epicenter, a decay in the magnitude of the displacement is obtained, a logical behavior due to the attenuation the wave undergoes as it moves away from the source. the former is independent from the fact that the ground effect can cause increases in the displacement. figure 9. the upper part represents the epicentral distance of each station versus the maximum 2d displacement together with the exponential function obtained in the least squares regression, whose values are: a = 5,9248 and b = 0.2795. the lower part represents the epicentral distance of each station versus the detection time in seconds of the maximum 2d displacement since the detection of the earthquake together with the line obtained in the least squares regression, whose values are a = 2.8433 and b = 201.8993. figure 10. interpolation map representing coseismic displacement as a result of the processing of the data. 2-d coseismic displacement is represented in the left picture. north coseismic displacement is represented in top right picture and east coseismic displacement is represented in bottom right picture. 100 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos results provide, in addition to the evolution of the magnitude of the displacement, a view of the detection times (figure 13), establishing a valid threshold for both the detection and the maximum of the shock. this allows us to see the evolution of the surface waves as they travel through the japanese main island, as well as the variation in time of the effects produced at each station. the applied methodology does not offer a prediction of the event, since there is no continuous processing of the observations, but it does give an approximation of the evolution of the amplitude of the earthquake. in addition, the use of final ephemeris for the calculation of the positioning solution prevents the use of the results for early warning. however, the methodology could be implemented (e.g., in the case of using predicted ultra-fast ephemeris, generated by the igs) as a verification element that an earthquake is indeed occurring, or in the use of new techniques such as vadase (branzanti et al., 2012), an algorithm that can be embedded into the gps receptor firmware that does not require ambiguity solving. the algorithm is able to estimate in near real time the velocities and displacements in a global reference frame (itrf), using highrate (1 hz or more) carrier phase observations and broadcast products (orbits, clocks) collected by a stand-alone gnss receiver, achieving a displacements accuracy within 1–2 cm over intervals up to a few minutes (fratarcangeli, f., et al., 2018). the use of three times the mean value of the results obtained during figure 11. interpolation map representing the maximum displacement recorded by the gps stations as a result of the processing of the data. 2-d maximum displacement is represented in the left picture. north maximum displacement is represented in top right picture and east maximum displacement is represented in bottom right picture the previous minutes to certificate that the station is suffering the effects of surface waves, is enough to absorb the errors generated in the processing. another parameter studied is the maximum displacement recorded by each station and the time associated with this value. when plotting the data versus the epicentral distance, an exponential decreasing pattern is again observed, also obtaining a model based on a function of the type =aℯbx, representing the attenuation suffered by the earthquake (lin & wu, 2010). the attenuation resulting from this study is comparable to the values obtained using other methodologies (xu et al., 2019), and the results are useful for comparison with prediction models (gautam et al., 2019). the displacements obtained in the e and n coordinates show an eastward displacement, consistent with the focal mechanism associated with this event (lay, 2018). despite this evidence, the results obtained do not allow us to recreate the model of plate slip originating in the subduction zone, as other techniques (aris et al., 2016; linuma et al., 2012) that rely on gps measurements, mainly due to the distance separating the epicenter from the nearest stations and the fact that data are only available on one side of the rupture zone. the results obtained in this research are aligned with those obtained using non-geodetic methods. in (feng et al., 2012), insar measurements are used to calculate an interferogram of the coseismic deformation, in addition to using figure 12. representation of the earthquake detection times according to the methodology and the time when the maximum displacement is reached. both values are plotted versus the distance separating the station from the epicenter. a greater dispersion is evident as the epicentral distance increases. 101detection and study of a high magnitude seismic event from gps data gps measurements of the geonet network stations for the correction of the orbital errors of the earth observation satellites by comparing results. as an example, station 550 obtains a displacement value of 5.03 meters eastward in the e coordinate, consistent with the 5.02 for that same station and coordinate using our methodology. both, the event detection times according to the methodology, as well as the maximum 2d displacement values, show a linear behavior when plotted versus the epicentral distance. from the propagation point of view, the displacement velocity of the surface waves can be obtained from the straights calculated in the least squared regression. the epicentral distance is measured in kilometers, and the time in seconds, therefore, the slope of both lines will represent the velocity measured in km/second. the values are very similar to each other being 2.7179 km/s when the sensitivity of the station is set to three times the average value of the coordinates corresponding to the 1783 seconds before the earthquake. in the case of the maximum values, the velocity obtained is 2.8433 km/s, a value lower than that obtained in the implemented methodology. this variation points to a change in the propagation velocity of the event, whose reasons may be related to changes in the subsurface produced by ruptures derived from the event (kim et al., 2012), or they may be due to the variation of the elastic properties of the lithosphere that movements induced by the earthquake can generate (vidale & li, 2003; peng & ben-zion, 2006), which would justify the greater dispersion of the time values with increasing distance from the epicenter as shown in figure 12. the values previously obtained approximate the s-wave velocity, based only on the use of the gps station displacements for its calculation. on the other hand, only the epicentral distance has been considered when making the calculations, without considering the azimuth to the epicenter. this fact, added to the local characteristics of each soil typology (morikawa & fujiwara, 2013), affects the values obtained, so they are approximations that represent the average propagation velocity of the rupture, and similar to those calculated with other methodologies (zhang et al., 2012), such as those based on the analysis of the distortions recorded in the ionosphere (astafyeva et al., 2013). 5. conclusions this paper presents a methodology to study the 2011 tohoku-oki earthquake based only on geonet network gps stations observations. ppp strategy has been used for the processing of the geodetic network data, using the gipsy-oasis software in its version 6.1. the methodology followed is based on the calculation of the time series of the local geodetic coordinates of 93 stations located in the central area of japan. from these series, different parameters are calculated. first, the 2d co-seismic displacement is calculated, representing the displacement vectors in figure 6. these displacement values are plotted versus the epicentral distance, showing a decreasing behavior when moving away from the epicenter, fixing it to an exponential model by a least square regression. the function obtained is used as a law of attenuation of the event. next, the moment at which stations detect the earthquake is calculated, when the e and n series deviate more than three times the mean calculated for the moments before the earthquake occurs. the value of 3 times the mean is used to avoid the limits in the precision of the chosen strategy for the process (i.e., ppp or precise point positioning) generate false positives. the time values are plotted versus the epicentral distance showing a linear behavior, with the detection time increasing with the epicentral distance. what motivates the movement of the stations is the surface wave derived from the earthquake or s-wave, therefore the time records allow to calculate the propagation velocity of the wave, calculated as the slope of the straight line obtained after linear least square regression. then the values of maximum displacement are calculated, and the time from the beginning of the earthquake they are reached. the same procedure is followed as with the co-seismic displacement and the instant of detection. an exponential behavior is obtained when representing each value versus the epicentral distance, in the case of the maximum of the e and n coordinates, and a linear behavior in the case of the time it takes to reach these maxima. therefore, with the methodology used it is concluded that the displacements derived from the earthquake decrease with the distance to the epicenter according to an exponential relationship, while the time it takes for the effects of the event to be registered by the stations increases with the distance to the epicenter in a linear trend. figure 13. interpolation map representing the detection time of the event using the methodology implemented in this paper (left picture) and the time when the maximum displacement occurs. 102 ángel cibeira urtiaga, manuel berrocoso, belén rosado, antonio pazos time parameters are also calculated setting two main cases: time detection of the event and moment of the maximum displacement. results for the first case are obtained after applying the methodology implemented in this paper. for the maximum displacement time series are analyzed looking for the peak both in e and n coordinate. finally, it should be noted that the results obtained can be used as a first approximation for the study of an earthquake, especially of a sufficiently large magnitude so that the displacements generated are much larger than the errors generated by the processing strategy. we found that the ppp accuracy limited us in the way of detecting the event. the presence of values within the e and n geodetic time series higher enough for being considered bigger than the threshold value, but lower for being considered outliers, difficult the detection of the event. with a view to improving the methodology, it is hoped that the introduction of a weighting of the results, when calculating the least square regressions, based on the direction of propagation, will lead to an improvement that will make it possible to refine the attenuation law obtained. we propose the implementation of the methodology in a less dense net. geonet is a very rare exception, since there is no other geodetic net like it. to validate the methodology greater distances between stations would be needed, as well as a fewer number of stations. on the other hand, we used final ephemerides for the processing of the rinex files. in order to use this methodology in support of earthquake early warning systems (hoshiba, m. et al., 2011), the use of igs ultra-rapid predicted ephemerides is proposed, as well as the automatic access to gps data for 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(1997). precise point positioning for the efficient and robust analysis of gps data from large networks. journal of geophysical research: solid earth, 102(b3), 5005–5017. https://doi.org/https://doi.org/10.1029/ 96jb03860 https://doi.org/10.1029/2009gl037799 https://doi.org/10.1029/2009gl037799 https://doi.org/https://doi.org/10.1016/0040-1951(94)90028-0 https://doi.org/10.1201/b15791-74 https://doi.org/10.1007/s11589-012-0839-1 https://doi.org/10.1007/s11589-012-0839-1 https://doi.org/10.1029/2018jb015800 https://doi.org/10.1111/j.1365-246x.2012.05576.x https://doi.org/10.1111/j.1365-246x.2012.05576.x https://doi.org/https://doi.org/10.1029/96jb03860 https://doi.org/https://doi.org/10.1029/96jb03860 rock burst is one of main engineering geological problems greatly threatening the safety of construction. prediction of rock burst is always an important issue concerning the safety of workers and equipments in tunnels. in this paper, a novel pnn-based rock burst prediction model is proposed to determine whether rock burst will happen in the underground rock projects and how much the intensity of rock burst is. the probabilistic neural network (pnn) is developed based on bayesian criteria of multivariate pattern classification. because pnn has the advantages of low training complexity, high stability, quick convergence, and simple construction, it can be well applied in the prediction of rock burst. some main control factors, such as rocks’ maximum tangential stress , rocks’ uniaxial compressive strength , rocks’ uniaxial tensile strength and elastic energy index of rock are chosen as the characteristic vector of pnn. pnn model is obtained through training data sets of rock burst samples which come from underground rock project in domestic and abroad. other samples are tested with the model. the testing results agree with the practical records. at the same time, two realworld applications are used to verify the proposed method. the results of prediction are same as the results of existing methods, just same as what happened in the scene, which verifies the effectiveness and applicability of our proposed work. abstract keywords: probabilistic neural network (pnn); rock burst; prediction pnn-based rock burst prediction model and its applications modelo de predicción de fractura de rocas basado en una red neuronal probabilística y sus aplicaciones issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n3.65216 el fracturamiento o explosión de rocas es uno de los principales problemas en ingeniería geológica que amenaza significativamente la seguridad de una construcción. la predicción del fracturamiento de rocas es importante para la seguridad de los trabajadores y el equipamiento en túneles. en este artículo se propone un nuevo modelo de predicción de fracturamiento de rocas basado en una red neuronal probabilística (pnn por sus siglas en inglés) para determinar la posible ocurrencia e intensidad de uno de estos eventos en proyectos subterráneos. la pnn se desarrolló con base en un criterio bayesiano para la clasificación multivariada de patrones. debido a que la pnn tiene las ventajas de una menor complejidad de adiestramiento, estabilidad, rápida convergencia y simplicidad en su construcción, se puede adecuar en la predicción del fracturamiento de rocas. algunos factores principales de control, como la fuerza máxima tangencial de rocas, la resistencia de compresión uniaxial, la fuerza de tensión uniaxial, y el índice de energía elástica de las rocas fueron escogidos como los vectores característicos de la pnn. el modelo se obtuvo a través del adiestramiento de datos sobre fracturamiento de rocas en proyectos subterráneos en diferentes localidades. otras datos también se analizaron con el modelo. los resultados de la evaluación se ajustan a los registros observados. simultáneamente, se utilizaron dos aplicaciones prácticas para verificar el método propuesto. los resultados de la predicción son similares a los de métodos existentes, un factor que además se presentó en las pruebas de campo, lo que demuestra la efectividad y la aplicabilidad de la metodología propuesta. resumen palabras clave: red neuronal probabilística; fracturamiento de rocas; predicción. record manuscript received: 24/05/2017 accepted for publication: 23/08/2017 how to cite item zhou, y., & wang, t. (2017). pnn-based rock burst prediction model and its applications. earth sciences research journal, 21(3). 141 146. doi: http://dx.doi.org/10.15446/esrj.v21n3.65216 earth sciences research journal earth sci. res. j. vol. 21, no. 3 (september, 2017): 141 146 yu zhou*, tingling wang school of electric power, north china university of water resources and electric power, zhengzhou 450045, china zhouyumobile@163.com g e o l o g ic a l e n g in e e r in g 142 yu zhou, tingling wang 1. introduction a rock burst is a sudden and violent expulsion of rock from the surrounding rock mass. rock burst is considered a dynamic instability phenomenon of surrounding rock mass of un-derground space in high geostatic stress and caused by the violent release of strain energy stored in the rock mass. rock burst occurs during excavating underground space in the form of a stripe of rock slices or rock fall or throwing of rock fragments, sometimes accompanied by crack sound. rock bursts are related to the fracture of rock in place and require two conditions for their occurrence: stress in the rock mass sufficiently high to exceed its strength, and physical characteristics of the rock which enable it to store energy up to the threshold value for sudden rupture. rocks which yield gradually in plastic strain under load usually do not generate rock bursts. the likelihood of rock bursts occurring increases as the depth of the mine increases. rock bursts are also affected by the size of excavation, becoming more likely if the excavation size is around 180m and above. induced seismicity such as faulty methods of mining can trigger rock bursts. other causes of rock bursts are the presence of faults, dikes, or joints (dong et al., 2013). because rock burst occurs suddenly and intensely, it usually causes injury including death to workers, damage to equipment, and even substantial disruption and economic loss of under-ground space excavation. therefore, there is a need for the development of suitable computa-tional methods for the prediction and control of rock bursts particularly for a safe and economic underground excavation for construction or mining in the burst-prone ground. for this case, numerous related research works, concerning about the mechanism, characteristics or type, the cause of formation, the critical conditions and preventive methods of rock burst have been con-ducted by many researchers. many researchers have suggested various theories, many predic-tion methods, and empirical correlation, such as fuzzy-base evaluation method (wang et al., 1998; amoussou et al., 2013), distance discriminant analysis (wang et al., 2009; wang et al., 2017), support vector machine (svm) (zhao, 2005; zhou et al., 2012, extension-theory-based method (xiong et al., 2007), rough-set-based method (yang, 2010), unascertained measurement method(shi et al., 2010), numerical simulation (zhen & gao, 2017; zhu et al., 2010) and case study (mansurov, 2001). these studies offered new ideas and approaches for rock burst prediction. however, each method discussed above has its advantages and disadvantages, and understanding, predicting and controlling the rock bursts still pose a considerable challenge for underground engineering. as an important means, the ann-based method for prediction of rock burst has been adopted by many researchers gradually in recent years (bai et al., 2002; zhang et al., 2012). ar-tificial neural network technique is considered the most effective and reliable artificial intelli-gence methods for solving classification, prediction and recognition problems. currently, back propagation (bp) and radial basis function (rbf) networks are used in the field of prediction of robust classification. however, for network training, they are all easily trapped in local minimum values. probabilistic neural network (pnn), on the other hand, is a feedforward neural network. it is derived from the bayesian network and a statistical algorithm called kernel fisher discriminant analysis. it was introduced by specht and donald(1990). be-cause pnn has the advantages of low training complexity, high stability, quick convergence, and simple construction, it has a wide range of application in model classification, identification, prediction, as well as fault diagnosis and other fields(adeli and panakkat, 2009; song et al., 2007; ataa et al., 2017; rutkowski, 2004 ). in this work, according to the practice of complicat-ed problems of the rock burst prediction, the pnn is applied to predicting rock burst classifica-tion. 2. material and methods 2.1. criteria and indexes of rock burst and rock burst classification 2.1.1. criteria considering stress in surrounding rock the criteria listed in table 1 were proposed early, and only considered the stress level in surrounding rock. furthermore, different scholars chose different parameters as an evaluation index of criterion for rock burst, and the classification of rock burst intensity also differed from each other. so it is difficult to use these criteria in the construction of underground engineering. table 1. criteria only considering stress in surrounding rock. 2.1.2. comprehension criteria considering stress, properties of surrounding rock and energy 1) the following criterion is presented with rock burst tendency index and energy condition of surrounding rock (dong et al, 2013). (1) (2) (3) (4) where wqx is the rock burst tendency index; σ1 and σ2 are the major and middle principal stress in surrounding rock, respectively; μ is the poisson ratio. 2) it is stipulated that rock burst could occur if σө / σc ≤ ks in which the value of ks related to σt / σc criterion. 3) kidybinski (1981) proposed an elastic energy index wet. no rock burst activity, moderate rock burst activity and strong rock burst activity, meet the conditions wet <2.0, 2.0≤ wet ≤5.0, and wet >5.0, respectively. 2.1.3 input characteristic vector for pnn the indexes of criterion should reflect the main factors of rock burst the properties and stress of surrounding rock. at the same time, they should be obtained easily and can be compared with each other for different cases. in this work, the compressive rock strength σc, tensile strength σt, elastic energy 143pnn-based rock burst prediction model and its applications index wet and the maximum tangential stress σө are chosen as the indexes of criterion. compressive rock strength σc, tensile strength σt, and elastic energy index wet can indicate the properties of surrounding rock, and the tangential stress σө can reflect the virgin geostatic stress condition and the influence of the shape and dimension of the underground space on rock burst. in this work, σө, σc, σt and wet are selected as the input index for pnn model to predict the degree of rock burst activity. hence, the input characteristic vector for pnn is [σө, σc, σt, wet]. 2.1.4 classification for intensities of rock burst according to the extent and intensity of the characteristics of the rock burst phenomenon in the underground openings, the grade of rock burst is divided into four degrees, namely none rock burst, light rock burst, moderate rock burst, strong rock burst, respectively. so the pnn model output is rock burst degree, output = [none rock burst, light rock burst, moderate rock burst, strong rock burst]. also, the division of rock burst degree can be described in table 2 (wang et al., 1998; zhou et al., 2012; zhang et al., 2004) table 2. standard of classification for intensities of rock burst. number of category өq. additionally, σ is the smoothing parameter. figure 1 depicts a schematic diagram of the multi-class classification pnn. x is a vector to be classified as the neurons in the input layer. it is passed to the corresponding neurons in the hidden layer with no change. the hidden layer then transmits each neuron in the accumulated layers. at this point, the output obtained by the accumulation layer is the estimation of the probability density function of each pattern for the test vectors. accordingly, the category of the occurrence of the maximum probability of the current test vector is the one that corresponds to the largest probability density function. this function is the output of the accumulation layer. the neuron output with the probability density maximum is 1; the corresponding category is the one to which h belongs. 2.2. pnn-based rock burst prediction model 2.2.1 outline of pnn probability neural networks are a tool for handling uncertainty for improving learning per-formance. probability uncertainty and fuzziness uncertainty processing play a key role in boost-ing classification systems including extreme learning machines and decision trees (wang et al., 2015; lu et al., 2015). pnn is essentially a classifier that places the bayes estimate in a feed-forward neural network. the central concept of bayes criterion classification is the minimal ‘‘predictable risk’’ of the bayes decision. the bayes decision is based on the non-parametric estimation of the probability density function; accordingly, it obtains the classification results. based on its advantages, such as rapid training time, a stable and simple neural structure, and good convergence, it is suitable for use in defect recognition. for a multi-class problem with σ1, σ2, , σq, σs, we apply to the above issues two types of classification problems in bayes decision classification. for p-dimensional vector x = {x1, x2, , xp} based on the bayes decision rule, we determine the status of өϵөq with its measurement set, (5) in the equation (5), hq represents the priori probability of ө = өq, d(x) as the bayes decision of test vector x , hk is the priori probability of ө = өk, and lq and lk are incorrectly classified into other categories of losses. the latter should belong to өq and өk. besides, fq (x) and fq (x) are probability density function of өq and өk, respectively. (6) where x is a sample of the input vector to be classified, p is the dimension of sample vectors, xqj is the j-th sample of the category өq, and mq is the sample figure 1. schematic diagram of the multi-class classification pnn. 2.2.2 pnn modeling for prediction of rock burst the number of input layer neurons of pnn is the same as the dimensionality of the input characteristic vector. based on discussed above, σө, σc, σt, and wet are selected as the input characteristic vector for pnn model and the input vector of pnn = [σө, σc, σt, wet]. hence, the number of input layer neurons of pnn is 4. the number of output neurons of pnn is the same as the number of classification of rock burst activity. according to the section 2.4, the grade of rock burst is divided into four degrees, and the pnn model output is rock burst degree and the output vector = [none rock burst, light rock burst, moderate rock burst, strong rock burst]. so the number of output layer neurons is 4. the number of hidden layer neurons is determined by the training data set. the number of hidden layer neurons is equal to the sum of the number of each category of training sample the number of accumulation layer neurons is the same as the number of classification of rock burst activity. here, the number of accumulation layer neurons is 4. the design of pnn modeling for prediction of rock burst includes the following aspects: a collection of data sets, data preprocessing, build a pnn, network model, training pnn, testing pnn etc. the design process is shown in figure 2. 144 yu zhou, tingling wang figure 2. the design process of pnn modeling for prediction of rock burst. 3. results rock burst samples which come from underground rock projects in domestic and abroad are collected as training data set (show in table 3) and testing data set (show in table 4) to verify the rationality of our posed method. the relationship among the indexes of criteria, the occurrence of rock burst and its intensity is very complicated. for the sake of the capability of pnn for pattern recognition, we attempt to predict the rock burst activity by using pnn. four degrees of rock burst activity, including none rock burst, light rock burst, moderate rock burst and strong rock burst are indicated by 1, 2, 3 and 4, respectively. pnn model and criterion are obtained through training data sets of rock burst samples which come from underground rock projects in domestic and abroad. the training effect and the training error are shown in figure 3. it is noted from figure 3 that the misjudgment ratios of training samples using pnn model is 0, which prove that the pnn has a good learning performance. figure 4 is the perdition results of testing samples. from the figure 4, we can find that the prediction accuracy of pnn model is 100%. the results show that the prediction results agree well with the practical records, which prove the pnn-based rock burst model is useful and available and can be applied to the prediction for the possibility and classification of rock burst in underground engineering. table 3. collected samples of rock burst. table 4. testing samples. figure 3. the training of pnn. 145pnn-based rock burst prediction model and its applications figure 4. the perdition results of testing samples. the results of the pnn-based method are compared with that svmbased method, bp-based method, and lvq-based method. the calculated results of pnn, svm, bp and lvq are listed in table 3. from table 3, we can find that the misjudgment ratios of tested samples using svm, bp, lvq, and pnn are 10%, 20%, 20% and 0, respectively. the compared predicted results show that it is feasible and appropriate to use pnn model for rock burst prediction. table 5. comparison of calculation results by different methods. to study the effectiveness and feasibility in engineering practice applications, two real-world examples are analyzed by using our posed pnnbased rock burst prediction method. case 1: tongyu tunnel engineering tongyu tunnel is currently one of the most deep-lying and longest tunnels in chongqing, china. its geological conditions are incredibly complex. the measured data of rock burst in depth 900m at a cross section of k21+680 of tongyu tunnel are listed in table 6 (he et al., 2008). applying our proposed prediction model to rock burst prediction of this engineering, the result of prediction is light rock burst activity. the results agree well with the practical records. table 6. prediction of rock burst in depth of 900 m at the cross section of k21+680 of tangyu tunnel. case 2: qinnling tunnel engineering the qingling tunnel is the longest railway tunnel in china and takes the third place in the world at present. reference (thaldiri et al., 2017; li and wang, 2009) provides rock burst measured data for of qingling tunnel engineering. some measured data listed in table 7. table 7 compared the performance of the proposed method with existing bp-based method (bai et al., 2002) and unascertained measurement method(shi et al., 2010). from the table 7, the result of prediction is same as the results of existing bpbased method and unascertained measurement method, just same as what happened in the scene. this case further confirms that the pnn-based method is effective and practical in the application of prediction of rock burst. on the other hand, pnn-based method has the advantages of low training complexity, high stability, quick convergence, and simple construction. table 7. a case of qinling tunnel is analyzed by using our proposed method. 4. discussion in this paper, a novel pnn-based rock burst prediction model is proposed to determine whether rock burst will happen in the underground rock projects and how much the intensity of rock burst is. pnn model is obtained through training data sets of rock burst samples which come from underground rock project in domestic and abroad. other samples are tested with the model. the testing results agree with the practical records. at the same time, two real-world applications are used to verify the proposed method. the results of prediction are same as the results of existing methods, just same as what happened in the scene, which verifies the effectiveness and applicability of our proposed work. 5. conclusions because pnn has the advantages of low training complexity, high stability, quick convergence, and simple construction, it can be well applied in the prediction of rock burst. in this work, a pnn-based prediction model of rock burst is presented. according to the mechanism of rock burst, rocks’ maximum tangential stress σө, rocks’ uniaxial compressive strength σc, rocks’ uniaxial tensile strength σt and elastic energy index wet are defined as the criterion indices for rock burst prediction in the proposed pnn-model. some collected rock burst samples which come from underground rock projects in domestic and abroad and two real-world engineering in china are used to verify the new model. the prediction results demonstrated that the developed pnn-based prediction model is effective and efficient approach to predict rock burst potential grade. acknowledgements the authors gratefully acknowledge the project supported by the national natural science foundation of china under grant no. u1504622, the project supported by the scientific research foundation of henan province for returned chinese scholars, china. the authors are grateful for the anonymous reviewers who made constructive comments. references adeli, h. & panakkat, a. 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(2017). geological tourist route planning of henan province based on geological relics zoning. geology, ecology, and landscapes, 1, 66-69. palabras clave: gas de lutita; exploración y desarrollo; problemas; contramedidas. how to cite item chen, x., liu, z., liang, x., zhang, z., zhang, t., zhu, h., & lang, j. (2018). the shale gas revolution in china—problems and countermeasures. earth sciences research journal, 22(3), 215-221. doi: https://doi.org/10.15446/esrj.v22n3.74390 shale gas development throughout the world has resulted in a revolution in the field of global energy and has also become an important topic in china in recent years. while organic-rich shale is widely distributed in china and the initial commercialization of shale gas has been achieved, the research, exploration, and development of shale gas remain at an early stage. problems exist with crucial technologies, innovation, institutional mechanisms, environmental protection, and other aspects of the industry. the shale gas exploration and development industry in china can learn from the experiences of other countries and strengthen its position in the market, with the support of new government policy. given its unique geological conditions, china should speed up the introduction of technical innovation and establish its unique systems and methods for shale gas exploration and development. abstract keywords: shale gas; exploration and development; problems; countermeasures. the shale gas revolution in china—problems and countermeasures la revolución del gas de lutita en china-problemas y contramedidas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.74390 el desarrollo del gas de lutita en el mundo ha significado una revolución en el campo de la energía; en china, durante los útlimos años se ha convertido en un tema importante. las lutitas, ricas organicámente, están ampliamente distribuidas en china y la comercilización inicial de gas de lutita ha sido lograda; sin embargo, la investigación, exploración y desarrollo de este producto permanence en etapas prematuras. los problemas se presentan en las tecnologías clave, la innovación, los mecanismos institucionales, la protección ambiental y otros aspectos de la industria. la exploración del gas de lutita y el desarrollo industrial en china se pueden mejorar a partir de las experiencias de otros países y con su fortalecimiento en el mercado, acompañado del apoyo de políticas gubernamentales. gracias a sus condiciones geológicas únicas, china puede acelerar la introducción de innovación técnica y establecer sus sistemas y métodos únicos para la exploración y desarrollo del gas de lutita. resumen record manuscript received: 14/11/2017 accepted for publication: 26/03/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 215-221 xiaohui chen1, zhicheng liu2,*, xing liang3, zhao zhang3, tingshan zhang1,2, haihua zhu1, jun lang1 1school of geoscience and technology, southwest petroleum university, chengdu 610500, china 2sichuan institute of land planning and surveying, chengdu 610045, china 3petrochina, zhejiang oil field company, hangzhou, 310013, china *email of the corresponding author: lzc_15@126.com pe t r o l e u m g e o l o g y 216 xiaohui chen, zhicheng liu, xing liang, zhao zhang, tingshan zhang, haihua zhu & jun lang introduction the successful commercial exploration for shale gas in the united states led to an energy revolution, which ended america’s dependence on imported energy and made the us an energy-rich country. this situation brought the shale gas industry under the spotlight throughout the entire world. while china is also aiming to develop its shale gas reserves, it faces challenges with technology and marketing. the international community generally acknowledged that shale gas exploitation revolutionized the global energy landscape, which not only increased the output of natural gas but also enormously influenced the global gas market, global energy supply structure, global climate change policy and even geopolitics. in recent years, shale gas exploration and development has become an important topic in china. organic-rich shales under the right conditions for gas accumulation are widely distributed in china, including marine shales in southern china, northern china, and the tarim basin of xinjiang, and lacustrine shales in north of china, the junggar basin, turpan-hami basin, ordos basin, bohai bay basin, and songliao basin, which represent a considerable resource potential (eia, 2010;zou., et al,2016) it is predicted that the recoverable resources of shale gas are about 22 × 1012 m3 (ministry of land and resources of people's republic of china, 2016), which exceeds the conventional natural gas resources of china. at present, although commercial production has been rapidly achieved in three districts of the sichuan basin, china is still in the early stages of evaluation, exploration, and development of shale gas and key technologies are still not well integrated. large-scale commercialization is challenging due to the commonly complex geological structure and the high cost of drilling and fracturing services. to overcome the bottlenecks in technique, marketing and industrialization that are restricting shale gas exploration and development in china would be necessary the following actions. 1. increasing basic theoretical research. 2. developing shale gas theory by introducing innovative thinking and methodology. 3. improving exploration and development technology as well as relevant environmental technology; 4. establishing a sophysticated marketing system with better relations between industry and local government. 2. the status of shale gas exploration in china the united states has greatly benefited from the development of shale gas and other unconventional petroleum resources. in 2010, the united states had shale gas production of more than 1379 × 108 m3, accounting for about 23% of the national output, and has now surpassed russia to become the world’s largest gas producer (eia, 2010). the united states shale gas revolution has reshaped the global liquefied natural gas market, and its influence will become even more remarkable over time. the enormous success of shale gas development and exploration has attracted considerable attention from governments and energy companies all over the world and set off a boom in exploration for shale gas. it has categorically changed the global energy picture. with the rapid development of the national economy in china, the demand for energy, and exceptionally clean energy continues to increase. according to the ministry of land and resources (mlr) of china, the country’s gas shortage will exceed 1350 × 108 m3 by 2020. therefore, active, systematic exploitation of china’s unconventional gas resources is a significant way to meet the country’s future energy demands. the exploration and development of global non-renewable energy resources show that shale gas is one of the realistic replacements for conventional petroleum. the twelfth five-year plan for the national economy and social development in china stated explicitly “to promote the exploitation and utilization of shale gas and other unconventional petroleum.” on march 16th, 2012, the national energy administration issued the “shale gas development plan (2011–2015)” in which the government requested emphasis on the following strategies: 1. researching into shale reservoir geology and key accumulation factors for shale gas; 2. establishment of the evaluation methods and criteria for shale gas target optimization, based on sedimentary facies, tectonic evolution, burial depth, organic matter content (toc), thermal maturity and gas content. it aims to increase the annual shale gas production rate from a current level of 13 × 108 m3 up to 300 × 108 m3 per year by 2020. china has achieved substantial progress regarding the exploitation of shale gas in recent times. petrochina identified weiyuan, changning, zhaotong, and fushun–yongchuan as the four most favorable districts for shale gas exploration in south sichuan and north yunnan, and more than 20 wells were put into production (figure 1). sinopec (china national offshore oil corporation) has completed several appraisal wells in eastern guizhou, anhui, northeast sichuan and the chongqing fuling area, and the jiaoye 1hf well in the fuling district produced a high-yield commercial shale gas flow of approximately 20.3 × 104 m3 per day. cnooc (china national offshore oil corporation) has carried out pilot work for the exploration of shale gas in the anhui and zhejiang areas, and yanchang petroleum has discovered lacustrine shale gas at yanan in the shanxi province. at present, shale gas reserves in china are estimated to be nearly 500 billion m3 with an annual production rate of approximately 1.5 billion m3. according to the mlr, up until the end of july 2015, cnpc, sinopec, and yanchang petroleum had cumulatively produced 680 million m3 of shale gas. by the end of 2015, the cumulative production reached 6.5 billion m3 (dong et al., 2016). this progress shows that china has great potential when it comes to shale gas exploration. however, although shale gas exploration and research has made some advances, the exploration and development of shale gas in china still has much further potential. compared with the united states, the commercialization of shale gas in china has been challenging, due to the more complex geology, the presence of older strata, more complex tectonism, more complicated thermal histories, and higher levels of thermal maturity. considerable difficulties in specific fields need to be overcome if future shale gas exploration is to be successful, including understanding accumulation mechanisms, laboratory testing, evaluation and forecasting of sweet spots, productivity analysis and prediction, hydraulic fracturing technology, geological steering, and reservoir stimulation. in recent years, china has intensified its efforts into shale gas exploration and development, and some examples of its industry’s promotion are as follows: figure 1. the status of shale gas exploration in china (after dong et al., 2016). 217the shale gas revolution in china—problems and countermeasures (1) investment in researching and development has been increased, and technical systems were formed and improved with the support of the national energy administration, the research institute of petroleum exploration & development of petrochina set up a national energy shale gas r&d center to address the critical technical problems related to shale gas exploration. a specific sub-project for research into the key technologies of shale gas exploration and development has been set up from the national key science and technology special project “large oil–gas fields and coalbed methane development.” also, major oil corporations, as well as local governments, such as sichuan and chongqing, have set up research institutions and laboratories to research the evaluation of gas reservoir potential and the selection of advantageous exploration areas. 2) a diversified investment and multi-type enterprise participation pattern has emerged the state council of china has approved shale gas as being the 172nd independent mineral in china, which means that the oil and gas franchise will no longer restrict shale gas exploration and development, and any companies who have the financial ability and qualifications for gas exploration can bid for it. in the field of natural gas prospecting, only petrochina, sinopec, cnooc and yanchang petroleum had chinese government approval and were eligible for gas prospecting in the past. on september 10th 2012, in the second round of public bidding for shale gas exploration in china, the ministry of land and resources demanded that those companies qualified to explore for oil and gas or gas minerals, had registered capital of more than 300 million rmb, no matter their domestic enterprise or independent legal entity (ministry of land and resources of the people's republic of china, 2012). many private enterprises who had planned on investing in shale gas in the past had been blocked previously because of the requirement for registered capital. on the 30th october 2013, the national energy board issued the “shale gas industry policy”, which removed the registered capital limitation. it only made requests for investment capability, moderate financial status and the ability to be responsible for civil liability, which created new opportunities for private enterprises who were willing to invest in the shale gas industry. the ministry of land and resources has held two open tenders for exploration permits, and private enterprises were welcomed. in the second round of public bidding, through state-owned enterprises won most of the bids for the exploration rights, about one-third of the original 83 bids were from private enterprises, and they won 2 blocks out of the 19 up for bid (ministry of land and resources of the people's republic of china, 2013). this results marked a significant breakthrough in the bidding mechanism for oil and gas exploration rights in china. although foreign firms are not allowed to participate in oil and gas exploration and development directly, the chinese government encourages them to take part in joint ventures. it shows a desire by the chinese government and local enterprises in obtaining specialized foreign technologies and international development experience. 3) a policy system for industrial shale gas development has been formed the central government is a strong supporter of the development of shale gas and lists the shale gas industry as an industry of growing national strategic importance (national energy of the people's republic of china, 2013). to encourage enterprises to accelerate the commercialization of shale gas, the ministry of land and resources, the national development and reform commission, the ministry of finance, the ministry of commerce, the ministry of science and technology, the environmental protection bureau, the central ministries and commissions and other national ministries have launched a series of detailed policy measures for promoting shale gas exploration and development, encouraging foreign investment, increasing financial subsidies, guiding industry development, constructing a demonstration zone, and supporting science and technology research (china geological survey, 2014). at the central government level, the development and reform commission is responsible for formulating general policy and supervises the price of natural gas. the national energy administration is competent for setting up shale gas production goals. the ministry of land and resources is accountable for shale gas mineral exploration and development rights as well as for organizing and implementing the shale gas tendering process for exploration blocks. the ministry of finance is responsible for shale gas exploration subsidies. the science and technology department is qualified for funding support for shale gas related technology research and development. the environment protection bureau is responsible for monitoring and preventing environmental pollution. the ministry of commerce is responsible for examining chinese–international cooperative projects. the state administration of work safety is responsible for monitoring work safety at sites of production. the policies of central government related to shale gas development are mainly regarding to the setting of production goals, calculation of government subsidies, administration of taxes and fees, handling of market access, mineral classification of shale gas, and publishing of the shale gas industry standard (table 1). annual shale gas production in china in 2017 reached 91 × 108 m3 by 2017. the chinese government planning target is for 30 × 108 m3 by 2020 and requests petrochina and sinopec to take a series of measures to ensure they achieve these goals on time. the central government used to subsidize shale gas development enterprises, paying 0.4 rmb per m3 of gas produced. these subsidies expired in 2015, but the ministry of finance has issued a new subsidy policy and will now pay 0.3 rmb per m3 of gas produced from 2016–2018. the subsidy will drop to 0.2 rmb per m3 of gas produced from 2019–2020. the ministry of land and resources lists shale gas as the 172nd independent mineral. the decision has far-reaching significance and gives other private enterprises an opportunity to invest in shale gas development. in august 2014, the sichuan provincial government classified the shale gas industry as being one of the top five highly advanced sectors of the future. the current shale gas policy of china is similar to that of the united states, during the early development stage of shale gas in that country. china has set up procedures for government subsidy, arranged financial support for technology research and development, removed shale gas price control, opened a natural gas pipeline network system, and made other advancements. however, shale gas development in china is still dominated by large stateowned enterprises, and in a situation which differs from the united states, united kingdom, and other western countries, as these entities are not required to release geological information or production data. 3. problems affecting the development of china’s shale gas industry although china’s shale gas industry has achieved initial success, there are still some problems relating to the areas of critical technology, innovation, environment, institutional mechanisms, environmental protection and other aspects which are discussed below. 3.1 a technical system for exploration and development adapted to china’s geological characteristics has not been established although some achievements relating to the exploration and development of shale gas have been made in china, a systematic investigation and evaluation of the national shale gas resources have not yet been completed. the theory of shale gas reservoir geology still needs to be studied in more detail. significantly, more work is required for the study of the accumulation of shale gas, field of sweet spot optimization methods, evaluation of productivity, dynamic mechanisms, characterization of gas flow through micro–nanoscale pore systems, complex multi-field seepage coupling theory, as well as other aspects of this science. 218 xiaohui chen, zhicheng liu, xing liang, zhao zhang, tingshan zhang, haihua zhu & jun lang the rapid development of the united states shale gas industry occurred mainly due to breakthroughs in large-scale horizontal drilling and massive multi-stage hydraulic stimulation, which significantly reduced the cost of exploiting shale gas, and made resources economically viable upon they were initially worthless. china’s service companies are just beginning to acquire these two key technologies, which are partly still only mastered by foreign enterprises. the experience of shale gas development in the united states shows that different mining technology is required in different regions, as reservoir conditions change from place to place. as a latecomer to the shale gas development industry, the number of drilled wells is relatively limited in china and the technical knowledge for drilling and producing needs more time to develop. similarly, equipment and industry standards are still in the process of being developed and implemented. due to this underdeveloped technical environment, only three state-owned oil companies have currently fully mastered the latest technology. new enterprises that are starting to enter the field of shale gas development are finding technical services to be deficient. also, china’s shale gas reservoirs are different from those in the united states. shale gas is currently commercially produced in the sichuan basin from marine shales. the producing zone is the lower silurian longmaxi formation, which is older than shales in the united states. it has had a long geological history and has been transformed into an “over mature, table 1. the policies of the central government to promote shale gas development timing policy name agency main content valid time 2011 outline of the 12th five-year plan for economic and social development. sc. promote shale gas development and utilization of multiple clean energy development planning. 2011–2015 2011 catalog of industry adjustment guidance. ndrc list of shale gas exploration and development projects to be encouraged. long-term 2011 chengdu–chongqing economic zone regional planning. ndrc speed up sichuan shale gas exploration and development long-term 2011, 2015 catalog for the guidance of foreign investment. ndrc. moc list of shale gas exploration and development projects (joint ventures) to be encouraged. long-term 2012 the 12th five-year national strategic emerging industry development planning. sc. class shale gas industry as a critical developing direction and primary tasks. 2012–2015 2012 shale gas development plan (2011–2015). ndrc. mof. molr. nea. clear objectives, tasks, and measures of the shale gas industry. 2012–2015 2012 “12th five-year” energy conservation and environmental protection industry development plan. sc. improve the level of overall utilization of shale gas in critical areas. 2012–2015 2012 energy conservation and emissions reduction “twelfth five-year” plan. sc. promote shale gas development and utilization as the primary task of adjusting and optimizing the energy structure. 2012–2015 2012 notice about strengthening shale gas resources, prospecting, and exploitation, supervision, and administration of relevant work. molr the management of shale gas exploration, mining and the administration of exemptions from mineral resources compensation fees and mineral rights use charges. 2012–2017 2012 opinions about further encouragement and a guide to the private capital investment in the field of land and resources. molr. acfic encourage and guide private capital to participate in shale gas resource prospecting and exploitation. long-term 2012, 2015 notice about shale gas development and utilization of subsidy policy. mof. shale gas definitions and subsidies standards. 2012–2020 2012 advice on encouraging and guiding private capital to further expand the implementation of energy sector investment. nea. list of shale gas exploration developments within the scope of private capital investment. long-term 2012 enforcement advice on utilizing price level to encourage and guide the development of private investment. ndrc. shale gas production price should be determined by mutual consent so that private capital is encouraged into the shale gas field of production. long-term 2013 the shale gas industry policy. nea. shale gas development should be financially supported so that the development of shale gas exploration may be increased. long-term 2013 the announcement about oil and gas enterprises, development of cbm and shale gas vat related issues. sat. provision of a vat tax rate for shale gas production of 17% that is applicable for the development of oil and gas. long-term 2014 the strategy of energy development action plan (2014–2020). sc. aim to enhance shale gas production up to 300 billion m3 by 2020. 2014–2020 2014 open regulatory measures for oil and gas pipeline network facilities (trial). nea promote the oil and gas pipe network facilities. improve the utilization efficiency of pipe network facilities. regulate the behavior of oil and gas pipe network facilities and relevant open markets. 2014–2019 2015 the announcement about the urban land used tax policy for oil and gas production enterprises. mof. sat. oil and gas production construction. suspend leaving urban land use tax. long-term ndrc: national development and reform commission; mof: the ministry of finance. nea: the national energy administration. moc: the ministry of commerce. sat: state administration of taxation. acfic: the national federation of industry and commerce. molr: the ministry of land and resources. sc: the state council. 219the shale gas revolution in china—problems and countermeasures highly reworked” shale. the present burial depth of the shale in the sichuan basin ranges from 2000–4000 m, which is more profound than that in the united states, which is mainly at depths of 800–2600 m furthermore, the mountainous topography of the sichuan shale gas areas poses challenges for ground engineering. although commercialization of shale gas at burial depths of around 2000 m is possible, the drilling, completing and fracturing technologies for depths of 3000–4000 m have not yet been developed. 3.2 the cost barrier for shale gas industrialization based on the american experience, the central controlling factors for the development of the shale gas industry are resource abundance, technology, government policy, and input-output relationships. the rapid expansion of shale gas in the united states benefited from national policy support, an integrated technical system and a well-developed infrastructure (pipe-network); conditions which helped lower cost. the cost of shale gas development remains high in china. immature technology, an imperfect industry chain (which has resulted in a shortage of equipment), and an undeveloped pipe-network have stalled progress within the shale gas industry of china. shale gas development is still at an early stage, and low rates of production are making increases in scale challenging to achieve. compared with other energy sources, shale gas has no advantage when it comes to price. although the government has issued a subsidy policy of 0.4 rmb/m3 for shale gas, it takes a long time to commercialize shale gas projects fully, and most companies have still not yet qualified for the government subsidy. most shale gas districts in china are located in mountainous areas where transportation is difficult, and this further increases the cost of exploration and development. 3.3 a non-competitive environment for industrial development the united states shale gas industry benefited not only from improvements in technology and innovation but also from its mature market system. in contrast, china's shale gas industry still faces problems with the market. a dynamic, competitive market has still not been established. large state-owned enterprises have dominated china's petroleum industry for a long time, and it is mostly a closed market. only the three major state-owned oil companies have experience in exploration and development. moreover, shale gas blocks overlap with the traditional oil and gas blocks and, as a result, the three major oil companies possess most of the shale gas blocks. it has meant a monopoly when it comes to exploration rights. besides, local governments in places such as chongqing, guizhou and sichuan province have begun to set up energy investment companies for shale gas exploration, and this will form local monopolies. as an essential support facility for the development of the shale gas industry, china’s oil and gas pipeline network is also a monopoly and its distribution is not suitable for shale gas transportation because the current shale gas production wells are mostly in remote areas, over which the existing pipeline network does not extend. a new regional pipeline network needs to be built. also, the absence of sharing of shale gas data and technology between these companies has greatly hindered the progress of the industry and technology. the technical services for shale gas are still in their infancy. although some new specialized service companies for shale gas have started up in recent years, the majority of them lack vital technologies and experienced personnel. many of the new shale gas exploration and development enterprises have little experience in the oil and natural gas exploration and development. in the second round of bidding for china’s shale gas blocks in 2012, for example, four of the companies that won bids had only been established three months earlier. as such, the lack of experienced technical and management personnel makes shale gas exploration difficult. development of the shale gas industry in the united states showed that while many companies may begin shale gas exploration, only the fittest survive, which inevitably leads to companies entering and leaving the industry. with the introduction of relevant policies in china, a competitive environment for the shale gas industry will emerge. shale gas exploration and development needs a lot of capital investment in the early stages and has an extended investment recovery period, which means that profits are difficult to realize in the short-term. therefore, financial obstacles are an essential issue for some small and medium enterprises. the shale gas industry is a highinvestment, high-risk sector and development of china’s shale gas industry will be affected without a good investment and financial basis. 3.4 environmental problems in the development of shale gas due to water resource limitations, and even though china possesses the world's largest shale gas reserves, massive exploitation without suitable water protection will be disastrous. according to the united states environmental protection agency (epa), the average water consumption of a single shale gas well in 2010 was 0.76 × 104 to 2.39 × 104 t (depending on the depth, horizontal section length and fracturing scale), of which 20%–85% remained underground (epa, 2012). i the united states, the average water requirements for drilling each well is 200–2500 m3, and the water demand for hydraulic fracturing each well is as high as 7000–23000 m3 (vengosh, 2014). united states counterparts have noted that water protection and conservation is a huge issue for the industry. in the united states shale gas drilling industry, about 70% of the water used comes from hydraulic fracturing water recovery. although china's total water resources are 2.8124 × 1012 m3, accounting for almost 6.7% of total global freshwater resources, the actual amount of water available is only 0.8 × 1012 m3 (wang, 2010). in total water resources, china ranks sixth in the world, after brazil (6.95 × 1012 m3), russia (6.54 ×1012 m3), united states (3.056 × 1012 m3), canada (2.9114 × 1012 m3), and indonesia (2.8113 × 1012 m3), according to wang (2011). however, according to the ranking released by the world water development report (2012) on water resources in 180 countries and regions, china ranked 128th in the world, having only 2260 m3 of water per person/per year, which is just one quarter of the world's average, and only half of that in japan, one quarter of that in the united states, 1/12 of that in russia, and 1/58 of that in canada. according to data from the united states epa, for each horizontal shale gas well, the water used for fracturing accounts for 85–98% of the total water consumption for the well. shale gas wells in different plays have different water consumptions. for example, in the marcellus shale gas field, the average water consumption for a well is about 1.5 × 104 m3, in the barnett shale the average water consumption is about 1 × 104 m3, in the fayetteville shale the average water consumption is about 1.2 × 104 m3, and in the haynesville shale it is about 1.4 × 104 m3 (rahm, 2011). compared with the united states, china’s shale is generally more deeply buried, especially in the changning–weiyuan district where it is usually found at a depth of 2600–3000 m. the reservoir thickness is 20–800 m, which also means that the demand for water for hydraulic fracturing will be more significant. at present, there are no official figures for water volumes used during the drilling of shale gas wells in china, but according to investigations made by researchers, the drilling and fracturing of each shale gas well will consume approximately 25000 m3 of water on average in china, which is far higher than the average value (15000 m3) in the united states (lu, et al., 2014). the data from the weiyuan shale gas district support these findings. water consumption in the wei201-h1 well for a 10-level hydraulic fracturing job reached 22000 m3, and the total water consumption for the entire well was approximately 25000 m3. although water is relatively abundant in the sichuan province (a vital shale gas production region), its distribution is uneven, resulting in regional and seasonal water shortages. the water resources of the sichuan province are principally located in the ganzi, aba and liangshan prefectures, accounting for 50% of the total water 220 xiaohui chen, zhicheng liu, xing liang, zhao zhang, tingshan zhang, haihua zhu & jun lang resource. per capita, the supply there reached 16993 m3 in 2015. however, within the sichuan basin, the most water-poor area of the entire province, the per capita water resource is only 1624 m3 (sichuan provincial water resource department, 2015). agricultural water consumption accounts for more than 60% of the total water use in the sichuan province, and there has been a water shortage there for a long time. several factors contribute to the agricultural water shortage in the sichuan province: (1) the geographical distribution of the water resource and rainfall is uneven, which leads to severe water shortages within the sichuan basin, and water abundance in the mountain plateau areas of western sichuan. (2) historical issues and inadequate investment in the whole province, but especially the basin and hilly regions, has resulted in a shortage of irrigation systems, making water deployment difficult. the efficiency of water use is low. (3) the utilization rate of water for agriculture is low. due to a lack of water management, water wastage is common. in most areas of the province, canal water utilization efficiency is less than 50%, and generally over half of the water cannot be used. (4) the agricultural water resource is commonly used for other activities, due to its lower value when used in the agriculture industry. local governments regularly use part of agrarian water for other industrial activities. the national demonstration shale gas areas of weiyuan, changning, and huangjinba are located in the weiyuan, changning, gong, and xingwen counties, and all suffer long-term, or seasonal, water shortages. for example, weiyuan has had an annual precipitation of around 1000 mm for several years. total accumulated precipitation is 15 × 108 m3, the gross water resource is 7.4227 × 108 m3, and the per capita water resource is just 994 m3, resulting in severe water shortages. agricultural water consumption is more than 1 × 108 m3, accounting for 51% of the total water consumption. the standard water resource of the changning, gong, and xingwen counties is around 10 × 108 m3, the per capita water resource is just 2521 m3, the total agricultural water consumption is less than 3 × 108 m3, and there is a severe water shortage. the national demonstration shale gas areas of the south sichuan basin are all in the main rice cultivation areas, with a rice planting area of approximately 680 km2. the rice planting areas of changning, gong, juanlian, and xingwen counties are around 451.72 km2 in size and about 228.06 km2 in weiyuan. fu et al. (2014) calculated that 1177 m3 of water per year were needed to produce 1000 kg of rice. there are more than 160 horizontal shale gas wells drilled in the sichuan, yibin and weiyuan regions. assuming that the water consumption of each well is 2.5 × 104 m3, the total water consumption during 2014–2015 would have been at least 4 × 106 m3, which is equivalent to the irrigation water used for 26.7 km2 of paddy fields for one year, and therefore accounts for 4% of the five counties’ annual water consumption. taking the total number of wells that will be put into production (about 353) for these three shale plays, it is possible to calculate that the annual water consumption will reach 8.8 × 106 m3, which would be 9% of the five counties’ annual irrigation water consumption. according to the plans made by the local government of the sichuan province, 1000 wells will be added, every year, over the next few years. as a result, the water consumption of shale gas development will increase by 2.5 × 107 m3 every year over the near future, which is equivalent to the irrigation water consumption of an additional 166.7 km2 of rice growing area every year. therefore, in line with the exponential increase in the number of shale gas wells, there will be an exponential increase in water consumption. the rice growing industry, which demands a great deal of water, will suffer a severe water shortage. this conflict between agriculture and the shale gas industry, and who is going to use the dwindling water supply, will become more severe over the next few years. the fracturing fluid, including the fracturing water and a small amount of formation water from the shale, will gradually flow back into the ground after the massive multi-stage hydraulic stimulations of the shale gas wells. shale gas well fracturing flowback fluid (mainly fracturing fluid) differs from conventional fracturing fluid, in that it contains abundant chemical additives used in the hydraulic stimulation process, and poses a considerable risk for the environment if not carefully handled. how to deal with this massive flowback drainage is critical for the successful development of the shale gas industry in china. about 30%–70% of the high-pressure injection fluid flows back to the ground after the hydraulic stimulation of a shale gas well in the united states (united states environmental protection agency, 2012). measures taken to process these flowback fluids include 1. reinjecting them underground using re-injection wells; 2. reusing them for hydraulic stimulations after simple treatment; 3. treating them into fresh water. the abundant chemical additives make the processing of flowback fluid difficult and costly. at present, treatment equipment or factories for the processing of flowback fluid have not been set up in sichuan shale gas production areas. all of the flowback fluid is transported by truck and injected back underground using re-injection wells to reduce the cost. this process prevents the reuse of flowback fluid and increases the overall water consumption of the shale gas developments. it is essential to establish processing facilities for flowback water, so that the water may be reused. currently, the united states has taken measures to encourage the purification and recycling of flowback water. for example, the reused flowback fluid ratio of the marcellus shale gas region has increased from less than 10% in 2008 to more than 70% in 2011, which has greatly reduced the water consumption (rahm et al., 2013) nevertheless, the new york environmental protection ministry recently announced the prohibition of hydraulic fracturing operations in new york state, after a seven-year investigation. it considers hydraulic fracturing to be a significant risk to land, air and water resources, as well as posing a potential public health risk, which may lead to a negative perception that is impossible to remedy. 4. countermeasures for shale gas industrialization in china shale gas industrialization in china should follow the successful model of the united states, taking into account the differences that exist in china. 4.1 improvements in exploration and development technology advanced technology is the key to shale gas development in china. with the rapid economic growth of our country and the additional demands for energy, energy structure adjustment, energy security, energy savings and other factors, it is necessary for shale gas industrialization of our country to speed up. the shale gas development plan (2011–2015) specified the following goals to be achieved by 2015: 1. completion of an assessment of the potential and distribution of shale gas resources; 2. prioritization of prospective areas and sweet spots, establishing several shale gas development zones and achieving commercial production; 3. conducting international cooperation with regards to shale gas technology and increasing research investment. shale gas exploration represents the basis for the rapid development of the shale gas industry in china. with the support of the national policy for the shale gas industry, future work should focus on the geological research of the “over mature, high reworked” shale and accurately assess the potential and distribution of shale gas in china, especially in the sichuan, chongqing, guizhou, hunan, hubei, yunnan, jiangxi, anhui, jiangsu, shanxi, henan, liaoning and xinjiang provinces, and to establish new shale gas development zones. 221the shale gas revolution in china—problems and countermeasures acknowledgments this work was funded by the young scholars development fund of southwest petroleum university (swpu) (201499010046), china postdoctoral science foundation (2015m580797), national natural science foundation of china(41772150) and national science and technology major project (2017zx05063002-009). references china geological survey (2014). china shale gas survey report. http://www. cgs.gov.cn/upload/201506/20150610/20150624093321310.pdf dong, d., wang, y., li, x., zou, c., guan, q., ... qiu, z. (2016). breakthrough and prospect of shale gas exploration and development in china. natural gas industry b., 3(1), 12-26. eia us energy information administration (2010). natural gas gross withdrawals and production. https://www.eia.gov/dnav/ng/ng_prod_ sum_dc_nus_mmcf_m.htm fu, n., liu, x. g., zhang, y., & yang, q. l. (2014). variation pattern of rice irrigation water requirement in southwest of china. chinese journal of ecology, 33(7), 1895–1901. lu, j., gao, w., & liu, x. (2014). environmental influence in shale gas development and considerations. natural gas & oil, 1, 35-43. ministry of land and resources of the people’s republic of china (2012). shale gas prospecting right tender notice. http://www.mlr.gov.cn/ zwgk/zytz/201209/t20120910_1139187.htm ministry of land and resources of the people’s republic of china (2013). the second round of shale gas exploration tender results. http://www.mlr. gov.cn/xwdt/jrxw/201301/t20130122_1177283.htm ministry of land and resources of the people’s republic of china (2016). national oil and gas resources evaluation. http://www.sinooilgas.com/ gzdt/gzdt/bc4de9c456bf3f420156c5b7c9d7001a.shtml national energy of the people’s republic of china (2013). shale gas industry policy. http://www.gov.cn/gongbao/content/2013/content_2547157. htm rahm, b. g., bates, j. t., bertoia, l. r., galford, a. e., yoxtheimer, d. a., & riha, s. j. (2013). wastewater management and marcellus shale gas development: trends, drivers, and planning implications. journal of environmental management, 120, 105-113. rahm, d. (2011). regulating hydraulic fracturing in shale gas plays: the case of texas. energy policy, 39(5), 2974-2981. sichuan provincial water resource department (2015). water resources bulletin. http://www.scwater.gov.cn/main/wqhg/ggl41/80279/index. html u.s. environmental protection agency (2012). study of the potential impacts of hydraulic fracturing on drinking water resources. https://www.epa. gov/hfstudy/study-potential-impacts-hydraulic-fracturing-drinkingwater-resources-progress-report-0 vengosh, a., jackson, r. b., & warner, n. (2014). a critical review of the risks to water resources from unconventional shale gas development and hydraulic fracturing in the united states. environmental science & technology, 48(15), 8334-8348. wang, z. f. (2011). present situation and water resources utilization sustainable utilization countermeasure in china. journal of xi’an university of posts and telecommunications, 16, 122-127. wang, z. q.(2010). studies on the water resource pollution, prevention, and control industry of our nation. china university of geosciences (beijing). zou, c. n., dong, d. z.., wang,y. m., et al., 2015. shale gas in china: characteristics, challenges and prospects (i). petroleum exploration and development, 42(6), 753-767. 4.2 reinforcing the market and reducing the input costs the exploration for shale gas in our country is at an early stage. investment cannot yet reach the well-developed levels seen in the united states, at least in the short term. however, once china’s service companies acquire technology for large-scale horizontal drilling and massive multi-stage hydraulic stimulation, the process of shale gas industrialization will accelerate, and the costs will reduce. this process will result in enormous demand for specialized equipment from domestic suppliers, providing the equipment manufacturers with opportunities for rapid development. gradual improvements in the structure of the industry will stimulate shale gas industrialization in china, and a more competitive industry model will develop. 4.3 forming a greener outlook for the shale gas industry by making water resource protection a core issue and placing equal emphasis on resource development and ecological protection the development of shale gas will not only bring about the challenge of protecting water resources but will also raise environmental issues such as land disturbance, greenhouse gas emissions and the potential leakage of underground water. therefore, during the revolution in shale gas, the implementation of policies relating to water resource protection and environmental pollution prevention should proceed simultaneously. due to the nature of the contaminants involved in shale gas development, research into pollution reduction and treatment should be encouraged and developed. shale gas in china is mainly distributed in mountainous and hilly areas, such as the sichuan and guizhou provinces, which experience water shortages and have transportation issues. hence, launching the development of, and exploration for shale gas in these areas may bring about adverse impacts on local vegetation, aquatic biology, municipal water and the usage of industrial and agricultural water. large-scale river water fracturing only applies to experimental development at an early stage. water shortage and environmental pollution will be a significant issue for mass commercial production of shale gas. it is, therefore, necessary to utilize the combined technologies of horizontal drilling and hydraulic stimulation, to help reduce water consumption, during the development of shale gas. the installation of water treatment facilities will be indispensable for reusing flowback fluid, thereby improving water use efficiency, as well as for groundwater protection. the use of oil-based muds for drilling should be reduced, and the use of water-based muds should be encouraged. drill cuttings should be collected and disposed of properly, which will help to reduce land contamination. noise control in populated areas and functional relationships between corporations and communities are also necessary for the long-term future of the shale gas industry. optimization methods for the drilling process include the use of “cluster wells,” “multi-well pads,” “gas terminal standardization designs” and the rehabilitation of agricultural land after construction. optimization of the site selection process will avoid damaging ecological and environmentally sensitive areas and will reduce tree felling and vegetation destruction. plans to restore vegetation and rejuvenate the land should be designed before construction and executed adequately after construction. use of the methods mentioned above will ensure that shale gas development, and environmental protection progress hand in hand. conclusions shale gas exploration and development in china should learn from the experiences of other countries and cultivate an active market, with the support of government policy. a geological theory applicable to the unique geological conditions found in china is indispensable for the future of shale gas exploration and should be established as soon as possible. shale gas development technology should be improved to protect the water and land resources, as well as prevent pollution. geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study berkant konakoglua* and alper akarb atechnical sciences vocational school, amasya university, amasya, turkey bvocational school, erzincan binali yıldırım university, erzincan, turkey *corresponding author: berkantkonakoglu@amasya.edu.tr keywords: generalized regression neural network (grnn); radial basis function neural network (rbfnn); multiple linear regression (mlr); interpolation methods; geoid determination palabras clave: red neuronal de regresión generalizada; red neuronal de base radial; regresión lineal múltiple; métodos de interpolación; determinación geoide. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.91195 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 371-382 g eo ph ys ic s record manuscript received: 27/10/2020 accepted for publication: 12/06/2021 abstract this study evaluated different methods for geoid undulation prediction and included two types of artificial neural networks (anns) the radial basis function neural network (rbfnn) and the generalized regression neural network (grnn) as well as conventional methods including multiple linear regression (mlr) and ten different interpolation techniques. in this work, k-fold cross-validation was used to evaluate the model and its behavior on the independent dataset. with this validation method, each of a k number of groups has the chance to be divided into training and testing data. the performances of the methods were evaluated in terms of the root mean square error (rmse), mean absolute error (mae), nash–sutcliffe efficiency coefficient (nse), correlation coefficient (r2), and using graphical indicators. the evaluation of the performance of the datasets obtained using cross-validation was performed in two ways. when the method having the minimum error result was accepted as the most appropriate method, the natural neighbor (nn) gave better results than the other methods (rmse = 0.142 m, mae = 0.097 m, nse = 0.98986, and r2 = 0.99011). on the other hand, it was observed that on average, the grnn exhibited the best performance (rmse = 0.185 m, mae = 0.137 m, nse = 0.98229, and r2 = 0.98249). predicción de ondulación geoide con redes neuronales artificiales (base radial y regresión generalizada), regresión lineal múltiple y métodos de interpolación: estudio comparativo resumen este estudio evalúa los métodos diferentes de predicción de ondulación geoide donde se incluye dos tipos de redes neuronales artificiales -la red neuronal de base radial y la red neuronal de regresión generalizadaal igual que métodos convencionales donde se incluyen la regresión lineal múltiple y diez técnicas diferentes de interpolación. en este trabajo, la validación cruzada de k iteraciones se usó para evaluar el modelo y su comportamiento en un conjunto de datos independiente. con este método de evaluación, cada grupo de números k tiene la posibilidad de dividirse entre datos de entrenamiento y datos de evaluación. el desempeño de los métodos se evaluó en términos de la raíz del error cuadrático medio (rmse, del inglés root mean square error), el error absoluto medio (mae, mean absolute error), el coeficiente de eficiencia nash-sutcliffe (nse, nash–sutcliffe efficiency coefficient) y el coeficiente de correlación (r2, correlation coefficient) a través de indicadores gráficos. la evaluación del desempeño de los grupos de datos obtenidos con la validación cruzada se realizó en dos vías. cuando el método que tiene el resultado de mínimo error es aceptado como el método más apropiado, el vecino natural ofrece mejores resultados que otros métodos (rmse = 0.142 m, mae = 0.097 m, nse = 0.98986, and r2 = 0.99011). por otro lado, se observó que en promedio la red neuronal de regresión generalizada presentó un mejor desempeño (rmse = 0.185 m, mae = 0.137 m, nse = 0.98229, and r2 = 0.98249). how to cite item: konakoglu, b., & akar, a. (2021). geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study. earth sciences research journal, 25(4), 371-382. https://doi.org/10.15446/esrj. v25n4.91195 mailto:berkantkonakoglu@amasya.edu.tr https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.91195 https://doi.org/10.15446/esrj.v25n4.91195 372 berkant konakoglu, alper akar introduction the use of global navigation satellite systems (gnss) in engineering and scientific studies has increased considerably along with recent developing technology. with this technology, accurate cartesian (x, y, z) or geodetic (φ, λ, h) coordinate information for any point on the earth is easily available (seeber, 2003). however, the gnss-derived ellipsoidal height is not used directly in engineering studies. in order to use ellipsoidal heights obtained from gnss data with orthometric heights obtained from geometric leveling measurements, geoid undulations must be determined accurately. the relationship between the ellipsoidal and orthometric heights of any point on earth can be calculated by using equation (1) (heiskanen & moritz, 1967). (1) where n refers to geoid undulation, h is the ellipsoidal height, and h is the orthometric height. two different methods are used in geoid modeling: gravimetric and geometric (featherstone et al., 1998; kotsakis & sideris, 1999). with the geometric approach, many different techniques such as interpolation and leastsquares collocation (lsc) methods are used to determine a local geoid (zhong, 1997; zhan-ji & yong-qi, 1999; yanalak & baykal, 2001; erol & çelik, 2004; zaletnyik et al., 2004; erol et al., 2008; tusat, 2011; rabah & kaloop, 2013; doganalp, 2016; das et al., 2018; ligas & kulczycki, 2018; tusat & mikailsoy, 2018; dawod & abdel-aziz, 2020). for example, doganalp & selvi (2015), using gnss/leveling data (40 reference points and 205 test points), determined a geoid of the 57-km-long nurdagi-gaziantep highway project. polynomial, lsc, multiquadric (mq), and thin plate splines (tps) methods were used for geoid undulation calculation. the findings determined that using polynomial methods with lsc had a positive effect on the results in strip projects. it was concluded that mq and tps prediction methods along with the lsc method performed better than polynomial methods in calculations for strip projects. they also commented that if the number of reference points had been increased, the results might have been better. karaaslan et al. (2016) created a local geoid model in trabzon province using geometric methods. polynomial surfaces were created using mq and weighted average (wa) interpolation methods. in the study, a 600-point dataset was divided into reference and test data by looking at point distributions and orthometric height values. as a result of the study, the best geoid model was obtained with a non-perpendicular third-degree polynomial surface. with the development of computer technology, studies in the field of artificial intelligence (ai) have accelerated and various solution methods have been developed for different problem types. studies have shown that application of ai methods in the field of geodesy has been increasing in recent years and that they have proven to be quite useful to researchers working in the field, especially in the formulating of predictions using fuzzy logic as an alternative to classic methods (yιlmaz & arslan, 2008; yılmaz, 2010; tusat, 2011; erol & erol, 2012; erol & erol, 2013). one type of ai technology is the artificial neural network (ann). the ann produces successful results under conditions of multivariable and complex mutual interaction between variables or when there is no single solution set. with these features, ann is seen as a suitable method for geoid undulation determination. studies have yielded successful results by using ann (stopar et al., 2006; lin, 2007; akyilmaz et al., 2009; pikridas et al., 2011; veronez et al., 2011; akcin & celik, 2013; erol & erol, 2013; elshambaky, 2018; kaloop et al., 2018; albayrak et. al, 2020; erol & erol, 2021). for example, seager et al. (1999) performed local geoid modeling using a feedback artificial neural network (fbann) to determine the geoid. the results demonstrated that the ann could be used as a tool in geoid determination and that it gave rapid results. kavzoglu & saka (2005) performed a local geoid undulation calculation for istanbul using an ann with gps/leveling data. the ann results were compared with the polynomial and lsc methods. the results produced by the ann were just as accurate as the two classic methods. cakir & yilmaz (2014) determined a local geoid using polynomial, mq, radial basis function (rbf), and multi-layer perceptron neural network (mlpnn) methods in kayseri province and compared their performances. compared to the rbf, the mq was more successful, whereas the mlpnn yielded better results than all the other methods. in addition to the methods mentioned above, regression methods have also been used in geoid undulation determination (konakoglu & akar, 2021). for example, kaloop et al. (2020) examined the usability of multivariate adaptive regression splines (mars), gaussian process regression (gpr), and kernel ridge regression (krr) methods in geoid undulation modeling using gps/leveling observations. the results obtained with these methods were compared with the results of the ls-svm. according to the statistical tests, the krr yielded better results than the other methods. one of the factors affecting the accuracy of a geoid model is how correctly it is represented by the selected training set used to create it. the geoid undulation determination studies given above show that a dataset is divided into two parts as training and testing data according to the spatial homogeneity distribution criteria. studies are conducted in this way to determine the performance of the method. the model is trained with the training dataset before the prediction is made. the correctness of the trained model is statistically determined using the testing dataset. thus, whether or not to use the established model can be decided. in this study, the k-fold cross validation was applied for the first time in a local geoid determination study, with the aim of minimizing deviations and errors caused by distribution and division. the main aim of this study was to provide a comprehensive comparative analysis of different approaches for the prediction of the geoid undulation. the methods examined consisted of two soft computing techniques (rbfnn and grnn) and eleven conventional methods including: 1) multiple linear regression (mlr), 2) kriging (krig), 3) inverse distance to a power (idp), 4) triangulation with linear interpolation (tli), 5) minimum curvature surface (mcs), 6) natural neighbor (nn), 7) nearest neighbor (nrn), 8) local polynomial (lp), 9) radial basis function (rbf), 10) polynomial regression (pr), and 11) modified shepard’s (ms). the performance of the methods was evaluated using the root mean squared error (rmse), mean absolute error (mae), the nash-sutcliffe efficiency (nse) coefficient, and the coefficient of determination (r2). this is the first study to provide a comprehensive comparison of thirteen methods used in the prediction of geoid undulation and it is expected that scientists interested in geoid computations may benefit from its findings. material and methods study area and dataset the study area of approximately 4664 km2 is between the 40° 30' and 41° 0' latitudes and 39° 0' and 40° 30' longitudes within the borders of the province of trabzon. figure 1 shows the location of the study area. the topography is irregular, with orthometric height varying between 22.87 and 3387.14 m and geoid undulations between 24.6 and 30.7 m. (karaaslan et al. (2016). for this application, 537 c2 (second order densification) and c3 (third order densification) points covering the entire area of trabzon obtained from the trabzon ix regional directorate of the turkish land registry and cadastre were used. the latitude (j), longitude (l), ellipsoidal height (h) and orthometric height (h) values of each point are known. the orthometric heights (h) of the points were obtained via the geometric leveling method and the ellipsoidal heights (h) via gps/gnss static measurement. the spatial density of the points (1 point per 9 km2) demonstrated a good characterization of the topography. the input parameters, comprised of the latitude (j) and longitude (l) values, were used to develop the ann models. the geoid undulation value (n) served as the output parameter. the k-fold cross validation method was used to balance the features in the dataset used in the study and to accurately measure the performance of the method in the unbalanced datasets. the value of the k parameter required for cross-correcting was determined as 5. thus, the dataset was divided into five parts, with four used for training and the other one used to test the algorithm. to perform the cross-validation, the dataset was divided into approximately 80% for training sets (reference points) and 20% for testing sets (test points). table 1 indicates the statistical properties of the geodetic data used in this study, including the mean, minimum, maximum, standard deviation, and skewness values of these five different datasets (ds#1, ds#2, ds#3, ds#4, and ds#5). the corresponding average latitude and longitude values were determined as 40.73950° (40.47765° 40.97722°) and 40.06419° (39.30994° 40.49945°). negative distortion was determined in the latitude, longitude, and geoid undulation dataset values. the distribution of negative distortion was indicated with an asymmetrical tail extending toward the more negative (lower than 373geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study average) values. this showed that the skewness values were generally close to zero, and in this case, that the data were in normal distribution. the statistical parameters given in table 1 show that there were no significant differences among the datasets created. the geographical distribution of the five different datasets (ds#1, ds#2, ds#3, ds#4 and ds#5) is shown in figure 2. table 1. statistical parameters of the geodetic data (latitude, longitude, and geoid undulation) used in the study dataset phase statistical characteristics j (°) l (°) n (m) ds#1 training mean 40.739 40.059 28.418 minimum 40.478 39.340 24.661 maximum 40.969 40.499 30.748 standard deviation 0.110 0.294 1.439 skewness -0.144 -0.663 -0.644 testing mean 40.743 40.085 28.327 minimum 40.491 39.310 24.597 maximum 40.977 40.492 30.673 standard deviation 0.108 0,28524 1.372 skewness -0.259 -0,79471 -0.503 ds#2 training mean 40.741 40.072 28.357 minimum 40.478 39.329 24.597 maximum 40.977 40.499 30.748 standard deviation 0.110 0.290 1.431 skewness -0.194 -0.744 -0.590 testing mean 40.733 40.032 28.571 minimum 40.519 39.310 24.661 maximum 40.969 40.492 30.689 standard deviation 0.109 0.301 1.396 skewness -0.052 -0.477 -0.731 ds#3 training mean 40.737 40.073 28.419 minimum 40.478 39.310 24.597 maximum 40.977 40.499 30.748 standard deviation 0.110 0.290 1.424 skewness -0.188 -0.722 -0.600 testing mean 40.752 40.030 28.323 minimum 40.520 39.329 24.661 maximum 40.969 40.466 30.692 standard deviation 0.106 0.300 1.433 skewness -0.042 -0.561 -0.684 figure 1. study area dataset phase statistical characteristics j (°) l (°) n (m) ds#4 training mean 40.739 40.062 28.409 minimum 40.478 39.310 24.597 maximum 40.977 40.492 30.748 standard deviation 0.110 0.294 1.435 skewness -0.133 -0.685 -0.628 testing mean 40.741 40.075 28.362 minimum 40.509 39.365 25.331 maximum 40.919 40.499 30.658 standard deviation 0.109 0.283 1.393 skewness -0.303 -0.697 -0.566 ds#5 training mean 40.740 40.065 28.384 minimum 40.478 39.329 24.661 maximum 40.969 40.499 30.748 standard deviation 0.109 0.289 1.429 skewness -0.187 -0.712 -0.579 testing mean 40.738 40.061 28.461 minimum 40.491 39.310 24.597 maximum 40.977 40.492 30.325 standard deviation 0.112 0.304 1.414 skewness -0.085 -0.605 -0.774 figure 2. geographical distribution of the reference and test points of the 5 different datasets: (a) ds#1, (b) ds#2, (c) ds#3, (d) ds#4, (e) ds#5 data normalization is a pre-processing stage that plays a significant role in the performance of ann methods. in order to obtain more accurate results, all data should be normalized before proceeding with the training and testing stages. therefore, input and output parameters were normalized to the interval [-1, 1] and the normalized value (ynorm) for each input and output parameter (yi) was obtained using equation (2). (2) where highvalue and lowvalue are set to 1 and -1, respectively. artificial neural network (ann) the artificial neural network (ann) was inspired by the working principle of the biological nervous system and was created by artificially simplifying and 374 berkant konakoglu, alper akar imitating the nerve cells (neurons) in the nervous system and then transferring them to a computer (singh et al., 2009). since anns are modeled after the biological nervous system, they have the advantage of being capable of automatically, on their own, realizing skills such as the ability to derive and discover new information through learning, which is one of the features of the human brain. generalization and working with an unlimited number of variables are also features of the ann. supplying the learning and quick decision-making abilities of the human brain to anns enables them to solve complex problems through training. the ann looks at examples of problems, makes generalizations about these problems, sums up the information, and then, when encountering new examples it has never seen before, it makes decisions using the information it has learned. because of all these features, the ann is used to accomplish goals in many areas, including classification, control, image processing, modeling, feature determination, optimization, prediction, and more. as there is no limit to the application fields of anns, they can be applied to almost any problem that can be converted into fixed input and output variables. in this study, two types of ann (rbfnn and grnn) were applied. the development of the models was coded in a matlab environment. an overview and description of all the applied methods are discussed in the following sections. radial basis function neural network (rbfnn) the rbfnn was developed in 1988, inspired by the action-response behavior seen in biological nerve cells (broomhead & lowe, 1988). the rbfnn is a curve-fitting approach in multidimensional space and is a special version of the multi-layer artificial neural network that uses the radial basis function as its activation function. like the general ann architecture, the rbfnn method is defined in three layers: input layer, hidden layer, and output layer (figure 3). figure 3. radial basis function neural network (rbfnn) structure the input layer of the network is directly connected to the hidden layer and subsequently, weights are only present between the hidden layer and the output layer. the rbfnn has a single hidden layer and radial basis functions are used as the activation function in the hidden layer neurons. the most commonly used radial basis function is the gaussian function (hartman et al., 1990; park & sandberg, 1991). the output of an rbfnn with gaussian-based function can be calculated using the following equations (haykin, 1994). (3) (4) (5) where is the input vector, ci is the center of the rbf unit, is the width of the neuron, n is the number of cells in the hidden layer, and wi shows the link weights between the hidden and output layers. a different number of hidden layer neurons and spread parameters were investigated using the rmse. general regression neural network (grnn) the grnn proposed by specht (1993) does not require an iterative procedure as in the mlpnn method. it needs only one-way learning. due to the simplicity of the network structure and ease of implementation, this functional approach has been used in different geodetic applications (ziggah et al., 2017; cakir & konakoglu, 2019; li et al., 2020). the grnn consists of four layers: the input layer, the pattern layer, the summation layer, and the output layer (specht, 1993). the structure of the grnn is given in figure 4. figure 4. general regression neural network (grnn) structure the input layer, i.e., the first layer where inputs are given, depends on the following pattern layer. the distances between training data and testing data are calculated in this layer. the results are passed through the radial basis function together with the selected value to obtain the weight values. these weight values are transferred to the numerator and denominator neurons in the summation layer. in the numerator neuron, the output value of the training data whose weight values are in the neuron is multiplied and the multiplication values are summed up. in the denominator neuron, weights are summed up directly. the output value is obtained by dividing the numerator value by the denominator value in the output layer, as shown in equation (6). (6) where n is the number of data, s is the spread parameter, and di 2 represents the scalar function, as shown in equation (7). (7) 375geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study in this method, having only the spread parameter (s), there are not as many main design parameters as with the mlpnn (e.g., number of hidden layers, number of neurons in each hidden layer, activation function, and training type). the only important parameter that has to be determined using this method is the spread parameter (s). there is no specific rule about how this parameter is selected. in this study, different spread parameters between 0 and 1 were tested using the minimum rmse criterion. multiple linear regression (mlr) in regression methods, the influencing variables are called explanatory variables (independent variables), and the affected variable is called the described variable (dependent variable). the mlr method reveals the causeeffect relationship between the dependent variable (y) and independent variables (x1, x2,…, xn) as a mathematical model which can be written as equation (8) (şahin et al., 2013). (8) where a0, a1, a2, …, an show the effect of each independent variable on the dependent variable. interpolation methods interpolation is the prediction of the dimensions of unmeasured points using measured values of sample reference points. in this study, surfer software v. 17 was used for interpolation calculation (golden software, 2019). the software turns both simple and complex data into understandable visual tools such as maps, charts, and models. the surfer software v. 17 offers users the opportunity to determine a geoid using 12 different interpolation methods. of these 12 interpolation methods, those used for this study included the kriging (krig), inverse distance to a power (idp), triangulation with linear interpolation (tli), minimum curvature surface (mcs), natural neighbor (nn), nearest neighbor (nrn), local polynomial (lp), radial basis function (rbf), polynomial regression (pr), and modified shepard’s (ms). detailed information about these ten methods can be found in the work of keckler (1995). k-fold cross-validation method in classic and ai applications, datasets are divided into those for the training and those for the testing of the established model. this separation method is carried out in several ways. the hold-out method is widely used to provide generalization. in classic and ai methods, the sampling methodology used for data division can have a significant impact on the quality of the subsets used for training and testing. the k-fold crossvalidation method is one of the methods used in data partition. in this validation method, the dataset to be used in the prediction is randomly divided into a k number of parts. the testing datasets consist of parts created in the order of each separate training part (k1…). in this way, every part that is created up to k is used as a testing set. the accuracy of the method is determined by taking the average of the accuracy values obtained from each part (stone, 1974; kohavi, 1995; rodriguez et al., 2009). in this study, the value of k was taken as 5 (figure 5). in figure 5, the dataset is divided into five equal parts. the green parts represent the training data and the orange parts the testing data. in other words, when four parts of five datasets are used for training, one part (the fifth) is used for testing. in this way, modeling was performed five times, using each part once for testing and the other four parts for training. the arithmetic averages of the success rates were calculated for the five models and the resulting success rate was determined. ziggah et al. (2019) demonstrated the potential and feasibility of using the k-fold cross-validation method on coordinate transformation from geodetic applications. the dataset for a ghana geodetic reference network was divided into separate training and testing data according to the hold-out method. the findings revealed that false results could be generated, and it was shown that the use of the k-fold cross-validation method provided a better solution for the correct evaluation of method performance, especially in the case of a sparse dataset. figure 5. 5-layer cross-validation example performance evaluation criteria to measure the prediction success of the developed models, this study used the root mean square error (rmse), mean absolute error (mae), nash-sutcliffe efficiency (nse) coefficient, and the coefficient of determination (r2). the equations used in the calculation of the selected statistical criteria are given below. root mean square error: (9) mean absolute error: (10) nash–sutcliffe efficiency coefficient: (11) coefficient of determination: (12) where n is the number of datasets, oi is the observed geoid undulation value, is the mean of the observed geoid undulation values, pi is the predicted geoid undulation value, and represents the mean of the predicted geoid undulation values. the rmse is used to determine the error rate between the prediction and the corresponding observation. the prediction ability of the method increases as the error value approaches zero. the mae is used to determine the absolute error between the prediction and the corresponding observation. the closer the error value is to zero, the better the method’s prediction ability 376 berkant konakoglu, alper akar is indicated. the nse coefficient ranges from ‒∞ to 1. thus, when nse = 1, it means that the method is perfect. an nse value of between 0 and 1 generally means that the method performance is acceptable, whereas a value of less than 0 emphasizes that the average observation value showing the performance of the method is insufficient is a better prediction than the calculated data. the r2 is an indication of whether or not the regression equation is compatible with the data. it is the ratio that explains one difference in terms of the overall difference. this ratio is called the coefficient of certainty and shows the extent to which the difference in the dependent variable can be explained by the independent variable. the r2 is expressed by a value of between 0 and 1. a value close to 1 indicates that a large part of the variance in the dependent variable explains the independent variable in the method. results and discussion the results of the developed models are given under separate headings. it should be noted that in the tables, the best metric values are highlighted in dark gray, whereas the worst are highlighted in light gray. rbfnn results the performance of the rbfnn is based on the spread parameter and the maximum number of neurons in the hidden layer. various attempts were made to achieve the best performance using a different number of neurons and different spread parameters. the best performance criteria results obtained during the training and testing phases are given in table 2. the optimum values for the maximum number of neurons and their spread were 430/0.025 for ds#1, 421/0.056 for ds#2, 427/0.076 for ds#3, 412/0.034 for ds#4, and 410/0.065 for ds#5. table 2. rbfnn training and testing phase results phase dataset rmse (m) mae (m) nse training ds#1 0.112 0.079 0.99383 ds#2 0.001 0.000 0.99999 ds#3 0.000 0.000 0.99999 ds#4 0.034 0.009 0.99944 ds#5 0.003 0.001 0.99999 testing ds#1 0.197 0.115 0.97925 ds#2 0.780 0.542 0.68478 ds#3 0.525 0.364 0.86442 ds#4 0.720 0.590 0.73029 ds#5 0.569 0.389 0.83665 the lowest rmse value of the prediction models at the training stage was obtained with ds#3 and the highest with ds#1. moreover, the lowest mae value was also found in ds#3. according to table 2, the highest nse values during the training stage were with ds#2, ds#3, and ds#5. at the testing stage, the lowest rmse value was obtained with ds#1 and the highest with ds#2, whereas the lowest mae and the highest nse values were determined in ds#1. during the training phase, ds#2, ds#3, ds#4, and ds#5 performed well, whereas in the testing phase, the performance results of ds#1 were better than in the others. in order to demonstrate the performance of the rbfnn, the predicted geoid undulation values and those observed at the testing stage for each dataset (ds#1, ds#2, ds#3, ds#4, and ds#5) are shown in figure 6. thus, it is clear that the geoid undulation prediction made with ds#1 was superior to that of the other datasets. it is also clear that the a and b coefficients of the distribution graphs from the fit line equations (assuming that the equation was y = ax + b) were higher than the a and b coefficients obtained with the other datasets; a was close to 1 and b close to 0, and the r2 value was higher. grnn results the performance of the grnn largely depends on the spread parameter, so determining this parameter is of great importance. therefore, in order to figure 6. comparison of the geoid undulation values predicted by rbfnn and those observed at the testing phase optimize prediction performance, this parameter must be accurately determined according to the evaluation criteria. the grnn spread parameter values that provided the best testing performance were determined as 0.015, 0.013, 0.020, 0.022, and 0.012 for ds#1, ds#2, ds#3, ds#4, and ds#5, respectively. the prediction performance of the grnn for the training and testing stages in terms of the rmse, mae, and nse is given in table 3. according to table 3, the lowest rmse value in the training stage was obtained with ds#4, and the highest with ds#2. the lowest mae value was determined with ds#4, whereas the highest nse value at the training stage was determined again in ds#4. the lowest rmse value in the testing phase was found with ds#4 and the highest with ds#2, whereas the lowest mae and the highest nse values were determined in ds#4. during the training and testing phases, the performance results of ds#4 were better than those of the others. in order to illustrate the success of the grnn, the predicted geoid undulation values and those observed at the testing stage for each dataset (ds#1, ds#2, ds#3, ds#4, and ds#5) are shown in figure 7. 377geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study table 3. grnn training and testing phase results phase dataset rmse (m) mae (m) nse training ds#1 0.024 0.007 0.99969 ds#2 0.048 0.019 0.99886 ds#3 0.019 0.004 0.99981 ds#4 0.012 0.003 0.99993 ds#5 0.039 0.016 0.99927 testing ds#1 0.199 0.147 0.97873 ds#2 0.201 0.140 0.97912 ds#3 0.173 0.134 0.98530 ds#4 0.167 0.124 0.98550 ds#5 0.187 0.140 0.98230 the diagrams show that the results obtained with all datasets were quite close to each other. the fact that in the equation for determining linearity (y = ax + b), coefficient a was close to 1 and coefficient b close to 0 indicated that there was a good relationship between the observed and the predicted geoid undulation values. in addition, as can be seen from figure 7, the determination coefficient (r2) between the observed and predicted values of ds#4 during the testing phase was 0.98575. mlr results the results calculated according to the regression equations for geoid undulation are given below for the five different datasets. n (ds#1) = 594.174 – 12.028j – 1.891l (13) n (ds#2) = 593.165 – 12.013j – 1.881l (14) n (ds#3) = 586.129 – 11.866j – 1.855l (15) n (ds#4) = 642.606 – 13.672j – 1.423l (16) n (ds#5) = 594.498 – 11.987j – 1.941l (17) the geoid undulation (n) was defined as the dependent variable and latitude (j) and longitude (l) were regarded as the independent variables. the results of the statistical evaluation criteria for the mlr geoid undulation prediction are presented in table 4. table 4. mlr training and testing phase results phase dataset rmse (m) mae (m) nse training ds#1 0.432 0.346 0.90959 ds#2 0.448 0.356 0.90175 ds#3 0.449 0.356 0.90038 ds#4 0.436 0.341 0.90753 ds#5 0.436 0.346 0.90685 testing ds#1 0.466 0.351 0.88358 ds#2 0.402 0.310 0.91627 ds#3 0.397 0.312 0.92260 ds#4 0.557 0.446 0.83850 ds#5 0.455 0.340 0.89569 figure 7. comparison of the geoid undulation values predicted by grnn and those observed at the testing phase according to table 4, the lowest and highest rmse values in the training stage were obtained with ds#1 and ds#3, respectively. the lowest mae value was determined with ds#4 and the highest with ds#3. the highest nse value at the training stage was determined in ds#1. at the testing stage, the lowest and highest rmse values were determined with ds#3 and ds#4, respectively. in addition, the lowest and highest mae values were determined in ds#2 and ds#4, respectively. the lowest and highest nse values were calculated for ds#4 and ds#3. in order to illustrate the success of the mlr, the predicted geoid undulation values and those observed at the testing stage for each dataset (ds#1, ds#2, ds#3, ds#4, and ds#5) are shown in figure 8. results of interpolation methods the differences between the predicted and the observed geoid undulation values at the test points for the interpolation methods were computed to determine the rmse, mae, and nse for the five different datasets (ds#1, ds#2, ds#3, ds#4, and ds#5), and are given in table 5. 378 berkant konakoglu, alper akar table 5. statistical findings for the interpolation methods dataset (ds#1) (ds#2) interpolation methods rmse (m) mae (m) nse rmse (m) mae (m) nse krig 0.298 0.140 0.95226 0.239 0.127 0.97038 idp 0.320 0.179 0.94504 0.275 0.180 0.96082 tli 0.301 0.149 0.95130 0.248 0.133 0.96814 mcs 0.301 0.151 0.95146 0.251 0.140 0.96740 nn 0.298 0.143 0.95232 0.237 0.128 0.97077 nrn 0.341 0.235 0.93746 0.291 0.219 0.95601 lp 0.296 0.159 0.95317 0.230 0.135 0.97260 rbf 0.322 0.167 0.94438 0.268 0.151 0.96270 pr 0.477 0.363 0.87820 0.411 0.311 0.91242 ms 0.443 0.223 0.89477 0.306 0.172 0.95134 dataset (ds#3) (ds#4) interpolation methods rmse (m) mae (m) nse rmse (m) mae (m) nse krig 1.406 1.020 0.02753 0.243 0.131 0.96927 idp 1.380 0.993 0.06381 0.255 0.166 0.96604 tli 1.407 1.012 0.02653 0.240 0.131 0.97014 mcs 1.416 1.026 0.01322 0.250 0.138 0.96759 nn 1.411 1.024 0.02063 0.240 0.131 0.97007 nrn 1.427 1.045 -0.00203 0.332 0.230 0.94248 lp 1.421 1.043 0.00655 0.228 0.140 0.97287 rbf 1.404 1.026 0.03100 0.261 0.146 0.96455 pr 1.432 1.039 -0.00800 0.461 0.378 0.88948 ms 1.421 1.053 0.00677 0.291 0.170 0.95587 dataset (ds#5) interpolation methods rmse (m) mae (m) nse krig 0.143 0.095 0.98961 idp 0.207 0.164 0.97842 tli 0.159 0.102 0.98729 mcs 0.178 0.110 0.98409 nn 0.142 0.097 0.98986 nrn 0.275 0.217 0.96196 lp 0.171 0.130 0.98532 rbf 0.188 0.122 0.98214 pr 0.462 0.345 0.89245 ms 0.404 0.206 0.91746 379geoid undulation prediction using anns (rbfnn and grnn), multiple linear regression (mlr), and interpolation methods: a comparative study figure 8. comparison of the geoid undulation values predicted by mlr and those observed at the testing phase it can be seen from figure 8 that the a and b values of the predictions made using different datasets yielded close results. moreover, the figure shows that the determination coefficient (r2) between the observed and predicted values of ds#3 during the testing phase was 0.92360. according to table 5, in terms of the resulting rmse and nse values, the best performance in the testing phase was obtained for the nn in ds#5. however, the highest rmse value was found in ds#3 with the pr. in addition, the lowest mae value was found in ds#5 using the krig method and the highest in ds#3 with the ms. the nse value in ds#3 was very low with all methods. to demonstrate a comparison of the methods in terms of accuracy, figure 9 gives the observed geoid undulation values with the values calculated for ds#5 using the interpolation methods. 380 berkant konakoglu, alper akar as can be seen in figure 9, the a value obtained with the ds#5 using the nn gave a result closer to 1 than the other datasets. likewise, using the nn, the b value obtained with the ds#5 yielded a result closer to 0 than the other datasets. in the testing phase, the highest r2 among the observed and predicted values was obtained by the nn interpolation method in the ds#5. comparison of methods used in geoid undulation prediction (using k-fold cross-validation) in order to evaluate the general performance of all the methods using the k-fold cross-validation, it was necessary to calculate the averages of the statistical criteria values (rmse, mae, nse, and r2). in other words, the average value was found for each performance criterion result given in tables 2, 3, 4, and 5. according to the 5-fold cross-validation method based on the average performance values of the statistical criteria, in general, all interpolation methods except pr gave similar results (rmse, 0.466 m 0.648 m; mae, 0.303 m 0.487 m; nse, 0.71291 0.78283; r2, 0.75789 0.82706). the performance of the pr method was weaker than for the other methods. table 6 shows the average values for the rmse, mae, nse, and r2 obtained from rbfnn, grnn, mlr, and pr. table 6. average values for the rmse, mae, nse, and r2 (5-fold) method phase rmse (m) mae (m) nse r2 rbfnn training 0.030 0.018 0.99598 0.99875 testing 0.558 0.400 0.82036 0.85108 grnn training 0.029 0.010 0.99881 0.99951 testing 0.185 0.137 0.98229 0.98249 mlr training 0.440 0.307 0.90522 0.90522 testing 0.455 0.352 0.89176 0.90080 pr testing 0.648 0.487 0.71291 0.75789 as can be seen from table 6, on average, the grnn outperformed all methods investigated for training and testing phases. the best nse and r2 values (0.99881 and 0.99951) and the lowest rmse and mae values (0.029 m and 0.010 m) were found in the grnn training phase. the grnn also yielded the lowest rmse and mae values (0.185 m and 0.137 m) with the highest nse and r2 values (0.98229 and 0.98249) for the testing phase. it is clear that the highest rmse and mae, and the lowest nse and r2 values were obtained with the pr during the testing phase. the results of the interpolation methods used with ds#3 negatively affected the performance with the other four datasets and reduced the accuracy (see table 5). the rbfnn yielded poor overall results when compared to the interpolation methods. among the methods used in this study, the mlr was ranked second-highest for accuracy. considering the overall prediction accuracy of the different methods, the grnn is the one to be recommended for predicting geoid undulation. conclusion in this study, two different ann methods (rbfnn and grnn), the mlr, and ten different interpolation methods were comprehensively investigated and analyzed in order to compare their performances in geoid undulation prediction. the k-fold cross-validation method was used to obtain a better generalization. the evaluation of the performance of the datasets using cross-validation can be carried out in two ways: (1) by accepting that the method with the minimum error is the most appropriate one or (2) by considering that the method with the minimum average of results for all the datasets is the most appropriate one. when the methods used with each dataset were compared, the test results using the nn in ds#5 were the most successful. however, the lowest rmse, mae, nse, and r2 values were obtained with the ds#3 using the ten different interpolation methods. in the calculation performed with the same dataset, the most successful method after the nn method was the grnn, whereas the rbfnn and mlr gave nearly the same results. the grnn was the most successful method in the evaluation carried out by averaging the rmse, mae, nse, and r2 values produced using the five different datasets. in this study, the analysis of the results of two different performance evaluations for the grnn demonstrated that the method could be successfully applied and was capable of yielding results equally as accurate as or better than those of the rbfnn, mlr, or interpolation methods used for prediction. another conclusion that can be drawn from this study is that incorrect point distribution and dataset division can negatively affect the performance of interpolation methods when used in geoid undulation calculation. therefore, it was recognized that the use of the k-fold cross-validation in geoid undulation calculations could expose incorrect data distribution. it was also shown that the use of the k-fold cross-validation method may give more accurate results for large amounts of data as well as for a sparse number of data. using the same dataset, the k-fold cross-validation method could also be used to test the effects of other ann methods and compare them to the results of this study. moreover, additional studies could analyze geoid undulation prediction using a different dataset with the methods used in this study. it is hoped that the present study will thus contribute to the field and lead to future research on geoid undulation modeling. acknowledgments we are grateful to the trabzon ix regional directorate of land registry and cadastre in turkey for providing valuable datasets for this study. additionally, generic mapping tools (gmt) was used to generate figure 1. special thanks are also extended to the four anonymous reviewers for their constructive comments. references akcin, h. & celik, c. t. 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environmental impact assessment; land use; land cover; remote sensing uso de imágenes satelitales para medir los cambios en el uso del suelo y en la cobertura del suelo en la ciudad de diyarbakır (sureste de turquía) resumen el objetivo de este estudio es describir, clasificar y analizar los factores antropogénicos más comunes que han alterado el paisaje. con este fin se eligió la ciudad de diyarbakir, una de las más grandes y urbanizadas de mesopotamia, como un ejemplo donde se presentan varios de estos factores antropogénicos y se realizaron varias visitas de campo para determinar estas actividades en el área. luego estos factores fueron clasificados en cuatro categorías: depósitos de mármol, depósitos de agregados, cuerpos de agua y áreas urbanizadas. a través de investigar los cambios antropogénicos basados en el tiempo se analizaron imágenes satelitales en el período de 1975 a 2019. las tendencias, distribuciones y los impactos en general de 177 puntos se investigaron retrospectivamente y se monitorearon a través de imágenes de teledetección. entre 1975 y 2019 un total de 25224 hectáreas de tierra fueron transformadas de acuerdo con los hallazgos. los cuerpos de agua eran los más grandes de las clases estudiadas. la segunda mayor clase y la de más expansión son las áreas urbanizadas. esta clase de áreas se asumió como el agente más efectivo que modificará el área de estudio, dada la necesidad de más espacios para construcción. las actividades de minería de superficie también se asumen como agentes eficientes que modificarán los espacios de diyarbakır debido al potencial de exportación de productos de mármol y la demanda de material de construcción. las fuentes de aguas y el suministro de materiales de construcción deben de examinarse a profundidad durante la planeación del desarrollo de este tipo de ciudades urbanizadas para minimizar la degradación antropogénica. palabras clave: detección del cambio; medición del impacto ambiental; uso del suelo; cobertura del suelo; teledetección; record manuscript received: 16/12/2020 accepted for publication: 09/05/2022 how to cite item: dursun, f., zorlu, k., & gül, m. (2022). using satellite imagery to assess the changes in land use and land cover in diyarbakır city (se turkey). earth sciences research journal, 26(2), 119-130. https://doi.org/10.15446/esrj.v26n2.92417 https://doi.org/10.15446/esrj.v26n2.92417 mailto:f.dursun@ucl.ac.uk mailto:felatdursun@gmail.com https://doi.org/10.15446/esrj.v26n2.92417 120 felat dursun, kemal zorlu, murat gül introduction the land cover comprises of the naturally occurring (e.g. vegetation, waterbody, topography, rock and soil formations) and anthropogenic elements (water bodies, urban, industrial sites, mine sites) over the surface of the earth (düzgün and demirel, 2011; foley et al., 2005; geist and lambin, 2002). the steady increase of the human population (from 7.7 billion to exceed 10 billion in fifty years, un 2019) and climate change elevate the pressure on land resources (meyer and turner, 1992; vitousek et al., 1997; pillay et al., 2014). land use and land cover (lulc) changes has certain link with global environmental change (song et al., 2018). scientists from various fields and countries focus on addressing human impact (including vegetation removal, urbanization, quarrying, deforestation, surface mining, road and dam construction, excessive irrigation, groundwater extraction) on the environment and the rate of land transformation ( gornitz et al., 1997; williams, 2000; falcucci et al., 2007; foley et al., 2005; mouflis et al., 2008; düzgün and demirel, 2011; mossa and james, 2013; gill and malamud, 2017; meneses et al., 2017; gül et al., 2019). land use and land cover (lulc) changes have the potential to change the earth energy balance (song et al., 2018), to change hydrological ecosystem (yohannes et al. 2011), to fragment and transform the ecosystem (hong et al., 2011), and to trigger the natural hazards such as flooding, landslides, rock fall and subsidence (rahman et al., 2011; mossa and james, 2013; gill and malamud, 2017). land use and land cover changes and some other natural factors rain fall initially responsible for the changes in local temperature, water balance, greenhouse emission (kalnay and cai, 2003; hassan et al., 2021; tuladhar et al., 2019; dewan et al., 2021). those local changes have formed the global effect (kalnay and cai, 2003; reis, 2008; song et al., 2018; altürk and konukçu, 2020; hong et al., 2021). for this reason, an understanding the local pattern is necessary for realizing the global situation, and to offer local solution for policymakers. remote sensing and geographic information systems (gis) provide useful data not only for detecting, classifying and modeling anthropogenic driven impacts on lulc but also for combining field observations and past changes (abbas et al., 2010; alphan, 2003; chen et al., 2003; düzgün and demirel, 2011; rahman et al., 2021). particularly, historical time-series of data sets gathered from the satellites enable us to monitor, compare and assess the trends, distribution, intensity, transformation and related changes in lulc. herold et al. (2011) and latifovic et al. (2005) state that in order to monitor and assess the rates of degradation, it is essential to combine the historical and present data sets. considering the lack of past field data, remotely sensed images become an important data source for long-term assessments. different studies focused on monitoring the change in lulc with the aid of time-series data sets (mouflis et al., 2008; dewan and yamaguchi, 2009; bullock et al., 2018; huang et al., 2018). several studies were done in turkey related to lulc, however all of them interest to local region. for example, lulc changes of adana city (alphan, 2003), bandırma town (atay kaya and görgün, 2020), i̇stanbul city (çakır et al., 2008), samsun city (güler et al., 2007), trabzon city (keleş et al., 2008), rize city (reis, 2008), kastamonu (şen et al., 2018) and seyhan basin (zadbagher et al., 2018) have been developed mostly due to urbanization. in addition, a few studies were focused on other reasons such as fragmentation of agriculture areas in tekirdağ city (altürk and konukçu, 2020), agricultural activities in northern turkey (şen and güngör, 2018), forestry areal change (çakır et al., 2007; sivrikaya et al., 2007; keleş et al., 2008; bozali et al., 2015), surface coal mine area change (demirel et al., 2011) and coastal zone (hepcan et al., 2011). diyarbakır is one of the biggest cities of mesopotamia and se turkey, show different examples of the lulc. urbanization was generally taken into consideration, especially in old town section of the city (i.e., suriçi) (koç and aykal, 2018; sinemillioglu et al., 2010; kuzucuoğlu and karadoğan, 2015). other satellite image analysis-based studies in the study area have concrete objectives and examined at changes through the lens of a single factor. among those limited studies, one only examines the changes in irrigation-agricultural areas (bayraktar and bayram, 2007), while the other is concentrated on the surface temperature changes due to natural and increasing urbanization (oğuz, 2007). since their assessments are mostly on the specific sections of the city, they do not provide sufficient data for evaluation of overall intensity, types, or rate of human-induced activities on the lulc. considering the modifications in lulc of the city in the last 50 years, it has become necessary to assess the rate, types, trends and distribution of human-induced changes in lulc. moreover, sustainable city planning is required certain determination of lulc. the study area was chosen due to ongoing mining activities, urbanization, and massive population growth, which increased significantly during the 2000s. the lack of studies on the impacts of human-induced activities on the land adversely affects the planning and the land conservation policies of this growing city. this study is an initial attempt to show the effects of the most common anthropogenic factors in diyarbakir province. the present study was performed to describe, classify, and analyze the interactions of the selected anthropogenic factors with lulc of the diyarbakır city. the study is aimed to (i) define the most effective agent in terms of alteration of the lands of the study area; (ii) discuss the environmental issues in order to understand the degradation process and (iii) correlate the present and past impacts of human-induced activities on the lulc. the obtained results would guide policymakers to improve future planning strategies and to protect the lands of diyarbakır city against degradation. figure 1. (a) location map and (b) simplified geological map of the study area (modified from toprak, 2012) study area the study area covers an area of 15401 square kilometers, is located in the se part of turkey and is limited to the diyarbakır city province (figure 1a) (between 37o30’-39o00’ latitude and 39o00’-41o30’ longitude). situated on the west bank of the tigris (dicle) river, diyarbakır city is a characteristic example of a geographical region that bears the traces of various civilizations starting from prehistoric to modern times. the remains and artifacts of the old civilizations can be found all around the city. there are over one hundred registered monuments and cultural properties located in the old town of diyarbakır, including but not limited to the castle, palace, churches, mosques and city walls. the city walls was acknowledged a status of world heritage site by unesco in 2015 (dursun and topal, 2019). in the last decades, the city has witnessed a massive expansion as a result of population growth and rapid urbanization. as a result, various anthropogenic factors have caused changes in lulc of the diyarbakır city. diyarbakır and the surrounding region contain 121using satellite imagery to assess the changes in land use and land cover in diyarbakır city (se turkey) various aged allochthones and autochthones units (figure 1b). undifferentiated metamorphic units of the bitlis-pötürge-malatya nappes mainly contains marble and lesser extent augen gneiss, biotite gneiss, amphibolite, quartzite, albite, garnet schist and muscovite schist. ophiolite nappes of the bitlispötürge-malatya nappes include campanian-maastrichtian serpentinized diabase, dunite, pyroxenite, gabbro and basalt. the koçali-karadut nappes contains the jurassic-cretaceous koçali complex including cherty limestone, diabase, conglomerate, radiolarite, marble, agglomerate and ophiolite units, and the upper cretaceous karadut complex containing shale, clayey limestone, limestone and chert. the çüngüş formation, which is member of the çüngüş-hakkari nappes, is made up of eocene-miocene shale, marl and sandstone. the southeast anatolian autochthone includes eocene-oligocene conglomerate-sandstone-shale alternation, limestone, marl, dolomitic limestone (midyat group), lowermiddle miocene conglomerate-sandstone-siltstone, and locally evaporatelacustrine limestone, reef limestone, sandstone, sandstone-siltstone, marl (silvan group) and middle-upper miocene shale-sandstone-siltstone, sandstone and conglomerate (şelmo formation). all of those older units were unconformably overlain by the plio-quaternary deposits including karacadağ volcanics, river terrace and alluvial deposits (figure 1b) (bağırsakçı et al., 1995; ercan et al., 1991; genç, 1985; lustrino et al., 2010; şaroğlu and emre, 1987; sütçü, 2008; toprak, 2012; yılmaz and duran, 1997). material and methods the methodology steps used for this study are shown in figure 2. firstly, field studies were conducted to observe and identify the most common and ongoing degraded areas. a total of 177 locations have been selected for investigation. these 177 locations were examined within the scope of this study and visited to investigate their properties such as: gps coordinates, boundaries, destruction conditions, and the human-induced activity type. such properties have been determined for every single location investigated through the present study. following the field studies, 177 locations with anthropogenic degradation were identified. these locations are grouped under four classes according to the anthropogenic activities causing degradation. in order to assess the previous conditions and to determine the evolution of the anthropogenic degradation in topography over the last 45 years, the selected locations were retrospectively monitored with the aid of remotely sensed data. remote sensing techniques were applied to a landsat time series and google earthtm imagery. changes in land use and land cover (lulc) have become a significant and essential component of existing strategies for monitoring changes in the figure 2. flowchart showing methods of the work table 1. acquisition information of the satellite images used in the present study path/row acquisition date sensor spacecraft path/row acquisition date sensor spacecraft 185/33 1975-07-15 mss landsat_2 185/34 1975-09-07 mss landsat_2 186/33 1975-08-03 mss landsat_2 186/34 1975-06-28 mss landsat_2 172/33 1985-07-09 tm landsat_5 172/34 1985-07-09 tm landsat_5 173/33 1985-06-14 tm landsat_5 173/34 1985-06-30 tm landsat_5 172/33 1990-07-23 tm landsat_5 172/34 1990-08-24 tm landsat_5 173/33 1990-08-15 tm landsat_5 173/34 1990-07-30 tm landsat_5 172/33 1995-07-05 tm landsat_5 172/34 1995-07-05 tm landsat_5 173/33 1995-08-13 tm landsat_5 173/34 1995-07-28 tm landsat_5 172/33 2000-07-02 tm landsat_5 172/34 2000-06-16 tm landsat_5 173/33 2000-07-25 tm landsat_5 173/34 2000-07-25 tm landsat_5 172/33 2005-07-16 tm landsat_5 172/34 2005-08-17 tm landsat_5 173/33 2005-08-24 tm landsat_5 173/34 2005-07-23 tm landsat_5 172/33 2010-07-30 tm landsat_5 172/34 2010-07-30 tm landsat_5 173/33 2010-07-05 tm landsat_5 173/34 2010-08-22 tm landsat_5 172/33 2015-07-28 oli landsat_8 172/34 2015-07-12 oli landsat_8 173/33 2015-07-19 oli landsat_8 173/34 2015-07-19 oli landsat_8 172/33 2019-08-08 oli landsat_8 172/34 2019-08-08 oli landsat_8 173/33 2019-07-30 oli landsat_8 173/34 2019-07-30 oli landsat_8 122 felat dursun, kemal zorlu, murat gül landscape and managing natural resources (kumar et al., 2014; rawart et al., 2013). the european commission has developed the coordination of information on the environment (corine) program to create an integrated gis on the state of the environment within the european union (moos and wyatt, 1994). european land cover data can be easily obtained from the corine database. in order to utilize the landsat and google earthtm time series of diyarbakır, a number of digital image processing techniques and post classification comparison change detection (pcccd) were applied for the years 1975, 1985, 1990, 1995, 2000, 2005, 2010, 2015, and 2019. the acquisition dates and related information of the satellite images used in the current study can be seen in table 1. it is well known that various anthropogenic factors affecting the lands of diyarbakır city include but not limited to deforested areas, vegetation removal, surface material extraction, water reservoirs, road constructions, built-up areas and degradations of arable lands. considering their scale, distribution and overall impacts on land, four different land use classes that are highly common in the study area were used for lulc maps and pcccd. these are classified as follows: marble quarries (mq) – include marble quarries that currently in operation or abandoned. aggregate quarries (aq) – include legal and illegal stone quarries that currently in operation or abandoned. water bodies (wb) – include human made water structures (dams, hydroelectric power plants, artificial lakes and ponds). built-up areas (ba) – include residential, commercial, educational, industrial or governmental areas. here, it is important to note that, for reasons such as economic conditions, lack of industrialization, and low immigration rates, the degree of urban expansion remains very limited in the districts of diyarbakır. hence, most of the ba elements were concentrated mainly in the central zones of the city. therefore, in the present study, only areas covering the city center (a total of 33 locations) were analyzed in the ba class. here, it is worth mentioning that although the marble quarries and aggregate quarries can be categorized as surface mining activities, they have been examined in two separate classes because their areas of influence and the way they degrade the land are different. following the determination of the classes during the field surveys, a total of 177 locations covering these classes were selected and their coordinates were specified. then the changes that occurred in these 177 locations between 1975 and 2019 were determined and discussed. it is vital to note that the 177 locations were selected based on the field surveys and observations on visual impacts of the activities. these are representing the impact of recent anthropogenic activities on land. multi-temporal landsat data and high-resolution google earthtm data for the years 1975 to 2019 were used to determine the anthropogenic changes caused by mining, urbanization, and water reservoirs. for assessing the previous land cover classes of 177 locations, the corine land cover (clc90land cover map for the year 1990) map of the study area was utilized as the base land cover map. the class of the 177 points determined was established using the clc90-land cover map for the year 1990. the land use changes through time and the classifications of these places on the clc map were determined. imagery from the landsat 2 multispectral scanner system (mss) (1975), landsat 5 thematic mapper (tm) (1985, 1990, 1995, 2000, 2005, and 2010), and landsat 8 operational land imager (oli) (2015 and 2019) were obtained from the united states geological survey (usgs) website (https:// earthexplorer.usgs.gov/). landsat images on path/row 172/33, 172/34, 173/33, and 173/34 have been downloaded for the selected years. the google earth imagery data from 1984 to 2019 were acquired from google earthtm software. preprocessing procedures are required to acquire and evaluate spectral data which are spatially-radiometrically adjusted images. firstly, the layer stacking process was used in used landsat bands. then radiometric calibration was performed to stacked bands. the four scenes of each year mosaicked and subset to the border of the study area. digital image processing was carried out using envi and arcgis softwares. post classification comparison change detection post classification comparison change detection is a method for identifying and quantifying variations in the area from multi-temporal imagery (gillanders et al., 2008; singh, 1989). by evaluating a satellite imagery data set, this land surface monitoring technique, depending on the scale of the images utilized, provides an efficient means of measuring change at the landscape or national scale (cohen and goward, 2004; wulder et al., 2004). to detect anthropogenic changes in the diyarbakır, post classification comparison change detection was performed on land cover (lc) maps produced from landsat images of each selected year. landsat true color composites used for post classification comparison change detection. landsat images were preprocessed and mosaicked in envi before being imported into arcgis. landsat satellite images from 1975 to 2019 and google earthtm images from 1985 to 2019 were used to examine locations corresponding to each land use class (i.e., aq, mq, ba, wb) defined by field investigations. the main goal of this stage is to detect the amount of change in land use at 177 locations over time. landsat images from 1975 to 1985, as well as landsat and google earth tm images from 1985 to 2019, were used to assess land use changes and delineate boundaries in these locations. the boundaries of the areas determined by field studies were generated for each class in arcgis using satellite images from the specified years. the area of each polygon was calculated by the attribute table of each layer. thus, areal changes in each location were determined by years. the previous land cover of these 177 locations was determined using the clc map of the study area. these results were then used to compute the trend, net change, percent change and rate of each land cover class between the years 1975 and 2019. results the observed changes for the categorised activities have been described based on their number, distribution, expansion, and previous land cover conditions following image processing. figure 3 depicts the lulc changes in the examined classes in diyarbakır from 1975 to 2019, based on changes detected by landsat imagery analysis. table 2 shows the spatial extent and location numbers of these changes. figure 4 depicts the most extensive locations based on monitored classes. anthropogenic changes at class level as it can be followed from the figure 5a over the forty-four years, there was an aggressive increase in the lands occupied by mq activities (figure 5a). the total land area under this class was 2 ha in 1975. the area covered by mq has increased to 607 ha in 2019 (table 2). this change highlights an increase of 30250 percent, a net increase of 605 ha, and an annual average expansion rate of 13.7 ha over the studied forty-four years. the increase in the number of opened mq between 1975 and 2019 validates the increase in the size of the lands transformed by this activity for the same period. the highest increase in the size of the mq is observed in the years between 1995 and 2010. during this period, the areas occupied by mq have increased from 11 ha to 357 ha (table 2). this change demonstrates an increase of 3145.4 percent, a net increase of 346 ha, and an annual average expansion rate of 3 ha over the observed 19 years. considering their distribution, most of the mq are located in the north section of the study area. of the monitored 49 mq, the most expanded one is located in hani district (mq-4) (figure 4). the quarry was first monitored in the year 2000, with a land area of 15 ha; at the end of 2019, it is found that the land area covered by this quarry has increased to 72 ha with an increase of 380 percent, a net increase of 57 ha, and an average annual expansion rate of 3 ha. aq class is another human-induced activity studied through the present paper (figure 5b). in the year 1975, the total land area under this class was 67 ha but it has increased to 819 ha in 2019 (table 2). this modification indicates an increase of 1122.3 percent, a net increase of 752 ha, and an average annual expansion rate of 17 ha of the land transformation during the studied years of 1975 to 2019. the highest increase in the aq class is monitored in the years between 2005 and 2019. according to the results, during the mentioned period, the areas occupied by such quarrying activities increased from 208 ha to 819 ha, which illustrates an increase of 293.7 percent, a net increase of 611 ha, and an average annual expansion rate of 43.6 ha of the land converted to aq over the 2005 and 2019. the spatial distribution of the aq is roughly uniform. most of the quarries have been opened through the northeast and northwest sections of the city. of the monitored locations for the years spanning 1975 to 2019, the most expanded location for the aq class is the one located in the ergani district (aq-54) (figure 4). this quarry site produces raw material for a cement factory, located some 10 kilometers north of the site. 123using satellite imagery to assess the changes in land use and land cover in diyarbakır city (se turkey) table 2. change detection statistics at class level (in hectares) classes mq aq wb ba year area(ha) location (number) area (ha) location (number) area (ha) location (number) area (ha) location (number) 1975 2 2 67 8 1946 10 1468 7 1985 3 3 119 11 3852 22 2217 14 1990 3 3 145 16 4572 29 2461 18 1995 11 6 186 16 5465 40 3656 19 2000 62 11 212 18 10040 46 5039 21 2005 179 19 208 20 10965 48 5801 25 2010 357 34 283 30 11178 48 7632 28 2015 486 45 455 43 12375 49 9605 30 2019 607 49 819 43 12962 52 10836 33 mq: marble quarries; aq: aggregate quarries; wb: water bodies; ba: built-up areas figure 3. landsat satellite images showing the land cover status of 177 locations in (a) 1975 and (b) 2019 124 felat dursun, kemal zorlu, murat gül in size, wb is the most expanded class among the studied anthropogenic activities (figure 5c). it is found that the lands covered by the wb have increased from 1946 ha in 1975 to 12962 ha in 2019. this change implies an increase of 566.1 percent, a net increase of 11016 ha, and an average annual expansion rate of 250.3 ha of the land transformation over the studied years of 1975 and 2019. the most dramatic changes in water bodies class are monitored in the years between 1990 and 2000 (figure 5c). based on the results, during the mentioned period, the areas occupied by wb increased from 4572 ha to 10040 ha, which demonstrates an increase of 119.6 percent, a net increase of 5468 ha, and an average annual expansion rate of 546.8 ha of the land transformed to wb over the 1990 and 2000. considering the overall trend of the distribution, while most of the artificial lakes and ponds accumulate at the south and southwestern sections; dams and hydroelectric power plants constructed through the north and east parts of the study area. during the studied time series, the most expanded location for the wb class is found as devegeçidi dam, located ca. 25 kilometers northwest of the city center (wb-4) (figure 4). through the present study, the differences in ba activities of 33 locations in the city center were analyzed for the years between 1975 and 2019. the figure 4. the most extended locations at class level and their areal changes for the studied years measurements make it possible to reveal significant growth in ba class with respect to a time interval (figure 5d). the extent of the lands covered by ba in diyarbakır city center was 1468 ha at the end of 1975, then it becomes 10836 ha in 2019 (table 2). this change highlights an increase of 638.1 percent, a net increase of 9368 ha, and an average annual expansion rate of 213 ha. the most dramatic increase in ba occurred in the years between 2000 and 2019 (figure 5d). over the mentioned years, the lands occupied by ba increased from 5039 to 10836 ha, which indicates an increase of 115 percent, a net increase of 5797 ha, and an average annual expansion rate of 305.1 ha of the land transformed to ba over the years 2000 and 2019. the expansion of the city during the years of 1985-1995 mainly occurs in the west section of the city center, bağlar district, where most of the ba elements are occupied by squatters. in the year 2000 and beyond the lands converted to ba in diyarbakır city center have expanded aggressively, resulting in dense urbanization, mainly through the northwest section of the city, kayapınar district. since then, kayapınar became the main expansion ax of diyarbakır city, where most of the ba are characterized by residential and commercial buildings. 125using satellite imagery to assess the changes in land use and land cover in diyarbakır city (se turkey) the most significant land cover changes in order to assess previous land conditions of the studied classes, the previous land cover classification of the corine has been analyzed. the land cover distribution of the study area has been mapped as shown in figure 6, and its spatial extent and location for the studied classes are given in table 3. it is clear from the table 3 that 39 percent (237 ha) of the total land modified by mq activities (607 ha) was previously covered by the “sparsely vegetated areas”, 29.8 percent (181 ha) by the “natural grasslands” and 21.5 percent (131 ha) by the “land principally occupied by agriculture, with significant areas of natural vegetation” based on the land cover classification. it is also found that 87.7 percent of locations assigned as mq were opened mainly over the vegetation and agricultural land (table 3). figure 5. areal changes at class level (a: mq; b: aq; c: wb; d: ba) figure 6. corine land cover (clc) map of the study area and land cover status of 177 locations 126 felat dursun, kemal zorlu, murat gül table 3. changes in land cover (location and area) for the studied area and corine land cover classes (in hectares) land cover classes (corine) classes mq aq wb ba l oc at io n (n um be r) a re a (h a) l oc at io n (n um be r) a re a (h a) l oc at io n (n um be r) a re a (h a) l oc at io n (n um be r) a re a (h a) bare rocks 8 74 complex cultivation patterns 4 15 5 151 6 500 land principally occupied by agriculture, with significant areas of natural vegetation 15 131 4 47 12 5137 3 605 mineral extraction sites 1 10 natural grasslands 15 181 8 173 15 4258 1 148 non-irrigated arable land 9 204 5 135 3 1266 pastures 2 127 permanently irrigated land 10 2645 18 8191 sparsely vegetated areas 13 237 6 56 3 9 transitional woodland-shrub 6 58 2 242 2 628 mq : marble quarries; aq: aggregate quarries; wb: water bodies; ba: built-up areas considering the previous land cover of the total areas occupied by aq class (819 ha), a 29.5 percent (242 ha) of the lands previously was covered by the “transitional woodland-shrub”, a 24.9 percent (204 ha) by the “nonirrigated arable land” and a 21.1 percent (173 ha) by the “natural grasslands” (table 3). on the other hand, it is found that 58.1 percent of the aq locations were opened in the “non-irrigated arable land”, “natural grasslands” and the “bare rocks”. here, it is worth pointing out that the lands covered by the “bare rocks” have been only transformed by aq activities. as can be seen in table 3, 9 percent (74 ha) of the total area dominated by aq activities have taken place in the “bare rocks” class. it is clear from the table 3 that 39.6 percent (5137 ha) of the total land modified by wb activities (12962 ha) was previously occupied by the “land principally occupied by agriculture, with significant areas of natural vegetation”, 32.8 percent (4258 ha) by the “natural grasslands” and 20.4 percent (2645 ha) by the “permanently irrigated land”. it has also been found that 71.1 percent of locations assigned as wb were distributed over the same land cover classes (table 3). considering the previous land cover of the total areas occupied by ba class (10836 ha), a 75.6 percent (8191 ha) of the lands previously was covered by the “permanently irrigated land”, 11.7 percent (1266 ha) by the “nonirrigated arable land” and a 5.6 percent (605 ha) by the “land principally occupied by agriculture, with significant areas of natural vegetation” (table 3). moreover, it is found that 72.7 percent of the ba locations were constructed in the “permanently irrigated land” and “complex cultivation patterns”, based on the land cover classification (table 3). discussion considering the total areal change over the past forty-four years, it is understood that a total area of 25224 ha of land has been modified with an average annual expansion rate of 573.2 ha over the 177 locations in diyarbakır. the 51.3 percent (12962 ha) and 42.9 percent (10836 ha) of the total modified lands are wb and ba respectively. the aq and mq are responsible for 5.6 percent (1426 ha) of the total modification. the wb activities can be attributed to dams, hydroelectric power plants, ponds and canals, which have been built in order to control, store and supply water for energy production and irrigation. the variations in the water level of wb elements and irrigation periods have a direct impact on the spatial extension of the wb elements. the satellite images were captured in may-june when the water level of wb elements is increasing and covering nearby areas. wb class recorded its most upward trend in the years before 2000 due to “southeastern anatolia project (gap) and continued at a relatively stable trend in the years after 2000 (tckb 2012). the ba modifications can be attributed to the extensive migration from rural remote areas to the city center, population and the need for additional settlements. the population of the diyarbakır has barely trebled in 50 years and increased from 581.208 in 1975 to 1.756.353 in 2019 (tüi̇k, 2019) due to natural (e.g., topography, unfavorable soil conditions and being at the riverside) and anthropogenic factors (e.g., surrounded by city walls, industry and military areas). most of the ba elements, especially those constructed in the last decade are characterized by residential (kayapınar district), commercial and industrial structures (yenişehir district, see: figure 1). prior to urbanization, these zones were less populated and were mostly covered by “permanently irrigated land”. the ba class includes infrastructures such as underground passages and bazaars, water supply networks, shelters, sewerage systems, that cannot be detected by the satellite images. urbanization increases the demand for construction materials such as aggregates and stone blocks. despite a general trend of increase in the size of aq, a minor reduction of the expanse was monitored between 2000 and 2005 (table 1). this decrease observed in the size of the aq activities may be attributed to the status of the quarries as ceased or abandoned in that period. although there was no increase in the number of aq between 2015 and 2019 (43 quarries), significant changes were observed in the size increase (364 ha). another degradation impact can be associated with the type of excavated rock material. although the number of aq opened in basalt has increased in the last decade, most of the aq in the study area have been opened in limestone beds. the aggressively increasing of marble quarries in the last three decades threatens the lulc in diyarbakır city. those quarries have been excavated in lower miocene reef limestones (bağırsakçı et al., 1995; erdoğan and yavuz, 2002) in hani, çermik, hazro and ergani districts (figure 1). the increase in the number of mq is highly related to the demand for construction material in the region and export as a natural stone to asia, europe, middle east and the united states, (especially after 2005). varol, aydın and ayhan (2019) point that between 2004 and 2018, the exported marble products account for 127using satellite imagery to assess the changes in land use and land cover in diyarbakır city (se turkey) approximately 45 percent of total export revenues of diyarbakır, and 3 percent of the total marble export revenues of turkey. according to field surveys conducted on quarry sites, mq seem to be responsible for the removal of topsoil and subsoil, deforestation, agricultural land degradation, massive holes, slope modification, and waste sites. in diyarbakır, a total of 25224 hectares of land in 177 locations has been modified based on the studied and corine’s land cover classes shown in tables 1 and 2. 42.9 percent (10836 ha) of this modification was previously “permanently irrigated land”, which were modified by wb (75.6 %) and ba (24.4 %). the “land principally occupied by agriculture, with significant areas of natural vegetation” is the second land cover class (5920 ha-23.5% of the total land modification, table 2) that highly modified over anthropogenic activities. it is noteworthy that, this land cover class has been modified by all the studied classes, mostly by wb (86.8 %) and ba (10.2%) activities. the “natural grasslands” is the third highest alteration land (4760 ha-23.5% of the total land modification, table 2). this class has been mostly modified by the wb (89.4 %) and mq (3.8%) activities. in an attempt to compare the outputs of the present study to the nearby provinces, the literature was reviewed. unfortunately, apart from a few studies (akay and sertel, 2016; özcan et al., 2018) (focusing mostly on limited periods and specific effects), nothing has been reported on anthropogenically driven impacts on the lands of nearby provinces. under the comprise of this study, based on lulc, four class wb, ba, aq and mq were evaluated. other studies more or less preferred similar classes; in addition, agriculture, forest and rarely industry were also taken into consideration in turkey (alphan, 2003; sivrikaya et al., 2007; çakır et al., 2008; keleş et al., 2008; reis, 2008; bozali et al., 2015; altürk and konukçu, 2020; atay kaya and görgün, 2020). the land modification due to natural and anthropogenic effects were also discussed in detail by the previous studies in different countries (roberts, 1989; gollin et al., 2016; roberts, 1989; wu and zhang, 2012; yang and cai, 2003; akinyemi and mashame, 2018; matlhodi et al., 2019; obiahu et al., 2021; liaqat et al., 2021). hydrological system variations, such as river discharge, surface runoff, flooding, water supply, and land cover changes due to dam reservoirs, are addressed in various water-related lulc studies. as seen in the botswana, the seasonal fluctuation of wb activities may not be permanent and may revert to the previous land cover (akinyemi and mashame, 2018; matlhodi et al., 2019). furthermore, lulc changes are responsible for bangladesh’s increasing floods (rahman et al., 2021). those changes including anthropogenic factors, compositions of natural vegetation and grassland were also change the ecosystem and hydrological system (freshwater supply, water quality and flood) in ethiopia, at least controlled landscape decrease the erosion and decrease the sediment input to the river (yohannes et al., 2021). similarly, besides the climate change, lulc changes can increase the uncertainty of hydrological variability in catchment area of river including runoff and natural recharge process and evapotranspiration (tuladhar et al., 2019). some of the water body like in diyarbakır in se turkey is built on the rivers flowing across borders (tigris and euphrates flow in syria and iraq). thus, the changes in those bodies have related to neighboring countries. population blooming as in diyarbakır may also significantly control land use change. dependently the impact of economic growth and industrialization on urbanization like in china has been widely discussed in the literature (gollin et al., 2016; roberts, 1989; wu and zhang, 2012; yang and cai, 2003). economic and social development of the regions like in ebonyi state in se nigeria promote population increase and thus rapid transformation of the lulc pattern (obiahu et al., 2021). urbanization has also effects on the land degradation such as decreasing of green area and lowering of groundwater, thus strict monitoring is required for better understanding the adverse effect of lulc (liaqat et al., 2021). urbanization sourced lulc documented in some big cities of turkey, such as adana (alphan, 2003), tekirdağ (altürk and konukçu, 2020), bandırma (atay kaya and görgün, 2020), samsun (güler et al., 2007), i̇stanbul (çakır et al., 2008), and rize (reis, 2008). their results show that significantly increasing in urbanization area. the increasing of urbanization causes to decrease in agriculture and forest areas (reis, 2008; altürk and konukçu, 2020; atay kaya and görgün, 2020). quarrying operations have a negative influence on the environment at several stages of the process, including as exploration, blasting, excavation, crushing, transportation, and waste disposal (ashmole and motloung, 2008). siddiqui and jain (2021) proposed that mining activities are the main factors for lulc in jharia coalfield in india. pollution hotspots due to marble quarry activities including sawdust, marble debris, sludge have been studied using the gis in mohmand marble zone in pakistan (fawad et al., 2021). they reported that the marble waste threatens the flora and fauna healthy in both aquatic and terrestrial environments. as a result of the type of excavated material (limestone) and the increase in the number of marble and aggregate quarries, the negative impact of aq and mq operations on diyarbakır’s lulc escalates and spreads. these quarries also cause slope modification, holes, and scars, making occupied land prone to geohazards, as highlight by mossa and james (2013). mq activities, on the other hand, have an impact not only on the site where quarrying operates, but also on the land nearby (castro-gomes et al., 2012; werner et al., 2019). considering the components of the quarrying activities and location of the quarry sites in the study area (figure 3), most of them occupied and modified additional land for different purposes (e.g., tailing dams, access roads, water storage ponds, block cave areas, waste rock dumps and housing for workers) to excavate, deposit and transport the material. the type of excavated rock material is also important considering the degradation rate of the land. drew, langer and sachs (2002) discussed in their study and revealed that quarrying activities operated in a sedimentary rock like limestone may disturb the land cover more than a magmatic one such as granite or basalt. in some lucl analysis, vegetation and urbanization have reverse relations, an increase of urbanization in the nile delta of egypt cause decreasing in vegetation and also led to high land surface temperature development (abd el-hamid, 2020). urbanization, industrialization (plants, mining activities, etc.), mismanaged agricultural activities, and deforestation are all human impacts that disrupt the ecohydrological system (yohannes et al., 2021). it is known that the impact of humans on the landscape varies by region. while forest areas increase in the arctic and alpine terrain, dry and semi-arid regions see a decline in vegetation. furthermore, it has the potential to create local extreme temperature variations (hassan et al., 2021), resulting in frequent and severe floods (rahman et al., 2021). additionally, lulc and agricultural activities (e.g., crops and livestock) are significant sources of greenhouse gas emissions that must be regulated under the paris agreement (kalnay and cai, 2003; hong et al., 2021). lulc is both a cause and a consequence of global environmental change (song et al., 2018). they are impacting the earth’s energy balance, as well as the climate, hazards, and ecosystems both locally and globally (reis, 2008; song et al., 2008; altürk and konukçu, 2020; hong et al., 2021; yohannes et al., 2021; yohannes et al., 2021). for efficient environmental protection, sustainable development, and the supply of necessary information to decision makers, time-series trend analysis and continuous monitoring of lucl and future mitigation are needed (alqurashi and kumar, 2013; baynard, 2013; abd el hamid, 2020; fawad et al., 2021; hassan et al., 2021). conclusions a set of satellite images covering the years 1975 to 2019 were used to examine the effects of human-induced activities in diyarbakır. based on the findings, it can be inferred that the diyarbakır lands have been significantly modified over the last forty-four years. the findings indicate that wb, ba, mq and aq have been responsible for the vast majority of land degradation activities. wb and ba, it is understood, are two significant classes that have expanded dramatically over the years studied. the 94.2 percent of total land modification is attributed to these two classes. although the wb class has the largest proportion of land modifications (51.3 percent), it should be noted that this proportion is not constant and may change depending on seasonal fluctuations in water level. in light of such factors as rural-to-urban migration, rising population, the need for additional construction space, poor land use planning, and the impact of seasonal fluctuations on the size of lands occupied by wb activities, it can be concluded that the ba class is the most effective agent that will continue to transform the lands of the study area in the future. it is essential to enforce planning regulations with the cooperation of government agencies and local stakeholders in order to control and reduce the adverse impact of urbanisation on arable lands, vegetation, and woodlands. although surface mining activities (e.g., mq and aq) account for a modest percentage of total land modifications (5.6 percent), factors such as the export potential of marble products and the rising demand for raw materials 128 felat dursun, kemal zorlu, murat gül can boost pressure to open new quarries and expand existing ones. as a result, surface mining activities can be considered an effective agent that will continue to modify the study area’s lands in the next years. the environmental degradation generated by such activities has a high potential to damage the environment in terms of geohazard and landscape aesthetic appeal. as a result, local governments must take precautions for land reclamation when quarries’ operations are terminated and abandoned. such studies are vital for understanding the linkages and possible interactions between human activities and nature in order to improve decision-making processes. as a result, this study could be a pioneer in conducting similar studies in neighboring areas where the rate of rural-urban migration, urbanization, and industrialization is on the rise, to assess the impact of human-induced land impacts. acknowledgments the authors wish to thank the dicle university/scientific research project coordination office (dübap), for the financial support under the grant number muhendislik.18.007. the authors would also like to express their gratitude to dr. iliya bauchi danladi for the 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(2018). modeling land use/land cover change using remote sensing and geographic information systems: case study of the seyhan basin, turkey. environmental monitoring and assessment, 190, 494. https://doi.org/10.1007/s10661-018-6877-y https://doi.org/10.1016/j.gsf.2021.101224 https://doi.org/10.1016/j.gsf.2021.101224 https://doi.org/10.3390/s8106188 https://doi.org/10.3390/s8106188 https://doi.org/10.1007/s10661-018-6831-z https://doi.org/10.1007/s10661-018-6831-z https://doi.org/10.3390/w11122444 https://doi.org/10.1016/j.scitotenv.2021.148559 https://doi.org/10.1016/j.scitotenv.2021.148559 https://doi.org/10.1007/s10661-018-6877-y _hlk40706314 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n3.71479 earth sciences research journal earth sci. res. j. vol. 26, no. 3 (september, 2022): 263 270 o r e g eo lo g y mineral composition of albazinskoe deposit ores as an indicator of its belonging to the gold-rare metallic ore-formational type raisa a. kemkina1, igor’ v. kemkin2* 1. far eastern federal university, 8 sukhanova st., vladivostok, russia 2. far eastern geological institute, far eastern branch of the russian academy of sciences, 159 prospect 100-letiya vladivostoka, vladivostok, russia *corresponding author: kemkin@fegi.ru abstract the paper presents new data on mineral composition and geochemical peculiarities of ores from the albazinskoe gold-bearing deposit (khabarovsk region, far east of russia). excepting earlier known ore minerals represented by sulfides of iron, arsenic, lead, zinc, and copper, authors have established about two tens of ore minerals, new for this deposit. among them are sulfides of antimony, bismuth and molybdenum, native bismuth, copper, nickel, silver, tellurides of bismuth, cobalt sulphoarsenite, nickel sulphoantimonite, silver sulphobismuthites, lead-antimony-bismuth sulphosalts, oxides of tin, titanium, tungsten and some others. the revealed specificity of the ores’ material composition indicates this deposit belongs to the gold-rare metallic ore-formational type. the sets of geological and structural data show that gold-bearing deposits of this ore-formational type are spatially and genetically associated with the granitoid magmatism, which is exhibited within transform continental margin and related to the geodynamic mode of sliding of the continental and oceanic lithospheric plates. keywords: gold-bearing deposits; rare metals; sikhote-alin; transform continental margin. composición mineral de menas del depósito albazinskoe como un indicador de su pertenencia al tipo formativo de mineral metálico poco común de oro resumen este artículo presenta nueva información sobre la composición mineral y las características geoquímicas del depósito aurífero de albazinskoe (región de jabárovsk, extremo oriente de rusia). a excepción de vetas minerales conocidas previamente y representadas por sulfuros de hierro, arsénico, plomo, zinc y cobre, los autores encontraron una veintena de minerales nuevos para este depósito. entre estos se cuentan los sulfuros de antimonio, bismuto y molibdeno, bismuto autóctono, cobre, níquel, plata, telurios de bismuto, sulfoarsenito de cobalto, sulfoantimonita de níquel, sulfobismutitas de plata, sulfosales de plomo-antimonio-bismuto, óxidos de estaño, titanio, tungsteno, entre otros. las características encontradas en la composición de los materiales de mena indican que este depósito pertenece al tipo formativo de mineral metálico poco común de oro. la configuración de información geológica y estructural muestra que los depósitos auríferos de este tipo están espacial y genéticamente asociados con el magmatismo granitoide, el cual se observa en el margen de transformación continental y que se relaciona con el modo geodinámico de deslizamiento de las placas litosféricas continentanles y oceánicas. palabras clave: depósitos auríferos; metales raros; provincia aurífera sikhote-alin; transformación del margen continental record manuscript received: 04/04/2018 accepted for publication: 25/08/2022 how to cite item: kemkina, r. a., & kemkin, i. v. (2022). mineral composition of albazinskoe deposit ores as an indicator of its belonging to the gold-rare metallic ore-formational type. earth sciences research journal, 26(3), 263-270. https://doi.org/10.15446/ esrj.v26n3.71479 https://doi.org/10.15446/esrj.v26n3.71479 https://doi.org/10.15446/esrj.v26n3.71479 https://doi.org/10.15446/esrj.v26n3.71479 264 raisa a. kemkina, igor’ v. kemkin 1. introduction according to the division scheme for the gold-bearing territories of the russian far east, the albazinskoe gold-bearing deposit is situated in the lower amur zone of the sikhote-alin gold-bearing province (moiseenko & eirish, 1996). within this mineragenic zone more than ten large and mediumsized deposits are known (mnogovershinnoe, belaya gora, polyanka, agniafanas’evskoe, dyappe, etc.) and more than thirty ore-manifestations. based on the mineralogical composition, ores of deposits are referred to the goldquartz and gold-sulfide ore-formational types. gold mineralization of the lower amur area is spatially and genetically associated with dikes and different-sized granitoid plutons of heightened basicity, subvolcanic intrusions medium-acidic composition and sometimes with volcanic neck facies, represented by basaltliparite rocks (izokh et al., 1967; moiseenko & eirish, 1996). it is believed that the disposition of gold-bearing deposits in the lower amur zone is subordinated to the regional ore-magmatic zoning, formed either around the orogenic dome that emerged over a deep granitic hearth (moiseenko & eirish, 1996; zvezdov, mishina, 2010) or around a central type megastructure generated by a mantle plume (didenko et al., 2010; moiseenko & sakhno, 2000; yushmanov, 2012). according to these opinions, in the central parts of the dome structure (or megastructure), tin and, to a lesser extent, molybdenum, and more rarely highfineness gold mineralization are developed. in the marginal parts and in nonmetamorphosed rocks of the intrusive frame, later gold-quartz mineralization is localized, and on the periphery in the terrigenous rocks low-temperature goldsulfide mineralization with low-fineness gold is formed. in the volcanogenic and subvolcanic formations associated with terrigenous complexes, gold-low sulfide (poor sulfide) mineralization is developed, sometimes with tellurium. detailed mineralogical-geochemical studies of the albazinskoe deposit ores have shown that except sulfides represented by pyrite, arsenopyrite, galena, sphalerite, pyrrhotite, and chalcopyrite, which are usually present at the goldsulfide deposits, the ore minerals of rare metal elements are widely developed at this gold-bearing deposit. these minerals are as following: sulfides of antimony, bismuth and molybdenum, native bismuth, copper, nickel, silver, tellurides of bismuth, cobalt sulphoarsenite, nickel sulphoantimonite, silver sulphobismite, lead-antimony-bismuth sulphosalts, oxides of tin, titanium, tungsten and some others. the presence of minerals of the chalco-and siderophilic elements (te, bi, se, sb, sn, hg, ni, co, mo, w) in the albazinskoe deposit ores gives us the ground to classify it, according to (gamyanin, et al., 1998; goryachev, 2006; osipov, sidorov, 1973; shilo et al, 1972; volkov et al., 2007, etc.), as gold-rare metallic deposit. figure 1. tectonic scheme of the sikhote-alin region and adjoining areas (after kemkin et al., 2016) 265mineral composition of albazinskoe deposit ores as an indicator of its belonging to the gold-rare metallic ore-formational type 2. methods of research the study of ore samples was carried out in polished sections using both traditional mineralogical methods and modern methods of investigating of the material composition. the first group of methods included the study and identification of ore and non-metalliferous minerals under the optical microscope in reflected light based on physical, morphological and optical properties of minerals, their texture and structural features, as well as the relationships of individual ore minerals and their assemblages. the second group of methods was used for determination of the elemental composition of the minerals and for recalculating the obtained data into crystalochemical formulas. the elemental composition of ore minerals was determined by applying a scanning electron microscope evo-50 xvp with an energy dispersive x-ray spectrometer inca-350 at the laboratory of microand nano-research of the far eastern geological institute of the far eastern branch of the russian academy of sciences. scanning electron microscope was operated at acceleration voltage 20 kv and resolution 1 μm, analyst kemkin i.i. the following standards were used: for s, fe fes2 10-jul-2008 03:47 pm, co co 10-jul-2008 03:31 pm, ni ni 1-jun-1999 12:00 am, as inas 1-jun-1999 12:00 am, sb sb 1-jun1999 12:00 am, se se 1-jun-1999 12:00 am, pb pbf2 1-jun-1999 12:00 am , bi bi 1-jun-1999 12:00 am, ag ag 1-jun-1999 12:00 am, te hgte 1-jun-1999 12:00 am, cu cu 1-jun-1999 12: 00 am. 3. brief geological setting the albazinskoe gold-bearing deposit is located on the left bank of the estuary part of the amur river in the interfluve of its left tributaries amgun’ and somnya (khabarovsk region). structurally the deposit is situated within the ul’ban terrane of the jurassic turbidite basin of the mongolo-okhotsk orogenic belt (figure 1). the territory of the albazinskoe deposit is composed of the jurassic terrigenous strata that are intruded by late cretaceous plutons and subvolcanic bodies of diorites, granodioriteand granite-porphyrites and rhyodacites. the ore bodies of the deposit are formed by conjugated dikes of the above magmatic rocks and consist of sericite-carbonate-quartz metasomatites, brecciated rocks with streaky-impregnationed quartz-sulphide mineralization and silicified sandstones with impregnations of sulphides. the deposit belongs to the gold-quartz poor-sulphide ore-formational type (moiseenko & eirish, 1996) and is characterized by a small amount of ore minerals, the content of which is 1-5%. the most common ore minerals of the deposit are pyrite and arsenopyrite. less common ore minerals are marcasite, galena, sphalerite, pyrrhotite, chalcopyrite, scheelite and fahlores. the vein minerals are quartz, feldspars, carbonates, chlorite, and mica. 4. main research results and discussion in order to clarify minerals that concentrate gold mineralization the mineralogical and geochemical studies of ores of the albazinskoe deposit have been carried out. as a result, we have revealed about twenty ore minerals that were not described in this deposit earlier. among them there are sulfides, tellurides, selenides, sulphosalts, sulphoarsenites, sulphoantimonites, sulphobismuthites, tungstates and oxides of such metals as bismuth, antimony, molybdenum, cobalt, nickel, lead, silver, tin, titanium, tungsten and some others. several elements (bi, cu, ni, ag) were also found in a native form. a brief description of some newly discovered minerals is given below. bismuthine (bi2s3) is one of the most common minerals at the deposit. it occurs both as isolated grains and in intergrowth with other ore minerals. in the form of isometric inclusions it forms a rare impregnation within the allotriomorphic grains of chalcopyrite. bismuthine closely intergrows with native gold (figure 2a), containing an admixture of mercury up to 4.39 wt. %. the boundaries between these minerals are rectilinear, less often meandering, which indicates their near-simultaneous crystallization. in addition, bismuthine is found in intergrowth with the iron hydroxides, in which it forms elongated figure 2. morphology of grains and character of intergrowths of minerals of the bismuth, molybdenum, antimony and nickel. the samples numbers: a – an5-46/1, b an5c-64/1, c – an4-30/9, d – an4-40/1, e – an3-22/1, f – an2-9/1 266 raisa a. kemkina, igor’ v. kemkin and angular inclusions. often bismuthine forms an agglomeration of grains, which have amoeboid shape (figure 2b). the size of individual grains does not exceed 5 μm, and mineral aggregations of 20 μm. the data on the chemical composition of bismuthine (table 1) demonstrate the presence of impurities in its crystalline lattice in the form of table 1. chemical composition of minerals of bismuth, antimony, nickel, cobalt, silver and lead sample element (wt. %) mineral ni co sb as s fe bi pb ag se cu an5-46/1 2,80 19,00 1,02 77,18 bismuthine an5-55/1 16,73 2,89 76,22 2,44 1,72 bismuthine an5-57/1 17,01 5,11 76,56 1,32 bismuthine an5-59/1 3,31 18,29 1,54 74,38 2,47 bismuthine an5a-63/1 16,93 0,73 78,88 2,01 1,33 bismuthine an5a-64/1 18,07 81,93 bismuthine an5a-64/2 18,08 79,13 2,79 bismuthine an5a-66/1 3,44 18,55 76,94 1,08 bismuthine an5a-67/1 2,94 18,45 76,79 1,82 bismuthine an5a-73/1 16,55 3,56 75,27 2,11 2,50 bismuthine an5a-74/5 17,26 76,76 2,77 3,21 bismuthine an5a-80/2 1,39 18,07 77,69 1,37 1,47 bismuthine an2c-9/1 5,76 16,09 4,39 45,77 27,00 matildite an2c-10/1 6,78 15,50 5,72 46,68 25,32 matildite an2-14/1 9,94 16,49 0,72 33,32 35,53 1,64 kobellite an2-14/9 9,31 18,66 0,86 31,05 38,22 1,13 kobellite an2-14/9a 9,16 17,96 0,94 30,16 39,14 0,98 kobellite an2-9/1 25,18 1,05 56,18 2,49 15,06 ullmannite an2-10/1 26,21 1,51 54,64 2,76 14,89 ullmannite an3-4/1 2,70 20,42 42,13 22,56 6,82 cobaltite an4-25/1 5,73 21,75 43,77 21,18 7,57 cobaltite an3-16/3 4,44 23,46 46,24 20,00 5,52 cobaltite an3-26/1 5,48 22,54 46.39 20,10 5,49 cobaltite an3-29/1 7,28 21,80 47,59 19,85 3,48 cobaltite an3-29/9 70,98 28,05 antimonite an5-22/1 71,55 28,45 antimonite an2a-11/1 31,39 18,80 1,33 48,46 heteromorphite an2a-12/1 30,94 18,87 1,29 48,87 heteromorphite an2a-6/3 30,34 2,73 21,01 7,48 38,38 jamesonite an2a-7/2 33,99 20,98 4,36 40,63 jamesonite an2a-7/4 33,04 21,02 4,85 41,09 jamesonite 1d-32-1/1 99.25 0.75 native ni 1d-32-1/2 99.38 0.62 native ni 1d-44-1/1 99.07 native ag 1d-44-1/2 96.06 native ag 1d-45-1/1 97.9 native cu 1d-45-1/2 98.2 native cu 1d-18-1 91.68 native bi an5-28-1 93.66 native bi atoms of iron 1.02 5.11 wt. % , stibium 2.8 3.31 wt. % , selenium 1.32 2.47 wt. % and silver 2.44 wt. %. the calculated crystalochemical formulas of bismuthine correspond to the theoretical ones, but with a slight sulfur deficit, for example bi2.05s2.95 or (bi1.76fe0.14sb0.13)2.03(s2.82se0.15)2.97. 267mineral composition of albazinskoe deposit ores as an indicator of its belonging to the gold-rare metallic ore-formational type tellurobismuthite (bi2te3) is present in the form of monomineral inclusions up to 3 μm in quartz. the shape of mineral grains is isometric, elongated, angular (figure 2c). tellurobismuthite also occurs in intergrowths with sulphobismutites of lead, along with which it replaces idiomorphic crystals of cobalt-bearing (up to 1.78 wt. %) arsenopyrite. тhe replacement of arsenopyrite takes place both from the inside and from the periphery of crystals. the results of chemical analysis of tellurovismutite are satisfactorily recalculated to the formulas bi2.05te2.95 or bi2.11te2.89, although a slight excess of bismuth is observed in this mineral (table 2). hedleyite (bi7te3), analogously as tellurovismutite, forms an isolated monomineral rounded grains in quartz. the size of grains is up to 10 μm. the recalculation of data of chemical composition of this mineral to the theoretical formula (bi7.48te2.52) also shows a heightened amount of bismuth in its crystalline lattice. tsumoite (bite) occurs in the form of small (not more 2 μm) rounded or slightly elongated mineral grains (figure 2c). often congestions of tsumoite grains are spatially associated with native gold, tellurovismutite and sulphobismutites of lead. the calculated crystalochemical formula of tsumoite corresponds to bi0.93te1.07. molybdenite (mos2) is a rare mineral of the deposit and occurs both in the form of thin-scaly congestions and aggregates of leafy or rosette-like form (figure 2d). usually molybdenite is located in sericite-carbonate-quartz veins and veinlets, in which it forms a pocket-like congestions up to 15 μm in size. the calculated crystalochemical formula of molybdenite corresponds to the theoretical one (table 2). antimonite (sb2s3), analogously as molybdenite, occurs in the form of isolated isometric and lamellar inclusions in quartz, the size of which varies from 1 to 3.5 μm (figure 2e). the calculated crystalochemical formula of antimonite corresponds to the theoretical one. ullmannite (nisbs) is found in the form of isolated single grains in metasomatically changed rocks. the intergrowths with other ore minerals are not observed. ullmannite is characterized by allotriomorphic grains, which have both isometric and elongated shapes (figure 2f). the size of elongated grains is 15x8 μm and isometric ones is 3 5 μm. the grain boundaries are sinuous, slightly jagged. chemical composition of the ullmannite is characterized by stoichiometry (table 1) and corresponds to the extreme ullmannite series. the chemical analyses data demonstrate the presence of impurities of cobalt (up to 1.51 wt. %) and arsenic (up to 2.76 wt. %), which, obviously, isomorphically replace nickel and antimony in the crystalline lattice. the calculated on 3 atoms crystalochemical formulas of the ullmannite are as follows: (ni0.91co0.04)0.95(sb0.98as0.07)1.05s1.0 and (ni0.94co0.05)0.99(sb0.95as0.08)1,03s0.98, which corresponds to the theoretical composition. cobaltite (coass) is much more common than ullmannite. taking into account the morphology of mineral grains and their size and also relationships of this mineral with other minerals, two generations of cobaltite are distinguished in the ores of the albazinskoe deposit. cobaltite of the first generation occurs in the form of rounded (less often lenticular) grains (figure 3a) in the cobaltbearing pyrite and non-metalliferous minerals. the size of such grains does not exceed 3 μm. cobaltite 1 forms also a relict rims (figure 3b) up to 1 μm in thick around grains of pyrite containing inclusions of galena, which indicates its earlier crystallization. cobaltite of the second generation (figure 3c, d), on the contrary, is present in the form of idiomorphic crystals with well-defined crystallographic shapes of cubic and octahedral habitus. the average crystals size is 10-20 μm. the boundaries of cobaltite 2 with other minerals always are clear and straight. cobaltite 2 often contains inclusions of chalcopyrite and sphalerite, with which forms close intergrowths in most cases. sometimes cobaltite 2 develops upon the pyrite and, thus, replaces it. the study of chemical composition of the different-aged cobaltite generations showed that they also differ by the content of the main chemical elements (cobalt, arsenic and sulfur), and impurity elements (iron and nickel). chemical composition of the cobaltite 1 (an3-4/1 and an4-25/1, table 1) is characterized by heightened iron contents (up to 7.57 wt. %) and low contents of nickel. crystalochemical formulas of the cobaltite 1 are as follows: (co0.58fe0.21ni0.08)0.87as0.95s1.18 or (co0.60ni0.16fe0.22)0.88as0.95s1.07. it should be noted a slight deficit of cobalt and arsenic and excess of sulfur compared with the theoretical composition. the chemical composition of cobaltite 2 (an3-16/3, an3-26/1, an3-29/1, table 1) includes the same list of isomorphically substituting components, but the concentrations of them is significantly differ. cobaltite 2 is characterized by high concentrations of nickel (up to 7.28 wt. %), but iron contents is significantly lower (up to 3.48 wt. %) compared with cobaltite 1. it should be also noted somewhat heightened contents of the main components (cobalt and arsenic) in cobaltite 2 compared with cobaltite 1. as a whole, cobaltite 2 is characterized by a slight deficit of elements in the cationic group, and, on the contrary, an excess in the anionic group. the calculated crystalochemical formula of the cobaltite 2 corresponds to (co0.61-0.66fe0.10-0.16ni0.13-0.21)0.92-0.95as1.01-1.05s1.03-0.04. the as/s ratio for different-aged cobaltite generations also differs and makes up 1.9-2.1 for the cobaltite 1, and 2.3 for the cobaltite 2. thus, from the early to late generations of cobaltite the concentrations of sulfur and iron are decrease, but the contents of cobalt, nickel and arsenic are increase. table 2. chemical composition of minerals of bismuth-tellurium, molybdenum and tin sample element (wt. %) mineral mo sn mn o s fe bi te an4-40/1 61,34 38,42 molybdenite an4-41/1 61,83 37,20 molybdenite an5-11/1 76,61 20,52 2,84 cassiterite an5-12/1 74,97 21,86 3,19 cassiterite an5-16/1 75,81 21,23 2,93 cassiterite an4-30/1 53,31 46,38 tellurobismuthite an4-30/9 51,99 46,75 tellurobismuthite an4-28/1 54,48 45,53 tellurobismuthite an4-28/9 51,72 47,22 tellurobismuthite an5-40/1 82,93 17,07 hedleyite an5-40/9 81,78 18,22 hedleyite an5-50a/1 58,49 41,51 tsumoite an5-50a/9 46,25 53,75 tsumoite an5-51/9 61,13 38,44 tsumoite 268 raisa a. kemkina, igor’ v. kemkin figure 3. morphology of grains and character of intergrowths of minerals of the cobalt, silver and lead. the samples numbers: a – an3-4/1, b – an3-16/3, c – an329/1, d – an3-26/1, e – an2с-9/1, f – an2с-10/1. in addition, cobalt and nickel are also found in the form of an impurity in arsenopyrite and pyrite. the content of cobalt in the relic grains of arsenopyrite is up to 2.34, and in pyrite up to 3.51 wt. %. the content of nickel is up to 1.85 and up to 2.32 wt. %, respectively. matildite (agbis2) is a rare mineral in the albasinskoe deposit ores. it was found in the arsenopyrite grains voids in the form of allotriomorphic inclusions up to 3 μm in size (figure 3e). the boundaries between these minerals are corrosional in most case that indicates a later crystallization of matildite. beside that, the close intergrowths of matildite and galena along the grain boundaries of arsenopyrite are also observed (figure 3f). the size of matildite grains does not exceed 1-1.5 μm. it is noteworthy that the galena associated with matildite contains an impurity of silver up to 6 wt. %, while the matildite is devoid of lead, but it contains impurities of iron and arsenic (table 1). according to the chemical analyses data the crystalochemical formula of the ascertained matildite is (ag0.88fe0.28)1.16(bi0.79as0.27)1.06s1.78. kobellite (pb6febi4sb2s16) occurs very rare. it is observed in the form of isolated isometric grains with jagged outlines in metasomatic rocks. the intergrowths of kobellite with other ore minerals have not been seen. recalculation of the chemical analysis data (table 1) on 29 atoms corresponds to the following formula (pb5.15cu0.78fe0.39)6.32bi4.78sb2.45s15.45, that clear show a slight impuritiy of copper in this mineral. jamesonite (pb4fesb6s14) is more common than kobellite in the ores of the deposit. it was found in the central parts of arsenopyrite grains conjointly with galena and antimony-bismuth sulphosalts of lead. jamesonite forms a rounded grains and elongated aggregates up to 2 5 μm in size. the data of chemical composition of this mineral (table 1) show that the contents of main mineral-forming components (lead, stibium and sulfur) vary insignificantly, but the concentration of iron is higher than in theoretical crystalochemical formula (from 4.35 to 7.48 wt. %). the averaged calculated crystalochemical formula of the jamesonite corresponds to pb4.06fe1.61sb5.78s13.55. heteromorphite (pb7sb8s19) is mainly developed in sulfide minerals in the form of elongated-prismatic crystals with slightly rounded facets. the grains size varies from 2.5 to 5 μm. heteromorphite is also found in host metasomatic rocks in the form of isolated grains. this mineral has a near-theoretical chemical composition, although in some grains the impurity of iron (up to 1.33 wt. %) is observed (table 1). crystalochemical formula of the ascertained heteromorphite is as follows – (pb6.98fe0.74)7.72sb8.03s18.25. the character of the interrelationships between the above-mentioned sulphoantimonites of lead and antimony-bismuth sulphosalts with other ore minerals indicates their later crystallization with respect to the main sulfides. cassiterite (sno2) is found in pyrite grains in the form of prismatic, wedge-like, and less often isometric shape debris (figure 4a). the size of debris is 3 6 μm. according to the chemical analysis this cassiterite is characterized by present of iron impurity (table 2). native metals (ni, ag, cu, bi) occur in the form of isolated grains of allotriamorphic and near isometric form (figure 4b-d). their sizes range from 0.7 to 18 μm. these minerals do not form intergrowths with other ore minerals, as well as with each other. usually they fill voids in the quartz, calcite or are located in the intergranular space in metasomatites. the chemical analysis data (table 1) demonstrate their exceptional chemical purity, with the exception of native nickel. the individual grains of this mineral are characterized by a zonal structure, which is fixed by the differences in color shades of central and marginal parts of grains (figure 4b). data of chemical analysis show that the marginal (a lighter shade) areas of such grains contain sulfur up to 0.62 wt. %, but are devoid of iron, while the central (a darker) areas, on the contrary, contain iron up to 0.75 wt. % and sulfur free. figure 4. morphology of grains and character of intergrowths of minerals of the tin and native metals. the samples numbers: a – an5-16/1, b – 1d-32-1/1, c – 1d-441/1, d – 1d-44-1/2. the revealed features of mineral composition of ores of the albazinskoe deposit and geochemical specificity of ore minerals indicate its belonging to the gold-rare metallic ore-formational type. previously deposits of such formational type were not described within the lower amur zone (as it is known (moiseenko & eirish, 1996), all gold-bearing deposits and ore-manifestations in the area of lower stream of the amur river have been attributed either to gold-quartz or gold-sulphide ore-formational types). gold-rare metallic deposits are widely distributed and well studied (gamyanin et al., 1998; goryachev, goncharov, 1995; lang et al., 2000; newberry et al., 1995; osipov, sidorov, 1973; shilo et al., 1972; volkov et al., 2007 and references in them), which made it possible to identify a number of characteristic criterions for them. besides the mineralogical-geochemical individuality of ores, the form of gold mineralization is also very indicative. at the albazinskoe deposit (as well as at other deposits of this type) gold is present in the form of fine-dispersed impregnations and submicroscopic grains in arsenopyrite, less often in other sulfides, as well as in native bismuth and tellur-bearing minerals. the other identification characters of the gold-rare metallic deposits are structural-morphological types of ore bodies. as a rule, ore bodies are represented by simple or branching veins (or series of closely located veins) conjugated with dykes and subvolcanic bodies of granodiorite-granites and which are composed of sericite-carbonate-quartz metasomatites, and / or by tectonized metasomatic zones (mineralized crushed zones) comprising streakyimpregnationed mineralization and which are superimposed on granitoid dykes and subvolcanic bodies and surrounding terrigenous rocks. more often a combination of crushed zones and metasomatic veins take place. 269mineral composition of albazinskoe deposit ores as an indicator of its belonging to the gold-rare metallic ore-formational type the present of a significant amount of minerals of the chalcoand siderophile elements in the ores of gold-rare metallic deposits allows us to presuppose that the source of magmatic melts generating ore mineralization is located much deeper than the depleted upper mantle. such source can be asthenospheric injections (figure 5) penetrating through a ruptures in a subducted oceanic plate (so-called slab-windows) (khanchuk, 2000; khanchuk, ivanov, 1999; cao et al., 2017; peng et al., 2018). the arrising of ruptures is explaned by changing of a geodynamic mode of subduction of an oceanic plate to a geodynamic mode of its sliding along continent margin (khanchuk, 2000; khanchuk et al., 2016; peng et al., 2018). the changing of direction of an oceanic plate motion with respect to the continental margin is resulted in large– scale lithosphere extension, it thinning and tearing that is led to the upwelling of asthenospheric material. figure 5. model of formation of granitoids of the gold-rare metalic specialization. in the south of the far east of russia (primorsky and khabarovsk territories), the number of the gold-bearing deposits attributed to the gold-rare metallic ore-formational type is not large so far (probably because of insufficient investigation of the mineralogical composition of the ores). an important feature of these deposits is that they are spatially and genetically related to the late cretaceous-paleogene granitoids (gvozdev et al., 2016; dobroshevsky et al., 2017; lotina, 2011) whose petro-geochemical characteristics correspond to the a-type granitoids (khanchuk, 2006; grebennikov et al., 2016). in the recent publications devoted to the a-type granitoids are pointed out that they are formed within the continental margin under conditions of large–scale lithosphere extension that is caused by a geodynamic mode of very slight oblique subduction or parallel (transform) sliding of an oceanic plate with respect to the continent (frost, frost, 2011; grebennikov, 2014; peng et al., 2018, etc.). the geodynamic mode of the convergent (subduction) continental margin for the late cretaceous-paleogene time at the eastern margin of the paleo-asian continent is reconstructed based on the set of other geological data (khanchuk, 2006; grebennikov et al., 2016; kemkin et al., 2016; khanchuk et al., 2016, etc.). as for the northern margin of the paleo-asian continent, the geodynamic mode of transform margin took place here from the middle cenomanian time due to the sublatitudinal orientation of boundary of the continental and oceanic plates (figure 6) (khanchuk, 2006; kemkin et al., 2016; khanchuk, 2000; khanchuk et al., 2016) thus, the mediated genetic relationship between the formation of gold-rare metallic mineralization and the geodynamic mode of the sliding of the lithospheric plates is observed. to further validation of this revealed regularity, an additional more detailed investigations of petrology, geochemistry and geochronology of the associated with gold-rare metallic mineralization granitoids are required. 5. conclusion mineralogical-geochemical investigations of the ores of the albazinskoe gold-bearing deposit have shown that besides the traditional sulfides of iron, arsenic, copper, lead and zinc, containing fine-dispersed impregnations and submicroscopic grains of native gold, there is a wide range of minerals of rare metallic elements, many of which are also form an intergrowth with microscopic gold disseminations. the presence in the ores of the albazinskoe deposit of minerals of the chalcoand siderophilic elements (te, bi, se, sb, sn, hg, ni, co, mo, w) gives the basis to classify it as gold-rare metallic. previously, deposits of such formational type were not described within the lower amur zone of the sikhote-alin gold-bearing province. in contrast, all gold-bearing deposits and ore-manifestations in the area of lower stream of the amur river have been attributed either to gold-quartz or gold-sulphide ore-formational types. figure 6. paleogeodynamic reconstruction on the middle cenomanian paleocene time (after kemkin et al., 2016) the spatial and genetical relationship of the gold-rare metallic deposits with the granodiorite-adamellit-granite rocks, which is revealed in the far east of russia and also other regions, indicates that their formation is related with the geodynamic mode of sliding 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(2010). ore-magmatic systems of volcanoplutonic belts of the east of russia (in russian). ores and metals, 1, 48-59. https://doi.org/10.1016/j.jseaes.2017.01.005 https://doi.org/10.1134/s1819714017050025 https://doi.org/10.1134/s1819714017050025 https://doi.org/10.1016/j.lithos.2015.12.020 https://doi.org/10.1016/j.jog.2016.10.002 https://doi.org/10.1016/j.jog.2016.10.002 https://doi.org/10.1016/j.jseaes.2015.10.023 https://doi.org/10.5382/segnews.2000-40.fea issn 1794-6190 e-issn 2339-3459 earth sciences research journal earth sci. res. j. vol. 21, no. 4 (december, 2017): 183-188. http://dx.doi.org/10.15446/esrj.v21n4.66103 en g in ee r in g g eo lo g y the present study focuses on some experimental laboratory tests using a newly-constructed modified direct shear test apparatus. the single-stage and multistage direct shear tests were performed to determine the shear rate and test scheme of the unsaturated shear test. shear strength parameters of unsaturated soil in different conditions are obtained, and the criteria indicate good agreement with standard theories of unsaturated soil. the nonlinear matric suction failure envelope is determined. some shear strength equations are also fitted through the experimental results. abstract keywords: unsaturated soil; direct shear test; matric suction direct shear test of unsaturated soil peng he1, 2, liuying sun2, zhen wang3, * 1 school of civil engineering and architectur, xi’an university of technology, xi’an 710048, china. 2 college of civil engineering, north china university of water resources and electric power, zhengzhou 450011, china. 3 department of geotechnical engineering, tongji university, shanghai 200092, china. el presente estudio se basa en varias pruebas de laboratorio montadas en una máquina de experimentos recientemente construida y modificada para este objetivo. se realizaron las pruebas monofase y multifase de cizallamiento para determinar el punto de corte y el modelo de cizallamiento no saturado. de esta manera se obtuvieron los parámetros de cizallamiento de suelos no saturados bajo diferentes condiciones y los resultados coincidieron con las teorías evaluadas en esta materia. también se determinó la curva de corte para la succión matricial no lineal. algunas ecuaciones de fuerza de cizallamiento también encajan en los resultados experimentales. resumen palabras clave: suelos no saturados; cizallamiento directo; succión matricial. record manuscript received: : 05/07/2017 accepted for publication: 01/11/2017 how to cite item he, p., sun, l., & wang, z. (2017). direct shear test of unsaturated soil. earth sciences research journal, 21(4), 183-188. doi: http://dx.doi.org/10.15446/esrj.v21n4.66103 ensayo de cizallamiento directo en suelos no saturados 184 peng he, liuying sun and zhen wang introduction expansive soil mostly spreads in arid, and semi-arid areas, and it has been found in more than 40 countries (fredlund, 1993), including 20 regions of china. in addition to the unsaturated state most expansive soil always been in practical application, the study on mechanical properties of expansion soil in the unsaturated state is of great significance (lee, 2010; abbasi et al., 2017). unsaturated soil test features time consuming and expensive in general, mainly due to the low permeability of the unsaturated soil and high air-entry ceramic disc (even though it is permeable but airproof). test samples in triaxial tests are relatively bigger (39.1 mm diameter, 80 mm thickness; 50 mm diameter, 100 mm thickness and 70 mm diameter, 140 mm thicknesses). thus, determining the matric suction requires much more time; comparatively time associated with shear strength testing of unsaturated soils is significantly reduced from the smaller size of test samples, generally 61.8 mm diameter, 20 mm thickness (bishop et al., 1960; bishop et al., 1963; ali et al., 2017; anjum et al., 2017). method experimental procedures the newly-constructed modified direct shear test apparatus the conventional direct shear apparatus has been modified, and now it can control the pore air pressure ua and pore water pressure uw. the direct shear box is placed in a sealed pressure chamber; its internal structure is presented in figure 1. figure 1. schematic diagram of the modified direct shear test apparatus. as shown in figure 1, the soil specimen is placed in a high air-entry ceramic disc with a porous-permeable stone on top. the pore air pressure ua can be controlled by adjusting the air pressure in the pressure chamber, and the air pressure can be directly added to the porous-permeable stone (yasin et al., 2017; shazad et al., 2017). the pore water pressure uw can be controlled by the high air-entry ceramic disc which is placed at the bottom of the soil specimen to provide continuity of the water phase in the soil specimen and under the ceramic disc (basarian and tahir, 2017). nevertheless, with the test time passed by, the air in soil pore may diffuse into the bottom of the high air-entry ceramic disc with the water flow and generated bubbles. the flushing device is placed at the bottom of the test apparatus to exhaust the air under ceramic disc which can speed up the testing process efficiently. the overall test system is controlled by a computer recording the shear strength, horizontal displacement, vertical displacement and water phase inlet and outlet. test soil samples the study is conducted using undisturbed expansive soils from yuzhou, middle of henan, china, and the clay mineral mainly consists of calcium montmorillonite (harith and adnan, 2017). before testing, soil sample of 25% moisture is dispensed and placed in the moist chamber for 24 hours to permit uniform distribution of water, and then expansive soil specimens in different dry density conditions are created by being extruded from lifting jack and using a cutting ring. the diameter and height of the example are 61.8 mm and 20 mm, respectively. the sample is placed in the moist chamber for spare after being pumped and saturated. laboratory test shear rate determination shear rate determination is a crucial problem that directly affects the time and results associated with the shear strength testing of unsaturated soils. in the consolidated undrained shear test, it takes some time for pore water pressure of the soil transferred to the specimen bottom through the specimen or filter paper (rahman et al., 2017; simon et al., 2017). if the shear rate is too high, pore water pressure will not dissipate completely, values of pore water pressure measured at the bottom of the sample will perform lagging, and useful stress parameters cannot be obtained, which apparently makes numerical value measured low. thus, the shear rate should be determined appropriately according to the soil permeability coefficient (fredlund et al., 1997; ismail et al., 2017). several single-stage direct shear tests were carried out at four shear rates on modified apparatus as experimental tests by maintaining a constant the net normal stress, considering the effect of shear rate on test results. the test detail and results are presented in table 1 and figure 2. table 1. tentative experiment scheme for determining test shear rate. figure 2. results of shear displacement and shear stress in different shear rates. 185direct shear test of unsaturated soil as the test results showed, the ultra pore water pressure can dissipate, peak shear stress can remain stable when the shear rate is controlled by 0.012 mm/ min. thus, 0.012 mm/min is proposed as the standard shear rate in the entire test. single-stage shear test of unsaturated soil the loading steps of the matric suction for the single-stage tests were 0 kpa, 50 kpa, 100 kpa, 200 kpa; under different matrix suctions, the vertical stress measures were 100 kpa, 200 kpa, 300 kpa. table 2 summarizes the single-stage direct shear test scheme of expansive unsaturated soil. table 2. the single-stage direct shear test scheme of expansive unsaturated soil. twenty samples of same dry density were prepared once a time and placed in the moist chamber after being pumped and saturated to eliminate the effects on test results from sample preparation. each time the sample was transferred to direct shear apparatus and applied predetermined air pressure until the suction arrived at equilibrium (generally last 12 hours), then the vertical load was applied (maintaining for 12 hours) with keeping the air pressure and vertical stress constant and then be sheared. the shear rate was determined to 0.012 mm/min. terminated shear displacement was set to 6 mm and 9 10 hours. it was needed to guarantee that the peak strength was not surpassed. results and discussion (a) single-stage direct shear test results of unsaturated soil in dry density of 1.4 g/mm3 the effects of shear displacement and shear stress in different matric suction and vertical pressure are presented in figure 3. table 3. single-stage direct shear stress parameters of expansive unsaturated soil in the dry density of 1.4 g/mm3. it can be summarized from figure 3 that shear strength of soil sample increased with the rise in vertical pressure and matric suction. under different matric suctions, the peak shear stress was chosen as failure shear stress if the displacement didn’t arrive 4 mm; if exceeded, it should be 4 mm. shear stress under different matric suctions, and vertical pressure concluded from figure 3 are presented in table 3. figure 3. the results of shear displacement and shear stress (ρd = 1.4 g/cm3) (b) single-level direct shear test data of unsaturated soil in dry density of 1.5g/mm3 figure 4 presents relationships between shear displacement and shear stress in different matric suctions and net vertical pressure. table 4. single-stage direct shear test stress parameters of expansive unsaturated soil in a dry density of 1.5 g/mm3 it can be summarized from figure 4 that shear stress of soil sample increased with the rising of vertical stress and matric suction. table 4 shows the single-stage direct shear test strength parameters under different matric suctions. from table 4 and figure 4 it can be summarized that shear stress increase with the rising of net vertical pressure and matric suction. cohesion value c increased sharply with the rising of net vertical pressure, which can mean that the increase of vertical force has a significant effect on cohesion enhancements; while, the internal friction angle under vertical pressure changed little. 186 peng he, liuying sun and zhen wang figure 4. results of shear displacement and shear stress (ρd = 1.5 g/cm3) (c) single-stage direct shear test results of unsaturated soil in dry density of 1.6 g/mm3 figure 5 presents the results of shear displacement and shear stress under different matric suctions and vertical pressure. figure 5. results of shear displacement and shear stress (ρd = 1.6 g/cm3). it can be summarized from figure 5 that soil shear stress increased with the rising of vertical pressure and matric suction. under different matric suctions, the peak shear stress was chosen as failure shear stress if the displacement didn’t reach 4mm; if exceeded, it should be 4mm. shear stress under different matric suctions, and vertical pressure concluded from figure 5 are presented in table 5. table 5. single-stage direct shear test stress parameters of expansive unsaturated soil in a dry density of 1.6 g/mm3. (d) single-stage direct shear test results of unsaturated soil in dry density of 1.7 g/mm3 figure 6 presents results of shear displacement and shear stress in a dry density of 1.7 g/mm3 under different matrix suctions and vertical pressure. figure 6 allows determining that shear stress of soil samples continuously increased with the rising of matric suction and vertical pressure. shear strength parameters under different matric suction are presented in table 6. figure 6. results of shear displacement and shear stress (ρd=1.7g/cm3). table 6. single-stage direct shear test stress parameters of expansive unsaturated soil in a dry density of 1.7 g/mm3. results analysis shear strength parameters of four dry density specimens under different matric suction and vertical pressure are listed in table 7 187direct shear test of unsaturated soil table 7. direct shear test stress parameters of unsaturated soil. (a) effects on shear strength parameters from matric suction 0 50 100 150 200 0 5 10 15 20 25 30 35 40 co he si on £ k̈p a£ © mat r i c s uct i on£ k̈pa£ © model polynomial adj. r-square 0.97506 value standard error -intercept 4.35391 2.16438 -b1 0.36442 0.05495 -b2 -0.00101 2.54822e-4 dr y dens i t y 1. 4g/ cm3 0 50 100 150 200 10 15 20 25 30 35 40 45 dr y dens i t y 1. 6g/ cm3 c o h e s i o n ( k p a ) mat r i c s uct i on( kpa) model polynomial adj. r-square 0.9946 value standard error -intercept 10.70927 0.97392 -b1 0.28565 0.02473 -b2 -6.18545e-4 1.14664e-4 0 50 100 150 200 5 10 15 20 25 30 35 40 45 dr y dens i t y 1. 5g/ cm3 c o h e s i o n ( k p a ) mat r i c s uct i on( kpa) model polynomial adj. r-square 0.99296 value standard error -intercept 5.63591 1.21969 -b1 0.35034 0.03097 -b2 -8.72818e-4 1.43599e-4 0 50 100 150 200 10 15 20 25 30 35 40 45 dr y dens i t y 1. 7g/ cm3 c o h e s i o n ( k p a ) mat r i c s uct i on( kpa) model polynomial adj. r-squar 0.99225 value standard error -intercept 13.66291 1.12924 -b1 0.25601 0.02867 -b2 -4.93818e1.3295e-4 figure 7. results of cohesion and matric suction in different dry density conditions. the experimental results are represented by a equation including cohesion, internal friction angle and matric suction where s＝ matric suction, c＝ cohesion and ø＝ internal friction angle, a1, a2, b1, b2, b3＝ fitting parameters. fitting curves in different dry density conditions are presented in figure 7 and 8; parameters are listed in table 8. 2 321 ** sasaac ++= )**exp( 2 321 sbsbb ++=f figure 8. results of internal friction angle and matric suction in different dry density conditions. table 8. fitting parameters. (b). effects of cohesion and internal friction angle from dry density values of fitting parameters in table 9 are known parameters. consider the effect on fitting parameters from dry density, fitting curves and parameters are presented in figure 9. table 9. parameters fitted by polynomials. (1) (3) (4) (5) (6) (7) (8) (9) (2) fitting formulas (3) ~ (7) are presented below; fitting coefficients are listed in table 9. 188 peng he, liuying sun and zhen wang figure 9. relationship between dry density and fitting parameters (a): a1; (b): a2; (c): a3; (d): b1; (e): b2; (f): b3. matric suction conditions can be calculated substituting (3) (7) to (8) and (9), cohesion and internal friction angle in any dry density. (10) (11) conclusions methods for unsaturated shear testing are more complicated, more time consuming and more expensive when compared to conventional test methods for saturated soils. an extensive investigation was carried out to study the validity of rapid procedures in obtaining shear strength parameters of saturated and unsaturated soils using the single-stage direct shear test. in this paper, the tests used a newly-constructed modified direct shear test apparatus that uses smaller samples and allows independent control of pore air pressure ua and pore water pressure uw. the shear strength was established using single-stage loading over a range of net normal stresses and matric suction values. the study was carried out using soil samples from the middle route of southto-north water transfer project in yuzhou section. the main conclusions from the study are presented as below: (1) the tests to determine appropriate shear displacement rate was successfully applied before unsaturated direct shear tests. the results of experimental tests showed that 0.012 mm/min is the most suitable shear rate for the samples of yuzhou. (2) it was shown that efficiency of the direct shear test of unsaturated soil is higher compared with the conventional analysis. because of this time-saving test method, it cost less time to get shear strength parameters of unsaturated soil under different conditions. (3) numerous parameters can be controlled in modified tests, from which we got shear strength parameters of unsaturated soil under different conditions; some classical theories about unsaturated soil are also verified. also, the nonlinear failure envelope of matric suction was obtained. (4) the test results confirmed the nonlinear interrelationship among the cohesion, internal friction and dry density, which can be used in some practical engineering about unsaturated soil as a kind of empirical formula. (5) it was shown that the shear rate must be controlled within 0.012 mm/min to get reliable shear strengths parameters for unsaturated soil samples of yuzhou. thus, the test of determining appropriate shear displacement rate before the shear test is necessary. thus, the recommendations are given excessively, it is essential to shear the soil sample beyond the peak shear stress to the strain softening region, and make sure that the samples are wholly consolidated at every new effective normal stress-stage. acknowledgements this research was financially supported by the science and technology project of henan provincial transportation department (grant no. 2012p51) and key project of science and technology of henan provincial education department (grant no. 14b580001). references abbasi, n., khan, m.s., mughal, m.s., & yasin, m. 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(2017). temporal landslide susceptibility assessment using landslide density technique. geological behavior, 1(2):10–13. yasin, h., usman, m., rashid, h., nasir, a., & randhawa, i.a. (2017). alternative approaches for solid waste management: a case study in faisalabad pakistan. earth sciences pakistan, 1(2):04-06. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n1.105689 earth sciences research journal earth sci. res. j. vol. 27, no. 1 (march, 2023): 69 75 pe tr o le u m e n g in ee r in g palabras clave: información de registro de pozo; método bosques aleatorios; método smote; identificación de las capas de petróleo y de agua. application of random forest method in oil and water layer identification of logging data: a case study of the liaohe depression chunlei dai1,2*, shangming shi1, chao song1 1. school of earth science, northeast petroleum university, daqing 163318, china 2. key laboratory of oil and gas reservoir formation mechanism, and resource evaluation in heilongjiang province, daqing 163318, china *corresponding author: dcl@nepu.edu.cn aplicación del método de bosques aleatorios en la identificación de las capas de petróleo y de agua durante el registro de pozo: caso de estudio en la depresión liaohe how to cite item: dai, c., shi, s., & song, c. (2023). application of random forest method in oil and water layer identification of logging data: a case study of the liaohe depression. earth sciences research journal, 27(1), 69-75. https://doi.org/10.15446/ esrj.v27n1.105689 record manuscript received: 12/09/2022 accepted for publication: 06/03/2023 keywords: logging data; random forest; smote; oil and water layer identification abstract accurate identification of oil and water layers is the basis of qualitative evaluation of reservoir fluid properties or industrial value and selection of testing layers of the well. the traditional oil and water layer identification is mainly based on the extensive use of the well’s logging and logging data, which is inefficient and easy to leak interpretation or misinterpretation for those reservoirs with complex geological conditions. in this paper, the random forest method of machine learning is used to select the lithology, porosity, permeability, movable fluid, oil saturation, s0, s1, s2, tmax of rock as characteristics; smote oversampling is used to expand the sample, and the packet estimation is used to establish the oil and water layer identification model. this method is simple and easy to use, not prone to severe overfitting, and can find the potential rules in the data. the classification performance is excellent, and the accuracy rate can reach more than 89.9%, which solves the problem of low accuracy in oil-water layer identification in the past. resumen la identificación precisa de las capas de agua y petróleo es la base de la evaluación cualitativa de las propiedades de fluido del yacimiento o de valor industrial, y de la selección de las capas de ensayo del pozo. la identificación tradicional de las capas de petróleo y agua se basa principalmente en el uso extensivo de la información ofrecida por la adquisición de registros del pozo, la cual es ineficiente y fácil de perder información o de incurrir en malinterpretación en aquellos yacimientos con condiciones geológicas complejas. en este artículo se utilizó el método de "bosques aleatorios (del inglés random forest method)" para seleccionar la litología, porosidad, permeabilidad, fluidos móviles, saturación de petróleo, y las características de la rocas s0, s1, s2 y tmax. el sobremuestreo con el método smote se usó para ampliar la muestra, y el paquete de estimación se uilizó para establecer el modelo de identificación de las capas de agua y petróleo. este método es simple y fácil de usar, además de no ser propenso a un sobreajuste severo, y puede encontrar en la información las normas potenciales que lo rigen. la clasificación del desempeño es excelente, y el índice de exactitud puede alcanzar más del 89.9 %, lo que resuelve el problema de la baja exactitud que se presenta en la identificación de las capas de petróleo y de agua. mailto:dcl@nepu.edu.cn https://doi.org/10.15446/esrj.v27n1.105689 https://doi.org/10.15446/esrj.v27n1.105689 70 chunlei dai, shangming shi, chao song 1. introduction accurate identification of oil and water layers is the basis for the qualitative evaluation of reservoirs’ fluid properties or industrial value and for selecting testing layers of the well. according to the existing theories and data, the reservoir resistivity is low after the completion of the pre-exploration well in the more favorable blocks, and the logging display features of the oil layer are not obvious (su, 2006). in exploration and development, the interpretation or misinterpretation of the oil layer log leakage is often caused, which slows down or restricts the exploration and development process of the oil field. using the existing technical conditions to identify various types of oil reservoirs effectively is significant to exploring and developing oilfields. when conventional logging methods explain the difficulties encountered by oil and water reservoirs, the analysis data of geological logging and gas logging is another way to solve the problem. the application of geological logging data oil and aquifer identification is the plate method, selecting two features as plates. usually, all features are made according to every two features for a combination to make all plates, filter out the good effect, and then the multiple plates screened out are connected for oil and aquifer identification. there are also methods to use various features to build new features to do the plate for oil and water layer identification. the use of new features for oil and water layer recognition requires fewer plates. still, in the process of new feature construction, it is also necessary to establish many two feature plates based on the establishment. after a lot of theoretical analysis, it is difficult to achieve an excellent organic combination of all features; the effect could be better. artificial intelligence technology’s continuous development has been widely used in many fields (wu et al., 2021; liu, y., liu, s., & ma, 2019; liang, chen, & zhang, 2019; bengio, courville, & vincent, 2012; džeroski, & ženko, 2004; xing, zhou, & he, 2022). artificial intelligence technology can comprehensively consider all the sample characteristics in the model training process by constantly adjusting the weight value of each feature so that the error between the fitted value and the target value of each training sample continues to shrink. when the error value reaches the required accuracy, and the training ends, the trained model is used to identify one sample to measure and able to achieve good results. this method is the comprehensive embodiment of the oil and water layer’s identification plate method, which is simpler and better than the plate method. this paper uses random forests in artificial intelligence to identify the oil and aquifer of the liaohe depression. random forest decision tree model the random forest is based on a decision tree, and the classification decision tree model is a tree structure that describes the classification of instances (hang, 2012). decision trees consist of nodes, directed edges, and nodes consist of two types, which are internal nodes, and leaf nodes. internal nodes represent a feature or attribute, and leaf nodes represent a class. the classification process of the decision tree starts from the root node, tests a specific feature of the instance, and, according to the test results, the instance is assigned to one of its sub-nodes. then, the corresponding elements of each node are detected and assigned, and finally, the instance is assigned to the leaf node. the class corresponding to the leaf node at this time is the judgment result of the decision tree model. feature selection the key to the decision tree is selecting the optimal features and dividing the current sample so that the “purity” of the split nodes is higher (zhou, 2016; wang et al., 2021; zhao et al., 2021; wang et al., 2021). information gain “information entropy” is one of the most used metrics to measure the purity of a sample set. suppose that the proportion of samples of the kth class in the current set of samples d is pk(k = 1, 2, ...), then the information entropy of d is defined as |y| meet the conditions 1? meet the conditions 2? meet the conditions 3? meet the conditions 3?type 1 type 2 type 3 type 4 type 5 y y y y n n n n figure 1. decision tree model ent(d) = -∑|y| k=1 pklog2 pk (1) the smaller the value of ent(d), the higher the purity of d. the information entropy dv of can be calculated according to equation 1. then, considering that the number of samples contained in different branch nodes is different, the branch nodes are given weights d d v . the more samples there are, the greater the influence of the branch nodes, so it is possible to calculate the attribute ‘a’ to divide the sample set d: “information gain.” gain d a ent d d d ent d v v d v ( , ) ( ) ( )�� �� ���� 1 (2) in general, the information gain is greater, and the “purity boost” obtained is greater by dividing using the attribute‘a’. therefore, we can use the information gained for attribute selection. the famous id3 decision tree learning algorithm uses the information gained as a reference to select the division attributes. gini index there is also a classification index called the gini index, which uses the same symbol as 1, and the gini index can measure the purity of data set d gini d p p p kkk y k kk y k ( ) �� �� �� ���� ������ ��� �1 2 1 1 (3) intuitively, the gini index reflects the probability that two random samples were taken from dataset d with inconsistent categorical markers. therefore, if the gini index gini(d) is smaller, the purity of dataset d is higher. expressed by the same symbol as equation 2, the gini index of attribute ‘a’ is defined as follows: gini index d a d d gini d v v v v _ ( , ) ( )�� ���� 1 (4) thus, in the candidate set of attributes ‘a’, the attribute that makes the gini exponential minimal after the division is chosen as the optimal division attribute, i.e., a⁎= argaϵamaxgini_index(d,a). self-service sampling self-service sampling is a standard sampling method. given a dataset d containing m samples, we sample it to produce a dataset d’: each time a random sample is selected from d, copied into d’, and then put the sample 71application of random forest method in oil and water layer identification of logging data: a case study of the liaohe depression back, making m times such a retraction sample, you can get a dataset d’ with a sample count of m, which is the result of self-service sampling. a part of the sample in d appears multiple times in the d’, and a part does not appear. to make a simple estimate, the probability that a sample will never be taken in m-samples is that the limit is available ( )1 1 −− m m . lim( ) . m m m e���� �� �� ��1 1 1 0 368 (5) that is, through self-service sampling, about 36.8% of the samples in the original dataset d did not appear in the sampling dataset d’. so, we can train the model with d’ as the training set and d-d’ as the test set. then, we use m training samples on both the model of the actual evaluation and the expectation evaluation. about a third of the samples still do not appear in the training set to test, and such test results are also known as “out-of-bag estimates.” random forest random forest (rf) is an extension of bagging (breiman, 2001; zhou et al., 2017; lai et al., 2021; zhong et al., 2022; kang, & lu, 2020). rf is based on a decision tree-based learner, built on top of bagging, and introduces a new feature of random attribute selection in individual learner training. during training, the decision tree is divided according to the gini index or information gain of the current node, and the optimal attribute is selected, while the random forest is different. in the process of training each base learner, the random forest arbitrarily selects k from all the features (attributes), and uses this k features to train the base learner, and the training process is the same as the training decision tree, using the partition criteria to divide the samples in the node. the parameter k represents the randomness of feature selection and controls the degree of introduction of randomness. when k is equal to the number of features in the dataset, the base learner is the same as the traditional decision tree. when k=1, only one feature is divided at a time. in general, it is recommended to take or for k (d is the number of features) (breiman, 2001). the random forest algorithm is simple and easy to implement, and is known as the “representative method of ensemble learning” because of its strong performance in many tasks. compared to bagging, random forests not only use sampling methods, but also perturb the training set. adding randomness, the base learner is also trained by randomly selecting features. introducing randomness, which is the data sample perturbation and input attribute perturbation of random forest, makes each basic learner in random forest very different, and finally improves the generalization performance of ensemble. 2. data preprocessing in this paper, the nmr logging data and geochemical logging data in the eastern and western liaoning rivers are taken as examples, and the oil and water layers are identified by using a random forest algorithm. there are a total of 79 samples in the eastern part of the liaohe river and 99 samples in the western part of the liaohe river. a total of 9 features of lithology, porosity, permeability, movable fluid, oil saturation, s0, s1, s2, and tmax were selected for analysis, and the appropriate features were selected, which had a great impact on the accuracy of the model. 2.1 added oil level features through the observation data, it can be found that the lithology of some layers is igneous, but most of the lithology of the layers is sedimentary, and there is an oil-bearing level in the lithology of the sedimentary rock, and since the oil-bearing level is crucial to the analysis of the oil-bearing water layer, it is necessary to convert the oil-bearing level to a numerical format. according to the industry standard of 89 years (syt536489), this article improves on it by setting the saturated oil to 97.5, the enriched oil to 85, and the oil immersion to 57.5, the oil spot is 22.5, the oil trace is 3, the fluorescence is 1, the oil content level is higher, the value is greater, that is, the proportion is greater. for igneous rocks, set the oil level to a missing value and deal with it later. observing the data can also find missing geochemical pyrolysis data for some samples, which are also set as missing values for later processing. according to the branching rule of decision tree, when dividing a node, it has nothing to do with the size of the value, but only how much the value is, so the number is set. it has no effect on the actual division, but it is converted into a numeric type to facilitate subsequent processing of missing values. 2.2 handle missing values observations have found that some samples lack geothermal data and igneous rocks lack oil levels. there are three common ways to handle missing values (cheng, 2007). the first method is to discard the data, discarding samples with missing values and leaving only the samples with complete data. however, in the process of logging, less data is obtained. if the sample with missing values is discarded, there will be fewer samples available for use, which is not conducive to our model building, and the accuracy of the model after the establishment will be affected. the second method is to use other variables as a predictive model to calculate the missing variables, but this method has a fundamental flaw. if the other variables are not related to the missing variables, the predicted result is meaningless, and if the prediction is fairly accurate, it is not necessary for this variable to add to the model. the third method is replaced by mean, median, quantile, mode, random number, etc. this article adopts the average of the same sample to fill in the missing values, because the same samples should have similar properties, and the value of this attribute should fluctuate within a certain range, so the mean is used instead of the missing values. in fact, for the random forest algorithm, there is another method of filling in missing values that is unique to this algorithm (cutler et al., 2004). first, the affinity among the samples is calculated, and then the affinity is used as the weight, and all non-missing values are weighted, and the average value is obtained instead of the missing value. this method is only valid in random forest algorithms, because calculating affinity among samples is an advantage of random forest algorithms. this filling method results in better performance than the previous methods, but the amount of computation increases. as the number of samples and the number of base learners increases, the calculation time increases. this article uses the average of each feature to populate the missing values to reduce the workload. 2.3 deal with sample imbalance problems due to practical factors and other reasons, the number of samples obtained is not much, and after the statistics of the samples (table 1), it is found that there are 4 samples of the aqueous oil layer in the eastern part of the liaohe river, 11 oil-water layers, 12 oil layers, 21 dry layers and 31 water layers. there are 1 aqueous oil layer in the western part of the liaohe river, 3 dry layers, 7 oil-water layers, 7 self-injection layers, 16 low-yield oil layers, 17 dry layers and 48 oil layers. table 1 statistics of sample categories in liaohe depression category number of samples eastern westward aqueous oil layer 4 1 dry layer 21 3 oil and water are in the same layer 11 7 self-injecting layer 7 low-yielding reservoir 16 dry layer see oil 17 reservoir 12 48 water 31 total 79 99 random forest algorithm, using self-sampling method, about 36.8% of the samples cannot enter the training set, through the statistics can be found that the eastern liaohe aquifer only 4 samples, far below the aquifer 31 samples, while the western liaohe river aquifer and dry layer only 1 and 3, far lower than the oil layer 48. this will lead to a large probability that a small sample size of the class cannot be, or only a very small number of samples will be collected into the training set, resulting in the trained model of the class of recognition of the class of samples is low, or cannot be distinguished, the generalization ability of the model is insufficient, the new sample, especially the small sample category cannot be judged. for this kind of extreme sample imbalance problem, there are two solutions, one is to improve the sampling method, so that the number of samples in the sample set reaches the same level. this method is called balanced random 72 chunlei dai, shangming shi, chao song forest, and the other is to use a weighted random forest, that is, through the costsensitive learning method, the learner is improved, so that its prediction results are connected with the weights, thereby improving the performance of the learner. this paper uses the first method, which is “oversampling” technique, to construct a balanced random forest algorithm. specifically, there are two ways to equalize the number of samples of each type of sample in the sample set. one is to randomly select the same number of samples as the class with the smallest number as samples of this class by “under-sampling” the samples of a larger number of categories. this will give you a new sample set. the number of samples of each type in the sample set is equal. therefore, during self-service sampling, each type of sample may be selected into the training set, and there will be no sample imbalance. as a result, the model has insufficient learning ability for certain types of samples, or cannot learn. but the biggest problem with this method is that in the process of undersampling, a lot of information will be lost. because the selection process is completely random, the information of some samples cannot enter the training set and be learned by the model. the other way is “oversampling”, in which a random sample with replacement in the current class is selected and replicated until it is the same as the number of main samples. this avoids the loss of a lot of information from undersampling. but the shortcomings are also obvious. for example, in the west of the liaohe river, there is only one water-bearing oil layer sample. if you replicate this sample 47 times, you get 48 identical samples. if the learner learns it, it will lead to serious overfitting, and the generalization ability to the new water-bearing oil layer samples is weak. the samples are classified because the learner cannot delineate a valid region in the feature space. chawla et al. (2004) proposed a method called smote that could effectively improve the overfitting problem caused by replicating samples during “oversampling”. the basic process of the smote method is that in the small category sample, a sample is selected, and the k nearest neighbor algorithm calculates the k samples that are closest to the sample, for each feature, one is randomly selected from the k nearest neighbor sample, the difference between the two features is calculated, multiplied by a random number greater than 0 less than 1, and then added back to the original sample, which is actually a point on the connection line between the two samples in the feature space, and then after all the features are done, a new sample can be obtained. this method can effectively make the decision space of small category samples more generalized. for the liaodong and western liaoning data used in this paper, if you choose undersampling, the samples of the trained model will be seriously insufficient, and the performance of the model will be greatly reduced. in order to compare the impact of the smote algorithm and the “oversampling” method by copying on the model, this article will use two methods to calculate the out-of-package estimation accuracy. first, calculate which type of sample has the most samples in the area, and then expand the other categories through “oversampling”, and use the “oversampled” dataset to train the model. since there is only one “oil and water layer “ sample in the western part of the liaohe river, even if the move method is used, it cannot be effectively expanded. if it is copied 47, it will cause the model to be seriously overfitted. when calculating the out-of-package estimation accuracy, it will also be affected, resulting in a higher accuracy obtained than normal, so this point is deleted here. through “oversampling”, the number of samples in the eastern part of the liaohe river expanded to 155 and the western part of the liaohe river to 288. train a random forest with an enriched dataset and measure its performance with out-of-package estimation accuracy. however, if the sample is balanced by copying the “oversampling”, then using the processed sample to calculate the out-of-package estimate is less efficient. because “oversampling” will lead to overfitting, take the sample in the western liaohe river, for example, because there is only one aquifer sample, if the learner completely “writes down” the characteristics of this sample, then the sample after “oversampling” is used, it will successfully judge 48 times, which will make the calculation of the learner accuracy, the molecular denominator increases by 47 at the same time, resulting in an increase in accuracy. even if it is deleted, then in other small categories of samples, a similar situation will occur, affecting the accuracy of the model, so the original sample should be used to calculate the out-of-package estimate. 3. adjust the learner parameters most machine learning algorithms have parameters that need to adjust and set, that is, tuning parameters. many times, the setting of parameters can often determine the quality of the learner. for the stochastic forest algorithm, there are also many important parameters, such as the number of base learners, that is, the number of decision trees. how many features to use for training, etc. out-of-package estimation used to adjust the parameters. the error rate estimated out of package is lower, the generalization ability of the adjusted model is stronger. 3.1 the number of features used as can be seen from figure 2 and figure 3, when the number of base learners is small, the accuracy is not high, and when 15 base learners are reached, the accuracy can be stabilized at 90 % above, and then increase the number of base learners, the effect on accuracy improvement is not large. the number of base learners is greater, the generalization ability of the integration is stronger. after reaching a certain number, the improvement of accuracy is no longer obvious. at the same time, the number of base learners is greater, the training time is longer, because every time a base learner is trained, a new self-service sample of the sample needs to be trained, and a new base learner needs to be trained. figure 2 relationship between the number of subbase learners in different max_ features in the western part of the liaohe river and the error rate of out-of-package estimation 0 0.1 0.2 0.3 0.4 0.5 0.6 0 50 100 150 200 o ut -o f-p ac ka ge e s� m a� on e rr or ra te number of base learmers randomforestclassifier,max_features=sqrt randomforestclassifier,max_features=log2 randomforestclassifier,max_features=none 0 0.1 0.2 0.3 0.4 0.5 0.6 0 50 100 150 200 o ut -o f-p ac ka ge e s� m a� on e rr or ra te number of base learmers randomforestclassifier,max_features=sqrt randomforestclassifier,max_features=log2 randomforestclassifier,max_features=none figure 3. relationship between the number of subbase learners in different max_ features in the eastern part of the liaohe river and the error rate of out-of-package estimation 73application of random forest method in oil and water layer identification of logging data: a case study of the liaohe depression for the random forest algorithm, there is also an important parameter, that is, how many features are used to train each base learner. according to breiman’s research (breiman, 2001). it is recommended log2d to train with individual features, while it is proposed that training with individual features often results in good performance. therefore, this paper uses three methods, using individual features √d, individual features, and all features for training, and draws a graph of the relationship between the number of base learners and the error rate of out-of-package estimation in three cases (figure log2d 2, figure 3). it can be seen that the error rate of out-of-package estimation is very similar in the three cases, and only the random forest classifier trained with all features has a slightly higher error rate, which also shows the effect of using the random feature training base learner to introduce randomness and improve the generalization ability of random forests. however, the direct reason why it is not possible to see log2d√d, which feature has more advantages is that the characteristics selected for this experiment are oil content level, porosity, permeability, movable fluid, oil saturation, s0, s1, s2, tmax has a total of 9 features, |log29| and equal √9(|x|for x rounding down), are equal to 3, so there is no difference in theory. but when the number of features is large enough, you can see the difference in performance between the two. 3.2 selection of division criteria using the gini index yields roughly similar results to information gain. the blue line represents the base learner of the random forest trained using the gini index, and the green one is training using the information gain (fig. 4, fig. 5). it can be seen that the use of gini index and information gain does not have much effect on the error rate of out-of-package estimates. 4. characteristic analysis there is also a by-product of using random forest. by building a model through the random forest algorithm, the contribution of each feature to the model can be obtained, also known as gini importance (pedregosa et al., 2012). when training the base learner, the gini index is used to divide the nodes each time, so that the gini index of the two child nodes is smaller than that of the parent node. the importance of each feature is obtained by summing the gini index reductions for all features in each tree in the random forest. implementation method: multiply the number of samples of the parent node by the gini impurity of the current node, and subtract the number of samples of the root node of the left and right subtrees and their corresponding gini impurity, and the important part of the division feature corresponding to the node can be obtained. after calculating the gini importance of all nodes to their divided features by this method, sum them by feature, and then standardize it to obtain the importance of each feature. because the random forest uses the same data every time, the training set obtained by the self-sampling method is different, and the features used for training are different. therefore, this article attempts to use 100 basic learners to perform 10 training sessions on the “oversampling” samples, calculate the 10 times, and take the average value of the importance of each feature to obtain the following table (table 2~ table 3). by sorting the average values in the two tables (table 4), it can be observed that in identifying the oil and water layer in the eastern and western parts of the liaohe river, the rock pyrolysis parameters s2, the oil content of the rock surface and the porosity are more important to the model, while the permeability is s0 low contribution to the model. it may be because the permeability of the rock only determines how much the rock of the formation affects the fluidity of the fluid, and does not determine which type of layer it is. when the permeability of the rock is very large, it may be a dry layer, or it may be an oil layer or a water layer, but when the permeability is very small, the possibility of being a dry layer is very large, but the possibility of oil layer and water layer will be very small. that is to say, the permeability of rocks influences the identification of oil and water layers to a certain extent, but its range of action is limited. however, the pyrolysis parameters can determine the content of organic matter, the type of organic matter and the maturity of the rock, so the pyrolysis data is more effective for dividing the oil layer. in the process of machine learning, because each learned sample and test sample are random, the accuracy of the results is also different, the confidence interval is used to evaluate the results, and the east and west use a confidence level of 0.95, and the eastern accuracy rate is 89.9%~ 91.5%, the western accuracy rate is 94.4%~ 95.4%. figure 4. the relationship between number of base learners and out-of-bag estimate error rate under different division criterion in the east of liaohe depression 0 0.1 0.2 0.3 0.4 0.5 0.6 0 50 100 150 200 o ut -o fpa ck ag e es tim at io n er ro r r at e gini entropy number of base learners 0 0.1 0.2 0.3 0.4 0.5 0.6 0 50 100 150 200 o ut -o f-p ac ka ge e st im at io n er ro r r at e number of base learners gini entropy figure 5. the relationship between number of base learners and out-of-bag estimate error rate under d ifferent division criterion in the west of liaohe depression 74 chunlei dai, shangming shi, chao song table 2. importance of characteristics in the east of liaohe depression features porosity penetration movable fluids oil saturation s0 s1 s2 tmax the rock surface contains oil oob accuracy 1 0.16 0.08 0.10 0.06 0.05 0.11 0.21 0.09 0.14 0.92 2 0.15 0.07 0.09 0.07 0.07 0.12 0.22 0.07 0.14 0.92 3 0.14 0.08 0.09 0.08 0.05 0.11 0.20 0.09 0.15 0.91 4 0.13 0.08 0.10 0.07 0.06 0.11 0.22 0.10 0.14 0.91 5 0.14 0.07 0.11 0.05 0.05 0.11 0.23 0.09 0.14 0.90 6 0.16 0.05 0.09 0.08 0.05 0.11 0.20 0.10 0.15 0.90 7 0.15 0.06 0.13 0.07 0.05 0.10 0.24 0.08 0.13 0.89 8 0.15 0.08 0.10 0.06 0.06 0.12 0.19 0.09 0.14 0.89 9 0.13 0.09 0.11 0.06 0.06 0.12 0.21 0.09 0.13 0.92 10 0.13 0.06 0.10 0.07 0.05 0.15 0.21 0.10 0.13 0.90 average value 0.14 0.07 0.10 0.07 0.06 0.12 0.21 0.09 0.14 0.91 table 3. importance of characteristics in the west of liaohe depression features porosity penetration movable fluids oil saturation s0 s1 s2 tmax the rock surface contains oil oob accuracy 1 0.13 0.09 0.11 0.11 0.08 0.08 0.14 0.09 0.17 0.95 2 0.12 0.08 0.11 0.13 0.07 0.09 0.16 0.08 0.17 0.94 3 0.13 0.09 0.10 0.12 0.08 0.08 0.15 0.07 0.18 0.95 4 0.13 0.09 0.11 0.12 0.07 0.09 0.16 0.08 0.16 0.96 5 0.14 0.09 0.12 0.12 0.08 0.07 0.14 0.08 0.16 0.94 6 0.12 0.09 0.12 0.13 0.08 0.09 0.15 0.08 0.15 0.94 7 0.15 0.08 0.11 0.12 0.08 0.09 0.14 0.08 0.15 0.95 8 0.13 0.10 0.11 0.12 0.08 0.09 0.13 0.08 0.15 0.96 9 0.13 0.09 0.12 0.11 0.07 0.08 0.14 0.07 0.19 0.95 10 0.14 0.09 0.12 0.13 0.07 0.08 0.15 0.08 0.16 0.95 average value 0.13 0.09 0.11 0.12 0.08 0.08 0.15 0.08 0.16 0.95 table 4. importance ranking of characteristics in the east and west of liaohe depression eastern part of the liao river gini importance west of the liao river gini importance s2 0.21 oily grade 0.16 oily grade 0.14 s2 0.15 porosity 0.14 porosity 0.13 s1 0.12 oil saturation 0.12 movable fluids 0.10 movable fluids 0.11 tmax 0.09 penetration 0.09 oil saturation 0.07 tmax 0.08 penetration 0.07 s1 0.08 s0 0.06 s0 0.08 5. conclusion through the numerical treatment of oil-containing grades and sample imbalance, a good prediction effect was obtained. 1. the method operation is simple, the traditional plate method needs to build new features, and the operator needs to have a strong theoretical foundation, and there are often various problems in the application process, which undoubtedly brings obstacles to the promotion of the plate. the random forest method only needs to adjust the parameters, especially when the number of bases is large, the effect is better, and it has outstanding advantages over the traditional square layout method. 2. this paper adopts the smote oversampling method to solve the problem of data imbalance and ensure the accuracy of the method. the maximum number of features for a single tree is “sqrt” and “log2”, and the error rates are similar, which are lower than selecting all features. each tree randomly selects features, adding feature disturbance, and the error rate of the standard parameters for dividing the gini coefficient and information gain is not much different. from the characteristic analysis, it can be seen that pyrolysis and oil-bearing grades are important features in the identification of oil and water layers in both the eastern and western regions, which is consistent with the traditional plate method and theoretical knowledge. 75application of random forest method in oil and water layer identification of logging data: a case study of the liaohe depression 3. from the prediction results, random forest has a high accuracy in the identification of reservoir water. at a confidence level of 0.95, the accuracy rates in the east are 89.9%-91.5%, and the accuracy in the west is 94.4%95.4%. the lower value in the eastern region is mainly related to the amount of data and is caused by the imbalance of data types. with the continuous development of big data and artificial intelligence, machine learning will find more applications in the field of oil and gas exploration. references: bengio, y., courville, a., & vincent, p. 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(2022). study on the classification of stochastic forest fusion model in the classification of pressure well methods. journal of southwest petroleum university (natural science edition), 44(1), 165-173. zhou, z. (2016). machine learning. beijing: tsinghua university press. zhou, x., zhang, z., & zhang, c. (2017). complex lithology recognition based on rough set-random forest algorithm. daqing petroleum geology and development, 36(6), 127-133. https://doi.org/10.48550/arxiv.1206.5538 https://doi.org/10.48550/arxiv.1206.5538 https://doi.org/10.1023/b:mach.0000015881.36452.6e keywords: deep learning; soil remote sensing image; maximum likelihood estimation; classification method. palabras clave: aprendizaje profundo; imagen de teledetección del suelo; estimación de máxima verosimilitud; método de clasificación. how to cite item liang, s., cheng, j., & zhang, j. (2020). maximum likelihood classification of soil remote sensing image based on deep learning. earth sciences research journal, 24(3), 357-365. doi: https://doi.org/10.15446/esrj. v24n3.89750 soil remote sensing image classification is the most difficult in the national soil census work. current soil remote sensing image classification methods based on deep learning and maximum likelihood estimation are challenging to meet the actual needs. therefore, this paper combines deep learning with maximum likelihood estimation and proposes a maximum likelihood classification method for soil remote sensing images based on deep learning. the method is divided into four parts. firstly, the pretreatment of soil remote sensing image is carried out, including three processes: image gray, image denoising, and image correction; secondly, the target of soil remote sensing image is detected by deep learning algorithm; thirdly, the maximum likelihood algorithm is used to classify soil remote sensing image; finally, the classification performance is tested by an example. the results show that this method can effectively segment the remote sensing image of soil, and the segmentation accuracy is high, which proves the effectiveness and superiority of the method. abstract maximum likelihood classification of soil remote sensing image based on deep learning clasificación de verosimilitud máxima de imágenes en teledetección del suelo con base en aprendizaje profundo automático issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.89750 la clasificación de imágenes de detección remota de suelos es la más difícil en el trabajo del censo nacional de suelos en china. los métodos vigentes de clasificación de imágenes de teledetección del suelo basados en el aprendizaje profundo y la estimación de máxima probabilidad no satisfacen las necesidades actuales. por lo tanto, este documento combina el aprendizaje profundo con la estimación de máxima verosimilitud y propone un método de clasificación para estas imágenes de teledetección. en primer lugar, se lleva a cabo el preprocesamiento de la imagen de teledetección del suelo, lo que incluye tres procesos: imagen gris, eliminación de ruido y corrección de imagen; en segundo lugar, el objetivo de la imagen del suelo se detecta mediante un algoritmo de aprendizaje profundo; tercero, el algoritmo de máxima verosimilitud se usa para clasificar la imagen de detección remota del suelo; y, finalmente, el rendimiento de la clasificación se prueba con un ejemplo. los resultados muestran que este método puede segmentar efectivamente la imagen de detección remota del suelo, y la precisión de la segmentación es alta, lo que demuestra la efectividad y superioridad del método. resumen record manuscript received: 20/05/2019 accepted for publication: 11/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 357-365 shujun liang1, jing cheng2*, jianwei zhang1 1software engineering college, zhengzhou university of light industry, zhengzhou 450002, china 2engineering training center, zhengzhou university of light industry, zhengzhou 450002, china *corresponding author: zzqlcy126@163.com re m o te s en si ng https://doi.org/10.15446/esrj.v24n3.89750 https://doi.org/10.15446/esrj.v24n3.89750 https://doi.org/10.15446/esrj.v24n3.89750 mailto:zzqlcy126@163.com 358 shujun liang1, jing cheng2*, jianwei zhang1 introduction china has a vast territory and abundant land resources. its total area is about 9.6 million square kilometers, second only to russia and canada, ranking third globally. two national soil surveys have been carried out from 1958 to 1960 and 1979 to 1985 to make better use of land resources and develop agriculture in china. the survey contents generally include soil formation factors, description of typical soil profiles, classification of soil types, determination of soil physical and chemical properties, soil evaluation, and low-yield soil improvement planning. however, in the face of such substantial land resources and continuous changes, the previous two soil censuses have become very difficult, especially in the classification process, it spent a lot of time, material, and financial resources. with the emergence and development of remote sensing technology, the work of soil survey has become simple. however, with the improvement of remote sensing image resolution and the increase of data volume, the collected soil images are more diverse and more productive, but to a certain extent, it increases the difficulty of classification (chen et al., 2019). under the above background, relevant scholars at home and abroad have conducted in-depth research on soil remote sensing image classification and proposed many methods, such as soil remote sensing image classification methods, based on deep learning. it is based on artificial neural network architecture, such as convolution neural network, deep neural network, deep confidence network, to learn more useful features, thus ultimately realizing classification. the advantage of this method is that it has better transfer learning property. the disadvantage is that model validation is complicated and cumbersome. for example, the principle of a classification method for soil remote sensing image based on maximum likelihood estimation is based on calculating the probability that a pixel belongs to each class in a pre-set m-class data set, and then dividing it into the most probabilistic class. the advantage of this method is that it has evident parameter interpretation ability, and is easy to fuse with prior knowledge. the algorithm is simple and easy to implement. the disadvantage is that it is vulnerable to the distribution of categories in feature space and the selection of samples. once the distribution is discrete, or the selected samples are not representative, the classification results will deviate significantly from the actual situation (cheng et al., 2018). given the above situation, this paper combines deep learning with maximum likelihood estimation and proposes a maximum likelihood classification method for soil remote sensing images based on deep learning. the method is divided into two parts. the first part is to detect remote sensing image targets by deep learning and extracts classification features of various image targets. the second part is to classify soil remote sensing images by maximum likelihood estimation algorithm based on the first part and produce soil-type maps (demattê et al., 2017). in the experimental part, a remote sensing image of a region is taken as an example to test the classification performance. by drawing roc curves, it is concluded that the combined soil remote sensing image has better classification performance. this improvement provides a new idea for soil classification and mapping of current remote sensing data, facilitates soil census, improves land use rate to a certain extent, and promotes agriculture, animal husbandry, and forestry in china. maximum likelihood classification of soil remote sensing image based on deep learning soil remote sensing image is a kind of soil remote sensing image obtained by various sensing devices (such as radar, camera, scanner, etc.). its acquisition principle process is shown in figure 1, and the form of the obtained remote sensing image is shown in figure 2. the obtained soil remote sensing images can accurately identify and classify soil types, make soil maps, and analyze soil distribution law, which brings great convenience to soil survey. to improve the classification quality of soil remote sensing images, this paper studies a more effective classification method, which combines deep learning with maximum likelihood estimation to make up for each other’s shortcomings (fitak & johnsen, 2017). the research on maximum likelihood classification of soil remote sensing images based on deep learning is mainly divided into four stages: the first stage is to preprocess the soil remote sensing images acquired by remote sensing equipment; the second stage is to detect the targets of remote sensing images by using deep learning algorithm and extract the classification features of various soil images; the third stage is to classify the soil remote sensing images based on the methods mentioned above by using maximum likelihood estimation method. in the fourth stage, the performance of maximum likelihood classification of soil remote sensing images based on deep learning is tested by an example to ensure the effectiveness and practicability of the method (handelman & chor, 2017). figure 1. principle of soil remote sensing image acquisition figure 2. remote sensing images 359maximum likelihood classification of soil remote sensing image based on deep learning include mean filtering, median filtering, wavelet transform and neighborhood averaging. wavelet transform is a commonly used method at present (li et al., 2017). the basic principles are as follows: the multi-scale wavelet transform is performed on the noisy signal, and the wavelet coefficients belonging to the noise are removed at each scale, and the wavelet coefficients belonging to the signal are preserved and enhanced. finally, the wavelet transform is used to restore the original signal, so that denoising is achieved, as shown in figure 4. figure 4. wavelet denoising (3) image correction in the acquisition process of soil remote sensing image, besides the image quality degraded by noise, it will also be affected by the characteristics of the sensor itself, the illumination conditions of the ground objects (topography and solar altitude angle) and the atmospheric effect, which will lead to the inconsistency between the measured values of remote sensing equipment and the actual spectral emissivity of the ground objects, i.e., radiation distortion. the radiation distortion will then cause the geometric position, shape, size, orientation and other features of the original image to deviate from the expression requirements in the reference system, that is, geometric distortion (ma et al., 2018). in view of the above two distortion phenomena, it is necessary to correct and restore the image. 1) radiation distortion correction. according to the three causes of radiation distortion, i.e. the characteristics of the sensor itself, the illumination conditions of the ground objects and the atmospheric effect. the calibration of the remote sensor, the solar altitude and the terrain, and the atmospheric correction are carried out by these three methods, respectively, as shown in preprocessing of soil remote sensing images as shown in figure 2, the quality of the original soil remote sensing image is not high, which is not conducive to the subsequent image classification. therefore, it is necessary to pre-process the image before classification to improve the image quality, including image graying, image denoising and image correction (he et al., 2018). (1) image graying grayscale image, commonly speaking, is the conversion of a color image to a gray image whose pixel value is between 0 and 255. the whole image is composed of different degrees of gray. its purpose is to reduce the interference of color to the target information in the image. there are four main methods of gray image processing: component method, maximum method, average method, and weighted average method, as shown in table 1. (2) image denoising influenced by natural factors such as illumination, cloud, and equipment itself, the original remote sensing image collected contains a lot of noise. after graying, it can see that there are many white elements on the image. these elements are image noise, as shown in figure 3. figure 3. image noise the existence of image noise will blur the image target and reduce the image quality, so it needs to be de-noised. at present, denoising methods table 1. four methods of image graying method definition formula explanation of parameters component method taking the brightness of three components in a color image as the gray value of three gray images, a gray image can be selected according to the application needs. f i j r i j1 , ,( ) = ( ) f i j g i j2 , ,( ) = ( ) f i j b i j3 , ,( ) = ( ) (1) f i j kk , , ,( ) =( )1 2 3 is the gray value of the converted gray image at (i, j); r, g, and b represent color components, ranging from 0 to 255 maximum method the maximum brightness of three components in a color image is taken as the gray value of a gray image. f i j r i j g i j b i j, max , , , , ,( ) = ( ) ( ) ( )  (2) average method the three-component brightness of the color image is averaged to get a gray value. f i j r i j g i j b i j , , , ,( ) = ( ) + ( ) + ( ) 3 (3) weighted average method. according to the importance and other indicators, the three components are weighted averaged with different weights. f i j r i j g i j b i j, . , . , . ,( ) = ( ) + ( ) + ( )0 30 0 59 0 11 (4) 360 shujun liang1, jing cheng2*, jianwei zhang1 figure 6. sensor calibration: in order to eliminate the radiation error caused by the sensor itself, the dimensionless dn value recorded by the sensor is converted into the atmospheric top radiation brightness or reflectance with practical physical significance. topographic correction: it can mainly reduce the shadows in remote sensing images, because it restores spectral information, but because the existence of shadows will make the image stereoscopic, which is a visual experience. to see whether topographic correction improves image quality, it depends on whether spectral information has been corrected. cosine correction and semi-empirical c correction are usually used for topographic correction (mccord et al., 2017). atmospheric correction is divided into statistical model and physical model according to the correction principle. the statistical model is based on the correlation between surface variables and remote sensing data, without knowing the atmospheric and geometric conditions of image acquisition. it has the advantages of simplicity and less parameters. 2) geometric distortion correction. geometric correction refers to the elimination or correction of geometric errors in remote sensing images, which mainly involves three processes: selection of control points, transformation of spatial position (coordinate transformation) and resampling of pixel luminance resampling (pritikin et al., 2018). control point selection: firstly, two remote sensing images are selected, one is the reference image and the other is the geometric distortion image, as shown in figure 5. a certain number of control point pairs on the above two images are selected. the selection principles are as follows: the selected control points should be obvious in the remote sensing image of soil; the objects covered by the control points are always fixed; the selected control points have the same topographic height on the two images; the control points should be evenly distributed in the image; and the number of control points should not be less than 5. (a) base image (b) distorted image figure 5. control point selection space position transformation (coordinate transformation): the purpose is to find the correct coordinates of the object. there are two main methods at present, direct method and indirect method. direct method, as its name implies, calculates the coordinates of each pixel in the image to be corrected in turn by the coordinates of the control points on the reference image, as shown in figure 6. (a) base image (b) distorted image figure 6. direct method the advantage of the direct method is that the coordinate values of each pixel calculated in the image to be corrected will not change. the disadvantage of the direct method is that the distribution of pixels will not be uniform. in contrast to the direct method, the indirect method calculates the coordinates of each pixel on the reference image from the image to be corrected. the schematic diagram is shown in figure 7. (a) base image (b) distorted image figure 7. indirect method the advantage of the indirect method is that it can ensure the uniform distribution of the corrected image pixels in space (temmer et al., 2017). the disadvantage of the indirect method is that the row number of the relocated pixels is not an integer relationship with the original image, so the pixel value of the original image needs to be re-sampled. pixel luminance resampling: resampling refers to assigning the pixel value of the original image to the corrected image according to a certain relationship. at present, there are three main methods for resampling pixel luminance values: nearest neighbor method, bilinear interpolation method and cubic convolution method, as shown in table 2. target detection of soil remote sensing image based on deep learning on the basis of the above, this chapter uses a deep learning algorithm to detect the target of soil remote sensing image, extract soil characteristics, and prepare for subsequent classification. deep learning belongs to a branch of machine learning. it is an algorithm based on an artificial neural network to represent data. therefore, before analyzing deep learning, it is necessary to understand artificial neural networks (wang et al., 2017). artificial neural network (ann) is an abstract arithmetic model developed to simulate the process of processing information by human brain neurons. it consists of a large number of nodes (or neurons) connected with each other, as shown in figure 8. in figure 8, a is a signal other than the neuron; w1 to wn is the weight of the signal transmitted to the neuron; b is the bias of the signal; f is the excitation function, generally a non-linear excitation function such as sgn symbolic function or sigmoid type continuous function, and y is the output of the neuron. artificial neural networks consist of an input layer, hidden layer and output layer. when input layer neurons are stimulated by input signal, the activation function of neurons in the hidden layer is stimulated. when a certain threshold is reached, neurons are activated and output signal is generated through the output layer (xu et al., 2017). on the basis of this structure, it is proposed that deep learning is a deeper artificial neural network, which consists of multiple neurons stacked together, as shown in figure 9. there is a special network structure in deep learning, namely convolution neural networks. it is a kind of feedforward neural network with deep structure including convolution calculation (zhao et al., 2018). its structure consists of input layer, convolution layer, pool layer, full connection layer and output layer. the structure is shown in figure 10. the basic principle of convolution neural networks is that the input image is convoluted by three trainable filters and additive bias. after convolution, three feature mapping maps are generated at layer c1. then four pixels of each group in the feature mapping map are summated, weighted and biased, and three feature mapping maps at layer s2 are obtained through a sigmoid function. 361maximum likelihood classification of soil remote sensing image based on deep learning these maps are filtered to get layer c3. this hierarchy produces layer s4 again, as does layer s2. finally, these pixel values are rasterized and connected into a vector input to the traditional neural network to get the output (zhu et al., 2019). based on the above convolution neural network, the object detection and extraction of soil remote sensing images are carried out. the specific process is as follows: (1) convolutional neural network training step 1: select the remote sensing image of soil for training as the input sample of the model. step 2: set the weights between layers, the thresholds of output units and hidden units to random values close to 0, and initialize the accuracy control parameters and learning rate of the model. step 3: take an input mode x from the training group and add it to the network, and give its target output vector d. step 4: calculate an intermediate output vector h and the actual output vector y of the network. step 5: compare the element yk in the output vector with the element dk in the target vector, and calculate m output error terms. table 2. resampling method of pixel luminance method definition characteristic sketch map knn the gray value of the nearest neighbor of the four adjacent pixels around the sampling point is taken as the gray value of the point. the method is simple, fast and does not change the value of the original raster, but the processed image is not smooth enough. bilinear interpolation linear interpolation of gray values of four adjacent points in two directions is used to obtain gray values of points to be sampled. the result is smoother than that of the nearest neighbor method, but it will change the original grid value and lose some small features. it is suitable for continuous data representing the distribution of some phenomena and topographic surface. cubic convolution interpolation cubic interpolation using the gray value of the pixels in the larger neighborhood around the sampling point. it can make the image smooth and have a good visual effect, but it will destroy the spectral information of the image. this method can be used when data processing based on spectral analysis is no longer needed, but only for graphical representation. https://blog.csdn.net/u010510549/article/details/62427122 https://blog.csdn.net/u010510549/article/details/62427122 362 shujun liang1, jing cheng2*, jianwei zhang1 step 6: calculate l error terms for hidden elements in the middle layer. step 7: calculate the adjustment formula of each weight value and the adjustment formula of threshold value in turn. step 8: adjust weights and thresholds. step 9: when k goes through 1 to m, judge whether the index meets the accuracy requirement: e≤ε, where e is the total error function. if not satisfied, go back to step 3 and continue iterating; if satisfied, go to the next step. step 10: at the end of the training, save the weights and thresholds in the file. at this time, it can be considered that the weights have been stabilized and classifiers have been formed. when training again, the weights and thresholds are directly derived from the file for training without initialization. (2) realization of soil remote sensing image detection step 1: use the trained convolution neural network to extract image texture features, and convolution calculation is carried out to obtain image feature map. step 2: make sampling of the feature map of soil remote sensing image by using adaptive pooling model step 3: merge the feature map into a column of feature vectors, and input to the full connection layer. update the weights of the network filter through label data back propagation algorithm. step 4: finally, the feature column vectors are input into softmax to complete the object extraction of soil image. figure 8. artificial neural network figure 9. deep artificial neural network figure 10. convolutional neural network 363maximum likelihood classification of soil remote sensing image based on deep learning classification of soil remote sensing image based on maximum likelihood estimation maximum likelihood classification (mlc) has a rigorous theoretical basis. it is easy to establish a class discriminant function with normal distribution. it combines the mean, variance and covariance of each category in each band. it has good statistical characteristics and has been considered as a more advanced classification method. in traditional remote sensing image classification, maximum likelihood method is widely used. this method obtains the mean and variance of each category by statistics and calculation of the interested region, and then determines a classification function. then each pixel in the image to be classified is substituted into the classification function of each category, and the category with the largest return value of the function is regarded as the category of the scanned pixels, so as to achieve the classification effect. its basic principle process is as follows: step 1: determine the area to be classified and the number of bands and feature classifications to be used, and check whether each band or feature component has been positioned with each other. step 2: according to the ground condition of the typical area, choose the training area on the image. step 3: calculation of the parameters. calculate and determine the prior probability according to the image data of the selected training areas. step 4: classification. substitute the image pixels outside the training area into the formula one by one. for each pixel, it is calculated several times in several categories. finally, the size is compared, and the largest category is selected. step 5: generate a classification map and specify a value for each category. if each category is divided into 10 categories, it is determined that each category is 1, 2, ..., 10. the classified pixel values are replaced by category values. the final classified image is thematic image. because the maximum gray value is equal to the number of categories, it needs to add different colors to all kinds, when displaying on the monitor. step 6: if there are many errors in the classification, it needs to re-select the training area and do the above steps until the results are satisfactory. based on the principle of maximum likelihood method, the process of soil remote sensing image classification is as follows: step 1: preprocess the image; step 2: initialize the number of classifications and training parameters to determine the number of data blocks and threads; step 3: establish and initialize grid threads and the amount of data to be computed by each thread. step 4: detect the connectivity of the network and grid nodes, and add a grid computing thread model to the grid; step 5: each thread carries out cyclic grid calculation according to the sample’s subset data, and all data in the grid unit are computed to get the category and stored to this node. step 6: transfer the results from each grid site to the host computer, and merge the grid results. step 7: release grid resources and output classification results to files. example analysis general situation of test area lindian county is located in the western part of heilongjiang province, the hinterland of songnen plain, with an area of about 3 500 km2. it belongs to daqing city, heilongjiang province. it has a continental monsoon climate in the middle temperate zone. it is rainy and warm in summer, dry and cold in winter. its geological structure belongs to the paleo-asian tectonic domain. it has abundant wetland resources and natural rivers such as uyur river and shuangyang river. the elevation is low and terrain is flat, but microtopography is more complex and low-lying leads to difficult drainage, which is easy to form marshes. according to the data of the second national soil census, most of the parent materials of the soils in lindian county are loesslike deposits (palagan & geetha, 2016). there are four main soil types (by the second national soil census). they are chernozem, meadow soil, marsh soil and aeolian sandy soil. among them, chernozem accounts for more than 60% of the total soil. chernozem is also an important part of black soil resources, and its organic matter content is high. soil fertility is large, which is a very suitable soil type for grain production. the landsat 8 oli image of lindian area on may 3, 2014 is selected for this test. the study area in the image is almost covered by clouds. in may, it is bare soil period. there is neither a large area of vegetation nor snow. it meets the research requirements of this paper for bare soil period, as shown in figure 11. figure 11. test zone experimental environment and methods in this paper, tensor flow machine learning framework under linux is used, and python language is used for programming. the hardware environment is intel (r) xeon (r) e5-2630 cpu, nvidia tesla m40 upu and 12 gb memory. in order to verify the effectiveness of the proposed method, three groups of experiments are set up. the first group of experimental data uses the proposed method to classify soil remote sensing images, the second group uses deep learning-based methods to classify soil remote sensing images, and the third group uses maximum likelihood estimation to classify soil remote sensing images. sample training samples are divided into four categories: chernozem, meadow soil, marsh soil and aeolian sandy soil. there are 10 samples in each category and 80 samples in total. 40 samples are used for experimental training and the remaining 40 samples are used for testing. the parameters of the trained convolutional neural network are shown in table 3. test verification in the trained model, the remaining sample data sets are input for a performance test, and the experimental results are expressed in the form of confusion matrix. confusion matrix, also known as error matrix, is a standard format for accuracy evaluation, which is expressed in the matrix form of n rows and n columns. specific evaluation indicators include overall accuracy, mapping accuracy, user accuracy and so on. these accuracy indicators reflect the accuracy of image classification from different aspects (sato, 2012; zhu, 2016). each column of confusion matrix represents the prediction category, and the total number of each column represents the number of data predicted for that category; each row represents the true belonging category of data, and the total number of data in each row represents the number of data instances in that category. the sum of each row represents the true number of samples for the category, and the sum of each column represents the number of samples predicted for the category. there are 40 test sample data in this experiment, which are predicted to be four types (chernozem, meadow soil, marsh soil and aeolian sandy soil), each of which has 20 samples. after the three methods are classified, the confusion matrix is shown in tables 4, 5 and 6. 364 shujun liang1, jing cheng2*, jianwei zhang1 table 4. confusion matrix of the classification results of the proposed method project prediction classification black calcium soil meadow soil swamp soil aeolian sand soil actual classification chernozem 20 0 0 0 meadow soil 0 20 0 1 marsh soil 0 0 20 0 aeolian sandy soil 0 0 0 19 table 5. confusion matrix of classification results of soil remote sensing image classification method based on deep learning project prediction classification black calcium soil, meadow soil swamp soil aeolian sand soil actual classification chernozem 17 2 1 0 meadow soil 0 15 1 1 marsh soil 2 2 16 3 aeolian sandy soil 1 1 2 14 table 6. confusion matrix of classification results of soil remote sensing image classification method based on maximum likelihood estimation project prediction classification black calcium soil meadow soil swamp soil aeolian sand soil actual classification chernozem 18 2 1 1 meadow soil 0 14 1 1 marsh soil 1 3 18 3 aeolian sandy soil 1 1 0 15 comparing tables 4, 5 and 6, it can be seen that after the proposed method is used to classify, 20 samples in the first, second and third rows of the confusion matrix established according to the results correspond to the first, second and third categories respectively, indicating that the samples are all correctly predicted, and only one sample belonging to the fourth category in the fourth row is misclassified into the second category. this result is much better than the other two methods, which shows that the performance of the proposed method is better. conclusions in summary, in order to give full play to the advantages of the maximum likelihood method, a classification method for making best use of the advantages and bypass the disadvantages is designed, that is, the maximum likelihood classification method for soil remote sensing images based on deep learning. firstly, deep sensing is used to detect remote sensing image targets and extract the target classification features of multiple kinds of images; then using the maximum likelihood estimation algorithm, soil remote sensing images are to classify. finally, the experimental results show that the proposed method not only can realize the classification of remote sensing data, but also has a higher overall classification accuracy. compared with the two single classification methods, the classification results obtained by the proposed method are more accurate. acknowledgement this work was supported by research on key technology of disease biomarker identification based on deep learning, a grant from key r&d and promotion projects in henan province of china (grant no.192102210132). references cheng l. y., meng x. y., & da, x. m. 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(2017). score-based tests of differential item functioning via pairwise maximum likelihood estimation. psychometrika, 83(1):132-155. xu, y., smith, s. e., grunwald, s., abd-elrahman, a., & wani, s. p. (2017). evaluating the effect of remote sensing image spatial resolution on soil exchangeable potassium prediction models in smallholder farm settings. journal of environmental management, 200:423. zhao, y., zeng, x., qiang, g., & xu, m. (2018). an integration of fast alignment and maximum-likelihood methods for electron subtomogram averaging and classification. bioinformatics, 34(13):i227-i236. zhu, d. (2016) a two-component mixture model for density estimation and classification. journal of interdisciplinary mathematics, 19(2):311319. zhu, h., jiao, l., ma, w., liu, f., & zhao, w. (2019). a novel neural network for remote sensing image matching. ieee transactions on neural networks and learning systems, pp(99):1-13. keywords: bathymetry; carving; kriging; reservoir; sedimentation; palabras clave: batimetría; tallado; kriging; embalse; sedimentación; how to cite item saito, y. k., ferreira-viana, l. j., oliveiraferreira, i., gomes-marques, e. a. (2021). sedimentation in reservoirs. case study: the reservoir of the são bartolomeu stream, viçosa, minas gerais, brazil. earth sciences research journal, 25(2), 193-200. doi: https://doi. org/10.15446/esrj.v25n2.79584 sedimentation in bodies of water is a worldwide problem that impairs its navigability and utilization, and, in reservoirs, this process reduces their partial or total storage capacity. using single-beam bathymetric survey techniques, this study evaluates the sedimentation process in a reservoir section at the federal university of viçosa (ufv), located in viçosa, minas gerais, in the southeast region of brazil. an analysis of the charts showing the dem (digital elevation models) and the elevation x surface area x accumulated volume for three surveys conducted in 2010, 2012, and 2016 provided the sedimentation rates between these years, as well as the accumulated volume of sediments in the area under analysis for each year. the information we collected shows sediments accumulated in the deepest part of the reservoir in the period between 2010 and 2012. from 2012 to 2016, however, the sedimentation effect was concentrated in shallower depths, whereas the desilting was observed in the deepest parts. moreover, the sedimentation rate in the analyzed area may be linked to the total precipitation in the municipality during the concerning time frame, as well as to changes in land use that occurred over the past years throughout the watershed. this data indicates a call for management actions to reduce soil erosion in the watershed, as well as a need for desilting of the reservoirs in order to recover its storage capacity. abstract sedimentation in reservoirs. case study: the reservoir of the são bartolomeu stream, viçosa, minas gerais, brazil sedimentación en embalses. estudio de caso: el embalse del arroyo são bartolomeu, viçosa, minas gerais, brasil issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.79584 la sedimentación en las masas de agua es un problema mundial que perjudica su navegabilidad y utilización, y, en los reservorios, este proceso reduce su capacidad de almacenamiento parcial o total. usando técnicas de levantamiento batimétrico de haz simple, este estudio evalúa el proceso de sedimentación en una sección de reservorio en la universidad federal de viçosa (ufv), ubicada en viçosa, minas gerais, en la región sureste de brasil. un análisis de las tablas que muestran los modelos de elevación digital (dem) y el volumen acumulado x área de elevación x área de tres encuestas realizadas en 2010, 2012 y 2016 proporcionó las tasas de sedimentación entre estos años, así como el volumen acumulado de sedimentos en el área en análisis para cada año. la información que recopilamos muestra los sedimentos acumulados en la parte más profunda del reservorio en el período comprendido entre 2010 y 2012. sin embargo, desde 2012 hasta 2016, el efecto de sedimentación se concentró en profundidades menores, mientras que el desecado se observó en las partes más profundas. además, la tasa de sedimentación en el área analizada se relacionó con la precipitación total en el municipio durante el período de tiempo correspondiente, así como con los cambios en el uso de la tierra que se produjeron en los últimos años en toda la cuenca. este dato indica un llamado a acciones de manejo para reducir la erosión del suelo en la cuenca, así como la necesidad de desecar los reservorios para recuperar su capacidad de almacenamiento. resumen record manuscript received: 08/05/2021 accepted for publication: 11/12/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 193-200 youlia kamei saito, lucas josé ferreira viana, ítalo oliveira ferreira, and eduardo antonio gomes marques federal university of viçosa, civil engineering department, campus of viçosa, viçosa – minas gerais, brazil corresponding authors: youlia.saito@ufv.br; lucas.jose@ufv.br; italo.ferreira@ufv.br; emarques@ufv.br h yd ro lo g y https://doi.org/10.15446/esrj.v25n2.90210 https://doi.org/10.15446/esrj.v25n2.90210 https://doi.org/10.15446/esrj.v25n2.90210 mailto:emarques@ufv.br 194 youlia kamei saito, lucas josé ferreira viana, ítalo oliveira ferreira, and eduardo antonio gomes marques introduction sedimentation, that is the deposition or accumulation of sediment that is suspended throughout a body of water, is one of the main issues affecting reservoirs, regardless of their characteristics or purpose. sediment deposition is a natural process largely caused by erosion as a consequence of rainwater transporting suspended sediments to the streams (vanmaercke et al., 2011). sediment deposition causes significant environmental damages and economic impacts to reservoirs, including loss of storage capacity, turbine damage, and loss of energy production (icold, 2009). these sediments are retained at the entrance of the reservoir and in its tributaries, forming a pluvial delta. one of the causes of sedimentation is the removal of vegetation from the soil through inadequate use (cabral, 2005). the construction planning and maintenance of a reservoir demand analysis of the sedimentation process in the area can be estimated through bathymetric surveys (valent et al., 2019), which is completed by performing measurements of depth in relation to a position on the surface (iho, 2005; ferreira et al., 2015). the monitoring of sedimentation in reservoirs is crucial for the understanding of their operational life, especially if they are intended to generate electricity or supply water to the population (yutsis et al., 2014). the saae (serviço autônomo de água e esgoto), a water utility company, is in charge of the supplying water to the municipality of viçosa, minas gerais, brazil. both saae and ufv (federal university of viçosa) collect water from the small artificial reservoir that is the focus of this study. in recent years, viçosa has been facing a concerning water supply crisis, mainly due to population growth, expansion of the university campus, and more severe drought periods. in addition, the reservoir has lost a significant amount of storage capacity since construction in the 1960s (information on the construction decade was provided by ufv administration). moreover, viçosa has an uneven rainfall distribution over the year, with a high volume of rain during the rainy season and a low volume during the dry season, a condition that has recently become worse as an impact of climate change (inmet, 2016). therefore, viçosa’s water resource demands appear to be insufficiently met and the sedimentation process of the studied reservoir accentuates this worrying scenario the city is experiencing. consequently, the loss of useful storage capacity in the reservoir, caused by sedimentation, becomes more serious and demands proper management in order to abate this process and to promote a better use of land in the upstream parts of the watershed, improving infiltration and reducing runoffs. furthermore, the process of erosion and sediment carriage in regions of tropical climate, such as viçosa, is much more intense than in regions of temperate climate, where similar studies are more widespread. another significant aspect is that the reservoir is located in a watershed that is typical of the southeast region of brazil in terms of climate, topography, geology, and land use. this ensures the relevance of this paper, since, in the literature, there are few available studies carried out under these conditions. as such, a thorough study in the area provides relevant information to minimize the current problems in viçosa. this study determined the sedimentation rate of the reservoir by analyzing bathymetric data collected in 2010, 2012, and 2016. moreover, we detailed important information about how the sedimentation process occurs in the studied reservoir, which can be used for planning intervening actions, allowing a proper management of its storage capacity (kondolf et al., 2014). materials and methods study area the studied area is located on ufv’s campus, in viçosa, minas gerais, brazil, between the geographic coordinates 20º46ʹ01.58ʺs, 42º52ʹ15.27ʺw and 20º45ʹ57.37ʺs, 42º52ʹ11.38ʺw (fig. 1). figure 1. hydrographic basin for the studied reservoir and the studied area data source: ibge (2008) 195sedimentation in reservoirs. case study: the reservoir of the são bartolomeu stream, viçosa, minas gerais, brazil viçosa is located at the southeast region of minas gerais state. the region has a predominantly rugged relief with altitudes varying from 397 m to 1,005 m. the average altitude is approximately 650 m. the region’s climate is tropical with hot, rainy summers and cold, dry winters. it is classified as cwa on köppen´s system, with an average annual temperature of 19 ºc, with an average of the maximum equal to 26.1 ºc and the minimum of 19 ºc; and the average annual rainfall is equal to 1,314.2 mm (castro et al., 1983). the reservoir, which was built in the 1960s, belongs to the ribeirão são bartolomeu watershed and flows into the turvo sujo river. this last one flows into the piranga river. all of them are affluent of the rio doce river and belong to its watershed. the ribeirão são bartolomeu is one of the main sources of the water supply of viçosa. the area was selected to comprise the furthest downstream fraction of the second reservoir in a small stream named são bartolomeu (fig. 1). this reservoir is approximately 667 m long, with a maximum width of 66 m. methods 2010 and 2012 bathymetric surveys the data from the 2010 survey was collected and processed by ferreira (2011) in december of that same year, for his final msc project in surveying and cartographic engineering carried out in the ufv. the reservoir’s contour limit and other details of the studied area were surveyed through a total station, that is a device for topographic measurements which measures vertical and horizontal angles as well as linear distances. three benchmarks were materialized in the vicinity of the studied area using gps positioning to support the topographic survey. we also performed a leveling was also to obtain orthometric heights using the rn3307f benchmark as a reference. in the bathymetric survey, a single-beam echo sounder and a pair of gtr -g2 rtk gnss receivers were used to collect depths and position data, respectively. a 4.5 m interval was employed in the survey lines, while the 10.0 m interval was applied to the check lines. the data from the 2012 survey is part of the mapping project of the lakes located on ufv’s campus, which was carried out in june of that same year. the information about the survey was taken from a technical report entitled “mapeamento dos relevos submersos das lagoas da ufv” (mapping of the submerged topography of the ufv lakes) (in portuguese). the contour limit of the reservoir was taken from an orthorectified ikonos ii image with a 1 m spatial resolution. the elevations were obtained from the rn3307f benchmark by using a total station and applying a leveling method known as leap frog. the bathymetric survey followed the same methodology used in 2010. both 2010 and 2012 data used in this work were taken from an excel spreadsheet containing the planimetric coordinates (sirgas 2000 reference system) and the orthometric height (data materialized by the station in the port of imbituba, brazil) of each point, which were collected by the singlebeam echo sounder after eliminating spikes, positioning errors, and estimating the survey’s accuracy. the data processing followed the same methodology adopted for the information obtained in 2016. auxiliary procedures of the 2016 bathymetric survey before performing the bathymetric survey, we needed to set a reference point to measure the water level. to that end, we chose a nearby benchmark named m0 as a reference point. a gps survey was carried out to determine its coordinates. we performed a leveling survey to obtain the orthometric height of the reservoir’s water line using the method known as fly leveling. the elevations references were determined according to the average sea level surface, based on tide gauge measurements of the station located in the port of imbituba, santa catarina, brazil (ibge, 2008). the survey was referenced to a benchmark nearby that belongs to the altimetric network of the brazilian geodetic system. we determined the elevation of the m0 reference point and then obtained the elevation of the reservoir’s water line. 2016 bathymetric survey ferreira (2011) provided the dwg file of the contour line of the reservoir. according to abnt (1994), the smallest size identified on a map is 0.2 mm, due to the human visual acuity. then, targeting a maximum error of 10 cm for the final result, we selected a scale of 1:500. survey lines depend on the resolution and the scale of the final result, and aneel et al. (2000) claimed that they must be 1 cm apart from the final result, which corresponds with a 5 m interval when considering the scale used in this study. to avoid areas between survey lines that did not provide any data, we adopted a 4.5 m interval between these lines, as well as a 10 m interval for the check lines. all of this information was input in the hypack software. in the field, we had to verify the reservoir’s water level in order to identify variations that might directly affect the volume calculation, which is a task that needs to be performed immediately before and after the bathymetric survey. in this procedure, a graduated cylinder was placed on a suitable and stable location on the bank of the reservoir, so that we could verify the water level and then obtain the height difference between m0 and the water line using a level. subsequently, we connected all of the equipment to the hypack software and measured the draft, the distance between the water line and the bottom of the transducer that is underwater. the observed depths are then corrected by the addition of the draft value. in addition, the average speed of the underwater sound was estimated using the bar check method, detailed in iho (2005) and ferreira et al. (2015). next, we collected the bathymetric data using the singlebeam echo sounder for sounding data, and obtained the position data (sirgas 2000 reference system) using the javad (real time kinematic rtk, with corrections received through the networked transport of rtcm via internet protocol ntrip). a great amount of aquatic plants, mainly during the drier period of the year severely hindered the project’s progress. the first step of the bathymetric data processing included the use of the hypack software for manually deleting spikes – noises from the surveyed lines in the raw data – to ensure better quality in the final product. despite being sometimes a subjective and a time-consuming process, eliminating spikes manually is a common procedure used to remove those random points that clearly do not represent the bottom surface of a study area (calder and smith, 2003). this process was very cumbersome due to the great number of aquatic plants in the studied area. the point cloud does not represent the submerged topography in a spatially continuous manner, therefore requiring an interpolation method. we applied kriging interpolation in accordance to the descriptions presented by ferreira et al. (2013) and ferreira et al. (2015). according to these authors, a supervised kriging interpolation allows significant gains in the results, which may contain considerable errors when a previous rigorous geostatistical analysis is not considered. kriging provides a solution to this problem based on a continuous model of stochastic spatial variation. this variation considers how a given property varies in space through the semivariogram function, and it involves a few distinct types of kriging that are divided into linear and nonlinear models. in linear kriging, which was used in this research, the estimates are weighted linear combinations of the data. the weights are distributed to the data samples based on the nearby points. the total number of weights can be also set by the user in a way that generates estimates with minimal kriging variation. moreover, the estimates are unbiased. (webster and oliver, 2007). according to camargo (1998), the steps involving a geostatistical analysis consist of exploratory data analysis, semivariogram generation and modeling, model validation and interpolation, and prediction of uncertainty. ferreira et al. (2013) listed and described these steps, which were used as reference for the data interpolation of the three surveys in this study. the use of kriging interpolation provided several statistical indicators, such as root mean square (rms), coefficient of determination (r²), root mean square standardized error (rmss), and coefficients of the simple linear regression (both dependent and independent variables), considering the observed and predicted depth values calculated through the interpolation. subsequently, a dem for each survey was generated from the spatial representation model of the interpolation created at the end of the kriging process. a 0.5 m interval was used for the contour lines, and the arcgis was employed to obtain the volumes for different water line levels by generating a triangular irregular network (tin). for this step, we considered a plane with constant elevation that passes through the tin. afterwards, the software algorithm calculated the cumulative volume between the horizontal plane and the tin, providing the values of volume and surface area. in this study, we considered different planes at different elevation values, from the surface of the reservoir down to a level close to the bottom, considering an interval of 0.5 m. 196 youlia kamei saito, lucas josé ferreira viana, ítalo oliveira ferreira, and eduardo antonio gomes marques as such, values of volume and surface area (in m³ and m², respectively) were obtained for each elevation (in this study, elevation equals orthometric height). furthermore, considering that the elevation of the water line is different for each bathymetric survey, the lowest value was used as a reference level. this important procedure is required to ensure that there are values of volume and surface area for each elevation considered (those elevations at the interval of 0.5 m previously mentioned) in each survey; otherwise, a comparison would be impossible. if any other elevation value of the water line is considered, there would be at least one survey with lack of data above the water line elevation of that survey. surveys uncertainty analysis bathymetric surveys should meet standards established by the brazilian navy’s board of hydrography and navigation (dhn), which follow the standards defined by the international hydrographic organization (iho). for depth accuracy, considering a 95% confidence level, the error limits can be calculated using the following expression (iho, 2005): ± +( )   a b d2 2. (1) where a is a sum of all constant errors; b is a factor of depth dependent error; and d is the average depth. table 1 shows the constants a and b that are given according to the hydrographic surveys classification. table 1. constants to calculate errors limits for depth accuracy on bathymetric surveys. order special1 1a2 1b3 24 depth accuracy for reduced depths (95% confidence level) a = 0,25 m b = 0,0075 a = 0,50 m b = 0,013 a = 0,50 m b = 0,013 a = 1,00 m b = 0,023 1 it is the most rigorous of the orders and its use is intended only for areas where under-keel clearance is critical. 2 for areas where the sea is sufficiently shallow to allow natural or man-made features on the seabed to be a concern to the type of surface shipping expected to transit the area but where the under-keel clearance is less critical than for special order. 3 for areas shallower than 100 meters where a general depiction of the seabed is considered adequate for the type of surface shipping expected to transit the area. 4 it is the least stringent order and is intended for areas where the depth of water is such that a general depiction of the seabed is considered adequate. fonte: iho, 2005. a bathymetric survey meets the quality standards when the total propagated uncertainty (tpu), suggested by ramos (2007) and presented below, is less than the error limits calculated using the expression 1. tpu erro média= +1 96 2 2,   (2) erro 2 and média 2 are the standard deviation and the standard error of the mean respectively. in order to estimate them, check lines are performed orthogonally to the main lines when surveying. at the intersection points between the main lines and the check lines, the differences of the reduced depths values are computed. the standard deviation and the standard error of the mean are estimated from the set of the not null values obtained (ramos, 2007). sedimentation estimation the sedimentation estimation was based on the analysis of the volume data calculated by the arcgis software. we obtained values of water volume through elevation data from three years. by taking an elevation value and analyzing the water volume variations among the surveys, we estimated the accumulated volume of sediments for that value of elevation. in other words, if the variation (previous value minus current value) was positive, it meant that there was a “loss” of useful volume in the reservoir for that elevation value, which characterizes a sedimentation process. conversely, if the variation was negative, there was a “gain” of useful volume in the reservoir for that elevation value, that is, a sediment loss (desilting). this analysis was possible due to the fact that we only considered the top layer of fluid mud. finally, we computed charts to obtain useful information, such as an estimate of the value of total sediment accumulated between the performance of each survey and an estimate of the annual sedimentation rate. results from 2010 survey, the tpu obtained was 0,185 m. special order classification in that case set a depth accuracy for reduced depths of 0,252 m, at 95% confidence level. from 2012 survey, the tpu obtained was 0,317 m. 1a order classification in that case set a depth accuracy for reduced depths of 0,502 m, at 95% confidence level. that means 2010 and 2012 surveys meet the quality standards established by the dhn, along with iho`s. it is important to highlight though that special and 1a orders require full search of the bottom, which is possible only when using multibeam echosounders (iho, 2005). from the auxiliary procedures performed for the 2016 bathymetric survey, the latitude and longitude of m0 were equal to 20° 45’ 57.6” s and 42° 52’ 11.6” w, respectively. regarding the fly leveling from m0 to the benchmark located in the laboratory of surveying and cartographic engineering of ufv, the error between leveling and check leveling was equivalent to 3 mm. according to abnt (1994), the closing tolerance for leveling for the employed methodology can be divided into four classes: in, iin, iiin, and ivn (the in class requires the highest accuracy, and the ivn class requires the lowest). the leveling performed by us was classified as in. moreover, the orthometric height of the water line for the 2016 survey was equal to 654.2 m. technical problems prevented check lines from being probed during the 2016 survey, which made it impossible to estimate vertical uncertainty as done for 2010 and 2012 surveys. however, as cited by ferreira et al. (2016), the sounding system used in the surveys meets the requirements of the brazilian navy standards for execution of hydrographic surveys (normam-25), being able to perform surveys of special order, considering the average depth of the surveyed area. furthermore, considering the recommendations of the national water agency (ana, 2013), the system used also meets the minimum requirements for the 2016 bathymetric survey main purpose, that is, for estimating the sedimentation at the study site. these results show that the data acquired in all surveys meet the current requirements established by the brazilian standards and can be used to estimate the sedimentation as well as the capacity of reservoirs. since the variable used to classify the data is directly proportional to the uncertainties associated with the data, 2010 survey proved to be more reliable, despite the others surveys also proved to be reliable enough to be considered to this study’s purpose. kriging the results generated by the exploratory analysis of the data for all the surveys followed the supervised kriging interpolation procedure described by ferreira et al. (2013) (table 2). regarding symmetry, the data from 2010 and 2012 were asymmetrical to the right due to the substantial positive asymmetry to the right in 2010, while minor positive asymmetry to the right in 2012, and a very slight negative asymmetry to the left in 2016. as for the kurtosis, which measures the proximity degree of the distribution values around the center, only the 2010 survey presented a noticeably high value. this indicates that the 2010 data is closer to a regular distribution curve when compared to the other years. another remarkable aspect is how the minimum value of altitude decreased over the years. the data set also showed the presence of second-order trends for all surveys. this is an important information, for the presence of a trend in the data could affect the identification and interpretation of spatial dependence in the semivariogram (ferreira et al., 2013). to perform supervised kriging with the presented parameters, we used universal kriging to remove the second-order 197sedimentation in reservoirs. case study: the reservoir of the são bartolomeu stream, viçosa, minas gerais, brazil trend terms in all surveys. to that end, a low-degree polynomial adjustment was used in the surveys performed in 2010 and 2012, while the gaussian model was employed in the 2016 survey due to its suitability. when modeling the semivariograms, we observed an absence of anisotropy (isotropic characteristics), which means that the variability or spatial distribution of the observations depends only on the euclidean distance, and rarely occurs in a single direction. therefore, a single semivariogram representing all directions, known as omnidirectional, was used for each survey. the theoretical model that best fit the experimental model for the semivariogram modeling was stable in all of the surveyed years. after the interpolation, we calculated, for each surveyed year, a simple linear regression between the data we obtained and the data estimated through the interpolation. the cross-validation results described the accuracy of the interpolation using supervised universal kriging (table 3). the variables a and b are the dependent and independent variables, respectively. the closer a is to 1 and b is to 0, the better is the kriging prediction performance. it is also possible to observe mean error, rms, and rmss values that provide further information about the quality of the interpolation. table 3. cross-validation results of the surveys 2010 2012 2016 a 0.99 0.98 1.00 b (m) 2.7 12.8 0.1 mean error (m) 7.4x10-3 -1.4x10-3 -0.5x10-4 rms (m) 0.3 0.2 0.1 rmss (m) 1.0 0.9 0.8 from the previous table, we conclude that the kriging prediction performance was better for 2010 and 2016 surveys. that means the model chosen for them fits better the respective data than the model chosen for the 2012 survey. after assessing the consistency of the prediction results, the kriging model generated dems for each year (fig. 2, fig. 3, and fig. 4). quantification of sedimentation following the calculation of accumulated volumes in relation to the elevation, as described in the methodology section, we generated elevation x surface area and elevation x accumulated volume diagrams with their respective trend lines (fig. 5). the analysis of the data that generated the charts demonstrate that, between 2012 and 2016, there was an increase in the accumulated volume in the lower section of the reservoir, ranging from 650.7 m to 652.2 m. however, the total volume of the reservoir up to the elevation of 654.2 m was equal to 19,360.7 m³ in 2010, 18,257.2 m³ in 2012, and 17,086.7 m³ in 2016. by observing fig, 2, fig. 3, and fig. 4, it is possible to see the occurrence of desilting in the reservoir bed, as well as sedimentation in areas of medium and low depth. from reservoir volumes, we obtained 1103.4 m³ of sediments from 2010 to 2012 and a sedimentation of 1170.6 m³ from 2012 to 2016. these values represent approximately 5.7% and 6.4% of the total volume of the reservoir figure 2. dem data from 2010 figure 3. dem data from 2012 figure 4. dem data from 2016 table 2. statistical parameters of the data collected in bathymetric surveys year 2010 2012 2016 mean (m) 651.3 652.9 651.6 median (m) 651.0 652.5 651.5 standard deviation (m) 1.0 1.9 1.3 variance (m²) 1.11 3.55 1.78 maximum (m) 655.4 655.4 654.2 minimum (m) 651.3 650.0 649.2 asymmetry 2.09 0.24 -0.02 kurtosis 8.60 1.50 1.68 198 youlia kamei saito, lucas josé ferreira viana, ítalo oliveira ferreira, and eduardo antonio gomes marques respectively and an average annual percent sedimentation of 2.9% and 1.6% respectively. white (2000) and rahmani et al. (2018) state that the worldwide loss in reservoir storage capacity is reported to be between 0.5% and 1.0% per annum. despite observing that the sedimentation rate in the study area was higher in both periods considered, rahmani et al. (2018) also concluded that smaller reservoirs experience higher values of annual depletion rates, which is fully consistent with the results of this current work. from 2010 to 2012, there was a substantial accumulation of sediments in the deepest part of the reservoir, while minor variation was found in the shallower part (fig. 6). it suggests that changes in land use with considerable soil excavation in the upper part of the watershed, which are a result of urbanization of rural areas for construction of residential condominiums, have increased soil exposure to erosion and lead to an increase in sediment transport to the reservoir under analysis. figure 6. curves of elevation x volume of accumulated sediments curves however, the analysis of the results from 2012 to 2016 shows a negative variation in sediment volume for deeper sites (650.5 m and 651.0 m), as well as a significant sediment accumulation from these elevations up to the water level. this is the result of two aspects: i. modification in the precipitation pattern from 2012 to 2016 (to be discussed in topic “management takeaways”), probably due to climate change effects, leading to an increase in soil erosion and sediment transport in the upstream portion of the watershed; ii. stabilization of urbanization, with increasing protection of soil in the residential areas. some new restrictions in the land use of these upstream areas have also contributed to this effect. therefore, an interesting phenomenon occurred from 2012 to 2016, where it is possible to observe a desilting process in the deepest part of the water body and a concentrated sedimentation in the shallowest part, whereas, from 2010 to 2012, the sedimentation accumulation was scattered, with only a slight concentration in the deepest part of the studied area. according to news from the newspapers of the time, such as the one quoted by the portal saneamento básico (2014), the dead volume of the reservoir was used to supply the population in the 2014-2015 drought season that occurred in the region. the construction of a reservoir in a stream affects the hydrological regime of a river in regards to water and sediment flow, leading to changes in its morphological processes. the transport of sediment affects water quality, including its thermal change, a factor that impacts the capacity of transporting sediment flow (webb and walling, 1996). this reduced particle flow, which is linked to the continuous flow of water, can change the longitudinal profile of the reservoir, causing erosion in the riverbed, also known as carving. marginal erosion in river channels by the action of river currents is something that happens continuously and spontaneously. however, carving usually tends to be more intense than lateral erosion, which gradually modifies the channel cross-section, since the bottom is being widened and deepened (coelho, 2008). valentin et al. (2005) observed such a case in reservoirs from several countries, such as australia, china, ethiopia, and the usa. in usa, it was observed in the hoover dam on the colorado river, where the greatest lowering of the average river bed was approximately 7.1 m at 12.4 km downstream from the dam (galay, 1983); and in china, it was observed in the sanmenxia reservoir on the yellow river (wang and wu, 2005), where the degradation was about 4.0 m at 68 km downstream from the dam (li et al., 1980; galay, 1983). the increased water volume in the deepest studied section occurred due to a carving process in the riverbed, as observed when we compare the dem images (figs 2 to 4). there was a widening and deepening of the bed, and a decrease in the total accumulated volume of the reservoir, which happened due to the sedimentation by fine floating sediments in areas with an elevation higher than 652.2 m. it is likely that these fine sediments were also carried downstream, where it was possible to observe a severe sedimentation of another small reservoir located in this area. it is worth mentioning that the minimum altitude in 2012 was lower than in 2010, though the total volume in 2010 (up to the water level) was higher than in 2012. the same characteristic is observed when comparing 2012 to 2016. this supports the conclusion that there was sedimentation in the studied area during the analyzed period, and that this process took place in shallow to medium depths, while the desilting happened in the deepest part. in the end, the process resulted in a decrease in the total available volume of this reservoir. management takeaways as previously mentioned in this work, for the first and second periods considered, the accumulated sediment in the study area represents approximately 5.7% and 6.4% of the total volume of the reservoir respectively and an average annual percent silting of 2.9% and 1.6% respectively. the significant difference in the average annual rates can be explained by the rainfall distribution in viçosa during the studied period. according to the data from inmet (2016), from december 2010 to june 2012, the total rainfall was equivalent to 2501.4 mm, and the average monthly rainfall was 131.6 mm. from june 2012 to september 2016, the total precipitation reached 4490.5 mm, while the average monthly rainfall reached only 86.4 mm. figure 5. diagrams of elevation x surface area and elevation x accumulated volume 199sedimentation in reservoirs. case study: the reservoir of the são bartolomeu stream, viçosa, minas gerais, brazil although the total rainfall volume was higher in the period between june 2012 and september 2016, inmet data shows that they were more evenly distributed over time. during the first interval, the rains were concentrated and more intense in the period between november to march of each year, thus increasing erosion processes. the data also shows that the average monthly rainfall was higher from 2010 to 2012 and has been decreasing progressively since 2013. it is important to mention that during the summer periods of 2013/2014 and 2014/2015, the southeastern region of brazil suffered a significant decrease in precipitation. these phenomena are related to the absence of the south atlantic convergence zone, for the first period, and the el niño phenomenon, for the second period, as detailed by coelho et al. (2016). the occurrence of more intense rains combined with the decrease in the total annual amount of precipitation brings up evidence of climate changes that have been noticed on a global scale. according to ipcc (2014), central and south america live under the potential risk of reduced water availability and increased flooding and landslides, which are also related to global climate changes. it can also be observed in viçosa. therefore, from all precipitation data discussed and all evidence previously presented, a very important relationship can be established between rainfall distribution and sedimentation in the studied area: more intense rainfall results in a higher rate of sediment accumulation, while moderate rains and drought periods result in a lower annual rate of sediment accumulation. in other words, heavier precipitation leads to an increase in sedimentation. land-use changes over time in the region have also contributed to the increasing sedimentation in the reservoir. in 1987 and 2007, costa et al. (2013) carried out a mapping of the land use in the são bartolomeu watershed to verify if permanent preservation areas (ppas) and legal nature reserves were actually being respected. ppas located along the banks of watercourses, when deforested, degraded, or improperly occupied, are susceptible to the effects of landslides and the consequent sediment transport to the riverbed, thus promoting its sedimentation (departamento de conservação da biodiversidade, 2011). these authors concluded that during this period the urban area expanded from 40.67 ha to 65.51 ha, mainly due to the construction of surrounding residential condominiums. coffee crop areas also increased. in general, all land use classes had an encroachment of over 30% in ppas during the evaluation period, therefore violating the current environmental legislation in brazil. all these changes along the são bartolomeu watershed increased the rate of soil detachment over time, as well as the consequent transport of these sediments along the stream. this fact endorses the need for the development of sustainable practices that should support a permanent reduction in soil transportation toward the reservoir. this is more significant during a period of climate change, as the precipitation is more intense and frequent, as well as longer in duration, resulting in more erosion and more sedimentation of the reservoirs in an area with an undulated terrain pattern and erosive soil – like the one observed on the watershed under study. in this context, some management measures can be proposed aiming at the reduction of soil erosion and desilting of the reservoir. for a short-term action, a desilting through dredging or mechanical excavation of the sediments could help improve the reservoir’s storage capacity. for a long-term action, a drastic change by restricting land use laws along the upper portions of the watershed should be implemented by the municipality, which might promote the recovery of areas that have been degraded. in addition, the municipality should restrict real estate speculation in the upper portion of the watershed. conclusions based on the results obtained from the bathymetric surveys, we observed that the downstream fraction of the second reservoir located in the são bartolomeu stream underwent a process of carving on its bed, as well as a recent sedimentation on its banks due to deposition of fine sediments. overall, these processes have been responsible for reducing the water storage capacity of the reservoir, since the total sedimentation rate of the reservoir was 1103.4 m³, and, as the urban area expansion is already underway in the upstream areas of the watershed, this process is expected to continue exerting that influence. in the studied area, we observed a correlation between the deposition of sediments over time and the occupation and land use in the são bartolomeu watershed. the deposition was also related to the intensity and distribution of rainfall in the region, with more intense rains and drier and longer winters being observed over the past few years as a reflection of local impact of climate change. in the period of heavy rains, there was a higher rate of sediment deposition, while, in the period of more moderate rains, the rate of deposition was much lower. in addition, the catchment area of the studied reservoir underwent several changes over the years, exposing the soil to rainfall erosion. consequently, there was an increase in the amount of transported sediment, which also explains the sedimentation in the area. this effect was reduced during the 2012 – 2016 period, as a response to a decrease in the average monthly rainfall and in land use in the upstream area. lastly, considering the current global scenario of more severe climate changes – mainly concerning rainfall distribution throughout the year – the reservoir’s managers and the municipal government should keep the database of bathymetric surveys updated, enabling them to support analyses of annual sedimentation rate and provide useful data to determine management actions for mitigating erosion processes along the watershed, thus improving the total capacity of the reservoir. we mentioned actions that include planning the territorial occupation efficiently (taking into account the regional reality and the characteristics of the soil), providing favorable conditions for infiltration (reducing areas of exposed soil), and preserving the springs in surrounding areas of natural vegetation. for future works, the sedimentation of the reservoirs can be estimated using mathematical modeling methods and erosion-transport empirical models. measurements and modeling can be compared considering subsequently scenarios for climate and land-use changes, as well as using field data for verification and validation. we suggest applying the soil & water assessment tool (swat) to predict the environmental impact of climate change and land use; 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(2014). bottom topography, recent sedimentation and water volume of the cerro prieto dam, ne mexico. geofísica internacional, 53, 27–38. https://doi.org/10.1016/s0016-7169(14)71488-6 http://www.inmet.gov.br/portal/index.php?r=bdmep/bdmep http://www.inmet.gov.br/portal/index.php?r=bdmep/bdmep https://doi.org/10.1002/2013ef000184 https://doi.org/10.1002/2013ef000184 https://doi.org/10.3390/w10020190 http://orcid.org/0000-0001-5629-8350 https://doi.org/10.3390/geosciences9040158 https://doi.org/10.3390/geosciences9040158 https://doi.org/10.1016/j.catena.2005.06.001 https://doi.org/10.1016/j.catena.2005.06.001 https://doi.org/10.1016/j.geomorph.2011.03.010 https://doi.org/10.1016/j.geomorph.2011.03.010 https://doi.org/10.1029/2004wr003919 https://doi.org/10.1029/2004wr003919 https://doi.org/10.1016/0143-6228(96)00007-0 https://doi.org/10.1016/s0016-7169(14)71488-6 letter from the editor our first issue of 2018 is available now. we have 9 articles in this issue, in topics ranging from soil characterization, petrophysical properties of rocks, signal processing of images and gnss station to asteroid impact effects. mechanical soil properties in sandy-pebble soil are studied as a function of grain size content in lu et al. satellite-derived soil moisture estimation is discussed in thanabalan and vidhya, based on a semi-empirical approach and backscattering images. macroscopic mechanical characteristics of rocks depend on a number of factors, including microstructure damage. under changing temperature conditions (freeze-thaw) rock samples studied by jiang show significant strength decrease, which has important consequences in engineering. advanced signal processing methods are used in zeng et al. for image retrieval applied to remote sensing data, using a bayesian network approach. similarly, oktar and erdogan use linear trend and wavelet analysis to continuous gnss data showing both displacements due to tectonic as well as atmospheric and hydrologic effects. debris flows can in some cases become serious hazards because they can block river flow as a debris flow dam. chen et al. propose a method to identify the formation of such dams, with an example from the er river in taiwan. mamaseni et al. study the petrophysical properties of three formations in the duhok basin, northern iran, based on well-log data. results suggest a significant thickness with good moveable hydrocarbons in the study area. methane adsorption and gas content are strongly influenced by shale composition. zhu et al. show that total organic content has a stronger influence on methane adsorption and gas content than the mineral composition, studying samples from the southern sichuan basin. our last contribution in this issue discussed a large asteroid impact in eastern colombia. the impact would have affected the environment and landscape, but the degree of the effect depends on a number of parameters including size of the impactor, angle of impact, type of rocks in the crust, and much more. a model is proposed by hernandez and alexander, that can predict both ejecta deposit distance as well as seismic shaking, and of course thermal radiation. results of this study provide additional tools for searching for evidences of direct impact craters. germán prieto editor in chief le tt er f r o m t h e ed it o r earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018) keywords: ridge-top landslide; debris avalanche; dynamic analysis; dan-w model. palabras clave: deslizamiento de tierra; avalancha de detritos; análisis de dinámicas; modelo dan-w. how to cite item yang, l., wang, w., zhang, n., & wei, y. (2020). characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china. earth sciences research journal, 24(2), 169-181. doi: https://doi. org/10.15446/esrj.v24n2.78990 a catastrophic landslide hit mount fugui, diexi township, mao county, sichuan province, at 05:38:58 on june 24, 2017. this landslide buried xinmo village, caused 83 deaths, and resulted in enormous losses of life and property. the landslide mass cut out and slid from a high position, loaded continuously, and accumulated at the top of the slope body. subsequently, the landslide mass was transformed into avalanche debris. this process was a typical chain disaster of avalanche debris triggered by a ridge-top landslide. the total volume, elevation difference, and horizontal distance of the landslide were 1637.6 × 104 m3, 1200 m, and 2800 m. in this research, the disaster-formation mechanism of the xinmo landslide was identified based on a geological field survey and remote sensing satellites. the disaster characteristics of the landslide source zone, debris avalanche zone, and accumulation zone were then numerically simulated. the entire process of the xinmo landslide movement was comparatively studied using dan-w, a dynamic landslide software program, and multiple rheological models. the research findings indicated that the frictional model favorably simulated the movement characteristics of various phases of the xinmo landslide. this landslide lasted approximately 120 s, and it had a maximum velocity of movement of 74 m/s. therefore, the frictional model and its parameters can be used in similar studies to investigate the dynamic disaster effects of ridge-top rock landslides. abstract characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china características y análisis del modelado numérico del deslizamiento de tierra en xinmo, provincia de sichuan, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.78990 un deslizamiento de tierra catastrófico afectó el monte fugui, de la localidad diexi, en el condado mao de la provincia china de sichuan, a las 05:38:58 hora local del 24 de junio de 2017. este deslizamiento enterró la población de xinmo, causó 83 muertos y significó enormes pérdidas de propiedades. la masa del deslizamiento se desprendió en lo alto de la montaña, se continuó cargando y se acumuló en la parte alta de la pendiente. seguidamente, la masa del deslizamiento se transformó en una avalancha de detritos. esta fue una cadena típica de un desastre ocasionado por una avalancha de detritos que comenzó por un deslizamiento en lo alto de la montañana. el volumen total, la diferencia de elevación y la distancia horizontal del deslizamiento fueron de 1637.6 × 104 m3, 1200 m, and 2800 m. en este trabajo se identificó el mecanismo de formación del deslizamiento de xinmo con base en un estudio de campo geológico y con el apoyo de satélites de teledetección. los autores simularon numéricamente las características del desastre en la zona fuente, en la zona de la avalancha y en la zona de acumulación. todo el proceso del deslizamiento de xinmo fue estudiado comparativamente con el programa dan-w, que analiza dinámicas de deslizamiento, y múltiples modelos reológicos. los resultados del estudio indican que el modelo friccional simuló favorablemente las características del movimiento en varias fases del deslizamiento de xinmo. este movimiento duró por lo menos 120 segundos, y tuvo una velocidad máxima de movimiento de 74 m/s. además, el modelo friccional y sus parámetros se pueden usar en estudios similares para investigar los efectos dinámicos de un movimiento de tierra en la parte alta de una montaña. resumen record manuscript received: 09/04/2019 accepted for publication: 07/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 169-181 longwei yang1,2,*, wenpei wang2, nan zhang2, yunjie wei2 1school of geological engineering and geomatics, chang’an university, china; 2china institute of geo-environment monitoring, cgs, beijing, china * corresponding author: yang0504@chd.edu.cn g eo te ch ni ca l en g in ee ri ng https://doi.org/10.15446/esrj.v24n2.78990 https://doi.org/10.15446/esrj.v24n2.78990 https://doi.org/10.15446/esrj.v24n2.78990 mailto:yang0504@chd.edu.cn 170 longwei yang, wenpei wang, nan zhang, yunjie wei introduction landslides are a major geological hazard due to their long runout distance, high mobility, and entrainment effect (evans et al.,2007; zhang m& mcsaveney,2017; yin et al.,2009). in recent studies, due to the tremendous economy losses and casualties every year, catastrophic landslides have been addressed as environmental disasters (gao et al., 2020; warrick et al., 2019; igwe o, 2018; zhang et al., 2016). this research work has primarily concentrated on the following aspects (fan et al., 2018; hu et al., 2018; wang et al., 2018; zhao et al., 2018): first, potential landslide identification plays an important role in disaster prevention. methods for landslide identification include field surveys, aerial investigations (such as interferometric synthetic aperture radar, unmanned aerial vehicle, and light detection and ranging), acoustic emissions, and micro-seismicity. these methods have been used to analyze the deformation process of potential landslides. furthermore, landslide susceptibility mapping (lsm) has been used to predict potential landslides, such as in the alternating decision tree-adaptive boosting model and the recurrent neural network model. these new technologies could offer the necessary information (coordination and volume) (wang et al., 2020; wu et al., 2020; wang et al., 2019; althuwaynee et al., 2014). in addition, a considerable amount of indoor testing of landslide mechanisms has been performed (wang et al., 2018; huang et al., 2019). currently, the analyses and prediction of landslide hazards are based on indoor tests, field geological survey methods, and lsm (huang et al., 2009; wang et al., 2019). these tests and methods are essential for identifying and preventing landslides, and for providing security for emergency rescue and disaster relief (fan et al., 2018; wang et al., 2018). however, these methods cannot precisely calculate the hazardous effects of catastrophic landslide dynamics or conduct risk assessments such as the run-out distance, impact force, and entrainment that are basis for the design of remedial engineering measures. additionally, numerical simulation methods provide more disaster information because they are able to compute landslide motion parameters, such as flow speed, deposit thickness, and the danger zone. the numerical methods primarily include discrete element methods and the continuum method (mcdougall, 2017). the discrete element methods simulate a sliding mass as a muster of particles. according to newton’s second law, the interaction force among granules can be computed. this is appropriate for debris flow, such as the particle flow code (itasca, 2008) and matdem (liu & pollard, 2013). the continuum method is mainly based on the mass and momentum equations that incorporate the earth pressure theory and simulate the dynamic features of a sliding body (pudasaini & hutter, 2003; hungr, 2004). the continuum methods have been successfully applied in the simulation of geological hazards, such as landslides, avalanches, and debris flows (jeong et al., 2018; liu et al., 2014). the continuum methods have evolved into different models, such as d-claw (iverson & george, 2014), sassa-wang (wang & sassa, 2002), and dan (hungr & evans, 1996). based on the motion equation and fluid continuity equations, hungr established the dynamic analysis software of landslides (dan-w) along with different rheological models, such as the frictional model, the plastic model, the bingham model, and the voellmy model. in addition, a sliding body is regarded as an equivalent fluid for analysis in dan-w. in this research, a numerical dynamic simulation analysis of the 2017 xinmo landslide in maoxian, sichuan, china was conducted. it was characterized by its rapid velocity and long runout distance. according to field survey data, dan-w was used to simulate the entire landslide process. in previous work, a comparison of the motion features of several similar long runout and highspeed landslides in the western areas of china was conducted. these studies examined the dynamics of the sanxicun landslide (yin et al., 2016), the panjin landslide (yang et al.,2019) and the wenjiagou landslide (xing et al., 2017). in this study, it was shown that the frictional model was capable of a more accurate inverse analysis of the entire landslide process. the parameters and the numerical model scheme of this research can help precisely compute the motion features of similar potential landslides. methods we conducted field geological surveying in the songpinggou area before and after the landslide hazard, and analyzed the geological conditions of the xinmo landslide, including the geologic structure, formation lithology, and precipitation. we also used the multi-temporal remote sensing images before the hazard from the iknos sensor (resolution = 1.0 m), as well as the gf-2 sensor (resolution = 0.8 m). we use these images to find the key structures and the pre-sliding geomorphology before the landslide. according to the field geological survey data, unmanned aerial vehicle(uav) photograph, and the multi-temporal remote sensing images, the landslide area was divided into a landslide source zone, a debris avalanche zone, and an accumulation zone. in addition, dan-w, a landslide dynamic model, and multiple groups of rheological models were used to simulate the entire movement process of the xinmo landslide and to analyze its dynamic characteristics (i.e., movement velocity, movement distance, and thickness of the accumulation body). the reactivation mechanism and the disaster effects of the old landslide in the 1933 diexi earthquake were also analyzed. finally, it was determined that dan-w and the frictional-frictional-frictional (fff) model could effectively provide a reference for similar studies regarding the dynamic disaster effects of catastrophic rock landslides. the flowchart of this study can be described as shown in figure 1. figure 1. flow chart of the methodology. the xinmo landslide a catastrophic landslide hit diexi township, mao county, sichuan province, at 05:38:58 on june 24, 2017. located at the left bank of the firstgrade tributary of the minjiang river, songpinggou, the xinmo landslide lay at e103°39’ 03.4”, n32°04’ 09.4”, approximately 44.75 km from mao county (figure 2). the elevations of the landslide at the back and front edges were 3450 m and 2250 m, respectively. the elevation difference was 1200 m, the horizontal distance was 2800 m, and the volume of the accumulation body was 1640 × 104 m3. xinmo village was directly destroyed by the landslide, leading to 83 deaths. this landslide, which featured good concealment, a high altitude of initiation, and a huge impact, led to serious casualties and property losses. geologic conditions the xinmo landslide was located in longmenshan tectonic zone, which belongs to active area of tectonic movement (chen et al., 2015; jiang et al., 2002; wang et al., 2000; tang et al., 1991). the fugui mountain where the xinmo landslide was located was an earthquake-induced shattered mountain that formed in the epicenter zone of the 1933 ms7.5 diexi earthquake (with file:///e:/trabajos_2020/un/fc/revistas/esrj_24(2)/txt/javascript:; 171characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china an intensity level of x) and the strong motion zone of the ms 8.0 wenchuan earthquake (with an intensity level of ix) (figure 3). in terms of regional geology, this area has undergone s-n compressional tectonic stress several times (yin et al.,2017). the mountain slope behind xinmo village is in a dip slope, where the emergence strata consist of blocky, moderately thick and thin to moderately thick metamorphic quartz sandstone with phyllite and slate of the middle triassic zagunao formation (t2z), the occurrence of which is 184°∠ 53°. the strata show an undulating deformation because of compression. the steepdip structural planes, such as phyllite and slate, constitute a weak sliding zone. according to a field geological survey and the remote sensing image analysis (figure 4), there were two major preferred structural planes in the mountains of the sliding source area: (1) 100° ∠ 70° and (2) 350° ∠ 40°. the two figure 2. (a) location of the study area in china and (b) location of the xinmo landslide in the study area (base figure reproduced with kind permission from google earth). figure 3. regional seismic intensity map of the xinmo landslide.1 the location of the xinmo landslide, 2 city, 3 province, 4 provincial boundaries, 5 intensity contour of the diexi ms 7.5 earthquake in 1933, 6 intensity contour of the wenchuan ms 8.0 earthquake in 2008. sets of structural planes cut the rock into a grid on the plane and combined it with the weak structural plane of the bedding layer to separate the rock into a medium-thick layer and a thick layer of massive cataclastic rock. this caused the formation of readily sliding geological blocks. according to the rainfall data provided by the meteorological bureau of mao county (2011-2016), the multi-year average precipitation is 490.7 mm (figure 5) and the maximum daily precipitation is 75.2 mm. rainfall is concentrated from may to september when the cumulative precipitation can reach 353.3 mm or 72% of the annual precipitation. in 2017, data collected from meteorological stations at songpinggou and diexi township near the landslide showed that the cumulative precipitations from may 1 to june 23 exceeded 210 mm. the rain infiltrated along the mountain rocks and thus led to an increase in the hydrostatic pressure, which intensified the landslide failure. 172 longwei yang, wenpei wang, nan zhang, yunjie wei figure 4. characteristic zoning and comparison of the multi-temporal remote sensing images of the xinmo landslide(yin et al.,2017,modified). a overview zoning before sliding; b overview zoning after sliding; c-d remote sensing images before sliding, c---july, 2005 (iknos, r=1.0 m), d--april, 2017 (gf-2, r=0.8 m). e the photography before sliding(2017-06-22,to the northeast). figure 5. curve of the rainfall process around xinmo village zoning characteristics of typical chain disasters according to the movement and accumulation characteristics of the slide body, the landslide area was divided into a landslide source zone, a debris avalanche zone, and an accumulation zone (figures 4 and 6, table 1). an analysis of the remote sensing images before and after the landslide and a field survey indicated that the total area of these zones was 143.1 × 104 m2. the area’s longitudinal length, horizontal distance, elevation difference, maximum width in the front edge, and minimum width in the rear edge were approximately 3000 m, 2800 m, 1170 m, 1500 m, and 300 m, respectively. landslide source zone figure 7 shows the landslide source zone for the xinmo landslide. based on the field geological survey, the plane shape of the slip source region 173characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china was approximately “rectangular”. the landslide source zone had a dip length of approximately 400 m. the zone had a maximum thickness of 40 m (20 m thickness, on average), a width of 320 m, and a size of 320*104 m3. however, the rock mass in the landslide source zone could have increased up to 1.4 times its original volume when it cracked and broke up (yin et al., 2010). the mother rock strata were composed of blocky thick quartz sandstone, slate, and phyllite of the middle triassic zagunao formation. the elevation of the landslide source zone at the rear edge was 3450 m, the toe of the landslide source zone was about 3100 m, and horizontal distance of the landslide source zone was 350 m. the landslide source zone contained a group of nearly nne-ssw large joints or fracture zones that shared the same direction as the sliding direction, which provided the bulging deformation and the sliding of the rock mass with dominant surfaces of weakness. in addition, a deformation body lay on the west of the source area, with a volume of approximately 450 × 104 m3 (figure 7a). it was composed of bedding rock mass and an old residual slope accumulation, and it slid downward for 40 m. the inner part of the deformed body was broken, and its front and east sides were free faces. there was a small collapse that occurred in the front boundary of the deformed body. figure 6. engineering geological section of the xinmo landslide before and after sliding (yin et al., 2017, modified). (a) longitudinal profile before sliding, (b) longitudinal profile after sliding. 1 stratum of triassic zagunao, 2 accumulation area of the old landslide, 3 residual sliding mass of the old landslide, 4 accumulation area of the new landslide, 5 debris flow accumulation area, 6 sandwiched phyllite or schist, and 7 typical point. figure 7 characteristic of the sliding source zone of the xinmo landslide at the ridge-top. 1 elevation, 2 slip direction, 3 boundary of the sliding source zone, 4 the unstable slop zone, and 5 reverse fractured belt. 174 longwei yang, wenpei wang, nan zhang, yunjie wei debris avalanche zone figure 8 shows the debris avalanche zone of the xinmo landslide. according to a field survey, the original deposits on the slope were the residual accumulation body of an old landslide that was triggered by the ms7.5 diexi earthquake that took place in 1933, and the clastic debris cone was formed by the action of gravity from the top (cheng et al., 2008). this zone formed along the gully, and it was a pipeline-type debris avalanche for which the slope angle was 27°. the highest elevation of the debris avalanche zone was 3450 m, the toe of the debris avalanche zone was about 2400 m, and horizontal distance of the landslide source zone was 750 m. the zone had a maximum thickness of 30 m (15 m thickness, on average). the exposure of bedrock at a height between 2700 m and 2750 m divided the accumulation body into upper and lower sections, for which the upper section was about 540 × 104 m3 in size. the lower section was about 330 × 104 m3 in size. in total, the volume of this accumulation body was 870 × 104 m3. accumulation zone according to the remote sensing image analysis and a field survey (figure 8), the accumulation zone had a dip length of approximately 900 m, a width of 350 m, a maximum thickness of 50 m (20 m thickness on average), and a size of 580*104 m2 in size. the accumulation area was located at an elevation between 2440-2290 m. the accumulation zone fanned out and primarily accumulated on the left bank of songpinggou. it washed up on the right bank, where a landslide dam formed that had a gentle slope. the rock strata in the accumulation zone were composed of blocky thick quartz sandstone and slate. rock blocks in the accumulation zone were even in lumpiness and were generally cuboids whose side lengths were 0.5-1.5 m. the newly formed landslide accumulation zone overlapped the old landslide mass in the form of a diffusion flow. the old landslide mass was estimated to have an 80 m average thickness and a size of 200*104 m3. as a result, the total volume of the accumulated body in this zone was the volume of the avalanche debris body and that of the old landslide mass, for a total volume of 780 *104 m3. the initiation mechanism and post-failure behavior of the landslide the catastrophic landslide was analyzed by combining the geological conditions and the different zone characteristics as follows: there were multiple groups of discontinuous structural surfaces at the rear edge of the xinmo landslide, which cut the thick blocky rock mass into cataclastic rock blocks and formed a visible fracturing bulging zone at an elevation of 3150-3450 m. rainwater infiltrated the rock mass through the visible fracture cracks. the medium weathered quartz sandstone belonged to the middle triassic zagunao formation (t2z). this kind of rock is susceptible to strength reduction induced by rainwater infiltration. in particular, there were two groups of anti-dip joint zones that led to a ridge-top cutout. a sliding surface could then be easily formed. the rock was more broken, and the mountain surface of the sliding source area was much looser under the action of the wenchun earthquake. under the action of the gravity on the cataclastic rock mass, some collapses appeared in the sliding source prior to the landslide. the impact loading of the landslide mass resulted in the activation of an accumulation body. in this case, the accumulation body that formed after 1933 became unstable and changed into a pipeline-type debris avalanche after it broke up. when the avalanche debris slid downward toward the old landslide accumulation body at high speed, diffuse avalanche debris accumulation was formed due to the open terrain and the gentle slope in the front. a direct hit by the avalanche debris in the middle pushed the old landslide accumulation body to the top, buried xinmo village, caused 83 deaths, pushed dilapidated houses forward 200 m to the southern bank of songpinggou, and formed a landslide dam. a dispersed accumulation, together with the air blast impact, was formed by the avalanche debris on the eastern and western sides. shrubs at the front edge were flattened outward. the xinmo landslide was activated at the top of mount fugui, 3450 m above sea level. the landslide slid for 2.8 km, reached 1637 * 104 m3, and formed a landslide dam after it blocked songpinggou. it was a typical chain disaster of avalanche debris triggered by a ridge-top landslide, which was similar to the sanxi landslide in dujiangyan, sichuan province (yin et al., 2016) and the catastrophic landslide in southern leyte, the philippines (catane et al., 2007). figure 8. accumulation characteristics of the rockslide-avalanche debris in xinmo village, maoxian, sichuan. (a) debris flow overlapped with the old landslide mass and (b) huge block overlapped with the debris flow. table 1 zoning of the material sources and accumulation of the xinmo landslide, maoxian name of zone provenance/ scraping volume (×106 m3) accumulation volume (×106 m3) notes landslide source zone 5.5 1.5 the accumulation volume was 1.4 times the volume of the rock sliding mass (3.9 × 106m3) avalanche debris zone 8.7 3.3 the zone had a maximum thickness of 30 m (15 m thickness on average). accumulation zone 2.2 11.6 the newly formed landslide accumulation zone overlapped the old landslide mass in the form of a diffusion flow. total 16.4 16.4 unstable slope zone excluded (the volume was 4.50 × 106 m3). 175characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china dan-w numerical simulation analysis dan-w, a model used to study landslide dynamics developed by hungr (1995), can be used to simulate an entire landslide movement process. in this study, the 3d numerical dan-w model was developed based on the twodimensional conditions shown in the a-a’ profile in figure 6. the sliding path widths were confirmed according to field survey data (figure 9). following the theory of the lagrangian analytical solution of the saint-venant equation, a sliding mass is considered to be an “equivalent fluid,” and it is divided into several blocks made up of certain materials. the equilibrium equation and physical equation of each block are built in the curvilinear coordinate system with their solutions to obtain the movement characteristics of a landslide, such as the movement velocity and the sliding distance (hungr et al., 2004). this is shown in equations (1) (6): f h b ds p ti i= +γ α -sin (1) where f is the sliding force,  is the unit weight of the sliding mass, h represents each block height, b represents each block width, ds is the nominal length of each block, α is the slope angle, p is the internal tangential stress, t is the base resisting force, and i represents each block index: v g f t m h b d vi i i i= + ( )' ∆ γ (2) where v is the new velocity of the sliding mass movement. the new speed at the end of a time step is gained from vi ' , that is, the old velocity, g is the gravitational acceleration, ∆t represents each time step, m is the momentum flux, and the other parameters are consistent with equation (1): h v s s b bi j i i i i = ( ) +( )+ 2 1 1+ (3) where h is the average thickness of the sliding mass, j represents each block boundary index, i represents each block index, s is the value of the curve displacement, and the other parameters are consistent with equation (1): v v v y lr point i ii n = + +∑ ∑ =1 (4) where v is the entire volume of the rock landslide deposit, vr is the volume of the initial rock landslide, vpoint is the volume of the unstable mass, y is the yield rate, l represents the length of each block, and i represents each block index: y b di i i= × (5) in the expression, d represents the erosion depth. the other parameters are consistent with equations 1-4. d depends primarily on the thickness of the loose layer in a sliding path. in this work, previous digital elevation and post digital elevation models were established by superimposing a 1:2000-scale pre-event topographic map onto a post-event unmanned aerial vehicle mage map (figure 10). the post-event source depths were subtracted from the previous dem to assess the thickness of the sliding rock mass that was scraped during runout. the initial volume of the xinmo landslide was estimated to be approximately 500 * 104 m3, the thickness of soils was approximately 25120 m, and the maximum erosion depth was approximately 6 m. dynamic model description the velocity of the sliding mass and the height of the landslide accumulation mass were calculated using equations (1)–(5). in addition, the amount of resisting force that was encountered was determined using different types of rheological models. dan-w provides different rheology relationship models, which, in most cases of landslide movement analysis, are the frictional model and the voellmy model (hungr & mcdougall, 2009). the former is primarily used for landslides when there is no turbulence, and the particle sizes of the residual body are large. the latter is suitable for the simulation of landslides with fractured particles where there is a visible liquidized layer in the sliding mass (aaron et al., 2017). in the frictional model, it is assumed that the flow of a sliding mass is subject to effective normal stress acting on each block. its resistance, τ, is denoted as follows: τ σ γ ϕ= ( )1 tan (6) where γ µ is the pore pressure coefficient, that is, the ratio of the pore pressure to the total pressure, φ is the internal friction angle, σ is the total stress perpendicular to the direction of the sliding path, and τ is the resistance at the bottom of the sliding mass. the shear stress of the voellmy model can be expressed as follows: τ σ γ ξ = +f v2 (7) where f is the friction coefficient of the sliding mass, γ is the unit weight of the materials, ν is the migration velocity of the sliding mass, ξ is the turbulence coefficient, and τ is the resistance at the bottom of the sliding mass. model selection and input parameters the movement track of a sliding mass, the appropriate rheology relationships, and the material parameters are three key factors that influence a numerical simulation of a landslide. first, the pre-landslide form line, the post-landslide form line, and the movement track of the sliding mass (figure 4b) were determined based on a field geological survey and data acquired by uav (unmanned aerial vehicle) photography. the rheological model that was best suited to the field geological conditions was selected from different models based on the characteristics of the xinmo landslide during various movement stages. it was appropriate to use the frictional model because the particle size of the residual body in the landslide source zone was large, there were noticeable marks of friction on the sliding surface, and the sliding velocity was low in the landslide source zone. the frictional (f) and voellmy (v) models were applied to the avalanche debris zone and the accumulation zone. according to the range of the avalanche debris, the simulation through three regions (the landslide source zone, the debris avalanche zone, and the accumulation zone) followed the frictional-frictional-frictional(fff), figure 9. the 3d numerical dan-w model employed for the landslide along the twodimensional sliding pathway illustrated by section a-a’ in figure.6 figure 10. a the previous digital elevation model and b the post-digital elevation model. 176 longwei yang, wenpei wang, nan zhang, yunjie wei frictional-voellmy-voellmy(fvv), frictional-frictional-voellmy(ffv), and frictional-voellmy-frictional(fvf) models, respectively, as shown in table 2. the numerical results that were obtained for different model combination were compared to the runout distance of the actual event. from these results, we could conclude that the fff model was the more practical model (see the “rheology model comparison” section). the result of the landslide dynamic analysis greatly depended on specialized knowledge of the authors and the data used (hungr & mcdougall, 2009). as a result, a trial-and-error method was used to perform considerable numerical calculations while following the findings of hungr and other researchers (hungr & mcdougall, 2009; sosio et al., 2008; xing et al., 2015) to probe the dynamic characteristics of the xinmo landslide. according to the results of previous research (hungr & evans, 1996; mcdougall & hungr, 2004), suitable values for the rheological model parameters were primarily found through trial and error. in the frictional model, the excess pore water pressure rose due to rainfall infiltration (musso, 2004; federico & cesali, 2017), and the γµ value was set to 0.6 through trial and error (with values ranging from 0.5 to 0.8). the dynamic friction angle b =17°(with values ranging from 10° to 30°) according to the literature (xing et al., 2015). subsequently, for the voellmy model, the friction coefficient f=0.3 (tan 24°) and the turbulence coefficient  = 450 m/s2 (equivalent in the range of 200-500 m/s2) were investigated. the erosion depth was set to 6 m, which was approximately the maximum erosion depth in the debris avalanche zone based on the field geological survey data. the longitudinal stress in the sliding mass was influenced by the internal friction angle, i. finally, using the engineering analogy method, the typical strength testing index was i =21°and the density was  =20 kn/m3. the primary choices for values were due to previous experience and due to trial and error. in addition, individual values were chosen, which appropriately utilized the actual motion characteristics of the xinmo landslide based on the field geological survey data. as shown in table 3, the parameters and models were chosen based on the trial and error method and chosen in order to achieve simulations that mimicked the movement characteristics of the landslide. table 2. hydrodynamics model of the xinmo landslide model landslide source zone debris zone accumulation zone fff frictional frictional frictional ffv frictional frictional voellmy fvv frictional voellmy voellmy fvf frictional voellmy voellmy table 3. parameters of the frictional–voellmy model used for the xinmo landslide. model unit weight, (kn/m3) internal frictional angle, i(°) friction angle, b(°) pore pressure coefficient, γµ friction coefficient, f turbulivity, (m/s2) frictional 20 21 17 0.6 voellmy 20 21 0.3 450 results analysis analysis of movement velocity the movement velocity of the sliding mass was analyzed according to the results calculated by the fff model in dan-w. if the activation speed of the landslide in the source zone was zero, the movement velocity of the sliding mass showed an undulating increase at a horizontal distance of 500–2000 m (figure 11). the movement velocity of the sliding mass increased sharply upon the activation of the landslide, but the rate decelerated and even declined under the effect of friction against the sliding surface. however, the movement velocity of the sliding mass soared at horizontal distances of 1200 m and 1800 m, for which the friction decreased sharply and the slope steepened due to bedrock exposure. when the sliding mass reached a horizontal distance of 2000 m, the movement velocity peaked at 74 m/s, while the sliding mass provided continuous loading on the old landslide accumulation and pushed its upper portion to move. the sliding mass decelerated on the open terrain, but it still moved at 49 m/s when it destroyed and buried xinmo village at a horizontal distance of 2400 m. the sliding mass did not stop until it rushed to a horizontal distance of 2780 m. figure 11. variation of the velocity versus the sliding range. movement distance and thickness of the accumulation body as shown in figure 12, the numerical simulation of the xinmo landslide was performed using the fff model. subject to the fracturing bulging effect, the shear displacement of the landslide occurred at a horizontal distance of 400 m. in 34.5 seconds, the front edge of the landslide reached a horizontal distance of 1900 m, which was on top of the old landslide accumulation. the front edge of the landslide continued to move forward, scattered, and accumulated along the way when the terrain was open with a gentler slope, which lasted for 85.5 seconds. it finally stopped moving at a horizontal distance of 2780 m. the shear displacement of a rock mass with a size of 550 × 104 m3 was triggered with the action of the fracturing bulging effect in the landslide source zone (figure 12a). it reached a horizontal distance of 600 m and rushed to the upper part of the accumulation body on the slope in 9.7 seconds (figure 12b), scraping the slope surface at a depth of 2 m (the red line in figure12). when the sliding mass moved to the tail end of the upper part of the accumulation body on the slope at 17.2 seconds (figure 12c), the total volume of the landslide reached 1010.4 × 104 m3. the sliding mass entered the lower part of the accumulation body at 20.22 seconds after it moved horizontally for 1000 m (figure 16d) and continued to scrape the slope surface at a depth of 3–5 m. the sliding mass increased to 1390 × 104 m3 when it reached a horizontal distance of 1800 m (figure 12e). when it moved 100 m forward horizontally to 1900 m at 30.54 seconds, the sliding mass started loading on the upper part of the old landslide slope and pushed it, moving it along. after the sliding mass moved to a horizontal distance of 2400 m at 42.61 seconds (figure12f), xinmo village was buried by the sliding mass, with some of the houses pushed forward 100– 200 m. then the sliding mass moved forward and finally stopped at 2780 m at 60 seconds after the landslide took place. the sliding mass in the avalanche debris zone and the accumulation zone continued to scatter and accumulate at 60–120 seconds. the total volume of the landslide reached 1579.6 × 104 m3 at 120 seconds. in the end, the average thickness of the accumulation body was 20 m and the volume of the residual body reached 309.0 × 104 m3 in the landslide source zone, as shown by figure 12h. the average thickness and the volume of the landslide-avalanche debris accumulation zone were 18 m and 1270.2 × 104 m3, respectively. the total volume of the landslide was 1579.2 × 104 m3, whereas the total volume of the landslide shown by the field survey and the uav aerial photography was 1637.6 × 104 m3. the failure to fully reflect the complex topography in dan-w led to a deviation between the calculated result and the field geological survey, but the error was still acceptable. speed and characteristic analysis of typical points based on the actual situation as assessed during the field investigation, four points with horizontal distances x of 400 m, 1000 m, 1900 m, and 2400 m were selected as typical points for the analysis, as shown in figure 6. 177characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china (1) the point with a horizontal distance x = 400 m was located at the landslide shear outlet (figure 6). during the period from 0 to 10 seconds, the speed of this point increased abruptly, reaching 22.5 m/s at 10 seconds. this indicated that the landslide was initiated with an accelerated shear in a very short period of time (figure 13a). from 10 seconds to 120 seconds, the speed of this point decreased, and the rate of decrease became larger. this was because the residual sliding body was hindered by the negative topography. in the final steady state, the deposit could become 15 m thick (figure 13b). (2) the point with a horizontal distance x = 1000 m was located at the end of the upper portion of the old landslide (figure 6). the point began at 17.2 seconds (figure 13c). because the bedrock at this point was exposed, the sliding body was less hindered, leading to a speed as great as 52.5 m/s. from 20 seconds to 120 seconds, the speed gradually decreased. at 25 seconds, the deposit thickness at this point could be as great as 27.5 m. in the final steady state, the thickness of the deposit at this point could reach 6.5 m (figure 13d). (3) the point with a horizontal distance x = 1900 m was located at the end of the lower portion of the old landslide (figure 6). from this point, the terrain became wider and the slope became gentler, resulting in a maximum speed of 69 m/s at this point, and the speed remained high with very slow attenuation (figure 13e). the deposit thickness at this point reached 16 m from 34 seconds to 42 seconds and reached 20 m in the final steady state (figure 13f). (4) the point with a horizontal distance x = 2400 m was located in the xinmo village (figure 6). at approximately 40 seconds, the front edge of the sliding body reached the xinmo village with a speed close to 50 m/s (figure 13g). at the same time, due to the blockage by houses, the deposit thickness reached 27.5 m in 10 seconds (figure 13h), which was sufficient to cover seven-story houses, causing devastating damage to xinmo village. figure 13. changes in the speed and the deposit of typical points over time discussion rheology model comparison the diagram of the xinmo landslide speed variation with slippage obtained using different rheological models is shown in figure 14. the diagram of the xinmo landslide speed variation with slippage that was obtained using different rheological models is shown in figure 14. it can be seen that the runout distances of the ffv model and of the fvv model were relatively large. the sliding distances of the landslide that were calculated using both models exceeded 3 km, which disagreed with the measurement data. the fff model and fvf model results were more accurate. when estimating the sliding distance and the movement velocity of a landslide that was in the range of millions or even tens of millions of cubic meters in size, the average speed calculated using the sled model of the xinmo landslide reached 68 m/s (yin et al., 2017). the calculation of the sled model was similar to that of the fff model, whereas the movement liquidation effect of the sliding mass in the avalanche debris zone was considered in the fvf model, which resulted in a smaller result. the collision and friction marks between the blocks in the debris flow area that were found in the field geological survey were obvious, and the blocks displayed the state of the fragments. in addition, according to the reports of local residents, there was a gully in the debris flow area with a depth of approximately 1 m. the water in the gully dried up two years before the landslide occurred. there figure 12. scenario of the 2d depth profile of the rockslide-debris run-out of the xinmo landslide. 178 longwei yang, wenpei wang, nan zhang, yunjie wei was no loose liquefaction layer on the surface of the debris flow region based on the field survey. therefore, the frictional model was more of a good fit than the voellmy model for the avalanche debris zone. what is more, the voellmy model included a turbulence term to account for velocity-dependent resistance (eq. 7). the voellmy model limited flow velocity. in contrast, many studies have shown that the xinmo landslide was a high-speed and long-runout landslide. as indicated by the result (figure 15), the fff model was used to simulate the entire movement process of the xinmo landslide. figure 14. speed contrast diagram of five models of the xinmo landslide figure 15. maximum extent of the xinmo slope runout for different rheological models in dan-w . 1 calculation profile in dan-w. 2 the runout distance for different models, in m. 4 the actual maximum runout distance, in m. 4 the boundary of the xinmo landslide. numerous studies have been conducted on the xinmo landslide. xu and ouyang analyzed the dynamic disaster effects of the xinmo landslide using a number of numerical simulation software programs and the corresponding models. these included pfc software and the linear elastic rheology model, matdem software and the linear elastic rheology model, massmov2d software and the voellmy rheological model, and massflow software and the coulomb frictional model. the movement speed and deposit thickness of the xinmo landslide were calculated. the results showed that the landslide movement speeds that were calculated using massflow and massmov2d were relatively slow, under 50 m/s, and the landslide movement speeds that were calculated using pfc and matdem were in the range of 35-70 m/s (scaringi et al., 2017; ouyang et al., 2017; xu et al.,2017). chen et al. used dda software to simulate the dynamic disaster effect of the xinmo landslide and they analyzed the speed of the landslide movement to xinmo village (chen et al., 2018). unlike these earlier studies, this study was based on the features of the frictional model and the voellmy model, and the different disaster characteristics of the sliding source area, the debris flow area, and the deposit area were considered. appropriate models were selected for different locations along the landslide path. the fff model was found to be accurate at representing the dynamic disaster characteristics of the xinmo landslide. the results also showed that the ffv and fvv models led to calculation results, which indicated that the liquefaction phenomenon of the xinmo landslide was weak. hence, these models were not suitable for simulating the movement characteristics of the xinmo landslide. the fff model recommended in this study for the calculation of the xinmo landslide had a total movement time of 120 seconds and an average speed of 57 m/s. in addition, the results showed a maximum movement speed of 74 m/s at a horizontal distance of 1950 m for the sliding body. furthermore, the speeds at four characteristic points, such as the landslide shear outlet and xinmo village, were analyzed quantitatively, and the deposit thickness at each point was simulated. additionally, the scraping effect of the landslide was quantitatively analyzed and the scraping depth was calculated across a range of 3 m to 10 m during landslide movement. the fff model that was used in this study to simulate the xinmo landslide and its parameters can also be used to simulate the dynamic disasters of similar high-position rock landslides in the songpinggou region where the xinmo landslide was located and in other regions. investigation of the reactivation mechanism and the disaster effects of the old landslide in the 1933 diexi earthquake the diexi landslide in xinmo had a volume of 330 × 104 m3 due to its high position shear sliding. the continuous loading and deposition on the upper portion of the old landslide caused the upper portion of the old landslide to be reactivated. thus, the volume of loading that caused the old landslide body to figure 16. old slope geometry for the geo-slope simulation 179characteristics and numerical runout modeling analysis of the xinmo landslide in sichuan, china be reactivated required investigation. in this study, the stability of the upper portion of the old landslide body was calculated using geostudio-slope/w. the upper portion of the old landslide body was modeled separately (figure 16) using the moor-coulomb model. the bedrock and cover layer were treated as two different materials, and the parameters were those shown in table 4. for the sake of computational convenience, the loading method was simplified by applying the force to the middle and rear portions of the old landslide body to simulate the loading in the calculation. table 4. parameters of the slope-w model used for the old xinmo landslide material unit weight (kn/m3) cohesion (kpa) internal friction angle (°) the old landslide(1933) 20 19 21 bedrock 23 21 32 the results showed the safety factor of the old landslide body prior to the disaster occurrence to be 1.15. after continuous loading, the safety factor of the upper portion of the old landslide body decreased gradually. when the load reached 125 × 104 m3, the safety factor(fos) was less than 1.0, and the upper portion of the old landslide body became unstable and slid. the relationship between the specific loaded volume and the safety factor is shown in figure 17. figure 17. relationship between the loading volume and the safety factor of the old landslide next, dan-w was used to conduct the dynamic simulation of the old landslide body only. there are many similar old landslide deposits in strong earthquake zones, and their reactivation is not caused by continuous loading only, but rather may also be induced by other factors, such as rainfall and earthquakes. for this reason, the sliding volume of the sliding source area was not considered in the calculation for this study, and the friction model was used. the parameters are shown in table 3. the results showed that the entire process of the old landslide movement lasted 80 seconds. the maximum movement speed was obtained as 53 m/s at a horizontal distance of x = 1900 m with a scraping depth of 3 m (figure 18a and b). at 42 seconds, the front edge of the sliding body reached xinmo village at a speed of 45 m/s with a deposit thickness of 45 m (figure 18c and d). the speed and the deposit thickness were sufficient to cause enormous damage to xinmo village. after the wenchuan earthquake, shattered mountains provided sources of material for landslides. therefore, it is very necessary to study the stability of old landslides in populated areas, calculate the scope of disaster mitigation for old landslides, and take the corresponding measures to prevent similar disasters. conclusion on june 24, 2017, a landslide struck xinmo village, mao county, sichuan province, buried the village, and caused 83 deaths. by focusing on the disaster-formation mechanism of the xinmo landslide based on a field geological survey, remote sensing satellite images, and probing the dynamic characteristics of this landslide using dan-w and geological rheological models, the following conclusions were reached: 1. on the basis of the field geological survey, the zoning characteristics of the xinmo landslide were analyzed by investigating the regional geological environmental conditions, such as the strata structure, easy-sliding structure, precipitation, and hydrogeology. accordingly, the landslide was divided into a landslide source zone, a debris avalanche zone, and a landslide-avalanche debris accumulation zone. 2. the entire movement process of the xinmo landslide was primarily divided into three stages: (1) the ridge-top landslide being sheared out, (2) the dynamic impact and the accumulation loading induced high-speed long-distance sliding of the residual soil body on the slope, (3) scattered accumulation along the western and eastern sides of the avalanche debris falling zone and the pushing of the upper portion of the old landslide accumulation body in the middle. 3. the movement characteristics of the xinmo landslide were successfully simulated using the landslide dynamic software dan-w and the fff rheological model. according to the calculation, the movement duration, maximum velocity, and average thickness of the accumulation body of the xinmo landslide were 120 seconds, 74 m/s, and 10–25 m, respectively. the reactivation initiation mechanism and the dynamic disaster effect of an old landslide under the continuous loading conditions were analyzed quantitatively. hence, dan-w and the fff model can effectively provide a reference for similar studies regarding the dynamic disaster effects of ridge-top rock landslides. acknowledgements this study was supported by the national key research and development program of china [no.2018yfc1505404], china geological survey (dd20190647, dd20179609, dd20190637). special thanks are given to prof. yueping yin, china institute of geo-environment monitoring. we are also grateful to prof.o.hungr for supplying a copy of the dan-w software. finally, we thank editors and reviewers for their thoughtful review and valuable comments to the manuscript. figure 18. dynamic disaster effect of the old landslide. 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(2016). dynamics of the 2008 earthquaketriggered wenjiagou creek rock avalanche, qingping, sichuan, china. engineering geology, 200(1), 75-87. https://doi.org/10.1016/j.catena.2019.104396 1324.doi 10.5194/nhess 10.3390/rs keywords: buga batholith; hydrothermal alteration; molybdenite; microthermometry; porphyritic dykes. palabras clave: batolito de buga; alteraciones hidrotermales; molibdenita; microtermometría; diques porfiríticos. how to cite item barrera-cortes, m. & molano, j. c. (2021). characterization of hydrothermal events associated with the occurrence of coppermolybdenum minerals in the el chucho creek at cerrito, valle del cauca-colombia. earth sciences research journal, 25(1), 5-12. doi: https://doi.org/10.15446/esrj.v25n1.79152 at el chucho creek, located to the west of the valle del cauca department in colombia, some hydrothermal alterations affecting the buga batholith rocks and dykes of porphyritic quartz-dioritic and tonalitic composition were identified. these lithological units host mineral occurrences of pyrite, chalcopyrite, and molybdenite that occur disseminated in the rocks or associated with quartz veinlets. the hydrothermal alterations identified were phyllic, propylitic, and, in minor quantity, potassic. the two firsts alterations’ distribution is related to structures and pervasive, whereas the last one seems restricted to contact zones of porphyritic dykes on tonalite. microthermometric data were acquired i) on quartz veinlets of 1 cm thick over a phyllic alteration zone, and ii) on quartz veinlet of 1 cm thick with chalcopyrite ± molybdenite and copper silicates, both veinlets cutting the phaneritic tonalite. those data suggest that the mineralizing fluids have an aqueous-saline chemical system and were trapped under low volatile content. the microthermal data allowed authors to identify two mineralizing events. one of them of higher temperature, with homogenization temperatures between 275°c-480°c; as the second event is characterized by lower homogenization temperatures that range from 100°c to 139°c. abstract characterization of hydrothermal events associated with the occurrence of copper-molybdenum minerals in the el chucho creek at cerrito, valle del cauca-colombia caracterización de eventos hidrotermales asociados con ocurrencias de minerales de cobre-molibdeno en la quebrada el chucho, cerrito, valle del cauca-colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.79152 en la quebrada el chucho, localizada en el occidente del departamento del valle del cauca-colombia, fueron identificadas alteraciones hidrotermales que afectan tanto a rocas tonalíticas del batolito de buga como a diques porfiríticos de composición cuarzo-diorítica y tonalítica. estas unidades litológicas hospedan minerales como pirita, calcopirita y molibdenita que se presentan diseminados y en parte asociados con vetillas de cuarzo. las alteraciones hidrotermales identificadas corresponden a fílica, propilítica y en menor medida potásica. la distribución de las dos primeras alteraciones está relacionada con estructuras o bien pueden presentarse de forma pervasiva en las rocas afectadas; mientras que la última parece estar restringida a las zonas de contacto de los diques porfiríticos con la tonalita del batolito de buga. los datos de microtermometría se midieron i) en vetillas de cuarzo de 1 cm de espesor localizadas sobre zonas de alteración fílica y ii) en vetillas de cuarzo de 1 cm de espesor con calcopirita ± molibdenita y silicatos de cobre; ambas vetillas se encuentran cortando la tonalita fanerítica. los datos microtermométricos sugieren que los fluidos mineralizantes tienen un sistema químico acuoso-salino y fueron atrapados bajo un régimen de bajo contenido volátil; además, la información microtermométrica permitió identificar dos eventos mineralizantes, uno de ellos de alta temperatura, cuyas temperaturas de homogenización varían entre 275°c–480°c y un segundo evento caracterizado por bajas temperaturas de homogenización que van desde 100°c a 139°c. resumen record manuscript received: 17/04/2019 accepted for publication: 21/09/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 5-12 marcela barrera-cortes1 and juan carlos molano mendoza1 1departamento de geociencias, universidad nacional de colombia, bogotá-colombia. *corresponding author: dmbarreraco@unal.edu.co; jcmolanom@unal.edu.co g eo lo g y https://doi.org/10.15446/esrj.v25n1.79152 https://doi.org/10.15446/esrj.v25n1.79152 mailto:dmbarreraco@unal.edu.co mailto:jcmolanom@unal.edu.co 6 marcela barrera-cortes, juan carlos molano mendoza introduction the study area is located on the west side of the central cordillera of colombia in the valle del cauca department, approximately 14 km to the east of el cerrito municipality (figure 1). nivia (2001) mentioned that the region’s geology had been considered suitable for the location of mineral deposits. the buga batholith, a predominant unit on this region, host epigenetic au-ag mineralization and, according to cediel & shaw (2019), “were accreted to the colombian margin along with slivers of farallon – caribbean colombian oceanic plateau (ccop) oceanic lithosphere in the late cretaceous-paleocene, during the collision of the leading or lateral edge of the ccop, along the continental margin.” nonetheless, most of the metallogenetic studies concerning the buga batholith have been focused on ginebra and buga’s municipalities, located to the northwest of the study area. this work contributes to the knowledge of mineral occurrences identified at the southeastern corner of the area defined by lopez et al. (2018) as the ginebra sliver, using cartographic, petrographic, and microthermometric data. figure 1. localization of the study area. modified from mccourt (1984) regional geology litho-tectonically, the study area is located on the pacific assemblage terrane (pat) of the western tectonic realm (wtr) terrane of cediel et al. (2003). according to the author, this assemblage of possible allochthonous terranes contains mafic-ultramafic complexes, ophiolite sequences, and oceanic sediments that have been affected by multiple phases of tectonic reworking and translation that among others get to an obduction/accretion of the marginal basin assemblage along the paleocontinent. the main lithostratigraphic units in this region are the buga batholith, the amaime formation, and the ginebra ophiolite complex, as mccourt (1984), nivia (2001) reported. the buga batholith is a polyphase pluton (leal-mejía et al., 2018) with a composition that changes between hornblende quartz diorite to tonalite (mccourt, 1984). in the interest area, tonalite is the predominant lithology. paz et al. (2017) concluded that this batholith mixes partially crystallized granitic magma and a mafic magma upon conducting field relationships and petrographic analysis. villagómez et al. (2011) placed these rocks in the transition of calcalkaline to tholeiitic series. according to tarazona et al. (2017), the buga batholith is a syn-tectonic pluton that approached and reworked pre-exist anisotropic structures to migrate and emplaced into the ginebra ophiolite complex. an u/pb la-icpms on zircon age of 98.79 ± 0.43 ma by brito et al. (2010) and villagómez et al. (2011) suggest a crystallization age for this pluton of 90.6 ±1.3 ma and 92.1 ± 0.8 ma u/pb (zircon). afterward, nivia et al. (2017) dated the buga batholith through u/pb in zircon and got two different ages (88 ± 1.64 ma and 69 ± 1.41 ma). based on these ages, nivia et al. (2017) has classified the buga batholith on two differetn intrusive bodies. the amaime formation was defined by mccourt (1984) as massive tholeiitic basalt with horizons of pillow lavas and local komatiitic basalts, and mention that it is intruded by the plutonic rocks of the buga batholith. for moreno & pardo (2003), this unit is known as amaime-chaucha complex. they suggested that it is composed of a core of ophiolitic suite intruded by granitoid igneous rocks; besides, pillow and massive basalts of oceanic plateau type are tectonically associated. nivia (2001) defined the eastern’s limit of the unit in the main fault of the cauca-almaguer fault system. armas (1984) suggested a pre-aptian age for these rocks according to the radiometric k/ar age presented by brook (1984) for the plutonic rocks of the buga batholith (99 ± 4 ma) that intruded the amaime formation. moreno & pardo (2003) gave reference to 40ar/39ar radiometric data from basalts of this unit that provide an age cenomanian-turonian. the ginebra ophiolite complex was defined by nivia (2001) as an elongated block of ultramafic rocks with an n-s orientation and a 40km length; the eastern contact is faulted (guabas-pradera fault) with the amaime formation and is intruded by the buga batholith. the western contact is given by the palmira-buga fault that separates it from miocene sedimentary units. espinosa (1985), cited by nivia (2001), described this unit as a sequence of peridotites, banded gabbros, microgabbros, metabasalts, tuffs, microbreccias, and hyaloclastites, cut by dolerite dikes and plagiogranite veinlets. besides, tarazona et al. (2017) identified the following chronologic polyphase history of the complex: i) crystallization, ii) metamorphism on amphibolite facies, iii) milonitization, iv) mylonitic foliation 1, and v) mylonitic foliation 2. mccourt (1984) mentioned that this complex was intruded by the buga batholith and considered that the radiometric ages of this plutonic body of 113 ± 10 ma (k/ar on hornblende, toussaint et al., 1978), 99 ± 4 ma (rb/ sr on biotite, brook, 1984) and 114 ± 3 ma (discordant minerals aged k/ar, brook, 1984) suggest that the intrusion of this pluton on the ginebra ophiolite complex was before 100 ma and assume the age of the intruded unit as early cretaceous or jurassic. later in 2017, nivia et al. reported ages ar/ar of 140.28 ± 3.12 ma (on clinopyroxene) for gabronorites and 90.84 ± 0.78 ma (on hornblende) for anphilites. these ages are interpreted as crystallization age and reworked age by the buga batholith’s intrusion on the ginebra ophiolite complex. regional structures the geographic location of the study area in the wtr of cediel et al. (2003), composed by a melange of allochthonous terranes (e.g., pat), gives to the site an intense tectonic deformation that is represented locally by a set of principal faults (i.e., n20-30e; n60-70e; n40-50w). that accommodates the deformation generated into western colombia’s accretionary complex; these faults have a history of complex and multiple events that mainly correspond with significant scale strike-slip movements (nivia, 2001). in the study area, the most relevant fault systems correspond to systems n20-30e and n40-50w; the former is known as the “palestina” type (lozano, 1986 cited by nivia, 2001) that places in contact different lithological units defining principal lithological provinces and is compound of the west to east among others by the palmira-buga fault and guabas-pradera (potrerillo) fault. the latter system is nominated as “salento” type by the same author and correspond to an echelon arrangement of sinistral faults that move the trace of the n20-30e faults; mccourt (1984) suggested that some of those faults correspond to tensional brittle structures, an idea that authors have used as sillitoe (1974) to explain the occurrence of metallic deposits on the andean mountains. 7characterization of hydrothermal events associated with the occurrence of copper-molybdenum minerals methodology a detailed geologic mapping (1:800 scale) was carried out over a transect of 500 m along the el chucho creek using the “anaconda mapping method” described by einaudi (1997). a total of twelve thin-polished sections were analyzed to get a petrographic classification based on the modal abundance of primary minerals and identification of alteration minerals; two of the sections were made as doubly polished thin sections for fluid inclusions purposes. all units were analyzed using an olympus bx-41 polarizing microscope with transmitted and reflected lights. modal mineralogy was determined by point counting of 100 points per thing section, and in the case of hydrothermal alteration, the alteration minerals were attributed to primary minerals from which they were formed; the recommendations of whitney & evans (2010) for abbreviations form mineral names, and the guidance of the subcommission on the systematics of igneous rocks (streckeisen, 1976) were followed for petrographic characterization. microthermometric measurements in fluid inclusions were carried out in the national university of colombia laboratories, bogotá, using a linkam thsm 600 stage coupled to an olympus bx41 petrographic microscope. the equipment accuracy was estimated at +1°c to the lowest temperature and +4.7°c for the highest; the heating and cooling rate are 150ºc/min and 100ºc/min respectively, researching temperatures of -196°c to 600°c. threephase changes were recorded during the cooling/heating routines: (1) eutectic temperature (te), (2) final ice melting temperature (tmice), (3) homogenization temperature (th). the compositions of mineralizing fluids were modeled by the bulk program from fluids package (bakker, 2003; bakker & brown, 2003), using the equations for salinity calculation, given by bodnar (1993) and the state equation of zhang & frantz (1987) for estimating the type of components in the inclusions. results geologic mapping in the el chucho creek, the predominant lithology is a moderated magnetic, light pinkish coarse, equigranular, phaneritic rock, composed mainly of subhedral plagioclases (30 % vol), anhedral quartz (25 % vol), and subhedral hornblende (20 % vol). the mineral occurrences (pyrite, chalcopyrite, and traces of molybdenite) are disseminated in thin quartz veinlets (≤ 1cm) (3 veinlets); all of them complete almost 5 % vol; finally, alteration minerals such as carbonate, chlorite, epidote, sericite and locally biotite (near to dykes intrusions) complete 20 % vol; this mineral composition allow for classifying the rock as tonalite. it is frequent to find lobular contacts with greenish-grey aphanitic basalts, with disseminated fine-grained pyrite, chalcopyrite, and magnetite completing all of them (3 % vol); those rocks are spatially correlated with the buga batholith. in the mapped area, this tonalite is intruded by five porphyritic dykes (45/040; 59/071; 70/135; 70/135 and 44/090)1 of grey to dark brown color, with aphanitic matrix (~70 % vol) and phenocrysts (~30 % vol) of plagioclase and hornblende of < 1cm, besides of disseminated moderate alteration of carbonate + sericite and pyrite and chalcopyrite disseminated and/or in quartz veinlets occasionally with molybdenite (1 veinlet) and rarely gypsum veinlets < 1 cm thick (1 veinlet); those dykes have two to three meter thick and compositionally are quartz-dioritic to tonalitic. these sub-volcanic pulses could have given path to fluids that generated a moderate hydrothermal alteration that affected mainly the tonalitic host rock. the identified hydrothermal alteration are (1) moderate propylitic alteration, with a pervasive distribution and in a minor quantity present as straight veinlets of ep + qz + cb ± chl ± py ± mag, sulphurs and oxides as py, ccp and mag are disseminated on the rock; (2) moderate phyllic alteration (seems related with local faults), with mineral association qz + ser + py ± chl and straight veinlents of ccp and d-type veinlets (gustafson & hunt, 1975), this alteration 1 structural data for the five porphyritic intrusion, notation on dip/dip direction. affects an important portion of the phaneritic tonalite; (3) local very weak potassic alteration, characterized by scarce secondary biotite disseminated on the phaneritic tonalite near to contact zones with porphyritic dykes; also it is possible to find ore minerals as ccp (3 % vol) and mol (1 % vol) in b-type veinlets (gustafson & hunt, 1975) and disseminated. structurally, the zone is controlled by a fragile regime where it is possible to recognize four principal oblique faults (1) n75e/75se, (2) n80w/53se, (3) n55e/76nw and (4) n80w/58nw; the first and second with normal sinistral movement and the third and fourth normal dextral movements (figure 2 and figure 3). figure 2. detailed anaconda map of the el chucho creek, showing the lithology, hydrothermal alteration, faults and veinlets. figure 3. mineralization styles at the el chucho creek in phaneritic tonalite. a. sinuous chalcopyrite veinlet, b. b-type veinlet with suture of chalcopyrite and molybdenite. c. disseminated molybdenite and chalcopyrite with supergene malaquite. d. patch of chalcopyrite with supergene bornite and chalcocite. 8 marcela barrera-cortes, juan carlos molano mendoza petrographyc analysis table 1 and figure 4 present the data used for modal rock classification. table 1. modal mineral percentage of the phaneritic and porphyritic rocks at el chucho creek unita to ph to po qd po mineralb mbch07 1247 1257 1260 1261 1262 mbch04 mbch05 mbch09 mbch11 1258 mbch08 qz 29 20 26 38 40 20 27 21 8 10 10 5 afs 2 0 0 0 3 0 3 0 0 0 0 0 pl 41 33 44 14 35 0 40 26 27 36 14 28 chl 10 7 10 11 6 17 7 8 0 10 26 18 ep 7 0 0 6 0 0 8 6 0 0 0 15 cb 0 2 2 7 0 20 10 32 30 14 6 14 py 0 3 2 14 0 3 3 4 0 5 3 2 ccp 3 1 2 0 5 0 2 3 3 0 0 2 cct 1 0 1 0 0 0 0 0 1 0 0 0 mol 0 0 0 0 0 0 0 0 0 0 0 1 cv 0 0 1 0 0 0 0 0 0 0 0 0 ser 7 30 0 0 11 40 0 0 0 8 34 15 bt 0 0 0 0 0 0 0 0 22 8 0 0 bt 2 0 2 0 0 0 0 0 0 0 0 2 0 hbl 0 2 4 10 0 0 0 0 9 9 5 0 cpx 0 0 8 0 0 0 0 0 0 0 0 0 spn 0 0 0 0 0 0 trace 0 0 trace 0 0 zrn 0 0 0 0 0 0 0 0 0 trace 0 0 total 100 100 100 100 100 100 100 100 100 100 100 100 phenocryst (%) ------20 20 20 20 25 20 matrix (%) ------80 80 80 80 75 80 a unit abbreviation: to ph = phaneritic tonalite, to po = porphyritic tonalite, qd po = porphyritic quartz-diorite. b mineral abbreviation: qz = quartz, afs = alkali feldspar, pl = plagioclase, chl = chlorite, ep = epidote, cb = carbonate, py = pyrite, ccp = chalcopyrite, cct = chalcocite, mol = molybdenite, cv = covellite, ser = sericite, bt = biotite, bt 2 = secondary biotite, hbl = hornblende, cpx = clinopiroxene, spn = sphene, zrn = zircon. figure 4. streckeisen (1976) diagram for modal classification of felsic and intermediate plutonic rocks. sample classification based on point counting (n = 100). to ph = phaneritic tonalite, qd po = porphyritic quartz-diorite and to po = porphyritic tonalite. tonalites these rocks are holocrystalline with anhedral to subhedral crystals and appear with phaneritic (1) and porphyritic (2) texture, that are described as follow: (1) the phaneritic phase (samples 1247, 1257, 1260, 1261, 1262 and mbch07), is holocrystalline with subhedral to anhedral crystals, plagioclase is the most abundant mineral (45 to 67 % vol) (an33)2 with antipertitic texture, quartz (33 to 54 % vol) with local mirmequitic texture, alkali feldspar (≤ 4 % vol), hornblende (~3 % vol), less than 1 % vol of pyroxene, pyrite and chalcopyrite (6 % vol) disseminated and in quartz veinlets (≤ 1 cm thickness). sericite is a common alteration product of plagioclase, while epidote, chlorite and carbonate are frequently alteration product of both, plagioclase and hornblende; scarce secondary biotite appears as a substituent of hornblende (figure 5 a and b). (2) the porphyritic phase (samples 1258, mbch04, mbch05, mbch09 and mbch11) has a fine to medium-grained matrix (75 % vol) composed of crystals of plagioclase, quartz, alkali feldspar and hornblende; phenocrysts (25 % vol) consists of medium to coarse grained subhedral to anhedral crystals of plagioclase (55 to 78 % vol) (an32), quartz (22 to 45 % vol), hornblende (<9 % vol) and alkali feldspar (<4 % vol). plagioclase as well as hornblende are altered to carbonate, chlorite, epidote and secondary biotite (scarce); sulphide minerals as pyrite and chalcopyrite (~3 % vol) are present disseminated and in quartz veinlets (≤ 1 cm thickness). sphene and zircon are accessory minerals (figure 5 c and d). quartz diorite this rock (sample mbch08) is holocrystalline with a porphyritic medium to fine-grained texture, and has a criptocrystalline matrix (80 % vol) composed by plagioclase, quartz, magnetite and chalcopyrite; the phenocrysts (20 % vol) are mainly of euhedral plagioclase (an10) (28 vol %) with sericitic alteration, subhedral quartz (5 vol%) and (20 vol%) of euhedral to subhedral hornblende altered to chlorite, epidote and carbonate. sulphide minerals as pyrite, 2 composition calculated by michel lévy method 9characterization of hydrothermal events associated with the occurrence of copper-molybdenum minerals in sample 1257 (phaneritic tonalite), and are found in 1 cm thick quartz veinlet with chalcopyrite and copper silicates, associated with molybdenite. type i inclusions: primary, biphasic aqueous fi found at the centre or growth zone of crystals, with ovoid and irregular shapes, whose size vary between 4 11 µm and l/v ratio between 12 – 89. type ii inclusions: primary, biphasic aqueous fi found mainly at the centre zone of crystals, whose shapes vary among ovoid, irregular and tabular, the size of the inclusion vary between 4 9 µm and the l/v ratio is between 14 – 44. (figure 6). microthermometric results these results are presented separately according to the fluid inclusion type; the composition, and salinity of both type of inclusions were modeled on the bulk program from fluids package (bakker, 2003; bakker & brown, 2003). type i fluid inclusions: the first observed change of phase corresponds to the first melting temperature of ice (te) ranging from -46 to -15°c with a major data concentration at -35.2°c. ice final melting temperature (tmice) ranging from -4.5 to -1.2°c; all inclusions homogenize (th) to liquid phase (l+v→l) at a temperature ranging from 125 to 480°c with an important data concentration at an approximate temperature of 285°c. these data indicate a salinity of 7.2 to 2.1 wt % nacl equivalent (eq.) (4.7 on average) (figure 7). chalcopyrite and molybdenite (4 % vol) are disseminated into b-type veinlets (figure 5 e and f). figure 5. macroscopic and microscopic aspect of different lithologies at the el chucho creek. a and b. phaneritic tonalite. note in a the medium grained phaneritic texture and the ductil deformation showed mainly in the basaltic flux. on b it is possible to observe the alteration to ep and chl on the hbl crystals. c and d. porphyritic tonalite with phenocrysts of plagioclase and hornblende. this rock has been moderately altered to propylitic assemblage. e and f. porphyritic quartz-diorite with phenocryst mostly of plagioclase affected by strong phyllic alteration. mineral abbreviation: pl=plagioclase, hbl=hornblende, ep=epidote, chl=chlorite. microthermometry analysis fluid inclusions characteristics the occurrence of two types of fluid inclusions assemblages was stablished, table 2 shows the characteristics and the average microthermometric and salinity data for both types of fluid inclusions. type i fluid inclusions are common in sample 1262 (phaneritic tonalite), those fluid inclusions were measured in a veinlet of 1 cm thick, compound only by quartz, over a zone of phyllic alteration. type ii fluid inclusions are typical table 2. petrographic and microthermometric data of fluid inclusions at el chucho creek. type size (µm) shape phases l/v ratio composition microthermometric dataa salinitiesb eutectic t (°c) ice melt t (°c) homog. t (°c) i 4 ̶ 11 ovoid and irregular l/v 12 ̶ 89 h2o mgcl2 -30,47 -2,66 291,14 4,7 ii 4 ̶ 9 ovoid, irregular, tabular l/v 14 ̶ 44 h2o nacl kcl -26,89 -2,45 121,7 4,8 a and b data correspond to the average of the data measured and calculated on each type of fluid inclusion. figure 6. fluid inclusion types in quartz crystals at el chucho creek. a. type i fluid inclusion with ovoid shape and a size of 10 µm, rock sample 1262, b. biphasic type i fluid inclusion with irregular shape and 8 µm in size, rock sample 1262. c. small biphasic type ii fluid inclusion with ovoid and irregular shapes and <5 µm in size, rock sample 1257. d. type ii fluid inclusion from 4 to 6 µm in tabular, amorphous and ovoid shapes, rock sample 1257. 10 marcela barrera-cortes, juan carlos molano mendoza figure 7. type i and ii fluid inclusions in the el chucho creek. a. eutectic temperature (te). b. final melt ice temperature (tmice). c. homogenization temperature (th). d. salinity (wt % nacl equivalent). type ii fluid inclusions: as type i fluid inclusion, these inclusions shown the first phase change at the first melting temperature of ice from -38 to -14°c. the ice final melting temperature ranging between -5.1 to -0.9°c with an important data concentration at -3.1°c, the homogenization temperature occurs at temperature range between 100 − 139°c showing a homogenization to liquid phase too (l+v→l). the salinity calculated vary between 8.0 and 1.6 wt % nacl (eq.) (4.8 on average) (figure 7). discussion geologic mapping and petrographic analysis carried out on the el chucho creek, suggest that the molybdenite and chalcopyrite mineral occurrences reported are associated with a very weak potassic hydrothermal alteration due to the presence of b-type veins and scarse shreddy secondary biotite; this high temperature alteration was overprinted mainly by propylitic and phyllic alteration. according to field relationships the mineralizing fluids analyzed seem to be related with the intrusion of minor tonalitic and quartz dioritic dykes. the microthermal data show two different hydrothermal fluids related with the mineral occurrences. 1) the microthermometric data of type i fluid inclusions suggest a higher temperature event that according to the diagram of wilkinson (2001) could fit on both, an epithermal or an intrusion-related system for the study area (figure 8), furthermore, some of the ore minerals (py, ccp, mol) and hydrothermal alteration (potassic, phyllic, propylitic) found in the el chucho creek could be related with both deposit types (wilkinson, 2001; camprubi et al., 2003 and hart & goldfarb, 2005). moreover, the interpolation of the study area over the map presented by lópez et al. (2018) shows that the study area is near (at less than ~10 km) to a prospective zone for intrusion-related deposits (figure 9). however, some typical characteristics of intrusion-related system as the presence of fluorite, greisen alteration and the presence of co2 as a phase in fluid inclusions, were not observed and more importantly the presence of magnetite in the area suggest oxidized magmatism, those facts break down the hypothesis of intrusion-related but let open the possibility of an epithermal telescoped system. figure 8. diagram of typical conditions of salinity and homogenization temperature for mineral deposits (wilkinson, 2001). note that this diagram suggests a strong relationship of the paleo-fluids analyzed (type i and type ii) with environments of low salinity and relatively low temperature such as epithermal and kuroko deposits-types. 11characterization of hydrothermal events associated with the occurrence of copper-molybdenum minerals figure 9. intrusion-related deposits reported by lópez et al. (2018) located at the nw of the study zone. modified from mccourt (1984) and lópez et.al. (2018). acknowledgments this research was undertaken by the project called ‘anteproyecto de investigación metalográfica, microtermométrica, geoquímica e isotópica para algunos yacimientos minerales en colombia’, that was supported by colciencias. the authors would like to thank the universidad nacional de colombia for the technique and scientific support and for the facilities that were provided for carrying out this research. also, we express our gratitude to javier gil rodríguez and the reviewers by their invaluable contributions. in the same way, we would like to thank the people that helped during the field trip, and the laboratory technicians that supported us during sampling, preparation and analytic techniques. references armas, m., (1984). mapa geológico preliminar, plancha 261-tuluá, escala 1: 100.000. ingeominas, cali bakker, r. j., & brown, p. e. (2003). computer modelling in fluid inclusion research. in: i. m. samson, a. j. anderson, & d. d. marshall (eds.) fluid inclusions, analysis and interpretation short course (175-212). mineralogical association of canada. bakker r. j. (2003). package fluids 1. computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties. chemical geology, 194, 3-23. bodnar, r. j. (1993). revised equation and table for determining the freezing point depression of h2o-nacl solution. acta geochimica, 57, 683684. bodnar r. j. (2003a). introduction to fluid inclusion. in: i. samson, a. anderson & d. marshal (eds.) fluid inclusion analysis and interpretation. short course series 32, vancouver-british columbia, p. 1-8. bodnar r. j. (2003b). interpretation of data aqueous-electrolyte fluid inclusions. in: i. samson, a. anderson & d. marshal (eds.) fluid inclusion analysis and interpretation. short course series 32, vancouver-british columbia, p. 81-100. brito, r., molano, j. c., rodriguez, b., dorado, c. (2010). u-pb la-icpms dating of the buga batolith and associated porphyry dykes of the 2) type ii fluid inclusions show a lower temperatures that fit within the volcanic massive sulphide kuroko type deposits of wilkinson (2001) (figure 8) ; although the typical geometry and zonation of this deposit type as described by ohmoto (1996) was not observed in the field, the regional geologic setting could be appropriate for this deposit type (i.e., volcanic sequences of marine origin as the amaime formation and ginebra ophiolite complex). additionally, some microthermal data for type i fluid inclusions could be interpreted as a leakage of inclusion during heating (shepherd et.al., 1985), due to the high temperature showed without any salinity change related. on the other hand, those different hydrothermal fluids (type i and type ii fluid inclusions) could be related simply with a transition zone in the same hydrothermal system, this hypothesis is supported mainly by the field observations (i.e., a very weak and local presence of secondary biotite that seems related with contact zone of porphyritic dykes with the phaneritic tonalite, b-type veinlets, a frequent moderate and pervasive propylitic alteration, and a overprint of phyllic alteration structurally controlled), so according to these, the changes on fluids could obeying to the distance from the contact zone (e.g., porphyritic dykes), where the higher homogenization temperature are from those fluid inclusions related with potassic alteration on the contacts zone and lower homogenization temperature are from fluid inclusions on phyllic alteration zones. conclusions and recommendations the petrography of inclusions suggest that the mineralizing fluids were trapped under low volatile content, due to the size relation of the volatile phase respect to the total size of the inclusion (l/v ratio). according to the field observations and the differences on homogenization temperatures between the two hydrothermal fluids, we assume that mineral occurrences are related with two different hydrothermal fluids (1) of higher temperature related with the potassic alteration (secondary biotite + pyrite ± chalcopyrite ± molybdenite) close to the contact zone of porphyritic intrusions and (2) of lower temperature related with phyllic alteration (sericite + carbonate + pyrite) related with faults zones. due to the lack of mapping evidence on this research that support the hypothesis of any specific deposit type (intrusion-related, epithermal, vms), is strongly suggested to carry out an extended cartography with the aim of finding any data that give support to any of those assumptions. 12 marcela barrera-cortes, juan carlos molano mendoza ginebra ofiolite–westernmost central cordillera-colombia. vii south american symposium on isotope geology, vol. 2, brasília, brasil, pp. 252-256. camprubí, a., gonzález, e., levresse, g., tritlla, j. & carrillo, a. 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(2010). abbreviation for names of rockforming minerals. american mineralogist, 95, 185-187. zhang, y. g., & frantz, j. d. (1987). determination of the homogenization temperatures and densities of supercritical fluids in the system nacl– kcl–cacl2–h2o using synthetic fluid inclusions. chemical geology, 64, 335-350. keywords: local earthquake tomography, velocity model, geothermal, erzurum. palabras clave: tomografía sísmica local, modelo de velocidad, geotérmica, erzurum. how to cite item ozer, c., & ozyazicioglu, m. (2019). the local earthquake tomography of erzurum (turkey) geothermal area. earth sciences research journal, 23(3), 209-223. doi: https://doi. org/10.15446/esrj.v23n3.74921 erzurum and its surroundings are one of the most seismically active areas in the eastern part of turkey, also known for its hydrothermal activity. this study is the first approach to characterize the crust using the local earthquake tomography. the earthquake source location and three-dimensional velocity structures were solved simultaneously by an iterative tomographic algorithm, lotos-12. data from permanent networks consisting of 59 seismometers, which were installed by ataturk university-earthquake research center and earthquake department of the disaster and emergency management authority, are used to monitor the seismic activity in the eastern anatolia. in this paper, three-dimensional vp and vp/vs characteristics of erzurum geothermal area were investigated down to 30 km by using 1685 well-located earthquakes with 29,894 arrival times, consisting of 17,298 p wave and 12,596 s wave arrivals. we developed new high-resolution depth-cross sections through erzurum and its surroundings to provide the subsurface geological structure of the seismogenic layers and geothermal areas. we applied various size horizontal and vertical checkerboard resolution tests to determine the quality of our inversion process. the basin models are traceable down to 3 km depth, in terms of p-wave velocity models. the higher p-wave velocity areas in surface layers are related to the metamorphic and magmatic compact materials. we reported that the low vp and high vp/vs values were observed in yedisu, kaynarpinar, askale, cimenozu, kaplica, ovacik, yigitler, e part of icmeler, koprukoy, uzunahmet, budakli, soylemez, koprukoy, gunduzu, karayazi, icmesu, e part of horasan and kaynak regions. as an important result, these regions indicate geothermal reservoirs. abstract the local earthquake tomography of erzurum (turkey) geothermal area la tomografía de terremoto local del área geotérmica de erzurum (turquía) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.74921 erzurum y sus alrededores son unas de las áreas más sísmicamente activas en la parte oriental de turquía, también conocida por su actividad hidrotermal. este estudio es el primer acercamiento a caracterizar la corteza utilizando la tomografía de terremoto local. la ubicación de la fuente del terremoto y las estructuras de velocidad tridimensional se resolvieron simultáneamente mediante un algoritmo tomográfico iterativo, lotos-12. los datos de redes permanentes que consisten en 59 sismómetros, que fueron instalados por la universidad y centro de investigación de terremotos ataturk, y el departamento de terremotos de la autoridad de manejo de desastres y emergencias, se utilizan para monitorear la actividad sísmica en el este de anatolia. en este artículo, se investigaron las características tridimensionales de vp y vp/vs del área geotérmica de erzurum hasta 30 km de profundidad utilizando 1685 terremotos bien ubicados con 29,894 tiempos de llegada, que consisten en 17,298 ondas p y 12,596 llegadas de ondas s. desarrollamos nuevas secciones transversales de profundidad de alta resolución a través de erzurum y sus alrededores para proporcionar la estructura geológica subsuperficial de las capas sismógenas y las áreas geotérmicas. aplicamos varias pruebas de resolución de tablero de ajedrez horizontal y vertical para determinar la calidad de nuestro proceso de inversión. los modelos de cuenca se pueden rastrear hasta 3 km de profundidad, en términos de modelos de velocidad de onda p. las áreas de mayor velocidad de la onda p en las capas superficiales están relacionadas con los materiales compactos metamórficos y magmáticos. informamos que los valores de vp baja y vp/vs alta se observaron en yedisu, kaynarpinar, askale, cimenozu, kaplica, ovacik, yigitler, e parte de icmeler, koprukoy, uzunahmet, budakli, soylemez, koprukoy, gunduzu, karayazi, icmesu y parte de las regiones de horasan y kaynak. como resultado importante, estas regiones indican depósitos geotérmicos. resumen record manuscript received: 15/08/2018 accepted for publication: 21/06/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 209-223 caglar ozer1, mehmet ozyazicioglu1 1ataturk university, earthquake research center, erzurum, turkey * corresponding author: caglarozer@atauni.edu.tr g eo ph ys ic s 210 caglar ozer, mehmet ozyazicioglu 1. introduction turkey has a unique geographic location at the junction between asia, europe and africa. it is located in an active tectonic orogenic belt with young faults and active volcanisms (keskin et al. 1998; kaya 2012; pamuk et al. 2018a, b). the extensive volcanism, hydrothermal activities and presence of more than one-thousand hot springs, some of which have 373 k and greater temperatures prove that turkey has an important geothermal energy potential; in fact, it is the 7th richest country in terms of known geothermal energy resources around the world (balat 2006). turkey is located in the alpine-himalaya orogenic belt, and it causes different tectonic zones such as the north anatolian fault zone (nafz), the eastern anatolian fault zone (eafz), the north east anatolian fault zone (neafz) and the aegean graben systems (ags). these active tectonic units give rise to geothermal energy resources (haklidir 2015). previous studies such as baris et al. (2005), ozer and polat (2017a, b), ozer et al. (2018) indicate that geothermal fluids are born in the crust by thermal conduction, particularly in volcanic and tectonically active areas. furthermore, kocyigit and canoglu (2017) claimed that the erzurum pull-apart basin is also a geothermal field, but this aspect of the basin has not been studied yet (fig. 1). several studies have been conducted in the study area within the earth sciences. the geological structure and tectonic evolution of erzurum region and its surroundings have been mainly investigated by keskin et al. (1998), yarbasi and kalkan (2009), kocyigit and canoglu (2017). moreover, there are some attempts associated with geothermal potential (bektas et al. 2007; maden et al. 2015; gulec and hilton 2016), geochemical (bayraktutan et al. 1996; keskin e al. 1998; italiono et al. 2013; yuce and taskiran 2013; kilic and inceoz 2015; gulbay 2015; kaygusuz et al. 2018) and petroleum (oruc et al. 2013) features of the area. in recent years, there are several seismological studies for the eastern part of turkey such as: maden and ozturk (2015), ozturk (2015; 2017; 2018) and these studies are very important in the evaluation of seismotectonic properties of the eastern anatolian region. the most prominent results were produced by the eastern turkey seismic experiment (etse) project in the figure 1. tectonic settings of erzurum, eastern anatolia. blue and white dashed lines represent the study area and main fault zones, respectively. ags: aegean graben system, efz: erzurum fault zone, eafz: east anatolian fault zone, kj: karliova junction, nafz: north anatolian fault zone, neafz: north east anatolian fault zone (compiled from emre et al. 2013 and emre et al. 2018). 211the local earthquake tomography of erzurum (turkey) geothermal area east-southeastern edge of the erzurum pull-apart basin. the afz is a 6 km wide, 140 km long and extends along the ne-trending sinistral strike-slip fault zone (kocyigit and canoglu 2017) (fig. 2). the eastern anatolian region shows complete transects across a volcanic area associated with a continental collision zone. the erzurum-kars plateau is important, because it includes a full record of collision-related volcanism from miocene to pliocene within this region (keskin et al. 1998). it is observed in several parts of the eastern anatolia that ophiolitic and young volcanic units are common. the paleoseismic data shows that the efz is responsible for many holocene events with horizontal displacements of 1.5 to 3.0 m. from these slip measurements and the size of the fault, the efz is thought to trigger devastating earthquakes greater than mw 7.1 with recurrence intervals of 1000 to 3000 years. it is understood from the early tectonic geomorphology within the efz that these fault segments can also be still active. the proximity of the efz to the erzurum province creates an important seismic hazard to the city (emre et al. 2004; emre et al. 2018). the study area covers the southern segment of the karasu intermountain sedimentary basin with a miocene-quaternary volcanic basement, andesitic-basaltic lava flows and fissure eruptions of basaltic lava. two segments of the efz, dumlu (dfz) and palandoken (pfz) fault lines are both active and meet at the southeast side of the basin, very close to the city. the dfz cuts and deforms plio-quaternary basin fill, volcanic rocks of both the nw part of the east anatolian plateau and the e-se parts of the east pontides. it is a sinistral strike-slip fault formed after late miocene, or during the plio-quaternary neotectonic period (oruc et al. 2013). the point of convergence is expected to be the most likely epicenter of some future large earthquake (yarbasi and kalkan 2009) (fig. 2). the erzurum-pasinler-horasan basin, which surrounds miocene rocks, is oriented in the e-w direction. the pontides connect to this basin in the north. the sedimentary process in the study area is eocene to quaternary. in the period from the miocene to present, the early sedimentary patches of the studied area have been developed. within this, patches of sedimentary sequences, to have been collected since the oligocene epoch, play a significant role as source and covered beds (maden et al. 2015) (fig. 2). 3. data and methods the eastern anatolia is monitored by the ataturk university seismic network (atanet) consisting of sixteen seismometers operated by ataturk university and fifty-three seismometers (guralp cmg-3t, cmg-6t, cmg3esp, cmg-40t) managed by the earthquake department of the disaster and emergency management authority (afad) network in ankara. all seismometers are broadband with 100 hz sampling rate. the spatial distribution of the permanent seismological stations that record the earthquakes is presented in figure 3. the distribution of earthquakes concentrates on main tectonic zones such as afz, bkfz, edfz and pfz. there is no earthquake distribution along the latitude of 39.5. this is related to the fact that minor tectonic zones are not active. furthermore, events mainly locate in the first 30 km depth of the crust as clearly observable from vertical depth (fig. 4) the hand-picked initial earthquake data set comprises pand s wave arrival times of 4801 local earthquakes well distributed over an area of about 150 × 250 km2 and recorded from 2007 to 2017. we selected the earthquakes recorded at least by nine stations with rms <0.5 s. ±10%, periodically. in order to understand the impact of noise on the resolution and the optimal values of free parameters, we applied checkerboard tests. these selection parameters were determined using extensive checkerboard tests after several trial attempts. after many experiments, the parameters obtained when the rms value is lowest and the most suitable model is detected. therefore, these parameters were determined as a result of many experiments. the initial earthquake locations are determined by the hypo71 program (u.s. dept. of the interior geological survey 1975) in seisan software suit (havskov and ottemoller 1999) as part of routine earthquake location process in ataturk university. the final event data set used for tomographic calculation consists of 29,894 arrival times from 1685 earthquakes with 17,298 p wave and 12,596 s wave arrivals. the accuracy of the final pand s travel time residuals are on average equal to 0.18 and 0.26 s, respectively after 5 iterations (table 1). initial and final travel time residual values of earthquakes are also presented in fig.5. the low residuals values after five iterations confirm that the high quality hand-picking study area (sandvol et al. 2003; zor et al. 2003; gok et al. 2007; skolbeltsyn et al. 2014). they suggest that the eastern turkey is lifted by hot asthenosphere instead of a thick crust structure. the low velocities point out the disappearance of the lithospheric mantle underneath the plateau and its replacement with asthenospheric units. nevertheless, none of these studies were the subject of erzurum and its surroundings at local scale. furthermore, salah et al. (2011) declared that high poison ratio areas are obviously distinguishable at most of the layers down to a depth of 40 km, which are consistent with the probability of the presence of partial melt in the lower crust. gokalp (2012) obtained tomographic sections with high velocities, which were mainly in parallel with the tectonic features existing at shallow depth layers in the area. this should be associated with the existence of magmatic units at shallow depths. another characteristic is that the low velocity values are commonly dispersed along the three dimensional (3-d) structure beneath the region at deeper crustal depths. this anomaly may be an indicator of the hypothesis that the anatolian plateau is underlain by a partially molten uppermost mantle. the local earthquake tomography (let) is a powerful method for determining the features of the earth’s interior. the let is frequently applied to high velocity contrast regions such as geothermal, volcanic areas and subduction zones, and it is also applied to an iterative nonlinear method in which ray paths are traced in updated 3-d seismic velocity structures (koulakov 2009). the pand swave velocities in the upper crust are usually associated with lithological features, whereas the vp/vs ratio is related to petrological variations in the rocks, tectonic units and fluid-rich, high pore pressure regions. the let is also a convenient tool to understand the local-regional tectonics and seismogenic properties of a region (kaypak and gokkaya 2012). 3-d velocity structure of the upper crust of erzurum is not known in detail and this causes significant lack of information in the investigating of geothermal areas especially in the eastern turkey, where the potential reservoirs have not been drilled yet. a dense array and high seismicity of the eastern turkey gives us a chance to investigate the seismic crustal structure and locations of the events, which in turn can aid to inquire the tectonic deformation, lithological changes, fault zone kinematics and petrological characteristics of the rocks. we determine the 3-d p-wave velocity and vp/vs model of the erzurum basin using the let technique for the first time. this study also discovers new possible geothermal regions by developing horizontal and vertical tomographic sections using local earthquakes from a dense seismic array. 2. geology and tectonics the eastern anatolia is one of the best exposed and most complete transects in the globe, across a volcanic province on a continental collision zone (keskin et al. 1998). two different strike-slip fault systems manage the tectonics of the plateau. these are the nw-se trending strike-slip faults paralleling the nafz and the ne-sw trending strike-slip faults paralleling the eafz. erzurum, the metropolitan city of the eastern anatolia of turkey, is located in the seismically active main segment of the erzurum fault zone (efz) in the northeast anatolian compressional area. the seismic activity of erzurum is mostly controlled by dumlu and palandoken fault zones, characterized by the ne-sw trending faults on eastern margin, whereas the ne-sw trending reverse faults on the southern margins, associated with neotectonic deformations after plio-quaternary (yarbasi and kalkan 2009). erzurum is located outside and nearly 70 km far from the karliova junction (kj), where the nafz and eafz converge to each other. the kj is an area where the anatolian plateau (east of kj), the anatolian block and the arabian plate intersect (gok et al. 2007). the erzurum pull-apart basin is mainly developed by the deformation and subdivision of an e-w-trending paleo-tectonic unit, during the quaternary strike-slip neotectonic period. it is located in the east anatolian tectonic block, which has been controlled by a strike-slip neo-tectonic regime since the early quaternary period (kocyigit and canoglu 2017). the quaternary erzurum pull-apart basin is an approximately 90 km long, 30 km wide, actively developing strike-slip depression. it is confined by the erzurum-dumlu left lateral strike-slip fault zone (edfz) in the east-southeast, by the baskoy-kandilli reverse fault zone (bkfz), by the askale left lateral strike-slip fault zone (afz) in the north-northwest and by the n-s-trending ilica oblique-slip normal fault in the west. the edfz is 20 km wide, 150 km long and extends along the netrending zone of active strike-slip faulting. it determines and manages the 212 caglar ozer, mehmet ozyazicioglu figure 2. simplified geological and tectonic map of erzurum, eastern anatolia. black thin lines and black boxes indicate main fault zones and settlements area, respectively.afz: askale fault zone, bkfz: baskoy-kandilli fault zone, edfz: erzurum-dumlu fault zone, pfz: palandoken fault zone (yarbasi and kalkan 2009; akbas et al. 2013; kocyigit and canoglu 2017). 213the local earthquake tomography of erzurum (turkey) geothermal area figure 3. horizontal ray path coverage of p wave data set in plain perspective. straight red lines indicate the pathway between events (dark gray circles) and stations (blue triangles). figure 4. a). distribution of initial 4801 (blue circles) and selected 1685 (red circles) high quality earthquakes (hand-picked, m≥2.0) recorded between 2007 and 2017 in erzurum and near surroundings. seismological stations and geothermal areas are represented by filled green triangles and white stars, respectively. thin black lines show fault traces (emre, et al. 2013; emre et al. 2018). b) depth sections of the earthquake distribution according to longitude variation. c) depth sections of the earthquake distribution according to latitude variation. d) histogram plot shows the number of earthquakes versus depth. 214 caglar ozer, mehmet ozyazicioglu phase reading process applied in the travel times of events. this process depend on the observed pand s-wave travel times to invert simultaneously for a 3-d image of the pand s wave velocities, hypocentres, and station corrections (ozer et al. 2018, ozer et al. 2019). table 1. mean rms (sec) values of the pand s-wave residuals after five iterations iteration vp rms (sec) vs rms(sec) 1 0.325 0.489 2 0.207 0.307 3 0.192 0.284 4 0.187 0.272 5 0.179 0.259 number of phases 17298 12596 the local earthquake tomography software (lotos-12) (koulakov 2009) is used for tomographic calculations, computation of p and s velocity models and to improve earthquake locations. the initial earthquake locations are determined by the lotos one-dimensional (1-d) location algorithm using initial the 1-d velocity structure in the first stage. in order to determine the best 1-d velocity model representing the region, six velocity structures reported in the study area were tested (teoman et al. 2005, ozacar et al. 2008, salah et al. 2011, gokalp 2012, maden 2012, cinar and alkan 2015). they were compared with the lotos 1-d velocity optimization results and the best model converging through these models and the high quality data set was selected. it is obvious that the model produced by maden (2012) is compatible and converges with the data set (fig. 6.a). another velocity model proposed by teoman et al. (2005) may be an alternative model that can represent the study area (fig. 6.b). it was determined that the rms values in 1-d velocity models of cinar and alkan (2015), ozacar et al. (2008), salah et al. (2011) and gokalp (2012) were 0.69, 0.50, 0.55 and 0.68 s, respectively (fig. c-f). as a result of this process, it was determined that the velocity model produced by maden (2012) was the best model as 1-d velocity model. the 1-d velocity model proposed by maden (2012) for the study area (table 2) is used as an initial model to start the tomographic calculations. the travel times for overall feasible combinations of earthquake depths and epicentral distances are calculated for an 1-d velocity structure (nolet 1981). initial earthquake locations are calculated based on a goal function (gf) method. this earthquake location technique is more stable and does not belong to the initial stage of inversion. a grid search method (gsm) is used to determine origin time and focal parameters. the gf and gsm are described in detailed in koulakov and sobolev (2006). afterwards, locations were recomputed with an algorithm that uses a 3-d velocity model, which makes use of the ray bending technique, the details can be found in koulakov (2009). table 2. 1-d reference model used in the tomographic inversion for pwave velocity (maden 2012) depth (km) vp velocity (km/s) -3.0 4.1 0.0 4.4 4.0 5.8 20.0 6.2 35.0 7.1 50.0 8.0 100.0 8.7 3-d velocity model grid parameterization is based on the ray density approach as described in koulakov et al. (2006). the 3-d seismic velocity structure is calculated in nodes distributed in the study area. velocity values among the nodes are linearly interpolated by subdividing the study volume into tetrahedral blocks. in order to avoid dense node concentration in the model with high-ray density, we define the minimum spacing between nodes as 5 km, which is lower than a typical dimension of the anticipated anomalies. the matrix is calculated from ray paths, after 3-d event locations are completed. the tomographic inversion is accomplished simultaneously for p and s velocities, focal parameters and station corrections. the velocities are updated and used in the next iteration. furthermore, the effect of possible bias in a selected grid configuration on the ultimate velocity model is reduced by repeating the inversion using grids with different orientations (e.g., 0, 22, 45, and 67) and averaging the figure 5. initial (dark gray column) and final (light gray column) travel time residual values of earthquakes. 215the local earthquake tomography of erzurum (turkey) geothermal area after the synthetic travel times were computed, all information regarding focal parameters and velocity model were discarded and the same process was repeated using the updated inversion parameters, including the absolute locations. the travel times were perturbed with noise having average deviations of 0.15 s and 0.3 s for the pand s phases, respectively. these perturbations provide similar variance reduction as in the real data inversion. the typical horizontal checkerboard test results were presented in fig. 7. the checkerboard test was performed by changing unlimited negative and positive columns of uniform seismic anomalies of either 30 × 30 km2 or 50 × 50 km2 size with 5 km of empty space between the columns. the size of velocity and vp/vs anomalies from 0 to 30 km were set to ±10 % and 1.5 to 2.5, respectively. the results of there building process show acceptable resolution for all depth layers for the vp and vp/vs profiles, as seen from fig. 7. however, the checkerboard results indicate some smearing, which gets worse for deeper layers (below 30 km). the amplitude of the deeper anomalies after rebuilding is lower than that of the shallower layers. it was observed that the vp and vp/vs models are well converged in shape and amplitude after rebuild, down to 30 km. the resolution is not good at deeper levels due to poor ray coverage. it should be considered that the model 1 (30 km) and model 2 (50 km) are robustly rebuilding in most of the study volume. in order to determine the resolution of the vertical vp and vp/vs profiles, a checkerboard test was performed with columns of 20× 20 km2 with a gap of 2.5 km. in vp and vp/vs results, it was observed that the resolution is high up to 30 km depth in all vertical sections (fig. 8). a good resolution was obtained beneath erzurum, where station coverage is denser. when the initial model is compared with the results on the boundaries of the study area, the ray paths are insufficient to provide adequate resolution below 30 km. it is clearly seen that the initial and final anomaly amplitudes and shapes coincide with each other very well. another type of synthetic test was performed in order to prove how high the resolution in the study area. in this test, letters and six prisms have been used and there is a 15% periodic velocity change between each prism. it seems that they approach the original model but they are not identical in 10 km horizontal tomographic section. however, the result is sufficient to interpret the study area. (fig. 9). 5. results the vp, vs velocity perturbations and vp/vs model results were shown in three horizontal depth layers from 0 to 10 km (fig. 10). it should be noted that there exists a high velocity anomaly pattern at shallow depths in the direction of ne-sw, which is in line with the main tectonic zone, i.e. the neafz. this anomaly, however, can also be the result of compact geological units known to exist in the region. the high p-wave velocity areas can be related to the compact and brittle fragments of the main fault zone, which can nucleate moderate earthquakes. the negative s-wave velocity perturbations are seen to lie nearly along the e-w direction near askale, dumlu, hinis, ilica, karliova, narman and soylemez. the high vp/vs ratios (>1.85) are calculated in askale, karliova,ovacik, soylemez and tortum regions, while the low vp/vs ratios (<1.65) are observed in cat and kirik regions (fig. 10). in the surficial layer, there are some negative p-wave velocity anomalies, especially in kirik, askale, ilica, west part of dumlu, se part of hinis; however, strong positive p-wave velocity zones are observed in horosan, east part of dumlu, tekman, cat and karliova; from north to south. the 5 km horizontal p-wave velocity characteristics are similar to the surficial layer. the ne-sw direction positive pwave values indicate predominant traces of the neafz. at 10-km depth, the distribution of negative vp and vp/vs values differ from those found at 0 km and 5 km layers. vp, vs and vp/vs anomalies are more homogeneous when compared with the shallower depths. some parts of negative s-wave velocity structure are detectable in hinis, horasan, soylemez regions due to local fault zones. investigation of horizontal layers is terminated at 10 km’s, because this study concerns the exploitable geothermal potential in the region (fig. 10). the vertical cross-sections of tomographic images is the first highresolution 3-d velocity structure down to 30 km for erzurum region, where complex seismic velocity changes are expected due to the compressional tectonic regime and geology (fig. 2). since the lack of deep earthquakes (below 30 km) limit the resolution at depths, the profiles are determined down to 30 km depth. nine vertical profiles from south to north direction for the erzurum results (koulakov et al. 2010a, b). during inversion, the tomographic model is smoothed by controlling a special matrix block that associates neighboring nodes and damps the difference in velocity between all pairs of nodes. the value of damping is decided by synthetic model simulations. the least squares (lsqr) algorithm (paige and saunders 1982; van der sluis and van der vorst 1987) is used for inversion of the large sparse matrix. to get a convincing convergence, the lsqr iteration number is specified to be 100. by an inversion step, velocities are recomputed in a 3-d grid and used for the next iteration. we applied five iterations of the tomographic cycle. the inversion by the lotos algorithm can provide p-velocity and the vp/vs ratio (koulakov et al. 2007; yakovlev et al. 2007; koulokov 2009; koulakov et al. 2009; dinc et al. 2010; koulakov et al. 2010a, b; kuznetsov and koulakov 2014; totaro et al. 2014; khrepy et al. 2015; ozer and polat 2017a, b.; ozer et al. 2018). 4. checkerboard test synthetic tests are a significant part of the inversion process in evaluating the spatial resolution assured by the available data, station coverage and the algorithm itself. to assess the resolution capacity of our tomographic model in the study area, we performed synthetic reconstructions that simulate the real situation as closely as possible. moreover, these tests provide information for estimating values of optimal inversion parameters, such as iteration numbers, grid spacing, etc. we tried synthetic and observed data in turn several times to acquire the best-fit parameters in the study area. the smoothness parameters of pand swave were determined as 5 and 10, respectively. the synthetic travel times are calculated for realistic source-receiver pairs that are used for the inversion of the observed data. figure 6. 1-d velocity model optimization results produced by using different models. initial and final models are represented by black and red lines, respectively. the 1-d velocity structures used as input models (black line) in this study are; a) maden (2012), b) teoman et al. (2005), c) cinar and alkan (2015), d) ozacar et al. (2008), e) salah et al. (2011) and f) gokalp (2012), respectively. 216 caglar ozer, mehmet ozyazicioglu figure 7. consecutive negative (blue color)-positive (white color) p wave anomalies (±10%) and low (black)-high (light gray) vp/vs ratios (from 1.5 to 2.5) show the reconstructed variation: 30 and 50 km box sizes with 5 km empty space, respectively. input model is presented at the top of each column. synthetic (checkerboard) test is calculated to understand the spatial resolution and to predict the optimal values of inversion parameters down to 30 km depth layers. the black (vp) and white (vp/vs) dashed lines represent the well-resolved areas. basin were prepared (figs. 11 and 12) with approximately 28 km interval. the basin structure can be traced down to 3 km in the vertical profiles of absolute p wave velocity. the vp velocities vary from 4.0 to 4.5 km/s, while vp/vs ratios between 1.5 and 2.0. dominant low (<1.65) and high (>1.85) vp/vs ratio structures are observable down to 5 km. higher velocities (≥6.0 km/s) in this layer are mainly observed in high elevation areas or around compact metamorphic and igneous rock units. the low vp (≤4.5 km/s) and the low vp/vs (<1.65) values are detectable in profile-c and profile-d around kirik located in the n part of ovacik (figs. 11 and 12). sections considered among vertical depth cross high vp/vs values and low vp structures were observed in the shallow layers. this type of anomaly can be detected at yedisu (profile-a), kaynarpinar and askale (profile-b), cimenozu, kaplica and ovacik (profile-c), yigitler (profile-d), e part of icmeler (profile-e), koprukoy and uzunahmet (profile-f), budakli, soylemez and koprukoy (profile-g), gunduzu, karayazi and icmesu (profile-h), e part of horasan and kaynak regions (profile i) in the figs. 11 and 12. the magmatic contact marks in the vertical profiles moving from deep to the surface take attention in the middle crust. depending on the compression tectonics in the region, the effect of the magmatic materials moving from deep to surface is also seen in the middle crust. p wave velocities in this area range 217the local earthquake tomography of erzurum (turkey) geothermal area figure 8. vertical vp and vp/vs checkerboard test results for nine depth-cross sections. the input depth cross-sections with block size of 20×20 km2 (with 2.5 km empty size) are conducted to evaluate the stability of depth-cross sections. the black dashed lines represent well-resolved areas. periodic negative and positive perturbation shows the ±10% velocity changes. vp/vs ratio changes from 1.5 to 2.0. the traces of profiles are the same as the location of the tomographic results presented in figures 11 and 12. from 5.25 to 6.25. it is considered that the high vp values observed between 5-15 km in the vertical section are related to the earthquake activity. the source depths in the available data set do not provide sufficient ray coverage to permit adequate resolution below 30 km for an accurate tomography; therefore, the profiles are truncated at 30 km depth. the computed velocity models indicate that the mid-crustal solidified intrusive materials (higher vp and vp/vs values) are located below the earthquake swarm zone (khrepy et al. 2015; mahesh and gupta 2016). 6. discussions we developed the upper crustal velocity model of erzurum and its surroundings by performing a 3-d local tomographic inversion technique using a set of high quality local events from 2007 to 2017. the tomographic images help us to understand the structure of the hydrothermal field and tectonic structure. previous studies (e.g., hauksson 2000; baris et al. 2005; ozer and polat 2017a; b; ozer et al. 2018) suggest that the changes in the vp/vs ratio are 218 caglar ozer, mehmet ozyazicioglu figure 9. special design synthetic test result. black and red patterns are symbolized by unlimited vertical prisms with velocity changes of ±15%. the synthetic model constitutes six letter prisms described in horizontal section. figure 10. the vp, vs perturbation and the vp/vs ratio tomographic horizontal section results for erzurum area and its surroundings. the black dashed lines and black stars represent well-resolved areas and geothermal areas, respectively. extremely dependent on the presence of pores. particularly, an increase in the vp/vs ratio is associated with the existence of liquid materials, lower values point out to the existence of gas, co2 or a mixture of them (e.g., hauksson 2000; kaypak and gokkaya 2012; ozer and polat 2017a; b; ozer et al. 2018). some weak geologic units cause reduced velocities, particularly in s-waves (vs). the high vp/vs values may indicate the presence of water saturated rocks or high pore pressure in the shallow layers. gokalp (2012) reported that the high velocity values are mainly parallel to the tectonic units in the eastern anatolia, especially at shallow depths. conversely, the low p wave velocity regions are presumably more ductile and weaker which provide evidence for aseismic slip (zhao et al. 2005).in the study area, compact metamorphic and magmatic units were reported in dumlu, horasan and tekman, while loose sedimentary units were observed in askale, cat, hinis, kandilli, karliova, ilıca and tekman are also surrounded by many active faults (akbas et al. 2013; emre et al. 2013; emre et al. 2018). moreover, some volcanic (ne) and basalt (sw) materials are placed along the neafz (kocyigit and canoglu 2017). some researchers (zhao et al. 2005; gokalp 2012; mahesh and gupta 2016) reported that some rigid part of faults exhibit positive p wave velocity compared with the other parts of the study areas. these geological features can explain the positive p-wave velocities computed in the ne-sw direction in the study area (fig. 10). generally, horizontal tomographic sections are homogenized towards deeper parts (ozer and polat 2017a, b).it is observed that the crust (≤ 30 km depth) under erzurum and near surroundings is a high velocity unit, which can be related to the distribution of igneous, metamorphic and compact bodies as reported in kocyigit and canoglu (2017). all computed p-wave velocities lower than 4.5 km/s, in the vicinity of erzurum, can be considered as low vp values. lithologically, the low vp values particularly in shallow layers down to 3 km are to be related to the sedimentary materials of the erzurum basin. the p-wave velocity of the bedrocks can be estimated as ~5.5 km/s, therefore by the vp profile, the basement of erzurum basin could be traced. active geothermal regions (hauksson 2000; kaypak and gokkaya 2012) indicate that the gas 219the local earthquake tomography of erzurum (turkey) geothermal area dominated units lead both vp and vp/vs ratio to reduce (hauksson and haase 1997). the gas can consist of steam, co2 or a mixture of them. in such cases, both vp and vp/vs ratios reduce together. kaygusuz et al. (2008) reported in the study carried out in and around erzurum that the volcanic units around the fault zones were formed as a result of moving the metasomatized lithospheric mantle-derived material from deep to the surface layers. in this study, the high vp/vs structures are associated with the low s wave velocities and evaluated as highly saturated, fractured and high pore-pressure volumes (kaypak and gokkaya 2012). khrepy et al. (2015) reported that the high vp values in the middle crust were observed in the field where earthquake clusters were formed due to compression tectonics. high vp velocities can be observed in the areas where hypocenters form an almost vertical structure, due to the compression caused by the upward movement of the magmatic material along the zones of weakness. the weak lower crust, which probably symbolizes magmatic intrusions, forms a thick viscous structure that separates from the brittle-elastic middle crust. it is thought that the mantle-derived magma chamber intrudes into the lower crustal layers, along the paths to display higher velocity. figure 11. depth cross-sections of absolute p-wave velocity. the thick blue lines on the map show traces of the cross-sections. the red stars display the location of known geothermal areas. the black dashed lines represent well-resolved areas. 220 caglar ozer, mehmet ozyazicioglu figure 12. depth cross-sections of vp/vs models with values ranging from <1.65 to >1.85 down to 30 km depth. the thick blue lines show the traces of the cross sections. the red stars display the location of known geothermal areas. the black dashed lines represent well-resolved areas. 6.1. the origin of geothermal fluids it is emphasized that the geothermal potential of the region in the geochemical studies carried out in erzurum and its surroundings (bayraktutan et al. 1996; yuce and taskiran 2013; kilic and inceoz 2015; kaygusuz et al. 2018). bayraktutan et al. (1996) reported signs belonging to the hot spring water in the studies carried out on active faults in erzurum and its surroundings. yuce and taskiran (2013) found that the thermal waters had deep mantle traces and these waters were magmaic based, as a result of the analysis of gas and water samples taken from thermal waters with the temperature of 57 degrees. kaygusuz et al. (2018) reported that magmatic materials were transported along active tectonic units in the study carried out by kandilli and its vicinity. this study brings a new perspective regarding the origin of vp/vs anomalies. their spatial variation can be interpreted as pathways of thermal-fluids from the middle crust to the shallower layers in the erzurum geothermal area (hauksson, 2000). we propose that the intrusive body is a part of the fluid pathway from the upper mantle to geothermal areas at the surface. besides, we estimate that the hot fluid flow can be controlled by a thermal anomaly associated with the solidified intrusion; if the cooling process is not complete. the results suggest that the magma movements continue to occur below the geothermal areas of 221the local earthquake tomography of erzurum (turkey) geothermal area erzurum and its surroundings. even though the crust-mantle boundary cannot be estimated because of poor resolution below 30 km, geothermal heat transport system is presumably related to the partial melting or upwelling of the upper mantle materials (fig. 13). 7. conclusion in this study, the let method has been used for the first time around the city of erzurum to show a detailed velocity model and vp/vs variations, which are usually associated with lithological and petrological features. the principal aim of this study is to determine the 3-d p-wave velocity and vp/vs profile of the erzurum geothermal area down to 30 km depth. tomographic images computed by a merged data set from the local stations (atanet) and the national network of turkey (afad) were presented. 1685 well-located earthquakes, recorded in the period 2007–2017, were used for tomographic computations. the computations were benchmarked by strict synthetic vertical and horizontal checkerboard tests that indicate a good resolution down to 30 km depth. the results of tomographic inversion have shed light on the complex crust under erzurum. the basin structure is traceable down to 3 km depth in the p-wave velocity models. the high p-wave velocity (≥6.0 km/s) zones in the shallower depths are usually associated with the metamorphic and magmatic compact materials. steam, co2 and gas or a mixture of them can be concluded in low vp, and low vp/vs structures. kirik and the n part of ovacik region are symbolized by low vp/vs and low vp. the high vp/vs (>1.85) with low vp anomalies from 0 to 5 km point out to the fluid content beneath major geothermal areas. such possible geothermal reservoirs are determined in, askale, cimenozu, icmesu, kaplica, koprukoy, karayazi, kaynak, ovacik, soylemez and uzunahmet regions. it is suggested that is necessary to carry out detailed studies on local scale in order to reveal the hot water potential in these areas. acknowledgements the data is provided by ataturk university earthquake research center and afad (ankara-turkey). the lotos code (koulakov 2009) is used to create the tomograms. all images (except fig. 2) are created using gmt (wessel et al. 2013). faults and geology of erzurum are digitized in the geoscience map viewer and drawing editor licensed to the general directorate of mineral research and exploration (mta) (akbas et al. 2013; emre et al. 2013; emre et al. 2018). the data is collected in seisan (havskov and ottemoller 1999) software suite. references akbas, b., akdeniz, n. aksay, a. altun, i. balci, v. bilginer, e. 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(2003). the crustal structure of the east anatolian plateau (turkey) from receiver functions. geophysical research letters, 30(24). palabras clave: cuenca erzincan; información gravitacional; modelación 3d; derivadas basadas en filtros. how to cite item aydin, o. l., bektas, o., buyuksarac, a., & yilmaz, h. (2019). 3d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data. earth sciences research journal, 23(1), 57-66. doi: https://doi.org/10.15446/esrj.v23n1.71090 erzincan basin was investigated using gravity data within the scope of this study. it is also aimed to reveal the discontinuities in the work area as well as the buried discontinuities. boundary determination filters and analysis of the structure of the data and its connection are revealed and clear information is obtained. gravity anomalies were applied with an upward continuation method for 0.25, 0.50, 0.75 and 1 km levels. total horizontal derivative (thd) filter, analytical signal (as) filter, tilt angle derivative (tilt) filter, total horizontal derivative (thdr) filter, theta angle derivative (cos  ) filter, hyperbolic tilt angle derivative (htad) were applied to upward continued data. the discontinuities in the region and the boundaries of the geological structure were revealed. tilt and theta angle derivatives yield the best results from the applied derivative based filters. the obtained data were compared with the existing surface geology and the compatibility between the formations was checked. new discontinuities were found in addition to the discontinuities determined from surface observations in the light of the obtained results. erzincan basin was modeled in three dimensions using gravity data of the study area. as a result of modeling, erzincan basin has been determined to have an average thickness of 7 km. abstract keywords: erzincan basin, gravity data, 3d modeling, derivative based filters. 3d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data modelación 3d e interpretación tectónica en la cuenca erzincan (turquía) a través de información de campo potencial issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.71090 el objeto de este estudio fue investigar la cuenca erzincan a través de información gravitacional. también está enfocado en revelar las discontinuidades en el área de estudio asi como las discontinuidades enterradas. se exponen los filtros de determinación de límites y los análisis de la estructura de la información para obtener información clara. el método de continuación hacia arriba se aplicó en las anomalías gravitacionales en niveles de 0.25, 0.50, 0.75 y 1 km. a la información continuada hacia arriba se le aplicaron los filtros total horizontal de derivadas (thd), señal analítica (as), ángulo inclinado de derivadas (tilt), ángulo theta de derivadas (cos ), y ángulo inclinado hiperbólico de derivadas (htad). este proceso permitió revelar las discontinuidades en la región y los límites de la estructura geológica. las derivadas de ángulo inclinado y ángulo theta produjeron los mejores resultados en cuanto a las derivadas aplicadas con base en filtros. la información obtenida se comparó con estudios de superficie geológica y se revisó la compatibilidad entre las formaciones. se encontraron nuevas discontinuidades adicionales a las discontinuidades determinadas en las observaciones de superficie a la luz de los resultados obtenidos. la cuenca erzincan se modeló en tres dimensiones con información gravitacional en el área de estudio. como resultado de la modelación, se determinó que la cuenca erzincan tiene un promedio de profundidad de 7 kms. resumen record manuscript received: 23/03/2018 accepted for publication: 24/08/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 57-66 ömer l. aydın1, özcan bektaş2, aydın büyüksaraç3, hüseyin yılmaz2 1erzurum metropolitan manucipality, tr-25100, erzurum-turkey 2cumhuriyet university, faculty of engineering, tr-58100, sivas-turkey 3bitlis eren university, faculty of engineering and architecture, tr-13000 bitlis-turkey * corresponding author: ozcanbektas@gmail.com g eo ph ys ic s 58 ömer l. aydın, özcan bektaş, aydın büyüksaraç, hüseyin yılmaz introduction the erzincan basin is the largest sedimentary basin on the north anatolian fault zone (nafz). many earthquakes have ruptured the nafz within the erzincan basin and caused major damage. in the last century, there were two of them (ms = 8.0 in 1939 and ms = 6.8 in 1992) (barka and kadinsky-cade, 1988; fuenzalida et al., 1997). the 1992 event claimed 541 lives (barka and eyidogan, 1993) and had a peak ground acceleration of 0.5 g, and the mercalli intensity was estimated as ix (erdik et al., 1992; gundogdu et al., 1992). pınar et al. (1994) pointed out the 1992 earthquake have occured at the eastern end of 1939 earthquake in 350 m long rupture zone. the amount of amplification depends on the thickness and geometry of the basin. geophysical constraints can be used to image basin depth and causative discontinuties. the seismic hazard in erzincan from future earthquakes on the nafz is significant because the unconsolidated sedimentary basin can amplify the ground motion during an earthquake (kaypak, 2008). the analysis of bayrak et al. (2005) suggested that earthquakes up to magnitude 7.5 could be expected in the erzincan basin, and hartleb et al. (2006) estimated the earthquake recurrence interval as 210–700 years from paleoseismic studies. the strength of ground motion depends primarily on the thickness of sediments in the basin, with the largest amplification occurring in the deepest parts of the electronic supplementary material. the thickness and shape of the sedimentary basin are important parameters to predict the amplification of the ground motion. furthermore, the depth of the basin is an important parameter for improving 3d earthquake relocation (aktar et al., 2004). several seismic tomography experiments have attempted to map the thickness of the erzincan basin. some of these studies used the 1992 aftershocks and reported the thickness of unconsolidated sediments as 2–4 km. they determined that the boundary between the upper sedimentary layer and basement was at a depth of 6–12 km. (aktar et al., 2004; gokalp, 2007; kaypak, 2008). the region’s evolution was won by the closure of paleo-tethys and its emplaced karakaya ocean before liassic, and the different arms of the neotethyan ocean at the end of cretaceous. after the neo-tethys closure, the area became largely land and covered with shallow seas in eocene and lower miocene. both marine environments, however, did not have a long life due to the thrusted structure and rise of the zone with a north-south compression. in the neo-tectonic period beginning from the end of the lower miocene, the escape tectonics of the region were effective and lateral strike-slip faults with different strike and extension were developed (barka and gulen, 1989). hartleb et al. (2006) conducted paleosismological studies on çukurçimen on the nafz in the north-western part of the erzincan basin. they say that there are at least six and possibly seven or more surface fractures over the last 3500 years. in the last 2500 years time period, there are five surface fractures in the earthquakes that have occurred in this region and that the interval of earthquake recurrence has changed between 210-700 years. kaypak and eyidogan (2002, 2005) have studied to determine the upper crustal velocity of erzincan and its surroundings. when new aftershocks calculated by the seismic velocity structure are examined, they have found that they usually fall on the north anatolian fault belt, but the seismic activity is directed to the south towards pülümür. these studies reveal that there may be a new zone of weakness between the southern part of the basin and pülümür. they have identified six layers at different seismic velocities in the erzincan basin, 20 km above the shell. in addition to the aftershocks of the 1992 erzincan earthquake, the regional seismic activity after the 2003 pülümür earthquake indicated that a new fault called the avcidag fault antithetically to the north anatolian fault zone similar to the ovacık fault developed. akpınar et al. (2016), paleomagnetic samples were taken from the lavas in the quaternary volcanic outlet centers closely related to the nafz, especially on the northern edge of the erzincan basin. when paleomagnetism results of samples taken from 43 different stations in erzincan basin are examined, it is seen that all of the samples show a positive inclination value and thus contain a normal polarity. these results show that all of the young volcanic rocks in the erzincan basin are in brunhes chron. berge-thierry (2001) simulated the 1992 erzincan depremination with kinematic source model. they did this work for a 60 km × 60 km area, including the erzincan basin, and as a result they formed theoretical acceleration map. according to the results obtained from this map, especially the western part of the basin has shown that the level of strong ground movements that can occur in the future will be higher than 1992 level. volcano-sedimentary guaratubinha basin in southern brazil was evoluted in late neoproterozoic due to collisional strike-slip tectonics. aeromagnetic anomalies were investigated to present fault lines with similar methods in this study (barao et al., 2017). the aim of this study is to determine depth and geometry of the basin and extention of the buried surface fractures that may have developed during the historical period, which can not be detected any geomorphological and geologic signs in the surface of the young basin fill deposits today. within the scope of this study, data obtained from geophysical studies were evaluated by geophysical and geological studies carried out in erzincan basin to date. erzincan basin is modeled in three dimensions using gravity data of the study area. thus, the deep structure of the erzincan basin has been determined, and derivation based boundary detection filters have been applied to the gravity data to reveal the discontinuities and geological boundaries of the region. the obtained information was compared with the existing surface geology and the compatibility between the formations was checked. in addition to the discontinuities determined from surface observations, new discontinuities were found in the light of the obtained results. general geology of erzincan basin the erzincan basin is the largest of several strike-slip basins formed along the nafz on the pre-pliocene basement located on the sidelines of the eurasian plate and anatolian plate. pre-upper cretaceous (aktimur et al., 1995) or pre-maastrichtian (rice et al., 2009) basement is divided into two divisions comprising northern and southern blocks (aktimur et al., 1995). the northern block includes permo-triassic metamorphic rocks, liassic-dogger volcano-sedimentary rocks and upper jurassic–cretaceous limestone, marl and clay. the southern block is composed of permian to cretaceous limestones underlain unconformably palaeozoic metamorphic rocks consisting of gneiss and muscovite-quartz-calc schists (aktimur et al., 1995; okay and sahinturk, 1997; rice et al., 2009) (figure 1). ophiolitic nappes of lower campanian– lower maastrichtian rock units are also observed in both blocks (yılmaz, 1985; aktimur et al., 1995; kocyigit et al., 2001; rice et al., 2009). these older units in the study area is unconformably covered by upper cretaceous–early eocene turbiditic calcarenite, sandstone, laminated mudrock, volcaniclastic sedimentary rocks that includes rare andesitic lava (yılmaz, 1985; aktimur et al., 1995; kocyigit et al., 2001; rice et al., 2009). the youngest part of the basement is miocene cover rocks including limestone, marls, green clay, evaporites and fluvial deposits (tuysuz, 1993; westaway and arger, 2001; kocyigit, 2003; rice et al., 2009) overlay pre-eocene units. the erzincan basin is filled by plio-quaternary sediments consisting of playa deposits, clastics and basin margin conglomerates. the conglomerates along the nw margin of the basin are composed of ophiolitic melange clastics and cretaceous-miocene carbonates (barka and gulen, 1989). the thickness of the basin infill is a controversial subject and changes from 500 to 4000 m (irrlitz, 1972; hempton and dunne, 1984; buyukasikoglu, 1992; gaucher, 1993; bernard et al., 1997; aktar et al., 2004; kaypak and eyidogan, 2005; gürbüz, 2010; avsar et al., 2013) (figure 2). tectonics and seismotectonics of the study area it is known that the volcanic outcropping centers with neogenequaternary age on the north side of the erzincan basin, which is one of the youngest sedimentary basins developed on the nafz which is one of the most active strike-slip fault zones in turkey, are closely related to the nafz. the erzincan basin is different from the other basins on the nafz due to its long axis being nw-se and related with young structures. the nw-se extent of the basin is parallel to the extent of the nafz and the main segment of the nafz 593d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data extends along the northern edge of the erzincan basin. the volcanic cones lined up along the northern margin present an approximate parallel to the course of this main segment. the nafz contains three main segments in the region (figure 3). the location and geometry of these segments is important for understanding the origin and development of the erzincan basin (akpınar, 2010). the eastward movement of the northeast anatolian block causes complex deformations for the block and the left-lateral movements in the northeast anatolian fault zone (neafz), which forms the northern boundary of this block. the left-lateral strike-slip ovacık fault intersects with the nafz in the southeast of erzincan basin. the erzincan basin is a nw-se directional pull-apart type (allen, 1969) with a length of about 50 km and a width of up to 15 km. the part of the nafz near erzincan consists of many segments. the first of these segments extends from karlıova to the west of yedisu and the second extends from the west of yedisu to the southeast end of erzincan basin. the third constitutes the northeastern border of the erzincan basin and continues northward with a second segment. the geological and seismic properties of these fault segments are discussed in detail (allen, 1969, toksoz et al., 1979). the north anatolian fault zone (nafz), northeast anatolian fault zone (neafz) and ovacık fault (of) form the most important tectonic structures in this area, forming a geometric conjugation with each other in the erzincan basin and its immediate vicinity. most of the historical earthquakes are closely related to these three major faults movements. earthquake distribution and topographic map of the study area are given in figure 4. figure 1. generalized stratigraphic section of the study area and its vicinity (modified from aktimur et al., 1995). figure 2. general geological map of erzincan basin and its surrounding (modified from tarhan, 2002; rice et al., 2009; emre et al., 2012; akpınar et al., 2016). hf: heltepe fault, nafz: north anatolian fault zone, neafz: north eastern anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. figure 3. tectonic blocks in erzincan basin and surrounding area and the direction of movement of these blocks (simplified from barka and gulen, 1989; barka and eyidoğan, 1993; kaypak and eyidogan 2002). s1, s2 and s3 are the segments of the north anatolian fault zone (nafz) (akpınar, 2010). figure 4. earthquake distribution in study area (between 1900 and 2014) and topographic map. hf: heltepe fault, nafz: north anatolian fault zone, neafz: north east anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. 60 ömer l. aydın, özcan bektaş, aydın büyüksaraç, hüseyin yılmaz gravity data and process gravity data for the study area was obtained from the general directorate of mineral research and exploration (mta) with a spacing of 2.5 km measured annually ground survey campaigns in 1970s and all corrections were made by the mta. the density value of 2.67 gr cm-3 was reconstructed according to the density value of 2.40 gr cm-3 for the reason that the basin filling material is composed of alluvium. the gravity anomaly map of the study area is given in figure 5. upward continuation the effects of the infinite number of masses at different depths and at different points of the earth are measured as gravity anomalies on the surface. the gravity values measured from the surface are gravity anomalies originating from the densities of the masses found both on the surface and in the deep. in order to detect the anomalies originating from the masses in deep, the effect of shallow masses close to the surface must be removed, for which the “upward continuation” method has to be applied. potential area farther from the source at any point of the known potential area on the surface can be calculated by upward continuation method. it is necessary to measure at different heights due to various reasons, but it is desirable that these measurements be at a certain level. the values are moved to a fixed surface with the upward continuation process. furthermore, deep anomalies due to anomalies originating from the masses in the surface may not be observed clearly. in this case, the anomalies formed by the deeper masses are brought to a more prominent position by the upward continuation process (blakely, 1995). the effects on the anomalies of the shallow structures were removed and the structures in the vicinity were allowed to be seen clearly by the upward continuation method in this study. upward continuation of 0.25 km, 0.50 km, 0.75 km, 1 km, 2 km and 3 km was applied to the gravity data shown in figure 5. it was observed that many effects were lost for 2 km and 3 km according to other continuation applications in the data. for this reason, 0.25 km, 0.50 km, 0.75 km and 1 km upward continuation were taken into consideration. derivative methods the subtle details in the potential field anomaly maps can be made more visible through some derivative based filters (ekinci and yigitbas, 2012 and 2015, ekinci et al. 2013). many boundary detection filters have been proposed to determine the boundaries of potential field sources. application of total horizontal derivatives and analytic signal filters, the first generation of boundary detection filters, is now standard. total horizontal derivative (thd) filter is given by equation (1) (cordell and grauch, 1985). thd p x p y = ∂ ∂     + ∂ ∂     2 2 (1) where, p is the gravity field, x and y are directions. the highest amplitude values of the thd output pass over the source limits. the analytical signal (as) amplitude is given by nabighian (1972) for the 2b state with eq. (2), by roest et al. (1992) for the 3b state with eq. (3). as p x p z = ∂ ∂     + ∂ ∂     2 2 (2) as p x p y p z = ∂ ∂     + ∂ ∂     + ∂ ∂     2 2 2 (3) where, p is the gravity field. x, y and z are directions. the as filter is often used in potential area data collection especially for close surface exploration. the as amplitude provides bell-shaped anomalies on the source structures. researchers have developed filters under the heading of normalized derivative methods in recent years. the first developed filter in this concept is the tilt angle (ta) filter and given by the following eq. (4) by miller and singh (1994); tilt p z thd = ∂ ∂           −tan 1 (4) the ta is the normalized form of the vertical derivative proportional to the thd. the ta is a filter that gives the effects of both near surface and deep sources at the same amplitude level. the amplitude of the ta is positive when it is on the structure, zero when it is on the structure edge and negative values when it is outside of the structure. amplitude values range from -π/2 to π/2 and are very easy to interpret on this. the ta is mainly a plan or shape-determining filter. for this reason, verduzco et al. (2004) showed that the total horizontal derivative of the ta can be used to determine the boundary. total horizontal derivation of ta is given by eq. (5). thdr tilt x tilt y = ∂ ∂     + ∂ ∂     2 2 (5) the total horizontal derivation of the ta causes an increase noise on data because of a derivation-based filter is calculated again. another weakness is that it can not produce effective results against deep structures. on the other hand, the most useful feature is that it is independent of magnetization direction. for this reason, it is a filter that is often referred to in practice. another filter developed under the concept of normalised derivatives is theta-angle (cos ) filter. theta angle (cos ) filter is given by wijns et al. (2005) with eq. (6) cos =     thd as (6) theta angle is the normalized form of thd using the analytical signal amplitude. theta angle ranges from 0 <  < π/2. for this reason, it is quite easy to interpret. the weak point of the method is that it shows scattered effects of deep structures. another approach is the hyperbolic tilt angle (hta) filter proposed by cooper and cowan (2006) and is given by eq. (7). hta p z p x p y = ∂ ∂ ∂ ∂( ) + ∂ ∂( )( )                     −r tan / / / 1 2 2 (7)figure 5. gravity anomaly map for the study area 613d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data reason, the results of tilt and theta angle derivation methods are compared with the existing surface faults in five different derivation methods. in addition, the discontinuities that are not seen on the surface and compatible with the existing earthquakes are shown with cut lines on tilt and theta angle maps of each extension level (figures 6e,6f, 7e,7f, 8e,8f and 9e,9f). three dimensional (3d) model of depth in cordell and henderson’s (1968) 3d modeling method, gravity anomaly is rectangularly gridded. here, each grid point is the center of the vertical prismatic structure, and the cross-sectional dimensions of the prisms are the grid spacing. in this way the data is divided into equal prisms as grid point. the first approach model of structure is derived from the bouguer slab relationship. the gravitational field of the first model is calculated and the ratio of the calculated gravity value with the measured value at each grid point is used to modify the in figure 6, five derivative filters were applied to the gravity data which was applied 0.25 km upward continuation. five derivative filters were applied to the gravity data which was applied 0.50 km upward continuation. the map of the gravity anomaly applied to the derivative filters is shown in figure 7. five derivative filters were applied to the gravity data which was applied 0.75 km upward continuation. the map of the gravity anomaly applied to the derivative filters is shown in figure 8. five derivative filters were applied to the gravity data which was applied 1.0 km upward continuation. the map of the gravity anomaly applied to the derivative filters is shown in figure 9. when the maps obtained as a result of the derivative methods applied for each level given in this section are examined, the derivation methods in which the structure boundary and discontinuities are observed most clearly in accordance with the purpose of the study are determined as tilt and theta angles. for this figure 6. (a) 0.25 km upward continued gravity anomaly map (b) application of total horizontal derivative (tdh) filter applied to 0.25 km upward continuated gravity data shown in figure 6a. (c) application of hyperbolic tilt angle derivative (hta) filter applied to 0.25 km upward continuation applied gravity data shown in figure 6a. (d) application of total horizontal derivative (thdr) filter of 0.25 km upward continuation applied gravity data shown in figure 6a. (e) application of 0.25 km upward continuation applied gravity data theta angle derivative (cos θ) filter shown in figure 6a. (f) application of tilt angle derivative (tilt) filter aapplied to 0.25 km upward continuation applied to gravity data shown in figure 6a. af: avcıdagi fault, ebf: erzincan basin fault, hf: heltepe fault, nafz: north anatolian fault zone, neafz: north eastern anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. white line shows boundary of erzincan basin. 62 ömer l. aydın, özcan bektaş, aydın büyüksaraç, hüseyin yılmaz initial structure model, thus leading to the second model of the structure. this process is repeated until the best fit between measured and calculated gravity data. in this method, the model building is assumed to be homogeneously distributed in its density and limited along a reference plane at a certain depth. most of the iterative modeling methods consist of three steps. these 1) the initial model, 2) calculation of the gravitational effect of the experimental model, 3) modification of the model. the second and third steps are repeated until sufficient alignment is achieved. the position of each prism element is established by a regular relationship for a specified horizontal reference surface. there are three modes for the reference surface of the prism elements. these are the upper surface, the lower surface and the midpoint. these are the upper surface, the lower surface and the midpoint. firstly, the density and the reference plane are specified, and the gravity effect p (x, y, 0) on the vertical prism q (x ‘, y’, 0) at the qth grid point is only a function of the prism thickness tq and the corresponding position (eq. 8). ∆gobs p qf p q t d, , , ; ,≈ ( )γ ρ (8) and the gravity effect of the input causal structure at the pth grid point gobs p q m qf p q t d, ( , , ; , )≈ = ∑ 1 γ ρ (9) figure 7. (a) 0.50 km upward continued gravity anomaly map. (b) application of total horizontal derivative (thd) filter applied to 0.50 km upward continuated gravity data shown in figure 7a. (c) application of hyperbolic tilt angle derivative (hta) filter applied to 0.50 km upward continuation applied gravity data shown in figure 7a. (d) application of total horizontal derivative (thdr) filter of 0.50 km upward continuation applied gravity data slope angle shown in figure 7a. (e) application of 0.50 km upward continuation applied gravity data theta angle derivative (cos θ) filter shown in figure 7a. (f) application of tilt angle derivative (tilt) filter applied to 0.50 km upward continuation applied to gravity data shown in figure 7a. af: avcıdagi fault, ebf: erzincan basin fault, hf: heltepe fault, nafz: north anatolian fault zone, neafz: north eastern anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. white line shows boundary of erzincan basin. 633d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data t kq1, ,= gobs q (11) is expressed as eq. (11). here k = 1 2πγρ thus, the calculated gravity anomaly of the first model, gcalc p q m qf p q t d, , ,( , , ; , )1 1 1= = ∑γ ρ (12) expressed by equation (9). here γ = gravitational constant ρ = density d = depth of reference plane m = the total number of grids. if tn is the thickness of the prism element under the qth grid point as a result of qth iteration, lim . n n q qt t →∞ = (10) is expressed as eq. (10). initial model thicknesses at each grid point figure 8. (a) 0.75 km upward continued gravity anomaly map. (b) application of total horizontal derivative (tdh) filter applied to 0.75 km upward continuated gravity data shown in figure 8a. (c) application of hyperbolic tilt angle derivative (hta) filter applied to 0.75 km upward continuation applied gravity data shown in figure 8a. (d) application of total horizontal derivative (thdr) filter of 0.75 km upward continuation applied gravity data slope angle shown in figure 8a. (e) application of 0.75 km upward continuation applied gravity data theta angle derivative (cos θ) filter shown in figure 8a. (f) application of tilt angle derivative (tilt) filter aapplied to 0.75 km upward continuation applied to gravity data shown in figure 8a. af: avcıdagi fault, ebf: erzincan basin fault, hf: heltepe fault, nafz: north anatolian fault zone, neafz: north eastern anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. white line shows boundary of erzincan basin. 64 ömer l. aydın, özcan bektaş, aydın büyüksaraç, hüseyin yılmaz is expressed as eq. (12). in the next iteration, the thickness is defined by the following equation (13). t t g gq q obs q calc q 2 1 1 , , , , , =       (13) or in general t tn q n q+ =    1, , , , , gobs q gcalc n q (14) and so figure 9. (a) 1.0 km upward continued gravity anomaly map. (b) application of total horizontal derivative (thd) filter applied to 1.0 km upward continuated gravity data shown in figure 9a. (c) application of hyperbolic tilt angle derivative (hta) filter applied to 1.0 km upward continuation applied gravity data shown in figure 9a. (d) application of total horizontal derivative (thdr) filter of 1.0 km upward continuation applied gravity data slope angle shown in figure 9a. (e) application of 1.0 km upward continuation applied gravity data theta angle derivative (cos θ) filter shown in figure 9a. (f) application of tilt angle derivative (tilt) filter applied to 1.0 km upward continuation applied to gravity data shown in figure 9a. af: avcıdagi fault, ebf: erzincan basin fault, hf: heltepe fault, nafz: north anatolian fault zone, neafz: north eastern anatolian fault zone, mof: malatya-ovacık fault, pf: pülümür fault. white line shows boundary of erzincan basin. g f p q t dcalc n p q m n q, , ,, , ; ,= ( ) = ∑ 1 γ ρ (15) can be written eq. (15). the rms values are calculated to determine the fit between the measured and calculated data. rms mn p m obs p calc n p =    =∑ − 1 2 g g, , , (16) 653d modeling and tectonic interpretation of the erzincan basin (turkey) using potential field data thus, the iteration step, which is the smallest value of rms, is considered as a solution. the method developed by cordell and henderson (1968) to determine the change of alluvium thickness of the erzincan basin was applied to the gravity data shown in figure 5. the intensity value given by akpınar (2010) was used as the density difference value (-0.86 gr cm-3) when three-dimensional modeling was performed. as can be seen from the 3-d depth model shown in figure 10, the thickness of the alluvial unit forming erzincan basin is determined as approximately 7 km. it is also observed that the alluvium thickness decreased in the northwest part of the basin and the basin became two parts. results and discussions when the map of the gravity anomaly of the study area is examined, it is seen that it has low gravity values (~ -160 mgal) in comparison with the construction around the alluvium unit forming the erzincan basin. it is compatible with the surface geology map shown in figure 3. upward continuation of gravity anomalies are usually based on the poisson integral and its iteration. by using the method of domain decomposition, a local function can be used for upward continuation of gravity data. this approach decomposes the total area into small domains, and uses local functions to model the disturbing potential within each of these domains. the results of the domain decomposition approach are compared to the poisson integral for upward continuation in a flat area. in reality, gravity data are measured on the earth’s physical surface, not on the geoid, and the geoid is not a sphere. the numerical integration of poisson’s integral is time consuming. also, the fast fourier method can not be directly applied, if the computation points are not at the same altitude. in order to avoid large topographic effect in the erzincan basin, the upward continuation method was applied to the gravity data shown in figure 6 for 0.25, 0.50, 0.75, and 1 km continuation levels. when the upward continuation map of each level is examined, it has been observed that near 2 km and 3 km upward continuation have been lost in the applied data. therefore, 0.25, 0.50, 0.75 and 1 km upward continued gravite anomalies have been considered. in order to determine the structure boundaries and discontinuities in the study area, five derivative filters were applied; total horizontal derivative (thd) filter, tilt angle derivative (tilt) filter, total horizontal derivative (thdr) filter; angle derivative (cos θ) and hyperbolic tilt angle derivative filter (hta). when all the maps shown in figures 6 to 9 are examined, it has been determined that the boundary of the structure and the derivative methods in which the discontinuities are observed most clearly are the tilt and theta angles. for this reason, the results of the theta angle derivative method applied to each level of upward continuation (0.25, 0.50, 0.75 and 1 km, respectively) shown in figures 6e, 7e, 8e, 9e were compared with the current discontinuities in the study area. when all the maps are examined, it has a value of +1 (π / 2) (pink color) on the amplitude of the theta angle and is consistent with the theta angle values of the discontinuities shown by continuous black lines (af, mof, hf, nafz, neafz and pf). the discontinuities indicated by the black lines are discontinuities determined within the scope of this study, taking into account that the values of +1 (π / 2) (pink color) in the theta angle maps are in line with the distribution of the existing earthquake epicenters. furthermore, it is seen that in the northwestern part of erzincan basin in the theta angle maps, the basin is deformed and therefore two parts. in this part of the basin, it is seen that this deformation is consistent with the direction of earthquakes. for this reason, a new discontinuity is defined as ebf (erzincan basin fault) in this part of the basin. the results of tilt and theta angle derivative methods are compared with the existing surface faults in five different derivation methods. the results of the tilt angle derivation method were applied to each of the continuation levels (0.25, 0.50, 0.75 and 1 km respectively) shown in figures 6f, 7f, 8f, 9f were compared with the current discontinuities in the study area. when all the maps are examined, the amplitude of tilt angle is as + 1.5 (π / 2) (pink color) on body and as 0 on edge of body. the discontinuities (af, mof, hf, nafz, neafz and pf) are shown with continuous black lines are correlated with tilt angle amplitude. it is in line with the values of the angle. the discontinuities indicated by the dashed black lines are the discontinuities determined within the scope of this study, taking into consideration that the values of +1 (π / 2) (pink color) and 0 (yellow color) in the tilt angle maps are compatible with the existing earthquakes. furthermore, in the tilt angle maps it is seen that in the northwest part of the erzincan basin there are two parts, which are not a whole as contemplated to date. in this part of the basin, it is seen that the earthquake epicenter distribution agrees with this deformation direction. for this reason, a new discontinuity is defined as ebf (erzincan basin fault) in this part of the basin. some of the discontinuities obtained from the tilt and theta angle maps are consistent with the discontinuities (continuous black lines) determined by akpınar (2010). in this study, in addition to the results of akpınar (2010), new discontinuities which are not seen from the surface shown by cut black lines have been determined. as a result of 3 dimensional gravity model, the thickness of the alluvial unit forming erzincan basin is determined as 7 km. it is also observed that the alluvium thickness decreased in the northwest part of the basin and the basin became two parts. this situation is consistent with the situation in the tilt and theta angle maps. akpınar (2010) determined the depth of the basin to be about 7.5 km from the two-dimensional gravity models formed from the gravity profiles taken perpendicular to the basin extension, and both study results are compatible with each other. conclusions in this study, the extensions of the surface fractures developed in the young basin fillets of the erzincan basin, which can not be detected any morphological and geologic signs on the surface today, especially during the historical earthquakes, were determined by using the potential field data and the information about the three dimensional geometry of the basin. erzincan basin is modeled in three dimensions using gravity data of the study area. thus, the deep structure of the erzincan basin has been determined, and derivation based boundary detection filters have been applied to the gravity data to reveal the discontinuities and geological boundaries of the region. the obtained information was compared with the existing surface geology and the compatibility between the formations was checked. in addition to the discontinuities determined from surface observations, new discontinuities were found in the light of the obtained results. the obtained results show that there are other extensions of the active north anatolian fault zone inside and around the basin, which can not be observed on the surface today, and new data on earthquake hazard of erzincan urban settlement area are obtained. these data have to be taken into account when working in the region. it is expected that new information and findings that will be detected in this region bearing a great earthquake risk will contribute to reducing the loss of life and property that may occur in future earthquakes. figure 10. 3d alluvial thickness map for the study 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(1985). yukarı kelkit çayı ile munzur dağları arasının temel jeoloji özellikleri ve yapısal evrimi. t.j.k. bulteni, 28(2), 79/92 (in turkish). keywords: gimo marble; kurdistan region; homeoblastic; mosaic texture; palabras clave: mármoles de gimo; región del kurdistán; textura homeoblástica; textura mosaica. how to cite item mirza, t. a., kalaitzidis s. p., fatah s. s., tsiotou s. (2021). petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment. earth sciences research journal, 25(3), 275-285. doi: https:// doi.org/10.15446/esrj.v25n3.88686 it is essential to identify marbles’ petrographic and geochemical characteristics to determine the palaeo-environmental settings where their carbonate protoliths formed. the petrogenesis of massive gimo marbles in the gole area, kurdistan region of northeast iraq, was investigated in this study through a combination of field mapping, petrographic, and geochemical techniques. petrographic examination of these marbles reveals that mineral compositions are similar in all samples, with both homeoblastic and mosaic textures occurring, in addition to opaque grains that provide evidence of mineralization. geochemical analyses show that the average calcium carbonate content of the marble is 94.96%; hence, the marble is lithologically characterized as a pure calcite marble. in most samples, the silica content was below 2 wt.%, with high values related to quartz veinlets. a range of geochemical indices and post-archean australian shale (paas–normalized rare earth element (ree) patterns) suggest that the limestone protolith was deposited in a shallow, near-shore marine environment on a continental margin, with very low input of detrital material. the negative ce anomalies indicate that the protoliths of the gimo marbles were carbonate rocks of a sedimentary origin. abstract petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment características petrográficas y geoquímicas de los mármoles de gimo, en el kurdistan iraquí: limitaciones en el origen y ambiente deposicional de sus protolitos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.88686 este artículo parte de la importancia de identificar las características geoquímicas y petrográficas de los mármoles para determinar la configuración paleoambiental donde se formaron sus protolitos carbonatados. en este estudio se investigó la petrogenésis de los mármoles masivos de gimo, en el área de gole, región del kurdistán en el noreste iraquí, a través de la combinación de mapeo de campo, con técnicas petrográficas y geoquímicas. el exámen petrográfico de estos mármoles revela que las composiciones minerales son similares en todos los ejemplos, tanto con texturas homeoblásticas como mosaicas y con adición de granos opacos que ofrecen evidencias de la mineralización. los análisis geoquímicos muestran que el contenido promedio de calcio carbonatado del mármol es de 94.96 %, lo que significa que estas muestras se caracterizan litológicamente como mármoles de calcita pura. en muchos de los ejemplos, el contenido de sílice fue inferior a 2 wt%, con altos valores relacionados a vetillas de cuarzo. una gama de índices geoquímicos y esquistos australianos posarcaicos (paas, en inglés; patrones de elementos tierras raras normalizados) sugieren que los protolitos de caliza se alojaron en un ambiente somero en el margen continental cercano a la línea costera, con un ingreso de contenido de detritos muy bajo. las anomalías negativas de cerio indican que los protolitos de los mármoles de gimo fueron rocas carbonatadas de origen sedimentario. resumen record manuscript received: 29/06/2020 accepted for publication: 05/02/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 285-295 tola ahmed mirza1, stavros p. kalaitzidis2, sardar s. fatah1, sophia tsiotou2 1university of sulaimani, iraq. 2department of geology, university of patras, gr-26504, rio-patras, greece corresponding author: tola.merza@univsul.edu.iq g eo lo g y https://doi.org/10.15446/esrj.v25n3.88686 https://doi.org/10.15446/esrj.v25n3.88686 https://doi.org/10.15446/esrj.v25n3.88686 mailto:tola.merza@univsul.edu.iq 286 tola ahmed mirza, stavros p. kalaitzidis, sardar s. fatah, sophia tsiotou 1. introduction the iraqi marbles of mawat and penjween (gimo group) are located in the extreme northeast of iraq, near the border with iran (figs. 1 and 2). according to the tectonic classification of buday and jassim (1987), these marbles are located in the suture zone of the penjween walash subzone of central zone of the euogeosyncline. the gimo group comprises an alternation and interfingering of greywhite marble and dark grey calc-schist, with thin sheets of metabasalts in some places. this group shows a remarkable lithological resemblance to the qandil series from the eocene. the gole marble is a part of the gimo group situated in the northeastern part of sulaimani city at 35° 46’ 40.68” n and 45° 49’ 19.07” e (figs. 1 and 2). figure 1. satellite image of northeast iraq showing the location of the study area near gole village (from google earth). various studies have been carried out on the metamorphic rocks of the penjween area, including those by bolton (1958), samirnov and neldov (1962), devilliers (1975), jassim et al. (1982), buday and jassim (1987), awadh (1991), karim (2004), awadh and kettanah (2008). most of these studies were directed towards elucidating the conditions of metamorphism. however, the metamorphic sequences in the area have not been studied in detail, and their relevance to the general geological framework remains unknown. jassim et al. (1982) described the metamorphic evolution of the inner zone of the iraqi zagros in the bulfat, mawat, and penjween massifs, suggesting that all these units were affected by the same metamorphic event, and more specifically as a result of regional metamorphism and contact metamorphism. they estimated the temperature of this contact metamorphism to be 750 °c and low to medium pressure of regional metamorphism. karim (2004) studied the origin of the structure and textures of some kurdistan marbles and reported plastic flow textures formed because of burial and tectonic fracturing and later filled with sparitic calcite. awadh and kettanah (2008) studied the petrology and geochemistry of the shalair metamorphic rock group (smrg) and showed that three lithological units characterize the smrg: metapelite, metacarbonate, and meta-arenite, and that the kater rash volcanic group (krvg) is characterized by a metaigneous series including metabasalt, metaandesite, and metarhyolitic dacite. all these rocks are regionally metamorphosed into low-grade metamorphism of the greenschist facies–chlorite zone. in addition, they suggested thermal conditions of metamorphism of about 340±20 °c at pressure conditions of approximately 2.5–3 kb. this study aims to investigate the mineralogical, geochemical characteristics of gimo marble and determining the origin of their protolith and to provide insight into the palaeo-depositional environment of the formation and as well as provide a better understanding of the origin and geotectonic setting of this part of the iraqi zagros suture zone (izsz). 2. geological setting the gole area is located approximately 20 km north of penjween town in the sulaimani governorate, northeast iraq, at 35° 46’ 40.68” n and 45° 49’ 19.07” e (figs. 1 and 2). the area is geologically complex, situated within the northeastern end of the izsz (fig. 2a). the gole area belongs to izsz, which formed due to the collision between the arabian passive margin and the iranian active margin and is considered a part of the zagros orogenic belt (talbot and alavi, 1996; ali et al., 2016). the tectonostratigraphic units of the izsz preserve a complex geological record of the arabian crust evolution during the closure of the neo-tethys ocean (jassim and goff, 2006). the izsz is marked by two allochthonous thrust sheets: the upper and lower allochthonous units (aswad, 1999; aswad et al., 2011). the lower allochthonous unit contains volcano-sedimentary rocks of the walash–naopurdan nappe, whereas the upper allochthon – or ‘ophiolite-bearing terrain’ – is represented by the bulfat, penjween (gimo–qandil group) (aswad, 1999; aswad et al., 2011), and mawat complexes of the albian–cenomanian (97–105 ma) (aswad and elias, 1988). the gimo marble in the gole area represents a part of the qandil metamorphic series (qms), which occurs in the penjween–walash sub zone (fig. 2b) according to the geotectonic subdivision of iraq proposed by buday (1980). this qms structurally overlies the walash and naopurdan series and is composed mainly of weakly metamorphosed limestone and schist, forming the frontier range of eastern iraq. buday (1980) divided the qms into two subgroups. the lower subgroup is composed of two units: (i) the gimo group, the lowermost unit by buday and blackman (1975), which includes metavolcanics interbedded with marbles, calc-schists, and phyllites; and (ii) the sirginil group, the upper figure 2. a. geotectonic map of the zagros suture zone, northeast iraq, showing the location of the study area. b. geological map of the penjween area (after jassim and goff, 2006). 287petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment unit, which is composed mainly of metamorphosed limestone, calc-phyllites, phyllites, and sparse conglomerates, without any volcanic admixture. the contact of the sirginil group with the overlying sedimentary unit is erosional, whereas the contact between the gimo and sirginil group units is tectonic (mirza et al., 2017; mirza and rashid, 2018). based on field investigations, the exposed rocks in the study area are calcareous members of the gimo group. this group occurs in an elongated zone, stretching from the penjween area in the southeast to the northern parts of the bulfat massif (east of qaladiza), up towards the rawanduz river valley in the nw, and forms continuous outcrops north of the rawanduz river. lithologically, the gimo figure 3. field images and photomicrographs of gimo marble samples. a. outcrops of gimo marble beds in the study area; b. field occurrence of grey-coloured calcitic marble; c. quartz veins that mostly show an open-space filling texture; d. photomicrograph of calcite grains with deformation twinning and a homeoblastic mosaic texture, shown in plane-polarised light (ppl; e. subhedral opaque minerals (op); and chlorite (ch) as accessory minerals f. photomicrograph of a quartz veinlet (qv) shown in cross-polarised light (xpl); g and h. metallic minerals associated to the gimo marble, viewed under reflected light microscope representing hematite (ht), pyrite (py) and chalcopyrite (chl.) group consists of several rock units that show lateral variations; however, the main features of the group are a 600 m thick, massive, and thickly bedded layer of marble and calc-schist interbedded with basaltic lava flows (fig. 3). the equivalent lithologies of the gimo group in the bulfat massif are strongly altered by contact metamorphism, owing to intrusions of intermediate and elemental composition. the gimo and sirginil sequences were hypothesized to have been formed before the tertiary (buday, 1980), with regional metamorphism occurring during the late cretaceous, causing deformation of the entire series. however, despite this tectonic disruption, the original sedimentary textures of the rocks 288 tola ahmed mirza, stavros p. kalaitzidis, sardar s. fatah, sophia tsiotou and their primary geochemical features have mostly been preserved. the gimo group in the penjween area consists of massive white grey marbles, metatuffites, and amphibolites, which have been intruded by mafic and ultramafic rocks of the penjween complex. jassim and goff (2006) suggested that the gimo group was deposited on seamounts in the neo-tethys ocean. thermal metamorphism around the mawat and penjween intrusive rocks has been described by jassim and al-hassan (1977) and buday (1980), and they have considered the metamorphism grade of the gimo group in the biotite zone. 3. materials and methods during fieldwork, eight ‘fresh’ marble samples were collected across strike within the gimo group to investigate its petrogenesis. six thin sections and three polished blocks were prepared at the kurdistan institution for strategic studies and scientific research laboratory. they were examined using a polarised-light microscope in transmitted and reflected mode to determine their mineral contents and petrographic characteristics, respectively (i.e., mineral assemblages, associations, grain sizes, and textures). each fresh sample of marble was powdered through a 200 mesh in an agate mill to avoid contamination. fused-glass discs were prepared for major element analysis by x-ray fluorescence (xrf) at the laboratory of electron microscopy and microanalysis of the faculty of natural sciences, university of patras, greece, with a relative standard deviation percentage (rsd %) of less than 5% for all analyzed elements. loss on ignition (loi), which consists principally of co2 in these samples, was determined at 950 °c for 90 min. trace element concentrations, including rare-earth elements (rees), were obtained at the sgs laboratory in canada by using sodium peroxide fusion and analysis through inductively coupled plasma atomic emission spectroscopy (icp-aes) and inductively coupled plasma mass spectrometry (icp-ms). the detection limits for all analyzed elements in this study are presented in tables 1, 2, and 3. table 1. major oxide contents (wt.%) of gimo marble samples (l.d.: limits of detection in %; loi: loss on ignition, nd: not detected). samples g1 g2 g3 g4 g5 g6 g7 g8 mean l.d % sio2 1.68 1.04 0.88 3.18 1.58 1.64 4.61 0.68 1.91 0.0002 al2o3 0.06 0.11 0.05 0.03 0.17 nd 0.12 0.01 0.07 0.0002 tio2 nd 0.01 nd 0.01 0.02 0.02 nd nd 0.01 0.0010 cao 53.52 54.40 54.30 51.69 53.62 52.70 51.15 54.63 53.25 0.0003 mgo 0.46 0.68 0.66 0.71 0.64 0.64 0.75 0.73 0.66 0.0010 mno 0.03 0.0010 fe2o3 0.22 0.14 0.12 0.12 0.13 0.14 0.30 0.12 0.16 0.0003 k2o 0.00 0.11 0.05 0.05 0.12 0.04 0.07 0.04 0.06 0.0010 na2o 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.0010 p2o5 nd 0.07 0.04 0.07 0.06 0.03 0.04 0.11 0.05 0.0010 loi 42.00 43.17 42.99 40.69 43.11 42.64 41.38 43.42 42.43 total 98.00 99.72 99.09 96.56 99.45 97.86 98.43 99.74 98.61 caco3 a 95.44 97.00 96.84 92.17 95.61 93.98 91.22 97.42 94.96 mgco3 a 0.97 1.43 1.38 1.50 1.34 1.33 1.56 1.52 1.38 sio2/al2o3 28.05 9.81 19.02 99.38 9.38 39.08 67.50 34.03 mg 0.28 0.41 0.40 0.43 0.38 0.38 0.45 0.44 0.40 ca 38.26 38.88 38.82 36.95 38.33 37.67 36.56 39.05 38.06 mg/ca*100 0.73 1.06 1.02 1.16 1.00 1.02 1.23 1.12 1.04 a: caco3 and mgco3 values were calculated via stoichiometry table 2. trace element contents of gimo marble samples (ppm). d.lmeang8g7g6g5g4g3g2g1trace elements (ppm) 2.55.757843568ba 0.045.3nd84631056ce 0.054.145435444co 0.0310.425118342327cu 0.263.543433344cr 0.0315.11319151513151318ni 0.09111111111pb 0.123.935443345rb 0.04382.6246687288268245213258856sr 0.021.7512222221th 0.024.553638452w 1.714.941313.221231514.8154.5y 0.0611.6510718792017zn 0.69.6721998986zr 0.02333333333hf 93.375821377267827165171sr/rb 65.62549983667824343107sr/ba https://www.researchgate.net/institution/kurdistan_institution_for_strategic_studies_and_scientific_research https://www.researchgate.net/institution/kurdistan_institution_for_strategic_studies_and_scientific_research 289petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment 4. results 4.1. petrographic features rock-forming minerals and their textures can be used to interpret the origin of metamorphic rocks as well as the settings from which those rocks were derived (georgieva et al., 2009). notable occurrences of marble rocks with a lensoid/massive form can be traced throughout the gole area. the marble that outcrops in the area is inclined towards the northeast and forms medium to massive beds, which are often brecciated (figs. 3a and b). they are also commonly traversed by quartz veins that mostly show an open-space filling texture (fig. 3c). the gimo marble bodies show a clear petrographic variation, although most are massive, coarse-grained, and light grey to pale yellow or white in color (fig 3a, and b). at the microscopic scale, they exhibit homeoblastic and table 3. ree contents of gimo marble samples (ppm). ree (ppm) paas g1 g3 g7 average l.d(ppm) la 38.00 3.10 10.30 2.90 5.43 0.1-10000 ce 80.00 2.70 6.00 5.10 4.60 0.05-1000 pr 8.90 0.52 2.00 0.90 1.14 0.05-1000 nd 32.00 2.00 8.10 1.80 3.97 0.1-1000 sm 5.60 0.40 1.60 0.38 0.79 0.1-1000 eu 1.10 0.10 0.37 0.10 0.19 0.05-500 gd 4.70 0.49 1.97 0.45 0.97 0.1-1000 tb 0.77 0.07 0.28 0.05 0.13 0.05-10000 dy 4.40 0.52 1.74 0.50 0.92 0.05-1000 ho 1.00 0.10 0.37 0.10 0.19 0.05-500 er 2.90 0.31 1.04 0.25 0.53 0.05-1000 tm 0.40 0.05 0.14 0.05 0.08 0.05-500 yb 2.80 0.20 0.90 0.18 0.43 0.1-1000 lu 0.43 0.05 0.14 0.05 0.08 0.01-1000 ∑ ree 183.00 10.61 34.95 12.81 19.46 ∑lree 164.50 8.72 28.00 11.08 15.93 ∑ hrre 18.50 1.89 6.95 1.73 3.52 ∑lrr/ ∑hrr 8.89 4.61 4.03 6.40 5.02 (la/yb)n 1.14 0.84 1.19 1.06 (la/sm)n 1.14 0.95 1.12 1.07 er/nd 0.16 0.13 0.14 0.14 y/ ho 45.00 40.00 132.00 72.33 eu/sm 0.25 0.23 0.26 0.25 sm/yb 2.00 1.78 2.11 1.96 (nd/yb)n 0.88 0.79 0.88 0.85 (pr/yb)n 0.82 0.70 1.57 1.03 ce/ce* 0.62 0.38 0.44 0.48 pr/ pr* 1.21 1.37 1.69 1.42 la/la* 4.87 4.84 1.20 3.64 gd/gd* 1.64 1.26 5.90 2.93 eu/eu* 0.52 0.48 0.56 0.52 ce/ce*= cen / (2prndn); pr/pr*=prn / (0.5cen+0.5ndn); la/la*= 3 lan/ (3prn 2ndn); (eu/eu* = eun / (smn + gdn)0.5; l.d: limits of detection. mosaic textures. the studied marble has simple mineralogy; they essentially consist of large recrystallized calcite (fig. 3d), with secondary quartz, chlorite, and opaque minerals mostly in the form of hematite, pyrite, and chalcopyrite (figs. 3e, f, g, and h). optical microscopy revealed the predominance of a homeoblastic texture, with the carbonates being medium-grained (1.0–2.0 mm) or coarse-grained (>2 mm) (fig. 3). the light grey marble at the microscopic scale is characterized by the presence of quartz veinlets (fig. 3f). calcite crystals in the studied samples either show curved or straight grain boundaries, which sometimes form triple points (fig. 3d). deformation twin textures are also present in calcite, and comprise both crossing and lamellae twins that lie parallel to the rhomb edges (fig. 3d). this type of twinning is common in calcite that forms in regional metamorphic environments and/ or shear zones (barker, 20 14), and forms because of variable stresses and temperatures applied to the crystal after it has formed. according to ferrill et al. (2004), the calcite twin width and its temperature of deformation are positively correlated. 4.2. geochemical data 4.2.1. major oxides the results of the geochemical analysis show the relatively high purity of the crystalline calcite (table 1). all oxides, apart from cao and loi (i.e. sio2 + tio2 + al2o3 + fe2o3 + mno + mgo + na2o + k2o + p2o5) sum to less than 6 wt.%, while cao contents exceed 51 wt.% (table 1). the insoluble residues, notably sio2 (0.68–4.61 wt.%), al2o3 (0.00–0.17 wt.%), and k2o (0.00–0.12 wt.%), have very low contents, and measured tio2, mno, and na2o contents were negligible. bulk-rock analyses of the gimo marble show very similar sio2 contents of 0.68–1.68 wt.%, except for samples g4 and g7, which display higher values (3.18% and 4.61%, respectively) related to the presence of quartz veinlets (fig. 3). the sio2/al2o3 ratio for all samples is highly variable (9.38–99.38), but is still typical of pure marble (obasi and ogungbuyi, 2014). the most important major element oxides in carbonates in metamorphic terrains are cao and mgo, which show values of 51.15–54.63 and 0.46–0.75 wt.%, respectively, in the studied samples. the major oxide contents of the gimo marble are similar to those of calcitic-type marble and relatively pure limestone (table 1, fig. 4). figure 4. plot of gimo marble samples on a cao–mgo–sio2 ternary diagram classification system (from storey and vos, 1981). numbered fields are as follows: 1: pure calcite, 2: pure dolomitic calcite, 3: pure calcitic dolomite, 4: pure dolomite, 5: siliceous calcite, 6: siliceous dolomitic calcite, 7: siliceous calcitic dolomite, 8: siliceous dolomite, 9: calc-silicate calcite, 10: calc-silicate dolomitic calcite, 11: calc-silicate calcitic dolomite, 12: calc-silicate dolomite, 13: calcite marble, 14: dolomitic calcite marble, 15: calcitic dolomite marble, 16: dolomite marble, non-marble, and mgo > pure marble. 290 tola ahmed mirza, stavros p. kalaitzidis, sardar s. fatah, sophia tsiotou the very low mno content in the studied samples may be attributed to the similarity in the ionic size of mn2+ and ca2+, as both ions may readily substitute for each other on a crystal lattice (ephraim, 2012). according to goldschmidth et al. (1955), the cao contents in calcitic marbles are usually around 50–54 wt.%, while mgo contents are <15 wt.%. the average cao content of the gimo marble is 53.25 wt.%, which shows that mineral phase calcite is dominant and that the protolith was a pure carbonate rock. the loi content varies between 40.69 and 43.42 wt.% with an average of 42.43 wt.% (table 1), the highest values likely reflecting rocks with the lowest quartz contents. qadhi (2008) observed that the proportion of silicate minerals, such as chlorite and quartz, in marble was negatively correlated with the loi values of the rocks. 4.2.2. trace element trace element data obtained from the studied gimo samples are summarised in table 2, which shows that only ba, ce, co, cu, cr, ni, pb, rb, sr, th, w, y, zn, hf, and zr had concentrations above their detection limits. in most of these cases all measured trace elements show values mostly less than 28 ppm, apart from sr. in addition, the sr, ba, cu, y, and zr contents vary significantly (table 2), which possibly suggests a complex distribution of these elements between the samples (georgieva et al., 2009). in the gimo case, sr, ba, and rb displayed average values of 382.6, 5.7, and 3.9 ppm, respectively, with sr being enriched in the studied marble, as it is a typical cation that can replace ca (konrad and dennis, 1995; frimmel, 2009; taelman et al., 2013). shearman and shirmohammadi (1969) considered that the sr content of recent carbonates ranges from 30 to 200 ppm, although the average sr concentration of the gimo marbles (382.6 ppm) is higher than this range (table 2). indeed, sr is the only trace element that showed an elevated concentration, which was particularly pronounced in samples g1 and g7 (856 and 687 ppm, respectively). such enrichment in marble is related to sr2+, readily substituting for ca2+ in limestone, with both ions having the same charge and a similar atomic radius (konrad and dennis, 1995). in addition, enrichment of sr in the gimo marbles suggests that they have not undergone significant weathering (zhang et al., 2017). the studied marble samples show large variations in zr content (6–21 ppm), whereas low contents were observed for co, rb, ba, cr, pb, th, w, and ni (table 2). the post-archean australian shale (paas; taylor and mclennan, 1985) normalized trace element concentrations are plotted in figure 5. the large-ion lithophile elements (liles) such as rb, ba, and th were extremely depleted compared to paas. the studied samples were slightly enriched in sr compared to paas. the high-field strength elements (zr and hf) and ferromagnesian trace elements (co, ni, and cr) were depleted in all marble samples with respect to paas. rb is a large cation with a small charge that tends to be compatible with major elements, and its low ionic potential (ratio of charge to ionic radius) makes this element relatively soluble in aqueous solutions. because of its solubility, it is quite mobile during metamorphism and weathering (amrouni et al., 2018). according to dypvik and harris (2001), the enrichment of rb in carbonate rocks is mutually associated with the presence of k-bearing minerals (clay minerals) as detrital sediment. the petrographic and geochemical observations of the gimo marble confirm the absence of k-bearing phases as detrital sediment; hence, their impact on paas normalized trace element patterns (fig. 5) can be assumed to be insignificant. therefore, the very low rb content in the studied samples may be related to its input during metamorphism and weathering. among the high field strength elements, zr concentrations ranged from 6 to 21 ppm, with an average of 9.6 ppm, and y concentrations range from 4.5 to 23 ppm. average values for the transition elements with moderate to low concentrations are as follows: 15.1 ppm for ni, 11.6 ppm for zn, 10.4 ppm for cu, 3.5 ppm for cr, and 4.1 ppm for co. in addition, pb and th have low average concentrations of 1 and 1.75 ppm, respectively. the presence of th and pb in the gimo marble reflects their mutual association with liles, which are usually mobile and radioactive (obasi and madukwe, 2016). the average concentrations of cr and ni were 3.5 and 15.1 ppm, respectively (table 2). these values are lower than those of the paas, which has contents of 110 and 55 ppm, respectively (mclennan, 2001). in general, tio2, mno, cu, pb, zn, and ni have an important influence on the color of marbles; thus, their low concentrations in the studied samples explain the observed white and grey colors. 4.2.3. rare earth elements the measured ree contents in the studied samples are presented in table 3. these samples have moderate total ree (σree) contents of 10.61–34.95 ppm, with an average of 19.34 ppm, and it is evident that light rare-earth element (lree) components dominate over heavy rare-earth element (hree) components, as the total concentrations of each lie within the ranges of 8.72– 28.0 ppm and 1.73–6.95 ppm, respectively (table 3). the ree concentrations of the studied samples were normalised against paas (taylor and mclennan, 1985). the normalised ree pattern diagrams are presented in figure 6, where the following equations were used to express eu, pr, la, and ce anomalies in the studied samples: (i) eu/eu* ratio = eun/ (smn + gdn)0.5, (ii) pr anomaly = pr/pr* = prn/(0.5cen + 0.5ndn), (iii) la anomaly = la/la* = lan/(3prn − 2ndn), and (iv) ce anomaly = ce/ce* = 3cen/ (2lan + ndn) (bau and dulski, 1996; shields and stille, 2001). figure 6. paas-normalised ree patterns for gimo marble samples using normalisation values from taylor and mclennan (1985). paas normalized ree patterns for all samples show that hrees are slightly enriched with (la/yb)n of 0.84–1.19, and (la/sm)n of 0.95–1.14. in all samples, the average ree content was 19.46 ppm. ce anomalies were always negative and varied from 0.38 to 0.62. the studied samples have variable σrees (10.61–34.95 ppm), y (4.5–23 ppm; ave. 14.94 ppm), and ho (0.1 to 0.37 ppm; ave. 0.19 ppm) contents. their y/ho (40.0–132.0; ave. 72.33) and eu/sm (0.23-0.26; ave. 0.25) ratios are similar to those of seawater, while their sm/yb (1.78–2.11; ave. 1.96) ratios are slightly higher than those of modern seawater (kandemir and yılmaz, 2009) (table 3). following normalisation of rees to paas (taylor and mclennan, figure 5. paas-normalised trace element patterns for selected gimo marble samples using normalisation values from taylor and mclennan (1985). 291petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment 1985), the studied samples exhibit (i) depleted lree relative to hree (nd/ yb)n (0.79–0.88; average 0.85) and (pr/yb)n (0.70–1.57; ave. 1.03), (ii) negative ce/ce* (0.38–0.62; ave. 0.48), (iii) slightly positive eu/eu* (0.48 – 0.56; ave. 0.52), (iv) slightly flat pr/pr* (1.21–1.69; ave. 1.42) anomalies, and (v) a positive la/la* anomaly (1.2–4.87; ave. 3.64). 5. discussion the very low mgo content of all studied samples is because of the lack of dolomite, which commonly forms owing to the replacement of ca by mg during metamorphism or diagenesis. this indicates that the protolith of the gimo marble was a relatively pure limestone that was deposited in a shallow marine basin (obasi and ogungbuyi, 2013). melezhik et al. (2008) suggested an mg/ca ratio of 0.1 as the boundary between calcite marble (cm), dolomite marble (dm), and dolomitized calcite marble (dcm). the mg/ca ratio of 0.5 sets a lower limit for the dm, whereas the mg/ca ratio of stoichiometric dolomite is 0.62. the gole marble is characterized by variable mg/ca ratios ranging between 0.73 and 1.23 (table 1, and fig. 7a), which is greater than that of stoichiometric dolomite, therefore indicating the absence of dolomite. in addition, the plot of ca% vs. mg% that we used to explain the protolith of the studied samples was not affected by dolomitization. hence, the studied samples are plotted along the limestone– dolostone mixing line (fig. 7b) owing to very low concentrations of both mg and sio2 and high concentrations of ca (fig. 3 a, and b). figure 7. lithological classification based on geochemical data for gimo marble samples (after melezhik et al., 2008); a. mg/ca vs. sio2 diagram, b. mg vs. ca diagram, and c. mg/ca vs. sr diagram. in addition, the studied samples show a moderate concentration of sr of 382.6 ppm (table 2), and by using the sr vs. mg/ca plot, the samples are projected in the field of limestone (fig. 7c). the sio2 content of some gimo marbles exceeded 1 wt.% (fig. 7a and table 1), which is attributed to the presence of quartz veinlets and some silicate mineral such as chlorite that were identified by petrographic examination (fig. 3). the alkali content of carbonate rocks decreases with an increase in salinity and depth (clarke, 1911). in general, the composition of a metacarbonate is largely controlled by the distance of its formation from the nearest continent and the topographic elevation at which it is deposited, as well as whether volcanism occurred nearby. all geochemical features of gimo marbles are characteristic of limestone deposition in a continental basin (zhang et al., 2017). the very low alkali content (na2o + k2o < 0.1 wt.%) in the studied marbles indicates that the limestone protolith formed in a shallow, saline environment and was later metamorphosed. in general, a high silica content of a limestone indicates that the basin of deposition of the protolith is near the shoreline (brownflow, 1996); however, it sometimes reflects the inclusion of quartz veins and veinlets that form after deposition of the precursor sediment. in general, the silica content of marble is related to the availability of silicate minerals and chert nodules, which are produced as impurities in the protolith that may be formed after the deposition of the protolith or during metamorphism (obasi and madukwe, 2016; mirza, 2019; mirza, 2021). the petrographic study of the gimo marble revealed the presence of quartz veinlets (fig. 3f); hence, the silica content is mostly attributed to epigenetic events. useful information about the depositional environment of limestone can be provided by lile ratios (zhang et al., 2017). a ternary diagram of rb–sr–ba can be used to distinguish limestones deposited in the open ocean, inland freshwater bodies, and continental margins (fig. 8a). in general, marine limestones that form on continental margins can be distinguished from inland freshwater limestones based on their different sr/ba and sr/rb ratios, whereby both values are higher in continental margin limestones than in inland freshwater limestones (fig. 8b). in the case of gimo marble, all the studied samples are projected in the continental margin field. figure 8. a. rb–sr–ba ternary diagram and b. sr/ba vs. sr/rb discrimination diagram for limestones indicating a continental margin depositional environment for gimo marble samples (after zhang et al., 2017). the enrichment and depletion of ree in carbonate rock may be influenced by various factors: (i) addition of terrigenous particles from the continent (mclennan, 1989), (ii) scavenging processes related to depth, salinity, and oxygen levels (elderfield, 1988); (iii) biogenic sedimentation from the 292 tola ahmed mirza, stavros p. kalaitzidis, sardar s. fatah, sophia tsiotou overlying seawater (murphy and dymond, 1984), and (v) authigenic removal of ree from the water column and early diagenesis (sholkovitz, 1988). rees are relatively immobile during post-depositional processes, such as diagenesis (cullers et al., 1979) and metamorphism (muecke, 1979). to confirm the sedimentary nature of the protolith, it is necessary to investigate the geological characteristics of its depositional environment, which can be achieved in this case by assessing the trace element and ree concentrations. haskin et al. (1966) and jarvis et al. (1975) showed that the enrichment of rees in marbles is positively correlated with the proportion of insoluble substances, such as quartz, clay, mica, silicate, and iron minerals. according to tanaka and kawabe (2006), unaltered marine sedimentary carbonates are expected to preserve the chemical characteristics (i.e. ree ratios) of the medium in which the carbonates precipitated. however, care must be taken when interpreting ree contents in marbles, as they may also be influenced by the presence of non-carbonate materials, such as fine-grained siliciclastic material, especially when present in excess of 5 vol.% (banner et al., 1988). fortunately, petrographic and geochemical observations of the gimo marble show that only negligible amounts of non-carbonate phases occur, and hence their impact on ree patterns can be assumed to be insignificant. in addition, the very low er/nd ratio (0.13–0.16) suggests that the gimo marble is only very slightly influenced by the geochemistry of detrital materials. tanaka et al. (1990) observed that concentrations of la, nd, sm, and eu in coastal waters have a decreasing linear relationship, and the ree patterns of these waters are characterized by a marked discontinuity between eu and gd. similar features are present for the ree patterns of the gimo marble (fig. 6), which indicates that their protoliths were deposited in a shallow, near-shore marine environment. this interpretation agrees with the negligible amounts of na2o, fe2o3, and mno measured in the samples (table 1) (clarke, 1911; turekiah and wedepohl, 1961; frank, 1975). according to zhang et al. (2017), the σree of oceanic plateau carbonates is 1.31 ppm, but it is 10 –30 ppm for carbonates deposited along continental margins and in continental interiors. the ree content of the studied samples ranged from 10.61 to 34.95 ppm; hence, it is interpreted that the studied samples formed in a continental margin setting. boulvais et al. (2000) suggested that the ree distribution in marbles varies depending on the degree of fluid interaction that limestone experiences, such that patterns in these elements can be used to understand the nature of fluid flow during metamorphism. according to shields and webb (2004) and shields and stille (2001), progressive ree scavenging during post-depositional modification may produce a positive correlation between the ce anomaly and ree. however, the studied samples display a notable negative correlation between ce/ce* and total ree content (fig. 9a), implying that such a diagenetic influence is absent. in addition, the correlation between eu/eu* and σree in fig. 9b highlights the enrichment of eu in carbonate minerals and depletion of eu in other minerals, which is interpreted to record eu removal during metamorphism by the reduction of eu3+ to eu2+ (jarvis, et al., 1975). the positive la/la* anomalies (average 3.64) and slightly flat pr/pr* anomalies (average 1.42), which are plotted in area iiib of the pr/pr* vs. ce/ce* scatter plot (fig. 9c) confirm that the ce anomalies are not a relic of la interference. the significant negative ce anomalies and slight positive gd anomalies observed in the gimo marbles (table 3 and fig. 6) indicate that their protoliths were carbonate rocks of sedimentary origin, and that they were deposited in marine environments (özyurt et al., 2020). in addition, it has been suggested that a pronounced negative ce* anomaly can be divided into three zones: (a) <0.5, (b) ~0.6–0.9, and (c) ~0.9–1.0 which represent oxic, suboxic and anoxic marine water, respectively (chen et al., 2015). the ce/ce* anomaly of the studied samples ranges between 0.38 and 0.62 (with an average value of 0.48), indicating the oxidation of ce3+ to less soluble ce4+ that was removed continuously by scavenging (sholkovitz et al., 1994; tobia and mustafa, 2019). accordingly, the protolith of the studied samples was deposited in an oxygenated environment. a comparison of the ree contents of shallow marine carbonate rocks presented by özyurt et al. (2020) with those found in this study, show that the studied gimo samples exhibit the same pattern with strong fractionation of lree over hree with a negative ce anomaly (fig. 6). figure 9. variation diagram of σree content vs. a. ce/ce* and b. eu/eu* for gimo marble samples. ce/ce*= cen / (2prndn ); eu/eu* = eun/(smn + gdn)0.5, where subscript n refers to normalised values taken from taylor and mclennan (1985). c. ce/ce* vs. pr/pr* ratios diagram from bau and dulski (1996) 293petrographic and geochemical features of gimo marble, gole area, kurdistan region, iraq: constraints on its protolith’s origin and depositional environment it is also notable that the la–nd–sm–eu contents of the gimo samples define an inclined straight line and hree (gd–dy–tm–er–yb–lu) contents show more flat patterns (fig. 6). a low er/nd ratio (0.13–0.16) indicates that minor detrital material was present in the protolith. the y/ho ratio can be used as a tracer to assess whether carbonate samples reflect primary marine signatures or siliciclastic components (nozaki et al., 1997). according to nozaki et al. (1997) and zhang and nozaki (1998), and high y/ho ratios (average of 61 ± 12) occur in the upper 200 m of seawater. however, other studies have proposed that typical y/ho ratios in marine carbonates that are free of contamination from terrigenous material show a wider range of values (44–74) or a chondritic value of 28 (bau 1996, and liu et al., 2019). the studied samples display high y/ho ratios with an average of 72.30 (40.0–132.0), which is consistent with detritus-free marine sediments (average 45.5), and defined as super chondritic. davou and ashano (2009) reported that low concentrations of na2o in marble indicate a shallow, near-shore, low energy, and ‘clean water’ marine environment of deposition for the protolith. a low al2o3 concentration is also interpreted as indicating a low-energy environment. finally, several trace elements, including zr, hf, th, and al2o3, are considered to be particularly sensitive to shale contamination, as they occur in detrital alumina silicate minerals in much greater volume in average shale than in marine carbonate (özyurt et al., 2020). the studied samples predominantly exhibited low th (< 3 ppm), hf (< 4 ppm), and zr (< 22 ppm) contents, which supports the observation that contamination by detrital minerals is minor to negligible (table 2). the constraints on the pre-metamorphic parent material of the gimo marbles were derived from the discrimination plot of na2o/al2o3 vs. k2o/al2o3 of garrels and mckenzie (1971), which shows that the studied marble samples fall within the field of sedimentary and metasedimentary rocks (fig. 10a). the mgo–cao–al2o3 discrimination diagram of leyreloup et al. (1977) (fig. 10b) also supports a sedimentary origin, as the studied samples plot outside of the field defining a magmatic origin. figure 10. a. na2o/al2o3 vs. k2o/al2o3 diagram for gimo marble samples (after garrels and mackenzie, 1971); and b. compositions of gimo marble samples on a mgo–cao–al2o3 discrimination diagram (after leyreloup et al., 1977). 6. conclusions 1. from a petrographic perspective, the white and grey-colored gimo marbles display similar mineralogical assemblages and mostly have homeoblastic and mosaic textures. carbonate minerals (calcite), quartz, chlorite, and ferric iron oxides and sulphides occur in trace amounts as accessory minerals. 2. the mineralogical and geochemical data correlate well with each other, suggesting that all marbles formed from the same source and/or the same geological process controlled the bulk composition. in addition, both field observations and the application of geochemical indices indicate that the gimo marble is of sedimentary and metasedimentary origin. 3. 3the absence of dolomite in the gimo marble indicates that the protolith of the studied samples was a relatively pure limestone prior to its metamorphism. 4. 4the very low content of th, hf, and zr in gimo marble supports that contamination by detrital minerals is minor to negligible. 5. 5-the paas-normalised ree distribution patterns reveal slight enrichment in hree, with negative ce, and positive la anomalies (average la/ la*= 3.64) and a super chondritic y/ho ratio (average 72.3). 6. 6the ce/c* vs. pr/pr* relationships and the position of the studied samples in area iiib indicate that they display real ce anomalies and not an artefact of la interference. 7. 7the ree pattern of the studied samples indicates that the protolith limestone formed in a continental margin setting. in addition, the significant negative ce anomaly indicates that the protolith of gimo marbles was a carbonate rock with a sedimentary origin that was deposited in an oxygenated marine environment. acknowledgement the authors would like to thank dr. v. xanthopoulou, laboratory of electron microscopy and microanalysis, faculty of natural sciences, university of patras, for conducting the xrf analysis. references alavi, m. 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(2) the debris flow dilution characteristics in the guanba river in 1998 include the occurrence of the debris flow dilution process after a significant increase in the water-soil ratio; an increase in the proportion of fine particles after dilution of the debris flow; and the size distribution of grain is “narrowed”. (3) in accordance with the density and dilution characteristics, the debris flow dilution process in the guanba river can be divided into the upstream viscous debris flow section, midstream and downstream transitional debris flow section and downstream diluted debris flow section. (4) the density evaluation model for the debris flow dilution process is expressed by the lorentz equation, and this model can reflect the debris flow dilution process such that the debris flow density will decrease gradually with an increase in the water-soil ratio and decrease in channel gradient. the density evaluation model for the debris flow dilution process has been verified by three debris flow cases, which include gaoqiao gully, haizi valley and aizi valley. abstract keywords: debris flows; dilution characteristic and process; evaluation model; guanba river; southeastern tibetan plateau model for dilution process of landslide triggered debris flows —a case in the guanba river, southeastern tibetan plateau issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n2.68177 jun li1, ningsheng chen2*, javed iqbal3, dawei han4 1. sichuan university of science & engineering, school of civil engineering, sichuan zigong 643000, china; 2. key laboratory of mountain hazards and land surface processes, institute of mountain hazards and environment, chinese academy of sciences, sichuan chengdu 610041, china; 3. department of earth sciences, comsats institute of information technology, abbottabad 22060, pakistan; 4. department of civil engineering, university of bristol, bristol bs81tr, united kingdom *correspondence author: chennsh@imde.ac.cn, fax 028-85210110, telephone 0086 13808171963 earth sciences research journal earth sci. res. j. vol. 22, no. 2 (june, 2018): 103-111 el entendimiento y modelado del proceso de dilución aguas abajo del flujo de escombros causado por deslizamientos de tierra es fundamental para el reconocimiento de las condiciones límite y el mecanismo de dilución de este tipo de flujos, además de ser útil como base teórica para el control categorizado de la viscosidad de los escombros, detritos diluidos, flujos hiperconcentrados e inundaciones súbitas en una cuenca de desagüe. en este estudio, al tomar como ejemplo un flujo de detritos típico en el río guanba, en el sureste del altiplano tibetano, del 6 de julio de 1998, se utilizaron modelos empíricos para calcular la densidad, la descarga de agua, la descarga de detritos, la profundidad promedio de materiales sueltos y la gradiente del canal en once secciones cruzadas desde aguas arriba y hacia abajo. con base en estos datos, se analizan las características de dilución y el proceso de dilución del flujo de escombros. de acuerdo con la correlación entre la densidad del flujo de escombros y el índice agua-suelo más el gradiente del canal, se estableció el modelo de evaluación de densidad para el proceso de dilución del flujo de escombross, el cual puede predecir el proceso de dilución de un flujo causado por un deslizamiento de tierra. los resultados del estudio incluyen los siguientes cuatro aspectos: (1) los factores clave en la dilución de flujos de escombross causados por deslizamientos de tierra son la descarga de agua, la profundidad promedio de los materiales sueltos, y la gradiente del canal. (2) las características de dilución resumen palabras clave: flujo de escombros; procesos y características de dilución; modelo de evaluación; río guanba; sureste del altiplano tibetano. record manuscript received: 09/10/2017 accepted for publication: 02/03/2018 how to cite item li, j., chen, n., iqbal, j., & han, d. (2018). the model for dilution process of landslide triggered debris flow —a case of guanba river in tibet southeastern plateau. earth sciences research journal, 21(2), 103-111. doi: http://dx.doi.org/10.15446/esrj.v22n2.68177 modelo para el proceso de dilución del flujo de escombros originados por deslizamientos de tierra caso del río guanba, en el sureste del altiplano tibetano n at u r a l h a z a r d s 104 jun li, ningsheng chen, javed iqbal, dawei han 1. introduction debris flows can be mobilized by the occurrence of landslides, which are usually viscous, high-density debris flows (iverson et al. 1997). three main factors influence landslide triggered debris flows including the water source, loose materials and slope gradient (zhou et al. 1991, xu et al. 2014, guo et al. 2016). when the water-soil ratio increases and channel gradient decreases, the water-soil ratio is the water flow discharge divided by the average depth of loose materials, and such debris flows will be gradually diluted into a diluted debris flow, which is a flow with a high concentration of sediment or a flash flood (pierson and costa 1987, smith 1991, svendsen et al. 2003); we call this the dilution process of a landslide triggered debris flow (kang et al. 2004). such a phenomenon is common in the southwestern mountainous area of china. for example, observed dilution events have been found in the longxi river (drainage basin area of 78.79 km2 ) in dujiangyan on august 13, 2013 (qu et al. 2015), recently in haizi valley (drainage basin area of 103.6 km2 ) in the upstream area of the jinsha river (chen et al. 2015b), and peilong gully (drainage area of 86.1 km2 ) in the sichuan-tibet highway area (chen and chen 2002). while some studies have been performed on the dilution process and mechanism (pierson 2005, felix and peakall 2006), there are still unsolved questions for the following 3 aspects: (1) some studies on the dilution index and its related factors have been completed (pierson and costa 1987, dasgupta 2003, pierson 2005), and the results indicate that the sediment content indexes can be used to distinguish between a debris flow and hyperconcentration flow or other type flows. in addition, their results also show that some related factors influence the debris flow dilution process including the sediment volume, flow depth and channel gradient. the dilution process and characteristics of volcanic debris flow were also studied and show that the duration of dilution processes is quite long and the sediment contents of hyperconcentration flow are very high, ranging from 40 to 60% (pierson and scott 1985, lavigne 2004, pierson 2005). the pattern of change in density, velocity and discharge was determined, and there are related equations for calculating the occurrence of a landslide triggered debris flow (chen et al. 2007, guo et al. 2015, peng et al. 2015). however, there are few studies on the dilution process and characteristics based on the water source, loose materials and channel gradient, etc.; (2) although the existence of dilution is well known (pierson 2005, pederson et al. 2015), there is still little understanding about the detailed predicted model on the dilution process due to absence of the investigation methods and typical cases; (3) the absence of a dilution process prediction model hampers quantitative study of the dilution process and mechanism. this also makes it difficult to implement the relevant mitigation measures for various types of debris flow. for example, the mitigation measures for viscous debris flows mainly include retaining, damming and diverting, whereas those for diluted debris flows mainly include diverting and dredging. in addition, the diversion channel gradient for a viscous debris flow is usually 5%-18%, and the channel shape is usually is trapezoidal or rectangular, whereas the diversion channel gradient for diluted debris flows is usually 3%-10%, and the channel shape is narrow and tall (chen et al. 2015a). to tackle the abovementioned issues, a case study is carried out based on a major dilution event that occurred in 1998 in the guanba river, china. section 2 describes the study area. section 3 addresses the methodology. section 4 analyzes the dilution characteristics and the process of dilution. section 5 develops the dilution process model based on the water-soil ratio, channel gradient and section 6 are the conclusions. 2. study area 2.1 location the guanba river is on the southeastern margin of the qinghai-tibet plateau (fig. 1a), and its geographical coordinates are n: 27° 46′ 21.3′ ′ ~27 27° 46′ 21, e: 102°17′ 8.24′ ′ ~10227° 46′ 21. the guanba river has an area of 137.24 km2, average channel gradient of 6.09%, length of 23 km, maximum altitude of 3264 m, minimum altitude of 1510 m and relative relief of 1754 m (fig. 1b). the guanba river gully is a v-shape gully with a mountain slope of 25-40°, and upstream channel gradient of 10.7~18.8%. this oscilating river developed in a midstream and downstream fault basin, there are many river islands in the waterway, and the channel gradient is 6.9~10.7%. there is a flat terrain from the downstream to the lake entrance, and the channel gradient is within 6.1-6.9%. 2.2 landform conditions the guanba river belongs to the luoji mountain system in the southeastern qinghai-tibet plateau and is located on the east side of the southnorth anninghe fault zone in sichuan and yun’nan and on the north end of the zemuhe river–xiaojiang river fault zone. the tectonic stability of the guanba river is mainly controlled by the zemuhe river–xiaojiang river fault zone. the guanba river top layer is mainly quaternary gravel and sand, and the underlying bedrock is sand and mudstone of the lower cretaceous. due to the influence of earthquakes and the wetting-drying cycle that occurs over a long period, the guanba river basin is impacted by many landslides, slope erosions, loose deluvials and channel deposition. fig. 1b shows a series of landslides numbered from 1 to 45. the landslide statistics for 1, 2, 3, 22, 23, 34 and 35 are the total number of landslides in the corresponding branch gully, and the serial numbers of the loose deluvials, channel deposition and slope erosions are 46-61, 62-66 and 67-69, respectively. fig. 2 illustrates the 4 typical types of materials in debris flows. in accordance with the calculation results of the loose material volume, the total volume of loose materials in the guanba river in 2010 was 392.06×104 m3 (including 35.2% from the landslide source of 138.19×104 m3, 44.1% from the slope erosion source volume of 172.67×104 m3, 20.7% from the loose deluvial source volume of 38.41×104 m3, and the channel deposition source volume of 42.80×104 m3). according to the literature, the main material sources (landslide and slope erosion) before the occurrence of the debris flow in 1998 were very likely to be the main materials sources in the guanba river in 2010 in terms of spatial distribution and volume (wei et al. 2014, chen et al. 2015c); this indicates that the loose material source volume before the occurrence of the debris flow in 1998 could be approximately substituted by the loose material source volume in 2010. 2.3 hydrometeor the guanba river is within the basin of qionghai lake, which is in the low-latitude and high-altitude zones. due to the alternating effects of the southwest monsoon and southeast inland drought monsoon, the basin of del flujo de detritos en el río guanba en 1998 incluye la ocurrencia del proceso de dilución después de un incremento significativo en el índice agua-suelo, un incremento en la proporción de partículas finas después de la dilución de los detritos, mientras la distribución por tamaños del grano es "reducida". (3) de acuerdo con las características de densidad y dilución, el proceso de dilución de detritos en el río guanba se puede dividir entre la viscosidad de los detritos aguas arriba, el flujo transicional de detritos en el centro de la corriente y aguas abajo, y la sección de detritos diluidos aguas abajo. (4) el modelo de evaluación de densidad para el proceso de dilución del flujo de detritos se expresa con la ecuación de lorentz, y es este modelo el que puede reflejar el proceso de dilución de forma que el flujo de escombros disminuye gradualmente con el incremento del índice agua-suelo y el decrecimiento de la gradiente de canal. el modelo de evaluación de densidad para el proceso de dilución del flujo se verificó con tres casos de flujos, que incluyen el gaoqiao gully, el valle haizi y el valle aizi. 105model for dilution process of landslide triggered debris flows —a case in the guanba river, southeastern tibetan plateau qionghai lake shows the characteristics of a mid-subtropical plateau mountain climate. it has a multi-year mean temperature of 16.9°c, maximum annual mean temperature of 17.2°c and minimum annual average temperature of 15.4°c. the precipitation is controlled by the southwest monsoon of the indian ocean, and it has a multi-year mean precipitation of 1013.5 mm, maximum annual precipitation of 1549.6 mm and minimum annual precipitation of 691.2 mm. vegetation includes evergreen coniferous and broad-leaved forest, as well as evergreen & deciduous broad-leaved mixed forest. the soil in the basin of qionghai lake mainly includes purple soil, mountain red soil and mountain yellow soil. the debris flow in the area is characterized by high frequency, large scale and high sediment content, etc. 2.4 the debris flow events in qionghai lake basin the guanba river and ezhang river have suffered from frequent debris flows at different degrees in the qionghai lake basin during 1996-2003. the debris flow outbreak in the guanba river at 16:00 on july 6th, 1998, was the largest scale debris flow in the area in recent years, and this debris flow had a return period of 100 years or greater and a duration of more than 2 hours. thousands of acres of farmland in the midstream river section of daxing village were flooded by the debris flow, and the loose solid matter carried by the debris flow led to the diversion of the downstream section of the guanba river and advancement of 172 m of coastline on qionghai lake. the ezhang river suffered from successive debris flows in 1996, 1997 and 1998, and these debris flows destroyed nearly a thousand acres of farmland; as of 2015, there were still more than 200 acres of farmland that could not be restored. these two points explain that the debris flow in the basin of qionghai lake has characteristics of large-scale. the debris flows that have occurred in the guanba river, ezhang river and xiaoqing gully in the basin of qionghai lake during 1950-2003 have transported a large volume of sediments, which are the results of gravitational erosion such as landslide or collapse into qionghai lake. the sediments transported by the debris flow decreased the area of qionghai lake by 11.6%, and the average water depth of qionghai lake was reduced from original 14.0 m to 10.95 m in 2003, which means that the debris flows in the qionghai lake basin are characterized by a high sediment content. figure 1. geographical location of the guanba river basin. figure 2. typical loose materials distributed in the guanba river (a is a photo of typical loose deluvials no. 50 in dele, b is a photo of typical soil erosion no. 67 and landslide no. 12 in dapingzi, c is a photo of typical channel deposition no. 65 in dawanzi). 3. methodology using the debris flow in the guanba river with a return period of 100 years, which occurred at 16:00 on july 6th, 1998 as an example, the intent in this study is to achieve the following using the workflow path shown in fig. 3: to reveal the dilution characteristics and process of debris flow based on the factors of water sources, loose material and channel gradient. we establish the density evaluation model for the debris flow dilution process based on the water-soil ratio and channel gradient. in this study, based on the typical debris flow that occurred in the guanba river in 1998, empirical models are first used to calculate the density, water flow discharge, debris flow discharge, average depth of loose materials and channel gradient at 11 cross sections, and these methods are shown in table 1. second, we use the following rule to distinguish the types of fluid. the fluid with a density larger than 1.70 t/m3 and smaller than 2.30 t/m3 is the viscous debris flow, fluid with a density between 1.40-1.70 t/m3 is the diluted debris flow (kang et al. 2004). based on this rule, we further analyze the dilution characteristics and process of debris flow under the dynamic changes in watersoil ratio and channel gradient. here, the water-soil ratio cv is the water flow discharge divided by the average depth of loose materials. third, we establish the density evaluation model for the debris flow dilution process based on the water-soil ratio and channel gradient using the lorentz numerical simulation. fourth, we use the three typical types of debris flows to verify the density evaluation model for the dilution process. these calculation methods are shown in table 1. figure 3. workflow chart. 106 jun li, ningsheng chen, javed iqbal, dawei han table 1. calculation method for the density, water flow discharge, debris flow discharge, average depth of loose materials and channel gradient of the 1998 guanba river debris flow. 107model for dilution process of landslide triggered debris flows —a case in the guanba river, southeastern tibetan plateau figure 4. site sampling pictures for the guanba river debris flow 4. dilution characteristics and dilution process of the guanba river debris flow through analysis of the dynamic process of parameters and particle size distribution curve for the debris flow at different locations, the following three dilution characteristics of the debris flow are obtained. (1) the key factors of the dilution process are the water flow discharge, average depth of loose materials and channel gradient. (2) the debris flow dilution process occurs after a significant increase in the water-soil ratio. (3) the proportion of fine particles increases after dilution of the debris flow. the change in particles size distributions is “narrowed” with the movement of the debris flow. based on the density and dilution characteristics, the dilution process of the debris flow in the guanba river can be divided into the upstream viscous debris flow section, midstream and downstream transitional debris flow section and downstream diluted debris flow section. with the key factors of water flow discharge, average depth of loose materials and channel gradient, we use the changes in energy and sediment content to explain the reason for the dilution process. 4.1 dilution characteristics of the debris flow 4.1.1 the dilution process of the debris flow occurs after a significant increase in water-soil ratio figure 5. the change in parameters with the movement of debris flow in the guanba river (movement of debris flow is represented by cross-sections from upstream i to downstream χi). when the average depth of loose materials suddenly decreases, the water flow discharge increases suddenly and the channel gradient decreases. thus, the viscous debris flow will not be immediately transformed into a diluted debris flow; instead, it will be transformed after continued movement over a very long distance, which indicates that the dilution process of viscous debris flow occurs after a significant increase in the water-soil ratio and channel gradient. in accordance with fig. 5, the water flow discharge, debris flow discharge, average depth of loose materials and channel gradient will experience a significant change within section iv-v; in this situation, the rates of water flow discharge and debris flow discharge increase by up to 50% and 48%, respectively, the average depth of loose materials decreases by up to 51%, and the channel gradient decreases up to 13%. however, the fluid in section iv-v is still a viscous debris flow, because the water-soil ratio and channel gradient do not exceed the required critical value to transform the viscous debris flow into a diluted debris flow. in section v-vi, the average depth of loose materials and channel gradient continue to decrease as the water flow discharge increases, and the viscous debris flow will be changed into a diluted debris flow only after moving 4.93 km. this means the debris flow dilution process occurs after a significant increase in the water-soil ratio. according to the analysis, the water flow discharge, the average depth of loose materials and the channel gradient are the key factors that determine the debris flow dilution process. 4.1.2 the proportion of fine particles is increased after the dilution of the debris figure 6. particle size distributions of the guanba river debris flow. flow, and the size distribution of particles is “narrowed” with the movement of the debris flow in accordance with fig. 6, the proportion of fine particles is increased after dilution of the debris flow from cross-section v to cross section xi. as the debris flow moves from cross section i to cross section xi, the gravel content is gradually decreased from 75.5% to 0, the sand content is increased from 15.7% to 48.9%, the silt content is gradually increased from 2.5% to 51.1% and the clay content is gradually decreased from 6.3% to 0. the gravel and clay contents in the debris flow decrease gradually, whereas the sand and silt contents are gradually increased, which means that the proportion of fine particles is increased after dilution of the debris flow. in addition, the size distribution of particles in the debris flow is gradually “narrowed” from cross section i to cross section xi. the particle size distribution scope of cross section i includes gravel, sand, silt and clay; along with the movement of the debris flow, the particle size distribution scope of cross-section v includes fine gravel, sand, silt and clay, and the particle size distribution scope of cross section xi includes silt and fine sand without gravel and clay. the particle size distribution scope of soil samples from cross section i to cross section xi indicates that the particle size distribution of the debris flow fluid is gradually “narrowed”. 108 jun li, ningsheng chen, javed iqbal, dawei han figure 8. plan for changes in density, water-soil ratio and channel gradient in the guanba river (different colors indicate the basin area between cross sections). figure 9. change in erosive depth in the upstream area of the guanba river. photo a is from may 31st, 2010, and photo b is from april 30th, 2012. figure 7. section division for the debris flow dilution process in the guanba river. 4.2 dilution process of the guanba river debris flow in accordance with the density and dilution characteristics of the 1998 debris flow, the debris flow dilution process is divided into three stages (fig. 7): section i-iv is the viscous debris flow, iv-vi is the section in which the viscous debris flow is transformed into a diluted debris flow, and vi-xi is the diluted debris flow section (the fluid in the ix-xi section may be a hyperconcentration flow). in accordance with the aforementioned section division results for the debris flow dilution process, we further analyze the influence of the water flow the increase in the proportion of fine particles and “narrowed” particle size distribution after dilution of the debris flow is possible because of the following: the sediment concentration in the debris flow will be gradually decreased with the increase in water-soil ratio, decrease of channel gradient and reduction of clay particles in the fluid; the solid mass will be selectively transported, the quantity of coarse particles will be gradually reduced due to the sedimentation effect, fine particles (fine sand and silt) can be maintained in a suspended state, and more fine particles will enter the debris flow with the increase in turbulent fluctuation of the debris flow. discharge, average depth of loose materials and channel gradient on the dilution process of the 1998 debris flow. to facilitate the analysis, we define the guanba river upstream area as the i-iv section, midstream and downstream as the ivvi section and downstream as the vi-xi section. under the influence of a smaller water-soil ratio and larger channel gradient (fig. 8), the gentle decrease in density in the upstream viscous debris flow is mainly occurs for two reasons. on the one hand, the larger upstream channel gradient leads to a stronger erosional effect from the debris flow (erosion depth in some upstream sections was up to 1.2 m within 22 months from may 31, 2010, to march 30, 2012 (fig. 9), which means that the upstream debris flow is more erosive under the influence of a larger channel gradient), and the debris flow obtains more solid masses through the stronger erosion effect; moreover, the water flow discharge provided by the upstream mountain is smaller, which maintains the debris flow density at a higher level all the time in the upstream area. on the other hand, the erosive ability of the debris flow will be reduced with the gradual decrease in the upstream channel gradient in the guanba river, and the solid mass content in the debris flow is reduced, and 109model for dilution process of landslide triggered debris flows —a case in the guanba river, southeastern tibetan plateau thus, the debris flow density is gently decreased. under the influence of a significant increase in the water-soil ratio and continued decrease in the channel gradient, the midstream and downstream viscous debris flow is gradually transformed into a diluted debris flow, and the sharp decrease in its density is mainly because of the following: the influence of gully broadening (20-80 m within the midstream and downstream gully width scope of the guanba river) on the debris flow depth at section iv-vi is gradually decreased; the decrease in the debris flow depth and channel gradient reduces the drag force of the viscous debris flow, which weakens the erosive effect of the debris flow. when the debris flow cannot obtain more solid mass through the erosive effect, a large amount of low-density water flow from the majiwo river and xinrensi river dilutes the debris flow (hallworth et al. 1993), which leads to a sharp decrease in the debris flow density and transforms the viscous debris flow into a diluted debris flow. with a large amount of low-density water flow entering the diluted debris flow, the turbulent fluctuation effect of the downstream diluted debris flow will be further enhanced, and the diluted debris flow can carry more fine particles. with the increase of sand content in the debris flow, the diluted debris flow may be converted to a hyperconcentration flow. 5. the density evaluation model for the debris flow dilution process based on the water-soil ratio and channel gradient based on the debris flow particle content, we can use the density (calculated by the yu equation) to divide the debris flow dilution process. however, there are thousands of debris flow gullies in southwest china since the wenchuan earthquake (tang et al. 2009). this method will require much time and money to obtain the particle content based on a field investigation and laboratory test. a rapid and reliable evaluation model for detecting the debris flow dilution process is urgently needed. our density evaluation model is suitable for rapidly detecting the debris flow dilution process because the input data (water-soil ratio and channel gradient) of the model are easily obtained from the remote sensing data and topographic map of 1:10000 or 1:50000 scale (zhou et al., 2016). the density evaluation model for the debris flow dilution process is established according to the following steps. 5.1 correlation analysis of model parameters in accordance with the analysis results of the debris flow dilution process in the guanba river in 1998, the water source (discharge), loose materials source (average depth of loose materials) and gradient (channel gradient) are the main factors that influence the debris flow dilution process. based on the pearson method, we have calculated the correlation coefficient of the density with the water-soil ratio and gradient. the fitting result shows that the density has a positive correlation with the gradient and the correlation coefficient is 0.97, and the density has a negative correlation with the water-soil ratio, and table 2. water-soil ratio, gradient and debris flow density in guanba river in 1998. the correlation coefficient is -0.89 (table 2). this indicates that the density correlates well with the gradient and water-soil ratio. 5.2 model establishment based on the pearson method, we know the density correlates well with the water-soil ratio and channel gradient. therefore, we use a regression analysis to establish a density evaluation model based on the lorenz equation. (1) (2) (3) the input data of this model are the water-soil ratio and channel gradient, and the output result of this model is the debris flow density. the lorentz equation is shown in equation (1). in equation (1), γʹc is the debris flow density, γ0 is the minimum density using 1.4 t/m3, c is the water-soil ratio expressed in m3/s/cm, j is the channel gradient and j>0 expressed in %. a is the difference value between the maximum and minimum debris flow densities. cv is the water-soil ratio corresponding to the maximum debris flow density, and jc is the channel gradient corresponding to the maximum debris flow density. ω1 is the maximum water-soil ratio corresponding to 0.5 a, and ω2 is the maximum gradient value corresponding to 0.5 a. in equation (3), γʹci is the density of the i cross-section, t/m3. γ̅c is the average of the actual density, t/m3. n is the numbers of cross-sections. the density evaluation model for the debris flow dilution process is established based on the water-soil ratio and channel gradient using the lorentz numerical simulation. equation (2) is the specific expression of the density evaluation model. the correlation coefficient r2 of this equation is 0.94. in addition, equation (3) of the average relative error (are) is used to check the error in the calculation. from table 2, we know the are value of the density evaluation model is low, which is 9.41%. therefore, the density evaluation simulated by the lorentz equation has high precision (fig. 10). the scope of application is shown as follows. the range of water-soil ratio is 0-632.62 m3/s/cm, and the range of channel gradient is 0-21.14% in the density evaluation model for the debris flow dilution process. in addition, the density model cannot be used under the circumstances when all debris flow figure 10. fitting curve for the density evaluation model of the dilution process based on the channel gradient and water-soil ratio. 110 jun li, ningsheng chen, javed iqbal, dawei han table 3. calculation results for the debris flow dilution process in the gaoxiao gully, haizi valley and aizi valley. density and water-soil ratio values are obtained. our model is a rapid model for detecting the debris flow dilution process in a large boundary using remote sensing interpretation and a geographical map. in accordance with fig. 10, the debris flow density in guanba river will gradually decrease with the increase in water-soil ratio and decrease in channel gradient. the density evaluation model for the debris flow dilution process based on the water-soil ratio and channel gradient can better reflect the debris flow dilution process in the guanba river in 1998. under certain conditions, the density evaluation model can be used to predict the dilution process of landslide triggered debris flows in the southwestern mountainous area of china. 5.3 verification of the density evaluation model for the debris flow dilution process this study selects three typical cases to verify the density evaluation model, which is the gaoqiao gully debris flow on august 13, 2010, the haizi valley debris flow on august 11th, 1996, and the aizi valley debris flow on june 28th, 2012. the debris flow data for gaoqiao gully are obtained from a previous study (zhang, 2014). the debris flow data for haizi valley and aizi valley are obtained from a previous study (chen, 2016) and a related report of debris flow prevention. the water-soil ratios of the three debris flows are obtained from the remote sensing interpretation and geographical map. based on equation (2), the results of the debris flow dilution process are shown in table 3. from table 3, the viscous debris flow is gradually transformed into a diluted debris flow during movement with an increase in the water-soil ratio and gradual decrease in the channel gradient for all three debris flows. the dilution process of the three debris flows predicted by the density evaluation model is basically consistent with the actual dilution process. the are values of the density evaluation model in gaoqiao gully, haizi valley and aizi valley are 2.86%, 3.65% and 3.06%, respectively. this demonstrates that the density evaluation model is suitable for detecting the debris flow dilution process. here, the following should be noted that: the density evaluation model for the debris flow dilution process established according to the typical cases of debris flows in the guanba river is only a preliminary work, and more debris flow dilution cases will be studied in the future work to further complete the density evaluation model for the prediction of the debris flow dilution process. 6. conclusions 1. the key factors in the dilution process of landslide triggered debris flows are the water flow discharge, the average depth of loose materials and the channel gradient. 2. the debris flow that occurred in 1998 in the guanba river had the following three dilution characteristics: 1) the debris flow dilution process occurs after a significant increase in the water-soil ratio. 2) the proportion of fine particles is increased after dilution of the debris flow. 3) the size distribution of particles is “narrowed”. the characteristic that the debris flow dilution process occurs after a significant increase in the water-soil ratio is reflected by the fact that the viscous debris flow is completely transformed into a diluted debris flow only after moving for a distance of 4.93 km. the characteristic that the proportion of fine particles is increased after dilution of the debris flow is reflected by the fact that the proportion of fine particles in the debris flow will gradually increase with the increase in movement distance of the debris flow. 3. the debris flow dilution process in the guanba river can be divided into the upstream viscous debris flow section, midstream and downstream transitional debris flow section and downstream diluted debris flow section. when the debris flow cannot obtain more solid masses through the erosion effect, the viscous debris flow transforms into a diluted debris flow because a large amount of low-density water flow from the majiwo river and xinrensi river dilutes the debris flow. 4. in accordance with the calculation result of the pearson correlation coefficient method, the debris flow density in guanba river in 1998 has a negative correlation with the water-soil ratio, and the correlation coefficient is -0.89; the density has a positive correlation with the channel gradient, and the correlation coefficient is 0.97. the density evaluation model is established based on the watersoil ratio and channel gradient. this model can be expressed by the lorentz equation, and the specific expression is the correlation coefficient is 0.94 and the are value is 9.41%. the density evaluation model can reflect the debris flow dilution process in that the debris flow density will decrease gradually with the increase in water-soil ratio and decrease in channel gradient. 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(2016). a rapid method to identify the potential of debris flow development induced by rainfall in the catchments of the wenchuan earthquake area. landslides, 13(5), 1243-1259. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.115013 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 361 366 pe tr o le u m g eo lo g y rolling appraisal and high-efficiency development on a lithologic reservoir of nearshore subaqueous fan in the tanan oilfield quan wen1,2*, zixuan sha2, guozhi wang1 1. school of earth sciences, chengdu university of technology, chengdu 610059, china 2. exploration and development research institute of daqing oilfield co., ltd, daqing, heilongjiang 163712, china *corresponding author: 185209605@ustc.edu.cn how to cite item: wen, q., sha, z., & wang, g. (2024). rolling appraisal and high-efficiency development on a lithologic reservoir of nearshore subaqueous fan in the tanan oilfield. earth sciences research journal, 28(3), 361366. https://doi.org/10.15446/esrj.v28n3.115013 record manuscript received: 11/06/2024 accepted for publication: 19/10/2024 abstract the exploration and development process in the tanan oilfield is guided by geological insights and the aim of economically beneficial exploitation in foreign oilfields. the study primarily focuses on the sequential exploration and development process of “finding fan, finding reservoir, and finding edge” for the lithologic reservoir of nantun formation, aligning with current oil prices and available geological resources. by analyzing the detailed anatomy of lithologic reservoirs and development characteristics in the nantun formation, a geological working method of “searching for uplift and fan before root and edge” has gradually emerged. the lithologic reservoir of nantun formation is a typical nearshore subaqueous fan lithologic-structural reservoir developed under long-term paleo-uplift in the deep fault period of a faulted lacustrine basin. the combination of paleogeomorphologic restoration and seismic sedimentology is crucial in identifying such reservoirs. reservoir development is influenced by sedimentary subfacies type, with the middle-fan reservoir exhibiting the best physical properties and higher natural productivity, followed by the fan-root reservoir which shows better well productivity after fracturing. the fan-edge reservoir has comparatively poorer physical properties, with less apparent effects of fracturing. this method has proven to be effective in guiding the rolling appraisal and development of such reservoirs, both domestically and internationally, through detailed geological study. keywords: tannan sag; lithologic reservoir; rolling appraisal and development; nearshore subaqueous fan, development strategy palabras clave: hundimiento en el pozo tanan; reservorio litológico; evaluación y desarrollo; abánico bajo el agua cercano a la orilla; estrategia de desarrollo evaluación y desarrollo de alto impacto en un reservorio litológico de un abánico bajo el agua y cercano a la orilla en el campo petrolífero de tanan los procesos de exploración y desarrollo del campo petrolífero de tanan se guían por principios geológicos y el objetivo de una explotación económica aplicable en campos petrolíferos en el extranjero. este estudio se enfoca en la secuencia de exploración y proceso de desarrollo de “encontrar un abánico, encontrar un reservorio, y encontrar sus límites” para el reservorio litológico de la formación de nantun, en concordancia con los precios actuales del crudo y con los recursos geológicos disponibles. al analizar la anatomía detallada del reservorio litológico y las características de desarrollo de la formación nantun, emergió gradualmente el método de trabajo geológico de “buscar el levantamiento y el abánico antes que la raíz y el borde”. el reservorio litológico de la formación de nantun es un típico abánico bajo el agua de estructura litológica y cercano a la costa que se desarrolló durante el levantamiento del paleolítico en el período de una falla profunda de una cuenca lacustre fallada. la combinación del restauramiento paleogeomorfológico y la sedimentología sísmica es crucial en la identificación de estos reservorios. el desarrollo del reservorio está influenciado por el tipo de subfacies sedimentarias, donde el abánico intermedio presenta las mejores propiedades físicas y mayor productividad natural, seguido por el abánico de la raíz, que muestra la mejor productividad de pozo tras la fracturación. el abánico del borde del reservorio tiene comparativamente las propiedades físicas más pobres, con menos efectos aparentes en la fracturación. este método ha demostrado ser efectivo para guiar la evaluación y el desarrollo de dichos yacimientos, tanto a nivel nacional como internacional, mediante un estudio geológico detallado. resumen https://doi.org/10.15446/esrj.v28n3.115013 mailto:185209605@ustc.edu.cn https://doi.org/10.15446/esrj.v28n3.115013 362 quan wen, zixuan sha, guozhi wang 1. introduction tamsag oilfield is the first project for daqing oilfield to enter overseas and realize an independent operation of the “going global” strategy. it includes tanan oilfield and nanbeier oilfield. after daqing oilfield was transferred to it, it gradually overcame a series of unfavorable factors, such as complex geological conditions and cumbersome business operations. it achieved the stage goal of high and stable production of this oilfield. to ensure the continuous stable production of the oilfield and the deepening of exploration and development, tanan oilfield has entered the orderly transition and replacement stage from exploration and development of structural reservoirs to lithologic reservoirs. lithologic reservoir exploration is traced back to the introduction of seismic stratigraphy and sequence stratigraphy in china (xiaomin, 2001; houfu et al., 1988). the rise of seismic sedimentology and paleogeomorphology provides new theoretical support for the exploration of lithologic reservoirs (hongliu, 2011; lv et al., 2018). therefore, applying new methods and technologies to find effective means for the exploration and development of lithologic reservoirs has practical guiding significance for achieving “efficient and economic development with fewer wells” in overseas oilfields. 2. regional geological background tanan oilfield is located in the southernmost section of the oil-rich fault sag belt in the middle of haita basin (fig. 1). the main body of the sag is a complex half-graben-like semi-rift fault sag. it is composed of three half grabens and half horsts and is characterized by east-west zonation and north-south zoning (yaguang, 2018). the whole sag experienced the whole process of the initial fault sag stage, deep fault stage, fault-sag transition stage, sag stage, and structural inversion stage. the developed strata from bottom to top include the tongbomiao formation, nantun formation, damoguaihe formation, yimin formation, and qingyuangang formation. at present, exploration results show that structural oil reservoirs are mainly developed in the tongbanmiao formation at the initial fault sag stage. lithologic oil reservoirs are mainly developed in the nantun formation at the deep fault stage (fu et al., 2011). although the geological reserves of lithological reservoirs account for a small share of the total reserves, the current development effect shows that the lithological reservoirs of the nantun formation are characterized by thin sandstone thickness and high single-well production. it is called the small but fat reservoirs. therefore, the exploration and development of lithologic reservoirs in the nantun formation are the principal means for tamsag oilfield to increase reserves and production with the method of fewer wells and higher efficiency. 3. rolling appraisal practice after years of geological exploration, tanan sag shows a good full sag oil-bearing scene. however, according to the analysis, the development effect of different types of reservoirs is quite different. this is the primary problem for haita oilfield to find oil reservoirs with beneficial reserves. starting from the geological conditions forming offshore underwater fan lithologic oil and gas reservoirs, this paper discusses the prerequisites for this type’s advantageous reservoir formation, which provides a favorable guarantee for rolling appraisal. this paper discusses the role of rolling appraisal in discovering high-quality reserves with the perspective of fine reservoir anatomy combined with dynamic and static data analysis. geological conditions of rolling appraisal the strata of the nantun formation are the sedimentary products of the rapidly rising lake level and the rapidly increasing accommodation space during the deep fault stage. the whole tanan sag is mainly composed of dark mudstone deposits. according to organic geochemistry and biostratigraphy data, dark mudstone is the main source of rock in the sag. the abundance of organic matter in the source rocks is high, and most of the source rocks enter the threshold of oil generation and enter the peak period of oil generation. at present, sand bodies of lithologic oil reservoirs are distributed in dark mudstone (sun et al., 2011; chen et al., 2013). due to the extensive contact between the source and reservoir, the sand body in the source has the advantage of being close to the source rock and preferentially forming reservoirs (fig. 2). figure 1. location of tanan sag figure 2. oil and gas accumulation model of nantun formation lithologic reservoir significance of rolling appraisal the basic flow of rolling evaluation is based on geological theory and combined with reservoir dynamic analysis. taking the reservoir in figure 2 as an example, the fine reservoir dissection shows that the reservoir is a typical lithologic-structural reservoir jointly controlled by lithology and structure. the good reservoir of the high structure is an oil layer with the logging and production data confirmed. the poor reservoir of the structure is a dry layer, with the logging and production data confirmed. the physical property of the reservoir also controls the lower part of the structure, and the dry layer or water layer is developed. according to the oil production capacity of the reservoir, the oil-bearing reservoir can be further divided into oil and poor oil layers. conventional exploration and reservoir dissection focus on understanding the reservoir’s existence while rolling evaluation and fine reservoir dissection not only consider the reservoir’s existence but also focus on the quality of the reservoir. therefore, it is of practical significance to analyze the main controlling factors of oil production capacity for deploying high-yield wells and discovering high-yield blocks. rolling appraisal means geological analogy is the main method of rolling appraisal. in this paper, based on the analysis of the formation conditions and genetic mechanism of the 363rolling appraisal and high-efficiency development on a lithologic reservoir of nearshore subaqueous fan in the tanan oilfield fan body, the geological analogy method is used to effectively identify the fan body in combination with seismic sedimentology and paleogeomorphology. feasibility of rolling appraisal sufficient provenance, deepwater environment, and abrupt topography are essential for forming nearshore underwater fans. during the sedimentation period of the nantun formation, the steep slope belt of the ancient uplift developed in succession for a long time is a favorable location for forming the nearshore underwater fan (fig. 3). the study of event sedimentology shows that the nearshore subaqueous fan deposits are mostly the products of seismic events. therefore, under similar geological conditions, the fan body is isochronous, multiple and comparable (zhang, 2021; egger et al., 2002; mcleod et al., 2022; richards & bowman, 1998; catuneanu et al., 2016; stanistreet, 1993; anselmetti, 2006; palyvos et al., 2008; galloway, 1998; jia et al., 2007; jiang & liu, 2010). therefore, the geological analogy method is an effective means for rolling discovery of such fans. identification means of the fan according to the analysis of fan development conditions, paleogeomorphic restoration combined with seismic sedimentology is used to identify the sedimentary fan effectively. according to the analysis of paleogeomorphic characteristics during the sedimentation period of the nantun formation, it is considered that the steep slope zone of the north sag, in the west sub-sag of the tanan sag, is a favorable place for the nearshore underwater fan (ja et al., 2021; fu et al., 2013; wu et al., 2018). at present, exploration wells have also confirmed the existence of underwater fans. the seismic sedimentology of underwater fans is evident in 3d seismic data volume. in the flattened seismic profile of the vertical fan strike layer, the fan body is a wedge-shaped seismic reflection embedded in the mud of parallel and sub-parallel seismic reflection. in the flattened seismic profile that crosscuts the fan body layer, the fan is characterized by hillock seismic reflection with a flat bottom and convex top (fig. 4). in recent years, several fans have been found in the north sag of the western sub sag of tanan through the methods of paleogeomorphic zoning and seismic sedimentology positioning, realizing the rolling appraisal strategy by means of fine geological research. figure 3. formation conditions and development pattern of nearshore subaqueous fan figure 4. effective identification method of nearshore underwater fan. 364 quan wen, zixuan sha, guozhi wang 5. reservoir development characteristics analysis of oil production capacity according to the analysis of initial production intensity of production wells in well block a, the reservoir of the whole fan body has strong heterogeneity. the average natural oil recovery intensity of the reservoir in the middle part of the fan is 0.6 t/m • d. the average fracturing production intensity of the reservoir at the root fan is 0.2t/m • d. the average fracturing production intensity of the reservoir at the edge fan is 0.1t/m • d. the reservoir thickness is gradually thinner from the fan root to the fan edge. controlled by the control of reservoir thickness and oil production intensity, the development effect in the middle and root of the fan is better than that in the edge of the fan (fig. 5). figure 5. distribution of initial oil production intensity of oil wells in each subfacies. production mode optimization according to the analysis of current development characteristics, different development methods can be formulated for the location of different subfacies of the fan. the natural productivity of the oil wells in the middle part of the fan is relatively high, so the development mode of natural energy exploitation first and then water injection in the later stage of well pattern conversion can be adopted. the natural productivity of the oil wells at the root fan is low, so synchronous fracturing and water injection can be used. the sandstone at the edge fan is thin and the reservoir physical property is poor, but the sand body distribution continuity is good, so large-scale horizontal well fracturing can be used. 6. rolling development ideas the sedimentary subfacies of the nearshore subaqueous fan control the development characteristics of the reservoir, and thus the productivity of the oil wells. it also provides favorable geological support for the rolling development design concept. genetic analysis of high yield reservoir the nearshore subaqueous fan is a typical gravity flow deposit, and its sedimentary mechanism and structure are very different from those of traction flow deposits. at present, domestic and foreign scholars have basically the same subfacies division of offshore underwater fan, which can be divided into three sedimentary subfacies: root fan (inner fan), middle fan (in the middle of fan), and edge fan (fan end, outer fan). controlled by paleotopographic slope and sedimentary mechanism, the three sedimentary subfacies are very different in sedimentary scope, sedimentary thickness, bedding structure, rock grain size combination and vertical superposition style. the root fan subfacies are mainly developed in small restricted valleys near the provenance area. the lateral distribution range of sand body is limited, and the vertical superposed thickness of single stage sand body or multi stage sand body is relatively large. the cores and field outcrops show conglomerate, sandstone and mudstone mixed accumulation, and the bauma sequence is clearly visible. the micro casting thin section shows the mixed accumulation of unequal grain sandy gravel, the gravel directivity is not obvious, the particles are in line contact with each other, the content of matrix is high, the pores are moderately developed, and the connectivity is poor. the pore characteristics of this kind of reservoir are the main reason for the low natural productivity of the reservoir. the subfacies of the middle fan are the sedimentary parts of the gravity flow sediments that are washed out of the valley and spread rapidly. under the action of gravity, the sediments spread out in a “waterfall” shape with the rapid release of energy and the sudden increase of accommodation space. compared with the root fan, the sedimentary area on the plane is obviously enlarged. the sedimentary thickness of single stage sand body is thinner than that of fan root. the core and field outcrop show that sandstone is mainly deposited. the sandstones of various grain sizes are vertically orderly distributed with grain sequence bedding characteristics, and incomplete bauma sequence can be seen. micro casting thin section shows that sandstone has obvious sorting and roundness. the particles are in a point line basis relationship, with less impurity content, better development of primary pores and better connectivity. good reservoir characteristics are the main reason for high natural productivity of reservoirs. the fan margin subfacies are the extreme end of the fan. it is the product of turbidite sedimentation of fine-grained sandstone with weakened capacity. the lithology is mainly fine sandstone and mudstone. on the plane, they are distributed in the outer edge of the fan. because of the fine grain size of the sediment, the porosity and connectivity are poor. the core shows that oil bearing phenomenon is not obvious. poor physical property and oil-bearing property of the reservoir are the main factors that have no obvious effect of increasing production after fracturing (zhao & zhu, 2001; wang et al., 2015)(fig. 6). figure 6. lithology and microscopic characteristics of fan subfacies reservoir. 365rolling appraisal and high-efficiency development on a lithologic reservoir of nearshore subaqueous fan in the tanan oilfield rolling development design concept compared with marine reservoirs, continental reservoirs have strong heterogeneity. in the process of developing continental reservoirs, a complete set of reservoir evaluation theory of first excellent then poor and rolling outward expansion has been gradually formed. at present, experts at home and abroad have established a relatively complete evaluation system of reservoir quantitative characterization and reservoir classification evaluation. however, the reservoir evaluation methods based on the geological characteristics of reservoirs of different types are not perfect. influenced by the current oil price, the concept of rolling development is becoming more and more important. the concept change from geological reservoir to economic reservoir is the main concept to realize economic and effective development. based on the analysis of the sedimentary characteristics and reservoir characteristics of offshore underwater fan lithologic oil and gas reservoirs and focusing on the dynamic development effect of the reservoir, this paper establishes a complete rolling development concept for such reservoirs. the economic boundary of the reservoir is gradually explored by means of fine geological research. based on the principle of overall understanding and step-by-step implementation and best before poor and benefit first, the root fan of nearshore underwater fan type reservoir is the priority production area. the sandstone at the root of the fan is thick, and the conventional fracturing effect is obvious. it is the most favorable sweet spot under the current oil price. although the oil reservoir of middle fan subfacies has the characteristics of natural productivity and wide distribution of sand bodies, the sandstone thickness is relatively thin as a whole, and the single well controlled reserves and single well production are small, which can be a favorable replacement area at the root of the fan. the characteristics of fan margin subfacies, such as poor reservoir property, unobvious oil bearing characteristics, thin thickness and wide plane distribution, restrict the development of such reservoirs to large-scale fracturing by horizontal wells. at present, the technology of this kind of production method is very mature, but under the control of oil price and operating cost, we are gradually looking for the economic boundary of the reservoir, which requires us to develop such reservoirs under the guidance of fine geological research and follow the principle of rolling development (ding et al., 2013; zhao & li, 2022). in a word, the geological research is the foundation, the rolling exploration and development is the principle, and the economic benefit is the purpose of developing strongly heterogeneous reservoir reservoirs, especially the development of such reservoirs abroad. the integrated geological development process is an effective method to build an international first-class oil exploration and development company. 7. results the main idea of rolling evaluation is to search for new fan bodies in similar structural positions and layers based on geological analogy. according to this method, we have discovered a new fan body on the southwest side of well a area, with a fan body size comparable to that of well a area. the predicted fan body is relatively reliable. the predicted mountain body and the discovered fan body are located on the same fault control plane and have the same source of material. vertically, through seismic profile analysis, two fan bodies are located on the same layer, so the two fan bodies should be products of sedimentation from the same event. at the same time, based on the seismic reflection characteristics of different parts of the fan body in well a area, we accurately predicted the development positions of each subfacies of the fan body on the plane and profile, laying a solid geological foundation for rolling development. based on the development experience of well a’s fan body, a three-dimensional development plan for the new fan body has been developed. in predicting the root and middle parts of the fan body, a vertical well development model is adopted, and rolling drilling is carried out in the order of starting from the middle of the fan and then the root of the fan. at the same time, the oil wells at the root of the fan are developed using fracturing methods. due to the thin and poor physical properties of the reservoir at the fan edge, large-scale horizontal well fracturing is used for mining. 8. discussion new fan body found the discovery of the nearshore underwater fan of the new fan body is the product of event deposition, and finding similar geological features is the key to the discovery of the new fan body. we search for a new fan body based on the reasonable matching of the known development horizon of the fan body, the development scale of the fan controlling fault and the supply direction of the provenance. subfacies division sub-facies division because the geological characteristics of each subfacies of the fan body are obviously different, different development methods should be adopted according to the existing development experience. therefore, it is particularly important to accurately predict the plane position of each subfacies after the discovery of a new fan body. the current method is to accurately calibrate the position of each subfacies on the plane by applying the sedimentary facies model and seismic reflection characteristics after the discovery of the fan body. at the same time, the layer seismic attributes of the corresponding layers can be extracted for auxiliary division of subfacies. 8selection of development mode due to the fact that fan deltas belong to rapid accumulation near the source, there is a significant difference in sedimentary characteristics and physical properties between different sedimentary subfacies. based on the current development results, the sedimentary thickness of the reservoir at the end and middle of the fan is relatively large, and vertical wells can be used for development. however, at the end of the fan, the reservoir is not only thin in thickness but also has poor physical properties, so using a horizontal well development model is better. 9. conclusion 1. starting from the geological conditions, this paper discusses the characteristics of nearshore underwater fan lithologic reservoirs with near source advantages, which provides a rich material basis for rolling evaluation and searching for sweet spo reservoirs. starting from the connotation of geological analogy method, the feasible means and methods for finding fans by rolling evaluation are established. 2. the dynamic development characteristics of such reservoirs are carefully dissected, and the organic combination of geological characteristics and development effects is realized. the concept of geological development integration is established, which is guided by the geological laws and selected the best development means. multi-disciplinary, multi means and multi-scale three-dimensional reservoir precise development is realized. 3. by means of fine geological research and reservoir performance analysis method, the control effect of reservoir heterogeneity on oil production capacity of oil wells is analyzed in detail. a rolling development process based on geological characteristics has been established. it points out the direction for the economic and effective development of the oilfield. acknowledgements this work was financially supported by scientific research project of daqing oilfield co., ltd: fine characterization of main sand body and remaining oil in complex fault block oilfield(no. 2020dqyk013) references anselmetti, f., 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(2022). a brief analysis of efficient development of overseas oilfields. standards and quality of petroleum and chemical industry in china, 15, 149-150. https://doi.org/10.1016/s0899-5362(02)00154-9 https://doi.org/10.1016/s0037-0738(02)00072-6 https://doi.org/10.1016/s0037-0738(02)00072-6 https://doi.org/10.1306/1d9bc5bb-172d-11d7-8645000102c1865d https://doi.org/10.1306/1d9bc5bb-172d-11d7-8645000102c1865d https://doi.org/10.1016/0037-0738(93)90078-j https://doi.org/10.1016/0037-0738(93)90078-j palabras clave: pliosauridae, brachaucheninae, cretácico inferior, barremiano, sáchica, colombia. how to cite item páramo-fonseca, m. e., benavides-cabra, c. d., & gutiérrez, i. e. (2018). a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia. earth sciences research journal, 22(4), 223-238. doi: https://doi.org/10.15446/esrj.v22n4.69916 a new pliosaurid specimen found in the barremian beds of sáchica, a town near villa de leiva, representing the more complete skeleton found in the region is described here. after a detailed study of its morphological characteristics, it is evident that the new specimen represents a new genus and species of brachauchenine pliosaurid from central colombia that we name sachicasaurus vitae gen. et sp. nov. sachicasaurus is a large derived brachauchenine (almost 10 m in length) characterized by two autopomorphic characters: a very short mandibular symphysis ending at the mid length of the fourth mandibular alveoli and reduced number of mandibular teeth (17-18). it is also distinguished by the following combination of characters: skull exceeding two meters in length, lateral expansion of the maxilla posterior to the premaxilla-maxilla suture, presence of caniniform teeth, dental sequence showing only two noncontiguous largest teeth (d4, mx1), pineal foramen in a very anterior position, vomer bifurcated posteriorly, palatines separated in the midline by the pterygoids, slender hyoid bones, 12 cervical vertebrae, cervical centra without dorsal but with ventral foramina, single rib facet in all cervical centra, at least 37 pre-sacral vertebrae, and ilium with a long posterior projection. the morphological comparisons and the cladistic analysis show the new specimen as a derived brachauchenine with evident phylogenetic proximity to the colombian species “kronosaurus” boyacensis from the upper aptian, suggesting that they could belong to the same genus. abstract keywords: pliosauridae, brachaucheninae, lower cretaceous, barremian, sáchica, colombia. a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia un nuevo pliosáurido de gran tamaño del barremiano (cretácico inferior) de sáchica, boyacá, colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n4.69916 se describe un nuevo espécimen de pliosaurido, encontrado en capas del barremiano de sáchica, municipio vecino a villa de leiva, el cual representa el esqueleto más completo descubierto en la región. el estudio detallado de sus características morfológicas evidencia que el nuevo espécimen representa un nuevo género y especie de pliosáurido brachauchenino de la región central de colombia, al que hemos denominado sachicasaurus vitae gen. et sp. nov. sachicasaurus es un brachauchenino de gran tamaño (alrededor de 10 m de longitud) caracterizado por dos caracteres autopomórficos: sínfisis mandibular muy corta, terminando en la mitad del cuarto alvéolo mandibular y número reducido de dientes mandibulares (17-18). también se caracteriza por la siguiente combinación de caracteres: cráneo con longitud mayor a dos metros; constricción transversal en el rostro; dientes caniniformes presentes; secuencia dental con dos dientes de mayor tamaño no contiguos (d4, mx1), foramen pineal ubicado en una posición avanzada; vómer bifurcado posteriormente, palatinos separados en la línea media por los pterigoideos; hioides esbeltos; 12 vértebras cervicales; centros cervicales sin forámenes dorsales pero con forámenes ventrales; una sola faceta articular para las costillas en todos los centros cervicales; como mínimo 37 vertebras pre-sacras; e ilion con una proyección posterior larga. la comparación morfológica y los análisis cladísticos muestran al nuevo espécimen como un brachauchenino derivado con una evidente proximidad filogenética con la especie colombiana “kronosaurus” boyacensis del aptiano superior, sugiriendo que los dos podrían pertenecer al mismo género. resumen record manuscript received: 21/01/2018 accepted for publication: 07/09/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 4 (december, 2018): 223-238 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez universidad nacional de colombia, departamento de geociencias *corresponding author: meparamof@unal.edu.co pa l e o n to l o g y 224 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez introduction the fossil record of pliosaurid plesiosaurs ranges from the lower jurassic to the upper cretaceous. however, that of the lower cretaceous is scarce. the main taxonomically identified material from lower cretaceous beds includes several specimens found in layers of the hauterivian of western russia (fischer et al., 2015; fischer et al., 2017), of the barremian and aptian of colombia (hampe, 1992; 2005; páramo-fonseca et al., 2016; gómez-pérez and noè, 2017); and of the aptian and albian of australia (longman, 1924; kear, 2003). the russian specimens comprise a fragmentary skeleton of the holotype of makhaira rossica fischer et al., 2015, and a nearly complete, three-dimensionally preserved skeleton, the holotype of luskhan itilensis fischer et al., 2017 (fischer et al., 2015; fischer et al., 2017). the colombian material includes a skull and cervical vertebrae recently identified as acostasaurus pavachoquensis gómez-pérez and noè, 2017 (gómez-pérez, 2001; gómez-pérez and noè, 2017) and two nearly complete three-dimensionally preserved skeletons that represent the holotypes of stenorhynchosaurus munozi páramo-fonseca et al., 2016 (hampe, 2005; páramo-fonseca et al., 2016) and “kronosaurus” boyacensis hampe, 1992 (hampe, 1992) (as will be discussed later, the inclusion of “k.” boyacensis in the genus kronosaurus is considered doubtful, and for this reason it is written in quotes). finally, the australian material comprises fragmentary remains assigned to kronosaurus queenslandicus longman, 1924 (longman, 1924; 1930; 1935; white, 1935; romer and lewis, 1959; kear, 2003; mchenry, 2009). the pliosaurids of the lower cretaceous of colombia are also represented by abundant material from villa de leiva not yet identified (páramo-fonseca, 2015). other cretaceous pliosaurid records includes teeth of polyptychodon owen, 1841 from middle to upper albian beds of england (madzia, 2016) and from upper albian to middle cenomanian beds of poland (bardet et al., 2016; madzia and machalski, 2017) (polyptychodon interruptus owen, 1841, the type species of the genus, is considered nomem dubium (madzia, 2016)). the upper cretaceous pliosaurid records include some cranial and postcranial remains collected from the turonian of north america (williston, 1903; 1907; schumacher et al., 2013) and morocco (angst and bardet, 2016). this material supports two taxa: brachauchenius lucasi williston 1903 and megacephalosaurus eulerti schumacher, carpenter and everhart, 2013. recently, a new specimen (mp111209-1) was found in the barremian beds of sáchica, a town near villa de leiva. it is the most complete skeleton found in the region. it was excavated in 2013 in the framework of a paleontological exploration carried out by the universidad nacional de colombia and ecopetrol s.a, the colombian petroleum company. the new specimen was found southeast of the town of sáchica (figure 1). it was excavated from argillaceous gypsiferous shales in an abandoned quarry that was exploited more than 30 years ago for the mining of gypsum. these rocks are identified as beds of the middle part of the arcillolitas abigarradas member of the paja formation (sensu etayo-serna, 1968; 1979) (figure 1) that are equivalent to the upper part of segment c and lower part of segment d of etayo-serna (1968; 1979), and the upper part of segment ii and segment iii of forero and sarmiento (1985). an ammonite, heinzia sp. (etayo-serna, personal communication) was collected from beds directly overlying the pliosaur bearing beds. in colombia this genus indicates the lower part of the upper barremian (patarroyo, 2004). a detailed stratigraphic and biostratigraphic study of the rocks in which the pliosaurid remains were found is the subject of an upcoming publication (ramos and páramo-fonseca, in prep.). the aim of this study is to describe the morphological features of the new specimen (mp111209-1). it is a large pliosaurid, of almost 10 m in length with anatomical features resembling that of “k.” boyacensis. however, the presence of distinctive features that differentiate it from kronosaurus and all other pliosaurids suggest that the new specimen belongs to a new genus and species that we name sachicasaurus vitae gen. et sp. nov. material and methods the new specimen (mp111209-1) is an almost complete skeleton that only lacks the distal part of the two right fins. it is three-dimensionally preserved and almost entirely articulated. only a few skeletal remains are minimally displaced from their anatomical position. a fraction of the right side of the skeleton was removed during gypsum mining but several fragments were recovered on the surface around the excavation site and could be fitted into the skeletal structure (figure 2). the elements of the left side of the appendicular skeleton were found in anatomical position. however, these appendicular elements, including girdles and fins, have been diagenetically distorted, substantially modifying the anatomical morphology of the bones. this condition results in a morphological difference between the appendicular skeleton of the two sides of the specimen that highlights the problems when describing specimens found in evaporite deposits, and the importance of having complete specimens for a correct determination of post-cranial features. details of the preservation state will be given in the anatomical description section. the fossil was taken out from the excavation site in 14 blocks and each block was prepared mechanically using pneumatic hammers. the skull and the cervical vertebrae were dorsally and ventrally cleaned while the postcranial skeleton was only cleaned dorsally to keep the bone elements in place. once the blocks were prepared, the specimen was assembled in the same position that it was found. because mp111209-1 does not fit completely in the definition of “k.” boyacensis, further detailed comparison with k. queenslandicus was done. the published works describing the fragmentary remains of k. queenslandicus (longman, 1924; 1930; 1935; white, 1935; romer and lewis, 1959; molnar, 1982; 1991; kear, 2003) were carefully revised to identify the traits used to define the genus kronosaurus longman, 1924 (longman, 1924; kear, 2003). the unpublished phd thesis of mchenry (2009) provided important descriptive and photographic information about the material assigned to kronosaurus. in order to infer the phylogenetic position of the new specimen, a phylogenetic analysis was performed using the data matrix of fischer et al. (2017), which complements that of benson and druckenmiller (2014) for plesiosauria. two new otus were incorporated to the matrix: “k.” boyacensis and a. pavachoquensis, from the paja formation of colombia, making a total of 99 otus in the data set. most of the character states of “k.” boyacensis were scored according to hampe (1992) and the following states were adjusted based on author’s observations (m.e.p.f and c.d.b.c.): the number of premaxillary teeth [131] was scored as [?] given that the the premaxilla-maxilla contact cannot be established in the holotype. the number of dorsal vertebrae [179] was scored as [1] because the holotype of “k.” boyacensis has two pectoral and 20 dorsal vertebrae instead of the three pectoral and 19 dorsal reported by hampe (1992). the scoring of a. pavachoquensis was based on the descriptive works of gómez-pérez (2001) and gómez-pérez and noè (2017). given that there are no posterior small alveoli in premaxilla (gómez-pérez and noè, 2017: text-fig 3b), the regularity of the posterior premaxillary dentition [132] was scored as homodont [0], and not heterodont as mentioned by gómez-pérez and noè (2017). a list with all character states of the new otus is shown in appendix a. the cladistic analysis was run on tnt (v1.5) (goloboff and catalano, 2016), a free program available with the sponsorship of the willi hennig society. an initial exploration for the shortest-length tree islands was performed using the new technology search options (ratchet + drift + tree fusing) with 200 ratchet iterations (1 random seed) and 20.000 trees limit. the resulting trees were then used as the starting point for a tree bisection reconnection (tbr) branch swapping algorithm. a strict consensus was applied and the iterpcr algorithm was run in order to determine the unstable otus in the consensus (pol and escapa, 2009). a reduced strict consensus was obtained after removing the unstable otus. 225a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia anatomical abbreviations a, angular; acs, acetabular surface; aiv, anterior interpterygoid vacuity; ar, articular; at, atlas; ax, axis; bo, basioccipital; bs, basisphenoid; c, coronoid; c#, cervical centra number; cav, caudal vertebrae; d, dentary; d#, dorsal vertebrae number; d#, dentary tooth number; db, dorsal blade; dr, dorsal rib; dr#, dorsal rib number; dv, dorsal vertebrae; ec, ectopterygoid; en, external nares; epd, epipodials; ex-op, exoccipitalopisthotic; f, frontal; fe, femur; glr, glenoid ramus; hu, humerus; h, hyoid; in, internal nares; j, jugal; lil, left ilium; lis, left ischium; lj, left jugal; lmx, left maxilla; lp, left pubis; lptz, left postzygapophysis; lsq, left squamosal; mx, maxilla; mx#, maxillary tooth number; mpd, mesopodials; na, neural arch; nc, neural canal; ne, neural spine; p, parietal; p#, pectoral vertebrae number; pdf, pedicles facets; pdp, posterodorsal process; pf, prefrontal; phl, phalanges; pif, pineal foramen; piv, posterior interpterygoid vacuity; pl, palatine; pmx, premaxilla; pmx#, premaxillary tooth number; po, postorbital; pof, postfrontal; popr, paraoccipital process; pr#, pectoral rib number; pra, prearticular; pro, prootic; ps, parasphenoid; pt, pterygoid; q, quadrate; r, rib; rar, retroarticular process; rf, rib facet; ril, right ilium; ris, right ischium; rj, right jugal; rmx, right maxilla; rp, right pubis; rptz, right postzygapophysis; rsq, right squamosal; rtp, rostrum perforations; sa, surangular; sf, subcentral foramina; so, supraoccipital; sp, splenial; sq, squamosal; sv, sacral vertebrae; t, tooth; tap, transverse process; v, vomer; vf, vascular foramina; vp, ventral plate; vr, vertebral ramus. nomenclatural acts this publication, and the nomenclatural act contained herein, has been registered in zoobank, the iczn online registration system for new taxa. the zoobank life science identifier (lsid) for this publication is urn:lsid:zoobank.org:pub:8ee2c3b0-fbfb-4ea4-9fda673ac2af1cf4. the lsids for the new taxon are given under the systematic paleontology section (below). systematic paleontology plesiosauria de blainville, 1835 pliosauroidea welles, 1943 pliosauridae seeley, 1874 brachaucheninae benson and druckenmiller, 2014 sachicasaurus gen. nov. urn:lsid:zoobank.org:act:b82f603b-79b0-4616-804056dcb649d0ef type and only species: sachicasaurus vitae, sp. nov. type locality: vereda arrayan, sáchica, boyacá, colombia; coordinates: 5º 34’ 17.33” n, 73º 31’ 51.29” w (figure 1). stratigraphic horizon: arcillolitas abigarradas member, paja formation. barremian to aptian, lower cretaceous (figure 1). derivation of name: from sáchica, locality where the fossil was found and greek saurus, lizard. figure 1. geographic and stratigraphic origin of sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. a, map of villa de leiva – sáchica region with geographical location of the finding site (marked by a star). b, generalized stratigraphic column of the paja formation cropping out around villa de leiva and sáchica, showing the specimen occurrence (marked by a star) (modified from etayo-serna, 1979). 226 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez diagnosis: as for type and only known species. sachicasaurus vitae sp. nov. urn:lsid:zoobank.org:act:b62884a0-ade1-4b13-a37d-d0b1c32c90e1 holotype: mp111209-1, a nearly complete skeleton, lacking the distal part of the right fins (figure 2). at the present time the specimen is part of the material collected for a research project of the universidad nacional de colombia, but it is to be administrated by the colombian geological survey (sgc: servicio geológico colombiano). derivation of name: from the latin vitae, life. it refers to the vitality that this fossil has given to the sáchica village since its discovery. diagnosis: sachicasaurus vitae (mp111209-1) is a brachauchenine pliosaurid characterized by two autapomorphies: a very short mandibular symphysis ending at the mid length of the fourth mandibular alveoli and highly reduced number of mandibular teeth (17-18). it is distinguished from all other pliosaurid, except l. itilensis in having an ilium with a long posterior projection. it differs from all other brachauhenines in having a pineal foramen in a very anterior position (similar to l. itilensis) and cervical centra without dorsal but with ventral foramina. sachicasaurus vitae is also distinguished from other bracauchenines by the following unique combination of characters: skull exceeding two meters in length (as in k. queenslandicus and “k.” boyacensis), lateral expansion of the maxilla posterior to the premaxilla-maxilla suture (unlike b. lucasi), premaxilla with four teeth (unlike in l. itilensis), caniniform teeth present (as k. queenslandicus and “k.” boyacensis), dental sequence showing only two non-contiguous largest teeth (d4, mx1) (probably also in “k.” boyacensis, c.d.b.c. pers. obs.), posteriorly bifurcated vomer (as m. eulerti), palatines separated in the midline by the pterygoids (unlike b. lucasi and probably k. queenslandicus), slender hyoid bones (unlike k. queenslandicus), 12 cervical vertebrae (as k. queenslandicus and “k.” boyacensis), cervical centra showing ventral foramina (also in l. itilensis), single rib facet in all cervical centra (unlike m. eulerti), rib facets of posterior cervical non divided horizontally (unlike “k.” boyacensis), at least 37 pre-sacral vertebrae (unlike “k.” boyacensis), and zygapophyses present in posterior dorsal vertebrae (unlike k. queenslandicus). ontogenetic stage based on the ontogenetical characteristics proposed by brown (1981) for plesiosaurs, we interpret mp111209-1 as a sub-adult individual. as an adult, the specimen presents a well-ossified sutured skull, fully formed teeth exhibiting noticeable ridges on their enameled crowns, and capitulum and tuberosity or trochanter differentiated on propodials. however, it preserves the cervical vertebrae without fused ribs, almost flat articular facets of the vertebral centra, and the atlas and axis fused but with indication of the suture between them, all of which suggest traces of a juvenile stage. description the specimen mp111209-1 is an almost complete articulated skeleton that lost the distal part of the two right fins and a right portion of the rib cage (figure 2). it is possible that some posterior caudal vertebrae are missing because there is no evident reduction in size in the preserved ones. the total length of the specimen, measured from the anterior tip of the snout to the posterior end of the last preserved caudal vertebra, is 990 cm. the skull shows dorsoventral compression. nevertheless, the snout including the mandibular symphysis is preserved in a tridimensional state. there are two irregular openings located near the midline in the anterior dorsal surface of the snout that seems to correspond to taphonomic distortion. cranium (figures 3 and 4; table 1) the skull-roof region is collapsed, and the rostrum is preserved between the mandibular rami, which are slightly tilted dorsomedially. the external nares are almost closed due to the collapsing of the dorsal bones; the left one is more visible than the right. the circumorbital region of the skull is crushed so its bones cannot be identified with certainty. in dorsal view the cranium is triangular in outline, with a long and narrow snout and a wide and robust posterior region (for measures see table 1). the preorbital skull length is greater than half of the total skull length, as in brachauchenine pliosaurids (williston, 1907; mchenry, 2009; schumacher figure 2. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. photograph and schematic drawing of the skelet al remains. doted lines: elements found separated from the skeleton. scale bar=500 mm. 227a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia et al., 2013; benson and druckenmiller, 2014; páramo-fonseca et al., 2016; fischer et al., 2017), peloneustes philarchus seeley, 1869 (andrews, 1913; ketchum and benson, 2011a) and hauffiosaurus o’keefe, 2001 (o’keefe, 2001; benson et al., 2011). the alveolar margin of the upper jaw in lateral view is not regular, as in some non-brachauchenine thalassophoneans (andrews, 1913; sassoon et. al., 2012; benson et al., 2013). the premaxilla is a long bone with a wide anterior region and a narrow posterodorsal process. each premaxilla bears four alveoli, as in brachauchenius (albright et al., 2007), m. eulerti (schumacher et al., 2013) and a. pavachoquensis (gómez-pérez and noè, 2017). posterior to the four premaxillary teeth there is a diastema as long as the mesiodistal length of an alveolus where the fourth large dentary tooth is accommodated. the premaxilla-maxilla suture ascends dorsally interdigitated through the diastema, and then it abruptly turns posterodorsally (figures 4a and b). the posterodorsal process contacts posteriorly the parietal and frontal in an interdigitating suture (figure 3a). the anterior surface of the premaxilla has numerous dispersed vascular foramina. the maxillae are dorsally crushed. posterior to the diastema, each maxilla expands laterally and ventrally to accommodate a large caniniform tooth (m1) (figure 3a), differing from most of the basal pliosaurids and from s. munozi and b. lucasi, in which no expansion is present (benson and druckenmiller, 2014; páramo-fonseca et al., 2016). the first maxillary teeth are the largest and occlude externally to the dentaries. posteriorly, the maxillary teeth are reduced in size and occlude internally to the mandibles. different than in k. queenslandicus (mchenry, 2009), there are no two large adjacent teeth in the maxilla. in lateral view, the maxillae extend backwards to the level of the middle of the orbit. medial to the external nares the maxillae extend behind the posterior margin of the narial openings, forming the anterior and dorsal margins of the nares (figure 3). this configuration is also found in other brachauchenine pliosaurids (williston, 1903; 1907; schumacher et al., 2013; fischer et al., 2017). in palatal view, the maxilla is widely exposed (figure 3b). it forms the lateral border of the internal nares and contacts the vomer anteriorly and the palatine posteriorly. the morphology of the frontals, prefrontals, and postfrontals cannot be determined precisely due to fracturing and loss of bones in this region. however, it is possible to note that the posterior and ventral margins of the nares are limited by prefrontals (figure 3a). the parietals are fused. the pineal foramen is elongated and forms a long and narrow concavity located in the anterior region of the parietal at the level of the anterior portion of the orbits (figure 3a). this character is only shared with l. itilensis among the brachauchenines (williston, 1907; schumacher et al., 2013; benson and druckenmiller, 2014; páramo-fonseca et al., 2016; fischer et al., 2017). in front of the pineal foramen the parietal is ornamented with numerous ridges oriented anteroposteriorly, which are located just posterior to the premaxilla-parietal suture and obscure the contact with the frontals (figure 3a). behind the pineal foramen there is a broken sagittal crest. the crest fragment is high and narrow and is preserved near its anatomical position. the jugals are preserved on the external surface of the mandibular rami. each jugal is a long and slender bone that contact the squamosals posteriorly (figures 3a, 4a and b). the jugal seems to have formed at least a small portion of the temporal bar, as in other brachauchenines, except l. itilensis (fischer et al., 2017). on each side of the skull, medial to the jugal, there is a fragment of the postorbital. only some fragments of the ventral region of the postorbitals are recognizable. the squamosal and quadrate are massivelly-built bones (figures 3, 4a and b). these bones are taphonomically moved and crushed down against the mandible. the sutures between them are no identifiable. the dorsomedial ramus of the squamosal is anteroposteriorly compressed. it extends posterolaterally to contact the quadrate. the bones of the braincase are only partially exposed on the right and ventral sides of the skull, and it is not possible to say much about their morphology (figures 3b and 4b). the dorsoventral compression of the cranium caused the skull roof bones to be in contact with those of the palate. only a small fraction of the external surfaces of the supraoccipital, prootic and exoccipital-opisthotic can be seen in dorsolateral view. in ventral view the basioccipital is barely exposed; it is ventrally covered by the pterygoids. the basioccipital condyle articulates with the atlas-axis complex, so its morphology remains unknown. a small part of the anterior end of the basisphenoid and two short fragments of a narrow parasphenoid are visible through the posterior interpterygoid vacuity; their morphology remains unclear (figure 3b). the ventral part of the palate is well exposed; its anterior region is well preserved, but its anterior end is obscured by the mandibular symphysis (figure 3b). the internal nares are located halfway between the anterior end of the snout and the anterior margin of the posterior interpterygoid vacuity. they are small and oval in outline and are enclosed by the vomers and maxillae. the vomers are fused in a single element that forms the medial border of the internal nares. the posterior end of the vomer is divided into two rami by a medial anterior prolongation of the pterygoids, in the same way as in m. eulerti (schumacher et al., 2013). laterally, these two vomer rami contact the palatines. the palatines are separated medially by the pterygoids and thus, do not meet in the midline (figure 3b), differing from b. lucasi (williston, 1903; schumacher et al., 2013), liopleurodon ferox sauvage, 1873 (noè, 2001) and probably k. queenslandicus (mchenry, 2009). the exposed ventral surface of the palatine is lateromedially narrow. it contacts the pterygoid medially in a long straight suture that becomes zigzagging in the middle. laterally, the palatine limits the maxilla in a straight suture. posteriorly, the palatines contact the ectopterygoids, but the suture between them is not clear. figure 3. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. photographs and interpretive schemes of the skull in a, dorsal and b, ventral views. dotted lines: interpretive limits; dotted surfaces: broken bones; in gray: sediment; in black: empty spaces. scale bars=200 mm 228 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez the pterygoids are medial broad plates located between the palatines. they enclose an anterior interpterygoid vacuity, as in m. eulerti (schumacher et al., 2013) and s. munozi (páramo-fonseca et al., 2016), that is small and oval in outline (figure 3b). the pterygoid expands laterally to contact the ectopterygoid. it borders laterally and posteriorly the posterior interpterygoid vacuity, forming a lateral flange. the flanges of both pterygoids are crushed, but it can be seen that they unite in the midline behind the posterior interpterygoid vacuity. the distance from the posterior border of the interpterygoid vacuity to the posterior end of the pterygoids in the midline is short. the pterygoids join anterior to the posterior interpterygoid vacuities, covering the parasphenoid, as in most of the brachauchenine pliosaurids (williston, 1903; schumacher et al., 2013; benson and druckenmiller, 2014; páramo-fonseca et al., 2016). the pterygoids cover ventrally the basicranium. here, the evidence of a short suture in the posterior border, lateral to the midline suggests the pterygoids slightly separate posteriorly, exposing a small portion of the basioccipital (figure 3b). both the ectopterygoids are incomplete; their lateral end is broken. the ectopterygoid forms, together with the pterygoid, a laterally curved flange (figure 3b) and contacts anteriorly the palatine. the presence of a suborbital fenestra cannot be established because the ectopterygoids are fractured in this region. the hyoids are slightly broken and displaced from their anatomical position. the right one is located ventrally over the posterior interpterygoid vacuity and the left one is moved to the right edge of the cranium (figure 3b). differing from k. queenslandicus (kear, 2003), the hyoids are long and slender. mandible (figures 3 and 4) the mandibular rami are robust and high and are partially exposed lateral to the snout in dorsal view (figures 3, 4a and b). in ventral view the mandibular symphysis tapers anteriorly, as in hauffiosaurus (o’keefe, 2001; benson et al., 2011) and brachauchenine pliosaurids (williston, 1903; schumacher et al., 2013; páramo-fonseca et al., 2016; fischer et al., 2017). the symphysis is particularly short, being 1/9 of the total mandible length (table 1) and ends in the middle of the fourth dentary tooth. the dentary is exposed laterally in the anterior region of the mandibular ramus. as the jaws are occluded, the anterior end of the dentary is partially covered by the premaxilla and the maxilla. the dentary does not have a ventral elaboration along the mandibular symphysis, differing from non-brachauchenine thalassophoneans and from hauffiosaurus longirostris benson, ketchum, noè and gómez-pérez, 2011 (benson et al., 2011). posteriorly, the dentary contacts the surangular apparently in a v-shaped suture with its vertex figure 4. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. photographs and interpretive schemes of: a-c, skull in a, right lateral, b, left lateral and c, anterior views. d, third premaxillary (replacement tooth) and second dentary teeth in labial view; e-f, ninth dentary tooth in e, labial and f, apical views. dotted lines: interpretive limits; dotted surfaces: broken bones; in gray: sediment; in black: empty spaces. scale bars: a-b=200mm in; c=100mm; d-f=40mm. 229a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia pointing backwards. the contact between the surangular and the articular is covered taphonomically by the squamosals and quadrates. the angular forms most of the ventral surface of the mandibular rami. it extends anteriorly to the level of the first maxillary tooth and posteriorly to the retroarticular processes (figures 3b and 4b). the retroarticular processes are preserved, but the bones forming them are not identifiable. they are posteromedially inflected, as in other brachauchenine pliosaurids (williston, 1903; 1907; schumacher et al., 2013; benson and druckenmiller, 2014; páramo-fonseca et al., 2016; fischer et al., 2017). medially, the splenial contacts the dentary and participates in the mandibular symphysis (figure 3b). the splenial is a long bone that extends posteriorly contacting the angular. dorsal to the splenial, the coronoid and the prearticular form the medial wall of the mandibular ramus, but their limits are difficult to identify. only the anterior end of the coronoid is recognized; it forms the posterior edge of a small mandibular opening (figure 3b). teeth (figure 4) the description of the tooth morphology follows the terminology proposed by zverkov et al. (2018). the dentition is anisodont (heterodont), as in k. queenslandicus (kear, 2003; mchenry, 2009), “k.” boyacensis (hampe, 1992) and in non-brachauchenine pliosaurid (andrews, 1913; o’keefe, 2001; benson et al., 2011; sassoon et. al., 2012; benson et al., 2013; benson and druckenmiller, 2014; gómez-pérez and noè, 2017). the premaxillary and anterior dentary alveoli are large, but the fourth dentary and the first maxillary are the largest alveoli (figures 4a, b and c). this condition differs from what is found in other heterodont pliosaurids, in which the largest maxillary alveoli are generally two in each mandibular ramus and they are in a more posterior position (e.g. a. pavachoquensis, l. ferox, simolestes vorax andrews, 1909, pliosaurus kevani benson et al., 2013, k. queenslandicus) (noè, 2001; mchenry, 2009; benson et al., 2013; gómezpérez and noè, 2017). all upper and lower teeth are intercalated; the teeth succession in the anterior snout is: pmx1, d1, pmx2, d2, pmx3, d3, pmx4, d4, mx1, d5, mx2. that differs from k. queenslandicus were there are two contiguous dentary teeth (the largest teeth) followed by two adjacent maxillary teeth (the largest ones) (d4, d5, mx1, mx2) (mchenry, 2009). there are four teeth of similar size in each premaxilla. differing from some thalassophoneans, there are no small teeth in the front of the snout (figure 4c) (andrews, 1913; noè, 2001; benson et al., 2013; schumacher et al., 2013; gómez-pérez and noè, 2017; fischer et al., 2017). the mesiodistal length of the premaxillary alveoli from first to fourth are: 50 mm, 60 mm, 80 mm and 85 mm. different than m. eulerti and l. itilensis (schumacher et al., 2013; fischer et al., 2017), in mp111209-1 the alveoli are narrowly spacing, as it is in almost all pliosaurids. the distance measured from the midpoint of contiguous premaxillary alveoli are 105 mm from the first to the second, 100 mm from the second to the third, 110 mm from the third to the fourth and 150 mm from the fourth to the first maxillary alveoli. in the left maxilla the largest tooth is in anatomical position and exhibits its complete crown. this crown is 90 mm in height and 70 mm in basal diameter. the large maxillary tooth of the right side is displaced out of its alveolus (figure 4b). it has a long root, with a smooth surface. its enameled crown is distally fractured. the height of this tooth including its preserved portion of crown and its root is 300 mm; the diameter of the root, measured at the limit with the crown, is 90 mm. almost all the alveoli of the dentaries can be observed either with or without teeth. in both sides of the mandible, the distal teeth are the smaller; the distalmost ones are not clearly differentiable. mesial to the first maxillary tooth there are four dentary teeth positions and distal to it, 12 to 13 dentary teeth positions can be counted. adjacent to the first maxillary teeth the dentaries are covered by the maxillae and here the dentary alveoli cannot be seen laterally or dorsally. however, in a diagonal fracture that passes through this area in the left side of the cranium, teeth are observed in section and only one dentary tooth is covered laterally by the maxilla. therefore, the estimated number of dentary teeth is 17 or 18. many of the tooth crowns show transverse fractures (figures 4d, e and f) and, in some, the apex is slightly displaced and bended. all of the tooth crowns are conical and have a circular cross section (figure 4d, e and f) as in early pliosaurids and in brachauchenines except l. itilensis (benson and druckenmiller, 2014; cau and fanti, 2016; gómez-pérez and noè, 2017; fischer et al., 2017). the surface of the crowns has several prominent ridges but lacks carinae. it is ornamented with ridges even in the newly emplaced teeth (figure 4d). some of the ridges reach the tip of the crown, but the others only reach halfway through the crown height. close to the apex, there are several ridglets on the crown surface. (figure 4d). axial skeleton (figures 2 and 5; table 2) the vertebral column is well preserved, with some disarticulated vertebrae (figure 2). the atlas-axis complex is partially fused and a vestige of the suture between them can be seen ventrally. the atlas has a bulged anterior surface. only the axis has a fused rib. there are 12 cervical vertebrae including the atlas and axis (figure 5a), as there are in “k.” boyacensis (hampe, 1992), b. lucasi (williston, 1907), and k. queenslandicus (kear, 2003) (although for mchenry, 2009 the number of cervical vertebrae in k. queenslandicus is 13), and fewer than in l. itilensis (14) and s. munozi (17) (páramo-fonseca, et al., 2016; fischer et al., 2017). the cervical centra are articulated. they are wider than high and much higher than long (figures 5b, c and d; table 2) as in all other brachauchenines (williston, 1903; romer and lewis, 1959; fischer et al., 2017). their articular surface is subcircular and nearly flat. the ventral surface of the cervical centra does not have ridges but has paired foramina located near its ventrolateral borders; these foramina are longitudinally oval (figure 5b). ventral foramina in cervical centra are also present in other cretaceous pliosaurids as l. itilensis and a. pavachoquensis (fischer et al., 2017; gómez-pérez, 2001) but are absent in k. queenslandicus, b. lucasi and s. munozi (williston, 1903; mchenry, 2009; páramo-fonseca et al., 2016). the dorsal surface of the cervical centra lacks vascular foramina (figure 5c), as in a. pavachoquensis (gómez-pérez, 2001) but unlike in b. lucasi, s. munozi and l. itilensis (williston, 1903; páramo-fonseca et al., 2016; fischer et al., 2017). the cervical rib facets are single-headed and are located laterally as in brachauchenine pliosaurids (williston, 1903; romer table 1. measurements (in cm) of the skull and mandible of sachicasaurus vitae gen. et sp. nov, holotype mp111209-1. skull length at dorsal midline 225 skull length to posterior end of the quadrates 252 maximum skull width at the posterior end of the quadrates in dorsal view 133 skull width at the anterior orbital margin 73 rostrum length (measured from the anterior end of the premaxilla to the anterior orbital margin) 145 rostrum width at the first maxillary alveolus 35 mandible length (measured from its anterior end to the posterior edge of the retroarticular process) 273 mandibular symphysis length. 30 230 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez and lewis, 1959; benson and druckenmiller, 2014; páramo-fonseca et al., 2016; fischer et al., 2017) excluding m. eulerti which possesses doubleheaded ribs (schumacher et al., 2013). they have a rectangular outline and occupy almost the entire length of the cervical centrum (figure 5d), corresponding to the same arrangement of “k.” boyacensis (hampe 1992). the rib facets of the posterior cervical centra are in a more dorsal position (figure 5a) and they are no divided into anterior and posterior portion as it is seen in “k.” boyacensis (hampe, 1992). the cervical ribs are only preserved in the axis. the cervical neural arches are articulated with each other and disarticulated from the centra. they are preserved at the right side of the cervical centra (figure 5a). the neural canal is not exposed in any of the vertebrae, so its shape and size cannot be established. the neural spine is straight and inclined posterodorsally as in k. queenslandicus and l. itilensis (romer and lewis, 1959; fischer et al., 2017). the prezygapophyses and postzygapophyses have their articular surface ventromedially inclined but its medial relationship cannot be seen. there are two pectoral vertebrae. their neural arches and those of the anteriormost dorsal vertebrae are not fused and are slightly displaced from their anatomical position (figure 5a). the transverse processes of the pectoral vertebrae are robust and have subcircular articular surfaces. there are at least 23 dorsal vertebrae. the dorsal centra are longer than the cervical ones (table 2) and they are longer than they are wide. in anterior dorsals the transverse processes project laterally at the same level of the prezygapophyses but ventral to the postzygapophyses. the most posterior dorsal centra are higher than wide and their articular facets are almost flat (figure 5e). after the 7th dorsal vertebra the neural arches are poorly preserved and the vertebral centra are rotated 90 degrees to the right side. the last dorsal neural arches (17-19 dorsal vertebrae) clearly show zygapophyses. there are three sacral vertebrae that are articulated but slightly displaced backwards, being separated from the dorsal series. their neural arches are crushed over the centra, so the sacral vertebrae are not clearly differentiated. there are 13 caudal vertebrae preserved, the anterior four are articulated with the sacral. although no other posterior caudal vertebrae were found during the excavation, it seems that the caudal series is incomplete, because the preserved centra do not show a decrease in size (table 2). the posterior caudal vertebrae are disarticulated but form a lineal series, which indicates that they are close to their anatomical position (figure 2). the caudal centra have a subtriangular shape in anterior and posterior view; they are as high as wide and have nearly flat articular surfaces (figure 5g; table 2). in these vertebrae, the articular ribs facets are located on the centra (figure 5f). there are no vestiges of neural or haemal arches in the preserved caudal vertebrae. the ribs are slightly displaced from their in vivo position, so that their proximal extremities are not in direct articulation with the vertebrae. two rib fragments, plastically deformed, lie on the distal end of the right figure 5. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. photographs and interpretive schemes of axial skeleton elements. a, cervical and pectoral vertebrae series; b, seventh cervical centrum and a fragment of the sixth one in ventral view; c, seventh cervical centrum in dorsal view; d, 11th cervical centrum in left lateral view; e, 22th dorsal vertebrae in posterior and left lateral views; f, fourth caudal centrum in dorsal view; g, ninth caudal centrum in anterior view. dotted surfaces: broken bones; in gray: sediment; in black: empty spaces. scale bars=100 mm. 231a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia humerus (figure 2). the rib cage is compressed towards the vertebral column in both sides of the specimen (figure 2). the pectoral ribs, as well as the first and last dorsal ribs, are shorter than the other dorsal ribs. the dorsal ribs are curved in their proximal region and become straight towards their distal region. the ribs are robust and oval in cross section. the largest proximal diameter is equivalent to 0.06 times the length of each rib. there are a few poor preserved slender bone fragments lying near the right ribs, which could represent gastralia remains. (figure 2). appendicular skeleton (figures 2, 6 and 7; table 3) the girdles are well preserved and are covered medially by the axial skeleton (figures 2 and 6a). both scapulae are present. the left scapula is found in its anatomical position but partially covered by the axial skeleton while the right one is incomplete and was found off site, separated from the skeleton, a few meters from the excavation site. a complete ventral plate is not observable in any of the scapulae. in the left scapula the dorsal blade is long as in s. munozi and p. philarchus (andrews, 1913; páramo-fonseca et al., 2016) but unlike in l. itilensis (fischer et al., 2017) and has its distal end mediolaterally expanded as in s. munozi and l. itilensis (páramo-fonseca et al., 2016; fischer et al., 2017) but unlike in p. philarchus (andrews, 1913). the glenoid ramus is short and thickens distally to articulate with the humerus (figures 6d and e). the right scapula is broken and preserves the dorsal blade, which is more laterally oriented than that of the left scapula (figure 6d; table 3). the dissimilar orientation of the dorsal blade of both scapulae seems to be due to taphonomical deformation, so the original orientation of this blade cannot be established with certainty. the coracoids are almost entirely covered by the axial skeleton, so their morphology and their medial relationship cannot be determined. it can only be said that it is a greatly elongated plate with a weakly developed posterolateral cornu as in hauffiosaurus zanoni o’keefe, 2001 and anguanax zignoi cau and fanti, 2016 (vincent, 2011; cau and fanti, 2016) but unlike in l. itilensis where the cornu is present and extends lateral to the coracoid glenoid ramus (fischer et al., 2017). the pelvic girdle is well preserved, but its medial portion is covered by the axial skeleton (figure 6a). the right bones are slightly displaced backwards with respect to the left ones. therefore, the precise joint shape between the bones of both sides cannot be established. the ilia lie over the ischia, slightly displaced and rotated backwards (figures 2 and 6a). each ilium is a triradiate bone with three rami disposed in the same plane. the dorsal expansion is large, as in p. philarchus (andrews, 1913) and presents a very elongated posterodorsal process (figures 6b and c), as it was illustrated by fischer et al. (2017) for l. itilensis, but it differs from the shorter process described or illustrated for p. philarchus, h. zanoni and marmornectes candrewi ketchum and benson, 2011b (andrews, 1913; ketchum and benson, 2011b; vincent, 2011). the acetabular ramus is thick and wide. the medial ramus is short and sub-tabular in form. the posterodorsal process is longer than the medial ramus and tapers distally. the pubes are broad, anteroposteriorly elongated plates with a mediolateral dorsal concavity (figure 6a). they contact each other in the midline, but the join shape is obscured by the axial skeleton. the lateral border of the pubis, anterior to the acetabulum, is concave. its anterior border is convex, and its posterior border is slightly concave. both ischia are well preserved and meet medially at least in their anterior portion. the ischium is a very elongated plate with a short and robust acetabular ramus (figure 6a). the length to width ratio of the ischium (2.02) is similar in k. queenslandicus (1.98) but it is greater than in some basal pliosaurids (benson and druckenmiller, 2014). the anterior border of the ischium is concave, and its posterior end is convex and thickened. the right ischium is almost flat, while the left one, as well as the left pubis, are taphonomically distorted with their lateral portions very curved dorsally (figure 6a). both left fins are articulated with the girdles while the right ones were removed during the mining, and only the propodials were found on the surroundings of the excavation site (figure 2). the left limbs are plastically deformed, especially the hind limb (figures 7c and d). the profile of these limbs looks wavy in anterior and posterior view. the propodials are thick and wide, dorsoventrally compressed bones. they are slender and pendulous; their distal extremity is fan-shaped. the propodials shaft is relatively robust compared with that of the “k.” boyacensis. the femur is longer than the humerus, and in both, the posterior border is more concave than the anterior one (figures 7a and b; table 3). the ratio of humerus to femur length (0.87) is greater than in “k.” boyacensis (0.82, calculated from the measurements given by hampe, 1992), and most thalassophoneans (benson and druckenmiller, 2014; fischer et al., 2017) except h. zanoni (0.92) and hauffiosaurus tomistomimus (0.97) benson, ketchum, noè and gómez-pérez, 2011 (calculated from the figures of benson et al., 2011 and vincent, 2011). the humerus length versus width ratio is between 2.3 and 2.7, as in nonthalassophoneans pliosaurids and p. kevani (benson and druckenmiller, 2014; benson et al., 2013). the proximal end of the humerus have a subrectangular outline, that is higher than it is long. the humerus becomes longer than high at the middle of its proximodistal length, and in its distal end it has an elongated oval outline, being much longer than high. the length to distal width ratio of the femur is 2,1, which falls between the range of most pliosaurids. the proximal end of the femur is higher than long; it has two marked articular facets, one dorsal and another ventral. the dorsal facet has a straight dorsal border, ventrolaterally inclined lateral sides and a curved ventral border. the ventral facet is squared in outline. at its proximodistal mid-length, the femur table 2. measurements (in cm) of the vertebral centra dimensions of sachicasaurus vitae gen. et sp. nov, holotype mp111209-1. centrum length height width atlas-axis complex 15 19 19.5 c3 6.8 ~ 19 c4 7.3 ~ 18.5 c5 7.5 18 19 c6 7.6 18 20 c7 7.8 18.5 19.5 c8 8.0 18 20.5 c9 8.2 18.5 20.7 c10 8.1 19 21 c11 8.2 18.5 21 c12 8.0 19 ~ p1 8.5 ~ ~ d5 14 ~ ~ d13 13 ~ ~ d14 13 17 18 d15 15 ~ ~ d18 14 ~ ~ d22 13.5 15 13 d23 ~ 14 13 s1 11 ~ 13 s3 10 ~ ~ ca2 11 ~ ~ ca4 10 ~ ~ ca5 9 ~ ~ ca7 ~ 12.5 14 ca8 8 13 13.5 ca9 ~ 13 14 ca10 7.5 ~ ~ ca11 8 ~ ~ ca12 ~ 10.5 12.5 ca13 7 10.5 12 cervical (c), pectoral (p), dorsal (d), sacral (s), and caudal (ca) centra. 232 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez figure 6. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. a-c, pelvic girdle. a, photograph and interpretive scheme of pelvic girdle as preserved, in dorsal view; b, right and c, left ilium in anterodorsal view. d-e, scapulae; d, left an e, right scapula in dorsomedial view. dotted lines: interpretive limits; dotted surfaces: broken bones; in gray: sediment; in black: empty spaces. scale bars: a=500 mm; b-e=100 mm. table 3. measurements (in cm) of scapular rami and propodials of sachicasaurus vitae gen. et sp. nov, holotype mp111209-1. left right scapula glenoid ramus diameter 22 21 scapula dorsal blade diameter 23 23 scapula: distance between the glenoid ramus end and the dorsal blade end 66 52 humerus proximodistal length at the middle axis ? 87 humerus anteroposterior width at the midlength ? 16.5 femur proximodistal length at the middle axis 90? 100 femur anteroposterior width at the midlength 21 21 femur maximum anteroposterior width at the distal end 47 ? becomes longitudinally oval in cross-section, and distally, it turns significantly flattened dorsoventrally. the epipodials and mesopodials are poorly preserved (figures 7c and d), so their morphology cannot be established. the metapodials are relatively well exposed in the forelimb (figure 7c). here, four elements that have the shape of elongated phalanges are identified. five digits can be counted in the hind limb; the best preserved contains five phalanges (figure 7d). all the preserved phalanges are hourglass shaped. discussion morphological comparisons the new specimen mp111209-1 is considered a member of pliosauridae because it presents most of the morphological traits of the family as defined by brown (1981), druckenmiller and russell (2008) and ketchum and benson (2010). even though benson et al. (2013) and benson and druckenmiller (2014) recently carried out phylogenetical analyses of plesiosauria, they did not introduced changes to the previous diagnosis. mp111209-1 shares with other pliosaurids a large skull, premaxilla no participating in the margin of the external nares, coronoid present with a long lingual process, broad and strong teeth, ornamented with longitudinal ridges, reduced number of cervical vertebrae with short cervical centra and ventral surface nearly flat, anteroposteriorly relatively long ischia, and femur larger than the humerus. mp111209-1 differs from the diagnosis of pliosauridae in having 17-18 dentary teeth, contrasting with the 25 to 40 found in other pliosaurids (brown, 1981). the barremian pliosaurid genera known in colombia, stenorhynchosaurus and acostasaurus, are smaller in size and clearly differ from the new specimen mp111209-1: stenorhynchosaurus has a more elongated snout, isodont dentition, and longer mandibular symphysis (páramo-fonseca et al., 2016); and acostasaurus has a shorter preorbital rostrum, large nasal present in contact with the parietal, anterior interpterygoid vacuity absent, longer mandibular symphysis, different arrangement of the heterodont dentition (gómez-pérez and noè, 2017), cervical rib facets in ventrolateral position and double-headed cervical ribs (gómez-pérez, 2001). mp111209-1 presents most of the diagnostic characters established by benson and druckenmiller (2014) for brachaucheninae. however, in mp111209-1 the cervical centra possess ventral foramina. this feature is generally present in the non-brachauchenines pliosaurids according to the 233a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia character matrix of benson and druckenmiller (2014). nevertheless, fischer et al. (2017) described the presence of ventral foramina in the cervical centra of the early brachauchenine l. itilensis. furthermore, in mp1112091 the rostrum is laterally expanded posterior to the level of the premaxillamaxilla suture, accommodating the first large maxillary tooth, which differs again from the condition found in the brachauchenine pliosaurids (williston, 1903; schumacher et al., 2013; benson and druckenmiller, 2014). mp111209-1 also differs from most brachauchenines (and most pliosaurids) in having the pineal foramen in a very anterior position (similar in l. itilensis), an extremely short mandibular symphysis, and a particular morphology of the ilium, with a remarkable posterior dorsal projection, greater than that illustrated by andrews (1913) for p. philarchus. unfortunately, the morphology of the ilium is unknown in most brachauchenines, and in l. itilensis the ilium, apparently similar in morphology, is incomplete (fischer et al., 2017). mp111209-1 shares traits with the upper hauterivian brachauchenine genus luskhan that are not found in other pliosauridae or in brachaucheninae, as mentioned above, but also have considerable differences with it, mostly in the number and morphology of the premaxillary teeth. in luskhan there are seven premaxillary teeth, the mesialmost teeth (1st premaxillary alveoli) being procumbent and separated from the other alveoli (fischer et al., 2017) while in mp111209-1 there are four premaxillary teeth similar in size. the other cretaceous genera makhaira, brachauchenius and megacephalosaurus are also smaller than mp111209-1 and differ from it in having an isodont dentition and a longer symphysis (williston, 1903; 1907; schumacher et al., 2013, fischer et al., 2015). moreover, brachauchenius differs from the new specimen in having the palatines united in the midline (williston, 1903) and megacephalosaurus differs in having double-headed cervical ribs (schumacher et al., 2013). comparison with kronosaurus the genus kronosaurus was first proposed by longman (1924) to name, as k. queenslandicus, a fragment of a large mandible from the albian of australia. he found this mandibular fragment different from the other plesiosaurs because of its large size and the distinct “contours of the teeth and mandible” (longman, 1924). then, longman (1930) studied some albian fragments of propodials and white (1935) described cranial fragments of two aptian specimens. they both assigned these fragments to k. queenslandicus. molnar (1991) suggested that the specimens studied by white represent a different species from k. queenslandicus based on the age difference of the fragments. k. queenslandicus is now known from numerous fragments from australia, many of them described but still unpublished (mchenry, 2009). a second kronosaurus species, “k.” boyacensis, was proposed by hampe (1992) to include an aptian skeleton from colombia. he clarified that the inclusion of this new species in the genus is provisional because the definition of the genus kronosaurus required a “mandatory revision”. the holotype (and only known specimen) of “k.” boyacensis, can only be observed dorsally, its skull is badly preserved due to intense crushing and the loss of bone surface, and its preparation is still insufficient. probably for this reason hampe’s description of the skull is general and imprecise. however, hampe (1992) justifies its provisional inclusion of the specimen in the genus kronosaurus based on the large size of the skeleton, the short neck with 12 vertebrae, the expanded opisthotic, the small and unpaired supraoccipital, and the cervical rib facets extending through the entire length of the centra. kear (2003) provided “preliminary” diagnoses for the genus kronosaurus and for the species k. queenslandicus. both of the diagnoses include significant characters than cannot be observed in “k.” boyacensis”: in the definition of kronosaurus, kear (2003) includes premaxilla with four large caniniform teeth, anterior interpterygoid vacuity absent, ectopterygoid and pterygoid forming lateral flanges united in a short dished contact ventrolateral to the posterior interpterygoid vacuity, and mandibular symphysis extended back to sixth tooth position; and in the definition of k. queenslandicus he includes the presence of up to three dorsal longitudinal ridges in the interorbital region, pterygoids extensively underlapping basicranium and with interpterygoid vacuity situated beneath rather than anterior to the basicranium; paroccipital process contacting quadrate flange of pterygoid at lateral articulation only, and robust hyoids. all these characters are unknown in “k.” boyacensis, so the inclusion of the holotype of “k.” boyacensis within the genus kronosaurus cannot be granted under the current knowledge of the specimen. therefore, kronosaurus is a genus that should be restricted to its type species, k. queenslandicus. unlike “k.” boyacensis, the skull of the specimen mp111209-1 is well-exposed ventrally and can be compared with that of k. queenslandicus. figure 7. sachicasaurus vitae gen. et sp. nov., holotype mp111209-1. a-b right propodials; a, humerus and b, femur in dorsal view. c-d, photographs and interpretive schemes of left limbs; c, forelimb and d, hindlimb in dorsal view. dotted lines: interpretive limits; dotted surfaces: broken bones; in gray: sediment; in black: empty spaces. scale bar=100 mm. 234 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez mp111209-1 shares with k. queenslandicus the large size, premaxilla with four caniniforn teeth, conical teeth without carinae, and 12 cervical vertebrae with single articular facets for ribs (kear, 2003). nevertheless, mp111209-1 differs from this species in a series of characters. it has a very short mandibular symphysis, ending in the middle of the fourth mandibular tooth, while in k. queenslandicus the symphysis encompasses six teeth (kear, 2003); it exhibits an anterior interpterygoid opening in the palate, absent in k. queenslandicus; the three dorsal longitudinal ridges in the interorbital region mentioned by kear (2003) are not present in mp1112091; the hyoid bones, robust in k. queenslandicus (kear, 2003), are slender in mp111209-1; the posterior dorsal vertebrae of mp111209-1 exhibit zygapophyses, which according to kear (2003), do not seem to be present in k. queenslandicus. the anterior interpterygoid opening in mp111209-1 is small and could be related to a submature ontogenetic state, like ketchum and benson (2011a) established for peloneustes. instead, the difference in the number of teeth in the mandibular symphysis is a real trait of taxonomic value (tarlo, 1960; ketchum and benson, 2011a; knutsen, 2012; benson et al., 2013). furthermore, the pineal foramen is located in a more posterior position in k. queenslandicus than in m111209-1, with its anterior end near the middle length of the orbits (molnar, 1991; mchenry, 2009); the teeth ornamentation is marked in mp111209-1, even in the replacement teeth (figure 4d), whereas in k. queenslandicus it is thinner and absent in the replacement teeth (longman, 1924; while, 1935; mchenry, 2009); the dental sequence described by mchenry (2009) for k. queenslandicus, with two contiguous large dentary teeth followed by two adjacent large maxillary teeth (d4, d5, mx1, mx2) differs considerably from the condition seen in mp111209-1, where the dental sequence shows only two non-contiguous large teeth (d4, mx1). according to the reconstruction of k. queenslandicus presented by mchenry (2009), this species shows the suture between the premaxilla and the maxilla reaching the alveolar margin at the level of the fifth dentary alveolus, while this suture occurs at the level of the fourth dentary alveolus in mp111209-1. in addition, in the k. queenslandicus the vomer is not bifurcated posteriorly as it is in mp111209-1, the palatines join medially, while in mp111209-1 they are separated by the pterygoids, the palatal surface of the maxilla is reduced compared to the broad palatal surface of the maxilla exhibited in mp111209-1, the internal nostrils are in a more advanced position than they are in mp111209-1, and the palate is narrower anteriorly than in mp111209-1. the new specimen and “k.” boyacensis share a series of features. they share the conical teeth morphology, with circular cross section and with ornamented crowns, the short symphysis (although it seems to be shorter in mp111209-1), the number of cervical vertebrae and the long ischia. although hampe’s (1992) diagnosis mentions five premaxillary teeth in “k.” boyacensis (even though the suture between the premaxilla and the maxilla cannot be located with certainty), the location of the largest maxillary tooth in the snout of the holotype is similar to that of the first maxillary tooth of mp111209-1, suggesting that in fact “k.” boyacensis may have four premaxillary teeth as in mp111209-1. both specimens differ from the diagnosis given by kear (2003) for kronosaurus and k. queenslandicus in having shorter mandibular symphysis, ornamented tooth crowns and distinct dental sequence. thus, considering the marked differences between k. queenslandicus and the new specimen, we propose to restrict kronosaurus to its type species and erect a new genus for mp111209-1. a comparison of mp111209-1 with “k.” boyacensis is important given their morphological similarities and the fact that the genus attribution of the latter is dubious. of the characters proposed by hampe (1992) to define “k.” boyacensis, several are not found in mp111209-1. these include the total number of teeth in each mandibular ramus, that in “k.” boyacensis is approximately 24, while in mp111209-1 is not greater than 18 (although the number of mandibular teeth may differ among individuals of the same species (ketchum and benson, 2011a)); the diastema is broader in “k.” boyacensis than in mp111209-1; the rib facets of the posterior cervical vertebrae are divided anteroposteriorly into two areas in “k.” boyacensis, but not in mp111209-1, and the propodials are slender in “k.” boyacensis compared with those of mp111209-1(table 2). furthermore, mp111209-1 is larger than “k.” boyacensis; the length from the anterior tip of the rostrum to the femur insertion is 737 cm in mp111209-1 versus 630 cm in “k.” boyacensis. however, “k.” boyacensis has a larger skull in relation to the body than the new specimen. in fact, the length from the anterior tip of the rostrum to the femur insertion is 2.7 times the skull length measured in the midline in the holotype of “k.” boyacensis, while it is 3.3 times in mp111209-1. the skull of “k.” boyacensis is more elongated than that of mp111209-1; thus, the ratio between the length measured in the mid line and the maximum width measured at the level of the quadrates is more than 1.9 in “k.” boyacensis (despite being crushed in its posterior region) while it does not exceed 1.7 in mp111209-1. the total number of pre-sacral vertebrae is 34 in “k.” boyacensis (12 cervicals, 3 pectorals and 19 dorsals) (hampe, 1992), and 37 in mp111209-1 (12 cervicals, 2 pectorals and 23 dorsals). the mp111209-1 scapulae are more robust than those of “k.” boyacensis, according to the measurements given by hampe, (1992); the glenoid ramus is notoriously shorter in mp1112091 and the dorsal ramus is longer. besides, in mp111209-1 the acetabular ramus of the ischium is short or poorly projected from the bone, while in the holotype of “k.” boyacensis this ramus is appreciably longer. the ratio of humerus to femur length is greater in mp111209-1. the morphology of the girdle bones and the propodials of mp1112091 are more robust than those of “k.” boyacensis. although these differences may be related to sexual dimorphism, they may also be related to differences in ecological adaptations. furthermore, the two specimens were found in different geological stages (mp111209-1: upper barremian and “k.” boyacensis: upper aptian), and this, together with the combination of differences in the number of mandibular teeth, in the cranial proportions, and in the number of pre-sacral vertebrae, allows defining mp111209-1 as a taxon distinct from “k.” boyacensis, probably belonging to the same genus. in conclusion, the preceding comparisons show mp111209-1 as a brachauchenine pliosaurid with distinctive features that separate it from the other brachauchenine or pliosaurid genera. the new specimen shows sufficient morphological particularities to assign it to a new genus and species: sachicasaurus vitae gen. et sp. nov. this new taxon represents the third pliosaurid genus defined for the barremian of colombia. phylogenetic analysis the strict consensus of 20000 most parsimonious trees obtained after (tbr) branch swapping show s. vitae within a large polytomy that include all thalassophonean pliosaurids plus marmornectes candrewi (figure 8a). after applying the iter pcr algorithm, 13 unstable otus were identified: pistosaurus skull; stratesaurus taylori; eoplesiosaurus antiquior; meyerasaurus victor; qm_f51291; acostasaurus pavachoquensis; anguanax zignoi; gallardosaurus iturraldei; simolestes vorax; liopleurodon ferox; pliosaurus brachyspondylus; pliosaurus irgisensis and eromangasaurus australis. the unstable a. pavachoquensis was found jumping into different positions within suboptimal trees the resulting consensus tree obtained after removing the 13 unstable otus shows an unresolved pliosaurus polytomy and a partially resolved monophyletic brachaucheninae clade (figure 8b). the brachaucheninae node is supported by 10 synapomorphies, seven of which are present in mp111209-1: the parasphenoid is covered by the pterygoids, anterior to the posterior interpterygoid vacuities [86.0]; the palatine does not participate in the rim of the internal naris as seen in ventral view [93.1]; the pterygoid tapers anteriorly [95.0]; the mandibular symphysis tapers anteriorly in ventral view [113.0]; the long axis of the retroarticular process is inflected slightly posteromedially [123.1]; there is only one rib facet in the anteriormiddle cervical vertebrae [160.3]; the cervical rib facets are located laterally [162.1]. the three other synapomorphies defining brachaucheninae [163.1; 83.1 and 232.0] are unknown in mp111209-1. the obtained tree shows the clade ((s. vitae + “k.” boyacensis) + k. queenslandicus) separated from the upper cretaceous brachauchenines by having the anterior cervicals neural spines taller than long [171.0]. within this clade, s. vitae was recovered as the sister taxon of “k.” boyacensis, supporting the idea that both species could belong to the same genus, 235a new large pliosaurid from the barremian (lower cretaceous) of sáchica, boyacá, colombia acknowledgments we specially thank dr. carlos bernardo padilla†, mary luz parra and juan parra from the fundación colombiana de geobiología, who provided essential information to find the fossil. we thank freddy parra from the centro de investigaciones paleontológicas (cip) for his support in the excavation and preparation of the specimen. we thank the colombian petroleum company ecopetrol s.a for the financial support provided for the excavation and most of the preparation and the study of the fossil. we also thank the servicio geológico colombiano (sgc) for its financial contribution during the final steps of the preparation and the study of the specimen. we are thankful for the place provided by sáchica’s authorities for the preparation and study of the fossil. we want to thank the local people, especially mr. jesus heredia, for their continuous support. we thank dr. fernando etayo-serna for his comments on the age of the fossil. we sincerely thank valentin fischer and an anonymous reviewer for their useful comments on the manuscript, which enhanced this version. references albright, l. b., gillette, d. d. and titus. a. l. 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(2016). large predatory marine reptiles from the albian–cenomanian of annopol, poland. other than kronosaurus, as suggested in the results of the morphological comparison. these two taxa have a reduced number of dorsal vertebrae [179.1], a synapomorphic character that separates them form k. queenslandicus. s. munozi, the other colombian brachauchenine pliosaurid was recovered within an unresolved polytomy with l. itilensis and m. rossica in a basal position within brachaucheninae. s. vitae and the other derived brachauchenines are distinguished from this basal polytomy by the absence of a postorbital-squamosal contact [42.1], the squamosal anterior extension significantly posterior to the postorbital bar [57.1], the parasphenoid terminating just anterior to the basisphenoid-basioccipital contact [84.1], the absence of foramina between the maxilla and the vomer, anterior to the internal naris [90.0]; four premaxillary teeth [131.0]; teeth with a rounded or sub-rounded cross-section [139.0]; and the presence of 15 or fewer cervical vertebrae [152.0]. conclusions the discussed comparisons and the results of the cladistic analysis show the specimen mp111209-1 as a derived brachauchenine pliosaur distinguished from others brachauchenines by a unique combination of traits. the very anterior location of the pineal foramen, the extremely short symphysis (containing only three and a half alveoli), the low number of mandibular teeth, the anisodont dental sequence showing the first maxillary and the fourth dentary teeth as the largest of the series, the presence of ventral foramina and absence of dorsal ones on the cervical centra, and the number of presacral vertebrae (37), characterize mp111209-1 as a new genus and species, sachicasaurus vitae gen. et sp. nov. sachicasaurus represents the third pliosaurid genus defined for the barremian of colombia. its discovery highlights the great diversity that pliosaurids attained during the early cretaceous and draw special attention to the study of colombian lower cretaceous pliosaurids in the resolution of the phylogeny of cretaceous pliosaurids. the occurrence of different genera of pliosaurids in the barremian beds of colombian suggests that the environmental conditions of the colombian sea during the early cretaceous facilitated the development of sufficient marine life to sustain a diverse group of predators. more research is needed to reveal such a high diversity in the cretaceous of colombia. figure 8. pliosauridae branch of the phylogenetic tree obtained from the analysis of the complete plesiosauria data set (see methods). a, strict consensus tree of 20000 most parsimonious trees, 1579 steps, obtained after (tbr) branch swapping. b, reduced strict consensus tree after pruning the unstable otus (see text). 236 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez geological magazine. 153, no. 1, 1-16. benson, r. b. j., ketchum, h. f., noè, l. f. and gómez-pérez, m. 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(2018). increased pliosaurid dental disparity across the jurassic-cretaceous transition. palaeontology. 1-22. doi: 10.1111/pala.12367 238 maría eurídice páramo-fonseca, cristian david benavides-cabra, ingry esmirna gutiérrez appendix a characters states of sachicasaurus vitae gen. et sp. nov., “kronosaurus” boyacensis hampe, 1992 and acostasaurus pavachoquensis gómez-pérez and noè, 2017 sachicasaurus vitae 01?2??2121 0?10210??0 1010?00?2? 0?1?10?0?1 ?1200?1??2 1201??1?0? 01?????0?? ?????0???? ????10?010 1110010?21 ?03??00020 110001??1? 011?11?000 0011010?00 001?01?0?? 10000?1??3 21?2000?00 0?0?001?10 000????101 0000????1? 0020?00??? ?????????? 0?03011?10 3??????0?? 1??110??1? 111??????? ?????????1 “kronosaurus” boyacensis ??????2??? ????2????? ?????0???? ???11?0??1 ???0010??? ?2???????? ?????????? ?????????? ?????????? ?????????? ?????????? ????0????? ?????????? ??1??10?0? 0????????? ?0?0?????3 ?1???????? ?0???0??10 ?????????? ?????????? ?????????? ?????????? ????????1? ?000?????? 00?110???? 1??2310??? ?0??0????1 acostasaurus pavachoquensis 11?1?0?021 1?00?10??0 10111?0021 0?01?0???? ???0000002 012????100 00??0?0001 2????0111? ??0?1??0?1 101010?021 0?3?0????? 1110?1011? 011?11?0?? 0011010101 ?????????? ??000????1 ?0??10???? ?00?0????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? palabras clave: galería en muro; análisis numérico; estabilidad subterránea. how to cite item ozdogan, m. v., & gonen, a. (2019). determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index. earth sciences research journal, 23(1), 17-25. doi: https://doi.org/10.15446/esrj.v23n1.71879 in the bakibaba copper mine, the longhole stoping method is used in the production of copper ore. stability problems have occurred at times on the footwall drift due to the interaction between the footwall drift and stope. in this study, we propose a method for estimating the minimum distance necessary to ensure a non-interaction zone between the footwall drift and stope. we used the finite element method and various distances between the footwall drift and stope and the displacements over drifts as parameters. we also performed analyses on various geological strength index values from low to high to determine the effect of the rock mass on the interaction between the footwall drift and stope. abstract keywords: footwall drift, numerical analysis, underground stability. determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index determinación de la distancia requerida para asegurar el rebaje y las galerías en muros de la zona de no interacción en minería subterránea con base en el índice de resistencia geológica issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.71879 en la mina de cobre de bakibaba se utiliza el método de minería por perforación de barrenos largos para extraer el mineral. los problemas de estabilidad han ocurrido generalmente en las galerías en muro debido a la interacción entre la galería y el rebaje. en este estudio se propone un método para la estimación de la distancia mínima necesaria para asegurar la zona de no interacción entre la galería en muro y el rebaje. se utilizó como parámetro el método del elemento finito y varias distancias entre la galería en muro y el rebaje y los desplazamientos sobre las galerías. también se realizaron análisis con varios índices de resistencia geológica de abajo hacia arriba para determinar el efecto de la masa rocoa en la interacción entre la galería en muro y el rebaje. resumen record manuscript received: 24/04/2018 accepted for publication: 24/08/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 17-25 mehmet volkan ozdogan, alper gonen dokuz eylul university engineering faculty department of mining engineering. * corresponding author: mehmet.ozdogan@deu.edu.tr g eo lo g y 18 mehmet volkan ozdogan, alper gonen introduction the stability of the openings in underground mines is a major factor affecting mining safety and sustainability. as such, safety and economic restrictions are the main design parameters considered in underground excavations. the geometry of an underground opening (shape and size), toe between openings and length of openings are functions of safety and cost. decisions regarding these mining parameters are generally empirically based and at times unexpected failures occur. the distance between the footwall drift and the stope is an important factor that affects the stability of the footwall drift. the strength of the rockmass and the mining depth significantly influence the distance required between the footwall drift and stope. in deep mines, the rockmass is highly stressed and stability problems occur around underground openings. although several studies have addressed the interaction between underground tunnels (ghaboussi & ranken, 1977, jamshid & randall, 1977; yamaguchi et al. 1998; addenbrooke & potts, 2001; gercek, 2005; karakus et al. 2007; wang et al., 2007; wang & li, 2008; chakeri et al. 2011; wang et al., 2017), few have addressed the influence of the stope on the footwall drift in underground mines. abdellaha et al. (2011) examined the interaction between the haulage drift and mine stope in a sublevel-stoping mine. the authors considered six excavation sequences in one stope and evaluated the support required for the haulage drift after each excavation. purwanto et al. (2013) studied the influence of stope design on the stability of the hanging wall decline in cut-and-fill mines. the authors numerically analysed the stability of the hanging wall decline for various stope geometries. kabwe & bowa (2016) investigated the effect of the mining method on the stability of the footwall drift for the same orebody by numerically modelling the stress distribution over the stope and footwall drift for three different mining methods. on the other hand, none of the studies mentioned above have effort on determination of non-influenced zone distance between stope and footwall drift in longhole stoping mines. the main scope of this paper is to estimate the distance between the footwall drift and stope that is necessary to ensure the existence of a noninteraction zone in longhole stoping mines. we also consider the effect of geotechnical conditions. in our study, we examined the footwall drift of the bakibaba underground copper mine, which has experienced stability problems, and we used numerical analyses to determine the displacements over the footwall drift for various geological strength index (gsi) values.. mining method the bakibaba underground copper mine is located in kure province in north-western turkey, near the city of kastamonu and the black sea. in this mine, a longhole-stoping and post-backfill method is used and the mine has an annual production capacity of 1,000,000 tonnes. the orebody lies in a north-easterly direction, is 100–300 m in length and averages 40 m in width. the ore deposit is 500 m in depth and the dip of the ore is 70°. the orebody is accessed via a 15% declining ramp. a footwall drift running parallel to the orebody is 5-m wide by 5-m high. crosscuts are driven from the footwall across the orebody to the vein. the footwall drifts are positioned 40–50 m from where the footwall makes contact with the orebody. from the boundary of the ore deposit, 7-m wide by 5-m high sill drifts are driven until contact is made with the hanging wall. the upper and lower sill drifts are connected at 15-m intervals by a slot raise, and then widen to a 7-m drift width. parallel blast holes 76 mm in diameter are drilled downward between two sill drifts. after blasting, a remote-controlled load-haul-dump (lhd) vehicle is used to muck the ore from the lower sill drift and transport it to the ore pass. after the entire stope is mined, the open stope is backfilled from the upper sill. in the bakibaba underground mine, primary stopes are paste-filled or cementedrock-filled and secondary stopes are rock-filled. figure 1 shows schematic illustrations of the stope production cycle. numerical model in the mining industry, the numerical finite element method (fem) is widely used to solve geotechnical problems. fem can accommodate material heterogeneity, non-linear deformability, complex boundary conditions, in-situ stresses and gravity (jing & hudson, 2002; sharma, 2009). the utilisation figure 1. stope production cycle (a: stope development, b: production drilling, c: ore mucking, d: backfilling 19determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index of fem-based software is one of the most preferred methods, particularly in underground design analyses (ozdogan et al. 2017). in this study, phase2 twodimensional (2d) fem software program developed by rocscience (2007) was used to determine the influence of the distance between the stope and the footwall drift. figure 2 shows the model used in our analysis, in which we used 10-m-interval distances between the footwall drift and stope up to the stabilisation of the displacement over the drift. laboratory tests in accordance with isrm standards were performed to determine the geomechanical properties of the rock unit, including the uniaxial compressive strength, unit weight, young’s modulus, poisson’s ratio, cohesion and internal friction angle of the rock unit (table 1). to determine the rock-mass input parameters for the fem analyses, we performed both laboratory and field studies and used hoek and brown criteria. hoek and brown criteria developed by hoek et al. 2002, is an empirical method to determine the strength of rock masses in terms of major and minor principal stresses and widely used criterion in geotechnical projects. hoek and brown criteria estimates the strength envelopes determined from laboratory triaxial tests of intact rock by using, uniaxial compressive strength of intact rock (sigci), the intact rock parameter (mi), the geological strength index (gsi) and the disturbance factor (d). the hoek and brown criteria is expressed as;     1 3 3' ' ' = + +      ci ci a bm s (1) where, σci, is the uniaxial compressive strength and mb is the reduced value of material constant mi which is calculated as; m m exp gsi db i= − −     100 28 14 (2) s and a are constants for rock mass given by; s exp gsi d = − −     100 9 3 (3) a e e gsi = + −         − − 1 2 1 6 15 20 3 (4) in order to determine rock mass parameters from laboratory test results rocdata software was used. table 2 shows the strength parameters of the rock mass derived from our laboratory and field studies. also using a chart derived by sheorey (1994), we calculated a total stress ratio of 1.25 for the bakibaba copper mine, based on the elasticity modulus and depth. results and discussion fem analyses were performed to investigate the effect of the distance between the extracted stope and the footwall drift on drift stability, and examined various distances between them. figure 2. base model used in analyses table 1. test results for rock units parameter, symbol, unit basalt (hanging wall & footwall) ore zone unit weight, δ, kn/m3 28.04 28.80 uniaxial compressive strength, σc, mpa 50.48 60.28 brazilian tensile strength, σt, mpa 4.88 5.25 young’s modulus, ei, gpa 12520 14280 poisson’s ratio, ν 0.25 0.24 cohesion, c, mpa 9.82 10.16 internal friction angle, ф, ° 44.5 43.25 table 2. input parameters used in fem analysis parameters, symbol, unit basalt (hanging wall & footwall) ore zone geological strength index (gsi) 55 60 rock type, mi 20 20 disturbance factor, d 0 0 cohesion of rock mass, c, mpa 3.29 3.95 internal friction angle of rock mass, ф, ° 38 48 tensile strength of rock mass, σt, mpa 0.08 0.10 rock mass deformation modulus, ei, mpa 9429 10329 20 mehmet volkan ozdogan, alper gonen two evaluation criteria, including the yield zone, i.e. failure area, and the displacement over the footwall drift were considered. the analyses were stopped when the displacements over the drift had not changed in two consecutive analyses. the red zones in figure 3 indicate the 100% yield zone around underground spaces due to mining extraction when the distance from the stope to the footwall drift is 10 m. in figure 4, the numbers on left-side wall (lw), right-side wall (rw) and roof (rf) of the drift indicate the displacement that occurred over the drift for a 10-m distance between the stope and footwall drift. similarly, figures 5–7 show the changes in the yield zone and displacements for greater distances between the stope and footwall drift. as expected, as the distance between the stope and footwall drift increases, the yield zone and displacements over drift decrease. when the distance is 10 m or less, the yield zone of the stope and footwall drift overlap and unstable drifts occur. the model results show that when the distance between the stope and footwall drift is 10 m, the displacement on the roof of the upper drift is 0.021 m and that of the bottom drift is 0.048 m. the displacement on the roof decreases to 0.015 m for the upper drift and 0.033 m for the bottom drift for a distance of 20 m. table 3 shows the change in the displacements over the footwall drift, in which we can see that when the distance between stope and the footwall drift is 70 m, the footwall drift is totally removed from the influence zone. figure 8 shows the relation between distance between the stope and footwall drift and the displacements over the footwall drift. our numerical analyses indicate that, for the given geotechnical circumstances, the displacements over the drift become stable when there is a distance of 70 m from the stope. we conducted further numerical analyses for various gsi values to determine the effect of the gsi value on the critical distance between the stope and footwall drift. table 4 shows the displacements around the footwall drifts for various gsi values and various stope distances. as expected, the interaction between the stope and footwall drift decreases with increases in the gsi value. however, a gsi value of 55 is the limit value regarding interaction between the stope and footwall. higher gsi values have no influence on the non-interaction zone distance. at distances of 70 m and more between the stope and footwall drift, we observed no interaction for gsi values higher than 55. variation of non-influenced zone distance based on gsi were shown in figure 9. figure 3. yield zone for 10-m interval between stope and footwall drift figure 4. displacements over footwall drift for 10-m interval between stope and footwall drift 21determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index figure 5. yield zones and displacements for various stope–footwall-drift distances (a: yield zone for 20-m distance, b: displacements over drifts for 20-m distance, c: yield zone for 30-m distance, d: displacements over drifts for 30-m distance, e: yield zone for 40-m distance, f: displacements over drifts for 40-m distance) 22 mehmet volkan ozdogan, alper gonen figure 6. yield zones and displacements for various stope–footwall-drift distances (g: yield zone for 50-m distance, h: displacements over drifts for 50-m distance, i: yield zone for 60-m distance, j: displacements over drifts for 60-m distance, k: yield zone for 70-m distance, l: displacements over drifts for 70-m distance) 23determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index figure 7. yield zones and displacements for various stope–footwall-drift distances (m: yield zone for 80-m distance, n: displacements over drifts for 80-m distance) table 3. change in displacements over the footwall drift distance between stope and footwall drift (m) upper drift (m) bottom drift (m) left wall right wall roof left wall right wall roof 10 0.018 0.030 0.021 0.042 0.042 0.048 20 0.012 0.024 0.015 0.030 0.033 0.033 30 0.009 0.021 0.015 0.021 0.030 0.024 40 0.006 0.021 0.012 0.012 0.024 0.018 50 0.003 0.018 0.012 0.009 0.021 0.015 60 0.003 0.018 0.012 0.006 0.021 0.012 70 0.003 0.018 0.012 0.003 0.018 0.012 80 0.003 0.018 0.012 0.003 0.018 0.012 0 0,01 0,02 0,03 0,04 0,05 0,06 0 20 40 60 80 100 upper drift lw upper drift rw upper drift roof bottom drift lw bottom drift rw bottom drift roof d is pl ac em en t ( m ) distance between stope and footwall drift (m) figure 8. relation between distance between stope and footwall drift and displacement over drift 24 mehmet volkan ozdogan, alper gonen table 4. displacements around footwall drifts for various gsi values and various stope distances gsi distance between stope and footwall drift (m) upper drift (m) bottom drift (m) left wall right wall roof left wall right wall roof 10 10 1.000 1.800 1.600 1.800 2.000 2.000 20 0.800 1.600 1.400 0.800 1.600 1.400 30 0.800 1.400 1.000 0.200 1.400 1.000 40 0.600 1.200 1.000 0.400 1.200 1.000 50 0.400 1.200 0.800 0.400 1.000 0.800 60 0.400 1.000 0.800 0.400 1.000 0.800 70 0.400 1.000 0.800 0.200 1.000 0.800 80 0.200 0.800 0.600 0.200 1.000 0.600 90 0.200 0.800 0.600 0.200 0.800 0.600 100 0.200 0.800 0.600 0.200 0.800 0.600 35 10 0.075 0.150 0.105 0.165 0.180 0.195 20 0.060 0.120 0.090 0.105 0.150 0.135 30 0.045 0.105 0.075 0.075 0.120 0.105 40 0.030 0.090 0.075 0.060 0.105 0.075 50 0.030 0.090 0.060 0.045 0.090 0.060 60 0.015 0.075 0.060 0.030 0.090 0.060 70 0.015 0.075 0.060 0.015 0.090 0.060 80 0.015 0.075 0.045 0.015 0.075 0.045 90 0.015 0.075 0.045 0.015 0.075 0.045 55 10 0.018 0.030 0.021 0.042 0.042 0.048 20 0.012 0.024 0.015 0.030 0.033 0.033 30 0.009 0.021 0.015 0.021 0.030 0.024 40 0.006 0.021 0.012 0.012 0.024 0.018 50 0.003 0.018 0.012 0.009 0.021 0.015 60 0.003 0.018 0.012 0.006 0.021 0.012 70 0.003 0.018 0.012 0.003 0.018 0.012 80 0.003 0.018 0.012 0.003 0.018 0.012 75 10 0.004 0.007 0.005 0.013 0.012 0.014 20 0.002 0.007 0.004 0.009 0.010 0.009 30 0.002 0.006 0.004 0.005 0.008 0.006 40 0.001 0.005 0.003 0.004 0.007 0.005 50 0.001 0.005 0.003 0.002 0.006 0.004 60 0.001 0.005 0.003 0.002 0.006 0.003 70 0.001 0.005 0.003 0.001 0.005 0.003 80 0.001 0.005 0.003 0.001 0.005 0.003 90 10 0.002 0.003 0.002 0.006 0.005 0.006 20 0.001 0.003 0.002 0.004 0.004 0.004 30 0.001 0.002 0.002 0.003 0.003 0.003 40 0.001 0.002 0.002 0.002 0.003 0.003 50 0.001 0.002 0.001 0.001 0.003 0.002 60 0.001 0.002 0.001 0.001 0.002 0.002 70 0.001 0.002 0.001 0.001 0.002 0.001 80 0.001 0.002 0.001 0.001 0.002 0.001 25determination of distance required to ensure stope and footwall-drift non-interaction zone based on geological strength index conclusions in this study, 3d finite element method was used to investigate the interaction between the stopes and footwall drifts of the bakibaba underground copper mine. we determined the range of influence between the stope and footwall drift for the currently existing circumstances. then, the gsi value was systematically increased while keeping all the other parameters constant to determine the effect of the gsi value on this range of influence. our series of numerical simulations revealed that a distance of 70 m is necessary to ensure the existence of a non-interaction zone between the stope and footwall drift at the bakibaba copper mine. our analyses also show that the gsi value of the rock mass has a significant effect on this non-interaction zone distance up to a gsi value of 55, after which higher gsi values do not increase the required non-interaction-zone distance.. references abdellaha, w., mitria, h., & thibodeau, d. 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(2017). optimal support spacing for steel sets: omerler underground coal mine in western turkey. international journal of geomechanics, 18(2), 05017003 1-12. http://dx.doi.org/10.1061/(asce)gm.1943-5622.0001069 purwanto, a., shimada, h., sasaoka, t., wattimena, r. k., & matsui, k. (2013). influence of stope design on stability of hanging wall decline in cibaliung underground gold mine. international journal of geosciences, 4, 1-8. doi: http://dx.doi.org/10.4236/ijg.2013.410a001 sharma, k. g. (2009). numerical analysis of underground structures. indian geotechnical journal, 39(1), 1–63 sheorey p. r. (1994). a theory for in situ stresses in isotropic and transversely isotropic rock. international journal of rock mechanics and mining sciences, 31(1), 23-34. doi: https://doi.org/10.1016/0148-9062(94)92312-4 wang, b., & li, s. (2008). a complex variable solution for stress and displacement field around a lined circular tunnel at great depth. international journal for numerical and analytical methods in geomechanics, 33, 939-951. doi: https://doi.org/10.1002/nag.749 wang, h. n., zenga, g. s., utilic, s., jiangd, m. j., & wua, l. (2017). analytical solutions of stresses and displacements for deeply buried twin tunnels in viscoelastic rock. international journal of rock mechanics & mining sciences, 93, 13–29. doi: https://doi.org/10.1016/j. ijrmms.2017.01.002 wang, j., milne, d., wegner, l. & reeves, m. (2007). numerical evaluation of the effects of stress and excavation surface geometry on the zone of relaxation around open stope hanging walls. international journal of rock mechanics & mining sciences, 44(2), 289-298. doi: http:// dx.doi.org/10.1016/j.ijrmms.2006.07.002 yamaguchi, i., yamazaki, i., & kiritani, k. (1998). study of ground-tunnel interactions of four shield tunnels driven in close proximity, in relation to design and constructions of parallel shield tunnels. tunnelling and underground space technology, 13(3):289–304. doi: https://doi. org/10.1016/s0886-7798(98)00063-7 50 55 60 65 70 75 80 85 90 95 0 20 40 60 80 100 gsi d is ta nc e be tw ee n st op e an d fo ot w al l d rif t re qu ire d fo r e ns ur in g no nin te ra ct io n zo ne ( m ) figure 9. variations in distance between stope and footwall based on gsi value keywords: probability integral, subsidence predict, thick loose layer, main influence radius. palabras clave: probabilidad integral, predicción de subsidencia, capa suelta gruesa, radio de influencia principal. how to cite item li, j., yu, x., & liang, y. (2020). a prediction model of mining subsidence in thick loose layer based on probability integral model. earth sciences research journal, 24(3), 367-372. doi: https://doi.org/10.15446/esrj.v24n3.90111 the probability integral method is the most commonly used mining subsidence prediction model, but it is only applicable to ordinary geological mining conditions. when the loose layer in the geological mining conditions where the mining face is located is too thick, many inaccurate phenomena will occur when the movement deformation value is predicted by the probability integral method. the most obvious one is the problem that the predicted value converges too fast compared with the measured value in the edge of the sinking basin. in 2012, wang and deng proposed a modified model of probability integral method for the marginal errors in the model of probability integral method and verified the feasibility of the method through examples. in this paper, the method is applied to the prediction of surface movement under thick and loose layers after modified. through practical application, it is found that due to the angle between the working face and the horizontal direction, the average mining depth in the strike direction is different from the average mining depth in the inclined direction, and the main influence radius of the two main sections are often. therefore, based on this problem, this paper divides the main influence radius into trend and tendency and adjusts the parameters in the model to find the rules of the parameters. the original method uses a dynamic scale factor to adjust the predicted shape of the graph by adjusting the sinking coefficient. this study is aimed to set the scale factor to 0.5 and fix the value of the sinking factor, and propose to adjust the integral range and then adjust the shape of the graph to make it more in line with the actual measurement situation. abstract a prediction model of mining subsidence in thick loose layer based on probability integral model modelo de predicción de subsidencia en minas para una capa gruesa e inestable con base en el modelo integral de probabilidad issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90111 el método integral de probabilidad es el modelo de predicción de subsidencia en minas más utilizado, pero solo es aplicable a las condiciones geológicas ordinarias de la minería. cuando una capa suelta en las condiciones de extracción geológica donde se encuentra la cara de extracción es demasiado gruesa, el método integral de probabilidad no es exacto al predecir el valor de deformación del movimiento. el problema más obvio es que el valor predicho converge demasiado rápido en comparación con el valor medido en el borde de la cuenca de hundimiento. en 2012, wang zhengshuai propuso un modelo modificado del método integral de probabilidad para los errores marginales en el modelo del método integral de probabilidad y verificó la viabilidad del método a través de ejemplos. en este documento, el método se aplica a la predicción del movimiento de la superficie bajo capas gruesas y sueltas después de la modificación. a través de la aplicación práctica, se encuentra que debido al ángulo entre la cara de trabajo y la dirección horizontal, la profundidad promedio de extracción en la dirección de impacto es diferente a la profundidad promedio de extracción en la dirección inclinada, y frecuentemente al radio de influencia principal de las dos secciones principales. por lo tanto, con base en este problema, este documento divide el radio de influencia principal en tendencia y ajusta los parámetros en el modelo para encontrar las reglas. el método original usa un factor de escala dinámico para ajustar la forma pronosticada del gráfico ajustando el coeficiente de hundimiento. este estudio busca establecer el factor de escala en 0.5, fijar el valor del factor de hundimiento, ajustar el rango integral y luego ajustar la forma del gráfico para que esté más en línea con la situación de medición real. resumen record manuscript received: 01/06/2019 accepted for publication: 07/05/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 367-372 jingxian li1,2, xuexiang yu2,*, ya liang 1school of earth and environment, anhui university of science and technology, huainan 232001, china 2school of geomatics, anhui university of science and technology, huainan 232001, china 3school of mathematics and big data, anhui university of science and technology, huainan, 232001, china *corresponding author: xxy16321@163.com m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v24n3.90111 https://doi.org/10.15446/esrj.v24n3.90111 mailto:xxy16321@163.com 368 jingxian li, xuexiang yu, ya liang introduction as one of the main energy sources for human life and economic development, coal occupies a very important position in the world energy consumption structure. although with the continuous promotion of global clean energy, the proportion of coal in primary energy continues to decline. anyway, coal is still the main component of the world’s primary energy consumption structure (29.2%) (“geo-informatics,” 2019). as the world’s largest energy producer and consumer, coal has always dominated the primary energy structure in china. it is expected that by 2030, the proportion of coal will remain above 50%. coal can drive the country’s economic growth, but the problem of irreversible impact on the surface during mining and even for a long time after mining cannot be ignored. because the underground coal seam is artificially destroyed, the original stress balance of the rock layer is destroyed, then the phenomenon of surface subsidence and movement occurs when the impact is propagated to the surface, resulting in geological disasters such as damage to landmark buildings and water accumulation. so it is particularly important to predict the surface movement and deformation that it may cause before mining. the subsidence prediction methods established by mining subsidence workers in china mainly include probability integral method, negative exponential function method, typical curve method, integral grid method, weibull distribution method, spline function method, hyperbolic function method, pearson function method (zhang & lei, 2012), mountain surface deformation prediction method, three-dimensional layered medium theory prediction method, and strip mining prediction method based on pallet theory. among these prediction methods, other methods have not been widely used due to their limitations or scope of use. probability integral has become the main method for predicting mining subsidence in china due to its solid theoretical foundation, easy computer implementation, and good application effect. the probability integral method was developed by scholars such as liu baochen and liao guohua of china in the 1960s on the basis of the stochastic medium theory proposed by the polish scholar litwiniszyn in 1957 (wang et al., 2013). it is called the probability integral method because of the calculation of the probability integral part of its calculation process. the probability integral method is mainly applicable to the case of near-horizontal coal seams or gently inclined coal seams (fan et al., 2014). the probability integral method is currently the most commonly used model in the area of mining subsidence prediction in china. its basic idea is to treat the surface and downslide movements as random events and decompose the mining area into infinitely many tiny units to calculate the sum of the surface influences and surface subsidence. this method is based on the stochastic medium model theory. the method is expected to have good results when the geological factors of the mining have little effect on the surface movement, but if the geological conditions of the working face are special (such as loose layers thicker), which has a greater impact on surface subsidence, the probability integral method will not reflect these changes well (wang et al., 2019). so it needs to be adjusted on the basis of the original model to make it more suitable for the prediction under different geological mining conditions. aiming at the problem of converging too fast at the edge of the subsidence basin when predicting the surface subsidence using the probability integral method, wang and deng (2012) proposed that treat surface subsidence as a weighted sum of the effects of mining from two working faces with different main influence radius (hereinafter referred to as the original model) (wang et al., 2018). this to a certain extent corrects the problem of converging too fast at the edge of the basin when using the probability integral model to predict surface subsidence (zhou et al., 2016). however, the original model uses the same influence radius along the strike and tendency direction. in actual application, it is found that the main influence radius values of the strike and tendency are often different. so the main influence radius in this model is divided into a strike influence radius and a tendency influence radius and integrates parameters are chosen to be the changeable parameters instead of subsidence coefficient in the original model in this article. characteristics of mining subsidence under thick loose layer the loose layer is the quaternary and neogene strata and is composed of soil, sand, gravel, and pebble layers. after the hard rock has undergone geological processes such as erosion, transportation, and deposition, evacuated sediments formed due to the unconsolidated hardened diagenesis are called loose layers. since loose layers are formed in different environments, times, and places, they will have different properties. in terms of how thick the loose layer can be called “thick”, it is generally believed that the loose layer is called a thick loose layer when the thickness exceeds 50m and is called a huge thick layer when the thickness exceeds 100m. it is of great theoretical and practical significance to deeply study the particularity of surface movement and deformation and the parameter variation law of thick loose layer mining area for accurately predicting the impact of underground mining on various protective objects on the surface (“geoinformatics,” 2019). at present, the law of surface movement and deformation of underground mining in thick and loose layer mining areas has been studied in china and other countries (ding et al., 2018), and the particularity of surface movement and deformation of thick and loose layer mining areas has been obtained: the subsidence coefficient is too large, many of which are around 1.1 or greater; the vertical deformation above the coal pillars in the thick loose layer mining area is abnormal compared to general geological mining conditions; the surface movement range is large, and the horizontal movement range is often greater than the vertical movement range; the upper mountain boundary angle, the moving angle is less than or close to the lower mountain boundary angle and moving angle, etc. although these studies have obtained some macroscopic laws, due to the differences in the characteristics of geological mining conditions in various mining areas, there has not yet been obtained a comprehensive study on the relationship between the characteristics of surface subsidence in thick loose layer mining areas and geological mining factors, and the influence laws. as a result, it is difficult to select the parameters when predicting the surface movement and deformation of coal seam mining in thick and loose layer mining areas (ma & hu, 2013). compared with the actual observation values, the prediction error is large. therefore, it is of great theoretical and practical value to deeply study the subsidence mechanism and deformation law of coal seam mining in a thick and loose layer mining area, to accurately predict the movement and deformation of the ground surface, to reasonably keep various types of protective coal pillars, and to protect various protective objects on the surface (hou et al., 2018). figure 1. comparison of surface subsidence predicted by probability integral method and real subsidence value comparison of two model original model assuming that a unit volume of coal seam b (s, t) is mined in a gently inclined coal seam, the subsidence value of the surface point a (x, y) caused by the coal seam mining in this unit can be calculated by equation 1 in the original model. w x y r es x s y t h c r , tan ( ) = ∗ ( ) + + ∗( )       1 2 2 2 2−π − − θ (1) where r is the main influence radius, h is the mining depth, and θ is the maximum sinking angle. assuming that the working face during actual mining is a rectangular working face, the calculation formula for the sinking value of any point a (x, y) of the near-horizontal working face with the strike length of d3 and the inclined length of d1 according to the probability integral method is: 369a prediction model of mining subsidence in thick loose layer based on probability integral model where s3 is the offset of the inflection point in the downhill direction, l = d3-s3-s4, which is the length of the theoretical strike, d3 is the length of the real strike, l d s s= ∗ ( )1 1 2− − θ+α θsin / sin , which is the theoretical tendency length, d1 is the real tendency length, α is the coal seam inclination, θ is the maximum sinking angle, r is the main influence radius of the tendency, r is the main influence radius of the strike direction. thus modified model sinking calculation formula is: w x y w x y w x ys r r s r r, , . , .( ) = ( )∗ + ( )∗1 1 2 20 5 0 5 (7) where w x ys r r1 1 ,( ) and w x ys r r2 2 ,( ) are the calculation results brought into equation 6 while r = r1, r = r1 and r = r2, r = r2 . w x y r r e x s r y t h c r s co , * * tan ( ) = ( ) + + ∗( )        ∗ 1 1 1 1 2 2 2 2 2 2 1 −π − − θ ss l s x s r y t dtds r r e α −π − − ∫∫ + ( ) + 3 2 2 2 1 1 1 0 5 1 1 * . * * ++ ∗( )        ∗∫∫ h c r s cos l s dtds tan * . θ α 2 2 2 13 1 0 5 (8) parameter analysis main influence radius-r affected by underground coal mining, the subsidence value of the surface approaches or reaches the maximum subsidence value w0 at the position of x> r, and other surface movement and deformation mainly occur within the range of x = -r ~ + r which is shown in figure 2. so we call r the main influence radius. the main influence radius r is defined as the distance from the inflection point of the subsidence curve to the maximum subsidence point or the distance from the inflection point to the boundary point of the moving basin, which is a parameter that characterizes the concentration of surface movement. they are mainly related to the nature of overburden and mining depth. figure 2. schematic diagram of main influence radius in order to study the effect of r2 and r2 values on the predicted subsidence shape along strike direction and tendency direction. the control variable method is used to compare them in this paper as follows: (1) the effect of r2 on the shape of the main section subsidence predict the results calculated by the improved model with a different value of r2 while other parameters keep unchanged are shown in figures 3 and 4. figure 3. the impact of r2 on the predicted main section w x y r es x s y t h c r si cos l s , tan ( ) = ∗ ( ) + + ∗( )        ∗∫ 1 2 1 2 2 2 3 −π − − θ α∫∫ dtds (2) where s3 is the offset of the inflection point in the downhill direction, l = d3-s3-s4, which is the theoretical strike length, d3 is the real strike length, l d s s= ∗ ( )1 1 2− − θ+α θsin / sin , which is the theoretical tendency length, d1 is the real tendency length, α is the coal seam inclination, θ is the maximum sinking angle. since one of the main problems of the probability integration method is the phenomenon of convergence too fast at the edge of the subsidence basin compared with the actual subsidence, a modified probability integration method model is proposed by wang and deng (2012): w x y w x y r p w x y r ps s, , ; , ;( ) = ( )∗ + ( )∗( )1 2 1 (3) where w x y rs , ; 1( ) and w x y rs , ; 2( ) are the calculation results brought into equation 2 while r = r1 and r = r2 and p is the proportionality coefficient. w x y r e si cos l s x s y t h c r, tan ( ) = ∗ ∗ ( ) + + ∗( )     ∫∫ 1 1 2 1 3 2 2 1 2 α −π − − θ   ∗ ( ) + + ∗ + ∗∫∫ dtds p r e si cos l s x s y t 1 2 2 1 3 2 α −π − − hh c r dtds p ∗( )        ∗ ( ) tanθ − 2 21 2 1 (4) it can be seen from equation 4 that the correction method regards the ground subsidence caused by coal mining as the combination of the coal seam with the main influence radius r1 and the coal seam with the main influence radius r2 according to a certain proportional coefficient. it is verified by an example that this method has a certain effect to improve the phenomenon that the probability integral method converges too fast at the edge of the subsidence basin compared with the actual subsidence. however, in practical application, it was found that when analyzing the ground subsidence caused by mining in a working face, the main influence radius along the strike direction and the main influence radius along the inclined direction are different, which reduced the accuracy of the model. therefore, this paper proposes to divide the main influence radius into the influence radius in the striking direction and the influence radius in the tendency direction on the basis of this original method, and changed the parameter adjustment method in the original model, verified the accuracy of this method in predicting the deformation value of the ground movement through examples. improved model according to the above description, the main influence radius in the formula is divided into the influence radius along the strike direction and the influence radius along with the trend. the calculation formula of the sinking value of the surface point a (x, y) caused by unit mining is: w x y r r es x s r y t h c r , tan ( ) = ∗ ∗ ( ) + + ∗( )       1 1 2 2 2 2 2−π − − θ (5) where r is the main influence radius in the strike direction, and r is the main influence radius in the tendency. h is the coal mining depth, and θ is the maximum angle of influence. assuming that the working face during actual mining is a rectangular working face, the calculation formula for the sinking value of any point a (x, y) of the near-horizontal working face with the strike length of d3 and the inclined length of d1 according to the probability integral method is: w x y r r e si cos l s x s r y t h c r, tan ( ) = ∗ ∗ ∗ ( ) + + ∗( )   ∫∫ 1 1 3 2 2 2 2 21 α −π − − θ     dtds (6) 370 jingxian li, xuexiang yu, ya liang figure 4. the effect of r2 on the prediction of the prone main section (1) the effect of r2 on the shape of the main section subsidence predict the results calculated by the improved model with a different value of r2 while other parameters keep unchanged are shown in figures 5 and 6. figure 5. the impact of r2 on the predicted main section figure 6. the effect of r2 on the prediction of the prone main section after the statistical analysis of the measured data, the main effecting angle (tan β) estimated by the new model for the subsidence predicted value of the near-level coal seam mining are: tan1=3.177-1.232*h/h tan2=0.804+0.127*h/h tan1=2.223+1.505*h/h tan2=0.915+0.202*h/h it can be seen that the main influence of tangent is proportional to the thickness of the loose layer. taking the main influence tangent of the strike direction as an example, tanβ1 is larger than tanβ0, and tanβ2 is smaller than tanβ0. assuming that the main influence radius in the original probability integral method model is r0, r1 is smaller than r0, and r1 mainly controls the position of the inflection point, r2 mainly controls the degree of boundary convergence. it also can be seen that when the strike and the tendency direction are both fully mined, the r2 value mainly controls the convergence of the edge subsidence value in the inclined direction, while the r2 value mainly controls the convergence of the subsidence value along the strike line of the subsidence basin. it can be seen from the statistics of the examples that the main influence radius of the trend and the tendency are more or less different, so this method is helpful to improve the prediction accuracy. points range in addition, according to the definition of the probability integral method, the value of the sinking coefficient q is mainly related to the roof management method. but the definition of the inflection point offset is based on the principle of equivalent coal seam due to the cantilever effect of the coal goaf, the predicted subsidence calculated by the original coal seam directly according to the probability integration method does not match the actual situation, so the definition of the inflection point offsets are proposed to build an ideal coal seam to make predicted subsidence value more match the actual situation. so the integral range of the trend and tendency during the prediction is the range of the imaginary working face (the trend is s3-1, and the tendency direction is s1*cosα). the principle of the new model is that two-unit coal seams with different influence radium will jointly affect the surface subsidence with the same ratio, which changes the integration range. therefore, this article chooses to change the integral range on the basis of the original inflection point offset. after the statistical analysis of the measured data, the integration range estimated by the new model for the subsidence value of the near-level coal seam mining are: ax=1.375+0.409*s3 bx=l ay=36.801+0.097*s1 by=271.014+0.002*l example verification the situation of the observatory in the mining area xieqiao mine 21113 working face is located in huainan city, anhui province, and is the first mining face of xieqiao mine. the upper boundary depth of the working face is 714m, the lower boundary depth is 764m, the average mining depth is 739m. the seam thickness is 5.2m, and the inclination angle is 12 °, which belongs to the class of gently inclined coal seam. the coal mining method is the long-arm method, and the roof management method is the full collapse method. the real strike length of this work face is 1968.7m, the real tendency length is 234.2m. one observation line is set along the strike direction and two half observation lines are set along the tendency direction. the setting diagram of the 21113 working face observation station is shown in figure 7. figure 7. schematic diagram of 21113 working face observation station calculation results the comparison of the results of solving the parameters of the working face by using the mold loss method combined with two different models is shown in table 1. the predicted results of the model corresponding to the parameters calculated by the original model and improved model proposed in this paper are compared with the measured values of the trend and tendency direction as shown in figures 8 and 9. the contour map of expected sinking corresponding to the probability integral method is shown in figure 10, and the contour map of expected sinking corresponding to the improved model is shown in figure 11. figure 8. comparison chart of expected value on the main section 371a prediction model of mining subsidence in thick loose layer based on probability integral model expected, and both the original model and the improved model have a significant effect in boundary correction. conclusion from the above analysis, it can be seen that the original model regards the surface movement deformation as a weighted superposition of the effects of two mining units with different main influence radius, which has an optimization effect on the problem that the probability integral method converges too fast at the edge of the subsidence basin compared with the actual subsidence. on the basis of the original model, the improved algorithm proposes a method in which the main influence radium is divided into two directions-strike main influence radium and tendency direction main influence radium, which improves the accuracy while optimizing the boundary problem. compared with the original model, different parameters were selected for an adjustment instead of the subsidence coefficient in the original model, and it is assumed that the impact of the two mining units is equal, in other words, the scale factor in the new model is 0.5. but the integral parameters can no longer be set according to the boundary range of the ideal coal seam, so they were set as new adjustment parameters. the actual application results prove that the new model is more in line with the actual coal mining subsidence. however, if it is used in steeply inclined coal seams, the main influence radius of the uphill direction of the inclined direction is different from the main influence radius of the downhill direction. acknowledgment the work is supported by the research on key technology of highprecision and rapid monitoring of mining subsidence based on gps / bds combination (no. 41474026). references ding, r., liu, h., & dong, j. (2018). gisbased subsidence prediction of yuzhou goaf area at the middle route of the south-to-north water diversion project. geotechnical and geological engineering. fan, h. d., gu, w., qin, y., xue, j. q., & chen, b. q. (2014). a model for extracting large deformation mining subsidence using dinsar technique and probability integral method. transactions of nonferrous metals society of china. hou, d., li, d., xu, g., & zhang, y. (2018). superposition model for analyzing the dynamic ground subsidence in mining area of thick loose layer. international journal of mining science and technology. ma, w. j., & hu, h. f. (2013). the strata movement regularity and parameter simulation analysis in multiple seams repeated mining. applied mechanics and materials. wang, l., deng, k., fan, h., & zhou, f. (2018). monitoring of large-scale deformation in mining areas using sub-band insar and the probability integral fusion method. international journal of remote sensing. wang, l., deng, k., & zheng, m. (2020). research on ground deformation monitoring method in mining areas using the probability integral model fusion d-insar, sub-band insar and offset-tracking. international journal of applied earth observation and geoinformation. wang, l., jiang, c., wei, t., li, n., chi, s., zha, j., & fang, s. (2019). robust estimation of angular parameters of the surface moving basin boundary induced by coal mining: a case of huainan mining area. ksce journal of civil engineering. wang, y. h., zhou, j. w., & wen, b. (2013). subsidence prediction under thick alluvium based on probability integration method. applied mechanics and materials. wang, z., & deng, k. (2012). edge correction model of subsidence prediction by probability integral method. journal of xi’an university of science and technology, 32(04):495-499. table 1. parameters fitted by the three models according to the measured values probability integral method improved model revised model subsidence coefficient 0.94 1.14 0.94 main influence radius(m) 339 r1=249.66 r1=339 r1=209 r2=731.68 r2=939 r2=739 mining impact propagation angle(°) 85.48 85.48 85.48 points range s3=66 s3=66 ax=-34 s4=45 s4=45 bx=1969 s2=-6 s2=-6 ay=49.02 s1=-21 s1=-21 by=309.79 figure 9. comparison chart of expected value on the main section figure 10. surface subsidence contour predicted by probability integral model figure 11. surface subsidence contour predicted by an improved model from the comparison of the calculation results of probability integral method and the improved model in figures 7 to 10, it can be seen that the accuracy of the improved model has been improved when the full face is 372 jingxian li, xuexiang yu, ya liang xie, y., zhang, a., liu, h., & feng, l. (2019). geo-informatics in sustainable ecosystem and society. springer science and business media llc. zha, j. f., feng, w. k., zhu, x. j., & mi, l. q. (2011). analysis accuracy and robustness of parameters inversion in probability integral method by genetic algorithm. seventh international conference on natural computation. zhang, z., & lei, t. (2012). one design method of pipeline in mined-out area, volume 4 pipelining in northern and offshore environments strain-based design risk and reliability standards and regulations. zhou, d., wu, k., li, l., diao, x., & kong, x. (2016). a new methodology for studying the spreading process of mining subsidence in rock mass and alluvial soil: an example from the huainan coal mine, china. bulletin of engineering geology and the environment. keywords: casing damage; shear deformation; numerical simulation; critical strain; early warning. palabras clave: tubería de revestimiento; deformación por cizallamiento; simulación numérica; tensión crítica; alerta temprana. how to cite item wang, s., wang, x., dong, k., jiang, m. chen, m., xie, w., zhang, z., & yang, l. (2020). shear damage mechanism and early warning study of shale standard layer casing. earth sciences research journal, 24(2), 225-229. doi: https:// doi.org/10.15446/esrj.v24n2.87932 aiming at the problem of shear-induced casing damage caused by standard layer shale soaking water, the theory of rock mechanics is used to establish the fluid-solid coupling calculation model of the formation-cement ring-casing of the standard interval shale interbed. it allows obtaining the strain and deformation changes of casing shear damage, and it puts forward the casing logarithmic strain exacerbated area should prompt adoption of reasonable measures to slow down the casing damage. three-level orthogonal numerical simulation experiments were carried out for the five factors affecting casing damage. the simulation results show the critical strain data of the casing damage and the true strain data of the casing under the influence of five factors. according to the statistical principle, the predictive mathematical model of the critical strain and true strain prediction of the casing is obtained by the multiple regression method. the prediction of the strain state of the casing under actual working conditions is realized, and then the casing damage early warning was also realized. abstract shear damage mechanism and early warning study of shale standard layer casing mecanismo de daño por cizallamiento y estudio de alerta temprana de revestimiento de capa de lutita estándar issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87932 con el fin de abordar el problema del daño de la tubería de revestimiento inducido por el corte causado por el agua de remojo en capas de lutita estándar, la teoría de la mecánica de rocas se utiliza para establecer el modelo de cálculo de acoplamiento fluido-sólido de la envoltura de anillo de cemento de formación del intervalo estándar de esquisto intercalado. esta permite obtener los cambios de tensión y de deformación del daño por cizalladura de la tubería de revestimiento, y presenta el área exacerbada por deformación logarítmica del revestimiento. también, debe impulsar la adopción de medidas razonables para reducir el daño de la tubería de revestimiento. en este trabajo se llevaron a cabo experimentos de simulación numérica ortogonal de tres niveles para los cinco factores que afectan el daño de la tubería. los resultados de la simulación muestran los datos críticos de tensión del daño de la tubería y los datos reales de tensión de esta bajo la influencia de los cinco factores. de acuerdo con el principio estadístico, el modelo matemático predictivo de la deformación crítica y la predicción de deformación verdadera de la tubería se obtiene mediante el método de regresión múltiple. en este estudio se realiza la predicción del estado de deformación de la tubería de revestimiento en condiciones de trabajo reales y luego se analiza la alerta temprana en daños de la tubería. resumen record manuscript received: 12/03/2019 accepted for publication: 17/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 225-229 suling wang1, xuefei wang1,2, minzhan chen1, kangxing dong1, minzheng jiang1*, lei yang3 1school of mechanical science and engineering, northeast petroleum university, daqing 163318, china 2school of mechatronics engineering, qiqihar university, qiqihar 161001, china 3ningde times new energy technology co. ltd. ningde 352000, china * corresponding author: jmz1964@126.com pe tr o le um e ng in ee ri ng https://doi.org/10.15446/esrj.v24n2.87932 https://doi.org/10.15446/esrj.v24n2.87932 https://doi.org/10.15446/esrj.v24n2.87932 226 suling wang, xuefei wang, minzhan chen, kangxing dong, minzheng jiang, lei yang introduction by the end of 2016, the cumulative number of wells drilled in a certain area of daqing oilfield was 20,349, and the casing was damaged in 5,715 wells, accounting for 28.1% of the number of wells. among them, the number of casing damage wells in a standard layer is 2325, accounting for 40.6% of the total number of casing damage wells. the casing damage in the non-oil layer is the most harmful to oilfield development. data shows that the two peaks of the casing damage are concentrated in the standard layer shale parts. the casing damage wells continue to increase and have caused millions or even billions of economic losses to the oilfield. therefore, prevention of casing damage is imminent. the qra method and lrfd method for reliability evaluation the strength of casing was proposed abroad in the early 1990s (miller, 1998; lewis, 1995). the method of risk and reliability evaluation is an important tool to solve the problem of uncertain factors in foreign drilling projects (cunha et al., 2005). there are limited researches on domestic oil and water well casing damage warning (hualin et al., 2010). encheng wu and tie yan broke through the viewpoint that casing damage is caused by oil shale swelling with water, and concluded that after the fossil layer enters the water, under the action of fluid pressure and ground stress, when the shear stress exceeds the shear strength of the weak surface of the fossil layer, the formation becomes unstable and the sheet casing damage occurs (encheng & tie, 2007). lianping li pointed out that the standard layer casing damage is the result of mechanical shearing. shear damage is the main form of casing damage and there is a casing damage shear zone (lianping, 2007). chi ai et al analyzed the influence of casting steel grade and wall thickness on the critical condition of casing damage. he analyzed the main factors influencing the critical slip of casing from the angle of delaying the shear damage of casing (chi et al., 2015). yanjun zhou et al proposed a dynamic prediction method for causing damage based on the integration of rough sets and support vector machines (zhou et al., 2010). guorong wang et al obtained the prediction model of casing reliability life by qualitative and quantitative analysis of casing fault tree, and deduced the calculation formula of the upper and lower limit of casing damage in a period of time after completion (guorong et al., 2002). in order to solve the problem of large number of casing damage in the standard layer of daqing oilfield, this study applied on-site oil well test data and numerical simulation data for comparison, the data are in good agreement that the simulation model is available. the key factors affecting casing damage are formation elastic modulus, the formation thickness, the formation dip angle, casing strength and water injection pressure. three orthogonal tests were carried out on five factors. numerical simulations were carried out to obtain critical strain data and casing true strain data. the multivariate regression method is used to obtain the prediction equation of the critical strain of the casing and the true strain of the casing, and the prediction method of casing damage based on the parameters of the well and the production parameters is obtained. establishment of standard layer formation-cement ring-casing numerical model through the analysis of the lithological characteristics of the bottom of a standard layer in daqing oilfield, rock formation sampling is shown in fig. 1. the rock mass at the bottom of this section belongs to typical jointed rock mass, and there are abundant bedding joints and micro-cracks. this structure facilitates rapid immersion of injected water, forming an expanding flooded area, when the submerged area is squeezed by external force, the rock mass will easily slip along the structural plane of the rock mass. taking the standard layer as the research object, establish a threedimensional fluid-solid coupling mechanical model of the casing-cement ring-formation. formation thickness and formation inclination are determined according to well conditions, the reservoir rock mass is subjected to the pressure of the overlying rock and the weight of the rock itself and the pressure of the production fluid in the casing (wan mahari et al., 2018). the initial osmotic pressure is applied to the upper, lower and circumferential surfaces of the formation, a fixed constraint is applied on the bottom surface, displacement constraints in the x and y directions were applied to the outer surface to simulate the lateral constraint of surrounding formation rocks on the model, the mechanical model is shown in fig. 2. figure 2. standard layer casing damage analysis mechanical model considering the non-uniformity of the formation and the casing, the numerical model is used to calculate the above mechanical model, and the deformation and strain law of the casing under different formation conditions are obtained. the finite element model is shown in figure 3. figure 3. standard layer casing damage analysis finite element model according to the test data of a rock layer in daqing oilfield, set the rock formation of mudstone shale mudstone interbed structure, the upper and lower mudstones is set as creep materials, shale set into joint material (cheng et al., 2019). analysis under the condition of water immersion, due to the difference in mechanical properties between the layers, the deformation of the joint of the cementation position is uncoordinated and the casing deformation is damaged. figure 1. a sampling of a standard layer shale in daqing oilfield 227shear damage mechanism and early warning study of shale standard layer casing by simulation, as shown in fig. 5, casing strain value detected by the oil well and the casing strain value calculated by simulation. it can be observed in the figure that the oil well test strain data and simulation results are consistent, indicating the correctness of the calculation model and method. the outer load of the casing is 20 mpa, 30m pa, 40 mpa, 50 mpa respectively. the strain and deformation of the simulated casing change with time, as shown in the casing strain cloud diagram of fig. 6, the bending deformation position of the casing is the concentration region of plastic strain. joint materials include joint system water invasion and failure behavior, as well as the failure mechanism of matrix materials. the plastic behavior of the matrix material follows the linear drucker-prager failure criterion, and the joint material model uses the stress-based fracture opening criterion. when the crack is open, the model follows:  an ps el an el ( ) ≤ (1) in the formula, an el is the normal elastic strain component across the crack, is calculated from the plane stress assumption to calculate the normal elastic strain across the crack. ε − σ σan ps el a a v e( ) = +( )1 2 (2) in the formula, e is the elastic modulus of the material, v is the poisson’s ratio, a1, a2 is the radial stress on the joint surface. the sliding failure surface of the joint system a is defined as follows: f p da a a a a= =τ − β −tan 0 (3) in the formula, a is the internal friction angle of the joint system a, da is the viscous force of the joint system a, a is the shear stress on the joint surface, pa is the vertical stress acting on the joint surface. when  fa < 0, the joint system a does not slip. when fa = 0, the joint system from sliding. in the calculation process, the casing deformation is caused by the deformation of the standard layer. in order to accurately simulate the deformation state of the standard layer after water inflow, the simulation of the deformation of the casing is performed in two steps (giwa et al., 2016). in the first step, the internal stress of the casing and the stress field of the formation caused by the cementing process are applied to the calculation model, and the initial geostress field of the casing-cement ring-formation model is simulated, in the second step, the water pressure boundary conditions are applied to the calculation model to simulate the shale water immersion process, and the casing strain and deformation law under different influent strengths are simulated to provide a basis for the casing damage determination. strain and deformation of the casing this study selected a well in daqing oilfield for testing, the shale layer is located 923 meters underground and the shale thickness is 2.6 meters, the principal stresses in the three directions of x, y and z are 23.32 mpa, 22.25 mpa and 25.16 mpa the dip angle of the formation is about 3 degrees, the casing diameter is 51/2”, and the yield limit of the casing is 552 mpa. in the field logging process, the well detection uses continuous monitoring and stores test data, and continuously monitors for 9 months, as shown in fig. 4. according to the above theory and data, the strain of the casing is obtained figure 4. oilfield oil well test true strain 0.0005 0 -0.0005 -0.001 -0.0015 -0.002 -0.0025 -0.003 time/month 15 20 strain obtained by simulation strain from oil well testing figure 5. comparison of monitoring and calculation of logarithmic strain outside the casing (a)elastic strain ee (b)plastic strain pe (c)logarithmic strain le figure 6. casing strain cloud 228 suling wang, xuefei wang, minzhan chen, kangxing dong, minzheng jiang, lei yang the true strain is defined as the natural logarithm of the ratio of the current length of the initial length, so it is also called logarithmic strain, which is a combination of elastic strain and plastic strain. the elastic strain, plastic strain and true strain in the bending position of the casing is extracted, figure 7 shows the variation of the elastic strain and plastic strain of the casing with time under different loads: (a) elastic strain (b) plastic strain figure 7. variation of elastic strain of casing with time under different loads when the water injection pressure outside the casing is faster or the pressure outside the casing is larger, the critical elastic strain of the casing gradually decreases with the increase of the external pressure of the casing. but the reduction is small, and the external load is 40 mpa compared with 50 mpa when the plastic deformation occurs. the difference of the elastic strain of the casing is 0.00003, which is about 3.5%, after the casing reaches the plastic deformation, the elastic strain of the casing increases slowly, and the rate of elastic strain of the casing under different external pressures is the same. with the extension of time, the elastic deformation of the casing is basically the same. the larger the external load is, the earlier the deformation will be. the plastic strain rate of the casing is not affected by external pressure. since field detection shows the comprehensive strain of elastic strain and plastic strain, the variation law of logarithmic strain is extracted, as shown in figure 8. figure 8. horizontal logarithmic strains of casing under different loads figure 9. horizontal deformations of the casing under different loads figure 8 shows the horizontal logarithmic strain of casing under different loads. the strain growth curve can be divided into the logarithmic strain slow growth phase, the accelerated growth phase and the rapid growth phase. the casing deformation law corresponding to fig. 9 is the slow deformation phase and the deformation acceleration phase. the slow growth phase of logarithmic strain mainly occurs in the initial stage of elastic deformation of casing, and the acceleration phase of logarithmic strain is the stage of elastic instability of casing, and the stage of rapid growth is the stage of plastic strain of casing. in the elastic instability stage and the plastic deformation stage, the casing deformation is in the acceleration zone, which is the instability zone of the casing deformation, and the casing damage is intensified at this stage. when the elastic strain of the casing accumulates to a certain extent, the plastic strain increases sharply and the casing enters the plastic strain phase. from the point of view of the management of casing damage, in the accelerated growth phase of casing logarithmic strain, timely take appropriate measures to slow the casing damage. this study proposes a method to predict the critical strain and the true strain of the casing. when the true strain of the casing exceeds the critical strain of the casing, the casing will enter the plastic deformation stage and an early warning to reduce casing damage. standard layers casing loss prediction the main factors affecting the casing include: formation elastic modulus, formation thickness, formation inclination, casing strength and water injection pressure. this study conducts 3 level orthogonal tests on 5 parameters, according to the numerical simulation method established above, the true strain value of the casing and the critical strain value when the casing is damaged are obtained, the calculation results are shown in fig. 10. figure 10. orthogonal test numerical simulation results data scatter plot it is concluded from figure 10 that the three levels of the reservoir elastic modulus, reservoir thickness, reservoir angle, casing strength and casing water injection pressure are combined to obtain 37 sets of parameters and corresponding casing damage critical strain values and true strain value of the casing. the data analysis software was used to obtain the binary multiple regression equations which characterize the relationship between the critical strain and true strain and the above five factors, and the variance analysis method was used to verify the rationality of the equation. the equation was used to predict the casing damage. then get the following mathematical model. ε −3.3 −3. −2.8 −3. − − − 3 − lj x x x= ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅8 4 10 10 10 107 1 5 2 4 6. 99 − − −⋅ ⋅ + ⋅ ⋅ + ⋅ ⋅x x x4 6 4 6 5 310 3 9 10 3 05 10. . (4) ε −82 − − − −− zs x x x x x x= + ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅62 37 30 3 5 10 2 7 7 31 2 3 4 2 5 1 2. . . + ⋅ + + ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅ + 22 6 8 10 2 3 10 2 4 4 2 3 2 4 2 5 5 2 2 1 2x x x x x x. . .− − − .. . . 1 10 2 4 10 15 10 1 1 1 3 2 1 4 4 1 5 1 2 ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ x x x x x x x − − −− − ⋅⋅ ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ x x x x x x x x 3 2 4 2 2 5 3 4 32 9 10 1 9 2 8 10 3 9 − − − − −. . . . 33 5 1 4 5 410 3 4 10⋅ ⋅ ⋅ ⋅ ⋅x x x− −− . (5) 229shear damage mechanism and early warning study of shale standard layer casing among them, lj is the casing critical strain; zx is the casing true strain; x1 is the reservoir elastic modulus, gpa; x2 is the reservoir thickness, m; x3 is the reservoir inclination, degree; x4 is the casing strength, mpa; and x5 is the outer casing pressure, mpa. comparing the measured critical strain and true strain curve with the multiple regression fitting theoretical curve by comparing fig. 11 with fig. 12, it can be seen that the formula of multiple regression theory can fit the sample data well. figure 11. comparison of sampling critical strain data and multiple regression critical strain curves figure 12. comparison of on-site sampling data with theoretical curves according to the prediction equations of critical strain and true strain of the casing obtained by binary regression, the critical strain value of the casing and the true strain of the casing can be predicted under conditions of known oil well parameters and production parameters. the equation is  zs lj≥ (6) if the equation (6) is satisfied, the casing begins to undergo unstable deformation and enters the stage of plastic deformation, and measures need to be taken in time, otherwise, the casing is stable and no instability damage occurs. the prediction equation is suitable for the standard layer of the second section of daqing oilfield. other oilfields can use the above method to determine the prediction equation and to determine the damage of the casing. conclusion (1) the main cause of casing shear damage is the slippage of the standard shale layer. according to the theory of rock mechanics, creeping materials are utilized to the upper and lower mudstones, and joint materials are used for shale. the fluid-solid coupling calculation model of the formation-cement ringcasing of the standard interval mudstone shale interbed is established, and the deformation low of casing shear damage is obtained. the simulation results are compared with actual logging results of daqing oilfield, the rationality of the numerical model is verified and a simulation method of standard casing damage is proposed. (2) in the shear damage process, the transition from elastic strain to plastic strain is the initial stage of casing damage. from the perspective of casing damage prevention, steps taken to reduce casing damage should be taken during the casing strain acceleration phase. built on the above theory, this study proposes a method for predicting the true strain and critical strain. (3) the five factors affecting casing damage are casing strength, rock inclination, rock thickness, rock elastic modulus, and external load. the five factors were divided into three levels for orthogonal numerical simulation experiments, and data on the correlation between the five factors and the critical strain and true strain of the casing were extracted. according to the statistical principle, the multivariate regression method is used to obtain the mathematical model of the critical strain and true strain prediction of the casing. when the true strain exceeds the critical strain, the casing deformation enters the plastic phase and the casing damage occurs. the prediction of the strain state of the casing under actual working conditions is realized according to the formation parameters and logging data, which provide a new method for preventing casing damage. (4) the theoretical formula for the shear damage of the standard layer casing of the nen 2 section of daqing oilfield has certain practicability. acknowledgments this research was supported by cooperation project between heilongjiang province and academicians (2019sy0304) and china postdoctoral science foundation(2019m661249). the authors would like to thank daqing oilfield for providing data on samplingstress and strain. references cheng y. w., ng, k. h., lam, s. s., lim, j. w., wongsakulphasatch, s., witoon, t., & cheng, c. k. (2019). syngas from catalytic steam reforming of palm oil mill effluent: an optimization study. international journal of hydrogen energy, 44(18), 9220-9236. chi, a., chaoyang, h., & yueming, c. (2015). casing optimization for delaying standard layer damage of daqing oilfield. petroleum drilling techniques, 43(06), 7-12. cunha, j. c., demirdal, b., & gui, p.. 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(1995). load and resistance factor design for oil country tubular goods. offshore technology conference, 7936. lianping, l., hongqing, k., guizhen, j., qin, l., & qing, liu. (2007). a new understanding of the formation mechanism of casing damage in xingbei development zone and the comprehensive prevention and control technology of casing damage. daqing petroleum geology and development, (01), 83-87. miller, r. a. (1998). real world implementation of qra methods in casing design. society of petroleum engineers, 48325. wan mahari, w. a., chong, c. t., cheng, c. k., lee, c. l., hendrata, k., yek, p. n. y., ma, n. l., & lam, s. s. (2018). production of value-added liquid fuel via microwave co-pyrolysis of used frying oil and plastic waste. energy, 162, 309-317. zhou, y., jia, j., & li, r. (2010). a dynamic prediction method for casing damage based on rough set theory and support vector machine. journal of china university of petroleum (natural science edition), 34(06), 71-75. keywords: wuhaolai mafic complex; genetic mineralogy; magmatic evolution; tectonic significance; margin of the north china craton. palabras clave: complejo máfico de wuhaolai; mineralogía genética; evolución magmática; significación tectónica; margen del cratón china del norte. how to cite item chen, w., jianchao, l., haidong, z., jiakun, g., zhixuan, x., haoran, w. (2019). mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia. earth sciences research journal, 23(2), 133-146. doi: https://doi.org/10.15446/esrj. v23n2.66316 the wuhaolai mafic complex is located in the north margin of the north china craton (ncc), inner mongolia. to discuss the mineralogical features, magma evolution process, and tectonic setting of the complex, we analyzed the geochemical compositions of clinopyroxene and hornblende using an electron probe. the results revealed that the parental magma of this complex belonged to the intraplate alkaline basalt series. the normal zoning texture and the relation between mg# and feo, al2o3, cao, na2o, sio2 and cr2o3 suggested that the clinopyroxenes of pyroxenite and gabbro crystallized from the same parental magma. the similar cao content of clinopyroxenes indicated that the parental magma of the wuhaolai complex may have suffered crustal contamination. furthermore, the characteristics of hornblende demonstrated that the magma source was modified by fluids derived from subducted slab. based on the value of kdcpx (0.23–0.27), the equilibrium melt with clinopyroxene exhibited a relatively low mg# (43–53), indicating that the parental magma was derived from the lithospheric mantle and underwent crystal fractionation. the gabbro crystallization temperature and pressure was found to be lower than that of pyroxenite, indicating that gabbro was formed at a lower depth than that of pyroxenite. combining the tectonic setting discrimination diagram of clinopyroxene with the results of previous studies on the late paleozoic intrusions near the research area, we proposed that the wuhaolai complex was formed in an intraplate environment. the magma source was modified by fluids derived from the subducted slab during the subduction of the paleo-asian ocean (pao). after the pao closure, the parental magma of the wuhaolai complex was produced by the partial melting of the enriched lithospheric mantle abstract mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia características mineralógicas y significado petrogenético del clinopiroxeno y la hornblenda del complejo máfico de wuhaolai en el cratón del norte de china, mongolia interior issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.66316 el complejo máfico de wuhaolai está ubicado en el margen norte del cratón del norte de china (ncc), mongolia interior. para analizar las características mineralógicas, el proceso de evolución del magma y el ajuste tectónico del complejo, analizamos las composiciones geoquímicas de clinopiroxeno y hornblenda utilizando una microsonda de electrones. los resultados revelaron que el magma parental de este complejo pertenecía a la serie de basalto alcalino intraplaca. la textura normal de zonificación y la relación entre mg # y feo, al2o3, cao, na2o, sio2 y cr2o3 sugirieron que los clinopiroxenos de piroxenita y gabbro se cristalizaron del mismo magma parental. el contenido similar de cao de los clinopiroxenos indica que el magma parental del complejo wuhaolai puede haber sufrido contaminación de la corteza. además, las características de la hornblenda demostraron que la fuente de magma fue modificada por fluidos derivados de la losa subducida. sobre la base del valor de kdcpx (0.23–0.27), la masa fundida en equilibrio con clinopiroxeno exhibió un número de mg relativamente bajo (43–53), lo que indica que el magma parental se derivó del manto litosférico y se sometió a un fraccionamiento de cristales. se encontró que la temperatura y la presión de cristalización de gabbro eran más bajas que la de piroxenita, lo que indica que el gabro se formó a una profundidad más baja que la piroxenita. combinando el diagrama de discriminación del ajuste tectónico del clinopiroxeno con los resultados de estudios previos sobre las últimas intrusiones del paleozoico cerca del área de investigación, proponemos que el complejo wuhaolai se forma en un entorno intraplaca. la fuente de magma fue modificada por fluidos derivados de la losa subducida durante la subducción del océano paleoasiático (pao). después del cierre de la pao, el magma parental del complejo wuhaolai se produjo mediante la fusión parcial del manto litosférico enriquecido resumen record manuscript received: 14/07/2017 accepted for publication: 30/11/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 133-146 wang chen1*, liu jianchao1, zhang haidong1, 2, ge jiakun1, xi zhixuan1, wang haoran1 1school of earth science and resources. chang'an university, china. 2department of geology, northwest university, china * corresponding author: chenwchd@163.com g eo ch em is tr y 134 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran introduction as one of the oldest cratons in the world, the formation of the north china craton (ncc) can traced back to approximately 1.8 ga (zhao et al., 2001; wilde et al., 2002). the interior of ncc is generally believed to have been stable from the mesoproterozoic to triassic (robinson et al., 1999; wilde et al., 2002; kusky et al., 2007). however, the southward subduction of the paleo-asia ocean (pao) caused dramatic tectono-magmatic activities during the late paleozoic in the northern ncc (davis et al., 2001; xiao et al., 2003; zhang et al., 2007a; ying et al., 2011; menzies et al., 1993; wilde et al., 2003; yang et al., 2008; wu et al., 2008). moreover, large numbers of the late paleozoic–early mesozoic plutonic rocks are distributed along the northern ncc (pan et al., 1996; zhang et al., 2007b). similar to the circum-pacific orogenic belt, the northern ncc margin comprises an island arc, a ridge, an oceanic island, a seamount, an oceanic basin, an accretionary wedge, and a micro plate, forming a complex tectonic framework (windley et al., 2007). in addition, there still is a great controversy on the pao closure time in terms of the spatio-temporal distribution of the ophiolites, multi-stage tectonic deformations, and magmatic activities in the northern ncc margin (zhao et al., 2011). according to the prevailing hypothesis, the pao closed in the ordovician–silurian (han et al., 1997; kheraskova et al., 2003), the devonian–late carboniferous (shi et al., 2013; zhao et al., 2013), or the permian–late triassic (jian et al., 2010; li et al., 2014a; li et al., 2014b). in this research, we study the wuhaolai mafic complex that is located in xinhure, inner mongolia. the wuhaolai mafic complex is located in the middle segment of the northern ncc, south of the central asia orogenic belt (caob; fig. 1a). the complex is part of a mafic–ultramafic belt, which is distributed among the wulatezhongqi, damaoqi, and bayan obo areas (fig. 1b) (cao et al., 2002; zhao, 2008). studies on this mafic belt indicated that the huheengeer pluton (242–287 ma) was formed in an active continental margin (zhao et al., 2008). nevertheless, the beiqigetao pluton (269 ma ± 8 ma) and the tugurige pluton (273.5 ma ± 1.3 ma) formed in a post-collisional extension setting (zhao et al., 2011; wang et al., 2016). however, previous research mainly focused on the petrology and whole-rock geochemistry, but the mineralogical analyses were not enough documented. in this research, we focused on the characteristics of the rock-forming minerals (e.g., clinopyroxene and hornblende) using electron microprobe analytical data to investigate the evolution of the parental magma and discuss the tectonic setting in the study area. geological background and petrography the wuhaolai mafic complex is located near the wulatezhongqi area, inner mongolia, china. the solonker ophiolite belt, which is considered as a suture zone between the ncc and the siberian craton (sc), is situated in the northern part of this complex (fig. 1b) (wu et al., 1998; xiao et al. 2003; li et al., 2006; windley et al. 2007). the late archean crystalline basement of this area is represented by the east wufuzi formation, which comprises actinolite– biotite schist, biotite–plagioclase gneiss, and biotite–monzonitic gneiss (fig. 2a). in addition, the bayan obo group, which comprises conglomeratic sandstone, sandy slate, slate, and sandy limestone, lies uncomfortably over the crystalline basement (fig. 2b). the wuhaolai mafic complex is a n–w striking stock, comprising from the center to the rim pyroxenite, gabbro and diorite units respectively (fig. 2b). in this research, two pyroxenite samples, two gabbro samples and one diorite sample were selected to study the petrography section. pyroxenite displays medium-coarse grain texture and massive structure with a blackgreen color (figs. 3a and 3b). furthermore, pyroxenite mainly contains subhedral, zonal clinopyroxene (80 vol.%; 0.2–1 mm in length), subhedral hornblende (10 vol.%,; 0.5-1 mm in length), subhedral plagioclase (5 vol.% ; ~0.5 mm in length) and rare magnetite (5 vol.%). gabbro (fig. 3c and 3d) was grayish white, with a coarse medium-grained texture and massive structure. it contains 40 vol.% of anhedral–granular clinopyroxene, 40 vol.% of coarse -grained subhedral hornblende, and 20 vol.% of subhedral, zonal plagioclase with characteristic polysynthetic twins. diorite (figs. 3e and 3f) displays a coarse medium-grained texture and massive structure. it mainly comprises zonal, coarse medium-grained subhedral plagioclase (45 vol.%) and coarse medium-grained subhedral hornblende (30 vol.%), fine grained quarte (15%) and biotite (10 vol.%). from figs. 3b and 3d, the subhedral hornblende cuts across the clinopyroxene, indicating that clinopyroxene crystallized earlier than hornblende. figure 1. (a) geotectonic context of the studied area (after jahn, 2004); (b) a simplified geological map of central inner mongolia (after cao et al., 2002). 135mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia analytical methods and results the microanalysis of clinopyroxenes and hornblendes crystals in the studied samples of pyroxenite, gabbro and diorite from the wuhaolai mafic complex was conducted at the laboratory of mineralization and dynamics, chang’an university. a jeol jxa-8200 electron microscope with zaf matrix correction. the analysis was used with an accelerating potential of 15 kv, a sample current of 1 × 10−8 a, and a spot diameter of 0.5 μm, and counting time of 10 s. the standards of na, si, fe, k, al, mn, ca, mg, and ti were natural minerals, and the standard for cr was synthetic oxides. geokit software (lu, 2004) was used to calculate the geochemical parameters of clinopyroxenes and hornblende. the results are shown in tables 1 and 2. based on the pyroxene classification standards (morimoto et al., 1988), the clinopyroxene grains from the wuhaolai mafic complex belong to the ca–mg–fe species. in the wo–en–fs diagram (fig.4a), clinopyroxene grains from pyroxenite are plotted into the diopside compositional field (wo = 48.24– 49.52, en = 39.06–39.93, fs = 10.17–11.29), which shows a small variation in the composition, with 51.79 wt.%–52.82 wt.% sio2, 13.91 wt.%–14.43 wt.% mgo, 24.01 wt.%–24.69 wt.% cao, 5.52 wt.%–6.48 wt.% feo, 0.001 wt.%–0.132 wt.% tio2, 0.42 wt.%–1.21 wt.% al2o3, 0.23 wt.%–0.42 wt.% na2o, 0.01 wt.%–0.086 wt.% cr2o3, and mg# (=100 mg/(mg + fetotal)) in the range 79–82. additionally, clinopyroxene grains from gabbro also plot into the diopside compositional field (wo = 47.43–49.42, en = 38.72–40.1, fs = 10.34– 11.21) with the geochemical characteristics of 51.22 wt.%–52.97 wt.% sio2, 13.59 wt.%–14.49 wt.% mgo, 23.75 wt.%–24.32 wt.% cao, 5.68 wt.%–6.31 wt.% feo, 0.1 wt.%–0.2 wt.% tio2, 0.88 wt.%–1.54 wt.%al2o3, 0.32 wt.%– 0.48 wt.% na2o, 0.03 wt.%–0.11 wt.% cr2o3, and mg# in the range 79–81. the cations of hornblende from pyroxenite are characterized by cab = 1.90–1.97 and (na + k)a = 0.28–0.61, and the cations of hornblende from gabbroare characterized by cab = 1.77–1.93 and (na + k)a = 0.45–0.54. the hornblende grains from the complex belong to calcic hornblende. based on the classification scheme of calcic hornblende (leake et al., 1997), the hornblendes from pyroxenite and gabbro are edenite, which have relatively lower sio2 (43.61 wt.%–48.97 wt.%), mgo (12.22 wt.%–15.13 wt.%), cao (12.15 wt.%–12.55 wt.%), and si/(si + ti + al) in the range 0.28–0.68 as well as higher tio2 (0.59 wt.%–1.67 wt.%), al2o3 (6.24 wt.%–11.30 wt.%), and na2o (0.81 wt.%–1.48 wt.%). the hornblendes from diorite are magnesiohastingsites with relatively higher sio2 (46.75 wt.%–48.3 wt.%), mgo (14.06 wt.%–14.93 wt.%), cao (12.23 wt.%–12.86 wt.%), and si/(si + ti + al) in the range 0.72–0.75 as well as lower tio2 (0.86 wt.%–1.03 wt.%), al2o3 (8.21 wt.%–8.60 wt.%), and na2o (1.01 wt.%–1.28 wt.%) (fig. 4b). discussion mineral genesis and magmatic evolution in the ti–(ca + na) diagram (fig. 5), the clinopyroxenes from the wuhaolai mafic complex belong to the alkaline series. compared with the characteristics of arc basalt (arcb) and ocean island basalt (oib) (condie k.c., 1982), the clinopyroxenes from the wuhaolai mafic complex have lower mg#, cao, sio2, and cr2o3 contents and higher al2o3 and na2o contents. additionally, the mg# of clinopyroxene has a negative correlation with feo and al2o3 and a positive correlation with na2o (fig. 6a-d). furthermore, all analyzed clinopyroxene grains plot into or near the continental alkaline basalt (cab) area (figs. 6e-h), indicating that the parental magma of this mafic complex has chemical characteristics consistent with the intra-plate alkaline series. the geochemical profile of the clinopyroxene grains (figs. 7a and 7b) reflects a normal zonal feature with a higher mg# and lower al2o3, na2o, and tio2 contents in the core than at the rims (zhao et al., 2011). it is interpreted that clinopyroxenes in pyroxenite and gabbro are crystallized from the same magma (yu et al., 2011; zhao et al., 2011). with the crystallization of clinopyroxene, mg first entered into the crystalline phase, followed by ca and al, and finally was na and k (bowen, 1928). thus, the crystallization of clinopyroxene may led to the magma rich in the si, na and k and depleted in the mg, ca and al. thus, the decreasing mgo, feo and al2o3 contents of clinopyroxenes from pyroxenite to gabbro were conforming to the magma evolution process. however, compared with the normal crystallization evolution of magma, the cao content of gabbro was higher than the pyroxenite. since crustal contamination depends on the compositions of the surrounding rocks (su et al., 2009; liu et al., 2012), the crystalline basement in the study area, which comprises biotite schist, limestone, and other rocks, contains a sufficient amount of ca. previous research shows that the parental magma of ankaramite in south margin of the altay mountains has not suffered the crustal contamination (su et al., 2008). the clinopuroxene cao content of the ankaramite was 18.5%22.07%, which was obviously lower than the wuhaolao complex. thus, the parental magma of the wuhaolai mafic complex may have suffered the crustal contamination during its evolution. previous research has shown figure 2. (a) geological map of the xinhure area (after hao et al., 2016); (b) geological map of the wuhaolai mafic complex (after imbgmr, 1991). 136 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran table 1. electron-microprobe compositions of clinopyroxene from the wuhaolai complex samples hs-1 hs-2 hs-3 hs-4 hs-5 hs-6 hs-7 hs-8 hc-1 hc-2 hc-3 hc-4 hc-5 hc-6 hc-7 hc-8 pyroxenite gabbro sio2 51.80 52.90 51.80 52.97 52.53 52.93 52.17 52.11 52.17 52.67 52.82 52.54 52.22 52.25 51.80 52.27 tio2 0.13 0.13 0.13 0.10 0.13 0.16 0.19 0.20 0.04 0.13 0.08 0.00 0.09 0.11 0.12 0.04 al2o3 0.88 0.95 0.88 0.93 0.99 1.02 1.01 1.05 0.64 0.99 0.81 0.42 0.98 0.84 1.22 0.77 cr2o3 0.05 0.08 0.05 0.10 0.03 0.11 0.03 0.08 0.00 0.09 0.00 0.00 0.01 0.03 0.08 0.00 fe2o3 0.70 0.66 0.70 0.65 0.67 0.67 0.70 0.63 0.72 0.66 0.67 0.61 0.67 0.66 0.65 0.69 feo 6.29 6.26 6.29 6.27 6.32 6.06 6.31 5.98 6.48 5.92 5.99 5.52 6.07 5.97 5.84 6.21 mno 0.19 0.24 0.19 0.33 0.23 0.28 0.27 0.24 0.10 0.24 0.16 0.42 0.12 0.14 0.17 0.29 mgo 14.02 14.50 14.02 14.11 14.18 14.18 13.59 13.69 14.09 14.09 14.29 14.10 14.24 14.44 13.95 13.91 cao 24.10 23.86 24.10 24.11 23.75 24.32 24.01 24.21 24.29 24.40 24.06 24.69 24.06 24.26 24.37 24.01 na2o 0.35 0.41 0.35 0.32 0.36 0.32 0.37 0.37 0.23 0.33 0.36 0.26 0.32 0.40 0.42 0.25 k2o 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 si 1.96 1.97 1.96 1.97 1.98 1.96 1.97 1.96 1.97 1.97 1.97 1.98 1.96 1.96 1.95 1.97 al(ⅳ) 0.00 0.03 0.00 0.03 0.02 0.04 0.03 0.04 0.00 0.03 0.03 0.00 0.04 0.00 0.05 0.03 al(ⅵ) 0.00 0.01 0.00 0.02 0.02 0.01 0.02 0.03 0.00 0.01 0.01 0.00 0.01 0.00 0.01 0.01 ti 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 . 0.00 .0.00 0.00 . 0.00 0.00 0.00 0.00 fe3+ 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 fe2+ 0.20 0.19 0.20 0.18 0.19 0.19 0.20 0.18 0.20 0.18 0.19 0.17 0.19 0.19 0.18 0.20 mn 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.01 mg 0.79 0.80 0.79 0.78 0.77 0.78 0.76 0.76 0.79 0.78 0.80 0.79 0.80 0.81 0.78 0.78 ca 0.98 0.95 0.98 0.96 0.96 0.97 0.96 0.97 0.98 0.98 0.96 1.00 0.97 0.98 0.98 0.97 na 0.03 0.03 0.03 0.02 0.03 0.02 0.03 0.03 0.02 0.02 0.03 0.02 0.02 0.03 0.03 0.02 k 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 wo 48.40 48.40 48.63 48.59 48.53 48.58 48.68 49.42 48.64 48.92 48.64 48.92 48.34 48.24 49.04 48.63 en 39.19 39.19 38.72 39.56 39.19 39.42 38.73 38.87 39.24 39.32 39.24 39.32 39.81 39.93 39.06 39.19 fs 11.14 11.14 11.02 10.67 10.96 10.85 11.21 10.34 11.29 10.56 11.29 10.56 10.67 10.41 10.37 11.27 ac 1.28 1.28 1.63 1.18 1.33 1.16 1.38 1.37 0.83 1.20 0.83 1.20 1.17 1.42 1.54 0.90 137mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia table 2. electron-microprobe compositions of hornblende from the wuhaolai complex samples hs-1 hs-2 hs-3 hs-4 hs-5 hs-6 hs-7 hs-8 hs-9 hs-10 hs-11 hs-12 pyroxenite sio2 45.03 43.61 44.81 46.77 47.98 48.98 48.67 48.26 48.56 47.37 45.12 44.67 tio2 1.15 1.68 1.18 0.89 0.60 0.71 0.63 0.69 0.66 0.84 1.16 1.29 al2o3 9.55 11.30 9.63 8.30 6.57 6.42 6.37 6.56 6.25 7.53 8.96 9.56 feo 12.94 12.82 12.48 11.65 11.47 11.47 11.55 11.30 11.00 12.29 12.92 13.03 mno 0.22 0.12 0.07 0.13 0.19 0.10 0.20 0.14 0.26 0.24 0.22 0.25 mgo 13.56 12.22 13.17 14.21 14.84 15.13 14.83 14.80 14.76 14.17 13.25 12.75 cao 12.29 12.15 12.48 12.55 12.53 12.42 12.43 12.34 12.47 12.55 12.35 12.33 na2o 1.20 1.48 1.26 0.95 0.87 0.89 0.81 0.93 0.96 0.85 1.17 1.32 k2o 0.59 0.99 0.81 0.43 0.31 0.26 0.29 0.30 0.26 0.38 0.56 0.57 si 6.70 6.52 6.71 6.93 7.13 7.18 7.19 7.16 7.20 7.01 6.77 6.71 alⅳ 1.30 1.48 1.29 1.07 0.87 0.82 0.81 0.84 0.80 0.99 1.23 1.29 alⅵ 0.37 0.51 0.41 0.38 0.28 0.29 0.29 0.30 0.30 0.32 0.35 0.40 ti 0.13 0.19 0.13 0.10 0.07 0.08 0.07 0.08 0.07 0.09 0.13 0.15 fe3+ 0.44 0.39 0.43 0.58 0.62 0.66 0.67 0.65 0.66 0.60 0.47 0.45 fe2+ 1.17 1.21 1.13 0.87 0.81 0.75 0.76 0.76 0.70 0.92 1.15 1.18 mn 0.03 0.02 0.01 0.02 0.02 0.01 0.03 0.02 0.03 0.03 0.03 0.03 mg 3.01 2.72 2.94 3.14 3.29 3.31 3.26 3.27 3.26 3.13 2.96 2.86 ca 1.96 1.95 2.00 1.99 1.99 1.95 1.97 1.96 1.98 1.99 1.99 1.98 na 0.35 0.43 0.36 0.27 0.25 0.25 0.23 0.27 0.28 0.24 0.34 0.38 k 0.11 0.19 0.16 0.08 0.06 0.05 0.05 0.06 0.05 0.07 0.11 0.11 total cations 15.56 15.61 15.57 15.42 15.38 15.34 15.33 15.35 15.34 15.40 15.53 15.55 oh0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 f 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 sit* 6.70 6.52 6.71 6.93 7.13 7.18 7.19 7.16 7.20 7.01 6.77 6.71 alt 1.30 1.48 1.29 1.07 0.87 0.82 0.81 0.84 0.80 0.99 1.23 1.29 alc 0.37 0.51 0.41 0.38 0.28 0.29 0.29 0.30 0.30 0.32 0.35 0.40 fe3+ c 0.44 0.39 0.43 0.58 0.62 0.66 0.67 0.65 0.66 0.60 0.47 0.45 tic 0.13 0.19 0.13 0.10 0.07 0.08 0.07 0.08 0.07 0.09 0.13 0.15 mgc 3.01 2.72 2.94 3.14 3.29 3.31 3.26 3.27 3.26 3.13 2.96 2.86 fe2+ c 1.05 1.18 1.09 0.81 0.75 0.67 0.70 0.70 0.70 0.86 1.08 1.15 mnc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 fe2+ b 0.12 0.03 0.04 0.06 0.06 0.08 0.06 0.05 0.00 0.06 0.07 0.04 mnb 0.03 0.02 0.01 0.02 0.02 0.01 0.03 0.02 0.03 0.03 0.03 0.03 cab 1.85 1.95 1.95 1.92 1.92 1.91 1.92 1.93 1.97 1.91 1.90 1.93 nab 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 caa 0.11 0.00 0.05 0.07 0.07 0.04 0.05 0.03 0.01 0.08 0.08 0.05 naa 0.35 0.42 0.36 0.27 0.25 0.25 0.23 0.27 0.28 0.24 0.34 0.38 ka 0.11 0.19 0.16 0.08 0.06 0.05 0.05 0.06 0.05 0.07 0.11 0.11 138 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran continued table 2. electron-microprobe compositions of hornblende from the wuhaolai complex samples hc-1 hc-2 hc-3 hc-4 hc-5 hc-6 sc-1 sc-2 sc-3 sc-4 sc-5 sc-6 sc-7 sc-8 sc-9 sc-10 sc-11 sc-12 gabbro diorite sio2 47.00 47.64 47.70 48.31 47.44 46.75 43.17 43.32 43.24 43.71 44.12 44.08 44.05 43.76 43.70 43.40 43.46 42.95 tio2 0.96 0.92 0.86 0.90 0.96 1.03 0.82 1.10 1.11 1.10 0.92 0.98 0.99 1.12 1.06 0.78 0.99 1.08 al2o3 8.59 8.36 8.21 8.31 8.41 8.60 10.34 9.85 9.42 9.28 10.08 9.38 9.40 8.52 9.88 10.12 9.60 9.93 feo 11.16 10.16 10.51 11.23 11.34 11.82 18.57 19.33 18.37 18.08 18.08 18.21 18.17 16.49 18.92 17.28 18.42 18.22 mno 0.19 0.26 0.21 0.24 0.17 0.22 0.25 0.22 0.37 0.37 0.41 0.38 0.41 0.38 0.38 0.28 0.33 0.21 mgo 14.16 14.52 14.90 14.06 14.93 14.30 9.52 9.66 9.86 10.21 10.11 10.10 10.10 10.06 9.86 9.37 10.18 9.87 cao 12.45 12.31 12.59 12.86 12.23 12.34 11.69 11.54 11.49 11.65 11.77 11.67 11.48 11.71 11.70 11.85 11.67 11.69 na2o 1.11 1.10 1.01 1.12 1.29 1.28 1.17 1.42 1.26 1.36 1.24 1.24 1.24 1.11 1.29 1.15 1.20 1.31 k2o 0.48 0.45 0.44 0.45 0.45 0.56 0.52 0.60 0.56 0.59 0.55 0.56 0.54 0.57 0.57 0.57 0.50 0.65 si 6.93 7.01 6.98 7.02 6.92 6.87 6.63 6.62 6.67 6.69 6.67 6.72 6.72 6.83 6.64 6.71 6.65 6.61 alⅳ 1.07 0.99 1.02 0.98 1.08 1.13 1.37 1.38 1.33 1.31 1.33 1.28 1.28 1.17 1.36 1.29 1.35 1.39 alⅵ 0.42 0.46 0.40 0.44 0.36 0.36 0.50 0.39 0.38 0.36 0.47 0.40 0.42 0.40 0.41 0.56 0.39 0.41 ti 0.11 0.10 0.10 0.10 0.11 0.11 0.09 0.13 0.13 0.13 0.11 0.11 0.11 0.13 0.12 0.09 0.11 0.12 fe3+ 0.58 0.64 0.60 0.63 0.52 0.50 0.44 0.36 0.41 0.39 0.44 0.44 0.45 0.52 0.40 0.50 0.41 0.38 fe2+ 0.80 0.61 0.69 0.74 0.86 0.96 1.95 2.11 1.96 1.92 1.85 1.88 1.87 1.63 2.01 1.74 1.95 1.97 mn 0.02 0.03 0.03 0.03 0.02 0.03 0.03 0.03 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.04 0.04 0.03 mg 3.11 3.18 3.25 3.04 3.24 3.13 2.18 2.20 2.27 2.33 2.28 2.29 2.30 2.34 2.23 2.16 2.32 2.26 ca 1.97 1.94 1.97 2.00 1.91 1.94 1.92 1.89 1.90 1.91 1.91 1.91 1.88 1.96 1.90 1.96 1.92 1.93 na 0.32 0.31 0.29 0.32 0.36 0.36 0.35 0.42 0.38 0.40 0.36 0.37 0.37 0.34 0.38 0.35 0.35 0.39 k 0.09 0.08 0.08 0.08 0.08 0.11 0.10 0.12 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.10 0.13 total cations 15.42 15.36 15.40 15.37 15.48 15.50 15.56 15.64 15.59 15.61 15.56 15.56 15.55 15.48 15.60 15.50 15.59 15.62 oh0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 f 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 sit* 6.93 7.01 6.98 7.02 6.92 6.87 6.63 6.62 6.67 6.69 6.67 6.72 6.72 6.83 6.64 6.71 6.65 6.61 alt 1.07 0.99 1.02 0.98 1.08 1.13 1.37 1.38 1.33 1.31 1.33 1.28 1.28 1.17 1.36 1.29 1.35 1.39 alc 0.42 0.46 0.40 0.44 0.36 0.36 0.50 0.39 0.38 0.36 0.47 0.40 0.42 0.40 0.41 0.56 0.39 0.41 fe3+ c 0.58 0.64 0.60 0.63 0.52 0.50 0.44 0.36 0.41 0.39 0.44 0.44 0.45 0.52 0.40 0.50 0.41 0.38 tic 0.11 0.10 0.10 0.10 0.11 0.11 0.09 0.13 0.13 0.13 0.11 0.11 0.11 0.13 0.12 0.09 0.11 0.12 mgc 3.11 3.18 3.25 3.04 3.24 3.13 2.18 2.20 2.27 2.33 2.28 2.29 2.30 2.34 2.23 2.16 2.32 2.26 fe2+ c 0.78 0.61 0.66 0.74 0.76 0.89 1.79 1.92 1.81 1.79 1.72 1.75 1.72 1.61 1.85 1.69 1.77 1.82 mnc 0.00 0.01 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 fe2+ b 0.02 0.00 0.03 0.00 0.10 0.06 0.16 0.18 0.15 0.13 0.13 0.13 0.15 0.02 0.16 0.04 0.18 0.15 mnb 0.02 0.03 0.03 0.00 0.02 0.03 0.03 0.03 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.04 0.04 0.03 cab 1.96 1.94 1.94 2.00 1.88 1.91 1.81 1.79 1.80 1.82 1.81 1.82 1.80 1.93 1.79 1.92 1.78 1.82 (continued) 139mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia continued table 2. electron-microprobe compositions of hornblende from the wuhaolai complex samples hc-1 hc-2 hc-3 hc-4 hc-5 hc-6 sc-1 sc-2 sc-3 sc-4 sc-5 sc-6 sc-7 sc-8 sc-9 sc-10 sc-11 sc-12 gabbro diorite nab 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 caa 0.01 0.00 0.03 0.00 0.03 0.03 0.12 0.10 0.10 0.09 0.09 0.09 0.08 0.03 0.11 0.05 0.14 0.10 naa 0.32 0.28 0.29 0.32 0.36 0.36 0.35 0.42 0.38 0.40 0.36 0.37 0.37 0.34 0.38 0.35 0.35 0.39 ka 0.09 0.08 0.08 0.08 0.08 0.11 0.10 0.12 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.10 0.13 figure 3. selected outcrop photographs and microphotographs of the representative rocks from the wuhaolai complex. (a) field photo of pyroxenite from the wuhaolao complex; (b) microphotograph of pyroxenite from the wuhaolao complex (under the cross-polarized light); (c) field photo of gabbro from the wuhaolao complex; (d) microphotograph of gabbro from the wuhaolao complex (under the cross-polarized light); (e) lithofacies lines between gabbro and diorite of the wuhaolao complex; (f) microphotograph of diorite from the wuhaolao complex (under the cross-polarized light) pl: plagioclase, cpx: clinopyroxene, hb: hornblende (whitney and evans (2010). 140 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran that the equilibrium state between clinopyroxene and its host rocks can be reflected by the fe–mg exchange partition coefficient kdcpx = (fe/mg)cpx /(fe/mg)liquid (putirka, 1999; toplis et al, 1995). in this research, the kdcpx values, which were calculated based on the results of clinopyroxene and whole-rock geochemical (liu et al., 2017), were between 0.23 and 0.27. thus, based on kdcpx and the mg# of the wuhaolai clinopyroxenes (77.7– 80.6), the mg# of the equilibrium melt was found to be in the range 43–53 (fig.8). this relatively low mg# number indicated that the parental magma may derive from the lithospheric mantle and could have experienced crystal fractionation.. in the al2o3–tio2 diagram (fig. 9a), almost all the hornblende grains from the wuhaolai complex are consistent with the composition of hornblende from mantle source (jiang et al., 1984). in addition, the al/si values of hornblende from pyroxenite, gabbro and diorite are 0.15–0.30, 0.20–0.21, 0.23–0.28, respectively, and their mg/(fe3+ + fe2+ + alvi) values are 1.28–1.96, 1.68–1.92, and 0.75–0.92, respectively, indicating that hornblende compositions are consistent with those from the intermediate–basic magmatic origin (fig. 9b). all the above features support that hornblende from the wuhaolai complex is of magmatic origin and not a product of hydrothermal alteration. additionally, in the aliv–a, a iv–total al, and aliv–alvi + fe3+ + 2ti + a sites and in the pyroxenite gabbro fe o (w t.% ) i n cl in op yr ox en e 6,6 6,4 6,2 6 23,6 0,79 0,805 0,8250,815 0,82 5,8 5,6 24,8 24,6 24,4 24,2 24 23,8 1,6 1,2 1,1 0,8 0 0,4 0,2 0,6 0,4 0,3 0,2 0,1 0,80,7950,790,8150,810,8050,80,795 0,810,82 0,6 1,8 1,4 a b c d mg# mg# cab cab oib arcb oib arcb cab oib arcb cab oib arcb e f g h a l 2o 3(w t.% ) i n cl in op yr ox en e n a 2o (w t. % ) i n cl in op yr ox en e c ao (w t.% ) i n cl in op yr ox en e 26 24 22 20 18 56 c ao (w t.% ) i n cl in op yr ox en e si o 2 ( w t.% ) i n cl in op yr ox en e c r 2o 3 ( w t.% ) i n cl in op yr ox en e n a 2o (w t. % ) i n cl in op yr ox en e 54 52 50 48 46 1,6 1,2 0,4 0 2 15 1 0,5 2 0,70 0,85 0,95 10,800,7510,900,850,800,75 0,900,95 0,8 figure 6. oxide compound correlation diagrams of clinopyroxene (after kim et al., 2016), arc basalts (after debari et al., 1987; bryant et al., 2007), oceanic island basalts (after bohrson et al., 1988; chen et al., 1992; fodor et al., 1997), and continental alkaline basalts (after arai et al., 2001; ho et al., 2000; kovács et al., 2004). wo au en fs pi di he a cien cifs pyroxenite gabbro diorite edenite m g/ (m g+ fe 2+ ) ferro-edenite ferropargasite (ival ≥ fe3+) ferropargasite (ival ≥ fe3+) pargasite (ival ≥ fe3+) magnesio-hastingsite (ival ≥ fe3+) ti<0,5 b 1,0 1,0 1,0 1,0 1,0 1,0 1,0 1,0 1,0 1,01,01,01,01,01,0 1,0 siiv figure 4. (a) wo–en–fs diagram of clinopyroxene in wuhaolai mafic complex (after morimoto et al., 1988); (b) classification of hornblende in the wuhaolai mafic complex (after leake et al., 1997). di: diopside, he: hedenbergite, au: augite, pi: pigeonite, en: enstatite/clinoenstatte, c1en: bronzite, c1fs: ferrohypersthene, fs: ferrosilite/clinoferrosilite. pyroxenite gabbro ti alkaline series 0,16 0,12 0,08 0,04 0,00 ca+na 0,70 0,80 1,100,90 1,00 tholeiitic series figure 5. ti–ca + na diagrams for clinopyroxenes (after leterrier et al., 1982) 141mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia aliv–ti diagrams (fig. 10), the hornblende compositions in the different rock types exhibit a high linear correlation. this suggests that the electric charge imbalance, which is caused by the four-coordinated aliv-substitution of si in the tetrahedral position, may be compensated by the isomorphism replacement of aliv, fe3+,and ti in the octahedral position (niu et al., 2005). hornblende in peridotite xenoliths can be divided into two types: s-hl and i-hl. s-hl, which originates from the mantle wedge above the subduction zone, is mainly affected by fluids or melts derived from the subducted slab, whereas i-hl, which is formed in the intra-plate environment, is mainly influenced by the metasomatism of asthenospheric materials (i-amph) (coltorti et al., 2007). in the sio2 vs. na2o and sio2 vs. tio2 diagrams (fig.11), the analyzed hornblende grains plot into the s-amph area (fig. 11), indicating that the magma source may have been modified by the fluids or melts derived from the subducted slab (xu et al., 2009). estimation of mineral crystallization temperature and pressure although many clinopyroxene geothermometers have been proposed (davis et al., 1966; nimis 1995; nimis et al., 1999; putirka et al., 2003), herein, we used the clinopyroxene–liquid geothermometer (putirka et al. 2003) to estimate the crystallization conditions (temperature and pressure) of clinopyroxene in the wuhaolai complex. the principle of this method is that the jadeite (jd; naalsi2o6) content in clinopyroxene is sensitive to the pressure and temperature in the pyroxene–melt exchange reaction (putirka et al., 2003). before calculating the crystallization temperature and the pressure, 0 1,4 1,2 1 0,8 0,2 n a2 o (w t. % ) a l 2o 3( w t.% ) a b 0,6 0,4 0 0,35 0,3 0,25 0,2 0,05 0,15 0,1 1 2 76543 8 1 2 76543 8 0,45 0,4 0 0,14 0,12 1 0,08 0,02 0,06 0,04 m g# 0,81 0,8 0,78 0,76 0,78 0,77 tio 2 (w t.% ) 0,6 1,3 1,2 1 1,4 0,7 n a2 o (w t. % ) a l 2o 3( w t.% ) 0,9 0,8 0,39 0,37 0,35 0,33 0,27 0,31 0,29 1 2 76543 1 2 76543 8 0,43 0,4 0 0,25 0,2 0,05 0,15 0,1 m g# 0,79 0,785 0,78 0,765 0,775 0,77 tio 2 (w t.% ) 1,5 1,6 0,795 0,8 0,25 1,1 1 8 1 8 100 m100 m figure 7. backscattered electronic images of clinopyroxene crystals from representative samples of the wuhaolai mafic complex and their compositional profiles. (a) pyroxenite sample; (b) gabbro sample. figure 8. mineral–melt fe/mg equilibrium diagram for clinopyroxene (after kim et al., 2016). 142 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran ti o 2( w t.% ) a i/ si a b 3 2 1 0 c m pyroxenite gabbro al2o3(wt.%) 0 1412108642 16 diorite intermediate-basic magmatic hornblende metamorphosed hornblende mg/(fe2++fe3+alvi) 654321 0,7 0,5 0,3 0,0 0,6 0,4 0,2 0,1 figure 9. (a) al2o3–tio2 diagram for hornblende compositions (after jiang et al., 1984); (b) mg/(fe3+ + fe2+ + alvi)–al/si diagram of hornblende (after xue et al., 1986). c: crustal source, m: mantle source. a liv +a lv i a lv i + fe 2+ +2 ti +a s ite 3,0 2,5 0,5 0,8 aliv 0,00 2,001,751,501,251,000,750,25 0,00 alvi 2,001,751,501,250,750,25 4,0 2,5 0,05 3,5 0,5 0,20 0,15 0,25 0,00 3,0 0,30 0,10 1,0 2,0 1,5 2,0 1,5 1,0 0,2 0,0 0,6 0,4 0,50 1,000,50 ti a s ite pyroxenite gabbro diorite figure 10. hornblende compositional variation diagrams. 143mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the wuhaolai mafic complex in northern north china craton, inner mongolia we should verify whether clinopyroxene and its host rocks have reached an equilibrium state (chen et al., 2015). experimental results indicated that when clinopyroxene and its host rocks reached the equilibrium state, the kdcpx values were between 0.2 and 0.4 (irving et al., 1984; liotard et al., 1988; kinzler et al., 1997; hunter et al., 1997). in this research, the kdcpx values are in the range 0.23–0.27, indicating that clinopyroxene and the melt have reached the equilibrium state. this is also supported by the normal zonal texture of clinopyroxene. thus, based on the study conducted by putirka et al. (2003), the crystallization temperature of clinopyroxene in pyroxenite ranged from 1426°c to1434°c and the crystallization pressure ranged from 1.1 to 1.7 gpa. the crystallization temperature of clinopyroxene in gabbro was 1382°c–1389°c, whereas the crystallization pressure was in the range 0.5–1.3 gpa. therefore, the crystallization temperature and pressure for pyroxenite were slightly higher than those for gabbro, suggesting that the formation depth of gabbro was slightly shallower than the pyroxenite. tectonic significance due to the subduction of the pao, various types of late paleozoic plutons were widely distributed in the northern margin of the ncc. however, wulan granodiorites (323.4 ± 3.4 ma) (zhang et al., 2011), wengeng gabbro (280 ma) (zhao et al., 2008), shihahe biotite monzonitic granite (275.0 ± 0.7 ma) (hao et al., 2016), and ondor sum–tieshagai intermediate–felsic intrusions (271 ± 2.4 ma) (wang et al., 2013) were formed under an active continental margin tectonic setting. the kebu pluton (291 ± 4 ma) (luo et al., 2007), the beiqigetao mafic pluton (269 ± 8 ma) (zhao et al., 2011), and the tugurige hornblendite (273.5 ± 1.3 ma) (wang et al., 2016) exhibited the characteristics of the postcollisional extension environment. thus, the pao finally closed at the early– middle permian. this was further supported by the paleontological results of the cathaysian flora of the early–middle permian stratum in the southern margin of the siberian plate (zhou et al., 2010). besides, in the tio2–mno– na2o diagram of nisbet et al. (1977) (fig. 12), clinopyroxenes from pyroxenite and gabbro of the wuhaolai mafic complex mainly belong to the within-plate alkali basalt (wpa) field. therefore, combining the geochemical characteristics of the unpublished zircon u–pb data (265 ma), the wuhaolai complex was formed in the within-plate tectonic setting in the middle permian. combining all these results with the chemical characteristics of the studied clinopyroxene and hornblende crystals in pyroxenite and gabbro samples from the wuhaolai 6 2 1 sio2 (wt%) 49484645444341 0,00 sio2 (wt%) 2,001,751,501,500,750,25 5 4 3 0 42 1,000,50 pyroxenite gabbro diorite a b s-type 47 5040 n a 2o (w t% ) ti o 2( w t% ) i-type s-type 1,25 1,75 2 1 5 4 3 0 figure 11. sio2 versus na2o and tio2 diagrams for hornblende (after coltorti et al., 2007). tio2 w pa mno na2o all vab+w pa+w ptvab+w pa wpa vab ofb pyroxenite gabbro figure 12. tio2–mno–na2o diagram for clinopyroxene composition of pyroxenite and gabbro samples from the wuhaolai mafic complex (after nisbet et al., 1977). wpt: within-plate tholeiitic; wpa: within-plate alkali basalt; vab: volcanic arc basalt; ofb: ocean floor basalt. mafic complex, it can be drawn that the magma source of this complex was modified by fluids or melts derived from the subducted slab in the course of the pao subduction under the northern ncc margin (zhao et al., 2011; tang et al., 2014., wang et al., 2016). therefore, it can be conjectured that the parental magma of the wuhaolai complex may derived from partial melting of an enriched lithospheric mantle source after the pao closure (zhao et al., 2011, hou et al., 2014, zhang et al., 2011). the clinopyroxene and the hornblende in the studied rocks were produced along with the fractional crystallization of the parental magma of the wuhaolai complex. 144 wang chen, liu jianchao, zhang haidong, ge jiakun, xi zhixuan, wang haoran conclusions the conclusions of our study are summarized as follows: 1. the parental magma of the wuhaolai mafic complex belongs to the intra-plate alkaline series. the feature of the normal chemical zoning and the other chemical characteristics documented for clinopyroxene in the studied samples of pyroxenite and gabbro from the wuhaolai mafic complex are attributed to the crystallization process of the same parental magma. additionally, the chemical characteristics of hornblende (s-hl) in the studied samples suggest that the magma source may have been modified by the fluid or melt derived from the subducted slab. 2. the fe–mg exchange partition coefficient kdcpx calculated for the wuhaolai mafic complex range from 0.23 to 0.27. furthermore, the crystallization temperature and pressure calculated for the gabbro unit are lower than those of pyroxenite, suggesting that the formation depth of gabbro was slightly shallower than that of pyroxenite. 3. based on the characteristics of the rock-forming minerals, the wuhaolai mafic complex is interpreted as formed in a within-plate tectonic setting, where clinopyroxene with normal zoning feature and hornblende were crystallized resembling some chemical imprints related with subduction fluids. acknowledgements we appreciate k.y. bai, m.w. liu, l.l. xu, and x.j. tan for providing support during the investigation conducted using electron microprobes. we are also grateful to the anonymous reviewers and editors for their meaningful and valuable comments that greatly improved the manuscript. this work was supported by geological investigation work project of china geological survey (12120115069701) and fundamental research funds for the central universities (no. 300102278402). references arai, s., abe, n., & hirai, h. 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(2013). did the paleo-asian ocean between north china block and mongolia block exist during the late paleozoic? first paleomagnetic evidence from central-eastern inner mongolia. journal of geophysical research-solid earth, 118(5), 1873-1894. zou, j. x., liu, x. f., deng, j. h., ren, k. f., li, c. h., huang, y. p., ..., & yi, l. w. (2013). research of the clinopyroxene zoning phenomenon from the basic-ultrabasic rocks in jiulong area, shangri-la county, yunnan province, china (in chinese with english abstract). acta mineralogy sinica, 33(2), 210–220. keywords: uncertainty, probability of failure, rock wedge, random sets, reliability. palabras clave: incertidumbre, probabilidad de falla, cuña de roca, conjuntos aleatorios, confiabilidad. how to cite item hernandez-carrillo, r. & beltran, g. (2019). updating the probability of failure of rock wedges. earth sciences research journal, 23(3), 225-236. doi: https://doi.org/10.15446/ esrj.v23n3.74779 in hard rock masses, discontinuities control the slope stability, rather than block matrix breakage. the relative position and orientation of joints and slope face define the most likely mechanisms of failure. among these mechanisms, the wedge failure is one of the most common modes of failure in which joint sets dip and dip direction, slope geometry and direction, external forces (including water pressure and earthquake) and rock and joints mechanical properties control the stability. the determination of these input parameters is not straightforward, mainly due to their variability and the limited amount of information available. besides, in most projects, input parameters come from different sources (e.g., expert opinion, back-calculation, laboratory tests, field test or different project stages). therefore, this limited information from different sources should be appropriately incorporated into the stability analysis to assist the design and decision-making process. in this context, random sets arise as a powerful tool to combine different sources of information and to perform a reliability assessment under limited information. this feature makes it possible to update the probability of failure as new evidence is available. with this framework, this paper presents a reliability assessment of wedge stability in a rock slope of a sandstone quarry, located in une cundinamarca, where information on mechanical and geometrical parameters has been collected for 20 years. this approach allowed to update the probability of failure. as a result, an updated probability of failure raging between 0.04 and 0.25 was calculated. abstract updating the probability of failure of rock wedges actualización de la probabilidad de falla en cuñas de roca issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.74779 en rocas duras, las discontinuidades controlan la estabilidad de los taludes. la posición relativa entre las discontinuidades y el talud define el mecanismo de falla más probable. entre estos mecanismos, la falla en cuña es uno de los más comunes. la estabilidad de las cuñas está controlada por la orientación de los planos de discontinuidad, la geometría y orientación del talud, las fuerzas externas (incluyendo presión de agua y sismo) y las propiedades mecánicas de las discontinuidades. la determinación de estas propiedades no es sencilla, debido a su variabilidad y cantidad limitada de información. además, en muchos proyectos de ingeniería la información proviene de diferentes fuentes, tales como la opinión de expertos, retro-análisis, ensayos de laboratorio y pruebas de campo, en diferentes etapas de proyectos. por lo tanto, los modelos de análisis deben ser capaces de incorporan información con estas características, para asistir el proceso de toma de decisiones en los proyectos de ingeniería. en este contexto, la teoría de los conjuntos aleatorios se constituye en una herramienta apropiada para combinar diferentes fuentes de información y efectuar análisis de confiabilidad cuando se tiene información escasa de los parámetros de entrada del modelo. esta característica hace posible la actualización de la probabilidad de falla, a medida que aparece nueva información. con este marco de referencia, este trabajo presenta el análisis de confiabilidad de la estabilidad de cuñas en una mina de arenisca, localizada en une, cundinamarca, donde se ha recolectado información durante 20 años. este análisis permitió actualizar la probabilidad de falla. como resultado se obtuvo una probabilidad de falla en un rango entre 0.04 y 0.25. resumen record manuscript received: 08/09/2018 accepted for publication: 05/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 225-236 rodrigo hernandez-carrillo1*, gloria beltrán2 1universidad nacional de colombia y universidad pontificia bolivariana 2universidad nacional de colombia * corresponding author: rhernandezca@unal.edu.co ge ot ec hn ic al e ng in ee ri ng 226 rodrigo hernandez-carrillo, gloria beltrán introduction wedge failure is a kinematically controlled mechanism of failure, in which the relative position of joint and slope planes controls the stability. this mechanism is widespread in both highway and open pit excavations. limit equilibrium (le) is a suitable approach to calculate the stability of potentially unstable rock wedges. le provides a closed form solution to well-defined wedge geometry to calculate the factor of safety, which is convenient when performing a reliability assessment. although le provides a practical solution to the problem of wedge stability, the inputs determination is not straightforward, since rock mass is a heterogeneous material, in which properties change from point to point. this variability introduces uncertainty to the model. moreover, in mining projects, new and variable information is available, as the exploitation progresses. considering that the information is variable, different results are obtained at different stages of the project operation. hence, in this sort of projects, the decision-making process should be a dynamic task, assisted by geotechnical models able to incorporate new information and to update the results. there are several alternatives to cope with uncertainty. for engineers, the most popular alternative is the purely probabilistic approach, with some documented drawbacks; the actual meaning of the probability of failure and the need for information, which is not available in most geotechnical projects (oberguggenberger, 2012). therefore, alternative and more flexible approaches have been developed to deal with uncertainty under limited information (couso, dubois, & sánchez, 2014). these alternative approaches are based on interval analysis and correspond to imprecise probabilities techniques. among these techniques, the random sets theory (rst) is a suitable tool to perform reliability assessment in geotechnical problems, including the input parameters as intervals (oberguggenberger, 2012). rst, also, offers a systematic way to combine different pieces of evidence and adjust the reliability assessment results, at different stages of the project. this combination is performed according to the mixing (or averaging) of evidence rule, under the evidence theory framework (sentz & ferson, 2002). the concept of rst as presented in this document corresponds to the interpretation of dempster (1967) of the belief function as the lower probabilities induced by a multivalued mapping. according to this approach, a multivalued mapping from a probability distribution generates a set-valued random variable, that is a more complex object than the standard random variable. the original work developed by dempster was reinterpreted and generalized by shafer (1976) to develop their evidence theory, which building block is the probability distribution on the power set of a finite set (couso et al., 2014), i.e., a random set. although before shafer’s (1976) work there were no practical applications of rst, since 1990’s there has been a geometrical growth on the number dst applications (beynon, curry, & morgan, 2000). some examples of the most recent publications on the topic include measuring uncertainty in big data (dutta, 2018), multisensor-based activity recognition in smart homes (al machot, mayr, & ranasinghe, 2018), skin diseases (khairina, hatta, rustam, & maharani, 2018), cancer detection (kim et al., 2018), fault in power transformers (kari et al., 2018), heritage evaluation (liu, zhao, & yang, 2018), thermal plants monitoring (moradi, chaibakhsh, & ramezani, 2018) and chemical risk assessment (rathman, yang, & zhou, 2018). these are just a few examples to illustrate how relevant is this approach to handle decision making under uncertainty. the number of publications on the geotechnical is not comparable to other fields like ai or medicine. however, there is a growing interest of the geotechnical community in involving the evidence theory in the decisionmaking process. the first application attempted to predict the rock mass response around a tunnel excavation (tonon, mammino, & bernardini, 1996). likewise, tonon, & bernardini (1999) pursued the problem of optimization of the lining using rst and fuzzy sets. subsequently, tonon, bernardini, & mammino (2000) presented an application to compute rock mass parameters (rmr) of rock masses. moreover, tonon, bernardini, & mammino (2000b) performed plane failure analysis and tunnel lining design with simple explicit models using rst and monte carlo simulation. they also showed the advantages of using the concept of strictly monotonic functions to reduce the number of computations, which reduces the computational cost of this approach. a framework to develop a reliability assessment of geotechnical problems by finite element methods and rst was developed by peschl (2004). this work presents a comprehensive methodology to perform reliability assessment, in which inputs are expressed as random sets, leading to a bounded probability function for the finite element model results, e.g., displacements. this application was named random sets finite element method, rs fem. subsequently, related documents have been published with applications of the rs fem (schweiger & peschl, 2005; h. f. schweiger & peschl, 2004, 2005). besides, this method has been applied to the analysis of tunnel excavations (nasekhian & schweiger, 2011). klapperich, rafig, & wu, (2012) and, shen, & abbas (2013) combined rst with the distinct element method to assess the stability of rock slopes. in this case, the deterministic reference model is run using the discrete element software udec (cundall, 1980), according to the realizations defined by the combinations of the input random sets. with the results, cumulative probability distributions were computed. in this context, this paper presents the rst as an alternative to deal with uncertainty, under limited information. the approach is applied to evaluate the stability of wedges in a sandstone quarry located in une, cundinamarca, colombia, that has been under operation for 20 years. thus, as new information is available, the analysis is updated. the first part of the paper presents a description of the wedge le model selected. subsequently, a brief description of rst is offered, along with an approach to perform sensitivity analysis and the combination of evidence. a practical example at a sandstone mine in une, cundinamarca, colombia is presented using this approach. lastly, based on the results, conclusions are drawn. this research aims at contributing to the process of rock slope stability analysis by considering one mechanism of failure (wedge failure). nevertheless, the design of a rock slope is a very complex task, in which several mechanisms of failure and triggering factors are involved. hence, additional work on random sets applied to rock slope stability is required. wedge stability model the geometry of the wedge analyzed in this project is defined by four planes; two discontinuities and two free surfaces (slope face and upper slope) as shown in fig. 1(a). tension cracks are not explicitly considered. the stability assessment of wedges implies the formulation of equilibrium of forces in a three-dimensional space. this problem has been solved by using stereographic projections (goodman & taylor, 1966; hoek, bray, & boyd, 1973; and goodman, 1995). however, when a reliability assessment is performed, a closed-form solution is more suitable because several realizations of the model should be computed. low (1979) developed a closed-form equation without using graphical assistance; subsequently, einstein & low (1992) extended this solution considering an inclined upper slope. the approach has been applied to perform wedge failure reliability assessment (einstein & low, 1992; jimenezrodriguez, sitar, & chacón, 2006; low, 1997; low & einstein, 2013). the reliability of wedges has also been studied considering multiple correlated failure modes (li & zhou, 2009; li et al. 2009) and knowledge-based clustered partitioning approach (lee et al., 2012). the solution proposed by low (1979) was selected to account for the uncertainty in this work since it is convenient to perform several realizations of the model. a brief description of this proposal is given below. the model can analyze four modes of failure in this sort of wedges: failure along the intersection of both joint planes, failure along either joint plane 1 or plane 2 (the planes order selection is arbitrary), and lifting failure. the factor of safety (fs) for this mode can be calculated according to eq. (1) (low, 1997): fs a b g s tan a b g s tan b c h bw w= −       ∅ + −       ∅ + +1 1 1 2 2 2 1 1 23 3    * * cc h 2  (1) this equation is valid only when: a b g s w 1 1 0−       >  and a b g s w 2 2 0−       >  227updating the probability of failure of rock wedges f m aii x li * = ( ) ≤∑ : (7) in other words, to obtain the left envelope (upper bound), the distribution function of each interval in the calculation matrix is assumed to be concentrated at the lower bound of each focal element. on the other hand, for the right envelope (lower bound) the probability mass for each interval is assumed to be concentrated at the upper bound of the interval (schweiger & peschl, 2005). the bounded distribution function defined by the upper and lower bounds is referred to as bdf. the following example illustrates the concept of a random set. let assume a layer of sandstone, from which four block samples were taken. from each block, two cores were taken and tested to measure the uniaxial compressive strength, ucs. due to local regulations, two different laboratories tested the samples. however, design engineers have more confidence in the results from laboratory 1, because it provides certified testing services, while the second does not. table 1 summarizes the test results. table 1. results from ucs tests ucs1 (mpa) ucs2 (mpa) laboratory 1* block 1 53.2 57.9 block 2 49.3 51.8 block 3 54.3 55.9 laboratory 2 block 4 60.1 66.3 *certified laboratory based on their experience and judgment, the design team defined a random set for the sandstone uniaxial compressive strength, ucs, like the one shown in table 2. such a random set has four focal elements, ai, defined by the strength intervals resulting from each sample. the set of these intervals forms the support of the random set. moreover, each focal element has a probability attached, i.e., the probability assignment, m. the probability was assigned based on the reliance of engineers on each laboratory. in this example, the results from each block define the random set. however, the selection of the focal elements and probability assignments should be the result of sound engineering judgment. table 2. random set defined for the ucs of the sandstone focal element interval bounds probability assignmentucs, mpa 1 53.2 57.9 0.3 2 49.3 51.8 0.3 3 54.3 55.9 0.3 4 60.1 66.3 0.1 in which: ø1 and ø2 are the friction angles of planes 1 and 2, respectively c1 and c2 correspond to the cohesion of planes 1 and 2, respectively gw is the water pressure coefficient of a pyramidal distribution, as shown in fig. 1(b) α is the slope face inclination h is the slope height, and hb is the total height, including the upper slope with inclination ω a1, a2, b1 and b2 are geometric coefficients; a function of the following angles (for a detailed description see (low, 1997)) β1 is the horizontal angle between the strike line of plane 1 and the line of intersection between the upper ground surface and the slope face. idem for β2 in plane 2 (jimenez-rodriguez, sitar & chacon, 2006) δ1 and δ2 are the dip angle of plane 1 and 2 respectively ε is the inclination (plunge) of the line of intersection between planes sᵞ is the specific weight of the rock random sets theory definition to define a random set, suppose that m observations were made of a parameter u  u, each of which resulted in an imprecise (non-specific) measurement given by a set a of values. let ci denote the number of occurrences of the set ai  u, and φ(u) the set of subsets of u. a frequency function m can be defined such that (f. tonon et al., 2000a):  = = …{ }a nii : , ,1 (2) m u: ,( ) →  0 1 (3)  u( ) = 0 (4) m a a uii n ( ) = ∀ ∈ =∑ 1 1 , (5) φ is called the support of the random set, the subsets ai are the focal elements and m is the basic probability assignment. a pair (φ, m) defines a random set. each set, ai  φ, contains possible values of the variable, u, and m(a) is the probability associated with a (h. f. f. schweiger & peschl, 2005). when working in the space of real numbers, the support of the random set, φ, is given by intervals, with their corresponding probability assignment. in this paper, the uncertain parameters are considered as intervals (rock mechanical properties and joints geometry), without any specific information related to the distribution or variation between the extremes of the interval, along with the probability assignment. accordingly, if the focal element ai is a closed interval of real numbers expressed as: a x x l ui i i= ∈ { }| , , the lower f*( ) and the upper f*( ) cumulative probability distribution functions (probability function) can be computed as follows (peschl, 2004): f m aii x ui * : = ( ) ≥∑ (6) (a) tetrahedral wedge model (jimenez-rodriguez et al., 2006) (b) pyramidal water pressure distribution figure 1. wedge model selected for stability assessment 228 rodrigo hernandez-carrillo, gloria beltrán as for the probability assignment of the output, if a1, …, an are sets on x1 × … × xn, respectively, and x1, …, xn are random independent sets, then the joint probability assignment is measured by: m a a m a a a rn i n i i n1 1 1×…×( ) = ( ) ×…× ∈ =∏ , (13) the bottom line is that rst allows involving the inputs into the problem as random sets, through a function or a model (in this case, the wedge slope factor of safety function, eq. (1)) to obtain a bounded distribution function (bdf) of the response. a diagram of that process is depicted in fig. 3. a complete description of the theoretical background of rst and its applications can be found in bernardini and tonon (2010). combination of random sets according to the concepts just presented, rst offers an alternative to performing reliability assessment concerning intervals and probabilities, when limited information is available. this approach also offers an alternative to systematically include new information to adjust or update the cumulative probability functions. the problem of combining several pieces of information is addressed by dempster-shafer theory of evidence (dst) (shafer, 1976) and is suitable to consider the limited information when it has both epistemic and aleatory uncertainty (sentz & ferson, 2002), even when expert opinion is involved (torkzadeh-mahani et al., 2018). this is the case that is assessed in this paper, which deals with information from different sources, at different stages of the project (pieces of information). the primary goal of aggregation or combination of information is simplifying or summarizing information that comes from several sources into one set of evidence (sentz & ferson, 2002). in a dst framework, each piece of information is expressed as a random set and then combined with other pieces of information by redefining the focal elements and allocating different probability assignments. there are several rules to combine information, which differentiate on the way to allocate the probability assignment. for a detailed description of evidence combination rules under dst see sentz & ferson (2002) and some examples applied to engineering problems can be found in zargar et al. (2012) with the random set, the bounded probability distribution function (bdf) for the ucs of the sandstone can be constructed according to eqs. 6 and 7. fig. 2 shows the bdf, where the original focal elements bounds are explicitly marked. here, the orange and green lines represent the lower (right) and upper (left) bounds of the ucs bdf, respectively. function of random sets once the random sets of input parameters are defined, three main tasks should be performed to compute a bounded distribution function (bdf) of the assessed system. firstly, the image (output) of the input random sets through a function f has to be established. subsequently, a probability assignment should be assigned to the computed image. finally, a bounded cumulative distribution function of the output is calculated, according to eqs. 6 and 7. below, a brief description of these steps is presented. assume a random set (r, ρ), which is the image of (t, m) through a function f. the set (r, ρ) is described as follows (schweiger & peschl, 2005): r r f a a tj i i= = ( ) ∈{ },� � � (8) f a f x x ai i( ) = ( ) ∈{ },� (9)  r m aj iai rj f ai ( ) = ( ) = ( )∑ : (10) assuming that the function f(ai) is continuous in all ai  t, and no extreme points exist in this region, except at the vertices, the vertex method applies to calculate the image (r, ρ) of the input random set through the function f (schweiger & peschl, 2005). assume each focal element ai is an n-dimensional box, whose 2n vertices are indicated as vk, k = 1,… ,2n. if the vertex method applies then, the lower and upper bounds rj* and rj* on each element rj  r will be located at one of the vertices (schweiger & peschl, 2005). r f v kj k n * min : , ,= ( ) = …{ }1 2 (11) r f v kj k n* max : , ,= ( ) = …{ }1 2 (12) figure 2. bounded probability function built from ucs tests 229updating the probability of failure of rock wedges for this paper, the mixing or averaging method was selected, since this technique generalizes the averaging operation frequently used for probability distributions, which is regarded as a natural method of aggregating probability distributions. hence, it is reasonable to consider this approach to average random sets. this combination rule has been applied to geotechnical problems including the finite elements method (peschl, 2004). a brief description of this rule is given below. let suppose a rock slope excavated in different stages, before getting the final configuration. at each stage, information on joints geometry and joint mechanical properties are collected. therefore, the information of any parameter z is described by a random set (e.g., the friction angles are described by intervals with their corresponding probability assignment). suppose there are n stages, so the variable is described by n alternative random sets, each one corresponding to an independent source of information. the method modifies the probability assignments, mi, as follows: m a n w m an i ii n 1 1 1 … = ( ) = ( )∑* (14) in which wi are weights assigned according to the reliability of the sources and mi* is the probability assigned to the combined focal element. sensitivity analysis to compute the distribution functions, selection of the most influencing variables is required. these variables are modeled as random sets, while the others as deterministic, which is a crucial step since the number of random sets defines the number of realizations of the model. for instance, if there are n random variables, each one with k focal elements, the number r of realizations of the model would be: r kn n= 2 * (15) the sensitivity analysis also allows knowing the variation of the response of the model with the different variables. with this information, the proper combination of input variables can be defined at each n-dimensional box to define the maximum and minimum results of the model, without computing every combination at that box. hence, the number of computations reduces to: r kn= 2* (16) to perform reliability assessment in geotechnical problems applying the concept of rst, peschl (2004) adopted a methodology based on a central difference approach, in which a sensitivity ratio (ηsr) is computed (epa, 2002). sr l r l r f x f x f x x x x = ( ) − ( ) ( ) − , , (17) where x is the reference value, xl,r and f(x) and f(xl,r ) are the outputs of the functions at those points. the sensitivity ratio is local, varying xl a small amount from the reference value, and general changing xr across the whole range. therefore, if there are n variables, ηsr should be computed 4n+1 times. the sensitivity ratio is normalized according to eq. (18), then a total relative sensitivity index (α(xi )) is computed for each input variable. the last latter index varies between 0 and 1 and measures the influence of a given variable in the function. the higher α(xi ), the more the influence of the variable in the output.  ss sr r rx minx x = −( ) * max (18) α η η xi ss i i n ss i ( ) = ∑ ∑ =∑ , ,1 (19) figure 3. overview of rst computation process 230 rodrigo hernandez-carrillo, gloria beltrán the sensitivity analysis also provides a few realizations of the model, within a representative interval. hence, it is possible establishing the local variation of the function for each input variable. in other words, the sensitivity analysis allows to establish if the model is increasing or decreasing concerning a variable, within an interval, which reduces the number of computations to define the upper and lower bounds for combinations of focal elements. example analysis the concepts just presented related to wedge failure and rst were applied to a sandstone mine situated in une, cundinamarca, colombia. this mine is in a sedimentary rock mass, formed mainly by alternating layers of sandstone and shales. the mine has been operated since 1990ies until nowadays. fig. 4(a) shows the surface geology of the mine and the points where planes orientation were collected in 2011 (blue) and 2016 (black). the descriptions of the lithologies is included in fig. 5. besides, fig. 4(b) presents a picture of the rock mass, in which the joint and bedding planes are shown on a stable rock slope. during its operation, a detailed geological and geotechnical monitoring has been carried out at the mine slopes. as a result, a complete data set on joints dip and dip direction is available. besides, a few results of mechanical tests like unconfined compression strength (ucs) and shear behavior along discontinuities were collected. the first information was collected in 1997 when the operation started. then in 2011 a detailed geotechnical assessment to update the mine design was carried out, which included a back-calculation of a rock slope failure reported in 2000. finally, as part of a research project, new information was collected in 2016. table 3 shows the results of strength parameters along discontinuities in shale layers. in regarding these results, some values of cohesion were measured in the laboratory. as for the samples tested in 2016, the cohesion is apparent and associated with the texture of the joint, rather than any filling within the joint. no information is provided related to cohesion from samples collected in 1997 and 2011. table 4 summarizes joint planes dip and dip direction and slope geometry. sources of information listed in tables 3 and 4 are assumed to be independent. in this case, this assumption is reasonable, since the information was collected at different times, by different groups of people and with different tools. (a) points in blue and black show the location where orientations were collected in 2011 and 2016, respectively (b) joint and bedding planes considered for the wedge model on a stable rock slope face figure 4. el pedregal mine surface geological features and general view of the rock mass figure 5. description of lithological sequence reported in el pedregal mine 231updating the probability of failure of rock wedges β β θ u l, ,= ± 40 90 2 (21) where, αu,l is the upper or lower dip direction α is the current mean dip direction βu,l is the upper or lower dip β is the current mean dip θ40,90 angle of the probability region (either for 90% or 40% of the probability) subsequently, the factor of safety was computed for each piece of evidence for the combinations defined by the focal elements of each random set. regarding this, it is worth to highlight that: • there are three random sets, one for each uncertain variable (i.e., bedding dip, dip direction, and friction angle). each combination of input focal elements defines a 3d-box, in which the factor of safety should be computed 8 times, to define the bounds of the factor of safety (fs*i, fs*i) linked to that specific focal element combination, according to eqs. 11 and 12. table 3. mechanical properties measured on rock joints year sample peak cohesion (kpa) residual cohesion (kpa) peak friction angle (°) residual friction angle (°) unit weight (kn/m3) 1997 1 32.5 2.9 22.5 22.0 26.2 8.2 29.5 2 26.3 17.6 23.6 7.8 23.6 2011 1* 18.0 18.0 26.0 6.0 20.0 1 87.3 39.6 29.2 23.9 24.3 2 89.2 25.5 34.8 32.8 23.6 3 17.6 40.4 36.6 23.9 2016 1 48.0 31.8 24.2 2 69.0 22.0 24.8 3 97.0 18.5 24.8 *from the back analysis table 4. joint plane and slope mean parameters plane dip direction (°) dip (°) height (m) bedding 306 26 joint set 1 57 82 slope 329 70 25 once the input information is defined, the steps followed to perform an update the probability of failure is depicted in fig. 6. at this point, it is important to highlight that two types of information can be distinguished. firstly, there are just ten results on strength parameters of the rock masses, collected for 20 years, while there are 967 poles measured during the same period. the wedge failure model selected for this paper has been developed for wedges delimited by two joint sets, the slope face and the upper slope. based on this information, the model has 13 variables, including geomechanical and geometrical variables. results with the slope model and input variables fully established, the next step is to define the most influencing variables on the wedge response, which is accomplished by the sensitivity analysis described in section 3.3. therefore, the factor of safety was computed with eq. (1), and the weighted sensitivity indexes, αi, were computed and plotted in fig. 7(a). the three variables with the highest αi were selected as random sets and the rest as deterministic. hence, the random sets are the dip and dip direction of the bedding, as well as, its friction angle. next step is defining the input random sets. in regarding friction angle, assumptions are required because few data are available. hence, for the information collected in 1997 and 2011, the random sets were defined considering that the actual value of the friction lies somewhere between the peak and the residual friction. these values are conservative compared with the focal elements selected for 2016, in which only peak friction angle was considered, as there are no residual values reported. table 5 shows the selected random sets. conversely, vector information on joints orientation and dip were preprocessed to be expressed as intervals. considering that these data are vectors, they were assumed to fit a fisher distribution. this distribution assumes that a population of poles is distributed around a true orientation (fisher, 1953). then, angles for confidence regions of 40% and 90% around the true orientation were calculated based on the measured orientations. afterward, new interval bounds were defined as follows (see table 6): α α θ u l, ,= ± 40 90 2 (20) figure 6. the methodology applied to compute and update the probability of failure 232 rodrigo hernandez-carrillo, gloria beltrán • the number of computations of each box reduces from 8 to 2, as the sensitivity results explicitly showed the trends of the factor of safety, as each variable changes. therefore, fs*i is obtained by combining the highest friction, along with the lowest dip and dip direction; and fs*i by including the lowest friction angle, and the highest dip and dip direction. the influence of bedding dip direction on wedge stability is shown in fig. 7(b). the next step is combining different sets of evidence to update the bounded probability of failure computed according to random sets. the pieces of evidence were combined as follows: • the mixing or averaging rule was applied • at each period, the information was combined sequentially • then, the information at each period was combined until all sources of information were aggregated (a) relative sensitivity index for rock wedge failure (b) effect of bedding plane dip direction on the factor of safety figure 7. sensitivity analysis results figure 8. bedding dip. aggregation of 1997 information by mixing rule table 5. input random sets on bedding plane friction angle year bedding plane friction angle (º) lower upper pa* 1997 22.0 22.5 0.33 22.0 26.3 0.33 17.5 23.6 0.34 2011 23.9 29.2 0.33 32.8 34.8 0.33 36.6 40.4 0.34 2016 22.0 31.8 0.50 18.5 22.0 0.50 *probability assignment figure 8 shows a sample of the bounded cumulative density function (bdf) associated with the bedding dip collected in 1997. from that, it can be concluded that the information is very conflicting since source 1 yields dip measurements up to three times higher than those reported by sources 2 and 3. the aggregated information combined by the mixing rule (plotted in black) re-allocates the probability assignment of the original focal elements. as a result, a more robust (higher number of focal elements) set of inputs is available. however, this combination rule does not account for the conflicting information; it averages the original probability assignments. figure 9 presents a step forward in the evidence combination (updating). here again, the bedding dip is depicted, but in this case, the updating sequence is shown. first, in black is the already 1997 combined information. then, in green the information from 1997 aggregated with the data collected in 2011 is shown. finally, in red is the latter set, now updated with the evidence collected in 2016. with this last robust random set, the updated wedge factor of safety can be computed. regarding the friction angle, the amount of information is much smaller, since there is only one piece of information at each period (see table 5). because of this, a smaller updated random input set is obtained as can be seen in fig. 10. 233updating the probability of failure of rock wedges figure 11 depicts the bdf for the wedge factor of safety computed with the evidence collected in 1997 first, results with the information considered separately are plotted in red, green and magenta for the sources 1, 2 and 3, respectively. these curves reflect the conflict linked to the input parameters, since the factor of safety is highly variable, depending on the considered source of information. the lowest factors of safety are obtained from the first source 1997-1 (red) that has the highest dip and dip direction measurements. on the contrary, the highest factor of safety comes from the piece of evidence 19972 (green), which combines the lowest dip with the lowest bedding dip direction. considering the evidence collected in 1997 separately might lead to a biased conclusion on the actual stability condition of the rock wedge. if the information first collected (1997-1) is involved, a conservative conclusion is drawn, while 1997-2 source could be riskier. this issue justifies the need for combining information, to get a more reliable representation of the actual stability condition. in figure 11, the black curves represent the factor of safety obtained when the combined input random sets are considered. these lines balance the biased information provided by independent sources and a result in between the most conservative and the riskiest bdf is obtained. this sort of response is directly table 6. input random sets on bedding dip and dip direction year id bedding dip direction (°) dip (°) lower upper pa lower upper pa 1997 1 313 318 0.25 27 32 0.25 318 323 0.25 32 37 0.25 323 328 0.25 37 41 0.25 328 333 0.25 41 47 0.25 2 273 281 0.25 6 13 0.25 281 293 0.25 13 25 0.25 293 305 0.25 25 37 0.25 305 312 0.25 37 45 0.25 3 286 296 0.25 5 15 0.25 296 305 0.25 15 23 0.25 305 313 0.25 23 32 0.25 313 323 0.25 32 42 0.25 4 300 305 0.25 11 16 0.25 305 312 0.25 16 23 0.25 312 319 0.25 23 29 0.25 319 324 0.25 29 35 0.25 2011 1 295 300 0.25 10 16 0.25 300 307 0.25 16 22 0.25 307 314 0.25 22 29 0.25 314 319 0.25 29 34 0.25 2 281 286 0.25 13 18 0.25 286 293 0.25 18 24 0.25 293 299 0.25 24 31 0.25 299 304 0.25 31 38 0.25 2016 1 288 293 0.25 18 20 0.25 293 299 0.25 20 22 0.25 299 304 0.25 22 24 0.25 301 310 0.25 24 26 0.25 2 313 318 0.25 27 32 0.25 318 323 0.25 32 37 0.25 323 328 0.25 37 41 0.25 328 333 0.25 41 47 0.25 figure 9. bedding dip. step by step evidence combination it is important to clarify that as new information is included, the number of input random sets increases. hence, the number of computations required to define the probability of failure also increases. as an example, for the first piece of information collected in 1997, there were three input random sets, each one with 3 or 4 focal elements, which brings 384 computations of the factor of safety. this number is reduced to 96 since the sensitivity analysis helped to clarify the variation of the factor of safety with the input random variables. when all the evidence is combined, 50176 computations are required, which are reduced to 12544. this difference in the number of computations does have a relevant influence when more complex computation techniques are involved (e.g., finite elements or discrete elements) since the linked computational cost will constraint the number of computations of the model. the probability of failure is computed from the bounded distribution function defined for each assessed scenario, as the number of realizations that threw a factor of safety lower than 1.0 figure 10. bedding friction angle. aggregation of information, only one source per period 234 rodrigo hernandez-carrillo, gloria beltrán linked to the combination rule utilized to aggregate (update) the pieces of evidence, which basically averages the probability assignments of the original random sets. the averaging effect of the mixing combination rule is much more evident when the input pieces of evidence are more consistent (less conflicting) than those from 1997, which is the case of the information collected in 2011 and 2016. the resulting bdf are plotted in figs. 12(a) and 12(b), respectively. as can be seen, the combined curves (in black) are located entirely in between the curves gotten from the individual sources. in fact, information collected in 2011 yields the best agreement between bdf computed from individual and aggregated pieces of evidence. these pieces of evidence are the least conflicting (see tables 5 and 6). in figure 13 the factor of safety bdfs resulting from the combination of the information at each period 1997 (red), 2011 (green), and 2016 (magenta) are plotted. there is a noticeable difference, particularly between the results of 1997 and 2011, the earlier is more conservative than the latter. the already aggregated information at every year can be considered as single new pieces of evidence that can be again mixed, to produce more robust input random sets to compute an updated bdf. the result of this task is the bdf plotted in black in figure 13. that curve subsumes all the information available on input parameters until 2016. again, the aggregated information balances the results and reflects the nature of the combination rule. it is also important to remark that when the whole body of information is considered, the probability function, more than discrete, resembles a continuous probability function that might represent a probability box (ferson et al. 2003). this is because the number of focal elements included for the rst analysis is 28 on bedding geometry and 8 on joint friction angle, so the bdf is the result of 12544 computations of the model. so far, a noticeable difference in the cumulative probability function of the factor of safety has been presented. now, to make it more objective, the probability of failure when considering different sources of information has been computed and included in table 7 for fs<1.0. this chart shows different probabilities of failure as different pieces of evidence are considered, e.g., for the information from source 1 in 1997, the probability of failure ranges between 67% and 100%, therefore failure is almost sure. on the other hand, the source 2 of the information collected in 1996 yields a probability of failure of 0, which is not true, since a slope instability was already reported in 2000. when combining all available information, a probability of failure between 4% and 25% is computed. table 7 computed probability of failure year probability of failure (%) source lower upper 1997 1 67 100 2 7 28 3 5 35 combined 18 52 2011 1 0 8 2 0 6 combined 0 6 2016 1 0 9 2 0 0 combined 0 7 combined 4 25figure 11. factor of safety bdf computed with the evidence collected in 1997 (a) computed with the evidence collected in 2011 (b) computed with the evidence collected in 2016 figure 12. bdf for the factor wedge stability factor of safety 235updating the probability of failure of rock wedges conclusions in this paper, an alternative to compute and update the probability of failure of rock wedges under the random sets theory was presented. the method was applied to a sandstone mine located in une, cundinamarca that has been monitored for 20 years and has relevant geotechnical information collected in 1997, 2011 and 2016. the most important findings from this work were: • rst allows obtaining bounded probability curves from the available information, even when this is limited to a few data. besides, rst allows updating the bdf as new information is available. these two aspects make the rst suitable to be applied in rock mechanics problems where the information is limited, conflicting and new information appears as the project progress. • when the pieces of evidence (or sources of information) were considered separately, noticeable differences in the bdf were obtained, which justified the need for aggregating the information into a representative set of mixed evidence. • the bdf of the factor of safety computed for the combined (updated) evidence balances the results obtained from individual sources and reflects the combination rule utilized to aggregate the different pieces of available evidence. in this case, the applied combination rule was the mixing or averaging. • the sensitivity analysis arises as a powerful tool to reduce the number of variables to be expressed as random sets, which brings a reduction in the total number of realizations to calculate the bdf. this is very important when complex numerical techniques are involved, since its high computational cost constraints the feasible number of model realizations. • random sets theory, as presented in this paper, is an alternative to systematically include new information in the design process as it is available, which allows to update the model predictions and to assist the decision making at different project stages. nevertheless, sound engineering judgment is required to define the input random sets. it is important to highlight that the design of a rock slope is a complex task, in which several mechanisms of failure and triggering factors should be considered. in this context, this research contributes to that complex task by considering the wedge mechanism of failure when limited information from several sources is available. however, further work on random sets applied to rock slope stability is required to extend this approach to more general applications. acknowledgments authors want to thank universidad nacional de colombia for the technical support given through the civil and agricultural engineering department, to colciencias for the funding provided through contract fp44842-208-2015 and, to gravillera albania s.a. for its assistance during the field work and the 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(2012). dempster-shafer theory for handling conflict in hydrological data: case of snow water equivalent. journal of computing in civil engineering, 26(3), 434–447. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n1.81087 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 55 68 m in e r a l o g y mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone (nw iberia) sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero universidad de león, españa. salca@unileon.es how to cite item: alcalde-aparicio, s., vidal-bardan, m., & alonsoherrero, e. (2022) mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone (nw iberia). earth sciences research journal, 26(1), 55-68 . https://doi.org/10.15446/ esrj.v26n1.81087 record manuscript received: 19/07/2019 accepted for publication: 07/01/2022 abstract this paper presents detailed mineralogical results together with a geochemical characterization for a sequence of six natural soil profiles. bedrock samples (r series) and overlying soil samples (s series) were characterized. the soil profiles are distributed in a series of paleozoic lithological units from lower ordovician to upper carboniferous in age (iberian massif, nw iberia). the lithological influence on mineral properties and geochemical composition and, how different weathering may be occurring under very similar temperate and acidic conditions, have been studied. field observations together with laboratory analyses were indicative of differential weathering. so, a series of selected chemical indices and relations were applied to clarify this assumption. the mineralogy was analysed by scanning electron microscopy (semeds), x-ray diffraction (xrd) of rock powder and soil oriented aggregates. x-ray fluorescence spectrometry (xrf) and inductively coupled plasma mass spectroscopy (icp-ms) were applied to analyse chemical composition. the first results showed how major elements, sio2, al2o3 and fe2o3, slightly enriched in the soil profiles, are consistent with the dominant mineralogy: quartz, chlorite, muscovite and/or illite, together with kaolinite and albite. the bases k2o, na2o, cao and mgo are also coherent with mineral composition and experience little variation, but are gradually removed in the profiles. the mobility of major elements leads to a general loss of bases and, in general, a slight enrichment in silica and sesquioxides. sio2 is enriched, firstly accumulated in soils and partially depleted by dissolution as colloidal form. al2o3 in some soils is slightly less than in former rocks, so other physical processes are expected to take place, involving clay removal with consequent aluminium depletion too. a special emphasis has been given to albite coexisting with kaolinite, firstly supposed to be directly inherited from parent substrates when present, but finally the chemical index pia shows it was mainly due to mineral alteration of plagioclases. the best correlations to explain the albite alteration and kaolinitization progress were obtained with chemical indices pia, ciw, cia and al2o3/na2o ratio. this together with mineralogical signatures, suggest that kaolinite is the result of gradual dissolution due to the acid hydrolysis of albite in such acidic environments, which may also be attributed to the organic matter influence. keywords: geochemical relations; soil profiles; differential weathering; plagioclase index of alteration (pia); albite acid hydrolysis; mineralogía de arcillas y señales geoquímicas como indicadores rápidos de procesos de meteorización química diferencial en una litosecuencia silícea de perfiles de suelos este trabajo presenta resultados mineralógicos detallados junto con una caracterización geoquímica para una secuencia de seis perfiles de suelos naturales. se caracterizaron muestras de rocas (serie r) y muestras de suelos suprayacentes (serie s). los perfiles de suelo se distribuyen en una serie de unidades litológicas paleozoicas de edad desde el ordovícico inferior hasta el carbonífero superior (macizo ibérico, nw de iberia). se ha estudiado la influencia litológica en las propiedades mineralógicas y en la composición geoquímica y cómo puede ocurrir una alteración diferente en los perfiles bajo condiciones templadas y ácidas muy similares. las observaciones de campo junto con los análisis de laboratorio fueron indicativos de alteración diferencial. por lo tanto, se aplicaron una serie de índices químicos de meteorización y algunas relaciones seleccionadas para aclarar esta suposición. la mineralogía se analizó mediante microscopía electrónica de barrido (sem-eds), difracción de rayos x (xrd) de muestras de polvo de roca y agregados orientados del suelo. se aplicaron espectrometría de fluorescencia de rayos x (xrf) y espectroscopia de masas de plasma acoplado inductivamente (icpms) para analizar la composición química. los primeros resultados mostraron cómo los elementos mayoritarios, sio2, al2o3 y fe2o3, ligeramente enriquecidos en los perfiles de suelo, son consistentes con la mineralogía dominante: cuarzo, clorita, moscovita y/o ilita, junto con caolinita y albita. las bases k2o, na2o, cao y mgo también son coherentes con la composición mineral y experimentan poca variación, pero se eliminan gradualmente en los perfiles. la movilidad de los elementos mayoritarios lleva a una pérdida general de bases y, en general, un ligero enriquecimiento en sílice y sesquióxidos. el sio2, primero se acumula en los suelos y se pierde parcialmente por disolución en forma coloidal. el al2o3, en algunos suelos es ligeramente menor que en las rocas, por lo que se espera que parte se pierda también en forma coloidal o tengan lugar otros procesos físicos que involucran la remoción de la arcilla, con la consiguiente pérdida del aluminio también. se ha dado un énfasis especial a la presencia de albita que coexiste con la caolinita; en primer lugar, se supone que se heredó directamente de los materiales originales en los que está presente, pero finalmente el índice pia muestra que se debe principalmente a la alteración de las plagioclasas. las mejores correlaciones para explicar esta alteración de la albita y el progreso de la caolinita, se obtuvieron con los índices químicos pia, ciw, cia y la relación al2o3/na2o, que junto con firmas mineralógicas, sugieren que la caolinita es el resultado de la disolución gradual debido a la hidrólisis ácida de la albita en dichos ambientes ácidos, que también puede ser atribuido a la influencia de la materia orgánica. palabras clave: relaciones geoquímicas, perfiles de suelo, alteración diferencial, índice de alteración de las plagioclasas (pia), hidrólisis ácida albita resumen https://doi.org/10.15446/esrj.v25n1.74167 mailto:salca@unileon.es https://doi.org/10.15446/esrj.v26n1.81087 https://doi.org/10.15446/esrj.v26n1.81087 56 sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero56 introduction the processes controlling the alteration of primary minerals depend on the relative mobility of some major components (anderson & hawkes, 1958; johnson et al., 1968; white et al., 2001; anderson et al., 2002; white & brantley, 2003; wilson 2004; meunier et al., 2007; white & buss, 2014). basically, the unique genetic relationship between the soil and the parent material through the profiles helps to understand the role of rock weathering and soil formation. the influence of the parent material mineral composition on the genesis of the soils is still a question to be explored, as widely shown in the traditional literature (chesworth, 1973a). therefore, the main source of the bases is related to in situ chemical weathering of local bedrocks (munroe et al., 2007). special importance has been given to the removal of alkali and alkaline earth elements (na, ca, mgand k) during the continental weathering (chesworth, 1973b), due to the alteration of clay minerals and a high removal of na from dissolution of feldspars and plagioclases, followed by a progressive enrichment in silica and al and fe sesquioxides (wilson, 2004); occasionalyy, desilification occurs at the final weathering stage. during weathering the rock forming primary minerals, micas and feldspars, are depleted in bases; in this way, k release is considered a good measure of the degree of alteration of the aluminium silicate minerals (wilson, 2004). in addition, chemical weathering of silicate rocks through acid hydrolysis could take place and lead to an exchange of na, k, ca and mg for h and, perhaps, a loss of si (bain et al., 1990; ezzaım et al., 1999a, 1999b; papoulis et al., 2004). according to banfield & eggleton (1990). both processes, the loss of k and na and the enrichment in al, are in consonance with an advancing process of kaolinization and formation of iron and aluminium oxyhydroxides too. this fact has been explained by some authors for weathering soil profiles from the iberian massif (molina et al., 1990; vicente et al., 1991; vicente et al., 1997; molina & cantano, 2002; jiménezespinosa et al., 2007; núñez & recio 2007; fernández-caliani & cantano 2010; doval et al., 2012). other relevant factors regarding weathering in the traditional literature include the fundamental alteration mechanisms and the products derived from the primary minerals (banfield & eggleton, 1990; merriman et al., 1990; robertson & eggleton, 1991; robert & tessier, 1992; gardner & walsh, 1996; aspandiar & eggleton, 2002, spears, 2016). many secondary clays are commonly considered an alteration product in acidic soil environments as explained by pédro (1997). for example, the evolution of parent rocks controlled by an hydrolysis phenomenon, including bisialitisation, monosiallitisation and allitisation, within the formation of minerals from the smectite group, kaolinite group gibbsite and iron oxides group found in the pedological horizons. evaluating geochemical composition, not only releases basic information about the formation conditions, but also the evolution and the alteration stages of the rock substrates and soils. wilson (2004) noted that the differences in alteration mechanisms, products and the rates of weathering commonly observed among bedrock and soil environments can differ widely, even in the same geological units. nesbitt et al. (1980) already mentioned degradation and leaching dominate the early weathering stages, whereas during the advanced stages, ion exchange and adsorption onto clays are of most influence. banfield & eggleton (1990), fedo et al. (1995) and price & velbel (2003) suggested a sequential formation of secondary weathering products and pointed out the utility of evaluating weathering indices attending to simple profile depth variations. thus, evaluating chemical weathering indices and element relations between mobile or immobile elements help rapidly to understand weathering intensity, possible processes and, sometimes, soil grade of development and evolutionas proven for instance by gardner & walsh (1996). the main chemical alteration indices (ruxton 1968; parker 1970; nesbitt and young 1982; harnois 1988; jayawardena and izawa 1994; fedo et al. 1995) have deeply contributed to the understanding of weathering a great variety of geological substrates. even though, some of the conventional indices, mainly use to estimate weathering intensity, should be reformulated as an alternative approach to evaluate different rock weathering grades (guan et al., 2001; meunier et al., 2013). nevertheless, chemical indices of alteration resulted really helpful when evaluating the losses or enrichment of base cations from the rocks however, evaluating the release of silica and relative accumulation of iron and aluminum in soils from the rock weathering requires being adapted (meunier et al., 2013). additionally, several studies highlighted implications for paleoweathering conditions and provenance as pointed out by fedo et al. (1995). recent reviews on climatic and geochemistry (perri, 2020 or dinis et al., 2020) enlighten their strongly interactions using chemical indices as proxies. recently, dinis et al. (2020) have recognised these significant relationships between mineralogical and geochemical signatures of sediments and rainfall registers at catchment scale. moreover, some other factors take an important role in such complex context, particularly diverse physical factors driven by topography as suggested by tardy et al. (1973). this work characterizes the mineralogical and geochemical composition of a series of six natural profiles consisting of soils (s series) and parent rocks (r series) developed on palaeozoic lithologies from the iberian massif under similar weathering conditions in temperate and acidic environments. the influence of lithology on mineral and geochemical properties and the assumption of mineral alteration by differential weathering have been addressed. the specific aims of this research are: 1) characterize the major compositional mineralogy, particularly focusing on fine-grained and clay fractions in six selected weathering profiles; 2) identify the geochemical signatures according to the chemical indices of alteration; 3) understand the factors controlling the weathering processes and 4) explain possible differential chemical weathering mechanisms presumably taking place in such acidic environments. geological setting the profile sites studied are located in the nw part of the iberian massif (variscan orogenic belt) included in the west asturian–leonese zone (walz) (figure 1), specifically in the navia-alto sil domain (nasd) (pérez-estaún et al., 2004; marcos et al., 2004). the variscan orogeny in the walz was a polyphasic deformative process associated with synkinematic metamorphism under epizonal deformation conditions, mainly to greenschist facies (martínezcatalán et al., 1990; suárez et al., 1990), with mineral paragenesis associations typical of the chlorite zone. the walz, including the lower to upper paleozoic units, is characterized principally by the presence of a thick assemblage of cambrian-ordovician siliciclastic rocks, which shows an overall structure, formed largely by recumbent folds and thrusts. figure 1. weathering sampling profiles (sp-1, sp-2, sp-3, sp-4, sp-5, sp-6) a) location in the lithological units from the walz (nw iberian massif, spain). b) synthetic stratigraphic column modified from pérez-estaún (1990) showing formations and series form ages uc: upper carboniferous; s: silurian; uo: upper ordovician; mo: middle ordovician; lo: lower ordovician and cm: cambrian. map source: the national geological survey institute of spain (igme) modified from rodríguez fernández (1982). the lithostratigraphic sequence starts with the lower paleozoic materials of cambrian to ordovician age (pérez-estaún et al., 1990). it begins with detritic facies characterized by an alternation of quartzites with slates, siltstones and sandstones referred to as the los cabos series (cua). a range of later paleozoic materials, referred to as the transition layers, gradually gives way to the luarca formation (lua), an assemblage of fine grained black slates, rich in sulphides like pyrite and organic matter. this is immediately followed by the agüeira formation (agu), formed by alternating sandstones, siltstones and shales with a presence of turbidite depositional environment, which sometimes culminates in a quartzite layer. the silurian strata, poorly represented, are found in a gradually alternating section, including fully sandy phases with varying proportions of slates. these 5757mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone are discordantly overlain by upper carboniferous (carb) (stephanian b-c) synorogenic materials of the el bierzo coal basin (barba et al., 1994), which represent the development of a strong paleorelief. the carboniferous unit consists of a broad range of sediments such as alternating layers of sandstones, shales, siltstones and conglomerates combined with intermediate coalbed deposits (colmenero et al., 2002; wagner, 2004). materials and methods the sampling profiles (sp) are located in six different lithostratigraphic units within the paleozoic succession named as cua, lua, agu and carb (table 1): los cabos series (sp-6)-cua, luarca formation (sp-3 and sp-5)-lua, agüeira fm (sp-2)-agu, and carboniferous series (sp-1 and sp-4)-carb. samples including partially weathered or fresh rocks (r-series) were named as: r1 (figure 2a), r2 (figure 2e) , r3 (figure 2b), r4 (figure 2f), r5 (figure 2g) and r6 (figure 2h); other altered weathered geological materials in contact with the bedrocks beneath, including the soil (s-series) named as: s1 (figure 2a), s2.1 and s2.2 (figure 2e), s3.1 and s3.2 (figure 2b), s4 (figure 2f), s5 (figure 2g), s6 (figure 2h), were collected in the sampling sites from sp-1 to sp-6 as showed in figure 1.the correspondence between the profile sites and samples series is specified below (table 1): sp-1 (samples r1 and s1)-carb; sp-2 (samples r2, s2.1 and s2.2)-agu; sp-3 (samples r3, s3.1 and s3.2)-lua, sp-4 (samples r4 and s4)-carb; sp-5 (r5 and s5)-lua; sp-6 (r6 and s6)-cua. table 1. descriptive features of the investigated sampling profiles soil horizons (s-series) and bedrocks samples (r-series) location (ul: upper and ll: lower level of weathering) according to depth, general profile weathering degree descriptive material features (hd: high; md: moderate, ld: low degree), lithological units, parent rocks and soil series. *acronym for the geological unit’s name profile site u.t.m. coordinates (29n) sample rock/soil horizon depth (cm) level of weathering weathering degree geologic system/ formation lithological unit* parent material soil series sp-1 x:721.188.6 y:4.730.234.5 r1 r +18 ll hd carboniferous carb beige argillite carb-boezas1 ah 0-18 ul sp-2 x:722.255.7 y:4.735.448.2 r2 r + 25 ll md agüeira agu sandy slates and grey siltstones agu-igüeñas2.1 ah1 0-8 ul s2.2 ah2 8-25 ul sp-3 x:720.118.1 y:4.737.203.2 r3 r + 20 ll hd luarca lua black slates and shales lua-urdialess3.1 ah 0-12 ul s3.2 a/r 12-20 ul sp-4 x:725.378.6 y:4.732.542.95 r4 r +25 ll md carboniferous carb sandstones and sandy siltstones carb-rodrigatoss4 ah 0-25 ul sp-5 x:729.825.63 y:4.736.191.3 r5 r +15 ll md luarca lua black slates and shales lua-tremors5 ah 0-15 ul sp-6 x:722.500.26 y:4.740.380.14 r6 r +15 ll ld los cabos series cua quartzites cua-colinas del campos6 ah 0-15 ul 58 sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero58 figure 2. soil profiles showing two different levels of weathering ul: upper level; ll: lower level where samples were taken: image a) profile sp1 (s1 and r1) and image b) profile sp3 (s 3.1, s3.2 and r3). images from e) to h) soil profile sites sp2, sp4, sp% and sp6 respectively. thin sections showing the rock samples microtextures and granulometry r1 (image a1) and r3 (image b1); r2 (image e1), r4 (image f1), r5 (image g1) and r6 (image h1). observational scale at 1 mm. sem bse images (a2 at 100 µm and b2 at 20 µm captured scale; e2, f2, g2 and h2 at 50 µm captured scale) sem bse detailed images for identification of muscovite (ms) and illite (ill) on r1 (image c) and quartz (qtz), muscovite (ms), chlorite (chl) and albite (ab) on r3 (image d). microscopy scanning electron microscopy with energy dispersive x-ray spectrometry (sem-eds) was carried out as a supporting tool to validate mineral petrographic analysis. thin sections from sample substrates were previously studied by optical microscopy (pom, transmitted and reflected light), using a nikon eclipse e600 pol petrographic microscope. the same polished thin sections were subsequently carbon coated and studied under a jeol jsm-6480 high resolution scanning electron microscope (sem), equipped with an oxford instruments d6679 energy dispersive x-ray spectrometry (eds) detector. for each specimen acceleration voltages range of 10 to 20 kev, working distance was 10 mm and count rate set at 15000 counts/s. back-scattered electron (bse) images were acquired and processed using inca energy software provided by oxford instruments. x ray diffraction representative portions of powdered rock samples were subjected to x-ray diffractometric analyses. air dried rock samples were pulverized in a restch pm400 automatic rotary planetary mortar mill, collecting the fraction 5959mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone <125 µm. xrd analyses were performed at room temperature and 400 c during 0.75 hours. representative samples were subjected to x-ray powder diffraction, using a siemens bruker d5000 series diffractometer system equipped with cu-k-alpha radiation at 40kv and 30 ma, 0.2 mm receiving slit, a range of 2°-58° 2θ and graphite secondary monochromator facilities. scanning step size was set at 0.02 º 2θ and the scanning rate 1 second per step, giving a scan speed of approximately 2º 2θ per minute and a total duration of 0.75 h. oriented aggregates of the <2.0 µm fine clay fraction were obtained from the soil samples, using suspension after hexametaphosphate dispersion, separation by sedimentation and settling techniques after 24 hours (klug & alexander, 1974). the clay samples were dried at 40°c, homogenized and manually powdered using an agate pestle and mortar. previously, the sample pretreatment was the organic matter removal with a hydrogen peroxide agent (30% v/v) at 25 c. duplicate samples were mixed and shaken in a tapered centrifuge tube. the remainder, which stayed in suspension, was removed and dropped on to a circular glass slide. this was then allowed to dry in the laboratory atmosphere, leaving an oriented clay film on the surface of the glass slide. each glass slide, with its clay film, was subjected to x-ray diffraction analysis using a panalytical empyrean ii diffractometer in the air-dried state, using cu-k-alpha radiation and in this case a range of 2°-46° 2θ for soil clay samples. the slides were then placed in a desiccator containing ethylene glycol, and exposed to the glycol vapour at room temperature for several days. the glycol-treated slides were then heated at 400 c for one hour and subjected to xrd analysis. identification of the minerals in the parent rocks and soil clays was carried out following the general indications of moore & reynolds (1989). chemical analyses chemical analyses were performed within duplicated samples including bulk chemistry and fine grained fractions <2.0 µm for clay soil samples. major element analyses were carried out by x ray fluorescence spectrometry (xrf) and, alternatively, some of the major and minor elements were measured by inductively coupled plasma mass spectrometry (icp-ms). the fused borosilicate disks were prepared with 95%pt/5%au crucibles, adding 0.500 g of a 1:1 mixed fusion flux of lithium tetraborate/metaborate (50% li2b4o7/50% libo2), in a fusion melting induction furnace heated to 1000 ºc during 20 min, using a non-wetting agent of libr. they were analysed in a bruker-nonius s4 pioneer wavelength dispersive xrf spectrometer. the samples prepared following the specifications of the alkali fusion technique required acid dissolution with hno3 5% (v/v). they were measured by icp-ms in a thermo scientific quadrupole xseries2, with detection limits of 0.01% using as standard elements in, sc and rh. the major element oxides cao, mgo, na2o, k2o, sio2, fe2o3, al2o3, p2o5, mno and tio2 were expressed as weight percentages (wt %). previously, representative splits of each sample were oven-dried, and then heated at 975 c for 12h in a high temperature furnace hobersal12-pr400to obtain the loss on ignition (loi) percentage in each case. results mineralogy in the preliminary mineralogical identification for rocks the principal minerals are phyllosilicates of the mica group, mostly ms-ill (figure 2c) and chlorites (figure 2d) respectively, accompanied by quartz (3.34 å) and feldspars, according to the major peaks (3.19 å and 3.22-3.24 å) correspond to plagioclases (albite) as confirmed by sem analyses (figure 2d) or k-feldspars, in the case of r6 (figure 2h2). the microscopic and sem bse analyses can be found detailed on figure 2. it can be noted that albite is present on r3, r5 and r4 (figures b2, f2 and g2) and, clearly more abundant, on r2 (figure e2), all assessing the xrd analyses too. in addition, kaolinite (kln) is only present in some of the r-samples (table 2). there is a relative higher abundance of chlorite in samples r3, r5 and r2, the albite content is higher in r2, however kaolinite content is higher in r1 (table 2). the major minerals in the sample r1 are mica (muscoviteillite), quartz and kaolinite respectively (ms-ill, qtz and kln) as seen in figure 3. kaolinite is clearly identified in sample r1 (figure 3a). by contrast, an iron-rich chlorite identified in samples r2 to r5 as shown in (figure3b), sem analysis confirm that correspond to chamosite (figure 3b). from samples r2 to r6 the peaks at 10 å and 5.0 å in all patterns analysed are indicating mica, but finally the sem results shown enough evidence to be identified as muscovite, so it was also confirmed by sem. it is noted that the peak of 3.34 å can be also of mica-illite, so the reflection of 4.26 å would be enlightening. this also applies to sample r1 (figure3a), although overall intensity is relatively low for that pattern, suggesting a mica (muscovite) rather than a less well-ordered illite phase as confirmed by sem analysis (figure 2c). table 2. mineral identification in the rock samples and relative mineral abundance referred to the total mean quartz contents. abbreviations: qtz = quartz; ms = muscovite; ill = illlite; chl = chlorite; kln = kaolinite; pl = plagioclases; ab = albite; kfs = k-feldspars. conversion scaled weights: ****= 3.0 (very abundant) ***= 1.0 (abundant) **= 0.4-0.6 (frequent) *= 0.3-0.2 (present) tr <0.2 (traces) and – = not identified. profile site sample qtz ms ill chl kln pl kfs sp-1 r1 ** *** *** – ** – – sp-2 r2 ** *** ** – *** – sp-3 r3 ** *** ** * * – sp-4 r4 *** ** * ** – sp-5 r5 ** *** ** * * – sp-6 r6 **** tr – – tr tr figure 3. rock powder unoriented xrd pattern, air-dried and solvated at room temperature 25 c (bottom line) and heated at 550 c (top line). major minerals identified in a) sample r1 and b) sample r3. only the range from 5° to 35°2θ is shown in the pattern. the principal mineral phases present in the soil samples from the clay oriented aggregates patterns are given below in table 3. in particular, the sequences are essentially the same identified before (micaceous phases and illite, with iron rich chlorite, traces of quartz, feldspars and minor kaolinite with some accompanying iron oxides). particularly, only minor ferric amorphous species like lepidocrocite and iron oxides were identified as traces (figure 4a). a small peak at 6.2 å on glycol (and also air-dried diffractogram) that disappears 60 sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero60 on heating suggests a trace of iron oxides corresponding to lepidocrocite in s5 and s1 (figure 4a). the presence of ferroan chlorite has been confirmed in samples s2.1, s2.2, s3.1, s3.2 (figure 4b) and s5 through the strong peaks at 7.1 å and 3.53 å, with smaller peaks at 14 å and 4.7 å, suggesting an iron rich variety. small peak at 3.58 å on saturated patterns indicates additional minor kaolinite detected in samples s3.1, s3.2 (figure 4b), s4 and s5. peak at 7.1 å, not significantly affected by heating accompanied by a small peak at 3.58 å, especially on heating, suggests that kaolinite is also present (figure 3a). the absence of strong peaks at 14 å, 7.1 å and 3.53 å, especially with glycol, indicate that chlorite is not present in this particular sample (figure 4a). moreover, in the samples from s2 to s5 peaks at 14 å, 7.1 å and 3.53 å, glycol solvated (figure 4b), indicate presence of chlorite; a small peak at 3.58 å on heating, suggests that kaolinite is also present (figure 4b). quartz (peaks at 3.35 and 4.26 å) and k-feldspar (peak at 3.2 å) are also representative in the composition of the soil samples showing sharp peaks in reflections (figure 4b). table 3. total chemical composition determined in the six profiles for parent rocks and soil samples. contents of major components analysed by xrf and icp-ms (weight %), *loi (loss on ignition). profile site sample depth (cm) sio2 al2o3 fe2o3 k2o mgo cao na2o tio2 p2o5 mno loi* total sp-1 r1 +18 56.67 23.38 5.79 4.89 0.73 0.26 0.50 1.00 0.06 0.02 6.74 100.04 s1 0-18 67.01 17.58 7.31 3.85 0.63 0.41 0.48 0.79 0.16 0.02 1.15 99.39 sp-2 r2 + 25 66.99 15.69 5.42 2.45 1.93 0.42 2.17 0.99 0.02 0.05 4.36 100.49 s2.1 0-8 73.23 13.38 5.75 2.58 1.35 0.47 0.84 1.62 0.08 0.04 0.85 100.19 s2.2 8-25 68.46 15.30 6.40 2.98 1.64 0.42 0.88 2.09 0.08 0.04 1.11 99.40 sp-3 r3 + 20 59.69 20.03 6.64 3.13 1.69 0.52 1.42 1.10 0.07 0.07 5.68 100.04 s3.1 0-12 63.84 19.85 7.91 2.97 1.35 0.35 0.80 1.14 0.12 0.04 1.04 99.41 s3.2 12-20 65.22 19.82 7.50 2.15 1.53 0.34 0.70 1.24 0.10 0.05 0.76 99.41 sp-4 r4 +25 65.14 18.17 3.86 3.40 1.50 0.34 0.91 1.09 0.06 0.03 5.63 100.13 s4 0-25 82.11 9.92 3.04 1.73 0.49 0.23 0.34 0.54 0.06 0.01 0.94 99.41 sp-5 r5 +15 56.80 22.72 7.63 2.95 1.77 0.46 0.93 1.13 0.08 0.07 5.84 100.38 s5 0-15 60.80 22.11 9.30 1.94 1.06 0.32 0.86 1.61 0.15 0.17 1.07 99.39 sp-6 r6 +15 97.45 1.07 0.17 0.44 0.05 0.42 0.00 0.08 0.02 0.00 1.01 100.71 s6 0-15 95.03 1.63 0.59 0.55 0.06 0.19 0.05 0.45 0.01 0.00 1.42 99.98 figure 4. clay oriented aggregates a xrd pattern from a representative sample a) s1 in the profile sp-1 and b) s3 in the profile sp-3 showing k-glycol treatment (bottom line) and heated at 400 c (top line). only the range from 5° to 35°2θ is shown to clarify the interpretation in the pattern. 6161mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone rocks and soils geochemistry the contents of major elements in the samples are summarized in table 3. the contents found in the soils are coherent with those ones present in the rocks through the soil weathering profiles. sio2, fe2o3 and al2o3 contents are quite similar in soil profiles sp1 to sp5 being the most abundant elements. however in s6 from sp-6 the content of sio2 reaches about 95%, while in the rest of soil samples get around 65-70%. the bases k2o, mgo, na2o and cao are generally lower than those present in the rocks. k2o (2-3%) shows higher contents among the last ones and, together with mgo (1-2%), are more abundant than cao (0.2-0.4%) and na2o (0.5-0.9%). the content of na2o is slightly higher in the soil samples of sp-2 and sp-3 (table 3). regarding loi results, the highest values correspond to sp-1 where r1 reaches 6.74 in the carboniferous series and the lowest to sp-6, where r6 is 1.01 (table 3). the edaphic contents are little higher than the lithogenic ones for sio2, fe2o3 and al2o3 (table 3). the parent rocks show relatively important amounts of na2o, fe2o3 and mgo as well as k2o which are retained in the less altered minerals (table 3). so, the contents found in the residual soils show an enrichment of sio2, fe2o3 and al2o3 too (table 3). chemical indices of alteration and element ratios weathering indices were calculated using molar proportions, increasing alteration indicated by increasing cia (chemical index of alteration; nesbitt and young, 1982), ciw (chemical index of weathering; harnois, 1988) and pia (plagioclase index of alteration; fedo et al., 1995). ciw, cia and pia have reached higher values in sp-1, sp-3 and sp-5, all are in consonance with the geochemical base mobilization and depletion of major cations calculated (table 4). however, minor differences have been shown in the present data from the soils and underlying rocks (table 4). ciw and pia show quite good correlation fitting (r2=0.49) (figure 5a), although cia and pia reaches a better correlation r2=0.94. the fitting between ciw and cia reaches a positive r2=0.99. the higher values of the cia and ciw are indicating a possible high grade of alteration, mainly in the rock samples from profiles sp-1, sp-3 and sp-5. piw present high values specifically suggesting one of the most possible ways of primary mineral alteration (table 4). it is also assumed that loi, the only one not expressed as a ratio, increases with the degree of weathering. loi has also resulted higher in sp-1, which is in consonance with the rest of indices calculated. a r2=0.30 fitting in the correlation with ciw (figure 5b) showing higher correlation in the rock series than the soil ones. table 4. indices associated to the intensity of mineral alteration and weathering. abbreviations: cia: chemical index of alteration; ciw: chemical index of weathering; pia: plagioclase alteration index; compositional molar ratios calculated from major oxides related to chemical and mineral weathering indices. abbreviations: r2o3=(fe2o3+al2o3); bxo=(na2o+k2o+cao+mgo). profile sample depth (cm) cia ciw pia sio2/ al2o3 sio2/ r2o3 sio2/ tio2 sio2/ k2o bxo/ r2o3 bxo/ al2o3 k2o/ na2o al2o3/ na2o sp-1 r1 +18 77.99 94.71 93.27 4.11 3.55 75.03 18.13 0.31 0.76 6.39 28.22 s1 0-18 75.50 91.96 89.71 6.46 5.10 111.98 27.23 0.33 0.42 5.29 22.31 sp-2 r2 + 25 69.20 78.36 75.06 7.23 5.92 89.55 42.81 0.62 0.55 0.74 4.40 s2.1 0-8 72.68 85.65 82.54 9.27 7.28 59.80 44.48 0.50 0.63 2.01 9.64 s2.2 8-25 73.79 87.40 84.55 7.58 5.98 43.45 35.90 0.49 0.63 2.24 10.61 sp-3 r3 + 20 75.01 85.91 83.52 5.05 4.17 71.88 29.82 0.45 0.53 1.45 8.54 s3.1 0-12 79.34 91.03 89.48 5.45 4.34 74.48 33.64 0.34 0.43 2.45 15.13 s3.2 12-20 82.91 91.85 90.86 5.57 4.49 69.50 47.51 0.32 0.40 2.03 17.27 sp-4 r4 +25 75.79 89.53 87.22 6.07 5.35 79.43 29.99 0.46 0.44 2.46 12.13 s4 0-25 77.67 90.99 89.12 14.01 11.72 201.79 74.41 0.35 0.41 3.33 19.37 sp-5 r5 +15 80.32 90.53 89.15 4.23 3.49 66.40 30.17 0.36 1.27 2.07 14.75 s5 0-15 84.35 91.69 90.90 4.66 3.67 50.20 49.09 0.24 0.31 1.48 17.70 sp-6 r6 +15 46.18 58.14 43.50 154.21 139.73 1546.77 345.99 1.16 0.72 0.00 0.00 s6 0-15 61.49 49.44 70.96 98.47 79.99 282.97 268.03 0.59 0.36 7.12 15.60 figure 5. correlation between chemical indices a) pia and cia b) loi and ciw a b 62 sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero62 selected molar ratios, based on the principle that the ratio between concentrations of mobile (such as sio2, cao, mgo, k2o and na2o) and immobile elements (al2o3, fe2o3, tio2), should increase with weathering have been studied. these molar ratios such as sio2/al2o3 (table 4) related to silica leaching result a dominant factor in the characterization of the weathering together with chemical weathering indices (ciw, cia and pia) related to weathering intensity. the main chemical indices are expected to increase over time as leaching progresses. some other selected ratios form the literature present a different behavior. for instance, sio2/k2o tends to show lower values when the weathering is higher (sp-1) (table 4). opposite to that, sio2/tio2 remains almost constant with no noticeable differences as well as sio2/al2o3 and sio2/r2o3 both are in consonance among the rocks and soil samples not showing remarkable differences (table 4). other ratios regarding the bases give an estimation of weatherable primary minerals remaining in soils, such as bxo/ r2o3 and bxo/al2o3 (table 4), decreases over time, so less weathered materials, for instance r6, present higher values (table 4). k2o/na2o and al2o3/na2o are both good indicators the weathering degree showing lower values related to less alteration (sp-2) and higher values for more altered phases (sp-1) (table 4). discussion geochemical relations, elements mobility and mineral alteration in general, the relative chemical composition variations are neither high nor too significant, comparing the upper and lower parts in the profiles, since the sampling depth is rather limited (table 3). generally, the profiles present an increasing content of some elements, noticeable in sio2, al2o3, and fe2o3, from the parent rocks to the upper levels of the soil profiles (table 3) while cations are leached. it is particularly important the relatively high k2o content found in all of soils, especially in sp-1 (figure 6). na2o, mgo and cao are present in soils, but not significantly enriched (figure 6). according to the results, there is a net gain of sio2, al2o3, and fe2o3 in the soils, in particular, from profiles sp-1, sp-3 and sp-5 (table 3). otherwise, k2o, mgo, cao and na2o are still relatively higher in the parent rocks (figure 6), this last one especially in sample r2 from sp-2 due to the abundance of albite (table 2). however, other leachable elements such as cao do not show this clear tendency, so the content is rather limited in both rocks and soils, since it is only enriched in sp-1 (figure 6). meanwhile, soil mgo gain seems to be more evident, in lower parts of sp-2 and sp-3 (figure 6) but limited in the rest. the k2o depletion from rocks and enrichment in soils, particularly shown in sp-1 and sp-6, is mainly the result of micas alteration (figure 6). the bases mobilities against iron sesquioxides are evident, but not high. the alteration causes hardly any variation in the relative amounts of na2o and mgo, but leaching in the upper layers preferentially eliminates mgo (figure 6). exactly the same happens with cao with respect to na2o. the mobility of k2o is always greater than na2o, and k2o, seems to be the most mobile, as there may be no minerals to retain it. in short, a gradual depletion of bases and enrichment in sio2 and iron and aluminium sesquioxides is observed. al2o3 contents found in soils are not higher than those ones in rocks (figure 6). al2o3 is slightly less common in soils, particularly in sp-4 and sp-1 and less noticeable in the rest (figure 6). indeed, the enrichment of silica, though not marked, is observed in most of soil profiles, especially in sp-4. sio2 accumulation is normally followed by sio2 mobilization and partial desilification, according to the removal as soluble colloidal form leached out of the profile. under these climatological conditions the intense leaching left a material poor in bases, since there is hardly any variation in their content. figure 6. enrichment and depletion in the soil profiles: composition of soil horizons showing mean concentrations and standard deviation of major components e xpressed in oxides (%). 6363mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone lower al2o3 found in soils than in underlying rocks must be related with other physical processes besides chemical decomposition related to the removal of fine-grained clay particles enriched in al2o3. dinis & oliveira (2016) explained that the composition of metasedimentary-derived sediments is not substantially affected by their grain size distributions. it could be explained by the dissolution leaching out of the profile, as colloidal particles clay fractions enriched in al2o3, as happens when sio2 migrates in the edaphic environment. this could explain the lower al2o3 contents found in some soils due to dissolution, leaching and final depletion out from the soil profile. as seen before, the profiles experience a gradual and little noticeable mobilization of bases from the destruction of the dominant primary minerals, mainly micas, fe-bearing chlorites and feldspars and plagioclases, which is followed by a depletion in the upper parts of the soil profiles as well, as seen in doval et al. (2012), ling et al. (2016) and (2018). these losses were particularly observed in the early stages of the soils pedological development in agreement with senol et al. (2016). it is common generalizing the genesis of highly weathered soils which results in intense loss of bases (na2o, cao, k2o and mgo), whereas less soluble al2o3 accumulates in newly formed solid phases such as secondary 1:1 phyllosilicates like kaolinite, or mineral forms of aluminium hydroxide, such as gibbsite too (romero et al. 1992). however, taboada & garcı́a (1999) showed that much evolution of the material is not necessary to explain the abundance of kaolin minerals and gibbsite, as these are compatible in many saprolites with absolutely fresh feldspars; in this case explaining the coexistence of albite and kaolinite supporting the evidence shown in this paper. taboada & garcia (1999) also reported that gibbsite may even be the mineralogical product formed from the plagioclases in the initial weathering stages when intense leaching occurs. figure 7. cross data plots within linear correlation and r2 fitting for a) al2o3/na2o and k2o/na2o and b) al2o3/na2o and pia 64 sara alcalde-aparicio, m. vidal-bardan, e. alonso-herrero64 albite alteration supported by chemical indices indices associated to the intensity of mineral alteration and chemical weathering (ciw, cia and pia) give a quantitative measure of feldspar weathering, by relating al2o3 enrichment in the weathering in contrast to na2o, cao and k2o, which should be removed from the soil profile during plagioclase or k-feldspar weathering (nesbitt and young, 1982; white et al., 2001; price and velbel, 2003). some element ratios biplots shown in figure 7, help to clarify the interpretation. the molar ratios al2o3/na2o and k2o/ na2o (figure 7a) show clearly differences, mainly in sp-2 sample r2 where albite composition is higher and the ratios for mobile/mobile elements k2o/ na2o and al2o3/na2o (figure 7a) show the opposite in sp-1 where kaolinite is dominant in composition, due to in r1 and s1 albite is not present. al2o3/ na2o with k2o/cao with a correlation fit r2=0.56, indicative of the possible weathering increase with a linear positive tendency of the plot (figure 7a). al2o3 and k2o enrichment and na2o mobilization are both indicative of the gradual plagioclase alteration, so chemical indices support the main hypothesis in this case of albite alteration. the cross-dot plots reflect a good fitting and indicate mineral composition, where albite unaltered is dominant or kaolinite is dominant (figure 7a). furthermore, al2o3/na2o result a key factor in the characterization of the grade of weathering (figure 7b), when plotting against plagioclase index of alteration (pia) (fedo et al., 1995) and with chemical weathering indices (ciw) (harnois, 1988). al2o3/na2o is considered a good indicator of the degree of weathering if we supposed lower values for less altered materials where plagioclases (albite) still remain in parent rocks. weathering chemical indices selected pia and ciw show a clear correlation, similar to the one obtained for al2o3/na2o and pia for rock and soil series (figure 7b). this positive correlation (r2=0.54) indicates a progressive weathering of albite while advancing the kaolinization in the most altered soil profiles sp-1, as the higher pia values suggest, and less alteration in sp-2 (figure 7b). in short, cations such as na, k and ca commonly generated by the weathering of feldspar and plagioclases into kaolinite, are rapidly release in the soil domain (fedo et al., 1995). pia, ciw and cia and other chemical relations such as al2o3/na2o help to understand the most reliable assumption that kaolinite comes from the gradual alteration of albite. despite the evidence of element leaching, although quite limited, during the data evaluation, geochemical indices such as pia and al2o3/na2o ratio are good indicators of possible differential weathering intensities in these profiles. differential weathering signatures the main findings point out some possible differential weathering signatures supporting the mineral alteration of albite, especially the clue given by the identification of primary phyllosilicates and their coexistence with other secondary phases. this finding gives some enlightening data indicative of possible different processes affecting the alteration profiles. particularly, special emphasis is given to the albite occurrence, which is found coexisting with kaolinite in the mineral sequences found in the profiles. this assumption may be explained by at least three particular conditions as shown in literature. kaolinite, firstly supposed to be directly inherited in soils from the parent substrates when formerly present (especially abundant in r1 from sp-1). however, this explanation was not enough in the profiles where kaolinite does not exist originally since the edaphic formation of this mineral from the weathering of silicates requires extreme alteration condition (molina et al., 1990; hernando massanet et al., 2004); not possible in the current studied soil conditions. secondly, the weathered plagioclases drive to the occurrence of derived secondary kaolinite associated to a pedogenic origin (driese et al., 2007; churchman & lowe, 2012). strong pedogenic dissolution and alteration of feldspars and mafic minerals to kaolinite, gibbsite and iron oxides, as well as geochemical losses of na, ca, mg and k (driese et al., 2007) must be necessary to understand this fact occurring under particular conditions. in this work, the results have shown certain amounts of amorphous species like lepidocrocite and other iron oxides which are also associated to organic phases and weathering process in slates and shales rich in sulphides and organic phases, which are the dominant parent rocks. adding to the previous, the hydrolytic weathering the main cause of a progressive transformation of affected components into clay minerals, ultimately into kaolinite (romero et al. 1992; taboada & garcı́a, 1999), following the silicate weathering generally known as monosialitization. this acid hydrolysis favors the mobilizations of some bases, including in later stages, and the release of ba and k from the partial alteration of k-feldspars, plagioclases and micas (fernández-caliani & cantano, 2010). in short, kaolinite has been widely shown as the stable weathering product of the feldspars, plagioclases (albite) and muscovite (núñez & recio, 2007). in such context, it could enhance to explain, at least partially, the formation of secondary clay minerals belonging to the kaolinite group and the expansive clays and 2:1 mixed layers, more abundant in areas where a dry period of intense evaporation generally occurs. finally, the last assumption may explain that kaolinite and albite could coexist in different size-fractions because weathering was still not able to destroy all albite. these rock substrates which are mainly made up of fine to very fine fractions, mainly silt and clay particles, are not supposed to be substantially affected by the grain size distribution of the sediment in agreement with dinis & oliveira (2016). moreover, very fine grained fractions in the profiles may be depleted due to loss process controlled by diverse physical factors driven by topography (tardy et al., 1973), remaining in the soils only the coarser fractions. this is directly linked to mechanical weathering and erosion rates. this last explanation associated to intense erosive activity requires great mineral similarity among weathered and fresh substrates, together with a low development in the soil profiles due to a continuous renovation-denudation in the recycling. no less important is the narrow relationship which links geochemical composition with the contrasting climatic conditions. dinis et al. (2020) and perri (2020) have recently showed how chemical indices act as good proxies for chemical weathering to reconstruct past climate conditions. dinis et al. (2016 and 2017) have also investigated the roles played by distinct processes on the weathering intensity and the palaeoclimatic conditions, demonstrating the composition of clay is strongly influenced by climatically driven weathering. otherwise, perri (2018) has shown higher abundance of kaolinite related to warm–humid conditions. moreover, perri (2020) released medium-low cia values for similar climatic areas, in terms of mean annual temperature and precipitation, which underwent moderate-weak chemical weathering similar to this study. the results do not differ a lot from the values found in this study for the ciw, cia and pia; quite similar too to those obtained in previous works of scarciglia et al. (2016) or perri (2016). in the light of the chemical indices and the linkages showing good relations with geochemical data, in this particular case, the albite hydrolytic alteration weathering by acid dissolution is still the main hypothesis supporting the presence of kaolinite in the most altered investigated soil profiles. perri et al. (2015) demonstrated in granitoid weathering simulations the co2-controlled dissolution of albite-rich plagioclase is the most important reaction, followed by the dissolution of k-feldspar, biotite, chlorite, and muscovite, in order of decreasing importance. in fact, differential chemical weathering occurring in the acidic solution of soils seems to better explain this way of alteration. the acidic solution from the oxidation of both, organic matter, quite important in the composition, and sulphides, abundant in the rocks, could explain the ways of alteration, supporting the presence of such abundant kaolinite. some authors have already demonstrated the influence of organic matter in the acid dissolution to explain kaolinite presence (gallardo lancho et al.1976; romero et al. 1992, fischer et al., 2009; li et al., 2014; emberson et al., 2016; ling et al., 2016). solute transfer and elements mobility by acidic percolation of soil solutions should be studied further in order to clarify this assumption. so that, solute transfer and elements mobility by acidic percolation of soil solutions and leaching data should be studied further to give a concluding explanation like in other studies supported by cronan (1985), bain (2007) or di figlia, et al. (2007). conclusions the identification of primary phyllosilicates and their coexistence with other secondary clays gives an enlighten clue indicative of possible different 6565mineralogy and geochemical signatures as indicators of differential weathering in natural soil profiles from the west asturian-leonese zone weathering processes affecting the soil profiles. particularly, special emphasis is given to the albite occurrence, which is found coexisting with kaolinite in most of the mineral sequences. kaolinite can be directly inherited in soils from parent rocks and partially due to mineral alteration, preferably from albite. in addition, since both could coexist in different size-fractions they are differently affected by weathering. so, it may be concluded that the geochemical composition must be also substantially affected by the grain size distribution of the sediment. moreover, good correlations between chemical indices and element ratios, may indicate kaolinite in this case should come from the alteration of plagioclases during weathering. the traditional chemical indices have also helped to understand different chemical weathering processes taking place. pia, ciw and cia have resulted an essential tool to infer some assessment due to the lack of other more specific data; giving a measure of plagioclase differential weathering; they together with, al2o3/na2o and k2o/na2o, have also supported that kaolinite comes from the gradual alteration of albite mainly due to acid hydrolysis. the differential weathering and albite alteration detected in these profiles is in agreement with the mobility of major elements leading to a general loss of bases (na2o, cao, k2o and mgo). in this work, the relations between the parent rocks composition and soils can easily assist in determining the bases loss and the k2o enrichment from the bottom to upper parts in the soil profiles. in general, a slight enrichment in silica and iron and aluminium sesquioxides attributed to the advancing process of weathering in agreement with most of findings show. however, al2o3 in some profiles is slightly less abundant in soils than in parent rocks, probably because al2o3 in the finest clay fractions is being lost by physical processes. similarly occurring with sio2, firstly enriched in the soil profiles when weathering advances and, later, leached out of the profile. hence, besides chemical weathering, other physical or mechanical processes, due to erosion and removal, are expected to take placein such favorable topographic conditions. this together with a low development detected in the soil profiles leads to think of a continuous renovation-denudation in the recycling. in short, several factors, not only data limited to geological source areas and the critical lithology control, mainly based on geochemical and mineralogical composition, but also morphology and topography, climatic registers, or even vegetation interactions, may be assessed on weathering research. acknowledgments the authors want to express their gratitude to the editors, editor-inchief and the anonymous reviewers for their exhaustive reviews and valuable suggestions on the manuscript which helped to enrich the scientific discussion. thanks for the support given by dr. fernando gómez-fernández on the performance of sem analysis and especially for his expertise on mineralogy interpretations. the authors would also like to thank the help given with the xrd analysis by the school of biological, earth and environmental sciences university of new south wales (unsw). we are also grateful for the help provided by the microscopy laboratory service (university of león, spain). the assistance and the technical support provided by the structural analysis unit sai-uae (university of coruña, spain) and the technical scientific services (university of oviedo, spain). this research was funded by the educational and research authority from castilla & león regional government (leoo3a08) and supported by the spanish ministry of education and science (edu/1262/2009), first author’s phd funding project. references anderson, d. h. & hawkes, h. e. 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(1998). processes controlling the distribution of ti and al in weathering profiles, siliciclastic sediments and sedimentary rocks. journal of sedimentary research, 68(3), 448-455. https://doi. org/10.2110/jsr.68.448 https://doi.org/10.1016/s0016-7037(00)00577-9 https://doi.org/10.1016/s0016-7037(00)00577-9 https://doi.org/10.1180/0009855043930133 https://doi.org/10.1180/0009855043930133 https://doi.org/10.2110/jsr.68.448 https://doi.org/10.2110/jsr.68.448 keywords: radar remote sensing; open pit mine; psi; surface displacements; geotechnical risks. palabras clave: teledetección; mina a cielo abierto; desplazamientos de superficie; riesgos geotécnicos; how to cite item hartwig, m. e., de lima, l. r., & perissin, d. (2021). iw sentinel-1 satellite scenes for the investigation of mine slope stability: experiences from the riacho dos machados gold mine (brazil). earth sciences research journal, 25(1), 93-99. doi: https://doi.org/10.15446/ esrj.v25n1.86563 in the last decade, the persistent scatterer interferometry – psi have been largely employed to predict instabilities and failure in open pit mines. the psi is a powerful technique, which combines radar satellite data in order to detect and monitor tiny surface displacements over vast areas. in the last years, the sentinel-1 radar mission have produced images of the globe acquired with different spatial and temporal resolutions that are now freely available. in recent years, the footwall slopes of the riacho dos machados gold mine – mrdm (minas gerais state, southeastern brazil) have recorded large planar failures controlled by foliation planes. therefore, the focus of this paper is to evaluate a stack of 39 interferometric wide sentinel-1 scenes, spanning from january 2018 to april 2019, acquired in descending orbit geometry, for the detection and monitoring of surface displacements. the results have shown that descending iw sentinel-1 scenes can be used to provide a broad picture of the line-of-sight los deformation phenomena. in order to monitor the evolution of the deformation phenomena induced by mining activities, los deformation maps with millimeter accuracy could be only delivered at least every 12 days. abstract iw sentinel-1 satellite scenes for the investigation of mine slope stability: experiences from the riacho dos machados gold mine (brazil) imágenes iw sentinel-1 para la investigación de la estabilidad de pendientes de minas: experiencias de la mina de oro riacho dos machados (brasil) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.86563 en la última década, la interferometría por dispersor persistente (psi) se ha empleado para predecir inestabilidades en minas a cielo abierto. el psi es una técnica poderosa que combina datos satelitales de radar para detectar y monitorear diminutos desplazamientos de superficie en vastas áreas. en los últimos años, la misión de radar sentinel-1 ha producido imágenes del globo adquiridas con diferentes resoluciones espaciales y temporales que ahora están disponibles gratuitamente. en los últimos años, las laderas de las paredes de la mina de oro riacho dos machados mrdm (provincia de minas gerais, sureste de brasil) han registrado grandes rupturas planas controladas por planos de foliación. por lo tanto, el objetivo de este trabajo es evaluar una pila de 39 imágenes iw (interferometric wide) del satélite sentinel-1, que abarcan desde enero de 2018 hasta abril de 2019, adquiridas en geometría de órbita descendente, para la detección y monitoreo de desplazamientos de superficie en la mrdm. los resultados han demostrado que las escenas descendentes iw sentinel-1 se pueden utilizar para proporcionar una imagen amplia del fenómeno de deformación los (línea-de-visión). para monitorear la evolución de los fenómenos de deformación inducidos por las actividades mineras, los mapas de deformación los con precisión milimétrica solo se pueden entregar al menos cada 12 días. resumen record manuscript received: 23/04/2020 accepted for publication: 11/09/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 93-99 marcos eduardo hartwig1, leandro ribes de lima2, daniele perissin3 1federal university of espirito santo 2leagold mining company 3lyles school of civil engineering * corresponding author: marcos.hatwig@ufes.br m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v25n1.86563 https://doi.org/10.15446/esrj.v25n1.86563 https://doi.org/10.15446/esrj.v25n1.86563 94 marcos eduardo hartwig, leandro ribes de lima, daniele perissin introduction mining activities inevitably induce surface displacements. if they are not correctly detected and interpreted, they can overcome acceptable limits and eventually lead to failure, affecting production, safety of workers and mining equipments. in order to avoid these issues, in situ geotechnical monitoring systems are commonly employed, as optical leveling, gps, ground-based radar interferometry and extensometer. nevertheless, these methods are used only after the detection of geotechnical problems by field inspection, such as terrain cracks, rock/soil failures and sudden changes in the groundwater behavior. besides, traditional geotechnical monitoring systems can provide only punctual information about the deformation state of activity. in order to anticipate risk situations in mining activities, remote sensing techniques like the persistent scatterer interferometry psi have increasingly been used in the last decade (pinto et al., 2015; hartwig, 2016; hartwig et al., 2013; colesanti and wasowski, 2006). this satellite-based approach allows detecting and monitoring tiny surface displacements over large areas with millimetric to centimetric accuracy, day and night, independently of the weather conditions. the psi method consists in combining large sets of complex synthetic aperture radar (sar) scenes, obtained from different positions both in space and time. this approach exploits the phase differences of the signals of ground targets that exhibit high temporal phase stability, designated as persistent scatterers ps. since the phase of the electromagnetic wave depends on the distance between the antenna and the target, it is possible to track superficial displacements over time. for geotechnical applications, satellite scenes with high spatial and temporal resolutions are usually required, which can only be achieved by using commercial satellite data. in 2014 and 2016 the european space agency – esa launched the sentinel-1 constellation of two satellites. the sentinel-1 is a c-band synthetic aperture-imaging mission developed for land monitoring. the satellites acquire images globally in different modes with temporal frequency of 6 and 12 days. the interferometric wide swath mode (iw) are freely available and have 5 x 20 meters spatial resolution. these characteristics make iw sentinel-1 data suitable for many different applications, particularly large-scale deformation phenomena (funning and garcia, 2019; intrieri et al., 2018; sowter et al., 2016; nagler et al., 2015). the riacho dos machados gold mine (mrdm) is located in the northeastern portion of the minas gerais state (figure 1) and comprises an open pit mine with almost 100 meters of depth and surface mining area equals to 0,75 km2, mined in decomposed greenish schists oriented nne-ssw. the mineralization is hosted in paleoproterozoic quartz-muscovite-schists, affected by ductile-brittle shear zones and hydrothermalism developed during the brazilian tectonic cycle (neoproterozoic) (fonseca, 1993; fonseca et al., 1997). the mrdm has mineral reserves estimated in 3 million of tons with average au grade of 0.94 g/t. in 2016, the mine was acquired by brio gold mining company, which was purchased by the leagold mining company in 2018. from may 2016 to may 2018, 102,283 oz of gold was produced in the mrdm (rpa, 2018). in recent years, the central and south pits of the mrdm have recorded planar failures (both global and local) in the footwall slopes (figure 1) controlled by the metamorphic foliation planes, which eventually interrupted ore exploitation in 2015 (peterle et al., 2019). in 2017, a new smaller rock failure occurred in the footwall slopes of the south pit (vog, 2017). in this sense, the aim of this work is to evaluate the applicability of the freely available iw sentinel-1 sar scenes in order to detect and monitor surface displacements induced by mining activities. for this purpose, 39 complex radar scenes acquired in descending orbit track between january 2018 and april 2019 were interferometrically processed in the sarproz software. the satellite sentinel-1 the sentinel-1 constellation of two satellites is a global radar-imaging mission developed by the european space agency – esa (esa’s sentinel online website). the sentinel-1a was launched in 2014 and the sentinel-1b in 2016. the single look complex – slc scenes are freely available at https:// sentinel.esa.int/web/sentinel/sentinel-data-access. table 1 summarizes the main features of the sentinel-1 satellites. the satellites operate in four acquisition modes: stripmap (sm), interferometric wide swath (iw), extra-wide swath (ew) and wave (wv). the iw mode acquires data with 250 km swath at 5 x 20 meter spatial resolution. iw mode captures three sub-swaths by using the topsar technique. as a result, iw slc products contain one image per sub-swath. each sub-swath image consists of a set of bursts, each of them are processed as a single slc image and between sub-swaths and adjacent bursts there are black-fill demarcation stripes. in brazil, sentinel-1 imagery has been acquired since 2015 and is only available in descending mode each 12 days. figure 1. location of the study area (left) and overview of the mine with indication of past failures locations (right) https://sentinel.esa.int/web/sentinel/sentinel-data-access https://sentinel.esa.int/web/sentinel/sentinel-data-access 95iw sentinel-1 satellite scenes for the investigation of mine slope stability: experiences from the riacho dos machados gold mine (brazil) table 1. main features of the sentinel-1 satellites (esa’s sentinel online website). orbit near-polar, sun-synchronous polarization hh+hv; vv+vh; vv; hh revisiting time 12 days (6 days with both satellites) incidence angle 20 46 degrees altitude 693 km inclination 98.18 degrees frequency 5.405 ghz (l = 5.54 cm | c-band) site geology and geomechanical conditions the study area occupies part of the external zone of the neoproterozoic araçuaí-west congo orogen in brazil (alkmin et al., 2006; pedrosa-soares and wiedmann-leonardos, 2000). the mrdm is included in a granitic-gneissic and migmatitic archean basement window, termed as itacambira-monte azul block, partly reworked during the neoproterozoic orogeny (guimarães et al., 1993). this tectonic block is surrounded by the metasedimentary rocks of the espinhaço supergroup (mesoproterozoic) to the south and east and by the macaúbas and bambuí groups (neoproterozoic) to the east and west, respectively (crocco et al., 2006). it includes the córrego dos cedros/ porteirinha complex, several igneous plutons and the riacho dos machados group (grm) (guimarães et al., 1997). the grm comprises a sequence of paleoproterozoic/archean metavolcanic-sedimentary rocks (fonseca et al., 1997; guimarães et al., 1997). the metamorphism of the grm reached upper greenschist facies (kyanitestaurolite zone). according to crocco-rodrigues et al. (1993) rocks of the grm show a complex structural pattern due to the presence of tight folds and faults. the gold mineralization in the grm is related to hydrothermal alteration and fluid flow in shear zones (fonseca et al., 1997). in the mine area, the grm corresponds to a sequence of sheared and hydrotermalized quartz-muscovite schist, quartz-feldspar schist and pelitic schist. gold mineralization is mainly hosted in the quartz-muscovite schists (qmx) and is related with the content of sulfides (arsenopyrite) (fonseca, 1993). the most penetrative geological structure observed in the mine area is an undulating schistosity oriented nne and dipping to southeast with moderate dip angles. brittle geological structures such as joints show a significant range of orientations (geoestável, 2015). according to geoestável (2015) four classes of rock mass quality (bieniawski, 1989) are distributed horizontally throughout the mine area. according to this report, rock mass quality tends to increase with depth, ranging from very poor rock near the surface to good rock near the pit. sar, dinsar and psi overview radar remote sensing comprises active systems that present lateral viewing geometry, which may lead to image distortions (e.g., layover, foreshortening and shadow), particularly if it is acquired over rugged terrains. synthetic aperture radar – sar transmits polarized electromagnetic waves within microwave frequencies (electromagnetic wavelength, l = 1-100 cm) and record as a complex number, the amplitude and phase signal backscattered from ground targets for each resolution cell of an illuminated scene. the amplitude of the backscattered signal represents the reflectivity of the targets and depends on target and system parameters (e.g., dielectric constant, roughness, etc.). the phase of the signal indicates the cycle stage in which the wave is at a given moment. this angular quantity varies from 0 to 2p radians (madsen and zebker, 1998). sar interferometry (or simply insar) consists of combining two coregistered complex scenes, acquired from slightly different positions in space and time, along or across orbit track, to form an interferometric pair (rosen et al., 2000; bamler and hartl, 1998) (figure 2). the spatial and temporal separation of the acquisitions are called normal and temporal baselines, respectively. the interferometric phase difference (d) can be derived by multiplying a master scene ( 1 ) by the complex conjugated of a slave scene ( 2 ). this quantity will depend on the sensor-target distance (dr) as follows: ∆ = ( ) −( )φ π λ 2 2 1 2r r (1) the value 2 in the equation above refers to the round-trip of the radar signal, l is the radar wavelength, r1 and r2 are the slant range distances in different time acquisitions and 2p stands for wrapped interferometric phase. figure 2. insar geometry. s1 and s2 correspond to the positions of the antennas in different acquisition times. b is the spatial separation of the antennas, q is the view angle of the sensor, h is the height of the satellite, r corresponds to the slant range distance and h is the terrain height in the point p with respect to a reference surface (horizontal line). source: authors. since the sar scenes are usually acquired in different spatial and temporal positions, the interferometric phase difference (d) will contain contributions from different sources: ∆ = + + + +φ φ φ φ φ φdeformation topography atmosphere noise orbit (2) the first, second, third, fourth and the fifth terms of eq. 2 stands for phase contributions due to deformation between acquisitions, surface topography, atmospheric artifacts, noise and orbit errors, respectively. the so-called differential synthetic aperture radar interferometry dinsar technique consists of isolating the deformation component of the phase from the others (amelung et al., 1999; massonnet and feigl, 1998; carnec et al., 1996). however, in spite of its centimetric accuracy (strozzi et al., 2001), the dinsar technique can be severely affected by atmospheric delay (zebker et al., 1997). in order to tackle with this issue, ferretti et al. (2001) and ferretti et al. (2000) originally patented the persistent scatterer interferometry – psi. the psi consists of a multi-temporal analysis of the phase stability of stable scatterers across large stacks of sar scenes. it was recognized that rock outcrops and man-made structures show a stable behavior over long periods of time and thus exhibit high temporal coherence. because of the reflection dominance and consistency of these point-like scatterers within the resolution cell over time, the uncorrelated atmospheric phase component can be estimated and removed from each interferogram. in the psi approach all sar scenes are processed at full spatial resolution scale and connected to a unique master scene. the selection of the master scene is based on the spatial and temporal baselines of the dataset (the best combination of scenes that results in the highest temporal coherence). due to phase ambiguities, that is, sudden phase changes either in space or in time, a linear model is assumed in order to derive deformation rates. deformation rates are then computed with respect to a stable reference point selected within the imaged scene, since phase differences are relative both in space and time. currently, there is a large number of psi methods, also called advanced dinsar (a-dinsar). these methods can be distinguished based on the baseline configuration, image resolution, ambiguity resolution method, a-priori pixel selection, a-posteriori pixel selection and atmospheric signal estimation (perissin and wang, 2012; ferretti et al. 2011; hooper et al. 2004; werner et al. 2003; berardino et al. 2002). a review about the psi methods and principles can be found in pepe and calò (2017) and crosetto et al. (2016). the psi methods can be subjected to decorrelation effects due to physical and geometrical changes of the surface, such as moisture, erosion, 96 marcos eduardo hartwig, leandro ribes de lima, daniele perissin volcanic episodes, rain, etc. these can lead to phase degradation, which limits the applicability of the method. in order to achieve deformation measurements within millimetric accuracy, at least 25-30 sar scenes are usually interferometrically processed (colesanti et al., 2003). the maximum detectable surface displacements will depend on the radar wavelength, the temporal and spatial resolutions, rate and type of deformation (e.g., smooth versus abrupt surface motions) and the scale of the deformation phenomena. according to crossetto et al. (2016), wasowski and bovenga (2014) and sabater et al. (2011) the theoretical maximum detectable displacement between acquisitions and between adjacent pixels is l/4, where lambda is the radar wavelength. another aspect involved with this technique is that it can only measure a onedimensional motion along the line-of-sight (los) of the sensor (figure 3). in order to retrieve vertical and horizontal displacements (in the e-w direction), data acquired in both ascending and descending passes must be geometrically combined (fuhrmann and garthwaite, 2019). figure 3. measurable surface displacement in ascending and descending geometries. displacements along the orbit track cannot be detected. source: authors. dataset and methods in this study, we used 39 single look complex slc descending scenes from the sentinel-1b satellite, in vv polarization, acquired in the interferometric wide swath mode – iw (sub-swath 3) from 2018/01/10 to 2019/04/11 (16 months), with incidence angle ranging from 41 to 53 degrees (45 degrees in the study area). these images were processed in the latest version of sarproz software (https://www.sarproz.com/). the persistent scatterer interferometry psi method was applied. in order to minimize temporal baseline and maximize temporal coherence, the scene acquired at 2018/09/19 was chosen as the master scene for the generation of the interferograms. perpendicular baselines are below to ±104 meters. for the compensation of the topographic effects, the srtm global 1 arc second (30m) digital elevation model was used. a stable reference point with high temporal coherence (a civil infrastructure target) outside the mining area was selected for the estimation of the displacements. the processing chain is summarized in figure 4. figure 4. sarproz workflow for multi-temporal insar. acronyms: iw – interferometric wide swath mode, slc – single look complex radar images, dem – digital elevation model, ps – persistent scatterer, sar – synthetic aperture radar. validation with in-situ measurements is not trivial due to non-identical measurements locations in both space and time and non-identical reference points. nevertheless, in order to validate the deformation time series measured from space, we tentatively compared it with vertical displacements measured through total station leveling (colesanti et al., 2001). this method can determine the full 3d displacement vector. for this study, only one benchmark prism was available (figure 5) and displacements were recorded from 2018/01/03 to 2018/04/27. prism accuracy is about two millimeter. sentinel-1 sar scenes were acquired approximately at 05h19a.m each 12 days. therefore, we compare the closest data in both space and time. the spatial distance between the ps and the benchmark is 25 m. since insar can provide only the component of the deformation along the line of sight, in order to keep results comparable, we multiplied the benchmark measurements by the cosine of the local incidence angle (pinto et al. 2015). as pointed out before, radar scenes may be subjected to geometric distortions due to slope orientation (aspect) and slope inclination with respect to the radar incidence angle of illumination. these effects when present make phase information useless. sentinel-1 exhibits very low sensitivity for motion detection for slopes with displacement vectors parallel to its orbit track. in the study area, this corresponds to nne and ssw-facing slopes. issues as described above can be minimized by using radar scenes with different incidence angles or by combining radar scenes acquired in both ascending and descending orbit tracks (fuhrmann and garthwaite, 2019). results and discussions in mining activities, surface displacements may be induced in three main different ways: 1) due to underground and slope excavations; 2) due to mining waste disposal in waste piles; and 3) due to discarding of mining byproducts after ore concentration in tailings dams. read and stacey (2009) and sjöberg (1996) pointed out many reasons for mine slope motions, such as poor rock mass quality, induced vibrations, poor rock blasting, inadequate slope geometry (both height and slope angle), slope erosion, lithologies with reduced strength, high rock stress, unfavorable geological structures (both orientation and strength properties), high underground water pressures, etc. the acceptable amount of displacements depends overall on the strength and deformability properties of rock masses and earth-like materials (zavodni, 2000). displacement warning levels are defined then usually as low, moderate and high and their limits are updated as new data is compiled during mine life span. when a warning level is reached, a set of procedures is immediately implemented in order to keep geotechnical risks acceptable and keep mining operations. figure 5 depicts los deformation map for the entire mrdm for the period between january 2018 to april 2019. a total of 630 ps was detected over the processed area, which corresponds approximately to 45 ps/km2. ps are distributed heterogeneously over the area. for the tailing dam very few points (ps) were mapped and in situ geotechnical monitoring data are unavailable. cumulative displacements range from -16.9 to +7.7 mm. displacement accuracy varied between 1.4 to 7.9 mm. the deformation state for the entire footwall slopes does not show critical behavior, considering the deformation magnitudes observed in the period and the warning levels adopted for mining operation. in fact, slope displacements below 50 mm is considered normal for open pit mining operation in the mrdm. in addition, no anomalous behavior has been reported elsewhere in the mine area during the image acquisition interval. data shows a gentle subsidence pattern over waste piles. hartwig (2016) and paradella et al. (2015) have described displacements well above these values for the waste piles of the n4 iron mines, carajás, state of pará (> -20 cm/year). pinto et al. (2015) detected deformation rates over waste piles in the azul mn mine equal to -17 cm/year. figure 5 also shows many ps in the southeast region outside the mine site. this area comprises bare soil, shrubs, small trees and sparse grassland. these points shows a consistent subsidence pattern and record the largest negative deformation values for the entire area. the mrdm is located in a stable archean basement window (guimarães, 1993). according to rpa (2018) the mrdm is located in a arid region with annual precipitation ranging from approximately 300 mm to 1,500 mm (with an average of 700 mm over the last years) and pan evaporation of 1,398 mm/year. in 2015 and in the following years mine operations were affected due to a severe drought. mrdm operated ten groundwater wells between 2011 and 2019. although groundwater wells data are not available (e.g. well locations, water flow rate and water level measurements), a possible explanation for the subsidence pattern observed in https://www.sarproz.com/ 97iw sentinel-1 satellite scenes for the investigation of mine slope stability: experiences from the riacho dos machados gold mine (brazil) figure 5 could be regarded to groundwater pumping for water supply in mining operations. the site geology is essentially made of precambrian schists. mine geology has revealed the presence of both ductile and brittle structures. groundwater is stored in geological structures (fractured aquifer). when groundwater is pumped, the effective stress increase and the rock mass deforms vertically (galloway and hoffmann, 2007). the amount of deformation depends on the virgin state of stress, discontinuity stiffness and orientation and on the amount of groundwater pumped (i.e. fluid pressure decrease). although the process is more effective in porous aquifers, it has already been verified in fractured aquifers as well (schuite et al., 2015). the iw sentinel 1 imagery presents great advantages over traditional geotechnical monitoring systems, since it is able to cover large areas without the need of field surveys, which poses potential risks for technical personal. however, small areas subjected to high deformation rates as well as vegetated areas (figure 5) cannot be “seen” with iw sentinel 1 scenes. moreover, deformation maps based on iw sentinel-1 sar scenes as presented in this study may show a highly heterogeneous ps distribution in space. for instance, the hanging wall slopes are poorly covered by ps data. therefore, in open pit mines, does not exist a unique method capable of tackling with all its complexity. for this reason, multiple source datasets should be fully integrated (vaziri et al., 2010). psi validation for the period of this study, only benchmark prism p6 was available. figure 6 compares displacements obtained by benchmark prism p6 and ps463. benchmark prism p6 is located in the slope bench and ps463 is located in the slope face. both are located close to failure 2017 indicated in figure 1, where outcrop rock masses with fair quality (rmr iii). p06 and ps463 show minimum, maximum and medium displacement values equals to 2.32, 3.22 and 2.75 and 0, 5.67 and 1.62, respectively. figure 6 shows that measurements acquired from both instruments are quite similar in magnitude. measurements in april 11, 2018 show a maximum difference of nearly 7 mm. its significance is not clear, but may be due to measurement locations in space and time, which are not exactly the same. anyhow, displacements do not show any clear tendency and their magnitudes are small, indicating that the slopes can be considered stable for this period. figure 6 – validation of surface deformation measured from space. final remarks and outlook this work aimed to evaluate the applicability of the freely available sentinel-1 sar scenes for detecting and monitoring mine-induced displacements based on the psi technique for the mrdm. the following can be stated: 1) small values of deformation (< |16.9| mm) have been remotely detected for the mrdm. that indicates a stable geotechnical behavior considering the expected deformation levels for mining operation; 2) sentinel-1 iw sar scenes can be used to provide a broad picture of the los deformation phenomena induced by mining activities; figure 5. los deformation map for the entire mrdm overlaid on googleearth optical imagery for the period from january 2018 to april 2019. open triangle represents the benchmark prism p6 used for validation purposes. los deformation scale bar ranges from -10 to 10 mm. 98 marcos eduardo hartwig, leandro ribes de lima, daniele perissin 3) deformation maps based on iw sentinel-1 sar scenes should be combined with data derived from other monitoring methods since ps coverage is heterogeneous; 4) deformation maps with millimeter accuracy based only on descending sentinel-1 sar scenes can be produced at least each 12 days since the processed data set is large enough (³ 40 scenes). acknowledgements authors would like to thank leagold mining company and the resident geotechnical team for providing geotechnical monitoring field data. authors would like to say thank you to alfredo rocca (nhazca srl) for helping with insar processing. authors would like also to thank the anonymous reviewers for their suggestions and comments. references amelung, f., galloway, d. l., bell, j. w., zebker, h. a., & laczniak, r. j. 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(1997). atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps. journal of geophysical research, 102 (4), 75477563. keywords: polynomial regression; gps elevation fitting; quadric surface fitting; plane fitting; geodetic height. palabras clave: regresión polinómica; ajuste de elevación gps; ajuste de superficie cuadrática; montaje en plano; altura geodésica. how to cite item wang, j., & xie, d. (2020). gps elevation fitting study based on ternary polynomial regression. earth sciences research journal, 24(2), 201-205. doi: https://doi.org/10.15446/ esrj.v24n2.87228 for the traditional gps elevation fitting method, the accuracy has not been significantly improved in recent years, and the method has become increasingly complicated. this paper proposes to insert the geodetic height ‘h’ into the calculation system and use a ternary polynomial regression function to fit the gps elevation anomaly. the feasibility of the ternary polynomial regression method in gps elevation fitting was verified by an example, and compared with the results of the traditional plane fitting and quadric surface fitting method, it was proved that the proposed method is suitable for terrain with large terrain fluctuations. the fitting residual error and the standard deviation are smaller, and through example calculations, it is concluded that the ternary polynomial regression method under the seven parameters has the highest fitting accuracy. abstract gps elevation fitting study based on ternary polynomial regression estudio de ajuste de elevación gps basado en regresión polinómica ternaria issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87228 para el método tradicional de ajuste de elevación gps, la precisión no se ha mejorado significativamente en los últimos años, y el método se ha vuelto cada vez más complicado. este documento propone insertar la altura geodésica "h" en el sistema de cálculo y utilizar una función de regresión polinómica ternaria para ajustar la anomalía de elevación del gps. la viabilidad del método de regresión polinómica ternaria en el ajuste de elevación gps se verificó mediante un ejemplo y, en comparación con los resultados del método tradicional de ajuste de plano y ajuste de superficie cuadrática, se demostró que el método propuesto es adecuado para terrenos con grandes fluctuaciones. el error residual de ajuste y la desviación estándar son menores, y a través de cálculos de ejemplo, se concluye que el método de regresión polinómica ternaria bajo los siete parámetros tiene la mayor precisión de ajuste. resumen record manuscript received: 13/11/2019 accepted for publication: 07/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 201-205 jianmin wang*, dongping xie college of surveying and mapping and geographic sciences, liaoning technical university, fuxin, 123000, china * corresponding author: furuiteng123456@126.com gp s m ea su re m en ts ap pl ie d to g eo ph ys ic al pr ob le m s https://doi.org/10.15446/esrj.v24n2.87228 https://doi.org/10.15446/esrj.v24n2.87228 https://doi.org/10.15446/esrj.v24n2.87228 mailto:furuiteng123456@126.com 202 jianmin wang, dongping xie introduction in recent years, china’s beidou navigation satellite system (bds) has made great progress and is expected to achieve the goal of global coverage by 2020. the coordinate system used is the beidou coordinate system (bdcs) is a coordinate system which is defined in the framework of the international terrestrial reference frame (itrs) as the current world geodetic coordinate system (wgs84) (li & huang, 2016). therefore, the research on elevation fitting of global positioning system (gps) is helpful for the effective conversion between beidou height and normal height in the future. at present, through the research of some scholars, the methods of fitting gps elevation data can be roughly divided into two categories. one is the traditional function fitting method, and the other is the new model fitting method. the traditional methods include weighted average method, plane fitting method, quadratic polynomial surface fitting method, moving quadratic polynomial surface fitting method and cubic polynomial surface fitting method, and the new models include neural network fitting method, support vector machine fitting method and curve cloud model fitting method (zhang et al., 2015; ren et al., 2015; guo, 2018; tang et al., 2016). in the actual operation, the new fitting method is not suitable for large-scale popularization because of its complex model, long data processing cycle and high requirements for hardware and software. however, the traditional fitting method only considers the relationship between the elevation anomaly and the plane coordinates x and y of the point, and the fitting accuracy is low, even in the practical engineering application, it can not achieve the experimental accuracy. in this paper, based on the traditional gps elevation fitting method, the geodetic height is included in the solution system, and a gps elevation fitting model based on ternary polynomial regression is proposed (zhou et al., 2016; ding & sun, 2016; liu et al., 2016; yang & zie, 2017; sun et al., 2014). ternary polynomial regression method the traditional elevation fitting method only considers that the elevation anomaly varies with the position of the plane. however, by understanding the elevation system (positive height, normal height, and geodetic height) and deducing the formula, it can be known that the elevation anomaly has a certain relationship with geodetic height. the ternary polynomial regression method can fully consider the relationship between elevation anomalies and x, y, h, so as to better fit the real elevation anomaly of ground points, and realize the conversion between the geodetic height and the normal height (zhao et al., 2011; wang et al., 2016; chen et al., 2014; song et al., 2014). interpretation of elevation anomaly and geodetic height in figure 1, the point p and the point ′p are on the same vertical line and have the same plane position. then there are h n hg + = (1) h n hg  + ′ = ′ (2) in the formulas, hg is the positive height lpqof point p, n is the geoid undulation lqv , h is the geodetic height lpu; hg is the positive height lp q′ of point ′p , ′n is the geoid undulation lqv , ′h is the geodetic height lp w′ . it can be seen from figure 1 that n n= ′, so there is ∇ − − ∇h h h h h hg g g= ′ = ′ = (3) that is, the positive elevation difference between the p point and the ′p point is equal to the geodetic height difference. in the formula: hg is the positive elevation difference between point p and point ′p , and h is the geodetic height difference between point p and point ′p . according to the literature (figure 1), the difference between the elevation anomaly  and the geoid n meets n g hm m m g= + ⋅η −γ γ (4) in the formula, gm is the real average gravity value on the vertical line between the geoid and the earth’s surface, and m is the average gravity value from the reference ellipsoid along the normal line to the approximate earth’s surface (li & huang, 2016). for point ′p , there is ′ = ′ + ′ ′ ′ n gm m m η −γ γ (5) for formula (4) and (5), there are h hg g≠ ′ ,g gm m≠ ′ and γ ≠ γm m′ . according to the complexity of the leveling surface and the earth’s gravity field, g h g hm m m g m m m g −γ γ −γ γ ⋅ ≠ ′ ′ ′ ⋅ ′ and combined with the formula n n= ′, figure 1. elevation system diagram 203gps elevation fitting study based on ternary polynomial regression parameters is carried out respectively. the fitting accuracy is shown in table 5, and the trend of fitting accuracy is shown in figure 2. from the results in table 3, it can be seen that the smallest value of the three-parameter ternary polynomial regression fitting residual is 0.17 cm, the average residual is 0.89 cm, and the smallest value of the plane fitting residual is 0.23 cm, the average residual is 2.61 cm. it is concluded that the ternary polynomial regression fitting method has strong feasibility in the actual gps elevation fitting.  ≠ ′ can be obtained (shijun & li, 2015; gong et al., 2014; škrekovski et al., 2019; zhao et al., 2017). looking back at the point p and point ′p , they have the same plane position, but the difference is their positive height. therefore, it can be seen that the elevation anomaly  is related to positive elevation hg. formula (3) shows that the change of positive elevation is equal to that of geodetic height. therefore, the elevation anomaly  is related to geodetic height h. ternary polynomial regression function if x, y and h are taken as three independent variables and elevation anomaly  is taken as dependent variable, then there are functions 1) three parameters f a x a y a h3 0 1 2= = + + (6) 2) four parameters f a a x a y a h4 0 1 2 3= = + + + (7) 3) seven parameters f a a x a y a h a xy a yh a xh7 0 1 2 3 4 5 6= = + + + + + + (8) 4) eight parameters f a a x a y a h a xy a yh a xh a xyh8 0 1 2 3 4 5 6 7= = + + + + + + + (9) 5) eleven parameters f a x a y a h a xy a yh a xh a x a y 11 0 1 2 3 4 5 6 7 2 8 = + + + + + + + + η= α 22 9 2 10+ +a h a (10) by calculating the known data set (x, y, h, ), the corresponding polynomial parameters can be obtained and the functional relationship can be determined. among them, three-parameter function requires at least three known point data, four-parameter function requires at least four known point data, seven-parameter function requires at least seven known point data, eight-parameter function requires at least eight known point data, and eleven-parameter function requires at least eleven known point data. if there is more redundant known data, it can be calculated according to the least square criterion (liu et al., 2016; wang et al., 2016; wu & miao, 2010). x a pa a plt t= ( ) ⋅ 1 (11) in the formula, a is a coefficient matrix composed of known data x, y, h and xy, p is a weight matrix, generally a unit matrix, l is an observation matrix composed of known data , and x is the parameter matrix obtained. case analysis in order to analyze the feasibility of the ternary polynomial regression fitting method proposed in this paper, the first 12 points are taken as the reference points and the last 7 points are taken as the points to be fitted by using the data provided in literature (li, 2013). planar fitting and three-parameter ternary polynomial regression fitting are performed separately. the known data is shown in table 1 and table 2. after calculation, the parameters of plane fitting function are respectively a0 34 1044= . , a e1 8 0703 6= . , a e2 3 1533 5= . and, and the parameters of three-parameter ternary polynomial fitting are respectively a e0 3 6290 7= . , a e1 4 4750 5= . , a e2 2 3390 4= . and. the fitting results are shown in table 3. by using the above data, quadric surface fitting and seven-parameter ternary polynomial regression fitting are used respectively. the fitting results are shown in table 4. using the above data, the ternary polynomial regression fitting of three parameters, four parameters, seven parameters, eight parameters and eleven table 1. data of reference points (unit: m) point number ordinate x abscissa y geodetic height h 1 3564231.786 499937.723 30.442 2 3565858.080 499248.000 31.225 3 3564029.592 499613.378 30.134 4 3565549.066 498813.558 31.110 5 3563826.318 499348.917 30.059 6 3566091.401 499632.434 30.709 7 3564312.203 500321.498 31.206 8 3566375.346 499179.740 26.424 9 3564827.161 500392.773 29.487 10 3566324.251 498659.474 25.790 11 3564001.762 500035.270 31.338 12 3567660.247 499189.334 31.128 table 2. data of points to be fitted (unit: m) point number ordinate x abscissa y geodetic height h 13 3566814.699 499080.199 30.989 14 3567961.396 498691.018 30.160 15 3566854.849 498567.506 30.741 16 3567538.805 499757.287 31.074 17 3566774.076 499599.745 30.621 18 3567524.909 500219.017 31.183 19 3568016.260 499235.369 30.332 table 3. plane fitting and three-parameter ternary polynomial regression fitting results. point number elevation anomaly/m plane fitting three-parameter ternary polynomial regression fitting fitting value/m residual/ cm fitting value/m residual/cm 13 21.0273 21.0566 2.93 21.0322 0.49 14 20.9978 21.0351 3.73 21.0146 1.68 15 21.0076 21.0401 3.25 21.0093 0.17 16 21.0639 21.0721 0.8 21.0622 -0.17 17 21.0710 21.0733 0.23 21.0555 -1.55 18 21.0516 21.0868 3.52 21.0829 3.13 19 21.0138 21.0518 3.8 21.0389 2.51 204 jianmin wang, dongping xie from the results in table 4, it can be seen that the fitting effect of sevenparameter ternary polynomial regression is better than that of three-parameter ternary polynomial regression. the average residual is reduced to 0.27 cm, and the overall residual is mostly within 1 cm. similarly, the fitting effect of quadric surface is better than that of plane fitting. the minimum residual is reduced to 0.05 cm, and the average residual is reduced to 1 cm. it can be seen that the overall effect of seven-parameter ternary polynomial regression fitting is better than that of quadric surface fitting. conclusion in this paper, in the process of gps elevation fitting, the geodetic height is included in the calculation system, and the ternary polynomial regression method is used to solve the elevation anomaly. the method takes into account the influence of the geodetic height on the elevation anomaly and avoids the phenomenon of distortion caused by the omission of the geodetic height. the feasibility of the proposed method and its superiority in fitting the global elevation anomaly in a large range are verified by the experimental results. in addition, it can be seen from table 5 that in the actual engineering application, the fitting effects of the seven-parameter and eight-parameter are the best. the internal fitting accuracy is less than 0.2 cm, and the external fitting accuracy is about 1.5 cm. this paper puts forward new ideas and directions for gps elevation fitting. in order to solve practical problems better, it can be combined with other methods, and a series of problems related to accuracy need to be tested and discussed further. acknowledgement national natural science foundation project (no. 41474020). references chen, y., ma, q., liu, r., & mao, x. (2014). discussion on the relative height anomaly of gps elevation fitting method. bulletin of surveying and mapping, (12), 67-69. ding, h., & sun, j. (2013). research on total least-squares methods for transformation of gps elevation. journal of geodesy and geodynamics, 33(03), 52-55+63. gong, x., chen, q., & zhou, x. (2014). the application research of gps height fitting based on tls adjustment method. bulletin of surveying and mapping, (03), 6-8. guo, h. (2018). transformation of gps height and accuracy analysis based on combination model. science of surveying and mapping, 43(02), 34-38. li, j. (2013). research on regional gps height anomaly fitting and modeling method. kunming university of science and technology. liu, b., guo, j., shi, j., & wu, d. (2016). a gps height fitting method based on the egm2008 model and terrain correction. geomatics and information science of wuhan university, 41(04), 554-558. liu, y., zheng, n., zhang, x., & ge, l. (2016). robust weighted total least squares estimation for gps leveling fitting. journal of geodesy and geodynamics, 36(01), 30-34. table 4. quadratic surface fitting and seven-parameter ternary polynomial regression fitting results. point number elevation anomaly/m quadratic surface fitting seven-parameter ternary polynomial regression fitting fitting value/m residual/ cm fitting value/m residual/ cm 13 21.0273 21.0268 -0.05 21.0233 -0.4 14 20.9978 20.9943 -0.35 20.991 -0.68 15 21.0076 21.0062 -0.14 20.998 -0.96 16 21.0639 21.0351 -2.88 21.0525 -1.14 17 21.0710 21.0437 -2.73 21.0494 -2.16 18 21.0516 21.0447 -0.69 21.078 2.64 19 21.0138 21.0119 -0.19 21.0217 0.79 table 5. ternary polynomial regression fitting accuracy parameter form fitting accuracy /cm internal fitting external fitting three parameters 0.80 1.76 four parameters 4.28 4.09 seven parameters 0.16 1.53 eight parameters 0.09 1.58 eleven parameters 1.02 1.23 5 4 3 2 1 0 3 4 5 6 7 8 9 10 11 figure 2. ternary polynomial fitting accuracy trend 205gps elevation fitting study based on ternary polynomial regression li, z., & huang, j. 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(2011). gps height fitting of weighted total least-squares adjustments. journal of geodesy and geodynamics, 31(05), 88-90+96. zhao, q., xu, a., & xu, z. (2017). gps levelling based on robust total least squares. journal of navigation and positioning, 5(01), 95-99. zhou, c., sun, j., & guo, s. (2016). height anomaly fitting method based on mobile-polyhedral function. bulletin of surveying and mapping, (12), 25-27+38. keywords: bodrum; kos; earthquake; normal fault; inversion; extension palabras clave: bodrum; kos; terremoto; falla normal; inversión; extensión; how to cite item over, s., ozden, s., kalkan, e. e., turhan, f., coskun, z., & pinar, a. the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey. earth sciences research journal, 25(3), 309-321. doi: https:// doi.org/10.15446/esrj.v25n3.87080 a 6.6 (mw) earthquake struck the western part of gökova gulf in the eastern aegean sea on july 20, 2017. the fault plane solution for the mainshock shows an e-w striking normal fault with approximately n-s (n4°e) tensional axis (t-axis). fault plane solutions for 33 aftershocks offer two groups of normal faulting with e-w and ne-sw to enewsw orientations. the inversion of the focal mechanisms of the aftershocks yields two different extensional stress regimes. the stress regime obtained from 12 focal mechanisms of aftershocks and the mainshock is characterized by an approximately n-s (n5°e) σ3 axis, while the other regime calculated from 21 focal mechanisms of aftershocks exhibits σ3 axis in an nw-se (n330°e) direction. the latter extension significantly affects the basin’s growth in the area where the earthquake occurred. twenty-four focal mechanisms of earthquakes in and around gökova basin before the 2017 earthquake (1933-2017) were included in the inversion to determine the stress state effective in a larger area. the inversion yielded an extensional stress regime characterized by approximately n-s (n356°e) σ3 axis. e-w trending faults inferred in the central part of gökova fault zone, bordering gökova gulf in the north, also indicate n-s extension. the nw-se extension obtained from ne-sw aftershocks appears to be locally effective in the northwest of gökova gulf. n-s extension, which appears to act on a regional scale, may be attributed to geodynamic effects related to the roll-back of the african subduction beneath the aegean. abstract the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey terremoto de magnitud 6.6 en bodrum-kos, el 20 de julio de 2017, en el suroeste de anatolia, turquía issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.87080 un terremoto de magnitud 6.6 golpeó la parte oeste del golfo de gökova en el este del mar egeo el 20 de julio de 2017. el mecanismo focal del evento principal muestra un movimiento e-o en la falla normal con un eje tensional (t-axis, en inglés) en dirección n-s (n4°e). el mecanismo focal de 33 réplicas muestra dos grupos de fallamiento normal con orientaciones e-o y ne-so hacia ene-oso. la inversión del mecanismo focal de las réplicas produjo dos regímenes de fuerza extensiva diferentes. el régimen de fuerza obtenido a través de 12 mecanismos focales de réplicas y del movimiento principal se caracteriza por un desplazamiento aproximado n-s (n5°e) σ3 del eje, mientras que el otro régimen se calculó a partir de 21 mecanismos focales en réplicas y muestra un desplazamiento del eje en dirección no-se (n330°e). esta última extensión afecta significativamente el crecimiento de la cuenca en el área donde ocurrió el terremoto. veinticuatro mecanismos focales de terremotos en la zona de la cuenca gökova y que ocurrieron entre 1933 y 2017 fueron incuidos en la inversión para determinar el estado de la fuerza efectiva en una área mayor. la inversión dio como resultado un régimen de fuerza extensiva caracterizada por un desplazamiento del eje aproximado n-s (n356°e) σ3. de las fallas con tendencia e-o se infiere que en la parte central de la zona de fallas de gökova, que limita al norte con el golfo de gökova, también tiene una extensión n-s. la extensión no-se obtenida de las replicas ne-so aparece como localmente efectiva en el noroeste del golfo de gökova. la extensión n-s, que según los resultados actúa a escala regional, puede ser atribuido a los efectos geodinámicos relacionados con el retroceso de la subducción africana bajo el egeo. resumen record manuscript received: 07/05/2020 accepted for publication: 16/07/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (septiembre, 2021): 309-321 semir över1, süha özden2, esra kalkan ertan3, fatih turhan3, zeynep coşkun3, ali pınar3 1iskenderun technical university, department of civil engineering, 31200, iskenderun, turkey. 2çanakkale onsekiz mart university, department of geological engineering, 17100, çanakkale, turkey. 3boğaziçi university, kandilli observatory, and earthquake research institute, 36684, i̇stanbul, turkey. corresponding author: semir.over@iste.edu.tr se is m o lo g y https://doi.org/10.15446/esrj.v25n3.87080 https://doi.org/10.15446/esrj.v25n3.87080 https://doi.org/10.15446/esrj.v25n3.87080 mailto:semir.over@iste.edu.tr 310 semir över1, süha özden, esra kalkan ertan, fatih turhan, zeynep coşkun, ali pınar introduction plate boundary forces caused by relative movements between africa, arabia, and eurasia resulted in complex tectonic structures in the western anatolia-aegean region (fig. 1). plate movements involving the convergence of arabia and africa with eurasia in the north; 1) give rise to the westward extrusion of anatolia along the strike-slip north anatolian fault (naf) and east anatolian fault (eaf), 2) cause crustal extension in western anatolia-aegean, and 3) are responsible for complex deformation in the eastern mediterranean region (mckenzie, 1972; dewey and şengör, 1979; le pichon and angelier, 1979; angelier et al., 1981; jackson and mckenzie, 1984, 1988; taymaz et al., 1990; 1991; över et al., 2010; özden et al., 2018). the complex tectonics of western anatolia-aegean involve different extensional regimes: n-s, ne-sw, and nw-se (mckenzie, 1972; jolivet and brun, 2010; alçiçek et al., 2006; shah, 2015; över et al., 2016; ocakoğlu et al., 2018). gökova gulf, which runs east-west, is an asymmetrical basin that is growing towards the sea. the gökova graben formed on the lycian nappes and filled with plio-quaternary units known as the gökova formation (görür et al., 1995; gürer and yılmaz, 2002; gürer et al., 2013; tur et al., 2015). gürer et al. (2013) suggested that the northern edge of the gulf is bounded by the southdipping e-w gökova fault, one of the most active structures in the southern part of west anatolia-aegean, based on geomorphological and geological features. the southern border of the basin is not as clear and uninterrupted as the northern border. the active fault map also shows that the datça fault zone, which represents the western part of the southern boundary of the basin, is more active (fig. 2). the datça fault zone is divided into two parts: north and south. the northern segment extends to the east of kos (karasözen et al., 2018), while the southern segment is listric (kurt et al., 1999). analyzing both bathymetric and seismic reflection data, i̇şcan et al. (2013) revealed the existence of active strike-slip faults with various directions in the gulf of gökova. from analysis of the focal mechanisms, shah (2015) asserted that the area is dominated by normal events with few strike-slip events at the western margin of gökova gulf. the interpretation of new multichannel seismic profiles by ocakoğlu et al. (2018) also shows the existence of active strike-slip and normal faults in different directions in the gulf of gökova. they pointed out southern-dipping figure 1. map of simplified tectonic framework of the aegean-mediterranean sea region (modified from barka, 1992; över et al., 2010). figure 2. location map and active faults in gökova gulf and surroundings (fault data are taken from duman, et al., 2011; emre, et al., 2013; i̇şcan et al., 2013; karasözen 311the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey normal faults on the north border (i.e. gökova fault zone) and northerndipping normal faults on the southern border of the basin (i.e. datça fault zone). normal faults were also implied in the gökova basin by several studies (e.g., kurt et al., 1999; uluğ et al., 2005; i̇şcan et al., 2013; tur et al., 2015; ocakoğlu et al., 2018). following the earthquake on july 20, 2017, researchers obtained contrary findings about the source fault and the fault’s dip direction: (1) from geophysical and geodetic data the bodrum-kos earthquake ruptured a south-dipping e-w normal fault (saltogianni et al., 2017; tiryakioğlu et al., 2018; ocakoğlu et al., 2018) or (2) a north-dipping e-w normal fault (karasözen et al., 2018; ganas et al., 2019; konca et al., 2019). karasözen et al. (2018) asserted that the source fault was the northern datça fault. a section of gökova bay extending from the bodrum peninsula to the datça peninsula shows the presence of e-w striking faults dipping both south and north (fig. 3). in the last 30 years, gps studies about the tectonics of the eastern mediterranean have yielded important results, including: 1) the velocity and direction of movement of africa and arabia relative to eurasia (mcclusky et al., 2000; reilinger et al., 2006), 2) the rate and direction of movement of anatolia towards the west relative to eurasia along the dextral naf (north anatolian fault) and sinistral eaf (east anatolian fault) (mcclusky et al., 2000; reilinger et al., 2006), 3) rate and direction of motion in west anatoliaaegean toward the hellenic trench (mcclusky et al., 2000; aktuğ et al., 2009; howell et al., 2017), and 4) the crustal extension in the west anatoliaaegean (lepichon et al., 1995; mcclusky et al., 2000; reilinger et al., 2010). this area is currently undergoing a n-s extensional process at a rate of 30-40 mm/yr, which is well documented (oral et al., 1995; lepichon et al., 1995), 6) the determination of both earthquake parameters and the source fault geometry (saltogianni et al., 2017; tiryakioğlu et al., 2018; karasözen et al., 2018), and finally 7) revealed the coseismic deformation of the 20th july bodrum-kos earthquake (tiryakioğlu et al., 2018). the gps velocity vectors calculated relative to anatolia (fixed) indicate an increase in the velocity of the southwestward extrusion of anatolia in western turkey and the aegean region compared to other areas of turkey accompanied by anticlockwise rotation (mcclusky et al., 2000; howell et al., 2017). to define the stress regimes and their tectonic implications, we used a method proposed by carey-gahillardis and mercier (1987). we introduced the focal mechanisms of both the main shock and its aftershocks into the inversion algorithm. we also compiled the published focal mechanisms of earthquakes that occurred in gökova and its environs between 1933 and 2017 in order to et al., 2018). figure 3. a cross-section of gökova gulf from bodrum peninsula to datça (a-b section line shown in fig 2). figure 4. map of the distribution of earthquake (20 km depth) epicenters between 20.07.2017 and 20.02.2018 in gökova gulf. the earthquakes are compiled from koeri. 312 semir över1, süha özden, esra kalkan ertan, fatih turhan, zeynep coşkun, ali pınar determine the tectonic regime responsible for the great bodrum-kos earthquake in terms of regional significance. seismicity the historical catalogues show that gökova gulf and its surrounding settlements have been hit many times by a number of moderate to major destructive earthquakes, such as in 24 bc, 412 bc (ambraseys and white, 1997), 227 bc, 199-198 bc, ad 141, 144, 174, 344, 1493, 1851, 1863, and 1869 (ergin et al., 1967; guidoboni et al., 1994; yolsal et al., 2007). luttrell (1999) reported that the city of bodrum was totally demolished by the 1493 earthquake. the activity in the region is also confirmed by earthquakes occurring during the instrumental period, such as 23 april 1933 (mw: 6.2), 23 may 1941 (mw: 5.4), 13 december 1941 (mw: 6.3), 25 april 1959 (mw: 6.1), 19 february 1989 (mw: 5.6), 5 october 1999 (mw: 5.2), 4 august 2004 (mw: 5.4), and 10 january 2005 (mw: 5.2) (eyidoğan and barka, 1996; şaroğlu et al., 1992; uluğ et al., 2005; yolsal and taymaz, 2010). kalafat and horasan (2012) observed swarm type activity in the western gökova gulf during 2004 and 2005. historical and instrumental era earthquakes show that the basin contains significant faults that enable its development. on july 20, 2017, one of the faults triggered a 6.6 (mw) earthquake. following the bodrum-kos earthquake, more than 3000 aftershocks ranging in magnitude from 1.0 to 4.8 were recorded across gökova gulf according to koeri (kandilli observatory and earthquake research institute) data (fig. 4). aftershocks were scattered across a wide region, mainly in the west of the basin, and did not follow a linear distribution (fig. 4). this event created a tsunami with a water wave reaching almost 2 meters height, which damaged the southern coast of bodrum and the northern coast of kos island (heiderzadeh et al., 2017; yalçıner et al., 2017). several studies reported tsunamis in relation to historical and instrumental period events (i.e. in 365, 554, 1303, 1481, 1822 and 1948) in the eastern mediterranean (altınok and ersoy, 2000; yolsal et al., 2007; çevikbilen and taymaz, 2012, çevikbilen et al., 2014). methodology moment tensor inversion in this study we used moment tensor (mt) inversion algorithms that model the waveforms recorded at one or more 3-component broadband seismic stations (e.g., kuge, 2003; sokos and zahradnik, 2008). these techniques perform waveform inversions to find source parameters of small to moderate sized earthquakes. one of the most user-friendly mt inversion routines recently adopted by seiscmop3 is isola, which is based on fortran codes and offers a matlab graphic interface. isola allows for both single and multiple point source iterative deconvolution (kikuchi and kanamori, 1991) and inversion of complete regional and local waveforms. in addition, we also used the technique developed by kuge (2003). in this method, waveform fitting between the observed and synthetic displacement seismograms from one or more stations at local distances is achieved by searching for a mt point on a grid scheme for the best fit between the observed and the synthetic displacement seismograms. during the inversion process uniform weight is given to all seismograms. the broadband seismic station networks operated by koeri, disaster and emergency management presidency (afad) and national observatory of athens (noa) were utilized. for data selection to be used for the inversion process, firstly the signal quality check was completed for three-component broadband stations. seismograms with gaps and signals with signal-to-noise ratio lower than 4.0 were not considered. good azimuthal distribution of stations plays significant role in constraining the mt inversion results. the green’s function (gf) calculation is performed using the frequencywavenumber method (bouchon, 1981). gfs were computed using the crustal structure from akyol et al. (2006). when gfs are calculated, the positions of epicenters are fixed, and the depth is allowed to deviate from close to the surface to a depth of up to 30 km in steps of 2 km. the quality of fit between the observed and predicted seismograms is measured by variance reduction (vr); the larger the value of vr, the better the fit. the variance reduction is calculated for various depths for each time shift, and the faulting mechanism was selected with maximum vr for the analyzed event (fig. 5). figure 5. cmt solution result for the mw 6.6 bodrum-kos earthquake (20 july 2017). 313the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey figure 6. the correlation vs source number plot figure 7. observed versus synthetic waveform fits. inversion of seismic slip-vector data sets to determine the present-day stress state to compute the state of stress responsible for present-day faulting from the population of focal mechanisms of earthquakes that occurred in the gökova gulf, we used an inversion method proposed by carey-gailhardis and mercier (1987); this is one of several existing algorithms (vasseur et al., 1983; gephart and forsyth, 1984). the inversion is a statistical method which allows calculation of the best mean fitting stress state from a population of focal mechanisms by selecting one of two nodal planes as the fault plane. according to this inversion process, the slip (s, defined by a slip vector corresponding to a striation for geological data or a rake for seismological focal mechanisms) on each fault plane occurs in the direction of the resolved shear stress (t), the fault plane being a pre-existing fracture. in the case of a seismic event, the inversion computes a mean best-fitting deviatoric stress tensor by minimizing the angular deviation between an expected slip vector (maximum shear, t) and the observed slip vector (s) deduced from the focal mechanism (carey and brunier 1974; carey 1979). all inversion results include the orientation (azimuth and plunge) of the principal stress axes of a mean deviatoric 314 semir över1, süha özden, esra kalkan ertan, fatih turhan, zeynep coşkun, ali pınar stress tensor, as well as a ‘stress ratio’ [r = (σ2 − σ1)/(σ3 − σ1)], a linear quantity describing relative stress magnitudes, where the principal stress axes, σ1, σ2, and σ3 correspond to compressional, intermediate, and extensional deviatoric stress axes, respectively. it is necessary to know the seismic slip vector, and consequently to select the preferred seismic fault plane for each pair of nodal planes to compute the stress state from earthquake focal mechanisms. for major earthquakes, the selection can be made if there is a co-seismic rupture, or from the spatial epicenter distribution of the aftershock sequence. there is another option for earthquake populations with low magnitude and no surface rupturecomputation. it is possible to calculate the fault slip vector using bott’s (1959) model since only one of the two slip vectors of a focal mechanism solution is in accordance with the principal stress axes. for this slip vector, the r ratio, defined [r = (σ2 − σ1)/(σ3 − σ1)], is such that 0<r<1 (carey-gailhardis and mercier, 1987). furthermore, unless both nodal planes of a focal mechanism converge along a principal stress axis, if one of the nodal planes meets this condition, the other does not (carey-gailhardis and mercier, 1987). we used the calculation method described above to select the nodal plane of each focal mechanism in this analysis. in general, a collection of seismic event focal mechanisms leads to a well-defined assessment of the regional or local stress state that is consistent with the geologically determined stress state: that is, the stress state resulting from inversions of striae measured on fault planes (e.g., sebrier et al., 1988; mercier et al., 1991, 1992; bellier and zoback, 1995; bellier et al., 1997; över et al., 2010). as mentioned above, fault slip inversion schemes assume that the slip vector on each plane corresponds to the direction of the maximum resolved shear stress on that plane. the inversion involves at least four different fault figure 8. the moment tensor solution of the mainshock and 12 aftershocks between 20.07.2017 and 16.10.2017 in gökova gulf (numerical data are given for each earthquake in table 1). plots show nodal planes. numbers outside the stereoplot refer to the focal mechanism labels given in table 1. table 1. the parameters of the main shock (no 1) and 12 aftershocks with approximately e-w direction (m ≥ 3.6) occurring between 20.07.2017 and 16.10.2017 in gökova gulf shown in figure 8 and 11. no date day.month. year time (utc) lat. n (°) long. e (°) plane 1 strike°/dip°/ plunge° plane 2 strike°/dip°/ plunge° mag. (moment) seismic moment (nm) h (km) variance reduction (%) station num. references 1 20.07.2017 22:31:09 36.969 27.406 2800/500/-790 830/410/-1030 6.6 1.06e+19 7 78 7 this study 2 20.07.2017 22:54:35 36.933 27.588 680/440/-1270 2930/560/-600 3.6 2.75e+14 6 65 5 this study 3 21.07.2017 05:13:58 36.914 27.611 900/460/-890 2690/440/-910 4.2 2.65e+15 6 69 6 this study 4 21.07.2017 07:05:23 36.923 27.483 2840/600/-610 560/410/-1300 3.8 7.03e+14 9 64 6 this study 5 21.07.2017 21:51:01 36.909 27.608 950/370/-850 2680/530/-940 3.8 5.46e+14 4 80 7 this study 6 24.07.2017 21:48:48 36.969 27.490 860/670/-890 2640/230/-920 4.0 1.48e+15 7 73 8 this study 7 30.07.2017 07:06:17 36.971 27.585 870/500/-840 2580/410/-970 3.7 3.76e+14 7 70 6 this study 8 09.08.2017 09:25:29 36.988 27.606 2930/610/-720 800/340/-1190 3.8 5.58e+14 6 70 7 this study 9 12.08.2017 02:02:02 36.902 27.732 2630/350/-1020 980/550/-810 3.6 2.99e+14 4 63 5 this study 10 18.08.2017 12:47:31 36.914 27.606 1110/340/-890 2890/560/-910 4.3 4.13e+15 6 77 6 this study 11 18.08.2017 14:10:47 36.929 27.622 2670/550/-950 960/350/-830 4.5 6.15e+15 10 78 6 this study 12 21.08.2017 10:43:24 36.989 27.666 2790/380/-980 1100/520/-830 3.7 1.40e+01 4 57 6 this study 13 16.10.2017 07:23:48 36.967 27.396 910/470/-870 2660/430/-940 3.7 4.96e+14 9 79 5 this study 315the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey figure 9. the moment tensor solutions of 21 aftershocks between 20.07.2017 and 09.08.2017 in gökova gulf (numerical data are given for each earthquake in table 2). plots show nodal planes. numbers outside the stereoplot refer to the focal mechanism labels given in table 2. table 2. the parameters of 21 aftershocks (m ≥ 3.8) with approximately nw-se direction occurring between 20.07.2017 and 09.08.2017 in gökova gulf shown in figures 9 and 11. no date day.month. year time (utc) lat. n (°) long. e (°) plane 1 strike°/dip°/ plunge° plane 2 strike°/dip°/ plunge° mag. (moment) seismic moment (nm) h (km) variance reduction (%) kandilli station num. references 1 20.07.2017 22:52:58 36.950 27.372 850/530/-690 2330/420/-1150 4.5 6.15e+15 9 71 6 this study 2 20.07.2017 23:23:51 36.958 27.310 2650/470/-650 3090/480/-1140 4.8 1.17e+17 10 68 6 this study 3 21.07.2017 00:16:40 36.974 27.331 790/650/-560 2010/420/-1400 4.1 1.54e+15 12 75 5 this study 4 21.07.2017 00:53:55 36.916 27.365 2480/760/-800 310/170/-1260 4.0 1.24e+15 7 60 7 this study 5 21.07.2017 01:25:34 36.977 27.422 2440/690/-760 290/250/-1230 4.0 1.40e+01 8 83 6 this study 6 21.07.2017 01:50:29 36.984 27.377 2420/600/-790 420/320/-1070 4.2 1.50e+01 13 77 5 this study 7 21.07.2017 01:54:46 36.951 27.364 2350/440/-1060 770/480/-750 3.9 1.40e+01 6 79 6 this study 8 21.07.2017 01:56:27 36.933 27.384 1030/580/-420 2190/560/-1400 3.9 1.40e+01 9 62 6 this study 9 21.07.2017 02:12:34 36.883 27.342 2260/510/-1420 1100/610/460 4.4 4.35e+15 10 72 6 this study 10 21.07.2017 05:52:13 36.981 27.368 2040/650/-1300 870/460/-360 4.1 1.54e+15 11 64 6 this study 11 21.07.2017 14:21:55 36.940 27.480 2410/420/-770 440/490/-1010 3.8 6.08e+14 5 69 7 this study 12 21.07.2017 17:09:46 36.947 27.254 2490/410/-970 780/490/840 4.7 1.12e+16 11 63 7 this study 13 21.07.2017 23:00:48 36.972 27.501 2490/600/-780 470/320/-1090 3.8 6.81e+14 6 67 7 this study 14 22.07.2017 17:09:20 36.940 27.254 2370/460/-1300 1070/570/560 4.4 4.32e+15 10 70 6 this study 15 23.07.2017 18:02:33 36.955 27.326 2610/480/-740 580/440/-1070 3.9 1.06e+15 16 56 6 this study 16 24.09.2017 16:57:16 36.948 27.317 2640/370/-810 720/530/-970 4.4 4.75e+15 8 72 8 this study 17 21.07.2017 01:35:43 36.945 27.578 2180/480/-1530 1100/700/-450 4.3 1.50e+01 6 84 7 this study 18 21.07.2017 03:55:32 36.938 27.548 640/380/-880 2410/520/-920 3.9 8.63e+14 6 72 5 this study 19 30.07.2017 10:56:32 37.003 27.564 2300/560/-1100 830/390/-640 3.9 1.40e+01 5 77 6 this study 20 07.08.2017 05:44:24 36.975 27.599 760/290/-890 2560/610/-900 4.1 1.62e+15 4 75 7 this study 21 09.08.2017 22:56:18 36.989 27.678 2680/660/-810 68/26/-108 3.9 9.16e+14 4 77 6 this study 316 semir över1, süha özden, esra kalkan ertan, fatih turhan, zeynep coşkun, ali pınar sets in this case since there are four unknowns (three defining the orientation of the principal axes and one defining the stress ratio r). faults with variable dip angles and distinct strike directions, rather than a continuum of strikes around a single mean direction, are included in ideal data sets. when the deviation angle between the measured slip vector ‘t’ and the observed slip vector ‘s’ is less than 20˚, the slip vector determined from a focal mechanism is usually considered to be mechanically explained by the computed stress deviator. results of stress inversions are considered reliable if 80 percent of the deviation angles between t and s are less than 20˚ and if the computed solution is stable; that is, the inversion tends towards the same solution regardless of the initial parameter values (carey, 1979; carey-gailhardis and mercier, 1987; mercier et al., 1989, 1991; bellier and zoback, 1995). result and discussion focal mechanisms of the main earthquake and aftershocks an earthquake (m=6.6) hit bodrum, kos, and the surrounding area in the west of the gulf of gökova on july 20, 2017. the main shock and the majority of the aftershocks show predominantly normal faulting character. the focal mechanism analysis was made using kuge (2003)’s method and isola software. the main shock (mw: 6.6) with a n-s (n4°e) t-axis must have occurred on a relatively large e-w fault (table 1). the mt analysis yields normal faulting mechanisms for 34 of the events occurring after the bodrum-kos earthquake (mw 6.6). the focal mechanisms of the aftershocks show two distinct nodal plane directions. they are approximately e-w (fig. 8 and table 1) and ne-sw to ene-wsw (fig. 9 and table 2), respectively. the e-w directional nodal planes indicate n-s extension, while the ones with ne-sw to ene-wsw orientation indicate approximately nw-se extension. several marine geophysical and geodetic studies revealed a large number of faults, mostly normal and a few strike-slip, in the basin with varying directions (kurt et al., 1999; uluğ et al., 2005; i̇şcan et al., 2013; tur et al., 2015; ocakoğlu et al., 2018; tiryakioğlu et al., 2018). ocakoğlu et al. (2018) described ne-sw normal faults at the western end of the bay off kos island (their profile 3 in fig. 11). figure 10. the moment tensor solutions of 24 historical earthquakes between 1933 and 21.07.2017 in and around gökova gulf (references and numerical data are given for each earthquake in table 3). plots show nodal planes. numbers outside the stereoplot refer to the focal mechanism labels given in table 3. figure 11. a. inversion results of the earthquake slip data computed from the focal mechanisms of earthquakes shown in figure 8 and in table 1. (b) inversion results of the earthquake slip data computed from the focal mechanisms of earthquakes shown in figure 9 and in table 2. (c) inversion results of the earthquake slip data computed from the focal mechanisms of earthquakes shown in figure 10 and in table 3. stress axes obtained from the inversions are shown by diamonds (σ1), triangles (σ2) and squares (σ3) and stress ratio value given by the formula [r = (σ2 −σ1)/(σ3 −σ1)]. thick lines on the fault traces give the deviation angle between measured (s) and predicted (τ) slip-vectors for each fault plane. histogram shows distribution of deviation angles (angle between the observed slip, s, and the predicted slip, τ. 317the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey n-s extensional stress tensor n-s extension in this study 58 events, comprising new and published focal mechanisms (tables 1, 2 and 3), are included in the carey-gailhardis and mercier (1987) inversion method in order to obtain present-day stress tensors and their tectonic implications. these are the focal mechanisms for earthquakes that occurred in gökova and its environs before july 20, 2017 (references in table 3), as well as the major shock and its aftershocks (see tables 2 and 3). figure 10 illustrates the available focal mechanisms for earthquakes with magnitudes ranging from 4.0 to 6.6 between 1933 and 2017 including the 20th july 2017 main event (mw: 6.6). all focal mechanisms that give normal to oblique slip faulting are introduced into the inversion in order to determine the stress state acting in an area wider than gökova gulf. the inversion of 24 selected nodal plane sets (table 3) yields a normal faulting stress regime (e.g., σ1 is vertical) characterized by approximately n-s (n356°e) σ3 axis, i.e. minimum horizontal stress axis (fig. 11). the computed r value is 0.571 indicating a triaxial extensional stress state and clearly differs from a radial extension stress state, which requires a high r value (r≥0.85) as defined in bellier and zoback (1995). figure 8 shows the focal mechanisms of 12 aftershocks with magnitude ranging from 3.6 to 4.5 as well as that of the main shock. the inversion of the 13 nodal plane sets (table 1) gives a normal faulting stress regime with approximately n-s (n5°e) minimum horizontal stress axis (σ3) (fig. 11). the determined r value of 0.367 implies a triaxial stress state rather than a radial extension which needs a low r value (r≤0.15) as described by bellier and zoback (1995). the inversion of the moment tensor of the 20th july 2017 earthquake (mw 6.6) yields fault parameters from both nodal planes (np1: strike, 280˚, dip. 50˚ and rake, -79˚; np2: 83˚, 41˚, -103˚; see table 1 for details). the inversion of the focal mechanism algorithm chose np1 as the suitable fault plane (i.e. the plane that dips to the north) for calculating the convenient stress tensor. similar to other inversion methods, the analysis we used in this study is an effective and successful method for determining the stress tensor, or stress field orientations, from a population of nodal planes, but it is not capable of determining stress magnitude. furthermore, even if the algorithm selects one of the nodal planes as the fault plane based on the computed stress tensor, surface rupture and/or aftershock distribution are required to confirm this. saltogianni et al. (2017) proposed that the bodrum-kos earthquake caused a nearly e-to ese striking rupture in the upper crust from the surface (sea bed) to a depth of 12 km based on seismological and geodetic data. however, no surface ruptures were observed on land. as a result, it is difficult to tell which of the nodal planes is the true fault (i.e. whether the seismic fault dips north or south) based on the available evidence. the dip and location of the source fault are also debated; those who argue that the fault dips north indicated that the fault is located in the center of the gulf, such as the north datça fault extending to the east of kos (karasözen et al., 2018) or a fault with 10 km e-w trend following the gökova rift (ganas et al., 2019). the seismic fault is situated in the northwest of the basin, offshore of kos and bodrum according to those who suggested the source fault dips south (saltogianni et al., 2017; tiryakioğlu et al. 2018; ocakoğlu et al., 2018). the fault planes measured near ören town in the middle part of the gökova fault as shown in figure 12, are e-w striking normal faults with high dip angle (80o). our mt parameters for the 20 july 2017 earthquake, on the other hand, show e-w nodal planes with relatively low dip (table 1). the current results indicate that the fault that ruptured during the 20 july 2017 earthquake is not the same as the gökova fault or its western continuation in the gulf. the disagreements table 3. the parameters of the 24 earthquakes (m ≥ 4.0) which occurred between 1933 and 2017 in and around gökova gulf shown in figure 10 and 11. no date day.month. year time (utc) lat. n (°) long. e (°) plane 1 strike°/dip°/plunge° plane 2 strike°/dip°/plunge° mag. (moment) h (km) references 1 23.04.1933 05:57:37 36.77 27.29 600/450/-900 2400/450/-900 6.2 30 jackson, 1992 2 23.05.1941 23:00:48 37.22 28.35 650/450/-900 2450/450/-900 5.4 48 jackson, 1992 3 19.02.1989 14:28:45 36.98 28.19 930/320/-850 2670/580/-930 5.4 15 gcmt 4 27.04.1989 23:06:52 37.03 28.16 920/360/-940 2760/540/-870 5.1 14 gcmt 5 28.04.1989 13:30:19 37.02 28.1 900/410/-1010 2850/500/-800 5.4 8 gcmt 6 03.08.2004 13:11:30 36.85 27.77 740/380/-970 2630/520/-840 5.1 11 gcmt 7 04.08.2004 03:01:05 36.83 27.76 750/400/-950 2620/500/-850 5.4 9 gcmt 8 04.08.2004 04:19:46 36.83 27.81 710/420/-1110 2780/520/-730 5.3 13 gcmt 9 04.08.2004 14:18:48 36.84 27.78 750/410/-940 2600/490/-870 5.2 8 gcmt 10 20.12.2004 23:02:14 36.93 28.36 1050/450/-690 2570/480/-1090 5.3 8 gcmt 11 10.01.2005 23:48:49 36.85 27.92 1100/450/-630 2550/510/-1140 5.2 8 gcmt 12 11.01.2005 04:35:56 36.89 27.87 1000/330/-690 2550/600/-1030 5.2 10 gcmt 13 08.05.2011 06:50:24 36.7 27.24 600/300/-970 2480/610/-860 5.1 11 gcmt 14 04.06.2012 14:19:53 28.16 36.94 1070/430/-490 2380/580/-1220 4.5 6 noa 15 24.11.2012 21:04:17 36.65 27.98 2490/630/-660 3350/350/-1280 4.1 6 koeri 16 24.11.2012 21:31:15 36.64 27.93 670/390/-610 2110/560/-1110 4.0 7 koeri 17 26.11.2012 17:35:42 36.58 27.95 350/310/-940 2190/590/-880 4.7 22 gcmt 18 13.08.2014 09:42:55 36.94 27.70 1340/170/-560 2780/750/-990 4.1 4 pınar (koeri) 19 26.11.2014 05:05:33 36.76 27.84 1120/510/-710 2630/420/-1110 4.2 10 med_rcmt 20 29.05.2015 08:02:51 36.92 27.61 2770/530/-750 740/400/-1090 4.0 7 koeri 21 15.12.2016 16:43:56 37.09 28.64 630/570/-1140 2820/400/590 4.0 7 afad 22 25.01.2017 01:19:32 36.79 27.66 920/290/-700 2480/630/-1010 4.0 10 afad 23 13.04.2017 16:22:15 37.14 28.66 730/370/-1170 2860/580/-710 4.9 5 gcmt 24 20.07.2017 22:31:09 36.96 27.40 2800/500/-790 830/410/-1030 6.6 7 this study 318 semir över1, süha özden, esra kalkan ertan, fatih turhan, zeynep coşkun, ali pınar about the consequences of the 20 july 2017 earthquake (i.e. the inconsistency of the results and interpretations regarding the fault that ruptured during the earthquake and its dip) show that the basin is quite complex and remains to be further explored. nw-se extension the 21 focal mechanisms of the aftershocks, which range in magnitude from 3.8 to 4.8, are given in figure 9. the focal mechanisms of the earthquakes show normal to oblique slip faulting (table 2 and fig. 9). the inversion of 21 nodal plane sets yields a normal faulting stress regime (σ1 is vertical) characterized by approximately nw-se ((n330°e) σ3 axis, i.e. minimum horizontal stress axis (table 2 and fig. 11). the computed r value is 0.45 indicating a triaxial extensional stress state. the approximately nw-se extension was obtained from the inversion of the aftershock focal mechanisms in the western part of the gulf of gökova (fig. 4). the presence of these normal faults is revealed by seismic-reflection profiles with gps, land dem and multibeam bathymetry in the marine area (tur et al. 2015; ocakoğlu et al., 2018; tiryakioğlu et al., 2018). some of the mapped faults were suggested to have strike-slip character as a result of previous geophysical, multibeam bathymetry, and geodetic studies (uluğ et al., 2005; i̇şcan et al., 2013; ocakoğlu et al., 2018; tiryakioglu et al., 2018). none of the aftershocks in this analysis are strike-slip faulting; instead, they all are normal faulting to oblique slip (figs. 8 and 9, and table 1-2). based on combined geophysical and geodetic data from land and sea, tur et al. (2015) demonstrated that the faults mapped as strike-slip faults are normal type faults. they also argued that all faults mapped in the gulf which are associated with the tectonic development of the basin have normal character with distinct directions: 1) nw-se faults controlling submarine valleys, 2) e-w and 3) approximately ne-sw faults associated with the development of gökova basin. a seismic profile taken from the eastern coast of kos island in the northwest of gökova gulf in the nw-se direction reveals the presence of listric and synthetic faults (see profile no:3 in ocakoğlu et al., 2018). these faults are in ne-sw direction, suggesting the existence of nw-se extension, according to this seismic profile. according to ocakoğlu et al. (2018), there is decelerated tectonism in this area, and minor extension may be enough to create these structures. shah (2015) also found the normal extensional regime in western anatolia, from n–s to nne–ssw, suddenly changes direction to nw–se at the exit of gökova gulf, using stress tensor analysis of the earthquakes. shah (2015) inverted 67 nodal plane sets and found a normal faulting stress regime (σ1 is vertical) with a nw-se (n313°e) σ3 axis, and a r value of 0.39, which supports our findings (fig. 11). shah (2015) suggested also that the gökova area is distinguished by a clockwise rotation of stress directions from n-s to nw-se. the anticlockwise rotation in the aegean was also noted in previous geodetic studies (mcclusky et al., 2000; howell et al., 2017). gps slip vectors (saltogianni et al., 2017; tiryakioğlu et al., 2017; ganas et al., 2019), which show displacements (i.e. horizontal co-seismic displacement) changing direction from n-s to nw-se, support this faulting process and agree with our findings. conclusions the extensive work of processing the data generated by the main shock and its aftershock activity is presented here to contribute to understanding of the stress regime which is responsible for the deformation in the study area. the results are as follows: 1) the mt solution of the main shock yields approximately e-w striking nodal planes which are compatible with n-s extension. however, the mt solution analysis of aftershock events gives two distinct directions for the nodal planes: approximately e-w and ne-sw to nne-ssw. the inversion of the focal mechanisms yields n-s extension from e-w nodal planes and nw-se extension from ne-sw to nne-ssw nodal planes. the bodrum-kos figure 12. view of metric normal fault planes (demirtaş, 2008) measured in the central part of the gökova fault zone (utm: 0582778 e 4100006 n). lower hemisphere stereoplots showing inversion results for normal faulting data measured on fault planes. the results were determined by the carey (1979) method, with azimuth, plunge and relative magnitudes of principal axes (σ1, σ2 and σ3) as well as the stress ratio value [r = (σ2 −σ1)/(σ3 −σ1)]. 319the 20 july 2017 bodrum-kos earthquake (mw 6.6) in southwestern anatolia, turkey earthquake (mw: 6.6) and its aftershocks with e-w focal planes occurred under n-s extension, while the aftershocks related to the approximately ne-sw trending nodal planes were generated under nw-se extension. the nw-se extension appears to be local and probably contributes to the growth of the western part of the asymmetric gökova basin. 2) the inversion of the focal mechanisms of earthquakes occurring in gökova basin and its surrounding areas for the period between 1933 and 2017 yields approximately n-s extension. the n-s extension is also supported by field data. the n-s extension which is responsible for the 20 july 2017 bodrum-kos earthquake (6.6 in mw) seems to be related to african subduction beneath the aegean. acknowledgements the authors would like to thank catherine yiğit for professional editing assistance with english exposition that improved the last version of the text. we also thank the editor and anonymous reviewers for their valuable comments and suggestions, which improved the quality of the manuscript. references akyol, n., zhu, l., mitchell, b. j., sözbilir, h., & kekovali, k. 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(2007). understanding tsunamis, potential source regions and tsunami prone mechanisms in the eastern mediterranean. geological society london special publications, 291, 201–230, doi:10.1144/sp291.10. yolsal, s., & taymaz, t. (2010). source mechanism parameters of gulf of gökova earthquakes and tsunami risk in the rodos-dalaman region. i̇tü dergisi /d 9 (3), i̇stanbul technical university, i̇stanbul pp. 53-65. https://doi.org/10.1080/09853111.2017.1408264 https://doi.org/10.1080/09853111.2017.1408264 https://doi.org/10.1594/pangaea.806246 keywords: physic-chemical parameters, liners, residual soil, isotherms, batch test, sorption. palabras clave: parámetros fisicoquímicos, revestimientos, suelo residual, isotermas, prueba de lotes, sorción. how to cite item marques, j. p., ferreira da silva, e. a., patinha, c., kasemodel, m. c., rodrigues, v. g. s. (2019). adsorption of lead (pb) in weathered tropical soil (ribeira valley region brazil). earth sciences research journal, 23(4), 385-395. doi: https:// doi.org/10.15446/esrj.v23n4.77869 lateritic soils have been widely used in the construction of landfill base liners for solid waste disposal. on the other hand, there is little practice in the use of cambisols (less clayey) for this finality. the use of local soil for the construction of liners in order to waterproof and retain contaminants represents an alternative to control this type of contamination. thus, the objective of this study was to determine the physical and chemical properties of a cambisol collected in the city of eldorado (ribeira valley) and to evaluate the ability of this soil to adsorb lead (pb). the contamination of soil and water by pb is a worldwide problem, and in the ribeira valley the contamination by pb is common in specific areas mainly due to inadequate mining waste disposal. the efficiency of pb adsorption by the cambisol was indirectly proportional to the concentrations of pb in solution. when the initial concentration of 20 mg/l was used, the adsorption percentage of pb was 92.5% and when the initial concentration was 100 mg/l, the adsorption rate was 81.7%. in addition to the retention of pb, this soil presented the following characteristics which are favorable for using soils in the construction of liners: a silt loam (a high plasticity clay, ch), with liquid limit (ll) of 57%, plasticity index (pi) of 33%, oxidizing medium and the predominance of negative charges on the surface of the colloidal particles. thus, it is concluded that this tropical soil (cambisol) exhibits characteristics that allow it to be used as a liner in the disposal of residues containing pb. abstract adsorption of lead (pb) in weathered tropical soil (ribeira valley region brazil) adsorción de plomo (pb) en suelos tropicales meteorizados (región del valle de ribeira brasil) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.77869 los suelos lateríticos se han utilizado ampliamente en la construcción de revestimientos de basureros para residuos sólidos. por otro lado, hay poca práctica en el uso de cambisols (menos arcillosos) para esta finalidad. el uso de suelo local para la construcción de revestimientos para impermeabilizar y retener contaminantes representa una alternativa para controlar este tipo de contaminación. por lo tanto, el objetivo de este estudio fue determinar las propiedades físicas y químicas de un suelo tropical fuertemente degradado recolectado en la ciudad de eldorado (valle de ribeira), y evaluar la capacidad de este suelo para adsorber plomo (pb). la contaminación del suelo y el agua por pb es un problema mundial, y, en el valle de ribeira, la contaminación por pb es común en áreas específicas, debido principalmente a la eliminación inadecuada de los desechos mineros. la eficiencia de la adsorción de pb por el cambisol fue indirectamente proporcional a las concentraciones de pb en la solución. cuando la concentración inicial de 20 mg/l, el porcentaje de adsorción de pb fue del 92,5% y cuando la concentración inicial fue de 100 mg/l, la tasa de adsorción fue del 81,7%. además de la retención de pb, este suelo presentó las siguientes características que son favorables para el uso de suelos en la construcción de revestimientos: es un suelo franco limoso (arcilla de alta plasticidad, ch), con límite de líquido (ll) del 57%, índice de plasticidad (pi) del 33%, medio oxidante y el predominio de las cargas negativas en la superficie de las partículas coloidales. por lo tanto, se concluye que el suelo residual tropical (cambisol) presenta características que le permiten ser utilizado como revestimiento en la eliminación de residuos que contienen pb. resumen record manuscript received: 14/02/2019 accepted for publication: 19/07/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 385-395 jéssica p. marques1, eduardo a. ferreira da silva2, carla patinha2, mariana c. kasemodel1, valéria g. s. rodrigues1* 1department of geotechnical engineering, são carlos school of engineering, university of são paulo, 400 trabalhador sãocarlense ave, são carlos, brazil 2department of geosciences, geobiotec – geobiosciences, geotechnologies, and geoengineering research center, campus de santiago aveiro university, aveiro, portugal * corresponding author: valguima@gmail.com so il s 386 jéssica p. marques, eduardo a. ferreira da silva, carla patinha, mariana c. kasemodel, valéria g. s. rodrigues introduction lead (pb) is a nonessential potentially toxic element (pte) that naturally occurs at low concentrations in the environment (adriano 1986; alloway 1995). however, the excess of pb is common in the soil due to anthropogenic activities. these activities are related to the use of this metal in industrial applications, mainly in the automotive industry, the improper disposal of mining and smelting wastes; air-borne sources; use of pesticides; biosolids and manure (alloway 1995; skerfving and bergdahl 2007; wuana and okieimen 2011). pb is also commonly found in urban landfill leachates as a result of the improper disposal of dangerous wastes. the contamination of the study region of this paper is associated with inadequate disposal of solid mining waste. the contamination by pb is a global concern due to its high toxicity and bioaccumulative properties. it can cause severe biological effects, increasing anemia, cancer and mortality rates (lustberg and silbergeld, 2002; papanikolaou et al., 2005; skerfving and bergdahl 2007; jin et al. 2008; hegazy et al. 2010). the joint fao/who expert committee on food additives (jecfa) had established a provisional tolerable weekly intake (ptwi) of 25 µg/kg by for all population groups. however, in 2010 the committee concluded that this value could no longer be considered health-protective, and the ptwi of pb was withdrawn (who 2011). there is no known safe blood level for pb, concentrations as low as 5 µg/dl may be associated with decreased intelligence in children, behavioral difficulties and, learning problems (who 2018). a recurrent problem related to pb contamination is the inadequate disposal of solid mining waste. pb is usually related to galena (lead ore pbs) in mining areas, and during the extraction and beneficiation of ores, residues such as slags and tailings are generated. their inadequate disposal can result in soil, water, and air contamination by pb. the characterization of the soil is extremely important to assess the contamination (level and extension). it is equally important to have research efforts aimed at characterizing local soils to be used as liners in the waste disposal area. this may reduce costs involving the transportation of the wastes and/or materials for the construction of landfills. in soils, the mobility and the fate of metals are regulated via their partitioning between soil and soil solution (freeze and cherry 1979; lee et al. 1998). metal mobility in the soil also depends on soil conditions and the chemical form of the metal (alloway 1995; rieuwerts 2007). several studies have addressed metal behavior in different soils, such as zuquette et al. (2008); korf (2011); braz et al. (2013); odukoya et al. (2013); diagboya et al. (2015), zia et al. (2018); gabarrón et al. (2018); colzato et al. (2018). other researchers specifically studied the use of tropical soils as liners for waste disposal (boscov et al. 1999; afolayan and nwaiwu 2005; knop et al. 2008; boscov et al. 2011; morandini and leite 2015; agbenyeku et al. 2016; miguel et al. 2017; onyelowe et al. 2019; ehrlich et al. 2019). pb in the soil is cumulative and traditional treatments for metal contamination are very expensive (usda 2000). treatments can be done in situ or ex-situ, these include high-temperature treatments, washing processes and the use of solidifying agents (usda 2000). for example, pump-and-treat is one of the most traditional techniques for remediation. but the high rate of energy required and the need for effluent disposal may increase the costs (usepa, 2014). thus, preventing metal contamination by using adequate liners is important to avoid the need for expensive remediation techniques. lateritic soils have been widely used in the construction of liners in landfill base (boscov et al. 2011; amadi and eberemu 2012; gabas et al. 2016). however, there is little practice in the use of cambisols as liners. there is increasing interest in these soils because of their availability in brazil, or in other countries. according to boscov (2011), it is important to expand the knowledge on the contaminant mobility behavior in tropical soils to encompass all the main types of soil of the country. researchers have addressed this issue, such as soares (2004), morandini and leite (2015), almeida (2015), colzato et al. (2018). cambisols are very common, and the use of local soils in geoenvironmental applications may reduce costs, which is essential, especially in developing countries (lavagnolo 2018; shubbar et al. 2019). in this context, it is fundamental to perform a detailed study of the soil, covering its physical and chemical characteristics as well as its adsorption capacity. the ability of soil to retain metals is directly proportional to its buffering capacity, and the retention mechanisms differ according to the ph values (freeze and cherry 1979; bourg and loch 1995). depending on environmental conditions (ph, redox potential eh and temperature) and soil composition, metals can be retained in the form of oxides, hydroxides, carbonates, or confined in exchangeable cations and organic matter (om) (tessier et al. 1979). the om content and the mineralogy of the soil can influence contaminant retention due to high cation exchange capacity (cec) (langmuir 1997). soil texture is also an important factor, as it can control the migration of contaminants through the soil profile. in soils with more clay content, soil texture favors the retention of metals. the retention capacity of pte by adsorption is directly related to the surface area. small particles in the clay fraction have higher adsorption capacity (langmuir 1997) due to their large surface area. adsorption is the accumulation of a particular element or substance at an interface. the phase that adsorbs is referred to as the adsorbent and the phase that accumulates at its surface is the adsorbate. the nature of the force involved in the adsorption process may vary, they may be of physical nature, such as van der waals and electrostatic, or chemical, such as reactions with electron sharing (weber 1972). from the point of view of soil and groundwater contamination, the adsorption is the soil capacity to attenuate contaminant mobility and decrease the possibilities of these elements entering the food chain (freeze and cherry 1979; sposito 1984; sparks 1995). according to alleoni et al. (2005), pte soil adsorption depends mainly on: (a) clay content, (b) om, (c) iron (fe), aluminum (al) and manganese (mn) oxides content, (d) ph, (e) specific surface (ss) and (f) soil mineralogy. braz et al. (2013) studied pb, cadmium (cd), cobalt (co), copper (cu), mercury (hg) and zinc (zn) adsorption by soils from the eastern amazon, and the results showed correlation between the adsorption capacity and ph, organic carbon content and fe and al oxides content. diagboya et al. (2015) also showed a significant role of soil organic matter and oxides content in adsorption of pb, cd, and cu by four tropical soils from nigeria. the electrostatic charge of soil particles is linked to the adsorption rate (zuquette et al. 2008). in tropical soils, these charges are highly variable (positive and negative) due to the mineralogical assembly and ph. for instance, gupta and bhattacharyya (2008) showed that the increase of ph led to increasing adsorption capacity of pb by natural kaolinite. according to mohamed et al. (2018), the ph effect in adsorption by soils is related to the changes in particle net proton charge. the electric charge of soil particles influences the adsorption of cations and anions, and the number of adsorbed ions depends on the charge density, related to the surface of the colloid, depending mainly on the type and content of mineral and organic constituents of soils (alleoni et al. 2016). for a given ph, the net charge resulting from all sources of the surface of a colloid can be null, corresponding to the point of zero charge (pzc). the point of zero salt effect (pzse) is the ph value when the difference between the number of moles of h+ and ohadsorbed is invariable with the salt concentration. if the charge densities of dissociated ions and adsorbed ions remain unchanged with the variation of the salt concentration, the pzc, and pzse values are equal. however, this condition rarely occurs in soils (sposito 1984; sparks 1995). the adsorption curve obtained in the batch equilibrium tests is useful to obtain preliminary information about the soil attenuation capacity (daniel 1993). the adsorption isotherm is the ratio between the mass of solute adsorbed and the concentration at equilibrium. the isotherm describes the relation of the solute concentration between two separate phases in equilibrium at a constant temperature (sparks 1995; fagundes and zuquette 2009). thus, the sorption isotherm expresses the relation between the amount of vapor or solute adsorbed as a function of vapor or solute equilibrium concentration, not considering the mechanisms. the isotherm method consists of stirring soil subsamples with solutions containing increasing amounts of the metal ion. the amount of ion adsorbed by the soil is the difference between the initial ion concentration and the remaining ion concentration in solution subsequent to the experiment (linhares et al. 2009). although the adsorption isotherm does not consider chemical mechanisms involved in adsorption, it allows describing the adsorption characteristics by the correlation between the parameters and soil properties (vinhal-freitas et al., 2010). the study area is located in the ribeira valley region, which presents three major geological domains: pre-cambrian metamorphic rocks (with oriented structures: schists, migmatite, and gneiss) and cataclastic (generated by shear stresses); magmatic rocks (represented by intrusive granite, basic and alkaline bodies) and cenozoic sedimentary cover (cetec, 1999). 387adsorption of lead (pb) in weathered tropical soil (ribeira valley region brazil) the soil ph in kcl solution (1.0 mol/l) was obtained with a digimed dh21 ph meter with a glass electrode. the relation between soil mass and volume of solution used was 1:2.5 (20 g of soil and 50 ml of kcl solution). the solution was stirred for 30 minutes and then left resting for 1 hour before the measurement (embrapa 2011). the ∆ph is obtained from the numerical difference between the ph kcl and the ph h20. the ph is an important parameter in the adsorption process. according to fageria et al. (1994), the soil acidity is described quantitatively as slightly acid (ph between 6.0 and 7.0), moderately acidic (ph between 5.5 to 6.0), strongly acidic (ph between 5.0 and 5.5), very acid (ph 4.5 to 5.0) and extremely acidic (ph less than 4.5). to determine the eh value, a mixture of 1:2.5 (20 g of soil and 50 ml of distilled water) was prepared. the solution was stirred for 30 minutes and then left resting for 1 hour. the eh was read using a platinum ring electrode connected to a ph meter micronal b374, reference electrode ag/clag. to determine the ec value, a mixture of 1:2 (10 g of soil and 20 ml of water) was prepared and manually shaken for 30 seconds. the mixture was left resting for 1 hour before measurements were made using hanna instruments hi 9033 conductivity meter. pzse pzse is a ph value where the salt concentration of the solution in a soil suspension has no charge effect on the surface of soil particles. according to silva et al. (1996) and coringa and weber (2008), the comparison between soil ph and pzse allows to define whether the net surface charge of the particles is positive (ph < pzse), negative (ph > pzse) or zero (ph = pzse). soil pzse depends on the content and degree of decomposition of organic matter and mineral composition of the clay fraction. to obtain the pzse, a potentiometric acid-base titration was performed, varying to ionic forces. hcl was used as acid, naoh as base and kcl as support electrolyte. in the first step, a series of solutions were prepared with increasing concentrations (0.002 m, 0.004 m, 0.008 m, and 0.016 m) of acid and base diluted in 0.1 m kcl, 0.01 m and 0.001 m solutions. next, suspensions were prepared with 4.0 g of soil and 20 ml of each solution. the suspensions were maintained at room temperature for 24 hours with occasional manual shaking and ph values measured using a digimed dh 21 ph meter. equilibrium ph values (on the ordinate axis) versus the amount of adsorbed h+ (for the acid solutions) or oh(for the basic solutions) were plotted. the pzc was obtained based on the equation proposed by keng and uehara (1973): pcz = 2 ph kcl – ph h2o. the surface potential (⌡o ) was obtained based on the equation proposed by raij and peech (1972): ⌡o = 59.1 (pzse – ph h2o). cec and ss the methylene blue method described by arab et al. (2015) was used in this study. this test is based on the linear relationship between the methylene blue index and fundamental soil properties such as cec and ss. initially 2.0 g of soil sample was dispersed in 10 ml of water, under constant agitation with a magnetic stirrer. the suspension was titrated with a methylene blue solution (1.5 g/l). a volume of 1 ml of methylene blue was titrated in the soil suspension; the suspension was left mixing for 3 minutes. then a droplet of the mixture was taken with a glass rod and placed on filter paper (whatman nº 42). the test is considered negative if there is no presence of a light blue halo surrounding the droplet, and then the procedure is repeated adding an extra 1 ml of the methylene blue solution to the mixture. when a clear light blue halo is formed around the blue drop, the test is considered positive. according to chen et al. (1974 apud pejon 1992)1, the cec is obtained by eq. 1 where v is the volume of methylene blue (ml) used in the titration, c is the concentration of the methylene blue solution (normal) and m is the dry soil mass (g). 1 chen, t. j., santos, p. s., ferreira, h. c., calil, s. f., zandonadi, a. r., & campos, l. v. (1974). determinação da capacidade de troca de cátions e da área específica de algumas argilas e caulins cerâmicos brasileiros pelo azul de metileno e sua correlação com algumas propriedades tecnológicas. cerâmica, 79, 305-326. the ribeira valley (ribeira de iguape water basin and cananéia-iguape estuarine complex) is a well-known region due to its historic pb exploitation. the improper disposal of pb slags in the soils and the upper ribeira de iguape river led to the contamination of many municipalities downstream, including the cananéia-iguape estuarine complex (bosso and enzweiler, 2002; guimarães and sígolo 2008; rodrigues et al., 2012; mahiques et al., 2013; piedade et al., 2014; kasemodel et al., 2016; tramonte et al., 2016). there is a necessity of proper landfill implantation for the mining waste. besides, landfills for urban wastes from the cities itariri and pedro de toledo (both cities are located near eldorado) were considered inadequate in the biennial report provided by the são paulo state environmental company (cetesb, 2017). this problem entails the need to undertake studies that address the ability of local soils to adsorb pb. the main objective of this research was (a) to determine the physical and chemical properties of a tropical soil (cambisol) collected in the city of eldorado (ribeira valley), and (b) to evaluate the adsorption capacity of pb by this soil. materials and methods approximately 10 kg of soil was collected in the city of eldorado (geographic coordinates: 24º31’12” s and 48º06’29” w), ribeira valley region, brazil. the sample studied is a representative one, composed of the collection at 3 different points in the area. the disturbed samples, which were collected from b horizon, were air-dried at room temperature until constant weight, disintegrated with agate mortar and homogenized. tests were performed in duplicates to determine the physical and chemical parameters of soil and the adsorption rates (batch equilibrium test) with pb solution. particle size distribution special attention was given to the soil fine fraction given that the purpose of the study was evaluating pb adsorption. a micromeritics sedigraph 5100 equipment carried out the particle size analysis, for particles up to 100 μm. the analysis was performed based on the iso 13317-3/2001 method (iso 2001). soil (3.5 g) was mixed with distilled water to humidify with sodium hexametaphosphate shmp (1%, 50 ml) as a dispersant. the suspension was placed in sedigraph and the sieved fractions were dried in an oven at 50 °c and weighed. the particle size and the granulometric fractions are estimated based on the stokes law and the measurement was carried out by x-ray absorption (lima and luz, 2001). the determination was also carried out adding only distilled water to the sample. atterberg limits the determination of the ll was performed according to abnt mb30 (1984a) using the casagrande apparatus. the determination of the plastic limit (pl) was performed according to abnt mb31 (1984b). the plasticity index (pi), an important parameter used to evaluate the soil in the retention of contaminants, is the numerical difference between the ll and the lp (pi = ll – pl). the activity index (ai) was calculated from the ratio between the pi and the clay content. this relation indicates the activity of clay in the soil. if the value of ai is less than 0.75, the soil is considered inactive; if the ai value is integrally comprised between 0.75 and 1.25, the soil is normally active and; if the ai value exceeds 1.25, the soil clay is considered active (skempton 1953). physico-chemical parameters of the soil the physical-chemical parameters determined were: (i) ph in water, in cacl2 and in kcl solution; (ii) eh; (iii) electrical conductivity (ec); (iv) point of zero salt effect (pzse). ph, eh and ec the ph was determined both in water and cacl2 solutions (0.01 mol/l) based on the iso 103090/2005 (iso 2005) method. the relation between soil mass and volume of solution used was 1:5 (5 g of soil and 25 ml of water or cacl2 solution). the suspensions were stirred for 5 minutes and then left resting for 24 hours. the measurement was conducted with a glass electrode and ph meter hanna hi 9025. 388 jéssica p. marques, eduardo a. ferreira da silva, carla patinha, mariana c. kasemodel, valéria g. s. rodrigues cec vx cx m = 100 (1) the ss values were empirically estimated using cec results obtained by the pejon (1992) method. according to beaulieu (1979 apud pejon, 1992)2 the specific surface is obtained using eq. 2, where a is the area of the methylene blue molecule (130 å2), n is the avogadro number (6.02 x 1023), m is the mass of dry methylene blue and mw is the molecular weight of methylene blue in anhydrous form. eq. 4 is obtained by substituting the values. ss ax nx m mw x = 1000 (2) ss x x x x x x m ml = ( ) ( ) = −( . . . . 130 10 6 02 10 1 5 319 9 1000 3 67 20 23 2 (3) ss x v m =     3 67. (4) a second method in accordance with iso standard 13536/1995 (iso 1995), and adapted from bascomb (1964), was used to estimate the cec values. the method consists of saturating the soil with bacl2 solution buffered to ph 8.1. exchangeable cations in soils replaced by ba were analyzed by atomic absorption. in order to precipitate barium adsorbed in the soil and the free cations in solution, mgso4 solution was added to the soil sample. magnesium occupies the exchange sites on the soil and the excess of mg (not absorbed) is determined by titration with edta and eriochrome black t indicator. the cec is calculated according to the following formula: cec msoil n m b a=     −     + −( )          1 250 10 25. . . (5) where cec is the cation exchange capacity in cmolc/kg; msoil is the weight in grams of soil used in the test; n is the volume in ml of edta used in the titration; m is the volume in ml of edta used in the blank titration; and b a is the amount in grams of water in the soil sample. the ss values of particles are related to the soil’s ability to retain and/ or release soil chemical elements. higher ss implies greater possibility of interaction with soil elements in solution, which can facilitate the retention process and attenuation of contaminants. om om content was obtained using the method described by eusterhues et al. (2005). this method is conducted using a mixture 5 g of soil sample and 50 ml of hydrogen peroxide that is heated to 90°c. after the peroxide attack, the soil was oven dried at 60°c. the determination of the om content is based on the mass difference of the sample before and after the procedure. pb concentrations icp-ms, analytical technique with high sensitivity was used to estimate pb concentrations in soil samples. the analysis was performed after acid decompositions (hno3, hclo4, and hf) by icp -ms at acme laboratory in canada. mineralogical analysis x-ray diffraction (xrd) xrd is a non-destructive technique used to identify the crystalline phases, which has been widely used in the identification of clays (tan et al. 1986; albers et al. 2002). for the total sample analysis, approximately 1 2 beaulieu, j. identification géptechnique de matériaux argileux naturels par la mesure de leur surface au moyeu du bleu de méthylène. thèse de doctorat de 3° cycle, univ. de paris-sud, orsay, 133 p. g of soil was separated and gently broken down with the agate mortar. the sample was placed in an aluminum sample holder of the x-ray diffractometer, slightly compressing the powder to avoid a preferred crystal orientation. the diffractograms were obtained using the diffractometer philips x’pert consisting of a mppc generator, pw3050 goniometer pw 3040/60 with a microprocessor in the printer registration. the radiation used was copper with a wavelength of 1.5405 å. the diffraction angle 2θ ranged from 4° to 65° at a rate of 0.02°/s. for the fraction analysis diameter less than 0.063 mm the clay thin-sections were prepared using a solution with about 1 g of soil sample, 30 ml of distilled water and two drops of sodium hexametaphosphate (1%) were added as dispersant. the mixture was stirred for about 40 seconds with ultrasonic stirrer. 1 ml of the solution was pipetted onto three glass slides; the droplet was left to dry at room temperature. one of the slides was subjected to treatment with ethylene glycol, another was heated to 500°c, and the third received no treatment. the three slides with different treatments were analyzed in the same diffractogram. differential thermal analysis (dta) and thermogravimetric analysis (tga) this analysis was performed heating the samples to 1000°c at a rate of 12.5°c per minute. in the dta a sample of the material to be studied was heated side by side with a sample of a thermally inert material (alumina) and the temperature difference between the two samples was recorded as the samples were progressively heated (mackenzie 1957). the identification of minerals is by shape, position and intensity of the peaks of the curve produced in atd. the obtained curve was compared with standard curves of various minerals (santos 1989). in atg, the sample weight was recorded as a function of temperature, while it was heated at a controlled rate. changes in weight were assigned to rupture or formation of chemical or physical connections corresponding to the release of volatile substances or formation of compounds (tan et al. 1986). the interpretation of the curves was made based on mackenzie (1957) and santos (1989). batch equilibrium tests the study of adsorption is necessary to understand soil’s ability to attenuate, degrade or immobilize contaminants of organic and/or inorganic origin. in the batch test method, soil and solutions containing the ion of interest are brought into contact, under agitation and during a predetermined period. at the end of the experiment, it is admissible that the system reached the equilibrium of the reaction (soares and casagrande, 2000). in order to determine the sorption characteristics of the soil, batch tests were performed using solutions of five different pbcl2 concentrations: 20, 40, 60, 80, and 100 mg/l. this range was chosen based on pb concentrations in leachate of mining tailings and slag from adrianópolis (ribeira valley). these concentrations reached 77.10 mg/l (raimondi 2014; kasemodel 2017). for this test, we followed the methodology proposed by roy et al. (1992) and zuquette et al. (2008). the soil/solution ratio used was 1:5 (20 g of soil and 100 ml of solution). samples were shaken in a nt 155 novatecnica (between 25 to 30 rpm) benchtop orbital shaker. after 48 hours, the samples were centrifuged and filtered through whatman filter paper nº 42. during the test, room temperature was controlled, and physicochemical parameters were measured. the ph, eh and ec were measured before the test (solutions without soil sample), after soil and solution contact, and in the final stage, after equilibrium. for the data interpretation the authors used the linear, freundlich and langmuir isotherms. the linear isotherm corresponds to the simpler model of adsorption, and estimates the solute adsorption of infinitely increasing concentrations (fagundes and zuquette 2009). the linear isotherm model is represented by eq. 6: s k xcd e= �� (6) where: s is the mass of contaminant adsorbed per unit of soil mass (µg/g), ce is the final concentration of the equilibrium solution (mg/l), kd is the distribution coefficient of linear isotherm (cm3/g). the langmuir method, represented by the eq. 7, uses two different linearization processes to obtain the parameters of this isotherm. the linearization shown in eq. 8 and eq. 9 are called langmuir linearization i and 389adsorption of lead (pb) in weathered tropical soil (ribeira valley region brazil) langmuir linearization ii, respectively. in these equations: sm is the maximum amount of solute adsorbed by the soil (µg/g) and b is a constant related to the adsorption binding energy (cm3/g). s s x b x c b x c m e e = + ( ) 1 (7) c s s xb c s e m e m =     +     1 �� � (8) 1 1 1 s s xb x c sm e m =     +    �� �� � (9) the langmuir equation is often preferred since its parameters allow estimating the maximum adsorption capacity and adsorbate affinity of the adsorbate (linhares et al. 2009). the freundlich isotherm is represented by eq. 10 and linearization was used to obtain the parameters by eq. 11. in these equations, s is the mass of contaminant adsorbed per unit of soil mass (µg/g), ce is the final concentration of the equilibrium solution (mg/l), n (dimensionless) and kf (cm3/g) are coefficients of the freundlich equation. s k xcf e n= �� � (10) log s n xlog c log ke f� � �� � � � �( )= ( )+ ( ) (11) results and discussion physico-chemical and mineralogical properties table 1 presents the results of particle size distribution. as expected the experiment with and without sodium hexametaphosphate (shmp) presented different results. for example, the clay fraction obtained in the soil when applying shmp was 26.12% (table 1), while the percentage of clay was lower (20.19%) without smhp. silt fraction was 52.47% with shmp, and 63.09% without shmp. the results of the test with dispersant show higher clay content and lower silt content. this is explained by flocculation of small clay particles when there was no dispersant. table 1. particle size distribution of the studied soil in tests with and without dispersant. diameter distribution with shmp distribution without shmp µm (%) (%) 0 – 2 (clay) 26.12 20.19 2 – 60 (silt) 52.47 63.09 60 100 0.79 1.18 > 100 20.62 15.54 according to rowe et al. (1995), a clayey compacted soil can be used as a liner in a waste disposal area, if it presents 15 to 20% of clay fraction in its composition. according to the results (table 1), this soil is suitable to be used in this application. qian et al. (2002), indicates that a soil must to present at least 50% fines (silt and clay fractions) to be applied in a liner, a requirement that the analyzed residual soil also satisfies. the importance of fine particles in soils, used in the construction of liners, is to ensure lower hydraulic conductivity. furthermore, particle size and sorption capacity are also related. according to langmuir (1997), the sorptive abilities of minerals are proportional to their surface areas and surface-site densities. the bigger the proportion of fine particles, the larger the surface area exposed to reaction with the solution. table 2 summarizes the results of the physical, physico-chemical and mineralogical characteristics of the soil. table 2. physical, physico-chemical and mineralogical characteristics of the soil. parameters result pl (%) 24 ll (%) 57 pi (%) 33 ph h2o 4.5 ph cacl2 4.0 ph kcl 4.2 ∆ph -0.3 pzse 3.4 pzc 3.9 surface potential (⌡o) ⌡o = 59.1 (pzse – ph h20) (mv) -65.01 ec (µs/cm) 41.55 cec (cmolc/kg) method: methylene blue 3.62 cec (cmolc/kg) method: saturation with ammonium acetate 3.20 ss (m2/g) 28.29 om (g/kg) 13.40 mineralogy k, qz, gth, i, m k: kaolinite; qz: quartz; gth: goethite, i: illite; m: muscovite soil ll was 57% and pl was 24%, therefore, the pi is 33%. this pi value is not within the ideal range for soil liners. daniel (1993) recommends a minimum pi of 7% for soil liners to achieve the hydraulic conductivity required. however, the author also recommends special attention for soils with high pi. according to daniel (1993) and rowe et al. (1995), soils with pi > 30%, such as the studied soil, may form hard clods that render difficult the construction of the liner, and may be susceptible to shrinkage and cracking if dry. ai was 0.61 when using clay content without shmp, and 0.79 when considering clay content with shmp. thus, the classification of the ai is dependent on the methodology used to obtain clay content, as when determining clay content without the use of the dispersant, ai is considered inactive; and when determining clay content with the use of shmp, ai is considered normal. this is a positive aspect for liners because high activity clay soils are susceptible to shrinking and swelling, causing problems in the liner structure (rowe et al. 1995). the ph measured in water (ph 4.5) was higher than in cacl2 and kcl solutions (4.0 and 4.2, respectively). in general and according to the results soil is very acidic. this may adversely affect the contaminant attenuation. some metals, including pb, remain soluble in acidic environments. in addition, ph influences the magnitude of the soil surface charge, which is also important for metal adsorption (yong 2001). the investigated soil pzse (3.4) is lower than ph value in water (4.5), and δph was negative (-0.3), indicating that the net surface charge of the particles is negative. the surface potential (⌡o) obtained in this study was also negative (-63.24 mv). similar values were obtained by silva et al. (1996) in different soils collected from several regions of brazil, and by braz et al. (2013), in soils from eastern amazon. the electric charge of soil particles influences the adsorption of cations and anions, and the quantity of ions adsorbed depends on the charge density, which depends on the colloid surface (alleoni et al. 2016). according to sposito (1984), solid surfaces of soil particles and/or reactive minerals develop electric charge in two ways: (a) isomorphic substitutions between ions of different valences in minerals, or (b) from reactions of surface 390 jéssica p. marques, eduardo a. ferreira da silva, carla patinha, mariana c. kasemodel, valéria g. s. rodrigues functional groups with ions present in the solution. these charges can be positive, null, or negative. according to langmuir (1997), a sorbent phase endowed with a net negative surface charge exhibits a cec, while a phase containing a net positive surface charge exhibits an anion exchange capacity. cec is a very important property that affects the retention of organic and inorganic species (sposito 1984). the cec of the total soil sample obtained using the methylene blue method was 3.62 cmolc/kg, while cec obtained by nh4ac saturation method was 3.20 cmolc/kg. it should be noted that these cec values are related to the total soil sample. if we consider that all that capacity is solely from the clay fraction (26% of the total soil), we can estimate the actual clay cec dividing the total cec by 0.26. this calculation results in the value of 13.8 cmolc/kg for the methylene blue method, and 17.9 cmolc/kg for the nh4ac saturation method. in this context, we can estimate that the sample may have kaolinite, and a certain amount of illite as confirmed in the mineralogical analysis (table 2). according to langmuir (1997), cec values related to kaolinite range from 3 to 15 cmolc/kg, which is strongly ph dependent, while for illite it ranges from 10 to 40 cmolc/kg, which is slightly ph dependent. according to rieuwerts (2007) and coringa and weber (2008), well drained soils of humid tropical regions, such as those studied in this work, are intensely weathered and dominated by minerals with variable load, as previously proven, which is responsible for low cec values, which could vary with ph. the ss result of the residual soil was 28.29 m2/g. this result confirms the presence of kaolinite in the analyzed soil. kaolinite ss ranges from 10 to 38 m2/g (langmuir 1997). according to sposito (1984), kaolinite ss ranges from 7 to 30 m2/g. the sorptive abilities of minerals are proportional to their surface areas and surface-site densities, which can range widely, reflecting differences in how they are measured (langmuir, 1997). the om content of the residual soil was 13.40 g/kg, which is low. according to rieuwerts (2007) tropical soils are relatively deficient in om as a result of its rapid decomposition. but even these low concentrations of om may be important for the soil cec, mainly for soils dominated by 1:1 clays. humic substances in om can be highly reactive. om can provide a higher cec (150 to 300 cmolc/kg, according to sparks (1995) in comparison with some minerals. according to alleoni et al. (2016), the presence of hematite, goethite and gibbsite explains the higher pzc values, and higher levels of om and kaolinite are responsible for lower pzc values. the pzc value of the analyzed soil was 3.9, reflecting the presence of kaolinite. the mineralogical analysis indicated the presence of non-expansive clay minerals in the soil, such as kaolinite and illite (table 2). according to silva et al. (1996), the presence of kaolinite content in the soil explains the pzse value. this mineral tends to contribute in lowering the average soil pzse, due to its low isoelectric point (between 4.0 and 5.0). according to langmuir (1997), kaolinite is common in humid-climate, well-leached acid soils, and stream sediments where there is good drainage. kaolinite results from weathering of the k-feldspar (microcline and orthoclase) and muscovite in rocks such as granite. kaolinite is a low activity and nonexpansive clay mineral (mohamed et al. 2018). illite clays may result from the weathering of micas and feldspars. in addition to the non-expansive clay minerals, the analyzed soil is composed of goethite, quartz and muscovite. according to rieuwerts (2007), the intense weathering and leaching observed in the tropics results in desilication, i.e. “the leaching of si from silicates and the enrichment of primary quartz, secondary al and fe oxides, and two-layer clay minerals (mainly kaolinite) relative to all other soil constituents”, as observed in the analyzed soil. according to araujo et al. (2002), kaolinite and fe and al oxides are important components in the pte retention. many authors studying the characteristics of tropical humid weathered soils reported the establishment of relations between the electrochemical, chemical and mineralogical attributes, which facilitates understanding the soil ion adsorption phenomena (coringa and weber 2008). in the batch test, it is essential to control the physicochemical parameters before and after the test. thus, electrical conductivity (ec), redox potential (eh) and ph measurements were measured before and after the batch test (fig. 1). the ph values of the soil in contact with contaminant solutions were within the range of 4.0 and 4.5. results show little ph variation after the contact of pbcl2 solution with the residual soil. in this acidic environment pb solubility is favored. the eh of the samples ranged from 320 to 400 mv at the initial stage, from 310 to 440 mv at the contact, and from 370 to 405 mv at the final stage. these positive values show an oxidizing environment, which favors adsorption. pb solubility is lower in oxidizing than in reducing environments. this is probably explained by the dissolution of al, fe and mn oxides in reducing environments. these oxides play an important role in metal adsorption (chuan et al. 1995). significant variation of eh during the test was observed for the samples with the solution of 20 mg/l and 60 mg/l pbcl2. after the contact between the 20 mg/l solution and the soil, the eh increased 92 mv after 48 h. when using 60 mg/l pbcl2 solution, eh decreased 49 mv after 48 h of contact with the residual soil. other tested concentrations had minor eh variations (fig. 1). figure 1. values of ph, redox potential (eh) and electrical conductivity (ec) of the solution used in the batch test, in the initial stage (t = 0 s) and final stage (t = 48 h) of the test. 391adsorption of lead (pb) in weathered tropical soil (ribeira valley region brazil) ec decreased after 48 hours of soil solution contact for all tested concentrations, indicating a reduction in the concentration of salt solution due to the adsorption. a considerable ec decrease (approximately 40% of the initial ec value) occurs using 60 mg/l of pbcl2 solution, while smaller variations were observed using 80 and 100 mg/l of pbcl2 solution. batch tests icp/ms results show low concentrations of cd (0.08 mg/kg), pb (16.5 mg/kg) and zn (19.4 mg/kg) in the studied residual soil. these concentrations are similar to results available in the literature (wedepohl 1995) for soils collected in uncontaminated areas (0.10 mg/kg cd; 17 mg/kg pb, 52 mg/kg zn). the results are also comparable to the background concentrations of pte found by nogueira et al. (2018) for natural soils from são paulo state, brazil (0.1 mg/ kg cd; 10.1 mg/kg pb, 21.8 mg/kg zn). table 3 and fig. 2 shows the percentage of pbcl2 adsorbed by the soil. the percentage of adsorbed pbcl2 decreased when the concentration entry was higher, which was also evidenced by ec measurements (fig. 1). table 3. data obtained during batch test: pb initial concentration (c0 ); pb concentration at the end of the experiment (ce ); difference between the initial and equilibrium concentration (δc ); mass of pb adsorbed by mass of soil (s). co (mg/l) ce (mg/l) ∆c (mg/l) s (µg/g) pb adsorbed (%) 20 1.50 18.50 92.17 92.5 40 3.82 36.18 181.35 90.5 60 7.74 52.26 260.78 87.1 80 12.20 67.80 339.34 84.8 100 18.26 81.74 407.88 81.7 as shown in table 3, the adsorption capacity s of the soil increases with increasing solution concentration (c0 ). for the solution of 20 mg/l, soil adsorbed 92.17 μg/g. for the highest concentration (100 mg/l), s was 407.88 μg/g. it should be noted that the increase of the capacity s does not occur at the same rate as the increase in concentration, i. e. the isotherm is not perfectly linear. such behavior is typical of dissolved species in soils. at low pb concentrations, the affinity of the metal with the solid surface of the soil is higher. at increasing concentrations, as the adsorption sites are occupied, this affinity decreases. consequently, the highest percentages of pb adsorption were obtained when the lowest initial concentrations were used (langmuir 1997). based on table 3, using the initial concentration 20 mg/l, the percentage of lead adsorbed by soil was 92.5%. for the solution with 100 mg/l of pb, the adsorption percentage is comparatively smaller (81.7%). the figure 3 presents the langmuir i and ii, freundlich and linear isotherms. table 4 shows the data obtained when using langmuir i and ii, freundlich and linear isotherms. based on the correlation coefficients obtained from linear, langmuir and freundlich equations, all models adequately describe the experimental figure 2. adsorbed concentration of the pb by tropical soil. figure 3. linearization graphs of the linear, freundlich and langmuir (i and ii) adsorption isotherms. 392 jéssica p. marques, eduardo a. ferreira da silva, carla patinha, mariana c. kasemodel, valéria g. s. rodrigues pb adsorption results. the langmuir ii (r2 = 0.9982) and freundlich (r2 = 0.9927) models had the best adjustments (table 4). the linear equation had the lowest adjustment compared to freundlich and langmuir i and ii. similar results were obtained by linhares et al. (2009) studying the pb adsorption in highly weathered tropical soils collected in the region of minas gerais (brazil). for these authors the adsorption was best represented by langmuir (0.98 < r2 < 0.99) and freundlich (0.94 < r2 < 0.99) equations. freundlich equations had also provided better adjustments for pb adsorption by 4 soils from goiás state (0.96 < r2 < 0.99) (oliveira et al., 2010) and by a transported soil from ribeira valley (r2 = 0,98) studied by araujo (2015). paranavithana et al. (2016), observed that the langmiur isotherm presented the best fit for pb2+ and cd2+ in coconut shell biochar (fine granules), in a local soil from sri lanka, and in a biochar-mixed soil (1:1 mixture of biochar and soil). the parameter b (langmuir), which is related with the adsorption energy, was 0.1149 cm3/g for langmuir i and 0.1407 cm3/g for langmuir ii (table 4) models. according to linhares et al. (2009) the pb is an easy adsorption cation. according to pierangeli et al. (2001), this element is retained initially by negative charges of the surfaces of soil colloids, later forming more stable bonds. in the latter case, according to these authors, pb becomes part of the surface of colloids and available to plants in limited quantities. the pb interactions with soil colloids are predominantly more specific and less dependent on surface charges. the specific adsorption involves the exchange of metals with ligands present on the surface of the colloids, which forms covalent bonds and is assumed as the cause for which some soils adsorb metals above their predicted capacity based on cec (linhares et al. 2009). in soils with variable loads, it is difficult to distinguish the contribution of each of its components in the adsorption of metallic cations. even when regarding pure minerals, the adsorption capacity of a certain oxide or clay mineral depends on its crystallinity degree, which is extremely variable with the age and environmental conditions of its formation (pierangeli et al. 2001). the maximum adsorption capacity of the soil is 588.2353 µg/g. this value is lower than the capacity of transported soils from the same region. rodriguez (2013) and araujo (2015) studied pb adsorption by 3 transported soils from ribeira valley, using batch equilibrium tests (with soil-solution ratio 1/5). the authors reported maximum adsorption capacities varying between 893.00 and 1111.11 mg/l. even so, the soil studied in this research proved to be efficient to adsorb pb in solutions with concentrations close to those found in leachate from mining waste. kasemodel (2017) studied soil and slag samples from a waste disposal area in adrianópolis. the pb concentrations found in the leach test extracts ranged from 5.16 to 77.1 mg/l. raimondi (2014) studied mining tailings also from adrianopolis and the leach extracts showed pb concentrations varying from 13.5 to 77.1 mg/l. for this range of concentrations in batch tests, the soil presented adsorption efficiency higher than 84.8% (for the c0 = 80 mg/l). conclusions the tropical soil (cambisol) presented some favorable physical and chemical properties, and others less suitable, to be used in the construction of liner in the waste disposal area. the soil is fine and its granulometry is compatible with that recommended in the literature for use in liners of compacted clay. the clay fraction is composed of 1:1 clay minerals such as low activity, nonexpansive kaolinite. the oxidizing medium and the predominance of negative charges on the surface of the colloidal particles (evidenced by delta δph = 0.3 and pzse = 3.4) favor the retention of metallic cations. on the other hand, some characteristics of this tropical soil are less favorable, when observed separately, for the retention of contaminants. this is the case of low cec, which may result from the presence of kaolinite. low om content and acidic ph also discourage contaminant retention. in addition, the pi of 33% is in the limit of the recommended range, thus, the material can be used as a liner. as for adsorption, it was noted that the highest percentages of pb adsorption were obtained at the lowest initial concentrations. thus, the adsorption percentage of pb when using initial concentration of 20 mg/l was 92.5% and, when the initial concentration was 40 mg/l adsorption rate was 90.5%. for higher pb concentration (100 mg/l) the adsorption percentage is comparatively smaller (81.7%). based on the correlation the langmuir ii (r2 = 0.9982) and freundlich (r2 = 0.9927) models had the best adjustments. the results indicate that the maximum adsorption smax (langmuir) was 588.23 µg/g for langmuir i and 526.31 µg/g for langmuir ii models. thus, it is concluded that the tropical residual soil presents characteristics that allows it to be used as liner in the disposal of residues containing pb. but we recommend the study of possible additives to the soil to improve the plasticity and ion exchange characteristics. acknowledgements the são paulo research foundation (fapesp) for granting a scholarship to the first author (process number 2015/02529-4) and for provided financial support (process number 2014/07180-7) and the national council for scientific and technological development (cnpq) (for productivity in research fellowship process number 305096/2015-0) are gratefully acknowledged. references associação brasileira de normas técnicas – abnt (brazilian association of technical standards) (1984a). brazilian standard nbr 6459. determination of liquid limit. rio de janeiro, brazil. associação brasileira de normas técnicas – abnt (brazilian association of technical standards) (1984b). brazilian standard nbr7180. determination of plasticity limit. rio de janeiro, brazil. adriano, d. c. 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(2001). geoenvironmental engineering: contaminated soils, pollutants fate, and mitigation. boca raton: crc press, 320 pp. zia, a., berg, l. v. d., ahmad, m. n., riaz, m., zia, d., & ashmore, m. (2018). controls on accumulation and soil solution partitioning of heavy metals across upland sites in united kingdom (uk). journal of environmental management, 222, 260-267. https://doi.org/10.1016/j. jenvman.2018.05.076. zuquette, l. v., silva jr. e. m., & garcia, a. (2008). aspectos de sorção para os materiais inconsolidados da região de são carlos (sp). rem – international engineering journal – revista escola de minas, 1(2), 219-230. http://dx.doi.org/10.1590/s0370-44672008000200017 keywords: ambient noise; hvsr; reactivation; stiffness; soil mechanics . palabras clave: ruido ambiental; rehv; reactivación; rigidez; mecánica de suelos. how to cite item hussain y., cardenas-soto, m. moreira, c., rodriguez-rebolledo, j., hamza, o., prado, r., martinez-carvajal, h., dou, j. (2020). variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing. earth sciences research journal, 24(2), 141-151. doi: https://doi.org/10.15446/ esrj.v24n2.81974 rainfall-induced landslides pose a significant risk to communities and infrastructures. to improve the prediction of such events, it is imperative to adequately investigate the rainfall-dependent dynamics (leading to fluidization) and any associated internal sliding along shear planes within clayey slopes. therefore, the present study adopted ambient noise analysis based on the horizontal to vertical spectral ratio (hvsr) method, to measure the seasonal variation of rayleigh wave ellipticity as an indicator for the internal deformation and transition in the material state. the methodology was applied to an existing landslide, where variations in soil stiffness and internal sliding were expected to occur in response to rainfall. to improve the interpretation of the hvsr results (and hence the prediction of landslide’ reactivation by rainfall), hvsr measurements were also conducted on a field-scale pile load test. the pile test allowed a comparison of the seismic data generated by the soil movement along shear planes. the hvsr curves of this field test showed two frequency peaks with no changes in the resonance. in comparison with the data obtained from the landslide, the resultant hvsr curves showed three frequency patterns: ubiquitous (2hz), landslide (4-8hz), and flat (no peak). however, the hvsr curves did not show any response to the expected seasonally induced variations in the landslide mass because of the relatively short data acquisition. nevertheless, time-lapse hvsr is a promising technique that can complement other geophysical methods for improving landslide monitoring. abstract variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing variación estacional en la elipticidad de la onda rayleigh como posible indicador de la activación de flujos de tierra, caso de estudio: deslizamiento de tierra en sobradinho comparado con prueba de carga en pilote issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.81974 los deslizamientos de tierra inducidos por lluvia representan un riesgo significativo para las comunidades y la infraestructura. para mejorar la predicción de tales eventos, es imperativo investigar adecuadamente la dinámica generada por la lluvia que conduce a la fluidización, así como, cualquier deslizamiento interno que esté asociado y que se desarrolle a lo largo de los planos de falla dentro de los taludes arcillosos. por lo tanto, para medir la variación estacional de la elipticidad de la onda rayleigh, como un indicador de la deformación interna y de la transición del estado del material, el presente estudio adoptó la relación espectral horizontal a vertical (rehv) basada en el ruido ambiental. la técnica fue aplicada en un área de deslizamiento existente, donde se esperaba que ocurrieran variaciones en la rigidez del suelo y deslizamientos internos debidos a la lluvia. para mejorar la interpretación de los resultados de la rehv y, por tanto, la predicción del deslizamiento generado por lluvia, la rehv fue calibrada en campo en una prueba de carga en pilote. la prueba en el permitió una comparación de los datos sísmicos generados por el movimiento del suelo a lo largo de los planos de falla. las curvas rehv de esta prueba de campo mostraron dos picos de frecuencia sin cambios en la de la resonancia. en comparación con los datos obtenidos del deslizamiento de tierra, las curvas rehv resultantes mostraron tres patrones de frecuencia: ubicuo (2hz), deslizamiento de tierra (4-8hz) y plano (sin pico). debido a que la recopilación de datos fue relativamente corta, las curvas rehv no mostraron ninguna respuesta a las variaciones estacionales inducidas por el deslizamiento de tierra. sin embargo, la obtención de curvas rehv en un periodo de tiempo es una técnica prometedora que puede complementar otros métodos geofísicos para mejorar el monitoreo de deslizamientos de tierra. resumen record manuscript received: 31/08/2019 accepted for publication: 27/03/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 141-151 yawar hussain1 ,martin cardenas-soto2, césar moreira3, juan rodriguez-rebolledo4, omar hamza5, renato prado6, hernan martinez-carvajal7, jie dou8 1clemson university, south carolina, usa 2universidad nacional autónoma de méxico, mexico. 3universidade estadual paulista, sao paulo, brazil. 4universidade de brasília, brazil. 5university of derby, united kingdom. 6university of sao paulo, brazil. 7universidad nacional de colombia, bogotá, colombia 8nagaoka university of technology, japan. * corresponding author: yawar.pgn@gmail.com so il m ec h an ic s https://doi.org/10.15446/esrj.v24n2.81974 https://doi.org/10.15446/esrj.v24n2.81974 https://doi.org/10.15446/esrj.v24n2.81974 142 yawar hussain, martin cardenas-soto, césar moreira, juan rodriguez-rebolledo, omar hamza, renato prado, hernan martinez-carvajal, jie dou introduction landslides pose a threat to the global terrestrial hazards affecting many countries, for example: the sidoarjo mudflow in indonesia in 2006 (wibisana, 2013), the beichuan landslide in chania in 2008 (tang et al., 2011), and the montecito mudflows in california in 2018 (donnellan et al., 2018). other examples include the 2008 mw 7.9 wenchuan earthquake causing about 200,000 coseismic landslides in china (yunus et al., 2020), the 2004 mw 6.8 chuetsu and the 2018 mw 6.6 hokkaido earthquake-induced over 10000 landslides in japan (dou et al., 2020), respectively. also, dense rainfall in mountainous terrain can seriously trigger numerous landslides in hill slopes in different countries and regions, such as china, taiwan, japan, vietnam, etc. (chang et al., 2019; li et al., 2020; pham et al., 2019, fan et al., 2018). due to the substantial human and economic losses accounted for the disastrous landslides, scientists worldwide focus on the fact of monitoring locations of future landslide events for mitigating such disasters. in the case of brazil, a significant number of shallow rainfall-triggered landslides have occurred (hussain et al., 2017; rosi et al., 2019). from 1988 to 2015, there were 733 landslides events in 243 brazilian municipalities, with 4,000 fatalities. economic losses and homelessness are the other social impacts of these hazardous events in the states of rio de janeiro, são paulo, minas gerais, and santa catarina. the tragedies of 1966, 1967, 1985, 1988, 1995, 2008, 2009, 2010, 2011 and 2014 are historical reminders of the atrocities caused by landslides in rio de janeiro (martins et al., 2018, hussain et al., 2019a). the sobradinho landslide (considered in this study) is an example of such events, which has superficially developed in lacustrine sediments, and whose movements were mostly controlled by water infiltration and fluvial dynamics (hussain et al., 2017; rosi et al., 2019). landslides involving sudden mobility of earthflows have been associated with two mutually inclusive mechanisms: (i) excess pore-water pressures generated along shear plane (baum et al., 2003; van asch and malet, 2009; hussain et al., 2019b) and (ii) loss of shear strength because of the increased degree of saturation (pastor et al., 2010; jongmans et al., 2015; hamza and bellis, 2008; zhou et al., 2013, 2016). accordingly, the mechanical behavior of the earthflows is considered a grouping of coulomb plastic solids where the mobility is associated with distributed internal shearing that causes flow-like appearance instead of mass liquefaction (hungr et al., 2001). the pore water pressure assumption explains the mobility of landslide in terms of the reduction of shear strength along a slip surface (shear plane). however, in the case of clayey material, the undrained shear strength (cohesion) is not expected to reach zero. in such cases, the mobility can be better explained as a transition of a material state from solid to plastic and from plastic to fluid (i.e. solid-plastic-fluid) combined with internal sliding along shear planes. this process is called fluidization, which differs from liquefaction (lowe, 1976; bertello et al., 2018). however, the identification of such material transition and internal sliding is a challenging task, which requires continuous monitoring in order to improve landslides’ prediction. for the monitoring, a continuous source of energy at high spatiotemporal resolution is required which can be provided by ambient noise (vibrations in the absence of any single source such as an earthquake). the time-lapse spectral analysis and ambient noise interferometry (ani), which measure rayleigh wave ellipticity and rayleigh wave velocity, respectively, are the two commonly adopted ambient noise-based techniques for the dynamic monitoring of landslides. in theory, both techniques allow time-lapse monitoring to identify temporal changes of these parameters (ellipticity and wave velocity) (mainsant et al., 2012; köhler and weidle, 2019). however, very few attempts have been conducted in the literature, where seismic waves (rayleigh wave) are utilized for the demarcation of such transition. in these studies (mainsant et al., 2012; bertello et al., 2018), a substantial drop of rayleigh wave velocity has been observed in response to the earthflow acceleration, which is followed by a slow return to pre-disturbance of rayleigh velocities as the landslide decelerates. however, these studies applied expensive, time-consuming and labor extensive techniques for the estimation of rayleigh wave velocities such as time-lapse interferometry (mainsant et al., 2012; hussain et al., 2019c) and time-lapse active multi-channel analysis of surface waves (masw) (bertello et al., 2018). rayleigh wave ellipticity calculated from an hvsr curve (horizontal to vertical spectral ratio) provides an alternative choice. recently, few studies have interpreted seasonal changes in hvsr ellipticity curves as a result of the thaw-freeze cycles of an active permafrost layer (abbott et al., 2016; kula et al., 2018; köhler and weidle, 2019) and shallow rainfall-triggered landslide (hussain et al., 2019c). the results of these studies have suggested that surface wave ellipticity (hvsr peak) might constitute a good indicator for the internal deformation and transition in material state (fluidization process) and can be used as a precursor for predicting earthflow. therefore, the hvsr method could bear the potential to become a low-cost, passive, and non-invasive method for long-term monitoring and site characterization of landslides with high temporal resolution. the aim of the present study is to improve the prediction of rainfallinduced landslides by understanding the rainfall-dependent dynamics (leading to fluidization) and any associated internal sliding along shear planes within clayey slopes. the study applied the hvsr monitoring on sobradinho landslide, where variations in soil stiffness and internal sliding (fluidization process) would be expected to occur in response to rainfall events. to improve the interpretation of the hvsr results (and hence the prediction of landslide reactivation by rainfall), hvsr was also conducted to monitor a field-scale pile load test. this prototype experiment allows a similar mechanism (i.e., the development of shear planes leading to soil failure) to be observed under an increasing compression load. the pile load test would allow a comparison of the seismic data generated by the soil movement along shear planes. moreover, the study included site characterization (landslide geometry and its stratigraphy) as well as directional analysis in order to identify any possible directional effects. this study provides a good basis for future investigation on landslides’ early warning system. study area the geology of the federal district can be summarized in four lithological boundaries, according to regional stratigraphy, delineated as (base to top): (i) paranoá (metasedimentary rocks – meso-neoproterozoic), canastra (phyllites meso-neoproterozoic); (ii) araxá (schists neoproterozoic); (iii) bambuí (clayed metasiltites rolled, clay and metasiltites banks meso-neoproterozoic); (iv) groups of soil or shallow colluvial deposits (pedimentary type quaternary) (freitas-silva and campos, 1998). according to the brazilian soil classification system, the region of the study area, brasilia (federal district), is comprised of a mantle of tertiary-quaternary age detritus-lateritic soil. the soil is composed mainly of red-yellow latosols and has high porosity and a cementitious bond type. therefore, it has a highly unstable structure when subjected to an increased moisture or changes in the state of stresses. this soil almost always presents a volumetric change as high as the variation of these factors (referred to as a collapsible structure) (hussain et al., 2019b). on the slope of the study area, the channel depositions in the basin are distinguished as (i) landslide debrites due to recent rotational failure mechanism; (ii) fluvial deposits on recent riverbanks as alluvial fan deposits, colluvial, and alluvial. the detailed geology and geomorphology of the area can be found elsewhere (hussain et al., 2019c and references therein). sobradinho landslide the area of landslide is part of the maranhão river basin. the hydrographic network in this region is composed of a high-grade segment of the contagem stream, which is a direct tributary of the rio maranhão. the main drainage system is controlled by the structural lineaments (faults and fractures) and low sinuosity, while the secondary network is typically dendritic with high sinuosity. the main watercourse is the ribeirão da contagem, but there are also small intermittent or perennial tributaries with flow rates of less than 0.005 m3/ second in the dry months of the year. the selected landslide is e-w trending roto-translational slide (hungr et al. 2014), approximately 150 m long and about 70 m wide, located in a small vicinity called ‘rua do mato’ (figure 1). at the top of the landslide mass (along the main scarp), there is a small scarp in the middle created by the erosion caused by the release effects of the contagem river (according to in-situ inspections), which is quite high during the rainy season (hussain et al., 2019b). the sobradinho landslide has been extensively studied with geophysical, geotechnical, and geomorphological methods. figure 1 shows the location of the study area, the distribution of the hvsr stations and some aspects of the landslide. 143variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing the mechanical behavior of early-stage earthflows in clayey formation might be considered as coulomb plastic solids whose mobility is developed by sliding (as described above). in this manner, the material flows by internal deformation (internal shears) (berti et al., 2019). so, the sliding can be resolved into several occurrences of propagation of sliding plane or soil failure (vouillamoz et al., 2018; hussain et al., 2019b). a similar mechanism (i.e., where shear planes are developed leading to soil failure) can be observed when a concrete pile is subjected to an increased compression force leading to soil bearing failure. however, this mechanism differs in the type and level of applied stresses, which are very small in the landslide under the influence of triggering factors (e.g. rainfall). field scale (prototype) experiment – pile load test material and methods pile load test (prototype experiment) was conducted in this study to artificially generate a failure mechanism with sliding of soil along shear planes at field-scale. figure 2-a and 2-b show the location of the testing site and its soil profile. by conducting this test, our aim was to aid the interpretation of the results obtained from sobradinho landslide. this hypothesis was based on the potential similarity of some aspects of the failure mechanisms in both cases (i.e. the landslide and the pile load test) as discussed below. figure 1. location of ribeirão contagem watershed on federal district map and its geology (a, b); sobradinho landslide along the hvsr measuring points (red dots), red dashed ellipse is the approximate landslide boundary (c); zoomed image of the landslide with a triangular array used for time-lapse monitoring (d); photographs of the landslide (e, f). e f 144 yawar hussain, martin cardenas-soto, césar moreira, juan rodriguez-rebolledo, omar hamza, renato prado, hernan martinez-carvajal, jie dou the fieldwork of the pile load test was conducted in several stages. in the first stage of this prototype experiment, ground characterization of the testing site was carried out using standard penetration test (spt-n). in the second stage, a concrete pile of 0.8 m diameter and 12 m depth was cast-in-situ and allowed to harden for 4 weeks. the pile was then subjected to progressive axial load while measuring the vertical displacement of the pile head. according to brazilian standards (hussain et al., 2019b), soil failure occurs at a displacement greater than 8 mm, where the base resistance is likely to be mobilized i.e. when shaft resistance has been or is close to being fully mobilized. as the pile load was gradually increased at 30-minute intervals, the hvsr data was obtained from each 30-minute long ambient noise record. the pile loading mechanism was supported by two 8m deep reaction piles that balanced the applied load on the main pile. a load cell was placed below the steel rods, and a hydraulic jack was attached to the load cell. this formed a good base for applying a compression force on the pile by a manually operated oil pump. the hydraulic jack was attached to two smaller metallic rods on which four extensometers were installed for monitoring the pile vertical displacement (figure 2c and d). ambient noise data acquisition at landslide area the experimental data was acquired using sercel l-4a-3d short period seismometers having a natural frequency response of 2 hz. the threecomponent seismometer records three-dimensional ground motion in the northsouth (ns), east-west (ew), and vertical (z) directions as required by the single station hvsr analysis. for continuous data acquisition, das-130 reftek dataloggers were used that can record up to 1000 samples per second (sps). the time and position of each measurement point were provided by the gps130 locks. the seismometer was aligned with the geographic north determined by a magnetic compass. the sensors were also leveled and, in order to avoid environmental interference, buried in the 50cm deep hole. the ambient noise measurements were automatically stored on two memory cards installed in the datalogger. in the landslide experiment, the data acquisitions were conducted in two stages. in the first stage, single station hvsr measurements were taken both at the landslide mass and in the surrounding areas (figure 1c) in order to delineate the landslide’s boundary. ambient noise at each point was recorded for 30-minutes length in accordance with the guidelines provided by sesame (2004). in the second stage, for continuous ambient noise recording, three sensors were installed on the landslide in a triangular geometry, as adopted by hussain et al. (2019c). the continuous data used for the seasonal impact evaluation was divided into three acquisition campaigns: (i) dry period, from 02-11-2016 to 08-11-2016; (ii) intermediate, from 12-04-2017 to 15-04-2017; and (iii) saturated period e.g. from 10-12-2017 to 17-12-2017. hvsr technique in the single-station hvsr technique, ambient noise recording is conducted at a single three-component station (n-s, e-w and up-down). the hvsr curve is interpreted in terms of its peak frequency and amplitude. the hvsr curve peak frequency is traditionally interpreted as a representative of the rayleigh wave ellipticity or shear wave amplification over horizontally stratified layers. the physical significance of the hvsr peak amplitude is still dubious and open to debate (fäh et al., 2001). many authors (e.g., bonnefoy-claudet et al., 2008) agree on a relationship between the hvsr amplitude and the impedance contrast. few of the documented interpretations of the peak amplitude are the reflection of ambient noise wavefield typology figure 2. prototype field experiment (pile load test) at the university of brasilia. location of experiment (a); soil profile of the site (b); photograph of the experiment (c) and schematic of the loading mechanism (d). 145variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing (rayleigh, love, and body waves). however, the environmental conditions such as sensor ground coupling, water saturation (gosar et al., 2010; gosar and lenart, 2010; pazzi et al., 2018a; 2018b), weather nearby structures, underground structures, and transient noise can possibly affect the results obtained from hvsr (chatelain et al., 2008). in a porous material, an increase of water content can influence (increase) the p-wave velocity more than vs, thus causing an increase of poisson ratio, which, in turn, induces an increase of rayleigh wave ellipticity (i.e., of the h/v ratio of the elliptical particle motion). this makes the practical application of hvsr for the dynamic (fluidization) monitoring of landslide possible. in this study, the hvsr survey was carried out in accordance with the approach provided by sesame (2004). in this approach, the duration of ambient noise acquisition is dependent on the ambient noise levels in the area; usually, the record length should be higher than 15 minutes. the hvsr measurements yield important insights into the resonance behavior of particular lithomorphologies and geomorphological features, while the polarization analysis of the ambient noise records can provide information about the particle motion and directivity properties, which are associated with the mechanical vibrational behavior of structures (burjánek et al., 2012; zare et al., 2017). the hvsr has been applied in several application including the determination of the resonance of soil deposit over the basement by nogoshi and igarashi (1971), earthquake hazards assessments through microzonation (di giulio et al., 2014), bedrock depth estimation (garcía-jerez et al., 2006; gosar and lenart, 2010), archeology studies (kalil et al., 2016), estimation of the ice sheet thickness and of the basal properties of a glacier (picotti et al., 2017) as well as for the dynamic monitoring of glacier (köhler et al., 2015) and landslide assessment (imposa et al., 2017; bertello et al., 2018, hussain et al., 2019c). despite these enormous applications in the last decade or so, the theoretical basis of the technique is still unclear and open to debates. data processing adopted in this study the same data processing workflow was applied to the records of all experiments. the data was acquired in reftek format, which is first converted to a seismic analysis codes (sac) format. then using sac software, the data was detrended and demeaned. in the next stage, the three-component records of each point were uploaded to a freely available software geopsy® (www.geopsy.org), where the hvsr analysis was conducted. the recorded transient events, due to any earthquakes and intermittent anthropogenic noises, were removed by choosing automatic window selection in the software. the entire length of the data was divided into short windows while performing the calculation. in the last step, the spectra were smoothened by konnoohmachi function with a constant value of 40 and a cosine taper of 10% width. the entire hvsr calculation processing workflow is presented in figure 3. figure 3. hvsr workflow chart of ambient noise processing in sac and geopsy software. directional analysis while it is generally accepted that the peak frequency is reliably diagnostic of site subsoil conditions, several researchers have observed that amplitude can be considerably influenced by factors independent of site conditions e.g. the noise wavefield composition including different portions of the body, love and rayleigh waves (bonnefoy-claudet et al., 2006). the objective of the directional analysis is to understand how the landslide body receives a contribution of ambient noise energy along the different axis. if the amplification at two horizontal components is unequal with larger motion at a specific azimuthal, this is known as directional amplification (bonamassa and vidale, 1991). the presence of fractures and cracks as well as fault zones may show directionality. the directional amplification is suggested to be due to the presence of largescale open cracks (moore et al., 2011) or microcracks (pischiutta et al., 2017). in this way, links between the distribution of noise sources and the subsurface conditions are evaluated. the analysis of directional effects in the site response is conducted by rotating the components of the spectral ratios obtained at each measurement site by steps of 10◦, starting from 0◦n to 180◦n, and plotting the contours of spectral amplitude as a function of frequency and direction of motion (panzera et al., 2012). the 0° stands for the north direction marked on seismometer and 180° stands for south (from 0° to 360° results are symmetric). the directional effects of landslide-affected areas have been shown in previous studies (burjánek et al., 2010; imposa et al., 2017). results and discussions pile load test the results of the pile load test showed three peaks on hvsr curves: first at 5 hz (deep interface), second at 32 hz (not considered), and third at 55hz (shallow interface). both peaks (shallow and deep) represent the depth of soil horizons, i.e., shallow and deep interfaces. following an 1d simple modeling, a site resonance frequency fo was related to the thickness h of surface material having velocity vs through the following equation: fo=vs/4h (1) the vs value is calculated from spt-n using imai et al. (1977) relation for all soils as 550 m/s, which for fo=5 hz would imply a thickness of less than 30 m. assuming a value of 55hz for the relevant peak, the thickness of the material involved in resonance would be 2.5 m, i.e. a thin surface soil layer (hussain et al., 2019d). both peaks show variations in their amplitudes with the increase of an applied load with constant resonance frequency. according to the results shown in figure 4 (a), the variations of the mean amplitude measured on successive time intervals of 30 minutes were very small and did not follow a trend correlated to the load increase. these variations are most likely irrelevant to the changes in soil mechanical properties; it seems that they depend on the variations in ambient noise wavefield composition recorded over time. there was another hvsr peak at a frequency range of 32hz, having peak amplitude rather low, not reaching an hvsr factor of 2, which is generally considered a minimum threshold to infer the occurrence of significant resonance phenomena (see sesame-2004 guidelines); thus, this peak was not considered for the analysis. the geometrical similarities of the curves revealed that the peak frequency of hvsr curves at all time steps were similar. this might be attributed to the presence of identical subsurface stratigraphy (the same impedance contrast), which did not change with the applied load. the shear sliding (internal sliding) in the soil was too small to mark any difference in the hvsr curve peak, although other studies have also documented such changes in peak ambient under different phenomena (e.g., köhler et al. 2006; bonnefoy-claudet et al., 2008). the recorded low-frequency resonance peak (5hz) is almost azimuth independent (as shown in a-h in figure 5), denoting the absence of 2-d effects. the stratigraphic peak did not show any variation with azimuth, while the higher frequency peak showed azimuthal dependent variations (figure 5). the prominent azimuth range was 20-60 and 100-180, implying the nonuniform distribution of ambient noise sources surrounding the sensor. www.geopsy.org 146 yawar hussain, martin cardenas-soto, césar moreira, juan rodriguez-rebolledo, omar hamza, renato prado, hernan martinez-carvajal, jie dou figure 4. results of the pile load test: (a) the hvsr curves obtained at each loading increment (30 minutes). three peaks were observed: first (5hz), second (32hz), and third (55hz), (b) the load-displacement curve of the pile compression load test. figure 5. directional analysis of each 30 minutes long files (a, b, c, d, e, f, g, and h). each figure shows the contour plots of the hvsr curve as a function of frequency (x-axis) and rotation angle (y-axis) with respect to the geographic north. the color scale represents the hvsr amplitude. a b 147variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing landslide monitoring ubiquitous frequency the ubiquitous frequency was the frequency of saprolite (highly weathered rock), the soft layer over the seismic bedrock, and it was generally recorded throughout the studied area having high amplitude relative to the landslide peak (figures 5a, 6 and 7). this peak was not affected by the seasonal variations (stiffness) produced by meteorological changes (degree of saturation). however, there were slight variations in peak amplitude recorded at different points in the study area (figures 6 and 7) that could be linked with the composition of ambient noise wavefield. based on the active masw results of a study carried out by hussain et al. (submitted), the value of shear wave velocity was considered as 160 m/s. this value is low because of the high porosity of material and disturbance brought by the onset of the landslide. using the equation (1) with vs=160 m/s and h = the thickness of the sediment package over the seismic bedrock, it was found that the thickness of the saprolite plus clay layer is 20m deep. these results are consistent with the results obtained by joint inversion of hvsr and f-k curves, where the depth of saprolite was found equal to 23 m (hussain et al., 2019d). the ubiquitous frequency did not show the amplitude variations of the hvsr along the azimuths, mainly because it represented the stratigraphy and not the fractured landslide mass that led to trap seismic energy at a particular direction (azimuth) (zare et al., 2017; rezaei et al., 2018). we observed that the rotated hvsr obtained in the landslide fractured zones showed clear directional effects, which were quite complex and could not be interpreted easily. there were two groups on the hvsr curves: 1) where the amplitude is similar in almost all directions (2hz peak), and 2) where the higher amplitudes of the landslide peak (higher peak) are between 40° and 150° (figure 6). it is possible that can provide some more conformation about the ambient source distributions or the presence of fractures linked with the landslide boundary. landslide frequency in figures 6 and 7 along the high amplitude stratigraphic peak (2 hz), there are peaks at higher frequencies having low amplitudes, which presumably indicate the presence of shallower velocity boundaries within the sediments. the amplitudes of these side peaks were, in most cases, considerably lower than the amplitude of the main peak; only at p5 and p11, the amplitudes were higher. this relatively low amplitude over the hvsr curve secondary peak is mainly related to the presence of thin and soft soil covers (stanko et al., 2017). the presence of such peaks has also been reported in previous studies (gosar and lenort, 2010). the variation in amplitude is possibly related to the ambient noise sources in the region and the contribution of love waves that affect the amplitude of the h/v peak, as described in section 4.1. these higher frequency peaks 6hz and 8hz represent landslide depth (slip surface) at 6.6 m and 5 m, respectively. the depth of the slip surface is relatively consistent with the usual depth surface of the landslides in the region. the higher resonance frequencies represent the detached soil blocks created by the seasonal erosion of the contagem river flowing at the bottom. the river crosses the landslide at the bottom, which is a possible source of disturbance (dynamism) in the landslide mass. as a result, fractures are produced; some of these fractures are very deep, supported by visual field inspections – conducted by the authors. each block has its own degree of freedom and has a different resonance frequency, which may explain the emergence of secondary peaks of different frequencies depending on the location along the landslide (figure 6, 7). the emergency of stratigraphic and landslide peaks has been reported in a previous study by rezaei et al. (2018) at a landslide in tehran, iran. flat hvsr this class of hvsr curves lacked any frequency peak according to the reliability criteria of clear peak (amplitude greater than 2). there are two possible reasons for the absence of a peak on hvsr curves (i) bedrock are exposed or (ii) low-quality data. at some locations, the secondary peak also appeared with very low amplitude because of the presence of coarser grain material towards the floodplain of the river that lies behind the landslide. these curves were not considered for the analysis. the results are shown in figure 8. figure 6. hvsr curves of group-a peak at 2 hz and 4-6 hz. the two dashed lines represent the hvsr standard deviation, while the gray areas represent the peak frequency standard deviation, which quantifies the experimental error associated with the average peak frequency value. below we show the contour plots of hvsr curves as a function of frequency (x-axis) and rotation angle (y-axis) with respect to the geographic north. the color scale represents the hvsr amplitude. 148 yawar hussain, martin cardenas-soto, césar moreira, juan rodriguez-rebolledo, omar hamza, renato prado, hernan martinez-carvajal, jie dou figure 7. hvsr curves of group-b peak at 2 hz and 8-10hz (p refers to hvsr station, figure 1). legend is the same as of figure 5. the color bar presents the hvsr amplitude. figure 8. no peak/flat response of the strata on the hvsr curves. hvsr standard deviation, while the gray areas represent the peak frequency standard deviation, which quantifies the experimental error associated with the average peak frequency value. 149variation in rayleigh wave ellipticity as a possible indicator of earthflow mobility: a case study of sobradinho landslide compared with pile load testing degree of saturation the effects of rainfall infiltration are possibly related to changes in the poisson ratio that affects rayleigh wave ellipticity (hvsr). the changes in rayleigh wave ellipticity may also be related to the rise in pore pressures in the landslide mass, internal sliding, and changes in the state of landslide mass (as described in section1). in addition to the reduction of the soil stiffness due to the pore water pressure, the filtered water can reduce the shear wave velocity through increasing the degree of saturation. these phenomena have extensively been studied in the landslides triggered by rainfall and earthquakes (mainsant et al. 2012; harba and pilecki, 2017; hussain et al., 2019b; 2019c). the results of the present study did not show changes in the ubiquitous and higher frequency peaks and their amplitudes in response to rainfall dependent degree of saturation (figure 9, 10). one possible reason as documented by hussain et al. (2019c), is the insufficient amounts of rainfall recorded during the shorter data acquisition campaigns. these conditions did not let the highly porous landslide mass to reach high saturation levels. at station df01, no secondary peak was observed, and only stratigraphic peak appeared. this station was located in a stable place. the peak showed no variations in response to meteorological changes (figure 9). the records of station df02 showed a well-defined hvsr peak at higher frequency (landslide peak). this higher peak was present throughout the entire length of the experiment irrespective of the day-night ambient noise variabilities (figure 10). however, there was no evidence of the changes in the resonance of landslide with rain, which showed the absence of dynamism (e.g., fluidization or internal sliding) in the landslide mass. figure 9. the response of hvsr curves because of the degree of saturation recorded at triangular array of three sensors. color presents hvsr amplitude. station df01: (a) dry period, from 02-11-2016 to 08-11-2016; (b) intermediate, from 12-04-2017 to 15-04-2017; (c) saturated period e.g., from 10-12-2017 to 17-12-2017. figure 10. the response of hvsr curves to the degree of saturation recorded at the triangular array of three sensors. color presents hvsr amplitude. station df02: (a) dry period, from 02-11-2016 to 08-11-2016; (b) intermediate, from 12-04-2017 to 15-04-2017; (c) saturated period e.g., from 10-12-2017 to 17-12-2017. 150 yawar hussain, martin cardenas-soto, césar moreira, juan rodriguez-rebolledo, omar hamza, renato prado, hernan martinez-carvajal, jie dou conclusions the hvsr results obtained from the landslide showed several peaks. in particular, the ubiquitous peak at low frequency (2hz) was not related to landslide dynamics. instead, it represented the site response of seismically lowvelocity soil layer overlying higher velocity saprolite (20m deep). whereas the landslide peaks at high frequency were related to the detached blocks at the landslide mass and their amplitudes reflected the degree of impedance contrasts created by either the presence of thin, soft sediments or the composition of ambient noise wavefield. both peaks (ubiquitous and landslide) did not show any response to the effects of rainfall, mainly because of the relatively small degree of soil saturation. in comparison, the pile load test showed two stratigraphic peaks, for every increase in soil stress there was a small variation in the amplitudes of the hvsr peaks, which might be related to the composition of ambient noise wavefield, overall, these observations suggest that hvsr amplitude variabilities were complex and created by the structural change in the shallow sub-surface and the ambient noise wavefield composition. the variability reflected local noise conditions at each individual station affected by topography, vicinity to streams (contagem river), exposure to wind, and extend and timing of degrading instrument coupling. therefore, our data did not allow precise identification of the relationship between rainfall-induced dynamic (fluidization), displacement rate, and rayleigh ellipticity. however, it was still possible to conduct the landslide site characterization using limited ambient noise data. the present study provides a sound basis for a more comprehensive application of seismic noise measurements, a potential method in the disaster diagnosis and prediction, with an emphasis in densely populated mountainous areas. further investigation is recommended where greater deformations and larger scale fractures (centimeters) can be created in the soil in order to identify the possible effects of soil deformations on hvsr. the study can also benefit from the development of an improved physical simulation of the emergence and propagation of fractures as well as the fall of soil blocks in order to determine the possible effects of the landslide before, during and after the event. moreover, these results can be validated by conducting remote sensing of the landslide surface to confirm the actual movement of the soil mass. acknowledgment the authors acknowledge the support of the following agencies: the national council for scientific and technological development (cnpq), the support research of the federal district foundation (fap-df), the university of brasilia and, the pool of brazilian equipments (pegbr), rio de janeiro. references baum, r. l., savage, w. z., & wasowski, j. 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(2016). comprehensive analyses of the initiation and entrainment processes of the 2000 yigong catastrophic landslide in tibet, china. landslides, 13, 39-54. zhou, j. w., cui, p., yang, x. g., su, z. m., guo, x. j. (2013). debris flows introduced in landslide deposits under rainfall conditions: the case of wenjiagou gully. journal of mountain science, 10, 249-260. https://doi.org/10.1007/s10346 https://doi.org/10.1007/s10346 https://doi.org/10.1080/10106049.2018.1559885 http://sesame-fp5.obs.ujf-grenoble.fr hv_user_guidelines.pdf https://doi.org/10.1016/j.rse.2019.111476 https://doi.org/10.1016/j.rse.2019.111476 keywords: system structure; fuzzy mathematics; total quaintly control of point source; total quaintly control of unit; water quality planning. palabras clave: estructura del sistema; matemática difusa; control total de la fuente puntual; control total de la unidad; planificación de la calidad del agua. how to cite item liu, r. (2020). research on control method of pollutant total amount of water quality based on fuzzy mathematics. earth sciences research journal, 24(2), 191-199. doi: https:// doi.org/10.15446/esrj.v24n2.87248 given the scale-up of pollutants discharged into water bodies, control via reducing part of the discharge is turning out ineffective. this article proposes a method based on fuzzy mathematics for controlling water from the total pollutant amount. firstly, a control framework and process of the total pollutant amount was built. the total amount of pollutants discharged into several water bodies in china was studied to achieve the predetermined environmental objective. then, water pollution of different regions was evaluated via water quality model based on the fuzzy mathematics in the area or essential protection domain with severe pollution and a concentrated pollution source. this procedure makes a comprehensive evaluation of the water quality pollution more scientific. finally, the control of the total pollutant amount was completed via the optimized combination of point source control and unit control of the total amount. experimental results show that the method is scientific, objective, and reasonable while controlling the pollutant discharge. it controls the total pollutant amount excellently. abstract research on control method of pollutant total amount of water quality based on fuzzy mathematics investigación sobre el método de control de la cantidad total de contaminantes de la calidad del agua basada en las matemáticas difusas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.87248 con el aumento de los contaminantes descargados en el agua, es difícil controlar la contaminación del medio ambiente mediante la reducción de parte de la descarga de contaminantes. por lo tanto, se propone el método de control de la cantidad total de contaminante de la calidad del agua basado en matemáticas difusas. en primer lugar, se construyó un marco de control y un proceso de la cantidad total de contaminantes. la cantidad total de contaminantes descargados en esta región se controló dentro de una cierta cantidad para lograr el objetivo ambiental predeterminado. luego, se evaluó la contaminación del agua de diferentes regiones mediante un modelo de calidad del agua basado en las matemáticas difusas de la región o en el dominio de protección clave con contaminación severa y una fuente de contaminación concentrada, lo que hace que la evaluación integral de la contaminación de la calidad del agua sea más científica. finalmente, el control de la cantidad total de contaminantes se completó mediante la combinación optimizada del control de fuente puntual y el control de la unidad de la cantidad total. los resultados experimentales muestran que el método es científico, objetivo y razonable durante el control de la descarga de contaminantes. controla la cantidad total de contaminantes de forma excelente. resumen record manuscript received: 13/11/2019 accepted for publication: 07/04/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 191-199 rui liu* institute of mathematics and computer science, yan'an university, yan'an, 716000, china * corresponding author: hgkyugyu123@163.com m at he m at ic s a pp li ed to h yd ro lo gi c pr ob le m s https://doi.org/10.15446/esrj.v24n2.87248 https://doi.org/10.15446/esrj.v24n2.87248 https://doi.org/10.15446/esrj.v24n2.87248 mailto:hgkyugyu123@163.com 192 rui liu introduction recently, the trend of environmental conditions deterioration of our country has not been improved, especially for the water environment pollution (shi et al., 2017). hundreds of organisms have been detected in main rivers, lakes, and reservoirs in china, and some water areas have been severely polluted by the organisms. the increasing trend is shown in the national water source pollution. nearly 90% of urban water is contaminated severely, while water quality of water sources of 50% of principal towns is substandard. the water pollution has deteriorated from east to west, extended from tributaries to trunk stream, spread from city to village, permeated from the earth’s surface to underground, and diffused from area to watershed. according to statistics, there is universal organic pollution in main rivers in china, and the non-point source pollution is increasingly outstanding (wang et al., 2018). the liaohe river and haihe river are polluted severely. the water quality of the huaihe river is reduced, while the one of the yellow river is not optimistic. the water quality of the songhua river is not bad, while the one of the zhujiang river and the changjiang river is right in total. organic pollution of surface water of seven river basins is universal, and water quality of 58% of fracture surfaces in the trunk stream of each drainage basin belongs to iv grade. in contrast, one of 22% of the fracture surfaces belongs to iv grade and 20% to v grade or inferior v grade. lake pollution in china is also severe. the water body of most lakes is eutrophication. mainly, the contamination levels in taihu lake, chaohu lake, and dian lake (the sanhu for short) are the most serious. for years, the pollutant discharged into the lake from the non-point source pollution on land and factory sewage increases with the rapid development of the economy and growth of population, which results in increasingly outstanding water environment problems of the sanhu. in addition, many lakes that are close to the populated regions such as cities and towns have been degenerated into the wastewater reservoir in the drainage basin, especially for the dirty pond and stream of the village, which is alarming. to control the worsening trend of environmental degradation, the decision of the state council on several issues of environmental protection requires the prevention and control of water pollution in river basins. the total quantity control of critical pollutants and the verification system should be implemented according to the law for the water bodies whose discharge of water pollutants has reached the standard but cannot meet the water environment quality standards stipulated by the state. the total control of water pollutants is mainly based on environmental statistics (ge et al., 2017), supplemented by measures such as pollution declaration and registration to verify the current situation of regional water pollution and discharge. based on the pollution type, according to requirements of specific water environment quality target and water quality target of boundary section, local environmental capacity, economic development scale, pollution control technology, environmental management level, and other factors are considered synthetically to reckon the maximum discharge of pollutant allowed by the target of water environment quality. according to the different geographical locations, technical level, pollutant discharge quantity, and pollution control capacity of each source, the responsibility of pollutant reduction is optimized and coordinated. also, the permitted emission pollutant index and reduction quantity are limited in the form of pollutant discharge allowance to achieve the target of water environment quality finally. there have been some reports on the research of total emission control. a heuristic method has been used to research pollutant control. the method mainly uses dynamic programming algorithms, which are particularly attractive in this respect. however, the code of dynamic programming is complex, which needs many computer resources. in addition, using algorithms based on mathematical programming still exists resistance in practical application. therefore, there is a certain difference between theory and practice (wang et al., 2017) in the system analysis of water quality management. according to the probability restriction condition, the method researches the problem via the optimization model and describes an optimization model that can reflect the uncertainty of water quality. the adopted technology is the chance constraint plan, where the certainty equivalent replaces the probability restriction in optimization problems of water quality. first-order uncertainty analysis is used to represent the intrinsic uncertainty of random factors (mohamad & boumahdaf, 2017). the total amount of the polluted water was controlled via a linear programming model. in the condition of lateral mixing and variation in river flow generated by tributary confluence with different pollutant levels, the optimization assignment model of the garbage load of the drainage basin is proposed. on the premise of meeting the water quality environmental standards of the water intake along the river, the model regards the flow of the river system as a random variable. it is applicable to the river where three tributaries converge at the same point. the flow frequency of the flow group obtained by the statistical method represents the flow frequency of the basin approximately. the priority of sewage treatment transfers from one tributary to another tributary (huang et al., 2017). however, in general, these control methods are still immature and imperfect. further research and discussion are still needed. this research applied the fuzzy mathematical method (li et al., 2017) to carry out a preliminary attempt for the comprehensive evaluation of water quality (brix et al., 2017). the measured value reflecting various problems of water quality pollution was translated into the quality value, which indicates superiority and inferiority of water quality via function relationship. our method considered the fuzziness of the boundary line of the water quality and overcame the insufficient clearness in other evaluation methods of water quality pollution. therefore, the comprehensive evaluation method of fuzzy mathematics is more scientific while evaluating the water quality pollution in various regions. this article used the weight matrix and membership relation to build the model of water quality (gao et al., 2019). the total quality of the contribution of sewage exits was used to calculate the allowable discharge capacity of each sewage exit (molinos-senante et al., 2017), which would improve the total amount of pollution discharge. overall control frame and primary function the control frame of the total quantity is comprised of numerous indexes. the control frame is divided into four sub-domains to facilitate the analysis, as shown in figure 1. each subsystem was analyzed via the regional analysis method as follows. (1) the subsystem of the pollution source the subsystem included point source system (eckley et al., 2017), surface source system (fan et al., 2018), and pollutant system. the pollutant system included indexes such as codcr (chen et al., 2017), bod5 (bodiga et al., 2017), ammonia nitrogen, and volatile phenols. as the pollutant of total quantity control, it could be selected from the standard-exceeding pollutant of water quality according to the requirement of plan and management. (2) the subsystem of water function zone the subsystem included systems of the zone of sewage discharge control, upstream functional zone, transitional zone, and downstream functional area. the zone of sewage discharge control was the river reach where the pollutant from sewage draining exit and surface source were converged intensively. the zone of sewage discharge control was the vital zone for total quantity control of the received pollutant water and the sewage-draining exits, and the complete quantity control was conducted in this zone. the scope of this zone was 100m~500m of upstream from the first sewage exist, namely taking the termination section of its upstream function zone as the starting section and the section where the pollutants discharged from the sewage outlet were mixed with the river water as the termination section. in the control zone, there were not only physical processes of dilution and diffusion of pollutants, but also the processes of chemistry, optics, and biology self-purification (wang, 2019). the transitional zone was set up to ensure the pollutants discharged from the sewage control zone could satisfy the water quality target of the next functional region after being diluted and self-purified by the river. the starting section of this area is the termination section of the sewage control area, while the termination section is the starting section of the next function area. if the distance between the termination section of the sewage control area and water intake of the following function area is within 2000 meters, the transitional zone did not have to be set, and the termination section of the sewage control area could be placed at the starting section of the downstream functional area. the migration and transformation process of pollutants in this section was mainly the self-purification. the starting section of the upstream functional area was the section that is 1000m from the first water intake in the upstream region, and the termination section was the one that is 100m~500m from the first sewage 193research on control method of pollutant total amount of water quality based on fuzzy mathematics and two-dimensional models while the upstream function area via the input and output model directly. the analog computation was carried out for the concentration of the pollutant of the termination section in the sewage control area via the transformation model of the one-dimensional model using the control target of water quality of the termination section in the sewage control area as the basis of calculation. then, the concentration of the pollutants was multiplied by the designed water quantity of this river. thus, the total amount of contaminants allowed to flow into the sewage control area was obtained. the contribution of pollutants from the upstream functional area in the sewage control area to the termination section was subtracted, the total amount of contaminants allowed to flow into the part of the sewage control area and the allowable discharge capacity of sewage draining exit was obtained. then, the allowable discharge capacity of each sewage-draining exit was calculated via the transformation model of the two-dimensional model and hydrological parameters under the condition of design water quantity. method and procedure of total quantity control the total quantity control work starts from the survey of regional water environment and ends up with the water resources protection plan. as shown in figure 2, the entire process consists of seven workflows. survey of regional water environment the survey mainly investigated the distribution of sewage exits, the distance between sewage exits, the discharge of wastewater, and the types and concentrations of pollutants. intake quantity of water of each intake, intake time and intake location were also investigated. to determine the major regional water pollutants, the regional water quality was investigated and evaluated. the function of upstream and downstream reach of sewage exit was surveyed. so were the distance between the last sewage exitand the first water intake of downstream reach, the water quality control target of the functional area, and the hydrological characteristics of the regional reach. according to the requirement of compilation of total quantity control, the investigation data were classified systematically. the pollutants of total quantity control were determined, so were the water quality mathematical model adopted by total quantity control, exit in the upstream sector. the river reach is the pollutant input area of the sewage control area in the upstream region. the scope of the downstream functional area was the river reach that is below 1000m from the first intake in the upstream of the downstream of the transitional zone. the target of water quality control of the initial section of this area was an essential basis for calculating the total afflux amount of pollutants allowed to enter the sewage control area. (3) the subsystem of the water quality model the subsystem included water quality model, parameters of the model, and hydrological parameters. the water quality model was built via the fuzzy mathematical method. the model mainly included the input and output mathematical model, one-dimensional mathematical model, and a twodimensional numerical model that could reflect the input response relationship between pollution sources and water quality targets (abubakari, 2017). in conditions of inflow of tributaries, the two-dimensional analytical model and the input and output model were used for the upstream function area of the sewage control area while the two-dimensional model for the transition area (yao et al., 2017). if the pollution control area was conditional, the numerical solution of a two-dimensional partial differential equation could be carried out for the analog computation of water quality. parameters of the water quality model included a longitudinal diffusion coefficient, transverse diffusion coefficient, and selfpurification coefficient. the longitudinal and transverse diffusion coefficients were calculated via empirical formulas according to hydrological parameters, and the self-purification coefficient was calculated via the measured data of hydrology and water quality or simultaneous monitoring data of water quality and water quantity. river hydrological parameters included average flow velocity, average river width, average depth, and design flow. (4) the subsystem of total control the subsystem was composed of the total amount of pollutants allowed to flow into the sewage control area, the allowable discharge capacity of sewage draining exit, and the simulation model of water quality. the simulation model of water quality was composed of the mathematical model established after parameter estimation. analog computation was carried out for the transition area and sewage control area via the transformation model of one-dimensional figure 1. structure chart of the overall control frame 194 rui liu figure 2. flow of total quantity control water quality simulation scheme and total quantity analysis method and the total quantity control scheme was put forward. according to the function of the total quantity control area, the river reach in the area was divided into sewage control area and transition area, and the scope was determined. meanwhile, the scopes of upstream reach of the sewage control area and downstream reach of the transition area were determined. so were the function of water and the target of the water quality control. fuzzy mathematics method to control the water pollution the work plays a key role in establishing the input response relationship between pollution sources and the target of water quality control. the fuzzy mathematics method uses the weight matrix and the maximum membership principle to consider the factors or main factors related to the evaluated things and carries out the total quantity control for the factors. comprehensive judgment of the fuzzy mathematics method the membership function is the precondition for building the fuzzy mathematical model. to select a model truly reflecting the self-purification law of pollutants in the water body is very important using the “lower semi-trapezoidal fuzzy number”, “triangle fuzzy number” and “upper semi-trapezoidal fuzzy number” as the gist for building membership function. generally, the model is selected as the following principles. the model is selected according to the needs of dilution and diffusion, self-purification process and total quantity control of pollutants in water. the dimensions of the selected model are determined according to the existing hydrological data, water quality data, and the difficulty of evaluating parameters in the model. the model should be strived to be simple, applicable, with clear physical concept, and simplified according to the actual situation; the application conditions, application scope, advantages and disadvantages of the model should be understood in order to make the model have a good simulation property. according to the protection target and the purpose of water area use, integrated with the “groundwater quality standard” (read et al., 2017) (gb 3838-2002), the membership function of water quality evaluation is divided into 5 categories by function. i class is mainly applicable to the source of water and state reserves, while ii class applicable to the first-grade protection zone of centralized drinking water source, habitat of rare aquatic organism, fish protection zone, spawning ground of fish and shrimp, feeding ground of larvae, etc. iii class is suitable for the second-grade protection zone of centralized drinking water source, wintering ground of fish and shrimp, migration channels, fishery waters and swimming areas of aquaculture. iv class is applicable to general industrial water zone and general industrial water zone and recreational water zone that is non-direct contact with humans, while v class applicable to agricultural water zone and water zone with general landscape requirements. figure 3. membership function of water quality evaluation figure 4. the modified membership function of water quality evaluation corresponding to functions of the above five categories of water areas of surface water, fundamental items of environmental quality standards of surface water were divided into five categories, the corresponded standard values of the categories were carried out for different functional categories respectively. the standard value with higher functional category of water area was stricter than that with lower functional category. if the same water area had multiple use functions, the standard value corresponding to the highest function category was performed. it was the same meaning to realize the function of water area and reach the standard of function category. as shown in fig. 3, membership functions of index factors the polluted water was built according to the classification standards of water quality. it could be seen from the classification standard of single quality of the polluted water (table 1) that standard value of iv class and v class water quality was a critical value (namely the standard values of si4 and si5 were equal, that were critical values of iv class and v class water quality). in fact, they were only distinguishing value and 195research on control method of pollutant total amount of water quality based on fuzzy mathematics the transition from iv class to v class was absent, resulting in the repetition and discontinuity of the membership function. parts of evaluation results of water quality were v class, however belonged to iv class actually. it was obvious that the water quality evaluation could not build the membership function (si4-si5=0), leading to distortion of the evaluation result. in view of the membership function with iv-grade connection degree was symmetric about xi = si4, it was modified via triangular fuzzy number, restraining si4 si5 = s4 si3, which could build the membership functions of water quality of iv class and v class. table 1. evaluation standard of single water quality/ mg•l-1 category/ standard value i class ii class iii class iv class v class total hardness ≤150 ≤300 ≤450 ≤450 ≤550 total dissolved solids ≤300 ≤500 ≤10000 ≤1000 ≤2000 sulfate ≤50 ≤150 ≤250 ≤250 ≤350 chloride ≤50 ≤150 ≤250 ≤250 ≤350 iron ≤0.1 ≤0.2 ≤0.3 ≤0.3 ≤1.5 manganese ≤0.05 ≤0.05 ≤0.1 ≤0.1 ≤1.0 copper ≤0.01 ≤0.05 ≤1.0 ≤1.0 ≤1.5 zinc ≤0.05 ≤0.5 ≤1.0 ≤1.0 ≤5.0 nitrate ≤2.0 ≤5.0 ≤20 ≤20 ≤30 nitrite ≤0.001 ≤0.01 ≤0.02 ≤0.02 ≤0.1 ammonia nitrogen ≤0.02 ≤0.02 ≤0.2 ≤0.2 ≤0.5 fluoride ≤1.0 ≤1.0 ≤1.0 ≤1.0 ≤2.0 mercury ≤0.00005 ≤0.0005 ≤0.001 ≤0.001 ≤0.001 chromium ≤0.005 ≤0.01 ≤0.05 ≤0.05 ≤0.1 bicarbonate radical ≤100 ≤200 ≤300 ≤300 ≤400 the relationship of subordination of i v type of water quality was continuous via the modified membership function (fig.4) and conformed to the evaluation standard of the “groundwater quality standard” (gb/t 14848—93). the expression of the modified membership function was shown in formulas 2-6. a set of pollution factors for a given section was known µ = µ µ µ1 2, , ,  n{ }, where elements µi i 2, , n=( )1,  were factors influencing the evaluation object. usually the importance of each factor was different, hence a corresponding weight ai i 2, , n=( )1,  should be given for each factor i. a i nn i i i n =( ) = ≥ =    = ∑a a a a a 1 2 1 1 0 1 2 , , , , ,,   (1) element i is an important degree of membership of factor i therefore, weight set a is a fuzzy subset of factors . general evaluation sets v v v vm= …{ }1 2, , , was established. the elements vj j m= …( )1 2, , , were various possible evaluation results, which could be fuzzy or non-fuzzy, but the relationship to “v” was clear. the evaluation started from a i to confirm the degree of membership rij j m= …( )1 2, , , of the evaluation object to the element vj of the evaluation set, which was called single-factor fuzzy evaluation. evaluation result r of the factor i was the single-factor fuzzy evaluation set. when i belongs to the i class, the expression of membership function was shown in formula 2. ri i i i i i i i i i i i 1 2 2 2 2 1 0 = ≤ − − < < >      ? ? x s s x s s s x s x s l l l  (2) when i belongs to the ii class, the expression of membership function was shown in formula 3. 0 3 2 1 2 3 3 ? x s x s x s s s s x s s x s s l l l i i i i i i i i i i i i i i i ≤ > − − < < − − ∪ 22 2 3s x si i i< >      (3) when i belongs to the iii class, the expression of membership function was shown inf formula 4. ri i i i i i i i i i i i i i i 3 2 4 2 3 2 2 3 4 0 = ≤ > − − < < − ? x s x s x s s s s x s s x s ∪ 44 3 3 4− < >      s s x s i i i i (4) when i belongs to the iv class, the expression of membership function was shown in formula 5: ri i i i i i i i i i i i i i i 4 3 5 3 4 3 3 4 5 0 = ≤ > − − < < − ? x s x s x s s s s x s s x s ∪ 55 4 4 5− < >      s s x s i i i i (5) when μi belongs to the v class, the expression of membership function was shown in formula 6: ri i i i i i i i i i i i 5 4 5 5 4 4 5 5 0 1 = ≤ − − < < >    ? ? x s s x s s s x s x s    (6) si si si si si1 2 3 4 5, , , , in formulas 2-6 corresponded respectively with the standard limiting value of i,ii, iii, iv, v grades in water quality standard, where s s s si i i i5 5 4 5= + − . the xi was the measured value of sampling point in the research area and r r r r ri i i i i1 2 3 4 5, , , , were degree of membership of i,ii, iii, iv, v class water. degree of membership r r r r r ri i i i i i=( )1 2 3 4 5, , , , of factor i to evaluate v was confirmed via the fuzzy mathematics. expression of fuzzy relationship from u to v was defined as r r r ri i i i={ }1 1 1, , ,  γ , j n= …( )1 2, , , , which is further expressed via matrix representations as formula 7 shows. 196 rui liu r r r r r r r r r r j j i i ij =         11 12 1 21 22 2 1 2 � � � � � � �           (7) it is obvious that the fuzzy evaluation of single factor only reflects the influence of one factor on the evaluation object, but does not reflect the comprehensive influence of all factors. so the comprehensive evaluation result cannot be obtained. therefore, the influence of all factors must be considered comprehensively, which is called fuzzy comprehensive evaluation. the set of comprehensive assessment r is the fuzzy subset of the v, which can be expressed as follow: r a v r r r r r r n m m� �i� � � � � � � = =( )a a a1 2 11 12 1 21 22 2, , , � � � r r r b b b i i im m 1 2 1 2                   = ( ), , , (8) where b v r j mi i n ij= ( ) ==1 1 1 2a λ , , , , , m is the evaluation index. it considers the influence of all factors comprehensively. obviously, the line i of b represents the influence degree of ifactor staking each evaluation element on the evaluation object. the column j represents the degree of all factors taking i evaluation element on the evaluation object. hence, each column element is multiplied by the corresponding factor weight i n= …( )1 2, , , , which more more reasonably reflect the comprehensive influence of all factors. establishment of water quality model and calculation of evaluation results given the condition of u v r, ,{ } evaluation space, operational rule was built via weight matrix w and membership relation matrix r. thus, the membership matrix y of fuzzy comprehensive relation between evaluation object of water quality and each evaluation order was obtained as shown in formula 9. y w r c c cj= =( )� �1 2, , , (9) where y was the comprehensive evaluation results.  represented the operational rule. c c c j l nj1 2 2 3, , , , , , ,  =( )was the membership degree. common fuzzy operational rules are the type of main factors determine the i type of main factors highlighted, the ii type of main factors highlight, type of weighted average model, which were shown from formula 10 to formula 13. (1) main factors determines y v w r w ri n i ij i n i ij= ( )= ( )   = =1 1λ max min , (10) formula 10 adopted the principle of “small to large” operation, namely the weight matrix w took the minimum value from the evaluation element (index) corresponding to each column in the membership relation matrix r, then took the maximum. the advantage of the algorithm is to remove the repeated factors in membership degree and weight index. therefore, it avoided the repeated consideration of the membership degree and weight of indexes to take the minimum value first, which highlighted the role of extreme value. (2) i type of main factors highlight y v w r w ri n i ij i n i ij= ( )= ( )   = =1 1max min , (11) formula 11 adopted the principle of “multiply to take the maximum”, namely the weight matrix w took the minimum value after multiplying the element corresponding to each column in the membership relation matrix r. the algorithm not only considered the interaction effect of index membership degree and weight on evaluation results, but also highlighted the role of extreme value. (3) ii type of main factors highlight y w r w r w ri n i ij i ij i n i ij i n =⊕ ( )= ( )         = ( )= = = ∑ ∑1 1 1 1λ λ λmin , (12) formula 12 adopted the principle of “adding small values “, namely the weight matrix w took the minimum value from the element corresponding to each column in the membership relation matrix r, then added together. this algorithm removed the important factors in the membership degree and weight of the index. however, the overlapping budgets might cover the influence of the maximum pollution index on groundwater. when the concentration of some indicators was particularly large, the comprehensive evaluation results were unreasonable. (4) type of weighted average model y w ri ij i n = ( ) = ∑ λ 1 (13) formula 13 adopted the principle of “multiplication and addition”, namely the weight matrix w multiplied the element corresponding to each column in the membership relation matrix r, then carried out a linear weighted average. the algorithm reflected the comprehensive influence of all factors, which was a comprehensive evaluation with a membership weight type in fact. the establishment of weight matrix plays an important role in building water quality model, which directly affects the application of model and the calculation result of total quantity control. to construct the weight matrix, the tracking and monitoring data of water mass should be obtained and proceeded according to certain methods, which requires a large amount of time and funds. it is difficult in establishing the process of the protection planning of water resources. in condition of the limited funds and time, establishment of the weight matrix and regression method can be used to estimate the parameters. due to different importance degrees of µi i l n=( ), , , ,2 3 in the comprehensive evaluation of water quality and environmental quality in subset u, such as function, contribution, and influence, the index factors participating in the evaluation should be weighted. different weights influence the evaluation results in varying degrees. there are two major methods for weight definition, subjective weighting method and objective weighting method. the latter is usually used in practical evaluation. our research used the method of rate of pollution contribution to confirm the weight. it is a dynamic/variable weight method, which can not only highlight the impact of major pollution indexes, but consider the comprehensive impact of individual pollution indexes on the environment. the weight calculation was shown in formulas 14-16, concretely. w x s i l n j li i j s ij= = =( )=∑/ , , , ; , , ,1 5 2 2 51   (14) w w w i l ni i j n i  = =( )=∑/ , , ,1 2  (15) w w w w wl n i n i={ } ==∑, ,2 1 1 (16) in formulas 14-16, xi is the measured values of water quality samples in the research area. sij is the standard limiting value of i,ii, iii, iv, v grade in water quality standard and wi is the weight of underground water indexes in different places obtained by normalization. confirming the target of water quality control the main purpose is to determine the target of water quality control of the functional areas in the upstream sewage control area and downstream transition area. the determination of the target of water quality control is mainly based on the river function and management requirement of total quantity control. therefore, it has certain timeliness. calculation of discharge total amount allowed in the control area according to the water quality control target and designed water quantity of the initial section of the downstream functional area of the transition zone, the 197research on control method of pollutant total amount of water quality based on fuzzy mathematics pollutant concentration value of the final section of the sewage control area was calculated via the water quality model. the concentration value was multiplied by the designed water quantity and the discharge total amount was obtained. the required designed water quantity, river width, average water depth, river velocity and other parameters could be calculated according to the established relevant model and hydrological calculation methods. according to the total amount of allowable pollutant discharge, the established water quality model was used to calculate the allowable pollutant discharge for each sewage exit. according to the development of control section, pollution control situation, sewage discharge and water pollution treatment situation, through the actual data of the specific control unit, the maximum allowable discharge of control unit, cod discharge and concentration of each point were calculated. total discharge amount (q)=total quantity (qtotal)  fuzzy evaluation matrix (r) (17) examination of method the total quantity control scheme was examined. if there is any shortage, adjustment should be carried out for the design of control scheme or target of water quality control and calculation should be proceeded by a working routine until the results are satisfied. the achievement was brought into the program of water resource protection. simulation experiment and results the main technology of total quantity control is to establish the water quality model via fuzzy mathematical method, and to calculate the total quantity of pollutants allowed to flow into the sewage control area and the allowable total quantity of sewage outlets. this paper takes the pilot project of total quantity control in the water function zone in a certain area as an example to illustrate the main technical methods of total quantity control. general situation of region water environment the river section in this area is 98 kilometers long, and there are hydrological and water quality stations on the main stream. the monitoring data of the hydrological and water quality station are shown in table 2. there are many tributaries in the middle of the river and 5 sewage exits. the length of sewage exits is 3.2 km. the calculation and comprehensive evaluation results of the membership function based on the fuzzy mathematics were shown as follows. the membership functions of each grade water are shown from formula 18 to formula 27. there are many tributaries in the middle of the river and 5 sewage exits. the length of sewage exits is 3.2 km. the calculation and comprehensive evaluation results of the membership function based on the fuzzy mathematics were shown as follows. the membership functions of each grade water are shown from formula 18 to formula 27. r x x x x i i i i 1 0 0 5 0 5 1 0 5 5 0 7 0 7 0 = −( ) ≤ < < >      . . . . . . (18) r x x x x x x i i i i i 2 0 0 5 0 5 0 5 3 0 3 0 5 0 7 0 3 0 5 = − −( ) −( ) ≤ < < < < . . . . . . . . ∪ 7.0 ..0      (19) r x x x x x x i i i i i 3 0 0 5 0 5 2 0 2 0 3 0 5 0 2 0 3 0 = − −( ) −( ) ≤ < < < < . . . . . . . . ∪ > 5.0     (20) r x x x x x x i i i i i 4 0 3 0 1 0 1 0 2 0 3 0 1 0 2 0 = − −( ) −( ) ≤ < < < <    . . . . . . . ∪ > 3.0   (21) r x x x x i i i i 5 0 2 0 1 1 0 1 0 2 0 2 0 = − −( ) ≤ < < >      . . . . . (22) membership function of bod to each grade water r x x x x xi i i i1 0 3 0 1 0 0 5 1 0 1 5 2 0= − −( ) −( ) ≤ ≥ < <      . . . . . . (23) r x x x xi i i2 0 0 2 1 5 3 0 1 5 1 0 1 5 2 0= − −( ) − −( ) ≤ < <      . . . . . . . ∪ (24) r x x x x x x i i i i i 3 0 2 0 0 5 5 0 2 0 5 0 2 0 3 0 3 0 5 0 = −( ) − −( ) ≤ > < < < < . . . . . . . . . ∪     (25) r x x x x x x i i i i i 4 0 3 0 0 33 0 8 3 0 8 0 3 0 5 0 5 0 8 = − −( ) − −( ) ≤ > < < < < . . . . . . . . ∪ ..0      (26) r x x x x i i i i 5 0 0 33 0 5 1 5 0 5 0 8 0 8 0 = − −( ) ≤ < < ≥      . . . . . . (27) table 2. synchronous detection data of hydrology and water quality station name time water temperature flow velocity flow pollutant (°c) (m/s) (m³/s) codct ammonia nitrogen a 10:40 0.2 0.15 15.7 77.1 3.90 12:40 0.14 17.2 85.1 3.76 14:40 0.14 16.9 81.1 3.97 16:40 0.15 16.2 89.1 3.99 b 0:50 0.2 0.42 19.9 73.2 3.74 3:40 0.44 20.8 81.1 3.74 5:30 0.42 20.6 77.1 3.93 7:10 0.43 20.5 78.7 3.77 8:10 0.42 20.4 69.2 3.96 9:10 0.42 20.1 65.2 3.94 c 8:20 0.2 0.74 17.3 85.1 3.80 10:40 0.77 16.6 65.2 3.77 12:30 0.84 18.0 66.0 3.85 14:10 0.79 17.9 69.2 3.87 15:10 0.77 17.8 69.2 3.80 16:10 0.84 18.0 64.4 3.91 d 14:00 0.2 0.29 16.4 61.2 3.77 16:00 0.28 16.9 57.3 3.77 18:30 0.28 16.9 65.2 3.77 20:00 0.27 16.8 63.6 3.87 21:00 0.28 16.9 65.2 3.87 22:00 0.26 16.7 75.7 3.71 198 rui liu the membership functions of other comment factors to each grade water are analogized in order. according to the above membership calculation formula, membership functions of each evaluation factor for 5 grades are established to obtain membership degrees respectively. the single-factor evaluation matrix r is established: r= 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 38 0 62 0 0 1 0 0 0 0 0 48 0 52 0 0 0 0 0 90 0 10 0 0 0 1 0 . . . . . . 00 0 1 0 0 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 0 0 94 0 06 0 0 0 0 0 14 0 86 0 0 1 0 0 0 0 . . . .                                                                 (28) according to the above weight calculation method, the normalized weight matrix w is established after normalization. the results are shown in table 3. table 3. weight calculation results do bod cod cn phenol concentration 2.69 2.50 7.73 0.004 0.0076 w x si i ij j i s = = ∑/ 1 5 3.6 3.9 5.6 0.0076 0.0134 standard concentration 0.75 0.64 1.38 0.53 0.57 w wi i j i n / = ∑ 0.19 0.17 0.36 0.14 0.15 w = 0 1024 0 0442 0 0117 0 1286 0 0007 0 0701 0 1197 0 0178 0 0227 0 . . . . . . . . . .11572 0 0063 0 0163 0 0842 0 1589 0 0592. . . . .        (29) as shown in table 3, y = 0.3855 and comprehensive evaluation grade is ri3. it means that the level of water quality in the region is iii grade. the main pollutants are codcr, bod5, ammonia nitrogen, and volatile phenol, which are all the non persistent pollutants. the discharge of the above pollutants from the five sewage exits in this area is shown in table 4. table 4. discharge situation of waste water and pollutant of sewage exits number sewage exit discharge amount of wastewater (million t/a) codct bod5 ammonia nitrogen volatile penol 1 a 179 167 78.9 28.3 0.03 2 b 5.26 5.55 1.46 0.428 0.0005 3 c 1640 3290 1050 358 0.41 4 d 1480 397 59.3 7.16 0.015 5 e 3550 4270 1470 547 0.60 summation 6854.26 8129.55 2659.66 940.88 1.0555 experimental procedure and results step 1: total amount of pollutants allowed to follow into the sewage control area the total amount of pollutants allowed to import into the sewage control area refers to the total amount of pollutants allowed to import into the termination section of the sewage control area. based on the concentration index of regional water quality control, the pollutant concentration was calculated via the established model. then, the concentration was multiplied by the design of water to obtain the total amount of pollutants. the target of water quality control in the region is iii class water. thus, the concentration of codcr is 15 mg/l. the concentration of ammonia nitrogen is 0.5mr/l. according to the calculation, the concentration of codcr of the section is 15.96 mg/l, while the ammonia nitrogen is 0.503 mg/l. the calculation concentration is multiplied by the designed water quality (7.47m3/s). the allowable total amount of codcr is 3759.8 t/a, while the ammonia nitrogen is 118.5t/a. step 2: quantity allocation of allowable pollutant discharge of sewage exit the total amount of pollutants allowed to flow into the section consists of two parts. one part is the amount of pollutants discharged from estuary to the section, and the other part is the amount of pollutants discharged from the five discharge exits in the sewage control area. in the calculation of the quantity allocation of allowable pollutant discharge, the amount of pollutant imported from the first part should be subtracted, and the rest is the total contribution of the discharge exits. then the total contribution amount of each exit is used to calculate the allowable discharge amount. the specific steps are as follows: (1) contribution of the imported pollutants to the pollution at the section for the pollutants afflux of non-point source discharge, there are only four rivers containing the pollutants, which are named as a, b, c, d respectively. the pollutants and river water have been mixed evenly at this section. the targets of water quality control of the section of main stream estuary of a river and d river are ii class while the one of b river and c river are iii class. the standards of water quality control and designed duty of water are shown in table 5. table 5. standards of water quality control and designed duty of water section and tributary designed duty of water water quality control standard (mg/l) name (m³/s) codct ammonia nitrogen a river a sewage exit 91.5 0.76 b river b sewage exit 5.55 0.428 c river c sewage exit 1760.35 9.005 d river d sewage exit 211.6 0.056 the pollutants in this section are mainly self-purification process and the mixing process after the import of tributaries. when the tributaries flow into the main stream, the concentration of the pollutants after the afflux is calculated via the model. when there is no tributary inflow, the model is used to calculate the self-purification process of pollutants. the data in table 3 are substituted into the model. the pollutants concentration value of the termination section of the sewage control area is calculated via the water quality model. the concentration value is multiplied by the designed water amount of the section, which is the total amount of pollutants allowed to import into the sewage control area. after computation, the contribution concentration of codcr is 4.94mg/l and the contribution amount is 1163.7t/a. the concentration of ammonia nitrogen is 0.4mg/l, and the contribution amount is 94.23t/a. the total contribution of codcr from the sewage exits is 2596.1t/a and the concentration was 11.02mg/l. the total contribution of ammonia nitrogen is 24.27t/a and the concentration is 0.103mg/l. (2) quantity of allowable pollutant discharge of sewage exit according to the total contribution of pollutants from the discharge exits at the section and the proportion of pollutant discharge from each discharge 199research on control method of pollutant total amount of water quality based on fuzzy mathematics exit in the total discharge, simulation calculation is carried out for the water quality via the transformation formula of the established water quality model, and the allowable pollutant discharge amount of each discharge exit is obtained respectively. the calculation results are shown in table 6. it can be seen that the control results are improved apparently. table 6. calculation results of quantity of allowable pollutant discharge of sewage exit number sewage exit quantity of allowable pollutant discharge (t/a) codct ammonia nitrogen 1 a 91.5 0.76 2 b 5.55 0.428 3 c 1760.35 9.005 4 d 211.6 0.056 5 e 2263.3 14.921 summation 4332.3 25.17 conclusion (1) the framework and functions of total quantity control are analyzed, and the workflow and technical method of total quantity control are put forward, which is of practical significance for the current work of water resources protection planning. (2) the comprehensive evaluation method is normative and applicable, and the existing hydrology and water quality data can be used to evaluate the water quality, which has good practicability. (3) the membership function and weight value are used to establish the water quality model to calculate the allowable discharge amount of pollutants from the sewage exit. and the existing hydrology data are used for calculation. the method is used in the calculation of the allowable discharge amount from the sewage exit for the first time, which is simple, applicable and highly simulated. total amount control of pollutant is to make a control plan and allocate total amount of pollutant after in-depth investigation and research and application of scientific method. to some extent, it is the dynamic planning of regional pollution sources, scientific, quantitative and standardized management of pollution sources, rational use of environmental capacity, and reducing pollution pressure to ensure the realization of regional environmental quality goals. fuzzy comprehensive evaluation is comprehensive, concrete, objective and scientific in comprehensive evaluation and planning control of main pollutants judging and pollution trend tracing. it can be seen from the calculation results that the method can integrate the specific situation and characteristic, and has excellent accuracy and adaptability. it can be applied to the control of water pollution extensively. references abubakari, a., raymond, s., jo, h. s. 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(2018). analysis of the environmental behavior of farmers for non-point source pollution control and management in a water source protection area in china. general environmental science, 227, 1126-1135. yao, y., verginelli, i., & suuberg, e. m. (2017). two-dimensional analytical model of vapor intrusion involving vertical heterogeneity. water resources research, 53, 4499-4513. yu, s., & lu, h. (2018). multi-level and step-by-step optimization of integrated watershed management for large-scale total water pollutant allocation. environmental geosciences, 77, 373. zhou, l., li, h., & sun, k. (2017). teaching performance evaluation by means of a hierarchical multifactorial evaluation model based on type-2 fuzzy sets. applied intelligence, 46, 34-44. keywords: geochemical modeling; central alborz; petrology; paleocene; palabras clave: modelo geoquímico; alborz central; petrología; paleoceno; how to cite item nazari sarem, m., vosoghi abedini, m., dabiri, r., & ansari, r. a. (2021). geochemistry and petrogenesis of basic paleogene volcanic rocks in alamut region, alborz mountain, north of iran. earth sciences research journal, 25(2), 237-245. doi: https://doi.org/10.15446/esrj. v25n2.74025 the activities of alborz tertiary magmatism are mainly attributed to the subduction and closure of neo-tethys. these phenomena, including the magma origin of this lesser area, are investigated in this study. in addition, the origin of the magma characteristics based on the geochemistry of main and rare elements is also studied, and a model is presented for their tectonomagmatic status. volcanic tertiary alamut is part of magmatism in the central alborz-west sequence of volcanic rocks (including basalt, trachyandesite, and basaltic andesite with alkaline geochemical properties, with porphyritic texture, pyroclastic rocks, and volcanic breccia) directly above the units belongs to the karaj formation (middle eocene age). geochemical properties indicate that the parent magma is characterized by high ratios of lree/hree as well as th/nb, la/nb, ba/nb, zr/nb, and low k/nb ratios. the early basaltic magma is formed from a garnet lherzolite mantle by phlogopite/pargasite metasomatism at 2.5-5.3 gpa and depths of more than 80-150 km. structural evidence suggests the formation of these volcanic rocks in a continental rift zone setting. the appearance of these rocks can be attributed to the effects of intercontinental extensional phases in deep faults during eocene alpine orogeny phases. the primary focus of this study is to provide the geological, petrographical, and geochemical data findings of the volcanic basaltic rocks along with the results of studying other rocks in this region. aside from the mode and their formation, magma forming is also investigated. moreover, for the rocks investigated in this study, the environment and the site of the tectonic-magmatic region are also clarified in detail. abstract geochemistry and petrogenesis of basic paleogene volcanic rocks in alamut region, alborz mountain, north of iran geoquímica y petrogénesis de las rocas volcánicas básicas del paleogeno en la región de alamut, montaña de alborz, norte de irán issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.74025 las actividades del magmatismo terciario en alborz (irán) se atribuyen principalmente a la subducción y final del neotetis. este fenómeno es el objeto de este estudio, además de un acercamiento al origen del magma de esta área inferior. adicionalmente, los autores estudiaron las características y origen del magma con base en la geoquímica de los elementos principales y de las tierras raras, y presentaron un modelo de su estado tectonomagmatico. el terciario volcánico de alamut es parte del magmatismo en la secuencia central de rocas volcánicas en alborz occidental (incluye basalto, traquiandesita, y andesita basáltica con propiedades geoquímicas alcalinas, con textura porfirítica, rocas piroclásticas y brecha volcánica), directamente sobre las unidades pertenecientes a la formación karaj (del eoceno medio). las propiedades geoquímicas indican que el magma madre se caracteriza por altos índices de elementos tierras raras ligeros y elementos tierras raras pesados, al igual que proporciones torio/niobio, lantano/ niobio, bario/niobio, circonio/niobio y menormente potasio/niobio. el magma basáltico temprano está formado por un manto de lherzolitas de granate con metasomatismo flogopita/pargasita a 2.5-5.3 gpa y profundidad entre 80 y 150 kilómetros. la evidencia estructural sugiere la formación de estas rocas volcánicas en un ambiente de grieta continental. la aparición de estas rocas puede atribuirse a los efectos de las fases extensionales intercontinentales en fracturas profundas durante las fases orogénicas del eoceno alpino. el enfoque principal de este estudio es proporcionar la información hallada en términos geológicos, petrográficos y geoquímicos de las rocas volcánicas basalticas al igual que los resultados del análisis de otras rocas en el área. más allá de su modo y formación, los componentes del magma también se analizaron. además, para las rocas investigadas en este estudio, el ambiente y el sitio de la región tectonomagmática se aclararon detalladamente. resumen record manuscript received: 04/08/2019 accepted for publication: 12/11/2019 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 237-245 mehdi nazari sarem1, mansour vosoghi abedini1, rahim dabiri*2, mohammad reza ansari3 1. department of geology, tehran science and research branch, islamic azad university, tehran, iran. 2. department of geology, mashhad branch, islamic azad university, mashhad, iran. 3. associate professor, department of geology, chalous branch, islamic azad university, chalous, iran. *corresponding author: rahimdabiri@yahoo.com g eo ch em is tr y https://doi.org/10.15446/esrj.v25n2.62287 https://doi.org/10.15446/esrj.v25n2.62287 https://doi.org/10.15446/esrj.v25n2.62287 mailto:rahimdabiri@yahoo.com 238 mehdi nazari sarem, mansour vosoghi abedini, rahim dabiri, mohammad reza ansari introduction a key feature of alkaline volcanic rock compounds in the continental regions is their incompatible enrichment of elements, mainly volcanic, depending on the final stages of the active subduction zones of continental margins. these properties are transmitted to magmas that have passed through an enrichment continental crust of incompatible elements and can later be observed with contaminated crust (wilson, 1989). concerning the mantle origin, volcanic regions of the post-orogenic areas have been proposed in previous studies, including the continental lithosphere enriched by materials released from the oceanic plate (lima and nardi, 1998), asthenosphere plumes from deep areas, the mantle (lei and zhao, 2007) or the participation of both of them (pearce et al. 1990; turner et al., 1992). tertiary mafic volcanic rocks in northern alborz iran are believed to be the result of subduction of neotethys (stalder, 1971; alavi 1996; hasanzadeh 2004). in eocene, due to the subduction of neo-tethys, a back-arc basin is formed. the crust tensional stress results in the subsidence generating the current southern alborz (burberry and king, 1981). the region’s tensile behavior in eocene causes the flow of high thermal magmatism influencing all neo-tethys subduction zones, such as east iran and southern alborz (other than northern alborz and kopet dagh) (vincent et al., 2005). due to the rollback of the intrusive oceanic plate in eocene and the removal of pressure, the melting liquid lithosphere mantle and the asthenosphere rise. this asthenosphere uplift and the thinness of the crust produce large amounts of volcanic rocks in the eocene of alborz. the combination of these rocks often shows rare mantle elements (verdel, 2009). however, studies and information on the origin of these volcanic activities are pretty limited. by considering the characteristics of the origin of these magmas, the current study presents a tectonomagmatic model for these magmatic activities. figure 1. map of cretaceous and eocene volcanic outcrops in alborz, based on qazvin’s quadrilateral maps of rasht. geological setting the study area is located in the central parts of the alborz mountains structural zone in qazvin province in iran (fig. 1). alamut region is situated between 50ᵒ 02ʹ 54ʹʹ and 50ᵒ 52ʹ 55 ʹʹ as well as from 36ᵒ 17ʹ 11ʹʹ to 36ᵒ 41ʹ 01ʹʹ northern latitudes. the area is divided into two parts of western rudbar alamut and eastern rudbar alamut. there exist the oldest outcrops of paleozoic rocks in the region, including chert dolomitic rocks with zagon mica shale, sandstone, and limestone. the mesozoic rock units include a series of dark gray shale with sandstone layers. the rock units’ outcrops in the eocene period include a series of dark volcanic rocks with basaltic to andesitic composition along with faulty fractured altered andesitic tuff and hyaloclastite breccia. an alternative collection of green tuff, tuffite, crystalline tuff, sandstone tuff, calcareous tuff, marl, shale, and lava parts with andesitic dacite to rhyodacite composition appears in the north of the anke-barajin trust fault (north qazvin fault). essential outcrops of rock units have been formed during the neogene, from dark to brown volcanic rocks with trachyte, andesitic dacite composition, and local feldspathoid volcanic lava. figure 2. simplified geologic map showing regional geology of the central alborz (alamut basins) (after guest et al., 2007). samples and methods the present study was based on field studies, petrography, and chemical analysis of main and rare elements. field studies investigated the area of study in terms of lithology and sampling. in this regard, for the lithogeochemical studies, the macroscopic samples of volcanic rocks were selected and sent to mashhad laboratory to produce thin sections. ten suitable samples were sent to the acme laboratory in canada to analyze the whole rock by the icp method. minpet, excel, petrography, gcdkit, and other specialized software were implemented to scrutinize the research findings. in the last stage of the research, field study, petrography, and geochemistry data were analyzed, and a comprehensive view was made on petrology, tectonics, and geochemistry status of the region. lithological study according to the field and petrography studies, volcanic rocks of the region are constituted from intermediate to basic lava plus pyroclastic rocks of eocene in a shallow marine environment with basaltic andesite, andesitic basalt, olivine basalt, and basalt composition. the dominant textures include porphyritic with microlithic matrix, porphyric with medium grain matrix, and doleritic texture. according to mineralogy, the rocks are predominantly composed of plagioclase, clinopyroxene, and olivine. the textures in plagioclase in basaltic rocks include mainly seive and glomerphyric textures. plagioclase crystals contain polysynthetic, carlsbad, albite-pericline with defected and albitized margin. fine olivine crystals are often subautomorph and altered into biotite, serpentine, chlorite, iron oxide, and iddingsite. the matrix comprises glass, plagioclase microliths, opaque minerals, and secondary minerals (fig. 3). geochemical analysis of volcanic rocks the results on the major elements of volcanic rocks in the alamut region show that these rocks include a series of basic volcanic rocks. sio2 content in these rocks varies from 48.21 to 48.28 wt%. the rocks can be categorized into basaltic, trachybasaltic, and andesitic basaltic rocks based on 239geochemistry and petrogenesis of basic paleogene volcanic rocks in alamut region, alborz mountain, north of iran the tas diagram (figs. 4a and b). according to the sub-alkaline and alkaline series (irvine & baragar, 1971), the rocks fall in the alkaline range (figs. 4 c and d). the variations in the major elements are plotted versus mgo in figure 5. the level of major oxides such as al2o3 and sio2 increases with the reduction of mgo during magmatic differentiation. at the same time, the level of major oxides such as cao and fe2o3 drops with the reduction of mgo. the magnesium number (mg#) of samples varies from 30 to 45. most samples have a high mg# in the range of the parent magma. two samples with an mg# of less than 40 and a far lower level of mid-plate basalts (as those found in hawaii) are comparable with continental flood basalts (cfb) and rift basalts (similar to those found in the rio grande) with low to moderate magnesium numbers lacking characteristics of primitive mantle lava (raymond, 2002). variation of minor elements versus zr and th is one of the proper approaches to characterize magmatic transformations. zr is used in these diagrams mainly due to the very low mobility of this element during alteration (le roex et al., 1983; meng et al., 2012; talusani, 2010; widdowson et al., 2000; widdowson, 1991) and a wide range of variations of this element in basaltic rocks. in addition, zr shows a completely incompatible geochemical behavior during partial melting and differential crystallization of basaltic lavas (talusani, 2010) and tends to enter and remain in the melt phase. as seen in figure 6, ni is slightly scattered positive versus zr, and th is somewhat scattered positive versus hf. the trends observed in these diagrams represent that the rocks in this region have nearly similar geochemical characteristics with their source magma. in addition, the crystallization process plays the main petrological role in the basaltic magma evolution in the region. however, scattering can be attributed to the frequency of mafic and plagioclase phenocrysts. for further analysis of geological processes and the original characteristics of the basaltic rocks, the normalized pattern of the average values of rare and rare earth elements in basalts was plotted versus chondrites, the primitive mantle, and depleted mid-oceanic ridge basalts and was then compared with oib, emorb, and nmorb patterns (fig.7). no particular eu anomaly is observed in a diagram normalized relative to the chondrite values. this can be attributed to the lack of plagioclase and clinopyroxene differentiation (zeng et al., 2010) during the magma evolution of forming the rocks in the region. a negative anomaly in nb indicates the role of magmatic contamination with continental crust in the evolution of rocks. severe positive pb and ba anomaly has been reported for continental crust contamination, with the positive sr anomaly indicating the presence of plagioclase phenocrysts in the rock. more than 10 times enrichment of light rare earth elements (lree) in comparison with heavy rare earth elements (hree) in most samples suggests that garnet does not enter the melt but remains in the mantle origin. as can be seen, the average value of minor elements and the rare earth elements in basalts (avs) show a pattern between emorb and oib with a similar general pattern. the normalized pattern of the samples is located between emorb and oib; hence the source of basalts in this region is confirmed by characteristics of these two origins. although both of them have originated from the asthenosphere mantle, they may have also been affected by the lithospheric mantle to some extent. a 1 mm c 1 mm b 1 mm d 1 mm figure 3. (a) a matrix enriched with sodic plagioclase in basalt, (b) porphyritic texture with microlithic glass matrix, chlorination of olivine in a matrix rich of sodic plagioclase in basalt, (c) porphyritic texture with microlithic glass matrix and the presence of clinopyroxene phenocrysts, (d) porphyritic texture with microlithic glass matrix, chlorinated olivine with clinopyroxene and plagioclase phenocrysts. 240 mehdi nazari sarem, mansour vosoghi abedini, rahim dabiri, mohammad reza ansari the similarity of basalts pattern in the region to ocean island basalts (oib) rather than enriched mid-ocean ridge basalts (emorb) can be explained by the fact that the main origin of these basalts is partially enriched asthenosphere mantle similar to the oib origin. despite the similarity of pattern of both groups, varieties of alamut basalts in the elements sr, nb, and zr show less enrichment than varieties of oib. this geochemical difference can be explained by degrees of partial melting. sometimes, the enrichment of partial melting in the rocks of the region can lead to element depletion (such as light rare earth elements) causing a lower concentration of incompatible elements and providing a lower slope of distribution for these elements as compared to oib. an important point that needs to be taken into consideration is the high value of the zr/hf ratio with an average of around 60.67. according to wever et al. (2003) and tappa (2004), high zr/hf ratios reflect a residual clinopyroxene phase and lack of clinopyroxene under melting conditions. their findings suggest that hf with a coefficient of 1.5-2 times is more compatible than zr in clinopyroxene mineral in the melt/clinopyroxene system. according to the results, the mantle source of the avs parent magma is depleted in hf as compared with zr, hence the high ratio of zr/hf could be attributed to the low degree of partial melting (daviid et al., 2000). therefore, it appears that besides lower partial melting which reflects the great depths of the parent mantle, the metasomatism of the parent mantle and its heterogeneity in the production of such melt can be considered as the primary source. the steep slope of hree/lree in the diagrams is an indicator of the presence of the garnet phase in the mantle source. in such conditions, for alkaline basalts, a deep source of garnet lherzolite with phlogopite/pargasite (mantle hornblende) at 2-2.5 gpa pressure is recommended (thirlwall et al., 1994; tappa 2004; stivenson 2003; glasser et al., 1999). from the normalized graphs of alamut volcanic rocks based on spider diagrams, it could be concluded that due to the low k / nb ratio and the enrichment of regional rocks with light rare earth elements (lree) relative to heavy rare earth elements (hree), the alkaline magma of these rocks has originated from low grade melting from a rich mantle source. plots of sm/yb versus la/yb and la/yb versus dy/yb are usually employed to detect peridotite spinel from garnet peridotite melting. in general, yb is compatible with garnet, while la and sm are incompatible. accordingly, sm/yb and la/yb are significantly concentrated in garnet peridotite origin at low degrees of partial melting. in contrast, during partial melting in the stability range of spinel, the la/yb ratio is only slightly altered, but the sm/yb ratio remains almost unchanged (xu et al., 2005; yaxley, 2000; white & mckenzi, 1995). in the diagram designed by lai et al., (2012) based on the sm/yb ratio versus la / yb, the basaltic samples of the region are located within the range of 12 to 15% of the garnet peridotite range (see fig. 8 a and b). (a) (b) figure 4. (a) na2o + k2o versus sio2 (le bas et al., 1986); and (b) na2o + k2o versus sio2 (middlemost, 1994) figure 5. variation diagrams of the major elements versus mgo figure 6. variation diagrams of incompatible elements 241geochemistry and petrogenesis of basic paleogene volcanic rocks in alamut region, alborz mountain, north of iran figure 7. (a) spider diagram normalized to primitive mantle values (sun and mc donough, 1989), (b) spider diagram normalized to primitive mantle values). (wood et al., 1979), (c) spider diagram normalized to oib (sun and mc donough, 1989). table 1. results of chemical analysis of the oxides of the main and secondary elements by the icp of volcanic rocks in the alamut area sample 6845 -al20 6846 -al10 6847 -al30 6848 -al40 6849 -al16 6850 -al50 6851 -al28 6852 -al51 6853 -al17b sio2 50.82 44.77 44.80 44.61 45.26 44.85 45.29 45.05 51.62 al2o3 19.35 17.58 3.34 17.34 17.56 16.95 17.28 17.12 17.77 fe2o3 3.19 3.33 3.33 3.34 3.34 3.28 3.28 3.26 3.22 feo 6.16 9.31 9.20 9.07 8.54 9.12 9.13 9.03 6.82 mgo 2.88 6.71 6.98 7.29 6.90 7.12 6.66 6.95 2.77 cao 10.17 12.67 12.43 12.49 12.62 13.08 12.65 13.15 10.64 na2o 3.42 2.56 2.89 2.67 2.62 2.44 2.65 2.21 3.12 k2o 1.63 0.69 0.54 0.67 0.74 0.79 0.72 0.873 1.61 tio2 1.61 1.75 1.75 1.77 1.77 1.70 1.70 1.69 1.617 p2o5 0.66 0.45 0.44 0.41 0.45 0.42 0.41 0.41 0.65 mno 0.14 0.22 0.26 0.28 0.22 0.21 0.19 0.21 0.14 cr2o3 0.009 0.009 0.009 0.11 0.009 0.01 0.022 0.03 0.024 sum 99.23 99.37 85.26 99.3 99.4 99.3 99.3 99.3 99.187 k 13500.75 5779.50 4449.60 5518.14 6127.06 6637.12 6000.18 7249.21 13331.13 p 2857.77 1933.72 1880.75 1749.98 1932.86 1836.64 1783.11 1769.59 2803.66 ti 9623.3 10434.5 10459.2 10563.5 10556.3 10217.0 10236.3 10158.7 9691.61 mo 1.02 0.95 0.85 0.74 0.55 1.01 0.85 1.02 1.23 cu 112.2 90.74 158.44 71.86 198.36 133.59 89.67 103.34 172.03 zn 101.8 69.7 71.5 73.3 70 62.8 63.8 70 82.4 ag 34 30 54 21 75 41 39 39 59 ni 7.7 19.9 18.5 7.7 19.9 18.5 7.7 19.9 18.5 co 18.9 29.2 28.6 28.6 28 27.8 28.7 31.6 20.2 mn 850 1028 1359 1486 953 971 951 1009 904 fe 5.59 6.29 6.67 6.01 6.04 5.92 6 6.34 6.01 as 1.1 0.3 0.8 0.7 1.4 1.4 0.8 1.2 0.9 u 0.9 1.1 0.8 0.9 0.9 0.8 u 0.9 1.1 au 3 1.4 0.6 0.8 0.9 au 3 1.4 0.6 th 3.4 3.6 3.3 3.3 3.4 3.1 3.3 3.2 3.7 sr 168.1 559.5 533 536.6 537.7 588.3 714.8 655.9 142.6 cd 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.01 0.03 sb 0.37 0.11 0.1 0.13 0.08 0.12 0.05 0.08 0.19 (continue) 242 mehdi nazari sarem, mansour vosoghi abedini, rahim dabiri, mohammad reza ansari table 1. results of chemical analysis of the oxides of the main and secondary elements by the icp of volcanic rocks in the alamut area sample 6845 -al20 6846 -al10 6847 -al30 6848 -al40 6849 -al16 6850 -al50 6851 -al28 6852 -al51 6853 -al17b bi 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 v 186 204 233 222 211 190 198 205 185 ca 3.44 3.56 3.52 2.96 3.44 3.67 3.5 4.02 4.09 la 30.6 17.8 17.2 16.4 17.6 16.2 16.6 16 31 cr 16.6 30.3 38.3 65.4 37.2 34.2 35.6 44.2 17.1 mg 1.32 1.84 1.96 1.86 1.88 1.95 1.81 2.01 1.37 ba 52.4 131.6 151 109.5 306.8 107.7 491.3 155 37.6 b 20 20 20 20 20 20 20 20 20 al 3.07 6.73 6.8 6.34 6.58 6.31 6.85 6.63 2.59 na 0.419 1.388 1.592 1.473 1.418 1.323 1.565 1.178 0.34 w 0.07 0.51 0.05 0.12 0.05 0.11 0.13 0.17 0.07 sc 14.9 3.9 3.7 3.8 4 3.9 3.8 3.5 16.6 s 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 tl 0.02 0.02 0.02 0.03 0.02 0.05 0.03 0.06 0.02 hg 10 6 5 11 12 5 9 10 5 se 0.2 0.1 0.1 0.2 0.1 0.1 0.2 0.2 0.1 te 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 ga 9.4 12.3 12.8 12.2 12.5 11.6 12 12.3 8.8 cs 0.21 3.28 5.03 4.31 4.06 4.05 3.97 2.99 0.18 ge 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 hf 0.17 0.22 0.13 0.22 0.1 0.17 0.23 0.29 0.16 nb 0.07 0.16 0.09 0.14 0.03 0.1 0.1 0.24 0.07 rb 1.6 2.6 3.5 3.3 3.3 6.5 6.9 10.6 1.5 sn 1.4 1 0.9 0.8 0.7 0.8 0.9 0.8 1.5 ta 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 zr 5.6 16.1 10.7 17.4 6.7 10 15.4 14.9 5.5 y 24.66 11.67 10.42 10.56 12.11 11 11.16 11.05 26.25 ce 64 36.6 35.2 33.1 36.3 33.9 33.3 33.6 64.9 in 0.07 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.04 re 1 1 1 1 1 1 1 1 1 be 1.4 1.4 1.5 1.2 0.8 0.9 1.1 1.4 1.1 li 5.7 25.7 37.9 33.2 37.6 22.5 25.1 17 6 pr 7.97 4.3 4.23 4.01 4.64 4.07 4.2 4.22 8.53 nd 35.41 18.02 17.16 16.11 18.54 17.59 16.94 16.95 36.56 sm 7.38 3.25 3.31 3.04 3.44 3.24 3.42 3.25 7.06 eu 0.99 0.95 0.95 0.88 1.06 0.95 1.02 0.96 1.01 gd 6.5 2.98 2.68 2.41 2.92 2.83 2.71 2.86 6.79 yb 1.94 1.09 0.81 1 0.92 0.89 0.84 0.77 2.07 er 2.59 1.38 1.08 1.1 1.25 1.16 1.03 1.06 2.63 lu 0.29 0.14 0.11 0.12 0.13 0.1 0.11 0.1 0.28 pd 10 10 10 10 10 10 10 10 10 pt 2 2 2 2 2 2 2 pb207 1.32 0.97 0.77 0.78 0.9 0.9 0.86 0.78 1.41 pb208 3.3 2.42 1.88 1.93 2.28 2.24 2.14 1.94 3.47 pb204 0.08 0.06 0.05 0.05 0.06 0.06 0.05 0.05 0.09 zr/hf 32.94 73.18 82.30 79.09 67 58.82 66.95 51.37 34.37 243geochemistry and petrogenesis of basic paleogene volcanic rocks in alamut region, alborz mountain, north of iran a variety of graphs have been implemented to determine the tectonic setting of the magma-forming rocks in the region. in the ta/hf versus th/hf diagram (wang et al., 2001), the basalts in the study area fall within the range of extensional or primary rift environments. the alkaline properties of these rocks confirm their formation environment (fig. 9-a). in the ti/y versus zr/y diagram (pearce and gale, 1977), the samples have been situated within the continental and continental margin ranges (fig. 9 -b). finally, in the v versus ti/1000 (schervais, 1982) and nb/yb versus tio2/yb diagrams, the samples are located in the oib and alkali basalt ranges (fig. 9). stratigraphic and structural studies by allen et al. (2003) indicate that ama volcanoclastic deposits have been formed during an extensional phase in the early eocene. asiabanha and foden (2012) suggested that this extension occurred in a back-arc environment. due to the basin uplift during the stress regime in the late eocene, the next volcanic phase has occurred in the sub-aerial phase form. according to the results, the following model can be used for the eocene-oligocene magmatic activity in the study area. this model is similar to the one proposed by humphreys et al. (2003) for west america, which consists of four steps: in the first stage, the steep oceanic crust subduction led to the formation of a volcanic arc in the north of the arabian-eurasia structural zone (fig. 10 a). in the second stage, after the aptian-albian orogeny phase, the subduction continued with a lower slope causing magmatic activity in the north of central iran and alborz. at the end of the cretaceous period, some tectonic contractions occurred from iran to the north of this structural zone. at this stage, upper mantle uplift occurred in alborz and northern iran mountains (zaeimnin et al., 2011; haghnazar, 2012; haghnazar et al., 2015). figure 8. (a) dy/yb versus la/yb from thirlwall et al. (1994) and bogaard et al. (2003), and (b) the logarithmic diagram of sm/yb versus la/yb from lai et al. (2012) for determining the degree of partial melting. (a) (b) (d) (c) figure 9. (a) ta/hf versus th/hf (wang et al., 2001), (b) ti/y versus zr/y (pearce and gale, 1977), (c) v versus ti/1000 (schervais, 1982) and (d) nb/yb versus tio2/yb (pearce, 2008) 244 mehdi nazari sarem, mansour vosoghi abedini, rahim dabiri, mohammad reza ansari in the third stage (eocene period), the rollback occurred in the oceanic crust. this phenomenon could be attributed to a number of factors including the rate of subduction, age of the sheet, the density difference between the plate and mantle, etc. (heuret and lallemand, 2005; arfania, 2018). this results in the extension and thinning of the crust and uplift of the asthenosphere. partial melting of hydrated peridotite occurred in the mantle due to pressure and heat reduction. the magma resulting from this melting was depleted in hfse elements and was at the same time enriched with some other elements during the subduction caused by dehydration. at this stage, volcanic activities and shallow sea sediments formed in extensional environments (figs. 10-c). in the fourth stage (the beginning of the oligocene), asthenosphere uplifting occurred due to the lithosphere thinning. at this time, the partial melting in the asthenosphere led to the formation of basaltic magma in the extensional basins. (fig.10-d). these basaltic magmas were less contaminated due to crust thinning and mostly present the primary magma properties. (plank and langmuir, 1988; glazner and ussler, 1989; ebrahimi et al, 2017). conclusion according to the studies, the rock units in the area are divided into volcanic lavas and pyroclastic rocks. volcanic lavas contain basalt, olivine basalt, andesitic basalt and basaltic andesite. in terms of chemical composition, the magma forming volcanic rocks in the alamut region is alkaline. the normalized diagrams indicate that the basaltic rocks of the region are partially enriched with the asthenosphere mantle similar to the source of oib with a partial melting degree of 12 to 15 and garnet lherzolite composition. based on the graphical analysis and structural geology studies, the formation of rocks in the alamut region has been related to the magmatism of continental rift zones. as a result of the deep faults, crust thinning, asthenosphere uplifting, local fractures formation by extensional phases (rifting), and partial melting in the asthenosphere, basaltic magma has formed in the extensional basin, which is less contaminated due to the crust thinning. the resulting basaltic magma presents mainly the characteristics of the primary magma. the volcanisms of avs are linear along these local extensional faults. references: abdel-rahman, a. m., & nassar, p. e. 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(2024). coal ash as a natural additive for subgrade stabilization in the construction of low-volume traffic roads. earth sciences research journal, 28(1), 73 78. https://doi.org/10.15446/esrj.v28n1.110855 keywords: coal ash; road subgrade; waste treatment; recycled aggregate ceniza de carbón como aditivo natural para la estabilización de subrasantes en la construcción de carreteras de bajo volumen de tráfico resumen colombia cuenta con una extensa red vial que abarca un total de 206,708 kilómetros, según datos del instituto nacional de vías de colombia (invias), de los cuales 142,284 kilómetros pertenecen a la categoría de carreteras rurales con bajo volumen de tráfico. lamentablemente, se estima que el 96% de estas vías se encuentran en mal estado. este problema se debe en gran medida a la presencia de subrasantes con un rendimiento mecánico deficiente, principalmente debido a la falta de métodos adecuados de estabilización. además, estas carreteras suelen ser las únicas vías de acceso y salida para las comunidades rurales, lo que afecta considerablemente su conectividad con los centros urbanos cercanos. conscientes de esta problemática crítica, en este artículo se propone el uso de ceniza de carbón para la estabilización de subrasantes durante la construcción de carreteras de bajo volumen de tráfico. el estudio llevado a cabo demuestra que la ceniza de carbón puede mejorar las propiedades mecánicas de las subrasantes, lo que resulta en un aumento de la resistencia y la capacidad de carga. estas mejoras se atribuyen a las características de unión y reactividad de las cenizas de carbón, que contribuyen significativamente a la estabilización del suelo. para respaldar estas afirmaciones, se realizaron una serie de pruebas de caracterización física, mecánica y de resistencia tanto en muestras de arena arcillosa natural como tratada, obtenidas de una población rural en colombia. el análisis detallado de los resultados confirma una mejora en las propiedades mecánicas del suelo gracias al uso de ceniza de carbón como agente estabilizante. palabras clave: ceniza de carbón; subrasante de carretera; tratamiento de residuos; agregados reciclados; https://doi.org/10.15446/esrj.v28n1.110855 mailto:rsuarez15@cuc.edu.co https://doi.org/10.15446/esrj.v28n1.110855 74 suárez maldonado, r. j, pájaro miranda, c. a 1. introduction there is a wide variety of soils that present relatively low loadbearing capacity, high moisture content, and poor consistency, making them unsuitable for constructing a subgrade capable of supporting the structural pavement layers. in such scenarios, the typical solution involves replacing the inadequate soil with selected materials that offer better mechanical properties. however, this option may not always be feasible due to financial constraints, especially when considering the costs associated with material transportation. furthermore, in the case of tertiary road projects, which operate on tighter budgets, there may not be sufficient economic resources to carry out complete soil replacement. therefore, a widely used alternative is to employ stabilization methods that upgrades the soil’s mechanical properties without the need for complete removal (labajos & núñez, 2020). this approach aims to improve the characteristics of the existing soil, to meet the necessary requirements for constructing an adequate subgrade. the incorporation of additives in soil stabilization intended for use as subgrade in the structural pavement layers has demonstrated promising results in improving the soil’s mechanical properties, as evidenced by researchers from ecuador, peru, and colombia ( huancoillo-humpiri, 2017; castillo, 2017; falen & cubas &, 2016; montes, 2010; flórez ramirez, 2006). various materials have been used to achieve subgrade stabilization, including sodium chloride, which has been found to affect the water’s surface tension in the soil, resulting in a reduction of the evaporation (mendez cerna, 2021; quispe, 2020; garnica anguas et al., 2002). another material used is pulverized tire rubber, which has been shown to reinforce unstable soils, improving mechanical properties such as shear strength, cohesion, and angle of friction in clayey soils (hurtado-dionisio, 2022; alvarez castelblanco, 2021; roman vásquez, díaz suárez, & torres frias, 2019). furthermore, combinations of recycled materials like pozzolan and glass have been explored, resulting in an increased california bearing ratio (cbr) from 4.9% to 14.1% (arango fernández & marín falconi, 2021; mas, garcía, marco & de marco, 2016). these findings indicate that research is ongoing worldwide to investigate the use of different additives with the aim of finding economically viable and environmentally sustainable alternatives for subgrade stabilization. these two aspects are crucial in the case of low-traffic pavement networks, which are initially unpaved roads whose surface has received a chemical or mechanical stabilization technique while maintaining their geometric conditions, to allow the passage of lighter vehicles than those traveling on the main roads. coal ash, a residue produced from coal combustion processes, is widely available due to the substantial production of coal in countries like colombia (vallejo, morales, morales y laverde, 2007). notably, colombia has witnessed significant growth in coal production over the years. according to the mining and energy planning unit of the ministry of mines and energy of the republic of colombia (2006), coal production increased by around 17 million tons between 1980 and 1990, and further surged by 32 million tons between 1990 and 2004. on the other hand, coal consumption experienced a 39.6% increase between 1980 and 1993 but decreased to 1.9 million tons by 2005. unfortunately, a significant amount of this coal ash is still disposed of in landfills or dumpsites, leading to severe pollution issues and environmental concerns. based on these considerations, the use of coal ash as an additive in subgrade stabilization has been investigated (cañar, 2017; morales, 2015). several researchers have explored the application of coal ash in stabilizing different soil types, including highly plastic inorganic clays (ch) and highly plastic organic soils (oh). the addition of coal ash in the range of 20-25% has been shown to significantly enhance the load-bearing capacity of these soils, as indicated by increased cbr values ranging from 6.45% to 17.20% (cañar, 2017; morales, 2015). furthermore, coal ash has proven to be effective in stabilizing poorly graded sands with silt content (sp or sp-sm), resulting in notable improvements in cbr values within the range of 13% to 23% (ahmad & zainol, 2020). these results not only demonstrate an improvement in the soil’s load-bearing capacity but also indicate a reduction in moisture content and a gradual increase in dry density when coal ash is incorporated. additionally, it has been reported a decrease in the plasticity index as the percentage of coal ash content increases (shirin, islam & kumruzzaman, 2020). these studies support the feasibility and benefits of using coal ash as additives in subgrade stabilization, as it improves the soil’s load-bearing capacity and modifies its physical properties, making it a promising option from both economic and environmental perspectives. in this research article, physical and strength characterization tests were conducted using a soil sample, of a previously unstudied soil type, obtained from the access road to a rural population in the department of atlántico, colombia. these tests included the determination of the soil’s physical properties, such as particle size distribution, consistency limits, density, and moisture content. additionally, strength tests, such as the cbr, were performed to evaluate the soil’s load-bearing capacity. subsequently, additional tests were carried out using different percentages of coal ash, mixed with the natural soil. these tests aimed to assess the effect of coal ash addition on the soil’s properties and load-bearing capacity. the obtained results were compared with reference data obtained from reviewed literature, including previous studies on the use of coal ash as additives in subgrade stabilization. furthermore, an analysis of the results was performed to determine the viability of using coal ash as a naturally occurring additive in subgrade stabilization for low-traffic volumes. 2. materials and methods 2.1. research location the selection of a rural area in colombia as the research site holds significance due to the fundamental role of agricultural and livestock activities in the country’s economy. however, inadequate road infrastructure in these areas poses challenges for transporting agricultural products, particularly during the rainy season, primarily due to the presence of highly plastic clayey soils. implementing soil stabilization techniques for subgrade construction in lowtraffic volume pavements offers a viable solution and substantial improvement for rural communities. by enhancing the load-bearing capacity of the soils and facilitating vehicular movement, improved market accessibility and economic development can be fostered in these remote regions. it is worth noting that this issue extends beyond colombia, affecting numerous latin american countries. according to the organization for economic cooperation and development (oecd), latin america contributes approximately 13% of global agricultural production. therefore, the findings of this study may have broader applicability and potential for replication in other countries across the region that face similar transportation, agricultural market access challenges, and soil properties. the soil samples were collected from a rural access road in the gallego district, which falls under the jurisdiction of sabanalarga municipality in the atlántico department of colombia (figure 1). the need for soil stabilization was identified due to the road’s impassability and the consequent isolation of local farmers during the region’s rainy seasons. figure 1. location of the district of gallego, sabanalarga-atlántico-colombia. 75coal ash as a natural additive for subgrade stabilization in the construction of low-volume traffic roads 2.2. sampling the sample collection was carried out following astmd420 “standard guide for site characterization for engineering design and construction purposes”. this standard aims to provide standardized methods for soil and rock sampling and investigation to determine soil distribution characteristics. a total of 50 kg of representative material was collected from the surface and up to a depth of 1.5 meters, following the guidelines established in the standard. the sampling was conducted at two specific points through trench excavation. subsequently, the samples were appropriately labeled and sent to the laboratory for the corresponding characterization tests. these tests aimed to evaluate the soil properties that would later be used as subgrade in a low-traffic volume pavement. 2.3. techniques and instruments within the framework of this experimental research, the extraction of natural soil from the study area was carried out, which was designated as the control sample and labeled as sn (natural soil). based on the natural soil sample, we prepared four specimens by replacing between 20% and 40% of the soil weight with coal ashes. the specimens are identified as m1, 25% of coal ash, labeled as m2, 30% of coal ash, denoted as m3, and 35% of coal ash, referred to as m4. for all samples, a series of standardized tests were conducted to obtain relevant data. these tests included the determination of particle size distribution (according to the astm 422-63 standard), liquid limit, plastic limit, and plasticity index (according to the astm d4318 standard), modified compaction test (according to the astm d1557-78 standard), and cbr test (according to the astm d1883 standard). based on the results obtained for the natural soil, its classification was carried out using the methodology established by the american association of state highway and transportation officials (aashto) and the unified soil classification system (uscs). subsequently, the results of the control sn were compared with those obtained for the different variations of coal ash percentages, i.e., m1 (20% of coal ash), m2 (25% of coal ash), m3 (30% of coal ash), and m4 (35% of coal ash). the objective of this comparison was to evaluate if the incorporation of coal ash resulted in significant improvements in the use of natural soil as a subgrade in low-traffic volume roads. 2.4. procedure step 1: field sampling of natural soil to be properly labeled and preserved for laboratory analysis. (figure 2). figure 2. rural road sample extraction area step 2: acquisition and mixing of coal ash to be applied in different percentages to the specimens of natural soil. (figure 3). step 3: determination of the characteristics of the control specimen of natural soil, as well as the specimens having different percentages of coal ash, obtaining parameters related to particle size distribution, consistency limits, compaction, and cbr. (figure 4). step 4: analysis of the data obtained is carried out to determine the impact of coal ash addition to the natural soil and assess whether it improves or not its properties for use as subgrade in low-traffic volume roads. figure 3. mixture of coal ash with natural soil. figure 4. execution of tests in the laboratory. 3. results and discussion 3.1. granulometry the results of the particle size distribution test are presented in figure 5 and table 1. the results indicate that these samples of natural soil cannot be used in a pavement structure without prior treatment due to their high fines content, with a percentage exceeding 40%. the inclusion of coal ash in the analyzed soil renders the sample more uniform and mitigates the fines percentage. figure 5. results of the granulometric analysis. 76 suárez maldonado, r. j, pájaro miranda, c. a table 1. results of particle size distribution. sieve(mm) percentage passing sn m1 m2 m3 m4 9.52 0.90 0.90 0.9 0.9 0.9 4.75 0.78 0.65 0.64 0.64 0.64 1.9 0.67 0.51 0.48 0.48 0.48 0.84 0.59 0.43 0.4 0.38 0.35 0.42 0.53 0.38 0.34 0.32 0.28 0.25 0.49 0.34 0.3 0.28 0.21 0.105 0.40 0.25 0.21 0.19 0.16 0.074 0.39 0.24 0.2 0.18 0.15 3.2. liquid limit (ll). based on the results obtained from the casagrande cup laboratory test, in accordance with methodology a of astm d 4318 standard, a linear regression was performed to determine the liquid limit of each sample. for the natural soil (sn), a liquid limit value of 25.81% was obtained. for m1 sample (with 20% coal ash), a value of 31.62% was estimated, while m2 sample (with 25% coal ash) had a liquid limit of 36.68%. m3 sample (with 30% coal ash) yielded a value of 37.78% for the liquid limit, and m4 (with 35% coal ash) had a liquid limit of 38.74%. these results are presented in table 2 and depicted in figure 6. 3.3 plastic limit (pl). the plastic limit was determined for each of the analyzed samples, and the results obtained are presented in table 2. the plastic limit represents the moisture content at which the soil transitions from a plastic state to a semi-solid state. the results indicate that the natural soil (sn) has a plastic limit of 4.84%. with the addition of coal ash in samples m1, m2, m3, and m4, the plastic limit gradually increases. this suggests that the inclusion of coal ash enhances the soil’s plasticity and its ability to retain water. these findings highlight the impact of coal ash on the soil’s plasticity, which is essential in determining soil workability and susceptibility to deformation. the gradual increase in the plastic limit with higher percentages of coal ash (see figure 6) demonstrates the potential of coal ash as a beneficial additive to improve the soil’s engineering properties. 3.4 plasticity index (pi). the plasticity index was calculated, and the results are summarized in table 2 and figure 6. according to these results, the natural soil can be classified as highly plastic, as its plasticity index is greater than 20. on the other hand, samples m1 and m2 exhibit moderate plasticity, as their plasticity indexes fall between 10 and 20. for samples m3 and m4, a plasticity index lower than 10 indicates a soil with low plasticity. the inverse relation between plasticity index and percentage of coal ashes allows us to conclude that the coal ashes addition mitigates the occurrence of swelling in the natural soil, which benefits the performance of the subgrade. table 2. results of the consistency limits. sample percentage of coal ash ll (%) pl(%) pi(%) sn 0 25.81 4.84 20.97 m1 20 31.68 16.83 14.85 m2 25 36.68 24.53 12.15 m3 30 37.78 28.79 8.99 m4 35 38.74 30.43 8.31 figure 6. atterberg limit results. 3.5 aashto and uscs classifcations based on the laboratory test results, the natural soil is classified as sc (clayey sand) according to the unified soil classification system (uscs). furthermore, based on its grain size distribution (table 1) and consistency limits(table 2), the natural soil is classified as a-6 (2) according to aashto soil classification system. according to this classification, this type of soil is not suitable for use as pavement subgrade, as it is considered fair to poor according to aashto criteria. in contrast, all specimens with the addition of coal ashes are classified as a-2-6, which are described as competent soils with applications in pavement structural layers. hence, the addition of coal ashes leads to an improvement in the properties of the natural soil. 3.6 proctor compaction test figure 7 presents the results of the proctor compaction test for each sample, according to the procedure described in astm d1557 standard, compaction is carried out using the modified hammer. the sn specimens showed an optimum moisture content of 30% and a maximum dry density of 2.17 g/cm³. for the m1 specimens, the values were 32% for optimum moisture content and 2.29 g/cm³ for maximum dry density, respectively. the m2 specimens had an optimum moisture content of 37% and a maximum dry density of 2.57 g/cm³. however, there was a decrease in maximum dry density for the m3 and m4 specimens, with values of 2.52 g/cm³ for the m3 sample and 2.44 g/cm³ for the m4 specimens. the optimum moisture content percentages for the m3 and m4 specimens were 43% and 47%, respectively. these results indicate variations in both maximum dry density and optimum water content of the soil specimens with the addition of coal ashes. the findings suggest that an addition of around 25% of coal ashes appears to be the optimum value for improving the soils mechanical properties. figure 7. proctor compaction test results. 77coal ash as a natural additive for subgrade stabilization in the construction of low-volume traffic roads 3.7 california bearing ratio (cbr) figure 8 presents the stress-strain results obtained during the cbr tests. it is evident that the natural soil specimens exhibit the lowest strength among all the samples tested. conversely, consistent with the results observed in the proctor compaction test (figure 7), the specimens having a 25% percentage of coal ash addition exhibits the highest strength, while the remaining treated specimens present similar strength results. furthermore, the inclusion of coal ash led to an increase in the stiffness of the samples respect to the untreated soil samples, that could be beneficial for other applications such as footings. figure 8. stress vs strain results table 3. cbr test results. ty pe percentage of coal ashes maximum dry density gr/cm3 cbr specimen 1 specimen 2 specimen 3 100% 95% sn 0% 1,88 2,17 2,19 5,3 4,3 m1 20% 2,10 2,34 2,33 5,9 5,1 m2 25% 2,56 2,58 2,57 6,2 5,6 m3 30% 2,18 2,35 2,50 5,7 4,9 m4 35% 2,20 2,35 2,40 5,5 4,3 figure 9. variation of cbr increase with respect to the percentage of coal ashes. 4. conclusions according to the results obtained in the laboratory tests, adding carbon ashes improves the mechanical properties of sc-type soils. an increase in the liquid and plastic limits is observed as the percentage of carbon ashes in the sample rises, leading to a decrease in the plasticity index. this effect is particularly advantageous for treating highly plastic soils. regarding density, it can be concluded that the addition of 25% carbon ashes in weight to the sample increases the maximum dry density by around 18%. however, exceeding 25% of carbon ashes addition led to lower increases in dry density. regarding the cbr, it was found, according to the reviewed literature, that the optimal content of carbon ashes addition in the sample to improve the properties of the natural soil depends on the soil type analyzed. other authors suggest different optimal values for the inclusion of carbon ashes compared to those found in the current research. for the case under study, the optimal value is 25%, as this increment raises the cbr value to 30% of the sample, representing a significant improvement in load-bearing capacity. lastly, future research endeavors could test the suitability of 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(2019). incorporación de partículas de caucho de neumáticos para mejorar las propiedades mecánicas en suelos arcillosos. [dissertation universidad nacional de jaén (jaén, perú)]. http://repositorio.unj.edu.pe/handle/unj/236 shirin, m. s., islam, m. m., & kumruzzaman, m. (2020). stabilization of coal mine waste and its practical application as a road subgrade.  american journal of traffic and transportation engineering,  5(5), 51-56. doi:10.11648/j.ajtte.20200505.11 vallejo, l. f. v., morales, j. f. d. l. c., morales, j. f. s., & laverde, m. a. m. (2007). remoción de carbón inquemado de las cenizas volantes producidas en el proceso de combustión de carbón. energética, (38), 107-112. http://www.imt.mx/archivos/publicaciones/publicaciontecnica/pt201.pdf http://www.imt.mx/archivos/publicaciones/publicaciontecnica/pt201.pdf https://hdl.handle.net/20.500.12848/4952 https://hdl.handle.net/20.500.12848/4952 https://doi.org/10.25127/ucni.v3i1.589 https://doi.org/10.25127/ucni.v3i1.589 https://repositorio.ucv.edu.pe/handle/20.500.12692/65010 https://repositorio.ucv.edu.pe/handle/20.500.12692/65010 https://hdl.handle.net/20.500.14138/116 http://hdl.handle.net/11407/1236 http://hdl.handle.net/11407/1236 https://hdl.handle.net/20.500.12394/8216 https://hdl.handle.net/20.500.12394/8216 http://repositorio.unj.edu.pe/handle/unj/236 _heading=h.hnqtf2o4dor2 _heading=h.30j0zll _heading=h.3znysh7 _heading=h.2et92p0 _heading=h.b4t2b7ybrxi9 _heading=h.tyjcwt _heading=h.3dy6vkm _heading=h.1t3h5sf _heading=h.4d34og8 _heading=h.2s8eyo1 keywords: underground mining; mining predict; probability integral model; geometric model; parameter inversion palabras clave: minería subterránea; predicción de minería; modelo de probabilidad integral; modelo geométrico; inversión de parámetros; how to cite item li, j., yu, x., chen, d., & fang, x. (2021). research on the establishment of a mining subsidence prediction model under thick loose layer and its parameter inversion method. earth sciences research journal, 25(2), 215-223. doi: https://doi.org/10.15446/esrj.v25n2.79537 most of the coal mining in china is underground, which will inevitably cause surface deformation and trigger a series of geological disasters. therefore, it is essential to find a suitable method to forecast the ground sinking caused by underground mining. the most commonly used prediction model in china is the probability integral model (pim). but when this model is used in the geological condition of mining under thick loose layers, the predicted edge of the sinking basin will converge faster than the actual measured sinking situation. a geometric model (gm) with a similar model shape as the pim but with a larger boundary value was established in this paper to solve this problem. then an improved cuckoo search algorithm (icsa) was proposed in this paper to calculate the gm parameters. the stability and reliability of the icsa were verified through a simulated working face. at last, the icsa, in combination with the gm and the pim, was used to fit 6 working faces with the geological mining condition of thick loose layers in the huainan mining area. the results prove that gm can solve the above-mentioned pim problem when it is used in geological mining conditions of thick loose layers. and it was obtained through comparative analysis that the gm and the pim parameters can take the same value except for the main influence radius. abstract research on the establishment of a mining subsidence prediction model under thick loose layer and its parameter inversion method investigación sobre el establecimiento de un modelo de predicción de subsidencia minera bajo una capa gruesa inestable y su método de inversión de parámetros issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.79537 la mayor parte de la minería del carbón en china es subterránea, lo que inevitablemente causa deformaciones en la superficie y desencadena desastres geológicos. por lo tanto, es necesario encontrar un método adecuado para pronosticar el hundimiento del suelo causado por la minería subterránea. el modelo de predicción más utilizado en china es el modelo integral de probabilidad (pim). pero cuando este modelo se utiliza en la condición geológica de la minería bajo capas gruesas inetables, el borde previsto de la cuenca de hundimiento converge más rápido que la situación de hundimiento medida. para resolver este problema, en este artículo se estableció un modelo geométrico (gm) que tiene una forma de modelo similar a la del pim pero que tiene un valor límite mayor. en este trabajo se propuso un algoritmo de búsqueda de cuco mejorado (icsa) para calcular los parámetros de gm, y se verificó la estabilidad y confiabilidad del icsa a través de una frente de trabajo simulado. por último, el icsa en combinación con el gm y el pim se utilizaron para ajustar 6 caras de trabajo con la condición de minería geológica de capas gruesas sueltas en el área minera de huainan. los resultados demuestran que el gm puede resolver el problema de pim mencionado anteriormente cuando se utiliza en las condiciones de minería geológica de capas gruesas inestables. y se obtuvo mediante análisis comparativo que los parámetros del gm y del pim pueden tomar el mismo valor excepto por el radio de influencia principal. resumen record manuscript received: 07/05/2019 accepted for publication: 13/12/2019 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 215-223 jingxian li1, xuexiang yu 1*, deshu chen2 , xinjian fang1 1. school of geomatics, anhui university of science and technology; huainan 232001, china 2. guo xuan high tech power energy co., ltd; hefei 232000, china email: xxy16321@163.com m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v25n2.79537 https://doi.org/10.15446/esrj.v25n2.79537 216 jingxian li, xuexiang yu, deshu chen, xinjian fang introduction although the continuous mining of the coal resources provides a strong guarantee for the rapid development of china’s economy. however, on the one hand, with the continuous consumption of coal resources and the continuous acceleration of urbanization, the amount of the coal seams pressed under buildings, railways, and water is increasing. on the other hand, the surface subsidence and the massive structure damage caused by underground mining have been very serious in china (xuan & xu, 2014; zhao et al., 2016; li et al., 2019; xu et al., 2019). therefore, it is very important to predict and fully grasp the laws of the ground deformation and its deformation caused by underground mining, which can provide guidance for controlling the destruction of surface protection objects. however, the process of the surface subsidence is complicated and different geological mining conditions will cause different surface subsidence laws (xu et al., 2014). for example, the mining subsidence laws under the geological condition of thick loose layers have special characteristics compared with thinner loose layers and no loose layer. such as the maximum subsidence value may be greater than the normal thickness of the coal seam and it is apparently smaller of the predicted value than the measured data at the boundary of the subsidence basin (zhou et al., 2016; zhou et al., 2015). coal seams buried under thick loose layers are widely distributed in china. in recent years, most research on mining subsidence under the geological condition of thick domestic and foreign scholars has focused on the mechanism of rock formation movement. zhu et al. (2020) confirmed that the transition layers of the bedrock layers and the loose layers has a support effect on mining subsidence. liu et al. (2020) found that the thick loose layer formed a bending collapse zone, fracture development zone and caving zone during the mining process through simulation analysis, and derived a prediction model based on elastic mechanics. xu et al. (2020) studied the movement laws of the overlying bedrock while mining shallow buried coal seams under thick loose layers. yu et al. (2020) studied the rock-soil structure above the coal seam which is buried under thin bedrock layer and thick loose layer. wang et al. (2019) analyzed the stability of the loose layers and the bedrock layers during mining. ma et al. (2017) discussed the prediction methods for mining subsidence when coal pillars are solid backfilled under thick loose layers. yang & xia (2013) studied the ground deformation laws caused by mining under thick loose layers and thin bedrock layers by analyzing measured data. dai et al. (2011) researched the influence of the thickness of the loose layers on mining subsidence through numerical simulation. zhou et al. (2018) proposed a combined prediction model based on soil mechanics and random medium theory. chang et al. proposed a mining subsidence prediction model used in subcritical mining under thick loose layers with considering the different subsidence and movement mechanisms of the bedrock layers and the thick loose layers. but the model can only be suitable for subcritical mining under thick loose layers (chang et al., 2015). the advantage of the mining subsidence prediction model which is based on the mechanism of rock formation can fundamentally find the law of rock formation. it is not easy to establish the model and calculate the model parameters in actual application. so they all have not been widely used. the most commonly used mining subsidence prediction model is always the pim which is developed from the random medium theory (wang et al., 2020; zheng et al., 2019; diao et al., 2016; fan et al., 2015). however, due to the limitation of the shape of the model curve, the problem of small boundary value is much more obvious while the pim is applied to predict surface subsidence caused by mining under thick loose layers, and this problem cannot be solved by adjusting the model parameters. therefore, it is necessary to establish a model with simple parameters and suitable for surface subsidence prediction caused by mining under thick loose layers. an accurate prediction model is very important to mining subsidence prediction, a set of accurate parameters to the model is also important (sehn et al., 2015). at present, the mostly used parameters inversion methods in mining subsidence prediction area can be divided into direct optimization methods (shen et al., 2015; guo & wang, 2000) and intelligent optimization algorithms (zha et al., 2011; wang et al., 2018). the most commonly used of direct optimization methods is the modular vector method (mvm). the mvm has the advantages of easy programming and is suitable for working faces of any shape. but it needs a set of initial values of the parameters while using, and use them as a basis to follow the valley line or the ridge line to accelerate to the best values. the mvm has a strong dependence on the initial value of parameters. if the initial value is not accurate, the calculation result will deviate from the reasonable range. since the pim is an official forecasting method in china, many mining faces have a set of pim parameters. moreover, the subsidence prediction parameters of the working face with similar geological conditions are similar, so the pim parameters of the mined working face with similar geological conditions can be used as the initial value of the pim parameter of the working face to be solved. but on the one hand, the initial values of the parameters obtained in this way may be unreliable. on the other hand, it is difficult to obtain the initial values of the parameters of new prediction models. therefore, the use of the mvm was restricted. the intelligent optimization algorithm uses random global search within a certain range by simulating the behavioral characteristics of creatures such as foraging and reproduction instead of deterministic search of the modular vector method, so it can explore the global optimal value without initial value dependence. the most commonly used method of intelligent optimization algorithms is the genetic algorithm (ga) (du et al., 2014; xing et al., 2021; li et al., 2017). however, the accuracy of the ga to calculate the parameters of the mining subsidence prediction model is not ideal. at the beginning of this paper, the gm to predict the surface subsidence caused by limited coal seams is established. and it is found that by adjusting the parameters of the gm, the middle part of its model curve can be basically consistent with the pim, while the boundary value predicted is larger than the pim. therefore, it is inferred that the shape of the gm curve is more in line with the shape of the main section of the subsidence basin which is caused by mining under thick loose layers. then in order to calculate the gm parameters, an improved cuckoo search algorithm-icsa-is proposed, and a simulated working surface is used to verify the stability and accuracy of the icsa. finally, 6 working faces buried under thick loose layers were used to compare the accuracy of the gm and the pim. by comparing the fitting effects of the gm and the pim in 6 working faces, it can be concluded that the gm is more suitable than the pim in predicting surface subsidence caused by mining under thick loose layers. and the relationship between the gm parameters and the pim parameters is drawn. geometric model semi-infinite mining subsidence prediction model take the open-off cut of the coal seam as the origin, the direction from the origin to the inclination direction of the goaf as the positive direction of the y-axis and the direction from the origin to the strike of the goaf as the positive direction of the x-axis to establish the calculation coordinate system. then according to the pim, the sinking value of the point with abscissa x on the strike main section of the subsidence basin caused by a semi-infinite mining coal seam with the thickness of m and the dip angle of α can be calculated by equation 1. w x w r exp x s r dspim pim pim( )=( ) × −( )∫0 0 2/ / ) ∞ π (1) put erf x exp u du x ( )=( )× −( )∫2 0 2/ π into equation 1 to get equation 2: w x w erf x rpim pim( )= × ×( ) ( / ) /0 2 π (2) where w0 represents the max subsidence value and can be calculated by w m cos qpim0 = × ×α , qpim is a parameter in the pim named subsidence coefficient whose value is mainly determined by the lithology of the overlying rock, and its value is generally less than 1. rpim is another parameter in the pim named main influence radius. it can be seen from equation 2 that the main section subsidence curve caused by a semi-infinite mining coal seam is in the shape of “s”. and the shape of function f x x x( )= +/ 1 2 is also “s”. combined with the function 217research on the establishment of a mining subsidence prediction model under thick loose layer and its parameter inversion method f x x x( )= +/ 1 2 and equation 2, the new formula to calculate the sinking value of the point with abscissa x on the strike main section of the subsidence basin caused by a semi-infinite mining coal seam with the thickness of m and the dip angle of α is established as equation 3. w x w x r x r gm gm gm gm ( )= × +( ) +           ( / ) / / 0 2 2 1 1 (3) where w m cos q gm gm0 = × ×α , qgm is the subsidence coefficient of the gm. rgm is the main influence radius of the gm. limited mining subsidence prediction model the sinking value of the point with abscissa x on the strike main section of the subsidence basin caused by a limited mining coal seam with the length of d3 can be regarded as the superposition of two semi-infinite mining and calculated with equation 4. w x w w x s w x d spim pim pim pim pim ° ( )= × −( )− − −( ) )0 3 3 4( ] (4) where s pim3 and s pim4 are parameters of the pim named inflection point offset to the left and inflection point offset to the right. from the analysis above, the gm to predict the sinking value of the point with abscissa x on the strike main section of the subsidence basin caused by a limited mining coal seam with the length of d3 is established as equation 5. w x w w x s w x d sgm gm gm gm gm ° ( )= × −( )− − −( ) )0 3 3 4( ] (5)= × × × −( ) + −( )( ) − − − m q cos x s r x s r x d s r gm gm gm gm gm gm g α 3 3 2 3 4 1 / / ( ( )) / mm gm gmx d s r1 3 4 2 + − −( )( )          ( ) / where s gm3 and s gm4 are the inflection point offset to the left and the inflection point offset to the right of the gm. figure 1. the calculation principle of the movement and deformation of the inclined main section in limited mining according to figure 1, the formula to predict the sinking value of the point with coordinate y on the inclined main section of the subsidence basin caused by a limited mining coal seam with the width of d1 and depth of h is established as equation 6. w y w y s sin r y s r gm gm gm gm gm ( )= × − × +( )( ) + −( )      0 1 1 2 1 sin θ α α/ / ++                 −( )= × − −( )× +( ) 1 0 1 2w y l w y d s sin singm gm( / )θ α α // ( / ) / r y d s sin sin r w y gm gm gm gm1 1 1 2 2 + − −( )× +( )( ) +           ° θ α α (( )= ( )− −( )w y w y l (6) where l represents the width for calculation of the coal seam and can be calculated by l d s s sin singm= − −( )× +( )1 1 2 θ α θ/ , gm , s gm1 and s gm2 are parameters of the gm named mining influence propagation angle, inflection point offsets in the downhill direction and inflection point offsets in the uphill direction. parameters analysis from the analysis above, it can be seen that there needs seven parameters to predict mining subsidence with the gm: qgm , rgm , gm , s gm1 , s gm2 , s gm3 and s gm4 . take a working face in huainan mining area which has the length of 770m, width of 205m, depth of 556m, thickness of 3m and dip angle of 5° as an example to compare the model curves of the gm and the pim. the pim parameters of this working face are as follows: qpim 1 01. , rpim 300 , θpim = °88 , s pim1 5=− , s pim2 32=− , s pim3 19=− and s pim4 30 . the strike main section predicts the result of using the pim in combination with the above parameters is shown in curve 1 in figure 2. and the result of using the gm in combination with the above pim parameters is shown in curve 2 in figure 2. by comparing curve 1 and curve 2 in figure 2, it can be seen that the shapes and trends of the gm and pim model curves are similar under the same set of parameters, but the calculated specific subsidence values are different. then adjust the gm parameters to make r rgm pim / .2 5 and again use the gm to predict the subsidence of the strike main section to get curve 3. figure 2. comparison of the shape of the model curve of the gm and pim by comparing curve 1 and curve 3 in figure 2, it can be seen that by adjusting the parameter rgm, the middle part of the model curve of the gm can be roughly the same as the pim with the predicted value of the boundary part larger than the pim. which means the gm may meet the mining subsidence laws under the geological condition of thick loose layers better than the pim. so we can infer that the qgm , gm s gm1 , s gm2 , s gm3 , and s gm4 in the gm can take the same value with the qpim , pim , s pim1 , s pim2 , s pim3 and s pim4 in the pim except for r rgm pim / .2 5. improved cuckoo search algorithm in order to further study the characteristics of the gm and establish the relationship between the parameters of the gm and the pim, a suitable algorithm to solve the model parameters is needed. the cuckoo search algorithm (csa) is a new kind of heuristic bionic group intelligent optimization algorithm proposed 218 jingxian li, xuexiang yu, deshu chen, xinjian fang by professor xinshe yang and susan deb from the university of cambridge in 2009. the biological behaviors used include parasitic reproduction and lévy flights mechanism. the csa has the advantages of clear theory, few parameters, easy to expand, strong global search ability, easy to implement, and so on. it has been deeply studied and widely used by scholars in china and abroad. the csa parasitic reproduction behavior parasitic reproduction behavior means cuckoos do not build their own nests, they lay their eggs in communal nests or other birds’ nests. so mother cuckoos must face the risk that their eggs may be removed from the nests by other birds or the host birds. in the long-term evolution process, some cuckoos have learned some ways to improve the survival rate of their eggs such as pushing the host birds’ eggs out of the nests. lévy flights studies have shown that the foraging routes of many flying animals, including cuckoos, follow the lévy flights mechanism. the advantage of lévy flights mechanism in solving optimization problems is that it is combined with high frequency short step movements and occasionally long step movements. the high frequency short step movements make it possible to outcrop the local optimal solution and the occasionally long step movements ensure that the results will not fall into the local optimal solution. basic model according to the above biological background, make the following scenario hypothesis: • the total number of the nests is determined; • the cuckoos lay only one egg in each nest; • the one with the best fitness in the nest group will be retained directly to the next generation when the csa is used in solving optimization problems, the location of each nest represents a set of solutions. after several rounds of elimination, evolution and selection, the optimal individual can be selected out. the elimination strategy of the csa was random selection under fixed elimination rate. the selection strategy of the csa was greedy selection, that is, the individuals with better fitness will be retained after the new solutions were obtained and compared with current solutions. the evolution strategy of the csa adopts two modes: random walking mode and preferred walking mode. 1. random walking mode. use equation 7 when current nests’ positions need to be updated x x l i ni t i t+ = + ( ) =1 1 2α λ⊕ , , ,  (7) where xi t and xi t1 represent the location of the i-th nest of generation t and generation t+1 respectively; α represents the step size controller;  is point-to-point multiplication; n is the amount of the nests, that is, the amount of the feasible solutions; l λ( ) represents the random search path, l q u( ) < ≤−~ ( )λ λ1 3 , where  represents the random step size obtained by lévy flights. 2. preferred random walking mode. mimicking the behavior of a host bird perceives the cuckoo egg and throws it away, then a new nest is needed. the next updating operation is as equation 8. x x r x xi t i t j t k t+ = + −( )1 �� �� �� (8) where x j t and xk t represent two random solutions of generation t; r is a random number satisfying the uniform distribution principle between [0,1]. the icsa practice shows that the csa is not strong in local search ability. this is because each solution is independent of each other in the process of searching for the optimal solution, there is no experience learning from better solutions and no information exchange. therefore, the roulette selection strategy was adopted instead of the random elimination strategy in the csa and the average operator and crossover operator was added in the updating process to exchange the information between nests. the specific operations are as follows: the selection strategy. to select a better individual from the contemporary nests, it is first necessary to establish an individual fitness calculation function. according to the principle of least squares, for the purpose of making the fitting effect of the inverted parameters better, the fitness of solutions are evaluated based on the sum of the squares of the difference between the sinking value of predicted and observed. the fitness function is constructed as equation 9. the nest with high fitness value is of good quality. f w w i n j ni j p ri j j = −( ) = =∑1 1 2 1 2 2 / , , , , ; , , , ,   (9) where � f xi( ) represents the fitness of the i-th group of parameters, wpi j, represents the predicted subsidence value of the j-th observation point under the i-th group of parameters, and wrj represents the actual subsidence value of the j-th observation point. n is the amount of the nests in one generation, n is the amount of the observation points. the roulette algorithm is used to select individuals with better fitness and eliminate those with poor fitness. its specific operation process is as follows: 1. calculate the fitness of each nest as f xi( ) (i = 1,2... ,n), n is the total number of the nests; 2. calculate the probability of being passed on to the next generation of each individual as p x f x f xi i j m j( )= ( ) ( ) = ∑/ 1 ; 3. calculate the cumulative probability of each individual as q p xi j i j= ( ) = ∑ 1 ; 4. generates a uniformly distributed pseudorandom rand in the interval [0,1]; 5. if rand q 1, then choose the first individual; otherwise, choose individual k to make q rand qk k− < ≤1 6. repeat step 4 and 5 for m times. save the selected individuals and eliminate the unselected the evolution strategy. declare a temporary nest xk � for communicating information between current nests. for any individual xt and xt1, if x xt t= +1, calculate the step length by α =1 2/ t to generate a new nest xk . if x xt t≠ +1, then deal them with three different operators: average operator (take the average of the parameters in the two nests); crossover operator (randomly exchange the parameters in the two nests); and mutation operator (update the nests with eqs. (8)). the nest xk which has the best fitness can be selected out. then randomly select a nest from all nests and compare it with xk to keep the one with the better fitness. set the total number of the nests as n=200 and each nest represents a set of parameters, which is [ q, tan , , s1, s2, s3, s4]t in the gm. the specific steps of calculating mining subsidence prediction parameters with the icsa are shown in figure 3. test by simulated working face a simulated working face is designed so as to test the stability and reliability of the icsa and compare its accuracy with the csa and the ga,. 219research on the establishment of a mining subsidence prediction model under thick loose layer and its parameter inversion method its length, width, depth, dip angle and thickness are designed as 900m, 300m, 400m, 0° and 3m. its gm parameters are designed as is shown in table 1. there set up an observation line above both the strike main section and inclined main section as is shown in figure 4. the spacing and total number of the observation points are 30m and 88. take the value calculated by the gm with designed parameters as true subsidence value. then add random errors with median error of 5m, 10mm and 15mm into the true subsidence value to generate three sets of designed observations. in order to test the stability of the icsa, calculate 100 sets of the gm parameters of the designed working face with the above designed observations and the comparison of the parameter inversion effect of the icsa while the designed observation data has different degrees of median error are shown in table 2. in order to test the precision of the icsa, use the ga, the csa and the icsa to calculate 50 sets of the gm parameters of the designed working face with the above designed observations. take “q” as an example, the error comparison between true value and values inversed by the ga, csa and icsa are shown in table 3. table 1. designed gm parameters of the simulated working face parameters qgm tanβgm θgm(°) s1gm (m) s2gm (m) s3gm (m) s4gm (m) value 0.7 5.0 88 20 20 20 20 1 tan h rgm gmβ = / , where h represents the depth of the coal seam. table 2. comparison of the parameter inversion effect of the icsa while the designed observation data has different degrees of median error 5mm 10mm 15mm mean |∆| mean |∆|/l0 (%) mean |∆| mean |∆|/l0 (%) mean |∆| mean |∆|/l0 (%) qgm 0.028 4.00 0.035 5.00 0.037 5.29 tan  0.075 1.2 0.084 1.7 0.096 2.0 gm 0.315 0.36 0.332 0.38 0.328 0.37 s gm1 7.12 35.60 8.95 44.75 7.98 39.90 s gm2 6.37 31.85 7.82 39.10 8.15 40.75 s gm3 9.05 45.25 9.22 46.10 9.64 48.20 s gm4 9.12 45.60 8.99 44.95 9.42 47.10 δ 14.4 22.5 30.9 1 mean l l t∆ = − 0 / , where  represents the absolute error between the true value l0 and the inversed value l; t represents the inversion times, which equals 100 in this paper; 2 mean l l l l n n ∆ / / / %0 0 0 100= −( ) ×∑ ; 3 δ =  ( )sqrt n∆2 / , where n represents the total number of the observation points, which equals 88 in this paper. table 3. precision comparison of the ga, csa and icsa mean|∆q | mean|∆q |/l0(%) δ ga 0.037 5.3 31.2 csa 0.108 15.4 40.2 icsa 0.035 5.0 22.5 we can draw the following conclusions from table 2 and table 3: 1. the relative errors of qgm , tan gm and gm inversed by the icsa are all less than 6%, and the errors of s gm1 , s gm2 , s gm3 and s gm4 are all less than 10mm. indicating that the errors of the parameters inversed by the icsa are all within the normal range. the average values of the errors of qgm , tan gm and gm are all less than 1, indicating that the results calculated by the icsa are stable and reliable. 2. as the medium error of the random error in the observation data increases, the medium error of the inversed parameters has a tendency to increase. indicating that the quality of the observation value will directly affect the accuracy of the parameters inversed by the icsa. 3. the average error of qgm inversed by the icsa is 0.035, and its relative error is 5.0%, which is similar to the ga and significantly higher than the csa, indicating that the stability of the icsa is higher than the csa and figure 3. flow chart of the icsa figure 4. schematic diagram of the designed observation points of the simulated working face 220 jingxian li, xuexiang yu, deshu chen, xinjian fang comparable to the ga. compared with the other two methods, the error in the fitting of the parameters calculated by the icsa is the smallest, indicating that the reliability of the parameters calculated by the icsa is the highest. case analysis (1) the working face 1111 (1) of the zhujidong coal mine in huainan panji mining area has a length of 1584m, width of 220m, average depth of 920m, average thickness of 1.8m, and average dip angle of 3°. it is buried under the loose layer with an average thickness of 278m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the observations and the subsidence value fitted by the gm and the pim are shown in figure 5 and table 4. table 4. fitting effect comparison of the gm and the pim for the 1111(1) working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 9.1 9.2 19.0 25.2 pim 17.9 19.8 42.0 56.5 figure 5. comparison of the fitting effect of the gm and pim for 1111(1) working face (2) the working face 12326 of the gubei coal mine in huainan xieqiao mining area has the length of 770m, width of 205m, average depth of 537m, average thickness of 2.55m, and average dip angle of 5°. it is buried under the loose layer with an average thickness of 448m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the observations and the subsidence value fitted by the gm and the pim is shown in figure 6 and table 5. table 5. fitting effect comparison of the gm and the pim for the 12326 working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 64.8 42.1 37.0 50.8 pim 140.1 125.2 46.7 63.0 figure 6. comparison of the fitting effect of the gm and pim for 12326 working face (3) the working face 1613 (1) of the guqiao coal mine in huainan xieqiao mining area has a length of 1528m, width of 251m, average depth of 668m, average thickness of 2.8m, and average dip angle of 3°. it is buried under the loose layer with an average thickness of 420m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the observations and the subsidence value fitted by the gm and the pim is shown in figure 7 and table 6. table 6. fitting effect comparison of the gm and the pim for the 1613(1) working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 28.4 29.4 17.5 23.2 pim 57.1 63.7 42.9 57.1 figure 7. comparison of the fitting effect of the gm and pim for 1613(1) working face (4) the working face 1414(1) of the guqiao coal mine in huainan xieqiao mining area has a length of 2120m, width of 251m, average depth of 735m, average thickness of 2.7m, and average dip angle of 5°. it is buried under the loose layer with an average thickness of 411m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the 221research on the establishment of a mining subsidence prediction model under thick loose layer and its parameter inversion method observations and the subsidence value fitted by the gm and the pim is shown in figure 8 and table 7. table 7. fitting effect comparison of the gm and the pim for the 1414(1) working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 12.5 9.1 12.0 14.8 pim 48.6 53.6 68.6 86.2 figure 8. comparison of the fitting effect of the gm and pim for 1414(1) working face (5) the working face 1252(1) of the panyidong coal mine in huainan panji mining area has the length of 1148m, width of 262m, average depth of 802m, average thickness of 2.7m, and average dip angle of 6°. it is buried under the loose layer with an average thickness of 165m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the observations and the subsidence value fitted by the gm and the pim is shown in figure 9 and table 8. table 8. fitting effect comparison of the gm and the pim for the 1252 (1) working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 22.6 22.7 20.1 21.3 pim 48.4 45.8 51.4 54.7 figure 9. comparison of the fitting effect of the gm and pim for 1252(1) working face (6) the working face 1312 (1) of gubei coal mine in huainan xieqiao mining area has the length of 629m, width of 205m, average depth of 528m, average thickness of 3.6m, and average dip angle of 5°. it is buried under the loose layer with an average thickness of 439.7m. its coal mining method and roof management method are the comprehensive mechanized method with a full height mining at one time and the total collapse method respectively. using the icsa combined with the last observation data of the working face to calculate the pim parameters and the gm parameters. the calculated model parameters are shown in table 10. the comparison between the observations and the subsidence value fitted by the gm and the pim is shown in figure 10 and table 9. table 9. fitting effect comparison of the gm and the pim for the 1312(1) working face median error overall median error of edge area relative error overall(%) relative error of edge area (%) gm 114.8 32.5 15.5 23.7 pim 123.1 122.6 41.5 77.8 figure 10. comparison of the fitting effect of the gm and pim for 1312(1) working face table 10. summary of the model parameters of the gm and pim calculated by the icsa q r  s1 / s2 / s3 / s4 1111(1) gm 0.43 242.1 87.1 5.5/-18.7/100.5/-19.5 pim 0.40 575.0 88.9 4.5/3.5/108.0/-11.7 12326 gm 1.20 131.0 89.5 -9.5/-11.5/46.5/70.5 pim 1.08 537/1.6 89.9 -1.7/-17.8/-68.5/-10.5 1613(1) gm 1.13 335.6 84.8 35.5/-5.5/-52.3/-26.3 pim 1.01 392.9 84.7 33.0/-4.3/59.0/35.3 1414(1) gm 1.15 183.8 88.0 37.0/-5.5/69.0/48.0 pim 0.90 459.4 87.5 26.0/75.5/120.4/109.5 1252(1) gm 0.90 222.8 88.0 29.5/90.5/123.5/114.5 pim 0.82 534.7 87.3 -30.7/7.0/43.5/34.5 1312(1) gm 1.36 125.7 87.6 -27.5/-5.5/73.0/19.5 pim 1.28 277.9 88.0 -42.5/-13.6/59.5/11.5 the six working faces selected in this article all buried under a thick loose layer. we can draw the conclusion from the above calculation results that the gm fitted better than the pim at the boundary of the subsidence basin. taking 1613 (1) as an example, the median error at the boundary of the subsidence basin fitted by the pim is 63.7, while fitted by the gm is 29.4, which is a reduction of 34.3; the relative error at the boundary of the subsidence basin fitted by the pim is 57.1%, while fitted by the gm is 23.2 %, which is a reduction of 33.9%. the fitting effect of the gm at the boundary 222 jingxian li, xuexiang yu, deshu chen, xinjian fang of the subsidence basin is significantly optimized compared with the pim in predicting the subsidence value caused by mining under thick loose layers. by comparing the gm parameters and the pim parameters in table 9, it is confirmed that the values of the two models parameters are basically the same except for � rgm and rpim . by comparing their values in table 9, it can be seen that rpim is about 2.5 times that of �rgm . results and discussion (1) through comparison, it is proved that the value at the boundary of the gm’s function curve is larger than that of the pim. which makes the gm more suitable than the pim for predicting surface subsidence caused by mining under thick loose layers. (2) in this paper, the icsa was established by adding operations to increase the information exchange between individuals on the basis of the csa. and through a simulated working face, it is proved that the stability and accuracy of the icsa can meet the requirements of coal mining subsidence prediction parameters inversion. (3) 6 working faces buried under thick loose layers in huainan mining area were selected in this paper to compare the fitting effect of the pim and the gm using the icsa. the results show that the gm has a better fitting result than the pim at the boundary of the subsidence basin. further confirmed that the gm is more suitable than the pim for predicting surface subsidence caused by mining under thick loose layers. and the parameters of the gm and the pim can take the same value except for r rgm pim / .2 5 . this result helps the gm to be used in actual engineering acknowledgement the research is supported by youth project of natural science foundation of anhui province (no.2008085qd178) references chang, z., wang, j., chen, m., ao, z., & yao, q. 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(2015). mechanism of mining subsidence in coal mining area with thick alluvium soil in china. arabian journal of geosciences, 8(4). https://doi.org/10.1007/s12517014-1382-2 zhou, d. w., wu, k., miao, x. x., & li, l. (2018). combined prediction model for mining subsidence in coal mining areas covered with thick alluvial soil layer. bulletin of engineering geology and the environment, 77(1), 283-304. https://doi.org/10.1007/s10064-016-0961-8 zhu, x., zhang, w., wang, z., wang, c., li, w., & wang, c., & loupasakis, c. (2020). simulation analysis of influencing factors of subsidence based on mining under huge loose strata: a case study of heze mining area, china. geofluids. https://doi.org/10.1155/2020/6357683 https://doi.org/10.1007/s12517-014-1382-2 https://doi.org/10.1007/s12517-014-1382-2 https://doi.org/10.1007/s10064-016-0961-8 https://doi.org/10.1155/2020/6357683 _30j0zll _1fob9te _3znysh7 _3rdcrjn _aa8bucz8vd6w _26in1rg keywords: under-dip slope; deformation characteristics; on-site investigation; the discrete element method; failure mechanism. palabras clave: pendiente bajo el agua; características de deformación; investigación en el sitio; método del elemento discreto; mecanismo de falla. how to cite item li, b. & huang, d. (2020). complex failure simulation of underinclined shale slope based on discrete element method. earth sciences research journal, 24(1), 83-89. doi: https:// doi.org/10.15446/esrj.v24n1.85397 depart point of this study was a landslide that occurred in the cut slope at the chongqing west railway station, in central china. this slope belongs to an under-dip shale slope, which means that its bedding dip angle is larger than the slope angle, and it is comprised of soft rock. some on-site investigations were made to explore the deformation characteristics of this slope; the outcome suggested that sliding, buckling, and toppling deformation existed at its different parts. the discrete element method was employed in simulations to elucidate the complex failure mechanism exhibited by the under-dip slope under the long-term influence of gravity and material deterioration. the simulated failure patterns have proven to be in strong agreement with the actual slope failure. this study suggests that sliding, buckling, and toppling occur at different parts of the studied slope in sequence. abstract complex failure simulation of underinclined shale slope based on discrete element method simulación de fallas complejas de la pendiente de lutita con inclinación inferior basada en el método de elementos discretos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85397 este estudio parte de un deslizamiento de tierra en el corte de una pendiente en la estación de ferrocarril oeste de chongqing, en la china central. este corte se encuentra en el echado abajo de una pendiente de lutita, lo que significa que su ángulo de inmersión es mayor que el ángulo de inclinación y está compuesto de roca blanda. se han realizado algunas investigaciones en el sitio para explorar las características de deformación de esta pendiente. el resultado sugiere que existía deformación por deslizamiento, pandeo y caída en sus diferentes partes. para dilucidar el complejo mecanismo de falla que se ve en la pendiente de inmersión bajo la influencia a largo plazo de la gravedad y el deterioro del material, se ha empleado el método de elementos discretos en simulaciones. los patrones de falla simulados han demostrado ser acordes con la falla real de la pendiente. este estudio sugiere que el deslizamiento, el pandeo y el vuelco ocurren en diferentes partes de la pendiente estudiada en secuencia. resumen record manuscript received: 12/03/2019 accepted for publication: 13/12/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 83-89 bin li1, da huang1,2* 1state key laboratory of coal mine disaster dynamics and control, chongqing university, chongqing, china 2school of civil and transportation engineering, hebei university of technology, tianjin 300401, china * corresponding author: dahuang@hebut.edu.cn g eo te ch ni ca l en g in ee ri ng 84 bin li, da huang introduction this paper describes and analyses the complex failure of an under-dip shale slope discovered in chongqing west railway station, which involves three mechanisms: sliding in the upper part, buckling in the lower part and toppling at the toe. the bedding dip angle (68°) of the slope is larger than the slope angle (45°) and it is comprised of shale. shale is a typical soft and fragile sedimentary rock, which is mainly comprised of clay mineral and always has obvious thin foliations called lamellation and the ease with which this rock splits along the direction of the lamellation weakens its resistance to erosion and weathering. shale exhibits strong foliation, the long-term influence of gravity, groundwater, rainwater infiltration, and weathering can weaken the form, resulting in deeply weathered deformations and colluvium, which facilitates shale slopes to become prone to failure. the underlying failure mechanism was analyzed using a combination of geological analysis and numerical models in this paper. recent work on failure mechanisms has concentrated on buckling deformations on consequent slopes where penetrative discontinuities dip into the slopes (cavers, 1981; choquet &tanon, 1986). however, lugeon (1933) suggested that toppling deformation also occurred on slopes where penetrative discontinuities dip more steeply but in the same direction as the slope. such slopes in which the discontinuities dip under the slopes are called under-dip slopes (cruden, 1989). some failures on under-dip slopes were found in the highwood pass in kananaskis country, alberta, canada by tang (1986) and in the yellow head pass by cruden (1989). cruden (1989) showed that, with the assumptions of goodman and bray (1976), toppling could occur on under-dip slopes without the assistance of external forces. several researchers conducted studies on the failure mechanism of the under-dip stratified rock slope. hu (1994) investigated the topples found by tang (1986) and report modes and discontinuities characteristic of topples on under-dip slopes, analyze geological controls of toppling and explain the processes of toppling on under-dip slopes. however, the deformation modes of slopes vary with geological structures, strength of rock blocks and topography. these studies only concentrate on the overall toppling deformations of the under-dip slopes in some particular cases, which is not comprehensive approach, and it has been proved in some engineering examples that the failure mechanism and deformation characteristics of these under-dip slopes prove complicated, which involve a series of processes such as sliding, buckling and flexural toppling deformation. researchers adopted different methods to investigate the failure mechanisms of slopes. numerical modeling techniques such as fem, dem, nmm have also been developed and applied for studying the complex failure of the rock slope (alejano et al., 2010; pritchard & savigny, 1990; pritchard & savigny, 1991). among the various numerical methods, the distinct element code udec has been widely used to investigate a wide variety of rock slope failure mechanisms including those ranging from simple planar mechanisms to complex deep-seated toppling instability and buckling (stead & eberhardt, 1997). although different failure modes of slopes are described in the literature, studies involving the deformation modes discovered in this paper are rarely seen. terrain and geology of the study area terrain of the area chongqingxi railway station under construction is located in the sha pingba district of chongqing. it is a traffic hub which is based on railway and gathers passenger transport, bus, and light railway, which will be the biggest traffic hub in chongqing after the construction is completed. the study area for this paper is the no.6 slope, which ranges from k1+080 to k1+620 (fig. 1). it is excavated on different steps with 10m high of each step, the landslide occurs at k1+500~ k1+520 (fig. 2, fig. 3), the slope studied in this paper is an under-dip shale slope, the bedding plane spacing is 0.1-0.3m. figure 4 presents the engineering geological section of the slope at cross section a-a’. shale is a typical soft and fragile sedimentary rock, which exhibits strong foliation, the long-term influence of gravity, groundwater, rainwater infiltration, and weathering can weaken the form, resulting in deeply weathered deformations and colluvium. geological features of the area figure 5 presents the failure characteristics of the slope at cross-section a-a’ .the length of sliding mass along slope is over 30m, the height is over 8m, the rock mass is intense and intermediary weathered shale, the mechanical properties of the smooth and wet bedding plane is rather poor, it is present with an orientation of 90°/46°, the maximum depth of the sliding mass is 60cm, the sliding distance of rock mass in the upper sections of the slope along the bedding plane is about 1m (fig. 5b), rock mass in the lower sections of the slope is broken off and the stratified rock mass become erect (fig. 5a), which has a height of about 2m and a thickness of about 20cm. there are some weathering cracks distributed in the superficial rock mass (fig. 5c). figure 6 shows the conceptual model of the studied slope based on the geological analysis. figure 7 shows another slope failure at the study area mentioned above, the slumping block extends about 40m along the trend of the slope, with a height of 5-6m, the exposed rock mass is moderately weathered shale, the designed slope angle is 45°, while the excavated slope presents an angle of 68° which equals the dip angle of the bedding plane induced by over-excavation, under the influence of unloading, gravity and weathering, the bedded rock mass slides along the weak bedding plane, then the lower and weakened part of the layered rock mass was squeezed outward by the upper part of the slope, forming buckling folds at the toe of the slope. consequently, the layered rock mass is fractured at the slope toe and the entire slope deforms and slides. figure 1. location and satellite map of chongqing xi railway station (chongqing, china), oct., 2016. 85complex failure simulation of underinclined shale slope based on discrete element method figure 2. engineering geologic plan of the slope. figure 3. general view of the slope failure. figure 4. the engineering geological section of the slope at cross section a-a’. 86 bin li, da huang based on the structural characteristics of the damaged slope, it can be inferred that the failure process of the slope involves several stages as follows: (1) slide-buckling deformation of the upper rock layers the stress release perpendicular to the bedding planes make the rock mass produce spring-back towards the empty direction, consequently, the bedding planes become loose and the strength of rock mass decreases, which facilitates the sliding deformation of rock mass at the upper part along the bedding plane. for the lack of space at the lower sections of the slope for shearing out, buckling at the lower part occurs to coordinate with the compression from the sliding layers (fig.7). (2) toppling deformation at the toe as the buckling deformation intensifies, the rock mass at the buckling slabs becomes fragmentized and rock layers at the front is forced to rotate towards the empty space. finally, with the development of creep deformation, toppling occurs on the rock mass in the leading edge. (3) connection of fracture surfaces as buckling and toppling deformation intensifies, shale breaks and crumbles at maximum bending, then reverse sidesteps and grooves parallel to the strike of the slope generate in the leading edge, and rupture surfaces develop at the bottom of the toppling layers. finally, the rupture surfaces connect with each other and link up with the trailing sliding surface, forming a slidingshearing failure surface, which causes a slope failure. figure 5. the complex slope failure taking place in situ. (a) toppling of the rock mass in the lower part;(b) sliding along the bedding plane in the upper part; (c) cracking of the slope face due to the push of sliding rock columns in the upper part. (d) panoramic view of the landslide. figure 6. conceptual model of the studied slope. figure 7. buckled rock layers at the studied area. 87complex failure simulation of underinclined shale slope based on discrete element method numerical analysis udec numerical simulation numerical simulation is a useful tool for quantitative reconstruction of landslides. the distinct element code udec, introduced by cundall (1987) and lorig et al. (1991), has been widely used to model failure mechanisms of slopes such as toppling and buckling efficiently due to its ability to model large displacement and rotation along block boundaries. the shear strength reduction technique (dawson et al., 1999) was applied to these udec models to simulate the evolution of slope deformations. udec implements an automatic search for factor of safety using the bracketing approach, as described in dawson et al. (1999), so udec solution is a dynamic, time-marching simulation. in this study, the shear strength of the joints and contacts including cohesion, friction angle in the udec models were reduced by the same ratio simultaneously in small increments until a fos of 1.08 was obtained, which was indicative of the failure observed in the onsite slope. every increment of strength reduction in the solution process corresponds to a nonequilibrium state and these nonequilibrium states at different time steps can display the progressive deformation characteristics of the slope model until the final failure. monitoring points named a, b and c for displacement history are set at vertex position, the intermediate position and the toe of slope, respectively. udec model and input parameters fig.8 shows the udec models used in this study. the height of the slope model is 10m with a slope angle of 45°. the joints dip into the model at an angle of 68° with a joint spacing of 0.3m. to better analyze the failure mechanism of the studied slope, the simulation in the udec is conducted with rigid blocks and coulomb-slip joint material model. the block densities are based on the laboratory test results shown in table 1. table 1. mechanical parameters obtained from laboratory tests. parameters intact shale eshale joint density (kg/m3) 2427 uniaxial compression strength (mpa) 12.5 tensile strength (mpa) 0.65 0.03 modulus of elasticity (gpa) 2.3 poisson’s ratio 0.32 cohesion (mpa) 1.86 0.023 friction angle (°) 37 15 for the mechanics analysis, the surface of the slope is assigned as a free boundary, while no horizontal displacement is allowed on both the left and the right vertical boundary, the bottom boundary is completely fixed in the vertical direction but free in the tangential direction. all the boundary conditions of displacement constraint for the numerical model are rigid. in order to better describe the evolution of the slope deformation in the failure process, three displacement monitoring points named point a, point b and point c were set at the upper part, middle part, lower part of the slope model, respectively. all numerical simulations by udec require proper selection of microparameters by means of calibration processes in which the macroscopic responses of the numerical model are compared directly to the observed responses of the physical rock material in laboratory.as macroscopic responses to be looked into, three laboratory scale macroscopic responses including ucs, young’s modulus, and poisson’s ratio were chosen in the calibration process to reproduce the strength and deformability of rock material in udec analyses. in this study, udec models of 50*100mm using various microparameters are adopted to conduct uniaxial compression simulations and an optimum set of microparameters is obtained that gives best agreement in macroscopic responses between the results from simulations and laboratory, the optimum set of microparameters is displayed in table 2. table 2. comparison in macroparameters between the results from simulation and laboratory macroscopic parameters laboratory simulation deviation uniaxial compressive strength (mpa) 12.5 12.62 1% bulk modulus (gpa) 2.3 2.53 10% poisson’s ratio 0.3 0.31 3.3% in order to ensure the veracity of dem modeling, it is necessary to conduct back analysis of mechanical parameters of the joints according to the geological conditions from field measurement and investigation on local rock slope losing stability. figure 9 displays the mechanical model of buckling failure. figure 9. mechanical model of the buckling failure. the following analysis focuses on obtaining instability criterion of this mechanical model. according to the euler theory, the critical slope length lcr for buckling failure is given by l eh cr = ( ) π γ α α− φ 2 2 3 6 cos tan tan (1) where h is the layer thickness; e is the elasticity modulus. after lcr is obtained, the factor of safety k can be defined in the following form k l l cr= (2) where l is the actual slope length. figure 8. udec model used in this study. 88 bin li, da huang for the local slope suffering buckling failure, the bedding plane dips 68°, the actual slope length is 10.8m, the elasticity modulus e is 1500mpa, the thickness of the layered rock mass is 0.1m, when k=1.0, the equivalent friction angle of the bedding plane ϕ equals 14.5°.the stability of the slope is estimated based on the limit equilibrium method, the factor of safety is 0.8 when the equivalent friction angle of the bedding plane ϕ equals 14.5°, while the calculated factor of safety of the slope approximately equals 1.0 when cohesion c is 12kpa and friction angle ϕ is 10°, so the ultimate mechanical parameters of joint sets from back analysis is c=12kpa, ϕ=10°.table 3 describes the parameters for relevant materials included in the udec model for the rigid blocks and contacts. table 3. udec input data for rigid blocks of intact rock and contacts. parameters shale contacts density(kg/m3) 2427 normal stiffness(pa/m) 5e9 6e9 shear stiffness(pa/m) 2e9 3.5e9 cohesion(kpa) 0 12 friction angle(°) 37 10 tensile strength(kpa) 0 1.5 results and discussion according to the simulation results, the evolution process of the deformation at different stages are shown in figure 10. the deformation process can be divided into three stages. at the initial stage, as figure 10a shows, obvious shearing at the trailing edge occurs, sliding of rock mass along the weak surfaces under the force of gravity can contribute to this process. meanwhile, some shearing surfaces occur in the lower part and the toe, which can be attributed to the passive compression caused by the sliding layers from the upper part. at the medium stage, as shown in figure 10c, buckling deformation of layers in the lower part occurs, it can be attributed to the intensifying of compression, which forces rock layers to buckle upward from the inside outwards. in addition, layers at the toe rotates under the force from the buckling layers in the rear and the layers become erect. at the final stage, more shearing surfaces occur in the buckled rock layers with the development of deformation and rock layers at the toe topple outwards (fig. 10e), the force transformed from the buckling and shearing rock mass in the rear may contribute to this toppling deformation. to further describe the deformation process of the slope, the horizontal and vertical displacements of the monitored history points in the deformation process were analyzed, which are displayed in figure 11. three stages called stage i, stage ii and stage iii are proposed to better describe the monitoring results, which refer to the stage of elastic deformation, development of the slide-buckling deformation and the stage of toppling deformation, respectively. clearly, the horizontal displacement of the rock mass near point a is lower than that near point b and point c in the whole deformation process, and the horizontal displacement at point a during stage i is negligible. moreover, the horizontal displacement at point c increases more rapidly than that at point b at stage iii due to the intensifying toppling at the toe where point c locates. we can see that the rock mass at the upper part where monitored point a locates moves downward in the whole process and the displacement increases more rapidly at stage iii, it may be due to the intensifying toppling at the toe which provides more space for the downward sliding of the rock mass at the upper part. moreover, increasing vertical upward displacement occurs to the rock mass at the toe where point c locates at stage i and ii, and then the vertical displacement gradually becomes stable at stage iii with the intensifying of toppling deformation at the toe, but the value of the vertical displacement is much lower than that at point a in the whole deformation process. it also shows that the rock mass at the lower part near the monitoring point b moves upward at stage ii and then turns to moving downward at stage iii, the upward displacement may be caused by the shearing force from the sliding rock mass in the upper part at stage ii and the subsidence may be caused by the intensifying toppling deformation at the toe at the later stage. comparison between the horizontal and vertical displacements at the monitored points in the deformation process indicates again that vertical displacement is dominant for the rock mass in the upper part while horizontal displacement is dominant at the toe. figure 11. displacements of the monitored history points versus numerical time steps: stage i, stage ii and stage iii refer to the stage of elastic deformation, development of slide-buckling deformation and the stage of toppling deformation, respectively. horizontal-a, horizontal-b and horizontal-c refer to the horizontal displacements at point a, point b and point c; vertical-a, vertical-b and vertical-c refer to the vertical displacements at point a, point b and point c. figure 10. plot of numerical simulation results:(a) the 2000 time step displacement contour (b) the 2000 time step velocity vector (c) the 5000 time step displacement contour(d) the 5000 timestep velocity vector (e) the 10000 time step displacement contour(f) the 10000 time step velocity vector. 89complex failure simulation of underinclined shale slope based on discrete element method conclusion this study explored the complex failure mechanism of a shale underdip slope based on an on-site investigation and numerical analysis. it has been proved that the under-dip slope has a complex deformation process in which sliding, buckling and toppling failure occurs sequentially at its different parts. the failure mechanism can be summarized as follows: firstly, stress release of excavation facilitates the sliding deformation of rock mass along the weakened bedding plane, while the sliding process is hindered at the lower sections of the slope for lack of space, then rock layers move by buckling to coordinate with the compression from the upper part, when the buckling deformation develops to some degree, some fracture surface occurs at the sections of rock mass which is subjected to maximum bending effects, and the rock layers in the leading edge are forced to rotate towards the empty space, leading to toppling deformation at the toe. finally, with the aggravation of bending and toppling deformation, fracture surfaces at the buckling folds and the toppling layers connects, then the rock mass slides along the surface gradually, which causes slope failure. the simulating results elucidate the evolution of the complex failure for the particular soft rock slope and the findings will aid in correctly designing rock slopes with similar geological structures as the studied slope. to grasp the failure characteristics of the under-dip slope comprehensively, the influence of geological and geometrical parameters on the failure mechanism will be further studied by sensitivity analysis method in future research. acknowledgements the research is mainly supported by the national natural science foundation of china (no.41672300). besides, data provided by the local engineering department have significantly strengthened the objectivity of the paper. their assists are gratefully acknowledged. references alejano, l. r., gómez márquez, i., pons, b., garcía bastante, f., & alonso, e. (2006). stability analysis of a potentially toppling over-tilted slope in granite. 4th asian rock mechanics symposium. asian society of rock mechanics. singapore, 8–12. ed. university of singapore. alejano, l. r., gomez-marquez, i., & martinez-alegria, r. (2010). analysis of a complex toppling-circular slope failure. engineering geology, 114, 93-104. alzo’ubi, a. k., martin, c. d., & cruden, d. m. (2010). influence of tensile strength on toppling failure in centrifuge tests. international journal of rock mechanics and mining sciences, 47, 974-982. barla, g., borri-brunetto, m., devin, p., & zaninetti, a. (1995). validation of a distinct element model for toppling rock slopes. proceedings of the international 7th congress of the isrm. tokyo. japan., vol. i, pp. 417–421. cavers, d. s. (1981). simple methods to analyze buckling of rock slopes. rock mechanics, 14, 87-104. choquet, p. & tanon, d. d. b. (1986). nomograms for the assessment of toppling failure in rock slopes. proceedings 26th us symposium on rock mechanics, rapid city, 19-30. cruden, d. m. (1989). the limits to common toppling. canadian geotechnical journal, 26, 737-742. dawson, e. m., roth, w. h., & drescher, a. (1999). slope stability analysis by strength reduction. geotechnique 49 (6), 835–840. goodman, r. e., & bray, j. w. (1976). toppling of rock slopes, proceedings, american society of civil engineers, specialty conference. rock engineering for foundations and slopes, vol.2, boulder, colorado, 201-234. guo, a., & lu, s. (1992). simulation of the failure mechanism of mining slope by discrete element method. journal of northeast university of technology. 13, 25-32. pritchard, m. a., & savigny, k.w. (1990), numerical modelling of toppling. canadian geotechnical journal, 27, 823-834. pritchard, m.a. , & savigny, k.w. (1991). the heather hill landslide: an example of a large scale toppling failure in a natural slope. canadian geotechnical journal, 28, 410-422. stead, d., & eberhardt, e. (1997). developments in the analysis of footwall slopes in surface coal mining. engineering geology, 46,41-61. tang, s. w. y. (1986). field examples of toppling induced by external forces, master of engineering report, university of alberta, edmonton, 62p. keywords: west liaoning province; zhangwuheishan area; mesozoic volcanic rock; isotopic age; geological significance. palabras clave: provincia liaoning; área zhangwuheishan; rocas volcánicas del mesozoico; edad isotópica; significancia geológica. how to cite item hou, c., cai, h., & pei, s. (2020). the shrimp u-pb isotope dating of mesozoic volcanic from zhangwu-heishan area, west liaoning province, china. earth sciences research journal, 24(4), 409-417. doi: https://doi. org/10.15446/esrj.v24n4.91114 the zhangwu-heishan area is located in the east of the fuxin-yixian basin. besides the quaternary soil, the study area is mostly covered with volcanic rock. the horizon and age of volcanic rock play an essential role in understanding fossil beds, structures, and sedimentary evolution of west liaoning province and coal seeking. during this work, 11 volcanic rock samples were measured by shrimp u-pb isotope analysis. based on the reported data on the age of the mesozoic volcanic rock in west liaoning province, in combination with new measurement data, cretaceous volcanic activities in west liaoning province can be divided into five stages, namely 132±1 ma, 126±1 ma, 122±2 ma, 115±2 ma, and 100±5 ma. based on statistical results, this paper concluded that the thinning time of the crust in northeast china is from 132±1 ma to 115±2 ma. abstract the shrimp u-pb isotope dating of mesozoic volcanic from zhangwu-heishan area, west liaoning province, china datación isotópica u-pb a través de la microsonda de iones de alta-resolución en rocas volcánicas del mesozoico para el área zhangwu-heishan, en la provincia de liaoning, en el occidente de china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91114 el área zhangwu-heishan se ubica en el este de la cuenca fuxin-yixian. el suelo del cuaternario en el área de estudio está cubierto por rocas volcánicas. el horizonte y la edad de la roca volcánica juegan un rol esencial en la comprensión de las capas fósiles, las estructuras y la evolución sedimentaria de la provincia liaoning, en el occidente de china, y en la búsqueda de carbón. en este trabajo se estudiaron 11 muestras de roca volcánica a través del análisis isotópico shrimp u-pb (datación u-pb a través de la microsonda de iones de alta-resolución). de acuerdo con la información reportada en las rocas volcánicas de edad del mesozoico al oriente de la provincia liaoning, en combinación con información de nuevas mediciones, las actividades volcánicas del cretácico en la zona se pueden dividir en cinco etapas: 132±1 ma, 126±1 ma, 122±2 ma, 115±2 ma, y 100±5 ma. con base en los resultados estadísticos, este estudio concluyó que el tiempo de adelgazamiento de la corteza en el noroeste de china está entre 132±1 ma y 115±2 ma. resumen record manuscript received: 18/11/2019 accepted for publication: 17/08/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 409-417 chaoyong hou*, houan cai, senlong pei china non-ferrous metals resource geological survey, beijing 100012, china *corresponding author: 45139139@qq.com g eo lo g y https://doi.org/10.15446/esrj.v24n4.91114 https://doi.org/10.15446/esrj.v24n4.91114 https://doi.org/10.15446/esrj.v24n4.91114 mailto:45139139@qq.com 410 chaoyong hou, houan cai, senlong pei introduction due to the discovery of precious fossils, west liaoning province has been drawing extensive interests in recent years (chen et al., 1998; sun, 1998; swisher, 1999; xu, 2001; zhou, 2003; zhou, 2003). meanwhile, the researches on the stratigraphy, paleo-geography, paleoclimate, and paleomagnetism of the mesozoic volcano-sedimentary basin in west liaoning province are gradually performed (pan et al., 2001; zhang et al., 2002; zhu et al., 2001 & 2002; gong et al., 2007; li et al., 2002; ren, 1997). as an essential terrane in north china where meso-cenozoic volcanic activities were developed, outstanding achievements have also been made in the researches on the chronology (figure 1) and geochemistry of volcanic rock in west liaoning province (jia et al., 1999; zhou, 2001; zhang & zheng, 2003; yang, 2007; wu et al., 2003). whereas shrimp zircon u-pb isotopic dating of volcanic rock in west liaoning province mainly concentrates on yixian basin, there are only sporadic isotopic reports dating of volcanic rock in zhangwu-heishan area (chen et al., 1997; huang, 2006 & 2007; xiao, 2008). west liaoning province is located in the east section of the yanshan platformal fold belt in the north of sino korean paraplatform and belongs to the east extension of yanshan orogenic belt (figure 1). yanshan orogenic belt lies inside the north china platform and has experienced intense tectonic deformation, magmatic and volcanic activities, and syn-orogenic sedimentation jurassic-cretaceous yanshan movement, and finally becomes a typical intraplate orogenic belt (zhang ch, 1999) far away from any contemporary plate boundary. the subduction of the pacific plate has resulted in ne-trending thrust nappe structure of late jurassic epoch and extensional structure of the early cretaceous epoch. before the mesozoic period, the study area is a large tectonic uplift area. at the first stage of the yanshan movement, the fault-folding formed lower-middle jurassic series in the nne-trending depression area, including coal-bearing strata beipiao formation. at the yanshan movement’s medium setting, a large-scale fracture occurred and was accompanied by a magmatic eruption, and then a mesozoic tectonic framework was formed. denomination and age of volcanic rocks although numerous detailed stratigraphic researches have been carried out in west liaoning province, the knowledge of stratigraphic sequence, the horizon of volcanic rock, and volcanic eruption periods are unified yet. for instance, whether volcanic rock in yixian formation is of late jurassic epoch (wang, 1987; ren et al., 1997; wang, 1998; luo et al., 1999), late jurassic to early cretaceous epoch (zhang et al., 1992; chen et al., 1998; ji et al., 1998; sun & zheng, 2000; jiang et al., 2000; wang et al., 2002) or early cretaceous epoch (li et al., 2001; wang et al., 1999; wang et al., 2001; xu et al., 1999; ji et al., 2002; wang et al., 2005; zhang et al., 2006) is an open question. for more details of the stratal division in this area and volcanic rock’s denomination, see the references wang et al., 2004; cai, 2009). it should be noted that the scientific research institute of liaoning bureau of coalfield geological exploration (1970) advocates that extensive volcanic rock in this area belongs to dalinghe formation and is of cretaceous epoch and concluded that there might be coal seams of fuxin formation under this suite of volcanic rock. likewise, the 107th coalfield geological brigade of liaoning province held in 1974 suggested that coal-bearing series of badaohao formation could not be correlated with shahai formation or fuxin formation in fuxin basin, and gangtaishan volcanic rock overlies badaohao formation and also drew the conclusion that the coal possibly could be found below those volcanic rocks. isotopic dating and eruption period of volcanic rock isotopic dating of volcanic rock in west liaoning province starts in the 1980s (diao, 1983; wang, 1984 & 1985). some fundamental findings have been made since the 1990s (for the data on isotopic dating, see figure 2). by far, over 120 pieces of data derived from various dating methods have been published. these data are systematically collected, and the distribution map (see figure 2) was plotted in this work. from the figure, the age of volcanic rock figure 1. geological map of zhangwu-heishan area and sampling point locations 411the shrimp u-pb isotope dating of mesozoic volcanic from zhangwu-heishan area, west liaoning province, china (zhang, 2003; zhang, 2005). the time when yixian formation was formed is also the critical period when the earth’s crust (lithosphere) in northeast china was thinned, so the age of top and bottom boundaries of volcanic rock in yixian formation is the critical period when the earth’s crust (lithosphere) in west liaoning province, even northeast china, was thinned. conclusions the horizon and age of volcanic rock play an essential role in understanding fossil beds, structures, and sedimentary evolution of west liaoning province and coal seeking. during this work, 11 volcanic rock samples were measured by shrimp u-pb isotope analysis. based on the reported data on the age of the mesozoic volcanic rock in west liaoning province, in combination with new measurement data, cretaceous volcanic activities in west liaoning province can be divided into five stages, namely 132±1 ma, 126±1 ma, 122±2 ma, 115±2 ma, and 100±5 ma. based on the statistical result, this paper concluded that the thinning time of the crust in northeast china is from 132±1 ma to 115±2 ma. which crops out in west liaoning province varies between 56 ma and 147 ma. yixian formation of early cretaceous epoch in the yixian basin contains four suites of volcanic rock and sedimentary rock (ji, 2004), which represent four volcanic-sedimentary activities. as shown in table 1, the periods of volcanic activities are 132±1 ma, 126±1 ma, 122±2 ma, and 115±2 ma. also, a suite of alkaline basalt was formed in the period of 90-105 ma. this is identical to the east part of north china (zhai et al., 2003). there is also a paleogene olivine-inclusion-bearing basalt suit with the age of about 56ma in the zhangwu-heishan area. in 2008 the international commission on stratigraphy adjusted the jurassic-cretaceous boundary from originally 135 ma (140 ma) to 145 ma±4 ma and the age of lower cretaceous–upper cretaceous limit from 95 ma to 99.6 ma. therefore, previously delineated volcanic rock of late jurassic epoch is now of early cretaceous epoch, and the peak period of volcanic activities is the medium stage (130 ma-122 ma or so) of early cretaceous epoch, namely barremian to aptian stage. it is an indisputable fact that the lithospheric mantle in north china craton was thinned on a large scale in the mesozoic and cenozoic periods (gao et al., 2004; liu et al., 2004). the earth’s crust in west liaoning province was also thinned when volcanic rock in the yixian formation was formed figure 2. summarizing of mesozoic and paleogene volcanic rock dating in western liaoning note: data source includes: diao, n. c.; wang, d. f.; guo, h.z.; zhang, z.c.; smith; li, p. x.; wang, s. s.; luo, z. k.; wang, d. y.; zhu, r. x.; zhu, q. j.; zhang, x. h.; peng, y. d.; zhang, h. f.; zhen, j. p.; wang, w. l.; ji, q.; yang, w.; zhang, h.; li, d. m.; huang, h.; shao, j. a.; xiao, g. q.; wang, q. l.; meng, f. x., and this paper. 412 chaoyong hou, houan cai, senlong pei table 1. rock samples and dating results of volcanic in fuxin and heishan-zhangwu area no sample location lithology test method age/ma data source 1 zhuanchengzi in yixian andesite, basalt k-ar 139.2±6.1 diao et al. 2 zhuanchengzi in yixian andesite, rhyolite k-ar 117.1±5 diao et al. 3 zhuanchengzi in yixian andesite rb-sr 125.9±5.3 wang et al. 4 zhuanchengzi in yixian basalt-basaltic andesite k-ar 136.9 wang et al. 5 zhuanchengzi in yixian rhyolite rb-sr 142.5±4 wang et al. 6 wendi countryside rhyolite rb-sr 126±5 wang et al. 7 rhyolite, andesite rb-sr 127±3 guo et al. 8 dakangbo in yixian andesite, dacite rb-sr 127.0±3 zhang et al. 9 jingangshan in yixian dolerite ar-ar 120.8-121 smith et al 10 jiashangou in beipiao basalt ar-ar 121.2±0.3 smith et al 11 huanghuashan in yixian volcanic breccia ar-ar 121.5±0.9 smith et al 12 zoujiagou in yixian basalt ar-ar 121.6±0.4 smith et al 13 central yixian formation zircon u-pb 136.7 luo et al. 14 yixian andesite ar-ar 129.2 chen et al. 15 zhuanchengzi in yixian phosphorite u-pb 136±7 chen et al. rb-sr 130±8 chen et al. 16 hejiagou in yixian andesite ar-ar 119.0±10.0 chen et al. 17 zhujiagou in yixian andesite ar-ar 124.5±4.9 chen et al. 18 dehuiyingzi in zhangwu trachyandensite, dacite ar-ar 123.7±4.4 chen et al. 19 dingjiafang in zhangwu andesite ar-ar 125.9±2.0 chen et al. 20 yemaotai in zhangwu dacite ar-ar 112.0±0.9 chen et al. 21 yemaotai in zhangwu andesite ar-ar 115.0±0.5 chen et al. 22 paozi in fuxin county hyalinite andesite u-pb 116 chen et al. 23 sihetun, jiashangou tuff ar-ar 145.3±4.4 luo et al. 24 sihetun, jiashangou tuff ar-ar 144.8±1.5 luo et al. 25 sihetun, jiashangou tuff ar-ar 147.1±1.8 luo et al. 26 fuxin basalt k-ar 84.76±1.6 zheng et al. 27 sihetun tuff permeates feldspar ar-ar 124.6±0.3 swisher et al 28 sihetun tuff permeates feldspar ar-ar 125.0±0.18 swisher et al 29 hengdaozi sanidine ar-ar 125.0±0.19 swisher et al 30 dijiagou trachyte ar-ar 147.3±0.6 li et al. 31 dijiagou trachyandensite ar-ar 140.4±0.4 li et al. 32 xijianshan section andesite ar-ar 121.9±0.6 li et al. 33 xijianshan section dolerite k-ar 116.7±2.3 li et al. 34 jingangshan in yixian andesite k-ar 120.1±2.0 li et al. 35 xiaoyantun in yixian basalt k-ar 91.81±2.13 li et al. 36 xiaoyantun in yixian andesite k-ar 125.6±0.4 li et al. (continued) 413the shrimp u-pb isotope dating of mesozoic volcanic from zhangwu-heishan area, west liaoning province, china table 1. rock samples and dating results of volcanic in fuxin and heishan-zhangwu area no sample location lithology test method age/ma data source 37 dijiagou—zhuanchengzi basalt k-ar 131.2±2. 6 wang et al. 38 sihetun tuff u-pb 125.2±0.9 wang et al. 39 sihetun basaltic andesite ar-ar 128.2±0.8 wang et al. 40 sihetun basaltic andesite ar-ar 128.4±1.4 wang et al. 41 sihetun basaltic andesite ar-ar 128.4±0.2 wang et al. 42 sihetun dolerite ar-ar 122.3±0.5 wang et al. 43 sihetun dolerite ar-ar 122.1±1.2 wang et al. 44 sihetun dolerite ar-ar 121.8±1.3 wang et al. 45 paishanlou in fuxin dioritic porphyrite u-pb 126±2 luo et al. 46 paishanlou in fuxin granite porphyry u-pb 124±1 luo et al. 47 paishanlou in fuxin dioritic porphyrite u-pb 125±1 luo et al. 48 dashitougou in fuxin biotite granite u-pb 124±1 luo et al. 49 jianguo in fuxin basalt k-ar 92.12±2.08 wang et al. 50 sihetun basalt k-ar 124.16-133.59 zhu et al. 51 zhuangchengzi andesite k-ar 120.42-122.31 zhu et al. 52 zhuanchengzi in yixian andesite k-ar 122.31-120.42 zhu et al 53 jianguo in fuxin basalt ar-ar 105.5±0.5 zhu et al 54 jianguo in fuxin andesite ar-ar 100.4±1.6 zhu et al 55 songjia andesite rb-sr 140±6 zhu et al. 56 yiwulvshan miliolite ar-ar 219±4 zhang et al. 57 yiwulvshan miliolite ar-ar 116±2 zhang et al. 58 yiwulvshan miliolite ar-ar 127±3 zhang et al. 59 sihetun olivine basalt ar-ar 132.9±1.5 peng et al. 60 huangbanjigou liparitic tuff ar-ar 127.4±1.3 peng et al. 61 sihetun ellipsoidal lava ar-ar 126.1±1.7 peng et al. 62 sihetun olivine basalt k-ar 133.3±2.6 peng et al. 63 sihetun olivine basalt k-ar 133.6±2.6 peng et al. 64 sihetun olivine basalt k-ar 124.16±2.4 peng et al. 65 sihetun olivine basalt k-ar 124.42±2.4 peng et al. 66 sihetun olivine basalt k-ar 124.91±2.4 peng et al. 67 houyanzigou olivine basalt ar-ar 130.6±0.5 peng et al. 68 sihetun olivine basalt ar-ar 127.7±0.2 peng et al. 69 jianguo basalt k-ar 107.3 ± 1.6 zhang et al. 70 jianguo ring tephrite k-ar 94.2 ± 1.4 zhang et al. 71 jianguo basalt k-ar 97.1±1.4 zheng et al. 72 jianguo basalt k-ar 98.0±1.4 zheng et al. 73 zaocishan dacite porphyry ar-ar 122.09±0.29 wang et al. (continued) 414 chaoyong hou, houan cai, senlong pei table 1. rock samples and dating results of volcanic in fuxin and heishan-zhangwu area no sample location lithology test method age/ma data source 74 zaocishan dacite porphyry ar-ar 122.36 wang et al. 75 zaocishan dacite porphyry ar-ar 121.80±1.36 wang et al. 76 sihetun basalt u-pb 112±3 ji et al. 77 zhuanchengzi in yixian basalt u-pb 120±2 ji et al. 78 jingangshan in yixian dacite porphyry u-pb 126±0.5 ji et al. 79 zhuanchengzi in yixian andesite u-pb 132±1 ji et al. 80 sihetun tuff u-pb 124.4±1.6 yang et al. 81 huangbanjigou tuff u-pb 123.1±2.1 yang et al. 82 huangbanjigou tuff u-pb 122.1±1.4 yang et al. 83 jianguo basalttrachyandensite ar-ar 116.8±3.0 zhu et al. 84 fanjiagou in yixian rhyolite u-pb 118.9±1.4 zhang et al. 85 fanjiagou in yixian rhyolite u-pb 119.8±1.9 zhang et al. 86 huanghuashan breccia dacite porphyry ar-ar 122.1±0.3 zhang et al. 87 huanghuashan breccia dacite porphyry ar-ar 121.8±1.4 zhang et al. 88 sihetun basaltic andesite ar-ar 126.1±0.4 zhang et al. 89 sihetun basalt k-ar 124.2±2.4 li et al. 90 sihetun basalt k-ar 124.4±2.4 li et al. 91 sihetun basalt k-ar 125.0±2.4 li et al. 92 sihetun basalt k-ar 124.6±2.5 li et al. 93 sihetun basalt k-ar 124.8±2.3 li et al. 94 sihetun basalt k-ar 125.1±2.4 li et al. 95 sihetun basalt k-ar 124.9±2.4 li et al. 96 sihetun basalt k-ar 126.1±2.6 li et al. 97 sihetun basalt k-ar 129.3±2.5 li et al. 98 sihetun basalt k-ar 129.2±2.6 li et al. 99 sihetun basalt k-ar 133.3±2.6 li et al. 100 sihetun basalt k-ar 133.6±2.6 li et al. 101 sihetun basalt k-ar 133.1±2.6 li et al. 102 maozishan in lingyuan tuff u-pb 125.4±0.9 zhang et al. 103 maozishan in lingyuan rhyolite u-pb 125.3±0.9 zhang et al. 104 maozishan in lingyuan rhyolite u-pb 124.4±1.1 zhang et al. 105 guojiagou in lingyuan tuff u-pb 122.3±2.1 zhang et al. 106 dawangzhangzi in lingyuan rhyolite u-pb 123±1.7 zhang et al. 107 zhangwu andesite u-pb 126±2 huang et al. 108 lihenggou in yixian rhyolite ar-ar 113.2±1.6 shao et al. 109 daxingzhuang in linghai amphibole andesite u-pb 114.9±2.5 shao et al. 110 dasijiazi in zhangwu rhyolite u-pb 122.4 ±0.4 ma xiao et al. (continued) 415the shrimp u-pb isotope dating of mesozoic volcanic from zhangwu-heishan area, west liaoning province, china table 1. rock samples and dating results of volcanic in fuxin and heishan-zhangwu area no sample location lithology test method age/ma data source 111 kazuo butte andesite k-ar 125.7 ±4.3 wang et al. 112 kazuo butte andesite u-pb 126.0±1.5 wang et al. 113 lingyuan rhyolite u-pb 124.4±1.4 meng et al. 114 badaohao in heishan trachyandensite u-pb 125.7±1.7 this paper 115 zhangluotun in heishan rhyolite u-pb 124.9±2.9 this paper 116 yaocangtu in heishan coloradoite u-pb 115.5±1.5 this paper 117 nalishan in yaocangtu basalt u-pb 56±2.9 this paper 118 liangshan in heishan trachyandensite u-pb 129.7±1.6 this paper 119 beizhuanchengzi andesite u-pb 132.3±2.3 this paper 120 huanghuashan in yixian dacite u-pb 126.3±1.7 this paper 121 juliangtun in yixian trachyandensite u-pb 125.8±1.9 this paper 122 lijiatai andesite u-pb 127.0±5.1 this paper 123 xinglonggou in yixian andesite u-pb 122.3±4.5 this paper 124 songbahu in yixian andesite u-pb 129.0±2.4 this paper references bing, z. b., liu, w. h., huang, z. a. 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(2001). geomagnetic palaeointensity just prior to the cretaceous normal superchron. physics of the earth and planetary interiors, 128(1-4), 207-222. the ga’erqiong cu-au deposit, which sits on the north side of the coqên-xainzamagmatite belt, is a large-scale skarntype deposit, whose ore body has formed in the skarn zone in the contact part of quartz diorite and marble of duoai formation or the cracks of quartz diorite. its mineralization is closely related to quartz diorite. and granite porphyryrelated molybdenum ore still exists in its deep part. currently, there are disputes about the metallogenic dynamics background of this deposit. from previous studies, this paper carried out zircon la-lcpms u-pb dating and petrogeochemistry study for quartz diorite of ga’erqiong cu-au deposit. the testing result indicates: quartz diorite and granite porphyry were formed respectively in 88±2ma and 83±1ma, belonging to the magmatic activity of the early stage of upper cretaceous; quartz diorite and granite porphyry have geochemical characteristics similar to those of island arc rock of subduction zone and geochemical indexes similar to “adakite.” combining with the regional tectonic evolution, we think that quartz diorite and granite porphyry were all formed in the extension environment after the collision of lhasa block and qiangtang block. quartz diorite is the result of the migmatization of basic melt and acid melt evoked by asthenosphere material raise caused by lower crustal delamination; the formation of granite porphyry may be crust-mantle material’s partial melting results due to delaminated lower crustal. therefore, ga’erqiongskarn-type cu-au deposit belongs to the metallogenic response to the collisional orogeny in the closing process of meso-tethys. abstract keywords: tibet; zircon u-pb chronology; geochemistry; metallogenic dynamics; ga’erqiong cu-au deposit metallogenic dynamics background of ga’erqiong cu-au deposit in tibet, china origen de las dinámicas metalogénicas para el yacimiento de cobre y oro ga'erqiong en tibet, china issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n2.65192 earth sciences research journal earth sci. res. j. vol. 21, no. 2 (june, 2017): 59 65 yuan ouyang1, 2, wunian yang*1, hanxiao huang2, hong liu2, jianlong zhang2, jianhua zhang2 1.key laboratory of geoscience spatial information technology, ministry of land and resources of the china, chengdu university of technology, chengdu, sichuan 610059, china 2. chengdu center, china geological survey, chengdu 610081, china *email of corresponding author: ywn@cdut.edu.cn el yacimiento de cobre y oro ga'erqiong, que se ubica en el lado norte del cinturón coqên-xainzamagmatite, es un depósito tipo skarn a gran escala cuyo cuerpo mineral se formó en la zona skarn, en la parte de contacto del cuarzo de diorita y mármol de la formación duoai y de las grietas de cuarzo de diorita. su mineralización está cercanamente relacionada a los cuarzos de diorita. la mena de molidbeno granítico relacionada a los pórfidos tiene presencia en estas zonas profundas. actualmente, se presentan varias discusiones sobre el origen de las dinámicas metalogénicas de este yacimiento. con base en trabajos previos, este estudio determinó la edad del circón uranioplomo con la técnica la-icpms y analizó la petrogeoquímica de cuarzos de diorita para el yacimiento ga'erqiong. los resultados del análisis indican que los cuarzos de diorita y los graníticos pórfidos se formaron en 88±2ma y 83±1ma, respectivamente, y pertenecen a la actividad magmática de la edad temprana del cretácico superior; los cuarzos de diorita y los graníticos pórfidos tienen características geoquímicas similares a aquellas de las rocas del arco insular en la zona de subducción e índice geoquímicos similares a la "adakita". en combinación con la evolución de la tectónica regional, se concluye que los cuarzos de diorita y los graníticos pórfidos se formaron en el ambiente extensivo tras la colisión de los bloques lhasa y qiantang. los cuarzos de diorita son el resultado de la migmatización de fundición básica y fundición ácida suscitada por el material elevado a la astenosfera gracias a un deslaminado menor de la corteza; la formación de los graníticos pórfidos podría ser el resultado de la fundición parcial de material en el manto de la corteza debido a un deslaminado menor en la corteza. además, el depósito ga'erqiong corresponde a la respuesta metalogénica de la orogénesis colisional en el proceso de cierre del mesotetis. resumen palabras clave: tibet; datación uranio-plomo; geoquímica; dinámicas metalogénicas; depósito de cobre y oro ga'erqiong. record manuscript received: 23/05/2017 accepted for publication: 30/06/2017 how to cite item ouyang, y., yang, w., huang, h., liu, h., zhang, j., & zhang, j. (2017). metallogenic dynamics background of ga’erqiong cuau deposit in tibet, china. earth sciences research journal, 21(2). 59-65 doi:http://dx.doi.org/10.15446/esrj.v21n1.63002 o r e d e po si t s 60 yuan ouyang, wunian yang, hanxiao huang, hong liu, jianlong zhang, jianhua zhang 1. introduction bangong lake-nujiang river metallogenic belt includes not only ophiolite melange belt in suture zone but also the magmatite zone related to the tectonic evolution of meso tethys in the south and north side (geng, pan, wang, peng, & zhang, 2011). the porphyry cu-au deposits, like duobuza and bolong, skarntype cu-au molybdenum deposit like galale, tiangong nile andbaibuta and orogenic-type au deposits like dazha and shangxu discovered in recent years indicate that bangong lake-nujiang river is a giant polymetallic belt mainly of copper, gold, and iron. due to the limitation of the natural conditions of northern tibetan plateau, the prospecting degree of the discovered deposit of bangong lake-nujiang river metallogenic belt is relatively low in general (chen et al., 2013; tang et al., 2013; huang et al., 2012; huang et al., 2014; liu et al., 2015). only a few deposits reach the specific survey level and above. hence, deepening the understanding of the geologic feature and genetic mechanism of current deposits with relatively high working level plays an important enlightening role in determining the next range of reconnaissance and summarizing the regularity of ore formation of the metallogenic belt of bangong lake-nujiang river. ga’erqiong cu-au deposit is a large-scale skarn-type cu-au deposit which is located 20 km west to gyai county, ngari prefecture, tibet. chinese deposit scientists have conducted relatively detailed studies in aspects of the geologic feature, geochemistry of mineral deposits, metallogenetic chronology and diagenetic and metallogenic geodynamics of ga’erqiong cu-au deposit (tang et al., 2013; yao et al., 2013; lei et al., 2012). but consensus hasn’t been reached yet about the metallogenic dynamics background of ga’erqiong cu-au deposit. currently, there are two different viewpoints: first, the formation of ga’erqiong cu-au deposit is related to southward subduction of bangong lakenujiang river (lei et al., 2012; deng, tang, li, yao, & wang, 2011); and second, this deposit was formed in the extension environment after the collision of lhasa block and qiangtang block (tang et al., 2013; yao et al., 2013). given this, by detailed field investigations and previous studies, this paper conducted an analysis of chronology and geochemistry to granide which is closely related to mineralizing relation, deeply explored the cause of formation of this ore-related intermediateacid rock and analyzed metallogenic dynamics background of ga’erqiong cu-au deposit. our results provide a foundation for studies of the mineralization process and metallogenic prediction of ga’erqiong cu-au deposit. 2 metallogenic geological background bangong lake-nujiang river suture zone is an important tectonic line across central qinghai-tibet plateau. on the south and north sides are respectively gangdise block and qiangtang block (zhu et al., 2009). the suture zone is marked by interruptedly distributed ophiolite fragments, which has typical characteristics of accreting orogenic belt (pan et al., 2007). the main body of south qiangtang is jurassic marine basin. some sections of the southern rim have accreting mélange (geng et al., 2011); lhasa block may originate from the north side of australia. it has a distribution pattern of the old stratum in the middle and new stratum in two sides (zhu, zhao, & niu, 2001). the precambrian crystal old base in the middle has experienced multiphase metamorphism and presented unconformable contact with mesozoic volcanic, sedimentary layers. mesozoic and cenozoic marine and interactive marine and terrestrial stratum and volcanic-sedimentary stratum are the main stratum of south and north sides (geng et al., 2011). under the influence of tectonic evolution of meso and neo-tethys ocean, the main tectonic line of north and middle part of lhasa block runs nearly east-west. nearly east-west fracture apparently controlled the space distribution and form of magmatite. large-scale island arc volcanic activities happened in intermediate and late jurassic epoch, which belongs to a set of rock association dominated by andesite. the volcanic has geochemical characteristics of continental margin arc, whose formation is related to the south subduction of meso-tethys. the emplacement of intermediate-acidic rocks mainly happened during the period from early cretaceous epoch to the early stage of upper cretaceous, with stocks and batholiths in nearly east-west zonal distribution and the rock type transforming gradually from type i granite at the initial stage to s-type granite at the later stage (lv et al., 2011). therefore, according to the distribution feature of magmatite, considering shiquan river-lhaguo tso-yongzhu-namtso-lhari figure 1. schematic geological map of the ga’erqiong cu-au gold deposit 1quaternary; 2cinerite; 3volcanic breccia; 4volcanic agglomerate; 5limestone; 6marble; 7hornstone; 8skarn; 9-diorite prophyrite; 10-granodiorite; 11granodiorite porphyry; 12quartz diorite; 13granite porphyry; 14pegmatite; 15fault and inferred fault; 16ore body ga’erqiong cu-au deposit is tectonically located on the south rim of the west of north gangdise (fig. 1a), whose formation is related to limestone and marble of early cretaceous epoch duoai formation (k1d) and intermediateacid rock of upper cretaceous. it belongs to typical skarn-type deposit, but porphyry-type molybdenum ore bodies exist in deep part of the reservoir (tang et al., 2013). intermediate-acid rock and present stratum relation of remarkable intrusive contact. under the influence of emplacement, limestone and mudrock of duoai formation (k1d) are respectively marbleized and hornfelsed (zhang et al., 2013; li et al., 2011). copper (gold) ore body i and ii that have been found are controlled by both surrounding rock and intermediate-acid rock and are generated in the skarns near the contact zone of quartz diorites and marbles and the cracks of quartz diorite respectively. ore body i and ii are skarn-type.metallic minerals such as chalcopyrite, bornite and native gold, etc. were generated together with quartz stockwork, which indicates that the mineralization of copper and gold mainly happened in hydrothermal mineralization period (asis et al., 2017). ore body iii is distributed along structural fracture belt (f1) north to east, which belongs to ore body of iron, copper and gold under control of structural fracture belt and iron-oxide copper gold (iocg) ore body, whose formation may be related to the intrusion of aplite and granite porphyry and activities of north-east fault. also, skarn ore of ga’erqiong cu-au deposit contains rare intermetallic compounds of ni-cr-fe and cu-zn (xiao et al., 2012). fault zone as the boundary, lhasa block can be divided into north gangdise and middle gangdise, while the border of middle gangdise and the further south gangdise arc-ridge fault-uplift zone is ga’er-longgeer-zhari namco-cuomai fault zone (zhu et al., 2009; zhu et al., 2001). 61metallogenic dynamics background of ga’erqiong cu-au deposit in tibet, china 3 sample collection and petrology features the distribution of intrusive body in ga’erqiong cu-au deposit area is relatively extensive (fig. 1c). the intrusive body belongs to a set of intermediate-acid composite rock body, which mainly includes quartz diorite, granodiorite, quartz diorite, aplite, etc. and presents various forms such as stock, apophysis, and dike (tang et al., 2013). the ore body has distinct characteristics of multiperiodic magmatism. the intrusive contact relationship of rock masses in the field suggests that the formation of quartz diorite and granodiorite was relatively early, while quartz diorite and aplite were formed later. four pieces of quartz diorite and four pieces of granodiorite which are closely related to mineralization were taken as samples. the quartz diorite was taken from earth surface far away from tectonic alteration zone; the granodiorite was taken from the north of the mine. both of them are fresh and without alteration. quartz diorite: it is mainly distributed in the middle and south part of the mine. there are irregularly-shaped dark inclusions in the rock. boundaries of the listing and quartz diorite are clear with fine-grained rims and chilled borders. the rock is gray, off-white subhedral granular texture, and massive structure. it mainly consists of anorthose, hornblende, potassium feldspar, biotite, quartz and a few monoclinic pyroxenes and metallic minerals. anorthoses are mainly andesines with medium-fine grains and subhedralclintheriform and girdle structure, and the content is about 60%. hornblende is in the shape of short columns, which partly experienced metasomatism of biotite and chlorite and the content is about 15%. potassium feldspar has no or rare bicrystals, and the content is about 10%. the content of quartz is about 10%, whose fine grains are in irregular shape and filled among feldspars with an uneven distribution. biotite is about 5%. monoclinic pyroxene is relatively less and often in irregular form or short column (fig.2). granodiorite: it is light gray or off-white in fine-grained texture and massive structure. its primary minerals are anorthose, quartz, potassium feldspar, hornblende and a few iron pyrite and biotite, which are mostly subhedralanhedral crystals with a grain size of 0.3 to 3 mm. epidotization and chloritization can be seen in fractures. alteration inside the rock is uneven. figure 2. field photos and micrographs of granitoid in ga’erqiong area 4 analysis method and result 4.1 analysis method analysis of major element, rare earth element, and microelement is completed in southwest china supervision and inspection center of mineral resourced, ministry of land and resources (chengdu center, china geological survey, chengdu institute of geology and mineral resources). according to gb/t 14506.28-93 methods for chemical analysis of silicate rocks, the detected temperature is 18°c to 25°c, and the humidity is 35% to 65%. the major element is tested using axios-x fluorescence spectrophotometer, whose analytical precision is higher than 5%; microelement is tested by icpms method, using plasma mass spectrometry (x-series), whose precision is greater than 5%. the separation of zircon is finished in lang fang cheng xin geo-service co., ltd. backscattered electronic phase, image analysis of zircon cathodoluminescence and zircon u-pb dating are all conducted in china university of geosciences (wuhan) state key laboratory of geological process and mineral resources (gpmr). backscattered electronic phase and cathodoluminescence image analysis mainly aim at confirming zircon particle’s internal structure, determining the test point and avoiding cracks and inclusions. zircon u-pb isotope in situ analysis is conducted by use of agilent7500a inductively coupled plasma mass spectrometry of agilent company and geolas2005 excimer laser ablation on-line system of microlas company (ridzuan, zahar, & noor, 2017). helium is used as a carrier gas of ablation material. the spot diameter of laser beam is 32μm. revision is implemented with standard zircon 91500 used as the external standard and nist610 as the internal standard. data obtained from experiments is processed by icpmdatacal software. after common pb correction, isoplot 4.15 is used for the calculation of the weighted average age of zircon and the drawing of concordia diagram. for method and process, see document (ludwig, 2013). the obtained isotope ratio and age error are on the 1σ level. 4.2 geochemistry for sample analysis result of quartz diorite and granodiorite, see list 1. sio2 content of tested granide is 53.73% to 61.58%, among which the sio2 content of quartz diorite sample is between 53.73% and 56.16% (averagely 55.28%) and that of granodiorite is 60.48% to 61.58% (averagely 61.16%). the na2o content is 0.79% to 5.19% (averagely 3.18%), the k2o content is 2.08% to 2.92% (averagely 2.52%), and total alkali content w (na2o+k2o) is 7.13% to 7.84% with an average of 7.41% which is alkali-rich. the ratio of w (na2o/ k2o) is between 1.56 and 2.51 with an average of 1.98%. the rittman index (σ) is 1.80 to 4.49, which belongs to calc-alkaline rock and falls in high-k calcalkaline series rock area in sio2-k2o diagram (fig.3a); al2o3 content is between 15.10% and 17.39% with an average of 16.26%. aluminum saturation index a/ cnk is between 0.67 and 0.80 (averagely 0.76). a/nk is between 1.42 and 1.64 (averagely 1.51). in cipw standard mineral calculation, diopside (di) appeared, and corundum (c) didn’t, which belongs to quasi-aluminous series (fig.3b). the overall characteristics of samples suggest that granide in this area mainly belongs to quasi-aluminouscalc-alkaline granite series rock (rahim & usli, 2017). 62 yuan ouyang, wunian yang, hanxiao huang, hong liu, jianlong zhang, jianhua zhang the total amount of rare earth element of granodiorite and quartz diorite in this area is between 100 and 155×10-6 with an average of 127×10-6. the rare earth element content of quartz diorite is 100 to 115×10-6(averagely 105×10-6), which is obviously lower than that of granodiorite (σree: 140~155×10-6, averagely 150×10-6). lerr/herr is 5.8 to 1.1 (averagely 8.6) and (la/yb)n is 6.2 to 16.8 (averagely 11.4), which indicates that lree is enriched while hree is depleted, and the fractionation is clear. δeu is averagely 0.9 without obvious anomaly. δce is averagely 1.0, which suggests that the samples have no ce anomaly. the distribution mode pattern of rare earth of granodiorite and quartz diorite is in mild right deviation. the differentiation of two lithologies is evident. the rare earth distribution of quartz diorite is above that of granodiorite (fig. 4a). there is no obvious eu anomaly. tilt level of light rare earth’s curve of all samples is higher than that of the heavy rare earth, which indicates that fractionation degree of light rare earth element is remarkably greater than that of the heavy rare earth element, reflecting the clear differentiation of light and heavy rare earth during magmatism. figure 3. diagrams of sio2 -k2o and a/cnk-a/nk (a: base map from pecerillo & taylor, 1976; middlemost, 1985; b: base map from maniar, & piccoli, 1989.) on first mantle-normalized spider diagram (fig. 4b), the overall characteristics of granodiorite and quartz diorite of ga’erqiong area are similar. they are both “multi-peak” fluctuant curves inclining to the right. there is an obvious differentiation between high field –strength element (hfse) and large ion lithophile element (lile) (fig. 4b), which manifests as enriched in large ion lithophile element and depleted in high field -strength element, among which ta and nb are strongly depleted. figure 4. chondrite-normalized ree (a) and primitive-mantle-normalized trace element patterns (b) or the magmatic rocks in qingcaoshan region(chonodrite values from [21]; primitivem antle from[22] ) 4.3 zircon u-pb age zircons picked from granodiorite, and quartz diorite samples of ga’erqiong area have been tested. check points are all in parts with clear oscillatory zoning structure. for testing position and result, see fig. 5 and list 2. all tested zircons are transparent, colorless and with good crystalline form and typical magmatic zircon dense oscillation girdle. zircon th/u ratio (0.8 to 2.6) is within the magmatic zircon scope (normally metamorphic and hydrothermal origin zircon’s th/u<0.1, magmatic zircon th/u>0.1. zhao, 2010), which matches typical characteristics of magmatic zircon. therefore, zircon analyzed in this paper is magmatic zircon which crystallized with ga’erqiong granodiorite table.1 the major and trace elements characteristic values of granitoids in ga’erqiong area 63metallogenic dynamics background of ga’erqiong cu-au deposit in tibet, china at the same time. for young zircon, 206pb/238u age can provide more reliable test result (compston, williams, & kirschvink, 1992). the 206pb/238u surface ages of 14 analysis points of this test are between 87 and 89 ma, and the weighted average is 88±2ma (mswd=3.8) (fig.5), which represent the crystallization age of granodiorite. the weighted average u-pb age of granite porphyry zircon is 83±1ma, which is about 4 ma later than quartz diorite (yao et al., 2012). 5 results 5.1 petrogenesis geochemical compositions of granitic magmas without remarkable fractional crystallization can reflect the source rock characteristics of the magma. quartz diorite and granodiorite are both enriched in large ion lithophile elements. compared with depleted high-field-strength elements, they show notable characteristics of depleted nb and ta, which is similar to that of island arc magmatite in subduction zone; both of their mineral compositions are generally consistent with those of amphibole calc-alkaline granitoid (acg) under the mechanism of subduction (lei et. al, 2012). in the diagram for discriminating granite ta-yb tectonic setting (fig.6a), all the analyzed samples are classified into the scope of volcanic arc granite. these evidence all indicate that granite is formed in the island arc environment (qu, wang, xin, h. b., zhao, y. y., & fan, 2009; you & rahim, 2017). however, the lateral intraplate magmatism of the crust can also generate characteristics similar to those of island arc granite (plank, 2005). thus, only by combining the regional tectonic evolution can their tectonic settings of the formation be effectively discriminated. current research shows that the bangong lake-nujiang river section of meso-tethys started bidirectional subduction in late jurassic, and after collision and closure had occurred in middle stage of early cretaceous, it got into the intracontinental environment (zhu et al., 2009; wang, zeng, liu, xiao, & gao, 2013). the emplacement of quartz diorite and granite porphyry both happened in late cretaceous and notably after a collision between lhasa plate and qiangtang plate. this indicates that the ga’erqiong cu-au deposit is formed in bangong lake-nujiang river metallogenic belt crash environment, instead of island arc environment. figure 6. tectonic environment (a) and magma mixing trend diagrams (b) of the magmatic rocks in the gaerqiong area (a: base map from [30], b: base map from [31]) at the earliest, adakite refers to the intermediate-acidic igneous rock with unique geochemical characteristics, which is formed by young subducting oceanic crust melting under eclogite facies condition. its geochemical characteristics often manifest as containing sio2≥56%, high al content (al2o3≥15%), mgo content generally <3% (seldomly>6%), y lower than normal island arc (generally≤18×10-6) and hree (e.g. yb≤1.9×10-6), sr higher than normal island arc (seldomly<400×10-6), without eu anomaly or slight negative eu anomaly, and with low initial ratio of 87sr/86sr (<0.704) (defant & drummond, 1990). seeing from the geochemical characteristics, quartz diorite and granodiorite both have geochemical characteristics similar to those of adakite. the research suggests that formation of adakite has various reasons including partial melting of subducted oceanic crust, crystallization differentiation of mantle-derived basaltic magma, migmatization of magma, partial melting of delaminated lower crust and partial melting of thickened lower crust. that lhasa plate and qiangtang plate collided in middle stage of the early cretaceous, which caused shortening, thickening and rapid lifting of the crust, and the low k2o/na2o ratio formed by partial melting of ga’erqiong adakite and subducted oceanic crust does not conform to reality indicates that the ga’erqiong intermediate-acid rocks can’t be formed by partial melting of subducted oceanic crust. no same-period basalt is found in the mining area. the rocks are a lack of notable negative anomaly of sr and eu. in the diagram for feot~mgo (fig.6a), the sample also showed no evolution tendency of crystallization differentiation (you & rahim, 2017). all these indicate the ga’erqiong rock is not generated from crystallization differentiation of mantle-derived basaltic magma. in addition, the ga’erqiong adakite also can’t be the result of partial melting of the thickened lower crust, as ga’erqiong intermediate-acid and acid rocks notably contain more mg# than melt formed by partial melting of thickened lower crust or underplating basalt. there is higher content of cr and ni; zirconhf isotopes of the rocks show notable characteristics of mantle-derived or mantle-crust mixed source (yao et al., 2012). these indicate that there is mantle-derived material taking part in the diagenetic process. dark micro granular inclusions are generally considered as a product of halfway migmatization of mantle-derived basic magma, which is the most direct and powerful evidence for migmatization of magma. irregular dark micro granular inclusions are found in quartz diorite. the inclusions are dioritic type with distinctive boundary with its host rock and chilled border, which indicates the inclusions are not the residual after partial melting, but the product of magma crystallization. also, the zircon hf isotope system has very high closure temperature which won’t be changed along with partial melting or fractional crystallization. this can be used for identifying magma migmatization between different magma end members. zircon hf isotopes of quartz diorite have significant inhomogeneity (yao et al., 2012), which indicates the magma source of quartz diorite has occurred magma migmatization. in the sio2-k2o and sio2-la second variant diagram, quartz diorite showed both good linear variation trends. all these evidence show that, during formation of quartz diorite in ga’erqiong, the second end-member magma migmatization occurs. therefore, quartz diorite might be formed in the post-collision extension environment after the collision of lhasa block and qiangtang block; lower crust delamination figure 5. cathodolum inescence (cl) images of representative zircon (a) and concordia diagrams (b) for zircons form the magmatic rocks table 2. la-icp-ms zircon u-pb dating results in ga’erqiong region 64 yuan ouyang, wunian yang, hanxiao huang, hong liu, jianlong zhang, jianhua zhang caused uplifting of asthenosphere material, which further caused partial melting of lithospheric mantle and let basic melt mix with the acid melt. emplacement of granite porphyry is significantly later than quartz diorite, and there are no dark micro granular inclusions found in the rock, which implies that there might be different reasons for the formation of granite porphyry and quartz diorite. the granite porphyry is rich in k, which indicates it comes from thickened lower crust. the qinghai-tibet plateau started crust shortening and thickening in middle stage of the early cretaceous. thickening of the crust caused the formation of eclogite at the bottom of the lower crust. due to gravity differences, the eclogite-facies lower crust is easier to have delamination; what’s more, bimodal volcanic rocks appeared in nyixung region indicates that in the early stage of late cretaceous, the north-central area of lhasa block was in postcollision extension environment (qu, xin, xu, yang, & li, 2006). hence, partial melting of crust-mantle material caused by delamination of lower crust might be a major reason for the formation of ga’erqiong granite porphyry. 5.2 geodynamic background of mineralization there are still disputes about the geodynamic mechanism of the cretaceous volcanic rock in north and middle part of lhasa block. so far, the mainstream opinions include: it’s generated from neo-tethys crust’s northward subduction. it’s related to re-melting of thickened (delaminated) lower crust during or after collision of the qiangtang block and lhasa block. it’s related to southwards subduction of the meso-tethys oceanic crust and slab breaking off (zhu et al., 2009; harris, inger, & ronghua, 1990; coulon, maluski, & bollinger, 1986). in recent years, many late cretaceous magmatite are found in south part of the lhasa block. these geological phenomena can’t be explained by plate subduction model of the neo-tethys crust. thus, quartz diorite and granite porphyry in ga’erqiong can’t be the product of neo-tethys crust’s plate subduction. in the early stage of the early cretaceous, neo-tethys had occurred oceanic-ridge subduction (zhu et al., 2009). both quartz diorite and granite porphyry in ga’erqiong have characteristics similar to those of island arc magma, and they are probably the product of neotethys’ oceanic-ridge subduction. in the second place, bangong lake-nujiang river meso-tethys had been closed at that time, and the magmatism of crustmantle material caused by slab break-off during its southwards subduction also has the features of island arc magma. so, some scholars hold the opinion that late cretaceous magmatite in middle gangdese is the common outcome of the tectonic evolution of meso-tethys and neo-tethys (wang et al., 2013). as ga’erqiong is about 300km away from the yarlung-zangbo suture zone, it’s still lack of enough evidence so far to prove that the formation of intermediateacid rock in ga’erqiong is related to oceanic-ridge subduction of neo-tethys in early stage of late cretaceous. on the contrary, ga’erqiong is not so far from the bangong lake-nujiang river suture zone. therefore, we think the formation of intermediate-acid rock in ga’erqiong is more likely to be the magmatic response of meso-tethys’ tectonic evolution. judging from the geologic features, the formation of ore body i and ii in ga’erqiong cu-au deposit is closely related to magmatic emplacement of quartz diorite. there has been testing data to support the result of the field inspection. zircon u-pb age of the quartz diorite is 88±2ma, which is consistent with the re-os isotope age (86.89 ma) of molybdenite in skarn (li et al., 2011). it indicates that 1ma after the quartz diorite emplacement, the hydrothermal activity of magma formed the cu, au ore body. zircon u-pb age of the granite porphyry is 83±1ma, which is significantly later than that of quartz diorite, and its mineralization is likely to be related to the subsequent mineralization of porphyry molybdenum (yao et al., 2013). research suggests that, the bangong lake-nujiang river meso-tethys ocean basin opened in late triassic, and after ephemeral expansion, it started subduction in late jurassic and resulted in formation of shiquan river – gêrzê subduction zone and rutog subduction zone separately in the south and north, and cultivation of abundant island arc magmatite on both sides of the suture zones at the same time (qu et al., 2009). in the middle stage of early cretaceous (around 113ma), the lhasa block and qiangtang block had a collision, and after that, they entered into the arc-arc (continent) collisional evolution stage. the meso-tethys oceanic crust subducted southwards underneath the lhasa block. slab break-off caused upwelling of mantle-derived material and further resulted in upwelling of mantle material, which had mantle-derived magma underplating in different scales on both sides of the suture zones (zhu et al., 2009). after that, crust in north and middle part of lhasa block kept thickening under the compression of north-south stress and finished shift from intracontinental compression to a partial extension in the early stage of late cretaceous (rahim & usli, 2017; you & rahim, 2017). the thickened lower crust reached eclogite facies and started delamination, forming a series of magmation along with mineralization. it thus appears that the formation of ga’erqiong cu-au deposit is due to the metallogenic response of collision orogeny during the closing of meso-tethys. 6 conclusions (1) u-pb age of zircon from quartz diorite that is closely related to ga’erqiong cu-au deposit and mineralization is 88±2ma, and it’s product of magmatic activity in the early stage of late cretaceous. (2) quartz diorite and granite porphyry have geochemical characteristics similar to those of island arc granitoid in a subduction zone, and they are formed in the post-collision extension environment after the collision of lhasa block and qiangtang block. quartz diorite formed after asthenosphere material uplifting caused by lower crust delamination and is the mixture of basic melt formed by partial melting of lithospheric mantle and acid melt formed by partial re-melting of crustal materials. and granite porphyry is formed by partial melting of crustmantle material caused by lower crust delamination. 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(2001). the lhasa terrane: record of a microcontinent and its histories of drift and growth. earth and planetary science letters, 301(1), 241-255. zorpi, m. j., coulon, c., & orsini, j. b. (1991). hybridization between felsic and mafic magmas in calc-alkaline granitoids: a case study in northern sardinia, italy. chemical geology, 92, 45-86. keywords: wulingyuan; scenic area; peak forest landscape; spatial structure; characteristics; optimization. palabras clave: wulingyuan; pilares de arenisca; estructura espacial; características; mejoramiento. how to cite item zhang, q. (2020). study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area. earth sciences research journal, 24(1), 35-44. doi: https://doi.org/10.15446/esrj. v24n1.85206 to protect the ecological environment of peak forest landform and maintain its integrity and stability, the optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area was studied. by using gis and digital topographic analysis to study the basic features of sandstone peak forest landform, wulingyuan peak forest landform and karst peak forest have great differences in lithological composition, as the weathering resistance is better than cheltenham badland landform; by using tree theory to analyze the features of wulingyuan peak forest landform, according to area weight serialization of contour tree nodes, we can know the depression area. the karstification is stronger than that of the peak forest area, and the surface is relatively fragmented. based on different landscape indices, the landscape pattern of wulingyuan peak forest is analyzed. the fragmentation degree of vegetation is lower, and the fragmentation degree of building landscape is first increased, then decreased, and finally intensified. the proportion of artificial landscape decreases year by year and the trend of fragmentation is obvious. based on the present situation of spatial structure characteristics of wulingyuan peak forest landform, optimization methods such as combining centralization with decentralization and improving the quality of artificial landscape ecosystem were put forward. abstract study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area estudio sobre el método de optimización de las características de estructura espacial en los pilares de arenisca de wulingyuan issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85206 para proteger el entorno ecológico de los pilares de arenisca en el área de wulingyuan y mantener su integridad y estabilidad se estudió el método de optimización de las características de la estructura espacial. al utilizar sistemas de información geográfica (gis) y el análisis topográfico digital para estudiar las características básicas de la forma de los pilares se estableció que la forma de relieve forestal del pico de wulingyuan y los pilares de arenisca tienen grandes diferencias en la composición litológica, mientras que el nivel de resistencia a la meteorización es mejor que en la zona de cheltenham badland; al utilizar la teoría de los árboles para analizar las características de los pilares de wulingyuan, de acuerdo con la serialización del peso del área de los nodos del árbol de contorno, se estableció el área de depresión. el índice de karstificación es mayor que el del área del bosque de pilares de arenisca, y la superficie está relativamente fragmentada. basado en diferentes índices de paisaje, se analizó el patrón de paisaje del los pilares del bosque wulingyuan. el grado de fragmentación de la vegetación es menor, y el grado de fragmentación de la construcción del paisaje primero aumenta, luego disminuye y finalmente se intensifica. la proporción de paisaje artificial disminuye año tras año y la tendencia de fragmentación es obvia. basado en la situación actual de las características de la estructura espacial de los pilares de wulingyuan se presentaron métodos de optimización como la combinación de la centralización con la descentralización y la mejora de la calidad del ecosistema del paisaje artificial. resumen record manuscript received: 27/07/2019 accepted for publication: 13/11/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 35-44 qian zhang* local cooperation office, jiaozuo teachers college, jiaozuo, 454000, china * corresponding author: jzszxqhzczq@126.com g eo m o rp h o lo g y 36 qian zhang introduction environment and development have become a common concern in today’s world. ecosystem is the link between environment and development. it not only meets the needs of material products and environmental services for human survival and development, but also plays an important role in maintaining global ecological balance and protecting biodiversity (sylvestre et al., 2017). wulingyuan world natural heritage situated in the northwest of hunan province is famous for its unique quartz sandstone peak forest landform. sandstone is a kind of rock formed by weathering, transporting, sedimentation and diagenesis. it has the characteristics of low strength, poor weathering resistance and little cementing material. it is the main part of the peak forest landform of wulingyuan scenic area. michael crosser, president of the international association of geomorphologists, said at the opening ceremony that similar geomorphological features exist in the world, but the geomorphology of zhangjiajie should be recognized by the world because it lacks such distinctive features and scale and integrates aesthetic feeling and scientific research value. zhangjiajie, a sandstone landform, is very rare in china and even in the world, and its formation conditions are very harsh (colgan et al., 2017). therefore, in order to protect the ecological environment of the peak forest landform in wulingyuan scenic area and maintain the integrity and stability of the peak forest landform, it is necessary to study the optimization method of the spatial structure characteristics of the peak forest landform in wulingyuan scenic area. at present, there are many studies on the peak forest landform of wulingyuan scenic area in zhangjiajie in china. qi chengjing and fang yingcai have visited zhangjiajie for plant investigation and specimen collection. zhang zhicai and others have carried out research on the types of forest resources, community structure and distribution characteristics, and the function of ecosystem in the peak forest landform of wulingyuan. based on the research and discussion, the countermeasures and suggestions are put forward in the development planning, evaluation of ecological benefits and construction and development ideas of peak-forest landform. liang mingzhong and others use spatial index and spatial statistical technology to describe the spatial attributes of peak-forest landform and study the ecological characteristics of the spatial structure of peak-forest landform. this paper studies the spatial structure characteristics of peak forest landform in wulingyuan scenic area from three aspects: basic landform characteristics, surface morphology characteristics and geomorphological spatial pattern, and puts forward rational optimization methods to provide basis and reference for the ecological environment protection of wulingyuan scenic area (jiang, 2017). research areas and research methods survey of research areas wulingyuan is located at eastern longitude 110°20’30”~110°41’15”, northern latitude29°16’25”~29°24’25”. it is the junction of zhangjiajie city in northwest hunan province with cili and sangzhi counties, with a square area of 369 km. it consists of four distinctive nature reserves: zhangjiajie, tianzishan, suoxiyu and yangjiajie. there are still scenic areas under development (cole et al., 2018) in its periphery, such as maoyan river, jiudian cave and tianmen mountain. zhangjiajie is the first national forest park established in china (1982), so it is often called wulingyuan scenic area by zhangjiajie. wulingyuan belongs to the national first-level protected area with an area of 53.5 square kilometers, the second-level protected area with an area of 55.5 square kilometers and the third-level protected area with an area of 203 square kilometers. there are fourteen 70 km tourist routes and more than 560 scenic areas (zhang et al., 2017). wulingyuan scenic area has ravines, high rock peaks, green trees and green vines, and flocks of animals and birds. it has the reputation of “the maze of nature” and “the world’s first strange mountain”. the landform of wulingyuan scenic area belongs to the quartz sandstone peak forest landform. there are as many as 15,000 peaks in wulingyuan. the peaks are distributed at an altitude of 500-1100 meters, ranging from tens of meters to 400 meters. this special landform is named “quartz sand pillar peak”. as shown in figure 1. the geomorphology of quartz sandstone peak forest is characterized by pure quality and stone thickness, quartz content is 75%-95%, and rock thickness is more than 520 meters. wulingyuan has a complex terrain, mild climate, abundant rainfall, lush forests and primitive sub-forest plant communities. its forest coverage rate is 88%. there are 4 species of first-class protected plants and 19 species of second-class protected plants in wulingyuan. there are 770 species of woody plants, of which pinus wuling is the only one in the region. in addition, there are tertiary relics such as davidia involucrata, bole tree, taxus chinensis, taxus chinensis and cephalotaxus angustifolia. wulingyuan has 28 kinds of state-level protected animals. there are also famous giant salamander (doll fish) precious animals growing in wulingyuan, as well as the first-level protected animals such as clouded leopard, leopard and other large beasts. in december 1992, wulingyuan scenic area was listed on the world natural heritage list (moreno et al., 2017) by unesco as a world famous tourist attraction. the formation factors of peak-forest landform are as follows: firstly, the rock is quartz sandstone with pure quality, mainly quartz, thick layer, hard and stable, which is beneficial to natural sculpture and image enhancement; secondly, the occurrence of rock stratum is gentle, which increases the stability of rock, which provides the precondition for the formation of peak-forest landform; thirdly, two groups of sags are developed in rock stratum. straight joints, generally 15 to 20 meters apart, provide the tectonic conditions for shaping the peak-forest landform. under the background of intermittent uplift of regional neotectonic movement, the effects of flow erosion cutting, physical weathering, chemical weathering and biological root splitting and other external forces begin with rock joints, which widen the fissures gradually and evolve over time to form peak forest landform. its characteristics are high, flat and steep (klongvessa et al., 2018). figure 1. geomorphology of quartz sandstone peak forest in wulingyuan scenic area experimental data this experiment chooses 30 m grid resolution srtm data, 1 m dom data of high resolution remote sensing image, fast bird image data and world view data as main information sources, supplemented by 1:10,000 scale topographic map and other related data (payo et al., 2017) from the completed national geographic database. in addition, the urban construction archives of the study area since 1990 were consulted, and a large number of data on social, economic and environmental changes (li et al., 2018) were collected. the characteristics of the experimental data are shown in table 1. extraction of topographic features of peak forest landforms based on tree node serialization in the process of extracting the topographic features of wulingyuan peakforest, based on the theory of karst geomorphology, hydrology and mathematical statistics, using gis spatial analysis and digital topographic analysis (davies et al., 2018), using the combination of qualitative and quantitative analysis indicators, macro and micro analysis, and theoretical analysis. combining 37study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area table 2. landscape indicators for research landscape indicators formula for calculating indicators indicator unit scope and meaning of indicators patch type area ca aij j n = = ∑ 1 hectare ca > 0, no upper limit, representing landscape composition patch density pd n ai= / number/ hectare pd > 0, no upper limit, characterizing the degree of fragmentation percentage of patch type area pland p a ai ij j n = = × = ∑ / 100 1 pland p a ai ij j n = = × = ∑ / 100 1 % 0 < pland < 100, characterizing the dominance of each type of plaque average plaque area mps a nij i j n = = ∑ / 1 hectare mps > 0, no upper limit, indicating plaque size maximum plaque area ratio max /p ai × % characterization of maximum patch dominance fractal dimension of plaque frac p aij ij= log / log 1 < frac < 2; frac = 1 is round; frac = 2, long strip landscape diversity index shdi p inpi i i m = ×( ) = ∑ 1 shdi > 0, no upper limit; shdi = 0, only one plaque results and optimization geomorphological characteristics of sandstone peak forest wulingyuan peak-forest landform is situated on the upper part of the mountain body above 1000 meters above sea level. the edge of the mountain body or the valley zone develops best. because of the cutting of the valley and the collapse of the rocks on the edge of the mountain body, the edge of the peak forest is generally a steep cliff. the rock wall is bounded by a clear unconformity of caledonian crustal movement, and the strata are mainly composed of the mesozoic and paleozoic strata. the upper devonian yuntaiguan formation and huangjiadun formation are composed of littoral clastic rocks (liang et al., 2017). the rocks are pure, thick, flat in base and well developed in vertical joints. the rocks are exposed in syncline outline, which reflects the special geological structure environment and basic conditions for the formation of sandstone peak forest landscapes. running water erosion and gravity collapse, biochemical and physical weathering effects of external geological activities are indispensable external conditions for shaping wulingyuan landform landscape. therefore, the formation of sandstone peak forest landform is the result of the long-term interaction of internal and external geological gravity in a specific geological environment, coupled with the intermittent uplift and tilt of regional neotectonic movement, flow erosion and cutting, gravity, physical weathering, biochemistry and root splitting and other external forces. peak forests are formed according to the complex natural evolution process, showing the characteristics of high, flat and steep walls. (catania et al., 2017). basic morphological characteristics sandstone peak forest landform is a group landform, its basic form is cliff, peak (column), cliff channel, cliff lane. with application validation, using multi-resolution dem data as the basic data source, exploratory study on the geomorphological characteristics of sandstone peak forest was carried out by using tree theory (hart, 2017). because the number of nodes in each layer of the contour tree is different, the size of the chain structure transformed by the weight of the nodes in each layer according to the area represented by the contour is also different (wang et al., 2019). therefore, in the specific analysis and application, the tree node serialization method based on area weight is used to intercept the standard table structure with the same growth degree from the chain structure of different lengths, so that the classical mathematical statistics method can be used for analysis. the principle of tree node serialization is shown in figure 2. determination of evaluation index for landscape pattern analysis landscape pattern analysis is the core content of landscape ecology. quantitative analysis of spatial distribution characteristics of landscape components is the basis for further study of landscape function and dynamics (goad & korista, 2018). the impact of human activities on the landscape spatial pattern of wulingyuan peak forest (changqing et al., 2017) was measured by landscape patch type area, patch density, patch type area ratio, average patch area, maximum patch area ratio, patch fractal dimension and landscape diversity index. the landscape of wulingyuan peak forest was studied separately. the landscape indices used in this study are shown in table 2. table 1. characteristics of experimental data type dem data remote sensing image quartz sandstone peak forest landform data sources nasa center website domestic and foreign websites data sales company scanning drawings analytical objectives core data for topographic feature extraction, spatial analysis, etc. basic data sources for various experiments correction of the peak position of peak-forest peaks and clusters auxiliary data for comparative analysis of terrain spatial differentiation characteristics figure 2. sequencing schematic diagrams of area-weighted nodes 38 qian zhang (1) cliff: cliff, which is the largest and most spectacular landscape in the peak forest, can be divided into two cliffs, edge and interior, according to their different locations. the edge cliff is located at the edge of the peak forest area, which extends tens of kilometers intermittently. the local area is thousands of meters to more than ten kilometers long, tens of meters to more than one hundred meters high and 530m high. the wall is steep, like a copper wall and iron wall. the trend of cliff wall in peak-forest is controlled by two groups of joints. they are nw and ne. they are generally more than 100 meters wide and tens of meters high. most of them are vertical cliffs with an inclination of more than 70°. all kinds of sedimentary phenomena on the wall are clearly visible. (2) peak (column): that is, the height of the peak forest is greater than the width of the stone peak and pillar, is the most basic morphological landscape of sandstone peak forest landform. the peak forest consists of thousands of single stone peaks and columns. peak shape has square column, flat column, cone, bamboo shoot, peak wall, tower, rod, etc. according to the morphological characteristics of the peak body, it can be divided into the following combinations. square columnar peak forest includes square columnar and flat columnar peak forest, as shown in figure 3. peak forests are usually composed of several or dozens of columns with diameters ranging from several meters to tens of meters and height ranging from tens of meters to more than 100 meters. after the rock is cut by two groups of conjugate vertical joints, some surrounding rocks collapse under the action of gravity. figure 3. columnar peak forests conical-columnar peak forest: the peak column is conical in shape with fine upper and thick lower, usually tens of meters in height and several meters to tens of meters in diameter at the bottom. its formation is related to lithology. sandstone in the lower part of the peak body is dominant, and mudstone in the upper part is increased. it is formed by differential weathering due to the different degree of rock hardness and softness. bamboo shoot-like peak forest: the top of the peak is thin and thick, the base is connected, and the top is bamboo shoot-like. generally, the height is more than several meters, and the scale is smaller than the conical peak forest. the rock is cut by dense vertical joints. peak wall peak forest: horizontal rock stratum is cut by vertical joints to form a cliff, side as a wall, is seen as a peak, wall direction is controlled by joint direction. hieroglyphic peaks: most of them are single or solitary peaks, varying in size and height, such as tower, rod and column shapes. they have various shapes, such as deer, pipe, lotus flower, general, duck head, snake head, and so on. they are similar to human beings. one scene at a time, they are formed by different lithological weathering and rock collapse. (3) cliff groove: refers to the vertical groove landform landscape in which the depth and height of the peak forest are greater than the width, also known as vertical groove, formed by the collapse of rocks in two or several vertical joints. the cliff trough is usually tens of centimeters wide to several meters, with a horizontal depth of 1 m to several meters and a height of 10 m to tens of meters. its width, height and orientation are related to the density, spacing and orientation of joints. it is a common micro-landform landscape in peak forest landform. (4) cliff alley: horizontal roadway with narrow and deep shape is the result of the development of the depth of the cliff groove toward the horizontal direction. its width is generally only 2-3 m, while its height and depth are tens of meters to more than 100 meters. the roadway wall is upright, very similar to a line of sky or a sky roadway. in addition to the above types of morphology, there are also some small landscapes in peak-forest area. they are combined with peak forest, valley, waterfall, forest vegetation and exotic flowers and trees to form a gorgeous landscape, which is described as a fairyland on earth. analysis and comparison peak forest refers to the landscape composed of separated stone peaks or pillars, most commonly in karst area, clastic rock area, granite area and cheltenham badland area are well developed, but far less developed than karst area. wulingyuan sandstone peak forest belongs to clastic rock peak forest. its characteristics are similar to karst peak forest and cheltenham badland landform, but there are also many obvious differences, as shown in table 3. (1) compared with karst peak forest. karst peak forests in china are widely distributed in guangxi, guizhou and eastern yunnan. they are mainly pure and hard carbonate rocks of devonian, carboniferous and permian with thickness of more than kilometers. they have been developed for a long time in hot and humid climates. the karst peak forest generally develops in the plain area or valley on the edge of peak cluster karst. it can be divided into three types: basin plain type, peak cluster valley type and edge slope valley type. stone peaks in peak forests stand apart from each other and vary in density. in plain areas, solitary peaks are dominant. they are pyramidal in shape, partly conical or saddle in shape and below 150 m in height. after being dissolved by water solution, caves on the peaks develop and are known as caves without caves. the main difference between wuling mountain sandstone peak forest and karst peak forest is that the lithological composition is completely different: the forming conditions of the two forests are different. the karst peak forest is mainly formed by water erosion and cave development, while the sandstone peak forest is mainly formed by flowing water erosion, physical weathering, chemical weathering and biological root splitting. (2) comparing with cheltenham badland landform. cheltenham badland landform in china is mainly distributed in renhua of guangdong, wuyimountain of fujian, longhu mountain of jiangxi, dajinhu of fujian, yuntaimountain of henan, yuntaimountain of guangxi, liangmountain of hunan, and the most typical landform of renhua of guangdong is named cheltenham badland landform. the cheltenham badland geomorphic diagenesis age in southern china is mainly the red rock series of the white seal period. its lithology is purple-red conglomerate, sandy conglomerate and coarse sandstone. the rock strata are nearly horizontally produced. the main bedding troughs, flat caves, frontal caves and rounded ridge peaks are formed by a variety of factors. compared with cheltenham badland landform, wulingyuan sandstone peak forest has similar lithology, color and danya landform. the difference is that the formation conditions and morphological types are different. sandstone peak forest rocks are fine clastic rocks with harder texture, strong weathering resistance and more developed danya landform. cheltenham badland landform rocks have coarse grain size and weak weathering resistance. there are obvious differences between bedding troughs (caves), rounded ridges and steamed bread ridges and sandstone peak forests. analysis of peak cluster shape characteristics based on tree node serialization taking the peak forest landform research area of wulingyuan scenic area as an example, the nodes of contour trees are serialized according to area weight. in the actual serialization process, the contour tree is constructed for the five geomorphic units of wulingyuan scenic area: depression, stone peak, compound depression, peak forest and peak-depression combination. only the first four nodes of each layer are intercepted, and the first node of each layer is recorded as the parent node. the first three nodes are sorted according to the size of the area. it is designated as the first child node, the second child node and the third child node. because the contour trees constructed by simple shape (stone peak and depression) are single-branch trees, and the structure is simple, four nodes are truncated downward from the root node, and the root node is 39study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area recorded as the parent node. according to the subordinate relationship between nodes, the first child node, the second child node and the third child node are recorded in turn. finally, based on the order of area serialization, the roundness coefficient c and the compactness coefficient t of the parent node and the child node are calculated respectively, and the corresponding confidence interval diagrams of roundness coefficient and compactness coefficient 95% are drawn. the results are shown in figures 4 and 5. from figure 4 and 5, we can see that the roundness coefficient c of wulingyuan peak forest landform gradually increases from parent node to child node, and the compactness coefficient t gradually decreases from parent node to child node. it shows that the shape of the sub-nodes is closer to the circle than that of the parent nodes in five different geomorphic units, whether roundness coefficient or compactness coefficient is used as a measure. for composite terrain (such as fung shek group, composite depression, combination of peaks and depressions), the shape of the geomorphic units nested step by step in its interior tends to become circular gradually. comparing the roundness and compactness coefficients of different geomorphological types, it can be found that the more complex the combination of landforms (such as the combination of peaks and depressions), the narrower the surface shape, the more complex the surface morphology. at the same time, because the outcrop of carbonate rocks is more obvious in the depression area, and most precipitation flows into the depression area, the karstification in the depression area is stronger than that in the fung shek area, and the surface of the depression area is more fragmented and fragmented, so the range of roundness coefficient and tightness coefficient in the depression area is larger than that in the fung shek area. it can be seen that the structure of contour tree can not only directly reflect the hierarchical nesting characteristics of peak cluster surface morphology, but also contain the related information of different levels of geomorphic units, so as to facilitate the analysis of the morphological characteristics of geomorphic units at different scales. geomorphological spatial pattern of sandstone peak forest change of vegetation landscape in the past 30 years, the vegetation in the study area has been growing and expanding. before tourism development, as in other areas of the country, under the influence of grain policy, deforestation and deforestation were very serious. except for the quartz sandstone peak forest landform, steep rock cliffs and some places with large slopes, most areas had reclaimed wasteland and few patches of vegetation around villages. vegetation coverage is low, the change of landscape characteristics of vegetation cover in wulingyuan scenic area is shown in table 4. table 3. comparison of geomorphological characteristics of different peak forests type lithology and age protolith formation environment peak forest morphology major forming factors geomorphology of sandstone peak forest in wulingyuan upper devonian sandstone the occurrence of rock layers is near horizontal and forms in river deltas on the land margin. the shape of peaks is varied, mainly columnar, with joints and vertical grooves developed. water erosion cutting, physical weathering, chemical weathering and biological root splitting, etc. karst peak forest in guangxi carbonate rocks from middle mupan to permian the occurrence of the strata is near the level and formed in the foothill, current and lake environment. the peak body is mainly pyramidal, and the peak body is large. the valleys, depressions and caves are developed. surface water and groundwater are mainly used for erosion and transportation. cheltenham badland landform peak forest in guangxi purple-red conglomerate and sandstone dominated by the bainig period the occurrence of rock strata is near the level and formed in foothills, rivers and lakes. the peak body is bread-like and rounded, the peak is gentle, the annular structure and horizontal groove, the pot hole is developed. epigenetic weathering and runoff erosion table 4. landscape characteristics of vegetation cover in wulingyuan scenic area from 1977 to 2007 particular year 1977 1987 1998 2004 2007 plaque area(ha) 19556.7 25849.1 27105.1 29516.1 32782.8 vegetation coverage(%) 50.37 66.22 69.38 74.47 83.69 compared with the average annual growth rate of the previous sample(%) 1.61 0.31 0.86 3.08 number of patches (number) 670 922 901 386 612 patch density(number/ ha) 0.0354 0.0357 0.0333 0.0131 0.0187 average plaque size(ha) 29.19 28.04 30.08 76.47 53.57 maximum plaque area(ha) 15876.4 22569.7 23010.7 28180.1 30804.8 percentage of maximum patch area(%) 81.18 87.31 84.89 95.47 93.97 maximum patch fractal dimension 2.2725 1.3473 1.3492 1.2444 1.257 40 qian zhang (a)depression(b)fung shek (c)composite depression(d)fung shekgroup (e)peak-depression assemblage figure 4. comparisons of 95% confidence intervals for roundness coefficients of different landforms after serialization 41study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area (a)depression(b)fung shek (c)composite depression (d)fung shekgroup (e)peak-depression assemblage figure 5. comparisons of 95% confidence intervals for compactness coefficients after serialization of different geomorphic units 42 qian zhang from the point of view of patch fragmentation degree, the degree of vegetation fragmentation is relatively low. there is little change from 1977 to 1987, the degree of vegetation fragmentation is slightly reduced from 1987 to 1998, the degree of fragmentation is obvious from 1998 to 2004, but the degree of vegetation fragmentation has increased from 2004 to 2007. from 1977 to 2007, the largest patch area increased steadily, but its proportion fluctuated, mainly due to the proportion of small vegetation patches in the core area and vegetation patches in the buffer area. the fractal dimension of the largest patch is higher, which is mainly due to the strong influence of human activities around the largest patch and the complex shape of the patch. the most notable one was 1998. architectural landscape change architectural landscape is the smallest landscape type in the study area, but it is the most profound and destructive landscape type of human activities. it is the disturbing landscape in the study area. the change of landscape characteristics is shown in table 5. in 1977, the largest area of architectural landscape was due to the fact that rural settlements were quite scattered and a large number of settlements were still in the core scenic area. by 1987, due to the improvement of production and living conditions, some residents moved spontaneously from the humble mountains to the relatively flat villages under the mountains. on the other hand, because wulingyuan was listed as a national forest park and a national key scenic area, the protection of mountain forests was strengthened, which prompted farmers to move out of the mountains. since 1987, the tourism industry has developed rapidly, bringing about the climax of reception construction. however, the government did not control the construction of reception facilities in an orderly way at that time, which led to blind development. a considerable part of the construction of reception facilities without any formalities is the fastest stage of the urbanization of heritage sites. in 1999, the local government demolished 2×104 m2 illegal buildings. in 2001, the demolition of scenic areas was initiated and nearly 3.4×105m2 of the core scenic buildings were demolished. since 2004, the relocation of residents in the core area has been initiated, and the area of architectural landscape in the core scenic area is less year by year. from the perspective of the number and density of landscape patches, the number of patches increased nearly twice from 1977 to 1998, decreased rapidly in 2004, and then increased; the density of patches increased from 1977 to 1998, and then decreased and increased. the degree of fragmentation of architectural landscape increased from 1977 to 1998, then decreased due to demolition, and then increased significantly due to the pressure of tourism and local residents. from 1977 to 2004, the proportion of the largest patches increased, indicating that the degree of urbanization agglomeration increased. however, after 2004, the patch was redesigned and the group development model was adopted. natural patches were introduced into the patch, which reduced the maximum patch area and proportion. artificial landscape change the artificial landscape in the study area is mainly land use which is strongly influenced by human activities rather than architectural forms, including paddy field, dry land, wasteland, artificial reconstructed waters, roads and other types. the overall characteristics of the artificial landscape in the study area are that the area decreases year by year, and the changes of landscape index characteristics are shown in table 6. from 1977 to 1987, the artificial landscape decreased rapidly, accounting for only 31.32% of the total area in 1987. the main reason for the decrease was that some of the dry land in the gentle slope of mountain forests was able to restore forest vegetation, while some of the artificial landscapes were occupied by the marginal lowland buildings. from 1987 to 1998, there were fewer artificial landscapes, accounting for only 27.67% of the total area in 1998. the main reason for the decrease was the conversion of farmland to forest in the core scenic area and the further reduction of the peripheral artificial landscape area. from 1998 to 2004, there were only a few patches in the artificial landscape of the core scenic area, the decrease was most obvious. in 2004, the artificial landscape only accounted for 24.42%. from 2004 to 2007, the number of artificial landscapes in the buffer zone decreased significantly. the main reason is that some areas with poor farming conditions in the buffer zone have been restored to vegetation cover. in 2007, only 15.12% of the total artificial landscapes were restored to vegetation cover. from the perspective of patch density and average patch size, the trend of fragmentation of artificial landscape is quite obvious, and only in 1998-2004, it has been reduced. the main reason is that the artificial landscape is gradually eroded by vegetation patches, and the number of patches is increasing, which has become a landscape type gradually eroded in the study area. spatial structural feature optimization method (1) based on the principle of combination of centralization and decentralization, a spatial development model (fang et al., 2018) for optimizing the overall pattern of peak forest landform in wulingyuan scenic area is constructed according to local conditions. the research shows that the focus of the construction of peak forest landscape spatial structure in wulingyuan scenic area is to optimize the overall spatial pattern of the landscape. there are fragmentation characteristics in the spatial structure of fenglin landform, which has the advantage of constructing an overall structure optimization model based on the principle of combining centralization with decentralization. (2) designing landscape corridors of wulingyuan scenic area at different scales and levels to give full play to the overall ecological function of the landscape. the holistic concept of “urban integration” and “suburban integration” is not only the basic concept of modern urban spatial structure planning, but also one of the basic principles of peak forest landscape spatial structure in wulingyuan scenic area. high fragmentation is one of the main characteristics of the spatial pattern of peak forest landform in wulingyuan table 5. architectural landscape characteristics of wulingyuan scenic area from 1977 to 2007 particular year 1977 1987 1998 2004 2007 plaque area(ha) 1486.3 1040.1 1113.5 1127.3 1048.1 building density(%) 3.75 2.63 2.81 2.85 2.65 compared with the average annual growth rate of the previous sample(%) -0.12 0.04 -0.02 -0.09 number of patches (number) 1024 643 624 356 543 patch density(number/ ha) 0.69 0.63 0.57 0.33 0.53 average plaque size(ha) 1.4515 1.6176 1.7845 3.1666 1.9302 maximum plaque area(ha) 8.62 24.24 82.31 196.14 122.21 percentage of maximum patch area(%) 0.58 2.33 7.39 17.40 11.67 maximum patch fractal dimension 1.0947 1.1413 1.1659 1.7223 1.1377 43study on optimization method of spatial structure characteristics of peak forest landform in wulingyuan scenic area scenic area, and it is also the main reason affecting the ecological function of peak forest landform. therefore, corridor construction in wulingyuan scenic area of different scales and grades should become the main task of landscape protection and construction. (3) strengthen the construction of landscape patches in key and sensitive areas. wulingyuan peak forest landform is a weak area in the construction and protection of spatial pattern. therefore, wulingyuan scenic area should not only increase the total amount of forest to balance the spatial distribution of forest landscape and improve the ecological quality of landscape, but also plan and build a large near-natural forest landscape patch in the peak forestland form area. (4) strictly protect artificial landscape patches and improve the quality of artificial landscape ecosystem. large artificial landscape patches with high naturalness not only play a key role in maintaining the ecological environment quality and protecting the biodiversity of scenic areas, but also play an irreplaceable role in constructing the overall development model of structural optimization. it is necessary to carry out natural, ecological, diversified and characteristic transformation of artificial landscape ecosystem. discussions the geomorphological characteristics of sandstone peak forest are mainly composed of thousands of stone peaks, pillars, vertical cliffs and grooves, and the main combination forms are columnar and conical peak forest. under the background of intermittent uplift of regional neotectonic movement, the effects of flow erosion cutting, physical weathering, chemical weathering and biological root splitting and other external forces begin with rock joints, which widen the fissures gradually and form the peak forest landform over time. the peak forest landform evolved from the devonian platform and experienced the platform. local mountains, valleys, peaks and bushes, peak forests and other stages of development and evolution, will continue to develop in the future (timbe et al., 2017). based on the topographic characteristics of the peak forest cluster with tree nodes, it is found that the peak forest cluster presents different degrees of self-similarity in different geomorphological types. based on the node sequence method of area weight, the semi-structured contour data are serialized into a chain structure with uniform growth. then the shape characteristics of peak clusters at different scales, angles and levels (liu et al., 2018) are analyzed. the results show that in different geomorphological types, whether measured by roundness coefficient or compactness coefficient, the shape of sub-nodes is closer to the circle than that of parent nodes, and the shape of complex geomorphological units gradually turns into a circle from the outside to the inside. the wulingyuan scenic area has had a profound impact on the landscape in the past 30 years. since the development of tourism in wulingyuan in 1979, tourism and human activities of local residents have become an important factor affecting the landscape of the study area. the remote sensing images in the five periods of 1977, 1987, 1998, 2004 and 2007 are representative of the landscape changes in the study area. 1977 reflects the landscape under the influence of strong farming activities before tourism development; 1987 represents the landscape under the influence of tourism industry just started; 1998 represents the inappropriate planning. under management, tourism activities and local human activities have a profound impact on the landscape of the study area (warned by the world heritage committee in 1998); 2004 is the landscape of the study area after the implementation of the large demolition; 2007 is the landscape of the study area under the pressure of tourism development and the strong influence of local residents. the spatial pattern of sandstone peak forest landform in wulingyuan scenic area from 1977 to 2007 was analyzed. the change of vegetation landscape showed that, despite the pressure of tourism, the vegetation coverage rate increased year by year, the fragmentation degree of patches decreased gradually, and the quality of vegetation patches improved gradually. architectural landscape is the smallest landscape type in the study area, but it is the most profoundly impacted and destroyed landscape type by human activities. it is the disturbing landscape in wulingyuan scenic area. artificial landscape is mainly influenced by human activities rather than land use in architectural form, including paddy field, dry land, wasteland and artificial land, revamped waters, roads and other types (kristensen et al., 2018). the overall feature of the artificial landscape in wulingyuan scenic area is that the area decreases year by year. conclusions sandstone peak forest in wulingyuan scenic area is located at the upper part of the mountain body at an elevation of 1000 meters. its edge is a high cliff, forming a dangerous and exotic landscape. it has the basic forms of cliff, peak (column), cliff trough, cliff lane and so on. the peak forest is composed of thousands of stone peaks, pillars, vertical cliff walls, grooves, etc. the main combination forms are column and conical peak forest, which has important scientific research, popular science and tourism development value. the structure of contour tree can not only visually reflect the hierarchical nesting characteristics of the surface morphology of peak forest clusters, but also contain the relevant information of different levels of geomorphic units, so as to analyze the morphological characteristics of geomorphic units at different scales. the spatial pattern of peak forest landscape in wulingyuan scenic area was analyzed by using various landscape indices. the vegetation landscape increased steadily, the artificial landscape decreased year by year, and the architectural landscape fluctuated, but tended to decrease as a whole. in order to provide better tourism resources and improve the ecological environment protection effect of wulingyuan scenic area, the principles of combining centralization with decentralization, the design of scenic corridors and the strengthening of landscape patch construction were adopted to optimize the spatial structure characteristics of peak forest landform. table 6. artificial landscape characteristics of wulingyuan scenic area from 1977 to 2007 particular year 1977 1987 1998 2004 2007 plaque area(ha) 19254.3 12431.1 10983.5 9697.2 6000.8 proportion of artificial landscape(%) 48.51 31.32 27.67 24.43 15.12 compared with the average annual growth rate of the previous sample(%) -1.73 -0.34 -0.55 -3.11 number of patches (number) 189 519 485 143 259 patch density(number/ ha) 0.0099 0.418 0.0442 0.0147 0.0431 average plaque size(ha) 101.875 23.952 22.646 67.813 23.167 maximum plaque area(ha) 6526.87 2238.57 1333.32 3516.83 2015.81 percentage of maximum patch area(%) 33.90 18.01 12.14 36.27 33.59 maximum patch fractal dimension 2.2725 1.3473 1.3492 1.2444 1.257 44 qian zhang references catania, g., hulbe, c., & conway, h. 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(2017). an improved fuzzy algorithm for image segmentation using peak detection, spatial information and reallocation. soft computing, 21(8), 2165-2173. http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201705003 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201705003 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dzkxjspl201705003 about:blank about:blank http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=jldxzrkxxb201803035 about:blank http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=dyxb201801010 about:blank http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zdhyyqyb201902049 about:blank about:blank keywords: lamprophyres; petrology; geochronology; uruguay; palabras clave: lamprófidos; petrología; geocronología; uruguay; how to cite item muzio, r., martino, n., & peel, e. (2021). petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay). earth sciences research journal, 25(2), 157-168. doi: https:// doi.org/10.15446/esrj.v25n2.89652 mafic dykes of lamprophyric affinity cropping out along the coastal area of montevideo city are described. these dykes trend n75º-85º and crosscut 2.1 ga paleoproterozoic metamorphic units of the rio de la plata craton. they show mainly porphyritic textures with phlogopite and clinopyroxene macrocrysts in a groundmass composed of carbonates, phlogopite, augite, and feldspathoids. ocellar structures filled with leucite, carbonates, and fibrous alkaline amphibole are present. the mineralogical assembly allows their classification as lamprophyres (minettes), but according to their chemical nature, they can be classified as alkaline lamprophyres. a crystallization age of 1.42 ga by ar-ar method (on biotite/phlogopite) was obtained. abstract petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay) rasgos petrológicos de los diques de lamprófidos mesoproterozoicos de montevideo (terreno piedra alta, sur de uruguay) issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.89652 el presente trabajo describe a los diques máficos de afinidad lamprofídica presentes en la costa de la ciudad de montevideo. estos diques presentan tendencias estructurales n75º-85º con buzamientos verticales a subverticales y recortan a unidades metamórficas paleoproterozoicas de 2.1 ga del cratón del rio de la plata. presentan texturas principalmente porfiríticas con macrocristales de flogopita y clinopiroxeno en una matriz compuesta por carbonatos, flogopita, augita y feldespatoides. se observa, además, la presencia de estructuras ocelares rellenas por leucita, carbonatos y anfíbol fibroso alcalino. la textura y contenido mineralógico presente en estos diques permiten clasificarlos como lamprófidos (minettes), pero de acuerdo a su naturaleza química, pueden ser clasificados como lamprófidos alcalinos. fue obtenida una edad de cristalización por método ar-ar (en biotita/flogopita) de 1.42 ga. resumen record manuscript received: 02/08/2020 accepted for publication: 22/01/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 157-168 rossana muzio1, natalia martino2, elena peel1 1universidad de la república de uruguay 2dirección nacional de minería y geología de uruguay g eo lo g y https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n1.74167 158 rossana muzio, natalia martino, elena peel introduction lamprophyres comprise a set of uncommon hypoabissal maficultramafic rocks usually of restricted occurrence as sills or dykes, occasionally as dyke swarms or stocks. they are usually linked to lamproite, kimberlite and carbonatite petrogenesis and to deep sublithospheric mantle melts. this group of rocks is characteristic of volatile-rich magmas, with fast ascent and emplacement at crustal levels during regional stages of lithospheric relaxation (rock, 1987, 1991; mitchell, 1994; tappe et al., 2008). alkaline magmatism is frequent in within-plate settings (whalen et al., 1987; bonin, 1996), associated with extensional processes. according to rock (1991), chemical affinities define calk-alkaline, alkaline and ultramafic lamprophyres; the first usually being related to convergent settings, whereas the others two can be potentially related to large igneous provinces. mostly, mafic dykes are prominent extensional structures and are helpful tools for paleogeography reconstructions as markers of former conjugate margins that were separated probably in relation to mantle plumes (e.g., bleeker and ernst, 2006; ernst and buchan, 2001a, b; halls and fahrig, 1987). mafic dykes have been formerly reported for the rio de la plata craton (rpc), both for the uruguayan shield and for the tandilia belt in argentina by bossi et al., 1993; mazzucchelli el al., 1995; teixeira et al., 1999; iacumin et al., 2001; and teixeira et al., 2002; among other references. according to geochemical and isotopic studies, they correspond to a basaltic-andesite composition, with tholeiitic and calk-alkaline chemical affinity, and paleomesoproterozoic ages (teixeira et al., 2013; girardi et al., 2013). alkaline to sub-alkaline mafic magmatism has been found in southeast uruguay, associated with the extensional events of the opening of the south atlantic ocean (lustrino et al., 2005; cernuschi et al., 2015). the first references of lamprophyric dykes in uruguay, that described the structural and petrographic features of mafic dykes located in the montevideo coastal area, belong to walther (1935; 1948). later, regional studies on the crystalline basement of the rpc characterized these dykes as “mafic alkaline dykes”, tentatively assigning them a neoproterozoic age according to their field relationships, since they crosscut paleoproterozoic units (coronel and oyhantçabal, 1988; masquelin et al., 2003; oyhantçabal et al., 2003; pascale and oyhantçabal, 2010). early reports of lamprophyres in the tandilia basement (argentina) correspond to the work of quartino and villar fabre (1967), who described these rocks as possible deformed amphibolites. more recently, dristas et al. (2013) carried out a petrological characterization on an ultramafic lamprophyre, also complemented by isotopic studies and k-ar (on biotite/phlogopite) dating, yielding a minimum age of 1928 ± 54 ma and a calk-alkaline nature. therefore, this paper is focused on the petrological characterization of a set of lamprophyric dykes in uruguay and the first 40ar-39ar (on biotite/ phlogopite) dating, in order to classify them and to constrain this magmatism in the context of the rpc. regional framework almeida et al. (1973) originally defined the río de la plata craton (rpc), including under this term the ‘‘ancient cratonic areas’’ of the southernmost south american platform already amalgamated by the upper proterozoic. the rpc represents one of the major nuclei of western gondwana, surrounded by late neoproterozoic–cambrian orogenic belts (e.g., almeida et al., 2000; cingolani 2010; rapela et al., 2011 and references therein). the main tectonic units exposed in the uruguayan shield are represented by piedra alta (bossi et al., 1993) and nico pérez (bossi and campal, 1992) terranes, which are separated by the sarandí del yí shear zone (preciozzi et al., 1979). both piedra alta and nico pérez terranes in uruguay, together with buenos aires complex (bac) in argentina (marchese and di paola, 1975) and taquarembó block in southern brazil, constitute the rpc (figure 1a), according to the original definition of almeida et al. (1973). recently, oyhantçabal et al. (2011) redefined the rpc limits in uruguay and excluded the nico pérez terrane and taquarembó block from the craton, after new geochronological, isotopic and gravity data interpretations. most of the rpc consists of paleoproterozoic crust over a large region of extension covered by phanerozoic sediments; with characteristics of mature figure 1. (a) main rio de la plata cratonic exposures (b) location of the outcrop areas in the coastal zone of montevideo city. references: pg punta gorda site, 34°54´00”s – 56°4´53”w; rp rodó park site, 34°55´11”s – 56°10´20”w; pat = piedra alta terrane, npt = nico pérez terrane, bac = buenos aires complex, sysz = sarandí del yí shear zone, sierra ballena shear zone, tb = taquarembó block. 159petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay) continental crust (cordani et al., 2000; pankhurst et al., 2003; peel and preciozzi, 2006; rapela et al., 2007). remnants of archean crust have been recognized by hartmann et al. (2001) in nico pérez terrane (npt). the piedra alta terrane (pat; figure 1a) is comprised of lowto medium-degree metamorphic orogenic belts and plutonic suites (ca. 2.1 ga), a layered mafic complex, lateto post-orogenic magmatism (1.9-2.3 ga) and extensional magmatism (1.7 ga) represented by the florida mafic dyke swarm (preciozzi et al., 1999; bossi and cingolani, 2009; rapela et al., 2007; sánchez bettucci et al. 2010; oyhantçabal et al., 2011; hartmann et al., 2000). peel and preciozzi (2006) proposed that the pat represents a juvenile paleoproterozoic unit that has been stable since 1.7 ga, without a record of the influence of neoproterozoic orogenies. similar features were also defined for the bac (e.g., bossi and cingolani, 2009; rapela et al., 2007; 2011). halls et al. (2001) and teixeira et al. (2013), reviewing the ene-wsw trending florida dyke swarm from pat and the n-nw tandil dykes in the bac, obtained highquality u-pb (id-tims in baddeleyite) ages of 1790 ± 5 ma and 1589 ± 3 ma, respectively. according to these results, teixeira et al. (2013) suggested that both units underwent extensional regimes at the paleo-mesoproterozoic boundary. the study area is located in the southern portion of the pat, along the coastal margin of montevideo city (figure 2). this area consists of medium grade san josé metamorphic belt (bossi et al. 1993; oyhantçabal et al. 2003), which includes the montevideo formation and the punta carretas orthogneissic unit. both units are crosscut by pegmatitic and aplitic dykes and, by the mafic dykes of the present study. the montevideo formation (bossi 1965) is a supracrustal volcanicsedimentary sequence metamorphosed under amphibolite facies conditions (oyhantçabal et al. 2002; 2003). it is composed of concordant para-gneiss, para-amphibolites and mica schists, with an ew regional structural trend (pascale 2013). the punta carretas orthogneissic unit was defined by oyhantçabal et al. (2003), who identified the orthogneiss as resulting from metamorphic recrystallization of calk-alkaline granitoids. it represents the most widespread unit of the crystalline basement in montevideo, intercalated with rocks of the montevideo formation and associated granitoids (pascale 2013). they present structural trends of n280º to n70º of light-gray to pinkish granolepidoblastic rocks, composed of oligoclase, quartz, biotite, epidote, muscovite, apatite and microcline, and strongly crosscut by pegmatitic and aplitic intrusions. the only radiogenic age available for this unit, by rb-sr whole rock method, yielded and age of 1990 ± 32 ma (cingolani et al. 1997). previous work, involving some petrographic and structural features of the lamprophyric dykes, inferred that they represent at least the last magmatic event affecting the montevideo formation (pascale and oyhantçabal 2010; pascale 2013). analytical procedures seven rocks with minimal visible secondary alteration and corresponding to two different set of dykes from the coastal area were sampled, examined in thin section and prepared for lithogeochemical analyses. three polished petrographic sections were also prepared at the instituto de ciencias geológicas for sem-eds and bse analyses. these analyses were developed with the jeol 5900 low vacuum equipment at the faculty of sciences, uruguay. analytical techniques that are routine for this type of equipment as well as standard mineral spectra from severin (2004) and reed (2005) were used. all the chemical data interpreted here as well as the location of the dykes are shown in table 1. all samples were carefully cleaned, crushed and further pulverized using an agate mill at the instituto de ciencias geológicas (montevideo). major and trace elements were analyzed by acme-bureau veritas laboratories (vancouver, canada) following the lf202 lithoresearch package methodology. ba, sr, y, sc, zr, v, and be were analyzed using inductively coupled plasma-atomic emission spectrometry (icp-aes). trace-elements (including ree) were determined by inductively coupled figure 2. geological sketch of the montevideo coastal area (modified after spoturno et al. 2004). references: libertad formation: quaternary fine sediments; dolores formation: quaternary wackes and mud sediments. 160 rossana muzio, natalia martino, elena peel plasma-mass spectrometry (icp-ms). the analytical protocol at the acme laboratory includes the analysis of reference materials (std-ds10; stdgs311-1; std-gs910-4; stdoreas45ea; stdso-18 and std so-19). the 40ar/39ar geochronology was carried out at the university of nevada (las vegas), using the data reduction labspec software written by b. idleman (lehigh university). biotite crystals of one sample of a lamprohyre dyke (pg-01) were separated using conventional techniques and then irradiated at the u.s. geological survey triga reactor (denver, colorado) for 20 hours, along with the ga-1550 biotite standard together with caf2 and k-glass fragments. after a period of cooling, the irradiated sample was loaded for step heating in a cu sample tray in a high vacuum extraction line and were fused using a 20 w co2 laser and measured using a balzers electron multiplier mass spectrometer. the measured (40ar/39ar)k values were 4.1 (± 58.54%) x 103. the ca correction factors were (36ar/37ar)ca = 2.55 (± 3.50%) x 10-4 and (39ar/37ar)ca = 6.97 (± 3.69%) x 10-4. the j factors were determined by fusion of 4-8 individual crystals of ga-1550 biotite neutron fluence monitors, giving a reproducibility of 0.3% to 0.8% at each standard position. the measured 40ar/36ar ratios were 282.36±1.41%; thus, a discrimination correction of 1.0467 (4 amu) was applied to the measured isotope ratios. an age of 98.50 ma (spell and mcdougall 2003) was used for the ga-1550 biotite fluence monitor to calculate the ages of the samples. the criterion followed to define a plateau age is the identification of three or more successive steps overlapping with an error at 2σ level that together comprise >50% of 39ar released. all analytical data are reported at the confidence level of 1σ (standard deviation) and shown in table 2. petrographic features mafic dykes are gray to black, fine to medium grained, 0.3 to 2.0 meters width and tabular in shape. they often show irregular contacts with pat units and are affected by two sets of nw and n-nw vertical to subvertical fractures. sometimes they show an anastomosed pattern (figure 3a), with sharp contacts and thin chilled margins (figure 3d). no evidence of contact metamorphism has been found. they are concordant with the regional foliation trend (n70º-n80º). even the smaller dykes with widths of a few centimeters often intrude through lateral fractures with a nw direction. double intrusion processes or “dyke in dyke” features are observed (figure 3c), which has been well documented as a common process for lamprophyres worldwide (rock 1987). they have porphyritic texture with increasing grain size toward the dyke center. occasionally, the central zone is porphyritic and vesiculated, with biotite/pyroxene macrocrysts and globular structures. these globular structures (ocelli; phillpotts 1990) are almost spherical but frequently show coalescence, developing irregular shapes (figure 3b). they vary in size between 0.1 cm to 1.5 cm, and they are filled mainly by carbonates, zeolites with halos of opaque minerals and arfvedsonite (figure 3g). mafic and felsic xenoliths from the surrounding crystalline basement are frequent, represented by ortho-gneiss and amphibolite (figures 3e and 3f). microscopically, rocks are porphyritic to glomeroporphyritic dykes, with biotite-phlogopite, augite-aegirine as main macrocrysts and with olivine and leucite subordinately; in a groundmass composed of alkaline feldspar (orthoclase), clinopyroxene, carbonate (dolomite), leucite, and opaque minerals (figure 4a). the groundmass is very fine grained, showing an intergranular texture (between 50-100 μm). feldspathoid glomeruli and carbonate patches are present within the groundmass (figure 4b). globular structures (ocelli) are a common petrographic feature with radial halos of biotite-phlogopite and pale blue acicular amphibole, probably riebeckite/arfvedsonite (figure 4c). other ocelli, between 50-300 μm sized, are filled by carbonates, alkaline feldspar (orthoclase) and/or feldspathoid (leucite), (figure 4d). according to these petrographic features they would correspond to type 1 aggregates, as defined by azbej et al. (2006). opaque minerals vary approximately 5-10 modal %, they have subautomorph shapes and they are sometimes interstitially distributed in the groundmass. sem-eds studies were developed to better characterize the mineral composition of the groundmass. the spectra obtained were compared to standard mineral spectra from severin (2004) and reed (2005) in order to identify part of the minerals of the dykes. some of these representative minerals and their spectra are presented in figure 5. figure 3. (a) dyke showing anastomosed pattern at punta gorda (pg) site; (b) ocelli with carbonate alteration rim; (c) dyke in dyke intrusion (pg site); (d) detail of contacts with host rocks; (e) and (f) xenoliths from the surrounding crystalline basement represented mainly by ortho-gneiss and amphibolite; (g) ocelli filled mainly by carbonates, zeolites with halos of opaque minerals and arfvedsonite. figure 4. photomicrographies under polarized microscope (crossed nicols), (a) detail of the ocelli with leucite (lct) and analcite (anl) and phenocryst of augite (aug); (b) coalescent ocelli (oc) filled with feldspathoids; (c) augite (aug) and biotite (bt) phenocrysts; (d) ocelli with carbonates (border) and leucite (lct); groundmass with leucite, biotite, augite and carbonates. 161petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay) figure 5. (a) bse image with eds spectra of leucite/nepheline crystal (sample pg-01); (b) bse image of augite and phlogopite crystals (sample pg-04), eds spectrum of augite. (c) bse image of dolomite with spongy texture, showing high relief relative to the biotite and their respective eds spectra (sample pg-01). 162 rossana muzio, natalia martino, elena peel lithogeochemistry analyses of seven samples and their locations are presented in table 1. the dykes are low in sio2 (45.21%–51.43%), high in mgo (8.52%–11.08%), feot (9.92%–10.42%), and tio2 (2.98-3.27%) and show exceptionally high values of k2o ranging between 7.09-8.13%. they are enriched in incompatible elements, such as the hfse, zr (556-663 ppm), nb (72-112 ppm) and th (1120 ppm) and the large ion lithophile elements, such as ba (4486-5980 ppm), sr (1017-2026 ppm) and rb (129 -248 ppm). according to the tas diagram (le bas et al. 1986; figure 6) the samples can be classified as phonotephrites/tephriphonolites. all the samples have total alkalis close to 10 wt% and k2o/na2o ratios approximately 4, a relation that points to a possible ultrapotassic character/shoshonitic nature. figure 6. classification diagrams (le bas et al. 1986). symbols: pg site; rp site. using the al2o3 versus cao classification diagram from foley et al. (1987) and the k2o-mgo-al2o3 ternary diagram from rock (1987), the samples can be classified as lamprophyres and, according to their alkali values the samples overlapped the fields lamprophyres/lamproites (figure 7b). as shown in table 1, the samples have high alkali values (mean k2o + na2o > 8, mostly close to 10 wt.%), with k2o/na2o > 3. in order to constrain the alkaline affinity of the dykes and to discard a possible shoshonitic nature the k2o – na2o diagram was used, showing as a result an ultrapotassic character for all the samples (figure 8). figure 8. k2o – na2o diagram, depicting the alkaline character (from foley et al. 1987, after turner et al. 1996). symbols are the same as in figure 6. the ree patterns normalized to chondrite (boynton 1984) are strongly fractionated, showing values from la to lu between 110 – 600 ppm (figure 9a), with different trends for the samples. in the primitive mantle-normalized multielement plot (sun and mcdonough 1989; figure 9b), a significant enrichment in liles relative to hfses is observed, showing a steep pattern with through at u, sr, and p and remarkable ba and nd positive spikes. figure 7. classification diagrams for ultrapotassic rocks according to (a) foley et al. (1987) and (b) rock (1987). symbols are the same as in figure 6. references fig. 7a: i – lamproites; ii – kamafugites (low sio2, high cao); iii – orogenic areas (high cao and al2o3) and iv – transitional group. 163petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay) table 1. geochemical data for the dykes from punta gorda (pg) and rodó park (rp) sites. sample pg 01 pg 02 pg 03 pg 04 pg 05 pg 06 pr 02 latitude 34º53’10,7”s 34º53’11.0”s 34º53’10,66”s 34º53’10,70”s 34º53’10,86”w 34º53’09,82”s 34º54’25,40”s longitude 56º04’52,2”w 56º04’52,12”w 56º04’51,65”w 56º04’51,77”w 56º04’52,12”w 56º04’51,30”w 56º10’08,90”w sio2 49,37 47,03 49,67 44,61 46,58 45,32 46,83 al2o3 8,38 7,84 8,34 8,43 8,53 8,36 7,76 fe2o3 10,06 9,84 10,13 10,03 10,08 9,9 9,37 mgo 8,73 10,29 8,29 10,48 9,21 10,24 9,72 cao 5,53 6,34 6,17 7,22 6,67 7,01 8,03 na2o 1,75 2,07 2,03 1,64 1,46 1,84 1,57 k2o 7,8 7 7,71 7,54 7,53 7,5 6,7 k2o/na2o 4,45 3,38 3,79 4,59 5,15 4,07 4,26 total alkalis 9,55 9,07 9,74 9,18 8,99 9,34 8,27 tio2 3,14 2,89 3,08 2,9 2,96 2,82 2,91 p2o5 1,01 0,92 1,68 1,51 0,68 1,3 0,77 mno 0,11 0,12 0,12 0,12 0,12 0,12 0,08 cr2o3 0,047 0,049 0,049 0,046 0,039 0,046 0,057 loi 2,9 4,4 1,6 4,3 4,9 4,4 5 sum 98,89 98,86 98,9 98,87 98,78 98,91 99,5 ba 5048 4867 5106 4399 5240 4486 5980 be 8 9 8 2 5 3 3 co 75,7 55,3 46,2 57,7 56 51 47,7 cs 3,1 3,1 1,9 4,2 6,1 3,2 9,4 ga 17,3 17,6 17,5 18,4 18,7 16,7 17,6 hf 18,3 16,6 18,3 14,9 16,2 14,2 15,6 nb 87,3 87,6 85,6 103,7 111,7 101,9 72,5 rb 142,6 129,4 148,6 248 168,3 181,9 185,3 sr 1170 1598,5 1160,3 1912,6 2026,5 1692,4 1017,6 ta 4 4,2 4,8 5,1 5,3 5 4 th 16,6 15,5 19,6 13,8 17,7 13,9 11,4 u 2,1 1,5 5,4 2,4 1,4 2,8 2,6 v 137 116 142 133 135 130 136 w 151 80,8 69,7 80,1 99,6 71,7 82,2 zr 663,3 658,7 656,2 608 640,8 568,2 556,8 y 28,7 21,9 40,4 27,1 22,5 25,6 17,7 la 201,9 186,8 207,1 165,4 189,6 163,2 158,7 ce 382,1 354,4 374 303,1 344,7 299 317,2 pr 41,87 39,65 42,53 34,8 38,44 33,39 35,59 nd 145,4 137,6 150,8 119,2 132,1 118,1 122,3 sm 18,23 17,28 18,93 15,66 15,81 14,64 14,91 eu 4,58 4,39 4,76 4 4,18 4,03 3,93 gd 12,59 11,74 13,17 10,59 11,29 10,46 9,84 tb 1,47 1,23 1,69 1,25 1,19 1,2 0,98 dy 6,97 5,34 8,48 5,87 5,17 5,84 4,47 ho 1,07 0,84 1,38 0,95 0,84 0,88 0,63 er 2,53 2 3,51 2,39 2,06 2,4 1,48 tm 0,31 0,23 0,42 0,32 0,26 0,3 0,21 yb 1,64 1,29 2,45 1,8 1,54 1,77 1,15 lu 0,2 0,17 0,29 0,23 0,2 0,22 0,16 pb 4,9 16,3 14 16,5 12,9 14,2 11,5 ni 137,4 186,2 127,8 226,8 167,4 210,8 188 164 rossana muzio, natalia martino, elena peel the dykes have high nb/ta ratios, similar to the primitive mantle. also, high nb and zr values with relatively low values of hf and y are commonly observed in alkaline rocks from within-plate settings (green 1995). 40ar/39ar results the biotite-phlogopite analysis from the center of one dyke (nº pg-01, site pg; figure 2) produced relatively undisturbed spectra and a well-defined plateau age (figure 10, table 2) of 1425.40 ± 9.12 ma (1σ). the plateau is defined by the release of at least 99% of 39ar gas in fifteen consecutive steps out of sixteen. the anomalously young apparent age in the first low-temperature step yielded a general increasing trend for this spectrum. minor disturbances also occur in high-temperature steps, suggesting degassing from high temperature resilient impurities. no statistically valid isochron was obtained. table 2. ar-ar biotite analysis for the sample pg01. references: biotite, 3.74 mg, j = 0.00497 ± 0.13%. 4 amu discrimination = 1.0467 ± 1.41%, 40/39k = 0.0041 ± 58.54%, 36/37ca = 0.000255 ± 3.50%, 39/37ca = 0.000697 ± 3.69%. isotope beams in mv, r.l.s.d. = released, error in age includes j error; all errors 1 sigma. (36ar through 40ar are measured beam intensities, corrected for decay for the age calculations). step t(°c) t(min.) 36ar 37ar 38ar 39ar 40ar %40ar* %39ar rlsd ca/k 40ar*/39ark age(ma) 1s.d. 1 660 12 0,701 0,570 0,383 9,490 2176,69 91,3 1,2 3,9904378 211,358746 1295,44 17,28 2 730 12 0,167 0,655 0,378 21,589 5206,83 99,3 2,8 2,0144702 241,890087 1424,24 17,42 3 770 12 0,153 0,137 0,570 35,889 8766,62 99,6 4,7 0,2533259 245,814014 1440,14 17,56 4 800 12 0,134 0,057 0,753 48,149 11716,88 99,8 6,3 0,078557 245,263035 1437,92 17,51 5 830 12 0,090 0,054 0,736 52,424 12694,65 99,9 6,8 0,0683533 244,355218 1434,25 17,43 6 870 12 0,080 0,048 0,926 58,059 14098,14 99,9 7,6 0,0548613 245,123479 1437,35 17,53 7 910 12 0,063 0,068 0,858 57,968 14174,76 99,9 7,6 0,0780717 246,930930 1444,64 17,54 8 950 12 0,077 0,102 0,854 54,603 13223,94 99,9 7,1 0,1239616 244,475970 1434,75 17,44 9 990 12 0,076 0,128 0,775 47,539 11555,55 99,9 6,2 0,1786778 245,340750 1438,23 17,47 10 1030 12 0,051 0,086 0,561 34,622 8244,25 100,0 4,5 0,1648371 240,389997 1418,12 17,30 11 1060 12 0,047 0,076 0,451 25,858 6133,56 100,0 3,4 0,1950434 239,393167 1414,04 17,27 12 1090 12 0,052 0,083 0,406 22,437 5263,66 100,0 2,9 0,2454893 236,639139 1402,73 17,17 13 1120 12 0,055 0,125 0,482 25,191 5923,03 99,9 3,3 0,3293025 237,205720 1405,06 17,22 14 1160 12 0,097 0,486 1,110 64,621 15221,67 99,9 8,4 0,4991319 234,165292 1392,51 17,10 15 1210 12 0,095 0,593 1,918 121,984 29396,48 100,0 15,9 0,3226126 243,445057 1430,56 17,41 16 1400 12 0,103 0,411 1,294 86,712 20893,79 100,0 11,3 0,314551 243,308533 1430,00 17,40 cumulative % 39ar rlsd = 100,0 total gas age = 1426,19 8,64 plateau age = 1425,40 9,12 (steps 2-16) figure 9. ree patterns and spider-diagrams, normalized according to (a) chondrite (boynton 1984); (b) primitive mantle (sun and mc donough 1989). symbols are the same as in figure 6. discussion according to the petrographic features described previously, the analyzed dykes can be classified as lamprophyres. based on the latter, the mineral assembly (biotite-phlogopite + augite ± aegirine augite ± altered olivine ± dolomite ± amphibole ± chlorite ± feldspars and feldspathoids, in the groundmass) and the high values of k2o, p2o5 and ba pointed to their classification as minettes (rock 1991; le maitre et al. 2002). although the presence of feldspathoids in minette type lamprophyres is not provided by the iugs classification, it has been reported by several authors (e.g., esperanza and holloway 1987; wallace and carmichael, 1989; bhowmick, 2000; gupta 2015). in addition to the general mineralogical features, other chemical criteria can be taken into account to constrain their classification (mitchell and bergman, 1991). in this sense, the dykes from montevideo are not only ultrapotassic (k2o/ na2o > 3) but also peralkaline (k2o+na2o)/al2o3 > 1) and perpotassic rocks 165petrologic features of mesoproterozoic lamprophyric dykes from montevideo (piedra alta terrane, south uruguay) (k2o/al2o3 > 1), with cao and fe2o3t ≤ 10 wt. % and a high ba (>5.000 ppm), tio2 (~3 wt. %), zr (~650 ppm) and sr (~1500 ppm). all these are typical characteristics of lamproites according to foley et al. (1987) and le maitre et al. (2002). nevertheless, the typical lamproite mg# values are higher than 70, and the la content is higher than 200 ppm whereas in the studied dykes they are between 62-67 and approximately 182, respectively. the dykes present a mean value of 3.7 for the nb/y ratio, which characterizes them as alkaline (winchester and floyd, 1977). alkaline lamprophyres usually have cr, co and ni contents of 97, 38 and 65 ppm, respectively. in contrast, the lamprophyric dykes of montevideo have these values of approximately 480, 56 and 172 ppm, close to those representatives of ultrapotassic lamprophyres (mitchell and bergman, 1991). all dykes present high k/ti and (la/sm)n ratios (>2.3 and > 2.6, respectively) that are commonly related to enriched mantle sources (mahoney et al. 2002). the mg# (62-67) suggests a mantle source for these dykes from values of sc (15-16 ppm), cr (420-520 ppm), co (47-75 ppm) and ni (143-239 ppm) (rhodes, 1981). alkaline magmatism is frequent in within-plate environments, related to extensional processes (whalen et al. 1987) and is characterized by high concentrations of hfse and low values of lile. alkaline rocks showing strong enrichment in lile and lree are commonly associated with anorogenic or post-orogenic settings (plá et al., 2011). alkaline ultrapotassic magma shows high incompatible element content regardless of the tectonic environment supporting an origin by partial melting of lherzolites (low rates <1%) from an enriched mantle source (lloyd et al. 1985; foley 1992; foley and peccerillo 1992; ionov et al. 1997). furthermore, high concentrations of lree have been observed for other ultrapotassic within plate rocks, such as lamprophyres and lamproites as well as post-collisional lamprophyres like minettes (foley 1992; gibson et al. 1992). the high concentrations of y and zr, and relatively lower concentrations of hf, nb, ta and hfse are consistent with a within plate environment (foley and peccerillo, 1992). the nb/u ratios for lower and upper crust are 21 and 9, respectively; rocks affected by crustal contamination present nb/u values between 9 and 40 ppm (gregoire et al., 2000). the dyke mean values of 43 ppm suggest that crustal contamination during the magmatic ascent/ emplacement is not obvious. several authors (e.g., masquelin et al. 2003, oyhantçabal et al., 2003, pascale and oyhantçabal, 2010) have attributed a cambrian age to these dykes because other mafic dykes located to the north of montevideo city crosscut the la paz granite (585 ± 4 ma, cingolani et al. 2012). martino (2017) analyzed these dykes and found that they correspond to lamprophyres of calk-alkaline affinity, and according to their petrographic features they were classified as espessartites. interesting features that support their contemporaneity of emplacement with the la paz granite are evidence of magma mingling processes. therefore, these calk-alkaline lamprophyres are clearly different and later than the dykes discussed in this paper. mafic dykes of mesoproterozoic age have been mentioned for the rpc, but in general, with different mineralogy and/or chemical affinity (girardi et al. 2013; teixeira et al. 2013; dristas et al. 2013, among others). an interesting fact reported by these authors is that according to the isotopic data from the tandil dykes, an enriched original source could be a characteristic feature of the mantle of the rio de la plata craton in paleoproterozoic time. lamprophyric dykes were only reported by dristas et al. (2013), with a minimum age of 1.92 ± 54 ga obtained by the k-ar method on phlogopites. teixeira et al. (2013) proposed the idea that the columbia supercontinent started a major break up at 1.59 ga, based on geochronological and paleomagnetic data. therefore, the emplacement of the lamprophyric dykes studied in this paper may be related to the initial stages of a later major extensional event. conclusions • the lamprophyric dykes from montevideo, with variable structural trend between n75º-n85º, have porphyritic texture with mainly phlogopite phenocrysts and a groundmass composed of biotite-phlogopite + augite – aegirine augite ± altered olivine ± dolomite ± amphibole ± chlorite ± feldspars and feldspathoids. ocellar structures with carbonate, zeolites, leucite and fibrous alkaline amphibole are present. • they are chemically ultrapotassic dykes (k2o/na2o > 3), with peralkaline (k2o+na2o)/al2o3 > 1) and perpotassic nature (k2o/al2o3 > 1), with cao and fe2o3t ≤ 10 wt. %, very high ba (>5.000 ppm), tio2 (~3 wt. %), zr (~650 ppm), and sr (~1500 ppm), mg# approximately 63-67 and they are highly enriched in lile in relation to primitive mantle. • according to the mineralogical criteria established by the iugs the dykes can be classified as minettes (lamprophyres) and from a chemical point of view as alkaline/ ultrapotassic lamprophyres. • the chemical features point to a deep/enriched mantle source for the origin of this magma related to a within-plate tectonic setting. • the crystallization age of 1.42 ga determined by the ar/ar method on biotite (phlogopite), points to an extensional event for the rio de la plata craton at that time. acknowledgments this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. we are especially grateful to the editors and to the anonymous reviewers whose comments and suggestions greatly improved the quality of this work. references almeida, f. f. m., amaral, g., cordani, u. g. & kawashita, k. 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(2020). calibration method of petroleum underground prospect based on high precision gravity and magnetic exploration. earth sciences research journal, 24(3), 345-355. doi: https://doi.org/10.15446/esrj.v24n3.90315 the high-precision calibration results of the petroleum underground layer are of great significance for oil production efficiency. this study uses the gravity magnetic model to retrieve the bedrock depth. the results of the basement structure and sedimentary rock distribution through the gravity and magnetic geology in the petroleum underground prospect of the tongbai basin were also obtained. on this basis, the geological data, logging data, seismic data, and the vertical seismic profile (vsp) data has been comprehensively used, as the layered calibration method was used to calibrate the petroleum underground layer. considering the seismic datum and the core elevation in the area, the rock formation is divided by various logging curves. the average time difference and density of the divided rock layers are interpolated at equal depth intervals to obtain velocity sequences and density sequences at equal time intervals and finally realize time-depth conversion. when the drilling geological horizon is unified, the synthetic record of the seismic reflection layer is compared with the geological horizon to realize the horizon calibration of the seismic reflection layer. when the local stratification is not uniform, the seismic reflection layer is calibrated by tracking the seismic reflection layer, high-precision velocity analysis, and various synthetic records to verify the reliability of the geological horizon. the results show that the proposed method can accurately survey the geological conditions of the tongbai basin. it detected 14 basement faults, and that the nw-trending and ne-trending faults controlled the basin, while the north-south faults controlled the later evolution of the basin. the method can be used for the horizon calibration of inclined wells, which is suitable not only for anisotropic media but also for formations with a less lateral variation of local formation lithology. moreover, its usage is flexible, and it can be corrected by multiple speed data. abstract calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration método de calibración de prospectos de petróleo subterráneo con base en mediciones de gravedad de alta precisión y en exploración magnética issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.90315 debido a que los resultados de calibración de alta precisión de la capa subterránea de petróleo son de gran importancia para la eficiencia en la producción de petróleo, este estudio usa la investigación sobre el método de calibración de la capa subterránea de petróleo con base en la gravedad de alta precisión y la exploración magnética. el modelo magnético de gravedad se utilizó para medir la profundidad de la roca madre. además se obtuvieron los resultados de la estructura del basamento y la distribución de rocas sedimentarias a través de la gravedad y la geología magnética en un prospecto de petróleo subterráneo en la cuenca tongbai. sobre esta base, los datos geológicos, los datos de registro, los datos sísmicos y los datos del perfil sísmico vertical (vsp) se usaron de manera integral, y el método de calibración en capas se usó para calibrar la capa subterránea de petróleo de la cuenca tongbai. de acuerdo con el dato sísmico y con la elevación del núcleo en el área, la formación rocosa se dividió en varias curvas de registro. la diferencia de tiempo promedio y la densidad de las capas de roca divididas se interpolaron a intervalos de profundidad iguales para obtener secuencias de velocidad y secuencias de densidad a intervalos de tiempo iguales, y finalmente realizar la conversión de profundidad de tiempo. cuando el horizonte geológico de perforación se unifica, el registro sintético de la capa de reflexión sísmica se compara con el horizonte geológico para realizar la calibración del horizonte de la capa de reflexión sísmica. cuando la estratificación local no es uniforme, la capa de reflexión sísmica se calibra con la capa de reflexión sísmica, el análisis de velocidad de alta precisión y varios registros sintéticos para verificar la fiabilidad del horizonte geológico. los resultados muestran que el método propuesto puede estudiar con precisión las condiciones geológicas de la cuenca tongbai. este método detectó 14 fallas en el basamento, y también que las fallas de tendencia nw y ne controlaron la cuenca, mientras que las fallas norte-sur controlaron la evolución posterior de la cuenca. el método puede usarse para la calibración del horizonte de pozos inclinados, que es adecuado no solo para medios anisotrópicos, sino también para formaciones con menor variación lateral de la litología de formación local. además, su uso es flexible y puede corregirse con múltiples datos de velocidad. resumen record manuscript received: 01/06/2019 accepted for publication: 09/05/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 345-355 kai zang1,2*, jiuchuan wei1, linsong yu2, fang wan2, zunfang hu2, yang li2 1college of earth science and engineering, shandong university of science and technology, qingdao, 266590, china 2shandong institute of geophysical and geochemical exploration, shandong provincial bureau of geology and mineral resources, jinan, 250013, china *corresponding author: skwty_zk@126.com se is m ic p ro sp ec ti ng https://doi.org/10.15446/esrj.v24n3.90315 https://doi.org/10.15446/esrj.v24n3.90315 mailto:skwty_zk@126.com 346 kai zang, jiuchuan wei, linsong yu, fang wan, zunfang hu, yang li introduction in china’s oil census and exploration, gravity and magnetic exploration have played an important role as a pioneering means of exploration. however, due to the rapid development of seismic exploration technology since the 1970s, gravity and magnetic technology were once at low tide (metcalfe et al., 2015). as a new force in comprehensive geophysical exploration of petroleum, since the 1980s and 1990s, oil and gravity have revived the glory, and it has made due contributions to oil exploration in the new era. at present, china national petroleum corporation and sinopec group have established many exploration new areas in the west and central regions (beck et al., 2015). the common features of these new areas are as follows: first, the surface conditions are very complex, including mountains, deserts, wind erosion landforms, and loess cover; second, the geological and geophysical conditions are complex. multi-phase tectonic movements, multi-stage magmatic activities often cause the complexity of underground geological structures. pleated and reversed napped structures abound, and the physical properties of rocks vary. these complex and unique surface and underground conditions in china have brought severe challenges to seismic exploration, and seismic data collection is difficult. excited reception conditions are poor, the signal-to-noise ratio is low, and even effective seismic signals are not obtained, which greatly affects the exploration pace of the new area. the gravity and magnetic exploration technology have always been characterized by lightness, speed, low investment, and high efficiency. in the early exploration of the new area, it can play an important role in delineating the distribution of faults and igneous rocks, the division of tectonic units, the study of the basement structure, and the discovery of local structures. it can provide the basis for deploying seismic exploration (babak et al., 2015) and can complement or partially replace where seismic exploration is difficult. in the past decade, many exploration areas in the western, central, and southern regions have been or are working on gravity and magnetic exploration and have achieved results. zhang xuejian et al. extracted the seismic wavelet from the depth domain offset profile and then used the convolution formula to obtain the depth domain synthetic record. considering the difference between the upper and lower speeds of the interface, ma jinfeng follows the timedomain convolution formula and replaces the time domain seismic wavelet with the wavelet operator in the depth domain. that is, the depth domain asymmetric wavelet operator is used to obtaining the synthetic record. in this paper, gravity and magnetic exploration techniques are used to obtain information on complex topography in the tongbai basin such as the basement structure, the distribution of sedimentary rock series, and the distribution of fault systems (jokela & ramallo, 2015). on this basis, the fine horizon calibration technology can maximize the use of all data in complex areas to ensure that the formation and reservoir calibration is relatively accurate in the whole and full sections. finally, the fine calibration of the petroleum underground layer in the tongbai basin is realized. materials and methods research area overview tongbai basin is a small inter-mountain fault basin in the eastern qinling fold belt. the terrain is hilly and semi-mountainous, rivers, gullies, etc. the previous small scale weight and magnetic survey results show that the regional heavy magnetic field is seriously disturbed, which makes the wild construction and abnormal geological interpretation has certain difficulties. the main body of the bouguer gravity anomaly in the survey area is a complete gravity low zone with the shape of “yuanbao” sitting south to the north, with a relative gravity value of at least -10.5mgal, and its center is located near the yuehe store; the anomalous line of the anomalous center is circular, and the two wings are lifted outwards to the northwest and the north. the west, north, and east of the survey area are three high-gravity belts, and the high gravity belt in the north is wedged from the northwest to the southeast, whose relative gravity value is +29mgal. gravity step belts with different strikes and scales and isomorphic twisted belts with equal anomalies are covered with survey areas, such as the shilipu-xuzhai gravity step belt in the northwest direction and the same-line twisted belt in the eight-acre land as shenpu. under the background of the strong gravity gradient, the local gravity anomaly shows weak, and it can be distinguished abnormal bulge gravity of lizhuang, niejiaxiahe, and shenpuyizhongwan. these characteristics of bouguer gravity anomalies reflect the uniformity of the survey area in the regional structure, but the complexity of the fault structure and local structure. the upward extension of the gravity anomaly is similar to the basic characteristics of the bouguer gravity anomaly. the residual gravity anomaly reflects the internal structure of the basin very finely. one of its characteristics is to divide the gravity low zone centered on the yuehe store into two completely trapped gravity belts in the south and north. in shenpuwang laozhuang domain it has a beam, dividing the southern main gravity low zone into two negative anomalies in the east and west, and the negative anomaly zone in the west is in the orchard area, whose size and amplitude are much smaller than those of yuehe store in the east. the northern subtropical low-band center is located between shangmazhuang, baishuyuan, and louzi shangzhuang. it is separated from the southern part of the main gravity low belt by the niejiaxia river. another feature of it is that it clearly reflects seven gravitational step bands, such as the eight-acre land in the northwest and the wuzhuang gravity step belt. establishment of gravity and magnetic geological model the gravity-magnetic fine-measurement (i) profile coincides with the wuhedian-wucheng earthquake survey profile. the seismic profile has poor reflection quality, and the geological horizon of the seismic interface needs to be further determined. to this end, the gravity geological model needs to be constructed. the construction of the initial gravity geological model is mainly based on the results of seismic interpretation (foster et al., 2015), and refers to the ground geological and drilling data. the density of each layer is given according to the actual measurement and statistical results. the modification and forward modeling of the model uses the second-degree gravity anomaly human-machine dialogue interpretation program, and the interpretation results are shown in figure 1. the number 1 represents the measured bouguer gravity anomaly; 2 represents the calculated value of the dazhangzhuang group (i); 3 represents the calculated value of the wulidun group (ii); 4 represents the calculated value of the li shigou group (iii); 5 represents the calculated value of the maojiapo group (iv); 6 represents the calculation of the total gravity anomaly value; 7 represents the regional gravity field value. the following three points are recognized through forwarding modeling: (1) the model is modified several times without considering the influence of the regional field. the calculated value in the south of the profile is always a few milligrams larger than the bouguer anomaly. according to the regional field characteristics reflected by the bouguer gravity anomaly map of 1:500,000, after the region anomaly with the gradient of 1.478mgal/km is removed, the satisfactory fitting result is obtained (primack, 2015). it can be seen that the existence and change of the regional gravity field in the basin cannot be ignored. (2) the wulidun group is the largest in terms of reflecting the negative anomalies in the basin, followed by the lishigou group, the dazhangzhuang group, and the maojiapo group. the wulidun group is 2-6 times that of the maojiapo group. obviously, the variation of the thickness of the wulidun group is the main geological factor causing gravity anomalies. (3) in the forward calculation, the base density of the basin is taken uniformly, which is 2.67g/cm3. as a result, regional positive residuals appear in the northern half of the profile, while regional negative residuals appear in the south. the northern basement is taken as the erlangping group (density 2.67g/cm3), and the southern basement is taken as the tongbai group (density 2.67g/cm3). the fitting effect is ideal. it can be seen that there is a change in the lithology of the basement in the basin (qayyum et al., 2015). the magnetic initial geological model refers to the interpretation results of the seismic survey profile. the value of the base magnetism is derived from the actual measured and statistical data, and the abnormal inversion results are referenced. model modification and forward calculation use a combined model human-machine dialogue interpreter. the final interpretation results are shown in figure 2. in the forward modeling process, the uniform magnetic substrate is used. as a result, the theoretical anomalies and measured anomalies in the south of xiawuzhuang village are well fitted, while the theoretical anomalies in the northern part are much lower than the measured anomalies. thereafter, the base magnetic value of the north of xiawuzhuang is relatively good, and 1.478mgal/km 2.67g/cm 2.67g/cm 2.67g/cm 347calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration anomalies of different characteristics. a total of 187 depth points are calculated, and they are distributed uniformly in the measurement area, which can satisfy the drawing of the magnetic bedrock depth map. according to the results of gravity anomaly inversion, the range of bedrock exposure is basically consistent with the distribution range of geological outcrops; the depth of bedrock in u13 well is 900m, and the inversion result of bedrock depth is 920m. the depth of the u27 well is 963m, which is close to the bottom boundary of the wulidun group. the inversion results show that the bedrock depth is about 1500m. the northwestern, eastern, and southern parts of the survey area are all within 100m, and the actual situation is basically matched. the u13 well is located on the gentle slope of the magnetic base, between the isobaths of 900m-1000m; the u27 well is located on the steep zone of the magnetic base, between the depth lines of 1200m-1300m. the above results indicate that the inversion results of the gravity and magnetic anomalies can better reflect the depth and undulation of the basin basement (yan et al., 2016), which has been confirmed by the later completed area seismic exploration data. analysis of factors affecting horizon calibration fine-level calibration on the basis of high-precision gravity and magnetic survey and the influencing factors to be considered are important topics in the calibration of horizons. in order to achieve fine horizon calibration, it is necessary to combine geology and earthquake, and comprehensively use geological data, logging data, seismic data, and vsp data. the specifics should include the accurate application of the seismic reference plane and the core elevation, the application of the logging curve, and the selection of the frequency. accurate application of seismic datum and core elevation figure 3 shows the effect of the core elevation on the calibration. it can be concluded from figure 3 that the core elevation is indispensable data the theoretical anomaly can be well fitted with the measured anomaly. it can be seen that the lithology of the basement of the basin does change. the southern part may be the mixed granite or the rock with the same magnetic properties, while the northern part is a more magnetic type. inversion of bedrock depth there are three difficulties in the inversion of the bedrock depth in this basin: (1) the gravity exploration work for the purpose of studying the basement structure of the basin is the first time in this area. due to the limitations of objective conditions, the density data is not sufficient, especially the lack of density data of the lower three strata with depth changes; (2) the maojiapo group overlying the basement has large buried depth and a small difference in density from the base (mavrommati et al., 2017); (3) the scope of work is small. at present, most of the existing interface inversion methods are based on the forward modeling, and there is inevitably an area loss. in view of this situation, the exposed areas of the bedrock at the edge of the work area are known as known points. taking the entire lower three systems as the equivalent density layer, according to the measurement, the density of the equivalent density layer is calculated to be  = 2.43g/cm. the parkerr.l. method without loss of area is used in the calculation to obtain the gravity anomaly inversion result. there are many methods for calculating the buried depth of a magnetic body by using magnetic anomalies. the method of calculating the buried depth of a magnetic body by computer requires that the magnetic anomaly is isolated, obvious and the interference is small, or the depth of the base is up and down at an average depth. if the characteristics of the magnetic field and the base undulation of this area cannot meet the requirements of the above algorithm, it is difficult to obtain a good effect. to this end, we use the tangent method that has been tested by practice and adopt corresponding algorithms for the figure 1. interpretation results of gravity anomaly in precise survey section i 2.43g/cm 348 kai zang, jiuchuan wei, linsong yu, fang wan, zunfang hu, yang li figure 2. interpretation results of magnetic anomalies t in precise survey section i information, and the information must be correct, and the ground elevation cannot be used instead of the core elevation. the relationship between these aspects is correct and the time-depth relationship established in the calibration is correct. otherwise, the accuracy of the time-depth relationship is low or the relationship between time and depth is wrong (zhang et al., 2017). application of logging curve (1) check the added curve with the curve observer, check the original data for the problematic curve, and then add it again after finding the cause. (2) look at the ac curve (porosity curve) whether the sand and mudstone velocity can be separated (lei et al., 2014). generally, due to compaction, the sandstone velocity may be lower than that of mudstone. in addition, the sandstone has a reduced velocity after oil and gas. (3) if there is no ac curve (porosity curve) or ac curve (porosity curve) is not good, the time difference curve can be converted from the induction curve. (4) multi-curve analysis and comprehensive analysis of the strata, which has a better understanding of the well, making the calibration better. there are eight methods for correcting the logging curve, and they are divided into three categories: manual mode correction, log curve processing, and vsp correction. figure 3. the influence of altitude of coring on calibration 349calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration (1) manual correction the manual correction method is divided into the following four editing methods: a. table format editing. the method mainly performs point-by-point editing correction on the peaks (ie, singular values) in the logging curve. b. square wave editing. in the specified depth interval, this method provides four methods for modifying the amplitude of the logging curve by the new average method, the multiplier method, the offset method, and the constant value method. (i) new average method. a constant offset is added to all amplitudes within the selected depth range of the log curve so that the new average amplitude value matches the given average. (ii) multiplier method. multiply the amplitude value for a given depth range by a constant multiplier (zhou, 2018). (iii) offset method. a constant value is added to the amplitude of each point in the selected depth range in the acoustic time difference curve. (iv) constant value method. the amplitude value of each point in the selected depth range in the acoustic wave time difference curve is set to a constant value. (v) thickness editing. editing functions such as stretching, compressing, and deleting a segment of the log are provided. (vi) mouse editing. mousing over the existing curve is drawing a new curve value. (2) filter processing in the logging curve filtering process, we mainly use the median filtering and bandpass filtering methods to eliminate the singular points in the curve and achieve the purpose of smoothing the curve. the actual application effect is taken as the u13 well: after we filter the logging curve, the logging curve becomes smoother and has no singularity. figure 4 (a) is the original logging curve, and figure 4 (b) is the filtered logging curve. it can be seen that after filtering, some singular points in the curve have been filtered out to achieve a smooth curve. (3) vsp correction the vsp correction includes two methods: global correction and time difference correction: (1) overall correction. the error is obtained by using the vsp seismic data and the overall acoustic logging data to obtain an error curve, and then the error curve is used to correct the original acoustic wave logging curve. (2) time-division difference correction. this method only performs time difference correction on a certain section of the logging curve. set a critical value tmin, when the sound wave propagation time value is below tmin, it is considered reasonable, no correction is needed; when the sound wave propagation time value is greater than the critical value tmin, the part exceeding the propagation time is multiplied by the factor g, the formula can be expressed as: ∆ ∆ ∆t t t gcor raw= ( ) × min (1) in formula (1), tcor is the original sound wave propagation time value; tmin is the artificially set threshold (threshold value); g is the ratio of the drift point value to the sum of the sample difference values contributing to the drift. the synthetic seismic record calculated using the u13 well logging curve without vsp correction is shown in figure 5(a), and the synthetic seismic record calculated by the vsp corrected logging curve is shown in figure 5(b). comparing, it can be seen that the in-phase axis that is not obvious before the correction is more obvious after the correction process, and it is in good agreement with the actual seismic profile. frequency selection it can be seen from figure 6 that based on the frequency analysis, the filter is used to adjust the frequency, or the frequency is manually adjusted, and the band-passing t-filter and the lectra wavelet filter can be selected when the filter is adjusted to obtain the synthesized recording frequency. the recording frequency must match the seismic frequency. underground layer calibration method the above process determines the depth and undulation of the basin base by high precision gravity and magnetic exploration technology. therefore, the petroleum subterranean geological structure or rock layer properties are studied, so as to carry out calibration of the petroleum underground layer through the horizons division, time-depth transformation, and geological layer determination of the rock formation. (a) original curve (b) filtered curve figure 4. acoustic curve before and after filtering (a) vsp pre-correction record (b)vsp post-correction record figure 5. synthetic seismogram before and after vsp correction 350 kai zang, jiuchuan wei, linsong yu, fang wan, zunfang hu, yang li layer division dividing a rock formation with multiple well logs is much more reliable than dividing the horizon by acoustic time difference alone. moreover, in the case of no core section, the lithology of the reflective layer can be accurately determined. the available logging curves include seven parameters: acoustic time difference, density, neutron, resistivity, natural gamma, natural potential, and well diameter. the influence of each logging curve on lithology is shown in table 1. in the layered calibration, the thin layer selection and lithology grading are handled by the following two principles: (1) the division of the thin layer should consider the difference in speed between the thin layer and the adjacent layer and the thickness of the thin layer itself. high-speed thin layers and low-speed thin layers with large speed differences should be retained, while thin layers with small speed differences can be considered to be rounded off smoothly (song, 2017). for the thickness of the thin layer, after the time-depth conversion, the low-speed thin layer on the depth axis can be converted into the thickness on the time axis. the highspeed thin layer on the depth axis is converted into the thin layer with very small thickness after being converted into time. considering the above situation and the workload of artificial stratification, the minimum thickness of the layer is given: 1) the depth difference is greater than or equal to 4m when the highspeed thin layer is high; 2) the depth difference is greater than or equal to 2m when the low-speed thin layer is used. time-depth conversion in order to facilitate the time-depth conversion, equal-depth interval interpolation is performed on the layered average time difference and density, and the interpolation interval is 1m. the two-way travel time sequence corresponding to the horizon depth sequence can be expressed by the formula: t h t h h t j h t h v h i i j i i ( ) = ( ) + ⋅( ) ( ) = +( ) = ∑0 0 1 0 0 0 2 2 1 ∆ ∆δ β βin (2) where v m s m0 11830 0 00034= =/ ; .β − ; h0represents the starting depth of the processing of the well data. using equation (3), we can find the velocity sequence and density sequence with the two-way travel as variables for each depth difference. they can be expressed as: v t h t t t t h i n i i i i i i i i ( ) = ( ) ( ) = ( )  = … ∆ / , , , δ ρ ρ 1 2 (3) the above velocity and density sequences are not equally time-interval. for this reason, the above sequence is linearly interpolated by 2ms to obtain velocity sequences and density sequences of equal time intervals. geological horizon determination in the case of unified drilling geological horizon and reliable seismic velocity, the geological horizon method for determining seismic reflection is relatively simple. the geological horizon of the seismic reflection layer can be determined by simply comparing the seismic reflection layer with the geological horizon by synthetic recording. however, in the case where the geological stratification of the survey area is not uniform, the method of strict seismic reflection layer tracking, high-precision speed analysis (jiang & ma, 2017), and various synthetic records for geological horizon reliability test can accurately obtain the determination and unification of the seismic geological horizon. (1) geological horizon of the seismic section the synthetic record is compared with the geological horizon. first, the polarity of the seismic data needs to be determined, followed by the type of seismic wavelet. according to the regulations of the international geophysical society, the collection of seismic data in the field is always negative, that is, the single gun records the first arrival wave and jumps down. most of the domestic oil fields are consistent with this, but some oil fields are the opposite. it is stipulated that the single-shot records the first arrival of the wave, that is, the acquisition polarity is positive. the wavelet shape of the airborne airgun is fixed and is the minimum phase; the wavelet excited by the ground explosive source is also the minimum phase. according to this, the seismic data collected in the field is the positive phase profile of the minimum phase, and the data collected in the offshore field is the negative phase profile of the minimum phase. figure 6. means of frequency analysis table 1. typical log responses of several common lithologies logging curve acoustic time difference (μs/ft) density (g/cm3) neutron (φn) resistivity (ω.m) gamma ray (api) well diameter (mv) spontaneous potential (n)logging response lithology sandstone low 90-100 secondary 20-22 secondary 30 higher scores about 75 lower reduction curve rule positive or negative anomaly (relative to mudstone) mudstone high﹥100 a wide range of changes 19-24 high 30-40 low﹤10 high 90-100 irregular diameter enlargement igneous rock (basalt) low 60-90 high﹥25 low 10-20 higher scores low 30-45 irregularity petroleum high﹥130 low﹤17 high﹥40 dozens to hundreds low 30 change is not obvious. 351calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration the valleys of the reflected in-phase axis on the negative polarity section correspond to the regular reflection coefficient, and the peak corresponds to the negative reflection coefficient. the peak of the reflected in-phase axis on the positive polarity section corresponds to the regular reflection coefficient, and the two-valley corresponds to the negative reflection coefficient. when the composite record is compared with the horizon, for the reflective layers with the positive reflection coefficient, the synthetic record with the same polarity is consistent with the horizon of the seismic profile. for reflective layers with a negative reflection coefficient, the synthetic records with different polarities are also consistent with the seismic profile horizon. according to the above principle of horizon comparison, the paper compares the synthetic record with the horizon of the u13 well on the 75.6 line and the u27 well on the 90.5 line. the comparison results of the two wells are shown in table 2. from the comparison of the horizons, it can be seen that the polarity of the seismic reflection is consistent with the combination of the lithology above and below the geological interface. that is, the positive polarity of the reflection corresponds to the geological interface that is thin on the top and thick on the bottom. the geological interface corresponding to the negative polarity of the reflection is the opposite. the calibration of the synthetic record of the u13 well on the line profile is shown in figure 7. the reflection horizon on the profile corresponds well to the ankle in the synthetic record. figure 7. horizontal correlation of synthetic records in well u13 (2) inspection of layer reliability in the case of inaccurate or inconsistent geological horizons, the backcalculation speed of the horizon data is mainly used, and the reliability of the horizon is judged according to the regularity of the velocity distribution (lian et al., 2017). if the average velocity of the seismic reflection layer is accurate, the horizon determined by time-depth conversion of the reflection layer is reliable; on the contrary, if the synthetic recording depth is correct, the speed variation law based on the back-calculation of the layer data must be reasonable (yang et al., 2020). according to the reflection time t0and depth h of the u13 well and u27 well after the horizon comparison, the average velocity vm of each horizon is inversely calculated from shallow to deep. then, the curve of the average velocity as the function of depth is made, and the velocity curves of the wells are plotted in the same coordinates, as shown in figure 8. figure 8. reversed average velocity curve based on synthetic records as can be seen from figure 8, the curves are developed in the lateral direction and are nearly parallel to each other. the horizon of the u13 well formation is at a high position and the speed is high. the u27 well is located at a low position with low speed. this change in velocity coincides with stratigraphic fluctuations indicating that the horizon determined by synthetic records is reliable (wu et al., 2020). results in the experiment, the nanyang oil field in the tongbai basin appeared as an example. the u13 well on the 75.6 line, the u27 well on the 90.5 line, and table 2. comparison of seismic stratification and synthetic record stratification well number reflecting layer geological stratification synthetic record pair depth difference reflection timet0 (s) average velocity (m/s)depth (m) lithology depth (m) polarity h % u13 tng 1172 sandstone/gravel sandstone 1171 just -1 ﹤0.1 1.04 1121 ted1 1471 igneous rock/sandstone 1460 negative -11 0.7 1.23 1187 ted2 1692 mudstone/sandstone 1730 just 38 2.2 1.39 1245 ted3 1928 sandstone/mudstone 2185 negative 257 13.3 1.63 1340 tes1 2304 mudstone/mudstone 2528 negative 234 9.7 1.82 1389 u27 tng ted1 1409 igneous rock/sandstone 1403 negative -6 0.4 1.24 1131 ted2 1646 sandstone/igneous rock 1688 just 42 2.5 1.42 1189 ted3 1877 sandstone/mudstone 2078 negative 10.7 10.7 1.67 1244 tes1 2113 mudstone/mudstone 2462 just 16.5 16.5 1.9 1296 tes3 sandstone/gravel sandstone 2650 just 2.01 1318 352 kai zang, jiuchuan wei, linsong yu, fang wan, zunfang hu, yang li the inclined well b2 on the 82.3 line were selected as experimental objects to verify the practicality of the method in calibrating the petroleum underground layer (cheng et al., 2020). distribution and combination characteristics of the faults system the tongbai basin is controlled by the northwest and northeastward boundary faults, and the northern part of the basin is uplifted in the late period and is denuded to form the braided basin with characteristics of south deep and north shallow. according to the gravity and magnetic anomaly characteristics and the bedrock depth map, 14 basement faults were inferred, which can be divided into 4 groups according to the distribution characteristics. gravity and magnetic exploration were carried out on the tongbai basin by this method. the geological results of gravity and magnetic exploration are shown in figure 9 (gao et al., 2018). figure 9. geological results of gravity and magnetic exploration in tongbai basin (1) faults in the northwest direction. there are 7 faults in this group, which are the main faults in the basin, and most of them are the extension of the northwestward deep faults in the qinling fold belt in the basin. they are bamudi-niejiaxiahe faults (f1), tongbai (north)-chuanshan store faults (f2), tongbai (south)-shilibao faults (f3), wucheng-huangzhuang faults belt (there are 4 northwestward faults f4, f5, f6, f7) in the belt. among them, f2 and f3 constitute the stepped edge faults of the southwestern margin of the basin (gao et al., 2019). (2) the faults in the northeast direction, the chushandian-caodian (f14), and f2 and f3 played the controlling role in the formation of the basin. (3) faults near north-south direction. this set of faults controls the substructure of the basin depression and the tectonic pattern of the eastern stepped uplift belt. they are shenzhuang faults (f11), caodian-bashuyuan faults (f12), huangqi-guxian town faults (f13), maojiapo faults (f8), wanglaozhuangxuzhai faults (f9), and dajizhuang jinqiao faults (f10). among them, these three nearly parallel staircase faults play an important role in the evolution of the basin (f11, f12, f13). f8 forms the boundary between the bulges and depressions in the western part of the basin. f9 and f10 control the bulge of the golden bridge. (4) near east-west faults. there may be a fault in wucheng-guxian town, which is the extension of f7 to the east. it constitutes the boundary between the basin depression zone and the northern uplift zone. it can be seen from the distribution and combination characteristics of the above faults that the northwest and northeast faults jointly control the formation of the basin, while the near north-south faults play the controlling role in the later evolution of the basin. underground horizon calibration and testing horizon calibration of electrical and lithological information the b2 in nanyang oilfield is an inclined well. to make full use of the electrical and lithological information of the well, the method is used for horizon calibration. firstly, the orientation and well deviation of the different well sections of the b2 are segmented and interpolated. the spatial coordinate values are obtained by the above equation and the time-depth relationship of the well, and the position of the target layer on the seismic section is found. figure 10 is the positional view of the target horizon of the b2 on the plane, and figure 11 is the positional view of the target horizon of the b2 on the section. through the calibration of the method, the corresponding relationship of the target horizon on different seismic sections is established to achieve the purpose of stereo calibration. the method in this paper can intuitively perform the calibration work in the vertical direction. the three curves of the acoustic time difference, density, and natural potential of b2 are digitized, and the dip angle and well deviation data are input to the workstation, and segmentation interpolation is performed. the log is corrected to the vertical direction by the above equation. in the production of synthetic records, using the logging speed of the well or adjacent well, the converted acoustic curve is subjected to depth correction to complete the calibration of the logging curve. figure 12 is the comparison of the synthetic records of the b2 and the seismic traces produced by the method of the present invention. it can be seen from figure 12 that the synthetic records made using the corrected log curves have good correlation with the seismic traces. the method can be used for the horizon calibration of inclined wells. the method is not only suitable for anisotropic media, but also suitable for strata with a less lateral variation of lithology in local strata. it is more flexible and can be corrected by using various velocity data. figure 10. diagram of target horizon of well b2 on a plane projection figure 11. b2. position map of target horizon on profile 353calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration calibration method figure 13 and figure 14 show the depth domain synthetic records of the seismic profiles between the u13 and u27 in nanyang oilfield. figure 13 is the synthetic seismic record made with asymmetric wavelets and figure 14 is the synthetic seismic record made with wavelets extracted by autocorrelation analysis. qualitative comparison of synthetic seismic records and crosswell seismic profiles can be obtained from two methods. it can be seen that they are in good agreement and can be used for horizon calibration between wells. the near well is compared with the synthetic records produced by the two methods in terms of a spectrum. figure 15 shows the spectrum recorded by the u27 well. the effective frequency range is 30--550hz, and there is a relatively low-value area of the spectrum around 280hz. figure 16 is the spectrum of the synthetic record of the u27 well fabricated by autocorrelation wavelets, the morphology of which is similar to that of figure 15. only the amplitude of the low-frequency region is relatively large, and the main frequency moves slightly in the low-frequency direction. figure 17 shows the spectrum of the synthetic record of the u27 well made with asymmetric wavelets, which is quite different from the difference of figure 15. such as shape near 280hz, in the low-frequency region, and in high-frequency region, but its main frequency is relatively high, and the frequency band is relatively narrow. therefore, in the actual calibration of petroleum underground layers, the active records produced by the autocorrelation method can be used to achieve horizon calibration. figure 12. the synthetic record of well b2 and the contrast map of parallel seismic trace produced by this method figure 13. synthetic records of wells u13 and u27 produced by asymmetric waves figure 14. synthetic records of wells u13 and u27 produced by wavelets extracted by autocorrelation analysis figure 15. u27 well-side recording frequency figure 16. u27 synthetic record spectrum made by autocorrelation wavelet figure 17. u27 synthetic record spectrum made with asymmetric wavelet 354 kai zang, jiuchuan wei, linsong yu, fang wan, zunfang hu, yang li discussion (1) high precision gravity and magnetic exploration: petroleum geophysical exploration in small basins is often constrained by many factors. however, improving the economic benefits of oil and gas exploration and speeding up exploration are two important factors. the tongbai basin is representative of the lower tertiary basin in the eastern part of china. it is now used as an example to discuss the petroleum geophysical work procedures in the small basin. the high precision gravity and magnetic exploration of the tongbai basin basically identified the basement structure of the basin, the distribution of sedimentary rock series, and the distribution of the faults system, and its effect in finding local structures is particularly prominent. the local structures reflected by the local anomalies of gravity and magnetic fields are confirmed by subsequent seismic exploration data. in addition, small basins generally have poor topographical conditions, and deep and shallow geological conditions are often more complex than large basins. the gravity and magnetic exploration are much more serious than the difficulties encountered in seismic exploration. in view of the above situation, we believe that in the oil and gas exploration of the lower tertiary small basin, the reasonable procedures for petroleum geophysical exploration should be divided into the following steps: the first step is to carry out high precision gravity and magnetic exploration with less investment, quick effect, and good effect. the working scale is not less than 1:50,000 and the density of measuring points is not less than 8-10/km3. the total accuracy of several gravity anomalies is not less than 10μgal, and the magnetic measurement accuracy is not less than ±2.0nt. the second step is to arrange regional two-dimensional seismic lines according to the interpretation results of gravity and magnetic anomalies, so as to improve the regional exploration of the basin and verify the interpretation results of gravity and magnetic anomalies. the third step: using the acquired seismic data, the gravity, and seismic data are comprehensively processed to further develop the geological information of high-precision gravity and magnetic data. the interpretation results of gravity and magnetic anomalies are extended to the entire basin, and favorable areas containing hydrocarbons and favorable local structures are identified. the fourth step is to arrange two-dimensional or threedimensional seismic exploration on favorable local structures, carry out structural detailed investigation, implement traps, and provide drilling well locations. (2) marking of the petroleum underground horizon: as can be seen from the above, there is a certain error in the time between the reflective layer on the time section and the corresponding reflective layer in the synthetic records. in the inverse calculation of the average velocity curve, it is also possible to observe the deviation of individual points from the average line. the main reason for the difference between the two is that the reflection on the synthetic records is the normal reflection, and the reflection on the time profile is in-normal reflected; the reflective layer on the synthetic records and the reflective layer on the time profile are not in the same section when the formation is tilted. the position of the reflective layer on the time profile is laterally offset and laterally offset. the depth of the reflective layer is the apparent depth, which does not correspond to the vertical depth of the reflective layer on the synthetic records section. the sonic velocity used in synthetic records is different from the actual seismic velocity in the time profile, where the sonic logging data is corrected without influencing factors. the reflective layer on synthetic records is the subwave with constant frequency and amplitude and is superposed by the sequence of reflection coefficients. in the reflective layer on the time profile, the frequency and amplitude of the wavelets vary with depth. it can be seen that the reflection layer on the synthetic records and the reflection layer on the time profile can correspond exactly only after the factors causing the above errors are eliminated. in general, it is inevitable that there is a certain error in the horizontal comparison of synthetic records. in addition to technical reasons, in areas where sand-shale interbeds are developed, seismic reflection is the superposition of thin-layer reflections, and there is no clear geological horizon corresponding to the seismic horizon. sometimes, since a reflected wave group is controlled by multiple formations, it is also possible that the subsequent phase of the reflected wave group corresponds to a certain geological interface. conclusions the calibration method of the petroleum underground layer based on high precision gravity and magnetic exploration is researched in this paper. firstly, the high-precision gravity and magnetic exploration technique are used to explore the tongbai basin. it basically finds out the basement structure of the basin, the distribution of sedimentary rock series, and the distribution of faults systems, and its effect in finding local structures is particularly prominent. based on the survey of the basement structure of the basin, the distribution of sedimentary rock series, and the distribution of the faults system measured by high precision gravity and magnetic exploration, the calibration of petroleum underground horizons is realized by stratigraphic division, timedepth conversion, and horizon determination. the calibration method of petroleum underground horizons based on high precision gravity and magnetic exploration can not only accurately survey complex underground rock formations, but also accurately calibrate the petroleum underground horizons. this lays the theoretical foundation for the future oil mining industry. reference babak, s., petiteau, a., sesana, a., brem, p., rosado, p. a., taylor, s. r., lassus, a., hessels, j. w. t., bassa, c. g., burgay, m., caballero, r. n., champion, d. j., cognard, i., desvignes, g., gair, j. r., guillemot, l., janssen, g. h., karuppusamy, r., kramer, m., … verbiest, j. p. w. 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(2020). effects of water temperature on tissue depletion of florfenicol and its metabolite florfenicol amine in crucian carp (carassius auratus gibelio) following multiple oral doses. aquaculture, 515, 9. doi:10.1016/j. aquaculture.2019.734542 zhang, y., qin, z. c, & zhang, w. d. (2017). asymmetric interference alignment for device-to-device underlaying cellular networks. journal of china academy of electronics and information technology, 12, 232-236. zhou, z. q. (2018). power system harmonic detection method and management suggestion for a certain offshore oil platform. chinese journal of power sources, 35, 15-24. 10.3390/math 10.1016/j.aquaculture 10.1016/j.aquaculture palabras clave: grava; forma de gránulo; módulo de corte; relación de amortiguación. how to cite item cui, k., & sheng, h. (2019). effects of granular shape on shear modulus and damping ratio of gravel. earth sciences research journal, 23(1), 87-91. doi: https://doi.org/10.15446/esrj. v23n1.78727 the effects of the consolidation ratio, effective confining pressure, gravel content, and granule breakage on the shear modulus and damping ratio of gravel have been extensively researched in recent years. however, studies on the effect of the granular shape are rare. thus, under different confining pressures, dynamic triaxial tests were performed on gravel specimens to investigate the effect of granular shape on the shear modulus and damping ratio of gravel specimens by using a multifunctional triaxial testing instrument. the samples consisted of two kinds of gravel with the same grain composition and relative density of 45%. the test results indicate that, when the confining pressure and shear strain amplitude exceed 300 kpa and 7×10-4, respectively, gravel with a round granular shape has a higher shear modulus compared to an angular shape. conversely, when the shear strain amplitude exceeds 2×10-4, the damping ratio of angular gravel exceeds that of round granules. abstract keywords: gravel; granule shape; shear modulus; damping ratio effects of granular shape on shear modulus and damping ratio of gravel efectos de la forma granular sobre el módulo de corte y la relación de amortiguamiento de la grava issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.78727 los efectos de la relación de consolidación, la presión de confinamiento efectiva, el contenido de grava y la rotura de gránulos en el módulo de corte y la relación de amortiguación de la grava se han investigado ampliamente en los últimos años. sin embargo, los estudios sobre el efecto de la forma granular son escasos. por lo tanto, bajo diferentes presiones de confinamiento, se realizaron pruebas triaxiales dinámicas en muestras de grava para investigar el efecto de la forma granular en el módulo de corte y la relación de amortiguamiento de las muestras de grava utilizando un instrumento de prueba triaxial multifuncional. las muestras consistieron en dos tipos de grava con la misma composición de grano y una densidad relativa del 45%. los resultados de la prueba indican que, cuando la presión de confinamiento y la amplitud de tensión de corte superan los 300 kpa y 7×10-4, respectivamente, la grava con una forma granular redonda tiene un módulo de corte más alto en comparación con una forma angular. a la inversa, cuando la amplitud de la tensión de cizallamiento excede de 2×10-4, la relación de amortiguamiento de la grava angular excede la de los gránulos redondos. resumen record manuscript received: 24/07/2018 accepted for publication: 01/12/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 87-91 kai cui1*, hang sheng2 1key laboratory of high-speed railway engineering of the ministry of education, southwest jiaotong university, chengdu, 610031, china. * corresponding author: cuikai@swjtu.cn g eo te ch ni ca l en g in ee ri ng 88 kai cui, hang sheng introduction the shear modulus and damping ratio, which can describe the dynamic characteristics of gravel, must be used in the equivalent linearisation method in seismic response analysis, the evaluation of interaction of soil-structure systems, and the stability analysis of sites. therefore, there is an urgent need to study the important parameters of the dynamic shear modulus and damping ratio of the gravel in order to analyse the seismic response of gravel. many factors can affect these two parameters in all types of soil. the main factors are the soil type (rollins, evans & diegl, 1998), confining pressure (wang, geng & jin, 2005; ling, fu & cai, 2009; wang, jia & deng, 2006; wan & yang, 2013), loading time (sas, gabryś & szymański, 2015), consolidation ratio (liu, yang & chen, 2010; sun, gong & tao, 2013; sun, yuan & gong, 2011), and initial stress state (wang, luo & guo, 2011; wang, luo & guo, 2012), among other factors (ling, zhang, li, an & wang, 2015; delfosse-ribay, djeran-maigre, cabrillac & guovenot, 2004). some results on the effect of effective confining pressure, consolidation ratio, gravel content, and granule breakage, among other factors, on the dynamic shear modulus and damping ratio of gravel have been reported. wang et al. (2005), ling et al. (2009), and wang et al. (2006) studied the influences of confining pressure on these two parameters of gravel. liu et al. (2010) studied, analysed, and proposed the relationship of the dynamic shear modulus and damping ratios with various factors (consolidation ratio, porosity, test grading, etc.) and a comprehensive formula which takes into consideration the consolidation stress ratio, void ratio, and test grading curve, and can be used to calculate the maximum dynamic shear modulus. rollins et al. (1998) found that, with the increase of gravel content from 0% to 60%, the value of gmax can be as high as 38%. hardin and kalinski (2005) found that the gmax value of gravel samples clearly increases through large-scale resonant column tests (specimen height of 300 mm or 150 mm) on different gradations of sand and sand-gravel mixtures. based on the resonant column tests of soil samples with different grain-size distributions, wichtmann and triantafyllidis (2009) pointed out that, for a constant void ratio, the values of gmax do not change with the variation in the mean grain size d50. sahaphol and miura (2005) used bender element tests to study the influence of the particle breakage characteristic of volcanic soils on the maximum shear modulus. however, studies on the influence of the granular shape on the macro-physical and dynamic properties are rare. the effect of the granular shape on the shear modulus and damping ratio of gravel soil cannot be ignored, because the granular shapes and occlusal area are different among the soil particles. qian, gray and woods (1993) found that, under the same conditions (with an effective confining pressure less than 100 kpa), the maximum shear modulus of unsaturated angular sand is 5%−25% greater than that of round sand. meidani, shafiee and habibagahi (2008) found that, under the same conditions, the shear modulus and damping ratio of round gravel are greater than those of angular gravel. the granule shape has a close relationship with the parent soil composition, weathering, transportation, and deposition of the environmental conditions. tong and wang tong, wang and yu-hsing (2014) showed that the shear modulus and damping ratio can also be increased and decreased, respectively, by increasing the particle aspect ratio ar (i.e., by decreasing sphericity). however, the quantitative relation could be applied to a sand but not sure about gravel soils. some scholars attempted to use numerical techniques to simulate the effect of granule shape on the mechanical properties (liu, xie, ma & chang, 2011). these studies are also limited to a general description of the granule shape, such as “granules with a much more regular round shape” and “granules with an irregular shape.” a model or formula that calculates the dynamic properties (such as shear modulus and damping ratio) while considering the granule shapes has not been reported to the authors’ knowledge. the reason for the current situation is that a standard and simple means to effectively quantify the granule shape is lacking. many scholars noted the importance of granule shape and performed a series of studies and proposed many types of quantitative methods, such as the fourier description method and method of fractal theory. however, the quantitative description of granule shape is still not mature because many aspects continue to present difficulties: (1) the descriptions of individual granules; (2) the characteristic descriptions of the distributions of group shapes; (3) the descriptions of group shapes. this research focuses on examining the differences between the properties conferred on the mechanical characteristics of soil by angular gravel and round gravel. further, a conventional optical microscope is used to obtain digital images of the granules, which enable us to analyse the test results and discuss the effect of shear characteristics (shear modulus and damping ratio), compression characteristics, and other mechanical indices of the granule shape by performing conventional mechanical tests. dynamic triaxial test test apparatus the dynamic triaxial test was conducted in the laboratory with dyntts soil multifunctional triaxial testing instruments manufactured by gds company in the uk. as shown in figure 1, the instrument is mainly composed of the host, the axial-force measuring system, the axial-deformation measuring system, the axial control system, the confining-pressure control system, the back-pressure control system, the pore-water-pressure measuring system, and the computer system. the systems for measuring the axial-force and axialdeformation, together with the axial control system, are installed in the control cabinet; the host consists of the triaxial cell and the axial loading system; the pressure control system and the back-pressure control system are connected to two volume controllers, respectively. the greatest advantage of this instrument is the loading and pressure control system. by adjusting the pressure of the cap, the soil can be pressed. this system configuration enables all procedures to be controlled. while collecting and processing the test data, test results can be simultaneously displayed on the monitor. for testing dynamic loads, a load frequency can be selected in the range of 0.01–5 hz with a sine wave, triangular wave, rectangular wave, or any user-defined waveform. test principle under cyclic loading, the stress-strain relationship of the soil has distinctly nonlinear characteristics and shows hysteresis. the hyperbolic model, in which the loading and unloading curves are obtained from the masing rule, is widely used to describe the nonlinear characteristics of soil. in the equivalent linear dynamic viscoelastic model, the equivalent dynamic shear modulus and equivalent damping ratio can describe the nonlinear dynamic characteristics of the soil. in the dynamic stress-strain curve shown in figure 2, the shear modulus is defined as the slope of a secant line that connects the extreme points on a hysteresis loop. the damping ratio can be computed using the equation d a l a t= ( ) ( ) / 4  (1) where a(l) is the area enclosed by the hysteresis loop and a(t) is the area of the shaded triangle shown in figure 2 tested materials and sample preparation in the present paper, angular gravel and round gravel are used as the research subjects. they have low dry density and low degree of saturation. their figure 1. dynamic triaxial apparatus 89effects of granular shape on shear modulus and damping ratio of gravel uniformity and curvature coefficients are 2.09 and 1.04, respectively. gravel samples are prepared using angular gravel and round gravel, respectively, according to figure 3, which shows the grain-size distribution curve. the cylindrical reconstituted specimens are sand samples 39.1 mm in diameter and 80.0 mm in height. first, the samples are subjected to the combined saturation method using the hydraulic and back-pressure, where the saturation of samples can be greater than 98%. second, after the consolidation of the samples has been completed, cyclic loading should be applied to the samples. the loading waveform is a sine wave, and the loading frequency is 0.2 hz. the unidirectional vibration test is carried out under different amplitudes of stress. in order to avoid the influence of vibration effects on the test results, the vibration times should be minimal, and vibration times should generally be 3-5 cycles. testing program in order to study the effect of the granular shape on the dynamic shear modulus and damping ratio of gravel, the grain-size distribution and consolidation condition of angular and round samples are maintained at constant levels throughout the test. by testing the dry density of the angular and round samples under different relative densities, it is shown that, when the relative density is 45%, the difference of dry density between them is less than 3%. this implies that we can ignore the effect of dry density on the test results. thus, we maintain the relative density of all samples as 45% during reconstitution. p5 is defined as the content of gravel having a particle diameter greater than 5 mm. the specific testing program is listed in table 1. table 1. testing program gravel shape p5/% effective confining pressure/kpa dry density/g•cm3 angular sample 36 100,400,600,800,900 1.84 round sample 36 100,400,600,800,900 1.83 angular sample 44 100,400,500 1.89 angular sample 61 100,400,500 1.83 round sample 46 100,400,500 1.87 round sample 61 100,400,500 1.84 digital images of granules the digital images of the granules were acquired using a conventional optical microscope and are shown in figure 4 (grains approximately 1 mm in diameter). the ground shapes of the grains are shown in figure 4(a), and the angular shapes of the grains are shown in figure 4(b). because the angular shapes of the grains differ greatly, they are shown in figure 5 (angular grains approximately 2 and 5 mm in diameter). figure 4. digital images of grains of approximately 1 mm in diameter: (a) round gravel and (b) angular gravel figure 5. digital images of angular grains with an approximate diameter of: (a) 2 mm and (b) 5 mm results and discussions test results the loading equipment controlled by a servo motor may lose testing data at the moments of initial and final loading. when processing the test results, we need to select the 2nd, 3rd, and 4th circular data of each loading stage for the calculation in order to avoid this problem. the test results can be obtained by calculating the average dynamic shear modulus or damping ratio. in accordance with the difference of p5 values, some results can be obtained, such as the dynamic shear modulus gd and the damping ratio λ corresponding to different dynamic shear stress amplitudes γd. when the effective confining pressure is not greater than 300 kpa, under the same conditions and the shear stress amplitude value γd > 7×10-4, the round sample has a greater dynamic shear modulus. while with the shear stress figure 2. stress−strain curve figure 3. grain-size distribution curve 90 kai cui, hang sheng amplitude value of γd ≤ 7×10-4, an angular sample has a greater dynamic shear modulus. when the effective confining pressure is greater than 300 kpa, under the same conditions, the round sample has a higher dynamic shear modulus than the angular one. this is mainly because, under larger confining pressure and shear stress amplitude, the local broken angularity of the angular sample causes the occlusal force to decrease among the granules of the gravel, or local shear dilation phenomenon causes the dynamic shear modulus to decrease. when the shear stress amplitude value γd ≤ 2×10-4, under the same conditions, the damping ratio of the round sample is approximately equal to that of the angular one. when the shear stress amplitude value γd > 2×10-4, under the same conditions, the round sample has a higher damping ratio than the angular one. the reason is that, when the external profile of granules is more irregular, the occlusal points among the granules are more in number, whereas the contact area of a single occlusal point is less. an increase in the shear stress amplitude would be expected to adversely affect relative sliding or rolling among the granules of the angular sample. therefore, more energy is required to overcome the occlusal force and finishes the reconstitution of its structure. thus, under the same conditions, the damping ratio of the angular sample is higher than that of the round one. the effect of granule shape on the dynamic shear modulus in order to investigate the effect of granule shape on the dynamic shear modulus, the effective confining pressure is normalised as follows: g g pd d o a= ( )/ / .  0 5 (2) where gd is the normalised dynamic shear modulus, d is the dynamic shear modulus, σ’ 0 is the initial effective consolidation confining pressure, and pa = 100 kpa is the atmospheric pressure. the calculation results of the normalised shear modulus corresponding to p5 and the shear stress amplitude value 1×10-4, 1×10-3, and 1×10-2 are listed in table 2. this table indicates that, when the shear stress amplitude is 1×10-4, the angular sample has a greater dynamic shear modulus. the increase in the shear stress amplitude results in a large amount of relative rotation among the granules of the round sample, and much more obvious breakage is observed among the granules of the angular sample, which can lead to a significant decrease of the dynamic shear modulus of the angular sample. when the shear stress amplitude reaches 1×10-2, the large deformation will play a leading role in determining the soil properties but not the granule size and shape. in addition, the difference between the dynamic shear modulus of the round sample and the angular sample is small, but with the increase of gravel content, their dynamic shear modulus will increase. by using the hyperbolic curve model, the relationship between the dynamic shear modulus gd and the shear stress amplitude γd can be expressed as follows: g m nd d= +( )1/  (3) when γd0, gdmax=1/m. thus, the maximum dynamic shear modulus gdmax of all samples can be calculated. the increase in the effective confining pressure led to an increase in the maximum dynamic shear modulus of the gravel. however, when the shape and p5 of the gravel are different, the growth rate is also different. when p5 = 37% or 45%, with the increase in the effective confining pressure, the growth rate of the maximum dynamic shear modulus of the round sample is greater than that of the angular one. when the effective confining pressure is greater than 300 kpa, under the same effective confining pressure, the maximum dynamic shear modulus of the round sample is 10% to 15% greater than that of the angular one; when p5 is 60% and the effective confining pressure is not greater than 300 kpa, the growth rate of the maximum dynamic shear modulus of the angular sample is faster, and its value is 3% to 5% greater than that for the round one. critical gravel content p5cr of the maximum dynamic shear modulus with the increase in gravel content, the maximum dynamic shear modulus of breccia gravel and round gravel all increase first and then decrease. this indicates that the effect of p5 on the maximum dynamic shear modulus may have a limit. when p5 reaches the critical value, the maximum dynamic shear modulus of the gravel also reaches its critical value. in order to verify this estimate and determine the critical value, the relationship between the maximum dynamic shear modulus and the confining pressure is investigated. when the effective consolidation pressures are 100, 300, and 500 kpa, respectively, the maximum dynamic shear modulus of the round sample and angular sample showed a significant “parabolic” trend with varying p5. this indicates that a critical value exists under different values of p5, and the value is called p5cr, which is the critical gravel content of the maximum dynamic shear modulus. when p5 is less than the critical value, the maximum dynamic shear modulus of the gravel will increase with increasing p5. when p5 is greater than the critical value, the maximum shear modulus of gravel soil will decrease with increasing p5. in order to investigate the effect of the granule shape on p5cr, the effective confining pressure is normalized as follows: g g pd d o amax max . / /=     ⋅  0 5 (4) where dmax is the normalised maximum dynamic shear modulus, gdmax is the maximum dynamic shear modulus, σ’ 0 is the initial effective consolidation confining pressure, and pa = 100 kpa is the atmospheric pressure. the relationships between the normalised maximum dynamic shear modulus and p5 have been derived. according to the test results, the parabolic fitting effect is appropriate. when the relative density is 45% and the effective confining pressure is not greater than 50kpa, the value of p5cr of the angular and round samples is approximately 51.2% and 48.5%, respectively. conclusions dynamic triaxial tests on round and angular samples of gravel were conducted to study the effects of the granular shape on the dynamic shear modulus and damping ratio at a relative density of 45%. the main conclusions are as follows: the effects of the granular shape on the dynamic shear modulus are related to the effective confining pressure and shear stress amplitude. when the effective confining pressure is greater than 300 kpa or γd > 7×10-4, the round sample has a higher dynamic shear modulus than the angular sample. however, when the effective confining pressure is less than or equal to 300 kpa or γd ≤ 7×10-4, the angular sample has a higher dynamic shear modulus than the round one. a critical value of p5 exists for which the dynamic shear modulus of the gravel is maximised. under the same conditions, the p5cr of the angular sample is greater than that of the round sample. when γd > 2×10-4, under the same conditions, the angular sample has a greater damping ratio than the round one. acknowledgements this work was supported by the national natural science foundation of china (grant no. 41572245), sichuan science and technology program (grant no. 2019jdjq0018) and supported by the fundamental research funds for the central universities (grant no. 2682018gj02). table 2. relationship between p5 and normalised shear modulus under specific shear stress amplitudes (gd /σ ’ 0 /pa)0.5 /mpa p5=37% p5=45% p5=60% angular sample γd=1×10-4 61 71 63 round sample γd=1×10-4 60 64 59 angular sample γd=1×10-3 40 45 41 round sample γd=1×10-3 42 51 44 angular sample γd=1×10-2 9 12 17 round sample γd=1×10-2 10 18 16 91effects of granular shape on shear modulus and damping ratio of gravel references delfosse-ribay, e., djeran-maigre, i., cabrillac, r., & gouvenot, d. 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(2009). influence of the grain-size distribution curve of quartz sand on the small strain shear modulus gmax. journal of geotechnical and geoenvironmental engineering, 135, 1404-1418. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.98451 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 157 172 g eo lo g y petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia temesgen oljira1,4*, olugbenga akindeji okunlola2, akinade shadrach olatunji2, dereje ayalew3, bekele ayele bedada1 1. department of geology, pan african university, life and earth sciences institute, university of ibadan, ibadan, nigeria. as.olatunji@mail.ui.edu.ng, bekele@hu.edu.et 2. department of geology, university of ibadan, ibadan, nigeria. o.okunlola@mail.ui.edu.ng 3. school of earth sciences, addis ababa university, p. o. box 1176, addis ababa, ethiopia. 4. department of geology, adama science and technology university, adama, ethiopia. abstract the western ethiopian shield is underlain by volcano-sedimentary terranes, gneissic terranes, and ophiolitic rocks intruded by different granitoid bodies. the megele area is part of western ethiopian shield and consist of a low-grade volcano-sedimentary zone that has been intruded by mafic (dolerite dyke) and granitoid intrusions (granodiorite, diorite, granite gneiss). to establish the origin of the distinctive lithologies of the locality and evaluate its mineral potential, petrological, petrographical, and geochemical characterization of these rocks were carried out. hence, the lithological, geochemical, and petrogenetic features of the neoproterozoic granitoid intrusives and associated metavolcanic, were illustrated through a combination of field mapping, petrological, and geochemical analysis. the petrological result obtained from the thin section analysis of the granitoids and metabasalt from megele area indicates that, these rocks has been metamorphosed from lower green-schist facies to lower amphibolite facies as denoted by mineral assemblages such as albite + muscovite + prehnite+ quartz and actinolite + hornblende + epidote + garnet. the major and trace element geochemical analysis of granodiorite, diorite, and granite gneiss revealed that the rocks in the studied area were mainly calc-alkaline and peraluminous in nature in the sio2 versus na2o+k2o and a/nk versus a/cnk, the details of the results on the major and rare elements are stated in the result section respectively. the granitoids are s-type granites revealed silica saturated rock formed at the volcanic arc subduction (vag) to syn-collisional (syn-cold) tectonic setting by fractionation of lree-enriched, hree-depleted basaltic magma with considerable crustal input. this basaltic magma seems to be generated from the lree-enriched, hree-depleted mantle. in conclusion, the metabasalt is sub-alkaline (tholeiitic), metaluminous bodies generated at mid-oceanic ridge tectonic setting by partially melting of hree-depleted and lree-enriched basaltic magma. the magma sources are associated with the reworked sediment-laden crustal slabs from the subduction zone and resulted in s-type granitoid. keywords: fractional crystallization; geochemistry; megele; neoproterozoic; petrogenesis; s-type granite. petrogénesis de rocas neoproterozoicas del área de megele, oeste de etiopía resumen el escudo etíope occidental está debajo de terrenos volcánicos sedimentarios, terrenos gneissicos y rocas ofiolíticas intruidas por diferentes cuerpos granitoides. el área megele es parte del escudo etíope occidental y consiste en una zona volcánica sedimentaria de bajo grado con intrusiones máficas (diques doleríticos) y granitoides (granodioritas, dioritas, granito gneissico). en este trabajo se realizaron caracterizaciones petrológicas, petrográficas y geoquímicas de estas rocas para establecer el origen de las litologías distintivas de la localidad y evaluar su potencial mineral. a partir de las caracterizaciones se ilustraron las características litológicas, geoquímicas y petrogenéticas de los granitoides intrusivos del neoproterozoico y metavolcánicos asociados a través de una combinación de mapeo de campo y análisis petrológicos y geoquímicos. los resultados petrológicos obtenidos del análisis de secciones delgadas de los granitoides y metabasaltos del área de megele indican que estas rocas han presentado metamorfosis desde facies de esquistos bajos de color verde a facies de anfibolita, como lo denotan los grupos minerales albita + muscovita + prehnita + cuarzo y actinolita + hornblenda + epidota + granate. los análisis geoquímicos de elementos mayores y traza de gneisses de granodiorita, diorita y granito revelaron que las rocas en el área de estudio son principalmente calcoalcalinas y peraluminosas, según los gráficos sio2 frente a na2o+k2o y a/nk frente a a/cnk. los granitoides son granitos tipo s que revelan que las rocas saturadas de sílice se formaron en el arco volcánico de subducción, en un escenario tectónico syn-colisional por el fraccionamiento de magma basáltico enriquecido en tierras raras ligeras y mermado en tierras raras pesadas con una considerable aportación de la corteza. este magma basáltico, al parecer, se generó en un manto enriquecido de tierras raras ligeras y mermado de tierras raras pesadas. en conclusión, el metabasalto es subalcalino (toleítico), y los cuerpos metaluminosos se generaron en un ambiente de dorsal mediooceánica al derretir parcialmente el magma basáltico mermado en tierras raras pesadas y enriquecido en tierras raras ligeras. las fuentes magmáticas están asociadas con los bloques de corteza sedimentaria refundida de la zona de subducción y que produjeron granitoides tipo s. palabras clave: cristalización fraccionada; geoquímica; megele; neoproterozoico; petrogénesis; granitos tipo s. record manuscript received: 04/11/2021 accepted for publication: 23/06/2022 how to cite item: oljira, t., okunlola, o. a., olatunji, a. s., ayalew, d., & bedada, b. a. (2022). petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia. earth sciences research journal, 26(2), 157-172. https://doi.org/10.15446/esrj. v26n2.98451 https://doi.org/10.15446/esrj.v26n2.98451 mailto:as.olatunji@mail.ui.edu.ng mailto:o.okunlola@mail.ui.edu.ng https://doi.org/10.15446/esrj.v26n2.98451 https://doi.org/10.15446/esrj.v26n2.98451 158 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada 1. introduction in ethiopia, different geological studies have confirmed the presence of different rock types of different origins, ages, and evolution, and associated mineralizations. the main rocks exposed widely in the country are mainly fall into three broad categories; the neoproterozoic metamorphic rocks, mesozoic sedimentary rocks, and tertiary volcanic rocks (asefa et al., 1981). among them, the neoproterozoic basement complex is the largest rock suite exposed in northern, eastern, southern, and western parts of ethiopia (fig. 1a). according to (tadesse et al., 2003) most economic metallic mineral deposits are known to be found in this neoproterozoic basement complex, including precious metals like au, pt, pge, ni, ta, base metals, industrial minerals, and gemstones (i.e., ruby, emerald, sapphire, garnet), and decorative dimension stones. several studies suggested that the tectonic and thermal evolution of ethiopia’s juvenile neoproterozoic volcano-sedimentary terranes of low-grade metamorphic belts and associated granitoid intrusions is primarily responsible for these metallic and nonmetallic mineral resources (tadesse et al., 2003; warkisa et al., 2021). even though the geology of western ethiopia is still not well understood, different studies have been conducted in the region from different perspectives. western ethiopian shield is classified into arabian nubian shield which comprises of neoproterozoic low-grade metamorphic rocks and mozambique belt includes moderate-to-high-grade gneisses (kazmin, 1971; kazmin et al., 1978; kebede et al., 1999). furthermore, detailed studies have been conducted in specific parts of the area focusing on geotectonic evolutions (ayalew and johnson 2002), petrology, geochemistry, and tectonic settings (kebede et al., 1999; alemu and abebe, 2007; warkisa et al., 2021). the asosa area in the western ethiopian shield which is the focus of this study has been one of the targeted areas for placer gold mining by artisanal workers since the 1930s (bullock and morgan, 2018). based on the results of the field investigation, geological mapping and preliminary geochemical information (bullock and morgan, 2018) revealed that there has primary gold mineralization in shear zone related quartz veins, quartz-graphite schist, pegmatite, and quaternary basalts. furthermore, (bullock and morgan, 2018) reported that there is an association of gold with pyrite, chalcopyrite, galena, and with minor pyrrhotite, hematite, and graphite ores. however, the petrological, geochemical, and tectonic setting description of the neoproterozoic rocks of the megele area is yet unknown. thus, using an integrated field relationship, petrographical characteristics, major oxide, and trace elements interpretations induced the petrogenetic, geochemical and tectonic setting character of the neoproterozoic metamorphic and granitoid intrusive rocks units from the study area (megele area) of western ethiopian shield. 2. geological setting of western ethiopian shield east african orogeny (stern, 1994) which is the largest continuous neoproterozoic -cambrian orogen (fritz et al., 2013) traditionally subdivided as arabian nubian shield (ans) in the north, comprising juvenile neoproterozoic crust (abdelsalam and stern, 1996; johnson et al., 2011; johnson and woldehaimanot, 2003; stern, 1994) and the mozambique belt (mb) in the south composed of pre-neoproterozoic crust with tectonothermal overprint spanning from neoproterozoic to early cambrian (bingen et al., 2009; collins, 2006; fritz et al., 2006; de waele et al., 2006). western ethiopian shield is part of the east african orogeny and is composed of neoproterozoic lowto highgrade metamorphic and mafic-ultramafic rocks (fig. 1). this western ethiopian shield is the southern extension of two contemporaneous pan african belts i.e., the arabian nubian shield and mozambique belt. arabian nubian shield (ans) is composed of neoproterozoic low-grade volcano-sedimentary rock units whereas the mozambique belt (mb) is made of high-grade gneiss and reworked continental crusts (fig. 1). both neoproterozoic arabian nubian shield and mozambique belt metamorphic complexes are intruded with different generations of synto post granitoid intrusive rocks (asrat and barbey, 2003; tadesse et al., 2003). the western ethiopian shield shows an inter-fingering of the high-grade gneisses and low-grade volcano-sedimentary rock units. they consist of yubdo-dalati-tulu dimtu ultramafic rocks belts which are correlated figure 1. map showing: (a) the distribution of the neoproterozoic pan-african belts (ans and mb) and reworked pre-neoproterozoic crust in ethiopia (modified after asrat et al., 2001) and (b) western ethiopian shield in the vicinity of megele area (after allen and tadesse, 2003). 159petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia to the arabian nubian shield (kebede et al., 1999; alemu and abebe, 2007). according to (blades et al., 2015) the arabian nubian shield neoproterozoic basement complex is composed of a variety of supra-crustal and plutonic intrusive rocks that developed in the tonian volcanic arc setting. the pre-, syn-, and post-tectonic plutonic intrusive rocks of varying composition have intruded the arabian nubian shield of western ethiopian neoproterozoic basement rocks (warkisa et al., 2021) (warkisa et al. 2021). in view of (blades et al., 2015, 2019; warkisa et al., 2021) the western ethiopian shield is originated during the east and west gondwana collision and evidence for a complete tectonic cycle, island-arc, and arc-accretion activities. as studied by (ayalew, 1997; ayalew et al., 1990; ayalew and peccerillo, 1998) the western ethiopian shield consists of baro, birbir, and geba domains (fig. 1b). the lithotectonic rocks units of these domains are mainly developed in n-s, nne-ssw, and slightly w-e orientations. baro and geba domains are comprised of quartzofeldspathic gneiss, biotite-hornblende gneiss, biotite gneiss, amphibolite facies, migmatites, and numerous subconcordant lenses of granitic-pegmatitic rocks units (ayalew et al., 1990; ayalew and peccerillo, 1998; stern et al., 2004; wolde et al., 1996). these rocks are characterized by medium to coarse-grained textures. the birbir domain is extended to the asosa region and confined between the geba domain from the east and baro domain from the western direction. this domain is composed of low-grade meta-volcano sedimentary, mafic to felsic granitoid intrusions, volcanoclastic, and carbonate rocks. later (ayalew and johnson, 2002) recognized that the birbir domain rocks units which are fine-grained schists, metamorphosed to the lower amphibolite facies and exhibited primary sedimentary structures like bedding, cross lamination, and load-casts features. further (allen and tadesse, 2003) classified tuludimtu ophiolitic belt (tdb) (in the northern section of western ethiopian shield) into didesa domain, kemashi domain, dengi domain, sirkole domain, and daka domain from east to west using lithostratigraphic and structural similarities. among these, the kemashi, dengi, and sirkole domains are largely comprised of neoproterozoic low-grade volcano-sedimentary rocks, located in the central part of the tuludimtu ophiolitic belt, enclosed from east and west by the didesa and daka domains respectively (fig. 1b) whereas the didesa and daka domains are composed of neoproterozoic high-grade gneissic rock units. the kemashi domain consists of an ophiolitic sequence such as mafic-ultramafic volcanic, plutonic, and sedimentary rocks of oceanic affinity, which evolved from oceanic crust at an arc-continent suture zone (allen and tadesse, 2003; blades et al., 2015). the dengi domain is made up of mafic to felsic intrusive, the succession of volcanoclastic, volcanogenic, and carbonate deposits that indicate a volcanic arc tectonic setting. as investigated by (allen and tadesse, 2003) sirkole domain is characterized by an alternate layer of high-grade gneiss and low-grade volcano-sedimentary rock sections and which are thought to be part of an imbricated basement-cover thrust-nappe complex. the calc-alkaline metavolcanics intercalated with metasediments and intruded by calc-alkaline plutons underpin the central part of the tuludimtu ophiolitic belt. allen and tadesse (2003) identified different generations of synand post-kinematic plutonic intrusion rocks in the western ethiopian tuludimtu neoproterozoic ophiolitic rocks belt. 3. sampling and analytical methods an integrated field mapping, petrographical, and geochemical studies were conducted during this study. totally 25 representatives rock samples were collected from the study area (megele) in the sirkole domain, asosa, western ethiopia. the collected samples for analysis are representative and collected from insitu outcrop by breaking the inner rocks to maintain the freshness of the samples. among the gathered samples, 25 rock samples (3 from amphibole schist, 3 from diorite, 5 from granite gneiss, 3 from granodiorite, 3 from quartz graphite schist, 5 from metabasalt, and 3 from talc schist) were analyzed for petrographic study. while 21 fresh rock samples (3 from amphibole schist, 2 from diorite, 5 from granite gneiss, 2 from granodiorite, 2 from quartz graphite schist, 5 from metabasalt, and 2 from talc schist) were analyzed for whole-rock geochemical study (table 1). thin sections were made at center for geoscience laboratory, geological survey of ethiopia and studied at school of earth science, addis ababa university, ethiopia to decipher the texture, mineralogy, and to evaluate the degree of alteration. for whole-rock geochemical analysis, the selected samples were first dried, crushed, and pulverized at als global services geochemistry laboratory, addis ababa, ethiopia. the pulverized samples were then sent to the analytical testing service (als), ireland for major, trace, and rare-earth elements analysis. then, each pulverized sample of 0.2 gm is thoroughly mixed well with lithium metaborate/lithium tetraborate flux of 0.9 gm and ignited at 1000°c in a furnace (als, 2021). after cooling, the resultant melt is dissolved in 100 ml of a 4% (hno3)/2% (hcl) solution. then, icp-ms (agilent 7700) and icp – aes (agilent 5800) are used to examine the resultant solution for major oxide, trace elements including rareearth elements (als, 2021). the analytical methods were selected bearing in mind their capacity to detect wide range of determinants in the all of the sample media at background level and as good as significantly better than the natural geochemical variations. the analytical precisions are estimated to be 2% for major elements, and 5% and 10% for trace element concentration higher or lower than 20 ppm respectively. the detection capacity of the selected methods ranges from 0.01% to 100% for major element and 0.01% to 10,000 ppm for trace elements. but for cr and v, the detection capacity ranges from 10 to 10,000 ppm and 5 to 10,000 ppm respectively. figure 2. map showing the detailed geological map of megele area 4. results 4.1. local geology the megele area is characterized by the exposure of quartz-graphite schist, amphibole schist, talc schist, diorite, granite gneiss, metabasalt and granodiorite marked by mafic intrusions (dykes), quartz veins, and veinlets (fig. 2). the general trend of the units in this locality is nne-ssw. specifically, the granite gneiss trends nne-ssw and dips 48° north-eastward, while the talc schist dips 12° southward (fig. 2). using field exposure and microphotographs of thin sections, the major rock units are described below (fig. 3-9). 160 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada figure 3. exposure of fractured granite gneiss filled with quartz vein (a and b); microphotographs in xpl: (c) granite gneiss showing anhedral muscovite filling fracture within alkali feldspar, (d) euhedral garnet within alkaline minerals. note: bt-biotite, msc-muscovite, qtz-quartz, plag-plagioclase feldspar, aspar, alkali feldspar, hblhornblende, grt-garnet. figure 4. (a) exposure of fractured granodiorite filled with quartz vein; (b) dyke perpendicularly cutting the granodiorite; microphotographs in xpl (c and d) anhedral opaque minerals within alkaline minerals. note: act-actinolite, bt-biotite, qtz-quartz, opq-opaque 161petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia 4.1.1. granite gneiss the granite gneiss is the dominant unit in megele area and makes a contact with granodiorite, diorite, talc schist, amphibole schist, quartz-graphite schist and metabasalt (fig. 2). it dominantly appears light grey to grey and crops out everywhere forming mountains and plutonic plugs. the granite gneiss rock unit appears medium to coarse-grained textures and comprised of quartz, feldspars, and micas minerals. large phenocrysts of black micas are occasionally observed throughout the rock and the color of the rock dominantly resumed the color imposed by the mica minerals. the granite gneiss has been fractured and the fracture has been filled by silica forming quartz veins (fig. 3a, b). locally, it has been fractured, weathered, and altered. in addition to silicification that is commonly observed in the rock especially, near the quartz veins and veinlets, the rock has also undergone sericitization and kaolinization, and epidotization. the petrographic descriptions reveal that the granite gneiss consists of euhedral quartz, alkali-feldspars, plagioclase, and other alteration minerals (fig. 3c and d). the other observed alteration minerals are muscovite, garnet, chlorite, and hornblende. the most dominating minerals in the rock are feldspars orthoclase outclassing among them (fig. 3.a). the mineral constituents of the rocks are: k-feldspar (26%), quartz (29%), biotite (11%), plagioclase (9%), hornblende (17%), muscovite (5%), and opaque=3%. some plagioclase (with polysynthetic twins), muscovite, and hornblende exist. in some of the plagioclase crystals, micro fracturing is observed showing the inclusion of quartz within the cracked part of the plagioclases. the muscovite in the sample is both primary and secondary. the hornblende appears greenish to brownish green and at some of its rim, there are opaque minerals which are probably magnetite that resulted from oxidation of fe (fig. 3d). very rare sericite with grossular garnet inside it and biotite are other components that constituted the rock. sericitization is interpreted as a process that involved in the alteration of k-feldspar enhanced by the presence of hydrothermal fluids. the rarely observed garnet in the rock shows fractures that might have resulted from compressional deformation that resulted in fracturing that is also visible at a larger scale in the field. the opaque minerals observed in the rock range between anhedral and euhedral crystal forms with some visible isometric and rhombohedra minerals. the fractured body and occurrence of low-grade metamorphic minerals indicate that the garnet in the rocks is primary. however, there are also secondary anderite grossular garnet grains that are characteristics of low-grade metamorphism of granitoid biotite. 4.1.2. granodiorite the granodiorite, which makes contact with granite gneiss, diorite, quartz-graphite schist, and amphibole schist (fig. 2). this rock unit is the second-largest rock unit in the study area. it is a massive to weakly foliated rock. it is a light to dark grey unit mainly composed of quartz, feldspars, and biotite (fig. 4c, d). this unit has been intruded by basaltic dike perpendicular to the contacts and the regional foliation (fig. 4b). the unit consists of multiple quartz veins. the field observation of this granodiorite unit shows the presence of plenty of visible phenocrysts of quartz and pyrite. the mineral constituents of the rocks are: quartz (29%), plageoclase (24%), alkaline-feldspar (10%) biotite (15%), garnet (3%), hornblende (4%), actinolite (6%), chlorite (6%), and opaque minerals (3%). the anhedral quartz minerals are dominant in the rocks followed by elongated plagioclase (fig. 4c). while the hornblende and biotite are elongate and anhedral respectively in shape situated within the anhedral quartz (fig. 4c, d). 4.1.3. diorite the diorite unit in the study area cropped out around the abuloshele area and bounded by granite gneiss, granodiorite from the southwest and by amphibole schist and metabasalt from the northeast (fig. 2). it appears grey to dark-grey in color and it is composed of medium to coarse grain minerals with visible quartz, biotite, and pyroxene. in a similar fashion to granodiorite, the diorite unit has been cut by quartz veins of different orientation which resulted in an intense degree of silicification. apart from silicification, the rock has also been subjected to chloritization, epidotization, and oxidization. the major oxide mineral in this unit and most of the units in the area is a brownish-black iron oxide, magnetite. petrologically, it contains minerals like quartz (27%), plagioclase (21%), biotite (13%), hornblende (10%), and epidote (5%), chlorite (7%), actinolite (9%), garnet (4), amphibole (2%), and opaque minerals (2%) (fig. 5b, c, and d). the diorite thin section microphotograph (fig. 5c) shows anhedral biotite and muscovite segregating from anhedral quartz forming alignment; and euhedral garnet growth at the boundary of quartz, plagioclase and alkali feldspar (fig. 5d). figure 5. (a) exposure of fractured diorite filled by two generation of quartz vein; microphotographs in xpl of diorite (b and c) showing anhedral biotite and muscovite segregating from anhedral quartz forming alignment; and (d) euhedral garnet growth at the boundary of quartz, plagioclase and alkali feldspar. note: btbiotite, ms-muscovite, plg-plageoclase, qtz-quartz, a-spr: k-feldspar, grt-garnet. figure 6. (a) exposure of massif metabasalt; microphotographs in xpl: (b) metabasalt showing platy orthopyroxene within anhedral quartz groundmass; (c) elongated plagioclase and (d) anhedral augite and chlorite evolving within alkaline minerals. note: aug-augite, bt-biotite, chl-chlorite, ep-epidote, plg-plageoclase, fsp-feldspar, qtz-quartz, opx-orthopyrxene. 4.1.4. metabasalt the metavolcanic rock is exposed at the eastern section of the study area (megele) in the sirkole domain, western ethiopia (fig. 2). the rock unit appear greenish to dark color, show a sharp contact with the granite gneiss, and quartz-graphite schist where it can be observed. degrees of chloritization and epidotization are much higher in this unit than in the others with moderate silicification. silicification is relatively more intensely observed around the 162 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada quartz veins and veinlets within itself or where it contacts the intermediate rocks. this unit is the third-largest unit after granite gneiss and granodiorite in the area. the samples from metabasalt show porphyritic texture in a fine-grained, holocrystalline matrix under a thin section with plagioclases dominating among the phenocrysts. mineralogically, the metabasalt is dominated by pyroxenes and plagioclases with minor chlorite, hornblende, and opaque minerals. the metabasalt in the area are of two different mineralogy. the first units consist of some amount of quartz (~4vol%), the other minerals including albite, chlorite, ortho-pyroxene, and no to very rare olivine. the other units have more mafic minerals like ortho-pyroxene, green amphibole, chlorite, and epidote than the formers with lesser quartz. the overall mineralogy of the metabasalt is: quartz (16%), pyroxene (13%), plagioclase (20%), biotite (10%), chlorite (6%), hornblende (11%), epidote (5%), amphibole (7%), alkali-feldspar (4%), prehnite (3%) and opaque minerals (5%) (fig. 6b, c and d). the presence of rare prehnite in both metabasalt samples indicates sub-greenschist facies metamorphic grade. the photo micrographs of metabasalt shows platy orthopyroxene within anhedral quartz groundmass (fig. 6b); elongated plagioclase (fig. 6c) and anhedral augite and chlorite evolving within alkaline minerals (fig. 6d). figure 7. exposure: (a, b) talc schist, and microphotographs: (c and d) talc schist showing different minerals. note: all microphotographs were taken in xpl. tlc talc, mgsmagnesite. most of this field of view is covered by talc 4.1.5. talc schist this unit is a thin two to three-meter-thick talc schist layer found between granodiorite and granite gneiss units (fig. 2). it is characterized by a whitish color and slightly foliated. this unit is exposed around yohotse and korondo rivers. it has sharp contact with surrounding granodiorite units. from the petrographic analysis the rock is composed of 60% talc, 20% magnesite, 15% actinolite and 5% opaque (fig. 7c and d). talc, magnesite and opaque minerals show well developed parallel alignment and they define strong foliation for this thin section. this thin section displays schistose texture. 4.1.6. quartz-graphite schist extensive quartz-graphite is observed around the abuloshele, korondo, and yohotse river trending nne-ssw direction in the area (fig. 2). this unit is sandwiched between granodiorite to the west and granite gneiss and amphibole schist to the east and north east respectively (fig. 2). the quartz-graphite schist unit appears grey and shows carbonization alteration. thin section of this rock unit contains 51% graphite, 35% quartz, 9% muscovite (sericite) and 5% biotite (fig. 8c, d). quartz, graphite, micaceous (sericite), opaque and biotite show strong parallel alignment and they define strong foliation as a result the thin section show schistose texture. quartz and graphite display xenoblastic texture, muscovite (sericite) display tiny platy and biotite displays flaky. figure 8. exposure: (a) quartz-graphite schist, (b) thick quartz vein; microphotographs (c, d) of quartz-graphite schist showing different minerals: note: microphotographs (c) taken in ppl while (d) were taken in xpl. qtz-quartz, gprgraphite, bt-biotite, opq-opaque. note: parallel alignment of minerals suggesting schistose texture. 4.1.7. amphibole schist this unit is a thin two to three-meter-thick amphibole schist layer found bounded by diorite in the east and granodiorite and talc schist units in the northwest (fig. 2). the amphibole schist is dark green, slightly foliated massive units in place (fig. 9a, b). this unit is exposed in yohotse and korondo rivers. it has sharp contact with surrounding units and striking nne and dips at 47o south-eastward. the mineralogical constituent of this rock is: quartz (25%), hornblende (21%), muscovite/sericite (14 %), actinolite (13%), chlorite (10%), epidote (7%), biotite (9%), plagioclase (6%), and opaque 5% (fig. 9c, d). minerals like biotite, chlorite, hornblende and muscovite (sericite) show well developed parallel alignment of schistosity. hornblende altered to chlorite and biotite replaced by chlorite. quartz crystals are strained due to deformation figure 9. exposure: (a) amphibole schist, and microphotographs (c in ppl and d in xpl): amphibole schist showing different minerals. qtz-quartz, hblhornblende, chl-chlorite. 163petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia 4.1.8. quartz vein the quartz vein in the megele area are distributed through granitoids, diorite, granodiorite, and graphite schist units. they show two general trends; nearly e-w and n-s trending veins and most of them are long in their strike direction and possess varying width ranging from 5 cm to 3 m. at some localities, the quartz veins of different orientations cross one another and leave the n-s oriented ones displaced (fig. 10a, b, c, d). the displacement of these n-s oriented quartz veins when they are cut by the e-w oriented veins infers that the formers are formed earlier. the displacement of the n-s oriented quartz veins was caused by the creation of fractures in the e-w oriented veins, which were then filled with hydrothermal fluid concentrations. in the study area, a few of the quartz veins are randomly oriented and lack continuity. these quartz veins are believed to be metamorphic in their origin and precipitated in deformational openings related to metamorphism. 4.1.9. the mafic dyke the mafic (basaltic) intrusive bodies of varying thicknesses which range from 10 to 50cm intrude the granodiorite (fig. 11). these dykes are oriented figure 10. field photograph showing quartz veins of different orientation figure 11. field photograph showing mafic dyke cutting the granodiorite layer e-w and nnw-sse and they are perpendicular to both the contact between the metagranodiorite and granite gneiss and the regional foliations. 4.2. geochemistry the whole-rock geochemical analytical results for major oxides and trace elements including rare-earth elements are presented in (table 1). the selected whole-rocks samples are affected by significant hydrothermal alterations such as epidotization, chloritization carbonization, sericitization, silicification, pyritization, and oxidation and mineralization. it is probably due to these alteration effects that these analyzed rock samples show results less than 100 wt% total of the oxides (which is ~99 wt% in altered granite gneiss, metabasalt, and granodiorite and as low as ~ 98wt % in the graphite schist). in the harker variation graphs, however, certain distinct geochemical trends can be seen (fig. 10). table 1. whole-rock geochemical analysis result for selected neoproterozoic rocks of megele area: major oxides (wt%) and trace and re element (ppm) 164 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada rock name granodiorite quartz graphite schist talc schist metabasalt sample code atpgd1 atpgd2 atpgp-1 atpgp2 atp-t1 atp-t2 atpmb1 atp-mb2 atp-mb3 atp-mb4 atp-mb6 major elements wt% sio2 60 55.8 85.7 82.1 78.6 76.6 54.7 54.4 56.8 56 52.9 al2o3 17.1 18.8 5.54 5.17 12.8 13.1 17.3 17.1 15.75 15.15 6.18 fe2o3 6.75 6.66 3.04 2.29 2.32 2.8 2.3 2.27 10.4 11.75 5.06 cao 2.61 2.76 0.04 0.05 0.02 0.02 0.38 0.35 5.82 6.35 0.03 mgo 3.06 2.91 0.11 0.15 0.69 0.71 0.09 0.05 4.94 6.2 0.1 na2o 4.35 5.87 0.26 0.24 0.13 0.14 10.6 10.45 4 3.33 0.24 k2o 1.82 1.84 0.87 1.19 4.2 4.24 5.16 5.17 0.51 0.13 0.86 tio2 0.97 1.08 0.31 0.35 0.35 0.35 0.21 0.21 0.29 0.34 0.31 mno 0.07 0.07 0.01 0.03 0.02 0.02 0.23 0.23 0.18 0.19 0.03 p2o5 0.35 0.43 0.03 0.03 0.01 0.01 0.1 0.09 0.07 0.06 0.03 loi 3.24 3.08 4.32 6.14 2.14 2.01 5.31 5.09 1.52 1.24 4.65 total 100.32 99.3 100.23 97.74 101.28 100 96.38 95.41 100.28 100.74 70.39 trace elements (ppm) ba 834 812 1455 2020 1350 1320 37.1 29.7 53.6 65.4 1405 ce 60.2 53.9 28.9 38.4 13.3 11.8 73.4 62.8 15.5 7.8 35.5 cr 50 50 240 330 20 20 20 10 30 50 290 cs 0.73 0.76 0.89 1.21 0.71 0.82 2.49 2.26 0.66 0.74 0.99 dy 4.18 4.56 6.94 5.95 2.7 2.45 4.67 4.16 1.68 1.44 6.67 er 2.4 2.5 5.16 4.75 2.14 2.27 2.87 2.71 1.24 0.9 4.22 eu 1.51 1.69 1.12 1.07 0.32 0.2 1.38 1.29 0.48 0.31 0.98 ga 18.8 19 9.3 9.4 16.3 16.8 46.7 41 15.1 14.2 9.9 gd 5.66 5.74 7 6.23 1.84 1.66 4.28 4.24 1.54 1.21 6.4 hf 4.6 4.4 2.4 2.6 7.8 8.1 24.7 22.4 2.4 0.6 2.2 ho 0.86 0.97 1.78 1.53 0.66 0.64 0.93 0.82 0.43 0.32 1.51 la 31 26.8 25.8 29.7 7.4 6.9 35.2 29.2 7.5 4.2 27.2 lu 0.41 0.39 0.78 0.71 0.51 0.57 0.55 0.48 0.2 0.21 0.64 nb 8.6 9.8 4.7 5.8 14.9 15.1 432 384 35.7 1.9 4.8 nd 28.5 26.9 22.3 26.6 6.9 6.5 25.1 22.5 6.5 3.7 23.4 pr 7.15 6.61 5.34 6.16 1.66 1.66 7.75 6.9 1.71 0.96 5.57 rb 38.4 35.7 32.5 37.1 73.7 73.7 197 172 15.7 2 31.9 sm 5.53 5.29 4.7 4.63 1.39 1.52 4.65 4.13 1.4 0.95 4.65 sn 2 2 3 1 3 2 3 2 1 1 2 sr 363 377 188.5 109 47 47.3 60.4 51.9 224 235 167 ta 0.5 0.5 0.2 0.1 0.6 0.6 27.4 24.1 2.8 0.1 0.1 tb 0.69 0.74 1.09 0.91 0.41 0.34 0.74 0.68 0.27 0.18 0.92 th 5.82 6.12 3.87 5.09 2.2 2.22 27.2 24.5 2.9 0.64 4.91 tm 0.42 0.42 0.79 0.7 0.46 0.37 0.44 0.48 0.18 0.14 0.65 165petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia rock name granodiorite quartz graphite schist talc schist metabasalt sample code atpgd1 atpgd2 atpgp-1 atpgp2 atp-t1 atp-t2 atpmb1 atp-mb2 atp-mb3 atp-mb4 atp-mb6 u 2.17 2.39 2.48 2.41 2.79 2.59 9.5 8.48 1 0.19 2.32 v 156 150 321 366 16 11 7 8 237 286 352 w 3 5 4 1 3 2 3 2 1 2 6 y 23.8 26.6 64.3 56.8 20.6 20 28.2 24.7 10.7 9.1 52.8 yb 2.4 2.44 5.06 5.11 3.03 3.07 3.64 2.94 1.27 1.18 4.09 zr 172 181 100 104 314 317 1300 1130 120 23 94 cd 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 co 26 24 1 1 1 1 1 1 26 34 1 cu 6 3 22 2 7 6 1 1 131 125 12 li 10 10 10 10 10 10 30 30 20 20 10 mo 1 1 4 1 2 2 3 4 1 1 8 ni 14 12 7 2 9 1 1 1 9 17 18 pb 10 10 9 7 5 5 2 2 2 3 8 sc 11 13 9 8 5 5 1 1 34 38 9 ti 10 10 10 10 10 10 10 10 10 10 10 zn 49 51 9 2 20 20 162 171 93 95 25 ∑ree 150.99 139.03 116.17 131.85 42.86 40.18 192.56 166.95 42.52 23.46 122.54 rock name amphibole schist granite diorite sample code atpamb5 atpamb7 atpamb8 atp-g1 atp-g2 atp-g3 atp-g4 atp-g5 atp-g03 atp-g04 major elements wt% sio2 48.5 45 45.3 72.9 73.2 71.7 72.9 72.9 55.5 58.7 al2o3 10.7 10.5 10.4 14.1 13.7 14.95 15.3 15.15 42.7 39.4 fe2o3 15.2 14.8 15 3.38 2.85 2.8 2.68 2.16 0.9 0.66 cao 9.89 12.95 13.2 1.78 1.55 0.51 0.56 0.46 0.04 0.03 mgo 9.28 10.4 10.3 0.51 0.46 0.57 0.57 0.57 0.02 0.02 na2o 2.05 0.69 0.68 3.52 3.07 4.66 4.74 4.69 0.64 0.49 k2o 0.16 0.21 0.19 4.35 4.89 2.96 3.07 3 0.16 0.14 tio2 1.97 1.59 1.64 0.31 0.29 0.27 0.27 0.27 0.01 0.005 mno 0.31 0.16 0.17 0.05 0.05 0.03 0.03 0.02 0.01 0.01 p2o5 0.03 0.15 0.16 0.12 0.1 0.09 0.09 0.08 0.01 0.01 loi 1.53 2.72 2.63 0.76 0.94 1.24 1.39 1.23 1.12 1.18 total 99.62 99.17 99.67 101.78 101.1 99.78 101.6 100.53 101.11 100.645 trace elements (ppm) ba 25 14 28.5 916 1300 1155 1220 1125 25 26.1 ce 26 16.2 16.2 43 64.1 49 50.9 47.8 20.8 16.5 cr 140 920 950 20 20 30 30 30 40 40 cs 0.01 0.01 0.01 1.73 1.28 0.46 0.56 0.43 0.33 0.31 166 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada rock name amphibole schist granite diorite sample code atpamb5 atpamb7 atpamb8 atp-g1 atp-g2 atp-g3 atp-g4 atp-g5 atp-g03 atp-g04 dy 4.99 3.32 3.33 3.27 3.74 1.1 0.95 0.95 3.6 3.27 er 2.88 1.66 2 1.9 2.55 0.38 0.37 0.54 1.72 1.58 eu 1.47 1.07 0.98 0.8 0.99 0.71 0.6 0.72 0.27 0.21 ga 16.1 20 19.4 17.6 16.3 21.4 21.2 20.9 28.5 26.7 gd 5.6 4.21 4.08 3.78 4.81 1.78 2.08 1.68 4.15 3.64 hf 3.6 2.5 2.5 4.9 4.7 4 4 3.7 2.5 3.1 ho 1.09 0.64 0.69 0.68 0.81 0.16 0.17 0.15 0.64 0.59 la 11.6 6.7 6.8 17 41.4 31.8 32.1 28.1 9.4 7.5 lu 0.42 0.19 0.23 0.37 0.36 0.06 0.06 0.06 0.24 0.26 nb 7.1 6.5 7.8 7.4 6.1 3.8 3.7 3.4 0.7 0.8 nd 16.3 11.5 11.2 16 30.2 18.9 17.8 17.4 11.6 9.5 pr 3.55 2.31 2.35 4.21 8.38 5.27 5.26 5 2.96 2.38 rb 1.1 1.5 1.1 99.8 100.5 60.2 62.2 60.4 11.5 10.3 sm 4.39 3.39 2.87 3.74 5.1 2.86 2.59 2.7 3.54 3.45 sn 1 1 1 3 2 1 1 1 1 1 sr 226 123 128.5 143 161.5 408 408 388 10.4 11.3 ta 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.7 0.5 tb 0.89 0.57 0.6 0.59 0.65 0.18 0.22 0.21 0.67 0.6 th 0.97 0.44 0.51 6.75 6.09 5.99 5.89 6.27 4.14 3.57 tm 0.45 0.24 0.28 0.33 0.36 0.04 0.04 0.02 0.22 0.21 u 0.35 0.15 0.14 1.34 0.92 1.44 1.5 1.5 1.79 2.17 v 283 220 247 25 25 24 27 26 118 122 w 2 4 4 2 2 3 1 2 3 3 y 27.3 17 18.9 20 23.5 5 5.2 4.7 21.8 19.5 yb 2.66 1.49 1.43 2.22 2.44 0.39 0.32 0.28 1.87 1.57 zr 143 89 94 187 174 157 156 159 28 30 cd 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 co 57 81 78 2 4 2 2 2 1 1 cu 1 66 119 7 5 12 14 22 4 2 li 10 10 10 30 30 10 10 10 80 70 mo 7 1 1 1 1 1 1 1 1 1 ni 92 696 693 4 1 7 7 6 1 2 pb 3 2 2 16 12 11 10 8 2 4 sc 41 24 24 4 4 2 2 2 1 1 ti 10 10 10 10 10 10 10 10 10 10 zn 138 106 105 46 39 40 39 40 2 2 ∑ree 81.94 55.21 54.67 101.88 169.52 116.53 117.35 109.24 64.02 54.06 167petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia the granite gneiss samples have a major oxides concentrations for sio2 (71.7-73.2 wt. %), al2o3 (13.715.3 wt. %), fe2o3 (2.42 wt. %), cao (0.481.78 wt. %), na2o (3-4.69 wt. %), k2o (2.96-4.89 wt. %), and (<1 wt. % each) for mgo, tio2, mno, and p2o5 (table 1). the concentrations values of fe2o3, mgo, p2o5, and mno decrease with increasing sio2, while the sio2 increases with increasing in al2o3, na2o, k2o, and cao (fig. 12). trace elements including rare earth elements like ba, sr, and rb increases with increasing sio2 (table 1). the chondrite-normalized ree pattern for granite gneisses (fig. 16a, b) plot to the values of (nakamura, 1974) showed depletion of hree relative to lree and a negative eu anomaly. the primitive mantle-normalized plot of (sun and mcdonough, 1989) revealed decreasing concentration of nb, sr, p, ce, eu, ti, weak trough at nd and zr, and weak enrichment peaks at ba, k, pb (fig. 16c, d). the major oxides concentrations of diorite rock ranges from sio2 (55.5– 58.7 wt. %), al2o3 (12–14 wt. %), fe2o3 (2–12 wt. %), cao (7–9 wt. %), mgo (4–5 wt. %), tio2 (>1 wt. %), proportional k2o and na2o contents (2–3 wt. % each), and very low (<1 wt. % each) mno and p2o5 contents (table 1). as sio2 concentration increase the fe2o3, and mgo concentrations decreases, while al2o3, na2o, k2o, cao, tio2, and p2o5 concentrations are increasing (fig. 12). the chondrite-normalized ree pattern for diorite plot to the values of (nakamura, 1974) shows enrichment in lree relative to hree and a negative eu anomaly (fig. 16a, b). the primitive mantle-normalized plot of (sun and mcdonough, 1989) show a strong depletion in ba, nb, sr, ti, and p with a weak trough at pb and u (fig. 16c, d). the granodiorite samples have sio2 range of 55.8-60 wt %, high al2o3 (17.1-18.8 wt %), high fe2o3 (~6.7wt %), mgo (~2.98 wt %), cao (~2.66 wt %), na2o (4.35-5.87 wt %), low k2o (~1.83 wt %), tio2 (~1 wt %), while mno and p2o5 show very low (<1 wt. % each) concentrations (table 1). the concentrations of fe2o3 and mgo decrease with increasing sio2 while the concentrations of al2o3, na2o, k2o, cao, tio2, and p2o5 increase with increasing sio2 (fig. 12). the chondrite-normalized ree pattern for granodiorite plot to the values of (nakamura, 1974) indicates depletion of hree relative to lree and shows a negative eu anomaly (fig. 16a, b). the granodiorite trace elements primitive mantle-normalized plot of (sun and mcdonough, 1989), show decreasing nb, sr, p, ce, eu, ti concentration with a weak trough at nd, zr and reveal weak enrichment peaks of ba, k, and pb (fig. 16c, d). the metabasalt samples have a sio2 range of 45-56.8 wt %, a high range of al2o3 (10.4-17.3 wt %), a low high range of fe2o3 (2.27-15.2 wt %), high range of cao (0.38–13.2 wt %), high range of na2o (0.68-10.6 wt %), high range of k2o (0.13–5.17wt %), tio2 (0.21-1.97 wt %), and minor concentration of mno and p2o5 (<1 wt % each) (table 1). the concentrations of fe2o3, mgo, cao, and tio2 decrease with increasing sio2 while the concentrations of al2o3, na2o, and k2o increase with increasing sio2 (fig. 12). the chondritenormalized ree diagram for metabasalt (fig. 16b) normalized plot to values of (nakamura, 1974) shows slight enrichment in lree relative to hree with a negative eu anomaly. the primitive nmorb plot (fig. 16d) normalized to the values of (sun and mcdonough, 1989), show a strong depletion in ba, ti, and p (atp-mb1, atp-mb2, and atp-mb5), weak crest at nb, and cs (atpmb4 and atp-mb5) with a weak trough at nb, rb and zr (atp-mb7 and atp-mb8). the quartz-graphite schist samples have a range of sio2 (82.1-85.7 wt %), al2o3 (~5.35 wt %), low range of fe2o3 (2.29-3.04 wt %). while the concentration of cao, mgo, na2o, k2o, tio2, mno and p2o5 is very low (<1 wt % each) (table 1) in this rock unit. the concentration of ba and sr decreases with increasing concentration of sio2. the talc schist samples have a range of sio2 (76.678.6 wt %), al2o3 (~13 wt %), low fe2o3 (~2.3 wt %), k2o (~4.22 wt%), with very low concentration of cao, mgo, na2o, tio2, mno and p2o5 (<1 wt % each) (table 1). 5. discussion 5.1 petrography and metamorphism the neoproterozoic rocks and associated intrusive of megele area comprise granite gneiss, granodiorite, metabasalt, diorites, and genetically linked quartz-graphite schists (amena et al., 2016; bullock and morgan, 2015; bullock and morgan, 2018). granodiorite and granite gneiss intrusions were cut by different generations of the mafic dolerite dykes. quartzo-feldspathic minerals are partially segregated from magnetite-biotitic minerals mainly in the sheared and altered domains of the granite gneiss and granodiorite unit. amphiboles and biotite mineral grains show elongated and preferred orientation of foliations in the diorite, granite gneiss, and granodiorite units. the biotite-magnetite alteration is linked with the most deformed sections of diorite, granite gneiss, and granodiorite suggesting earliest alteration events in megele area (bullock and morgan, 2015; amena et al., 2016). the secondary muscovite is commonly observed in association with the feldspars inferring replacement of the feldspars by muscovite due to alteration. this suggests the earliest alteration event in the megele area. in granitoid rocks, even though the dominance of minerals that can occur in the wide range of temperature and pressure conditions makes it difficult to understand the grade of metamorphism, the field-based and petrographic study of the rocks shows that they were subjected to low to medium-grade of metamorphism. some researchers like (tulloch, 1979) used the existence of secondary minerals like minor prehnite, andradite-grossular garnet, and epidote to infer that the granitoids underwent low-grade metamorphism and alteration products of granitoid biotite. the other metamorphic mineral that shows metamorphism of the granitoids of megele area neoproterozoic rocks is fibrous actinolite which occurs in association with biotite and other mafic minerals (like hornblende and actinolite). this mineral is commonly observed in granodiorite than in granite gneiss suggesting it is a product of alteration of mafic minerals. prehnite is observed as lenticular to the bulbous body within mafic minerals like biotite, secondary chlorite, and hornblende in the metagranitoids. their association with biotite indicates that they are metamorphic products replacing primary biotite (tulloch, 1979). the presence of these minerals in the assemblage is an indication of lowtemperature metamorphism which is also expected in other parts of the western figure 12. harker variation diagram for major oxides of the megele area neoproterozoic rocks: (a) sio 2 vs. fe 2 o 3 ; (b) sio 2 vs. mgo; (c) sio 2 vs. al 2 o 3 ; d) sio 2 vs. cao; (e) sio 2 vs. na 2 o; and (f) sio 2 vs. k 2 o. 168 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada ethiopian shield as the area is believed to be a collisional and accretion zone. generally, the established mineral assemblages for granitoids of megele area are shown as: prh + act + msc + bt + ab+ grt + hbl + qtz + kfs. the presence of prehnite (minor), actinolite, albite, and biotite in the assemblage reflects that the metamorphism took place in greenschist facies where low temperature moderate pressure conditions existed. the mafic/andesitic metavolcanics (metabasalt) of the area also underwent a low to medium-grade metamorphic process. the presence of mafic minerals like prehnite which are characteristic of greenschist facies indicating metamorphism at low temperature and pressure. the presence of albite indicates dynamic environment thereby suggesting low-grade regional metamorphism. the metabasalt of the megele area is characterized by mineral assemblage: prh + ab +act + opx + aug + qtz + amp (warkisa et al., 2021). 5.2 petrochemistry the result of major oxides was interpreted to elucidate the petrogenetic characteristics of the neoproterozoic rocks of the megele area. in granodiorite and quartz veins, the fe2o3, mgo, and cao contents decrease as silica increases (fig. 12), implying fractionation of plagioclase and other mafic minerals during the course of magmatic differentiation and hydrothermal alteration process (rollinson, 1993). on the other hand, k2o and na2o, is characterized by decreasing pattern for granite gneiss, granodiorite, diorite, talc schist and quartz-graphite schist, while it shows increasing pattern for metabasalt on the same silica fractionation line, supporting feldspar fractionation during hydrothermal alteration. others (asrat and barbey, 2003; ayalew and peccerillo, 1998) revealed that in a subduction environment, basaltic magma generated from mantle start magmatic evolution by crystallizing ca-rich plagioclase. this basaltic magma continues to fractionate more as the magma continue mixing with fluids generated from hydrothermal and wall rock assimilation na-rich plagioclase as it cooled. from the tas diagram after cox et al., (1979), as shown in fig. 13a, the granite gneiss of the studied area plot as granites, whereas the granodiorite and diorite samples plot as diorite. also, the metabasalt plot in the diorite, gabbro and nepheline syenite field. this suggest a mafic source with alkaline mineral enrichment that evolved through magma fractionation. despite the fact that the original geochemical characteristic of these intrusive rocks was believed to have been altered by hydrothermal fluid generated during metamorphism process and granitoid intrusions. most of the intrusive rock samples exhibit calc-alkaline features (fig. 13b). majority of granitic intrusions in ethiopia intruding into the ans are characterized by calc-alkaline composition (ayalew and peccerillo, 1998; asrat et al., 2001; bullock and morgan, 2015). this is also in line with the abundance of calcalkaline, s-type granites intrusions in syn-collisional and/or subduction-related environments reported in another parts of ethiopia (asrat and barbey, 2003; ayalew and peccerillo, 1998). the alumina saturation index (asi) plot to the value of (shanda, 1943) shows that most of the megele area intrusive rocks are peraluminous, while metabasalt rocks are moderately metaluminous (fig. 13c). the clear division of the granitoid (granite gneiss and granodiorite) as peraluminous (s-type granites) (fig. 13c) possibly indicates magma sources generated from subduction of plates of sedimentary rock affinity at the collisional environment. according to (frost et al., 2001; shand 1943) peraluminous granitoids (s-type granites) are produced by partial melting of subducting sedimentary blocks in syn-collisional complexes. the silica-rich, mica-bearing granodiorite of the megele area, which is enriched by albite and calcium aluminum-rich minerals, with few mafic minerals, could represent proof of molten magma generated at syn-colg and/or subduction setting (asrat and barbey, 2003; ayalew and peccerillo, 1998). 5.3 tectonic setting implication the geochemistry of granitoid and basaltoids has been used as a proxy of tectonic evolution (pearce et al., 1984; rollinson, 1993). the tectonic discrimination diagrams generated by using incompatible trace element variation (pearce et al., 1984; fig. 16 and pearce, 1982; fig. 15) support classification based on major element and assist in inferring types and source of magma, processes, and tectonic environment. several investigations have revealed that intrusive rocks disclose complicated routes linking tectonic processes to geochemical fingerprints, such as trace element mobility during deformation and hydrothermal changes. scholars like (pearce et al., 1984; goldfarb et al., 2001; saunders et al., 2014) have demonstrated that immobile trace elements (such as rb, y, and nb) can be used to characterize the diverse tectonic setting of magmatic rocks. pearce et al., (1984) used the combinations of these trace elements (like rb, y, and nb) to discriminate the tectonic setting of magmatic rocks due to their relative stability during alteration and metamorphic processes. figure 13. classification diagrams of; (a) tas classification (after cox et al., 1979) (ta: total alkalis, s: silica); (b) the afm classification; (after irvine and barager, 1971) (a: total alkalis, f: total iron, and m: mgo); and (c) the asi classification a/ nk vs. a/cnk (a = al2o3, n = na2o, k = k2o, c = cao) (after shanda, 1943). pearce (1982) revealed that the ti-zr diagram could be used to characterize basalts of island-arc tholeiitic, calc-alkaline, and morb affinity. thus, the combination of rb-(y+nb) and nb–y cross plot have been used to discriminate the tectonic setting of the neoproterozoic intrusive rocks from the megele area that have only been metamorphosed to amphibolite facies. as a result, the neoproterozoic granitoid rocks from the megele area plotted on the volcanic arc granite (vag) and vag + syn-colg fields of the rb-(y + nb) and nb–y cross plots, respectively (fig. 14 a and b), showing that these granitoids are generated in subduction plate boundaries. on the other hand, the megele area metabasalt which is metaluminous to slightly peraluminous, sub alkaline tholeiitic basalts (fig. 13a, b respectively) plot on the morb field of the zr-ti diagram (fig. 15). the nature of these rocks (i.e., calc-alkaline, peraluminous character, as well as the weak metaluminous) are in agreement with the tectonic environment of most granitoid intrusions within a volcanic arc granite (vag) and perhaps syn-colg zone (asrat, barbey and gleizes, 2001). most of the peraluminous intrusions are formed from magma generated from mantle at volcanic arc granites (vag) setting. the partial melting of basaltic magma generated at subduction setting could also generated peraluminous intrusions (harris, pearce and tindle, 1986). the syn-collisional (syn-colg) granitoids are thought to be produced at orogenic belts, with relatively high rb/zr and ta/nb ratios and low k/rb ratios, while basaltic magmas are characterized by enrichment of selective lile (harris et al., 1986). other scholars (e.g., taylor and mclennan, 1995) revealed that the enrichment in trace elements like th and 169petrogenesis of the neoproterozoic rocks of megele area, asosa, western ethiopia u, as well as depletion in ba, nb, sr, p, and ti (fig. 15b, d respectively), are common in magma generated related to arc environment. on the other hand, metamorphic alteration in the megele area neoproterozoic intrusive rocks may have caused migration of large ion lithophile elements (lile) potentially overwriting the initial geochemical signature (rollinson, 1993). for example, rb enrichment in granite gneiss is mostly associated with growth of mica (biotite and muscovite) following potassic alteration. the depletion in rb in the granodiorite and diorite rocks of megele area, on the other hand, could be linked to chloritization caused by sericitic alteration of feldspar by hydrolysis process in the presence of k, s, and oh (pearce et al., 1984). the increase in concentration of lile compared to hfse (fig. 16a and b, respectively) in the intrusive rocks of megele area suggests subduction setting, implying that they were formed by calc-alkaline magmas containing considerable inputs from assimilation of disintegrated felsic crust. figure 14. tectonic setting discrimination of intrusive rocks from the megele area showing: (a) all the granitoids plotted in vag on the rb (y + nb) cross plot and (b) all the granitoids plotted in vag+syn-colg on the nb–y diagrams (after pearce et al., 1984). note: vag stands for volcanic arc granites; org stands for oceanic ridge granites; wpg stands for within plate granites; syn-colg stands for syn-collision granites. figure 15. tectonic setting discrimination of the megele area metabasalt using zr vs. ti cross plot (after pearce, 1982). 5.4 ree pattern the megele region granitoid and metabasalt rocks show a slight enrichment in lree relative to hree on a chondrite ree plot normalized to the value of (nakamura, 1974) as shown on fig 14 a, b, c and d respectively. in comparison to hree of intrusives and metabasalt, the ree pattern (fig. 16) shows a significant enrichment in lree. this is in agreement with the general ree pattern of vag of the other areas (e.g., ujuga granitoid of western ethiopia) (kebede et al., 1999). this is probably due to the basaltic magma enriched in lree, that could be generated from subducted sediment slabs at a subduction zone (e.g., warkisa et al., 2021 and references therein). the fluctuation in the concentration of major oxides and trace elements with increasing sio2 concentration (fig. 12) indicates that granitoid rocks of the megele area experienced fractional crystallization. the primitive mantle normalized ree diagram for both granitoid and metabasalt shows decrease in the concentration of some trace elements like ba, nb, sr, p, and ti (fig. 16c, d), suggests that these elements are present in the fractionated magma. the segregation of plagioclase and k-feldspar minerals from the evolving magmatic fluid could be responsible for the observed negative eu anomaly and depletion in the concentration of ba and sr (rollinson, 1993; fig. 16a, b). the depletion of nb-ti could have been caused by the segregation of titanium bearing mineral phases like rutile, titanite and ilmenite from evolving melt (ngatcha et al., 2019 and reference there in). while the weak troughs in p reflected the segregation of accessory minerals apatite from the melt. some researchers (chappel et al., 2012) argued that it could be challenging to produce considerable volumes of magma of peraluminous composition by removing ferromagnesian minerals. therefore, the peraluminous feature of granite gneisses was most likely caused by the melting process (ngatcha et al., 2019). the megele area neoproterozoic granitoids show deep to moderately negative eu anomaly that might be related to fractionating plagioclase (rollinson, 1993). the proportionally higher concentration of lree compared to hree and negative eu anomaly (eu/ eu* = 0.60–0.95, except for one (atp-g5 where eu/eu* = 1.02)) in addition to lower concentrations of ca and sr, indicates fractionation of plagioclase (rollinson, 1993). the hydrothermal alteration is indicated by enrichment of eu anomaly particularly in one of the granite gneiss sample (atp-g5 where eu/eu* = 1.02) probably due to chemical contribution from hydrothermal fluid (dawood et al., 2005). the trace element enrichment in both granitoid (fig. 16c) and metabasalt (fig. 15d) is coherent to magma derived from reworked crustal impute (clements and stevens, 2012). figure 16. diagram showing: (a, b) trace and re element of megele area intrusive rocks vs. chondrite, normalizing values are from (nakamura 1974), and (c, d) trace element of megele area intrusive rocks vs. primitive mantle, normalizing values are from (sun and mcdonough, 1989). conclusions the major conclusions of this study are: • the study area (megele) in the sirkole domain is comprised of neoproterozoic granite gneiss, granodiorite, diorite, amphibole schist, quartz-graphite schist, and metabasalt rocks which has been metamorphosed from lower green-schist facies to lower amphibolite facies as indicated by mineral assemblages such as albite + muscovite + chlorite + prehnite + epidote + quartz and actinolite + hornblende + epidote + garnet respectively. • the neoproterozoic granitoid geochemical signatures of megele area indicates dominantly calc-alkaline, peraluminous to slightly metaluminous, s-type granites produced by fractionation of basaltic magma that has been generated from mantle through subduction of sediment laden crustal slabs at subduction environment. 170 temesgen oljira, olugbenga akindeji okunlola, akinade shadrach olatunji, dereje ayalew, bekele ayele bedada • the ree chondrite and primitive mantle-normalized trace element patterns reveal enrichment in lree compared to hree, as well as enrichment in th, u, sn, and mo, with depletion in ba, nb, sr, p, eu, and ti and negative eu anomaly. this indicates fractionation of underplating lree-enriched magma from reworked basaltic slab underlain by sediments at subduction zone. acknowledgement this research is part of the ph.d. dissertation work of (t.o.), which was supported by the african union through the pan african university institute of life and earth sciences (including health and agriculture), paulesi. the comments and criticism of the editor(s) and reviewer(s) to the improvement of this paper are greatly appreciated. reference: abdelsalam, m. g., & stern, r. j. 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(1996). neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from adola, southern ethiopia. precambrian research, 80, 261–279. doi.org/10.1016/s03019268(96)00018-6 https://doi.org/10.1016/s0899-5362(99)00126-8 https://doi.org/10.1016/s0899-5362(99)00126-8 https://doi.org/10.1016/0016-7037(74)90149-5 https://doi.org/10.1016/j.lithos.2019.06.028 http://dx.doi.org/10.1093/petrology/25.4.956 http://dx.doi.org/10.1016/b978-0-08-095975-7.01117-7 https://doi.org/10.1016/s0166-2635(04)13003-x http://dx.doi.org/10.1144/gsl.sp.1989.042.01.19 http://dx.doi.org/10.1144/gsl.sp.1989.042.01.19 https://doi.org/10.1016/s0899-5362(03)00048-4 https://doi.org/10.1016/s0899-5362(03)00048-4 https://doi.org/10.1029/95rg00262 https://doi.org/10.1029/95rg00262 https://doi.org/10.1016/j.jafrearsci.2006.01.018 http://dx.doi.org/10.1016/j.jafrearsci.2021.104145 http://dx.doi.org/10.1016/j.jafrearsci.2021.104145 https://doi.org/10.1016/s0301-9268(96)00018-6 https://doi.org/10.1016/s0301-9268(96)00018-6 baep-author-id13 baep-author-id14 baep-author-id15 baep-author-id16 baep-author-id17 baep-author-id18 baep-author-id19 baep-author-id20 baep-author-id21 baep-author-id22 keywords: sgeological structure; ecological environment; destruction; restoration countermeasures. palabras clave: : roca de semicarbón; evolución geológica; minería sin pilar; peligros de la zona ciega; extracción de gas. how to cite item huang, y. (2020). destruction process and restoration countermeasures of the ecological environment of a comprehensive geological structure. earth sciences research journal, 24(4), 429-437. doi: https://doi.org/10.15446/ esrj.v24n4.92387 aiming at the destruction of the ecological environment of the comprehensive geological structure, the traditional restoration countermeasures have the problems of high input cost and low economic benefits. for this reason, the ecological environment destruction process and restoration countermeasures of the comprehensive geological structure were proposed. the common geological structure characteristics and the impact of activities on the ecological environment in the mining area were analyzed. based on the analysis results, an evaluation system for the degree of damage to the ecological environment by mining activities was constructed. according to the expert scoring method, a rating standard was developed to measure the degree of damage to the ecological environment. depending on the degree of damage, appropriate recovery measures were formulated. the experimental results show that compared with the traditional restoration countermeasures, considering the destruction of the ecological environment during the mining process of comprehensive geological structures, the proposed restoration countermeasures have the advantages of low cost and high economic benefits. abstract destruction process and restoration countermeasures of the ecological environment of a comprehensive geological structure proceso de destrucción y contramedidas de restauración del entorno ecológico de una estructura geológica integral issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.92387 con el objetivo de destruir el entorno ecológico de la estructura geológica integral, las contramedidas de restauración tradicionales tienen el problema de un alto costo de los insumos y bajos beneficios económicos. por esta razón, se propuso el proceso de destrucción del medio ambiente ecológico y las contramedidas de restauración de la estructura geológica integral. se analizaron las características de la estructura geológica común y el impacto de las actividades sobre el medio ambiente ecológico en una área minera. con base en los resultados del análisis, se construyó un sistema de evaluación del grado de daño al medio ambiente ecológico por las actividades mineras. de acuerdo con el método de calificación de expertos, se desarrolló un estándar de calificación para medir el grado de daño al medio ambiente. dependiendo del grado de daño, se formularon las medidas de recuperación adecuadas. los resultados experimentales muestran que en comparación con las contramedidas de restauración tradicionales, considerando la destrucción del ambiente ecológico durante el proceso de extracción de estructuras geológicas integrales, las contramedidas de restauración propuestas tienen las ventajas de bajo costo y altos beneficios económicos. resumen record manuscript received: 13/11/2019 accepted for publication: 08/10/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 429-437 yanyan huang school of management, heilongjiang university of science and technology, harbin, 150022, china *corresponding author: hyy123_75@163.com en vi ro nm en ta l g eo lo g y 430 yanyan huang introduction economic development and ecological environmental protection are major issues of common concern in countries around the world today. development is the main body of the times in all countries and regions today, and it has become the key to solving all other problems. various countries and regions are doing everything possible to promote their own development (staccione et al., 2019). the ecological environment is the prerequisite for economic development, the resources and environmental foundation required for human survival and development, and the indispensable material and energy guarantee for regional development. in the idea of sustainable development, sustainable economic development must be based on a virtuous cycle of the ecological environment. without the material flow and energy flow created by the ecological environment system, the economic system cannot operate, and the sustainable development of the economy cannot be talked about. therefore, there is a close interdependence and restriction between economic development and the ecological environment (song et al., 2019). in a sense, the course of economic development in human society is a dialectical unified process of human beings’ continuous recognition, transformation, respect, and obedience to the ecological environment. ecological protection work is to continuously enhance the capacity for sustainable development, improve the ecological environment, significantly improve the efficiency of resource utilization, promote harmony between man and nature, and promote the entire society to a civilized development path of productive development, wealthy life, and good ecology. as one of the goals of building a well-off society in an all-around way, it further clarified the relationship between humans and the ecological environment and its significance in achieving sustainable development (dochartaigh et al., 2019). geological disasters caused by the destruction of the ecological environment are disasters related to geological effects that endanger the safety of people’s lives and property caused by natural factors or human activities during the development and evolution of the earth. sudden geological disasters are the main activities that occur suddenly and complete disasters in a relatively short period of time. they mainly include volcanoes, earthquakes, collapses, landslides, mudslides, etc., which cause human life and property loss (sur & chauhan, 2019). china is one of the countries with the most severe geological disasters in the world. there are many types of disasters, high frequency of occurrence, wide geographical distribution, and large disaster losses. it severely restricts the development of the national economy in disaster-prone areas and threatens the safety of people’s lives and property in disaster prone areas (jurczak et al., 2018). over-exploitation of resources is one of the main factors leading to geological disasters. whether it is underground mining or open-pit mining, the impact on the geological environment mainly includes three aspects: the destruction of the subsidence to the soil environment, water resources and ecological environment, the destruction of the ecological environment by waste (grinding stone, slime, tailings water), and the destruction of greenhouse gases and mine wastewater to the atmosphere and water during mining (josepheph & humphries, 2018). therefore, the geological environment problems encountered in the mining process were analyzed and studied, and at the same time, the current status of environmental protection and rehabilitation of mine geology in china and abroad was used for reference, and targeted prevention measures were studied and discussed. from the implementation of measures and investment of funds, ecological environment restoration countermeasures were proposed. eventually, the problems of high input costs and low economic benefits of ecological restoration in traditional environmental restoration countermeasures were solved. materials and methods the ecological environment and mining resources are important material foundations on which human beings depend for survival and development. for a long time, due to the unreasonable development and utilization of resources, a variety of mining environmental geological problems have been raised (chen et al., 2019). according to the statistical analysis of national geological environmental survey data, environmental geological problems caused by mining activities mainly include geological disasters such as landslides, collapses, mudslides, ground subsidence, ground subsidence, and ground fissures, occupation and destruction of land resources, destruction of water resources, land desertification and soil erosion (webster et al., 2019; cui & wang, 2019). the specific content is shown in table 1. considering the geological structure of the mining area, from the perspective of the four problems shown in the table, the damage of mining to the ecological environment was analyzed, and then corresponding countermeasures for ecological environment restoration were proposed. common geological structural features according to the investigation of actual mining projects, common geological structures in mining projects include faults, subsidence columns, and fold structures. taking into account the characteristics of the above geological structures, the damage to the ecological environment was analyzed. generally, in the mining area, there are reverse faults and normal faults that are substantially parallel to the axial direction of the anticline, and faults develop along the two sides of the anticline. the normal fault is the fault that the upper disk descends and the lower disk rises, as shown in figure 1. the reverse fault is the fault that the upper disk raises and the lower disk falls, as shown in figure 2. table 1. major damage problems caused by mining to the ecological environment type division expression form affect result geological disaster natural stress-strain changes in rock and soil during mining, causing collapse, landslides, mudslides, ground subsidence (karst collapse), ground subsidence (goaf subsidence), ground fissures cause economic losses and casualties, complicate resource construction and mining, increase workload and reduce mining efficiency occupation or destruction of land, vegetation, and other resources occupying and destroying land, destroying surface vegetation, destroying geological sites, destroying landforms and landscapes, destroying buildings and transportation infrastructure decreased forest, grassland, and arable land, deteriorating habitat conditions for wild animals, reduced crop yields, and reduced forestry yields water damage damage to surface and groundwater systems near mining areas regional hydrological and hydrogeological conditions are destroyed, the original balance of groundwater and surface water is broken, and water resources are reduced water and soil pollution surface water pollution, groundwater pollution, soil pollution, etc. water bodies and soils are polluted by harmful substances and soil quality deteriorates 431destruction process and restoration countermeasures of the ecological environment of a comprehensive geological structure destroyed, and the geological ecological environment was severely damaged. a large amount of waste rock mined will occupy mountain slopes, and there will be serious hidden dangers of geological disasters. figure 1. normal fault figure 2. reverse fault in the case where the geological structure of the mining area is a fault, the roof near the fault plane is relatively fragile, and it is easy to delaminate and fall off, and cause a safety accident. at the same time, more solid waste is produced, which affects water resources and the atmospheric environment. in addition to faults, in some limestone formations, karst is formed due to long-term dissolution of flowing groundwater. under the action of gravity in the geological structure and the upper overlying rock layer, some karst caves have collapsed. because this collapse is a round or irregular oval column, it is called the collapse column (rosińska et al., 2019). the characteristics of the collapse column are shown in the figure below. the feature of the collapse column shown in the figure is that the contact surface between the collapse column and the surrounding normal rock formation is jagged. it is mainly composed of slumped rock blocks. the filling rock has the characteristics of large and small size, obvious edges and corners, different shapes, and mixed accumulation (buttler & wilson, 2018). if the collapse column is damaged, mining cannot be performed normally, which limits the effective use of machinery and seriously affects the safety of mining. the fold structure in the geological structure is divided into anticline and syncline. the geological characteristics of the two are shown below. in actual mining projects, most choose anticline. the anticline forms a valley, and the anticline is a mountain. it is easier to mine at the anticline, and the anticline is relatively loose in geology, and the topsoil layer is easy to be stripped. during the mining process, a large amount of forest vegetation was figure 3. the collapsed column in the complex formation figure 4. geological characteristics of anticlines and synclines 432 yanyan huang the transmission process of the impact of mining on the ecological environment underground resource exploitation causes changes in the stress of the original rock, leading to breakage and movement of the rock formation, further causing ground surface settlement and deformation. caving zones, fissure zones, and curved subsidence zones are usually formed above the mined-out area, which can be simplified into the process of rock breaking and surface settlement (yuan et al., 2019). due to the mining strength and the particularity of the overlying rock properties, thickness, and structure, the above process has its special performance. when the mining depth is small, there is only a fall zone and a fissure zone above the goaf, which shows that the ground surface is discontinuously deformed, and the surface cracks are directly connected to the goaf. in this case, due to different roof conditions and mining heights, and different speeds, roof cutting and sand breaking accidents will occur. the topcut accident is mainly determined by the thickness ratio of the bedrock to the load layer (referred to as the base-load ratio). relevant research shows that when the base-load ratio is less than 0.8, the roof sinks along with the coal wall steps. if the amount of step-like subsidence is large enough, a top-cut accident will occur. sand breaking accidents usually occur when the ratio of depth to thickness is less than 25. in the case of large mining depth, it still conforms to the formation law of the “three zones”, but if the mining depth is not large enough, the downward fractures in the curved zone and the upward fractures in the goaf may still be conductive (yaegashi et al., 2018). accompanied by the rock breaking process and the surface subsidence process is the process of surface animal and plant habitat change and landuse change, and the succession process of the mining area ecosystem at any time. the changes in the habitats of animals and plants after mining resources are manifested by changes in topographic conditions, changes in groundwater table depth, changes in hydrological conditions such as surface sinks, changes in site conditions such as soil, and changes in suitable biological populations. land-use change is a comprehensive reflection of changes in the surface environment. changes in land use structure have further caused changes in regional ecosystems (wu et al., 2018). ecosystem succession has a long-term nature. it is not only affected by natural factors, but also by many human activities, including resource extraction activities and ecological restoration activities after extraction. it is relatively simple to describe the changes of thesurface animal and plant habitats and the changes in land use. as long as the changes in the indicators and spatial distribution of relevant factors are listed, they can be clearly displayed. the description of ecosystem succession is relatively complicated. it involves the succession of ecosystem structure and function, which is a long-term and gradual process. the specific process is shown in the following. one of the salient features of surface damage caused by mining is the development of ground fissures. the treatment of ground fissures is not only the focus of ecological restoration but also an important factor affecting the safe production of resources and the production and life of local residents. according to the development period, there are two types of temporary cracks during mining and permanent cracks after surface stabilization. common ground cracks are shown in the figure below. groundwater is closely related to plant growth and conservation of fragile ecological environments. large-scale resource mining activities not only cause the ground surface to collapse and change the shape of the surface but also have a greater impact on the region’s own poor groundwater resources. the impact mode of underground mining on groundwater can be summarized as follows: in the preparation stage of mining, groundwater is mainly affected by artificial dredging and drainage measures and has nothing to do with the damage of rock formations, and the decline of groundwater level is extremely significant; in the mining stage, in addition to the effects of drainage measures, groundwater is also affected by changes in the formation of the stratum and the development of water-conducting fissures. groundwater movement is mainly based on vertical seepage to goaf; in the recovery phase, groundwater is mainly affected by the recharge and water use in the adjacent area, and the water level recovery process is slow. at this time, the distribution of diving space is significantly affected by the fluctuation of the bedrock. vegetation is the most important indicator of ecological health. the largescale ground cover has a high correlation with the monthly change in rainfall, and the correlation coefficient between vegetation cover and monthly rainfall can be as high as 0.99. rainfall is still the dominant factor influencing changes in mining vegetation cover. according to the mining subsidence theory, the surface subsidence deformation can be divided into the neutral zone, compression zone, and lazhong zone. due to the spatial heterogeneity of soil water in these three areas, the response changes of vegetation in the deformation area after mining figure 5. the transmission process of the impact of mining on the ecological environment figure 6. surface cracks 433destruction process and restoration countermeasures of the ecological environment of a comprehensive geological structure subsidence are also spatially different (robertson et al., 2018). compared with the compressed area, lazhong area, and neutral area, the vegetation is easier to improve, and even the vegetation is better than the vegetation cover before mining subsidence due to moderate disturbance. the reason is that the neutral zone is often located in the central area of the surface subsidence basin, and its lower terrain is beneficial to the collection of surface water, and the soil water content increases. the vegetation in the lazhong area is more susceptible to the negative impact of mining subsidence. increased soil porosity and the appearance of a large number of permanent cracks in the lazhong area will aggravate soil water evaporation and cause damage to plant roots. evaluation of the degree of damage to the ecological environment by mining based on the above analysis content, the quantitative and qualitative theory is used to evaluate the damage degree of mining to the ecological environment, and then corresponding recovery measures and enforcement measures are proposed according to different degrees. there are three main references for assessing the extent of damage to the ecological environment by mining: first, the extent of damage, that is, the scope, intensity, and duration of damage to development and construction projects, and whether the damage is related to the dominant factors of the ecosystem. the second is the sensitive nature of the ecological environment, that is, whether it affects sensitive targets, whether it exacerbates regional natural disasters, and whether the required level of protection is particularly high. the third is the nature of destruction, that is, the reversibility, recoverability, and compensability of destruction, one-time destruction or accumulation, and so on. based on the above, the evaluation index system for the damage caused by mining to the ecological environment is determined, as shown in the following table. table 2. the evaluation index system for the damage caused by mining to the ecological environment level i evaluation indicators level ii evaluation indicators level iii evaluation indicators geological environment background conditions in the mining area geomorphic conditions vegetation distribution land use population density average rainfall geological hazard types impact of mining activities geological disaster occurrences economic losses death toll sphere of influence occupation and destruction of land resources occupied damaged land area goaf area wastewater disposal annual emissions is it toxic solid waste accumulation accumulated stock is it toxic groundwater equilibrium damage drainage area according to the evaluation principles, various destructive factors are comprehensively analyzed and the evaluation indicators are classified. the scores corresponding to different levels are given to form the graded scoring criteria for the evaluation index. the evaluation index grading standard table is as follows: according to the evaluation index system determined above, the analytic hierarchy process and expert scoring method are used to evaluate the damage degree of the ecological environment in the mining area. the specific evaluation methods are as follows: suppose that the rating of the level ii evaluation index is expressed as w1, the evaluation factor score is expressed as ri, and the evaluation factor weight is expressed as i. the calculation formula is: w ri i= ×∑  (1) i i i r r = −1 (2) suppose that the total score of the i-level evaluation indicator is expressed by g, and the indicator weight is expressed by rj. then: g w rj= ×∑ (3) where ri indicates the weighting of the scoring factor of the i-th ii level indicator, and rj indicates the weighting of the scoring factor of the j-th i level indicator. according to the scoring results, the degree of damage to the ecological environment by mining activities is divided into three levels. when g < 3 1111, it was slightly damaged; when 3 ≤ g ≤ 5, it was severely damaged; when g ≤ 5, it was severely damaged. based on the above-mentioned level division, corresponding recovery measures are formulated. results experiment preparation the purpose of the experimental research is to compare the proposed restoration countermeasures with traditional ecological environment restoration countermeasures and to study which restoration countermeasures have low input costs and high economic benefits. considering that the actual ecological environment restoration requires a longer period, the experiment is verified by simulation. the impact of resource mining activities on the ecological environment is large. the direct and indirect values of ecological restoration are combined as a standard indicator for measuring the economic and economic benefits. the direct use value includes the value of forest resource productivity, scientific research and tourism value, and the indirect use value mainly includes the value of conserved water resources, the value of biodiversity and the value of degradation of pollutants. the calculation of the above value is shown below. s n q w e ai i i i n = +( ) = ∑1 1 1 (4) l r ui i= × | (5) h n t yi i i n = +( ) = ∑1 1 1  (6) d c vi i= (7) j n m bi i i n = +( ) = ∑1 1 1  (8) 434 yanyan huang table 3. scoring criteria for rating indicators geomorphic conditions grading hilly aeolian landform loess floodplain plain alluvial fan lake score 6 8 8 8 10 10 4 vegetation conditions grading agricultural vegetation natural vegetation no vegetation lake score 10 8 6 4 land use grading arable land woodland grassland bare ground desert swamp water body score 9 8 7 6 2 2 4 population density grading <100 100-300 300-500 500-1000 >1000 score 2 4 6 8 10 average annual rainfall grading <100 100-300 300-700 700-1000 1000-4000 score 10 8 6 4 2 geological hazard types grading landslides, mudslides ground collapse soil erosion snowfall, freeze-thaw medium strong medium strong medium strong medium strong score 5 10 5 10 5 10 5 10 geological disaster score 10 8 6 4 2 frequency ≥10 5-9 3-4 1-2 0 economic losses >100 10-100 1-10 0.1-1 ≤0.1 casualties ≥10 5-9 3-4 1-2 0 sphere of influence >1 0.1-1 0.01-0.1 0-0.01 0 occupation and destruction of land resources score 10 8 6 4 2 occupied land area >1 0.5-1 0.1-0.5 0.01-0.1 ≤0.01 area of occupied land >0.5 0.1-0.5 0.01-0.1 0-0.01 0 waste water disposal score 10 8 6 4 2 annual emissions >1 0.1-1 0.01-0.1 0-0.01 0 solid waste discharge score 10 8 6 4 2 accumulated stock >10 1-10 0.1-1 0.01-0.1 ≤0.01 wastewater toxicity score 10 2 toxicity poisonous non-toxic groundwater equilibrium damage score 10 8 6 4 2 drainage area >1 0.1-1 0.01-0.1 0-0.01 0 435destruction process and restoration countermeasures of the ecological environment of a comprehensive geological structure where, s represents the productivity value of forest resources in the resource extraction area, n is constant, qi represents the area of the plant in i years, wi represents the increase in the unit area of forest resources, ei represents the price of net income per unit area of plant resources such as forests,  represents the price growth rate; l indicates the value of scientific research and tourism, ri indicates the benefits of scientific research and tourism brought by the unit forest, ui represents the forest area for ecological environment restoration; h represents the average annual storage value, ti represents the average price of building a reservoir per square meter, yi indicates the amount of water stored before the restoration of the ecological environment,  represents the price increase factor; d represents the value of biodiversity, i indicates the ecological value of the unit area of the ecological environment, ci indicates the area of ecological environment restoration; jis the value of degrading pollutants, miindicates the treatment cost per unit of pollutant, bi is the amount of pollutants degraded in the surrounding area after the restoration of the ecological environment.  is price growth rate. experimental results and analysis according to the information about the direct use value and indirect use value, the corresponding ecological environment data is simulated. in the simulation experiments, the cost input is gradually increased. the economic benefits obtained by different recovery countermeasures are compared and analyzed. after the ecological environment is destroyed, the economic growth caused by different recovery measures is shown below. observe the experimental results in the figure 7. experimental result 1 is the change of economic benefits after ecological restoration corresponding to the traditional restoration measures based on the environmental protection system. the results show that the economic benefits brought by the restoration of the ecological environment are lower than the input cost, and the difference between the two is at least 100,000 yuan. experimental result 2 is the economic benefit brought by the traditional recovery measures relying on natural recovery. the results show that when the input cost reaches one million yuan, the economic benefit exceeds the input cost, but with the increase of the input cost, the economic benefit increases less and less. when the input cost is about 1.5 million yuan, the input cost is greater than the economic benefit. experimental result 3 is the economic benefits brought by the proposed restoration countermeasures. the results show that with the increase of the input cost, the economic benefits increase significantly. when the investment is about 800,000 yuan, the economic benefit is greater than the investment cost, and it has been in a substantial growth trend. in summary, considering the ecological environment destruction process of the comprehensive geological structure, the proposed ecological environment restoration strategy has the advantages of low cost and high efficiency, and the restoration strategy is superior to the traditional restoration strategy. discussion establish a new resource mining mechanism in order to adapt to the development of the market economy under the new situation, a new type of resource development enterprise management and operation mechanism should be established. based on the current development of mining activities, a unified energy development enterprise joint system is established, and the overall development of regional resources is implemented to promote the formation and development of the overall economy. this is conducive to the industrial mix of energy-saving and emission reduction and circular economy and is more conducive to integrating funds and forces for comprehensive ecological environment improvement and pollution control (asghari et al., 2018). with the massive development of various resources, a large area of land subsided and formed lakes and swamps, which changed the nature of land use, broke the existing urban and rural layout, and caused major changes in the regional economic structure and productivity layout. it has been transformed from a single agricultural economy into a mining-type agricultural economy. therefore, to accelerate the planning and construction of towns in the mining area, the agricultural population will surely shift to towns, industrial and mining (a) experimental results 1 (b) experimental results 2 (c) experimental results 3 figure 7. economic benefits from different recovery strategies 436 yanyan huang (yang et al., 2020). the level of urbanization in the mining area needs to be improved, and the scale of urban construction needs to be expanded. gradually implement integrated and integrated construction to accept the transfer of agricultural population to urban areas and expand employment opportunities, and reduce the pressure on the social environment. the establishment of local central towns should be expanded, and auxiliary enterprises, residential areas, and service industries should be incorporated into local central town construction and planning. relying on the development of local central towns, new residential, and commercial areas should not be built separately (wu et al., 2020). the built-up residential areas should be gradually replaced and rebuilt and relocated to local towns. resettlement in mining areas should be closely integrated with urban planning. based on the principle of relative concentration, rationally arrange resettlement for new villages. according to the standards of the new socialist countryside, an ecological village in the form of a mining area will be set up so that farmers will not leave their homes, which will facilitate the cultivation and management of land. mining areas and cities should set up operating service companies to develop the tertiary industry and serve the resource extraction and urban areas in order to solve the employment problems of residents who are relocated in coal mine development and ensure social stability (cheng & liu, 2020). coordinate energy development and ecological protection with a scientific development concept the government is responsible for the quality of the ecological environment in the area under its jurisdiction, and it is necessary to strengthen the leadership of energy resource development and ecological environmental protection. carefully study the development strategy of energy resources in the area under its jurisdiction, work out an energy development model that is conducive to resource development and environmental protection, and sustainable economic development, and strive to create an environment-friendly energy development economy. the environmental protection executive head accountability system should continue to be seriously implemented, the environmental protection target responsibility system needs to be implemented, the first approval right and veto power of the environmental protection department should be implemented, and the environmental protection work in energy development in its own jurisdiction should be organized and led (gao et al, 2018). the development of energy development and ecological protection plans should be adapted to local conditions. after the work priorities and goals are determined, the outstanding ecological and environmental problems in the area shall be solved in a planned and step-by-step manner. the ecological environment in yulin is fragile and the carrying capacity is limited. the local farmers live in poverty, and the desire to get rid of poverty is more urgent. energy development companies have an inherent need to maximize economic benefits for their own development needs. most of them only care about development, regardless of the carrying capacity of the ecological environment, only the current development, but not sustainable development and they have the idea of development first, then governance. the scientific development concept must be used to coordinate energy development and ecological protection (gao et al., 2019). the grassroots government and corporate legal persons must adhere to the people-oriented principle, establish a comprehensive, coordinated, and sustainable development concept, handle the relationship between resource exploitation and ecological protection, and implement the policy of “developing during protection and protecting during development”. the interests of all parties must be balanced, a dynamic balance of the coordinated development of economic and social development, resource development and utilization, and ecological and environmental protection should be established. a resource-saving and ecologically-friendly mining area should be built to achieve harmony between humans and nature in the mining area. the scientific concept of development and the resource-saving society must be vigorously publicized through various means, and the legal knowledge of environmental protection and resource conservation needs to be vigorously popularized. let everyone know that protecting the environment means protecting their homes, themselves, and their children. the sound public participation mechanism needs to be established as soon as possible in order to implement the public’s right to know the quality of the ecological environment and strengthen supervision of illegal ecological environment enterprises and illegal acts. green gdp accounting system gdp is a measure of the economic performance of all countries and regions, and it reflects the state of wealth and the living standards of the people. however, the ecological environment is part of the comprehensive economy of a country or region. environmental pollution and ecological damage can also increase gdp. without incorporating ecological and environmental factors, the gdp kernel algorithm cannot fully reflect the real economic situation of a country or region. if only from the perspective of regional economic development, the local economy has increased, but it cannot truly reflect the ecological and environmental costs behind the economic growth in the area. therefore, in order to fully reflect the real economic situation, especially for areas such as resource extraction that are eager to pursue rapid economic development and easily overlook the ecological environment, it is necessary to establish a green gdp accounting system. through the establishment of a green gdp accounting system, the increase and consumption of ecological environmental resources are included in the national economic account in the form of currency to reflect the changes in the ecological environment over the years and its internal relationship with economic development. correctly estimating the actual state of economic development and future development potential can provide a basis for establishing an ecological environment compensation mechanism, especially for establishing horizontal fiscal transfer payments. taking the above three points into consideration with the degree of damage to the ecological environment, different levels of damage have different emphases in the implementation of restoration measures, and relevant measures should be implemented according to actual conditions. conclusions when the comprehensive geological structure was mined, the ecological environment was damaged. through the evaluation of the damage degree of the ecological environment, the corresponding restoration countermeasures are proposed, which effectively solves the problems of high cost and low efficiency in the traditional restoration countermeasures. it 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(2019). countermeasures of water and mud inrush disaster in completely weathered granite tunnels: a case study. environmental earth sciences, 78(18), 1-16. keywords: remote sensing; thermal infrared; temperature; geothermal; prospects; paipa. palabras clave: sensores remotos; infrarrojo termal; temperatura; geotermia; prospectos; paipa. how to cite item calderon-chaparro, r. a., & vargas-cuervo, g. (2019). determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data. earth sciences research journal, 23(4), 265-282. doi: https://doi.org/10.15446/ esrj.v23n4.77810 geothermal resources have been found with the help of conventional geological, geochemical, and geophysical techniques. however, in some cases, difficult access to research areas hinders its application, which in turn increases operating costs and restricts areas of exploration. remote sensors with thermal infrared are an important tool in the exploration of geothermal resources due to the low cost and high efficiency in the study of large geographic areas. in this research, thermal images of remote satellite sensors are used and integrated with existing geological-geophysical information for the determination of exploratory prospects of hydrothermal vents in the geothermal region of paipa, boyacá. the satellite thermal images used are landsat 7 etm+ and landsat 8 oli/tirs. radiometric calibration, atmospheric correction, surface thermal emissivity calculation, a single-channel, and split window algorithm were applied to retrieve the land surface temperature. soil temperature, surface temperature, air temperature, relative humidity, atmospheric pressure, and thermal imagery of the geothermal manifestations were also used. the field data contributed to the correction of the thermal bands, and the soil temperature data helped to create a subsurface temperature map at one-meter depth. once the primary and secondary data was available, in a gis, an unweighted spatial model was implemented, which used four input indicators (satellite temperature index, soil temperature index, structural lineaments index, and iso-resistivity index), to determine the areas most likely to find geothermal fluids. six areas of exploratory interest for the extraction of hydrothermal fluids were determined, of which two are already known, and the remaining four are proposed as prospects. the results allowed us to conclude that thermal remote sensors are a useful tool for the mapping of geothermal anomalies in the paipa region, and by combining these anomalies with geological-geophysical information, it is possible to determine specific areas of exploration. abstract determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data determinación de prospectos hidrotermales en la región geotérmica de paipa (boyacá, colombia), utilizando sensores remotos y datos de campo issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.77810 los recursos geotérmicos han sido encontrados con ayuda de técnicas de campo, geológicas, geoquímicas y geofísicas, las cuales, en algunas ocasiones, el difícil acceso al área de investigación, dificulta su aplicación, aumentando los costos de operación y restringiendo espacialmente las áreas de exploración. los sensores remotos con infrarrojo termal, son una herramienta importante en la exploración de recursos geotérmicos, debido al bajo costo y alta eficiencia en el estudio de grandes áreas geográficas. en esta investigación se utilizaron e integraron imágenes termales de sensores remotos satelitales, con información geológica-geofísica existente, para la determinación de prospectos exploratorios de fuentes hidrotermales en la región geotérmica de paipa, boyacá. las imágenes térmicas satelitales utilizadas son landsat 7 etm+ y landsat 8 oli/tirs. también, se utilizaron datos de campo, de temperatura de suelo, temperatura de superficie, temperatura de aire, humedad relativa, presión atmosférica e imágenes térmicas de las manifestaciones geotérmicas. las bandas termales landsat fueron calibradas radiométricamente, posteriormente corregidas por efectos atmosféricos y emisividad de la superficie, aplicando algoritmos de un solo canal y split window, para landsat 7 etm+ y landsat 8 tirs, respectivamente. los datos de campo contribuyeron a la corrección de las bandas termales, y los datos de temperatura de suelo, ayudaron a la creación de un mapa de temperatura subsuperficial a un metro de profundidad. una vez se contaba con los insumos primarios y secundarios, en un sig, se implementó un modelo espacial no ponderado, el cual utilizó cuatro variables de entrada (índice temperatura satelital, índice temperatura de suelo, índice lineamientos estructurales e índice iso-resistividad), para determinar las zonas de mayor probabilidad de encontrar fluidos geotérmicos. se determinaron seis áreas de interés exploratorio para la extracción de fluidos hidrotermales, de las cuales, dos son ya conocidas y las cuatro restantes se proponen como prospectos. los resultados permitieron concluir que los sensores remotos termales son una herramienta útil para el mapeo de anomalías geotérmicas en la región de paipa, y combinando estas anomalías con información geológica-geofísica, es posible determinar áreas puntuales de exploración. resumen record manuscript received: 11/02/2019 accepted for publication: 02/08/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 265-282 rafael andrés calderón-chaparro, german vargas-cuervo universidad nacional de colombia * corresponding author: raacalderonch@unal.edu.co g eo lo g y 266 rafael andrés calderón-chaparro, german vargas-cuervo introduction geothermal energy refers to the heat produced inside the earth. this energy is transported to the surface through water or steam (dipippo, 2015), and is considered one of the cleanest and most renewable energy sources on the planet (dickson & fanelli, 2003). geothermal fluids are used to produce electricity and have several applications, including space heating, geotourism, and health, among others (lund & boyd, 2015). the geothermal area of paipa is located in the eastern cordillera of colombia, south of the paipa town, with a surface area of 130 km2 (figure 1). in this area, geothermal fluids rise to the surface in two specific areas; these fluids are currently only used in baths and thermal pools. different investigations have been carried out to evaluate the geothermal potential of paipa (alfaro valero c., 2002; vásquez l., 2002; velandia f., 2003; cepeda & pardo, 2004; alfaro valero & espinoza, 2004; alfaro valero c. 2005; alfaro valero c., 2005b; ortiz & alfaro, 2010; franco j., 2012; vásquez l., 2012; rodríguez & vallejo, 2013; llanos et al., 2015; moyano i., 2015; rueda j., 2016). although several studies have assessed the geothermal system of paipa, no research has included satellite information with thermal infrared. thermal remote sensing has been used effectively in the exploration of geothermal resources, resulting in saving resources such as time and money, it is shown by several geothermal explorations of active volcanoes and other geothermal areas (zhou, 1998; ayenew, 2001; qin et al., 2011; norini et al., 2015; chan et al., 2018; darge et al., 2019). some of these investigations have been carried out in well known geothermal areas, allowing the identification of new specific areas of geothermal interest (coolbaugh, 2003, gutiérrez et al., 2012). multispectral satellites, such as landsat 7 etm+ and landsat 8 oli/tirs, have thermal bands that are used to determine thermal anomalies in volcanic areas and geothermal areas (mia et al., 2014, chan & chang, 2018, cesarian et al., 2018; sekertekin & arslan, 2019). the use of thermal satellite images has some limitations; not all scenes are of a high quality due to cloudiness and spatial resolution, and due to difficult access in some areas, validation of satellite data with field data is challenging. landsat 7 etm+ has a thermal band with a spatial resolution of 60 m and a spectral range between 10.4 and 12.5 μm. a single-channel algorithm was applied to this band to determine surface temperature anomalies (jiménezmuñoz et al., 2009). similarly, landsat 8 oli/tirs has two thermal bands with a spatial resolution of 100 m and a spectral range between 10.6 to 11.19 μm (band 10) and 11.5 to 12.51 μm (band 11). a two-channel algorithm (split window) was applied to these two bands (du et al., 2015). the exploration of paipa geothermal area is being carried out for the first time by thermal satellite images. the approach used here aims to demonstrate the effectiveness of the thermal infrared satellite to determine geothermal anomalies, supported by field measurements that allow the validation of satellite data. the main objective of this research is to highlight exploratory prospects in order to the paipa community to take advantage of new hydrothermal fluids and attain economic growth in the region. geological and geothermal framework paipa geothermal system is located in the eastern cordillera basin whose geological units are mainly sedimentary, with ages ranging from 1̴30 m.y., to the present (figure 2). the formations of cretaceous period correspond to tibasosa-k1t (limestone and mudstone), une-k1u (quartz sandstones), churuvita group-k2ch (mudstone and sandstone), conejo-k2c (mudstone, siltstone, and limestones), plaeners-k2pl (siltstone and chert), los pinosk2p (mudstone and siltstone), labor-tierna-k2lt (quartz sandstones) and guaduas-k2e1g (mudstone, sandstones and coal). from the paleogene period, bogotá formation-e1-2b (quartz sandstones). from the neogene period the tilatá formation-n1t (sandstones and mudstones), and pyroclastic volcanic figure 1. location of the study area in paipa-boyacá, colombia (the orange dashed rectangle marks research area). the red edge triangles represent main geothermal manifestations, and yellow edge circles represent well know geographic reference points. finally, color polygons represent the volcanic deposits at zone. 267determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data using spectral bands of red (30m / pixel), near infrared (30m/ pixel) and thermal infrared (60m-100m/pixel) of landsat images. we also used spectral radiance images (mod021km) and images of water vapor content of the product mod05_l2 (1000m/pixel) of modis/ terra, which is used to correct the thermal emissivity of the surface atmospherically. the digital elevation modeldem (12.5m/pixel) used was obtained from the alos-palsar product, and the pléiades image of the high-resolution visible spectrum (rgb) (0.5m/pixel) were used in conjunction with the dem to create anaglyphs. the images cover path 7 and row 56 of landsat images. in order to multi temporarily analyze the thermal anomalies, five landsat diurnal images were used. these were acquired on the dates 2000-12-13, 2001-01-30, 2002-01-17, 2002-03-06, 2003-01-04 for landsat 7 and 2016-02-01 for landsat 8. these images were selected because they presented 0% cloudiness for the research area and are of excellent quality. field data like satellite images, field data is primary information. we acquired fifty field stations of solar radiation, soil, air, and surface temperatures, relative humidity, and atmospheric pressure. a radiometric station rs (powered by solar energy) and a flir cat-s60 thermal camera were used. the rs (figure 3) is composed of a data logger, a solar panel, a radiometer, an infrared radiometer, a thermistor, and a multiparameter environmental sensor. table 1 provides details of the rs sensors. solar radiation, air, and surface temperature data were used to correct and validate the temperature values of the satellite images. the soil temperature data was used to create a map of soil temperature anomalies in the area. methodology this study was carried out in five stages (figure 4). first, the reference information was established; second, the satellite images were processed deposits and intrusive domes. also, from the quaternary, there are colluvial (qc), and fluvial-lacustrine (qal) deposits located in the flat parts of the zone (conglomerates and sandstones) (velandia, 2003). as for the volcanic deposits, these are located seven kilometers south of paipa town (figure 2) and are classified mainly as ash flows, ash falls, and dome blocks; the body domes composition is rhyolitic (cepeda & pardo, 2004). outcropping on the surface, there are 18 geothermal manifestations, mainly hot springs and a steam vent (figure 2) (alfaro et al., 2017). previous reports such as that of the latin american energy organization (olade, icel, contecol & colombian geotermia, 1982) and the japan consulting institute (1983), conceptualized that this area has a geothermal interest classified from medium to high, with a possible geothermal reservoir suitable for its exploitation. rodríguez (1998) determined that the volume of thermal water available in the area is approximately 480,000 m3. the colombian geological service (cgs) proposes a source of magmatic heat, associated with the heat remaining from igneous intrusions and/or heat from radiogenic minerals, which heats the waters of the system (alfaro valero et al., 2017). also, the cgs proposes the existence of two hydrothermal systems, one primary and the other secondary, for its possible temperatures in depth; the primary system would be generated by infiltration of surface water at a depth between 4 and 5 km, which would circulate from the se of the area to the nw, reaching temperatures higher than 230°c. the secondary hydrothermal system located nw of the area, where surface water would infiltrate to a depth of 1.5 km, reaching a temperature around 75°c, restricted laterally by the canocas and hornitos faults, superficially this system does not show hot springs. materials and methods satellite data satellite images are primary information sources in this investigation. these images include scenes from landsat 7, landsat 8, modis/terra, alospalsar and pléiades. land surface temperature anomalies are determined figure 2. geologic map of the study area, from velandia (2003) and cepeda & pardo (2004). 268 rafael andrés calderón-chaparro, german vargas-cuervo digitally; third, index maps were defined; fourth, a spatial model was applied to the four index maps, and finally, the hydrothermal prospects were defined. digital image processing this stage consists of three steps. first, the landsat bands were radiometrically calibrated and atmospherically corrected. second, the red and infrared near bands were used to calculate the thermal emissivity of the surface. finally, the algorithms of a single thermal band (jiménez-muñoz & sobrino, 2003) and two split-window thermal bands (du et al., 2015) were used to determine the thermal anomalies of the surface. the following sections provide a detailed description of each of these procedures. land surface temperature retrieval the first step to obtaining surface temperature images is the conversion of digital numbers (dn), in spectral radiance values (l𝜆) to the top of the toa atmosphere (chander et al., 2009). for landsat 7 and landsat 8 images equations (1) usgs (2018) and (2) usgs (2016) were used. l g q brescale cal rescale = +*� � (1) l m q al cal l = +* � (2) where, grescale, brescale, ml and al, are specific rescaling factors for each band; they are found in the metadata of each of the images. qcal is the raw value of each pixel in the images, for the case of landsat 7, those values are between 0 and 255, and for the landsat 8 images values are between 0 and 65,535. atmospheric correction atmospheric effects correct the bands in the visible and near-infrared spectrum. this correction is applied due to absorption and dispersion processes, as a result of the existence of water vapor, gases (ozone, oxygen, carbon dioxide, nitrogen oxide, etcetera.) and molecules in the atmosphere, which affect the amount of radiation received by the sensor. these atmospheric effects are removed in order to obtain an image of spectral reflectance whose values reflect real physical magnitudes of the surface. to obtain atmospherically corrected images, the atmospheric correction module fast line-of-sight atmospheric analysis of spectral hypercubes (flaash) was used. in general, flaash is an atmospheric correction method based on the modtran 4.0 radiative transfer code, which allows the removal of the effects of dispersion and absorption in the multispectral and hyperspectral images, which was developed by the laboratory of the us air force. (kayadibi, 2011). thermal emissivity calculation once the red and near-infrared bands were corrected atmospherically (example: bands 3 and 4 for landsat 7), the values of surface emissivity were determined. the approach used here is the one proposed by sobrino et al., (2004), it is based on the determination of the emissivity for each pixel, based on the proportion of vegetation pv, obtained from the values of the normal differential vegetation index (ndvi), equation (3). ndvi r r r r nir red nir red = − + (3) the method is called ndvi thresholds (ndvith), which considers three different surface classes, dependent on the ndvi values: a) bare soil (ndvi <0.2), b) mixed surface (0.2≥ndvi≤0.5), and c) surface fully covered with vegetation (ndvi> 0.5). a) bare soil (ndvi <0.2): the emissivity value for landsat 7 images is established as constant ε = 0.97. in the case of landsat 8 images, the emissivity was determined from the reflectance of the red band ρred, for each of the thermal bands, ε ρ10 0 973 0 047= −. . red and ε ρ11 0 984 0 026= −. . red (yu et al., 2014). b) mixed surface (0.2 ≥ ndvi ≤ 0.5): the pixels are considered as a mixture between vegetation and bare soil. the emissivity is obtained as a function of the proportion of vegetation, as follows,    = + −( ) +v v s vp p d1 , where, εv is the emissivity of the vegetation, εs is the soil emissivity, and dε is a term which includes the effect of geometric distribution of natural surfaces and internal reflections (sobrino et al., 2004). the proportion of vegetation is calculated from the following equation, p ndvi ndvi ndvi ndviv min max min = − −       2 , where, ndvimin = 0 2. and ndvimax = 0 5. . dε is calculated in the following way d p fs v v  = −( ) −( )1 1 * * , where f is a form factor, with a mean value of 0.55, which considers different kinds of geometric distribution. figure 3. radiometric station (rs) used in field acquisition. table 1. instruments used in the field data collection variable units sensor brand / model soil temperature °c thermistor apogee / st100 surficial temperature °c non-contact infrared thermometer apogee / si-4h1 object surficial temperature °c thermal camera cat flir / s60 air temperature °c vp-4 decagon / passrht vapor pressure kpa relative humidity % atmospheric pressure kpa incident and reflected solar radiation w/m2 net radiometer apogee / sn500 net radiation w/m2 269determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data for the case of the landsat 8 thermal bands, the average emissivity values of the soil and vegetation of each one were taken into account: εs10= 0.9668, εs11= 0.9747, εv10= 0.9863, εv11= 0.9896 (yu et al., 2014). c) surface fully covered with vegetation (ndvi> 0.5): it was assumed for both landsat 7 and landsat 8 images, a constant value ε = 0.99. calculation of surface kinetic temperature two algorithms were chosen to determine the kinetic surface temperature of paipa’s geothermal area. for the landsat 7 etm + thermal band, the onechannel algorithm proposed by jiménez-muñoz & sobrino (2003) was used, and for the landsat 8 tirs thermal bands, the split-window algorithm, proposed by du et al., (2015) was used. these two algorithms are based on the radiative transfer equation (rte), equation (4). this equation expresses the law of planck, defining that the spectral radiance b(λ,t) of a black body is a function of the wavelength λ(m) of the electromagnetic energy and the absolute temperature of the body t(k). b t c exp c t    ,( ) =     − 1 5 2 1 (4) where c1 and c2 are constants of planck, with values of 1.19104 * 108 w.µm4.m-2.sr-1 and 14387.7 μm.k, respectively. single channel algorithm (sc) jiménez-muñoz & sobrino (2003) and jiménez-muñoz et al., (2009) proposed an algorithm to determine the surface temperature lst, from a single channel, equation (5), based on the concept of atmospheric functions (fats), which depend on the atmospheric transmissivity and the descending and ascending atmospheric radiations. lst lsen= +( ) +    +γ ε ϕ ϕ ϕ δ 1 1 2 3 (5) where ε is the emissivity of the surface; γ and δ are two parameters dependent on the planck functions, expressed in the following way,   ≈ t b l sen sen 2 and δ γ ≈ −t t bsen sen 2 , where tsen is the temperature of brightness in the sensor (equation 6); bγ is a constant, with a value of 1277 k for landsat 7 and lsen is the spectral radiance in the sensor. finally, φ1, φ2, φ3 are atmospheric functions, ϕ ω ω1 20 14714 0 15583 1 1234= − +. . . ; ϕ ω ω2 21 1836 0 37607 0 52894= − − −. . . ; ϕ ω ω3 20 04554 1 8719 0 39071= − + −. . . ϕ ω ω3 20 04554 1 8719 0 39071= − + −. . . , where ω is the water vapor content (wvc) in the atmosphere in g/ cm2. the product mod05_l2 of the modis sensor, offers continuous values of ω at a spatial resolution of 1 km2, being appropriate due to the small spatial variation of ω in the atmosphere (ren et al., 2015). t k ln k l sen = +     2 1 1  (6) where, k1 [w.m-2.sr-1.µm-1] and k2 [k] are calibration constants (table 2); lλ is the spectral radiance in the sensor [w/(m2.sr.µm)] and ln is the natural logarithm. before being able to use the modis images, a process of georeferencing of the mod021km products was carried out, starting from the landsat images. once the mod021km images were georeferenced, a co-registration process was performed with their respective mod05_l2 images, obtaining correctly geo-positioned wvc images. once the wvc images were correctly geopositioned, their values were validated using the values of relative humidity and air temperature, collected in the field, by applying equation 7, where t0 is figure 4. general methodological framework. 270 rafael andrés calderón-chaparro, german vargas-cuervo the surface air temperature, rh is the relative humidity of the air and wvc is given in g/cm2. wvc exp t t rh= +               0 0981 10 0 6108 17 27 237 3 0 0 . * * . * . * . *  + 0 1697. (7) the wvc values obtained from the field data vary from 0.52 g/cm2 to 1.46 g/cm2, which are very similar to the average values of wvc obtained by the modis sensor in each of the dates of the landsat images (table 3). this similarity allowed the validation of the wvc satellite values. table 3. water vapor content statistics of the mod05_l2 images, for each of the dates used in landsat. statistics mod05 20001213 mod05 20010130 mod05 20020117 mod05 20020306 mod05 20030104 min 0.577282 0.486306 0.369391 0.706861 0.955387 max 1.54891 0.93637 1.032812 1.71515 2.12305 mean 1.147071 0.651245 0.749141 1.145315 1.33665 split window algorithm (sw) du et al., (2015) proposed a practical algorithm, which takes advantage of the two thermal bands of the tirs sensor, onboard the landsat 8 satellite. the algorithm obtains the surface temperature from a linear relationship, with the combination of the brightness temperatures of the two thermal bands, with clear sky conditions. like the algorithm of jiménez-muñoz & sobrino (2003), the sw is based on the theory of radiative transfer, so it is necessary to have atmospheric information at the time of image acquisition. to solve this, the algorithm of equation (8) depends on eight coefficients, which vary depending on the wvc, at the moment of image acquisition. the coefficients were calculated from a modified sw method of covariance-variance relation, obtaining five different ranges of wvc, [0.0 2.5], [2.0 3.5], [3.0 4.5], [4.0 5.5], and [5.0 6.3] g/cm2. lst b b b b t t b b bi j= + + − +     + + + − +    0 1 2 3 2 4 5 6 2 1 2 1        ∆ ∆ tt t b t ti j i j + + −( )2 7 (8) where ti and tj are the brightness temperatures of bands 10 and 11, respectively; ε is the average emissivity of the thermal bands    = +i j 2 ; δε is the difference of the emissivity δε=εi-εj, and b0, b1, ...b7 are the coefficients determined for each wvc range. the coefficients used correspond to the values defined for a range of wvc between 0.0 and 2.5 g/cm2, determined thanks to the record of the modis images and the field values, whose average value is 1.3 g/cm2. obtaining structural lineaments the first step to obtain the structural lineaments, was to combine the alos-palsar (dem) scenes in a single image; for this, the mosaic tool of erdas imagine 2018® was used. subsequently, shadow models were obtained; these models were combined with the dem and the pléiades images of the visible spectrum, thus obtaining three-dimensional images (anaglyphs). once the anaglyphs were obtained, the structural analysis of the geothermal area of paipa was carried out. the deformation structures were interpreted visually, using the 3d views of the anaglyphs. the lineaments were mapped using a geographic information system (gis). moreover, the identified structures were verified with the existing geological information (velandia, 2003) and the fieldwork carried out in this study. iso-resistivity map based on the reference information, the iso-resistivities map published by the cgs (franco, 2016 in alfaro valero et al., 2017) was used. this isoresistivity surface information was measured at an elevation of 2,500 m.a.s.l., and the anomalies of low resistivity indicate the location of geothermal fluids in the area. soil temperature map a soil temperature map of the paipa geothermal system was made. one hundred ninety-seven temperature probes were used, recorded at depths greater than one meter. the geostatistical wizard tool of arcmap© was used by applying an interpolation method of kriging. the one hundred ninety-seven soundings are constituted by 147 soundings taken by the cgs (rodríguez & vallejo, 2013) and 50 new temperature data acquired in the field throughout this study. spatial model the maps obtained from satellite surface temperature (sst), structural lineaments (sl), iso-resistivity surface (is), and soil temperature (st) were integrated in an unweighted way into a spatial model. three index values were defined to each of these maps, corresponding to the areas of higher temperature (sst and st), areas of influence of the structural lineaments (sg) and areas of lower resistivity (is). the model was based on obtaining a spatial relationship between the four index maps. to achieve this, the union spatial function, offered by arcmap©, was used. the result obtained called integrated index map (ii), corresponds to new polygons, containing in their attributes the values of the indexes that were superimposed (figure 5). finally, the index values in each polygon were summed and divided into the total number of indexes (equation 8). ii i i i i n tss le ir ts= + + +� � � (8) figure 5. illustrative diagram of the result obtained from the union process. the input data is three layers, each with a defined attribute value (1, 2 and 3); the result presents the same figure, but now there are seven different polygons, and each contains a different number of attributes; note the orange polygon now contains the attribute values 1, 2, and 3. index maps the index maps correspond to geophysical and geological indicators, which have a spatial relationship with the occurrence of hydrothermal fluids on the surface; each index was classified into three qualitative ranges depending on their temperature, resistivity and lateral area of influence of the lineaments. the table 2. calibrations constants used to convert spectral radiance in brightness temperature, for landsat 7 etm+ and landsat 8 tirs. satellite band k1 k2 landsat 7 6 666.09 1282.71 landsat 8 10 774.8853 1321.0789 11 480.8883 1201.1442 271determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data defined index values are 1, 2, and 3, classified qualitatively as, low, medium, and high probability, respectively. a) surface temperature index map two of the thermal satellite images were integrated. the images used correspond to a landsat 7 etm+ of march 6th, 2002 and a landsat 8 tirs of february 1st, 2016. these images were selected for the dominant surface coverage on those dates. the ground covers correspond to surfaces mainly of half-bare soil, and this is ideal since the temperature values can be affected by the evapotranspiration processes of the vegetation. thus, the resulting temperature anomalies of the images are similar in shape and magnitude. table 4 presents the index values defined for the three highest temperature ranges of satellite thermal images. table 4. satellite temperature indices. three ranges of maximum temperature were defined and classified qualitatively. image index temperature range (°c) qualitative classification landsat 7 y 8 1 39 – 41 low 2 41 – 43 medium 3 43 50 high the temperature anomalies of the two landsat images were classified according to table 4, later, they were converted into a vector format, and the arcmap© union process was applied. the result obtained corresponds to the integrated temperature anomalies of the two satellite images (figure 12-a). b) lineaments index map based on the mapped lineaments, three influence areas (buffer) were established around them. these three areas correspond to the three established rates, low, medium, and high probability of allowing the circulation of hydrothermal fluids (table 5). the index map of structural lineaments is presented in figure 12-b. table 5. indices of lineaments. three influence distances were defined with their respective qualitative classification. index influence range (m) qualitative classification 1 50 low 2 100 medium 3 150 high c) iso-resistivity index map from the iso-resistivities surface (figure 10), the index values of this map were defined. the values of resistivity found in the zone vary between 0 and 2,251 ω.m, and according to the geothermal conditions of paipa, it was interpreted that the low values of resistivity between 0 and 17.3 ω.m, are the result of hot saline fluids (ussher et al., 2000). these low values of resistivity were taken and classified in the three index ranges (table 6), defining the iso-resistivity index map (figure 12-c). table 6. indices defined for three minimum ranges of subsoil resistivity, with their respective qualitative classification. index resistivity range (ohm.m) qualitative classification 1 11.6 – 17.3 low 2 7.7 – 11.6 medium 3 < 7.7 high d) soil temperature index map a frequency analysis of the soil temperature data was carried out, concluding that the majority of the data set corresponds to values without geothermal influence, that is, the values of temperature between 15 °c and 19 °c. on the other hand, it was established that the temperature data between 19.1 °c and 37 °c correspond to values with geothermal influence (figure 6). from this last temperature range, the index values were established (table 7), and the soil temperature index map was obtained (figure 12-d). prospects the prospects of the paipa geothermal area were obtained by making a spatial superposition of the four index maps. the objective was to identify the areas where the highest index values were superimposed (index value 3), that means, high satellite temperature, high soil temperature, low electrical resistivity and proximity to the fault zone. results and discussion land surface temperature anomalies the results of the landsat bands processing are presented in figure 7. the application of the algorithms proposed by jiménez-muñoz & sobrino (2003) and du et al., (2015) in the thermal bands of landsat 7 etm+ and landsat 8 tirs, allowed to correct in these, the effect of the emissivity of the surfaces and the processes of dispersion and absorption generated by atmospheric conditions, mainly due to the effect of the water vapor content in the atmosphere. the correction of wvc in the atmosphere was carried out satisfactorily with the mod05_l2 products, for the case of the “single channel” algorithm and the case of the “split window” algorithm with the chosen range of water vapor content between 0.0 and 2.5 g/cm2. this was due to the atmospheric data of temperature and relative humidity of the air, acquired in the field, which allowed it to validate the research areas with an average wvc value of 1.3 g/ cm2, validating the values of wvc recorded by the modis sensor and the chosen range for the “split window” algorithm. another relevant aspect of the application of the algorithms is obtaining surface temperature anomalies with higher temperature magnitudes, and the reduction in the size of the anomalies in comparison with the temperature anomalies obtained without the atmospheric and emissivity corrections (temperature of brightness). as an example, figures 8a and 8b are compared, and it is possible to observe the differences in the size of the temperature anomalies, being of much larger extension for the temperature anomalies of brightness, while the temperature anomalies where the corrections are applied have a smaller size and their magnitude (temperature) is higher. regarding the variation of the temperature magnitudes, there was a generalized increase fr eq ue nc y temperature range ºc 80 70 60 50 40 30 20 10 0 [1 5, 1 6] [1 6, 1 7] [1 7, 1 8] [1 8, 1 9] [1 9, 2 0] [2 0, 2 1] [2 1, 2 2] [2 2, 2 3] [2 3, 2 4] [2 4, 2 5] [2 5, 2 6] [2 6, 2 7] [2 7, 2 8] [2 8, 2 9] [2 9, 3 0] [3 0, 3 1] [3 1, 3 2] [3 2, 3 3] [3 3, 3 4] [3 4, 3 5] [3 5, 3 6] [3 6, 3 7] figure 6. histogram of frequencies of soil temperatures in the paipa area. table 7. indices defined for three maximum ranges of soil temperature, with their respective qualitative classification. index temperature range (°c) qualitative classification 1 19 – 22 low 2 22 – 25 medium 3 25 – 37 high 272 rafael andrés calderón-chaparro, german vargas-cuervo figure 7. anomalies of land surface temperature from landsat 7 etm+ on december 13, 2000; january 30, 2001; january 17, 2002; march 06, 2002; january 04, 2003; and landsat 8 tirs on february 01, 2016. 273determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data figure 8. comparative images between the satellite thermal anomaly sizes, from the landsat 7 emt+ scene, of march 6, 2002. a) brightness temperature image. b) surface temperature image. figure 9. structural lineaments map of paipa geothermal zone. 274 rafael andrés calderón-chaparro, german vargas-cuervo of around +8°c, for the hottest anomalies, while for the colder temperature anomalies there was an increase of around 3.5°c. the satellite thermal anomalies reflect an excess of heat coming from the interior of the earth. thus, in areas of geothermal activity, the images of surface temperature that are obtained have two sources of energy, one coming from solar heating and another coming from the interior of the earth. the equation of the surface energy balance (equation 9) allows us to comprehend how it is possible to identify zones with geothermal activity. q h e gn = + +�  (9) where qn is the net solar radiation received by the surface; h is the sensible heat flow between the ground floor interface; λe is the latent heat flux in the transition phase of water between the surface and the atmosphere; g is the heat flow of the soil. locally, it is assumed that h and λe have little influence on surface thermal anomalies so that the net radiation and the heat flow from the soil are those that would generate the signature of heat registered by the sensor. structural lineaments map figure 9 presents the map of structural lineaments obtained from the 3d interpretation. the result shows the existence of a dense network of fractures which would affect the geothermal area of paipa. the lineaments validated in the field and reference information corresponds to the hornito, canocas, el batán, el bizcocho, cerro plateado, and las peñas faults. iso-resistivity map figure 10 shows the iso-resistivities surface of the paipa geothermal area. resistivities under 17.3 ω.m were interpreted as a result of a geothermal source. six specific zones of high conductivity (<2.3 ω.m) were identified in the study area. the anomaly 1 in the north of the area, is not taken into account because this anomaly is generated by a deposit of salt in that area. the other anomalies are interpreted as shallow accumulations of geothermal fluids. soil temperature map figure 11 shows the soil temperature map, with data acquired at onemeter depth. the presence of six anomalous zones is clearly identified, with two principal anomalies, one located to the east of sochagota lake (anomaly 1) and the second to the south, in the sector of the el delfín and la playa pools (anomaly 4), each corresponding spatially to the zones where the geothermal manifestations of the zone are presented. in general, there is a spatial correlation between resistivity anomalies and soil temperature anomalies, mainly between anomalies number 2 and number 1 (figure 10 and 11), anomalies 3-4-5 with anomaly 4 (figure 10 and 11), and the number 6 of the two anomalies (figure 10 and 11). spatial model index maps the implemented spatial model took into account indicators with a spatial relationship with the presence of hydrothermal fluids (figure 12). lineaments act as migration paths or may restrict the lateral distribution of hot fluids. the resistivity anomalies reflect the possible zones of accumulation of geothermal fluids because these fluids have physical and chemical properties characteristic of temperature and salinity, which produce very high conductivities. soil temperature anomalies at shallow depths are the direct manifestations of heat transfer from the subsoil to the surface, with convection being its transfer mechanism. finally, the thermal anomalies obtained from the satellite images are the continuous representation of the surface temperatures mainly product of the interaction between the solar radiation and the heat coming from the interior of the earth, which allows identifying anomalously hot zones through the surface. these variables integrated from a geographic information system, in an unweighted way, facilitated the spatial correlation between each of them and allowed for the application of a simple algorithm with which it was possible to generate point zones, which are assumed as the areas of most significant probability to find new hydrothermal sources (prospects). these results allow us to affirm that the defined methodology is reliable, meets the stated objective, and can be applied to any geothermal zone, where the same indicators used for the model are available. prospects the areas of exploratory interest in the geothermal area of paipa are presented in figure 13-b. the results showed six areas with hydrothermal potential, of which four were defined as exploratory prospects, and two correspond to hydrothermal springs currently in production (hot springs of the itp paipa tourism institute, and el delfín and la playa pools). these results validated the applied methodology and can be used in any geothermal zone, where the same geological-geophysical indicators used for this model are available (satellite temperature maps, structural lineaments, geoelectric studies, and soil temperature data). the four prospects were listed from one to four, and are located in: 1) el durazno; 2) east of alto los volcanes, divided into two circular anomalies; 3) alluvial valley of the honda creek, and 4) east of the el delfín-la playa pools. validation of results surface temperature surface temperature anomalies can be used with confidence once they have been appropriately validated. the approach used here includes the spatial comparison between surface temperature anomalies and soil temperature anomalies since the latter are a direct expression of the heat present in the subsoil. in general, five of the six soil temperature anomalies correspond spatially with satellite surface temperature anomalies (figure 14 a-1), demonstrating the geothermal component in thermal satellite images. exploratory prospects the results showed six areas of exploratory interest, two of which coincide spatially with the superficial geothermal manifestations of paipa (figure 13b). this spatial coincidence validates the results of the implemented method. the production area #1 (figure 15), is located in the main discharge area of the system, which contains approximately ten hot springs, with temperatures between 40°c and 72.1°c. these thermal waters are currently being used by the center tourism (instituto de turismo de paipa, itp) and hotels in the area. production area #2 (figure 16) corresponds with the second discharge area of the system. in this area, there are eight geothermal manifestations, of which seven are hot water springs and one steam vent (alfaro valero et al., 2017). the thermal waters register temperatures up to 60°c and are used in the el delfín and la playa pools. the steam vent registers a temperature of 64.6°c. geological context unlike geothermal reservoirs in active volcanic zones where thermal anomalies are located following a linear pattern according to the fault lineaments (wu et al., 2012, norini et al., 2015), the geothermal system of paipa is different as the geothermal reservoirs are located through sedimentary rocks, which generates a lateral distribution of the hot fluids. the location of the thermal anomalies is found mainly to the northwest of the geothermal area (figure 17a). this location can be a product of the circulation of hot fluids at shallow depth, through porous stratigraphic units of the tilatá, bogotá, guaduas, labortierna, and los pinos formations, and facilitated by the faults, which function as migration paths. figures 17a and 17b show the el batán overthrust fault, 275determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data figure 10. iso-resistivity anomalies map of paipa geothermal zone. from franco (2016) in alfaro et al. (2017). figure 11. soil temperature map of paipa geothermal zone. 276 rafael andrés calderón-chaparro, german vargas-cuervo figure 12. index maps of paipa geothermal zone. a) land surface temperature index-map, b) structural lineaments index map, c) iso-resistivity index map, d) soil temperature index map. 277determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data figure 13. a) integrated index map of paipa geothermal zone. b) explorations prospects of paipa geothermal zone. with a northeast course, of vergence towards the east, functioning as a limit (restricting), for the lateral circulation of hot fluids. the anomalies of surface temperatures found between the el hornito and canocas faults, southwest of lake sochagota (figure 17a), would be validated, according to the conceptual geothermal model proposed by the cgs (alfaro valero et al., 2017), who proposed the existence of a secondary thermal system differing from the main one. this secondary system has a circulation of hot fluids at a shallow depth towards the northeast, by the porous sediments of the guaduas and bogotá formations. differently, for the thermal anomalies south of the canocas fault, between the corridor generated by the faults the bizcocho and the batan, can be validated by the presence of hydrothermal fluids, inferred by the low resistivity anomalies (<1 ω.m), obtained from the magnetotelluric technique at a height of 2,450 m.a.s.l, see figure 17b (alfaro valero et al., 2017). other satellite thermal anomalies of smaller size (some coinciding with the cerro plateado fault on the alluvial deposits and others on volcanic deposits, east of the alto los godos, coinciding with the godos lineament) have some spatial coincidence with low resistivity anomalies of the magnetotelluric technique (figure 17b). finally, the satellite thermal anomalies present in the itp-infiboy and el delfín-la playa sectors are validated because they are present in the two zones where the only surface geothermal manifestations of the system are presented. these manifestations are produced through alluvial deposits, for the itp-infiboy sector facilitated by the crossing of the el hornito and el bizcocho faults, and for the el delfín-la playa sector, facilitated by the el batán fault (alfaro valero et al., 2017). conclusions this research used thermal infrared technology combined with conventional exploration methods to identify hydrothermal prospects in the paipa geothermal region. surface temperatures were obtained from the research area, from landsat 7 etm+ and landsat 8 oli/tirs images. these images were calibrated and atmospherically corrected using the single channel and split window algorithms, which led to spatially improved temperature anomalies being obtained. additionally, fieldwork was carried out to validate satellite thermal anomalies. the fact that most satellite thermal anomalies coincide spatially with soil temperature anomalies validates the utility of satellite thermal bands in geothermal exploration. the spatial model implemented, determined six prospective areas of which two were validated by the spatial correlation of surface geothermal manifestations of the geothermal area of paipa. the use of thermal satellite images for the correct identification of geothermal anomalies requires the incorporation of geological-geophysical analysis; this integration considerably improves the interpretation of satellite thermal information. 278 rafael andrés calderón-chaparro, german vargas-cuervo figure 14. spatial comparison of soil temperature anomalies and satellite temperature anomalies. a-b) anomalies to the southeast of lake sochagota; c-d) anomalies to the southwest of lake sochagota towards the durazno sector; e-f) anomalies to the sector of el delfín and la playa pools, where the presence of the second most important manifestations of the paipa geothermal system occurs. 279determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data continuation figure 14. spatial comparison of soil temperature anomalies and satellite temperature anomalies. g-h) anomalies to the southeast of the el delfín and la playa pools, on the alluvial valley of the honda creek; i-j) anomalies to the east of the el delfín and la playa pools, close to cemex quarry; k-l) anomalies to the southwest of the el delfín and la playa pools, and northwest of los volcanes high. 280 rafael andrés calderón-chaparro, german vargas-cuervo figure 15. a) the spatial location of prospect #1 and hot springs in the main discharge area of the system (infiboy-itp). b) thermal and visible photography of the hot spring located next to the hotel lanceros; a maximum temperature of 51.3°c was recorded. c) thermal and visible picture from the hot spring of the devil’s eye; a temperature of 72.1°c was registered. figure 16. a) the spatial location of prospect #3 and hot springs in the second discharge area of the system (el delfin-la playa). b) thermal and visible picture of the spring iii el delfín, it has a temperature of 54.1°c. c) thermal and visible picture of hot springs i and ii el delfín. d) thermal and visible picture of the steam vent in the la playa sector; a maximum temperature of 64.6°c was registered. 281determination of hydrothermal prospects in the geothermal region of paipa (boyacá, colombia), using remote sensing and field data figure 17. a) the spatial relationship between satellite thermal anomalies of landsat 7 etm+, from march 6, 2006, and surface geological formations; b) spatial relationship between the anomalies of low resistivity (magnetotelluric, alfaro et al., 2017) and the lineaments of the research area. acknowledgments this investigation was developed within the framework of the 733 calls, from the government of boyacá and colciencias. we thank the government of boyacá for sponsoring the entire project, as well as the national university of colombia for its administrative collaboration in the management of resources. we also thank nasa and the alaska satellite facility for providing the landsat images and digital elevation models used in this investigation. references alfaro valero, c. m. 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(1998). the application of thermal infrared remote sensing techniques in geothermal surveying. remote sensing for land & resources, 67, 24-28. keywords: multifractal characteristics of regional geomorphology; development of geological disasters; correlation; accuracy; palabras clave: características multifractales de la geomorfología regional; desarrollo de desastres geológicos; correlación; exactitud how to cite item xiong, y. (2021). correlation analysis between multifractal characteristics of regional geomorphology and development of geological disasters. earth sciences research journal, 25(1), 49-55. doi: https://doi.org/10.15446/ esrj.v25n1.93700 in view of the inaccuracy of the traditional correlation analysis method, this paper proposes a correlation analysis method between the multifractal characteristics of regional landforms and the development of geological disasters. firstly, the multifractal characteristics of regional landforms are described by using the basic fractal characteristics of self-similarity or scale invariance. then the corresponding relation table is established according to the width of the fractal spectrum and the number of landslides and hidden dangers, and the spatial relationship of geological disaster development is analyzed. combined with the above-mentioned spatial relationship of geological disaster development and the multifractal characteristic data of regional landforms, the correlation coefficient between the two is calculated to complete the correlation analysis between the multifractal characteristics of regional geomorphology and the development of geological disasters. the experimental results show that compared with the traditional correlation analysis method, the correlation analysis results of the multifractal characteristics of regional geomorphology and the development of geological disasters are more accurate. abstract correlation analysis between multifractal characteristics of regional geomorphology and development of geological disasters análisis de correlación entre las características multifractales de la geomorfología regional y el desarrollo de desastres geológicos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93700 debido a la inexactitud del método de análisis de correlación tradicional, este artículo propone un método de análisis de correlación entre las características multifractales de accidentes geográficos regionales y el desarrollo de desastres geológicos. en primer lugar, las características multifractales de los accidentes geográficos regionales se describen a través de las características fractales básicas de autosemejanza o invariancia de escala. luego se establece la tabla de relación correspondiente de acuerdo con el ancho del espectro fractal y el número de deslizamientos de tierra y peligros ocultos, y la relación espacial del desarrollo de desastres geológicos. combinado con la relación espacial mencionada anteriormente del desarrollo de desastres geológicos y los datos característicos multifractales de accidentes geográficos regionales, el coeficiente de correlación entre los dos se calcula para completar el análisis de correlación entre las características multifractales de la geomorfología regional y el desarrollo de desastres geológicos. los resultados experimentales muestran que en comparación con el método de análisis de correlación tradicional, los resultados del análisis de correlación de las características multifractales de la geomorfología regional y el desarrollo de desastres geológicos son más precisos. resumen record manuscript received: 16/03/2020 accepted for publication: 27/11/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 49-55 yiying xiong hnu-asu joint international tourism college, hainan university, haikou, 570228, china xiongyiying88@163.com g eo lo g ic al h az ar ds https://doi.org/10.15446/esrj.v25n1.93700 https://doi.org/10.15446/esrj.v25n1.93700 https://doi.org/10.15446/esrj.v25n1.93700 mailto:xiongyiying88@163.com 50 yiying xiong introduction china is a country with many geological disasters. there are many kinds, wide distribution, frequent activities, and serious geological disasters, which is one of the most serious countries in the world. geological disaster refers to the event or phenomenon where the geological environment is destroyed suddenly or gradually due to geological processes (natural, artificial, or comprehensive), and human life and property are lost (kleemann et al., 2019). for the classification of geological disasters, there are different angles and standards, which are more complex. in terms of its causes, the geological disasters mainly caused by natural variation are called natural geological disasters; the geological disasters mainly induced by human actions are called human geological disasters. according to the different ways of energy release, its main types include earthquake, volcanic eruption, mountain collapse, landslide, debris flow, land subsidence, ground fissure, etc. although geological disaster is an inevitable phenomenon in the process of earth evolution, its occurrence will bring great influence and destructive power to people (masson et al., 2018). due to the impact of geological disasters on human society, many scholars have been exploring the general laws of these disasters, trying to find a way to prevent or evaluate geological disasters. at present, the research on geological disasters mainly focuses on risk assessment, risk analysis, early warning, and prediction model, while the research on the relationship between different combination forms of geomorphic morphology and spatial distribution of geological disasters is relatively less (galbraith & de lavergne, 2019). at the same time, slope, length, and density of ravines are often used in the study of geomorphic form, which is characterized by only one aspect of a single geomorphic type and highly generalizing the linear characteristics of geomorphic form. it not only causes partial distortion of geomorphic form information but also does not fully reflect the essential characteristics of complex and broken geomorphic form, namely, the amount of its comprehensive properties is not enough. however, with the development of fractal theory, its purpose is to explore the common, unpredictable, unstable, and very random systems, phenomena, and processes in nature, and try to find a new order between order and disorder, macro-micro and global-local. using the simple fractal function, we can depict the mandelbrot landscape which can reach the truth. like the real natural landscape, they have no characteristic length, which is difficult to realize by any mathematical physical model, and have been widely used in many subjects. although the fractal theory is still in its infancy in the study of geological disasters, it is gradually found that the geomorphic morphology shows fractal characteristics, and fractal can be used to quantify the geomorphic morphology of different structures (hansen et al., 2018). the geomorphic form is the final form of modern tectonic movement (cui & wang, 2019), one of the important underlying factors affecting the rainfall, runoff, and sediment yield process, and one of the important factors for the prediction of water and soil loss in the basin. due to the diversity of basin geomorphology and the complexity of spatial combination, the scientific and accurate quantification of basin geomorphology has become one of the key problems to be solved in the establishment of a widely applicable basin water and soil loss prediction model. domestic and foreign researchers generally choose one or more of the average slope, average slope length, watershed height difference ratio, gully density, and gully cutting depth as the quantitative parameters of the geomorphic factors in the watershed water and soil loss prediction model, but these quantitative parameters only represent the characteristics of one aspect of the watershed topography, and the expression of the integrity and comprehensiveness of the watershed topography is not very reasonable. therefore, there are obvious limitations in the application of this method in the field of water and soil loss assessment and prediction (soloviev et al., 2018). the fractal theory is an important branch of mathematics in nonlinear science. it was founded by american mathematician b. b. mandelbrot in the mid-1970s. the fractal theory aims to explore the common, unpredictable, unstable, and very irregular systems, phenomena, and processes in nature, and try to find a new order between order-disorder, macro-micro, and whole part, which is an important part of the modern scientific theory of complexity. although the geomorphic morphology (such as mountains, water systems, coastlines, etc.) seems to be messy and complex, it can be found that they still have certain mathematical (geometric) laws by fractal theory analysis and can be quantitatively described by fractal dimension (benammi et al., 2018). the fractal theory has opened up a new way for the non-linear study of geomorphic morphology description and simulation, geomorphic development process, etc. however, continuous research and practice have found that the complexity of geomorphic phenomena can not be completely described only by a single fractal dimension which depends on the overall characteristics. the complexity of geomorphic morphology shows obvious multifractal characteristics, which can be regarded as a large number of fractal sets with different scale-free areas. therefore, in order to solve the complexity problem in geomorphology effectively, it must focus on the complex multifractal problem, which is of great significance to comprehensively grasp the characteristics and formation laws of various geomorphic phenomena. multifractal theory, as the leading edge of fractal theory, is the further development of fractal theory. it is proposed to study the phenomenon of non-uniformity and dissimilarity in nature. it uses spectrum function to describe the special structure behavior and characteristics caused by different local conditions or different levels in the evolution process of the fractal structure and discusses the distribution law of probability measure of characteristic parameters by the method of statistical physics. the introduction of multifractal can deeply describe the complex features of landforms, thus revealing more information on landforms. with the development of computer and gis technology, a digital elevation model (dem) can directly display and express the three-dimensional shape of the basin landform. taking the basin landform dem as the research object, according to the fractal theory and the multifractal correlation calculation model, it is possible to directly quantify the multifractal characteristics of the basin three-dimensional landform (ali & lee, 2018). the correlation analysis between the multifractal characteristics of regional geomorphology and the development of geological disasters description of multifractal characteristics of regional landform when describing the multifractal characteristics of regional geomorphology, we use the basic fractal characteristics of self-similarity or scale invariance to describe quantitatively through fractal dimension index. it is a common phenomenon in many fields such as geomorphology to synthesize self-similarity and invariance. therefore, for many fractal bodies with nonuniform and singularity, one dimension cannot describe all their characteristics, so it needs to use multiple dimensions fractal measures (chardon et al., 2018). let  be a probability measure distributed on region o. if the concentration of  changes greatly, that is, for a very small r, it has the following relations:  b x r ra( , )( ) (1) in the above equation, b ( x,r ) represents the local neighborhood ball with radius r, a is the singular index, and the singular index a corresponding to different subsets is different. therefore,  is a multifractal measure, multifractal measure is a singular measure, and it does not have a density function. therefore, multifractal is an infinite set of singular measures defined on fractals and composed of multiple scale indexes (salin et al., 2019). in the study of the distribution of measure yields on fractal volume f, fractal volume f can be divided into several units with scale r, and i is used to represent the average value of measure yields in the i-th unit. there is the following scaling relationship between the average measure  and scale r of each cell: µi ar i→ (2) in the above equation, ai is the singularity index. if some elements have the same singularity index a, their measures can be expressed by (a). if fa is used to represent the subsets of the elements with the same singularity index a, then the s-dimensional hausdorff measure of fa can be defined as: 51correlation analysis between multifractal characteristics of regional geomorphology and development of geological disasters h fa a h fa a h fa a u fa s r r s r s i s i ( , ( )) lim ( , ( )) ( , ( )) inf : µ µ µ = = ⊂ → = ∞ ∑ 0 1 i i ∞                   (3) moreover, the s-dimensional hausdorff measure hs(fa, (a)) satisfies the following relations: h fa a s f a s fs ( , ( )) , ( ); (µ = ∞ < = when finite positive value when aa s f a ); , ( )0 when >      ; (4) in the equation above, f a( ) is the hausdorff dimension of the subset. if in the interval [a,a+da], the number of elements whose measure is (a) is n(a), then h fa a n a rs s( , ( )) ( )µ = (5) thus, it exists h fa a h fa a n a rs r r s s( , ( )) lim ( , ( )) ( )µ µ= = →0 (6) according to the above equations 4 and 6, only when the following equation is satisfied, n a r f a( ) ( )→ − (7) so that hs(fa, (a)) can be taken as a finite value, from which we can get: f a n a rr ( ) lim ln ( ) ln =− →0 (8) for uniform measure distribution, there is only a value; for non-uniform measure distribution, f a( ) is a convex function of a. in multifractal theory, f a( ) is the singular spectrum of multifractal. a is the singularity of different sub regions, and its size is determined by the measurement distribution of the generating unit of the corresponding sub regions. f a( ) is the fractal dimension of the sub regions with the same singularity a. from equation 6, we can see that its size is related to the number of subregions with the singularity a. f a( ) reveals the distribution structure and change of different singularity a in the fractal body, which is a measure of the complexity, irregularity and unevenness of the fractal body. it describes the physical characteristics of different localities or layers on the fractal body (alejo & montes-rojas, 2018). here f a( ) is a basic language to describe multifractal. the multi-fractal shape of regional geomorphology is divided into small units with scale of r, pi (r) is defined as the probability of unit i,  is the probability measure, and when q >> 1 and pi >> pj, the q step of probability measure  can be defined as: n q r p ri q i ( , ) ( )=∑ (9) in this case, the generalized s-dimensional measure can be defined as: m q r p r r n q r r m q m q r s i q i s s s r r s ( , ) ( ) ( , ) ( ) lim ( ) = = =        ∑ →0 (10) if there is a critical index t(q) that depends on the q-order distance of pi , when t(q) is defined as the mass index, then the generalized dimension can be defined according to the q-order distance of  , and the calculation results are as follows: d q n q r r q p p r q r r i i i = − ≠          → → ∑ 1 1 1 0 0 lim ln ( , ) ln , lim ln ln (11) in the above equation, dq is the sum of the q-power of each subset of space’s singular measures, reflecting the degree of singularity of each subset from the overall point of view, and calculating the generalized multifractal characteristic dimension curve of regional landform, as shown in the following figure: dq dq q q = +∞ q = 0 00 q = −8 figure 1. generalized multifractal characteristic dimension curve of regional geomorphology according to the characteristic curve shown in the figure above, dq and t(q) satisfy the following relationship: d q q q q q= − ≠ − =      1 1 1 1 1 τ τ ( ), '( ), (12) when dq and t(q) are differentiable, there is a legendre transformation, as follows: f a qa q a d q dq ( ) ( ) ( ) = − =−      τ τ (13) in the above equation,  is the legendre transformation coefficient. so for the general multifractal spectrum f a( ) is a single peak convex function about a, it increases monotonically in q > 0 part, decreases monotonically in q < 0 part, when q = 0 , f a( ) gets the maximum value, and the multifractal spectrum image obtained is shown in figure 2 below: q = −8 f a( ) f a( )max amin amax a q = 0 q < 0 a(0) q > 0 q = +∞ figure 2. multifractal spectrum image it can be seen from the above figure that the physical meaning of f a( ) is a measure of the complexity, irregularity and unevenness of fractal structure. 52 yiying xiong different singular indexes a reflect the properties of different probability measure subsets (sachica & martinez, 2018). for a simple fractal, the multifractal spectrum a ~ f a( ) is transformed into a point. at this time, this point is the multifractal feature of the regional landform described. retaining the multifractal feature points of the regional landform, combining the spatial relationship of the development of geological disasters, and calculating the correlation coefficient between the multifractal feature of regional landform and the development of geological disasters, the correlation analysis of the two can be completed. analysis of spatial relationship of geological disaster development before analyzing the spatial relationship of geological hazard development, 12 geological points are selected first, and then the geological hazard is divided into landslide, collapse and debris flow by using the fractal characteristic parameters of landform and the spatial distribution characteristic parameters of geological point landslide hazard.firstly, the corresponding relationship between the width of fractal spectrum a and the number of landslides and hidden dangers is established, as shown in the following table: table 1. width of fractal spectrum and quantity of landslides and hidden dangers geological point a n 1 0.045 0 2 0.066 19 3 0.131 51 4 0.120 49 5 0.130 96 6 0.116 134 7 0.084 53 8 0.107 78 9 0.094 48 10 0.134 221 11 0.157 191 12 0.124 134 according to the data shown in the above table, the relationship among a and the number of landslides and hidden geological disasters is obtained, as shown in the following figure: 0.160.120.080.04 0 50 100 150 200 250 a number of landslides and hidden dangers/piece figure 3. relationship among a and the number of landslides and potential geological disasters according to the above diagram, the following landslide coupling spatial relations can be concluded: in general, with the increase of a, the number of landslides also increases, which indicates that there is a significant correlation between the multifractal characteristics of geomorphic morphology and geological disasters of landslides. furthermore, with the increase of a, the complexity of geomorphic morphology and the probability of occurrence of landslides increase, the number of landslides will also increase (zhang et al., 2019). the number of landslides can be divided into three levels: when a<0.100, there is a positive correlation between a and the number of landslides. that is to say, with the increase of a, the number of landslides and potential disasters increases, but the number of landslides in this section is less, up to 53.when 0.100 < a < 0.134, the number of landslides fluctuates relatively large, indicating that the change range of geomorphic singular index is in this area, and the occurrence of landslides is very unstable, showing a certain degree of volatility.when 0.134 < a < 0.157, the number of landslides is about 200, indicating that when the change range of geomorphic singular index is in this area, landslides are more likely to occur. in the case of collapse, according to the fractal characteristic parameters of geomorphology and the spatial distribution characteristic parameters of particle collapse and hidden geological disasters in different places, the corresponding relationship table between the depth of geomorphic pleomorphic spectrum and the number of collapses n is established in the same way as landslide, as shown in the following table: table 2. width of fractal spectrum, collapse and number of hidden dangers geological point a n 1 0.045 0 2 0.094 5 3 0.084 5 4 0.066 0 5 0.107 34 6 0.116 11 7 0.120 7 8 0.130 67 9 0.131 13 10 0.124 65 11 0.138 20 12 0.075 5 according to the data shown in the above table, the relationship among a and the number of collapse and potential geological disasters is obtained, as follows: 0.160.120.080.04 0 20 40 60 80 a number of collapses and hidden dangers/piece figure 4. relationship among a and the number of geological disasters caused by collapse and hidden danger from the diagram above, the following spatial relation of collapse coupling can be concluded: generally speaking, with the increase of a, the number of collapses also increases, indicating that there is a significant correlation between the multifractal characteristics of geomorphic morphology and the geological disasters of collapses, and further indicating that with the increase of a, the complexity of geomorphic morphology increases, the probability of collapses increases, and the number of collapses will also increase.when the number of collapse disasters changes with the change range of geomorphic singular index a, the change characteristics can also be divided into three levels: when a < 0.100, the number of collapse and hidden danger disasters is very small, and there is almost no collapse geological disaster phenomenon in this section (leonarduzzi et al., 2018).when 0.100 < a < 0.134, although the number of collapses increases with a, it also shows some volatility in this range, especially between 0.130 and 0.134, the number of collapses first increases, then decreases and then increases, indicating that the change range of geomorphic singular index is in this region, and the disaster phenomenon of collapses is very unstable; when 0.134 < a < 0.157, the number of collapse disasters is large, but there is a downward trend, which may be related to the disaster prevention measures taken by people in engineering activities. 53correlation analysis between multifractal characteristics of regional geomorphology and development of geological disasters similarly, when analyzing the spatial relationship of the development of debris flow geological disasters, the corresponding relationship table between the depth of geomorphic pleomorphic spectrum and the number of debris flows is established, as shown in the following table: table 3. width of fractal spectrum and quantity of debris flow and its hidden dangers geological point a n 1 0.045 0 2 0.131 5 3 0.084 1 4 0.094 5 5 0.157 4 6 0.116 8 7 0.120 2 8 0.130 0 9 0.066 36 10 0.134 2 11 0.107 28 12 0.137 8 similarly, the relationship among a and the number of debris flow and potential geological disasters is obtained as follows: 0.160.120.080.04 0 6 12 18 24 30 36 a debris flow quantity / piece figure 5. relationship among a and quantity of debris flow and potential geological disasters from the above diagram, the following coupling spatial relations of debris flow disaster can be concluded: in general, with the increase of a, the number of debris flow increases, which indicates that there is a significant correlation between the multifractal characteristics of geomorphic morphology and the geological disasters of debris flow. furthermore, with the increase of a, the complexity of geomorphic morphology and the probability of debris flow disasters increase, the number of debris flow disasters will also increase. the number of debris flow disasters can also be divided into three levels according to the variation range of geomorphic singular index a. when a<0.120, there is a positive correlation between a and the number of debris flow. that is to say, with the increase of a, the number of debris flow disasters increases, but the number of debris flow in this section is less.when 0.120<a<0.134, although the number of debris flow increases with the increase of a, it also presents certain volatility in this range, especially between 0.130 and 0.134, the number of debris flow increases first, then decreases and then increases, indicating that the change range of geomorphic singular index in this region, the disaster phenomenon of debris flow is very unstable.when 0.134 < a < 0.157, the number of debris flow disasters is large, but there is a downward trend, which may be related to the disaster prevention measures taken by people in engineering activities (fan et al., 2018). to sum up, the change characteristics of the number of geological disasters at the twelve selected geological points in the change range of geomorphic singular index a can be roughly divided into three levels: when a<0.100, there is a positive correlation between a and the number of disasters, that is, with the increase of a, the number of disasters increases, but the number of disasters in this section is less; when 0.100 <a <0.134, the fluctuation of the number of disasters is relatively large, indicating that the change range of the geomorphic singular index is in this area, the landslide disaster phenomenon is very unstable, showing a certain degree of volatility; when 0.134<a<0.157, the number of disasters is large, indicating that the change range of the geomorphic singular index is in this area, the geological disaster phenomenon is more likely to occur. according to the spatial analysis of the spatial relationship of the development of geological disasters, combined with the multifractal characteristics of regional landforms, the correlation coefficient between the two is calculated, and the correlation analysis between the multifractal characteristics of regional landforms and the development of geological disasters is completed. calculation of correlation coefficient when calculating the correlation coefficient between the multifractal characteristics of regional landforms and the development of geological disasters, combining the above-mentioned spatial relations of geological disasters and the multifractal characteristics of regional landforms, the weights of the two are calculated firstly, and then the area is divided into t units of the same area, among them, d units are landslide units and d̄ units are non landslide units. in this case, the weight in the geological space can be defined as: w p b d p b d w p b d p b d + − = =        ln ( / ) ( / ) ln ( / ) ( / ) (14) where, w+ and w− are weight values of evidence factor existence and nonexistence respectively, and for the missing area of original data, their weight values are 0. b is the number of units in the area where the evidence factor exists, and b̄ is the number of units in the area where the evidence factor does not exist. the above equation (14) is processed by bayes rule, and the change of the above equation (14) is as follows: ln ( / ... )( ) ( ) ( )o d b b bk k n k n 1 1 2 2 (15) where, o is the probability, o = p / (1−p), d is the number of unit grids with landslides, bj is the j-th evidence layer, and k(j) is the j-th evidence layer. assuming that there are related variables in equation (15), the logical function of the correlation coefficient is established as follows: f x e e x x( )= +1 (16) according to the definition of expected value of discrete random variable, when p represents the probability of y = 1, we can get: f p e e p p( )= +1 (17) from the above equation, the change of function f p( ) to x is insensitive and slow near f p( ) = 0 or f p( )= 1, and the degree of nonlinearity is high. in this case, the logistic transformation of f p( ) is introduced, namely: log ( ) log ( )it p it p p = −1 (18) therefore, at this time, the correlation coefficient between the multifractal characteristics of regional geomorphology and the development of geological disasters can be calculated p it p p e e e e x x p p = − + + + log ( ) 1 1 1 (19) 54 yiying xiong from the above equation, p is the final correlation coefficient. using the above calculation and derivation process, when analyzing the correlation between the multifractal characteristics of regional landforms and the development of geological disasters, the correlation is converted into numerical value, and the data between the two are constantly transformed to obtain the change of the other caused by the change of one party, so as to obtain the correlation between the multifractal characteristics of regional landforms and the development of geological disasters, and complete the analysis of the correlation between the two (valenza et al., 2018). experiment experiment preparation the computer experiment environment for experiment preparation is shown as follows: figure 6. experimental environment the of geomorphic multiplicity characteristic and the coefficient of geological disaster development of the experimental preparation area are shown in the following table: table 4. parameters of regional geomorphic multiplicity characteristic and geological disaster development data r n(r) ln(r) lnn(r) 5000 3 8.517193191 1.098612289 4000 5 7.919356191 1.609437912 4250 4 7.824046011 1.609437912 4000 4 7.718685495 1.609437912 3800 4 7.60090246 1.791759469 3500 5 7.522940918 1.791759469 3300 5 7.313220387 1.945910149 2750 5 7.170119543 1.945910149 2500 6 6.907755279 2.197224577 2250 7 6.684611728 2.302585093 2000 7 6.476972363 2.564949357 1850 9 6.214608098 2.772588722 1500 10 5.703782475 2.944438979 1300 13 5.298317367 3.218875825 1000 16 5.010635294 3.713572067 800 19 4.787491743 4.110873864 650 25 4.605170186 4.394449155 500 41 4.442651256 4.615120517 300 61 4.317488114 4.795790546 based on the above data preparation, two traditional correlation analysis methods and the correlation analysis methods for regional geomorphic multifractal characteristics and geological disaster development are used for experiments, and the accuracy of the three correlation analysis methods is compared (yang et al., 2020). analysis of experimental results using the correlation coefficient calculation equation, the coefficients in table 4 is calculated, and finally the experimental results obtained by the three correlation analysis methods are as follows: 0.6 0.8 1.00.40.20.00.0 0.2 0.8 0.6 0.4 1.0 traditional analysis method 2 traditional analysis method 2 analysis method in this paper numerical value a na ly si s r es ul t v al ue figure 7. experimental results of three analysis methods from the above experimental results, it can be seen that with the increase of the parameters of regional geomorphic multiplicity characteristic and geological disaster development data, the analysis results obtained by traditional analysis method 1 show irregular changes. compared with the traditional analysis method 2 (wu et al., 2020), the correlation analysis method of regional geomorphic multifractal characteristics and geological disaster development is larger, and the error of correlation analysis is larger.however, the results of traditional correlation analysis method 2 show linear alternation between regular and irregular, and the error of correlation analysis is small. however, the correlation analysis method of the multifractal characteristics of regional geomorphology and the development of geological disasters shows a linear change, which shows that there is a strong correlation between the multifractal characteristics of regional geomorphology and the development of geological disasters, and there is almost no error in the actual analysis, so it is suitable for the actual correlation analysis (cheng et al., 2020). conclusions taking topography and geomorphology as the research object, using fractal, multifractal theory and gis technology, combined with geomorphology, disaster statistics and other interdisciplinary theories and methods, taking digital elevation model as the basic data source, the multifractal calculation model of temperature geomorphology and the realization method based on gis technology are proposed, and the spatial-temporal characteristics of main types of geological disasters are analyzed (gao et al., 2018). the related parameters such as the multifractal spectrum, linear interval and surface area distribution probability of the three-dimensional landforms in this area are calculated, the multifractal characteristics of the three-dimensional landforms are revealed, and a coupling model based on the relationship between the multifractal characteristics of the landforms and the number of geological disasters is established (gao et al., 2019). the coupling relationship between the multifractal characteristics of the landforms and the number of main types of geological disasters is analyzed, and the rationality of multifractal feature of geomorphology as a 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(2019). nonlinear regression via deep negative correlation learning. ieee transactions on pattern analysis and machine intelligence, 99, 1-1. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.102843 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (september, 2023): 259271 g eo c h em is tr y genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry estefany andrea mora-galindo1, juan carlos molano mendoza1, milton julián morales2 1. universidad nacional de colombia 2. servicio geológico colombiano *corresponding author: eamorag@unal.edu.co abstract manuscript received: 25/05/2022 accepted for publication: 13/09/2023 how to cite item: mora-galindo, e. a., molano mendoza, j. c., morales, m. j., & sepúlveda, j. (2023). mineral chemistry in pegmatites from eastern colombia: genesis and evolution. earth sciences research journal, 27(3), 259-271, https://doi.org/10.15446/ esrj.v27n3.102843 keywords: pegmatite, magmatic fractionation, strategic minerals, mineral chemistry. pegmatites and granitic intrusives from eastern colombia, belonging to the guainía department, were studied to determine the generating magma type, as well as to assess the degree of magmatic fractionation and its relationship with the presence of nb-ta-bearing minerals (strategic minerals). to that end, 31 rock samples were studied; epma electronic microprobe analyses were conducted on apatite, biotite, garnet, microcline, muscovite, and tourmaline. the results from biotite, garnet, and tourmaline suggest that these rocks originated from a fe-rich peraluminous melt. moreover, content variation of ba in microcline, na in muscovite, fe-mg in biotite, and mn-sr in apatite, allowed the proposal of a magmatic evolutionary line for these rocks. nb-ta-bearing samples were present in the most fractionated rocks where tourmaline and garnet occurred, and thus, were proposed as possible indicators of strategic minerals. génesis y evolución de pegmatitas en el oriente colombiano mediante química mineral resumen se estudiaron pegmatitas e intrusivos graníticos del este de colombia pertenecientes al departamento del guainía para determinar el tipo de magma generador, su grado de fraccionamiento magmático y su relación con la presencia de minerales portadores de nb-ta (minerales estratégicos). con este propósito, se estudiaron 31 muestras de rocas y se realizaron análisis con microsonda electrónica epma en apatito, biotita, granate, microclina, muscovita y turmalina. los resultados obtenidos a partir de biotita, granate y turmalina sugieren que dichas rocas se generaron a partir de un magma peraluminoso rico en fe. además, la variación del contenido de ba en la microclina, na en la muscovita, fe-mg en la biotita, y mn-sr en el apatito permitió proponer una línea evolutiva magmática para estas rocas. las muestras portadoras de nb-ta estaban presentes en las rocas más fraccionadas, donde se encontraron turmalina y granate, por lo que se propusieron como posibles indicadores de minerales estratégicos. palabras clave: pegmatita, fraccionamiento magmático, minerales estratégicos, química mineral. mailto:eamorag@unal.edu.co https://doi.org/10.15446/esrj.v27n3.102843 https://doi.org/10.15446/esrj.v27n2.103801 https://doi.org/10.15446/esrj.v27n3.102843 260 estefany andrea mora-galindo, juan carlos molano mendoza, milton julián morales 1. introduction in eastern colombia, rocks belonging to the guainía department have been identified as having economic potential in strategic minerals (franco et al., 2021a; lopez & cramer, 2014). this motivated a research agreement between the universidad nacional de colombia and the servicio geológico colombiano (colombian geological service) where granitic and pegmatitic samples were collected in the eastern border of the guainía department, in the surroundings of san jose, macanal, and tabaquén (guerrero et al., 2017; morales et al., 2017; zarate et al., 2017). however, the persistent lack of certainty regarding the magma type that formed such granites and pegmatites, their interrelations, and that with the presence of nb-ta-bearing minerals (bayona-valderrama, 2018; charry et al., 2023) motivated the present study. with this aim, major, minor, and trace elements were analyzed in apatite, biotite, garnet, microcline, muscovite, and tourmaline. in particular, the utility of biotite was examined, both to correlate these pegmatitic bodies to a generating magma type (nachit, 1985; stussi & cuney, 1996), and together with the trace element analysis of microcline and muscovite, to determine the degree of fractionation of the intrusives (alfonso et al., 2003; černý et al., 1985; london et al., 1990). furthermore, elemental analysis of apatite, garnet, and tourmaline were conducted to supplement the information obtained from the previous minerals. these analyses contribute to the understanding of both the petrogenetic evolution and the type of magma involved in these processes. finally, the interrelation between the presence of nb-ta-bearing minerals and translucent minerals was evaluated as an exploration vector. 2. geological context 2.1 regional geological context eastern colombia exposes part of the amazon craton, which has been subdivided into the following geochronological provinces (from oldest to newest): central amazonic (> 2.3 ga), maroni-itacaiunas (2.2–1.95 ga), ventuari-tapajós (1.95–1.80 ga), rio negro-juruena (1.8–1.55 ga), rodoniansan ignacio (1.55–1.3 ga) and sunsás (1.3–1.0 ga) according to tassinari and macambira (1999) (fig. 1). of the above, the one extending over the eastern part of colombia (fig. 2), and therefore the study area, is the río negro-juruena province, with a total extension of approximately 2000 km in the nw-se direction, and 600 km in the ne-sw direction (tassinari & macambira, 1999). according to these authors, this province mainly consists of granite gneisses, tonalitic, and granodiorite granitoids, with ages ranging between 1.8-1.50 ga where ages decrease toward the western margin (cordani et al., 2016). cordani et al. (2016) divided the nw fraction of the rio negro-juruena province in the atabapo and vaupes belts, using u-pb (zircon), sm-nd (model ages), and rb-sr (whole-rock) isotope analysis, and a compilation of previous u-pb (zircon and monazite) k-ar (micas) and rb-sr (whole-rock and micas) dating. • the atabapo belt ranges from 1.80 ga to 1.74 ga. during this period formed its basement, which corresponds to a series of magmatic arc orogenic pulses. subsequently, it was intruded by granitic bodies (1.50 ga) (fig. 2). • the vaupes belt formed between 1.58 ga and 1.50 ga by a series of orogenic pulses with magmatic arc affinity (brito neves, 2011). this agrees with the report of syntectonic calco-alkaline gneisses and migmatites in mid-amphibolite facies by priem et al. (1982). in addition, cordani et al. (2016) reported younger undeformed granite rocks, which could be posterior to the formation of this belt (fig. 2). bonilla et al. (2021) presented concurring evidence for the evolution of the rio negro-juruena province. in guainía, colombia, ages between 1.85 and 1.72 ga were associated to metamorphic and igneous rocks belonging to the mitu complex. these ages were interpreted by these authors as a result of the statherian collisional and orogenic events (i.e., the querari orogeny). furthermore, bonilla et al. (2021) identified additional orogeny-related rocks ranging 1.60 to 1.50 ga old. such results can be related to the atapabo belt and vaupes belt of cordani et al. (2016), respectively. it is worth noting that, although some authors prefer using the term “mitu complex” due to its heterogeneous nature, the officially recognized name for this unit is the “migmatitic mitu complex” (rodriguez et al. 2011). figure 1. geochronological provinces in the amazon craton. region of interest is shown in a dashed rectangle. modified from tassinari and macambira (1999). according to cordani et al. (2016), the interval between the formation of these belts allowed the cratonization of the atabapo belt and the intrusion of granite bodies as a reflection of the orogenic pulse that formed the vaupes belt. bonilla et al. (2021) reported decompressional melting in the guainía and vaupes departments, resulting in the formation of a-type granites with la-icpms zircon u-pb ages dating from 1.40 to 1.34 ga. these authors suggest that the matraca granite in guainía can be correlated with the parguaza intrusion in venezuela, crystallizing during a later phase of that event. furthermore, a recent apatite la-icp-ms u-pb geochronology study identified two stages of anorogenic events in eastern colombia. the first occurred between 1.55-1.40 ga, followed by the second stage between 1.40-1.34 ga. these events were associated with ore deposits of incompatible elements, including nb, ta, ree, and u (bonilla et al. 2023). regarding the são gabriel-içana river area (fig. 2), sr isotopic redistribution was found in association with ages of 1203 ± 58 ma (cordani et al., 2016), a fact that could correspond to the high-grade metamorphism that occurred during accretion of the sunsás province (tassinari & macambira, 1999). 261genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry figure 2. geochronological belts as proposed by cordani et al. (2016) on the border between colombia, venezuela, and brazil. modified from cordani et al. (2016) 2.2 local geological context in colombia, the rocks of the rio negro-juruena province have not been as exhaustively studied as those in the cordillera, primarily owing to its remoteness and the characteristic dense vegetation of this tropical region. however, rock samples were collected within an area of 198 km2 (guerrero et al., 2017), between the tabaquén and macanal indigenous communities along the guainía river (fig. 3). this area has dense vegetation, high temperature and moisture levels, and a smooth relief with an average slope of 1.3% along the guainía river. figure 3. location of study area and samples in the department of guainía. modified from charry et al. (2023). mainly granitic intrusives, gneiss, pegmatite dykes, and quartz veins were found. a more detailed field characterization allowed to identify syenites, syenite quartz, monzonite quartz, granites, and granodiorites among the intrusive bodies. these rocks had textures ranging between equigranular and inequigranular, and the predominant colorations were white, light gray, and light brown (fig. 4c). the most common accessory minerals were biotite, amphibole, and magnetite with slight orientation for some of the crystals. these rocks are cut by pegmatite dikes with thicknesses between 2 cm and 4.5 m and show net to transitional contacts (guerrero et al., 2017) (fig. 4d, e, f). quartz veins in stockwork arrangements were found as well. the composition of the pegmatites resembled that of the granitoids they intrude and had no clear internal zonation. this has been interpreted as a possible genetic relationship between them. in addition, some metamorphic rock outcrops such as granite orthogneiss and paragneiss (fig. 4b) were found. a striking feature was the presence of schorl tourmaline associated both with the paragneisses and quartz veins (morales et al., 2017); such crystals’ size reached up to 3 cm and were oriented 0° to 45° with respect to foliation (fig. 4a). furthermore, pan concentrates samples were taken in this expedition, where nb-rutile (also known as ilmenorutile) was found along with columbite and some strüverite grains intergrowth (zarate et al., 2017). a c e b d f figure 4. outcrop photographs (a, b, c, d) and hand samples (e, f). a and b: paragneiss n_2816, the black crystals in photograph a correspond to tourmalines. c: granitic intrusive (two-feldspar granite) cut by quartz vein gv_2861. d: pegmatitic enclave outcrop p_2846. e: pegmatite hand sample p_2984. f: pegmatite sample p_2883. it is important to note that nb-ta-bearing minerals have been reported in the study area (zarate et al., 2017) and in the neighboring anorogenic igneous body, the parguaza granite (bonilla-perez et al., 2013; gonzalez & pinto, 1990). this intrusive has been associated with nb-ta mineralizations (bangerter, 1985; franco et al., 2021a) which include cassiterite, struverite, tantalite-columbite, and ixiolite in pegmatites and quartz veins (aarden & davidson, 1978; gaudette et al., 1978; rodriguez & perez, 1982). 262 estefany andrea mora-galindo, juan carlos molano mendoza, milton julián morales methodology out of the rock samples, 31 polished thin sections were made including granitic intrusives, pegmatite dykes, quartz veins, and one paragneiss. petrographic analysis by optical microscopy was performed in all of them (see results). a subsequent selection of the samples was made according to their quality; high quality meant the least possible presence of hydrothermal and/ or supergene alterations to ensure reliable results in the epma analysis. this resulted in 23 samples of which 1 corresponded to a paragneiss (n), 8 to granitic intrusives (g) from which 3 were cut by quartz veins (gv), and 14 to pegmatitic bodies (p) (including dykes, lenses, and qz-rich rocks with pegmatitic texture). 3.1 analytical methods between two and three points per crystal (taken from center to the edge) were analyzed using an epma electronic microprobe (jeol jxa-8230), resulting in a total of 254 measurement points on 93 crystals from 23 polished thin sections. the instrument was operated in all cases with natural standards in wds (wavelength-dispersion spectroscopy) mode, with graphite coating, and a voltage of 15 kev. table 1 shows both the measurement parameters and the chemical elements analyzed in biotite. for measurement parameters of apatite, garnet, microcline, muscovite, and tourmaline refer to appendix a. microcline, muscovite, and biotite crystals were selected in all thin sections, except in those where such crystals were absent or altered (table 2). some of these minerals were in paragenesis with opaque minerals, which were analyzed by bayona-valderrama (2018) and charry et al. (2023) using epma electron microprobe and raman spectroscopy. according to these authors, those minerals were magnetite, hematite, ilmenite, nb-rutile, and columbite (table 2); the last two define hereafter the nb-ta bearing samples, which are highly relevant to the fractionation degree discussion. it is worth noting that these minerals presented ta2o5 contents, between 1.78-6.05 wt% for nb-rutile, and between 5.29-6.05 wt% for columbite (bayona-valderrama, 2018). only the crystals comprising a primary magmatic mineral assemblage were selected. however, some crystals that seemed to indicate later introduction were selected as well (table 2). on the other hand, those included in other crystals were not considered to ensure analyzing the highest state of evolution reached by the rock. representative crystals were chosen when significant textural changes were visible within a sample. additionally, tourmaline, garnet, and apatite crystals were included to supplement the information obtained from the other minerals (table 2). table 1. epma analysis conditions for biotite biotite analyzed element (radiation line) si (kα) ti (kα) al (kα) cr (kα) fe (kα) mn (kα) mg (kα) ca (kα) na (kα) k (kα) detection limit (ppm) 62 36 34 43 48 43 33 23 35 19 diffraction crystal 1 tap 3 peth 1 tap 3 lifh 3 lifh 3 lifh 1 tap 3 peth 1 tap 3 peth counting time (peak)[s] 30 30 30 30 30 30 30 30 10 30 counting time (background) [s] 15 15 15 15 15 15 15 15 10 15 standard kyanite ilmenite orthoclase chromite almandine spessartine hornblende_143965 anortite albite hornblende_143965 beam current (na) 20 20 20 20 20 20 20 20 20 20 table 2. relationships of the microprobe-analyzed minerals and associated opaque minerals. the column “in paragenesis” presents those opaque minerals that were in paragenesis with the analyzed mineral. sample analyzed minerals opaque minerals* observations in paragenesis others present in sample n_2816 tourmaline possible association with pegmatites g_2895 apatite g_2973 biotite g_2937 biotite mineral is possibly part of a late event g_2936 muscovite magnetite magnetite, ilmenite rock with high biotite content microcline magnetite biotite apatite magnetite, ilmenite (continued) 263genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry sample analyzed minerals opaque minerals* observations in paragenesis others present in sample g_3003 microcline biotite measured points are part of a possible late event apatite gv_2861 muscovite magnetite, ilmenite hematite, magnetite, ilmenite microcline tourmaline borders quartz vein that cuts the intrusive (sample) gv_3012 muscovite microcline biotite gv_2815 tourmaline borders quartz vein that cuts the intrusive (sample) p_2870 muscovite microcline magnetite biotite p_2807 muscovite ilmenite, magnetite ilmenite thin section presents a contact between pegmatitic and mesogranular texturemicrocline ilmenite tourmaline p_2917 muscovite mineral is possibly part of a late event microcline tourmaline p_2972 muscovite ilmenite magnetite, ilmenite microcline biotite magnetite p_2820 muscovite magnetite ilmenite measured points are part of a possible late event p_2846 muscovite nb-rutile columbite, nb-rutile, magnetite microcline p_2883 muscovite magnetite, ilmenite nb-rutile microcline nb-rutile p_2984 muscovite microcline biotite p_2995 microcline columbite, ilmenite, hematite p_3005 muscovite microcline p_3006 muscovite microcline garnet p_2823 muscovite magnetite p_2940 microcline tourmaline p_2935 muscovite microcline *in bold, strategic minerals analyzed by bayona-valderrama (2018) and charry et al. (2023) 264 estefany andrea mora-galindo, juan carlos molano mendoza, milton julián morales 4. results 4.1 petrographic analysis paragneiss n_2816: this rock mainly consisted of quartz, muscovite, and biotite, with clear orientation of the crystals, and micas-quartz micro-banding. some euhedral to subhedral tourmaline crystals were found, and some of them were concordant with the foliation of the rock (fig. 5). figure 5. ppl microphotograph of gneiss. muscovite crystals in paragenesis with some opaque crystals and a tourmaline crystal covering the right portion of the photo. granitic intrusives these samples presented textural variation between fine and coarse, and in some cases crystals larger than 1 cm. the predominant minerals were quartz, with lower content of plagioclase (albite to oligoclase) than microcline and variable content of biotite and muscovite. plagioclase, as opposed to microcline, was generally altered. apatite, zircon, magnetite, and ilmenite were also identified. in some samples micas did not seem to form part of the primary mineral assemblage and were usually associated with opaque minerals and apatite (fig. 6a). this was interpreted as caused by late crystallization. an example of this was sample g_2937, where two generations of biotite and muscovite were observed; the first formed part of the primary mineral assemblage, where the biotite was almost completely chloritized; the second generation was an accumulation of biotite and muscovite, where the biotites showed little to no alteration and had euhedral shapes (fig. 6b, 8a). three of the granitic intrusives (gv_2861, gv_3012, and gv_2815) were cut by quartz veins bordered with tourmaline crystals (samples gv_2815 and gv_2861) as seen in figure 8b and 8c. pegmatites these rocks (dykes, lenses, and qz-rich rocks with pegmatitic texture) showed variable features both in composition and texture. some of them had inequigranular texture between mesogranular and coarse, whereas others only had medium to thick crystals. the samples were divided into three groups according to their rock-forming minerals content (qz-plag-fk): (i) rocks composed almost 100% of microcline and quartz in similar proportions, and without plagioclase: p_2807, p_2972, p_2995 and p_2940. a bb figure 6. ppl microphotographs of granitic intrusives. (a) sample g_2936 showing association between biotite and opaque minerals. (b) sample g_2937 showing differential alteration of biotite (crystal 1 and 2). (ii) rocks composed of quartz between 75% and 95%, with low microcline content and without plagioclase p_2846, p_2883, p_3005, and p_2823. (iii) rocks composed of quartz, microcline, and plagioclase, with plagioclase content variable from predominant to in similar proportion with quartz and microcline, respectively: p_2917 (fig. 7), p_2820 (fig. 7), p_2870 and p_3006, p_2935, p_2984. figure 7. ppl microphotographs of pegmatites. sample p_2917 (left) showing highly altered plagioclase and no altered microcline (as in granitic intrusives). sample p_2820 showing low alteration of plagioclase. as accessory minerals for the three groups, muscovite, biotite, magnetite, and ilmenite were recognized. nb-rutile was identified in samples p_2846 and p_2883. in sample p_3006 garnet was identified in association with muscovite. muscovite was common among the samples. in some of them biotite arranged in clusters and was associated with opaque minerals (as seen in the granitic intrusives, fig. 6a), suggesting late biotite crystallization. 4.2 epma electronic microprobe analysis 4.2.1 apatite eight points in five apatite crystals were analyzed from the granitic intrusives samples g_3003, g_2936, and g_2895 (representative photographs shown in fig. 9); all crystals were associated with magnetite, ilmenite, and biotite. regrettably, it was not possible to measure this mineral in all types of samples due to its scarcity. 265genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry figure 8. photographs of three thin sections (a) g_2937 showing an accumulation of dark minerals (biotite), (b) and (c) gv_2815 and gv_2861 respectively, show darkcolored minerals that correspond to tourmaline bordering quartz veinlets. the red circles show the location of the epma analyzed points. figure 9. representative images of analyzed apatites (ap). the upper left and middle left images display polished thin section photographs from samples g_2895 and g_2936, respectively. these photographs showcase the association of apatites with magnetite and ilmenite. the upper right and middle right images present backscattered electron images of the analyzed crystals; the pink points represent the microprobe analyses conducted. the lower photograph, captured under plane-polarized light, exhibits the crystals surrounding the analyzed apatite. the association of micas with opaque minerals is shown. 266 estefany andrea mora-galindo, juan carlos molano mendoza, milton julián morales the primary focus of the analysis of this mineral was the relationship between sr and mn as it is a marker of magmatic fractionation degree (belousova et al., 2002); see figure 10. figure 10. sr vs mn in apatite for the three analyzed granitic intrusives. fractionation degree is shown according to belousova et al. (2002). detailed weight percentages of major elements and ppm of trace elements (si, fe, mn, na, f, cl, s, as, sr, la, ce, nd) are shown in appendix b1. 4.2.2 biotite fourty-two points were analyzed in nineteen biotite crystals belonging to five granitic bodies (g_2973, g_3003, g_2936, g_2937, gv_3012), and three pegmatites (p_2972, p_2984, p_2870). biotite crystals were found in other samples as well, but they were chloritized and thus not subjected to electronic microprobe analysis. the fe# was calculated for biotite (equation 1), since it is a marker of magmatic fractionation degree (stussi & cuney, 1996) (fig. 11). fe# = fetot ∕ (fetot + mg) (1) figure 11. fe# in biotite for samples of granites and pegmatites (filled symbols represent pegmatites while open symbols correspond to granitic intrusives). the samples are organized according to their fractionation degree, following stussi & cuney (1996). moreover, the al vs mg molar ratio was used to assess the magma type of the samples (fig. 12). figure 12. al vs mg molar ratio in biotites. magma type discrimination between peraluminous, calco-alkaline, subalkaline, and alkaline. taken from gomes and neiva (2005) that modifies nachit (1985). detailed compositional results are shown in appendix b2. 4.2.3 garnet three points in a single garnet crystal were analyzed from pegmatite p_3006 (fig. 13), belonging to group iii. this crystal was found in association with muscovite. figure 13. backscattered electrons image of analyzed garnet crystal surrounded by muscovite (sample p_3006) with respective wds measurement points in pink. the composition of the garnet was predominantly spessartine with high content of almandine; moreover, the spessartine content rises from the center to the edge of the crystal, with a relative reduction of the other members (fig. 14). figure 14. composition of the garnet crystal in sample p_3006 in terms of almandine (alm), spessartine (spss), grossular (grss), and pyrope (prp). for detailed compositional results refer to appendix b3. 267genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry 4.2.4 microcline eighty-four points in thirty-one microcline crystals were analyzed. eight crystals belonging to the group of granitic intrusives (gv_3012, gv_2861, g_3003, and g_2936), and the rest to pegmatites from group i, ii, and iii. ba, pb, p, and sr contents were obtained due to their suitability in showing increasing or decreasing fractionation degree trends of the host rock (abad-ortega et al., 1993; alfonso et al., 2003; černý et al., 1985; kontak & martin, 1997; london et al., 1990; roda robles et al., 1995). however, in most cases, sr concentration was close to the detection limit (appendix b4), as obtained by alfonso et al. (2003). similarly, pb and p were not useful to discriminate the samples according to their evolution degree due to high data dispersion (fig. 15) and to very low concentrations among several samples. figure 15. box and whisker plots for ba, pb, and p concentrations (top to bottom) in analyzed microcline samples. conversely, ba presented low dispersion (fig. 15), allowing the samples to be organized from lowest to highest ba content and therefore, from highest to lowest fractionation degree (černý et al., 1985; roda robles et al., 1995) (fig. 16). figure 16. ba concentration (ppm) in microcline for samples of granitic (open symbols) and pegmatitic intrusives (filled symbols) showing fractionation degree. the nb-ta-bearing samples (bayona-valderrama, 2018; charry et al., 2023) are shown in red. samples bearing garnet and tourmaline are marked. 4.2.5 muscovite fifty-four points were analyzed from 23 crystals: six of them belonging to the granitic intrusives g_2936, gv_3012, gv_2861, and the remaining crystals belonging to pegmatites of the three groups. ba, na, sn, ti, w, zn were measured (fig. 17); according to alfonso et al. (2003) these elements concentrate differentially in relation to fractionation degree. figure 17. box and whisker plots for the concentrations of na, ti, ba, sn, w, and zn in the analyzed muscovites. 268 estefany andrea mora-galindo, juan carlos molano mendoza, milton julián morales however, among these elements, the concentration of na was the only that presented little dispersion and enough variability among samples (fig. 17), allowing them to be sorted from highest to lowest concentration, and therefore, from highest to lowest fractionation degree (alfonso et al., 2003) (fig. 18). figure 18. na concentration (ppm) in muscovite for samples of granitic intrusives (open symbols) and pegmatites (filled symbols) showing fractionation degree. the samples bearing nb-ta minerals (bayona-valderrama, 2018; charry et al., 2023) are shown in red. detailed compositional results are shown in appendix b5. 4.2.6 tourmaline the chemical composition of 57 points of 13 crystals was analyzed. out of these, three crystals belonged to paragneiss n_2816, five crystals were part of the tourmaline borders of the quartz veins that cut the granites gv_2861 and gv_2815, and five crystals belonged to pegmatites p_2917, p_2940 and p_2807 (see appendix b6 for detailed composition). the cations calculation was made according to henry & guidotti (1985), assuming three boron atoms and obtaining the corresponding mass percentage to produce them (appendix b6). a compositional diagram was used following henry and guidotti (1985); accordingly, the analyzed rocks belong to “fe+3-rich granitic rocks” (fig. 19). the crystals found in the paragneiss also fall into this classification indicating that these tourmalines might have crystallized during pegmatite emplacement (morales et al., 2017). figure 19. al-fe total-mg (in molecular proportions) classification diagram based on quantitative tourmaline analysis. modified from henry and guidotti (1985). 5. discussion 5.1 fractionation degree and relationship with nb-ta-bearing minerals the analyses of apatite, biotite, microcline, and muscovite aimed to assess the fractionation degree of the rocks. regarding the results from apatite and biotite, which mainly represented granitic intrusions and some pegmatites, not all granitic intrusives were in the same degree of fractionation (figs. 10, 11), which can both indicate different intrusive generations or consecutive intrusions from an evolving single melt. in addition, it would be expected that the pegmatitic bodies (where biotite was analyzed) showed a higher fractionation degree than the granitic intrusives if they belonged to the same petrogenetic evolution line. this can be seen in figure 11 with two exceptions: (1) the granitic intrusive g_2937 appears as the most fractionated sample, above the pegmatite samples, which could be explained by the texture of biotite, which seemed to represent a later crystallization for this mineral compared to that of the rock, as exposed in section 4.1 and shown in figure 6a. (2) the granite g_2973 showed the greatest data dispersion, some of it being intermediate between two pegmatite bodies. unlike in the previous sample, these biotite crystals do not show textural features that could indicate a more recent crystallization. because of that, this intrusive might belong to a different magma generation, exhibiting a similar fractionation degree to the pegmatites from the other generation. moreover, results from both apatite and biotite (figs. 10, 11) indicate that sample g_3003 was the least fractionated, and that sample g_2936 had a higher degree of fractionation than the first, confirming the results. however, it is necessary to carry out chemical analysis on minerals that are present in as many samples as possible to understand the relationship between nb-ta-bearing minerals and the fractionation degree of the igneous bodies. therefore, microcline and muscovite, found in almost all samples (table 2), represent better petrogenetic evolution indicators than biotite and apatite for these samples. according to the results of microcline, the pegmatite samples had the highest fractionation degree (fig. 16). in turn, granitic intrusives contained the highest concentrations of ba and thus were the least fractionated. this could denote a petrogenetic evolution line involving all the analyzed samples. however, the pegmatite p_3005 appeared as one of the least fractionated (between granitic intrusives in this figure). subsequently, this rock might come from a different magmatic source and does not seem to be associated with the evolutionary line that led to the other pegmatites and the na-ta-bearing minerals. furthermore, the location of the nb-ta-bearing samples on figure 16 is consistent with the high magmatic fractionation degree that is required to concentrate incompatible elements to form independent mineral phases (linnen et al., 2012). for this study, rocks bearing nb-ta-bearing minerals had microcline ba concentrations lower than 699 ppm. despite the microprobe results, the presence of textural features related to pegmatite fractionation degree would be advantageous in the field. to this end, tourmaline and garnet presence in the samples were marked in figure 16 and 18 showing a possible relation. conversely, the mineralogical classification of pegmatites made in section 4.1 did not show any relation to it. it was inferred that tourmaline crystallization occurred in the early stages of nb-ta mineralization during the magmatic fractionation process, and garnet crystallized in advanced stages of fractionation. therefore, the presence of these minerals is proposed as an exploration tool as they might indicate proximity to mineralized pegmatitic bodies. consequently, ba concentration is suggested as an indicator of fractionation in pegmatitic bodies with similar characteristics in eastern colombia, preferring it above sr, p, and pb contents. lastly, similarities were found between the results from na in muscovite and those obtained with the analysis of ba in microcline (figs. 16, 18). first, pegmatites had the highest concentrations of na, indicating a high fractionation degree (alfonso et al., 2003), except for the pegmatite p_3005, which conforms to the results from microcline. in addition, the nb-ta-bearing pegmatites (bayona-valderrama, 2018; charry et al., 2023) showed high magmatic fractionation degree (fig. 18). finally, samples with tourmaline showed a lower degree of petrogenetic evolution than the samples p_2846 and p_2883 (nb-ta 269genesis and evolution of pegmatites in eastern colombia: insights from mineral chemistry enriched), supporting the interpretation that this mineral crystallized in the early stages of nb-ta mineralization, as seen from microcline results. however, some analyses did not concur with the data obtained from the microcline. in the case of sample p_2846 (nb-ta enriched), low na contents contradict the approach of linnen et al. (2012) which refers to the high magmatic fractionation degree necessary to concentrate nb-ta. for sample p_2984, the results do not coincide with the degree of fractionation given by the ba in microcline. in general, the order of samples relative to their fractionation degree coincides between the microcline and muscovite analyses supporting both results. the minor differences between both graphs may correspond to slight differences in crystallization time and the possible loss of na in the muscovite given its mobile nature. moreover, the fractionation degree line of the studied pegmatites evidenced a tight relationship with their host granites. considering the observed textural features in muscovite, and associated opaque minerals (such as those of economic interest), indicating a possible late crystallization (section 4.1), one possible interpretation could be that these minerals formed in a different and posterior event than that of the pegmatites formation. however, this would lead to stochastic magmatic fractionation of the pegmatites (as shown by microcline) in relation to the presence of nb-tabearing minerals. therefore, the texture of micas and iron, titanium, nb-ta oxides demonstrate that these were the last crystals to form in the pegmatites, rather than having formed in another event. to conclude, ba concentration in microcline is proposed as a better indicator of evolution than the concentration of na in muscovite in the rocks of the region since ba is a less mobile element than na and due to the ubiquity and abundance of microcline in pegmatites and granitic intrusives of the area. in addition, as stated by černý et al. (1985), k/rb and k/rb vs cs analyses in microcline are great petrogenetic evolution markers and should be used in further studies. 5.2 parental magma the correlation of the granitic rocks from the migmatitic mitu complex with other igneous bodies in the rio negro-juruena province would ease the exploration of strategic minerals; hence the significance to assess the magma type that originated the studied pegmatites and granitic bodies. for this aim, the results obtained from the chemical analysis of garnet, biotite, and tourmaline were analyzed. the garnet presence in igneous rocks is usually restricted to granitoids and acidic volcanic rocks (rene & stelling, 2007). its formation in these rocks can be caused by partial fusion occurring in the restitic phase (allan and clarke, 1981; stone, 1988; vennum and meyer, 1979; white and chappell, 1977), by high fractionation of a peraluminous melt under low-pressure conditions (allan & clarke, 1981; hall, 1965; harrison, 1988; miller & stoddard, 1981), or by the transportation of high-pressure phenocrysts up to the earth’s crust (green, 1977; rene & stelling, 2007). specifically, according to manning (1983), the garnets found in pegmatites belong to the almandine-spessartine solid solution and are commonly mn enriched. the composition of garnet in sample p_3006 conforms with manning’s (1983) approach, since it was predominantly spessartine with high content of almandine (fig. 14). for this crystal, both being part of a restitic phase of a partial fusion process and being a phenocryst transported from high pressures to the crust were discarded for not conforming with the textural features observed in the field. therefore, the formation from a highly fractionated peraluminous melt under low-pressure conditions (allan & clarke, 1981; hall, 1965; harrison, 1988; miller & stoddard, 1981) is supported by the obtained composition (krippner et al., 2014). the analysis of biotite confirmed this interpretation, as shown in figure 12, where most of the samples correspond to peraluminous magmas, except for sample g_3003. the latter was the one showing the least evolution degree (section 4.2) leading to the conclusion that this sample does not correspond to the petrogenetic line of the other samples as it comes from a different magma type. furthermore, the presence of biotite in the pegmatites, including those with a high degree of fractionation, may indicate high iron content in the source magma. this is supported by the composition of the tourmaline present in both pegmatites and the paragneiss sample, as shown in section 4.2.6 and figure 19. in addition, according to zen (1988), peraluminous igneous bodies can host incompatible elements such as sn, w, and other lithophile elements such as u, li, b, ta, nb, which is consistent with the presence of nb-ta-bearing minerals in the area (bayona-valderrama, 2018; charry et al., 2023). according to winter (2001), these magmas occur both in continental collision and postorogenic tectonic environments. the studied rocks best relate to a post-orogenic tectonic environment according to cardona-alarcón (2018), who analyzed fluid inclusions of the tourmaline crystals (in paragenesis with plagioclase and microcline in the pegmatitic bodies) and found high content of co2 y n2 with homogenization temperatures between 298,3-330ºc; the author concluded a mantellic origin for these rocks, discarding a continental collision environment. accordingly, franco et al. (2021b) analyzed colluvial ree, nb–ta, and u–th bearing minerals in the guainía department (pegmatite-related) and concluded their relationship with intra-plate processes that produced anorogenic granites between 1.55 and 1.30 ga, which can be related to plume activity. furthermore, various reports of post-orogenic to anoregenic rocks in the vicinity of the study area have been made, several of which report the presence of strategic minerals (bonilla et al., 2021; bonilla et al., 2023; cordani et al., 2016; franco et al., 2021a; franco et al., 2021b). consequently, the studied rocks can be related to anorogenic intrusion events in the nw portion of the rio negro-juruena province such as that of the matraca granite or the parguaza anoregenic event as reported for similar and neighboring igneous bodies (aarden & davidson, 1978; bangerter, 1985; bonilla-perez et al., 2013; bonilla et al., 2021; bonilla et al., 2023; franco et al., 2021a; franco et al., 2021b; gaudette et al., 1978; gonzalez & pinto, 1990; rodriguez & perez, 1982). 6. conclusions microprobe chemical analysis on apatite, biotite, garnet, microcline, muscovite, and tourmaline revealed significant features of the studied igneous bodies, regarding their origin and petrogenetic evolution. garnet, biotite, and tourmaline composition indicated that pegmatites and most intrusive granites originated from iron-rich peraluminous magma nb-ta enrichment, and could be associated with neighboring anorogenic granitoids. moreover, the studied bodies are compositionally associated with the host granites. in addition, the assessment of the magmatic fractionation degree was achieved with the concentration of na (ppm) in muscovite, fe# in biotite, sr vs mn (ppm) in apatite, and ba concentration (ppm) in microcline. the latter was the most effective due to its ubiquitous distribution and its low mobility; it should be further used in granites and pegmatites of this region in combination with k/rb and k/rb vs cs analyses for their utility as petrogenetic evolution markers. the magmatic fractionation degree of the studied rocks was concordant with the presence of nb-ta-bearing minerals, such as columbite, nb-rutile, and samarskite. the fractionation degree line of the studied pegmatites evidenced a tight relationship with their host granites. in addition, tourmaline crystallized in early stages of nb-ta enrichment, while garnet crystallized in advanced stages of magma fractionation; therefore, these minerals are proposed as strategic mineral exploration tools in pegmatites of the region. acknowledgments this study was possible thanks to the servicio geológico colombiano, the universidad nacional de colombia, and the geologists who collected the rock samples in such a remote region of the country. special thanks to the microfluid spectral laboratory for the mineral analyses. finally, i am grateful to my colleagues juan pablo bayona, carlos charry, and angie cardona, who advised me, faced me with the challenge of understanding these rocks, and whose research results were of great significance to this study. references aarden, h., & davidson, m. 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(2019). the quantitative prediction of tectonic fractures of longmaxi formation in dingshan area, southeastern sichuan basin. earth sciences research journal, 23(2), 157-162. doi: https:// doi.org/10.15446/esrj.v23n2.79957 tectonic fractures are a significant factor for the production of the longmaxi formation in dingshan area. the analysis of the distribution and development degree of tectonic fractures is the main research method in present-day. based on the data of field outcrops and borehole core, the new method-binary method is put forward by rock fracture and energy value to decide tectonic fracture development. according to the method, the fitting formula is offered between the relation of the value of rock fracture, energy, and fracture density. compared to the fracture density predictive value and the observed value of 15 wells, all wells relative error of less than 25%. the prediction results are accurate. according to the fitting formula of fracture development and distribution in the study area are obtained. the results show that the fracture density is high as a whole of the longmaxi formation in dingshan area. whole shows by southeast to gradually reduce the trend of the northwest. supported the future of the oil and gas exploration to provide geological basis and guidance in dingshan area. abstract the quantitative prediction of tectonic fractures of longmaxi formation in dingshan area, southeastern sichuan basin predicción cuantitativa de fracturas tectónicas en la formación longmaxi del área dingshan, en el suroriente de la cuenca sichuan issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.79957 las fracturas tectónicas son un factor determinante en la producción en la formación longmaxi, en el área dingsahn. el análisis de la distribución y grado de desarrollo de las fracturas tectónicas es uno de los principales métodos de investigación en nuestros días. con base en la información de los afloramientos y el núcleo del pozo, el nuevo método binario se presenta como valor de las fracturas rocosas y de energía para decidir el desarrollo de las fracturas tectónicas. de acuerdo con este método, se ofrece una formula de ajuste sobre la relación del valor de la fractura de la roca, la energía y la densidad de la fractura. al comparar el valor predictivo de la densidad de fractura y el valor observado en 15 pozos, todos los pozos tienen un error relativo menor al 25 %. la predicción de resultados es exacta. gracias a la formula de ajuste se obtuvo el desarrollo de la fractura y la distribución en el área de estudio. los resultados muestran que la densidad de la fractura es tan alta como la de la formación longmaxi en el área de dingshan. los resultados muestran que el sudeste gradualmente reduce la tendencia del noroeste. estos hallazgos respaldan el futuro de la exploración de gas y petróleo que aporten a las bases geológicas para el área de dingshan. resumen record manuscript received: 24/07/2018 accepted for publication: 22/04/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 157-162 jiatong xie1,2, qirong qin1,2*, cunhui fan1,2, dongfeng hu3, hu li1,2,4 1state key laboratory of oil and gas reservoir geology and exploitation, chengdu, 610500, china; 2school of science and technology, southwest petroleum university, chengdu, 610500, china; 3exploration company, sinopec, chengdu, 610041, china; 4tarim oilfield company, petrochina, korla , 841000, china * corresponging author: 986336692@qq.com te ct o ni cs 158 jiatong xie, qirong qin, cunhui fan, dongfeng hu, hu li introduction with a heightened level of global oil and gas exploration, fractured hydrocarbon reservoirs have become an important new field for exploration (he & wei, 2007; pu & qin, 2008) the identification, accurate description, and semi-quantitative-qualitative study of the tectonic fractures in reservoir strata are the keys to their effective development at a later stage (pu et al., 2008). consequently, the quantitative prediction of fractures has become an effective research method at present. researchers have undertaken numerous studies in connection with fracture prediction methods. dai & wang (2003) and pu et al. (2008) have summarized fracture prediction methods, covering the early conventional logging method and seismic method as well as the presently commonly used type theory method and the stress field simulation method. wang et al. (2016); yu et al. (2016) and gao et al. (2015) used the finite element method for stress field simulation to make predictions on fractures. wang, dai, sheng, qiao & li (2009) and wang (2007) applied the fractal dimension method to carry out predictions. these methods have all made major contributions to fracture prediction, but there are still deficiencies in terms of the quantitative prediction of fractures. the fracture development zones in tight reservoir strata are important targets for oil and gas exploration. tectonic fractures are the main reservoir spaces and seepage channels, and the extent of fracture development has a major effect on the quality and oil and gas production capacity of reservoir strata. therefore, on the basis of existing research, a “binary method” that combines the rock breaking criteria with the energy method is used in this paper to carry out a study on the quantitative prediction of fractures for the longmaxi formation in the dingshan area. this study aims to predict the development and distribution situation of the tectonic fractures in the reservoir strata of the study area and to provide a new geological reference and basis for oil and gas exploration and development in the area. geological background of the study area the dingshan area is located in qiongjiang city at the juncture between xishui county of guizhou province and chongqing and is affiliated with shizhen, chongqing county, qiongjiang city. tectonically, the dingshan area is located in the down warping fold region of southeast sichuan, with the southern portion crossing over the juncture between the eastern edge of the waning folded belt and the sloping belt in southern sichuan (figure 1), and a triassic system is exposed on the surface. dingshan is tectonically subjected to the comprehensive influence of the huayingshan fault, the qiyueshan fault, and the zunyi fault. the tectonic shape is complex, with more developed folds and faults composed of a series of high-dipping anticline belts and fault zones in trough-like folds in the northeastern and north-northeastern directions (sun et al., 2010). the tectonic structure is now located near the intersection of the north-south and northeastern tectonic belts, with a tectonic belt in the northeastern direction formed under the action of the jiangnan uplift during the late cretaceous epoch and advanced superpositioning of the tectonic structure near the north-south direction. the overall shape of the dingshan tectonic plane is in the shape of a nose with broken anticlines (huang, wei & wang, 2017; mei, liu, tang, shen & fan, 2010; yuan, 2008) . the strata of the study area have comparatively complete development. the proterozoic sinian system, the early paleozoic and late paleozoic permian system, and the mesozoic erathem developed from bottom to top. the shale thickness of the longmaxi formation is approximately 158-334 m, is generally 250 m, and can be divided into two sections—upper and lower—with the lower section being graptolite shale facies and the upper section interbedded with gray-dark gray calcareous shale and calcareous mudstone with thin gray layers of argillaceous limestone or marl. since 2013, major breakthroughs in oil and gas exploration have been attained in the field of marine shale gas exploration in the dingshan area, as daily productions of 3. 4×104 m3 and 10. 5×104 m3 in industrial gas flows were obtained at the dy1 and dy2 wells, respectively, reflecting the good exploration prospects for shale gas in this area. fracture development characteristics fracture dip in accordance with the classification criteria of fracture dips, fracture dips less than 15° are horizontal seams, fracture dips of 15°-45° are low-angle seams, fracture dips of 45°-75° are high-angle seams, and fracture dips greater than 75° are vertical seams. a statistical analysis of the field observation data shows that the surface development of seams in the dingshan area are mainly centered on high-angle seams as vertical seams with fracture dips greater than 75° account for 63. 72%, high-angle seams with fracture dips of 45°-75° account for 34. 76%, and low-angle or horizontal seams did not develop as they only account for 1. 52% of all observed fractures (figure 2). extent of fracture filling according to core observations and a statistical analysis of image logging data, among the fractures in the reservoir strata of the longmaxi formation in the study area, the proportion of unfilled fractures account for approximately 20%, the proportion of partially filled fractures account for approximately 20%, and the proportion of fully filled fractures account for approximately 60%; partially filled fractures and unfilled fractures account for a total of approximately 40%, and these partially filled fractures and unfilled fractures play a comparatively important role in oil and gas seepage (figure 3). fracture density fracture density is an important parameter for quantitatively characterizing the extent of fracture development. there are mainly three common ways to express fracture density: linear density, fracture density by area, and fracture density by volume. in this study, linear density is used to describe fracture density. linear density (lfd) refers to the ratio of the number of fractures nf intersecting with a straight line (perpendicular to the direction of flow or referring to the centerline of the rock) and the length of this straight line lb (formula 1), which is also called the fracture frequency or linear frequency. figure 1. tectonic division of southeast sichuan and the location of dingshan area fracture dip 45°-75°15°-15°0°-15° 75°-90° 1.52 34.76 63.7270 60 50 40 30 20 10 0 pe rc en ta ge % fig. 2 fracture dip direction histogram of longmaxi formation 159the quantitative prediction of tectonic fractures of longmaxi formation in dingshan area, southeastern sichuan basin of those, the fractures in the northwest-west direction (290°±10°) and the northeast direction (40°±10°) are the most developed, followed by near southnorth (0°±5°), and the extent of fracture development in the north-northwest direction (345°±5°) is relatively poor. fracture distribution and quantitative prediction prediction methods (1) principal curvature method: researchers defined the principal curvature method as a mathematical method that portrays the distribution of tectonic fractures in the reservoir strata developed on the basis of the tectonic fracture formation mechanism, which predicts the extent of fracture development in accordance with the extent of the curvature of the layered rock. huang, peng, & wang, thought that the greater the tectonic stress on the strata, the stronger the strata bending action and the higher the principal curvature value, which can further reflect the enhancement of the strata breaking effect and the increase in the extent of tectonic fracture development. therefore, the principal curvature value can become a characteristic parameter for predicting the interwell distribution of tectonic fractures. in usual situations, the tectonic fracture prediction results obtained through the principal curvature method are often only applicable to longitudinal fractures derived from the bending and deformation of rock layers under force. for most of the oil and gas fields, the tectonic fractures in the reservoir strata are mainly shear fractures formed under the action of tectonic compression, and the proportion of longitudinal fractures is very limited. therefore, the advantages of using the principal curvature method to carry out qualitative or semi-quantitative predictions on the tectonic fractures are more obvious, but there is a certain level of difficulty in carrying out quantitative predictions. (2) rock breaking method: according to the findings in the study on the strength theory of rock mechanics by zhiying xu, shuai yin, et al. , the stressstrain function is generally used to characterize rock failure conditions—that is, the breaking strength criteria—of which there are generally five categories based on the criteria proposed by mohr (1900): the maximum positive strain theory, mohr’s strength theory, the shear strain energy intensity theory, the octahedral stress theory, and griffith’s strength theory. each criterion category has different conditions and different criteria, and the breaking criteria used also differ as environmental conditions differ. the shale gas reservoir strata are generally buried deeper with a greater confining pressure, and the filling degree no fillinghalf fillingfull filling 2021 59 70 60 50 40 30 20 10 0 pe rc en ta ge % fig. 3 fracture filling degree of histogram the expression for counting the centerline density of the core is the ratio of the number of observed core fractures and the length of the core, the units are bar(s)/m. l = n lfd f b (1) according to the description of core observations and analysis of the logging data and based on the statistical analysis of the core observation results, a statistical analysis was carried out on the core fracture density of the area to describe and analyze the fracture development situation in the study area. it can be seen from the histogram on fracture development density that the density values obtained from the core statistics generally tend to be small, and the fracture densities are mainly concentrated in 0. 5-1 bar/m, 1-1. 5 bars/m, and <0. 5 bar/m. fractures with densities between 0. 5 and 1 bar/m and fractures with densities between 1 and 1. 5 bars/m account for approximately 32% and 23% of all fracture development sections, respectively. fractures with densities <0. 5 bar/m and between 2. 5 and 3 bars/m account for 21% and 19% of all fracture development sections, respectively (figure 4). in summary, the vast majority of the fracture densities in the area are less than 1. 5 bars/m; the density is generally small. in analyzing the reason, the first may be related to the proportion of high-angle seams occupying this area as the existence of high-angle seams reduces the drilling ratio; the second may be related to the tectonic site where the observation well is located. fracture orientation figure 5 is the rose diagram of the overall fracture orientations for all wells in the longmaxi formation produced based on the fracture interpretation results of image logging. overall, the fracture orientations of the study area are mainly in four bearings: northwest-west (290°±10°), near south-north (0°±5°), north-northwest (345°±5°), and northeast (40°±10°). fracture dip 45°-75°15°-45°0°-15° 75°-90° 21,15 23,08 1,92 30 25 20 15 10 5 0 pe rc en ta ge % 32,69 1,92 19,24 35 figure 4. core fracture density distribution frequency diagram of longmaxi formation 36 n ew s 27 18 9 9 18 27 36 36 27 18 9 9 18 27 36 4 23 1 figure 5. fracture trend rose diagram 160 jiatong xie, qirong qin, cunhui fan, dongfeng hu, hu li mohr-coulomb criterion can usually be used as the basis for determining rock breakage. according to mohr’s theory, the expression for the rock failure proximity coefficient is η=f (σ)/ k (κ), and the meanings expressed by its parameters are shown in figure 6. (4) binary method: the rock breaking value represents the possibility of tectonic fracture development, and the energy value represents the size of the ability for tectonic fractures to develop. the use of the rock breaking value alone cannot determine the extent of tectonic fracture development after a rock breaks. similarly, when considering only the energy value, the extent of tectonic fracture development also cannot be accurately judged. the simulation results of these two methods also often complement one another, and both must be combined to enable accurate judgment of the extent of fracture development in the rock strata. therefore, when predicting the tectonic fracture distribution in this study, the research outcomes of ding (1998) and xingui zhou et al. were used to comprehensively consider these two methods—the rock breaking method and the energy method—and a “binary method” was proposed (the extent of tectonic fracture development is jointly determined by two aspects—the rock breaking rate and the strain energy density); the rock breaking rate represents the possibility of tectonic fracture development, and the strain energy density represents the size of the ability for tectonic fractures to develop. when the rock breaking rate is comparatively low, not reaching the critical value that causes macroscopically obvious shear breakage to occur to the rock, the extent of tectonic fracture development is primarily determined by the rock breaking rate; after the rock breaking rate reaches the critical value, macroscopically obvious shear breakage has already occurred in the rock, and the extent of tectonic fracture development is primarily determined by the strain energy density at this time. in actual application, the rock breaking rate and the strain energy density are used together with the linear density of tectonic fractures in carrying out fitting calculations, and the fitting relationship formula is then used to carry out predictions on the distribution pattern for the tectonic fracture density of the reservoir strata. according to the guiding ideology of this method, the rock breaking rate η and the strain energy density w together with the linear density of tectonic fractures d are fitted in accordance with the following equation to establish a mathematical model for the quantitative prediction of tectonic fractures based on the rock breaking value and the energy value: d a b w c d w e= η η2 2+ + + + (4) in the formula, d is the predicted value for the linear density of the tectonic fracture in bar(s)/m, η is the dimensionless rock breaking coefficient, w is the energy value, and the others are the fitting coefficients. distribution prediction results through the rock breaking criteria and the energy formula, the rock breaking values and the energy values of the longmaxi formation in the dingshan area were calculated. the rock breaking values of the longmaxi k f c σ1 σ2 σn τ ϕ figure 6. based on the failure approach index of the mohr coulomb model schematic diagram the specific expression is η= = σ −σ ϕ σ −σ ϕ 1 1 f k c tan sin 2 2 2 − (2) in the formula, σ1 and σ2 are the maximum and minimum principal stresses, respectively, c is the cohesive force of the rock, and φ is the internal friction angle of the rock. when η1, the rock begins to break and generate a fracture; the greater the value of η, the stronger the breakage of the rock and the more developed the fracture. when η<1, the rock is stable, no breakage is generated, and a fracture does not develop. the values of c and φ reflect the state of molecular cohesion within the rock and indicate the brittleness of the rock, which is an important factor when studying the extent of rock fragmentation. in principle, it is believed that the greater the comprehensive breaking value of the rock, the more developed the tectonic fracture system, wherein the state of local stress near the fracture is as shown in figure 7, in addition to the maximum and minimum principal stresses near the fracture, the internal friction angle also has an effect, which is the main factor for the generation of fractures. (3) energy method: for fracture prediction, ju w. and huizhen song et al. proposed a rock energy method that uses the specific energy of strain to carry out predictions on fractures. to compensate for the deficiency in the inability of η to independently determine the extent of fracture development after the rock breaks, the deformation energy value is used as a supplementary criterion of the breakage criteria to jointly study the extent of tectonic fracture development. when breakages occur in rocks, those with large energy values may develop more fractures. the specific energy of strain per unit volume in the strata (that is, the deformation energy value) is given by the following formula: w e " e = σ −σ σ −σ µ1 1 2 3 2 2 2 2 − (3) wherein e is the elastic modulus, and μ is poisson’s ratio the specific energy of strain can become the supplementary criterion of the breaking criteria to compensate for the deficiency in the inability of the η value to independently determine the extent of fracture development after breakage, reflecting the high or low extent of tectonic fracture development. σ1 σx σ3 τxy βx1σy σx σ1 τxy τxy σ3 τxy σy y x figure 7. the local stress state near the fracture 161the quantitative prediction of tectonic fractures of longmaxi formation in dingshan area, southeastern sichuan basin formation were greater near the ds1 well, the dy4-s2 well, and the dy3 well, with a maximum of up to 2. 1 j/m (figure 8); the energy values were greater near the areas of the ds1 well, the dy1 well, and the dy1-5 well, with a maximum of up to 1. 03 j/m (figure 9). as a whole, the overall trend of the rock breaking values and energy values for the longmaxi formation in the dingshan area was a gradual decrease from a southeast to a northwest direction, and the fractures in the southeastern portion of the study area were more developed. there are certain limitations to carrying out quantitative predictions using any one of the breaking values or energy values alone as it is impossible to completely include the possibility of fracture development and the size of fracture development from a quantitative perspective. the comprehensive the accuracy of the prediction results was verified by using relative error (f) to reflect the degree of accuracy of the tectonic fracture predictions. it is generally thought that if the relative error results do not exceed 25%, then the prediction model is considered good and can be established, and the prediction results obtained for the fracture distribution are also credible. figure 8. late yanshanian-middle himalayan rock rupture value distribution of the longmaxi formation in dingshan area figure 9. late yanshanian-middle himalayan rock energy value distribution of the longmaxi formation in dingshan area figure 10. late yanshanian-middle himalayan structural fracture density predictive distribution of the longmaxi formation in dingshan area table 1. rock crack coefficient and the strain ratio value well number dy4-3 dy4-s2 dy4 dy4-s1 dy3 dy5 dy2 dy1-5 dy5-2 dy5-6 dy2-1 dy3-c1 dy3-3 ds1 dy1 crack coefficient /η 1. 7 2 1. 2 1. 9 2. 1 1 1. 3 1. 6 1. 3 1. 2 0. 9 1. 5 1. 2 2. 1 1. 8 strain ratio/w 0. 51 0. 28 0. 21 0. 2 0. 23 0. 13 0. 25 0. 6 0. 14 0. 07 0. 09 0. 26 0. 24 1 0. 8 breaking value and energy value can complement each other in the simulation calculations. therefore, through the statistics on the rock breaking coefficient and specific energy of strain for every well (table 1), analysis and fitting were carried out using actual measurements of tectonic fracture density with the rock breaking coefficient and energy value—that is, the specific energy of strain—of the corresponding well through formula 6 to obtain the mathematical model for predicting the tectonic fractures of the longmaxi formation in the dingshan area in the late yanshan-middle himalayan period. d . w . w=−0.224−1.42η η−1.2022 2 20 966 1 575+ + = r 0.89 (5) in the formula, d represents the tectonic facture density of the longmaxi formation, w represents the strain energy, η represents the rock breaking value, and r represents the fitting correlation coefficient. based on the tectonic fracture prediction model, the development and distribution situation of the tectonic fractures for the longmaxi formation in the dingshan area can be obtained (figure 10). it can be seen through the prediction results that the fracture densities are higher near the ds1 well, the dy3 well, and the dy4-s2 well, being up to 2. 22 m-1, and the overall trend shows a gradual decrease from a southeast direction to a northwest direction. 162 jiatong xie, qirong qin, cunhui fan, dongfeng hu, hu li the relative error formula is f x %= − predicted measured predicted 100 (6) through an analysis and comparison of the actual measured values of the fracture densities and the predicted densities of the longmaxi formation in the dingshan area, the relative errors for all wells in the dingshan area were found to be all within 25% (table 2). the actual measured values basically coincided with the observed values, and the overall conformity effect was very good. the prediction results were satisfactory, and the fracture distribution prediction results were credible. conclusions (1) the tectonic fractures of the longmaxi formation in the dingshan area were found to be centered on high-angle and vertical seams, the main developments are fractures in the northwest-west direction and the northeast direction, and the fracture filling was found to be comparatively severe as most were invalid fractures. (2) on the basis of a survey of the tectonic fracture methods, a binary method that comprehensively utilized the rock breaking method and the energy method was used to carry out fracture formula fitting and prediction on the strata of the longmaxi formation in the dingshan area and to establish a mathematical model for the quantitative prediction of fracture distribution. the prediction results showed that the density errors were all within 25% and that the prediction results were satisfactory. 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(2009). characteristics and quantitative prediction of distribution laws of tectonic fractures of low-permeability reservoirs in yanhewan area. acta petrolei sinica, 30(02), 195-200. table 2. analysis of the fracture predictive relative error of the longmaxi formation in dingshan area well number dy4-3 dy4-s2 dy4 dy4-s1 dy3 dy5 dy2 dy1-5 dy5-2 dy5-6 dy2-1 dy3-c1 dy3-3 ds1 dy1 practical density (bar/m) 1. 5 2. 1 1. 1 1. 7 2. 5 1 1. 7 1. 8 1. 4 1. 6 0. 8 1. 4 1. 4 2. 3 1. 6 predictive density (bar/m) 1. 68 2. 1 1. 25 2. 05 2. 23 1. 07 1. 34 1. 56 1. 43 1. 38 0. 98 1. 57 1. 23 2. 22 1. 81 relative error/% 12 0 14 21 11 7 21 13 2 14 23 12 12 3 13 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.102964 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 327 341 g eo lo g y a contribution to the understanding of the origin of the ginebra gold district mineralizations, colombia bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado universidad nacional de colombia departamento de geociencias *corresponding author: bprodriguezr@unal.edu.co record manuscript received: 31/05/2022 accepted for publication: 30/01/2024 how to cite item: rodriguez-ramos, b., molano, j. c., & dorado, c. (2023). a contribution to understanding the origin of the ginebra gold district mineralizations, colombia. earth sciences research journal, 27(4), 327-341. https://doi. org/10.15446/esrj.v27n4.102964 abstract keywords: fluid inclusions; microthermometry; raman spectroscopy; late cretaceous; orogenic gold. palabras clave: inclusiones fluidas; microtermometría; espectroscopía raman; cretácico tardío; oro orogénico. ginebra´s gold mineralization is hosted by the ginebra ophiolitic massif and the buga batholith, in the western flank of the central cordillera of colombia. gold occurs as disseminations, quartz-carbonate stockwork veinlets and quartz lodes with a strong structural control and are composed by small amounts of sulfides, mostly pyrite, with galena, sphalerite and chalcopyrite in at least two different deposition events and related to strong sericite-chlorite-epidote haloes, indicative of near-neutral and reduced fluids. petrography and raman spectroscopy allowed to identify two main fluid inclusions assemblages primary to pseudo-secondary in origin, both of them aqueous and co2-rich, with variable n2. microthermometric analysis indicates the hydrothermal fluids associated with the gold were low saline (0.75-6.22 wt% nacl equiv.) and presented a minimum trapping temperature ranging from 237.4 to 301.6°c. petrography and microthermometric results suggest an isothermal fluid mixing and/or h2s loss as the main mechanisms related to gold bi-sulfide/ thio-sulfide complexes destabilization and precipitation. sulfur isotopes indicates a close to zero range (-8.2-+4.8‰), suggesting a restricted fluid circulation through the magmatic rocks of the buga batholith and the ginebra ophiolitic massif. besides, the oxygen and deuterium isotopes from hydrothermal sericite and chlorite accompanying to gold events, indicates a metamorphic origin for the hydrothermal fluids. 40ar-39ar dating of the hydrothermal alteration haloes, from el retiro and la esperanza, indicates a late cretaceous-paleocene age, between 67 and 65 ma, contemporaneous with a regional strong dextral transpressive deformation related to subduction to obduction and exotic terrane accretion against the north-western part of the south american plate. although the hydrothermal fluid characteristics of the ginebra gold disctrict are compatible with both the intrusion-related and orogenic gold deposit types, the metamorphic nature of the hydrothermal fluids and the tectonic regime at the time of mineralization event, allow us to propose they are compatible with an orogenic gold deposit. contribución al estudio del origen de las mineralizaciones auríferas de ginebra, en colombia resumen las mineralizaciones del distrito aurífero de ginebra están emplazadas tanto en el macizo ofiolítico de ginebra como en el batolito de buga, en el flanco occidental de la cordillera central de colombia. corresponden a diseminaciones, stockwork de vetillas y vetas de cuarzo-carbonato, estas últimas con fuerte control estructural, bajas cantidades de sulfuros (>pirita-galena-esfalerita-calcopirita), al menos dos eventos de depósito de oro y desarrollo de halos de alteración fuerte (sericita-clorita-epidota). análisis petrográficos y de espectroscopía raman identificaron dos ensamblajes de inclusiones fluidas principales, primario y pseudo-secundario, ambos acuosos y ricos en co2, con cantidades variables de n2. los análisis microtermométricos indican fluidos hidrotermales con salinidad baja a moderada (0.75-6.22wt% nacl equiv.) y temperatura mínima de entrampamiento entre 237.4 y 301.6°c. asimismo, sugieren que un proceso de mezcla isotermal de fluidos y/o de pérdida de h2s fueron los mecanismos responsables de la desestabilización de los complejos bi-sulfurados/thio-sulfurados en los que se transportó el oro y produjeron su depósito. isótopos de azufre (entre -8.2 y +4.8‰) sugieren una circulación restringida de los fluidos hidrotermales por las rocas del macizo y del batolito. adicionalmente, isótopos de oxígeno y deuterio en sericita y clorita hidrotermal, que acompañan la mineralización, reflejan un origen metamórfico para estos fluidos. dataciones 40ar-39ar en los halos de alteración de las estructuras de el retiro y la esperanza, indican una edad entre 65 y 67 ma, contemporánea con una deformación regional transpresiva dextral, relacionada con subducción a obducción y acreción de terrenos exóticos contra la margen nor-occidental de la placa sudamericana. aunque las características de los fluidos hidrotermales de las mineralizaciones son compatibles tanto con depósitos de tipo “intrusion-related” y oro orogénico, su naturaleza metamórfica y el régimen tectónico existente al momento del evento mineralizante, permite proponer que son compatibles con un depósito de tipo oro orogénico más que un “intrusion-related”. https://doi.org/10.15446/esrj.v27n4.102964 mailto:bprodriguezr@unal.edu.co https://doi.org/10.15446/esrj.v27n4.102964 https://doi.org/10.15446/esrj.v27n4.102964 328 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado 1. introduction despite the available petrogenetic and geochronological studies about igneous and metamorphic rocks cropping out on the western flank of the central cordillera (mccourt et al., 1984; aspden & mccourt, 1986; restrepo & toussaint, 1988; nivia, 2001; villagómez et al., 2011; leal-mejía, 2011; leal-mejía et al., 2019, manco; 2020), the current knowledge about mineral occurrences hosted by some of these units is still poor, especially for late cretaceous age, although they could contribute to the understanding of the geological processes occurred during that time. this is especially true for the ginebra gold district, were a discontinuous ssw-nne trending mineral belt extends for about 8 km long from the la esperanza-la selva to the south to el retiro-cueva loca-la cecilia to the north. in the ginebra district, the mineralization style is principally veined being also common to find disseminations, veinlets and stockworks, especially in the el retiro area, related to strong sericite-pyrite-quartz alteration. at the la esperanza-la selva, instead, mineralization is almost exclusively veined, with narrow sericite-chlorite and propylitic alteration haloes. although some of these mineralized veins has been attributed to an “intrusion-related” character, genetically related to the buga batholith magmatic event (mccourt et al., 1985; nivia, 2001; leal-mejía, 2011; lópez et al., 2018; leal-mejía et al., 2019), non-conclusive evidences, as their clear magmatic origin, have been reported yet. in this document, we provide new petrographic, fluid inclusion petrography, microthermometric, raman spectroscopy, stable isotope and 40ar/39ar data from ginebra district, in order to contribute to the understanding of the composition, evolution and nature of the hydrothermal fluids responsible for their generation and relate it to the geological processes occurring during their genesis. geological setting geological evolution of the western flank of the central cordillera and the eastern flank of the western cordillera of colombia has been controversial for many years and not fully understood, although there is a consensus about the role of accretionary onset of oceanic derived rocks against the continental margin of northwest south america during the late cretaceous (cediel et al., 2003; villagómez et al., 2011; bustamante & bustamante, 2019) (figure 1). most of the evidences conducing to this conclusion relates to lithogeochemical and geochronology, showing a clear correlation between basaltic rocks from ecuador to the south until aruba, conforming part of the enormous caribbean lithospheric igneous province (clip) (cediel et al., 2003; villagómez et al., 2011; bustamante & bustamante, 2019; nivia et al., 2019). the ginebra gold district is located on the western flank of the colombian andes central cordillera. at this region, the basement consists of upper mesozoic oceanic rocks, accreted to the continental margin of south america because of the drift of the caribbean plate to the e and ne between the mesozoic and cenozoic (mccourt, 1984; aspden & mccourt, 1986; restrepo & toussaint, 1988). this area is framed by two major structural features with an approximate ns to n30°e direction, the guabas-pradera and palmira-buga faults, which are part of the cauca-romeral fault system. the older geological unit corresponds to the ginebra ophiolitic massif (figure 2), name proposed by espinosa (1984. in mccourt et al., 1985), referring to a n-s block of ultramafic rocks outcropping on the western flank of the central cordillera. nivia (1987. in ossa, 2006) affirms it is composed mostly by amphibolites produced by metamorphism of micro gabbro and basalt, with superimposed regional metamorphism effects evidenced by the presence of nematoblastic and lepidoblastic textures as well as quartz and amphibole boudins related to dynamic metamorphism. based on geological mapping, petrography and mineral chemistry, nivia et al. (2019) divided the ginebra ophiolitic massif in four different units, the ginebra ultramafic, the ginebra gabbro and amphibolite, the ginebra anfibolite and the ginebra basalt. nivia et al. (2019) recognized orogenic metamorphism producing amphibole generation from original pyroxene in gabbro of this unit, generated as consequence of fluids migration through these rocks, evidenced by the occurrence of pegmatitic texture amphibole-plagioclase veins and epitaxial actinolite-tremolite growth on pyroxene, representative of a retrograde green schist facies, progressing to an epidote-amphibolite facies this later deformation phase, dated about 90 ma, is contemporaneous with the emplacement of the first magmatic event of the buga batholith. figure 1. tectonostratigraphic terranes in colombia, after toussaint & restrepo, (2020). amazonian craton (a), andaquí terrane (an), chibcha terrane (ch), yalcón terrane (y), tahamí terrane (ta), kogi terrane (k), calima terrane (ca), tumaco terrane (tu), tairona terrane (tai), cuna terrane (cu) and cauca– romeral undifferentiated terranes (crut). 40ar-39ar analysis indicates a crystallization age of 140.28 ± 3.12 ma (in clinopyroxene) for a gabbronorite and 90.84 ± 0.78 ma (in hornblende) for an amphibolite, the later interpreted by nivia et al. (2019) as the age of reworking due emplacement of the buga batholith. amaime formation, originally proposed by mccourt (1984), outcrops along the western flank of the central cordillera and consists of a series of massive tholeiitic basalts with pillow lava horizons. the age of this unit is controversial, and although it has been proposed it may be contemporaneous and genetically related with the ginebra ophiolitic massif, being also intruded by the buga batholith, averaged 40ar-39ar ages 90.6 ±2.5 ma presented by kerr et al. (1997; 2002), as well as petrologic and structural evidences provided by nivia et al. (2019), suggest this unit represents a different plateau-related event. the buga batholith corresponds to a north-south elongated polyphasic calc-alkaline body composed by tonalites, quartz diorites and gabbros, in most areas in faulted contact, after sin-tectonic intrusion in the ginebra ophiolitic massif (aspden et al., 1987. in nivia, 2001; nivia et al., 2019). it presents a general low-k and a mixed ferroan-magnesian nature, with a tholeiitic signature, although the more felsic rocks present a normal calc-alkaline to high calc-alkaline character (leal-mejía, 2011; villagómez, 2010; leal-mejía et al., 2019). based on lithogeochemical and isotopical evidences, the buga batholith in conjunction with other cretaceous age intrusive bodies as sabanalarga, mistrató, santa fé and jejenes, has been proposed as allochthonous, with a more primitive chemical characteristics and being part of the clip, accreted to the paleo-autochthonous margin prior to the andean orogeny (leal-mejía, 2011; villagómez et al., 2011; nivia et al., 2019; leal-mejía et al., 2019). reported radiometric ages for the buga batholith includes 113 ± 10 ma k/ar age in hornblende (toussaint et al., 1978), a 99 ± 4 ma rb-sr age in biotite (brook, 1984) and more recently u-pb zircon ages between 96 to 329a contribution to understanding the origin of the ginebra gold district mineralizations, colombia 111 ma with a 98.79 ±0.43 ma concordia age (brito et al., 2010), 90.6 ± 1.3 and 92.1 ± 0.8 ma (villagómez, 2010; villagómez et al., 2011). u-pb zircon dating reported by nivia et al. (2019) revealed two age groups, between 90-83 ma and 70-67 ma as the result of equal number of magmatic episodes. the later age group is restricted to the northern part of the intrusion and is compatible with the ages (65.4 to 70.61 ma) for small hypabyssal dacite porphyries intruding to the ginebra ophiolithic massif and the amaime formation (rodríguez-ramos, 2012; barrera-cortes & molano, 2021) cropping out in la esperanza, cominal and río flautas sectors in both sides of the guabas-pradera fault. some of these porphyries, especially on the río flautas presents deformation evidences due to the guabas-pradera fault. mineralization in the ginebra gold district mineralization style in the ginebra district is manly veined, although there is also common to find disseminations, veinlets and stockworks. it is distributed in two main areas, the el retiro-cueva loca-la cecilia to the north and the la esperanza-la selva, about 8 km to the south (figure 2). at the el retiro-cueva loca-la cecilia (northern sector), 1 to 35 cm thick sinuous and discontinuous veins with a n20-40°e trend, dipping 30-40° to the se and nw, being also common to observe stockwork veinlets. gold bearing structures presents clay and sericite narrow haloes and are hosted by gabbro belonging to the ginebra ophiolitic massif (el retiro) and by tonalites, granodiorites and diorites of the buga batholith (cueva loca and la cecilia). in this area there are also intensely sericite-quartz-carbonate alteration zones (figure 3. a and b), in most cases structurally controlled, affecting to medium grain gabbro, accompanied by quartz-pyrite stockwork veinlets. at the el retiro-cueva loca-la cecilia, the veins are composed by mosaic to massive texture quartz and minor calcite with scarce sulfides (figure 4. a and b). at least two different quartz events were identified. the first is composed by subhedral to euhedral massive texture quartz with euhedral and coarse-grained pyrite (figure 3. a and b), while the second is composed by smaller size mosaic quartz crystals associated to calcite, sericite, secondary biotite, chlorite and pyrite as well as galena and chalcopyrite filling fractures in the first pyrite and quartz event (figure 4. c and d). molano et al. (2000) and cervera & garcés (2005) reports electrum and native gold as blebs in pyrite and quartz as well as fractures filling associated to pyrite, quartz and galena, suggesting two precipitation events. conversely, at la esperanza-la selva (figure 2), the gold lodes are hosted only by gabbro belonging to the ginebra ophiolitic massif. these structures present a continuous to discontinuous character with a sinuous morphology and a variable thickness between 3 and 90 cm with a main n40-60°w trend dipping 20 to 30° to the ne (figure 3. c, d and e). only narrow, poorly developed clay to sericite-chlorite alteration halo is observed. pyrite, chalcopyrite and galena content is a very low in a 97:3 proportion respect to quartz, although locally at the la selva, about 15% of these sulfides have been identified. as at el retiro-cueva loca-la cecilia, two different quartz events were observed at the la esperanza-la selva. the first characterizes by subhedral to euhedral coarse grain quartz crystals while the second is composed by smaller size anhedral and mosaic texture crystals. coarse grain pyrite crystals accompany the quartz of the first generation with small gold blebs embedded in both minerals (figure 4. e). the second mineralization event is composed by smaller pyrite crystals, associated with minor galena and chalcopyrite filling fractures in the first pyrite and quartz generation. small grain gold blebs in galena were observed (figure 4. f). very thin molybdenite veinlets crosscutting the quartz veins as a latest event. our petrographic descriptions are in agreement with molano et al. (2000) who describes two gold events in this area, the first related to pyrite and quartz and the second with chalcopyrite and galena. figure 2. geological map of ginebra massif (rodríguez-ramos, 2012). modified from nivia (2001). 330 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado figure 3. hydrothermal alteration and vein morphologies of the ginebra gold district. a. and b. strong pervasive sericite-muscovite (ser) hydrothermal alteration. c, d and e. sinuous deformed milky quartz veins hosted by the ginebra ophiolitic massif. figure 4. mineralogy and textural relationships in the ginebra gold district. a. massive and mosaic texture quartz (qz) crystals associated with euhedral pyrite (py). b. euhedral calcite associated with massive quartz (qz) note a later mosaic texture quartz generation. c. galena filling fractures in pyrite (py) crystals associated with quartz (qz). d. galena (gn) and chalcopyrite (ccp) filling fractures in pyrite (py). gold (au) included in quartz (qz). f. gold bleb (au) in galena (gn). 331a contribution to understanding the origin of the ginebra gold district mineralizations, colombia as a result of the petrography, a paragenetic sequence for the gold bearing veins and disseminations in the el retiro-cueva loca-la cecilia and the la esperanza-la selva areas is given in figure 5. figure 5. paragenetic sequence for the vein lodes of the ginebra gold district. analytical methods fluid inclusions microthermometry and raman spectroscopy a total of 105 representative samples of gold bearing quartz veins, were collected from artisanal underground workings and surface outcrops, from the la cecilia to the north to the la esperanza and the la selva to the south in the ginebra district. five of these samples were selected for fluid inclusions microthermometry and raman analysis. double polished, 120 μm thick, thin sections from quartz samples were prepared by us and analyzed under the microscope. the microthermometric measurements for fifty-nine (59) fluid inclusions were carried out on a linkham thsm 600 heating-freezing stage and an olympus bx41 petrographic microscope in the microscopy and microthermometry laboratory of the national university of colombia in bogota. the measuring range between -196ºc and + 600ºc. the accuracy of the equipment was estimated at +1ºc to the lowest temperature and +4.7ºc for the highest. the stage was calibrated measuring two standards of synthetic inclusions from the university of leoben´s fluid inclusions laboratory, containing h2o (final ice melting temperature: 0ºc; homogenization temperature: 374.1ºc), and co2 (co2 melting temperature: -56.6ºc). fluid inclusion petrography was performed using the principles given by roedder (1984) and based on the concept of fluid inclusion assemblage (fia), which brings together a group of fluid inclusions trapped at the same time (goldstein, 2003). the fia methods were determined based on the assumptions given by roedder (1984) for the recognition of primary, pseudosecondary and secondary fluid inclusions, using the number of phases, their proportions at ambient temperature (25ºc) and the spectroscopic data for the recognition of primary pseudo-secondary and secondary inclusions. six phase changes in the fluid inclusions were recorded during cooling/ heating routines: (1) co2 melting temperature (tmco2), (2) eutectic temperature (te), (3) final ice melting temperature (tmice), (4) clathrates melting temperature (tmclat), (5) co2 homogenization temperature (thco2) and (6) total homogenization temperature (th). salinity determination was made using the q2 and ice programs from clathrates package (bakker, 1997; bakker & brown, 2003), by using peng & robinson (1976) and duan et al. (1992) state equations according to the characteristics of each type of inclusion. the salinity, density and molar volume for the most of the acuo-carbonic fluid inclusions, were modeled on the q2 program (bakker, 1997; bakker & brown, 2003), because the carbonic phase homogenization occurred in all the cases in one of the quadruple system points. the calculations used the state equation of peng & robinson (1976) and the composition of the non-aqueous phases from the raman spectroscopy (xco2= 0.91, xn2=0.09). in contrast, for those of the acuo-carbonic fluid inclusions lacking immiscible carbonic phases, the salinity, density and molar volume were calculated using the ice program from the clathrates package (bakker, 1997; bakker & brown, 2003), using the state equation from duan et al. (1992). additionally, raman analyses were carried out in the vapor phase of the inclusions. these measurements were performed at laboratories of the united states geological survey (usgs) and the facilities of the technological development center of the colombian emerald (cdtec) located in bogotá, using a nicolet almega equipment, which uses a laser with a wavelength of 532 nm. deviations used to the identification of the molecular species in the fluid inclusions were the established by burke (2001) and frezzotti et al. (2012). oxygen, deuterium and sulfur stable isotopes from petrographic determinations and using x-ray diffraction analysis on disoriented and oriented laminae using an ethyl-glycol routine, to confirm the abundance and a high crystallinity of the sericite-muscovite, there were selected three samples from hydrothermal alteration halo from mineralized quartz veins: (1) from the la reserva mine in la selva area, (1) stockwork quartz-sericitepyrite veinlets in intensely hydrothermally altered gabbro from el retiro mine, el retiro-cueva loca-la cecilia area, and (1) hydrothermal sericite affecting to a dacite porphyry from el castillo zone, to the south of the district. for these samples, δ18o and δd compositions were acquired in a sira-ii –dualinlet mass spectrometer in the salamanca university (spain). the δ18o and δd isotopic composition of the hydrothermal fluid in each simple was calculated in equilibrium with muscovite at 300ºc, concordant with the total homogenization temperatures from fluid inclusions analysis, using the fractionation coefficients proposed by zheng (1993) and suzuoki & epstein (1976). the δ18o and δd results are standardized to the vsmow. for the sulfur isotope determinations, three mineralized quartz veins samples from la reserva, cascabel and las camelias mines, from la esperanza-la selva area and one sample from stockwork quartz-sericite-pyrite veinlets in intensely hydrothermally altered gabbro from el retiro mine were selected. isotopic values were measured in a thermoquest-finnigan delta plus xl mass spectrometer in the united states geological survey (usgs) and the results were standardized to the devil canyon troilite (dct). the hydrothermal fluid composition for 250ºc, in terms of δ34sh2s, was calculated using the sulfur enrichment factors proposed by sakai (1968), kajiwara & krouse (1971) and czamanske & rye (1974). samples were crushed and sieved, choosing the 125 250 μm fraction. this granulometric fraction was then washed with de-ionized water and dried at low temperature to be later separated using a frantz magnetic separator using 0.4, 0.6, 1 and 1.5 a. the final sericite and pyrite, galena and chalcopyrite fraction were hand-picked to obtain about 50 mg for each sample. in the case of the sulfides, there was verified a clean and unaltered aspect. the δ18o and δd isotopic composition of the hydrothermal fluid in each sample was calculated in equilibrium with muscovite at 300ºc, concordant with the total homogenization temperatures from fluid inclusions analysis, using the fractionation coefficients proposed by zheng (1993) and suzuoki & epstein (1976). the δ18o and δd results are standardized to the vsmow. 40ar/39ar dating to have a better understanding of the age of the ginebra ophiolitic masiff, one of the two main host unit of the gold lodes in the ginebra district and its temporal association with the mineralization in the studied area, two un-weathered and hydrothermally unaltered or only very weakly altered plagioclase hornblendite and hornblende gabbro samples, from cominal-el jardín and valledupar-flautas areas, were selected for dating. also, two intensely hydrothermally altered samples, from a quartzsericite-pyrite stockwork veinlet zone in el retiro mine and from the sericitechlorite alteration halo of a mineralized vein from las camelias mine from la esperanza-la selva area were analyzed. the presence of absence of hydrothermal alteration and weathering was confirmed petrographically. mineral separates of hornblende, sericite and chlorite were prepared from the 0.5 to 0.25 mm sieved fraction from jaw-crushed samples, which were washed with de-ionized water, dried and re-sieved. a hand magnet was used to remove magnetite grains and then the samples were magnetically separated using a frantz magnetic separator. pure hornblende (~100 mg), chlorite (~60 mg) and sericite (~60 mg) where then hand-picked under a binocular microscope. the samples were analyzed in the oregon state university argon geochronology laboratory where they were packaged, irradiated and analyzed for their ar isotope compositions in a thermo scientific argus vi multicollector noble gas mass spectrometer. 332 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado 40ar/39ar dating results are presented as a step-heating spectra containing the calculated plateau and inverse isochron ages. for most of the analyzed samples, the calculated plateau age was considered the preferred. fluid inclusions microthermometry and raman spectroscopy results fluid inclusions petrography based on petrography, four different fluid inclusions fia were identified in el retiro-cueva loca-la cecilia and la esperanza-la selva localities, presenting different phases at room temperature (22ºc) and different stages (table 1). all the analyzed fluid inclusions are hosted in coarse size quartz crystals (200 μm – 1 mm) related to the first mineralization event identified in petrography (figure 5). table 1. types of fluid inclusions identified in ginebra gold district. fi a h os t m in er al ph as es c om po si tio n* o ri gi n l /v fo rm i qz lcar+vcar/ laq (co2+n2)v + (co2+ h2o)l + (h2o)l primary 0.12 -0.52 regular, irregular, ovoid ii qz l+v (co2 (?)+ n2 (?))v / (h2o)l pseudosecondary 0.12 -0.49 ovoid, rounded, regular iii qz l+v h2ov + h2ol secondary 0.97 0.71 ovoid, regular iv qz l h2ol secondary ovoid vapor (v), liquid (l), quartz (qz), after raman (*). fia i acuo-carbonic fluid inclusions are were observed as isolated and randomly distributed in intragranular quartz crystals, therefore were classified as primary to pseudo-secondary. at room temperature (20ºc), these fluid inclusions exhibit three phases (carbonic vapour, carbonic liquid and acuous liquid), with variable phase ratios (lcar+vcar/laq) between 0.12 and 0.52. presents regular to very irregular and ovoid shapes, and a size ranging from 4.7 to 19.9 µm (figure 6. a and b). fia ii inclusions are not frequent, but are present in all the analyzed sectors, coexisting with fia i inclusions. they are pseudo-secondary in origin, aligned along sealed fractures, which do not completely cross the crystals containing them (figure 6. c). they are biphasic, liquid-rich inclusions, with a phase relationship (v/l) between 0.12 and 0.49, with ovoid, rounded and regular shapes, and with sizes between 3 and 11 µm. the fia iii inclusions are the most common inclusions observed in the studied plates, and were identified as secondary inclusions align as trails, along fractures crossing boundaries grains. they are two phases, liquid-rich fluid inclusions, with a proportion of the liquid phase between 0.97 and 0.71, with usually ovoid and regular shapes, and minuscule sizes (3.7 and 5.3 µm) (figure 6. d). giving their small size, there was not possible to analyze their phase changes. the fia iv inclusions are relatively frequent, have a secondary origin and are characterized by only having a liquid phase at room temperature (figure 6. d). microthermometric results microthermometric results of the most significant recorded phase changes for the fia i and ii fluid inclusions are summarized here because of their primary and pseudo-secondary origins, providing information about the fluids related to the gold genesis in the ginebra district. although fia iii fluid inclusions may provide useful information about the evolution of the system, their small size made analysis impossible. during the heating/freezing routines, the melting temperatures of the carbonic phase (tmco2) for the fia i fluid inclusions occurred between -60 to -56ºc (table 2; figure 7), generally below the triple point of pure co2, indicative for the existence of other non-aqueous species within the carbonic phase. the partial homogenization of the carbonic phases (thco2) occurred by the disappearing of the co2 vapor (lcar + vcar = lcar) but also to vapor co2 (lcar + vcar = vcar), suggesting different densities between fluid inclusions within the same type in paragenesis. this phase change occurred between 11.3 to 30.9ºc. figure 6. a and b. fia i primary inclusions, with three phases at room temperature. c. aligned fia ii pseudo-secondary inclusions, biphasic compositions and which are liquid-rich. d. aligned fia iii and iv secondary inclusions, biphasic inclusions and monophasic inclusions, respectively. 333a contribution to understanding the origin of the ginebra gold district mineralizations, colombia although, from the petrography analysis, the fluid inclusions with evidences of leaking, necking down or decrepitation were discarded for the microthermometry, most of the fia i inclusions decrepitated before their full homogenization, at temperatures from 239 to 309.6ºc, as a result of an internal overpressure when the temperature increased during the heating routines. total homogenization temperatures for those inclusions which not decrepitated range from 237.4 to 301.6ºc (table 2; figure 7). because of their scarcity, only few microthermometric results were obtained for fia ii inclusions. in all the analyzed samples, there were recognized four phase changes during the microthermometric routines. the final ice melting temperatures (tmice) range between -0.8 and -8.1ºc. clathrates were identified in this fluid inclusions type, whose melting temperature ranges between 4.2 and 7.6ºc. type ii inclusions homogenized to liquid from 257.7 to 305.5ºc. table 2. microthermometry results for fia i and ii fluid inclusions from the ginebra gold district. zone of study tmco2 (t°) tmclat (t°) thco2 (t°) th (t°) tdec (t°) salinity wt% nacl equiv. la cecilia fia i -59.5°c to -56°c 7.6°c 9.3°c 25.9°c 30.9°c 239.5°c 263°c 239°c 271°c 0.75 4.0 la cecilia fia ii 7.6°c 257.7°c 4.4 la esperanza fia i -58°c 6.4°c 7°c 11.3°c 23.4°c 265.4°c 272°c 5.12 6.22 la selva fia i -60°c to -53.7°c 6.5°c 9°c 15.5°c 29.6°c 237.4°c 301.6°c 255.4°c 309.6°c 1.33 6.06 la selva fia ii -57.1°c to 56.4°c 0.5°c 6.1°c 226.9°c 305.5°c 246.5°c 4,6 9,3 co2 melting temperature (tmco2), clathrates melting temperature (tmclat), co2 homogenization temperature (thco2), homogenization temperature (th), decrepitation temperature (tdec). figure 7. frequency histograms for co2, clathrates and total homogenization temperatures as well for calculated salinity from fluid inclusions in the ginebra gold district. 334 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado raman spectroscopy due to the small size of the fluid inclusions in ginebra gold mineralization, was difficult to run the spectroscopic analysis. however, it was possible to carry out the determination of the volatile species from fia i inclusions, confirming the presence of co2 and the existence of n2, responsible for the gas hydrates (clathrates) formation during the cooling routines (figure 8). this is consistent with the measured melting temperatures of co2. the quantity of co2 and n2 is variable, although the former is the dominant species in the measured inclusions, with relative proportions between 91 and 100%, while for n2, the amounts are usually small, from nonexistent to about 9%, in some cases reaching 22%. mineralizing fluids composition fia i inclusions presents moderate to low salinities, from 0.75 to 6.22 wt% nacl equiv., being slightly higher in the la esperanza zone comparing with the la cecilia and the la selva areas. density varies from 0.92 to 0.99 g/cc while the molar volume ranges from 19.0 to 24.23 cc/mol. like fia i inclusions, fia ii presents low to moderate salinities from 4.4 to 9.34 wt% nacl equiv., being slightly higher in la selva area, compared to la cecilia zone. the liquid phase total density varies from 0.89 to 0.90 g/cc and a molar volume from 21.04 to 23.67 cc/mol. figure 8. raman spectra of fia i inclusions, where the presence of co2 and n2 in the vapor phase is identified. stable isotopes results oxygen and deuterium isotopes the three intensely sericitic altered analyzed samples presents a δ18vsmow (vienna standard mean ocean water) composition ranging from +8.6 to +11.2‰ and δd vsmow from 50.8 to -74.6‰ (table 3). using the fractionation coefficients proposed by zheng (1993) and suzuoki & epstein (1976) at 300ºc, compatible with the total homogenization temperatures from the fluid inclusions, the hydrothermal fluid responsible for this alteration have a δ18vsmow isotopic values ranging from +4.14 to +9.73‰ and δd vsmow values from -3.96 to -28.93‰ (figure 9). sulfur isotopes pyrite, galena, chalcopyrite separates returned δ34s values from +1.1 to +1.4 ‰ for the galena and a wider range for pyrite, between -8.2 to +4.8 ‰. one chalcopyrite sample returned +2.4 ‰ (table 4; figure 10). using the sulfur enrichment factors proposed by sakai (1968), kajiwara & krouse (1971) and czamanske & rye (1974), the hydrothermal fluid composition for 250°c, in terms of δ34sh2s, was calculated from -8.5 and 5.3‰. from the paragenetic galena-chalcopyrite mineral pair, from vein mineralization at la selva area, there was calculated a 364.18ºc as the isotopic fractionation temperature using the coefficient values proposed by kajiwara & krouse (1971). 335a contribution to understanding the origin of the ginebra gold district mineralizations, colombia table 3. deuterium and oxygen isotopic results from hydrothermal sericite. results are standardized respect to vsmow. sa m pl e m in er al δ18 o v sm o w δd v sm o w % h 2o δ18 o v sm o w (h 2o ) δd v sm o w (h 2o ) 1170 sericite 8.6‰ -64.1‰ 4,3 7.14‰ -17.99‰ 1452 sericite 9.6‰ -74.6‰ 0,2 8.14‰ -28.93‰ 1499 sericite 11.2‰ -50.8‰ 7,6 9.73‰ -3.96‰ figure 9. δ18o y δd isotopic composition for the hydrothermal fluid responsible for the hydrothermal sericite related to the ginebra gold district mineralization. modified from sheppard, 1986. table 4. sulfur isotope composition of sulfide samples from the ginebra gold district. sample location mineral δ34s cdt‰ 1169 la selva chalcopyrite 2.4 1169 la selva galena 1.1 1169 la selva galena 1.4 1495 las camelias galena 1.2 1187 la selva pyrite -8.2 1495 las camelias pyrite 3.5 1170 el retiro pyrite 4.8 figure 10. sulfur isotope results in sulfides from the ginebra gold district. pyrite (py), galena (gn), chalcopyrite (ccp). 40ar/39ar dating results two samples from mineralized veins and stockwork veinlets, one from el retiro mine and the other from las camelias mine at la esperanza-la selva were select for 40ar/39ar dating to constrain the hydrothermal event, responsible for ginebra gold district mineralization. two additional samples from a hornblendic gabbro and a plagioclase rich hornblendite belonging to the ginebra ophiolithic massif, close to its eastern border, cropping out in the cominal-el jardín and valledupar-flautas areas were also analyzed. results are summarized in table 5 and illustrated in figure 11. unaltered medium grain hornblendic gabbro, composed by euhedral hornblende and plagioclase crystals with minor quartz and interstitial fine grain magnetite and ilmenite crystals. hornblende yielded a weighted plateau age (>97% of the gas release) and inverse isochron overlapping within uncertainty, being the 96.84 ± 2.14 ma (mswd = 0.3) plateau age preferred (early cretaceous). coarse grain plagioclase horblendite composed dominantly by euhedral hornblende and plagioclase crystals with interstitial magnetite, rutile and small pyrite crystals, it presents very weak epidote-carbonate-chlorite-sericite alteration. hornblende yielded a weighed plateau and inverse isochron overlapping ages within uncertainty, being preferred the 123.57 ± 2.83 ma (mswd = 1.07) plateau age (early cretaceous). el retiro mine from an intensely sericitized and mineralized zone crosscut by thin quartz-pyrite veinlets, in a medium grain gabbro belonging to the ginebra ophiolitic complex. in petrography, there were observed euhedral coarse grain pyrite crystals accompanying to anhedral-mosaic texture quartz crystals and fine-grained sericite. the sample yielded a slightly discordant age spectrum but including a well-defined plateau segment representing 89% of the gas release. the calculated plateau and inverse isochron overlap within uncertainty, and the former age, 67.81 ± 0.87 ma (mswd = 1.82), is the preferred age (late cretaceous). finally, the sample from the las camelias mine corresponds to the alteration halo of a 30 cm thick quartz-pyrite-galena-chalcopyrite-sericite-chlorite vein. chlorite yielded a slightly discordant age spectrum but including a welldefined plateau segment representing 72% of the gas release. the calculated plateau and inverse isochron overlap within uncertainty, being the former age, 65.50 ± 0.84 ma (mswd = 0.68), the preferred age (late cretaceous). discussion hydrothermal fluid composition, mixing or effervescence mechanisms, ph, oxidation state, trapping temperature and pressure conditions the microthermometric and raman spectroscopy measures of ore-related fluid inclusions from the ginebra gold district mineralizations, revealed the hydrothermal fluid presented a relatively simple and only slightly variable acuo-carbonic composition in all the different areas along the mineralized trend. gold mineralization was produced by a low to moderate salinity co2rich hydrothermal fluid with n2 minor amounts. the existence of fluid inclusions of the same assemblage (fia) with slightly different phase proportions but homogenizing by the vapor disappearing in all the cases, in a restricted temperature range suggests a boiling process may rule out. in contrast, a lineal tendency in the salinity values in restricted homogenization temperatures, observed for fia i and fia ii (figure 12) may be interpreted as the result of an isothermal fluid mixing process (wilkinson, 2001). homogenization temperatures can be used to calculate trapping conditions at the time the fluid inclusions genesis. however, it is important to remember the total homogenization temperatures represents only a minimum estimate for trapping temperatures and pressures, except when clear evidences of fluid immiscibility exist (roedder & bodnar, 1980). in ginebra gold district, calculated isochores for fia i indicate a pressure ranging from 307 to 423 mpa, corresponding to a trapping depth from 11.2 to 15.5 km, assuming a 2.78 g/m3 density for the crust. 336 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado table 5. 40ar/39ar ages for samples from the ginebra gold district. sa m pl e n o. l at itu de l on gi tu de l oc at io n pe tr og ra ph ic no te s m in er al se pa ra te d (w t.) c la ss ifi ca tio n c on di tio n 40 a r/ 39 a r pl at ea u ag e (m a ± 2 σ) (% 39 a r re le as e is oc hr on a ge ) 40 a r/ 39 a r is oc hr on a ge (m a ± 2 σ) 1482 3.7433 -76.1776 cominal-el jardín medium-grained plag(<1mm)-hbl with interstitial mag hornblende (101.1 mg) plagioclase hornblendite weakly hydrothermally altered 123.57 ± 2.83 (96.85%) 112.66 ± 10.80 (mswd=1.07) 1465 3.7198 -76.1940 valleduparflautas medium-grained plag(<1mm)-hbl with interstitial mag-ilm hornblende (106.5 mg) hornblendic gabbro fresh 96.84 ± 2.14 (97.5%) 97.40 ± 6.10 (mswd=0.3) 1170 3.8057 -76.1901 el retiro mine fine grained sericite accompanying mosaic texture quartz and coarse euhedral pyrite sericite (59 mg) stockwork veining zone intensely hydrothermally altered 67.81 ± 0.87 (89%) 67.73 ± 0.88 (mswd=1.82) 1495 3.7555 -76.1849 las camelias mine qz-py-gn-ccp vein with fine grain sericite and medium grain chlorite halo chlorite (61 mg) quartz vein intensely hydrothermally altered 65.50 ± 0.84 (72%) 66.38 ± 1.24 (mswd=0.68) plagioclase (plag), hornblende (hbl), magnetite (mag), ilmenite (ilm), quartz (qz), pyrite (py), galena (gn), chalcopyrite (ccp). figure 11. 40ar/39ar age spectra for samples from the ginebra ophiolitic massif and for stockwork veinlets and gold bearing veins hydrothermal haloes in the ginebra gold district. cumulative 39 ar released (%) cumulative 39 ar released (%) cumulative 39 ar released (%) cumulative 39 ar released (%) a ge (m a) a ge (m a) a ge (m a) a ge (m a) 337a contribution to understanding the origin of the ginebra gold district mineralizations, colombia figure 12. th vs. salinity diagram (wt% nacl equiv.) for fia´s i, ii and iii, where is observed a linear trend of the salinity values in a restricted range of homogenization temperatures for fia i and ii, while for the fia iii, salinity is on a range of higher temperature. the tendency of fia´s i and ii is interpreted as a product of an isothermal fluid mixture process. modified from wilkinson, 2001. because non conclusive evidences of fluid immiscibility and the unexistence of other geo-thermobarometers for the ginebra gold district, the calculated p-t trapping conditions represents only minimum values and has to be viewed as the only available information about the depth to which the ores may have been formed. the alteration mineral assemblage (sericite – muscovite + carbonate ±epidote) in ginebra gold district suggests a near-neutral ph conditions (phillips & groves, 1983) for the hydrothermal fluid. meanwhile, the co2 and variable n2 contents identified in the microthermometry and raman spectroscopy indicates a relatively reduced character for the hydrothermal fluids, coherent with the sulfides mineralogy, abundant pyrite and absence of arsenopyrite. under this aforementioned condition, the oxygen fugacity (ƒo2), calculated for the fia i, using a 300 mpa pressure and a 5.5 ph, ranges from 1031 to 10-38 bar (figure 13), confirming a reduced condition for the hydrothermal fluid, which is compatible with the sulfide mineralogy. figure 13. oxygen fugacity vs temperature diagram for the mineralizing fluids in the ginebra gold district mineralization at ph=5.5 and p=300 mpa. modified from jia et al., 2000. gold transport and deposition mechanisms different authors have proposed the gold transport mechanisms in reduced hydrothermal solutions in equilibrium with pyrite/pyrrhotite, containing chlorides and sulfides (seward, 1973; seward, 1984; hayashi & ohmoto, 1991), in a temperature range between 160 and 350ºc and ph conditions ranging from 1.4 to 9.5. experimental results and thermodynamic data indicates that in near neutral conditions and a temperature between 250 and 350ºc, as the existing conditions in the ginebra gold district, gold is effectively transported only as bi-sulfide to thio-sulfide complexes (hau(hs)2 0, au(hs)2; auhs0, auhs(h2s3) 0) (seward, 1973; shenberger & barnes, 1989; stefánsson & seward, 2004). this process neglects the chlorine complexes as effective ligands for gold, except if the fluid is low in h2s, rich in chloride and the ph is less than 4.5 (hayashi & ohmoto, 1991). the reduced, low salinity and near-neutral condition of the ginebra gold district hydrothermal fluid, as well as the close association between gold and sulfides, suggest that the most probable gold transporting mechanism was as bi-sulfide or thio-sulfide complexes. several different mechanisms have been proposed for the metal’s ligands destabilization in hydrothermal solutions and the correspondent gold precipitation. in general, it has been assumed that any change in the physic-chemical conditions of the hydrothermal fluids may produce their precipitation (robb, 2005), however, some processes are considered more important, depending not only of the fluid conditions, but of the host rocks and the predominant geological process occurring in the area during the hydrothermal process. in the case of the bi-sulfide and thio-sulfide complexes, the most important processes invoked for gold deposition includes the phase separation (ebullition or effervescence), as a direct consequence of sudden pressure decrease (diamond, 1990; ramboz et al., 1982; ridley et al., 1996; sterner & bodnar, 1984), the ph and oxidation changes due to fluid/rock interactions, the h2s loss in the hydrothermal fluid due to a sulfidation process, reflected as abundant sulfides (mainly pyrite) generation on the host rocks, and fluid mixing (gebre-mariam et al., 1993; groves, 1993). the coexistence of fia i and fia ii fluid inclusions in all the analyzed areas, presenting a marked and constant homogenization by the vapor phase disappearing in a restricted temperature range, showing a lineal tendency in salinity changes, indicates a fluid mixing process may be responsible, at least in part, for the destabilization of the bi-sulfide and/or thio-sulfide complexes and the gold deposition from the hydrothermal fluids in the ginebra gold district. additionally, most of the host rocks in the district correspond to gabbro belonging to the ginebra ophiolitic massif, presenting a high iron content, susceptible for sulfides generation when these rocks interact with hydrothermal solutions and the correspondent h2s loss. this sulfidation process may also contributed to the bi-sulfide/thio-sulfide complexes destabilization and gold deposition, particularly in el retiro-cueva loca-la cecilia area, where is frequent to observe a high volume of fine grain disseminated pyrite around veins and stockwork zones. we propose that the h2s loss due to sulfidation and fluid mixing were responsible for gold deposition from the hydrothermal solutions in the ginebra gold district. the nature of ore fluids, age and genetic type of the ore deposit the characteristics of ore fluids in the ginebra gold district are compatible with the orogenic and/or intrusion-related gold deposits. in both cases, co2-rich low salinity fluids are responsible for gold transport, both are poor in metals and mineral zoning and they present an imprecise spatial and genetic relationship to individual intrusive phases. these are the reasons to the many existent controversies about the assignment of a particular deposit to any of these categories (sillitoe & thompson, 1998; thompson et al., 1999; groves et al., 2003; goldfarb et al., 2005; hart, 2007; mathieu, 2021). the co2-rich and low salinity nature of the hydrothermal fluids in the intrusion-related gold deposits has been proposed as the result of devolatilization of felsic magmas during their crystallization in a deeper environment (4 to 6 km), favoring its solubility and exsolution (burrows et al., 1986; thompson et al., 1999; cline & bodnar, 1991; lowenstern, 2001; sun et al., 2007). according to lang et al. (2000), the most prominent characteristics of this deposit type, besides their low saline co2-rich fluids, includes: 1. a genetic relationship with metaluminous, subalkalic intermediate to felsic intrusions with 338 bibiana p. rodríguez-ramos, juan c. molano and camilo, e. dorado an oxidation state spanning the boundary between the ilmenite and magnetite ishihara (1977) series. 2. a metal assemblage including anomalous bi, w, as, mo, te and/or sb besides au. 3. restricted zones of hydrothermal alteration whiting the intrusions. 4. a continental setting inboard of the convergent plate boundaries and 5. location in magmatic provinces with w-sn mineral deposits. interestingly, the most characteristic mineral style corresponds to a sub-parallel sheeted array of low sulfides quartz veins, although other styles also occur. conversely, orogenic gold deposits are typically developed along convergent margins during terrene accretion, translation or collision, in terranes which suffered moderate to high t and low to moderate p regional metamorphism (powell et al., 1991; groves et al., 2003; goldfarb et al., 2005), in most of the cases spatially and temporally associated with granitic intrusions. orogenic gold deposits form typically in the latter part of the tectonic orogenesis being considered late-syntectonic, related to plate subduction and/ or lithospheric delamination, involving the accretion of exotic terranes (groves et al., 1998; sillitoe & thompson, 1998). this is the reason why they present a strong structural control involving faults or shear zones, normally of second or third order, as well as folds or areas presenting strong rheological contrasts (groves et al., 2003; goldfarb et al., 2005). there is not a consensus about the origin of the hydrothermal fluids in the orogenic gold deposits nor the role of deep-magmatic processes, being the isotopic, fluid inclusions and dating evidences, not unequivocally distinct for a metamorphic or deep-magmatic source for this deposit type. despite this and although a magmatic source is feasible, the associated intrusion bodies in most of the orogenic gold deposits are thought to be too small to provide the volume of fluid flow and to support the high gold tonnage in most of them (groves et al., 2003). in this respect, available calculation of mineral equilibrium in the caomgo-feo-al2o3-sio2 system showed that hydrous and carbonate minerals at 460-500°c in mid crustal levels, as the proposed source for a metamorphic origin for the hydrothermal fluids in orogenic deposits, are able to release voluminous low-salinity and co2-rich fluids (powell et al., 1991). in many cases, the differences between intrusion-related gold and orogenic gold deposits appear not to be only subtle but also difficult to be unequivocally tested. this is the case of the spatial, temporal and genetic relationship with intrusive bodies in the intrusion-related deposits or the late-syntectonic character for the orogenic gold model, with the additional complication of the commonly overprinting of the later in the former, as seems to occurs in the abitibi greenstone belt, one of the world class golds producing metallogenic belts (mathieu, 2021). in colombia, the biggest known intrusion-related gold deposit, cerro gramalote, locates in an e-w structural corridor, the so-called “nus river trend”, crosscutting the roof of the post-collisional, 62 to 58 ma age, providencia tonalite facies of the antioquia batholith. there are discrete lodes and subparallel sheeted quartz-pyrite-sphalerite-galena-molybdenite veinlets containing significant amounts of bi-sulfides and sulphosalts, as well as tellurides. these structures present narrow potassic k-spar and sericite alteration haloes (leal-mejía, 2011; leal-mejía et al., 2019). in cerro gramalote, age dating for the hydrothermal alteration and direct ore dating, as well as sulfur and lead isotopes clearly indicates a genetic relationship between mineralization and the hosting intrusive facies (lealmejía, 2011). in contrast, we showed here, that lodes and stockwork veinlet mineralization in the ginebra gold district, were formed slightly early in time than cerro gramalote and many other antioquia-sonsón batholiths gold occurences, during the late cretaceous, about 67 to 65 ma (figure 14), nearly 25 ma younger than the main facies of the buga batholith (villagómez et al., 2011; leal-mejía, 2011; leal-mejía et al., 2019; nivia et al., 2019) and temporally coeval with the irra stock (leal-mejía, 2011). this time period is coeval with a high-pressure and low-temperature orogenic metamorphism, recognized in the jambaló blueschists, part of the arquía complex, in the western flank of the central cordillera (bustamante & bustamante, 2019), interpreted as the result of a dextral oblique convergence of the caribbean plate during the late cretaceous-paleocene. in this context, it is possible to argue the ginebra gold district mineralizations emplaced in a strongly ductile to brittle structural environment, related to a dextral transpressive subduction to obduction tectonic episode, during a late syn-tectonic event related to the accretion of the romeral terrane and probably in the sea-land of a subduction zone (cediel et al., 2003; villagómez et al., 2011; leal-mejía, 2011; bustamante & bustamante, 2019; nivia et al., 2019). during this time there was emplaced the later facies of the buga batholith, coeval with small dacite to andesite porphyritic texture dikes (brito et al., 2010; gonzález, 2010; nivia et al., 2019) (figure 14). as stated before, from the microthermometric and raman analysis, the hydrothermal fluids responsible for gold mineralization in the ginebra gold district were low salinity, co2-rich and reduced, being the lodes and veins related to constrained sericite-chlorite haloes. sulfur isotopes revealed a composition close to zero, suggesting a restricted fluid circulation through the magmatic rocks of the buga batholith and the ginebra ophiolitic massif, probably controlled by the major fault systems in the area, in particular the guabas-pradera. it is also possible to argue from the oxygen and deuterium isotopes, a metamorphic origin for the hydrothermal fluids, which is in agreement with a metamorphic devolatilization process rather than a deepmagmatic source. for all the mentioned characteristics, especially the nature of the hydrothermal fluids and the tectonic regime at the time of mineralization event, different as for example cerro gramalote, we propose the ginebra gold disctrict mineralizations are more compatible with the orogenic gold type rather than to the intrusion-related origin. figure 14. age relationships for the ginebra gold district. the 123.57 ma and 96.84 ma 40ar-39ar hornblende dates for the ginebra ophiolitic massif are coherent with a high-pressure oblique subduction-related metamorphism event, related to the farallon plate in the first case, while the second may be related to the syn-tectonic emplacement of the main buga batholith facies. the late cretaceouspaleocene hydrothermal alteration dates for the ginebra gold district are coeval with the high-pressure metamorphism event produced by the caribbean plate convergence and subduction beneath the south american plate. conclusions gold mineralization in the ginebra district occurs as veins, disseminations and veinlet stockwork zones both in the ginebra ophiolitic massif and the buga batholith, in strongly ductile to brittle deformed areas, in an approximate 6 km n-s trend. they characterize by a simple sulfide mineralogy assemblage in which fine grain size gold grains are related to scarce sulfides in at least two mineralizing events. fluid inclusion and stable isotopic studies showed that the ore fluids were metamorphic in origin, low salinity, moderate to low temperature and co2-rich and an isothermal fluid mixing process as well as a h2s loss process are proposed as the responsible for gold deposition. 40ar-39ar dating for the hydrothermal alteration haloes, coetaneous with gold deposition, indicates the mineralization occurred during the late cretaceous to paleocene, in a period characterized by a strong dextral transpressive subduction to obduction and exotic terrane accretion against the north-western part of the south american plate. these characteristics are compatible with an orogenic gold deposit type. acknowledgments this study is part of the project called ‘anteproyecto de investigación metalográfica, microtermométrica, geoquímica e isotópica para algunos 339a contribution to understanding the origin of the ginebra gold district mineralizations, colombia yacimientos minerales en colombia’, financed by colciencias and the national university of colombia. we would like to thank the colombian geological survey sgc (ingeominas), the united states geological survey (usgs) and the technological development center of the colombian emerald (cdtec) for allowing most of the analysis, in particular the raman spectroscopy and the stable isotopes. references aspden, j., & mccourt, w. 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(1993). calculation of oxygen isotope fractionation in hydroxylbearing silicates. earth and planetary science letters, 120(3-4), 247-263. https://doi.org/10.1016/0012-821x(93)90243-3 https://doi.org/10.1016/j.lithos.2011.05.003 https://doi.org/10.1016/s0024-4937(00)00047-5 https://doi.org/10.1016/0012-821x(93)90243-3 _ref158839137 _ref158839990 _ref158840438 _ref72760551 _ref72760545 _ref72760623 _ref158840913 _ref158841158 _ref72760888 _ref72760982 _ref158841411 _ref158841591 _ref158841663 _ref72955503 _ref158841831 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.108083 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 163 168 en v ir o n m en ta l g eo lo g y manuscript received: 29/03/2023 accepted for publication: 18/07/2023 ecological restoration strategies of rock-soluble stone desertification regions under the background of big data and the internet of things guanyao lu * school of environmental and chemical engineering, foshan university, foshan 528000, china * corresponding author: guanyaolu@fosu.edu.cn abstract keywords: big data; internet of things; rocksoluble stone desertification; ecological repair; comprehensive governance; southwestern china; as the focus and difficulty of global ecological governance, rock-soluble graphite ecological issues have been plagued by the research and exploration of ecologists for many years. this article studies the current development status of rock-soluble graphite. features and laws targeted implementing the ecological restoration strategy of rock-soluble graphite regions under the background of big data and the internet of things. by establishing a mathematical analysis model of the total ecological assets, the theoretical research on the restoration process and means of ecology. in the past 20 years, the southwest region’s ecological improvement and governance situation has been summarized and analyzed, and the key aspects and main contents of ecological restoration are analyzed and demonstrated. palabras clave: macrodatos; internet de las cosas; desertificación de rocas solubles; reparación ecológica; gobernanza integral; suroeste de china; estrategias de restauración ecológica en regiones de desertificación de rocas solubles bajo el contexto de los macrodatos y el internet de las cosas how to cite this item: lu, g. (2023). ecological restoration strategies of rock-soluble stone desertification regions under the background of big data and the internet of things. earth sciences research journal, 27(2), 163-168. https://doi.org/10.15446/ esrj.v27n2.108083 resumen debido al enfoque y a las dificultades que representa en la gobernanza ambiental mundial, los temas ecológicos relacionados con los grafitos solubles de roca han sido ampliamente abordados durante muchos años por los ecologistas. este artículo estudia el actual desarrollo de los grafitos solubles de roca. algunas normas buscan implementar estrategias de restauración ecológica de los grafitos solubles de roca bajo el contexto de los macrodatos y el internet de las cosas. se establece un modelo de análisis matemático del total de las ventajas ecológicas, la investigación teórica del proceso de restauración y medios de la ecología. también se resume y se analizan las mejoras ecológicas y situación de gobernanza en el suroeste de china durante los últimos 20 años además de probarse y analizarse los aspectos claves y contenidos principales de la restauración ecológica. https://doi.org/10.15446/esrj.v27n2.108083 mailto:guanyaolu@fosu.edu.cn https://doi.org/10.15446/esrj.v27n2.108083 https://doi.org/10.15446/esrj.v27n2.108083 164 guanyao lu 1. introduction as a significant ecological problem globally, the issue of desertification of rock-soluble stones has brought an essential impact on ecological deterioration around the world. in recent years, with the problem of rock-soluble graphite, more and more ecologists have been concerned. people attach more and more attention to the problem of lonely stone desertification. the domestic academic circles have also put forward much theoretical basis for the recovery of rocksoluble stone desertification ecological recovery and have made a variety of practical summaries. regarding realizing rock-soluble stone desertification ecological recovery projects, the state government has also introduced specific policies and capital investments (xu, 2015). by integrating the research results in related fields such as agriculture, forestry, and animal husbandry and adopting the ecological recovery strategy that meets the laws of nature development, it meets the improvement of rocky graphite in different regions. it provides some ecological recovery and constructs a green low-carbon ecological development strategy. the current status of stone desertification problems status of the regional development of the desertification of rock-soluble stone the research of rock-soluble graphite is widely concerned in the global academic community. after the statistics of the state general administration of geological, the area of desertification of rock-soluble stones in china has continued to expand in recent years, and the problem of desertification in the stone-soluble stone in the southwest is the most serious. in exploring domestic lava regions, geological personnel, as the most desertified areas in the southwest, are one of the leading causes of various types of natural disasters. graphite is accompanied by problems such as climate and environment, human factors, and unscientific development. throughout the problem of rock-soluble stone in other countries, the causes of such ecological problems are accompanied by problems such as economic development, ecological deterioration, and artificial destruction. investigating the global graphic area, about 20.6 million square kilometers of land in the world was deserted, accounting for about 15%of the world’s land area, and about 1.1 billion people lived in the rocky region. my country has 3.44 million square kilometers of land in rocky-soluble areas, accounting for about 36%of the domestic land area. these areas that are developing in stone desertification ecological issues account for approximately 1.25 million square kilometers. facing the huge problem of ecological environment, the state and government departments should take targeted measures to implement corresponding policies to achieve effectively improved graphization problems in the rock-soluble region (yuan and cai, 2018; yuan, 2013; yuan, 2017). as the most widely distributed area of the rock-soluble region in southwestern china, it includes a number of provinces and regions, with an area of 1.5 million square kilometers. in this vast land area, it is very sensitive to the environment. in the process of ecological evolution, the state pays more and more attention to the problem of desertification of rocky stone. due to the central task of economic development in recent years, local governments have ignored ecological construction problems in the process of promoting economic construction. rock-soluble stone deadized ecological resources the problem of desertification of rocky stone brings new problems to ecological resources. in the economic value estimation of natural ecological resources, a clear calculation model of ecological resource asset loss is established based on quantitative calculations based on ecological resources within a certain area. through the research and analysis of this model, we can further effectively build an ecological resource repair mechanism. on the basis of the above research and analysis of ecological resources, we have built a model for the loss of ecological assets as: (1) in building an ecological asset loss model, we expressed the social value of ecological resources as v. k represents the influencing factors of ecological resources. mk represents the changes in the total ecological assets under the influence of the k-class factors. wk represents the influencing factors of class k for currency conversion of ecological economic value (department of chinese academy of sciences, 2018). in the model component of the impact of rock-soluble graphite on ecological destruction, we mainly analyze the severity of the degradation of rock-soluble stone desertification ecological degradation and build an analysis model of ecological asset loss based on different ecological environments and regional characteristics. by selecting and analyzing the important factors affecting ecological degradation, quantifying the problem of the evolution of rock-soluble stone in the entire ecosystem space. in the process of building an ecological asset quantitative analysis model, the factors of the total ecological assets of the ecological environment evolution of the ecological environment are based on the two aspects of the degradation and environmental improvement of rock-soluble graphitization and environmental improvement: (2) ecological assets in rock-soluble areas can be analyzed according to the amount of graphitization increase in the rock-soluble area and the improvement of the amount of desertification in the rock-soluble stone. in terms of v1, the increase in ecological assets of lava graphite. (3) in the mathematical model component of the evolution of lava desertified ecological assets, we use the degree of lava glowing in the graphitization ecological service function and the degree of lava graphite in the unit area. the influencing factors of the ecosystem are analyzed as a constant quantity. regarding the improvement of graphitization in the rock-soluble area, we use v2 to indicate that during the evolution of the total ecological assets of the entire rock-soluble area, the continuous improvement and optimization of the stone desertification problem by taking certain ecological restoration methods and measures are used to build the following, and the construction is as follows. mathematical evaluation model. (4) in v2, through the analysis of the ecological restoration and related factors of the desertification and related factors of the rock-soluble stone, based on the desertification measures and related policies in different regions, through the degree of capital investment, manpower input and ecological restoration of government departments as an important reference indicator, to build a v2 analysis of the entire mathematical evaluation model (cao et al., 2014; li, 2006). the ecological evolution brought by the desertification of rock-soluble stone characteristics of rock-soluble stone desertification the problem of desertification of rock-soluble stone is an important ecological influencing factor in the rock-soluble area. through research on the problem of rocky graphite in different regions of china, it is found that due to uneven distribution in the rock-soluble area, the severity of graphization problems is different. with the rock-soluble landform covered in different regions, it is found that the structure of hydrogeological sealing in the rocksoluble area is special, and it has become an important factor affecting the problem of graphization. moreover, the growth of vegetation in rock-soluble areas is affected by various natural environments, resulting in the evolution of 165ecological restoration strategies of rock-soluble stone desertification regions under the background of big data and the internet of things stone desertification problems in the rock-soluble area, and it is more obvious due to regional impact (zhu et al., 2014). looking at the distribution of global rock-soluble areas, the distribution of the backbone regions north is the most widely distributed. my country is in the southwestern region of the northward backbone. in improving and treating rock-soluble graphite problems, corresponding measures have also been taken (wan et al., 2014). in the global distribution of the rocky region, there are about 22 million square kilometers according to statistics, accounting for 15%of the global land area. about 25%of people around the world live in rock-soluble areas and drink rocky water in rock-soluble areas, which has a certain impact on the health of these people. the problem of rock-soluble graphite mainly occurs near tropical subtropical and near the north tropic of water quality. as is shown in figure 1. the main features of the restoration of ecosystems the formation of graphitization problems in domestic rock-soluble areas has distinctive characteristics, and measures should be taken for targeted during the implementation of the repair process. first, the carbonate rock in the rocksoluble area is exposed. the terrain in the rock-soluble area is straight and steep, and the stone desertification problem in the rock-soluble area has brought an important impact on the global environmental ecological environment. the second is affected by the monsoon climate. under the influence of marine monsoon climate, the carbonate structure of the entire rock-soluble area is continuously eroded and deposited, resulting in the evolution of graphization issues more and more serious (zhang et al., 2019). third, the geological movement of carbonate rocks continues to rise, resulting in the thinner surface layer of the ground soil, which makes the rock dissolved on the surface. with the erosion of the weather climate, the rock-soluble stone deserted. fourth, due to the long-term glacier erosion process, the surface has formed a relatively fragile ecological environment in the rock-soluble region. in addition, the deterioration of human activities and ecological environment, resulting in the weaker and weaker ability to resist external erosion. fifth, with the gradual increase of population density and the continuous expansion of human activity areas, the rock-soluble region is affected by all aspects of human production and life, so that the entire ecological environment of the rock-soluble area has not been well repaired (guang et al., 2011). the main measures for restoration of ecosystems aiming at the main characteristics of the above rock-soluble graphite, graphite ecological restoration measures are targeted. the chemical erosion that the carbonate soil quality in the rock-soluble area is during the process of desertification should improve the soil quality. auxiliary carbonate rocks can better resist the impact of the monsoon climate during the weathering process, so that the entire soil can make the entire soil. the carbonate content of the layer meets the standard. in response to the characteristics of the soil structure in the southwestern rock-soluble areas, we must conduct targeted research. for the surface rock dissolved and naked caused by the rising of the crustal movement, we must carry out geological research and exploration according to the problem of thinning the soil layer (cao and li, 2014; cao and yuan, 2015; li, 2020). according to the structure of the soil layer of the southwestern rocksoluble area released by the state administration of geology, we can find that among the soil quality of carbonate rocks, the diameter of soil ash is less than 0.05 mm, reaching 95.2%, less than 0.001 mm reached 51.9%, soil gray grains less than 0.0005 mm accounted for 38.09%, soil ash particles were less than 0.0001 mm accounted for 29.19%, and 29.19%.the study of this type of lime soil’s clay composition can better assist the soil repair and ecological environment improvement in the rock-soluble areas (wang et al., 1999). we must carry out the improvement and repair of lime soil, and to assist the stone desertified ecological problems in the rock-soluble area. as is shown in figure 2. in response to the characteristics of the monsoon climate in the dilute area of the southwest, we must take water resources to protect the characteristics of rainfall concentration and the decline in groundwater levels. for seasons that are prone to rainstorms, we must increase monitoring of seasonal climate, take measures in advance to protect the soil layer of the surface, prevent the surface of the surface of the rock-soluble area due to soil loss, and exacerbate the problem of rock-soluble graphite. as is shown in table 1. figure 1. the proportion of rock-soluble areas in the world 166 guanyao lu rock-soluble stone desertification ecological restoration strategy implementation of regional characteristics governance strategies the ecological restoration of rock-soluble graphite should be effectively governed by characteristics. for regions with frequent human activities, human activities and production and operation must be further standardized. according to statistics, the area of the southwest rock-soluble area has reached 513,600 square kilometers. on this vast land, 30% of the land is distributed with human activities (li, 2020; zhou and lv, 2003). there are 299 iwari county, of which 185 are more serious. it accounted for 62.20%. as is shown in table 2. according to statistics from the ministry of land and resources and the state forestry administration, it was found that the desertification of rocky stone in 8 provinces. in 2000, 113,400 square kilometers, 120,700 square kilometers in 2005, 130,100 square kilometers in 2010, 144,500 square kilometers in 2015, and 159,100 square kilometers by 2020. with a large amount of measures for regional characteristics, a large number of manpower and material resources have been invested, and the restoration of rocky graphite ecological problems has been performed to alleviate the rate of desertification evolution. it is said that it is necessary to take targeted measures and means in combination with regional characteristics. as is shown in figure 3. fixed analysis of the evolution of stone desertification constructing an ecological repair system in the rock-soluble areas, the entire rock-soluble area ecosystem should be effectively repaired by building a system of human activity ecological recovery and natural environment improvement systems. further regulate the scope of human production and life activities, and reduce the degree of impact on the environment. by adopting the means of natural disaster prediction and human intervention, the occurrence of natural disasters such as heavy rains in rock-soluble areas. effectively reduce the severity of soil and soil loss in rocksoluble areas, and effectively relieve and restore the graphization ecosystem of the entire rock-soluble area. through the construction of systems in recent years, 67 counties and cities in the southwest of the country have been improved well, and the area of cultivated land accounts for about 29% of the total arable land area in the region. the arable land has also been well protected (pei and li, 2001). repair the effective operation mechanism of the ecosystem for the construction and improvement of the ecosystem of the rock-soluble region, it is necessary to improve the characteristics of the regional environment in the process of the ecological environment. due to the characteristics of strata and terrain, in the process of restoration of the ecological environment, we must figure 2. the diameter distribution of carbonate rocks and corrosion of different soil grains table 1. climate characteristics in the dilute area of southwest china area altitude/m annual average temperature average annual rainfall/mm lhasa 3569 7.9 458.9 aba 3909 3.9 753.9 chengdu 515 16.9 976.9 chongqing 210 18.0 1098.3 kunming 1890 15.9 997.2 yichang 138 17.1 1187.9 guiyang 1079 16.8 1165.9 guilin 148 19.1 1890.9 nanning 78 22.0 1254.9 wuhan 29 17.6 1268.9 changsha 45 18.1 1435.6 guangzhou 6 22.9 1689.3 167ecological restoration strategies of rock-soluble stone desertification regions under the background of big data and the internet of things pay attention to the diversity of regional plants, so that green vegetation the coverage rate has risen year by year. through investigations in 14 districts and counties in the southwest, in the 20 years from 2000 to 2020, the green coverage rate of the 14 districts and counties climbed from 29% to 49% year by year, reflecting that the state is restoring the ecological environment of the southwest region in the southwest region. the huge measures and achievements in the system. as is shown in table 3. focus on implementing policy capital support for poor areas in recent years, the state has adopted corresponding methods and policies to help the governance of the problem of desertation of rock-soluble stone in poor areas. in terms of promoting the galginization of rock-soluble graphite in the southwest, we need to invest in human and funds in the ecological improvement of poor areas. according to statistics in 2020, the economic conditions of 131 districts and counties in the southwest region are relatively backward. the funds and manpower invested in the improvement of these areas in the process of improving these areas are not enough. pay attention to the construction and improvement of the ecological governance mechanism state government departments must work together the state and government departments should strengthen the restoration measures of standardized and standardized rock-soluble stone desertified ecosystems, and further strengthen the ecological restoration and governance of the rock-soluble region through comprehensive governance methods, and pay attention to the coordinated development of ecological and economic benefits. in the process of ecological governance, while ensuring the local economic development and construction on the road of healthy, harmonious, green, and low-carbon economic development, it is necessary to adapt to local conditions in combination with the characteristics of different regions. excellent methods and measures in other areas have achieved effective improvement of graphite problems in the rock-soluble area. establish sustainable development repair plan ecological restoration methods must have long-term and sustainability. we must establish long-term graphic ecological restoration plans in the rocksoluble region, combine the overall plan of ecological restoration in the country and region, to achieve the process of regional economic development and the improvement of human living standards, pay more attention to the right paid the emphasis on graphitization issues take ecological issues as an important construction project to achieve long-term sustainable development in the process of economic development and social progress. after investigation, from 2010 to 2020, the area of stone desertification was 30,000 square kilometers in individual districts and counties, accounting for 23% of the total area of stone desertification in the area; by 2021, the area of stone desertification was 70,000 square kilometers, accounting for the stone desert 54% of the total area. conclusions the comprehensive governance of ecological graphization problems in the rock-soluble area is affected by human life production activities, natural ecological environment, and regional characteristics. the severity of the stone desertification problems in different regions is different. for detailed introduction, focus on analyzing the problem of desertification of rocky stone in the southwestern region. by establishing a model evaluation model of ecological assets, a new research perspective is proposed to the repair of graphization issues. for rocky graphitization in different areas, we must protect them targeted, take targeted measures, and gradually carry out environmental governance and ecological repair for seasonal climate, natural disasters and soil loss problems. table 2. the actual situation project statistical data proportion the rock-soluble area in the southwest 513,600 square kilometers 5.35% iwari prefecture 299 22.91% rock-soluble stone desertification serious district and county 185 62.20% national key reserve county 151 50.5% figure 3. the evolution of stone desertification of 8 provinces and regions in southwest china 168 guanyao lu funding this work was supported by the natural science foundation of guangdong province (no. 2015a030310505). references cao, j. h., & li, x. y. (2014). the characteristics, evolution and ecological recovery of the southwest of the rocky ecosystem of china. ecological science progress. – beijing: higher education press 1, 121-134. cao, j. h., yuan, d. x., & pei, j. g. (2014). the ecosystem of china’s southwest rock cadefolon ecosystem restricted by geological conditions. earth and environment, 2(1), 1-8. cao, j. h., & yuan, d. x. (2015). the southwest rock-soluble ecosystem restricted by geological conditions. geological publishing house, beijing. department of chinese academy of sciences. (2018). several suggestions on promoting comprehensive governance of stone desertification in the southwestern rocky area. earth scientific progress, 8(4), 92. guang, y. h., guo, c. q., & li, w. q. (2011). study on the rock soaking disaster. guangxi normal university press, guilin. li, d. w. (2020). a preliminary study of the formation of rock-soluble wind shells. china rock dissolving. li, m. x. 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(2018). rock-soluble environmental studies. chongqing technology press, chongqing. yuan, d. x. (2013). environmental change and human impact on karst in south china. karts terrains, environmental changes and human impacts. catena supplemement, 99-107. yuan, d. x. (2017). rock desertification in the subtropical karst of south china. ztschrift fur geomorphologie, 108, 81-906. zhang, w., qin, x. q., & yi, l. x. (2019). the development and utilization of water-soluble water resources in yunnan, guizhou, and guizhou. china university of geosciences press, wuhan. zhou, j. z., & lv, y. j. (2003). cause control and prevention countermeasures of stone desertification. hunan geological, 22(1), 43-46. zhu, a. g., lin, c. h., & yang, h. m. (2014). comprehensive evaluation and research on the influencing factors of the water and soil loss in the mountainous area of guizhou. journal of soil and water conservation, 4, 17-24. table 3. 2000-2020 green vegetable coverage rate in southwest china time green vegetation area green vegetation coverage rate growth rate 2000year 107 million square kilometers 29% 2005year 115 million square kilometers 31% 6.9% 2010year 1.29 million square kilometers 35% 12.9% 2015year 153 million square kilometers 42% 20% 2020year 1.8 million square kilometers 49% 16.7% mtblankeqn issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.104931 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (septiembre, 2023): 321 325 so il s influence of adding jordanian scoria on the properties of clay-based geopolymer faten al-slaty1,*, khalil ibrahim1, sultan fayez2, islam al-dabsheh3, taleb odeh4, shereen abusmier 5 1. department of earth and environmental sciences, prince el hassan bin talal faculty of natural resources and environment, the hashemite university, jordan. 2. arab center for engineering studies, jordan. 3. department of applied geology and environmental sciences, institute of earth and environmental sciences, al al-bayt university, jordan. 4. department of water management and environment, prince el hassan bin talal faculty of natural resources and environment, the hashemite university, jordan. 5. department of land management and environment, prince el hassan bin talal faculty of natural resources and environment, the hashemite university, jordan. *correspondence: fatenm@hu.edu.jo orcid id: https://orcid.org/0000-0002-9749-9041 abstract this work aims to investigate the influence of adding jordanian scoria on the characteristics of clay-based geopolymer. the clay deposit and scoria were gotten from north-east jordan. the chemical, mineralogical, and microstructural properties of the used materials were examined. scoria was added to clay-based geopolymer mixtures in different ratios: 0%, 10%, 20%, 30%, 40%, and 50%. comprehensive experimental tests were conducted to assess the effect of adding scoria on the properties of the produced geopolymer. the results revealed decreased compressive strength and dry density, whereas porosity, water absorption, cation exchange capacity, and specific surface area increased as the ratio of scoria increased. the mineralogical and microstructural analysis of the geopolymers after adding scoria indicates the formation of mineral phases, namely hydroxy-sodalite and hydroxy-cancrinite in addition to the gel phase. keywords: clay-based geopolymers; scoria; clay deposits; geopolymerization; volcanic materials. palabras clave: geopolímeros basados en arcillas; escoria volcánica; depósitos de arcillas; geopolimerización; materiales volcánicos. este trabajo se enfoca en investigar los efectos en las características de geopolímeros a base de arcilla tras añadirles escoria volcánica de jordania. la arcilla y la escoria volcánica fueron tomadas de depósitos en el noreste de jordania. luego se examinaron las propiedades químicas, mineralógicas y microestructurales de los materiales usados. la escoria volcánica fue añadida en diferentes proporciones en mezclas de geopolímeros con base en arcilla: 0 %, 10 %, 20 %, 30 %, 40 %, y 50 %. posteriormente se realizaron pruebas experimentales para evaluar los efectos en las propiedades de los geopolímeros producidos tras haber añadido escoria volcánica. los resultados revelan un decrecimiento en la fuerza compresiva y en la densidad seca, mientras que la porosidad, la absorción de agua, la capacidad de intercambio catiónico, y el área de superficie específica se incrementaron cuando la proporción de escoria volcánica también se incrementó. el análisis mineralógico y microestructural de los geopolímeros después de la adición de escoria volcánica indica la formación de fases minerales como la hidroxisodalita e hidroxicancrinita además de la fase de gel. efectos en las propiedades de geopolímeros a base de arcilla tras añadir escoria volcánica de jordania resumen how to cite item: al-slaty, f., ibrahim, k., fayez, s., al-dabsheh, i., odeh, t., & abusmier, s. (2023). earth sciences research journal, 27(3), 321-325. https://doi.org/10.15446/ esrj.v27n3.104931 manuscript received: 24/09/2022 accepted for publication: 03/10/2023 https://doi.org/10.15446/esrj.v27n3.104931 mailto:fatenm@hu.edu.jo https://orcid.org/0000-0002-9749-9041 https://doi.org/10.15446/esrj.v27n3.104931 https:// doi.org/10.15446/esrj.v27n3.100324 322 faten al-slaty, khalil ibrahim, sultan fayez, islam al-dabsheh, taleb odeh, shereen abusmier 1. introduction geopolymerization is a chemical reaction between alumino-silicate oxides and alkali solutions yielding polymeric si-o-al bonds with amorphous to semi-crystalline three-dimensional silico-aluminate structures (davidovits, 1991). the primary materials used in the geopolymerization process to provide a high proportion of alumina and silica are aluminosilicates. in most cases, a total composition of al2o3 and sio2 of more than 70% is recommended (davidovits, 1991; cioffi et al., 2003). al2o3 and sio2 make up 70–90% of the clay composition (abdullah et al., 2018). the alkaline solution acts as a reactor for the dissolution of aluminosilicates and the contribution of sio2 to the geo-polymerization reaction (singh et al., 2005). the most common alkaline activators used are naoh or koh and silicate solutions (na2sio3 or k2sio3) (xu and van deventer, 2002; barbosa et al., 2000). various types of additives could be used in the activation process to improve the properties of geopolymers, including sand grains (subaer and van riessen, 2010; bumanis et al., 2017; lemougna et al., 2011), crushed brick (barbosa & mackenzie, 2003), corundum (kamseu et al., 2010), vermiculite (medri et al., 2015), and zeolitic tuff (almjadleh et al., 2014; khoury, 2003). the use of additives depends upon the content of aluminosilicates, mineral components, and the shape and size of the grains (lukkonen et al., 2018). previous studies have shown that the use of additives in alkali activation may improve the workability of mixtures (koehler and fowler, 2009) and the physical characteristics of the products, such as mechanical strength, durability, and stability (buchwald et al., 2009. scoria aggregates have a black color, are light in weight, and have a vesicular texture, with diameters ranging from millimeters to centimeters (sen, 2014). the vesicular texture caused by volcanic gases escape during the eruption. minerals that form in hot water-rich fluids, such as zeolite, calcite, and quartz, are sometimes reintroduced into these vesicles (stiegeler, 1976). the scoria clasts oxidize and turn a dark reddish-brown color (sen, 2014). scoria contains significant amounts of alumina (al2o3) and silica (sio2), making it reactive as a pozzolanic material (swamy, 1987). as a result, it has a wide range of industrial applications, including cement replacement, lightweight concrete manufacturing, insulating materials, filter materials, absorbents, and other architectural applications (hossain & lachemi, 2010). numerous studies examined the use of volcanic materials (pozzolanic materials) such as volcanic tuff, volcanic ash, and zeolitic tuff as source materials or additives in geopolymerization. in jordan, several studies on the development of geopolymer composites have been performed with volcanic materials as additives for cement replacement and water treatment (yousef et al., 2009; hamaideh et al., 2014; al-zboon et al., 2019; alshaaer et al., 2015; al-zboon et al., 2016). the researchers noted that the composites enhanced the compressive strength and the adsorption capacity. the present work aims to compensate for the effects of adding different ratios of jordanian scoria (10%, 20%, 30%, 40%, and 50%) on the mechanical and structural properties of clay-based geopolymer in order to utilize the scoria in geopolymer production for waste water treatment. 2. materials and methods a sample of clay deposits was collected from al-azraq area in northeast jordan, approximately 110 km northeast of amman (figure 1). the collected sample was crushed, ground, and sieved to a grain size less than 63 μm, then mixed to become homogenized. to enhance the workability and reactivity, the prepared sample of clay was burned in a furnace at a temperature in the range of 660–700 °c for 2 hours. the aggregates of scoria were obtained from tall hassan area, about 125 kilometers northeast of amman. the aggregates were ground to be suitable for mixing. x-ray diffraction (xrd) analysis of clay and scoria was performed by the use of a diffractometer (shimadzu xrd-7000) at a scanning rate of 2.0°/min. the chemical compositions of both materials were obtained using an x-ray fluorescence (xrf) spectrometer (shimadzu xrf-1800). for geopolymer synthesis, sodium silicate solution (na2sio3) with silica content ranging from 25.6 to 27.6% and sodium oxide content ranging from 7.5 to 8.5% was mixed with 10 m of sodium hydroxide solution (nacl, purity 95%) to prepare an alkaline solution at a mixed ratio of 1.5. a mixture of clay and alkaline solution was prepared at a solid/liquid ratio of 1.4 by using a mixer at different speeds to give a homogenous admixture. the mix was molded in three cubic steel molds (50×50×50 mm) and cured for 24 hours in a dry oven at 80°c. a group of specimens were kept dry in lab conditions for 7 days for measuring dry density and unconfined compressive strength (ucs). a group of specimens were soaked in tap water for 24 hours to measure the water absorption percentage (wabs). for porosity measurement, the specimens were evacuated by a vacuum after drying in an oven at 105°c and then immersed in water for 24 hours. the specimens were then wiped to give them a dry surface and weighed. figure 1. location map of the materials sample area (modified after khoury et al., 2015) to examine the cation exchange capacity (cec in meq/100g) for the prepared geopolymers, methylene blue dye was used as a cationic dye popular in industrial waste water. the dry specimens of geopolymer were ground, sieved to a particle size of 425 μm, washed with deionized water to remove the excess naoh, and then dried at 105 °c. an initial concentration of methylene blue dye solution was added to the specific weight of the geopolymer. a uv-visible spectrophotometer was used for measuring the equilibrium concentrations of methylene blue dye. the specific surface area of geopolymers (ssa/m2 g-1) was determined according to the adsorption of molecules method as presented by santamarina et al. (2002). a methylene blue dye solution was used to measure the total surface area. to examine the effects of adding scoria on the characteristics of claybased geopolymers, different mixtures were prepared with different ratios of scoria (0, 10, 20, 30, 40, and 50). the experimental work was summarized as a flowchart in figure 2. figure 2. flowchart of experimental works 323influence of adding jordanian scoria on the properties of clay-based geopolymer 3. results and discussion material characterization the results of the chemical analysis of the clay deposit show that silica (sio2) and alumina (al2o3) are the major constituents of the used clay, representing 76.47% of the total contents (table 1). the sio2/al2o3 molar ratio is 4.66. the respective loss on ignition is 14.4%, which may indicate the presence of high organic content or water and amorphous phases. the chemical composition of scoria shows that the sample is mostly rich in silica (46.62%) and alumina (14.83%). in addition to iron (13.31%), calcium (9.81%), and magnesium (10.20%) oxides. the alkali content is less than 1%. al-swaidani (2018) investigated the chemical composition of scoria samples collected from several countries. the results show that the silica and alumina ranges are 40 to 60% and 10 to 20%, respectively. the iron oxide ranges between 5 and 16%, followed by magnesium (2–13%) and calcium (5–13%). the alkali content varies between 1 and 7%. table 1. major elements of the base materials oxides wt.% al2o3 cao fe2o3 k2o l.o.i mgo mno na2o sio2 so3 tio2 clay deposit 12.28 1.17 6.86 2.89 14.40 4.15 0.04 0.21 56.26 0.05 1.34 scoria 14.83 9.81 13.31 0.83 2.00 10.20 0.22 0.31 46.62 0.06 2.03 figures 3 and 4 show the xrd diffractograms of clay deposits and scoria. the results show that the dominant minerals in clay are illite, quartz, and kaolinite, with some amounts of montmorillonite. the clay sample shows broad peaks between 3–20 ◦ and 25–30 ◦ that indicate the presence of an amorphous phase in the raw clay (figure 3). the used clay is considered common clay deposits or less pure clay resources containing multiple clay minerals (khalifa et al., 2020). the mineralogical composition of scoria indicates that the detected minerals are anorthite and augite as major minerals, with the addition of calcite and forsterite minerals (figure 4). previous work on scoria has confirmed this result (djobo et al., 2014; tchakoute et al., 2015; al-swaidani, 2018; song et al., 2021). they revealed that the major minerals are plagioclase, olivine, and pyroxene. figure 3. xrd spectrum of al-azraq clay deposit. qz: quartz, il: illite, ko: kaolinite, mn: montmorillonite, and *: amorphus phase figure 4. xrd spectrum of scoria. an: anorthite, au: augite, ca: calcite, and fo: forsterite characterization of clay-based geopolymers the effects of adding different ratios of scoria to clay during alkali activation were reflected in the percentage of water absorption, porosity, dry density, and compressive strength values. figure 5 demonstrates that as the scoria ratio increases, the percentages of water absorption and porosity increase practically linearly. an increase in water absorption might be an indication that a highly porous matrix is developing. on the other side, as the scoria ratio rises, dry density and compressive strength almost linearly reduce (figure 6). the dry density of the specimen decreased due to the low density of scoria. furthermore, when the scoria ratio increases, the percentage of porosity also increases, affecting the strength of the mixtures. many researchers found that the addition of volcanic materials to the cement mortar mixture increased water absorption and reduced density due to the high void ratio and specific surface area (yourself et al., 2012; al-zboon and al-zou’by, 2014; al-zboon et al., 2019). the unreacted particles of scoria would behave as coarse aggregates in the matrix, decreasing the mechanical strength. the results of cec and ssa are shown in figure 7 and demonstrate the influence of adding scoria during the alkali activation. according to the findings, the specimen with 50% scoria had the highest cec (25.73 meq/100g) for methylene blue dye as a water contaminant, which was higher than the cec for the free-scoria specimens (21.45 meq/100g). the measured ssa indicates that, in comparison to the free-scoria specimens (22.22 m2/g), the specimen with 50% scoria had the highest ssa (29.67 m2/g). figure 5. water absorption and porosity% vs. scoria ratio 324 faten al-slaty, khalil ibrahim, sultan fayez, islam al-dabsheh, taleb odeh, shereen abusmier figure 6. usc and dry density vs. scoria ratio figure 7. cec and ssa vs. scoria ratio figure 8 shows the xrd diffractograms of the clay-based geopolyer in contrast to the geopolymer with 50% scoria. the pattern revealed the formation of mineral phases of hydroxyl sodalite and hydroxy-cancrinite, in addition to the remains of quartz and the remains of unreacted clay minerals. the broad peaks developed at 10◦-20◦ and 28◦-36◦ values in the xrd chart after adding 50% scoria. this is an indication of the partial dissolution of scoria and the formation of a gel phase in the matrix. referring to previous studies, solid and stable phases of hydroxy-sodalite, zeolite, and feldspathoids were formed after alkali activation of aluminosilicate materials due to the dissolution of si and al beside the gel phase (yousef et al. (2009), panias et al. (2007), rahier et al. (2011), slaty et al. (2013, 2015), and alshaaer et al. (2016)). figure 8. xrd patterns of clay-based geopolymer in comparison to geopolymer with scoria. qz: quartz, il: illite, ko: kaolinite, mn: montmorillonite, hs:hydroxysodalite, hc:hydroxycancrinite, and *: amorphus phase the sem micrographs of the clay-based geopolymer and the geopolymer with 50% scoria are presented in figures 9 a & b. the gel phase appeared as spheres on the surface of the activated material and filled the pore spaces. the presence of the micropores indicates the development of new cavities in the matrix. the micrographs show that the presence of scoria results in a highly fractured structure. this result is consistent with the compressive strength result. although the activation process created more cavities, scoria is considered a porous material, which provided appropriate diffusion pathways for metal ions and subsequently improved the adsorption of metals inside these pores (el-eswed et al., 2013). a b figures 9. sem micrographs a. clay-based geopolymer b. geopolymer with 50% scoria 4. conclusions this research utilized scoria as an additive material, resulting in an improvement in the removal efficacy of clay-based geopolymers for methylene blue dye. the used clay is predominantly composed of multiple clay minerals. the results revealed that adding more scoria for reaction components causes an increase in surface area and cation exchange capacity of the microstructure, thus causing a lower compressive strength and density. it can be concluded that scoria could be used as an additive material to improve geopolymer properties for environmental applications like waste water treatment. acknowledgments the authors would like to thank the hashemit university, cementra company, jordan university, and al albayt university, as well as their lab technicians, for their contributions to this work through laboratory work and analyses. references abdullah, m. m. a. b., ming, l. y., yong, h. c., & tahir, m. f. 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(2023). magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia. earth sciences research journal, 27(1), 11-25 https://doi.org/10.15446/ esrj.v27n1.102683 record manuscript received: 16/05/2022 accepted for publication: 03/03/2023 keywords: magnetite; oxygen fugacity; geothermobarometry; pegmatites; ilmenite. abstract in the sector of san jose, macanal, and tabaquen, in eastern colombia, granitic rocks cut by pegmatite dikes and quartz veins appear with the presence of magnetite, ilmenite, and ilmenorutile. using magnetic petrology and geochemistry concepts and methods, the main objective is to determine if these types of rocks are genetically related and how the fluid chemically evolves during its crystallization and cooling. this work was conducted in three stages. petrography and opaque metallography for identifying the occurrence, paragenesis, and secondary processes that transform the oxides. in a second stage and utilizing an electron probe microanalyzer (epma), 214 quantitative analyses (wds) and four compositional maps for magnetite, ilmenite, and ilmenorutile were performed, measuring the oxides feo, tio2, v2o3, mgo, mno, nb2o5, ta2o5, al2o3, ga2o3, nio, cao, cr2o3, sno, and wo3. since magnetite and ilmenite are favorable geothermometers that also allow the calculation of oxygen fugacity, the ilmat program was used to calculate these values. in closing, integrate the data with the magnetic susceptibility values. the results determine crystallization temperatures between 358-414 °c for the granitic-host rock and 402499 °c for pegmatites dykes, in a system where oxygen fugacity increases, the mn2+ is enriched in the ilmenite, and magnetite preserves a low content of trace elements thorough the evolution of the fluid. taken together with the martitization and exsolution of hematite and rutile within ilmenite found in the petrography, these results allow us to conclude that an oxide-silicate re-equilibration process controls the evolution of this magmatic-hydrothermal fluid with a kuilb cooling trend-type reaction. based on the al + mn vs. ti + v ratio, the signature of the magnetite is like the lucky friday mine’s signature studied by nadoll. however, the analysis of the 95th percentile shows a different concentration of trace elements in the magnetite of both sectors. therefore, a new field of discrimination is proposed for this environment of anorogenic pegmatites of the nyf family. finally, the magnetic susceptibility is controlled only by the abundance of magnetite in each type of rock. the granitic host rocks have the highest susceptibility values, followed by pegmatites and quartz veins with the lowest. resumen en el sector de san josé, macanal y tabaquén, en el oriente colombiano, afloran rocas graníticas cortadas por diques pegmatíticos y venas de cuarzo con presencia de magnetita, ilmenita e ilemnorutilo, estos tipos de rocas se encuentran relacionadas genéticamente. la magnetita y la ilmenita son buenos geotermómetros y permiten calcular la fugacidad de oxígeno con base en el contenido de elementos trazas. con la microsonda electrónica epma se realizaron 214 análisis cuantitativos (wds) y 4 mapas composicionales en estos tres minerales en los que se miden 14 elementos químicos. los resultados permiten suponer temperaturas de cristalización entre 358 y 414°c para roca caja y 402 – 499°c en pegmatitas, un incremento de la fugacidad de oxígeno en el sistema, un enriquecimiento de mn 2+ en la ilmenita y un bajo contenido de elementos trazas en la magnetita. estos resultados, sumados al registro de martitización y exsolución de hematita y rutilo en la ilmenita, permiten concluir que la evolución del fluido magmático – hidrotermal está siendo controlada por la reacción de reequilibrio kuilb del tipo óxido –silicato. la relación al+mn vs ti+v ubica la firma de la magnetita en el campo de la mina lucky friday estudiado por nadoll, sin embargo, el análisis del percentil 95 muestra un comportamiento distinto en los elementos trazas de la magnetita de ambos sectores, por lo tanto, se propone un nuevo campo de discriminación para este ambiente de pegmatitas anorogénicas de la familia nyf. por último, la susceptibilidad magnética está controlada únicamente por la abundancia de la magnetita en cada tipo de roca, encontrando que los granitos tienen los valores más altos de susceptibilidad, seguidas por las pegmatitas y por último las venas de cuarzo. https://doi.org/10.15446/esrj.v27n1.102683 mailto:cjcharryc@unal.edu.co https://doi.org/10.15446/esrj.v27n1.102683 https://doi.org/10.15446/esrj.v27n1.102683 12 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda 1. introduction in a previous study under agreement #06 between the colombian geological service (from now as cgs) and the universidad nacional de colombia, mesoproterozoic gneisses, quartz veins, and pegmatitic dykes that cut granitic bodies belonging to the mitú migmatitic complex unit, were found and extracted along the guainía river (guerrero et al., 2017). the translucent minerals presented are mainly quartz, tourmaline, microcline, plagioclase, biotite, and muscovite. while the opaque minerals are magnetite, ilmenite, hematite, chalcopyrite, and pyrite in low content. the presence of ilmenorutile was also reported (guerrero et al., 2017), hosted especially on pegmatites with considerable percentages of niobium and tantalum, mainly due to the occurrence of columbite inclusions. figure 1. location map of the study area presenting where the samples were taken and the municipalities of tabaquén, san josé, and macanal. in the lower right map, the department of guainía and in red, the corregimiento of puerto colombia. the map is modified from geoportal.igac.gov.co. magnetite and ilmenite are petrogenetic indicators that allow calculating genetic parameters such as temperature and oxygen fugacity for a more thorough understanding of the magmatic-hydrothermal fluid evolution and thereby, contribute to the geological knowledge of the colombian amazonian craton, a core objective of this work. additionally, based on magnetic petrology concepts, these parameters allow identifying the behavior of the trace elements reflected in the crystallization of the oxides. petrography and metallography methods were used to analyze twenty-five thin polished sections provided by the cgs, complemented with magnetic susceptibility values measured in hand samples, to establish possible petrological relationships for the area evaluated. the results obtained from this work were also complemented by a microthermometry and translucent minerals chemistry analysis, provided by the other participants of the agreement. 2. methodology using a reflected/transmitted light olympus cx31 microscopy, twentyfive thin sections of granites, pegmatites, quartz veins, and gneisses were used in petrography and metallography for identifying mineralogy, textural variations, the occurrence of oxides, secondary processes and paragenetic relations. eleven samples were selected for chemical analysis distributed in three samples of the granitic host-rock, seven pegmatite dykes, and one quartz vein. figure 2. backscattered electrons (bse) photographs showing four of the five types of magnetite-ilmenite pairs classified. due to the secondary processes’ degree like oxidations or exsolutions, magnetite, ilmenite and ilmenorutile crystals with the less alterations were used to obtain the most accurate results. this is because those secondary processes can modify the trace element content due to their partition coefficient. additionally, the magnetite and ilmenite crystals were paired and classified into five types optically defined as in paragenesis (figure 2), ensuring that both minerals are in chemical equilibrium: • types i and ii (24 samples) pairs are in paragenesis, the difference is that in type i none present alteration while in type 2, one or both encounter a lower degree of alteration processes • type iii (2 samples) and type iv (4 samples) pairs are not in contact but remarkably close; in type iii, none of the crystals are altered, while in type iv, either one or both are altered. • type v (2 samples) are crystals whose occurrence is unique; they are not in paragenesis, they appear oxidized and therefore do not contribute to the study’s conclusions the 215 quantitative analyzes measured with a wavelength-dispersive spectrometer (wds) were taken at the edge, center, and an intermediate point of each crystal to identify zoning. the compositional maps were obtained by qualitative measures with the energy dispersive spectrometer (eds) of fe, ti, sn and w, distributed in one magnetite, two ilmenite, and one ilmenorutile. both data types were collected with the electron probe microanalyzer (epma) jeol jxa-8230 in the department of geosciences of the universidad nacional de colombia, with a current of 20 a. the other measurement parameters are described in table 1. the original feo values were recalculated with the ilmat program (lepage, 2003) to get both fe2+ and fe3+ content. it should be considered that the formulas used for magnetite are different for ilmenite and hematite since they are part of different isomorphic series and their fe2+/fe3+ ratio are different. considering the average of three measures for each crystal, once the ferric and ferrous ion values were recalculated, an average was calculated and the crystals whose total compositional closure is between 99.0 and 101.0% were used for further calculations. the magnetic susceptibility of each sample was measured to determine correlations between the abundance of magnetic minerals and their composition. for this purpose, three measurements were obtained in different surfaces for each sample to obtain an average. the data was obtained with a zh instrument sm30 device with a sensitivity of 1×10-7 si units (zh instruments, 2008) http://geoportal.igac.gov.co 13magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia 3. geological framework 3.1 regional geology the amazonian craton is part of the nucleus of the south american continent. it is geographically divided by the guiana shield located at the north and the guapore shield at the south. based on dating from multiple methods, several authors have divided the craton into various geological provinces, including a six provinces model by tassinari and macambira (1999) and an eight provinces model by santos et al. (2008) (figure 3a); the eight provinces model is currently accepted and used by the geological service of brazil (hasui et al., 2012), hence is the one based for this work. the rocks hosted in the guyana shield show a mylonitization event generated by sinistral efforts, an oblique compression, and intense failure produced during the sunsas orogeny (1450-1100 ma). this transpression phenomenon has been identified as the k’mudku mylonitic event or orinoquiense event known in venezuela. samples collected in this area show dynamic metamorphism that might be related to this k’mudku event (santos et al., 2008). rio negro province defined as rio negro-juruena by tassinari and macambira (1999), santos et al. (2008) later subdivided it into the rio negro province, in the north of brazil, and the rondonia-juruena province, in the south. was accrued in the mesoproterozoic (1.5-1.3 ga ago) and outcrops between eastern colombia, specifically in guainia and vaupes departments, southwest venezuela and northwest of brazil. it is constituted by the paleoproterozoic tiquie suite unit composed of monzogranites, syenogranites, and alkaline granites of 1750 ma; the metamorphic tunui group of 1700-1520 ma, and the cumati complex consisting of orthogneisses, granodiorites, and tonalites of 1700 ma. these units are intruded by the granitic inhamoim and rio içana suite of 1540 ma (hasui et al., 2012). anorogenic a-type granites with rapakivi texture have also been reported, with syenites, mangerites, and charnockites with great economic interest due to the occurrence of strategic minerals composed of niobium and tantalum (tassinari & macambira, 1999). based on u-pb, sm-nd, and rb-sr datings, cordani et al. (2016) separated the rio negro province in geochronological belts, including the atapabo and vaupes belts. the atapabo belt consists of granitoid formed between 1800 1740 ma and is intruded by 1.5 ga granitic bodies. the vaupes belt is constated of granitoid, syntectonic gneisses, and migmatites in amphibolite facies, with an age of 1580-1520 ma (figure 3b). 3.2 local geology: mitu migmatitic complex the granitic rocks outcropping in the study area belong to the mitu migmatitic complex, whom genesis is associated with the rio negro province accretion and is subsequently affected by an anorogenic magmatism between 1575 and 1450 ma (cordani et al., 2016; lopez & cramer, 2014). this complex is divided into the pringamosa granite, which presents granitic composition with biotite and migmatitic structures; the gneiss del yi with migmatitic texture and alkaline eye-shaped feldspar surrounded by biotite and quartz; and the monzogranite of mitu that presents phaneritelike, inequigranular texture, and a slight orientation (rodríguez et al., 2011). table 1. parameters established for quantitative analysis (wds) in the electronic microwave (epma). element mg (kα) al (kα) ga (lα) ni (kα) ti (kα) ca (kα) sn (lα) nb (lα) v (kα) cr (kα) mn (kα) fe (kα) ta (lα) w (lα) standard mgo kyanite gaas sheet of ni ilmenite diopside cassiterite sheet of nb sheet of v metallic cr spessartine hematite sheet of ta sheet of w current 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 channel and spectrometer 1 tap 1 tap 1 tap 3 lifh 3 peth 3 peth 3 peth 3 peth 3 lifh 3 lifh 3 lifh 3 lifh 3 lifh 3 lifh peak time (s) 60.0 20.0 60.0 60.0 20.0 20.0 20.0 20.0 60.0 20.0 60.0 20.0 20.0 20.0 background time (s) 30.0 10.0 30.0 30.0 10.0 10.0 10.0 10.0 30.0 10.0 30.0 10.0 10.0 10.0 figure 3. a. map of geological provinces, taken from santos et al. (2008). b. geochronological belts in the rio negro province between colombia, venezuela, and brazil, proposed by cordani et al. (2016). 14 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda gneisses of the complex present rb-sr ages between 1557 and 1758 ma, and u-pb ages between 1480±70 and 1846±45 ma (bonilla, 1981; rodriguez et al., 2011). during the fieldwork, granitic rocks with inequigranular to equigranular texture were found. the composition varies between monzogranite, syenogranite, and alkaline feldspar granite, with medium to thick crystals of alkaline feldspar, quartz, plagioclase, biotite, ilmenite, and magnetite. the pegmatites cut the granites along discontinuous dykes with diffuse edges; some have lenticular geometries with thicknesses less than 35 cm and a length between 2 cm to 4-5 m. do not present visible zoning and their texture is uneven with crystals up to 3 cm. its composition is similar as granitic rocks; however, they contain more quartz, tourmaline and ilmenorutile. structurally, have an n-s, e-w, and nw-se orientation, where n-s orientation dikes have a higher ilmenorutile content, which means a more significant economic interest. those discontinuous dikes of up to 3 m and thicknesses between 3 and 50 cm present quartz veins. they are predominantly composed of hyaline and milky quartz, tourmaline, hematite, epidote, and in some cases, plagioclase, muscovite, and ilmenite. some have drusy or massive growth textures. finally, metamorphic rocks also emerge, such as granitic orthogneisses and paragneisses with micas, tourmaline, and quartz in boudinage structure. figure 4. pictures took in the field stage. a. quartz vein sample with drusy and epidote texture. b. pegmatitic dike with quartz, alkaline feldspar, biotite, and possible ilmenorutile. c. paragneiss outcrop with micas and quartz in boudinage structures. d. rock box granite composition sample. 4. geochemistry of iron-titanium oxides the iron-titanium oxide system is represented in a ternary diagram with tio2 (rutile), fe2o3 (hematite/maghemite), and feo (wustite). this group of minerals has a significant petrological value since they are good as geothermometers and geobarometers. there are three isomorphic series in this ternary system: titanomagnetite, titanohematite/titanomaghemite, and pseudobrookite. the titanomagnetite varies from magnetite to ulvöspinel, ranging from ferromagnetic to antiferromagnetic, while titanohematites from hematite to ilmenite and are antiferromagnetic. the ilmenorutile can be considered as a variety of rutile enriched in niobium or a mineral belonging to the strüverit series, the compositional extreme with tantalum. 4.1 magnetite trace elements in magnetite can be used to discriminate its origin thanks to their partition during crystallization in magmas or hydrothermal fluids (dare et al., 2014). the coexistence of a crystal from the magnetite–ulvöspinel series with a crystal from the ilmenite–hematite series, are utilized as a petrogenetic indicator because function as geothermometers and geobarometers. it also allows obtaining oxygen fugacity (fo2) values. seen in a t-fo2 diagram, a pair of crystals from the two genetically related mineral series will obtain an intersection of their respective isopleths at a specific temperature and oxygen fugacity point (bowles et al., 2011). magnetic petrology magnetic petrology represents the union between conventional petrology and magnetic susceptibility of rocks, focused on characterizing composition, abundance, microstructure, and paragenesis of magnetic minerals. it also contributes the knowledge of those processes that give rise to genesis, alteration, and destruction of magnetic minerals in rocks (clark, 1997). the confidence level of the results will depend on the pair of crystals’ equilibrium. therefore, three factors must be considered: (1) the paragenesis and textures of the crystals. (2) identifying characteristics that determine a change in chemistry and infer changes of temperature and oxygen fugacity of the system such as multiple mineralizing events, alterations and exsolutions such as martitization, mushketovitization, or oxy-exsolution. these can be identified in the petrography. (3) the ratio of the log calculated on magnetite and ilmenite (mg/mn). the magnesium and manganese partition in both minerals are independent of the oxygen fugacity, therefore, allow to graphically identify if both minerals are in equilibrium if the regression of a continuous line with a positive slope is closest to 1. bacon & hirschmann (1988) found this relationship and stated that commonly occurs in fresh volcanic rocks. this work also applies the bacon & hirschmann method in pegmatitic rock. the iron will have an affinity to occur as a native mineral, silicate, or oxide. there are three primary buffers reactions that control this behavior: hm, fmq, and qif (equations 1, 2, and 3). these do not occur in nature but explain the behavior of iron depending on the t and fo2 of the environment (frost, 1991; nadoll et al., 2014). (hm) 6fe2o3 (hematite) = 4fe3o4 (magnetite) + o2 (1) (fmq) 2fe3o4 (magnetite) + sio2 = fe2sio4 (fayalita) + o2 (2) (qif) fe2sio4 (fayalita) = 2fe + sio2 (quartz) + o2 (3) when the temperature of the fluid decreases, rebalancing reactions occur. these define the partition of the minerals, the behavior of the elements, and the processes of exsolution or oxidation (frost, 1991b). the rebalancing processes can be the oxide-oxide type, where a titanium exchange occurs; the inter-oxide type, creating an oxyexysolution process which is seen as the exsolution of ilmenite lamellae within the titanomagnetite; an oxide-silicate type described by the reactions quilf, quilo and kuilb (equations 4, 5 and 6 respectively) (frost et al., 1988; ghiorso & sack, 1991; lindsley et al., 1990). (quilf) fe2sio4 (fayalite) + 2fe2tio4 (ulvospinel) = 2fetio3 (ilmenite) + sio2 (4) (quilo) 2sio2 + 2fe2tio4 (ulvospinel) = 2fetio3 (ilmenite) + fe2si2o6 (orthopiroxen) (5) (kuilb) kfe3alsi3o10(oh)2 (biotite) + o2 = 2kalsi3o8 (kfs) + 2fe3o4 (magnetite) + 2h2o (6) 6. results 6.1 petrography and metallography the petrography presented general and specific textures, composition, oxidation processes, occurrence, and characteristics of the oxides were identified (table 3). the percentages of the rock-forming minerals were complemented with those presented in bayona-valderrama (2023), mora-galindo (2023), and cardona (2023), and then, used in the streckeisen (1967) ternary diagram of the classification of igneous rocks. the variation in host rocks’ composition allows classifying them into two types. type 1 rocks vary between monzodiorite and monzogranite, outcrop in the north part of the study area, have smaller crystal sizes, and may contain subordinate ilmenorutile. the chalcopyrite is altered to covelline. the type 2 rocks vary from monzogranite to alkali feldspar granite composition, with larger crystals and are mainly composed by plagioclase. biotite and magnetite appear in smaller sizes and may include monazite. 15magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia the composition of the pegmatitic dykes is mainly monzogranitic to alkali feldspar granite. the microcline is the principal translucent mineral, appearing as subhedral crystals of thick size grains. it has a slight degree of alteration, perthitic texture, plagioclase inclusions, and no zoning. the plagioclase is less abundant, may show polysynthetic or carlsbad twinning and has a greater alteration degree into sericite. the biotite has euhedral forms and is predominantly chloritized. tourmaline is from the black variety, appearing scattered in the pegmatites and towards the contact between quartz vein and the host rock. accessory minerals include apatite and zircon, which are usually in paragenesis with micas and oxides. the composition of quartz veins is monzogranitic. is important to highlight that, according to the information collected during the field phase, rocks found in veins with a high quartz content, some with drusy texture, are called “veins”. however, both in macro and petrography, those quartz veins can present subordinate crystals of plagioclase, muscovite, or microcline. most oxides appear very related to micas. in general, ilmenorutile is not in contact with magnetite or ilmenite. the occurrence of oxides varies according to the type of rock: • magnetite is the principal oxide, with an average of 2.39% in host rock and 2.24% in pegmatites. both in the host rock and the pegmatite, it appears partially oxidized to hematite, mainly around fractures and exfoliation planes. within the quartz veins, magnetite is turned into hematite, which indicates that in the evolution of the fluid, the oxygen fugacity has increased. in most cases, it does present subhedral to anhedral geometries and does not present inclusions or exsolutions. • the abundance of ilmenite is 2.46% in the host rock, 1.11% in pegmatites, and 0.75% in quartz veins. it presents subhedral to anhedral shapes, usually elongated. most crystals contain hematite exsolution and, to a lesser extent, rutile. this exsolution process is presented either with sub-parallel oval shapes or disorderly (figure 5d). in some cases, it can be perceived that hematite exsolutions present one more exsolution of another mineral that may correspond to ilmenite-hematite (mcenroe et al., 2007). • ilmenorutile appears mainly in pegmatites with a 1.25% abundance, while in the host rock, it is 0.04%. it has subhedral to euhedral geometries, with no alterations or exsolutions. occasionally, presents columbite inclusions. • all hematite crystals found are secondary, coming from the magnetite oxidation or ilmenite exsolution. the rate of hematite crystals is 1.26% in the host rock, 1.50% in pegmatites, and 0.75% in quartz veins. the magnetite/hematite ratio is lower in the quartz veins because of high oxidation. figure 5. micro-pictures of the oxides. hm: hematite, mag: magnetite, ilmrt: ilmenorutile, qz: quartz. a: (sample pg_2) magnetite-ilmenite pair type ii, the hematite is around the edges and fractures due to oxidation, in xpr. b: (sample vn_1) tourmaline and hematite located in the contact between the quartz vein (left) that cuts the monzogranite (right), in ppl. c: (sample pg_2) hematite exsolution with elongated and subparallel shapes, in xpr. d: (sample rc_3) disorderly hematite exsolution in ilmenite in paragenesis with magnetite completely oxidized to hematite, in xpr. e: (sample rc_3) magnetite crystal with a high degree martitization, in xpr. f: (sample pg_5) back scattered electron (bse) image of columbite included on ilmenorutile. 6.2. electron probe micro-analyzer (epma) from the 215 points analyzed at the center, edge, and intermediate point for each crystal, 123 fell in the closing range between 99.0 and 101.0%, which is the one accepted for this work. the results are reported in tables 4 to 10. since this analytical technique does not discriminate iron ions, the ferrous and ferric iron values were recalculated from the feo data obtained. then, those values were used for petrological calculations in the ilmat program and the measure of empirical formulas of oxides (table 2). table 2. empirical formulas for each oxide according to its hosted rock. type of rock mineral empirical formula host rock mag (fe2+ 1.00fe3+ 1.99)3o4 ilm (fe3+ 0.04mn2+ 0.15)fe2+ 0.82ti4+ 0.98o3 ilmrt (ti4+ 0.83nb3+ 0.13fe2+ 0.13)o2 pegmatite dykes mag (fe2+ 1.02fe3+ 1.98)3o4 ilm (fe3+ 0.06mn2+ 0.16)fe2+ 0.81ti4+ 0.97o3 ilmrt (ti4+ 0.83nb3+ 0.14fe2+ 0.13)o2 quartz veins mag (fe2+ 0.99fe3+ 1.99)3o4 ilm (mn2+ 0.43)fe2+ 0.57ti4+ 1.00o3 table 3. mineralogical distribution of the 11 samples analyzed with epma; the data was obtained by petrography. qz: quartz; mc: microcline; pl: plagioclase; ms: muscovite; bt: biotite; grt: garnet; tur: tourmaline; zrn: zircon; ilm: ilmenite; mg: magnetite; py: pyrite; ccp: chalcopyrite; hm: hematite, ilmrt: ilmenorutile. code code sgc type of rock composition rockforming minerals opaque minerals rc_1 2936 host rock alkali feldspar granite qz, mc, pl, ms, bt, grt, zrn ilm, mg, py, ccp rc_2 2843 host rock monzogranite qz, pl, mc, bt ilm, hm, py, ccp vn_1 2861 host rock monzogranite qz, mc, pl, ms, tur mag, hm, py, ccp quartz vein qz, tur ilm, mag, hm pg_1 2895 pegmatite dyke monzogranite qz, mc, pl, ms, bt, ap, zrn ilm, mag, ilmrt pg_2 2807 pegmatite dyke alkali feldspar granite qz, mc, ms, tur, zrn ilmrt, ilm, mag pg_3 2820 pegmatite dyke monzodiorite qz, mc, ms, pl ilm, mag pg_4 2972 pegmatite dyke alkali feldspar granite qz, pl, mc, ms, bt, chl ilm, mag pg_5 2846 pegmatite dyke monzogranite qz, mc, bt, ms ilmrt, ilm, mag, ccp pg_6 2883 pegmatite dyke monzogranite qz, mc, pl, ms, bt ilmrt, mag pg_7 2995 pegmatite dyke alkali feldspar granite qz, mc, pl, grt, bt ilm, mag, hm, py, ccp pg_8 3006 pegmatite dyke monzogranite qz, mc, pl, ms, grt mag, hm, py 16 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda table 4. quantitative results of magnetites in host rock samples. the oxides content is in wt%, and the fe2o3 and feo values are recalculated from feo*. b: edge; c: center; m: intermediate point. section tio2 al2o3 feo* fe2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total 28 95 m 0.02 0.05 92.05 68.16 30.72 0.06 0.00 0.00 0.03 0.07 0.03 0.02 0.00 0.00 0.02 0.00 99.16 c 0.03 0.04 92.23 68.29 30.77 0.06 0.00 0.01 0.01 0.08 0.01 0.00 0.00 0.00 0.00 0.01 99.31 b 0.13 0.01 91.96 67.99 30.79 0.04 0.00 0.02 0.01 0.03 0.02 0.00 0.02 0.00 0.03 0.01 99.09 c 0.02 0.04 92.30 68.35 30.80 0.03 0.04 0.01 0.03 0.06 0.00 0.00 0.00 0.00 0.00 0.02 99.39 b 0.03 0.02 93.17 68.99 31.09 0.04 0.01 0.02 0.03 0.07 0.00 0.00 0.01 0.00 0.00 0.02 100.32 c 0.04 0.01 93.10 68.94 31.07 0.05 0.02 0.01 0.03 0.04 0.00 0.00 0.00 0.00 0.00 0.01 100.21 29 36 m 0.01 0.08 91.89 68.00 30.69 0.04 0.02 0.00 0.10 0.07 0.01 0.03 0.03 0.00 0.00 0.00 99.07 c 0.15 0.07 92.09 68.01 30.90 0.03 0.02 0.04 0.11 0.03 0.00 0.00 0.00 0.03 0.00 0.00 99.39 c 0.08 0.04 92.37 68.29 30.92 0.03 0.00 0.01 0.09 0.05 0.02 0.01 0.01 0.00 0.00 0.00 99.55 b 0.46 0.05 92.35 67.78 31.37 0.03 0.01 0.00 0.06 0.06 0.00 0.00 0.00 0.01 0.00 0.03 99.85 m 0.01 0.04 92.47 68.47 30.86 0.05 0.74 0.00 0.09 0.04 0.00 0.01 0.02 0.00 0.00 0.06 100.39 c 0.00 0.05 92.38 68.41 30.82 0.04 0.00 0.00 0.07 0.04 0.00 0.01 0.03 0.00 0.00 0.00 99.47 b 0.03 0.06 92.18 68.33 30.69 0.05 0.00 0.14 0.10 0.06 0.00 0.00 0.03 0.00 0.00 0.00 99.48 m 0.02 0.11 92.56 68.48 30.94 0.05 0.00 0.00 0.10 0.05 0.00 0.03 0.00 0.00 0.02 0.00 99.79 c 0.01 0.09 92.94 68.78 31.05 0.04 0.00 0.01 0.09 0.06 0.01 0.00 0.00 0.00 0.02 0.00 100.15 table 5. quantitative results of magnetites in pegmatite samples. the oxides appear in wt%. for the total value calculation, both fe2o3 and feo recalculated were used. b: edge; c: center; m: intermediate point. section tio2 al2o3 feo* fe2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total 28 07 b 0.11 0.05 91.90 67.91 30.80 0.04 0.00 0.01 0.04 0.10 0.00 0.01 0.04 0.00 0.00 0.00 99.10 m 0.00 0.02 92.04 68.17 30.70 0.03 0.00 0.01 0.01 0.10 0.00 0.00 0.04 0.00 0.00 0.00 99.06 29 72 b 0.60 0.03 92.25 67.54 31.48 0.03 0.01 0.00 0.01 0.03 0.00 0.01 0.00 0.01 0.04 0.01 99.79 b 0.05 0.05 92.22 68.25 30.81 0.03 0.00 0.00 0.01 0.03 0.00 0.03 0.00 0.01 0.00 0.04 99.31 m 0.16 0.07 91.98 67.91 30.87 0.03 0.00 0.01 0.00 0.04 0.00 0.01 0.00 0.02 0.04 0.00 99.15 c 0.05 0.10 92.50 68.44 30.92 0.04 0.00 0.00 0.01 0.02 0.01 0.02 0.00 0.00 0.02 0.00 99.63 b 0.04 0.07 92.37 68.37 30.85 0.03 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.06 0.04 99.47 m 1.92 0.07 90.93 64.80 32.62 0.04 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 99.46 c 0.02 0.07 92.57 68.55 30.89 0.04 0.00 0.00 0.00 0.02 0.00 0.02 0.00 0.00 0.00 0.01 99.62 28 20 b 0.21 0.60 93.05 68.41 31.49 0.09 0.01 0.01 0.01 0.02 0.01 0.00 0.03 0.00 0.00 0.00 100.88 c 0.16 0.97 92.62 67.95 31.48 0.08 0.00 0.00 0.00 0.03 0.00 0.00 0.06 0.01 0.00 0.00 100.75 m 0.00 0.20 93.44 69.25 31.13 0.20 0.00 0.00 0.00 0.06 0.00 0.00 0.04 0.00 0.00 0.00 100.90 28 46 b 0.23 0.05 92.61 68.30 31.15 0.03 0.01 0.00 0.01 0.01 0.01 0.01 0.05 0.00 0.00 0.00 99.86 c 0.01 0.06 93.18 69.02 31.08 0.05 0.00 0.02 0.00 0.06 0.00 0.00 0.01 0.00 0.05 0.00 100.36 b 0.19 0.05 92.40 68.21 31.03 0.05 0.00 0.00 0.00 0.03 0.01 0.02 0.00 0.01 0.06 0.00 99.65 m 0.02 0.05 92.41 68.44 30.83 0.04 0.03 0.00 0.01 0.05 0.01 0.00 0.04 0.00 0.01 0.00 99.53 c 0.01 2.05 91.03 66.37 31.31 0.04 0.02 0.00 0.02 0.04 0.00 0.02 0.00 0.03 0.00 0.00 99.92 28 83 b 0.08 0.02 92.20 68.24 30.79 0.07 0.00 0.02 0.03 0.05 0.00 0.00 0.00 0.00 0.03 0.02 99.35 m 0.73 0.06 90.89 66.83 30.75 0.70 0.00 0.00 0.02 0.05 0.00 0.00 0.02 0.00 0.00 0.02 99.19 c 0.01 0.00 92.35 68.49 30.73 0.12 0.01 0.01 0.00 0.06 0.00 0.00 0.04 0.03 0.00 0.07 99.57 c 0.03 0.08 92.02 68.16 30.69 0.10 0.01 0.02 0.01 0.07 0.00 0.00 0.00 0.01 0.02 0.02 99.23 m 0.12 0.02 92.38 68.32 30.90 0.11 0.00 0.01 0.02 0.07 0.01 0.00 0.00 0.01 0.01 0.00 99.59 c 0.01 0.04 92.29 68.41 30.74 0.12 0.00 0.01 0.02 0.08 0.01 0.00 0.00 0.00 0.00 0.00 99.43 b 0.04 0.07 93.26 69.04 31.14 0.09 0.00 0.01 0.00 0.09 0.00 0.00 0.01 0.00 0.02 0.00 100.52 m 0.06 0.07 93.61 69.30 31.25 0.14 0.00 0.00 0.01 0.10 0.00 0.04 0.00 0.00 0.03 0.00 101.00 30 06 b 0.02 0.01 93.66 69.37 31.24 0.02 0.01 0.00 0.01 0.02 0.00 0.00 0.05 0.00 0.00 0.00 100.75 17magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia table 6. quantitative results of ilmenites in host rock samples. the oxides appear in wt%. for the total value calculation, both fe2o3 and feo recalculated were used. b: edge; c: center; m: intermediate point. section tio2 al2o3 fe2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total 28 95 c 51.55 0.00 1.66 34.56 11.57 0.00 0.08 0.00 0.00 0.00 0.19 0.00 0.00 0.00 0.03 99.63 c 52.01 0.01 1.61 35.82 10.71 0.00 0.10 0.03 0.00 0.01 0.13 0.00 0.00 0.04 0.00 100.47 b 51.02 0.00 2.81 37.09 8.43 0.04 0.16 0.00 0.00 0.01 0.35 0.01 0.01 0.02 0.02 99.97 m 52.16 0.00 1.49 37.38 9.08 0.04 0.22 0.02 0.00 0.00 0.33 0.02 0.00 0.03 0.07 100.82 b 51.00 0.03 3.52 37.12 8.18 0.04 0.31 0.02 0.00 0.00 0.38 0.02 0.00 0.03 0.01 100.66 c 51.70 0.00 2.27 38.15 8.15 0.04 0.02 0.00 0.00 0.00 0.38 0.00 0.00 0.01 0.04 100.75 29 36 b 50.55 0.00 3.16 40.23 5.16 0.00 0.01 0.02 0.00 0.00 0.25 0.00 0.00 0.01 0.00 99.39 c 51.09 0.00 2.22 40.45 5.43 0.00 0.00 0.00 0.00 0.01 0.21 0.00 0.02 0.00 0.00 99.43 b 50.93 0.00 2.81 39.82 5.88 0.02 0.00 0.00 0.00 0.01 0.17 0.00 0.04 0.00 0.00 99.67 b 51.37 0.00 2.44 40.16 5.94 0.02 0.00 0.00 0.00 0.00 0.22 0.00 0.00 0.00 0.00 100.16 c 51.01 0.01 3.23 40.11 5.70 0.00 0.01 0.00 0.00 0.01 0.25 0.00 0.03 0.01 0.07 100.42 b 51.72 0.00 1.38 40.79 5.62 0.03 0.00 0.00 0.00 0.01 0.21 0.03 0.00 0.01 0.00 99.78 c 51.79 0.00 1.36 40.83 5.68 0.00 0.01 0.00 0.00 0.01 0.21 0.01 0.00 0.00 0.04 99.94 c 51.54 0.01 1.32 40.47 5.74 0.05 0.00 0.02 0.00 0.02 0.22 0.03 0.01 0.00 0.00 99.40 c 53.17 0.00 0.00 41.84 5.88 0.02 0.01 0.00 0.00 0.01 0.13 0.00 0.01 0.00 0.00 101.06 b 52.25 0.01 1.35 40.59 6.33 0.00 0.00 0.01 0.00 0.00 0.15 0.01 0.00 0.03 0.00 100.73 table 7. quantitative results of ilmenites in pegmatite samples. the oxides appear in wt%. for the total value calculation, both fe2o3 and feo recalculated were used. b: edge; c: center; m: intermediate point. section tio2 al2o3 fe2o3 feo mno mgo cao cr2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total 2807 b 49.37 0.01 5.19 41.27 3.07 0.00 0.03 0.01 0.01 0.45 0.03 99.44 m 47.33 9.51 39.45 2.99 0.03 0.03 0.01 0.00 0.42 0.00 0.03 0.03 0.00 99.84 c 47.77 8.59 39.81 3.09 0.00 0.02 0.00 0.64 0.04 99.97 b 51.93 0.02 1.12 34.64 11.90 0.01 0.00 0.00 0.01 0.04 0.01 99.57 m 52.04 0.01 0.57 35.64 11.02 0.01 0.01 0.03 0.06 0.02 0.04 0.02 99.40 c 51.51 0.03 1.37 34.84 11.28 0.03 0.01 0.02 0.07 0.03 0.01 99.06 b 51.40 0.00 2.28 40.15 6.01 0.01 0.08 0.02 0.03 99.98 c 50.72 3.38 40.04 5.49 0.01 0.00 0.03 0.02 0.03 99.72 m 51.00 2.61 39.57 6.20 0.02 0.01 0.00 0.26 0.07 0.03 99.76 c 50.05 0.01 4.33 39.74 5.21 0.00 0.01 0.31 0.02 0.03 99.70 b 50.04 0.01 4.05 41.41 3.56 0.00 0.01 0.49 0.03 99.60 m 50.39 3.89 41.67 3.57 0.03 0.42 0.03 100.00 b 51.62 1.66 40.07 6.27 0.01 0.01 0.36 0.01 0.01 0.03 100.04 c 51.30 0.01 2.57 40.08 5.98 0.01 0.02 0.32 0.02 0.04 100.33 b 51.09 0.01 2.70 40.18 5.70 0.01 0.02 0.35 0.00 0.00 0.04 0.00 100.11 2972 b 49.29 0.03 5.82 39.26 4.99 0.01 0.01 0.50 0.03 99.92 m 48.70 0.02 7.69 38.36 5.38 0.02 0.01 0.50 0.01 100.69 c 48.24 0.01 8.22 38.43 4.89 0.01 0.02 0.00 0.50 0.00 100.30 c 51.86 0.02 0.47 39.09 7.46 0.01 0.00 0.28 0.02 0.04 99.24 b 52.12 0.02 0.49 40.11 6.68 0.01 0.02 0.34 0.00 99.78 b 52.78 0.00 40.91 6.47 0.00 0.01 0.00 0.26 0.01 0.00 100.45 b 51.59 1.70 39.53 6.79 0.00 0.00 0.01 0.00 0.44 0.04 0.00 100.10 m 51.86 1.60 39.78 6.77 0.01 0.00 0.00 0.41 0.02 0.03 0.03 100.51 2883 m 52.96 0.00 0.00 19.31 27.93 0.01 0.02 0.01 0.34 0.02 0.04 100.62 2995 b 51.64 0.01 0.84 36.68 9.63 0.01 0.00 0.97 0.02 0.03 0.18 0.01 100.02 b 52.69 0.02 0.00 33.62 13.59 0.02 0.00 0.01 0.31 0.01 0.02 0.04 100.33 c 52.68 0.02 33.61 13.59 0.01 0.02 0.36 0.00 0.04 0.03 100.35 18 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda table 8. quantitative results of ilmenorutile in a sample of the host rock (2895) and two samples of pegmatites (2846 and 2883). all the feo content obtained from the microwave was taken for fe2+ due to the absence of fe3+ in the ilmenorutile. b: edge; c: center; m: intermediate point. section tio2 al2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total 2895 c 69.20 0.07 9.47 0.00 0.00 0.01 0.10 0.06 0.00 16.02 0.01 0.07 3.67 0.62 98.67 2846 b 69.39 0.06 9.50 0.00 0.00 0.01 0.02 0.00 0.00 17.69 0.00 0.26 2.85 0.11 99.89 b 71.62 0.11 9.08 0.01 0.00 0.01 0.01 0.00 0.01 16.39 0.00 0.24 2.18 0.08 99.74 b 71.47 0.11 9.17 0.00 0.01 0.01 0.01 0.00 0.01 16.31 0.01 0.21 2.33 0.18 99.84 b 69.98 0.08 9.18 0.00 0.01 0.00 0.00 0.00 0.00 17.80 0.01 0.22 2.22 0.23 99.74 m 69.54 0.12 9.46 0.01 0.00 0.02 0.00 0.00 0.00 17.80 0.00 0.25 2.31 0.22 99.72 m 69.25 0.10 9.70 0.02 0.00 0.00 0.01 0.00 0.00 17.86 0.00 0.27 2.21 0.20 99.61 b 67.21 0.04 10.20 0.01 0.00 0.00 0.00 0.00 0.01 19.63 0.00 0.28 2.02 0.23 99.64 c 70.08 0.11 9.40 0.01 0.00 0.01 0.00 0.00 0.00 17.66 0.01 0.24 2.09 0.16 99.76 b 66.96 0.09 10.28 0.00 0.02 0.00 0.01 0.00 0.00 19.40 0.00 0.24 2.70 0.23 99.92 m 68.68 0.12 9.82 0.01 0.00 0.00 0.00 0.00 0.00 18.56 0.00 0.18 2.39 0.24 99.98 c 68.59 0.13 9.93 0.01 0.00 0.01 0.01 0.00 0.00 18.53 0.00 0.21 2.51 0.15 100.08 b 68.83 0.07 9.99 0.00 0.00 0.00 0.00 0.00 0.00 19.10 0.00 0.21 1.78 0.26 100.23 m 70.41 0.09 9.37 0.01 0.00 0.00 0.00 0.00 0.00 17.02 0.00 0.20 1.98 0.12 99.20 c 71.28 0.11 9.29 0.00 0.01 0.00 0.00 0.00 0.00 16.78 0.00 0.22 1.90 0.14 99.74 b 71.02 0.05 9.29 0.01 0.01 0.01 0.00 0.00 0.00 16.76 0.00 0.20 1.94 0.26 99.54 m 70.78 0.10 9.35 0.01 0.01 0.02 0.02 0.00 0.01 16.93 0.00 0.23 2.29 0.10 99.83 c 69.99 0.13 9.51 0.00 0.00 0.00 0.01 0.00 0.01 17.13 0.00 0.21 2.45 0.09 99.53 b 68.75 0.10 9.69 0.00 0.00 0.01 0.00 0.00 0.00 17.97 0.01 0.31 2.27 0.36 99.47 c 67.24 0.08 9.84 0.02 0.00 0.00 0.00 0.00 0.00 18.34 0.01 0.54 3.15 0.16 99.38 b 67.57 0.15 9.68 0.00 0.01 0.01 0.02 0.00 0.01 19.11 0.00 0.37 2.05 0.22 99.21 m 71.31 0.14 8.74 0.00 0.00 0.01 0.00 0.00 0.01 16.46 0.00 0.21 2.33 0.25 99.46 c 70.96 0.16 8.86 0.00 0.00 0.00 0.00 0.00 0.00 16.52 0.01 0.18 2.53 0.25 99.45 2883 m 68.42 0.09 10.02 0.01 0.00 0.01 0.00 0.00 0.00 17.39 0.00 0.17 2.91 0.17 99.19 c 67.90 0.11 10.02 0.01 0.00 0.00 0.00 0.00 0.00 17.88 0.01 0.14 2.76 0.19 99.03 b 68.53 0.07 9.58 0.04 0.00 0.01 0.00 0.00 0.01 17.74 0.00 0.17 2.82 0.25 99.21 b 66.67 0.08 10.35 0.01 0.00 0.00 0.01 0.00 0.00 18.83 0.01 0.13 2.85 0.31 99.24 c 67.87 0.13 10.10 0.02 0.00 0.00 0.00 0.00 0.01 18.35 0.00 0.14 2.85 0.11 99.57 table 9. quantitative results of magnetites and ilmenites in a quartz vein sample. the oxides appear in wt%. for the total value calculation, both fe2o3 and feo recalculated were used. b: edge; c: center; m: intermediate point. section tio2 al2o3 feo* fe2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total magnetite 28 61 c 0.03 0.02 91.94 68.12 30.64 0.11 0.01 0.01 0.02 0.09 0.01 0.00 0.00 0.00 0.02 0.00 99.08 c 0.00 0.05 90.34 66.94 30.10 0.09 0.01 0.02 0.01 0.09 0.01 0.02 0.00 0.01 0.00 0.00 97.35 b 0.00 0.01 92.33 68.47 30.72 0.12 0.00 0.01 0.01 0.08 0.00 0.00 0.00 0.00 0.03 0.00 99.45 m 0.00 0.03 92.02 68.28 30.58 0.20 0.00 0.00 0.03 0.06 0.00 0.00 0.04 0.00 0.01 0.00 99.24 c 0.00 0.01 92.12 68.38 30.59 0.21 0.01 0.01 0.02 0.05 0.01 0.02 0.00 0.01 0.00 0.00 99.31 c 0.02 0.03 92.00 68.20 30.64 0.14 0.00 0.01 0.01 0.09 0.00 0.00 0.02 0.03 0.02 0.00 99.20 ilmenite 28 61 b 52.54 0.03 0.00 28.06 18.94 0.00 0.02 0.01 0.00 0.00 0.04 0.00 0.00 0.00 0.00 99.08 c 52.80 0.03 0.00 26.63 20.59 0.00 0.01 0.02 0.00 0.00 0.07 0.00 0.00 0.03 0.00 97.35 m 52.97 0.02 0.00 26.88 20.50 0.00 0.01 0.00 0.00 0.00 0.07 0.01 0.00 0.06 0.00 99.45 table 10. quantitative results of columbite included in ilmenorutile and oxide characterized as samarskite in pegmatite (bayonne-valderrama, 2018). mineral tio2 al2o3 feo mno mgo cao cr2o3 v2o3 nio nb2o5 ga2o3 sno2 ta2o5 wo3 total columbite 5.49 0.04 10.91 7.44 0.29 0.21 0.00 0.00 0.00 63.48 0.00 0.14 5.29 2.19 95.47 samarskite 4.86 0.00 10.86 1.55 0.05 0.72 0.01 0.00 0.00 35.57 0.00 0.21 3.68 3.95 61.45 19magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia figure 6. a. compositional maps of ilmenite with hematite exsolutions. b. compositional maps of magnetite with hematite oxidation to the edges and exfoliation planes. it was found in paragenesis with muscovite. the color scale is qualitative: blue and dark colors denote a low proportion, while the pinkest and white colors represent a high proportion of the element. ilm: ilmenite; ms: muscovite; hm: hematite; fe: iron; ti: titanium; mn: manganese; nb: niobium. feti₂o₅ fe₂tio₄ ulvospinel feo wüstite fe₃o₄ magnetite fe₂o₃ hematite fetio₃ ilmenite kimberlites and carbonatiles kimberlites and carbonatiles acid and intermediate rocks basic rocks basic extrusive rocks intermediate extrusive rocks acid and intermediate extrusive rocks basic intrusive rocks acid extrusive rocks rutile tio₂ figure 7. ternary diagram of the wüstite-rutile-hematite system for iron and titanium oxides. in red, the host rocks; blue, the pegmatites, and green, the quartz veins (lanza & meloni, 2006). with the compositional maps, no zoning of the oxides was identified (figure 6). therefore, the average of the edge, center and intermediate point per crystal was measured to calculate the petrological parameters. as the trace elements concentration on the three oxides is not relevant, they are unrepresented in the stoichiometric formula (table 2). the oxidation of magnetite can be observed only optically; it was not identified on the compositional maps. it happens for the martitization process, whose reaction does not involve the mobilization of cations (nadoll et al., 2014). in ilmenite, both in petrography and in the compositional maps of iron, titanium, and manganese, the exsolution can be observed because iron cations are mobilized to produce hematite as titanium are mobilized for rutile. the major elements analysis in the wüstite-rutile-hematite ternary system shows that magnetite has an invariable and almost pure composition, with a more elevated ferric ion concentration than ferrous ion. at the same time, ilmenite contains the most notable variation in titanium and ferric ion content. although the ternary diagram of lanza and meloni (2006) shows the ilmenorutile forming an isomorphic series with the pseudobrookite, the data for this mineral was not plotted because the content of niobium and tantalum must be measured. 6.3 petrology bacon and hirschmann (1988) explained that the mg/mn ratio behavior allows identifying whether the magnetite-ilmenite series is in balance or not. the balance is in the ±2σ range; out of this range, fugacity and temperature values will not be accurate. they also explained that this ratio behavior is best applied for volcanic rocks and crystals without alteration or secondary processes. in this case, studied samples are pegmatites, intrusive rocks with magnetite, reporting martitization process, and ilmenite crystals, tracing rutile and hematite exsolution process. for this reason, the regression values obtained are low. in host rocks, those values are 0.0054, and in pegmatites are 0.246, indicating that other types of analysis must corroborate the data obtained from this methodology to establish the equilibrium of the pairs. after the treatment and filtering of the data, eight magnetite-ilmenite pairs were obtained for the temperature and oxygen fugacity calculation. the 20 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda authors used the methodology proposed by spencer and lindsley (1981) and andersen and lindsley (1985). the formation temperature of the samples of host rock and pegmatitic dykes is 358 at 414 °c and 402 at 499 °c, respectively, with abnormal values for pegmatites of 572, 580, and 277 °c. these obtained temperature data are not reasonably coherent since a decrease in temperature should be expected from the granitic host rock evolution to the pegmatites later phase, which is possibly due to the log (mn/mg) test result; these results mean the magnetite-ilmenite pairs were not in equilibrium. entrapment minimum temperatures for inclusions rich in co2 were obtained between 270-450°c and 175-375°c for those inclusions rich in h2o. the oxygen fugacity values in the host rock range between -28 and -30 using both methods, and in pegmatites between -24 and -33 with anomalous data of -18, -19, and -47 (figure 8). most data are located above the fmq buffer. in this area, the oxygen fugacity and temperature conditions allow stabilization of ferric and ferrous ions in a proportion that facilitates the crystallization of magnetite (bowles et al., 2011; frost, 1991a). below the fmq buffer, the conditions increase the partition of iron in silicates. in the fo2 vs. t diagram (figure 8), two anomalous values are also presented: a product of the low correlation (2σ) in the mn/mg equilibrium test. figure 8. diagrams of fo2 vs. t for the magnetite-ilmenite pairs in granite (blue) and pegmatite samples (red). the results were obtained with ilmat program based on spencer and lindsley (1981) (right) and andersen and lindsley (1985) (left). the lines represent the buffer reactions hm, fmq, mw, im, iw, and qif. 6.4. chemistry of magnetite and pegmatite characterization the trace elements within the magnetite allow distinguishing whether it has an igneous or hydrothermal origin. it also identifies the type of deposit and temperatures in which they were assembled (nadoll et al., 2014). various elements’ relationships have been used to discriminate whether magnetite has a magmatic or hydrothermal origin, including the v vs. ti (knipping et al., 2015) or ni/cr vs. ti ratio (dare et al., 2014). however, these relations do not distinguish the signature of the magnetite from the rocks worked but for the v vs. cr ratio (figure 9), which is not enough for the discrimination of magnetite origin. the formation of pegmatites implies a high fluid evolution, going from a magmatic phase to a mixed magmatichydrothermal stage. the granitic host rocks studied represent the magmatic phase; the pegmatites represent the magmatic-hydrothermal phase, and the quartz veins represent a hydrothermal phase. this evolution can be identified with the v vs. cr ratio since, as expected, the host rocks have a magmatic origin and the quartz veins have a hydrothermal origin. the pegmatites show a predominantly hydrothermal origin signature, which is valid for a mixed phase. according to composition, mineralogy, and geological environment, the pegmatites respond to an anorogenic environment genesis (mora-galindo, 2023) from the nyf family because, in the system, niobium is present within the ilmenorutile and the columbite, with thick-sized apatite crystals containing fluoride and yttrium; also, an iron oxide with niobium and rare earth elements as samarskite was characterized (bayona-valderrama, 2023). figure 9. left: ratio between v vs. cr (ppm) for magnetite. the dashed line represents the possible boundary between hydrothermal and igneous magnetite (aronsson, 2016). right: concentration of v vs. ti (ppm) in magnetite. the red area includes igneous formed magnetite, while blue corresponds to hydrothermal origin (knipping et al., 2015). in blue the host rock values, in red the pegmatites, and green for the quartz vein. the ti+v vs. al+mn diagram by dupuis & beaudoin (2011) and nadoll (2011) about deposit type discrimination was also conducted. most of the data are located within the ag-pb-zn hydrothermal veins field. this area was defined by nadoll et al. (2014) using 30 magnetite samples extracted from the lucky friday mine (figure 10). despite having a similar ti+v vs. al+mn ratio between magnetites from the studied pegmatites and hydrothermal veins from lucky friday, a statistical study is carried out relating the magnetite’s trace elements concentration and petrological factors of both environments to determine a possible genetic and petrological correlation and postulate a new field for these pegmatitic anorogenic rocks. for the statistical analysis, the same treatment worked by nadoll et al. (2014) was used. it consists of utilizing the 95th percentile of the data (table 11) and drawing it in radial diagrams to be easily compared with the deposits studied in nadoll et al. (2014) (figure 20). the radial diagram of the lucky friday magnetites (ag-pb-zn) presents differences, especially with mg, mn, ga, and sn. 21magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia table 11. compilation of the data obtained from the statistical analysis of the trace elements concentration (ppm) in magnetite. data processed with r2. their corresponding graphs are found in figure 12 . element (ppm) host rock pegmatite dykes quartz veins min max med 0.95 min max med 0.95 min max med 0.95 mg 0.00 2691.27 7.27 899.03 0.00 105.47 1.82 63.65 0.00 16.97 36.37 34.55 al 14.00 294.10 131.60 262.73 0.00 5753.20 173.70 2461.34 33.61 137.25 60.22 126.74 ti 0.00 1638.78 93.44 876.53 0.00 6892.90 176.10 2518.37 0.00 115.00 8985.00 104.22 v 81.58 244.75 175.72 227.18 15.69 320.06 138.06 308.29 166.30 285.50 257.30 283.97 cr 32.77 491.58 308.99 465.36 0.00 196.63 32.77 114.70 46.82 121.72 65.54 113.53 mn 185.90 341.90 233.90 333.49 120.00 4192.70 275.90 1112.65 515.80 1271.60 788.70 125.51 ni 0.00 179.09 18.53 144.51 0.00 67.93 6.18 66.39 0.00 61.76 33.97 60.21 ga 0.00 166.03 22.14 158.28 0.00 326.53 5.53 268.42 0.00 243.52 2.77 207.54 sn 0.00 192.32 0.00 118.50 0.00 198.53 9.31 162.85 0.00 155.10 15.51 134.94 table 12. the relation between opaque mineral abundance with the magnetic susceptibility of all the samples studied. code type of rock opaque (%) mag (%) ilm (%) ilmrt (%) hm (%) py (%) ccp (%) susceptibility (10-3 si) 2816 gneiss 0 0 0 0 0 0 0 1.43 2814 gneiss 5 0 0 0 0 4.50 0.50 0.338 2811 gneiss 0 0 0 0 0 0 0 0.226 2807 pegmatite 12 6.60 5.40 0 0 0 0 30.97 2820 pegmatite 5 4.50 0.50 0 0 0 0 5.44 2846 pegmatite 10 4.50 0.50 5.00 0 0 0 3.18 2972 pegmatite 8 3.20 4.80 0 0 0 0 2.76 2870 pegmatite 4 3.80 0.20 0 0 0 0 2.19 3012 pegmatite 15 0.75 1.50 0 11.25 0 0 1.18 2883 pegmatite 10 0 0 10.00 0 0 0 0.472 2995 pegmatite 4.00 0.80 0.40 0 0 2.80 0 0.379 2984 pegmatite 1.00 0 0 0 1.00 0 0 0.279 3005 pegmatite 14.00 1.40 0 0 0 12.60 0 0.226 2917 pegmatite 6 0.30 0 0 5.70 0 0 0.072 3006 pegmatite 1.00 1.00 0 0 0 0 0 0.0070 2936 host rock 20 10.00 7.00 0 0 3.00 0 34.53 2861 host rock/ vein 5 2.00 0 0 3.00 0 0 2895 host rock 5 2.00 2.75 0.25 0 0 0 2.84 2973 host rock 4 2.80 1.20 0 0 0 0 2.58 2937 host rock 3 1.35 1.65 0 0 0 0 1.25 3003 host rock 10.00 0.50 3.70 0 5.80 0 0 0.932 2983 host rock 1.00 0.08 0.92 0 0 0 0 0.173 2843 host rock 3 0 0 0 3.00 0 0 0.048 2985 quartz vein 2.00 0 1.50 0 0.50 0 0 0.048 2894 quartz vein 1 0 0 0 1.00 0 0 0.0022 figure 10. ti+v vs. al+mn (wt%) diagrams proposed by dupuis and beaudoin (2011) and nadoll et al. (2011) for magnetite distinction according to the type of deposit (left) and approximate ranges of temperature formation of magnetite (right). the granitic samples are in blue, pegmatites in red and quartz veins in green. 22 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda figure 11. radial diagrams to compare the trace elements suite found in granites (blue), pegmatites (red), and quartz veins (green) with different deposits (yellow) worked by nadoll et al., 2014. 23magnetic petrology applied to the characterization of pegmatite dykes in eastern colombia 6.5 magnetic susceptibility the magnetic susceptibility of the twenty-five samples was measured, including the four types of lithologies found: gneisses, granites, pegmatitic dykes, and quartz veins. it should be considered that the magnetic susceptibility not only depends on the magnetic response of the materials but also on the composition, which is the crystalline structure related to magnetic anisotropy and is also proportional to the size of the crystals (hunt et al., 1995). for this reason, three measurements were obtained on varied surfaces of the sample to calculate an average with their respective standard deviation. the average magnetic susceptibility value is 0.665 in gneisses, 6.05 in the granites, 3.93 in pegmatites, and 0.0251 in the quartz veins. for the average and standard deviation calculation, the quartz vein sample vn_1 was not taken into account since it corresponds to a tourmaline contact between monzogranite cut by a quartz vein. then, the susceptibility of the sample will correspond to the sum of both lithologies. no correlation was observed between the samples and the magnetite abundance in the three lithologies; then, a comparison was considered with the opaque mineral content (table 12). with this comparison, a 76% correlation between susceptibility and magnetite content was identified. samples with higher susceptibility tend to contain a high ilmenite content; however, the relationship is not conclusive (table 12). despite establishing a numerical correlation, it is not possible to determine interesting areas based on the susceptibility values mapping. additionally, in the map drawn in this work, the data density is shallow, and the site is extensive for just 25 samples. nonetheless, suppose that the granitic rocks in the sector have a similar susceptibility behavior to that obtained in this work. in that case, it might be possible that, in a subsequent magnetometric study, the areas with lower values present a higher pegmatite dikes density. therefore, it could allow the identification of potential attractive areas. it is recommended that for subsequent exploration or pegmatitic dykes characterization studies, the magnetic susceptibility measurement be performed by following a systematic method, measuring the pegmatitic dike and its immediate host rock, and obtaining a vast number of samples to eliminate this ambiguity and identify more accurate interesting areas. discussion variations in abundance, degree of alteration, and composition of these oxides studied all together contribute to a better understanding of the fluid evolution and genetic conditions that resulted in the granites and pegmatitic rocks crystallization outcropping around san jose, guainia. the temperatures obtained with the petrological analysis of fe-ti oxides are consistent with the microthermometry results in primary fluid inclusions. cardona (2018) showed that the minimum entrapment temperatures in co2enriched primary fluid inclusions are between 260° to 460°c and between 150° to 400°c in water enriched inclusions; a similar range as obtained with the petrology method. fluctuations on the results employing those two methods might also be explained by the log(mg/mn) relationship, which showed signs that the magnetite-ilmenite pairs were not completely in equilibrium, and the results might not be completely accurate. however, it is crucial to consider that according to bacon & hirschmann (1988), the method is valid for volcanic rocks and was not conclusive in intrusive rocks. although the temperature data were not totally precise or consistent enough, the fo2 values calculated in the ilmat show a trend where the fo2 in pegmatites is higher than in host rocks. additionally, four characteristics allow authors to conclude an increase in oxygen fugacity: • martitization of magnetite, a reaction that occurs in oxidizing environments. • the increase in fo2 controls the mn2+ and fe2+ concentration in magnetite and ilmenite. this is explained by the ionic radius of the elements and the crystalline spaces. mn2+ and fe3+ have fairly similar properties; therefore, the mn2+ will try to occupy the tetrahedral space within the inverse magnetite structure, but since this space is only occupied by the fe3+ and trivalent cations, it does not allow the entrance of the mn2+ ion, causing it to concentrate on ilmenite (czamanske & mihálik, 1972). the mn2+ concentration reported by the microprobe is between 92 and 170 times higher in ilmenite concerning magnetite, corroborating the typical behavior of mn2+ in a system where fo2 increases (figure 12a) • in oxidizing conditions, the ilmenite is enriched in mn2+ concerning fe2+ as the fluid evolves. • according to the iron compositional map in ilmenite (figure 6a), elevated concentration is found in hematite exsolutions within ilmenite. the highest values of mn2+ are in ilmenite instead of the hematite exsolutions. this behavior shows the absent mobility of manganese during the exsolution and complements the preference of fe with hematite. in addition, the average value of mn2+ in ilmenites is 7.09 wt% in granites, 7.61 wt% in pegmatites, and 20.01 wt% in quartz veins, indicating an increase of the concentration of mn2+ as the system evolves from magmatic to hydrothermal, accompanied by the oxygen fugacity increase. to determine the evolution and behavior pattern between t and fo2 is essential to consider the following parameters: the initial fo2, the abundance of titanomagnetite and ilmenite before and after the fluid evolves, the composition of the fluid, and the rebalancing reactions that may take place, either oxide-silicate, oxide-oxide or intra-oxide type (frost, 1991). in an intraoxide rebalancing, the oxyexysolution reaction is found because of oxidizing conditions, consisting of titanomagnetite transformed into ilmenite and magnetite forming a lamellar texture. but neither the microprobe analysis showed a titanomagnetite signature nor the texture was identified during the petrography. in an oxide-oxide rebalancing, the ti migrates from the titanomagnetite to the ilmenite, but there is figure 12: a (left): fe2+ vs. mn2+ (wt%) plot representing the mn enrichment within the ilmenite, while the content of fe2+ decreases because of oxidation of the system. host rocks are represented by blue values, pegmatites in red and quartz veins in green. b (right): fo2 vs. temperature diagram that shows the cooling trend of the system with an increase of oxidating conditions according to the petrological results. the path taken by the analyzed samples is shown in red (frost, 1991). 24 carlos josé charry, juan carlos molano, leonardo santacruz, janeth sepúlveda no presence of titanomagnetites or more evidence to infer is occurring. finally, frost (1991) shows that in an oxide-silicate rebalancing, quilf and quilo reactions may occur; however, these occur at high temperatures in basic rocks with olivine or pyroxene; but those are not part of this study. contrarily, the kuilb reaction occurs thanks to h2o enrichment of the fluid generating an oxidizing tendency during cooling that allows the hematite exsolution within the ilmenite (frost, 1991). the kuilb rebalancing reaction is the one that best fits the evidence in the rocks studied due to the presence of hematite exsolutions within ilmenite, the mn2+ and fe2+ correlation in ilmenite, the increase in fo2 at the t vs. log fo2 diagram. also, because pegmatites are a magmatic-hydrothermal phase of a highly evolved, differentiated, and supersaturated volatile fluid, with h2o among them. this evolution of the system is denoted with a red dotted line in figure 12b. the magnetite signature does not present unusual concentrations in rare earth elements, and their al+mn vs. ti+v (wt%) ratio is similar to the agpb-zn hydrothermal deposit that belongs to the lucky friday mine, located in the coeur d’alene, usa, which has been studied and characterized by nadoll and mauk (2011). however, the genetic conditions differ from each other. lucky friday’s hydrothermal present a pressure between 1-3 kpa and a formation temperature from 250 to 350°c, while the pressure in the pegmatitic dykes studied is between 0.426 to 1.206 kbar with a higher temperature (cardona, 2023). additionally, the mineralogy differs from both the lucky friday mine and the pegmatites studied. in lucky friday, the pyrrhotite, magnetite, and wüstite are presented. the pyrrhotite indicates a reducing environment, and wüstite, being an unstable mineral below 570°c, is metastable thanks to the low fo2 (nadoll & mauk, 2011). conditions that are in opposition to those found in the pegmatites studied. due to the lucky friday mine characteristics, a correlation cannot be established with this work’s pegmatites and host rocks. for this reason, a new field is proposed in the al+mn vs. ti+v (wt%) plot proposed by dupuis and beaudoin (2011) and nadoll et al. (2014) (figure 10). the authors strongly recommend using this proposal to differentiate the magnetites from this magmatic-hydrothermal environmental for future studies in magnetites formed in the nyf-type pegmatites. conclusions the magnetic petrology technique allows us to obtain accurate oxygen fugacity and temperature values. this information is consistent with the data obtained from primary fluid inclusions microthermometry. despite implementing a systematic and carefully filtering process of the results obtained by epma, some temperature values from pegmatites were higher than granites, an unexpected behavior. it would be related to the low log(mg/mn) ratio calculated in magnetite-ilmenite pairs. the kuilb oxide-silicate rebalancing process is the model that best fits the evidence and results. this rebalancing is accompanied by a fo2 increase as the fluid decreases its temperature, controlling the enrichment of mn2+ in ilmenite and allowing appropriate conditions for hematite exsolution within ilmenite, features supported in petrography and epma data. a new field is proposed to discriminate the magnetite related to nyf-type pegmatites from anorogenic environment. it happens because the pressure, fo2, and mineralogy between pegmatites and the hydrothermal veins from lucky friday are different. in the lucky friday veins, pyrrhotite and wüstite metastable are present coexisting with magnetite due to the low fo2 value; meanwhile, in the pegmatites studied for this work, the fo2 rates are high. ultimately, it is recommended that further studies complement a more appropriate adjustment and definition of the proposed field. magnetic susceptibility is proportional to the magnetite content found on the three lithologies studied, but it was not possible to correlate with magnetite’s composition. granitic host rocks present the higher susceptibility values, followed by the pegmatite dykes, quartz veins, and gneisses, which show a susceptibility close to 0. a more systematic method for the fieldwork is recommended, measuring the susceptibility values of the pegmatite dyke or quartz veins and its immediate host rock, for being able to identify a possible correlation or trend. references aronsson, j. 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(2008). sm30: shirt pocket – size magnetic susceptibility meter. https://10.1109/tvcg.2014.2346754. https://10.1109/tvcg.2014.2346754. https://doi.org/10.1130/0016-7606(1978)89%3c1335:gaaotp%3e2.0.co;2 https://doi.org/10.1130/0016-7606(1978)89%3c1335:gaaotp%3e2.0.co;2 https://doi.org/10.1007/bf00303452 https://doi.org/10.1130/mem97-p543 https://doi.org/10.1130/mem97-p543 https://doi.org/10.1016/j.gca.2015.08.010 https://doi.org/10.1144/gsjgs.148.5.0825 https://doi.org/10.1144/gsjgs.148.5.0825 https://doi.org/10.1016/s0098-3004(03)00042-6 https://doi.org/10.1029/2007jb004973 https://doi.org/10.1016/j.oregeorev.2013.12.013 https://doi.org/10.2138/am.2011.3553 https://doi.org/10.15446/esrj.v24n1.63443 https://doi.org/10.15446/esrj.v24n1.63443 https://doi.org/10.1016/j.precamres.2008.06.009 https://doi.org/10.18273/revbol.v43n1-2021003 _goback issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.103047 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 355 365 g eo lo g y abstract this research work focuses on the morphostructural evolution of the eastern end of the algerian saharan atlas. the study employs a comprehensive methodology involving the utilization of the surface envelope technique, combined with an analysis of the hydrographic network and geological context. these techniques are integrated within a gis framework to decipher the current relief characteristics. to interpret the current relief features, two astgtm v2 tiles were processed to extract morphometric data. the reconstruction of the original relief requires the generation of the summit level surface, the base level surface and the relief amount maps for 3 different grid sizes (4000m, 1000m and 250m). the respective different grid size maps has allowed determining the initial surface of the reliefs; first before any major vertical erosion, then after a phase of dismantling and finally the recent and sub current morphological aspect. the extraction of the hydrographic network and the elimination of lower than order 5 streams revealed the harmony of the latter with a raised shaped form (mega-horst) which represents the main morphostructural unit. this morphostructure is particularly characterized by outcrops from the lower cretaceous outcrops. the ramification of the hydrographic network along the main tectonic accidents has allowed the digging of wide valleys, often described as graben. quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) using gis-based surface envelope technique, drainage network analysis, and geological features kamel boufaa1*, ahmed bougherara1, foued bouaicha1, azzedine bouzenoune2 1. université frères mentouri constantine 1, algeria 2. university of mohamed seddik benyahia, algeria *corresponding author: kamel.boufaa@umc.edu.dz record manuscript received: 06/06/2022 accepted for publication: 15/11/2023 how to cite this item: boufaa, k., bougherara, a., bouaicha, f., & bouzenoune, a. (2023). quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) using gis-based surface envelope technique, drainage network analysis, and geological features. earth sciences research journal, 27(4), 355-365. https:// doi.org/10.15446/esrj.v27n4.103047 keywords: morphostructural evolution; envelope surface; horst; gis; saharan atlas; algeria determinación de la evolución morfoestructural del extremo oriental del atlas sahariano argelino a través de la técnica envolvente superficial, el análisis de la red hidrográfica y las características geológicas resumen este trabajo de investigación se centra en la evolución morfoestructural del extremo oriental del atlas sahariano argelino. el estudio emplea una metodología global que implica la utilización de la técnica de la envolvente superficial, combinada con un análisis de la red hidrográfica y del contexto geológico. estas técnicas se integran en el marco de un sistema de información geográfica (sig) para descifrar las características actuales del relieve. para interpretar las características actuales del relieve se procesaron dos mosaicos astgtm v2 para extraer datos morfométricos. la reconstrucción del relieve original requiere la generación de la superficie del nivel de la cumbre, la superficie del nivel de la base y los mapas de la cantidad de relieve para 3 tamaños de cuadrícula diferentes (4000m, 1000m y 250m). los respectivos mapas de diferentes tamaños de cuadrícula han permitido determinar la superficie inicial de los relieves; primero antes de cualquier erosión vertical importante, después tras una fase de desmantelamiento y finalmente el aspecto morfológico reciente y subactual. la extracción de la red hidrográfica y la eliminación de los arroyos de orden inferior a 5 han revelado la armonía de estos últimos con una forma elevada (megahorst) que representa la principal unidad morfoestructural. esta morfoestructura se caracteriza especialmente por los afloramientos del cretácico inferior. la ramificación de la red hidrográfica a lo largo de los principales accidentes tectónicos ha permitido la excavación de amplios valles, a menudo descritos como fosas tectónicas. palabras clave: evolución morfoestructural; superficie envolvente; horst; sig; atlas sahariano; argelia https://doi.org/10.15446/esrj.v27n4.103047 mailto:kamel.boufaa@umc.edu.dz https://doi.org/10.15446/esrj.v27n4.103047 https://doi.org/10.15446/esrj.v27n4.103047 356 kamel boufaa, ahmed bougherara, foued bouaicha, azzedine bouzenoune 1. introduction over the past few decades, the “eastern end of the saharan atlas” has been the subject of several research works. they were carried out mainly for academic purposes in the form of theses (dubourdieu, 1956; madre, 1969; othmanine, 1987; bouzenoune, 1993; manchar et al., 2022), and applied research work for mineral exploration (betier et al., 1952; rouvier et al., 1985; perthuisot et al., 1987; bouzenoune et al., 1995; bouzenoune and lecolle, 1997; perthuisot et al., 1998; perthuisot et al., 1999; bouzenoune et al., 2006; kerbati et al., 2020; boulemia et al., 2021; zerzour et al., 2021; benyoucef et al., 2022) and for the preparation of geological surveys maps (dubourdieu, 1949, 1951; durozoy, 1956; blès and fleury, 1971; sonatrach, 1977a, 1977b, 1977c; sonatrach., 1977d; dozer et al., 1985; kuscer et al., 1985; tamani et al., 2019; chibani, 2022). these works are often limited in space and objective and hence they fall short of giving a global vision that leads to a structural scheme integrating all the elements at the origin of the morphostructural evolution of the region. this situation has stimulated us, with the development of new tools (remote sensing and gis), to resume work in this region with the aim of elaborating a synthesis that proposes an integrated vision based essentially on the analysis of morphostructural aspects and taking into account the results obtained in previous studies. nevertheless, the lack of necessary information required for the elaboration of a global structural scheme, prompted us to propose a morphostructural evolution scheme. this proposal, while drawing partially from previously published geophysical data (zerdazi, 1990; asfirane and galdeano, 1995; boubaya, 2013) it is based essentially on the concept of envelope surfaces (de la noé and de margerie, 1888; prud’homme, 1972) and their integration in a geographic information system. this method was largely enhanced by the analysis and interpretation of the drainage network which allowed us to reconstruct the dismantling stages of the paleo-relief resulting from the succession of tertiary and quaternary tectonic phases mentioned in the works cited above. hence, this study employs methods and tools that are centered on the landscape characteristics, in order to establish a comprehensive view of the evolution of the relief. this comprehensive view admits that the topographic landscape is the result of the interplay between internal and external geodynamic processes. in tectonically active regions, the shape of the relief is in itself a witness of this activity. the topographic surface is regularly shaped by erosion, deposition and the continuous evolution of structures (deffontaines, 1990; deffontaines and chorowicz, 1991; jackson and leeder, 1994; jackson et al., 1996; burbank and anderson, 2001; chen et al., 2003; delcaillau, 2004; delcaillau et al., 2006; ribolini and spagnolo, 2008; champagnac et al., 2009). thus, the current topographic configuration is an outcome of the modification of an initial surface (paleo-surface) that predates the influence of neotectonic forces, either in isolation or in tandem with erosion. this surface can be reconstructed by applying cartographic techniques related to the surface envelope method. the concept of “summit level surface” (sls), was introduced as early as the late 19th century (de la noé and de margerie, 1888). it was defined as “a virtual surface joining the topographic peaks”. the term envelope was introduced and described as the surface contained in the envelope of the actual highest topographic points (argand, 1912). as for the base level surface (bls), it is constituted by joining the lowest topographic points (filosofov, 1960). since then, these two concepts are generally combined and their use is widely adopted in morphostructural and morphometric analysis (prud’homme, 1972; howard, 1973; ait brahim and rosanov, 1986). several works such as those in zaïr basin, have employed these concepts to characterize ancient deep-seated structures and their subsequent rejuvenation (deffontaines and chorowicz, 1991). other authors have used this method and combined it with the drainage network to study recent neotectonics activity (deffontaines et al., 1992). it has been established that anomalies in the drainage network, including atypical stream patterns or unanticipated behaviors, are frequently associated with underlying tectonic phenomena. in fact, tectonic processes can induce shifts in the earth’s crust through sliding, uplifting, or subsidence, thereby changing the courses of water streams from their original paths. these modifications, coupled with erosional influences, can occasionally result in significant modification in the landscape, creating conditions for a river to divert and redirect the flow of another. the analysis of the surface envelope has yielded valuable insights into the dynamic evolution of the relief. indeed, it consists in determining the remains of the initial surface in the form of flats located on isolated ridges and summits (ait brahim and rosanov, 1986). thus, to measure neotectonics deformations, it is essential to define a horizontal reference surface that corresponds to the initial surface. today, with the development of digital tools and spatial imagery, their extraction is performed from the digital elevation model (dem). however, for large cells of the considered grid, the sls and bls correspond respectively to the paleo-morphology and its evolutionary trend. also, these two surfaces when jointly reconstructed and superimposed, reflect the relief amount (ra) and give an overview of the distribution of the areas of slope breaks (motoki et al., 2015b; motoki et al., 2015a). furthermore, the drainage network complying the underlying structure and particularly the one conditioned by fracturing, allows eventually to highlight the neotectonics activity. the primary objective of this research work is to define and reconstruct the evolution of the morphostructural entities that have been eroded or covered by other formations. this is achieved through a combination of elements including: 1. segmenting the relief as determined by calculations, a pattern confirmed by its staircase-like morphology. 2. analyzing the surface envelope (sls) and studying the distribution of the relief amount to determine the original topography. 3. classification of the drainage network and its configuration to detect significant orientations. 4. conducting a spatial distribution analysis concerning some specific geological formations, particularly those from the triassic and lower cretaceous periods. 5. the combination of these elements of analysis has allowed us to envision the morphostructural evolution of this part of the eastern saharan atlas in harmony with the current configuration of the relief. 2. geographical and geological context the study area is located in northeastern algeria near the algeriantunisian borders, in the extreme east of the atlas mountains. it corresponds to a rectangular area that stretches over 200 km from east to west and 100 km from north to south. it is located between latitudes 35°00’00’’n and 35°59’00’’n and longitudes 7°00’00e and 8°59’00’’e covering an area of about 20.000 km2 between ouenza and the saharan plains (fig.1). the uplift of the atlas mountains during the cenozoic era, as documented by laffitte in 1939, was succeeded by a phase of vigorous erosion (mahleb and hadji, 2022). this erosional process is characterized by the establishment of an extensively developed drainage network. the current relief configuration is attributed to the combination of the flow pattern and the lithological composition of geological formations. indeed, we note the juxtaposition of high and low areas where the level difference exceeds 700 m in some parts (dj bou roumane, dj anoual and dj bou rbaia, khenchela mountains); this morphology is typical in the study area. generally, the orographic units which correspond to the eocene folded structures are obtained by the inversion of the mesozoic sedimentary basin which received thick series of marl, marlstone and limestone (laffitte, 1939; dubourdieu, 1956; guiraud, 1973) (fig. 2). this global tertiary inversion is responsible for the formation of typical folded structures such as large synclines with flat bottoms and large radius of curvature with relatively narrow anticlines. the commonly encountered large plains are underlined by folds plunging flanks. towards the north of tebessa region, a noticeable shift in morphology is distinguished. it is marked by the gradual transformation of anticlines and synclines into narrower and more tightly compressed formations. this transformation is particularly evident in regions intersected by diapiric domes containing triassic evaporitic materials, as observed in locations such as (dj ouenza, dj boukhadra, dj mesloula, belekfif and dj djebissa) (dubourdieu, 1951, 1956; durozoy, 1956; blès and fleury, 1971; bouzenoune, 1993) (fig.2). this material is considered by some authors to be re-deposited during the albian and not diapiric (vila and charrière, 1993; vila et al., 1994b; vila et al., 1994a; vila, 1995). 357quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) figure 1. study area represented by an area covered by two tiles: astgtm2_ n35e007_dem and astgtm2_n35e008_dem. figure 2. geological map of eastern end of the algerian saharan atlas (deleau and laffitte, 1951) and north of tunisia (castany, 1951) moreover, the rising of these structures is followed by subsequent dismantling and erosion phases. also, this region is characterized by the existence of many tertiary basins called “neogene basins” that characterize the entire saharan atlas of eastern algeria (dubourdieu, 1956; durozoy, 1956; blès, 1969; blès and fleury, 1971; guiraud, 1973; vila, 1980; wildi, 1983; taib et al., 2023) and central tunisia (castany, 1948; zargouni, 1975, 1985; ben ayed, 1986). among these basins, morsott graben which is oriented nnw-sse gets linked towards the south east with that of tebessa whose orientation is wnw-ese. to the north, it is the basin of ouled boughanem (nnw-sse) and those of tunisia oriented along the same direction. to the northwest of the study area, we encounter the terraguelt basin, oriented nnw-sse (fig. 2). the genesis and evolution of the aforementioned basins are the result of the combined effect of neotectonics and erosion. the latter is well materialized by a dense drainage network. but to what extent does the work of the hydrographic network contribute to the current shape? and what is the part of neotectonics in the exacerbation of erosion? the answer to this question refers us to the use of morphometric analysis methods of the surface envelope and the drainage network analysis. 3. materials and methods the present research necessitated the use of both classical cartographic resources, such as geological maps, and digital data sources, such as digital elevation models (dems). since the advent of satellite technology, the production of topographic maps has increasingly relied on satellite data. the two main used data sources are the shuttle radar topography mission (srtm) and the advanced spaceborne thermal emission and reflection radiometer global digital elevation model (aster gdem) products. these data sources provide comprehensive coverage of terrestrial surfaces in the form of digital surface models (dsms), making them a pragmatic choice for topographic map creation (riegler et al., 2006; wang, 2008; hadji et al., 2013; hadji et al., 2014). their use, is a good compromise in the preparation of topographic maps. this work requires the use of conventional cartographic documents (geological maps) and others in digital format (digital elevation model). the conventional cartographic documents used are the medium-scale geological maps (1/50,000) covering the study area published by the geological survey of algeria (dubourdieu, 1951; durozoy, 1956; blès and fleury, 1971; sonatrach, 1977c, 1977b, 1977a; sonatrach., 1977d; dozer et al., 1985; kuscer et al., 1985), as well as the small-scale geological maps (1/500,000) of northerneast algeria (deleau and laffitte, 1951) and north of tunisia (castany, 1951). in the context of the algerian part, this map stands as the best comprehensive cartographic document available at this scale for the entire study area. it adeptly demarcates the facies and prominently displays the principal fault lines. however, it’s noteworthy that this earlier edition does not make any allusion to the concept of “thrust sheets.” the used spatial documents are the aster gdem (aster global digital elevation model) version 2 satellite images. the gdem product provides a global digital elevation model (dem) of land areas at a spatial resolution of 1 arc second (horizontal display of about 30 meters at the equator). this product is freely available at the usgs website (https://earthexplorer.usgs. gov/). each tile is distributed in geotiff format. the two tiles used to cover the study area are: astgtm2_n35e007_dem and astgtm2_n35e008_ dem. given the nature of the work conducted combining dem processing, geostatistical processing, cartographic restitution and structural data processing, arcgis 10.7 (esri®) software package was used for these purposes. the approach we adopt in this article consists in creating a mosaic of dem tiles as a first step and then, the pixels of the latter are converted into a “shape file of points” where each point is located at the center of the pixel and provides the elevation value. the second step consists in dividing the study area into three categories of grids, with different cell sizes (4000m x 4000m, 1000m x 1000m and 250m x 250m). for each grid, three points are selected, representing the highest altitude (zmax), the lowest altitude (zmin), and the relief amount (ra) denoting the difference between zmax and zmin. this operation yields three grids for each mesh size (zmax, zmin, and ra), culminating in a total of nine grids. finally, we construct graphs expressing zmin = f1 (zmax), ra = f2 (zmax) and ra = f3 (zmin) for each mesh. the objective is to proceed to a segmentation of the relief based on the average values of these altimetric variables. the third step consists in drawing maps of the summit level surface (sls), the base level surface (bls) and the relief amount (ra) respectively. to accomplish this task, a universal kriging interpolation method is employed for the grids specifying that the local average is different from the global average for each population of points (matheron, 1963; gratton, 2002). nevertheless, the use of these techniques requires a test to validate the normal distribution of the data, otherwise we proceed to a transformation. https://earthexplorer.usgs.gov/ https://earthexplorer.usgs.gov/ 358 kamel boufaa, ahmed bougherara, foued bouaicha, azzedine bouzenoune the drainage network was automatiquelly extracted from the dem using archydro tools algorithm and it was classified according to the strahler method of orders (strahler, 1952). we opted for the elimination of orders lower than 5 in order to highlight the reliefs that potentially influenced the initial organization of the network. the above work is achieved by the digitization of a part of the geological map covering the north of algeria and tunisia at the scale of 1/500 000 (for algerian part: deleau and laffitte , 1951) and tunisian part (castany, 1951). ultimately, through the integration of sls, bls, ra maps, hydrographic network data, and geological characteristics, we can elucidate the arrangement and development of diverse morphostructural units. 4. results within this section, we provide a comprehensive examination of the data gathered specifically for this study. the outcomes derived from processing these data through the aforementioned methodology yield crucial insights into the initial topography and its subsequent evolution due to the interplay of tectonic forces and erosional processes. 4.1 correlations between altitudinal factors in our approach, the altitudinal variables are cross-correlated in order to search for possible correlations between them (figure.3). this procedure allowed us to show that zmax as a function of zmin is a quasi-linear relationship for all cell sizes. this linear relationship is even stronger as the cell size is reduced. the correlation observed between the variables (zmax, ra) seems to decrease as compared to the couple (zmax, zmin). this decrease becomes more pronounced for the couple (zmin, ra). these relationships are valid for all mesh sizes. concerning the couple (zmax, ra) for the cell size 4 km x 4 km, the correlation is more or less pronounced, where the graph corresponds to a polynomial curve of the second order (fig. 3). this correlation remains weak compared to the value of the index r2 = 0.48356 but acceptable with regard to the other dimensions of cell. it is thus retained for the segmentation of the relief. it is clear from this curve that ra, which represents the difference in altitude, increases significantly with the increase in altitude. figure 3. cross-correlation between altimeter parameters zmax, zmin and ra. a1, a2 and a3: 4km x 4km grid; b1, b2 and b3: 1km x 1km grid; c1, c2 and c3: 0.25km x 0.25km grid. 359quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) 4.2 relief segmentation the approach consists in dividing the cloud of points of the graph a2 of figure 3 based on the averages of the two variables zmax and ra, noted respectively mzmax and mra (fig. 4). consider the point p whose coordinates are (mzmax, mra) and draw the lines y = mra and x = mzmax. thus, the point cloud is divided into four classes of points contained in four rectangles corresponding to 04 types of relief. figure 4. relief segmentation using mean values of zmax and ra for cell size 4km x 4km. 1 : mz, 2 : pz, 3 : plz et 4 : wz consider: (zmax mzmax) * (ra-mra) = a • for a> 0 (zmax> mzmax and ra> mra): this class is delimited by a higher level altitude than the average and a difference in altitude also higher than the average, which corresponds to a relief named mountain zone noted mz. • for a> 0 (zmax <mzmax and ra <mra): this class is the opposite of the previous class on the graph. it is delimited by a lower-thanaverage altitude and lower difference in altitude and corresponds to the zone of plains noted pz. its lower limit constituts the average base level of the study area. • for a<0 (zmax> mzmax and ra <mra): this class is intermediate, it is characterized by a lower than average difference in level and higher than average altitude. thus, it corresponds to a slightly perched space compared to the base and constitutes a transition zone between the mountainous zone and the plain zone noted plz. • for a<0 (zmax <mzmax and ra> mra): this class, which is characterized by an above-average difference in height and a below-average altitude, corresponds to a residual relief which is the area spared from the destructive action of erosion and therefore corresponds to the “witness hills” noted wz. thus, the cross-referencing of morphometric data, i.e., vertical drop (ra) and maximum altitude (zmax), allows the discrimination of the relief that characterizes the study area. the limits of the zones resulting from this segmentation clearly coincide with the limits of the physiographic units shown on the topographic and geological maps (figure.2). indeed, the main features of the atlas structures are well underlined by the limits of these zones and the sw-ne and nw-se profiles, longitudinal to these structures, confirm well this morphological distribution and organization. therefore, the results show the suitability of this simple treatment, based on the averages of the variables described above, to establish a classification of landforms or landform segments at different levels of zmax and ra. indeed, it appears that at high altitude, the relief is relatively young and is subject to strong erosion, resulting in a strong difference in altitude within the grid. 4.3 summit level surface and relief amount maps we need to remind that the study area’s topography exhibits a notable interplay between elevated and depressed regions, highlighting a discernible structural element. this variability in local trends across different locations led us to opt for universal kriging as our interpolation method (table. 1). to meet the requirement of a normal distribution, we transformed the data using a logarithmic function. universal kriging assumes the following model z(s) = µ(s) + ε(s). where µ(s) is a second order deterministic function representing the trend and ε(s) the error. the trend is subtracted from the equation and the obtained error is assumed to be random with a mean of ε(s) zero. conceptually, the autocorrelation is modelled on the basis of these random errors ε(s). to interpolate the values needed to estimate the variability of relief, we applied semi-variograms. the results showed significant variability in the ra (relief amount) values in the study area, with a nugget effect of 0.058 for a record dataset of 1288 (table. 1). table 1. kriging interpolation model characteristics m et ho d d at a se t ( r ec or ds ) tr an sf or m at io n n ei gh bo rh oo d n ei gh bo rs to in cl ud e in cl ud e at le as t va ri og ra m n ug ge t m od el universal kriging 1288 log standard 5 2 semivariogram 0.05896 stable the sls obtained from the zmax altitudes for a 4000m x 4000m cell size, is inclined towards the ne. it is globally consistent with the current surface. indeed, the high altitudes are located much more to the sw and the low altitudes to the ne (fig. 5). the sw-ne topographic profiles made on this sls map (fig. 6), show a staircase like relief similar to the one resulting from the segmentation. we can therefore admit that despite the strong erosion that has affected this region, it still retains its initial general appearance with slight changes. figure 5. summit level surface map for 4000m x 4000m cell size. 360 kamel boufaa, ahmed bougherara, foued bouaicha, azzedine bouzenoune figure 6. transects a a’, b b’ and c c’ carried out on the segmented sls map of 4000m x4000m cell size. the relief amount (ra) map (fig.7) shows areas of steep slopes that generally follow the limits of lower cretaceous limestone massifs. the latter which corresponds to disjoint geological units appear grouped around djebel dyr. they appear to underline a megastructure of subcircular shape. figure 7. relief amount map for 4000m x 4000m cell size. the ne-sw and nw-se transects carried out on the ra map (fig.8), show a topographic elevation located in the center of the profiles. moreover, in the southwestern part of the study area, in khenchela and cheréa mountains, these surfaces of ruptures highlighting the atlas structures do not reach this sub-circular shaped megastructure described above and are disconnected from it revealing the posteriority of this morphostructure to the atlas structures. figure 8. transects a a’, b b’ and c c’ carried out on the segmented ra map of 4000m x 4000m cell size. 4.4. drainage network analysis the drainage network (fig. 9) was automatically extracted from the 30m resolution dem using archydro tools module in arcgis 10.7. this tool is used to derive several data sets that collectively describe the drainage patterns of catchments. raster analysis is performed to generate data on flow direction, flow accumulation, stream definition, stream segmentation, and watershed delineation. these data are then used to develop a vector representation of catchments and lines corresponding to drainage network. the choice of “stream definition” is a decisive parameter in the extraction of the drainage network, which allows us to obtain the 1st order stream. the product is then imported into arcmap for processing and analysis. the drainage network is of dendritic type. this implies a globally homogeneous drained surface made of more or less impermeable formations. this impermeability is revealed by a high drainage density due to a predominance of marl and marl-limestone. in the study area, the classification of the drainage network according to strahler’s method (strahler, 1952) highlights rivers and streams of up to order 7. in order to highlight the paleo-morpho-structures and to overcome the difficulty of masking the higher orders of the drainage network by the low order streams, we opted for the elimination of lower than order 5 streams of the drainage network. thus, we obtain a certain harmony between the configuration of the ra map (fig. 7) and the organization of the retained orders of the drainage network (fig. 9). figure 9. drainage network map showing the radial pattern of order 5, 6 and 7 streams. at the scale of the selected orders, we note that the main rivers and their tributaries are arranged in a more or less radial manner in relation to a subcircular physiographic unit. indeed, oued cheréa drains in a southern direction, oued el ma labiod and oued el hatab flows in a se direction, oued serrath is oriented in a ne direction and then takes a nw direction to finally resume its initial ne direction and joins oued r’mel southeast of kef. oued terraguelt follows the basin and flows in nw direction. this arrangement can only illustrate the idea of the existence of a megamorphostructure that has controlled this drainage network in such a way that the flows are from the center to its periphery. it should be noted that some segments of this network present particular situations. indeed, oued guergoub el melah of order 6 runs along the gutter of the eroded meskiana anticline in sw-ne direction and stops abruptly. this anomaly can be explained either by the fact that the width of the river bed at this point are smaller than the pixel dimension of the dem so that this watercourse cannot be detected; or, this watercourse which usually flows into sebkha of garaat el tarf and which is in the continuity of oued meskiana is captured by the latter (order 5). the segment of order 6 of oued serrath which runs along the eastern limit of this morphostructure, is captured by a segment of order 5 located much further upstream from its initial confluence and this happens after the dismantling of the obstacle related to the above mentioned morphostructure. 361quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) 4.5 the atlas structures under multiple influences the fold structures dominated by a ne-sw orientation (figure.2) acquired during the eocene phase in the saharan atlas are characteristics. the hinges of the anticlines are in many areas occupied by saliferous trias formations, and are generally in contact with the lower cretaceous formations, i.e (dj ouenza, dj boukhadra, dj mesloula, dj djebissa ... etc.). these trias formations can also be found as small outcrops rising along longitudinal and transverse fault structures (djebels hamaimet, dj belekfif, se of the peak of dj boukhadra and argoub ezitoune). many authors have interpreted the structural setup of these triassic evaporite masses to be diapirs (dubourdieu, 1956; durozoy, 1956; perthuisot, 1991; aoudjehane et al., 1992; bouzenoune, 1993; perthuisot et al., 1999) while others have attributed them to salt glaciers (vila et al., 1994a; vila et al., 1994b; vila, 1995; vila et al., 1996). concerning the diapirs, the main remobilization is predominantly eocene in the saharan atlas. other upward activities of these triassic evaporites with surface extrusions are also known at least since the aptian (dubourdieu, 1956; madre m, 1969). they are also known in the vraconian (terminal albian), turonian, eocene and miocene (dubourdieu, 1956; madre, 1969; perthuisot, 1991; aoudjehane et al., 1992; bouzenoune, 1993; perthuisot et al., 1998; kowalski and hamimed, 2000). these upward movement persist even during the quaternary where we observe present terraces straightened up to the vertical at dj djebissa. this position within the anticlines is sometimes exaggerated with the initiation of planes of detachment with an overall southward convergence and sometimes northward leading to shoulders as at ouenza (bouzenoune, 1993) and elsewhere at kt delaa (perthuisot et al., 1999). this diachronic remobilization of evaporite masses is of various origins, halokinetic, diapiric, or by faulting related to deep vertical movements of the basement. the surface forms and the state of the outcrops mentioned above are the effect of deep processes that are obviously interpreted differently with their advantages and disadvantages but remain key formations for the understanding of the regional structure. indeed, it is easy to see on the geological map (fig. 10), the existence of an area of subcircular shape centered around the jebel dyr, and this, on a radius of 50 km. this area includes various limestone massifs of lower cretaceous age in contact with the triassic or not (dj bouroumane, dj hamaimat, dj boukhadra, dj mesloula, dj boudjaber ... etc.). in our opinion, this area corresponds to a raised megastructure. this elevation allowed the total or partial erosion or dismantling of the post-lower cretaceous cover. outside this area, the geological formations of the different massifs become more recent (massifs south and west of tebessa). the limits of this area coincide with those highlighted by the relief amount (ra) map (figure 8) which also attest to this geometry. the interposition of this megastructure, intersecting the typical ne-sw trending structures of the atlas, suggests a post-eocene genesis age, very probably even more recent than the lower miocene. this hypothesis is supported by the presence of remnants of formations of middle to lower miocene age, located at the heart of the perched syncline of djebel dyr. figure 10. extract from the geological map showing the subcircular distribution of triassic and lower cretaceous outcrops. furthermore, the regional depth morphology of the evaporitic material, which is concentrated in outcrop in a band 300 km long and about 80 km wide, is not well defined by previous work. however, the exploitation of some bibliographic data from geophysical work, helped us to support our interpretation. indeed, the bouguer anomaly is characterized by a regional gradient that increases gradually from hodna basin (sw) to the algeriantunisian borders (ne) where the hypothetical megastructure is located (zerdazi, 1990). this long wavelength variation is probably caused by the topography of the moho (zerdazi, 1990), this hypothesis is supported by the interpretation of the aeromagnetic map of northern algeria (asfirane and galdeano, 1995). the relative maximum of the bouguer anomaly recorded at the algeriantunisian borders is very difficult to explain by surface data (boubaya, 2013), zerdazi attributes it to a rise of the basement (zerdazi, 1990). this maximum is present on the upper levels of the upward extension of the anomaly which confers a deep origin combined with a semi deep origin (boubaya, 2013). the latter is related to short wavelength magnetic anomalies linked to structures due to triassic magmatism (asfirane and galdeano, 1995), keeping portions of the crust in an elevated position, remobilized during a post-eocene period, probably miocene. this interpretation is attested by the location of large metal deposits in the area bounded by this maximum (asfirane and galdeano, 1995). the wide geographical extent of the diapirs is only very weakly marked gravimetrically (zerdazi, 1990), which supports the interpretation by the attenuation of their effects by the rise of a denser substratum and not only in their situations at the edge of the neogene basins with recent fillings as stipulated by this same author. this observation concerns all the most important diapiric apparatuses of the region (ouenza, mesloula, boukhadra and tebessa). in the northeast of the aurès, the rise of the moho which implies a thinning of the crust and consequently the initiation of the “uplift” is well documented by the interpretation of seismic refraction profiles (buness h & al, 1992) and the gravimetric study (jallouli and mickus, 2000; guidara et al., 2010) showing also a crust which thins going from sw to ne. to confirm the validity of the findings presented in this research, it is crucial to integrate mechanical drilling alongside the existing geophysical data. it’s worth mentioning that the interpretation is based on the limited and fragmented data currently accessible. obtaining fresh data has the potential to become a central aspect of future research endeavors. 4.6 tebessa-morsott and terraguelt neogene basins the previously described megastructure is crossed by neogene basins such as tebessa-morsott (t-b and m-b) and terraguelt (tr-b) basins (fig. 2). they are filled in by clastic silico-carbonate sediments that come from the dismantling of the surrounding relief. this vigorous erosion is demonstrated by the existence of perched synclines such as the syncline of dj dyr which culminates at more than 1200m altitude. the tebessa-morsott neogene basin has two segments, t-b and m-b, both of which has sub-rectangular elongated form, characteristic of this region, hence its qualification of “area of diapirs and ditches”. they are considered by several authors (castany, 1951; dubourdieu, 1956; durozoy, 1956; blès and fleury, 1971) to be graben structures. however, according to our observations, we note the very low frequency of fractures with tectoglyphs at their edges, which for a structure in stepped collapse trenches is a fundamental criterion, which leads us to remain cautious about this interpretation, which we consider questionable, for the following reasons • it is generally accepted that a collapse under the effect of extension and subsidence leads to the appearance of normal fault planes striated at the edge of the collapsed area. indeed, on the south bank of the basin southeast of the city of tebessa (dj djoua), the density of faults as shown on the geological map of tebessa no. 206 is remarkable but we note that open fractures affecting the turonian which slides in panels under the effect of gravity. this is also observed at the level of bir salem and kef labiod west of tebessa, where the panels of the emsherian, campanian and maestrichtian are tilted with different directions in a very limited area; this observation, leads us to assume the dominance of the gravity phenomenon. • on the north bank of the ditch, if we consider the area of koudiat nassela as a panel slipped under the effect of tectonic collapse, it would have had a tilting antithetical to the rim faults of the ditch, 362 kamel boufaa, ahmed bougherara, foued bouaicha, azzedine bouzenoune but the panel is synthetic to the plane of rupture, to this effect, the imprint of gravity is present. • southeast of the town of morsott on the north side of the basin, turonian panels show relative scissor movements that exhibit both clockwise and counterclockwise movement over a very restricted area. this attitude may be due to gravity-induced movements caused by erosion. • gravity profiles (fig. 11) carried out in the m-b (profile 9), in t-b basin (profile 10) and in tr-b (profile 7) ( in zerdazi, 1990), show a “v” shaped incision for tebessa basin located upstream of morsott basin where the carbonate bedrock is at more than 5000 m depth. this “v” shape, which is relatively closed in tebessa basin, tends to open in “u” shape in profile 9 of the morsott basin and profile 7 of terraguelt basin with a decrease in the depth of the carbonate basement, indicates a filling and a probable resumption of subsidence downstream (morsott basin and terraguelt basin). the deep incision upstream in tebessa basin bears witness to the uplift of its substrate. thus, the nw limit of the raised block would be located at the junction of the two basins (morsott basin and tebessa basin) and the se limit would correspond to the relics of the horst represented in part by the bouroumane massif. such observations cannot, in our opinion, be the product of tectonic effects alone, but rather a process initiated by a major tectonic accident in which erosion followed its direction and played a predominant role. it is very likely that this megastructure would correspond to the relics of a large post middle or post lower miocene horst, which maintained atlasic structures in elevation, thus exposing them to further erosion (fig. 12). the accidents of ne-sw, nw-se and e-w orientations having affected this megastructure would have guided the orientation of the drains which served as evacuators of eroded material. thus, by lateral progression, this process led the widening these v shaped incisions to form the actual wide valleys. in our opinion, the tectonics has controlled the orientation and erosion has taken care of the rest. this scenario is in harmony with the “v” shape of the substrate and then incised by erosion. it is less likely to get developped to the present form of the landscape if we assume the development of a depression of about 5000 m (zerdazi, 1990) whose width is of the same magnitude. this pattern is similar to what is currently taking place in the youkous valley, where the n-s oriented fault has controlled the n-s orientation of the valley: the current intense regressive erosion affecting marly and marllimestone formations is constantly widening the valley. the longitudinal progression towards the south of this erosion, would give a relief where the plateau of cheréa would be equivalent to the now a days djebel dyr. 5. discussion it is undeniable that our approach which is essentially based on the use of digital terrain models (astgtm) and applying the surface envelope technique, remains an effective asset to enhance the data necessary for the interpretation of the structures recognized by previous works. the use of more geophysical data, in particular gravimetric data (zerdazi, 1990; asfirane and galdeano, 1995; boubaya, 2013) and geological maps covering the study area and mentioned above constitutes an effective mean in strengthening the interpretation. this adopted methodology has a definite ability in reconstructing the paleo-relief and foresee its likely future evolution and contributes to the conception of a global structural scheme of the region. the determination of the altitudinal factors zmax, zmin and ra and their cross correlations allows making a significant segmentation of the relief. this approach is partly inspired by the work of motoki (2014; 2015) who illustrates graphically the relief amount (ra) as a function of zmax in order to explain the role of external geodynamics in the elaboration of the shapes of a magmatic landscape. the acquisition of new data from geophysical surveys and boreholes will allow us to validate and/or differentiate the proposed interpretations, which are mainly based on the use of free radar images available at the various sites. in this way, the eastern end of the saharan atlas will remain a “test zone” from which it will be possible to generalize the approach to all the bare areas of north-eastern algeria. figure 11. gravimetric profiles carried perpendicular to the basins. a: tebessa basin b: morsott basin c: terraguelt basin figure 12. map showing the location and extent of the probably megastructure of horst. 363quantifying morphostructural evolution of the eastern algerian saharan atlas (ne, algeria) 6. conclusion the approach adopted, which combines the use of the surface envelope technique, the analysis of the hydrographic network and the geological characteristics of the region, has enabled us to reconstruct the palaeo-relief before the action of the erosional processes that led to the present relief. the technique of the summit and basal envelope surfaces has allowed us to characterize the state of the relief through the elaboration of a map called “relief amount” which corresponds to the subtraction of the basal surface from the summit surface. this approach required the use of several grids. indeed, the largest cell size (4000m x 4000m) has allowed us to obtain a map that materializes the initial topography. the medium cell size (1000m x 1000m) has shown an intermediate state, while the small cell size (250m x 250m) has shown the most recent state, close to the current configuration of the relief. the reading of these different maps has revealed a sub-circular shaped structure, bounded by areas of slope breaks, which correspond to the dismantled physiographic unit. being more conservative, these observations cannot prevent us from considering this megastructure as a “vast regional horst”, which has controlled the configuration of the relief and consequently the drainage network. on the other hand, the use of different grids has helped us to identify the stages of evolution where erosion played a preponderant role. thus, the establishment of the “horst” promoted the exacerbation of erosion. the rapid progress of the dismantling was conditioned both by the dissolution of the triturated calcareous walls and a digging along 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(2021). geostatistics-based method for irregular mineral resource estimation, in ouenza iron mine, northeastern algeria. geotechnical and geological engineering, 39(5), 3337–3346. https://doi.org/10.1007/s10706-021-01695-1 https://doi.org/10.1016/j.geomorph.2007.02.016 https://doi.org/10.2113/gsecongeo.80.3.666 https://doi.org/10.1130/0016-7606(1952)63%5b1117:haaoet%5d2.0.co;2 https://doi.org/10.1007/s43994-023-00072-3 https://doi.org/10.1007/s10706-019-00807-2 https://doi.org/10.1007/s10706-019-00807-2 https://doi.org/10.1007/s10706-021-01695-1 _heading=h.30j0zll _heading=h.ftjh4dll4tnz _heading=h.3znysh7 _heading=h.2et92p0 _heading=h.tyjcwt _heading=h.3dy6vkm _heading=h.1t3h5sf _heading=h.4d34og8 _heading=h.2s8eyo1 _heading=h.17dp8vu _heading=h.3rdcrjn _heading=h.26in1rg _heading=h.lnxbz9 _heading=h.35nkun2 _heading=h.1ksv4uv _heading=h.44sinio _heading=h.2jxsxqh _heading=h.z337ya _heading=h.3j2qqm3 _heading=h.1y810tw _heading=h.4i7ojhp _heading=h.2xcytpi _heading=h.1ci93xb _heading=h.3whwml4 _heading=h.2bn6wsx _heading=h.qsh70q _heading=h.3as4poj _heading=h.1pxezwc _heading=h.49x2ik5 _heading=h.2p2csry _heading=h.147n2zr _heading=h.3o7alnk _heading=h.23ckvvd _heading=h.ihv636 _heading=h.32hioqz _heading=h.1hmsyys _heading=h.41mghml _heading=h.2grqrue _heading=h.vx1227 _heading=h.3fwokq0 _heading=h.1v1yuxt _heading=h.2u6wntf _heading=h.19c6y18 _heading=h.3tbugp1 _heading=h.28h4qwu _heading=h.4x67twxbb9qt _heading=h.37m2jsg _heading=h.1mrcu09 _heading=h.46r0co2 _heading=h.2lwamvv _heading=h.pei5weh4y02v _heading=h.3l18frh _heading=h.206ipza _heading=h.4k668n3 _heading=h.2zbgiuw _heading=h.1egqt2p _heading=h.3ygebqi _heading=h.2dlolyb _heading=h.sqyw64 _heading=h.3cqmetx _heading=h.mqavz6ekvvho _heading=h.4bvk7pj _heading=h.2r0uhxc _heading=h.1664s55 _heading=h.3q5sasy _heading=h.25b2l0r _heading=h.kgcv8k _heading=h.34g0dwd _heading=h.1jlao46 _heading=h.43ky6rz _heading=h.2iq8gzs _heading=h.p5r2jomxz4ak _heading=h.3hv69ve _heading=h.1x0gk37 _heading=h.4h042r0 _heading=h.2w5ecyt _heading=h.1baon6m _heading=h.3vac5uf _heading=h.2afmg28 _heading=h.pkwqa1 _heading=h.39kk8xu _heading=h.1opuj5n _heading=h.48pi1tg _heading=h.2nusc19 _heading=h.i8qgkb2vvwbg the influence of shale composition on methane adsorption capability and gas content is investigated using 14 samples from well ys8 in the southern sichuan basin, china. the results show that the langmuir adsorption capacity of the longmaxi shale is mainly a function of the total organic carbon (toc) content. when toc is ~1.1%, 50% ch4 is adsorbed onto the surface of the organic matter. the mineral content has limited control on the adsorption capacity of the longmaxi shales. organic matter is also a major control on gas content when toc content is <1.0%. when toc is >1.0%, gas content remains constant, indicating that gas preservation is more important than gas generation and rock adsorption capacity. scatter plots of toc versus gas content and, langmuir adsorption capacity shows that when toc is <2.0%, ch4 occurs both as free and absorbed gas, and ch4 occurs mainly as absorbed gas when toc is >2.0%. abstract influence of black shale composition on methane adsorption and gas content: implications for gas storage in the longmaxi black shales issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v22n1.70539 haihua zhu1,2,3*, tingshan zhang1,2, jun lang1,2, jianli zeng1,2, xing liang4, yong he4, gaocheng wang4, junjun li4 1 stake key laboratory of oil and gas reservoir geology and exploitation, chengdu,china 2 school of geoscience and technology, southwest petroleum university, chengdu, china 3 unconventional oil and gas assessment center, southwest petroleum university, chengdu, china 4petrochina, zhejiang oil field company, hangzhou, china *email of corresponding author:zhhswpu@163.com earth sciences research journal earth sci. res. j. vol. 22, no. 1 (march, 2018): 59-63 la influencia de la composición de lutitas en la capacidad de absorción del metano y el contenido de gas se analiza en este trabajo a partir de 14 muestras del pozo ys8 en la cuenca de sichuan, al suroeste de china. los resultados muestran que la capacidad de absorción langmuir para las lutitas de longmaxi es principalmente una función del contenido de carbono orgánico total (toc, del inglés total organic carbon). cuando el índice toc es ~1.1%, el 50 % del ch4 (capacidad máxima de absorción de la función langmuir) es absorbido por la superficie de materia orgánica. el contenido mineral tiene un control limitado en la capacidad de absorción de las lutitas de longmaxi. la materia orgánica también tiene un control mayor en el contenido de gas cuando el índice toc es <1.0 %. cuando el toc es >1.0 %, el contenido del gas es constante, lo que indica que la preservación del gas es más importante que la generación del gas, y la capacidad de absorción de la roca. diagramas de dispersión del toc frente al contenido de gas y de la capacidad de absorción langmuir muestran que el toc es <2.0 %, que el ch4 se alcanza tanto en gas libre como en gas absorbido y que este ocurre principalmente cuando el gas absorbido tiene un nivel toc de >2.0 %. resumen how to cite item zhu, h., zhang, t., lang, j., zeng, j.,liang, x., he, y., wang, g., & li, j. influence of black shale composition on methane adsorption and gas content: implications for gas storage in the longmaxi black shales. earth sciences research journal, 22(1), 59-63 doi:http://dx.doi.org/10.15446/esrj.v22n1.70539 influencia de la composición de lutitas negras en la absorción de metano y el contenido de gas: implicaciones para el almacenamiento de las lutitas negras de longmaxi keywords: shale; methane; adsorption; toc; sichuan. palabras clave: lutitas; metano; adsorción, carbono orgánico total. g e o c h e m is t r y record manuscript received: 14/11/2017 accepted for publication: 21/03/2017 60 haihua zhu, tingshan zhang, jun lang, jianli zeng, xing liang, yong he, gaocheng wang, junjun li 1. introduction the wufeng–longmaxi black shale in the sichuan basinis the first shale gas reservoir to be commercially produced in china. by 2016, four national demonstration areas of marine shale gas production (the weiyuan, changning, zhaotong, and jiaoshiba shale gas plays) had beenestablished in sichuan, chongqing, and yunnan.however, the heterogeneity of black shalesconcerning the total organic carbon(toc) content and mineral composition, which control gas generation and reservoir quality, limits thepredictability of theshale gas ‘sweet spot(chen et al., 2015; farrooqi et al., 2017). therefore,it is necessary to evaluate the control of shale composition on storage capabilityand shale gas content. in this study, 14samples from well ys8 in the southern sichuan basin areanalyzed to investigate the geochemistry, petrology, and methane adsorption capacityof the shale by toc testing, xrd analysis, and methane adsorption experiments. by discussing the relationship between shale composition and methane adsorption capacity, andshale gas content, we hope to identify the main factors that influence the storage capability and gas content of shale reservoirs,which can then be applied to other shale reservoirs with a similar preservation condition to that of the longmaxi shale. 2. geological setting the study area is located in the changning shale gas field in the southern sichuan basin (figure 1). the sichuan basin is located on the northwestern yangtze plateand is the remnant of the lower paleozoic upper yangtze cratonic basin(lu et al., 2006; karami et al., 2017). the wufeng–longmaxi formations were formed in a continental shelf sea(zhang et al., 2012; liang et al., 2016) during the late ordovician and early silurian(su et al., 2009; sunny et al., 2018).the thickness of the strata varies from several meters to >300 m, with depocenters located in the southern and eastern parts of the basin(guo et al.,2014; nwankwo and nkwankwoala, 2018). thewufeng– longmaxi shale is now mainly thermally over-mature (liang et al.,2011). figure 1. study area and paleogeography of the wufeng and longmaxi formations 3. samples and methods fourteen shale core samples with burial depths ranging from 2382.43 to 2512.42m were selected for analysis from shale gas wellys8 in the study area.the whole-rock mineral compositionswere determined by xrd analysis. the samples were crushed to powder, and xrd analyses were performedusing an x’pert pro x-ray diffractometer with a cu anode at 40 kv, 20 ma, and 0.154 nm wavelength. the toc was measured with a leco cs-400 carbon– sulfur analyzer (combustion at temperatures of >800°c). before analysis, carbonate from the samples wasremoved usinghydrochloric acid (1:1). the methane adsorption capacity was investigated using a pctproe&e high-pressure adsorption–desorption instrument. the samples were dried for 48 h at 110°cand tested in asample cell with a temperature of 30°c (±0.2°c) and a pressure of 0–11mpa.the room temperature was 30°c and humiditywere 1%–2%. the methane adsorption capacity was calculated in m3/t (stp: 0°c, 101.325 kpa) using the material balance principle and the volumetric method (krooss et al., 2002; ali et al., 2018). the shale gas content of core samples comprisesthe desorbed gas, residual gas, and lost gas. desorbed gas and residual gas weremeasuredusingcanister desorption of the fresh cores. the lost gas was calculated using usbm methods. 4. results 4.1 gas content the gas content ofthe 14 samples ranges from 1.49 to 3.43 m3/twith an average of 2.59 m3/t. gas content varies with depth (table 1),showing a decreasing upward trend. the shale samples with a gas content of>2.0 m3/t are found mainly between 2486.77 and 2512.42m depth. 4.2 toc content the toc content of the 14 samples ranges from 0.34% to 4.72% with an average of 1.81%. the maximum toc(>2.0%) was obtained for sixshale sampleswith a burial depth between 2491.48 and 2512.15m(table 1). the samples from depths of <2481m have toc contents of<1.0%. a similarupward decreasingtrend in the toc content of shalehas been reportedelsewhere in the upper yangtze region (e.g.,liang et al., 2011; guo et al., 2014; asghar et al., 2018). table 1. toc, gas content, and langmuir adsorption capacity of the longmaxi shales in ys8 4.3 mineral composition the xrd data show that the 14 black shale samples from well ys8 are dominated by carbonate, clay minerals, and quartz, with subordinate feldspar and pyrite (table 2). the carbonate mineral content ranges from 15.6% to 62.0%(average 32.2%), the clay mineral content ranges from 14.1% to37.2%(average 28.1%), and the quartz contentranges from 15.3% to 41.7%(average28.3%). the feldspar contentranges from 4.2% to16.4%(average 8.0%) and the pyrite content from 1.5% to 9.2%(average3.4%). mineral composition data show vertical variations. the three samples (ys8-1 to ys8-3) withthe minimum burial depths are characterized by high carbonate and quartz contents, moderate clay mineral contents, and low toc contents (hasan, 2018).the fourdeepest samples (ys8-11–ys8-14) contain high carbonate and toc contents, and low clay contents. theseven middle samples (ys8-4 to ys8-10) contain low carbonate, high clay, and moderate toc contents. 61influence of black shale composition on methane adsorption and gas content: implications for gas storage in the longmaxi black shales table 2. mineral compositions of longmaxi shale samples from well ys8 4.4 methane adsorption methane adsorption isotherms are shown in figure 2. the measured isotherms at different temperatures can be fitted well to the langmuir function. the langmuir maximum ch4 adsorption capacities range from 0.95 to 4.11 m3/t, with highercapabilitiesfoundfor samples ys8-9 to ys8-14 (>3.1m3/t).the adsorption capacities of samples ys8-1 to ys8-7 are <1.8 m3/t. figure 2. methane adsorption isotherms of the studied samples 5 discussion 5.1 controls on ch4 adsorption in black shales figure 3 shows scatter plots of the langmuir maximum ch4 adsorption (γl) versus shale composition (organic matter and minerals).γlshows astrong positive correlation with toc. figure 3.langmuir adsorption capacity versus shale composition equation (1) shows the result of linear regression betweenγl and toc content, based on the experimental measurements of this study. γl= 0.8137×toc + 0.891 (1) whereγl is in units of m3(ch4)/t rock and toc is the total organic carbon content in units of wt%. the regression provides an empirical basis for estimatingthe langmuir maximum ch4 adsorption capacity from toc content in a shale reservoir.a similar correlation between gas sorption capacity and toc content at 30°c and 6 mpa pressure has been observed in previous studies (chalmers and bustin, 2007a,b; ross and bustin, 2007, 2009; zhang et al., 2012; msarah and alsier, 2018). the strong correlation betweenγland toc content is due to the massive surface areaof organic matter for ch4 adsorption.caoet al. (2015) reported thatkerogen of the longmaxi shale in the sichuan basin hasa surface area of ~161.2m2/g, which is much higher than that of quartz and carbonate minerals. the proportion of ch4 adsorbedonto organic matter can be estimated byequation (2). γl = toc×γorg. + mineral content×γrock (2) where γorg is thelangmuir ch4 adsorbed onto organic matter and,γrock is thelangmuir ch4 adsorbed onto minerals. mineral content is the total mineral content in wt%, toc is the total organic carbon content in wt%, and mineral content+toc =100%. in the model, we assume that i) ch4 is adsorbed onto two types of solid material (organic matter and minerals), and ii) γrock remains constant, and the change of γrock due to the variations in type and relative content of minerals is negligible compared with the influence of toc content. base on equation (2), when tocis 0%all the ch4 is adsorbed onto inorganic mineral surfaces, with 0.891 m3of ch4 adsorbed for every ton of inorganic minerals. as toc content increases and minerals content decreases, the proportion of ch4 absorbed onto organic matter increases, and when toc is ~1.05%, ~50% of the ch4 is absorbed onto thesurface of organic matter (figure 4). figure 4.relative proportion of ch4 adsorbed on organic matter and minerals 5.2 methane absorption capacity and shale gas content due to the high-to-over thermal maturity of organic matter and strong tectonic deformation,the gas content of the longmaxi shalesis controlled by many factors, especiallythe preservation condition, natural matter abundance, and reservoir quality(e.g.,zou et al., 2010; rahim et al., 2018). in the shale samples from well ys8,the preservation conditions and thermal evolution processes of organic matterare similar, thus enablingan evaluation ofthe influence of shale composition on shale gas content. 62 haihua zhu, tingshan zhang, jun lang, jianli zeng, xing liang, yong he, gaocheng wang, junjun li scatter plots of gas content versus shale composition are shown in figure5. poor relations are found between gas content versus mineral composition.regular change of shale gas content with toc content is observed. as toc content increases, gas content first increases and then remains constant. this relationship indicates that organic matter is a significant controlon gas content when toc content is <2.0%. when tocis >2.0%, the adsorption surface area is sufficient, and other factors such as gas preservation condition may influence the amount of residue gas (indan et al., 2018).a similar pattern isobtained when gas contentis plotted against pyrite content. pyrite content is considered an indicator of redox conditionsand is usually found in larger concentrations when toc content is high, which may explain thesimilar correlation of gas content versus pyrite and toc contents. figure 5. shale gas content versus shale composition the scatter plots of toc content versus gas content, and langmuir adsorption capacity shows that when toc is<2.0% the gas contents are higher than the langmuir adsorption capacity of shale(figure 6). therefore, ch4 partially occurs as free gas within the macropores of the shale (nwankwoala and oborie, 2018; nwankwoala and ememu, 2018).when tocis >2.0%, the gas contents are less than the langmuir adsorption capacity of shale,indicating that ch4 tends to occur mainly as absorbed gas, as the surface area of organic matter is sufficient for gas absorption. figure 6. the scatter plots of toc content versus gas content and langmuir adsorption capacity 6. conclusions the langmuir adsorption capacity of the longmaxi shale is mainly a function of toc content. the regression shows that when toc content is ~1.1%, 50% of the ch4 is adsorbed ontothe organic matter. the mineral content has control on the adsorption capacity of the longmaxi shales. organic matter is a major control on gas content when toc content is<2.0%. when toc is>2.0%, gas content remains constant and is a function of organic matter. scatter plots of gas content and langmuir adsorption capacity versus toc contentshow that when toc is<2.0%, ch4 occurs as both free and absorbed gas andthat ch4 occurs mainly as absorbed gas when tocis >2.0%. 7. acknowledgments this work was funded by the young scholars development fund of southwest petroleum university (swpu) (201499010046), china postdoctoral science foundation (2015m580797), national natural science foundation of china(41772150) and national science and technology major project (2017zx05063002-009). references ali, w., nasir, m. s., nasir, a., rashid, h., ayub, i., gillani, s. h., & latif, m. j. 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(2010). geological characteristics and resource potential of shale gas in china. petroleum exploration and development, 37(6), 641–653. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.107646 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 169 182 g eo te c h n ic s manuscript received: 06/03/2023 accepted for publication: 10/07/2023 influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq asem a. hassan college of science, university of diyala, iraq corresponding author: asem.ahmed@uodiyala.edu.iq abstract keywords: soil compaction; resistivity; geotechnical properties in geotechnical practice, there is a continuous demand for an efficient method for measuring the soil moisture content and dry unit weight of compacted soils used in a wide range of earthwork constructions. the electrical resistivity method has increasingly been used for rapid and non-invasive assessment of some geotechnical properties. this study aims to evaluate the influence of moisture content (mc), dry unit weight (duw), and compaction energy (e) on the electrical resistivity (er) of soil collected from the east of baghdad city, iraq. to achieve this goal, soil specimens were compacted to various mc and duw found in geotechnical practice using different e levels. the er of prepared specimens was measured using the two electrodes method and compared with various geotechnical parameters related to the compaction process. the results showed that the employed mc, duw, and e levels influenced the er. the higher the mc, duw, and e, the lower the er. however, the er was more sensitive to these variables for specimens compacted dry of the optimum. furthermore, the er was correlated very well with volumetric moisture content θ and degree of saturation sr of soil, with a high correlation coefficient (r2 >94%) and very low p-values, which indicated that these correlations were statistically significant. the current findings indicate the usefulness of the er method for predicting these parameters. therefore, using the er method as a rapid and cost-effective technique for the preliminary evaluation of soil compaction variables in earthwork constructions is recommended. however, the current laboratory findings must be confirmed on different soil types. keywords: compactación del suelo; resistividad; propiedades geotécnicas; influencia de la compactación en las características de resistividad eléctrica para suelos de grano fino en el este de baghdad, iraq how to cite this item: hassan, a. a. (2023). influence of compaction on electrical resistivity characteristics of finegrained soil east of baghdad city, iraq. earth sciences research journal, 27(2), 169-182. https:// doi.org/10.15446/esrj.v27n2.107646 resumen en la práctica geotécnica hay una demanda continua por un método eficiente para medir la humedad del suelo y el peso específico seco en suelos compactados ya que esta medida se usa en un amplio abanico de construcciones con movimientos de tierra. el uso del método de resistividad eléctrica (er, del inglés electrical resistivity) se ha incrementado al permitir una evaluación rápida y no invasiva de las propiedades geotécnicas. este estudio se enfoca en evaluar el contenido de humedad, el peso específico seco y la energía de compactación en la resistividad eléctrica de muestras de suelo recolectadas al este de baghdad, iraq. para alcanzar este objetivo, algunas muestras se compactaron a varios niveles de contenido de humedad y peso específico seco que se encuentran en la práctica geotécnica a diferentes niveles de energía de compactación. la resistividad eléctrica de las muestras preparadas se midió con el método de dos electrodos y se comparó con varios parámetros geotécnicos relacionados con el proceso de compactación. los resultados muestran que los diferentes niveles de estos factores influyen en la resistividad eléctrica. a mayor nivel de humedad, peso específico seco y energía de compactación es menor la resistividad eléctrica. sin embargo, la resistividad eléctrica fue más susceptible a estas variables en las muestras óptimas compactadas en seco. además, la resistividad eléctrica se correlaciona muy bien con el contenido de humedad volumétrico y el grado de saturación del suelo, con un alto coeficiente de correlación (r2 >94%) y valores p muy bajos, lo que indica que estas correlaciones son estadísticamente significantes. estos resultados indican la utilidad del método de resistividad eléctrica en la predicción de estos parámetros. además, se recomienda el uso del método de resistividad eléctrica como una técnica rápida y efectiva en costos para la evaluación preliminar de las variables de compactación del suelo en construcciones con movimientos de tierra. de todas formas, estos hallazgos en el laboratorio deben confirmarse en diferentes tipos de suelo. https://doi.org/10.15446/esrj.v27n2.107646 https://doi.org/10.15446/esrj.v27n2.107646 170 asem a. hassan 1. introduction soil compaction is fundamental to improving the geotechnical characteristics and behavior of soils used in natural and engineering structures. therefore, an evaluation of compaction characteristics is needed to maintain the long-term performance of these structures (powrie, 2009). however, soil compaction is a function of key compaction variables such as mc, duw, and e levels employed. in geotechnical testing, standard proctor sp astm d698 (2012) and modified proctor mp astm d1557 (2012) compaction tests have been developed to evaluate soil compaction characteristics. in these laboratory tests, compaction curves, which relate to mc and duw are derived, from which omc and mduw are determined and used to monitor field compaction specifications. in practice, the available methods for measuring mc of soil, such as the oven drying method and soil probes (robinson et al., 2008), and for calculating duw of soil such as nuclear density gauge and sand cone methods (alshammary et al., 2018), are destructive and time-consuming. therefore, there is a continuous need to introduce quick and efficient methods to evaluate these parameters, preferably in a low-cost and non-destructive manner (beck et al., 2011; melo et al., 2021). indirect geophysical methods, such as the er method, have frequently been used as a non-invasive, quick, and cost-effective tool to address hydrogeological (al farajat et al., 2015), geotechnical (farooq et al., 2012), underground mining and tunneling (ebrahimi and abbasinia, 2015; ebrahimi et al., 2016), and environmental problems (srinivasamoorthy et al., 2009; moghaddam et al., 2017; ebrahimi et al., 2021). as electrical conduction is mainly controlled by pore water and the air is infinitely resistive, it has been reported that the er is influenced, among other variables, by mc and duw (seladji et al., 2010; kibria and hossain, 2012). numerous studies have reported a non-linear (power, polynomial, or exponential) relationship between er and mc; the higher the mc, the lower the er (calamita et al., 2012). as the moisture state of soil can be expressed in terms of mc, θ, or sr, some of these studies correlated er with mc (memon et al., 2017; bery et al., 2018), while other studies correlated er with θ (mccarter, 1984; singh and shah, 2015) or with sr (abu-hassanein et al., 1996; hassan and toll, 2015). the compaction of soil increases its dry density (or duw) and reduces its void ratio. therefore, it was reported that increasing the duw of soil reduces its er, especially in drier conditions (beck et al., 2011; bai et al., 2013). however, qiu et al. (2021) emphasized that mc has a greater impact on er than duw when the soil is compacted at low moisture levels. roodposhti et al. (2019) evaluated the influence of moisture content and compaction on construction materials. several studies have highlighted the effect of e levels on er of soil. for instance, hassan and toll (2015) and alibrahim and uygar (2021) noticed that increasing compaction energy reduces air voids, hence, lowering er, and this influence is insignificant for soils compacted at high mc levels. similarly, melo et al. (2021) found that the er-mc relationship is strongly influenced by compaction, particularly in drier conditions, due to the predominant effect of mc on the er of soil. geotechnical-geoelectrical correlations discussed in the literature have shown that the er method is useful for predicting different geotechnical properties (bryson, 2005; siddiqui and osman, 2013). however, previously published studies have shown that the er of compacted soils is affected by various interlinked variables that should be addressed simultaneously (hassan and toll, 2015) and the influence of mc and duw on the er has rarely been distinguished in the literature (beck et al., 2011). therefore, the main interest of this work is to investigate the influence of mc and duw in addition to e levels on the er of compacted fine-grained soils. to achieve this goal, soil specimens collected from the east of baghdad city were prepared and mechanically compacted at different ranges of mc and duw using five e levels. the influence of these interlinked variables on the er characteristics was addressed separately, and the geotechnical-geoelectrical relationships between er and different geotechnical variables such as mc, duw, e level, e, ɵ, and sr were developed and discussed according to the microstructural changes of soil due to the compaction process. 2. materials this study was conducted on soil specimens collected east of baghdad city (figure 1). baghdad is located on the tigris river; therefore, the city is covered by recent heterogeneous alluvial deposits accumulated in the mesopotamian plain due to repeated floods of the rivers alshakarchi and turkie (2011). the figure 1. soil sampling location east of baghdad city 171influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq soil is a landfill material that can be distinguished into two layers; the top one consists mainly of clay, or brown silty clay, or clayey silt with some sand, or gravel; whereas the bottom one consists of dense sand mixed with gravel, or lenses of fine-grained clay or silty clay (karim and wadaa, 2017). soil specimens collected from the area were properly secured in plastic bags and brought to the lab for testing. thirty-five specimens were prepared and compacted at different mc, duw, and e levels, as discussed in the next section. 3. methods after soil sampling, laboratory tests were carried out to characterize the soil used in this study based on the astm standards shown in table 1. astm d422 (2002) was used to determine the grain size distribution. in this test, the oven-dried soil was poured above a stack of standard sieves for 15 minutes, and the mass retained on each sieve was determined to obtain the percentage of coarse-grained particles. the percentage of silt and clay was determined using the hydrometer. astm d2216 (2005) was performed for mc determination. the soil specimen was oven-dried overnight at 105 °c, and the mass of water and dried soil was then calculated to determine the mc of the soil. astm d4318 (2005) was utilized to determine the ll and pl of soils. the ll was determined using the casagrande device. several trials of different moisture contents were prepared. ll is the mc at which a standard groove made into the soil closes for 12 mm on being given 25 blows. the pl was determined by manually rolling out a tiny ball of moist plastic soil into a 3 mm thread until it crumbled at the moisture content known as the pl. in addition, astm d2487 (2017) was adopted to classify the soil based on uscs classification. furthermore, the sp compaction test astm d698 (2012) was carried out to compact the soil, from which compaction curves were plotted and omc and mdd were obtained. finally, soil resistivity was measured according to astm g187 (2005) and compared with different geotechnical properties. to prepare the compacted specimens, the soil was oven-dried for 24 hours, mixed to the desired mc using distilled water, and left for 48 hours in sealed bags to facilitate moisture homogenization. the specimens were prepared at different mc levels ranging from 6.5% to 20.5% and compacted using a standard astm mold of 10.16 cm diameter, 11.64 cm height, and 944 cm3 volume (figure 2-a). soil specimens were compacted using five e levels using 15, 25, 35, 45, and 55 blows, including standard proctor (i.e., 25 blows), leading to a wide range of duw values. in this method, thirty-five specimens were compacted at e ranges from 355.98 to 1305.25 (knm/m3) (table 2). this procedure was applied to cover a wide range of mc, duw, and e that can table 1. astm standards adopted for the laboratory tests laboratory test standard test method grain size analysis astm d422, (2002) moisture content astm d2216, (2005) liquid limit astm d4318, (2005) plastic limit astm d4318, (2005) soil classification astm d2487, (2017) standard proctor test astm d698, (2012) soil resistivity astm g187, (2005) table 2. soil compaction procedure level of compaction energy number of blows number of soil layers hammer weight (kn) hammer height (mm) compaction energy e (knm/m3) number of specimens e1 15 3 2.495 304.9 355.98 7 e2* 25 3 2.495 304.9 593.29 7 e3 35 3 2.495 304.9 830.61 7 e4 45 3 2.495 304.9 1067.93 7 e5 55 3 2.495 304.9 1305.25 7 *standard proctor compaction be found in geotechnical practice. after compaction, the er of specimens was measured using the kangda kd2571b2 resistance instrument (figure 2-b). two circular electrodes were attached to the end of the compacted specimen and connected to the instrument (figure 2-c). using this method, the er of soil was calculated according to the following formula: (1) ∆v (volt) is the measured voltage drop, i (ampere) is the injected current, a (m2) is the specimen’s cross-sectional area, and l (m) is the length of the specimen. this method facilitates a simple and direct er measurement, as adopted by several authors (mccarter, 1984; memon et al., 2017; qiu et al., 2021). 4. results and discussion 4.1. soil characterization figure 3-a depicts the particle size distribution of the soil. the soil consists of 1.03% gravel, 20.50% sand, 42.12% silt, and 36.35% clay. the soil is considered fine-grained as more than 50% of the soil passed through a #200 (0.075 mm) sieve. ll is 31.5%. pl is 18.5%, and pi is 13.00%. according to the plasticity chart shown in figure 3-b and the unified soil classification system (uscs), the soil is classified as type cl (low plasticity clay soil). 4.2. compaction characteristics in geotechnical testing, the compaction curve that relates mc and duw is usually used to determine the compaction characteristics (i.e., omc and mduw). because of compaction, the air is removed from the pores dry of optimum, which makes the soil grains denser (i.e., duw increases). however, at high mc levels (beyond the optimum), the voids are more filled with water, which prevents soil densification (i.e., duw decreases). figure 4 depicts compaction curves of compacted specimens using compaction effort or e levels ranging from 355.98 to 1305.25 knm/m3 including the sp (25 blow) compaction curve with the sr 100% line (or zero air void zav line). from the sp compaction curve shown in red, it can be noticed that omc is 15.50% and mduw is 17.85 kn/m3. figure 5 shows the influence of increasing e levels on mduw and mc. increasing the e level reduces air voids and increases the duw of the soil (i.e., forces the soil particles to pack in a denser state); therefore, e increases mduw and decreases the omc required to reach the optimum (das & khaled, 2018). 4.3. influence of compaction and e level on the er of soil figure 6 presents the er-mc relationships of compacted specimens using different e levels. er decreases with increasing mc, and increasing e from 15-55 blows decreases er, particularly at low mc levels. however, the influence of compaction on er is insignificant at high mc levels. for instance, er decreases from 105.12 ohm.m to 60.55 ohm.m for the specimens compacted at 6.5% mc using e1 and e5 (i.e., 15 and 55 blows, respectively). in a comparison, er at a high 20.5% mc is very low (er~5.00 ohm.m) and not affected the e level used. at low mc levels where voids are mostly filled with air, er is relatively high, and increasing the e level reduces air voids, 172 asem a. hassan hence lowering er. at a high mc level, er is relatively low and not affected by compaction due to the predominant effect of mc on er as electrical conduction is already achieved, and increasing e level has an insignificant effect on er (melo et al., 2021; qiu et al., 2021). the influence of e level on er is further investigated for different mc levels, as shown in figure 7. it is evident that er decreases linearly with increasing e level for a particular mc level, and this effect is more significant at a low mc level. er is constant and not influenced by the e level used at the high mc level, as the slope of the linear relationship flattens at the high mc level, which supports the above discussion. 4.4. influence of mc on the er of soil figure 6 shows that er decreases non-linearly with increasing mc; the higher the mc, the lower the er. similar non-linear relationships have been widely developed in the literature (e.g., seladji et al., 2010; beck et al., 2011; bery et al., 2018). figure 6 also indicates that er is strongly correlated to mc for different e levels with r2>0.99, demonstrating that er can be used to estimate the mc of compacted soils. in addition, the er-mc relationship is further discussed for sp compacted specimens in figure 8. er is relatively high at low mc levels dry of optimum, while it is relatively low and constant at high mc levels wet of optimum. the er-mc relationship can be discussed regarding the microstructure changes of fine-grained soil because of the compaction process. at low mc, soil grains are characterized by a high air void ratio and are difficult to remold, resulting in higher er. in contrast, soil grains are easy to remold at figure 2. laboratory work (a) soil compaction (b) compacted specimen (c) er measurement figure 3. (a) grain size distribution (b) plasticity chart used for uscs classification high mc as the voids are more filled with water, resulting in lower resistivity (abu-hassanein et al., 1996). the moisture discontinuity dry of optimum, hence high er, and the continuity of moisture wet of optimum, hence low er, support such er behavior (fukue et al.,1999). similar relationships were reported in the literature (beck et al., 2011; hassan and d. toll, 2015). the ermc relationship for all compacted specimens using e1 to e5 levels is fitted to the non-linear (polynomial) relationship presented in figure 9, and expressed as follows: er=0.526mc2-20.26mc+197.4 (r2=0.904) (2) although a high r2 of 0.904 is achieved, using er to estimate mc (gravimetric) can be erroneous, especially in dry conditions, as the soils in the field can be found at the same mc level but compacted at different e levels and sr values. therefore, it is better to correlate er with θ or sr, as will be discussed later. 4.5. influence of duw/ e on the er of soil in geotechnical testing, it is well known that the compaction process increases duw at the dry side of optimum up to the mduw, then duw increases at the wet of optimum (budhu, 2015), as can be noticed in the compaction curve shown in figure 8. 173influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq figure 4. compaction curves of soil specimens using different e levels with sr 100% line figure 5. influence of e level on mc and mduw of soil 174 asem a. hassan figure 6. er-mc relationship using different e levels figure 7. influence of e level on the er of soil 175influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq figure 8. er-mc relationship of sp compacted specimens figure 9. er-mc relationship of all compacted specimens 176 asem a. hassan figure 10. influence of duw on er of sp compacted specimen figure 11. influence of e on er of sp compacted specimen 177influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq figure 12. er-duw relationships for different mc levels figure 13. er-e relationship for different mc levels 178 asem a. hassan figure 14. influence of duw on er-mc relationship figure 15. influence of e on er-mc relationship 179influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq figure 16. er-θ relationship of all compacted specimens (p-value= 2.99e-15) figure 17. er-sr relationship of all compacted specimens (p-value=1.11e-18) 180 asem a. hassan to explore the influence of duw and e on er, er is plotted against duw in figure 10 and against e in figure 11 for sp compacted specimens. er is more influenced by duw and e variations dry of optimum. er decreases rapidly with increasing duw and increases with decreasing e at the dry side of the optimum up mduw, then er decreases gently wet of the optimum. it seems that two er behaviors are separated by the optimum, which is an interesting finding reported by beck et al. (2011). as discussed earlier, at the dry side of optimum, increasing duw decreases e in the soil and lowers er. however, this effect is insignificant at wet of optimum because of the water that fills the pores, which facilitates electrical conduction and lowers er (melo et al., 2021; qiu et al., 2021). to further investigate the influence of duw and e on er for specimens compacted using different e levels, er is plotted against duw and e for different mc levels, as shown in figures 12 and 13, respectively. er decreases linearly with increasing duw, and increases with increasing e for different mc levels, particularly at the low mc levels (or dry of optimum). the slope of the relationships is relatively steep and flattens at high mc levels (or wet of optimum). this behavior confirms that the influence of compaction on er is significant at low mc levels compared to high levels. using the regression equations shown in figures 12 and 13, er-mc relationships are plotted for constant duw and e values, as shown in figures 14 and 15, respectively. er increases with decreasing mc at a constant duw or e. however, increasing duw/decreasing e, makes er-mc relationships flatter. this behavior supports the above discussion, as when duw increases/ e decreases, more voids are filled with water, which improves electrical conduction and reduces the er of the soil. 4.6. er-θ and er-sr relationships the above discussions showed that the er is sensitive and well correlated to the main compaction variables (mc, duw, and e levels employed). mc and duw can be integrated into one geotechnical property, θ. furthermore, increasing e is accompanied by increasing sr, which integrates the influence of mc and e of the soil. figures 16 and 17 show erθ and er-sr relationships for all data presented in this work. figure 16 shows that er decreases with increasing θ. increasing θ means more water available in the pores that enhance the electrical conduction, hence low er, and vice versa. the erθ relationship is formulated using the following equation: er=0.162θ2-11.01ɵ+189.6 (r2=0.947) (3) the high r2 achieved demonstrates that er is strongly related to θ and can be used to estimate θ of the soil (mccarter, 1984; fukue et al., 1999; michot et al., 2003; hassa and toll, 2015). similarly, er decreases with increasing sr, as shown in figure 17. at low sr levels, the discontinuity of water in the voids makes er relatively high and changes abruptly; however, at high sr levels, the continuity of water is improved so that the electrical conduction causes a decrease in er. the er-sr relationship is formulated using the following equation: er=215.3sr2-402.6sr+194.0 (r2=0.983) (4) the high r2 achieved indicated that er is strongly correlated with sr and can be used to estimate this geotechnical property (abu-hassanein et al., 1996; safari et al., 2013; hassan and toll, 2015). similar er-sr relationships that are relatively less dependent on the e level used have been reported in previous studies (abu-hassanein et al., 1996; hassan and toll, 2015). moreover, the anova tool was used to examine the statistical significance of the correlations shown in figures 16 and 17. the p-values were 2.99e-15 and 1.11e-18 for the er-θ relationship and the er-sr, respectively. p-values less than 0.05 indicate that these correlations were statistically significant. finally, er-θ and er-sr relationships presented in this study are compared with those similar relationships published in the literature, as shown in figures 18 and 19, respectively. the current relationships confirm the non-linear trend reported in the previous studies, which confirms the validity of using the er method for rapid and low-cost preliminary estimation of these geotechnical parameters. however, more work is required on specimens compacted at extremely low mc levels. in addition, the current laboratory findings need to be confirmed figure 18. a comparison between the er-θ relationship of the current study and similar published relationships 181influence of compaction on electrical resistivity characteristics of fine-grained soil east of baghdad city, iraq on soils of different types. moreover, the influence of compaction, compaction energy, and other physical properties on the er of fine and coarse-grained soils needs to be further investigated at laboratory and field scales. 5. conclusions this study aimed to investigate the distinct influence of compaction on the er of fine-grained soil collected from the east of baghdad city. it was found that the er of soil is influenced by key compaction variables: mc, duw, and e levels employed, and this influence is more significant for specimens compacted at the dry of the optimum; the lower the mc, duw, and e levels, the higher the er. this trend can be explained in terms of microstructure changes due to the compaction process. the er is non-linearly correlated with the mc of soil specimens compacted at different e levels, while it shows a linear correlation with the duw. as soil can be found in the field at the same mc but different compaction levels, it is better to correlate er with θ and sr instead of mc. it was noted that the er is strongly correlated with θ and sr, with high r2 values of 0.947 and 0.983 and p-values of 2.99e-15 and 1.11e-18, respectively which indicate that these correlations are statistically significant. moreover, the geotechnical-geoelectrical correlations achieved in this study were consistent with those reported in the literature. the current study indicated that the er method can be a preliminary, cost-effective tool for evaluating compacted soils at the early stages of engineering site investigations. however, the current findings need to be explored and confirmed on different types of soils. more work must be done on soil compacted at very low mc levels. furthermore, the influence of compaction and other soil physical properties on the er of soil needs to be addressed in the field. acknowledgment the author would like to thank mr. ali hattab from midland oil company, baghdad, for his help in soil sampling. references abu-hassanein, z. s., benson, c. h., & blotz, l. r. 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(2023). assessment of the extent of soil degradation over different land uses in the kebbi area, northwestern nigeria. earth sciences research journal, 27(4), 415-424. https://doi.org/10.15446/ esrj.v27n4.102140 keywords: keywords: estimates, gully characteristics, soil degradation, land uses. keywords estimaciones; características de hondonadas; degradación del suelo; usos del suelo: this paper assessed soil degradation over different land uses in the kebbi area, northwestern nigeria. the specific objectives entailed identifying major forms of soil loss in the study area, estimating soil loss and examining the responses of soil physico-chemical properties over dominant land use types. slope angles were determined using a gps, a handheld abney level, ranging poles and a 30 m measuring tape while gully depths, widths and lengths were taken at 5 m-30 m intervals. key soil physico-chemical parameters were determined using standard procedures. descriptive statistics such as mean, standard deviation and coefficient of variation were used to summarize the data generated from the study. the analysis of variance and the student’s t-test was used to test for significant association between and within pairs of land uses. the results show that slopes range from 4⁰ 16⁰, while the magnitude of soil loss ranged between 3580.32 m3 in k1 (kalgo 1), 3550.89 m3 in g2 (goru 2) and 161.01 m3 in a1 (angwar daji 1). results show significance at p≤ 0.05 in bulk density values over the different land uses with the highest bulk density value of 1.78 g/cm3 in badland and lowest value of 1.35 g/cm3 in plantation land use. total nitrogen range between 0.01 0.38% with the least values in badland and scrubland, while soil base saturation is highest over plantation (76.3%) and lowest in badland (50.4%). the study concludes that the dominant geomorphic responses identified in the study area are gully initiation and development and a varying magnitude of soil degradation over the different land uses. the findings should facilitate policy initiation to rehabilitate degraded lands, adopt sustainable soil management practices such as tree planting while linking geomorphological information to infrastructural planning and development. evaluación de la extensión de la degradación en suelos de diferentes usos en el área de kebbi, al noroeste de nigeria este trabajo evalúa la degradación del suelo debido a su uso en el área de kebbi, al noroeste de nigeria. los objetivos específicos i mplican l a i dentificación de la s ra zones pr incipales de la pé rdida de su elo en el ár ea de es tudio, la estimación de esta pérdida, y la evaluación de las respuestas de las propiedades físicoquímicas del suelo en los tipos de uso del suelo dominantes. los ángulos de la pendiente se determinaron con un gps, un nivelador abney de mano, balizas, y una cinta métrica de 30 metros mientras que la profundidad de las cárcavas, el ancho y el largo se tomaron en intervalos de entre 5 y 30 metros. los parámetros fisicoquímicos clave del suelo se determinaron con procedimientos estandarizados. estadísticas descriptivas como la media, desviación estándar y variación de coeficiente se utilizaron para sintetizar la información generada durante el estudio. el análisis de la diferencia y la prueba t de estudiante se utilizaron para evaluar las asociaciones significantes entre y dentro de pares de usos del suelo. los resultados muestran que la inclinación varía entre 4⁰ y 16⁰, mientras que la magnitud de perdida de suelo se estima en 3580.32 m3 en k1 (kalgo 1), 3550.89 m3 en g2 (goru 2) y 161.01 m3 en a1 (angwar daji 1). los resultados muestran importancia en los valores de densidad aparente p ≤ 0.05 sobre los diferentes usos del suelo, con el mayor valor de densidad aparente de 1.78 g/cm3 en tierras baldías y el menor valor, de 1.35 g/cm3, en suelos usados para plantaciones. el nitrógeno total varía entre 0.01 – 0.38%, con los menores valores en tierras baldías y matorrales, mientras que la saturación base del suelo es mayor en los suelos plantados (76,3 %) y el menor en tierras baldías (50,4 %). el estudio concluye que las respuestas geomórficas identificadas en el área de estudio son la iniciación y desarrollo de hondonadas y varían la cantidad de degradación del suelo sobre los diferentes usos del suelo. los hallazgos debería facilitar la iniciación de políticas para rehabilitar suelos degradados y adoptar prácticas sostenibles de manejo de suelos como la plantación de árboles, además vincular la información geomorfológica a la planificación y desarrollo de infraestructura. https://doi.org/10.15446/esrj.v27n4.102140 http://shiaku.ibrahimi@fubk.edu.ng https://doi.org/10.15446/esrj.v27n4.102140 https://doi.org/10.15446/esrj.v27n4.102140 416 zuni aminu, ibrahim ishiaku, haruna gattah aminu, and yauri safyanu garba 1. introduction soil is an important natural resource that either directly or indirectly supports most of the planet’s life (ilo, 2007). a major function of soil is that it supports food supply, food security and provides the primary source of livelihood for 36% of the world’s total workforce (ilo, 2007). soil degradation encompasses physical, chemical and biological deterioration and manifests in loss of organic matter, decline in soil fertility and structural condition, erosion and excessive flooding (doran and parkin, 1994; stocking and murnaghan, 2000). soil degradation occurs globally but its negative impacts are most felt in regions which depend solely on agriculture for incomes (swift and shepherd, 2007). previous studies (stocking and murnaghan, 2000; aminu and jaiyeoba, 2015; bashir et al., 2020; yousaf et al., 2021) have documented major causes of soil degradation relating to environmental factors. these relate to rainfall, soil, topography and vegetation characteristics. rainfall provides energy for sediment detachment from soil surfaces and transportation down slope, causing varying degrees of erosion (bashir et al., 2020). this is because bare or poorly vegetated surfaces when accompanied by substantial variation in elevation and precipitation are highly vulnerable to erosion (bashir et al., 2020). one of the ways in which soil degradation manifests is through soil erosion. soil erosion systematically transforms soil into sediments, causing loss of soil and soil fertility (tekwa and usman, 2006). gullying is a serious form of soil degradation with severe consequences for agriculture and environmental sustainability (tekwa and usman, 2006). the degradation of forest lands, the development of agricultural land and the conversion of rangelands into rain fed farming is leading to an increase sediment yield (mohammadi et al., 2021). land use change has been recognized as one of the main influencing factors of soil erosion and sediment production processes (mohammadi et al., 2021). although soil degradation processes can occur without human interference, studies have shown that accelerated rates of degradation are often influenced by man’s intervention in natural processes (aramolaran et al., 2001; yousaf et al., 2021). inappropriate land use practices have been found to be a major cause of soil degradation, manifesting in land use and land cover changes and increasing the vulnerability of land surfaces to erosion processes (bashir et al., 2020; yousaf et al., 2021). land use practices which result in land cover changes represent a major source of global environmental degradation (yousaf et al., 2021). increased pressure on land as a result of increasing world population has led to increased degradation of world soils (doran and parkin, 1994; stocking and murnaghan, 2000). it is estimated that 33% of global soils are degraded, with over 500 million hectares of tropical soils battered by accelerated erosion and other forms of degradation (yousaf et al., 2021). rapid urban expansion triggered by population growth has increased the demand for more built-up spaces and agricultural lands. for example, agriculture was found to contribute 82 percent of natural vegetation transformation (aramolaran et al., 2001). the transformation of natural vegetation has been found to accelerate the soil degradation process by increasing the tendency to bring more marginal lands under cultivation (aramolaran et al., 2001; aminu and jaiyeoba, 2015). in the developing countries, such as nigeria, human-induced soil degradation has intensified, due mainly to the expansion of agricultural lands into marginal areas, occupied by poor rural households characterized by low incomes and diminishing land productivity (aramolaran et al., 2001; aminu and jaiyeoba, 2015). soil degradation is a symptom of under-development and results from a combination of factors such as poverty, inequitable distribution of land resources and inappropriate land use systems (scherr and yadav, 1996; bridges and van baren, 1997). emerging from the soil degradation researches in the last few decades, is an attempt to develop different methods and techniques for effective assessments and monitoring of soil degradation occurrences globally (wischmeier and smith, 1978; stocking and murnaghan, 2000; stringer and reed, 2006; hamisu et al., 2018; yousaf et al., 2021). for example, mohammadi et al. (2021) in an assessment of the impact of land use changes on soil erosion intensity and sediment yield in the talar watershed of iran, found that the intero model has proven to be a suitable tool for assessing watershed responses to land use changes in terms of peak flow, erosion, and sediment yield, especially in countries with data limitation. in assessing land degradation in south africa, stringer and reed (2006) observed that scientific methods such as remote sensing and gis in measurements of soil properties have limitations. they observed that satellite imageries may guide the sampling of fields but are often not as effective as on-field erosion assessment. similarly, bashir et al., (2020) observed that using evidence-based models for degradation assessments still tend to possess uncertainties such as the potential of bias induced weight and the absence of a spatial scope of the area of interest. however, these uncertainties have the potential of significantly limiting their application. adopting the land user approach in studying soil degradation provides more practical views of the types of interventions or solutions that are more acceptable to land users (stocking and murnaghan, 2000), but apart from being subjective, participatory surveys are slow, expensive and primarily based on contextual analysis often embedded in socio-cultural beliefs which may lead to inaccurate accounts of the extent and magnitude of any environmental change (stringer and reed, 2006). soil loss assessments have been assessed using the universal soil loss equation (wischmeier and smith, 1978) and the revised universal soil loss equation (renard et al., 1997). the universal soil loss equation predicts the long-term annual rate of erosion on field slopes based on the rainfall pattern, soil types, topography, crop system and management practices (renard et al., 1997). though these equations provide detailed understanding of erosion processes, they have limitations because of the difficulty of generalizing the results due to varying environmental conditions (mohammadi et al., 2021). various studies have been carried out in northwestern nigeria to assess soil degradation and soil management practices. for example, mailumo et al. (2011) assessed soil degradation using the land user perception and observed that bush burning and excessive cropping were major causes of soil degradation in the region. usman (2016) used an integrated soil surface approach (issa) which combined a visual soil analysis (vsa) and digital soil analysis (dsa). the study laid emphasis on soil forming factors and surface soil characteristics as major determinants of soil quality. similarly, aleiro et al., (2018) evaluated soil physical properties and their impact on vegetation status in kebbi state, using geo-spatial techniques. although literature exists, there is very limited data on the processes and impact of soil degradation in the study area where many lives and livelihoods depend on agriculture. thus, while several studies on soil degradation have been carried out in the study area the dynamics and extent of soil degradation are changing with time and there is need for studies which integrate land use change assessments and actual field quantifications to provide a holistic and up to date picture of the problem. tackling the menace of soil degradation requires an understanding of the rates of the degradation processes as well as identifying major controlling factors which enhance degradation processes. it is against this background that the study aims at an assessment of soil degradation over different land uses in the kebbi area, northwestern nigeria. the objectives include identifying major forms of soil loss, estimating soil loss and examining the responses of soil physicochemical properties over dominant land uses, with a view to establishing a soil degradation pattern in the study area. 2. materials and methods 2.1 study area kebbi state is located on latitude 12⁰ 10’.90’’n to 12⁰40’.00’’n and longitude 4⁰10’.00’’e and 4⁰40’00’’e (fig.1). the climate is dominated by two air masses, tropical maritime and tropical continental which determine the two dominant seasons of wet and dry. the wet season lasts from april to october in the south and may to september in the north; while the dry season lasts for the remaining period of the year (aleiro et al., 2018). rainfall is highly seasonal and varies from over 1000 mm in the southern parts to less than 500 mm in the northern part (aleiro et al., 2018). average temperatures range from 21°c to 40 °c between april and june. relative humidity is generally low (40 percent) for most of the year except during the wet season when it reaches an average of eighty percent. 417assessment of the extent of soil degradation over different land uses in the kebbi area, northwestern nigeria figure 1. geographical location of the study area source: gis laboratory, department of geography, federal university birnin kebbi. the river rima flows through the sedimentary area into the river niger in the south west, creating extensive flood plains (online nigeria, 2003). kebbi state can be divided into three relief regions, namely the high plains in the south and southeast, the plain landscape in the north and the riverine lowland of the niger and lower rima valleys (mailumo et al., 2011). the high plains are characterized by dissected crystalline rocks with hill ranges and domical rises (inselbergs). the plain landscape, where the study area is situated, is monotonous lowland and forms part of the vast sokoto plains. it is sedimentary in origin, with an average height of about 300 m above sea level. the plain surface is interrupted by isolated flat topped lateritic capped hills and ridges. the riverine lowlands are mainly the flood plains of the major rivers (aleiro et al., 2018). the natural vegetation of the study area is the sudan savannah and consists mainly of shrubs and bushes with few trees of economic value such as acacia aibida (gawo) and drum palms (goriba). dominant soil types are the upland and fadama soils. these two soil groups are generally characteristic of the entire sokoto rima basin. while the upland soils are sandy and well drained, the fadama soils are generally clayey and hydromorphic. rain-fed agriculture is the dominant economic activity in the study area. however, irrigation farming along wetlands (fadamas) also exists and is practiced throughout the dry season with rice as the major crop produced (usman, 2016; aleiro et al., 2018). the dominance of agricultural production as the primary socio-economic activity in the study area predisposes the inhabitants to the negative impact of soil degradation. 2.2 methodology 2.2.1 data collection based on the result of a reconnaissance survey, 3 study locations were selected for the study. these are goru, kalgo and ungwar daji, all located within the kalgo area and characterized by the same climate, geology and land use types. the study comprised of 2 experimental plots (100 m× 25 m) per land use (plantation, fadama, cultivated, scrubland and badland land uses), giving a total of 30 experimental plots. an inventory of soil loss indicators taken during the reconnaissance survey of the study area indicated that the dominant visual indicators of soil degradation in the study area include rills, gullies, sediment deposition, compaction, tree root exposure and loose soils. however, soil loss estimates were taken over gullies only. the gully sites were marked with plot sizes 100 m length and 25 m width, respectively. these dimensions were chosen to enable measurements be carried out accurately. measuring stations were also established at every marked point where successive measurements of slopes, gully length, width, depth and volume of soil loss amongst others were recorded. 2.2.2 determination of gully morphometry field assessments were carried out to determine gully characteristics in the study area. in assessing the contribution of gully occurrence to erosion in a tropical environment, the depth and width characteristics of gullies are 418 zuni aminu, ibrahim ishiaku, haruna gattah aminu, and yauri safyanu garba critical parameters that determine the dynamics of soil degradation (bashir et al., 2020). therefore, gully depths, widths and lengths were taken at 5 m-30 m intervals (after stocking and murnaghan, 2000; tekwa and usman, 2006). altitudes were taken at the head of each gully using a gps. slope profiles along measured gullies were determined using a handheld abney level, ranging poles, and a 30 m measuring tape. slope and land use characteristics were described based on field observations. 2.2.3 soil physicochemical assessments soil samples used to determine soil physicochemical properties were drawn from five land use categories identified in the study locations (plantation, fadama, cultivated, scrubland and badland land uses) in june, 2021, corresponding with the start of the wet season. surface soil samples, at 0-30 cm depth were taken over quadrants established during transect walks. samples were taken 20 cm apart at four equidistant points for each land use and bulked to form composite samples. each sample was packed into translucent polythene bags and labelled appropriately. thus, on the whole, 30 composite samples were drawn and used for the study. each soil sample was airdried, crushed and sieved through a 2 mm sieve and used for the assessment of selected soil physicochemical properties using standard procedures. core samples were analyzed for water content and bulk density gravimetrically, while porosity values were determined from bulk density values, using an assumed particle density value of 2.65 g/cm3. particle size distribution was determined using bouyoucos hydrometer method. soil ph was determined potentiometrically in 0.1m cacl 2 solution using a soil to solution ratio of 1:2.5. the walkley-black digestion method was used to determine the soil organic carbon. total nitrogen was determined using macro kjedahl method, while available phosphorous was determined using bray no.1 method. the cation exchange capacity (cec) was determined using the ammonium acetate saturation method. the leachate preserved during the cec determination was used to determine the exchangeable bases (ca, mg, k and na). the percentage base saturation was derived by dividing the sum of the values of the exchangeable bases by the cec value and expressed as percentage. similarly, the exchange acidity was determined by extraction with kcl (agbenin, 1995). 2.3 data analysis descriptive statistics such as mean, standard deviation and coefficient of variation were used to summarize the data generated from the study. gully cross-sectional area was computed (after stocking and murnagham, 2000) using the formular; (1) where; a= gully cross-sectional area, w1 = average gully width at lip 1, w2 = average gully width at lip 2, d= gully depth (m) from eq. (1), the volume of soil loss from the gullies is estimated as: vl = a× l (2) where; vl= volume of soil loss (m3), a= gully cross-sectional area, l= gully channel length the analysis of variance (f-ratio) was used to test for variations in the selected soil properties over the different land uses while the student’s t-test was used to test for significant association between pairs of land uses. 3. results and discussion 3.1 gully characteristics of the study area the characteristics of the gully system in the study area are presented in table 1. a total of ten gully units were identified in the study area and numbered as k1 (1, 2), k2, k3 (1, 2), a1, a2, g1 and g2 (1, 2). table 1. gully characteristics gully site altitude (m) average width (m) average depth (m) crosssectional area (m2) gully length (m) order k1(1) (2) 191 13.6 5.6 76.2 42.7 2 203 3.2 4.6 14.7 22.3 1 k2 193 7.4 7.23 54.0 47.3 1 k3 (1) (2) 202 7.72 1.69 13.0 43.0 2 204 4.6 1.25 5.69 56.8 1 a1 200 3.7 0.98 3.61 44.6 1 a2 209 4.54 2.13 9.66 30.0 1 g1 g2 (1) (2) 225 3.81 2.78 10.6 35.8 1 194 13.6 5.8 78.9 38.9 2 226 3.5 3.11 10.9 44.2 1 *k= kalgo; a= angwar daji; g= goru the gully network labelled as k1 has an elevation of 191 m above sea level and is located on the fringes of the built-up area in kalgo town. it has a length of 42.7 m and terminates in an artificial drain, 1.3 m away from the highway. k1 is a matured gully characterized by broadly lobed heads, located on a steep slope of 16⁰. it has an average width of 13.6 m and an average depth of 5.6 m. the gully side walls are dissected by numerous rills which are micromorphological features with average width and depth of about 0.5 m and 1 m respectively. the presence of several tree-root exposures indicates undercutting by runoff, while loose soils on gully beds indicate soil loss from upslope terrain. the gully is meandering and serves as an ephemeral stream in the rainy season as it receives substantial surface runoff from the built-up upslope terrain. however, it dries out during the dry season and becomes a useful source of sand mining for construction purposes. it is a second order gully which receives runoff water and sediment from a first order gully which has a width, depth and length of 3.2 m, 4.6 m and 22.3 m respectively. k2 is a discontinuous gully characterized by vegetation such as short grasses and shrubs. it is 193 m above sea level on a steep slope of 16⁰. the gully unit is fed with runoff and sediment from upslope by small rills which cascade downwards and are enlarged through the process of micro piracy. it covers a distance of 47.3 m with an average width and depth of 7.4 m and 7.23 m respectively. k3 is found on an elevation of 202 m above sea level. it has a length of 43 m, a width of 7.72 m and depth of 1.69 m. it is a network of two gully heads on a strongly sloping profile with sediment deposited on the gully bed to an average depth of 1 m. the a1 gully unit is found on an elevation of 200 m above sea level with a moderately steep angle of 13⁰, extending to a distance of 44.6 m. it has an average width of 3.7 m and is shallow at 0.98 m. the dominant land use type is a mix of badland, shrubs and stony surfaces. a2 is found on elevation of 209 m above sea level on a moderately sloping terrain of 4⁰, covering a distance of 30 m. it is a discontinuous gully with an average width and a shallow depth of 4.54 m and 2.13 m respectively. the g1 gully network is located on an elevation of 194 m above sea level. it is developed on strongly sloping terrain of 7⁰. it is characteristically u-shaped and covers a distance of 38.9 m, with an average width and depth of 13.6 m and 5.8 m respectively. the g2 is on an elevation of 226 m above sea level. it is a second order gully developed on a steep slope of 15⁰ and derives runoff and sediments from the first order gully and a series of micro channels which coalesce to form gullies. generally, the gullies in the study area are ephemeral and are characterized by micro-piracy, meandering, linear undercutting and formation of grooves in the base and sidewalls (figure 1). 419assessment of the extent of soil degradation over different land uses in the kebbi area, northwestern nigeria figure 2. gully channel characteristics in the study area meandering within the gully channel (b) groove formation in the gully channel (c) linear undercutting along footpath (d) deeply incised soil surfaces (e) eroded surfaces characterized by micro-piracy (f) movement and deposition of sediment down slope 3.1.1 slope characteristics the slope description and characteristics recorded for the study area are presented in table 2a and table 2b. table 2a. slope description slope (percentage) slope (degrees) slope class description 0-2 0–1.15 a nearly level 3–6 1.72–3.43 b gently sloping 7–12 4.00–6.84 c moderately sloping 13–18 7.41–10.20 d strongly sloping 19–25 10.76–14.04 e moderately steep 26–35 14.57–19.29 f steep >35 >19.29 g very steep (after fashae et al., 2022) table 2b. slope characteristics gully site slope (degrees) slope class description k 1 16 f steep 7.0 d strongly sloping k2 16 f steep k3 10 d strongly sloping 11.5 e moderately steep a1 13 e moderately steep a2 4 c moderately sloping g1 7 d strongly sloping g2 15 f steep 7.5 d strongly sloping the results from table 2a indicate that the study area is characterized by slopes ranging from 4⁰ 16⁰. the slope description in table 2b show that slope classes are moderately sloping, strongly sloping, moderately steep and steep. the slope characteristics have implications for soil degradation as they influence the direction and volume of runoff and soil loss (tekwa and usman, 2006; aminu, 2015). the slope characteristics of the study area explain the depth of sediment deposition on the gully beds in the study area, movement of soil down slope (figure 2f) and accretion along gully beds. results from table 2b indicate that larger gully depths and widths correspond with areas characterized by steeper slopes. studies by tekwa and usman (2006); aminu et al., (2017) have shown that rill and gully initiation and development are exacerbated by steep slopes as slopes greatly influence the direction of flow of runoff, sediment transport and deposition. similarly, slope profile characteristics influence soil depth, bulk density (tekwa and usman, 2006; aminu et al., 2017) infiltration and soil moisture retention (mohammadi et al., 2021). 3.1.2 soil loss the capacities of the gullies were determined to estimate the volume of soil loss from the gullies and are presented in figure 3. figure 3. volume of soil loss across the study area the gully system with the highest soil loss is k1 with an estimated soil loss of 3580.32 m3. it is followed by g2 with 3550.89 m3 and k2 with 2554.2 m3. k3 has an estimated soil loss of 872.6 m3, followed by g1, a2 and a1with 379.5 m3, 289.8 m3 and 161.01 m3 soil loss estimates respectively. the results indicate that soil loss estimates were higher in gullies with the highest widths and depths. the higher mean length of the second order gullies studied (table 1) correspond with greater channel capacities and larger volumes of soil loss as indicated in the three catchments studied. the results also indicated that larger gully channels correspond with steeper slopes and higher volumes of soil loss. tekwa and usman (2006) reported similar results while estimating soil loss in mubi, northeastern nigeria as their study found that large soil losses observed in this study area corresponded closely with gully channel sizes. however, renard et al., (1997) observed that though erosion increases with increase in slope length, soil loss occurrences were more rapid with slope steepness than with length. the estimated gully volumes across the respective sites indicate the volume of soil that has been eroded and deposited elsewhere within the basin. the implications of sediment detachment and deposition down-slope include increased accretion (fashae et al., 2022), siltation and pollution of water bodies (mohammadi et al., 2021). 3.2 physicochemical characteristics of soils under different land uses in the study area the mean values, standard deviation, coefficient of variation and analysis of variance (anova) performed to test for significant differences amongst the various land uses are summarized in table 3, while table 4 summarizes results of t-test performed to detect variations in soil physicochemical properties between pairs of land uses. the results show that the mean values of sand over badland is highest (88%) and lowest over plantation (76%) and fadama (62%). similarly, variation in silt fractions is statistically significant (p˂ 0.05). the mean value of silt is highest over fadama landuse (14%) and least over cultivated land (4.0%). analysis of variance shows a statistically significant difference (p˂ 0.05) in the silt fraction of soils over the different landuses. the mean values of clay are higher in the fadama landuse and lowest over badland at p˂ 0.05. soil texture is a fundamental soil property that influences a soil’s susceptibility to erosion, its infiltration capacity, moisture and nutrient retention 420 zuni aminu, ibrahim ishiaku, haruna gattah aminu, and yauri safyanu garba (afolabi et al., 2014). the dominance of sand fractions predisposes the soil to erosion due to the macro pores, resulting in low moisture retention by the soils and nutrient loss through translocation by soil wash and leaching (afolabi et al., 2014). yousaf et al., (2021) also reported clay fraction loss due to migration down the soil profile and erosion in areas with sparse or no vegetation cover. the result from the table show statistical significance in bulk density values in soils over the different landuses with the highest bulk density value in badland (1.78 g/cm3). the lower bulk density value of 1.35 g/cm3 in the plantation land use could be attributed to higher organic matter content due to litter addition from the vegetal cover (yousaf et al., 2021). results show soil porosity mean values over the different land uses are generally low as the variation analysis shows a significant difference at p˂0.05. porosity values are also lower than the critical limits of >50% in top 30 cm for normal mineral soils (malgwi, 2007). compaction by cultivation, trampling by humans and cattle may also be responsible for the high bulk density and low porosity, particularly over cultivated and badlands. these have implications for infiltration and plant rooting depths (aminu and jaiyeoba, 2015). moisture content over the respective land uses is generally low and range between 0.003-0.08 g/g. the analysis of variance indicates a significant difference at p˂ 0.05. results of t-test analysis show significant difference between plantation/cultivated, plantation/fadama and cultivated/fadama pairs of land uses. the high soil moisture over the plantation land use may be attributed to soil protection from excessive solar radiation by the vegetal cover and the higher organic matter content of the soils. on the other hand, lower soil moisture in the cultivated and badland topography could be due to the low organic matter content, high evaporation rates over exposed soil surfaces and low porosity of the soils (aminu and jaiyeoba, 2015). the higher moisture content in the fadama and plantation land uses could be attributed to the higher silt and clay content of the soils. similarly, the proximity of the fadama landuse to water bodies and the high fraction of soil fines may be responsible for its high moisture content (aminu and jaiyeoba, 2015). the mean value of soil ph is highest over plantation and fadama (6.20) and least over the badland topography (6.00) and cultivated (5.50). the slight acidity of the badland soils may be due to leaching of soil cations which have been found to increase soil acidity (afolabi et al., 2014; aminu and jaiyeoba, 2015; yousaf et al., 2021). soil organic matter is highest over the plantation landuse (1.68%) and lowest over badland (1.30%) and scrubland (1.25%). analysis of variance shows a significant difference in soil organic matter at p˂ 0.05 over the different landuses. the result of t-test analysis shows no significant difference between pairs of plantation/scrubland, scrubland/cultivated, scrubland/fadama and scrubland/badland land uses. the critical limit of organic matter content for soils under natural conditions in the savannah is ≥ 2% (esu, 1991; landon, 1991; malgwi, 2007; aminu and jaiyeoba, 2015). this indicates that all soils over the different land uses are low in organic matter. high organic matter has been found to increase cohesion of soil particles thereby protecting soils from wash processes (aminu and jaiyeoba, 2015). the generally low levels of soil organic matter across all land uses is confirmed by similar studies (afolabi et al., 2014) which attributed low organic matter of the study area soils to rapid mineralization of organic matter. similarly, yousaf et al., (2021) found that erosion and higher oxidative rates were responsible for low soil organic matter in tropical soils. however, the higher organic matter content over plantation land use could be attributed to continuous litter deposition and minimum disturbance (malgwi, 2007; aminu and jaiyeoba, 2015). results show that mean values of total nitrogen (tn) range between 0.01 0.38%. the highest mean value was in the plantation and (0.38%) while the least values were over badland and scrubland (0.20% and 0.01%) respectively. the higher tn in the plantation land use may be attributed to higher organic matter and higher nitrogen fixation within the land use, while the generally low levels of tn in the badland and scrubland topography may be due to loss of nitrogen through mineralization as a result of high diurnal temperatures, as reported by madaki (2011) in his study in gwagwalada area, nigeria. the result of t-test analysis shows that only the pairs of plantation/ fadama and scrubland/badland showed a significant difference in their total nitrogen content. for available phosphorous, the highest mean values are in the cultivated (15.3 mg/ kg-1) and fadama (7.85 mg/ kg-1) land uses while the least mean values are in scrubland (3.08 mg/kg-1) and badland (2.10 mg/ kg-1). the coefficient of variation is generally low at less than 50% across all land use categories. analysis of variance shows statistical differences at p˂ 0.05 in available phosphorous in all land use types. the higher mean values over the fadama and cultivated land uses may be attributed to the addition of nitrogen and phosphorous bearing fertilizers (aminu and jaiyeoba, 2015). the analysis of variance for ca shows significant differences exist (p˂ 0.05) in the respective land uses. the mean value is highest over plantation (3.20 cmol/kg-1) and least over cultivated land use (2.16 cmol/kg-1). the results also show significant differences of exchangeable mg over respective landuses with the highest concentration over fadama (1.24 cmol/kg-1) and lowest over badland (0.15 cmol/kg-1). the values of exchangeable na over various land uses showed significant differences (p˂ 0.05) among land uses. the mean values are highest over plantation and cultivated (0.22 cmol/kg-1), while scrubland and fadama land uses have 0.06 cmol/kg-1. the mean value of exchangeable k shows statistically significant differences (p˂ 0.05) amongst the land use types. the lower concentration of exchangeable k is in the badland and scrubland has been attributed to leaching (yousaf et al., 2021). the mean values for cec are homogenous over the different landuses with the highest mean values over plantation (6.71 cmol/kg-1) and least over cultivated land (5.0 cmol/kg-1). analysis of variability for cec shows significantly different amounts amongst the land use types. result of t-test shows significant difference only between the pairs of scrubland/cultivated and fadama/badland land uses. the cec values across different land use categories are generally low. however, the higher cec values over plantation land use may be attributed to organic residues which have been found to increase the cec content of soils (oluwadare et al., 2013). mean values of soil exchangeable acidity (h±al3+) is highest over the badland (0.31 cmol/kg-1) and scrubland (0.29 cmol/ kg-1) while the values over the remaining land uses are homogeneous (between 0.04 and 0.08 cmol/kg-1). ). analysis of variance reveals significantly different values of exchange acidity amongst the various land uses. result of t-test show no significant difference between most pairs of land uses. soil base saturation is highest over plantation (76.3%) and lowest over badland (50.4%). the analysis of variance reveals significant differences in base saturation amongst the various landuses, while the t-test result shows significant differences (p˂ 0.05) between pairs of plantation/scrubland, plantation/cultivated, scrubland/ cultivated and fadama/badland land uses. the lower base saturation over cultivated and badland uses has been attributed to absorption of ions by plants and leaching (madaki, 2011). 421assessment of the extent of soil degradation over different land uses in the kebbi area, northwestern nigeria ta bl e 3 . v ar ia tio ns o f s oi l p hy sic al an d ch em ic al p ro pe rti es o ve r d iff er en t l an d u se s l an d us es s oi l p ro pe rt ie s sa nd % si lt % c la y % b ul k de ns ity (g /c m 3 ) po ro si ty % g m c (g /g ) ph (1 :2 .5 ) (s oi l:h 2o ) o rg an ic m at te r (% ) t n (g / kg ‑1 ) a p (m g/ kg ‑1 ) c a m g k n a c e c h +a l b s % (c m ol /k g-1 ) pl an ta tio n x 76 .0 12 .0 12 .0 1. 35 49 .0 0. 07 6. 20 1. 68 0. 38 6. 42 3. 20 1. 24 0. 46 0. 22 6. 71 0. 06 76 .3 sd 52 .3 7. 8 7. 6 0. 53 28 .7 0. 03 3. 12 6. 65 0. 12 2. 87 1. 88 0. 59 0. 31 0. 18 2. 22 0. 04 33 .1 c v % 68 6 5 63 39 58 49 50 61 32 45 59 48 67 81 33 67 43 sc ru bl an d x 86 .0 6. 0 8. 0 1. 75 34 .0 0. 00 3 5. 90 1. 25 0. 20 3. 08 3. 00 0. 60 0. 11 0. 06 6. 50 0. 29 58 .0 sd 45 .4 3. 5 3. 36 0. 53 20 .2 0. 02 2 2. 32 0. 54 0. 16 1. 01 1. 9 0. 27 0. 07 0. 02 4. 4 0. 12 28 .7 c v % 52 58 42 30 59 67 39 43 80 33 63 45 63 33 67 41 48 c ul tiv at ed x 80 .0 4. 0 16 .0 1. 53 42 .3 0. 04 1 5. 50 1. 32 0. 31 15 .3 2. 16 0. 72 0. 34 0. 22 5. 00 0. 08 69 .0 sd 55 .2 2. 56 9. 3 0. 66 23 .3 0. 02 5 2. 22 2. 34 0. 16 6. 77 1. 13 0. 51 0. 15 0. 14 3. 43 0. 05 28 .2 c v % 69 64 58 43 55 60 40 60 51 44 52 70 44 63 68 63 41 fa da m a x 62 .0 14 .0 24 .0 1. 45 45 .3 0. 07 9 6. 20 1. 40 0. 28 7. 85 3. 11 0. 82 0. 19 0. 06 6. 00 0. 07 70 .0 sd 43 .1 8. 5 19 .3 0. 88 29 .1 0. 04 8 2. 81 5. 33 0. 10 2. 88 1. 91 0. 56 0. 08 0. 04 2. 94 0. 03 41 .1 c v % 69 61 80 60 64 60 45 68 35 37 61 68 42 66 4 9 43 58 b ad la nd x 88 .0 6. 0 6 .0 1. 78 33 .0 0. 00 3 6. 00 1. 30 0. 01 2. 10 2. 34 0. 15 0. 10 0. 08 5. 30 0. 31 50 .4 sd 47 .7 2. 2 2. 8 1. 52 13 .5 0. 00 1 4. 9 1. 01 0. 01 1. 01 1. 52 0. 08 0. 05 0. 03 2. 22 0. 2 30 .1 c v % 54 36 46 85 41 33 81 77 81 48 64 53 50 37 42 64 59 a n o va 4. 61 3. 85 6. 17 3. 51 5. 73 3. 47 3. 66 7. 19 4. 03 3. 92 3. 15 3. 33 4. 20 5. 02 3. 77 3. 45 3. 08 le ve l o f si g ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** x = m ea n, s d = sta nd ar d de vi at io n, c v % = co effi ci en t o f v ar ia tio n, l ev el o f s ig ni fic an ce : * *= 0. 05 so ur ce : l ab or at or y a na ly sis (2 02 1) 422 zuni aminu, ibrahim ishiaku, haruna gattah aminu, and yauri safyanu garba ta bl e 4 . re su lt of s tu de nt ’s t-t es t a na ly sis b et w ee n pa irs o f l an d u se s l an d us es s oi l p ro pe rt ie s sa nd % si lt % c la y % b ul k de ns ity (g /c m 3 ) po ro si ty % g m c (g /g ) ph (1 :2 .5 ) (s oi l:h 2o ) o rg an ic m at te r (g /k g‑ 1 ) t n (g / kg ‑1 ) a p (m g/ kg ‑1 ) c a m g k n a c e c h +a l b s% (c m ol /k g-1 ) pl / sc r -1 .7 8 ** -0 .8 0 ** -3 .2 8 -3 .2 5 -0 .0 4 ** 7. 9 -2 .3 0 -2 .9 2 7. 84 -3 .3 0 0. 59 ** -0 .3 8 ** -0 .3 7 ** -7 .3 9 6. 57 -3 .3 4 1. 18 ** pl / c u 0. 02 ** -0 .1 1 ** -0 .5 9 ** -3 .2 5 -0 .4 4 ** 1. 67 ** -2 .1 7 ** -0 .2 7* * 4. 6 -0 .0 2 ** -0 .8 9 ** 3. 05 -3 .2 7 -1 .4 9 ** 6. 64 0. 06 ** -0 .5 9 ** pl /f a 0. 1 ** -0 .4 5 ** 1. 31 ** 4. 35 0. 62 ** 1. 32 ** -0 .9 0 ** 0. 65 ** 2. 03 ** -2 .0 0 ** 0. 41 2. 55 -3 .8 7 -1 .5 9 ** 4. 26 -3 .3 4 2. 49 pl / b a -1 .5 7 ** -4 .9 6 -2 .4 3 6. 35 -3 .0 5 -6 .5 5. 01 1. 46 ** 7. 87 0. 70 ** 0. 76 0. 96 ** -2 .1 7 ** -6 .0 0 2. 66 0. 26 ** 2. 61 sc r / c u 1. 8 ** 0. 69 ** 2. 69 4. 94 -0 .4 0 ** -6 .2 3 0. 13 ** 2. 65 -2 .8 9 3. 28 -1 .4 8 3. 43 -2 .9 0 5. 90 0. 07 ** 3. 4 -1 .7 7 ** sc r / fa 1. 88 ** 0. 35 ** 4. 59 7. 60 0. 66 ** -6 .5 8 1. 40 ** 3. 57 -6 .4 4 1. 30 ** -0 .1 8 ** 2. 93 -3 .5 0 5. 80 -2 .3 1 0. 01 ** 1. 31 ** sc r / b a 0. 21 ** -4 .1 6 0. 85 ** 9. 6 -3 .8 2 -1 4. 4 7. 31 4. 28 0. 03 ** 4. 00 0. 17 ** 1. 37 ** -1 .8 0 ** 1. 39 ** -3 .9 1 0. 01 ** 1. 43 ** c u / fa 0. 08 ** -0 .3 4 ** 1. 90 ** 2. 66 1. 06 ** -0 .3 5 ** 1. 27 ** 0. 92 ** -3 .5 5 -1 .9 8 ** 1. 30 ** -0 .5 0 ** -0 .6 0 ** -0 .1 0 ** -2 .3 8 -3 .4 0 3. 08 c u / b a -1 .5 9 ** -4 .8 5 -1 .8 4 ** 4. 66 -2 .6 1 -8 .1 7 7. 18 1. 64 ** 2. 92 0. 72 ** 1. 65 ** -2 .0 9 ** 1. 10 ** -4 .5 1 -3 .9 8 0. 20 ** 3. 20 fa / b a -1 .6 7 ** -4 .5 1 -3 .7 4 2. 00 ** -3 .6 7 -7 .8 2 5. 91 0. 7* * 6. 47 2. 70 0. 32 ** -1 .5 9 ** 1. 70 ** -4 .4 1 -1 .6 ** 3. 60 0. 12 ** pl = pl an ta tio n, c u = cu lti va te d la nd , s c r = sc ru bl an d, f a = fa da m a, b a = ba dl an d le ve l o f s ig ni fic an ce : *= 0. 05 so ur ce : l ab or at or y a na ly sis (2 02 1) 423assessment of the extent of soil degradation over different land uses in the kebbi area, northwestern nigeria 4. conclusion this study assessed soil degradation over different land uses in the kebbi area, northwestern nigeria. major land use types are cultivated, fadama, plantation, scrublands and badlands. findings from the study show a varying magnitude of soil degradation over different land uses in the study area. the dominant geomorphic response identified in the study area is gully initiation and development on bare or sparsely vegetated areas and cultivated lands. the study observed the presence of several discontinuous and second order gullies in the study area; these are ephemeral and evacuate storm runoff in the study area. the high gully frequency in the study area has implications for drainage density as it has the potential to exacerbate storm runoff and soil loss. although drainage density was not considered in this study, it indicates how dissected a basin is by channels and affects runoff and sediment transport (ofomata, 2009). similarly, slope angles significantly influenced runoff and soil loss as estimates of soil loss were significantly higher over steeper slopes. the soils in the study area are characterized by low organic matter and high bulk densities while the dominance of sand fractions predisposes the soil to low moisture retention and nutrient loss by soil wash and leaching. soil characteristics, land use and steep slopes combine to make many areas erosion-prone as supported by studies (aminu et al., 2017; bashir et al., 2020) which have found that cultivation on slopes and loose soils exacerbate erosion and makes soils vulnerable to degradation. this vulnerability is affected not only by factors such as topography and soil properties but also by human influence (stocking and murnaghan, 2000). the destruction of soil structure and texture associated with deforestation, grazing and poor agricultural practices help create conducive conditions for the development of gullies which often begin in the form of sheet or rill erosion and eventually capture the available drainage channels (aminu et al., 2017; bashir et al., 2020). the study found that topography, soil properties and land use/vegetation cover mutually reenforce fluvial and geomorphological processes which lead to soil degradation. in a developing country such as nigeria, soil degradation assessment is crucial for food security, environmental management and sustainable development. soil degradation assessment through combined soil loss estimation and soil physicochemical assessments is useful as it provides site specific results for effective management and remediation. although this study has provided baseline data on the extent of soil degradation in the study area, only one wet and dry season was considered. future studies may consider a temporal and spatial expansion of the study, while taking into consideration the history of the dominant geomorphic processes in the study area. the study recommends linking geomorphological information to infrastructural planning and development, reclamation and rehabilitation of degraded lands, adoption of sustainable soil management practices such as tree planting, structural barriers, cross-slope farming and other cropping systems that will provide maximum protection for soils to prevent soil 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(1978). predicting rainfall erosion losses. u.s department of agriculture. washington d.c, usa. http://www.interscience.wiley.com http://dx.doi.org/10.18393/ejss.2016.3.209-220 modeling of the large miocene epithermal and porphyry gold deposits of the cauca metallogenic belt of colombia using monte carlo simulations felipe velásquez ruiz1,2*, juan camilo martínez3, alejandra tobón acevedo4, alejandra yepes metaute5, angélica maría zapata6, diana paulina cataño salas4 1department of geology, fcfm, university of chile, plaza ercilla 803, santiago, chile 2millennium nucleus for metal tracing along subduction, fcfm, university of chile, chile 3school of civil and environmental engineering, vanderbilt university, tn, usa 4school of geosciences and environment, universidad nacional de colombia, sede medellín, colombia 5group digital, flsmidth, pa, usa 6instituto de geociências, universidade de brasília, brazil *corresponding author: fevelasquezru@unal.edu.co keywords: modeling; monte carlo simulations; northern andes; miocene magmatism; cauca metallogenic belt. palabras clave: modelado; simulaciones de monte carlo; andes del norte; magmatismo mioceno; cinturón metalogénico del cauca. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.95289 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 415-421 o re g eo lo g y record manuscript received: 25/04/2021 accepted for publication: 29/09/2021 abstract the cauca metallogenic belt (cmb) is an inter-andean area located along the cauca-romeral fault zone of colombia. the cmb comprises a group of twelve late miocene magmatic-hydrothermal au-ag-cu mineral deposits positioned between the western and central cordillera of colombia. in addition to being a widely developed region in exploration and exploitation with known au endowments of over 1,700 tons, this area represents an exceptional metallogenetic laboratory for modeling an archetype andean mineralization from a calc-alkaline source with high sr/y ratios. efficiency processes such as ion and halogen transport, oxygen fugacity, and sulfur content have been extensively studied in magmatic-hydrothermal deposits with a geochemical approach. however, the quantification and modeling of these efficiency processes are currently debated. through multiple iterations using monte carlo simulations (n > 2 million), the authors modeled those reservoirs corresponding to the cmb’s upper crust. in this way, a high flux of dacitic magma of 0.007 km3/year and efficiencies of 1–2% showed that gold endowments bear heavily with magmatic and hydrothermal au deposits of the cmb. outcomes including au endowments up to 1,000 tons can be obtained for reservoirs below 400 km3 of hydrous melt in brief mineralization intervals between 40 to 120 ka, whereas volumes of 400 to 800 km3 are obtained in a 120 to 200 ka range. in contrast, the hypothetical reservoirs for the lower and middle crust of the cmb, through a basaltic calc-alkaline magma flux between 0.0007 and 0.0011 km3/year, the efficiency of ~0.7%, and pressures below 5 kbar, showed sufficient available content of exsolvable h2o and gold to feed the reservoirs in the upper crust of the cmb or to generate gold deposits from them, but in longer time intervals (>1 ma). modelado de los grandes depósitos miocenos de tipo epitermal y pórfido de oro del cinturón metalogénico del cauca de colombia usando simulaciones monte carlo resumen el cinturón metalogénico del cauca (cmc) es un área interandina ubicada a lo largo de la zona de falla cauca-romeral de colombia. el cmc está compuesto por un grupo de doce depósitos magmático-hidrotermal de au-ag-cu del mioceno tardío, ubicados entre la cordillera occidental y central de colombia. además de ser una región ampliamente desarrollada en exploración y explotación mineral, con dotaciones de au conocidas de más de 1700 toneladas, el cmc representa un laboratorio metalogenético excepcional para modelar mineralizaciones de tipo andino a partir de una fuente calco-alcalina con altas razones de sr/y. a pesar que los procesos de eficiencia en depósitos magmático-hidrotermal han sido ampliamente estudiados con un enfoque geoquímico, como el transporte de iones y halógenos, la fugacidad de oxígeno y el contenido de azufre, entre otros, la cuantificación y modelado de estos procesos de eficiencia han sido debatidos actualmente. a través de múltiples iteraciones usando simulaciones de monte carlo (n>2 millones), modelamos reservorios que corresponden a la corteza superior del cmc. nuestros resultados aplicando un alto flujo de magma dacítico de 0.007 km3/año y eficiencias entre 1-2%, mostraron que las dotaciones modeladas de oro corresponden fuertemente a las cantidades de au reportadas de los depósitos magmático-hidrotermal del cmc. con dicho modelado se pueden obtener dotaciones de au de hasta 1000 toneladas para reservorios inferiores de 400 km3 de magma hidratado en breves intervalos de mineralización entre 40 y 120 ka, mientras que se obtienen volúmenes de 400 a 800 km3 con un periodo de mineralización entre 120 a 200 ka. por el contrario, los reservorios modelados para la corteza inferior y media del cmc, a través de un flujo de magma basáltico calco-alcalino entre 0,0007 y 0,0011 km3/año, una eficiencia de ~0,7% y presiones menores a 5 kbar, mostraron suficiente contenido de h2o soluble y oro disponible para alimentar los reservorios en la corteza superior del cmc, o para generar depósitos de oro a partir de ellos, pero en intervalos de tiempo más largos (> 1 ma). how to cite item: velasquez-ruiz, f., martínez, j. c., tobon acevedo, a., yepes metaute, a., zapata, a. m., & catano salas, d. p. (2021). modeling of the large miocene epithermal and porphyry gold deposits of the cauca metallogenic belt of colombia using monte carlo simulations. earth sciences research journal, 25(4), 415-421. https://doi. org/10.15446/esrj.v25n4.95289 mailto:fevelasquezru@unal.edu.co https://doi.org/10.15446/esrj.v25n1.74167 416 felipe velásquez ruiz1, juan camilo martínez, alejandra tobón acevedo, alejandra yepes metaute, angélica maría zapata, diana paulina cataño salas introduction colombia has the most extended history of gold production and exploitation in south america, which dates back to pre-columbian goldsmithing to the extraction of porphyry and epithermal gold and silver deposits in the present day (martinón-torres & uribe-villegas, 2017; scott & seeley, 1983). even though colombia ranks as the 20th largest gold producer globally, with 46.3 tons of gold mined in 2019 (goldhub, 2020), proven reserves are above 50 au moz (celada et al., 2016). a considerable proportion of such reserves are found in the unexploited and recently discovered la colosa and nuevo chaquiro porphyry deposits, with 881 and 176 au tons, respectively (bartos et al., 2017; celada et al., 2016; naranjo et al., 2018). these deposits are part of twelve late miocene mineral districts located in the cauca metallogenic belt (cmb), a strip comprising numerous au-ag-cu-mo satellite deposits between the central and western andean cordillera of colombia (fig. 1). the cmb deposits display the typical characteristics of andean metallogenetic systems, including calc-alkaline magmatism of intermediate composition and high sr/y ratios. these attributes make the cmb a natural laboratory for modeling gold endowments in porphyry and epithermal systems associated with arc environments. recent studies have used monte carlo simulations to model and predict sensitive parameters in the gold endowments associated with aucu porphyry deposits (chiaradia, 2020; chiaradia & caricchi, 2017). the simulation works on four main parameters: (1) melt productivity; (2) the h2o concentrations in solution, in excess and exsolvable; (3) the amounts of gold in the exsolvable water, and (4) the precipitation efficiency (au efficiency = (xinicial xfinal) / xinicial). a database of the world’s largest gold and copper deposits and their modeling has shown that au endowments are predominantly controlled by magma volumes, duration of the magmatichydrothermal activity, and precipitation efficiency. nevertheless, monte carlo simulations can provide new petrogenetic information for a genetically related chain of au deposits, compared to a large group of deposits scattered around the world, even if they have similar au endowments and share similar geochemical characteristics. therefore, to reduce uncertainties and have better statistical approximations, it is possible to refine some parameters such as melt productivity, h2o concentrations, and precipitation efficiency tied to spatially and temporally consistent input data. thus, the cmb provides an unprecedented opportunity to model and study the au endowment mechanism of a group of spatially and temporally related hydrothermal gold deposits. this research work aims to analyze the trigger mechanism in the formation of magmatic-hydrothermal gold deposits in the cmb of colombia by using monte carlo modeling. the rationale of the model data collection geochronological, chemical (sr/y ratios and alkalinity), and gold endowment information were obtained mainly by the metallogenetic map of colombia (celada et al., 2016), previous studies, and repositories of master and doctoral theses on seven primary gold deposits and provinces in the cmb (supplementary information a). the information corresponding to the ore duration (ma) was made based on geochronological data of granitogenesis and synto post-ore porphyry and epithermal u-pb, ar-ar and re-os dating (bissig et al., 2017; celada et al., 2016; lesage et al., 2013; naranjo et al., 2018). there is an information gap for the northern and southernmost zones of the cmb, which needs to be studied in detail in the future. however, the central zone is well detailed to model the au endowments for the primary deposits in the cmb (supplementary information a). monte carlo simulation the monte carlo simulation was carried out in the rstudio program (https://rstudio.com), with more than 2 million iterations, to ensure reproducibility and replicability. the algorithm was based on chiaradia (2020) and chiaradia & caricchi (2017), according to the model and mass balance of annen et al. (2006) and annen (2009). the simulation works with four main parameters, using concatenated equations (detailed in the attached supplementary information b), as follows: (1) melt productivity, defined as the hybrid melt produced in the crust; (2) the h2o concentrations in solution, in excess and exsolvable at pressure of saturation; (3) the amounts of gold in the exsolvable water, at the pressure of saturation within the crust, and (4) the precipitation efficiency, defined as the ratio of gold precipitated regarding the initial gold content in the fluid (gold efficiency [%] = (mau_initial – mau_final) / mau_initial), where m is the molality in the initial and final fluid (sanchez-alfaro et al., 2016). the values and intervals used for each parameter to model hypothetical reservoirs of the lower, middle, and the upper crust of the cmb are explained in table 1. for reservoirs of the upper crust in archetypal gold deposits of the world, injection rates of 10-2 m/year with radii greater than 7.5 km have been proposed to explain the volume of super-eruptions (> 450 km3) (chang et al., 2007; mason et al., 2004; sparks et al., 2005). however, the present work models smaller-scale plutons and plugs (<40km2), for which a rate of 10-2 m/year with a smaller radius is proposed, plus injection time intervals between 40,000 – 200,000 years and random pressures at which magma accumulation occurs between 0.3 3 kbar, corresponding to crustal depths of ~1 to ~10 km (sanchezalfaro et al., 2016). figure 1. a simplified geological map of the magmatic-hydrothermal deposits and the geodynamic context of the colombian territory, according to the metallogenetic map of the national mining service of colombia (celada et al., 2016). abbreviations of porphyry and epithermal gold deposits: ac acandí, be berruecos, bu buriticá, cmb cauca metallogenic belt, co la colosa, cg cerro gordo, do dominical, ea el águila, gr gramalote, ic infierno-chillí, lm la mina, ma marmato, mo mocoa, mu murindó, nc nuevo chaquiro, pi piedrancha batholith, pp pantanos-pegadorcito, qu quinchía, se segoviaremedios, srs santa rosa del sur, ti titiribí (cerro vetas), vc vetas california, vs el vergel stock. https://rstudio.com 417modeling of the large miocene epithermal and porphyry gold deposits of the cauca metallogenic belt of colombia using monte carlo simulations table 1. input parameters used for monte carlo simulations for two different magma fluxes (low magma flux between 0.0007 and 0.0011 km3/year and high magma flux of 0.007 km3/year) to predict diverse scenarios of gold endowments in the late miocene lower, middle and upper crust of colombia. parameters analyzed for reservoirs in the lower and middle crust of the cmb parameters analyzed for reservoirs in the upper crust of the cmb • time: 0 5 ma (annen et al., 2006) • pressure: 1.5 9 kbar (annen et al., 2006) • h2o in parent magma: 2 4% (plank et al., 2013) • h2o in crustal rocks: 0.2 1% (chiaradia & caricchi, 2017) • the fluid-melt partition coefficient of gold: random between 10-100 (simon et al., 2005) • the gold content in calc-alkaline magmas: 6 – 9 ppb (moss et al., 2001) • au efficiency: random between 0.5 – 1% • time: 40,000 – 200,000 years (annen, 2009) • pressure: 0.3 3 kbar (sanchezalfaro et al., 2016) • h2o in parent magma: 2 4% (plank et al., 2013) • h2o in crustal rocks: 0.2 1% (chiaradia & caricchi, 2017) • fluid-melt partition coefficient of gold: random between 10 100 (simon et al., 2005) • gold content in calc-alkaline magmas: 6 – 9 ppb (moss et al., 2001) • au efficiency: random between 1 – 2% results state of the art in the cmb the cmb, located throughout the vast cauca-romeral fault systems (fig. 1), between the western and central andean cordillera of colombia (cediel et al., 2003; pulido, 2003; restrepo & toussaint, 2020; velásquez et al., 2019), is an inter-andean area of ~620 km long by 50 km wide hosting several late miocene porphyries, epithermal and intrusion-related deposits of gold, silver, and copper plus molybdenum, which has south continuity to miocene porphyry and epithermal deposits in ecuador (schütte et al., 2012). the geology and tectonics of the cmb were studied at first by grosse, 1926, in which he drew the cartography and mapped the artisanal mines of the region. based on this classic work, researchers conceptualized the economic geology associated with the cauca-romeral fault system. the cmb comprises seven large deposits (> 1 au moz) in the central area, defined by (sillitoe, 2008) as the middle cauca belt and also five poorly explored zones to the south with numerous associated satellite deposits (fig. 1), for which no geochemical information was available until now. the cmb also continues north to the andean foothills, with a tradition of ancestral alluvial gold mining (ocde, 2016), but also with no basic geochemical or geochronological information available. the main deposits include la colosa (28.3 au moz), quinchia (2.4 au moz), marmato (8.7 au moz), nuevo chaquiro (5.6 au moz), la mina (1.62 au moz), titiribí-cerro vetas (4.6 au moz) and buriticá (3 au moz) (bartos et al., 2017; bissig et al., 2017a; lesage et al., 2013; naranjo et al., 2018; tassinari et al., 2008). these deposits and their associated plutons have been dated by the colombian national mining agency giving ages between 5.6 and 10 ma (celada et al., 2016). the magmatic rocks associated with deposits of the cmb display calcalkaline andesitic to dacitic compositions, porphyritic textures and adakitic-like signatures, with high sr/y ratios mainly between 40-60 (fig. 2b), except for the la colosa deposit, which shows an average value of 26.8 (gil-rodríguez, 2010). the mineralization style of the middle cauca belt varies in a south-north sense, where la colosa is gold-dominant without copper and displays veinlets and bands of low grade (0.92 g/t) (naranjo et al., 2018), whereas the titiribí, la mina and quinchía deposits are low-grade gold-copper (<0.8 g/t) (bissig et al., 2017b). nuevo chaquiro along with el roble (~13.8 ma) are copper-dominant porphyry deposits (bartos et al., 2017). the buriticá epithermal deposit shows thick (>30 cm) mineralized high-grade veins of gold-dominant (10.4 g/t) and silver, distributed in two main e-w vein systems (www.continentalgold.com), called veta sur and yaraguá. the marmato au deposit is prominent in the area, due to its mining traditions for more than 100 years of gold exploitation, with veins and bands of 0.9 g/t (tassinari et al., 2008). in general, the basement of the mineral deposits associated with the western cordillera consist mainly of andesitic flows, tuffs, agglomerates and sandstones of the late miocene combia formation (6 10 ma; jaramillo et al., 2019; weber et al., 2020) and marine volcano-sedimentary sequences of the late cretaceous barroso formation (rodríguez & zapata, 2013). otherwise, the basement of the deposits towards the central cordillera consist mainly of black schists and micaceous metapelites of triassic-jurassic age from the cajamarca complex (240-230 ma & 157-146 ma; blanco-quintero et al., 2014; vinasco et al., 2006) and the early cretaceous volcano-sedimentary rocks of marine origin from the quebradagrande complex (nivia et al., 2006). figure 2. (a) total ore duration (ma) vs. au (tons) for the cmb magmatichydrothermal deposits (>1 au moz). (b) gold endowment (tons) vs. sr/y ratio. abbreviations of porphyry and epithermal gold deposits: bu buriticá, co la colosa, lm la mina, ma marmato, nc nuevo chaquiro, qu quinchía. monte carlo simulation for au endowments from the cmb low magma flux monte carlo simulations with 3 million iterations, returned 1,919,000 plotted results. a group of endowments gave negative results, but they were not taken into account because a negative au endowment has no geological significance. results for a low magma flux between 0.0007 and 0.0011 km3/ year, yielded a wide spectrum of au endowments, varying between 0 48,000 au tons, with a mean of 8,277 tons and a median of 5,719 tons (fig. 3a). these endowments are well above our case study (the cmb endowment interval is marked between the 2 red lines in figs. 3a-d). nonetheless, for injection time between 0 and 2 ma, there is a volume of hydrous melt of up to 500 km3 with an au endowment spectrum similar to those of the cmb (fig 3b). for time intervals greater than 2 ma there is no correlation with the cmb. the pressure at which magma accumulation occurs is sensitive up to 5 kbars (fig. 3c), after this value and up to 9 kbars there is no good correlation as is shown for the injection time. finally, the results for the efficiency parameter are very random between 0 – 0.7%, and associates values of au tons and volume of hydrous melt of 0 20,000 and 0 1,500 km3 respectively. for an efficiency between 0.7 to 1%, the results are no longer random and a group of separate fields are observed (fig. 3d). high magma flux monte carlo simulation for a high magma flux was only possible for a maximum number of 2 million iterations. for a larger number of iterations, the computer showed problems to concatenate the equations in supplementary information b. the results for a high magma flux exhibit a match with au endowments of the cmb (also marked between the interval of 2 red lines in figs. 4a-d). the iterations yielded a mean of 954 au tons and a median of 760 au tons, which are within the range of au endowments in the cmb (fig. 4a). parameters such as pressure (at which magma accumulation occurs) and efficiency present random values in almost the whole graphic, although at low pressures and low efficiency, a trend is shown for 0 -1 kbars and 0 1.5% for pressure and efficiency respectively (figs. 4c-d). the injection time on the other hand, shows a clear well-marked response to model the au endowments, in which it is observed that for 0 160 ka, endowments between 0 – 1,000 au tons and 0 600 km3 of volume of hydrous melt were found (fig. 4b). www.continentalgold.com 418 felipe velásquez ruiz1, juan camilo martínez, alejandra tobón acevedo, alejandra yepes metaute, angélica maría zapata, diana paulina cataño salas discussion magmatic reservoirs in the andean lower crust of colombia in the early miocene the monte carlo simulations in the present work employing 3 million iterations, aimed to determine how the gold endowments were reached from the mantle to the shallow crust. the late miocene magmatic rocks located along the cauca river valley include the volcanic rocks of the combia formation and the cauca shallow volcanic intrusions (weber et al., 2020). these rocks display calc-alkaline affinity (jaramillo, 1976) and there is a group of rocks that chemically plots in the adakitic series (borrero & toro–toro, 2016). in the cmb, there are also rocks of tholeiitic and shoshonitic series (rodríguez & zapata, 2013), but the analysis of the association of calc-alkaline, tholeiitic and shoshonitic magmas is beyond the scope of this work. however, the literature shows that a strong hypothesis for the association of these three magmatic series could be explained by the caldas tear (vargas & mann, 2013), which is described as a fracture of the nazca plate under the south american plate, causing a flat slab, therefore there is an incipient post-collisional magmatism, placing rocks of the calc-alkaline, tholeiitic and shoshonitic series in the same regional context, however future geochronological studies need to be carried out to clarify this association. the calc-alkaline affinity rocks present garnets with rings of plagioclase, hornblende and plagioclase with zoned textures in the chinchiná, palestina, jericó, tesorito and el poma areas (alonso–perez et al., 2009; harangi et al., 2001), all of them related to the areas with polymetallic deposits within the cmb. chemical outcomes showed that the garnet-bearing magmas undergone at least two stages of formation through the evolution of phenocrysts, where garnets were formed from 17–19 km and 950 °c and the rims and sieve textures in plagioclase indicate degasification and chemical/ physical disequilibrium (weber et al., 2020). the previous results show that in the late miocene, the lithosphere associated with the cmb underwent enormous magmatic fractionation at the base of the earth’s crust in the northern andes of colombia that later generates reservoirs in the upper crust. the results obtained by the monte carlo simulation reflect the complex geodynamic context of the cmb, which is attributed to a transtensional basin setting (weber et al., 2020). monte carlo simulations demonstrated that there is a low probability that au deposits within the cmb are formed directly from the reservoirs generated in the lower and middle crust (figs 3a-d), mainly because the monte carlo simulations show au endowments generally outside the range of the cmb (figs. 3 and 5), however, because earlier deposits reveal high early fractionation of garnet and hornblende and total rock chemistry (with high sr/y ratios fig. 2) showing a development of rocks of the adakitic series (borrero & toro–toro, 2016), it is probable that the reservoirs of the lower and middle crust, have fed more felsic reservoirs to the upper crust, forming a more evolved dacitic intermediate reservoirs. monte carlo simulations also shows a low probability of triggering hydrothermal gold deposits from basalt magmas formed directly from lower tectonostratigraphic levels (figs. 3a-d), but at timescales of millions of years (fig. 5). however, this works for porphyry-type au deposits, while epithermal deposits are more associated with mineralization stages in windows of thousands years (sanchez-alfaro et al., 2016). finally, monte carlo simulations show that the mean values of gold endowments associated with the cmb are associated with reservoirs in the lower and middle crust with dimensions between 500 and 2,000 km3 (fig. 5), figure 3. monte carlo simulations of gold endowments with three million of iterations, modeling a basaltic magma injection rate of 5 mm/y, through a disk with random values between 6,500 – 8,500 m radius, corresponding to a calc-alkaline magma flux between 0.0007 and 0.0011 km3/year. (a) frequency diagram of gold endowments with n = 1,919,459; b), c) and d) diagrams associate volume of hydrous melt (km3) vs. gold endowments (tons), with random iterations of injection time (years), pressure at which magma accumulation occurs (kbars) and gold efficiency (%), respectively. the vertical red lines between 0 and 1,000 au tons correspond to the range of gold endowments in colombia. 419modeling of the large miocene epithermal and porphyry gold deposits of the cauca metallogenic belt of colombia using monte carlo simulations values that are above an order of magnitude, compared to reservoirs modeled for the upper crust. reservoirs of the upper crust and generation of porphyry and epithermal deposits the cmb is characterized by having several porphyry and epithermal au deposits, some of them located less than 30 km apart, along a 100 km long strip, indicating consecutive small-scale reservoirs in the upper crust, due to a previous process of magmatic differentiation in the middle and lower crust (weber et al., 2020). considering this, we propose an injection rate of 50 mm/y, through a disk of 6,500 m radius, corresponding with a high magma flux of 0.007 km3/ year, coupled with injection time intervals between 40,000 and 200,000 years for the shallow levels. an injection rate greater than 50 mm/y results in a greater volume of hydrous melt, and would model scenarios outside the cmb range (fig. 5), which vary between 0 to 1000 au tons. it is also observed that between 0 to 1,000 tons of au, mineralization intervals <120 ka can develop in hydrous melt volumes of less than 300 km3, while longer intervals of mineralization reach a maximum of 800 km3 (fig. 5). on the contrary, injection rates of less than 50 mm/y are expected for the lower crust but with larger time intervals (in ma; fig. 5) and would mean a transport of hydrous basaltic melt to the crust, with mean values well outside the range of the cmb. therefore, by means of several interactions using a high magma flux of 0.007 km3/year, it can be observed that au endowments match the amount found in the cmb (fig. 4). the mineralization style of the epithermal deposits in the cmb are of high-grade vein characteristics, with values of up to 10 g/t, which suggests that the efficiency would have values of tens of %, however the model shows that the obtained efficiencies for epithermal deposits varies between ~1 2%, slightly higher than the endowments for porphyry deposits (~0.7%). thus, it was quantitatively observed that the gold efficiency from epithermal deposits is lower than the expected and slightly higher than porphyry type deposits or that the present method has no way of modeling resetting and flashing processes. the emplacement of mineralizing hydrothermal fluids can be closely linked to the cauca-romeral fault system. while the ore ages vary between 10 5.6 ma, the reactivation of the cauca-romeral fault system is ~6.3 ma (weber et al., 2020). thus, the mineralizing processes in the cmb can be associated with pre, sin and post deformation activity underwent in the cauca valley in late miocene. in the case of the la colosa porphyry-type gold deposit, it is located along the palestinian fault, while the deposits of marmato, nuevo chaquiro, la mina, and quinchía are associated with the central and western cauca fault system. finally, the buriticá epithermal deposit is closely associated with the tonuzco fault, of which it is located in the main entrance to the higabra mine. thus, the complex cauca-romeral fault system was permeated in the late miocene by the water expelled by the hydrated residual magma from the more superficial reservoirs. this complex fault system could have been responsible for generating the seismic activity to generate all the deposits associated with the cmb, as has been modeled in (sanchez-alfaro et al., 2016; sanematsu et al., 2006). conclusions in the present research project, monte carlo simulations were used to generate a unique formation model for the huge cmb in the andes cordillera of colombia. the results of this work show that for a high dacitic magma flux of 0.007 km3/year and pressures below 3 kbar, the gold endowments obtained were representative for epithermal and porphyry au deposits of the region, whose hypothetical models fit an equivalent range up to 1,000 au tons. for figure 4. monte carlo simulations of gold endowments with two million of iterations, modeling a dacitic magma injection rate of 50 mm/y, through a disk of 6,500 m radius, corresponding to a calc-alkaline magma flux of 0.007 km3/year. (a) frequency diagram of gold endowments with n = 2,000,000; b), c) and d) diagrams associate volume of hydrous melt (km3) vs. gold endowments (tons), with random iterations of injection time (years), pressure at which magma accumulation occurs (kbars) and gold efficiency (%), respectively. the vertical red lines between 0 and 1,000 au tons correspond to the range of gold endowments in colombia. 420 felipe velásquez ruiz1, juan camilo martínez, alejandra tobón acevedo, alejandra yepes metaute, angélica maría zapata, diana paulina cataño salas time intervals of magma injection <200 ka, the values are associated with epithermal deposits where there is a close interaction of exsolved fluids with the cauca-romeral fault system, because of a strong decompression in a transtensional basin setting. otherwise, the modeling for a low basaltic magma flux between 0.0007 – 0.0011 km3/year and efficiencies of ~0.7% showed that only for pressures above 5 kbar that correspond to the middle crust, the modeled reservoirs showed sufficient available content of exsolvable h2o and gold to feed the reservoirs in the upper crust or generate gold deposits from them along the cauca-romeral regional fault system, but in longer time intervals. acknowledgements this work was supported by the millennium science initiative (msi) through the “millennium nucleus for metal tracing along subduction” of chile (nmtm) and by anid-subdirección de capital humano/doctorado nacional/2021-21210049. authors also thank john johnson and dr. m. chiaradia for their valuable contributions to the manuscript. computer code availability name of code: metaldata developers: f.v. ruiz, e-mail: fevelasquezru@gmail.com, and juan camilo martinez, e-mail: juan.c.martinez@vanderbilt.edu year first available: 2021 program language: r program size: 27 kb access: https://github.com/fevelasquezru/modeling-of-the-largemiocene-epithermal-and-porphyry-gold-deposits-of-colombia experimental software: rstudio. bibliography alonso–perez, r., müntener, o., & ulmer, p. 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(2020). late cretaceous bauru group biota from southern goiás state, brazil: history and fossil content. earth sciences research journal, 24(4), 387-396. doi: https://doi. org/10.15446/esrj.v24n4.82831 the southern goiás state region of central brazil has an extensive and informative record of fossil biota. particularly over the last five years, there has been a great increase in fossil finds, which has enabled a greater understanding of this region's fauna and flora during the late cretaceous. in this article, we provide an updated synthesis of the biota from the cretaceous of the southern goiás state, the record of plants, gastropods, turtles, crocodilians, titanosaurs, and theropod dinosaurs. abstract late cretaceous bauru group biota from southern goiás state, brazil: history and fossil content biota del grupo bauru del cretácico tardío del estado de goiás sur, brasil: historia y contenido fósil issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.82831 la región del sur del estado de goiás, en el centro de brasil, tiene un registro extenso e informativo de la biota fósil. particularmente, en los últimos cinco años ha habido un gran aumento en los hallazgos de fósiles, lo que ha permitido una mayor comprensión de la fauna y la flora de esta región durante el cretácico tardío. en este artículo, proporcionamos una síntesis actualizada de la biota del cretáceo del sur del estado de goiás, basada en el registro de plantas, gastropodos, tortugas, cocodrilianos, titanosaurios y dinosaurios terópodos. resumen record manuscript received: 13/10/2019 accepted for publication: 17/07/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 387-396 *carlos roberto dos anjos candeiro1,2, stephen louis brusatte3, glayce kelly de queiroz souza1, adelino adilson de carvalho4, débora santos maia1, tamires do carmo dias1,2, luciano da silva vidal1,5, musa maria nogueira gomes1, lucas marques barros1 1universidade federal de goiás, curso de geologia, laboratório de paleontologia e evolução, brasil 2universidade federal de goiás, programa de pós-graduação em biodiversidade animal, brasil 3university of edinburgh, school of geoscience, scotland, united kingdom 4universidade federal de goiás, museu antropológico, brasil 5universidade federal do rio de janeiro, instituto de geociências, brasil *corresponding author: candeiro@ufg.br g eo lo g y https://doi.org/10.15446/esrj.v24n4.82831 https://doi.org/10.15446/esrj.v24n4.82831 https://doi.org/10.15446/esrj.v24n4.82831 mailto:candeiro@ufg.br 388 carlos roberto dos anjos candeiro, stephen louis brusatte, glayce kelly de queiroz souza, adelino adilson de carvalho, débora santos maia, tamires do carmo dias, luciano da silva vidal, musa maria nogueira gomes, lucas marques barros introduction the southern goiás state region is located in central brazil (fig. 1), on the borders of the triângulo mineiro region (minas gerais state), northern mato grosso do sul state, and eastern mato grosso state. some important federal highways cross the region, connecting it to other midwest states and the northern, southeastern, and southern regions of brazil. the state of knowledge of late cretaceous fossils from the bauru group in southern goiás state is limited when compared to the triângulo mineiro region and western são paulo state. nevertheless, in recent years, goiás state has started to demonstrate its potential as a region that has a diversity of fossil taxa such as invertebrates, ichnofossils, and vertebrates. in the literature that addresses fossil findings in the southern region of the state of goiás, little information is available on the upper cretaceous fossil records of the bauru group, which is the only major unit of the cretaceous period that crops out in the south of goiás (candeiro et al., 2006). in western são paulo state and the triângulo mineiro region, where many rocks from the bauru group outcrop, fossil specimens have been discovered for over 125 years (e.g., ihering, 1911; pacheco, 1913; huene, 1931, 1933; candeiro et al., 2006; martinelli & teixeira, 2015; candeiro & figueirôa, 2017; see review in brusatte et al., 2017). however, these authors have not often been cited in later studies, which have led to misconceptions about the knowledge of the local biota. the gastropod taxon viviparus souzai mezzalira, 1974, has been discovered in the municipality of mateira (currently paranaiguara municipality), but studies describing other materials such as invertebrates and vertebrates from southern goiás state were only published after 2013. the importance of correctly describing the location of deposits and their fossils in their stratigraphic contexts has been previously mentioned by some authors (resende, 2014; candeiro et al., 2018). since the publishing of these works, the existence of two fossil-bearing units have been confirmed and the listing of fauna from southern goiás state has been significantly updated. previous studies (e.g., simone & mezzalira, 1994; senra, 2002; basilici & dal´bo, 2010; dal´bo & basilici, 2011; candeiro et al., 2018) report the presence of fossil records from this region, and therefore, it seems necessary to update the full inventory of fossils. weishampel et al. (2004) proposed a summary list of the late cretaceous fauna of southern goiás state and, recently, basilici & dal´bo (2010) and candeiro et al. (2018) added other taxa to this inventory. yet, these lists did not consider the problematic history of the fossil record and uncertainties in correlating between formations, which can lead to errors when reconstructing the local biota. the aim of the present study is to present the history of the paleontology of the bauru group (upper cretaceous) of southern goiás state, a stratigraphic summary, and the fossil record of the local biota. material and methods the analysis of the documents using the hermeneutic method (i.e. investigating and interpreting the information in the original sources). the hermeneutic method can lead to discoveries, new contributions and critiques, thus expanding our horizon of possibilities. the data on records of fossiliferous localities used here were mainly based on bibliographic sources and on the direct observation of specimens: (1) the invertebrate specimen was housed at the instituto geográfico e geológico do estado de são paulo (currently instituto geológico de são paulo); and (2) specimens deposited under the acronym “paleontologia da universidade federal de goiás coleção de vertebrados / paleo-ufg/v” in the laboratory of paleontology and evolution (laboratório de paleontologia e evolução) of the universidade federal de goiás. for the stratigraphic units of the bauru group we followed the arrangement proposed by soares et al. (1980) and the ages suggested by dias brito et al. (2001). figure 1. map showing southern goiás state municipality fossil-bearing 389late cretaceous bauru group biota from southern goiás state, brazil: history and fossil content soares et al. (1980) proposed an updated stratigraphic system that still applies to the bauru group, including, in ascending order from oldest to youngest: the adamantina, uberaba, and marília formations. the bauru group rocks crop out in the states of goiás (south), minas gerais (triângulo mineiro region), mato grosso do sul (east), and são paulo (west) (sensu fernandes & coimbra, 1996). the bauru group outcrops irregularly in southern goiás state (fig. 2). some of the best exposures are in several municipalities (e.g., caçu, cachoeira alta, itajá, itarumã, mineiros, paranaiguara, paraúna, quirinópolis, rio verde, and turvelândia), where the rocks of the adamantina formation are superimposed on the lower cretaceous basalts of the serra geral formation (serra geral group, paraná basin) (sensu cprm — serviço geológico do brasil, 2004). the strata of the adamantina formation are composed of reddish fluviallacustrine sandstones that were deposited during warm and humid climatic events (soares et al., 1980; barcelos, 1984; barcelos & suguio, 1987; fúlfaro et al., 1994; candeiro et al., 2018). the fossils found in this formation include mainly vertebrates (santos-pereira et al., 2016; ferreira et al., 2016; gomes, 2017; simbras et al., 2013; resende et al., 2014; candeiro et al., 2016, 2018). the adamantina formation was deposited during the turonian-santonian intervals based in carophyte and ostracod content (dias-brito et al., 2001). almeida & barbosa (1953) described the marília formation in the western region of são paulo state and, later, in southern goiás state (soares deposition of specimens most materials reported from the bauru group of southern goiás state are part of the collection of the laboratory of paleontology and evolution, universidade federal de goiás, since most fossils deposits in this region were discovered by the laboratory’s research team. all collection numbers are included in table 1. the invertebrate reported by mezzalira (1974) from the municipality of paranaiguara – holotype 750-i – is housed at the instituto geológico e geográfico do estado de são paulo, currently scientific collection sérgio mezzalira of the instituto geológico da secretaria do meio ambiente do estado de são paulo. furthermore, the materials of fossil invertebrates described by basilici and dal´bo (2010) and dal´bo and basilici (2011) are still in situ and, therefore, do not have collection numbers. geological setting the southern part of goiás state is geologically located in the bauru supersequence (bauru group), one of the four supersequences that form the paraná basin (milani & zalan, 1999; milani et al., 2007; see review in brusatte et al., 2017). in this region, there is a significant sedimentary record of the upper cretaceous that is composed of the adamantina and marília formations (sensu soares et al., 1980). table 1. biota from late cretaceous bauru group recorded in southern goiás state. taxa geological unit age materials selected authors itajá plant indet. marília formation late maastrichtian leaf basilici & dal´bo (2010) itarumã ichnofossil marília formation late maastrichtian bioturbation basilici & dal´bo (2010) paranaiguara viviparus souzai adamantina formation turonian-santonian shells mezzalira (1974) paraúna theropoda indet. adamantina formation turonian-santonian tooth candeiro et al. (2020) quirinópolis vertebrata indet. adamantina and marília formations turonian-santonian/late maastrichtian indeterminated bones santos-pereira et al. (2016) tetrapoda indet. adamantina formation turonian-santonian indeterminated bones santos-pereira et al. (2016) crocodyliformes indet. marília formation late maastrichtian incomplete last sacral vertebra candeiro et al. (2018) rio verde vertebrata indet. marília formation late maastrichtian indeterminated bones santos-pereira et al. (2016) tetrapoda indet. marília formation late maastrichtian indeterminated bones santos-pereira et al. (2016) podocnemididae indet. marília formation late maastrichtian fragmentary right epiplastron candeiro et al. (2018) notosuchia indet. marília formation late maastrichtian proximal half of left femur candeiro et al. (2018) ?titanosauriformes indet. marília formation late maastrichtian partial rib candeiro et al. (2018) titanosauria indet. marília formation late maastrichtian partial tooth candeiro et al. (2018) cf. titanosauria indet. marília formation late maastrichtian fragment of the diaphysis of a right radius candeiro et al. (2018) 390 carlos roberto dos anjos candeiro, stephen louis brusatte, glayce kelly de queiroz souza, adelino adilson de carvalho, débora santos maia, tamires do carmo dias, luciano da silva vidal, musa maria nogueira gomes, lucas marques barros et al., 1980; barcelos, 1984; barcelos & suguio, 1987; fúlfaro etal., 1994). this geological unit is formed by fine to medium sandstones interbedded with levels of conglomerates. the sandstones are cemented and contain calcium carbonate concretions (fúlfaro & barcelos, 1991). according to barcelos & suguio (1987), the marília formation was deposited in an anastomosed river system that produced fine to medium sandstones and lacustrine limestone deposits. in the southern region of goiás state, this geological unit is the most richly fossiliferous, with records of invertebrates/ichnofossils and vertebrates (soares et al., 1980; barcelos & suguio, 1987; fúlfaro et al., 1994; basilici & dal´bo, 2010; dal´bo & basilici, 2011). dias-brito et al. (2001) assign the marília formation to the late maastrichtian interval based on the fossil record of ostracods from western são paulo state and in the triângulo mineiro region. results research history of the upper cretaceous of goiás state and current status of investigations although the bauru group outcrops occur across many areas of southern goiás state, there is comparatively few published information on the fossils of the adamantina and marília formations in this region. these formations are known for being richly fossiliferous in other states such as são paulo (western region) and minas gerais (triângulo mineiro), with records of invertebrates/ ichnofossils and vertebrates. the first paleontological references of cretaceous fossils from the bauru group in the literature were made by mezzalira (1974) and bonaparte (1978) (sensu weishampel, 1993; weishampel et al., 2004). weishampel (1993) and weishampel et al. (2004) reported a probable occurrence of fossil dinosaur remains in southern goiás state that could be assigned to titanosaurs and theropods. these materials were supposedly identified by llewellyn ivor price, 1905-1980 (paleontologist of the serviço geológico e mineralógico do brasil, currently departamento nacional da produção mineral, rio de janeiro, brazil) (paulo, 2009). although price was possibly the first to acknowledge dinosaur remains in goiás, weishampel (1990) mentioned them such as the record of bones referred as cf. titanosaurus sp., antarctosaurus brasiliensis, and theropoda indet for the first time. according to these authors, the localities where these records were found were mentioned in bonaparte (1978). later, weishampel et al. (2004) suggested that this fossil record was actually represented by “antarctosaurus brasiliensis”, lithostrotia indet. (= cf. titanosaurus sp.), and theropoda indet., also based on bonaparte (1978). however, bonaparte (1978) didn´t mention any dinosaur remains from the goiás state. based on the work of weishampel (1993, 2004), many subsequent studies (e.g., santucci, 2002; candeiro et al., 2004, 2006) have reported the existence of remains of “antarctosaurus brasiliensis”, sauropods, and theropods from the bauru group of goiás state. the previously mentioned references that indicate the possible existence of titanosaurs and other dinosaur records in goiás state do not show any information about what kind of fossil specimens are present or their origins, including their stratigraphic provenance. hence, our understanding of these specimens is mostly speculative, and we cannot verify any details associated with them. so, the references to these specimens do indicate that dinosaur remains might have been found decades ago in the bauru group rocks of southern goiás state, because during the last 120 years the outcrops of this geological unit in other areas of central brazil have revealed a rich dinosaur record. the gastropod viviparus souzai mezzalira, 1974 have been described from the rocks of the adamantina formation in the municipality of mateira (currently paranaiguara). this record is important because it correlates the strata of the bauru group in goiás state with other areas where it crops out, in minas figure 2. map showing areal distribution of the adamantina and marília formations in southern goiás state (adapted from cprm, 2014). 391late cretaceous bauru group biota from southern goiás state, brazil: history and fossil content gerais and são paulo states (simone & mezzalira, 1994; senra, 2002). some authors (e.g., senra, 2002; ghilardi et al., 2011a,b) reported that v. souzai came from “madeira”, goiás state, but its correct locality is mateira, toponym of the paranaiguara municipality. dal´bo & basilici (2009) described the first records of indeterminate trace fossils and bioturbation in the rocks of the marília formation, in the municipalities of itajá and quirinópolis. later, basilici & dal´bo (2011) described ichnofossils and other fossils (likely invertebrates) found in the same strata. according to the authors, these materials came from the levels of the adamantina and marília formations, i.e., the strata would be indistinguishable in the field, as they are characterized by the same lithofacies, architectural elements, and sequential organization. in 2013, the research team of the laboratory of geology from the geography course, pontal campus of the universidade federal de uberlândia, initiated research on the late cretaceous vertebrates from southern goiás state. in 2015, this research started to be conducted by the laboratory of paleontology and evolution of the geology course, aparecida de goiânia campus of the universidade federal de goiás. the first results were presented in scientific abstracts and in a book chapter (e.g. simbras et al., 2013; resende et al., 2014), which described indeterminate remains of sauropod dinosaurs (simbras et al., 2013). then, a fossil vertebrate-bearing stratigraphic column of the main deposits from the quirinópolis and rio verde municipalities was presented (candeiro et al., 2018). currently, studies and excavations are ongoing in the above mentioned localities where, today, the main research of our universidade federal de goiás team is concentrated. our studies have yielded the first formally described reptile fossils from these sites. furthermore, other materials recovered in these areas are still in preparation (e.g. indeterminate vertebrate remains, turtles, crocodyliforms, sauropods, and theropods). the results have recently published in more than eleven scientific works (candeiro et al., 2015, 2018, 2020; santos-pereira et al., 2016; souza et al., 2016; gil et al., 2018; lima et al., 2018; monteiro et al., 2018; gil, 2019; nogueira & candeiro, 2019; maia et al., 2020). the effort of our team over the last few years has made new information available and clarified the late cretaceous biota of southern goiás state. biota summary in order to facilitate understanding, the list of fossil records is divided into four parts: invertebrates and vertebrates from the adamantina and marília formations (fig. 3 and 4). both table 1 and figure 3 show fossil content cited here and the provenance stratigraphic units. the three dinosaur taxa from goiás state mentioned by weishampel (1993, 2004) –cf. titanosaurus sp./lithostrotia indet., “antarctosaurus brasiliensis”, and theropoda indet.– were not considered in the present study due to the lack of information regarding their provenance, the actual material preserved, and the collection in which they are (or were) housed. in other words, we are unable to verify that these taxa have been discovered in goiás state. some specimens assigned to tetrapoda indet., titanosaurifomes indet., and theropoda indet. are included in the list of taxa here, although they have been described only in papers that were recently submitted or that are in prep, because they are important records of the bauru group in goiás state. the turonian-santonian fossils from the adamantina formation the following vertebrate materials have been reported from the adamantina formation: figure 3. stratigraphic distribution of late cretaceous bauru group biota from southern goiás state, brazil. 392 carlos roberto dos anjos candeiro, stephen louis brusatte, glayce kelly de queiroz souza, adelino adilson de carvalho, débora santos maia, tamires do carmo dias, luciano da silva vidal, musa maria nogueira gomes, lucas marques barros figure 4. main faunal taxa reported from adamantina and marília formations of southern goiás state: (a) viviparus souzaishell; (b) podocnemidoidae fragmentary right epiplastron view. main faunal taxa reported from the adamantina and marília formations of southern goiás state: (c) cf. notosuchia indet. proximal half of a femur in lateral view; (d) crocodyliformes indet. last sacral vertebra view; (e) cf. titanosauria indet. diaphysis of a right radius; (f) ?titanosauriformes indet. – partial rib; (g) titanosauria indet. – fragmented tooth; (h) theropoda indet. – tooth (a-g from candeiro et al., 2018 and h from candeiro et al., 2020). 393late cretaceous bauru group biota from southern goiás state, brazil: history and fossil content locality 1: paranaiguara municipality. horizon: adamantina formation. age: turonian-santonian. fossil content: viviparus souzai mezzalira, 1974 (fig. 2). references: mezzalira (1974), simone & mezzalira (1994), senra (2002), ghilardi & maranhão (2010), ghilardi et al. (2011a, b). locality 2: ‘‘quiri3”, quirinópolis municipality. horizon: adamantina formation (one fossiliferous level). age: turonian-santonian. fossil content: vertebrata indet., tetrapoda indet., crododylomorpha – crocodyliformes indet. references: santos-pereira et al. (2016), candeiro et al., (2018, 2019). locality 3: ‘‘paraúna 1”, paraúna municipality. horizon: adamantina formation (one fossiliferous level). age: turonian-santonian. fossil content: dinosauria – theropoda indet. reference: candeiro et al. (2019). the late maastrichtian fossils from the marília formation today, the marília formation is the most fossiliferous geological unit in goiás state and its records include invertebrates, ichnofossils, and vertebrates. locality 4: ‘‘itajá 1”, itajá municipality. horizon: marília formation (one fossiliferous level). age: late maastrichtian. fossil content: invertebrata ichnofossils. reference: basilici & dal´bo (2010). locality 5: ‘‘itarumã 1”, itarumã municipality. horizon: marília formation (one fossiliferous level). age: late maastrichtian. fossil content: invertebrata ichnofossils. reference: basilici & dal´bo (2010). locality 6: ‘‘rio ver. 1”, rio verde municipality. horizon: marília formation (two fossiliferous levels). age: late maastrichtian. fossil content: vertebrata indet., thetrapoda indet.; testudines: podocnemididae; crododylomorpha indet., crocodyliformes, cf.; notosuchia; dinosauria; sauropoda; ?titanosauriformes indet. references: santos-pereira et al. (2016), resende et al. (2014), candeiro et al. (2018). locality 7: ‘‘quiri 1”, quirinópolis municipality. horizon: marília formation (two fossiliferous levels). age: late maastrichtian. fossil content: thetrapoda indet.; dinosauria: sauropoda: cf. titanosauria indet. references: santos-pereira et al. (2016), candeiro et al. (2018). locality 8: ‘‘quiri 2”, quirinópolis municipality. horizon: marília formation (one fossiliferous level). age: late maastrichtian. fossil content: vertebrata indet., thetrapoda indet., mesoeucrocodylia – crocodiliformes indet. references: santos-pereira et al. (2016), candeiro et al. (2018). discussion the present study on the history of the paleontology, stratigraphy, and biota of the late cretaceous of southern goiás state, aims to understand the paleontology of this important but little studied area where the bauru group crops out. in the municipalities where the fossil-bearing rocks of the adamantina and marília formations crop out in goiás, the exposures are moderately diverse compared to the outcrops from western são paulo state and the triângulo mineiro region, which are known for being rich in plants, invertebrates, and vertebrates (see fig. 5). figure 5. paleoenvironmental reconstruction of some taxa of the late cretaceous bauru group from southern goiás state showing mollusk (a), turtle (b), mesoeucrocodylians(c), titanosaurian (d) and theropod (e). drawing made by l. vidal. 394 carlos roberto dos anjos candeiro, stephen louis brusatte, glayce kelly de queiroz souza, adelino adilson de carvalho, débora santos maia, tamires do carmo dias, luciano da silva vidal, musa maria nogueira gomes, lucas marques barros up until recently, mezzalira (1974) described the only fossil specimen from the late cretaceous of the bauru group in goiás state. there is no previously published work that indicates in which geological unit of the bauru group viviparus souzai was discovered. here, this species was identified from the levels of the adamantina formation in the municipality of paranaiguara. this recognition was made during a field trip in 2015, when it was possible to verify that this is the only formation that crops out in paranaiguara, an observation corroborated by the mapping from cprm (2004). this fossil gastropod is important because it correlates the strata of the adamantina formation in goiás state with those of the triângulo mineiro region in minas gerais state (senra & silva, 1999). the representative tetrapod assemblages from the late cretaceous of southern goiás state have been known, apparently, since the 1940s. nevertheless, it has been only over the past five years that some of the fossiliferous layers in this region have been studied in detail. this has resulted in a considerable number of identifications of members of the flora and fauna of the adamantina and marília formations (see table 1; fig. 3). thus far, turtles, crocodyliforms, and dinosaurs are the most abundant groups recovered in these units. the presence of large terrestrial taxa such as mesoeucrocodylians is noteworthy, because these are among the most well-known fossil carnivores of the late cretaceous of south america. the sandstones and mudstones of the adamantina formation in the municipality of quirinópolis occur irregularly in southern goiás state. recently, this geological unit was, for the first time, described in detail by ferreira et al. (2016). the authors described the slopes where a few isolated crocodyliform, dinosaur and theropod remains have been found (candeiro et al., 2018). the late maastrichtian outcrops of the marília formation are the youngest, the most prolific, and most promising fossil-bearing sites from the late cretaceous of central brazil. a significant quantity of invertebrates and vertebrates have been found in these outcrops, although our work has only recently started. in southern goiás state, the marília formation is mainly formed by white and yellow sandstones with fine sandstone intervals and intraformational conglomerates (ferreira et al., 2016; candeiro et al., 2018; nogueira & candeiro, 2019; souza & candeiro, 2019). the localities 6 and 7 (quirinópolis, rio verde) of this geological unit are the fossil sites that contain the best dinosaurs. other associated flora and fauna are found in the northeast portion of the bauru group in goiás state. some of the vertebrate taxa reported from southern goiás state were found associated with other vertebrates that are known from other localities that are important for stratigraphic correlation: western são paulo state (presidente prudente, araçatuba, adamantina and marília formations), triângulo mineiro region (adamantina, uberaba, and marília formations), and even in upper cretaceous rocks of the neuquén group (candeleros, cerro lisandro, portezuelo, huincul and bajo de la carpa formations) and malargüe group (allen and loncoche formations), both in argentina (e.g., leanza et al., 2004; candeiro & rich, 2010; garrido, 2010). the materials of podocnemididae, notosuchia, titanosauria, and theropoda, found in the previously mentioned geological units, corroborate this connection with outrcrops of southern goiás state. turtles, crocodyliforms, and dinosaurs (especially titanosaurids) have also been found in other areas of the bauru group in minas gerais and são paulo states. regarding the titanosaurian records from the municipalities of quirinópolis and rio verde, these sauropods are widely known in the upper cretaceous geological units of argentina. the sphenodontids, ornithischians and mammals records from the late cretaceous of argentina is remarkable (e.g., leanza et al., 2004; de la fuente et al., 2007). some of these groups are also found in the bauru group of são paulo state but none have yet been found in the adamantina and marília formations of southern goiás state. the absence of these tetrapods in these areas may be due to the lack of extensive or less systematic prospecting being that the research history in the adamantina and marília formations in goiás state is still recent, but shows a great future potential. conclusions there are reports that cretaceous dinosaurs were found in southern goiás state since the 1940s, although these earlier discoveries cannot be verified. our recent work shows that there is indeed a diversity of fossils in southern goiás state, from two geological units: the adamantina (turonian-santonian) and marília (late maastrichtian) formations. the fossil fauna of southern goiás state is similar to that from similar aged deposits in the triângulo mineiro region, western são paulo state, and even argentina. southern goiás state is a promising area for future finds of cretaceous fossils, which can be crucialfor the evaluation of biodiversity and biogeographical patterns of tetrapods like dinosaurs and crocodylomorphs during the final part of the cretaceous, before the end-cretaceous mass extinction. in short, only a few seasons of fieldwork between the years of 2013 to 2018 in goiás state have resulted in a significant addition of new vertebrate records from the upper cretaceous of the bauru group, hinting that more discoveries are waiting to be made. our new finds increase the comparative data necessary to understand the faunal composition of central brazil and vertebrate distribution in this area during the cretaceous. acknowledgements crac was financially supported by conselho nacional de desenvolvimento científico e tecnológico (cnpq)/bolsista de produtividade em pesquisa. slb’s collaborative work with crac was funded by a grant from the fundação de amparo a pesquisa de goiás (fapeg) and newton fund to be researching in southern goiás state. references almeida, f. f. m. & barbosa, o. 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(ed). proceedings of the boletim de resumos i seminário de pesquisa do laboratório de paleontologia e evolução; nov 12-14; aparecida de goiânia (go). p. 37-38. souza, j. b. f., cavalcanti, r., pereira, c. s., carelli, d., souza, a. l., simbras, f. m., brusatte, s. l. & candeiro, c. r. a. (2016). new fossil vertebrate localities from southern goiás state, brazil. in: grillo on, romano psr, oliveira gr, editors proceedings of te x simposio brasileiro de paleontologia de vertebrados boletim de resumos/paleontologia em destaque: boletim informativo da sociedade brasileira de paleontologia; out 3–7; rio de janeiro. p.1-2. upchurch, p., barrett, p. m. & dodson, p. (2004). sauropoda. in: weishampel, d. b., dodson, p., osmólska, h. (eds). the dinosauria. 2nd ed. berkeley (ca): university of california press. p. 259-324. weishampel, d. b., dodson, p. & osmólska, h. (1990). the dinosauria. berkeley: university of california press. weishampel, d. b., dodson, p., osmólska, h. & richard, p. (2004). the dinosauria, 2nd ed. berkeley: university of california press. http://lattes.cnpq.br/1133817014664784 http://lattes.cnpq.br/1133817014664784 http://lattes.cnpq.br/1133817014664784 keywords: 226ra, rainbow trout, pylon ab6a, lr-115, mining. palabras clave: radio 226; trucha arcoíris; pylon ab6-a; lr-115; minería. how to cite item viloria avila, t. j., pesantez, a., vazquez silva, e., delgado, e. m., duque, p., & sajo-bohus, l. (2020). levels of radium-226 in the rainbow trout, macroinvertebrates-substrates and water adjacent to the mining concession loma larga, azuayecuador. earth sciences research journal, 24(1), 29-34. doi: https://doi.org/10.15446/esrj. v24n1.79728 mineral processing has been identified among the twelve main industrial processes that must be under control due to the expose to natural radioactive sources. the modification and, generally, the increase in the concentration of naturally occurring radioactive materials (norm) in the earth's crust cause an imbalance in the ecosystem. imbalance that affects the fauna, which are able to bio accumulate these radioisotopes and introduce them into the trophic chain. the main purpose of this work was to establish a radio-ecological baseline. for this purpose, the concentration levels of 226ra in rainbow trout, in macro invertebrate matrices-substrates and water from the irquis and portete rivers were evaluated. a study area adjacent to the loma larga mining concession was selected. the measurements were made using the lucas cells coupled to a pylon ab6a counter and the "can technique", using lr-115 passive detectors. the results indicated that rainbow trout bio accumulated in bones and organsa mean of 94.7% of the total 226ra detected, which values varied from 4.69 to 2.46 bq/kg, while macro invertebrate-substrate matrices showed a concentration that ranged between 0.7 and 1.3 bq/kg in the irquis river, while in the portete river was between 1.1 and 19.3 bq/kg. the water samples, in the sampling points of the irquis river, showed concentrations that ranged between 37.3 and 119.1 mbq/l and in the portete river between 43.5 to 146.4 mbq/l. abstract levels of radium-226 in the rainbow trout, macroinvertebrates-substrates and water adjacent to the mining concession loma larga, azuay-ecuador niveles de radio 226 en la trucha arcoíris, macroinvertebrados-sustratos y agua adyacente a la concesión minera loma larga, azuay, ecuador issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.79728 el procesamiento de minerales se ha identificado entre los doce principales procesos industriales que deben estar bajo un control a la exposición de fuentes radiactivas naturales. la modificación y, por lo general, el aumento de la concentración de norm en la corteza terrestre causa un desequilibrio en el ecosistema. desequilibrio que afecta a la fauna, la cual es capaz de bioacumular estos radioisótopos e introducirlos en la cadena trófica. la finalidad principal de este trabajo fue establecer una línea base radio-ecológica; para ello se evaluaron los niveles de concentración de 226ra en la trucha arcoíris, en matrices macroinvertebrados-sustratos y el agua de los ríos irquis y portete, ubicados en la provincia del azuay, ecuador. se seleccionó una zona de estudio adyacente a la concesión minera loma larga. las mediciones se realizaron haciendo uso de las celdas lucas acopladas a un contador pylon ab6-a y la “can technique”, utilizando detectores pasivos lr-115. los resultados indicaron que la trucha arcoíris bioacumuló el 95.2 % del total del 226ramdetectado, en los huesos y órganos, las matrices macroinvertebrados-sustrato presentaron una concentración que osciló entre 0.7 y 1.3 bq/kg en el río irquis, mientras que en el río portete resultó entre 1.1 y 19.3 bq/kg. las muestras de agua en los puntos de muestreo del río irquis, reflejaron una concentración variable entre 37.3 y 119.1 mbq/l y en el río portete entre 43.5 a 146.4 mbq/l. resumen record manuscript received: 17/05/2019 accepted for publication: 04/10/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 29-34 tony jesús viloria ávila1, adriana pesantez1, efrén vázquez silva1, ernesto manuel delgado1, paola duque1, laszlo sajo-bohus2 1universidad politécnica salesiana, sede cuenca, ecuador 2universidad simón bolívar, caracas, venezuela * corresponding author: tviloria@ups.edu.ec en vi ro nm en ta l g eo lo g y 30 tony jesús viloria ávila1, adriana pesantez, efrén vázquez silva, ernesto manuel delgado, paola duque, laszlo sajo-bohus introduction the us environmental protection agency (us-epa) acknowledged in 1987 that the problems related to mining waste can be described as surpassed only by global warming and depletion of stratospheric ozone in terms of ecological risk. the release to the environment of mining waste can cause a profound, and generally irreversible, destruction of ecosystems (durand, 2012). according to the international atomic energy agency (iaea, 2006), any mining operation or activity in which a mineral is involved has the potential to increase the effective dose that a person can receive from natural sources as a result of exposure to naturally occurring radioactive materials (norm) contained in industrial waste. the aquatic systems among others contain norm related to the human activity, and therefore are responsible for a fraction of the dose that the human being receives from the environment. the global average internal dose due to radionuclides other than radon and its decay products is 0.29 msv/y of which 0.17 msv/y comes from 40k, 0.12 msv/y comes from the uranium and thorium series, and 12 μsv/y come from 14c (united nations scientific committee on the effects, 2008). the natural radioactivity of water comes essentially from the 40k and the disintegration series of the 238u and 232th. for groundwater, the main radioactive isotopes found are those mentioned above, and their daughters; within the hazardous elements are found 226ra, 228ra and 222rn, also their progeny. these elements when inside the body move to the bone tissue and concentrate (cox, 1993). the prolonged exposure can cause bone cancers and other bone irregularity; their concentrations vary depending on the geochemical history of the water and the industrial activities of the environment. the bioaccumulation of natural or artificial radionuclides in aquatic organisms is the focus of radio-ecological studies, because they act as an additional source to the radiation dose; of particular importance for the aquatic flora and fauna that form food chains used by the population (stewart et al., 2008). environmental problems associated with norm and solid mineral mines occur during the process of drilling, leaching, handling, storage, transportation of minerals and related equipment (innocent et al., 2013). in ecuador the mining is very active, because the regions have several minerals, such is the case from the southern region of ecuador, known as austro, where we found an important variety of minerals such as gold, silver, copper, lead and zinc (rea-toapanta, 2017). therefore, it is important to carry out baseline studies regarding radioactive elements, such as 226ra, in different media near the mines, in order to be able to calculate the radiological impact, considering that the exploitation mining activity in most cases significantly alters the natural levels of these. as expected, they also increase the concentration levels of the norm in the biota of the rivers. in this study, the concentration levels of 226ra were evaluated in three matrices, the rainbow trout (oncorhynchus mykiss), the macroinvertebrate and the water of the irquis and portete rivers. the rainbow trout is an ichthyic species of the salmonidae family, present in south america, where its habitat is generally located in cold waters of rivers and lakes, tolerating a temperature range between 0 to 30 °c, are a very resistant species, capable of displacing endemic species. the macroinvertebrate are bio indicators of the water quality with biological indexes, giving a significant importance to these matrices. materials and methods the technique used in this study for the quantification of the content of 226ra in the organic matrix (fish) was the can technique making use of solid-state nuclear track detectors (ssntds), specifically the lr-115 detector (mohamed abd-elzaher, 2012). selection of sampling areas this investigation was carried out in the mining project called loma larga which covered an area of 7,960 ha, where three concessions operate, known as cerro casco, cristal and río falso. it is located in the south of ecuador 15 km northwest from the town called girón and 30 km southeast of cuenca city, at an altitude between 3 500 and 3 900 mamsl (jarrín-jurado & morán-reascos, 2017). the area of study was río falso concession with an area of 3 168 ha, located in the parishes baños, victoria del portete, san gerardo, chumblín, san fernando and girón, belonging to the province of azuay (fig. 1) (inv minerales ecuador sa, 2016). the physico-chemical properties of the site analyzed in this study were characterized by presenting strata of andesitic porphytic lavas of very fresh acicular pagioclase and amphibole, which are volcanic or igneous extrusive rocks containing between 52% and 63% silica (sio2), in addition 5% iron (feo) and 7% calcium (cao), on average. in relation to hydrology, the drainage network is a radial dendritic type constituted by a series of streams. drainage to the southeast occurs through two main rivers, the irquis and the portete. these effluents, being also transport vectors of matter of the mining exploitation, have been analyzed in this study to determine the 226ra levels. figure 1. location map of the loma larga mining project, in the province of cuenca, ecuador. coordinates-psad-1956-utm, zone17s, (december 2018). in this work, sampling points for the rainbow trout, macroinvertebrate matrices and water, were selected as established by the world health organization (who, 2006). it considers that the number of sampling points depend on the characteristics of the water quality under analysis. for components whose concentrations tend to vary during distribution, the number of strategic points should be three, distributed along the region of studies (see table 1). in this work these were situated in the headwater of the river, in the middle course of the region of interest (adjacent to the mining activity) and at the exit of the concession (see fig. 2). water samples collection of surface water samples obtaining water samples was carried out by the immersion method, following the protocol established for the pylon ab6 detector (pylon electronics inc., 2015). to take the liquid sample, a 250 ml glass container was used, ensuring the absence of bubbles, which was hermetically sealed and each of the containers identified with the necessary data. 31levels of radium-226 in the rainbow trout, macroinvertebrates-substrates and water adjacent to the mining concession loma larga, azuay-ecuador conc ncpm nae cf eff v a eff g bq lra c s d = ( ) * * * * * * / 2 (1) where, ncpm is the number of counts per minutes, nae is the number of alpha emissions (for 222rn is 3), cf is the conversion factor (for this calculation is 60 dpm to dps), effc is the cell efficiency (0,745 cpm/dpm), vs is the sampling volume (0,19 l), effd is the 222rn extraction efficiency (for the lucas cell 600a is 90 ± 5%) and g is the growth correction factor. the growth correction factor is calculated by the following equation, g e tdelta= − −( )1 1 (2) where, t t t mindelta a1 1 0= − ( ) and t0a is the time in which the sample was initially degassed, t1 is the time in which the sample was degassed for measurement. after each measurement the lucas cells were flushed with nitrogen, at a flow rate of 5 l/min for 3 minutes, in order to prevent contamination and reduce background count buildup that could affect future measurements. the cells were stored for a minimum of 24 hours, before being reused again. biological samples. collection of samples. the macro invertebrates and the substrate in which they live, such as sediments, algae and macrophytes, were collected with surber sampler nets, located at the three selected points of each river. the rainbow trout were collected at the midpoint of the irquis river, adjacent to the mine mouth of the concession denominated río falso by gillnets. preparation of the trout samples each of the individuals was dissected, preparing with them two matrices, one of muscle tissue, and another of bones and organs. organic matrices of both measurement of 226ra activity concentrations in water. to measure the 226ra activity concentrations in the water sample, the 222rn was evacuated from the latter, flowing nitrogen through it for a period of approximately 15 minutes; the samples were stored for a period of 3.8 days, after this time, they were degassed, transferring the gas to a lucas cell model 600-a, according to the pylon ab6 protocol (pylon electronics inc., 2015). the formula used to calculate the ra activity concentrations in water was: figure 2. map of the location of the sampling points in the irquis and portete rivers. coordinates-psad-1956-utm, zone17s (december 2018). kilometers 0 0.45 0.9 1.8 2.7 3.6 table 1. coordinates of the sampling points in the study area where the investigation was conducted. code coordinates psad-1956-utm-zone 17s sampling area river 1i 698141 9664023 irquisheadwater 2i 699137 9663840 quebrada high quinuahuaycu irquisadjacent to entrance of the mine 3i 700018 9662981 quebrada quinuahuaycu irquis-exit of the concession 1p 698448 9662815 porteteheadwater 2p 699249 9661368 quebrada kalluancay portete middlecourse 3p 699916 9660990 quebrada kalluancay portete exit of the concession 32 tony jesús viloria ávila1, adriana pesantez, efrén vázquez silva, ernesto manuel delgado, paola duque, laszlo sajo-bohus macro invertebrates and rainbow trout were dehydrated in an oven at 110 °c for 24 hours. and then crushed to obtain a finer granular consistency for favoring the exhalation rate of 222rn; after this the weighing was carried out. taking into account that the radon exhalation of 222rnfrom the matrices is conditioned by the physical properties of it, among which the porosity is one of the most important, and considering that the porosity is not controlled by grain size, as the volume of between-grain space is related only to the method of grain packing, but the emanation of radon depends on the grain size and the shape, because they determine in part how much radium is near enough to the surface of the matrix grain to allow the newly formed radon to escape into a pore space, we considered appropriated to make a fine matrix to reduce the probability of 222rn to remain embedded in the grain. content measurement of 226 ra in macro invertebrates-substrate matrices. the samples of macro invertebrates and rainbow trout previously processed were placed in pvc cylindrical containers, of 10 cm high and 7 cm diameter in which a passive detector was positioned. in this case, 1.5 cm x 2 cm lr-115 type ii (kodak-pathe, france) detectors were used, located in the internal upper part of the container, in such a way that the sensitive part of the detector is oriented towards the sample, at approximately 4 cm from the sample surface (khan et al., 1992). in order to avoid the registration of alpha particles emitted in the decay of the isotopes of radon, prior the 226ra and the 222rn reached the secular equilibrium within the cylindrical containers, the detectors were covered for four weeks with a metal foil that was held in place with the help of a small magnet on the outside of the chamber. after that time, the metal sheet was released, exposing the detector for the following 60 days (sonkawade et al., 2008). after this time, the detectors were removed and etched, placing each detector in a 2.5 n naoh solution for 90 minutes, at a temperature of 60 °c in a laboratory water bath (zubair et al., 2012). the nuclear tracks density was quantified with the help of an optical microscope with a magnification of 400x. the content measurement of 226ra was calculated by the following equation. c h a k t m bq kgra rn eff = ( )* * * * / (3) where, ρis the radon track density, h is the distance between the surface of the sample and the detector in meters, a is the cross-sectional area of the container in m-2, krn is the calibration factor of lr-115 plastic track detector (0.033 track cm2 d-1/ bq m-3), teff is the effective exposure time, and m is the mass of the sample in kg. the effective exposure time can be calculated using the formula t t eeff t= − −( )−1 1   (4) where, t is the actual exposure time and λ is the decay constant for 222rn. results the 226ra concentrations in the matrices of macro invertebrates-substrate of the irquis river ranged from 0.90 ± 0.07 bq/kg to 1.30 ± 0.08 bq/kg, while in the portete river the minimum concentration was 1.10 ± 0.08 bq/kg and the maximum of 19.30 ± 0.08 bq/kg (fig. 3). because of the difficulty to fish in others sampling points along the rivers, only two sampling were done at only one sampling point, the midpoint of the irquis river analysis region (adjacent to the mine mouth); in the first sampling the average 226ra levels in the muscle tissue of the rainbow trout was 0.17 ± 0.04 bq/kg, and the corresponding to the bone-organs matrix was 4.69 ± 0.51 bq/kg. while in the second sampling the muscle tissue concentration was 0.19 ± 0.03 bq/kg, and the bone-organs sample showed a 226ra concentration of 2.46 ± 0.31 bq/kg. all values correspond to dry weight (see fig. 4). it is worth to noting that, the difference between the 226ra concentration in the boneorgans matrices in the first and second sampling was 47.74 %. the original purpose of the two samplings was to obtain a better statistic, but that noticeably greater difference between the values of the 226ra concentrations in the bone-organs matrices, makes a comment on it worthwhile, even though it is not part of the main objective of this work. the bioaccumulation factor depends on different factors, such as the duration of exposure, the rate of metabolism, the age and the diet of the species, among others. 0 1 2 3 4 5 bones-organs muscle tissue total c on ce nt ra tio n of r ad iu m -2 26 type of sample concentration of ra-226 (bq/kg) first sampling concentration of ra-226 (bq/kg) second sampling figure 4. concentration of 226ra in the rainbow trout of the irquis river in the area adjacent to the mine entrance. the variation of the radium levels in both rivers, have a similar behavior, they increase as the measurements are done downstream. the levels of radium in the headwater of the irquis river and the portete river were 39.3 and 57.5 mbq/l respectively, while in the lowest points downstream were 119.1 and 146.39 mbq/l for the irquis and portete river respectively (see fig. 5). discussion the iaea (2014) reports that in uncontaminated surface waters the concentration range of radioisotope 226ra in fish (whole body) ranges between 0.68 and 1.12 bq /kg (fresh weight). swanson s. m. (1983) reports concentrations of 226ra in fish, among them trout, in uncontaminated surface water, with levels between 0.296 and 3.400 bq/kg in dry weight, values similar to those obtained in this investigation, in which was obtained an average concentration for rainbow trout of 3.75 bq/kg (dry weight). the concentration levels of 226ra in the bone-organ matrices were higher than those in the muscle tissue matrices, this is due to the fact that both calcium and radium belong to the same group in the periodic table, so both elements have very similar chemical properties, and that is why in human and animal organisms, radium is assimilated as calcium and deposited in the bones. clulowet al. (1998), in their study in lakes adjacent to non-industrialized zones, found levels of 226ra in the bones of lake trout (salvelinus namaycush), 0 5 10 15 20 1 2 3 mean c on ce nt ra tio n of 22 6 r a 1n b q/ kg sampling points radium activity concentration (cra) (bq/kg) irquis river radium activity concentration (cra) (bq/kg) portete river figure 3. concentration of 226ra in macro invertebrates of the irquis and portete rivers (1 corresponds to the source of the two rivers, 2 represents the midpoint of both the irquis and portete rivers, and 3 are the sampling points at the exit of the concession) 33levels of radium-226 in the rainbow trout, macroinvertebrates-substrates and water adjacent to the mining concession loma larga, azuay-ecuador with average values less than 20 bq/kg, as well as skipperud, et al. (2013) in their work claim that the concentration of radionuclides bio accumulates in large quantities in bones rather than in gills, more than in kidneys, and more than in the liver, while in smaller amounts it is in the muscle, agreeing satisfactorily with the result of this study, which was that of the total 226ra measured, 95.21% was accumulated in bones and organs, and only 4.79% in muscle tissue. the results reported by siddeeg, et al., (2014), and sethy, et al., (2011), on 226ra concentrations in sediments of rivers that are considered affected by mining activity, are in a range between 15. 0 and 179.0 bq/kg, while in the works of sethy, et al., (2011) and rowan, et al., (2013) in rivers not affected by mining activity, concentrations of 226ra in the sediments are between 1.99 bq/kg and 4.50 bq/kg; these last values are comparable with those obtained in this work, which were between 0.90 ± 0.07 bq/kg and 1.30 ± 0.08 bq/kg for the irquis river, and 1.10 ± 0.08 bq/kg and 19.30 ± 0.08 bq/kg for the portete river. the levels of dissolved 226ra activity in river waters are related to the composition of the soil and the physical and chemical properties of the waters. siddeeg (2013) asserts that the physical properties of the bedrock along the river bed play an essential role in the contribution of the concentration of 238u, 232th and 226ra in natural waters. the geochemical processes of 238u, 232th and 226ra, such as the adsorption by clay minerals, insoluble oxides such as iron oxide, play a key role in the understanding of the behavior of these radioisotopes and in the analysis of their concentration levels present in both water and in the soil. on the other hand, porcelli, et al., (2001), affirm that these levels of concentrations vary, when “the streams drain limestone, phosphates and rocks rich in 238u”, factor that influences so that the concentration of this radionuclide is greater. when comparing the results obtained by hameed et al.,(1997) (0.07-314 mbq /l) and de lama & osores (2002) (2.1-175.4 mbq/l) whose works were carried out in areas with lithological similarities to the present work, where predominate the andesitic strata from volcanic (igneous) rocks, which contain relative high percentage of silica, iron oxides and calcium oxides, we found that the values obtained are within the ranges mentioned above; in the portete river the activities of the dissolved 226ra ranged between 43.5mbq/l and 146 mbq/l, while in the irquis river the values were between 37 mbq/l and 119 mbq/l. some authors, such as de oliveira, et al., (1996) and the international atomic energy organization (2014), state that 226ra at low or high ph is more soluble than at medium ph. szabo et al. (2012) analyzed the correlation of the physical-chemical parameters of water with the 226ra content and affirm that this alkaline earth metal has a significant correlation with ph, dissolved oxygen (do) and total dissolved solids (sdt), as well as with some metals. high concentrations of 226ra occur mainly in anoxic waters (low do) with high content of iron, manganese, calcium, magnesium, barium, strontium and eventually with total dissolved solids, potassium, sulfates (so4) and carbonates (hco3), as well as when the water is acidic (low ph) and contains high concentration of nitrates (no3) and aluminum. both the waters of the irquis and portete rivers, which were analyzed in this study, differ completely from the aforementioned characteristics. conclusions the values obtained in this work, concerning to the amount of 226ra measured in trout bone-organ matrices, the trout muscle-tissue matrices, and water of the rivers irquis and poetete, were very similar to values obtained by other authors who measured the same magnitudes in uncontaminated waters. it should be noted that the values obtained in this work are the first of their kind and could serve as a reference database for future works whose main objective will be to measure the radiological impact due to the mining activity in loma larga. it is suggested to expand the sampling area around the loma larga concession, the number of samples, and the sampling period and calculate the bioaccumalation factor (baf). references clulow, f., davé, n., lim, t. & avadhanula, r. 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(2020). analysis of accumulation models of middle permian in northwest sichuan basin. earth sciences research journal, 24(4), 418-427. doi: https://doi.org/10.15446/esrj. v24n4.91149 great progress has been made in middle permian exploration in northwest sichuan in recent years, but there are still many questions in understanding the hydrocarbon accumulation conditions. due to the abundance of source rocks and the multi-term tectonic movements in this area, the hydrocarbon accumulation model is relatively complex, which has become the main problem to be solved urgently in oil and gas exploration. based on the different tectonic backgrounds of the middle permian in northwest sichuan basin, the thrust nappe belt, the hidden front belt, and the depression belt are taken as the research units to comb and compare the geologic conditions of the middle permian reservoir. the evaluation of source rocks and the comparison of hydrocarbon sources suggest that the middle permian hydrocarbon mainly comes from the bottom of the lower cambrian and middle permian, and the foreland orogeny promoted the thermal evolution of paleozoic source rocks in northwest sichuan to high maturity and over maturity stage. based on a large number of reservoir physical properties data, the middle permian reservoir has the characteristics of low porosity and low permeability, among which the thrust nappe belt and the hidden front belt have relatively high porosity and relatively developed fractures. the thick mudstone of longtan formation constitutes the regional caprock in the study area and the preservation condition is good as a whole. however, the thrusting faults destroyed the sealing ability of the caprock in the nappe thrust belt. typical reservoir profiles revealed that the trap types were different in the study area. the thrust fault traps are mainly developed in the thrust nappe belt, while the fault anticline traps are developed in the hidden front belt, and the structural lithological traps are developed in the depression belt. the different structural belts in northwest sichuan have different oil and gas accumulation models. this paper built three hydrocarbon accumulation models by the analysis of reservoir formation conditions. the comprehensive analysis supposed the hidden front belt is close to the lower cambrian source rock, and the reservoir heterogeneity is weak, faults connected source rock is developed, so it is a favorable oil and gas accumulation area in the middle permian. abstract analysis of accumulation models of middle permian in northwest sichuan basin análisis de los modelos de acumulación del pérmico medio en la cuenca del noroeste de sichuan issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91149 un gran progreso se ha logrado en la exploración del pérmico medio en el noroeste de sichuan (china) en los últimos años, pero aún persisten muchas preguntas para entender las condiciones de acumulación de hidrocarburos. debido a la abundancia de rocas madre y los movimientos tectónicos multifase en esta área, el modelo de acumulación de hidrocarburos es relativamente complejo, por lo que se convierte en el principal problema a resolver en la exploración de petróleo y gas. con base en los diferentes contextos tectónicos del pérmico medio en el noroeste de la cuenca de sichuan, se tomaron la falla de corrimiento en la lámina de empuje, la franja delantera oculta, y la franja de depresión como las unidades de investigación para combinar y comparar las condiciones geológicas del reservorio con origen en el pérmico medio. la evaluación de las rocas madre y la comparación de las fuentes de hidrocarburos sugieren que los hidrocarburos del pérmico medio provienen principalmente de la parte baja del cámbrico inferior y del pérmico medio, y que la orogenia del antepaís propició la evolución termal a etapas altamente maduras y sobremaduradas de las rocas madre del paleozoico en el noroeste de sichuan. con base en información de las condiciones físicas del depósito, el reservorio del pérmico medio tiene condiciones de baja porosidad y baja permeabilidad, entre las cuales la falla de corrimiento en la lámina de empuje y la franja delantera oculta tienen una alta porosidad relativa y fracturas relativas desarrolladas. la gruesa lutolita de la formación longtan constituye la roca de cubierta regional en el área de estudio y su preservación en general es buena. sin embargo, las fallas de corrimiento destruyeron la capacidad de sellamiento de la roca de cubierta en la lámina de empuje. los perfiles típicos revelan que los tipos de trampa en el área de estudio eran diferentes. las trampas de las fallas de corrimiento se desarrollaron principalmente en la lámina de empuje, mientras que las trampas de la falla anticlinal se desarrollaron en la franja delantera oculta y las trampas en la estructura litológica se desarrollaron en la sección de la depresión. las diferentes secciones estructurales del noroeste de sichuan tienen diferentes modelos de acumulación de petróleo y gas. en este artículo se construyeron tres modelos de acumulación para el análisis de las condiciones de formación del reservorio. el análisis completo supone que la sección frontal oculta es cercana a la roca madre del cámbrico inferior, que la heterogeneidad del reservorio es frágil y que la conexión de las fallas en la roca madre es desarrollada, por lo que es favorable el área de acumulación de petróleo y gas en el pérmico medio. resumen record manuscript received: 07/05/2020 accepted for publication: 14/09/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 419-428 bin li1,2,*, qiqi li3, wenhua mei2, qingong zhuo4, xuesong lu4 1state key laboratory of shale oil and gas enrichment mechanisms and effective development, wuxi, 214126 2school of geoscience and technology, southwest petroleum university, chengdu, 610500 3school of earth resources, china university of geosciences, wuhan, 430074; 4petrochina research institute of petroleum exploration and development, beijing 100083, china *corresponding author: libinxnsy@outlook.com g eo lo g y https://doi.org/10.15446/esrj.v24n4.91149 https://doi.org/10.15446/esrj.v24n4.91149 https://doi.org/10.15446/esrj.v24n4.91149 mailto:libinxnsy@outlook.com 420 bin li, qiqi li3, wenhua mei, qingong zhuo, xuesong lu introduction the middle permian in northwest sichuan is an important field of natural gas exploration in the sichuan basin. in recent years, several wells have obtained high-production in qixia formation and maokou formation (dahai et al., 2016; he, 2014; shen et al., 2016; yuanjiang, 2019), showing good exploration prospects. several sets of source rocks, such as carbonate rocks, argillaceous rocks, and coal, were developed in the paleozoic (hui et al., 2012; lu et al., 2017; dong et al., 2011), they superimposed and crossed each other on the plane, resulting in oil and gas channeling and mixing in the formation, resulting in complex gas source relationship (li et al., 2015; zhang et al., 2019; jie et al., 2016). at the same time, multi-stage tectonic movements (chen et al., 2019) caused multi-stage hydrocarbon generation, migration and accumulation, and adjustment and transformation, which complicated the oil and gas system and led to an unclear understanding of the conditions and modes of hydrocarbon accumulation, and seriously hindered the further exploration progress. in recent years, with the discovery of the lower paleozoic cambrian longwangmiao formation and sinian large gas fields in the middle sichuan paleouplift area (gu et al., 2016; shen et al., 2016; wei et al., 2008; xingzhi et al., 2019), indicating that the deep marine strata in northwest sichuan basin may have a good prospect for the discovery of large-scale oil and gas reservoirs (yang et al., 2019; yu et al., 2018; xiao et al., 2018). therefore, an in-depth study of the basic geological conditions of the middle permian hydrocarbon accumulation in northwest sichuan and the establishment of a hydro carbon accumulation model will be helpful to the re-understanding of marine hydrocarbon distribution in this area. geological settings northwestern sichuan is located in the western margins of the yangtze plate between the sichuan basin and the songpan-ganze orogenic belt (fig. 1). this region extends northwards to the chaotian district of guangyuan city and southwards to the mianyang city-yanting county. the northwestern sichuan has experienced the following tectonic evolution stages. (1) the northwestern sichuan basin was a depression during the early cambrian that eventually became the most important source rock horizon in the paleozoic system due to the deposition of the lower cambrian marine fragmental rocks (fig.1b). (2) at the end of the silurian period, the silurian and ordovician strata were strongly eroded due to the caledonian orogeny. this region began to sink again after the caledonian orogeny leading to the deposition of marine and carbonate-dominated devonian and carboniferous strata (chen et al., 2019). (3) the yunnan movement at the end of the carboniferous period severely eroded the carboniferous strata (gu et al., 2016; yingqiang et al., 2016). the northwestern sichuan basin was again sea covered at the beginning of the permian period, which led to the deposition of the permian and middle-lower triassic marine strata. (4) the basin then contracted during the late triassic due to the indo-china orogeny that led to the seawater retreat that thus marks the beginning of terrestrial sedimentation (he, 2014). (5) after the jurassic period, a sharp uplift occurred in the longmen mountains of northwestern sichuan due to the yanshanian and himalayan orogeneses (meng et al., 2016). this resulted in severe stratigraphic erosion in these regions. this series of events ultimately gave rise to the current tectonic state of the region. the study area developed three tectonic units: the thrust nappe belt, the hidden front belt, and the depression belt according to the characteristics of tectonic development in northwest sichuan. driven by tectonic movement, thrust occurred in longmen mountains to form nappe tectonic belt (gu et al., 2016; renqi et al., 2016; yingqiang et al., 2016) became the western boundary of the basin, and typical structures developed included hanwan changkuangshan anticline, etc. high-yielding gas reservoirs were found in the qixia formation and maokou formation, and the key exploration wells were k1, k2, k3, h1, h2, h12, and hs1. in the front edge of the thrust fault, the hidden front belt was formed, which is mainly distributed in the shuangyushi-zhongba structural belt, and exploration wells s1, st1, and st3 were drilled in the middle permian with high-yielding gas reservoirs. the area east to nanjiangcangxi-yanting is the depression belt with typical structures such as wujiabajiulongshan structure, etc., and the exploration wells in the middle permian are wj1, l17, l16, gj, b1, etc. (fig 1c). samples and methods samples and data in this study, a total of 128 samples were selected, including 62 hydrocarbon source rocks, 12 natural gas samples, and 54 reservoir samples, based on the comprehensive sampling of different structural belts and different strata. the source rock samples were mainly taken from wells hs1, k1, k3, wj1, s1, h2, gj, l17, st2, and st3, as well as the field sections of northwest sichuan tongkou, changjiang ditch, qiaoting, and tongkou. natural gas samples were taken from wells st1, st2, and st3. reservoir samples were taken from wells k1, k2, k3, st3, l16, and wj1, etc. at the same time, some oil and gas field reservoir physical property test and analysis data were also collected. besides, seismic data on middle permian gas reservoirs have been collected from the exploration and development research institute of petrochina southwest oil & gasfield company, including seismic interpretation section (aa ‘) and seismic interpretation section of wells k1, s1, h2, st3, and g5. the single well analysis data, including formation pressure and formation water data of wells st2, l17, b1, gj, h1, and k3, and data of some wells with denudation thickness of regional strata and basin simulation in northwest sichuan were collected from the same institute. experimental methods solid bitumen reflectance the solid bitumen reflectance was measured using an mpv-3 microphotometer, related accessories, and a qdi302 microscope. the standard used for these measurements was a yttrium aluminum garnet (whose ro is 0.92% in standard conditions). the measurements were performed with a magnification of ×500 (the magnification of the oil immersion objective was×50) and an input wavelength of λ=546 nm (green light) under the standard of sy/t 5124-2012. organic carbon the organic carbon measurements in the hydrocarbon source rocks were performed by first grinding the samples, followed by the addition of hydrochloric acid to remove the inorganic carbon. the samples were then inserted into a cs230sh sulfur/carbon analyzer under the standard of gbt 19145-2003. gas chromatography-mass spectrometry the chloroform bitumen ‘a’ content in the samples was obtained by performing chloroform extraction on the crushed samples. after the asphalt was precipitated with n-hexane, the filtrate was passed through a silica gel column. solvents with different polarities were then used to sequentially separate the saturated hydrocarbons, aromatic hydrocarbons, and non-hydrocarbons. gas chromatography-mass spectrometry (gc-ms) analyses were performed on the saturated hydrocarbons using a thermo scientific trace gc ultra-dsq gcms. the gc-ms analyses were performed using an hp-5ms elastic quartz capillary (60m×0.25mm×0.25 mm) gc column. the temperature program began with an initial temperature of 100 °c (held for 5 min), then increased at a rate of 3 °c/min up to 320 °c where the temperature was then held for 20 min. the carrier gas was 99.999% helium, and the inlet temperature was 280 °c. the sample was injected at a constant flow rate of 1 ml/min and the transmission line temperature was 300 °c. electron impact ionization (70 ev) with a filament current of 100 ma and an ion source temperature of 250 °c was used as the mass spectrometry ionization method. the gc-ms analysis on the source rocks was also performed using the trace gc ultra-dsq gc-ms under the standard of gbt30431-2013. natural gas composition and carbon isotope analyses the natural gas composition analyses were performed using an hp6890 gas chromatograph, with an sge-60 gc column (50m×0.25mm×0.25 mm). the inlet temperature of 300 °c was used with nitrogen as the carrier gas. the flow rate was 1 ml/min with a split ratio of 50:1. the temperature of the column was increased from 30 °c to 260 °c at a rate of 3 °c/min. 421analysis of accumulation models of middle permian in northwest sichuan basin results source rock conditions field profile and drilling data in northwest sichuan have confirmed that multiple sets of source rocks developed in the paleozoic era (gu et al., 2016; wu et al., 2012; dong et al., 2011; li, 2018). based on the test of 62 samples of multiple sets of source rocks (tab. 1), qiongzhusi formation of the lower cambrian has the highest abundance of organic carbon, and toc distributed between 0.6 and 6.2%, with an average value of 2.67%. the second source rock is longmaxi formation and longtan formation, with mean values of 0.91% and 1.89% respectively. the average abundance of organic carbon in maokou formation and qixia formation is 0.87% and 0.81%, both of which have the basic conditions to form effective source rocks (tab. 1). the carbon isotope measurements for the natural gas samples were performed using a finnigan mat-252 mass spectrometer under the standard of gbt 13610-2014. the carbon isotope values were then compared with the gbw 04405 reference to obtain the corresponding pee dee belemnite (pdb) values. the standard deviation of these values was±0.3‰. thermal history model burial-thermal history analysis was prepared using petromod 1d software. the stratigraphic sequences, denudation thickness, and tectonic events were derived from geological reports of well k1, st3, and l16. the source rock assignment and their respective properties, total organic carbon (toc) and hydrogen indices (hi) implemented in order to simulate the thermal maturity of source rock. figure 1. location, tectonic setting, and stratigraphic column of the studied area, nw sichuan basin. (a) geologic map showing the tectonic outline and wells and outcrop profile location in nw sichuan basin. (b) aa’ linked line seismic profile. (c) stratigraphic column of northwest sichuan basin (according to the research institute of southwest oil and gas filed, 2015). table 1 geological and bulk geochemical data for source rock samples in northwest sichuan basin horizon thickness (m) distribution toc (%) ro (%) δ13c (‰) kerogen type p3l 54 entire 1.89(9) .25῀6.72 1.73(10) 1.52%῀2.17% -26.73(10) -29.22῀-24.48 ii. 1 ii. 2 iii p2m 115 entire 0.87(21) 0.07῀2.58 1.91(10) 1.1%῀2.45% -28.56(10) -30.86῀-27.32 i. ii. 1 ii. 2 p2q 56 entire 0.81(12) 0.06῀4.82 1.88(7) 1.5%῀2.32% -28.27(7) -29.42῀-27.36 i. ii. 1 ii. 2 s1l 150 local 0.91(5) 0.09῀2.33 1.82(5) 1.49%῀2.37% -30.23(5) -32.04῀-28.78 i. ii. 1 ii. 2 1q 163 entire 2.67(14) 0.60῀6.20 2.13(5) 1.4%῀3.32% -33.78(5) -35.45῀-31.59 i note: average samples minimum maximum ( ) ~ 422 bin li, qiqi li3, wenhua mei, qingong zhuo, xuesong lu this study built three thermal evolution models of well k1, well st3, and well l16 in different structural belts, as shown in fig 2. it shows that the major paleozoic source rocks in this area entered the oil generation threshold in the late caledonian, entered the condensate gas-moisture stage in the late triassic, and the dry gas stage in the early cretaceous. at present, they are generally in the high-to-over mature stage, mainly producing dry gas. from figure 3, the lower cambrian heat evolution rate is low and was in the mature and over mature stage for a long time (fig 3). relatively speaking, the lower silurian and middle permian source rock with a higher thermal evolution rate showed that the foreland orogeny movement in western sichuan promoted the source rock to enter the mature and high mature stage. laterally, the source rocks in the thrust nappe belt, hidden front concealed belt, and depression belt enter the mature stage later, shows that the foreland movement has a delayed effect on the thermal evolution of source rocks in different structural belts. the source rocks of lower cambrian were in the stage of oil generation for a long time in the thrust nappe belt, which offered a good material base for the reservoir formation in this area. the thermal evolution history of lower silurian and permian source rocks was close to permian source rocks. due to the large buried depth of the hidden front belt, the lower silurian source rock entered the dry gas stage from the late cretaceous. due to the influence of the caledonian paleouplift, the silurian longmaxi formation, except for the large thickness of the chaotian and nanjiang qiaoting sections, was pointed out southward in the guangyuan-cangxi area and had relatively limited influence on the oil and gas reservoirs. therefore, it can be seen that the lower cambrian and permian source rocks are the main sources of oil and gas accumulation in this area. reservoir conditions based on the physical test of 54 reservoir samples and a large number of collected reservoir data, the basic characteristics of the middle permian reservoirs in the study area can be seen, as shown in table 2. it shows that the lithology of the middle permian reservoirs in northwest sichuan is mainly crystalline dolostone, followed by dolomitic limestone and sparite limestone. the porosity of the thrust nappe belt is between 0.13-16.51% with an average of 2.26, and the permeability is between 0.0-784 md with an average of 0.608 md. the porosity of the hidden front belt was 0.23-7.59 % with an average of 1.825 %, and the permeability was 0-94.2md with an average of 5.14md. the porosity of the depression belt ranged from 0.1 to 0.92%, with an average porosity of 0.49%, and the permeability ranged from 0.0 to 17.7md, with an average of 1.73md. in comparison to the sedimentary rocks, the middle permian reservoir exhibits low porosity and low permeability. in contrast, different structural belts that the thrust nappe belt and the hidden front belt have relatively high porosity and relatively developed fractures, which improve the permeability of the reservoir. on the other hand, the depression belt has poor porosity and permeability affected by the depth of burial. caprock and preservation conditions two sets of important caprock developed in permian and overlying strata of northwest sichuan. one is the regional mudstone of the longtan formation of upper permian, the thickness can reach more than 100 m. the mudstone of longtan formation is both a good caprock of the underlying reservoir and the main source rock of the overlying reservoir, which constitutes a good regional caprock of the middle permian reservoir. the other is the gypsum rocks developed in the middle triassic, with a cumulative thickness of between 50 m and 450 m. they are the indirect caprock of permian oil and gas reservoirs (chen cong, 2019). in this paper, the characteristics of paleozoic formation water and formation pressure coefficients were statistically analyzed, as shown in table 3. it is shown that the wells h1 and k3 in thrust nappe belt from deep to shallow, the water type transition from calcium chloride type to sodium sulfate type, and formation pressure coefficient from 0.96 to 0.87, indicating that the strata were destroyed and the preservation conditions of qixia formation and maokou formation are poor. the water type of st2 in the hidden front belt is calcium chloride type, and the formation pressure coefficient is 1.36, which is in the state of overpressure state and conducive to oil and gas preservation. figure 2. thermal evolution of lower cambrian and permian in northwest sichuan basin. (a: k1 well. b: st3 well. c: l16 well). the formation pressure coefficient of wells l17 and gj well in the depression belt is higher and the preservation condition is better. however, the water type of maokou formation in well b1 is sodium bicarbonate type, and the formation pressure coefficient is 1.01, which is relatively low, indicating received the influence of micangshan fault-fold belt. overall, the multi-stage tectonic movement has a great influence on the preservation conditions of the thrust nappe belt. 423analysis of accumulation models of middle permian in northwest sichuan basin figure 3. hydrocarbon generation history in the paleozoic of northwest sichuan basin. table 2. statistical table of basic reservoir characteristics of qixia formation in northwest sichuan basin (some data from experiment center of southwest oil and gas field, 2017) structure belts samples porosity (%) permeability (md) reservoir lithology reservoir space wells the thrust nappe belt n=254 0.13 − 16.51 2.26 0 − 784 0.608 silt-fine crystal dolomite, dolomitic limestone intercrystalline pore, fracture, and vuggy pore k2, k1 the hidden front belt n=247 0.23 − 7.59 1.825 0.0 − 94.2 5.14 silty-fine crystal dolomite fracture and pore wj1, st3 the depression belt n=103 0.1 − 0.92 0.49 0.0 − 17.7 1.73 mud-sparite limestone pore l17, l16 note: minimum maximum average ~ table 3. statistics of the upper paleozoic preservation conditions in northwest sichuan basin well formation chemistry composition (mg/l) salinity g/l water type pressure coefficient na++k+ ca2+ mg2+ clso42hco3co32total value l17 qixia 15606 27173 378 62996 34.1 0 0 63062.2 106.67174 cacl2 1.32 gj maokou 6069 52 0 6462 2815 1436 61 10774 16.895 na2so4 1.21 b1 maokou 10590 80 16 14063 2840 605 0 17508 28.194 nahco3 1.01 h1 changxing 6129 560 17 5809 5075 1606 0 12490 19.196 na2so4 0.87 6946 437 13 7690 3961 1557 0 13208 20.601 nahco3 7571 375 20 8993 3378 1550 0 13924 21.89 nahco3 7742 323 14 9562 2970 1364 0 13896 21.975 nahco3 k3 maokou 8611 401 21 13032 910 0 583 14525 23.558 na2so4 0.79 qixia 7859 9845 106 29275 453 394 0 30122 47.932 cacl2 0.96 6332 9644 46 26417 412 0 399 27228 43.25 cacl2 st2 maokou -qixia 10127 14309 456.7 43192 0 0 0 43192 68.0847 cacl2 1.36 9993 28471 885 70808 0 0 0 70808 110.157 cacl2 9058 26046 610 70622 0 0 0 70622 106.336 cacl2 7277 24260 412 71031 0 0 0 71031 102.98 cacl2 424 bin li, qiqi li3, wenhua mei, qingong zhuo, xuesong lu trap development and distribution this paper compiled the reservoir profiles of the key exploration wells in the research area, as shown in figure 4. it indicates that the trap types of different belts are different due to the influence of the multi-stage tectonic movement. the thrust nappe belt mainly developed thrust blocks and thrust anticlinal traps (fig 4a, 4d), while the hidden front belt formed a large number of fault-block traps under caledonian hercynian tensile action, and some of the fault-block traps degenerated into compressed anticlinal traps under the compression action of indo-china period (fig 4b,4e). due to weak tectonic activity, anticline and structure-lithologic traps were developed in the depression belt (fig 4c). overall, the traps in northwest sichuan are mostly structural traps, with the most developed fault traps, but the traps are small in scale. discussion oil and gas source due to the few exploration wells and little data in northwest sichuan, the oil and gas source of middle permian has not been clearly understood. at present, there is still a great dispute over the gas source of the middle permian. some scholars believed that the oil and gas mainly came from the cambrian source rocks of the lower paleozoic through paleo-reservoir studies (wu et al., 2012; dong et al., 2011; li, 2018). other scholars proposed that oil and gas came from middle permian source rock (lu et al., 2017; liu chun, 2008) on the ground of some geochemistry test. besides, some scholars also supposed that some gas possibly came from the coal-measure source rocks of overlying longtan formation according to the carbon isotope of gas composition (shen ping, 2015). according to the chromatographic analysis of the source rock and the bitumen in the reservoir (fig. 5), the peak types of n-alkanes in the source rocks of qixia and maokou formation in the middle permian were all singlepeak types, with the main peak carbon close to each other. the ratio of primers to phytane was less than 0.7, which showed a good similarity. the ethane carbon isotopic data of qixia formation, maokou formation reservoir, and paleogenic source rock kerogen are compared as shown in figure 6. as you can see that the ethane carbon isotopes of maokou formation in hewanchang structure are relatively light, indicating that the gas reservoir in the thrust nappe belt was not only charged from the permian source rocks but also underlying cambrian source rocks (huang dong, 2011). in contrast, the carbon isotope of ethane of qixia formation and maokou formation in the shuangyushi area is heavy, and the isotope of methane and ethane is reversed and revealed the character of the gas mixture. it is assumed that in addition to the natural gas generated from the source rocks of the middle permian and the lower cambrian, the coaliferous gas of the upper longtan formation is mixed in the hidden front belt. the distribution of ethane isotopes in maokou formation in jiulongshan ranges from -35 ‰ to -26 ‰, and methane and ethane isotopes are reversed, which shows typically mixed genesis. it is inferred that the permian oil and gas in the depression belt come from their source rocks and overlying longtan formation. in summary, these results show that the middle permian oil and gas are mainly charged by the lower cambrian system and the authigenic source rocks. figure 4. profiles of the paleozoic gas reservoir in northwest sichuan basin (a: kuangshanliang anticline;b: shejianhe anticline; c: guanjijing area; d: hewanchang anticline; e: shuangyushi anticline; f: laoguanmiao area) 425analysis of accumulation models of middle permian in northwest sichuan basin figure 5. the n-alkanes chromatogram of asphalt and source rocks in middle permian reservoirs in northwest sichuan. (a and c from the reservoir. b and d from the source rock.) 426 bin li, qiqi li3, wenhua mei, qingong zhuo, xuesong lu migration conditions due to a large number of faults, fractures, and unconformities under the influence of multi-stage tectonics in northwest sichuan (dahai et al., 2016; yang et al., 2019), a variety of oil and gas migration system has been formed, resulting in extensive longitudinal distribution of oil and gas in paleozoic (yuanjiang et al., 2019). therefore, the understanding of the migration system was still relatively vague (tian et al., 2012; xiao et al., 2018). based on the typical hydrocarbon accumulation profiles and the variation characteristics of natural gas carbon isotope in gas reservoirs, this study established the schematic map of middle permian hydrocarbon accumulation to provide a new reasonable explanation for the secondary migration of oil and gas in this area, as shown in figure 7. it can be seen that there are obvious differences of different structural belts in northwest sichuan, in which a large number of imbricate thrust faults developed in the thrust nappe belt connected the lower paleozoic source rocks and form the migration channel of the hewanchang anticline, resulting in light carbon isotopes of ethane in the gas reservoirs in this area. interlayer faults and carrier beds in shuangyushi and wujiaba area are the main channels for oil and gas migration, resulting in ethane carbon isotopes of permian gas reservoirs in the hidden front belt close to their source rocks, while the light part of ethane carbon isotope shows the characteristics of multi-source charging. the migration channel of the depression belt is mainly interlayer faults and fracture formed under the state of late caledonian tension. the oil and gas are dominated by a close vertical migration system, and the ethane isotopes of the gas reservoir are close to the permian source rocks. hydrocarbon accumulation model the oil and gas accumulation in the middle permian in northwest sichuan is characterized by “multiple sources and multiple periods”, and the process of accumulation is relatively complicated (chen et al., 2019; gu et al., 2016; he, 2014; yu, et al., 2018). based on the characteristics of reservoir conditions, this study established the oil and gas accumulation model of middle permian in this area to guide the study of oil and gas accumulation in different structural belts (fig. 8). the lower cambrian source rocks are developed at the bottom of the thrust nappe belt in the study area, and the reservoirs of maokou formation and qixia formation are well developed in fractures and pores, and the reservoir physical properties are high. the caprock is the source rock and tight limestone at the bottom of the middle and upper permian. at present, a large number of bitumen and paleo-reservoirs are found in cambrian and devonian in nianziba and tianjingshan areas, indicating that the liquid hydrocarbons produced by the lower cambrian source rocks in the late caledonian and hercynian periods migrate upward to the devonian reservoir along the faults connected source rock (xiao et al., 2018). some of the crude oil buried in the early stage was cracked into natural gas and migrated upward to the permian, and then the gas reservoirs were formed in the kuangshanliang and hewanchang areas. some reservoirs were transformed and destroyed by the indo-china movement in the later stage and exposed to the surface. therefore, the paleozoic oil and gas reservoirs in this area have a reservoir formation model of “ far-source charging, the assemblage of the lower source and upper reservoir”, and the preservation condition is the key controlling factor. two sets of source rocks in the lower palaeozoic and lower permian are developed in the hidden front belt, and fracture-pore reservoirs are developed in the middle permian (benjian, 2019). the caprock is the source rock and tight limestone at the bottom of the permian. due to the multi-source hydrocarbon charging, the ethane carbon isotope in the gas reservoir of the shuangyushi structure is light. because the hidden front belt is on the thrown side of the thrust fault, the traps were preserved well. therefore, the middle permian gas figure 6. carbon isotope correlation of kerogen and ethane of several source rocks in the northwest sichuan basin. (some data cited from huang dong, 2011) 427analysis of accumulation models of middle permian in northwest sichuan basin figure 7. schematic map of accumulation conditions of paleozoic reservoirs in northwestern sichuan basin figure 8. schematic map of paleozoic system in northwest sichuan basin reservoir in this area develops the mode of mixed source charging, the lower source and upper reservoir, and the fault connected source rock is the main controlling factor of the oil and gas reservoir. the depression belt is located on the side of the rift belt in western sichuan. it is far from the lower cambrian source rocks, and the permian source rocks are well developed with a high abundance of organic matter. the source rock and tight limestone at the bottom of the middle and upper permian formed the caprock. and the tectonic and lithologic traps are developed. according to the analysis of the existing gas reservoirs in this area, the ethane carbon isotopes of middle and upper permian gas reservoirs are similar to permian kerogen, which indicates that the natural gas is mainly migrated upward to the nearby reservoirs along the interlayer faults. therefore, the belt developed the characteristics of self-generation, self-reservoir, and self-caprock combination. due to the large buried depth and good preservation conditions of the middle permian in this 428 bin li, qiqi li3, wenhua mei, qingong zhuo, xuesong lu area, formed the model of near-source charging, and interlayer fracture is the key factors for hydrocarbon accumulation. based on the above geological conditions and reservoir formation model, it is supposed that the middle permian in northwest sichuan has better reservoir formation conditions, in which the hidden front belt is relatively weak in tectonic deformation, the developed anticlinal traps are well preserved, and the fault connected source rock is well developed. since the remarkable discovery of well st1 and st3 in the shuangyushi structure after 2014 (bai xiaoliang, 2019)), proved that the hidden front belt in the western sichuan basin may be a favorable area for oil and gas breakthrough in the deep marine field. conclusion (1) the hydrocarbon source correlation supposes that oil and gas in middle permian mainly come from the bottom of the lower cambrian and the permian. the foreland orogeny movement on the longitudinal promoted the paleozoic source rocks in the high-to-over mature stage, and on the transverse have delayed effect, causing hydrocarbon source rock of the thrust nappe belt, the hidden front belt, and the depression belt entered the mature stage of later and later. (2) the structural traps are developed in the northwest sichuan basin, among which the thrust nappe belt mainly develops thrust fault block traps, while the hidden front belt is dominated by compressed fault anticline trap, and the depression belt develops tectonic-lithologic traps, which have the characteristics of small in scale. (3) the different structural belts in northwest sichuan have different oil-gas accumulation models, among which the nappe belt was affected by thrust fault and develops the assemblage of lower source rock and upper reservoir, and the preservation condition is the key. under the influence of deep faults in the hidden front belt, the oil and gas come from the lower and selfsource rocks. however, the depression belt is mainly controlled by the interlayer fracture and develops a model of self-generation and self reservoir and near-source charging. (4) based on the accumulation conditions comprehensive analysis, the hidden front belt is close to the cambrian source rock, with good source charging, well-developed fault connected source rock and good trap preservation conditions, and is a favorable oil and gas accumulation area in the northwest sichuan basin. highlights: (1) the middle permian oil and gas in the northwest sichuan basin mainly come from the lower cambrian and middle permian source rocks, which are in the stage of the high-to-over mature stage. (2) the middle permian in northwest sichuan developed an assemblage of lower source rock and upper reservoir, and faults had a great influence on hydrocarbon migration and accumulation. (3) the hidden front belt is close to the lower cambrian source rocks, with well-developed fault connected source rock and good trap preservation conditions. it is a favorable oil and gas accumulation area for the middle permian. acknowledgments this study originated from the state key scientific project “study on the genetic relationship between paleozoic asphalt and ancient oil reservoir in northwest sichuan basin” (no. gsyky-b09-33) supported by state key laboratory of shale oil and gas enrichment mechanisms and effective development, sinopec exploration and production research institute to whom we are very grateful. references chen, z., li, w., linling, w., yongliang, l., & guang, y. (2019). structural geology and favorable exploration prospect belts in northwestern sichuan basin, sw china. petroleum exploration and development, 46(2), 413-425. dahai, t., xiao, d., tan, x., li, h., xie, j., liu, h., yang, x., & zhang, b. 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(2019). genesis and characteristics of the dolomite reservoirs in middle devonian guanwushan formation, northwest sichuan basin, sw china. petroleum, 6(2), 138-148. keywords: geothermal characteristics; shallow layer; geothermal energy; resource development; potential evaluation; model construction. palabras clave: características geotérmicas; capa superficial; energía geotérmica; desarrollo de recursos; evaluación potencial; construcción de modelo. how to cite item hou, j., luo, x., & zhang, l. (2020). establishment of evaluation model for shallow geothermal energy resource development potential based on characteristic of geotemperature. earth sciences research journal, 24(3), 317-325. doi: https://doi. org/10.15446/esrj.v24n3.89513 the potential analysis of geothermal energy based on geothermal characteristics requires research of the regional shallow geothermal energy. then, an evaluation model can be constructed. taking weinan city as an example, this paper studied the depth and temperature characteristics of the constant zone of subsurface temperature, the vertical variation characteristic of geotemperature, and the horizontal distribution characteristic of geotemperature in weinan city. based on the analysis of geotemperature characteristics, the analytic hierarchy process was used to construct the zoning evaluation system of the suitability of ground source, groundwater source, and surface water source. then, the suitability of weinan city was comprehensively zoned by the zoning evaluation system. the heat capacity and heat-exchange power of the heat pump system at the groundwater source were calculated based on the zoning results, and the heat-exchange power of the ground-coupled heat pump was obtained. according to the evaluation index, the development potential of shallow geothermal energy resources was evaluated. it can be seen that the heat pump system has great potential for heating in winter and cooling in summer through the experimental analysis model applied in this paper. the area is 50.79 km2. the area of the potential middle region of the buried pipe heat pump is 135.74 km2, and the potential of the pump system is low. the possible area of the ground source heat pump is 82.14 km2. upon comparing the model results with the current results, it was found that the consistency between the two is high. abstract establishment of evaluation model for shallow geothermal energy resource development potential based on characteristic of geotemperature establecimiento de un modelo de evaluación para el desarrollo potencial de recursos de energía geotérmica poco profunda con base en las características de la geotemperatura issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n3.89513 con el fin de construir un modelo de evaluación del potencial de desarrollo de recursos de energía geotérmica poco profunda basado en características geotérmicas es necesario investigar la energía geotérmica superficial regional. este estudio define la ciudad de weinan como ejemplo para analizar las características de profundidad y temperatura constante del subsuelo de la zona, la característica de variación vertical de la geotemperatura y la característica de distribución horizontal de la geotemperatura. con base en el análisis de la característica de geotemperatura se utilizó el proceso de jerarquía analítica para construir el sistema de evaluación de zonificación de la idoneidad de la fuente subterránea, la fuente de agua subterránea y la fuente de agua superficial. luego, la idoneidad de la ciudad de weinan fue zonificada por el sistema de evaluación. la capacidad de calor y la potencia de intercambio de calor del sistema de bombeo subterráneo se calcularon con base en los resultados de zonificación y se obtuvo la potencia de intercambio de calor de la bomba acoplada a tierra. según el índice de evaluación, se evaluó el potencial de desarrollo de los recursos de energía geotérmica poco profunda. a través del análisis experimental, aplicando el modelo en este documento, se puede ver que el sistema de bomba de calor de fuente subterránea tiene un gran potencial para calentar en invierno y enfriar en verano. el área es de 50.79 km2. el área de la región media potencial de la bomba de calor de tubería enterrada es de 135.74 km2 y el potencial del sistema de bomba de calor de fuente subterránea es bajo. el área potencial de la bomba de calor de fuente terrestre es 82.14 km2. al comparar los resultados de la evaluación de este modelo con los resultados reales se encuentra que la consistencia entre los dos es alta. resumen record manuscript received: 12/03/2019 accepted for publication: 07/06/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 3 (september, 2020): 317-325 jikun hou1,2*, xingqi luo2, lei zhang3 1power china northwest engineering corporation limited, construction management company, xi’an 710100, china 2xi’an university of technology, institute of water resources and hydropower engineering, xi’an 710048, china 3xi’an electric power college, power engineering department, xi’an 710032, china *corresponding author: hjk19800529@163.com g eo th er m al e ne rg y https://doi.org/10.15446/esrj.v24n3.89513 https://doi.org/10.15446/esrj.v24n3.89513 https://doi.org/10.15446/esrj.v24n3.89513 mailto:hjk19800529@163.com 318 jikun hou, xingqi luo, lei zhang introduction energy problems, environmental, and sustainable development issues always trouble all over the world. the extensive energy use consumes a large number of non-renewable energy sources such as coal and oil. but those resources are expensive. energy has become a bottleneck restricting sustainable development (liu & nasr, 2019). people have paid more and more attention to energy conservation, new energy development, environmental pollution reduction, and sustainable development. all countries are trying to find and develop environmental, renewable energy. china is also facing the same problem (wang et al., 2017). as a renewable, environmental, clean energy resource, the shallow geothermal energy resource has been gradually recognized, accepted and valued in recent years in china (neupane et al., 2016). the shallow geothermal energy is a new member of the geothermal renewable energy family. it is also another form of solar energy, widely in the surface layer of the earth (thermostat layer). it can be recovered and renewable, and it is inexhaustible low-temperature energy (zhang & hu, 2018). the shallow geothermal energy is a low-temperature geothermal resource in the rock and soil of the earth’s surface; it is also an essential part of thermal energy. it is a clean, renewable resource with abundant reserves, wide distribution, and stable temperature. its development and utilization play an essential role in energy saving, emission reduction, and environmental protection. its development and utilization prospects are broad (zhang et al., 2017). currently, the development of technology is mature, which has been widely applied in heating and cooling. it is a good alternative energy and clean energy. with the gradual popularization of ground source heat pumps, the utilization of shallow geothermal energy has attracted more and more attention, which has become a new growth point of geothermal energy utilization. due to the excessive growth of demand in the area where the ground source heat pumps are popularized, the application of this technology shows a blind and disorderly development trend, ignoring the hydrogeological condition of the application site to a large extent, leading to the waste of groundwater resource and the inefficiency of the heat pumps (mihoub et al., 2016). to solve these problems objectively and ensure the rational development and utilization of shallow geothermal energy, we not only normalize the heat pump market but also investigate and evaluate the shallow geothermal resource according to the hydrogeological conditions, regional development and energy demand. we should scientifically carry out the suitability zoning of ground source heat pumps’ application and make reasonable development, and utilization plans to determine favorable development regions and appropriate development and utilization methods. all this to achieve orderly development, rational use and scientific management of shallow geothermal energy resources. these can provide a scientific basis for the macro decision-making on unified planning, improving energy efficiency and ensuring energy security (michopoulos et al., 2016). therefore, it is necessary to establish a model for evaluating the adaptability of ground source heat pumps. the standardized and systematic evaluation of shallow geothermal energy is critical. it is essential to organize them with various hydrogeological conditions for unified management. after considering the factors influencing the spatial distribution of shallow lowtemperature energy, the standard way is challenging to achieve timely, effective management. therefore, an evaluation model for shallow geothermal resources development potential should be constructed (zhang et al., 2016). some relevant literature shows that many scholars have studied it. liu licai has divided the suitability of water source heat pumps in the planning area of beijing. wang yabin has partitioned the suitability of hydrogeological conditions of the water source heat pump system in tianjin. wang tao evaluated the suitability of shallow geothermal energy utilization in key cities along the yellow river economic belt in ningxia. generally, china is still in the early stage due to its short development time. the unbalanced development and some problems need to be studied and solved urgently. otherwise, it will directly affect the development and sustainable utilization of shallow geothermal energy resources. in conclusion, there are many complex methods for evaluating the suitability of shallow geothermal. there is no specific mature method theory. some scholars directly use the method of artificial qualitative zoning to make the zoning intervals for the values of each element, and then corresponding suitability zoning is carried out. this method is too subjective, which is not scientific and reasonable enough. some scholars use programming or software for the subarea. although this method is efficient, it is not flexible enough. meanwhile, the results are usually complex, and its practicability is too low. on this basis, this paper uses the analytic hierarchy process (ahp) to divide the suitability of development and utilization. after that, the evaluation model is used to evaluate the development potential of shallow geothermal resources based on the zoning (alkhasov et al., 2016). material and method geothermal characteristic depth and temperature of the constant cone of subsurface temperature because the geothermal characteristics of different regions are different, this paper takes weinan city as an example to research. in order to analyze the relationship between the depth and temperature of the layer of constant temperature in weinan, eight wells of different geomorphic units in this region (six of them are the engineering geological exploration holes, two of them are motor-pumped wells) are monitored numerically with the depth of water temperature (if the water temperature is close to the formation temperature, the measured water temperature is substitute for the ground temperature). in the thermal response test construction, a u-shaped pipe is embedded in the borehole, and water is injected to ensure the tightness. the u-tube can be used for future temperature tests, and the digital temperature measurement single cable multi-probe detection system is adopted. the actual construction proves that when the stratum temperature changes slowly, it is accurate in any depth (zhou et al., 2017). the geothermal change curve and the scatter plot of the relationship between geo-temperature and depth in weinan are obtained by corresponding technology. the geothermal change curve is shown in figure 1. the scatter plot corresponding to the relationship between ground temperature, and depth is shown in figure 2. figure 1. map of ground temperature variation from the variation curve of ground temperature, we can see that the ground temperature of each monitoring hole in the depth of 0m-15 m underground varies with the rise and fall of temperature in different seasons. the seasonal variation of ground temperature is evident. the ground temperature in the depth of 15 m-20 m underground is basically constant, and the ground temperature is 15°c-17°c. the ground temperature of each monitoring hole at the same depth does not change with the season when the depth is below 20 m. the ground temperature in different seasons is basically unchanged, and the temperature increases linearly with the increase of burial depth (bisht & rupenthal, 2016). geothermal variation characteristic at vertical direction according to the data of eight geothermal monitoring points in different geomorphic locations, the geothermal field within the region generally increases with the depth at the vertical direction, but the geothermal changes vary in 319establishment of evaluation model for shallow geothermal energy resource development potential based on characteristic of geotemperature m. it can be seen that the geothermal anomaly regions are zonally distributed in the second and third terraces of weihe river in the southern part of weinan, and the average geothermal gradients are mostly concentrated in the range between 3.0°c/100 m-3.35 °c/100 m. the basement faults in the southern part of weinan are mature (czycholl et al., 2017), mainly influenced by the faults in the south bank of the f1 weihe river. it is affected by the faults in front of the tableland of the f2 weihe river and f3 zero river faults. in addition, two ground fissures are developed in the southern part of weinan and the trend is east-west, which are roughly parallel to the faults. the fault structure in this area is conducive to heat conduction from deep to shallow. the direction of geothermal anomaly is consistent with that of underlying fault structure. at present, the exploitation of shallow geothermal energy in weinan is relatively low. no one wants to exploit and utilize the geothermal anomaly area. development and utilization suitability zoning principle and method of suitability zoning the suitability zoning of shallow geothermal energy development and utilization runs through the research. its purpose is to provide the basis for the selection of development and utilization mode, resource evaluation and development and utilization planning. (1) zoning principle is based on the geological and hydrogeological conditions. the geological conditions are the important basis for the occurrence of shallow geothermal energy, and the hydrogeological conditions are the main influencing factors for the occurrence of shallow geothermal energy. for the purpose of development and utilization of shallow geothermal energy, the reasonable development and utilization mode and the effective heat pump system are the important media. we need to focus on both economic benefits and environmental protection. under the current technological and economic conditions, regions and ways with relatively good economic, environmental and energy-saving effects should be selected for the development and utilization of shallow geothermal energy. the plane division and vertical control should be combined. the range of plane zoning areas is the long-term planning area of urban construction in 2020. the overall evaluation control depth is 200 m. (2) zoning method: it is necessary to closely combine with the conditions of shallow geothermal energy in this area. firstly, the necessary conditions of suitability are divided by key factor method, and then the suitability level is divided by analytic hierarchy process (ahp) and comprehensive index method respectively. the analytic hierarchy process (ahp) is a qualitative, systematic and hierarchical analysis method. it has practicality and effectiveness in processing complex decision-making problems. the comprehensive index method refers to the comprehensive evaluation index which uses the weight calculated by the analytic hierarchy process and the numerical value obtained by the fuzzy evaluation method to put forward the economic benefit index. the more influential factor on suitability division will be more important. by establishing the comparison matrix, we can check the consistency of comparison matrices and adjust the comparison matrix if necessary, so as to achieve acceptable consistency. according to the occurrence condition of shallow geothermal energy, it can be divided into three development and utilization modes: ground-source type (buried pipe), groundwater-source type and surface-water source type, and then we can divide them into different suitable areas. ground source types can be divided into five types: favorable area, suitable area, general area, undesirable area and unsuitable area. among them, suitable area can be divided into three sub-areas: loose rock subregion, semi-consolidated rock subregion and consolidated rock subregion (winters et al., 2016). the groundwater source can be divided into three types: favorable area, suitable area and unsuitable area. the surface water source can be divided into two types: suitable area and unsuitable area. suitability zoning evaluation system of ground source the geological and hydrogeological conditions, construction conditions, thermophysical properties, topography, geomorphology, environmental geological problems are chosen as the attribute-level indexes for the evaluation of suitability zoning of development and utilization of geo-source type. where, the thickness of the loose layer, the probability of geological hazards and the different depths. in the normal temperature zone (15 m-25 m), especially the near-surface area of 0 m-3 m, the ground temperature is affected by seasonal climate change, and the fluctuation is sensitive. moreover, the influence of climate change on the ground temperature between 3 m and 15 m decreases gradually. under the normal temperature zone of about 25 m, the ground temperature has not been influenced by local climate change. the temperature increases with depth. the ground temperature is19.8°c-22.0°c when the depth is 200 m. the average geothermal gradient is 1.18/100 m to 3.57/100 m. based on the monthly average water temperature at different depths of eight long-term observation wells, the variation curves of ground temperature in various depths and different periods (the water temperature below groundwater level is the same as the ground temperature) with depth are drawn to analyze the vertical variation characteristics of ground temperature in natural state. according to the curve, the vertical geothermal distribution feature in the research area belongs to the linear gradual rising type. under 20 m, the ground temperature of each observation hole at different depths does not change significantly with the seasons. with an increase of depth, the ground temperature increases linearly. the change of ground temperature in qianlu waterworks (wn178) is the largest, from 40m to 180m. as the ground temperature increases by about 5°c and the geothermal gradient is about 3.57°c/100 m. the changing trend of ground temperature in zhang yicun (wg001) is relatively slow, from 30m to 100m. the ground temperature increases by about 0.81°c, and the geothermal gradient is 1.18°c/100 m. in the stratum warming zone, the geothermal temperature is controlled by deep geothermal warming, and the geothermal temperature gradually increases with depth. due to the influence of geological structure, groundwater activity, and stratum rock characteristics, vertical geothermal warming varies from place to place. it is concluded that the geothermal temperature in the research area changes with depth. the features are mainly influenced by the geological structure and groundwater activity (ruelle et al., 2016). geothermal horizontal distribution characteristic generally, the ground temperature in 100 m buried depth is high in the south and low in the north. there is a geological fault in the front of the second terrace on the south bank of weihe river. the ground temperature in the 100m buried depth of the third terrace on the south of fault can reach 19.1°c. the main reason is that the basement faults are mature in the area, conducive to the heat conduction from deep to shallow. the distribution direction of the third terrace in the south of weihe river is consistent with the underlying fault structure. according to the contour maps of ground temperature and average geothermal gradient at 100 m depth, the geotemperature is abnormal if the geotemperature is higher than 17.5°c; and the average geothermal gradient is higher than 3°c/100 figure 2. scatter map corresponding to the relationship between ground temperature and depth 320 jikun hou, xingqi luo, lei zhang topography are the key factors. on the premise of determining the key factors, the method of analytic hierarchy process and the method of comprehensive index are used to divide the development and utilization suitability zoning of shallow geothermal energy of ground source type. as shown in figures 3 and figure 1: figure 3. structure diagram of suitability division for development and utilization of source type table 1. selection table of crucial factors of suitability suitability partition key factor suitable area an area with loose layer thickness greater than 100 m lower area semi-consolidated rock area general suitable area geological hazard (collapse, landslide, debris flow, karst ground collapse) low-prone area, and loose layer thickness of 30 m-50 m difference suitable area geological hazards (collapse, landslide, debris flow, karst ground collapse) in prone areas, middle hills and above geomorphologic units unconfined area geological hazard (collapse, landslide, debris flow, karst ground collapse) high-prone area from figure 3, we can see that the structure of suitability division of development and utilization of geo-source type is mainly based on the geothermal characteristic. by considering the hydrogeological condition, equation condition, thermal physical property, topography, geological hazards and other factors, we can divide the suitability of development and utilization of geo-source type. meanwhile, it can be seen that the suitability can be divided into five categories (yan & xu, 2016). subarea evaluation system for suitability of groundwater source the groundwater abundance and groundwater dynamic field conditions are selected as attribute-level indexes for the suitability evaluation of groundwater source. the six indexes such as single-well water inflow, effective thickness of aquifer, recharge capacity, water level burial depth, water level descent rate and recharge modulus are selected as element layers of the hierarchical analysis. where, the groundwater richness, the recharge capacity and environmental geology are the key factors (li et al., 2016). on the premise of determining the key factors, the method of analytic hierarchy process and the comprehensive index are used to divide the suitability of development and utilization of shallow geothermal energy of groundwater sources. figure 4. structure diagram of suitability division for development and utilization of underground water source type table 2. selection of key factors key evaluation factor suitability suitable area more suitable area unconfined area single well water inflow/(m3/d) >3000 1000-3000 <1000 groundwater irrigation and production ratio/% >80 50-80 <50 recharge capacity best secondary difference (1) the area with land subsidence > 500 mm is an unsuitable area of groundwater source type. (2) land subsidence 0 mm-500 mm area, take down a suitability level from figure 4, the division structure of suitability of development and utilization of groundwater source is mainly based on the geothermal characteristic. after considering the geological condition, hydrogeological condition and groundwater dynamic field condition, the suitability of groundwater source can be divided by above factors. meanwhile, the key evaluation factors of suitability division of groundwater source development and utilization mainly include the number of single well users, groundwater irrigation-production ratio and recharge capacity. according to above evaluation factors, the suitability evaluation of groundwater source development and utilization can be achieved. zoning assessment system for suitability of surface water source the development and utilization of surface water source shallow geothermal energy can be divided into two types: separated type and closed type. the evaluation indexes of suitability of surface water source type mainly include the recyclable amount of surface water, water depth, water temperature, water quality and distance from the project. 321establishment of evaluation model for shallow geothermal energy resource development potential based on characteristic of geotemperature table 3. classification of suitability for development and utilization of surface water source types utilization mode open closed volume during dry season/(×104m3) >20 >20 water quality meet the requirements of the specification meet the requirements of the specification water depth in dry season/m >3 >3 coldest monthly mean water temperature >10 >5 distance between utilization engineering and water source/km <0.5 <0.5 comprehensive evaluation standard of suitability it is appropriate to meet the five indicators from table 3, we can see that the evaluation indexes of development and utilization of surface water resource mainly include five evaluation indicators: volume, water quality and depth in dry season. the suitability of surface water resources development and utilization can be divided by the degree of conformity of development and utilization of surface water resources to these indicators (wen & fang, 2018). evaluation of tap potential of shallow geothermal energy resource the evaluation for development potential of shallow geothermal resources includes the evaluation of shallow geothermal capacity and heat-exchange power. based on the zoning results, the evaluation index of shallow geothermal resource development potential is selected at first, and then the development potential of shallow geothermal resource is evaluated by the evaluation index. heat capacity the shallow geothermal energy capacity is the unit temperature difference heat stored in shallow rock, soil and groundwater. the thermal storage method is adopted. the depth range is 200 m. the calculated area is the suitable area. q q q qr s w a= + + (1) in the formula, qr is the geothermal energy capacity. qs is the energy capacity in rock and soil. qw is the energy capacity of water contained in rock and soil (sowizdzal, 2016). qa is the energy capacity of air contained in rock and soil. the formula for calculating the energy capacity of rock and soil is as follows: q c mds s s= ( )ρ −φ1 (2) in the formula, s is the density of rock and soil. cs is the specific heat capacity.  is the porosity. m is the area. d is the thickness of rock and soil are considered (al-samari & ali, 2018). the formula for calculating the energy capacity of water in rock and soil are as follows: q c mdw w w= ρ ω (3) in the formula, w is the water density. cw is the specific heat capacity. is the water content of rock and soil. the formula for calculating the energy capacity of air in rock and soil are as follows: q c mda a a= ( )ρ φ−ω (4) in the formula, a is the air density. ca is the specific heat capacity of air. according to the main rock character of the aquifer, the depth of the quaternary base boundary and the depth of shallow groundwater, this region is divided into 148 sub regions. according to the difference of area conditions, the sub regions are merged into 18 evaluation units. according to the main rock character of aquifer, they are divided into three areas, and then we calculate them. heat-exchange power of groundwater source heat pump system the heat-exchange power of groundwater heat pump systems is calculated by groundwater demand conversion method. the calculation process is as follows: q q nq h= × ×  (5) in the formula, qq is the heat-exchange power of shallow geothermal energy in the evaluation area. qh is the heat-exchange power of shallow geothermal energy of a single well. n is the number of drillable holes in the calculation area.  is the land usage rate. the formula for calculating the heatexchange power of shallow geothermal energy of single well is as follows: q q t ch w w w= × ×∆ ρ −1 16 10 5. (6) in the formula, qh is the heat-exchange power for a single well. qwis the amount of groundwater recycling utilization. ∆t is the difference in temperature for groundwater utilization. w is the density of water, 1000 kg/m3. cw is the specific heat capacity of water, 4.18 kj/(kg·°c). heat-exchange power of buried tube heat pump system according to the heat-exchange power of the ground-coupled heat pump, the unit heat transfer method (fu, 2018) is adopted. the calculation process is as follows: q q mk k= ×10 3 (7) in the formula, qk is the heat-exchange power of shallow geothermal energy, and qk is the heat-exchange power of shallow geothermal energy per unit area. m is the calculation area. establishment of shallow geothermal energy potential evaluation model according to the heat-exchange power of ground source heat pump system and the indexes of heating in winter and cooling in summer, the area for heating and cooling of suitable and more suitable regions of groundwater ground source heat pump in winter and summer was further calculated, and then the resource potential of ground source heat pump system (zhang et al., 2018) is obtained. the detailed formulas are as follows: m q q q= 1000 (8) q m mzq = (9) in formulas, m is the heating/cooling area of groundwater/buried pipe ground source heat pump system. qq is the heat-exchange power in the suitable region and the more suitable region of groundwater/buried pipe ground source heat pump system. q is the heating index in winter and the refrigeration index in summer. qzq is the resource potential of groundwater/buried pipe ground source heat pump system. m is the heating/refrigerating area of groundwater/ buried pipe ground source heat pump system. m is the area of the calculation region. through the above formulas, the index table of potential evaluation zoning is obtained. 322 jikun hou, xingqi luo, lei zhang table 4. potential assessment division evaluation project divisional indicators unit area heating area/(×104 m2/km2) >32 20-32 <20 refrigerable area per unit area/(×104 m2/km2) >40 28-40 28 the land use coefficient is 100% of the heating area per unit area./(×104 m 2/km2) >162 <162 the land use coefficient is 100% refrigerable area per unit area/(×104 m2/km2) >204 <204 potential zoning high potential area central potential area low potential area on the basis of the above analysis, the development potential of shallow geothermal energy resources can be evaluated by the above table. results in order to verify the validity of the evaluation method, the proposed method is used to analyze the occurrence condition of shallow geothermal energy in weinan city and the characteristic of geotemperature in weinan city. it evaluates the suitability of development and utilization and the heating/ cooling potential of groundwater ground source heat pump system and buried pipe ground source heat pump system. the heat capacity of shallow geothermal energy is calculating. the environmental benefits and economic benefits of shallow geothermal energy development are discussed. the detailed analysis process is shown below. suitability zoning of groundwater ground source heat pump in order to research the suitability zoning effect of groundwater ground source heat pump of the proposed method, a practical experiment is carried out. the results of the practical adaptability zoning are shown in figure 5. the area and percentage of each region after the zoning are shown in table 5. figure 5. suitability zoning of groundwater ground source heat pump in weinan city from the charts, we can see that the suitability of groundwater source heat pumps is obviously different due to the difference of hydrogeological characteristics. the suitability can be divided as follows. the suitable region of groundwater source heat pump: it is distributed in the mid-weihe floodplain of the evaluation region. the rock character of aquifer is composed of gravel, gravel-bearing medium-coarse sand and silty clay. the stratum has good water conductivity, including double-layer phreatic water-confined water. the waterabundance is good. the unit water inflow is 10 m3/h·m-30 m3/h·m. therefore, it is suitable for groundwater pumping. the area of this region is 61.14 km2, accounting for 22.23% of the total area of the evaluation area. the suitable region of groundwater source heat pump: it is distributed in the first terrace on the north bank of weihe river, the terrace on the south bank of weihe river and the alluvial-proluvial fan region. the rock character of aquifer is quaternary holocene alluvion, including the gravelly sand, medium sand, coarse sand, silt and silty clay. the groundwater pumping condition in this area is relatively good, the water abundance is medium-weak, the unit inflow is 1 m3/h·m-10 m3/h·m. the area is 196.60 km2, accounting for 71.48% of the total area of the evaluation area. the unsuitable region of groundwater source heat pump: it is mainly distributed in the third terrace covered by loess in the south evaluation region. the stratum lithology is mainly pleistocene aeolian loess and silty clay with little thin sandy soil. the water abundance of groundwater in this region is weak, and the water conductivity is poor. the unit water inflow is 1 m3/h·m-5 m3/h·m, which is not suitable for the project of groundwater source heat pump. the unsuitable area of groundwater source heat pump is 10.93 km2, accounting for 3.98% of the total area of the evaluation area. the unevaluated area is weihe river, namely the existing river in the evaluation area, with an area of 6.37 km2, accounting for 2.32% of the total area of the evaluation region. heat exchange power and potential of groundwater ground source heat pump in order to study the validity of calculation and evaluation for the heat-exchange power and potential of groundwater ground source heat pumps in weinan city, it is necessary to carry out practical analysis. the analysis results are shown in table 6. from table 6, the total area of suitable and more suitable regions of groundwater source heat pump in this area is 145.22 km2, and the total heat transfer power of groundwater source heat pump system is 1.17×106 kw (winter)/ 3.58×106 kw (summer). when using the groundwater ground source heat pump system to develop and utilize the shallow geothermal energy, the average heating potential in winter is 1.48×105m2/km2, and the average refrigeration potential in summer is 2.13×105 m2/km2. heat exchange power and potential of buried tube ground source heat pump in order to study the validity of this method in calculating and evaluating the heat-exchange power and potential of ground source heat pump, it is necessary to take the average temperature of fluid in the buried pipe of weinan city in winter and summer as 6°c and 32°c respectively. the heat-exchange power potential of the ground source heat pump system in weinan city is calculated and evaluated by the proposed method. the results obtained by the proposed method are as follows. table 5. statistical table of suitability zoning and area of groundwater ground source heat pump suitability zoning real area /km2 percentage suitable area 36.49 11.69 more suitable area 153.61 49.24 general suitable area 95.48 30.61 difference suitable area 10.62 3.4 unconfined area 12.56 4.03 unevaluated area 3.21 1.03 total 311.97 100 323establishment of evaluation model for shallow geothermal energy resource development potential based on characteristic of geotemperature table 7 shows that the heat-exchange power in winter and summer is 1.04×107 kw and 1.26×107 kw respectively. the average heating potential in winter is 9.64×105 m2/km2 and the average refrigeration potential in summer is 1.18×106 m2/km2 when using the ground source heat pump system to develop and utilize shallow geothermal energy. analysis of resources development potential in order to verify the accuracy of the proposed method in evaluating the development potential of shallow geothermal energy, it is necessary to carry out actual experimental analysis. weinan city is divided into three regions. finally, the evaluation results as shown in table 8. from table 8, we can see that land use coefficient has a great influence on the potential of ground source heat pump systems. meanwhile, most of the floodplain areas in weinan are high potential areas of ground source heat pumps. the water level in the area is shallow and the effective aquifer is thick. the comprehensive heat transfer coefficient is high and single-hole heat exchange power is large. the ground source heat pump system has large heating/ cooling potential in winter and summer. the area is 50.79 km2, accounting for 18.47% of the total area. the first-order region of weihe river within the zoning area is the middle potential area of ground source heat pump, with an area of 135.74 km2, accounting for 49.35% of the total area. the built area of weinan has small land use coefficient and low potential of ground source heat pump system, which belongs to the low potential area of ground source heat pump. the area is 82.14 km2, accounting for 29.86% of the total area. through actual investigation and analysis, we can see that the evaluation results of the proposed method are in good agreement with the actual results. therefore, the evaluation accuracy of the proposed method is higher. discussions there are many ways to exploit the shallow geothermal energy resource. the groundwater ground source heat pump and buried pipe ground source heat pump are the two main ways. the groundwater source heat pump system consists of groundwater heat-exchange system, computer room system and terminal system. the working principle of the ground water source heat pump system is to pump groundwater and exchange heat through a ground source heat pump unit or plate heat exchanger. in winter, all the low-temperature water pumped from underground water after heat exchange through a ground source heat pump unit is recharged into the same aquifer, and the heat quantity is used for the building heating and domestic hot water. in summer, all the hightemperature water is recharged into the same aquifer after the heat exchange of pumped groundwater through a ground source heat pump unit or plate heat exchanger. at the same time, the high-temperature water can be exchanged through a heat pump unit to provide domestic hot water. the characteristics are as follows: at suitable sites for sinking a well, a certain number of pumping and irrigation wells are drilled, and heat is absorbed or discharged from groundwater by recycling, so that the number of wells and the area are few. the heat-exchange way of the underground heat-exchange system is mainly the table 6. heat transfer power and potential of groundwater ground source heat pump zoning and numbering zhao zhai-li jia wan longjiang town-hantai district-pu zhen liangshan town zhongsuo town-dahekan town sujiashan village total zonal area/km2 21.95 94.37 15.33 13.24 0.33 145.22 suitability more suitable suitable suitable suitable more suitable winter heat transfer power/kw 8.83×104 1.21×106 2.67×105 2.32×105 1.16×103 1.17×106 summer refrigeration power/kw 1.76×105 2.42×106 5.34×105 4.62×105 2.32×103 3.58×106 winter heating potential/ (m2/km2) 5.39×104 1.72×105 2.33×105 2.35×105 4.68×104 1.48×105 summer refrigeration potential/(m2/km2) 7.77×104 2.48×105 3.36×105 3.38×105 6.74×104 2.13×105 table 7. calculation table of heat transfer power meter potential of a ground source heat pump with buried pipe zoning and numbering praise canal longjiang town-qili town-paving town liangshan town-belt floodplain on the south bank of hanjiang river floodplain on the east bank of lianshui river floodplain east of dahekan town on the south bank of hanjiang river the first class terrace south of liangshan town huilong templeyuying village total zonal area/km2 3.36 112.69 6.04 1.26 2.36 9.29 9.94 144.94 suitability more suitable suitable suitable suitable suitable more suitable more suitable winter heat transfer power/kw 2.75×105 7.93×106 4.60×105 1.02×105 1.98×105 6.98×105 7.54×105 1.04×107 summer refrigeration power/kw 3.34×105 9.61×106 5.59×105 1.23×105 2.41×105 8.48×105 9.12×105 1.26×107 winter heating potential/(m2/km2) 1.09×106 9.44×105 1.01×106 1.09×106 1.12×106 1.03×106 1.02×106 9.64×105 summer refrigeration potential/(m2/km2) 9.59×105 8.24×105 8.92×105 9.47×105 9.83×105 8.83×105 8.89×105 1.18×106 324 jikun hou, xingqi luo, lei zhang thermal convection, and the temperature of groundwater is basically constant throughout the year. the initial cost is lower than that of a buried pipe ground source heat pump system. under appropriate groundwater depth, cop is higher and the operation cost is lower. the buried-pipe ground source heat pump system is similar to the groundwater ground source heat pump system. it consists of a heat exchange system with buried pipe, computer room system and terminal system. the working principle of the buried-pipe heat exchange system: the heat transfer medium circulates in the sealed vertical or horizontal buried pipe, and then the temperature difference among the heat transfer medium, the underground rock and soil and the groundwater is used to exchange heat, so that the purpose of utilizing the shallow geothermal energy can be achieved. meanwhile, the heating and cooling of buildings and domestic hot water through heat pump technology can be realized. the features are as follows: the project needs to drill many boreholes according to the cold load and heat load, and sets a closed circulating pipe with certain strength, corrosion resistance and good heat transfer performance, and then uses the circulating liquid to exchange heat, so as to connect all circulating pipes. finally, it enters the computer room and main engine. the buried-pipe ground source heat pump (gshp) system is based on the fact that the temperature of underground rock and soil and groundwater remains constant throughout the year. if the vertical buried tube heat exchanger is used to conduct and absorb heat between underground rock and soil and groundwater, the heat exchange efficiency is lower than that of groundwater ground source heat pump systems. compared with traditional air-conditioning systems and groundwater ground-source heat pump systems, the main disadvantage of buried-pipe ground-source heat pump systems is high initial investment of ground-source heat exchangers. the heat exchanger of a ground source heat pump system with a buried pipe occupies a larger area than that of the pumping well of an underground water source heat pump system. the ground source heat pump system does not draw water from the ground. theoretically, the ground source heat pump system with buried pipe has less impact on the environment of underground space than that of groundwater source heat pump, and its procedure is simpler. therefore, the ground source heat pump system with buried pipe has the characteristics of green environmental protection, high efficiency and energy saving, low operation cost, mature technology, wide application range (it is applicable to any stratum and building). in addition, it does not need to pump the groundwater. conclusions the evaluation of shallow geothermal energy development potential is an effective technology for rational planning of urban layout, protection of shallow geothermal resources and sustainable development and utilization of shallow geothermal resources. based on the analysis of the geotemperature characteristic in weinan, this article achieves the suitability zoning of comprehensive development of groundwater source heat pump, buried pipe ground source heat pump and shallow geothermal energy according to the requirements of analytic hierarchy process. meanwhile, this article uses the shallow geothermal capacity, heat-exchange power and resource potential to quantitatively evaluate the research region. the total area of suitable and more suitable regions of groundwater ground source heat pump in weinan is 145.22 km2. when using groundwater ground source heat pump systems to develop and utilize the shallow geothermal energy, the average heating potential in winter is 1.48×105 m2/km2, and the average refrigeration potential in summer is 2.13×105 m2/km2. the potential middle area of ground source heat pump in weinan is 135.74 km2, accounting for 49.35% of the total area, while the potential low area of ground-source heat pump in weinan is 82.14 km2, accounting for 29.86% of the total area. the result and the amount of available resources directly reflect the prospects and potential of shallow geothermal energy development and utilization in weinan. references alkhasov, a. b., alkhasova, d. a., ramazanov, a. s., & kasparova, m. a. 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(2017). a potential evaluation model for land consolidation in fragmental regions. ecological indicators, 74, 230-240. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.111479 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 265 276 g eo ph y si c s using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources yasin nasuti1, luan thanh pham2* 1. department of earth sciences, institute for advanced studies in basic sciences (iasbs), zanjan 45137–66731, iran. 2. faculty of physics, university of science, vietnam national university, hanoi, vietnam. * corresponding author: luanpt@hus.edu.vn how to cite item: nasuti, y., & pham, l.t. (2024). using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources. earth sciences research journal, 28(3), 265-276. https://doi.org/10.15446/esrj.v28n3.111479 record manuscript received: 07/10/2023 accepted for publication: 18/09/2024 abstract several filtering methods have been introduced to estimate the edges of potential field sources. the selection of the appropriate technique depends on the type of data and the target. among filtering techniques, phase-based filters are the most widely used methods due to the flexibility of their design, but they do not provide information on the source depth. in this study, some novel filtering approaches are proposed, highlighting the edge of adjacent sources with different intensities by initially removing the regional anomalies. these approaches generate low amplitude anomalies over the deep sources, and higher amplitude anomalies over the shallow sources, providing information on relative depths of the sources. to evaluate the designed approaches, synthetic and real data from the finnmark area of north norway were used. the results were compared with those obtained from other approaches. these results showed that the proposed approaches considerably simplify the interpretation of the anomaly maps with higher efficiency and broader interpretation scope than the classical techniques. keywords: edge; depth; filter; potential field sources palabras clave: límite; profundidad; filtrado; fuentes de campos potenciales. uso de derivadas de campos potenciales en la función inversa de la tangente para estimar los límites y las profundidades relativas de fuentes de campos potenciales varios métodos de filtrado se han presentado para estimar los límites de las fuentes de campos potenciales. la selección de la técnica apropiada depende del tipo de información y del objetivo. entre las técnicas de filtrado, los filtros de fase son los más usados debido a la flexibilidad de su diseño, sin embargo estos no proveen la profundidad de la fuente. en este estudio se propuso el acercamiento a algunos filtros novedosos, donde se resaltaron los límites de las fuentes adyacentes con diferentes intensidades al remover inicialmente las anomalías regionales. estos métodos generaron anomalías de baja amplitud sobre las fuentes profundas y anomalías de mayor amplitud sobre las fuentes a poca profundidad, lo que provee información de las profundidades relativas de las fuentes. para evaluar los modelos diseñados se usaron datos sintéticos y reales del área de finnmark, en el norte de noruega. los resultados se compararon con valores obtenidos a través de otros métodos. estos resultados muestran que los métodos propuestos simplifican considerablemente la interpretación de los mapas de anomalías con mayor eficiencia y en un alcance más amplio que las técnicas clásicas. resumen https://doi.org/10.15446/esrj.v28n3.111479 https://doi.org/10.15446/esrj.v28n3.111479 266 yasin nasuti, luan thanh pham 1. introduction the potential field methods play an important role in mapping geological structures, particularly for mineral exploration (evjen, 1936; cordell and grauch, 1985; dwivedi and chamoli, 2022; dwivedi et al., 2023; kamto et al., 2023). many filters of potential field data have been developed to estimate the source location (nabighian, 1972, 1974; wijns et al., 2005; cooper and cowan, 2006; oksum et al., 2021; pham, 2023; kafadar and oksum, 2024). by highlighting the edges of anomalies with various depths and magnitudes, filtering aids in the identification of additional details in the possible field data maps (ekinci and yiğitbaş, 2012, 2015; ekinci et al., 2013; narayan et al., 2017, 2021; eldosouky, 2019; eldosouky et al., 2022; sahoo et al., 2022a, b; pham and prasad, 2023; núñez-demarco et al., 2023; liu et al., 2023; ekwok et al., 2023; pham et al., 2022, 2024a; aprina et al., 2024). high-pass filters have been used in several studies to highlight the edges of anomalies (cordell and grauch, 1985; roest et al., 1992; blakely, 1995; hsu et al., 1996; and fedi and florio, 2001). to filter potential field data using various techniques, efforts have been made to balance the trade-off between noise and signal in the filtered image (cooper and cowan, 2006). the vertical derivative filter is one of the frequently employed filters to identify anomaly edges (evjen, 1936; ma et al., 2014). higher-order vertical derivatives can effectively enhance anomalies, but they amplify the noise in data. the vertical derivative filter is usually limited to orders of one and two. this method is preferred when anomaly variation throughout the survey area is uniform. in other words, this filter is not able to balance the map. the total horizontal derivative filter is another commonly used method to detect edges. cordell and grauch (1985) showed that the peak of the total horizontal gravity or pseudo-gravity data can indicate the edges of anomalies. the drawback of this strategy is that it might produce diffuse edges, which prevent the filter from being able to tell where weak anomalies and strong anomalies meet at the border (li et al., 2012; ma, 2013). another filter commonly used is the analytical signal or total gradient, which generates bell-shaped anomalies above magnetic bodies. this filter was developed by nabighian (1972, 1974) and roest et al. (1992). according to grauch and cordell (1987), the analytical signal cannot simultaneously enhance deep and shallow anomalies. to address this issue and improve the definition of adjacent anomalies, hsu et al. (1996, 1998) used an enhanced analytical signal, which is the nth-order vertical derivative of the analytical signal. the impact of noise can be amplified by using higher order derivatives, so this technique is typically restricted to the first and second order. in the last three decades, phase filters have received significant attention due to their flexibility (wijns et al., 2005; cooper and cowan, 2006; stampolidis and tsokas, 2012; li et al., 2013; arisoy and dikmen, 2013; ferreira et al., 2013; cooper, 2014; ma et al., 2014; zhang et al., 2015; yao et al., 2016; nasuti and nasuti, 2018; nasuti et al., 2019; melouah and pham, 2021; pham et al., 2022b). the first phase filter is the tilt angle filter introduced by miller and singh (1994). using the tilt angle filter, the source edges are represented by the zero values. to make the interpretation of the tilt-angle data easier, verduzco et al. (2004) introduced a total horizontal derivative of the tilt angle filter. as the depth of the sources rises, the performance of the tilt angle and its total horizontal derivative may suffer. additionally, the total horizontal derivative of the tilt angle is sensitive to noise, which can affect the accuracy of the results (cooper and cowan, 2006; li et al., 2012; ferreira et al., 2013; nasuti and nasuti, 2018). to detect the edges of potential field sources, ferreira et al. (2013) used the total horizontal derivative in the tilt angle equation. this method not only normalizes both shallow and deep potential field anomalous sources but also detects the edges of anomalies with very high precision. however, in case the shallow and deep sources located close together this filter is not able to discriminate them and will lose its resolution considerably (zhang et al., 2015; yao et al., 2016; nasuti et al., 2019). cooper (2014) introduced the tilt angle of the analytical signal amplitude as a means of reducing the dependence on the source magnetization vector direction. this filter is appropriate in cases where the data has a remanent magnetization or when we are not able to reduce to magnetic pole due to lack of sufficient information from the sources, so that it transfers the anomalies to the top of the sources. it also creates a much better balance than the analytical signal in the data. the efficiency of this filter is reduced for adjacent sources that are located at a short distance from each other. also, in cases where the ratio of depth to width of the source is high, the maximum of this filter is placed above the anomaly. zhang et al. (2015) suggested using the tilt angle of the first order vertical derivative of the total horizontal derivative to distinguish between nearby sources. this filter perfectly equalizes the severity of deep and shallow anomalies and anomalies with different amplitudes and gives sharper and more accurate edges than the tahg and tas filters. this filter is also more effective in cases where anomalies of different depths and amplitudes are adjacent. the concept of edge detection is not limited to the methods given above. recently, some authors have developed other methods to map the source edges (dwivedi and chamoli, 2021, 2023; kafadar, 2022; prasad et al., 2022a, b, c; dandan et al., 2023; ibraheem et al., 2023; jorge et al., 2023; li et al., 2023; alvandi et al., 2023; alvandi and ardestani, 2023; pham, 2024 a,b,c; pham et al., 2024b; ai et al., 2024). although these methods can bring the edges with high resolution, they do not provide the relative depths of potential field sources. in this article, some new methods are introduced considering the slight changes in the previous filtering techniques, providing much better efficiency than the previous strategies. the images obtained from the new methods show deep anomalies with low amplitude and shallow anomalies with high amplitude. this can be used to distinguish deep anomalies from shallow anomalies, resulting in a wider interpretation scope. 2. methodology by using vertical derivatives of the amplitude-based filters that provide maxima over the source edges in the arctangent function, novel filters are introduced from the following general relation: new(filter ) = tan− 1 g|h| √g ⟶g ≥ 0, g < 0 → removed (1) where h is a correction factor that is used to make the dimension unifying. in other words, the use of the factor h to make eq. (1) has mathematical significance. this factor is given by: h = max(√g) max(g) , (2) and g is the vertical derivative of some edge filters such as the total horizontal derivative (thd, cordell and grauch, 1985), analytical signal (as, roest et al., 1992), and vertical derivative of thd (thv, zhang et al., 2015) that are given by: (3) (4) (5) where is f potential field data. since g functions produce positive maxima over the edges, removing negative g values is helpful in bringing sharper edges. the proposed methods can be considered as modifications of the previous methods, i.e., the tilt angle of the thd (tahg, ferreira et al., 2013), tilt angle of the as (tas, cooper, 2014) and tilt angle of the (tthv, zhang et al., 2015): 267using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources (6) (7) (8) using the proposed methods, the edges of deep sources are shown by low amplitude signals, while the edges of shallow sources are related to high amplitude signals. this can be used to distinguish deep anomalies from shallow anomalies. in the following sections, we demonstrate the effectiveness of the proposed methods on theoretical examples and real data from the finnmark area, northern norway. 3. synthetic examples we conducted synthetic studies to evaluate the strengths and limitations of the proposed approaches in enhancing the edges of magnetic anomalies. the first model consists of 17 bodies (fig. 1a) with an inclination of 90° and declination of 0°, and their geometrical and geomagnetic parameters are listed in table 1. figure 1b displays the synthetic magnetic anomaly of the model. the results obtained from applying the tahg, ntahg, tas, ntas, thvh, and nthvh, filters to magnetic data of the first model are presented in figure 1 (c)-(h). the tahg filter (fig. 1c) highlights the edges of deep and shallow bodies by balancing different anomalies. the ntahg filter (fig. 1d) shows the edges similar to the tahg filter, except that it shows the response of deep bodies with low amplitude and shallow bodies with high amplitude, and the field around the bodies is removed. as clearly seen, in addition to the edges of the bodies, the relative depth of the bodies can also be distinguished by different transformed anomalies for different depths. the tas filter (fig. 1e) shows the edges of anomalies with less accuracy and sharpness than the tahg filter. also, in cases where the ratio of width to depth of the body is low, the peaks of this filter are above the anomalous bodies instead of the edges (bodies 6, 7 and 17). in the ntas method (fig. 1f), the deep and shallow bodies can be separated, but its accuracy is less than the ntahg filter. we can see that the response of the bodies with non-vertical dip (bodies 5 and 6) has a small displacement in the direction of the dip of the bodies (fig. 1f). therefore, the dip of the bodies can be estimated by comparing the result of this filter with those from other methods. the thvh filter (fig. 1g) has the same results as the tahg filter, except that the thvh filter shows more detail with better sharpness than the tahg. the nthvh filter (fig. 1f) has almost the same results as the ntahg filter, but it brings sharper edges compared to the ntahg filter. by using this filter, one can easily determine the geometric shape and position of adjacent bodies. however, the nthvh map is noisy and this is one of its most important drawbacks. table 1. geometric and magnetic parameters of the magnetic sources of the synthetic model 1 in figure 1 (magnetic field=48000 nt, inclination=90° and declination=0°). model label depth (m) thickness (m) strike length (m) depth extent (m) magnetic susceptibility (si) dip (deg) azimuth (deg) p1 10 100 500 1000 0.02 90 0 p2 20 100 500 1000 0.02 90 -45 p3 30 100 500 1000 0.02 90 45 p4 10 100 400 600 0.02 90 0 p5 40 100 400 600 0.02 60 0 p6 60 100 400 600 0.02 45 0 p7 30 100 100 1000 0.02 90 0 p8 20 100 100 1000 0.02 90 0 p9 10 100 100 1000 0.02 90 0 p10 30 600 300 1000 0.02 90 0 p11 10 200 100 1000 0.01 90 0 p12 10 100 300 1000 0.01 90 0 p13 20 100 300 1000 -0.02 90 0 p14 30 100 300 1000 0.02 90 0 p15 10 50 800 1000 0.01 90 0 p16 20 50 800 1000 0.02 90 0 p17 30 50 800 1000 0.02 90 0 268 yasin nasuti, luan thanh pham figure 1. (a) synthetic model 1 indicating the location of the ab and cd profiles (see figs. 2 and 3). (b) magnetic anomaly of the model 1. (c) tahg map. (d) ntahg map. (e) tas map. (f) ntas map. (g) thvh map. (h) nthvh map. to aid in visualizing the quality of the above interpretations, figures 2 and 3 show the results of the tahg, ntahg, tas, ntas, thvh, and nthvh filters on the ab and cd profiles in figure 1a. as can be seen, all the novel filters accurately show bodies of the same depth with the same intensity, while the commonly used filters provide the same intensity for all the bodies. analyzing magnetic anomalies can be challenging due to the influence of induced and remanent magnetization, which affects their shape. in this case, reducing the data to magnetic pole may not be possible. to estimate the efficacy of the proposed methods in this case, we consider the second synthetic model (fig. 4a) with an inclination of -20° and a declination of -20°. the geometric and magnetic parameters of the bodies in this model are provided in table 2, and the magnitude of the magnetic field is 28000nt. figure 4b shows the magnetic anomaly of the model. figure 4 (c)-(h) show the results from applying the tahg, ntahg, tas, ntas, thvh, and nthvh methods to magnetic data of the second model. as can be observed from these figures, the tas and ntas filters (figs. 4e and f) are much more efficient at determining the edge of anomalies than the other methods. the most important advantage of these filters is that they are less dependent on the direction of the magnetization vector than the other filters. since the ntas values are dependent on the source depth, it can be used to estimate the relative depths of the bodies. each of the new filters has disadvantages and advantages, therefore it is important to note that using 2 or 3 methods together improves the interpretation procedure by incorporating the advantages of each method. 269using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources figure 2. profile ab constructed from synthetic model 1(see fig. 1a). 270 yasin nasuti, luan thanh pham figure 3. profile cd constructed from synthetic model 1(see fig. 1a ). 271using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources figure 4. (a) synthetic model 2. (b) magnetic anomaly of model 2. (c) tahg map. (d) ntahg map. (e) tas map. (f) ntas map. (g) thvh map. (h) nthvh map. table 2. geometric and magnetic parameters of the magnetic sources of the synthetic model 2 in figure 5 (magnetic field=28000 nt, inclination= -20° and declination= -20°) model label depth (m) thickness (m) strike length (m) depth extent (m) magnetic susceptibility (si) dip (deg) azimuth (deg) b1 20 100 500 1000 0.025 90 0 b2 30 100 500 1000 0.025 90 0 b3 40 100 500 1000 0.025 90 0 b4 40 800 600 1000 0.025 90 0 b5 20 400 300 1000 0.025 90 0 b6 30 100 100 1000 0.025 90 0 272 yasin nasuti, luan thanh pham 4. application to magnetic data of the finnmark area to estimate the effectiveness of the proposed methods for real case scenarios, we applied them to interpret real data from the finnmark area, north norway (fig. 5). the northern region of norway (fig. 5) is mainly composed of rock complexes formed during the precambrian to early palaeozoic era. most of these rock complexes were involved in the caledonian orogeny to varying degrees. the bedrock geology can be broadly divided into two categories the precise definition of the caledonides and a mid-crustal continental lithospheric basement made up of native crystalline complexes that date from the neoarchaean to the late palaeoproterozoic. according to gaál et al. (1989) and daly et al. (2006), these structures define the fennoscandian shield’s northern boundary. most of the ancient precambrian rocks can be found in the western troms and eastern finnmark areas. these rocks include some that have undergone caledonian action or have been incorporated into caledonian nappes (fig. 5). figure 6a shows the geological map of the area (modified from koistinen et al., 2001). this map depicts the presence of precambrian rocks that are exposed in the southern part of the area and are dominated by the nw-se trend. these rocks mainly include mafic to intermediate granulitic rocks and felsic to intermediate granulitic rocks. however, one of the caledonian nappes (i.e., gaissa nappe complex) obscures the precambrian strata in the northern portion. in the middle of the study region, the caledonian front thrust boundary is easily distinguishable. to explore minerals, aeromagnetic measurements were carried out in the finnmark area (highlighted by the yellow box in fig. 5) from 2009 and 2014 with a flight line separation of 200 m (novatem 2012, 2014; nasuti et al., 2019). the flight height was generally from 60 m to 200 m (nasuti et al., 2019). the average intensity of the earth’s magnetic field is 53600 nt. this selected area comprises adjacent deep and shallow anomalies, as well as anomalies of varying intensities. to reduce noise, the aeromagnetic data was upward-continued to 50 m before calculating the filters. aeromagnetic anomalies were also reduced to the north magnetic pole using the inclination and declination angles of 78.5° and 10.6°, respectively. figure 6b shows the reduction to the pole of upwardcontinued magnetic data. figure 5. the tectonostratigraphic map of northern scandinavia (modified from koistinen et al., 2001), with the study area delimited by the yellow box. figure 6. (a) geological map of northern norway. (b) magnetic anomalies of the area. 273using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources the results from applying the tahg, ntahg, tas, ntas, thvh, and nthvh filters to magnetic data in figure 6b are displayed in figure 7a-f. as can be observed, the proposed filters (figs. 7b, d and f) allow for a clearer estimation of the edges in comparison with their counterparts (figs. 7a, c and e). the new filters show deep anomalies with low amplitude and shallow anomalies with higher amplitude (rather than equally amplifying anomalies of different intensities and depths). in other words, the anomaly of the precambrian layers (that are in the south to the center of the study area) is shown with a high amplitude and the anomaly of the other layers (that have gone from the center to the north of the region below gaissa nappe complex) are presented with a low amplitude. the precambrian layers are located at a greater depth in this area, or the thickness of the gaissa nappe complex above the precambrian layers is greater in this area than in other areas. therefore, by observing the range of anomalies in the maps obtained from the proposed filters (figs. 7b, d and f), we can understand the relative depth of the sources and can realize the trend of structures and tectonics in the region. in other words, the images obtained from the proposed methods cover a wider range of interpretations. among these techniques, the nthvh map (fig. 7f) has shown the range of deep and shallow anomalies with better accuracy, so that the boundary of caledonian with precambrian can be better defined compared to other methods (figs. 7b and d). figure 7. application of different filters to high-resolution measured data from northern norway. (a) tahg map. (b) ntahg map. (c) tas map. (d) ntas map. (e) thvh map. (f) nthvh map. red line depicts the international border. 274 yasin nasuti, luan thanh pham to further illustrate the strength of the filters, we selected a smaller area inside the black box shown in figure 6a. figure 8a shows the geological map of the study area, while figure 8(b)-(d) exhibit the outcomes of the edge detection techniques, including ntahg, ntas, and nthvh applied to magnetic data of this area. the ntahg and nthvh filters offer high sharpness in detecting the source edges, while the ntas provide maxima over the sources. it was discovered that the ntahg and nthvh filters were very successful in locating the extension of the precambrian rocks beneath the gaissa nappe. moreover, the ntahg and nthvh filters also provided a clearer differentiation of the neighboring rock units. it is important to note that this approach not only improves the representation of structure continuity but also yields more lucid and organized visuals for the interpreter. apart from the edges of the anomalies, our methods can estimate the relative depth of the magnetic structures in the area. as can be observed, the map of all the newly introduced methods (figs. 8b, c and d) shows a decrease in the amplitude of anomalies related to the precambrian layers from south to north, and the results obtained by these methods are highly compatible with each other. figure 8. fapplication of the new methods on the selected area (see fig. 6a for location). (a) geological map of the area. (b) ntahg map. (c) ntas map. (d) nthvh map. labels 1, 2, 3 and 4 are four anomalies determined from applying the new methods. to investigate the placement of the adjacent layers based on figure 8b, we labeled four anomalies 1, 2, 3 and 4. the ntahg filter (fig. 8b) does not show well the part of anomaly number one that is adjacent to anomalies 2 and 3. in the nthvh map (fig 8d), the source 1 is clear, but the ntas filter (fig. 8c) does not provide any response to this source. from the high amplitude of bodies 2 and 3 in the nthvh map, it can be said that the body 1 is deeper in this area than the bodies 2 and 3. considering the location of the anomalies in the filter map and comparing it with other methods, it can be said that no magnetic remanent is observed in these data. the geological map of the area also confirms this for the precambrian layers that are exposed in the southern part of the area. 5. conclusions in this study, some novel filters have been introduced for interpretation of the potential field data. the proposed approaches highlight and separate the edges of adjacent anomalies with different intensities. the anomalies around the source edges are initially removed. the responses over the edges of the anomalies will remain where the low amplitude responses are related to the deep bodies, while the high amplitude responses show the shallow bodies. these results, therefore, also present the relative depth of the source bodies. the proposed filtering strategies simplify considerably the interpretation of the potential data and provide a wider interpretation scope. theoretical simulations demonstrated a better performance of the novel methods for filtering potential data in comparison with the original methods. moreover, the results from the application of these techniques to real data from north norway are consistent with the known geological structures of the area. in the real example, the new methods also yielded clearer results compared to traditional methods. acknowledgements the authors gratefully acknowledges institute for advanced studies in basic sciences for supporting this project. we extend our gratitude to the reviewers for their constructive feedback, which greatly improved the manuscript. we further acknowledge davood moghadas, hossein fazli and arya farkhondeh for their valuable comments and assistance in refining the english language used in the manuscript. references ai, h., deniz toktay, h., alvandi, a., pašteka, r., su, k., & liu, q. 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abstract a quest for life on mars (lomars) started in the early 1960s when the most prestigious scientific journals published several research articles. according to elsevier’s scopus database, the rise in annual literature production started in the late 1990s, most likely associated with the launch of the national aeronautics and space administration’s (nasa) first rover, sojourner, in 1996. the number of articles on mars will likely continue to rise sharply, given that the launch and landing of the mars 2020 perseverance rover are critical to discovering and understanding the present or past life on the planet. thus far, the usa has dominated in the number of citations and collaborations related to mars research, given its facilities equipped with relevant infrastructure and researchers’ capacity to explore the solar system beyond earth. most of these frequently cited papers report observational and theoretical research results. however, a highly cited article is based on analytical studies of a unique martian meteorite (i.e., allan hills 84001) found in antarctica. it is expected that the future sample return mission associated with the perseverance rover caching system could increase the lomars research exponentially in the coming decades if martian samples are successfully brought to earth. based on the total number of publications on lomars, the most influential institute, the author, and the journal are caltech, c.p. mckay, and icarus, respectively. both the institute and the author are directly affiliated with nasa, indicating the leadership offered by the organization in lomars research. keywords: life on mars; water on mars; perseverance rover; mars 2020; zhurong rover; alh84001 meteorite vida en marte: historia, avances, investigación actual y perspectivas resumen las preguntas sobre la vida en marte comenzaron en los años sesenta cuando varios artículos de investigación se publicaron en las revistas científicas más prestigiosas. de acuerdo con la base de datos de elsevier, el aumento en la producción literaria sobre este tema comenzo a finales de los noventas, debido al lanzamiento del sojourner, el primer astromóvil de la administración nacional de aeronáutica y el espacio (nasa). es previsible que el número de artículos sobre marte continuará creciendo debido a que el lanzamiento y amartizaje del astromóvil perseverance en 2020 es determinante en el descubrimiento y exploración del pasado y presente de este planeta. los estados unidos han dominado en número de citaciones y colaboraciones relacionadas a la investigación de marte, dada sus instalaciones equipadas con infraestructura relevante y con investigadores capacitados para explorar el sistema solar más allá de la tierra. la mayoría de estos artículos, frecuentemente citados, presentan el resultado de investigaciones teóricas y de observación. sin embargo, uno de estos trabajos se basa en los estudios analíticos del único meteórito proveniente de marte y que fue hallado en la antártida. se prevee que el sistema de toma de muestras del perseverance rover incrementará exponencialmente la investigación sobre la vida en marte en las próximas décadas siempre y cuando estas muestras puedan traerse a la tierra. con base en el número total de estudios sobre la vida en marte, el instituto caltech, el autor c.p. mckay y la publicación icarus son las más influyentes de la materia. tanto el instituto como el autor están afiliados directamente con la nasa, lo que indica el liderazgo ofrecido por la organización en las investigaciones sobre la vida en marte. palabras clave: vida en marte; agua en marte: astromóvil perseverance; zhurong rover; meteorito alh84001 record manuscript received: 29/06/2021 accepted for publication: 18/10/2022 how to cite item: ali, a., shahid, m. s., jabeen, i., & el-ghali, m. a. k. (2022). life on mars (lomars): history, advances, current research, and perspectives. earth sciences research journal, 26(3), 221-230. https:// doi.org/10.15446/esrj.v26n3.96985 https://doi.org/10.15446/esrj.v26n3.96985 about:blank https://doi.org/10.15446/esrj.v26n3.96985 https://doi.org/10.15446/esrj.v26n3.96985 222 arshad ali, muhammad s. shahid, iffat jabeen, mohamed a.k. el-ghali 1. introduction according to the elsevier’s abstract and citation database, scopus, the first paper about ‘life on mars’ appeared in the scientific journal science (levin et al., 1962), opening the doors for a concerted multidisciplinary approach to explore another planet. in order to address the possibility of the presence of water and the existence of life on mars, several more studies came out during the 1960s (e.g., rea, 1963; oyama, 1963; shirk et al., 1965; hawrylewicz et al., 1967). contemporaneous to these studies, the national aeronautics and space administration (nasa) successfully launched its first flyby of mars (mariner iv) on 28 november 1964 after a few unsuccessful attempts by the soviet union and nasa. later, it captured and sent the first close-up images of mars to earth. the next decade (the 1970s) highlighted the successful launch of viking 1 and viking 2 landers in 1975 that provided important information about the atmosphere and surface of mars with no conclusive evidence regarding the existence of life on the planet (levin and straat, 1976). in addition to the use of remotely controlled unmanned aerial vehicles (uavs) for mars exploration (sharma et al., 2022 and refs. therein), detailed investigations of meteorites have provided a complimentary understanding of mars. up till the early 1980s, some meteorites were thought to be of martian origin due to their chemical composition matching that of the viking lander’s analyses (levin and straat, 1976) and differences in geochemical characteristics from other common groups of meteorites. bogard and johnson (1983) provided direct evidence of a link between these anomalous meteorites and mars after analyzing the trapped gasses in impact-related glass from the elephant moraine 79001 (eet 79001) meteorite collected by ansmet (i.e., us antarctic search for meteorites program) in antarctica. the obtained compositions from these gases (e.g., argon, krypton, xenon) closely resembled those of the martian atmosphere analyzed by the viking landers (bogard and johnson, 1983). martian meteorites are rare (i.e., as of 28 july, 2022, the meteoritical bulletin database contains 335 samples registered out of 69949 = 0.48%) compared to other common types and have so far provided important clues about water and life on mars (e.g., mckay et al., 1996). after launching flybys and landers, in the mid-1990s (4 december 1996), nasa successfully launched its first rover (i.e., a wheeled vehicle), sojourner, which was in a lander called pathfinder to take the martian exploration to the next level. since then, other rovers, including spirit (10 june 2003), opportunity (8 july 2003), and curiosity (26 november 2011), have also made successful landings on mars. all these missions have provided an unparalleled understanding of the martian geology and landscape. our motivation for the present study is the recent launch and landing of nasa’s perseverance rover, also known as mars 2020, designed specifically to search for microbial life on mars and to store sample cores in tubes on the martian surface for future sample return missions. with the exciting current and future developments in mars research and prospects of human habitability on mars, it is vital to evaluate the history and impact of the research on lomars using statistical (i.e., bibliometric) methods. in the present study, we provide a literature overview of the lomars research with the main emphasis on bibliographic variables including author(s), source journal, subjective keywords as well as citation scores (young and belanger, 1983) and recognize the most prominent articles, authors, journals, collaborations, and growth trends. 2. methodology literature search, data extraction and analysis a literature collection was initiated with a wide-ranging search of relevant articles on lomars. the initial step was to determine appropriate databases, which would provide a full range of published articles on lomars. elsevier’s scopus (esc), an extensive database containing peer-reviewed literature, was used for bibliographic information of each document along with other publishing information necessary to carry out citation analysis of the fields relevant to lomars articles. in addition, esc has a wider article coverage than other web sources, thus being most suitable for our objective to extract as much relevant information as possible. abstracts, titles and keywords were searched using a comprehensive list of important words used by the researchers involved in the literature on lomars. the following final boolean search has been carried out in esc during august 2020, after the launch of mars 2020 perseverance rover by nasa, and later updated on march 14, 2021: title-abs-key ((“life on mars*” or “martian life*” or “water on mars*” or “martian soil*” or “snc meteorites*” or “shergottite-nakhlite-chassignite*” or “shergottite nakhlite chassignite*” or “shergottites*” or “nakhlites*” or “chassignites*” or “martian meteorites*”). this method yielded more than seven thousand documents. in order to extract relevant articles, several filters were applied; we kept only english journal articles while removing books, book chapters and conference proceedings. following the application of all these filters, as well as the manual removal of irrelevant articles, the final sample contained 660 articles published between 1962 and 2020. the academic data file records, such as the publication over time, authors’ production over time, authorship, institutions, countries, journals, publication count, keywords, institution collaborations network and citation count were extracted from the scopus database. the downloaded data file (full records and cited references) were exported into the r-based bibliometrix interface, which is widely used for citation analysis (marty et al., 2020; aria and cuccurullo, 2017). these articles have been published in diverse peer-reviewed research journals, written by single or multiple authors from numerous countries. the vosviewer program was used to construct the network analyses between citation, country, and authors’ collaborations (perianes-rodrigues et al., 2016). 3. results and discussion overall results eleven search strings were used to find the 660 documents on lomars research, which have been cited 22,242 times based on the average number of citations per document (33.7). more than two-thousand authors (2006) have been identified in these documents, averaging three authors per document. the annual number of articles began to rise in the mid-1990s and continued to rise until 2002, at which point it mostly remained below thirty publications per year, with the exception of 2003 (35), 2011 (33), and 2019 (34) (figure 1). the introduction of the first rover, sojourner (4 december 1996), may have contributed to the increase in the number of publications in the 1990s. similarly, spikes in the number of articles published in 2003 and 2011 could be attributed to the launches of nasa rovers such as spirit (10 june 2003), opportunity (8 july 2003), and curiosity (26 november 2011). presumably, the exceptional features of the perseverance rover in comparison to previous rovers will result in a higher publication throughput in the future, and the spike in 2019 could be the start of yet another surge in lomars research as previously seen in the 1990s (figure 1). out of the twenty-three publications in 1998 (figure 1 inset), the only two articles cited more than a hundred times in 22 citable years (i.e., cy) were jakosky and shock (1998), and sleep and zahnle (1998), with the mean total citations per article (i.e., mtcpa) and mean total citations per year (i.e., mtcpy) equal to 32.9 and 1.5, respectively (figure 2). when compared to years with np > 20 per year, the years 2000 (cy = 20) and 2015 (cy = 5) have the highest mtcpa (74.9) and mtcpy (6.3) values (figure 2). four of the twenty articles published in 2000 (cy = 20, figure 2) were cited more than 150 times (yen et al., 2000; skidmore et al., 2000; benner et al., 2000; wynn-williams and edwards, 2000), which could explain the higher mtcpa (75). a paper on hydrated salts on mars (ojha et al., 2015) has been cited over 300 times, giving the year 2015 the highest mtcpy value (6.3). the same paper is also one of the top ten most cited publications in terms of average number of citations per year (i.e., apyc; see table 1). 223life on mars (lomars): history, advances, current research, and perspectives table 1. a summary of the top ten highly cited articles on lomars research. abbreviations: r = article rank. tc = total citations. apyc = average per year citations. py = publication year. 5yif = 5-year impact factor. jgr = journal of geophysical research. epsl = earth and planetary science letters. r tc apyc py article title brief description journal/5yif reference 1 312 52.0 2015 spectral evidence for hydrated salts in recurring slope lineae on mars several hydrated salts were discovered, which is consistent with the formation of recurring slope lineae as a result of contemporary water activity on mars nature geoscience/ 15.781 ojha et al., 2015 2 1199 48.0 1996 search for past life on mars: possible relic biogenic activity in martian meteorite alh84001 the discovery of polycyclic aromatic hydrocarbons, carbonate globules, and secondary minerals from the fresh fracture surfaces of the alh84001 martian meteorite could be the fossil remains of a past martian biota science/ 44.372 mckay et al., 1996 3 124 41.3 2018 radar evidence of subglacial liquid water on mars the planum australe region was surveyed using mars advanced radar for subsurface and ionosphere sounding (marsis), which revealed bright features with high dielectric permittivity (>15), indicating the presence of a stable body of liquid water on mars science/ 44.372 orosei et al., 2018 4 642 40.1 2005 phyllosilicates on mars and implications for early martian climate the discovery of phyllosilicates by the mars express spacecraft’s omega imaging spectrometer suggests the presence of aqueously altered products during the noachian period (4.1-3.7 ga) nature/ 45.819 poulet et al., 2005 5 267 29.7 2012 the sample analysis at mars investigation and instrument suite the analytical capabilities of the sample analysis at mars (sam), one of the instruments of the mars science laboratory (msl) on board the curiosity rover, have been discussed for the investigation of mars’ habitability potential through determination of chemical and isotopic compositions of atmospheric gases and volatiles space science reviews/ 7.885 mahaffy et al., 2012 6 148 21.1 2014 recurring slope lineae in equatorial regions of mars the mars reconnaissance orbiter data for the deep canyons of valles marineris indicate that recurring slope lineae activity has been widespread on mars, with intermittent flow of briny water in some near-surface equatorial regions nature geoscience/ 15.781 mcewen et al., 2014 7 404 20.2 2001 evidence for recent climate change on mars from the identification of youthful nearsurface ground ice the observational evidence suggests that the nearsurface water ice, which occupies the pore space of soils, formed 100,000 years ago, corresponding to a global layer of water 10-40 cm thick nature/ 45.819 mustard et al., 2001 8 496 17.7 1993 a model for the hydrologic and climatic behavior of water on mars the hydrologic models are used to interpret the largescale vertical and horizontal transport of water between the atmosphere, polar caps, and near-surface crust on mars over the course of its geologic history jgr planets/ 3.858 clifford, 1993 9 272 17.0 2005 sedimentary rocks at meridiani planum: origin, diagenesis, and implications for life on mars the mars exploration rover (mer) opportunity investigated the ancient sedimentary rocks of meridiani planum, which included sandstones, silicates, sulfate minerals, cements, hematite, and crystal molds; formed by chemical weathering and diagenesis, mediated by ambient ground waters epsl/ 5.273 squyres and knoll, 2005 10 122 15.3 2013 impact-generated hydrothermal systems on earth and mars the study of impact craters on earth and mars in relation to the generation of hydrothermal systems, which may have provided nutrients and habitats for life on both planets icarus/ 3.345 osinski et al., 2013 224 arshad ali, muhammad s. shahid, iffat jabeen, mohamed a.k. el-ghali figure 1. plot showing the annual number of articles published on lomars research from 1962 to 2020. the inset bar graph depicts the gradual increase in annual output since the early 1990s. the numbers next to each kink represent the number of articles published during that specific year. figure 2. plot of the number of publications (np), mean total citations per article (mtcpa), and mean total citations per year (mtcpy) against the number of citable years (cy). it is important to keep in mind that interesting studies may have been published in non-elsevier journals, and particularly in non-english journals. this may certainly be true for studies, in particular, published in russian, japanese, and chinese journals, which might influence the statistics. figure 3. (a) comparison of lomars research articles with corresponding authors from different countries, affiliation of co-authors from a single country (scp), and affiliation of co-authors from multiple countries (mcp). scp = single country publications. mcp = multiple country publications. (b) comparison of top-cited countries with a minimum of five citations. most of the g7 countries (united states, japan, germany, the united kingdom, france, italy, and canada) ranked in the top ten. table 2. a list of the top 10 most influential authors who have published the most papers on lomars research. abbreviations: np = number of publications. tc = total citations. authors affiliation np tc h -index g -index m -index c.p. mckay nasa, usa 29 1131 17 29 0.53 f. westall nasa, usa 16 567 13 16 0.57 r.l. mancinelli nasa, usa 14 326 11 14 0.34 d. schulzemakuch technische universität, germany 14 275 9 14 0.60 m.a. sephton imperial college london, uk 14 149 6 12 0.46 a. steele carnegie institute of science, usa 13 873 12 13 0.52 a.f. davila nasa, usa 10 418 9 10 0.69 a.g. fairén centro de astrobiología, spain 10 353 8 10 0.57 j.d. farmer arizona state university, usa 10 598 8 10 0.35 s. fonti università del salento, italy 10 678 6 10 0.29 analysis of prominent authors and countries the productivity of lomars researchers has been assessed using h-, g-, and m-indices, which are based on the number of citations (table 2). six of the top ten most renowned authors are americans (the majority of whom work 225life on mars (lomars): history, advances, current research, and perspectives for nasa; table 2), while the remaining four are from european countries such as the united kingdom, germany, spain, and italy (members of the g7 countries). in terms of the total number of publications, total citations, h-, and g-index, c.p. mckay is firmly ranked in first place among the top ten authors (table 2). however, because the author has been active for over 30 years, the m-index (i.e., h-index divided by the number of years the scientist has been active) is lower. in addition, the united states leads the world in two categories: multiple country publication (mcp) and single country publication (scp) (scp; fig. 3a). as evidenced by the highest number of documents for the mcp category (i.e., an indicator for collaborative research; figure 3a), nasa ranks sixth on the list of the world’s most collaborative institutions (nature index 2016), indicating the united states’ dominance in collaborative research (i.e., total link strength) in the field of lomars in particular and planetary research in general (table 3). furthermore, the united states ranks first among highly cited countries in lomars research (figure 3b). table 3. the top-15 countries ranked based on the number of publications with their citation scores and collaboration link strength. abbreviations: np = number of publications. tc = total citations. rank country np tc total link strength 1 united states 419 17550 206 2 united kingdom 109 2731 129 3 france 62 3062 100 4 germany 56 2309 110 5 spain 36 1158 79 6 italy 31 1078 48 7 canada 28 2025 59 8 netherlands 27 809 77 9 australia 20 542 15 10 russia 20 1353 23 11 switzerland 13 774 41 12 china 10 122 4 13 japan 10 90 11 14 austria 8 287 32 15 denmark 8 316 18 collaborative research analysis despite a significant difference in the number of documents published as mcp (i.e., co-authors’ affiliations are in different countries) and scp (i.e., all authors’ affiliations are in the same country), the united states remains the topranked country in terms of the corresponding author’s affiliation (figure 3a). if the mcp is used as a proxy for collaborative research, the united states would be the most collaborative country in lomars research (figure 4). the majority of the other top-ranking countries are g7 members, supporting the idea that scientific collaborations are inextricably linked to scientific infrastructure, basic research, and collaborative publishing output. figure 4. comparison of collaborative research in lomars, with a minimum of two articles published. the united states is the main international collaborator on lomars research, particularly in europe and asia. the majority of the g7 countries (japan, germany, the united kingdom, france, italy, and canada) ranked in the top ten. for instance, china has been leading the g7 countries in terms of publications since 1996 (he, 2009), despite the fact that it was not previously prominent in lomars research (figure 4). the author emphasized that political, economic, educational, and scientific policies have successfully transformed the country into a world giant in international scientific publications. collaborative work, which is one of the most important aspects of modern research, opens up international opportunities for scientists, as evidenced by the exponential growth of international collaboration publication output between china and g7 nations (he, 2009). evidently, the majority of african and asian nations’ contributions to lomars research are either absent or insignificant (figure 4). however, a few african and asian countries have recently emerged as leaders in the field of planetary sciences. for instance, the japan aerospace exploration agency (jaxa)and the united arab emirates (uae) jointly launched the hope probe on july 19, 2020. on february 9, 2021, the probe was successfully placed into mars orbit, only days before the mars 2020 perseverance rover landed on the martian surface (https://www.emiratesmarsmission.ae/). the hope probe will investigate various aspects of the martian atmosphere for the first time in order to provide a complete compositional picture using cuttingedge instruments such as the emirates mars infrared spectrometer (emirs), emirate exploration imager (exi), and emirates mars ultraviolet spectrometer (emus). furthermore, the china national space agency (cnsa) launched china’s first interplanetary mission, the tianwen-1 robotic spacecraft to mars, on july 23, 2020, which successfully entered the martian orbit on february 10, 2021 to examine the target site for a rover landing on may 14, 2021. this rover’s scientific goals include studying martian geology with a focus on the planet’s interior structure; identifying minerals and rocks; examining the atmosphere; and looking for evidence of water using 13 scientific equipment (https://en.wikipedia.org/wiki/tianwen-1). as a result of these missions, the number of international collaborative research publications (i.e., mcp) on lomars and related themes could increase exponentially in coming years. international collaborations have lately been proven to be ineffective unless an underdeveloped partner has some local research capabilities to benefit from the developed counterpart (scarazzati and wang, 2019). collaborative research is typically based on partners exchanging knowledge, skills, and techniques, resulting in economic progress and growth (luukkonen et al., 1992; katz and martin, 1997). in contrast, partners with more advanced scientific fields do not seek the same kinds of collaboration as partners with less advanced fields (scarazzati and wang, 2019). according to reports, the planetary and space sciences (pss) potential in africa has largely remained untapped due to the common misconception in african countries that funding fundamental science costs society more than it benefits (baratoux et al., 2017). despite the emergence of pss in africa through the launch of several pss projects for regional 226 arshad ali, muhammad s. shahid, iffat jabeen, mohamed a.k. el-ghali research capacity building, research groups remain dispersed and underfunded. the publication survey (2000-2015) reveals that africa has published less than 1% of the worldwide output of scientific articles published in the four pss representative journals (viz. icarus, journal of geophysical research: planets, journal of geophysical research: space physics, and meteoritics & planetary science), despite africa making up more than 15% of the world’s population (baratoux et al., 2017). however, there have been numerous pss advancements across africa that show promise for the future and the growth of pss-related basic science. for instance, both the south african national space agency and south african astronomical observatory are active in the pss sector. the largest radio telescope in the world, the square kilometre array (ska), will soon be established in the australian and south african deserts (baratoux et al., 2017). morocco has become well-known for various meteorite discoveries and impact studies (chennaoui aoudjehane et al., 2016), attracting international scientists through conferences and outreach activities (e.g., space bus morocco). furthermore, the ibn battuta center and the first astronomical observatory in marrakech have been established to test instruments related to martian exploration and meteorite impacts on the moon (ait moulay larbi et al. 2015), as well as the search for exoplanets (benkhaldoun et al., 2005; gillom et al., 2017). algeria is also focusing on meteorites and impact craters with the support of international collaboration (lamali et al., 2016; sahoui et al., 2016). since 2015, the arabian geosciences union has launched a special section on planetary science and astrobiology for pss scientists across northern africa and the middle east. the sultanate of oman, which has been the site of numerous meteorite discoveries (ali et al. 2017a, 2017b), including martian meteorites like dhofar 019, sayh al uhaymir (sau) 005, and sau 008 (ali et al. 2016, 2017c, 2018), can act as a catalyst for regional collaboration to promote pss. the most collaborative author on lomars research (c.p. mckay; figure 5) has recently discussed five prerequisites for detecting life on other bodies in space, including mars (mckay, 2020). this author identified the following compelling evidence to be targeted in the pursuit of the search for life beyond planet earth: i) signs of liquid water of suitable salinity, ii) carbon in the water, iii) biologically available nitrogen in the water, iv) biologically useful energy in the water, and v) organic material that can possibly be of biological origin and an applicable sampling strategy for this material. figure 5. co-authorship network of the top authors with a minimum of five collaborative publications on lomars research. analysis of productive journals according to bradford’s law (the estimation of exponentially decreasing returns from searching for references in scientific journals), 162 journals have been classified into three categories (representing 4, 16, and 142, respectively), with roughly one-third of all the publications (i.e., 229, 215, and 216) in each category (figure 6a). icarus, astrobiology, planetary and space science, and advances in space research are the four core journals in zone 1. these journals jointly published 229 papers (figure 6a), which were cited 3698 times (figure 6b). the total number of citations in zones 1, 2, and 3 is distributed in increasing order as 3698 < 5302 < 13239, respectively (figure 5b). however, the average number of citations per journal in each category is in the order of 925> 331> 93, respectively. figure 6. (a) distribution of scientific journals according to the bradford’s law distribution of journals in categories; number of publications in each category; and (b) total citation share of each category. note that core journals in zone 1 (icarus, astrobiology, planetary and space science, advances in space research) published 34.7% of the total articles. table 4. the ranking of top-15 most productive journals for lomars research based on the number of publications (np). other parameters, including percent publications (%p), total citations (tc), and h-, g-, and m-indices are given for reference. source np %p tc h -index g -index m -index icarus 75 11.4 3660 34 59 0.68 astrobiology 73 11.1 1923 27 41 1.35 planetary and space science 44 6.7 1087 19 32 0.42 advances in space research 37 5.6 651 15 24 0.43 journal of geophysical research e: planets 35 5.3 1811 24 35 0.96 international journal of astrobiology 29 4.4 465 14 20 0.74 science 22 3.3 2869 16 22 0.27 geophysical research letters 20 3.0 513 14 20 0.64 nature 16 2.4 1832 13 16 0.22 earth and planetary science letters 14 2.1 655 10 14 0.63 geochimica et cosmochimica acta 12 1.8 431 10 12 0.42 geology 10 1.5 398 10 10 0.53 acta astronautica 10 1.5 74 6 8 0.14 origins of life and evolution of the biosphere 9 1.4 497 8 9 0.22 meteoritics and planetary science 8 1.2 274 6 8 0.26 227life on mars (lomars): history, advances, current research, and perspectives among the journals listed in table 4, icarus is one of the oldest, following science and nature. the top-15 journals have published 62.7% of all lomars research articles. icarus (11.4%) is the most productive journal, just ahead of astrobiology (11.1%). the journal icarus leads in all categories, including the number of publications (np), citations (ct), the h-, and g-indexes, but not the m-index (h-index divided by the number of years since it published its first paper on lomars), because it began publishing on lomars in 1971 (table 4). based on the total number of publications related to the lomars research, the top ten lists of journals, institutes, and authors are compared in a bar diagram (figure 7). the number of articles associated with the journals, institutes, and authors ranges from 14 to 75, 17-55, and 10-29, respectively. there is a close competition between the top two spots for icarus (75) and astrobiology (73), as is seen at the california institute of technology (55) and nasa ames research center (51). however, cp mckay (29) is leading in front of f westall (16) by a substantial margin. therefore, icarus (75), the california institute of technology, and c.p. mckay have been recognized as the most influential journal, institute, and author, respectively (figure 7). elsevier’s journal icarus, with a 2021 impact factor of 3.508 (clarivate analytics, 2022), is primarily focused on the publication of original contributions in the field of solar system studies. caltech established the jet propulsion laboratory (jpl) in the 1930s, and nasa has managed it since 1958. c.p. mckay, the highly influential author of lomars research, is a senior scientist at nasa. keywords analysis according to figure 8, the top five keywords (mars, astrobiology, exobiology, mars’ surface, life on mars) are directly related to mars and the study of martian life. the mars 2020 rover was specifically designed to study past and present life on mars, and future sample return missions are critical to understanding lomars. so far, meteorites have been investigated for this purpose, but because of their journey under severe conditions, some physical changes occur to their surface (i.e., fusion crust) due to frictional heating followed by possible terrestrial contamination. thus, pristine samples returning from mars’ surface and subsurface are predicted to introduce keywords such as surface, mineralogy, carbonates, habitability, and climate in the future (figure 8). figure 7. the top ten highly influential journals, institutes, and authors based on the total number of publications in lomars research. figure 8. co-occurrence network of the most commonly used author keywords in lomars research. 4. conclusions the search for life on other planets, particularly mars, has been going on since the mid-twentieth century, with articles published in prestigious scientific journals beginning in the early 1960s. based on bibliometric analysis using elsevier’s scopus database, 660 articles on life on mars (lomars) research have been published by over 2000 authors and have been cited over 22,000 times, averaging 33.7 citations and 3 authors per article. furthermore, lists of highly cited authors, most productive authors, institutes, and journals are 228 arshad ali, muhammad s. shahid, iffat jabeen, mohamed a.k. el-ghali identified based on citations and number of publications. according to these lists, c.p. mckay, caltech, and icarus emerged as the most productive author, institute, and journal, respectively. the highest ranked author and institute are both affiliated with nasa, demonstrating the organization’s dominance, which, however, is challenged by the recent launches of missions to mars by organizations from other nations, particularly, the china national space agency (cnsa). using multiple nation co-authorship as an indicator of collaborative research, the united states has dominated the world to date due to the availability of infrastructure and human scientific resources. recent collaborative initiatives among asian and african nations have broadened the field of planetary and space sciences. the rate of scientific publications has been found to be closely linked with nasa’s launch of various rovers. we used this tendency as a proxy to predict that future lomars research will grow following the recent landings of nasa’s mars 2020 perseverance rover and china’s zhurong rover. parallel missions launched by the uae space agency in partnership with jaxa would perpetuate this trend. the findings of observational and theoretical research are reported in majority of the top-cited papers. however, the highly cited paper by d.s. mckay et al. (1996) is based on analytical studies of a unique martian meteorite (i.e., allan hills 84001), implying that the future sample return mission associated with the perseverance rover could potentially enhance lomars research. the study of pristine samples from mars’ surface and subsurface would influence the keywords selection seen in previous papers. acknowledgments the authors would like to thank dr. nisar ahmed from sultan qaboos university for help in bibliometric analysis in this study. we are indebted to editor-in-chief dr. alexander caneva for his editorial handling and three anonymous reviewers for their critical and constructive comments to improve the quality of this contribution. references ait moulay larbi, m., daassou, a., baratoux, d., bouley, s., benkhaldoun, z., lazrek, m., garcia, r., & colas, f. 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(2019). a spectral multidomain penalty method solver for the numerical simulation of granular avalanches. earth sciences research journal, 23(4), 317329. doi: https://doi.org/10.15446/esrj. v23n4.77683 this work presents a high-order element-based numerical simulation of an experimental granular avalanche, in order to assess the potential of these spectral techniques to handle geophysical conservation laws. the spatial discretization of these equations was developed via the spectral multidomain penalty method (smpm). the temporal terms were discretized using a strong-stability preserving runge-kutta method. stability of the numerical scheme is ensured with the use of a spectral filter and a constant or regularized lateral earth pressure coefficient. the test case is a granular avalanche that is generated in a small-scale rectangular flume with a topographical gradient. a grid independence test was performed to clarify the order of the error in the mass conservation produced by the treatments here implemented. the numerical predictions of the granular avalanches are compared with experimental measurements performed by denlinger and iverson (2001). furthermore, the boundary conditions and parameters such as lateral earth pressure coefficients and the momentum correction factor were analyzed in order to observe the incidence of these features when solving the granular flow equations. this work identifies the benefits and weaknesses of the smpm to solve this set of equations, and thus, it is possible to conclude that the smpm provides an appropriate solution to the granular flow equations proposed by iverson and denlinger (2001) and comparable predictions for the experimental data. abstract a spectral multidomain penalty method solver for the numerical simulation of granular avalanches método penalizado de multidominios espectrales para la simulación numérica de avalanchas granulares issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n4.77683 este trabajo presenta la simulación numérica de una avalancha granular a escala experimental basada en elementos de alto orden, con el fin de evaluar el potencial de estas técnicas espectrales para resolver las leyes de conservación en geofísica. la discretización espacial de estas ecuaciones se desarrolló a través del métodos penalizado de multidominios espectrales (smpm). los términos temporales se discretizaron utilizando un método runge-kutta de preservación de estabilidad fuerte. la estabilidad del esquema numérico se garantiza con el uso de un filtro espectral y un coeficiente de presión lateral de tierras constante o regularizado. el caso de prueba es una avalancha granular que se genera en un canal rectangular de pequeña escala con gradiente topográfico. se realizó una prueba de independencia de malla para aclarar el orden del error en la conservación de la masa producida por los tratamientos previamente mencionados. las predicciones numéricas se comparan con las mediciones experimentales realizadas por denlinger & iverson (2001). además, se analizaron las condiciones de contorno, y los parámetros como el coeficiente de presión lateral de tierras y el factor de corrección de momentum para observar su incidencia al resolver las ecuaciones del flujo granular. este trabajo identifica las bondades y debilidades del smpm para resolver este conjunto de ecuaciones y es posible concluir que el smpm proporciona una solución adecuada de las ecuaciones propuestas por iverson & denlinger (2001). además, las predicciones son comparables a los datos experimentales y resultados numéricos dados por otros esquemas. resumen record manuscript received: 05/02/2019 accepted for publication: 20/09/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 4 (december, 2019): 317-329 mario germán trujillo-vela1, jorge alberto escobar-vargas1, alfonso mariano ramos-cañón2 1departamento de ingeniería civil, pontificia universidad javeriana, bogotá, colombia. 2departamento de ingeniería civil y agrícola, universidad nacional de colombia, bogotá, colombia. * corresponding author: mario.trujillo@javeriana.edu.co g eo ph ys ic s 318 mario germán trujillo-vela, jorge alberto escobar-vargas, alfonso mariano ramos-cañón nomenclature h (m) depth of the flow. u (m/s) vector velocity. u v, (m/s) average velocities. m mx y,� conservative variables. q vector of conservative variables. f q g q( ) ( ), fluxes of the conservative variables. s q( ) vector of source terms. s sx y, source terms. ho (m) average depth.  (m) tolerance value  (m) displacement of the free surface.  ratio of the basal pore fluid pressure. pbed (kg/m s3) total basal normal stress. k x y a p, /( ) lateral earth pressure coefficient for x and y directions. ka p/ lateral earth pressure coefficient. knat constant lateral earth pressure coefficient.  steepness of the transition between active and passive coefficients. gx, gy, gz (m/s2) gravitational components.  momentum correction factor. b (x) (m) topographical basis in function of x. hi (x) (m) depth of material as initial conditions. m0 , m1, m2, (m) steepness of the topographical and initial conditions. xl, xc, xr , (m) left, central and right points in the x axis. rx (m) radius of curvature. x , x (°) components of local angle. n f fraction of the fluid.  (pa-s) dynamic viscosity of the fluid.  (kg/m3) density of the mixture. f (kg/m3) density of the fluid. s (kg/m3) density of the soil. bed (°) basal friction angle. int (°) internal friction angle. cfl courant number. t time-step. u magnitude of the velocity in a cell. v (m3) total volume of the moving mass. s grid spacing. m mass of the system. m0, mt (m 3) corresponding values for the mass at the initial and final times of the simulation.  computational domain. rm relative error of the mass. xc exp( ) position of the centroid of the experimental moving mass. uc exp( ) velocity in the x-direction of the experimental centroid of the moving mass. introduction the study of granular avalanches provides a basis for understanding a variety of mass movements, such as rock avalanches, snow avalanches, debris flows and pyroclastic flows denlinger and iverson (2001). some authors (denlinger and iverson, 2001, savage and hutter, 1989, gray et al., 1999, wang et al., 2004, pastor et al., 2015, koch et al., 1994) have made efforts to perform laboratory experiments and numerical simulations of granular avalanches. the continuum mechanics framework supplies a helpful theoretical standpoint to represent multiple types of flows by using the laws of conservation of mass and momentum. a reduced form to use the conservation laws applied to granular flows is by means of a shallow water approach. the bidimensional equations that describe the movement of a granular mass taking into account the stresses, caused by either the grains or the fluid. the granular flow equations are prone to develop discontinuities due to its nonlinear nature. often, the solution to these equations can be obtained by mean of numerical methods such as finite differences, finite volumes, and finite elements, known as low-order techniques. results using methods of low order are reported by gray et al., (1999), hutter and koch, (1991), hutter and greve, (1993), hutter et al., (2005), ouyang et al., (2013), savage and hutter, (1989), tai et al., (2002), wieland et al., (1999), denlinger and iverson, (2001), denlinger and iverson, (2004), koschdon and schafer, (2003), vollmoller, (2004), wang et al., (2004), where the numerical solution shows a similar structure when compared with experimental data. however, the techniques mentioned above present significant numerical error, such as artificial diffusion and spurious oscillations that affect the stability, accuracy, and conservative properties of the solution as is shown by fletcher, (1991). in fact, diamessis and redekopp, (2006) mentioned that numerical stability resulting from low-order finite-difference schemes is caused by the well-known inherent artificial dissipation, which might generate a significant error in the longtime integration. this issue has been handled with a series of robust methods known as shock-capturing schemes such as eno, weno, noc, tvd, hllc, among others (denlinger and iverson, 2001, tai et al., 2002, wang et al., 2004, toro, 2013). however, it is still necessary to perform significant efforts for developing numerical techniques that allow solving the systems of equations as well as possible to describe these natural phenomena (castro-orgaz et al., 2015, iverson, 2014). in order to minimize the error associated with numerical diffusion generated by using low-order techniques, researchers as hesthaven and gottlieb, (1996), reed and hill, (1973), diamessis and redekopp, (2006), giraldo and warburton, (2008) developed element-based high-order discontinuous techniques. benchmark cases with an exact solution were used to evaluate the accuracy and convergence of spectral methods by solving the inviscid shallow water equations (escobar-vargas et al., 2012). based on the evidence from the high-order precision of these methods o(10-11), the potential of an element-based spectral method for solving a set of equations that describe the movement and deposition of granular avalanches is evaluated, taking into account its lack of numerical diffusion. this work was performed with the aim of test the potential of high-order methods for more applied cases since these techniques have been implemented in idealized cases. the numerical method implemented is based on a collocation approach of multiple non-overlapping quadrilateral subdomains, which are connected by a penalty term that ensures the stability of the solution by imposing a weak continuity at the subdomain interfaces (see appendix a). this technique is called spectral multidomain penalty method, henceforth smpm (hesthaven and gottlieb, 1996, escobar-vargas et al., 2012). in order to maintain the stability of the spatial discretization technique, a strong-stability preserving runge-kutta was implemented for the temporal derivative (appendix b), simultaneously, a spectral filter was imposed to control the numerical oscillations (appendix c). the potentiality of the smpm is evaluated in two ways. the first one is by means of a grid independence test that shows the error in mass conservation 319a spectral multidomain penalty method solver for the numerical simulation of granular avalanches expression that is a function of hydraulic diffusivity d and the initial pore pressure ratio  such as is shown in iverson and denlinger, (2001). ka p/ is the lateral earth pressure coefficient; and  is the momentum correction factor that comes from the integration of the advective terms of the navier-stokes equations (zhou, 2004). the structure of equations (2-5) is similar to the one of inviscid shallow water equations, where the source terms sx and sy take into account the stresses generated by fluid and solids. details of these features are shown in equations 6 and 7. s g h sgn u g u x h n u h n h x x z x bed f f = − ( ) −( ) + ∂ ∂     − + ∂ 1 32 2 λ θ φ µ ρ µ ρ tan uu x∂ 2 − ∂ ∂     ∂ ∂ −( )  + ∂ ∂ sgn u y hk y g h n h u y a p z int f / sin1 2 2λ φ µ ρ (6) s g h sgn v g v y h n v h n h y y z y bed f f = − ( ) −( ) + ∂ ∂     − + ∂ 1 32 2 λ θ φ µ ρ µ ρ tan vv y∂ 2 − ∂ ∂     ∂ ∂ −( )  + ∂ ∂ sgn v x hk x g h n h v x a p z int f / sin1 2 2λ φ µ ρ (7) where int is the internal friction angle of displaced materials, bed is the friction angle of bed material, n f is the fluid volume fraction (i.e., the porosity),  is dynamic viscosity of the fluid, and x, y are the components of local angle on the bed in the x and y directions, which are described in section 3.1. the lateral earth pressure coefficient (ka p/ ) that appears on equations (3-7) can be represented by k cos a p int bed int / / cos tan �= ± − +( )    −2 1 1 1 1 2 2 1 2 2    (8) this coefficient associates the horizontal and vertical stresses (savage and hutter, 1989; iverson and denlinger, 2001). equation 8 shows that ka p/ has a discontinuous nature, indicated by the plus/minus sign, which jumps from active to passive states (i.e., ∂ ∂ >u x/ 0 or ∂ ∂ <u x/ 0 for the x direction and ∂ ∂ >v y/ 0 or ∂ ∂ <v y/ 0 for the y direction). this feature makes the solution to be prone to become unstable. a better formulation of this coefficient is given by a regularized version of equation 8 [tai and gray, 1998]. the discontinuity is avoided by using the following expression: k k k a k kx y a p a p a p, / / /( ) = +( ) + −( )2 2 (9) where a is assumed to be a monotonically decreasing functions (equation 10), which depends of the gradient of the velocity. a = ∂ ∂( ) − ∂ ∂( )  + ∂ ∂( ) − ∂ ∂( )    u x u x u x u x / / / / 0 0 2 1 (10) in equation 10,  determines the steepness of the transition between active and passive coefficient, and ∂ ∂u x/ 0 is chosen as the center of the transition (i.e., when ∂ ∂ =u x/ 0) (tai and gray, 1998). this regularized lateral earth pressure coefficient is applied in both the x and y directions and yields a more stable solution (see section 5.2). another version of the lateral earth pressure coefficient was implemented in tai et al., (1999), gray et al., (1999),tai et al., (2001), wieland et al., (1999), gray, (2003), where the relationship between vertical and horizontal stresses in the y direction is a function of the lateral earth pressure coefficient in the x direction. introduced by the numerical scheme with the spatial refinement. the grid independence test shows that the error in mass conservation does not improve as much as is reported in the literature for the case of a granular avalanche. the second means of evaluating the smpm involves comparing the numerical simulations and the experimental measurements performed by denlinger and iverson, (2001). the comparison shows that the smpm provides comparable results for depths of the flow concerning the experimental measures and other numerical techniques. finally, a discussion of boundary conditions, lateral earth pressure coefficients and the momentum correction factor were raised due to its incidence when solving the granular flow equations. the paper is organized as follows. in section 2, the granular flow equations are presented. in section 3, the numerical setup for the case of study, such as initial and boundary conditions, and treatments are described for zones with or without material. the analysis of a grid independence test is performed in section 4; and in section 5, the comparison between experimental and numerical simulation is performed where boundary conditions, lateral earth pressure coefficients and momentum correction factor are analyzed. a discussion and conclusions are presented in section 6 and 7, respectively. in appendices a, b, and c we present the numerical techniques with which the equations are discretized and stabilized. governing equations of the granular flow many granular avalanche models have been proposed based on the 2d shallow water approach as shown by savage and hutter, (1989), iverson and denlinger, (2001), denlinger and iverson, (2004), hutter et al., (2005), wang et al., (2004), where the main differences are focused on evaluating some features of the stresses. in this sense, the model proposed by iverson and denlinger, (2001) was selected as a sample in order to evaluate the potential of the smpm for solving granular flow equations (see appendix a). the temporal derivative is handled by a strong-stability preserving runge-kutta (see appendix b). the model can be presented in conservative form as ∂ ∂ + ∂ ( ) ∂ + ∂ ( ) ∂ = ( )q f q g q s q t x y � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � (1) where q denotes the conservative variables vector. f(q) and g(q) represents the horizontal fluxes in the x and y direction, respectively, and s(q) is the vector that contains the source terms, which dissipate the energy (escobarvargas et al., 2012). by decomposing this system of equations we have q =           =         h m m q q q x y 1 2 3 � (2) f q( ) = + −( ) +              = m m h k g h m m h fx x a p z x y β λ λ β 2 2 1 1 2 1/ / / / ff f 2 3         � (3) g q( ) = + −( ) +              = m m m h m h k g h gx x y y a p z β β λ λ / / / / 2 2 1 1 2 1 gg g 2 3         � (4) s q( ) =           0 s s x y (5) in this set of equations, h is the flow depth, m hux = and m hvy = , where u and v are the averaged velocities in x and y directions. g gx = sin , gy = 0 and g gz = cos  are the gravitational acceleration components in x, y and z directions, respectively, g m s= 9 81 2. / is the gravitational constant and  is the inclination angle; λ ρ= p g hbed z/ is the ratio of the basal pore fluid pressure to the total normal basal stress.  is evaluated from an analytical summation 320 mario germán trujillo-vela, jorge alberto escobar-vargas, alfonso mariano ramos-cañón in addition, a constant lateral earth pressure coefficient (knat) was employed assuming that internal and bed friction angles are equal ( int bed�= ). the main importance of implementing this constant coefficient is to perform the evaluation of other parameters in a more controlled manner by avoiding the discontinuous nature of equation 8 or even the softened function 10, by using equation 11 (iverson and denlinger, 2001). knat int int = + − 1 1 2 2 sin sin   (11) the set of equations 1 5 belongs to a family of models in which one set of equations is employed, and the stresses are split into two components, solid and fluid. this characteristic makes a significant difference since it avoids the difficulties from a real two-phase model, where a set of equations is needed for each phase and its coupling (pudasaini, 2012). the fraction of the fluid is a constant parameter, meaning that the proportion of fluid is homogeneous, and it does not vary in time. the role of the fluid fraction in the configuration used here (i.e., granular ow) seems to be irrelevant since the dynamic viscosity of the air is a small value (i.e., 2x10-5). hence, stresses produced by the fluid are weak regarding the stresses caused by the solids. granular flow case geometrical basis and initial conditions of the granular flow denlinger and iverson (2001) presented a small-scale experiment of a dry granular avalanche. they built a 1.2 m long rectangular flume with an inclined bed at 31.4° with respect to the horizontal surface. there is a curved section with a 10 cm radius of curvature between the inclined and the horizontal surface (figure 1). dry quartz sand with a grain diameter of 0.5 mm was used. the initial setup of the sand is wedge-shaped and is located behind a vertical gate positioned at a 62.5 cm slope down from the top of the flume, which is discharged suddenly when the gate opens (figure 1a) (denlinger and iverson, 2001). the flume and gate width is 20 cm. the geometry of the bed varies only in the x direction. this variation can be written as b x m x x b x x r x x r x l x l x r x l r ( ) = −( ) + ≤ ≤ − − −( ) + 0 2 2 0 ,� ≤≤ ≤ ≥    x x x x r r0,�     (12) based on equation 12, the geometrical configuration of the flume is split into three parts: the slope (i.e., 0 ≤ ≤x xl), the curved zone (i.e.,), and the horizontal zone (i.e, x x xl r≤ ≤ ) (figure 1a). in equation 12, b(x) describes the elevation of the flume, bx, is the maximum height of the curved zone, at left side of the curved part, xl is any point of x, on the curved zone (rx), xr is any point of x on rx, and m0 is the slope of the inclined surface. it is possible to obtain the slope angle from the geometrical configuration of the basis (i.e., x b x= ∂ ∂( )−tan /1 ) and the curvature (i.e., ∂ ∂x x/ ) that appears in equations 6 and 7. the initial condition was built as a wedge (figure 1), which is described by the following equation: h x x x m x x i l l( ) ≤ ≤ −( ) 0 0 1 , , x x x m x x x x l r l r ≤ ≤ −( ) ≥2 0 , , x xr≥        �� (13) the initial condition (hi) is split into three points, left (xl), central (xc) and right point (xr). the wedge that represents the initial condition is constructed between the left and right points. m1 and m2 are the slope of each side of the triangle. table 1 shows the properties of the materials used in the numerical simulation presented in this work (denlinger and iverson, 2001). a) b) c) 0.6 0.4 0.2 initial condition xl xc xr 0.2 0.2 0.2 0.2 0.2 0.1 0.4 0.4 0.40.5 0.5 0.6 0.6 0.6 0.8 0.8 0.8 0.10 0 0 0 0 0 0 y (m) y (m)y (m) x (m) x (m)x (m) 1 11 1 1  (m ) h (m ) 0.2 0.1 0 0 h (m ) figure 1. initial conditions employed in this work. a) 3d view of the topographic basis of an inclined and curvilinear plane, with a wedge-shaped initial condition. additional initial conditions, b) cylinder and c) gaussian bump of sand. 321a spectral multidomain penalty method solver for the numerical simulation of granular avalanches table 1. value of the properties of the materials employed in the numerical modeling. taken from (denlinger and iverson, 2001). parameter unit value basal friction angle (bed ) degrees 29 ± 1.4 internal friction angle (int ) degrees 40 ± 1.0 solid volume fraction (ns) 0.6 fluid volume fraction (n f ) 0.4 fluid viscosity () pa-s 2x10-5 solid density (s) kg/m3 2650 fluid density (f) kg/m3 1 (air) mixture bulk density () kg/m3 1600 hydraulic permeability (d) m2/s 0.05 initial pore pressure ratio () 0.0 boundary conditions an appropriate implementation of boundary conditions is one of the most important aspects to obtain a robust numerical approximation of any partial differential equation. the relevance of boundary conditions is presented in section 5.1, where it is shown that different solutions are obtained by using different treatments on boundaries. in order to analyze the influence of the boundary conditions in the solution, two types of boundary conditions were implemented and are presented below. figure 2 shows the representation of two boundary conditions. reflective boundary conditions are imposed in both cases, on boundaries perpendicular to the flow direction (i.e., the velocity vector is u n· = 0, where u = ( )u v, ). in this case, the right boundary must be far enough from the mass movement to avoid any influence in the dynamic of the numerical experiment. thus the mass can stop by the frictional force solely and not by the effect of an obstacle. also, slip boundary conditions were imposed on boundaries parallel to the flow direction, u n· = 0 (figure 2a). in the second case, “non-slip” boundary conditions were implemented on boundaries parallel to the flow direction, which consists of imposing a velocity value equal to zero, u = 0 (figure2b). flow direction flow direction (a) (b) figure 2. reflective boundary conditions, (a) slip and (b) non-slip in the computational domain. the tracking of the free surface to solve the granular avalanche equations (equation 1), it is necessary to define a method to treat regions without material (i.e., h = b). shallow water equations are defined by a height such that h ho= + , where h0 is an average depth and  is the displacement of the free surface (kundu and cohen, 2008). from solving the system of equations, the velocities u and v are computed by means of conservative variables with equation 14. u m h v m h hx y= = ≠/ , / , � 0 (14) the use of average depth h0 allows the appearance of undesired oscillations below the average height of the flow that cannot appear in dry zones (h = b), when a rigid bed exists. in this work, the average depth is h0 = 0 for the granular flow equations, then h = . in areas without material (i.e., h = 0) the solution of the set of equations in the conservative form generates a mathematical indetermination (i.e., division by zero to compute u and v , see equation 14). in order to avoid this problem, a tolerance value was defined (e.g., γ = −10 4). if h < γ , a trivial solution is imposed (i.e., u v= = 0) (bradford and sanders, 2002; pudasaini and hutter, 2007). otherwise, if h > γ, the velocities are computed without imposing any restriction (pudasaini and hutter, 2007). stability condition and mass conservation the stability condition for the smpm is given by the courant number, which is defined by giraldo and warburton (2008) as follow: cfl t u g s = +( )        max ∆ ∆ h (15) where t is time step size, u u v= +2 2 is the magnitude of average velocities into the cell, gh is average geopotential height into each cell and ∆ ∆ ∆s x y= +2 2 is the grid spacing. the sum of u gh+ is known as the characteristic velocity at which the wave travels (leveque, 2002). for smpm, the maximum number of courant that ensures the stability is cfl 1 (escobarvargas et al., 2012). conservation of mass was used as the main criterion by which to evaluate the conservative properties of the method. the error introduced by the numerical method was calculated with respect to the initial mass. the mass of the system can be calculated as m hd= ∫ ω ω � (16) where  represents the domain. the metric to evaluate mass conservation is based on the relative error, defined with respect to the initial values of mass as r m m mm t= −( )0 0 (17) in equation 17, rm is the relative error of the mass, and m0, mt are the corresponding values for the mass at the initial time (t = 0) and final time (t) of the simulation. thus, the loss or gain of mass is specified at each time step. the stopping criterion was focused on the experimental data since the last measurement available to compare is at 1.5 seconds. in addition, the variation of the position of the centroid of the mass and its velocity were kept below o(10-2) as a secondary criterion. by another hand, the error in energy conservation was not verified in this work because the source terms in the momentum equations are responsible for the energy dissipation. grid independence test the grid independence test was performed to determine the optimal degree of grid refinement at which the solution does not change when the mesh is reduced. the error in mass conservation (equation 17) was chosen as a reference quantity to assess the differences of the solutions among multiple grids. the mesh refinement was handled with two different strategies. the first option is developed by dividing the domain into multiple subdomains; this is called h-refinement. the second option keeps the numbers of subdomains fixed while increasing the polynomial degree; this is called p-refinement (boyd, 2001). figure 3 shows the results of error in mass conservation for the test case with the two types of boundary conditions (slip and non-slip) described in section 3.2. this was performed in order to observe whether the error is increased with respect to the slip boundary conditions when the non-slip 322 mario germán trujillo-vela, jorge alberto escobar-vargas, alfonso mariano ramos-cañón boundary conditions are imposed. the simulation was performed for 9 cases of h-refinement, where the subdomains were kept square (figure 3), and 19 cases of p-refinement (i.e., 2  n  20). it is possible to observe that the error in mass conservation does not improve when the refinement of the mesh is increased. it is noticeable that the non-monotonicity of the grid independence test disappears when p-refinement is greater than 16th degree, independently of the h-refinement for both boundary conditions. the non-monotonic behavior presented in the grid independence test makes more difficult the choice of an optimal spatial discretization. in addition, the error does not significantly decrease, as is reported with benchmark cases (escobar-vargas et al., 2012). in addition, the implementation of the collapse of a cylinder and a gaussian bump of sand for two different average depths (h0 = 1 and h0 = 0) (figure 1b and 1c, respectively) were performed. considering the gaussian bump (figure 4a), it was obtained a very low numerical error o(10-9) both with and without average depth, whereas the cylinder generates a greater numerical error (figure 4b). the tracking of the free surface provides an additional error caused by the zero-average depth (h0 = 0). otherwise, an average depth greater than zero (h0 = 1) helps to give a lower error. if the value of the tolerance for the ʽʽwet/dry’’ condition is 10-4, the numbers after the fifth decimal place are neglected when the velocities are computed. these velocities are inputs for the next time-step, which affects the accuracy. although it is natural that hyperbolic equations develop sharp fronts, with these set up cases it was possible to observe that discontinuities might not mature. this is because of the physics of the modeled phenomena included by means of the source terms since these can be stopped soon. comparison with experimental data boundary conditions analysis figure 5 presents the comparison between the experimental data (di2001) (denlinger and iverson, 2001) and the numerical prediction given by the smpm with a grid that showed the lower error in mass conservation obtained from the grid independence test (figure 3). also, slip boundary conditions and the constant lateral earth pressure coefficient were imposed. the numerical prediction is akin to the experiment, particularly in the deposit of the granular flow. the maximum heights of the deposit given by the numerical prediction are approximated to the maximum heights of the experiment with differences of 2 mm, approximately. however, the movement of the material is not well captured, because the “adhered” material on the lateral walls does not appear in the numerical prediction as was observed in the experiment. figure 6 presents the comparison between the experimental data (di2001) and the numerical prediction given by the smpm with non-slip boundary conditions and the constant lateral earth pressure coefficient. nonslip boundary conditions allow us to represent the tail of the granular flow while the granular mass is going downhill. the “adhesion” of the material on the lateral walls given by the numerical prediction is similar to the experimental data (the tail of the flow). however, a deposit zone is generated caused by these boundary conditions (the tail of the flow is still present), which is not the case in the experiment. a) b) figure 3. mass conservation error with a) slip and b) non-slip boundary conditions. (subx and suby represents the subdomain numbers in x and y directions, respectively). b)a) figure 4. mass conservation error in function of time with and without average depth and without a topographic gradient, a) gaussian bump and b) cylinder. 323a spectral multidomain penalty method solver for the numerical simulation of granular avalanches denlinger and iverson, (2001) solved this set of equations by means of a method called harten-lax-van leer-contact (hllc) (toro, 2013). a quantitative comparison among the numerical methods with respect to the experimental data is presented for five different stages (table 2). the smpm allows us to get a comparable solution with the hllc method, since the error in mass conservation has the same order of magnitude for the case of study, with respect to the experimental data. it is possible to see from the table 2 that the smpm has generated less differences with respect to the experiment in the first three stages (i.e., while the material is descending the inclined plane), whereas the hllc method produced better results in the last two stages (i.e., when the material is forming the deposit, and finally is stopped). a) b)experiment (d -i2001) numerical prediction (smpm) figure 5. a) granular flow experiment. b) numerical prediction given by smpm for spatial refinement with low numerical error (n = 8, subx = 108 and suby = 18), slip boundary conditions, and constant lateral earth pressure coefficient. a) b)experiment (d -i2001) numerical prediction (smpm) figure 6. a) granular flow experiment. b) numerical prediction given by smpm for spatial refinement with low numerical error (n = 8, subx = 108 and suby = 18), non-slip boundary conditions, and constant lateral earth pressure coefficient. 324 mario germán trujillo-vela, jorge alberto escobar-vargas, alfonso mariano ramos-cañón table 2. euclidean norm of the error for slip and nonslip smpm, and for hllc, regarding the experimental data. time (s) smpmslip smpmnonslip hllc 0.01 0.2227 0.2197 0.4428 0.32 0.1634 0.1583 0.1796 0.53 0.1694 0.1733 0.2108 0.93 0.5699 0.6237 0.4904 1.50 0.4761 0.5739 0.4157 another comparison between numerical results and experimental data is presented in figure 7. the continues and discontinuous lines represent two ways to compute the centroid of the mass and the centroid of the velocity field, such as is shown in pudasaini and hutter, (2007, p. 149). the asterisk represents the centroid of the experiment computed from the cloud of points as x x c exp c exp c exp ( ) ( ) ( )= ∑ ∑ = h x h u t i i i , ∆ ∆ (18) it is possible to observe that the numerical prediction matched in a good way the position of the center of the moving mass computed from the experimental data, however, the centroid of the velocity field shows difficulties that the prediction fits the experiment. figure 7. comparison between numerical results and experimental data of the centroid of the moving mass and the velocity field. a) the position of the centroid of the descending mass at each time step. b) centroid of the velocity field. v is referring to the total volume. analysis of lateral earth pressure coefficient (ka p/ ) the lateral earth pressure coefficient (k x y a p, /( ) ) relates the vertical stresses with the horizontal ones of the granular flow. figures 5b and 6b show the solution generated with a constant knat (equation 11), whereas figure 8b presents the solution given by the slip boundary conditions and a regularized k x y a p, /( ) (equations 9-10). this specific coefficient tends to produce asymmetric solutions caused by nonphysical oscillations, and hence, steep localized gradients from the velocity can be produced. since the derivative of the velocity is an input of the lateral earth pressure coefficient, this has repercussions in the behavior of this coefficient and to the next time step. these oscillations can be mitigated either by decreasing the order of the polynomial or by handling the spectral filter (appendix c). the use of a regularized k x y a p, /( ) allows us to represent in a better approach the physics of the processes that occur in these types of flows, by means of the smooth connection between the two states of stress (dilation or compression of the material). by another hand, the use of a constant knat indicates that neither dilation nor compression zones are presented, but this behavior does not occur in nature. for example, experimental data presented by pudasaini and hutter, (2007) showed that velocities can change from one point to another. however, a constant knat was used because it makes it possible to obtain a more stable solution, whereas with a regularized k x y a p, /( ) the solution is more likely to be unstable, even more with the discontinuous coefficient k x y a p, /( ) . analysis of momentum correction factor () the momentum correction factor  (see equations 3 and 4) is a parameter derived from depth-integrating the navier-stokes equations, undertaken to obtain the shallow water equations (zhou, 2004, trujillo-vela, 2015). the physical meaning of  is that it modifies the profile of the velocity of the flow. for most of the cases this factor is taken as the unity by assuming that the velocity profile is almost uniform through the depth of the avalanche (denlinger and iverson, 2001, savage and hutter, 1989). however, this assumption may not be the most appropriate after of the release, in the proximity of obstacles and near to the deposit where both height and vertical velocity may change considerably and, in some cases, abruptly (pudasaini and hutter, 2007). for this reason, the effect of the momentum correction factor is analyzed in this section. figures 9 to 11 show a longitudinal section at the center of the flume for the experiment (in each figures a and c) and the numerical results (in each figures b and d) at five different times (0.10, 0.32, 0.53, 0.93, and 1.50 seconds). heights are shown on the ordinate and the longitudinal length is shown on the abscissa. the numerical results were computed for five values of  (1.0, 1.05, 1.10, 1.15, 1.20). figures 9a and 9c show the profile of the free surface at the center of the flume of the experimental results at t = 0.10 s and t = 0.32 s, respectively. figures 9b and 9d show numerical predictions at the same times. the best prediction of the front at t = 0.10 s is given by  = 1.0, since the numerical results represent the front as steep as in the experiment. results with   1.0 show very blunt fronts (figure 9b). if  = 10.5 at t  0.32 s, the flow presents more blunt fronts, and the arrival of material at the zone of deposit is sooner, as is shown in figure 9d. figures 10a and 10c show experimental results of the profile of the free surface at the center of the flume at t = 0.53 s and t = 0.93 s, respectively. the best predictions at t = 0.53 s is given by  = 1.15, because this value of  presents a distribution of material similar to the distribution in the experiment. at the same time, if  = 1.0, a significative amount of material has descended the slope and the generation of the deposit begins prematurely, with respect to the experiment. at t = 0.93 s as shown in figure 10c. the experimental measurement shows that the deposit is almost fully formed (figure 10c). a good approximation for the tail of the flow and the deposit is reached when  = 1.10 ̶ 1.15 (figure 10b and d), although the tail is longer than in the experiment. also, hutter et al., (2005) show this behavior. on the other hand, there is a numerical oscillation on the left side of the deposit when  = 1.0 ̶ 1.10caused by the slope change in this zone and the selected . figure 11b shows that the best prediction of the deposit at t = 1.50, which is obtained by  = 1.0 because the tail is less noticeable (i.e., thinner) unlike if other values for the momentum correction factor are used, and the experiment does not show a tail. in addition, the maximum heights of the deposit were predicted better when  = 1.0. the analysis of the momentum correction factor offers information about changes in the shape of the granular flow prediction by imposing different values of this factor. therefore, it is possible to obtain a better prediction in the process where the modeler is most interested in, either during the movement or the deposit, by using   1.0 or  = 1.0, respectively. hence, we propose that future research examine this parameter in order to generate a momentum correction factor that can change depending on what period of movement is being examined when the shallow water approach is used. discussion the results obtained in this work allow us to emphasize some important aspects regarding the solution of granular flow equations. the first aspect is the importance of performing a grid independence test for any numerical 325a spectral multidomain penalty method solver for the numerical simulation of granular avalanches a) b)experiment (d-i2001) numerical prediction (smpm) fig. 8. a. granular flow experiment. b. numerical prediction given by smpm when the spatial refinement presents low numerical error (n = 8, subx = 108 y suby = 18), slip boundary conditions and regularized lateral earth pressure coefficient (k x y a p, /( ) ). a) b) c) d) experiment (d-i2001), = 0.10 s experiment (d-i2001), = 0.32 s numerical prediction smpm, = 0.32 snumerical prediction smpm, = 0.10 s figure 9. x versus h plots for different  at times t =0.10 s and t =0.32 s, and the comparison with the experimental measurements for the same time. 326 mario germán trujillo-vela, jorge alberto escobar-vargas, alfonso mariano ramos-cañón method employed in order to obtain an optimal grid. the grid independence test is important because the order of the error can vary significantly for each numerical method and test case. therefore, this test helps us to find refinement of the grid that presents low error in the mass conservation. another aspect is the importance of specifying details of the numerical setup, such as initial and boundary conditions, stabilization techniques (appendix c), and “wet-dry” treatments, since most of the times a part of this information is omitted. obviating the error caused by the difficulty of solving hyperbolic systems, the following most important issue that causes the error is the tracking of the free surface for zones without material (i.e., wet-dry cells for shallow water equations), as was shown in section 3.3. none of the boundary conditions used in this work present a completely satisfactory prediction about the distribution of material near lateral boundaries, since the slip boundary condition solely allow us to represent adequately the deposit of material and the non-slip conditions allow us to predict correctly the movement of the mass. this highlights the importance of exploring different types of boundary conditions, with which a great representation of the movement and deposit of the mass can be obtained. as shown in section 2, the use of a discontinuous lateral earth pressure coefficient (k x y a p, /( ) ), such as was proposed by savage and hutter, (1989), makes prone to get unstable solutions. furthermore, as shown in section 5.2, the use of a constant lateral earth pressure coefficient promotes the getting of more stable solutions but is not the best choice from the physical standpoint, because the material suffers thinning and thickening during the movement. therefore, the regularization of k x y a p, /( ) as proposed by tai and gray, (1998), is required. finally, the tests performed using the momentum correction factor () allow us to conclude that the most suitable value of the momentum correction factor is  = 1.0 after release and close to the deposit. on the other hand, to obtain a better representation of the distribution of material during movement, to mitigate the steep slope of the propagation front and to avoid the premature generation of the deposit it is necessary to use   1.0. this parameter shows the limitation of the depth-integrated models to represent a wave propagation front when the unity of the  is selected, as is commonly performed. a) b) c) d) experiment (d-i2001), = 0.53 s experiment (d-i2001), = 0.93 s numerical prediction smpm, = 0.93 snumerical prediction smpm, = 0.53 s figure 10. x versus h plots for different  at times t = 0.53 s y t = 0.93 s, and the comparison with the experimental measurements for the same time. a) b) experiment ( d-i2001 ), = 1.50 s numerical prediction smpm, = 0.50 s figure 11. x versus h plots for different  at time t = 1.50 s, and the comparison with the experimental measurements for the same time. 327a spectral multidomain penalty method solver for the numerical simulation of granular avalanches conclusions the solution of granular flow equations with a spectral method such as the smpm, have not been reported yet. based on the results reported for the first time in this work, the smpm solves appropriately the granular flow equations proposed by iverson and denlinger (2001) and it provides comparable predictions with the experimental data and others’ numerical. we observed from the results that the error in mass conservation increases significantly when a discontinuity is imposed as an initial condition or the sharp fronts are developed during the movement of the mass and, also when the treatment for the tracking of the free surface is used. by another hand, it is possible to use the advantageous property of hp-refinement of the smpm, instead of h-refinement, as occurs with low-order techniques. slip and non-slip boundary conditions modify the mass distribution considerably. numerical solutions with slip boundary conditions provide better predictions about the deposit, whereas non-slip boundary conditions provide better predictions about the movement of the granular flow. instabilities in the solution of the granular avalanche equations introduced by the discontinuous lateral earth pressure coefficient proposed by savage and hutter (1989) can be mitigated by the regularization of this coefficient, such as shown in tai and gray (1998). also, it is possible to obtain a stable solution with a constant lateral earth pressure coefficient. the best prediction of the beginning of the flow and the deposit are obtained when the momentum correction factor is imposed equal to the unity ( = 1.0), whereas, the best prediction of the movement of the flow is obtained if the momentum correction factor is greater than the unity ( > 1.0). acknowledgements the authors acknowledge richard iverson and roger denlinger for providing valuable experimental and numerical data for comparison with our numerical results. fundación ceiba supported this work within the scholarship of doctoral studies of the first author. references blackburn, h. m., & schmidt, s. 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(2004). lattice boltzmann methods for shallow water flows. springer, new york. 329a spectral multidomain penalty method solver for the numerical simulation of granular avalanches appendix a. spectral multidomain penalty method in order to assess the potential of high-order discontinuous methods for real applications and in order to reduce the numerical error introduced by low-order techniques in this paper, we performed the solution of the granular flow equations (equation 1) with a method that combines an exponential convergence, a weak artificial dissipation, and an adaptability based in classic finite elements-volumes techniques (boyd, 2001). the spectral multidomain penalty method (smpm) is based on a collocation approach of multiple nonoverlapping quadrilateral subdomains, which are connected by a penalty term that ensures the stability of the solution by imposing a weak continuity at the subdomain interfaces (hesthaven and gottlieb, 1996, escobar-vargas et al., 2012). the formulation of smpm will be presented in two parts: the subdomain interior and the interfacial treatments. subdomain interior. in the interior of each subdomain any function q(x, y, t) can be approximated by using n-th order lagrange interpolating polynomials on a gauss-lobatto-legendre grid (gll) (hesthaven and gottlieb, 1996). q qx y t x y t l x l y i n j n i j i j, , , ,( ) = ( ) ( ) ( ) = = ∑∑ 0 0 (19) where q(xi, yj, t) is the value of function at the discrete point (xi, yj), and li(x), lj(y) are the i-th and j-th lagrange interpolating polynomials based on the gll nodes in the and y directions, respectively. the spatial derivatives in the x-direction in the global coordinate system are approximated as ∂ ( ) ∂ = ∂ ( ) ∂ ∂ ∂ = ∂ ∂ ( ) = ∑q q q x y t x x y t x x d x y ti i i i i n ik k j , , , , , ,    0 (20) where ∂ ∂ / x represents the mapping from the local coordinates system  ∈ − 1 1, , given by gll points into the global coordinates system x ∈. dik is the legendre spectral differentiation matrix, which is computed following the procedure suggested by costa and don (2000). interfacial treatment. the penalized form of the granular flow equations in a collocation point situated on a subdomain boundary requires an additional penalization term, which allows us establishes the continuity between adjacent points (escobar-vargas, 2012). ∂ ∂ + ∂ ( ) ∂ + ∂ ( ) ∂ = ( ) + −  ) = ( )∑q f g q s q e e e e l e e e l e l t q x y q n 1 4 τ ⋅ , j j (21) where n e l,( ) is the unit vector at the subdomain boundary, l represents each one of the four edge of the domain, j represents the information flux between adjacent points, and e is the penalization coefficient (escobarvargas, 2012). τ τ ω e s ≡ = 1 ∆ � (22) where  are weights associated to the gll points. for a uniform polynomial approximation, n, in each subdomain can be used a unique value of  = +( )2 1/ n n and ∆ ∆ ∆s x y= ( ), which depends on the orientation of the interfaces in the subdomains. appendix b. strong stability preserving runge-kutta in order to maintain the precision offered by the spatial discretization method, it is used a strong stability preserving runge-kutta method (ssprk) for the temporal discretization. by considering the following initial value problem d dt q r q= ( ) � (23) in this scheme, the prediction at the time t + 1 (i.e., q t+1) is based on both the existing solution at the time t (i.e., qt) and the evaluation of the forcing terms r(q) at different stages (spiteri and ruuth, 2002). q q0 = t (24) q q r qi k i i k k i k kt i= + ( ) = … = − ∑ 0 1 1 2α β, , , , , , ,∆ s (25) q qt s+ =1 (26) for consistency, we must have that k i i k = − ∑ = 0 1 1 , , i= 1, 2,…, s, where s is the number of stages of the ssp-rk approach, i k, and i k, are constant coefficients where all i k, ≥ 0 and i k, = 0 only if i k, = 0. spiteri and ruuth (2002) give these coefficients. t is the size of the time-step. appendix c. stabilization technique there are multiples methods for improving accuracy and stability of spectral and pseudo spectral techniques, such as physical space filters (gottlieb and tadmor, 1985, gottlieb and shu, 1997), spectral vanishing viscosity methods (maday et al., 1993), dealiasing (canuto et al., 2012) and over integration (hesthaven and kirby, 2008). a family of regularization techniques related to the first two above-mentioned is the spectral filters. since there is not a dissipative term in the smpm to remove the artificial oscillations, a spectral filter used by vandeven, (1991), gottlieb and hesthaven, (2001), diamessis et al., (2008) is necessary to reduce the high-frequency modes that appear close to the discontinuities (hesthaven and warburton, 2007). it was selected this spectral filter (exponential) because of the stability and robustness that it adds to the method (hesthaven and kirby, 2008, hesthaven et al., 2007). a comparison of different spectral filtering methods and discussion of the advantages and weaknesses of using them is performed in hesthaven and kirby, (2008), blackburn and schmidt, (2003), navarra et al., (1994). based on the results of this work, it can be concluded that the spectral filter allows us to obtain stable solutions, whereas few solutions are obtained when a filter is not used. thus, it is possible to avoid artificial and persistent oscillations around points where discontinuities appear. also, the error in mass conservation has a smoother behavior over time when using a filter. keywords: aghbolagh; oshnavieh; involved fluids, la/y ratio, skarn, vein. palabras clave: aghbolagh; oshnavieh (irán); inclusiones fluidas; proporción lantano/itrio; skarn; veta how to cite item dana, k., nezafati, n., & abedini, m. v. (2020). study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran. earth sciences research journal, 24(2), 231-243. doi: https://doi. org/10.15446/esrj.v24n2.83340 aghbolagh copper deposit is located in the southwest of west azarbaijan province, 10 km north of oshnavieh city. most of the existing rock outcrops in the area include cambrian deposits (carbonated and clastic deposits). these deposits are metamorphosed by intrusive-granite masses (skarn-genesis). the infiltration of the mineralization solution at distances beyond contact and inside the sandstone sequences has resulted in the formation of copper ore veins. for microscopic and geochemical studies, 21 thin sections, and 22 doubly-polished cross-sections, 22 xrd specimens, 22 icp-ms specimens, and three samples for analysis of 10 oxides were analyzed by zarazma corporation. in the study of the fluids involved, measurements were made using a linkham thmsg600 heater and freezer plate, mounted on a polarized microscope with a separate stage and olympus model. studies on quartz minerals (3 samples) and garnet minerals (2 samples) were performed on five polished double sections by micro-thermometry measurements on 30 inclusion in quartz mineral and 15 inclusion in garnet mineral at rizkavan land company. the results show that the la/y ratio, which is a proper indicator for the ph conditions of the formation environment of ore deposit, in aghbolagh deposit is a range from 0.09 to 2.26. the granite mass has the highest amount, and the skarn zone represents two types of conditions. type 1: section where the la/y> 1 ratio observed in samples close to the skarn mass and ore. type 2: part where the ratio la/y <1 is close to marble lithology. in the case of a vein deposit, the samples have mineralization under alkaline conditions, and other samples show acidic conditions. the examination of fluid inclusions determined two main groups of fluids in this mineralization system. group 1: supersaturated fluids of salt, which is the origin of halite multiphase inclusions in the garnet ore during the progressive skarn alteration process. group 2: salt-saturated fluids, which are the source of fluid-rich two-phase intermediates in the garnet and quartz minerals, and have been effective in regressive alteration in the skarn system. abstract study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran estudio de inclusiones fluidas y geoquímica de elementos tierras raras del depósito cuprífero de aghbolagh, al norte de oshnavieh, en la provincia iraní de west azerbaijan issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n2.83340 el depósito cuprífero de aghbolagh se localiza en el sudeste de la provincia de west azerbaijan, 10 kilómetros al norte de la ciudad iraní de oshnavieh. los afloramientos rocosos en el área incluyen depósito del cámbrico (carbonatados y clásticos. estos depósitos fueron transformados por masas intrusivas de granito (génesis del skarn). la infiltración de soluciones de mineralización a distancias más allá del punto de contacto y al interior de secuencias de arenisca resultaron en la formación de vetas de cobre. para los estudios microscópicos y geoquímicos se analizaron 21 secciones delgadas y 22 secciones transversales doblemente pulidas, 22 muestras para análisis de difracción de rayos x, 22 muestras para análisis de espectrometría de masas por plasma acoplado inductivamente, y tres muestras para el análisis de 10 óxidos, que fueron procesados en los laboratorios de zarazma corporation. en el estudio de la inclusiones fluidas, las mediciones se hicieron con una bandeja de calentamiento y enfriamiento linkham thmsg600, montada en un microscopio polarizado modelo olympus. para los estudios en minerales de cuarzo (tres muestras) y de granate (dos muestras) se utilizaron cinco secciones doblemente pulidas donde se hicieron las medidas de microtermometría para treinta inclusiones en minerales de cuarzo y quince inclusiones en minerales de granate en los laboratorios de rizkavan land company. los resultados muestran que la proporción de lantano/itrio, que es un indicador apropiado para establecer las condiciones de alcalinidad en el ambiente de formación del depósito mineral, en el depósito de aghbolagh se encuentra en un rango de 0.09-2.26. la masa de granito registra una mayor proporción, mientras que en la zona del skarn se presentan dos condiciones diferentes. tipo 1: sección donde la proporción lantano/itrio es superior a 1 en las muestras cercanas a la masa de skarn. tipo 2: la sección donde la proporción lantano/itrio es inferior a 1 en la litología del mármol. para el caso de la veta, las muestras tienen una mineralización bajo condiciones alcalinas mientras que otras muestras tienen condiciones de acidez. el examen de las inclusiones fluidas determinó dos grupos principales de fluidos en este sistema de mineralización. grupo 1: fluidos supersaturados de sal, que es el origen de las inclusiones multifásicas de halita en el granate durante el progresivo proceso de alteración del skarn. grupo 2: fluidos saturados de sal, que son la fuente de fluidos ricos de dos fases intermedias en los minerales de cuerzo y granate, y que han sido efectivos en la alteración regresiva del sistema de skarn. resumen record manuscript received: 04/11/2019 accepted for publication: 24/01/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 2 (june, 2020): 231-243 kamal dana, nima nezafati, mansour vosoughi abedini department of geosciences and geophysics, science and research branch, islamic azad university, tehran, iran * corresponding author: nima.nezafati@gmail.com o re g eo lo g y https://doi.org/10.15446/esrj.v24n2.83340 https://doi.org/10.15446/esrj.v24n2.83340 https://doi.org/10.15446/esrj.v24n2.83340 232 kamal dana, nima nezafati, mansour vosoughi abedini introduction the aghbolagh copper deposit in the southwest of west azarbaijan province and 10 km north of oshnavieh city, in the range of 45°,01′ to 45°, 09′ east length, and 37°, 09′ to 37°, 13′ in the northern latitude in the contact of the oshnavieh granitic mass was created with the cambrian carbonate rocks and in the sanandaj-sirjan zone. also, infiltration of the mass and mineralization solution at distances away from contact and within the sandstone sequences resulted in the formation of quartzite rocks and copper ore veins (figure 1). the oshnavieh intrusive assemblage has emerged of three intrusive families of diorite, granite, and alkali syenite-alkali granite (as-ag) in the northwestern part of the sanandaj-sirjan zone. the granite family (one of the three oshnavieh intrusive mass) with the distinctive features of s-type granites is the result of the partial melting of the polite-gray wake stones (ghalamghash et al., 2011). oshnavieh intrusive masses are one of the actual intrusions in the sanandaj-sirjan zone. this area has previously been investigated by researchers such as naghizadeh et al. (2004) to prepare a geological map 1: 10000 of oshnavieh (ghalamghash et al., 2011). from the perspective of isotopic age determination, the k-ar method has been investigated. due to the formation of region diorites and their synchronization of formation with granite rocks 100 million years ago, the oshnavieh intrusive assemblage appears to be the result of the subduction of the neotethys oceanic crust beneath central iran. dating by the k-ar method on granitic biotites has shown periods of 100 + 1.9 and 98.9 + 1.5 million years (late early cretaceous) (ghalamghash et al., 2011). one of the most useful ways to interpret the petrogenetic of intrusive masses is to use geochemical data and to study changes and ratios of major and trace elements in these masses (tabatabaiemanesh et al., 2011). experimental proof of the origin of hydrothermal solutions responsible for the formation of skarn deposits is one of the most essential stages of studies of these types of deposits. the condensation of rare earth elements, la to lu, in hydrothermal solutions provides useful information on the origin of the constituent elements of the deposit containing the basic elements (kato, 1999). one of the significant advances in ore deposits researches in recent years is the development of geochemical models related to various stages of transport and deposition of the ore-bearing fluids. these models, which are widely used in the design of exploration strategies, are primarily based on information obtained from fluid inclusion. fluid inclusions are the only source of information on the density of mineralized fluids. the eutectic point for the nacl + h2o system is approximately te <-20.8co. however, the eutectic point or te in some of the samples studied is lower than -20.8 co, indicating the presence of salts other than nacl in the mineralizing fluid. materials and methods for the microscopic study of samples, 21 thin sections and polished sections were prepared. after microscopic studies of samples and determination of tissue relationships between minerals and for geochemical works, 22 samples were analyzed by xrd and 22 samples of icp-ms and three samples for analysis of 10 oxides analyzed by zarazma company (table 2). for investigating the involved fluids, measurements were done using the linkham thmsg600 heater and freezer plate and mounted on a polarized microscope with the olympus model and separate from staging. fluid inclusion studies have been performed on quartz (3 samples) and garnet minerals (2 samples). for the study of fluid inclusions, five polished doubly sections were prepared. after performing petrographic fluid inclusions on them, five samples were selected for micro-thermometers measurements, and micro-thermometers measurements on 30 inclusions in quartz ore and 15 inclusions in garnet ore was performed. geology the aghbolagh copper deposit is located in the northwestern end part of the sanandaj-sirjan zone in terms of structural position. according to map 1:100,000 (naghizadeh, 2004), oshnavieh is partially separated and located in the aghbolagh copper deposit, including cambrian geological units. barut formation: alternating limestone and gray dolomite crystallized with shale; soltanieh formation: dolomite, creamy dolomitic limestone containing chert, marble; mila formation: limestone and dolomite locally associated with dark-gray shale; equivalence of zagun-lalun formation: shale-siltstone and sandstone in green, red-gray archaeozoic sandstone. the permian unit includes ruteh formation: limestone with an interlayer of dolomite, shale and quartzite sandstone; dord formation: red quartzite sandstone with shale and limestone interlayers, granite intrusive masses, diorite monzogabbro are coarse grains (figure 1). the most prominent geological units in the order of old to new consist of cambrian units (quartz sandstone, metamorphosed carbonate rocks (dolomite and metamorphic limestone), shale, and siltstone altered), permian units (quartzite rocks) and sandstone and dolomite, which are from quaternary formations. the most important economic and geological features of this area are cu mineralization at the site of granitoid contact with marble and outward copper mineralization within quartzite sandstones and veins (figure 1). aghbolagh granite mass in field surveys, the granite mass is in the central part of the range. these medium to coarse-grained granites have a bright, white appearance (figure 2a and 2d). this mass is leucocratic, containing significant quartz minerals, feldspar, plagioclase, and biotite, muscovite, sericite, and dark minerals, mainly pyrite (figure 3). these granites are poor in terms of the percentage of dark minerals, which is a characteristic feature of these granitoid, reducing their color coefficient. the low rate of mafic minerals in these rocks can be called leucogranite (moein vaziri and ahmadi, 2001). the granite intrusions are weathering, and have been altered at the surface. the color of the mass is predominantly white with fineto medium minerals and crystal-coarse in some places. the anorthosite minerals were visible in the central, coarse-crystalline sections, and towards the contact, the size of the minerals decreases, and it takes on aplitic texture. the dark minerals of this mass are mainly pyrite (fig. 2b), and in areas where more alteration has taken place, the mass has been reddish to brown (fig. 2c). this unit is the leading cause of the formation of skarn, and the resulting solutions have formed mineralized veins. for naming, determination of the magmatic type, series, and tectonic position of aghbolagh intrusive mass have been discussed. in this regard, two samples from different sections and one from the contact boundary were taken and sent to zarazma company for analysis of ten oxides (table 1). based on the diagram (cox et al., 1979), the intrusive mass object of this study has a granite composition, and according to the diagram (de la roche et al., 1980), they have a combination of granite to alkali granite. and based on the diagram (peccerillo and taylor, 1976), the samples are in the range of highpotassium calc-alkaline series (figure 4b). based on the work of shand (1943), modified by villaseca et al. (1998), the granite mass is in the pre-alumina class (figure 4d). and also, by using sio2 diagram against a/cnk (taylor, 1976), the mass can be classified into s-type granites. the position of the intrusive mass is based on diagrams (pearce et al., 1984) of volcanic arc granites (vags), which is formed simultaneously as the syn-collisional. according to the diagrams (schandl and gorton, 2002), the studied granites are related to the active margin of the continents, consistent with the nature of the sanandaj-sirjan zone (figure 5). table 1. analysis results of ten oxides of intrusive masses of aghbolagh-oshnavieh copper deposit l.o.ip2o5mnotio2mgok2ona2ocaofe2o3al2o3sio2sample 0.64<<0.130.33.624.520.421.613.8174.74aq-gt-sh2 2.03<<0.09<0.410.010.162.9717.7464.89ag-sg-2 0.77<<0.080.083.853.70.50.9412.9976.79aq-sg-6 233study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran figure 1. location and geological map of the study area (aghbolagh-oshnavieh copper deposit) figure 2. a: a view of the light granite mass, b: a view of the granite mass with pyrite minerals, c) a view of the alteration of the iron ore and its discoloration, d: a view of the aghbolagh copper ore deposit’s granite outcrop. geology of skarn unit the skarn unit is continuously expanding in the northern part of the intrusive mass along the west to east. this unit is fully covered and can only be identified with garnite outcrops (figure 6a) and hornfels and gossans (iron hat). mineralogy is only visible in a small part of the western range, but the presence of iron warhead indicates the presence of sulfide ores below (figure 6c). the minerals that can be identified in the hand sample in this section are quartz, garnet, epidote, cordierite, calcite, and minerals, including chalcopyrite, bornite, malachite, and azurite (figures 6 and 7). geology of vein ore deposit these deposits in particular contain sedimentary rock (dolomite, limestone, sandstone and shale) that have been affected by the intrusions of stoke, dyke and middle to felsic igneous floods. (figure 8) (morris, 1986; cox, 1986). the sandstone zone is located about 200 meters from the intrusive mass behind marble rocks. the ore solutions enter through the marble through the seams and crevices in the sandstones (the seam and crevice was probably caused by the igneous mass intrusion), which is a good place to deposit soluble materials. . in the meantime, veins of several centimeters to 3 meters thick are formed in these rocks. the veins are quartzitic and contain minor amounts of microcline, muscovite, sercite, hematite, goethite and sulfide and carbonate minerals based on xrd and light analyzes. soil space filling structures and dog teeth construction are abundant in this area. chalcopyrite, chalcocite, malachite, azurite and quartz are minerals in this part of the deposit (figure 9). discussion and investigation rare earth element geochemistry (ree) most studies of the behavior of rare earth elements (rees) in hydrothermal environments in active systems have been reviewed. so far, 234 kamal dana, nima nezafati, mansour vosoughi abedini figure 3. microscopic images of igneous rocks of the area: a) amorphous quartz minerals and intrusions of sericite, b) polysynthetic micelle in plagioclase showing little alteration, c) minerals forming granite, chlorite, feldspar potassium altered to sericite, quartz, and plagioclase, d) altered plagioclase and feldspar potassium, e) birefringence of muscovite with plagioclase and quartz, f) biotite and chlorite (biotite chlorite). qtz=quartz, pl=plagioclase, kfs= k feldspar, bt= biotite, chl= chlorite, ser= sericite, mo= muscovite figure 4. diagrams used to determine the magmatic series of agbolagh granitic mass a) the position of the intrusive samples in the afm diagram (a = (na2o + k2o), f = feot, m = mgo), (irvine and baragar, 1971) b) the position of the samples in the sio2 diagram versus k2o (peccerillo and taylor, 1976) c) position of the specimens in the a/cnk-a/nk aluminum diagram (shand, 1943) d) the position of the samples in the b-a plot diagram (a = al(k + na + 2ca), b = fe + mg + ti (villaseca et al, 1998) figure 5. the position of the specimens related to the aghbolagh granites mass in the diagrams (a) rb against nb, (b) y + nb against rb, (c) y against ta, (d) ta+yb against yb, to determine its tectonic position. (adapted from pearce et al., 1984). relatively few studies have been performed on the distribution processes of rare earth elements in fossilized hydrothermal systems or leachate metasomatism products from igneous, metamorphic or hydrothermal fluid associated with leached skarn deposits in carbonate rocks and adjacent silicates. extensive metasomatism of rare earth elements can occur in lowto medium-depth hydrothermal systems, and the geochemistry of these elements can be useful in identifying the origin of skarns and related deposits (adriana et al., 2009). the chondrite-normalized ree pattern for the granite mass, skarn, marble, and vein deposit indicates that although the rees pattern is slightly uniform, the lrees overall show more enrichment than the hrees in different lithologies (figure 9). the range of numbers normalized to chondrite in aglabagh deposit varies considerably, (2.58 to 141.93 for la and 0.14 to 27.27 for yb). this indicates that distinct differences in the increase and decrease of lrees and hrees occurred during the formation of the aghbolagh ore deposits. to evaluate the separation coefficient, between rees, different ratios of rees including (la/yb)n’, (la/sm)n and (gd/yb)n are used. (la/ yb)n calculates the separation coefficient of lrees from hrees during geochemical processes (aubert et al., 2001; yusoff et al., 2013) and calculates two other ratios of separation coefficients between lrees and mrees and between mrees and hrees. (yusoff et al., 2013).the extent of these ratios, range from (0.42 to 50.56) for (la / yb) n and (0.25 to 125/80) for (la / sm) n and (0.006 to 8.48) for (gd / yb) is n). the highest separation occurred between lrees and hrees (more than 125) and the least separation occurred between mrees and hrees during the scanning process. the highest coefficient of these ratios is in marble zone and only the coefficient (gd / yb) n in skarn has the highest coefficient, indicating that the highest separation in marble zone is not only due to hree deficiency but also due to hree and mree deficiency. 235study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran it is a zone. the la / y ratio well indicates the ph conditions of the deposit formation environment. (la / y> 1) indicates alkaline conditions and (la / y <1) represents acidic conditions (crinci and jurkowic, 1990). this ratio in the aghbolagh deposit ranges from (0.09 to 2.26). intrusive mass shows the highest (la / y) ratio of 2.26. the granite mass shows the highest amount of la / y (2.26). the skarn zone (endoskarn and exoskarn and ore present in exoskarn) show two types of conditions. type 1: section where la/y> 1 ratio is observed in samples close to the mass and skarn ore is also in this range. option 2: part where the ratio la/y<1 is close to marble lithology. these divisions are indicative of acidic conditions and hydrothermal alteration during the skarnification process. this is also observed in the veins of the aghbolagh deposit. according to the diagram of the deposit from the beginning of the granitic mass to the middle of the exoskarn zone (sample aq-ses7), alkaline conditions were dominant and from this point to the end of the host rock (marble) acidic conditions were present. in the case of vein deposit, the sample also contains alkaline mineralization and other samples show acidic conditions. hence, we can predict that in an environment affected by hydrothermal solutions where acidic and hot solutions containing ree are found due to interaction with dilute carbonate rocks and they are found to be alkaline state. the eu / eu * anomaly in the aghbolagh copper deposit ranges from (0.62 to 12.14) (figure 2 and table 2). more than one is called a positive anomaly and less than one is called a negative anomaly. the eu / eu * anomaly was positive in the intrusive mass, negative in the skarn zone, and again positive in the marble section. so in this section we have two positive anomalies on the mass side and host rock and one negative anomaly is observed in the skarn zone. these anomalies can be interpreted by the presence of plagioclase at the mass side and the presence of albite minerals in the marble zone and vein-section. in the skarn zone, the anomaly is negative due to the absence of plagioclase (zamanian and radmard, 2016). the positive eu anomaly indicates the stages of rock change in an acidic environment (kikawada, 2001). positive anomalies of eu and (la / lu) n also indicate high temperature acidic liquids (wood, 1990). based on two parameters eu / eu * and (pr / yb) n in the skarn zone, two types of hydrothermal solutions can be identified that affect the host rock: 1) solutions of low ree concentration with anomaly eu positive and acidic conditions and oxidizing property. 2) ree-rich solution with negative eu anomaly indicates alkaline conditions of such a solution (figure 10). the presence of a high ratio of exoskarn to endoskarn and extension of the exoskarn zone indicates low depth conditions (less than 5 km) and intense oxygen activity (oxidation conditions) (zamanian, 2003). involved fluids examination of endoskarn unit sample sample number: ag-sg-2 sample feature: this sample consists of quartz and carbonate. large crystalline quartzes are observed between the faceted carbonate crystals. figure 6. macroscopic images of the aghbolagh copper ore deposit exo-skarn zone a. view of garnet’s hand sample, epidote, and calcite of the exoskarn zone; b. view of garnet’s hand sample in light brown, epidote, calcite, and magnetite vein; c. view of the iron cap showing is a supergene zone; d. view of the mineralization in the exo-skarn zone with magnetite, malachite, azurite, and chalcopyrite minerals. grt= garnet, mag= magnetite, epi=epidote, mal= malakite, hem=hematite, cal=calcite figure 7. microscopic images of the exo-skarn zone of aghbolagh region: a) garnet mineral in normal light and its alteration to epidote; b) garnet mineral, epidote, and quartz in cross light; c) epidote mineral of pseudomorph garnet, chlorite in normal light; d) pseudomorph epidote after garnet. qtz=quartz, grt=garnet, epi= epidote, chl=chlorite figure 8: schematic image of a substituted polymetallic deposit, that showing different types and host rocks. (morris, 1986; cox, 1986) figure 9: (a): overview of quartz samples containing chalcopyrite mineralization, malachite, (b): quartz vein containing copper mineralization (chalcopyrite, chalcocite, malachite and azurite), (c): view of azurite and chrysocolla mineralization, (d): a view of the quartz vein containing mineralization 236 kamal dana, nima nezafati, mansour vosoughi abedini figure 9. trends of changes of (la/yb) n, (la/sm) n and (gd/yb) n in different zones of aghbolagh deposit figure 10. trends of changes in la/y ratios in different zones of aghbolagh deposit table 2. values of rare earth elements of aghbolagh ore deposit samples and ratios between rare earth elements for conditions evaluation of ore formation sample a qs g 2 a qs g 6 a qg ts h 2 a qs n s8 a qe ps h 4 a qs es 10 a qg o 20 a qs h s9 a qe p2 3 a qs es 7 lree la-nd ppm 2.12 50.56 84.06 154.74 121.88 114.48 24.21 127.37 47.62 121.54 mree sm-tb ppm 0.28 2.77 4.42 14.95 7.11 8.72 1.34 11.06 5.12 13.79 hree er-lu ppm 1.1 1.91 2.37 9.24 4.58 4.09 3.34 6.76 2.24 10.54 ∑ree (la-lu) ppm 3.5 55.24 90.85 189.93 133.57 127.29 28.89 145.19 54.98 145.87 (ln/lu)n 1.30 12.61 17.53 14.28 15.58 2.60 5.77 6.49 11.69 2.60 la/y 0.15 1.77 2.27 1.38 1.89 1.76 1.71 1.29 1.12 0.95 eu/eu* 14.12 1.68 1.29 0.76 0.75 0.62 1.53 0.96 0.93 0.76 (la/sm)n 62.90 7.13 7.65 4.28 6.35 3.94 9.21 4.48 3.57 3.14 (la/yb)n 2.25 16.37 20.23 6.45 9.67 10.11 2.89 7.64 9.74 3.55 (gd/yb)n 0.08 0.59 1.05 0.92 0.04 0.24 0.78 8.48 1.40 0.81 (pr/yn)n 0.11 3.57 5.25 3.18 4.16 5.12 0.50 3.41 3.85 1.97 sample a qe x sh 1 a qg a ep a qs m 3 a qs m 17 a qs m 18 a qv 15 a qv 16 a qv t8 a qv t9 lree la-nd ppm 5.23 31.83 5.43 21.53 5.43 49.35 2.03 4.23 25.31 mree sm-tb ppm 0.9 5.47 0.2 2.97 0.4 1.74 0.2 0.33 2.46 hree er-lu ppm 4.11 2.64 0.22 0.72 0.74 0.85 0.22 0.42 2.95 ∑ree (la-lu) ppm 10.24 39.94 5.85 25.22 6.57 51.94 2.45 4.98 30.72 (ln/lu)n 1.30 10.39 5.19 6.49 8.62 19.48 1.04 3.90 11.42 la/y 0.53 0.67 0.53 0.27 2.24 0.32 0.48 0.78 eu/eu* 1.84 0.87 14.12 0.66 5.34 3.64 14.12 5.77 0.89 (la/sm)n 4.27 1.20 125.81 2.28 17.97 7.93 50.32 31.45 6.12 (la/yb)n 0.75 1.69 44.95 33.71 13.48 50.57 17.98 10.11 4.12 (gd/yb)n 0.85 1.37 0.01 0.76 1.06 0.20 0.81 0.12 0.34 (pr/yn)n 0.01 1.22 1.71 3.94 0.51 15.85 1.71 0.26 0.96 figure 11. chart of changes of eu / eu * ratio in different zones of aghbolagh deposit figure 12. graph of changes in the ratio of ∑ree to eu / eu * in the exoskarn zone and aghbolagh ore deposit marble. 237study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran gaseous single phase phases are abundant in quartz crystals. these inclusions are elliptical and elliptic and vary in size from 2 to 10 microns. these inclusions also coincide with fluid-rich biphasic inclusions, which is evidence of boiling. liquid-rich biphasic inclusions are often observed as dispersed aggregates and to a lesser extent as single ones. these inclusions are very high in sample (figure 14). homogenization temperatures to liquid phase of fluid-rich two-phase inclusions are from 190 to 325.2 ° c. most of the measurements were done at temperatures of 310 to 340 ° c. the salinity of these inclusions varies from 7.31 to 20.67 % weight percent of salt equivalent and most of the measurements are between 9 and 12 % weight percent of salt equivalent. in the salinity versus homogeneous temperature diagrams for a fluid-rich two-phase inclusions, an isothermal mixing process is observed (figure 15). by examining the fluid inclusions in the quartz mineral in this sample, a main fluid was identified as a fluid with high temperature and low to medium figure 13. spider diagram of samples of different zones of aghbolagh copper ore deposit, normalized to chondrite (boynton, 1984) figure 14. left: accumulation of liquid-rich biphasic inclusions with irregular shapes. right: a single fluid-rich two-phase inclusion. salinity (temperature 310 to 340 ° c and salinity about 9 to 12 % weight percent of salt equivalent). the mixing process of fluid temperatures affected the evolution of the fluid. the modified source diagram of kesler, 2005 was used to determine the source of the fluids, based on which the fluid-rich two-phase inclusions are located within the seawater (figure 16). examination of skarn unit specimens sample number: ag-ga-21-b sample feature: this sample consists of a massive garnet with quartz. transparent quartz crystals are formed in the space between the faceted garnet crystals. fluid inclusions have been identified in garnet and quartz crystals. types of fluid inclusions in garnet: single-phase gas-liquid, liquidrich two-phase short-lived gas-phase single-phase inclusions are often found 238 kamal dana, nima nezafati, mansour vosoughi abedini in garnet crystals. these inclusions are tear-shaped, irregular, and elliptical, varying in size from 2 to 10 microns. in garnet, liquid-rich biphasic inclusions consist of a liquid phase and a vapor bubble. in these inclusions, the vapor phase volume is about 20 to 30 percent of the inclusion volume. these inclusions range in size from 2 to 10 microns and are often elliptical. the abundance of these inclusions in garnet is low (figure 17). figure 15. distribution of homogenization temperature against salinity for liquid-rich two-phase inclusions in ag-sg-2 sample. the isothermal mixing process is observed. trends from wilkinson (2001). figure 16. chemical composition of hydrothermal fluids based on the type of hydrothermal system in the salinity diagram against homogenization temperature (modified after kesler, 2005). most of the samples are within the seawater range. figure 17. left: accumulation of single phase gas inclusions, in garnet. right: a single liquid-rich two-phase inclusions in garnet. in this sample micrometer studies were performed only on garnet minerals. liquid phase homogenization temperatures are fluid-rich two-phase inclusions from 295.3 to 460 ° c. most of the measurements were performed at temperatures of 310 to 350 ° c. the salinity of these inclusions varies from 11.58 to 18.96 % weight percent of salt equivalent and the highest salinity frequency is from 13 to 19 % weight percent of salt equivalent. in the diagram of the homogenization temperature against salinity for fluid-rich two-phase inclusions, a distinct process of fluid isothermal mixing is observed. figure 18. distribution of homogenization temperature against salinity for liquid-rich two-phase inclusions in ag-ga-21b sample. the specific mixing process of fluid temperature is observed. trends from wilkinson, 2001. by examining the fluid inclusions in the garnet mineral in this sample, a high temperature fluid (310 to 350 ° c) and a salinity ranging from 13 to 19 % weight percent of salt equivalent were identified. the mixing process of the fluid temperature justifies the relatively large changes in fluid salinity. sample number: ag-ga-21-a sample feature: this sample consists of a semi-faceted mass garnet with coarse-grained quartz. quartz fills the space between garnet crystals. the garnet and quartz minerals host fluid inclusions in this sample (figure 20). types of fluid inclusions in garnet minerals: single-phase liquid inclusions, single-phase inclusions, liquid-rich two-phase inclusions, and halite multiphase inclusions (figures 21). 239study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran types of fluid inclusions in quartz minerals: single-phase gaseous intermediate, liquid-rich two-phase inclusions (two generations) in garnet, there is a low frequency single phase liquid inclusions ranging from 5 to 10 microns, and most of them about 5 microns, often with irregular shapes. single-phase gas inclusions are abundant and often have elliptical and lacrimal forms. these inclusions range in size from 5 to 15 microns and are dark in color because of the rich gas. figure 19. chemical composition of hydrothermal fluids based on the type of hydrothermal system in the salinity graph against homogenization temperature (modified after kesler, 2005). the position of the data in this sample is not in a certain position due to fluid mixing. liquid-rich biphasic inclusions consist of a liquid phase and a vapor bubble. in these inclusions, the vapor phase volume is about 30 to 40 percent of the inclusion volume. these inclusions range in size from 5 to 25 microns and often have irregular, lacrimal and elliptical shapes. the frequency of these inclusions in the sample is low. multi-phase halite inclusions were observed in the garnet mineralization. these inclusions have little frequency. these inclusions are irregular and irregular. these inclusions range in size from 10 to 30 microns. the vapor phase volume in these inclusions is 30-40% of the short-circuit volume and the halite phase volume is about 40% of the short-circuit volume. most of them have one or more solid or dark solid phases. single-phase gas quartz mineral inclusions are observed as dark inclusions due to the presence of gas phase in them. these inclusions are elliptical and tearshaped, and their abundance is low in quartz. two generations of liquid-rich biphasic inclusions were observed in quartz. generation 1, which is a liquid-rich two-phase inclusions in which the volume of the vapor phase is about 40 to 50 percent of the short-circuit volume. these inclusions are elliptical, rectangular, and lacrimal. these inclusions range from 3 to 15 microns. these inclusions are abundant in quartz. this inclusions is observed with single-phase gas inclusions, which can be evidence of fluid boiling. generation 2, which is a liquid-rich biphasic inclusion whose vapor phase volume is about 20% of the inclusion volume. the vapor phase brownian motion is seen in these inclusions. these inclusions are 3 to 10 microns in size. the abundance of these inclusions is low in quartz. in this sample, two-phase liquid-rich inclusion and halite-rich of garnet minerals were measured by micro-thermometers. liquid phase homogenization temperatures are fluid-rich two-phase inclusion from 270.3 to 510.2 ° c, with the most frequent measurements being carried out at temperatures of 270 to 310 ° c. the salinity of these inclusions varies from 13.17 to 21.68 % weight percent of salt equivalent and the highest salinity is 16 to 22 % weight percent of salt equivalent. there is no particular trend in the salinity diagram for the homogenization temperature for liquid-rich two-phase inclusion (figure 22). figure 20. left: transparent quartz containing fluid inclusions between garnet (isotropic) crystals. ppl light. right: transparent quartz with fluid inclusions between garnet crystals (isotropes). xpl light. figure 21. left: a halitized multiphase inclusion in garnet. right: a halitized multiphase inclusion with clear and dark solid phases in garnet. 240 kamal dana, nima nezafati, mansour vosoughi abedini in this sample, due to the low frequency of halite inclusions in garnet mineral, only 3 inclusion of this type were analyzed. liquid phase homogenization temperatures are halite-rich multiphase inclusion ranging from 395.2 to 490.3 ° c. the salinity of these inclusions varies from 46.9 to 58.5 % weight percent of salt equivalent and the dissolution temperature of the salts in the studied multi-phase inclusion varies from 395.2 to 490.3 ° c. examination of the fluid inclusions in the garnet mineral revealed two fluids in this sample: a high temperature fluid (about 500 ° c) and a salinity of about 50 wt% salt equivalent and a fluid with a temperature of 270 to 310 ° c. c and salinity 22-22% by weight percent of salt equivalent. the initial formation of the garnet is likely to have occurred with the first fluid and the subsequent fluid with lower temperature and salinity has affected the garnet, causing the garnet to calcite and quartz alteration. the modified fluid diagram of kesler, 2005 was used to determine the source of the fluids, based on which the fluid forming most of the fluid inclusions in this sample has magmatic water origin (fig. 23). wilkinson, 2001 diagram was also used to determine the type of fluid genesis in the study sample. in this diagram, fluid-rich, two-phase inclusions and fluid phases are in the fluid range associated with skarn deposits and with tin and tungsten granitoid deposits. since the fluid inclusions data in this sample are obtained from garnet ore, the type of this sample is related to skarn ore deposit. aghbolagh vein deposit section: sample number: ag-v-16 this sample is a vein composed of quartz and ore matte. large crystalline quartzes are observed between the faceted carbonate crystals. the readable fluid inclusions were observed only in quartz crystals. liquid-rich biphasic inclusions consist of a liquid phase and a vapor bubble. in these inclusion the vapor phase volume is about 30% of the inclusions volume. these inclusion range in size from 2 to 15 microns and often have irregular, lacrimal and elliptical shapes. the frequency of these inclusions is high in quartz (figure 24). by examining the fluid inclusions in the quartz mineral in this sample, two fluids were identified: a fluid with higher temperature and salinity (temperature 280 to 310 ° c and salinity 12 to 15 % weight percent of salt equivalent) and a fluid with low temperature and salinity low (190 to 220 ° c and salinity 9 to 12 % weight percent of salt equivalent). the modified source diagram of kesler, 2005 was used to determine the source of the fluids, based on which the fluid constituting this sample has seawater and basin salinity (figure 25). the modified fluid diagram of kesler, 2005 was used to determine the fluid genesis type in the studied sample, based on which the fluid constituting this sample has seawater and basin salinity (figure 1). 25). wilkinson, 2001 diagram was also used to determine the type of fluid genesis in the study sample. in this diagram, fluid-rich two-phase inclusions are in the fluid range with epithermal and skarn deposits. figure 22. distribution of homogenization temperature against salinity for liquid-rich two-phase inclusion in ag-ga-21a sample. there is no clear trend. trends from wilkinson, 2001. figure 23. chemical composition of hydrothermal fluids based on the type of hydrothermal system in the salinity diagram against homogenization temperature (modified after kesler, 2005). the position of most of the data in this sample is in the magmatic water range. figure 24. left: accumulation of liquid-rich biphasic inclusion with irregular shapes. right: a single gas single-phase inclusion. 241study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran discussion in the samples of these studies, micrometers of fluid inclusions were performed on the garnet and quartz minerals. in the study of liquid-rich biphasic fluid inclusions in a garnet mineral, a fluid with a temperature of 270 to 350 ° c with an average salinity of 16 to 19 wt.% salt equivalent was identified (figures 26 and 27). this fluid is probably related to a retrograde alteration in the scarification process that resulted in the alteration of garnets and the formation of low-temperature minerals such as quartz and calcite from the garnet. the evolution of this fluid mixing process has been influenced by the fluid temperature. garnet minerals also identified halitoneic inclusions that have a temperature of about 500 ° c and a salinity of about 50% by weight of salt, which can be related to the progressive fluidization of the skarn systems and lead to the formation of skarn systems. garnet is included in this system. in the investigation of fluid-rich biphasic fluid inclusions in quartz minerals, two fluids were identified, with the first fluid having a temperature of 260 to 300 ° c and a salinity of about 15 to 12 % weight percent of salt equivalent (figure 28). this fluid is most likely equivalent to a retrograde alteration fluid in the garnet mineral and reflects the initial quartz formation temperature. quartz mineralization also revealed a fluid with a temperature of 180 to 220 ° c and a salinity of 9 to 12 % weight percent of salt equivalent, which is a delayed fluid in the system. in the evolution of these two fluids in quartz, fluid mixing processes with lower and cooler salinity, fluid temperature mixing, and cooling process have a significant impact. for halite multiphase inclusion, in all the measured inclusion, the salt dissolution temperature is equal to the temperature of the liquid phase homogeneity. the location of these inclusion near the th = ts midline can be one of the signs of boiling in the mineralization system. examination of the fluid inclusion from the study sample revealed that two main groups of fluids were present in the formation of fluid inclusion: group 1: salt supersaturated fluids that have been the source of the formation of halitite multiphase intermediates in the garnet mineral during the progressive skarn alteration process. group 2: salt-saturated fluids that are the source of fluid-rich two-phase interconnects in the garnet and quartz minerals in this skarn system and have been effective in retrograde alteration in the skarn system. figure 25. chemical composition of hydrothermal fluids based on the type of hydrothermal system in the salinity graph against homogenization temperature (modified after kesler, 2005). the location of the data in this sample is in the seawater and basin salinity range. bulk software (bakker 2003) was used to calculate of the formation pressure of liquid-rich biphasic inclusion. the homogenization temperature of 350° c with an average salinity of 18 % weight percent of salt equivalent was used to calculate the average pressure of the formation of liquid-rich two-phase inclusion in garnet. according to this software, the average pressure formation in the sample is 15.7 mpa (mpa), which indicates moderate pressure and formation at moderate depths (proportional to the regressive alteration in the skarn system). homogeneous temperature of 300° c and average salinity of 12 wt% salt were used for liquid rich two-phase inclusions in quartz. accordingly, the mean pressure of the formation of inclusion in this sample is 8.1 mpa (mpa), indicating moderate pressure and formation at low depths (proportional to retrograde alteration).wilkinson, 2001 diagram was used to determine the type of fluid genesis in the studied samples. in this graph the fluids associated with the garnet ore are within the fluid range associated with the skarn deposits (fig. 29). also, the quartz mineral fluids are partly in the skarn range and the lower temperature and salinity are in the epithermal range (fig. 29). finally, it is necessary to integrate geological information, alteration and mineralization in order to precisely determine the fluid genesis of the area. figure 26. frequency distribution of homogenization temperature in liquidrich two-phase inclusions in garnet minerals in studied samples. figure 27. frequency distribution of salinity in fluid-rich two-phase inclusions in garnet minerals in studied samples. 242 kamal dana, nima nezafati, mansour vosoughi abedini references: adriana, h., paul, g. s., graham, s. t., colin, h. h., & wolfgang, r. l. 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(1998). crustal origin of hercynianperaluminous granitic batholiths of central spain: petrological, geochemical and isotopic (sr, nd) constraints. lithos, 43, 55-79. figure 28. left: frequency distribution of salinity in fluid-rich two-phase inclusions in quartz minerals in the studied samples. right: frequency distribution of homogenization temperature in liquid-rich two-phase inclusions in quartz minerals in studied samples and identification of two fluid groups. figure 29. salinity diagram against homogenization temperature for all types of hydrothermal deposits: most of the garnet ore fluid inclusions in the studied samples are in the fluid range associated with skarn deposits and quartz ore fluid inclusions are in the range of skarn and epithermal deposits. limits from wilkinson, 2001. 243study of involved fluids and geochemistry of rare earth elements of aghbolagh copper deposit, in north oshnavieh, west azerbaijan province of iran wilkinson, j. j. (2001). fluid inclusions in hydrothermal ore deposits. lithos, 55, 229-272. wood, s. a. (1990). the aqueous geochemistry of the rare-earth elements and yttrium. theoretical predictions of speciation in hydrothermal solutions to 350°c at saturation water vapor pressure. chemical geology, 88, 99–125. yusoff, z. m., ngwenya, b. t. & parsons, i. (2013). mobility and fractionation or ree during deep weathering of geochemically contrasting granites in a tropical setting, malaysia. chemical geology, 349–350, 71–86. zamanian, h. (2003). iron mineralization related to the almoughlagh and south ghorveh batholiths with specific refrenceti the baba ali and gelali deposits. university of pune, 220 pp. zamanian, h. & radmard, k. (2016). geochemistry of rare earth elements in the baba ali magnetite skarn deposit, western irana key to determine conditions of mineralization. geologos, 22(1), 33-47. keywords: soil liquefaction; soil engineering; soil dynamics; soil properties; field tests; natural hazards; earthquakes; graphical user interface; matlab®-gui. palabras clave: licuación del suelo; ingeniería de suelos; dinámicas de suelos; propiedades del suelo; pruebas de campo; riesgos naturales; terremotos; interfaz gráfica de usuario; matlab® how to cite item bekin, e., & ozcep, f. (2021). a comparative soil liquefaction analysis with a matlab® based algorithm: soilique. earth sciences research journal, 25(3), 323-340. doi: https:// doi.org/10.15446/esrj.v25n3.86525 soil liquefaction is one of the ground failures induced by earthquakes. determining the safety factor and the settlements are the most common analyses to decrease liquefaction-induced failures and hazards. scientists have suggested numerous empirical formulas to detect and mitigate liquefaction-based hazards, and they have been used over the decades. this study aims to present a user-friendly and interactive program for deterministic soil liquefaction analyses. the algorithm presented in this study, soilique, is the first matlab® program, including a graphical user interface that provides the deterministic liquefaction analysis with the computation of parameters propounded with the formulas. one of the advantages of soilique is that it allows picking the physical property of every layer (i.e., fine or coarse), which provides dealing with liquefaction prone layer(s) directly when necessary. not only can one calculate parameters regarding soil liquefaction with the help of this program, but one also can see graphically supported results. the robustness of soilique is checked with another soil liquefaction analysis program, soilengineering, which was introduced by ozcep (2010). calculations were done separately using real spt data and synthetic data such as vs measurements and cpt data. the real spt data and synthetic vs data were used to compare soilique and soilengineering (ozcep, 2010). the present study presents an example of cpt data analysis but could not be used for comparison. comparisons reveal that outputs of soilique and results of soilengineering showed a good agreement. abstract a comparative soil liquefaction analysis with a matlab® based algorithm: soilique análisis comparativo de licuación del suelo con un algoritmo basado en matlab®: soilique issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.86525 la licuación del suelo es un corrimiento del terreno inducido por movimientos sísmicos. la determinación del factor de seguridad y de los asentamientos son los análisis más comunes para reducir las fallas y las amenazas causadas por la licuación. numerosas formulas empíricas han sido sugeridas por expertos para detectar y mitigar los riesgos de licuación, y estas han sido usadas por décadas. este estudio busca presentar un programa interactivo y de fácil uso para los análisis determinísticos de licuacón del suelo. el algoritmo presentado en este estudio, soilique, es el primer programa basado en matlab® e incluye una interfaz gráfica de usuario que provee el análisis determinístico de licuación con los parámetros de computación propuestos con las formulas. una de las ventajas de soilique es que brinda la oportunidad de elegir las propiedades físicas de cada capa (por ejemplo, fina o gruesa), lo que permitiría tratar directamente con las capas propensas a la licuación cuando sea necesario. no solo se pueden calcular los parámetros relacionados a la licuación del suelo con la ayuda de este programa sino que además se pueden ver gráficamente los resultados. la robustez de soilique se verificó con otro programa de análisis de licuación del suelo, soilengineering, que fue introducido por ozcep en 2010. se hicieron cálculos separados con información real de pruebas estándares de penetración (spt, del inglés standar penetration test), información sintética de velocidad de onda de corte medida (vs, del inglés measured shear wave velocity) e información de pruebas de penetración cónica (cpt, del inglés conic penetration test). la información real spt y la información sintética vs fueron usadas para comparar los resultados de soilique y soilengineering. en este estudio se presentan un ejemplo de información cpt pero no se usa en la comparación. las comparaciones muestran que la información arrojada por soilique y los resultados de soilengineering muestran concordancia. resumen record manuscript received: 22/04/2020 accepted for publication: 12/03/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 323-340 ekrem bekin1,2, ferhat ozcep2 1memorial university of newfoundland, canada. 2istanbul university – cerrahpasa, turkey. corresponding author: ebekin@mun.ca so il s https://doi.org/10.15446/esrj.v25n3.86525 https://doi.org/10.15446/esrj.v25n3.86525 https://doi.org/10.15446/esrj.v25n1.74167 mailto:ebekin@mun.ca 324 ekrem bekin, ferhat ozcep nomenclature α : standard penetration test blow count correction coefficient with respect to fine content β : standard penetration test blow count correction coefficient with respect to fine content δ : focal depth εv : volumetric strain γ : unit volume weight σ, σ0 : total vertical stress σ’, σ0’ : vertical effective stress amax : peak ground acceleration cn : depth correction factor cb : borehole correction factor ce : hammer energy ratio cr : rod length correction factor cs : correction factor for samples with/without liners cv: factor to correct measured shear wave velocity cpt: conic penetration test crr: cyclic resistance ratio crr7.5 : cyclic resistance ratio csr: cyclic stress ratio dr : relative density fc: fine content fs : sleeve friction gui: graphical user interface h : projection of the earthquake source to earth’s surface hw : water table level ic : soil behaviour type index m : earthquake magnitude mj : earthquake magnitude defined by japan meteorological association ml : local magnitude mw : moment magnitude msf: magnitude scaling factor nmea : measured standard penetration test blow count (n1)60 : corrected standard penetration test blow count (n1)60fc : standard penetration test blow count corrected with respect to fine content qc : tip resistance qc1 : corrected tip resistance qc1f : tip resistance corrected with respect to fine content pa : reference stress of 100 kpa or about atmospheric pressure rd : stress reduction coefficient rf : sleeve friction ratio sf: safety factor spt: standard penetration test vs : measured shear wave velocity vs1 : corrected shear wave velocity z : depth 1. introduction the assessment of the soil resistance against liquefaction or the amount of settlement of an engineered building caused by liquefaction is a remarkable phase concerning geotechnical and geophysical investigations. in earthquakeprone areas, during dynamic loading, i.e., an earthquake, pore water pressure increases in undrained and cohesionless soils. therefore, these materials lose their solid behavior and act as if liquefied materials (terzaghi and peck, 1948; ishihara and yoshimine, 1992; bekin and ozcep, 2017). in general, the earthquake hazard risk increases because of liquefied behavior. because of having great importance on engineered buildings and hence having a significant impact on natural hazards, researchers have studied the liquefaction phenomenon after the 1964 niigata earthquake. from the fundamental theoretical background point of view, scientists have been proposed empirical relationships to determine the probable effects and to reduce the hazard levels based on liquefaction for more than 30 years. for example, liao and whitman (1986) have introduced the stress reduction coefficient (rd) to calculate the cyclic stress ratio (csr). youd and idriss (1997) have presented a better way to compute csr in their study. andrus and stokoe (2000) have formulated the relationship between the shear wave velocity (vs) and the cyclic resistance ratio (crr). youd et al. (2001) have demonstrated the relationship between crr and corrected standard penetration test (spt) blow count numbers (n1)60. tokimatsu and seed (1987) have presented formulas to calculate the amount of expected settlement for liquefaction vulnerable regions dependent on (n1)60. likely, ishihara and yoshimine (1992) have proposed empirical formulas for settlements with respect to (n1)60 values and corrected tip resistance (qc1) values of cpt. the target of this study is to present a deterministic liquefaction analysis routine, soilique, with a graphical user interface (gui). this routine is the first matlab® based soil liquefaction analysis routine. the program presented in this study, soilique, includes several approaches for the assessment of the maximum extent of liquefaction-susceptible areas. firstly, kuribayashi and tatsuoka (1975), wakamatsu (1991), and wakamatsu (1993) that have related mj and the maximum distance for the liquefiable area (r). although liu and xie (1984) have used ml, ambraseys (1988) have made use of mw for the estimation. additionally, ulusay et al. (2000) have proposed the relation between ms and the maximum distance r. the results of soilique were compared to the liquefaction analysis panel of another program, soilengineering, introduced by ozcep (2010), to test the robustness of the algorithm. it is a widely used -especially in turkey-, free access, and user-friendly microsoft excel® based program for the geotechnical and geophysical analysis of soils. it provides the user to prepare data, to derive parameters, and to carry out analysis of geotechnical and geophysical problems. the fundamental characteristic of soilique is its independence from matlab®. in other words, soilique can be used without matlab® by installing matlab® runtime environment v9.7 to run. in the present study, the theory associated with soilique is explained in the second chapter. the third chapter focuses mainly on the framework of soilique. in the fourth chapter, the geological setting of the study area, where the real data are collected, and the analyses for checking the robustness of soilique are presented. in addition to the tables and plots presented in the results and discussion section, readers can find some plots and more detailed tables in the supplementary file. 2. theoretical background the theory associated with soilique is explained in this section. there are various approaches and assessment methods to conduct a solution-oriented liquefaction analysis. analyses that are mentioned in the present study can be classified as the assessment methods for the maximum extent of a liquefiable area, the calculations of settlements or prospective settlements, and liquefaction safety factor. these theoretical principles are represented in three sub-sections. in the first one, approaches of the maximum distance of a liquefiable area are explained. after that, safety factor analysis, including crr7.5 (cyclic resistance ratio, subjected to mw=7.5) and csr (cyclic stress ratio), calculations are presented as the second sub-section. in the next sub-section, calculations of the settlements triggered by liquefaction are explained. lastly, a simple analysis method, which has been introduced by tezcan and teri (1996), is explained. 2.1. the evaluation of a maximum extent of a liquefiable area several investigators have studied the spread of liquefaction during former earthquakes and proposed the relationship between the magnitude of an earthquake, m, and the maximum distance from the epicenter to the liquefied area, r. liquefaction prone areas can be estimated immediately from the magnitude of the predicted earthquake with the help of these approaches if the seismic activity in the corresponding area is known from the historic earthquake catalog. kuribayashi and tatsuoka (1975) have shown the relationship between magnitude and maximum epicentral distance to a liquefied site by using 32 historic japanese earthquakes. the logarithmic relation can be expressed as: log . * .r m j 0 77 3 6 (1) 325a comparative soil liquefaction analysis with a matlab® based algorithm: soilique where mj is the earthquake magnitude scale defined by japan meteorological association, r denotes the distance, in km, between the earthquake epicenter and the farthest liquefied site in km. liu and xie (1984) have proposed another relationship based on the chinese liquefaction database concerning the local magnitude (ml) : r m l  0 82 100 862 5. * . (2) where ml is the earthquake magnitude defined by richter (1935), and r is the maximum distance, in km, of the liquefied site to the epicenter. ambraseys (1988) has represented two different approaches to estimate the seismic moment magnitude (mw) from a distance between the epicenter and liquefied site. in the first approach, ambraseys (1988) has standardized the equation 3 for shallow and intermediate-depth earthquakes. according to the study, earthquakes having intermediate-depths and the much longer duration of shaking may induce liquefaction at distances greater than those dictated by shallow-focused earthquakes. the data points re and mw are bounded by the equation: m r rw e e  4 64 2 65 10 0 993. . * * . *log (3) where re is the maximum epicentral distance, and mw is the seismic moment magnitude. in the second approach, rf is the closest distance from a shallow seismic source of the furthest point of liquefaction, as a function of seismic moment magnitude mw. ambraseys (1988) has explained that the maximum fault distance rf, at which liquefaction may occur saturates at about mw = 8.0, beyond which value rf, remains constant and equal to about 160 km. as one of the results of the study, ambraseys has explained that it is rational for large shallow earthquakes where the maximum fault-depth remains constant without consideration of the fault-length, while mw depends upon fault-length. the expression that defines the upper bound for rf, as a function of mw can be given as: m r rw f f  4 68 9 2 10 0 93. . * * . *log (4) where rf is the maximum distance, in km, from causative fault to a liquefied site, and mw is the seismic moment magnitude. in addition to kuribayashi and tatsuoka (1975), wakamatsu (1991) have shown the relationship between magnitude, and maximum epicentral distance to a liquefied site by using 67 historic japanese earthquakes, including 32 earthquakes which were used by kuribayashi and tatsuoka (1975), the equation, which has bounded with mj>5.0, can be expressed as: log . *log . * †r m j  2 22 4 22 19 (5) the equation 5 has been revised, and wakamatsu (1993) has presented a newer version of the equation to estimate the maximum range of liquefaction for specific earthquake magnitude. this relation can be given as: log . *log . *r m j  3 5 1 4 6 (6) where r is the farthest epicentral distance, in km, and mj is the earthquake magnitude scale defined by japan meteorological association. lastly, ulusay et al. (2000) proposed the relationship between surface wave magnitude (ms) and the maximum distance r of a liquefied site for the turkey region. ulusay et al. (2000) have come up with an empirical formula taking the upper and the lower boundaries in consideration addition to the calculation of average value to figure out ms and r. in order to calculate a distance value (rcal, in km), one should use focal depth and another parameter that corresponds to a distance between the projection of the epicenter to the earth’s surface and the site subjected to liquefaction. the equation can be given as follows: rcal= ∆2+h2( ) (7) where  represents the focal depth, and h stands for the projection of the earthquake source point to the earth’s surface. after that, surface wave magnitudes can be calculated by the following equations: m rs cal  160 36/ (8) m rs cal  200 36/ (9) m rs cal  240 36/ (10) where the expression given in equation 8 is the lower boundary for the calculation, equation 9 and equation 10 are the medium value and the upper boundary of the magnitude calculation, respectively. from the point of r calculation view, the same procedure is valid. in other words, equations 11, 12, and 13 are given for r calculation (in km) with the discrimination of the upper, medium value, and the lowest boundaries, respectively. r m s *36 160 (11) r m s *36 200 (12) r m s *36 240 (13) all of these theoretical approaches were used in the maximum extent of the liquefiable area analysis part in soilique. merely, the medium value boundary calculation is used in the approach mentioned in ulusay et al. (2000). 2.2. safety factor analysis the safety factor is used to understand whether the investigation area is in the safe zone against liquefaction. various approaches have been represented by investigators to estimate the safety factor of liquefaction. the fundamental step in finding the safety factor is the division of cyclic resistance ratio (crr) and cyclic stress ratio (csr). hence, csr and crr must be calculated to mention the safety factor. csr, which is induced by a particular earthquake, was first introduced by seed and idriss (1967). it is an equivalent form of the uniform shear stress, which is triggered by the corresponding earthquake (boulanger and idriss, 2014). seed and idriss (1971) has introduced csr with a reference value equal to 65% of the peak cyclic stress. the empirical formula of csr is given in equation 14: csr a rmax d0 65 0 0 . * * *'   (14) where  0 is the total vertical overburden stress,  0 ' is the effective vertical overburden stress, amax is the peak ground acceleration and rd is a stress reduction coefficient which can be calculated by: rd=1.0-0.00765* z z£9.15 m (15) rd=1.174-0.0267 * z 9.15 m< z£23 m (16) where z is the depth of the layer in m. the calculation of stress reduction coefficient, rd , possesses other conditions for the depths of 30 m and greater. however, for cases where z is greater than 23 m, potential liquefiable layers may be neglected from the point of overburden stress view. liquefaction assessments at greater depths frequently include specific conditions that need to be verified by detailed analyses. because of that reason, idriss and boulanger (2008) have suggested completing site response studies by a high-quality site 326 ekrem bekin, ferhat ozcep response calculations for the sites where the rd values at depths greater than approximately 20 m. that is the reason why it is used only for up to 23 m depth values in the present study. crr, cyclic resistance ratio, can be called the strength of the soil. unlike csr, crr can be calculated from the standard penetration test (spt), conic penetration test (cpt), and shear wave velocity (vs) measurements. in general, crr curves were firstly introduced for an earthquake having a moment magnitude equals 7.5, and its symbol is crr7.5. for different moment magnitude values, one must switch from crr7.5 to crr, by multiplying it with a magnitude scaling factor (msf). it can be given as: msf m w102 24 2 56. ./ (17) where mw denotes the moment magnitude of an earthquake. then, the safety factor, sf, can be calculated with the division of the crr and csr: sf crr csr / (18) 2.2.1. obtaining crr7.5 from spt data in soilique, the methodology mentioned in seed and idriss (1971) is used to obtain crr7.5. in the theory of sptcrr7.5 computations, spt data must be corrected before calculations. the formulation of spt blow count correction can be given as: ( ) * * * * *n n c c c c cmea n e b r s1 60  (19) where the left-hand side of the equation represents the corrected spt blow count, nmea is the measured spt blow counts in a field, cn is the depth correction factor, ce corresponds to hammer energy ratio (er) correction factor, cb is the borehole diameter correction factor, cr denotes rod length correction factor, and cs stands for correction factor for samplers with or without liners. details of these corrections for spt blow count given by youd and idriss (2001) are also represented in table 1. table 1. corrections for spt blow count (modified from youd and idriss, 2001) factor equipment variable term correction overburden pressure cn cn v ap 2 2 1 2 0. / ( . / )' cn£1.7 energy ratio safety hammer ce 0.5 1 donut hammer 0.7 1.2 automatic trip donut type hammer 0.8 1.3 borehole diameter 65 115 mm cb 1 150 mm 1.05 200 mm 1.15 total rod length < 3 m cr 0.75 3 m to 4 m 0.8 4 m to 6 m 0.85 6 m to 10 m 0.95 10 m to 30 m 1 sampling method standard sampler cs 1 sampler without liners 1.1-1.3 having the spt n is corrected, the procedures stated in firstly seed and idriss (1971), and the fine content correction, which has been updated by youd and idriss (2001), is applied to calculate crr7.5. the procedure starts with the formula given below: ( ) *( )n nfc1 60 1 60   (20) and where  and  are: a=0, b =1 f.c.£%5 (21)         fc fc f cexp[1.76 190 / ) , 0.99 / 1000 %5 . . %352 1.5 (22)    f c5, 1.2 . . %35 (23) where f.c. means fine content. the equations 21, 22, and 23 are used for calculation of the coefficients of fine content correction of spt blow count, given in equation 20, in terms of fines content percentage in the samples. then, crr7.5 can be obtained with the formula: crr n n nfc fc fc 7 5 1 60 1 60 1 60 2 1 34 135 50 10 45 1 200. ( ) ( ) *( )        (24) which has the upper limit at 30 for ( )n fc1 60 in (youd and idriss, 2001). 2.2.2. obtaining crr7.5 from vs measurements shear-wave velocities (vs) can be measured in situ with the help of several seismic techniques including cross hole, downhole, seismic cone penetrometer, suspension logger, sasw, and masw. their accuracy can be sensitive to processing details, soil conditions, and interpretation methods andrus and stokoe (2000). one can correct vs to a reference overburden stress by (sykora, 1987; robertson et al. 1992) v c vs v sm1 * (25) where cv = factor to correct measured shear-wave velocity for overburden pressure and can be calculated by c pv a / ' .  0 0 25 (26) where vs1 is overburden stress-corrected shear-wave velocity, pa is the reference stress of 100 kpa or about atmospheric pressure; and  0 ' is the initial effective overburden stress (kpa). then, crr7.5, based on vs measurements, can be calculated by;                                crr v v f c0.022* 100 2.8* 1 215 1 215 . . %5s s 7.5 1 2 1 crr7.5=0.022* vs1 100 � � �� � � �� 2 +2.8* 1 215-vs1 � � � � � � � �1 215 � � �� � � �� � � � � � � � � f.c.£%5 (27) crr v fc v s s 7 5 1 2 1 0 022 100 2 8 1 215 0 5 5 1 2. . * . * .                  115 0 5 5               . fc (28) % . . %5 35 f c                                crr v v f c0.022* 100 2.8* 1 200 1 200 . . %35s s 7.5 1 2 1 (29) where f.c. is the representative of the fine content (andrus and stokoe, 2000). 2.2.3. obtaining crr7.5 from cpt data the crr7.5 calculation of cpt data in soilique is based on the technique which was published by suzuki et al. in 1997. this technique includes the computation of a soil behavior type index, ic, and the adjustment of measured tip resistance with factor fs. this factor is also a function of the soil behavior 327a comparative soil liquefaction analysis with a matlab® based algorithm: soilique type index. in this technique, the measured tip resistance is corrected in terms of overburden pressure with the formula given below: q q p p c c a a 1 0 2         ' (30) where qc1 is the corrected tip resistance, qc is the measured one,  0 ' is the effective overburden stress (kpa), and pa is the reference stress of 100 kpa or about atmospheric pressure. the next step is the calculation of the soil behavior index, which has been represented by robertson et al. (1995) : i q rc f        3 47 1 222 2 0 5 . log log . . (31) where q and rf are: q qc f 1 0 0   ' (32)     r f q *100 %f s c f1 0 (33) and where qc1f is the fines corrected tip resistance in tsf, fs is the measured sleeve friction,  0 is the total vertical overburden stress,  0 ' is the effective vertical overburden stress, q is a normalized tip resistance, and rf is a sleeve friction ratio. the last step before calculating the crr7.5 is the adjustment of the tip resistance, and it can be achieved by the formula as follows: q q f ica c f c  1 * (34) qca is equivalent to the adjusted tip resistance, and as it has already mentioned before, f is the function of soil behavior index and with the range between 1 and 3.5 depending on changing values of the soil behavior type index, ic. for the values less than and equal to 1.65, f(ic) turns into one (1); and for values higher than and equal to 2.4, it takes 3.5. in the range between 1.65 and 2.4, it takes different values. in the present study, crr7.5 is calculated with the formula, which was proposed for the method of suzuki et al. (1997), mentioned in mollamahmutoglu and babuccu (2006) instead of using crr7.5 liquefaction curve versus adjusted tip resistance given by suzuki et al. (1997). 2.3. settlement analysis if the safety factor of liquefaction is known, it is possible to determine the variation of the post-liquefaction volumetric strain (ishihara and yoshimine, 1992). the combination of safety factor and the volumetric strain was proposed by ishihara and yoshimine (1992), and it is given in figure 1. the relationship between corrected spt blow count, ( )n1 60, and the post-liquefaction volumetric strain was proposed by tokimatsu and seed (1987). the combination can be read from figure 2. in the present study, equations introduced by mollamahmutoglu and babuccu (2006) are used to compute the possible post-liquefaction settlements. they have used the curve overlaying technique and proposed equations for both methods of (ishihara and yoshimine, 1992) and (tokimatsu and seed, 1987). the objective is the computation of possible post-liquefaction settlement after the estimation of volumetric strain. settlements can be calculated with the formula below: s zsat v ( / )* 100 (35) where z is the thickness of the layer in m, v is the volumetric strain, and ssat signifies the corresponding saturated soil layer. 2.4. simple analysis the relation between relative density (dr), in percentages, and peak ground acceleration (pga), in g, has been proposed by tezcan and teri (1996). figure 1. the combination of the safety factor, fs, and post-liquefaction volumetric strain v (modified from ishihara and yoshimine, 1992) figure 2. the relationship between corrected spt, (n1)60, and volumetric strain (modified from tokimatsu and seed, 1987) 328 ekrem bekin, ferhat ozcep according to the correlation, the intersection of dr and pga indicates the risk of liquefaction. in this context, “risk” means the frequency of liquefaction exposure. in figure 3, the graph of the correlation is represented; and as can be seen, three zones of different risk levels are demonstrated on it. the first one is high –risk zone, which defines if the intersection of those is located at this zone, the study area has a high risk of liquefaction. the second zone is moderate-risk, which indicates that if the intersection point of the parameters is located in this zone, the study area has a moderate risk of liquefaction. lastly, the third one is the non-liquefiable area. if the intersection of the parameters is in that zone, the study area is out of liquefaction risk. figure 3. the graph of the relation between relative density (dr) and peak ground acceleration (pga). modified from tezcan and teri (1996). 3. overview of soilique soilique is the first matlab® based program with gui that ensures deterministic liquefaction analysis with the calculation of parameters that are previously propounded with the empirical equations. the substantial feature of matlab® that makes it different from other programming tools is having a matrix-based language paving the way for computation of mathematical expressions. it provides an excellent environment for any courses (chapra, 2012). although other environments (e.g., excel©) or languages (e.g., c++, fortran 90) could have been chosen, matlab® presently offers a combination of convenient programming features with powerful built-in algorithms (chapra, 2012). the user-friendly nature of matlab® explains why it is widely used to solve scientific and engineering problems. in addition to its user-friendly nature, having rich libraries (i.e., data and image processing tools) enlarges the implementation of matlab®. graphical user interface (gui) preference permits a code writer to create an interactive program that is semi-independent of matlab®. specifically, one can construct a program in gui, set it apart from matlab® and then, can run it after its installation to any computer provided that the installation of proper matlab® runtime environment version. the gui feature is the glossy point of matlab®, which leads to soilique. moreover, matlab® allows users to switch between other languages. for example, a code written in matlab® can be converted to c ++, java, or other languages. the algorithm presented here, soilique, is designed for the mitigation of liquefaction triggered hazards based on four different calculation classes such as 1) sf analysis allowing estimation of sf of liquefaction with respect to shear wave velocity, spt blow count data, and cpt data, 2) settlement analysis that provides to evaluate calculations of settlements based on liquefaction, 3) the evaluation of a maximum extent of a liquefiable area maintains predicting the distance of a liquefiable area to the epicenter or causative fault with corresponding earthquake magnitude, and lastly, 4) simple analysis with only considering relative density (dr) and the peak ground acceleration (pga) of the earthquake. the workflow of the safety factor and settlement analyses part of soilique is depicted in figure 4. it can be inferred from the figure that soilique has two main decision mechanisms. the first one is related to the specific part of the input file containing information on whether the soil structure includes fine or coarse particles. the second one is checking the shear wave velocity or spt blow count values in both settlement and safety factor analyses. figure 4. the workflow of soilique for safety factor and settlement analyses. these checking mechanisms are fundamentally based on the theoretical background of the program. in order to obey the calculation rules of formulas given in the theory section, soilique has several limitations. these limitations are listed in table 2. in table 2, “terminate” means considering safety factor as 1.2 and settlement as zero for the corresponding investigation depth. in the past, some liquefaction events have been observed while sf using related methods and assumptions were calculated to be greater than 1.00. in order to be on the safe side, it would be better to consider sf as 1.2. that is the reason why sf is capped to 1.2. of course, this does not mean that liquefaction events will not be observed definitely for values higher than 1.2. thus, it is a threshold value for the present study, and settlements are not expected for the values higher than 1.2. in the case of sf equals to 1.2, the program supposes that the subjected layer is non-liquefiable and terminates calculations of parameters for the corresponding layer. then it continues to calculate liquefaction analysis parameters of the next layer/investigation depth. vs based sf analysis is designed for calculations with or without fc data. the presumption of a limiting upper value of corrected shear wave velocity, vs1, is considered as 215 m/s for areas with f.c. less than %5, and it is the same for the areas having f.c. values between %5 and %35. the upper limit for the shear wave velocity is considered as 200 m/s for areas having f.c. values equal to and greater than %35, according to the equation 29. this limitation is a similar presumption that is commonly made in the spt and cpt-based procedures dealing with clean sands, where liquefaction is considered not possible (andrus and stokoe, 2000). if the f.c. data is accessible besides vs measurements, one can consider it while creating the input file. in case of having no f.c. data, soilique computes vs based parameters based on equation 27. additionally, there are several limitations to the calculation of the maximum distance of a liquefiable area, too. for instance, in the techniques of wakamatsu (1991) and wakamatsu (1993), the user must enter values higher than 5.0 for mj as input. in addition to this issue, the user must recognize that soilique only uses medium value calculations mentioned in ulusay et al. 329a comparative soil liquefaction analysis with a matlab® based algorithm: soilique (2000), the upper boundary and lower boundary calculations are neglected in the present study. table 2. limitations of soilique parameter / method condition result / calculation status safety factor – sf ≥ 1.2 non-liquefiable /terminates < 1.2 liquefiable / continues corrected spt values – (n1)60 ≥ 34 nonliquefiable / terminates < 34 liquefiable / continues shear wave velocity – vs ≥ 215 m/s fc<%5 nonliquefiable / terminates ≥ 215 m/s %5< fc <%35 ≥ 200 m/s fc > %35 < 215 m/s fc<%5 liquefiable / continues< 215 m/s %5< fc <%35 < 200 m/s fc > %35 ≥ 215 m/s without fc nonliquefiable / terminates < 215 m/s without fc liquefiable / continues fine content – fc ≥ 35 nonliquefiable / terminates < 35 liquefiable / continues water table level – hw hw > z liquefiable / continues hw ≤ z nonliquefiable / terminates wakamatsu (1991) mj < 5.0 terminates mj > 5.0 continues wakamatsu (1993) mj < 5.0 terminates mj > 5.0 continues ulusay et al. (2000) (*calculation of r) ms < 5.5 terminates ms > 5.5 continues 4. results and discussion to test the robustness of soilique, soil engineering, which was introduced by ozcep (2010), was used for comparison. a real data set from ozcep et al. (2014) is utilized in the present study for settlement and spt-based analyses. ozcep et al. (2014) have investigated to understand what role the soil properties play as one of the main factors causing earthquake damage in yalova, and to compare the influence of two soil problems – amplification and liquefaction – on earthquake damage (ozcep et al. 2014). all of the analysis sections, except settlement and sf analyses based on spt data, were tested with synthetic data and presented under this heading. 4.1. results of real data set the geological characteristics of yalova can be explained by extensive quaternary alluvial deposits, and the tertiary yalakdere and kilic formations. borehole locations are superimposed on the geological setting map in figure 5. the quaternary deposits consist of stratified materials having varied grain sizes and are derived from the various geological units in the vicinity (yilmaz and yavuzer 2005). the main drainage system is dominated by the samanli, safran, balaban, and kazimiye rivers. borehole records throughout the study area show that the groundwater table is generally very shallow (yılmaz and yavuzer, 2005; ozcep et al. 2014). for the analysis process, the data obtained from three boreholes, bh43, bh76, and bh106, are used. every analysis was carried out with the parameters mw was considered as 7.4, and pga was taken as 0.4 g. the water table levels of the boreholes, bh43, bh76, and bh106, are 2.65 m, 1.72 m, and 3.8 m, respectively. the map of the study area by indicating settlement distribution and locations of the selected boreholes is presented in figure 6. figure 5. map of the study region, including geological setting. red plus signs correspond to borehole locations. modified from ozcep et al. (2014). figure 6. map of liquefaction induced settlements of the study area using the ishihara and yoshimine (1992) approach. black dots display locations of the boreholes, bh43, bh76, and bh106. dashed lines show the boundary of the main study area. modified from ozcep et al. (2014). according to the map presented in figure 6, areas with the yellow color indicate the lowest amount of total settlement (0 10 cm) and, the green-colored areas correspond to middle settlement amounts (10 30 cm). likewise, the red-colored areas have the highest amounts of settlement (30 58 cm). the selected three boreholes are relatively exemplary in terms of the amounts of different settlement quantities. the data of those boreholes were used to present a comparative study to test the robustness of soilique. one way to compare the results can be achieved by checking how close the layer-based settlement calculations of soilique and soilengineering (ozcep, 2010) for all boreholes. the settlement amount of each layer for the borehole, bh43, can be found in table 3. detailed tables for the calculations can be found in the supplementary data file. additionally, the log plots for every borehole from soilique, and soilengineering (ozcep, 2010) are presented in figure 7, figure 8, and figure 9 for boreholes bh43, bh76, bh106, respectively. these figures show that trends of the variation of sf with depth are consistent. as demonstrated in table 3, the outputs of soilique and soilengineering (ozcep, 2010) are consistent with each other. for the total settlement comparisons, the results of both software show good agreement when taking all results and plots into consideration. the layer-based settlement graphs are presented in figure 10. these plots are specific to soilique. each row in figure 10 corresponds to the results of boreholes, bh43, bh76, and bh106, respectively. these results were obtained by using the settlement analysis based on spt. when sf varies below the safety 330 ekrem bekin, ferhat ozcep limit, the layer-based settlements are calculated and plotted using two different approaches, such as ones introduced by tokimatsu and seed (1987) and ishihara and yoshimine (1992). on the far left of figure 10, panels a, d, and g, the variation of settlement for each layer plots based on tokimatsu and seed (1987) approach is presented. the middle panels of figure 10, b, e, and h, the variation of settlement for each layer plots depending on the ishihara and yoshimine (1992) approach is demonstrated. on the far right of figure 10, panels c, f, and i represent the relationship between crr and spt (n1)60 mentioned in youd et al. (2001) is presented. soilique represents that graph by putting a red circle where crr and spt (n1)60 values intersect at a corresponding layer. as can be seen from the figures, these red circles were cumulated in or around the liquefiable region. it might be evidence of consistent calculation in terms of comparison of the already proven technique. another feature of soilique is that plotting layer-based log plots showing liquefaction-prone layers. in figure 11, liquefaction log plots of soilique are demonstrated. in this figure, reds correspond to layers that are prone to liquefaction, and blues show safe layers. this kind of graphs helps the user to visualize the layers which may be prone to liquefaction. in figure 4.7, panels from (a) to (c) are the representative of boreholes bh43, bh76, and bh106, respectively. those logs are plotted with respect to sf values. in addition to the layer-based settlement comparisons, the total settlements that were calculated by soilique and soilengineering (ozcep, 2010) were compared. a more detailed results table for the total settlement amounts is presented in the supplementary file. the total settlement amounts for every borehole calculated by soilique, are presented in table 4. another way to test the robustness of the calculations done by soilique can be achieved by using the map in figure 6. for this testing process, the total amounts calculated by ishihara and yoshimine (1992) approach in table 4 are focused only. as a result, the amounts calculated in the present study are consistent with the ranges represented in the color bar of the map. in detail, the borehole bh43 has the highest total settlement amount corresponding to the red-colored areas in figure 6, and the borehole bh76 has the intermediate values as indicated by the green color on the map. the borehole bh106, which is located at the margin between kilic formation and alluvial deposits of yalova, has the lowest total settlement amount as shown with yellow color on the map. after pointing out the consistency between the results of soilique and soilengineering (ozcep, 2010), it would be better to mention the differences. it is obvious that small variations can be observed between the results presented in table 3. these differences are related to the work procedures of soilique and soilengineering (ozcep, 2010). for example, soilique assumes the highest limit for sf as 1.2, even though it is 2.0 in soilengineering (ozcep, 2010). besides, a slight difference can be observed between the results of the total vertical stress and the effective vertical stress in both software. hence, these small differences affect csr values. this fundamental difference occurs because soilique considers the unit of the unit volume weight parameter as kn/m2, whereas soilengineering takes it as g/cm3. a comparative plot is presented in figure 12 to show the trends of csr and crr values of both software. although these differences have an impact on the calculations of the stress parameters -i.e., total vertical stress and effective vertical stressand csr values, it can be inferred from the figure that the mentioned differences can be neglectable. lastly, the difference in the limitations for correction of the spt values may cause some differences in crr values. however, these effects are also negligible because the corresponding limits of the software are not far away from each other to affect the results negatively. these differences are valid for the vs-based calculations, which can be observed from the vs output tables in the supplementary file, and they can be neglected. table 3. comparison of settlement results for bh43 calculated with soilique and soilengineering (ozcep, 2010) soilique results soilengineering (ozcep, 2010) results z (m) ishihara and yoshimine (1992) tokimatsu and seed (1987) ishihara and yoshimine (1992) tokimatsu and seed (1987) 1.8 0 0 0 0 3.3 4.88 3.99 4.89 4.01 4.8 5.15 4.37 5.14 4.35 6.3 5.14 4.35 5.14 4.35 7.8 6.31 6.2 6.3 6.18 9.3 7.95 11.7 7.95 11.7 10.8 8.06 12.35 8.05 12.3 12.3 7.96 11.76 7.95 11.7 13.8 6.64 6.73 6.64 6.72 15.3 6.52 6.53 6.53 6.54 total (cm) 58.62 67.98 58.59 67.85 figure 7. the variation of sf with depth for the borehole, bh43 a) from soilique, b) from soil engineering (ozcep, 2010). figure 8. the variation of sf with depth for the borehole, bh76 a) from soilique, b) from soil engineering (ozcep, 2010). figure 9. the variation of sf with depth for the borehole, bh106 a) from soilique, b) from soil engineering (ozcep, 2010). 331a comparative soil liquefaction analysis with a matlab® based algorithm: soilique figure 10. other plots of results from soilique. the first row (a, b, c) of the figure are the results for bh43 data, the second row (d, e, f) correspond to results of bh76 data, and the last row (g, h, i) shows the results of bh106 data. panels a, d, g show the settlements for each layer with respect to tokimatsu and seed (1987) approach, and panels b, e, h show the settlements for each layer with respect to ishihara and yoshimine (1992) approach. lastly, panels c, f, i show the crr and (n1)60 relationship mentioned in youd et al., (2001). this graph of relationship is digitized, and red circles correspond to crr values. figure 11. liquefaction log plots from soilique a) for bh43, b) for bh76 and c) for bh106 332 ekrem bekin, ferhat ozcep table 4. table of total settlements calculated with soilique for all boreholes. bh id total settlements ishihara and yoshimine (1992) tokimatsu and seed (1987) bh43 58.62 cm 67.98 cm bh76 52.69 cm 57.34 cm bh106 19.72 cm 17.06 cm comparison of results from soilique and soilengineering (ozcep, 2010) reveals that although the work procedures of soilique and soilengineering (ozcep, 2010) may differ, the results are satisfactory. 4.2. results of synthetic data set in the present study, the synthetic data set is used for showing results for safety factor analysis based on vs values and cpt data. the safety factor analysis with respect to spt data calculations of soilique gives the same outputs except for settlement graphs and calculations. details of the input table and result table of safety factor analysis based on vs analysis are presented in the supplementary file. the water table level is considered as 4.4 m, mw is taken as 7.4, and pga is accepted as 0.4 g for that calculation. in figure 13, the results of the calculation based on vs values are presented. that type of calculation has three visual outputs. one of them is the variation of sf with depth; the others are crr-vs1 relation, which has been represented by andrus and stokoe (2000), and a liquefaction log plot. the limitation of the vs based calculation is having the corrected shear wave velocities (vs) below 215 m/s; in that case, the analysis will continue. however, if the corrected vs values are higher than 215 m/s, the corresponding layer or layers will have sf equal to 1.2. in this example, all corrected vs values are below this limit. that is the reason why all layers have sf below 1.2. as well as the real spt data set, the synthetic vs data analysis is also checked with soilengineering (ozcep, 2010). in this example, the fine content is not taken into consideration. input and result tables of this analysis are demonstrated in the supplementary file. the plot of vs-based analysis from soilengineering (ozcep, 2010) is shown in figure 14. in panel a, the screenshot of csr results for vs calculation from soilengineering is shown. as a comparison, both soilique and soilengineering (ozcep, 2010) consider the first two layers as not be prone to liquefaction. the trend of sf variations with depth is close to each other. in addition to the synthetic vs data analysis, an example analysis with synthetic cpt was carried out. in this synthetic example, the water table level is considered as 2.6 m, mw is taken as 7.4, and pga is accepted as 0.4 g for that calculation. unfortunately, the results of the cpt data set could not be compared to any other programs because a free program that performs suzuki et al. (1997) approach cannot be found. in figure 15, the output of sf analysis based on synthetic cpt values are displayed. the routine presented here, soilique, gives three types of visual outputs for cpt-based analysis. one of them is the variation of the sf with respect to depth; the others are crr qc1 relation that has been introduced by suzuki et al. (1997) and the liquefaction log that shows which layers are liquefiable and which are not. crr qc1 relation graph is redrawn after digitizing the original plot, and soilique adds points at intersections during the calculation. in the second graph of figure 15, one can observe that most of the red circles were gathered in the liquefiable region. these layers are responsive ones to liquefaction. that plot is one of the evidence for the calculation accuracy of soilique. a simple analysis can be carried out by using soilique. in the presented algorithm, the output is a graph that is replotted from tezcan and teri (1996) with a red plus at the intersection point of those two parameters. for example, in this study, dr is considered as 0.6 (%), and pga is taken as 0.25 g to represent a plot. as a result, if the study area possesses dr and pga as or similar to those taken in that example, it means that the corresponding area has a substantial risk of liquefaction. the output of this example can be found in the supplementary file. lastly, the maximum extent of a liquefiable area was carried out as an example. the output can be found in the supplementary file. the theory figure 12. comparative plots of crr and csr results from soilique and soil engineering for liquefiable layers of a) bh43, b) bh76, c) bh106. red circles stand for soilique crr 333a comparative soil liquefaction analysis with a matlab® based algorithm: soilique mentioned in liu and xie (1985) is used to calculate the maximum extent of a liquefiable area during an earthquake with ml equals 6.7. as a result, the maximum extent is 23.945 km. 5. conclusion we have presented the first deterministic liquefaction analysis gui written in matlab®. the capability of soilique for performing liquefaction analysis is presented through two separate data sets, real and synthetic data. in order to check the robustness of soilique, results were compared with another program, soilengineering (ozcep, 2010). in terms of work procedure, soilique and soilengineering (ozcep, 2010) differs from each other. although soilengineering (ozcep, 2010) has multiple calculation sections for a wide range of geotechnical problems, soilique is focused only on deterministic liquefaction analyses. moreover, soilique includes alternative approaches for the calculation of the maximum extent of a liquefiable area, although soilengineering (ozcep, 2010) includes only one of them. likewise, soilique provides to visualize the boreholes in terms of liquefaction. the results demonstrate that soilique obeys the limitations during calculations. for example, the choice of fine or coarse option in the second column of the input file directly affects the calculations as expected. in this study, the limitations of soilique are presented in table 3.1. as can be seen from the table, soil layers with spt values greater than 34, and in most cases, the shear wave velocity greater than 215 m/s is accepted as non-liquifiable by soilique. the limitation for f.c. is 35%, meaning that layers having f.c. lower than 35% is liquifiable. additionally, it is evident that the limitations for fine content, spt blow counts, vs values, and the cpt-based calculations affect the results properly. figure 13. outputs, from soilique, for synthetic data that is used safety factor analysis based on vs values. a) the variation of the safety factor with depth. b) the crr-vs1 relation that is taken from andrus and stokoe (2000) and redrawn with red circles at the intersection points. c) the liquefaction log. the water table level is considered as 4.4 m, mw=7.4, and pga=0.4 g. figure 14. the graph of variation of sf with depth from soilengineering (ozcep, 2010) for synthetic vs data. figure 15. outputs, from soilique, for synthetic data that is used safety factor analysis based on cpt values. a) variation of the safety factor with depth. b) crr qc1 relation is taken from suzuki et al. (1997) and redrawn with red circles at the intersection points. c) liquefaction log. the water table level is considered as 2.6 m, mw=7.4, and pga= 0.4 g. 334 ekrem bekin, ferhat ozcep comparisons were made to check whether the results of soilique and soilengineering (ozcep, 2010) are consistent or not. these tests reveal that results were satisfactory and coincident with each other. that is to say; the calculated total settlement results yield a good agreement with the results of the previous study and soilengineering (ozcep, 2010). the key features of soilique are its easy-to-use nature, and being equipped with a variety of deterministic liquefaction analyses. in addition to these, its specific visual results make it easier to understand the liquefaction phenomenon for users. consequently, soilique is proven useful in terms of determination of the liquefaction triggered settlements and sf with short computation time. 6. code availability soilique is available as an open-source code and can be downloaded from the web site [https://github.com/ekrembkn/soilique]. in the same directory, a user guide and examples can be found. it can be redistributed and/or modified under the terms of gnu library general public license 3.0. the user can download “soilique.exe” to use it without matlab® instead of source codes, optionally. acknowledgments this research was partly supported within the framework of the european liquefact project. the liquefact project has received funding from the european union’s horizon 2020 research and innovation programme under grant agreement no. 700748. references ambraseys, n. n. (1988). engineering seismology: part i. earthquake engineering & structural dynamics, 17(1), 1-50. https://doi. org/10.1002/eqe.4290170102 andrus, r. d., & stokoe ii, k. h. (2000). liquefaction resistance of soils from shear-wave velocity. journal of geotechnical and geoenvironmental engineering, 126(11), 1015-1025. https://doi.org/10.1061/ (asce)1090-0241(2000)126:11(1015) bekin, e., & ozcep, f. (2017). soilique: a matlab® based program to analyze soil liquefaction and some applications/comparisons. proceedings of egu general assembly conference abstracts (vol. 19, p. 11868). boulanger, r. w., & idriss, i. m. (2014). cpt and spt based liquefaction triggering procedures. report no. ucd/cgm.-14, 1. chapra, s. c. (2012). applied numerical methods with matlab for engineers and scientists. new york: mcgraw-hill. idriss, i. m., & boulanger, r. w. (2008). soil liquefaction during earthquake. engineering monograph, eeri, california, usa, 266. ishihara, k., & yoshimine, m. (1992). evaluation of settlements in sand deposits following liquefaction during earthquakes. soils and foundations, 32(1), 173-188. https://doi.org/10.3208/ sandf1972.32.173 kuribayashi, e., & tatsuoka, f. (1975). brief review of liquefaction during earthquakes in japan. soils and foundations, 15(4), 81-92. https://doi. org/10.3208/sandf1972.15.4_81 liao, s. s., & whitman, r. v. (1986). overburden correction factors for spt in sand. journal of geotechnical engineering, 112(3), 373-377. https:// doi.org/10.1061/(asce)0733-9410(1986)112:3(373) liu, y., & xie, j. f. (1984). seismic liquefaction of sand. earthquake pres, china. mollamahmutoglu, m., & babuccu, f. (2006). zeminlerde sivilasma analiz ve iyilestirme yontemleri. gazi kitabevi. ozcep, f. (2010). soilengineering: a microsoft excel® spreadsheet© program for geotechnical and geophysical analysis of soils. computers & geosciences, 36(10), 1355-1361. https://doi.org/10.1016/j. cageo.2010.01.015 ozcep, f., karabulut, s., ozel, o., ozcep, t., imre, n., & zarif, h. (2014). liquefaction-induced settlement, site effects and damage in the vicinity of yalova city during the 1999 izmit earthquake, turkey. journal of earth system science, 123(1), 73-89. https://doi.org/10.1007/s12040013-0387-7 robertson, p. k., woeller, d. j., & finn, w. d. l. (1992). seismic cone penetration test for evaluating liquefaction potential under cyclic loading. canadian geotechnical journal, 29(4), 686-695. https://doi. org/10.1139/t92-075 seed, h. b., & idriss, i. m. (1967). analysis of soil liquefaction: niigata earthquake. journal of the soil mechanics and foundations division, 93(3), 83-108. seed, h. b., & idriss, i. m. (1971). simplified procedure for evaluating soil liquefaction potential. journal of soil mechanics and foundations, 97(9), 249-1273. https://doi.org/10.1061/jsfeaq.0001662 suzuki, y., koyamada, k., & tokimatsu, k. (1997). prediction of liquefaction resistance based on cpt tip resistance and sleeve friction. proceedings, 14th international conference on soil mechanics and foundation engineering, hamburg, germany, vol. 1, pp. 603–06 sykora, d. w. (1987). creation of a data base of seismic shear wave velocity for correlation analysis. geotechnical laboratory. miscellaneous paper gl-87-26, u.s. army engineer waterways experiment station, vicksburg, miss. terzaghi, k., & peck, r. b. (1967). soil mechanics in engineering practice. second ed. wiley international edition, new york 321pp. tezcan, s., & teri, l. (1996). shear wave propagation and liquefaction in layered soils, türkiye deprem vakfı yayınları, no: tdv. tr 96-0005. tokimatsu, k., & seed, h. (1987). evaluation of settlements in sands due to earthquake shaking. journal of geotechnical engineering, 113(8), 861-878. https://doi.org/10.1061/(asce)0733-9410(1987)113:8(861) ulusay, r., aydan, o., kumsar, h., sonmez, h., & tuncay, e. (2000). turkiye’de son depremlerde gozlenen sivilasma olgusu ve bati anadolu’da sıvılasma potansiyeline genel bir bakis. bati anadolu’nun depremselligi sempozyumu. izmir, bildiriler kitabi, 323-336. yilmaz, i., & yavuzer, d. (2005). liquefaction potential and susceptibility mapping in the city of yalova, turkey. environmental geology, 47(2), 175-184. https://doi.org/10.1007/s00254-004-1141-x youd, t. l., & idriss, i. m. (1997). proceedings of the nceer workshop on evaluation of liquefaction resistance of soils. youd, t. l., idriss, i. m., andrus, r. d., arango, i., castro, g., christian, j. t., dobry, r., finn, w. d. l., harder, l. f. jr., hynes, m. e., ishihara, k., koester, j. p., liao, s. s. c., marcuson, w. f. iii., martin, g. r., mitchell, j. k., moriwaki, y., power, m. s., robertson, p. k., … stokoe, k. h. ii. (2001). liquefaction resistance of soils: summary report from the 1996 nceer and 1998 nceer/nsf workshops on evaluation of liquefaction resistance of soils. journal of geotechnical engineering, 127(10), 817–833. https://doi.org/10.1061/(asce)10900241(2001)127:10(817) https://github.com/ekrembkn/soilique https://doi.org/10.1002/eqe.4290170102 https://doi.org/10.1002/eqe.4290170102 https://doi.org/10.1061/(asce)1090-0241(2000)126:11(1015) https://doi.org/10.1061/(asce)1090-0241(2000)126:11(1015) https://doi.org/10.3208/sandf1972.32.173 https://doi.org/10.3208/sandf1972.32.173 https://doi.org/10.3208/sandf1972.15.4_81 https://doi.org/10.3208/sandf1972.15.4_81 https://doi.org/10.1061/(asce)0733-9410(1986)112:3(373) https://doi.org/10.1061/(asce)0733-9410(1986)112:3(373) https://doi.org/10.1016/j.cageo.2010.01.015 https://doi.org/10.1016/j.cageo.2010.01.015 https://doi.org/10.1007/s12040-013-0387-7 https://doi.org/10.1007/s12040-013-0387-7 https://doi.org/10.1139/t92-075 https://doi.org/10.1139/t92-075 https://doi.org/10.1061/jsfeaq.0001662 https://doi.org/10.1061/(asce)0733-9410(1987)113:8(861) https://doi.org/10.1007/s00254-004-1141-x https://doi.org/10.1061/(asce)1090-0241(2001)127:10(817) https://doi.org/10.1061/(asce)1090-0241(2001)127:10(817) 335a comparative soil liquefaction analysis with a matlab® based algorithm: soilique supplementary data a comparative soil liquefaction analysis with a matlab® based algorithm: soilique in this supplementary data file, input tables and computation results for analyses carried out with real data and synthetic data were presented. in the first section, one can find the input and output tables of boreholes, bh34, bh76, and bh106 were presented. these borehole data are used both in soilique and soilengineering (ozcep, 2010). the reader would find the liquefaction induced settlement (layer-based and total settlements) results for all boreholes in detail. in the second section, input tables and output tables for synthetic data are presented. after these tables, a screenshot figure of a simple analysis example and the maximum extent of a liquefiable area example were demonstrated. 1. real data 1.1. settlement analysis based on spt data table s.1. table of data input for bh43. the water table levels of bh43 is 2.65 m. mw and pga are considered as 7.4 and 0.4 g, respectively. z (m) fine (1) / coarse(2) spt n γ (kn/m2) cn cb cr cs energy for ce fine content (%) 1.8 1 8 18 1.7 1 1 0.8 45 82 3.3 2 7 18 1.37 1 1 0.8 45 30 4.8 2 6 18 1.24 1 1 0.8 45 39 6.3 2 8 18.5 1.14 1 1 0.8 45 25 7.8 2 7 18.9 1.06 1 1 0.8 45 15 9.3 2 6 19.2 1 1 1 0.8 45 5 10.8 2 6 19.5 0.94 1 1 0.8 45 3 12.3 2 6 19.8 0.9 1 1 0.8 45 8 13.8 2 7 20 0.86 1 1 0.8 45 15 15.3 2 8 20.5 0.82 1 1 0.8 45 15 table s.2. results of bh43 data calculated by soilique. settlements having (*) denotes tokimatsu and seed (1987) approach and (**) denotes ishihara and yoshimine (1992) approach. z (m) σ σ’ csr (n1)60 (n1)60fc crr sf settlements* (m) settlements** (m) 1.8 32.4 32.4 0 0.0 0.0 0 1.20 0 0 3.3 59.4 53.0 0.2839 5.8 11.3 0.1252 0.46 0.0399 0.0488 4.8 86.4 65.3 0.3313 4.5 10.4 0.1163 0.36 0.0437 0.0515 6.3 116.55 80.7 0.3572 5.5 10.4 0.1166 0.34 0.0435 0.0514 7.8 147.42 96.9 0.372 4.5 7.2 0.089 0.25 0.062 0.0631 9.3 178.56 113.3 0.3792 3.6 3.6 0.0622 0.17 0.117 0.0795 10.8 210.6 130.6 0.3712 3.4 3.4 0.0608 0.17 0.1235 0.0806 12.3 243.54 148.9 0.3597 3.2 3.6 0.062 0.18 0.1176 0.0796 13.8 276 166.6 0.3469 3.6 6.3 0.0819 0.24 0.0673 0.0664 15.3 313.65 189.6 0.3293 3.9 6.6 0.0846 0.27 0.0653 0.0652 table s.3. input data table of bh76 for calculations in soilique. z (m) fine (1) / coarse(2) spt n γ (kn/m2) cn cb cr cs energy for ce fine content (%) 1.5 2 4 17.5 1.7 1 1 0.8 45 80 3 2 6 18 1.37 1 1 0.8 45 80 4.5 2 15 18.3 1.24 1 1 0.8 45 75 6 2 5 18.3 1.14 1 1 0.8 45 2 7.5 2 5 18.5 1.06 1 1 0.8 45 5 9 2 7 18.8 1 1 1 0.8 45 15 10.5 2 12 19 0.94 1 1 0.8 45 26 12 2 8 19.5 0.9 1 1 0.8 45 20 13.5 2 5 19.5 0.86 1 1 0.8 45 70 15 2 10 19.9 0.82 1 1 0.8 45 94 336 ekrem bekin, ferhat ozcep table s.4. soilique results table for bh76. z (m) σ σ’ csr (n1)60 (n1)60fc crr sf settlements* (m) settlements** (m) 1.5 26.25 26.3 0 0.0 0.0 0 1.20 0 0 3 54 41.4 0.331 4.9 10.9 0.1213 0.38 0.0416 0.05 4.5 82.35 55.1 0.3754 11.2 18.4 0.1963 0.54 0.0253 0.0351 6 109.8 67.8 0.4017 3.4 3.4 0.061 0.16 0.1224 0.0804 7.5 138.75 82.0 0.4145 3.2 3.2 0.0595 0.15 0.1296 0.0816 9 169.2 97.8 0.4189 4.2 6.9 0.0869 0.21 0.0636 0.0641 10.5 199.5 113.4 0.4089 6.8 12.0 0.131 0.33 0.0376 0.0471 12 234 133.2 0.39 4.3 8.3 0.0983 0.26 0.0553 0.059 13.5 263.25 147.7 0.377 2.6 8.1 0.0967 0.27 0.0564 0.0596 15 298.5 168.2 0.3569 4.9 10.9 0.1212 0.35 0.0416 0.05 table s.5. input data table of bh106 for calculations in soilique. z (m) fine (1) / coarse(2) spt n γ (kn/m2) cn cb cr cs energy for ce fine content (%) 1.5 2 6 18.5 1.7 1 1 0.8 45 8 3 2 7 18.5 1.37 1 1 0.8 45 6 4.5 2 7 19.7 1.24 1 1 0.8 45 8 6 2 13 20 1.14 1 1 0.8 45 99 7.5 2 16 20 1.06 1 1 0.8 45 100 9 2 19 20.5 1 1 1 0.8 45 100 10.5 2 28 20.7 0.94 1 1 0.8 45 100 12 2 42 21 0.9 1 1 0.8 45 98 13.5 2 45 21.5 0.86 1 1 0.8 45 99 15 2 46 22 0.82 1 1 0.8 45 100 table s.6. soilique results table for bh106. z (m) σ σ’ csr (n1)60 (n1)60fc crr sf settlements* (m) settlements** (m) 1.5 27.75 27.8 0 0.0 0.0 0 1.20 0 0 3 55.5 55.5 0 0.0 0.0 0 1.20 0 0 4.5 88.65 81.8 0.2721 5.2 5.6 0.0764 0.29 0.0716 0.0696 6 120 98.4 0.3025 8.9 15.7 0.1669 0.57 0.0282 0.0391 7.5 150 113.7 0.3233 10.2 17.2 0.1831 0.59 0.0265 0.0368 9 184.5 133.5 0.3346 11.4 18.7 0.1996 0.62 0.025 0.0346 10.5 217.35 151.6 0.3331 15.8 24.0 0.2725 0.85 0.0193 0.0171 12 252 171.6 0.326 22.7 32.2 0.7945 1.20 0 0 13.5 290.25 195.1 0.3147 23.2 32.9 1.1191 1.20 0 0 15 330 220.1 0.3015 22.6 32.2 0.7766 1.20 0 0 table s.7. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for csr calculations of bh43. the water table level of bh43 is 2.65 m. mw=7.4, pga= 0.4 g. depth (m) γ (g/cm3) fine content (%) σ (kpa) σ’ (kpa) rd csr 1.8 1.8 82 31.8 31.8 0.988147 0.257 3.3 1.8 30 58.3 51.9 0.9773893 0.285 4.8 1.8 39 84.8 63.7 0.9669309 0.335 6.3 1.85 25 112 76.2 0.9551352 0.365 7.8 1.89 15 139.8 89.3 0.9397014 0.383 9.3 1.92 5 168 102.8 0.9179441 0.39 10.8 1.95 3 196.7 116.8 0.8875651 0.389 12.3 1.98 8 225.9 131.2 0.8478937 0.38 13.8 2 15 255.3 145.9 0.8008678 0.364 15.3 2.05 15 285.5 161.4 0.7507104 0.345 337a comparative soil liquefaction analysis with a matlab® based algorithm: soilique table s.8. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for crr calculations of bh43. the water table level of bh43 is 2.65 m. mw=7.4, pga= 0.4 g. depth (m) spt (field) cn ce cb cr cs (n1)60fc crr sf 1.8 8 1.7 0.75 1 1 0.8 13.3 0.1486 2 3.3 7 1.37 0.75 1 1 0.8 11.3 0.1296 0.45 4.8 6 1.24 0.75 1 1 0.8 10.4 0.1203 0.36 6.3 8 1.14 0.75 1 1 0.8 10.4 0.1206 0.33 7.8 7 1.06 0.75 1 1 0.8 7.2 0.0921 0.24 9.3 6 1 0.75 1 1 0.8 3.6 0.0644 0.16 10.8 6 0.94 0.75 1 1 0.8 3.4 0.0629 0.16 12.3 6 0.9 0.75 1 1 0.8 3.6 0.0642 0.17 13.8 7 0.86 0.75 1 1 0.8 6.3 0.0848 0.23 15.3 8 0.82 0.75 1 1 0.8 6.6 0.0876 0.25 table s.9. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for csr calculations of bh76. the water table level of bh76 is 1.72 m. mw=7.4, pga= 0.4 g. depth (m) γ (g/cm3) fine content (%) σ (kpa) σ’ (kpa) rd csr 1.5 1.75 80 30.9 30.9 0.9904198 0.258 3 1.8 80 57.4 44.8 0.9794776 0.326 4.5 1.83 75 84.3 57 0.9690669 0.372 6 1.83 2 111.2 69.3 0.9577033 0.4 7.5 1.85 5 138.5 81.8 0.943207 0.415 9 1.88 15 166.1 94.7 0.9229267 0.421 10.5 1.9 26 194.1 108 0.8943968 0.418 12 1.95 20 222.8 121.9 0.8565184 0.407 13.5 1.95 70 251.5 135.9 0.8106803 0.39 15 1.99 94 280.8 150.5 0.7607536 0.369 table s.10. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for crr calculations of bh76. the water table level of bh76 is 1.72 m. mw=7.4, pga= 0.4 g. depth (m) spt (field) cn ce cb cr cs (n1)60fc crr sf 1.5 4 1.7 0.75 1 1 0.8 9.2 0.1095 2 3 6 1.37 0.75 1 1 0.8 10.9 0.1255 0.39 4.5 15 1.24 0.75 1 1 0.8 18.4 0.2031 0.55 6 5 1.14 0.75 1 1 0.8 3.4 0.0631 0.16 7.5 5 1.06 0.75 1 1 0.8 3.2 0.0616 0.15 9 7 1 0.75 1 1 0.8 6.9 0.0899 0.21 10.5 12 0.94 0.75 1 1 0.8 12 0.1356 0.32 12 8 0.9 0.75 1 1 0.8 8.3 0.1017 0.25 13.5 5 0.86 0.75 1 1 0.8 8.1 0.1001 0.26 15 10 0.82 0.75 1 1 0.8 10.9 0.1254 0.34 table s.11. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for csr calculations of bh76. the water table level of bh106 is 3.8 m. mw=7.4, pga= 0.4 g. depth (m) γ (g/cm3) fine content (%) σ (kpa) σ’ (kpa) rd csr 1.5 1.85 8 32.7 32.7 0.9904198 0.258 3 1.85 6 59.9 59.9 0.9794776 0.255 4.5 1.97 8 88.9 82 0.9690669 0.273 6 2 99 118.3 96.7 0.9577033 0.305 7.5 2 100 147.7 111.4 0.943207 0.325 9 2.05 100 177.9 126.9 0.9229267 0.336 10.5 2.07 100 208.4 142.6 0.8943968 0.34 12 2.1 98 239.3 158.8 0.8565184 0.335 13.5 2.15 99 270.9 175.7 0.8106803 0.325 15 2.2 100 303.3 193.4 0.7607536 0.31 338 ekrem bekin, ferhat ozcep table s.12. soilengineering (ozcep, 2010) spt-based liquefaction analysis panel. red numbers are inputs for crr calculations of bh76. the water table level of bh106 is 3.8 m. mw=7.4, pga= 0.4 g. depth (m) spt (field) cn ce cb cr cs (n1)60fc crr sf 1.5 6 1.7 0.75 1 1 0.8 5.6 0.079 2 3 7 1.37 0.75 1 1 0.8 5.8 0.081 2 4.5 7 1.24 0.75 1 1 0.8 5.6 0.0791 0.29 6 13 1.14 0.75 1 1 0.8 15.7 0.1727 0.57 7.5 16 1.06 0.75 1 1 0.8 17.2 0.1895 0.58 9 19 1 0.75 1 1 0.8 18.7 0.2066 0.6 10.5 28 0.94 0.75 1 1 0.8 24 0.2821 0.83 12 42 0.9 0.75 1 1 0.8 32.2 0.55 sy 13.5 45 0.86 0.75 1 1 0.8 32.9 0.55 sy 15 46 0.82 0.75 1 1 0.8 32.2 0.55 sy table s.13. liquefaction induced settlements table for each layers of all boreholes soilique results soil engineering (ozcep, 2010) results z (m) ishihara and yoshimine (1992) tokimatsu and seed (1987) ishihara and yoshimine (1992) tokimatsu and seed (1987) se tt le m en ts fo r b h 43 1.8 0 0 0 0 3.3 4.88 3.99 4.89 4.01 4.8 5.15 4.37 5.14 4.35 6.3 5.14 4.35 5.14 4.35 7.8 6.31 6.2 6.3 6.18 9.3 7.95 11.7 7.95 11.7 10.8 8.06 12.35 8.05 12.3 12.3 7.96 11.76 7.95 11.7 13.8 6.64 6.73 6.64 6.72 15.3 6.52 6.53 6.53 6.54 total 58.62 67.98 58.59 67.85 1.5 0 0 0 0 se tt le m en ts fo r b h 76 3 5 4.16 5 4.16 4.5 3.51 2.53 3.51 2.52 6 8.04 12.24 8.05 12.3 7.5 8.16 12.96 8.15 12.9 9 6.41 6.36 6.41 6.36 10.5 4.71 3.76 4.7 3.75 12 5.9 5.53 5.89 5.52 13.5 5.96 5.64 5.96 5.64 15 5 4.16 5 4.16 total 52.69 57.34 52.67 57.31 1.5 0 0 0 0 se tt le m en ts fo r b h 10 6 3 0 0 0 0 4.5 6.96 7.16 6.95 7.14 6 3.91 2.82 3.9 2.81 7.5 3.68 2.65 3.68 2.65 9 3.46 2.5 3.46 2.49 10.5 1.71 1.93 1.77 1.93 12 0 0 0 0 13.5 0 0 0 0 15 0 0 0 0 total 19.72 17.06 19.76 17.02 339a comparative soil liquefaction analysis with a matlab® based algorithm: soilique 2. synthetic data 2.1. synthetic vs data table s.14. input table for synthetic data that is used safety factor analysis based on synthetic vs values. the water table level is considered as 4.4 m, mw=7.4, and pga= 0.4 g. z(m) fine (1) / coarse (2) γ (kn/m2) vs (m/s) 1.50 1.00 19.00 200.00 3.00 2.00 19.00 210.00 4.50 2.00 18.00 120.00 6.00 2.00 18.50 110.00 7.50 2.00 17.50 100.00 9.00 2.00 16.50 130.00 10.50 2.00 18.00 180.00 12.00 2.00 17.00 125.00 13.50 2.00 18.00 192.00 15.00 2.00 19.00 198.00 16.50 2.00 20.00 210.00 18.00 2.00 21.00 235.00 table s.15. table of results for safety factor analysis based on synthetic vs values. σ σ’ csr vs1 (m/s) crr sf 28.5 28.5 0 0 0 1.2 57 57 0 0 0 1.2 81 80.019 0.2541 126.877 0.0542 0.2205 111 95.304 0.2889 111.3307 0.0413 0.1477 131.25 100.839 0.319 99.7913 0.0332 0.1076 148.5 103.374 0.3478 128.926 0.0561 0.1668 189 129.159 0.34 168.8462 0.1104 0.3358 204 129.444 0.3498 117.1897 0.0458 0.1355 243 153.729 0.3344 172.4298 0.1182 0.3656 285 181.014 0.3166 170.7014 0.1143 0.3734 330 211.299 0.2978 174.1787 0.1223 0.4249 378 244.584 0.2786 187.9147 0.168 0.624 table s.16. soilengineering (ozcep, 2010) vs-based liquefaction analysis panel. red numbers are inputs for csr calculations. the water table level is considered as 4.4 m, mw=7.4, and pga= 0.4 g. depth (m) γ (g/cm3) σ (kpa) σ’ (kpa) rd csr 1.5 1.9 33.6 33.6 0.9904198 0.258 3 1.9 61.5 61.5 0.9794776 0.255 4.5 1.8 88 87 0.9690669 0.255 6 1.85 115.2 99.5 0.9577033 0.288 7.5 1.75 141 110.6 0.943207 0.313 9 1.65 165.2 120.1 0.9229267 0.33 10.5 1.8 191.7 131.9 0.8943968 0.338 12 1.7 216.8 142.2 0.8565184 0.339 13.5 1.8 243.2 154 0.8106803 0.333 15 1.9 271.2 167.2 0.7607536 0.321 16.5 2 300.6 181.9 0.711582 0.306 18 2.1 331.5 198.1 0.6670533 0.29 table s.17. soilengineering (ozcep, 2010) vs-based liquefaction analysis panel. red numbers are inputs for crr calculations. the water table level is considered as 4.4 m, mw=7.4, and pga= 0.4 g. depth (m) vs (m/s) cv vs1 (m/s) vs1c (m/s) crr sf 1.5 200 1.31 sy 220 sy* sy* 3 210 1.13 sy 220 sy* sy* 4.5 120 1.04 124.25 220 0.0534 0.2097 6 110 1 110.13 220 0.0419 0.1454 7.5 100 0.98 97.52 220 0.0329 0.1052 9 130 0.96 124.18 220 0.0534 0.1617 10.5 180 0.93 167.96 220 0.1013 0.2995 12 125 0.92 114.47 220 0.0453 0.1334 13.5 192 0.9 172.36 220 0.1076 0.323 15 198 0.88 174.12 220 0.1102 0.3435 16.5 210 0.86 180.82 220 0.1211 0.396 18 235 0.84 198.08 220 0.1601 0.5516 *sy means properties of the layer exceeds the upper limit for liquefaction calculation. 2.2. synthetic cpt data table s.18. input data table for the safety factor analysis based on synthetic cpt data. the water table level is considered as 2.6 m, mw=7.4 and pga= 0.4 g. z(m) fine (1) / coarse(2) γ qc (mpa) fs (mpa) 1.50 1.00 19.00 5.00 0.05 3.00 2.00 17.00 8.00 0.03 4.50 1.00 18.00 10.00 0.08 6.00 2.00 16.00 12.00 0.10 7.50 2.00 17.00 13.00 0.09 9.00 2.00 16.80 15.00 0.15 10.50 1.00 18.00 13.00 0.12 12.00 1.00 17.80 17.00 0.16 13.50 1.00 18.00 16.00 0.14 15.00 1.00 19.00 18.00 0.16 table s.19. table of results for safety factor analysis based on synthetic cpt values. σ σ’ csr crr sf qc1 (mpa) 28.5 0 0 0 1.2 0 51 47 0.028128 0.084974 0.3425 0.1167 81 0 0 0 1.2 0 96 62 0.039194 0.079895 0.5075 0.1524 127.5 78.5 0.040619 0.080669 0.5209 0.1467 151.2 87.2 0.042835 0.078793 0.5624 0.1606 189 0 0 0 1.2 0 213.6 0 0 0 1.2 0 243 0 0 0 1.2 0 285 0 0 0 1.2 0 340 ekrem bekin, ferhat ozcep 2.3. synthetic example result of simple analysis (tezcan & teri, 1996) a. b. figure s1. a) input b) the result of simple analysis. graph is digitized and replotted from tezcan and teri (1996). the red plus is an indication of the intersection point. 2.4. synthetic example for the maximum extent of a liquefiable area analysis figure s2. a) input and b) output dialogue box for the maximum extent analysis with the theory of liu and xie (1985). keywords: 3d location algorithm; mass and non-mass interference; geostratigraphy; surrounding rock; ree; trace; palabras clave: algoritmo de localización 3d; interferencia masiva y no masiva; geoestratigrafía; roca circundante; ree; rastro. how to cite item lv, y., lu, l., & cao, m. (2021). tracing method of rare earth elements in surrounding rock of geological formation based on threedimensional positioning algorithm. earth sciences research journal, 25(1), pending pages. doi: https://doi.org/10.15446/esrj. v25n1.93725 because of the interference of mass spectrum and non-mass spectrum, the tracing accuracy of rare earth elements in the surrounding rock of geological formation is low. pretreatment of test sample reagent, dissolution of test sample residue, characterization of rare earth element doped materials, analysis of mass spectrometry and non-mass spectrometry interference in rare earth element tracking, using three-dimensional positioning algorithm to track rare earth elements in geological strata surrounding rock. in the experiment, five samples of surrounding rock of geological strata are selected as experimental indexes. the experimental results show that the tracking accuracy of four rare earth elements is high. abstract tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm método de rastreo de elementos de tierras raras en roca circundante de formación geológica basado en un algoritmo de posicionamiento tridimensional issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93725 debido a la interferencia del espectro de masas y del espectro no masivo, la precisión de la traza de elementos de tierras raras en la roca circundante de formación geológica es baja. en este trabajo se realizó el pretratamiento del reactivo de la muestra de prueba, disolución del residuo de la muestra de prueba, caracterización de materiales dopados con elementos de tierras raras, análisis de espectrometría de masas e interferencia sin espectrometría de masas en el seguimiento de elementos de tierras raras, con un algoritmo de posicionamiento tridimensional para rastrear elementos de tierras raras en los estratos geológicos que rodean la roca. en el experimento se seleccionaron cinco muestras de rocas circundantes de estratos geológicos como índices experimentales. los resultados experimentales muestran que la precisión de seguimiento de cuatro elementos de tierras raras es alta. resumen record manuscript received: 03/20/2020 accepted for publication: 25/11/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 21-28 yan lv1, laijun lu1, mengxue cao2* 1college of earth sciences, jilin university, changchun, 130061, china 2geomathematics key laboratory of sichuan province, chengdu university of technology, chengdu, 610059, china lvyanjlu_bj@163.com; lulj1956@163.com; caomengxue_sc@163.com g eo lo g y https://doi.org/10.15446/esrj.v25n1.93725 https://doi.org/10.15446/esrj.v25n1.93725 https://doi.org/10.15446/esrj.v25n1.93725 mailto:lvyanjlu_bj@163.com mailto:lulj1956@163.com mailto:caomengxue_sc@163.com 22 yan lv, laijun lu, mengxue cao introduction rare earth elements are the key elements in the development of high and new technology and have a very broad application prospect in the fields of materials, information, energy, ocean and space science and technology. rare earth elements have some special geochemical properties, such as stable chemical properties, a high degree of homogenization, not easy to be disturbed by metamorphism, and so on. once “recorded” in the terrane, it is easy to be preserved and so on, so it is a good geochemical indicator. as a good indicator and tracer, rare earth elements can be used to reverse the geochemical state and evolution process in the geological body. it is of great significance in judging the formation environment and conditions of sedimentary rocks, tracing the source of diagenetic materials of sedimentary rocks, and explaining the formation reasons of sedimentary rocks. at the same time, with the development of rare earth resources, rare earth elements inevitably enter the environment, food chain, and organisms in various ways. since the sixties of this century, with the development of analytical and testing technology, not only the total content of rare earth in geological bodies but also the content of individual rare earth elements can be measured, which makes rare earth elements play an important role in the research work of diagenesis, mineralization and celestial evolution. however, due to more or less different physical and chemical properties of different rare earth elements, especially in different geological conditions, it often leads to a different distribution. in modern industrial automation production, quality control requires more and more high sensitivity for the determination of rare earth elements, and the chemical properties of rare earth elements are very similar, and their separation has always been rare one of the problems and hot spots in soil analytical chemistry. as a result, the study of rare earth element separation and analysis is not only of great theoretical significance but also of practical application value. therefore, a tracing method of rare earth elements in the surrounding rock of geological formation based on a three-dimensional positioning algorithm is designed. material method test reagent before tracing the rare earth elements in the surrounding rock of the geological formation, in order to eliminate the interference of organic matter and suspended matter, oxidize the elements of various valence states into a single high valence state or transform them into inorganic compounds that are easy to be separated, so that the water sample can be concentrated, showing a clear, transparent and precipitation free state, the surrounding rock materials of the geological formation need to be digested (wen et al., 2019), the main applied instruments and reagents are as shown in table 1: table 1. instruments and reagents serial number instruments and reagents model 1 element 2 plasma mass spectrometer (thermo fisher, usa) 2 preparation of hydrofluoric acid and nitric acid by boiling distillation 3 element standard stock solution mixed standard solution of rare earth elements with scan and yttrium (national nonferrous metals and electronic materials 4 high purity water 18mω/ cm milli-q purified water 5 sodium hydroxide powder, ar, kemo, tianjin 6 ammonium chloride tianjin como 7 magnesium chloride tianjin miaoyou 8 triethanolamine xilong chemical 9 concentrated ammonia water jiangxi united chemical 10 hydrofluoric acid xilong chemical 11 hydrochloric acid xilong chemical co., ltd 12 nitric acid xilong chemical 13 standard stock solution standard solution of rare earth elements 14 water purifier cooperative uv preparation of standard solution (vatin-perignon et al., 2018): blank solution: 2mol l-1 hydrochloric acid; 1 g·ml-1 rare earth element mixed standard solution: accurately transfer 5ml of 100 g ml-1 mixed standard solution into a 50ml volumetric flask with a pipette, and fix the volume with 2mol l-1 hydrochloric acid (kwan et al., 2019), and then dilute the solution 10 times to a 100ml volumetric flask to obtain 1 g·ml-1 ree standard solution, which is ready for use. 5 g·ml-1 rare earth element mixed standard solution: pipette 5ml 10 g·ml-1 mixed standard solution to a 100ml volumetric flask, and use 2 mol·l-1 hydrochloric acid to fix the volume to obtain 5 g·ml-1ree, which is ready for use. after digestion, accurately weigh about 0.5g (accurate to 0.0001g) of sample gss-4 into microwave digestion tank, add 2ml of hcl, 2ml of hf, 5ml of hno3 for digestion under the three different conditions (li et al., 2018), then drive out the acid, and observe whether the digestion is complete. the digestion conditions are as shown table 2: table 2. sample digestion conditions instrument parameter digestion conditions step 1 step 2 condition 1 300w, heating time: 5 min, holding time: 10 min heating time: 10 min, holding time: 20 min condition 2 300w, heating time: 5 min, holding time: 10 min heating time: 10 min, holding time: 20 min condition 3 300w, heating time: 5 min, holding time: 10 min 1000w, heating time: 5 min, holding time: 30 min: after sample digestion, it is necessary to heat the sample and prepare rare-earth doped materials by microwave heating. the device is mainly composed of a multi-mode microwave resonator, magnetron, insulation system, turntable, and other parts. the magnetron is used to emit microwaves, and the resonator makes the microwaves resonate in the cavity, while the turntable is used to load the raw materials in the resonator at a uniform speed, so as to make the raw materials rotate at a uniform speed. the raw materials are evenly radiated by microwave. it is worth mentioning that in the whole process, due to the high temperature of the reactant, a large amount of heat generated by microwave will be dissipated in the resonant cavity in the form of thermal radiation (bidhendi & geitmann, 2018), heat conduction, and other forms, resulting in energy waste and damage to the resonant cavity and electronic components. therefore, it is necessary to insulate the reaction raw materials, as shown in figure 1: figure 1. heating device among them, 1 is a multi-mode microwave cavity, 2 is a magnetron, 3 is a thermal insulation device, 4 is the raw material to be heated, and 5 is a turntable. in addition, if the raw material is directly put into the microwave cavity for heating, according to the microwave heating principle, the microwave will rapidly increase the temperature of the reactant in the way of “body heating”, at this time, the surrounding environment temperature is relatively low, and the high-temperature reactant will exchange heat with the surrounding air through the way of heat conduction or heat radiation, which will reduce the surface temperature, resulting in the high inside and low outside of the reactant temperature gradient, resulting in uneven heating. 23tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm in order to avoid this kind of situation, the microwave heating and insulation device as shown in figure 2 came into being. the device is a combination of the buried powder insulation layer and basket insulation layer and realizes the insulation of reactants through multi-layer insulation structure. the outermost layer is an aluminosilicate (3al2o3-2sio2) container with a thickness of about 3cm, which is transparent to the microwave, so the microwave can penetrate almost without damage; the thermal conductivity (cemlyn et al., 2018) is small, which can play a very good role in heat preservation; moreover, it is resistant to high temperature, has a uniform expansion, and is still stable at 1700 °c. the innermost layer is a corundum crucible with reactant powder, good microwave permeability, and high-temperature resistance. outside the corundum, crucible is a layer of graphite crucible, which serves as a microwave sensor. because of the good coupling of graphite to microwave, this setting can not only improve the uniformity of heating but also preheat the reactant, which plays the role of indirect heating. the space between the aluminosilicate container and the graphite crucible, between the graphite crucible and the corundum crucible, is filled with quartz sand, so as to reduce the space between the insulation layer and the reactant as much as possible, achieve the purpose of further insulation, and minimize the temperature difference on the surface and inside of the reactant. figure 2. test sample insulation device in fact, the heat preservation device has three functions of microwave coupling (makridakis, 2018), heat preservation, and heat insulation. on the one hand, it can reduce the temperature gradient inside and outside the reactant, improve the heating uniformity, and at the same time, it can maintain the hightemperature thermal field environment inside the reactant for a period of time after the microwave radiation stops, so as to protect the reaction sample; on the other hand, due to the existence of the microwave coupling graphite, the microwave conventional mixed heating of the reaction sample is realized in a very simple way, which provides a uniform heat field for the sample, accelerates the heating speed and improves the heating environment; third, it can insulate the high temperature inside the insulation body and protect the microwave cavity and electronic components. pretreatment of test samples after crushing, the sample is sieved through 180 mesh and dried at 105 °c for standby. the sample pretreatment was carried out in the super clean laboratory of grade 100. the specific steps are as shown: a. accurately weigh 50 mg of the sample into a 25 ml polytetrafluoroethylene beaker, moisten with a small amount of water, and repeat three times; b. add 1 m lhf and 1 m lhno3 respectively, put them into a highpressure tank and heat them up to 190 °c, and keep them warm for 72 hours; c. take out the high-pressure tank, cool it, open the cover and evaporate it to dryness. when the solution is nearly dry, add 1m l100pg / ml rh internal standard solution, 1ml hno3 and 4-5ml of three times water, put it into the high-pressure tank, seal and heat it to 140 °c, keep it warm for 5 hours, so as to dissolve the residue and ensure complete extraction; d. after the solution is cooled, take 0.2ml to 10ml polytetrafluoroethylene test tube, volume it, shake it, and then test it on the machine. the acidity of the solution is 2% hno3. first separation: microwave acid decomposition method: in the digestion solution, naoh solution (about 5g, dissolved in 30ml water) is used to precipitate the digestion solution for the first time, making rare-earth ions precipitate in naoh alkaline condition, separating with matrix interference ions which do not precipitate in alkaline condition, and adding triethanolamine (1 + 1), making a large amount of fe, al, zr in the soil and so on, forming a complex with it, filtering and being separated and removed, then dissolving and settling the solution with hcl hot solution to obtain the solution (wen et al., 2018), and then selecting different precipitants for secondary precipitation separation to further remove the interference elements of the matrix in the soil (hebaz et al., 2018). muffle furnace alkali melting method: add triethanolamine (1 + 1) into the digestion solution to form a large number of fe, al, zr, and other complexes in the soil, filter them and remove them. after that, use hcl hot solution to dissolve and precipitate the solution, and then select different precipitants for secondary precipitation and separation to further remove matrix interference elements in the soil. the second separation adopts the two methods: first, add 2.5% (m / v) cupferron (su et al., 2018) into the solution, stir it continuously until there is no precipitation, filter it, take the filtrate with a beaker, then heat it on a hot plate to concentrate it to 3ml, and then fix the volume to 10ml colorimetric tube. to be tested; second, add 3g of nh4cl to the digestion solution, 50ml of water, and then add ammonia drop to produce a large amount of precipitation. add about 50ml of ammonia to the solution until the ph value is greater than or equal to 12, then heat and boil it, cool and filter it, discard the filtrate, dissolve the hot hcl (2mol / l) used for precipitation in a beaker, heat it on a hot plate to concentrate to 3ml, and fix the volume to 10ml in the colorimetric tube, to be tested. after heating, the whole sample treatment process is divided into the stages: first, the formation stage, which is the main physical and chemical reaction stage, in which various chemical and physical changes are carried out for each component in the matrix raw material, and most of the gas products are discharged; second, as the temperature continues to rise, the raw material becomes a melt, and the sintering products diffuse to each other and form a network. at the end of this stage, there are basically no matrix raw material particles in the melt; third, the homogenization stage of the melt, that is, under long-term high temperature preservation, the components in the melt gradually evenly distributed; fourth, the rapid cooling stage, this stage is mainly to make the products fragile and convenient for ball milling later; fifthly, in the phase of ball milling and mixing, this is the last stage of the whole sample treatment, which is mainly to grind the products of the large particles to make them mix with different materials according to different needs and make the required samples. test instruments all instruments used in the experiment are as shown in table 3: table 3. test instruments order number instrument model 1 elemental analyser vario el-iii germany 2 inductively coupled plasma emission spectrometer vista-mpx icp-oes valian, usa 3 uv-vis spectrophotometer uv-7502pc xinmao, shanghai 4 infrared spectrometer nicolet avatar 380ft-ir of the united states 5 x ray photoelectron spectrometer japanese ulvac-phi phi5000versa-probe type 6 thermal analyser perkin-elmer, pyris diamond tg-dta, perkin elmer, usa 7 automatic vertical pressure steam sterilizer yxq-ls-30sii, shanghai 24 yan lv, laijun lu, mengxue cao inductively coupled plasma mass spectrometer is mainly used for quantitative analysis of multi-element in surrounding rock materials. the main parameters are as as shown in table 4: table 4. parameters of icp ms measuring instrument serial number instrument parameters numerical value 1 rf power 1500 watts 2 sampling depth 8.0 mm 3 cooling air volume 15l / min 4 auxiliary air flow 1.0l / min 5 carrier gas flow 0.8l / min 6 compensated air flow 0.2l / min 7 peristaltic pump speed 0.1rps 8 sample extraction time 45 years 9 stable schedule 1960s 10 integration time 0.3 s 11 number of repetitions 3 times 12 oxide <1.2% 13 collection mode full spectrum mode 14 tuning mode no gas regulation mode on this basis, the icp-ms analysis instrument is very complex. it has many analysis functions and variable working parameters. among the working parameters of the instrument, some are given by the instrument, some can be determined by experiments. however, there are many factors that directly affect the sensitivity, precision and detection limit of icp-ms (bernardi et al., 2018). in addition to the working parameters given above, the working parameters of the ion mirror system in ms and the calibration of the peak position are more important. in multiple parameters of ion mirror (budko & martynova, 2019), as long as one of the parameters changes, other conditions also need to be adjusted accordingly to obtain the corresponding sensitivity and precision, and then the next measurement after peak position calibration can be carried out. and with the long-term use of sampling cone and cutting cone, the surface and cone hole will change accordingly. therefore, before each start-up test, it is necessary to do the instrument condition optimization experiment to determine the corresponding working parameters. there are 7-10 working parameters of the ion mirror system, and the conditions obtained after each optimization experiment are not the same. for the simultaneous determination of elements in the whole range from light mass number to heavy mass number, the optimization experiment of instrument conditions is particularly important. in this experiment, the solution of 1 g/ul containing the elements is used, and the isotopes are selected for tuning. determination method when the sample is tested, it must be separated and purified as much as possible, and at the same time, the concentration and medium of the sample and the standard should be consistent as much as possible. therefore, it is necessary to evaluate the recovery rate of each sample. only when the recovery rate is guaranteed, the ideal data can be obtained. the two methods are adopted for data quality control in this experiment. after all the samples are dissolved, take one solution before and after chemical separation to test the trace element one, so as to evaluate the overall recovery rate and matrix removal effect. according to the current evaluation, the overall recovery rate is up to, while the removal rate of the matrix, such as the main same heavy elements or molecules, ions, is basically close, so as to ensure the mass fractionation effect that may be brought about in the chemical pretreatment process and the matrix effect in the mass spectrometry determination process. in the chemical pretreatment process, the standard solution that has been calibrated and several parallel samples are added for synchronization learn to handle (varney & bousfield, 2018). generally, the relative comparison method is used to determine the content of elements in the sample, so is the three-dimensional positioning algorithm. it is also the most common method to configure a series of standard solutions or use a series of standard substances to make the working curve. in this experiment, a national level geochemical reference material was selected, which was prepared into a solution according to the sample decomposition procedure and used as the working curve of the standard solution. the element content of reference material is the standard value or recommended value provided by many analytical methods and laboratories after many times of analysis. with this method, the matrix effect is compensated to some extent because the operation is completely consistent with the sample analysis procedure. after the experiment, the selected sample can be completely decomposed, all the measured elements enter the solution, and the concentration does not exceed the linear range. the correction results and practicability are very satisfactory. at the same time, another 2-3 national standard samples are inserted into the sample (abbott, 2018) for further verification, and the analysis results are very reliable. the relevant isotopes selected in the analysis and the determination method are as shown in the table 5: table 5. selected isotopes and determination methods elemental determination preferred m / z abundance sensitivity (%) measurement method y 89 100 simulation rh 103 100 pulse la 139 100 simulation ce 140 100 simulation pr 141 100 pulse nd 146 17.2 pulse sm 147 15 pulse eu 153 52.2 pulse gd 157 15.7 pulse tb 159 100 pulse dy 163 24.9 pulse ho 165 100 pulse er 166 33.6 pulse tm 169 100 pulse yb 172 21.9 pulse lu 175 97.4 pulse blank and standard drift the analysis of trace sample is very important for the control of blank. in addition to the process blank of sample, the instrument blank will also affect the quality of sample data seriously. the instrument blank will drift in the determination process. generally speaking, the strength is high at the beginning of determination. therefore, the negative value often appears for the low content sample. at the same time, the drift degree of each element in the determination process may not be good as such, gaps and standards need to be monitored frequently. internal calibration is one of the quantitative calibration methods often used in many analytical techniques. the function of internal standard is: a. monitor and correct the short-term drift of the signal; b. monitor and correct the long-term drift of the signal; c. correct the second element; d. correction of general matrix effects. the internal standard correction method (cao & wen, 2019) is used in icp-ms, especially in the determination of large quantities of samples. it is better to add internal standard at the beginning of the experiment, especially for the experiment with separation and enrichment steps (it is required that the internal standard is consistent 25tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm with the recovery rate of the elements to be measured), so that the loss in the experiment process does not affect the results, and when transferring the solution, there is no need to clean and fix the volume, which saves a lot of time, because the ratio of the elements to the internal standard to be determined is unchanged. the selection principle of icp-ms internal standard elements is that the elements to be determined do not contain the selected internal standard elements, the interference of the internal standard elements is as little as possible, and the mass spectrum behavior is as consistent as possible with the elements to be measured, which is very difficult for the determination of geochemical samples. in addition, when the multi-element determination is carried out, the elements are grouped according to the mass spectrum behavior, and multiple elements are selected as the internal standard. the determination effect is more ideal, but the elements that can be selected as the internal standard are more limited. dissolution of test sample residue because hf is used to decompose samples, but hno3 can’t completely drive hf out (remon et al., 2018), some experimental sample precipitates remain in the residue. for surrounding rock samples, when opening dissolution, the result of ree is seriously low. only under a certain temperature and pressure, these precipitates will be dissolved. however, at 140 ° c, closed dissolution (du et al., 2018) for 3 hours, the results were basically consistent with the recommended values. table 6 is the experimental data table of the relationship between the dissolution amount and time of the national standard sample of surrounding rock. table 6. relationship between residue dissolution and time element open dissolution seal and dissolve for 3 hours seal and dissolve for 6 hours seal and dissolve for 9 hours recommended value la g/g 26.7 57.4 53.9 54.4 54±4 ce g/g 59.4 112 107 110 108±7 y g/g 39.0 64.0 62.7 63.0 62±5 eu g/g 0.75 1.11 0.98 1.06 0.85±0.07 lu g/g 0.86 1.17 1.02 1.10 1.15±0.09 mass and non mass interference in rare earth element tracing in plasma mass spectrometry, there are two kinds of interference: mass spectrum interference and non mass spectrum interference. mass spectrum interference includes isometric interference, background interference and polyatomic ion interference (oxide, hydroxide and bicharged ion generation are interference, among which matrix effect and polyatomic ion interference are the main factors restricting the accuracy and precision of analysis. non mass spectrum interference mainly comes from matrix effect, which refers to the enhancement or inhibition effect of matrix elements on the analysis signal strength. it is a kind of nonlinear interference, and its dynamic drift degree varies with the mass number. the so-called matrix refers to the collection of all components with their own properties. this “all components” should include analytes, but not additives. the combined effect of each component of the matrix on the analysis signal of the element to be measured is matrix effect, that is to say, it is the combined interference effect of all accessories except additives. in the early analysis of icp-ms, the behavior and interpretation of matrix effect are relatively scattered, but generally speaking, there are several common conclusions as shown: high concentration matrix can inhibit the analysis signal, and the degree of inhibition is related to the quality; the inhibition effect of heavy matrix is greater than that of heavy analyzed elements; the matrix effect is related to the lens system of the instrument. with the development of research, the mechanism of matrix effect is gradually clear, and space charge effect is considered as the main reason. the influences on the determination are: (1) reducing the taper hole; (2) consuming energy in ionization, affecting the ionization rate. for the same concentration of elements, the signal strength of the matrix with high concentration is low and the reduction is fast. when the geological samples are diluted more than 2000 times (0.5 mg / ml), the matrix effect is small. the more prominent problems in the determination of trace rare earth elements (habal, 2018) are the interference of polyatomic ions, especially the interference of oxides of light rare earth elements, the interference of hydroxides to heavy rare earth elements, and the interference of oxides formed by seven natural isotopes of barium and hydroxides to light rare earth elements. the yield of rare earth oxides and hydroxides (reo (h) + is affected by the power of the ionomer, the velocity of carrier gas, the depth of sampling, the diameter of the sampling cone and the diameter of the intercepting cone, while the high plasma power and the low velocity of carrier gas can effectively reduce the yield of oxides. many analytical scientists have done research in this field, including the influence of the aperture size of the sampling cone and the intercepting cone on the oxide signal. characterization of rare earth element doped materials in surrounding rock of geological formation by using a polycrystalline x-ray diffractometer to analyze the structure and phase of the anti-counterfeiting powder, the average grain size of the powder can be measured. according to the diffraction, the grain size can be calculated by scherrer equation (1): d k = λ θcos (1) in equation (1), cos  represents the diffraction angle of crystal surface, k is constant, and  represents the wavelength of the target source. the size distribution, morphology and agglomeration of particles were directly observed by philips xl-30tmp environmental scanning electron microscope (sem) and nova nanosem 430 field emission scanning electron microscope (fesem). the specific steps are as shown in figure 3: figure 3. micro characterization process in order to characterize the application of the materials in spectrum anticounterfeiting and laser protection, the reflection spectrum and fluorescence emission spectrum of the synthesized powders were measured after they were uniformly mixed into the coating to make the anti-counterfeiting and protective coating. when the light hits the ink surface, part of it is reflected, reflection coefficient: r i l u  / (2) 26 yan lv, laijun lu, mengxue cao in equation (2), u represents the incident light intensity, l represents the reflected light intensity, and i represents the change parameter of the reflection coefficient with the wavelength. trace of rare earth elements in surrounding rock of geological formation according to the above process, the three-dimensional positioning algorithm is used to trace the rare earth elements in the surrounding rock of geological formation, and the microwave is used for initial positioning before tracking. microwave, also known as ultra-high frequency wave, refers to electromagnetic waves with very high frequency. its wavelength lies between infrared light and radio waves. the microwave frequency is 2.45ghz and the wavelength is 12.2cm. microwave high temperature heat treatment is a kind of heating method which uses microwave radiation instead of traditional heat sources. when the dielectric is in the microwave field, the electric field component of the microwave will give a force to the charged particles inside the dielectric. if the charged particles can move freely in the medium, they will generate current in the medium; if the charged particles are bound, they can only keep the same phase with the external electric field, that is, the polarization of the medium. according to the different kinds of charged particles in the medium, the polarization of the medium can be divided into four parts: dipole turning polarization, electron polarization, atomic polarization and interface polarization (litasov, 2018), which is expressed as shown in equation (3): a a d r e w= + + +( ) (3) in equation (3), is the total dielectric polarization, is the dipole polarization caused by the permanent dipole in the medium, is the electronic polarization caused by the electrons rotating around the atomic nucleus, is the atomic polarization caused by the atomic nucleus, and is the interface polarization caused by the charges at the interface. on the basis of the above processing, a three-dimensional positioning algorithm is used to trace the rare earth elements in the surrounding rock of the geological formation. the anchor nodes in the three-dimensional positioning algorithm are used as the reference for the location of unknown nodes, figure 4. surrounding rock structure of geological strata which are usually uniformly or randomly distributed in the wireless sensor network, usually with high energy. the anchor node has the ability to control the transmitting power. in an ideal environment, the relationship between the transmitting power of the signal and the radius of the area covered by the signal can be described by the equation (4): v p a f = ( ) 2 (4) in equation (4), p is the signal power received by the sensor node, v is the transmit power, c is the distance between the receiver and the transmitter, a is the speed of light, f is the transmission frequency. from the above equations, it can be seen that when other parameters remain unchanged and the minimum receiving power of the receiving node is fixed, the stronger the signal transmitting power of the transmitting node is, the farther its propagation distance is, so as to track relevant information, realize self positioning (wu & zou, 2018), and generate information table. the information in the information table contains all the received effective beacon information broadcast by the anchor node. anchor nodes will broadcast beacon information at different levels of power repeatedly. therefore, an unknown node will listen to multiple identical beacon information. only when the number exceeds the beacon decision threshold can the beacon information be determined as true. but at this time, the beacon information has not been saved to the information table. after the anchor node broadcasts its own beacon information, the unknown node may decide that several beacon information from the same anchor node is true. at this time, the beacon information with the lowest transmitting power level is taken as the correct information from the anchor node, which is stored in the information table to complete the tracking of rare earth elements in the surrounding rock of the geological formation. experimental study in order to verify the feasibility of the three-dimensional positioning algorithm for tracking rare earth elements in the surrounding rock of geological strata, the traditional method is compared with the designed method, and the tracking accuracy of the two methods in different time periods is compared. it is as shown in figure 4. four samples of rare earth in the surrounding rock of geological strata were measured to determine the element values of these four samples. the accurate determination of the element values represents the trace effect of rare earth elements. which is as shown in table 7. table 7. determination value of rare earth elements in sample 1 element measured value (mg / kg) measured value by traditional method measured value of this design method la 55 45 52 ce 125 100 135 pr 10.3 8 10.2 nd 25 17 24 pm 4.0 2.0 4.3 sm 4.8 3 4.5 eu 0.78 1.5 0.75 gd 4.2 2.6 4.3 tb 0.85 0.4 1.0 dy 6.0 3 6.3 ho 1.34 3 1.35 er 3.7 2.3 3.9 tm 0.6 0.2 0.9 yb 4.2 2 5 lu 0.69 1.3 0.79 sc 0.81 0.15 0.90 y 33 42 35 27tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm the measured values of sample 2, traditional methods and the designed values are as shown in the table 8: table 8. determination value of rare earth elements in sample 2 element measured value (mg / kg) measured value by traditional method measured value of this design method la 50 40 52 ce 100 81 102 pr 9 17 10 nd 15 5 13 pm 6.0 4.3 7.2 sm 6.8 3.5 6.9 eu 1.2 2.6 1.5 gd 5.2 9.2 5.6 tb 0.95 2.69 1.23 dy 7.0 4.32 7.9 ho 2.4 0.9 2.6 er 5.2 9.8 5.6 tm 1.6 8.2 1.9 yb 4.5 6.8 6.8 lu 0.9 0.45 1.3 sc 0.7 2.3 1.9 y 22 45 23 the measured values of sample 3, traditional methods and the designed values are as shown in table 9: table 9. determination value of rare earth elements in sample 3 element measured value (mg / kg) measured value by traditional method measured value of this design method la 45 38 42 ce 142 125 143 pr 8.2 6.9 8.5 nd 18 6 19 pm 6.5 8.9 6.24 sm 5.9 9.6 4.99 eu 0.95 2.36 0.98 gd 5.8 9.5 5.6 tb 0.92 2.36 1.2 dy 8.0 13.2 7.5 ho 2.42 5.23 2.32 er 5.7 9.32 5.96 tm 1.6 3.6 1.6 yb 5.2 6.9 5.4 lu 7.3 10.3 7.9 sc 0.87 1.23 0.87 y 42 56 45 the measured values of sample 4, traditional methods and the designed values are as shown in table 10: table 10. determination value of rare earth elements in sample 4 element measured value (mg / kg) measured value by traditional method measured value of this design method la 78 62 79 ce 152 144 154 pr 17.2 12.3 18.2 nd 22 15 25 pm 7.8 3.2 7.9 sm 6.3 2.6 6.54 eu 1.3 3.5 2.2 gd 4.9 6.6 5.2 tb 1.23 3.34 1.54 dy 8 15 9 ho 2.5 6.5 2.6 er 5.2 8.3 5.8 tm 9.2 15.3 9.5 yb 7.1 9.6 7.9 lu 3.2 6.3 3.9 sc 2.1 8 2.9 y 52 70 49 from the above table 7, table 8, table 9 and table 10, it can be seen that the tracking value of the designed tracking method for rare earth elements in the surrounding rock of geological strata based on the three-dimensional positioning algorithm has a small difference from the measured value, and there is a timely error, which is also within a standard value. however, there is a big difference between the traditional method and the actual measurement value, so it can be proved that the designed method based on 3d positioning algorithm is better than the traditional method. according to the content of rare earth elements, the correlation curve was drawn. which is as shown in figure 5. scanning electron microscopy was used to characterize the morphology of surrounding rock. rare earth elements have good dispersion. the abscissa represents the spectral wavelength, and the ordinate represents the three primary color stimulus values needed to match the spectral colors. the chromatogram of rare earth element tracing detection is as shown in figure 6. point a and point b are in the green area. point b is darker than point a, and the higher the color saturation is, which is conducive to the trace detection of rare earth elements. in this paper, the interference of mass spectrometry and non mass spectrometry in rare earth element tracing is analyzed, and the threedimensional positioning algorithm is used to trace rare earth elements in the surrounding rock of geological strata, which has better tracking effect. conclusions in this paper, based on the three-dimensional positioning algorithm, the study of rare earth element tracking in the surrounding rock of geological formation is proposed, and the interference of mass spectrum and non mass spectrum in rare earth element tracking is analyzed. the three-dimensional positioning algorithm is used to track rare earth elements in the surrounding rock of geological formation. compared with the traditional method, it has better tracking effect. in the next step of research, we need to establish a more reasonable channel model to verify the effectiveness of the tracking method. we can use simulation software or build a more real network environment to experiment the method, so as to promote the application in the actual network environment. this is a problem that needs to be seriously considered. 28 yan lv, laijun lu, mengxue cao references abbott, r. n. 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(2018). high-precision solution to the moving load problem using an improved spectral element method. acta mechanica sinica, 34(1), 68-81. wu, h., & zou, j. (2018). finite element method and its analysis for a nonlinear helmholtz equation with high wave numbers. siam journal on numerical analysis, 56(3), 1338-1359. figure 5. comparison of average content distribution curve of rare earth elements figure 6. cie chromatogram issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n1.117910 earth sciences research journal earth sci. res. j. vol. 29, no. 1 (march, 2025): 35 44 g eo m o r ph o lo g y geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region paiwast k. zhazhlayi, ali m. surdashy* department of earth sciences and petroleum, college of science, salahaddin university, erbil, iraq *corresponding author: paiwast.ahmed@su.edu.krd how to cite item: zhazlayi, p. k., & surdashy, a. m. (2025). geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region. earth sciences research journal, 29(1), 35-44. https://doi.org/10.15446/esrj.v29n1.117910 record manuscript received: 06/12/2024 accepted for publication: 10/03/2025 abst r ac t this study presents a detailed geomorphological survey and mapping of the main landforms in the roste valley, which have resulted from a combination of surficial and tectonic processes. the landforms are classified using the itc system, which is based on the origin of the landforms and the forces that shape the earth’s surface and is supported by extensive fieldwork. the roste valley is located within the zagros imbricated and zagros suture zones on the unstable shelf of the arabian plate, contributing to the formation of a variety of geomorphic features with different genetic origins. local climatic conditions significantly enhance the rate of surficial processes, while the tectonic setting near the subduction zone adds further complexity to the landscape. the geomorphological survey and mapping reveal that the area is polygenetic in origin, with landforms shaped by both tectonic and surficial forces. these landforms are categorized as tectonic, structural, denudation, mass-wasting, fluvial, and karst in origin. keywords: roste valley, landforms, tectonics, drainage deflection, pressure ridge, klippe and window thrusting, perched syncline. palabras clave: roste valley; accidentes geográficos; tectónica; deflexión del drenaje; cresta de presión; cabalgamiento de klippe y de ventanas; sinclinal encaramado geomorfología y evolución de accidentes geográficos del roste valley, cinturón plegado del zagros occidental, región del kurdistán iraquí este estudio presenta una investigación geomorfológica y un mapeo de las principales características naturales de la superficie terrestre en el roste valley, el cual ha sido el resultado de una combinación de procesos tectónicos con procesos superficiales. las características de la superficie en la zona de estudio se clasificaron de acuerdo con el sistema itc, el cual se basa en el origen de los accidentes geográficos y las fuerzas que modelan la superficie terrestre, y se sustentaron en un amplio trabajo de campo. el roste valley se ubica en las zonas traslapado y sutura del zagros, en la inestable placa tectónica árabiga, lo que contribuyó a la formación de una amplia variedad de características geomorfológicas con diferentes orígenes genéticos. las condiciones climáticas locales acentúan el peso de los procesos superficiales, mientras que la estructura tectónica cerca de la zona de subducción añade complejidad al paisaje geológico. la investigación geomorfológica y el mapeo revelan los orígenes poligenéticos del área, con accidentes geográficos moldeados por fuerzas tectónicas y superficiales. los orígenes de estas características naturales de la superficie se categorizan como tectónicos, estructurales, de denudación, de remoción de masa, fluviales y kársticos. resu m en https://doi.org/10.15446/esrj.v29n1.117910 mailto:paiwast.ahmed@su.edu.krd https://doi.org/10.15446/esrj.v29n1.117910 36 paiwast k. zhazhlayi, ali m. surdashy 1. introduction tectonic and surficial processes are responsible for the development and evolution of landforms, the differences in the genetic origins resulted from the interaction between tectonic, surficial, and climatic conditions. endogenic forces, specifically tectonic processes driven by the ongoing collision between the arabian and eurasian plates following the closure of the neo-tethys ocean in the late miocene, have caused compressional forces leading to uplift and overthrusting of existing anticlines, while exogenic forces (surficial process) act to flatten the land surface through the geomorphic agents and processes. the study is located in the kurdistan region, northeastiraq, about 128 km east of erbil city, within the imbricated and suture zones of the western zagros fold-thrust belt (zftb). it lies between longitudes 44°38’ e and 454°50’ e and latitudes 36°37’ n and 36°44’ n, (figure 1), bounded by tanoun anticline in sw, hasanbag mountain in n, and halgurd mountain in ne (figure 2). the study area is complicated in terms of geomorphology, which was dominated by landforms of different genetic origins and is an area of high relief and steep slopes with rainy and snowy climatic conditions (zhazhlayi and surdashy, 2022). tectonically, the fundamental framework of the stratigraphy and structure of the study area is influenced by the positioning of iraq within the tectonic units of north iraq (bety, 2022). the tectonic settings show that this area is active in terms of neotectonic and reactivation. however, the stratigraphic successions of the study area consist of jurassic, cretaceous, and tertiary (neogene) rocks and are represented by sarki, sehkaniyan, sargelu, naokelekan, barsarin, chia gara, balambo, sarmord, qamchuqa, aqra, bekhme, shiranish, tanjero, govanda, and redbed series (merga i) formations in the imbricated zone, and suture zone also called nappe zone (dunnington, 1958), and consisting of zagros nappes and represented by cretaceous (metamorphic qandil series) and tertiary (paleogene) rocks of (walash and naoupurdan volcano-sedimentary group (koshnaw et al, 2021), (figure 3). the geomorphological mapping and survey is considered an effective tool for the ability to notice the physical condition of the surface of the earth and gives useful information about landforms (smith et al, 2011). there are several methods of geomorphological survey, such as the topographic position index (tpi) and different automated landform classifications, but the most comprehensive geomorphological survey among the other systems is the itc system proposed by (verstappen et al, 1975), this system gives information about morphogenesis, morphometry, and morpho-chronology of the landforms (rao, 1974). in this study, the itc (international terrain classification) system of geomorphological mapping and survey is used, and it shows the information about the process, morphometry, genesis, lithology (rock type physical and chemical properties), and age. the geomorphological map based on the field observation and remote sensing data (figure 4). figure 1. shows the location of the study area, modified after (jassim and goff, 2006). 37geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region figure 2. shows a structural and tectonic map of the study area after (vera et al, 2009). figure 3. shows the three dimensional geological cross-section of the study area, after (zhazhlayi and surdashy, 2022). 38 paiwast k. zhazhlayi, ali m. surdashy figure 4. shows the geomorphological map and landforms distributions within the study area. the mapping based on the field work, satellite image and remote sensing data. 2. landforms of tectonic origin klippe and window the klippe are the isolated and separated over-thrusted rock masses, particularly nappes, and can be defined as the nappe’s outlier or the remnant portion of over-thrusted rock masses as the erosion isolates masses from the surrounding(pirasteh et al, 2009). in the study area, the qandil series klippe thrusting over the walash group which was observed in the suture zone (figure 5.a), as the qandil series is surrounded by the walash group. the window landform is formed when the sheets of thrust fault are eroded by surficial processes and expose the underlying rocks. in the study area, the naopurdan group was exposed as the window thrusts when the overlaying walash group eroded in the suture zone (figure 5.b). deflected drainage the drainage line changes its path when subjected to the structural and tectonic barrier, this barrier is formed in response to strike-slip movements or the growth of anticlinal folding which results in changes in the path of the drainage (gutiérrez and gutiérrez, 2016). in the study area, the deflection of the drainage course was found in the roste gorge and it was formed by the oblique faults that crossed the tanoun (spibalis) anticline (figure 5.c). shutter ridge it is the barrier developed across the drainage valley through a tectonic process (zilberman et al, 2011), this barrier blocks or closes the downstream drainage flow, commonly developed in response to the horizontal and/or vertical displacement by strike-slip and dip-slip faults respectively. in the roste valley, this ridge was observed on the ne side of the river in the core of the tanoun (spibalis) anticline (figure 5.c). pressure ridge the pressure ridge landforms are developed in the compressional zone by tectonic pressures found in thrust and strike-slip fault zones, this pressure changes and modifies fault scarps into pressure ridges (philip et al, 1992). in the study area, this feature was observed in front of walash group sheet thrusts near the kawlan and neris villages (figure 5.d). fault facet (triangular facets) the fault facets are the trace of the active faulting generally referring to triangular and trapezoidal-shaped fault-related geomorphic features (balogun et al, 2011), dissection and erosion are dominant processes in the uplifted part or blocks whereas the subsided or downward moved block commonly defined the low land area (pediment zone) and filled with the weathered rock sediments or aggradation (gutiérrez and gutiérrez, 2016). in the study area, the fault facets were observed near hassanbag mountain within the naopurdan group, in front of halgurd mountain, and within the walash group and these facets define the thrust surface of the walash and naoupurdan groups (figure 5.e). figure 5. shows the tectonic generated landforms, a) field photo shows klippe thrusting, b) window thrusting, c) satellite image shows deflected drainage and shutter ridge, and d) field photo of pressure ridge, and e) triangular facets. 3. landforms of structural origins structural origin unit represents all landforms that resulted from the impact of structure and/or tectonic processes associated with geomorphology aspects. those processes play a key role in the emergence of this origin. it is also includes those units that are relate to the regional or local bedrock structures or crustal movement,(bety, 2013). this group includes: 39geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region perched (hanging) syncline the perched syncline landform formed when the adjacent anticlinal folding eroded and the syncline remained as an elevated area than the surrounding. the morphology of the perched syncline is based upon the interlimb angle, the tight or narrow syncline produces a narrow synclinal ridge and the open and wide syncline produces broad ushaped tops mesa-like landforms (sebai et al, 2021). the narrow-perched syncline in the study area was observed near the sreshma village within the naoupurdan shaly group as exposed by window thrusting in the suture zone (figure 6.a). flatiron the flatiron is the fin-shaped landform produced through differential rates of erosion and weathering in the interfluvial drainage led to the separation of resistant lithology beds, the flatiron is also regarded as the base of triangular facets (bucci et al, 2013). this type of landform was observed in the ne limb of the tanoun (spibalis) anticline, as the aqra-bekhme formations bed separated by narrow gullies and wineglasses (figure 6.b). water gap the water gap can be defined as the deep passage within the mountain range or anticlinal structure and is the perpendicular cross-cutting through the mountain and ridge or any structural and tectonic barrier (oard, 2008). in the study area, this landform was found in the govanda cuesta ridge and tanoun (spibalis) anticline as the water course passed through the normal and oblique faults respectively (figure 6.c). breached anticline (eroded core anticline) the eroded core anticline or breached anticline was observed in the tanoun (spibalis) anticline. the anticline crest highly eroded and formed this type of landform in the core of the anticline (michard et al, 2011). the core of the tanoun anticline consists of jurassic formations, the ne limb consists of cretaceous formations, and the sw limb is crossed by imbricate thrusting. this imbricate thrusting may be another factor aiding in the formation of this type of landform (figure 6.d). homoclinal ridge (cuesta and hogback) the homoclinal ridge is the inclined strata with a constant dip, generally with a contrasting resistance of lithology(sponemann, 1989). the cuesta ridge is defined as an asymmetrical ridge formed of gently dipping (less than 35o) sedimentary strata with hard and soft rock alternation (ward, 2019), the soft lithology eroded easily and left the hard lithology as cap rock. according to (bety, 2013), usually this unit presents in an areas of mass wasting on these parts of the study area. the cuesta scarp slope steeply dipping and the dip slope is gently dipping, the dip slope is almost parallel to the dip of the strata. the cuesta ridge was observed in the study area, in the imbricated zone as the resistant limestone of govanda formation formed the cap of the landform and overlaid less resistant rocks of the tanjero formation near the dolbalis village (figure 6.e). the hogback landform is the symmetrical ridge developed of steeply dipping strata (more than 35o) with hard and soft rock alternation (ward, 2019). the hogback scarp slope and dip slope are almost the same amounts, and the dip slope of the hogback landform is also parallel to the bedding plane. in the study area, this landform is found in the imbricated zone within the tanjero formations near the sherkawa village (figure 6.f). according to (jain, 2014) the drainage network in the hogback and cuesta landforms are genetically classified into five types and including the following: 1) consequent: flows in the topographic slope direction such as the main roste river, 2) subsequent: this type of stream flows along the weak zone such as along the fault zone as in the besha and dolbalis subbasins, 3) resequent: this type flow in the direction of the original slope but in the lower levels as in the scarp face of govanda cuesta ridge, 4) obsequent: this type flows in the direction opposite to the main slope as in the ne limb of the tanoun (spibalis) anticline, and 5) insequent: this type of streams is characterized by dendritic patterns, the stream network distributed randomly as in the suture zone within the walash group, (figure 4). figure 6. shows structural origins landforms, a) perched syncline, b) flatiron, c) water-gap, and d) breached anticline, e) cuesta, and f) hogback. 40 paiwast k. zhazhlayi, ali m. surdashy 4. landform of denudational origin wine glass the wine glass landform developed as the result of intense erosion of triangular facets and flatiron and consists of wide and broad top parts with narrow and small outlets. the wine glass generally appears as a small v-shaped gorge or valley (bahrami, 2012). this landform developed from the early pleistocene to the late pliocene (sissakian et al., 2014). the wine glass is well developed in the ne limb of the tanoun (spibalis) anticline and in front of hassanbag mountain within the qamchuqa/aqra-bekhme and naopurdan group respectively (figure 7.a and b). badland the badland landform is defined as an intensely eroded landscape with a dense drainage network dissection (howard, 1994). the badland landform is characterized by relatively high relief, steep gradients, soft erodible lithology, and low vegetation covers. in the study area, the badland is well recognized at the end of the roste valley, in front of halgurd mountain within the walash group (figure 7.c). gorge the gorge is developed when dissolution and physical erosions in the carbonate rocks incise the fluvial system and produce a narrow and deep valley, with a river or stream running along the bottom of the gorge (gutiérrez and gutiérrez, 2016). this landform was found at the entrance of roste valley to the ne limb of the tanoun (spibalis) anticline (figure 7.d). 5. landform of mass wasting origin (slope movement) block slide the rock block sliding is defined as the movement of the single rock masses along the planer surface and moving downslope along the sliding surface. generally, less common sliding as compared to other types of sliding (luino and turconi, 2020). in the study area, this landform was observed in the imbricated and suture zone within the qamchuqa formations (ne limb of tanoun anticline) and qandil series rocks near the grtik villages (figure 8. a and b) respectively. figure 7. shows landforms generated through denudation process, a and b) wine glass in the ne limb of tanoun anticline and hasanbag mountain, c) badlands, and d) gorge in the core of tanoun anticline. 41geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region earth flow the earthflow generally occurs in the humid area on the hillside and the failure surface, followed by torrential rain or snow melting. the fluvial process of heavy rain and snow melting moving the fine-grained material (silt and clay) in the elongate and narrow transportation zone and depositing them downslope, the earthflow is generally taken the shape of the narrow tongue. the activity of earth flow is continuous for a long time (conforti et al, 2020); (jain, 2014). in the roste valley the earthflows are well developed within the soil covers above the qandil series in the suture zone, near the piromar village (figure 8.c), the climatic condition (heavy rain and snow) and the topographic form (mountains and hill slopes) in the study area aids the movement of fine-grained soil cover, the material transported downslope and formed the lobate-shaped depositional feature. rockfall rockfall is defined as the bounding or free movements of loose and fragmented rock downward along the slope surface under the action of gravity (luckman, 2013). in the study area, the rock fall was found in several parts because of high relief and climatic conditions, the climatic factor aids in rock fragmentation (weathering), and the steep slope and gravitational process moves the fragmented rock downslope (figure 8.d and h). in the imbricated zone the rock fall was observed near the besha village within the redbed series (merga i) and in the core of tanoun anticline, in the suture zone was found within the qandil series and walash group near the roste and grtik villages. transitional slide a transitional slide is a type of mass wasting developed when the masses of the coherent rock or soil move and displace along the failure surface of the plane or the shear zone. the transitional landslide occurs on the planer surface with the linear displacement whereas the rock mass in the rotational sliding moves along the concave and curved surface. the failure surfaces are generally caused by preexisting stratigraphic or structural discontinuities such as contact between weak and soft cover with bedrock, bedding, faults, and joints (morino et al, 2022). according to (gomberg et al, 1995), the faulting in landslides is the same and similar to the crustal rock faulting, both including the displacement of masses along the planer failure surface. in the study area, the land-sliding occurred near hasanbag mountain within the naopurdan group (figure 8.e and f). toppling the rotation in the forward direction of rock about an axis located below the gravity center of the mass movement is known as topples. toppling generally occurs on steep slopes and cliffs and consists of slabs or rock columns separated by steeply dipping vertical discontinuous planes such as bedding planes, joints, and cracks due to the unloading of masses. the progression of forward rotations results in losing basal supports of rock slabs or blocks and the toppling starts to fall. the toppling is well developed in the ne limb of the tanoun (spibalis) anticline within cretaceous rocks (figure 8.g). figure 8. shows landforms produced by mass wasting process, a and b) block slide in the suture zone and core of tanoun anticline respectively, c) earth flow, d and h) rock fall, e and f) transitional slide in the hasanbag mountain the red line indicate seemingly sliding surface, and g) toppling in the core of tanoun anticline. 42 paiwast k. zhazhlayi, ali m. surdashy 6. landforms of fluvial origins river terraces the river terraces are commonly developed in response to tectonic and/ or climatic controls, in high tectonic settings continuous uplifting can develop river terraces hundreds of meters higher than the present elevation of the channel (daley and cohen, 2018). terraces commonly represent the old channel locations now abandoned by the migration of the channel. in the study area, the river terraces are well developed in the core of the tanoun (spibalis) anticline, when the roste river migrated across the oblique faults leaving behind 7 levels of terraces within the jurassic rock (figure 9.a). abandoned alluvial fan alluvial fans are morphologically fan-shaped landforms, developed as the result of the accumulation of sediment and clastic rock materials at the outlets of the river which flows from the mountain range in narrow valleys into the low-land area (haldar, 2020). the sediment and clastic material from the apex of the fan flows out radially into planes, the size ranges from small to large alluvial fans. in the study area, the abandoned or old alluvial fan was found in the ne limb of the tanoun (spibalis) anticline (figure 9.b). floodplain the flood plain is a flat area adjacent to the drainage stream, mainly consists of alluvium (unconsolidated) deposits, and is periodically subjected to stream flooding. floodplains are generally developed by point bar deposition and sometimes may be formed by overbank flow deposition during the flooding period (panizza and goudie, 2013). the floodplain in the study area is narrow and small located in the middle of roste valley near simelan and mama khatybian village (figure 9.c). gully and rill the gully erosion is formed as the result of several rainfall events (overland flow) morphologically linear, small, and deeply eroded channels with steep slope sides (divide), and ephemeral gullies may be continuous or discontinuous depending on the climatic conditions (gao, 2013). the rill erosion is developed as the consequence of repeated and periodic rainfall and the overland flows, and morphologically is small incised channels on slope surfaces. the depth and width are tens of centimeters, formed on a bare surface with little or no vegetation (berger et al, 2010). in the study area, the gully and rill erosion were recognized in different parts of the valley, particularly within the naopurdan shaly group in the suture zone, near the gazna village (figure 9.d). figure 9. landforms developed through fluvial process, a) river terraces, b) abandoned alluvial fan in the ne limb of tanoun anticline, c) flood-plain, and d) gulley and rill erosions. 7. landform of solutional origin karstification (cave) a cave is defined as a natural ground opening, formed in a variety of lithology, particularly in carbonate rocks (limestone) and resulting from the dissolution process spatially in limestone rocks. the size ranges from small caves (room-size) to the large interconnecting caves passage that extends to several km in length (morgan, 1991). in the study area, a cave landform was found in the ne of tanoun (spibalis) anticline within the aqra-bekhme formations, near the chambarok village. the caves in the study area are formed by the dissolution process in the carbonate rocks (figure 10.a). karren field and solution pits the karren landform is formed by the dissolution process in bare rocks and is typically formed in the areas where the carbonate rocks (limestone and dolomite) are dominant and may develop in the evaporitic rocks. the channel depth ranges from millimeters to one meter or more and is several centimeters wide. the solution pits are the type of circular karren formed as the result of dissolution typically in the carbonate rocks (figure 10.b). the size of the pit is up to 5 cm and is elliptical or circular in shape (lundberg, 2019). in the study area, the karren field and solution pits were developed within the qamchuqa limestones in the truncated syncline near the imbricate thrusting in the core of the tanoun (spibalis) anticline. figure 10. karstic origin landforms, a) cave, and b) karren field and solution pits in the ne limb and core of tanoun anticline respectively. 43geomorphology and landforms evolution of roste valley, western zagros fold-thrust belt, iraqi kurdistan region conclusion finally, the geomorphological survey and mapping of the roste valley emphasize the study area’s complex polygenetic nature, with a diverse range of landforms formed by various geological processes. these include tectonic landforms generated by the compressional and extensional stresses started from the closure of the neo-tethys ocean in the middle miocene, and the tectonic movement is still ongoing to the present day. structural features developed by faulting and folding in combination with the erosion process. surficial processes and climatic conditions of the study area aid erosion and transportation processes, resulting in the formation of different denudation landforms fluvial landforms resulted from river action; the roste river, from the end of the miocene period, changed in its path, velocity, and intensity, hence shaping different landforms. furthermore, because of high relief and steep slopes, mass-wasting processes have led to the formation of different landforms. the solution features are especially evident in the tanoun anticlines in the 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(2011). the evolution of the northern shutter ridge, mt. carmel, and its implications on the tectonic activity along the yagur fault, ministry of national infrastructures, geological survey of israel, gsi/14/2011 pp. 26. http://dx.doi.org/10.1130/l1056.1 http://dx.doi.org/10.46717/igj.55.2e.3ms-2022-11-17 http://dx.doi.org/10.46717/igj.55.2e.3ms-2022-11-17 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n2.118095 earth sciences research journal earth sci. res. j. vol. 29, no. 2 (june, 2025): 195 -199 pa la eo n to lo g y abstract first record of crocodyliformes osteoderm from the upper cretaceous bauru group, state of goiás , brazil. thainara aparecida divina arruda de paula1,2, carlos roberto dos anjos candeiro1*, tamires do carmo dias1, luciano vidal3, paulo victor luiz gomes da costa pereira² 1. laboratório de paleontologia e evolução, curso de geologia, campus aparecida de goiânia, universidade federal de goiás, estrada municipal s/n quadra e área lote 04 – bairro fazenda santo antônio, 74971-451 – aparecida de goiânia – brasil. 2. sendo programa de pós graduação em biodiversidade animal, universidade federal de goiás 3. departamento de geologia, instituto de geociências, universidade federal do rio de janeiro, ilha do, av. athos da silveira ramos, 274 – cidade universitária, 21941909, rio de janeiro – rj, brasil. *corresponding author: candeiro@ufg.br record manuscript received: 20/12/2024 accepted for publication: 25/05/2025 how to cite this item: paula, t. a. d. a., candeiro, c. r. d. a., dias, t. d. c., vidal, l., & pereira, p. v. l. g. d. c. (2025). first record of crocodyliformes osteoderm from the upper cretaceous bauru group, state of goiás, brazil. earth sciences research journal, 195-199. https://doi. org/10.15446/esrj.v29n2.118095 the crocodyliformes from the late cretaceous of south america were one of the main components of the tetrapod faunas found in rocks of the bauru group at goiás, minas gerais and são paulo states, brazil. the presence of osteoderms has been reported in articulated specimens or found isolated in the palaeontological record. in the present work, we describe the first osteoderm of mesoeucrocodylia from the campanian-maastrichtian of the marília formation (bauru group) of the state of goiás. paleo-ufg-114 has a quadrilateral shape, a rough external texture with radially oriented corrosion, an internal facet with the presence of intersecting grooves, foramina in the internal and external region and the presence of a poorly developed keel. these features differentiate them from the osteoderms of titanosaur dinosaurs found in the rocks of this geological unit. the presence of paleo-ufg-114 in the state of goiás reaffirms that mesoeucrocodylians were important components of the reptile fauna, and contributes to increasing knowledge about the group’s geographic distribution in this poorly-explored region of central brazil. keywords: osteoderm, mesoeucrocodylia, late cretaceous, bauru group, central brazil. primer registro de un osteodermo crocodyliformes del cretácico superior del grupo bauru, estado de goiás resumen los crocodyliformes del cretácico tardío de américa del sur fueron uno de los componentes principales de las faunas de tetrápodos halladas en rocas del grupo bauru en los estados de goiás, minas gerais y são paulo, brasil. la presencia de osteodermos ha sido reportada en especímenes articulados o encontrados aislados en el registro paleontológico. en el presente trabajo describimos el primer osteodermo de mesoeucrocodylia del campaniano-maastrichtiano de la formación marília (grupo bauru) del estado de goiás. el espécimen paleo-ufg-114 tiene una forma cuadrilátera, una textura externa rugosa con corrosión orientada radialmente, una faceta interna con presencia de surcos intersectados, forámenes en las regiones interna y externa y una quilla poco desarrollada. estas características los diferencian de los osteodermos de dinosaurios titanosaurios encontrados en las rocas de esta unidad geológica. la presencia de paleo-ufg-114 en el estado de goiás reafirma que los mesoeucrocodilianos fueron componentes importantes de la fauna de reptiles y contribuye a aumentar el conocimiento sobre la distribución geográfica del grupo en esta región poco explorada del centro de brasil. palabras clave: osteodermo; mesoeucrocodylia; cretácico tardío; grupo bauru; brasil central. https://doi.org/10.15446/esrj.v29n2.118095 mailto:candeiro@ufg.br https://doi.org/10.15446/esrj.v29n2.118095 https://doi.org/10.15446/esrj.v29n2.118095 196 thainara divina arruda de paula, carlos dos anjos candeiro, tamires do carmo, luciano vidal, paulo victor luiz gomes introduction the bauru group outcrops at areas of the states of goiás, mato grosso do sul, minas gerais and são paulo (fernandes & coimbra, 1996; batezelli et al., 2019). recently studies have proposed a new lithostratigraphic arrangement for this geological unit composed of the adamantina, uberaba, marília and serra da galga formations (batezelli et al., 2019; soares et al., 2020). these layers have a rich fossil content regarding by plants, invertebrates and vertebrate remains found mainly in the adamantina, marília and serra da galga formations at the minas gerais and são paulo states (ihering, 1911; pacheco, 1913; von huene, 1931, 1933; candeiro et al., 2006; martinelli & teixeira, 2015; santos-pereira et al. 2016; candeiro & figueirôa, 2017; see review in brusatte et al., 2017; candeiro et al., 2018; candeiro et al., 2020). records of crocodyliforms have been reported since the beginning of the last century for these states (price, 1945; buffetaut, 1980; brandt neto et al., 1991, 1992; carvalho and bertini, 1998, 1999, 2000). crocodyliforms from the bauru group are known from complete skeletons and isolated teeth (carvalho et al., 2004; candeiro et al., 2018). records of osteoderms have been described and are important elements found of this group (marinho et al., 2006), being present on baurusuchidae, conforme table 1. these osteoderms, along with associated cranial and postcranial remains, are often preserved with structural integrity. several baurusuchid specimens, for instance, exhibit well-preserved anatomical features and low degrees of weathering and transport, suggesting favorable taphonomic conditions within the depositional environments of the bauru group (bandeira et al., 2018). table 1. crocodyliformes groups of bauru group with osteoderms used as comparisons. taxa selected authors peirosauridae such as montealtosuchus arrudacamposi carvalho, vasconcellos, tavares, 2007 peirosaurus torminni price, 1955 uberabasuchus terrificus carvalho, ribeiro, avilla, 2004 itasuchidae as itasuchus jesuinoi price, 1955 isolated osteoderms e.g., torres et al., 2001 baurusuchidae marinho et al., 2006 the south of the state of goiás presents outcrops of sedimentary rocks belonging to the adamantina and marília formations, with great fossiliferous potential with records of invertebrates (mezzalira, 1974), plants (basilici & dal’bo, 2010; candeiro et al., 2022), fish, turtles, crocodyliforms, sauropods and theropods (candeiro et al., 2018, 2020, 2024). this region has a great fossiliferous potential for crocodyliforms material findings, which could increase the knowledge about de diversity and distribution of this group at central brazil. in 2022, unpublished materials of crocodyliforms and dinosaurs were collected from marília formation in the municipality of quirinópolis, state of goiás by laboratório de paleontologia e evolução (curso de geologia, universidade federal de goiás) researching team. this article aims to describe the first crocodyliform osteoderm for the state of goiás and thus contribute to records of this taxon on the region. geological setting the municipality of quirinópolis is located in the south of the state of goiás (brazil), where rocks from the adamantina and marília formations of the bauru group are exposed (fig. 1; soares et al., 1980). ages are estimated to vary between campanian-maastrichtian to adamantina formation and aged as maastrichtian to marília formation (sensu batezelli et al., 2019). the marília formation in the state of goiás is the most prolific in terms of cretaceous vertebrate remains in the state, but with records concentrated in the neighboring municipality of rio verde (see review in candeiro et al., 2020). institutional abbreviation paleo-ufg: coleção de paleontologia, laboratório de paleontologia e evolução, universidade federal de goiás, aparecida de goiânia, state of goiás, brazil. ufg: universidade federal de goiás figure 1. lithostratigraphic map of the bauru group and the study area in the municipality of quirinópolis, goiás, brazil (adapted from sigas, 2003). morro da antena rural outcrop in quirinópolis municipality (marília formation) fine-grained deposits, mainly claystones, siltstones and fine sandstones, form tabular to cuneiform bodies up to 8 m thick and are laterally extensive and intercalated between dune units (fine-grained sandstones with cross bedding) (fig. 2). internally, these units are characterized by massive to laminated claystones from 4 to 22 cm thick and siltstones where does the osteoderm studied here come from, this level presents abundant rootlets and desiccation cracks. in addition, there are beds from 2 to 17 cm thick, composed of laminated claystones interspersed with fine sandstones, mainly massive or with asymmetrical subcritical climbing ripples. a very striking feature of these deposits is that both the upper and lower contacts are non-erosive abrupt. figure 2. (a) stratigraphical profile of morro da antena outcroup of the marília formation; (b) locality of morro da antena in rural area of the municipality of quirinópolis, southern goiás state, indicating the level at which the osteoderm paleo-ufg/114 was found (black silhouette). materials and methods paleo-ufg/114 consists of an osteoderm in a good state of preservation from morro da antena, municipality of quirinópolis, state of goiás, brazil. the material is housed at laboratório de paleontologia e evolução, universidade federal de goiás of city aparecida de goiânia, state of goiás, brazil. the morphological description followed the criteria outlined by marinho et al. 2006, considering the overall shape, external and internal ornamentation patterns, and the orientation and position of the keel. identification of the specimen as pertaining to mesoeucrocodylia was based on comparisons with published descriptions and figures of similar osteoderms from the bauru group and other upper cretaceous crocodyliforms. these comparisons were conducted using images and diagnostic features available in the literature. this approach allowed the recognition of shared morphological traits consistent with mesoeucrocodylian taxa. paleo-ufg/114 was collected manually in the field in 2023 and prepared in 2023 and 2024 at the laboratório de paleontologia e evolução, curso de geologia, universidade federal de goiás. measurements were recorded using 197first record of crocodyliformes osteoderm from the upper cretaceous bauru group, state of goiás a mitutoyo caliper from the 500 series, absolute aos digital model with absolute technology, with a capacity of 150mm/6. the views were taken using a leica model ez4 binocular stereomicroscope and the photographs were taken using a canon eos rebel t4i camera with a canon 18-55 lens. the measurement accuracy is on average ±0.02mm. samples and reference and comparison figures came from publications. results systematic palaeontology crocodylomorpha walker, 1970 crocodyliformes hay, 1930 (sensu clark, in benton & clark, 1988) mesoeucrocodylia whetstone & whybrow, 1983 notosuchia gasparini, 1971 material o specimen and provenance: the material consists of an isolated osteoderm of crocodyliformes found in the maastrichtian marília formation, bauru group and is listed in the laboratório de paleontologia e evolução of the universidade federal de goiás under number paleo-ufg/114. description the fossil is well preserved and has signs of breakage on the sides. the preservation and slight abrasion of the material indicate that it was rolled from a previous location until the collection site. paleo-ufg/114 has a nearly flat internal face and a slightly convex external face, measuring 3 cm in length, 0.62 cm in depth and 2.15 cm in width. it has a quadrilateral shape and is compressed dorsoventrally (fig. 3). the external surface has a rough texture, with grooves of different sizes well distributed throughout the region and a slightly centralized keel (fig. 3a and b), not extending beyond the edge of the specimen. the internal face exhibits a range from smooth to finely pitted bone, pierced by numerous vascular foramina and intersecting grooves consistent with the condition described for mesoeucrocodylia (marinho et al., 2006). lateral margins bear semi‐rounded, polished breakage surfaces, indicating transport abrasion prior to final burial or by other biostratinomic interactions such as predation or necrophagy, although we believe this is less likely due to the aspects of wear seen throughout the osteoderm. figure 3. paleo-ufg/114, a: view of the external region; b: line drawing of the external region. the red line indicates the reconstructed keel. the gray shapes indicate abrasion on the surface; c: view of the internal region; d: detail of the internal region showing foramina [arrow 1] and intersecting grooves [arrows 2 and 3]; e, f and g: side view showing the slightly arched keel. scale a, b = 2cm. when compared to osteoderms of baurusuchus albertoi nascimento & zaher, 2010, which are arranged in two parasagittal rows and show progressive enlargement from small cervical plates (~1 cm long) to large, imbricating caudal shields with pronounced midline crests and reticulate microtexturing (nascimento & zaher, 2010), paleo-ufg/114 differs in keel development (more subdued and centrally confined) and lacks true imbrication. relative to mariliasuchus amarali carvalho & bertini, 1999, whose dorsal osteoderms are quadrangular with a continuous, prominent principal crest flanked by secondary crests and irregular grooves fading toward the border (nobre & carvalho, 2013), paleo-ufg/114 presents a more irregular network of sulci, a weaker crest, and a dorsolateral plate geometry rather than the broad, tilelike arrangement seen in mariliasuchus. finally, the pattern of internal vascular canals and secondary remodeling in paleo-ufg/114 most closely matches that of montealtosuchus arrudacamposi carvalho, vasconcellos & tavares, 2007, whose osteoderms (13–53 mm long) bear an elevated median keel, shallow grooves, and punctate vascular canals within a predominantly woven–parallelfibered bone matrix (tavares et al., 2015) discussion and conclusions osteoderms are not exclusive for crocodylomorphs, being present also in titanosaur sauropods (e.g., d’emic et al. 2009). comparing with other groups that have titanosaurian osteoderms, paleo-ufg/114 differ from those due to the lack of a rough internal surface, with an interlaced bone pattern, the presence of a bulb and root separated by a cingulum, according to morphology regarded by d’emic et al. (2009). the identification of paleo-ufg/114 as a crocodyliforms osteoderm is based on: (1) its rough external texture with radially oriented pitting; (2) an internal facet, with the presence of intersecting grooves that may have been caused by contact with underlying tissue, with the presence of foramina in this region; (3) presence of a keel whose elevation differs according to the region to which the osteodems belongs (marinho et al., 2006). when dealing with crocodyliforms, osteoderms are separated into nuchal (cervical), dorsal (thoracic), caudal, gastral (ventral/abdominal) and appendicular parts of the body (ross and mayer, 1983; frey, 1988; salisbury and frey, 2001). hill (2010) subdivided the dorsal osteoderms into paravertebral and dorsal accessory regions, the latter of which may be the region belonging to the osteoderm described here in this work because they are quadrilateral, dorsoventrally compressed, similar to plates in general structure and with the presence of a slight keel that does not exceed the margin of the osteoderm. also according to hill (2010), this type of osteoderm can vary in its dimensions depending on the region in which it is found, being the closest to the dorsal accessory wide than longer and those located more laterally can be the equal or longer and the keel also presents variations in its development, especially as it is positioned more laterally on the dorsal dermal shield (filippi et al. 2013). on the inner surface of paleo-ufg/114, there are intercrossing grooves reported for other osteoderms of notosuchia, mesoeucrocodylia, such as itasuchus (marinho et al., 2006) and montealtosuchus (tavares et al., 2015). the notosuchians form a clade of crocodyliforms of varying sizes and habits known, mainly from the upper cretaceous strata of south america (carvalho, et al. 2004; tavares et al., 2015; candeiro et al., 2018). recently, postcranial remains belonging to mesoeucrocodilians were reported from the strata of the marília formation at rio verde municipality (candeiro et al., 2018) and the first skeleton of caipirasuchus from the state of minas gerais, brazil, described by martinelli et al. (2018). paleo-ufg/114 is the first osteoderm occurrence of this type of material in the state of goiás. therefore, the discovery presented reinforces the morphological diversity of these elements and their importance for the paleoecological and paleobiogeographic interpretation for the region. additionally, it contributes to the group’s knowledge in the state of goiás, indicating the fossiliferous potential of the location and increasing the understanding of the cretaceous paleofauna in central brazil. acknowledgements our collaborative project was funded by a grant from the fundação de amparo à pesquisa de goiás (fapeg, 2023/24). references basilici, g. & dal’ bo, p. f. f. (2010). anatomy and controlling factors of a late cretaceous aeolian sand sheet depositional system the marília and the adamantina formations, nw of the bauru basin, brazil. sedimentary geology, 226:71-93. batezelli, a., ladeira, f.s.b., nascimento, d.l. & silva, m.l. 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(2015). osteoderms of montealtosuchus arudacamposi (crocodyliformes, peirosauridae) from the turonian-santonian (upper cretaceous) of bauru basin, brazil. cretaceous research 56: 651-661. torres, s. r.; avilla, l. s.; abrantes, e. a. l. & bergqvist, l. p. (2001). dinosaur osteoderms from the adamatina formation, upper cretaceous of são paulo state, brazil. anais da academia brasileira de ciências (2002) 74(2):361-370 von huene, f. (1931). verschiedene mesozoische wirbeltierreste aus südamerika. neues jahrbuch für mineralogie, geologie und paläontologie, abteilung a, 66, 181-198. von huene, f. (1933). ein versuchzur stammesgeschichte der krokodile. central blatt für mineralogie, geologie und paläontologie, abteilung b, 11, 577-585. _heading=h.mob5joc845h1 _heading=h.ce1zbdqwo1xb _heading=h.4j5t3hyhvniy _heading=h.k538p1j8z39v _heading=h.6zz0ztrqq77l issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.112804 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 93 101 m in in g e n g in ee r in g how to cite this article: sun, c., hou, k., wang, s., & qian, s. (2024). analytic hierarchy process-fuzzy comprehensive evaluation method-based depletion assessment study of xinshan iron ore mine. earth sciences research journal, 28(1), 93-101 https://doi.org/10.15446/esrj.v28n1.112804 abstract: taking the xinshan iron ore mine as an example, this paper, based on collecting and analyzing the actual production data and similar simulation test data of this iron ore mine, analyses various factors affecting ore depletion by bottomless column segmental chipping method by using hierarchical analysis method (ahp) and fuzzy comprehensive evaluation method (fce), and establishes an evaluation system for comprehensively assessing the depletion of the ores. the results show that structural parameters, blasting parameters, loading parameters, and geological conditions are the main factors affecting ore depletion. the structural parameters are the most important factors, accounting for 35%. with the increase of the released amount, the released grade gradually decreases, the depletion rate gradually increases, and the comprehensive evaluation value gradually decreases. the released body is an approximate ellipsoidal block with a wide upper and narrower lower part. the end wall plays an obstructive role in the flow of the bulk body, which makes the end of the released grade higher and the middle of the released body higher. at the same time, due to the influence of blasting and shovel loading, the particles in the release body show some sorting phenomena. this paper provides a scientific basis and reference for predicting and controlling ore depletion in the bottomless column segmental chipping method. analytic hierarchy process-fuzzy comprehensive evaluation method-based depletion assessment study of xinshan iron ore mine chentao sun1,2,*, kepeng hou1,2, shining wang3, shanguang qian1,4 1. school of land and resource engineering, kunming university of science and technology. 2. key laboratory of development and utilisation of blue mines and special underground space in yunnan province. 3. school of nursing, kunming medical university, china; 4 school of architecture and engineering, kunming metallurgical higher and specialist university *corresponding author: 20212201043@stu.kust.edu.cn record manuscript received: 07/11/2023 accepted for publication: 20/04/2024 evaluación del agotamiento de mena en la mina de hierro de xinshan con base al proceso de análisis jerárquico y la evaluación integral difusa keywords: bottomless column segmental chipping method; ore depletion; hierarchical analysis method; fuzzy comprehensive evaluation method; discharge grade resumen con la mina de hierro de xinshan como un ejemplo y con el fin de recolectar y analizar la producción de información y los datos de prueba de simulaciones similares en esta mina, este trabajo analiza varios factores que afectan el agotamiento de la mena a través del método de astillado segmentario de pilar sin base por el proceso de análisis jerárquico (del ingles ahp, analytic hierarchy process) y por la evaluación integral difusa (del inglés fce, fuzzy evaluation method, y establece un sistema de evaluación para medir ampliamente el agotamiento del recurso. los resultados muestran que los parámetros estructurales, los parámetros de explosión, los parámetros de carga y las condiciones geológicas, son los factores principales que afectan el agotamiento de la mena. estos parámetros estructurales son los factores más importantes y significan el 35 %. con el incremento de la cantidad de material explotado, el grado de explotación decrece gradualmente, el índice de agotamiento se increment y el valor de la evaluación integral desciende. la cantidad de material explotado es aproximadamente un bloque elipsoidal con una parte superior amplia y una parte inferior más estrecha. la pared del fondo juega un papel obstructivo para el flujo del grueso del mineral, la cual incrementa el grado de explotación en el fondo y a la mitad del cuerpo. al mismo tiempo, debido a la influencia de las explosiones y de la carga de la excavación, las partículas en el material explotado muestran algunos fenómenos de clasificación. este artículo proporciona una base científica y una referencia para la predicción y el control del agotamiento del mineral en el método de astillado segmentario de columna sin fondo. keywords: método de astillado segmentario de columna sin fondo; agotamiento de mineral; proceso de análisis jerárquico; evaluación integral difusa; grado de descarga. https://doi.org/10.15446/esrj.v28n1.112804 https://doi.org/10.15446/esrj.v28n1.112804 mailto:20212201043@stu.kust.edu.cn 94 chentao sun, kepeng hou, shining wang, shanguang qian 1. introduction bottomless pillar segmental chipping is a mining method that uses selfweight to sink the overburden rock, chipping the ore body and releasing it through the discharge holes to the surface or underground storage (wang et al., 1988). this method is suitable for metal deposits with large inclination, large thickness, high hardness and low grade, and has the advantages of simple process, high degree of mechanization and good safety (jin et al., 2017). however, the method also suffers from serious ore loss and depletion, which affects the resource utilization and economic efficiency of the mine (zeng, 2020). ore depletion is the reduction or loss of ore grade during the mining process due to various reasons (wu et al., 2012). ore depletion of bottomless pillar segmental chipping method mainly occurs in the process of ore release, i.e., in the process of ore release under the overburden rock, due to the mixing effect of the surrounding rock and the ore body, which makes the grade of the ore release lower than the original grade. there are many factors affecting the ore depletion of the bottomless pillar segmental chipping method, mainly including the structural parameters of the quarry, blasting parameters, loading parameters and geological conditions (yang, chen, & wang, 2017). the selection of the technical evaluation methodology has a significant impact on the accuracy of the evaluation results. currently, common evaluation methods at home and abroad include the data envelopment analysis method (peykani et al., 2020), life cycle assessment approach (yang & wang, 1998; chen et al., 2020), grey correlation analysis (wang, guo, & lian, 2005; yu & zhang, 2014), delphi method (brady, 2015), hierarchical analysis method (ahp) (guo, zhang, & sun, 2008; zhou et al., 2021; sun et al., 2013), fuzzy comprehensive evaluation (fce)(xiong, & xian, 2003; yu et al., 2020) and portfolio evaluation methodology (feng & sun, 2020). the evaluation model that combines ahp and fce is the analytical hierarchy process fuzzy comprehensive evaluation (ahp-fce)(han, mei, & lu, 2004). it has been widely studied in various fields such as performance appraisal (dai, 2019), management software selection (chen, 2019), environmental evaluation (zhang & fan, 2020), education quality evaluation (dai & li, 2016), tourism service quality evaluation (zhang, liu, & xia, 2016), and programme preference (luan, qi, & kou, 2020). this paper takes xinshan iron ore mine as an example, which is mined by the bottomless column segmental avalanche method. based on collecting and analyzing the actual production data of similar iron ore mines and similar simulation test data, the ore depletion evaluation system of the bottomless column segmental avalanche method is established by using the hierarchical analysis method and fuzzy comprehensive evaluation method. a comprehensive evaluation of the exudate body in this iron ore mine is carried out. the purpose of this paper is to explore a scientific and effective ore depletion evaluation method of bottomless column segmental chipping method, and to provide some reference bases for similar metal mines to improve the grade of the exudate body and reduce the depletion rate. 2. experimental analysis methods the analytical methods in this paper is twofold: firstly, the hierarchical analysis method (ahp), which is used to determine the weights of the factors affecting ore depletion in the bottomless column segmental disintegration method; and secondly, the fuzzy comprehensive evaluation (fce), which is used to carry out a comprehensive evaluation of the releasing body based on the weights of the factors and the evaluation indexes. 2.1 experimental step the principle and methodology of the similarity simulation tests were referred to the literature (zhang, 2023), and the main steps are as follows: 1. selection of mineral rock particles of similar composition and size geometry and broadly similar mechanical properties to the crumbled ore rock at the site, and mixing them in certain proportions to form a bulk; 2. make a model geometrically similar to the on-site quarry structure and ore release system, reducing the size to a certain scale, and setting up ore release holes and observation holes; 3. populate the model with the bulk and perform the ore release operation in a certain order, releasing a certain amount of bulk at a time; 4. after each release, parameters such as grade, depletion rate, morphology, and particle distribution of the released body are measured through observation holes and data are recorded; 5. repeat the above steps until all the bulk is released. figure 1. flow chart of xinshan iron ore release simulation experiment figure 2. ore of each particle size during the experiment table 1. particle size volume parameter particle name grain size volume fraction ore particles <20cm 82.5% 20~40cm 10% >40cm 7.5% waste stone granules 60~70cm 100% there are two main sources of data for this paper: the actual production data from this iron ore mine and the similar simulation test data from this iron ore mine. the actual production data were provided by the iron ore mine, and the similar simulation test data were obtained by the authors from tests conducted in the laboratory. table 2. main mining structure parameters of sinzan iron ore mine parameters numerical value segment height(m) 20 inlet spacing(m) 15 approach width(m) 5 approach height(m) 5 avalanche step(m) 5 ore release hole diameter(mm) 200 depth of ore release hole(m) 20 95analytic hierarchy process-fuzzy comprehensive evaluation method-based depletion assessment study of xinshan iron ore mine (a) front of mine release model (b )side of mine release model figure 3. front and side view of mine release model 2.2 weighting 2.2.1 modelling the hierarchy the principles and steps of hierarchical analysis method and fuzzy comprehensive evaluation method were referred to the literature ( [26], and firstly, the hierarchical structure model of ore depletion evaluation by bottomless column segmental disintegration method was established. 2.2.2 constructing a judgement matrix as shown in figure 3, the expert scoring method was used to make the determination of the degree of two-by-two matrix comparison, and the fuzzy linguistic variables from 1 to 9 were used to indicate the relative importance between the factors, as shown in table 6, and the scoring resulted in table 5; table 3. classification of importance of indicators c on si de ra tio ns st ru ct ur al pa ra m et er s b la st in g pa ra m et er s l oa di ng pa ra m et er s g eo lo gi ca l co nd iti on structural parameters equal importance slightly important slightly important slightly important blasting parameters clearly important equal importance slightly important slightly important loading parameters clearly important clearly important equal importance slightly important geological condition clearly important clearly important slightly important equal importance table 4. 1 to 9 fuzzy language variables fuzzy linguistic variable hidden meaning trigonometric fuzzy function 1 equal importance (1,1,1) 3 slightly important (1,2,3) 5 clearly important (3,4,5) 7 important (5,6,7) 9 very important (7,8,9) 2 midpoint (1,1.5,2) 4 midpoint (2,3,4) 6 midpoint (4,5,6) 8 midpoint (6,7,8) ore depletion by the bottomless column segmental disintegrations method target level structural parameters upper section central section lower section blasting parameters loading parameters geological parameters normative layer programme layer figure 4. ahp model diagram 96 chentao sun, kepeng hou, shining wang, shanguang qian table 5. judgment matrix after scoring c on si de ra tio ns st ru ct ur al pa ra m et er s b la st in g pa ra m et er s l oa di ng pa ra m et er s ge ol og ic al co nd iti on structural parameters (1,1,1) (1/3,1/2,2/3) (1/2,2/3,1) (1/2,2/3,1) blasting parameters (3/2,2,3) (1,1,1) (2/3,1,3/2) (2/3,1,3/2) loading parameters (1,3/2,2) (2/3,1,3/2) (1,1,1) (4/5,1,5/4) geological condition (1,3/2,2) (2/3,1,3/2) (4/5,1,5/4) (1,1,1) 2.2.3 solve for the vector of evaluation indicator weights calculate the maximum eigenvalue and eigenvector of the judgement matrix, get the weights of each factor, and carry out the consistency test to ensure the reasonableness of the judgement matrix; since the judgement matrix is composed of fuzzy numbers, it is necessary to calculate the maximum eigenvalue and eigenvector using the algorithm in fuzzy mathematics, and use fuzzy consistency ratio to test the consistency; 1. calculate the fuzzy product of the elements of each row of the judgement matrix, i.e., multiply the elements of each row by columns to obtain: = 1 • 2 • 3 • 4( ), = 1, 2, 3, 4 where ai1,ai2,ai3,ai4 denote the four elements of row i of the judgement matrix and denotes the multiplication operation of fuzzy numbers, i.e.. , ,( ) • , ,( ) = , ,( ) 2. calculate the fuzzy weighted average of the elements of each row of the judgement matrix, i.e., obtained by dividing the elements of each row by the median of their fuzzy product: = 1 , 2 , 3 , 4( ), = 1, 2, 3, 4 where mi denotes the intermediate value of ai, i.e., mi = (ai)2 and / denotes the division operation of fuzzy numbers, i.e.. =, ,( ) 3. calculate the maximum eigenvalues and eigenvectors of the judgement matrix, i.e., sum the elements of each column and normalise them to obtain: ⎝ ⎠ where ∑ denotes the summation symbol and + denotes the addition operation of fuzzy numbers, i.e.. as if: ;, ,( ) + , ,( ) = + , + , +( ) ,λ = 4. 05, 4. 25, 4. 45( ) ω = 0. 35, 0. 25, 0. 20, 0. 20( ) 2.2.4 conducting consistency tests the specific steps and formulas for calculating the fuzzy consistency ratio to test consistency are as follows: 1. calculate the fuzzy product of the judgement matrix with its eigenvectors, i.e., multiply each row element with its corresponding weight and sum the results by rows to obtain: = 1 • ω 1 + 2 • ω 2 + 3 • ω 3 + 4 • ω 4( ) , = 1, 2, 3, 4 where n denotes the order of the judgement matrix. 2. calculate the fuzzy difference between the judgement matrix and its largest eigenvalue, i.e., subtract each row element from its corresponding largest eigenvalue and take the absolute value to obtain: = 1 − λ 1| | , 2 − λ 2| | , 3 − λ 3| | , 4 − λ 4| |( ) , = 1, 2, 3, 4 where l denotes taking the absolute value sign and denotes the subtraction operation of fuzzy numbers, i.e.. , ,( ) − , ,( ) = − , − , −( ) 3. calculate the fuzzy quotient of the judgement matrix with its largest eigenvalue, i.e., divide each row element with its corresponding largest eigenvalue and take the reciprocal to obtain: = λ( ) = λ 1 1 , λ 2 2 , λ 3 3 , λ 4 4 ( ) , = 1, 2, 3, 4 continuing to calculate the fuzzy quotient of the judgement matrix with its largest eigenvalue yields. = ( ) , = 1, … ; = 1, … ; = 1, … 4. calculate the fuzzy consistency index of the judgement matrix, i.e., the fuzzy difference of the elements of each row is added to its corresponding fuzzy quotient and the minimum value is obtained: = +( ) = +( ) , = 1, … ; = 1, … 5. calculate the fuzzy consistency ratio of the judgement matrix, i.e., divide the fuzzy consistency index of each row element with its corresponding random consistency index and take the minimum value to obtain: = ( ) = ( ) , = 1, … ri table 6. random consistency index ri n 1 2 3 4 5 6 7 8 9 10 11 ri 0 0 0.58 0.90 1.12 1.24 1.32 1.41 1.45 1.49 1.51 where ri denotes the random consistency index, when n = 4.ri=0.9, cr= λ −( ) − 1( ) , catch: cr= 0.0925 < 0.10 ,adopted by consensus. 2.3 establishment of an evaluation system 2.3.1 identification of evaluation indicators the experimental data from the second segment was used to calculate the depletion because the second segment had good ore release. table 7. calculation of the amount of ore released at different crumbling steps in the second section av al an ch e ste p/ m 1. 0 1. 5 2. 0 2. 5 3. 0 3. 5 4. 0 to tal am ou nt of o re re lea se d/ g 866.89 1253.94 1630.22 1979.12 2334.44 2657.84 2713.36 am ou nt of w as te ro ck /g 31.07 35.91 42.59 34.12 36.07 50.11 48.12 am ou nt of p ur e or e/g 835.82 1218.03 1587.63 1945 2298.37 2607.73 2665.24 th eo re tic al pu re o re am ou nt /g 835.82 1253.73 1671.64 2089.55 2507.46 2925.37 3343.28 th eo re tic al or e r es id ue /g 0 35.7 84.01 144.55 209.09 317.64 678.04 th eo re tic al or e re sid ue /% 0 2.93 5.29 7.43 9.10 12.18 25.44 97analytic hierarchy process-fuzzy comprehensive evaluation method-based depletion assessment study of xinshan iron ore mine figure 5. the relationship between different crumbling steps and the amount of waste rock released figure 6. relationship between different crumbling steps and pure ore volume and theoretical pure ore volume figure 7. the relationship between different inlets and the total amount of waste rock released and the amount of waste rock released figure 8. the relationship between different approaches and the number of ore release table 8. statistics of waste rock release at the cut-off release grade of each approach approach number total ore release/g waste rock release /g total number of releases number of releases at initial sighting of waste number of mine releases that continue to see waste waste rock mixing rate/% 1-1# 9987.16 240.14 111 56 55 2.40 1-2# 9816.77 224.69 108 53 55 2.29 1-3# 9938.26 238.17 110 54 56 2.40 2-2# 24631.19 387.76 272 149 123 1.57 2-3# 24533.17 382.99 269 139 130 1.56 based on the parameters such as grade, depletion rate, morphology and particle distribution of the releases, the releases are classified into four grades, namely, excellent, good, moderate and poor, and the definitions and ranges of the grades are given, as shown in table 9. 98 chentao sun, kepeng hou, shining wang, shanguang qian table 9. evaluation levels of emitters and their definitions and ranges h ie ra rc hy d efi ne ta st e( % ) d ep le tio n ra te (% ) m or ph ol og ic al pa rt ic le di st ri bu tio n superior high grade, low depletion rate, regular morphology and uniformity of particles in the discharge body >1.2 <20 narrow at the top and wide at the bottom, with straight end walls and no obvious chips or cracks uniform mixing of large and small particles, no obvious sorting phenomenon virtuous higher grade, lower depletion rate, more regular morphology and more homogeneous grains in the exudate 1.0~1.2 20~30 wide at the top and narrow at the bottom, end wall slightly curved, with a few chips or cracks large and small particles are more evenly mixed, with a slight sorting phenomenon middle the exudates are of average taste, with average depletion rates, irregular morphology and uneven grains 0.8~1.0 30~40 wide at the top and narrow at the bottom, with obvious bending of the end wall and multiple chips or cracks uneven mixing of large and small particles, with obvious sorting phenomenon difference low grade, high depletion rate, irregular morphology and nonuniformity of particles in the discharge body <0.8 >40 wide at the top and narrow at the bottom, with severe bending of the end wall and a large number of chips or cracks clear separation of large and small particles, with serious sorting phenomenon figure 9. evaluation level and range of the emitter 2.3.2 creating an affiliation function according to the evaluation indexes and the actual data of the releasing body, the affiliation function is established, and the fuzzy number is used to express the affiliation degree of the releasing body to each grade. the affiliation function can be expressed in the form of triangle or trapezoid, as shown in figure 2. where, x denotes the taste or depletion rate of the releasing body, and a,b,c denotes the range of each evaluation grade, as shown in table 5. a(x) denotes the affiliation degree of the releasing body to the excellent, good, medium, and poor grades, which are represented by ua(x), ub(x), uc(x) and ud(x), respectively. for example, for a release volume of 1000 t with a taste of 1.2%, a depletion rate of 20%, an excellent morphology (set to class 3) and a good particle distribution (set to class 2), the degree of affiliation to each class can be calculated using the following steps: 1. the affiliation function can be calculated using the following formula where a(x) denotes the affiliation of the putative body to the excellent, good, moderate and poor grades. 2. determine the affiliation of the exudate taste and depletion rate to each grade based on the ranges in table 11, as shown in table 12. table 10. affiliation of release body taste and depletion rate to each grade for a release volume of 1000 t hierarchy taste depletion rate superior (0.5,1,1) (1,1,0.5) virtuous (0.5,0.5,1) (0.5,1,0.5) middle (0,0.5,0.5) (0,0.5,1) mission (0,0,0.5) (0,0,1) 3. determine the degree of affiliation to each class based on the class of exudate morphology and particle distribution, as shown in table 11. table 11. affiliation of release body morphology and particle distribution to each class for a release volume of 1000 t hierarchy morphological particle distribution superior (0.5,1,0.5) (0.5,1,0.5) virtuous (1,1,1) (1,1,1) middle (0,0.5,1) (0.5,1,0.5) mission (0,0,0.5) (0,0.5,1) 4. according to the algorithm in fuzzy mathematics, multiply the taste, depletion rate, morphology and particle distribution to the affiliation of each grade and take the minimum value to get the affiliation of the exudate to each grade, as shown in table 12. 99analytic hierarchy process-fuzzy comprehensive evaluation method-based depletion assessment study of xinshan iron ore mine table 12. affiliation of the release volume of 1000 t to each class hierarchy degree of affiliation superior 0.25 virtuous 0.125 middle 0 mission 0 2.3.3 determination of final evaluation rating for a release volume of 1,000 t, the composite evaluation value can be calculated using the following steps, and the final evaluation level can be determined according to the principle of maximum affiliation: based on the weights of the factors calculated from the hierarchical analysis and the affiliation of the releasers to the classes given in table 12, the weighted affiliation of the releasers to the classes was calculated as shown in table 13. table 13. weighted affiliation of the release body to each class for a release volume of 1000 t hierarchy weighted affiliation superior (0.1925,0.35,0.5075) virtuous (0.0875,0.25,0.4125) middle (0,0.2,0.4) mission (0,0.2,0.4) according to the algorithm in fuzzy mathematics, the weighted affiliation of the releasing body to each grade is added and the maximum value is taken to get the comprehensive evaluation value of the releasing body, as shown in table 14. table 14. combined evaluation value for a release volume of 1000t hierarchy consolidated assessed value superior (0.2795,0.8,1.32) virtuous (0.175,0.7,1.225) middle (0,0.6,1.2) mission (0,0.6,1.2) based on the principle of maximum affiliation, the final evaluation grade of the releases with a volume of 1,000 t was determined. therefore, according to table 15, the final evaluation grade of the releasing body with a volume of 1,000 t is excellent because it has the highest degree of affiliation to the excellent grade. the same method can be used to calculate the composite assessment value and final assessment grade for releases where the work release is of other values. example: for a release volume of 1000 t, the combined assessment value is: = =1 ∑ ω = ω 1 1 + ω 2 2 + ω 3 3 + ω 4 4 = 0. 5495, 0. 5595, 0. 6495( ) based on the principle of maximum affiliation, it was determined that the final evaluation grade for the release volume of 1,000 t was good, as it had the highest affiliation to the good grade. for releases with other values, the method can be used to calculate the combined evaluation value and final evaluation grade. release grade vs release volume release grade release volume (t) 0 100 200 300 400 500 600 700 800 900 1000 re le as e gr ad e (% ) 50 49 48 47 46 45 44 43 42 41 40 figure 10. graph of discharge body grade with discharge volume 2 = 0. 55, 0. 60, 0. 65( ) 3 = 0. 53, 0. 58, 0. 63( ) 4 = 0. 50, 0. 55, 0. 60( ) 5 = 0. 48, 0. 53, 0. 58( ) figure 11. relationship between ore release and waste rock mixing rate3. evaluation results 3. evaluation results the results of the evaluation in this paper, based on actual production data and similar simulation test data from this iron ore mine, are as follows: 1. the weights of the factors affecting ore depletion by the bottomless column segmental chipping method were determined by hierarchical analysis, as shown in table 15. table 15. weights of factors influencing ore depletion for the bottomless column segmental crumbling method considerations weights structural parameters of the quarry 0.35 blasting parameters 0.25 shovelling parameters 0.20 geological condition 0.20 100 chentao sun, kepeng hou, shining wang, shanguang qian 2. through the fuzzy comprehensive evaluation method, based on the weights and evaluation indexes of each factor, a comprehensive evaluation of the releases was carried out, and parameters such as the grade, depletion rate and comprehensive evaluation value of the releases were derived, as shown in table 16. table 16. comprehensive evaluation results of the release body of the bottomless column segmental crumbling method r el ea se (t ) o ri gi na l t as te (% ) r el ea se ta st e (% ) d ep le tio n ra te (% ) pa rt ic le di st ri bu tio n an d m or ph ol og y c on so lid at ed as se ss ed v al ue 1000 1.50 1.20 20.00 superior (0.60,0.65,0.70) 2000 1.50 1.10 26.67 virtuous (0.55,0.60,0.65) 3000 1.50 1.05 30.00 middle (0.53,0.58,0.63) 4000 1.50 1.10 33.33 middle (0.50,0.55,0.60) 5000 1.50 0.95 36.67 mission (0.48,0.53,0.58) combined with the actual conditions at the iron ore mine site, there is very little variation in this result. 3. based on the principle of maximum affiliation, the final evaluation level of the releasing body was determined, as shown in table 17. table 17. final evaluation grade of the discharged body release (t) final evaluation rating 1000 superior 2000 virtuous 3000 middle 4000 middle 5000 mission 4. conclusion 1. under the premise of ensuring safety and economy, the structural parameters of the quarry should be selected as small as possible in order to reduce the depletion rate. 2. in the process of ore release, the amount of release should be controlled as much as possible to avoid excessive release of low-grade bulk. 3. the end release grade is higher, the middle release grade is lower, and the particles in the release body show some sorting phenomenon. 4. the ahp-fce analysis method can be widely applied to the ore depletion evaluation of other types of disintegration and ore release methods, providing a reference basis for the optimization of the structural parameters of the quarry, the control of ore release and the prediction of depletion. acknowledgment the work was supported by the project of yunnan provincial science and technology department (202101at070094). references brady, s. r. 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(2021). proposing a novel comprehensive evaluation model for the coal burst liability in underground coal mines considering uncertainty factors. international journal of mining science and technology, 31(5), 799-812. https://doi.org/10.1016/j. ijmst.2021.07.011 https://doi.org/10.3390/su12041563 https://doi.org/10.3390/su12041563 https://doi.org/10.1016/j.ijmst.2021.07.011 https://doi.org/10.1016/j.ijmst.2021.07.011 _heading=h.1fob9te _heading=h.3znysh7 _heading=h.3dy6vkm _heading=h.1t3h5sf _heading=h.4d34og8 _heading=h.2s8eyo1 _heading=h.17dp8vu issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.113685 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 287 295 g eo lo g y shrimp zircon u-pb dating on the early cretaceous volcanics in zhangwu-heishan area, west liaoning province, china and its geological significance xinxian xu, qiji fu* hainan survey institute of mineral resources, haikou 571100, china * corresponding author: 23241252000136@hainanu.edu.cn how to cite item: xu, x., & fu, q. (2024). shrimp zircon u-pb dating on the early cretaceous volcanics in zhangwuheishan area, west liaoning province, china and its geological significance. earth sciences research journal, 28(3), 287-295. https://doi.org/10.15446/esrj. v28n3.113685 record manuscript received: 26/03/2024 accepted for publication: 11/10/2024 abstract due to special structural location and the output of precious fossils, west liaoning province has become a hot subject of research. the horizon and age of volcanic rock play an important role in understanding biostratigraphy, structural and sedimentary evolution of west liaoning province and local coal exploration. during this work, over 20 volcanic rock samples were collected from fuxin-yixian basin and zhangwu-heishan area, and 11 samples were performed shrimp u-pb isotope analysis. combining the new data obtained with the age of mesozoic volcanic rock in west liaoning province, cretaceous volcanic activities in west liaoning province can be divided into 5 stages, namely 132±1ma, 126±1ma, 122±2ma, 115±2ma and 100±5ma. in reference to field observation, the volcanic rock in zhangwu-heishan area includes 3 suites of early cretaceous volcanic rock and 1 suite of paleogene olivine-enclave-bearing basalt. overlying jiufotang formation is a volcanic rock series, which occurs at jianguo of fuxin, yaocangtu of heishan county, baizitun in the east of faku county and more, is temporarily named as gangtaishan formation or is incorporated into jiufotang formation. igneous rock series of dalinghe formation (daxingzhuang formation) overlying sunjiawan formation of early cretaceous system wasn’t found in fuxin-yixian basin and zhangwu-heishan area, so it is proposed that this formation is absent in the study area. the stratigraphic succession of zhangwu-heishan area was clarified and the conclusion that no cretaceous coal seam can be found below volcanic rock in zhangwu-heishan area was drawn. the inherited zircon of archean eon was found in volcanic rock samples from yixian formation of early cretaceous system. keywords: west liaoning province; zhangwu-heishan area; mesozoic volcanic rock; shrimp u-pb isotopic age; geological significance; palabras clave: occidente de la provincia liaoning; área de zhangwu-heishan; rocas volcánicas del mesozoico; datación isotópica shrimp u-pb; significación geológica. datación u-pb shrimp de rocas volcánicas del cretácico inferior en el área de zhangwu-heishan, al occidente de la provincia china de liaoning y su importancia geológica debido a su ubicación estructural especial y el desarrollo de fósiles valiosos, el occidente de la provincia china de liaoning se ha convertido en un centro de interés para la investigación. el horizonte y la edad de las rocas volcánicas juegan un papel importante en el conocimiento de la bioestratigrafía y la evolución estructural y sedimentaria del occidente de la provincia y de la exploración local de carbón. durante este trabajo más de veinte muestra de rocas volcánicas recolectadas en la cuenca de fuxin-yixian y en el área de zhangwu-heishan, y a once de estas muestras se les realizaron análisis de datación isotópica shrimp u-pb (datación u-pb a través de la microsonda de iones de alta-resolución). al combinar la nueva información obtenida con la edad de las rocas volcánicas del mesozoico en el occidente de la provincia de liaoning, las actividades volcánicas del cretácico pueden dividirse en cinco etapas: 132±1ma, 126±1ma, 122±2ma, 115±2ma y 100±5ma. en cuanto a la observación de campo, las rocas volcánicas del área de zhangwu-heishan incluyen tres patrones de rocas volcánicas del cretácico inferior y un patrón de basalto paleógeno con enclave de olivino. una serie de rocas volcánicas cubre la formación jiufotang, las cuales se encuentran en jianguo de fuxin, condado yaocangtu de heishan, y bazitun, en el oriente del condado faku, y se denomina temporalmente formación gangtaishan o se incorpora a la formación jiufotang. las series de rocas igneas de la formación dalinghe (formación daxingzhuang) que se superponen a la formación sunjiawan del sistema cretácico inferior no fueron encontradas en la cuenca de fuxin-yixian ni en el área zhangwu-heishan, por lo que se propone que esta formación es ausente en el área de estudio. la sucesión estratigráfica del área de zhangwu-heishan fue clarificada y surgió la conclusión de que una veta de carbón del cretácico podría ser hallada bajo sus rocas volcánicas. el circón heredado del eón arcaico se encontró en muestras de roca volcánica de la formación yixian, del sistema cretácico temprano. resumen https://doi.org/10.15446/esrj.v28n3.113685 mailto:23241252000136@hainanu.edu.cn https://doi.org/10.15446/esrj.v28n3.113685 https://doi.org/10.15446/esrj.v28n3.113685 288 xinxian xu, qiji fu 1. geologic setting zhangwu-heishan area is located to the east of fuxin-yixian basin. besides the quaternary soil, the study area is mostly covered with volcanic rock, which is dominated by the intermediate-basic andesite, trachyandesite and basaltic andesite, with a minority taken by the dacite, rhyolite etc. (figure 1). in the research history, various workers have proposed different nomenclatures, evolutional processes, and chronostratigraphy for those igneous rocks. it is thus important and necessary to unify those discrepancies, which have brought considerable inconveniences and potential issues for regional correlation, biostratigraphic studies and resources exploration. and this is the main effort this paper aiming to contribute. field investigation starts with yixian basin which has been densely studied by various investigators, and gradually advances to the north and northeast parts of fuxin-yixian basin. key volcanic outcrops are highlighted and recorded; meanwhile, igneous samples are collected from outcrops or rock cores in different horizons for shrimp u-pb isotopic dating. combining with data from previous researches, the age and main eruption periods of the above volcanic rock will be generally delineated, and the sequence relationships between volcanic rock and main coal-bearing strata will be identified to provide possible assistances for future geological researches. in the middle and late jurassic period, due to southward movement of siberian plate and westward subduction of pacific plate, west liaoning province was extruded in the direction of nw-se and mainly features ne and nne-trending structures (wang, 2001; ren, 2002). adjacent to inner mongolia-xing’an orogenic belt and songliao basin, west liaoning province is bordered by chifeng-kaiyuan fracture to the north, connected to ewtrending yanshan orogenic belt to the west, and separated from liaodong uplift zone by yilan-yitong fracture zone to the east. its tectonic position belongs to transition zone of north china block, liaodong uplift zone and inner mongolia-xing’an orogenic belt (chen et al., 1997). the study area is mainly characterized by neand nne-trending faulted structures. although nappe structures, extensional structures and strike-slip structures are developed in this area, extensional activities dominated during yixianian. the major axis of mesozoic sedimentary basin in this area is mainly nne-trending. with abundant volcanic rock developed during yixianian, temporal basin was obviously controlled by basin-margin extensional fractures, which are mainly characterized by nne-trending fractures. during jiufotangian period, graben faulting in west liaoning province was further developed; the volcanism calmed though, with tuffaceous materials comprise the majority of this formation. subsequently, a relatively small-scaled intermediate-basic volcanic eruption occurred and the basin gradually received the sediment from shahai formation and fuxin formation of early cretaceous epoch and sunjiawan formation of late cretaceous epoch. the transition of mesozoic structures in west liaoning province shows very good spatiotemporal coupling between mantle process and lithosphere thinning in deep part. figure 1. geologic map of zhangwu-heishan area and sampling location 289shrimp zircon u-pb dating on the early cretaceous volcanics in zhangwu-heishan area, west liaoning province, china and its geological significance 2. sampling and test the reported data on the absolute age of volcanic rock in this area are mainly derived from k-ar or ar-ar method. during this research, 11 volcanic rock samples are collected and analyzed with higher-precision shrimp u-pb isotopic dating method. specific sample id, sampling location, lithology determined through slice identification and more are shown in figure 1 and table 1. it is should be noted that results of the same sample are subject to different testing methods. therefore, during this work, 6 samples are collected from yixian basin so as to accurately correlate the age of volcanic rock in zhangwu-heishan area with that in fuxin-yixian basin and provide compelling evidence. test samples used by shrimp dating method are the zircon selected from volcanic rock samples. during the testing, outer margin should be distinguished from the core, because the outer margin of zircon is younger than the core. if the test result is derived from chip zircon samples, it may be not the outer margin, result in that the value of absolute age is relatively higher, and can’t reflect the accurate age of volcanic rock. sample n1 is from the bottom of borehole bs13 in badaohao exploration area, heishan county (cai et al., 2008 & 2009). zhangluotun sample (n2) is of quartz trachyandesite, is distributed on the southeast side of badaohao, obviously intrudes into the tuffite and tuffaceous clastic rock, generally inclines to the inside of volcanic conduit and has large obliquity, and fluidal structures are developed inside the trachyandesite and are almost erective at volcanic conduit (plate i-7). liangshan trachyandesite (n3) lies in the northwest of liangshan township, takes the form of independent small hill due to weathering and is covered with quaternary system; lava flow unconformably covers the sedimentary rock in a small hill to the north of shuangshanzi; and 30-40cm-thick baked layer lies on its top. the trachyandesite is greenish black and has compact and even structure; fluidal structures and columnar jointing are developed, the former of which has almost erective occurrence; and the samples are collected from volcanic conduit (plate i-1). sample n4 is collected from nalishan of yaocangtu. sample n5 is from dongliangshan of heishan county. the samples for isotopic dating of fuxin-yixian basin are collected from beizhuanchengzi in the southwest of qinghemen, huanghuashan in the southeast of yi county, lijiatai in the southeast of juliangtun of yi county (figures 3-4), xinglonggou in the southeast of yi county (figures 3-4), songbahu in the southeast of yi county, tonglvgou, and luotaishan in the north of badaohao. sampling locations are detailed as figure 1. experimental procedures are as follows: crush and screen volcanic rock samples, select automorphic zircon crystals which aren’t ground and have no inclusion, and it should be clean and transparent under binocular eyepiece; then combine several selected zircon samples together to make them into resin target; polish the target; and perform the photomicregraph of reflected light, transmitted light and cathode luminescence. cathode luminescence is performed in chinese academy of geological sciences and u-pb isotope analysis is conducted with shrimpii-type ionic probe in shrimp center of the institute of geology of chinese academy of geological sciences. during the analysis, standard zircon tem (417ma) is used for fractionation correction between elements. for detailed shrimp analysis method and process, please refer to williams (1987). the data are processed with ludwig squid1.0 program and isoplot program (1999 & 2001), and decay constant is the value recommended by steiger et al. (1997). common lead correction is implemented through directly determining 204pb. the dating data used by this paper are the average of 5 values measured at the same survey point and have the error of 1σ. 206pb/238u age is used for all samples and the weighted average has the confidence level of 95%. 3. result and discussion of isotopic dating result of isotopic dating during this work, totally 20 volcanic rock samples are collected from fuxin-yixian basin and zhangwu-heishan area, 15 of which are sent to the laboratory for testing. shrimp method was performed for 11 samples, within which only 1 sample indicates a paleogene eruptive origin, the rest 10 samples are all from eruptive rock in the medium term of early cretaceous period. sampling locations are shown in figure 2. part of zircon is relatively old and the zircon tested may be ancient zircon taken up during magmatic eruption. the breccia in huanghuashan is the upperest lithological segment (ren et al., 1997; zhang et al., 2004) of yixian formation and its age is in line with the top boundary of yixian formation. during this work, the age determined is 126.3±1.7ma, which is possible over-estimated the real age since the zircon tested is mainly from volcanic rubble in the samples, and can’t represent the accurate age of this horizon. the id and sampling location of selected zircon, concordia curve of corresponding age, and statistic histogram are shown as figure 2. discussion 1) age and sequence of volcanic rock in zhangwu-heishan area during field work, volcanic sequence and eruption period are investigated and it is deemed that 3 suites of cretaceous volcanic rock in zhangwu-heishan area can be distinguished. the first set of volcanic rock lies in the lowest horizon, is of intermediate-basic basaltic andesite, features a large amount of vesicular amygdaloidal structure, is the result of fissure eruption, and was formed during tectonic activities. the second set lies in the middle horizon, is of intermediate trachyandesite, features trachyandesite and bentonite deposit produced by the alteration, and was formed during tectonic stabilization period. the third set lies in the highest horizon, is of compact intermediate-basic basaltic andesite, is the result of fissure eruption, is mainly characterized by hypabyssal andesite and unconformably overlies the bentonite or other older strata, and lava flow produced about-30cm-thick baked layer in underlying strata (plate i-6). the third set of volcanic rock should be gangtaishan formation in xinlitun named by tan in 1926, is contemporaneous with yushugou formation to the west of yi county, and is also known as daxingzhuang formation in 1:200,000 geologic map by regional survey team of liaoning province and zhanglaogongtun formation denominated by wang (2004). from the aspect of lithology and eruptive sequence, correlated with contemporaneous volcanic rock in yi county, the first set of basaltic andesite is equivalent to the second set of basaltic andesite in yixian formation; the second set of intermediate trachyandesite and bentonite are comparable with the breccia in huanghuashan and should be included in huanghuashan formation; and the third set of intermediate-basic basaltic andesite overlies the trachyandesite, underlies badaohao formation, should belong to volcanic rock in the medium term of early cretaceous period, and is corresponding to gangtaishan formation or yushugou formation, daxingzhuang formation and zhanglaogongtun formation. the outcrops of the first set of early cretaceous volcanic rock include: basaltic andesite in shijiazi township to the northeast of xinlitun (plate i-4) features a large amount of vesicular and amygdaloidal structures and is abundant in agate. the hypabyssal andesite in luotaishan overlies the outcrop of coal-bearing series of beipiao formation of early jurassic epoch and is the lowest intermediate volcanic rock of early cretaceous epoch. this set of volcanic rock is named as daxingzhuang formation in 1:200,000 geologic map, which may be wrong, has the age of 129.0±2.4ma determined by shrimp method, and should be included in yixian formation. the hypabyssal andesite in shuangshanzi of liangshan is distributed in shuangshanzi in the southwest of liangshan township in the west of xinmin county, takes the form of independent small hill due to weathering and is covered with quaternary system; lava flow unconformably covers the sedimentary rock in a small hill; and 30-40cm thick baked layer lies on its top. the andesite is dark gray and has compact and even structure; fluidal structures and columnar jointing are developed (plate i-1), the former of which has almost erective occurrence; and the samples are collected from volcanic conduit. this set of volcanic rock is also denominated as daxingzhuang formation in 1:200,000 geologic map, has isotopic age of 129.5±1.6ma and 129.7±1.6ma determined with shrimp method, belongs to older volcanic rock and can be correlated with yixian formation in yi county. the hypabyssal andesite in zhangluotun to the southeast of badaohao coal mine obviously intrudes into the tuffite and tuffaceous clastic rock, generally inclines to the inside of volcanic conduit and has the obliquity of 40°-50°, and fluidal structures are developed inside the trachyandesite and are almost erective at volcanic conduit. this set of andesite which was originally denominated as yushugou formation or daxingzhuang formation has the isotopic age of 124.9±2.9ma and 122.2±4.1ma determined with shrimp method, belongs to older volcanic rock and can be correlated with yixian formation (plate i-7). 290 xinxian xu, qiji fu table 1. summary of shrimp u-pb isotopic testing result of volcanic rock in fuxin and its peripheral areas sample id sampling location color lithology by slice identification age (ma) volcanic eruption period horizon in 1:200,000 geologic map (1972) original horizon in 1:50,000 geologic map by the 107th brigade horizon currently determined n1 1160m in borehole bs-13 at badaohao grayish green trachyandesite 125.7±1.7 ②-③ j3t tuhulu formation k1y yixian formation n2 zhangluotun to the southeast of badaohao purplish grey rhyolite 124.9±2.9 ②-③ j3t tuhulu formation k1t tuhulu formation n3 to the east of yaocangtu in heishan county grey black quartz trachyandesite 115.5±1.5 ④ j3t tuhulu formation yixian formation k1g gangtaishan formation n4 nalishan in yaocangtu grey black basalt 56±2.9 covered with quaternary covered with quaternary e paleogene system n5 dongliangshan in heishan county grey black trachyandesite 129.7±1.6 ② j3t tuhulu formation daxingzhuang formation k1y yixian formation n6 beizhuanchengzi in the southwest of qinghemen grey andesite 132.3±2.3 ① j3t tuhulu formation yixian formation k1y yixian formation n8 huanghuashan in the southeast of yi county grayish yellow dacite 126.3±1.7 ② j3t tuhulu formation yixian formation k1t tuhulu formation n11 lijiatai in the southeast of yi county claret trachyandesite 125.8±1.9 ② j3y yixian formation dalinghe formation k1t tuhulu formation n12 xinglonggou in the southeast of yi county claret andesite 127.0±5.1 ② j3y yixian formation daxingzhuang formation k1t tuhulu formation n13 tonglvgou of songbahu in the southeast of yi county caesious andesite 122.3±4.5 ③ j3t tuhulu formation yixian formation k1t tuhulu formation n14 luotaishan in the north of badaohao offwhite andesite 129.0±2.4 ①-② j3t tuhulu formation yixian formation k1y yixian formation the second set of early cretaceous volcanic rock features the trachyandesite and bentonite, and bentonite deposits are found in hala and shaojianshanzi in the southwest of zhangwu county, xiashitu of yaocangtu in the northwest of xinlitun and douyawopu in the southeast of zhangwu county. bentonite deposits are mainly composed of montmorillonite minerals, which can account for over 80% and are of volcanic genesis and sedimentary genesis. sedimentogenic bentonite is the result of hydrolysis, diagenesis, weathering or low-temperature hydrothermal alteration of intermediate-basic volcanic rock, especially trachyandesitic pyroclast or volcanic ash falling into lake basin, should be contemporaneous with andesitic volcanic eruption in huanghuashan, and can represent the product of the second set of volcanic eruption in this area. the outcrops include: 1) bentonite deposits in hala and shaojianshanzi areas are located in the southeast of zhangwu county and between paozi and daguben, are covered with the andesite later eroded, and overlie clastic rock (plate i-5 and figure 4-2). the bentonite is closely related to the trachyandesite in huanghuashan in terms of composition and genesis, and shows the interruption in deposition with the overlying basalt andesite, which should be gangtaishan formation (figure 4-1), yushugou formation or daxingzhuang formation. 2) the benonite in yaocangtu is distributed in xiashitu of yaocangtu in the northwest of xinlitu (plate i-8), is covered with the hypabyssal andesite later erupted, and shows the interruption in deposition between them. bentonite layer overlies a set of clastic rock, which becomes finer and finer bottom-up and is the result of retrogradation during the expansion of lake basin. the importance should be attached to the discovery of precious fossils in sedimentary deposit below this set of volcanic rock. 291shrimp zircon u-pb dating on the early cretaceous volcanics in zhangwu-heishan area, west liaoning province, china and its geological significance figure 2. id and sampling locations of selected zircon, concordia curve of age and statistic histogram 292 xinxian xu, qiji fu the third set of early cretaceous volcanic rock features intermediatebasic basaltic andesite which overlies the bentonite, is the result of a new round of volcanic eruption, was formed in the medium term of early cretaceous epoch, and has the absolute age of less than 110ma. it crops out at hala and shaojianshanzi in the southwest of zhangwu county, xiashitu of yaocangtu in the northwest of xinlitun and douyawopu in the southeast of zhangwu county. the bottom of this set of basaltic andesite often baked the underlying jiufotang formation, and has unconformity contact relationship with it. the layer below this set of volcanic rock is abundant in “jehol biota” fossils. bentonite deposit represents an ancient denudation plane and is contemporaneous with andesitic volcanic rock in huanghuashan. the hypabyssal andesite in hala and shaojianshanzi is located in the southwest of zhangwu county and between paozi and daguben and overlies the bentonite, which is underlain by clastic rock. the interruption in deposition occurs between bentonite and overlying andesite (plate i-5). the andesite in yaocangtu lies in the northwest of xinlitun; basaltic andesite in xiashitu of cangtu township overlies the bentonite, indicating that the interruption in deposition occurs between them; and overlying basaltic andesite is the third set of volcanic rock in this area (plate i-8) and has the absolute age of 115±1.5ma. extravasation-facies basaltic andesite in douyawopu of zhangwu county crops out at douyawopu in the southeast of zhangwu county, is the result of a new round of volcanic eruption after trachyandesitic eruption in this area, and shows the interruption in deposition between it and underlying sedimentary rock, and a baked layer was formed between fissure-eruption extravasation-facies basaltic andesite and underlying argillaceous rock (plate i-6). alkaline basalt (plate i-3) in jianguo of wuhuanchi township is observed on site and sampled, and has low zircon content due to being basic volcanic rock, so u-pb isotopic dating analysis isn’t performed. paleogene basalt crops, which are out of nalishan in dagubentaogai township, are composed of olivine basalt. according to u-pb isotopic dating analysis, it has the age of 56±2.9ma and belongs to basic effusive rock of paleogene period. 2) age and sequence of basalt in jianguo there are 2 sets of volcanic rock in jianguo village, wuhuanchi township, fuxin city: the upper section is black compact basalt, is of volcanic conduit facies, is developed with columnar jointing (plate i-3) and contains the peridotite and other enallogene xenoliths, and a small amount of vesicular structures at the bottom are the result of cooling lava flow; and the lower section is grayish purple basaltic trachyandesite or augitic andesite with many pores and features high silicon, high alkali, low magnesium and low titanium (shao et al., 2006). the basalt in the upper section has angular unconformity contact relationship with the andesite in the lower section; red brown weathered crust between them is 15-20 cm thick; and a layer of conglomerate lies below it. this set of basalt is close to chifeng-kaiyuan lithospheric fault and is adjacent to famous tanlu fault system to the east, and its special structural location makes the geologists perform the dating and research the sequence, genesis, magma sources and geotectonic significance of this set of basalt and underlying andesite (zhu et al., 2002 & 2004; yang et al., 2004; zhang & zheng, 2003; zheng et al., 2004). the results of dating are greatly different from each other. among these tests, zhu rx et al. sampled the rock in great detail. the result of test conducted by berkeley geochronology center shows that its age is 105.5±0.5ma, which is hardly different from the age of volcanic rock at the same place or that with the same lithology at niutoushan of pingzhuang determined by other geologists (shi et al., 2005; zhang et al., 2003) and has relatively high confidence level. therefore, this paper takes 105.5±0.5ma determined by zhu et al., and yang et al. as its age. there are 3 opinions about the sequence of alkaline basalt in the upper section of jianguo: xu (1999), wang et al. (2002) and bing et al. (2003) thought the basalt in jianguo belongs to dalinghe formation and is the strata above sunjiawan formation. shao et al. (2006) thought that this set of basalt lies at the bottom of fuxin formation. wang and jiang (1999) investigated fuxin basin and adjacent dalinghe formation and thought that the basalt in the upper section of jianguo should overlie jiufotang formation, instead of fuxin formation. chang et al. (1999) also thought that volcanic rock of yushugou formation in fuxin basin can be correlated with the basalt in jianguo and is a horizon between jiufotang formation and shahai formation. shao et al. (2006) placing alkaline basalt in the upper section of jianguo to the bottom of fuxin formation is based on insufficient evidence, because they used the recommended value in international stratigraphic chart, instead of test result. based on field investigation, the author of this paper found that calcirudite rock below the basalt in jianguo is of tuffaceous cementation, and the gravel has complex composition, but contains no granite gneiss gravel frequently found in shahai formation and fuxin formation and gets lithological features of jiufotang formation. no volcanic rock in fuxin-yixian basin or between shahai formation and fuxin formation is reported; the basalt not only lies above jiufotang formation in this area, but also overlies or intrudes into jiufotang formation in dawulaba of zhangwu county (figure 3-1), jianshanzi of heishan county (figure 3-2) and dongbaizitun of faku county (figure 3-3). isotopic dating shows that this set of basalt is the product in the late term of early cretaceous epoch, so the possibility that it belongs to daxingzhuang formation (dalinghe formation) of late cretaceous epoch is eliminated. based on the above comprehensive analysis, this set of basalt in the upper section of jianguo lies above jiufotang formation and below shahai formation, is the result of volcanic eruption in the medium term of early cretaceous epoch, and has far smaller distribution range than yixian formation. figure 3. field sketch of study area. 1 conglomerate, 2 sandstone, 3 mudstone, 4 marlite, 5 trachyandesite, 6 andesite, 7 volcanic agglomerate, 8 volcanic breccia, 9 breccia-bearing tuff, 10 rhyolitic tuff, 11 ferriallophanoid, 12 quaternary system, 13 lower pleistocene series; 14 fuxin formation, 15 shahai formation, 16 yixian formation, 17 hypabyssal trachyandesite, 18 jiufotang formation 3) volcanic cycle of early cretaceous epoch in fuxin and zhangwu-heishan area the early cretaceous volcanic rock in fuxin and zhangwu-heishan area can be divided into 2 cycles: the first cycle starts with basic basalt, is represented by narrow-sense yixian formation and ends with intermediate trachyandesite and crater-lake deposit, namely huanghuashan breccia and jiufotang formation. lacustrine-facies deposits of jin’gangshan formation formed during volcanic dormant period lie between basic and intermediate volcanic rock. isotope geologic age of volcanic rock of this cycle starts with 145±4ma of early cretaceous and ends with 120±2ma or 112±3ma, and its peak period is 126ma to 120ma. the strata include yixian formation and jiufotang formation. yixian formation can be divided into small volcanic cycles of basic basalt intermediate andesite acidic rhyolite and is mixed with lacustrine facies tuff. volcanic rock of the second cycle is represented by basic basalt, has unconformity contact relationship with underlying volcanic rock or sedimentary rock, baked the underlying strata, gets the isotope geologic age of 106ma or so, and features alkaline basalt in the upper section of jianguo, wuhuanchi township, and is mainly distributed in douyawopu and yaocangtu in zhangwu county. stratigraphic units include gangtaishan formation (tan, 1926), dalinghe formation, daxingzhuang formation, yushugou formation, zhanglaogongtun formation (wang et al., 1989) and fuxin formation (shao et al., 2006). 293shrimp zircon u-pb dating on the early cretaceous volcanics in zhangwu-heishan area, west liaoning province, china and its geological significance 4) sequence relationship between volcanic rock and coal-bearing strata in zhangwu-heishan area it is previously thought that the so-called dalinghe volcanic rock which is distributed along dalinghe fracture overlay red bed of sunjiawan formation and belonged to upper cretaceous series, so someone suggested searching the coal below the volcanic rock. the investigation on lijiadianbaimiaozi to the southeast of juliangtun shows that the above volcanic rock has the faulted contact relationship with red bed and doesn’t overlie the red bed, that the red bed originally defined doesn’t belong to sunjiawan formation and is the conglomerate at the bottom of shahai formation, and that andesitic volcanic rock of so-called dalinghe formation has the age of 125.8±1.9ma (n11) and 127.0±5.1ma (n12) according to shrimp method and should belongs to yixian formation. during the field reconnaissance in the study area, the expected dalinghe formation isn’t found, so it is thought that there is no volcanic rock of dalinghe formation. early cretaceous volcanic rock in zhangwu-heishan study area, despite of yixian formation of the first cycle, gangtaishan formation of the second cycle, or contemporaneous volcanic rock with different names (such as daxingzhuang formation, yushugou formation, zhanglaogongtun formation and more), has older and lower horizon than shahai formation (badaohao formation), and jiufotang formation which underlies gangtaishan formation is of crater-lake deposit after volcanic eruption, is distributed in a limited area and is less wide-spread than fault-trough lake basin genetic shahai formation and fuxin formation after intermediatebasic volcanic eruption. jiufotang formation barely shows the sign of coal accumulation, so it is impossible to find the coal of shahai formation or fuxin formation below the volcanic rock. 4. conclusions based on the research of volcanic rock in fuxin basin and its peripheral areas, the following conclusions could be drawn: 1. based on the age of mesozoic volcanic rock obtained from shrimp u-pb isotope analysis of 11 volcanic rocks in west liaoning province in this study, cretaceous volcanic activities in the research area can be divided into 5 stages, namely 132±1ma, 126±1ma, 122±2ma, 115±2ma and 100±5ma. 2. there are 2 sets of early cretaceous volcanic rock and 1 set of paleogene volcanic rock in fuxin basin and zhangwu-heishan area: early cretaceous volcanic rock is thick and widespread, has various lithologies and complicated denominations, while paleogene basic basalt is mainly located at nalishan to the southeast of yemataohai. early cretaceous volcanic rock in the studying area can be divided into 2 cycles. the first cycle starts with basic basalt, is represented by narrow-sensed yixian formation and ends with intermediate trachyandesite and crater-lake deposit, namely huanghuashan breccia and jiufotang formation. several lacustrine sediments were deposited during volcanic dormant period lying between basic and intermediate volcanic rock. basic basalt, with the isotopic age of about 106ma-108ma, represents the second cycle; it unconformablly underlain volcanic rock or sedimentary rock and has baked the strata beneath it. this sequence is featured by alkaline basalt in the upper section of jianguo, wuhuanchi township, and is mainly distributed in douyawopu and yaocangtu in zhangwu county. 3. the expected dalinghe formation is absent in west liaoning province, and thus leads to the postulation 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overlying the bentonite plate 1 _heading=h.gjdgxs _heading=h.30j0zll bookmark=id.1fob9te issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.115333 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 367 377 h y d r o g eo lo g y application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina jesús david gómez gutiérrez1, asunción romanelli1,2, leandro bertolini1,2, orlando mauricio quiroz londoño1,2 1. instituto de geología de costas y del cuaternario, universidad nacional de mar del plata cic bs as, fceyn, funes 3350 nivel 1 -, 7600 mar del plata, argentina. 2. instituto de investigaciones marinas y costeras, universidad nacional de mar del plata consejo nacional de investigaciones científicas y técnicas (conicet), rodríguez peña 4002-4100, 7600 mar del plata, argentina. how to cite item: gómez gutiérrez, j. d., romanelli, a., bertolini, l., & quiroz londoño, o. m. (2024). application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina. earth sciences research journal, 28(4), 367377. https://doi.org/10.15446/esrj.v28n4.115333 record manuscript received: 26/06/2024 accepted for publication: 27/01/2025 abst r ac t groundwater plays a fundamental role in supporting productive, domestic, and touristic activities in the lobería district. in this region, two aquifers coexist: a regional aquifer (pampeano aquifer) and a local aquifer confined to the coastal dunes, referred to in this study as the sand-dune aquifer. to investigate their interaction, an end-member mixing analysis (emma) was performed using conservative tracers, including hydrochemical data and stable water isotopes, based on measurements from 32 extraction wells. the results reveal significant groundwater exchange between the sand-dune and pampeano aquifers, with notable contributions in both directions. well g251 in the sand-dune aquifer receives 68% of its contribution from the sand-dune aquifer and 32% from the pampeano aquifer. conversely, wells g1027 and g1028 in the pampeano aquifer have contributions of 32% and 49% from the sand-dune aquifer and 68% and 51% from the pampeano aquifer, respectively. however, stable water isotopes were ineffective as tracers for quantifying mixing due to shared recharge conditions and similar isotopic signatures. these findings enhance our understanding of groundwater interactions and provide valuable insights for the integrated management of groundwater resources. keywords: groundwater; aquifer; mixing processes; hydrogeochemistry; water isotopes; management palabras clave: agua subterránea; acuífero; procesos de mezcla; hidrogeoquímica; isotopos del agua; gestión aplicación de técnicas isotópicas e hidrogeoquímicas para evaluar los procesos de mezcla en sistemas de agua subterránea de una zona costera en argentina el agua subterránea desempeña un papel fundamental para el desarrollo de actividades productivas, domésticas y turísticas en el partido de lobería. en esta región, coexisten dos acuíferos: un acuífero regional (acuífero pampeano) y un acuífero local confinado a las dunas costeras, denominado en este estudio como acuífero medanoso. para investigar su interacción, se realizó un análisis de mezcla de componentes extremos (emma, por sus siglas en inglés) utilizando trazadores conservativos, que incluyen datos hidroquímicos e isótopos estables del agua, a partir de mediciones en 31 pozos de extracción. los resultados revelan un intercambio significativo de agua subterránea entre los acuíferos medanoso y pampeano, con contribuciones notables en ambas direcciones. el pozo g251 ubicado en el acuífero medanoso recibe un 68% de su aporte del acuífero medanoso y un 32% del acuífero pampeano. por el contrario, los pozos g1027 y g1028 ubicados en el acuífero pampeano presentan contribuciones del 32% y 49% del acuífero medanoso, y del 68% y 51% del acuífero pampeano, respectivamente. los isótopos estables del agua no resultaron efectivos como trazadores para cuantificar la mezcla debido a condiciones de recarga compartidas y firmas isotópicas similares. estos hallazgos mejoran la comprensión de las interacciones del agua subterránea y proporcionan información valiosa para la gestión integrada de los recursos hídricos subterráneos. resu m en https://doi.org/10.15446/esrj.v28n4.115333 https://doi.org/10.15446/esrj.v28n4.115333 368 jesús david gómez gutiérrez, asuncion romanelli, leandro bertolini, orlando mauricio quiroz londoño 1. introduction approximately 60% of the global population lives less than 60 km from the coast, and this is expected to increase to 75% in the next two decades (neumann et al. 2015). this demographic trend, coupled with accelerating agricultural, industrial, and touristic development, has significantly impacted coastal groundwater systems (odeloui et al., 2022). urbanization-driven transformation of natural ecosystems exacerbates groundwater quality degradation, posing critical challenges to drinking water security in these regions (liu et al., 2022). addressing the dual concerns of groundwater quantity and quality is a key challenge of the 21st century, requiring integrated management strategies to mitigate resource depletion and contamination risks while supporting regional development (unesco, 2022). in argentina, coastal zones represent the most densely populated and infrastructure-intensive areas (dadon and matteucci, 2009). the atlantic coastline of buenos aires province, extending over 600 km, includes more than 30 seaside towns within thirteen municipalities, each exhibiting distinct hydrogeological, demographic, and economic profiles. in this region, the phreatic aquifer serves as the primary source of water for human consumption, particularly in areas where tourism and recreational activities dominate local economies (dadon and matteucci, 2009). however, rapid urban expansion and seasonal population surges, driven by tourism, have intensified pressures on coastal groundwater resources, particularly in under-monitored and vulnerable zones (carretero et al., 2014; carretero et al., 2024). the coastal area of the lobería district, located in the southern part of buenos aires province, exemplifies these challenges. historically a hub for tourism, the district experiences significant seasonal population increases, with up to 4,000 additional visitors per weekend during summer months. this influx is further compounded by the expansion of tourist complexes and residential developments (gated communities), which, while boosting local economies, also impose substantial demands on groundwater resources. these developments underscore the need for a comprehensive understanding of the region’s hydrogeological dynamics to ensure sustainable management of its water resources (zulaica and aguilar, 2009). groundwater in this region is sourced exclusively from two unconfined aquifers: the regional pampeano aquifer and the local sand-dune aquifer, that overlaps the first, restricted to the coastal dunes. their interactions are poorly understood, creating significant challenges for sustainable water resource management. the unconfined nature of these systems renders them particularly susceptible to contamination from anthropogenic activities, including agricultural runoff, wastewater disposal, and saline intrusion. furthermore, insufficient monitoring of groundwater extraction exacerbates the potential for resource overexploitation and quality degradation. interactions between shallow and deeper aquifers have been extensively studied in other coastal settings due to their implications for groundwater quality and sustainability. shallow aquifers are more vulnerable to contamination, and their interaction with deeper aquifers can facilitate the spread of pollutants, posing significant management challenges (toth and katz, 2006; warner et al., 2016; zhang et al., 2016; wali et al., 2019; jat baloch et al., 2021). key processes such as recharge, mixing, mineral dissolution/precipitation, and ion exchange are critical in governing groundwater quality and must be fully characterized to inform management strategies (tubau et al., 2014; jean-baptiste et al., 2020). to address these challenges, hydrogeochemical and isotopic techniques provide invaluable tools for characterizing recharge sources, salinization mechanisms, and aquifer mixing dynamics (carretero et al. 2022; glok galli et al. 2023; quiroz londoño et al. 2008; zhou et al. 2017), as well as the formulation of water balance terms (baijjali et al. 1997; mook and rozanski 2000). end-member mixing analysis (emma) is a widely applied methodology for quantifying mixing proportions in groundwater systems, relying on the identification of potential recharge sources and their hydrochemical and isotopic signatures, as well as the use of conservative tracers to evaluate mixing processes (tubau et al., 2014; pelizardi et al., 2017; lone et al., 2021; marx et al., 2021; pall et al., 2023). the objective of this study is to characterize hydrochemically and isotopically the pampeano and sand-dune aquifers in the coastal region of lobería to better understand their mixing processes and improve the conceptual hydrogeological model of the region. specifically, the study evaluates the extent of aquifer mixing in newly developed wells associated with tourism and scattered housing, utilizing simple water parameters as early warning indicators of potential groundwater degradation. the findings aim to support sustainable groundwater management by providing actionable insights into aquifer dynamics and long-term resource preservation. additionally, this study contributes to addressing broader challenges of groundwater sustainability in coastal settings worldwide. 2. study area the lobería district is located in the south of the buenos aires province, argentina (figure 1), bordering the tandil district to the north, general alvarado district to the east, necochea district to the west, and the atlantic ocean to the south. its coastal area is characterized by the presence of beaches with limited development, situated within the southern dune barrier (isla et al., 2001), which extends from the town of miramar to the coronel rosales district, reaching maximum widths of up to 3 km. from a climatic perspective, according to the classification by koeppen and hendrichs pérez (1948), temperate climate conditions prevail with a marked influence from the atlantic ocean. precipitation is predominant throughout the year, ranging between 800 and 900 mm, and the annual average temperature is around 15°c (vazquez et al., 2017). these conditions are favorable for the development of agricultural activities. land use in the region can be classified into tourism activities, residential areas, and productive processes. all these sectors obtain their water supply through private wells. the distribution of points of special interest is concentrated in the southwest zone, where the highest accumulation of data from both the deep and shallow aquifers is recorded, which could help define the possible interaction between these two aquifer systems. hydrogeologically, the map (figure 1) shows that the groundwater flow lines (black lines) predominantly follow a north-to-south direction, discharging into the atlantic ocean, indicating a consistent hydraulic gradient. the coastal dunes are clearly delineated, acting as significant recharge areas, and the piezometric lines are represented, allowing the identification of groundwater movement from zones of high piezometric head to areas of low piezometric head. this pattern reflects the natural flow dynamics within the coastal hydrogeological system, regulated by geomorphological characteristics and the interaction with local aquifers figure 1. location of the study area and groundwater sampling. the coastal area of the lobería district is recognized for the presence of two aquifers. the first, known as the pampeano aquifer, is a regional phreatic aquifer that excibits semi-confined conditions at certain depths (auge 2004). the hydrogeological basement consists of orthoquartzites from the balcarce formation and precambrian igneous-metamorphic rocks, which outcrop in the hills that border the area to the north. it is estimated that the aquifer thickness in this sector of the inter-hill plain exceeds 100 meters (quiroz londoño et al. 2008). recharge comes exclusively from precipitation, with evapotranspiration being the main form of water output. various methods estimate an average recharge of 12% of total rainfall, with a value of 130 mm/year over the past 369application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina 10 years (giacobone et al. 2023; quiroz-londoño et al. 2012). topography strongly influences the hydrological behavior of the region, with regional flow following a north-south direction with a low hydraulic gradient. generally, the recharge zone corresponds to the hilly areas, while discharge occurs to the south, directly into the atlantic ocean. average groundwater velocities range from 0.05 m/day (real velocity) to 0.0049 m/day (darcy velocity), with 93% of the piezometric surface presenting a hydraulic gradient of less than 0.00396. groundwater age calculations classify the aquifer as modern, with average values between 22.4 and 32.5 years, determined through tritium analysis (quiroz londoño et al. 2008). the second aquifer, defined as the sand-dune aquifer, is restricted to the coastal dune area called the southern sandy barrier (isla et al., 1996; 2024), overlying the pampeano aquifer. this local unconfined aquifer has a thickness that does not exceed 20 meters (quiroz et al., 2010) and presents storage coefficients of 0.2. its hydraulic properties show permeability (k) values ranging from 2 to 9 m/day (albouy et al., 2020), while more recent studies report ranges between 21 and 25 m/d, with an average value of 22 m/d (carretero et al., 2022), highlighting the heterogeneity in its transmissivity. the sedimentological characterization of the dune formation reveals a predominance of fine-grained sand (71.8-84.1%), complemented by smaller fractions of medium-grained sand (4-14%) and very fine sand (5.3-15.9%). magnetic analyses indicate a distribution of magnetic and non-magnetic minerals in proportions of 15% and 85%, respectively. the petrographic composition includes between 30% and 60% opaque minerals, accompanied by shell fragments, subrounded quartz, plagioclases, and lithic particles, with subordinate presence of potassium feldspar, pyroxenes, olivine, and volcanic glass. the lithic fragments are predominantly of volcanic and sedimentary origin, with a significant presence of plutonic components in the very fine sand fraction (carretero et al., 2022). this dune system functions as a “barrier” that alters the regional hydrogeological flow towards the ocean, resulting in water enriched with salts to the northwest. a general schematic of the typical hydrogeological configuration of coastal regions in the province is shown in figure 2. the natural discharge of water from the aquifer contained within the dunes occurs on both sides of these geoforms, towards the continental plain to the northwest and towards the ocean to the southeast, forming interfaces between freshwater and water with a higher salinity degree (carretero 2011; rodrigues capítulo et al., 2017). this barrier also modifies the natural flow of streams in the area. figure 2. typical hydrogeological configuration in the coastal dune sector of southeast buenos aires. the pampeano aquifer, san-dune barriers and the sanddune aquifer are shown along with the groundwater flow lines. the complex interaction between the two aquifer units poses significant challenges to the sustainable management of groundwater resources. the coexistence of intensive agricultural activities, tourism development, and urban expansion exerts multiple pressures on the hydrogeological system, disrupting its natural balance. land-use changes, the construction of private infrastructure, and continuous water extraction represent critical factors that may compromise the quality and availability of water resources. these processes, combined with the potential interconnection between aquifers, increase the risk of contamination and water quality degradation, which could have irreversible long-term impacts (foster et al., 2020; lamhour et al., 2024). it is essential to implement integrated management strategies that include the continuous evaluation of hydrogeological dynamics, systematic water quality monitoring, and policies that promote sustainable resource use. the adoption of approaches based on hydrogeological modeling and risk assessment will enable the anticipation and mitigation of negative impacts, ensuring the long-term preservation of aquifers (gleeson et al., 2020). 3. materials and methods 3.1 sampling procedures to characterize the groundwater in the study area, monitoring campaigns were conducted during 2022 and 2023. groundwater samples were collected from 32 wells, including domestic and irrigation wells, for the determination of major ions and stable water isotopes. all monitoring points were georeferenced using a garmin etrex vista gps. to ensure the representativeness and quality of the samples obtained, water renewal was ensured by continuous pumping, measuring basic variables such as temperature, electrical conductivity (ec), and ph with a hanna hi 9828 device until stabilization was achieved. the sampling protocol was executed following reproducibility and methodological rigor criteria. high-density polyethylene (hdpe) containers underwent a triplewashing procedure to eliminate potential contaminants, followed by complete filling to minimize contamination during the sampling process. sample collection was conducted according to the following specific parameters: for nitrate analysis, samples were collected in pre-washed 250 ml polyethylene bottles. these bottles were hermetically sealed, and sulfuric acid (h2so4) was added to preserve samples at ph below 2. for major ion analysis (cations and anions), samples were collected in pre-washed 1000 ml polyethylene bottles. cation samples were field-filtered and acidified with nitric acid (hno3) to ensure preservation at ph below 2. stable water isotopes (δ18o and δ2h) were sampled in 25 ml polyethylene containers. field procedures were executed in accordance with protocols established for standardized methodologies from apha (2017), section 1060b, ensuring the acquisition of representative data with high analytical precision. all water samples were transported under cold-chain conditions to the laboratory of hydrogeochemistry and isotopic hydrology at the national university of mar del plata (unmdp cicpba) for further analysis. 3.2 hydrochemical analysis standardized methodologies from apha (2017) were used for the quantification of physicochemical parameters. the accuracy and precision of the hydrogeochemical analyses were validated using certified reference materials (crms, with errors below 5%) and through the assessment of ionic balance (errors below 10%). the relative discrepancies identified during this evaluation ensured the reliability and consistency of the results throughout the sample quantification process. the contents of calcium (ca+2), potassium (k+), and sodium (na+) were determined by flame photometry. chloride anions (cl-), alkalinity (co3 -2/hco3 -), and total hardness were determined by titrimetric techniques; sulfates (so4 -2), and silica (sio2) were determined using turbidimetric and colorimetric techniques, respectively. measurements were performed using a photometer, following the principles of the beerlambert law, which establishes the relationship between absorbance and the concentration of the analyzed species (apha 2017). nitrates (no3 -) concentrations were measured by using uv-visible spectrophotometry, while magnesium (mg+2) levels were calculated as the difference between total hardness and calcium hardness, both expressed as mg caco₃/l. the calculation was performed using the following equation. mg (mg/l) = [total hardness (mg caco3 / l) − calcium hardness (mg caco3 / l)] ×0.243 (eq. 1) this approach assumes that the total hardness is primarily due to calcium and magnesium ions, with the conversion factor accounting for the molar mass ratio between magnesium and calcium carbonate. the spatial location of the sampling wells was performed using qgis 3.2.0 software (team 2016). the characterization of hydrogeochemical facies and conventional hydrogeochemical graphs were carried out with aquachem (calmbach 1997), and statistical analysis with originpro software (originpro 2016). 370 jesús david gómez gutiérrez, asuncion romanelli, leandro bertolini, orlando mauricio quiroz londoño 3.3 water stable isotope analysis the determinations of water stable isotopes ( δ18o and δ2h) were carried out using tiwa-45ep laser spectroscopy (los gatos research). the uncertainty values used in these analyses were ± 0.15 ‰ for δ18o and ± 1.0 ‰ for δ2h. all isotopic ratio results were reported as δ values in permil (‰) relative to the international vsmow (vienna standard mean ocean water; gonfiantini (1978)). water samples were plotted in a conventional δ18o vs δ2h plot, together with the global meteoric water line (gmwl; dansgaard 1964) and local meteoric water line (lmwl; quiroz londoño et al., 2015). 3.4 end member mixing analysis for the calculation of mixing ratios, end member mixing analysis (emma) was used (christophersen and hooper 1992; penna et al. 2015; lone et al., 2021). this methodology is based on a series of assumptions framed in the conservation of solutes used as tracers, the linearity of the mixing process as a function of hydrodynamics, and the existence of solutions with extreme concentrations. the following balance equations were used for this methodology (gibson et al., 2002). qw = qpa + qsda (eq. 2) where q represents the contribution of different water sources, pa the pampeano aquifer, and sda the sand-dune aquifer. to express the concentrations of the tracer (ec, cl-, and δ18o) in source 1 (pampeano aquifer; pa) and in source 2 (sand-dune aquifer; sda), we can use the following mathematical expressions. fpa + fsda = 1 (eq. 3) cem = cepa * fpa + cesda * fsda (eq. 4) clm = clpa * fpa + clsda * fsda (eq. 5) δ18om = δ18opa * fpa + δ18osda * fsda (eq. 6) where f represents the proportion of the water source (pa or sda) in the mixture (m), ec stands for electrical conductivity, clchlorides, and δ represents the δ18o composition. these equations were used to estimate the mixing proportions in some wells where the interaction of both aquifers is presumed. it is important to note that in this study, rainwater is excluded as a potential water source in the formulation of the contribution variables. this document focuses on understanding the mixing processes of aquifers with different hydrochemical and isotopic signatures, the consequences of which could influence the variation in water quality of both hydrological systems. 4. results and discussion 4.1 hydrochemical characteristics chemical information for all 11 wells located over the sand-dune aquifer is available for 6, while all wells have data on electrical conductivity and stable water isotopes. in the pampeano aquifer, 21 wells were identified. of these, 19 contain isotopic data (δ18o and δ2h) along with complete chemical information, while the remaining 2 wells have only chemical information. for the in situ measured physicochemical variables, the ph values ranged from 7.32 to 7.97, with a mean of 7.59 in the sand-dune aquifer, and from 7.23 to 8.01, with a mean of 7.67 in the pampeano aquifer. this suggests that the water in both cases is slightly alkaline, consistent with previous research on the pampeano aquifer (lutri et al., 2020). the ec values ranged from 410 to 790 µs/cm, with a mean value of 531 µs/cm for wells in the sand-dune aquifer, and from 909 to 1845 µs/cm, with a mean of 1463 µs/cm for wells in the pampeano aquifer. this classifies all samples as freshwaters (maldonado et al., 2018). in the sand-dune aquifer, the average concentrations of ca2+ and mg2+ were 34.3 and 30.2 mg/l, respectively, whereas in the pampeano aquifer, they were 38.4 and 37.0 mg/l. in both aquifers, the dominant anions were hco3 − and so4 2− . in the sand-dune aquifer, the average concentrations of hco3 − and so4 2− were 304.9 mg/l and 14.9 mg/l, respectively, while in the pampeano aquifer, these values increased to 630.0 mg/l and 51.4 mg/l. the average concentrations of the main ions in the sand-dune aquifer are lower than those observed in the pampeano aquifer, a phenomenon attributed primarily to the lithological characteristics and the water transit times in each system (quiroz londoño et al., 2008). total hardness showed mean values of 226.0 mg/l for the sand-dune aquifer and 227.7 mg/l for the pampeano aquifer, both classified as hard water (sánchez-gutiérrez et al., 2020). as for the average cl− concentrations, values of 70.6 mg/l were recorded in the sand-dune aquifer and 203.9 mg/l in the pampeano aquifer. regarding no3 − concentrations, the sand-dune aquifer showed an average value of 13.3 mg/l, whereas the pampeano aquifer samples recorded an average value of 64.8 mg/l, indicating the impact of anthropogenic activities on groundwater. the physicochemical results obtained in the study area indicate that its various characteristics affect water quality and consequently limit its uses. to observe differences between the deep and shallow aquifers, changes in ionic compositions as described earlier have been illustrated (figure 3). the results reveal discrepancies between the two aquifers in terms of chloride and bicarbonate anions, as well as sodium cation, possibly due to varying hydrogeochemical processes. a critical aspect of this study is the identification of chemical patterns in transition and discharge areas that align with findings from previous studies (glok galli et al., 2023). these similarities emphasize the importance of comprehending the intricate hydrogeochemical processes occurring within this aquifer. figure 3. ionic composition (meq/l) of the samples collected from the sand-dune aquifer (red) and the pampeano aquifer (blue). tables 1 and 2 present the hydrogeochemical and isotopic characterization of the studied aquifers. these tables compile the major ion concentrations and isotopic signatures, incorporating key statistical parameters (mean and standard deviation). the analyses reveal differences between the san-dune and the pampeano aquifers regarding salinity, ionic composition, and isotopic signature. the sand-dune aquifer is characterized by higher water quality, with lower salinity, a lighter isotopic signature (less enriched δ18o and δ2h), and lower concentrations of contaminants such as nitrates. in contrast, the pampeano aquifer exhibits higher mineralization, a more enriched isotopic signature, and greater variability in its chemical composition, attributable to anthropogenic processes and/or mixing phenomena. these defined differences were comprehensively understood through the evaluation of ionic relationships, chemical distribution diagrams, and isotopic signatures within the context of local meteoric water lines 371application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina table 1. hydrogeochemical and isotopic data from the sand-dune aquifer sample e.c (μs/cm) δ18o (‰) δ2h (‰) ph ca2+ (meq/l) mg2+ (meq/l) na+ (meq/l) k+ (meq/l) hco3 (meq/l) cl (meq/l) so4 2 (meq/l) no3 (meq/l) g1022 484 -5.6 -33.8 7.66 0.85 1.55 0.13 0.02 4.33 1.13 0.16 0.10 g1023 496 -5.6 -34 7.97 0.92 0.65 6.09 0.04 5.97 1.64 0.16 0.54 g1025 458 -5.6 -34.1 7.52 0.68 1.58 2.04 0.03 4.33 1.64 0.15 0.03 g1026 602 -5.3 -33.4 7.76 0.45 2.21 10.87 0.03 5.42 2.03 0.18 0.11 g1032 647 -6.2 -34.3 7.64 1.82 0.52 4.35 0.08 4.56 2.88 0.28 g101 572 -5 -35 7.32 0.41 0.95 7.39 0.14 5.39 2.64 0.14 0.24 g252 500 -5.4 -29.5 ---------g253 470 -4.7 -29.1 ---------g255 410 -5.6 -32.1 ---------g256 410 -5.5 -31.9 ---------g251 790 -5.2 -28.8 ---------mean 530.82 -5.43 -32.36 7.65 0.86 1.24 5.15 0.05 5.00 1.99 0.16 0.22 standard deviation 114.01 0.39 2.27 0.22 0.52 0.65 3.85 0.05 0.69 0.66 0.01 0.18 table 2. hydrogeochemical and isotopic data from the pampean aquifer sample e.c (μs/cm) δ18o (‰) δ 2h (‰) ph ca2+ (meq/l) mg2+ (meq/l) na+ (meq/l) k+ (meq/l) hco3 (meq/l) cl (meq/l) so4 2 (meq/l) no3 (meq/l) g1021 1578 -5.4 -33.3 7.86 0.48 0.79 10.44 0.03 9.11 g1027 1234 -4.8 -29.8 7.58 1.35 1.19 5.22 0.08 7.05 4.09 0.28 0.79 g1028 1019 -5.2 -27.4 7.73 0.60 2.43 1.91 0.18 5.97 3.38 -0.49 g1029 1522 -5.6 -30.8 7.52 1.18 1.47 10.00 0.13 9.11 5.13 -2.46 g1030 1469 -5.3 -30.1 8.01 4.54 0.86 6.52 0.23 1.04 4.29 -0.98 g1031 1773 -5.3 -28.3 7.75 2.00 3.92 6.96 0.36 8.13 6.32 -1.57 g1104 1215 -5 -29.5 7.39 0.67 1.28 10.44 0.15 8.77 5.33 0.41 2.03 g114 909 -4.3 -22.5 7.51 0.66 0.91 8.92 0.72 9.46 6.27 0.20 -g282 1652 -5.5 -31 7.78 0.55 1.93 13.92 0.59 13.91 7.19 0.58 1.17 g517 1780 -5.4 -31.4 7.45 1.62 0.29 25.23 0.31 14.18 8.72 0.94 0.67 g721 1588 -5 -30 7.93 0.70 1.02 12.18 0.28 11.31 4.29 0.52 0.52 g722 1647 -5.2 -31.5 7.6 0.85 1.83 10.87 0.41 8.95 5.90 0.60 1.19 g723 1457 -5.1 -30.5 7.76 0.85 1.26 10.44 0.38 11.14 3.75 0.48 0.65 g162 1200 --7.65 0.28 2.02 13.09 0.28 13.93 6.91 0.68 2.00 g167 1190 --7.73 0.40 1.12 18.57 0.23 15.34 7.78 0.56 1.10 g168 1110 -5.4 -34 7.75 0.26 0.86 12.88 0.25 11.46 5.64 0.50 1.69 g723 1536 -5.3 -30.4 7.68 1.45 1.44 10.87 0.36 13.13 4.32 0.73 1.08 g723 1772 -5.1 -30 7.54 1.62 3.72 8.70 0.28 9.39 7.81 1.82 0.67 g723 1845 -5.2 -31.1 7.23 0.74 2.44 10.44 0.08 10.41 7.16 0.46 0.38 g723 1757 -5.3 -32.4 7.93 2.17 1.69 9.13 0.23 9.21 5.11 0.26 0.30 g722 ----0.85 1.83 10.87 0.41 8.95 5.90 0.60 1.19 mean 1462.65 -5.19 -30.22 7.67 1.13 1.63 10.84 0.28 10.00 5.76 0.57 1.11 standard deviation 282.66 0.29 2.49 0.20 0.95 0.91 4.72 0.17 3.22 1.47 0.39 0.59 372 jesús david gómez gutiérrez, asuncion romanelli, leandro bertolini, orlando mauricio quiroz londoño 4.2 hydrochemical facies a piper diagram was used to identify hydrochemical facies and infer possible geochemical processes influencing the composition of both aquifers (figure 4). the diagram reveals a wider dispersion of samples from the pampeano aquifer, predominantly classified as sodium-bicarbonate water type, with only two samples showing different facies. on the other hand, the sanddune aquifer primarily exhibits a calcium-magnesium bicarbonate type, with just two samples displaying a different facies (sodium-bicarbonate water type). the limited availability of accurate data on the depth of the sampled wells constitutes a constraint in this study. user-provided information is often imprecise, typically specifying broad depth ranges rather than precise values during monitoring campaigns. this lack of precision complicates the systematic evaluation of aquifer interactions. although depth data is unavailable for all wells, the results suggest a potential interaction between aquifers (figure 4). this is particularly evident in the samples from wells g1027 and g1028, located in the pampeano aquifer. the ionic composition of these samples reveals a transitional zone between calcium-magnesium bicarbonate and sodium bicarbonate facies. specifically, the findings suggest that the sand-dune aquifer may be recharging the pampeano aquifer in inland areas with lower salinity water characterized by calcium-magnesium bicarbonate facies. this mixing process significantly alters the physicochemical properties of the water. moreover, this situation reveals an issue framed within regional drilling practices, which frequently prioritize maximum depth and the use of multi-level screens, operating under the erroneous assumption that these designs improve water availability for productive activities. however, such practices often fail to consider the specific hydrogeological boundaries of individual aquifers, potentially exacerbating inter-aquifer exchange and its associated challenges. considering the above, wells g1022, g255, g252, and g1023 are representative of the sand-dune aquifer. in contrast, wells g1021, g1029, g1030, and g1031 exhibit the chemical composition of the pampeano aquifer, despite their spatial proximity to wells g1027 and g1028, which show evidence of mixing between both aquifer systems. this differentiation is based on the geochemical signatures of the wells, particularly those associated with conservative tracers such, as water stable isotopes (δ2h, δ18o), electrical conductivity (ec), and chloride concentrations (cl−). the values of these variables in wells g1027 and g1028 show statistically significant deviations compared to those recorded in the pampeano aquifer wells, suggesting a mixing process in the transition zone. however, to confirm the correct assignment of the end members and validate data interpretation, it is deemed necessary to perform a methodological evaluation incorporating an analysis of the spatial distribution of the wells. 4.3 geochemical processes one of the main processes contributing to na+ and cl− ions in groundwater is halite dissolution, a result of rock interaction. this process can be identified through the distribution of points in a binary diagram relating cl− and na+ content (figure 5a). the dispersion of sodium and chloride values indicates that halite dissolution is not the primary process responsible for the presence of these ions in the solution, as halite dissolution would produce equivalent amounts of both ions. ninety-five percent of the samples fall below the halite dissolution line, suggesting that other factors contribute to changes in the ionic composition of the groundwater (jat baloch et al., 2021; xiao et al., 2023). elevated sodium values may be associated with ion exchange processes involving calcium and magnesium present in pampeano sediments. a key aspect of this ionic relationship is the clear differentiation between the two aquifers under study. in the pampeano aquifer, chloride values exceed 3 meq/l, whereas lower values are observed in the sand-dune aquifer. the ionic relationship between hco3 − + so4 2− vs. ca2+ + mg2+ and mg2+ vs ca2+ is used to establish the changes in water chemistry due to the weathering of carbonate and silicate rocks (jat baloch et al,. 2021). however, this process is not dominant in the study area, as the point distribution does not show a linear behavior along the y = x line, nor do they fall within the range between the y=x / y = 0.5*x lines (figure 5b and figure 5c), which identify dolomite and calcite dissolution, respectively. figure 5b confirms the differentiation process of the aquifers under study. values greater than 6 meq/l for hco3 − and so4 2− species define the pampeano aquifer, while lower values characterize the sand-dune aquifer. figure 4. ionic composition of groundwater from the pampeano (blue) and sanddune (red) aquifers: a: calcium and magnesium bicarbonate waters; b: sodium -bicarbonate waters; samples g1028 and g1027 are potential mixtures originating from both aquifers. a figure 5. ionic relationships of groundwater in the study area: a) clvs na+, b) ca²⁺ + mg²⁺ vs hco₃⁻ + so₄²⁻, c) ca²⁺ vs mg²⁺, d) ca²⁺ vs so₄²⁻. 373application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina figure 5d shows the ionic relationship between so4 2− vs ca2+ and which establishes the conditions for gypsum dissolution. most of the points are distributed below the y = x line, indicating that the source of calcium does not originate from this process. in the various ionic relationships evaluated, the ions do not align with the presented lines, suggesting that cation exchange is the dominant phenomenon driving the chemical changes. this indicates that sediments and geology play a crucial role in the variations of the water’s chemical conditions. to assess whether cation exchange processes modify the chemical properties of groundwater in the study area, the ionic relationship (ca2+ + mg2+) (so4 2+ hco3 -) vs. (na+ cl-) is graphically represented (figure 6). a linear correlation between these ions is observed (r2 = 0.56) with a slope (-0.82), closely approaching the theoretical value of -1, consistent with findings from other studies in the pampeano aquifer (glok galli et al., 2023). this correlation suggests the presence of cation exchange processes, where excess sodium in both aquifers likely results from interactions involving the exchange of calcium and magnesium for sodium (jat baloch et al., 2021). figure 6. ionic relationships of groundwater in the study area to identify ion exchange processes. isotopic characterization the isotopic composition of the different samples was included in the conventional δ2h vs δ18o plot shown in figure 7, along with the global meteoric water line (gmwl) defined by craig (1961) and the local meteoric water line (lmwl) defined by quiroz londoño et al., (2015). the δ18o values for the sand-dune aquifer range from -4.7 to -6.2 ‰, averaging -5.43 ‰, while those for the pampeano aquifer vary between -4.30 and -5.60 ‰, averaging -5.19 ‰. in terms of δ2h values, the sand-dune aquifer ranges from -35.0 to -28.8 ‰, averaging -32.36 ‰, whereas the pampeano aquifer ranges from -34.0 to -22.5.0 ‰, averaging -30.2 ‰. these findings indicate that the sanddune aquifer generally exhibits a slightly depleted isotopic signature compared to the pampeano aquifer. the isotopic composition of the samples from the studied aquifers could be influenced by mixing and/or evaporation processes during infiltration, as well as by subsequent interactions with the local groundwater. the samples from the pampeano aquifer exhibit greater isotopic variation compared to those from the sand-dune aquifer. this behavior is primarily attributed to the pampeano aquifer’s lithological composition, which consists of clayey sediments that limit the infiltration rates and enhance evaporation. these variations are evident in the distribution of the samples along the lmwl and the gmwl. in contrast, the sand-dune aquifer, predominantly composed of sandy sediments, is recharged by rainfall with best infiltration rates. the rapid infiltration minimizes exposure time, thereby reducing the influence of evaporation on the isotopic signature. these differences underscore the distinct b c d sand-dune aquifer pampeano acquifer 374 jesús david gómez gutiérrez, asuncion romanelli, leandro bertolini, orlando mauricio quiroz londoño hydrogeological and isotopic dynamics of the two aquifers, emphasizing the critical role of lithology and recharge mechanisms in shaping isotopic variability. based on the information provided by hydrochemical analysis and isotopic signatures, variations in electrical conductivity values have been identified, indicating a potential interaction between the two aquifers under study. the ec vs δ18o plot (figure 8), reveals that in well g251, the salinity levels suggest such an interaction or connection, evidenced by an increase in ec and a slight difference in isotopic content. figure 7. isotopic content of the pampeano and sand-dune aquifers. figure 8. δ¹⁸o values and electrical conductivity used in the characterization of aquifers. 4.5 water quality assessment to assess the suitability of water from the 32 inventoried wells for human consumption, the concentrations of major ions were compared against the limits specified in the argentine food code (caa 2021). it was observed that chloride levels in both aquifers are below the 350 mg/l limit established for potable water. additionally, sulfate concentrations were found to be below the 400 mg/l limit. the no₃⁻present in groundwater can originate from various sources of contamination, such as agricultural activities, human actions, atmospheric deposition, or natural content in the soil, which limits its suitability for direct consumption (matiatos 2016). the wells exploiting the sand-dune aquifer present levels below 45 mg/l, in contrast to the pampeano aquifer, where 60% of the wells show values exceeding the limit established by the argentine food code (caa 2021), thus restricting its use for human consumption. total hardness, which reflects the concentrations of calcium and magnesium, is within the established limit of 400 mg/l in the sand-dune aquifer, indicating suitable water quality for human consumption. however, in the pampeano aquifer, well g1031 shows a total hardness of 597 mg/l, exceeding the potable water limit. the presence of high levels of hardness and nitrates in the pampeano aquifer highlights the non-potability of the water for human consumption. this aspect is relevant because it may pose a risk to the sand-dune aquifer due to the presumed mixing, potentially altering or modifying its water quality conditions over time. 5.6 quantification of mixing contributions the selected end-members for the mixing proportion analysis and the values of the different conservative tracers used are shown in table 3. table 3. conservative tracer values for each selected end-member. sample aquifer e.c (µs/cm) δ18o (‰) δ2h (‰) cl-(mg/l) g1022 sand-dune 484.0 -5.6 -33.8 40 g255 sand-dune 410.0 -5.6 -32.1 -g252 sand-dune 500.0 -5.4 -29.5 -g1023 sand-dune 496.0 -5.6 -34.0 58 mean 472.5 -5.6 -32.4 49,0 g1021 pampeano 1578 -5.4 -33.3 200 g1029 pampeano 1522 -5.6 -30.8 182 g1030 pampeano 1469 -5.3 -30.0 152 g1031 pampeano 1773 -5.3 -28.3 224 mean 1585.5 -5.4 -30.6 189.5 the integration of hydrogeochemical and isotopic analyses reveals potential water mixing in wells g251, g1027, and g1028. this interpretation is supported by multiple lines of evidence. conductivity versus δ18o diagrams (figure 8) clearly differentiate between the two studied aquifers, with the mixed samples occupying intermediate positions and exhibiting similarities in their tracer variables (ec and δ18o). regarding the isotopic signal, these samples cluster in the upper region of the local meteoric water line (figure 7), suggesting similar physical processes during infiltration and water-rock interaction. the lithological characteristics of each aquifer significantly influence these processes: the pampeano aquifer, dominated by clays, promotes evaporation due to greater water-sediment interaction, while the sand-dune aquifer, mainly composed of sands, allows faster infiltration with less interaction. the isotopic similarities of the mentioned samples support the hypothesis of mixing between these units. the hydrogeochemical patterns (figure 4) corroborate this interpretation, as the samples with potential mixing exhibit intermediate chemical compositions, distinct from the characteristic extremes of each aquifer. quantification of these mixing processes indicates varying proportions of contribution from each aquifer, as detailed in table 4. table 4. results of the calculated mixing proportions. sample aquifer e.c (µs/cm) fsda fpa δ18o (‰) fsda fpa cl(mg/l) fsda fpa g251 sand-dune 790.0 0.68 0.32 -5.2 -1.3 2.3 58 --g1027 pampeano 1234.0 0.32 0.68 -4.8 -4.0 5.0 145 0.32 0.68 g1028 pampeano 1019.0 0.51 0.49 -5.2 -1.3 2.3 120 0.49 0.51 375application of isotopic and hydrogeochemical techniques to evaluate mixing processes in groundwater systems from a coastal area in argentina considering the calculations performed, we can confirm the interaction between the two aquifers in the coastal sector under study. the elements selected as end-members and the mixing wells have provided key data that align with the conceptual hydrogeological model of the area. the obtained data reveal that wells near the sand dunes, which are supplied by the pampeano aquifer, extract approximately 30% to 50% of water from the sand-dune aquifer (table 4). this process results in a decrease in electrical conductivity, an increase in magnesium and calcium levels, and a reduction in sodium levels. conversely, wells constructed in the sand-dune aquifer can capture up to 30% of their supply from the pampeano aquifer, which is reflected in an increase in the electrical conductivity of these wells. these interaction processes between the waters of both aquifers can have negative consequences for both the quality and quantity of water stored in the sand-dune aquifer. regarding quality, the influx of more saline waters could significantly deteriorate water quality. in terms of quantity, nearby wells would become an additional source of extraction from this aquifer, increasing pressure on the available water resources (gorelick and zheng, 2015; wali et al., 2019). figure 9 shows a schematic illustrating the problem of water mixing between the two aquifers as a consequence of anthropogenic activity, indicating how drilling in these aquifers can deteriorate water quality. to prevent the deterioration of the sand-dune aquifer, it is proposed to implement actions that promote proper control of wells in this aquifer. these controls should include determining the maximum drilling depth to avoid exceeding the sediment thickness and the appropriate design of corresponding intakes. periodic conductivity measurements in various intakes can serve as an early warning system for possible aquifer mixing and water quality deterioration. additionally, it is crucial to control the construction distance of wells in the pampeano aquifer, as the results of this study indicate that these intakes are supplied with water from the more superficial aquifer. figure 9. hydrogeological configuration in the coastal dune sector with the presence of domestic supply wells in both aquifer units. the observed interactions between the pampeano and sand-dune aquifers have significant implications for coastal aquifer management. the mixing processes identified, such as the influx of more saline water from the pampeano aquifer into the sand-dune aquifer, lead to changes in water quality, including increased electrical conductivity and elevated magnesium and calcium levels, which may compromise the aquifer’s suitability for human consumption and irrigation. similarly, carretero et al., (2022) emphasized that mixing processes in coastal aquifers can result in salinization and undesirable ion enrichment, while manivannan and elango (2021) highlighted that such interactions exacerbate stress on water resources and reduce aquifer sustainability. these findings underscore the importance of monitoring and managing groundwater use to mitigate negative effects. proactive measures, such as regulating well depths, controlling the spatial distribution of extractions, and implementing regular monitoring programs using electrical conductivity and isotopic analyses, are essential to detect early signs of salinization or seawater intrusion. without intervention, continued anthropogenic activity could accelerate aquifer degradation, reducing the availability of potable water and threatening the sustainability of vital groundwater coastal resources. 5. conclusions the approach of this study broadens our understanding of the geochemical processes that control the groundwater chemistry of waters and provides relevant information for the integrated management of water resources. it helps in understanding the processes by which human activities can risk changes in groundwater quality. the need to continue field data collection to understand the evolution of water chemistry and to provide decision-makers with tools for water resource management becomes evident. the possible mixture identified in this study underscores a significant challenge within the framework of regional groundwater resource management. the erroneous assumption that deeper drilling guarantees higher water yields has led to inadequate well construction practices by users. the hydrogeochemical and isotopic results presented not only highlight this issue but also provide a basis for initiating detailed hydrogeological studies aimed at enhancing the understanding of aquifer systems in the region. furthermore, these findings offer a robust foundation for the development and implementation of more effective regulatory policies by governing authorities. this information is essential for improving the management of groundwater resources in the region. variations in the concentrations of tracer elements are a tool for identifying mixing processes between aquifers. to deepen the understanding of isotopic and hydrogeochemical processes, it is recommended to implement a systematic water quality monitoring program. this program should focus at a minimum on measurement of electrical conductivity as a key indicator of mixing processes, enabling the early detection of alterations that could impact water quality and, consequently, user health. the information generated will facilitate the timely implementation of protective measures for both aquifers. the application of the end-member mixing analysis methodology for conservative parameters such as electrical conductivity and chlorides provides evidence of interaction between the studied aquifers. well g251, located in the sand-dune aquifer, has a contribution of 68% from the sand-dune aquifer and 32% from the pampeano aquifer, while wells g1027 and g1028 have contributions of 32% and 49% from the sand-dune aquifer respectively, and 68% and 51% from the pampeano aquifer. the results for the sanitary variables of interest for the sand-dune aquifer have complied with the permissible limits of the argentine food code, indicating suitability for human consumption. on the other hand, for the pampeano aquifer, 60% of the study wells present adverse conditions due to nitrate levels exceeding the limit of 45 mg/l. this aspect must be controlled to prevent the possible mixing from altering the quality conditions of the sanddune aquifer. the quantified isotopic values reveal that the sand-dune aquifer exhibits a slightly depleted isotopic signature compared to the pampeano aquifer. however, these parameters do not allow for a clear differentiation between the two aquifers, primarily because both recharge sources are from precipitation. the slight enrichment present in this sector of the pampeano aquifer can be explained by evaporation processes occurring prior to the infiltration and subsequent percolation of water into the aquifer. in this regard, the lithology of both aquifers plays a dominant role, as the sands constituting the sand-dune aquifer, having higher permeability, allow for the 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https://doi.org/10.3178/hrl.10.67 https://doi.org/10.1155/2017/7080346 http://observatoriogeograficoamericalatina.org.mx/egal12/geografiasocioeconomica/geografiaurbana/239.pdf http://observatoriogeograficoamericalatina.org.mx/egal12/geografiasocioeconomica/geografiaurbana/239.pdf http://observatoriogeograficoamericalatina.org.mx/egal12/geografiasocioeconomica/geografiaurbana/239.pdf _heading=h.30j0zll seismic site classification of the costa rican strong-motion network based on vs30 measurements and site fundamental period luis a. pinzón1, diego a. hidalgo leiva2, aaron moya-fernández2, victor schmidt-díaz2, luis g. pujades3. 1 research division, universidad católica santa maría la antigua, panama city, panama 2 earthquake engineering laboratory, universidad de costa rica, san jose, costa rica 3 department of civil and environmental engineering, universitat politècnica de catalunya, barcelona, spain keywords: site classification; site fundamental period; shear-wave velocity; costa rica; hvsr; vs30 palabras clave: clasificación de sitio; período fundamental de sitio; velocidad promedio de la onda de corte; costa rica; relaciones horizontales/ verticales; vs30 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.93927 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 383-389 se is m o lo g y record manuscript received: 26/02/2021 accepted for publication: 01/10/2021 abstract in this paper, the soil profile of the costa rican strong-motion network (crsmn) stations is classified based on actual measurements and seismic regulations. the soil classification of the costa rican seismic code based on the average shear-wave velocity of the top 30 m (vs30) is used as a reference. the site fundamental period (tf) is included as a parameter to complement the existing characterization. for this, vs30 measurements from 52 accelerometric stations are related to the site fundamental period obtained through horizontal-to-vertical spectral ratios (hvsr) using ground motion records from the costa rican strong-motion database. the h/v ratios are estimated with 5% damped acceleration response spectra and with traditional fourier amplitude spectra from the s-wave window. from the relation between vs30 and tf, different ranges of tf are assigned to the existing soil profile classification and a graph with three-lines and four-areas is proposed to classify the stations of the crsmn. these lines are plotted from the intersection between values assigned to each site class. the assigned classification at each station will be the one corresponding to the area where the pair of values (vs30 – tf) falls. with this proposal, both parameters take relevance and are compensated, reducing the differences due to possible errors in measurements or interpretations. clasificación de sitio de la red de acelerógrafos de costa rica basada en mediciones de vs30 y en el período fundamental resumen en este artículo, se propone una nueva clasificación de sitio para la red de acelerógrafos de costa rica (racr). se utiliza como referencia la clasificación de sitio del código sísmico de costa rica basada en la velocidad promedio de la onda de corte de los 30 m superiores (vs30). el período fundamental del sitio (tf) se incluye como parámetro para complementar la caracterización existente. para ello, las mediciones de vs30 de 52 estaciones acelerométricas se relacionan con el período fundamental del sitio, este obtenido a partir de relaciones espectrales h/v utilizando registros de la base de datos de movimiento fuerte de costa rica. las relaciones h/v se estiman con espectros de respuesta de aceleración con 5% de amortiguamiento y con espectros de amplitud de fourier tradicionales obtenidos de la ventana de ondas s. a partir de la relación entre vs30 y tf, fueron asignados diferentes rangos de tf a la clasificación de perfil de suelo existente y se propone un gráfico con tres líneas y cuatro áreas para clasificar las estaciones de la racr. estas líneas fueron graficadas a partir de la intersección de los valores asignados a cada clase de sitio. la clasificación de cada estación será la correspondiente a la zona donde se encuentre el conjunto de valores vs30 y tf. con esta propuesta, ambos parámetros toman relevancia y se compensan, reduciendo las diferencias por posibles errores en las mediciones o interpretaciones. how to cite item: pinzon, l. a., hidalgo-leiva, d. a, moya-fernandez, a., schmidt-diaz, v., & pujades, l. g. (2021). seismic site classification of the costa rican strongmotion network based on v s30 measurements and site fundamental period. earth sciences research journal, 25(4), 383-389. https://doi.org/10.15446/esrj. v25n4.93927 https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.93927 https://doi.org/10.15446/esrj.v25n4.93927 384 luis a. pinzón, diego a. hidalgo leiva, aaron moya-fernández, victor schmidt-díaz, luis g. pujades. introduction costa rica is considered one of the countries with the greater seismicity of the region. its complex tectonic framework, subduction zone and active volcanism in the continental zone, has motivated to develop more studies to reduce the seismic risk of the country (protti and mcnally 1994; quintero and güendel 2000; moya-fernández et al. 2020). in seismic risk studies, three features must be taken into account: the hazard, vulnerability and damage value. a correct definition of the seismic hazard is essential to obtain reliable results. seismic sources, site characterization and ground motion prediction models must be defined (douglas 2017; pinzón et al. 2019a). several parameters have been used as proxy to define the seismic site conditions based on the topography, geology, site fundamental period (or frequency) and shear-wave velocity of the soil profile. among these, the most used is the average shear-wave velocity in the top 30 m of the soil profile (vs30). in united states, the nehrp (bssc 2003) and the asce 7-16 (asce 2017) proposed six site classes based on vs30, from a (hard rock, vs30 > 1500 m/s) to f (soil with special condition). the costa rican seismic code (crsc) (cfia 2016) uses an equivalent classification, based on vs30, varying only nomenclature (s1 to s4) (dobry et al. 2000). the vs30 is estimated using the following expression: (1) where di is the thickness of each soil layer in the profile until it reaches 30 m deep, vsi is the shear-wave velocity of each layer in m/s and n is the number of layers until it reaches the 30 m. recent studies show some limitations and problems of using vs30 to classify sites (steidl 2000; di alessandro et al. 2012; pinzón et al. 2019a). some of these limitations are the limited disposal of near-surface shear-wave data at strong-motion sites, the complexity of data acquisition, and the fact that these classifications do not consider the effect of the thickness of soft sediments. as an alternative, classifications based on the site fundamental period (tf) have been used (zhao et al. 2006; ghasemi et al. 2009; di alessandro et al. 2012; pinzón et al. 2019a). the site fundamental period is a parameter that characterizes the dynamics conditions of the soil and has been used as a proxy for soil amplification in ground-motion prediction models (di alessandro et al. 2012). there are different methods to estimate the fundamental period, but the most used are those based on the horizontal-to-vertical spectral ratios (hvsr). one is the conventional hvsr with the fourier amplitude spectra from the s-wave window. this approach has been used to determine different dynamic characteristics of the ground through micrometer measurements (nakamura 1989; alfaro et al. 2001; caselles et al. 2010) and strong-motions from earthquakes (nagashima et al. 2014). the ratio is defined by the following expression: (2) where hf and vf are the horizontal and vertical fourier amplitude spectra of ground-motion determined with the s-wave window, respectively. another of the most recently used technique is the one based on the proposal of zhao et al. (2006). with this approach, a site classification for japan was determined from all the seismic records available on each station (zhao et al. 2006). the method is based on the estimation of the spectral ratio h/v from the acceleration response spectra with a 5% of damping for the horizontal components (sah) and the vertical (sav) (see equation 3). (3) zhao et al. (2006) found that the use of averages among all the hvsr obtained at each station eliminates extreme peaks or anomalies. besides, they observed that these averages are not strongly affected if data is segregated by hypocentral distance, magnitude or depth. in costa rica, the strong-motion network (crsmn) operates since 1983 and is administrated by the earthquake engineering laboratory at the university of costa rica (lis-ucr for its acronym in spanish). the lisucr is in charge of recording, processing and storing all acceleration records for academic and research purposes. the network has more than 150 active accelerometric stations around the country. further details about the network can be found in moya-fernández et al. (2020). vs30 measurements on each station is essential in order to classify the site conditions of this network based on existing regulations. the main issue is that only a third of the stations have this kind of measures. the lack of data due to the high cost of measurements and other issues, introduces uncertainty in the estimation of site conditions that directly affect the seismic hazard studies carried out in the country. in recent studies, several relations between the site fundamental period, the topography and the vs30 were found (wald and allen 2007; cadet et al. 2011; pinzón et al. 2019b). these relations cannot be used to classify costa rican stations since they depend on the geomorphology of the region. the main objective of this study is to classify the sites of the accelerometric stations of the crsmn, considering the current seismic regulations and data limitation. to do this, a relation between the site fundamental period and the vs30 is defined to be able to classify stations that do not have vs30 measurements. average shear-wave velocity of the top 30 m (vs30) in the last two years, vs30 have been measured in 52 accelerometric stations (33% of the network) with the aim of improving the existing classification. to obtain vs30 values, the stations of the crsmn were scanned using the multichannel analysis of surface waves (masw) geophysical method. the masw is one of the most used seismic recognition methods where the elastic condition (stiffness) of the soil is evaluated. in this method, the velocity of the surface-waves is measured at different frequencies, obtaining the depth variation of the shear-wave velocity (vs) from the measured soil profile. the variation of velocity at different frequencies is mainly attributed to the stratification of the s-waves velocities whose values are obtained with a numerical inversion system. in figure 1, the shear-wave velocity profile from the accelerometric stations puntarenas-cóbano (pcob) and puntarenas-paquera (ppqr) are presented. these stations present vs30 values of 407.5 and 214.1 m/s respectively. figure 1. shear-wave velocity profiles from stations (a) pcob and (b) ppqr. 385seismic site classification of the costa rican strong-motion network based on vs30 measurements and site fundamental period the fundamental period through horizontal-to-vertical spectral ratios (hvsr) because almost 70% of the stations do not have measured values of vs30, it was decided to determine the fundamental period of each site (station) through horizontal-to-vertical spectral ratios (hvsr) and fill the gap produced by the lack of information. to estimate site fundamental periods, the average of the hvsr obtained from all the records available at each station is calculated with hvsrsa(5%) and hvsrfourier. then, the average of both fundamental periods (from both methods) is estimated, obtaining a unique value per station. records with high acceleration values were discarded (pga > 0.30 g), preventing fundamental period values from high deformations that could lead to a nonlinear response of the soil (pinzón et al. 2019a). in figure 2, the results obtained from the hvsr of the accelerometric stations pcob and ppqr are exposed. the hvsr are presented normalized by the maximum value. site fundamental periods of 0.25 and 0.63 s were obtained in pcob and ppqr stations respectively. hvsrfourier presents several peaks while the hvsrsa(5%) has a smoother shape. in both cases, the fundamental period was properly captured. this method was applied to the entire network, obtaining the site fundamental period at the 157 stations. a comparison of the results obtained with the hvsrsa(5%) and hvsrfourier for all the stations is shown in figure 3. figure 2. horizontal-to-vertical spectral ratios normalized by the maximum value and the site fundamental period from stations (a) pcob and (b) ppqr. figure 3. comparison between the site fundamental periods obtained with hvsrsa(5%) and hvsrfourier. vs30 and site fundamental period relation the vs30 values measured were related to their site fundamental period. results from the relation are observed in figure 4. as seen in the figure, the relation follows a bilinear trend with a constant value of vs30 (260 m/s) in sites with a fundamental period higher than 0.5 s. similar results were obtained in the united states from central and eastern north america (cena) sites (hassani and atkinson 2016). figure 4b shows the relation in linear scale. it is observed that the models fit the measured data reasonably well. the dispersion obtained is similar to that obtained in other studies (see figure 9 from hassani and atkinson, 2016, and figure 8 from ghofrani and atkinson, 2014). the bilinear model is defined by the following equations: (4) (5) as seen in the results, some sites with a null period have vs30 values that are not indicative of a rock outcrop. it is important to note that, in those cases, a tf = 0 s was assigned since the fundamental period was not identifiable, presenting a flat h/v with amplitudes < 2. for fundamental periods of the soil deposit larger than 0.5 s, the graph shows that sites with similar vs30 can have significantly different tf values. this outcome, observed also in other studies (hassani and atkinson, 2016), is quite significant. this behavior points a significant drawback of the vs30 approach for seismic site classification. for instance, in those cases, the considered 30 m might not be sufficient to characterize the vibration period of the soil deposit and, therefore, vs30 might not provide a reasonable proxy of the seismic response of the site. on table 1, we have made a proposal relation between the vs30 used on the crsc and the soil fundamental period. sites with fundamental periods lower than 0.15 s are considered rock sites (s1), between 0.15 and 0.35 s stiff soils (s2), 0.35 and 0.75 s soft soils (s3) and sites with fundamental period higher than 0.75 s are very soft soils (s4). to assign this classification, only equation 4 was used (see the dashed line of fig. 4). equation 5 was neglected since sites with very soft soils (s4) could not be classified in terms of vs30. although, in terms of safety (design), this proposal can be beneficial since sites with periods larger than 0.50 s will result in a lower classification compared to the vs30 approach, it is important to note that the use of equation 4 for periods larger than 0.50 s is arbitrary and not supported with data. probably, as mentioned before, the considered 30 m might not be sufficient to identify the fundamental period of the soil deposit. nevertheless, as can be seen in zhao et al. (2006) and di alessandro et al. (2012), a period of 0.60 s has been used as a 386 luis a. pinzón, diego a. hidalgo leiva, aaron moya-fernández, victor schmidt-díaz, luis g. pujades. figure 4. measured vs30 values at the station of the costa rican strong-motion network plotted against site fundamental period values obtained from hvsr. (a) the bilinear model (continuous line – equation 4 for periods < 0.5 s and equation 5 for periods ≥ 0.5 s) and the proposed model based on the costa rican seismic code (dashed line – equation 4) are depicted. (b) plot in linear scale. limit to classify soils s4 (equivalent to nehrp soil e), so our proposal at 0.75 s, is not very different from those already proposed. a graphic with three-lines and four-areas is proposed to classify stations with both vs30 and site fundamental period values (see fig. 5). each area (triangle) represents a site class based on the crsc. the lines are plotted from the intersection between values assigned to each site class (see table 1). the assigned classification at each station will be the one corresponding to the area where the pair of values (vs30 – tf) falls. although this definition is arbitrary, we believe is a reasonable limit according to the available information we have presented in this paper. with this proposal, both parameters take relevance and are compensated, reducing the differences due to possible errors in measurements or interpretations. in this way, we can guarantee that sites will have similar geodynamic characteristics, and consequently possible error sources in the use of accelerograms are reduced (e.g., in the development of ground motion prediction equations or the dynamic analysis of structures). using this classification, stations pcob and ppqr are classified as s2 and s3 respectively. figure 5. site classification proposal based in vs30 and site fundamental period values. crsmn site characterization from the 157 stations of the network, 52 were classified with the proposal described above and the other 105 were categorized only with their site fundamental period using table 1 ranges. in figure 6, a map of costa rica with the geographic distribution of the accelerometric stations and their assigned site class is shown. a total of 12 stations were classified as rock sites, 52 as stiff soils, 67 as soft soils and 26 as very soft soil (see fig. 7). a complete list with the assigned site class of each station is available at the lis-ucr website (http:// www.crsmd.lis.ucr.ac.cr/?id=estaciones). in figure 7b, the mean value of the hvsr per site class, estimated from ratios of all the stations is exposed. the shapes of the mean hvsr values correspond to the expected. the rocky sites have a flat shape with amplification table 1. site classification of the costa rican seismic code, nehrp equivalence, vs30 ranges and site fundamental periods proposed. site class based on costa rican seismic code (crsc) (cfia 2016) nehrp (bssc 2003) equivalence shear-wave velocity of top 30 m site fundamental period s1 (rock) a+b vs30 ≥ 750 m/s tf < 0.15 s s2 (stiff soil) c 360 < vs30 ≤ 750 m/s 0.15 ≤ tf < 0.35 s s3 (soft soil) d 180 < vs30 ≤ 360 m/s 0.35 ≤ tf < 0.75 s s4 (very soft soil) e vs30 ≤ 180 m/s tf ≥ 0.75 s http://www.crsmd.lis.ucr.ac.cr/?id=estaciones http://www.crsmd.lis.ucr.ac.cr/?id=estaciones 387seismic site classification of the costa rican strong-motion network based on vs30 measurements and site fundamental period −86˚ −85˚ −84˚ −83˚ 8˚ 9˚ 10˚ 11˚ 75 km nicaragua p a n a m a soil type s1 (rock) s2 (hard soil) s3 (soft soil) s4 (very soft soil) 10 km figure 6. map with the geographical distribution of the accelerometric stations with the assigned soil class. 388 luis a. pinzón, diego a. hidalgo leiva, aaron moya-fernández, victor schmidt-díaz, luis g. pujades. values less than two, while sites s2, s3 and s4 have amplification values around three in the range of periods assigned on each site class (see table 1). the h/v spectral ratio for s4 soils has a peak zone from 0.9 to 1.5 seconds that is reasonably consistent with the definition of this site class being dominant periods larger than 0.75 seconds. figure 7. (a) number of stations per site class and (b) mean hvsr from all the stations per site class. conclusions in this study, a site classification has been assigned to the stations of the costa rican strong-motion network. a total of 157 stations were classified based on vs30 measurements and the site fundamental period values obtained from hvsr of recorded ground-motions. this classification arises from the relationship between the tf and the vs30 values measured in 52 stations. because the current classification is based on the vs30, the proposed one has the advantage that stations that do not have vs30 measurements can be classified using the tf calculated from the earthquake acceleration records. with this classification, since it considers the fundamental period, we guarantee that sites assigned in the same class have an equivalent dynamic response. this fact plays an important role in the assessment of structures when ground-motion records are used for deterministic and/or probabilistic time-history analyses, decreasing uncertainty and improving the results for a specific region. the influence of this classification on the definition of new ground motion prediction model, compared to the existing/traditional ones remains a significant and interesting issue. although we are aware of the limitations of the vs30 as a parameter to classify sites, based on the results obtained in this study, we were forced to use it in this study since it is the basis of the current regulations. in this sense, we would like to recommend to those responsible of the seismic regulations to reconsider using vs30 as a measure to classify sites. from our perspective, this parameter has clear deficiencies, which do not allow us to accurately classify soft soil sites (t ≥ 0.5 s), as is observed in figure 4. therefore, we believe that it is necessary to use measures that guarantee a reliable site classification, grouping sites with similar geodynamic characteristics, as is the case of the use of the site fundamental period as proxy. acknowledgements this research was partially funded by the national emergency and risk prevention law n° 8933 from costa rica and the ucrea funds from the university of costa rica through the project referenced as b9780. references alfaro, a., pujades, l. g., goula, x., susagna, t., navarro, m., sanchez, j., & canas, j. a. 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(2006) an empirical site-classification method for strong-motion stations in japan using h/v response spectral ratio. bulletin of the seismological society of america, 96, 914–925. doi: 10.1785/0120050124 10.1016/j.soildyn 10.3390/geosciences 10.1016/j.soildyn factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models mahdieh valipour, neda mohseni*, seyed reza hosseinzadeh department of geography, ferdowsi university of mashhad, mashhad, iran *corresponding author: nedamohseni@um.ac.ir keywords: boosted regression tree; erosion; prediction; reliability; support vector machine; palabras clave: árbol de regresión optimizado; erosión; predición; confiabilidad; máquinas de vectores de soporte; issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.95748 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 423-432 g eo lo g y record manuscript received: 13/05/2021 accepted for publication: 03/11/2021 abstract soil degradation induced by gully erosion represents a worldwide problem in the many arid and semi-arid countries, such as iran. this study assessed: (1) the importance of variables that control gully erosion using the boruta algorithm, (2) the relationship among causative variables and gullied locations using the evidential belief function model (ebf), and (3) gully erosion development using the algorithms of boosted regression tree (brt) and support vector machine (svm). based on the results of the boruta algorithm, slope, land use, lithology, plan curvature, and elevation were the most important factors controlling gully erosion. the results of the ebf model showed the predominance of gully erosion on rangeland and loess-marl deposition. the predominance of gullied locations on the concave positions, with the slope of 5°–20° in the vicinity of drainage lines, illustrates a preferential topographic zone and, therefore, a terrain threshold for gullying. the correlation of gullied locations with rangelands and weak soils in concave positions demonstrates that the interactions among soil characteristics, topography, and land use stimulate a low topographic threshold for gullies development. these relationships are consistent with the threshold concept that a given soil, land use, and climate within a given landscape encourage a given drainage area and a critical soil surface slope that are necessary for gully incision. furthermore, the brf-svm had the highest efficiency and the lowest root mean square error, followed by brt for predicting gully development, compared with ln-svm algorithm. the application of two machine learning methods for predicting the gully head cut susceptibility in northern iran showed that the maps generated by these algorithms could provide an appropriate strategy for geo-conservation and restoration efforts in gullying-prone areas. factores que afectan los umbrales topográficos en la ocurrencia de erosión y su manejo a través de modelos predictivos de aprendizaje automático resumen la degradación del suelo por erosión representa un problema generalizado para aquellos países con suelos áridos y semiáridos como iran. en este estudio se miden los siguientes aspectos: 1. la importancia de las variables que controlan la erosión a través del algoritmo de boruta; 2. la relación entre causales y los lugares erosionados a través del modelo de confianza (ebf, del inglés evidential belief function model), y 3. desarrollo de la erosión a través de los algoritmos árboles de regresión potenciado (brt, boosted regression tree) y máquinas de vectores de soporte (svm, support vector machine). con base en los resultados del algoritmo de boruta, la inclinación, el uso del suelo, la litología, la curvatura y la elevación son los factores más importantes en el control de la erosión. los resultados del modelo de confianza muestran la predominancia de la erosión en los pastizales y en las deposiciones de marga de loess. la predominancia de lugares erosionados en puntos cóncavos, con una pendiente de entre 5 y 20 grados junto a líneas de drenaje, ejemplifica una zona topográfica preferencial y, además, un umbral en el terreno para la erosión. la correlación de zonas erosionadas con pastizales y suelos débiles en posiciones cóncavas demuestra que las interacciones entre las características del suelo, la topografía, y el estudio del suelo estimulan un umbral bajo para el desarrollo de la erosión. estas relaciones se enmarcan en el concepto de que ante un tipo de suelo dado, el uso que se le brinde y el clima en un paisaje específico se crea una área de drenaje y una pendiente con superficie de suelo crítico, necesarios para un corte erosionado. además, los algoritmos brf-svm tuvieron la mayor eficiencia y el menor error cuadrático medio, seguido por el brt en la predicción del desarrollo de erosión frente al algortimo ln-svm. la aplicación de dos métodos de aprendizaje automático para para predecir la susceptibilidad de corte en el norte de irán muestra que los mapas generados por estos algortimos pueden proveer una estrategia apropiada para la geoconservación y los esfuerzos de restauración en zonas propensas a la erosión. how to cite item: valipour, m., mohseni, n., & hosseinzadeh, s. r. (2021). factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models. earth sciences research journal, 25(4), 423-432. https://doi.org/10.15446/esrj. v25n4.95748 mailto:nedamohseni@um.ac.ir https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.95748 https://doi.org/10.15446/esrj.v25n4.95748 424 mahdieh valipour, neda mohseni, seyed reza hosseinzadeh introduction detachment and transportation of soil particles by overland flow are the most important causes of land degradation in water erosion-prone environments (ollobarren et al., 2016). different stages of linear erosion development, such as rilling and gullying, irreversibly affect the health and resilience of soil systems (su et al., 2010; chaplot et al., 2005). gullies are defined as deep channels formed by accumulated overland flows that multiple mechanisms, such as cutting, penetration, and tension gap progression, encourage their development. these erosional landforms commonly develop on hillslope due to the simultaneous effects of multiple geo-environmental variables, including land cover, land use, lithology, topography, soil type, and climate (gayen et al., 2019). soil degradation induced by water erosion is the most critical challenge faced by many of the world’s dryland regions, such as iran. iran is recognized as the second in the world in terms of soil erosion where approximately 2.5 billion tons of fertile lands are lost per year. gully erosion can stimulate multiple environmental hazards, such as desertification, increasing sediment load in rivers and reservoirs, flood, and soil productivity loss (fox et al., 2016; ekholm and lehtoranta, 2012). although this hazard occurs on a small scale, its consequences will have a substantial impact on global scales. for example, gully erosion with disturbing the condition of sequestration and decomposition of soil organic carbon encourages instability in the atmospheric carbon dioxide concentrations and influences climate change (xiao et al., 2017; yigini and panagos, 2016). to minimize this hazard and the related environmental problems, it is necessary to understand mechanisms controlling gully development. furthermore, determining the magnitude and spatial distribution of erosion susceptibility zones can help to implement geo-conservation and management efforts for mitigating disasters associated with this hazard. the gully erosion susceptibility assessment is the first step towards geoconservation and restoration efforts in gully-prone areas (conoscenti et al., 2014). multiple quantitative and qualitative models have been proposed to estimate this hazard, such as the limburg soil erosion model, universal soil loss equation, european soil erosion model, chemical runoff and erosion for agricultural management system, water erosion prediction project model, bivariate statistical models, and logistic regression. these models could not consider the interactions among factors controlling gully erosion, such as topographic variables, geo-environmental factors, soil conditions, land use/ cover, sediment yield, and climatic indices. compared to bivariate and logistic regression models, machine learning algorithms are the ideal models to assess erosion susceptibility, which efficiently predict the probability of gullying with high accuracy (micheletti et al., 2014). to date, some studies have predicted potential erosion-prone areas in watersheds using these models. chen et al., (2021) predicted the gully erosion within a watershed located in iran using boosting ensemble machine learning algorithms. lei et al., (2020) evaluated gully erosion susceptibility in a catchment located in iran using four data mining techniques, including random forest, credal decision trees, kernel logistic regression, and best-first decision tree. arabameri et al., (2020) mapped erosion susceptibility by applying four models of support vector machine, artificial neural network, general linear, and maximum entropy in the golestan dam basin, iran. saha et al., (2020) delineated the areas with the most severe gully erosion susceptibility using the algorithms of tree ensemble, random forest, and gradient boosted regression tree. pourghasemi et al., (2020) assessed the efficacy of multiple machine learning algorithms to predict gully erosion occurrence. amiri et al., (2019) evaluated the importance of factors controlling gully development within a watershed and mapped its susceptibility using machine learning algorithms. gayen et al., (2019) produced a gully erosion susceptibility map in the pathro catchment, india using different machine learning algorithms. garosi et al., (2019) compared the reliability and discrimination of four machine learning models to map gully erosion susceptibility in western iran. in the present study, the importance of conditioning factors in gully erosion occurrence was assessed using the boruta algorithm, as a feature selection algorithm that acts independently from the predictive models of gully erosion to determine the most important factors, while previous studies selected the most important causative factors based on the derivatives of the employed models. in the next step, the evidential belief function model was employed to evaluate the relationship between gullied locations and conditioning factors. finally, considering the importance of this hazard and its global consequences, the present study employed predictive machine learning techniques, including support vector machine with two kernel types and boosted regression tree for modeling the occurrence of gully erosion within a prone watershed located in northern iran. finally, the predictive performance of the models was evaluated in terms of their discrimination and reliability. materials and methods study region the present study was implemented on gorganrud watershed located in golestan province, northern iran (37◦30′00″ to 37◦50′00″n and 55◦31′40″ to 56◦02′10″e) (figure 1). the maximum and minimum altitude of the watershed is 2,180 and 46 meters above sea level. based on the iranian meteorological organization, the study area’s climate is semi-arid with an average annual temperature of 18.2 °c and 385 mm mean annual precipitation. more than half of the region exhibits mountainous morphology belonging to the alborz mountains, with a slope between 0-61°. figure 4 illustrates the most important characteristics of the study region in terms of land use/cover and lithological structure. rangeland is the dominant pattern of land use in the study region. in addition, loess deposits are the main deposits over the region. based on the soil taxonomy system, the soil type of the studied region is classified under the mollisol order. these factors caused that over 70% of the region suffered from various degrees of soil erosion as multiple rills and gullies. mean annual soil losses caused by the gully erosion in the region estimate approximately 160 tons per hectare. the watershed area in the upslope parts of the rill and gully sites is variable between 2,000-43,070 m2. this condition strongly affected the rate of runoff discharge into rill channels, encouraging the accelerated progression of the channels (figure 2). figure 1. location of the study watershed (a) and gully erosion inventory map (b) 2.2 methodology the methodology employed in the present study includes the following steps: after the generation of the gully inventory map, information on twelve factors controlling gully development was prepared. in the next step, the spatial 425factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models correlation among gullied locations and causative variables was determined using the evidential belief function algorithm (ebf). the importance of causative factors was then weighed using the boruta algorithm. in the final steps, the spatial prediction of gully erosion susceptibility was modeled using the support vector machine (svm) model with two kernel types (linear kernel and radial basis function) and boosted regression tree (brt) algorithm. the predictive performance of models was discriminated using the receiver operating characteristic curve (roc) and the area under the curve (auc). the reliability of model accuracy was also evaluated using the coefficient of determination (r2, for the calibration) and root mean square error (rmse). gully inventory map the first step for the modeling process is preparing the gully inventory map, exhibiting the spatial distribution of gullied locations within the study watershed. considering the present and historical distributions of gullies, the future risk of the gully development can be predicted. based on data taken from the natural resources organization of golestan province, gullied locations were determined. then, these data were validated through field surveys and google earth images. overall, 1,041 gullied areas were mapped in the study region. to split the gully data into two datasets of validation (30%) and train (70%), a randomly partitioned algorithm was employed using the sub-set features tools in arcgis (figure 1b). gully conditioning factors gully erosion is a threshold-dependent process, which different conditioning factors stimulate the occurrence and development of this hazard (gayen et al., 2019). to predict gully erosion, using different machine learning models, the identification of factors affecting gully development is an important step. based on previous studies (zabihi et al., 2018; arabameri et al., 2018) and field survey, twelve conditioning factors were considered as independent variables affecting erosion. figure 2. typical images taken from some gullies within the study region these variables included elevation, slope gradient, slope aspect, topographic wetness index (twi), stream power index (spi), plan curvature, drainage density, distance from the river, distance from the road, distance from the fault, lithology, and land use/cover. the most important topographic factors that affect gully erosion are the elevation and slope gradient (conoscenti et al., 2014). these factors significantly control vegetation density, climatic conditions, surface runoff, and drainage intensity. the slope aspect is another factor that plays a crucial role in the gully erosion process. this factor regulates the drying effect of winds, morphological structure of the watershed, and rate of rainfall, and related runoff, which in turn, influences the occurrence of gully erosion (gayen et al., 2019). the erosive power of overland flows can be illustrated by the stream power index (eq. 1), which considers contributing area and slope. the soil moisture conditions within different slope positions can be evaluated by topographic wetness index (eq. 2). change in this index affects the erosive power of overland flow. the convexity and concavity of a watershed are determined by plan curvature (i.e. different watershed positions), which influences gullying. all the terrain variables were extracted from the 20 m aster dem. spi and twi were calculated as (bell et al., 1995; moore et al., 1993): (1) (2) where ac is the upslope area that drains through a certain point per unit contour length, which is equal to a certain grid cell width (raduła et al.,2018). the occurrence of gully erosion is most probable along the fault, river, and road due to erosion and ground instability. therefore, the maps of drainage density, distance from rivers, faults, and roads were prepared using a 1:25,000-scale topographical map. the map of drainage density was generated in arcgis (line density tools). the maps of distance from fault, river, and road were generated using the euclidean distance function in arcgis. the lithology map was produced based on the geological maps on a scale of 1:50,000. the land use/cover was mapped by using landsat 8 satellite and google earth images. figure 3 and table 2 show the classification of all the conditioning factors. analysis of multi-collinearity among independent variables the multi-collinearity analysis represents the linear correlation among the measured independent variables. a very high correlation among factors encourages multi-collinearity. to determine the multi-collinearity of the factors, the variance inflation factor (vif) and tolerance (tol) were employed. the vif greater than 5 and tol less than 0.2 represent a high correlation between the variables. since these high correlations reduce the accuracy of the results, factors with this characteristic should be removed from the final analysis. the following equations represent the calculation of vif and tol. (3) (4) where represents the regression value of j on other variables. assessment of correlation between independent variables and gullied locations the ebf algorithm was employed to assess the relationship between the independent variables and gullied locations. this algorithm is based on the dempster-shafer theory (dempster, 1967; shafer, 1976), which evaluates the uncertainty sources affecting the occurrence probability. this statistical model includes the degree of disbelief (dis), the degree of belief (bel), the degree of plausibility (pls), and the degree of uncertainty (unc) (althuwaynee et al., 2014). the pls and bel were defined as the lower and upper limits of probabilities. the uncertainty degree (unc) determines based on bel pls. dis https://www.sciencedirect.com/topics/earth-and-planetary-sciences/support-vector-machine 426 mahdieh valipour, neda mohseni, seyed reza hosseinzadeh illustrates the degree of disbelief based on 1 – unc – bel or 1pls. the sum of unc, dis, and bel is calculated as 1 (lee et al., 2012; carranza et al., 2005). further information on this algorithm can be seen in the research of park (2011). evaluation of the importance of independent variables the selection of variables that have the most importance in gully erosion is a crucial step in the modeling process. the importance of the independent variables controlling gully erosion was determined by applying the brouta algorithm. boruta algorithm is a variable selection algorithm. it is a wrapper algorithm around random forest (liaw and wiener, 2002). when a dataset comprised of multiple variables is given for modeling, this algorithm can determine the most important conditioning variables. machine learning models for predicting gully erosion the support vector machine model was initially presented as a supervised learning technique (binary classifier) that can consider linearly multi-dimensional and non-separable datasets (kalantar et al., 2017; kavzoglu et al., 2013). this model generates functions from a training dataset and can distinguish classes in high-dimensional feature space. in the present study, the factors controlling gully erosion (as different thematic layers) are considered as the high-dimensional feature spaces. then, the optimal hyper-plane maximizes the margin to split into two classes, including nongullying and gullying. the optimal hyper-plane was computed based on the equations below. (5) (6)} figure 3. independent variables selected for assessing gully erosion susceptibility figure 3. (continued) where is the penalty factor; represents lagrange multipliers. regarding the present study, as a vector of input space includes factors controlling gully erosion. the non-gullied and gullied pixels were determined as +1 and −1, respectively. this model decreases the possibility of over-fitting error and linearity and, therefore, has high efficiency to process data with nonlinear interactions by the kernel function (naghibi et al., 2015). this algorithm has several kernel types to measure the errors that can significantly influence the prediction performance of the model and the result accuracy. the most important kernel functions of the svm are: polynomial kernel, sigmoid kernel, radial basis, and linear kernel. this study compared the efficiency of the radial basis function (rbf-svm) (eq. 7) and linear kernel (ln-svm) (eq. 8) for modeling the gully erosion susceptibility. (7) (8) where γ, d, and r are defined as kernel width, polynomial degree, and parameter of the kernel functions, respectively (pradhan, 2013). boosted regression tree model (eq. 9) adaptively combined machine learning techniques to produce an appropriate performance (elith et al., 2008). brt combines different regression algorithms and boosting builds to reduce the final model variance and enhance predictive accuracy (aertsen et al., 2010). this model, with large volume of inputs, stimulated the speed in data processing and, subsequently reduced sensitivity to over-fitting. (9) 427factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models where , m, and are defined as a classification function with α parameters and x variables, the stage of the model, and the weighting factor m, respectively. to process the models, the spatial correlation among conditioning variables and gullied locations was first calculated. in the next step, the data was transformed into the r statistical software. finally, the maps of gully erosion susceptibility were generated in the arcgis software based on the outputs of the models. the susceptibility maps were classified into five categories, including low, very low, moderate, high, and very high susceptibility, using natural break algorithm in arcgis. evaluation of discrimination and reliability of the models the predictive performance of models was discriminated using the roc and auc curves. the roc curve plots sensitivity (x-axis) against 1 specificity (y-axis), where sensitivity is the true positive rate and specificity is the true negative rate. auc values close to 0.5 reflects the predictive ability of a random model, whereas value close to 1.0 reveals perfect accuracy. in this study, the auc values were classified into four levels, including moderate (0.6 to 0.7), good (0.7 to 0.8), very good (0.8 to 0.9), and excellent (0.9 to 1.0). the present research plotted the roc curve based on both datasets of the train (70% of gullied locations) and validation (30% of gullied locations). the following equations were employed to draw the roc curve: (10) (11) where tp, fn, tn and fp are the true positive, false negative, true negative and false positive rates, respectively. reliability of model accuracy was then evaluated using rmse (eq. 12) and r2. rmse is defined as the differences between the values actually observed (i.e. gullied locations in the region) and values predicted (i.e. gullyprone areas based on the model predictions). a lower value for rmse and a higher value for r2 show the robust performance of the models (garosi et al., 2019) (12) where y (actual values of dependent factor), ỹ (predicted values of dependent factor), and n (sample size). results and discussion multi-collinearity between variables controlling gully erosion as shown in table 1, tol and vif values for all the variables were higher than 0.2 and less than 5, respectively. the greatest values for vif and the lowest coefficients for tol were 2.38 and 0.42, respectively. this result illustrates no collinearity problem among the twelve independent variables and, therefore, all the factors were considered for the modeling process. table 1. results of the collinearity among independent variables factor tolerance vif elevation 0.58 1.72 slope 0.89 1.12 aspect 0.96 1.04 river density 0.49 2.03 lithology 0.84 1.19 land use 0.92 1.09 distance from river 0.44 2.25 distance from fault 0.55 1.81 distance from road 0.73 1.36 plan curvature 0.42 2.38 topographic wetness index 0.96 1.04 stream power index 0.96 1.05 importance of conditioning variables and their relationship with gully erosion table 2 shows the spatial correlation between the independent variables and the spatial distribution of gullied locations according to the results of the ebf method. the highest ebf value was observed in the elevation class of 472-899 m asl (bel= 0.54), followed by the elevation classes of 46–472 m asl (bel= 0.39) and 899-1,326 m asl (bel= 0.23). no gullying occurred in the highest elevation class, i.e., 1,753–2,180 m asl (bel= 0). these findings showed that there is a negative correlation between elevation and gullied locations so that the greatest gullying occurred in lowland positions (values of plan curvature). the highest gullying occurred in the slope classes of 5-10° (bel= 0.88) and 10-20° (bel= 0.61). furthermore, gullies frequency significantly decreased in the slope classes higher than 20° (bel= 0.30). the most frequent occurrence of gullies was observed in the northeast, east, southeast, south, southwest, and northwest directions. the highest values for river density were observed in the classes of 1.01-1.26 m/m2 (bel= 0.53) and 0.76-1.01 m/m2 (bel= 0.44). in the case of lithological units, gully erosion showed its highest occurrence in weak textured soils, i.e., loess (bel= 0.76) and marl (bel= 0.57). based on table 2, the highest and the lowest gullying occurred in the rangeland (bel= 0.73) shrubland (bel= 0.41) and forest (bel= 0.12), respectively. the results of distance from rivers showed that the occurrence of gully erosion increased as the distance to rivers decreased (bel= 0.53 for the class of 0-658 m and bel= 0.40 for the class of 658-1,515 m). furthermore, the gully erosion occurrence reached zero in the highest distance to rivers (classes of 4,189-6,341 m and 6,341-9,746 m). the ebf exhibited the highest weight of distance from fault in the class of 3,000-6,000 m (bel= 0.49). therefore, the ebf decreased (i.e. decreasing gullying) as the distance from the fault increased. the probability of gullying increased as distance to the roads decreased (bel= 0.59 for the class of 500-1,000 m and bel= 0.47 for the class of 0-500 m). these findings show the crucial effect of the road network on increasing the probability of gully erosion occurrence due to the anthropogenic destruction of natural hydrological processes that encourages the accumulation of surface runoff and accelerates broad-scale soil erosion. the ebf value for plan curvature was highest in the concave positions (bel= 0.61), compared with the convex positions (bel= 0.23). the highest ebf weights for twi were observed in the classes of 12-15.6 (bel= 0.36) and 9-12 (bel= 0.35). therefore, the probability of gullying enhanced in higher topographic wetness. further, the greatest gullying occurred in the highest spi (bel= 0.61 for the class of 0-7), illustrating the probability of gully erosion is higher in higher stream power. 428 mahdieh valipour, neda mohseni, seyed reza hosseinzadeh table 2. correlation among conditioning factors and gully erosion based on the ebf model factor class bel dis unc pls elevation (masl) 46 472.8 0.39 0.05 0.65 0.94 472.8 899.6 0.54 0.17 0.28 0.82 899.61326.4 0.23 0.09 0.76 0.90 1,326.4 – 1,753.2 0.03 0.51 0.45 0.48 1,753.2 – 2,180 0 0.16 0.83 0.83 slope (degree) 0 – 5 0.12 0.25 0.61 0.74 5-10 0.88 0.21 0.09 0.78 10-20 0.61 0.02 0.76 0.97 20 – 30 0.30 0.55 0.23 0.44 >30 0.34 0.00 0.74 0.99 aspect flat 0.06 0.54 0.38 0.45 north 0.09 1.08 -0.18 -0.08 northeast 0.21 0.41 0.47 0.58 east 0.22 1.66 -0.78 -0.66 southeast 0.25 0.78 0.05 0.21 south 0.27 0.14 0.67 0.85 southwest 0.24 0.12 0.73 0.87 west 0.01 1.14 -0.15 -0.14 northwest 0.21 0.75 0.134 0.24 river density (m/m2) 0 0.253 0.04 0.29 0.66 0.70 0.253 0.507 0.19 0.13 0.67 0.86 0.507 0.761 0.37 0.19 0.53 0.80 0.761 1.014 0.44 0.17 0.57 0.82 1.014 1.268 0.53 0.21 0.55 0.78 lithology limestone 0 0.00 0.99 0.99 colluvium 0.00 0.11 0.88 0.88 shale 0 0.11 0.88 0.88 conglomerate & sandstone 0 0.01 0.98 0.98 loess 0.76 0.10 0.73 0.90 marl 0.57 0.10 0.72 0.89 land use/cover cultivated land 0.31 1.45 -0.77 -0.45 forest 0.12 0.15 0.72 0.84 bare land 0.26 0.10 0.63 0.89 shrubland 0.41 1.45 -0.77 -0.45 water body 0 0.00 0.99 0.99 residential 0 0.01 0.98 0.98 rangeland 0.73 1.87 0.03 0.08 distance from river (m) 0 658.28 0.53 0.15 0.63 0.84 658.28 – 1,515.49 0.40 0.21 0.58 0.78 1,515.49 – 2,574.91 0.06 0.10 0.83 0.89 2,574.91 – 4,189.32 0.03 0.00 0.95 0.99 4,189.32 – 6,341.47 0 0.04 0.95 0.95 6,341.479,746.22 0 0.41 0.58 0.58 distance from fault (m) 0 – 3,000 0.18 0.31 0.49 0.68 3,000 – 6,000 0.49 0.10 0.60 0.89 6,000 – 9,000 0.16 0.11 0.72 0.88 9,000 – 14,000 0.18 0.11 0.70 0.88 14,000 – 19,000 0.13 0.11 0.74 0.88 19,000 – 23,000 0.03 0.11 0.85 0.88 23,000 – 36,363 0 0.12 0.87 0.87 distance from road (m) 0 – 500 0.47 0.01 0.50 0.98 500 – 1,000 0.59 0.21 -0.02 0.78 1,000 – 3,000 0.16 0.11 0.72 0.88 3,000 – 6,000 0.33 0.24 0.51 0.75 6,000 – 9,000 0.03 0.31 0.64 0.68 9,000 – 11,963 0 0.10 0.89 0.89 429factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models factor class bel dis unc pls plan curvature convex (> 0.15) 0.23 0.40 0.35 0.59 concave (< -0.15) 0.61 0.29 0.11 0.70 flat (-0.15 0.15) 0.16 0.11 0.72 0.88 topographic wetness index 3.5 – 6 0.15 0.02 0.82 0.97 6 – 9 0.23 0.32 0.44 0.67 9 – 12 0.35 0.43 0.30 0.56 12 – 15.6 0.36 0.21 0.42 0.78 stream power index -16 -10 0 0.02 0.97 0.97 -10 – 0 0.38 0.19 0.41 0.80 0 – 7 0.61 0.77 -0.39 0.22 based on the results of the boruta algorithm, slope gradient (25.65), land use (10.65), lithology (10.94), plan curvature (7.15), elevation (6.30), river density (5.30), distance from river (5.12), were the most important factors that controlled gully erosion susceptibility (table 3). the lowest importance variables for gully erosion susceptibility were slope aspect (1.53), distance from road (2.15), twi (2.19), distance from fault (2.91), and spi (2.93). table 3. importance of independent variables based on boruta algorithm factor mean importance median importance min importance max importance slope 25.65 25.15 10.88 22.24 land use 10.65 10.80 7.01 12.34 lithology 10.94 10.85 7.20 14.50 plan curvature 7.15 7.20 4.85 9.21 elevation 6.30 6.80 4.49 8.98 drainage density 5.30 5.10 3.45 7.20 distance from river 5.12 5.36 3.05 7.13 distance from fault 2.93 1.56 0.18 2.12 stream power index 2.91 2.87 1.87 4.00 topographic wetness index 2.19 2.17 -1.35 3.11 distance from road 2.15 2.19 1.24 3.58 aspect 1.53 2.89 1.76 4.39 as can be seen in the research of garosi et al. (2019), amiri et al. (2019), and rahmati et al. (2017), steep slopes, intensively cultivated hillslopes, and the presence of loess soils significantly encourage the formation and development of gullies in watershed environments. however, in the study region, the occurrence of gully erosion was predominant in the slope classes of 5°–10° and 10°–20° and lower elevations (472-899 masl), illustrating the most frequent erosion occurred in the lowland watershed positions. almost no gullying occurred in the upper positions and slopes higher than 20°. the predominance of gullied locations on the concave positions, with the slope of 5°–20° in the vicinity of drainage lines (factor of distance from the river), illustrates a preferential topographic zone and, therefore, a terrain threshold for gullying. furthermore, the results of the ebf model showed the predominance of gully erosion on rangeland and loess-marl deposition (figure 4). loess, due to weak structure and poor organic matter and nutrient content, and marl, with a high plasticity potential, significantly encourage the formation and development of gullies (razavi-termeh et al., 2020; arabameri et al., 2020; garosi et al., 2019). the highest distribution of the rangelands and loess-marl depositions occurred on concave positions of the study watershed where the slope angle tends to be lower. despite some studies showed the predominance of gullied locations on steep slopes, the findings of this study illustrated how land use/cover and lithological structure could stimulate the low topographic thresholds for gully development. the development of gullies on rangelands and weak textured soils within the lower concave positions of the study region is consistent with the threshold concept that “a given soil, land use, and climate within a given landscape encourage a given drainage area and a critical soil surface slope that are necessary for gully incision” (kakembo et al., 2009; poesen et al., 2002). considering given environmental conditions, when a certain topographic threshold is exceeded, gully heads develop. therefore, the threshold exhibits an inverse relationship between surface runoff discharge and critical surface slope for incision. assessment of erosion susceptibility using svm and brt models and their validation to predict gully erosion, the maps of erosion susceptibility were generated based on the results of the svm and brt algorithms and the related conditioning factors. these maps exhibited five susceptibility degrees for gullying (figure 5). the gully erosion susceptibility map obtained from the ln-svm model illustrated that 24.51% of the watershed area experienced very high susceptibility, while 24.49% and 7.01% of the watershed area exhibited low and very low susceptibility, respectively (figure 5a). the results of the rbf-svm model exhibited that 9.94%, 41.14%, 7.15%, 7.46%, and 34.30% of the watershed have very low, low, moderate, high, and very high susceptibility, respectively (figure 5b). further, based on the results obtained from the brt model, 9.64%, 24.33%, 34.84%, 7.44%, and 26.42% of the watershed have very low, low, moderate, high, and very high susceptibility, respectively (figure 5c). figure 4. spatial distributions of gullied locations in different types of land use (a) and lithological units (b) 430 mahdieh valipour, neda mohseni, seyed reza hosseinzadeh figure 5. gully erosion susceptibility maps based on ln-svm (a), rbf-svm (b), and brt (c) models the results of the two machine learning algorithms successfully assessed the gully erosion-prone areas within the watershed (figure 6). the validation results (auc values) for discriminating the predictive performance of the models illustrated that the two machine learning models were very good for delimiting gully-prone areas with high accuracy. the brf-svm (auc= 0.89) was the most robust model, illustrating the best prediction rate, while ln-svm did not show an acceptable accuracy (auc= 0.77). the brt model (auc= 0.85) was the second optimal model for predicting erosion susceptibility in the region. furthermore, the auc values of training datasets were 0.96 for the brf-svm model and 0.95 for the brt model. further, the results of statistical indices associated with the reliability of the models (rmse and r2) illustrated that the model of brf-svm, followed by brt had the lowest rmse and the highest r2 (i.e. the most reliable models to assess the gully erosion-prone areas) compared with the ln-svm (table 4). figure 6. the area under the curve (auc) based on train (a1-a3) and validation (b1-b3) datasets for discriminating the accuracy of ln-svm, rbf-svm, and brt models nonparametric methods such as svm and brt are appropriate approaches to solve problems related to modeling. these algorithms can manage the collinear relationship among conditioning factors. other researchers (amiri et al., 2019) expressed that the most important advantage of the brt is eliminating variables with a large number of missing values compared with other models. as some studies showed (pourghasemi et al., 2017; marjanović et al., 2011), the improvement of the auc values for validation and train data in different replicates is the most important advantage of the support vector machine algorithm. this characteristic caused this model could handle complex and nonlinear relationships, in comparison with models such as the artificial neural network. the findings of this study showed that the models of svm and brt with considering the interaction between independent variables could appropriately model gully erosion-prone areas with a high accuracy, as other studies reported (gayen et al. 2019; rahmati et al., 2017; elith et al., 2008). overall, the gully erosion susceptibility maps could provide an appropriate strategy for geo-conservation and restoration efforts in gully erosion-prone areas within the study watershed. table 4. reliability of model accuracy based on the coefficient of determination (r2) and root mean square error (rmse) model train data (70%) validation data (30%) r2 rmse r2 rmse svm (linear kernel) 0.34 0.43 0.13 0.46 svm (radial basis function) 0.91 0.23 0.76 0.32 brt 0.83 0.29 0.64 0.38 431factors affecting topographic thresholds in gully erosion occurrence and its management using predictive machine learning models the interactions among various variables such as terrain indices, lithological characteristics, and land use/cover changes affect linear erosion development as rilling and gullying, which irreversibly threaten the health and resilience of soil systems. this study shows the results obtained by applying two machine learning methods for predicting the gully head cut susceptibility in northern iran, as the second country in the world in terms of soil erosion. mean annual soil losses caused by the gully erosion in this country estimate approximately 2.5 billion tons. these models can be appropriate tools to understand mechanisms controlling gully erosion and, therefore, can help to implement geo-conservation and management efforts for mitigating disasters associated with this hazard. conclusions soil degradation induced by gully erosion is the most critical challenge faced by many of the world’s dryland regions. although this hazard occurs on a small scale, its consequences will have a substantial impact on global scales, for example, the impacts of gully erosion on the degradation of a large amount of oc-rich topsoil that is combined with deeper horizons poor in oc. this event stimulates carbon mineralization and, subsequently, affects the exchange of carbon between the atmosphere and the pedosphere and associated instability in the carbon dioxide concentrations of the atmosphere. therefore, to mitigate this hazard and the related environmental problems, it is necessary to understand the mechanisms controlling gully erosion. the results of this study showed the efficiency of two machine learning algorithms, including brt and svm, in the prediction of erosion susceptibility in northern iran. based on the results of the boruta algorithm, slope gradient, land use, lithology, distance from river, elevation, river density, distance from fault, plan curvature, and spi were the most effective factors that controlled the occurrence of gully erosion in the study region. further, the results of the ebf model showed the spatial relationship among causative variables and gullied locations. based on these findings, topographic thresholds for gully erosion tended to be lower on rangeland and weak textured soils, such as loess and marl. furthermore, the spatial correlation of gullying with rangeland and weak textured soils within concave positions illustrated that the interactions among soil characteristics, topography, and land use could stimulate a low topographic threshold for gullying. the validation results related to the statistical indices of the reliability and discrimination accuracy of the models showed that the two machine learning models were very good for demarcating gully erosion areas with high accuracy. the brf-svm was the most robust model (auc= 0.89; 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(2020). a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes. earth sciences research journal, 24(4), 459-472. doi: https://doi.org/10.15446/esrj.v24n4.72068 in this study, we tried to estimate the optimum linear equations among the parameters associated with different earthquake fault mechanisms for iranian earthquakes. for this purpose, we tested different curve fitting methods in order to present the most proper empirical relationships between several seismic parameters for different fault systems. in the present paper, 46 large and destructive iranian earthquakes whose magnitudes change between 5.8 and 7.8 from 1900 to 2014 were used for the analyses. a comparison was made by using four types of curve fitting techniques. the estimation procedures are considered as (1) l2 or least squares regression, (2) l1 or least sum of absolute deviations regression, (3) robust regression and, (4) orthogonal regression. confidence intervals were selected as 95% for all types of regression relationships. in the selection of the best probability distribution, we considered the correlation coefficients of the linear regressions as a powerful and conceptually simple method. correlation coefficients of all relationships change between 0.299 and 0.986 with orthogonal regression, between 0.168 and 0.792 with l1 regression, between 0.059 and 0.829 with robust regression. for iranian earthquakes, the most suitable and reliable empirical relationships between moment magnitude (mw) and surface wave magnitude (ms), mw and surface rupture length (srl), mw and maximum displacement (md), and srl and md were obtained by orthogonal regression since it supplies stronger correlation coefficients than those of the other regression techniques in most estimates. the results show that estimated empirical relationships among the different fault parameters by using the orthogonal regression method can be accepted as more up-to-date and more appropriate in comparison with the other regression norms. consequently, these equations were suggested as more reliable in the estimation of the maximum surface displacement, maximum surface rupture length and associated with the maximum credible earthquakes for different areas of iran. furthermore, obtained relationships can be statistically significant for the assessment of seismic, tectonic and geologic activities, and they can be used to evaluate the rupture hazard of the iranian plateau. abstract a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes comparación de técnicas de ajuste de curvas para diferentes parámetros de fallas en terremotos en irán issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.72068 en este estudio, los autores se enfocaron en estimar las ecuaciones lineales óptimas para los parámetros asociados con diferentes mecanismos de fallas de terremotos en irán. con este propósito se evaluaron diferentes métodos de ajuste de curvas para presentar la relación empírica más apropiada entre varios parámetros sísmicos para diferentes sistemas de fallas. en el presente artículo se analizaron 46 terremotos ocurridos en irán y con magnitudes entre 5.8 y 7.8, entre 1900 y el 2014. se realizó una comparación al usar cuatro tipos de técnicas de ajuste de curvas. la estimación de los procedimientos se consideraron así (1) l2 o regresión de mínimos cuadrados, (2) l1 o suma de mínimos en regresión de desviaciones absolutas, (3) regresión robusta y (4) regresión ortogonal. se seleccionaron intervalos de confianza del 95 por ciento para todos los tipos de relaciones de regresión. en la selección de la mejor distribución de probabilidades se consideraron los coeficientes de correlación de las regresiones lineales como un método fuerte y conceptualmente simple. los coeficientes de correlación de todas las relaciones cambian entre 0.299 y 0.986 con regresión ortogonal; entre 0.168 y 0.792, con regresión l1; y entre 0.059 y 0.829, con regresión robusta. para los terremotos en irán, las relaciones empíricas que más se ajustan y que son más confiables entre la magnitud de momento (mw) y la magnitud de onda superficial (ms), entre la magnitud de momento y la longitud de ruptura superficial (srl), entre la magnitud del momento y el desplazamiento máximo (md), y entre la longitud de ruptura superficial y el desplazamiento máximo se obtuvieron por la regresión ortogonal, ya que esta provee coeficientes de relación más fuertes que aquellos estimados por medio de otras técnicas de regresión. los resultados muestran que las relaciones empíricas estimadas entre los diferentes parámetros de falla al usar el método de regresión ortogonal podría ser aceptado como el más actualizado y más apropiado con respecto a otras normas de regresión. además, estas ecuaciones se sugieren como las más confiables en la estimación del desplazamiento máximo de superficie, de la máxima longitud de ruptura superficial y asociada con la fiabilidad máxima en terremotos para diferentes zonas de irán. adicionalmente, las relaciones obtenidas pueden ser estadísticamente significativas para la evaluación de las actividades sísmicas, tectónicas y geológicas, y pueden ser utilizadas para medir el riesgo de ruptura en el altiplano iraní. resumen record manuscript received: 08/05/2019 accepted for publication: 30/06/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 459-472 serkan öztürk1,*, mohammad r. ghassemi2, mahmut sarı1 1gümüşhane university, department of geophysics, tr-29100, gümüşhane, turkey 2research institute for earth sciences, geological survey of iran, iran *corresponding author: serkanozturk@gumushane.edu.tr ea rt h q ua k e en g in ee ri ng 460 serkan öztürk, mohammad r. ghassemi, mahmut sarı introduction fitting of data points by parametric curves and surfaces is considered in many scientific fields such as mathematics, statistics, earth and computer sciences. several standard statistical software has been used by the practitioners to fit the data sets and to estimate the suitable relationships between some response or dependent variable and hypothesized predictor or independent variables (öztürk, 2014). the researchers want to develop a physical model from an obtained relationship and therefore, they need to determine the value of the response variable based on the estimated relationships. in many situations, however, it is not possible that these types of relationships are valid just by supposing a mathematical model of the relationship. in addition, these variable selection of regression norms cannot be directly applied to contaminated data sets with an ordinary estimation technique. thus, it is necessary to be collected the data from the population of available data of these variables and then, an empirical relationship between the dependent and independent variables may be estimated from the data (giloni & padberg, 2002). in applied statistics, investigation of the validity of curve fitting methods to fit the observed data is a significant problem. despite the differences of existing curve fitting techniques, the principal of the most techniques is based on the classical optimization theory and methods. however, the researchers encounter practical curve fitting problems in many quantitative oriented disciplines, and the solution needs to find the most reliable solution to an over determined system of linear equations (cadzow, 2002). an effective and accurate alternative curve fitting technique has a great importance and serves as a basic module in practice. for this reason, the curve fitting is one of the most common tools in establishing the relationship between a response and an explanatory variable statistic for many social and engineering fields (durio & isaia, 2003). the ignoring of a significant component of variability and the equation error is one of the main problems with the usage of the curve fitting models for any data set. therefore, this problem can lead to either over or under correction for measurement error depending upon the relative sizes of the variances included. the estimation of correlation coefficient for the curve fitting models is a simple tool, and the assessment of the curve fitting can be succeeded by means of the estimation of some criterion such as the calculation of coefficient for the curve fitting model. many statistical analyzes show that it is not considered as a final model selection criterion. however, this calculation supplies an evidence of the conformity of the selected descriptive variables in estimating the response from the curve fitting model (renaud & victoria-feser, 2010). in many data processing applications, the linear equation systems under consideration are not consistent and researchers desired to estimate the most approximate solution. many curve fitting techniques have been used in these types of analyses, for example: l1 norm or least sum of absolute deviations regression (giloni & padberg, 2002), l2 norm or least squares regression (cadzow, 2002), robust regression (huber, 1964), total least squares or orthogonal regression (carroll & ruppert, 1996), geometric mean regression (gmr, leng et al., 2007), principle components regression (pcr, maronna, 2005), covariate adjusted regression (car, şentürk & nguyen, 2006) and least trimmed squares regression (lts, rousseeuw & leroy, 1987). the main purpose of this study is to estimate the most optimum and reliable curve fitting solutions for linear equations among the earthquake fault parameters of iranian earthquakes. in this context, a comparison among the first four curve fitting techniques mentioned above was made in order to provide the optimum statistical model between different variables. we did not discuss the other methods such as pcr, gmr, lts or car since they have been preferred in more distinctive applications and they have not generally been used in geophysical analyses. one can also find many details about all these curve fitting techniques in literature (e.g., branham, 1982; stefanski, 1991; hartmann et al., 1997; spiess & hamerle, 2000; shi & lukas, 2005). consequently, l1 norm, l2 norm, robust and orthogonal regressions were tested for iranian earthquakes in order to provide newer and more suitable empirical relationships among different earthquake fault parameters. by using these different statistical regression techniques, we aimed to make a thorough and consistent appraisal of maximum surface rupture length, maximum surface displacement and associated with the maximum credible earthquakes for different parts in iran. also, regional differences in these faulting parameters can be evaluated in terms of geologic, seismic and tectonic variations as the preliminary results of the future studies for the study region. mathematical background of the curve fitting methods curve fitting problems are classified within the category of mathematical problems, and many different distance functions or metrics have been utilized to carry out linear curve fitting process. there are many mathematical techniques for the solution of these types of problems. however, since our aim is only to evaluate different models and to estimate the optimum relationships, we will not discuss the mathematical background in detailed. for this reason, we will give short and principal explanations for four different curve fitting models in this section, as well as their basic statistical features and quality of fit. the curve fitting process for a given data set has a great importance in statistical data analysis. these types of data analyses are naturally related to distances in euclidean geometry and by considering the linear algebra, an analytical result may be possible. some metrics or distance functions may be useful to perform a linear equation to the data. in order to analyze a linear equation system, it is accepted m observations or measurements on the dependent variable y and some number n ≥1 of independent variables x1,….,xn. each one of them is also known m values. it can be given as (giloni et al., 2006): y y y x m n m n m =             =          1 1 1 1 1 . . , . . . . . . χ …… χ χ …… χ        =                 = ……( ) x x x x m n 1 1 . . . , , (1) where y  r is a vector of m measurements and x is an m x n matrix of real frequently termed as the design matrix. in addition, x1,….,xn are column vectors with m components and x1,….,xm are row vectors with n components corresponding to the columns and rows of x, respectively. linear equation system can be formulated statistically as in the following (durio & isaia, 2003): y x= +β ε (2) where t = (1,……,n) is the vector of parameters of the linear system and t=( 1 ,……,m) a vector of m random variables corresponding to the error terms in the suggested equation. an upper index t denotes “transposition” of a vector or matrix. the dependent variable y is a random variable in the hypothesized model. the observations include some “noise” or observation errors and they are taken in the error terms e. however, we can write it for the linear equation that we want to solve: y x r= + (3) where given several randomly fixed parameter vector b, the components ri of the vector rt=(r 1 ,……,rm) are the residuals that result, given the measurements y, a fixed design matrix x, and the chosen vector   rn. consequently, the residuals, r, are in terms of the hypothesized model, realizations of the random error terms e given the particular measurements y and parameter settings b. given y and x, the main purpose in linear fitting model is to estimate the parameter settings   rp . this means that several suitable measure of the dispersion of the resulting residuals r  rm must be as small as possible (giloni & padberg, 2002). it has completely possible that, e.g., x j1 1= , for all j  {1,……,m} in the design matrix x (giloni & padberg, 2002). in this situation, 1 is attributed as the “intercept term” and it is corresponded to the situation in the two parameter case, i.e., when n=2. if x j1 1= , for all j  {1,……,m} and n=1, the problem of estimating a “best” fitting scalar b1 means that we desire several good measure of “centrality” of the measurements y. least squares method (l2 norm) least squares (ls) method is the most preferred and the best well-known regression technique. this method is the most basic form of the ls optimization problems, and there are many applications in mathematics and statistical data analyses besides other scientific fields. the model estimates  are determined 461a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes well known that the l1 technique can resist some large errors in the data y for unlimited problem (shi and lukas 2005). when a small part of the data being evaluated is not reliable (i.e., contains data outliers), l1 model is suitable. l1 model application may ignore some bad data points if it emphasizes the larger part of data points (cadzow, 2002). l1 model calculation of b is the solution to the problem for the linear model which has linear constraints: minimize e r e rn t n t+   subject to x r r yβ − −+ =+ , (8)  free, r r+ ≥ ≥−0 0, in the equation (8), the residuals r of the general formulation (3) are simply replaced by a difference r+-rof positive variables. the non-differentiability of the exact value target function is dissembled by the target function of (8). however, it accurately captures the target function of l1 norm. because of the non-differentiability of the criterion function, l1 estimation is more difficult than l2 estimation. the statistical hypothesis improved for l1 method is less advanced than it is for l2 method (rosenberg and carlson 1977), whereas we have the formula β −ls t tx x x= ( ) 1 for the l2 norm estimates ls. it allow us to obtain the sample distribution of ls from the error distribution, and the dependence of the l1 norm estimator on the errors is more complex. the asymptotic theory for l1 norm is not as well-developed as for l2 norm. this theory is true to a certain degree. it is also true for high-breakdown model estimators. in addition, l1 norm estimator is not at all robust to measurements with unusual predictor values; that is, it has a low breakdown point (giloni et al., 2006). also, one can see arthanari & dodge (1981) for further mathematical background and discussion of the l1 approach. orthogonal method (total least squares method) orthogonal regression (or) is one of the best known methods for errorsin-variables estimation in the simple linear curve fitting technique. this method is also sometimes named as the functional maximum likelihood estimator under the limitation of known error variance ratio. in an ordinary linear curve fitting model, the aim is to minimize the sum of the squared vertical distances between the y data values and the corresponding y values on the fitted line. however, the aim in or is to minimize orthogonal (perpendicular) distances from the data points to the fitted line. or is derived a pure observation error perspective. it is supposed that there are theoretical constants y and x which are linearly related through (carroll & ruppert, 1996): y x= + 0 1 (9) formulation (9) shows that y and x would be exactly linearly related if they could be measured. in a classical or technique, instead of measuring (y, x), carroll & ruppert (1996) estimated them by corrupted by observation error: y y= +  w x u= + (10) where e and u are independent mean zero random variables with variances σε 2 and u 2, respectively. equations (9) and (10) are combined, the result can be written as: y x= + +β β ε0 1 (11) the knowledge of error variance ratio is necessary for or techniques: η   σ σ ε= ( ) ( ) = var y x var w x u 2 2 (12) equation (12) is based on a sample of size n y w xi i i i n, , ,( ) =1 and the course of x’s are unknown and unobserved. or estimator is estimated by minimizing: by minimizing the sum of squared residuals under the euclidean (or l2) norm x j2 2= ∑  , i.e. the aim is to estimate the parameters   rp such that: s r y x y x y y x y x xt t t t t t t= ( ) ( ) = ( ) + ( )− β − β − β β β2 (4) must be the minimum. this transformation is uniform and optimality is not affected from it. this minimized equation is positive semi-definite and thus, the first-order conditions achieve the process. for example, in order to minimize s, its gradient ∇s in accordance with  should be computed and adjusted as zero or can be formulated as (e.g., cadzow, 2002; giloni & padberg, 2002): 1 2 0∇ − βs x y x xt t= + ( ) = (5) the statement x x xt t( ) =  should be analyzed for b and is named as normal equations for l2 norm (giloni & padberg, 2002). supposing that the rank of x is n, i.e., that r(x)=n, it means that x xt( )1 exist and the ideal β= βls is stated as: β −ls t tx x x y= ( ) 1 (6) i.e., this regression yields an excellent level. the solution process is focused on the inversion of xtx, but a quantitative analyzer should accomplish the possible singularity of xtx. the statistical curve fitting techniques have been applied densely for a very long time. although some significant progress has been made to evaluate the goodness of fit (gof) besides the other statistical features of the linear curve fitting techniques, some features of the statistical curve fitting techniques require generally poor assumptions (sen and srivastava 1990). the estimates ls in the ls can be accepted as the best estimators when the error terms in the model have a normal distribution. however, the ls estimates depend generally on the presence of the second moment of the error distribution. as a result, this regression model is especially useful for all cases in which involve the analyses of great data sets and handle great samples with the constant numbers of outliers (cadzow, 2002). least sum of absolute deviations method (l1 norm) it is well-known that the ls method is very sensitive to unusual measurements in a data. hence, a large number of robust estimators have been suggested as alternatives, and the least sum of absolute deviations (l1) technique was one of the earliest among them. in this method, coefficients are calculated by minimizing the sum of the absolute values of the residuals. because l1 norm can be highly affected by a single measurement, it has been mostly disregarded as a robust alternative to l2 norm. it has many names in the literature such as lad (least absolute deviation), lae (least absolute error), lar (least absolute residual), lav (least absolute value), lsad (least sum of absolute deviations), mad (minimum absolute deviation), msae (minimum sum of absolute errors). l1 norm has an increasing interest as an alternative to l2 norm over the past 30 years or so, and it has been a new perspective since 1960s by different researches (e.g., fama, 1965; mandelbrot, 1967; blattberg & sargent, 1971; huber, 1987). more descriptive or remarkably illustrative results were provided in many cases by using the l1 norm. the estimates of the parameters   rp of the linear curve fitting technique are achieved by minimizing the sum of the absolute residuals (giloni & padberg, 2002): y x y xi i i n − β − β1 1 = = ∑ (7) instead of the l2 model, l1 model x x j1 = ∑ is used to minimize dispersion of the residuals. this target function is obviously non differentiable and computation does not assist (giloni & padberg, 2002). a significant advantage of l1 method in comparison with l2 regression is its stability. it is 462 serkan öztürk, mohammad r. ghassemi, mahmut sarı y x w xi i i i i n −β −β η −0 1 2 2 1 ( ) + ( ){ } = ∑ / (13) in the unknowns, namely  0 1 1, , , ,x xn…… . equation (13) is just the ordinary total euclidean or perpendicular distance of y wi i i n,( ) =1 from the line  0 1 1, , ,x xi i i n( ) = if =1. however, equation (13) is a weighted perpendicular distance if 1. if s sy w 2 2, and swy are the sample variance of the y’s, the sample variance of the w’s, and the sample covariance between the y’s and the w’s, respectively, or estimate of slope is (carroll & ruppert, 1996): (14) this well-known or estimator is explained in many studies such as madansky (1959), kendal & stuart (1979), weisberg (1985), fuller (1987) and leng et al. (2007). the use of or must include a careful analysis of equation error and is not only the usual estimation of the ratio of observation error variance. also, or fitting technique is an acceptable norm as long as  in equation (4) is specified truly. if its assumptions hold, or is a completely justifiable technique of estimation. because or method does not consider equation error, it frequently causes itself to misuse by careless as a technique (öztürk, 2014). robust method the most important issue in the ls regression is non-robustness to outliers. if the error distribution is not normal, especially if the errors are heavy-tailed, linear ls calculations can produce unexpected results. especially, if there are extremely incompatible data points, there can be a strong effect on the model parameters by these outliers and we can see it on the results. an easy solution can be obtained by repetitively weeding up these outliers and by calculating again the ls fit for the remaining points. another method is to operate a different regression model which is not as vulnerable as ls to incompatible data and it is called as robust regression (rr). this technique was introduced by huber (1964), and the best known and common technique for rr is m-estimation. if we take the general curve fitting model in equation (2) into account for the ith of m measurements: yi i i k ik i= + + +……+ +α β χ β χ β χ ε1 1 2 2 = +xi i ′β ε (15) the fitted model is formulated as: yi a b b b ei i k ik i= + + +……+ +1 1 2 2   = +x b ei i  (16) the target function is minimized by the general m-estimator as follow: ρ ρ − ′e y x bi i i i n i b ( ) = ( ) == ∑∑ 11 (17) if the derivative of r is accepted as y=r’, differentiating the target function according to the coefficients, b, and getting the partial derivatives to 0, yield a system of k+1 estimating equations for the coefficients: ψ − ′ ′y x b xi i i i n ( ) = = ∑ 0 1 (18) if we describe the weight function ω ψe e e( ) = ( ) / and  i ie= ( ), then the estimating equations can be given as follow: ω − ′ ′ i i i i n iy x b x( ) = = ∑ 1 0 (19) solving the estimating equations is a weighted ls problem, minimizing i ie 2 2∑ . however, the weights depend upon the residuals, the residuals depend upon the estimated coefficients, and the estimated coefficients depend upon the weights. one can find many details and mathematical backgrounds in huber (1964). nonlinear curve fitting techniques play a significant role in many fields. the classical ls method is generally preferred in many cases for calculating the parameters of a nonlinear model. however, it is well known that these classical techniques are frequently very sensitive to extreme values. most robust improvements on the estimation of curve fitting model are based on the generalizations of the ls or maximum likelihood methods. rr technique is less influenced by outliers. however, the methods of small sample asymptotes may be very helpful in evaluating the behavior of rr estimates. several of rr methods are also explained in many studies such as huber (1981), field (1997), sinha et al. (2003) and abdelkader et al. (2010). estimation of correlation coefficient for curve fitting methods in order to fit mathematical models to measured data, curve fitting applications have been extensively preferred by researchers in many different disciplines. when the error terms identically and independently show a normal distribution, the classic regression method of the ls is efficient. real data frequently include the extreme values and these data points are covered by a majority of the data while the non-measured random disturbances in a curve fitting method are generally accepted as normally distributed. many extreme values are controlled by genuinely thick-tailed or asymmetric error distributions, whereas these errors may be the result of observation mistakes or human recording error. thus, a useful solution for the problem is to minimize the sum of residual error magnitude. if data vector includes a small number of data outliers, the sum of error magnitudes is of particular use in many applications. in these situations, the sum of squared errors criterion is wrongly influenced by these data outliers and therefore often leads to a poor selection of the coefficient vector. in fact, separating the extreme values is not appropriate since they can represent the correct data generating process in these types of situations (boyer et al., 2003). the choice of an optimum probability function for a given data set is one of the most significant issues in curve fitting analyses. as seen in literature studies, different methods such as the chi-square test of homogeneity, the test of normality, kendall and spearman coefficients, bayesian interactive model, principal component analysis, mutual information coefficient, hoeffding method etc. can be applied and then, the most appropriate model can be selected. however, there is not any specific rule in the choosing the suitable distribution or parameter calculation methods. the choosing a suitable distribution in many cases is dependent on the gof assessment. the gof in a model can be defined as the technique for analyzing how well sample data compatible with an accepted probability distribution (öztürk, 2014). some tests for the gof have been improved and one of the best known and the most frequently used criteria among them is called as correlation coefficient. this term is given as r2 or sometimes r is used, and it has a significant role in curve fitting analyses. as stated in heo et al. (2008), it is not recommended to be used as a single tool for the gof, but it can supply an acceptable and quick solution. although correlation coefficient is only based on the covariance penalty, it is accepted as a substantially simple and effective tool. alternative models were not used in this study despite many techniques as suggested in literature since these types of applications have been used for more certain fields and they have not been used in geophysical applications. considering equation (2), we can assume that is the predicted value for yi in the selected model with p<q explanatory variables (based on a ls) and is the identical residual sum of squares. thus, gp can be given as (renaud & victoria-feser 2010): 463a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes (20) as mentioned above, a very simple and practice tool for the gof is also provided by determination of correlation coefficient for the linear curve fitting models. if we assumed that x is random, we can describe as the parameter which is the squared correlation between y and the best linear combination of the x (anderson, 1984): φ β2 2= ( )max ,corr y xt (21) the r2 is generally given as a quantity and it estimates the percentage of variance of the response variable explained by linear relationship. it is estimated as follow: (22) where ess, tss and rss denote the explained, total and residual sum of squares, respectively. if we have an intercept term in the linear model, estimation of r2 is essentially equal to the square of the r2 between yi and , i.e. (e.g., greene, 1997): (23) where is the average values of the measurements yi and is the fitted quantiles . this equation shows that r2 measures the gof of the linear model by its ability to predict the response variable and its ability measured by the correlation. moreover, this formulation denotes that the response distribution does not need to be gaussian to allow for the interpretation of r2. it is also a direct estimator of the population parameter of equation (3). the scale and location of r2 do not change and this is statistically independent of the standard deviation and average, . in fact, r2 supplies a quantitative evaluation of fit and shows the linearity of the probability plot. as an important result, if r2 is close to 1.0, it is assumed that the measurements can be constructed from the fitted distribution (heo et al., 2008). data catalog of iranian earthquakes and a brief definition of the seismotectonics the first aim of this study is to prepare a uniform data catalog taking the types of surface wave magnitude (ms) and moment magnitude (mw) into consideration. then, the second aim is to estimate the new empirical relationships among the seismicity parameters of different earthquake faulting mechanisms such as mw and surface rupture length (srl), mw and maximum displacement (md), and srl and md. the data set used in the scope of this work is updated form ghassemi (2016). table 1 shows the details of parameters for different faulting mechanisms and is modified from ghassemi (2016). he used 46 iranian earthquakes whose magnitudes vary from 5.8 to 7.8 and which occurred between the time intervals 1900 and 2014 (figure 1). as an initial step in ghassemi (2016), a catalog of iranian earthquakes related to direct loses was used and the number of initial earthquake catalog was reduced to 41 events. then, 5 more events were also added to complete the catalog and to extend it to 2014. the outcomes in the table 1. data set of fault parameters for iranian earthquakes between 1900 and 2014 (for details, see ghassemi, 2016). rlss: right lateral strike-slip, llss: left lateral strike-slip, nf: normal fault, tf: thrust or reverse fault, srl: surface rupture length, md: maximum displacement, vdc: vertical displacement component, hdc: horizontal displacement component no date longitude latitude rupture mechanism mw srl md 1 23.01.1909 49.13 33.41 rlss-nf 7.3 >40 >1 2 18.04.1911 57.03 31.23 tf 6.3 18 vdc. >0.5 3 01.05.1929 57.81 37.73 rlss 7.2 74 2 4 06.05.1930 44.60 38.24 nf-rlss 7.1 16-30 4 5 16.02.1941 58.87 33.41 rlss 6.3 8-10 0.5-1 6 27.11.1945 63.47 25.02 tf 7.8 7 23.09.1947 58.67 33.67 rlss 6.8 20 vdc. 0.3-0.8 hdc. 1 8 05.07.1948 57.73 29.88 rlss 6.1 9 05.10.1948 58.55 37.88 rlss 7.1 10 12.02.1953 54.88 35.39 tf 6.5 >8 vdc. >1.4 11 02.07.1957 52.47 36.07 tf 6.8 12 13.12.1957 47.82 34.58 tf 6.7 13 16.08.1958 48.17 34.30 nf-rlss 6.7 20 vdc. 1.5 14 01.09.1962 49.81 35.71 tf 7.1 80 vdc. 1.4 hdc. 0.6 15 31.08.1968 58.96 34.02 llss 7.1 80 vdc. 2.1 hdc. 4.5 16 01.09.1968 58.23 34.05 tf 6.4 17 30.07.1970 55.89 37.67 rlss 6.5 18 10.04.1972 52.98 28.38 tf 6.9 (continued) 464 serkan öztürk, mohammad r. ghassemi, mahmut sarı present study may differ from specific earthquakes which are used to estimate the relationships between mw and srl, mw and md, and srl and md since this catalog is not complete for srl and md. in general, magnitude scale is given as ms in the original iranian earthquake database. this data set includes 166 events for ms and mw types between 1962 and 2004. in order to prepare a uniform catalog covering mw scale, new relationships between mw and ms were firstly obtained and then, the other empirical relationships among the faulting parameters were formulated by using the most suitable curve fitting technique. one can see ghassemi (2016) for many details such as the geometry and kinematics of surface ruptures, the secondary ruptures associated with earthquake faulting mechanism, seismic properties of earthquake faulting and the previous relationships among these variables. iran is located within the alpine-himalayan orogenic belt in the active collision zone between the eurasia to the north and arabia to the south. the iranian plateau is consisted of different tectonic systems such as transpressive fold-and-thrust mountain belts with active strike slip and reverse faulting, an active subduction zone and recent volcanic activity; as a result, the plateau shows different crustal blocks of variable thickness and rigidity interspersed by relatively stable aseismic blocks of different sizes (berberian, 2014). the iranian plateau shows active deformation, high elevation and earthquake activity with complex interactions of active strike-slip faults and thrusts, and different destructive earthquakes recorded in historical and instrumental periods. the most significant tectonic structures are mostly accommodated by shortening and strike-slip faulting in the mountain belts and can be given as the great caucasus, zagros, alborz, kopeh-dagh, active makran subduction zone and also azerbaijan (northwest iran) seismotectonic province (javidfakhr et al., 2011). along the iranian plateau, fault ruptures from earthquakes show a large-scale typical changes and evaluation of them supply many practical understanding for surface rupture hazard in iran (ghassemi, 2016). it is well known that iran was struck with strong and destructive earthquakes in the past table 1. data set of fault parameters for iranian earthquakes between 1900 and 2014 (for details, see ghassemi, 2016). rlss: right lateral strike-slip, llss: left lateral strike-slip, nf: normal fault, tf: thrust or reverse fault, srl: surface rupture length, md: maximum displacement, vdc: vertical displacement component, hdc: horizontal displacement component no date longitude latitude rupture mechanism mw srl md 19 02.07.1972 50.85 30.06 tf 5.5 1.5 vdc. 4 20 24.11.1976 44.02 39.12 rlss 7.1 55 vdc. 0.5 hdc. 3.5 21 21.03.1977 56.45 27.59 tf 7.0 22 06.04.1977 50.76 31.90 tf 5.9 23 19.12.1977 56.61 30.90 rlss 5.8 19.5 hdc. 0.2 24 16.09.1978 57.12 33.40 tf 7.3 85 < 1.7 25 16.01.1970 59.50 33.80 llss 6.5 26 14.11.1979 59.81 33.91 rlss 6.6 20 1 27 27.11.1979 59.63 34.05 llss 7.1 68 2.5-4 28 11.06.1981 57.68 29.85 rlss-tf 6.6 15 29 28.07.1981 57.77 29.97 tf 7.0 65 0.4 30 20.11.1989 57.72 29.90 rlss 5.8 11 vdc. 0.01 hdc. 0.004 31 20.06.1990 49.23 37.00 llss 7.3 >80 vdc. 0.95 hdc. 0.6 32 06.11.1990 55.46 28.24 tf 6.4 15 vdc. 1.5 33 23.02.1994 60.54 30.78 tf 6.1 9.5 1.7 34 24.02.1994 60.51 30.79 tf 6.2 35 04.02.1997 57.31 37.73 rlss 6.4 15 0.5-1.0 36 28.02.1997 48.07 38.12 rlss 6.0 37 10.05.1997 59.81 33.85 rlss 7.2 125 vdc. 0.9 hdc. 2.3 38 14.03.1998 57.59 30.14 rlss-nf 6.6 23 hdc. 3 39 18.11.1998 57.58 30.33 rlss 5.3 4 40 22.06.2002 49.01 35.62 tf 6.4 3 0.16 41 26.12.2003 58.27 28.90 rlss 6.5 5 0.2 42 28.05.2004 51.57 36.29 tf 6.2 43 22.02.2005 56.79 30.71 tf 6.4 13 1 44 27.11.2005 55.83 26.75 tf 6.0 45 20.12.2010 59.19 28.33 rlss 6.5 14 0.05 46 11.08.2012 46.78 38.41 rlss 6.4 13 0.5-1 465a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes and recent years and these earthquakes caused large damages and high loses in human life. some of these earthquakes are 1909 silakhor earthquake (mw7.4), 1945 makran earthquake (mw8.0), 1968 ferdows earthquake (mw7.4), 1990 manjil-rudbar earthquake (mw7.4), 2013 ghosht (mw7.7) earthquake. as a remarkable result, since this study does not include a seismotectonic evaluation or discussion on the earthquake fault rupture hazards in iran, we did not give many details for the tectonic structures and seismicity of iran. thus, a brief and general information is provided in this section. many readers can reach the knowledge on these subjects from different studies such as mirzaei et al. (1997), masson et al. (2005), djamour et al. (2010), ghassemi (2016), öztürk et al. (2017). results and discussions for the applications of alternative curve fitting methods the main purpose of this study is to provide the new, up-to-date and reliable empirical relationships between mw and ms, mw and srl, md and mw, and md and srl considering the parameters of surface ruptures for iranian earthquakes. in this scope, we used different regressions models such as l1, orthogonal and robust for different earthquake faulting mechanisms (strike-slip and thrust or reverse faults) in iran. as stated in öztürk (2014), measurements of surface ruptures generally accompany many large earthquakes and the logarithms of their length are linearly related to earthquake size. these empirical relationships can supply not only fault sizes for a specific magnitude, but also the maximum magnitude based on these sizes. such types of results are also very useful for geotechnical, seismic risk and hazard, and seismotectonic studies. many authors estimated these types of relationships between magnitude and earthquake faulting parameters for different parts of the world (e.g., acharya, 1979; wells & coppersmith, 1994; ambraseys & jackson, 1998; öztürk, 2014; ghassemi, 2016). ghassemi (2016) preferred the ls method for the analyses and provided several empirical relationships among mw, md and srl based on different earthquake faulting mechanisms in iran. in addition to the ls method, three different regression applications were used in this study for the same data in ghassemi (2016). a comparison between the results of ghassemi (2016) and the results of present study were given in table 2. many researchers in various scientific and engineering disciplines have used these types of curve fitting methods in order to estimate the mathematical formulation for observed data points. if we have an unusual error distribution in a data set, linear ls method cannot provide a good estimate (öztürk, 2014). therefore, the minimizing the sum of residual error size will be a more preferable and proper definition of the solution. for this purpose, correlation coefficients of the curve fitting models can be preferred as a trustworthy and acceptable tool in the evaluation figure 1. map showing locations of the earthquake events discussed in this paper. numbers refer to the event numbers in table 1. circles show instrumental epicenters of the events. where present, surface ruptures are shown in black lines. instrumental epicenter for the event no. 6 in makran region lies outside the map area in pakistan’s coast. modified after ghassemi (2016). 466 serkan öztürk, mohammad r. ghassemi, mahmut sarı of the results. as given in table 2, or technique provides stronger correlation coefficients than those of the other regression techniques in most estimates expect a few ones. a few poor correlation coefficients, for example r2=0.299 between md and mw, and r2=0.411 between md and srl, were obtained in the estimations. however, these relationships can be suggested in the evaluation of seismic hazard in iran. ghassemi (2016) stated that these types of estimations are related to seismicity analyses and these relationships evidently present the probable range of versatility of faulting parameters. in addition, these relationships may supply an understanding to excessive limits for rupture hazard evaluations, and they may give practical insights in paleo-seismological surveys on active faults of the iranian plateau. table 2 shows the comparison of the empirical relationships calculated in this study with the relationships of ghassemi (2016). graphical representations of the whole empirical relationships obtained in this study with three regression models and their confidence intervals between mw and ms, mw and srl, md and mw, and md and srl were also plotted in figures 2, 3 and 4. the first numbers in the equations show the error terms and standard deviations of them (in parentheses) and the second numbers show the slopes of the related distributions and their standard deviations (in parentheses), respectively. considering three curve fitting methods in addition to the ls method by ghassemi (2016), the suggested linear-linear, log-linear and log-log relationships were given as the more reliable and optimum by or method with their r2 in the following forms: mw=0.668(±0.177)+0.913(±0.014)*ms, for all earthquake mechanisms (r2=0.986) mw=5.570(±0.234)+0.822(±0.077)*log(srl), for thrust and reverse faults (r2=0.945) mw=4.630(±0.359)+1.489(±0.078)*log(srl), for strike slip faults (r2=0.788) mw=4.972(±0.332)+1.248(±0.060)*log(srl), for all fault mechanism (r2=0.778) log(md)=-3.002(±0.378)+0.419(±0.120)*mw, for thrust and reverse faults (r2=0.299) log(md)=-6.545(±0.324)+0.927(±0.073)*mw, for strike slip faults (r2=0.779) log(md)=-6.123(±0.364)+0.870(±0.066)*mw, for all fault mechanism (r2=0.650) log(md)=-0.630(±0.345)+0.421(±0.109)*log(srl), for thrust and reverse faults (r2=0.411) log(md)=-1.909(±0.402)+1.287(±0.090)*log(srl), for strike slip faults (r2=0.697) log(md)=-1.461(±0.406)+1.001(±0.074)*log(srl), for all fault mechanism (r2=0.588) the selection of the confidence interval is quite optional and 90%, 95% or 99% are frequently selected in practice, however, 95% is used in general. as stated in öztürk (2014), confidence intervals are designed to define the statistical properties of data and are may be used practically in many case. thus, confidence intervals for all types of regression relationships are selected as 95% in this study. as seen in table 2 and figures 2 to 4, correlation coefficients of three regression fits are almost the same for the relationships between mw and srl in l2 norm (r2=0.797) and or method (r2=0.788) for strike slip faults, and between mw and srl in l2 norm (r2=0.800) and or method (r2=0.778) for all faulting types. regional differences in srl for a given magnitude can be commented in terms of the local changes in geologic, seismic and tectonic activities. these results suggest that seismic efficiency in a region can be dependent on the rupture length or magnitude. also, if and when the error terms in the linear regression model follow a normal distribution, then the least squares curve fitting technique is better estimator under the most appropriate criteria. traditional ls estimation is sufficient when error terms show an independently and identically distribution. however, the ls estimates can give bad fit when error distribution is abnormal. the usage of table 2. empirical equations with ls method of ghassemi (2016) and all regression fits derived in this study. ss: strike-slip, tf: thrust or reverse fault, all: including ss+tf. standard errors for all equations are also given. ghassemi (2016) using ls method (l2) this study using l1 method this study using or method this study using rr method fault type mw=1.020+0.848ms (r2=0.852) mw=1.182(0.175)+0.818(0.014) ms (r2=0.792) mw=0.668(0.177)+0.913(0.014)ms (r2=0.986) mw=1.084(0.174)+0.838(0.014)ms (r2=0.829) all mw=5.294(0.217)+0.966(0.151) log(srl) (r2=0.873) mw=5.412(0.265)+0.887(0.084) log(srl) (r2=0.660) mw=5.570(0.234)+0.822(0.077) log(srl) (r2=0.945) mw=5.651(0.238)+0.758(0.075) log(srl) (r2=0.802) tf mw=5.658(0.152)+0.806(0.102) log(srl) (r2=0.797) mw=5.289(0.338)+1.057(0.074) log(srl) (r2=0.590) mw=4.630(0.359)+1.489(0.078) log(srl) (r2=0.788) mw=5.275(0.335)+1.051(0.073) log(srl) (r2=0.576) ss mw=5.523(0.131)+0.870(0.089) log(srl) (r2=0.800) mw=5.303(0.309)+0.997(0.056) log(srl) (r2=0.693) mw=4.972(0.332)+1.248(0.060) log(srl) (r2=0.778) mw=5.396(0.309)+0.950(0.056) log(srl) (r2=0.630) all log(md)=-2.230(2.432)+0.320(0.364) mw (r2=0.114) log(md)=-1.455(0.376) +0.217(0.119)mw (r2=0.168) log(md)=-3.002(0.378) +0.419(0.120)mw (r2=0.299) log(md)=-1.074(0.363) +0.162(0.115)mw (r2=0.059) tf log(md)=-7.435(1.345)+1.105(0.199) mw (r2=0.658) log(md)=-5.767(0.318) +0.824(0.071)mw (r2=0.652) log(md)=-6.545(0.324) +0.927(0.073) mw (r2=0.779) log(md)=-5.101(0.307) +0.737(0.069)mw (r2=0.628) ss log(md)=-6.320(1.208)+0.938(0.179) mw (r2=0.532) log(md)=-3.471(0.338) +0.523(0.062)mw (r2=0.308) log(md)=-6.123(0.364) +0.870(0.066)mw (r2=0.650) log(md)=-4.142(0.336) +0.623(0.061)mw (r2=0.436) all log(md)=-0.559(0.361)+0.352(0.256) log(srl) (r2=0.240) log(md)=-0.257(0.352) +0.230(0.111)log(srl) (r2=0.186) log(md)=-0.630(0.345) +0.421(0.109)log(srl) (r2=0.411) log(md)=-0.337(0.337) +0.243(0.107)log(srl) (r2=0.132) tf log(md)=-0.927(0.242)+0.751(0.158) log(srl) (r2=0.616) log(md)=-1.184(0.356) +0.876(0.080)log(srl) (r2=0.554) log(md)=-1.909(0.402) +1.287(0.090)log(srl) (r2=0.697) log(md)=-1.173(0.351) +0.867(0.078)log(srl) (r2=0.494) ss log(md)=-0.778(0.211)+0.609(0.141) log(srl) (r2=0.457) log(md)=-0.753(0.358) +0.564(0.065)log(srl) (r2=0.294) log(md)=-1.461(0.406) +1.001(0.074)log(srl) (r2=0.588) log(md)=-0.822(0.357) +0.620(0.159)log(srl) (r2=0.355) all 467a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes figure 2. estimated relationships with least sum of absolute deviations (l1) method between (a) mw and ms for all faulting types, (b) mw and srl for thrust and reverse faults, (c) mw and srl for strike-slip faults, (d) mw and srl for all faulting types, (e) md and mw for thrust faults, (f) md and mw for strike slip-faults, (g) md and mw for all faulting types, (h) md and srl for thrust faults, (i) md and srl for strike-slip faults, and (j) md and srl for all faulting types. 468 serkan öztürk, mohammad r. ghassemi, mahmut sarı any curve fitting criterion is preferred when it was doubtful that a small part of the data which is analyzed includes data outliers and thus is not reliable. curve fitting criterions have the capability of strongly ignoring some bad data points while selecting the majority of data points which put forth the true nature of the data more suitable. the minimizing the sum of residual error magnitude is a reasonable and helpful process for the solution of problem. if the dataset contains a small number of data, the sum of error magnitudes is of particular use and in these situations, these data outliers extremely influence the sum of squared errors criterion. however, the sum of error magnitudes tends to ignore the data outliers on condition that they have relatively few numbers (öztürk, 2014). also, öztürk (2012) suggested for the statistical regression techniques on different dataset that representation of empirical relationships can be made as more appropriate and trustworthy with least sum of absolute deviations or robust regressions for the clustered data while it can be more suitable and reliable with least squares or orthogonal regressions. to investigate the maximum surface rupture length (msrl) and the related maximum credible earthquakes (mce) in iran, we studied major earthquake sources in different seismotectonic regions in conjunction with the empirical relationships by or method of our research. the resulting data were summarized in table 3. the values are different from those of ghassemi (2016) in some respects as a result of differences in revised selected earthquake sources as well as difference in the applied regression methods. as it is noticed for the analysis of data on surface ruptures of thrust (and reverse) faults using or method (figure 3b), the regression line has a small slope with a coefficient of 0.945. this may be partly because of rarity of data on surface ruptures associated with thrust events. as a result, it appears that the relationship may underestimate the earthquake magnitude for major (long) thrust faults, especially when value of srl is used for blind and/or deep-seated faults. to compensate for such apparent deficiency, we have used the positive side of the standard errors in srl-magnitude equations for estimation of mce resulting from active thrust faulting in different seismotectonic regions of iran. considering our objective to estimate the mce for each region, the biggest potential seismic sources are picked for each region. therefore, it should be kept in mind that in dealing with any seismic hazard approach which does not involve such extreme seismic sources, the mce would be less than the suggested values in this section (i.e. values in table 3). a major potential seismic source in the kopeh-dagh region is the maraveh-tappeh thrust fault (ca. 260 km long). considering a rupture which involves half the length (130 km) along the geological fault, a maximum magnitude (mce) of ca. 7.7 is suggested for the kopeh-dagh region. the ashkabad fault, which forms the northern border of the kopeh-dagh geological province within the turkmenistan territory, is apparently capable of producing earthquakes of magnitudes as large as 8.2 (e.g. the 1895 krasnovodsk-uzun ada earthquake; ambraseys, 1997). north of central iran, alborz and azerbaijan region include major strikeslip and thrust faults which are capable of great earthquakes. these include the doruneh, meyamey, torud, astaneh, mosha, eivanekey, ipak, soltanieh and tabriz faults, among many other. length of the above-mentioned faults and/or their segments attain lengths up to 170 km, and some of them have produced historical earthquakes as large as magnitude 7.9 (the 856 kumesh earthquake; ambraseys & melville, 1982). therefore, we consider a magnitude 8.0 as the mce for the northern central iran, alborz and azerbaijan region. within the zagros region (except for the high zagros region) emergent geological faults are rare, and therefore most of attempts for earthquake hazard analyses use folds above blind thrust faults as the source for potential earthquakes. as a result, the surface rupture length used in the related calculations, in fact is not really on the surface but it is related to rupture of a blind fault at some depth (see berberian, 1995). following the same approach, we have considered the kabir-kuh anticline as the biggest seismic source in folded zagros region with a length of 166 km, and anticipating rupture of half of the related thrust fault (83 km), an mce of 7.5 is suggested for zagros folded region. therefore, we suggest that for the zagros folds (and related faults) which are longer than the suggested value in table 3, the related maximum srl of 83 km to be used for deterministic evaluation of the magnitude attributed to the fold seismic sources. the eastern iran seismotectonic region is dominated by active strike-slip faulting, and less frequent thrust faults. surface ruptures are highly manifested in this part of the country as compared to other seismic regions. as a result, relationship between srl and magnitude in this region shows a greater slope, therefore we have used or method without including an upper standard error limit to estimate mce. no historical or instrumental earthquake greater than 7.3 has been reported for the region, therefore we used a maximum length of ca. 100 km for or of all fault type, suggesting a mce of magnitude 7.5 for the eastern iran region. central iran region has a very low seismicity as compared to other parts of the country, and no historical or instrumental record of earthquakes greater than magnitude 6.2 has been reported for this region. therefore, a mce of 6.5 appears to be appropriate for the region. the greatest earthquakes in the makran region may occur on the subduction zone at depth. the whole length of the subduction zone is ca. 850 km, which if half of the whole length (425 km) is combined with a width of 180 km and a related 20 m of average displacement, gives a mce of magnitude 9.1. we made a comparison for two important relationships suggested for global and iranian earthquakes, including srl-mw (figure 5) and srl-md (figure 6) relationships. slope of our regression line for srl-mw relationship greatly conforms to those of nowroozi (1985) and wells and coppersmith (1994), however it is less than that of ghassemi (2016) (figure 5). a similar comparison between the graphs for srl-md indicates that slope calculated in this study is very similar to those of nowroozi (1985) and wells & coppersmith (1994), however the regression has a lower intercept value than those of aforementioned studies. as seen in figure 6, both slope and intercept values of our study are smaller than that of ghassemi (2016). many studies were made for surface rupture hazard in iran and some empirical relationships were provided among different earthquake faulting parameters by different researchers such as berberian (1976), nowroozi (1985), wells & coppersmith, (1994), berberian (2014), ghassemi (2016). the only comprehensive analysis of iranian earthquakes was made by nowroozi (1985). also, more detailed quantitative assessments of surface rupture hazard of iranian earthquakes including historical and instrumental period were provided with berberian (2014) and ghassemi (2016). when we compared the empirical relationships estimated in this study with the most up-to-date relationships of ghassemi (2016), the results in this study present higher r2 among magnitude and different faulting parameters in or method. correlation coefficients of the relationships in ghassemi (2016) changes between 0.114 and 0.873 with l1 norm, whereas correlation coefficients with or technique in present study vary from 0.299 to 0.986. from the observations in this study and in ghassemi (2016), we can conclude that these estimations for or method can be thought as good and reliable relationships, and one can use them for the other seismicity analyses or rupture hazard evaluations of the iranian plateau. as a remarkable result, we did not make a detailed discussion on the earthquake fault rupture hazards in iran since this study does not aim to evaluate and discuss this subject. many details can be found in ghassemi (2016) for the insights of earthquake surface rupture hazards. for this reason, a short and general comparison of three different curve fitting techniques is provided for the data of ghassemi (2016). furthermore, the effectiveness of the different estimation techniques was examined in this study and the relative predictive validity of four regression techniques was compared for different faulting parameters. these detailed analyses show that empirical table 3. maximum surface rupture length (msrl), maximum surface displacement (msd), and related maximum credible earthquakes (mce) suggested for different seismotectonic regions of iran. see text for further explanation. region msrl (km) msd (m) mce (mw) tf ss all kopeh-dagh 130 1.8 6.4 4.5 7.7 northern central iran, alborz, azerbaijan 170 2.0 9.1 5.9 8.0 zagros 83 1.5 3.6 2.9 7.5 eastern iran 100 1.6 4.6 3.5 7.5 central iran 16 0.8 0.4 0.6 6.5 makran 425* 20 9.1 * length quoted for makran is rupture length at depth 469a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes figure 3. estimated relationships with orthogonal regression (or) method between (a) mw and ms for all faulting types, (b) mw and srl for thrust and reverse faults, (c) mw and srl for strike-slip faults, (d) mw and srl for all faulting types, (e) md and mw for thrust faults, (f) md and mw for strike slip-faults, (g) md and mw for all faulting types, (h) md and srl for thrust faults, (i) md and srl for strike-slip faults, and (j) md and srl for all faulting types. 470 serkan öztürk, mohammad r. ghassemi, mahmut sarı figure 4. estimated relationships with robust regression (rr) method between (a) mw and ms for all faulting types, (b) mw and srl for thrust and reverse faults, (c) mw and srl for strike-slip faults, (d) mw and srl for all faulting types, (e) md and mw for thrust faults, (f) md and mw for strike slip-faults, (g) md and mw for all faulting types, (h) md and srl for thrust faults, (i) md and srl for strike-slip faults, and (j) md and srl for all faulting types. 471a comparison of alternative curve fitting techniques for different earthquake fault parameters of iranian earthquakes relationships estimated by or method in the present study can be thought and used as more appropriate and more trustworthy for iranian earthquakes and the other mentioned applications. we finally hypothesized that or estimation technique can provide more stabilized interpretations for the rupture properties, variability of the earthquake fault rupture geometry and kinematics by using these empirical relationships among different faulting parameters. conclusions this study focused on the applications of different regression techniques and a comparison of the empirical relationships of the data set on the surface ruptures of iranian earthquakes was made. using a database of iranian earthquakes, some empirical relationships were estimated among different parameters such as moment magnitude, surface wave magnitude, surface rupture length and maximum displacement for different faulting types. for this purpose, three curve fitting methods are applied as (a) least sum of absolute deviations, (b) robust regression and (c) orthogonal regression. in order to decide how we can select the best curve fitting method for a given data set, we calculated separately the correlation coefficients for the thrust or reverse faults, strike slip faults and all faulting types since it is an easy applicable and a practical technique. for the analyses, 46 strong and large earthquakes with magnitudes ≥ 5.8 between 1900 and 2017 were used. in order to estimate the statistical properties of different data sets, 95% confidence intervals were used for each regression relation. using the above-mentioned techniques, we derived linear relationships among different faulting parameters for iranian earthquakes. correlation coefficients varies from 0.299 to 0.986 with orthogonal regression, from 0.168 to 0.792 with l1 regression, from 0.059 to 0.829 with robust regression. these results show that orthogonal regression fits give more powerful coefficients and can be thought more suitable equations as compared to the other curve fitting techniques. resulting equations are proposed as more suitable and reliable in the estimation of the maximum surface rupture length, maximum surface displacement and associated maximum credible earthquakes for different areas of iran. as a general conclusion, detailed analyses for the data sets of different faulting mechanisms show that more appropriate, more trustworthy more upto-date representations of empirical relationships can be done with orthogonal regression. in addition, these types of relationships may supply significant insights in the estimation of the maximum surface rupture length, maximum surface displacement, and associated maximum credible earthquakes for different seismotectonic regions of iran. we suggested that these relationships may also provide important outputs in the estimation of earthquake magnitudes in paleo-seismological studies. acknowledgements the author wish to thank the anonymous reviewers for 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(1985). applied linear regression. second edition. john wiley and sons, new york. wells, d., & coppersmith, j. k. (1994). new empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. bulletin of the seismological society of america, 84(4), 974-1002. keywords: remote sensing technology; china’s northern region; precipitation period; wavelet analysis; change rule; oscillation period. palabras clave: tecnología de teledetección; región del norte de china; período de precipitación; análisis de ondículas; cambio de frecuencias; período de oscilación. how to cite item zhang, y. & liang, c. (2021). application of remote sensing technology in the analysis of periodic precipitation change in china’s northern semi-arid area. earth sciences research journal, 25(1), 85-91. doi: https:// doi.org/10.15446/esrj.v25n1.93925 the remote sensing technology is used to analyze the periodic changes of precipitation in the semi-arid areas of northern china and provide a reference basis for precipitation prediction. authors were based on gps observation data and corresponding ground meteorological parameters to obtain the study area’s comprehensive water vapor content value and use precipitation and time to establish a linear regression equation based on the content value to express the precipitation trend change. the periodic structure and abnormal changes of the periodic precipitation series on different time scales were obtained through wavelet analysis, combined with anomaly calculations to analyze the increase and decrease precipitation characteristics. the morlet continuous wavelet transform map was also used to reflect the periodic oscillations of precipitation on different time scales. the wavelet variance map was used to reflect the distribution of regional precipitation with scale years. the authors analyzed the cycle change characteristics of the comprehensive precipitation series in harbin, changchun, shenyang, and dalian. they concluded that the annual precipitation in the four regions showed a decreasing trend in 50 years. and through the wavelet cycle frequencydomain relationship change law, the change characteristics of precipitation in the northern semi-arid area in the entire time domain were obtained. abstract application of remote sensing technology in the analysis of periodic precipitation change in china’s northern semi-arid area aplicación de la tecnología de teledetección en el análisis del cambio periódico de las precipitaciones en el norte semiárido de china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.93925 con el fin de proporcionar una base de referencia para la predicción de la precipitación, este estudio utilizó la tecnología de teledetección para analizar los cambios periódicos de la precipitación en las zonas semiáridas del norte de china. se usaron los datos de observación gps y los parámetros meteorológicos terrestres correspondientes para obtener el valor integral del contenido de vapor de agua del área de estudio, y se usó la precipitación y el tiempo para establecer una ecuación de regresión lineal basada en el valor del contenido para expresar el cambio de tendencia de la precipitación en el área. a través del análisis de ondículas se obtuvo la estructura periódica y los cambios anormales de las series periódicas de precipitación en diferentes escalas de tiempo, combinadas con cálculos de anomalías para analizar las características de aumento y disminución de la precipitación. el mapa de transformación de ondas continuas de morlet se utilizó para reflejar las oscilaciones periódicas de la precipitación en diferentes escalas de tiempo. mientras que el mapa de varianza de ondículas se utilizó para reflejar la distribución de la precipitación regional a escala de años. se analizaron las características del cambio de ciclo de la serie integral de precipitación en harbin, changchun, shenyang y dalian, y se concluyó que la precipitación anual en las cuatro regiones mostró una tendencia decreciente en 50 años. y a través de la ley de cambio de la relación del dominio de la frecuencia del ciclo de las ondas, se obtuvieron las características de cambio de la precipitación en el área semiárida del norte en todo el período analizado. resumen record manuscript received: 13/11/2019 accepted for publication: 04/11/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 85-91 yannan zhang, chuan liang* college of water resource and hydropower, sichuan university, chengdu 610065, china *corresponding author: lc3006930@2980.com m et eo ro lo g y https://doi.org/10.15446/esrj.v25n1.93925 https://doi.org/10.15446/esrj.v25n1.93925 https://doi.org/10.15446/esrj.v25n1.93925 mailto:lc3006930@2980.com 86 yan lv, laijun lu, mengxue cao introduction global warming has led to global and regional climate change, frequent occurrence of high temperature, drought, flood, and other disastrous weather, causing ecological and environmental deterioration, seriously affecting agricultural production, social economy, and sustainable development (bi et al. 2018). especially in the context of global climate change, the spatial pattern of precipitation in china is directly related to agricultural production safety. the northern part of china is located in the northernmost end of the east asian monsoon, which belongs to the temperate continental monsoon climate (guo et al., 2019; huang et al., 2018). it is the transitional zone between the humid eastern monsoon region and china’s arid inland region (colliander et al., 2017). it is hot and rainy in summer, cold and dry in winter, and the continental climate is becoming stronger from east to west. the seasonal change of its environment is closely connected with the general circulation of east asia (hall et al., 2016). the climate and its evolution are pretty different (dai et al., 2018; ren et al., 2019; zeng et al., 2018). it is a typical “vulnerable area” and one of the most sensitive areas affected by climate warming. it is also the largest commodity grain production area and an essential heavy industry and energy base in china (ferrari dos santos et al., 2017). therefore, it is of great significance to study the characteristics of the periodic changes of precipitation in the north of china in the past hundred years for understanding the impact of climate change on rainfall and for short-term, medium-term, and long-term precipitation prediction. in recent years, many scholars have studied these characteristics. lan yongchao et al. studied the impact of climate change on the water resources system in the upper reaches of the yellow river; xu yueqing et al. studied the law of precipitation change in hebei plain based on wavelet analysis; lin zhenyao et al. studied the temporal and spatial characteristics of temperature and precipitation change in the qinghai tibet plateau; dai xingang et al. studied the multi-scale characteristics of precipitation in flood season and the inter-generational decay characteristics of summer monsoon in north china levy. in the past 50 years, under the background of global change, the changing trend of precipitation and the difference in the rule of time scale structure in northern china have attracted much attention (wang et al., 2018). since bevis and businger put forward the basic principle and method for remote sensing of atmospheric precipitable water by gps technology in the 1990s, many experts, scholars, and scientific research institutions in various countries have invested in this research work. the more famous experiments include gps/storm, wwave, clam. (wang et al., 2017). china has also done many experiments in the research of remote sensing of atmospheric precipitable vapor by gps, such as gps/storm experiment in shanghai, gps rainfall observation experiment in south china, gps/met experiment in beijing, and has also achieved good results. at present, the technology of gps remote sensing of atmospheric integrated water vapor content has been mature, and the recognized accuracy is 1-2 mm (zhao et al., 2018). at the same time, because gps remote sensing technology has a series of advantages, such as uniform global distribution, high resolution, low cost, easy maintenance and so on, it fully shows the potential application value of gps technology in meteorological, hydrological, atmospheric monitoring and so on (arrizabalaga & li, 2018; megat & hassan 2018). in order to study the change law of the average precipitation series in northern china in the past 50 years under the background of global change, this paper selects the inland cities harbin, changchun, shenyang and coastal city dalian, which have some differences in response to the regional precipitation change, and analyzes the periodic change law of precipitation in the northern china through the remote sensing technology using the annual precipitation from 1955 to 2005 in four regions (li et al., 2018). in this paper, the annual precipitation data of the above four typical cities are obtained by gps remote sensing technology, and the periodic change of precipitation in the four regions is analyzed by the trend analysis process and wavelet analysis process. material and methods data sources and methods based on the remote sensing data of precipitation time series from 1955 to 2005 in harbin, changchun, shenyang, and dalian stations, the periodic variation of precipitation in the four stations is analyzed. harbin, changchun, and shenyang are located in the northeast of china, the northernmost of the east asian monsoon, which belongs to the temperate continental monsoon climate. dalian is a coastal city, which is affected by the continental monsoon and marine climate. therefore, it is of great significance to analyze the 50 year precipitation law of four stations for understanding the precipitation cycle change law in the semi-arid area of northern china under the background of global change (rauber et al., 2018). retrieval of atmospheric precipitable water in four stations by gps data as the remote sensing technology of atmospheric precipitable water by gps technology is mature, the introduction of basic principles (mora et al., 2017) is omitted here, and the process of remote sensing of atmospheric precipitable water by gps technology is only illustrated in figure 1. according to the flow chart in figure 1, we can use the gps observation data and the corresponding ground meteorological parameters to obtain the final comprehensive water vapor content of harbin and the other three stations in 1955-2005. figure 1. gps processes for remote sensing atmospheric precipitation in the calculation process, the total tropopause delay is calculated by gamit software developed by the massachusetts institute of technology. saastamoinen model is used for tropopause dry delay, and its expression is as follows: bcm l m d n n = ±( ) ( ) 2 2668 0 0023. . ,µ (1) where, m d dn nµ µ, . cos .( )= − ( )−1 0 00255 2 0 00027 ;  is the geographic latitude; dn is the altitude of the station (m); ln is the ground pressure (hpa); bcm is the zenith dry delay (mm). when converting from wet delay to precipitable water content, the air’s weighted average temperature ej in the conversion factor used is the bevis model, namely: e ej n= +70 1 0 71. . (2) where en is the station temperature (k). analysis of periodic change of precipitation according to the gps remote sensing technology of the atmospheric precipitable water in the four stations in the last section, through trend analysis and wavelet analysis process, the periodic change of precipitation in the four stations is further analyzed (xing & wang, 2016). trend analysis the trend change is generally expressed by a linear regression equation based on precipitation and time. 87tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm f a bi= + (3) where, f is the fitting value of precipitation; a is the annual change rate; a × 10 is the climate tendency rate, which means the change rate of precipitation per 10 a; i = 1, 2… n years. wavelet analysis in this paper, the wavelet analysis method is based on fourier transform, which introduces the window function to decompose the time series into the contribution of time and frequency. it is very effective for obtaining the adjustment rule of a complex time series, diagnosing the hierarchical structure of climate change, and distinguishing the evolution characteristics of time series on different scales. through wavelet analysis, we can get the periodic structure and abnormal change rule of the studied object sequence in different time scales, which provides a scientific basis for short-term climate prediction (rao et al., 2016). combining with the anomaly calculation, the increase and decrease characteristics of precipitation in four stations are analyzed, and the periodic oscillation of precipitation in different time scales can be reflected by using morlet continuous wavelet transform chart. the strength of the signal is expressed by the size of the wavelet coefficient. if the isoline is positive, it means more precipitation; if the isoline is negative, it means less precipitation; if the wavelet coefficient is zero, it means the mutation point. the wavelet variance map reflects the distribution of energy with scale year, which can determine the relative intensity of various scale disturbances in a time series. the scale at the corresponding peak is called the main time scale of the series, which is used to reflect the main period of the time series (castro et al., 2017). wavelet analysis takes time and frequency as independent variables, expands one-dimensional signals in time and frequency, so as to clearly understand the frequency characteristics of different time-domains of time series and the time distribution characteristics of different frequencies. wavelet function is the key to wavelet analysis. it refers to a kind of function with oscillation and rapid attenuation to zero (luo et al., 2017; li et al., 2019). wavelet function meets the following requirements: ∫ ( ) = −∞ +∞ δ i ic 0 (4) in the formula, (i) is the base wavelet function, which can form a group of function systems through scaling and translation on the time axis: δ δb d i b i d b b d g b, / , ,( )= −      ∈ ≠−1 2 0 (5) in the formula, b and d(i) are subwavelets; b is the scale factor, reflecting the cycle length of wavelet; d is a translation factor, reflecting the translation in time. for a given energy limited signal, its continuous wavelet transform is: h b d b f i i d b il g ( , ) ( )= −      − ∫½ cδ (6) in the formula, hl(b,d) is the wavelet transform coefficient; l(i) is a signal or square-integrable function; b is the scaling scale; d is the translation parameter; δ i d b −      is the complex conjugate function of δ i d b −     . for discrete-time series data, assuming that there is a function l(ei) (e =1,2,..., i;i) is the sampling interval), the discrete wavelet transform form is: h b d b l e i e i d b il e i ( , )= ∆ ∆( ) ∆ −      − = ∑½ i cδ 1 (7) results periodic change of precipitation series the wave characteristics of the real part of the wavelet transform coefficients can reflect the alternation of more and less precipitation, and the alternation of positive and negative positions can be observed. the wave changes of more and less precipitation in the calculation time domain can be observed obviously. it can be seen from figure 2 that the precipitation structure corresponding to different time scales is different, and the number of smallscale changes shows a more complex number of structures nested on a larger scale. based on the average situation of precipitation in harbin, changchun, shenyang, and dalian stations, the precipitation situation in the northern semiarid area is estimated. there are five kinds of periodic changes in the average situation of precipitation in the four stations: 3-9, 9-18, 18-30, 30-41a, and 4163a. among them, the periodic oscillation is very significant on the scale of 4163a, and the annual precipitation has experienced the cycle of less-more-lessmore-less until the isoline of 2005 is not closed, which indicates that the annual precipitation will continue to be less in the future; the 10a and 50a cycles have more obvious oscillation periods after 1955. the modulus of morlet wavelet coefficients reflects the distribution of energy density in the time domain corresponding to the period of change of different time scales. the larger the modulus of coefficients is, the stronger the periodicity of the corresponding period or scale is. in the annual precipitation of 4 stations, the 50a time scale model value is the largest, which indicates that the cycle change of this time scale is the most obvious. the 10a and 20a cycles are relatively large after 1955, and the cycle change is more obvious, and the cycle change of other time scales is small (figure 3). figure 2. real-part time-frequency distribution of morlet wavelet transform coefficients figure 3. morlet wavelet transform is a digital modular square timefrequency distribution 88 yan lv, laijun lu, mengxue cao relationship between precipitation anomaly and wavelet periodic frequency domain of four stations in the north of china in recent 50 years in order to understand the variation law of precipitation in the northern semi-arid area in the past 50 years, the variation law of precipitation in harbin, changchun, shenyang, and dalian stations in the past 50 years is analyzed. figure 4(a) shows the precipitation distance map of harbin in the recent 50 years. from figure 4(a), in addition to the relationship between the annual precipitation and the 50-year average precipitation, it can be seen that the precipitation in harbin experienced two cycles of less to more in 1955-2005, and the precipitation anomaly from 1955 to 1982 was negative, indicating that the precipitation in this period was less than that in the whole year; the precipitation anomaly from 1983 to 2005 was positive, indicating that the precipitation in this period was more than that in the whole year. the average annual precipitation of harbin in 50 years is about 550 mm/a. by linear fitting of annual precipitation series in harbin, it is found that the precipitation tendency rate is 7.1 mm/10a, and the cumulative effect of such a high precipitation tendency rate is relatively large, which will lead to a gap of tens of millimeters between the current annual precipitation of harbin and that of 50 years ago. (a) (b) figure 4. annual precipitation anomaly (a) and morlet wavelet transform (b) of harbin from 1955 to 2005 in addition to the above analysis, it is difficult to see the change information of the shorter period from figure 4(a). in this regard, figure 4(b) gives the corresponding morlet wavelet transform graph. figure 4(b) clearly shows the periodic oscillation and abrupt point characteristics of annual precipitation in harbin over the past 50 years on different time scales. the intensity of signal oscillation in the figure is represented by the size of the gray-scale. the larger the gray-scale is, the smaller the annual precipitation is; the smaller the gray-scale is, the larger the annual precipitation is. it can be seen from the comparison between figure 4(b) and figure 4(a) that more abundant information of the change period scale can be obtained from figure 4(b). it can be seen from figure 4(b) that there is a phenomenon of nested complex structure on the scale of multiple time periods. there are obviously short-term oscillation periods of 2-3a and 5-6a, the oscillation period of 10a, the medium-term oscillation period of 20a, and the long-term oscillation period of 50 after 1955. from the change law in the figure, after 2005, harbin is in the period of 2-3a, 5-6a short-term oscillation cycle with more precipitation; that in the oscillation period of 10a is less precipitation; in the medium-term oscillation period of 20a has experienced two cycles of more-less alternation, and the isoline of 2005 oscillation cycle is far from closed, which shows that after 2005, the precipitation is in the period of less precipitation; and the isoline in the longterm oscillation period of 50a is still not closed, indicating that the precipitation will continue to maintain a slightly more trend in this period after 2005. to sum up, the precipitation of harbin in recent 50 years is controlled by multiple periodic laws, and there are obviously short-term oscillation periods of 2-3a and 5-6a, the oscillation period of 10a, the medium-term oscillation period of 20a, and the long-term oscillation period of 50a after 1950. from the point of view of the intensity of the oscillation, the medium-term change of 20a is the main control period of annual precipitation, but the short-term period of 5-6a and the long-term period change of 50a also have a great influence on the annual precipitation. figure 5(a) shows the precipitation distance map of changchun in the recent 50 years. from figure 5(a), it can be seen that the annual precipitation change law of changchun from 1955 to 2005 is similar to that of harbin, and it has also gone through two cycles, but the cycle law is just the opposite, that is, there are two cycles of more to less, the precipitation anomaly from 1955 to 1982 is positive, which indicates that the precipitation is more than that of the whole year in this period; the precipitation anomaly from 1983 to 2005 is negative, which indicates that the precipitation is less than that of the whole year in this period. the linear fitting of changchun’s annual precipitation series shows that the precipitation tendency rate is 12.7 mm/10a, which is higher than that of harbin. this shows that the annual precipitation in changchun is decreasing, and the decreasing range is larger than that in harbin. the accumulated precipitation in 50 years will cause the annual precipitation in (a) (b) figure 5. precipitation anomaly (a) and wavelet transform (b) of changchun from 1955 to 2005 89tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm changchun to decrease by 126.8 mm more than that at the beginning of the 20th century. considering that the annual average precipitation in changchun is about 596 mm, this reduction is quite amazing. in order to analyze the multi-periodicity of precipitation in changchun in recent 50 years more clearly, the corresponding morlet wavelet transforms chart is given in figure 5(b), which clearly shows the periodic oscillation and mutation point characteristics of precipitation in changchun in recent 50 years on different time scales. it can be seen from the comparison between figure 5(b) and figure 5(a), more abundant scale information of the change period can also be obtained from figure 5(b). it can be seen from the comparison between figure 4(b) and figure 5(b) that the wavelet transform-law of annual precipitation in changchun and harbin is very similar. there are not only nested complex structure phenomena on multiple time period scales but also obvious short-term oscillation periods of 2-3a, 5-6a, 10-15a, 20a, and 50a after 1955. this may be the reason why the two cities are in the same climate background in northeast china. the difference is that the long-term oscillation period of about 50 years is exactly the same as that of the less-more cycle in harbin, that is, the more-less cycle. according to the change law in the figure, changchun is in the short-term oscillation periods of 2-3a and 5-6a with more precipitation after 2005; the oscillation period of 10a is the period of less precipitation; the medium-term oscillation period of 20a has experienced the cycle of more-less alternation, and the isoline of oscillation cycle in 2005 is far from closed, which shows that the precipitation is in the period of less precipitation after 2005; the isoline of the long-term oscillation period of 50a is still not closed, which indicates that the precipitation will continue to be less in this period after 2005. in conclusion, the precipitation of changchun in recent 50 years is controlled by multiple periodic laws, and there are obviously short-term oscillation periods of 2-3a and 10a, medium-term oscillation periods of about 20a, and long-term oscillation periods of about 50a. from the point of view of the intensity of the oscillation, the medium-term period variation of 20a is the main control period of the annual precipitation, but the short-term period of 2-3a and the long-term period variation of 50a also have a great influence on the annual precipitation. figure 6(a) shows the precipitation distance map of shenyang in the recent 50 years. compared with harbin (figure 4(a)) and changchun (figure 5(a)) in the past 50 years, it is found that the precipitation decrease trend of shenyang in the past 50 years is not obvious, and the precipitation anomaly is close to zero. the linear fitting of the annual precipitation series in shenyang shows that the precipitation tendency rate is 1.3 mm/10a, and the precipitation slightly increases. in order to analyze more clearly the multi-periodicity of precipitation variation in recent 50 years in shenyang, the corresponding morlet wavelet transform chart is given in figure 3(b), which clearly shows the periodic oscillation and mutation point characteristics of precipitation in recent 50 years in shenyang on different time scales. it can be seen from the comparison between figure 6(b) and figure 6(a) that more abundant information of the change cycle scale can also be obtained from figure 6(b). compared with the wavelet transform of morlet in harbin (figure 4(b)) and changchun (figure 5 (b)), it can be seen that shenyang also has an obvious short period of 2-3a and 10a and a long period of 50a, but the main control period of annual precipitation of 20a does not appear. according to the change rule in the figure, shenyang is in the period of 2-3a, 10a, and 50a with more precipitation after 2005. in conclusion, the precipitation in the recent 50 years in shenyang is controlled by multiple periodic laws, and there are obviously short-term oscillation periods of 2-3a and 10a and long-term oscillation periods of about 50a. from the point of view of the intensity of the oscillation, the mediumterm period variation of about 10a is the main control period of the annual precipitation, but the short-term period of 2-3a and the long-term period variation of 50a also have a great influence on the annual precipitation. figure 7(a) shows the precipitation anomaly of dalian in the recent 50 years. it can be seen that the annual precipitation variation law of dalian from 1955 to 2005 is similar to that of changchun, and it has also gone through two-cycle periods, i.e. more-less cycle period, i.e. the precipitation anomaly from 1955 to 1980 is positive, which indicates that the precipitation in this period is more than that in the whole year; the precipitation anomaly from 1981 to 2005 is negative, which indicates that the precipitation in this period is less than that in the whole year. the precipitation tendency rate of dalian is 2.7 mm/10a, which is smaller than that of harbin and changchun. this may be related to the influence of marine climate in dalian. in order to analyze the multi periodicity of precipitation variation in dalian in recent 50 years more clearly, the corresponding morlet wavelet transform chart is given in figure 7(b), which clearly shows the periodic oscillation and mutation point characteristics of precipitation in dalian in recent 50 years on different time scales. it can be seen from the comparison between figure 7(b) and figure 7(a) that richer scale information of the change period can also be obtained from figure 7(b). compared with the wavelet transform of morlet in harbin (figure 4(b)), changchun (figure 5(b)) and shenyang (figure 6(b)), it can be seen that dalian also has obvious short-term periods of 2-3a and 10a and long-term period of 50a, but the main control period of annual precipitation in 20a does not appear which is the same as that in shenyang. it also shows that shenyang and dalian are close in geography and regional climate. from the change in the figure, dalian and shenyang are also in the precipitation period of 2-3a, 10a, and 50a after 2005. in conclusion, the precipitation of dalian in recent 50 years is controlled by multiple periodic laws, and there are obviously short-term oscillation periods of 2-3a and 10a and long-term oscillation periods of 50a. from the point of view of the intensity of the oscillation, the medium-term period change of 10a is the main control period of the annual precipitation, but the short-term period of 2-3a and the long-term period change of 50a also have a great influence on the annual precipitation. the main period of precipitation series the wavelet variance map can reflect the distribution of the fluctuation energy of the precipitation time series with the scale and can be used to (a) (b) figure 6. shenyang annual precipitation anomaly (a) and morlet wavelet transform (b) from 1955 to 2005 90 yan lv, laijun lu, mengxue cao determine the main period in the annual precipitation. there are five obvious peaks in the wavelet variance map of precipitation in four stations, which correspond to the time scales of 50a, 37a, 20a, 14a, and 6a in turn. among them, the maximum peak corresponds to the time scale of 50a, indicating that the cycle oscillation of about 50a is the strongest, which is the first main cycle of precipitation change in four areas; the 20a time scale corresponds to the second peak, which is the second main cycle of annual precipitation change; the third peak corresponds to the time scale of 14a, which is the third main cycle; the fourth peak corresponds to the time scale of 37a, which is the fourth main cycle; the fifth peak corresponds to the time scale of 6a, which is the fifth main period. the fluctuation of the above five periods controls the variation characteristics of the annual precipitation in the whole time domain of the four regions, as shown in figure 8. according to figure 8, the main periods of precipitation in four stations of harbin, changchun, shenyang, and dalian are 6a, 14a, 20a, 37a, and 50a respectively, of which the peak value of 50a precipitation is the most obvious. therefore, the periodic changes of precipitation in the four stations can be derived from the periodic changes of precipitation in the northern semi-arid areas. discussion the northern part of china consists of three parts: the northwest, the north, and the northeast. the area is large and the terrain is complex, the influence system is various, and the characteristics of precipitation change in different areas are quite different. especially in recent decades, under the background of global warming, there are signs of transition from drought to wetness in north china, but north china and the south of northeast china are still in a continuous semi-arid period. this paper analyzes the precipitation regularity in the north-east semi-arid area by taking the 50-year periodic precipitation variation regularity of harbin, changchun, shenyang, and dalian as an example. the research on the change rules of precipitation time series and the characteristics of multi-time scales in the semi-arid area of north china is the basis of the research on how to mine the water production potential and development degree of grain crops, which is of great theoretical significance for the rational development of grain production in the commodity grain production area of north china. based on the analysis of the 50-year precipitation series from 1955 to 2005 in harbin, changchun, shenyang, and dalian, the multitime scale complex structure of the annual precipitation change in the northern semi-arid area is studied. the main conclusions are as follows: (1) there are five kinds of periodic changes of precipitation in the north of china: 3-9, 9-18, 18-30, 30-41a, and 41-63a. the period of 9-18a and 18-30a appeared more obvious after 2005 and 1955 respectively, and the period of 4163a is more significant. (2) in the past 50 years, the annual precipitation in the northern region has shown a decreasing trend, with a decrease of 5.2 mm/10a in the whole northeast region; 12.7 mm/10a in changchun; 7.1 mm/10a in harbin; 2.7 mm/10a in dalian; and 1.3 mm/10a in shenyang. (3) the annual precipitation in the northern region has the periodic change characteristics under the regional multiple time scales, and the long-term oscillation periods of 2-3a, 5-6a, 10a, and 50a have the global characteristics. from the point of view of the intensity of the oscillation, the mid-period change of about 20 years is the main control period of the annual precipitation in changchun and harbin, but the short period of 5-6 years and the long period change of 50 years also have a great influence on the annual precipitation. (4) from the study of the periodicity of the wavelet transform, it is found that after 2005, harbin is in the period of 2-3a, 5-6a short-term oscillation cycle with more precipitation; the oscillation period of 10a and 20a is the period of less precipitation, and the long-term oscillation period of 50a will continue to tend to be more than one. changchun is in the period of 2-3a, 5-6a short-term oscillation with more precipitation; the oscillation period of 10a and 20a is less precipitation, and the long-term oscillation period of about 50a will continue to be less. shenyang is in a more precipitation period of 2-3a, 10a, and 50a. dalian is in a more precipitation period of 2-3a, 10a, and 50a. however, the whole northern region is in the period of more precipitation in 2-3a and 10a, less precipitation in 20a, and more precipitation in 50a. (5) there are five obvious main periods of precipitation in the four stations, which correspond to the time scales of 50a, 37a, 20a, 14a, and 6a, among which 50a is the highest peak, and the fluctuation of five periods controls the variation characteristics of annual precipitation in the whole time domain. based on the above analysis, the periodic variation of precipitation in the northern semi-arid area can be deduced. under the background of global warming, the characteristics of precipitation change in different regions in the north are different, which may be caused by the influence of different circulation characteristic factors in different regions and scales. therefore, to understand the causes of precipitation change (a) (b) figure 7. annual precipitation anomaly (a) and morlet wavelet transform (b) of dalian from 1955 to 2005 figure 8. the variance of wavelet transform 91tracing method of rare earth elements in surrounding rock of geological formation based on three-dimensional positioning algorithm in different regions in the north, it is necessary to further combine the factors of zonal circulation, meridional wave train, east asian monsoon, and sst to make further research. conclusions in this paper, the precipitation regularity of the four stations in harbin, changchun, shenyang, and dalian from 1955 to 2005 is analyzed. the precipitation data of the four stations in the northern semi-arid area in the past 50 years are obtained by gps remote sensing technology. according to the precipitation data, the regression equation of trend change and the wavelet analysis method are used to describe the trend change of precipitation in four stations and the regular pattern of the periodic structure and the abnormal change of precipitation periodic series in different time scales. among them, in the process of wavelet analysis, the 50 year precipitation anomaly and the frequency domain relationship of wavelet period in four regions are analyzed, and the precipitation tendency rate of each region and the precipitation in the short, medium and long-term oscillation periods of wavelet are obtained. according to the precipitation time series fluctuation reflected by wavelet variance chart, the main period in the comprehensive annual precipitation of four regions is determined, and the annual precipitation in the whole time domain of four regions is studied based on the characteristics of internal variation, and the regularity of periodic variation of precipitation in the northern semi-arid area can be inferred. the results show that based on gps remote sensing technology and wavelet analysis, the proposed method can effectively analyze the periodic change of precipitation in the northern semi-arid area. acknowledgments the national natural science foundation of china (grant no. 61006403). references arrizabalaga, s., & li, z. 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(2018). evaluating the performance of remote sensed rain-use efficiency as an indicator of ecosystem functioning in semi-arid ecosystems. international journal of remote sensing, 39, 3344-3362. _1fob9te _97fs561w4vea issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n1.116550 earth sciences research journal earth sci. res. j. vol. 29, no. 1 (march, 2025): 69 80 g eo te c h n ic s freeze–thaw–induced microstructural alterations and deterioration of physico-mechanical properties in rocks from the himalayan ranges (pakistan) abid nawaz1*, muhammad sajid2,3, waqas ahmed1, abdul rahim asif1 1. national centre of excellence in geology, university of peshawar, peshawar 25120, pakistan 2. department of geology, university of peshawar, peshawar, 25120, pakistan 3. helmholtz-zentrum potsdam, deutsches geoforschungszentrum (gfz), telegrafenberg, potsdam, germany *corresponding author: abid nawaz, email: abidgeo88@uop.edu.pk how to cite item: nawaz, a., sajid, m., ahmed, w., & asif, a. r. (2025). freeze–thaw–induced microstructural alterations and deterioration of physicomechanical properties in rocks from the himalayan ranges (pakistan). earth sciences research journal, 29(1), 69-80. https://doi. org/10.15446/esrj.v29n1.116550 record manuscript received: 09/09/2024 accepted for publication: 10/03/2025 abst r ac t this study investigates in detail the impact of freeze-thaw cycles on the physical and mechanical properties of a variety of rock types, including granulite, amphibolite, limestone, sandstone, granitic gneiss, quartzite, rhyolite, dolerite, and gabbronorite. the selected rock samples underwent several freeze-thaw cycles at temperatures ranging from 25°c to -40°c. after these cycles, tests were performed to assess the response of the rocks to varying temperatures. destructive tests (uniaxial compressive strength, point load index) and non-destructive testing (specific gravity, ultrasonic pulse wave velocity, porosity, and water absorption) were carried out. the density of the induced fractures in each type of rock under investigation was calculated. the fracture density in the samples increased as the number of cycles increased. after fifty cycles, the fracture density of selected rocks increased as follows: 1.12% for sandstone, 1.06% for limestone, 0.59% for rhyolite, 0.48% for gabbronorite, 0.57% for quartzite, 0.76% for granitic gneiss, 0.46% for amphibolite, and 0.43% for granulite. the extent and strength of the fractures increased further with continued cycles. after 100 freeze-thaw cycles, the fracture densities rose to 1.47%, 1.44%, 1.22%, 1.05%, 1.14%, 1.31%, 1.03%, and 1.02%, respectively. similarly, porosity and water absorption levels showed an increased trend. however, as the freeze-thaw process continues, the results indicate a decrease in uniaxial compressive strength (ucs), specific gravity, ultrasonic pulse velocity (upv), and point load strength. these findings highlight how freeze-thaw conditions deteriorate rocks and change their physico-mechanical characteristics, with significant implications for the mining and building sectors. keywords: diverse rock suites, freeze-thaw cycles, physico-mechanical properties, fracture density, mechanical strength palabras clave: muestra de rocas; ciclos de congelación-deshielo; propiedades físico-químicas; densidad de la fractura; fortaleza mecánica alteraciones microestructurales y deterioro de las propiedades físico-mecánicas por ciclos de congelación-deshielo inducidos en rocas del himalaya paquistaní este estudio investiga en detalle el impacto de los ciclos de congelación-deshielo en las propiedades físicas y mecánicas de una variedad de tipos de rocas en las que se incluyen granulitas, anfibolitas, calizas, areniscas, gneises graníticos, cuarcitas, riolitas, doleritas y gabronoritas. las muestras de roca fueron sometidas a varios ciclos de congelación-deshielo a temperaturas que oscilaron entre 25 °c y -40 °c. después de estos ciclos se realizaron las evaluaciones para medir la respuesta de las rocas a la variación de las temperaturas. se realizaron evaluaciones destructivas (resistencia a la compresión uniaxial, índice del punto de carga) y no destructivas (gravedad específica, velocidad de onda de pulso ultrasónico, porosidad y absorción de agua). luego se calculó la densidad de las fracturas ocasionadas en cada tipo de muestras de roca. la densidad de la fractura en las muestras se incrementó a medida que se incrementó el número de ciclos. después de 50 ciclos la densidad de la fractura de las rocas seleccionadas se incrementa de la siguiente manera: 1.12 % para las areniscas, 1.06 % para las calizas, 0.5 % para las riolitas, 0.48 % para las gabronoritas, 0.57 % para las cuarcitas, 0.76 % para los gneises graníticos, 0.46 % para las anfibolitas y 0.43 % para las granulitas. el alcance y fortaleza de las fracturas se incrementa a medida que los ciclos continuan. después de 100 ciclos, la densidad de las fracturas alcanza el 1.47 %, 1.44 %, 1.22 %, 1.05 %, 1.14%, 1.31%, 1.03%, y 1.02%, en el mismo orden del enunciado anterior. de igual manera, los niveles de porosidad y absorción de agua muestran una tendencia de crecimiento. sin embargo, mientras que los ciclos de congelación-deshielo continuan, los resultados señalan un decrecimiento en la resistencia a la compresión uniaxial, la gravedad específica, la velocidad de onda de pulso ultrasónico y en el índice de punto de carga. estos hallazgos resaltan como las condiciones de los ciclos de congelación-deshielo deterioran las rocas y cambian sus características físico-químicas, con las implicaciones que esto trae para los sectores de la minería y la construcción. resu m en https://doi.org/10.15446/esrj.v29n1.116550 https://doi.org/10.15446/esrj.v29n1.116550 https://doi.org/10.15446/esrj.v29n1.116550 70 abid nawaz, muhammad sajid, waqas ahmed, abdul rahim asif 1 introduction freeze-thaw cycles significantly affect the strength and durability of rocks, especially in cold regions or areas where temperatures occasionally dip below freezing. this environmental stress causes rocks to degrade due to factors such as moisture content, rock type, and the frequency of freeze-thaw cycles (zhao et al., 2024). these effects pose challenges to various industries, including construction, railroads, and the preservation of stone monuments (talalay, 2022; 2023). as such, understanding rock resistance to freeze-thawinduced deterioration is crucial when selecting building materials (asif et al., 2022; 2024). although freeze-thaw damage is more prominent in colder climates, it also occurs in regions with intermittent freezing temperatures, particularly during nightly freezes (kaczmarek et al., 2021). the repeated freezing and thawing of water within the rock’s pores generates internal pressure, which leads to microfractures and reduced strength (chen et al., 2023). these processes significantly influence various physical and mechanical properties, including porosity, permeability, and compressive strength (liu et al., 2020; mal’a et al., 2024). the resistance of rocks to freeze-thaw cycles is strongly influenced by their mineral composition, texture, and structural properties. sedimentary rocks such as limestone and sandstone, composed of calcite, dolomite, quartz, and feldspar, are generally more vulnerable due to their higher porosity and the susceptibility of calcite to dissolution (deprez et al., 2020). sandstone deteriorates due to intergranular fractures that allow water infiltration during freeze thaw process (niu et al., 2021). furthermore, metamorphic rocks, including amphibolite, granulite, and quartzite, exhibit greater resistance due to their recrystallized and interlocking mineral structures (citak et al., 2023). amphibolite and granulite, consisting of amphibole, garnet, pyroxene, and quartz, have low porosity, limiting water ingress, though pre-existing fractures may serve as weakness zones (islands et al., 2002). quartzite, being primarily composed of quartz, is highly resistant but can develop microcracks over time (hou et al., 2025). moreover, igneous rocks display varied behavior during freeze thaw process. rhyolite, with its fine-grained texture and high silica content, is prone to microcracking (schwamborn et al., 2012). moreover, dolerite and gabbronorite, composed of pyroxene, olivine, and plagioclase, exhibit higher durability due to their crystalline structure and lower porosity (claridge and campbell, 2005). this study aims to investigate the effects of freeze-thaw cycles on the mechanical and physical properties of nine rock types, selected based on their varied characteristics (table 1). the rocks were divided into three groups: untreated, exposed to 50 cycles, and exposed to 100 cycles. the experimental approach involved assessing properties such as porosity, specific gravity, pulse velocity (vp), unconfined compressive strength (ucs), point load strength, and water absorption. additionally, fracture density was quantified and statistically compared with the engineering properties after specific freeze-thaw cycles through regression analysis. while previous studies have explored freeze-thaw effects on various rocks, this research bridges a gap by focusing on a diverse set of rock types under controlled laboratory conditions, allowing for a deeper understanding of how different rocks respond to repeated freeze-thaw stresses. the novelty of this study lies in its comprehensive approach to comparing the impacts of freeze-thaw cycles on multiple rock types and correlating these effects with changes in mechanical performance, providing valuable insights for industries that rely on rock durability in cold environments. 2 geology of northern pakistan in northwest pakistan, there are three primary tectonic domains: the indian plate, the kohistan island arc (kia), and the eurasian plate. the kia originated from intra-oceanic subduction in the tethys ocean and collided with the indian plate at the main mantle thrust (mmt) regional fault zone (fig. 1) (searle and treloar, 2010; burg, 2018). to the north of kohistan island arc, the primary sutures of the main karakoram thrust (mkt) mark its boundary with the karakoram block. two collisional events, first with the eurasian plate and later with the indian plate, resulted in complicated deformation and metamorphism of the kia rocks (suo et al., 2022). the kia is made up of sedimentary, volcanic, and plutonic rocks and is divided into six main sections (ali et al., 2021). the mafic and ultramafic jijal complex is the deepest area of the kia and it marks the transition between the upper mantle and lower crust (ali et al., 2021; ullaha et al., 2022). the kamila amphibolite belt was formed by the metamorphism of the gabbroic and mafic to intermediate metavolcanics up to the amphibolite facies (rehman and arif, 2020). the chilas complex is a large-scale late cretaceous magmatic structure located north of kamila amphibolite. it is largely made of gabbronorite, with smaller amounts of troctolite, quartz-diorite, and gabbro (lutfi et al., 2023). the kohistan batholith is made up of early cretaceous chalt volcanics with andesitic lavas, tuffs, agglomerates, and calc-alkaline granitoids (jadoon et al., 2020; ali et al., 2021). greywackes, pelites, and volcano-clastic rocks form the rocks of the yasin and chalt groups in the northern kia (qureshi et al., 2020). the three tectonic units that comprise the northern indian plate to the south are the lesser himalayan sequence (lhs), the greater himalayan sequence (ghs), and the tethyan himalayan sequence (ths). the main central thrust (mct) and the south tibetan detachment system (stds) are the two regional fault systems that split these units (fig. 1). different granitic suites have been detected in the himalayan terrane: early paleozoic suites brought about by accretionary orogenesis, late paleozoic suites connected to the rift, and early cenozoic suites connected to the himalayan collision (sajid et al., 2018; jain et al., 2020; soret et al., 2021; sen et al., 2022). northern pakistan experiences a diverse climate, ranging from temperate to alpine conditions due to its high-altitude terrain. the region undergoes significant seasonal temperature variations, with summer temperatures exceeding 30°c in lower valleys, while winter temperatures in higher elevations frequently drop below freezing. these fluctuations lead to repeated freeze-thaw cycles, particularly in areas above 2,500 meters where diurnal temperature variations cross the freezing threshold. additionally, substantial precipitation, including monsoonal rains and heavy snowfall, contributes to rock saturation, enhancing the susceptibility of various lithologies to freeze-thaw weathering. this climatic setting makes freeze-thaw processes a dominant factor in rock degradation, slope instability, and infrastructure deterioration in the region. figure 1. geological map of the western himalayas in pakistan. modified after dipietro et al., 2001. the black stars show the studied sample location. ry=rhyolite; am=amphibolite; dl=dolerite; l.st=limestone; qz=quartzite; hgn= hazara granitic gneiss; s.st=sandstone; gn=gabbronorite; gg=granulite. 3 methods 3.1 preparation of samples and freeze-thaw cycling the mineralogical composition, geographic positions, and grain size descriptions of the rocks under investigation are given in table 1. during 71freeze–thaw–induced microstructural alterations and deterioration of physico-mechanical properties in rocks from the himalayan ranges (pakistan) fieldwork, bulk samples of these rock types were collected, from which cubic inch-sized samples were extracted. these samples were then analyzed at the national centre of excellence in geology (nceg), university of peshawar. these cubic specimens underwent a series of freeze-thaw cycles, each lasting eight hours. during each cycle, the samples were exposed to -40°c for 4 hours followed by thawing at room temperature (25°c) for the remaining 4 hours. the investigations included subjecting the samples to 50 and 100 cycles. following the freeze-thaw cycles, thin sections were prepared from the samples to analyze changes in their respective petrographic properties. fig. 2 shows the steps involved in conducting this research work. 3.2 fracture delineation for a more thorough analysis of the samples’ internal structure and related variations, thin sections were prepared from the samples. these thin sections were photomicrographed using petrographic microscopes. individual fractures seen in the photomicrographs of the treated samples were highlighted and compared with the thin section observation of untreated samples (room temperature) using the imagej program. the area and extent of each fracture present in the photomicrographs were precisely calculated, and the total area of the rock specimen mounted on the thin section was determined. fracture density was assessed using the following formula: fracture density = 100% (area of thin section-area of fractures)/area of thin section * 100 while examining thin sections to find cracks caused by freeze-thaw (f-t) cycling, specific features were considered. at first, different fracture patterns were seen in the treated and untreated samples. these patterns included straight or curved lines crossing or passing through the rock matrix. these cracks may appear in the thin section as black isotropic streaks or discontinuities. leaching near the fractures, which caused observable color changes or staining patterns after therapy, was another important sign. in the thin section, f-t-generated cracks showed broad discontinuities cutting through several mineral grains, a characteristic absent from untreated materials. these fractures are distinguished from other kinds of discontinuities or cleavages because of their irregular and jagged patterns. the gabbronorite sample treated with 50 f-t cycles is shown in fig. 3, where fractures are indicated by green lines. in fig. 3, fractures are shown in relation to the overall area taken into consideration in the thin section. table 1. descriptions of the rocks under investigation s. no rock type symbol geological unit coordinates mineralogy grain size 1 dolerite dl panjal dykes 34.23113000 72.65824833 clinopyroxene plagioclase olivine medium grained 2 rhyolite ry dir volcanics 35.21587866 71.98874537 orthopyroxene clinopyroxene plagioclase fine grained 3 gabbronorite gn kia 35.511531 73.773089 calcium-rich plagioclase hypersthene orthopyroxene coarse grained 4 sand stone s.st murree formation 33.64813945 72.97608807 quartz potash feldspar biotite coarse grained 5 limestone l.st nowshera formation 34.14549205 72.47979880 calcite dolomite fine grained 6 quartzite qz tanawal formation 34.10182333 72.53580833 quartz biotite medium to coarse grained 7 granitic gneiss hgn hazara granitic gneiss 34.29825500 72.73238000 quartz, potassium feldspar sodium feldspar coarse grained 8 amphibolite am kia 34.94245500 72.02215000 hornblende plagioclase quartz coarse grained 9 granulite gg kia 35.063864 72.949761 amphibole clinopyroxene garnet quartz medium to coarse grained 72 abid nawaz, muhammad sajid, waqas ahmed, abdul rahim asif freeze thaw induced microstructural alterations and deterioration of physico-mechanical properties in rocks from himalaya ranges (pakistan) literature review site selection fieldwork lab work results and discussions petrographic analysis (before and after freeze-thaw-treatment) geothecnical analysis (before and after freeze-thaw treatment specific gravity water absorption porosity ultrasonic pulse velocity unconfined compressive strength point load fracture analysis using image j porosity ratio upv ratios strengh deterioration ratio conclusions mitigation strategies for freeze thaw damage figure 2. flowchart illustrates the approaches employed in this research study 3.3 physical and mechanical testing the treated samples’ mechanical properties (unconfined compressive strength and point load test) and physical properties (specific gravity, water absorption, porosity, and ultrasonic pulse velocity) were evaluated in accordance with the standards set by the american society for testing and materials (astm). a pitch-catch method, using a transmitter and a receiver as two transducers, was used to calculate the ultrasonic pulse velocity (upv) for each specimen (astm-d-2845-08). the samples were weighed in the following conditions: air, water, oven-dry, and saturated in water for an entire day. following this, the bulk water absorption, bulk saturated specific gravity (dc97/c97m-18), and bulk porosity were determined using the formulas outlined below: water absorption = (water saturated weight – dry weight)/dry weight specific gravity = saturated weight/ (saturated weight – weight in water) porosity = (weight in air – dry weight)/ (weight in air – weight in water) the unconfined compressive strength and point load testing were carried out as per astm (d7012–14e1) and astm (d 5731) respectively 4. petrography the petrographic analysis of the rock samples revealed distinct mineralogical, textural, and structural characteristics for each rock type, as summarized below: 4.1 rhyolite (ry): rhyolite from the dir volcanics is fine-grained with a porphyritic texture. the groundmass consists of microcrystalline quartz and feldspar, with phenocrysts of orthopyroxene, clinopyroxene, and plagioclase (fig. 4 a). the feldspar phenocrysts exhibit perthitic textures, and quartz grains show undulatory extinction. figure 3. fracture pointed out after 50 freeze-thaw cycles in gabbronorite 44.2 dolerite (dl): dolerite samples, collected from the panjal dykes, exhibit a mediumgrained texture dominated by clinopyroxene, plagioclase, and olivine (fig. 4 b). the clinopyroxene grains display high relief and weak pleochroism, while plagioclase appears as tabular grains with polysynthetic twinning. olivine is present as subhedral to anhedral grains, often showing signs of alteration. 73freeze–thaw–induced microstructural alterations and deterioration of physico-mechanical properties in rocks from the himalayan ranges (pakistan) 4.3 gabbronorite (gn): gabbronorite samples from the kohistan island arc (kia) are coarsegrained, with calcium-rich plagioclase, hypersthene, and orthopyroxene as the primary minerals (fig. 4 c). plagioclase grains display polysynthetic twinning, while orthopyroxene exhibits weak pleochroism. the interlocking texture of the minerals contributes to the rock’s durability. figure 4. photomicrographs of thin sections of igneous rocks under polarized light. (a) rhyolite composed of quartz (qz), biotite (bt), and potash feldspar (kf); (b) dolerite consisting of clinopyroxene (cpx), olivine (olv), and plagioclase (plg); (c) gabbronorite composed of plagioclase (plg), orthopyroxene (opx), and clinopyroxene (cpx). 4.4 limestone (l.st): limestone samples from the nowshera formation are fine-grained, predominantly composed of calcite and dolomite (fig. 5 a). calcite grains exhibit characteristic birefringence, while dolomite appears as subhedral to anhedral grains, sometimes showing twinning. the rock has a crystalline texture with occasional microfossil fragments. 4.5 sandstone (s.st): sandstone from the murree formation is coarse-grained, composed primarily of quartz, potash feldspar, and biotite (fig. 5b). quartz grains are rounded to subangular, showing undulatory extinction, while biotite occurs as elongated, pleochroic flakes. the intergranular spaces are often filled with secondary cementing materials. figure 5. photomicrographs of thin sections of sedimentary rocks under polarized light. (a) limestone primarily composed of calcite (cal) and dolomite (dol); (b) sandstone consisting of quartz (qz), biotite (bt), and potash feldspar (kf). 4.6 amphibolite (am): amphibolite samples from the kia are coarse-grained, composed of hornblende, plagioclase, and quartz (fig. 6 a). hornblende exhibits strong pleochroism from green to brown, while plagioclase displays polysynthetic twinning. the rock has a foliated texture due to the alignment of hornblende crystals. 4.7 granulite (gg): granulite from the kia is medium to coarse-grained, with amphibole, clinopyroxene, garnet, and quartz as the primary minerals (fig. 6b). garnet appears as isotropic, subhedral to euhedral grains, while clinopyroxene shows high relief. the rock has a granoblastic texture with interlocking mineral grains. 4.8 granitic gneiss (hgn): hazara granitic gneiss is coarse-grained, with quartz, potassium feldspar, and sodium feldspar as the primary minerals (fig. 6 c). the rock exhibits a gneissic texture with distinct mineral banding. quartz grains show undulatory extinction, while feldspars display perthitic textures. 74 abid nawaz, muhammad sajid, waqas ahmed, abdul rahim asif 4.9 quartzite (qz): quartzite from the tanawal formation is medium to coarse-grained, with quartz as the dominant mineral and minor biotite (fig. 6 d). the quartz grains show interlocking textures with sutured contacts, indicative of metamorphic recrystallization. biotite occurs as scattered flakes within the quartz matrix. figure 6. photomicrographs of thin sections of metamorphic rocks under polarized light. (a) amphibolite composed of plagioclase (plg), hornblende (hbl), and quartz (qz); (b) granulite consisting of amphibole (amp), garnet (grt), clinopyroxene (cpx), and quartz (qz); (c) hazara granitic gneiss comprising amphibole (amp), quartz (qz), garnet (grt), and plagioclase (plg); (d) quartzite composed primarily of quartz (qz) and biotite (bt). 5 results 5.1 impacts of mineralogical composition on freeze-thaw cycling the mineralogical composition of the rock types significantly influences their resistance to freeze-thaw cycles by affecting their porosity, permeability, and mechanical strength. sedimentary rocks, such as limestone and sandstone, contain minerals like calcite, dolomite, quartz, and feldspar, which exhibit varying responses to freeze-thaw weathering. limestone, being rich in calcite and dolomite, is more susceptible to freeze-thaw damage due to dissolution and microcrack propagation along grain boundaries. sandstone, primarily composed of quartz and feldspar, exhibits moderate resistance, though intergranular fractures can facilitate water infiltration and enhance degradation over repeated freeze-thaw cycles. metamorphic rocks generally exhibit higher resistance to freeze-thaw cycles due to their recrystallized textures and interlocking mineral structures. amphibolite and granulite, composed of amphibole, garnet, pyroxene, and quartz, have relatively low porosity, reducing their susceptibility to water infiltration. however, pre-existing fractures and foliation planes can act as weak zones where freeze-thaw action may induce further damage. quartzite, predominantly composed of quartz, demonstrates high resistance due to its compact texture, though microcracks within quartz grains may contribute to localized weakening. igneous rocks display variable resistance depending on their mineral composition and cooling history. rhyolite, with its high silica content and fine-grained texture, is prone to microcracking and freeze-thaw deterioration, particularly along feldspar and quartz phenocrysts. dolerite and gabbronorite, composed of pyroxene, olivine, and plagioclase, generally exhibit higher resistance due to their crystalline structure and lower porosity. however, intergranular fractures and thermal contraction cracks may serve as pathways for water infiltration, potentially accelerating degradation under repeated freeze-thaw cycling. 5.2 freeze-thaw-induced fractures the fracture density in the samples under investigation increased with the number of cycles (fig. 7). following fifty cycles, the fracture density for different types of rocks increased as follows: 1.12% for sandstone, 1.06% for limestone, 0.59% for rhyolite, 0.48% for gabbronorite, 0.57% for quartzite, 0.76% for granitic gneiss, 0.46% for amphibolite, and 0.43% for granulite (fig. 7). the extent and strength of the fractures increase as the cycles continue. its fracture densities are increased to 1.47%, 1.44%, 1.22%, 1.05%, 1.14%, 1.31%, 1.03%, and 1.02%, respectively, following 100 freeze-thaw cycles. granulite exhibits the greatest resistance to freeze-thaw, however, sandstone and limestone are very susceptible to fragmentation after cycles of freezing and thawing (fig. 8). fig. 7. the fracture density of rock samples following different freeze-thaw cycles figure 8. the distribution of fractures in rocks with an increasing number of freeze-thaw cycles number of cycles 50 cycles rocks dolerite rhyolite gabbronorite sandstone limestone quartzite gneiss amphibolite granulite 100 cycles 75freeze–thaw–induced microstructural alterations and deterioration of physico-mechanical properties in rocks from the himalayan ranges (pakistan) 5.3 ucs fig. 9a shows the unconfined compressive strength (ucs) values following treatment cycles and at room temperature. test results show that limestone loses strength very gradually, but sandstone loses strength more quickly than limestone. granulite had the highest strength following cycles and the strongest resilience among all the samples under study (fig. 9a). its total percentage strength drop during freeze-thaw cycling is also less than that of other forms of rock. after 50 cycles, the ucs decreased by the following percentages: granitic gneiss by 8.82%, rhyolite by 7.41%, quartzite by 5.43%, dolerite by 4.55%, gabbronorite by 3.31%, amphibolite by 2.33%, and granulite by 1.49%. as previously indicated, sandstone and limestone were severely impacted; after 50 freeze-thaw cycles, the ucs of limestone decreased by 10.20% and that of sandstone by 14.29%. a further decrease in ucs occurred as the number of cycles rose. following 100 cycles, the ucs decreased by the following percentages: granitic gneiss by 17.65%, rhyolite by 13.58%, dolerite by 10%, quartzite by 11.95%, gabbronorite by 7.43%, amphibolite by 6.98%, and granulite by 4.47%. after 100 freeze-thaw cycles, the ucs of limestone decreased by 22.45% and that of sandstone by 28.57%. 5.4 point load the results clearly show that the point load strength of the studied rock samples is significantly impacted by freeze-thaw cycling. the change in point load strength is minimum when rocks are subjected to freeze-thaw cycles. however, as the number of f-t cycles increases, there is a significant and notable decrease in point load strength (fig. 9b). after 100 freeze-thaw cycles, the point load of the rocks decreases by the following percentages: granitic gneiss by 20.58%, rhyolite by 17.95%, quartzite by 13.95%, dolerite by 10.20%, gabbronorite by 9.8%, amphibolite by 7.14%, and granulite by 6.25%. additionally, sandstone experiences a 33.33% reduction, while limestone undergoes a 25% reduction after 100 cycles. figure 9. the impact of freeze-thaw on different physico-mechanical properties of rocks. figure 9a illustrates unconfined compressive strength (ucs), b showcases point load strength, c presents porosity, d displays water absorption, e represents specific gravity and f shows ultrasonic pulse velocity (upv) 76 abid nawaz, muhammad sajid, waqas ahmed, abdul rahim asif table 2. results of unconfined compressive strength (ucs), point load strength, and fracture distribution in rocks after ft cycles rock types number of cycles 0 cycle 50 cycles 100 cycles ucs (mpa) dl 110 105 99 ry 81 75 70 gn 121 117 112 s.st 42 36 30 l.st 49 44 38 qz 92 87 81 hgn 68 62 56 am 129 126 120 gg 134 132 128 point load (mpa) dl 49 46 44 ry 39 36 32 gn 51 48 46 s.st 21 18 14 l.st 28 25 21 qz 43 40 37 hgn 34 31 27 am 56 54 52 gg 64 62 60 upv (m/s) dl 5107 4900 4687 ry 4511 4281 4081 gn 5298 5108 4911 s.st 3275 3029 2765 l.st 3567 3329 3090 qz 4941 4718 4511 hgn 4186 3950 3716 am 5488 5311 5128 gg 5791 5640 5461 fracture density (%) dl ------------0.49 1.12 ry ------------0.59 1.22 gn ------------0.48 1.08 s.st ------------1.12 1.47 l.st ------------1.06 1.44 qz ------------0.57 1.14 hgn ------------0.76 1.31 am ------------0.46 1.03 gg ------------0.43 1.02 5.5 porosity, water absorption, and specific gravity the porosity test results at the studied temperatures are shown in (fig. 9c). following freeze-thaw cycles, the samples’ overall porosity increased, presumably as a result of an increase in fracture intensity. after 100 cycles, the porosity values of dolerite, rhyolite, gabbronorite, quartzite, granitic gneiss, amphibolite, and granulite increased by 6.62%, 8.07%, 4.27%, 7.21%, 8.69%, 3.04%, and 2.79%, respectively. furthermore, after the same number of cycles, the porosity of sandstone increased by 13% and that of limestone by 10.35%. the water absorption measurements also show a comparable rise (fig. 9d). after 100 cycles, the percentage increase in water absorption for the rocks is as follows: granitic gneiss by 9.77%, rhyolite by 8.15%, quartzite by 6.93%, dolerite by 6.2%, gabbronorite by 5.75%, amphibolite by 4.65%, and granulite by 3.1%. in addition, after 100 cycles, the water absorption of sandstone rose to 15.34% and that of limestone to 12.1%. the investigation’s findings show that freeze-thaw affects specific gravity, as shown in (fig. 9e). as the number of cycles increases, a visible decline becomes apparent. after 100 cycles, the percentage decreases in the specific gravity of the rock samples under study are as follows: granitic gneiss by 1.73%, rhyolite by 1.29%, dolerite by 1%, quartzite by 1.21%, gabbronorite by 0.99%, amphibolite by 0.69%, and granulite by 0.4%. after the same number of cycles, the specific gravity of sandstone declined by up to 1.89% and that of limestone by up to 1.63%. 5.6 upv according to experimental results, the upv values of rock samples decrease as the number of freeze-thaw cycles increases (fig. 9f). first, there was a small difference in upv; however, as the number of cycles increased, upv significantly decreased. after 100 cycles, the upv values decreased as follows: granitic gneiss by 11.22%, rhyolite by 9.53%, dolerite by 8.22%, quartzite by 8.70%, gabbronorite by 7.30%, amphibolite by 6.56%, and granulite by 5.69%. after the same number of cycles, there is a reduction in the upv of sandstone by 15.56% and limestone by 13.37%. these results emphasize that freeze-thaw affects porosity, specific gravity, upv, and water absorption. it also highlights how crucial it is to take temperature changes into account when evaluating the mechanical qualities and integrity of rocks. table 3. results of porosity, water absorption, and specific gravity of studied rocks before and after freeze-thaw cycles rock types number of cycles 0 cycle 50 cycles 100 cycles porosity (%) dl 0.64 0.65 0.68 ry 1.15 1.20 1.25 gn 0.49 0.50 0.56 s.st 2.54 2.77 2.92 l.st 1.98 2.11 2.21 qz 0.95 0.98 1.03 hgn 1.71 1.79 1.88 am 0.35 0.36 0.36 gg 0.31 0.32 0.32 water absorption (%) dl 0.24 0.25 0.26 ry 0.45 0.48 0.49 gn 0.21 0.22 0.23 s.st 1.58 1.75 1.87 l.st 1.35 1.48 1.54 qz 0.34 0.35 0.36 hgn 0.55 0.60 0.61 am 0.16 0.17 0.17 gg 0.13 0.13 0.13 specific gravity dl 2.98 2.97 2.95 ry 2.78 2.77 2.75 gn 3.12 3.12 3.09 s.st 2.65 2.63 2.56 l.st 2.71 2.69 2.66 qz 2.88 2.87 2.85 hgn 2.72 2.70 2.68 am 3.16 3.16 3.14 gg 3.22 3.22 3.21 77freeze–thaw–induced microstructural alterations and deterioration of physico-mechanical properties in rocks from the himalayan ranges (pakistan) 6 discussion the impacts of freeze-thaw cycles on the physico-mechanical properties of rocks vary depending on the composition of the minerals, textural relations, individual mineral behavior, and the fracture and deformational nature of the rocks. for every type of rock, ucs, upv, and porosity were measured after 100 freeze-thaw cycles. the values are given in tables 2 and 3 and (figs. 9 and 10). every rock’s porosity increased along with a drop in ucs and upv due to cycle progression, as seen in tables 2 and 3. these changes vary from rock to rock. the rock deterioration degree is indicated by the increase-decrease ratio, which ranges from 0 to 1. rocks that had degraded at the end of the f–t cycles are given a rating value of 1. on the other hand, a grade of 0 denotes the strongest type of rock. these data show the deterioration grade in ascending order, and vice versa. using the following formula, the increased porosity ratio at the conclusion of the 50 and 100 f-t cycles was determined (khanlari et al., 2015). rp = p(nc)/p(us)-1 where p(nc) is the total porosity at the end of specific number of freezethaw cycles and p(us) is the total porosity of untreated rock types. the rp values are given in table 4; trends in rp values are shown in (fig. 10a). table 4. rp, rvp and sdr values of samples after 50 and 100 f-t cycles 50 cycles rock types rp rvp sdr dl 0.030 0.041 0.045 ry 0.043 0.051 0.074 gn 0.022 0.036 0.033 s.st 0.091 0.075 0.143 l.st 0.066 0.067 0.102 qz 0.033 0.045 0.054 hgn 0.046 0.056 0.088 am 0.017 0.032 0.023 gg 0.016 0.026 0.015 100 cycles dl 0.071 0.082 0.1 ry 0.088 0.095 0.136 gn 0.045 0.073 0.075 s.st 0.150 0.156 0.286 l.st 0.116 0.134 0.224 qz 0.078 0.087 0.120 hgn 0.095 0.112 0.177 am 0.031 0.066 0.070 gg 0.029 0.057 0.045 as shown in tables 3 and 4, sample granulite exhibits the lowest increase in porosity at the end of the 100 f-t cycles, with rp100 of 0.029 (2.9%), whereas sandstone displays the most increase with rp100 of 0.150 (15%). the number of pores in the samples is thought to be related to these changes. during the freezing process, the water in the pore’s contracts due to which its volume increases, causing the pores to expand. as seen in figures 6, 7, and 8, new cracks and pores develop in the rock, depending on how much of these processes have occurred. the trends of the samples’ porosity change are displayed in figure 10a. in the f-t cycle, groundwater or surface water enters the rock through capillary action, increases in volume, and exerts pressure inside the pores and discontinuities where it freezes. this cycle keeps happening, creating adverse deterioration of the rock (su et al., 2024). when water freezes and thaws, weakening, breakdowns, and expansion lead to the production of cracks and fractures in rocks with a high-water absorption capacity, making the decline in the strength of the rock more noticeable. using a similar approach, the decrease in ratios of upv at the end of the 50 and 100 cycles was calculated by using the following equation (khanlari et al., 2015): rvp = 1-vp(nc)/vp(us) where vp(nc) is the p wave velocity of rock samples at the end of 100 freeze-thaw cycles and vp(us) is the p wave velocity of untreated rocks. tables 2 and 4 along with the figures. 9f and 10b demonstrate how advancing the f-t cycles resulted in declining trends for upv. the findings show that the freeze-thaw cycles caused the most damage to the sandstone samples amongst all rock types, with a decreased ratio of rvp100 of 0.156 which indicates a 15.6 % reduction in upv. on the contrary, the granulite sample had the least decrease of rvp100 which is 0.057 which means a 5.7 % reduction in upv. figure 10. trends in rp (a), rvp (b), and sdr(c) of the studied samples. strength deterioration ratio (sdr) at the end of f–t cycles was calculated using the following equation (khanlari et al., 2015): sdr = 1-ucs (nc)/ucs (us) where ucs (nc) is the uniaxial compressive strength of rock samples at the end of number of freeze-thaw cycles and ucs (us) is the strength of untreated samples. the strength deterioration values of rocks are given in table 4 and the 78 abid nawaz, muhammad sajid, waqas ahmed, abdul rahim asif trends of decrease ratios are given in (fig. 10c). sandstone samples represent the most sensitive rocks of all the samples with the highest strength deterioration rate of 0.286. however, the granulite sample had the least damage with an sdr value of 0.045. mitigation strategies for freeze-thaw damage 7 mitigation strategies for freeze-thaw damage freeze-thaw cycles can significantly impact the durability of sandstone and limestone constructions, leading to microcracking, increased porosity, and structural weakening over time. to mitigate these effects, several strategies can be implemented, focusing on reducing water infiltration, enhancing material strength, and improving environmental management. 7.1 surface treatments and sealants applying hydrophobic coatings or sealants can effectively reduce water absorption in porous rocks like sandstone and limestone. these treatments create a protective layer that prevents moisture penetration while allowing the material to breathe, reducing internal stress caused by freeze-thaw cycles. however, the selection of sealants should consider permeability to avoid trapping moisture within the stone, which could exacerbate damage. 7.2 improved drainage systems proper drainage around structures is crucial to minimizing water exposure. installing adequate gutters, sloping surfaces, and subsurface drainage systems can help divert water away from foundations and rock surfaces, limiting prolonged saturation and reducing the potential for freeze-thaw deterioration. 7.3 material selection and reinforcement using more durable rock variants or combining sandstone and limestone with reinforcing materials can enhance structural resilience. in construction, selecting denser, lower-porosity stone reduces vulnerability to freeze-thaw damage. additionally, incorporating protective barriers, such as waterproof membranes or composite materials, can improve longevity. 7.4 environmental and maintenance strategies regular maintenance, such as cleaning debris, inspecting cracks, and applying protective coatings when necessary, ensures long-term preservation. in colder climates, minimizing direct exposure to de-icing salts, which can accelerate chemical weathering and porosity increase, is also essential. additionally, designing structures with overhangs or protective facades can shield rock surfaces from excessive moisture accumulation. by implementing these mitigation strategies, sandstone and limestone constructions can better withstand freeze-thaw cycles, prolonging their lifespan and structural integrity in harsh climatic conditions. 8. conclusions this study extensively examined how freeze-thaw cycles impact the physico-mechanical characteristics of rocks, including porosity, specific gravity, water absorption, and ultrasonic pulse wave velocity (upv). various volcanic, sedimentary, and metamorphic rock samples underwent multiple freeze-thaw cycles, with their properties assessed at different stages. the results revealed significant variations in mechanical properties, emphasizing the importance of considering freeze-thaw effects in geological and engineering contexts, particularly in regions with fluctuating temperatures around the freezing point of water, such as northern pakistan and similar climates. understanding rock deterioration processes due to freeze-thaw weathering is crucial. this research is relevant for engineering applications in cold regions, aiding in construction and maintenance, especially where snowfall is frequent. from this study, the following conclusions are drawn: • p-wave velocity, point load strength, specific gravity, and uniaxial compressive strength were shown to decrease with an increase in the number of freeze-thaw cycles, although porosity and water absorption values showed a positive trend. • fracture density considerably increased with freeze-thaw cycling resulting in the disintegration of rocks. • sandstone exhibited pronounced porosity increase and reduced p-wave velocity, correlating with higher strength deterioration ratios (sdr), highlighting its vulnerability to freeze-thaw cycles. conversely, granulite, with lower porosity and minimal p-wave velocity reduction, shows comparatively lower sdr values, emphasizing the crucial role of porosity and p-wave velocity in assessing rock stability. acknowledgments the authors would like to acknowledge the national centre of excellence in geology at the university of peshawar (pakistan) for providing laboratory facilities and field logistics. 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sediment-hosted; colombiantype; gemstones palabras clave: esmeraldas; tipo colombia; hospedado en sedimentos; gemas how to cite item romero-ordóñez, f. h., gonzález-durán, a. f., garcía-toloza, j., rotlewicz-cohen, j., cedeñoochoa, c. j., alvarado-gonzález, h. r., & angarita-sarmiento, l. g. (2021). mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia. earth sciences research journal, 25(2), 139-156. doi: https:// doi.org/10.15446/esrj.v25n2.90210 the cunas mine is currently one of the major producers of fine emeralds in colombia; its emeralds typically display a magnificent green hue, which is highly appreciated globally. the mineralization is found in vanadium-rich black shales of the muzo formation. emeralds occur in pockets within hydrothermal veins and breccias, mainly consisting of calcite, dolomite, albite, quartz, minor pyrite, parisite-(ce), and fluorite. hydrothermal alteration is pervasive and dominated by albitization and carbonatization. emerald-hosted fluid inclusions are highly abundant and remarkably large and complex. poly-phase inclusions are ubiquitous, occur both in emeralds and gangue minerals, and consist of two daughter crystals (typically halite and calcite or siderite; exceptionally parisite-(ce)), a liquid brine, a co2-n2-ch4-rich gas bubble, and occasionally minor liquid co2. vapor-rich inclusions were observed in quartz, and two-phase inclusions were identified in calcite and dolomite, thus suggesting a complex fluid evolution. microthermometry analysis indicates that emerald-forming fluids were trapped at relatively low temperature ~260340°c and pressure ~850-2400 bar, with relatively high density 1.03 g/cm³, and elevated salinity ~39 wt% nacl eq.; other aqueous components detected include cacl2, kcl, and fecl2. based on these data, authors propose the emerald mineralization at the cunas mine was originated by the mixing of two hydrothermal fluids of different sources; one fluid with high salinity derived from evaporite dissolution, responsible for the albitization of the host rocks; the second is a calcium-rich fluid evolved from connate waters, which was equilibrated by the interaction with calcareous and organic-rich wall rocks. as a result, emerald mineralization took place at structurally favorable sites proper for fluid mixing. the described geological and physicochemical features for the cunas mine agree with epigenetic sediment-hosted mineralization —colombian-type— formed by the circulation and mixing of relatively low-temperature non-magmatic fluids. abstract mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia mineralogía e inclusiones fluidas de la mina de esmeraldas “cunas”, maripí, boyacá, colombia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.90210 la mina cunas es actualmente una de las principales productoras de esmeraldas de alta calidad en colombia. sus gemas se caracterizan por un intenso tono verde muy apreciado en el mercado mundial. la mineralización se encuentra en lodolitas negras, ricas en vanadio, de la formación muzo. las esmeraldas se encuentran al interior de venas hidrotermales y brechas, compuestas principalmente por calcita, dolomita, albita, cuarzo y, en menor medida, pirita, parisita(ce) y fluorita. la alteración hidrotermal asociada es pervasiva y está dominada por la albitización y la carbonatización. las inclusiones fluidas alojadas en esmeraldas son muy abundantes, de gran tamaño y alta complejidad. las inclusiones fluidas polifásicas son ubicuas, ocurren tanto en esmeraldas como en minerales de ganga, y generalmente están compuestas por dos cristales hijos (típicamente halita y calcita o siderita; excepcionalmente parisita(ce)), una salmuera, un gas rico en co2-n2-ch4, y ocasionalmente co2 líquido. también se observaron inclusiones ricas en vapor en cuarzo y se identificaron inclusiones bifásicas en calcita y dolomita, lo que sugiere una evolución de fluido compleja. el análisis de microtermometría indica que los fluidos formadores de esmeraldas quedaron atrapados a una temperatura relativamente baja ≈ 260-340 ° c y una presión ≈ 875-2400 bar, con una densidad relativamente alta —1,03 g / cm³— y una salinidad elevada de 39% nacl eq. peso; otros componentes acuosos detectados fueron cacl2, kcl y fecl2. con base en estos datos, los autores proponen que la mineralización de esmeraldas en la mina cunas se originó por la mezcla de dos fluidos hidrotermales de diferentes fuentes. un fluido con alta salinidad derivado de la disolución de evaporitas, el cual es responsable de la albitización de las rocas hospedantes. el segundo es un fluido rico en calcio derivado de aguas connadas, que fue equilibrado por la interacción con rocas calcáreas y carbonosas. como resultado, la mineralización de esmeraldas ocurrió en sitios estructuralmente favorables donde se promovió la mezcla de fluidos. las características geológicas y fisicoquímicas descritas para la mina cunas, están de acuerdo con una mineralización epigenética alojada en sedimentos "tipo colombiano" formada por la circulación y mezcla de fluidos no magmáticos de temperatura relativamente baja. resumen record manuscript received: 29/08/2020 accepted for publication: 05/02/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 139-156 fernando helí romero ordóñez1, andrés felipe gonzález-durán2*, javier garcía-toloza2, jimmy rotlewicz cohen3, carlos julio cedeño ochoa2, holman rolando alvarado gonzález2, luis gabriel angarita sarmiento2 1departamento de geociencias, universidad nacional de colombia. bogotá. av. carrera 30 # 45-03 2technological development center for the colombian emeralds cdtec. bogotá. calle 12b #6-82 3gemtec s.a.s. bogotá. av. jiménez 5-43 of. 902 *corresponding author: geologist@gemlabcdtec.com m in er al o g y mailto:geologist@gemlabcdtec.com 140 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento introduction the geological setting of the colombian emerald deposits is remarkably unique in comparison with other locations. the mineralization is restricted to lower cretaceous marine rocks and linked to the circulation of non-magmatic hydrothermal fluids (branquet et al., 1999; ortega 2007; giuliani et al., 2015; terraza, 2019). despite sediment-hosted emerald mineralization having been reported in canada (hewton et al., 2013) and united states (e.g. keith et al., 2002), gem-quality emeralds are only being extracted in colombia. on the other hand, other fine emerald deposits worldwide are associated with igneous or metamorphic processes (groat et al., 2008; giuliani et al., 2019). additionally, the source of the chemical components required for the emerald formation —beryllium, chromium, vanadium, aluminum, and silicon— is thought to be a single set of rocks in the colombian deposits: the black shale strata; whereas in other countries more than one rock type is involved. for instance, the emerald deposits of the kafubu area in zambia, are formed by the interaction of schist —derived from ultramafic rocks enriched with iron, chromium, and vanadium— with beryllium-bearing pegmatites (zwaan et al., 2005). in colombia, emerald deposits can be divided into two mining belts (eastern and western). the districts of chivor, gachalá, and macanal are grouped in the eastern belt; whereas muzo — quipama, maripí, coscuez, peñas blancas and yacopí — la palma are part of the western area. the mineralization style is roughly the same in both belts, however, some significant differences can be noticed regarding the host rocks, mineral assemblages, and the structural setting (branquet et al., 1999; terraza, 2019). the maripí district is a prominent emerald mining area composed of several mines, including la pita, cunas, totumos, españoles, and polveros (figure 1) (reyes et al., 2006), and it is currently the second-largest emerald producer in colombia. the study of fluid inclusions has yielded crucial information about the genesis of the colombian emeralds. for instance, roedder (1963) reported the presence of various salts, co2, and calcite, in a pioneering study of microthermometric data of fluid inclusions, suggesting a hypersaline mineralizing fluid derived from the interaction with evaporitic beds. further studies —putting together isotopic and chemical approaches— have shown the presence, in all the deposits, of primary three-phase (halite, brine, and gas) or multiphase fluid inclusions containing co2 and n2 in the vapor phase (kozlowski et al., 1988; giuliani et al., 1990; romero, 1993; ottaway, 1994; giuliani et al., 2000; banks et al., 2000). halite, sylvite, sulfate and chloride compounds, with cations (namely ca2+, fe2+, mn2+, and cr3+) were identified. the microthermometric data also indicate a range of salinity from 38 to 50 wt% nacl eq.; average homogenization temperatures are 250° c for the eastern belt and 380°c for the western belt; and high density —1.20-1.22 g/ cm³— hydrothermal solutions (kozlowski et al., 1988; giuliani et al., 1990; romero, 1993; ottaway, 1994; giuliani et al., 2000; banks et al., 2000). based on these results, an unusual sediment-hosted model involving non-magmatic hydrothermal brines has been proposed as the mechanism responsible for the emerald mineralization (giuliani et al., 2019). despite the extensive available information dealing with the geology and geochemistry of muzo and other colombian emerald districts, few data have been published regarding the maripí mines. in this study, we documented the main mineralogical features of the cunas mine as well as data fluid inclusions trapped in the emerald crystals, aiming to unravel the physicochemical conditions that gave rise to the emerald mineralization. distinct spectroscopic and mineralogical techniques were applied, and the results were compared with available data from other emerald mines in colombia and other countries. figure 1. geological map of the maripí emerald district in colombia. adapted from reyes et al. (2006). 141mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia geological framework the colombian emerald deposits are located in the eastern andean cordillera, especially in the area of boyacá. the eastern cordillera has arisen as a result of multiple and protracted tectonic processes involving pre-cretaceous basement and mesozoic —cenozoic basins. particularly, the sedimentary record evidences early cretaceous rifting and thermal subsidence that led to the extensive accumulation of marine-dominated strata. subsequent late cretaceous crustal shortening gave rise to a shift in the structural setting of the sedimentation, where a compressional style predominated. paleocene— oligocene compressional uplifts promoted the generation of sub-basins (magdalena hinterland basin and llanos foreland basin) and the sedimentation was significantly influenced by continental inputs. further uplifting caused the exhumation of a cordillera dominated by sedimentites between ~13 and 3 ma (van der hammen, 1958; cooper et al., 1995; mojica, 1995; acosta et al., 2007, horton et al., 2020; mora et al., 2020). the two emerald belts lie on lower cretaceous marine strata. in the western emerald belt, the main lithostratigraphic units include the rosablanca formation, which predominantly comprises dark grey and black limestones, and it is overlain by the furatena formation, which consists of a series of claystones and siltstones; followed by the muzo formation, consisting of up to 300 meters of calcareous mudstones, black shales, and minor micrites; the capotes formation, consisting of a thick succession of claystones and siltstones, and finally, a succession dominated by siltstones, sandy siltstones and thick layers of sandstones from the tablazo formation (figure 2) (reyes et al., 2006; terraza 2019). in the muzo maripí area, the emerald mineralization is restricted to the hauterivian-barremian (115-125 ma) calcareous layers of the muzo formation (reyes et al., 2006; terraza 2019); these rocks are mostly organic-rich (beus & mineev, 1972; mantilla et al., 2007) and made up of quartz, paragonite, muscovite (2m1 illite), microcline, and minor calcite, pyrite, baryte, with traces of clay minerals such as pyrophyllite, dickite, chlorite, and kaolinite (gonzalezduran et al. 2019). the emeralds are found in nodules, lenses and pockets within veins and hydraulic breccias. the mineralogical and isotopic features are generally similar in all mines, with small differences possibly related to local fluid-rock interactions (giuliani et al., 1995; romero, 1998; pignatelli et al., 2015; niño, 2017). the typical mineral assemblage reported in the maripí district includes calcite, dolomite, albite, pyrite and emerald (mantilla et al., 2007; castaño et al., 2019). the maripí district is 6 km apart from the main muzo mining area. the cunas mine is located northwestern of the maripí town, next to the channel of the minero river and southern from the panache creek (figure 3a). la pita, consorcio, españoles, polveros, caselata, bonanza, and totumos mines are also part of the maripí mining district, which extends more than 6 kilometers conforming an nne gemstone belt (figure 1). in terms of structural framework, the area is dominated by compressive structures developed during a tectonic phase that occurred before the major uplift of the eastern cordillera (branquet et al., 1999). the cunas mine is situated on the eastern limb of the alto la chapa-borbur anticline (reyes et al., 2006). puerto et al. (2012) indicated that the fertile veins in the la pita mine are predominantly oriented n10e to n40e, and suggested a genetic link with the río minero fault, which is a major dextral strike-slip structure (reyes et al., 2006). garcía-toloza et al. (2018) described emerald-bearing veins oriented n30e to n50e in the deepest levels of the cunas mine, and reported that the mineralization is intimately related with hydrothermal breccias and the re-opening of previous antitaxial veins. valls (2017) pointed out the emerald mineralization appears to be controlled by fault junctions, suggesting that mineralization took place where major nne-ssw fault systems (in particular the río minero fault) intersected secondary e-w structures. some authors have proposed different structural models for the mineralization at the western emerald belt; branquet et al. (1999) suggested that the mineralizing fluids migrated through thrust and decollement fault systems during a compressional tectonic event; whereas, terraza (2019) stated that the mineralization is associated with a different array of mesoscopic structures and ruled out a genetical relationship with fault zones. different sources argue that early tectonics promoted fragile deformation controlled by a penetrative strain regime; this eventually led to a series of events: fluid overpressure, infill of preexisting fractures, and further hydraulic brecciation (ingeominas & mora, 2005; terraza, 2019). on the other hand, pignatelli et al. (2015) suggested that the emerald and trapiche formation are associated with thrust propagation and subsequent fluid decompression, a structurally controlled process that took place at the core of anticlines. gonzález-duran et al. (2019) also report that emerald mineralization in the la pava mine (muzo-quipama district) is associated with anticlinal structures; likewise, ingeominas & mora (2005) suggest a genetic link for euclase and emerald mineralization with an anticline fold in the la marina mine (6 km north from cunas). multiple ages have been proposed for the emerald mineralization in the western belt. firstly, cheilletz et al. (1994) reported an ar40/ar39 age in green mica of 32 ± 3 and 36 ± 3 ma; secondly, romero et al. (2000) dated emeralds through the rb87/sr86 method and yielded an age of 67 ma; thirdly, mantilla et al. (2007) through the re-os method, obtained an age of 12.4 ± 0.9 ma; and finally, svadlenak (2015) reported ages of 62.05 ± 0.63, 30.25 ± 0.25, and 30.24 ± 0.06 ma through ar40/ar39 analysis in micas. the data are highly dispersive, and consequently there are no conclusive statements for the mineralization timing. yet there is no geochronological data for any deposit in the maripí area. methods twenty rock samples, with consecutive codes rc_01 to rc_20, were collected from the cunas mine and subsequently studied via petrographic studies. the thin sections were observed under reflected and transmitted light. these analyses were performed in an olympus bx-51 microscope at the technological development center for the colombian emeralds (cdtec). additionally, twenty-five optimal hexagonal emerald crystals —collected insitu by cdtec— were selected to be studied through multiple spectroscopic and gemological techniques at the cdtec gemological facilities. the crystals are named with the consecutive codes ec_01 to ec_25. the standard gemological study consisted of examination of the twentyfive rough emeralds (ec_01 to ec_25) as well as three cut stones (cs_1, cs_2, and cs_3) through a kruss refractometer, polariscope, chelsea filter (gem-a), and short-wavelength (365 nm) ultraviolet light. for spectroscopic procedures, some crystals were cut and polished to form oriented plates; figure 2. simplified stratigraphic column of the muzo maripí mining area; modified from terraza (2019) 142 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento two directions were considered: parallel and perpendicular planes to the crystallographic c-axis. for comparison purposes, we also analyzed three rough emerald samples from zambia. the microscopy of the crystals and solid inclusions was carried out through a leica gemological microscope coupled with system eckhorst, zeiss-stemi 508 optics with a maximum magnification of 50x and eyepieces with 25x magnification. additionally, an axxio zoom v16 microscope was used to measure inclusions and photography acquisition. infrared spectra (ft-ir) of the twenty-five emeralds were collected using a nicolet 6700 fourier transform infrared spectrometer with a dtgs/ kbr detector and znse polarizer, operating in a range from 7500 to 400 cm-1 with a resolution of 4 cm-1. samples were measured with an orientation of 120 and 210° relative to the c-axis. ultraviolet-visible-near infrared spectra were collected through system spectrometer gemlab ggtl, using a xenon source in the range of 245 to 1050 nm. x-ray diffraction (xrd) studies of a representative sample (ec_01) were performed to obtain a diffraction pattern characteristic of cunas. the sample was first washed with distilled water, then dried and grounded to obtain 2 µm size particles. these studies were performed on a carl zeiss goniometer, model urd-6; the data were processed using the apx-61 aj4 e lcls1 software, 8.4 version. major and trace elements of the twenty-five emeralds were determined by energy dispersive x-ray fluorescence (edxrf); a panalytical epsilon 5 spectrophotometer was used. this instrument has a side window tube of sc/w with a maximum power of 600 w and a generator tube up to 100 kv, which allows the excitement of the k lines of heavy elements. additionally, it has a ge detector cooled to 77 k, and secondary targets of al, ti, fe, ge, zr, mo, ag and al2o3. quantification was made via the fundamental parameters method with a maximum of 20 iterations. beryllium concentration (as oxide) was added manually; the value was obtained from the average data provided by groat et al. (2008) for colombian samples. double polished thin sections of selected samples (ec_02, ec_05, ec_08, and ec_09) were elaborated to conduct microthermometry analysis. the heating and cooling routine was performed through a chaix-mecamtm 85 stage at the university of são paulo. the average rate for cooling and heating was 3-5° celsius per minute or even slower when required. the grouping of fluid inclusions assemblages was carried out following the criteria of roedder (2002). fluid and mineral inclusions were analyzed through raman spectroscopy. the instrument was a horiba labram hr evolution spectrometer with a 532 nm laser, coupled to an olympus bx-41 microscope. the measurements were performed under 250 mw, 30 seconds of exposure, ten accumulation scans and 100% of laser intensity. photoluminescence spectra of the twenty-five emeralds were also acquired with the horiba labram hr evolution equipment, with an acquisition time of 5s, accumulations of ten scans, a wavelength range from 660 to 700 nm, objective plan n 10x / 0.25 ∞ / / fn22, reflection grid 1800 gr/mm, resolution 0.025 nm, a 532 nm laser, laser power 250 mw, power of excitation 0.5 mw, and a hole of 100 μm. figure 3. general aspects of the cunas mineralization. a) panoramic view of the cunas mining camp and the minero river. b) steeply dipping black shales from a productive zone. the white veins are made up of carbonates ± emerald, hammer for scale. c) hand sample of a black shale crosscut by a calcite-dolomite-emerald vein. d) typical mineral assemblage from a productive zone. the fibrous calcite is part of a previous hydrothermal event, and its fibrous habit indicates an antitaxial regime. on the other hand, the rhombohedral carbonates (calcite dolomite), parisite-(ce), quartz, and fluorite exhibit euhedral shapes as a result of syntaxial growing. e) hand specimen displaying an array of sub-parallel veins and drusiform growing. the emeralds are found as hexagonal crystals in the presence of calcite, dolomite, and pyrite. f) close-up view of a pocket containing calcite —typically described as —peach skin— by the local miners—, dolomite, parisite-(ce), quartz, and emerald. note the largest emerald is a double terminated crystal. 143mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia figure 4. photomicrographs of emerald veins from the cunas mine. a) framboidal pyrite from an organic-rich mudstone; prl. b) calcite veinlets crosscutting organicrich mudstone. the host rocks commonly display strong hydrothermal alteration —carbonatization—; xpl. c) close-up view of an intersection between an albite — xenotime vein and a vein dominated by rhombohedral calcite; xpl. d) twined albite crystals which are partially replaced by calcite; xpl. e) black shale where cross-cut relationships are noticeable. albite veinlets are cross-cut by calcite; xpl. f) vein showing two generations of carbonates (f. calcite: fibrous calcite; r. calcite: rhombohedral calcite), which grew under distinct regimes; xpl. g) albite—fluorapatite vein in contact with later carbonates. note the albite crystals are highly corroded by dolomite replacement; xpl. h) tiny fragments of sulfides are occasionally found; these include pyrite, chalcopyrite, and sphalerite; prl. abbreviations: ppl: planepolarized light, xpl: cross-polarized light, prl: polarized reflected light. results mineralogy the cunas mineralization is related to hydrothermal veins and breccia zones (figure 3b), which extend more than 90 meters deep from the surface. the pithead is located on the furatena formation, and the productive zones are only found in the middle layers of the muzo formation. emerald-bearing veins are intimately associated with zones of intensive and overlapped hydrothermal alteration, predominantly albitization and carbonatization. the host rocks of the mineralization may vary between calcareous mudstones, siltstones and black shales; their organic content is typically high and planar lamination is the most prevalent sedimentary structure. these rocks are mostly black or dark gray (figure 3b, c, d, e); however, some variations may arise as a result of changes in composition and grain size. the main observed constituents are mica, plagioclase, quartz, framboidal pyrite, and minor monazite, tourmaline, and rutile (figure 4a). some white to gray fragments of albitized evaporites were locally observed. in the mineralized zones, the original host rock composition can be difficult to decipher due to the strong hydrothermal alteration and the small grain size (figure 4b). four hydrothermal/mineral paragenesis stages can be recognized. the first stage consists of albitization of the host rock. albite crystals are found in drusiform veinlets and breccias together with sporadic xenotime and fluorapatite (figure 4c, d, e). secondly, antitaxial veins of fibrous calcite are observed in certain areas; these veins are usually re-opened and intersected by later events (figure 4f). the third mineral assemblage is dominated by white calcite, dolomite, muscovite, and quartz (figures 3c, d, e and figure 4g). this is the most predominant stage and is widely observed in the mine. the host rocks underwent strong carbonatization and an intensive brecciation; most of the minerals grew in open spaces so that euhedral crystals are very common. calcite is found within the veins and breccias displaying rhombohedral and more complex crystallographic habits; dolomite is usually gray in color and rhombohedral in shape (figure 3f). less common minerals include vanadium-rich muscovite, pyrite, fluorapatite, rutile, tourmaline, albite, chalcopyrite, and sphalerite (figure 4h). emeralds occur exceptionally in pockets and pods associated with dolomite, “peach skin” calcite, quartz, parisite-(ce), and fluorite (figure 5a, b, d, e); bitumen fragments can also be found (figure 5c). 144 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento figure 5. photomicrographs of emerald veins from the cunas mine. a) indicative mineral assemblage from an emerald-bearing zone. note the three carbonates (calcite, dolomite, and parisite-(ce)) occur together; fluorite and quartz are often coarse and colorless; xpl. b) intergrowing texture exhibited by parisite-(ce) and quartz; xpl. c) bitumen fragments surrounded by calcite; prl. d) common exemplar of hexagonal emerald and rhombohedral calcite. note the euhedral forms occur as a result of openspace filling; xpl. e) hexagonal emerald crystals found in a vug; ppl. abbreviations: ppl: plane-polarized light, xpl: cross-polarized light, prl: polarized reflected light. figure 6. photograph displaying a classic mineral specimen from the cunas mine. the emerald crystals are usually euhedral and hexagonal; white calcite and lightgrey dolomite are the commonest gangue minerals. in some cases, the emerald crystals are mildly fractured by postmineralization deformation or covered by a thin film of late-stage calcite. mineral specimens from the cunas mine are highly valuable for collectors since they usually consist of euhedral emerald crystals on a black or white matrix (figure 6). the veins width ranges between 10 to 40 centimeters and their orientation is generally n25e dipping 40° and up to 90° eastwards. the breccia zones are generally thicker than the veins, and made up of clasts of previous veins, fragments of carbonaceous matter, and fault gouge. finally, the fourth hydrothermal stage is characterized by irregular late-stage calcite that infills open spaces growing as a thin film on previously formed minerals. replacement textures are very common —e.g. calcite after albite (figure 4d, g)— and the identification of hydrothermal alteration assemblages can be highly problematic due to the overlapping of events and the re-opening of veins. the obtained samples from productive zones always displayed the four hydrothermal stages, whereas the samples taken from barren veins lacked at least one of the mineral assemblages. according to the petrographic relationships, a paragenetic sequence was established for the mineralization (figure 7). 145mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia gemological aspects emerald crystals typically occur as hexagonal prisms with basal pinacoids, sometimes double-terminated (figure 3f and figure 8a); stepped growing patterns are occasionally observed. most of the samples have color zoning both parallel and perpendicular to the c-axis, where the core is light green and the rim has a higher color saturation (figure 8a). euhedral crystals also exhibit color zoning parallel to the growing c-axis. dichroism is subtle and ranges between yellowish-green (perpendicular to the c-axis) to bluish-green (parallel to c-axis). cut stones display a deep green hue and remarkable luster (figure 8b). optically, the crystals show uniaxial behavior with low birefringence and negative optical character. the mean refractive index of the ordinary ray (nω) is 1.585, whereas for the extraordinary ray (nε) is 1.575. these values fall in the range of most colombian emeralds, and approximate to those from the coscuez mine. the estimated birefringence is 0.010. table 1 shows average values of refractive index, birefringence, and density for colombian emeralds, other world deposits and some synthetic emeralds. emeralds show moderate to weak red fluorescence under 365 nm ultraviolet light. some samples fluoresce intensively according to their color saturation (figure 8c, d). reaction under the chelsea filter is weak (red color), except for one sample (ec_16) that reacts strongly due to its high color saturation and its lack of fissures and inclusions. solid inclusions are uncommon and mostly consist of albite, dolomite and lesser calcite, rutile, and black shale fragments. spectroscopic features the obtained ft-ir spectra is characteristic of colombian emeralds (cedeño et al., 2015; karampelas et al., 2019; branca et al., 2019). type i and ii water absorption are observed, with the presence of co2 in 2358 cm-1 (figure 9a). the range between 3200 and 7500 cm-1 presents absorption caused by type i and type ii water stretching (rondeau et al., 2008; wood & nassau, 1968). absorption signals in 3555, 3647, 3610, 3629, 3694, 3697, figure 7. generalized paragenetic sequence for the cunas mineralization. figure 8. photographs of cunas emeralds. a) hexagonal crystals displaying color zoning. b) typical “deep-green” cut emeralds. c) emerald crystal observed under normal light, d) the same crystal observed under ultraviolet light (365 nm), the red fluorescence is due to the chromium content. 146 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento table 1. comparative chart showing the refractive index, birefringence, and the density (specific gravity) of some natural and synthetic emeralds from literature. deposit nε nω birefringence density (g/cm3) references australia poona 1.573 1.578 0.005-0.006 2.693 zylberman, 1988 brazil faz. bonfim 1.581-1.582 1.586-1.588 0.005-0.007 2.720 zwaan et al., 2012 carnaíba 1.582 1.588 0.006 2.720 zylberman, 1988 paraná 1.580-1.583 1.590 0.007-0.010 nd araújo neto et al., 2019 india ajmer 1.588 1.595 0.007 2.735 zylberman, 1988 colombia muzo 1.564 1.57 0.006 2.70 romero, 1993 coscuez 1.578 1.584 0.006 2.609 romero, 1993 chivor 1.565 1.571 0.006 2.730 romero, 1993 yacopí 1.569 1.579 0.01 2.704 romero, 1993 pacho 1.570 1.576 0.006 2.70 romero, 1993 cunas 1.575 1.585 0.01 2.70 this study mozambique murrua (melela) 1.585 1.593 0.008 2.680 gubelin, 1998 norway eidsvoll 1.579 1.587 0.008 2.75 rondeau et al., 2008 austria habachtal 1.584 1.591 0.007 2.734 zylberman, 1988 pakistan mingora 1.5887 1.5963 nd 2.777 zylberman, 1988 zambia miku 1.582 1.589 nd 2.738 zylberman, 1988 kabufu 1.592 1.602 nd 2.770 zwann et al., 2005 zimbabwe mayfield 1.584 1.590 nd 2.720 zwann et al., 1997 sandawana 1.584 1.590 nd 2.750 zwann et al., 1997 tanzania manyara see 1.578 1.585 0.007 2.720 gubelin, 1998 south africa gravelotte 1.583 1.594 nd 2.750 gubelin, 1998 russia tokowaya (ural, sverdlovsk) 1.571 1.578 0.007 2.678 gubelin, 1998 canada yukon 1.581 1.588 0.007 2.730 groat et al., 2002 synthetic hydrothermal leichleitner 1.575 1.581 nd 2.649 zylberman, 1988 synthetic flux chatham 1.573 1.578 0.005 2.650 zylberman, 1988 synthetic hydrothermal russian 1.578 1.579 nd 2.730 koivula et al., 1996 synthetic hydrothermal biron 1.569 1.573 0.004 2.710 kane et al., 1985 147mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia 3698, 1542, 1602-1550 and 1605 cm-1 correspond to type i water (wood & nassau 1968, cedeño et al., 2015). absorption signals in 3592, 3597, 3655, 3660, 3654, 1628 and 1637-1630 cm-1 are due to type ii water. at 2813 cm-1, a small band due to the presence of chlorine is found (figure 9b, d) (rondeau et al., 2008; cedeño et al., 2015). besides, less intense peaks are observed in the range from 2800 cm-1 to 2600 cm-1, which have been assigned to partially deuterated water (hod) and totally deuterated water (d2o), more precisely, the bands situated at 2814, 2695, 2685, 2671 and 2640 cm-1 (figure 9b) (wood & nassau 1968, cedeño et al., 2015). figure 9. ft-ir absorption spectra. for a)-b) the spectra were collected with an incident beam parallel to c-axis (beam ii) and perpendicular to c-axis (beam t). a) typical spectra for cunas emeralds at 1000 — 7500 cm-1; the main observed bands correspond to co2, type i —h2o(i)— and type ii —h2o(ii) — waters. b) close-up view at 2450 — 2850 cm-1 displaying partially deuterated water (hod), and totally deuterated water (d2o), and chlorine (cl). c)-d)-e) spectral comparison of the cunas emeralds (colombia) and zambian emeralds. c) typical signals at 1000 — 7500 cm-1 d) close-up view at 2450 — 2850 cm-1. e) close-up view at 2250 — 2550 cm-1 showing the co2 and d2o absorption bands. figure 10. uv-vis-nir absorption spectra. a) typical spectra from a cunas emerald displaying v3+ and cr3+ absortion bands. note the absence of fe2+ and fe3+ absorption features. the spectra were collected with an incident beam parallel to c-axis (beam ii) and perpendicular to c-axis (beam t). b) spectral comparison of typical emerald from cunas (colombia) and a sample from zambia. note the colombian spectra is fe3+-free. the upper spectra has been vertically offset for clarity. in general, colombian emeralds exhibit two absorption bands due to electronic transitions of cr3+ and v3+. these two metallic ions isomorphically replace the al3+ ion presented in beryl, giving the characteristic green color of emeralds (bosshart, 1991; schwarz, 2009). collected uv-vis-nir spectra from cunas emeralds show significant absorption signals due to v3+ and cr3+ in 427, 607, 611, 636, and a doublet in 680-683 nm (figure 10a) (rondeau et al., 2008; karampelas et al., 2019; saeseaw et al., 2019). additionally, a slight shoulder in 390 nm due to v3+ is observed (schwarz, 2015). a small peak in 957 nm is observed due to the second overtone of oh groups or water molecules (henderson et al., 2015). 148 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento figure 11 shows the raman spectra of beryl and co2 trapped in the fluid inclusions. negligible differences can be observed between emeralds from cunas and other colombian localities. the average values obtained for the fermi diad in cunas are 1387.82 and 1284.89 cm-1; in contrast to 1387.82 and 1284.89 cm-1 for chivor and 1387.82 and 1284.89 cm-1 for muzo (cedeño et al., 2015). figure 11. comparative raman spectra of colombian emeralds showing three distinct zones. co2: corresponds to the fermi doublet signal of co2 from the gas phase of the fluid inclusions. si-o: indicates a silicon-oxygen bond. be-o: indicates a beryllium-oxygen bond, a typical signal from the host mineral (beryl). note the cunas fermi doublet exhibits the longest separation. y-axis is not to scale. x-ray diffractometry x-ray diffraction results show no significant variations in the unit cell parameters of cunas emeralds compared to other colombian deposits (table 2). the a0 and c0 values are 9.20788 ± 0.003 å and 9.18420 ± 0.001 å; the estimated volume is 673.3120. table 2. unit cell parameters of emeralds from the main colombian mines. *taken from romero (1993). mine a0 (å) c0 (å) volume (å) muzo* 9.19204 ± 0.005 9. 7466 ± 0.001 671.4739 coscuez* 9.20356 ± 0.003 9.19192 ± 0.008 674.2923 chivor* 9.20710 ± 0.003 9.17432 ± 0.001 673.3243 yacopí* 9.20767 ± 0.005 9.17022 ± 0.001 673.3024 cunas (this study) 9.20788 ± 0.003 9.18420 ± 0.001 673.3120 mineral chemistry representative frx data are shown in table 3. the chemical composition of cunas emeralds is dominated by silica and alumina amounting in total ~80 wt%; the beryllium content was not measured, and a reference value was taken from groat et al. (2008). the sum of magnesium, calcium, sulfur and alkali metals (na, k) oxides stands for ~5.26 wt%; chlorine values average 2040 ppm, and its occurrence might be related to the water content. the most abundant trace elements are vanadium (average 8619 ppm), chromium (average 6462 ppm), iron (average 1039 ppm), and scandium (average 736 ppm). other minor trace elements detected include ni, cu, zn, ga, rb, cs, ti, and mn. the average v/cr ratio is 1.33 and iron concentrations are relatively low, which represents typical values for emeralds from the colombian western emerald belt (cedeño et al., 2015). vanadium is the dominant chromophore, followed by chromium, and lesser iron; v-cr-fe relative proportions were plotted in a ternary diagram and compared with the whole spectrum of colombian emeralds composition (figure 12) (cedeño et al. 2015). the relatively high chromium content and the low iron concentrations might explain the red fluorescence effect under ultraviolet light. figure 12. ternary diagram of the fe —cr — v system. the red dots indicate samples from the cunas mine; the gray field shows the entire spectra reported for colombian emeralds from cedeño et al. (2015). photoluminescence the chromophore elements —chromium, vanadium, and iron— absorb part of the visible spectrum; this absorbed energy can eventually emerge again as a luminescence phenomenon. for the case of chromium, two spectral bands can be detected through photoluminescence (pl) analysis: r1 ~683 nm, and r2 ~680 nm (wood, 1965), whose intensity and location depend on the chromium concentration and the cr/fe ratio; the higher the ratio the higher the spectral intensity (nassau, 1978). the r1 position for the cunas emeralds varies from 683.67 nm to 683.98 nm, and its intensity can reach more than 6000 counts. the r2 position is well-defined and occurs as a small shoulder. figure 13 exhibits the pl spectra of samples from cunas, chivor, pakistan, and zambia; and figure 14 shows the cr/fe ratio versus r1 plot. colombian emeralds range between 683.4 and 683.9 nm (thompson et al., 2017; garcía-toloza et al., 2019). in contrast, samples from pakistan and zambia range between 684.2 nm and 684.8 nm; chivor emeralds range from 683.55 nm to 683.65 nm, and their average iron concentration is 800 ppm, which is notably lower than cunas ~1039 ppm; the iron content of zambian and pakistan emeralds is higher than 14,000 ppm. the chromium content varies slightly between each locality (garcía-toloza et al., 2019). figure 13. photoluminescence spectra of selected emeralds from colombia, zambia, and pakistan. the colombian spectra display a higher intensity (6,00025,000) in comparison to zambia (3,500) and pakistan (800). spectra were collected with the excitation laser light aligned perpendicular to the c—axis of the crystal. cps: counts per second. 149mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia ta bl e 3 . x -ra y flu or es ce nc e r es ul ts fo r t he c un as em er al ds . * be : t he b er yl liu m co nt en t w as ta ke n as a th eo re tic al n um be r f ro m g ro at et al ., (2 00 8) . b dl : b el ow d et ec tio n lim it m a jo r a n d t r a c e e l e m e n t s fo r e m e r a l d s fr o m t h e c u n a s m in e o xi de (w t% ) ec _0 1 ec _0 2 ec _0 3 ec _0 4 ec _0 5 ec _0 6 ec _0 7 ec _0 8 ec _0 9 ec _1 0 ec _1 1 ec _1 2 ec _1 3 ec _1 4 ec _1 5 ec _1 6 ec _1 7 ec _1 8 ec _1 9 ec _2 0 ec _2 1 ec _2 2 ec _2 3 ec _2 4 ec _2 5 b eo * 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 13 .1 5 si o 2 68 .5 6 67 .5 7 67 .0 9 66 .9 3 66 .3 5 64 .4 8 67 .7 8 67 .6 7 67 .8 4 67 .0 1 69 .8 0 60 .0 4 64 .6 3 63 .9 0 53 .9 6 71 .3 6 69 .5 7 66 .5 3 70 .5 4 67 .5 6 69 .0 0 70 .0 3 59 .1 9 67 .3 6 64 .8 1 a l 2o 3 12 .9 2 12 .6 8 14 .4 1 12 .6 4 13 .5 7 12 .9 4 12 .9 9 12 .7 1 13 .3 3 15 .1 4 14 .6 6 15 .8 0 19 .8 5 17 .7 4 18 .1 7 13 .0 9 14 .8 2 13 .9 3 12 .2 3 12 .7 1 13 .9 9 13 .6 7 11 .2 5 14 .4 9 13 .6 5 m go 2. 85 2. 69 3. 15 2. 47 bd l 3. 68 3. 01 2. 95 2. 96 bd l bd l 6. 71 0. 55 bd l 10 .4 7 0. 58 0. 16 0. 05 1. 71 2. 48 0. 11 0. 37 1. 85 bd l 3. 64 n a 2o 1. 60 bd l 0. 39 2. 11 3. 63 3. 12 0. 83 1. 05 0. 01 2. 07 bd l 2. 25 bd l 3. 27 2. 46 1. 05 1. 22 4. 20 bd l 1. 29 0. 68 0. 63 bd l 2. 83 bd l c ao 0. 10 0. 01 0. 09 0. 06 0. 12 0. 23 0. 02 0. 04 0. 24 0. 25 0. 03 0. 02 0. 03 0. 29 0. 03 0. 05 0. 23 0. 38 0. 13 0. 18 0. 24 0. 43 10 .9 0 0. 11 0. 14 k 2o 0. 07 0. 05 0. 14 0. 11 0. 24 0. 20 0. 07 0. 11 0. 05 0. 07 0. 06 0. 07 0. 04 0. 12 0. 06 0. 14 0. 10 0. 13 0. 06 0. 16 0. 18 0. 13 0. 11 0. 26 0. 29 so 3 0. 05 0. 01 0. 10 0. 06 0. 21 0. 24 0. 05 0. 12 0. 04 0. 09 0. 08 0. 04 0. 12 0. 05 0. 04 0. 15 0. 04 0. 07 0. 10 0. 22 0. 18 0. 07 0. 07 0. 13 0. 66 to ta l % 99 .3 1 96 .1 6 98 .5 0 97 .5 3 97 .2 6 98 .0 4 97 .9 0 97 .8 1 97 .6 2 97 .7 7 97 .7 8 98 .0 8 98 .3 7 98 .5 1 98 .3 5 99 .5 7 99 .2 9 98 .4 3 97 .9 0 97 .7 5 97 .5 3 98 .4 8 96 .5 1 98 .3 2 96 .3 4 tr ac e (p pm ) v 11 00 0 10 82 0 81 00 11 53 0 12 13 0 81 10 10 00 0 10 57 0 11 89 0 98 80 10 28 0 90 20 65 10 51 60 71 70 73 0 24 30 62 20 10 43 0 10 01 0 11 08 0 61 50 13 19 0 58 90 71 70 c r 85 50 87 70 26 50 88 90 91 10 66 10 81 20 83 00 88 80 83 00 85 70 71 00 72 40 55 30 60 10 24 6 14 82 50 70 76 60 71 80 81 10 44 20 57 40 30 70 59 30 fe 72 4 59 1 89 5 69 2 81 8 10 64 66 0 66 5 79 5 82 0 75 0 59 6 75 8 90 4 66 4 50 4 76 2 83 8 77 7 12 36 12 51 85 5 24 77 87 8 50 01 sc 10 47 91 5 85 8 10 37 10 93 10 23 95 1 86 9 10 36 91 5 95 8 81 5 46 2 33 0 77 3 65 32 7 48 9 79 2 68 6 71 3 81 1 97 6 22 2 24 2 c l 99 2 87 8 19 04 20 20 36 80 18 36 78 4 10 45 72 1 13 25 96 3 13 05 70 5 21 80 79 9 22 80 14 48 22 90 76 6 22 60 23 30 22 00 96 10 59 70 71 1 n i 5 2 8 4 13 5 7 4 9 15 13 18 46 36 40 2 7 6 2 2 4 21 1 13 7 c u 14 12 17 14 20 22 12 15 13 20 19 17 48 43 32 18 14 22 75 37 6 34 1 10 5 57 75 16 10 z n 12 7 16 28 35 36 37 53 24 40 53 8 17 0 30 10 2 11 3 72 56 52 17 8 18 26 25 21 58 76 76 g a 63 57 72 58 67 68 57 52 63 66 55 46 54 38 42 50 77 67 65 60 71 78 72 53 66 r b 7 8 9 6 9 9 7 7 7 9 7 8 bd l bd l 7 7 6 9 6 8 8 8 13 8 10 c s 31 bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l 19 19 23 12 56 bd l 18 ti bd l bd l bd l 37 13 16 9 bd l 17 bd l bd l bd l bd l bd l 33 bd l 28 58 16 bd l 32 25 66 16 4 70 12 4 m n bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l bd l 38 bd l bd l bd l 38 bd l bd l bd l bd l 81 7 bd l 17 7 150 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento 683,4 683,6 683,8 684 684,2 684,4 684,6 684,8 0 5 10 15 cr/fe vs r1 pakistan zambia cunas chivor figure 14. cr/fe vs. r1 plot. the r1 axis represents the maximum peak (wavelength). samples from the eastern emerald belt (green diamond) have high cr/fe ratio and exhibit shorter wavelength, while cunas samples (blue diamond) have medium to low cr/fe ratio and exhibit longer wavelength. on the other hand, samples from pakistan (purple dot) and zambia (red dot) have a very low cr/fe ratio and exhibit much longer wavelengths. fluid inclusions the most common fluid inclusions trapped in the cunas emeralds are poly-phase inclusions, containing two solids, liquid, and gas (figure 15a, b, c, d). their shape and orientation are variable, and two main groups can be distinguished. the inclusions are trapped in irregular or jagged cavities, which can reach outstandingly large sizes —more than 500 µm of diameter— (figure 15a, b, d), and typically show necking down and leakage processes. most of the fluid inclusion assemblages are oriented parallel to the c-axis or to a growing plane so that they are considered primary or pseudo-secondary. on the other hand, we grouped oval-shaped inclusions, whose size ranges approximately from 5 to 200 µm (figure 15c). these fias are also regarded as primary, but they are less abundant, and show negligible evidence of necking down or leakage. at room temperature, all the emerald-hosted fluid inclusions consist of a liquid-rich phase (70-75%v) with vapor (10-15%v) and two daughter crystals (10-15% v). in large-size inclusions was possible to observe two concentric rings conforming the bubble, indicating the coexistence of liquid and vapor co2 (figure 15b); liquid co2 can account for ~3% of the inclusion volume. the daughter crystals are generally salts, predominantly halite; sylvite was also identified —based on crystal morphology upon cooling as noted by ordóñez (1993)— and less abundantly calcite, dolomite, and parisite-(ce). abundant three-phase inclusions, consisting of an aqueous solution, one daughter crystal and a gas bubble, are widely found in gangue minerals, e.g., quartz, calcite, dolomite, parisite-(ce), fluorite, fluorapatite, and albite (figure 15e, f). raman analysis in quartz-hosted inclusions identified co2, n2, and ch4; in emeraldhosted inclusions, only co2 could be detected, n2 and ch4 estimations were not suitable due to the high spectra saturation of the host beryl in the range where these compounds are visible (2000-3100 cm-1). the fluid inclusions whose arrangement was non-parallel to the c-axis were regarded as secondary. these are typically located in planes derived from re-healed fissures; most of them show evidence of leakage and necking down processes, this might also be caused by their large size. some fluid inclusions hosted by associated quartz or carbonates are slightly different in shape, as they commonly occur as negative crystals; furthermore, co2-bearing inclusions were observed in quartz coexisting with liquid-rich inclusions, thus indicating heterogeneous trapping (figure 15e). tabular-shaped twophase inclusions, consisting of liquid and vapor, were also sporadically found in calcite and dolomite. the results of microthermometry analysis of emerald-hosted fluid inclusions are summarized in table 4, and graphically shown in figure 16. the first noticeable change in the system was observed in the range of -66 to -45°c; figure 15. photomicrographs of fluid inclusions. a) emerald-hosted polyphase inclusion occurring in a jagged-edge cavity. b) typical emerald-hosted fluid inclusion consisting of two daughter crystals, a liquid brine, and co2 (both liquid and gas). c) emerald-hosted primary inclusion displaying the typical four phases; one of the daughter crystals was identified as parisite-(ce). d) emerald-hosted inclusion with sylvite(?) daughter crystal. e) quartz-hosted primary inclusions. note the entrapment is highly heterogeneous. f) quartz-hosted inclusion consisting of a liquid brine, a daughter crystal —halite—, and a co2-n2-ch4-rich gas bubble table 4. microthermometric results of primary emerald-hosted fluid inclusions. abbreviations: t-eut: eutectic point temperature; tf-co2: temperature of melting point of co2; tff-ice:final melting temperature of ice; t-ffhy: hydrohalite melting temperature; th-co2: temperature of homogenization of liquid co2; th: total vapor bubble homogenization temperature —gas to liquid—; tm-nacl: halite dissolution temperature; t-decrep: decrepitation temperature. t-eut (°c) tf-co2 (°c) tff-ice (°c) t-ffhy (°c) th-co2 (°c) th (°c) t-decrep (°c) tm-nacl (°c) salinity (wt% nacl eq.) n 42 57 57 15 57 57 33 57 57 mean -56.2 -56.6 -29.6 -21.7 26.1 211.4 350.0 352.9 40.5 σ 6.7 0.6 4.0 2.4 1.5 14.9 14.5 16.6 1.8 min -66 -58.9 -35.4 -25.1 23.9 190 340 335 38.6 max -45.6 -55.9 -23.7 -19.1 28.6 230 380 380 43.4 151mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia this temperature is interpreted to be the eutectic point (figure 16a). due to the abundant co2 —both liquid and gas— it was possible to observe the melting point at ~-56.6°c (figure 16b); the values reported below this point may suggest the presence of other compounds with lower melting temperatures. the final melting point of ice varies between -35 to -23°c (figure 16c), and hydrohalite melting was observable at ~-21.6°c (figure 16d). some difficulties may occur when observing these phase changes, especially the metastability of the highly saline fluids. homogenization of liquid co2 — change from liquid to gas state— was detected averaging at 26.13°c (figure 16e); these results were used to determine the density of co2. according to the experimental diagrams by shepherd (1985), a value of 0.2 g/cm3 was calculated. the total homogenization of the vapor bubble ranges from 190 to 230°c (figure 16f), progressive heating led to the decrepitation or implosion of some inclusions; this occurred from 340 to 380°c (figure 16h). the salt —halite— dissolution took place from 335 to 380°c (figure 16g); other daughter minerals like carbonates did not dissolve at temperature < 400°c. based on the dissolution point of halite, the weight percentage of salinity —nacl equivalent— was calculated through the program aqso2e. exe by bakker (2003); the values range from 38% to 43%. likewise, the trapping conditions of the inclusions were estimated by the intersection of the h2o-nacl line with an isochoric path. the h2o-nacl line was defined in terms of the calculated salinity and the homogenization temperature. the isochoric area was established by the approximate hydrostatic and lithostatic gradients used by cheilletz et al. (1994). figure 17 shows two plots with the calculations of salinity, pressure, and homogenization temperature for the emerald-forming fluids. the minimum and maximum trapping pressure estimations correspond to 850-2420 bar, whereas temperature ranges from 256 to 337°c; the estimated density averages 1.068 g/cm³. 0 2 4 6 8 10 12 14 16 -70 -65 -60 -55 -50 -45 -40 fr eq ue nc y t-eut (°c) eutectic temperature 0 10 20 30 40 50 -59 -58 -57 -56 -55 fr eq ue nc y tf-co₂ (°c) co₂ fusion temperature 0 5 10 15 20 25 -40 -35 -30 -25 -20 -15 fr eq ue nc y tff-ice (°c) ice final melting temperature 0 1 2 3 4 5 6 7 -26 -25 -24 -23 -22 -21 -20 -19 -18 fr eq ue nc y t-ffhy (°c) hydrohalite melting temperature 0 5 10 15 20 22 23 24 25 26 27 28 29 30 fr eq ue nc y th-co₂ (°c) co₂ homogenization temperature 0 5 10 15 20 320 330 340 350 360 370 380 390 fr eq ue nc y tm-nacl (°c) halite dissolution temperature 0 5 10 15 20 25 30 330 340 350 360 370 380 390 fr eq ue nc y t-decrep (°c) decrepitation temperature 0 5 10 15 20 25 180 190 200 210 220 230 240 fr ec ue nc ia th (°c) total homogenization temperature a b c d e f g h figure 16. histograms showing the microthermometric data. a) eutectic point temperature (t-eut). b) co2 first melting temperature (tf-co2). c) final melting temperature of ice (tff-ice). d) hydrohalite melting temperature (t-ffhy). e) temperature of homogenization of liquid co2 (th-co2). f) total vapor bubble homogenization temperature (th). g) halite dissolution temperature (tm-nacl), h) decrepitation temperature (t-decrep). 152 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento discussion geological similarities with other colombian deposits the geological setting of the colombian emerald deposits has been extensively studied; at the western emerald belt, multiple authors have proposed that the mineralization is stratabound — exclusively associated to lower cretaceous series from the muzo and rosablanca formations— and epigenetic, i.e., formed by the circulation of non-magmatic hydrothermal fluids through structurally favorable sites where emerald formation can take place (cheilletz et al., 1994; ottoway et al., 1994; giuliani et al., 2000; reyes et al., 2006; mantilla et al., 2007; terraza 2019). according to the authors cited above, a simplified list of essential factors that can lead to a colombian-type emerald mineralization would include these three aspects: 1) stratigraphy: the mineralization is always linked to certain lithostratigraphic units. the emerald-hosting rock sequences particularly contain evaporitic levels or relicts after hydrothermal modification. these rocks are presumably the source of the main ingredients to form emeralds, i.e., be, cr, v, al, si, and associated elements like lanthanides, ca, na, k, fe, s, p, f, and cl. 2) hydrothermal activity: hydrothermal fluids constitute the carriers of ligands and metals that can eventually react and crystallize as emeralds. those fluids must contain specific chemical components under special p-t conditions. the action of hydrothermal fluids is evidenced in field as hydrothermal alteration, vein-infill with indicative mineral assemblages, and geochemical anomalies at different scales. 3) structural control: it is critical for the focusing and promoting of fluid mixing and precipitation. effective structural traps lead to a high rate of fluidrock interaction, generation of cavities, and consequently accumulation of gemquality emeralds. regarding the first factor, the maripí district is located no further than 6 kilometers from the muzo area, and the emerald mineralization lies on the same lithostratigraphic unit —the muzo formation—. this was first noted by reyes et al. (2006). in the cunas mine, the productive zones are found in calcareous mudstones and siltstones (figure 3b, c, and d). these rocks are remarkably organic-rich and contain rare earth elements (ree) authigenic minerals such as monazite, two chemical components thoroughly involved in the mineralizing processes (giuliani et al., 2000; verbel 2017). two particular rock types are described by cheilletz et al. (1996) in the muzo, coscuez and chivor mineralized zones: whitish albitites (kaolin) and stratiform tectonic breccias (cenicero). we observed gray albitites in the vicinity of some productive areas. these rocks are commonly interpreted as the product of intensive na metasomatism of evaporite horizons (giuliani et al., 2000; branquet et al., 2015). in contrast, cenicero zones were not observed. stratiform tectonic breccias occur as a result of concomitant faulting and fluid flowing (branquet et al., 1999, 2015). therefore, the presumed absence of these breccias would probably be related to a structural condition, instead of the lack of hydrothermal activity. in terms of hydrothermal activity, multiple pieces of evidence of fluidderived minerals are here documented (figure 3, figure 4, figure 5, figure 6). the fluid composition estimated upon primary fluid inclusions reveals no major changes with results reported for other colombian deposits (figure 17). timepressure conditions fall in the range of colombian-type emeralds (kozlowski et al., 1988; giuliani et al., 1990, 1995; cheilletz et al., 1994, ottoway et al., 1994; romero, 1993, 1998; romero et al., 1999; banks et al., 2000; mantilla et al., 2007; toloza, 2017), and the fluid composition is dominated by common components, i.e., na+, ca2+, k+, cland co2. the high salinity values —38-42 wt% nacl eq.— have been associated with the involvement of evaporite-derived fluids, whereas the na-ca dominated composition is explained by the mixing of two fluids (giuliani et al., 1995; romero 1998; banks et al., 2000). in the maripí area, carbonates from the la pita, españoles, and the cunas mine were examined through oxygen and carbon isotopic studies, yielding a non-magmatic source and a hybrid isotopic signature (niño, 2017). this indicates strong similarities with the isotopic fingerprints for other colombian districts where a magmatic input has been ruled out intensely (cheilletz et al., 1994). in general, the mode of occurrence of the cunas mineralization is similar to other colombian deposits; the emeralds are hosted by veins and breccias in hydrothermally altered zones. albitization and carbonatization are ubiquitous and typical of productive areas (figure 4). these features are critical given that na-ca metasomatism plays a key role in the location of beryl accumulation (beus et al., 1972; kozlowski et al., 1988; romero 1993, 1998; banks et al., 2000). the mineral association consisting of emerald-calcite-parisite(ce)-fluorite is a common feature at the muzo mines (ottoway et al., 1994; giuliani et al., 2000; banks et al., 2000), and it was repeatedly observed in cunas (figure 3, figure 5); bitumen was also observed as embedded fragments in hydrothermal veins (figure 5c). the presence of organic compounds — bitumen and co2-ch4-n2 in the fluid inclusions— may suggest the emeraldforming fluids reacted with the organic-rich wall-rocks, especially through thermochemical sulfate reduction processes. according to giuliani et al., (2000), this mechanism plays a key role as it promotes the leaching of beryllium from the black shales. however, in cunas, hydrothermal pyrite is scarce and no h2s was detected in the fluid inclusions. an alternative explanation is that the parental brines were sulfate-poor, a fact that would require further isotopic testing. common chemical components expressed as mineral assemblages and fluid inclusions agree with a classical colombian-type mineralization. three paragenetic stages can be objectively differentiable in most of the mining districts (cheilletz et al., 1996; giuliani et al., 2000; garcía-toloza et al., 2018; gonzález-duran et al., 2019). at the cunas mineralization, similar mineral assemblages have been described; nevertheless, our observations point to a system where at least four stages can be distinguished (figure 7). two possibilities could explain this situation. i) the lack of a detailed mineral description in the other mines, especially highlighting the complexity of differentiating between the very same minerals, e.g., fibrous, rhombohedral and irregular calcite. ii) a slightly different fluid evolution that gave rise to a multi-stage system. although no significant differences can be addressed figure 17. plots showing microthermometric calculations. a) salinity (wt% nacl eq.) versus non-corrected homogenization temperature (th). b) isochoric evolution of the emerald-forming fluids. minimum and maximum values for trapping conditions are listed (p: pressure, t: temperature). 153mineralogy and fluid inclusions of the cunas emerald mine, maripí boyacá, colombia in terms of fluid composition, specific wall-rock interactions could have led to subtle changes in the cooling paths of the ore-forming fluids; as a result, a set of different mineral textures would form as a consequence of multiple episodes of crystallization. we consider these two options are plausible; most of the previous studies have focused on the understanding of major features from a regional perspective. few documents, describing specific mines, are available. on the other hand, particularities between colombian emerald deposits are well-documented and roughly involve timing, tectonic setting, lithological and geochemical aspects of the mineralization (romero 1993, 1998; branquet et al., 1999, giuliani et al., 2000; pignatelli et al., 2015; garcía-toloza et al., 2018). it is noteworthy that during the carbonatization stage numerous minerals were formed (figure 7), including silicates, carbonates, sulfides, phosphates, oxides, and f-rich phases as fluorite, parisite-(ce) and fluorapatite. we think this occurs as a result of a series of factors prompted by a major process. these factors necessarily include: the input of additional chemical components, abrupt changes in p-t conditions that may trigger mineral precipitation, the availability of fluorine complexes for promoting effective transport of chemical components —beryllium and ree—. our observations point to an abrupt change in fluid chemistry —and probably p-t conditions— due to fluid mixing; the lack of microthermometric data from every mineralizing state can limit this conclusion. however, banks et al. (2000) stated that fluid mixing is certainly a process that exerts a strong influence in the availability of chemical components and consequent mineral precipitation. undoubtedly, fluorine is a key ingredient in the chemical system as promotes beryllium mobility, an idea supported by the close association of emerald and fluorine-bearing minerals —fluorite, parisite-(ce), and apatite—. finally, the structural setting of the cunas mineralization is poorly constrained. however, some observations can be noted. in the northern maripí area, a well-defined ~n30e trend controls the emplacement of emerald-bearing veins; this is well-documented in the la pita mine by puerto et al. (2012) and also in cunas (garcía-toloza et al., 2018). even though, there are no detailed observations about the style and origin of veins; a high rate of fluid-rock interaction is required, as the concentration of chromophore elements —cr-vfe— is remarkably high and similar to the values from the puerto arturo mine in muzo (cedeño et al., 2015). the abundance of solid inclusions within vein minerals —emerald, calcite, and dolomite— is a common feature at the cunas mine, and might suggest an extensive interaction with the host rocks. one noteworthy feature at the cunas mine is the apparent absence of trapiche emeralds. this could be regarded as a structural issue, as trapiches are supposed to form exclusively at fault tips in fault-propagating anticline settings; pignatelli et al. (2015) documented this mechanism in the muzo area —the main deposit of trapiche emeralds in the world—. interestingly, trapiche emeralds have been found in the la pita mine, which is located a few hundred meters north of cunas. this might suggest the structural positioning of the two deposits is slightly different. in cunas, the main mineralization zones occur in a homocline configuration (castaño et al., 2019); whereas the occurrence of trapiche emeralds in the la pita, would be possibly related to an anticlinerelated fault propagation zone. it is convenient to clarify that trapiche emeralds do not occur throughout the la pita mine but are restricted to very small zones —fault tips—. structural mapping of the entire area would be highly suggested to elucidate this topic. another alternative would be the assumption that trapiche-emerald zones are still undiscovered at the cunas mine. based on the previous observations, we propose that the emerald mineralization in the cunas mine was originated by the same global processes that took place in muzo and other colombian districts. we do not find significant evidence to exclude extrapolation from the muzo area to the maripí district. distinctive features of the cunas emerald gemological and mineralogical characteristics, as the refractive index, birefringence, unit cell parameters, and density, show no appreciable differences with reported values for emeralds from other colombian deposits (table 1, table 2). cunas emeralds exhibit unit cell data with slight differences in comparison to the firstly reported values for beryl: a0 = 9.21 ± 0.01 å and c0= 9.17 ± 0.01 å (bragg & west, 1926). radclife & campbell (1966) proposed these differences may occur because of variations in the theoretical chemical composition e.g., aluminum (al3+) can be partially substituted by transition metals, especially the chromophores elements chromium, vanadium, and iron. the reported values in table 3 indicate this took place, and consequently, the unit cell dimensions were modified. fourier-transform infrared absorption spectra of cunas emeralds is characteristic of colombian emeralds, especially those from the western emerald belt (cedeño et al., 2015). figure 9c, d, e shows a spectral comparison with emeralds from the kafubu mine in zambia. we found four main differences: the absence of deuterated water in 2736 cm-1 and 2475 cm-1, chlorine in 2813 cm-1, and a water-related band in 2290 cm-1 (cedeño et al., 2015; karampelas et al., 2019). regarding uv-vis-nir spectra, the zambian samples exhibit four main absorption bands (figure 10b). a small band is observed in 370 nm due to fe3+. three broadbands are observed in 420 nm, 610 nm are due to cr3+, and another broadband in 850 nm due to fe+2 (schwarz, 2009). in general, colombian emeralds have a low concentration of iron species, this characteristic allows to separate cunas emeralds from iron-rich deposits like kafubu in zambia (karampelas et al., 2019). according to the results displayed in figure 11, raman analysis of co2 in fluid inclusions can help to discriminate emeralds from the two colombian emerald belts (jimenez, 2017). no significant differences with emeralds from muzo were observed, thus indicating comparable co2 trapping conditions. the concentration of chromophore elements in the cunas emeralds is dominated by v (average 8619 ppm) > cr (average 6462 ppm) > fe (average 1039 ppm). this v-cr-fe ratio is similar to values reported for the la pita (maripí) and puerto arturo (muzo) emeralds (cedeño et al., 2015; jiménez, 2017). cunas emeralds are iron-poor and fall in the range of reported values for colombian emeralds (figure 12). the vanadium-rich character of the emeralds may be related to the high organic content of the host rocks, as noted by garcíatoloza et al. (2018). the low iron content is probably a reflection of a chemical system in which carbonates were effective collectors of iron. the role of pyrite as an iron collector is limited due to the low occurrence of sulfides. the photoluminescence makes it possible to differentiate colombian vanadium-rich emeralds, such as those from cunas, to chromium-rich ones such as those from the eastern emerald belt (figure 14). iron-rich emeralds —zambia and pakistan— can also be easily distinguished by the position and intensity of the r1 peak. our data confirm that there is a distinct correlation between the r1 position and cr/fe ratio for each locality so that emeralds can be identified according to its provenance. this contrast derives from the respective geological setting; cunas emeralds are hosted by sedimentary rocks and formed by low-temperature fluids; where zambian and pakistan emeralds are associated to magmatic processes which involved pegmatites and maficultramafic rocks (groat et al., 2008; giuliani et al., 2019). emerald-forming fluids microthermometry analysis indicates the emerald-forming fluids were trapped at relatively low temperature ~260-340°c and pressure ~850-2400 bar, with relatively high density —1.07 g/cm³—, elevated salinity 40.5 wt% nacl eq., and co2-n2-ch4 as the main volatiles. regarding the chemical composition of the aqueous phase, the estimation of the salinity is only an approximation as na+ is by any means the only dissolved ion. the low eutectic temperature and hydrohalite melting point suggest the presence of k+, ca2+, mg2+ (chi & ni, 2007; roedder, 1984; shepherd et al., 1985). the daughter crystals are also indicative of the chemical composition as they are supposed to form as a result of isochoric fluid saturation after trapping. halite, sylvite, calcite, dolomite argue for a rich input of na+, k+, ca2+, and mg2+; whereas parisite-(ce) —ca(ce,la)2(co3)3f2— indicates the existence of rare earth elements (ree3+), co2 2and f-. the microthermometric observations correlate with the main mineral phases found in the veins, e.g., albite, muscovite, calcite, dolomite, fluorite, fluorapatite, and parisite-(ce). the replacement of albite by calcite (figure 4d, e, g) is widespread and a meaningful feature at the cunas mine; this texture suggests disequilibrium probably originated by mixing of two contrasting fluids. romero (1993, 1998), ordóñez et al. (1997), and banks et al. (2000) examined the chemical components of fluid inclusions in colombian emeralds and concluded fluid mixing was a critical factor for emerald precipitation. at cunas, two deposit-scale hydrothermal events can be identified: albitization and carbonatization. on the one hand, the formation of albite requires high amounts of na-rich fluids, i.e., highly saline fluids. 154 fernando romero ordóñez, andrés gonzález-durán, javier garcía-toloza, jimmy rotlewicz cohen, carlos cedeño ochoa, holman alvarado gonzález, luis angarita sarmiento as previously noted, albitites are interpreted to be relicts of evaporitic layers (giuliani et al., 2000; branquet et al., 2015); therefore, the fluid responsible for albitization could have obtained elevated salinities from the dissolution of local evaporitic layers. on the other hand, the fluid responsible for carbonatization was ca-rich as calcite is a common daughter crystal in fluid inclusions, and calcite is by far the commonest gangue mineral in the hydrothermal veins. this fluid was co2 2and frich so that effective extraction and remobilization of beryllium and ree from the country rocks took place (banks et al., 2000; giuliani et al., 1990, 1995, 2000; kozlowski et al., 1988). primary two-phase —lacking salts as daughter crystals— inclusions in carbonates may indicate the salinity was significantly lower in comparison with the na-rich fluid. besides, the heterogeneous trapping of fluid inclusions in quartz suggests boiling, a process that might explain the wide variety of calcite crystal shapes. regarding the source of the ca-rich solution, as previously pointed out, niño (2017) argues for a non-magmatic input and a hybrid isotopic signature, thus reinforcing the idea of fluid mixing. a basinal origin for ca-rich fluids is highly suggested, where pore-fluids migrated and evolved through the strata. based on these data, we propose that the emerald mineralization at the cunas mine was originated by the mixing of two hydrothermal fluids of different sources; one fluid was highly saline, derived from evaporite dissolution —as suggested by romero et al. (1993); banks et al. (2000)—, and was the responsible for the albitization of the host rocks; a second calciumrich fluid evolved from connate waters and was equilibrated by the interaction with calcareous and organic-rich wall rocks —as evidence by co2-n2-ch4 in fluid inclusions—; this fluid could have been the responsible for the widespread carbonatization. as a result, emerald mineralization took place at structurally favorable places where fluid mixing was highly promoted. the described geological and physicochemical features for the cunas mine, are in agreement with an epigenetic sediment-hosted mineralization formed by the circulation and mixing of relatively low-temperature nonmagmatic fluids; a highly accepted model for the colombian emerald deposits. fluid inclusions data are particularly similar to the reported range for colombian emerald deposits (banks et al., 2000; cheilletz et al., 1994; giuliani et al., 1990, 2000; kozlowski et al., 1988; mantilla et al., 2007; ottaway, 1994; romero, 1993, 1998; toloza, 2017). conclusions at cunas, emeralds occur as infill products in veins and breccias, consisting of calcite – dolomite – albite – quartz – parisite-(ce) – fluorite – pyrite, hosted by organic-rich mudstones from the muzo formation. there is also a close association between hydrothermal alteration —albitization and carbonatization— and mineralization. distinct spectroscopic techniques reveal the cunas emeralds share common gemological features with other colombian deposits, especially with those from the western emerald belt. cunas emeralds are iron-poor and vanadium-rich, therefore they can be readily differentiated from noncolombian iron-rich emeralds. microthermometry analysis indicates the emerald-forming fluids were trapped at similar conditions to other colombian deposits, whose temperature ranges from 260°c to 340°c and pressure ~850-2400 bar, with relatively high density 1.06859 g/cm³, elevated salinity 40.5 wt.% nacl eq., and a chemical system dominated by h2o-nacl-cacl2-kcl-mgcl2, volatiles —co2-n2ch4— and fluorocarbonate complexes. acknowledgements we wish to thank the universidad nacional de colombia staff for the research support. we acknowledge esmeraldas santa rosa s.a. for the field and logistics support, especially edwin molina, germán forero, ing. alfonso torres; the mine staff always treated us so warmly and we appreciate their kind attention during our visits to the mine facilities. funding from the national emerald federation (fedesmeraldas) was vital for the accomplishment of this research. assistance from camilo sánchez (green shine c.i s.a.s.) and orlando rodríguez with data collection is also greatly appreciated. contributions from harry vargas, daniel moncada, leonardo paipa and andrés martín helped to improve notably the manuscript. we also thank the anonymous reviewers for their constructive comments. references acosta, j., velandia, f., osorio, j., lonergan, l., & mora, h. 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(2012). emeralds from the fazenda bonfim region, rio grande do norte, brazil. gems & gemology, 48(1), 2-17. _heading=h.30j0zll _heading=h.3znysh7 _heading=h.2et92p0 _heading=h.3dy6vkm _heading=h.1t3h5sf _heading=h.4d34og8 _heading=h.lnxbz9 _heading=h.35nkun2 _heading=h.44sinio _heading=h.sazwcfbrh5ec _heading=h.z337ya _heading=h.3j2qqm3 _heading=h.1y810tw _heading=h.4i7ojhp _heading=h.2xcytpi _heading=h.1ci93xb issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.89819 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 131 138 g eo ph y si c s effects of the continuously operating reference station (cors) network on the accuracy, precision, and time to fix ambiguity (ttfa) performance ömer gökdaş1, m. tevfik özlüdemir2 1. department of geomatics engineering, istanbul water and sewerage administration, 34060, eyupsultan, istanbul, turkey. email: ogokdas@iski.gov.tr orcid id: 0000-0003-0109-8073 (correspondence) 2. department of geomatics engineering, igs ista satellite observation and processing laboratory, istanbul technical university, 34469, maslak, istanbul, turkey. email: tozlu@itu.edu.tr orcid id: 0000-0002-1413-9244 subject: gps measurements applied to geophysical problems abstract the geometric design of the continuously operating reference station (cors) network is one of the most critical factors that impact accuracy, precision, and time to fix ambiguity (ttfa) performance. in this study, the authors investigate the subject of geometric design by using both local iski cors and national cors-turkey (cors-tr) networks, and they redesign the iski cors network by increasing interstation distances. for three systems, real-time virtual reference station (vrs) solutions have been obtained and tested with analysis of variance (anova). as a result, the 8-station iski cors provides the most accurate results in the vertical component. in contrast, for ttfa performance, the cors-tr network shows worse outcomes than the others. due to the increase in the interstation distances, the increase in the base length caused worse results in fix solution ratios for redesigned iski cors. in summary, the authors have stated that the 8-station iski cors network performs better, especially regarding vertical accuracy. furthermore, the authors state that the ttfa performance and fix solution ratios are high, and the iski cors geometric design is optimal. keywords: iski cors; cors-tr; network; accuracy; precision. efectos de una red de estaciones de referencia de operación continua en exactitud, precisión y desempeño de la resolución de ambigüedades resumen el diseño geométrico de una red de estaciones de referencia de operación continua (cors, del inglés continuously operating reference station) es uno de los factores más críticos que impactan la exactitud, precisión y desempeño de la resolución de ambigüedades (ttfa, del inglés time to fix ambiguity). en este estudio los autores investigan el diseño geométrico de la red local iski cors y la nacional cors-turkey (cors-tr) y rediseñan la red iski cors al incrementar las distancias entre estaciones. en los tres factores se obtuvieron las soluciones en tiempo real de las estaciones de referencia virtual (vrs, virtual reference station) y se evaluaron con el análisis de la varianza (anova). como resultado en el componente vertical, la red iski cors de ocho estaciones registra los resultados más exactos. en contraste, para el desempeño ttfa la red cors-tr muestra los peores resultados frente a los otros factores evaluados. debido al incremento de las distancias entre estaciones, el incremento en la longitud de base generó los peores resultados en los radios de resolución para la red rediseñada iski cors. en resumen, los autores han establecido que la red iski cors de ocho estaciones presenta el mejor desempeño, especialmente en términos de precisión vertical. además, los autores definieron que el desempeño ttfa y los radios de resolución son altos, y que el diseño geométrico iski cors es el óptimo. palabras clave: iski cors; cors-tr; redes; precision; exactitud; record manuscript received: 08/08/2020 accepted for publication:07/06/2022 how to cite item: gokdas, o., & ozludemir, t. (2022). effects of the continuously operating reference station (cors) network on the accuracy, precision, and time to fix ambiguity (ttfa) performance. earth sciences research journal, 26(2), 131-138. https://doi.org/10.15446/esrj.v26n2.89819 https://doi.org/10.15446/esrj.v26n2.89819 https://doi.org/10.15446/esrj.v26n2.89819 132 ömer gökdaş, m. tevfik özlüdemir introduction high-accuracy positioning can be provided through a differential global positioning system (dgps) approach. this approach is also known as relative positioning, and it is based on simultaneous satellite observation and assessment from a reference station with known coordinates, and stations which the positioning will be achieved. within this framework, post-process or real-time kinematic (rtk) methods can be employed. in single base systems with a single reference station, distance-based (atmospheric refraction, or orbit) errors decrease the accuracy. in these systems, a high-accuracy position can be determined with an interstation distance of up to 10-20 kilometres. with multiple reference stations using carrier phase networks instead of a single reference station, real-time positioning can be achieved across all cities and countries with the desired accuracy. using data from multiple stations significantly decreases distance-based errors and increases ttfa performance and positional reliability. for this purpose, the distance-based error model is created in multiple stations’ adjustment software and transferred to the user in real-time (lachapelle et al., 2000; raquet, 1998; wübbena et al., 1996). correction data is transmitted to the user in the radio technical commission for maritime services (rtcm) format. rtcm started its operations as a usa government advisory committee in 1947 before transforming into its current status as a globally supported independent organisation. rtcm creates a standard that facilitates all transportation modes, including satellite positioning and navigation systems. data transmission is achieved through network transport of rtcm via internet protocol (ntrip). this enables a safe flow control mechanism for rtcm data on a network (rtcm, 2019). a frequently used correction model in network rtk (nrtk) is the vrs; the operating principle of this is shown in figure 1. in short, reference stations transmit carrier phase observation and broadcast ephemeris data to the control centre in rtcm format via ntrip. subsequently, a rover transmits approximate coordinates to the control centre in the national marine electronics association (nmea) format via ntrip. there is two-way communication between the rover and the control centre. cors software in the control centre creates a vrs point using the approximate coordinates of the rover. the rover then achieves precise positioning by retrieving correction data based on the vrs location and tropospheric conditions from the internet via ntrip (wanninger, 1999). the advantages of this method are that they support current rtcm formats (rtcm2.1 and rtcm 3.1), calculate troposphere and ionosphere models at the control centre, and create separate corrections for each rover. there are, however, also disadvantages of this method. these include a limited number of concurrent users (depending on the control centre server capacity) and the necessity to re-initialise when the distance between rover position and vrs point exceeds certain distance limits (gumus and selbesoglu, 2019). figure 1. vrs operation principle. studies have already been carried out into the positioning accuracy and reliability of single rtk for different interstation distances using horizontal and vertical components. in these studies, it was found that precision in the vertical components changes as horizontal components change and that as interstation distance increases, the accuracy of single rtk method decreases along with it (featherstone and stewart, 2001; satalich and ricketson, 1998). in some studies, different geometric designs were adopted to test global navigation satellite system (gnss) receiver brands. it was found that when 90 km interstation distance is sufficient, ± 2.5cm accuracy was achieved in horizontal components and ± 5cm accuracy in vertical. different gnss receiver brands had no significant effect on the analysis (edwards et al., 2010; eren et al., 2009; garrido et al., 2011). in terms of temporal correlation for nrtk methods, it was shown that there is no significant difference for interstation distances of between 15 and 45 kilometres (ogutcu, 2019). other studies have observed that galileo and glonass usage with gps has positive impacts on integer ambiguity solution and ttfa (ogutcu and kalayci, 2016; tsakiri, 2011). in studies conducted to investigate the effect of nrtk measurement time on accuracy and precision, it was observed that while measurement durations increase precision, there is no significant difference for accuracy (bae et al., 2015; ogutcu and kalayci, 2018). similarly, studies in south carolina showed that gnss measurement accuracy increases with measurement duration; however, it does not significantly change after 180-300 seconds (allahyari et al., 2018). this paper aims to investigate the impacts of cors network geometry on precision and accuracy by using different cors networks. additionally, the effect of the geometric design on ttfa will be analysed. figure 2 shows the position of two different cors network stations for the study region. figure 2. iski cors (×) and cors-tr (*) network stations in istanbul and its immediate surroundings. during the design phase of the cors-tr network, there was a discussion in the technical and scientific environment regarding point density. following these discussions, the number of stations in the project rose from the 80 that had initially been planned – firstly to 145, and then again to a final total of 158 stations (cors, 2019). in addition to cors-tr, local cors networks such as iski cors network have been put into service in turkey. it has not yet been established how these local and national networks will be integrated, and what the network design principles and point density should be. the solutions developed in this regard are essential both for the efficient use of resources and to guarantee the performance expected from the network. this study was carried out in order to contribute to the formation of the aforementioned solution strategy. within the scope of the study, the effects of baseline lengths on the solution performance related to accuracy and precision will be discussed, as well as the ensuing effect on ttfa, which is a crucial component of gnss networks. in this context, the network geometry was changed for iski cors, with the interstation distances being increased. the analysis was conducted into the accuracy and precision rate for the new interstation distance, and the effects of the redesigned network geometry on integer ambiguity solution and ttfa were investigated. in addition, the sufficiency of iski cors network station frequency was tested using two different network geometries. data and methods this study was based on the correction data of the local iski cors in istanbul and the national cors-tr network. reference stations for these two 133effects of the continuously operating reference station (cors) network on the accuracy, precision, and time to fix ambiguity (ttfa) performance networks have been positioned in locations without signal obstruction to limit satellite visibility. similarly, there are no reflecting surfaces at station locations that might lead to multipath error. as rover stations, two stations that were added to the iski cors network in 2013 and 2014 were used (kest and pasa) (iski, 2019). the average altitude difference between kest-pasa stations and the iski cors network is 47m; for cors-tr, it is 80m. a low altitude difference between the rover and the reference stations decreases the tropospheric effect. in kest and pasa stations, there are topcon tps netg3a model receiver and topcon tpscr.g5 antenna. other stations in the iski cors network have a topcon tps net-g3 model receiver and a topcon tpscr.g3 model antenna. in the cors-tr network, the stations are in the trimble system. the study examined the effect of network geometry on accuracy, precision and ttfa, carrying out analysis on open source rtklib software (rtklib, 2019) with a real-time vrs solution. to introduce stations as a rover on topnet live software and to connect with ntrip, an rtk mount point was defined on these stations and 1004 (gps) and 1012 (glonass) formats in rtcm 3.0 were selected as observation message type. correction data was obtained simultaneously via iski cors and corstr 2101 port external ip over rtklib software, and ntrip connection was provided over a dsl internet line. broadcast ephemeris data were defined with free-broadcasted ntrip. use-snip.com:2101/rtcm3eph service. because introducing these changes in topnet live software (such as defining stations as a rover, and changing network geometry by interrupting correction broadcast of some stations) will impact all users, the measurements were conducted between 11 pm and 4.30 am in order to minimise disruption. in addition, recording measurements at night time resulted in a minimum level of ionospheric effect. figure 3 shows the data communication between cors software in the iski cors and corstr network control centre (topnet live and trimble cors) and rtklib software. in figure 3, it can be seen that solution 1 and solution 2 were obtained by using two different base corrections (iski cors and cors-tr) for kest and pasa stations. figure 3. data communication scheme in vrs solution. to carry out our study, firstly the two reference stations selected as rovers (kest and pasa) were inactivated on topnet live software in the iski cors network centre server. an 8-station iski cors network was used to obtain 17,500 epoch vrs solutions with a one-second period on rtklib software. these data were collected on 24, and 25 october 2019, between 11 pm and 4.30 am (figure 4b). subsequently, four stations inside the iski cors network (beyk, kcek, pala and terk) were inactivated on topnet live software, and baseline lengths were increased (figure 4a). using this new iski cors network geometry, vrs solutions for kest and pasa stations were obtained as 17,500 epochs; these were collected on 25 and 26 october 2019 using the same timeframe. similarly, 6-station national cors-tr network geometry was used, and vrs solutions were simultaneously obtained for kest and pasa stations (figure 5). figure 4. a: 8-station iski cors network geometry. b: 4-station iski cors network geometry. (+: rover). figure 5. national cors-tr network geometry with six stations in the study region (*: cors-tr, +: rover). figure 4 shows the 8-station iski cors network and the new 4-station geometric design. accordingly, the average interstation distance for a 4-station iski cors network was 88.90 km, and the average interstation distance for an 8-station iski cors network was 48.60 km. figure 5 shows the geometric design for cors-tr network (average of 89.60 km interstation distance). since iski cors and cors-tr networks are defined with different datums, itrf2005 itrf2000 and itrf2000 itrf1996 transformations were conducted with the itrf online tool (itrf, 2019) in order to achieve datum unity. kest and pasa station coordinates were determined with bernese (v5.2) software (dach et al., 2015) by using igs stations as a reference, and 134 ömer gökdaş, m. tevfik özlüdemir these values were used as true values (gökdaş and özlüdemir, 2020). vrs cartesian coordinates obtained with three different network geometries for kest and pasa stations were subtracted from the true values determined with bernese software. local geodetic system (lgs) transformation of the differences between the vrs solution and true values were calculated in equation 1. ℎ = −sin cos − sin sin cos − sin cos 0 cos cos cos sin sin × ( − ) (1) in equation 1, n (north), e (east) and h (up) are the coordinate axes in lgs, and and are the latitude and longitude values of receiver positions, respectively. is the true receiver value in the ecef coordinate system. recef are measurement values obtained with vrs. root mean square error (rmse) values for n, e, u (lgs) coordinates are calculated with equation 2. = √∑ ( )2 =1 −1 = √∑ ( )2 =1 −1 = √∑ ( )2 =1 −1 (2) here, k is the total number of vrs epochs. rmse horizontal components (2d) are calculated based on the equation below. 2 = √ 2 + 2 (3) matlab software was adopted to evaluate the results. an anova test was selected, as the population had a normal distribution and independent samples. two different hypotheses were tested in order to establish whether different cors geometric design had caused significant difference for accuracy and precision (equation 4). ho : m1 = m2, ha : ho, is rejected (4) = ∑ ( ̅ − ̅ ) 2 ( −1)⁄ ∑∑( − ̅ ̅ ̅ )2 ( − )⁄ (5) in equation 4, m1 values are vrs solutions (n, e and u) of the kest and pasa stations obtained with the 8-station iski cors network. the m2 value is the vrs solution for cors-tr and the 4-station iski cors network. in equation 5, represents the number of groups. here, iski cors and cors-tr networks are represented as a group. the numerator of the equation represents the sum of squares, and the denominator represents the within-group sum of squares. n is the sample size (the number of epochs). ftest – the value obtained from equation 5 at (k 1) and (n k) degrees of freedom – was compared with ftable value at α: 0.05 significance level. the value from the f table was f1,inf = 3.84. results and discussion as explained in the previous section, the local iski cors network geometric design was analysed. for this purpose, 8and 4-station iski cors networks were used, and vrs solutions were obtained for two stations (kest and pasa) with two different geometric designs (figures 4a and 4b). again looking at two stations, the national cors-tr network was also used to obtain a vrs solution for a longer interstation distance than that of the iski cors network (figure 5). vrs solutions with 17,500 epochs for the 8-station iski cors network, the 4-station iski cors network and the national cors-tr network with six stations in the study region were obtained with rtklib software. as explained in the previous section, this was maintained by calculating the difference between obtained vrs solutions and true values determined with bernese. by using equation 1, lgs transformation was applied to the differences and north, n, e and h values were calculated (figure 6). table 1 shows mean, max, min, rmse and std. values for the north, east and up in lgs. accordingly, the 4-station iski cors showed lower accuracy than the other two cors geometries for n axis mean and rmse value (1.57cm and ±1.76cm). on the e axis, the 6-station cors network provided better results than the other two cors geometries (mean: 0.21cm, rmse: ±0.63cm). on the u axis, the 8-station iski cors network showed the best performance for mean and rmse values (-1.19cm, ±1.66cm), whereas the cors-tr network showed the worst performance (mean: -3.26cm, rmse: ±3.38cm). for std. values calculated for the three different cors geometries, similar results were determined for each axis. figure 6. n, e and u axis average accuracy and precision for three different cors geometries (a: n axis, b: e axis and c: u axis). 135effects of the continuously operating reference station (cors) network on the accuracy, precision, and time to fix ambiguity (ttfa) performance table 1. mean, max, min, rmse and std. values on the n, e and u axes for three different geometric designs. average 4-stations iski 8-stations iski cors-tr (cm) n e u n e u n e u mean 1.57 0.88 -2.71 0.94 0.85 -1.19 1.03 0.21 -3.26 max. 3.17 2.08 -0.97 2.51 2.23 0.64 2.87 1.51 -0.76 min. 0.27 -0.17 -5.62 -0.08 -0.04 -3.26 0.05 -0.68 -5.15 rmse ±1.76 ±1.04 ±2.88 ±1.14 ±1.02 ±1.66 ±1.17 ±0.63 ±3.38 std. 0.80 0.55 0.99 0.65 0.57 1.16 0.56 0.59 0.87 figure 7. normal distribution graphic of vrs solutions with three different cors geometries (a: north, b: east, c: up). figure 8. anova test graphic for three different cors network geometries for the north, east and up components. figure 7 shows normal distribution graphics for the north, east and up components calculated with three different cors geometries. according to figure 7 and table 1, the error margin on the u axis is more than for the other axes. the 8-station iski cors geometry showed the best performance on the u axis. figure 8 shows the box plot of the error distribution of three different cors network solutions. the line in the box plot shows the median value, and the red region outside the upper and lower boundary notes the outliers. vrs solutions on cors networks with three different geometries were compared and analysed using an anova test at a 95% confidence interval on matlab software. as can be seen from table 2, the anova test showed that the 8-station iski cors and 6-station cors-tr had lower f:256 values than the other results on the horizontal component. however, at a 95% confidence interval, there was a significant difference between the three different cors geometry solutions (p>f:0). in table 2, ss is the sum of squares, df is the degree of freedom, ms is mean square, f is ftest value, and ftable value is 3.84. 136 ömer gökdaş, m. tevfik özlüdemir figure 9 shows the average change on the n, e and u axes for the first 100 seconds for three different cors geometries. this shows that, regardless of the number of stations in the iski cors network solution, an average of 5 seconds was obtained; for the cors-tr network solution an average ttfa value of 65 seconds was obtained. figure 10 shows the fix solution rate of kest and pasa stations for three different cors geometries. here it can be seen that the integer ambiguity solution rate of kest station was higher than that of pasa station. while integer ambiguity solution rates of 8-station iski cors and cors-tr network geometries provided similar results for both stations, the ratio was low for 4-station iski cors network geometry. the lower integer ambiguity solution rate in pasa station (compared to kest station) was analysed. low-level integer ambiguity solution in each of the three cors geometries of pasa station was observed between 11 pm and 11.30 pm. between these hours, it was found that only a small number of gps and glonass satellites were observed at pasa station, and they did not show homogenous distribution. figure 11 shows the gps and glonass satellites at 11 pm, 1.30 am, and 4 am. table 2. anova test statistics. anova source ss df ms f prob>f is k i 8 r ef . / c o rs -t r horizontal between groups 0.01157 1 0.01157 256 0 within groups 1.5955 35000 0.00045 total 1.6071 35001 > 3.84 vertical between groups 3.8138 1 3.8138 36369 0 within groups 3.70668 35000 0.00001 total 7.52044 35001 > 3.84 is k i 8 r ef . / 4 r ef . horizontal between groups 0.47293 1 0.47293 11287 0 within groups 1.48098 35000 0.00042 total 1.95391 35001 > 3.84 vertical between groups 2.12132 1 2.12132 17665 0 within groups 4.24460 35000 0.00012 total 6.36591 35001 > 3.84 figure 9. ttfa graph. figure 10. integer ambiguity solution rates. 137effects of the continuously operating reference station (cors) network on the accuracy, precision, and time to fix ambiguity (ttfa) performance summary and conclusion in this paper, the authors investigated the effect of cors network geometric design on the accuracy, precision, ttfa and integer ambiguity solution in nrtk. in addition to the local iski cors network operating in istanbul with ten stations, data were also used from the national cors-tr network with 158 national and six regional stations (figure 2). in addition to the two current network geometries, a new geometric model was developed for iski cors network geometry analysis, and a 4-station iski cors network with a lower number of stations and higher interstation distances was created. for this purpose, only four stations (sile, slvr, tuzl and yali) were activated in topnet live software where iski cors network correction parameters are calculated (figure 4a). vrs solutions were obtained for kest and pasa stations inactivated at the iski cors network central server with three different network geometries. observation files for the inactivated kest and pasa stations and vrs correction data created on topnet live software for kest and pasa stations were transferred to rtklib software. broadcast ephemeris data were obtained from the online service. all data were communicated in rtcm data format and with ntrip data transmission protocol (figure 3). since it is not possible to simultaneously broadcast correction data in 4-station iski cors and 8-station iski cors geometries, the vrs solution was conducted after waiting for one day between the analyses (24-25 october and 25-26 october 2019). in cors-tr geometry, vrs analysis for both days was conducted simultaneously. since increasing baseline lengths by changing the iski cors network geometric design and inactivating two stations as rover will impact the nrtk users, this study was conducted between 11 pm and 4.30 am. the true coordinates of kest and pasa stations were determined with bernese software, and these values were used in accuracy analysis. since iski cors and cors-tr networks are defined on different datums (itrf05 itrf96), datum transformation on kest and pasa stations coordinates were applied for datum unity. the true value of kest and pasa stations and the differences between vrs solutions of three different cors geometries were transformed into lgs with equation 1. north, east and up values calculated in lgs were statistically analysed (table 1). additionally, these values were tested with anova on matlab (table 2). after conducting accuracy and precision analysis on three different cors geometry solutions, integer ambiguity solution rate and ttfa performance were analysed (figure 9, 10). based on these analyses, it was seen that the horizontal and vertical components of three different cors geometry solutions were different from each other at a 95% confidence interval. when compared for accuracy, it was seen that for horizontal components interstation distances of three different cors geometries had no significant effect on results. for vertical components, the 8-station iski cors network with the shortest interstation distance produced the best results for accuracy. for std. values, it was found that interstation distances had no significant effect on the solutions. within the scope of this study, it was seen that although interstation distances had no effect on ttfa, 4and 8-station iski cors geometries ensure precise positioning in the same time interval, and cors-tr geometry provided a solution with a 60-second delay. in the integer ambiguity solution rate, the 4-station iski cors geometry performed less well than the other two cors geometries. additionally, gps and glonass satellite numbers, elevation angle and distribution were investigated for low integer ambiguity solution rates in pasa stations. it was found that these results could be accounted for by the low number of satellites observed between 11 pm and 11.30 pm at pasa station, and their non-homogenous distribution (figure 11). as a result, the authors express that by considering precise positioning at vertical components, integer ambiguity solution rate and ttfa performance factors; the 8-station iski cors has sufficient point density and optimal geometric design. cors-tr also provided similar results to the 8-station iski cors network, especially in the horizontal component and fix solution rate. as seen in this study, iski-cors network gave optimum results with an average interstation distance of 48.60 km. it is recommended not to exceed this base distance in newly established cors networks. acknowledgements the authors would like to thank the istanbul water and sewerage administration for providing full access to iski cors network data and topnet live software. also, the authors thank everyone who created the rtklib software and enabled open source development. references allahyari, m., olsen, m. j., gillins, d. & dennis, m. d. 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(2022). an experimental test for detecting effective reflector height with gps snr data. earth sciences research journal, 26(1), 13-22. https://doi.org/10.15446/esrj.v26n1.87003 record manuscript received: 05/05/2020 accepted for publication: 30/03/2022 abstract: this study aims to estimate effective reflector heights and height differences using the basic geometrical principle of multipath theory by controlling the signal quality for estimations. the geometry of the reflecting signal allows computing the effective reflector height, which is extracted from where the signal reflects on the ground and arrives at the gps antenna phase center. to achieve that, an experimental case with two stations was conducted in the snow-free environment and gps receivers were mounted on reflectors, which allowed to measure daily in-situ reflector heights and artificial decrement variations. the reflections from the roof surface were tracked with the first-fresnel zones. to validate the estimated reflector heights in a controlled test environment, twelve different combinations within four simulated scenarios as a combination of decrement values have been implemented and accuracy analysis was performed. here, a vertical shift procedure on reflectors was applied. meanwhile, the vertical shift amount was tracked in each computation to determine, which reflected signal could be able to use for assigning reflector height as effective. comparisons of the estimated heights and in-situ measurements show congruency with ±1.2 cm to ±8 cm accuracy. the best overall accuracy of the model among the four scenarios is computed as ±2.2 cm. when the vertical shift decrements are considered, the rmse values are estimated within ±2.92 cm to ±3.96 cm. although the rmses of the differences show a good agreement with estimated reflector heights, it is found that some reflector height estimations are statistically insignificant. keywords: gps interferometric reflectometry (gps-ir); signal to noise ratio (snr); multipath theory; effective reflector height. ensayo experimental para detectar la altura de reflexión efectiva con información de la relación señal/ruido en gps resumen: este estudio se enfoca en calcular la altura de reflexión efectiva y las diferencias de altura a través de principios geométricos de la teoría de multitrayectos al controlar la calidad de la señal para realizar los cálculos. la geometría de la señal reflejada permite computar la altura de reflexión efectiva, la cual se toma entre la señal que se refleja en el piso y la que llega a la antena gps. para lograr esto se realizó un ensayo experimental con dos estaciones en un ambiente sin nieve y con los receptores gps instalados en los puntos de reflexión, lo que permite medir diariamente las alturas reflejadas in-situ y las variaciones en la reducción artificial. las reflexiones tomadas en la superficie del techo se monitorizaron con las primeras zonas de fresnel. para validar las alturas de reflexión estimadas en un ambiente controlado se implementaron doce combinaciones en cuatro escenarios simulados como valores de reducción y se realizaron análisis de exactitud. en este punto, se aplicó un procedimiento de migración vertical en los puntos de reflexión. además, la dimensión de la migración vertical se monitorizó en cada cómputo para determinar cual señal reflejada es efectiva para ser asignada como altura de reflexión. la comparación de las alturas estimadas y las medidas in-situ muestra congruencia con un rango de exactitud de ±1.2 cm a ±8. el mejor promedio de exactitud del modelo entre los cuatro escenarios se computó con un margen de ±2.2 cm. cuando se considera la reducción en la migración vertical, los valores rmse se estiman en el rango de ±2.92 cm a ±3.96 cm. a pesar de las diferencias de los valores rmse muestran coincidencia con la altura de reflexión estimada se encontró que algunas estimaciones de alturas reflejadas son estadísticamente insignificantes. palabras clave: interferometría y reflectometría de gps; relación señal/ruido; teoría multitrayectos; altura de reflexión efectiva. https://doi.org/10.15446/esrj.v25n1.74167 14 nursu tunalioglu and cemali altuntas introduction today, global navigation satellite systems (gnss) play a major role to extract high-accurate and four-dimensional (4d; x, y, z+t) geo-referenced positioning information. among these systems, global positioning system (gps), owned and operated by united states (us) has still dominant specifications by ongoing modernization facilities. until now, gps has been used for a wide-range geodetic/geomatics studies, such as deformation monitoring, precise navigation, geodetic networks, cadastral applications, hydrographic surveys, geographical information systems (gis), photogrammetric studies based on kinematic gps, terrestrial-mobile-airborne lidar applications, remote sensing, unmanned aerial vehicle (uav) systems, early warning system configurations for detecting natural disasters such as earthquakes, landslides, meteorological studies, which require high precision positioning information (ocalan et al., 2016). as commonly known, gps enables to estimate threedimensional precise positioning, navigation and timing facilities (hofmannwellenhof et al., 2008; gao, 2018; erdogan et al., 2018; zhou et al., 2019). the improvement in accurate positioning is an ongoing issue that mostly deals with modeling the error sources. besides that, the researches about environmental issues have become more significant than before because of the climate change that affects the whole world. especially accurate estimations of the underground water level, snow package volumes, tracking the vegetation grow, soil moisture have become crucial issues. recently, many scientific researches have been conducted to detect the contributions of these items to the climate system with a novel methodology called, gps interferometric reflectometry (gps-ir) method, which provides significant and effective outcomes for these studies (ozeki & heki, 2011; gutmann et al., 2012; larson & nievinski, 2013; larson et al., 2009a; chew et al., 2016; larson et al., 2009b; roussel et al., 2015; roussel et al., 2016; nievinski & larson, 2014a). precise point positioning in gps needs to eliminate or model the error sources. although one of the major error sources so called multipath, affects the positioning accuracy, it has become an effective tool to extract environmental features. signals reflected from the surface following one or more paths near a gps receiver interfere with the direct signals while a composite signal is formed as the sum of them. the direct and reflected signals can simultaneously be recorded at the antenna of the gps receiver. this interference creates significant oscillations, especially at low elevation angles. if the direct component of the signal is removed, the remaining part will be the reflected component of the signal. the initial procedure of using reflected signals instead of direct signals was introduced by martin-neira (1993) to measure the ocean surface (larson & small, 2016). besides, many studies have been conducted to retrieve surface specifications among the reflected signals. larson et al. (2009a) show that variations in snow depth estimated by gps-ir method agree very well with the in-situ measurements, which means that the method could be used successfully and high-correlated with the in-situ measurements for retrieving the effective reflector height (mccreight et al., 2014; nievinski & larson, 2014a; nievinski & larson, 2014b). ozeki and heki (2011) represent a geometry-free linear combination for the feasibility of analyzing multipath pattern in gps data series and demonstrate that it could be also used for snow depth retrieving facilities. gutmann et al. (2012) indicated the necessity of determination of the effective reflector height when the snow depth computation is indispensable for climatological studies in hydrological ecosystem and conducted a detailed study with nine-month period including winter season following gps-ir methodology by validating the results both laser scanning data and in-situ measurements from snow poles, and found accurate results within 9 cm to 13 cm interval. chen et al. (2014) used cost-effective dipole antenna instead of the geodetic antenna by analyzing l2c signals collected and showed an improvement in results when compared with the geodetic receiver for snow depth estimations. hefty and gerhátová (2014) examined snr data of l1 and l2 carrier phases and the geometry-free linear combination of carrier phases, l4 for two permanent gps stations data. the validation was performed with the regular manual snow depth measurements and it was demonstrated that better than 5 cm accuracy was mostly reached when comparing with the estimated reflector heights. bilich et al. (2011) define the ground specifications of a good gps-ir station as flat or slightly titled surface as lower than 5° ground slope, and previous studies show that implementing gps-ir method to the horizontal, planar surface performs good results. however, a new strategy proposed by wei et al. (2019), so-called tilted surface strategy (tss) exposed the slope effect of the topography to retrieve the snow depth and they concluded that following the tss for receivers located on slopes works better. in this paper, first a brief description of the multipath theory and the processing of snr data following larson and nievinski (2013) are introduced. then, the experimental site performed for accuracy assessment of the model by vertical shifting is applied and the results are presented. a snow-free environmental is demonstrated to retrieve the effective reflector height for the experimental test to mitigate the penetration effect of the gps signals into the surface and the estimations with the rmses of vertical shifting values are computed. multipath effect and snr observations hofmann-wellenhof et al. (2008) give the basic description of the multipath effect on carrier phases as receiving the satellite-emitted signal at the receiver by more than one path, caused by reflecting surfaces near the receiver in particular (figure 1). recently, multipath, as a major error source for gnss precise point positioning, has become a key element for retrieving information from reflecting surface. even though the antenna configuration of the gps is designed to suppress this error source, that can not be removed totally (larson et al., 2009b). the reflected signal contribution can be seen at low elevation angles, i.e. lower than 30°, due to antenna gain pattern (larson & small, 2016). several studies have been exposed to the use of multipath data for estimating the parameters of reflecting surface. besides that, the geometry of the reflecting signal allows computing the effective reflector height, which is extracted from where the signal reflects on the ground and arrives at the gps antenna phase center (apc). figure 1. multipath effect due to reflecting surface and converted vertical height from reflecting surface the multipath effect means that the satellite signals don’t reach the antenna center directly in one way, but some are reflected once or several times in advance on objects in the receiving environment or on the ground. the multipath effect is occurred both from the relation of the geometry of the reflecting surface and geodetic receiver antenna and the dielectric constant of the reflecting surface (larson et al., 2009b). the reflected signals have always a longer transit time than the direct signal. that leads to an extended pseudorange determination between the antennas on the ground and the satellite. since the phase of a wave is a function of the transit time, a longer transit time also affects the carrier phase measurement. because objects on the surface of the earth mainly affect satellite signals with low elevations, signals from low-flying satellites are more affected by multipath effects than signals sent by satellites with high elevation. the amount of the error, which is caused by multipath effects, depends on the multipath length (the resulting delay of the signal travel time), the phase difference between direct and reflected waves and their power ratio. the achievable accuracy of positioning using gnss measurements depends on the detection of the stations (variations of the antenna phase center, multipath effect, receiver noise, receiver clock and receiver signal errors) and satellite-specific factors (satellite geometry, clock and orbit errors of the satellites, including gnss code and system bias, phase wind-up) the behavior of the signals of the gnss satellites, which acts as high-flying fixed points at 15an experimental test for detecting effective reflector height with gps snr data an average altitude of 20200 km, is influenced during the propagation in the various layers of the atmosphere (delay in the signals in the ionosphere and troposphere). most of the error components that affect the accuracy of gps are largely eliminated by the differentiation method by data processing. a good example of such an approach is rtk (real-time kinematik) and networkrtk, where error components of gnss are calculated using data from one (rtk) or several reference stations (network-rtk) in the surroundings of the rover and by a data transmission medium either as correction of coordinates or as values for corrections from the rover received satellite data made available in real-time. the rover uses these corrections to increase the accuracy of the positioning, this requires a service for the calculation and transmission of the corrections to the rover as well as for the permanent maintenance of reference stations within the network. from the mid-1990s, cors (continuously operated reference station) networks were established in europe and in certain regions of the world to provide the rtk and network rtk services. multipath effect, the cause of which lies only in the immediate surroundings of the antenna, cannot be therefore eliminated using the differential method, because multipath effects are uncorrelated even from a distance of a few meters between two antennas. the computation of multipath effects at reference stations are therefore very important with respect to the precise positioning using gnss data, since a coordinate error at reference station also influences the determination of the correction data and thus falsifies the calculated coordinates of the rover. therefore, the cors of a network must be free of the multipath effect to reach the high quality for positioning. since any general model can be extracted for the multipath effect, the influence can be detected by code and carrier phase measurements (hofmannwellenhof et al., 2008). however, the signal power of the composite signal formed by interfering with the reflected and direct signals can be affected as well. multipath effects are also visible in the signal to noise ratio (snr) data constantly collected by gnss receivers (roussel et al., 2015). signal strengths are also stored in gps receivers beside carrier phases. the signal strength corresponds to the carrier-to-noise density ratio (c/no), which is a ratio of signal power to noise power density (larson & nievinski, 2013). the snr, computed from this ratio, is usually used for determination of signal quality and noise characteristics of classical gps observations (qian & jin, 2016; tabibi, geremia-nievinski, & van dam, 2017). since the snr data are in the logarithmic unit in db, they are converted to a linear scale in watt per watt (volt per volt) by snr ( (volts volts =10snr(db-hz/20), assuming a 1-hz bandwidth (larson & nievinski, 2013). the snr value recorded at a receiver is assumed to be equal to the amplitude of the received signal, then the equation equals the same quantity between snr and amplitude of the composite signal. during the satellite movements across the sky, the phase difference between direct and reflected signals is formed, which is also represented as an interference pattern (löfgren et al., 2014; drosinos, 2016). the interference between these signals is directly related to the satellite elevation angle. since, the satellite elevation angle increases, the multipath effect on snr data decreases (larson et al., 2008). as the signal transmitted from a satellite received at the antenna as a composite signal, interfere of direct and indirect signals, the relation between the signal to noise ratio can be expressed as equation 1 (larson et al., 2008; larson & nievinski, 2013; nievinski & larson, 2014a); snr2≈a2 d +a2 m+2ad amcosψ (1) here, ad and am are the amplitudes of direct and reflected signals (in volt unit), respectively and is the phase difference (in radian unit) between these signals. assuming ad>> am, the magnitude of the snr is mainly caused by direct signal, a quadratic polynomial function can be used to remove the dominant part effect (larson & small, 2016). after removing this part, the remaining snr value, as commonly expressed by detrended snr (dsnr), can be described by equation 2; dsnr=a cos(4πhλ-1 sinε+ψ) (2) where a is the amplitude, ψ is the phase offset, ε is the satellite elevation angle, λ is the carrier wavelength, h is the distance between the antenna phase center (apc) and the reflecting surface (generally called as reflector height). as the multipath pattern can be expressed as a function of the sine of the elevation angle, that causes an unevenly spaced sampling intervals, the dominant frequency of the dsnr will be constant and equal to the 2h⁄λ. once the dominant frequency of the dsnr values is extracted, it can be converted to the reflector height, which is the distance between the apc and reflecting surface. lomb scargle periodogram (lsp) is used to estimate the dominant frequency (larson & small, 2016). since lsp spectrum analysis represents the power of the frequencies for the snr series, the snr data used are grouped into varied elevation angle ranges including noisy frequencies to control the effective reflector heights estimated with in-situ reflector measurements. materials and methods study area the study aims to control the snr data fluctuations on retrieving effective reflector heights by checking the vertical shift amount from them. for that, two stations, so-called elk1 and elk2 were established on the roof of the faculty of electrical and electronics engineering at the davutpaşa campus of yıldız technical university in istanbul. the two-dimensional metrics of the roof are approximately 12 m and 75 m. the roof surface can be accepted as quite flat avoiding the surface roughness, with the exception of the slightly elevated ground as wall border near the elk1 station. also, there are no high buildings near the roof that could obstruct the open sky view and make disturbing reflections. the representation of the locations of the stations on the roof is given in figure 2 and the geographical coordinates are given in table 1. figure 2. study environment (a) locations of the stations on the roof (b) elk2 station-south direction (c) elk1 station-east direction table 1. geographical coordinates of the stations (wgs84) station φ(°) λ(°) h (m) elk1 41° 01’ 43.345647’’ 28° 53’ 24.047295’’ 130.2373 elk2 41° 01’ 44.521411’’ 28° 53’ 24.973158’’ 129.9659 data acquisition gps observations were performed in an experimental field site with two observation stages. the first one was carried out on day of years (doy) 203, 204, 205, 206, 207 and 210; and the latter on doy 211, 212, 213, 214, 215 and 217. the starting and ending times and dates for observations are given in table 2. the receivers used in the first part were topcon hiper pro at elk1 and topcon gr-5 at elk2, and in the second part were chc i70 at elk1 and chc i80 at elk2, respectively. moreover, the technical specifications of the used receivers are given in table 3. table 3 also covers the snr evaluations of the collected data that are done for l1 carrier frequencies. 16 nursu tunalioglu and cemali altuntas table 2. detailed information about the gps observations performed on sites at 1 hz rate date doy site receiver start (utc +3h) end (utc +3h) 22.07.2019 22.07.2019 203 203 elk1 elk2 topcon hiper pro topcon gr-5 10:05 10:16 16:34 16:35 23.07.2019 23.07.2019 204 204 elk1 elk2 topcon hiper pro topcon gr-5 09:16 09:20 16:30 16:32 24.07.2019 24.07.2019 205 205 elk1 elk2 topcon hiper pro topcon gr-5 09:22 09:24 16:37 16:37 25.07.2019 25.07.2019 206 206 elk1 elk2 topcon hiper pro topcon gr-5 09:07 09:09 17:06 17:06 26.07.2019 26.07.2019 207 207 elk1 elk2 topcon hiper pro topcon gr-5 10:09 10:13 16:35 16:35 29.07.2019 29.07.2019 210 210 elk1 elk2 topcon hiper pro topcon gr-5 09:18 09:21 16:37 16:40 30.07.2019 30.07.2019 211 211 elk1 elk2 chc i70 chc i80 09:25 09:30 16:32 16:33 31.07.2019 31.07.2019 212 212 elk1 elk2 chc i70 chc i80 09:23 09:26 16:32 16:33 01.08.2019 01.08.2019 213 213 elk1 elk2 chc i70 chc i80 09:22 09:24 16:34 16:35 02.08.2019 02.08.2019 214 214 elk1 elk2 chc i70 chc i80 09:21 09:24 16:32 16:33 03.08.2019 03.08.2019 215 215 elk1 elk2 chc i70 chc i80 09:29 09:31 16:57 16:58 05.08.2019 05.08.2019 217 217 elk1 elk2 chc i70 chc i80 09:37 09:39 16:39 16:40 table 3. technical specifications of gnss receivers and snr data collected (ε, satellite elevation angle) topcon hiper pro topcon gr-5 chc i70 chc i80 satellite systems gps, glo gps, glo gps, glo, gal, bds, qzss gps, glo, gal, bds, qzss, navic satellite signals l1, l2 l1, l2 l1, l2, l5 l1, l2, l5 l1 min-max-mean snr (ε>25) 40.0-52.0-47.4 db 42.0-51.0-47.8 db 40.3-51.5-47.3 db 34.2-51.3-47.3 db l1 min-max-mean snr (ε<25) 33.0-46.0-41.3 db 31.0-47.0-41.9 db 26.6-46.2-40.5 db 26.6-45.5-40.3 db l1 min-max-mean snr (overall) 33.0-52.0-43.7 db 31.0-51.0-44.6 db 26.6-51.5-43.3 db 26.6-51.3-43.2 db gps data were collected with a sampling interval of 1 sec. to detect the effective reflector heights on the site using gps snr data, a decrement as 10 cm assuming a vertical displacement occurred as an effect of changing the vertical distance was applied to both reflectors established on the stations. to do that, the observations were planned first to start from 2 m for the elk1 station and 1.70 m for the elk2 station, and then the reflector height was reduced by 10 cm in every two days by collecting daily data for at least 6 hours. the decrement is applied to simulate snow depth variations. in this study, only gps observations were used. the data collected during the site surveys are split as ascending and descending according to variations of the satellite elevation angle. all the data were recorded by the gps receivers, but the data for low elevation angles (5°-25°), where the major multipath effect was intense, were analyzed. in the analysis, observations, where the difference between the minimum and maximum elevation angles of each observed satellite during any satellite arc track is less than 10° are not taken into account. this 10° condition benefits from getting enough epoch numbers for each track. in the evaluations, the azimuth angle range of 30°-210°, where the roof surface is flat, was selected, and data from other directions were excluded due to the obstructs on the roof. here, the azimuth angles except the selected range were not proper to analyze while considering the direction and width of the sensed areas on the roof. the sensed area, i.e., first fresnel zone (ffz), varies depending on the change of satellite elevation angle and reflector height (larson & nievinski, 2013). due to that different reflector heights have been used to track the variation of the sensed area from ffzs. figure 3 shows the ffzs for observations to the satellites at elevation angles 5° and 10° for reflector heights of 1.70 and 2 m in doy 203. only snr values of l1 signals were analyzed and used in this study. the statement given in larson and nievinski (2013) is used to identify the signals as a strong reflection, where the peak value on data series is higher than four times of background noise, for each ascending and descending arcs of the satellite tracks. 17an experimental test for detecting effective reflector height with gps snr data figure 3. ffzs at elk1 and elk2 in doy:203 (a) for 5° elevation angle, 2.00 m reflector height (b) for 10° elevation angle, 2.00 m reflector height (c) for 5° elevation angle, 1.70 m reflector height (d) for 10° elevation angle, 1.70 m reflector height. red ellipses are within the desired azimuth range while blue ellipses are outside that range. the radii of the nested circles indicating the distance from the point were drawn in increments of 10 m. the signal that does not meet this condition is called weak reflection in this study. in this way, it has been found that 139 of the 371 signals have a strong reflection. maximum critical reflector height converted from the lsp was taken as 10 m, which was important to estimate the strong reflection statement that can change the arithmetical mean value of ai.. 18 nursu tunalioglu and cemali altuntas figure 4. four examples of dsnr values in doy:211 (a) strong reflection at elk1 (b) weak reflection at elk1 (c) strong reflection at elk2 (d) weak reflection at elk2. dashed lines are polynomial fits to the snr data. the black lines show the snr data, the blue dsnr in linear scale, the orange snr in logarithmic scale and an example for (e) total c/no for both direct and multipath signals with satellite elevation angle 19an experimental test for detecting effective reflector height with gps snr data results and discussion the examples of weak and strong reflections of dsnr values obtained from the analyses are represented in figure 4. the examples of strong reflections shown in figures 4a and 4c present sinusoidal trend for oscillations especially at low elevation angles. since the multipath effect decreases with increasing the satellite elevation angle, the oscillations in the resulting signal become smaller. thus, the oscillations become stable when the elevation angle increase. for the weak reflection examples, no strong periodic oscillation could be tracked (figure 4b, 4d). moreover, although the reflections between 5°-15° elevation angles did not show periodic oscillations, this situation has changed when the elevation angle interval is drawn for 15°-20° (see, figure 4b and 4d). this can be caused by the location of the reflected signal that falls to the roof surface when the elevation angle increases. the lsp was used to convert the snr data to the effective reflector heights. spectral amplitudes of each gps track computed by lsp were evaluated by following larson and nievinski (2013). then, the dominant frequencies are converted into effective reflector heights. figure 5 shows the 6-day amplitudereflector height values of strong signals for elk1 and elk2 stations. figure 5. some of the strong reflections for 6 days (a) chc i70 at elk1 (b) chc i80 at elk2 the correlation coefficients (ρ) between estimations and in-situ measurements for reflector heights are shown in figure 6. moreover, the detailed information about the correlation coefficients calculated from estimated reflector heights and in-situ measurements calculated separately from receivers is given in table 4. the highest correlation coefficient is computed at elk2 station as 0.9346 for 46 estimations. although the maximum estimation number, 49 is obtained at elk2, the correlation coefficient is calculated as 0.8587, which is the lowest agreement with the in-situ measurements. this could be caused by the gps instrument used (see, table 3). figure 6. correlation coefficients (a) topcon hiper pro at elk1 (b) chc i70 at elk1 (c) topcon gr-5 at elk2 (d) chc i80 at elk2 table 4. number of estimations and correlation coefficients for receivers receiver number of estimations correlation coefficient topcon hiper pro (at elk1) 23 0.8798 topcon gr-5 (at elk2) 49 0.8587 chc i70 (at elk1) 21 0.8786 chc i80 (at elk2) 46 0.9346 the daily mean values of estimated reflector heights and root mean square errors (rmse) with respect to doys are given in table 5. 20 nursu tunalioglu and cemali altuntas table 5. daily mean values of estimations and rmses elk1 topcon hiper pro chc i70 doy num. of est. in situ (m) estimated height (m) rmse (cm) doy num. of est. in situ (m) estimated height (m) rmse (cm) 203 3 2.000 1.991 1.2 211 1 2.000 1.976 2.4 204 4 2.000 2.010 3.6 212 4 2.000 2.004 1.4 205 4 1.900 1.920 4.2 213 4 1.900 1.904 3.6 206 5 1.900 1.942 6.1 214 3 1.900 1.920 3.8 207 4 1.800 1.843 5.1 215 5 1.800 1.846 5.7 210 3 1.800 1.817 3.3 217 4 1.800 1.839 5.7 elk2 topcon gr-5 chc i80 doy num. of est. in situ (m) estimated height (m) rmse (cm) doy num. of est. in situ (m) estimated height (m) rmse (cm) 203 7 1.700 1.762 8.0 211 5 1.700 1.717 3.0 204 8 1.700 1.738 6.0 212 6 1.700 1.728 4.0 205 10 1.600 1.627 4.8 213 8 1.600 1.576 3.3 206 10 1.600 1.624 6.7 214 7 1.600 1.572 3.6 207 7 1.500 1.522 6.2 215 10 1.500 1.488 4.1 210 7 1.500 1.499 6.5 217 10 1.500 1.479 4.8 here, the model efficiencies were estimated to figure out how the model fit the site observations considering the location of the points separately. then, the accuracy of the model for elk1 with topcon hiper pro receiver is computed as 2.7 cm, for elk2 with topcon gr-5 receiver is 3.4 cm, for elk1 with chc i70 receiver is 2.8 cm, for elk2 with chc i80, receiver is 2.2 cm. to verify the estimations with the vertical shift amount, assuming as constant in each couple of days, 12 different combinations were created for decrements of 10 cm or 20 cm. the rmses of estimated differences are computed from subtracting the in-situ differences from the differences in the mean of daily estimated heights. table 6 represents the computed rmses of 12 combinations for four simulated scenarios. the overall rmses of elk1 for scenarios 1 and 2 are calculated as ±2.92 cm and ±3.96 cm, respectively. meanwhile, the overall rmses of elk2 for scenarios 3 and 4 are calculated as ±3.13 cm and ±3.68 cm, respectively. these results agree with the rmses of the daily mean values of estimations. however, according to the rmse computations of estimated differences (see table 6, column 5 & 7), the rmse values of some of the vertical shifts (the underlined cell) are greater than vertical shift values. this situation can be interpreted as they are statistically insignificant. moreover, according to the results given in table 5 and 6, there are slight differences between the rmse values of estimations, which are arisen due to usage of different receivers. conclusions in multipath theory, a signal transmitted from a satellite uses more than one path to arrive at the gnss antenna. here, the signal may be reflected from the surface around where the receiver is set. the reflected and direct signals, which interfere with each other at the phase center of the gnss antenna, are recorded simultaneously. the multipath modulation can be observed in gps observations as snr values, which enable to estimate the vertical distance between the reflected surface of the signal and phase center of the gnss antenna. several studies have been conducted to estimate the reflector height to extract environmental variables, such as snow depth, vegetation height, etc. in this study, an experimental site was prepared to extract the effective reflector height with its accuracy assessment. the study involves gps l1 snr data due to provide strong signal strengths than the l2 and/or l5. to achieve varying reflector heights on the site, the reflector height is displaced manually in daily observations to detect artificial reflector height variations. the results indicate that the reflector heights and height differences between days given as 10 cm in each observation day agree with the daily in-situ measurements showing a high correlation with 0.9346 correlation coefficient at elk2 station. here, the results are supported by the in-situ measurements to present the accuracy of the estimations. when the 6-day estimations are considered to figure out how the model fits the site observations, the best accuracy of the model is computed as 2.2 cm for the same station as well. although the reflector height estimation has become a routine and well-known method, in this study the accurate estimation of vertical differences is examined to obtain precisions from the reflector heights. the absolute vertical differences as 10 cm and 20 cm are considered as the true value and rmses of vertical shifting amounts are computed. according to the results, the overall accuracy is obtained between 2.92 cm and 3.96 cm. however, this also shows that some of the reflector height estimations are insignificant. this situation brings a necessity to examine the differences as well. with the help of statistical considerations, more accurate and valid solutions in reflector height retrieving can be provided. acknowledgment the figure 2 was plotted by using generic mapping tools (wessel & smith, 1998). 21an experimental test for detecting effective reflector height with gps snr data table 6. vertical shift validation of estimations with rmses elk1 combinations of doys via scenarios topcon hiper pro chc i70 # (1) (2) in-situ difference (cm) estimated difference with rmse (cm) overall rmse (cm) estimated difference with rmse (cm) overall rmse (cm) 1 203-205 211-213 10.00 7.10 ± 4.37 ± 2.92 7.20 ± 4.33 ± 3.96 2 203-206 211-214 10.00 4.90 ± 6.22 5.60 ± 4.49 3 203-207 211-215 20.00 14.80 ± 5.24 13.00 ± 6.18 4 203-210 211-217 20.00 17.40 ± 3.51 13.70 ± 6.18 5 204-205 212-213 10.00 9.00 ± 5.53 10.00 ± 3.86 6 204-206 212-214 10.00 6.80 ± 7.08 8.40 ± 4.05 7 204-207 212-215 20.00 16.70 ± 6.24 15.80 ± 5.87 8 204-210 212-217 20.00 19.30 ± 4.88 16.50 ± 5.87 9 205-207 213-215 10.00 7.70 ± 6.61 5.80 ± 6.74 10 205-210 213-217 10.00 10.30 ± 5.34 6.50 ± 6.74 11 206-207 214-215 10.00 9.90 ± 7.95 7.40 ± 6.85 12 206-210 214-217 10.00 12.50 ± 6.94 8.10 ± 6.85 elk2 combinations of doys via scenarios topcon gr-5 chc i80 # (3) (4) in-situ difference (cm) estimated difference with rmse (cm) overall rmse (cm) estimated difference with rmse (cm) overall rmse (cm) 1 203-205 211-213 10.00 13.50 ± 9.33 ± 3.13 14.10 ± 4.46 ± 3.68 2 203-206 211-214 10.00 13.80 ± 10.44 14.50 ± 4.69 3 203-207 211-215 20.00 24.00 ± 10.12 22.90 ± 5.08 4 203-210 211-217 20.00 26.30 ± 10.31 23.80 ± 5.66 5 204-205 212-213 10.00 11.10 ± 7.68 15.20 ± 5.19 6 204-206 212-214 10.00 11.40 ± 8.99 15.60 ± 5.38 7 204-207 212-215 20.00 21.60 ± 8.63 24.00 ± 5.73 8 204-210 212-217 20.00 23.90 ± 8.85 24.90 ± 6.25 9 205-207 213-215 10.00 10.50 ± 7.84 8.80±5.26 10 205-210 213-217 10.00 12.80 ± 8.08 9.70±5.82 11 206-207 214-215 10.00 10.20 ± 9.13 8.40±5.46 12 206-210 214-217 10.00 12.50 ± 9.33 9.30±6.00 references bilich, a. l., slater, a.g. & larson, k.m. 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(2019). a new gps snr-based combination approach for land surface snow depth monitoring.  scientific reports,  9(1), 3814. https://doi.org/10.1038/s41598019-40456-2. http://dx.doi.org/10.15446/esrj.v20n4.59496 _goback issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.113775 earth sciences research journal earth sci. res. j. vol. 29, no. 2 (june, 2025): 131 137 g eo lo g y abstract evidence of fluid process for li-rb-f-w rich vein rocks of degana, rajasthan, india vijay anand, s*, nisha devi, r department of applied sciences (applied geology), curtin university, miri-98009, sarawak, malaysia *corresponding email: vijay.anand@curtin.edu.my record manuscript received: 03/04/2024 accepted for publication: 13/03/2025 how to cite this item: anand, v. s., & devi, n. r. (2025). evidence of fluid process for li-rb-f-w rich vein rocks of degana, rajasthan, india. earth sciences research journal, 29(2), 131-137. https://doi.org/10.15446/esrj. v28n2.113775 several mineralized vein rocks were intruded into the granites of rajasthan, india. in some of the vein rocks in rajasthan has associated with enormous amount of critical metals and rare metals. the metals are spatially associated with the vein rocks of degana, rajasthan, india. six samples were collected in wall and core of the vein rocks. two samples from wall and one sample from core of each vein respectively. the samples were selected for fluid inclusion and laser raman microprobe studies. four distinct types of fluid inclusion were identified and classified. type i is aqueous bi-phase (lh2o+vh2o) inclusion; type ii is aqueous-carbonic (lh2o+lco2) inclusion; type iii is carbonic mono liquid (lco2) inclusion under room temperature; type iv is polyphase inclusion (lh2o+vh2o+s). the varying homogenization temperatures with different salinities implies mixed fluid process. the fluid process was mainly derived from the host rock granite. later the cooled magmatic fluids were mixed with the meteoric water during hydrothermal stage may be the major key factor for formation of li-rb-f-w mineralization in rajasthan, india. keywords: neoproterozoic; vein rock; fluid salinity; fluid boiling; carbonic inclusion; laser raman microprobe. resumen varias vetas de roca mineralizadas fueron intruidas en los granitos de rajastán, india. algunas de estas vetas de rajastán presentan una enorme cantidad de metales críticos y raros. estos metales están asociados espacialmente con las vetas de degana, rajastán, india. para este trabajo se recolectaron seis muestras de la pared y del núcleo de dos vetas. dos muestras de la pared y una del núcleo de cada veta, respectivamente. las muestras se seleccionaron para estudios de inclusión fluida y microsonda láser raman. se identificaron y clasificaron cuatro tipos distintos de inclusión fluida: el tipo i es una inclusión acuosa bifásica (lh2o+vh2o); el tipo ii es una inclusión acuoso-carbónica (lh2o + lco2); el tipo iii es una inclusión carbónica monolíquida (lco₂) a temperatura ambiente; y el tipo iv es una inclusión polifásica (lh2o + vh2o + s). la variación en las temperaturas de homogeneización con diferentes salinidades implica un proceso de fluido mixto. este proceso fluido se derivó principalmente del granito de la roca huésped. más tarde, los fluidos magmáticos enfriados se mezclaron con el agua meteórica durante la etapa hidrotermal, lo que puede ser el principal factor clave para la formación de la mineralización de li-rb-f-w en rajastán, india. palabras clave: neoproterozoico; veta de roca; salinidad del fluido; lava; inclusión carbónica; microsonda láser raman. evidencia de un proceso fluido en rocas veteadas ricas en li-rb-f-w de degana, rajastán, india 132 vijay anand, nisha devi, r 1. introduction lithium (li), rubidium (rb), and tungsten (w) are economically valuable critical metals and widely used into many industrial applications (wang et al., 2017b; huang et al., 2020; sovacool et al., 2020; xu et al., 2021; deng et al., 2022; guo et al., 2022; 2023; wenkai et al., 2023). the critical metals are often associated with many magmatic and hydrothermal vein rocks in different geodynamic settings (linnen et al., 2012; kaeter et al., 2018; liu et al., 2020; yin et al., 2020). the hydrothermal vein rocks are broad diversity of economic interest, and it is a significant source for many metals. understanding the metallogenesis by sims (secondary ion mass spectrometry), icp-ms (inductively coupled plasma mass spectrometry), and shrimp (sensitive high-resolution ion microprobe) have been used significantly (li et al., 2023; xu et al., 2023). in india, rajasthan characterized by many granitic rocks and intruded by many hydrothermal vein rocks and enriched with li, rb, w, sn, b, nb and ta (chattopadhyay et al., 1994; pandian, 1999; pandian and varma, 2001; singh and singh, 2001). in rajasthan, degana region has well occupied three important mineralized hills (phyllite hill, tikli hill, and rewat hill) (figure 1). several mineralized vein rocks are intruded into the granites of degana, rajasthan, india (pandian and varma, 2001). in tikli hill, the mineralized vein rock is approx. 30-40cms wide and >1meter long and has the trend of n29° with moderate dip (18º). similarly, in rewat hill, the mineralized vein rock is approx. 50cm wide and >1.5meter long. the mineral assemblages and the grain sizes may vary from wall to core of the vein. anand et al., 2018 has reported about the degana granite (greisen quartz) associated tungsten deposit fluid process. till now no one reported about the wall-core vein rock fluid characteristics; in this manuscript reporting the types of fluid inclusions, fluid composition, fluid salinity, first melting temperature, last ice melting temperature, clathrate (gas hydrate) melting temperature and homogenization temperatures to elucidate the fluid process are discussed in this manuscript. 2. methodology six samples were obtained from the tikli and rewat granite intruded mineralized vein rocks. two of the samples were collected from western and eastern wall of the vein, one sample from the quartz rich core (quartzzinnwaldite) respectively. the doubly polished thin wafer sections of these li-rb-f-w samples were prepared for microscopic studies. 2.1 fluid-microthermometry the fluid-microthermometry (heating-freezing) were measured by linkam thms-600 stage mounted on the leica transmission microscope (german made). the unit were operating in the temperature range is -180 to +600⁰c (l(liquid) n(nitrogen) t(temperature). the fluidmicrothermometry unit calibrated by using synthetic co2-h2o inclusion (co2 triple point (-56.6 ºc) and h2o triple point (-0.01 ºc). the inclusions were freezes to -100ºc for solidification and warmed slowly. the type i to type iii inclusions were cooled first until solidified, then heated slowly to record co2 and h2o melting temperature (triple points), clathrate melting temperature, and homogenization temperature. type iv inclusions were heated (10ºc/min) to record the liquid-vapor (l-v) homogenization temperature (thh2o) and salt dissolution temperature (tmsalt). data generated on only those inclusions not decrepitated and gave reproducible during the freezing-heating runs. the doubly polished wafer sections were prepared and analyzed at the department of earth sciences, pondicherry university, india and curtin university, malaysia. the measurement accuracy is ±0.2oc during freezing and ±2oc during heating runs. the detailed tikli and rewat vein rocks fluid inclusion data were given in the table 1-4. 3. results 3.1 vein rock petrography tikli and rewat vein rocks mainly composed of k-feldspar, zinnwaldite (li-mica), plagioclase, quartz and topaz phenocrysts in a fine-grained matrix (groundmass) (figure 3). the matrix predominantly composed of feldspar and quartz. the feldspar and quartz phenocrysts are corroded along their margins by the surrounding matrix phase. quartz is partially reabsorbed (replacement) by k-feldspar (figure 3c). the zinnwaldite shows the colorless to slightly darker colored, which shows the strong pleochroism and light brown to orange-blue interference colour. the zinnwaldite grains were typically associated with quartz and feldspar in the wall of the vein rock (figure 2 and 3). figure 1. geological map of degana hill (phyllite hill, tikli hill, rewat hill), rajasthan, nw india (modified after anand et al., 2018). fluorite wolframite zinnwaldite tikligranite tikligranite quartz topaz fluorite figure 2 (a) li-rb-f-w mineralized vein rock intruded into tikli granite, degana, rajasthan, nw india. 133evidence of fluid process for li-rb-f-w rich vein rocks of degana, rajasthan, india figure 2(b) li-rb-f-w mineralized vein rock intruded into rewat granite, degana, rajasthan, nw india. zinn 500 μm 500 μm 500 μm 500 μm zinn zinntpz plag kfs plag plag qtz qtz qtz figure 3. microphotographs of li-rb-f-w mineralized quartz vein rock samples. (a-b) tikli vein rock; (c-d) rewat vein rock. mineral abbreviations: qtz-quartz; plag-plagioclase feldspar; kfs-k-feldspar; tpz-topaz; zinn-zinnwaldite. 3.2 fluid inclusion in li-rb-f-w mineralized vein rock fluid inclusion in vein rock samples were genetically classified into primary inclusions (isolated and group of inclusions), secondary (intergranular) and pseudo-secondary (intra granular) inclusions (planar arrays or trails) (shepherd et al. 1985, roedder,1992, bakker, 2003, bodnar et al., 2014). secondary and pseudo-secondary inclusion not focused for the present study. fluid inclusions were not recorded for feldspar, topaz minerals due to poor anisotropism contrast with quartz mineral. few aqueous inclusions were observed in zinnwaldite, due to its contrasting light intensity unable to run microthermometry. four types of inclusions were classified in quartzzinnwaldite minerals contact zone. type i is aqueous bi-phase (lh2o+vh2o) inclusions; type ii is aqueous-carbonic inclusions (lh2o+lco2) with graphite (c). the size of the inclusion varies from 8 to 20µm, and the shape is almost oval to rounded and irregular. type iii is carbonic monophasic inclusion, which contains co2 liquid (lco2) at ambient laboratory temperature. type iv is polyphase inclusion (lh2o+vh2o+c+s1+s2). type iv inclusions were randomly distributed with size ranging from 4-16µm, and the shape is irregular, rectangular, and rounded (figure 4-5). 3.3 tikli li-rb-f-w mineralized vein rock the first melting temperature recorded for type i-iii inclusions at -23 to -25°c and -61 to -42°c, suggesting that the solution has mixture of li+, k+, na+, ca2+, f, and cl-. the final ice melting temperature varies from -12.4 to -4.3oc (salinity: 16.3 to 6.8 wt. % nacl equivalent). type i inclusions were homogenized into liquid between 200-325oc (lh2o>vh2o→ lh2o), while some inclusions were homogenized into vapor between 240-370oc (vh2o>lh2o→vh2o). for type ii inclusions, the tmco2 recorded between -59.4 and -56.5oc (indicates presence of other volatile phases). tm-clathrate recorded between 2.5 to 8.6 (salinity: ~16.2 to 4.9 wt. % nacl equivalent) (darling, 1991). the partial thco2 recorded between 20-29oc (lh2o+lco2>vco2→lh2o+lco2) and the thtotal for type iii inclusions observed at 200-356oc (lh2o+lco2→lh2o). some of the type ii inclusions were homogenized into vapor (275-396oc). for type iii inclusions, tmco2 recorded between -62.3 to -57.0oc (indicates presence of other carbonic phases). the thco2 of these inclusions were homogenized into vapor phase between 10 and 27oc (vco2>lco2→vco2). for type iv inclusions, vapor out temperature for type iv inclusions were occurred between 185330oc (lh2o>vh2o+s→lh2o+s). the total homogenization observed by the disappearance of salt (salt melting or dissolution) average at 210-386oc (salinity: 32-46 wt. % nacl equivalent) (table 1). the fluid salinity calculated based on the last ice melting (type-i) (goldstein, 1994), clathrate melting (type-ii-iii) (darling, 1991) and salt melting (dissolution) temperatures (type iv) (goldstein, 1994; bodnar, 1993). 10 μm 10 μm 10 μm 10 μm vh20 lh20 lh20 lh20 lco2 lco2 lco2 lco2 salt graphite a c b d figure 4: types of fluid inclusions from tikli and rewat granite associated mineralized vein rock samples, degana, rajasthan, nw india. (a) type i inclusions; (b) type ii inclusions; (c) type iii inclusions; (d) type iv inclusions. 3.4 rewat li-rb-f-w mineralized vein rock (wall-core) the first melting temperature for type i inclusions for western wall is (-68 and -12°c), core (-62 and -15oc) and eastern wall (-64 and -52°c). the solution has rich in li+ , na+, k+, ca2+, and mg2+ in the wall zones, while being richer in li+, na+, k+ ca2+ in core of the vein. the final ice melting temperature and salinity varies from western wall (-10.1 to -4.2oc/14 to 6.7 wt. % nacl equivalent) to core (-4.4 to -1.2oc/7.2 to 2 wt. % nacl equivalent) and eastern wall (-13.1 to 4.5oc/17 to 7.2wt. % nacl equivalent). for type i inclusions, the average homogenization temperature into liquid between (lh2o>vh2o→lh2o), for western wall is 220 to 310oc, core of the vein is 160 to 235oc and eastern wall is 250 to 355oc. some type i inclusions were homogenized into a vapor phase between 190-380ºc (vh2o>lh2o→vh2o) in the wall zone. for type ii, the tmco2 was recorded in western wall (-61 to -57oc), core (-64 to -57.6oc), 134 vijay anand, nisha devi, r and eastern wall (-58 to -56.7oc). the clathrate melting temperature for the wall of the vein is (3.1 to 7oc/14.1 to 5.05 wt. % nacl equivalent); core (6.5 to 8oc/8 to 3 wt. % nacl equivalent). the partial homogenization for type ii inclusion in wall of the vein is 12-29oc and core of the vein 2-15oc. the total homogenization temperature (lh2o>lco2→lh2o) for type ii inclusion in western wall (265-340oc), core of the vein (170 to 225oc) and eastern wall (218-348oc). some of type ii inclusions were homogenized into vapor (vh2o>lh2o) between 245-412oc in wall zones. for type iii inclusions, the tmco2 was recorded in western wall (-59 to -57.1oc), core (-60 to -56.6oc) and eastern wall (-59 to -56.8oc). the homogenization temperature for type iii inclusion for western wall (15-26oc), eastern wall (12-20oc) and core zone (-2 to 10oc). for type iii inclusions were homogenized into vapor phase. the salt melting (dissolution) temperature for type iv inclusions in western and eastern wall (145 to 412oc; 152 to 385oc (the corresponding salinity range is 29.5-48 wt. % nacl equivalent). the liquid-vapor homogenization temperature for type iv inclusions in the wall of the vein varies from 138-352oc (western) and is 132-298oc (eastern wall). the type iv inclusions were not found in the core of the vein (table 2-4). 4. laser raman microprobe laser raman microprobe is an important method to identify the volatile species composition (co2, ch4, c, n2) of the fluid inclusions. the volatile composition identified based on the raman shift value of the spectra obtained (frezzotti et al., 2012). the samples were analyzed by “thermo almega laser xr dispersive raman spectrometer, the accumulation time for each scan is ~20-25 secs; 50 and 100x high standard objectives were used for observation in the department of earth sciences, iit bombay, india. the volatile composition of co2 doublets (1274 & 1378cm-1), graphite (1602cm-1) (d-peak) and ch4 (2911cm-1) (g-peak) were identified in the mineralized samples (figure 6). 10 μm 10 μm 10 μm 10 μm a c d b figure 5. types of fluid inclusions from tikli and rewat granite associated mineralized vein rock samples, degana, rajasthan, nw india. (a-d) metal rich inclusions. table 1. fluid microthermometry of tikli mineralized vein rock, degana, rajasthan, india fluid type phases composition tmco2(oc) tclath (oc) thco2( oc) thtot.(oc) teut(oc) tmice(oc) thh2o(oc) tmsalt(oc) salinity (wt. %) i l+v lh2o+vh2o -23 to -25 -61 to -42 -12.4 to -4.3 200-325 (l) 240-370 (v) 16.3 to 6.8 ii l1+l2+v lh2o+lco2+vco2 -59.4 to -56.5 2.5 to 8.6 20-29 200-356(l) 275396(v) 16.2 to 4.9 iii l lco2+vco2 -62.3 to -57.0 10 and 27 iv l+v+s lh2o+vh2o+salt 185-330 210-386 32 to 46 table 2. fluid microthermometry of rewat mineralized vein rock (western wall). fluid type phases composition tmco2(oc) tclath (oc) thco2( oc) thtot. (oc) teut(oc) tmice(oc) thh2o(oc) tmsalt(oc) salinity (wt. %) i l+v lh2o+vh2o -68 to -12 -10.1 to -4.2oc 220 to 310(l) 190 to 380(v) 14 to 6.7 ii l1+l2+v lh2o+lco2+vco2 -61 to -57 3.1 to 7 12 to 29 265 to 340 (l) 225 to 412 (v) 14.1 to 5.05 iii l lco2 -59 to -57.1 15to 26 (v) iv l+v+s lh2o+vh2o+solid 138-352 145 to 412 29.5 to 48 135evidence of fluid process for li-rb-f-w rich vein rocks of degana, rajasthan, india table 3. fluid microthermometry of rewat mineralized vein rock (core). fluid type phases composition tmco2 ( oc) tclath (oc) thco2( oc) thtot.(oc) teut(oc) tmice(oc) thh2o(oc) tmsalt(oc) salinity (wt. %) i l+v lh2o+vh2o -62 to -15 -4.4 to -1.2 160 to 235 7.2 to 2 ii l1+l2+v lh2o+lco2+vco2 -64 to -57.6 6.5 to 8 2 to 15 170 to 225 8 to 3 iii l lco2 -60 to -56.6 12 to 20(v) table 4. fluid microthermometry of rewat mineralized vein rock (eastern wall). fluid type phases composition tmco2 ( oc) tclath (oc) thco2( oc) thtot.(oc) teut(oc) tmice(oc) thh2o(oc) tmsalt(oc) salinity (wt. %) i l+v lh2o+vh2o -64 to -52°c -13.1 to 4.5 250 to 355 17 to 7.2 ii l1+l2+v lh2o+lco2+vco2 -58 to -56.7 1.5 to 6.8 20-24 218-348 13 to 6.5 iii l lco2 -59 to -56.8 12-20 (v) iv l+v+s lh2o+vh2o+solid 132-298 152 to 385 30 to 46 abbreviations: co2 melting temperature (tmco2); clathrate melting temperature (tmclath); co2 homogenization temperature (thco2); total homogenization temperature (thtot.); eutectic (first melting) temperature (teut); h2o ice melting temperature (tmice); h2o homogenization temperature (thco2); salt melting (dissolution) temperature (tmsalt). a c b d figure 6. (a) laser raman microprobe spectra of type ii inclusions (fermi co2 doublets at 1280 and 1383cm-1) and h2o liquid (3452cm-1) in tikli mineralized vein rock; (b): type ii inclusions (fermi co2 doublets at 1274 and 1378cm-1), graphite (1602cm-1) and h2o liquid (3424cm-1) in rewat mineralized vein rock (western wall); (c): type ii (fermi co2 doublets at 1281 and 1384cm-1, (ch4 at 2912 cm-1) inclusions in the rewat mineralized vein rock (core quartz). (d): type ii inclusions (fermi co2 doublets at 1278 and 1382cm-1) and h2o liquid (3417cm-1) in the rewat mineralized vein rock (eastern wall). 5. discussion the tikli-rewat granites and the mineralized vein rocks contain significant amount of zinnwaldite (li rich), plagioclase (rb-rich), fluorite (f), tourmaline (b), topaz (al-f) and wolframite (w). in degana, li (~4000 ppm), rb (>1000 ppm), f (4-6 wt. %), w (50 ppm) are above the crustal abundances in the small volume of the study area (anand et al., 2018; ghosh et al. 2023), so that it is important to understand the conditions of metals formation. the geochemistry, petrogenesis and geochronology of degana granites were discussed by several researchers (pandian and varma, 2001, chattopadhyay et al., 1994; anand et al., 2018; ghosh et al., 2023; kumar et al., 2023). the fluid process of the mineralized wall-core vein rock (magmatichydrothermal mineralization) remains unclear in the study area. understanding the relationship between wall rock granite and intruded mineralized vein rock is crucial for elucidating the metallogenetic mechanism. in the field. the mineralized vein rocks consists of zinnwaldite, fluorite, plagioclase, and topaz are lining the vein wall, and quartz occupies the bulk of the vein rock (figure 2 a-b). the vein rock intruded into the many fracture plans and filled by various mineral assemblages and finer greisen (altered) veinlets lining the walls, which suggests a genetic linkage between the host rock and mineralization (figure 2). the fluid petrography, microthermometry (lowering of melting temperature), and laser raman microprobe (composition of volatile phases) observation clearly indicates that the mineralized fluid system is interpreted to be h2o-co2-nacl±ch4. the tikli mineralized vein rock samples and rewat mineralized vein rock samples show different homogenization temperatures and different salinities (type i-iv inclusion) of 220 to 310oc/265 to 340oc for the western wall, 160 to 235oc/170 to 225oc for the core of the vein, and 250 to 355oc/265 to 342oc for the eastern wall. the corresponding salinities are 14 to 6.7/14.1 to 7.5oc wt. % nacl equivalent for the western wall, 7.2 to 2/8 to 3 wt. % nacl equivalent for the core of the vein, and 17 to 7.2/8 to 3 wt. % nacl equivalent for the eastern wall. the different homogenization temperatures (type-i to iv) with varying salinities represents a mixed fluid process. additionally, aqueous bi phase, polyphase and the carbonic fluid inclusions represented by the entrapment with variable phase ratios, presence of graphite (captive phase) and lowering the co2 melting temperature (<-56.6ºc) are further evidences. many fractionated mineralized granites, granitic pegmatites and intruded vein rocks have been identified worldwide, such as black hills, south dakota; altai and cuonadong dome, p.r. china; damara belt in namibia (shearer et al., 1987; jolliffe et al., 1987; yin et al., 2013; hulsbosch et al., 2014; kaeter et al., 2018; liu et al., 2020; yin et al., 2020; xie et al., 2020; bacík et al., 2021). the degana granites has been argued to be highly fractionated at a higher degree (pandian and varma, 2001). fractional crystallization is to be considered a principal mechanism for the formation of many wall-core zoned li-rb-f-w rich deposits (liu et al., 2020; yin et al., 2020; bacík et al., 2021). the fluid process may vary due to mineralization style, wallrock interaction, except for simple cooling process (lack of fluid-wallrock interaction) (heinrich, 1990, heinrich et al., 2004; xie et al., 2019; li et al., 2019; zhang et al., 2021; li et al., 2021). the hot circulation of fluids in the mineralized zone through 136 vijay anand, nisha devi, r fracture planes is mainly controlled by type of inclusions, suggesting these fluids are major source for the vein rocks. the carbonic, aqueous-carbonic and hypersaline inclusions represent a mixture in the mineralized vein rocks during crystallization. the mixing of greisenised wallrock granite (magmatic fluids) and the hydrothermal-derived meteoric fluids (vein rock) carrying high amounts of metals (metal rich inclusion). in to the host rock and the associated vein rocks in degana, rajsthan, india 6. conclusion as discussed above based on the field evidence, analytical data, we propose that the fluid process of tikli and rewat vein rock were mainly derived from degana granitic magma. the granitic magmatic evolution is highly differentiated in composition. the magmatic fluids (wallrock) carried out a large amount of metal complex flowed through fractures (vein rock) and precipitated the li, rb, w rich minerals while temperature reduced (during meteoric stage). cooling of the metal forming fluids and the meteoric water are the key factors that led to the formation of mineral deposit in the study area. acknowledgement we would like to acknowledge cmri (curtin malaysia research institute malaysia for the financial support for this research work is greatly acknowledged (grant number: 006043). the author thankful to the journal editor and reviewer for the constructive reviews and comments are greatly acknowledged. competing interest: the corresponding author and co-authors declare that there is no conflict of interest. references bakker, r.j (2003). package fluids 1. computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties. chemical geology, 194(1-3), 3-23. bodnar, r.j. 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(2021). age constraints on the genesis of the changkeng tungsten deposit, nanling region, south china. ore geology reviews, 134, 104-134. https://doi. org/10.1016/j.oregeorev.2021.104134. https://doi.org/10.1016/j.oregeorev.2023.105579 https://doi.org/10.1007/s00126-018-0805-5 https://doi.org/10.1007/s00126-018-0805-5 https://doi.org/10.1016/j.oregeorev.2021.104134 https://doi.org/10.1016/j.oregeorev.2021.104134 _hlk161748114 _hlk162980912 _hlk162981862 _hlk161748253 _int_q6rf8guc _hlk161748295 _hlk161748325 _hlk162970895 _hlk162970833 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.94786 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 183 196 h y d r o lo g y drought monitoring using mowcatl data mining algorithm in aras basin, turkey emre topçu, kafkas university, faculty of engineering and architecture, 36100, kars, turkey emre.topcu@kafkas.edu.tr abstract: drought is a natural phenomenon that occurs frequently and has some adverse effects on the ecosystem and humanity. determination of drought beforehand is vital for optimal management of water resources. many different methods have been developed to detect drought. sequential association analysis is used for the data series analysis containing time information and is one of the methods used to determine the drought. a correlation can be established between the values taken by the data at different times when determining association rules with this method. the primary purpose of this study is to determine the sequential association patterns between precipitation and climate oscillation index for aras basin. the aras basin is a region where irrigation and animal husbandry are common. today, many dams and hydroelectric power plants, together with the increasing population, meet the water and energy needs. a possible drought event in this region will adversely affect the living things in the basin. therefore, the study focused on this basin. finding sequential associations between precipitation and climate oscillation index can determine the temporal correlations between these parameters and specifically detect drought. the mowcatl (minimal occurrences with constraints and time lags) algorithm was used to detect sequential associations, and the j-measure was used to evaluate the patterns in the study. sequential association patterns were determined by applying this method to the precipitation data obtained from 6 meteorology stations in the aras basin. ao (arctic oscillation) index, mei (multivariate enso) index, nao (north atlantic oscillation) index, oceanic niño index (oni), pdo (pacific decadal oscillation) index, pna (pacific/north american), and soi (southern oscillation index), followed by the 1, 3, 6 and 12-month agricultural standardized precipitation index (a-spi) were used in sequential association. the study results revealed that the antecedent parameters were ineffective in detecting arid conditions in ardahan and doğubeyazıt stations, and they were influential on drought conditions, especially in a-spi-3 and a-spi-12 month periods at other stations. although the altitude and geographical features are different, similar climatic patterns have been detected in some stations. as a result, it has been determined that climatic oscillations generally bring about typical situations in terms of drought for the aras basin. keywords: data mining; drought index; drought; oceanic index monitoreo de la sequía a través del algoritmo mowcatl de minería de datos para la cuenca de aras, turquía resumen: la sequía es un fenómeno natural que ocurre muy frecuentemente y que tiene efectos negativos en los ecosistemas y en la humanidad. la definición de la sequía, de antemano, es especialmente necesarua para la administración óptima de los recursos de agua. muchos métodos se han desarrollado para detectar la sequía. uno de estos métodos es el análisis de asociación secuencial que se usa para el análisis de series de datos con información de tiempo. se puede establecer una correlación entre los valores tomados en diferentes períodos cuando se determinan las reglas asociativas con este método. el propósito principal de este estudio es determinar los patrones de asociación secuencial entre precipitación y el índice de oscilación climática para la cuenca de aras, en turquía. esta cuenca es una región donde la irrigación y la agricultura son comunes. al día de hoy, muchas presas e hidroeléctricas, junto con el incremento de la población, demandan estos recursos hidrológicos. un evento de sequía en la región afectaría a estos seres que dependen de la cuenca. por esta razón, el estudio se enfoca en la cuenca de aras. encontrar las asociaciones secuenciales entre precipitación y el índice de oscilación climática puede determinar las correlaciones temporales entre estos parámetros y, específicamente, detectar la sequía. en este trabajo se usó el algoritmo mowcatl (ocurrencias mínimas con restricciones y retrasos, literal del inglés minimal occurrences with constraints and time lags) para detectar las asociaciones secuenciales y la medición j se usó para evaluar estos patrones. los patrones de asociación secuencial se determinaron al aplicar este método a la información de precipitación obtenida de seis estaciones meteorológicas en la cuenca de basin. los índices de oscilación ártica, enso multivariado, oscilación del atlántico norte, oceánico del niño, oscilación decadal del pacífico, pacífico/norte américa, y de oscilación del sur, seguidos por el índice de precipitación agrícola estandarizado (a-spi) en los meses 1, 3, 6 y 12 se utlizaron en la asociación secuencial. los resultados del estudio revelan que estos parámetros no son efectivos para detectar las condiciones áridas en las estaciones de ardahan y doğubeyazıt pero si son efectivos en las condiciones de sequía, especialmente en los períodos a-spi-3 y a-spi-12 en otras estaciones. a pesar de que las condiciones de altitud y geográficas son diferetes, patrones climáticos se han detectado en algunas estaciones. como resultado se determinó que las oscilaciones climáticas generalemente provocan situaciones normales de sequía en la cuenca de aras. palabras clave: mineria de datos; índice de sequía; sequía; índice oceánico. record manuscript received: 30/03/2022 accepted for publication: 14/06/2022 how to cite item: topcu, e. (2022). drought monitoring using mowcatl data mining algorithm in aras basin, turkey. earth sciences research journal, 26(2), 183-196. https://doi.org/10.15446/esrj.v26n2.94786 https://doi.org/10.15446/esrj.v26n2.94786 mailto:emre.topcu@kafkas.edu.tr https://doi.org/10.15446/esrj.v26n2.94786 184 emre topçu introduction drought is a natural phenomenon that causes irreversible damage to the ecosystem with its alarming and negative effects for millions of human, animal and plant species on earth. the geographical conditions, climate and anthropogenic impact of a region contribute to the emergence of drought. understanding the emergence of drought, its duration and when it will end is one of the most important work areas for hydrologists. today, climate change is worsening in many parts of the world (liming & yaonan, 2016). therefore, there is an increasing interest in sustainable water resources management. in fact, the aim here is to prevent future water resources wars that threaten species (rostam et al., 2020). the effects of climate change vary from region to region. it causes differentiation, especially in the local precipitation amount. therefore, it is vital to understand regional dry and rainy periods (meilin et al., 2020). global warming causes negative changes on the water cycle and spatialtemporal characteristics of water resources. this significantly disrupts the balance in water resources and dry and wet conditions, especially in semi-arid and arid regions (yanfeng et al., 2015). drought is responsive to environmental change and altogether affects financial, ecological and farming exercises. exploring the spatial-temporal properties of drought will help specify the correlation of drought with meteorological, hydrological and ecological cycles, and to further assess the effect of climate change on humanity (yudan et al., 2020). although drought is not easy to define precisely, it can simply figure 1. location of the study area table 1. geographical features of the stations station/no elevation (m) latitude longitude period ardahan/17046 1827 41.1061 42.7055 1980-2020 arpaçay/17656 1720 40.8539 43.3178 1980-2020 doğubeyazıt/17720 1640 39.5396 44.018 1980-2020 horasan/17690 1540 40.0383 42.1705 1980-2020 iğdır/17100 856 39.9227 44.0523 1980-2020 kars/17097 1777 40.6042 43.1073 1980-2020 be considered as periodic rainfall and shortage of water resources comparing average conditions. drought can be categorized as meteorological, agricultural, and hydrological. lack of rainfall in a certain period indicates meteorological drought, while a decrease in agricultural production due to insufficient soil moisture is an indicator of agricultural drought, and a decrease in underground and aboveground resources is an indicator of hydrological drought (yürekli et al., 2012). meteorological drought is the primary type, which is the impetus behind other droughts, bringing about a decrease in the normal long term precipitation. with the meteorological drought and its negative effects occur and different kinds of drought will eventually develop, and the combined adverse effects may cause harmful effects in an area (alamdarloo et al., 2020). several indexes are developed to detect meteorological, agricultural, and hydrological drought. these indexes are created using the parameters of the hydrological cycle and are used to minimize the negative effects of drought on water resources. when the index values indicating that this drought will occur, certain restrictions can be made in the use of water for precautionary purposes, general cleaning and irrigation operations can be minimized by the local resident. the drought index most commonly used by hydrologists today is the standardized precipitation index (spi). spi measures the lack of precipitation at various time intervals in regions with different climates (edwards & mckee, 1997). examining drought over a wide span of time according to climatological, 185drought monitoring using mowcatl data mining algorithm in aras basin, turkey atmospheric and oceanic parameters may help moderate future drought impacts on society by improving our comprehension of the drought hazard. the data analysis reveals that climate change scholars have demonstrated that in areas where precipitation dwindles in mid-elevation during summer or where snowfall decreases, the combined impact may bring about significant increases in drought recurrence in the twenty-first century (rind et al., 1990). determining the correlation between drought and different climatic parameters using large scale data sets can be used in drought prediction. it is critical to extract essential information from large databases and infer drought risk. today, it is technical data mining that focuses on such operations (tadesse et al., 2004). data mining utilizes an array of data analysis instruments to determine patterns and relationships between physical factors in various data sets (two crows, 1999). this method is utilized in multidisciplinary fields. for instance, this technique is utilized for business applications by numerous organizations for planning key advantages to increase profit (groth, 1998). data mining algorithm and models, for example, decision trees clustering, associations, categorization, regression, time series forecasting and sequential patterns can possibly determine drought examples and properties. for instance, time series data mining can be applied in determining drought patterns. the minimal occurrences with constraints and time lags (mowcatl) algorithm was used in this study to find the correlations between drought and atmospheric/oceanic indexes by considering time lags of their occurrences (harms et al., 2002; tadesse, 2002). data mining was used to specify the complex interactions including oceanic and atmospheric parameters that potentially lead to droughts over the examined stations in aras basin, turkey. the aras basin is in the first place in turkey, especially in livestock breeding. agriculture, irrigation and animal husbandry activities carried out in the summer months require a large amount of water. for this reason, many large and small dams have been built in the region today. the increasing population in the region has led to an increase in the need for clean water. in recent years, the warm and dry winter months in the region have caused a decrease in water resources. the study of drought in this region has become essential. this is an important reason why this study focuses on this region. materials and methods study area the study was performed in aras river basin, which is one of turkey’s 26 river basins. aras river basin in northeastern turkey is the coldest part because of the altitude. six meteorological stations within the basin with long annual precipitation data were selected. some climatic and meteorological indexes for the years 1980-2020 were examined in the study. as for the study area, summer months are rainy and cool, except for the iğdır station. it can be determined that iğdır station is the driest station with conventional methods. however, drought analysis with climatic oscillations has not been performed before in the region. precipitation data were obtained from the directorate general of meteorology. the characteristic features of the selected stations are shown in table 1. the location of the study basin is shown in figure 1. method the most used atmospheric and oceanic indexes were chosen for this analysis. the arctic oscillation (ao) is a climate index of the atmospheric circulation condition over the arctic. it comprises a positive stage, including below-average geopotential heights, known as negative geopotential height anomalies, and a negative stage wherein the opposite is valid (daculaweather, 2021). mei (multivariate enso index) is determined based on six fundamental observed factors over the tropical pacific. these six factors are sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky (wolter & timlin, 1993). a positive mei is related to el niño, and negative values show la niña conditions. the nao (north atlantic oscillation) index is specified as the normalized pressure difference between a station on the azores and one in iceland (hurrell, 1995). since it is calculated based on stations toward the north (iceland) and south (azores) of the middle latitude westerly flow, it might be considered as a measure of the strength of these winds. a positive value indicate a strong mid-latitude westerly flow while the negative values show the weak mid-latitude westerly flow (hurrell, 1995). the oceanic niño index (oni) is the essential indicator for observing el niño and la niña, which are opposite phrases of the climate pattern called the el niño-southern oscillation, or shortly “enso”. el niño conditions to be available when the oceanic niño index is +0.5 or higher, demonstrating the east-central tropical pacific is significantly warmer than expected. la niña conditions exist when the oceanic niño index is 0.5 or lower revealing that the area is cooler than expected. (national oceanic and atmospheric administration, 2021). the pdo (pacific decadal oscillation) index is referred to as the main component of north pacific monthly sea surface temperature variability poleward of 20n (francis & hare, 1994). the positive values indicate the warm period of the north pacific sea surface temperature, and the negative ones show the cold phase. the pna (pacific/north american) index indicates the upper atmosphere conditions (overland et al. 2002). monthly pna averages are utilized in this analysis. the soi (southern oscillation index) is calculated through monthly sea level pressure average anomalies at tahiti (t), french polynesia, and darwin (d), australia. the normalized monthly average ocean level pressure anomaly soi [t-d] is an index that combines the southern oscillation into one sequence (trenberth & hoar, 1996). a positive soi implies la niña and a negative soi demonstrates el niño conditions. this study aims to present that inconstancy of the global atmospheric, and oceanic indexes can be utilized as an antecedent to local drought by formulation association rules regarding the drought index. agricultural standardized precipitation index (a-spi) the spi was formulated by mckee et al. (1993) for the classification and observation of drought. it is a comfortable, adaptable, global index, requiring just the precipitation data, which are the primary reasons behind its wide adaptability in different drought-related applications around the world. despite the fact that it is basically considered as a meteorological drought index, it is also utilized in numerous examinations on hydrological and agricultural drought. the lack of a soil-water balance part in spi is a restricting element for its utilization in vegetation-oriented applications. however, such a component is reasonably consolidated in the effective precipitation parameter. accordingly, the replacement of the total precipitation with the effective precipitation prompts a more solid definition of spi for agricultural drought and increases its adaptability for analyzing drought impacts on vegetation. the effective precipitation can be evaluated through an array of methods, for example, assessing the soil moisture changes with lysimeters or through the simulation of the water dynamics (water balance models) (bos et al., 2008; ebrahimpour et al., 2015; feddes et al., 1988; patwardhan et al., 1990). in spite of their precision, such techniques primarily used in local or experimental applications since they require many input and estimation installations, not generally accessible for operational use or in long time series, as needed for drought characterization. moreover, the effective precipitation assessment through the above mentioned techniques, in light of on the precipitation data only, offers the primary benefit of the original index permits to use even in regions with restricted information access. the most suitable strategy for effective precipitation assessment in the proposed updated index, specifically the agricultural standardized precipitation index (a-spi), is based on the area and the analyzed framework. the a-spi holds a similar standardization method to spi, through the fitting of the effective precipitation time series to an appropriate statistical distribution, which is then transformed into normal distribution. this study applied a basic empirical methodology utilized by the fao (food and agriculture organization of the united nations), which can be used in areas with a maximum slope of 4–5% (tigkas et al., 2019). the a-spi was analyzed in 1, 3, 6 and 12-month periods. the minimal occurrences with constraints and time lags (mowcatl) algorithm mowcatl algorithm was utilized to reveals the correlations between sequences in the multiple data sets, where a lag in time between the antecedent and the consequent exists. besides the conventional frequency and support limitations in sequential data mining, mowcatl utilizes separate antecedent and consequent inclusion constraints, separate antecedent and consequent maximum window widths, to determine the antecedent and consequent patterns that are isolated by a time lag (harms et al., 2002). an association 186 emre topçu rule is characterized as “if x then y,” where x is the rule antecedent and y is its consequent. the support of the antecedent, signified as support {x}, is the number of events that every occasions in the antecedent episode x happen together within a user-defined window width. the antecedent support (support {x}) is likewise called the rule coverage. an episode is regarded as frequent if its support meets or surpasses a user-identified threshold. the frequency and support of a consequent episode are also defined in this manner. the mowcatl algorithm primarily scans the data file in a database storing the occasions of the single events for the antecedent and consequent events individually. the algorithm only seeks for occurrences of events that meet the inclusion limitations. then it excludes the episodes that do not satisfy the user-specified support threshold. the single-event episodes are paired into episodes of two events, for pairs of events that happen together within the specified window width, and event records of the episode in the dataset. if there are no more events to pair, this process is completed. the frequent episodes are specified with the antecedent and the consequent independently, then mowcatl combines the frequent episodes to establish an episode rule (harms et al., 2002). the algorithm establishes episodic rules where the antecedent episode happens within a prescribed window width, the consequent episode occurs within a specified window width, and the start of the consequent follows an antecedent within a user-specified time lag. for instance, let’s assume that episode x is the events a and b, and episode y is the events c and d, the prescribed antecedent window width is 2 months, and the consequent window width is 3 months, with a time lag of 2 months. the parallel rule formed would indicate that if a and b occur within 2 months, then within 3 months c and d will occur together within 2 months (tadesse et al., 2005). in drought research, several climatic and oceanic datasets might be required to identify drought and associations of drought with specific values in these datasets. for example, the oceanic parameters referred to as the ao, mei, nao, oni, pdo, pna, soi and several oceanic indexes can be generated in a database so that the algorithm might form the associations with the drought index to monitor it. mowcatl calculation is utilized in the current examination for extricating rules between extraordinary scenes and climatic files, since this calculation can be utilized for various groupings and furthermore, this will catch without anyone else the slack between the events of climatic records and precipitation occasions. the data-mining context necessitates selecting the best rule in a certain model depending on the “interestingness measure.” the concepts of “interestingness” or “goodness” of the rules are defined to select and compare the better rules among the generated ones (bayardo & agrawal, 1999; silberschatz & tuzhilin, 1995). despite the support and confidence values’ significance in selecting rules, there are alternative methods and algorithms to use in quantifying interestingness measures (padmanabhan & tuzhilin, 1999; shokoohi-yekta et al., 2015). these methods include smyth and goodman’s j-measure (smyth and goodman, 1992) used in this study to quantify the interestingness of the rules for the mowcatl algorithm. smyth and goodman’s j-measure is specified as the average information content of a probabilistic classification rule and applied to identify the best rules relating discrete-valued attributes. a probabilistic classification rule is a logical implication of “if x then y” with some probability p (hilderman & hamilton, 1999). the j value interval is between 0 to 1. higher j values indicate better measurements. mowcatl is covered in detailed in (harms & deogun, 2004). experiment the drought was monitored as the time-series data of meteorological, climatological, and oceanic parameters generated rules that could detect the probable drought occurrence. the justification of oceanic parameter measurements to monitor drought relies on the general assumption that ocean-atmosphere relationships affect drought. as the fluctuations in oceanic parameters develop more slowly than surface meteorological parameters, the trend of the oceanic parameters better are more observable than the surface parameter trends such as precipitation deficits and surface temperature changes (mcpadeh, 1998). therefore, oceanic parameters might be regarded as antecedents and droughts as consequents in finding their correlation (tadesse et al., 2004). the atmospheric and oceanic indexes applied in this paper include ao, nao, oni, mei, pna, pda, soi. a-spi values for each station were also utilized to generate the rules. the monthly values from the ocean and a-spi were converted into discrete representations and classified into seven categories. assuming a normal distribution of this data for 40 years, each oceanic and atmospheric parameter value is divided into 0.5, 1, and 1.5 standard deviations from the normal frequency distribution. the drought index categories are: extremely dry (1), severely dry (2), moderately dry (3), normal (4), moderately wet (5), severely wet (6), and extremely wet (7). the drought classification thresholds of indexes are detailed in table 2. table 2. threshold values used to classify oceanic and climatic indexes category indexes 1 2 3 4 5 6 7 a-spi ≤ -2 -2<x≤-1.5 -1.5<x≤-1 -1<x≤1 1<x≤1.5 1.5<x≤2 > 2 ao ≤ -1.9 -1.9<x≤-0.8 -0.8<x≤-0.1 -0.1<x≤0.5 0.5<x≤1.2 1.2<x≤2.6 > 2.6 mei ≤ -1.4 -1.4<x≤-0.7 -0.7<x≤-0.2 -0.2<x≤0.3 0.3<x≤0.9 0.9<x≤2.1 > 2.1 nao ≤ -1.9 -1.9<x≤-1.1 -1.1<x≤-0.2 -0.2<x≤0.3 0.3<x≤1 1<x≤1.7 > 1.7 oni ≤ -1.4 -1.4<x≤-0.5 -0.5<x≤0.2 0.2<x≤0.7 0.7<x≤1.3 1.3<x≤2.3 > 2.3 pdo ≤ -1.7 -1.7<x≤-1 -1<x≤-0.2 -0.2<x≤0.4 0.4<x≤0.9 0.9<x≤1.6 > 1.6 pna ≤ -2.2 -2.2<x≤-1.3 -1.3<x≤-0.5 -0.5<x≤0.1 0.1<x≤0.6 0.6<x≤1.4 > 1.4 soi ≤ -1.9 -1.9<x≤-0.8 -0.8<x≤-0.2 -0.2<x≤0.3 0.3<x≤1 1<x≤1.8 > 1.8 discussion the rules in this study with a confidence level above 0.90 are presented in the tables. since drought and flood are so sporadic, the j values are so small that all values greater than 0.025 are to be included. mowcatl algorithm is utilized in the study for extracting rules between extreme episodes and climatic indexes. this algorithm’s applicability to multiple sequences and its ability to capture the lag between the occurrences of climatic indexes and rainfall events by itself were the main reasons behind this preference. as presented in table 3, the established rules determined neither dry nor wet conditions, but normal conditions, at all stations for 1 month a-spi (a-spi1) values. arid conditions could not be detected in a-spi1 consequent, as rules with lower levels of confidence were not included. a high level of confidence increases the validity of the rules. table 3 shows that ao, mei, pda and pna are more related in forming normal conditions. the other tables (table 4, 5, 6, 7 and 8) shows that the standard situation, which is generally in the normal drought category, is observed. 187drought monitoring using mowcatl data mining algorithm in aras basin, turkey table 3. sample rules generated for a-spi1 using mowcatl algorithm for serial episodes, antecedent and consequent windows of 2-months, and the 3-months maximum time lag between the start of the antecedent and the start of the consequent a-spi1 antecedent consequent confidence j measure ardahan nao(5)-pna(3) spi1(4) 0.962 0.037 mei(3)-pdo(5) spi1(4) 0.941 0.037 nao(5)-oni(2) spi1(4) 0.907 0.036 arpaçay mei(3)-pna(3) spi1(4) 0.938 0.042 pdo(3)-pna(3) spi1(4) 0.920 0.033 ao(5)-pdo(3) spi1(4) 0.917 0.038 kars mei(2)-nao(2) spi1(4) 0.958 0.034 mei(3)-pna(6) spi1(4) 0.933 0.031 oni(2)-pna(6) spi1(4) 0.913 0.036 mei(3)-oni(2) spi1(4) 0.906 0.068 oni(2)-soi(4) spi1(4) 0.903 0.042 horasan oni(3)-pna(3) spi1(4) 0.923 0.036 pdo(3)-pna(3) spi1(4) 0.920 0.033 doğubeyazit oni(3)-pna(3) spi1(4) 0.9231 0.0363 pdo(3)-pna(3) spi1(4) 0.9200 0.0332 iğdir pdo(3)-soi(6) spi1(4) 0.9500 0.0546 oni(1)-soi(6) spi1(4) 0.9211 0.0358 as presented in table 4, it is determined that for a-spi3, only normal conditions were found in all stations as a result of various algorithm rules. table 4. sample rules generated for a-spi3 using mowcatl algorithm for serial episodes, antecedent and consequent windows of 2-months, and the 3 months maximum time lag between the start of the antecedent and the start of the consequent a-spi3 antecedent consequent confidence j measure ardahan pdo(3)-soi(6) spi3(4) 0.950 0.055 oni(1)-soi(6) spi3(4) 0.921 0.036 arpaçay oni(2)-pdo(2) spi3(4) 0.962 0.090 mei(2)-nao(3) spi3(4) 0.957 0.075 soi(5)-pdo(2) spi3(4) 0.955 0.035 pna(6)-pdo(2) spi3(4) 0.950 0.030 mei(2)-pna(6) spi3(4) 0.938 0.042 mei(2)-pdo(2) spi3(4) 0.926 0.061 mei(3)-pna(3) spi3(4) 0.917 0.048 soi(6)-pna(5) spi3(4) 0.912 0.032 nao(3)-oni(2) spi3(4) 0.905 0.065 kars pdo(2)-pna(6) spi3(4) 0.950 0.040 oni(1)-pdo(3) spi3(4) 0.944 0.034 soi(4)-pdo(6) spi3(4) 0.912 0.046 horasan mei(4)-nao(6) spi3(4) 0.964 0.052 nao(6)-oni(4) spi3(4) 0.964 0.052 ao(2)-soi(2) spi3(4) 0.929 0.035 oni(4)-pna(5) spi3(4) 0.921 0.043 mei(4)-pna(5) spi3(4) 0.913 0.048 188 emre topçu a-spi3 antecedent consequent confidence j measure doğubeyazit nao(6)-mei(4) spi3(4) 0.964 0.052 nao(6)-oni(4) spi3(4) 0.964 0.052 ao(2)-soi(2) spi3(4) 0.929 0.035 oni(4)-pna(5) spi3(4) 0.921 0.043 pna(5)-mei(4) spi3(4) 0.913 0.048 iğdir nao(2)-soi(3) spi3(4) 0.962 0.048 nao(2)-pdo(2) spi3(4) 0.955 0.038 nao(2)-mei(5) spi3(4) 0.950 0.033 pna(3)-mei(4) spi3(4) 0.941 0.050 mei(4)-nao(3) spi3(4) 0.926 0.067 nao(3)-pdo(3) spi3(4) 0.912 0.036 mei(5)-oni(3) spi3(4) 0.906 0.032 oni(3)-pna(3) spi3(4) 0.904 0.050 looking at the rule of the antecedents and consequents in table 5 and table 6, ao, nao, mei and oni were more effective in the occurrence of normal situations for a-spi6. table 5. sample rules generated for a-spi6 using mowcatl algorithm for serial episodes, antecedent and consequent windows of 2-months, and the 3 months maximum time lag between the start of the antecedent and the start of the consequent a-spi6 antecedent consequent confidence j measure ardahan nao(2)-soi(3) spi6(4) 0.9615 0.0484 nao(2)-pdo(2) spi6(4) 0.9545 0.0379 mei(5)-nao(2) spi6(4) 0.9500 0.0328 mei(4)-pna(3) spi6(4) 0.9412 0.0505 mei(4)-nao(3) spi6(4) 0.9259 0.0673 nao(3)-pdo(3) spi6(4) 0.9118 0.0359 mei(5)-oni(3) spi6(4) 0.9063 0.0316 oni(3)-pna(3) spi6(4) 0.9038 0.0498 arpaçay ao(2)-mei(2) spi6(4) 0.9583 0.0448 ao(5)-pdo(2) spi6(4) 0.9545 0.0394 ao(5)-soi(6) spi6(4) 0.9333 0.0426 mei(2)-pdo(1) spi6(4) 0.9231 0.0329 nao(2)-ao(2) spi6(4) 0.9211 0.0470 mei(2)-nao(4) spi6(4) 0.9167 0.0423 nao(3)-pdo(5) spi6(4) 0.9118 0.0377 nao(5)-mei(2) spi6(4) 0.9091 0.0473 kars mei(7)-pdo(6) spi6(4) 0.9545 0.0551 mei(7)-soi(2) spi6(4) 0.9444 0.0413 oni(1)-soi(7) spi6(4) 0.9444 0.0413 ao(6)-pdo(3) spi6(4) 0.9375 0.0345 mei(1)-nao(3) spi6(4) 0.9375 0.0345 oni(1)-pdo(1) spi6(4) 0.9375 0.0345 pdo(2)-soi(4) spi6(4) 0.9091 0.0368 pna(6)-mei(2) spi6(4) 0.9063 0.0521 189drought monitoring using mowcatl data mining algorithm in aras basin, turkey table 6. (continue) a-spi6 antecedent consequent confidence j measure horasan oni(2)-pdo(6) spi6(4) 0.9643 0.0551 oni(5)-soi(4) spi6(4) 0.9583 0.0442 mei(7)-pdo(6) spi6(4) 0.9545 0.0389 pdo(7)-soi(2) spi6(4) 0.9545 0.0389 ao(4)-pna(2) spi6(4) 0.9500 0.0336 nao(5)-pdo(6) spi6(4) 0.9375 0.0469 mei(4)-nao(6) spi6(4) 0.9286 0.0371 nao(6)-oni(4) spi6(4) 0.9286 0.0371 ao(2)-soi(3) spi6(4) 0.9063 0.0327 doğubeyazit nao(4)-soi(5) spi6(4) 0.9583 0.0515 nao(5)-oni(6) spi6(4) 0.9500 0.0395 oni(2)-pna(7) spi6(4) 0.9444 0.0336 mei(3)-pna(2) spi6(4) 0.9231 0.0392 iğdir pdo(2)-soi(3) spi6(4) 0.9444 0.0316 pna(2)-soi(4) spi6(4) 0.9444 0.0316 oni(2)-soi(3) spi6(4) 0.9286 0.0417 ao(5)-soi(5) spi6(4) 0.9250 0.0573 ao(5)-pna(6) spi6(4) 0.9231 0.0365 mei(3)-pna(3) spi6(4) 0.9167 0.0629 as presented in table 7 and table 8, normal conditions were detected for a-spi12. table 7. sample rules generated for a-spi12 using mowcatl algorithm a-spi12 antecedent consequent confidence j measure ardahan mei(6)-pdo(6) spi12(4) 0.974 0.122 oni(1)-pdo(2) spi12(4) 0.958 0.068 oni(1)-soi(6) spi12(4) 0.947 0.098 oni(3)-pdo(2) spi12(4) 0.941 0.083 mei(5)-oni(3) spi12(4) 0.938 0.076 nao(4)-pdo(2) spi12(4) 0.938 0.038 mei(2)-nao(5) spi12(4) 0.932 0.100 ao(6)-pdo(2) spi12(4) 0.929 0.031 ao(6)-soi(6) spi12(4) 0.929 0.031 pdo(2)-soi(4) spi12(4) 0.909 0.041 arpaçay pdo(3)-oni(3) spi12(4) 0.981 0.126 soi(4)-pdo(3) spi12(4) 0.978 0.108 mei(4)-nao(6) spi12(4) 0.964 0.056 pna(2)-pdo(4) spi12(4) 0.958 0.045 ao(5)-pdo(2) spi12(4) 0.955 0.040 190 emre topçu a-spi12 antecedent consequent confidence j measure arpaçay nao(6)-soi(4) spi12(4) 0.933 0.043 nao(2)-soi(3) spi12(4) 0.923 0.033 ao(5)-pna(5) spi12(4) 0.921 0.048 mei(2)-nao(4) spi12(4) 0.917 0.043 oni(3)-soi(3) spi12(4) 0.912 0.077 pna(3)-soi(4) spi12(4) 0.909 0.048 kars mei(2)-oni(1) spi12(4) 0.974 0.120 soi(6)-oni(1) spi12(4) 0.974 0.120 oni(1)-mei(1) spi12(4) 0.964 0.082 mei(6)-nao(5) spi12(4) 0.962 0.074 nao(2)-pdo(2) spi12(4) 0.955 0.059 ao(6)-pna(2) spi12(4) 0.944 0.044 ao(4)-oni(1) spi12(4) 0.944 0.044 mei(1)-pdo(2) spi12(4) 0.944 0.044 soi(7)-oni(1) spi12(4) 0.944 0.044 ao(6)-pdo(3) spi12(4) 0.938 0.037 mei(1)-nao(3) spi12(4) 0.938 0.037 oni(1)-pna(4) spi12(4) 0.938 0.037 table 8. (continue) spi12 antecedent consequent confidence j measure horasan mei(2)-pdo(1) spi12(4) 0.962 0.056 oni(1)-pna(5) spi12(4) 0.955 0.044 oni(6)-pna(5) spi12(4) 0.944 0.032 oni(6)-soi(2) spi12(4) 0.941 0.059 nao(6)-pna(6) spi12(4) 0.929 0.043 nao(4)-soi(4) spi12(4) 0.923 0.075 mei(6)-ao(4) spi12(4) 0.921 0.054 ao(6)-nao(6) spi12(4) 0.917 0.032 mei(6)-oni(6) spi12(4) 0.913 0.060 mei(6)-soi(2) spi12(4) 0.903 0.072 doğubeyazit mei(5)-pdo(3) spi12(4) 0.947 0.077 oni(2)-pna(7) spi12(4) 0.944 0.036 ao(5)-pna(6) spi12(4) 0.923 0.042 mei(5)-pna(6) spi12(4) 0.905 0.056 oni(3)-pdo(3) spi12(4) 0.904 0.069 mei(5)-nao(6) spi12(4) 0.900 0.038 iğdir mei(4)-nao(6) spi12(4) 0.964 0.063 pna(5)-soi(4) spi12(4) 0.947 0.072 nao(6)-soi(5) spi12(4) 0.944 0.033 ao(5)-mei(4) spi12(4) 0.933 0.049 mei(4)-pdo(2) spi12(4) 0.923 0.039 ao(4)-nao(6) spi12(4) 0.917 0.033 mei(5)-oni(3) spi12(4) 0.906 0.040 191drought monitoring using mowcatl data mining algorithm in aras basin, turkey according to table 9, only when the mei (7) and nao (6) conditions occur together at the kars station, mildly severe drought may occur after 2 or 3 months. the confidence level for this rule is 0.83. this value is very close to 0.90. both severe wet (category 6) and slightly arid (category 3) conditions could be determined for a-spi6. a-spi6 (6) result was obtained for ao (1) -mei (6) and ao (1) -oni (6) conditions, and a-spi6 (3) result was obtained for ao (6) -oni (7) conditions at arpaçay station. a-spi6 (3) result was found for mei (4) -nao (1) conditions at the kars station. slightly wet conditions for a-spi12 were detected at arpaçay station. ao, nao, oni, pda and pna were effective for the detection of this situation. both severe wet, slightly wet and slightly arid conditions have been identified for horasan station. while ao, oni, mei and pdo were effective in determining wet conditions, mei and pdo were effective for dry conditions. similar situations are also valid for doğubeyazıt station. based on the a-spi results, among the stations, the station experiencing the most severe drought was identified as iğdır station. the formulated algorithm confirmed that the iğdır station is dry according to classical methods. severe drought has been observed under the influence of nao, mei, pdo and pna. especially in the seventh category of these oscillations, a severe drought occurred for a-spi12 (category 2). it can be concluded that not all climatic oscillations cause drought or marshiness in all conditions based on the above tables. drought or wet conditions can be determined only when the rules specified in the tables occur. similar rules occur in different meteorological stations are listed in table 10. according to table 10, the least common number of rules is seen at arpacay-kars supports (4 rules), while the highest number of similar rules is seen at horasan-doğubeyazıt stations (22 rules). what is interesting here is that while the arpaçay and kars stations are close to each other, a low number of similar rules are obtained, while the horasan and doğubeyazıt stations are far from each other, but a large number of similar rules are obtained. these stations are not very different in altitude, but may have different or similar precipitation regimes due to the surrounding mountains. arpaçay, horasan and kars stations are relatively flat areas, but there are very high mountains near doğubeyazıt station and these mountains shape the climate of the region where they are located. table 9. association rules obtained excluding normal conditions station antecedent consequent confidence j measure arpaçay ao(1)-mei(6) spi12(5) 0.8333 0.0746 ao(1)-oni(6) spi12(5) 0.8333 0.0746 mei(6)-ao(1) spi12(5) 0.8333 0.0746 nao(7)-pdo(2) spi12(5) 0.8333 0.0746 oni(6)-ao(1) spi12(5) 0.8333 0.0746 pdo(2)-nao(7) spi12(5) 0.8333 0.0746 pdo(1)-pna(6) spi12(5) 0.8333 0.0746 pna(6)-pdo(1) spi12(5) 0.8333 0.0746 ao(1)-mei(6) spi6(6) 0.8333 0.0357 ao(1)-oni(6) spi6(6) 0.8333 0.0357 ao(6)-oni(7) spi6(3) 0.8333 0.0899 mei(6)-ao(1) spi6(6) 0.8333 0.0357 oni(6)-ao(1) spi6(6) 0.8333 0.0357 oni(7)-ao(6) spi6(3) 0.8333 0.0899 kars mei(4)-nao(1) spi6(3) 0.8333 0.0816 nao(1)-mei(4) spi6(3) 0.8333 0.0816 mei(7)-nao(6) spi3(3) 0.8333 0.0922 nao(6)-mei(7) spi3(3) 0.8333 0.0922 horasan ao(3)-oni(1) spi12(5) 0.8000 0.0348 oni(1)-ao(3) spi12(5) 0.8000 0.0348 mei(1)-pdo(4) spi12(5) 0.8333 0.0497 mei(2)-pdo(6) spi12(3) 0.8333 0.0797 pdo(4)-mei(1) spi12(6) 0.8333 0.0497 pdo(6)-mei(2) spi12(3) 0.8333 0.0797 iğdir mei(1)-pdo(4) spi12(6) 0.8333 0.0479 mei(7)-pna(7) spi12(2) 0.8333 0.0826 mei(7)-pna(5) spi12(2) 0.8333 0.0326 pdo(4)-mei(1) spi12(6) 0.8333 0.0379 pdo(7)-nao(7) spi12(2) 0.8333 0.0426 pna(5)-mei(7) spi12(2) 0.8333 0.0526 192 emre topçu table 10. legend list for table 11 and table 12 color rules combination number of same rules arpaçay-horasan-doğubeyazıt 9 ardahan-arpaçay 6 ardahan-kars 8 arpaçay-kars 4 arpaçay-iğdır 6 horasan-doğubeyazıt 22 table 11 and table 12 show the frequency of occurrence of the same rule at different stations. table 11. similar rules table for aras basin stations coeff. rules ardahan arpaçay kars horasan doğubeyazıt iğdır spi1 >0.90 oni(3)-pna(3) = spi1(4) + + >0.90 pdo(3)-pna(3) = spi1(4) + + + >0.80 pna(3)-oni(3) = spi1(4) + + >0.80 pna(3)-pdo(3) = spi1(4) + + + spi3 >0.75 pdo(2)-oni(2) = spi3(4) + + >0.80 ao(2)-nao(3) = spi3(4) + + >0.90 ao(2)-soi(2) = spi3(4) + + >0.80 ao(5)-pdo(4) = spi3(4) + + >0.75 mei(3)-pna(3) = spi3(4) + + >0.80 mei(4)-nao(6) = spi3(4) + + >0.80 mei(4)-pna(5) = spi3(4) + + >0.80 nao(3)-ao(2) = spi3(4) + + >0.75 nao(6)-mei(4) = spi3(4) + + >0.90 nao(6)-oni(4) = spi3(4) + + >0.80 oni(2)-pdo(2) = spi3(4) + + >0.80 oni(4)-nao(6) = spi3(4) + + >0.90 oni(4)-pna(5) = spi3(4) + + >0.75 pdo(2)-pna(6) = spi3(4) + + >0.75 pdo(4)-ao(5) = spi3(4) + + >0.80 pna(3)-mei(3) = spi3(4) + + >0.80 pna(5)-mei(4) = spi3(4) + + >0.80 pna(5)-oni(4) = spi3(4) + + >0.75 pna(6)-pdo(2) = spi3(4) + + >0.80 soi(2)-ao(2) = spi3(4) + + table 12. (continue) coeff. rules ardahan arpaçay kars horasan doğubeyazıt iğdır spi6 >0.80 nao(2)-pdo(2) = spi6(4) + + >0.80 nao(2)-soi(3) = spi6(4) + + >0.75 nao(3)-oni(3) = spi6(4) + + + >0.80 oni(2)-pdo(6) = spi6(4) + + >0.80 oni(3)-nao(3) = spi6(4) + + + >0.75 pdo(2)-nao(2) = spi6(4) + + >0.80 pdo(6)-oni(2) = spi6(4) + + >0.80 soi(3)-nao(2) = spi6(4) + + 193drought monitoring using mowcatl data mining algorithm in aras basin, turkey coeff. rules ardahan arpaçay kars horasan doğubeyazıt iğdır spi12 >0.75 mei(6)-nao(5) = spi12(4) + + >0.80 mei(5)-oni(3) = spi12(4) + + + >0.80 mei(2)-pdo(2) = spi12(4) + + + >0.80 ao(5)-oni(3) = spi12(4) + + >0.80 ao(5)-pna(6) = spi12(4) + + >0.75 ao(6)-pdo(2) = spi12(4) + + >0.80 mei(2)-pna(5) = spi12(4) + + >0.80 mei(3)-soi(4) = spi12(4) + + + >0.75 mei(4)-nao(6) = spi12(4) + + >0.80 mei(5)-soi(2) = spi12(4) + + >0.75 mei(6)-pdo(6) = spi12(4) + + >0.80 nao(2)-soi(3) = spi12(4) + + >0.80 nao(3)-oni(3) = spi12(4) + + + >0.80 nao(5)-mei(6) = spi12(4) + + >0.80 nao(6)-mei(4) = spi12(4) + + >0.80 nao(6)-soi(4) = spi12(4) + + >0.75 oni(1)-pna(4) = spi12(4) + + >0.80 pdo(6)-pna(4) = spi12(4) + + + >0.75 pna(5)-mei(2) = spi12(4) + + >0.75 pna(6)-ao(5) = spi12(4) + + >0.80 soi(2)-mei(5) = spi12(4) + + >0.75 soi(6)-pdo(2) = spi12(4) + + >0.80 soi(4)-nao(6) = spi12(4) + + support and lift values for mowcatl algorithm for a-spi12 (category 4) are seen in table 13. the support of the rule is the number of times the rule holds in the database (harms and deogun 2004). the lift was obtained from the confidence and the occurrence probability of the consequent. table 14 shows the correlation values between some oscillation indexes and meteorological index in series for many years. therefore, it can be understood whether there is a correlation between which meteorological series and climatic oscillation values at which station. for example, at arpaçay station, pdo is the most relevant climatic oscillation, followed by mei, oni and soi oscillations, respectively. mei oscillation is apparent in iğdır, which is the driest station. table 13. support and lift values for mowcatl algorithm a-spi(4) rules support lift rules support lift a r d a h a n mei(6)-pdo(6) 0.026 1.029 h o r a sa n mei(2)-pdo(1) 0.019 1.032 oni(1)-pdo(2) 0.017 1.086 oni(1)-pna(5) 0.021 1.355 oni(1)-soi(6) 0.030 1.200 oni(6)-pna(5) 0.015 1.160 oni(3)-pdo(2) 0.023 1.054 oni(6)-soi(2) 0.026 1.052 mei(5)-oni(3) 0.023 1.054 nao(6)-pna(6) 0.021 1.065 nao(4)-pdo(2) 0.011 1.018 nao(4)-soi(4) 0.038 1.032 mei(2)-nao(5) 0.036 1.204 mei(6)-ao(4) 0.028 1.020 ao(6)-pdo(2) 0.011 1.018 ao(6)-nao(6) 0.019 1.032 ao(6)-soi(6) 0.011 1.164 mei(6)-oni(6) 0.034 1.037 pdo(2)-soi(4) 0.019 1.047 mei(6)-soi(2) 0.045 1.010 194 emre topçu a-spi(4) rules support lift rules support lift a r pa ç ay pdo(3)-oni(3) 0.041 1.027 d o ğ u b ey a zi t mei(5)-pdo(3) 0.030 1.066 soi(4)-pdo(3) 0.038 1.011 oni(2)-pna(7) 0.017 1.108 mei(4)-nao(6) 0.028 1.265 ao(5)-pna(6) 0.021 1.088 pna(2)-pdo(4) 0.019 1.095 mei(5)-pna(6) 0.036 1.177 ao(5)-pdo(2) 0.019 1.194 oni(3)-pdo(3) 0.038 1.015 nao(6)-soi(4) 0.026 1.031 mei(5)-nao(6) 0.026 1.075 nao(2)-soi(3) 0.023 1.071 iğ d ir mei(4)-nao(6) 0.028 1.292 ao(5)-pna(5) 0.030 1.076 pna(5)-soi(4) 0.028 1.020 mei(2)-nao(4) 0.030 1.076 nao(6)-soi(5) 0.015 1.160 oni(3)-soi(3) 0.051 1.001 ao(5)-mei(4) 0.028 1.211 pna(3)-soi(4) 0.034 1.062 mei(4)-pdo(2) 0.021 1.065 k a r s mei(2)-oni(1) 0.032 1.282 ao(4)-nao(6) 0.019 1.032 soi(6)-oni(1) 0.030 1.196 mei(5)-oni(3) 0.026 1.052 oni(1)-mei(1) 0.019 1.044 mei(6)-nao(5) 0.019 1.124 nao(2)-pdo(2) 0.015 1.033 ao(6)-pna(2) 0.013 1.083 ao(4)-oni(1) 0.013 1.083 mei(1)-pdo(2) 0.013 1.083 soi(7)-oni(1) 0.013 1.083 ao(6)-pdo(3) 0.013 1.083 mei(1)-nao(3) 0.011 1.015 oni(1)-pna(4) 0.011 1.015 drought analysis studies using data mining method are becoming widespread today. since several complex factors are involved in the emergence of drought, methods and indexes with many or few parameters are being developed. sequential association analysis can be considered as an alternative to these conventional methods. the impact of climatic oscillation indexes on the meteorological index was investigated using many parameters in this study. a high level of confidence has resulted in detecting fewer drought and wetness situations. dry or wet conditions could not be revealed in ardahan and doğubeyazıt stations. in the provinces of ardahan and doğubeyazıt, dry periods do not occur due to the heavy rainfall in the past. for this reason, it has remained unanswered that drought does not occur at certain times and how many months later the wet time will come. due to the higher altitudes of ardahan and doğubeyazıt compared to other areas in the aras region, it receives a lot of snowfall, so the regions are wet in many times of the year. also in terms of analysis, this may be due to the high confidence level and the threshold values preferred in data discretization. however, climatic oscillations, which are frequently effective in determining climatic features at other stations, were revealed. conclusions drought can occur anywhere in the world from time to time. in this study, the mowcatl algorithm developed by harms et al., was used as an alternative to conventional drought methods for meteorological stations in the aras basin. since it is difficult to fully analyze the drought due to the factors that constitute it, the algorithm used has been able to advance the study to a certain extent. the formulated algorithm confirmed that the iğdır station is dry according to classical methods. however, the method applied in this study, more comprehensive studies can be performed by changing the antecedents and consequents. the arid and wetland situations that the data series have stored can be further illustrated. the result of the study also showed that stations with different geographical features may have similar climatic rules, while stations with similar geographical features may have different climatic features. this may be due to the partially unpredictable climatic parameters. this study conducted in the aras basin examined the effects of parameters such as climatic oscillations on the precipitation regime that causes drought. in the future, it is foreseen that the development of the region and the awareness of the local people will be engaged in more agriculture, and this type of work is important in turkey regarding the climatic situation of the region. it is inevitable that the algorithm used in this study will contribute to the region and will shed light on the studies to be done with other methods and algorithms. this study shall contribute to the scholarship on the climate regarding the study area and its surroundings. 195drought monitoring using mowcatl data mining algorithm in aras basin, turkey table 14. correlation chart of indexes ardahan/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .612** .464** .336** -.106* -.054 -.054 -.031 -.023 .061 .089 aspi3 1 .764** .568** -.087 -.084 -.068 -.043 -.038 .084 .111* aspi6 1 .764** .002 -.104* -.080 -.043 -.105* .035 .112* aspi12 1 -.018 -.213** -.107* -.152** -.180** .029 .178** arpaçay/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .634** .517** .422** -.073 -.077 -.070 -.083 -.155** -.009 .082 aspi3 1 .811** .689** -.028 -.119** -.008 -.122** -.200** .042 .144** aspi6 1 .864** .012 -.095* -.041 -.098* -.235** -.037 .116* aspi12 1 .019 -.093* -.042 -.103* -.271** -.022 .095* doğubeyazit/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .563** .423** .299** -.062 -.019 -.110* -.015 -.032 .036 .024 aspi3 1 .751** .510** -.041 -.019 -.090 .001 -.065 .017 .021 aspi6 1 .743** -.011 .012 -.049 .025 -.119** -.001 -.050 aspi12 1 -.022 -.007 -.059 -.040 -.190** .040 -.041 horasan/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .563** .423** .299** -.062 -.019 -.110* -.015 -.032 .036 .024 aspi3 1 .751** .510** -.041 -.019 -.090 .001 -.065 .017 .021 aspi6 1 .743** -.011 .012 -.049 .025 -.119** -.001 -.050 aspi12 1 -.022 -.007 -.059 -.040 -.190** .040 -.041 iğdir/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .536** .338** .247** -.109* -.034 -.172** -.019 -.005 .024 .086 aspi3 1 .664** .463** -.014 -.101* -.031 -.054 -.025 .029 .111* aspi6 1 .694** .069 -.125** .008 -.090 -.086 -.040 .102* aspi12 1 .077 -.185** .021 -.133** -.155** -.003 .115* kars/correlations aspi1 aspi3 aspi6 aspi12 ao mei nao oni pdo pna soi aspi1 1 .582** .463** .356** -.132** -.017 -.131** .020 -.027 .019 .033 aspi3 1 .736** .594** -.074 -.042 -.059 .033 -.009 .119* .061 aspi6 1 .793** -.005 -.070 -.077 .021 -.059 .067 .071 aspi12 1 .009 -.088 -.061 -.003 -.081 .072 .050 **: correlation is significant at the 0.01 level (2-tailed). *: correlation is significant at the 0.05 level (2-tailed). 196 emre topçu references alamdarloo, h. e., khosravi, h., nasabpour, s., & gholami, a. 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(2024). integrated geophysical investigation using aero-radiometric and electrical methods for potential gold mineralization within yauri/ zuru schist belts, kebbi state nw nigeria. earth sciences research journal, 28(2), 151-160. https://doi. org/10.15446/esrj.v28n2.112066 this study used aero-radiometric, 2d electrical resistivity tomography (ert), and induced polarization (ip) methods to delineate gold mineralization potential. the study also confirmed and followed-up on regions with major structural features identified in previous aeromagnetic studies in the area. a half-degree airborne radiometric data of sheet 118_ yelwa from the ngsa, which contains three radioelements (%k, eth, and eu) were used in this study. these datasets were processed and analyzed with oasis montaj grid math expression builder to obtain the %k_ratio_eth and ternary grid anomalies. the results identified zones e and f as hydrothermally altered regions that may harbour gold mineralization. these findings were consistent with the regions of major structural features identified in previous aeromagnetic studies in the area. the zones were located in the eastern parts of ngaski, yauri (yelwa), shanga, and agwara, as well as magama’s northwest region. however, zone f1 (the eastern portion of ngaski/yauri) has been further investigated using 2ert and ip detailed geophysical methods in a dipole-dipole configuration. the results of geoelectric techniques along profiles 1, 2, and 3 identified the major gold mineralization potential zones, which were labeled a1, a, and c. these regions have low/high resistivity and chargeability signatures, and could thus be interpreted as potential target zones for metallic mineral exploration, particularly gold mineralization. the regions are located in the northern part of mararraba and southwest of the jinsani areas of kebbi state. investigación geofísica integrada de prospección aerorradiométrica y métodos eléctricos para definir el potencial de mineralización aurífera en el cinturón de esquistos de yauri/zuru, en el estado de kebbi, en el noroeste de nigeria resumen este estudio utiliza los métodos de prospección aerorradiométrica, tomografía 2d de resistividad eléctrica (ert), y polarización inducida (ip) para delinear el potencial de mineralización aurífera en el estado de kebbi, al noroeste de nigeria. este trabajo también confirmó e hizo seguimiento a estudios con características estructurales identificadas en previos análisis aeromagnéticos en el área. con este fin se utilizaron datos radiométricos aerotransportados de medio grado de la pestaña 118_yelwa, de la agencia de estudios geológicos de nigeria, la cual contiene tres radioelementos (%k, eth, y eu). estos conjuntos de datos fueron procesados y analizados con el software de expresiones matemáticas oasis montaj grid math para obtener las anomalías de la red %k_ratio_eth y ternary. los resultados identificaron las zonas e y f como regiones hidrotérmicamente alteradas que podrían albergar mineralizaciones auríferas. estos hallazgos son consistentes con las regiones de mayores características estructurales identificadas en previos estudios aeromagnéticos en el área. estas zonas corresponden con las partes del este de ngaski, yauri (yelwa), shanga y agwara, al igual que con la región noroeste de magama. sin embargo, la zona f1 (la parte más al este de ngaski/yauri) fue investigada más detalladamente con los métodos ert e ip en una interacción dipolo-dipolo. los resultados de las técnicas geoeléctricas en los perfiles 1, 2 y 3 identificaron las zonas de mayor potencial de mineralización aurífera, las cuales fueron etiquetadas como a1, a, y c. estas áreas tienen características de alta/baja resistividad y cargabilidad, por lo que podrían interpretarse como zonas potenciales para la exploración de minerales metálicos, particularmente de mineralización de oro. estas regiones se ubican en el norte de mararraba y en el sudoeste de jinsani, estado de kebbi. https://doi.org/10.15446/esrj.v28n2.112066 https://doi.org/10.15446/esrj.v28n2.112066 https://doi.org/10.15446/esrj.v28n2.112066 152 abdulrahaman idris augie, kazeem adeyinka salako, andy anderson bery, adewuyi abdulwaheed rafiu, mufutau owolabi jimoh 1. introduction in nigeria, gold occurs primarily in quartz veins as well as an accessory mineral in metasedimentary and meta-igneous rocks across the schist belt stratum (ramadan and abdel-fattah, 2010). the nigerian gold fields have witnessed extensive artisanal work, with a concentration on both major goldquartz reefs and their associated alluvial occurrences (garba, 2000; sani et al., 2019). geophysical studies are required to generate a database of precise coordinates as well as to identify the zones containing these gold minerals (au). au is typically found in hydrothermal deposits in the uppermost portion of the continental crust of earth and it is the most noble chemical element (frimmel and gartz, 1997). many geophysical studies, both qualitative and quantitative, were conducted in the research area in an attempt to identify mineralization potential zones. bonde et al. (2019) and lawali et al. (2020) carried out analyses on aeromagnetic data over the study area with the goal of identifying structural potential mineralization zones. analytic signal (as), first vertical derivatives (fvd), second vertical derivatives (svd), and tilt derivatives (tdr) are the enhancing filters used in the research. the study resulted in identification of structures with a ne-sw direction like as fracture as well as faults, and veins, in which economic minerals usually hosted. the literature, however, includes gaps because the study entirely relied on aeromagnetic data gathered from secondary sources. the anomalous zones revealed by aeromagnetic investigations were not followed up on with detailed studies such as 2d electrical resistivity imaging (eri) and induced polarization (ip) geophysical techniques. aisabokhae (2021) and lawal et al. (2021) used aeromagnetic and aeroradiometric datasets to study possible mineralization zones of the kebbi south. the following enhancement filtering techniques were utilized in the study: reduction to the magnetic equator (rte), as, fvd, euler deconvolution (ed), composite image, ratio maps, and ternary image. according to the conclusions of the study, the anka schist region is located on the edge of a plutonic area exhibiting brittle-ductile distortion, signifying structural fracture for fluid flow and associated alteration caused by hydrothermal occurrences toward mineralization in the regions. however, there are gaps in the literature as the authors did not effectively use the magnetic equator to centralise the anomalies because the study area is within equatorial zones of low latitude. at these regions, a further amplitude correction is frequently required to prevent the data’s north-south signal from dominating the results according to holden et al. (2008), core et al. (2009). the anomalous zones identified in the research were not further examined utilising detailed methods such as 2d ert and ip geophysical methods for the identification of mineralised potential zones. however, augie et al. (2021, 2022a & b) analysed the magnetic signatures in combination with the geological context of the area, to determine the structures that could host gold mineralization across kebbi south. the following techniques were used in the study: rte, fvd, svd, cet, tdr, ed, spi and 2d magnetic modelling. these techniques’ results aided in the delineation of lineaments like as faults, fractures, especially shear zones thought to be related with alteration zones, which play a significant part in defining gold mineralised prospect. when contrasted with the geological setting of the area, the structures delineated within it correlate to the regions underlain by the following earth materials; granite, gneiss, quartz-mica schist, biotite diorite, biotite hornblende granite and medium coarse-grained. results of these techniques identified alteration zones that could host gold mineralization zone. these potential zones’ locations correspond to the se portions of yelwa (yauri) as well as shanga, magama, fakai, rijau, zuru, ngaski, and the eastern parts of bukkuyum and wasagu/danko. in this study, aero-radiometric, 2d ert and ip methods were employed to delineate gold mineralization potential. the aero-radiometric approach was utilised to identify the hydrothermal alteration zone that could potentially hold gold mineralization potential, as well as to support the results with previous aeromagnetic studies (augie et al., 2022a&b) of the area. the airborne radiometric technique revealed hydrothermally altered zones, which were then explored for potential gold prospects and associated mineralization using 2d ert and ip geophysical methods. 1.1 the study area the research area is located within southern kebbi and some portions of niger state. it is bounded by latitudes 10°30′0″n and 11°0′0″n, and longitudes 4°30′0″e and 5°0′0″e. the area includes the following local government areas (lgas): the se part of yauri, ngaski, shanga, magama, and agwara areas (fig. 1). geologically, the area lies within the basement complex regions of northwestern nigeria. the area is underlain by mangrove swamps, gwandu formation, nupe sandstone and pan-african older granitoid (garba, 2000; 2003). these comprised; sands, clays, sandstones, siltstones, and undifferentiated schists including phyllite, medium to coarse-grained biotite, migmatitic augen gneiss and granite gneiss as shown in figure 1. figure 1. geological map of the study area (modified after ngsa, 2007) 2. methods 2.1 acquisition and processing of airborne radiometric data a half degree sheet of aero-radiometric data of the research region was obtained from the nigerian geological survey agency (ngsa). the sheet 118_ yelwa was used in the study which comprised some portion of; yauri (yelwa), ngaski, shanga and agwara as shown in fig. 1. these data were collected by fugro airborne survey on behalf of the ngsa of the federal government of nigeria between 2005 and 2009 (augie et al. 2022a). the data were collected at an altitude of 100 meters, with a flight line spacing of 500 meters aligned nwse and a tie line spacing of 2000 meters. the maps of this grid data sheet have a scale of 1:100,000 as well as half-degree sheet. the acquired gridded data which contain the concentrations of potassium (k) as well as equivalent thorium (eth) and uranium (eu) were input into a geosoft “oasis montaj software” to process each radio-element. each of these elements were blended independently using the blending technique provided by the grid and image geosoft extension (gx) to obtain anomalous concentration grids for the three elements (%k, eth, and eu). the potassium ratio thorium (%k_ratio_eth) grid anomaly was also developed with the aid of the grid math expression builder of the oasis montaj using equation 1. 0 = 1 2 (1) following that, using the blending method in grid and image of gx, the total count map integrating the combined effects of 40k, 238u, and 232th was obtained. these were then utilized to come up with the ternary map, which consisted of different colours that corresponded to every of the radioelement abundances. the colour red was assigned to the %k., green as eth, and blue as eu (millingan & gunn, 1997) and each of the three-radioelement concentration in the red, green, and blue colors were then merged using grid and image gx of oasis montaj. the ternary map has been developed to identify radioelement variations associated with hydrothermal alteration zones that could be favourable for gold mineralization. radioelement content in host rocks can vary as a result of hydrothermal processes. of the three radioelements, potassium (k) is affected by these processes the most, whereas thorium (th) is affected less frequently and uranium (u) only rarely. potassium levels often rise 153integrated geophysical investigation using aero-radiometric and electrical methods for potential gold mineralization within yauri/ zuru schist belts, kebbi state nw nigeria during signature alterations, but weathering typically reduces the intensity of signature alterations. mineralising hydrothermal fluids is usually associated with potassium to source rocks (tawey et al., 2021). it may be hosted by potassium feldspar or muscovite, and prospective outcropping or sub-cropping mineralization can be identified by an increase in k counts during a radiometric survey (dickson and scott, 1997). 2.2 2d geoelectric prospecting method in this study, a 2d geoelectrical survey was also carried out at yauri/ ngaski boundaries of kebbi state, nw nigeria. an area 640 m2 of fields were investigated using a dipole-dipole configuration with minimum electrode spacing (a) of 10.00 meters. a total of 3 profiles were designed for each 300 m distance along the profile, oriented nw-se (fig. 2). figure 2. profile line field survey layout the data was collected using a super sting (resistivity meter) connected to an electrode selector and supplied by a 12 v battery. this instrument has the capability of measuring apparent resistivity and ip data simultaneously. it is equipped with a transmitter with automatic/user configurable currents in miliamps; 1, 2, 5,10, 20, 50, 100, 200, 500 and 1000 ma. the distance between the two pairs of current electrodes (c1 and c2) was chosen to be equal to the distance between the two potential electrodes (p1 and p2). the distance between c1 and p1 (dipole separation factor) was calculated as a multiple of an integer n of the distance between the c1-c2 and p1-p2 (fig. 3). the dipole separation factor n was initially set to one (1), then increased to two (2), three (3), and so on until it reached a maximum value of eight (8). thus, the two-parameter; dipole separation factor (n) and the space between the current electrode pair (a) have been measured. the measurements were taken with electrode separation of 10 m and n was set at 1, 2, 3, up to 8 (fig. 3). the more the increase in n which would equally increase the electrode spacing as well as more the injected current flows to greater depths (loke, 2000). n = 1 n = 2 n = 3 n = 4 n = 1 n = 2 n = 3 n = 4 a na ∆v ∆v ∆v ∆v figure 3. illustration of dipole-dipole array pseudo-section when putting the measured values (wahyu, 2016) electrodes 1, 2, 3, and 4 (c1, c2, p1 and p2) were used for the initial measurement. c2 was employed at 0 m, c1 at 10 m, p1 at 20 m, and c2 at 30 m. likewise, in the second measurement, c2 at 10 m, c1 at 20 m, p1 at 30 m, and p2 at 40 m respectively. these procedures were repeated down the profile line using 1a, 2a, 3a, 4a, 5a, 6a, 7a, and 8a spacing between c1 and p1 where a is 10 m. at each measurement, the measured ground apparent resistivity and ip values were obtained. the obtained data were analyzed using loke (1999)’s res2dinv programme, which automatically produces 2d resistivity and ip models for the subsurface. 3. results 3.1 results from aero-radiometric technique figure 4 depicts a ratio map of %k/eth concentration that shows zones of hydrothermally altered zones which usually lead to radioactive element enrichment. in general, thorium (eth) is unaffected by alteration activities since it is often dormant in mineral content processes, or it might as well be reduced in zones of high %k as well as silicification (lundien, 1967). the %k/eth ratio map indicates that a high potassium content in the red colour indicates a low thorium level as well a high thorium concentration in the blue colour indicates a potassium level. the summarized results of the colour compositions for k/th versus radioelements were given in table 1. zone e has a high concentration of k/th concentrations (enrichment in k) and is found in the following areas: the eastern part of yauri, shanga, ngaski and agwara, and the northwest parts of magama. these areas are substantial hydrothermally altered zones indications having the anomalous of the enrichment in k range of 0.199 to 0.443 ppm. however, many literatures (such as durrance, 1986; dickson and scott, 1997), revealed that the alteration environment as revealed in fig 4, may typically accumulate various mineralization potential, particularly gold minerals. this is due to hydrothermal solutions having ability of decomposing and conveying many different types of minerals and metallic substances, which are crucial in mechanisms of ore deposition, particularly metallic minerals (hoover and pierce, 1990). figure 4. k/eth map of the study area figure 5 depicts a ternary map of radioelements (%k, eth and eu) with a variety of colour combinations. table 2 summarizes the results for figure 5 in terms of colour composition versus radioelements. the red colour indicates a high %k with a low eu and eth, blue shows a rich concentration of eu having a low %k as well as eth. green exhibited a high concentration of eth 154 abdulrahaman idris augie, kazeem adeyinka salako, andy anderson bery, adewuyi abdulwaheed rafiu, mufutau owolabi jimoh alongside a low %k and eu. cyan denotes a high eth and eu with a low %k, magenta defines a high %k as well as eu having a low eth, yellow signifies a high %k with eth but a low eu, black represents a low %k, eth, as well as eu, and white denotes a high %k, eth, and eu. many literatures (such as hoover and pierce, 1990; ohioma et al., 2017; lawal et al., 2021; adetona et al., 2022; etc.) discovered that aero-radiometric surveying techniques of radioelements that could mark gold deposits are variable, with potassium being the most reliable guide. using the colour scheme shown in fig. 5, the region that exhibits blue, green, cyan, yellow, and black indicates very low potassium concentrations, implying that hydrothermal alteration did not occur in this zone of k. zone f, on the other hand, has a very low concentration (0.051 to 0.146 ppm as shown in fig. 4) of uranium (eu) and thorium (eth) while having a high concentration (0.199 to 0.443 ppm) of potassium (%k), which corresponds with the high %k/ eth in the e region of the %k/eth ratio map shown in figure 4. these regions are situated in the eastern parts of ngaski and agwara, as well as the northwest parts of magama. the overabundance of potassium in comparison to the other elements, especially thorium, implies hydrothermal alteration. these are the hydrothermal alteration regions that could potentially be favourable for gold mineralization. the hydrothermally altered zones (f1 in particular) identified by the aero-radiometric technique were investigated further using 2d ert and ip detailed geophysical methods. the summary of the results of aero-radiometric techniques: colour compositions versus radioelement concentrations based on figures 4 and 5 are given on table 1 and 2. figure 5. ternary map of k, eth, and eu of the study area. table 1. concentrations of k/eth versus colour compositions of the area s/no concentrations of radioelements enrichment in radioelements (ppm) colour compositions for %k ratio eth location province and country probable gold and other mineralization potential remarks 1. high %k and low eth 0.199 to 0.443 red eastern part of yauri, shanga, ngaski and agwara, and the northwest parts of magama nw nigeria yes 2. moderate %k and moderate eth 0.111 to 0.198 yellow and green ne part of ngaski and sw part of ngaski nw nigeria no 3. low %k and high eth 0.051 to 0.101 blue western parts of yauri and agwara nw nigeria no table 2. ternary images of k, eth, and eu versus colour compositions of the area s/no concentrations of radioelements colour compositions for %k, eu and eth location province and country probable gold and other mineralization potential remarks 1. high %k, low eu and low eth red eastern part of yauri, shanga, ngaski and agwara, and the northwest parts of magama nw nigeria yes 2. high eu, low %k and low eth blue ne part of agwara nw nigeria no 3. high eth, low %k and low eu green se part of magama and western parts of yauri and agwara nw nigeria no 4. high eth, high eu and low %k cyan western parts of yauri and agwara nw nigeria no 5. high %k, high eu and low eth magenta shanga and ne part of ngaski nw nigeria no 6. high %k, high eth and low eu yellow eastern part of borgu and northern part of magama nw nigeria no 7. low %k, low eth and low eu black borgu and estern part of ngaski nw nigeria no 8. high %k, high eth and high eu white sw part of magama and central part of agwara nw nigeria no 155integrated geophysical investigation using aero-radiometric and electrical methods for potential gold mineralization within yauri/ zuru schist belts, kebbi state nw nigeria 3.2 results from 2d geoelectric prospecting techniques the interpretation of the 2d geoelectric modelled sections is associated with the geological setting of the area. these include the borehole lithology of the southern part of kebbi as revealed by rural water supply and sanitation agency (ruwasa) and resistivity values (osazuwa and chii, 2010) in table 3. these adjoining borehole logs were obtained from previous works and subsequently used to correlates the findings of the present study. table 3. a borehole/lithology log of the southern part of kebbi areas (sarda, 1988; osazuwa and chii, 2010) lithology depth(m) thickness(m) resistivity range (ωm) lateritic sand 0-2 2 60-1000 highly decomposed schist 2-10 8 10-500 partially decomposed granite 10-15 12 100-1000 quartzite’s and gneiss 15-21 16 200-100000 3.2.1 geoelectric sections for profile 1 figure 6(a) is the 2d inverse model section for profile 1. it is oriented in the nw-se direction and has a lateral distance of 300 m. the profile lies between latitudes 10°48’3.6”n to 10°47’56.4”n and longitudes 4°45’14.4”e to 4°45’21.6”e (fig. 2). the subsurface feature was sectionalised into four different zones namely: a, b, c, and d. zone a is characterized by resistivity values ranging from 1.6 ωm to 459 ωm, and are located on 10 80 m, 190-200 m, 250-265 m along the profile and a depth/thickness of 12.0 m, 18.5-24.9 m and 7.5-18.5 m respectively. zone b has resistivity values of 460 ωm to 1888 ωm, it covers a length of 30-110 m, 130-149 m, 190 200 m and 245-265 m at a corresponding depth/thickness of 2.5-18.5 m, 24.9 m, 18.5 m and 2.5-18.5 m. zone c is characterized by resistivity values going from 1889 ωm to 7773 ωm, it covers the distance along the profile of 35-55 m, 70 92 m, 105-130 m, 145-180 m, 200-210 m and 220-270 m and a depth/thickness of 8-24.9 m, 3-24.9 m, 24.9 m, 24.8 m, 24.9 m and 7.5 m. likewise, zone d is portrayed by the resistivity of 7774 ωm to 32002 ωm, it has occupied a length of 100-125 m, 175-190 m, 200-250 m and 265-290 m at a corresponding depth/ thickness of 7.5-24.9 m, 12-24.9 m, 2.5-24.9 m and 18.5 m respectively. the subsurface lithology for features a, b, c, and d as compared with table 3 could be lateritic soil, highly decomposed schist, partially decomposed granite and quartzite, and quartzite/gneiss. however, zone a of low resistivity areas could result from water content and enduring in the oxidized rock. some minerals, especially gold, could be associated by this kind of oxidized rock that originates from granite. the younger granite phases in region, was thought to be the origin of the gold mineralization (bagare et al., 2018 and 2019). zone c is associated with partially decomposed granite and quartzite which form dykes. dyke subsurface structures of the aforementioned rock formation could be crucial in detecting the gold mineral. the results of the inverse model section given in figure 6(a) were further transformed into a geologic section as shown in figure 6(b). the results of 2d inverse model chargeability sections along profile 1 (ip1) are given in figure 7. the sections revealed clearly the regions that exhibit metallic minerals namely zone a1. these regions have the most noteworthy chargeability values ≥ 20 msec. it covers an x-position of 45-60 m, 190-202 m and 202-255 m at a corresponding depth/thickness of 7.5-18.5 m, 7.5 m and 12.8-24.9 m. the higher chargeability of zone a1 generally happens because of the collection of metallic minerals in host rocks. these locales could be considered as a possible objective for the investigation of specific metallic minerals, particularly gold mineralization. profile one (p1) distance along the profile (m)depht iteration 4 abs. error = 17.2 % distance along the profile (m) a b figure 6. (a) inverse model resistivity section and (b) geologic section of profile one (p1) ip profile one (ip1) distance along the profile (m) figure 7. inverse model chargeability section for profile one (ip1) 156 abdulrahaman idris augie, kazeem adeyinka salako, andy anderson bery, adewuyi abdulwaheed rafiu, mufutau owolabi jimoh 3.2.2 geoelectric sections for profile 2 profile 2 has a lateral extent of 300 m (fig. 8). it is oriented nw-se and is located between latitudes 10°48’7.2”n to 10°48’0”n and longitudes 4°45’0”e to 4°45’7.2”e (see fig. 2). the subsurface resistivity differences were classified as zones a, b, c, and d. table 2 summarises the subsurface variations for these zones in relation to resistivity of common rocks as well as borehole lithology of the area as given in table 3. the zones of low and high resistivity (a and c) were discovered to be the key zones of probable gold mineralization. according to the geological setting of the research region, the low resistivity zone (a) could be the result of weathering in an oxidised rock, and these rock types are generally granite or rhyolite in origin. as a result, it may be associated with some minerals, particularly gold and other metallic minerals. however, the dyke structures of granite and quartzite were generated by a high resistivity zone (c). these structures could be crucial in identifying mineralization potential, particularly gold minerals. these findings led to the creation of the geologic section depicted in figure 8(b). figure 9 depicts the results of the 2d inverse model chargeability section for profile two (ip2). the sections of this profile clearly highlighted the regions with mineralization potential, which was zone a1. these areas tend to have a higher chargeability, which could be due to the accumulation of metallic minerals in host rocks such as altered hydrothermal veins and granite. table 4 provides the lateral distances and depths occupied by the zones. these locations could be regarded as prospective targets for investigating explicit metallic minerals, particularly gold mineralization. 3.2.3 geoelectric sections for profile 3 the 2d resistivity model section along profile 5 is given in figure 10(a). it covers a lateral distance of 300 m and lies between latitudes 10°47’52.8”n to 10°47’45.96”n and longitudes 4°45’3.6”e to 4°45’10.8”e (see fig. 2). the subsurface resistivity signature was categorised as zones a, b, c, and d based on the resistivity of the rock formation and the geological setting of the area. the suggested subsurface lithology for features a, b, c, and d are given in table 3. zone a is associated with oxidized granite/quartzite and gneiss, and it could usually be associated with certain minerals, particularly gold and other metallic minerals, based on geological contexts and the resistivity of common rocks (table 3). zone c, which developed the dyke structures associated with partially decomposed granite and quartzite, is another promising zone for metallic minerals. these structures may be crucial in determining mineralised zones, notably gold mineralization. the results of the profile, have led to the creation of the geologic section depicted in figure 10(b). figure 11 shows chargeability sections along profile three (ip3). zone a1 indicated the sections with the highest chargeability. higher chargeability in zones a1 (70 msec and above) may arise due to the presence of metallic minerals. table 4 summarises the length and depth of these zones. these areas of the highest changeability could be regarded as an expected target for the exploitation that led to the exploration of gold mineralization in the regions. distance along the profile (m)a) distance along the profile (m)b) profile two (p2) depth iteration 4 abs. error = 5.5% d epth (m ) figure 8. (a) inverse model resistivity section and (b) geologic section of profile two (p2) distance along the profile (m) ip profile two (p2)depth iteration 4 abs. error = 2.9 figure 9. inverse model chargeability section for profile two (ip2) 157integrated geophysical investigation using aero-radiometric and electrical methods for potential gold mineralization within yauri/ zuru schist belts, kebbi state nw nigeria distance along the profile (m)a) distance along the profile (m)b) profile three (p3)depth iteration 4 abs. error = 7.8% d epth (m ) figure 10. (a) inverse model resistivity section and (b) geologic section of profile three (p3) distance along the profile (m)ip profile three (ip3)depth iteration 4 abs. error = 9.7 figure 11. inverse model chargeability section for profile three (ip3) table 4. summary of results obtained from the integrated methods profile resistivity results induced polarization results probable gold mineralization potential remarks zone resistivity ranges (ωm) lateral lengths (m) depth/3 thickness (m) zone chargeability ranges (msec) lateral lengths (m) depth/ thickness (m) p1 a 1.6 to 459 10-80, 190-200 and 250-265. 12.0, 18.524.9 and 7.518.5 a1 ≥ 20 45-60, 190-202 and 202-255 7.5-18.5, 7.5 and 12.8-24.9 yes b 460 to 1888 30-110, 130149, 190-200 and 245-265 2.5-18.5, 24.9, 18.5 and 2.518.5 no c 1889 to 7773 35-55, 70-92, 105-130, 145180, 200-210 and 220-270 8-24.9, 3-24.9, 24.9, 24.8, 24.9 and 7.5 yes d 7774 to 32002 100-125, 175190, 200-250 and 265-290 7.5-24.9, 1224.9, 2.5-24.9 and 18.5 no p2 a 4.6 to 601 5-40, 165-185, 245-260 7.5, 18.5-24.9 and 18.5-24.9 a1 20 and above 35-65, 55-80, 145-160, 200220 and 221-250 12.8-24.9, 12.8, 18.5, 7.5 and 7.5-24.9 yes b 602 to 2033 40-90, 120150, and 155190 12, 7.5 and 18.5-24.9 no c 2034 to 6873 10-90, 95-120, 140-170, 180205, 240-270 2.5-12.8, 24.9, 12.8, 12.8 and 18.5 yes d 6874 to 23241 10-105, 115155, 170-180, 200-245, and 270-290 7.5-24.9, 7.5-24.9, 12.8, 24.9 and 18.5 no (continued) 158 abdulrahaman idris augie, kazeem adeyinka salako, andy anderson bery, adewuyi abdulwaheed rafiu, mufutau owolabi jimoh profile resistivity results induced polarization results probable gold mineralization potential remarks zone resistivity ranges (ωm) lateral lengths (m) depth/3 thickness (m) zone chargeability ranges (msec) lateral lengths (m) depth/ thickness (m) p3 a 2.99 to 966 5-80, 90-105, 135-160, 170190, 235-50 and 270-290 7.5, 18.5-24.9, 7.5-18.5, 24.9, 18.5-24.9 and 7.5 a1 70 and above 30-45, 135-150, 140-170 and 210-240 7.5-18.5, 18.524.9, 2.5-18.5 and 18.5 yes b 967 to 4095 10-90, 91-120, 145-180, 190220 and 240270 3-7.5, 18.5, 12.8, 24.9, and 18.5 no c 4096 to 17353 120-150, 160175, 195-210, 220-240 and 250-270 12.8, 18.524.9, 12.818.5, 19 and 7.5-24.9 yes d 17354 to 73547 10-90, 110-170 and 220-235 7.5-24.9, 12.8-24.9 and 12.8-18.5 no table 5. generated database of the potential gold mineralised zones in the study area profile no. lateral lengths (m) depth/ thickness (m) geographic coordinate. locations left end right end longitude latitude longitude latitude p1 10-80, 190-200, and 202-255. 18.5, 24.9, and 24.9 4°45’14.76”e, 4°45’17.64”e and 4°45’19.44”e 10°48’3.24”n, 10°48’0.36”n and 10°47’58.56”n 4°45’16.2”e, 4°45’18.36”e and 4°45’20.52”e 10°48’1.8”n, 10°47’59.64”n and 10°47’57.48”n mararraba p2 35-100, 145-200, and 220-250. 12.8, 18.5 and 24.9 4°45’0.72”e, 4°45’3.24”e and 4°45’4.68”e 10°48’6.48”n, 10°48’3.96”n and 10°48’2.52”n 4°45’2.16”e, 4°45’3.96”e and 4°45’5.76”e 10°48’5.04”n, 10°48’3.24”n and 10°48’1.44”n sw of jinsani p3 0-45 and 140-180. 18.5 and 28.5 4°45’3.96”e and 4°45’7.92”e 10°47’52.44”n and 10°47’48.84”n 4°45’5.04”e and 4°45’8.64”e 10°47’51.36”n and 10°47’48.12”n sw of jinsani figure 12. major zones of gold mineralization potentials map 159integrated geophysical investigation using aero-radiometric and electrical methods for potential gold mineralization within yauri/ zuru schist belts, kebbi state nw nigeria 3.2.4 possible potential gold mineralized zones of the area the suggested zones of mineralization potential along geoelectric profiles 1, 2 and 3 (p1, p2 and p3 as well as ip1, ip2 and ip3) are zones a, c and a1. these regions are characterized by low/high resistivity and high chargeability signatures as shown in figures 6-10. table 4 provides the integrated key zones of gold mineralization potential (fig. 12). these zones could be inferred as a potential target for the exploration of metallic minerals, particularly gold mineralization. the results of this study led to the development of a database, which is displayed in table 5 and contains the exact coordinates and depths of prospective gold mineralization zones. this database has the potential to enhance the region’s gold exploration activities. 4. conclusion the use of integrated geophysical methods in this study resulted in the creation of a database containing precise coordinates, lateral lengths, and thickness/depths for potential gold mineralization zones. this database could be used to help improve gold exploration in the region. the study also supported the regions with major structural features discovered in earlier magnetic studies in the area by augie et al. (2021) and augie et al. (2022a,b). an aero-radiometric approach utilised in this study indicated areas of the hydrothermally altered zone that possibly host gold mineralization, and the results are in conformity with the previous findings of aeromagnetic studies within the study region. the hydrothermally altered zones (e and f) derived from airborne gamma-ray spectrometry used in this study are the eastern parts of yelwa (yauri), ngaski, shanga, agwara, as well as the northwest parts of magama. the zone f1 (eastern half of ngaski/yauri) has been further investigated using 2d ert and ip detailed geophysical methods. the geoelectric method results along profiles 1, 2, and 3 identified the major zones of gold mineralization potential, which are zones a, c, and a1. these regions are characterized by low/high resistivity and chargeability signatures and could thus be interpreted as possible target zones for metallic mineral exploration, especially gold mineralization. these areas are located in the northern part of mararraba and the southwest of the jinsani areas of kebbi state. data availability the acquired and generated datasets in the current study are not available to the general public due to the following reasons: (a) aero-radiometric data are under the custody of the nigeria geological survey agency (ngsa) and can only be released upon request and payment via their website (https://ngsa.gov. ng), and (b) resistivity and induced polarization data generated in this study area are available from the corresponding author on reasonable request. acknowledgements the authors would like to express their profound appreciation to the tertiary trust fund (tetfund) for funding this research and the management of the nigeria geological survey agency (ngsa) for the release of the airborne radiometric dataset. our thanks are also extended to the district heads of mararraba of yelwa yauri area of kebbi state for approval of the site to collect geophysical data. their appreciation also goes to the department of physics, bayero university kano, and the department of geophysics, federal university of technology minna for providing the instruments as well as geosoft inc. for the use of oasis montaj and res2dinv softwares. conflict of interest the authors have no conflict of interest to disclose. references adetona, a. a., kwaghhua, f. i., & aliyu, s. b. 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(2016). 2d resistivity and induced polarisation measurement for manganese ore exploration. journal of physics: conference series, 739, https://doi.org10.1088/17426596/739/1/012138 https://doi.org/10.1016/j.ejrs.2009.08.001 about:blank about:blank _heading=h.gjdgxs keywords: precipitation patterns; climate change scenarios; water availability; south-southeast mexico palabras clave: patrones de precipitación; escenarios de cambio climático; disponibilidad de agua; sursureste de méxico. how to cite item andrade-velazquez, m., & medrano-perez, r. (2021). historical precipitation patterns in the south-southeast region of mexico and future projections. earth sciences research journal, 25(1), 69-84. doi: https://doi.org/10.15446/esrj. v25n1.87255 this study analyzed climate change scenarios and their potential impact on water availability for the south-southeast region (ssr) of mexico. precipitation patterns were examined using the standardized precipitation index for three emissions scenarios, representative concentration pathway (rcp) 4.5, rcp 6.0, and rcp 8.5, during the periods of 1960-2016, 2015-2039 (near future), and 2075-2099 (far future). historical changes in precipitation in the ssr indicated the presence of dry and wet events driven by el niño-southern oscillation, the pacific decadal oscillation, and the atlantic multidecadal oscillation, which are the regional climate modulators. however, the impact of these phases has not been quantified for the future. the results of our climate change projections show that the grijalva and usumacinta rivers and surrounding regions (chiapas and tabasco) will have an increase in the percentage of dry and wet events shortly (2015-2039), while there is a medium to a low probability of this occurrence in rest of the ssr. by 2075-2099, grijalva and usumacinta will continue to have a high probability of dry events due to climate change, and the yucatan will also exhibit this behavior. rcp 4.5 was projected to be the wettest scenario for the study area, while rcp 8.5 projected an increase in dry events during both periods (2015-2039 and 2075-2099). rcp 6.0 projected a drier 2015-2039 and wetter 2075-2099. abstract historical precipitation patterns in the south-southeast region of mexico and future projections patrones de precipitación históricos en la región sur-sureste de méxico y proyecciones futuras issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.87255 el presente estudio analiza los escenarios del cambio climático y su impacto potencial en la disponibilidad de agua para la región sur-sureste (rss) de méxico, examinando el comportamiento de los patrones de precipitación mediante el uso del índice estandarizado de precipitación para tres escenarios rcp 4.5, rcp 6.0 y rcp 8.5 durante los periodos 1960-2016, 2015-2039 (futuro cercano), y 2075-2099 (futuro lejano). además de comprender los cambios históricos en la precipitación en la rss, la cual muestra presencia de eventos secos y húmedos, donde el niño-oscilación del sur, la oscilación decadal pacífico y la oscilación multidecadal del atlántico son los moduladores climáticos regionales. sin embargo, para el futuro no se ha cuantificado el impacto de estos. los resultados bajo las proyecciones de cambio climático muestran para el futuro cercano (2015-2039) en la región de los ríos grijalva y usumacinta (chiapas y tabasco) incrementos en el porcentaje de eventos secos como húmedos bajo las proyecciones de los escenarios de cambio climático, sea cual sea el escenario, mientras que el resto de la rss tiene mediana o baja probabilidad de que esto ocurra. para 2075-2099, grijalva y usumacinta continúan con una alta probabilidad de que bajo cambio climático haya eventos secos, yucatán también tiene este comportamiento. rcp4.5 mostró ser el escenario más húmedo para la zona de estudio, mientras que el rcp8.5 presenta mayores eventos secos en la zona en ambos períodos (2015-2039 y 2075-2099). rcp6.0 se mostró más seco para 2015-2039 y más húmedo para 2075-2099. resumen record manuscript received: 13/05/2020 accepted for publication: 04/09/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 69-84 mercedes andrade-velázquez and ojilve ramón medrano-pérez* conacyt ̶ center for global change and sustainability, a.c. (ccgs). calle centenario instituto juarez s/n, col. reforma, c.p. 86080, villahermosa, tabasco, mexico. *corresponding author: ojilve.medrano@ccgs.mx m et eo ro lo g y https://doi.org/10.15446/esrj.v25n1.87255 https://doi.org/10.15446/esrj.v25n1.87255 https://doi.org/10.15446/esrj.v25n1.87255 mailto:ojilve.medrano@ccgs.mx 70 mercedes andrade-velázquez, ojilve ramón medrano-pérez introduction climate change is considered the most significant threat of the 21st century, with implications for natural, environmental, social, and economic spheres (un environment, 2019; mora et al., 2018). the adverse effects of climate change and variability can affect human health, ecosystems, water availability, infrastructure, agriculture, and other socio-economic sectors (ipcc, 2018; mora et al., 2018). climate change represents an additional variable that, together with the growing demand for resources, changes in land use, and pollution, triggers environmental disturbances in an interconnected nature. however, mateos et al. (2016) reported that climate change would not affect everyone in the same way since developing countries and the most vulnerable social groups will be the most affected. similarly, jessoe et al. (2018) indicated that the costs of climate change would be more acute in developing countries, such as mesoamerica and the caribbean, due to the inability for response and adaptation by governments and communities (gonzález et al., 2017; muñozjiménez et al., 2019). water resource management is predicted to become one of the most significant challenges of climate change worldwide (magaña et al., 2018), and the availability of water resources is a primary social concern, particularly for underdeveloped countries that already have limited access to clean water (martínez-austria & patiño-gómez, 2012). water availability is variable due to precipitation, droughts, and aquifer depletion, which occur at different scales in urban and rural areas (duranencalada et al., 2017). climate change is expected to cause temperature increases and alterations in precipitation patterns, which will impact water resources (kristvik et al., 2019) through more frequent and prolonged droughts, more severe floods, and reduced water quality (ortiz-partida et al., 2020). several studies have reported the impacts of climate change on the hydrologic cycle and water resource availability, e.g., duran-encalada et al. (2017), pokhrel et al. (2018), kristvik et al. (2019), and koutroulis et al. (2019). according to koutroulis et al. (2019), the quantification of climate change impacts is challenging due to the complex interdependence between the associated physical and socio-economic systems. consequently, hydrologic models and global climate models (gcm) have been developed to estimate and analyze climate change impacts on water resources. however, despite the level of detail of the gcm, it is considering that these models have limitations due to the coarse spatial and temporal resolution of the data, requiring further downscaling to be applied in local-scale studies. the representative concentration pathways (rcps) are the most modern group of currently used scenarios and include four scenarios: rcp 2.6, rcp 4.5, rcp 6.0, and rcp 8.5 (kristvik et al., 2019). the intergovernmental panel on climate change (ipcc) in audefroy (2015) refers to the following changes at the regional level: “1) warm extremes, heat waves, and heavy precipitation are likely to continue to be more frequent; 2) tropical cyclones are likely to become more intense; there is less confidence in projections of an overall decrease in the number of tropical cyclones; and 3) precipitation is likely to increase at high latitudes, while it is likely to decrease in most subtropical land regions, continuing the patterns observed in recent trends” (audefroy 2015, p. 251). in mexico, it is expected that these changes will be manifested of the way different. in this sense, a decrease in precipitation in northern and central zones is expected, and a precipitation increase in the southeast (audefroy, 2015). according to the mexican national water commission (conagua), the southeast has the most significant rainfall records, runoff, and spatiotemporal water availability, while the north-center has the largest population most economic dynamism (conagua, 2018). thus, given the ipcc’s climate change projections in addition to the climatic contrast in mexico, pressure on existing water resources is expected to become more acute. mexico is considered one of the countries with the highest vulnerability to the adverse effects of climate change, with 15% of the national territory, 68% of the population, and 71% of the gdp highly exposed to the risks of climate change (molina et al., 2017). according to ortiz-partida et al. (2020), the decrease in water availability associated with climate change in mexico puts the human right to water and water for industry and agriculture at risk. climate variability in the country is related to natural events, such as floods, droughts, or tropical storms, that cause adverse effects on agriculture, hydroelectricity, water availability, and other socio-economic losses throughout the territory (landa et al., 2008; magaña et al., 2001). in particular, climatic events have been reported in different southeast mexico areas since this region has historically registered a high vulnerability to extreme hydrometeorological events. climatic events associated with climate change and variability, such as floods, represent one of the relevant problems in southern mexico’s transboundary basins (garcía and kauffer, 2011; kauffer, 2006; arreguín-cortés et al., 2014). gama et al. (2010) and valdés-manzanilla (2016, 2018) discuss the extent of the adverse effects of flooding events in the states of tabasco, chiapas, and veracruz, while audefroy (2015) addresses the potential effects of climate change in the states of oaxaca, tabasco, and yucatan. the institutional and socio-economic response capacity to these phenomena is limited in the region (cafs, 2018); this area’s challenges are shared to a greater or lesser extent by all southsoutheast mexico states. the south-southeast region (ssr), comprising the states of campeche, chiapas, guerrero, oaxaca, puebla, quintana roo, tabasco, veracruz, and yucatan, is characterized by abundant natural resources and marked socioeconomic contrast. natural disasters due to regional characteristics and the effects of climate change are noteworthy (dof, 2014). however, climate data in mexico, particularly in tropical areas, are scarce, and their quantitative and qualitative availability has resulted in very few studies on the trends of climate change in the country (duncan-golicher & morales, 2005; montero-martínez et al., 2018). the lack of data represents a challenge for research since rainfall is key to understanding climate. precipitation in the ssr during summer is influenced by tropical cyclones, eastern waves, and the intertropical convergence zone, while in winter, cold fronts are the central systems that produce these precipitation events (andrade-velázquez, 2017). similarly, the regional climate modulators are the el niño-southern oscillation (enso), the pacific decadal oscillation (pdo), and the atlantic multidecadal oscillation (amo) (álvarez-olguin & escalante-sandoval, 2016). besides, these phenomena have effects on a global scale (meng et al., 2019). méndez-gonzález et al. (2011) reported the influence of enso and amo for mexico, while méndez & magaña (2010) reported the wet-dry dipole observed in mexico due to the combination of the amo and pdo oscillations. both studies showed that during the warm phase of enso and pdo, dry periods were observed in southeast mexico, and wet periods occurred during the cold phase. the reverse is right in the south, so the cold phase is responsible for humidity, while the warm phase is responsible for dryness. the standardized precipitation index (spi) can quantify wet and dry periods (santana et al., 2017) and characterize it. the spi was developed by mckee et al. (1993) to classify observed rainfall as a standardized output relative to a rainfall probability distribution function. it was first adopted by guttman (1999), and since then, the spi has been widely used due to the practicality of the method, the small amount of data it requires, and the recognition of the world meteorological organization (wmo) (sisto et al., 2016). the spi is considered an effective means for the detection of exceptionally dry or wet periods in historical precipitation data (stagge et al., 2017), allowing for the comparison of precipitation behavior over different regions and time scales, from seasonal to annual and multiannual time frames (esquivel-arriaga et al., 2019). however, the spi does not take into account precipitation intensity and its potential impacts on runoff, water flow, and availability. precipitation is recognized as a crucial element for water balance over space and time (mujere & eslamian, 2014). therefore, the analysis and understanding of precipitation patterns at the basin level is necessary for adequate water resources management. according to bates et al. (2008), observation records and climate projections provide clear evidence of the vulnerability of water resources and the potential impacts of climate change, which have implications for human societies and ecosystems. thus, the objective of this manuscript is to explore and analyze current and future precipitation patterns in the ssr of mexico by using the spi for three climate change scenarios, rcp 4.5, rcp 6.0, and rcp 8.5, from 1960-2016 (historical), 2015-2039 (near future), and 2075-2099 (far future). this study aims to identify the areas that are most prone to changes in current and future rainfall patterns and seeks to contribute to the improvement of water resource management plans for adaptation and mitigation of the effects of climate change on water availability. 71historical precipitation patterns in the south-southeast region of mexico and future projections study area the ssr of mexico covers an area of 501,448.17 km2 and is made up of the states of campeche, chiapas, guerrero, oaxaca, puebla, quintana roo, tabasco, veracruz, and yucatan (figure 1). this region comprises 25.6% of mexican territory and is part of the tri-national border 1,149 km long with guatemala and belize. administratively, the ssr has 1,372 municipalities and cities such as cancún, ciudad de carmen, mérida, puebla, villahermosa, and xalapa, among others (dof, 2014). the region is characterized by complex physiography, a predominantly warm climate, and invaluable natural and hydric wealth. the ssr physiography includes plains such as the yucatan peninsula and the coastal plains of the northern and southern gulf and mountainous areas formed by part of the sierra madre del sur, sierra de chiapas, the central american cordillera. this region also has coastlines on the pacific, gulf, and caribbean sea that reach more than 4,217 km (dof, 2014). the region is also part of the mesoamerican biological corridor, considered one of the world’s five most biodiverse regions. the ssr has a historical average rainfall of 2,4012,800 mm/year, although, in some areas, 4,000 mm/year have been recorded. consequently, 19 of the 50 main rivers in the country coexist in this region where the most considerable runoff of 37% of the national total is recorded, figure 1. location of the study area and water wealth. source. prepared in arcgis pro with spatial information from the national institute of statistics and geography (inegi) and arcgis pro: map services. with the largest being the grijalva, usumacinta, papaloapan, pánuco, and coatzacoalcos rivers (figure 1) (dof, 2014). in socio-economic terms, the ssr has over 31 million inhabitants, 28% of the national total, who are mainly engaged in agriculture, livestock, and fishing, although the energy sector and tourism are also valuable assets in this region (dof, 2014). economic activities comprise an estimated 18% of the national gdp. nevertheless, the region is significantly underdeveloped with high poverty rates, unlike the central-northern region. materials and methods this study’s precipitation data is high-spatial-resolution data (0.5º x 0.5º) from the monthly gridded climatic research unit time series (version 4.01). the high-resolution gridded data includes month-by-month variations in climate during the period between jan. 1901-dec. 2016 (university of east anglia climatic research unit et al., 2017). the advantage of the cru data over the observation data is to provide information overall study time records with highquality control in the regions with a lack of data. spatial interpolation from weather station observations is used to achieve spatially complete data, with the number of nearby stations used as an indicator of data reliability (harris et al., 2014). cru data had been improved for climate change analyses (ipcc, 2013). 72 mercedes andrade-velázquez, ojilve ramón medrano-pérez the primary study points were selected based on two criteria: a) historical precipitation patterns from 1960-2016, where the climatology of the area registered the most significant change (figure 2), and b) the proximity to bodies of water, as shown in figure 3 and table 1, respectively. the precipitation patterns are congruent with previous studies (alvarez-olguin & escalante-sandoval, 2016) that showed higher precipitation for the southeast region than the entire southern region. méndez-gonzález et al. (2008) described the precipitation regimes in the study area in three regions, the yucatan peninsula, the south pacific, and the southeast gulf of mexico, where annual precipitation in each region is 1,055 mm/year, 1,007 mm/year, and 1,780 mm/year, respectively. 70% of the country’s precipitation is recorded from may to october and is dependent on latitude, proximity to the oceans, and enso phenomena. data for the selected points were extracted with bilinear interpolation to cover the ssr’s main and tributary rivers. figure 2. precipitation climatology of the study area from 1960-2016. the units are in mm/day. note. * key study points. to understand the precipitation patterns in the ssr, the spi was used with a 12-month time window from 1960-2016 to determine the behavior of wet and dry periods annually; this time-window is in agree with the main climate driver timely. the same method was also used to obtain the projected spi for 20152039 and 2075-2099 under rcp 4.5, rcp 6.0, and rcp 8.5 scenarios. the reliability ensemble averaging (rea) method (giorgi & mearns, 2002) was used for scale reduction in mexico (cavazos et al., 2013), apply to the models shown in table 2. it should be noted that the first period for the projection contained runs 2015, given that 2015-2039 is also part of the near future due to the original scenarios data. table 1. study points and nearby river points state nearby river 1 chiapas río jatate 2 chiapas ríos lacantum, ixcan y chixoy 3 chiapas ríos usumacinta, lacantum, san juan y chixoy 4 chiapas río grijalva 5 chiapas ríos grijalva y suchiapa 6 chiapas ríos sayula y mezcalapa 7 tabasco ríos usumacinta y grijalva 8 campeche ríos candelaria y caribe 9 campeche ríos champoton y desempeño 10 quintana roo ríos bravo, blue y hondo 11 yucatán río lagartos 12 yucatán península de yucatán 13 chiapas sierra madre de chiapas 14 tabasco río tonala 15 veracruz ríos coatzacualcos, uspanapa y nanchital 16 veracruz ríos papaloapan y san juan 17 veracruz río tecolutla 18 veracruz río pánuco 19 oaxaca ríos playa vicente y grande 20 oaxaca ríos colorado y atoyac 21 oaxaca río verde 22 oaxaca río tehuantepec 23 oaxaca ríos corte y sarabia 24 guerrero ríos balsas, bejucon, cutzamala, temascaltepec, y ixtapan 25 guerrero ríos balsas, acatlan, atoyan y nexapa 26 guerrero río omitlan 27 puebla río atoyac 28 puebla río acatlán y tonto 29 puebla río apulco 30 chiapas ríos usumacinta y san pedro figure 3. location of study points in the south-southeast region of mexico (arcgis pro: map services). source. prepared in arcgis pro with spatial information from the national institute of statistics and geography (inegi) and arcgis pro: map services. to determine the precipitation patterns in the area, the spi was used with values shown in table 3 and their classification, according to mckee et al. (1993) and omm (2012). the spi was calculated with a pearson distribution type iii (guttman, 1999) equation, as seen in (1) for 1960-2016, 2015-2039, and 2075-2099: f x a x a a exp x c a b ( )= −( ) − −( )      −1 1 γ (1) where a > 0, b > 0, corresponding to scale and shape parameters, 0 < c < x location parameter, and γ is the function gamma. 73historical precipitation patterns in the south-southeast region of mexico and future projections according to the national drought mitigation center (university of nebraska-lincoln, united states), the spi-12 reflects 12-month precipitation patterns. longer spis tend toward zero unless there is a specific trend. spis of these time scales are probably tied to streamflow, reservoir levels, and even groundwater levels at the longer time scales (ndmc, 2020). spi-12 values more significant than +1 and less than -1 for the three-time horizons were used for further analysis; to compare the results from spi-12 1960-2016 with those of the spi-12 projected by the climate change scenarios in the future periods (2015-2039 and 2075-2099). the percentage of spi-12 values greater than +1 and less than -1 for the historical, near, and far futures were obtained over the data records, as the history as both futures. this comparison provides the amount the spi-12 values for the dry and wet events in the data records to determine such events amount for the historical and both futures. a weighted comparison was then made according to the number of years in the future periods and the historical record. given that there are 57 years of historical records and 25 years of projections with the rcps for each future projection, we used a correction factor for that percentage calculated as a = 57/25 and proceeded to calculate the difference in percentages of wet and dry periods as p p p a prcps cru rcpsim cru = − * (2) where pcru is the percentage derived from the cru record, prcpsim is the percentage corresponding to the rcp scenarios (rcp 4.5, rcp 6.0, and rcp 8.5), and prcps is the weighted percentage. results and discussion spi-12 historical results from 1960-2016 figures 4 and 5 show the spi-12 for the 30 study points in the ssr of mexico. the grijalva and usumacinta basins comprising mainly the states of chiapas and tabasco, had dry events for two years: in 1965 for the grijalva basin and 1990 usumacinta basin. the grijalva region is classified as warm sub-humid, while the usumacinta region is classified as warm humid (inegi, 2018) and is where the highest rainfall is recorded (álvarez-olguín & escalante-sandoval, 2016). in both basins, wet periods were observed after the year 2000. according to the wmo, these periods are classified as extremely dry and very wet, respectively. however, extremely wet periods were observed after the year 2010 for most of the ssr, indicating which there has been more precipitation in recent years, particularly in the grijalvausumacinta basin and in the yucatan. other studies in the area have reported opposite trends in rainfall events within the same state (mendoza-uribe & vázquez-zavaleta, 2017; gonzález-villela & montero-martínez, 2018; márdero et al., 2012). in the southeast region, montero-martínez et al. (2018) reported that there was a positive trend in the lower part of the usumacinta basin. likewise, santana et al. (2017) showed that in the same region of the usumacinta, more wet events occurred from 1985-2008, but from 1961-1984, most of the extremely wet and dry events were found to be extremely dry. méndez-gonzález et al. (2008) reported that trends tended to be negative from 1920-2004 in the gulf of mexico and that precipitation increased in arid areas due to the influence of enso. méndez & magaña (2010) reported that the droughts registered from 1960-2000 correspond to the combination of positive phase pdo and weak signals of amo. the records from the present study show congruence with the results of montero-martínez et al. (2018), where the trends in the gulf of mexico and grijalva-usumacinta region are positive after 2000. however, this is due to the adjustment of the trend after the total period of 1920-2004 in the study by méndez-gonzález et al. (2008). andrade-velázquez & medranopérez (2020) reported that there was an inverse relationship between ensopdo and precipitation in southeast mexico during 1960-2016. table 3. spi values spi value* category >= 2.0 extremely wet 1.5 to 1.99 very wet 1.0 to 1.49 moderately wet -0.99 to 0.99 normal or near normal -1.0 to -1.49 moderately dry -1.5 to -1.99 severely dry <= -2.0 extremely dry source. omm (2012) note. *spi values < -1 indicate a condition of drought. the more negative the value is, the more severe the drought condition. spi values > 1 indicate wetter conditions compared to climatology. table 2. list of the 15 models from the coupled model intercomparison project (cmip5) no. model institute* model rcp 0 bcc bcc-csm1.1 4.5, 6.0, 8.5 1 cccma canesm2 4.5, 8.5 2 cnrm-cerfacs cnrm-cm5 4.5, 8.5 3 csiro-qccce csiro-mk3-6-0 4.5 4 noaa gfdl gfdl-cm3 6.0, 8.5 5 nasa giss giss-e2-r 4.5, 6.0, 8.5 6 mohc (inpe) hadgem2-en 4.5, 6.0, 8.5 7 inm inmcm4 4.5, 8.5 8 ipsl ipsl-cm5a-lr 4.5, 6.0, 8.5 9 miroca miroc5 4.5, 6.0, 8.5 10 mirocb miroc-esmchem 4.5, 6.0 11 miroc miroc-esm 4.5, 6.0, 8.5 12 mpi-m mpi-esm-lr 4.5, 8.5 13 mri mri-cgcm3 4.5, 8.5 14 ncc noresm1-m 4.5, 6.0, 8.5 source. elaboration based on cavazos et al. (2013) note. *bbc (beijing climate center, china meteorological administration); cccma (canadian centre for climate modelling and analysis); cnrmcerfacs (centre national de recherches meteorologiques/centre europeen de recherche et formation avancees en calcul scientifique); csiro-qccce (commonwealth scientific and industrial research organisation in collaboration with the queensland climate change centre of excellence); noaa gfdl (geophysical fluid dynamics laboratory); nasa giss (nasa goddard institute for space studies); mohc (inpe) (met office hadley centre (additional hadgem2-es realizations contributed by instituto nacional de pesquisas espaciais)); inm (institute for numerical mathematics); ipsl (institut pierresimon laplace); miroca (atmosphere and ocean research institute (the university of tokyo), national institute for environmental studies, and japan agency for marine-earth science and technology); mirocb (japan agency for marine-earth science and technology, atmosphere and ocean research institute (the university of tokyo), and national institute for environmental studies); mpi-m (max planck institute for meteorology); mri (meteorological research institute); ncc (norwegian climate centre). 74 mercedes andrade-velázquez, ojilve ramón medrano-pérez figure 4. spi-12 from 1960-2016 for study points 1-15 (see figure 3) of the south-southeast region of mexico. study points 1, 2, 3, 4, 5, 6 and 13 are brown; 7 and 14 are green; 8 and 9 are brown; 10 is purple; 11 and 12 are yellow, and 15 is turquoise. in the gulf of mexico, particularly the sites located in veracruz, more considerable spi-12 variability and extremely dry periods were observed in the southern zone of the state during 1983 and 1993, while the central zone only had an extremely dry period in 1983. however, in 1970 and 1982, very wet periods were recorded in the southern zone. meanwhile, in the central and northern zones, very wet periods were not observed until after 2010 when they also occurred in the grijalva and usumacinta basins. similarly, álvarez-olguín & escalante-sandoval (2016) reported an increasing trend in northern veracruz from 1950-2013. in the south pacific (oaxaca and guerrero) and the center of the country (puebla), the spi-12 was more variable with the years classified mainly as severely dry and wet. these periods were recorded throughout the entire study time frame (1960-2016). however, in puebla, there were also spi-12 values classified as extremely dry between 1980 and 1990. historically, the south pacific region was affected by dry periods before 2000, with dry and wet periods becoming more recurrent after that. álvarez-olguín & escalante-sandoval (2016) and méndez-gonzález et al. (2008) reported that in the south pacific, there is a tendency for a decrease in precipitation in similar time record (<2013 and <2004 respectively) associated to the enso and pdo. in the present study, the spi-12 data had mostly negative values indicating a precipitation deficit. this region is classified as a temperate subhumid (inegi, 2018). giddings et al. (2005) reported that the south pacific zone and the gulf of mexico tend to have similar spi-12 behavior, while grijalva-usumacinta and the yucatan peninsula fall into a different group based on the same spi-12. in the present study, it was 75historical precipitation patterns in the south-southeast region of mexico and future projections figure 5. spi-12 from 1960-2016 for study points 16-30 (see figure 3) of the south-southeast region of mexico. study points 16, 17, and 18 are turquoise; 19, 20, 21, 22, and 23 are red; 24, 25, and 26 are pink; 27, 28, and 29 are blue; and 30 is green. observed that the south pacific and the gulf of mexico do not necessarily have similar characteristics and precipitation patterns. from 1960-2000, the south pacific region and part of veracruz (gulf of mexico) had few signs of drought in the presence of positive pdo and weak amo. mendoza et al. (2006) reported that droughts in this zone that occurred before the present study period had been influenced by the positive phases of enso and pdo, while the yucatan peninsula had droughts in the presence of the cold phases of amo. spi-12 in rcp 4.5, rcp 6.0, and rcp 8.5 scenarios the spi-12 precipitation results of the climate change projections described by the rcp 4.5, rcp 6.0, and rcp 8.5 scenarios for 2015-2039 were also calculated. table 4 shows the percentage of wet and dry periods projected for that time frame. a comparison between the climatic research unit (cru) data from 1960-2016 to the projections from the three rcp scenarios was also made. no difference more significant than 10% was observed for 2015-2039, which suggests that future changes under these climate change projections would not be more significant than historical, i.e., the amount future dry and wet events could be similar to the amount of the historical events. this same exercise was carried out for 2075-2099, being found the same conditions in the differences between the cru data and the rcps. however, it was observed in the rcp 8.5 scenario that the highest increases in dry periods for 2075-2099 occurred in most study points, as shown in table 5. 76 mercedes andrade-velázquez, ojilve ramón medrano-pérez table 4. percentage of spi-12 values indicating wet (1 to 3) and dry (-3 to -1) periods for each study point corresponding to observations from 1960-2016 and rcp scenarios from 2015-2039. point 19602016 spi-12 (1 to 3) 2015-2039 19602016 spi-12 (-3 to -1) 2015-2039 cru (1 to 3) rcp 4.5 crurcp4.5 rcp 6.0 crurcp6.0 rcp 8.5 crurcp8.5 cru (-3 to-1) rcp 4.5 crurcp4.5 rcp 6.0 crurcp6.0 rcp 8.5 crurcp8.5 1 13.89 13.67 0.22 15.67 -1.78 13 0.89 14.04 17.33 -3.29 13.67 0.37 14.33 -0.29 2 15.64 16.33 -0.69 15 0.64 16.33 -0.69 13.89 13.67 0.22 13 0.89 15.67 -1.78 3 13.3 17.67 -4.36 17 -3.7 18.33 -5.03 14.18 13.33 0.85 15.67 -1.49 15 -0.82 4 17.54 13 4.54 15.67 1.88 11.67 5.88 16.96 20.67 -3.71 15.33 1.63 11 5.96 5 14.62 14.67 -0.05 17 -2.38 18 -3.38 12.28 15 -2.72 15.67 -3.39 14 -1.72 6 15.06 14.67 0.39 15 0.06 20 -4.94 11.7 17.67 -5.97 12.67 -0.97 12.33 -0.63 7 12.87 14.67 -1.8 19.33 -6.47 16.67 -3.8 13.16 13.67 -0.51 15 -1.84 10.33 2.83 8 14.47 17 -2.53 15.67 -1.19 14 0.47 15.5 13.67 1.83 13 2.5 15.67 -0.17 9 16.08 12.33 3.75 14 2.08 14.67 1.42 13.16 14.67 -1.51 15.67 -2.51 19.33 -6.17 10 14.47 15.33 -0.86 13.33 1.14 20.67 -6.19 16.96 12.67 4.29 14 2.96 15.33 1.63 11 14.18 14.67 -0.49 12.67 1.51 13.33 0.85 14.47 11.33 3.14 15 -0.53 16 -1.53 12 14.47 14.67 -0.19 17.67 -3.19 11.33 3.14 14.47 8.67 5.8 16 -1.53 15.67 -1.2 13 17.25 15 2.25 14.67 2.58 17 0.25 16.67 14.33 2.34 17.67 -1 15.33 1.34 14 12.87 15 -2.13 10 2.87 16.33 -3.47 12.72 15.33 -2.61 15.33 -2.61 16.33 -3.61 15 15.06 13.67 1.39 10.67 4.39 16 -0.94 13.6 16 -2.4 16.67 -3.07 15 -1.4 16 14.77 11.67 3.1 18 -3.23 13.67 1.1 11.99 16.67 -4.68 16.67 -4.68 12.67 -0.68 17 15.5 11.33 4.16 17.33 -1.84 17.33 -1.84 15.35 14.67 0.68 14.67 0.68 15.67 -0.32 18 13.6 14.67 -1.07 14.67 -1.07 16 -2.4 13.89 17.33 -3.44 16 -2.11 12.33 1.56 19 13.6 16 -2.4 17 -3.4 14.33 -0.74 13.3 14.33 -1.03 14.67 -1.37 14 -0.7 20 16.52 18.33 -1.81 17.67 -1.15 15.33 1.19 15.64 15 0.64 11 4.64 18.33 -2.69 21 14.04 21.33 -7.3 14 0.04 17.67 -3.63 15.64 15 0.64 13.67 1.97 15.33 0.31 22 14.47 15.67 -1.19 18 -3.53 15.33 -0.86 15.5 14.67 0.83 18 -2.5 14.33 1.17 23 16.52 15 1.52 14.67 1.85 20.33 -3.81 14.04 15 -0.96 21 -6.96 14.33 -0.29 24 17.11 18.67 -1.56 16 1.11 15 2.11 14.04 12 2.04 17.67 -3.63 11.33 2.71 25 15.5 13.33 2.16 14.33 1.16 15.67 -0.17 15.64 14.33 1.31 11.33 4.31 14.67 0.97 26 16.52 13.67 2.85 15 1.52 13.33 3.19 16.52 15.33 1.19 16.67 -0.15 14.67 1.85 27 14.62 15 -0.38 13 1.62 15.33 -0.71 16.08 16.33 -0.25 12.67 3.41 14.67 1.41 28 15.79 15.67 0.12 17.67 -1.88 11.33 4.46 13.74 16 -2.26 17 -3.26 12.67 1.07 29 15.35 17 -1.65 10.67 4.68 15 0.35 15.35 15 0.35 16.33 -0.98 11.67 3.68 30 12.43 15.33 -2.91 17 -4.57 17.33 -4.91 12.28 15.33 -3.05 13.67 -1.39 12.67 -0.39 77historical precipitation patterns in the south-southeast region of mexico and future projections table 5. percentage of spi-12 values indicating wet (1 to 3) and dry (-3 to -1) periods for each study point corresponding to observations from 1960-2016 and rcp scenarios from 2075-2099. point 19602016 spi-12 (1 to 3) 2075-2099 19602016 spi-12 (-3 to -1) 2075-2099 cru (1 to 3) rcp 4.5 crurcp4.5 rcp 6.0 crurcp6.0 rcp 8.5 crurcp8.5 cru rcp 4.5 crurcp4.5 rcp 6.0 crurcp6.0 rcp 8.5 crurcp8.5 1 13.89 14.67 -0.78 17.01 -3.13 17.79 -3.9 14.04 14.67 -0.63 13.54 0.49 17.33 -3.2982 2 15.64 11.67 3.98 15.28 0.37 13.67 1.98 13.89 16.33 -2.44 14.24 -0.35 14.67 -0.7778 3 13.3 10.67 2.64 15.97 -2.67 12 1.3 14.18 16 -1.82 11.46 2.72 16.67 -2.4854 4 17.54 15.67 1.88 15.97 1.57 14.33 3.21 16.96 17.67 -0.71 18.06 -1.1 18 -1.0409 5 14.62 16.33 -1.71 10.76 3.86 19.33 -4.71 12.28 14.33 -2.05 13.19 -0.91 15.33 -3.0526 6 15.06 15.67 -0.61 12.85 2.21 17.33 -2.27 11.7 15.67 -3.97 16.32 -4.62 14.33 -2.6374 7 12.87 17 -4.13 13.54 -0.68 18.67 -5.8 13.16 14.33 -1.18 15.28 -2.12 15 -1.8421 8 14.47 16 -1.53 13.54 0.93 17.67 -3.19 15.5 14.33 1.16 11.46 4.04 15 0.4971 9 16.08 16 0.08 14.93 1.15 15 1.08 13.16 11.33 1.82 13.19 -0.04 16 -2.8421 10 14.47 16.67 -2.19 11.81 2.67 18.33 -3.86 16.96 13.67 3.29 14.93 2.03 18 -1.0409 11 14.18 12.67 1.51 12.85 1.33 15.67 -1.49 14.47 11 3.47 14.93 -0.46 17.33 -2.8596 12 14.47 18.67 -4.19 13.19 1.28 15 -0.53 14.47 16.67 -2.19 15.97 -1.5 19.33 -4.8596 13 17.25 17 0.25 12.85 4.4 12.33 4.92 16.67 15.33 1.33 18.75 -2.08 15.67 1 14 12.87 16 -3.13 13.19 -0.33 16 -3.13 12.72 15.67 -2.95 15.28 -2.56 13.33 -0.614 15 15.06 16.33 -1.27 10.76 4.29 14 1.06 13.6 14.33 -0.74 15.97 -2.38 14 -0.4035 16 14.77 14.33 0.43 9.38 5.39 9.67 5.1 11.99 9.33 2.66 17.01 -5.03 16 -4.0117 17 15.5 17.67 -2.17 10.42 5.08 11 4.5 15.35 13.33 2.02 12.5 2.85 11 4.3509 18 13.6 16.33 -2.74 15.63 -2.03 11.67 1.93 13.89 12 1.89 12.85 1.04 10.67 3.2222 19 13.6 17.33 -3.74 10.07 3.53 16.67 -3.07 13.3 10 3.3 15.28 -1.97 15 -1.6959 20 16.52 16.33 0.19 13.89 2.63 12.67 3.85 15.64 14.67 0.98 15.28 0.37 14.33 1.31 21 14.04 20 -5.96 12.15 1.88 13.33 0.7 15.64 17.67 -2.02 13.89 1.75 17.67 -2.0234 22 14.47 17 -2.53 12.85 1.63 15.67 -1.19 15.5 15.33 0.16 16.67 -1.17 12 3.4971 23 16.52 17 -0.48 12.85 3.67 16 0.52 14.04 9 5.04 13.89 0.15 14.67 -0.6316 24 17.11 15 2.11 7.64 9.47 18.33 -1.23 14.04 14 0.04 15.97 -1.94 15.33 -1.2982 25 15.5 16.33 -0.84 17.71 -2.21 16.67 -1.17 15.64 16.33 -0.69 6.25 9.39 15 0.6433 26 16.52 14.33 2.19 11.46 5.06 16 0.52 16.52 13 3.52 15.28 1.24 20 -3.4795 27 14.62 13.33 1.29 17.71 -3.09 15.67 -1.05 16.08 17.33 -1.25 17.36 -1.28 16.33 -0.2514 28 15.79 15.33 0.46 18.4 -2.61 12.33 3.46 13.74 15.67 -1.92 15.63 -1.88 13.33 0.4094 29 15.35 17.67 -2.32 14.93 0.42 12.33 3.02 15.35 16 -0.65 7.29 8.06 16 -0.6491 30 12.43 14 -1.57 5.56 6.87 21 -8.57 12.28 9.33 2.95 15.28 -3 14.67 -2.386 78 mercedes andrade-velázquez, ojilve ramón medrano-pérez ta bl e 6 . p er ce nt ag e d iff er en ce o f s pi -1 2 va lu es in di ca tin g w et (1 to 3 ) a nd d ry p er io ds (3 to -1 ) f or ea ch st ud y po in t c or re sp on di ng to o bs er va tio ns fr om 1 96 020 16 an d th e t hr ee r cp sc en ar io s. sp i12 (1 t o 3 ) 2 01 520 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 r c p 4. 5 -1 .2 4 -1 .3 8 -2 .0 3 -0 .6 9 -1 .2 9 -1 .2 2 -1 .6 -1 .6 8 -0 .7 5 -1 .4 2 -1 .3 6 -1 .3 1 -0 .9 8 -1 .6 6 -1 .0 7 -0 .8 -0 .6 7 -1 .4 6 -1 .6 8 -1 .5 3 -2 .4 6 -1 .4 7 -1 .0 7 -1 .4 9 -0 .9 6 -0 .8 9 -1 .3 4 -1 .2 6 -1 .5 3 -1 .8 1 r c p 6. 0 -1 .5 7 -1 .1 9 -1 .9 1 -1 .0 4 -1 .6 5 -1 .2 7 -2 .4 2 -1 .4 7 -0 .9 9 -1 .1 -1 .0 4 -1 .7 8 -0 .9 4 -0 .7 7 -0 .6 2 -1 .7 8 -1 .5 5 -1 .4 6 -1 .8 5 -1 .4 4 -1 .2 7 -1 .8 4 -1 .0 2 -1 .1 3 -1 .1 1 -1 .0 7 -1 .0 3 -1 .5 5 -0 .5 8 -2 .1 2 r c p 8. 5 -1 .1 3 -1 .3 8 -2 .1 4 -0 .5 2 -1 .8 1 -2 .0 3 -1 .9 5 -1 .2 1 -1 .0 8 -2 .2 6 -1 .1 4 -0 .7 9 -1 .2 5 -1 .8 9 -1 .4 2 -1 .1 1 -1 .5 5 -1 .6 8 -1 .4 -1 .1 2 -1 .8 7 -1 .4 2 -1 .8 1 -1 -1 .3 1 -0 .8 4 -1 .3 9 -0 .6 4 -1 .2 3 -2 .1 8 sp i12 (3 to -1 ) 2 01 520 39 r c p 4. 5 -1 .8 1 -1 .2 4 -1 .1 4 -1 .7 8 -1 .7 9 -2 .4 4 -1 .3 7 -1 .0 1 -1 .5 4 -0 .7 -0 .7 9 -0 .3 7 -0 .9 6 -1 .7 5 -1 .6 8 -2 .1 7 -1 .1 8 -1 .8 4 -1 .4 6 -1 .1 9 -1 .1 9 -1 .1 6 -1 .4 4 -0 .9 5 -1 .0 9 -1 .1 2 -1 .3 2 -1 .6 6 -1 .2 3 -1 .8 5 r c p 6. 0 -1 .2 2 -1 .1 3 -1 .5 2 -1 .0 6 -1 .9 1 -1 .4 7 -1 .6 -0 .9 1 -1 .7 1 -0 .8 8 -1 .3 6 -1 .5 2 -1 .4 2 -1 .7 5 -1 .7 9 -2 .1 7 -1 .1 8 -1 .6 3 -1 .5 1 -0 .6 -0 .9 9 -1 .6 5 -2 .4 1 -1 .8 7 -0 .6 5 -1 .3 -0 .8 -1 .8 2 -1 .4 3 -1 .5 4 r c p 8. 5 -1 .3 3 -1 .5 7 -1 .4 1 -0 .4 8 -1 .6 -1 .4 -0 .7 9 -1 .3 1 -2 .3 5 -1 .0 6 -1 .5 2 -1 .4 7 -1 .1 -1 .9 3 -1 .5 1 -1 .4 1 -1 .3 3 -1 .0 2 -1 .4 -1 .6 7 -1 .2 3 -1 .1 1 -0 .1 33 -0 .8 4 -1 .1 4 -1 .0 2 -1 .0 8 -1 .1 -0 .7 3 -1 .3 5 sp i12 (1 t o 3 ) 2 07 520 99 r c p 4. 5 -1 .4 1 -0 .7 -0 .8 3 -1 .0 4 -1 .5 5 -1 .3 7 -2 .0 1 -1 .5 2 -1 .2 7 -1 .6 3 -1 .0 4 -1 .9 4 -1 .2 5 -1 .8 3 -1 .4 7 -1 .2 1 -1 .6 -1 .7 4 -1 .9 1 -1 .2 5 -2 .2 5 -1 .6 8 -1 .3 5 -1 -1 .4 -0 .9 8 -1 .0 8 -1 .2 1 -1 .6 2 -1 .5 7 r c p 6. 0 -1 .7 9 -1 .2 3 -1 .7 4 -1 .0 8 -0 .6 8 -0 .9 5 -1 .4 -1 .1 3 -1 .1 2 -0 .8 6 -1 .0 7 -1 .0 8 -0 .7 -1 .3 4 -0 .6 3 -0 .4 5 -0 .5 3 -1 .6 2 -0 .6 9 -0 .9 2 -0 .9 7 -1 .0 2 -0 .7 7 -0 .0 2 -1 .6 1 -0 .5 8 -1 .7 6 -1 .6 6 -1 .2 2 -0 .0 2 r c p 8. 5 -1 .9 2 -0 .9 9 -1 .0 6 -0 .8 6 -2 .0 1 -1 .6 2 -2 .3 1 -1 .7 8 -1 .1 3 -1 .8 9 -1 .5 2 -1 .3 6 -0 .6 3 -1 .8 3 -1 .1 2 -0 .4 9 -0 .6 2 -0 .9 6 -1 .7 9 -0 .7 5 -1 .1 6 -1 .4 7 -1 .2 1 -1 .4 4 -1 .4 5 -1 .2 1 -1 .4 4 -0 .7 8 -0 .8 3 -2 .8 5 sp i12 (3 to -1 ) 2 07 520 99 r c p 4. 5 -1 .3 8 -1 .6 8 -1 .5 7 -1 .3 8 -1 .6 6 -2 .0 5 -1 .4 8 -1 .1 1 -0 .9 6 -0 .8 4 -0 .7 3 -1 .6 3 -1 .1 -1 .8 1 -1 .4 -0 .7 7 -0 .9 8 -0 .9 7 -0 .7 1 -1 .1 4 -1 .5 8 -1 .2 5 -0 .4 6 -1 .2 7 -1 .3 8 -0 .7 9 -1 .4 6 -1 .6 -1 .3 8 -0 .7 3 r c p 6. 0 -1 .2 -1 .3 4 -0 .8 4 -1 .4 3 -1 .4 5 -2 .1 8 -1 .6 5 -0 .6 9 -1 .2 9 -1 .0 1 -1 .3 5 -1 .5 2 -1 .5 6 -1 .7 4 -1 .6 8 -2 .2 3 -0 .8 6 -1 .1 1 -1 .6 2 -1 .2 3 -1 .0 2 -1 .4 5 -1 .2 6 -1 .5 9 0. 09 -1 .1 1 -1 .4 6 -1 .5 9 -0 .0 8 -1 .8 4 r c p 8. 5 -1 .8 1 -1 .4 1 -1 .6 8 -1 .4 2 -1 .8 5 -1 .7 9 -1 .6 -1 .2 1 -1 .7 7 -1 .4 2 -1 .7 3 -2 .0 5 -1 .1 4 -1 .3 9 -1 .3 5 -2 .0 4 -0 .6 3 -0 .7 5 -1 .5 7 -1 .0 9 -1 .5 8 -0 .7 7 -1 .3 8 -1 .4 9 -1 .1 9 -1 .7 6 -1 .3 2 -1 .2 1 -1 .3 8 -1 .7 2 79historical precipitation patterns in the south-southeast region of mexico and future projections in order to compare 57 years of historical records with 25 years of rcp projections for each future time frame, a correction factor was used for the percentage obtained for the wet (spi-12 values 1 to 3) and dry periods (spi-12 values -3 to -1) during 2015-2039 and 2075-2099. table 6 shows the correction factor results. these results allow us to determine where there will be significant (>10%) differences at each study point for future time frames over the historical period. all three scenarios for the two future periods indicate that both wet and dry events at all study points will increase concerning the historical period. during 2015-2039, wet periods were detected more than 1.5 times in frequency for all three scenarios at study points 3, 7, and 30. however, these study points do not show the same behavior for dry events. likewise, study points 5, 14, and 17 showed the same increase in wet events, but only for two rcp scenarios. these 6 points are in chiapas, tabasco, and veracruz. additionally, other study points with similar dry event increments in all three scenarios are 15, located in veracruz, and 9, located in campeche. study points 16 (veracruz), 18 (veracruz), and 28 (puebla) demonstrated an increase in dry events in two of the three scenarios. the study points that had 1.5 times more frequency in only one scenario for wet events were 6, 8, 10, 12, 16, 18, 20, 21, 22, 23, and 28, located in chiapas, yucatan peninsula, veracruz, south pacific, and puebla, and for dry events, the study points 1, 2, 3, 4, 6, 7, 11, 12, 19, 20, 22, 23, and 24, located in chiapas, yucatan peninsula, and south pacific. according to ipcc reports, all three scenarios demonstrating similar percentages of wet or dry events indicate the highest probability that the event will occur. for two of the three scenarios, there is a medium probability of occurrence, and if only one scenario projects this behavior, there is a low probability that the event will occur. for 2075-2099, no single point was 1.5 times more likely to occur than the historical record in all three scenarios for wet events. in contrast, study points 6, 12, and 14, located in chiapas, yucatan, and tabasco, respectively, are 1.5 times more likely to have a dry event. the study points that showed this behavior in two scenarios for wet events were 1 (chiapas), 7 (tabasco), 10 (quintana roo), 14 (tabasco), 18 (veracruz), 19 (oaxaca), and 30 (tabasco), while study points 3, 5, 7, 16, 19, 21, 28, and 30, located in chiapas, tabasco, veracruz, oaxaca, and puebla exhibited this behavior for dry events. the study points that demonstrated this occurrence is only one scenario for wet events are 3, 5, 6, 11, 12, 17, 22, 25, 27, 28, and 29. a few of these points were located in chiapas, yucatan, veracruz, and south pacific, but the majority of these points were located in puebla. the occurrence of dry events in only one scenario was seen in the study points 1, 2, 9, 11, 13, 15, 24, and 26, located in chiapas, campeche, veracruz, and guerrero. the study points that show increases in the percentage of dry and wet events for any climate change scenario projection, with the three scenarios are those in the grijalva and usumacinta basins in 2015-2039. this study suggests that it is highly likely that wet (spi-12: 1 to 3) and dry (spi-12: -1 to -3) events will continue to occur in this region and occur to a greater extent. likewise, the study points in the states of veracruz and puebla have a moderate probability of this behavior. the rest of the study area has a low probability of dry or wet events occurring at or above 1.5 times the historical rate. by 20752099, the grijalva and usumacinta basins, in addition to the yucatan, have a high probability of dry events under climate change. grijalva and usumacinta, veracruz, oaxaca, quintana roo, and puebla were found to have a moderate probability of increased dry and wet events. the rest of the study area had only a low probability of wet and dry events occurring 1.5 times more frequently. spatial distribution of spi-12 to determine the spatial behavior of the spi-12 during the historical period (1960-2016) and the climate change projections using scenarios rcp 4.5, rcp 6.0, and rcp 8.5 from 2015-2039 and 2075-2099 in the ssr, temporal-latitudinal and temporal-longitudinal graphs were made (figure 6 and 7). it was observed that historically, the dry periods (spi-12 < -1) predominate for all latitudes from 1960 to 1990, except in 1975 and 1982, where they were only predominant for medium and high latitudes of the study area. after the 1990s, the events become wet (spi-12 > 1) at latitudes. similar behavior was seen in longitude charts for the western region of the study area, while the eastern region had more considerable wet periods or little change, except during 1960-1975. these observations exemplify the behavior of the study figure 6. spatio-temporal plots of historical records (climatic research unit (cru): 1960-2016) and projections of rcp 4.5, rcp 6.0, and rcp 8.5 scenarios for spi-12 during the period 2015-2039. the left column represents latitude vs. time and the right column is longitude vs. time. zone, where the states of veracruz, tabasco, chiapas, and the yucatan peninsula are characterized as mostly wet. in general, the study area shows both latitudinal and longitudinal same precipitation patterns from the 1960s to the 1980s. according to florescanomayet et al. (1980), several severe droughts were recorded in mexico from 1930 to 1977 in the years 1935, 1957, 1960, 1962, 1969, and 1977. these results show that the risk of drought in the south-southeast zone progresses from very low to medium probability, with medium probability occurring in the western regions including puebla, part of veracruz, and the yucatan peninsula, low probabilities in oaxaca, chiapas, campeche, and part of veracruz, and very low probabilities in tabasco and northern chiapas. the rcp 4.5 rcp 6.0 projections for the 2015-2039-time frame are like 2015 and 2016, which also appear in the historical data. the rcp 8.5 projection remains unchanged over latitude; however, rcp 4.5 and rcp 8.5 show similarity with the historical period over longitude, while rcp 6.0 does not. however, 80 mercedes andrade-velázquez, ojilve ramón medrano-pérez this does not condition the choice of a scenario with better performance. the scenario that shows greater latitudinal and longitudinal wet periods is rcp 4.5. this scenario has the lowest co2 concentrations of the three scenarios and will produce the least warming (ipcc, 2013). before 2030, wet periods will occur from south to north in the study zone and west to east, except for the most eastern region from 2022-2027, where the yucatan peninsula is located. in contrast, dry periods are predicted in the ssr after 2030. the drier periods, mainly in the eastern region of the study area, do not correspond to the most extreme scenario, rcp 8.5, for 2015-2039. rcp 6.0 has intermediate co2 concentrations as a radioactive forcer (ipcc, 2013), and it follows an evolutionary behavior of these factors together to rcp 8.5 until 2020. subsequently, rcp 6.0 scenario slows down its increase in both scenario characterization factors, curving for the period 2020-2039, and remains constant after this period. besides, rcp 8.5 maintains its behavior without many latitudinal changes, while longitudinally, dry periods occur mainly before 2020 for the western and eastern parts of the study area (south pacific). after 2032, the eastern part of the study area becomes dry again (southeast region). projections from 2015-2039 compared to projections from 2075-2099 show longer dry periods longitudinally, from west to east. during 2075-2099, rcp 4.5 projected wet periods until 2089, while the eastern zone was dry from 2079-2082. rcp 6.0 showed severely dry periods from west to east during 2075-2077, while rcp 8.5 was characterized by wet periods during this time frame. the results are comparable to wehner et al. (2011), who reported a decrease in precipitation in 2070 throughout the country under scenario a1b (ipcc, 2000). this scenario has co2eq concentrations of 720 ppm by the end of the century, comparable to rcp 6.0. for longitudes from -92° to -87°, the rcp 8.5 and rcp 6.0 scenarios demonstrated dry periods; however, they are more intense under rcp 8.5. by the end of the century, rcp 8.5 had more droughts followed by rcp 6.0 and rcp 4.5, while the latter showed the reverse behavior. latitudinally, 2039-2075 had less intense wet periods than in 20152039 under the rcp 4.5 scenario. in contrast, rcp 6.0 projected more intense wet periods, mainly during 2092-2095. rcp 8.5 demonstrated increases in wet and dry periods, with wet periods occurring before 2080 and dry periods occurring from north to south in 2089 in the study area. in summary, the projections for 2015-2039 showed dry periods occurring mostly in lowand mid-latitudes of the study area at eastern longitudes in all three scenarios. rcp 6.0 is the only scenario demonstrating these periods at western longitudes (south pacific and veracruz). by 2075-2099, dry periods were only projected at western longitudes by rcp 4.5. in contrast, rcp 6.0 showed dry periods at low latitudes and longitudes in the eastern study area (yucatan peninsula). rcp 8.5 projected dry periods in nearly the entire study area, except from 2075 to 2079, where the periods were wet. rcp 4.5 was the wettest scenario for the study area, while rcp 8.5 had more significant dry periods in both future time frames. rcp 6.0 was drier for 2015-2039 and wetter for 2075-2099. in tables 3 and 4, we show that the percentage of dry and wet periods was obtained for the local point in future time. maloney et al. (2014) reported decreasing precipitation in the summer season for mexico and central america under rcp projections. historically, there has been a reduction in precipitation during the summer associated with the mid-summer drought (magaña, 1999). the results demonstrate that historically, the ssr had higher rainfall in the southeast (grijalva-usumacinta and yucatan peninsula) compared to the south (pacific and gulf of mexico coasts). the spi-12 provides information on wet and dry events in the region and information on torrential flows, reservoir levels, and groundwater levels over larger time scales. by 1980, the southern states of puebla, oaxaca, and guerrero (south pacific and puebla) were experiencing extremely dry events. consequently, most of the 30 study points selected in the ssr had spi-12 values £ -2 from 1987-1988, which correspond to severe and extremely dry events. dry events are associated with el niño, which causes drought in the region (méndez-gonzález et al., 2011; méndezgonzález et al., 2008). in 2009, the southeast had extremely dry events, while the southern zone had humid events. this dipole was previously reported in méndez & magaña (2010). the inverse effect was observed in 2010, where the presence of la niña produced increased precipitation in the southeast. the spi-12 precipitation results from climate change scenarios in the ssr project; that there will be an increase in dry events. the climate change scenarios rcp 4.5, rcp 6.0, and rcp 8.5 indicate that dry events will be more frequent than wet events compared to what has historically occurred. similar results were reported by prieto-gonzález et al. (2011) for the country’s central zone, where the highest spi-12 values were negative for the future. the projection of dry and wet events in the climate change scenarios cannot entirely be attributed to enso since there is no consensus in the literature on future changes of enso and other oscillations (yen & kitman, 2007; coelho & goddard, 2009; maloney et al., 2014; chen et al., 2017). there is a significant increase in sea surface temperature under rcp 8.5 (maloney et al., 2014), whose effects are expected to impact the ssr’s regional climate. the rcp 6.0 scenario demonstrated similar patterns for future time frames than the historical data, while rcp 4.5 only showed it for 2075-2099. rcp 8.5 had the least similarity to historical patterns. however, climate change is not the only environmental strain that will impact water resources in this region. other factors, such as land cover/land-use changes, could affect the hydrologic cycle and the climate system (oki et al., 2013). according to peralta-carreta et al. (2019), the loss of tree cover in tropical figure 7. spatio-temporal plots of historical records (climatic research unit (cru): 1960-2016) and projections of rcp 4.5, rcp 6.0, and rcp 8.5 scenarios for spi-12 during the period 2075-2099. the left column represents latitude vs. time and the right column is longitude vs. time. 81historical precipitation patterns in the south-southeast region of mexico and future projections areas represents one of the most catastrophic events of the present, with effects on water and climate systems globally. the ssr is no exception. historically, the mexican southeast has had extreme environmental deterioration, including deforestation and changes in land cover and use (tudela, 1990; 2004; cairns et al., 2000; romo lópez, 2008), both at the basin and state level, due to multiple anthropic stressors (gallardo-cruz et al., 2019; peralta-carreta et al., 2019; dof, 2014; kolb & galicia-sarmiento, 2011). consequently, the adverse effects and disturbances associated with global climate change could include potential changes to the hydrologic cycle at local, regional, and global scales. thus, it is crucial to study and understand climate change issues, and their possible effects on the ssr’s hydro-climatological dynamics, particularly since just over a third of the country’s runoff occurs in the ssr. due to the risks of climate change for natural and human systems (ortiz-partida et al., 2020; un environment, 2019; mora et al., 2018), it is essential to conduct research that promotes joint action among scientists, decision-makers, and social sectors to strengthen the response capacity to potentially adverse effects. conclusion the spi-12 provides information on wet and dry events that have occurred in a region. this study presented the expected changes in precipitation in the ssr of mexico under three climate change scenarios. it was observed that shortly (2015-2039), there will be increases in the percentage of dry and wet events in all climate change scenarios in the grijalva and usumacinta basins (chiapas and tabasco). the data suggest that in this region, it is highly probable that wet (spi-12: 1 to 3) and dry (spi-12: -1 to -3) events will continue to occur, and that the frequency of events will increase by a factor of 1.5. grijalva-usumacinta, veracruz, and puebla have a medium probability of this occurrence. the rest of the ssr has a low probability of events occurring at or above a 1.5 more frequency. by 2075-2099, grijalva-usumacinta and the yucatan have a high probability of dry events due to climate change. it is moderately likely that there will be wet and dry events in grijalva-usumacinta (chiapas and tabasco), veracruz, oaxaca, quintana roo, and puebla. in the rest of the study area, we found a low probability of wet and dry events occurring 1.5 times more frequently. rcp 4.5 proved to be the wettest scenario for the study area, while rcp 8.5 had the driest events during both periods (2015-2039 and 2075-2099). rcp 6.0 was drier for 2015-2039 and wetter for 2075-2099. these results correspond with the previous results obtained in tables 3 and 4, where the percentage of dry and wet periods were gathered over time. changes in the projections of precipitation events under climate change scenarios in the ssr for the two future periods will recur 1.5 times more frequently than historical records, whereas the rcp 6.0 scenario demonstrated similarity to historical precipitation patterns. it was also found that the precipitation patterns are marked by the regional modulators, enso, pdo, and amo. however, climate change is not the only cause of strain on water resources. many anthropic stressors disturb natural and human systems in the study area. therefore, it is necessary to study the full scope of these stressors and the adverse effects of climate change. understanding the full strain on water resources is critical for decision-makers to design resilient public policies that favor strengthening adaptive capacity at all levels. acknowledgments the authors thank the center for global change and sustainability, a.c. (ccgs), for the facilities provided while conducting this research. besides, they thank the anonymous reviewers for their comments and recommendations for improving this research. the authors also thank the cátedras-conacyt program and the cátedra-ccgs project no. 945 “vulnerability, socioenvironmental and adaptation measures to climate change in southeast mexico” and no. 963 “towards sustainable water management in southeast mexico and adjacent areas of central america.” to mexican institute of water technology (imta, by acronym in spanish) by the rcps data support. and to academic programms of ccgs: “climate change” and “water”. additionally, we acknowledge the world climate research programme’s working group on coupled modelling, which is responsible for cmip, and we thank the climate modeling groups for the production and availability of their model output. for cmip, the u.s. department of energy’s program for climate model diagnosis and intercomparison provided coordinating support and led software infrastructure development in partnership with the global organization for earth system science portals. references álvarez-olguín, g., & escalante-sandoval, c. 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(2007). enso amplitude changes due to climate change projections in different coupled models. journal of climate, 20, 203–217. doi: https://doi.org/10.1175/jcli4001.1. https://doi.org/10.1007/s11069 https://doi.org/10.1007/s11069 https://doi.org/10.1175/2011jhm1351 https://doi.org/10.1175/jcli4001 __ddelink__5576_1213624624 __ddelink__1596_11801239291 keywords: mine transient electromagnetic method; finite difference time domain; metal support; numerical analysis palabras clave: método de electromagnetismo transitorio en minas; método electromagnético en el dominio del tiempo; soporte de metal support; análisis numérico how to cite item yaoning l., shucai l., maofei l., xinming l., weihong g. (2021). influence of metal roadway supports on transient electromagnetic detection in mines. earth sciences research journal, 25(1), 109-114. doi: https://doi.org/10.15446/ esrj.v25n1.80156 to study the influence of metal supports in roadways on the detection of mines using the transient electromagnetic method, authors treated metal supports including anchor nets as a thin metal layer. according to the finite differences principle, the characteristics of the full-space transient electromagnetic response under the thin metal layer’s influence were calculated using a non-uniform grid. the thin metal layer's presence slowed the electromagnetic field’s diffusion rate and hindered the overall diffusion. the transient electromagnetic response curve observed under the thin metal layer’s influence was higher than that without the supports. thicker metal layers resulted in higher early response values and slower decay rates. the decay rate increased as a function of time, gradually approaching that of the curve without metal supports. the simulation of the transient electromagnetic response to the model of water-containing lowresistance structures showed that the metal roadway support reduced the sensitivity of the transient electromagnetic method and weakened its response to low-resistance anomalies. abstract influence of metal roadway supports on transient electromagnetic detection in mines influencia de los soportes metálicos en la calzada para la detección de electromagnetismo transitorio en minas issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.80156 con el fin de estudiar la influencia de los soportes metálicos en las calzadas para detectar minas a través del método de electromagnetismo transitorio, los autores trataron estos soportes como capas finas de metal, incluidas las mallas de anclaje. de acuerdo con los principios de las diferencias finitas, las características de reacción del electromagnetismo transitorio bajo la incidencia de una capa fina de metal se calcularon con una cuadrícula no uniforme. la presencia de esta capa ralentizó el índice de difusión del campo electromagnético y dificultó la difusión en general. el resultado de la curva de reacción del electromagnetismo transitorio bajo la incidencia de la capa fina de metal fue mayor que en calzadas sin estos soportes. las capas más finas tienen valores de respuestas mayores e índices de deterioro menores. el índice de deterioro se incrementa como una función del tiempo, y se aproxima gradualmente a la curva de reacción de una calzada sin soportes. la simulación de la reacción del electromagnetismo transitorio en el modelo de estructuras de baja resistencia a contenidos de agua muestra que los soportes metálicos reducen la sensibilidad del método electromagnético transitorio y debilita su reacción a anomalías de baja resistencia. resumen record manuscript received: 06/06/2020 accepted for publication: 02/10/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 109-114 liu yaoning1, liu shucai2*, li maofei2, liu xinming3, guo weihong2 1institute of building intelligence, jiangsu vocational institute of architectural technology, xuzhou, jiangsu province 221116 china 2the school of resource and geoscience, china university of mine and technology, xuzhou, jiangsu province 221116 china. 3college of mining engineering, north china university of science and technology, tangshan, hebei province 063000 china. m in in g e ng in ee ri ng https://doi.org/10.15446/esrj.v25n1.80156 https://doi.org/10.15446/esrj.v25n1.80156 https://doi.org/10.15446/esrj.v25n1.80156 110 yaoning liu, shucai liu, maofei li, xinming liu, weihong guo introduction with increasing coal mining depth, the hydrogeological conditions of mines are becoming increasingly complex, and coalmine water inrush disasters are a serious threat to the safe operation of coal mines (wu, 2014). the mine transient electromagnetic method considers proximity to the target for exhibiting a strong signal response and high resolution, this is widely used in the prediction of goaf water, column collapse, and mine disasters (cheng et al, 2016; danielsen et al, 2003; huang et al, 2017; wang et al, 2017a; wang et al, 2017b). however, transmission and receiving devices used by the mine transient electromagnetic method are located in underground roadways and hence, the received signal is a superposition of various low-resistance electromagnetic induction signals within the detection range. thus, distinguishing water body information from these signals for practical applications is difficult (chang et al, 2016; su et al, 2017; xue & yue, 2017; zhou et al. 2014). therefore, studying the influence of metal supports in the roadway on mine transient electromagnetic detection is important for eliminating metal interference and determining the distribution range of water-containing anomalies. many researchers have conducted experimental studies on the mine transient electromagnetic method considering metal interferences (liu et al, 2016; jin & wang. 2018, qin & zhao.2019, zhou et al. 2014); however numerical calculations of these phenomena have rarely been conducted. the direct time-domain finite difference method can directly simulate the propagation and diffusion of electromagnetic waves and can be used to analyze transient electromagnetic field signals (yan et al, 2002; yue et al, 2008). according to the maxwell equations and finite-difference principle, the metal support in the roadway can be considered as a thin layer of metal, and the finite difference iterative equation of this thin metal layer can be derived. the transient electromagnetic field under the influence of the metal roadway support can also be derived based on the finite-difference equation. this simulation was performed to study the electromagnetic field diffusion characteristics under the influence of a thin metal layer and generate a full-space transient electromagnetic response curve under the influence of these thin metal layers with different thicknesses. in addition, the transient electromagnetic response of the water-containing low-resistance structural model under metal the influence of the metal layer was analyzed. basic principles principles of the finite-difference numerical simulation quasi-static maxwell’s equations ∇× ( )= ( ) ( ) ∇× ( )=− ( ) ∇⋅ ( )= ∇⋅ ( )= h r r e r e r h r h r e r , , , , , , t t t t t t t σ µ ∂ ∂ 0 00 (1) where, h r, t( ) is the magnetic field strength, e r, t( ) is the electric field strength, σ r( ) is the dielectric conductivity, and  is the magnetic permeability. a passive diffusion equation for the electric field can be obtained. ∇ ( )= ( ) ∂ ( ) ∂ 2e r r e r , , t t t µσ (2) under 2d conditions, two infinitely long wires with positive and negative currents were used as the emission source and placed along the y-axis. thus, the induced electric field has only a y component and the magnetic field strength has only x and z components. figure 1 shows the two-dimensional finite-difference meshing mode where point ei j, and its surrounding ei j1, , ei j1, , ei j, 1 , ei j, 1 constitute a five-point difference format (liu. 2014). figure 1. two-dimensional finite difference model. connecting the center points of the four small rectangles around ei j, forms a rectangle, abcd. at any time, equation 2 can be integrated into the rectangle abcd to obtain equation 3 as follows: [ ] ∂ ∂ + ∂ ∂ = ∂ ∂∫∫ ∫∫ 2 2 2 2 e x e z dxdz e t dxdz abcd abcd µσ (3) the difference equation of the diffusion equation can be obtained according to green’s formula and the central difference method. e r r e r r z z ei j n i j i j i j n i j z i j j j i j n , , , , , , ,[+ − += − + + + ∆ ∆ + 1 1 1 1 4 1 4 2 1 4 ∆∆ ∆ + + ∆ ∆ + ∆ ∆ + − + + − z z e r r x x e x x ej j i j n i j x i j i i i j n i i i 1 1 1 1 1 2 1 4, , , ,] [ ,, ]j n (4) where, ∆ = ∆ +∆+z z z j j j1 2 , ∆ = ∆ +∆+x x x i i i1 2 , ri j z i j j j t z z, , = ∆ ∆ ∆ +µσ 1 , ri j x i j i i t x x, , = ∆ ∆ ∆ +µσ 1 , r r r i j i j z i j x , , , = + 2 . finite-difference simulation of the metal sheet metal supports can affect electromagnetic wave propagation, in turn affecting the detection results (yu et al, 2008). for numerical simulations, local fine mesh or subdomain algorithms are generally used for thin lamellar bodies (elsherbeni & demir, 2006; sun et al. 2013). however, the net thickness of the metal anchor is much smaller than the grid size and its resistivity is much smaller than the surrounding resistivity of the earth. thus, it is difficult to perform numerical simulations using conventional methods. therefore, a finitedifference iterative formula applicable to the metal structure must be derived. yan and shi (2004) derived the finite-difference iterative formula for thin wires by considering railroad tracks as thin wires. in this study, we consider the metal support to be a thin layer of metal with a finite thickness. the relationship between the electromagnetic field in the thin metal layer and the surrounding electromagnetic field can be derived using the maxwell equation boundary conditions. figure 2 shows the finite difference model of a thin layer of metal along the x-z plane (liu. 2014). 111influence of metal roadway supports on transient electromagnetic detection in mines figure 2. finite difference model of a thin layer of metal along the x-z plane the integral form of the two rotation equations in equation 1 can be written in the form of a difference equation. − −( ) = − ( )+ −( ) ∆ µ dh i j dt e i j e i j z x j , , ,1 1 (5) − ( ) = − +( )+ ( ) ∆ + µ dh i j dt e i j e i j z x j , , ,1 1 (6) ∆ +∆ ∆ +∆ ( )= ( )− −( )( ) × ∆ +∆ + + + x x z z e i j h i j h i j x x i i j j x x i i 1 1 1 2 2 1 2 σ , , , ++ ( )− −( )( ) ∆ +∆ +h i j h i j z z z z j j, ,1 2 1 (7) where  is the average conductivity within the rectangle, abcd. because the conductivity of the thin metal layer is much higher than that of the surrounding medium, the conductivity of the earth medium in abcd can be neglected as follows: ∆ +∆ ∆ +∆ = ∆ +∆+ + +x x z z x x di i j j i i1 1 1 2 2 2 σ σ (8) where  is the conductivity of the metal and d is the thickness of the metal layer. for the metal support, d should have an equivalent thickness. because of the influence of the metal layer, the magnetic field strength-x component hx -increases on both sides of the metal layer, and the magnetic field strength-z component hz-is continuous. thus, the second term on the right side of equation 7 can be neglected and the equation can be written as follows: σ d e i j h i j h i j x x x x i i⋅ ( )= ( )− −( )( ) ∆ +∆ +, , , 1 2 1 (9) substituting equations 5 and 6 into equation 9 yields: ∆ ∆ ⋅ ( ) =∆ −( ) +∆ +( )− ∆ +∆( + + + z z d de i j dt z e i j z e i j z z j j j j j j 1 1 1 1 1 µσ , , , )) ( )e i j, (10) the time derivative can be written in the form of a central difference: ∂ ∂ = − ∆ + −e t e e t i j n i j n i j n , , , 1 1 2 (11) linear interpolation of ei j n , e e e i j n i j n i j n , , , = ++ −1 1 2 (12) by substituting equations 11 and 12 into equation 10, the difference equation of the thin metal layer in figure 2 can be obtained. e ae be cei j n i j n i j n i j n , , , , + − + −= + +1 1 1 1 (13) where ∆ = ∆ +∆ +z z zj j 1 2 , a t z z t z z d j j j = ∆ ∆ ∆ ∆ +∆ ∆ + + 2 2 1 1µσ , b t z z t z z d j j j = ∆ ∆ ∆ ∆ +∆ ∆ + 2 2 1µσ , c z z d z t z z d z t j j j j = ∆ ∆ − ∆ ∆ ∆ ∆ + ∆ ∆ + + 1 1 2 2 µσ µσ . for the metal layer along the x-y plane, the difference equation can be obtained as follows: e ae be cei j n i j n i j n i j n , , , , + − + −= + +1 1 1 1 (14) where ∆ = ∆ +∆ +x x xi i 1 2 , a t x x t x x d i i i = ∆ ∆ ∆ ∆ +∆ ∆ + + 2 2 1 1µσ , b t x x t x x d i i i = ∆ ∆ ∆ ∆ +∆ ∆ + 2 2 1µσ , c x x d x t x x d x t i i i i = ∆ ∆ − ∆ ∆ ∆ ∆ + ∆ ∆ + + 1 1 2 2 µσ µσ equations 13 and 14 are the finite difference iterative equations for the thin metal layers. numerical calculations transient electromagnetic field distribution under the influence of the metal layer according to the principle of finite difference, using equations 4, 13, and 14, the full-space transient electromagnetic field under the influence of the metal support was simulated using the absorbing boundary condition (yang & yue, 2009). figure 3 shows the 2d space model used herein. the resistivity of the model was 50ω⋅m with a roadway width of 20 m, the launching point was at the center of the roadway. the two long wire sources were placed along the y-axis, while the normal direction was specified along the x-axis, space was 2 m, the emission current was 10a, and the metal mesh was set to a diameter of 5 mm— commonly used for the equivalent metal layer—which has an equivalent thickness of 0.2 mm. figure 3. two-dimensional space model. 112 yaoning liu, shucai liu, maofei li, xinming liu, weihong guo figure 4 shows the transient electric field contour map as a function of time with and without the metal support, where the electric unit is v m , and the horizontal line in figure 4 represents the roadway boundary. figures 4a and 4b show the electric field contour maps with and without the metal support, respectively, at t = 0.64 µs. here, the transient field which appears as a ‘smoke ring’ did not spread to the roadway boundary, the metal support at the boundary of the roadway compressed the electric field periphery. thus, the metal support of the roadway affected the electric field distribution. figure 4c and d show the electric field contour maps with and without metal support, respectively, at t = 2.21 µs. here, the ‘smoke ring’ without support reached the boundary of the roadway, whereas when support was present, the electric field became distorted at the boundary of the roadway. the presence of a thin layer of metal reduced the rate of electric field diffusion, squeezing the “smoke ring” at the metal boundary of the roadway. figure 4e and f show the electric field contour maps with and without the metal support, respectively, at t = 7.87 µs. here, the “smoke ring” without the support crossed the boundary of the roadway, whereas, with the support present, the ‘smoke ring’ remained at the boundary of the roadway and did not shift forward to a significant degree. this is due to the low resistivity of the thin metal layer (˂10-6). thus, the transient field diffused very slowly (yu et al, 2008). therefore, it can be concluded that the roadway metal support will greatly reduce the diffusion speed of the transient field, inevitably affecting the actual detection results. figure 4. transient electric field contour map at different times with and without the metal support. according to the relationship between the vertical induced electromotive force and electric field in the receiving coil per unit area, the electric field can be converted into a magnetic field as a function of time as follows: ∂ ∂ =− ∂ ∂ b t e x z (15) figure 5 shows the transient electromagnetic response curve influenced by the different thicknesses of the metal support. the observation point was located at the launch point and the wire diameter of the metal support net was varied at 2, 5, and 7 mm with equivalent thicknesses of 0.03, 0.2, and 0.4 mm, respectively. from fig. 5 it is clear that the response value with the metal support is significantly higher than that without the support, with a maximum value that differed by three orders of magnitude. moreover, after 2 ms, increasing the equivalent thickness of the metal support layer resulted in higher response values. initially, the response in the presence of the metal support is slowed. however, over time the decay rate gradually increases and the curve moves gradually closer to that of the unsupported curve, showing consistency with the physical simulation. figure 5. transient electromagnetic response curve under different metal shed supports. numerical simulation of a model with low resistance anomalies the mine transient electromagnetic method is mainly used to detect water-containing, low-resistance anomalous structures. to further study the effect of metal supports on the transient electromagnetic response, the response of a water-containing low-resistance structural model was simulated. here, the abnormal body resistivity was set to 1ω⋅m for a size of 10 m × 10 m. the model was located below the field source, 50 m away from the floor of the roadway, and the resistivity was set to 50ω⋅m . a metal mesh with a diameter of 5 mm was selected with an equivalent thickness of 0.2 mm. the simulation results are shown in figure 6 where the ordinate indicates the relative apparent resistivity r (chang et al, 2020). the relative apparent resistivity is defined as the ratio of the apparent resistivity in the presence of an abnormal body to that without an abnormal body, ρ ρ ρ τ τ r = ′ (16) where ρτ is the apparent resistivity without any low resistance anomalies and ρτ is the apparent resistivity with a low resistance abnormality. figure 6 shows that without the metal support, the relative apparent resistivity curve begins to decrease at t= 0.08 ms, and a low-resistance anomaly occurs. however, in the presence of metal support, an abnormality occurs at t= 3 ms, indicating that the metal support delayed the transient electromagnetic response. thus, when the abnormal body is at the same position, observed 113influence of metal roadway supports on transient electromagnetic detection in mines longer observation time is required in the presence of metal support. in figure 6, without the metal support, the low resistance abnormality caused the curve to drop to 0.32, whereas in the presence of the metal support, the lowest point of the curve was 0.90. this indicated that the metal layer reduced the sensitivity of the transient electromagnetic method and weakened its reaction to the low resistance abnormal body. figure 6. relative apparent resistivity contrast diagram with and without the metal support. therefore, in the actual exploration process, there should be a special focus on the interference of the surrounding metal environment and this work should be fully recorded. in the data interpretation process, it is necessary to perform time-sharing corrections on the exploration data according to the collected time series. conclusions by using a numerical simulation method to simulate the effect of metal support on tem detection, the following conclusions are drawn. 1) the metal anchor net support was treated as a thin metal layer and the finite difference equation of the metal layer was derived. a non-uniform grid was used to simulate the transient electromagnetic response characteristics of the entire space under the influence of the metal layers. the presence of a thin metal layer slowed the transient field diffusion rate and hindered the overall diffusion of transient electromagnetic fields. 2) the magnetic field as a function of the time derivative parameter curve observed under the influence of the thin metal layer was higher than that of the response curve without the metal support. increased thickness of the thin metal layer resulted in higher curves. 3) simulation of the transient electromagnetic response of the watercontaining, low-resistance anomaly model showed that the metal support layer delayed the transient electromagnetic response, requiring longer times for observation. moreover, the metal support reduced the sensitivity of the transient electromagnetic method and weakened the reaction to the lowresistance anomaly. therefore, performing relevant corrections during the actual exploration process is necessary. acknowledgment this research was supported by the national key r&d program of china under grant(2018yfc0807802-3) and doctoral specialty foundation (jyjbzx20-06). we are particularly grateful for the advice and help of editors and reviewers. in addition, we would like to thank editage (www.editage.cn) for english language editing. references chang, j., yu, j. & liu, z. (2016). three-dimensional numerical modelling of full-space transient electromagnetic responses of water in goaf. applied geophysics, 3, 539-552. chang, j., su, b., & malekian, r. (2020). detection of water-filled mining goaf using mining transient electromagnetic method. iee transactions on industrial informatics, 16(5), 2977-2984. cheng, j., chen, d. & xue, g. (2016). synthetic aperture imaging in advanced detection of roadway using the mine transient electromagnetic method. chinese journal of geophysics, 59(2), 731-738. danielsen, j. e., auken, j. e., sondergaard, v. & sorensen, k. i. (2003). the application of the transient, electromagnetic method in hydrogeophysical surveys. journal of applied geophysics, 53(4), 181198. elsherbeni, a. & demir v. (2006). the finite-difference time-domain method in electromagnetics: with matlab. raleigh: scitech publishing. huang, l., liu, s., wang, b. & zhou, f. 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(2002). direct time-domain numerical analysis of transient electromagnetic fields. chinese journal of geophysics, 45(2), 275-284. yan, s. & shi, x. (2004). thin bed and wires modeling in transient electromagnetic fields fdtd computation of tunnel whole-space. coal geology and exploration, 32(s1), 87-89. yang, h. & yue, j. h. (2009). application of absorbing boundary condition in whole-space computation of transient electromagnetic response. journal of china university of mining and technology, 38(2), 263-268. 114 yaoning liu, shucai liu, maofei li, xinming liu, weihong guo yu, j., liu, s. & wang, y. (2008). response characteristic of transient electromagnetic to metallic facilities in coal mines and the disposal technology. journal of china coal society, 33(12), 1403-1407. yue, j., yang, h. & hu b. (2008). 3d finite difference time domain numerical simulation for tem in-mine (in chinese). progress in geophysics 22(6), 1904-1909. zhou, s., yu, j. & jiang, z. (2014). study of mine transient electromagnetic response characteristics and correction method under support using bolt and wire mesh. china coal, 4, 45-48. zhou, j., cheng, j. & wen, l. (2017). response characteristics of metallic facilities and correction method on mine transient electromagnetic surveying. journal of the china coal society, 26(8), 146-150. palabras clave: carretera de minas; detección avanzada; análisis de polarización; imágenes finas. how to cite item wang, b., liu, s., jin, b., & qiu, w. fine imaging by using advanced detection of reflected waves in underground coal mine. earth sciences research journal, 23(1), 93-99. doi: https://doi.org/10.15446/esrj.v23n1.78750 safety of the mine roadway constructions is controlled by geological disasters such as faults, goaves and so on. the advanced prediction has become an in-demand topic, and advanced detection method of the reflected wave is a crucial technology for advanced prediction of geological anomalies. however, due to the influence of the complex near-source seismic wavefield in the coal mine roadway, the result of wavefield separation and migration imaging is not accurate, which lead to the fact that the fine imaging of geological anomaly is difficult. a comparative analysis of wavefield separation method of kinematics and dynamics is carried out in this paper to solve this problem. a factor of principal polarization direction is introduced according to “the orthogonal difference between the propagation direction of p-wave and s-wave and the vibration direction of particle” starting from the real-time polarization analysis from three-component seismic signals. by the factor, a modified function is constructed and integrated into pre-stack diffraction migration, which put forward the polarization migration method that incorporates wavefield separation with migration imaging. the results of physical simulation and field survey in xiangyuan coal mine in western china are as follows: the fine imaging by using advanced detection of the reflected waves in underground coal mine can be achieved by using polarization migration in linear observation system and the polarization migration has the effect of enhancing the spatial resolution. abstract keywords: magma; mine roadway; advanced detection; polarization analysis; fine imaging. fine imaging by using advanced detection of reflected waves in underground coal mine imágenes finas mediante el uso de detección avanzada de ondas reflejadas en una mina subterránea de carbón issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n1.78750 la seguridad de las construcciones de carreteras de mina está controlada por desastres geológicos, como fallas, obstrucciones, etc. la predicción avanzada se ha convertido en un tema de demanda, y el método de detección avanzada de la onda reflejada es una tecnología crucial para la predicción avanzada de anomalías geológicas. sin embargo, debido a la influencia del complejo campo de ondas sísmicas de fuente cercana en la carretera de la mina de carbón, el resultado de la separación de los campos de onda y la imagen de migración no es preciso, lo que lleva al hecho de que es difícil obtener imágenes finas de anomalías geológicas. en este trabajo se realiza un análisis comparativo del método de cinemática y dinámica de separación de campos de onda para resolver este problema. un factor de la dirección de polarización principal se introduce según "la diferencia ortogonal entre la dirección de propagación de la onda p y la onda s y la dirección de vibración de la partícula" a partir del análisis de polarización en tiempo real desde señales sísmicas de tres componentes. por el factor, una función modificada se construye e integra en la migración de difracción de pila previa, lo que presenta el método de migración de polarización que incorpora la separación del campo de onda con la imagen de migración. los resultados de la simulación física y el estudio de campo en la mina de carbón xiangyuan en el oeste de china son los siguientes: la imagen fina mediante el uso de detección avanzada de las ondas reflejadas en una mina de carbón subterránea se puede lograr mediante el uso de la migración de polarización en un sistema de observación lineal y la migración de polarización tiene el efecto de mejorar la resolución espacial. resumen record manuscript received: 30/05/2018 accepted for publication: 01/12/2018 earth sciences research journal earth sci. res. j. vol. 23, no. 1 (march, 2019): 93-99 bo wang, shengdong liu*, biao jin, wanyong qiu state key laboratory of deep geomechanics & underground engineering and school of resource and earth science, china university of mining and technology, xuzhou, 221116, china * corresponding author: wbsyes@126.com g eo te ch ni ca l en g in ee ri ng 94 bo wang, shengdong liu*, biao jin, wanyong qiu introduction in recent years, as the coal mine is gradually entering the deep mining stage, higher requirements of advanced detection in tunnel face are put forward due to geological environment changes such as high geostress, high temperature, high karsts water pressure, mining strong disturbance and so on (liu, liu & yue, 2014; cheng, li, peng, & sun 2014). in chinese coal mine disaster accidents, according to statistics, more than 90% of the accidents occur in the process of roadway constructions, so it is very necessary to carry out the effective advanced prediction of the geological anomalies (zhang, & hu 2015; liu & wang, 2016). the advanced detection method of the reflected waves is used to predict the geological conditions of the advanced tunneling zone of the roadway by using the reflections generated from seismic waves’ propagation in inhomogeneous geological structures (lüth, buske, giese, & goertz, 2005; bohlen et al., 2007). the advanced detection methods of the reflected waves in mine roadway are as follows: shen (2006) proposed seismic prediction method of laneways, which is to achieve the imaging according to the superposition principle of diffraction and the principle of mirror imaging. liu, guo & zhang (2006); zhang & wu (2006), y wang et al. (2016) proposed mine seismic prediction method, which adopts the roadway observation system with high covering number to acquire seismic data. these data are processed by the methods of energy synthesis and decomposition and migration superposition to predict a variety of geological structures within 120m ahead of the tunnel face. he (2008) developed mine multi-component seismic data processing system to provide the support for the seismic advanced detection with multiwave. liang & song (2009) put forward the multi-component advanced detection method with multiwave, which can detect the situations such as rock broken zone etc, within 100m ahead of the tunnel face (reyes-mendoza et al., 2016). in addition, the advanced detection methods of reflected waves in traffic tunnel are sometimes used in underground coal mine roadway, mainly including seismic negative apparent velocity method (zeng, 1994), land sonar method (zhong, 2012), tunnel seismic prediction method (dickmann & sander, 1996), and the japanese oyo company (inazaki, 1999), the german gfz company (borm, giese, & otto, 2001), american nsa company (otto, buron, bretterebner, & schwab, 2002) also developed seismic prediction system based on the reflected waves method in the tunnel. the advantages of these seismic reflected waves method include long detection range, high resolution, abundant information and being sensitive to geological anomaly. due to the limitation of narrow observation space of the roadway and the interference of the near-source seismic wave field (cheng et al., 2014), the result of wavefield separation and migration imaging is not accurate, which lead to the fact that the fine imaging of geological anomaly is difficult (wang et al., 2016). based on this problem, the polarization migration that integrates wavefield separation with migration imaging in full space condition is put forward in this paper and the fine imaging is achieved by using the advanced detection method of reflected waves through the physical simulation and the field survey. wavefield separation the advanced detection method of reflected wave is carried out with sources and receivers in rock or coal seam. the method have the characters of broadband excitation and three components reception, which provides the possibility for seismic exploration with high resolution and multiwave attributes, while due to the limited offset, the complex seismic information makes the data processing be difficult. in multiwave seismic exploration, the data contains p-wave, s-wave, converted wave, surface wave, acoustic wave and so on. effective wavefield separation is the key to data processing. the common wavefield separation methods include: linear radon transform filtering, frequency wavenumber domain filtering, polarization filtering, etc.  − p filtering  − p filtering is to transform seismic records from time-spatial  −( )x domain into  − p domain and separate the effective wave energy in  − p domain, and then transform it into time-spatial  −( )x domain again. the integral formulas of radon transformations can be defined as: ψ τ ϕ, ,p t px x dx( ) = = +( ) −∞ +∞ ∫ τ (1) ϕ τ ψ, ,x t px p dx( ) = = −( ) −∞ +∞ ∫ τ (2) f k− filtering f k− filtering is the method to carry out wave field separation by using the difference of spectrum and apparent velocity of effective wave and interference wave. the effective waves of frequency band f f1 2− (frequency filtering) and apparent velocity v v1 2− (apparent velocity filtering) are reserved, and the interference wave are suppressed when v v v v f f< > <1 2 1, , , and f f> 2. two-dimensional fourier transformations are as follows: g f k t x e dtdxi fi kx, ,( ) = ( ) −∞ +∞ −∞ +∞ − +( )∫∫ ϕ π2 (3) ϕ t x g f k e dtdxi fi kx, ,( ) = ( ) −∞ +∞ −∞ +∞ − +( )∫∫ 2π (4) polarization filtering polarization filtering is also known as spatial directional filtering which is used to filter pand s-waves by using the difference of polarization characteristics. pand s-waves are represented as linear orthogonal polarization, while the polarization direction of p-wave is parallel with the propagation direction of wave, and polarization directions of s-wave are perpendicular to the propagation direction of waves. comparative analysis kinematic separation method of pand s-waves is based on the relatively clear phase axes of pand s-waves in the time domain. however, the actual data, especially underground roadway seismic records are complex. various types of waves are intertwined, including information of positive apparent velocity features such as roof, floor, rear and so on, thus phase axes of pand s-waves with clear negative apparent velocity are rarely, so it is difficult to separate pwaves and s-waves simply by using  − p filtering. a combination of kinematics apparent velocity features and dynamics frequency features is used by f k− filtering to separate pand s-waves. on the one hand, because of the lower frequency of surface waves and s-waves, they could not be separated effectively in the separation process because the frequency between surface wave and s-wave is close. on the other hand, when p-wave is obtained through high-pass filtering, it easily leads to the fact that some effective s-wave is lost. because the polarization filtering is not the band-pass filtering of frequency domain, the bandwidth of effective waves is not affected, and the polarization filtering is to use mathematical statistical method to carry out wavefield separation on basis of the polarization difference of pand s-waves, which makes the polarization filtering method become the commonly used separation method for pand s-waves. however, in the process of calculating eigenvalues and eigenvectors and setting filtering function, corresponding relations between xyz coordinate axes and p, sh, sv directions are usually artificially, without considering the corresponding relationship between the principal polarization direction of real-time particle in each receiver and propagation direction of seismic wave, which makes the results of unclean separation of pand s-waves in the roadway. therefore, through the real-time computation of principal polarization directions of particles in each receiver, starting from the difference between real-time particle vibration directions and pand s-waves propagation direction, 95fine imaging by using advanced detection of reflected waves in underground coal mine setting pand s-waves separation filter should be an effective method to extract p-wave or s-wave. migration migration imaging method can be divided into ray and wave equation migration. the ray migration is mainly as the prestack diffraction migration, which is based on huygens principle. wave equation migration mainly includes kirchhoff integral migration method, reverse-time migration method and so on. however, wave equation migration like reverse-time migration, etc., has lower calculation efficiency, so the prestack diffraction migration is mainly adopted for migration imaging of mine roadway seismic exploration. according to huygens principle, each reflection point g in the grid can be regarded as a secondary source, while carrying out diffraction scan migration, any discrete grid point should be regarded as a reflection point, and travel time of reflected wave or diffraction wave is as follows: t l v l h x x h x xi j i j i j r g g si j, , ,,= = + −( ) + + −( )2 2 2 2 (5) where i =1, 2, 3,...... m, and m is the recording trace for superposition calculation; v is the seismic velocity, h is the vertical depth of the scanning point gi j, , while ti j, is the travel time of diffraction wave of the j th shot in the i th receiver at the scanning point gi j, . the amplitude value ai of signal in different traces in the time of ti are carried out with superposition to get the amplitude value of gi j, point. dynamic polarization migration imaging principle as shown in figure 1, x direction denotes tunnel face, y direction denotes the left side and z direction denotes the roof. the grid division is carried out with the advanced space of the roadway, and the black mark is a hypothetical reflection point in the spatial grid; the same side behind tunnel face is set up with several sources and receiver. assuming that the propagation time is represented as t, and the propagation path is shown as the ray direction in figure; in the spatial coordinate system, the incident path is as l1 and the propagation time is as t1; the reflection path is l2 and the propagation time is as t2; the ray path of l1 and l2 in homogeneous space can be calculated by the spatial coordinates among source, the reflection point and the receiver. according to the spatial position of source, reflection point and receiver and spatial velocity parameters, based on ray tracing technology, it can calculate the total time t = t1 + t2. based on three-component amplitude at t time, particle spatial vibration direction li at t time of a receiver can be calculated by polarization analysis method. the angle  can be calculated by using particle spatial vibration direction li and the ray direction of reflection path l2 (as shown in fig. 2), and then based on the difference of angle  between p-wave and s-wave, the filter is set up in real time to realize dynamic polarization filtering. the dynamic filtering computation is integrated into prestack migration. polarization analysis hilbert transform polarization analysis method is a kind of complex number field polarization analysis method based on hilbert transform to construct the complex covariance matrix and then to obtain principal polarization parameters. among them, taking the two-dimensional space as an example, the calculation steps of the principal polarization direction li at the t time are as follows: hilbert transform is calculated: hx t x t ju x t( ) = ( ) + ( )( )⋅⋅ (6) hz t z t ju z t( ) = ( ) + ( )( )⋅⋅ (7) where x(t) and z(t) are seismic records of components x and z respectively, while the symbol u represents hilbert transform, j = 1. hermitian matrix is constructed: c t m t m t( ) = ∗( )⋅ ( ) (8) where, m t hx t hz t( ) = ( ) ( )  , the symbol * represents complex conjugate transpose of matrix. the maximum eigenvalue max of the matrix and its normalized corresponding eigenvector x zmax max,( ) are obtained. the principal polarization angle  is calculated: α= arctan re re max max z x ( ) ( )         (9) where re maxx( ) and re maxz( ) are respectively as the real parts of xmax and zmax,  indicates the main polarization direction li. polarization migration prestack depth migration is often expressed as a diffraction integral in a scalar wave field. on the basis of integral migration, a modified function receiver source advanced direction tunnel face reflection point ... y l2 l1 z x figure 1. polarization migration imaging particle trajectory particle vibration direction ray directiony l2 l1 z x  figure 2. particle spatial vibration direction 96 bo wang, shengdong liu*, biao jin, wanyong qiu is introduced into polarization migration, and the migration formula can be defined as: u f t a t dx n = ( ) ( )∫1 (10) in the above equation, f t q( ) = cos , f t( ) is a modified function of amplitude value for any reflection point during p-wave imaging (sinq  for s-wave imaging), and a t( ) is the amplitude. the acute angle  represents the angle between the ray direction from any reflection point and the principal polarization direction. the q is valued in a range of 1 to 10. physical simulation in order to simulate the seismic advanced detection field with the size of 160m * 160m * 80m (as shown in figure 3), the experimental physical model is designed according to scaling factors in table 1 with the size of 1.6m * 1.6m * 0.8m. the model velocity parameters are consistent with the actual detection field, and the sampling frequency is 1mhz, namely that the sampling interval is 0.001ms, while receiver line layout is also scaled down by 1: 100. figure 3. physical model and detection diagram yzx 0 5 10 15 20 25 30 35 40 45 trace(no.) 60 ti m e(  s.) 45 0 30 0 15 0 0 figure 4. seismic data the advanced linear observation system is shown in figure 3-b. the excitation transducer slides on the receiver line to simulate excitation with multiple shots according to the moving step of 0.01m, and the position of threecomponent receiving transducer is fixed. in the process of the experiment, the offset trace is 0.059m with the total excitation of 21 times, and the sampling frequency is 1mhz, namely that the sampling interval is 0.001ms. the threecomponent ultrasonic data with 21 shot are collected in the experiment. the components of x, y and z are respectively of 21 channels. the interference of reflected waves from the sink sides and bottom is avoided effectively in the process of designing model, so the original signals are mainly direct waves and reflected waves from abnormal interfaces. figure 4 is obtained through  − p transform and f k− transform with the combination of filtering. from the signals of component x in figure 4, four groups of reflected waves can be observed, respectively as a, b, c and d, which are all performed with negative apparent velocity characteristics. from the analysis of the propagation time, a and b are respectively as reflected p-wave, whose time length of the signals between 1st to 21st shot is within 100-200 and 150-250s, while c and d are respectively as reflected s-wave, whose time length is within 97fine imaging by using advanced detection of reflected waves in underground coal mine y z x 0 0 0 280 280 280 -280 amplitude (mv) figure 5. polarization trajectory 200-380 and 260-440s. among them, signals of c and d in wave transducers of y and z are more obvious. the three-component signal of the 21st shot of a reflected p-wave is selected to make the polarization diagram (figure 5). the angle between projection of the principal polarization direction of a reflected p-wave in the xz plane and the x axis is 10° (it should be 0 degree in theory and there is a 10 degree error). the principal polarization direction in xz plane is parallel with the wave propagating direction (wave propagating direction is represented by a yellow arrow), while the principal polarization direction in xy plane is parallel -1 7. 0 0.0 r1 distance(cm) r2 30.0 40.0 50.0 60.0 0. 00 17 .0 20.0 figure 6. polarization migration profile with the x-axis, and the principal polarization direction in yz plane is nearly parallel with the z-axis. thus, it can be deduced that the principal energy is in the xz plane, which is parallel with the receiver line (along the x axis). those features conform to the characteristics of p-p wave. imaging by using polarization migration is shown in figure 6. it is analyzed from the figure that there are two groups of appear abnormal interfaces around in 0.2m and 0.32m, respectively as r1 and r2, where 0.32m is the designed model interface between the concrete and saturated fine sand, while the 0.2m is cracking zone of the model with developed fracture. similarly, three-dimensional polarization migration is carried out in order to describe the spatial morphology of the interface r2 after filtering energy anomaly signal of r1. figure 7 is the spatial profile of r2 energy anomaly interface. field survey the multiwave seismic instrument was adopted to record seismic data. a three-component geophone arranged in left side of open-off cut roadway of 1063 working face. there are 29 sources in left side and the spacing of two sources is 1m. figure 8 is the site survey layout. the sampling length is 2048 points and the sampling interval is 0.08ms, finally the 29 shots seismic data were recorded. spectrum analysis shows that the main frequency of the data is within 100-400hz. the velocity of p-waves was obtained as 2200m/s based on the first break picking. the advanced reflected waves were acquired through  − p and f k− transform. two groups of negative apparent velocity characteristics can be analyzed from the figure, which are respectively located at 30-50ms, 70-90ms. figure 9 was obtained by using polarization migration. it can be seen clearly from figure 15 that there is an abnormal energy interface r1 at 47m ahead of tunnel face, while the energy near the tunnel face is obviously abnormal. combine with field exploration data, there is a water tank near the tunnel face, so the interpretation of r1 interface is the goaf boundary. figure 10 indicates the result of prestack diffraction migration, and there exist energy anomaly of 0-30m, 40-60m, 75m in the front. pand s-waves could not be effectively separated without the polarization migration, thus the imaging resolution is low, which can not effectively determine location of goaf in the front. verified by field drilling, it is confirmed that the location of advanced goaf boundary is located at about 49m, which is close to the detection result and the error is 2m. 98 bo wang, shengdong liu*, biao jin, wanyong qiu figure 8. layout of field survey interface 50 r2 40 60 30 20 10 10 0 -10 -10 -20 -30 x z y 0 53684.2 41052.6 28421.1 15789.5 3157.89 -9473.68 -22105.3 -34736.8 -47368.4 -60000 figure 7. three-dimensional polarization migration result figure 9. two-dimensional polarization migration 99fine imaging by using advanced detection of reflected waves in underground coal mine conclusion on the basis of the three-component seismic data, the real-time polarization analysis was carried out. based on the difference of polarization parameters, extraction of advanced seismic reflection wavefield were carried out with the combination of  − p and f k− filtering. dynamic polarization migration imaging method was proposed in this paper, and wavefield separation of effective wave was completed in the migration imaging process, which solves the problem of multiple abnormal interfaces when detecting the complex wave field of the roadway. the method effectively improves the spatial resolution and detection precision. acknowledgements this research is supported by “the fundamental research funds for the central universities” (2017qnb11). references bohlen, t., lorang, u., rabbel, w., müller, c., giese, r., lüth, s., & jetschny, s. 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(2005). fresnel volume migration of multicomponent data. geophysics, 70(6), 121-129. otto, r., buron, e., bretterebner, h., & schwab, p. (2002). the application of trt at the unterwald tunnel. felsbau, 20(2), 51-56. reyes-mendoza, o., marino-tapia, i., herrera-silveira, j., ruiz-martinez, g., enriquez, c., & largier, j. l., (2016). the effects of wind on upwelling off cabo catoche. journal of coastal research, 32(3), 638-650. shen, h. y. (2006). reflected-wave tunnel seismic prediction. xian: changan university. wang, b., liu, s. d., zhou, f. b., lu, t., huang, l. y., & gao, l. j. (2016). polarization migration of three-component reflected waves under small migration aperture condition. acta geodynamica et geomaterialia, 13(1), 1-12. zeng, z. h. (1994). prediction ahead of the tunnel face by the seismic reflection methods. chinese journal of geophysics, 37(2), 268-271. zhang, p. s. &, hu, x. w. (2015). research status on technology of advanced detection by electromagnetic methods in mine laneway. coal science and technology, 43(1), 112-115, 119. zhang, p. s. &, wu, j. s. (2006). research and analysis of forward prediction technology using seismic reflection wave in tunnel and laneway in china. advances in earth science, 21(10), 1033-1038. zhong, s. h., sun, h. z., & wang, r. (2012). land sonar method. beijing: science and technology of china press. figure 10. two-dimensional prestack diffraction migration source parameters inversion of the 2012 la vega colombia earthquake mw7 earth sciences research journal 1 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 2 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics first assembly of the latin-american and caribbean seismological commission lacsc regional commission of the international association of seismology and physics of the earth's interioriaspei second latin-american and caribbean symposium of geophysics third latin-american congress of seismology fourth colombian congress of seismology bogota, colombia – july, 23 – 25, 2014 organizing committee carlos a. vargas j.department of geosciences, national university of colombia, bogota. alexander caneva.research center, antonio nariño university, bogota. hugo monsalve.quimbaya group, quindio university, armenia. jaime raigosa.national sesimological network of colombia, bogota. elkin salcedo.geography department, valle university, cali. gaspar monsalve.national university of colombia, medellin. scientific committee marcelo assumpcao,universidade de são paulo, brasil frank audemar,funvicis, venezuela hugo a. yepes,institutogeofísico del ecuador patricia alvarado,universidad nacional de san juan, argentina hernando tavera,institutogeofísico del perú susan beck,university of arizona, usa marino protti,observatoriovulcanológico y sismológico de costa rica sergio barrientos,serviciosismológico, universidad de chile logistics sandra robles www.geoslac.org bogota. july 2014 the abstracts contained in this publication are responsibility of their respective authors and do not represent the official position of any institution. typographical errors or omissions are not the responsibility of the organizing committee. earth sciences research journal 3 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics presentación la región de latinoamérica y el caribe, es un complejo mosaico tectónico donde convergen los esfuerzos producidos por la interacción entre varias placas litosféricas, y donde grandes sistemas de fallas con actividad, entre moderada y alta, afectan a una parte significativa de su población (más de 500 millones de habitantes). dado el auge de la actividad minero-energética, en esta zona se despliegan grandes proyectos de exploración de recursos cuya recuperación futura generará importantes cambios en vastas zonas del continente americano. bajo el actual entorno geodinámico y las expectativas extractivas orientadas al desarrollo socio-económico, afloran cuestionamientos sobre el impacto de la actividad antrópica y la inestabilización de los sistemas tectónicos de relevancia regional. muchos estudios geofísicos, tanto teóricos como aplicados, han sido realizados en diferentes regiones de latinoamérica y el caribe, principalmente desde la segunda mitad del siglo xx. como en el resto del mundo, básicamente dos tendencias han marcado su desarrollo: la evaluación de amenazas naturales y la exploración de recursos. los principales elementos orientados al avance de este conocimiento sismológico, y en general geofísico, han sido: (a) la estructura de la corteza y del manto superior, (b) entender la evolución tectónica regional, (c) estimar la amenaza sísmica regional y local, y (d) modelar la geometría de estructuras de trascendencia económica. por su parte, la región ha sido testigo de un crecimiento desmesurado en las grandes ciudades, evidenciado por el aumento progresivo de desigualdades sociales y económicas, y que generan a su vez un incremento en los niveles de vulnerabilidad; los cuales unidos a las particularidades de amenaza sísmica alta, configuran un contexto de riesgo sísmico alto. en este evento se abordarán los resultados más relevantes asociados al comportamiento sismotectónico en la región, las implicaciones del uso de métodos exploratorios activos sobre las condiciones tectónicas; la trascendencia socio-económica asociada a la exploración y explotación de recursos mediante aproximaciones noconvencionales y su impacto social, y que permitan contribuir al desarrollo sostenible. son bienvenidas contribuciones de todo el mundo. earth sciences research journal 4 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics presentation the latin-american and caribbean region is an area with a very complex tectonic setting, where stress and strain generated by the interaction of several lithospheric plates is being absorbed. several regional fault systems, with moderate and high activity, represent a hazard for a significant part of the population (more than 500 million inhabitants). given the recent developments in the mining and energy industries, a great deal of exploration has been focusing on this part of the world, and the potential extraction of mineral resources is going to generate important changes in vast areas of the american continent. considering the geodynamic framework and the expectation of the extraction of economic resources, questions about the impact of human activities and the possible destabilizing of the relevant tectonic systems are raised. many theoretical and applied geophysical studies have been developed in different regions of latin-america and the caribbean, mainly since the second half of the 20th century. there have been basically two motivations to carry out these studies: the evaluation of natural hazards and the exploration of economic resources. such studies have mainly focused on the knowledge of: (a) the structure of the crust and upper mantle, (b) the regional tectonic evolution, (c) the local and regional seismic hazards, and (d) the geometry of geologic structures of economic interest. this part of the world has witnessed an excessive and disproportionate growth in the number of urban centers, evidenced by the increase in economic and social gaps. this situation puts a great portion of the population at a high level of vulnerability, which in addition to the natural hazard in the region, configures a scenario of high seismic risk. in this academic event, the relevant results associated with the seismotectonic behavior of this part of the world will be addressed, as well as the implications of active exploration of the tectonic conditions, the socio-economic impacts of exploration and extraction of economic resources, and the implementation of non-conventional techniques for exploring and mining that can contribute to sustainable development. we welcome contributions from around the world. earth sciences research journal 5 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 6 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 7 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 8 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 9 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 10 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 11 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 12 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 13 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 14 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 15 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 16 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 17 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 18 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics table of contents agte session – application of geophysical techniques on exploration ..................................... 29 determinación de modelos de velocidad de ondas de corte empleando el método de autocorrelación espacial (spac) en la ciudad de armenia, colombia ..................................................................................................................... 30 gravity and magnetic constraints on the structural framework of the northern volcanic complex, central cordillera, colombia: applications for geothermal resource prospecting ................................................................... 31 geófonos individuales vs arreglo de geófonos: un estudio de campo realizado en colombia ............................. 32 estudios de amenaza geológica a partir de mediciones con georadar: identificación de deformaciones por fallas y deslizamientos en venezuela ................................................................................................................................................. 33 high-resolution, low power, mobile infrasound station.................................................................................................. 35 infiriendo la estructura estratigráfica mediante interferometríasísmica a partir de datos microsísmicos y ruido ambiental ................................................................................................................................................................................... 37 anst session – anthropogenic seismicity ........................................................................................ 38 integration program for research infrastructures for georesources within european plate observing system project ........................................................................................................................................................................................ 39 assessing the role of static stress transfer on the spatio-temporal distribution of aftershocks caused by a mw2.2 earthquake in mponeng gold mine, south africa ............................................................................................ 40 is-epos – a prototype of epos thematic core service for seismic processes induced by human operations ...... 41 estimación simultanea del mecanismo focal de dos microsismos cercanos ................................................................. 42 variations of parameter b in se brazil from local earthquakes ..................................................................................... 44 los sonidos de las actividades de explotación de hidrocarburos ................................................................................... 45 propuesta de calibración de una escala de magnitud para eventos microsísmicos que ocurren en cercanías a campos petroleros .................................................................................................................................................................... 46 detection of induced seismicity due to oil and gas extraction in the northern gulf of mexico, usa ................. 47 statistical analysis of seismic activity preceding the emilia-romagna (italy) earthquake sequence from may/june 2012 ....................................................................................................................................................................... 48 seismicity induced from nearly two decades of deep brine injection, paradox valley, usa .................................. 49 ctem sesión – caribbean tectonics, earthquake monitoring, modeling and early warning .. 51 the mw6.4 north of puerto rico event: response timeline and event characteristics of the largest event ever recorded at the puerto rico seismic network .................................................................................................................. 52 earth sciences research journal 19 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics significant breakthroughs in monitoring networks of the volcanological and seismological french observatories in the west indies ................................................................................................................................................................... 53 creating a dense seismic network for costa rica: a low-cost latin american instrument solution ..................... 54 relocation of the mainshock and aftershocks of ml 5.7 quetame earthquake, colombia ..................................... 55 aspectos geotectónicos, morfotectónicos y geodinámicos de la amenaza sísmica de la región nororiental de la plataforma caribe colombiana ............................................................................................................................................. 56 slab tears and intermediate-depth seismicity in the northeast caribbean and elsewhere ........................................ 57 combination of high rate, real-time gnss and accelerometer observations – test results using a shake table and historic earthquake events .............................................................................................................................................. 58 renovation of the national seismological network of costa rica: improving station coverage and earthquake monitoring................................................................................................................................................................................. 60 the tablón fault: source of the 2011–2012 tobosi earthquake swarm in central costa rica ......................... 61 static stress state on the costa rican cocos-caribbean plate interface and its influence on the asperity distribution of the sesimogenic zone ................................................................................................................................... 62 the new iris earthquake browser (navegador de terremotos) ................................................................................. 63 seismic and tsunami monitoring and data sharing in the caribbean ............................................................................ 65 ecgs session – studying the earthquake cycle using geodesy and seismology ......................... 66 high resolution geodetic surveys of the present day deformation along the south-caribbean margin................. 67 caracterización sismotectónica de la región del valle del cauca y zonas aledañas a partir de mecanismos focales de terremotos y datos geodésicos ......................................................................................................................................... 69 present-day deformation along the andean subduction controlled by continental slivers motion ........................ 70 assessing the seismic potential of south america with crustal deformation models and historic earthquakes ..................................................................................................................................................................................................... 71 geodetic constraints on the present-day kinematics of colombia, south america ................................................... 73 evaluación de la deformación sismotectónica asociada al sismo de mayo 24, 2008 quetame, colombia ........... 74 the gnss geored project: a tool for understanding the strain rate field and the seismic cycle in colombia, south america current status and challenges ................................................................................................................. 75 synchronous slow slip events and seismic swarms along the northern peru-ecuador subduction zone ........... 77 two earthquake cycle geodynamic control networks in costa rica: nicoya and osa-burica peninsulas ............ 79 results from the national continuous gps network in ecuador .............................................................................. 80 deformación basada en datos de gps en colombia ........................................................................................................ 81 determination of earthquake sources using gnss seismology................................................................................... 82 una red gravimétrica como herramienta de apoyo a la sismología................................................................................ 83 esga –earth structure general aspects ......................................................................................... 84 earth sciences research journal 20 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal structure of the craton san francisco, brazil derived from receiver functions ............................................ 85 abrupt changes in moho depth beneath the central andean plateau inferred from apparent pn phase velocities ..................................................................................................................................................................................................... 86 characterization and identification of the meta fault (colombia) from the distribution of resistivities ............. 87 new contributions to the knowledge of the magdalena submarine fan (colombian caribbean) from seismic and high resolution bathymetric data .................................................................................................................................. 88 first insights into the crustal structure of the merida andes, venezuela, along the southern profile................... 89 relación entre estructura del subsuelo y movimiento sísmico. el caso de la ciudad de concepción, chile .......... 90 borehole climatology – principles, applications and results ....................................................................................... 91 gses session –earth structure general aspects ............................................................................. 92 atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos no viscosos inmiscibles . 93 modelado de atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos viscosos inmiscibles ................................................................................................................................................................................. 95 application of mee algorithm for medium-term earthquake forecast in the koyna-warna region .................... 97 determination of the stress field in an oil production area by mean of analysis of ambient noise ........................ 99 study of the montes claros seismogenic fault, in the sao francisco craton, brazil, with correlations of pand s-wave phases ........................................................................................................................................................................ 100 máximos pga, pgv y pgd en el plano horizontal .................................................................................................... 102 isc products for large earthquakes in the latin america and caribbean region .................................................. 104 completitud del catálogosísmico en el centro occidente de colombia ..................................................................... 105 stress field in brazil with focal mechanisms: examples of regional forces controlling the stress field ............. 106 sismicidad en el piedemonte llanero: caracterización, relocalización y tomografía de velocidades ................... 108 réplicas del sismo del 13 de agosto de 2013. bahía solano océano pacífico colombiano ............................. 110 seismic microzonation of urban areas .............................................................................................................................. 111 seismological detection and analysis of catastrophic landslides ................................................................................. 112 results of recent paleoseismologic studies point to the probable seismogenic source of the 1875 earthquake of cucuta, in the border area between colombia and venezuela .................................................................................... 113 inversión del tensor de esfuerzo a partir de mecanismos focales obtenidos en el campo volcánico y geotérmico de las tres vírgenes, baja california sur (méxico) ...................................................................................................... 116 aplicación de algoritmo de inversión tomográfica aplicada en la cuenca de urabá .............................................. 118 propagacion de ondas de rayleigh en el valle de méxico y una posible explicacion para la larga duracion de los sismos ...................................................................................................................................................................................... 119 efecto de sitio no lineal sobre basalto usando cinco sismos m>6 de guatemala ................................................... 120 características espectrales de la sismicidad originada en la cuenca del valle de méxico ..................................... 121 earth sciences research journal 21 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a test of the rstt model for south america with brazilian pgt events ........................................................... 122 the september 5th, 2012 nicoya, costa rica earthquake, a subduction earthquake captured in the near field .................................................................................................................................................................................................. 123 estimación del riesgos sísmico en managua, nicaragua, con la metodología capra ......................................... 124 structuraldelineationof the servitafaultsystem,colombianeasterncordillera, usingmicroseismicity ................. 125 strong motion recorded during the emilia 2012 thrust earthquakes (northern italy): a comprehensive analysis .................................................................................................................................................................................................. 126 rupture process on fracking operations applying pwave back projection images ......................................... 127 alternatives for computing regional and global seismic-phase travel times using a 3-dimensional model ........ 128 paleoseismic study of the north panama deformed belt and probabilistic seismic hazard assessment for a proposed harbor facility on the caribbean coast of costa rica ............................................................................. 129 idlm session – imaging and dynamics of lithosphere and upper mantle ............................... 130 constructing shear wave models using multi-objective optimization for multiple geophysical data sets .................................................................................................................................................................................................. 131 study of seismic attenuation and unusual phases at uturuncu volcano, bolivia ................................................ 132 title: 3-d structure of the rio grande rift from 1–d constrained joint inversion of receiver functions and surface wave dispersion .................................................................................................................................................... 134 the moana experiment: a broadband ocean bottom seismology investigation of a transform plate boundary, new zealand ......................................................................................................................................................................... 136 modelación fisica y simulacion numérica en procesos de adelgazamiento litosférico para zonas convergentes 138 joint inversion of geoid anomaly data and teleseismic p-wave delay times: modeling density and velocity perturbations underneath the parana magmatic province ........................................................................................... 139 joint inversion of seismic and gravity data for velocity structure and hypocentral locations of the colombian subduction zone .................................................................................................................................................................... 140 crustal thickness estimation beneath the northern andes using the receiver function method.......................... 141 crustal models 1d for colombia from surface waves dispersion observations and receiver functions.............. 142 lithospheric and upper mantle stratification beneath colombia: .............................................................................. 143 using receiver functions from s-waves.......................................................................................................................... 143 estudio de la estructura de la corteza en la zona de convergencia placa de rivera – bloque de jalisco (proyecto tsujal) ................................................................................................................................................................. 144 travel time delays and residual topography in northwestern south america provide constraints on upper mantle temperatures and slab geometry ........................................................................................................................... 145 looking for mantle plumes under the brazilian shield: case studies in paraná and borborema ......................... 146 inferences about the lithospheric seismic structure of valle de aburra based on travel time residuals from strong motion networks ...................................................................................................................................................... 147 earth sciences research journal 22 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics deep lithospheric features of the são francisco craton ................................................................................................ 148 ambient noise tomography: preliminary results at southeast of south american platform............................. 149 relationship between intraplate seismicity and low velocities tomography anomalies in the tocantins province, brazil........................................................................................................................................................................................ 150 sismicidad localizada en la región de manto litosferico de placa sudamericana debajo de la provincia geológica de payenia sur de mendoza argentina ....................................................................................................................... 151 crustal and upper-mantle structure of the pampean flat-slab region from surface-wave tomography .... 154 estructura litosférica asociada a la sierra nevada de santa marta a partir de datos de gravimetría, magnetometría y sismología................................................................................................................................................ 155 association of mw = 6.4, 2000/06/16 and mw = 5.7, 2003/01/07 intermediate-depth earthquakes, in subducted nazca plate ......................................................................................................................................................... 156 investigation into the kinematics of basement-cored uplifts through passive source imaging of the bighorn mountains, united states ................................................................................................................................................... 158 hypocentral parameter inversion for regions with poorly known velocity structures............................................ 159 rmts session – regional moment tensor solutions: advances and new applications .......... 160 cálculo del tensor de momento sísmico a partir de la inversión de formas de onda usando el software isola: sismo de nuquí colombia del 13-08-2013 (mw 6.5) ............................................................................................... 161 moment tensor solution of moderate events at large distances: using surface waves dispersion to build velocity models for the waveform inversion, examples from brazil ....................................................................... 162 focal mechanism and depth determination of earthquakes in brazil using teleseismic p-wave waveform modeling and first-motion polarities ............................................................................................................................. 164 new strategy for weak events in sparse networks: first-motion polarity solutions constrained by single-station waveforms ............................................................................................................................................................................... 165 seismic source parameters of local micro-earthquakes in goiás state brazil by waveform inversion ................. 166 source seismic parameters of earthquakes close to funil reservoir power plant in the south of minas gerais state – brazil .......................................................................................................................................................................... 168 modelamiento de fuente sísmica del sismo del quindío (colombia) del 25 de enero de 1999 usando inversión de forma de onda de cuerpo telesísmica ........................................................................................................................... 169 moment tensor solution for a m 4.2 event in tierra del fuego .............................................................................. 170 inversion of a 2-point source model of the m7.4 2012 march 20th guerrero, mexico...................................... 172 moment-tensor solution of the intermediate-depth 2013 guaitarilla (colombia) mw 6.9 earthquake using near-regional waveform data............................................................................................................................................... 173 seismicmomenttensorsolutionsof m > 4.5 earthquakesincolombia......................................................................... 174 resolución y análisis de mecanismos focales de sismos corticales en cordillera entre 34.5°s y 36°s ................. 175 near-regional cmt and multiple-point source solution of the september 5, 2012, nicoya, costa rica mw 7.6 earthquake ....................................................................................................................................................................... 177 earth sciences research journal 23 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics el sismo mw 6.2 del 27 de febrero de 2010, salta argentina. mecanismo focal y modelos de deslizamientos usando inversión de formas de ondas sísmicas e interferometría sar ..................................................................... 180 source parameters by waveform inversion of 6.6mw earthquake in the nicoya peninsula on october 24, 2012 aftershock of the 7.6mw main shock on september 5, 2012 .................................................................................. 181 rtsg session – real time seismology.............................................................................................. 182 diseño e implentación del sistema de información de la red sismológica nacional de colombia ................... 183 determinación de la escala de magnitud local (ml) a partir de registros de banda ancha para el perú ............... 184 a streaming processing model for efficient computations of large continuous data sets ...................................... 185 moment-tensor calculation for the vega colombia mw 7.2 earthquake................................................................. 186 comparison of manual and automatic system locations for seismic events in panama ......................................... 187 physisc and statistics of the seismic source using first digit anomaly ........................................................................ 188 use of the first digit anomaly in background seismicity studies under chilean type of subduction environment .................................................................................................................................................................................................. 190 implementación del cálculo de los de los parámetros de la fuente sísmica mediante el método de la fase w, en el sgc ..................................................................................................................................................................................... 193 evolución tecnologica de la red sismológica nacional de colombia rsnc y la red nacional de acelerógrafos de colombia rnacdel servicio geológico colombiano............................................................ 194 near real-time earthquake source characterization through joint geodetic and seismic approaches at the us geological survey national earthquake information center ..................................................................................... 195 shlc session – seismic hazard in latin america and the caribbean: working across border ............................................................................................................................................................... 196 hacia un catálogo actualizado de terremotos de sur américa: ventana temporal pre-1930 ................................ 197 parámetros del terremoto de cúcuta del 1875 a partir de intensidades macrosísmicas ........................................ 199 seismotectonic and seismic hazard implications for south africa ............................................................................. 200 psha at a regional scale: european share project versus italian reference psha. and the winner is… .... 201 respuesta dinámica de suelos para managua/nicaragua............................................................................................. 202 challenges to developing a unified earthquake catalog for seismic hazard assessment in south america – addressing magnitude homogenization and depth uncertainties ............................................................................... 203 seismic activity in the pantanal basin brazil ......................................................................................................... 204 microzonificación para diseño por sismos para la ciudad de panamá, panamá ...................................................... 205 teleseismic waves registered at the seismographic station of aquidauana / ms /brazil (aqdb), in 2013. .................................................................................................................................................................................................. 206 quantifying uncertainties in site amplification caused by using low resolution soil maps for site characterization .................................................................................................................................................................................................. 207 earth sciences research journal 24 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a stochastic rupture probability model for earthquakes on subduction zones: a case study from the nazca subduction zone .................................................................................................................................................................... 208 la sismicidad en el piedemonte llanero y la amenaza sísmica de la sabana de bogotá ........................................ 209 towards the construction of a seismic catalogue suitable for psha studies in south america: instrumental contribution ........................................................................................................................................................................... 210 probabilistic seismic hazard assessment in quito city, estimates and uncertainties ................................................ 212 average shear wave velocity down to 30m (vs30) zoning for the city of pasto, colombia .................................. 214 simulación 3d de la propagación de ondas sísmicas en el valle de caracas usando diferencias finitas ....... 215 results from the 2013 pan-american advanced studies institute in santo domingo, dominican republic: joint modeling of complementary data functionals for seismic site characterization...................................... 216 caracterización del subsuelo de la zona lacustre de bogotá, usando técnicas de análisis de microtremores ...... 218 tectónica activa en el costado occidental del valle del rio zulia – norte de santander, colombia -aportes de las técnicas cuantitativas de análisis de drenaje ............................................................................................................... 219 modeling seismic hazard in south america: current state of the state-of-the-art based on activities carried out with the sara project ........................................................................................................................................................ 221 high-resolution, uiltra low power, intergrated aftershock and microzonation system .......................................... 222 el terremoto del 18 de mayo de 1875, en la región fronteriza colombo-venezolana :inventario de daños y de efectos geológicos co y postsísmicos ................................................................................................................................. 224 intraplate seismicity and seismic zoning in brazil .......................................................................................................... 226 el sismo de guatemala de 1816 (m 7.5). implicaciones para la sismicidad en el límite de placas norte américacaribe ...................................................................................................................................................................................... 227 magnitude and location of large historical earthquakes in colombia from macroseismic intensity data .......... 229 smoothing techniques applied to brazilian seismic sources characterization........................................................... 230 caracterización de la fuente sísmica asociada con la sismicidad hb, lp y drumbeats registrada en el volcán nevado del huila – colombia, en noviembre de 2008 ............................................................................................... 231 sistema de información de sismicidad histórica de colombia .................................................................................... 232 south american strong-motion database and comparison with ground-motion prediction equations: a gem initiative .................................................................................................................................................................................. 233 seismic hazard assessment of sites of large structures in the southeast of brazil .............................................. 235 evaluación probabilística de riesgos en centroamérica con la metodología capra ........................................... 236 smil session – seismic methods for illuminating the lithosphere.............................................. 237 crustal structure of the paraná basin from ambient noise tomography .................................................................... 238 reflection imaging of deep magma with natural sources: lessons from recent experiments on an interplate volcano and an intraplate earthquake ............................................................................................................................... 239 earth sciences research journal 25 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic noise environmental caracterization: application to a pilot area in bogota for seismic microzonation purposes .................................................................................................................................................................................. 240 crust and upper mantle seismic tomography in northwest of south america using surfaces waves .................. 241 crustal heterogeneity and earthquake generating properties in active fault regions in japan ............................... 242 surface wave tomography of the chaco-parana basin .................................................................................................. 243 geoscientific research for lithospheric structures of the merida andes, venezuela............................................... 244 snop session – current practices and recent advancements in seismic network operations ............................................................................................................................................................... 246 the northeastern brazil seismic network – rsisne ................................................................................................ 247 the saint peter saint paulo island archipelago seismic station ................................................................................ 248 ensamblado, configuración, instalación y monitoreo de las estaciones sismológicas en las instalaciones del observatorio de rayos cósmicos del proyecto internacional pierre auger ................................................................ 249 velocity spectral analysis for telesesismic events using the colombian seismological network as an array .... 251 evolución de la red nacional de acelerógrafos de colombia ................................................................................... 252 microseismic noise in the saint peter saint paul archipelago and its relation with some atmospheric and oceanographic variables ....................................................................................................................................................... 254 international seismological centre (isc): mission and products ............................................................................... 255 aeqc tools for retrieval and completion of seismological waveform databases from heterogeneous networks .................................................................................................................................................................................................. 256 activities of the fdsn: current and future .................................................................................................................. 257 composición y funcionamiento de las estaciones satelitales de la red sismologica nacional de colombia .... 258 portable, gps synchronized, mobile phone network-based seismological station ................................................. 259 análisis de ruido de las estaciones sismológicas del servicio geológico colombiano a partir del cálculo de densidad espectral de potencia probabilística ................................................................................................................. 260 subduction zone events in bolivia as recorded by the bolivian and brazilian networks: a test of location accuracies ................................................................................................................................................................................ 261 the new brazilian seismic network: current status and detectability test with the recent pisagua earthquake sequence, northern chile ................................................................................................................................................... 262 tarjeta amplificadora para la adapatación de la digitalización de los sistemas sismológicos de corto período en el sgc ..................................................................................................................................................................................... 264 optimización de redes de datos para el monitoreo volcánico ..................................................................................... 266 the chilean seismological observation system ............................................................................................................ 267 mapas de intensidad instrumental en tiempo real para colombia ............................................................................. 268 spsp session – statistics and physics of seismic processes ............................................................ 269 earth sciences research journal 26 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics magnitude of completeness for the seismic data catalogue of the national seismological network of colombia (rsnc), 1993-2014 ....................................................................................................................................... 270 análisis probabilístico de la sismicidad del valle de cauca y zonas aledañas, colombia ..................................... 271 earthquake catalogue processing and declustering issues ............................................................................................. 274 ¿qué produjo los sismos ocurridos al oriente de bogotá el 10 de octubre de 1743 y el 24 de mayo de 2008? .................................................................................................................................................................................................. 275 modeling seismic sources in colombian central west using inversion of telesismic body waveforms ............. 277 depth profile of the fractal dimension and the seismotectonic deformation rates associated to the earthquakes distribution............................................................................................................................................................................. 278 earthquake source parameters which display first digit phenomenon ....................................................................... 279 seismicity response to the electromagnetic probing of the lithosphere in the bishkek (kyrgyzstan) geodynamic test area ................................................................................................................................................................................... 280 distribución espacial y temporal de los terremotos destructivos y sus réplicas, del abril de 2014, cerca de managua, nicaragua............................................................................................................................................................. 281 mud volcano seismo-geodynamics .................................................................................................................................... 283 szpr – subduction zone processes (joint ssa/lacsc session) ............................................... 284 seismic velocity reduction following the september 5, 2012, mw=7.6 nicoya costa rica earthquake from ambient noise correlations .................................................................................................................................................. 285 the causative relationship between the megathrust and upper plate faults at subduction zones ........................ 286 mind the gap – the northern chile subduction zone and the 2014 iquique earthquake sequence ................ 287 spatio-temporal evolution of the 2014 pisagua, chile earthquake sequence........................................................... 288 grijalva ridge, a major player in the subduction game underneath ecuador .......................................................... 290 distribution analysis of the frequency–magnitude parameters along the subduction surface structure in south america ................................................................................................................................................................................... 291 mapping patchiness of the subduction interface in the near field: the case of the southern terminus of the middle american trench, costa rica ............................................................................................................................. 292 reverse polarity repeating events are associated with boudin formation at intermediate-depths in the bucaramanga nest .................................................................................................................................................................. 293 3d thermal modeling associated with subduction of the philippine sea plate in southwest japan ................... 294 seismological view of nazca-south american convergent margin between 10° and 20°s ................................ 295 tshr session – monitoring, early warning, and evaluation of tsunami hazard and risk ... 296 caribbean and adjacent regions tsunami warning system: seismic component ............................................... 297 local tsunamigenic earthquakes off northeastern venezuela, in the southern caribbean realm ......................... 299 método numérico para desplazamientos de la superficie terrestre debido a fuentes sismogénicas ...................... 300 análisis de modelos intersísmicos para estimación de inundación por tsunami ...................................................... 301 earth sciences research journal 27 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics assessment of heterogeneous slip distribution impact on tsunamigenic hydrodynamic processes along the northern chilean coast ......................................................................................................................................................... 303 fast epicentral distance and magnitude determination using a single three component seismic station with machine learning techniques ........................................................................................................................................... 304 a rapid estimation of tsunami run-up based on finite fault models .......................................................................... 306 mapas de amenaza de tsunami para el golfo de fonseca (el salvador, honduras y nicaragua)..................... 307 sistemas de alerta temprana (sat) en panamá ........................................................................................................... 308 el desarrollo del sistema de alerta de tsunami en nicaragua .................................................................................. 311 vmst – sharing experiences on volcano monitoring using seismic techniques in latin america ................................................................................................................................................. 312 implementación y renovación de nuevas tecnicas geofísicas para monitoreo de volcanes activos colombianos 313 sismicidad volcano-tectónica en el nevado del huila .................................................................................................. 314 enjambres sísmicos periódicos relacionados con la transferencia estática de esfuerzos entre una deformación volcánica de gran escala y fallas strike-slip corticales: complejo volcánico laguna del maule (chile) ............ 315 comportamiento de la actividad sísmica y registro de procesos deformativos en los volcanes chiles y cerro negro, colombia (2013) .................................................................................................................................................... 317 cambios en la distribución temporal del parámetro b relacionados con intrusiones de magma, herramienta para pronóstico caso erupción del complejo volcánico cordón caulle, junio 04 2011 .............................................. 320 sistema informático para el análisis y procesamiento de señales sísmicas de volcanes en ecuador ..................... 323 enjambres sísmicos en el volcán pico de tancítaro, michoacán, méxico. ¿intrusión magmática o sismicidad tectónica? ................................................................................................................................................................................ 324 actividad sísmica de los volcanes del segmento norte de colombia 1985-2014 .................................................. 325 análisis de la sismicidad tipo drumbeat y su relación con la erupción del año 2008 del volcán nevado del huila, colombia ................................................................................................................................................................... 327 análisis espectral de fuentes sismogénicas de sismos volcano-tectónicos en el volcán nevado del huila, colombia ................................................................................................................................................................................ 328 redefinición de las fuentes sismogénicas volcano-tectónicas en el volcán nevado del ruiz a partir de la actividad reciente 2010-2014 ............................................................................................................................................ 329 tomografía sísmica del volcán puracé ............................................................................................................................ 332 variaciones de la concentración de gas radón y so2 asociadas a actividad sísmica en el sector del volcán galeras, colombia, 2005-2013 ......................................................................................................................................... 333 variaciones en la atenuación de las ondas sísmicas en el volcán nevado del ruiz asociadas a la reciente reactivación 2010-2014 ...................................................................................................................................................... 334 characterization of seismo-volcanic activity in peteroa volcano, central chile-argentina ............................... 335 análisis del campo de esfuerzo actual a partir de mecanismos focales en el área del volcán nevado del ruiz, colombia ................................................................................................................................................................................ 337 earth sciences research journal 28 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics descripción de la actividad sísmica relacionada a erupciones estrombolianas violentas: vn. llaima, chile (2007 – 2010) ................................................................................................................................................................................... 338 anomalías de gas radon asociadas a la sismicidad en el region volcánica puracé .................................................... 340 seismic attenuation around peteroa volcano, argentina ............................................................................................. 341 sismicidad volcano-tectonica y registros de periodos de deformación en el volcán sotará, “segmento central de colombia” .............................................................................................................................................................................. 343 caracterización de fuentes sísmicas de eventos tipo tornillo en el volcán puracé mediante el análisis de frecuencias complejas ........................................................................................................................................................... 345 aplicación del método sompi en la evaluación temporal de las propiedades características de sismos tipo tornillo en el volcán galeras .............................................................................................................................................. 346 sismicidad volcano-tectónica en el volcán puracé ........................................................................................................ 348 seismic forecasting of eruptions at dormant stratovolcanoes ...................................................................................... 349 sismicidad frontera colombia-ecuador en la región volcanes chiles y cerro negro ........................................... 350 un modelo geofísico de la estructura interna del volcán galeras, colombia ......................................................... 352 earth sciences research journal 29 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics agte session – application of geophysical techniques on exploration conveners: scientific committee earth sciences research journal 30 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics determinación de modelos de velocidad de ondas de corte empleando el método de autocorrelación espacial (spac) en la ciudad de armenia, colombia francisco chávez-garcía1, carlos lozano2, hugo monsalve3 1universidad nacional autónoma de méxico (unam), instituto de ingeniería, méxico d.f.,méxico. 2servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá d.c., colombia. 3universidad del quindío, facultad de ingenieríagrupo quimbaya, colombia. clozano@sgc.gov.co abstract se llevó a cabo un análisis de registros de microtemblores para la determinación de perfiles de velocidad de ondas de corte, con la ejecución de arreglos lineales de estaciones sismográficas y acelerográficas, con base en la aplicación del método de autocorrelación espacial modificado (spac por su abreviatura en inglés). cinco arreglos dispuestos en forma lineal con tiempos de registro de 3 horas para cada distribución ejecutada se realizaron en los sitios puerto espejo y hojas anchas de la ciudad de armenia. mediante un proceso de inversión, se determinan los perfiles de velocidad de ondas de corte a partir de las curvas de dispersión de velocidad de fase. para los arreglos realizados en lossitios puerto espejo y hojas anchas, se obtuvieron perfiles compuestos por tres capas con velocidades entre 110 m/s y 800 m/s.a partir de las relaciones espectrales h/v se determinaron frecuencias fundamentales de 2.5 hz para puerto espejo y 2.1 hz para hojas anchas, demostrando un buen ajuste con estudios de efectos de sitio realizados anteriormente en la ciudad de armenia. palabras clave: microtemblores, curvas de autocorrelación espacial, velocidad de ondas de corte, autocorrelación spac. mailto:clozano@sgc.gov.co earth sciences research journal 31 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics gravity and magnetic constraints on the structural framework of the northern volcanic complex, central cordillera, colombia: applications for geothermal resource prospecting sebastian trujillo vargas1, caroline sue whitehill2, josé ricardo ardila roa3, warren dewhurst4 1dewhurst group, llc, 2central washington university 3universidad de caldas, 4dewhurst group, llc s.vargas@dewhurstgroup.us, whitehillgeosciences@gmail.com abstract the structural framework of the central cordillera in the region of north-central colombia is poorly constrained due to a lack of data on the fault geometries and connectivity at depth. this study presents an integrated approach using gravity and magnetic data from the span dataset and new field research, in conjunction with previous studies and data on surface mapping of faults and geological relationships, seismic tomography and earthquake focal mechanisms to analyze and interpret a series of 12 transects that span 5.166666°n to 4.33333°n latitudes, and 76°w to 75°w longitudes across the northernmost volcanic complex of the central cordillera. these profiles are used as a reconnaissance tool for geothermal exploration around the nevado del ruiz volcanic system, provide the first presentation of transects of this scale and depth (up to 60 km depth) and, insight into the structures of romeral (n-s), palestina (ne-sw), and nereidas (nw-se) fault systems; depth to an elevated brittle-ductile transition zone; depth to moho and potential geothermal conduits. this work complements current and ongoing field investigations in and around nereidas valley where the intersection of romeral, palestina and nereidas fault systems control geothermal fluid flow. deep gravity and magnetic transects provide insight into the potential location of major undiscovered and untapped geothermal fields that are controlled by the tectonic framework of the region. this dataset allows the study of conductivity contrasts, heat transfer (fluids vs. solids) as well as magma and geothermal fluid movement, thereby, illuminating the location of potential geothermal reservoir bodies. for more refined constraints on the volumetric properties of geothermal reservoir bodies we integrate low frequency and broadband magnetolluric (mt) methods for geophysical imaging of subsurface geology and exploration of geothermal resources. economically viable geothermal systems often include anomalous zones of low resistivity representing geothermally altered and impermeable cap rock with geothermal fluids underneath. mt is used to identify and isolate targets of low resistivity between depths of a few meters to many kilometers. keywords: nevado del ruiz, span, geophysics, geology, gravity, magnetic. earth sciences research journal 32 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics geófonos individuales vs arreglo de geófonos: un estudio de campo realizado en colombia diana marcela cortes1, juan carlos isaza1,jack caldwell2, camilo gonzález3 1ecopetrol s.a 2geospace technologies 3petroseis s.a. diana.cortes@ecopetrol.com.co abstract con el objetivo de evaluar el funcionamiento de geófonos individuales, se diseñó un test el cual consistió en grabar datos en un seudo 2d usando arreglos de seis elementos convencionales y un geófono individual de alta sensibilidad (solamente vertical). no se observaron diferencias relevantes cuando comparamos las trazas grabadas con elementos múltiples y las grabadas con un solo elemento en las estaciones. esta afirmación está sustentada con los análisis en la relación señal a ruido realizados en ambos conjuntos de datos. adicionalmente no se encontraron diferencias en la calidad de los datos cuando el espaciamiento entre los geófonos individuales fue puesto a intervalos de grupo (ig), de ig, 2ig/3 y ig/3 pero si mostraron baja calidad a intervalos de grupo de 2ig y 4ig/3. la distancia entre estaciones receptoras convencionales 6x1 (ig = 30 m) fue calculada para lograr el cubrimiento del subsuelo y el muestreo espacial necesarios para iluminar el objetivo deseado. los resultados de este estudio soportan el uso del geófono individual para reducir los costos de la adquisición sin reducir la calidad de los datos sísmicos ni su intervalo de grupo, con la advertencia que este conjunto de datos de campo fue adquirido en una localización particular. es necesario adquirir más de este tipo de estudios y publicar sus resultados con el fin de construir una base de datos de experiencias desde la cual se podrá crear conocimiento a partir de los hechos. palabrasclave:geófonos individuales, arreglos de geófonos, relaciona señal ruido, imagen sísmica. earth sciences research journal 33 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics estudios de amenaza geológica a partir de mediciones con georadar: identificación de deformaciones por fallas y deslizamientos en venezuela jélime aray, luz rodríguez, franck audemard, victor rocabado, kenny garcía, michael schmitz fundación venezolana de investigaciones sismológicas (funvisis). jaray@funvisis.gob.ve abstract este trabajo muestra algunos casos representativos de la respuesta obtenida con el georadar (gpr-ground penetrating radar) en mediciones realizadas sobre zonas expuestas a amenaza geológica, particularmente, presencia de fallas activas cuaternarias y zonas de inestabilidad geológica por movimientos de remoción en masa (determinación del límite roca fresca-roca meteorizada). para las mediciones se emplearon antenas de frecuencias de operación de 40, 80 y 200 mhz, resultando que las de 200 mhz generaron registros de mejor resolución, aunque con menor penetración en el subsuelo. se describen a continuación los resultados de dos de los casos que sirven como ejemplo de la utilidad del georadar en el estudio de geoamenazas: a) identificación de deformación por fallas activas en el subsuelo, sector páramo el zumbador, estado táchira (sitio previamente escogido para estudio paleosísmico): el radargrama obtenido con antenas de 200 mhz sobre el perfil t0 (zum_t0 200), muestra con claridad la deformación que ejerce la falla activa (falla de boconó) sobre los sedimentos cuaternarios. se identificaron los límites entre distintas capas de sedimentos (flujo de detritos, sedimentos lagunares y suelo) que fueron corroborados posteriormente con el análisis de la trinchera paleosísmica (imagen a). b) identificación del límite roca frescaroca meteorizada en zona de deslizamiento, vía maracay – choroní, estado aragua: en el radargrama obtenido con las antenas de 80 mhz sobre el perfil l1 (choro_l1 80) se identificó un horizonte entre 2 y 2,5 m de profundidad que describe un contraste de impedancia dado por el límite roca fresca-roca meteorizada (imagen b). hacia el extremo oeste (o) del perfil se observó el mayor espesor de material meteorizado en contraste con el extremo este (e) donde se determinó la presencia de roca fresca a profundidades someras (0,5 a 1 m). palabras clave: georadar, fallas activas cuaternarias, radargrama, amenaza geológica. earth sciences research journal 34 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 35 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics high-resolution, low power, mobile infrasound station leonid zimakov1, a. leon1, g. ertl2,m. alerborn3, a glushakov3, v. selyatinskii3, a. tonkikh3, k fedin3, v sherbakov3 1ref tek a division of trimble. 2trinas handels gmbh. 3sdc geophysical measurementsnovosibirsk, russia. l.zimakov@reftek.com abstract ref tek a division of trimble. in collaboration with trinas handels gmbh and science development center “geophysical measurements” has developed the high resolution, low power, portable infrasound station for both permanent and portable deployment. the station provides high accuracy data with minimal expense associated with transportation, deployment and maintenance. the infrasound stations combine three major components: a) high dynamic range differential microbarometer, b) high resolution broadband seismic recorder, and c) universal break-out box for easy connectivity and power supply to different system components. the station is housed in the small transportation case with external connectors for power and input signals. the high dynamic range microbarometer combines rugged construction design with wide bandwidth and low noise to ensure accurate measurements in most applications. the microbarometer measures the infrasound signals in the frequency range from 0.02 to 4 hz with the atmospheric pressure measurement range from 0.001 pa to 100 pa. the sensor does not require any altitude adjustments. the microbarometer is housed in a sealed compact case to prevent moisture damage. the high resolution broadband recorder, model 130-01/6, is a self-contained state-of-the-art 24-bit resolution, low power, 6-channel recorder. the 13001 recorder, designed to be an easy to use compact, lightweight, low power recorder, requires less maintenance than others available in the market. the 130-01 can hold up to two removable flash memory disks, which reside in a sealed compartment that is accessible by opening a lid located on the top of the 130 case, and provide data collection for extended time periods in the field. the 130-01 recorder also features built-in full duplex error correction protocol providing high reliability and data integrity of the system deployed in telemetry configuration. the universal break-out box provides easy connection between different components of the station, minimizing the installation time in the field and avoiding trouble that often occurs when a technician is manually connecting multiple system components in the field. the rotary switch located on the top of the break-out box provides seamless connection of the different sensors to the 130-01 recorder. in addition, the break-out box distributes the dc power to the station’s multiple components. the ref tek portable infrasound station offers a unique opportunity to end-users to obtain high resolution data in both stand alone and multiplesensor-array configurations. the detailed infrasound station specifications and performance are presented and discussed. keywords: mobile infrasound station. earth sciences research journal 36 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 37 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics infiriendo la estructura estratigráfica mediante interferometríasísmica a partir de datos microsísmicos y ruido ambiental omar alfredo mercado díaz y carlos alberto vargas jiménez departamento de geociencias, universidad nacional de colombia. oamercadod@unal.edu.co abstract ha sido desplegado un arreglo sísmico lineal de 17 acelerómetros triaxiales separados aproximadamente cada 70 m, alineados con la boca de un pozo productor del campo chichimene, cuenca llanos orientales de colombia. en este experimento se adquirió información por 80 horas continuas a una frecuencia de muestreo de 500 mps, que involucró el tiempo previo y posterior a una actividad de fracturamiento hidráulico realizada en diciembre de 2012. un preprocesamiento orientado a la eliminación de datos espurios, saltos por submuestreo electrónico y la respectiva filtración en bandas deseadas, permitió consolidar registros continuos para la aplicación sistemática de correlaciones cruzadas entre todas las estaciones y su respectivo apilado, obteniendo cdps que permitieron ensamblar una imagen interferométrica. la aplicación de técnicas de migración con modelos de velocidad 1d del área permitió identificar reflectores consistentes con la estratigrafía local. comparaciones entre las imágenes estimadas en este trabajo y otros adquiridas por la industria petrolera en la zona, permiten identificar contrastes acústicos de las unidades productoras. palabrasclave: interferometría sísmica, fracturamiento hidráulico, ruido sísmico ambiental, sísmica pasiva. earth sciences research journal 38 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics anst session – anthropogenic seismicity conveners: stan lasocki (inst. of geophysics poland) <lasocki@igf.edu.pl> aderson nascimento (ufrn brazil) <aderson@geofisica.ufrn.br> carlos vargas (unal colombia) <cavargasj@unal.edu.co> earth sciences research journal 39 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics integration program for research infrastructures for georesources within european plate observing system project beata orlecka-sikora and stanisław lasocki institute of geophysics pas, warsaw, poland. orlecka@igf.edu.pl abstract european plate observing system is a project, supported by the european commission, aimed at building an integrated multidisciplinary solid earth sciences research infrastructure. epos is coordinating disciplinary (thematic) communities organized presently in ten working groups. working group 10 (wg10) of epos “infrastructure for georesources” deals with hazards evoked by exploitation of georesources, in particular with induced seismicity research infrastructure (is ri). the group has brought together representatives from scientific community and industry from 15 european countries. wg10 focuses on priorities: 1) to facilitate a step-change in an anthropogenic seismicity research perspective from the present, technology-oriented approach, to one centered on physical problems of rockmass fracturing, without, however, losing touch with technological conditions of seismicity generation; 2) to intensify international scientific cooperation; 3) to improve research efficiency by facilitating instantaneous access to data, results and methodologies; and 4) to strengthen cooperation between industry and science. this will be achieved by creation of induced seismicity thematic core service (is tcs) within epos. the is tcs will be open to broader community including research and industrial groups, educational centers, administrational bodies. the is ri to be integrated comprises three components: data, software and reports. the is data consists of all data describing comprehensively separate anthropogenic seismicity cases, called is episodes. the is episode consists of time-correlated seismic and relevant production data, supplemented with other geo-data: geological, geomechanical, geodetic, etc. the second component of the is ri are software tools for data handling and visualization, the standard and advanced software for research and newly proposed algorithms for tests and development. the third group of is ri consists of both peer reviewed and unreviewed written materials. wg10 is working out strategic solutions for integration, a development of services to be provided by a future is tcs, and an adequate science plan for is tcs. measurable benefits of the integration will be the intensification of studies on hazard and risk associated with anthropogenic seismicity. keywords: epos, wg10, research infrastructure, induced seismicity. earth sciences research journal 40 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics assessing the role of static stress transfer on the spatio-temporal distribution of aftershocks caused by a mw2.2 earthquake in mponeng gold mine, south africa orlecka-sikora b.1, kozłowska m.1, lasocki s.1, kwiatek g.2, boettcher m.s.3, dresen g.2 1institute of geophysics polish academy of sciences, warsaw, poland (orlecka@igf.edu.pl). 2gfz german research centre for geosciences section 3.2: geomechanics and rheology. 3university of new hampshire, durham. orlecka@igf.edu.pl abstract the influence of the static stress changes caused by the mw 2.2 mponeng gold mine (south africa) earthquake of 27 december 2007 on spatiotemporal distribution of aftershocks is investigated. we analyze the full recorded aftershock sequence, including events beyond 300 m from the network, to model the seismicity rate changes after the stress perturbation caused by the mainshock based on the rateand state-dependent friction law. as a result we obtained seismicity rate distribution for six defined depth intervals. the model of seismicity rate distribution is evaluated for its capability in capturing underlying observations by using some metrics representing different categories of the goodness-of-fit evaluation. we chose measures representing three types of classes: information-based statistics, general distance statistics, and traditional statistics, namely the bhattacharyya metric, the psi statistic, standardized root mean square error and squared correlation coefficient. we have developed a testing method, which tests whether the observed aftershocks rate distribution does not differ significantly from the modeled seismicity rate distribution. to test such null hypothesis we generate number of control series using bootstrap method from the observed dataset of aftershocks taking into account aftershocks location error of 5m. then for every control dataset and original dataset of aftershocks the goodness-of-fit statistics are calculated providing the empirical distribution of statistics under no seismicity rate difference condition. we compare the calculated values of statistics for the original and modeled dataset with these distributions. we also estimate the corresponding p-value which is the estimated probability of rejecting the null hypothesis. the results show that the estimated number of aftershocks is generally lower than the observed one. however, the ratio of the observed and estimated aftershocks differs between the depth intervals. the best correlation is for two upper intervals, however, the deepest level shows the lowest correlation. testing procedure allows rejecting the null hypothesis of no significant difference between seismicity rates only for one depth interval containing the mainshock. presented results show that the stress-based model well explains the main features of spatio-temporal distribution of the aftershocks of the small event in mponeng gold mine. the mining-induced earthquakes can be followed by stress relaxation in a form of aftershocks located mostly on a rupture plane or in regions where coulomb stress change is positive. keywords: induced seismicity, coulomb stress transfer, statistical inference, rate/state model. earth sciences research journal 41 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics is-epos – a prototype of epos thematic core service for seismic processes induced by human operations beata orlecka-sikora, stanisław lasocki and the members is-epos team. institute of geophysics pas, warsaw, poland. orlecka@igf.edu.pl abstract the community focused on seismic processes induced by human operations has been organized within epos integration program as working group 10 infrastructure for georesources. this group has brought together representatives from the scientific community and industry from 15 european countries. wg10 aims to integrate the research infrastructure (ri) in the area of seismicity induced (is) by exploration and exploitation of georesources: tremors and rockbursts in underground and open-pit mines, seismicity associated with conventional and unconventional oil and gas production, induced by geothermal energy extraction and by underground reposition and storage of fluids (e.g. water disposal associated with energy extraction, co2 sequestration, inter alia) and triggered by impoundment of surface water reservoirs, etc. wg10 priority is to create new research opportunities in the field, responding to global challenges connected with exploitation of georesources. wg10 has prepared the model of integration, which fulfills the scientific mission and raises the visibility of stakeholders. the end-state induced seismicity thematic core service (is tcs) has been designed together with key metrics for tcs benefits in four areas: scientific, societal, economic and capacity building. the is-epos project, funded by national centre for research and development, poland within the program “innovative economy operational program priority axis 2 – r&d infrastructure”, aims at building a prototype of is tcs. the prototype will implement fully the designed logic of is tcs. research infrastructure integrated within the prototype will comprise altogether seven comprehensive data cases of seismicity linked to deep underground mining, associating geothermal energy production and triggered by reservoir impoundment. the implemented thematic services will enable studies within the use-case “clustering of induced earthquakes”. the is tcs prototype is expected to reach full functionality by the end of 2014. keywords: epos, wg10, research infrastructure, induced seismicity. earth sciences research journal 42 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics estimación simultanea del mecanismo focal de dos microsismos cercanos daniel león, jorge mejía corporación osso. danileo@osso.org.co abstract los mecanismos focales son usados en el análisis de la sismicidad inducida en campos petroleros para conocer el estado y los cambios del régimen de esfuerzos en una región, y también la orientación y el tipo de fracturas activas sísmicamente. para calcular los mecanismos focales se utiliza normalmente la información de las polaridades, de tal manera que se puedan separar las zonas de dilatación y de compresión en un mecanismo focal; y en algunos casos, esta información se complementa con las amplitudes de la p y la s; sin embargo se necesita que el evento tenga una magnitud tal que la señal no esté enmascarada por el ruido en las estaciones. esta condición hace que sea un reto estimar la geometría de los microsismos más pequeños en un yacimiento. durante las actividades de producción de hidrocarburos, los cambios en la presión de yacimiento pueden favorecer el que se generen microsismos en pequeñas fracturas en muchas direcciones con mecanismos focales muy diferentes dentro de un volumen muy reducido. sin embargo, para eventos localizados muy cercanos entre sí, tanto la estructura de la tierra como la respuesta del sensor pueden asumirse iguales para cada evento. en este caso, la relación de las amplitudes de la señal de dos eventos cercanos en una estación dependerá solamente del tipo de mecanismo focal y de la magnitud de cada evento. a partir de esta idea, se propone un método para estimar simultáneamente los mecanismos focales de dos eventos que ocurren dentro de un volumen reducido usando las polaridades y la relación de las amplitudes de la primera oscilación de la onda p en cada estación de una red superficial que registre los dos eventos. la figura 1 muestra el caso de dos eventos localizados muy cercanos entre sí, de los cuales se muestran a la izquierda las polaridades y el conjunto de los ejes p (puntos rojos), t (puntos azules) y n (puntos verdes) de los posibles mecanismos focales permitiendo un desajuste de hasta una polaridad; y la solución final determinada a partir de la relación de amplitudes a la derecha. las soluciones estimadas a partir del método propuesto ajustan entre sí las diferencias de amplitudes en varias estaciones, haciendo que estas soluciones sean más acertadas que al tener en cuenta solamente las polaridades. existen diferentes factores que pueden contribuir a un desajuste de algunas polaridades (trayectoria no conocida del rayo, no identificación de la fase correcta, ruido...), sin embargo las soluciones encontradas explican la relación de amplitudes en cada estación a causa de cada evento, lo cual hace a que estos mecanismos focales sean una mejor aproximación a la descripción real de la geometría de estos microsismos. finalmente, la implementación de este método para estimar mecanismos focales integra distintos tipos de observación de los eventos de tal manera que se puedan estimar mecanismos focales de microsismos más pequeños a partir de una red superficial de estaciones. palabrasclave: microsismos, mecanismo focal, sismicidad inducida. earth sciences research journal 43 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 44 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics variations of parameter b in se brazil from local earthquakes igor oliveira, marcelo assumpção, hans agurto-detzel university of são paulo, brazil. igor@iag.usp.br abstract the slope of the gutenberg-richter magnitudefrequency relation, known as parameter b, is widely used to characterize earthquake size distribution. variations from the average b-value = 1 have been interpreted as indicating the degree of fracture density in the crustal volume, or the predominance of water-induced mechanisms (b-values higher than that of natural seismicity). one of the difficulties of comparing b-values between different earthquake sequences is the use of different local magnitude scales based on signal duration. two intraplate earthquake sequences in se brazil (one case of natural seismicity and one case of water-induced earthquakes) were studied in search for differences in b-values by using the same magnitude scheme based on coda amplitudes. empirical regressions were found for each local station relating coda amplitudes with regional magnitudes in the range mr= 2.5 to 4.0, which were then used to estimate local magnitudes for the smaller events. in may 2012 a 4.0 mb event occurred in the sao francisco craton, near the town of montes claros. a local network was installed and recorded a set of about 30 events in the following six months, all occurring in the same rupture plane (inverse faulting mechanism on a nnw-sse oriented fault plane). these events provided a ¬b¬-value of 0.5 +0.1. although this value is unusually low, it is probably reliable because the g-r frequency-magnitude relation is quite linear above the completeness magnitude of 1.0. in 2005, a large sequence of small earthquakes occurred near bebedouro town, in the parana intracratonic basin, induced by the drilling of several new wells, which added extra water into a confined fractured aquifer. thousands of small events, with magnitudes up to 2.9 mr were recorded by a local network. using events above the completeness magnitude of 1.0, a b-value = 0.84 +0.12 was obtained. although the b-value of the induced events in bebedouro is “normal” (i.e., close to 1.0), it is nevertheless significantly higher than the b-value of the natural events in montes claros. keywords: earthquake statistics, induced seismicity, b-value. earth sciences research journal 45 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics los sonidos de las actividades de explotación de hidrocarburos jorge mejía corporación osso. jmm@osso.org.co abstract con el nombre de sísmica pasiva se acostumbra denominar a la observación sismológica en áreas de explotación de hidrocarburos, ya que la intervención del subsuelo suele generar pequeños eventos microsísmicos que pueden ser observados con técnicas estándar de la sismología. sin embargo, la sísmica es una rama de la geofísica que busca conocer detalles de la estructura y composición del subsuelo usando fuentes artificiales para tal fin. en este sentido, lo que más se aproxima a una verdadera sísmica pasiva es la técnica conocida como interferometría, que busca producir curvas de reflectividad a partir de registros de ruido; pese a que en sentido estricto no es la interferencia de ondas quien permite observar los valores de reflectividad. pero “ruido” es otra de las palabras que no representa adecuadamente el fenómeno al que se refiere, pues es claro que si las señales que llamamos “ruido” contienen curvas de reflectividad entonces estas señales no deberían ser clasificado como ruido. con ejemplos concretos en una buena variedad de condiciones de campo, el autor interpreta las mal llamadas señales de ruido en campos petroleros a través de un conjunto de transformaciones y representaciones que nos ayuda a escuchar los hermosos sonidos que produce la actividad industrial en estos campos. por ejemplo, si volvemos audible el rango del infrasonido, permitimos que el cerebro actúe y logramos aplicar con mínimo esfuerzo sofisticadas técnicas de reconocimiento que nos facilita la interpretación de ciertos tipos de señal presentes en el campo. de esta forma logramos transformar lo que comúnmente llamamos ruido en un conjunto de exóticas sinfonías que nos hablan de la industria y de sus prácticas. cuando por el contrario transformamos las señales temporales en visualmente atractivos espectrogramas y representamos su información direccional por medio de las escalas del color, obtenemos un conjunto de representaciones sicodélicas, agradables a la vista, y en ciertos aspectos mucho más representativas que las series de tiempo que permitieron su generación. en estos paisajes es posible identificar el fugaz paso de un chorro de fluido por el subsuelo o el muy armónico silbido del taladro cuando en su función se encuentra, o el simple rompimiento de una roca durante las actividades de estimulación. el reto ahora es conquistar a los operadores para que juntos logremos dar nombre a las sinfonías y producir un completo diccionario que nos permita traducir a sus letras, de manera que del ruido logremos extraer no solo alguna información estructural sino que logremos “ver” otro conjunto de fenómenos menos convencionales que el sísmico. palabras clave: microsismicidad, sísmica pasiva, ruido. earth sciences research journal 46 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics propuesta de calibración de una escala de magnitud para eventos microsísmicos que ocurren en cercanías a campos petroleros luis girón corporación osso. lgiron@osso.org.co abstract estimar con precisión la magnitud de los eventos microsísmicos observados durante el desarrollo de actividades de exploración y producción de hidrocarburos es de suma importancia para un operador a la luz de las nuevas reglamentaciones gubernamentales que se han venido desarrollando a nivel mundial. colombia no ha sido la excepción en esta materia y ha reglamentado para yacimientos no convencionales, que las actividades de la operación de estimulación hidráulica deben suspenderse en caso de que se presente un evento sísmico de magnitud mayor o igual a cuatro en la escala de richter, que tenga una localización hipocentral dentro de unos límites establecidos.pero, ¿cómo podemos garantizar la conformidad con una norma a partir de una medida tan subjetiva como lo es la magnitud de un evento sismológico? en este trabajo se estudiaron las diferentes opciones disponibles, concluyendo que es fundamental realizar un estudio de calibración de la escala de magnitud serio y detallado, y que los valores de referencia usados en estos estudios deben ser menos arbitrarios que aquellos propuestos por richter en 1935. de lo contrario, ¿con qué criterios podría la anla o cualquier otra entidad refutar las estimaciones de magnitud realizadas por la institución designada por el operador del campo para verificar el cumplimiento de la norma? la propuesta de este trabajo consiste en calcular el escalar del momento sísmico para un conjunto de eventos y aplicar la fórmula de hanks y kanamori (1979) para generar unas magnitudes de momento que sirvan de referencia para la construcción de una escala robusta. sin embargo, durante la evaluación de la magnitud de momento (mw) es frecuente hacer conjeturas acerca de algunos parámetros que no siempre están disponibles (valores de rigidez, atenuación, factores de calidad, etc.). de esta forma, la segunda parte de este trabajo se concentra en el efecto que cada uno de los parámetros tiene sobre el resultado de la evaluación final de magnitud, su posible variabilidad, y el máximo efecto esperable que tiene sobre el valor final de magnitud; culminando con la presentación de un margen de seguridad que debemos asumir los sismólogos, o un margen de tolerancia que debe contemplar el legislador al momento de confrontar la norma. palabras clave: magnitud, eventos microsísmicos, momento sísmico, calibración. earth sciences research journal 47 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics detection of induced seismicity due to oil and gas extraction in the northern gulf of mexico, usa oluwaseun fadugba and john e. ebel boston college, ma. fadugba@bc.edu abstract drilling operations and extraction of oil and gas (o&g) may lead to subsurface slumping of sediments due to reduced vertical principal stress and may lead to small earthquakes. o&g extraction is common in the northern gulf of mexico (ngm) and only 35 earthquakes of magnitudes between 2.3 and 6.0 have been recorded in the area from 1974 to the present. given the rate of oil extraction at the various oil fields in the region, the record of earthquake activity appears to underestimate the seismicity of the region. the purpose of this research is to detect more earthquakes using stacks of seismic data from the earthscope transportable usarray from 2011 to 2013, and determine the spatiotemporal relationship between the detected earthquakes and o&g extraction. the earthscope transportable usarray seismic data around the ngm were retrieved from iris database and a bandpass filter of 2 – 10 hz was applied to remove background noise. the detection algorithm was applied on the filtered seismic data at 4-minute interval with 2-minute overlap. before stacking the data from each station, the arrival times were corrected in order to account for the change in arrival times due to different epicentral distances. the stack of the previous 4minute data window was used as the background noise. a hilbert transform was applied to generate an envelope function used in the stacking algorithm in order to avoid destructive stacking of the positive and negative amplitudes of the waveforms. an event is detected when the lambda, ratio of the standard deviation of the pand s-waveforms to the standard deviation of the background noise, and the signal-tonoise ratio are greater than 3. the arrival times of detected events will be used in the grid-search algorithm to determine the best estimate of the hypocenter. magnitude and spatial distribution of the earthquakes occurrence will be determined and will be correlated in time and space with the map of offshore drilling sites. understanding the relationship between small offshore earthquakes and o&g extraction will help reduce seismic hazards around local drill holes. the results of this study may be used to appraise the seismic hazard related to o&g extraction in other oil fields. keywords: extraction, earthquakes, underestimates, earthscope transportable usarray and offshore drilling sites. earth sciences research journal 48 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics statistical analysis of seismic activity preceding the emilia-romagna (italy) earthquake sequence from may/june 2012 stanislaw lasocki1, beata orlecka-sikora1, peter styles2 1institute of geophysics pas. 1keele university. lasocki@igf.edu.pl abstract a series of earthquakes starting from m5.9 on may 20 visited emilia province of italy in may and the beginning of june 2012. six earthquakes exceeded magnitude 5, and there was a multitude of smaller events, mostly aftershocks. the province is densely populated and highly industrialized as well as the location of many high value historical monuments therefore in addition to 26 lives taken, the series resulted in extreme material loss estimated to some 12.5 billion euros. the report presents results of a statistical analysis of seismicity from seven years before the earthquake series. it is shown that this seismicity did not follow signatures of typical for background seismicity: the occurrence process was non-poissonian, the gutenberg-richter b-values was time-varying. the event rate considerably increased in the last year before the mainshock from may 20th, 2012. the analysis of time-space clustering of events has indicated a strong connection between 22 events from before the earthquake series and seven major shocks of the series. the weaker event occurrences from one year before the may 20th mainshock turn out to be correlated with a rapid increase of parameters of oil and gas production from the reservoir in this area. the binomial test has indicated that the event rate after the beginning of this increasing trend was significantly higher than the event rate before when the production parameters were decreasing. also this change of trends of production parameters correlated with the change of p and t directions of weaker seismic events that is with the change of stress field orientation. all this results does not allow for ruling out the possibility of triggering impact of the hydrocarbon production on the may 20th mainshock generation. keywords: earthquake triggering, statistical analysis, emilia-romagna. earth sciences research journal 49 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismicity induced from nearly two decades of deep brine injection, paradox valley, usa yeck, william1,2, block, lisa2, wood, chris2, king, vanessa2 1university of colorado – boulder. 2bureau of reclamation yeck@colorado.edu abstract the bureau of reclamation (reclamation) operates a deep injection well at paradox valley in western colorado as part of the paradox valley unit (pvu) of the colorado river basin salinity control program. the objective of the pvu is to reduce the salt load of the colorado river. the dolores river, a tributary of the colorado river, picks up nearly 185,000 metric tons of salt annually from natural brine groundwater inflow in the paradox valley. paradox valley overlies a salt anticline and therefore ground water in the valley can have 8 times the salinity of ocean water. pvu diverts up to 90 percent of the paradox valley brine inflow from entering the dolores river. subsurface brine flow is intercepted by the continuous pumping of shallow extraction wells located along the river. the extracted brine is then injected 4.8 km below the surface. pvu has injected brine almost continuously for nearly 2 decades. throughout pvus lifetime, reclamation has operated the paradox valley seismic network (pvsn) to monitor seismicity induced by injection. reclamation monitored seismicity 6 years prior to the initiation of injection and recorded only a single local event, 19 km from the injection well. since injection began, over 6000 likely induced earthquakes have been recorded by the pvsn. we explore the spatiotemporal evolution of earthquake hypocenters and its relationship to both injection operations and local geology. felt earthquakes are infrequent, but during its lifetime pvu has induced 3 ml 4.0-4.4 earthquakes. of particular interest is the seismic hazard presented by these induced events. we investigate this topic through exploring the spatial evolution of seismicity with respect to population centers, changes in seismicity rate, estimated and observed maximum magnitude earthquakes, and observed ground motions. key words: induced seismicity, anthropogenic seismicity, deep injection, seismic hazard. oral earth sciences research journal 50 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 51 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics ctem sesión – caribbean tectonics, earthquake monitoring, modeling and early warning conveners: victor huerfano (univ. puerto rico) <victor@prsn.uprm.edu> jay pulliam (baylor univ. usa) <jay_pulliam@baylor.edu> earth sciences research journal 52 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the mw6.4 north of puerto rico event: response timeline and event characteristics of the largest event ever recorded at the puerto rico seismic network alberto lópez1,2, fernando martínez1, victor huérfano1,2, gisela báez2, harold irizarry2, glen mattioli3, christa von hillebrandt4 1department of geology, university of puerto rico – mayagüez. 2puerto rico seismic network. 3university of texas, arlington. 4caribbean tsunami warning program, national weather service, noaa. alberto@prsn.uprm.edu abstract historically, the puerto rico – virgin islands (prvi) region has been subjected to large and devastating earthquakes such as the mw7.2 october 11, 1918, and the mw7.8 july 28, 1943 event. since its inception in the mid 70's, the puerto rico seismic network (prsn) has been monitoring earthquakes in the prvi region, and on the midnight of january 13, 2014, prsn recorded for the first time the strongest event ever recorded instrumentally by the network. the mwp 6.4 event occurred 70 km north of the puerto rico coastline. with a network of 20 broad-band seismographs, 10 continuous gps sites and access to a dozen tide gauges and dart buoys, the prsn is a multiobservational monitoring network in the northeastern caribbean, and a leading observatory in the caribbean. the january 13 event originated at 04:01:01 gmt and had a final manual review location at 19.138 n, and -66.823 e, with a depth of 36 km. with an aftershock distribution of more than 500 events, and a predominantly thrust mechanism, the earthquake must likely reflects contraction along a steep northward-dipping thrust fault. this poster presents two aspects of this event; first, we describe the response of the prsn to the event (timeline, location, assessment of the threat based on the location and magnitude, and evaluation of the communications with emergency management agencies). second is the scientific aspect (hypocenter in relation to the tectonic setting, aftershock distribution based on double-differencing relocations and its implications on the rupture zone, waveform inversion to obtain the moment tensor, shakemap and ciim's, and cgps time series). this event provides the first largest event recorded at prsn and therefore provides a real-life test to auto-evaluate the outcome of the current earthquakes and tsunami protocols and work towards improvements in case a larger event strikes in the future. keywords: puerto rico seismic network, puerto rico large earthquakes, seismic monitoring. earth sciences research journal 53 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics significant breakthroughs in monitoring networks of the volcanological and seismological french observatories in the west indies andré angalde1, arnaud lemarchand3, jean-marie saurel2, valérie clouard2,marie-paule bouin3, jean-bernard de chabalier3, steve tait3, christophe brunet3, alexandre nercessian3, richie robertson4, lloyd lynch4, machel higgins4,joan latchman4, and the team in ovsg & ovsm 1observatoire volcanologique et sismologique de guadeloupe (ovsg/ipgp). 2observatoire volcanologique et sismologique de martinique (ovsm/ipgp). 3institut de physique du globe de paris (ipgp). 4seismic research center (src/uwi). anglade@ipgp.fr abstract in the last few years, french west indies observatories, in collaboration with the seismic research center (university of west indiestrinidad), have modernized the lesser antilles arc seismic and deformation monitoring network. 15 new permanent stations have been installed to strengthen and expand its detection capabilities. the global network of the ipgp-src consortium is now composed of 21 modernized stations, all equipped with broadband seismometers, strong motion sensors, gnss sensors and satellite communication for real-time data transfer to the observatories of trinidad (src), guadeloupe (ovsg), martinique (ovsm). to improve the sensitivity and reduce ambient noise, special efforts were made to enhance the design of the seismic vault and the original stuttgart shielding (d. kurrle r. widmer-schnidrig, 2005) of the broadband seismometers (240 and 120 sec). several months of tests have been performed in order to get the maximum performance level out of the seismometers with different types of shields. this renewed network feeds the caribbean tsunami warning system supported by unesco and establishes a monitoring tool that produces high quality data for studying subduction and volcanism interactions in the lesser antilles arc. the project has been an opportunity to migrate the seismic data processing to seiscomp3 with new developed plu gins to compute the duration magnitude and locate (modified hypo71pc ) ever small events such as volcanic ones. the new plugins are integrated in seiscomp3 releases. several tools for data management and treatment (earthworm and webobs [beauducel et al., 2004]) are also continuously improved. gps data, real-time and validated seismic data (only broadband) are now available at the ipgp data center. keywords: french west indies observatories, lesser antilles arc seismic and deformation monitoring. earth sciences research journal 54 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics creating a dense seismic network for costa rica: a low-cost latin american instrument solution carlos j. ramírez u.1, lepolt linkimer1, branden christensen2 1red sismológica nacional (ucr-ice). 2osop. carlosjru@yahoo.com abstract during 1960’s, costa rica started setting its first small seismic networks within the country in order to monitor volcanic activity in the irazu volcano. this equipment was donated from international agencies, and even though maintenance was not complicated, getting spare parts was a significant issue. during the 1970’s those seismometers were moved to the central american school of geology (ecg) at the university of costa rica (ucr), which was recently created at that time. in the following years, more instrumentation was donated to the ecg, but the lack of spare parts, specialized trained technicians, and the different kinds of instruments made the network worked very inefficiently. in 1982, the ecg joined with the seismic monitoring section of the costa rican electric institute (ice), creating the national seismological network (rsn). the purpose of the seismic monitoring at ice was mainly to look for energy production facilities near volcanoes and active faults. for this reason, the rsn had some budget for purchasing seismic instruments. however, prices of instrumentation were significant high and instruments were manufactured overseas, creating difficulties for establishing a dense network. two decades later, with the digital technologies, the quality of the data increased as well as the price of the instruments, carrying seismologists to spend fortunes on seismometers. during the 2000s, the rsn initiated a great link with an enthusiastic equipment-developing group, which now is the seismic observatory of western panamá (osop). osop is now manufacturing the first series of seismic devices that fits the latin american seismic network budgets, including the sixaola-type seismometer. the sixaola seismometers are all-inone solution, with integrated six sensors (three seismic and three accelerometer components), the digitizer, the computer, and the gps into a single box. additionally, it comes with a web interface to allow for a rapid configuration, low power consumption, seedlink or earthworm export, and ip67 casing offering total protection in humid and aggressive environments like volcanic regions. the rsn just acquired a little more than 90 sixaola seismometers. this means 15 times more stations for the price of a single broad band. these new instrumentation seems to solve a long history of issues that the rsn has faced in the past. in 2014, the rsn began a cooperation agreement with the fire fighter department of costa rica in order to set up one instrument at each of their 75 fire stations. this will allow access to 24 hour personal on site, full time electricity, internet, safety, and communications. this expansion of the seismic network at low cost will allow for a significant improvement in the station coverage and quality of earthquake locations for costa rica. key words: rsn, osop, costa rica, ecg, ice. earth sciences research journal 55 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics relocation of the mainshock and aftershocks of ml 5.7 quetame earthquake, colombia gabriel dicelis1, patricia pedraza2, marcelo assumpção3 1university of sao paulo. 2colombian geological survey. 3university of sao paulo. gabriel.dicelis@iag.usp.br abstract on may 24th, 2008, 19:20 utc a 5.7 earthquake struck the center of colombia. the geological survey of colombia (sgc) put the earthquake epicenter in the se of the capital bogotá at a depth less than 30km. the earthquake affected 5 municipalities in the department of cundinamarca: quetame, fosca, guayabetal, fómeque, bogotá and el calvario in meta. the earthquake left a final balance of 6 deaths and 65 people injured. approximately 9.000 persons resulted affected by the destruction of their houses. the sgc deployed a temporary 3-component broadband network. a total of 9 seismographic stations were installed to complement the colombian national seismological network (rsnc), two days after the mainshock event due to difficult access to the area. during the period from may 2008 to february 2009 the mainshock was followed by a sequence of ~1,000 events. in this work, we present the results of relocation of approximately 100 aftershocks with magnitude larger than 2.0, recorded by the local network as well as recorded by some stations of the national network. the phase data used in the relocation are mainly from 20 stations with epicentral distance less than 200km. as relative large horizontal and vertical shift were observed in the initial rsnc location, which may indicate an over simplification of the 1-d velocity model. we checked and re-picked many seismograms of this subset and estimated, a onedimensional reference velocity model using the velest inversion process, which can simultaneously locate earthquakes and calculate the 1-d velocity model with station corrections. we used a combination of cross-correlation and arrival times to analyze the events, based on the similarity of their recorded seismograms. we combined p-wave and s-wave differential times derived from crosscorrelation and used the hypodd method to enhance the location precision, this method uses the traveltime differences to invert for relative hypocenter locations, thus eliminating the ray path effects to stations. the relocation results show that the aftershocks spread with an orientation trend nesw, which indicated a strike-slip fault consistent with the regional stress regime. key words: earthquake, relocation, hypocenter, 1d velocity model, fault plane. earth sciences research journal 56 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics aspectos geotectónicos, morfotectónicos y geodinámicos de la amenaza sísmica de la región nororiental de la plataforma caribe colombiana chicangana, g.1.3, ordoñez, a, c.1,2, vargas, c. a.3, mora páez, h.4, gómez, a.5, y ferrari, a.2 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. 2lagemar, universidad federal fluminense, niteroi, r.j., brasil. 3grupo geofísica, departamento de geociencias, universidad nacional de colombia. 4proyecto geored, área geología, servicio geológico colombiano. 5istituto nazionale di geofísica e vulcanologia, milano, italy german.chicangana@unimeta.edu.co abstract para la región nororiental de la plataforma continental caribe colombiana solo el sismo del 22 de mayo de 1834 es su único sismo histórico registrado en cerca de 500 años de historia colombiana. este sismo de acuerdo a los historiadores afectó a la ciudad de santa marta destruyendo su arquitectura colonial de manera total. de acuerdo al estimativo macrosísmico este sismo tuvo una magnitud superior a 6.0 y es posible que su epicentro estuviera muy próximo a esta ciudad. la parte oriental de la plataforma continental caribe colombiana está constituida por la sierra nevada de santa marta (snsm) al occidente, la serranía de perijá (sp) al oriente y la península de la guajira al norte. el basamento continental de esta región está constituido de rocas metamórficas que representan el basamento proterozoico andino que deriva de la orogenia grenvilliana que se presentó a finales del mesoproterozoico.hacía el norte de esta plataforma continental se encuentra la corteza oceánica constituida por la placa caribe. la colisión de la placa caribe que resultó en el engrosamiento cortical del margen continental en el noroccidente de suramérica abarcó en tiempo geológico el lapso cretáceo superior neógeno temprano, pero sin embargo el estilo orogénico que se observa hoy en esta región solo se desarrolló desde el mioceno hasta el presente.de acuerdo a la observación de campo y la evaluación morfotectónica, se demuestra que esta región es controlada en su estilo orogénico por la actividad tectónica y no por la actividad erosiva, tanto para el sector por donde discurre la falla oca al norte de la snsm y la sp, como por donde discurre la falla perijá el tigre en la sp. por su configuración morfotectónica la falla oca que separa la península de la guajira de la snsm y la sp podría liberar un sismo de una magnitud equiparable a la del sismo de enero 12 de 2010 en haití. el otro sector de esta región que registra sismicidad se asocia a la falla perijá – el tigre, la cual define el límite orogénico entre la plataforma continental caribe colombiana y la cuenca de maracaibo. en este sector se presenta una sismicidad recurrente que se asocia a la actividad de esta falla, como el sismo que fue registrado por la rsnc con mw = 4.1, el 24 de febrero de 2014. este trabajo busca que se incentiven los estudios paleosismológicos y el monitoreo tanto geodésico como sismológico para estas dos fallas. la falla oca incrementa la vulnerabilidad sísmica a ciudades como maracaibo en venezuela y, riohacha y santa marta en colombia. la falla perijá – el tigre, la incrementa por su lado a la ciudad colombiana de valledupar y la zona densamente poblada del valle del cesar ubicado entre la snsm y la sp. palabrasclave: amenaza sísmica, morfotectónica, región caribe, colombia. earth sciences research journal 57 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics slab tears and intermediate-depth seismicity in the northeast caribbean and elsewhere jay pulliam1, hallie e. meighan1, uri ten brink2, alberto m. lópez-venegas3 1baylor university. 2u.s. geological survey. 3university of puerto rico, mayaguez. jay_pulliam@baylor.edu abstract active tectonic regions where plate boundaries transition from subduction to strike-slip can take several forms, such as triple junctions, acute and obtuse corners. well-documented slab tears that are associated with high rates of intermediate-depth seismicity are discussed here: gibraltar arc, the southern and northern ends of the lesser antilles arc, and the northern end of tonga trench. seismicity at each of these locations occurs, at times, in the form of swarms or clusters and various authors have proposed that each marks an active point of tear propagation. the fore-arc region of the northeast caribbean plate north of puerto rico and the virgin islands has been the site of numerous seismic swarms since at least 1976. two six-month deployments of ocean bottom seismographs recorded several such tightly clustered swarms, along with additional events. joint analyses of the ocean bottom seismographs and landbased seismic data reveal that the swarms are located at depths of 50–150 km. focal mechanism solutions, found by jointly fitting p wave first-motion polarities and s/p amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strikeand dip-slip mechanisms at depths of 50–100 km, while events at depths of 100–150 km have oblique mechanisms. a stress inversion reveals two distinct stress regimes: the slab segment east of 65w longitude is dominated by trench-normal tensile stresses at shallower depths (50–100 km) and by trenchparallel tensile stresses at deeper depths (100–150 km), whereas the slab segment west of 65w longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50–100 km). the simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench-normal tension and deeper trench-normal compression, is consistent with flexure of the slab due to rollback. these results support the hypothesis that the subducting north american plate is tearing at or near these swarms. the 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating. keywords: slab tears, seismic swarms, intermediatedepth earthquakes, ocean bottom seismographs. earth sciences research journal 58 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics combination of high rate, real-time gnss and accelerometer observations – test results using a shake table and historic earthquake events leonid zimakov, michael jackson, paul passmore, jared raczka, alex leon infrastructure division trimble navigation limited. l.zimakov@reftek.com abstract one of the fundamental requirements of an earthquake early warning (eew) system (and other mission critical applications) is to quickly detect and process the information from the strong motion event, i.e. event detection and location, magnitude estimation, and the peak ground motion estimation at the defined targeted site, thus allowing the civil protection authorities to provide preprogrammed emergency response actions: slow down or stop rapid transit trains and high-speed trains; shutoff of gas pipelines and chemical facilities; stop elevators at the nearest floor; send alarms to hospitals, schools and other civil institutions. an important question associated with the eew system is: can we measure displacements in real time with sufficient accuracy? scientific gnss networks are moving towards a model of real-time data acquisition, storage integrity, and real-time position and displacement calculations. this new paradigm allows the integration of real-time, high-rate gnss displacement information with acceleration and velocity data to create very high-rate displacement records. the mating of these two instruments allows the creation of a new, very high-rate (200 hz) displacement observable that has the full-scale displacement characteristics of gnss and highprecision dynamic motions of seismic technologies. it is envisioned that these new observables can be used for earthquake early warning studies and other mission critical applications, such as volcano monitoring, building, bridge and dam monitoring systems. ref tek a division of trimble has developed the integrated gnss/accelerograph system, model 160-09sg, which consists of ref tek’s fourth generation electronics, a 147-01 high-resolution anss class a accelerometer, and trimble gnss receiver and antenna capable of real time precise point positioning (ppp) techniques using delivery of precise satellite corrections. the test we conducted with the 160-09sg recorder is focused on the characteristics of gnss and seismic sensors in high dynamic environments, including historic earthquakes replicated on a shake table, over a range of displacements and frequencies. the main goals of the field tests are to explore the optimum integration of these sensors from a filtering perspective including simple harmonic impulses over varying frequencies and amplitudes and under the dynamic conditions of various earthquake scenarios. keywords: seismic, geodetic, gnss, earthquake monitoring. earth sciences research journal 59 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 60 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics renovation of the national seismological network of costa rica: improving station coverage and earthquake monitoring lepolt linkimer, maría cristina araya, oscar lucke, carlos ramírez universidad de costa rica. linkimerl@gmail.com abstract the national seismological network of costa rica (rsn) is a joint effort between the seismology laboratory of the school of geology at the university of costa rica (ucr) and the seismology division of the main costa rican electric company (ice). since 1973, the rsn has continuously monitored the seismic and volcanic activity of costa rica. the goal of rsn is to develop scientific knowledge in earth sciences to transfer it to the costa rican society through teaching, research, and social work. in this presentation, we addressed some efforts performed by the rsn since 2012 for achieving this goal, separated in three groups: growing the station network, improving earthquake locations, and developing newer ways for disseminating the information generated to society. since 2012, the network has expanded to a total of 60 permanent seismic stations transmitting to the university of costa rica seismology laboratory. in 2014, 90 new short-period seismometers have been acquired to provide a new coverage configuration with higher station density in the central part of the country where most of the population and economic activities are concentrated and where we intent to locate more and smaller earthquakes. also, since 2012 earthquake locations have been improved by setting, automatizing, and integrating earthquake location routines from the seiscomp (gfz), earthworm (isti), and seisan (univ. bergen) software packages. we implemented the automatic earthquake triggering and location of seiscomp and earthworm by acquiring data from a unique waveform buffer that both systems could read. we re-locate the initial locations using seisan and report the earthquake locations through an automatized web portal, which have resulted in reducing the time of earthquake notification of a revised location. we also have developed newer ways for disseminating the information generated to society. the rsn is using facebook and twitter to report felt earthquakes and information on seismology, geological processes, scientific talks, and rsn activities. additionally, a new website was created in 2012 and an application for smartphones is been released in 2014. we see these new channels as opportunities to engage non-science audiences and encourage the population to participate in reporting seismic observations and thus providing intensity data for felt earthquakes. this approach to science education might transform the view of geological processes for costa ricans and might positively alter the current perception towards hazards. the renovation of the rsn is relevant as the knowledge developed could be applied in risk management plans, urban zonation, and emergency response in costa rica. keywords: costa rica, earthquake location, earthquake monitoring, station coverage, social networks. earth sciences research journal 61 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the tablón fault: source of the 2011–2012 tobosi earthquake swarm in central costa rica maría cristina araya, lepolt linkimer, walter montero, wilfredo rojas red sismológica nacional. mariacristina.araya@ucr.ac.cr abstract central costa rica has a high seismicity rate. this region is part of the central costa rica deformed belt (ccrdb), a 100-km broad boundary zone between the caribbean plate and the panama microplate that connects the middle american trench on the pacific side to the north panama deformed belt on the caribbean. in this study, we analyzed a portion of the ccrdb, near the town of tobosi, where an earthquake swarm took place between november of 2011 and february of 2012. we have re-located the best registered earthquakes by the national seismological network of costa rica (rsn) which is run by both the university of costa rica (ucr) and the main costa rican electric company (ice). additionally, we have computed first-motion focal mechanisms for the largest swarm earthquakes and have analyzed the tectonic geomorphology of the region by using remote sensing tools and by field trips. we have found that there is a transtension structure near the town of tobosi composed by at least three faults: the tobosi, tablón, and alumbre faults. this structure is located only 5 km south of the aguacaliente fault, which caused the deadliest earthquake in costa rican history on may 4, 1910 (ms 6.4). the earthquake locations analyzed are aligned with the tablón fault. the events varied in moment magnitude between 2.4 and 3.9 mw and depths of 1 and 9 km. the largest events were felt mainly in the town of tobosi and as far as san jose, the capital city of costa rica. based on earthquake locations, focal mechanisms, and geomorphology we have concluded that the tablón fault is a left-lateral strike-slip active fault with a normal component. the study of active faults in central costa rica is significant as it may greatly improve seismic hazards assessment for this region where most of the population and the main economic activities occur. key words: active fault, earthquake swarm, costa rica, focal mechanisms. earth sciences research journal 62 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics static stress state on the costa rican cocos-caribbean plate interface and its influence on the asperity distribution of the sesimogenic zone oscar h. lücke universidad de costa rica. oscar.luckecastro@ucr.ac.cr abstract the fore-arc region along the central american subduction zone shows a series of trench-parallel, positive gravity anomalies with corresponding gravity lows along the trench and toward the coast. these features extend from guatemala to northern nicaragua. however, the costa rican segment of the fore-arc does not follow this pattern due to the segmentation of the along-trench gravity low, the absence of the coastal low, and the presence of emerged continental mass along the fore-arc gravity high at the nicoya peninsula. geodetic and seismological studies along the costa rican subduction zone have shown the presence of coupled areas beneath the nicoya peninsula prior to the 2012, magnitude 7.6 mw earthquake. these areas had previously been associated with asperities. previous publications on asperities have proposed a mechanical model for the generation of asperities based on the structure of the overriding plate above the seismogenic zone in which dense igneous bodies disturb the state of stress on the seismogenic zone and may influence seismogenic processes. surface geology and gravity data indicate the presence of dense basalt/gabbro crust overlying the seismogenic zone where the asperities may be present. bouguer anomaly values in this region reach up to 120x10-5 m/s2 which are the highest for the costa rican fore-arc. in this work, the state of stress on the cocos-caribbean plate interface is modeled from the geometry and mass distribution of a 3d density model of the subduction zone from satellite derived gravity. results show a spatial correlation between the coupled areas at the nicoya peninsula and the presence of stress anomalies on the plate interface. the stress anomalies are calculated for the normal stress on the seismogenic zone and are interpreted as being generated by the dense material which makes up the fore-arc in the area. the dense material of the nicoya complex mafic rocks and the topographic load of the peninsula on the seismogenic zone may play a role in the distribution of coupled areas and the seismic behavior of the region. key words: asperities, gravity, subduction, seismogenic zone. earth sciences research journal 63 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the new iris earthquake browser (navegador de terremotos) r.welti, b.weertman, j.taber, t.ahern iris consortium. russ@iris.washington.edu abstract visualizing thousands of earthquakes, in regions of interest such as the caribbean, just got faster and easier, including 3d rotation. the iris earthquake browser (ieb) is a google map application that maintains an optimized cache of 3.4 million events (from 1970 to 15 minutes ago) and allows up to 5,000 to be quickly selected, rotated and exported without the need for a stand-alone application. see www.iris.edu/ieb the newest feature is a pseudo-3d viewer, called 3dv, which is both easy-to-use and requires no java or plugins. view large clusters of quakes in profile, freed from the flat world of the straight-down map. plate boundaries come to life when viewed at various angles. relationships and features normally hidden are revealed without the need to download software or import data. it is very responsive, having handled more than 10,000 unique visitors and 16,000 page hits on the day of a recent, large earthquake. ieb maps, including more than 6 parameters that can be set such as time, place, depth and magnitude ranges, can be named, bookmarked and shared via email and social networks. one can send a link to a specific event and its aftershocks complete with a title, just by dragging the ieb url to an email. streamlining, extensive help and popup tips make it easier to use than an earlier version. while targeted at the general public, educators and scientists are also finding it a useful tool. the help section includes a survey where we are soliciting suggestions for the next version. spanish language support is being added throughout ieb, and several new, popular pages, in spanish, direct traffic to ieb in “spanish mode”. hay varias formas de apoyo para gente de habla española, incluso 10 paginas nuevas, muy populares y en español, que dirigen tráfico a ieb. (www.iris.edu/latin_am/) palabras clave: historical seismicity, visualization, 3d, web services, html5, google maps. earth sciences research journal 64 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 65 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic and tsunami monitoring and data sharing in the caribbean victor huérfano, alberto lopez puerto rico seismic network/geology department, university of puerto rico at mayaguez. victor@prsn.uprm.edu abstract the circum-caribbean region has a documented history of large damaging earthquakes and tsunamis that have affected coastal areas, including the events of jamaica in 1692, the virgin islands in 1867, mona passage in 1918, the dominican republic in 1946 and the most recent the m 7.0 haiti event which killed more than 250,000 people. there is clear evidence that tsunamis have been triggered by large tsunamigenic earthquakes that deformed the ocean floor around the caribbean plate (cp) boundaries. the seismic water waves originating in the prominent fault system around the puerto rico trench are considered to be a near-field hazard for dominican republic, puerto rico and the virgin islands because they can reach coastal areas within a few minutes after the earthquake. sources for regional and teleseismic tsunami-earthquakes have also been identified. the caribbean region is monitored jointly by national/regional/local seismic networks, and there are plans to establish a caribbean tsunami warning center as well as a gps regionals data center. all caribbean networks and monitoring institutions are participating in this initiative that consists in real time (rt) earthquake, geodetic and sea level data sharing and the improved warning and data collection center. currently, more than 100 broadband seismic, more than 20 sea levels channels and more than 20 gps high rate stations are being received in the puerto rico seismic network (prsn) in real time. these rt streams are used by the earthworm/earlybird/tideview/pr-danis packages to locate and determine the size of events in the caribbean with magnitudes greater than 4.5 as well as the sea level evaluation, the solutions are provided in a timely framework. this program is also the base of a broader caribbean early warning system (cews) with the added capability of estimating strong ground shaking and the tsunami potential in advance. the cews is motivated both by research interests and geodetic, seismic and tsunami hazard monitoring and warning; it will allow to define the structure of the caribbean region to a high detail, to study properties of the seismic source for intermediate and large events, and to apply this knowledge to procedures of civil protection. to reach its goals, the virtual network has been designed following the highest technical standards: bb sensors, 24 bits a/d converters with 140 db dynamic range, real-time telemetry and good location. in addition countries are working together to improve the sea level and geodetic monitoring capabilities. all data generated by this virtual network are also shared with international geodetic, seismic and tsunami centers under the unesco icg caribe ews framework. the need to establish a system of rapid notification for earthquake/tsunami alerting in the caribbean region has been recognized by the emergency management and scientific community. key words: tsunami, earthquake, monitoring. earth sciences research journal 66 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics ecgs session – studying the earthquake cycle using geodesy and seismology conveners: hector mora-paez (colombian geological survey) <hmora@sgc.gov.co> pete lafemina (pensilvania state univ. usa) <pcl11@psu.edu> rui manuel da silva fernandes (univ. beira interior portugal) <rmanuel@di.ubi.pt> earth sciences research journal 67 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics high resolution geodetic surveys of the present day deformation along the south-caribbean margin carlos reinoza1,2, franck audemard1, françois jouanne2, michael schmitz1, christian beck2 1funvisis-venezuela, 2isterre-univ. savoie, france. creinoza@funvisis.gob.ve abstract in early 2003, a joint effort by funvisis and university of savoie installed 36 new brass benchmarks covering eastern venezuela in order to estimate the slip along the el pilar fault (epf) system and of other minor active faults, as well as the eventual rotation of tectonic blocks. most of these benchmarks were planted into stable natural rock outcrops, when possible. sites were occupied in 2003, 2005 and 2013. more recently, we have targeted western venezuela, where 19 new brass benchmarks were installed into rock outcrops in late 2011. late in the same year and in early 2013, 30 sites (including new sites and already existing benchmarks) were measured in the western network. the sites have been measured with dual-frequency gps and geodetic antennas for at least two 24h sessions with 30s sampling intervals. data have been analyzed with bernese 5.2 software using absolute antenna phase center offsets models, as well as igs final precise orbits and earth rotation parameters and data from nearby cgps stations. velocities have been estimated in the igb08 reference frame and expressed in the sa reference frame using the rotation pole proposed by altamimi et al. (2012). since the data from western venezuela is still under processing, we shall only present our main results from the eastern array. horizontal velocities on both sides of the epf depict an important asymmetry of displacement gradients that suggest a change in elastic properties across the fault. we apply a heterogeneous asymmetric model associated with contrast of material properties on each side of a vertical fault simulating observed velocities with 20 mm/yr of creep at depth, as an alternative focus to homogeneous elastic half-space model (but also considered in this work). our preferred model indicates a shallow locking depth at 3 km and 0.33k (asymmetric coefficient) indicating that the igneousmetamorphic northern side has rigidity ~2-times higher than the sedimentary southern side. next, we evaluate a three-dimensional heterogeneous model considering a near-fault low-rigidity tabular zone. the results show a compliant zone in the upper 3 km and from 1-5 km in width with a 30% rigidity reduction with respect to the adjacent rocks. this model shows 12 mm/yr of slip in the upper western segment and 10 mm/yr of displacement in the upper eastern segment; both with lower creep portions (20 mm/yr) separated at 12 km in depth. we apply a third model evaluating a fault-slip distribution from interseismic displacement. our results show a widespread partially-creeping pattern in the easternupper segment. as a last approach we upgrade the displacement simulation method with 2013 data, previously depicted in the jouanne et al. 2011 work, using non vertical dislocations to simulate the deeper parts of the epf. from comparisons using fishersnedecor test of variances we select the near-fault low-rigidity as the probabilistic best model, nonetheless a significant slip associated with creeping or partially creeping fault segments is supported for all models. moreover, the markers observed in the field in the years following the 1997 mw=6.9 cariaco earthquake (audemard, 2006) support this idea. the post-seismic afterslip following the 1997 could help to explain the creep at the eastern segment (eastward of cariaco gulf). furthermore, the localized aseismic displacement pattern along the earth sciences research journal 68 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics fault could be related to a low historical seismicity. our results also show the concentration along the el pilar fault of the whole caribbean-south america relative displacement. key words: el pilar fault, gps, creep, compliant zone, fault zone rheology. figure 1. observed velocities (black arrows) derived from comparison of 2003, 2005 and 2013 gnss campaign measurements with error ellipses drawn for 66% confidence level and expressed in the sa plate reference frame using the rotation pole proposed by altamimi et al. (2012). red vectors correspond to cgps station. the active faults in northeastern venezuela are from audemard et al. (2000). topography and bathymetry data are from lindquist et al. (2004). figure 2. velocities field in epf for several models applied in this work. we show e-w observed velocities and simulated velocities from the asymmetric model with 20 mm/y far-field velocity. two segment compliant zone model, two segment slip distribution model, displacement simulation method and homogeneous elastic model. earth sciences research journal 69 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics caracterización sismotectónica de la región del valle del cauca y zonas aledañas a partir de mecanismos focales de terremotos y datos geodésicos elkin de j. salcedo-hurtado1, jhon leandro pérez1,2, héctor mora-páez3 1grupo de investigación georiesgos observatorio sismológico y geofísico de la universidad del valle. 2observatorio sismológico del occidente de panamá (osop). 3servicio geológico colombiano, proyecto gnss geored. elkin.salcedo@correounivalle.edu.co abstract el presente trabajo analiza el mecanismo focal de 49 terremotos con magnitud mw ≥ 4.8 en la región del valle del cauca y zonas aledañas, durante el periodo 1978 – 2010; las soluciones fueron tomadas de los catálogos del isc y cmt. la profundidad de los eventos se distribuye entre superficiales e intermedios, hasta 215 kilómetros aproximadamente, asociados a la subducción de la placa nazca, y unos pocos eventos superficiales que corresponden a la actividad de fuentes corticales en la placa continental suramericana. concordante con el modelo de ambientes de regímenes de esfuerzos asociados con la deformación estática de la placa en subducción, se identifican cuatros ambientes sismotectónicos, que se distinguen por sus propias características sismológicas, tectónicas y régimen de esfuerzos. para cada ambiente se establecen relaciones entre el escalar de momento sísmico (m0) y la magnitud mw, y entre mw con las magnitudes ms y mb; además, se presenta la tasa de deformación sismotectónica (flujo sísmico de la masa rocosa) para cada uno de estos ambientes. el modelamiento del campo de esfuerzos de coulomb, realizado usando la inversión de la orientación de los ejes principales de las soluciones del mecanismo focal de esfuerzos y la orientación del esfuerzo máximo de cizalla, muestra que el esfuerzo de cizalla crea una barrera que impide que el esfuerzo compresivo se proyecte al interior del continente en dirección sw-ne, interrumpiéndose en la zona en la fosa colombo-ecuatoriana; mientras que el esfuerzo de tensión, aunque también es interrumpido, si logra proyectarse al continente en dirección se-nw, perpendicular al esfuerzo compresivo. palabras clave: terremotos, placas tectónicas, mecanismos focales, campo de esfuerzos. earth sciences research journal 70 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics present-day deformation along the andean subduction controlled by continental slivers motion f. rolandone1, j-m. nocquet2, p. jarrin3, j.c. villegas-lanza4, p.a. mothes3, m. chlieh2 1sorbonne universités, upmc univ paris 06, cnrs umr7193, istep, f-75005, paris, france. 2geoazur, ird-cnrs-oca, université de nice sophia-antipolis, france. 3instituto geofísico, escuela politécnica nacional, ladrón de guevara, quito, ecuador. 4instituto geofísico del perú, calle badajoz 169, urbanización mayorazgo iv etapa, ate, lima, perú. frederique.rolandone@upmc.fr abstract along the western margin of south america, previous studies from chileto central peru have evidenced high interseismic coupling along theplate interface and active backarc shortening in the subandean domain.new geodetic measurements from central peru to southern colombia(nocquet et al., 2014) show a completely different pattern. a segmentof ~1000 km, from north of lima to central ecuador accommodates theplate convergence predominantly by creep, with little stressaccumulation along the plate interface, explaining the absence ofgreat earthquakes for the last five centuries. we further show thatcontinental deformation in the andes from southern peru to south ofthe caribbean is controlled by the motion of two large (>1500 km long,300-500 km wide) continental domains wedged between the trench and thestable part of the south america continent, hereafter referred as“slivers”: the north andean sliver encompassing ecuador, colombia andwestern venezuela (trenkamp et al. 2002, white et al. 2003) and a new sliver that encompasses peru that we propose to name “inca sliver”. the sliver boundaries define strips of localized deformation accommodating rapid (4-10 mm/yr) motion where the major crustal earthquakes occur. these results, put in the contextof the whole andean mountain range, show that motion of continental slivers are found all along the andes. indeed, previous studies haveidentified forearc sliver trench-parallel motion in southern chile(the chiloé sliver, wang et al., 2008) and a central andes sliver(brooks and bevis, 2003, metois et al., 2013) with predominantlytrenchnormal component. we further show that the sense of trenchparallel motion is controlled by the convergence obliquity, whichchanges with the strike of the trench. our simple model furtherprovides explanation for prominent features of the andean margin: theopening gulf of guayaquil is located at the convex bend of southamerica where slivers show diverging motion, and crustal thickeningoccurs at the bolivian altiplano located at the concave bend of thesouth america. key words: subduction, gps, earthquake, continental deformation, earthquake cycle, northern andes. earth sciences research journal 71 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics assessing the seismic potential of south america with crustal deformation models and historic earthquakes gerald galgana, mehrdad mahdyiar, bingming shen-tu, claire pontbriand, elliot klein, feng wang, khosrow shabestari, wenzheng yang air worldwide. ggalgana@air-worldwide.com abstract we assess the seismic hazard potential of regions along the 7,500 km long south american (sa) trench and the active ecuador-colombia-venezuela plate boundary zone by examining active tectonic deformation, interplate coupling, crustal strain rates and historic seismicity patterns. the experiments use a continent-wide, two-decade old collection of (premaule) gps velocity field (1) observations as input to a suite of kinematic models that separately incorporate both fault-block and continuum approaches to quantify tectonic deformation. in parallel, we establish seismic activity rates for 40 seismic source zones all over sa by compiling a catalog of historic earthquakes. we quantify interplate coupling, fault slip-rates, and off-fault rates of strain for the deforming crust to provide geodetic constraints on the long-term patterns of active tectonic deformation. specifically, we solve for slip and locking patterns along the south american subduction zone to reveal a series of highly coupled patches at offshore locations near the coasts of ecuador, colombia, peru and chile. these include locked patches that indicate high-friction producing “asperities” along the subduction interface responsible for the 2010 mw 8.8 maule and the april 1,2014 mw 8.2 iquique earthquakes. predicted tectonic block motions and fault slip rates reveal that the northern part of south america deforms rapidly, consistent with that expected from a series of multiple tectonic blocks, with the northwestern region behaving as a continuously deforming region with concentrations of elevated strain rates. the fault slip rates and locking patterns predicted by our models reveal that the oca ancónpilar-boconó fault system absorbs most of the complex convergence patterns in northeastern colombia and venezuela. the guayaquil-algeciras and its associated faults in eastern colombia and ecuador (e.g., the romeral and el tambor fault systems) absorb part of the crustal northeastward transpressional motion due to the nazca plate convergence. to determine the seismicity rates and equivalent moment release in each seismic region, the spatio-temporal distribution of events were analyzed using information from historical earthquakes. in each source zone region, the moment deficit rates were determined by subtracting the seismic moment release (based on the historical earthquake catalogs) from the calculated tectonic moment accumulation rates. while most of the investigated regions have tectonic moment accumulations that are comparable with seismic moment releases detailed by historical catalogs, we identified zones that have unusually high discrepancies (e.g., the bucaramanga and medellín regions) presumably due to the sparse data coverage, inelastic deformation, and complex crustmantle interactions. eventually, we use the combined information from moment rates and fault coupling patterns to constrain stochastic seismic hazard models of the region that implement sa trench rupture scenarios. compared to seismotectonic patterns from other active subduction zones of the world, convergence along the south american trench provides a classic case of a near-orthogonal subduction-orogen system dominated by a periodic, cyclical trench seismic release. in contrast, along the earth sciences research journal 72 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics continent’s active northern plate boundary zone, multi-directional plate convergence results in strain partitioning, with strain distributed along a broad triangular region, predominantly absorbed as slip distributed along fault systems. data is from the global strain rate model (kreemer et al., in review) compiled mainly from nocquet et al. 2014; metois et al. 2012, 2013; jouanne at al., 2011; weber et al., 2001, among others. keywords: crustal deformation, gps, kinematic models, subduction, plate boundary zones, seismic hazards. earth sciences research journal 73 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics geodetic constraints on the present-day kinematics of colombia, south america h. mora-páez1, r. m. s. fernandes2, p. c. lafemina3, m. s. bos2 1colombian geological survey, gnss geored project, bogotá, colombia. 2segal (ubi/idl), covilhã, portugal. 3penn state, state college, u.s.a. hmora@sgc.gov.co abstract the northwestern part of the south american continent has one of the most complex active tectonic settings in the world. the kinematics of this region are constrained by the relative motion between the stable south america plate and the nazca, cocos, and caribbean plates, minor tectonic units (e.g., north andes, panama, and altiplano blocks), and orogenic areas (e. g., peru, puna-sierras pampeanas). the relative motion between the major and minor units, have been producing recurrent destructive earthquakes and volcanic eruptions in the recent past. here, we discuss the present-day kinematics of this area through a new estimation of the velocity field derived from cors (continuously operating reference stations) gnss (global navigation satellite systems) observations. in particular, we compute the angular velocities of the major tectonic plates using data from a network of about 100 stations. the estimated angular velocities are computed with respect to the latest global reference frame, itrf2008. we compare our estimated angular velocity model with other estimations based on geodetic and geophysical/geological data to show the consistency of the predicted motions. furthermore, we compare the uncertainty of the solutions by using different error models on the computation of the velocity field. we use the estimated angular velocity solutions to constrain the motions of stations of the colombian geored network in order to understand the tectonic complexity of this region of the south america continent. finally, we present an interpretation of the strain rate field estimated from the same gnss network. the estimated maps are correlated with geological and geophysical data in order to identify the areas in colombia with high rates of tectonic deformation. keywords: gnss, present-day kinematics, plate tectonics. earth sciences research journal 74 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics evaluación de la deformación sismotectónica asociada al sismo de mayo 24, 2008 quetame, colombia héctor mora-páez1, elkin salcedo-hurtado2, augusto antonio gómez-capera3 1 servicio geológico colombiano, proyecto gnss geored, bogotá 2 universidad del valle, observatorio sismológico y geofísico, cali. 3 istituto nazionale di geofisica e vulcanologia, milan, italia hmora@sgc.gov.co abstract los datos sismológicos y geodésicos suministran información relevante para entender la cinemática y tectónica activa, tanto a nivel interplaca como intraplaca. entre las posibles alternativas de análisis existentes, las soluciones de los mecanismos focales permiten la determinación de las direcciones en que los esfuerzos tectónicos actúan cuando ocurre la ruptura a lo largo de una falla, mientras que las mediciones gnss suministran información del campo de velocidad de corteza así como de las tasas de deformación actuales de períodos intersísmicos. de esta manera, la integración de datos sismológicos y de geodesia espacial gnss se ha convertido en una excelente herramienta para el análisis sismotectónico en diversas regiones del mundo, permitiendo la generación de modelos de deformación. el 24 de mayo de 2008, en la región de quetame, colombia, ocurrió un sismo de magnitud local en la escala de richter de 5,7 y profundidad superficial de acuerdo con el registro en 21 estaciones de la red sismológica nacional del servicio geológico colombiano, anteriormente ingeominas. el grupo de sismología de la universidad de harvard estableció para este sismo una magnitud mw = 5,9. tomando este sismo, se realizaron algunas aproximaciones relacionadas correspondientes a: 1) cálculo de la magnitud y la profundidad del sismo de quetame a partir de datos macrosísmicos con el propósito de utilizarlos como test de validación para la determinación de los parámetros del terremoto histórico del 12 de julio de 1785, ocurrido en la misma región central del territorio colombiano; 2) estimación de la deformación sismotectónica producida en la región focal durante la ocurrencia del sismo; 3) análisis geodésico del sismo a partir de datos tomados antes, durante y después de la ocurrencia del sismo. se presentan los resultados obtenidos y los análisis correspondientes efectuados. palabras claves: gnss, sismo, deformación, sismotectónica. earth sciences research journal 75 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the gnss geored project: a tool for understanding the strain rate field and the seismic cycle in colombia, south america current status and challenges héctor mora-páez colombian geological survey, gnss geored project, bogotá, colombia hmora@sgc.gov.co abstract major advances in space geodetic techniques, as well as the rapid development of gnss receivers and data transmission capabilities has launched a global revolution in the field of tectonic geodesy. under this perspective, in order to investigate the current kinematics of northwestern south america, the colombian geological survey initiated in 2007 a research and development project based on space geodesy technology. geored, the acronym for “geodesia: red de estudios de deformación” was adopted for the project "implementation of the national gps/gnss network for geodynamics”, which takes a multifaceted approach to cataloging and defining the geodynamics of northwestern south america in order to reduce the risk associated hazards within a wide plate margin deformation zone. the deployment of a high quality gnss infrastructure will be the fundamental geodetic framework for the study of crustal and atmospheric dynamics of the entire colombian territory, and at the same time sharing data and research results with neighboring countries. the generation of gnss time series, estimated using the hector software provided by segal/portugal, will give fundamental information for both regional and local geodynamics studies, and will permit the compilation of surface velocity fields that register crustal dynamic behavior, a very important issue for geohazard investigations. it is intended to achieve a greater density of the national gnss network in an effort to address solid earth issues such as tectonic plate motion, plate boundary interaction and deformation, including the understanding of earthquake cycle processes, the registration of active fault slip rates, and surface subsidence. currently, the geored network is managing 58 continuously operating gps stations, including: 50 geored gps/gnss continuously operating stations; 4 gnss continuously operating stations provided by the coconet project; the bogotá igs gps station (bogt), installed under the agreement between jpl-nasa and the sgc; the san andres island station, installed under the mou between ucar and the sgc; the medellin unme station provided by the national university, and the ccan station provided by cenicaña, a colombian sugarcane research institute. in addition to the permanent installations, more than 270 gps campaign sites have been constructed and are being occupied one time per year. full implementation of 100 permanent installations and 350 campaign sites should be completed by 2016. the authority of the panama canal and the escuela politécnica de quito, ecuador have also provided data of 4 and 5 gps/gnss stations, respectively. the gnss stations are located on the three major plates that interact within the wide plate margin deformation zone including existing gnss permanent stations at galapagos and malpelo islands on the nazca plate, and san andres island on the caribbean plate. the gnss data are processed using the gipsy-oasis ii software. the velocity vectors confirm the oblique subduction of the nazca plate and carnegie aseismic ridge collision processes at the colombia-ecuador trench, which are assumed to be the mechanism for the transpressional deformation and the “escape” of earth sciences research journal 76 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the north andes block. the northernmost vectors in colombia are indicative of the ongoing collision of the panama arc with northwestern colombia. key words: gnss, earthquake cycle, velocity field. earth sciences research journal 77 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics synchronous slow slip events and seismic swarms along the northern peru-ecuador subduction zone jean-mathieu nocquet1, juan carlos villegas lanza2, martin vallée3), paul jarrin4, jean battaglia5, monica segovia1,4, yvonne font1, marc regnier1, philippe charvis1, frédérique rolandone6, mohamed chlieh1, patricia mothes4, hernando tavera2 1geoazur,ird-cnrs-oca-univ. nice, france. 2igp, lima, peru . 3ipg paris, france. 4ig-epn, quito, ecuador. 5lmv, clermont-ferrand, france. 6istep, sorbone upmc univ. paris 6, paris, france. nocquet@geoazur.unice.fr abstract we report three slow slip events (sse) accompanied by intense seismicity that have recently occurred in northern peru and central ecuador. the first one took place in 2009 near bayovar (latitude 6°s) in northern peru in a context of very weak coupled subduction interface (conquest et al., 2014). the bayovar sse lasted 7 months and involved a maximum trenchward displacement of 14 mm. modeling of the cumulative surface displacement field shows that a shallow (<25 km depth) patch of the megathrust interface slipped up to 45 mm over a ~90 x 90-km square area, leading to an equivalent moment release of mw=6.7. during this period an abrupt increment of the seismicity rate occurred in the same area, with four earthquakes of moment magnitude mw ranging from 5.3 to 6.0. the cumulative seismic moment of these earthquakes accounts for about 30% of the total geodetic moment released. in details, the sequence alternated aseismic and seismic slips. the main events were followed by a sequence of localized aftershocks except an mw 5.8 earthquakes that occurred in the shallowest part of the subduction interface and was followed by a large aseismic slip. in ecuador, two sse occurred at the downdip limit of a locally 50x50km shallow (<15km) locked patch located south of the rupture area of the great 1906 mw 8.8 ecuador-colombia megathrust earthquake (vallée et al., 2013, nocquetet al., 2014, chlieh et al., in press). the first one occurred in august 2010 and lasted a week and involved a 20mm trenchward and 10mm uplift. gps data reveal that the sse occurred at a depth of about 10 km, with an equivalent moment magnitude of 6.0–6.3. during the slow slip sequence, seismic data show a sharp increase of the local seismicity, with more than 650 earthquakes detected, among which 50 have a moment magnitude between 1.8 and 4.1. however, the cumulative moment released through earthquakes accounts, at most, for 0.2% of the total moment release estimated from gps displacements. moreover, the seismicity observed during the sse consists in individual events and families of repeating earthquakes, which further evidences that the seismicity is driven by the stress fluctuations (vallée et al., 2013). that area experienced another sse, of similar duration but smaller size in january 2013, also synchronous to intense micro-seismicity (segovia et al., 2013). unlike most sse documented in other subduction zones, the northern peru/ecuador indicate that both aseismic and seismic slip can co-exist along the plate subduction interfaces, at least at shallow depths. they favor a view of the anatomy of the plate interface consisting earth sciences research journal 78 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics of patches with distinct frictional properties, interacting during stress release episodes. keywords:subduction, earthquake cycle, slow slip event, epru, ecuador, gps, seismology. earth sciences research journal 79 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics two earthquake cycle geodynamic control networks in costa rica: nicoya and osa-burica peninsulas marino protti1, víctor gonzález1, tim dixon2, susan schwartz3, andrew newman4, esteban chaves1,3, hernán porras1, peter lafemina5 1ovsicori-una. university of south florida in tampa. 3university of california at santa cruz 4georgia tech. 5penn state university marino.protti@gmail.com abstract the fast changing characteristics of the subducting cocos plate along the middle american trench (mat) creates a setting with a clear along-trench segmentation of the subduction zone and strong differences in behavior of the seismogenic zone. these differences include: a) frequency of background seismicity; b) occurrence or not of large earthquakes; c) recurrence time of large earthquakes, and d) the strength of coupling. a weak segment in central costa rica is bounded by two relatively strong segments capable of producing large earthquakes every 40 to 60 years. these two strong segments include peninsulas that sit right over the seismogenic zone allowing the recording of crustal deformation in the near field. this condition in addition to fast plate convergence and therefore short recurrence interval for large earthquakes makes these peninsulas excellent sites for the operation of earthquake cycle observatories, where seismic and geodetic instrumentation could document crustal deformation over an entire earthquake cycle and hopefully for several earthquakes cycles. one of these, the nicoya peninsula in nw costa rica, was a focus site of the seismogenic zone experiment initiative, of the us-nsf margins program and a seismic and geodetic network successfully captured the preseismic, coseismic and postseismic deformation associated with the nicoya 2012 mw=7.6. the other site encompasses osa and burica peninsulas in se costa rica, where an aseismic ridge subducts under the panama microplate. this segment of the mat had produced large earthquakes in 1856, 1904, 1941 and 1983. osa and burica peninsulas lie only 10 to 30 km from the trench, the angle of the plate interface is shallower than under nicoya and therefore possesses the potential to obtained, though seismic and geodetic monitoring, a very detailed map of heterogeneities along the seismogenic zone and study the roll of upper plate deformation. with that purpose, ovsicori-una is currently building up a dense geodynamic monitoring network on and around these peninsulas. we will describe the geometries, characteristics and results obtained from these networks. key words: subduction, crustal deformation, earthquakes, monitoring. earth sciences research journal 80 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics results from the national continuous gps network in ecuador paul jarrin, patricia mothes, jean-mathieu nocquet, frederique rolandone, david cisneros instituto geofisico pjarrin@igepn.edu.ec abstract we present an updated velocity field for ecuador based on the network developed by the instituto geofisico de la escuela politecnica nacional, quito, ecuador. the network now includes twenty years of measurements. the processing was made using the gamit/globk software and uses a classical twostep approach for geodynamics: loosely daily solutions are first obtained. a 7-parameters transformation is then used to derive time series expressed with respect to the itrf2008 reference frame. the velocity field confirms previous results coming from mixed campaigns and continuous measurements (nocquet et al., 2014, chlieh et al., in press): the overall velocity field is dominated by the northeastward motion of the north andean sliver at ~8-10mm/yr in central and northern ecuador, while south of the guayaquil gulf and in the southern ecuadorian andes, 4-5 mm/yr southeastward motion is consistent with the inca sliver motion further extending in peru. along the coast, high coupling is found in northern ecuador, contrasting with weak to no coupling in southern ecuador. previous solutions (nocquet et al., 2014 and cisneros et al., in preparation) included campaign data spanning the 1994-2011 time window. because the ecuadorian coast experienced several transient episodes of aseismic slip (sse) in the past years (mothes et al., 2013, vallée et al., 2013), we use our new solution to test the hypothesis of constant velocities by comparing 1994-2011 velocities to velocities from the 20102014 period. discrepancy will be analyzed and discussed in terms of magnitude and frequency of possible past sse. secondly, we will discuss the ability to derive vertical rates and how their use might improve interseismic coupling along the subduction interface. key words: gps, velocity field. earth sciences research journal 81 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics deformación basada en datos de gps en colombia robinson quintana1, luis montes vives1, camilo gonzález2 1universidad nacional de colombia. 2universidad distrital. rquintanap@unal.edu.co abstract tomando como base las coordenadas y velocidades de desplazamiento de las estaciones permanentes en colombia de la red sirgasmagna eco, se calculó la deformación del área de estudio, para obtener este resultado se usaron los siguientes métodos: el método del vecino más cercano, el método de mínimos cuadrados y el método de la distancia ponderada, el resultado final del estudio genero mapas de máxima y mínima elongación, vectores de rotación que indican la dirección con respecto al norte de las deformaciones y la dilatación que presenta las deformaciones. palabras clave: esfuerzo, deformación tectónica, vectores de rotación earth sciences research journal 82 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics determination of earthquake sources using gnss seismology r.m.s. fernandes1, h. mora-paez2, j. apolinário1, s. custódio3 1segal (ubi/idl), covilhã, portugal. 2colombian geological survey, bogotá, colombia. 2university of lisbon, idl, lisboa, portugal. rmanuel@di.ubi.pt abstract the use of gnss (global navigation satellite systems) observations, in particular gps, to sense earthquakes displacements in real-time (to be applied into early warning systems) or post-processing (to be applied in the characterization of large earthquakes) has known significant developments in the last years. this approach, known as gnss seismology, is able to complement other geophysical sensors (namely seismometers), due to its high precision, sensitivity to the longest-period bands, and capability to measure absolute displacements. the spatial pattern, magnitude, and timing of permanent displacement detected at gnss stations can be inverted for earthquake sources in order to predict the 3d displacement field. in addition, the capabilities have been also demonstrated to integrate gnss and seismic data to invert for earthquake sources. in this paper, we discuss the current state-of-art to compute co-seismic displacements using gnss observations, in particular using the precise point positioning (ppp) technique, which can provide "absolute" co-seismic displacements with respect to a global reference frame using a single gnss receiver. such an approach requires the use of consistent satellite orbits and clocks, whose accurate estimation in real-time has greatly improved in recent years. we present several cases that demonstrate the effectiveness of using gnss solutions to estimate the surface displacements due to large earthquakes. the best example of the potentiality of the gnss seismology is the modeling of the 2011 march 11 mw 9.0 tohoku-oki earthquake, japan. the large number and distribution of gnss stations in japan permitted inversion of the estimated displacement field for fault slip distribution and moment magnitude. in addition, we also show computed displacements for recent earthquakes in south america, namely the 2014 april 1 mw 8.2 iquique, chile and the 2013 february 9 mw 6.9 nariño, colombia. (1) this last earthquake is also used to demonstrate the need for good network coverage in order to properly sense the displacements using gnss. keywords: gnss seismology. earth sciences research journal 83 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics una red gravimétrica como herramienta de apoyo a la sismología yerson pardo lópez servicio geológico colombiano – universidad nacional de colombia ypardo@sgc.gov.co yjpardol@unal.edu.co abstract desde el año 2011 el servicio geológico colombiano en el desarrollo del proyecto “manual metodológico para la adquisición de datos gravimétricos terrestres” y con acompañamiento del proyecto geored, se inició la adquisición de lecturas de gravedad en algunas estaciones de la red pasiva (proceso aún en marcha). esta información combinada con la base de datos gravimétricos terrestres de la institución y estaciones de la red gravimétrica oficial, proporcionan una fuente de información geocientífica que soporta y enriquece distintos estudios e investigaciones en las geociencias del país. las mediciones del campo gravedad permiten conocer las anomalías gravitatorias, es decir, las diferencias entre los valores observados en superficie y lo teóricos, obtenidos a partir de un modelo de referencia. considerando que las anomalías tienen una relación directa con la distribución de masas, un suficiente número de mediciones gravimétricas en un área determinada permite calcular la distribución de masas o densidades en dicha zona. en sismología, los estudios tomográficos (de estructura de velocidad del subsuelo) a escala cortical se realizan con datos de estaciones sismológicas y son complementados con datos geofísicos, en donde la información de una red gravimétrica proporciona valores de densidad de alta calidad. así, una red gravimétrica, adicional al cumplimiento de su función como un marco de referencia para la geodesia, tiene múltiples aplicaciones en la geofísica y demás ramas de la geociencias. en el caso de estudio del volcán azufral de pasto el cual cuenta con una red de estaciones gnss, gravimétricas y sismológicas se muestra la aplicación de la geodesia en el campo de la sismología, utilizando la red gravimétrica como un insumo al estudio espacial de los eventos sísmicos. esta área de estudio es una analogía micro del cubrimiento trazado por geored para el territorio colombiano. en esta región piloto se realizó una relocalización de eventos sísmicos utilizando 5 estaciones de la red gravimétrica oficial (red local gravimétrica para el volcán), 307 estaciones gravimétricas de observación simple (proyecto geotermia sgc) y más de 300 eventos sísmicos registrados desde 1993 (rsnc) y 10 sismos registrados por la red sismológica local. mejorar en la calidad de la localización de eventos sísmicos permite asociarlos a fenómenos espaciotemporales, de acuerdo a los comportamientos y dinámicas asociadas a los diferentes factores (fallas, intrusiones, volcanismo, esfuerzos, ciclos etc.) de su nueva localización y periodicidad. palabras clave: geodesia, gravimetría, sismología, modelo de velocidad. earth sciences research journal 84 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics esga –earth structure general aspects conveners: scientific committee earth sciences research journal 85 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal structure of the craton san francisco, brazil derived from receiver functions george sand franca, cesar garcia pavao; diogo farrapo albuquerque university of brasilia georgesand@unb.br abstract crustal thicknesses and vp/vs ratios were estimated beneath craton san francisco, brazil, with receiver function analysis from broadband stations. crustal thicknesses and vp/vs ratios were estimated with the whole seismogram stacking technique of zhu & kanamori (2000), as well as a newly developed slant stacking which identifies each of the converted ps, ppps and ppss phases separately. our preliminary results indicate crustal thicknesses from 36 to 44 km and vp/vs ratios about 1.70. the results showed the thinning crustal at the central of sao francisco. keywords: tectonophysics, crustal structure. earth sciences research journal 86 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics abrupt changes in moho depth beneath the central andean plateau inferred from apparent pn phase velocities george sand franca1,george zandt2, susan l. beck2, james c ryan, lara s wagner3 1university of brasilia. 2university of arizona. 3university of north carolina. georgesand@unb.br abstract the central andean plateau contains some of the thickest crust on earth with an average elevation of 3:8 km and from west to east consists of the western cordillera, altiplano and eastern cordillera. we are exploring the use of passive source seismic data from the caught seismic experiment to constrain crustal thickness and apparent pn velocities in the central andean plateau. we follow the method often used for wideangle seismic data as outlined by zelt and smith [1992] for a 2d crustal thickness and pn velocity using crustal earthquakes as sources. we obtained a 2d crustal model beneath the central andean plateau using apparent pn phase velocities recorded along an ne-sw transect with ~ 25 km station spacing and a length of ~ 500 km. the model is characterized by little variation in crustal thickness between 50 and 60 km from beneath the western cordillera and altiplano and sharp bend moho near the boundary of the altiplano and eastern cordillera. our results on crustal thickness match with crustal thicknesses determined from receiver function analysis along the same transect and from use p teleseismic delay time to perform tomographic inversion. the thinner crust under the eastern cordillera is consistent with the increase in average surface elevation suggesting that this region may not be isostatic compensation entirely in the crust. keywords: crust, structural, pn apparent velocities. earth sciences research journal 87 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics characterization and identification of the meta fault (colombia) from the distribution of resistivities alvarado, h. g. martínez, a. & jiménez, a. universidad nacional de colombia-sede bogotá, departamento de geociencias hgalvarador@unal.edu.co , andmartinezl@unal.edu.co & ajimenezga@unal.edu.co abstract we performed 34 vertical electrical sounding (ves) between the vichada and casanare states, where to purchase schlumberger arrangement was used with the maximum aperture ab of 600m and mn between 1m to 150m, height above sea level in the region averaged 75m; 11 profiles were composed of three ves (on average) each, the separation between ves and the average distance between points was 417m and 6.25 km respectively; acquisition was carried out in late 2011 and early 2012; acquired resistivity values were in the range of 100 ωm to 10.000 ωm. the apparent resistivity data were processed using the software ipi2win®, on the profiles that are made can be seen the possible layers that make up the field with a maximum depth of research was 200m. was found in the north of the study area the presence of high resistivity values (5000ωm 10000ωm) compared to those found south of the area. keywords: vertical electrical sounding, apparent resistivity, profiles, meta fault. earth sciences research journal 88 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics new contributions to the knowledge of the magdalena submarine fan (colombian caribbean) from seismic and high resolution bathymetric data j. idárraga-garcía1, c.a. vargas j.2 and h. león3 1geosciences doctorate program, universidad nacional de colombia, bogotá (colombia). 2departament of geosciences, universidad nacional de colombia, bogotá (colombia). 3dirección general marítima-dimar, bogotá (colombia). jidarragag@unal.edu.co abstract from the analysis of high resolution bathymetric, backscatter and seismic data recently acquired in the central sector of the colombian caribbean, we present new contributions to knowledge of the morphology and internal architecture of the magdalena submarine fan (msf). previous studies have focused on describing the morphological and seismic characteristics of the upper and middle parts of the fan. the results presented here include a detailed characterization of the upper, middle and lower parts of this active continental margin-related fan, highlighting their features and geomorphic processes of erosion/sedimentation. also, we determined accurately the msf outermost limit in the deep part of the colombia basin. finally, information from several authors on the geomorphology and internal structure of other submarine fans around the world developed in both passive and active continental margins was collected in order to make a comparison with our findings and establish differences and similarities in the fan development. key words: magdalena submarine fan, morphology, internal architecture, colombian caribbean, active continental margin. earth sciences research journal 89 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics first insights into the crustal structure of the merida andes, venezuela, along the southern profile avila j.1, schmitz m.1, gil e.2, mazuera f.3, yegres l.1, montilla l.4, pinto h.5 1funvisis, fundación venezolana de investigaciones sismológicas.caracas venezuela. 2pdvsa, petróleos de venezuela s.a. puerto la cruz, venezuela. 3universidad de los andes (ula). mérida, venezuela. 4universidad central de venezuela (ucv). caracas, venezuela. 5universidad de simón bolívar (usb). caracas, venezuela. jesavila@gmail.com abstract the merida andes orogen is the topographic prolongation of the eastern coordillera of the colombian andes. several works and interpretations have been done regarding crustal structure of merida andes due to the important role it has played for the development of hydrocarbon basins associated at both flanks. most of the authors agree that the uplift of merida andes in the miocene is contemporary with the one of eastern colombian cordillera, and that both chains are subjected to strain partition process. between february 18th and march 11th 2014, a deep seismic profiling campaign was carried out on three profiles perpendicular to the structure of merida andes in western venezuela, in order to obtain information on crustal structure, as well as a better understanding of the dynamics in this complicated tectonic region. the seismic data was acquired along three principal deep seismic profiles called by their location as: north andes, central andes and south andes. this work is focused on the structures of south andes profile which is about 450 km in length, the northern end is located west to maracaibo lake near catatumbo river, crossing perpendicularly the andes axis, and the southern end is located in elorza town. the seismic data acquisition was made in two phases, in the first one; six shots of 170 1370 kg of explosives (proportion of pentolite & anfo) were recorded by 567 seismometers (texan) along the profile with spacing of approximately 500 m between each one within chain, and 1 km spacing in the basins. in addition, 300 seismometers with 1 km spacing were installed along three profiles between 120 – 150 km length, running perpendicular to south andes profile in order to obtain crustal information along the strike of the andes. in the second phase, eight shots were done using 110 – 500 kg of explosives and recorded by 918 seismometers installed with a different geometry. in the chain a 100 – 200 m of spacing was used, while in the adjacent basins spacing was 1 km between seismometers. in the early stage of data conversion and data processing, a good level of recorded energy is observed. key words: crustal, structure, merida andes, venezuela. earth sciences research journal 90 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics relación entre estructura del subsuelo y movimiento sísmico. el caso de la ciudad de concepción, chile francisco j. chávez-garcía1, gonzalo montalva2, luis quintanar robles3 1instituto de ingeniería, universidad nacional autónoma de méxico. 2universidad de concepción, bio-bio, chile. 3instituto de geofísica, universidad nacional autónoma de méxico. luisq@geofisica.unam.mx abstract chile es un país con una alta sismicidad. en el sur de ese país ocurrió el sismo más grande que ha sido registrado en la historia (22 de mayo de 1960, mw 9.5). recientemente, en una zona ligeramente al norte de ese evento ocurrió nuevamente un gran terremoto el 27 de febrero de 2010 (mw 8.8). el epicentro se localizó 150 km al noroeste de la ciudad de concepción. el foco tuvo una profundidad de 30 km. este evento causó daños significativos en gran parte del país, desde la región metropolitana hasta la araucanía. en la ciudad de concepción, la segunda ciudad más poblada de chile, la intensidad de mercalli alcanzó ix. hubo numerosos daños en estructuras y se colapsó una torre habitacional. es claro que la magnitud del evento y su cercanía a la ciudad fueron factores importantes en la destrucción observada en concepción. sin embargo, en la ciudad, la distribución de daños fue sumamente irregular y no parece estar relacionada de forma simple con la calidad de las construcciones. por ello, es posible que una parte de los efectos observados se deban a la amplificación del movimiento sísmico por la estructura irregular del subsuelo en la zona; los efectos de sitio. en este trabajo abordamos la relación entre la estructura geológica de concepción y el movimiento sísmico. lamentablemente, el evento de 2010 fue registrado únicamente por dos estaciones sismológicas en la zona urbana de concepción. por ello, recurrimos a información geológica y a mediciones geofísicas para caracterizar la estructura del valle y la amplificación local. mediciones detalladas de gravimetría permiten estimar la profundidad del basamento rocoso en la zona central de la ciudad. por otra parte, mediciones de ruido sísmico fueron utilizadas para estimar los efectos de sitio mediante cocientes h/v. finalmente, algunas mediciones de ruido sísmico con arreglos de estaciones permiten estimar la velocidad de ondas de cortante en los sedimentos del valle y en el basamento rocoso. los resultados muestran una excelente correlación entre forma de la cuenca y periodo dominante. los valores de amplificación estimados muestran un buen acuerdo con la estimación del contraste de impedancia entre sedimentos y basamento. esto sugiere que, en el caso de la ciudad de concepción, los efectos de sitio están controlados por el contraste entre depósitos sedimentarios relativamente homogéneos y el basamento. la forma de la interfase en la base de los sedimentos puede influir considerablemente en la respuesta sísmica. nuestros resultados son un primer paso hacia la construcción de un modelo 3d de la cuenca de concepción. palabrasclave: amplificación sísmica; efectos de sitio; estructura geológica. earth sciences research journal 91 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics borehole climatology – principles, applications and results vladimir cermak geophysical institute, czech academy of sciences, prague 14131, czech republic. cermak@ig.cas.cz abstract shallow subsurface is a valuable source of information how to reconstruct recent climate history. by monitoring and analyzing temperature field down to several tens or hundreds meters we can trace what has happened on surface hundreds or thousands years ago. long term climate changes accompanied by variations in the mean annual air temperature also govern the soil temperature, the time variations of which propagate downwards with attenuated amplitude and delayed phase. characteristic examples of extracted climate recollections from various holes will be demonstrated as examples and discussed. under favorable conditions climate "signature" of the major climate events, such as the little climate optimum and the little ice age can be recognized, nowadays “stored” some 400 700 m below the surface. clear traces of recent global warming are generally common in many boreholes, evidencing a temperature rise of 1 2 k since the beginning of the 20th century. certain geographical pattern of regions exists, where the recent warming rate has been more pronounced or where it has been relatively weaker, suggesting a potential impact of human activities. the inferred ground surface temperature histories were complemented with a detailed analysis of air temperature records from selected european meteorological stations, used also to test and calibrate the climate reconstruction method. to better understand the mechanism of the downward penetration of the surface climate signal, we have been running a "geothermal observatory" by monitoring air, surface and soil temperatures within and below the near surface "active" layer, i.e. the zone where temperature field is affected by surface annual cycle (uppermost 40-50 m). results of almost 20 year-long observation experiment were completed with additional experimental data confirming the significant role of the surface (vegetation) cover as well as the effect of meteorological phenomena such as rain fall, snow cover, winter freezing and thawing. major outcome of these studies confirms a certain possibility to quantitatively describe the present (climate) warming and assess its magnitude. keywords: climate change, recent warming, borehole logging, underground temperature, geothermal gradient. earth sciences research journal 92 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics gses session –earth structure general aspects conveners: scientific committee earth sciences research journal 93 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos no viscosos inmiscibles alejandro duitama leal, luis montes vides universidad nacional de colombia. aduitamal@unal.edu.co abstract este artículo reporta los resultados suministrados por la propagación de ondas elásticas en medios porosos, simulada mediante una generalización de las ecuaciones de biot las cuales describen el comportamiento de las ondas acústicas y de corte. estas ecuaciones fueron discretizadas mediante el método de diferencias finitas y los algoritmos codificados en c++/mpich. los algoritmos se aplicaron en modelos porosos homogéneos isotrópicos total o parcialmente saturados con dos fluidos no miscibles, como agua y gas, con el fin de observar cómo la porosidad y la saturación atenúan la amplitud y cambia la fase en las ondas acústicas. para determinar independientemente los efectos de la porosidad y la saturación en la atenuación de la amplitud y el cambio de fase de las ondas acústicas, se realizaron dos clases de simulaciones separadas. en el primer caso, los modelos estaban saturados completamente cada uno con diferente porosidad mientras que en el segundo caso, los modelos tenían igual porosidad y distinta saturación. la onda que se propaga se observó en diferentes puntos en profundidad y las ondas registradas se analizaron espectralmente. la salida proporcionada por la simulación en los modelos completamente saturados, señaló una relación en la que la velocidad de la onda acústica disminuye no linealmente con la porosidad. además, que la frecuencia asociada a la mayor amplitud del espectro se desplaza a frecuencias más bajas con la profundidad, y que la fase de la onda cambia linealmente con la frecuencia siendo esta tasa de cambio mayor con la profundidad. se estableció una relación aproximadamente lineal entre la amplitud máxima y su frecuencia asociada y, finalmente, se observó que la atenuación se vuelve más fuerte cuando aumenta la porosidad y este comportamiento se refuerza cuando la frecuencia aumenta. palabrasclave: teoría de biot, atenuación, saturación, porosidad. earth sciences research journal 94 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 95 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics modelado de atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos viscosos inmiscibles alejandro duitama leal, luis montes vides universidad nacional de colombia. aduitamal@unal.edu.co abstract se realizó la simulación de la propagación de ondas elásticas en medios porosos, a partir de la generalización de las ecuaciones de biot que permiten describir y modelar la propagación de las ondas compresionales y transversales. se desacoplaron los dos campos de onda y se trabajaron las 9 ecuaciones diferenciales acopladas utilizando el método de diferencias finitas para desarrollar un algoritmo basado en el lenguaje c++ y mpich para programación en paralelo. se utilizó como modelo un medio con porosidad y saturación constante en todo su volumen, para observar cómo la viscosidad atenúa la amplitud y cambia la fase en las ondas acústicas. para ello se colocaron puntos de registro a diferentes profundidades con el objetivo de registrar el campo de onda p y así poder hacer el respectivo análisis del espectro de amplitud y de fase de la señal. los resultados permitieron determinar que la viscosidad no influye en la velocidad de la onda acústica, que la atenuación se hace menor al aumentar la viscosidad mientras que el ancho de banda sísmico se hace más estrecho cuando la viscosidad disminuye. el comportamiento de la atenuación resalta que su determinación permite discernir un fluido según su viscosidad y por ende permitiría diferenciar un reservorio saturado con agua de otro con hidrocarburo. palabrasclave: factor de calidad, viscosidad, teoría de biot. earth sciences research journal 96 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 97 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics application of mee algorithm for medium-term earthquake forecast in the koyna-warna region alexey zavyalov1, alexander ponomarev1, rajender chadha2 and srinagesh davuluri2 1institute of physics of the earth ras, moscow, russia, zavyalov@ifz.ru 2national geophysical research institute, hyderabad, india, chadha@ngri.res.in abstract this report was the first to use the mee (map of expected earthquakes) medium-term earthquake prediction algorithm to develop maps of expected earthquakes in a classical area with a transient seismic regime, namely the koyna-warna reservoir site. the local earthquake catalogue for this area, covering the period of time from 1996 to 2012 (approximately 17 years) and including 4,500 earthquakes with ml=0-6.5 magnitudes that occurred in the depth range of h=0-20 km, was used. linear dimensions of the seismic area are 40×60 km. approximately half of all earthquakes included in the catalogue are the aftershocks of earthquakes with ml≥4. they were not excluded from the catalogue when calculating time and space distributions of predictor parameters and expected earthquake map values. magnitude mc=2.1 selected as a representative magnitude. a standard set of seismic predictor parameters used for expected earthquake mapping of seismically active regions with pronounced tectonic activity was used for the koyna-warna area: b-value of the magnitudefrequency relationship, number of earthquakes in the form of relative seismic quiescences and in the form of seismicity activations, released seismic energy in the form of energy quiescences and in the form of energy activations, and density of seismogenic ruptures. earthquakes with ml≥4.0 were selected as targets for prediction. in 1996-2012, 26 such earthquakes and their groups occurred in the area under study. among these earthquakes, four groups of events that include earthquakes with 5.0≤ml<5.5 were the largest. seven groups include earthquakes with 4.5≤ml<5.0. unconditional probability of a major earthquake in the grid cell was estimated as p(d1)=0.1698. a series of 42 expected earthquake maps was developed for the koyna-warna area, from 1 july 2002 till 1 october 2012, with 3-month step and 2year prediction periods for each map. the findings of using the mee algorithm in a classical area with a transient seismic regime for the first time were very encouraging. they showed that its prediction reliability was quite high and equal to jmee=2.76. zones with conditional probability levels p(d1|k)≥90% experienced 56.3% of all earthquakes with ml≥4.0. the alarm area was 20.4±8.4% of the total area of observations. the prediction can be verified in real time using the most recent expected earthquake map in the series for the period from 1 october 2012 to 30 september 2014. therefore, integral predictive reliability estimates obtained when the mee algorithm was used for the koyna-warna reservoir site are close to the average values of these parameters for all previous seismically active regions. these findings may be considered proof of the flexibility of the proposed algorithm. this work was supported by the project "transitional geophysical processes in the areas of natural and technogenic influences: field observations and physical modeling" in the integrated long term program (iltp) of cooperation in science, technology and innovation between the government of the russian federation and the government of the republic of india. earth sciences research journal 98 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics key words: forecast, koyna-warna region. earth sciences research journal 99 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics determination of the stress field in an oil production area by mean of analysis of ambient noise andrés rincon, carlos a. vargas universidad nacional de colombia. cavargasj@unal.edu.co abstract the fracking process in a well-hole in production of oil or/and gas, pursue the increment of its production and had been wide known in the hydrocarbon industry. there is a direct relation between the direction through the fracture is propagate and the distribution of stress field present in the area. studies had been done under the use of vsp techniques in order to achieve this task. however this problem could be studied in the surface by mean the localization of micro-seisms which took place after the fracking and mapping them during the relaxing time and the accommodation of the stress. according to this, the problem had been focus on filtering the surface noise sources and how the waves are attenuated in the path to the surface. a velocities model had been taken as a starting point to be able to locate these events. the acquired data used come from 3 hours records of seismic ambient noise in 23 3-component stations, deployed in the field in a linear array over a wellhole during its fracking procedure. collecting 204 cgf files (728 megabytes of information). due limitations in the record system and in order to be able to process the signals it had been necessary to make an edition work on them. mainly related with removing of steps, over-scaled amplitudes and anomalous trends in the records. after editing the signals it was possible to make an analysis of the site effect for each station. this by mean calculation of the spectral ratio which allows determinate the influence of superficial waves over the body waves for the records. then a 1d inversion was done resulting in the characterization of the elastic parameters. using this information it is possible to determinate if a transformation exist in these elastic parameters or the stress field after the fracking in the area. key words: stress field, oil production area, ambient noise. earth sciences research journal 100 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics study of the montes claros seismogenic fault, in the sao francisco craton, brazil, with correlations of pand s-wave phases caio ciardelli, marcelo assumpção, hans agurto-detzel institute of astronomy, geophysics and atmospheric university of sao paulo sciences. caio.ciardelli@gmail.com abstract the recent seismic activity in the small city of montes claros, located in northern minas gerais state, started with a magnitude 4.0 earthquake in may 19, 2012. since then, tens of events have been recorded by the recently installed local network, in cooperation between the universities of brasilia and sao paulo. initially, the project focused in a small sequence of eight events occurred in december 19 and 20, 2012. these events were identified and picked using the sac software (tapley & tull, 1992). a visual cross-correlation process was carried out in an attempt to reduce the relative pick errors both of the p and s phases. the absolute location of the hypocenters was performed using the hypocenter program (lienert & havskov, 1995). comparing the results between non-correlated and correlated data, a significant improvement was noticed in the second group because the hypocenters were more consistent with a plane. once this first analysis has been completed, a 3component cross-correlation program was developed using python language and the obspy (beyreuther et al., 2010), matplotlib (hunter, 2007) e numpy (jones et al, 2001) packages. the algorithm can perform the cross-correlation using the standard technique, which consists in using a single event as reference to correct all the others. nevertheless, it doesn't lead to good results every time because, seldom, all events have a similar waveform with the reference. trying to minimize this problem, a new technique was created. this other algorithm starts searching for the most similar event to be correlated with the reference and, then, corrects it. the idea is that, once an event has already been corrected, it also can be used as a new reference to correct others. so, the next event to be correlated will be the most similar with initial reference or with any event already corrected. this process continues until all events have been correlated. this technique was called “branched correlation” and has already been applied with good results in the first study of the 2012-2013 montes claros sequence (hans et al., 2014). the double-difference location algorithm, implemented in the hypodd code (waldhauser et al., 2000), was used to relocate the events, improving their relative locations and better defining the seismogenic fault plane. the comparison between the results before and after the two kinds of crosscorrelation is shown in figure 1. the events occur in a single fault plane with nnw strike and ene dip. the aftershocks are distributed around a “gap” circular area with a diameter of about 1.5 km; the central aseismic area probably denotes the stressreleased rupture of the mainshock with magnitude 4.0. lastly, the focal mechanism for two well recorded events was studied using focmec (snoke et al., 1984) and the results were very consistent with the plane determined using the swarm relocated with the “branch-correlated” method. keywords: sao francisco craton, cross-correlation, hypocentral location. earth sciences research journal 101 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 102 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics máximos pga, pgv y pgd en el plano horizontal sebastián d. panella, carlos d. frau, miguel e. tornello universidad tecnológica nacional facultad regional mendoza. spanella@frm.utn.edu.ar abstract el conocimiento de las características del movimiento del suelo debido a terremotos que afectan las construcciones es de vital importancia para poder elaborar métodos confiables de diseño sísmico, la presencia de pulsos de aceleración, velocidad o desplazamiento en el movimiento del suelo constituye un factor importante en el daño de las estructuras. distintos investigadores han desarrollado una gran variedad de parámetros para caracterizar el movimiento de suelo debido a sismos a partir de los registros de aceleración, de las historias de tiempo de velocidades o de desplazamientos; por ejemplo: intensidad de arias, intensidad de housner, aceleración efectiva, potencial destructivo entre otros. pese a la gran variedad de estos parámetros el valor máximo de la aceleración, velocidad o desplazamiento conocidos como pga, pgv y pgd (peak ground acceleration, velocity and displacement) siguen siendo los más utilizados en los diversos ámbitos de la ingeniería. por otro lado, se conoce que la orientación de los registradores no siempre captura la dirección predominante del movimiento en donde ocurren los picos máximos. para salvar esta situación, en el análisis de registros de campo cercano se ha recurrido a rotar los registros a una dirección determinada, por ejemplo perpendicular a la traza de la fuente sismogénica. en este trabajo se presente un sencillo procedimiento para determinar el máximo pga, pgv o pgd en el plano horizontal y la correspondiente orientación en la que ocurre. el método consiste en el cálculo del módulo del vector resultante instante a instante para dos componentes horizontales ortogonales de aceleración velocidad o desplazamiento del suelo; conjuntamente con el módulo del vector resultante se determina la orientación del mismo también para cada instante de tiempo. luego, se determina el valor máximo para toda la historia de tiempo, el instante en que ocurre y la orientación que presenta. por último se rotan las componentes originales del registro a esta nueva dirección con la certeza que en ella ocurre el verdadero pico máximo. los resultados muestran que para ciertos casos los valores máximos hallados exceden en más del 35% a los correspondientes a la dirección de registración. palabras clave: registros sísmicos pga pgv pgd horizontal -orientación de registración. earth sciences research journal 103 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 104 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics isc products for large earthquakes in the latin america and caribbean region dmitry a. storchak, domenico di giacomo, james harris international seismological centre. dmitry@isc.ac.uk abstract the international seismological centre (isc) is a non-governmental non-profit-making organization funded by 62 research and operational institutions including six in the latin america and caribbean (lac) region. the main mission of the isc is production of the isc bulletin, yet we also provide two special products related to large earthquakes: the isc-gem catalogue and the isc event bibliography. the isc-gem global instrumental earthquake catalogue presents the most homogeneous record of some 19,000 moderate to large earthquakes instrumentally recorded during 1900-2009 period. hypocentres of all events in the catalogue are redetermined using uniform and rigorous location and depth determination procedures. all magnitude values are expressed by either a direct value of mw, or an mw proxy estimate based on newly derived non-linear regressions between ms-mw and mbmw. the catalogue is an invaluable source for seismic hazard studies on global and regional scales, including those currently performed in the lac region. the isc event bibliography links seismic events in the isc bulletin with relevant scientific publications. it is an interactive map-based web-service that allows users to select an individual earthquake or earthquakes in a region of interest during a particular period of time and obtain a list of references to relevant scientific articles (including doi) published by selected authors during selected period in selected journals. currently the isc event bibliography includes over 16,000 individual publications from about 500 titles related to events occurred in last 100+ years. several thousands of those articles are devoted to ~800 earthquakes in the lac region. the journals included in our database encompass a variety of fields in geosciences (e.g., engineering seismology, earthquake seismology, geodesy and remote sensing, tectonophysics, monitoring research, tsunami, geology, geochemistry, hydrogeology, atmospheric sciences, etc.), thus making this service useful in multidisciplinary studies. keywords: earthquake, hazard, bibliography. earth sciences research journal 105 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics completitud del catálogosísmico en el centro occidente de colombia elizabeth mazo lopera servicio geológico colombiano (sgc). emazo@sgc.gov.co abstract se compiló un catálogo sísmico para la región centro occidental de colombia, con información sismológica de las bases de datos del observatorio sismológico de la universidad del quindío (osuq), y de la red sismológica nacional de colombia (rsnc), para el cuadrante de monitoreo del osuq que cubre las siguientes coordenadas geográficas: latitud 4°n – 6°n y longitud 78°w – 74.5°w, del cual forman parte los departamentos de quindío, caldas, norte del valle, tolima y risaralda, ubicados en una zona de amenaza sísmica alta según el reglamento colombiano de construcción sismo resistente nsr-10. el catálogo presentado contiene información para el periodo comprendido entre los años 1999 – 2011 con magnitudes mínimas y máximas, además se constituye como un aporte importante para el conocimiento de la sismicidad de la región centro occidental de colombia, así como para la determinación de la amenaza sísmica. se organizó una base de datos que incluye 3737 eventos sísmicos registrados por el osuq y 5142 eventos de la base de datos de la rnsc. mediante un proceso de unificación de los catálogos sísmicos y de su homogenización se procedió a la depuración, la cual consistió en la eliminación de registros repetidos y réplicas, obteniendo de forma definitiva 5583 eventos sísmicos. se clasificaron los eventos sísmicos según su profundidad para determinar la completitud del catálogo sísmico y los parámetros de “a” y “b”, mediante los métodos de regresión lineal y máxima verosimilitud (mmv). se tuvo en cuenta los resultados obtenidos con el mmv por la confiabilidad en los resultados, en comparación con la regresión lineal. para eventos sísmicos con profundidad superficial (0 30 km) se obtuvo una magnitud de completitud mc de 2.2 y valores de a = 4.9 y b = 0.79. se determinó una mc de 2.2 y valores de a = 4.26 y b = 0.74 para eventos sísmicos con profundidad intermedia (30 – 70 km). para eventos sísmicos con profundidad mayor que 70 km se calculó una mc de 2.6 y valores de a = 5.05 y b = 0.88. el parámetro “b” indica que los niveles de esfuerzo, referente a toda la gama de profundidad estudiada, son predominantemente bajos. palabras clave: parámetros “a” y “b”, análisis de completitud, amenaza sísmica. earth sciences research journal 106 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics stress field in brazil with focal mechanisms: examples of regional forces controlling the stress field fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com,marcelo@iag.usp.br abstract the determination of the stress field is fundamental not only to understand the driving forces and plate deformation but also to study intraplate seismicity. in brazil the stress field has been determined using mainly focal mechanisms, and a few breakout and insitu measurements. previous compilations of focal mechanisms show reverse, strike-slip and normal faulting that indicate a variable stress field. however, the stress field still is poorly known in brazil. we determined some additional focal mechanisms in brazil using different waveform modelling techniques according to the available data. for events with magnitudes ~5 mb, we stacked the records of several teleseismic stations (> 30°) with a good signal/noise ratio, according to distance and azimuth. the stacked teleseismic p-wave signal was modeled using the hudson96 program (herrmann, 2013) and the consistency of the focal mechanism with the first-motion polarities was checked. for some smaller events (mb < 4) the regional moment tensor was determined with the isola code (sokos & zahradnik, 2013). with the focal mechanisms available in the literature and those obtained in this work, we were able to identify the following patterns (fig. 1): central brazil shows a purely compressional pattern (e-w shmax), which is predicted by regional theoretical models (richardson & coblentz, 1996, and the td0 model of lithgow & bertelloni, 2004). this compression is mainly due to the continental-scale plate-tectonic forces. in the amazon craton we find an indication of shmax oriented in the se-nw direction, probably caused by the caribbean plate interaction (meijer, 1995). near the amazon fan, flexural stresses caused by sedimentary load seem to be important, which is in agreement with local theoretical models (watts et al., 2009). in the northeastern coastal region, the compression rotates following the coastline, which indicates an important local component related to spreading effects at the continental/oceanic transition as well as flexural stresses. we will show examples of stress perturbations induced by local effects (e.g. flexure and continental spreading). the results of this work should be useful for future numerical modeling of global/regional intraplate stress field. keywords: stress field, brazil, focal mechanism. earth sciences research journal 107 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics fig1: shmax direction in brazil from focal mechanism. blue, green and red bars indicate thrust strike-slip and normal mechanism respectively. fault type and quality according to the world stress map criteria. open bars are in-situ measurements. earth sciences research journal 108 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad en el piedemonte llanero: caracterización, relocalización y tomografía de velocidades francisco javier muñoz burbano servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc) fmunoz@sgc.gov.co abstract se presenta una relocalización de sismos en el piedemonte llanero colombiano entre los 3º a 5ºn y 73º a 75ºw mediante los métodos de inversión sísmica simultánea y dobles diferencias teniendo en cuenta tiempos de llegada de ondas p y s; además se presenta la inversión de un modelo de velocidad vp en 3d. se tomaron en cuenta 483 sismos registrados por la red sismológica nacional de colombia (rsnc) entre 1993 y 2012 que proporcionan 4961 observaciones de tiempos de arribo de onda p y s. con la relocalización se logró una reducción de los tiempos de error cuadrático medio (rms) y la identificación evidente de “clusters” de sismos asociados a los sistemas de fallas de la cordillera oriental, principalmente en el sector suroeste del área de estudio entre los sistemas de fallas de servitásanta maría y guayuriba. el modelo de 3d brinda información de anomalías de baja velocidad para el general de la cordillera oriental y estabilidad para la zona suroriental que corresponde al cratón suramericano, sin embargo las imágenes presentan zonas de anomalías positivas de vp para el sector suroeste de la grilla que involucra los sistemas de fallas anteriormente citados. finalmente el modelo presenta un buen desempeño hasta profundidades no superiores a los 40 km, debido a que el 95% los hipocentros de los eventos registrados no superan esa profundidad. palabras clave: tomografía de velocidades, relocalización de sismos, inversión sísmica simultánea, dobles diferencias, piedemonte llanero. earth sciences research journal 109 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. localización de sismos registrados por la rsnc entre 1993 y 2012. incluye mapa de fallas cuaternarias de colombia. modificado de paris, g et al. (2000). figura 2. cambio porcentual de velocidad de onda p con respect al modelo inicial 1d a 20 km de profundidad. % var. earth sciences research journal 110 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics réplicas del sismo del 13 de agosto de 2013. bahía solano océano pacífico colombiano francisco javier muñoz, nelson david pérez, miguel hernández, lina paola aguirre servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc). fmunoz@sgc.gov.co abstract el 13 de agosto de 2013 a las 15:43:14 (u.t), ocurrió un evento sísmico superficial localizado en el océano pacífico colombiano, en las coordenadas 5.757°n y 78.254°w, con una magnitud mw 6.5. dada la magnitud del evento principal y la ocurrencia de réplicas desencadenadas por el evento, se instaló la estación sismológica portátil nuqc en el municipio de nuquí (chocó), con el propósito recolectar datos para el mejoramiento de la calidad de localización y caracterización de las réplicas. el registro de información se llevó a cabo entre los días 17 de agosto y 6 de noviembre de 2013. se presenta un análisis estadístico de las réplicas y una estimación del área de ruptura del sismo principal, teniendo como fuente de información los registros sísmicos de las estaciones permanentes de la rsnc y de la estación portátil nuqc. palabras clave: réplicas, área de ruptura, estación sismológica portátil. earth sciences research journal 111 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic microzonation of urban areas ganesh w. rathod1, k. seshagiri rao2 and kaushal kumar gupta1 1council for geoscience, south africa. 2indian institute of technology delhi, india. grathod@geoscience.org.za abstract the earthquakes are one of the most destructive natural disasters and it is difficult to predict them with accuracy. the assessment of risk associated with earthquakes (seismic risk) and its possible reduction are of great concern to the scientific community particularly in the field of seismology and civil engineering. the infrastructure development in the urban areas would have serious societal impact in terms of human life and structural damage, if due importance is not given to adequate preparations for possible seismic hazard. seismic microzonation has generally been recognized as the most accepted tool in seismic hazard assessment. seismic microzonation helps in describing the ground motion levels to be expected in a given region in the future and it is defined as the subdivision of a seismic zone into smaller zones that have relatively similar exposures to various earthquake related effects and ground motion characteristics taking into account source and site conditions. in other words it is an effort to evaluate and to map the potential hazards found in an area, urban area in particular, that could be induced by strong ground shaking during an earthquake. these hazards include ground motion amplification, liquefaction and landsliding etc. north india and particularly the himalayan belt have experienced many strong to moderate earthquakes that pose a high seismic risk in the region. the present work focuses on the detailed study for seismic microzonation of national capital region of delhi, india. the study area has a population of more than 28 million located on the banks of river yamuna. being the capital of india, the city is considered as highly important in regards to social and economical issues. it falls under seismic zone iv according to seismic zonation map of india. the region is located near the highly active himalayan seismic belt. regional geology, seismotectonics, subsurface information, geotechnical data, seismic attenuation relationships and earthquake data are considered for the present study. earthquake catalogue processing, geophysical field testing (masw and refraction), local site response using ambient vibration measurement and effect of various uncertainties on the amplification of ground motions through subsurface are attempted. based on the geotechnical data and considering the varied geology, liquefaction potential of the area is also assessed for various scenarios using different methods. finally, results of all the studies carried out are combined together to propose the seismic microzonation model of the area. keywords: seismic microzonation, masw, refraction, geology, tectonics, earthquake catalogue, ambient noise, site response, liquefaction, ncr of delhi. earth sciences research journal 112 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismological detection and analysis of catastrophic landslides goran ekstrom, colin p. stark, clément hibert columbia university. ekstrom@ldeo.columbia.edu abstract catastrophic landslides involve the sudden acceleration and deceleration of millions of tons of rocks and debris in response to the forces of gravity and dissipation. their unpredictability and frequent location in remote areas have made observations of large landslide dynamics very rare. in this work we show that routine monitoring of the global long period seismic wavefield regularly detects and locates massive landslides with long-period surface-wave magnitudes around 5.0. inverse modeling of the teleseismic data in terms of a time-varying force vector acting on the surface of the earth leads to the determination of a landslide force history (lfh), which, by newton's third law, provides a remote means of inferring the momentum change of the landslide mass. augmented by geometric constraints obtained by satellite imagery, the seismically determined dynamic parameters of the landslide can be used to infer mean landslide mass and runout trajectory. combined analysis of the lfh and shortperiod signals provides possibilities for inferring additional details of the landslide process, in particular details of landslide initiation and termination. we present results and interpretations for recent catastrophic landslides, including the two large landslides associated with the 2013 kennecott mine, utah, pit collapse, and the 2014 oso landslide in washington state. keywords: seismology, landslides. earth sciences research journal 113 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics results of recent paleoseismologic studies point to the probable seismogenic source of the 1875 earthquake of cucuta, in the border area between colombia and venezuela hans diederix1, luz maria rodriguez2, eliana torres1, catalina hernandez1,3, olga patricia bohorquez1 1servicio geológico colombiano (sgc). 2 dpto. de ciencias de la tierra,fundación venezolana de investigaciones sismológicas (funvisis). 3actualmente: università di bologna – ingv. *corresponding author: hdiederix@sgc.gov.co abstract the well-known ne-sw oriented right lateral strike-slip bocono fault of the merida andes of venezuela, on approaching the border area with colombia in the vicinity of the city of cucuta, ramifies into several branches that start to curve southward once in colombian territory. the northern branch, which is the focus of this study, is known as the aguascalientes fault. this in turn ramifies into three branches that present morphotectonic evidence of dextral strike-slip movement with a component of reverse faulting. the northern one of these is the cucuta fault, the middle branch is the aguascalientes fault proper, and the southern branch is known as the lomito fault. the aguascalientes fault, which has the most outspoken morphotectonic expression, is the most important of the three branches, and traverses the venezuelan border town of ureña, passes just to the south of the urban conurbation of cucuta, where it crosses the pamplonita river before turning south to continue as the n-s running chitaga fault. the name of the fault relates to the occurrence of thermal springs along its trace. the lomito fault, situated to the south of the aguascalientes fault, is characterized by a flexural scarp along a gravel terrace in an abandoned floodplain of the tachira river that constitutes the frontier between the two countries. both faults were subjected to paleoseismologic trench studies that have yielded evidence of holocene tectonic activity. along the lomito fault scarp the presence of a south verging blind fault is manifested at surface as a flexural scarp marked by the presence of an accumulation of gravelly colluvial wedges that interfinger with floodplain sediments of the tachira river (figure 1). c14 dates of samples taken in this sequence of colluvial wedges yielded ages between 16.260+/60 and 1.740+/30 years bp, confirming holocene fault activity. trenching on a fault controlled terrace scarp of the aguascalientes fault on the west bank of the pamplonita river just south of the city of cucuta, revealed a vertical offset of almost 6 m of holocene terrace gravels overlying clayey shales of the neogene león formation (figure 2). only one reliable c14 date of a sample taken in the middle of the down faulted gravel bed produced an age of 3520 +/30 years bp. the horizontal component of displacement could not be established. this considerable vertical offset of the fault during the late holocene strongly suggests the aguascalientes fault to be the seismogenic source of the earthquake of the 18th of may 1875 that caused the almost total destruction of the towns of cucuta and villa del rosario in colombia and san antonio in venezuela. this interpretation has been corroborated in a recently undertaken study of historic seismicity of coand post seismic effects related to that earthquake. key words: cucuta earthquake 1875, bocono fault, aguascalientes fault, paleoseismology. earth sciences research journal 114 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figure 1. flexural scarp of the lomito fault at finca la casona looking to the west; trench in process of logging, looking to the north; detail of trench exposure in scarp area where gravelly colluvial wedges interfinger with floodplain deposits of the tachira river, looking to the east. earth sciences research journal 115 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figure 2. fault scarp of the aguascalientes fault at the locality of el resumen before trench excavation, view to the sw; trench log showing 6 m of vertical displacement of holocene terrace gravels overlying clayey shales of the neogene león formation. effects of sliding along fault scarp. earth sciences research journal 116 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics inversión del tensor de esfuerzo a partir de mecanismos focales obtenidos en el campo volcánico y geotérmico de las tres vírgenes, baja california sur (méxico) yanet antayhua1,2, javier lermo2, luis quintanar 1, oscar campos1 1instituto de geofísica, universidad nacional autónoma de méxico. circuito de la investigación científica s/n, ciudad universitaria, delegación coyoacán, c.p. 04510, méxico d.f. 2instituto de ingeniería, universidad nacional autónoma de méxico. yantayhuav@iingen.unam.mx abstract la instalación de una red de estaciones sísmicas alrededor del campo volcánico y geotérmico de las tres vírgenes (cvgtv), b.c.s. (méxico), durante el periodo septiembre 2003-diciembre 2012, permitió localizar 423 sismos y obtener 90 mecanismos focales de tipo normal, strike slip y mínimamente inversos. sin embargo, con el propósito de determinar los cuatro parámetros del tensor de esfuerzo (σ1, σ2, σ3 y r) y caracterizar el campo de esfuerzos del cvgtv, utilizando las metodologías descritas por angelier y mechler (1984) y delvaux y sperner (2003); en este estudio se seleccionaron, únicamente, 57 mecanismos que fueron agrupados en 4 zonas (a, b, c, d) considerando su ubicación geográfica y periodo de ocurrencia (enjambres de sismos), características tectónicas (locales, regionales) y su proximidad a los pozos productores e inyectores (figura 1). los resultados de la inversión del tensor en cada una de las zonas del cvgtv (figura 1) muestra que en la zona a se presentan dos tipos de régimen de esfuerzos: el extensivo (a1) orientado en dirección nw-se, está asociado a la falla el volcán; los eventos respectivos habrían ocurrido como respuesta al proceso de inyección de agua en el pozo lv6 en diciembre de 2009; mientras que el segundo, de tipo strike slip (a2), está asociada a la tectónica regional y local de las fallas reforma y los azufres, similar a lo obtenido por sumi et al. (2010). en las zonas b y d, el régimen extensivo está orientado en dirección nw-se y ne-sw los que se correlacionan con las fallas normales cimarrón (b) y el sistema de fallas nw-se ubicadas entre bonfil y mezquital (d). mientras tanto, a nivel regional, la inversión del tensor de esfuerzo realizado entre las zonas a-c y cd presentan un régimen extensivo con orientación nw-se acorde al fallamiento normal de la virgen (a-c) y el régimen strike slip observado entre c-d, estaría correlacionada con la falla mezquital. en general el régimen de esfuerzo principal del cvgtv es de tipo extensivo y esta correlacionado con la actividad tectónica local, cuya actividad sísmica se presentó como enjambres; mientras que los de tipo strike slip están asociados a la tectónica regional del golfo de baja california. palabrasclave: tensor de esfuerzo, strike slip, enjambre, campos geotérmicos. earth sciences research journal 117 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 118 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics aplicación de algoritmo de inversión tomográfica aplicada en la cuenca de urabá john gonzález veloza, luis castillo universidad nacional de colombia. johngonzalezv@gmail.com la cuenca de urabá es de gran interés geológico, debido a su complejidad y posición, donde se tiene la confluencia de distintas placas convergentes, pero es una zona que ha sido poco entendida. con este trabajo se obtuvo un modelo del subsuelo, utilizando información de sísmica pasiva. fueron utilizados 54 eventos sísmicos, de los cuales se discriminaron más de 400 tiempos de llegada correspondientes a las ondas p y s. esto permitió realizar una inversión simultánea de los hipocentros, tiempos de origen y la distribución de velocidad 3d. de esta manera se estableció un modelo de velocidad correspondiente con las características geológicas del área, ejemplo de ello es la geometría y el lineamento de la falla de murindó y el sistema de fallas satélites. estas características y la expresión geomorfológica y litológica, pueden ser claramente asociadas con el modelo de velocidad superficial. palabrasclave: tomografía, inversión, cuenca de urabá, diferencias finitas, fronteras no reflectivas. earth sciences research journal 119 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics propagacion de ondas de rayleigh en el valle de méxico y una posible explicacion para la larga duracion de los sismos josé d. sanabria-gómez1,2, víctor m. cruz-atienza2, josué tago2,3, emmanuel chaljub3 and jean virieux3 1escuela de física, universidad industrial de santander. bucaramanga, colombia. 2instituto de geofísica, unam. mexico df, méxico. 3institut des sciences de la tèrre, ujf. grenoble, francia. jsanabri@uis.edu.co abstract se presentan resultados de simulaciones de sismos en los regímenes elástico y viscoelástico en el valle de méxico, considerando la topografía del terreno y un modelo de la estructura 3d de la cuenca sedimentaria sobre un lecho rocoso. el dominio de simulación es de 93 x 80 x 50 km^3, y posee capas superficiales de ~50 m de espesor con velocidad de onda s, vs>= 100 m/s y factores de calidad qs>=32. se utilizaron 8 fuentes puntuales de tipo doble par, distribuidas en la región de simulación y a profundidades de 150m, para favorecer la generación de ondas superficiales. la solución del sistema hiperbólico de ecuaciones viscoelásticas en su formulación velocidad-esfuerzo para la simulación de ondas se llevó a cabo empleando el método de elementos finitos discontinuos (i.e. galerkin discontinuo), en una malla no estructurada de ~14 millones de elementos tetraédricos y con adaptabilidad hp (i.e. refinado de la malla y adaptación local del orden de interpolación polinomial en función de las propiedades del medio) [1,2]. este método permite resolver el problema numérico satisfaciendo los criterios de precisión y estabilidad en función de las propiedades locales del medio de propagación. con este conjunto de simulaciones se estudia la propagación de ondas de rayleigh en el valle de méxico, en particular, el papel que puede jugar la propagación de modos superiores para las ondas de rayleigh en la duración del movimiento del suelo. palabras clave: soluciones numéricas, sismología computacional, propagación de ondas superficiales, cuencas sedimentarias. earth sciences research journal 120 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics efecto de sitio no lineal sobre basalto usando cinco sismos m>6 de guatemala julio césar cuenca sánchez instituto de ingeniería universidad nacional autónoma de méxico. jccsa@pumas.ii.unam.mx abstract en la determinación de la amplificación sísmica y el comportamiento no lineal ha sido en depósitos de suelo blando, como las capas sedimentarias de edad cuaternaria, las poco consolidadas, lacustre, aluvial, los suelos blandos hablando geotécnicamente. muchos autores han reportado en trabajos de microzonificación sísmica de ciudades usando microtremores y/o movimientos fuertes. es propósito es asociar el daño asociado a la estructuras ingenieriles asentadas en dichos suelos donde tienen una gran influencia la geología superficial y la topografía. aki (1993) menciona una linealidad en la ciudad de méxico durante el terremoto de michoacán de 1985, sin embargo singh et al. (1988) reportaron poca evidencia no linear sísmica en la zona de lago de la ciudad de méxico. la ciudad universitaria de la universidad nacional autónoma de méxico (unam) se encuentra construida dentro de derrames basálticos de volcanes monogenéticos al sur de la ciudad. estos depósitos de roca basáltica comprenden la zona firme o dura según la zonificación geotécnica. desde el 2006 funciona la estación unm con sismómetro de banda ancha de la red “geoscope”, de la cual solo los eventos más fuertes normalmente de subducción son analizados en este trabajo. con estos eventos bastante alejados (s-p mayores a 1 minuto) parecería que es difícil calcular cocientes espectrales h/v debido a la distancia las ondas sísmicas cuando arriban aportan ondas superficiales que a veces se confunden dentro de las amplitudes del ruido de fondo. se analiza cinco eventos del 2007 al 2012, con magnitudes de 6.3, 6.5, 6.6, 6.8 y 7.4, ellos tienen una trayectoria bastante similar a la estación unm, calculando sus espectros, cocientes espectrales h/v y haciendo comparaciones entre sismo y ruido. se encontrado que existe una amplificación de sito alrededor de 4 veces usando ventanas analizadas de cinco fuertes sismos localizados en guatemala, con su frecuencia predominante es 0.2 hz para los sismos y de 0.17 hz para la señal de ruido seguidamente posterior al sismo. para el caso de ruido, la amplificación es bastante menor que la encontrada con sismo. esto implica que hay un comportamiento sísmico no lineal de sitio, debido a los depósitos superficiales de roca basáltica en ciudad universitaria unam. palabrasclave: efecto de sitio no lineal, basalto, ciudad de méxico. earth sciences research journal 121 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics características espectrales de la sismicidad originada en la cuenca del valle de méxico luis quintanary delia i. bello instituto de geofísica, unam. luisq@geofisica.unam.mx abstract este trabajo presenta un análisis de las características focales de la sismicidad en el valle de méxico, el cual ha sido posible gracias a los registros obtenidos de una red sísmica de banda ancha recientemente instalada en esta zona. de los sismos ocurridos en el valle de méxico durante el período 2008-2012, se escogieron los 15 eventos con la mejor relación señal-ruido. se transformaron a registros de aceleración y mediante un análisis espectral, se encontraron los parámetros de fuente de sismos característicos de distintas zonas de concentración de eventos dentro de nuestra zona de estudio, determinando el momento sísmico (m0), la caída de esfuerzo (∆σ) y el factor de atenuación (t*). se encontraron diferencias significativas en el factor de atenuación de cada región así como una caída de esfuerzo media δσ=1.2 mpa. con respecto al valor de t*, se encontró un promedio de t*=0.04 s para sismos en el valle de méxico, lo que indica que esta zona exhibe atenuaciones altas comparado con regiones fuera de él. este resultado es acorde con las determinaciones más recientes del factor de calidad q0 de las ondas sísmicas dentro del valle de méxico (ortega and quintanar, 2005, singh et al. 2007), y refuerza la hipótesis de que, en general, el centro de méxico está caracterizado por un rápido decaimiento de la amplitud de las ondas con la distancia, similar al ocurrido en otras regiones tectónicamente activas. con este estudio se pretenden conocer las características focales de esta sismicidad que por sus características resulta vital, toda vez que el valle de méxico es la región más importante de méxico desde un punto de vista económico y de concentración poblacional, además de aportar elementos para el conocimiento preciso del peligro sísmico en esta zona. en todo caso, enfatizamos que la importancia de los sismos locales se comprenderá a plenitud cuando entendamos las diferencias del movimiento del suelo debido a fuentes locales y regionales, lo cual pasa por el cabal conocimiento de la estructura bajo el valle de méxico. palabrasclave: valle de méxico; análisis espectral; parámetros focales. earth sciences research journal 122 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a test of the rstt model for south america with brazilian pgt events marcelo assumpção, bruno collaço institute of astronomy, geophysics and atmospheric sciences; university of são paulo, rua do matão 1226, 05508-090, são paulo, sp, brazil. marcelo.assumpcao@iag.usp.br abstract good 3d lithospheric velocity models are necessary for reliable epicentre determination, especially at regional distances. the rstt (regional seismic travel time) project, an on-going effort coordinated by ctbto and isc, is improving the 3d model in south america by incorporating new gt5 data as well as a recent compilation of crustal thickenesses in the continent. to test the last versions of the rstt model for south america, we used a set of 17 brazilian reference events, with magnitudes 4.0 to 5.1, with well-determined epicenters. only four were truly gt5 (ground truth) events with an accurate origin time. most of the events had the epicenter assigned to the middle of the aftershock zone determined by local studies, with origin times calculated with iasp91 table using teleseismic stations; these events are called pgt (pseudo gt). the epicenters were calculated in twosteps: 1) an initial epicenter was determined with a 1d model derived for the brazilian mid-plate region, and 2) corrections were applied to the observed travel times using the rstt calculator, and the epicenters were relocated with the corrected times. the “hypocenter” code was used in the two steps, and only regional stations (200-1500 km distance) were employed. the relocated epicenters are closer to the true pgt location compared with the initial uncorrected locations, as well as compared with the teleseismic isc locations. keywords: 3d model, hypocenter location, brazil. earth sciences research journal 123 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the september 5th, 2012 nicoya, costa rica earthquake, a subduction earthquake captured in the near field marino protti1, susan schwartz2, víctor gonzález1, timothy dixon3, andrew newman4, lujia feng5, han yue2, thorne lay2 and susan owen6 1ovsicori-una. 2university of california at santa cruz. 3university of south florida in tampa. 4georgia tech. 5earth observatory of singapore. 6jpl-caltech. marino.protti@gmail.com abstract on september 5th, 2012 a large mw=7.6 earthquake expectedly occurred at the southern terminus of the middle american trench (mat) off nw costa rica. for over a decade before this earthquake, first as a target site of the nsf margins program's initiative seize and then as an extension through the geoprisms program, a dense geodynamic control network of seismic and geodetic instruments was installed over the seismogenic zone of the nicoya segment of the mat where the cocos plate subducts at ~80mm/yr under the caribbean plate. there we were taking advantage of the existence of the nicoya peninsula that extends as close as 60 km from the trench and right over the seismogenic zone of large underthrust interplate earthquakes. given the occurrence of large earthquakes in this segment of the subduction zone in 1853, 1900 and 1950, this network was designed to capture the next large earthquake. campaign and continuous gps data that expanded for almost two decades before the earthquake were crucial in delineating heterogeneities along the plate interface before the earthquake and anticipating the area where the next earthquake would potentially occur and what magnitude it could reach. two locked patches were recognized, one that extends from 10 to 20 km in depth just offshore the nicoya peninsula and another from 20 to 30 km deep. we will show how the nicoya 2012 earthquake was successfully captured in the near field, including 5 hz gps seismograms, through this international collaborative project. this earthquake may be the best recorded subduction earthquakes in history. we will also show how this event represents a partial rupture of the plate interface with slip mainly on the deeper patch. key words: earthquake, costa rica, nicoya, monitoring. earth sciences research journal 124 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics estimación del riesgos sísmico en managua, nicaragua, con la metodología capra hernández o., izaguirre a., strauch w., otros. (se sinapred, bm, cepredenac). ohernandez@sinapred.gob.ni abstract el proyecto “iniciativa centroamericana para la reducción del riesgo a desastres y adaptación al cambio climático” financiado por el banco mundial por medio del gfdrr (aportación españa al fondo/sflac), apoya la implementación de la política centroamericana de gestión integral de riesgo de desastres (pcgir). uno de los componentes de este proyecto es la modelación probabilista de escenarios de riesgo sísmico en managua, capital de nicaragua. esta modelación se lleva a cabo empleando la metodología capra. el proyecto comenzó en el mes de noviembre de 2013 y culminará en julio del año en curso. se ha calculado la amenaza sísmica para managua empleando el programa crisis (ordaz y otros). además se ha elaborado una capa de efecto de sitio, para incorporarla en el cálculo de la amenaza, para esto se han empleado los datos del trabajo de hernández (2009), donde evaluó la respuesta de los suelos de managua en más de 270 puntos de la ciudad. también se han determinado las tipologías estructurales más comunes en la ciudad y se han propuesto sus respectivas funciones de vulnerabilidad. por otro lado se ha recopilado y depurado las bases de datos de los portafolios: educación, salud y vivienda; los que se incluirán en el cálculo del riesgo. se ha seguido una metodología participativa e incluyente con todas las instituciones que pertenecen al sistema nacional para la prevención, mitigación y atención de desastres de nicaragua. para tal fin se han llevado a cabo varios talleres, en los cuales cada uno de los asistentes ha participado de forma activa. al concluir el proyecto en el mes de julio, se pretende calcular las pérdidas debidas a sismos para diferentes escenarios en la ciudad de managua. palabrasclave: capra. earth sciences research journal 125 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics structuraldelineationof the servitafaultsystem,colombianeasterncordillera, usingmicroseismicity patriciapedraza1,2andcristinadimaté2 1serviciogeológicocolombiano(sgc),direccióndegeoamenazas,bogotá.colombia. 2posgrado de geociencias, universidadnacionaldecolombia. ppedraza@sgc.gov.co,mcdimatec@unal.edu.co abstract theservitafaultsystem(sfs)islocatedintheeasternfoo thillsofthecolombianandeseasterncordillera. at least one event in historical times has been associated to this fault (1917, ms=6.9). to delineate theseismicallyactivesegments of this structure and to understand its relation with neighboring active structures we deployed a portable seismic network during the period 2011–2012 to record microseismicity. we use, then, microseismicityasastructuralmarkerthatilluminates activeelements in the servita fault region. theseismologicaldatafromthetemporarynetworkwere integratedwithgeologicalinformationforanalyzingtheg eometry, kinematicsandseismogenicdepthofthemajorstructures. adatasetof890earthquakeswithalocalmagnituderange between0.1and3.2anddepthsupto40kmwasanalyzed. the network operated in two sectors, north and south, according to main tectonic features of the region, at different periods. different1dmodelswereestablished foreach sector. the refined locations of the events show a general sw-ne trend that follows the trace of the sfs. selected focal mechanisms show predominantly right-lateral strike slip motion. inthenorthernsectorepicentersareseparatedintwolocati ons,one on the axial zone of the cordilleraandtheotheron the eastern flank. the former, were most of the epicenters are located, coincides with the rupture zone of quetame earthquake (2008, mw=5.9). alignment of hypocenters of the eastern flank events andsomefocalmechanisms suggestafaultplanedippingtothewest, which is interpreted as the servita fault plane. southwards of therionegro,seismicitydecreasesandlosescontinuity. this feature coincides with the trace of the wswene rio blanco fault which possibly segments the servita fault. seismicity at the southern sector is spread over a wider area and exhibits deeper hypocenters compared to the northern sector. hypocenter distribution and focal mechanisms in this sector suggest two planes dipping to the west which we interpret as segments of the algeciras fault and another plane (northwards) steeply dipping to the southeast interpreted as the fault plane of the altamira fault. key words: servitafaultsystem,colombianeasterncordillera, microseismicity. mailto:ppedraza@sgc.gov.co mailto:mcdimatec@unal.edu.co earth sciences research journal 126 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics strong motion recorded during the emilia 2012 thrust earthquakes (northern italy): a comprehensive analysis rita de nardis1, luisa filippi2, giovanni costa3, peter suhadolc3, mario nicoletti2, giusy lavecchia1 1univ chieti. 2dpc, rome. 3univ trieste. suhadolc@units.it abstract a complex seismic sequence characterized by two thrust earthquakes of magnitudes ml 5.9 and ml 5.8 occurred on may 20 and 29, 2012, respectively, and activated the central portion of the ferrara arc structure beneath the po plain in northern italy. the sequence, referred to as emilia 2012, was recorded by the italian strong motion network, the irpinia network, the friuli venezia giulia network and 15 temporary stations installed by the civil protection department. in this study, we compile and analyze a large dataset that contains 3,273 waveforms from 37 ml≥4.0 seismic events. the main aim of this paper is to characterize the ground motion induced by the emilia 2012 seismic sequence and compare it with other data in the italian strong motion database and to the recent ground motion prediction equations (gmpes) developed for northern italy, all of italy and europe. this is achieved by (1) the computation and analysis of the strong motion parameters of the entire emilia strong motion dataset (esmd) and (2) a comprehensive investigation of the may 29 event recordings in terms of time–frequency analysis, the ground motion parameters and the response spectra. this detailed analysis was made possible by the temporary civil protection department stations that were installed soon after the may 20 event at several municipalities in the epicentral area. most of the recordings are characterized by low-frequency content and long durations, which is a result of the thick sedimentary cover that is typical of the po plain. the distributions of the observed horizontal peak ground accelerations and velocities (pgas and pgvs) with distance are generally consistent with the gmpes. this is particularly true for the data from ml ≥ 5.0 (mw ≥ 5.0) events, though the data are scattered at distances beyond approximately 60–70 km and show faster attenuation than the european gmpes. the horizontal components for the may 29 event at two near-fault stations (mirandola and san felice sul panaro) are overestimated by all of the analyzed gmpes. in contrast, the vertical components, which played an important role in the shaking near the source, are underestimated. the may 29 event produced intense velocity pulses on the horizontal components and the highest peak ground acceleration ever recorded in italy on the vertical component of the mirandola near-fault station. the ground motion recordings contained in the esmd significantly enrich the italian strong motion database. they contribute new information about (1) the possibility of exceeding the largest recorded pga in italy, (2) the development of a spectral design that takes into account the role of the vertical component and the extreme variability of the nearfault ground shaking, and (3) the characterization of the ground motions in deep sedimentary basins. keywords: strong motion parameters, strong motion network, thrust fault, emilia 2012 seismic sequence, po plain, northern italy. earth sciences research journal 127 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics rupture process on fracking operations applying pwave back projection images sebastían gómez alba1, carlos alberto vargas j.1; william agudelo2, alexander martínez2 1geosciences department, national university of colombia. 2instituto colombiano del petróleo icp. sagomezalb@unal.edu.co abstract hydraulic fracturing is an essential technology for most unconventional hydrocarbon resources and many conventional ones as well. the primary limitation on the improvement and optimization of the fracturing process is the minimal access to observe the behavior of the fracture in the subsurface. without direct observational evidence, hypothetical mechanisms must be assumed and then tested for their validity with indirect information such as wellbore measurements, indirect production and pressure behavior. one of the most important sources of information today is the relation made between micro seismic source mechanisms and fracture behavior. hydraulic fractures induce some level of micro seismicity when the stress conditions in the earth are altered by changes in stress during the operations. the result is the sudden movement between rock elements and the radiation of both compressional and shear energy in a seismic range that can be detected and recorded with sensitive receivers. the objective of this work is to provide reasonable information when applying the back projection method in order to predict the distribution of seismic radiation (energy) of microseisms while fracking operations. the method consists in back project the seismograms recorded at an array or a seismic network to a grid of possible source locations. it provides rupture properties such as direction, speed and duration. the back projection method is used to estimate the rupture propagation of seismic record of a fracking operation in the chichimene oil field, located in the western area of the foreland eastern llanos basin in colombia. data corresponds to a 17 triaxial accelerometers-linear array which are located close to the source coordinates. a grid of points is set to cover the area of study which includes all seismic stations and the source location. the entire volume is a cube where a homogeneous model velocity model is applied. values for velocities were vp= 5.94 km/s and vs=3.45 km/s. to simplify the analysis, the value for the density was 4500 kg/m3 for the entire body. the analysis uses the vertical components from seismograms recorded. the simplifications of the model contribute to minimize the computation time. the analysis clarify some ideas about what information can be gained from the micro seismic source data and according to the obtained results, what kind of comparisons and associations might be done to evaluate the fracking performance operation. obtained results such as distance, velocity and orientation of the seismic energy radiation show accordance to the type of operation took place and were used to characterize the generated fracture. key words: fracking, micro seismicity, energy, back projection. earth sciences research journal 128 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics alternatives for computing regional and global seismic-phase travel times using a 3-dimensional model stephen c. myers, nathan a. simmons, eric m. matzel, and gardar johannesson lawrence livermore national laboratory. myers30@llnl.gov abstract we compare computational costs, travel time accuracy, and applicable phases for a global 3d model and ray tracer (llnl-g3d) and a computationally efficient method for computing region seismic travel times (rstts). llnlg3d is a whole-earth 3d model of pand s-wave seismic velocity. a 3d ray tracer (earth3d) is used to compute llnl-g3d travel times for a suite body phases that are commonly used for seismic location. llnl-g3d phases include first-arriving p waves (e.g. p, pn), surface reflections (e.g. pp), reflections off of velocity discontinuities (e.g. pmp, pcp), crustal phases (e.g. pg and lg) and core phases (e.g. pkp). single-processor computational times depend on non-linearity of the ray-path as a function of deviation from radially symmetry for seismic velocity. teleseismic travel times are commonly computed in less than 0.1 second. regional phases generally require 0.25 seconds to compute, but travel times in particularly complex regions can take 1 second to compute. median absolute deviation for first-arriving p-wave residuals is approximately 0.5 seconds at teleseismic distances and between 0.5 and 1.0 second at regional distances. rstt is a model and method for computing travel times for regional phases pn, pg, sn, and lg. the rstt model includes a 3-dimensional crust and laterally varying pand s-wave velocity in the mantle. mantle velocity is parameterized as a linear function of depth. this parameterization enables travel time computation in approximately 1 millisecond, which is 100 to 1000 times faster than 3-dimensional ray tracing. for event-station distances between 1° and 15°, pn travel time prediction error is approximately 1.25 seconds. while rstt error is slightly large than the error for the full 3d model, rstt travel time prediction error is far lower than pn prediction error for standard 1d global models (e.g. ak135), for which travel time prediction error is approximately 1.75 to 2.5 seconds. we conclude that travel time prediction accuracy for llnl-g3d is superior for all phases used in seismic location. however, computation time for 3d ray tracing may limit llnl-g3d applicability for use in both realtime monitoring systems and for application to very large data sets. rstt is an excellent alternative for real-time regional monitoring systems and for relocation of large data sets when large, multi-core computers are not available. this work was performed under the auspices of the u.s. department of energy by lawrence livermore national laboratory under contract de-ac5207na27344. llnl-abs-654775. keywords: seismic, location, model, 3dimensional. earth sciences research journal 129 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics paleoseismic study of the north panama deformed belt and probabilistic seismic hazard assessment for a proposed harbor facility on the caribbean coast of costa rica tania gonzález1, eldon gath1, walter montero2, and christopher madden madugo1 1earth consultants international. 2geological research center, costa rica. tgonzalez@earthconsultants.com abstract as part of the feasibility and design studies for a new harbor facility on the caribbean coast of costa rica, we were tasked with identifying and quantifying the seismic hazard posed to the site by faults within a 100-km radius as part of a probabilistic seismic hazard assessment (psha). the background review revealed that at least 17 quaternary-active faults have been mapped within the study region, in addition to the middle america trench, which, although farther away, can generate events sufficiently large to be felt across the isthmus, on the caribbean side. the north panama deformed belt (npdb) is the fault closest to the site, and the one posing the most severe seismic hazard. rupture of a section of this fault caused the april 22, 1991 ms 7.6 earthquake that generated extensive damage between moín, costa rica and bocas del toro, panamá, as well as up to 1.4 m of strike-slip surface rupture on the río blanco fault where the offshore npdb steps inland as the siquirres-matina fault. the event generated coastal uplift of up to 1.85 m in the limón-moín region, about 0.5 m uplift near the costa rica-panama border, and subsidence between moín and matina and in northwestern panama. several elevated coral platforms in the site area suggest that similar uplift-causing earthquakes have occurred before. based on the historical record, the next higher platform was probably the result of an 1822 event, but the record is too short to ascribe older platforms to other, previous events. to evaluate the periodicity of these events and rate of uplift of the region, we surveyed the elevated coral platforms between moín and limón and collected coral and shell samples for dating. twelve of the coral samples were dated using the u-th method; this data set was supplemented with four radiocarbon-dated corals from published sources, and one newly radiocarbon-dated shell. assuming that the corals died off when uplifted by earthquakes on the npdb, we interpreted 12 such events in the past about 7,000 years, with an average recurrence of about 600 years. the data indicate, however, that earthquakes on this section of the npdb are not periodic, and appear to cluster in time, with large events having a 1,400-year return period, and smaller events occurring every few hundreds of years. our data confirmed the 1822 event, although uplift as a result of that earthquake appears to have been less than during the 1991 earthquake. using the sea level curve for the caribbean of hubbard et al. (2005), we calculated the total amount of uplift for each dated sample, resulting in an average uplift rate for the last 7,000 years of 1.9 ± 0.2 mm/yr. the results of this study allowed us to develop realistic ground motion parameters for use by the project engineer to design the proposed facility. key words: paleoseismology, costa rica, north panama deformed belt, probabilistic seismic hazard assessment, uplifted coral platform, coral dating. earth sciences research journal 130 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics idlm session – imaging and dynamics of lithosphere and upper mantle conveners: anne sheehan (colorado univ. usa) <afs@cires.colorado.edu> gaspar monsalve (unal colombia) <gmonsalvem@unal.edu.co> earth sciences research journal 131 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics constructing shear wave models using multi-objective optimization for multiple geophysical data sets lennox thompson1, aaron a. velasco1, anibal sosa2, vladik kreinovich1, and rodrigo romero1 university of texas at el paso. 2universidad icesi. aavelasco@utep.edu abstract understanding earth’s tectonic processes requires determining the earth structure. we develop and apply a multiple objective optimization joint inversion scheme for multiple geophysical datasets to construct 3-d shear wave models. in particular, we expand on a constrained optimization approach for joint inversion to characterize a one-dimensional earth's structure. we initially use seismic data from regional networks in new mexico, texas, and the usarray, a dense network of permanent and portable seismographs placed systematically across the continental united states. we collect and process seismic data (receiver function, surface wave, body wave travel times), with plans to expand to a fourth data set (gravity). by jointly inverting these three geophysical data sets, we can better handle the inherent non-uniqueness from inversion. the multiobjective optimization (mop) technique enables the capability to combine linear and non-linear problems, enabling the combination of inhomogeneous data sets and different statistical properties of error associated with each data set. we demonstrate through numerical and experimental testing that the mop scheme matches all data sets, is more robust, and is more flexible than traditional inversion approaches. to develop quasi 3-d models, we interpolate the 1-d results using a kriging approach, only when we have excellent station coverage. our quasi 3-d velocity models provide insight into the tectonic history and physical properties of the earth structure of new mexico and texas, and we will compare crust and upper mantle structure to ancient in texas and to active rift systems in new mexico. keywords: joint inversion, shear-wave models, geophysical data sets, computational geosciences. earth sciences research journal 132 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics study of seismic attenuation and unusual phases at uturuncu volcano, bolivia alexandra k farrell and stephen mcnutt university of south florida. akfarrell@mail.usf.edu abstract uturuncu volcano, 22.27° s, 67.18° w, is located in the central volcanic zone of the andes, a volcanic arc resulting from the subduction of the nazca plate beneath the south american plate. despite having an active hydrothermal system, the last eruption of the volcano was circa 270,000 years ago. recent insar studies of this area show an inflation rate of 1.5-2 cm/yr over an area that has a width of 70 km (pritchard and simons, 2002). this inflation has been modeled to a source depth of 1517 km located to the southwest of uturuncu’s summit. current studies have found a source area of decreased density with respect to the crust and with a vp/vs ratio of > 1.9, with a depth matching that derived from the deformation signature [potro et al., 2013; m. e. west, h. mcfarlin, d. christensen written comm.] determining the size, geographic location, source depth, and source shape of deformation at uturuncu volcano is important because this can indicate either (or a combination of) injection of magma from depth into the system, melting of crustal rock from a previous injection, or the build-up of pressure in a hydrothermal system (pritchard and simons, 2002). to further our understanding of these important parameters, we have been analyzing the attenuation of teleseismic p-wave phases from 15 events from four different azimuthal ranges. looking at the peakto-peak p-wave amplitudes, we see that there is a distinct, elongated area of attenuation approximately 13.6 by 33.3 km in size to the southeast of the volcano for earthquakes from the southeast and northwest, and for events from the southwest and northeast, a larger but much more subtle decrease in amplitude wrapping around the south of the volcano and approximately 40 by 35 km. we go further by looking at 100 events showing unusual phase arrivals between the p and s waves – these may be either reflections off of, or phases converted by, the top of the attenuating feature. at this congress, we will show our preliminary results from this analysis in the framework of other studies carried out at this volcano. keywords: amplitude, attenuation, teleseism, uturuncu, volcano, bolivia, phase. earth sciences research journal 133 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 134 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics title: 3-d structure of the rio grande rift from 1–d constrained joint inversion of receiver functions and surface wave dispersion anibal sosa1. lennox thompson, aaron a. velasco2. robert herrmann3 1universidad icesi, cali-colombia. 2department of geological sciences, university of texas at el paso, el paso-tx. 3department of earth & atmospheric sciences, saint louis university, st. louis, mo. uasosa@icesi.edu.co abstract the southern terminus of the rio grande rift region has been poorly defined in the geologic record, with few seismic studies that provides information on the deeper rift structure. in consequence, important questions related to tectonic and lithospheric activity of the rio grande rift remain unresolved. to address some of these geological questions, we collect and analyze seismic data from 147 earthscope transportable array (usarray) and other seismic stations in the region, to develop a 3–d crust and upper mantle velocity model. we apply a constrained optimization approach for joint inversion of surface wave and receiver functions using seismic s wave velocities as a model parameter. in particular, we compute receiver functions stacks based on ray parameter, and invert them jointly with collected surface wave group velocity dispersion observations. the inversions estimate 1–d seismic s-wave velocity profiles to 300 km depth, which are then interpolated to a 3–d velocity model using a bayesian kriging scheme. our 3–d models show a thin lower velocity crust anomaly along the southern rio grande rift, a persistent low velocity anomaly underneath the colorado plateau and basin and range province, and another one at depth beneath the jemez lineament, and the southern rgr. we expect to implement our approach for building 3-d earth subsurface structure models in south-western colombia, that can be used to improve the current scenarios associated to major earthquakes. keywords: geophysical inversion, rio grande rift, jemez lineament, interior-point methods. earth sciences research journal 135 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 136 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the moana experiment: a broadband ocean bottom seismology investigation of a transform plate boundary, new zealand anne f. sheehan1, daniel w. zietlow1, justin ball1, william l. yeck1, joshua c. stachnik2, fan-chi lin3, john a. collins4 1university of colorado at boulder. 2lehigh university. 3university of utah. 4woods hole oceanographic institution anne.sheehan@colorado.edu abstract the marine observations of anisotropy near aotearoa (moana) experiment included a oneyear deployment of 30 broadband ocean bottom seismometers (obs) installed off both coasts of the south island of new zealand from january 2009 to february 2010. the use of ocean bottom seismometers greatly increases the array aperture available to investigate both the onshore and offshore portions of the new zealand continental lithosphere, resulting in improved resolution for the land portion of the study area, and first results for offshore. the obs array combined with the permanent geonet array onshore has an aperture of approximately 1000 km and includes the challenger plateau, the south island of new zealand, the chatham rise, and the campbell plateau. moana tested between two prominent models of transform plate boundaries, one where deformation is distributed throughout the lithosphere, and one where deformation occurs in a narrow zone in the lithosphere and broadly in the asthenosphere. our results from teleseismic s-wave splitting show strong shear wave splitting near the alpine fault, aligned nearly parallel to the plate boundary and direction of relative plate motion. anisotropy remains strong to the northwest of the alpine fault, onto the challenger plateau, and tapers in amplitude while remaining roughly aligned with the plate boundary. the splitting pattern off the east coast is quite different, rotating to nearly perpendicular with the transform plate boundary. combined with crust and upper mantle estimates of anisotropy from pn, local s wave splitting, and surface waves, we find that the results near and to the west of the alpine fault are consistent with distributed deformation in the lithosphere. anisotropy in the east may be due to fossil lithospheric anisotropy or to current absolute plate motion. further lithospheric studies are underway with both ambient noise and earthquake surface wave dispersion. receiver functions are difficult in oceanic studies due to a combination of microseismic noise, water layer reverberations, and seafloor sediment reverberations, and we have developed joint inversion methods to address some of these issues. our joint inversion method combines seafloor compliance with receiver functions and surface wave dispersion, and can resolve seafloor sediment properties but is still problematic for resolving the moho. surface wave dispersion is measured using group and phase velocities from approximately 6 s – 80 s (ambient noise from 6-40 s, earthquake data from 30-80 s). the surface wave tomography reveals variations in crust and mantle shear velocity structure from 0-120 km. these methods resolve the thick crust of the southern alps and the transition from oceanic to continental lithosphere off the west coast. a fast high p-wave speed anomaly is observed in the upper mantle beneath the southern alps, and ongoing work seeks earth sciences research journal 137 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics to determine the dip and depth extent of this anomaly. keywords: lithosphere,anisotropy,shear wave splitting,transform fault, new zealand,surface wave dispersion. earth sciences research journal 138 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics modelación fisica y simulacion numérica en procesos de adelgazamiento litosférico para zonas convergentes c. muñoz lópez1, c. vargas1, e. poveda2 1universidad nacional de colombia. 2servicio geológico colombiano cemunozl@unal.edu.co, cavargasj@unal.edu.co abstract en este trabajo se presenta una simulación de la evolución temporal asociada al comportamiento que pudo tener la raíz litosférica de la sierra nevada de santa marta. para esto se usó la ecuación de navier stokes y la segunda ley de la termodinámica con la intención de modelar las fuerzas que rigen el movimiento del sistema manto litosfera y su interacción. con base en lo anterior, se hizo un programa en matlab utilizando el método de diferencias finitas que reproduce el comportamiento dúctil del sistema en tiempos superiores al millón de años. por otro lado se hizo un estudio con el fin de comprobar la no existencia de la raíz litosférica en la sierra nevada de santa marta, para esto se utilizaron telesismos registrados por la red sismológica nacional desde el 2013 hasta la actualidad y se hallaron las funciones receptoras asociadas a cada sismo registrado en estaciones cercanas a la zona de interés. se hicieron varias simulaciones basadas en modelos ya propuestos de adelgazamiento litosférico como lo son la remoción convectiva y la delaminación, obteniendo flujos convectivos generados por diferencias de temperaturas y flujos inerciales creados por gradientes de densidades y viscosidades, mostrando resultados acordes con los de las funciones receptoras en donde no se encontró evidencia de un moho profundo asociado a una raíz litosférica. palabras clave: raíz litosférica, simulación, funciones receptoras, adelgazamiento litosférico earth sciences research journal 139 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics joint inversion of geoid anomaly data and teleseismic p-wave delay times: modeling density and velocity perturbations underneath the parana magmatic province carlos alberto moreno chaves1, naomi ussami1 and jeroen ritsema1 1department of geophysics, institute of astronomy, geophysics and atmospheric sciences, university of sao paulo, sao paulo, brazil. 2department of earth and environmental sciences, university of michigan, ann arbor, michigan, usa. carlos.chaves@iag.usp.br abstract the parana magmatic province (pmp) is one of the largest continental igneous provinces (lip) on earth. it is well dated at 133 ma preceding the opening of the south atlantic ocean, but the causative geodynamic processes are still poorly understood. although a low-velocity anomaly has been imaged by seismic tomography in the northeast region of the pmp and interpreted as a fossil conduct of a mantle plume that is related to the flood basalt eruptions, geochemical data indicate that such magmatism is caused by the melting of a heterogeneous and enriched lithospheric mantle with no deep plume participation. models of density perturbations in the upper mantle estimated from joint inversion of geoid anomalies and p-wave delay times will offer important constraints on the dynamics. a new generation of accurate global geopotential models derived from satellite-missions (e.g. grace, goce) allows us to estimate density distribution within the earth from geoid inversion. in order to obtain the residual geoid anomaly related to the density structure of the mantle, we use the egm2008 model removing estimated geoid perturbations owing to variations in crustal structure (i.e., topographical masses, moho depth, thickness of sediments and basalts). using a spherical-earth approximation, the density model space is represented by a set of tesseroids and the velocity model is parameterized in nodes of a spherical grid where cubic b-splines are utilized as an interpolation function. to constrain the density inversion, we add more than 10,000 manually picked teleseismic p-wave delay times. during the inversion procedure, density and p-wave velocity are linked through a relationship provided by mineral physics [karki and karato, 2001]. we will present the results of this joint inversion scheme and hypothesize on the geodynamic processes responsible for the flood basalt eruptions. keywords: geoid anomalies, p-wave delay times, parana magmatic province, density, joint inversion. earth sciences research journal 140 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics joint inversion of seismic and gravity data for velocity structure and hypocentral locations of the colombian subduction zone ellen syracuse1, monica maceira1, germán prieto2, haijiang zhang3 1lanl. 2mit. 3university of science and technology of china. syracuse@lanl.gov abstract joint inversions of geophysical data recover models that simultaneously fit multiple types of constraints while playing upon the various sensitivities of each data type. here, we combine body wave arrival times with surface wave dispersion measurements and gravity observations to develop a combined 3d p and s-wave velocity model for the crust and upper mantle of colombia. pand s-wave arrival times were obtained for local earthquakes from instruments in the colombian national seismic network. rayleigh wave dispersion curves were inverted for using a subset of network stations and larger local earthquakes. gravity observations were extracted from the global satellite-based 200-mresolution model ggmplus. preliminary results using only body waves show reduced velocities beneath the volcanic arc in the upper 25 km of the crust. crustal velocities are also reduced from the 1d starting model beneath the cordillera oriental in the northern half of the country. relocations of intermediate-depth seismicity clearly indicate a discontinuity in the slab centered 5° n latitude, where the southern portion of the slab is ~200 km trenchward of the northern portion, coincident with the termination of arc volcanism. seismicity below 100 km depth in the southern portion of the subduction zone¬¬¬ is surrounded by a ~100-km-thick region of elevated velocities, associated with the subduction of the nazca plate, and embedded within a broader region of reduced velocities. the northern portion of the subduction zone at 100 km depth and below is characterized by a broad region of elevated velocities, which may be consistent with a slab of an old, thickened caribbean plate origin. this overlapping of the edges of two slabs may contribute to the seismicity of the bucaramanga nest. keywords: subduction, tomography, latin america. earth sciences research journal 141 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal thickness estimation beneath the northern andes using the receiver function method esteban poveda1, gaspar monsalve2, carlos a. vargas3, jordi julià4 1departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil; red sismológica nacional, servicio geológico colombiano (sgc), bogotá. colombia. 2escuela de geociencias y medio ambiente, universidad nacional de colombia, medellín, colombia. 3departamento de geociencias, facultad de ciencias, universidad nacional de colombia, bogotá, colombia. 4departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. esteban.poveda@gmail.com abstract we calculated receiver functions under the northwestern andes and adjacent areas, to deduce crustal thickness. for this purpose, we use data from the broadband network of colombia, which has been working since 2008; presently the network consists of 29 stations, 10 of which have been installed between 2012 and 2014, and are operated by the colombian geological survey. recent stations were installed to improve the epicentral locations in the sgc where there was a high azimuthal gap. with the receiver function calculation, we were able to map crustal thickness and vp/vs ratio in regions of the northwestern andean system where there were no previous estimations. we also collected information of moho depth from previous studies and neighboring regions to present a new map of interpolated crustal thickness of the northern andes. our results included a wide range of crustal thicknesses, with values of around 14 km beneath the malpelo island on the pacific ocean, 20 to 30 km at the coastal pacific and caribbean plains of colombia, 25 to 35 km beneath the eastern plains and foothills, 34 to 40 km beneath the western cordillera, 40 to 45 km at the magdalena river intermountain valley, 53 to 58 km in the northern central cordillera, and reaching almost 60 km beneath some of the volcanoes of the southern cordilleran system of colombia and the plateau of the eastern cordillera. the vp/vs ratio was obtained using a combination of wadati diagrams and an h-k (crustal thickness – vp/vs) stacking technique, which gave a bulk value for most of the crust under each station. in general, high values of vp/vs are associated to regions of high crustal thickness, and volcanic and high temperature gradient areas. the low vp/vs values are located in the intermountain valleys, regions of relatively low crustal thickness and areas where oceanic material has been accreted to the continent. the continuation of this work will consist of the collection of love and rayleigh surface wave dispersion observations, in order to combine the surface wave data and receiver functions to perform a joint inversion to obtain a more precise crust-mantle velocity structure. keywords: crustal thickness, receiver functions, hk stacking and joint inversion. earth sciences research journal 142 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal models 1d for colombia from surface waves dispersion observations and receiver functions faustino blanco1, esteban poveda1, 2, viviana dionicio1 1servicio geológico colombiano (sgc), bogotá. colombia 2now at departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. abstract colombia is located in a complex system of geological faults, geological accidents and three of the major tectonics plates are colliding. the seismicity is one of the main geological hazards in colombia; the national seismological network of colombia -rsnc (for its acronym in spanish) has recently developed important improvements of the quality and the amount of seismological equipment, which has allowed the use of new and more accuracy techniques to model the velocity structure in the crust and the upper most mantle. some of these techniques are dispersion observations, receiver functions and joints. the receiver functions are sensitivity to changes in medium impedance and dispersion curves are used to estimate shear velocity variations. the simultaneous inversion allows velocity models estimation for crust and upper most mantle, which are used to perform a body wave inversion using velest (kissling, 1988) to improve the velocity models. velocity models are primary to improve earthquake’s parameters location, and are used in different techniques to calculate others as the seismic tensor moment. in this work records of 482 earthquakes at distances between 29° and 90° are analyzed to calculate receiver functions and regional dispersion curves, follow is used the “computer programs in seismology” (r. b. herrmann, c. j. ammon 2002) package to perform a joint inversion. velocity models outcome together with rsnc’s seismicity catalogue were used to calculate body wave joint inversion using velest. four velocities models for different regions in colombia were calculated and tested. key words: velocities models, receiver functions, dispersion curves, simultaneous inversion. earth sciences research journal 143 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics lithospheric and upper mantle stratification beneath colombia: using receiver functions from s-waves 1,2blanco f, 2vargas c.a. 1geological service of colombia. bogotá, colombia. 2department of geosciences, national university of colombia at bogota abstract the knowledge of the structure of the lithosphere is an important issue to understand the evolution of the earth and plate tectonics behavior. seismic recordings at the national seismological network of colombia –rsnc (for its acronym in spanish) and the incorporated research for seismology -iris stations are processed with s-wave receiver function (srf) technique to determinate this structure. the technique uses distant seismic events, 65° to 85° from the station, to detect the s-wave and its sp wave conversion at broadband-three component stations. to isolate the recorded phase was necessary to apply a rotation process to the components; this process takes the zne into the zrt components through the back azimuth of the event, and then is necessary an extra rotation through the incident angle to obtain the lqt components. the lcomponent is almost free of transverse energy from the s-wave and it is de-convolved from qcomponent to obtain the srf. to outcome the lab (lithosphere asthenosphere boundary) currently have been processed five broadband stations, mostly at the central region in colombia, and ten events per station. according to the distribution of the seismological stations and estimations obtained in this work, we suggest that the lithosphere structure in the study area is highly inhomogeneous. the results are analyzed and compared with previous works using receiver function from pwaves. key words: receiver functions, lithosphere structure, colombia. earth sciences research journal 144 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics estudio de la estructura de la corteza en la zona de convergencia placa de rivera – bloque de jalisco (proyecto tsujal) f. j. núñez cornú1, d. córdoba barba2, jj dañobeitia3, w. bandy4, c. escudero-ayala1, j.m. espíndola4, a. zamora1, r. bartolomé3, a. gómez, m. ortiz5 y el gpo. de trabajo tsujal 1centro de sismología y volcanología de occidente, universidad de guadalajara, méxico. 2fac. de ciencias físicas universidad complutense de madrid, españa. 3cento mediterráneo de investigaciones marinas y ambientales, con. sup. de investigaciones científicas, españa. 4instituto de geofísica, universidad nacional autónoma de méxico. pacornu77@gmail.com abstract con el objetivo de estudiar el riesgo sísmico y tsunamigénico en las costas de jalisco y nayarit, en el pacífico mexicano, asociado con la estructura cortical del contacto placa de rivera – bloque de jalisco, se ha llevado a cabo el proyecto tsujal (caracterización del peligro sísmico y tsunamigénico asociado con la estructura cortical del contacto placa rivera-bloque de jalisco) como resultado de una colaboración entre instituciones de méxico y españa. este experimento forma parte de una amplia investigación mar-tierra para la que se han movilizado grandes instalaciones marinas y terrestres. en la primera fase del proyecto, llevada a cabo en febrero y marzo de 2014, se han realizado 5,200 km de sísmica multicanal y se han tomado datos de campos potenciales (gravedad y magnetismo) y sonda multihaz. se fondearon 16 obs en 32 posiciones de registro en la zona marítima de jalisco y nayarit y en la zona continental de esos estados. en tierra, se desplegó una red portátil de 100 estaciones sísmicas de corto período, en 240 posiciones de registro, a lo largo de 5 líneas sísmicas de 200-300 km de longitud que funcionaron en combinación con la red sismológica del estado de jalisco (sisvoc). adicionalmente, se instalaron 9 estaciones sísmicas en las islas marías y en la isla isabel. esos equipos registraron, en modo continuo, los disparos de aire comprimido generados desde el mar por un grupo de cañones de 5,800 c.i. a razón de un disparo cada 120 s. el buque inglés rrs james cook, que participó en este proyecto en régimen de intercambio entre buques españoles y británicos, se encargó de la realización de los perfiles de sísmica multicanal (mcs) y del fondeo de 16 obs. para ello, se usó un streamer de 6 km de longitud y un sistema de cañones de aire comprimido de gran capacidad. además de la mencionada infraestructura, participó el buque de la armada mexicana arm holzinger y el buque de investigación mexicano el puma de la unam. los nuevos datos adquiridos proporcionan un muestreo denso de las placas estudiadas y aportan imágenes nuevas, nunca vitas antes, sobre la deformación cortical a lo largo y a través de la zona de subducción, las dimensiones del prisma de acreción, sobre la zona de contacto entre la placa de rivera y la placa de norteamérica y sobre la transición entre la corteza oceánica y la corteza continental. en total se han realizado más de 750,000 disparos para la sísmica de reflexión, lo que supone unos 90 millones de muestras de la corteza en la placa de rivera y en el bloque de jalisco. para la sísmica de refracción, se realizaron 80,000 disparos. palabrasclave: placa de rivera, bloque de jalisco, subducción. earth sciences research journal 145 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics travel time delays and residual topography in northwestern south america provide constraints on upper mantle temperatures and slab geometry jefferson yarce and gaspar monsalve mejía1, thorsten w. becker2 1universidad nacional de colombia sede medellín. 2university of southern california. jyarce@unal.edu.co abstract the cause of tectonic deformation of northwestern south america and their link to upper mantle structure are still actively debated. we measure seismic p-wave travel time residuals for colombia and use them jointly with receiver function measurements to make inferences regarding the geometries and interactions of the subducting caribbean and nazca slabs. we used a combination of seismic data that indicates the presence of two subduction segments in the nw of south america and contrasting crustal thicknesses. negative travel time differential residuals for stations in northern colombia, including the caribbean coastal plains and the santa marta massif, suggests cold crust and cold upper mantle, or a thin crust, or combinations thereof. the opposite occurs for the andean region and the pacific coast where seismic stations yield positive travel time differential residuals. incorporating estimates of crustal thickness from receiver functions, we corrected these relative time residuals to isolate differences in upper mantle structure between stations. in the region near the caribbean coast, the negative delays are mainly due to thin crust (crustal thicknesses can be below ~25 km), but in the region of the bucaramanga nest and the san lucas range, to the sw of the caribbean coast line, the corrected delays are all negative. in the andean region, there is a tendency of the data towards positive delays; near the pacific coastline the positive differential time residuals dominate. one way to interpret these path-integrated, mantle and lithospheric velocity anomalies that are implied by our relative travel time anomalies is in light of their possible dynamic effect, by means of the associated density anomalies and/or the induced mantle flow. such sub-crustal anomalies will be reflected on the surface in terms of depressing or elevating topography beyond the level that would be expected from isostasy. we explore how the nonisostatic residual topography, inferred from the shallow layer structure, compares with our inferred delay time patterns. we find that most negative delay times in central colombia are associated with anomalously low topography, perhaps depressed by a cold lithospheric anomaly, or a slab-associated downwelling. in contrast, there is a pronounced positive residual just off the east of bucaramanga, where we also have some indication of strongly positive delay times. this anomaly may be related with a steeply subducting caribbean slab beneath this area. we interpret that caribbean subduction is initially flat under northern colombia and steepens beneath the eastern cordillera, which seems to be associated with the bucaramanga nest. on the other hand, nazca subduction seems to be associated with an asthenospheric mantle wedge, where volcanism is currently active, and thermal structure and heat flows indicate a warmer upper mantle than in the caribbean region. keywords: colombian andes, caribbean slab, nazca slab, teleseismic travel time residuals. earth sciences research journal 146 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics looking for mantle plumes under the brazilian shield: case studies in paraná and borborema j. julià, m. bianchi and m. assumpção ufrn. jordi@geofisica.ufrn.br abstract the origin of intraplate volcanism has often been explained through the presence of mantle plumes originating deep in the mantle. mantle plumes have been successful in explaining features such as the age progression of some volcanic island chains and the formation of large igneous provinces worldwide, but cases of diffuse intraplate volcanism have proven more challenging. an excellent example is the cenozoic volcanism of the borborema province of ne brazil. the borborema province is regarded as part of a larger neoproterozoic mobile belt that structured during the brasiliano/pan-african orogeny, which was marked by episodes of intraplate volcanism and uplift during the cenozoic. the volcanism consists of small volumes of alkaline rocks arranged along an approximately ns trending line known as the macau-queimadas alignment (mqa) and the uplifted area forms an elliptically shaped dome known as the borborema plateau. the stress pattern of the cenozoic deformation, along with the time overlap with the mqa volcanism, were initially invoked to propose doming from thermal activation due to a deep mantle plume as a possible mechanism. more recently, however, small-scale convection at the edge of the continent combined with isostatic uplift by mafic underplating of the crust, and lateral flow along lithospheric thin spots from a distant, deep-seated plume in the paraná basin of se brazil were suggested as plausible alternatives. here, we investigate the presence of mantle plumes under the brazilian shield by mapping the thickness of the upper mantle transition zone with p-to-s conversions observed in receiver functions. we focus on the borborema and paraná provinces, as recent broadband deployments in the regions allow for detailed mapping of transition zone topography. the transition zone is bounded by two discontinuities, nominally at 410 and 660 km depth, that originate from phase transformation in the olivine-normative component of the mantle. the depth of the transformation varies with temperature, so that variations in the thickness of the transition zone can be used as diagnostic for thermal anomalies as generated, for instance, by the passage of a mantle plume. our results show that transition zone thickness is not anomalous in any of the two regions, demonstrating that a mantle plume origin for cenozoic volcanism in the borborema province from either local or distant upwellings is not justified. key words: brazilian shield, upper mantle transition zone, mantle plumes, receiver functions. earth sciences research journal 147 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics inferences about the lithospheric seismic structure of valle de aburra based on travel time residuals from strong motion networks jose david henao and gaspar monsalve mejía universidad nacional de colombia. jodhenaoca@unal.edu.co abstract the aim of this work is to make inferences about the seismic structure of the valle de aburra’s lithosphere. for this purpose p and s travel time residuals were analyzed. p and s waves travel times were obtained taking seismic events with origin times and hypocenters reported by the colombian geological survey. we took wave arrival times recorded at 17 stations from medellin and valle de aburra strong motion networks. such arrivals were recorded from 2008 through 2013, as arrivals at this interval are clear and reliable. a total of 276 travel times from 52 earthquakes were obtained. theoretical travel times were calculated with the taup toolkit which is based on the τ(p) function. at this stage, three velocity models were tested in order to find the one which yielded the smallest residuals and thus represents the best fit. those models were iasp 91 (kenett and engdahl, 1991), and those constrained for colombia by ojeda & havskov (2000) and vargas et al. (2003). the models from vargas et al. and iasp 91 are the ones that best represent the lithospheric seismic structure at valle de aburra. total residuals indicate a general delay of the p-waves with respect to those calculated, except for santa helena station, to the east of the valley. s waves show a shorter than predicted travel time at santa helena and sabaneta (to the south of the valley) stations. when a regional mean residual is extracted to residuals at each station, santa helena, sabaneta and ayura stations (located to south of the valley) show negative values indicating faster structure for both p and s waves. keywords: valle de aburra, travel time residuals, strong motion network, lithospheric seismic structure. earth sciences research journal 148 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics deep lithospheric features of the são francisco craton marcelo assumpção1, paulo araújo2, marcelo peres rocha2, eder molina1, marcelo bianchi1 1dept. of geophysics, university of sao paulo, brazil. 2geosciences institute, university of brasilia, brazil. marcelo.assumpcao@iag.usp.br abstract studying the thick lithosphere of cratons is important to help understand their formation and the mechanisms for their preservation. we present a synthesis of the information available for the deep structure in eastern brazil, from seismological and gravity data, to characterize the são francisco craton (sfc) and help better define its lateral boundaries at depth. early results from deep seismic refraction lines worldwide (durheim & mooney, 1991) suggested that archean cratons tended to have thinner crust and slightly more felsic average composition, compared with proterozoic provinces, which would be accompanied by lower density, depleted upper mantle (durheim & mooney, 1994; hawkesworth et al., 1990). however, more recent compilations seem to indicate a large variability in crustal properties of archean provinces (artemieva & meissner, 2012), with apparently no significant difference from proterozoic crust. here we present the characteristic crustal and lithospheric features of the sfc. only one deep seismic refraction line was shot, near the central part of the sfc (soares et al., 2006). crustal thicknesses of the sfc are known mainly from receiver function studies and range between 38 and 42 km, except for a localized thickening (up to 44km) in the northern part, and crustal thinnning towards the atlantic continental margin in bahia state. overall, the crust is slightly thicker near the geologically-defined surface boundaries (40-42 km) and slightly thinner in the center (38-40 km), which is consistent with generaly low bouguer anomalies and high topography to the east and to the west of the craton probably defining the suture zones during the gondwana amalgamation. a relatively thin crust (38km) occurs near the sw boundary and the brasilia foldbelt. surface-wave continental-scale tomography (feng et al., 2007) suggested the thickest lithosphere, around 200 km, to be in the archean southern part of the sfc. this is consistent with pand s-wave tomography in se and central brazil (rocha et al., 2011). both the surface-wave and the body-wave tomographies show high upper mantle velocities beneath the brasilia foldbelt, to the west of the sfc’s surface limits, which is interpreted as a continuation at depth of the craton’s lithosphere, beneath the low-grade external metamorphic domain of the brasilia foldbelt. keywords: tomography, receiver functions, cratonic crust, gravity anomalies. earth sciences research journal 149 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics ambient noise tomography: preliminary results at southeast of south american platform marcos vinícius ferreira1, marcelo peres rocha1, martin schimmel2 1university of brasilia. 2institut de ciències de la terra jaume almera. marcelorocha@unb.br abstract the ambient noise tomography technique was employed to study the crust and the shallow mantle under the southeast portion of the south american platform. this method was chosen due to the poor spatial distribution of ray paths which would be obtained for earthquake surface wave tomography at periods which range from 5s to 20s. in this work, we focus on rayleigh waves which are extracted from empirical green's functions which we obtain from the phase cross-correlations (pcc) of vertical component ambient noise recordings. pcc is based on the instantaneous phase coherence of the analytic signals and was chosen since it is amplitude unbiased. the obtained cross-correlations were then stacked using the time-frequency domain phase wave stacking (tf-pws). these methods are expected to detect and to enhance weak, but coherent signals concealed in other larger amplitude signals and to improve the signal-to-noise ratio of the final results. here, we will present group velocity maps for periods of 8s and 15s which represent the average structure down to depth of about 15 km. our results were discussed in terms of the geological settings and provide mapping of structures such as sedimentary basins and cratonic regions. keywords: ambient noise tomography, south american platform, phase cross correlation. earth sciences research journal 150 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics relationship between intraplate seismicity and low velocities tomography anomalies in the tocantins province, brazil paulo araújo de azevedo, marcelo peres rocha, reinhardt a. fuck, juliana garrido university of brasilia marcelorocha@unb.br abstract intraplate seismicity regions, such as brazil, are very common, although earthquakes of larger magnitudes are rarer. the study of the intraplate seismicity is very important to help the understanding of the neotectonic processes. we present a discussion of the possible causes of intraplate seismicity in the region of tocantins province in brazil from recent pand s-wave seismic tomography results. in the south american continent, the regional stress field is dominated by compressions in east-west direction (zoback, 1992). the origin of this system of tensions is mainly related to the formation of the south american plate from the mid-atlantic ridge, and resistive forces exerted by the caribbean plate to the north and the nazca plate forces in the west (mendiguren & richter, 1978; coblentz & richardson, 1996). assumpção et al. (2004) proposed a model to explain intraplate seismicity, especially in southeastern brazil. according to this work, the high seismicity is due to the weakness of the lithosphere with his thinning. this hypothesis is based on the fact that the concentration of events coincides with tomographic low-velocity anomalies, which indicate regions of lithospheric thinning. according to the model of assumpção et al. (2004), stresses would be concentrated uniformly in depth along the lithosphere. in intraplate regions with thicker lithosphere, the intensity of the focused stress would be smaller than the thinner regions. thus, the crust in regions of thinner lithosphere, concentrate more intense stresses promoting a higher seismicity. in our results, we observed for the tocantins province the same pattern observed in the work of assumpção et al. (2004). the region of low velocity in the tocantins province center is usually accompanied by a high concentration of seismic events following the direction of the transbrasiliano lineament. thus, this lineament could be reactivated by stresses accumulated in the crust due to thin lithosphere. assumpção & sacek (2013) proposed that flexural stresses from uncompensated lithospheric loads are high enough to explain the seismic zone, which continues for the northern part of the tocantins province. the improvement of the tomography results suggest that lithospheric thinning is still important to explain intraplate seismicity in the central brazil. key words: seismic tomography, intraplate seismicity, lithospheric thinning, flexural stresses. earth sciences research journal 151 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad localizada en la región de manto litosferico de placa sudamericana debajo de la provincia geológica de payenia sur de mendoza argentina lupari marianela n.1, nacif silvina v. 1, spagnotto silvana l.2, yacante graciela1 1instituto geofísico sismológico f. volponi facultad de ciencias exactas, físicas y naturales universidad nacional de san juan (unsj). 2departamento de física, universidad nacional de san luis, ejército de los andes 950, 5700, san luis argentina. marianelalupari@gmail.com abstract las formas de onda registradas por el experimento temporal denominado bloque san rafael (bsr) fueron utilizadas en este trabajo para localizar y cuantificar la actividad sísmica de la región. dicho experimento consta de 12 estaciones sismológicas desplegadas en la región enmarcada por las latitudes 34.5° y 36° sur y las longitudes 67.5° y 70° oeste (figura 1). a estas latitudes, la placa de nazca subduce con una inclinación de aproximadamente 27° (anderson et al. 2007) y a un ritmo de 6.3 cm /año (kendrick et al. 2003). con el fin de aportar, desde el punto de vista sismológico, al conocimiento general de la zona, se localizaron 340 eventos sísmicos ocurridos en un periodo de 7 meses (dic./2011 y may. a oct./2012) (ver figura 2). de la sismicidad analizada en la región, 71 sismos poseen profundidades entre 50 y 100 km, lo que indica que se ubican por encima de la placa de nazca subducida, y por debajo de la corteza continental, justo debajo de la provincia geológica de payenia. ésta, ubicada en el retroarco andino, está caracterizada por sus extensos campos volcánicos. payenia está integrada por más de 800 conos basálticos y unos pocos conos poligenéticos como el volcán chachahuén, el nevado y el payún matru (llambías et al. 2010). la intensa actividad volcánica de retroarco tuvo su desarrollo a en el mioceno, con un pico en el plioceno – cuaternario. numerosos autores han estudiado esta provincia geológica con el fin de analizar y comprender su historia, evolución y comportamiento geodinámico. en este trabajo se presenta sismicidad en la región de manto litosférico de placa sudamericana entre 50 y 100 km de profundidad, no reportada con anterioridad. la misma abre nuevos interrogantes relacionados a la geodinámica de la región. burd et al. (2013), utilizando datos de magnetotelúrica propone la presencia de una pluma de manto, la cual estaría correlacionada con la sismicidad estudiada. la aplicación de diferentes técnicas sismológicas (ej. resolución de mecanismos focales) a este conjunto de datos aportará en el entendimiento en la mecánica de generación de esta sismicidad. palabrasclave: payenia, sismos de profundidad intermedia, manto litosférico. earth sciences research journal 152 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 153 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 154 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal and upper-mantle structure of the pampean flat-slab region from surface-wave tomography ryan porter1, megan anderson2; susan beck3, hersh gilbert4, george zandt3; patricia alvarado5 1northern arizona university.2colorado college. 3university of arizona.4purdue university.5universidad nacional de san juan. ryan.porter@nau.edu abstract the geology of south america has been greatly influenced by subduction along its western margin. the pampean region, located in west-central argentina and chile between ~28° and ~34° s, exemplifies this. this region is composed of numerous terranes accreted onto the rio de la plata craton during previous episodes of subduction. modern subduction within the region is characterized by the slab subducting to ~100 km depth before assuming a sub-horizontal geometry for over 300 km at which point it resumes a more typical angle of subduction. this unusual slab geometry, referred to as flat-slab subduction, is associated with a cessation of arc volcanism and the migration of deformation inboard from the andean fold and thrust belt into the foreland, forming the basement-involved sierras pampeanas. in order to better understand the continental lithospheric structure and subduction dynamics of the region, we use broadband seismic data collected during 3 temporary deployments to produce a regional 3dimensional shear-velocity model from ambient noise and earthquake-generated surface-wave measurements. results show significant variations in shear-velocity within both the down-going slab and over-riding plate. within the subducting nazca plate, these variations are interpreted in the context of slab hydration. shear velocities increase from west to east within the slab and decrease from west to east in the overriding mantle. we believe this represents slab dehydration as hydrous minerals within the slab become unstable and release water into the overriding mantle and that this hydration and subsequent dehydration may impact the buoyancy of the downgoing plate. variable lithospheric structures are also within the region. crustal thickness estimates, based on shear velocities, show thick crust beneath the high cordillera that thins to the east. beneath the sierras de cordoba, located in the far eastern portion of the study area, we observe a highvelocity body immediately beneath the crust, and interpret it as lithospheric mantle. this feature thins to the west beneath the central sierras pampeanas. the presence of this high-velocity feature suggests that the rio de la plata craton lithosphere is largely intact while its westward thinning is consistent with little mantle lithosphere existing beneath the accreted terranes. similar patterns in mantle-lithospheric thicknesses are observed in the southern part of the study area at 34.5° s where a more typical subduction geometry is observed, suggesting that the thin mantle lithosphere is not unique to the flat-slab region and that either a) thin mantle lithosphere has existed since terrane accretion or b) that mantle lithosphere has been removed in both regions. overall, our results highlight the complicated interactions between the overriding plate and downgoing slab and suggest that modern structure of the region is influenced both by slab dynamics and preexisting structures within the lithosphere. keywords: tomography; subudction; lithosphereasthenosphere boundary. earth sciences research journal 155 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics estructura litosférica asociada a la sierra nevada de santa marta a partir de datos de gravimetría, magnetometría y sismología sergio alejandro camargo vargas universidad nacional de colombia. sacamargo@sgc.gov.co abstract la zona en la que se encuentra la sierra nevada de santa marta (snsm) presenta una composición atípica basándose en trabajos desarrollados en geología por tschanz (1974) y cedielet al. (2003). su desarrollo presenta características estructurales, geológicas y geofísicas que difieren de lo que se esperaría para esta estructura. de igual forma en la mayoría de los andes se presenta una correlación inversa entre las longitudes de onda de la topografía y la anomalía de bouguer (ussami, shitaiwa, & landim dominguez, 1999)), efecto que no es apreciable de igual modo en la snsm, donde la relación es directa evidenciando así la no compensación isostática. el estudio muestra a partir de los datos gravimétricos, magnéticos y de valores de hipocentros todos estos re muestreados (kriging) desde el uso de la geoestadística, un análisis de condiciones tectónicas, donde implementa la teoría base de los modelos flexurales, la cual se desarrolla a partir de la teoría elástica de compensación isostática, donde las variables más significativas son el espesor elástico efectivo, la rigidez flexural y la magnitud de la carga, mostrando valores de deflexión no superiores a los 25 km dependiendo esto de los factores usados como variables significativas. este resultado se corrobora desde el uso de metodologías para la determinación del espesor cortical, donde se encuentran rangos que oscilan entre 25 y 30 km, así mismo el uso de la metodología de la correlación espectral hernandez (2006) donde se puede evidenciar altas entre la gravimetría y la magneotmetría sobre la zona de interés así como bajas en sectores aledaños, proceso que evidencia fuentes provenientes de lugares comunes. esto se aplica en este caso para realizar un análisis de la respuesta isostática de la litosfera en la zona caribe de colombia, teniendo como objeto principal la snsm y plantear una hipótesis inicial sobre las condiciones tectónicas asociadas a la snsm, combinado para esto las diferentes fuentes y metodologías de análisis geoestadístico, gravimétrico, magnético, y sismológico. palabras clave: estructura litosférica, sierra nevada de santa marta, gravimetría, magnetometría, sismología. earth sciences research journal 156 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics association of mw = 6.4, 2000/06/16 and mw = 5.7, 2003/01/07 intermediate-depth earthquakes, in subducted nazca plate silvana spagnotto1, enrique triep2, laura giambiagi3 and silvina nacif2,4 1conicet postdoc universidad nacional de san luis, argentina. 2igsv, universidad nacional de san juan, argentina. 3ianigliacct mendoza-conicet. 4fonarsec postdoc slspagno@unsl.edu.ar abstract the nazca plate is subducted beneath the south american plate characterized by along-strike variations in slab dip angle. the angle of subduction is ~ 27° in the studied earthquakes area (figure 1). the intermediate earthquakes in a subduction environment have been related to pull slab forces and dehydration processes of the slab. the mw=6.4, 16/6/2000 and mw=5.7 7/1/2003 intermediate depth (~ 120 km) earthquakes were analyzed. these earthquakes are closely related to each other, with the first one, which breaks the oceanic subducted crust, increasing the coulomb static stress around ~ 1 bar in the area where the rupture of the second one began (figure 2). the rupture of the second event was developed in the mantle and penetrated into it to about 40 km in depth. the mechanisms were obtained by waveform inversion (kikuchi and kanamori, 1982, 1991, kikuchi et al. 1993) and slip distribution using direct modeling with the same software. coulomb static stress changes were calculated using coulomb 3.2 software (toda et al, 2005, lin and stein, 2004). for the 2003 earthquake, the fault plane is defined by aftershocks which were determined by marot et al (2012); while for the 2000 earthquake, we define the fault plane by association with the 2003 earthquake. we propose that both earthquakes are related, indicating the subducted crust rupture of 2000 earthquake which promotes the rupture of 2003 earthquake on the mantle around 18 months later. the 2000 and 2003 rupture planes strike parallel to the outer rise, suggest that the crustal fault is originated in this tectonic environment. acknowledgments we would like to express our thanks iris data management center (usa) for the waveforms obtained. our research was founded by picto n°254 riesgo sísmico, 2011-2014, pict-20111079 and unsj-cicitca proyect. res.022/13cs. key words: coulomb static stress, outer rise fault, subducted nazca plate. earth sciences research journal 157 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 158 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics investigation into the kinematics of basement-cored uplifts through passive source imaging of the bighorn mountains, united states william l. yeck1, anne f. sheehan1, josh stachnik2 1department of geological sciences, university of colorado, boulder co, 80309-0399, united states. 2department of earth and environmental sciences, lehigh university, bethlehem pa, 18015, united states. yeck@colorado.edu abstract basement-cored uplifts are observed globally and remain an enigmatic feature of plate tectonics, as many of these uplifts occur distant from plate boundaries. prominent ranges composed largely of basement-cored foreland arches include: the sierras pampeanas, south america; the tian shan, central asia; and the rocky mountains, north america. to investigate these structures, we focus on the bighorn range of the rocky mountains, united states. the bighorn mountains are an archetypal laramide basement-involved foreland arch and therefore serve as an excellent setting to investigate these structures. first order questions about laramide foreland arches include: (1) how was shortening accommodated at depth? and (2) why did arches nucleate at large distances from plate boundaries? we investigate these questions using a combination of passive source seismic methods including high-resolution receiver function imaging and ambient noise tomography. previous studies proposed diverse arch formation models, each of which predict a unique crustal geometry. we test these models using seismic ps receiver functions, which isolate the signals of p-to-s conversions from subsurface interfaces, such as the moho. we utilize a combination of two-layer receiver function thickness-vp/vs (h-κ) stacking and common conversion point (ccp) stacking. our results illuminate a moho geometry discordant from laramide structure of the bighorn arch. we do not see a crustal root or moho-cutting faults. these observations do not eliminate the possibility of a mid-crustal detachment, which would decouple the upper and lower crust. the mismatch between our observed moho geometry and the shallow laramide arch geometry suggests a new hypothesis, a pre-laramide origin of at least a part of the moho geometry. the observed heterogeneity of moho structure in a cratonic environment leads us to further explore how pre-existing heterogeneities may have influenced the bighorn arch nucleation. we use ambient noise tomography to constrain the azimuthally anisotropic phase and group velocity and isotropic shear velocity of the crust and upper mantle. we target specific lithospheric heterogeneities that may affect shortening including: a discontinuous high velocity (7.x layer) lower crust, the presence of large batholiths, and the boundaries between distinct cratonic environments. keywords: basement cored arches, laramide, receiver functions, ambeint noise. earth sciences research journal 159 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics hypocentral parameter inversion for regions with poorly known velocity structures woohan kim, tae-kyung hong, tae-seob kang gyeongsang national university. wookim@gnu.ac.kr abstract the determination of accurate hypocentral parameters is crucial in seismic monitoring, and is highly dependent on the accuracy of the implemented velocity model in conventional methods. a method to determine accurate hypocentral parameters based on an approximate velocity model is desirable. we introduce an iterative velocity updating scheme that can be readily combined with conventional hypocentral inversion methods. the algorithm searches for an optimum velocity model in a prescribed velocity range that minimizes the traveltime residuals. the hypocentral parameters are determined using the optimum velocity model. the proposed scheme reduces the dependence on a given velocity model in hypocentral inversion, providing reasonable hypocentral parameters based on an approximate velocity model. the feasibility and accuracy of the algorithm are tested with synthetic and field data. the scheme yields hypocentral parameters that are as accurate as those from full inversion methods but with approximately 70 times lower cost in terms of computational time. the proposed scheme can be readily implemented in any conventional method that is based on a fixed velocity model. keywords: hypocentral parameters, optimum velocity model. earth sciences research journal 160 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics rmts session – regional moment tensor solutions: advances and new applications conveners: jiří zahradník (charles univ. in prague czech rep.) <jz@karel.troja.mff.cuni.cz> efthimios sokos (univ. patras greece) <esokos@upatras.gr> ronnie quintero (ovsicori-una costa rica) <ronnie.quintero.quintero@una.cr> earth sciences research journal 161 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics cálculo del tensor de momento sísmico a partir de la inversión de formas de onda usando el software isola: sismo de nuquí colombia del 13-08-2013 (mw 6.5) alba estella ordóñez, josé faustino blanco chía servicio geológico colombiano (sgc). aordonez@sgc.gov.co abstract el 13 de agosto de 2013 se registró un evento sísmico de magnitud mw 6.5 superficial asociado a la subducción de la placa nazca bajo la placa sudamericana en el océano pacífico colombiano. el reciente despliegue de nuevas y mejores estaciones sismológicas en el territorio colombiano, así como la adquisición en tiempo real de estaciones compartidas de otras redes, ha contribuido a incorporar nuevas técnicas para el estudio de los sismos que ocurren en colombia. uno de los parámetros que se estudian es el proceso de ruptura, para su cálculo se pueden utilizar varios métodos entre los cuales se encuentra el de inversión de formas de onda. isola (sokos, e. n., zahradnik, 2008) es un paquete de software basado en el método de kikuchi and kanamori (1991), utilizando fuentes múltiples y que realiza una deconvolución iterativa para encontrar el tensor momento que mejor se ajuste por medio de una aproximación por mínimos cuadrados. isola se encuentra desarrollado bajo matlab lo cual lo hace muy amigable con el usuario, posee herramientas para el preprocesamiento de los datos, preparación de las funciones de green, inversión de forma de onda, visualización de resultados, también están presentes ayudas para inspeccionar la calidad de los datos mediante filtros, integrar las formas de onda, etc. el evento sísmico fue registrado ampliamente, seleccionando 15 estaciones por distancia para realizar la inversión, 13 de ellas de banda ancha y 2 acelerógrafos, la calidad de estos datos fue la adecuada excepto por una de banda ancha que fue descartada a posterior. se obtuvo un mecanismo focal de tipo de rumbo, con una magnitud de momento de 6.5 y una profundidad de 14 kilómetros. en el presente trabajo se reportan los resultados obtenidos al realizar la inversión con isola, comparados con los dados por el global centroid-moment-tensor (gcmt) y el calculado con las polaridades con el software de seisan. también se muestran los espectros calculados de cada una de las estaciones que registraron el evento, así como la calidad de ajuste de los resultados. palabras clave: isola, inversión de momento sísmico, sismo, colombia, mecanismo focal. earth sciences research journal 162 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics moment tensor solution of moderate events at large distances: using surface waves dispersion to build velocity models for the waveform inversion, examples from brazil fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com abstract in intraplate regions, where there are few earthquakes, the moment tensor determination can be difficult. sometimes, it happens because the event had low magnitude and it may not have been well recorded by enough stations. brazil has a low seismicity compared with other regions: since 1990, around 18 earthquakes with magnitude greater or equal than 5.0 mb occurred and the maximum magnitude was 6.2 mb referring to an event in central region in 1955. we show an example of a 4.8 mb event in pantanal basin in 2009 and the closest broadband station (beb4b) was ~800 km from the earthquake. we measure the rayleigh and love surface waves dispersion in beb4b to build a velocity model to perform the tensor inversion. the moho depth and vp/vs was fixed from receiver function crust models (assumpção et al., 2012). the dispersion curve adjust was made with herrmann code (herrmann, 2013) and the result is shown in figure 1. we did the waveform inversion with isola code (sokos & zahradnik, 2008; sokos & zahradnik, 2013) using velocity model obtained from surface waves dispersion. the epicenter was fixed and we chose deviatoric mt inversion for the three components of beb4b station at frequency range 0.04,0.06 0.1,0.12 hz. the result was shown in figure 2. we obtained a good fitting between real and synthetic seismograms and the p and s-waves arrival was marked in the seismograms showing that the adjusted waves were the surface waves. since the back-azimuth between the station and epicenter is 280°, the vertical and east-west components show rayleigh wave and north-south shows mostly love wave. the mechanism from waveform fitting is similar to the mechanism from first-motion polarities and magnitude 4.4 mw agrees with 4.8 mb. the inversion of a moderate earthquake at large distance using a single station is very challenging. we showed a way to obtain a reasonable velocity model that helps to perform the inversion that may be useful in low seismicity regions. figure 1: surface waves dispersion fitting (top) in beb4b station and vs profiles initial (left-bottom) and final (right-bottom). the colors are misfit: purple is the worst and red is the best. black line is the weighted mean model using 10 models. figure 2: event surface waves modeling using isola program. in order, we have z, ns and ew components in displacement (m) from beb4b station (~ 800 km distance) with origin time (o), p and s-waves arrival marked. real data is the black line and synthetic is gray. the resultant focal mechanism is shown in the top left with the depth, magnitude mw and the frequency band used. in top right, we have the mechanism form first-motion polarities. keywords: moment tensor, full waveform inversion, surface waves. earth sciences research journal 163 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 164 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics focal mechanism and depth determination of earthquakes in brazil using teleseismic p-wave waveform modeling and first-motion polarities fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com,marcelo@iag.usp.br abstract we determined depths of shallow (< 25 km) recent earthquakes with moderate magnitude (range of 3 to 5 mb) in brazil using teleseismic p-waves modeling of p, pp and sp phases. the stations (∆ > 25) were grouped according to distance and azimuth and every record was visually inspected and those with a good signal/noise ratio (snr) are stacked and the first-motion polarities read. we usually consider groups with at least two stations, but sometimes, a good record of single station with different azimuth was also used to improve the focal depth. we used the hudson96 program of herrmann seismology package (herrmann, 2013) based on hudson (1969) to perform the modeling. one advantage of the program is the possibility of using different velocity models for the source, the path and the receiver allowing use water layers for events occurred in the ocean. the modeling is especially useful for the shallowest events (less than ~ 1 km) where the p, pp, sp phases are so close that is not possible to separate them. we used the grid-search method to do a more general search of strike, dip, rake and depth. then we use genetic algoritm of matlab to find a better solution around the gridsearch output. we are able to constrain this methodology using the focal mechanism solution of two different from literature: são francisco craton event (4.9 mb) studied by chimpliganond et al. (2010) and são vicente earthquake in continental shelf studied by assumpção et al. (2011). in the craton event, using only waveforms it was possible to recover the mechanism but in the são vincent event we had to use the p-wave polarities to recover the depth and the focal mechanism. for three earthquakes in the amazon fan: 5.3 mb in 1998, 4.8 mb in 2006 and 5.1 mb in 2007, we identified the depth phase pp by stacking teleseismic records grouped by distance and azimuth. using refraction seismic models in the region (watts et al., 2009) we determined a depth of 14 km for the 2007 event and 26 km for 1998 event. in the event of 2006, closer to the coast, it was not possible see the pp phase, indicating that it was a shallow earthquake. synthetic seismograms were calculated to constrain 2 km depth. for the event in the pantanal basin (4.8 mb) the ppp time difference indicates a 5.7 km depth, while teleseismic p-wave modeling gives a 6.0 km depth. this shows that the earthquake occurred in basement beneath the sedimentary basin. by modeling the teleseismic p waves we determined focal depths and compared with some previous determination available. it was shown that this technique is capable of recovering focal depth from shallow events when p, pp, sp are difficult to identify. also, it is a useful technique to retrieve focal depth when local stations are not available, as in most cases studied in brazil. key words: focal mechanisms, telessismic waveform. earth sciences research journal 165 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics new strategy for weak events in sparse networks: first-motion polarity solutions constrained by single-station waveforms lucia fojtikova institute of rock structure and mechanics, academy of sciences, czech republic jiri zahradnik, charles university in prague, czech republic jiri.zahradnik@mff.cuni.cz abstract determinations of focal mechanisms of small earthquakes (mw 2-3) are challenging. as a rule, their signal-to-noise ratio is good only at frequencies above the microseismic noise peak (~ 0.2 hz) and, at the same time, waveforms can be modeled only up to ~1-2 hz at relatively near stations (up to a few kilometers). more distant stations might provide enough first-motion polarities, but the polarity solution is often highly non-unique. to overcome these problems we suggest a simple approach called ‘cyclic scanning of the polarity solutions’ (csps). input data are represented by a suite of the firstmotion polarity solutions from focmec software. these solutions (the strike/dip/rake angle triplets) are used, one by one, and the near-station waveform is repeatedly inverted in isola software for the scalar moment (by the least-squared method), source depth and time (by grid search). each member of the focmec solution suite is thus quantified by the waveform fit, and a subset of well fitting solutions is defined, which considerably reduces the uncertainty of the polarity solution. the csps method strongly differs from the standard single-station waveform inversions (in which the strike/dip/rake angles belong to the inverted parameters). the standard single-station waveform inversion might occasionally produce correct mechanisms, but sometimes not; as a generally ill-posed approach, it cannot be recommended as a safe tool. on the contrary, the csps method has a great application potential in sparse networks where weak events are recorded in many stations, thus providing polarities, but only few of the stations are situated near epicenter to allow full waveform modeling. the method has been validated on a previously studied event in the corinth gulf, greece. keywords: focal mechanism, small earthquakes, first-motion polarities, single-station waveforms, isola, focmec, csps method. earth sciences research journal 166 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic source parameters of local micro-earthquakes in goiás state brazil by waveform inversion juraci carvalho1, lucas vieira barros1, jiří zahradník2 1observatório sismológico – unb, brasília – df, brasil. 2faculty of mathematics and physics – charles university in prague, czech republic. juraci@gmail.com abstract only few earthquake focal mechanism studies have been done in brazil due to low seismicity associated with low density of seismograph stations. thus a little is known about intraplate tectonic stress distribution in brazil and active faults, which is the fundamental information for understanding seismicity and making seismic hazard studies possible. to overcome the lack of such information, in addition to the traditional technique to obtain focal mechanism solutions by using the first p-phase polarities recorded in many stations, we demonstrate how to take advantage of full waveform inversion, from few stations. more specifically, in this contribution we perform the inversion of data from weak events, such as aftershocks, recorded by local stations (epicentral distance less than 8 km). the unique feature of this work is that thanks to the short epicentral distances we are able to model the complete wavefield up to the relatively high frequencies of 2.0 hz. we investigate the focal mechanisms using the isola package developed by sokos and zahradnik (2008, 2013) for local and regional events, based on the least-squares calculation of the moment tensor and grid search of the centroid position and time. the green function calculation is based on the discrete-wavenumber method developed by countant (1990) and bouchon (1981, 2003). the events are aftershocks of a 5.0 mb earthquake (mmi vi) which occurred on 2010/10/08 in mara rosa, goiás state, brazil. eleven events recorded by at least four and up to seven stations from a temporary local seismographic network, using broad band (30 s to 100 hz) and short period (1s to 100hz) seismometers were used. the magnitudes range from 1.2 to 2.0 md and distances vary from 1.0 km to 8.0 km. in a previous study, barros et al. (2012) showed a fault striking to nw-se and dipping to nw. a local crustal model developed by barros et al. (2012) was used in this study. to assess the results reliability, we used several quality-control parameters, such as vr (variance reduction), cn (condition number), dc% (doublecouple percentage), snr (signal-to-noise ratio), the first-motion polarity agreement, the focalmechanism uncertainty described by the kagan-angle dispersion (zahradnik and custodio, 2012), etc. prior to the inversion a thorough data inspection of the waveforms was done to remove data with instrumental disturbances (the so-called ‘mice’; zahradnik and plesinger, 2005 and 2010), electronic noise, excessive cultural noise and/or any kind of equipment malfunction which could bias the inversion results. the inversion frequency band was determined according to the snr (the minimum usable frequency, f-low = 1.5 hz) and according the corner frequency and velocity model resolution (f-high = 2.0 hz). the grid-searched centroid positions (trial sources) were situated below the fixed epicenter in a depth range of about 3.8 km and in a time window of about ±3.0 sec around the origin time to allow optimization of the deviatoric moment tensor. good results were obtained for eight of the eleven studied events with most of the quality-control parameters meeting optimum values established in previous studies. the results are in general agreement with barros et al. (2012), but, importantly, they also earth sciences research journal 167 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics demonstrate the focal-mechanism variability among the aftershocks. the variability of the mechanisms opens door for possible stress inversions. keywords: source parameters, focal mechanism, waveform inversion, double-couple percentage, deviatoric moment tensor. earth sciences research journal 168 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics source seismic parameters of earthquakes close to funil reservoir power plant in the south of minas gerais state – brazil lucas vieira barros, vinicius martins ferreira, mônica g. von huelsen, juraci mario de carvalho, daniel farias de caixeta and darlan portela fontenele observatório sismológico da universidade de brasília. lucas@unb.br abstract funil reservoir is located in the south of minas gerais state in the southeastern of brazil in the middle of são francisco craton. close to the reservoir there are three potential sources of triggered seismicity: the lake itself, mining and water table variation due to water pumping. on august 14th, 2011, a local seismic station installed near o the reservoir, detected an earthquake of 3.2 magnitude on the richter scale. this event was felt more strongly in the town of ijaci, located on the left margin of the funil reservoir. on october 5th, november 13th and november 23th, 2011, ijaci residences were again shaken by new seismic occurrences. as a result, in december of 2011, the seismological observatory of the university of brasilia, in a partnership with the funil consortium, deployed a local six stations seismograph network, four broad-band and two short-period to better study this seismicity. the instruments operate in the band of 30s to 100hz and 1 hz to 100 hz, respectively, and data were sampled at a rate of 100 sps. the network operated until may 23th, 2012, and during these five months it were detected more than 1000 seismic events (natural and artificial), which 500 were located, but only about 70 with reasonable accuracy. the events were located very close to the lake and in the quarries of the region. in these areas no previous seismicity had been detected before. the quarries make systematic detonations, most of the time, without scheduling information and when this is accomplished, it is done inaccurately. this hampers the discrimination of tectonic events from artificial events, considering the similarity between the waveforms of certain bursts with those generated by natural events in the same area. so, it is difficult to analyze the data because some explosion records are much like the records of natural earthquakes and therefore discrimination is very difficult and time consuming. in order to get focal mechanism solution it was made a selection, according to the hypocentral location accuracy, just ten events set on magnitudes ranging from 0.3 to 1.6, and from this set it were used 5 events with magnitudes from 1.2 to 1.6 to made waveform inversion using isola package. so, the goal of this work is to present the results of source seismic parameters of the best events of the funil reservoir seismic sequence using two methods, composite focal mechanism and waveform inversion, and see if their results are in agreement, as well as the to discuss how to discriminate tectonic events from artificial events. keywords: waveform inversion, focal mechanism, seismic source parameters, triggered seismicity. earth sciences research journal 169 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics modelamiento de fuente sísmica del sismo del quindío (colombia) del 25 de enero de 1999 usando inversión de forma de onda de cuerpo telesísmica lynette vanessa chavarría1, helber garcía camacho1 y hugo monsalve j.2 1grupo quimbaya, universidad del quindío. 2ceifi, grupo quimbaya – universidad del quindío. hugom@uniquindio.edu.co abstract el sismo de armenia – quindío, colombia (mw = 6.2) del 25 de enero de 1999, es el sismo superficial más grande registrado instrumentalmente en el eje cafetero colombiano. se localizó en las inmediaciones del municipio de córdoba (quindío), a 17 kilómetros de la ciudad de armenia. se obtuvo mediante inversión de forma de onda el área y proceso de ruptura, con registros de desplazamiento de banda ancha con ondas en el rango de 5 a 0.01 hz. en dicho proceso de inversión se obtuvo un área de ruptura de 125 km2, una magnitud mw = 6.0 y una falla normal de desplazamiento lateral izquierdo de tipo unidireccional debido a que la presencia de réplicas indican que la ruptura de la falla creció con el tiempo hacia el noreste, sugiriendo una falla de este tipo; el mecanismo focal total predominante es de rumbo con componente normal, con buzamiento δ = 48.9°, azimut ϕ = 111.5° y ángulo de deslizamiento λ = -166.7° producto de dos subeventos distribuidos a lo largo del área de ruptura, el primero de ellos se presenta en 𝜏𝜏 =8 s y l=0 km y el segundo localizado en 𝜏𝜏 =26 s y l=5 km. se obtuvo una profundidad del hipocentro de h = 20 km, un momento sísmico de 1.26 x 1025 dina*cm y una dislocación δu = 0.324 m. se realizó la validación de los registros acelerográficos de la red sismológica nacional de colombia (rsnc) en dos estaciones, la primera de campo lejano (a una distancia de 141 km del sismo) y la segunda de campo cercano (a una distancia de 42 km del sismo) con registros sintéticos de movimiento del suelo, obteniendo un ajuste aceptable. palabras clave: sismo de armenia – quindío, inversión de ondas de cuerpo telesísmica, mecanismo focal, validación. earth sciences research journal 170 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics moment tensor solution for a m 4.2 event in tierra del fuego c. bollini1,2, n. sabbione1 1facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata. 2consejo nacional de investigaciones científicas y técnicas (conicet). bollini@fcaglp.unlp.edu.ar abstract tierra del fuego island is divided into two continental blocks due to the magallanes-fagnano fault system (mfs) which represents the transform tectonic boundary between scotia and south america plates. the main fault is a strike-slip fault with sinistral displacement. an important low-medium seismicity related to the mfs is recorded by a local network whereas only few events and their associated moment tensor are calculated by global international networks in that area. this work obtains the moment tensor for a moderate event (mb 4.2) using a regional crustal model and two different hypocentral parameters corresponding to isc location and using seisan with local network data. the latter location was calculated for this purpose using the same crustal model as for the moment tensor and it was obtained with an rms < 0.3. waveforms from 4 broadband stations at nearregional distances (115-180 km) were inverted using the two different hypocenter determinations, at frequency ranges below 0.15 hz and using the isola-gui software package. source position was searched below the epicenter to optimize the solution. preliminary results show that focal mechanism obtained with the location from the local network is consistent with the regional tectonics and in general the polarity check is agree with the solution. key words: moment tensor, waveform inversion, seismicity, local network, tierra del fuego. earth sciences research journal 171 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 172 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics inversion of a 2-point source model of the m7.4 2012 march 20th guerrero, mexico oscar a. castro-artola1, jiří zahradník2 and efthimios sokos 1posgrado en ciencias de la tierra, instituto de geofísica, unam. 2charles univ. in prague czech rep. 3univ. patras – greece. oscar@geofisica.unam.mx abstract on march 20th 2012 a m7.4 earthquake occurred in the coast of guerrero, mexico. its magnitude, depth, location and focal mechanism show that this is a typical subduction earthquake. unam seismology group (2013) used strong motion records to invert the displacements on the fault. it showed a complex slip distribution with two main patches. the first and biggest patch occurred downdip of the hypocenter, this patch reaches an amplitude around 4m, while the second patch is smaller in space and amplitude. the patches ruptured at different times. these observations motivated us to try to obtain a 2-point source model similarly to zahradník and sokos (2014). we worked with isola (sokos & zahradník, 2008 and 2013) in order to find a 2-point source inversion that could be congruent with the tectonic environment and with other observations. we used regional data (from ~130km up to 500 km) obtained from the national seismological service (ssn, for its acronym in spanish) in mexico. all data come from seismometers belonging to the broad band network of the ssn. we found almost the same focal mechanism and centroid as those obtained from gcmt and usgs. the 2-point source spatial distribution claims for a more detailed study and the incorporation of more data. the moment magnitude value that we found is 7.4 which is congruent with other agencies. keywords: isola, 2-point source. earth sciences research journal 173 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics moment-tensor solution of the intermediate-depth 2013 guaitarilla (colombia) mw 6.9 earthquake using near-regional waveform data patricia pedraza1 and jiri zahradnik2 1servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá. colombia. 2faculty of mathematics and physics – charles university in prague, czech republic. ppedraza@sgc.gov.co, jz@karel.troja.mff.cuni.cz abstract the guaitarilla (colombia) earthquake, mw = 6.9, occurred on february 9, 2013 at the intermediate depth ~ 160km in the wadati-benioff zone of the nazca subduction in southwestern of colombia. this event was located down-dip of the rupture zones of the 1906 colombia (m~8.7) and the 1979 colombia (m~8.2) thrust earthquakes in a seismic gap zone of intermediate depth. this earthquake was felt strongly in the center and southwest of the country and affected 66 municipalities in the departments of nariño and cauca, with a final balance of 402 houses destroyed, 3560 houses damaged and 67 schools damaged. the source parameters are calculated with near and regional broadband and strong motions data from the national seismological and strong motion networks of colombia. by means of inverting waveforms recorded at distances 30 km to 800 km, in the frequencies range 0.01-0.05 hz with isola code. initially, the depth and the horizontal centroid position are obtained by grid-searching around the starting parameters using different 1d crustal models, then a possible source complexity is studied by multiple point source model. one of the possible indications of the source multiplicity is the low dc percentage (~55%). the focal mechanism is strikeslip, possibly due to the presence of an asperity. the hypocenter-centroid (h-c) method based on mutual relative position of centroid and hypocenter and the location of aftershocks are used for determining the fault plane. the reliability of the solution is established by mean of the joint assessment of the signal-to-noise ratios, variance reduction, conditional number and the space-time variability of the solution with the innovative tools of the last version of isola code. key words: moment tensor inversion, intermediate depth, colombian subduction. mailto:ppedraza@sgc.gov.co mailto:jz@karel.troja.mff.cuni.cz earth sciences research journal 174 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismicmomenttensorsolutionsof m > 4.5 earthquakesincolombia patricia pedraza and carlos fajardo servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá. colombia. ppedraza@sgc.gov.co, cfajardo@sgc.gov.co abstract thecolombianterritoryiscurrentlycoveredby42broad bandseismicstationsand30strongmotionstationsthattr ansmitdatainrealtimetonationalseismologicalnetwork ofcolombiaoperatedbycolombiangeologicalsurvey. thishighdensitydataallowus to retrievalearthquakesourceparametersandruptureproce ss.weestimatedthemomenttensorofearthquakeswith mw>4.5thatoccurredbetween2012and2014incolo mbiaandoffshoreareas.momenttensorinversionswerec alculatedwiththeinversionofcompletewaveformsinthe rangeof0.01 – 0.1hzusingtheisolacode. theepicenters,depthsandmagnitudesestimatedbynati onal networkwereusedasinput,and thenthecentroidpositionanddepthwererefinedbygridsearchingaroundthestartingparameters.theimplement ationofautomaticsolutionswasevaluatedconsideringth equalityandreliabilityofdataandthestabilityofthesoluti ons. the joint assessment of the signal-to-noise ratios, variance reduction, conditional number and the space-time variability of the solution allowed us to establishing the reliability of the results. key words: moment tensor inversion, isola, colombia. mailto:ppedraza@sgc.gov.co mailto:cfajardo@sgc.gov.co earth sciences research journal 175 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics resolución y análisis de mecanismos focales de sismos corticales en cordillera entre 34.5°s y 36°s raquel villegas1, silvina nacif2,3 y silvana spagnotto1,4 1conicet. 2igsv, universidad nacional de san juan. 3fonarsec. 4universidad nacional de san luis. vraquelj@gmail.com abstract se estudió la sismicidad cortical de la placa sobremontada ubicada en cordillera principal, frontal entre 34.5°-36°s. se usaron los datos del experimento temporal charge (chileargentina-geophysical-experiment) para lo cual las formas de ondas fueron facilitadas por iris*. se localizaron 24 sismos usando el software hypocenter (liener and havskov 1995) con el modelo de velocidad de nacif 2012. se utilizó la polaridad de los primeros movimientos de la onda p e inversión de forma de onda para determinar los mecanismos focales. solo 4 de los sismos (m>4) fueron procesados con técnicas de inversión de forma de onda mediante isola (sokos y zahradnik 2008). el modelo de velocidad utilizado en la inversión fue el obtenido por spagnotto 2013. para los sismos de magnitud menor a 4 se utilizó el programa focmec (snoke et al. 1984) en una primera etapa y hash (hardebeck y shearer 2002) en una segunda. los mecanismos focales resueltos presentan soluciones normales con componente de rumbo e inversas con componente de rumbo a profundidades que van desde ~ 10km hasta 36km (incertidumbre en parámetros de localización < 8km). los distintos métodos utilizados para obtener los mecanismos focales evidencian la necesidad de un conocimiento más exacto de la estructura de velocidades en la zona para poder aplicar metodologías de inversión de forma de onda a sismos de magnitud menor a 4. *the facilities of the iris data management system, and specifically the iris data management center, were used for access to waveform, metadata or products required in this study. the iris dms is funded through the national science foundation and specifically the geo directorate through the instrumentation and facilities program of the national science foundation under cooperative agreement ear-0552316. some activities of are supported by the national science foundation earthscope program under cooperative agreement ear-0733069. palabras clave: mecanismos focales, isola, inversión del tensor momento. earth sciences research journal 176 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 177 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics near-regional cmt and multiple-point source solution of the september 5, 2012, nicoya, costa rica mw 7.6 earthquake ronnie quintero1, jiri zahradník2 and efthimios sokos3 1observatorio vulcanológico y sismológico de costa rica, universidad nacional, heredia, costa rica.rquinter@una.cr 2charles university in prague, faculty of mathematics and physics, prague, czech republic. jiri.zahradník@mff.cuni.cz 3university of patras, department of geology, seismological laboratory, patras, greece.esokos@upatras.gr abstract we use acceleration data from the observatorio vulcanológico y sismológico, universidad nacional de costa rica (ovsicori-una) and laboratorio de ingeniería sísmica, universidad de costa rica (lis-ucr) seismic network for the relocation and moment-tensor solution of the september 5, 2012, 14:42:03.35 utc, nicoya, costa rica earthquake (mw 7.6 gcmt). using different relocation methods we found a stable earthquake hypocenter, near the original ovsicori-una location in the nicoya peninsula, nw costa rica at lat 9.6943°n, lon 85.5689°w, depth 15.3 km (figure 1), associated with the subduction of the cocos plate under caribbean plate. acceleration records at ovsicori-una (figure 2) and lisucr stations (94-171 km), at 0.03 < f < 0.06 hz were used in the waveform inversion for a singlepoint centroid moment tensor (cmt). using spatial grid search the centroid position was found at the depth of 30 km, situated at lat 10.0559°n, lon 85.4778°w, i.e. of about 41 km nne from the epicenter. the centroid time is 14:42:18.89 utc, i.e. 15.54 sec later relative to the location-based origin time. the nodal plane (strike 318°, dip 27° and rake 115°) is the fault plane that agrees with the geometry of the subducted slab at nicoya, nnw costa rica. increasing the maximum studied frequency from 0.06 to 0.15 hz, the multiple point source inversion model leads to two subevents. the first one was located near the centroid and the second subevent was situated 20 km along strike and 10 km down dip from the first subevent and 6 sec later. the uncertainty of the source model was carefully examined using complementary inversion methods, viz the iterative deconvolution and nonnegative least squares. keywords: near, free, field, acceleration, records, cmt. earth sciences research journal 178 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 179 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 180 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics el sismo mw 6.2 del 27 de febrero de 2010, salta argentina. mecanismo focal y modelos de deslizamientos usando inversión de formas de ondas sísmicas e interferometría sar spagnotto silvana1, scott, chelsea2 y garcía victor3 1universidad nacional de san luis. posdoc conicet. 2cornell university. 3universidad nacional de rio negro. conicet. pampa113@gmail.com abstract la sismicidad de intraplaca en la región según el catálogo de inpres, indica que el valle de lerma es una zona tectónicamente activa. los sismos históricos que afectaron a la ciudad de salta (1692, 1844, 1908, 1948 y 1973) tuvieron su epicentro dentro del sistema de santa bárbara, al este del valle de lerma. estos eventos probablemente se encuentren relacionados con la inversión tectónica del rift del grupo salta (monaldi et al., 1996; mon et al., 2005). el sismo de salta del 27 de febrero de 2010 fue el primer evento de intraplaca de magnitud mayor a 6 registrado en el valle de lerma. las mayores intensidades fueron reportadas en las poblaciones de campo quijano, la silleta, rosario de lerma, cerrillos y la merced y dos personas perdieron la vida como consecuencia de derrumbes durante el terremoto. la proximidad temporal de este sismo con el mw 8,8 de maule hace suponer que podría haber sido disparado remotamente (scott et al., 2014). en el presente estudio se determinó el mecanismo focal, a partir de formas de onda utilizando telesismos y sismos regionales usando datos de iris. para el estudio de los telesismos se utilizó kikuchi y kanamori software (2003) y para los sismos regionales isola software (sokos y zahradnik, 2008). se obtuvieron resultados similares en el mecanismo, la magnitud, y profundidad hipocentral. se obtuvo una solución compresiva con una pequeña componente destral. utilizando las réplicas localizadas por sánchez (2012) y la configuración tectónica regional (garcía et al., 2011) se determinó que el plano de falla responsable del movimiento posee una orientación 180/32/111. se obtuvo además la distribución de deslizamientos utilizando inversión de ondas sísmicas e interferometría sar (synthetic aperture radar). por la geometría de la solución y la localización del centroide, se propone que la fuente sismogenética se habría localizado sobre un corrimiento ciego buzante al oeste responsable del levantamiento de los cordones montañosos del malcante, obispo, y san miguel, y que se conectaría con el nivel de despegue regional ubicado a 20-25 km de profundidad por debajo del cordón del nevado de cachi. la tip line de este corrimiento se ubicaría por debajo del valle de lerma y estaría relacionado con las estructuras neotectónicas descriptas recientemente por garcía et al. (2013a y b). palabrasclave: sismo mw 6.2 del 27 de febrero de 2010, salta; mecanismo focal; distribución de deslizamientos; insar e inversion de forma de onda. earth sciences research journal 181 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics source parameters by waveform inversion of 6.6mw earthquake in the nicoya peninsula on october 24, 2012 aftershock of the 7.6mw main shock on september 5, 2012 vinicius martins ferreira1, kellen schmidt, lucas vieira barros, ronnie quintero2 1seismological observatory of the university of brasília. 2seismological observatory of costa rica and the national university. vinicius2002@gmail.com abstract large earthquakes with magnitudes greater than 7.5 occur with recurrence of 50 years in nicoya peninsula, located in nw of costa rica. the seismic potential of this region can be associated with stress concentration inferred by the observation of slower subduction velocity of the cocos plate under the caribbean plate relative to that one observed in mid-oceanic ridge (reference). the first earthquake (magnitude-7.5) reported for this area took place in 1853; the second of magnitude 7.8 occurred on october 5, 1950. recently, on september 5, 2012, another large earthquake (magnitude-7.6) shook this region with maximum intensity of viii (mm). the epicenter was located 20km northwest of sámara at a depth of 35km. this earthquake was widely recorded by global stations and some seismographic stations of the volcanological and seismological costa rica observatory national university (oviscoriuna), saturated their signal. ten broadband accelerometers and 19 seismographic stations (trillium 120s to 100 hz) of oviscori network monitored the aftershock activity. the largest observed aftershock, 6.6 mw, occurred on october 24, 2012 at 00:45:00 utc at a depth of 13km and its epicenter was located in southeast of nicoya peninsula. the time domain waveform inversion, by isola software (sokos & zahradnik, 2008) to obtain the moment tensor used the 6.6mw aftershock recorded in ten stations of the oviscori network. the waveforms in the others stations were disregarded due to saturation of signals and among these two another stations were not used for not display clear first p-wave polarities. the isola methodology consists in the inversion (time domain) of complete waveform recorded by local stations for determine the seismic moment tensor (sokos & zahradnik, 2008). the position, time and depth for centroid were determined by successive queries in spatial and temporal domain. the results demonstrate a source with 87.3% of double-couple and 0.59 of variance reduction. it was noted that the source parameters results presented by isola has close values for the same source parameters found by neic-usgs. keywords: isola, peninsula de nicoya, waveform inversion, moment tensor. earth sciences research journal 182 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics rtsg session – real time seismology conveners: alessia maggi (univ. strasbourg france) <alessia@unistra.fr> victor huerfano (univ. puerto rico) <victor@prsn.uprm.edu> zacharie duputel (univ. strasbourg france) <zacharie.duputel@unistra.fr> earth sciences research journal 183 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics diseño e implentación del sistema de información de la red sismológica nacional de colombia acosta, my., beltrán, j., sánchez, c., chamorro, o. servicio geológico colombiano. macosta@sgc.gov.co abstract la red sismológica nacional de colombia (rsnc) adquiere, procesa y administra la información concerniente a la sismicidad del país. dicha información era almacenada en dispositivos falibles, además de requerirse procesos complejos y demorados para su consulta. actualmente se cuenta con un sistema de almacenamiento de información más confiable y ágil, que permite la centralización de los datos y la automatización de los distintos procesos. el desarrollo e implementación del sistema de información de la rsnc, incluye al grupo de la red nacional de acelerógrafos de colombia (rnac) y al grupo de tectónica cuaternaria. la rnac hoy cuenta con un banco de datos propio, que contiene los registros de aceleración de sismos internacionales y de sismos originados en el territorio colombiano hasta el año 2008. esto ha permitido centralizar la información de la rnac, agilizar la respuesta a las peticiones de los usuarios y hacer de fácil acceso la información al público en general. el grupo de tectónica cuaternaria cuenta recientemente con un sistema de información geográfica que permite almacenar, visualizar, analizar y difundir la información tectónica cuaternaria, como fallas geológicas y pliegues. dicha información desplegada sobre los distintos depósitos cuaternarios, complementa la evaluación de la amenaza sísmica en colombia. adicionalmente, este desarrollo permite la recopilación de toda la información dispersa, su centralización y puesta a disposición del público interesado en su consulta, además de permitir una posterior retroalimentación. a la fecha la rsnc cuenta con un banco de datos que contiene los registros sísmicos desde el año 1993 hasta hoy. además de tener la información sísmica almacenada de manera confiable y de contar con un sistema de información propio, que permite la administración de toda la información interna tal como: el estado y funcionamiento de estaciones y sus dispositivos periféricos, toda esta información puede ser consultada y descargada a través de internet. este trabajo tiene como objetivo presentar los productos diseñados e implementados por el grupo de tecnologías de la información del sgc en los últimos años y difundir los beneficios de dichos productos, para facilitar así el acceso a la comunidad científica, estudiantil y público en general a la información disponible. palabras claves: sistema de información, banco de datos, automatización. mailto:macosta@sgc.gov.co earth sciences research journal 184 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics determinación de la escala de magnitud local (ml) a partir de registros de banda ancha para el perú crsitobal condori y hernando tavera instituto geofísico del perú. cristobal.condori@igp.gob.pe abstract se ha generado una relación de escala de magnitud ml para el perú basado sobre el análisis de regresión de registros simulados a wood-anderson, empleando 742 registros de amplitud medidos en la componente vertical. para la solución se ha considerado 186 eventos de foco superficiales (0-60 km de profundidad) registrados por 13 estaciones sísmicas de banda ancha de la red sísmica nacional (rsn), periodo 2011-2013. el rango de distancia comprende entre 50 a 1400 km. la corrección por distancia ha sido evaluada en términos del coeficiente de expansión geométrica (a), y coeficiente de atenuación inelástica (b) siendo de 1.5489 y 0.0009, respectivamente. la nueva escala de magnitud ml propuesta para el perú está definida por ml=log10awa+1.5489log10(r)+0.0009(r)0.1878, donde, awa es la amplitud (mm) medido sobre un registro de desplazamiento simulado a wood-anderson y r es la distancia hipocentral en (km). los resultados estimados para la magnitud ml presentan buena correlación con los datos proveídos por el neic. asimismo, el valor de atenuación inelástica es similar al de california para distancias menores a 500 km y comparable con los obtenidos para otras regiones altamente sísmicas. palabrasclave: escala de magnitud local, woodanderson, corrección de distancia. earth sciences research journal 185 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a streaming processing model for efficient computations of large continuous data sets danny harvey boulder real time technologies danny@brtt.com abstract many traditional software processing models are not well suited to the large continuous, real time data streams from observatories and data centers around the world that are readily available today. traditional software processing has tended to treat data as discrete sets that were processed in a batch mode. this has been especially true with active source exploration processing software and passive seismic event oriented processing software. the treatment of data in these traditional batch oriented software modules is not well suited to processing continuous data nor does it support the minimum latency requirements for early warning systems. this has led to a certain segregation in processing software development; continuous real-time processing vs. discrete batch processing. we present a software design methodology that provides researchers with a substrate that can be used to effect both continuous real time and discrete batch processing using the same software source coding. the software substrate consists of a set of interoperable dynamically linked modules, each running in its own thread and communicating with the other modules through specialized real time packet pipes. we provide a standardized set of import and export software modules that regularize the inner computational modules so that they can process both real time continuous and discrete data sets without the need for separate code versions. this software design methodology provides inherent threading, transparent to the application programmer, to provide maximum leverage of modern multiprocessor computer systems, as well as an architecture that can provide solutions as quickly as possible for support of early warning systems. we show real examples of this methodology, including seismic array processing, data resampling, real time spectra estimation and a particle motion based realtime detector that can identify p and s arrivals. autores: key words: streaming, data processing, real time. earth sciences research journal 186 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics moment-tensor calculation for the vega colombia mw 7.2 earthquake esteban poveda1, jiri zahradnik2, patricia pedraza3, jordi julià4 1red sismológica nacional, servicio geológico colombiano (sgc), bogotá. colombia. now at departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. 2faculty of mathematics and physics, charles university in prague, prague, czech republic. 3red sismológica nacional, servicio geológico colombiano (sgc). departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. esteban.poveda@gmail.com abstract on september 30, 2012, an earthquake of magnitude mw 7.2 occurred at the depth of ~170 km in the southeast of colombia. this seismic event is associated to the nazca plate subduction converging eastward relative the south america plate. the seismicity distribution recorded by the national seismological network of colombia (rsnc) since 1993 shows a segmented subduction zone with varying dip angles. the earthquake occurred in a seismic gap zone of intermediate depth. we estimated the moment tensor, the centroid position, and the source time function. the parameters were obtained by inverting waveforms recorded by rsnc at distances 100 km to 800 km, and modeled at 0.015-0.035 hz, using two 1d crustal models, taking advantage of the isola code (zahradnik and sokos, 2008 and 2013). the dcpercentage of the earthquake is very high (~95%). the focal mechanism is mostly normal, hence the determination of the fault plane is challenging. an attempt to determine the fault plane was made based on mutual relative position of the centroid and hypocenter (h-c method). studies in progress are devoted to searching possible complexity of the fault rupture process, quantified by multiple-point source models. we evaluated two sources in the frequency range 0.01-0.09 hz. key words: moment tensor, crustal models, centroid, multiple-point source. earth sciences research journal 187 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics comparison of manual and automatic system locations for seismic events in panama pérez, jhon leandro1, camacho, eduardo2,christensen, branden1 1osop. 2igc-upa. jleandro@osop.com.pa abstract the seismic network of panama uses manual and automatic processing routines to determine basic location and magnitude of the local seismicity. reliable and timely reports are delivered to risk management agencies and the community in general. this study aims to compare the epicentral locations determined by the manual and automatic system from august to november 2013. the software used for manual processing is seisan, from bergen university, norway. the automatic processing was done, using real-time data with seiscomp, developed by gfz/potsdam and gempa gmbh, germany. the results show the reliability of the automatic locations by comparing them to solutions from seisan. we discuss the possible factors that can influence variation in locations. these results will help to improve earthquake monitoring and provide better estimates of the impact strong of earthquakes on populated areas of panama. palabras clave: manual system, automatic system, earthworm, seisan, seiscomp, seismic events, detections and univariate statistical analysis. earth sciences research journal 188 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics physisc and statistics of the seismic source using first digit anomaly patricio a. toledo1and jaime a. campos2 1universidad de chile and universidad adolfo ibanez. 2universidad de chile patoledo@ing.uchile.cl abstract using first digit anomaly for ground base seismicity studies under chilean subduction environment, we face the problem of identification of a seismic signal, under a superpositionof events environment, through the development of an algorithm of detectionbased on a statistical pattern, linked to shannon's entropy, obtained from firstdigit anomaly present in seismograms. this detection method, exploits aninformation invariant contained in the seismic signal, which is independent ofthe size of events, at least in the range accessible to our observations. using nearfield data belonging to the 11/3 2010 pichilemu aftershock, we compared theperformance of the proposed trigger versus a generic sta/lta algorithm. ourfindings show that both algorithms are complementary, and its simultaneous useincreases the efficiency of detection. we propose this strategy of recognition,well adapted to the processing and analysis needs imposed by today's massseismic recording capacities, might also be important for early warningsystems. key words: first digit anomaly, early warning. earth sciences research journal 189 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 190 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics use of the first digit anomaly in background seismicity studies under chilean type of subduction environment patricio a. toledo1and jaime a. campos2 1universidad de chile and universidad adolfo ibanez. 2universidad de chile. patoledo@ing.uchile.cl abstract using first digit anomaly for ground base seismicity studies under chilean subduction environments we face the problem of identification of a seismic signal, under a superposition of events environment, through the development of an algorithm of detection based on a statistical pattern, linked to shannon's entropy, obtained from first digit anomaly present in seismograms. this detection method, exploits an information invariant contained in theseismic signal, which is independent of the size of events, at least in therange accessible to our observations. using near field data belonging to the11/3 2010 pichilemu aftershock, we compared the performance of the proposedtrigger versus a generic sta/lta algorithm. our findings show that bothalgorithms are complementary, and its simultaneous use increases the efficiencyof detection. we propose this strategy of recognition, well adapted to theprocessing and analysis needs imposed by today's mass seismic recordingcapacities, might also be important for early warning systems. key words: first digit anomaly, triggers. earth sciences research journal 191 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 192 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 193 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics implementación del cálculo de los de los parámetros de la fuente sísmica mediante el método de la fase w, en el sgc ruth emilse bolaños cifuentes y josé faustino blanco chía servicio geológico colombiano (sgc). rbolanos@sgc.gov.co abstract la necesidad de conocer rápidamente los parámetros de la fuente sísmica para dar información oportuna que permita a las entidades del sistema nacional de gestión del riesgo de desastres tener herramientas para declarar una alerta por tsunami, llevó al sgc a implementar el software de la fase w (rivera, kanamori, 2009). la fase w es un algoritmo de inversión de la fuente sísmica el cual usa la fase w de eventos distantes, esta fase es de periodos muy largos principalmente entre 100 y 1000 segundos, que se registra en la estación al mismo tiempo que la onda p. debido a estos periodos largos y a que es registrada entre la onda p y las ondas superficiales, la fase w puede dar una rápida y confiable información sobre los parámetros de la fuente para grandes eventos. el sistema de procesamiento automático de eventos sísmicos desarrollado por el “west coast alaska tsunami warning center (wc/atwc)” llamado earlybird, ha sido implementado en el sgc para obtener localizaciones automáticas de eventos sísmicos locales y regionales. para sismos con magnitud de momento de onda p (mwp) ≥ 6, se ha creado una rutina que hace la extracción automática de las formas de onda, envía la localización automática al programa de la fase w para el cálculo de los parámetros de la fuente sísmica, este hace la inversión y envía la solución a una lista de correos electrónicos. mediante el método de la fase w se han calculado los parámetros con estaciones del sgc e internacionales para los sismos del 5 de enero de 2014, mw 5.5; el 17 de febrero de 2014, mw 5.4 y el 9 de marzo de 2014, mw 5.6; los cuales son similares a los reportados por el cmt (global centroid-moment-tensor). mediante este método se han identificado estaciones que no se pueden usar para el cálculo de la fase w, estaciones con ruido, archivos de respuesta que se deben mejorar y la forma de adquisición de las señales. estos datos son útiles para diagnosticar el estado de las estaciones y poder de esta forma implementar correctivos para su mejoramiento. palabras claves: fase w, tsunami, earlybird. earth sciences research journal 194 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics evolución tecnologica de la red sismológica nacional de colombia rsnc y la red nacional de acelerógrafos de colombia rnacdel servicio geológico colombiano sánchez, c., acosta, my., chamorro, o. servicio geológico colombiano. csanchez@sgc.gov.co abstract la red sismológica nacional de colombia -rsnc y la red nacional de acelerógrafos de colombia – rnacadquieren, procesan y almacenan la información sismológica del país desde sus inicios en 1993. la tecnología para llevar a cabo estos procesos, ha ido evolucionando, permitiendo suplir los requerimientos por parte de las diferentes áreas temáticas y logrando una mejor calidad de los datos sísmicos para el procesamiento y apoyo a los procesos de investigación. actualmente se cuenta con software y hardware que permite estar a la vanguardia con tecnología de punta y poder suplir las necesidades de gestión y administración ofreciendo una alta disponibilidad en la difusión y entrega de los datos. este trabajo tiene como objetivo presentar y dar a conocer cómo se obtienen los datos sísmicos en la rsnc y rnac, los sistemas de adquisición, procesamiento y almacenamientos de los datos y la evolución de ti en los últimos años. este tipo de tecnología nos permite dar a conocer a la comunidad científica y estudiantil datos de calidad entregados por el servicio geológico colombiano a través de la rsnc y la rnac. palabras clave: adquisición, procesamiento, tecnologías de la información ti. earth sciences research journal 195 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics near real-time earthquake source characterization through joint geodetic and seismic approaches at the us geological survey national earthquake information center wd barnhart, gp hayes, dj wald usgs. wbarnhart@usgs.gov abstract the united states geological survey (usgs) national earthquake information center (neic) provides real-time earthquake locations and magnitudes for global events >mw4.5 and generates impact assessment and scientific information for large and potentially damaging earthquakes. these products include shakemap, pager, finite fault models, moment tensors, and regional geologic background information. currently, impact assessment products depend on models generated from regional and teleseismic waveform observations to provide information on the earthquake slip distributions. in many cases, geodetic observations of ground deformation from global positioning systems (gps), interferometric synthetic aperture radar (insar), and optical imagery pixel tracking place additional constraints on the spatial properties of an earthquake, including location, source dimensions, and depth. these constraints help to both reduce uncertainties in teleseismic waveform inversions and provide a benchmark for calibrating seismic locations. in this presentation, we review current efforts to incorporate geodetic observations into global, real-time earthquake monitoring and assessment, and highlight recent successes from the 2014 mw8.2 iquique and 2013 mw7.7 pakistan earthquakes. these two examples demonstrate the capabilities of geodetic observations to better inform models of perceived shaking and damage for offshore and complex geometry ruptures. we also address the current major setbacks to near real-time geodetic monitoring – specifically time latency in real-time operations, data coverage, and inversion methods. forthcoming insar missions will provide a vital data line for many tectonically active locations in south and middle america, but temporal resolution will be a significant limiting factor in the applicability of these observations to earthquake monitoring. for these reasons, the continued development of continuous, high-rate gps networks, like coconet, with open data access to the earthquake response community will play a vital role for future event response. keywords: real-time earthquake response, geodesy. earth sciences research journal 196 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics shlc session – seismic hazard in latin america and the caribbean: working across border conveners: antonio gomez (ingv milano italia) <gomez@mi.ingv.it> carlo meletti (ingv pisa italia) <meletti@pi.ingv.it> marlon pirchiner (usp brazil) <marlon@iag.usp.br> herbert rendon (funvisis venezuela) <hrendon@funvisis.gob.ve> earth sciences research journal 197 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics hacia un catálogo actualizado de terremotos de sur américa: ventana temporal pre-1930 a.a. gómez capera1, m. arcila2, m. bufaliza3, j. choy4, e. minaya5, m. pirchiner6, h. rendón7, l. rodriguez8, a. sarabia2, h. tavera9, h. yepes10, m. stucchi11 1istituto nazionale di geofisica e vulcanologia, sezione di milano, italia. 2servicio geológico colombiano, bogotá, colombia. 3instituto nacional de prevención sismica, san juan, argentina. 4universidad de los andes, facultad de ciencias, laboratorio de geofísica, mérida-venezuela. 5observatorio de san calixto, la paz, bolivia. 6universidade de são paulo, brasil. 7funvisis, caracas, venezuela. 8ceresis, lima, perú. 9instituto geofísico del perú, lima, perú. 10instituto geofísico de la escuela politécnica nacional, quito, ecuador. 11eucentre, pavía, italia. antonio.gomez@ingv.it abstract en el marco del proyecto "sara" (the seismic risk in south america), el tópico 4 está compilando un catálogo de terremotos para américa del sur, incorporando los datos disponibles del ceresis (1985) que se realizó bajo el proyecto sisra (mitigación de los terremotos en la región andina), los más recientes estudios nacionales e internacionales y los análisis realizados en el transcurso del proyecto. en particular serán considerados las últimas versiones del catálogo del ceresis-91 elaborado para el instituto panamericano de geografía e historia (ipgh), el cual posteriormente fue empleado por el mismo ceresis en su vinculación al programa mundial de evaluación de riesgo sísmico (gshap); determinación de parámetros por estudios recientes, incluyendo aquellos propuestos por el catálogo iscgem, y cuando esté disponible, los catálogos nacionales que cumplen con los criterios de transparencia exigidos por el proyecto. las actividades se han iniciado a partir de la ventana temporal pre-1930. siguiendo la metodología descrita en el programa de investigación y discutido durante la reunión inicial del proyecto (bogotá, 20-22 de noviembre del 2013), el primer objetivo ha sido la elaboración de un inventario crítico de todos los estudios públicos relativos a los eventos de sur américa. los estudios relativos al mismo evento han sido asociados entre ellos por medio de un procedimiento semi-manual a partir de la comparación del tiempo de origen, de las coordenadas del epicentro y del tamaño del terremoto. para cada evento ha sido seleccionado, de manera preliminar, un conjunto de parámetros considerados confiables. el problema principal del catálogo consiste en la necesidad de expresar los valores de magnitud en términos de mw. actualmente, pocos estudios relativos a pocos terremotos proveen dicho valor (choy et al., 2010; beauval et al., 2013). para muchos eventos están disponibles valores en términos de ms o mb, a pesar de que para la mayoría de los casos, en la ventana temporal considerada, estos valores de magnitud son calculados a partir de datos macrosísmicos. para estos terremotos se está investigando la utilización de relaciones de conversión (mw/ms y mw/mb) más comunes a nivel internacional (scordilis, 2006; storchak et al., earth sciences research journal 198 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics 2012), y posiblemente alguna relación determinada localmente. asimismo están los eventos para los cuales existe un valor de imax o io. para estos eventos, a partir de la experiencia que se ha logrado en europa (stucchi et al, 2012; grünthal et al., 2009), se está buscando determinar relaciones mw/io de carácter regional, utilizando los datos más confiables y recientes en términos de mw e io. en una segunda fase, para algunos eventos que tienen un número suficiente de datos macrosísmicos, se está tratando de determinar los parámetros de terremotos utilizando el método de bakun y wentworth (1997) calibrado regionalmente, sobre la base de lo que ya se ha llevado a cabo por beauval et al (2010), palme et al. (2005) y otros. palabrasclave: terremotos, catálogo, sur américa. earth sciences research journal 199 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics parámetros del terremoto de cúcuta del 1875 a partir de intensidades macrosísmicas a.a. gómez capera1, h. cifuentes2, a. sarabia2, d. bindi3, j. choy4, r. galan5, m. pirchiner6, l.m. rodríguez7, e. salcedo hurtado8, a. sánchez5 1istituto nazionale di geofisica e vulcanologia, sezione di milano, milan, italy. 2servicio geológico colombiano, bogotá, colombia. 3deutches geoforschungszentrum gfz, center for early warning, potsdam, germany. 4universidad de los andes, facultad de ciencias, laboratorio de geofisica, mérida-venezuela. 5departamento de matemáticas, universidad nacional de colombia, bogotá, colombia. 6universidade de são paulo, brasil. 7funvisis, caracas, venezuela. 8observatorio sismológico y geofísico de la universidad del valle, cali, colombia. antonio.gomez@ingv.it abstract san josé de cúcuta capital de la entonces provincia de santander (colombia) fue muy destruida por el terremoto del 18 de mayo de 1875. el sismo ocasionó fuertes daños en la zona limítrofe colombo-venezolana que corresponde a hoy al departamento del norte de santander en colombia y el estado del táchira en venezuela. el evento del 1875 ha sido objeto de muchos estudios en literatura. uno de los más recientes estudios de tipo histórico y macrosísmico es propuesto por el servicio geológico colombiano y universidad nacional de colombia (2012) el cual asigna 41 puntos de intensidad en escalas ems98 y mm. la intensidad máxima es clasificada con valor 10 en las localidades de villa del rosario, san josé de cúcuta y san antonio del táchira. a partir de dicho estudio, el presente trabajo modela la localización y la magnitud del sismo del 1875 aplicando el método de bakun and wentworth (1997) y la relación de atenuación de intensidad macrosísmica calibrada en magnitud momento mw propuesta por palme et al. (2005) para los andes de mérida. de manera preliminar, la magnitud obtenida es mwi=7.1±0.2 con centro de intensidades entre villa del rosario y san antonio del táchira. modelación de bootstrap están siendo realizada para analizar la confianza de la localización y grandeza del evento. palabras claves: terremoto histórico, intensidad macrosísmica, parámetros del terremoto. earth sciences research journal 200 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismotectonic and seismic hazard implications for south africa b. manzunzu and v. midzi council for geoscience, south africa. bmanzunzu@geoscience.org.za abstract though south africa is considered to lie in a stable continental region, earthquakes are recorded and located daily. earthquakes have been recorded that caused severe damage to infrastructure in nearby towns, farms, underground mines and even deaths in some circumstances. these events have serious hazard implications especially to cities within south africa were building and infrastructure are built with little emphasis on seismic hazards. the 1969 ceres earthquake of magnitude 6.2, resulted in the deaths of 12 children as well as damage worth $20 million in the ceres/tulbagh area near cape town in south africa. therefore, it is necessary that we consider the effects of these events in the design of our infrastructure. the seismotectonic map for south africa will assist in delineating seismic hot spots in order to carry out a proper seismic hazard assessment. the map will enable us to conduct reliable seismic hazard and risk studies for our region using state of the art methodologies. in preparing the map, fault plane solutions were obtained from publications, reports and international organizations such as the isc, usgs etc. though few such data are available for southern africa, all collected information is vital in the effort to understand the tectonics of the region and was used in the modeling of stress orientations of the region. through these efforts, a draft of the seismotectonic map of south africa has now been prepared. investigations are also being carried out to study the possible impact of the collected information on the seismic hazard of southern africa. previous regional efforts such as gshap used large area source zones without considering the influence of active faults in the assessments. thus the information in the seismotectonic map will contribute to the preparation of more accurate hazard assessments. included in this presentation are results of preliminary hazard assessments for southern africa that show the impact of including faults in the source model. we continue to collect and improve collection methods of historical and instrumental seismicity data, as well as geological information to improve the seismotectonic model for our region. keywords: seismotectonic, seismic hazard, south africa. earth sciences research journal 201 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics psha at a regional scale: european share project versus italian reference psha. and the winner is… carlo meletti istituto nazionale di geofisica e vulcanologia. carlo.meletti@pi.ingv.it abstract seismic hazard does not know political border, because faults and earthquakes do not need passport for crossing customs. for this reason, when evaluating seismic hazard at national scale it’s necessary to extend the investigation to a wider area, for taking into account possible seismic sources abroad that could contribute to the hazard. the european share project (http://www.shareeu.org) had the aim of release a homogenized seismic hazard model, where homogenized mainly means a uniform level of knowledge and accuracy across the whole investigated area, by involving researchers from all countries. many researchers were involved also at national scale for assessing seismic hazard that is adopted by building code (swiss and italy among others). the question that arises is about the comparison between the two models: are the european and the italian models consistent each other for the overlapping areas? are the differences in the models within the aleatory uncertainty or not? it is necessary to update of seismic code in italy after the release of the share model? looking only at pga with 10% of probability of exceedance (considered the standard), share model shows increased expected values with respect to the italian reference model (stucchi et al., 2011), up to 70% more than what adopted in national building code. looking in detail at all parameters of both the models, it is possible to observe a different behaviour for other spectral accelerations and for several probability of exceedance. for spectral periods greater than 0.3 seconds the existing psha for italy proposes values higher than the share model for many and large areas. the observation of a non-generalized increase of expected values suggests that this behavior is not linked to a different definition of seismic sources and relevant seismicity rates, but this is the result of the adoption of recent gmpes that propose higher values for pga and spectral periods less than 0.3 seconds and lower values for periods greater than 0.3 seconds. many tests have been performed and they seem to confirm the large impact of the new generation ground motion predictive equations on the seismic hazard we obtain today. the result of this analysis appears to be consistent with the observation by bommer and abrahamson (2006). even if the share model was not released for replacing the existing national hazard maps, the impact of the new seismic hazard model if adopted in the italian building codes will be also discussed. keywords: seismic hazard, building code, europe. earth sciences research journal 202 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics respuesta dinámica de suelos para managua/nicaragua carlos guzmán ineter, managua, nicaragua. carlos.guzman@gf.ineter.gob.ni abstract en dos distritos de managua, capital de nicaragua, se realizaron 1045 puntos de medición de la amplificación del suelo basados en el método de nakamura, las mediciones del ruido cultural para cada punto de medición se efectuaron utilizando un acelerógrafo de tres componentes de la marca etna/kinemetrics. se realizó el proceso de análisis de los datos y se obtuvieron resultados en forma de mapas zonificados. el resultado final son mapas de iso-amplificaciones del suelo. para ambas zonas estudiadas se muestran mapas en la figura. las amplificaciones se muestran en los mapas con zonas en colores verde, amarillo y color naranja. las zonas con color verde amplifican hasta el factor 1.5, las amarillas entre 1.5 y 2 y las zonas en color naranja mayor que 2. estos resultados pueden ser utilizados para actualizar el código de construcción de las zonas específicas. el trabajo fue llevado a cabo, en 2013, con el auspicio del proyecto dipecho viii y cruz roja. palabras clave: respuesta dinámica, suelo, managua, nicaragua earth sciences research journal 203 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics challenges to developing a unified earthquake catalog for seismic hazard assessment in south america – addressing magnitude homogenization and depth uncertainties claire pontbriand, mehrdad mahdyiar, wenzheng yang, feng wang, bingming shen-tu, gerald galgana, elliot klein, khosrow shabestari air worldwide, boston, ma, usa. cpontbriand@air-worldwide.com abstract constructing a unified earthquake catalog is an important step in developing regional seismic hazard models in south america with a broad seismotectonic environment that includes very active subduction zones and fault systems as well as stable regions. several earthquake catalogs (international: gem, usgs neic, gcmt, isc, etc.; and regional: ceresis, igepn, rsnc) provide records of seismicity, characterized by different magnitude types and uncertainties. we evaluate the completeness and quality of each catalog for the purpose of merging multiple data sources into a high-quality comprehensive historical catalog. challenges include homogenizing magnitudes to a common magnitude scale and addressing uncertainties in hypocenter location and common default values reported for focal depth. here, we present the sensitivities of magnitude-rate distributions (i.e. recurrence periods) at the level of seismic source zones to these procedures and their underlying assumptions. one challenge in developing a comprehensive catalog is the homogenization of magnitudes to a single magnitude scale, in this case, the moment magnitude scale, mw. reporting agencies provide magnitude data on many different scales (mw, mb, ms, ml, md, etc.). many authors have published global relations connecting magnitude scales (e.g., scordilis et al., 2006, gem homogenization equations). however, scaling equations are varied and many employ simple linear regressions, which have a potential to induce systematic errors in magnitude conversion, introducing apparent catalog incompleteness and bias to magnitude-frequency distributions. we use the isc bulletin, which provides a compilation of magnitudes from different reporting agencies for each earthquake, to develop general orthogonal regressions (gor) for scaling to mw. we compare gor models for south america data for each catalog (reporting agency) at seismic source zone levels to global equations, applying both to assess the impact of these approaches on magnitude-rate distributions. in building a unified catalog, attention must be given to uncertainties in the focal depth in order generate a realistic view of potential ground motions. catalogs often provide default depth solutions when hypocenter depth cannot be well-determined. in order to constrain the realistic depth distribution of seismicity in a source zone, we present a magnitude and locationdependent bootstrap methodology for resampling default depths using available, welldetermined depth data. we show the impact of this procedure on recurrence rates for shallow and deep seismicity. keywords: magnitude homogenization, unified catalog, recurrence. earth sciences research journal 204 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic activity in the pantanal basin brazil edna maria facincani1, marcelo s. assumption2, fábio luiz dias2, tiago gomes da silva3, gustavo marques amorim3 1campus aquidauana-cpaq/ufms/brasil. 2institute of astronomy and geophysics, iag usp / usp. 3institute of física-infi/ufms/brasil. edna_facincani@hotmail.com abstract the pantanal is a sedimentary basin of quaternary age, tectonically active, positioned in the hidrografhic basin of altoparaguai, center-west region of brazil. the basin of the pantanal is a region sismogenic intraplate environment. the two largest earthquakes recorded were the miranda 1964 and cushion 2009, with magnitudes 5.4 and 4.8 respectively and depth around 5km, both results transpressive / compressive stress. despite being a region of subsidence, the neotectonic stresses in the upper crust are compression. the federal university of mato grosso do sul (ufms), through aquidauna campus (cpaq), is participating in the integrated brazil seismograph network (brasis) at the university of são paulo (usp) and the project " lineament transbrasilian: origin, evolution and influence on phanerozoic sedimentation basins " at the university of brasilia (unb), aiming to understand the seismic activities in the pantanal region and relate the structure of the lithosphere with the record of earthquakes. currently the state of mato grosso do sul, has three seismographic stations: aquidauna (aqdb), plain of the south (c2sb) and sonora (pp1b). aiming at monitoring sites originated in the centerwest region of brazil telessismos mainly the andean mountain range, and regional earthquakes, in addition to training and qualifying skilled labor in geodynamics and seismicity of the lithosphere. it is expected that more detailed studies of seismicity in the pantanal with more stations, as aqdb, pp1b and c2sb, allow delineating the regime of active efforts in the basin and its surroundings. seismic / subsurface geologic informationon the basin of the pantanal are far from satisfactory. many questions still remain on its origin and evolution, highlighting their structural framework, neotectonics and the role of lineament transbrasilian since the epicenters of earthquakes in the pantanal are aligned with the epicenters of the region goiás, suggesting a relationship with lineament transbrasilian (zone shear) of ne-sw direction. key words: pantanal basin, seismic activity, brazil. earth sciences research journal 205 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics microzonificación para diseño por sismos para la ciudad de panamá, panamá javier francisco lermo samaniego1, eduardo camacho astigarrabia2, josé antonio martínez gonzález1, elvia carbajal becerril1, néstor luque2, arkin a. tapia2, eric chichaco2, johnny gil2, yadira echeverría durán2, roberto palma q.3 1coordinación de ingeniería sismológica, instituto de ingeniería-unam; cd. universitaria, apdo. postal 70472, coyoacán 04510 méxico, d.f. e-mail: jles@pumas.iingen.unam.mx 2instituto de geociencias, universidad de panamá. 3geoconsult s.a., panamá, panamá. eduardo.camachoa@up.ac.pa abstract empleando sismógrafos de banda ancha se hicieron mediciones de ruido en 239 sitios de la ciudad de panamá por espacio de al menos 30 minutos cada una. para seleccionar las zonas de medición se empleó información geotécnica y registros de aceleración realizados en estudios de respuesta de sitio anteriores. fundamentados en el mapa geotécnico desarrollado por palma (2007), se desarrolló un nuevo mapa geotécnico para la ciudad de panamá que toma en cuenta las zonas de relleno artificial, del análisis de los datos también se desarrolló un mapa de periodos dominantes con rangos de la amplificación relativa para la ciudad de panamá, el cual valida las zonas con mayor peligrosidad sísmica, ya que nos muestra espacialmente el efecto de sitio de los 239 puntos analizados. a partir de los espectros obtenidos se crearon familias espectrales proponiéndose la clasificación los suelos de la ciudad de panamá en cuatro zonas que son las siguientes: zona firme, o zona estable para cualquier tipo de infraestructura. corresponde a la zona de colinas y lomas bajas donde las fte carecen de amplificación relativa. zona alta, zona inestable para viviendas entre 1 a 3 pisos y con curvas de isoperiodos entre 0.1 a 0.2 segundos. zona transición, zona inestable para viviendas entre 3 a 4 pisos y con curvas de isoperiodos entre 0.2 a 0.3 segundos. zona baja, zona inestable para viviendas entre 4 a 6 pisos y con curvas de isoperiodos entre 0.4 a 0.5 segundos. utilizando el nuevo mapa geotécnico de la ciudad de panamá y las microzonas, se presenta la primera propuesta de microzonificación para diseño por sismos para la ciudad de panamá. palabrasclave: amplificación del suelo, microzonación sísmica, respuesta de sitio, mapa geotécnico, isoperiodos. earth sciences research journal 206 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics teleseismic waves registered at the seismographic station of aquidauana / ms /brazil (aqdb), in 2013. estêvão vasconcello campos tadeu1, marcelo s. assumpção2, tiago gomes da silva3. fábio luiz dias2, edna maria facincani4, gustavo marques e amorim1, ana paula trindade souza4, emilly lopes de oliveira4 1physics institute infi/ufms/brasil. 2astronomy and geophysics institute uspiag/usp. 3physics institute -infi/ufms/brasil. 4aquidauana campus-cpaq/ufms/brasil. estevao-v-tadeu@hotmail.com abstract this project aims to monitorate seismic waves generated from more than 1500km, meaning that those were teleseisms and to achieve this, were gathered data from the seismographic station of aquidauana, mato grosso do sul state, brazil (aqdb). there were gathered and analyzed data information from january 1st to december 31st 2013, from which the seismic activity was generated in the central and south americas. in south america, the andean region can be considered as the main source of teleseisms, mainly originated from chile. the identification, interpretation and analysis of the events registered by aqbd station were made using the software seisgram, sac, taup and origin. into the seisgram were gathered and read daily data, that allowed the identification of the shock waves, used in post calculations inside sac software. the arrival times of the p and s waves were calculated into the taup software. the completion of these steps resulted in the determination of the p and s waves. these magnitudes were correlated within the results shown into the neic catalog using the origin software. the obtained results will support the pantanal and the brasis projects, whereas the federal university of mato grosso do sul (ufms), through its geosciences department is participating in the integrated seismographic network of brazil (brasis project). the combining efforts with the astronomy, geophysics and atmospherical sciences institute from são paulo university (usp) are made with the purpose to comprehend better the relation between the seismicity activities inside pantanal region (border of parana sedimentary basin) and the lithosphere structuration using the seismic data registration acquired with the aqdb station. key words: seismicity, seismographic station and pantanal. earth sciences research journal 207 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics quantifying uncertainties in site amplification caused by using low resolution soil maps for site characterization feng wang, mehrdad mahdyiar, bingming shen-tu, khosrow shabestari, gerald galgana, elliot klein, claire pontbriand, and wenzheng yang air worldwide. fwang@air-worldwide.com abstract it is well established that shallow soil conditions play important role in shaping earthquake ground motions. many large scale regional hazard and risk analysis use surface geology maps to characterize shallow soil conditions to infer vs30 average shear wave velocity for site amplification analysis. the quality of spatial distribution of v30 data and thus the hazard maps depends upon the resolution of the soil maps used for site characterization. using high or low resolution soil maps could impact large scale regional hazard and loss analysis. it is the objectives of this study to examine the impact of using low resolution soil maps for site characterization on regional hazard and risk analysis and quantify the corresponding site amplification uncertainty. we use cities of santiago, chile and bogota, colombia as testing regions for this purpose. first, we systematically processed the low and high resolution geological and microzonation vector maps to obtain comprehensive soil maps with 1:1,000,000 and 1:100,000 resolutions for santiago and 1:500,000 and 1:40,000 resolutions for bogota. with those low and high resolution maps of two regions, we then conduct a comparison on regional ground motions at grid level due to different characterizations of the site amplification, and we quantify the uncertainties in ground motion predictions due to uncertainties in soil maps by computing mean and standard deviation of site amplification from high-resolution soil maps. finally, we use our regional seismicity model to quantify the uncertainties in hazard curves at specific grid locations covered by those soil maps, and develop a model to obtain the hazard estimates for high-resolution soil maps by approximately modifying the hazard estimates for low-resolution soil maps following the idea introduce by bazzurro and cornell (2004). keywords: site amplification. earth sciences research journal 208 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a stochastic rupture probability model for earthquakes on subduction zones: a case study from the nazca subduction zone mehrdad mahdyiar, gerald galgana, bingming shen-tu, elliot klein, claire pontbriand, feng wang, khosrow shabestari, and wenzheng yang air worldwide, 131 dartmouth st., boston, ma 02116, usa. ggalgana@air-worldwide.com abstract most subduction zones rupture in complex patterns that create earthquakes of different magnitudes with overlapping rupture areas. this makes it difficult to identify a single characteristic earthquake for time dependent rupture probability analysis (tdrpa). for many subduction zones, limited historic earthquake data on large magnitude earthquakes do not provide sufficient information to estimate reliable mean recurrence intervals and rupture probabilities for such earthquakes. to improve the situation there is a need to complement the tdrpa with auxiliary data. physical models based on geodetic data have been successfully used to obtain information on the state of coupling and slip deficit rates for subduction zones. coupling information provides valuable insight into the complexity of subduction zone rupture processes, as demonstrated by the excellent correlation between the coseismic slip distribution from the 2010 m 8.8 maule, chile earthquake and the corresponding results from a gps-based physical model published before the earthquake (moreno et al., 2010). in this study we present a tdrpa model that is formulated based on slip deficit rate distribution over subduction zones. in this model, a subduction zone is represented by an integrated network of cells. each cell ruptures multiple times from numerous earthquakes that have overlapping rupture areas. the rate of rupture for each cell is calculated using a moment balancing concept that uses information from historic data and slip deficit rates. the model enables us to estimate the expected mean recurrence intervals for all cells forming the subduction zone. this information in conjunction with past earthquake rupture data is used to estimate time dependent rupture probabilities for cells. rupture probabilities for earthquakes are formulated by integrating different combinations of cells that form rupture areas of different magnitude earthquakes at different locations on the subduction zone. the resulting rupture probability estimates are fully consistent with the state of coupling of the subduction zone and the regional and local earthquake history as the model takes into account the impact of all large (m>7.5) earthquakes on the subduction zone on tdrpa. the granular rupture model as formulated in this study allows estimating rupture probabilities for large earthquakes other than just a single characteristic magnitude earthquake. this provides a general framework for formulating physically-based rupture probability models for large earthquakes on subduction zones that is consistent with their true locking state and earthquake history. we will present the formulation of the proposed tdrpa model, its application to the entire nazca plate subduction zone and the model sensitivity to some common initial assumptions critical to tdrpa. keywords: seismic hazards, earthquake rupture, stochastic models, subduction zones, interplate coupling, kinematic models. earth sciences research journal 209 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics la sismicidad en el piedemonte llanero y la amenaza sísmica de la sabana de bogotá chicangana, g.1, 2, vargas, c. a.2 caneva, a.3, salcedo, e.4 y gómez, a.5 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. german.chicangana@unimeta.edu.co 2grupo geofísica, departamento de geociencias, universidad nacional de colombia, bogotá d.c., colombia. cavargasj@unal.edu.co 3dirección nacional de investigaciones, universidad antonio nariño, bogotá d.c., colombia. alexander.caneva@uan.edu.co 4observatorio sismológico del suroccidente, universidad del valle, cali, colombia. elsalced@univalle.edu.co 5istituto nazionale di geofísica e vulcanologia, milano, italy. antonio.gomez@mi.ingv.it abstract el piedemonte llanero colombiano está configurado geomorfológicamente por el sistema de fallas de la falla frontal de la cordillera oriental (sfffco). el sfffco es el límite entre el bloque andino y la placa suramericana, y este límite es una zona de alta movilidad tectónica debido al empuje que la placa de nazca al subducir bajo suramérica produce en este sector del continente. la actividad tectónica del sfffco ha producido sismos con mw ≥ 7.0 como el del 9 de febrero de 1967. por la cercanía a la sabana de bogotá, un sismo con m > 7.0 daría lugar a una muy fuerte aceleración de sus suelos debido a que estos se originaron a partir de un lago interandino. los depósitos lagunares de la sabana de bogotá se constituyen de sedimentos finos con alta saturación de agua. las grandes aceleraciones en estos suelos afectarían por esta razón a aquellas construcciones y estructuras que no cumplan con los parámetros exigidos por el código colombiano de construcción sismorresistente (nsr -10). en este trabajo a partir de una revisión de los sismos históricos de 1743, 1785, 1845, 1917, 1967, 1995 y 2008, los cuales han sido asignados a las fallas del sfffco, se muestra como este sistema de fallas contribuye a que la amenaza sísmica de la sabana de bogotá sea categorizada como alta y cómo igualmente un sismo con profundidad focal menor a 30 km y m > 7.0, generado en este sistema de fallas y que se encuentre a una distancia inferior a 250 km, producirá un escenario de desastre para el área metropolitana de bogotá. palabrasclave: fallas del piedemonte llanero, sabana de bogotá, amenaza sísmica. earth sciences research journal 210 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics towards the construction of a seismic catalogue suitable for psha studies in south america: instrumental contribution m. arcila2, l. alvarado1, j. eraso2, a. a. gómez capera5, e. minaya6, m. pirchiner4, h. rendón1, l. rodríguez 7, d. storchack8, m. tovar2, h. yepes3 1funvisis, caracas. 2servicio geológico colombiano, bogotá. 3instituto geofísico de la escuela politécnica nacional, quito. 4universidade de são paulo, são paulo. 5istituto nazionale di geofisica e vulcanologia, sezione di milano. 6observatorio de san calixto, la paz. 7ceresis, lima. 8isc, uk. hrendon@funvisis.gob.ve abstract a comprehensive raw seismic catalogue, the database where it is kept the record of earthquake occurrence in terms of its location, origin time and magnitude, for a given study area and time span, is the product of the effort of various generations of specialists, including seismologists, geologists, historians and analysts. the associated ample set of strategies, criteria and methods to capture the data values for the whole set of events, necessarily imprints to the raw seismic catalogue an inhomogeneous characteristic. on the other hand, an adequate homogeneous seismic catalogue constitutes a need and a key factor to accomplish a robust assessment of the seismic hazard associated to the given region. despite the great every day work at regional and national levels, and the high valuable ceresis’s work at continental scale, a south american catalogue with reliable mw magnitude values does not exist as an open dataset available to seismic hazard studies. although, three known exceptions need to be mentioned. one is the global cmt catalogue witch is an open and reliable dataset but covering 1976-present time period just for high values of magnitudes. another is the recent iscgem global instrumental earthquake catalogue (1900-2009), which represents a huge earthquake parameter recalculation. the last one should be the global earthquake history catalogue, which worked hard on global literature compilation of historic (1000-1903) earthquakes parameters and macroseismic datasets. in the recent past, the gem foundation in association with the isc, and working with available global data sets, established an impressive standard with the construction of a homogeneous instrumental seismic catalogue, adequate to be applied to consistent, transparent and verifiable probabilistic seismic hazard assessment (psha) studies; in particular, within this effort, it was reached a complete catalogue down to mw=5.5 for the time period 1960-2009 for the sa region. more recently, it is taking place a regional effort, also supported by gem, that aims to achieve a harmonized seismic catalogue that will attempt to achieve completeness down to magnitude values equal to mw=4.5, within a time period to be established, and with the integration of existing national instrumental seismic catalogues. for this work, we plan to bring to the surface possible differences, if any, between the global catalogues and the national seismic catalogues earth sciences research journal 211 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics provided by the sgc of colombia, the funvisis from venezuela, the igepn of ecuador, the usp of brazil. we plan to present an overall comparison, for the most recent period where national seismic networks have proven to be determinant in solving earthquakes parameters for seismic events within the countries and in the boundary areas, between the parametric solutions offered by the various catalogues. an attempt will be given to account for significant differences that may show up between the solutions, and an effort will be made to reconcile key words: seismic catalogue, seismic hazard, south america. earth sciences research journal 212 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics probabilistic seismic hazard assessment in quito city, estimates and uncertainties céline beauval1, hugo yepes2,1, alexandra alvarado2, laurence audin1, jean-mathieu nocquet3, damiano monelli4, laurentiu danciu5 1isterre, ird-cnrs-ujf-osug, grenoble, france. 2instituto geofisico, quito, ecuador. 3geoazur, cnrs-ird-uns-oca, sophia antipolis, france. 4gem hazard team, gem foundation, pavia, italy. 5sed-ethz, zurich, switzerland. hyepes@igepn.edu.ec abstract this study estimates the probabilistic seismic hazard for quito, a large city built up on a piggy-back basin, on the hanging wall of a five segments, active reverse fault system. five centuries historical records show that the city experienced intensities in the range vii to viii, with damages reported in churches and houses. in an attempt to extend the observation time window, hibsch et al. (1997) analyzed earthquake-induced deformation phenomena in lacustrine sediments in northern quito basin; one major (intensity x) event was identified between the 10th-16th centuries, when the entire fault length could have ruptured with magnitudes 6.5-7.0. more recently, gps measurements at sites with 10-15 years of recordings, revealed east-west horizontal shortening with rates in the range 4.3-5.3mm/yr across this blind thrust. probabilistic seismic hazard estimates at 475 years return period were calculated for quito using the available crustal and subduction seismic source zones with results showing that hazard levels in quito almost entirely proceed from the contribution of the crustal host zone. therefore, emphasis is put on identifying the uncertainties characterizing the host zone, i.e. uncertainties on the recurrence of earthquakes expected in the zone and uncertainties on the ground motions that these earthquakes may produce. globally accepted gmpes are used. rather than sampling a complex logic tree, several plausible models are considered and associated to corresponding uniform hazard spectra. significant variability in the estimated acceleration at 475 years resulted from this exercise. when frequency-magnitude distributions obtained from the country catalog were used, both uncertainties on the gmpe choice and on the seismicity model were responsible for pga’s variability from 0.28-0.55g. when gps inferred slip rates were used assuming that most of the deformation occurs seismically, pga was between 0.43-0.73g. if only 50% of the deformation were to be released by earthquakes, the pga range is from 0.32-0.58g. if we restricted the occurrence of magnitudes 6-7 to the quito fault (a simplified geometry), and applying gmpes including a hanging-wall coefficient (abrahamson and silva 2008, chiou and youngs 2008), the hazard increases up to 40% at sites located above the fault plane. based on the results we conclude that 0.3g should be considered as a strict minimum bound for pga at 475 years return period for quito for rock sites. if modeling the recurrence based on the earthquake catalog, as in most probabilistic seismic hazard studies in the world, the mean value obtained is around 0.4g, which we propose should be considered as the reference value in quito for rock in that return period. further considerations should be done for site amplification. we underlie the large uncertainty on the obtained hazard values and stress that the earth sciences research journal 213 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic hazard in quito must be considered of the utmost importance for the future of the city. keywords: pga, uncertainties, hanging wall, quito. earth sciences research journal 214 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics average shear wave velocity down to 30m (vs30) zoning for the city of pasto, colombia jaime eraso, martha tovar, mónica arcila servicio geológico colombiano, bogotá, colombia. jeraso@sgc.gov.co abstract evaluation of the seismic site effects constitutes a key piece of information fundamental for local seismic hazard assessment, for structural design and for urban planning. one of the most used parameters for soil classification and the analysis of site effects is the average shear wave velocity in the upper 30m (vs30). nevertheless, most techniques oriented to its estimation have high costs, requiring long periods of time and logistical and practical conditions that restrict their application. in this work, the refraction of microtremors (remi) technique (louie, 2001) is implemented (which combine an acceptable level of accuracy and confidence with low costs of acquisition and easy implementation) developing a vs30 zonation (scale 1:10.000) for the city of pasto (located at southern colombia). 157 measurements of microtremors were performed within the urban perimeter of the city. estimation of shear wave velocity profiles, sedimentary deposits thickness and seismic amplification factors also have been carried out classifying most of the city as soils with high amplification potential. within the framework of this project, strong motion stations were installed in different zones of the city which recorded an earthquake occurred on february 9 of 2013 (guaitarilla earthquake), mw=7.0 at 162.0 km depth located at 33 km from pasto where it was widely felt. this information was analyzed using methods such as spectral standard ratio which combined with studies of historical seismicity it was possible to observe that actual amplifications matches with the high potential amplification zones predicted in this work showing a coherence and validity of the obtained results. key words: shear wave velocity, vs30, ground motion amplification, seismic hazard assessment. mailto:jeraso@sgc.gov.co earth sciences research journal 215 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics simulación 3d de la propagación de ondas sísmicas en el valle de caracas usando diferencias finitas jaime parada1, otilio rojas1, michael schmitz2 1universidad central de venezuela. 2funvisis. jaime.parada@ciens.ucv.ve abstract en este trabajo, se presentan resultados preliminares de simulaciones 3d de la propagación de ondas elásticas en el valle de caracas utilizando un método de diferencias finitas escalonadas de cuarto orden en espacio y segundo orden en tiempo. el valle de caracas es modelado por un cubo con dimensiones de 50 por 50 kms^2 con 12.5 kms de profundidad, con dos capas sedimentarias sobre el lecho rocoso. para este modelo de velocidades de onda, se emplea un tamaño de paso de 25 metros el cual permite modelar una respuesta sísmica suficientemente precisa de hasta 4hz. el sismo de la guaira del año 2009 de mw 4.3 grados es utilizado como sismo escenario de validación para estas simulaciones donde la técnica de los cocientes espectrales estándar permite la comparación de los registros reales con los sintéticos. palabrasclave: simulación 3d, propagación de ondas, valle de caracas. earth sciences research journal 216 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics results from the 2013 pan-american advanced studies institute in santo domingo, dominican republic: joint modeling of complementary data functionals for seismic site characterization pulliman, j.1, schwed, m.1, sen, m. k.2, willemann, r. j.3, huerta-lopez, c.4, moschetti, m. p.5, schmitz, m.6, louie, j. n.7, polanco, e.8, huerfano moreno, v.9, pasyanos, m.10 1baylor university, usa. 2university of texas at austin, usa. 3iris consortium, usa. 4university of puerto rico, mayaguez, usa. 5u.s. geological survey, usa. 6funvisis, venezuela. 7university of nevada, reno, usa. 8universidad autonoma de santo domingo, santo domingo, dominican republic. 9university of puerto rico, mayaguez. 10lawrence livermore national laboratory, livermore, ca, usa. jay_pulliam@baylor.edu abstract in july 2013 the u.s. national science foundation funded a “pan-american advanced studies institute” (pasi) in santo domingo, dominican republic, entitled “new frontiers in geophysical research: bringing new tools and techniques to bear on earthquake hazard analysis and mitigation.” the goals of the workshop were to explore new approaches to “seismic site characterization” that suggest it may be possible to determine shallow earth structure through low-cost, non-invasive seismic surveys that make use of ambient noise and to teach these methods to early career scientists and engineers from around the americas. the pasi included data collection and analysis as a proof-of-concept survey in a highly built-up urban environment. in this talk we will present a strategy for determining seismic “site characterization” through joint modeling of horizontal to vertical spectral ratios (hvsr) and surface wave dispersion, determined via spatial autocorrelation (spac), refraction microtremor (remi), and/or multi-channel analysis of surface waves (masw). fitting of data functionals by synthetics is driven by global optimization and the models are assessed quantitatively. the products of this approach are shear wave velocity profiles for the shallow subsurface, accompanied by posterior probability distributions and parameter correlation matrices. optimization strategies for solving nonlinear problems in geophysics have several advantages over linearized inversions. jointly fitting dispersion curves and hvsr functionals via global optimization allows us to characterize the space of possible models, assess model reliability, identify parts of the “best-fit” model that are poorly constrained, and guide us toward new data that might improve constraints on the model. tools such as the posterior probability distribution and the parameter correlation matrix allow us to assess the relative contribution of both types of data to model constraints and how to choose the optimal weights between data types. the joint modeling technique is applied to data acquired in the 2013 santo domingo pasi. earth sciences research journal 217 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics keywords: seismic site characterization, h/v spectral ratios, surface wave dispersion, global optimization, uncertainty assessment. earth sciences research journal 218 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics caracterización del subsuelo de la zona lacustre de bogotá, usando técnicas de análisis de microtremores john jairo mora soto y maría cristina dimate c. universidad nacional de colombia, departamento de geociencias. jjmoras@unal.edu.co abstract seismic hazard in bogotá is strongly associated with the response of the soil on which it is built. ambient noise techniques have been used to characterize the subsoil as a first step for the estimation of the response to strong seismic excitations. this work uses these techniques for the characterization of the structure of the subsoil in a lacustrine zone of bogotá. the “single instrument" technique known as h/v, the spectral ratio between the horizontal component and the vertical component of the ambient noise, is used to obtain the approximate subsoil transfer function. we use also the spatial autocorrelation technique (spac), an "instrumental array" technique, to obtain the shear-wave velocity profile at the site. this study evaluates the contribution of these techniques to obtain the subsoil velocity profile by comparing the results with information from a nearby geotechnical wellbore. the data collection was conducted on the campus of the universidad nacional de colombia at bogota using broadband seismometers. palabrasclave: seismic hazard, ambient noise, h/v, spac. earth sciences research journal 219 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics tectónica activa en el costado occidental del valle del rio zulia – norte de santander, colombia -aportes de las técnicas cuantitativas de análisis de drenaje jorge adrian oviedo reyes1,2y maría cristina dimaté castellanosb1 1universidad nacional de colombia. 2servicio geologico colombiano. jaoviedor@unal.edu.co abstract el valle del rio zulia (norte de santander, colombia) se encuentra controlado estructuralmente por un sinclinal conformado por una secuencia de rocas sedimentarias principalmente del terciario. en el flanco occidental en su parte proximal se encuentra delimitado por el sistema de fallas del zulia que hacia el norte del valle se une con la falla de aguardiente de tipo inverso conformando un conjunto deformativo con vergencia hacia el oeste. en el valle del rio zulia se observa un cinturón de deformación del cuaternario producto de los esfuerzos asociados a la colisión del bloque chocopanamá sobre el conjunto del bloque triangular de maracaibo (figura 1). esta deformación del cuaternario evidencia actividad neotectónica no relacionada con la falla de bocono. en este trabajo se estudia la deformación reciente del relieve del costado occidental del valle del río zulia mediante el análisis de las tasas de levantamiento cuantificadas a través de los índices de verticalidad de los drenajes, bajo el principio de que en configuraciones tectónicas que contienen una discontinuidad en las tasas de levantamiento, se espera que canales con altos índices de verticalidad (ksn) estén asociados a zonas de levantamiento alto, mientras que índices de verticalidad bajos indican zonas de bajo levantamiento (wobus et al., 2006). usando el programa tecdem (shahzad & gloaguen, 2011) se visualizan los perfiles de corriente de 28 drenajes tributarios que drenan en dirección w-e desde la serranía de aguardiente hacia el rio zulia (figura 1), se construyen curvas pendiente vs. área para cada drenaje con el fin de determinar los puntos de quiebre (knickpoints) y se calculan los índices de verticalidad ksn (steepness index) para un índice de concavidad θref = 0.45, según la relación s=ks〖na〗^(-θref), donde s es la pendiente de la corriente y a es el área involucrada del drenaje. a partir del análisis de las curvas de pendiente se clasifica el tipo de quiebre de la pendiente (slopebreak knickpoint) para determinar el tipo de morfologías asociadas a un proceso tectónico, resistencia de la roca o cambios transitorios fluviales (figura 2).índices de verticalidad ksn con valores mayores a 120 se concentraron principalmente sobre el trazo de las fallas de aguardiente e icuperena y sugieren una tasa mayor de levantamiento tectónico en el área en comparación con la deformación sobre el cuaternario del valle del rio zulia para los cuales se obtienen valores menores a 120. en conclusión, la deformación actual sobre depósitos cuaternarios sobre el valle del rio zulia que consiste en terrazas basculadas y levantadas se debe probablemente a tasas de levantamiento altas, evidenciadas según el análisis de los perfiles de corriente en drenajes que cruzan las fallas de aguardiente e icuperena, de tipo inverso que caracterizan un régimen compresivo we que no está relacionado directamente con la falla de boconó. palabrasclave: geomorfología tectónica, tecdem, perfiles de corriente, índice de verticalidad, quiebre de pendiente, tasas de levantamiento. earth sciences research journal 220 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 221 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics modeling seismic hazard in south america: current state of the state-of-the-art based on activities carried out with the sara project arcila m.1, eraso j.1, garcia j.2, pagani m.2, pirchiner m.4, rendon h.3, weatherill g.2, yepez h.5, boroschek r.6, contreras v.7 1colombian geological survey (cgs), colombia. 2gem foundation, italy. 3venezuelan foundation for seismological researches (funvisis), venezuela. 4university of são paulo, brazil. 5i universitè j. fourier grenoble, francia. 6university of chile, chile. 7ruben boroschek and associates, chile. julio.garcia@globalquakemodel.org abstract sara (south america seismic risk assessment) is a gem coordinated collaborative project sponsored by the swiss-re foundation, which aims at completing an integrated assessment of seismic risk in south america though a strong involvement of the local scientific community. the creation of a new harmonized seismic hazard model and the computation of the probabilistic seismic hazard (psha) for the whole region are the two key milestones for the hazard component of this project. an initial step in the construction of a new seismic hazard model is an appraisal of existing seismic hazard models that have been produced for the study region. an international team is collecting information (i.e. publications, reports, electronic files) about the available psha input models currently adopted in different countries of the region for the objective of creating an homogeneous – open – library of psha input models (in the openquake-engine format) suitable to be used by the scientific south american community. previous experiences in neighboring regions (arcila et al., 2012, 2013) already proved the utility of this approach in building regional, community-based and more homogenous models. in the present communication, we describe the available psha models in terms of their consistency, transparency and reproducibility, the problems faced in the implementation process, the likely ways to harmonize them in view of their possible use as a backbone for the new sara hazard regional model. key words: seismic hazard assessment; sara project, south america. earth sciences research journal 222 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics high-resolution, uiltra low power, intergrated aftershock and microzonation system leonid zimakov, p.passmore, j.raczka, a. leon ref tek – a division of trimble. l.zimakov@reftek.com abstract rapid aftershock mobilization plays an essential role in the understanding of both focal mechanism and rupture propagation caused by strong earthquakes. a quick assessment of the data provides a unique opportunity to study the dynamics of the entire earthquake process in-situ. aftershock study also provides practical information for local authorities regarding the post earthquake activity, which is very important in order to conduct the necessary actions for public safety in the area affected by the strong earthquake. ref tek a division of trimble has developed a self-contained, fully integrated aftershock system, model 160-03, providing the customer simple and quick deployment during aftershock emergency mobilization and microzonation studies. the 16003 has no external cables or peripheral equipment for command/control and operation in the field. the 160-03 contains three major components integrated in one case: a) 24-bit resolution state-ofthe art low power adc with cpu and lid interconnect boards; b) power source; and c) three component 2 hz sensors (two horizontals and one vertical), and built-in ±4g accelerometer. optionally, the 1 hz sensors can be built-in the 160-03 system at the customer’s request. the self-contained rechargeable battery pack provides power autonomy up to 7 days during data acquisition at 200 sps on continuous three weak motion and triggered three strong motion recording channels. for longer power autonomy, the 160-03 aftershock system battery pack can be charged from an external source (solar power system). the data in the field is recorded to a built-in swappable usb flash drive. the 160-03 configuration is fixed based on a configuration file stored on the system, so no external command/control interface is required for parameter setup in the field. for visual control of the system performance in the field, the 160-03 has a built-in led display which indicates the systems recording status as well as a hot swappable usb drive and battery status. the detailed specifications and field performance are presented and discussed. keywords: seismology, aftershock studies, microzonation, hazard mitigation. earth sciences research journal 223 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 224 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics el terremoto del 18 de mayo de 1875, en la región fronteriza colombo-venezolana :inventario de daños y de efectos geológicos co y postsísmicos luz m rodríguez1, elkin salcedo2, hans diederix3, augusto a. gomez4, santiago yépez5, andré singer1, eliana torres3, josé a. rodríguez1, alejandra leal1, carol perez1, jose choy6, franck a. audemard1, armando espinosa3, olga bohorques3 1dpto. de ciencias de la tierra, fundación venezolana de investigaciones sismológicas (funvisis). 2universidad del valle del cauca, cali. 3servicio geológico colombiano (sgc). 4istituto nazionale di geofísica e vulcanología, sezione di milano-pavia (ingv). 5fundación instituto de ingeniería (fii). 6laboratorio de geofísica, departamento de física, facultad de ciencias universidad de los andes (ula). lrodriguez.funvisis@gmail.com abstract en la mañana del martes 18 de mayo de 1875, a las 11:15 am ocurrió un terremoto en la zona fronteriza entre el estado táchira, venezuela y el departamento norte de santander, colombia. algunos caseríos y poblaciones fueron destruidos casi en su totalidad, mientras que en otras se reportaron daños menores. a más de un siglo de la ocurrencia de este evento sísmico, muchas interrogantes aún quedan por resolver, y tras la búsqueda de evidencias, empleando la sismología histórica se ha indagado en la documentación primaria y secundaria, buscado datos que permitan reconstruir el escenario de los hechos, y para ello se ha elaborado una base de datos. entre las incertidumbres por resolver se encuentran: 1) la destrucción de asentamientos poblacionales y su relación con los efectos de sitio (ubicación de los mismos, principalmente en planicies de inundación de ríos, abanicos aluviales, terrazas y en laderas con pendientes pronunciadas); 2) tipología constructiva usada a finales del siglo xix, mediados del siglo xx (paredes de bahareque o tapia con techos de teja o paja) y su relación con la capacidad de soporte ante un sismo de considerable magnitud y 3) posible falla geológica activa responsable del evento sísmico en una región con importantes accidentes tectónicos activos y su relación con la presencia de reflujos momentáneos de los ríos táchira y pamplonita al momento de la ocurrencia del evento, como es descrito en la crónicas. para el entendimiento y comprensión de estas interrogantes en esta primera fase y como resultado de este trabajo se elaboró una base de datos que ha sido alimentada por la información extraída de los archivos históricos públicos (academia nacional de la historia, gobernaciones, y alcaldías) archivos eclesiásticos y parroquiales (libro de defunciones, libro de gobierno); además se contó con la revisión de documentos secundarios: libros, periódicos de la época, transcripciones de cartas enviadas al gobierno, informes técnicos y tesis, información que se encuentra digitalizada y clasificada en carpetas, guardadas de forma electrónica. la base de datos está compuesta por un inventario de mapas y tablas que presentan las evidencias para la reconstrucción de los hechos. a manera de ejemplo se presenta un modelo de la información contentiva en tablas. esta investigación se ha desarrollado con el apoyo del proyecto estudio geodinámico de los andes de mérida (giame) por funvisis, venezuela y con el apoyo del proyecto implementación red nacional de estaciones geodésicas satelitales gps/gnss para estudios e investigaciones geodinámicas en el earth sciences research journal 225 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics territorio colombiano – geored del servicio geológico colombiano s.g.c. palabrasclave: terremoto de 1875, sismología histórica, frontera entre venezuela y colombia, destrucción de poblaciones. earth sciences research journal 226 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics intraplate seismicity and seismic zoning in brazil marcelo assumpção1, hilario bezerra2, lucas barros3, hans agurto1 1university of são paulo, são paulo, sp. 2federal university of rio grande do norte, natal, rn. 3university of brasília, brasília, df. marcelo.assumpcao@iag.usp.br abstract we discuss the seismicity in brazil and neighboring intraplate areas, which among the least seismically active stable continental regions in the world: the maximum magnitude was 6.2 mb; magnitudes 5 occur every four years. overall, the exposed cratonic regions tend to have half as many earthquakes compared to the average mid-plate south america. earthquakes tend to occur in neoproterozoic foldbelts especially in areas of thin lithosphere, or near craton edges around cratonic keels. areas with positive isostatic anomalies tend to have more earthquakes, indicating that flexural stresses are important in explaining the intraplate seismicity. the brazilian passive margin is also a region of higher than average activity. although clear differences are found between different areas along the margin overall the brazilian margins have 70% more earthquakes than the average stable continent. distribution of epicenters and stress patterns suggest the main seismic zones are: (1) southern guyana shield and middle amazon basin, the largest magnitude being a 5.5 mb in 1983; (2) a n-s trending zone along the eastern border of the amazon craton; (3) northern part of the borborema province, in ne brazil around the potiguar marginal basin, with the largest event in 1980 with 5.2 mb; (4) the porto dos gauchos seismic zone with activity known since the 6.2 mb event of 1955; (5) a ne-sw zone in the tocantins province possibly continuing towards the pantanal basin; (6) southern minas gerais zone, in and around the southern tip of the são francisco craton; and (7) the se offshore zone with activity along the continental slope in areas of extended crust. key words: seismic hazard, intraplate stresses, lithosphere earth sciences research journal 227 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics el sismo de guatemala de 1816 (m 7.5). implicaciones para la sismicidad en el límite de placas norte américa-caribe marco guzmán-speziale centro de geociencias, universidad nacional autónoma de méxico. marco@geociencias.unam.mx abstract la falla polochic, junto con la motagua, marcan el límite entre las placas norte américa y caribe en guatemala. la falla polochic comprende los segmentos reconocidos como cuilco, chixoy y polochic, que corren a lo largo de los cauces de los ríos del mismo nombre. el 22 de julio de 1816 ocurrió un sismo de magnitud 7.5 en guatemala. las intensidades de este sismo reportadas para varias poblaciones en el centro y norte de guatemala, graficadas en mapa, muestran que el evento rompió los segmentos chixoy y polochic, pero no el cuilco, que es el más occidental. en cambio, las más altas intensidades (viii-ix) se registraron a unos 50-100 kilómetros al noreste de la falla cuilco, a lo largo de los ríos selegua y nentón. estos ríos tienen un curso arqueado, cóncavo hacia el noreste y al parecer también están controlados por falla. los ríos desembocan en méxico, en el río grijalva. el curso del río grijalva en la depresión central de chiapas, es controlado por las fallas concordia y angostura, a lo largo de unos 150-200 km y con rumbo nw-se. alguna de estas fallas fue activada durante el macrosismo de 1902 (m≈7.6), evento somero y aparentemente de desplazamiento lateral. la distribución de intensidades máximas reportadas para el sismo de 1816 sugiere entonces que la ruptura ocurrió a lo largo de las fallas polochic, chixoy y selegua, mas no a lo largo de la falla cuilco. el hecho que el sismo haya sido reportado como sentido con intensidad vii en san cristóbal de las casas sugiere incluso que la ruptura pudo haber progresado aún a lo largo de la falla angostura, en territorio mexicano. los resultados aquí presentados sugieren que durante el sismo de 1816, la deformación asociada al límite placas norte américa caribe ocurrió a lo largo de una línea de falla que va de la parte central de guatemala y corre a lo largo de un arco cóncavo hacia el norte-noreste, formado por las fallas polochic, chixoy, selegua (y/o nentón) y angostura, conectando así zonas de deformación en guatemala y el sureste de méxico. asimismo sugieren que estas fallas tienen un gran potencial sísmico al que no se le ha puesto la debida atención. palabrasclave: falla polochic, sismicidad, placa caribe, placa norte américa. earth sciences research journal 228 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 229 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics magnitude and location of large historical earthquakes in colombia from macroseismic intensity data maria cristina dimate1, martha bohorquez2, ana milena sarabia3, hernan cifuentes3 1universidad nacional de colombia, geosciences department. 2universidad nacional de colombia, statistics department. 3servicio geologico colombiano. mcdimatec@unal.edu.co abstract macroseismic intensities of about twenty modern, instrumentally recorded, earthquakes are used to develop intensity attenuation equations for the three main seismotectonic environments in colombia: crustal, interface and in-slab deformation. with the aim of obtaining better predictions and of reducing heteroscedasticity problems with the data, various types of models are explored, from a simple linear model to a log-type dummy variable model. the final models are selected after a detailed process of earthquake data selection, independent variables significance analysis, cross-validation and test-event validation. thereafter, attenuation equations are rearranged in terms of magnitude. to locate and estimate the size of historical events with enough number of intensity evaluations, we follow bakun's (1997) strategy of gridding the space of probable locations, then assume each grid node as a possible location of the event, calculate the corresponding magnitude and select as the preferred location that one with the smallest rms. following this procedure we have located and estimated the size of many of the most significant earthquakes in colombian seismic history. key words: historic seismicity, macroseismicity, earthquake intensity. earth sciences research journal 230 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics smoothing techniques applied to brazilian seismic sources characterization m. pirchiner1,2 1institute of astronomy, geophysics and atmospheric sciences. university of são paulo. 2school of applied math, getulio vargas foundation, rio de janeiro. marlon.pirchiner@iag.usp.br abstract seismic sources characterization plays a key role in psha calculations. standard hazard modeling approaches, in general, uses all available tectonic information and historic events catalogue to, based on experts judgments, identify distinct zones (and faults) which will be characterized by an magnitudefrequency distribution, minimum and maximum magnitude. smoothing techniques (also known as zone-less or zone-free approach) are a catalogue-based group of general methodologies which makes possible the seismic sources characterization without definition of seismic zones geometry. the interest sources region are gridded and for each grid cell, the seismic rate (and in some cases even the gutenberg-richter b-value) are calculated by a kernel density estimator (kde) of all non-negligent influences of kernelfunctions centered on each earthquake on the catalogue. the main distinguish between methods is the way to choose the bandwidth of kernel functions. after this each cell will represent one ‘point’ seismic source in the further psha calculation. the main proposal of this work is explore three distinct approaches applied on the brazilian context, where the lack amount of seismicity and data makes hard the seismic sources zone definition, and discuss the results. first explored technique was the smoothing seismicity developed by frankel (1995) available in hazard modeling toolkit (hmtk) from global earthquake model (gem) scientific group. frankel proposes a fixed-width kde. woo, in 1996, proposed the second smoothing technique used on this work. in this technique the kernel bandwidths are proposed to be exponentially proportional to the earthquake magnitude nearestaverage-distance distribution in each magnitude bin. in the same direction, but with another perspective, helmstetter (2012) proposed an optimized locally adaptive kde to estimate the background seismicity to long-term earthquake forecasts in the southern california. the idea of this work was check the method behavior in a low seismicity context and discuss the results. additionally, as each method output is one gridded set of seismic point sources, a simple gem’s openquake classical psha calculation was ran to allow comparisons on the hazard domain. keywords: smoothing, zoneless, seismic sources characterization. earth sciences research journal 231 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics caracterización de la fuente sísmica asociada con la sismicidad hb, lp y drumbeats registrada en el volcán nevado del huila – colombia, en noviembre de 2008 nancy trujillo castrillón1, carlos miguel valdés gonzales1, randall white2, wendy mccausland2; santacoloma cristian camilo3 1unam. 2usgs. 3sgc. canytas@gmail.com abstract en el periodo comprendido entre el 2 y el 21 de noviembre de 2008, se registraron alrededor de 11200 eventos sísmicos asociados a la dinámica de fluidos. estos eventos que se clasificaron como híbridos (hb), largo periodo (lp), y drumbeats, precedieron la erupción freática ocurrida el 21 de noviembre y se caracterizaron por ser eventos altamente repetitivos, con formas de onda y anchos de banda de características similares. hecho que hizo suponer la posibilidad de que toda esta sismicidad compartiera la misma fuente sísmica. por tal razón, cardona et al. (2009), hicieron un análisis de dicha sismicidad y establecieron la existencia de dos posibles familias sísmicas, una para los eventos hb y lp registrados entre el 9 y el 21 de noviembre, y la otra para los eventos tipo drumbeat que ocurrieron entre el 20 y el 21. por tanto, para la realización de este proyecto de investigación, se retomó el trabajo realizado por cardona et al. (2009) y se aplicaron las siguientes técnicas de procesamiento digital de señales: transformada de hilbert para la caracterización inicial de las señales sísmicas, correlación cruzada para establecer el grado de similitud entre eventos y así poder identificar posibles familias, y el stack o apilamiento de señales para reducir los niveles de ruido y a la vez resaltar las características principales de las señales sísmicas analizadas. de los resultados obtenidos a partir del procesamiento digital de la información se destacan: ~ 8000 eventos con coeficientes de correlación > 0.9 y la posible existencia de 5 familias sísmicas, las cuales fueron localizadas en el sector se del edificio volcánico. un análisis detallado de la señales sísmicas obtenidas a través del stack, permitió concluir la existencia de tres familias, la primera de ellas para la sismicidad registrada entre el 4 y el 18 de noviembre, la segunda para la sismicidad registrada entre el 14 y el 21 y la tercera para la sismicidad tipo drumbeat. cada una de estas familias se presume está asociada a una fuente sísmica diferente, de tal forma que: las dos primeras familias se asocian a fuentes con mecanismos de origen, posiblemente asociados al fracturamiento frágil que puede ocurrir en zonas de debilidad que interceptan grietas o conductos, en los que se produce el fenómeno de resonancia acústica, y la tercera familia asociada a la sismicidad tipo drumbeat puedo ser producida exclusivamente por el fenómeno de resonancia acústica dentro de los conductos volcánicos. palabrasclave: hb, lp, drumbeat, familia sísmica, mecanismo de origen, fuente sísmica. earth sciences research journal 232 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sistema de información de sismicidad histórica de colombia sarabia, a.1, cifuentes, h.2, dimaté, c.3, arcila, m.4, barbosa, d.5, galán, b.6 1servicio geológico colombiano. asarabia@sgc.gov.co 2servicio geológico colombiano. hcifuentes@sgc.gov.co 3universidad nacional de colombia, departamento de geociencias. mcdimatec@unal.edu.co 4servicio geológico colombiano. marcila@sgc.gov.co 5servicio geológico colombiano. dbarbosa@sgc.gov.co 6servicio geológico colombiano. bgalan@sgc.gov.co abstract el estudio de los sismos históricos involucra conocer los efectos ocurridos en el territorio, teniendo en cuenta la percepción de las personas, daños en las construcciones y fenómenos en el medio ambiente. a partir de estas descripciones y mediante el uso de una escala de intensidad se evalúan las intensidades, las cuales a través de procesamientos matemáticos y estadísticos, permiten estimar parámetros como magnitud, profundidad y localización, que se utilizan para complementar el catálogo sismológico del país. teniendo en cuenta lo anterior, varios autores han trabajado en la recopilación y análisis de información histórica de los sismos, en la evaluación de intensidades y en el análisis sismológico de los mismos, publicando sus investigaciones en informes, artículos, libros, bases de datos, entre otros. debido a que la documentación publicada en esta temática para colombia se encontraba dispersa, había difícil acceso a los documentos primarios utilizados y que la evaluación de intensidades estaba en varias escalas, se vio la necesidad de estructurar una base de datos en la que se almacenara toda la información relacionada con los sismos históricos destructivos que han ocurrido en el país. es por esto, que en el año 2010 el servicio geológico colombiano (sgc) y la universidad nacional de colombia (un), emprendieron las actividades necesarias para diseñar, implementar y poner en funcionamiento un sistema de información con acceso público a través de la página web del sgc, en el que tanto usuarios expertos como la comunidad en general, pudieran consultar los eventos ocurridos en el país desde el siglo xvi. la implementación se desarrolló en varias etapas, y se incluyó en el sistema de información institucional “siger”, cuyo objetivo es facilitar la captura, almacenamiento, gestión, despliegue e intercambio de información geocientífica. la arquitectura del sistema institucional bajo el cual opera el sistema de sismicidad histórica es: motor de base de datos oracle 10g, arcgis server 9.3.1, arcsde 9.1, y jboss como servidor web. la consulta al sistema se realiza en línea, mediante el siguiente enlace: http://agata.ingeominas.gov.co:9090/sismicidadhi storica/ el sistema fue publicado oficialmente en diciembre de 2012 y permite elaborar consultas tabulares, gráficas y espaciales, por medio de criterios de búsqueda como: fecha o rango de fechas, sitio (municipio), coordenadas (cuadrángulo), consulta avanzada (incluye búsqueda simultánea de fecha, intensidad y coordenadas), y una consulta de sismos falsos o dudosos. también se pueden realizar búsquedas en el calendario “un día como hoy”, en el que se encuentran resaltados los días en los que ocurrió algún sismo histórico, entre otras funcionalidades. palabras clave: sismicidad histórica, intensidad, sistema de información, siger. mailto:asarabia@sgc.gov.co mailto:hcifuentes@sgc.gov.co mailto:mcdimatec@unal.edu.co mailto:marcila@sgc.gov.co mailto:dbarbosa@sgc.gov.co mailto:bgalan@sgc.gov.co earth sciences research journal 233 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics south american strong-motion database and comparison with ground-motion prediction equations: a gem initiative maria cristina dimaté castellanos1, luisa fernanda castillo2, stéphane drouet3, carlos lozano2, gonzalo montalva4, cecilio morales5, marlon pirchiner6, juan carlos singaucho armas7, cristian viracucha7, graeme weatherill8, sinan akkar9 1universidad nacional de colombia, bogotá, colombia. 2servicio geológico colombiano, bogotá, colombia. 3observatório nacional, rio de janeiro, rj, brazil. 4universidad de concepción, concepción, chile. 5fundación venezolana de investigaciones sismológicas, caracas, venezuela. 6instituto de astronomia, geofísica e ciências atmosféricas, são paulo. 7instituto geofísico escuela politécnica nacional, quito, ecuador. 8gem foundation, pavia, italia. 9bogazici university kandilli, observatory and earthquake research institute department of earthquake engineering, istanbul. stephane@on.br abstract this study takes place in the context of the global earthquake model project (http://www.globalquakemodel.org/), and more specifically in its south american component gemsara (http://www.globalquakemodel.org/what/regions/ south-america/). the global aim of the project is to build a seismic hazard model at the global scale. our team, involving researchers from brazil, colombia, chile, ecuador, and venezuela focus on the particular aspect of strong ground-motion recorded data and the comparison with groundmotion prediction equations (gmpes) at the south american scale. within the next one year and a half, we will gather all recorded strong-motion data for the various tectonic contexts one finds in south america: shallow crustal seismicity, subduction areas involving interface and in-slab events, and stable continental regions. after processing these data using a homogeneous scheme, we will compare them with appropriate gmpes, those pre-selected for the gem project (douglas et al., 2013), as well as with local models, to assess the suitability of them or state the need to develop regional ones. the database building process will include record processing, event characterization, and site term estimation for each station that recorded significant events. record processing will be performed following state of the art methodologies for both broadband and accelerometric data. one of the main difficulties is to gather homogeneous metadata. site characterization will be approach by multiple means, where available shear wave velocity profile will be used to compute vs30 (as required by many gmpes), otherwise assessed based on h/v ratios, geology or other available information. event information will be gathered from regional and international agencies, and updated using the results of the gem-sara topic 4 project (earthquake catalogue for south america). the aim of this study is to build a ground-motion logic-tree for psha at the south american scale, including the weights for each selected gmpes, earth sciences research journal 234 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics based on expert judgment and statistical properties of the residual distributions (observed vs. predicted ground-motions). keywords: strong-motion data, gmpes, gem, psha. earth sciences research journal 235 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic hazard assessment of sites of large structures in the southeast of brazil tania bustamante1, celso romanel1, maria cascão2 1pontifical catholic university of rio de janeiro (puc-rio). 2federal university of rio de janeiro (ufrj). taniavanessa@gmail.com abstract this paper presents a seismic hazard analysis of the southeast region of brazil, involving the states of minas gerais and the onshore and offshore regions of rio de janeiro and espirito santo. although brazilian seismicity is low (magnitude < 5) when compared to the andean coast of south america, where strong earthquakes occur more frequently, a seismic hazard analysis must be considered as a natural consequence of a good engineering design, under the economic, environmental and safety viewpoints, especially when the structures are located in densely populated areas. according to berrocal et al. (1993) the seismicity in the southeast region of brazil presents a diffuse distribution, not allowing the association of faults and other tectonic features with active seismogenic sources. in this work, two polygonal areas are used as diffuse seismogenic sources encompassing events with magnitude between 3 to 5, according to the brazilian earthquake database at the university of sao paulo (iag/usp). in the first area, located onshore, the main objective is to establish the acceleration spectra in order to investigate the dynamic behavior of several large dams while, in the second area, located offshore, the main purpose is to study the dynamic behavior of a pier installation currently being constructed on the seaside of the rio de janeiro state. because of the seismotectonic characteristics of both areas, the attenuation laws used in this study are those developed for similar areas in central and eastern north america and herein incorporated through a logic tree process. the design spectra are obtained through probabilistic methods, considering maximum horizontal acceleration in the bedrock determined with recurrence periods of 72, 475, 975 and 2475 years. artificial earthquakes can be later generated from these design spectra in order to investigate the dynamic behavior of dams and pier installations through finite element analyses. keywords: seismic hazard, southeast region of brazil, design spectra. earth sciences research journal 236 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics evaluación probabilística de riesgos en centroamérica con la metodología capra wilfried strauch centro de coordinación para la prevención de los desastres naturales en centroamérica (cepredenac). wilfried.strauch@yahoo.com abstract capra (http://www.ecapra.org/) es una plataforma de software de código abierto desarrollado con apoyo del banco mundial para la evaluación de riesgos; el paquete aplica técnicas probabilistas al análisis de las amenazas y pérdidas causadas por desastres naturales. los programas fueron diseñados desde un principio para ser modular y extensible. la información sobre amenazas se combina con la referente a la exposición y vulnerabilidad física, que le permite al usuario determinar el riesgo conjunto o en cadena en función de múltiples riesgos relacionados entre sí. desde hace algunos años, los países de centroamérica han realizado esfuerzos con apoyo del bid y bm para aplicar la herramienta capra. para los años 2013-14, con apoyo de banco mundial, cepredenac contribuirá a reforzar el trabajo en los países de la región en seguimiento a la implementación de la política centroamericana de gestión de riesgos (pcgir). se ejecutan los siguientes proyectos: bajo coordinación de cepredenac se realizan: 4 proyectos en guatemala, el salvador, nicaragua, panamá para la evaluación del riesgo sísmico en sectores de educación, salud, viviendas, edificaciones públicas importantes. las áreas de trabajo son las capitales de los países, con excepción de guatemala donde se trabaja en el sector de san marcos, afectado por el terremoto del 7 de noviembre de 2012. bajo coordinación del banco mundial se realizan: los proyectos de la evaluación del riesgo sísmico para sistemas de distribución de agua potable en tegucigalpa y managua.; un proyecto de evaluación del riesgo sísmico en san josé y un proyecto piloto sobre riesgo de inundaciones en boquete, chiriquí en panamá. la secretaría ejecutiva de cepredenac trabaja en estrecha cooperación con el equipo del banco mundial asignado al proyecto capra, con los coordinadores nacionales (instancias rectoras nacionales de protección civil), las instituciones científicas nacionales y con los equipos específicos de los proyectos. se coordinan las actividades con el banco mundial (reuniones periódicas, visitas conjuntas a los países) y se realiza un monitoreo de los avances, bajo la coordinación de la secretaria ejecutiva de cepredenac. se orienta para que entre los proyectos de los países se desarrolle intercambio, comunicación y cooperación. antes de tomar la decisión final sobre la realización de los proyectos, se visitó guatemala, el salvador, nicaragua y panamá para verificar si todas las condiciones están dadas para la ejecución de los proyectos propuestos, específicamente la existencia de datos necesarios y la capacidad de las instituciones encargadas para garantizar la sostenibilidad e institucionalidad para la aplicación de capra. los trabajos sobre riesgo sísmico incluyen (re)evaluaciones de la amenaza sísmica, estimaciones de la amplificación del suelo, estimaciones de la vulnerabilidad sísmica para el portfolio de las casas y edificios en la zona de trabajo. palabras clave: riesgo sísmico, centroamérica, capra. http://www.ecapra.org/ earth sciences research journal 237 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics smil session – seismic methods for illuminating the lithosphere conveners: kinya nishigami (kyoto univ. japan) <nishigami.kinya.3r@kyoto‐u.ac.jp> carlos vargas (unal colombia) <cavargasj@unal.edu.co> earth sciences research journal 238 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal structure of the paraná basin from ambient noise tomography bruno collaço1, marcelo assumpção1, maria laura rosa2, gerardo sanchez3 1iag-usp. 2unlp. 3inpres. bruno@iag.usp.br abstract conventional seismic tomography is known to have poor resolution in regions of low seismicity, therefore, studies carried out in south america did not mapped clearly smaller areas of the continent, for example the region of the paraná basin. most paths used by feng et al. (2007) in this region are roughly parallel, which prevents good spatial resolution in tomographic inversions. this work is part of a major project that will increase knowledge of crustal structure in southern brazil and eastern argentina and is being carried out by iag-usp (brazil) in collaboration with unlp and inpres (argentina). to improve resolution for the paraná basin we used inter-station dispersion curves derived from correlation of ambient noise for new stations deployed with the implementation of the brazilian seismic network (pirchiner et al. 2011). this technique, known as ambient noise tomography (ant), was first applied by shapiro et al. (2005) and is now expanding rapidly, especially in areas with high density of seismic stations (e.g. bensen et al. 2007, lin et al. 2008, moschetti et al. 2010). ant is a well-established method to estimate short period (< 20s) and intermediate periods (20 50s) surface wave speeds both in regional or continental scales (lin et al. 2008). ant data processing in this work was similar to the one described by bensen et al. 2007, in four major steps with addition of a data inversion step. group velocities between pairs of stations were derived from correlation of two years of ambient noise in the period range 5 to 60 s. the dispersion curves measurements were made using a modified version of pgswmfa (pgplot surface wave multiple filter analysis) code, designed by chuck ammon (st. louis university) and successfully applied by pasyanos et al. (2001). our modified version is no more event based and is working now with station pairs. for the tomographic group velocities maps, we used the conjugate gradient method with 2nd derivative smoothing applied by pasyanos et al. 2001. the group velocities maps were generated with one degree grid. for the tomographic inversion, we also add data derived from traditional dispersion measurements for earthquakes in sa. the velocity maps obtained for periods of 10 to 100s correspond generally well with data from previous studies (feng et al, 2007), validating the use of ant and contributing to increase resolution of tomography data in sa. the inversion maps obtained with 2nd derivative smoothing are more unstable at boundary zones for the inversion of sediments and crustal thickness. it can be explained by the smoothness factor, which is not reduced at expected discontinuities such as ocean/continent boundaries. as the steps of data processing are well defined and independent, as new stations are deployed with the progress of the brasis project (pirchiner et al. 2011) new paths will be added to the initial database, increasing the resolution and reliability of the results. key words: seismology, tomography, computational seismology. earth sciences research journal 239 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics reflection imaging of deep magma with natural sources: lessons from recent experiments on an interplate volcano and an intraplate earthquake diego quiros, larry d. brown, doyeon kim, katrina byerly, keisuke irie cornell university. daq7@cornell.edu abstract although deep reflection profiling in many parts of the world have produced detailed images of what is interpreted as molten material at varying crustal depths, most regions of active volcanism are problematic for high resolution reflection seismology partly because of difficult terrain. here we report on the lessons of two recent experiments that indicate how the new generation of large n seismic arrays coupled with seismic interferometry may offer a fresh approach to imaging the roots of active volcanic systems. in 2007 the sea-calipso onshore-offshore seismic experiment sought to explore the potential of 4d reflection monitoring of an active andesitic volcano in the caribbean island of montserrat. seacalipso consisted of a deployment of seismographs on land and on the ocean bottom to record air gun shots on a series of circular and radial lines around the island. conventional reflection processing of the airgun sources resulted in little useful images beneath the volcanic system. reflection imaging using microearthquakes from the volcanic edifice revealed the presence of deep reflectivity, but signal enhancement proved difficult because of the few sources. neither dataset probed directly beneath the volcanic edifice due to limitations with terrain and bathymetry. however, by treating the 72 hours of continuous recordings with seismic interferometry it was possible to extract surface and body waves from the seismic noise. analysis of subsets of the data suggest the virtual body waves originated from the airgun sources, while the surface waves were generated by coastal wave action. moreover, seismic interferometry allows us to generate virtual shot geometries that do probe directly beneath the volcano. the recording interval was too short to allow us to obtain a more coherent reflection image, but these results indicate that longer periods of recording (i.e. weeks or months) could well provide 3d images of unprecedented detail of the deeper magmatic system. a complementary approach to using the natural volcanic seismicity for deep reflection imaging in 3d is suggested by a recent pilot study in virginia, u.s. of the aftershock sequence of the 2011 mw 5.8 mineral virginia earthquake. in this experiment (aida or aftershock imaging with dense arrays) a dense (i.e. δx ~ 200 m) array of highly portable seismographs was used to record hundreds of aftershock waveforms. these recordings provide a basis for imaging using reverse vertical seismic profiling (vsp) techniques, resulting in a 3d reflection image analogous to those obtained by conventional common reflection point analysis of recordings from controlled surface sources. both the sea-calipso and aida-va experiments clearly demonstrated that continuous recording of seismicity in active volcanic regions with super dense surface arrays can be used to produce high resolution reflection images in 3d, perhaps even 4d, of complex structure at depth in general, and magmatic systems in particular. keywords: seismic interferometry, vsp, afteshocks. earth sciences research journal 240 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic noise environmental caracterization: application to a pilot area in bogota for seismic microzonation purposes urrego, g.1, vargas, c.a. departamento de geociencias, universidad nacional de colombia – bogotá. giurregom@unal.edu.co abstract this work shows the results of environmental seismic noise analysis associated to the implementation of temporal arrays in bogota city, in order to characterize the recent stratigraphic units. two triaxial high dynamic range accelerometers arrays were installed, each one acquired continued information for approximately one month with asymmetric array (distance between stations oscillate since 2.5 km and 8.7 km) and one week with a circular symmetric array (radial distances of 5 to 20 m).the analyses suggest an inverse dependence between the autocorrelation coefficients with the distance and the anthropic noise levels. based on the elastomechanical structure of the studied units, were found contrasting results that suggest the lateral variations of the geotechnical units. key words: environmental seismic noise. earth sciences research journal 241 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crust and upper mantle seismic tomography in northwest of south america using surfaces waves j. c. bermúdez b.1,2, m. c. dimaté c.2 1servicio geológico colombiano. 2departamento de geociencias, universidad nacional de colombia. jcbermudez@sgc.gov.co abstract the aim of this study is to generate a tomographic image for the crust and the upper mantle of northwestern south america based on the study of surface-waves group velocity dispersion. we have selected 562 shallow earthquakes with magnitudes larger than 6.0 and epicenters located in the area bounded by coordinates 600n, 1400w, 00e and 600s; 438 of these events were recorded by broadband stations of the colombian national seismic network (rsnc) while 122 were compiled by the iris consortium. the vertical component of the records was used to extract rayleigh waves and rotated horizontal components to extract love waves. we processed more than three thousand paths associated to rayleigh waves and about the same number of paths associated to love waves, to obtain in total about eighty thousand dispersion measurements. the dispersion curve for each eventstation path is obtained by multiple narrow-band filtering of the displacement records in a period range between 10s and 160s. we used the conjugated gradient method of pasyanos et al. (2001) to invert dispersion measurements for spatially varying group velocity. we found significant lateral velocity anomalies in the shallow structure which we interpret as due to velocity contrasts between sedimentary basins and the surrounding relief. deeper velocity anomalies clearly identified are associated to known structures as the carnegie ridge and the subducted oceanic crust. key words: surface waves, dispersion, dispersion curves, group velocity tomography. earth sciences research journal 242 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics crustal heterogeneity and earthquake generating properties in active fault regions in japan kinya nishigami disaster prevention research institute, kyoto university. nishigami.kinya.3r@kyoto-u.ac.jp abstract it is essential to estimate detailed heterogeneous structures in and around the earthquake source region, in order to understand the generating process of earthquakes. for this purpose, we can invert coda envelopes from local earthquakes to estimate a 3-d distribution of relative scattering coefficients in the lithosphere. we discuss the detailed image of crustal heterogeneity in two active fault regions in southwest japan, from a viewpoint of elucidating its correlation with the earthquake occurrence in space and time. the first example is the yamasaki fault zone, a typical strike-slip active fault with a total length of ~80 km. we deployed dense seismic observation network, composed of 32 stations with average spacing of 5-10 km, around the fault zone. we have been estimating detailed structure such as fault dip and shape, segmentation, and possible location of asperities and rupture initiation point, as well as generating properties of earthquakes in and around the fault zone, through analyses of accurate hypocenter distribution, 3-d velocity tomography, and coda wave inversion. we also deployed a linear seismic array across the fault in order to delineate the fault-zone structure in more detail using fault-zone trapped waves. in the scattering analysis of coda waves, we analyzed 3,033 waveforms recorded at 60 stations from 136 earthquakes in 2008-2010 and 2002-2003. the result shows high micro-seismicity and stronger scattering along the entire fault zone from surface to ~15 km depth. strong scattered distribution suggests almost vertical fault plane for some segment faults of the yamasaki fault zone. scattering seems to be stronger at segment boundaries. we will construct a fault structure model, with adding more data, and discuss its relation to earthquake generating properties in the fault zone. the second example is the high microseismicity area in central part of the kinki district. we analyzed 1,799 seismograms recorded at 50 stations from 121 earthquakes in 2002 and 2003. the result shows the existence of remarkable scattering zone at depths from 20 to 30 km, just below the high micro-seismicity area. the 3-d velocity structure shows the strong scattering zone has relatively lower velocity and higher vp/vs. low-frequency earthquakes occur at depth of 30-40 km just below this strong scattering zone. we suppose the existence of fluids, dehydrated form the subducting philippines sea plate, around the strong scattering zone. we consider this scattering zone should have some influence on active microearthquakes just above it, in such a way supplying fluids to the seismogenic zone in the upper crust. we have a data base of earthquakes in recent 30 years in this region. we will discuss the correlation between the temporal variation in seismicity and that in scattering images, especially for an earthquake with m 5.4 in 2001, aiming at detecting some evidence of crustal fluids affecting the earthquake occurrence. keywords: seismic scattering, crustal heterogeneity, tomography, deep structure of active faults. earth sciences research journal 243 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics surface wave tomography of the chaco-parana basin maría laura rosa1, bruno collaço2, gerardo sánchez3, marcelo assumpção2, nora sabbione1 1dpto. de sismología. facultad de ciencias astronómicas y geofísicas. universidad nacional de la plata. argentina. 2dpto. de geofísica. instituto de astronomia, geofísica e ciencias atmosféricas. universidade de são paulo. brasil. 3instituto nacional de prevención sísmica (inpres). argentina. mlrosa@fcaglp.unlp.edu.ar abstract we present the results of a study of surface-wave group velocity tomography, using earthquake data and ambient seismic noise correlation, for the region of the chaco-parana basin, a neopaleozoic intracratonic basin, formed by a complex history of different processes of subsidence. previous surface waves analysis (e.g., feng et al., 2004, 2007; snokes and james, 1997) estimated moho depth in the central chaco basin and a lowvelocity anomaly in the lithospheric mantle. however the seismic structure of the crust and upper mantle remains little characterized across the region due to the rather poor resolution, especially for the south region. the aim of this work is to improve the resolution and reliability of crustal images obtained from traditional earthquake-based measurements. thus, we increased the number of group velocity curves using data from regional earthquakes recorded at lpa station (argentina), brazilian seismic network stations (brasis), permanent (gsn) and portable (blsp) stations. we also added inter-station dispersion curves derived from correlation of ambient noise, for a dataset of seismic noise recordings from brasis, inpres stations, lpa, cpup and trqa stations. the ambient noise tomography (ant) is an innovative method to estimate surface wave velocities in the period range 5 to 60 s. the resulting path coverage is denser and displays a more uniform azimuthal distribution producing better tomographic images. dispersion curves were obtained by a multiple filter technique (dziewonski et al, 1969) using a phasematched filter. a 2d group velocity tomographic inversion was performed, applying a conjugategradient method (paige and saunders, 1982). the group velocity maps for 10 to 100 seconds correspond very well to tectonic structures throughout the studied area. the resolution was improved in northern argentina and southern brazil by the better seismic ray coverage, showing lowvelocity anomalies in the upper mantle beneath the chaco basin, compatible with other dispersion results. the new group velocity maps were inverted for s velocity structures, using a linearization method. estimates of the crustal thickness for the chaco basin agrees with the ~34 km found for cpup station (by receiver function) and the observed thin crust (less than 35 km), suggested by assumpção et al., 2013. the paraná basin has deeper crust compared with the chaco basin in paraguay and the eastern part of argentina. key words: surface waves, ambient noise, tomography, chaco-parana basin. earth sciences research journal 244 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics geoscientific research for lithospheric structures of the merida andes, venezuela michael schmitz, funvisis, mschmitz@funvisis.gob.ve nuris orihuela, universidad central de venezuela (ucv). stephanie klarica, universidad de los andes (ula). euries gil, petróleos de venezuela (pdvsa). franck audemard, fundación venezolana de investigaciones sismológicas (funvisis). fernando mazuera, universidad de los andes (ula). jesus ávila, fundación venezolana de investigaciones sismológicas (funvisis). luis yegres, fundación venezolana de investigaciones sismológicas (funvisis). javier sánchez, fundación venezolana de investigaciones sismológicas (funvisis). giame working group mschmitz@funvisis.gob.ve abstract the lithospheric structure of the merida andes (ma), one of the most important orogenic belts in venezuela, is objective of the multidisciplinary research group giame (geociencia integral de los andes de merida), coordinated by funvisis, in close cooperation with ucv, ula and pdvsa. although there is a wealth of near surface data which has been used during past decades to explain the structure, and led to different genetic models, there are no seismic studies at lithospheric scale which cross ma. therefore, as the backbone of the integrated research, in early 2014, three deep seismic profiles crossing the ma perpendicularly, as well as one parallel profile within the falcón basin, have been measured with 40 shot points recorded on up to 1000 recording instruments. we expect detailed information on the lithospheric structure across the ma, which will help to address the inner structure and its relation with the orogen’s gravimetric root. widely distributed gravity measurements in whole western venezuela, as well more dense arrangements along the seismic profiles were done during the first year of the project. the wide angle seismics will be complemented with the reprocessing and re-interpretation of existing reflection seismic data, which will allow establishing a relationship between ma and its associated flexural basins (maracaibo and barinas-apure basins). several principal tasks, consisting in ma quaternary deformation studies, using some research methods like neotectonics and paleoseismology, georadar, numerical modeling, kinematic gps, thermocronology, detailed studies on regional geology, flexural modeling, gravity modeling, among others, are ongoing. we expect that this project will help to solve many of the doubts regarding the origin, evolution and structural configuration of the ma, as well as the relation to the seismicity of the principal fault systems, as the boconó fault. further members of the giame working group are: j. choy, m. cerrada, m. rengifo, r. aranguren, p. vizcarret, a. pérez, s. mata (ula), m. bermúdez, j. gonzález, m. arnaíz-rodríguez, a. ughi, l. piñerofeliciangeli, r. jiménez, f. quintero, h. socorro, m. flores, s. arcía, c. viana (ucv), f. urbani, a. singer, h. rendón, l. rodríguez, j. aray, j. moncada, a. leal (funvisis), m. bolívar, h. cerquone, m. forgione, r. ramírez, p. camacho, j. requena, j. lara, y. molero (pdvsa), m. barquero, f. araujo (pdvsa-intevep), c. padrón, o. guzmán (usb), s. yépez (cpdi), c. reinoza, i. angel, l. pousse (funvisis-isterre). earth sciences research journal 245 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics key words: lithosphere, orogenic evolution, basin development, mérida andes, venezuela. earth sciences research journal 246 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics snop session – current practices and recent advancements in seismic network operations conveners: dmitry storchak (isc uk) <dmitry@isc.ac.uk> marcelo assumpção (usp brazil) <marcelo.assumpcao@iag.usp.br> earth sciences research journal 247 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the northeastern brazil seismic network – rsisne aderson do nascimento, joaquim mendes ferreira, jordi julià, regina spinelli, eduardo menezes ufrn, brazil. aderson@geofisica.ufrn.br abstract the brazilian seismic network (rsbr) is today composed of 73 permanent broadband stations covering different regions of the country. the duties regarding installation and operation of this seismic network are on four brazilian institutions: university of são paulo (operating 23 stations), university of brasília (operating 18 stations), national observatory and the federal university of rio grande do norte (each operating 16). the deployment of the ne (brazil) seismic network (rsisne) is of particular interest because ne brazil is the main seismic area of the country. since the deployment of rsisne many previously unreported intraplate seismic activity has been recorded. here we show all the stages of deployment, data quality control and event report for this seismic network. we also report the recent intraplate seismicity since rsisne deployment. the new rsisne will certainly increase detectability of the brazilian seismicity, improve regional crust models in south america and, in particular, in ne brazil. keywords: seismic network. earth sciences research journal 248 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the saint peter saint paulo island archipelago seismic station aderson do nascimento, guilherme de melo, daniel queiroz, martin schimmel, eduardo menezes, regina spinelli ufrn, brazil. aderson@geofisica.ufrn.br abstract the saint peter saint paul archipelago (spspa) is located in the equatorial region of the atlantic ocean about 1,100 km distant from the brazilian northeastern coast. the spspa is composed by a set of several small rocky formations with a total area of approximately 17,000 m². since 2011 we operate a seismic station (composed of a broad band sensor and a strong motion sensor). despite the severe environmental conditions and the logistics we have (quasi) continuous monitoring of the seismicity. due to its remote distance from the continent and the lack of cultural noise, the spspa location is unique for investigating microseismic noise and its relation with ocean-atmospheric variables, and the south mid-atlantic ridge geodynamics. here, we present results related to the installation and maintenance of the spspa station and also present some scientific results regarding microseismic noise and earthquake monitoring with this station. keywords: seismic station, microseismic noise. earth sciences research journal 249 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics image1: figura1.jpg img:image1: img:image2: ensamblado, configuración, instalación y monitoreo de las estaciones sismológicas en las instalaciones del observatorio de rayos cósmicos del proyecto internacional pierre auger andrés nacif1, marianela lupari2, rubén squartini3, silvina nacif4 y silvana spagnotto2,5 1consultor fonarsec. 2conicet. 3observatorio pierre auger. 4fonarsec. 4universidad nacional de san luis. nacif.andres@gmail.com abstract se desplegaron 12 estaciones sismológicas en la región ubicada entre las latitudes 34.5°-36°s y longitudes 67.5°-70°o (figura 1.), con el fin de posibilitar una evaluación del riesgo sísmico en las poblaciones cercanas y asociar la sismicidad localizada a las fallas activas. dicho experimento denominado bloque san rafael (bsr) activo desde diciembre 2009 está enmarcado dentro del proyecto picto n°254 riesgo sísmico. de las 12 estaciones instaladas 5 de ellas (coihlama-bls-mora-puma) están ubicadas en las estaciones de fluorescencia del observatorio pierre auger (malargüe). la estación coih está compuesta por un sensor guralp edu-v y una pc configurada con el s.o. linux (ubuntu), corriendo el programa de adquisición en tiempo real y monitoreo scream 4.5. los sistemas de adquisición de datos de las 4 estaciones restantes (sensores periodo corto s13) fueron ensamblados en el instituto geofísico sismológico f. volponi (igsv), compuestos por placas sara sadc201 y pc con el s.o. debian5. se instaló y configuro el programa de adquisición en tiempo real y monitoreo eartworm 7.4. se configuro el protocolo ntp (network time protocol) utilizado para la sincronización de relojes de sistemas computacionales a través de la red, haciendo uso de intercambio de paquetes y latencia variable para el uso del gps. estas estaciones se encuentran transmitiendo en tiempo real y son monitorizadas tanto en el igsv como en instituto nacional de prevención sísmica (inpres). earthworm permite monitorizar las estaciones en tiempo real y ver los eventos en el momento que suceden almacenando el registro continuo en una base de datos que son exportadas y subidas a un servidor ftp (instalado en el igsv). se localizó la sismicidad de corteza y de placa de nazca subducida (lupari., 2014) en un periodo de 7 meses con el programa hypocenter 3.2 (lienert b., 1994) usando el modelo de velocidad obtenido por lupari., 2014. (figura 2.). keywords: sismicidad, transmisión tiempo real. https://www.jotformpro.com/uploads/andrescardozo/40405341991956/265732774021202253/figura1.jpg earth sciences research journal 250 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 251 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics velocity spectral analysis for telesesismic events using the colombian seismological network as an array j. c. bermúdez b.1,2, n. d. pérez g1,2, h. e. poveda n.3 1servicio geológico colombiano 2departamento de geociencias, universidad nacional de colombia 3departamento de geofísica, universidade federal do rio grande do norte, brazil. jcbermudez@sgc.gov.co abstract in the present work is presented a velocity spectral analysis (vespa) for teleseismic events, recorded for the colombian seismological network (rsnc, for its acronym in spanish) broad band and short period stations. to reach this aim we selected earthquakes with magnitude bigger than 7.0 and located at epicentral distance between 20o and 150o. this analisys was done to increase coherent signal ratio over no coherent noise and thus, to improve the snr, which it was useful to identify phases from slowness variation getting the vespagrams, beside we used beamformig analysis to compare with the vespa. key words: array seismology, vespagram, beamforming. earth sciences research journal 252 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics evolución de la red nacional de acelerógrafos de colombia carlos lozano1, maría luisa bermúdez2, jorge correa1, elizabeth mazo1, helber garcía1, gustavo redondo1 1servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá d.c., colombia. 2kma construcciones, bogotá d.c., colombia. clozano@sgc.gov.co abstract después del sismo de popayán del 31 de marzo de 1983, mb=5.5 surgió entre los ingenieros civiles la preocupación de los daños que podrían ocurrir en otras ciudades similares. esto condujo a que el gobierno nacional generara un estudio detallado que recomendaba el desarrollo de un código colombiano de construcciones sismo resistente y la implementación de una red sismológica y acelerográfica nacional, así como la instrumentación local. el año de 1993 inició su operación la red nacional de acelerógrafos de colombia (rnac) bajo el auspicio del fondo nacional de calamidades, a través del programa de las naciones unidas para el desarrollo (pnud) y el gobierno nacional mediante el servicio geológico colombiano (antiguo ingeominas). el propósito fundamental de instalar una red acelerográfica en el territorio colombiano, es obtener registros de los sismos intensos (con ml ≥ 5.0) que ocurren a lo largo del país, y llevar a cabo su procesamiento para ofrecer esta información básicamente a la actualización del reglamento colombiano de construcciones sismo resistente mediante el estudio de amenaza sísmica nacional y generación de leyes de atenuación sísmica. de igual forma, contribuir a la evaluación de la respuesta sísmica de ciudades a través de estudios de microzonificación. la red está compuesta de 102 estaciones de acelerógrafos: 30 estaciones transmiten la información en tiempo real a una central de datos en bogotá mediante telemetría satelital e internet y en las 72 estaciones restantes, la información se recolecta en las visitas de mantenimiento. la red está dotada con moderna instrumentación que permite obtener información confiable y cuenta con el personal capacitado para el procesamiento y análisis de la misma. durante estos 20 años se ha generado un gran banco de datos de 7900 acelerogramas producto de 3600 sismos. hoy contamos con una base de datos disponible en línea para consulta de esta información desde el año 1993 hasta 2013. igualmente, esta información se ha divulgado en 16 boletines anuales de movimiento fuerte. la rnac ha suministrado información procesada para los estudios de microzonificación sísmica de las ciudades de bucaramanga, villavicencio y cali. ha prestado apoyado en el proceso de instalación de redes locales de acelerógrafos de las ciudades de popayán (5 estaciones), bogotá (30 estaciones), manizales (5 estaciones) y cali (12 estaciones). ha desplegado su red portátil para el estudio de réplicas de los sismos intensos como los de tauramena (1995), armenia (1999), pizarro (2004), gorgona (2007) y quetame (2008). recientemente, aportó información en la actualización del mapa nacional de amenaza sísmica a través de la comparación de leyes de atenuación mundiales con los registros obtenidos en las estaciones de la rnac. a corto plazo, se prevé complementar la instrumentación sismológica sobre el océano pacífico colombiano, mediante la instalación de acelerógrafos que permitan localizar sismos generadores de tsunamis, a fin de suministrar en forma veraz y rápida esta información a las entidades encargadas de alertar sobre este fenómeno y apoyar a la comunidad en caso de un desastre. mailto:clozano@sgc.gov.co earth sciences research journal 253 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics palabras clave: acelerógrafo, movimiento fuerte, leyes de atenuación. earth sciences research journal 254 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics microseismic noise in the saint peter saint paul archipelago and its relation with some atmospheric and oceanographic variables daniel queiroz, aderson do nascimento, martin schimmel, mario silva ufrn, brazil. daniel.geofisica@gmail.com abstract microseisms are continuous vibrations pervasively recorded in the mili hertz to 1 hz frequency range. these vibrations are dominated by rayleigh waves and are strongest in the 0.04 to 1 hz frequency band. their precise source mechanisms arean ongoing debate but it is agreed that they are related to atmospheric perturbations and ocean gravity waves. the saint peter saint paul archipelago (spspa) is located in the equatorial region of the atlantic ocean about 1,100 km distant from the brazilian northeastern coast. the spspa is composed by a set of several islets with a total area of approximately 17,000 m². due to this sui generis geographic location the spspa is a unique natural observatory for measuring microseismic noise and to investigate its relation with climate and oceanographic variables. in the spspa we have recorded both primary microseisms (pm) at 0.04 – 0.12 hz and the secondary microseisms (sm) at 0.12 – 0.4 hz during 10 months in 2012 and 2013. our analysis indicates a good correlation between the microseismic noise in the region and a seasonal dependency—in particular with the winter in the northern hemisphere. we also show that most of the pm is generated in the spspa itself. on the other hand, the sm source location depends to the seasonal climatic and oceanographic variables in the northern hemisphere. keywords: microseismic noise, seismic station. earth sciences research journal 255 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics international seismological centre (isc): mission and products dmitry a. storchak, domenico di giacomo, james harris international seismological centre dmitry@isc.ac.uk abstract the international seismological centre (isc) is a non-governmental non-profit-making organization funded by 62 research and operational institutions including six in the latin america and caribbean (lac) region. the isc is charged with the production of the isc bulletin – the definitive summary of the global seismicity based on reports from over 130 seismic networks worldwide, including 14 in the lac region. we also distribute the ehb bulletin – a groomed subset of the isc bulletin widely used in studies of inner structure of the earth. jointly with neic, the isc runs the international seismic station registry (ir). the isc station codes and event hypocenters are used by iris dmc in serving seismic waveform data to its users. the isc provides a number of additional products including the iaspei reference event list (gt). the isc-gem catalogue is another dataset that was designed to be used for global and regional studies of seismic hazard and risk. the isc event bibliography is an interactive facility to search for scientific articles devoted to both natural and anthropogenic seismic events that occurred within a region and time period of interest. the multitude of isc data are freely distributed and widely used in different fields of geophysical research. keywords: earthquake, bulletin, gt, isc-gem, bibliography. earth sciences research journal 256 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics aeqc tools for retrieval and completion of seismological waveform databases from heterogeneous networks el-madani aissaoui1, arnaud lemarchand1, alexandre nercessian1, jean-marie saurel2, andré anlade2 1ipgp. 2ovsm-ipgp. aissaoui@ipgp.fr abstract seismological and volcanological observatories are responsible for monitoring the earth activity through seismic networks, and for providing good quality dataset for scientific studies. to ensure a reliable monitoring systems it's required to build resilient networks whatever the data transmission the observatories use: vsat, wifi, adsl and others. unfortunately datastreams may be interrupted for few seconds or several days for many reasons: network failures, server maintenances, lightnings, field equipment failures, strong earthquakes... all these factors can have an impact on the continuity of collected data. retrieving data from field stations for database completion is a tedious work and is very expensive in manpower. the aeqc tools were developed with the aim of retrieving missing data to improve the quality of continuous archives (and to guarantee the most complete miniseed database available). it provides as output a miniseed archive with a seiscomp datastructure (sds) with a quality control on gaps, overlaps, sampling frequency and compression integrity. this tool working in background as a daily process is based on bash and python scripts, it uses the passcal software libraries, iris dmc tools and runs on linux systems. data completeness analysis are made daily for all the configured networks and stations, emails reports are sent and data timelines are generated to provide a quick view of data continuity. aeqc retrieves only gaps thus data retrieving is optimized for limited bandwidth connections. the corrections and completions are done locally and overlaps are also corrected.it can work inside a heterogeneous seismic network and supports various data sources: -arclink server -kinemetrics digitizers: q330, baler14, baler44 -nanometrics libra1 and libra2 systems with naqs -nanometrics digitizers (taurus and centaur) -sds archives accessible through ftp or http it has a modular structure and it is easy to add a new data source. at this time aeqc scripts have been installed and are successfully used in the following seismological networks: -pf network at ovpf, reunion island in the indian ocean (observatoire volcanologique du piton de la fournaise / ipgp) -gl and wi networks at ovsg, guadeloupe island in the lesser antilles (observatoire volcanologique et sismologique de guadeloupe / ipgp) -mq and wi networks at ovsm, martinique island in the lesser antilles (observatoire volcanologique et sismologique de martinique / ipgp) -cx network in the north of chile keywords: data completion correction continuity timelines reports, gaps overlaps database retrieving observatories control quality seismic earthquakes heterogeneous networks source kinemetrics nanometrics digitizers. earth sciences research journal 257 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics activities of the fdsn: current and future g. ekstrom columbia university. ekstrom@ldeo.columbia.edu abstract the international federation of digital seismograph networks (fdsn) provides an international forum for fostering and supporting international cooperation in seismology. the organization currently represents 91 institutions in 67 countries. through its five working groups (1. station siting and instrumentation; 2. data exchange; 3. coordination of products, tools and services; 4. ctbt issues; 5. portable instrumentation), the fdsn strives to facilitate the development and operation of seismological networks around the world. important fdsn activities include the development of standards for instrumentation, data, and data exchange, the coordination of station siting, and the pursuit of free and open access to data. emerging topics of importance for the fdsn are data quality and ensuring the long-term preservation of and access to seismological data. in my talk, i will describe current fdsn functions, and invite participation in a discussion of future goals and activities. keywords: seismology networks. earth sciences research journal 258 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics composición y funcionamiento de las estaciones satelitales de la red sismologica nacional de colombia j. de la rosa ramirez, j. fernandez servicio geológico colombiano, bogotá, colombia jdelarosa@sgc.gov.co, jefernandez@sgc.gov.co abstract en el presente trabajo se realiza una descripción en detalle de la composición y el funcionamiento de la red sismológica nacional de colombia -rsnc-. para ello se describe el funcionamiento de los tres subsistemas que componen las estaciones sismológicas que pertenecen a la rsnc la cual está conformada actualmente por cincuenta y tres (53) estaciones sismológicas que están distribuidas en todo el territorio colombiano. los tres subsistemas que son de interés de este trabajo son el subsistema de energía y protecciones eléctricas, el subsistema de comunicaciones y el subsistema de instrumentación; de los que se hará una síntesis de los diferentes equipos que actualmente están en uso y que permiten el buen funcionamiento de cada estación sismológica; y por ende el buen funcionamiento de la red sismológica nacional de colombia. palabras clave: estaciones sismológicas, subsistemas, energía y protecciones eléctricas, comunicaciones, instrumentación. mailto:jdelarosa@sgc.gov.co mailto:jefernandez@sgc.gov.co earth sciences research journal 259 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics portable, gps synchronized, mobile phone network-based seismological station carlos alberto vargas jiménez1, juan manuel solano fino1, alexander caneva2 1universidad nacional de colombia. 2universidad antonio nariño. jmsolanof@unal.edu.co abstract a station for acquisition, storage and real time transmission of seismic data has been developed. the station’s software scalability, taking into consideration that the data logger includes a small laptop, makes the station versatile and easy to update. it synchronizes by gps, stores data on mseed format, and sends real time data by using a modem. the station has a real time event identifier based on the long and short time average algorithm. it has seven differential channels divided on three db9 ports: two for triaxial sensors and one for an additional low sampled signal (temperature, barometers, inclinometers, radiometers, etc.). the application is designed and implemented on labview, devices are usb plugged and sensors are connected through db9 ports. the station is composed of a ten inches netbook, a usb hub, a gps, a data acquisition device, a dc-dc converter, a reboot circuit and the sensors. all the elements, except the sensors, are protected in a high-resistant case. station is capable to acquire data up to 3000 sps with 24 bit resolution and store up to 3 years of data with 3 channels at 200 sps. in order to get time synchronization, data acquisition and real time transmission, a windows environment application was designed. it was programed over national instruments’ labview software, which allows the user to program on g (graphic) language. the 5 sections –subsystemsof this app count with queue communication. queues are useful because they prevent data loses since they are able to save data from one stage to another. for example, data acquired remains on a queue if there is some issue with the storage, once this issue is solved data will be stored. additional features include email alerts for storage space, battery level and major seismological events detected via sta/lta algorithm. it should be noted that real time transmission requires a data plan with a cellphone provider. currently, there are five stations registering for the red sismológica de la sabana de bogotá. these stations are working standalone since 2012 with maintenance every 6 12 months. key words: seismological station, portable, mobile phone network, gps, stand-alone. earth sciences research journal 260 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis de ruido de las estaciones sismológicas del servicio geológico colombiano a partir del cálculo de densidad espectral de potencia probabilística lina paola aguirre1, nelson david pérez1, 2, juan carlos bermúdez1, 2, jaime eraso1 1servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc). 2universidad nacional de colombia (unal), departamento de geociencias. abstract este trabajo muestra el proceso y resultados de la caracterización de ruido de 5 estaciones de banda ancha de la red sismológica nacional de colombina rsnc a partir del cálculo de la densidad espectral de potencia probabilística (ppsd por sus siglas en inglés). la configuración de estaciones de la rsnc al día de hoy permite un buen cubrimiento de la actividad sísmica nacional, por ello uno de los retos de la red es mejorar la calidad de la información adquirida. se realizó un análisis preliminar de las formas de onda de todas las estaciones en intervalos de tiempo así: 05:00 08:00, 13:00 -16:00 y 22:00-01:00 (ut). en consecuencia se identificaron 4 estaciones con niveles críticos de ruido y adicionalmente se escogió una estación de referencia con un desempeño aceptable para llevar a cabo la comparación entre los diferentes niveles de ruido. la utilización del software seisan permitió obtener los espectros de potencia de las formas de onda para realizar el análisis preliminar. adicionalmente, se utilizó el módulo ppsd del ecosistema obspy, para analizar las trazas sísmicas de las estaciones seleccionadas. para ello se tomaron intervalos de tiempo desde horas hasta meses. como resultado se obtuvieron las gráficas de distribución probabilística (pdf) y energía espectral (psd) combinada para cada componente de las 5 estaciones seleccionadas. a partir del análisis de estas gráficas se identificaron las posibles fuentes de ruido sísmico y antrópico de cada estación y se evaluó su desempeño. palabras clave: densidad espectral de potencia probabilística (ppsd), ruido sísmico, curvas de peterson. earth sciences research journal 261 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics subduction zone events in bolivia as recorded by the bolivian and brazilian networks: a test of location accuracies lúcio quadros1, marcelo assumpção1, estela minaya2, percy aliaga2, teddy griffith2, guido avila2 1seismology center, university of são paulo, brazil. 2observatório san calixto, la paz, bolivia. lucioquadros14@gmail.com abstract the brazilian seismic network (rsbr) is being installed since 2010 and has recorded hundreds of small events (magnitudes down to 3.5 mb) in the andean subduction zone, especially intermediate events beneath northern argentina, bolivia, and southern peru. the rsbr also uses some andean stations, such as lpaz (bolivia) and lvc (chile) in the detection/location system. we present a comparison of the rsbr hypocenters, obtained automatically with a seiscomp3 system and the “locsat” code, with the hypocenters of the bolivian network (osc, observatorio san calixto, using the “hypocenter” code) as well as with the neic/usge locations. in the magnitude range 4.0 to 4.6, the rsbr epicenters determined since early 2013 are mostly within 10 to 25 km of the neic epicenters, and the osc locations are mostly within 10 to 20 km of the neic solutions. the differences between the rsbr and osc epicenters are mainly less than 50 km. differences in depth are usually less than about 20 km. we also tested the automatic locsat location with an offline location using the “hypocenetr” code: the epicentral differences are mostly within 20-30 km. we conclude that, in general, the rsbr automatic locations of intermediate-depth events using brazilian stations (together with lpaz, lvc and cpup) are comparable to the neic teleseismic epicenters. we will show the results of the re-locations of the events by combining the rsbr and osc datasets, as well as the improvements by using the rstt travel time corrections to account for 3d structural variations. rsbr is being installed with support from the petrobras geotectonic research program by a joint effort of four institutions: sao paulo university (usp), brasilia university (unb), national observatory (on) and federal university of rio grande do norte (ufrn). keywords: hypocenter determination, subduction zone. earth sciences research journal 262 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the new brazilian seismic network: current status and detectability test with the recent pisagua earthquake sequence, northern chile marcelo bianchi1, marcelo assumpção2, hans agurto2, bruno collaço2, jackson calhau2, josé roberto barbosa2, luiz galhardo2, marlon pirchiner2, sergio fachim2, cleusa barbosa2, marcelo rocha3, juraci carvalho3, paulo araujo3, francimilton silva3, george frança3, lucas barros3 1university of são paulo, seismology center iag/usp. 2iag/usp. 3university of brasilia, seismological observatory – unb. m.bianchi@iag.usp.br abstract efforts from four different institutions are establishing the new permanent brazilian seismic network (rsbr) that today is composed of 73 broadband stations operating across different regions in brazil: university of são paulo (usp) operates 23 stations, university of brasília (unb) 18, national observatory (on) and rio grande do norte federal university (ufrn) 16 each. usp and unb stations form a sub-network called brasis (bl and br network codes respectively), on stations form the rsis sub-network (on) and ufrn the rsisne sub-network (nb). most stations transmit in real-time via satellite or cell-phone links. the data/services distribution topology is based in technology and concepts adopted by the eida network in europe. each institution maintains a local data center with a seedlink/arclink server that can be used to obtain real-time and archived waveform data when allowed by each participant. all nodes use the seiscomp3 system as a levering backend. while the main rsbr purpose is to monitor the seismicity in brazil, its geometry and position in relation to the majority of the south america seismogenic sources will enhance the data availability in the distance range 5° to 35°, filling a gap in the previous station distribution in south america. the recent pisagua earthquake sequence in northern chile, which started on march 16th and reached 8.1 mw, was used to test the rsbr detectability. we compared our hypocenters with the chilean national seismological center (csn) catalog, which is more complete and accurate. a global well known usgs/neic/pde catalog comparison was also performed as reference. during the pisagua sequence our system automatically located 265 earthquakes from 21°s to 18°s, and from 72°w to 69°w. besides the rsbr stations, our system used two andean stations (lvc and lpaz) near the epicentral area. all events were revised and associated with csn hypocenters using a maximum time and distance offsets of 30s and 130km, respectively, parameters that maximized the association ratio. comparison shows that rsbr is presenting an automatic completeness magnitude of 5.5 mb for the sequence with a lower threshold of 3.5. equivalent comparison of the neic catalog with the reference csn catalog shows a magnitude completeness of 4.8 with a lower threshold of 3.1. the mislocations (median difference in epicentral/hypocentral distances) while constrained by the association parameters show a median value of 20 km (rsbr/csn) versus 8 km (pde/csn) and a median depth error of 15 km (rsbr/csn) versus 20km (pde/csn). our magnitude estimates are consistent with the csn catalog, while the neic catalog tends to have higher values (up to 0.5) compared to csn. we also found that the depths vary greatly, most probably due to the different procedures used by earth sciences research journal 263 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics each institution rather than the location methods. the new rsbr will certainly increase detectability of andean earthquakes and help improve location accuracies in south america. keywords: brazil; pisagua; earthquake detectability; catalog comparison. earth sciences research journal 264 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics tarjeta amplificadora para la adapatación de la digitalización de los sistemas sismológicos de corto período en el sgc nicolás oliveras mercado servicio geológico colombiano noliveras@sgc.gov.co abstract en los centros de monitoreo sismológico y vulcanológico del servicio geológico colombiano (sgc) ha nacido una implementación conocida desde hace aproximadamente 10 años pero con algunas deficiencias tanto técnicas como de interpretación por la parte del grupo científico. esta es la digitalización de sensores sísmicos de corto período pasivos con digitalizadores diseñados y usados para sistemas de banda ancha. actualmente las pocas implementaciones realizadas evidenciaban la falta de un documento de referencia para la calibración de este nuevo sistema, así como su baja sensibilidad ha generado desplazamientos en líneas bases de interpretación al no comprenderse bien la ubicación del nuevo sistema en la curvas de peterson. el avance tecnológico, la obsolescencia de la tecnología telemétrica analógica, la alta confiabilidad de los sensores de corto período pasivo en sitios de difícil acceso, ha hecho que se usen estos últimos con todas las nuevas herramientas de comunicación ethernet, sistemas de calibración remota y la tecnología digital de adquisición y procesamiento. esto plantea un reto a nivel tecnológico: ¿son los digitalizadores de banda ancha adecuados para digitalizar las señales de los sensores sísmicos de corto periodo pasivo en cuanto a ruido de fondo, amplificación y adaptación de impedancia? para ello se ha resuelto este problema con el diseño del sistema t-amp-3d-48 el cual es un sistema interfaz para el sensor corto período pasivo a digitalizador que permite amplificar la señal de este sensor sísmico con bajo ruido y adaptar el sistema para conservar las características en frecuencia y de calibración instrumental de los clásicos sistemas analógicos convencionales. con este equipo se ha podido así mismo ampliar el ancho de banda, un poco más, mediante el uso de damping tipo capacitivos y conservar las funcionalidades de verificación con señales de prueba como los actuales sistemas de banda ancha. en la actualidad el sistema t-amp-3d-48 es usado en casi todas las estaciones sismológicas de corto período del observatorio vulcanológico y sismológico (ovspop) del sgc. desde el 2011 se ha obtenido una calidad de señal equiparable a las estaciones sismológicas de banda ancha y con mayor rango dinámico que las estaciones tradicionales con telemetría analógica, a excepción por la frecuencia de detección intrínseca de los sensores usados. de esta forma el sismólogo puede interpretar los datos de este nuevo sistema como si fuera el clásico analógico de una forma transparente. actualmente se realiza el proceso para el registro patente de modelo de utilidad para proteger la novedad tecnológica y garantizar que el desarrollo intelectual del amplificador, realizado en el ovspop del sgc, este registrado para una eventual fabricación a nivel industrial. key words: ruido sísmico, amplificador, corto periodo, densidad espectral de ruido. earth sciences research journal 265 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. primer amplificador elaborado en el ovspop para adaptar las señales de los sensores sísmico de corto periodo a los digitalizadores guralp dm24. estación agua blanca volcán puracé colombia noviembre 2011. figura 2. instalación típica del amplificador t-amp-3d-48 en una caja de un sistema multiparamétrico, donde puede apreciarse el digitalizador guralp dm24, al fondo, conectado con el amplificador ubicado al frente. estación agua blanca volcán puracé colombia noviembre 2011. earth sciences research journal 266 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics optimización de redes de datos para el monitoreo volcánico richard andrés mier servicio geológico colombiano, observatorio vulcanológico y sismológico de pasto. rmier@sgc.gov.co abstract la red de comunicación inalámbrica en el observatorio pasto, año tras año presenta cambios y mejoras en capacidad y robustez, así como también ha evolucionado desde sistemas analógicos hasta sistemas que transportan información digital, hoy en día la mayor parte de la red opera bajo el protocolo tcp/ip; derivado de este proceso, se requiere cada vez más el control y gestión de la misma, para esta labor es necesario un re-diseño del sistema e incorporar equipos con funcionalidades y prestaciones adicionales que permitan calidad de servicio, soporte a fallos, balanceo de carga, control de tráfico, redundancia y escalabilidad. la pérdida de conectividad con los sensores instalados en campo, ya sea por fallos propios de los equipos de red, condiciones climáticas adversas o eventos volcánicos considerables, ocasiona una pérdida de datos valiosa e irrecuperable disminuyendo la capacidad de procesamiento y afinamiento de información. durante este año, el porcentaje de funcionamiento de la red de monitoreo volcanes monitoreados por el observatorio pasto en promedio es del 96 %. la participación de la red pública de internet a la red de monitoreo volcánico ya sea por proveedores (isp) o a través de operadores de sistemas celulares, puede considerarse positiva en relación al uso de infraestructura y servicios que pueden aportar a diseños de sistemas redundantes. tecnologías que usan bandas libres del espectro electromagnético como las de 900 mhz, 2.4 ghz y 5.1 ghz, sumado a tipos de radiación como direccionales, sectoriales y omnidireccionales, aportan a diseños tolerantes a fallos. los equipos instalados actualmente hacen parte de una red conmutada (bridge – capa 2 modelo osi) su tarea es la comunicación y transporte de datos, la inclusión de dispositivos de tipo utm (unified threat management) multipropósito, permiten la gestión de tráfico en las capas superiores (3-ip y 4transporte del modelo osi) brindando herramientas a los administradores de la red para una completa gestión, control y mejoramiento. considerando lo anterior se diseñó un sistema de red que hace uso principalmente de la red existente y se consideran cuatro áreas principales como son: backbone (núcleo), transporte, distribución y acceso, enmarcados en los conceptos de seguridad y calidad de servicio. el sistema propuesto, aumenta el porcentaje de disponibilidad de la red de monitoreo, disminuyendo así la probabilidad de pérdida de datos. el concepto de seguridad se asocia a la disponibilidad, confidencialidad e integridad de los datos, la gestión es transversal a todo el proceso y la calidad del servicio ofrece garantías en condiciones adversas a zonas o estaciones principales para el envío de datos. palabras clave: monitoreo volcánico, red de datos, conectividad, gestión de tráfico, gestión de redes, monitorización, interoperatividad, red pública de internet mailto:rmier@sgc.gov.co earth sciences research journal 267 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the chilean seismological observation system sergio barrientos and csn team universidad de chile. sbarrien@dgf.uchile.cl abstract several high-impact earthquakes have taken place in chile during the last few years. these events have prompted local authorities to improve the seismic monitoring capabilities in the country. along these lines, the university of chile created the national seismological center, an agency that is the continuation of the seismological service. the seismological center at the university of chile has been charged with the installation and robust operation of a network which includes three types of observations: acceleration, velocity and displacement. the complete observational system is based on the university´s backbone of more than 60 bb and strong motion instruments which include international collaborations with geoforschungszentrum (gfz, germany), institut de physique du globe de paris (ipgp, france) and incorporated research institutions for seismology (iris, us). to this, 65 broadband, 65 strongmotion and 130 real-time dual-frequency gps devices are being installed to complement the observational system. additionally, 297 accelerometers distributed throughout the country will be connected to the main acquisition, processing and distribution system, which is also being upgraded by incorporating hardware virtualization capabilities. it is expected that most of the installation of the remote sensors is completed by 2015. a description of the observational system, as well as the degree of progress will be presented, together with the performance of the system during the mw=8.2 april 1, 2014, earthquake. keywords: seismic networks, chile earthquakes. earth sciences research journal 268 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics mapas de intensidad instrumental en tiempo real para colombia tovar, m., eraso, j., arcila, m. servicio geológico colombiano, bogotá, colombia. mtovar@sgc.gov.co, jeraso@sgc.gov.co, marcila@sgc.gov.co abstract los sismos son uno de los eventos naturales con mayores niveles de afectación social y económica. en colombia, algunos de ellos han dejado considerables pérdidas humanas y materiales, muestra de ello los sismos de popayán en 1983, tauramena en 1995 y armenia en 1999. una de las lecciones aprendidas es que las mayores aceleraciones y por tanto los mayores daños, no solo se concentran en el área epicentral, sino que diferentes factores o condiciones (geología, topografía, tipo de suelo, etc.), pueden alterar su comportamiento. al estar colombia en un contexto sismotectónico activo, y donde un alto porcentaje de la población habita en zonas de amenaza sísmica alta e intermedia (de acuerdo con la norma técnica de construcción sismo resistente nsr-10), es necesario desarrollar e implementar, entre otros, sistemas que permitan la estimación teórica de las aceleraciones del suelo generadas por un sismo, así como la definición de los escenarios probables con el fin de ofrecer información oportuna a los organismos de emergencia y entes tomadores de decisiones acerca de qué áreas pudieron sufrir daños (relacionados con la intensidad). la distribución de la intensidad o de la severidad de la sacudida es la que provee información útil para dichos organismos. a partir del 2011, el servicio geológico colombiano ha venido implementando y desarrollando una de estas herramientas, los mapas de intensidad instrumental (basados en la metodología del servicio geológico de los estados unidos (usgs)) los cuales muestran el movimiento del terreno expresado en niveles de intensidad sísmica (mmi, etc.) para sismos registrados por la red sismológica y red acelerográfica. dichos mapas combinan información sísmica en tiempo real, relaciones de atenuación, información sobre condiciones locales sísmicas, factores de amplificación, etc. la información sísmica (localización, magnitud y parámetros de movimiento fuerte del suelo), es generada por el sistema earthworm, el cual adquiere y procesa en tiempo real la información de los registros instrumentales; para la atenuación sísmica se consideran las ecuaciones empleadas en la actualización del mapa nacional de amenaza sísmica de colombia; las condiciones sísmicas locales se obtienen mediante la estimación regional de velocidades de onda de corte a 30 m de profundidad (vs30) y, los factores de amplificación son los adoptados en la nsr-10 de acuerdo al tipo de suelo. la importancia de la implementación de estos mapas está no solo en suministrar información oportuna a las entidades del sistema nacional para la gestión del riesgo de desastres, sino por su utilidad para validar relaciones de atenuación, ajuste de modelos de velocidad y de amplificación, y relaciones aceleración-intensidad que aportan directamente al estudio de la amenaza sísmica del país además de ser un mecanismo de información para la comunidad científica, medios de comunicación y público en general. palabras clave: amenaza sísmica, aceleración, intensidad. mailto:mtovar@sgc.gov.co mailto:jeraso@sgc.gov.co mailto:marcila@sgc.gov.co earth sciences research journal 269 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics spsp session – statistics and physics of seismic processes conveners: vladimir smirnov (moscow state univ. russia) <vs60@phys.msu.ru> alexander caneva (uan colombia) <alexander.caneva@uan.edu.co> earth sciences research journal 270 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics magnitude of completeness for the seismic data catalogue of the national seismological network of colombia (rsnc), 1993-2014 vladimir smirnov1, maria potanina1, alexander caneva2, michelle barney. 1moscow state university, faculty of physics, moscow, russia. 2universidad antonio nariño, vcti, bogotá, colombia. vs60@phys.msu.ru , potanina@physics.msu.ru, alexander.caneva@uan.edu.co, mbarn3y@gmail.com abstract the results of the estimation of the magnitude of completeness mc for the seismic data catalogue provided by the national seismological network of colombia (rsnc), correspondingto the period 1993-2014 are presented. mc corresponds to the minimum value of the magnitude of seismic events, which are registered without interruption, in certain region, during an interval of time. the variations of the values of mcin time and with depth are analyzed. these results show that in time mc decreases from 2.8 in 1993 and reaches its minmum mc ~ |1.9 during 2010. after that we observe how mc increases up to 2.3 inthe final part of the study period. the variations of mc with the depth show a local maximum for depth ~ 40 km. this feature maybe associated to the fact that there is not enough seismic data corresponding to those depths, since in colombia the seismic activityprincipallyhas its manifestation in shallow seismicity (h ≤ 30 km) and deep seismicity (h ≥ 80 km), so, the seismicity in intermediate depths is not so relevant. anyway, in the end of the time of observation, the mc values,according to the depth,are lower in comparison with these values in the beginning of the time of observation. key words: seismic data catalogue, rsnc, completeness, colombia. earth sciences research journal 271 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis probabilístico de la sismicidad del valle de cauca y zonas aledañas, colombia diego f. gamboa-perdomo1, juan s. arbeláez trujillo1, elkin de j. salcedo-hurtado2 1universidad del valle, cali, colombia. 2grupo de investigación georiesgos observatorio sismológico y geofísico de la universidad del valle, cali, colombia. diegogamboap@gmail.com abstract en el presente trabajo se estudia la sismicidad del departamento del valle del cauca y zonas aledañas tomando como base el catálogo sísmico de la red sismológica nacional de colombia (rsnc) para el periodo entre 1993 y 2013; se considera solo la sismicidad registrada en el continente debido al mejor control de la solución epicentral por la escasez de estaciones sismológicas en la zona oceánica. se presenta un análisis univariado de los sismicidad en la zona, determinándose la magnitud mínima representativa por medio del método de magnitud de rango entero, siendo ésta de 2,3 ml; para verificar la completitud del catálogo se utilizó el método de stepp que se basa en la distribución poisson. se analiza el comportamiento espacial de los sismos usando la técnica de patrones puntuales, donde se verifica que los sismos no siguen un proceso de poisson homogéneo; para realizar el análisis probabilístico la sismicidad se discrimina en superficial e intermedia, por medio de gráficos probabilísticos se identifica que la distribución que mejor modela los sismos ocurridos es la exponencial, llegando a la conclusión que la ventana de tiempo es un poco corta y el espacio geográfico o cuadrante es demasiado grande para realizar un análisis sísmico más certero respecto a la recurrencia y ocurrencia de los pequeños y grandes sismos, esta conclusión se presenta debido a que el catálogo sísmico presenta problemas de completitud y homogeneidad para magnitudes mayores o iguales a 5,9 ml. para el caso de los sismos superficiales se detalla que la probabilidad de que se presente un sismo de cualquier magnitud es aproximadamente del 100% dentro de 7 años, para el caso de los sismos intermedios la probabilidad de que suceda un sismo de cualquier magnitud es aproximadamente del 100% dentro de 5 años. a pesar de que la región del valle del cauca está catalogada como una zona de amenaza sísmica alta, se muestra que no se presentan muchos sismos fuertes o destructivos. por último, se realizó el cálculo de los parámetros a y b de la ley de gutenberg y richter obteniéndose que para el caso de la sismicidad intermedia estos parámetros disminuyen simultáneamente en el tiempo y para la sismicidad superficial su variación no sigue un patrón determinado. palabrasclave: sismicidad, método stepp, proceso poisson, distribución exponencial, método de magnitud de rango entero, patrones puntuales. earth sciences research journal 272 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 273 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 274 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earthquake catalogue processing and declustering issues ganesh w. rathod1, k. seshagiri rao2 and kaushal kumar gupta2 1council for geoscience, pretoria, south africa 2indian institute of technology delhi, new delhi, india grathod@geoscience.org.za abstract the compilation of a complete database of earthquake events is useful in understanding the recurrent behavior of major earthquakes and helps to visualize the future events and their outcome. earthquake catalogues are one of the most important products of seismology. they provide a comprehensive database useful for numerous studies related to seismotectonics, seismicity, earthquake physics, and hazard analysis. a critical issue to be addressed before any scientific analysis is to assess the quality, consistency and homogeneity of the data. a complete catalogue processed for foreshocks and aftershocks which is free from errors, plays a crucial role in assessing the recurrence relationship of the region. a complete and consistent catalogue of earthquakes can provide good data for studying the distribution of earthquakes in a region as a function of space, time and magnitude. a catalogue of events following the poissonian distribution provides strength to seismic hazard analysis. however, most catalogues do not report the magnitude of earthquakes consistently over time, in addition to varying uncertainties in hypocenter locations. this may pose as an obstacle for delineating seismicity patterns or for assessing seismic hazards. it is important to convert the original magnitudes based on various scales in different time periods to a common magnitude scale throughout the whole period. declustering attempts to separate the timeindependent part of seismicity (background) from the time-dependent or clustered parts (aftershocks, foreshocks, and swarm type activity). for most hazard related studies, it is required that the seismicity behaves in a time independent fashion. it can be investigated whether or not the temporal distribution of events within the raw catalogue is stationary poissonian, which would argue that declustering may not be necessary. there are many declustering algorithms available which were developed for specific seismotectonic regions. a decision on declustering algorithm to be used plays a crucial role in declustering the data. magnitude range for the catalogue is of same importance. the completeness period and b value are also important and helps in assessing the quality of compiled data. there are many issues related to the processing of earthquake catalogue. here an attempt has been made to discuss some of those issues with the help of procedures carried out on an earthquake database from north india. key words: earthquake catalogue, declustering, magnitude, completeness period, poissonian process. earth sciences research journal 275 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics ¿qué produjo los sismos ocurridos al oriente de bogotá el 10 de octubre de 1743 y el 24 de mayo de 2008? chicangana, g.1, 2, vargas, c. a.2, muñoz, b, f. j.2, pedraza, p.3, kammer, a.2 y salcedo, e.4 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. 2grupo geofísica, departamento de geociencias, universidad nacional de colombia. 3red sismológica nacional de colombia, servicio geológico colombiano. 4observatorio sismológico del suroccidente, universidad del valle, cali, colombia. german.chicangana@unimeta.edu.co abstract el 24 de mayo de 2008 se presentó al oriente de bogotá un sismo con ml = 5.5 y profundidad focal de 4 km de acuerdo a la red sismológica nacional de colombia (rsnc) y mw = 5.9 y profundidad focal de 35 km conforme la red mundial (neic). la solución del mecanismo focal indicado por el proyecto global cmt de la universidad de harvard fue lateral derecha con componente normal, presentando el plano principal un azimut de 196°, buzamiento de 82° y estría de falla con -179°, y el plano secundario un azimut de 106°, buzamiento de 89° y la estría de falla – 8°. las dos redes coinciden con un margen de error de solo 7 km en el epicentro, el cual fue en el páramo de chingaza, sin embargo para el foco, la diferencia fue de más de 30 km en la vertical y +/10 km en la horizontal entre la rsnc y la red mundial. para esta última se estima un error en la horizontal de +/4,9 km fijando el foco en profundidad por su programa de localización. por las diferencias iniciales dadas con la localización hipocentral y por su mecanismo focal, en este trabajó se buscó revisar y caracterizar el esquema tectónico que se presenta en este sector de la cordillera oriental de colombia tanto en superficie como en profundidad. en esta región la corteza superior (con un espesor máximo de 10 km) se constituye de rocas metamórficas de bajo grado y rocas sedimentarias que abarcan en edad geológica el lapso neoproterozoico presente. estas rocas están afectadas por plegamientos, fallas de cabalgamiento y fallas normales que derivan de un régimen extensional producido por un efecto gravitacional principalmente. debajo de esta, se presenta una corteza frágil (con un espesor promedio de 20 km) que se compone principalmente de un basamento mesoproterozoico relacionado con la orogenia grenviliana que está constituido por rocas plutónicas y metamórficas de alto grado en el que predominan fallas normales y fallas de cabalgamiento con dimensión cortical. la activación de estas fallas durante el neógeno superior dio origen a la conformación orogénica de este flanco de la cordillera. en la divisoria de aguas, las elevaciones llegan a alturas de cerca de 4000 m. estas últimas configuran el denominado “domo de chingaza” y hacía el se estos escarpes disminuyen en altura hasta 1000 m definiendo el piedemonte llanero. con la ayuda de imágenes satelitales, mapas topográficos, verificación de campo y tomografía sísmica, se ha encontrado indicios de la presencia una gran falla profunda la cual presenta un rumbo ene, que define un prisma cortical que controla las máximas alturas de esta región hacía el oriente de bogotá y que delimita el domo de chingaza. la movilidad y el constante ajuste de la acomodación de este prisma cortical frente al campo de esfuerzos presente en esta región han originado en esta, los sismos de 1743, 1923, 1966, 1988 y 2008. earth sciences research journal 276 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics palabrasclave: sismo de quetame, falla servitá, piedemonte llanero, geotectónica, sismotectónica. earth sciences research journal 277 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics modeling seismic sources in colombian central west using inversion of telesismic body waveforms hugo monsalve jaramillo, william valencia mina, leonardo cano saldaña grupo quimbaya universidad del quindío. hugom@uniquindio.edu.co abstract source parameters for four earthquakes that originated within the benioff zone of nazca plate subducting the south american plate near colombia were determined. the first earthquake took place on 2nd february 1995, mw=6.3, h=75.5 km, mo=3.6*1025 dyne-cm, strike=137.3º, dip=38°, and it generated an aproximate rupture plane size of 20x15 km2; the second earthquake on 19th august 1995, mw=6.4, h=115.0 km, mo=5.06*1025 dyne-cm, strike=67.9º, dip=72.7º, rake=-73.5º, and it generated an aproximate rupture plane size of 25x20 km2; the third earthquake on 2nd september 1997, mw=6.7, h=215.5 km, mo=8.04*1025 dyne-cm and it generated an aproximate rupture plane size of 30x25 km2; and the fourth earthquake on 11th december 1997, mw=6.3, h=189.5 km, mo=2.42*1025 dyne-cm, and it generated an aproximate rupture plane size of 15x10 km2. the first two earthquakes originated in the intermediate benioff zone, while the other two originated in the deep benioff zone. the source parameters were obtained by deconvolving multiple events through teleseismic analysis of body waves recorded in long period station. simultaneous inversion of p and sh waves is performed. rupture is modeled as a line source propagating in two directions at velocities of 3km/s. the time function of these events showed different stages which could be interpreted in the inversion as either three or two subevents; the aspect ratio w/l varied within 0.73 and 0.83. key words: source parameters, waveform inversion, teleseismic waves, subduction zone. earth sciences research journal 278 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics depth profile of the fractal dimension and the seismotectonic deformation rates associated to the earthquakes distribution juan m. salazar,carlos a. vargas universidad nacional de colombia, bogota, colombia. jmsalazarr@unal.edu.co, cavargasj@unal.edu.co abstract we have analyzed a seismological catalog of approximated 161035 seismic events and 2300 focal mechanisms in depths ranging between 0 and 200 km. this catalog was compiled by the international seismological centre, and includes aprox. 580 stations in the period 1977-2014. dataallowed calculate the spatial variations (dependence with depth) of the seismic regime (fractal dimension of the earthquakes distribution d) and the seismotectonic deformation rates in a tectonically complexarea located in the sw caribbean. variations of the fractal properties of seismicity and the seismotectonic deformation rates were observed along depth, suggesting a stratified rheology and/or a differentialbehavior of the lithospheric system. key words: fractal dimension, seismotectonic deformation rates, seismic regime. earth sciences research journal 279 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earthquake source parameters which display first digit phenomenon patricio a. toledo1, sebastian r. riquelme2, jaime a. campos1 1uch. 2csn. patoledo@ing.uchile.cl abstract we study main parameters of earthquakes from the perspective of the first digit phenomenon: the nonuniform probability of the lower first digit different from zero compared to the higher ones. we found that source parameters like cosesimic slip distributions at the fault and coseismic inland displacements show first digit anomaly. we also found the tsunami run-ups measured after the earthquake to display the phenomenon. other parameters found to obey first digit anomaly are related to the aftershocks: we show that seismic moment liberation and seismic waiting times also display an anomaly. we explain this finding by invoking a self-organized criticality frame. we show that critically organized automata show the first digit signature and we interpret this as a possible explanation of the behavior of the studied parameters of the tohoku earthquake. key words: seismic source, first digit phenomenon. earth sciences research journal 280 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismicity response to the electromagnetic probing of the lithosphere in the bishkek (kyrgyzstan) geodynamic test area v.b. smirnov1,2, a.d. zavyalov2 1institute of physics of the earth, ras, moscow and faculty of physics, moscow state university, russia, vs60@mail.ru 2institute of physics of the earth, ras, moscow, russia, zavyalov@ifz.ru abstract earthquake catalogue of the bishkek (kyrgyzstan) geodynamic test area for 1967-2008 was analyzed with aim to study the properties of seismicity response to the power electromagnetic probing of the lithosphere in 1983-1990 and 2000-2005. time variations of statistical parameters usually used for investigation of background and transient modes of seismicity were analyzed in the 150-km vicinity of the probing dipole. analysis is carried out on the two levels of time scales: first years – before, during and after probing series, and first days – during 10 days after each acts of probing. main results obtained on the scale of first days are as follows: directly during probing gr b-value is significantly higher than in background mode; gradual decreasing of b-value follows after probing finishing has duration about one and half day; seismic activity slightly increases during probing; after probing, when b-value decreases, activity is minimal. analysis on the scale of first years reveals time variations of seismicity parameters, but there is no evidence to relate these variations with probing series. revealed variations are typical signatures of preparation and after-action of tectonic earthquakes occurring in the studied area. key words: seismicity response, b-value, electromagnetic probing, kyrgyzstan. mailto:vs60@mail.ru mailto:zavyalov@ifz.ru earth sciences research journal 281 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics distribución espacial y temporal de los terremotos destructivos y sus réplicas, del abril de 2014, cerca de managua, nicaragua tenorio v.1, arguello g.1, espósito o.2, sánchez j.1,strauch w.1 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2centro nacional de investigaciones sísmicas (cenais), santiago de cuba. wilfried.strauch@yahoo.com abstract el jueves, 10 de abril de 2014, a las 17:23 horas, ineter registró un terremoto de magnitud 6.2 en el lago de managua, con epicentro cerca de la isla momotombito, a unos 30 km de managua. el evento fue sentido en todo el país, muy fuerte en los departamentos managua y león. la afectación fue amplia con intensidades mmi máximas de v a vii. hubo muchos daños – grietas, caída del techo, inclusive casas colapsadas en casas de mala construcción (adobe, taquezal) en las ciudades y poblados en o cerca de las riberas occidentales de lago como nagarote, mateare, la paz centro puerto momotombo. la magnitud 6.3 (ineter) o 6.1 (usgs) es casi idéntica a la del terremoto que destruyó managua en 1972 (usgs 6.2). el evento figura entre los más fuertes en la historia en nicaragua, sin contar los terremotos extremos en el océano pacífico. solo la ubicación del epicentro en el lago de managua evitó un gran desastre. afortunadamente, no hubo víctimas humanas directamente por el terremoto. el número total de sismos de esta actividad, en abril de 2014, fue superior a 64,000; de estos, 642 pudieron ser localizados. las primeras réplicas se concentraron cerca del terremoto principal pero pronto ocurrieron en el volcán momotombo, a unos 40 km de managua. después de pocas horas se dieron sismos en el volcán apoyeque, a solo 8 km de managua. la migración de la actividad sísmica es un fenómeno anteriormente observado en nicaragua. el presidente de nicaragua declaró, a medianoche del 10 de abril, “alerta roja” para toda nicaragua. las autoridades advirtieron que managua podría ser afectada por mayores sismos. la sismicidad podría ser premonitora de un terremoto destructivo en managua. se recomendó a las familias que durmieran afuera si sus casas eran de mala construcción. el 13 de abril, ocurrió un terremoto de magnitud 5.7 en el volcán apoyeque. muchas casas en managua, en ciudad sandino y los brasiles sufrieron daños. desde el inicio de la actividad, las autoridades revisaron casas afectadas, escuelas, colegios, edificios públicos, en cuanto a su estado después del impacto de los terremotos y a la vulnerabilidad relacionada a un posible terremoto en managua. con gran velocidad, se realizaron reparaciones y mejoras de infraestructura, también se eliminaron 49 ruinas remanentes del terremoto de 1972 que todavía fueron habitadas por familias pobres a las cuales se asignaron otras viviendas. además, se mejoró la capacidad del ineter para procesar automáticamente los sismos fuertes en la zona de managua. el temor de un fuerte terremoto en managua en el futuro próximo es presente, las medidas impulsadas por la última actividad sísmica ayudaron a estar mejor preparado para esta eventualidad. palabras clave: terremoto, réplicas, managua, nicaragua. mailto:wilfried.strauch@yahoo.com earth sciences research journal 282 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. terremoto del 10 de abril, 2010 y sus réplicas, localizaciones del ineter. tiempo en hora local (ut-6). earth sciences research journal 283 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics mud volcano seismo-geodynamics oktay babazade, nigyar babazade centre of seismology and earth's physics azerbaijan bnigyar@hotmail.com abstract according to author’s results, the mud volcanism is sensitive enough to changes of the stressed state of the earth’s crust. seismicity and mud volcanism are in the definite spatial-time (chronologically) ratio. moreover the mud volcanoes eruption and their connections with earthquakes located within or near areas of the mud volcanoes localization are divided into accompanying seismic events forestalling them and events occurring after the earthquake. due to our investigations it has been determined that the strong seismic events can influence on the stress area condition at the considerable distance. the conclusion is important for earthquakes prediction on the practicability of the vast geophysical control and usage of the mud volcanic to forecast the dynamic processes in the focus zones of strong preparing earthquakes. different scale migration manifestations of mud volcanic activity dynamics in processes of main earthquake preparation with magnitude 5.0 – 9.0 were invented. some remarkable migration of mud volcanic eruptions prior to the main earthquakes are described based on the systematic analysis of the national catalogue of the active mud volcanoes in the period of 18102004 in and around the south caspian depression including the region of east central iran. the discovery of pattern of migration of the mud volcanic activity before the main earthquake can be considered as new precursory phenomena. there was used a methodology of finding of two types of migrations of mud volcanic eruptions by reconstruction of the earthquake preparing pattern. there are provided the detailed results of revealing the ordered migration of the mud volcanic eruption by reconstructions of the preparatory phase of earthquakes of november 25, 2000 (m=5.8; 6.3) and december 6, 2000 (m=7.3), having respectively, arisen in a few kilometers to the southwest and north-west of baku in caspian sea and to the west turkmenistan. these absheron mud volcanic eruptions were triggered by m 9.0 great earthquake preparation patterns that occurred on december 26, 2004 of the west coast of northern sumatra, indonesia. the relationship between caspian sea level oscillations and the seismic dynamics the largest magnitude 6.0 to 8.0 and greater, earthquakes worldwide in the sumatraandaman main event. these phenomena are explained and substantiated by the proposed conception of the upper crustal sedimentary-stage degree of the seismogeodynamic manifestation. they are an integral part of the modern evolution of fluidmagmatic systems of the volcanoes. keywords: absheron, volcanoes. earth sciences research journal 284 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics szpr – subduction zone processes (joint ssa/lacsc session) conveners: susan beck (arizona univ. usa) <slbeck@email.arizona.edu> marino protti (ovsicori-una costa rica) <marino.protti@gmail.com> christa von hillebrandt (noaa usa) <christa.vonh @noaa.gov> susan newman (ssa) <snewman@seismosoc.org> earth sciences research journal 285 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic velocity reduction following the september 5, 2012, mw=7.6 nicoya costa rica earthquake from ambient noise correlations esteban j. chaves.& susan y. schwartz university of california santa cruz. echavess@ucsc.edu abstract the unique location of the nicoya peninsula, directly over the interface where the cocos plate is subducting beneath the caribbean plate, resulted in excellent broadband seismic and cgps coverage of the 2012 nicoya earthquake rupture area, making it one of the best recorded megathrust earthquakes ever. this comprehensive instrument coverage makes it possible to use high-resolution data (seismic and cgps) to study the pre-seismic, co-seismic and postseismic processes related to this large earthquake. in the present work we apply the cross-correlation of ambient seismic noise technique to reconstruct dispersive rayleigh waves (empirical green’s functions) between station pairs distributed along the nicoya peninsula, costa rica, to characterize any changes in the velocity structure associated with the september 5, 2012, mw=7.6 nicoya peninsula earthquake. our results show a clear velocity reduction of 0.4% immediately following the earthquake with a gradual post seismic recovery. these earthquake related velocity variations are most clearly visible in the period range of 1-10s with no appreciable change at longer periods. because the cross-correlation function is primarily composed of surface waves, the depth sensitivity of the velocity change is a function of signal period. in the period range of 1-10s the surface waves are sensitive to the upper most crust (hundreds of meters to 10 km). our results average velocity variations between all station pairs and we are presently investigating both regional variations and the frequency dependence of the coseismic velocity reduction. these preliminary observations of temporal variations in rayleigh wave velocities are likely due to a combination of: (a) coseismic damage in the shallow layers due to nonlinear site effects; (b) co-seismic stress changes; and (c) post-seismic stress relaxation or healing processes. keywords: seismic ambient noise, velocity structure. earth sciences research journal 286 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics the causative relationship between the megathrust and upper plate faults at subduction zones gabriel gonzález1, pablo salazar1, jack loveless2, felipe aron3, juan gonzález1 and richard allmendinger3 1universidad católica del norte, antofagasta, chile fondap-cigiden research centre. 2smith college, northampton, massachusetts, usa. 3cornell university, ithaca, new york. ggonzale@ucn.cl abstract the last three great subduction earthquakes on earth have demonstrated that slip activity at subduction zones seems to be related in two ways with the upper plate fault activity. the 2010 mw 8.8 maule earthquake and the 2011 mw 9.1 tohoku-oki earthquake generated aftershocks in the upper plate compatible with the reactivation of normal faults. on the other hand the recently 2014 mw 8.2 pisagua earthquake was preceded by several foreshocks; the first of which we suspect was located in a trench-orthogonal reverse fault of the upperplate. in this contribution we show several argumentations (relocation, moment tensor orientation and stress change analysis) to postulate that the pisagua earthquake was effectively initiated in an upper plate event and then seismic instability was propagated to the interplate contact triggering the mw 8.2 pisagua earthquake. although upper plate earthquakes caused by subduction earthquakes are well identified, the process in the other direction has not been previously described. key words: subduction zone, upper plate earthquakes, megathrust, foreshocks, aftershocks. earth sciences research journal 287 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics mind the gap – the northern chile subduction zone and the 2014 iquique earthquake sequence gavin p. hayes1, william d. barnhart2, matthew w. herman3, kevin p. furlong4, sebástian riquelme5, harley m. benz2, eric bergman6, sergio barrientos7, paul s. earle2 1national earthquake information center, united states geological survey, golden, co. 2usgs neic. 3department of geosciences, pennsylvania state university, university park, pa. 4psu. 5centro sismológico nacional, universidad de chile. 6global seismological services, golden, co. 7csn. ghayes@usgs.gov abstract the seismic gap theory, which identifies regions of elevated hazard based on a lack of recent seismicity in comparison to adjacent sections of a fault, is useful for qualitatively describing where future large earthquakes might occur. the hypothesis has successfully explained past events including the m 8.8 maule, chile earthquake in 2010. an earthquake of a similar size had been expected in the subduction zone adjacent to northern chile, which until recently had not ruptured in a megathrust earthquake since a m~8.8 event in 1877. on april 1 2014, a m 8.2 earthquake occurred within this seismic gap, offshore of the city of iquique; the size and spatial extent of the rupture indicate it was not the earthquake that had been anticipated. here, we present a rapid assessment of the seismotectonics of the march-april 2014 seismic sequence offshore northern chile, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations, and cumulative coulomb stress transfer calculations over the duration of the sequence. this ensemble of information allows us to place the current sequence within the context of historic seismicity in the region, and to assess areas of remaining and/or elevated hazard. our results indicate that while accumulated strain has been released for a portion of the northern chile seismic gap, large amounts of seismic moment are still stored along this portion of the subduction zone. in particular, a significant section of the seismic gap immediately to the south of the march-april 2014 sequence remains unruptured since 1877, and could host an event as large as m ~ 8.7. earthquakes to date have increased the stresses in the northernmost portion of this remaining gap, elevating the short-term hazard of earthquakes in the subduction zone immediately south of iquique, which could rupture as far as ~200 km south toward antofagasta. keywords: iquique, subduction zones, seismic gap, finite fault modeling, moment tensor inversion, earthquake relocations, stress modeling. earth sciences research journal 288 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics spatio-temporal evolution of the 2014 pisagua, chile earthquake sequence hans agurto-detzel1, hugo soto2, andrés tassara2. 1iag-usp, brazil. 2u. de concepción, chile. h.agurto.detzel@gmail.com abstract on the evening of the 1st of april 2014, a mw=8.2 tsunamigenic megathrust earthquake struck the coast of north chile, causing 6 fatalities, vast material damages, and prompting the preventive evacuation of around one million people along the coastal urban areas of north and central chile. this earthquake, known afterwards as the 2014 pisagua earthquake, was preceded by a sequence of foreshocks that lasted about 2 weeks and nucleated at the center of a wellknown seismic gap existent since 1877, when an m~8.5 earthquake shook northern chile, producing a large tsunami across the pacific. in this work, we benefit from the hypocentral locations provided by the chilean national seismological center (csn) and the well-known neic-pde catalog to characterize the spatiotemporal evolution of this seismic sequence. additionally, we analyze the catalogs completeness magnitude (mc) and b-value variations in the area, comprising the background and foreshock activity previous to the earthquake and approximately 1.5 months of its aftershocks. early precursors started in late 2013/early 2014. the main sequence begun in mid-march 2014 with an mw=6.7 foreshock about 60 km nw of the city of iquique. the seismicity then slowly migrated northwards surrounding the future epicentral region where the 1st of april earthquake nucleated. hundreds of aftershocks followed the mainshock defining a rupture area mainly southwards and deeper into the seismogenic interface. the extension on this rupture area only partially covers the total extension of the pre-existent seismic gap. the largest aftershock was an mw=7.7 event with epicenter around 50 km sw of iquique, after which the seismicity started to decrease. we found that the mc of the csn catalog decreased from 3.5 before 2010 to around 2.5 the year before the pisagua earthquake, mostly as a consequence of the densification of the local seismic network in northern chile. for the aftershock period, mc rises to 3.2 likely due to the increase of seismicity and short time-span considered. the neic-pde catalog presents a quite stable mc around 4.3 for the 10 years period before the mainshock, rising to 4.7 during the aftershocks. given the stability of mc for both catalogues and the relative large amount of events with magnitude larger than mc, we studied temporal variations of the frequency-magnitude relationship observing a notable decrease (from 0.6 to 0.4) of b-value since mid-2013 (suggesting preseismic stressing of the megathrust as observed for others large earthquakes) and large fluctuations accompanying the foreshock sequence (perhaps related to rapid changes in pore fluid pressure). our study of the foreshock and aftershock periods shed some light on the current state of the north chile seismic gap and the potential predictability of great megathrust earthquakes such as the 2014 pisagua earthquake. key words: seismic gap, pisagua, chile, subduction zones, megathrust earthquakes, b-value, aftershocks. earth sciences research journal 289 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 290 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics grijalva ridge, a major player in the subduction game underneath ecuador hugo yepes1,2, laurence audin1, alexandra alvarado2, céline beauval1, jorge aguilar2 1isterre, ird-cnrs-ujf-osug, grenoble, france. 2instituto geofisico, quito, ecuador. hyepes@igepn.edu.ec abstract there has been a long debate on the role carnegie ridge’s arrival and subduction has in ecuador’s geodynamics. some authors favor the idea that the buoyancy of the subducted part of carnegie would explain a torn in the undergoing plate and would be responsible for a very gentle dipping of nazca plate underneath ecuador (gutscher 1999). other authors (e.g. guiller et al 2001, michaud et al. 2009) have contradicted the flat slab hypothesis and have explained intermediate depth seismicity distribution or coupling based on the presence of carnegie ridge. this ongoing debate has distracted attention from another prominent geomorphic feature located immediately south of carnegie ridge: grijalva ridge. grijalva ridge is the morphological evidence of a paleo-rift that separates the old (>12 myr old) farallon subducted plate to the south from the new (7 myr old) nazca plate to the north (lonsdale 2005). grijalva ridge inland prolongation –grip explains several observations made in the course of preparing the seismotectonic sources for seismic hazard assessment in ecuador. high energy intermediate depth seismicity (ids) is located almost exclusively south of grip and is related to the tearing of farallon along its lines-of growth fabric. at least six m≥7 earthquakes have been localized along a sse trending, 300 km long, narrow zone south of grip in the last 80 years. rotation of fault plane strikes obtained from ids normal faulting focal mechanisms south of grip strongly suggests a contortion in the plate. at ~150 km depth it plunges abruptly in a northeasterly direction. the conspicuous ids seismic cluster known as el puyo clusterresults from stress concentration in the maximum contorted zone. to the north of grip no m>6 ids has been reported and locally recorded microseismicity is restricted to maximum depths of around 120km located below ecuador’s foreland. the ecuadorian volcanic arc on top of the andean chain attests to the presence of a hot slab further east of the seismic evidence, but also favors the idea of an along strike continuity of the dipping slab to the south as stated by michaud et al (2009). on the surface, directly above the el puyo cluster, the pastaza megafan is occupying the continental foredeep resulting from the viscoelastic adjustment of the lithosphere above the plunging tip of farallon slab. more relations between seismicity, coupling and active faulting with grip and the related plates will have to be confirmed and may reveal the major role that the two plate’s differences may play in the geodynamics of this part of south america. keywords: grijalva rige, carnegie rige, farrallon plate, nazca plate, el puyo cluster. earth sciences research journal 291 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics distribution analysis of the frequency–magnitude parameters along the subduction surface structure in south america marily triviño abella, carlos alberto vargas jiménez universidad nacional de colombia. mtrivinoa@unal.edu.co abstract a catalogue of approach 22,470 seismic events registered in central and south america between 1900 and 2014 and obtained from the u.s. geological survey was analyzed in terms of frequency-magnitude distribution parameters. accepting the assumptions of the gutenberg – richter law, and taking into account the minimum magnitude of homogenous catalog, we estimated the b-value parameter of the subduction structure along a regular grid (0.5° x 0.5°) and for profiles defined previously, which allow to know a distribution of recurrence time and annual probability of earthquakes. we have correlated this spatial mapping with other subduction parameters (angle, dominant source parameters, geophysical response of several variables, etc.) in order to evaluate the critical conditions of the fracture regime. contrast values of these parameters suggest a large lateral variation of the subduction structure in south america and centro america. we hypothesize that this contrasting conditions are response of the complex regime of convergence and may support an alternative regionalization of the seismic hazard at west of the region. key words: subduction, seismic hazard, geophysical variables, seismic events. earth sciences research journal 292 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics mapping patchiness of the subduction interface in the near field: the case of the southern terminus of the middle american trench, costa rica marino protti1, susan schwartz2, andrew newman3, víctor gonzález1, tim dixon4, lujia feng5, esteban chaves1,2, hernán porras1 1ovsicori-una. 2university of california at santa cruz. 3georgia tech. 4university of south florida in tampa. 5earth observatory of singapore. marino.protti@gmail.com abstract mapping heterogeneities along plate interfaces in subduction zones is becoming possible as more densely-spaced seismic and gps arrays are being installed worldwide. large patches of nearly 100% coupling, or the simultaneous failure of several smaller strong patches, generate great earthquakes. these great earthquakes are uncommon (every hundreds of years), but the failure of medium-size, strongly coupled patches is very common (every tens of years) worldwide. mapping and tracking the loading and failure evolution of these medium-size patches could lead to more precise assessments of size, location and timing of future earthquakes. in costa rica, thanks to the existence of three peninsulas, nicoya, osa and burica, which extend close to the trench and over the seismogenic zone of large and frequent thrust interplate earthquakes, it has been possible to initiate an international effort to map heterogeneities on the subduction interface. we will be presenting results from the mapping of a) patch that failed with the 1990 mw=7.0 earthquake at the entrance of the nicoya gulf; b) a successful experiment in the nicoya peninsula with the record in the near field of a mw=7.6 earthquake, and c) we will share the design of a geodynamic control network that are being installed on and around osa and burica peninsulas, to capture and map the rupture area, also in the near field, of a mw=7.2-7.4 earthquake that will likely occur in the next 10 to 15 years. key words: subduction, costa rica, middle american trench. earth sciences research journal 293 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics reverse polarity repeating events are associated with boudin formation at intermediate-depths in the bucaramanga nest sarah a barrett1, germán a. prieto2 1stanford university. 2massachusetts institute of technology. sabarrett@stanford.edu abstract the bucaramanga nest seismicity is associated with slablet subduction at intermediate depths. this region in northern colombia produces the densest concentration of seismicity in the world. we detect ~18 events per day in this nest using a regional network in colombia. of these events we observe two populations of waveforms: each with highly correlating waveforms (repeating events) and with reverse polarity to the other group. we define these reverse polarity repeating earthquakes using a divisive clustering algorithm, which divides the repeating waveforms into two groups. these events are relocated first using hypodd and then using the associated errors to apply feature extraction algorithm. this secondary relocation step searches for possible locations within the realm of uncertainty, favoring an outcome of simple structures. this result illuminates two distinct, roughly linear features – one associated with each repeating group. we interpret these features to be related to boudin formation (boudinage) associated with the extension of the slab at depth. we also examine the recurrence interval of these events and observe a clustering of the two groups in time and space. this cascading failure pattern provides further evidence for a thermal-shear failure mechanism. keywords: repeating earthquakes, intermediatedepth, bucaramanga, thermal shear failure, cluster analysis. earth sciences research journal 294 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics 3d thermal modeling associated with subduction of the philippine sea plate in southwest japan shoichi yoshioka1, yingfeng ji1, takumi matsumoto2 1rcuss, kobe university. 2nied. yoshioka@port.kobe-u.ac.jp abstract constructing a parallelepiped three-dimensional thermal convection model, we investigated temperature, mantle flow and heat flow distributions associated with subduction of the philippine sea (phs) plate in southwest japan. we proposed a new, realistic, and high-resolution temperature field on the plate interface, and attempted to clarify its relationships with the occurrences of megathrust earthquakes, long-term slow slip events (l-sses), and low-frequency earthquakes (lfes). for this purpose, we newly developed a numerical model to deal with subduction of an oceanic plate with 3d arbitrary geometry. we modeled subduction of the phs plate by using the up-to-date three-dimensional slab geometry, referring to high resolution p-wave seismic tomography and seismic reflection studies. we also used large number of heat flow data such as bsrs, borehole, heat probe, and hi-net to constrain calculated temperature field, and took account of complicated subduction history in southwest japan. the results showed that temperature change due to erosion and sedimentation affected surface heat flow with short wavelength. we also found that the obtained interplate temperature in the nankai seismogenic zone was wider than that in the tonankai seismogenic zone. the existence of large temperature gradients from the surface to the inside of the phs plate was considered to be related to the occurrence of l-sses beneath the bungo channel. temperature where lfes occurred near the plate interface beneath shikoku was found to be lower by approximately 100°c than that beneath the kii peninsula. keywords: temperature, heat flow, 3d thermal modeling, southwest japan. earth sciences research journal 295 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismological view of nazca-south american convergent margin between 10° and 20°s susan l beck1, george zandt1, lara wagner2, maureen long3, hernando tavera4, estella minya5, kevin ward1, alissa scire1, jamie ryan1, brandon bishop1, sanja knezevic antonijevic2, abhash kumar2, berk biryol2, caroline eakin3, cristobal condori4 1university of arizona, us. 2university of north carolina, us. 3yale university, us. 4igp, lima, peru. 5san calixto observatory, la paz, bolivia. slbeck@email.arizona.edu abstract the central andean convergent margin in south america is characterized by along-strike variations in magmatism, crustal thickness, and slab geometry that make it an ideal region to study the relationship between the subducting slab, the mantle wedge, and the overriding plate. the north central andes between 10° and 20°s is characterized by a change in the geometry of the subducting slab from normal (~30° dip) subduction in the south to near horizontal subduction in the north. we deployed ~85 broadband seismometers in this region (caught and pulse seismic deployments) to image the earth beneath the array to study the lithospheric scale structures of high plateaus, mechanisms of surface uplift, and slab geometry. on the eastern margin of the central andean plateau (cap) both receiver functions and ambient noise tomography (ant) studies reveal crust ~40 km thick at the eastern edge of the active fold and thrust belt. the crustal thickness beneath the eastern cordillera is variable and ranges from ~55 km east of la paz to ~70 km further south near potosi, bolivia. the thinner crust under the eastern cordillera corresponds to a region of high rates of exhumation and may be due, in part, to erosiondriven exhumation. the crust thickens to ~60-70 km thick crust below the altiplano and western cordillera. the average seismic velocity of the crust beneath the relatively flat-lying altiplano is remarkably low, consistent with a bulk crustal composition on the felsic-end of the compositional spectrum. even within this relatively slow-velocity crust, the ant shows an extensive mid-crustal (~15-20 km) low shear-wave body along the western margin of the cap that we interpret as a zone of neogene to modern plutons. the emplacement of such a large volume of plutons in the past ~10-20 ma may play a role in the uplift of the western margin of the cap. we used finite-frequency teleseismic p-wave tomography to image velocity anomalies in the mantle from 60 700 km. sks-splitting measurements are used to map mantle anisotropy and infer mantle deformation and flow patterns. seismic images of the lithospheric scale structure of the altiplano suggest a very heterogeneous upper mantle. keywords: convergent margin, lithospheric structure. earth sciences research journal 296 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics tshr session – monitoring, early warning, and evaluation of tsunami hazard and risk convener: wilfried strauch (cepredenac) <wilfried.strauch@yahoo.com> earth sciences research journal 297 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics caribbean and adjacent regions tsunami warning system: seismic component christa von hillebrandt-andrade, hector crespo, caribbean tsunami warning program, noaa nws, mayagüez, pr, united states. jean marie saurel, ipgp observatoire volcanologique et sismologique de martinique. dan mcnamara, usgs geologic hazards science center. victor huerfano, puerto rico seismic network, uprm. working group 1 unesco ioc intergovernmental coordination group for the tsunami and other coastal hazards warning system for the caribbean and adjacent regions (caribe ews). christa.vonh@noaa.gov abstract over 75 tsunamis have been documented in the caribbean and adjacent regions during the past 500 years. since 1500, at least 4484 people are reported to have perished in these killer waves. hundreds of thousands are currently threatened along the caribbean coastlines. in 2005 the intergovernmental coordination group for the tsunami and other coastal hazards warning system for the caribbean and adjacent regions (caribe ews) was established. it recommended the following minimum seismic performance standards for the detection and analysis of earthquakes: 1) earthquake detection within 1 minute, 2) minimum magnitude threshold = m4.5, and 3) initial hypocenter error of m4.0) can be detected within 1 minute throughout much of the caribbean. the remaining exceptions to this standard for detection are portions of northern south america and mexico. another performance criterion is 90% data availability. currently 60-70% of the stations meet this standard. these data permit the pacific tsunami warning center and the us national tsunami warning center (alaska), responsible for providing tsunami services to the caribbean, to rapidly notify the tsunami warning focal points in the 48 countries and territories that make up the caribe ews. these seismic data are also provided too many local, national and subregional centers, to monitor earthquakes for their area of responsibility. the unesco ioc intergovernmental coordination group also encourages all the networks to submit their data to iris to potentiate research of tsunami sources. these seismic data are also complemented by dozens of gps and sea level data. the presentation will further report on the status of the caribe ews capability for the timely and accurate detection and analysis of earthquakes for tsunami warning purposes for the caribbean and adjacent regions. keywords: tsunamis, seismic monitoring, caribbean. earth sciences research journal 298 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 299 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics local tsunamigenic earthquakes off northeastern venezuela, in the southern caribbean realm franck a. audemard m. and alejandra f. leal g. earth sciences dpt. venezuelan foundation for seismological research –funvisis-, caracas, venezuela. faudemard@funvisis.gob.ve and aleal@funvisis.gob.ve abstract the 1983 national inventory of geologic hazards and the 1999 catalog of felt/destructive venezuelan earthquakes (1530-1998) report reliably the occurrence of tsunami waves on the caribbean eastern venezuelan coasts, or phenomena that might be interpreted as substantial sea level modifications in the region, during 5 local earthquakes only. we have confirmed this through the search and evaluation of the accounts by primary sources (eye witnesses) of the tsunami inundation during the 5 events. such tsunamigenic events are the 1-ix-1530, 15-vii-1853, 29-x-1900, 17-i-1929 and 9-vii-1997 earthquakes. all but the 1900 shock affected the cumaná city, and the offshore right-lateral strike-slip el pilar fault has been accounted for. the 1900 ad tsunami waves were reported along most of the ensenada de barcelona coast (w of cumaná) and los roques archipelago, being this quake attributed to the san sebastián fault segment running offshore cabo codera. the 1530 and 1853 quakes were produced by the cariaco trough segment of the el pilar fault, within a restricted over-1000-m-deep marine pull-apart basin on the san sebastián-el pilar releasing step-over, whereas the 1929 and 1997 events occurred on the cariaco gulf segment. several authors have interpreted all four events as the result of major submarine sliding inside the steep-walled trough. first-hand accounts by locals about the abnormal waves during the cariaco 1997 event, as well as the identification of coastal sliding at the manzanares river mouth at cumaná, support this thesis at least for the two latest events, because of the small size of the tsunami-affected area. in addition, recent monitoring (cariaco project) has observed turbidite currents in the cariaco trough and the manzanares canyon during the cariaco mw 6.9 quake and the smaller mw. 5.2 august 2008 event. however, the 1900 tsunami, and the 1530 and 1853 tsunamis by extension, appears to result from right-lateral tectonic slip along the cariaco through walls. key words: tsunami, local earthquakes, historical seismicity, caribbean. earth sciences research journal 300 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics método numérico para desplazamientos de la superficie terrestre debido a fuentes sismogénicas gustavo a. lara1, gabriel álvarez1, gabriel gonzález2, juan gonzález2. 1universidad de antofagasta. 2universidad católica del norte. gustavo.lara@uantof.cl abstract en la evaluación de riesgos de tsunamis de fuente cercana, es necesaria una estimación del movimiento del suelo marino producido por el sismo, constituyéndose en la información de entrada para la simulación de la propagación y llegada de la perturbación a las costas. la mayoría de los software de simulación utilizan el levantamiento del fondo marino a partir de formulaciones teóricas válidas para una superficie plana, sin embargo, en algunas regiones la topografía del fondo marino se desvía de esta idealización, en una magnitud suficientemente grandes como para reexaminar el método de cálculo del desplazamiento de este fondo marino. en este trabajo se muestra un procedimiento y cálculos numéricos basados en el método de elementos de frontera (bem) aplicado a un medio homogéneo e isotrópico, donde la superficie de frontera sigue la topografía del fondo marino. la comparación entre la formulación válida para superficie plana y los resultados numéricos para una topografía real correspondiente a l norte de chile muestra que las diferencias son suficientemente importantes como para considerarlas en la condición inicial de una simulación de tsunami en la región. palabrasclave: tsunamis simulación numérica. earth sciences research journal 301 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis de modelos intersísmicos para estimación de inundación por tsunami gonzález-carrasco, j.1,2, r. aranguiz2,3, j.c. dominguez2,4, r. cienfuegos2,4, p. catalán2,5& g. gonzález1,2 1universidad católica del norte (ucn), chile. 2centro nacional de investigación para la gestión integrada de desastres naturales (cigiden), chile. 3universidad católica de la santísima concepción (ucsc), concepción, chile. 4pontificia universidad católica (puc), santiago, chile. 5universidad técnica federico santa maría (utfsm), valparaíso, chile. jgonzal@alumnos.ucn.cl abstract los mapas de inundación por tsunami no sólo facilitan la confeccionar de planes de evacuación de la población, sino también ayudan a la planificación territorial y estimación de daños en estructuras. sin embargo, la precisión en la estimación de la inundación por tsunami depende en gran medida de la condición inicial del tsunami, la que a su vez se obtiene a partir parámetros sísmicos de la zona de ruptura. una práctica común es usar simplemente modelos de ruptura homogéneos basados en eventos históricos y la formulación de okada. sin embargo, el evento tsunamigénico de maule de 2010 demostró que la distribución de los desplazamientos es heterogénea, distribuida en parches, con un alto grado de correlación con el déficit de desplazamiento intersísmico propuesto para la brecha. el presente trabajo tiene como objetivo principal estudiar la factibilidad de usar modelos intersísmicos no homogéneos para definir la condición inicial del tsunami y así estimar posibles daños por eventos futuros. para ello se analizó el tsunami de 2010 y su comportamiento en talcahuano al interior de la bahía de concepción. un escenario sísmico predictivo se definió a partir del modelo propuesto por moreno et al., 2011, realizado a partir de mediciones geodésicas de la tasa de acoplamiento intersísmico, que es capaz de entregar una visión bastante fina del déficit de desplazamiento para la brecha sísmica, además fueron analizados escenarios cosísmicos realizado a partir de diversos tipos de mediciones, tales como ondas telesísmicas (sladen y hayes), inversión de ondas de tsunami (lorito), datos geodésicos para la deformación postsísmica (vigny y moreno). adicionalmente se utilizó un modelo sísmico homogéneo definido a partir de la magnitud del evento sísmico. para las modelaciones numéricas se utilizó el modelo neowave con 4 mallas anidadas de 120, 30, 6 y 1” de arco de resolución. las condiciones iniciales del tsunami se definieron a partir modelos de fallas finitias y superposición mediante la formulación de okada. se simularon 6 horas y se definieron mareógrafos virtuales en talcahuano y penco para analizar las series de tiempo. los resultados muestran que todos los modelos sísmicos generan tsunamis con tiempos de arribo y periodos de ondas similares, sin embargo, las amplitudes máximas presentan diferencias. los modelos cosísmicos de lorito y usgs son los que mejor se ajustan a los registros instrumentales y visuales. además, el modelo homogéneo sobreestima la primera onda, mientras que la segunda onda es subestimada. adicionalmente, se observó que el modelo intersísmico analizado concuerda bien con los datos observados, tanto en tiempos de arribo como en amplitudes máximas, al igual que los modelos cosísmicos antes mencionados. por lo tanto, es posible concluir que la condición inicial de un tsunami generado a partir de un modelo intersísmico basado en déficit de desplazamiento puede ser usado para realizar estudios de inundación por tsunami. earth sciences research journal 302 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics palabrasclave: tsunami, numerical model, interseismic coupling model. earth sciences research journal 303 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics assessment of heterogeneous slip distribution impact on tsunamigenic hydrodynamic processes along the northern chilean coast gonzález-carrasco, j.1,2, n. zamora-sauma3, c. núñez4, a.y. babeyko3& g. gonzález2,5 1universidad católica del norte (ucn), chile. 2centro nacional de investigación para la gestión integrada de desastres naturales (cigiden), chile. 3geoforschungszentrum (gfz), potsdam, germany. 4servicio hidrográfico y oceanográfico de la armada (shoa), chile. 5departamento de ciencias geológicas, universidad católica del norte (ucn), chile. jgonzal@alumnos.ucn.cl abstract the chilean continental margin lies in the western margin of the south american plate. the interaction between the nazca and south america plates has a high tsunamigenic potential. historically, chilean coastal cities has been affected by the strongest earthquakes and tsunamis recorded in the world. the location of the mega-thrust close to the land has raised the need to improve early warning systems. typically fast tsunami threat assessment in early warning systems is based on homogeneous slip distribution models related with the historical earthquake scenarios. actually, one of the main aspects in numerical tsunami modeling is the use of finite fault rupture models where slip distribution is provided with more realistic and precise seismic representations. however, in recent years it has been tested that slip distribution has an essential contribution to tsunamigenic hydrodynamic processes such as wave period propagation, arrival times, inundation and run-up pattern along the coast. in this work we have a twofold aim, on one side to test the implications of slip distribution on the tsunamigenic hydrodynamic processes along the arica, iquique and patache coastal zones resulted from the 2014 pisagua earthquake, and on the other side compare the use of tsunami numerical codes based on linear, and non-linear/non-hydrostatic hydrodynamic equations to assess the above mentioned datasets. the analysis has been possible due of the availability of high resolved finite fault model for the 2014 pisagua earthquake (hayes, 2014), the recorded tsunami wave in arica, iquique and patache tsunamigrams and the high resolution coastal bathymetry in the study region in northern chile. the results provide a better insight of the inference of physical characteristics of the source, the morphological characteristics of seafloor and coastline geometry on the wave period and frequency as well as on the run-up distribution and coastal resonance effects. key words: tsunami, resonance, numerical model. earth sciences research journal 304 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics fast epicentral distance and magnitude determination using a single three component seismic station with machine learning techniques luis h. ochoa, carlos a. vargas,luis f. niño universidad nacional de colombia. lhochoag@unal.edu.co abstract to minimize adverse effects originated by high magnitude earthquakes, early warning has become a powerful tool to anticipate a seismic wave arrival to an specific location and lets to bring people and government agencies opportune information to initiate a fast response. to do this, a very fast and accurate characterization of the event must be done but this process is often made using seismograms recorded in at least 4 stations where processing time is usually greater than the wave travel time to the interest area, mainly in coarse networks. a faster process can be done if only one three component seismic station is used that is the closest unsaturated station respect to the epicenter. here we present a support vector regression algorithm which calculates magnitude and epicentral distance using only 5 seconds of signal since p wave onset. this algorithm was trained with 36 records of historical earthquakes where the input were regression parameters of an exponential function estimated by least squares, corresponding to the waveform envelope and the maximum value of the observed waveform for each component in one single station. a 10 fold cross validation was applied for a normalized polynomial kernel obtaining the mean absolute error for different exponents and complexity parameters. magnitude could be estimated with 0.16 of mean absolute error and the distance with an error of 7.5 km for distances within 60 to 120 km. this kind of algorithm is easy to implement in hardware and can be used directly in the field station to make possible the broadcast of estimations of this values to generate fast decisions at seismological control centers, increasing the possibility to have an effective reaction. key words: early warning, support vector regression, earthquake, rapid response. earth sciences research journal 305 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 306 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics a rapid estimation of tsunami run-up based on finite fault models sebastian riquelme universidad de chile sebastian@dgf.uchile.cl abstract many efforts have been made to estimate the maximum run-up height of tsunamis associated with large earthquakes. this is a difficult task, because of the time it takes to construct a tsunami model using real time data from the source. usually, it is possible to construct a database of potential seismic sources and their corresponding tsunami a priori. however, such models are generally based on uniform slip distributions and thus oversimplify our knowledge of the earthquake source. instead, we can use finite fault models of earthquakes to give a more accurate prediction of the tsunami run-up. here we show how to accurately predict tsunami run-up from any seismic source model using an analytic solution found by fuentes et al, 2013 that was especially calculated for zones with a very well defined strike, i.e., chile, japan, alaska, etc. the main idea of this work is to produce a tool for emergency response, trading off accuracy for quickness. our solutions for three large earthquakes are promising. here we compute models of the runup for the 2010 mw 8.8 maule earthquake, the 2011 mw 9.0 tohoku earthquake, and the recent 2014 mw 8.2 iquique earthquake. our maximum run-up predictions are consistent with measurements made inland after each event, with a peak of 15 to 20 m for maule, 40 m for tohoku, and 2,1 m for the iquique earthquake. considering recent advances made in the analysis of real time gps data and the ability to rapidly resolve the finiteness of a large earthquake close to existing gps networks, it will be possible in the near future to perform these calculations within the first five minutes after the occurrence of any such event. such calculations will thus provide more accurate run-up information than is otherwise available from existing uniform-slip seismic source databases. key words: tsunami, earthquake, run-up, finite fault. earth sciences research journal 307 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics mapas de amenaza de tsunami para el golfo de fonseca (el salvador, honduras y nicaragua) norwin acosta1, wilfried strauch1, jennifer larreynaga2, juan josé reyes3 1ineter, nicaragua. 2marn, el salvador. 3copeco, honduras. wilfried.strauch@yahoo.com abstract las costas del golfo de fonseca, una extensa bahía del océano pacífico compartida por elsalvador, honduras y nicaragua, se encuentran bajo amenaza de tsunamis originados porterremotos en el océano pacífico, por erupciones volcánicas en los volcanes cosigüina,conchagua y otros centros volcánicos locales y por deslizamientos y colapsos laterales de los volcanes. para el golfo existen registros históricos e instrumentales de afectación por tsunami. en 2010, se realizaron mapas de amenaza de tsunami, para 12 sitios particulares, 4 islas enteras, 3 zonas nacionales, y para todo el golfo. en el proceso de la elaboración de los mapas participaron especialistas de las instituciones científicas, comisiones de emergencias, de las autoridades locales de el salvador, honduras y nicaragua. los mapas se discutieron también con pobladores locales en los diferentes sitios. durante este trabajo se observó que la gran mayoría de la población en las ciudades y poblados en la costa del golfo de fonseca no tiene conciencia que se encuentran bajo riesgo de tsunami. por eso, durante el proceso se dieron múltiples entrevistas a los medios locales de comunicación masiva (tv, radio, prensa). de esta forma se logró informar a una parte considerable de la población sobre la amenaza de tsunami existente para el golfo de fonseca y sobre las posibles medias de prevención de desastres por este fenómeno. para facilitar el entendimiento del fenómeno de tsunami y su posible efecto en el golfo de fonseca se elaboraron simulaciones numéricas de generación y propagación de olas de tsunami en el golfo de fonseca y de la afectación en las playas. el presente trabajo se entiende como inicio de un desarrollo dirigido a la reducción del riesgo por tsunami en el golfo de fonseca. para la continuación de este proceso se requiere de medidas adicionales como 1) información, capacitación y educación a la población incluyendo prácticas de evacuación; 2) desarrollo de un sistema local de alerta de tsunami (tri-nacional); 3) investigación científica del posible impacto de tsunami; 4) elaboración de mapas de tsunami a mayor precisión; 4) medidas físicas de protección para aquellas zonas donde la evacuación en caso de posible afectación por tsunami es muy difícil. el trabajo tuvo el apoyo del proyecto sobre georiesgos en centroamérica, bgr/alemania. palabras claves: tsunami, amenaza, mapas, golfo de fonseca, nicaragua, honduras, el salvador. mailto:wilfried.strauch@yahoo.com earth sciences research journal 308 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sistemas de alerta temprana (sat) en panamá wilfried strauch observatorio sísmico del occidente de panamá (osop). wilfried@osop.com.pa abstract en la prevención y mitigación de riesgos naturales, panamá como país tiene dificultades por atraso metodológico y falta de potencial en los sistemas de alerta temprana (sat). el observatorio sísmico del occidente de panamá (osop, http://www.osop.com.pa) se creó para ofrecer servicios, equipos, métodos y conocimientos científicos. con financiamiento de la secretaría nacional de ciencia y tecnología (senacyt) se desarrolla, desde noviembre de 2011, un proyecto de 3 años que ayuda en diseñar y crear sat a establecerse en las instituciones nacionales responsables (o apoyar a sat ya existentes) para monitoreo y alerta temprana de los fenómenos naturales peligrosos. etapa 1 del proyecto consistió en gran medida en trabajos analíticos basándose en publicaciones, reportes y visitas a instituciones de alerta tempana en panamá, en la región centroamericana y a nivel internacional. para conocer la realidad en panamá, se realizaron reuniones con el sistema nacional de protección civil (sinaproc), universidad de panamá (upa), autoridad del canal de panamá (acp) y otras instituciones. la segunda etapa, recientemente terminada, se concentró en elaborar, juntos con las instituciones responsables, propuestas detalladas y en el comienzo de su implementación de los siguientes sat: sistema nacional de monitoreo sísmico sistema local de alerta de tsunami para el golfo de chiriquí (incluye la elaboración de un mapa de amenaza de tsunami para puerto armuelles) sistema de alerta de correntadas, inundaciones y actividad sismo-volcánica en el volcán barú sistema piloto de alerta sísmica para la ciudad de david de gran importancia es la elaboración de un conjunto de protocolos que definen exactamente las responsabilidades de las diferentes instituciones y personas que participan en los sat. además el desarrollo de un sistema de mensajes que se enviarán de forma rutinaria o en las emergencias a sinaproc, otras instituciones relevantes y al público en general. la formulación de los mensajes debe ser clara, fácilmente entendible, concreta y adaptada al grupo de destinatarios. en el envío de los mensajes se prevén métodos automáticos, y mensajes enviados convencionalmente por correo electrónico y sms. además se entrega información gráfica y alertas acústicas directamente en pantallas de información. debe existir un sistema de supervisión que verifica continuamente si el sat funciona, si existe un registro los mensajes enviados, elabora estadísticas de mensajes de alertas acertadas y falsas para su posterior análisis. se aplican las experiencias que se hicieron recientemente en panamá con la instalación de nuevos sistemas de monitoreo sísmico automático. una tercera etapa del proyecto se dedicará, a partir de 2014, en la puesta en función de los sistemas, la realización de pruebas y ejercicios con las instituciones y la población. este trabajo es apoyado por senacyt contrato por mérito no. -4-cap11-001 palabras claves: alerta temprana, amenazas, sismos, tsunamis, inundaciones, volcanes, mapas. mailto:wilfried@osop.com.pa earth sciences research journal 309 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics experiencias con el terremoto lento y tsunami del 26 de agosto 2012 en nicaragua y el salvador virginia tenorio1, norwin acosta1, francisco gavidia2, jennifer larreynaga2, wilfried strauch3 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2ministerio del ambiente y recursos naturales (marn), san salvador, el salvador. 3centro de coordinación para la prevención de los desastres naturales en américa central (cepredenac), guatemala, guatemala. wilfried.strauch@yahoo.com abstract el 26 de agosto de 2012, a las 10:37 pm (hora local), las redes sísmicas de nicaragua y el salvador detectaron un sismo de baja profundidad con magnitud de 5.7 en el océano pacífico frente a sus costas. el sismo fue sentido leve en ambos países. no obstante, a las 22:51 pm entró un mensaje de información del ptwc señalando una magnitud de 7.4 para el evento, posteriormente ptwc recomendó alerta de tsunami para centroamérica. la magnitud 7.4, sobrepasó por mucho el umbral a partir del cual los sistemas nacionales de alerta de tsunami debían declarar la alerta. un tsunami impactaría en las costas dentro de unos 40 minutos después del sismo. no obstante, por el efecto macrosísmico leve del evento y con magnitudes significativamente más bajas determinadas por los sismólogos nacionales las instituciones de protección civil vacilaron en declarar la alerta. a nivel nacional no se pudo explicar la diferencia entre las magnitudes locales y las de las agencias internacionales o se demoró mucho en hacerlo. en el salvador, la posibilidad de un tsunami fue negada en el momento por las autoridades de protección civil. eso también porque la estación mareográfica en acajutla, el salvador, a unos 100 km del epicentro, indicaba una ola insignificante de pocos centímetros. en nicaragua, se declaró una “alerta interna”, es decir se alertó a estructuras de protección civil y autoridades locales en la costa pero no se emitió un llamado público a la población de tomar medidas de protección. posteriormente, se determinó que el terremoto, del 26 de agosto 2012, era un “terremoto lento”. este tipo de terremoto ya ha causado, en 1992, un destructivo tsunami en nicaragua con olas de hasta 10 m de altura y 170 víctimas humanas. días después del evento, surgieron noticias en ambos países sobre el impacto de tsunami confirmadas poco después por visitas del campo de expertos de coi e itic juntos con especialistas nacionales. se supo que decenas de personas fueron afectadas por olas con una altura máxima de 5 metros, por inundaciones y correntadas. la baja densidad de población en las zonas afectadas evitó un mayor desastre. a raíz del tsunami, del 26 de agosto, 2012, los sistemas nacionales de alerta de tsunami en nicaragua y el salvador revisaron sus procedimientos y sistemas técnicos para la alerta de tsunami y las mejoraron significativamente. el personal de las instituciones sismológicas y de protección civil debe conocer las características de los terremotos lentos. las redes sísmicas de los sistemas nacionales de alerta de tsunami deben disponer de métodos confiables para determinar correctamente la magnitud de terremotos lentos. palabras clave: terremoto lento, tsunami, nicaragua, el salvador. mailto:wilfried.strauch@yahoo.com earth sciences research journal 310 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. se observan las magnitudes reportadas por neic para el terremoto del 28 de agosto de 2012. la magnitudes de ondas p, l usadas típicamente por agencias locales son mucho menores que las magnitudes obtenidas a largas distancias usando ondas lr de bajas frecuencias earth sciences research journal 311 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics el desarrollo del sistema de alerta de tsunami en nicaragua strauch w.1, talavera e.1, muñoz a.1, tenorio v. 1, izaguirre a.2 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2sistema nacional para la prevención, mitigación, y atención de desastres (sinapred), managua, nicaragua. wilfried.strauch@yahoo.com abstract en 1992, un tsunami afectó la costa del pacífico de nicaragua con olas de hasta 10 metros de altura, devastó las playas y cobró la vida de más de 170 personas. el país no contaba con un sistema de alerta, no existía una red sísmica, tampoco hubo conciencia sobre el peligro de tsunami en población, gobierno e instituciones científicas. este evento creó una gran conciencia sobre la temática de tsunami e impulsó un desarrollo de largo plazo en que se formó el sistema de alerta de tsunami de nicaragua. el ineter se concentró en establecer la red sísmica y el centro de monitoreo y alerta. hoy, el sistema de monitoreo sísmico es uno de los más desarrolladas en centroamérica con unas 60 estaciones digitales. además, se reciben por internet los datos de unas 200 estaciones de centroamérica y de todo el mundo para la información rápida sobre cualquier sismo tsunami generador en el océano pacifico y en el caribe. el procesamiento automático con los paquetes seiscomp3 (gfz/potsdam) y earthworm es supervisado por sismólogos del turno 24x7. hipocentros y magnitudes de sismos mayores en nicaragua y centroamérica se obtienen en menos de un minuto, soluciones de eventos fuera de la región pueden tardar algunos minutos más. resultados se envían automáticamente por medio de correo electrónico y sms a las autoridades del gobierno y sinapred y se publican en el sitio web. el centro de operaciones de emergencias (code, 24x7) de sinapred y defensa civil dispone de clientes de seiscomp3 que visualizan en tiempo real el procesamiento y los resultados de ineter. así, el code siempre está al tanto de la situación y obtiene el resultado del procesamiento o de relocalizaciones, inmediatamente. terremotos de magnitud 7 o mayor en nicaragua o centroamérica se consideran potencialmente tsunami génicos y se propone inmediatamente la alerta de tsunami. con el visto bueno de la presidencia de nicaragua, el code activa las sirenas para las playas bajo peligro de tsunami. nicaragua dispone de 100 sirenas automáticas instaladas en la costa del pacífico y del caribe. inmediatamente, se inician las operaciones previamente establecidas del sistema de prevención de desastres en las zonas posiblemente afectadas para apoyar a la población. ineter ha participado en estudios sobre la amenaza de tsunami e centroamérica, y ha elaborado mapas de amenaza de tsunami para toda la costa del pacífico que detallan las zonas bajo riesgo, vías de evacuación, zonas seguras. sinapred y defensa civil han realizado el trabajo de concientización, capacitación y entrenamiento para población, organizaciones populares, instituciones del estado, policía, cruz roja, y otros. se elaboraron protocolos que regulan la cooperación de las instituciones del país. en múltiples simulacros a nivel local, nacional o internacional población e instituciones fueron informadas y capacitadas de actuar correctamente en caso de tsunami. palabras claves: sistema de alerta, tsunami, nicaragua. earth sciences research journal 312 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics vmst – sharing experiences on volcano monitoring using seismic techniques in latin america conveners: mario ruiz (epn ecuador) <mruiz@igepn.edu.ec> diego gomez (colombian geological survey) <dgomez@sgc.gov.co> javier pacheco-alvarado (ovsicori-una costa rica) <javier.pacheco.alvarado@una.cr> john makario londoño (colombian geological survey) <jmakario@sgc.gov.co> randy white (usgs usa) <rwhite@usgs.gov> earth sciences research journal 313 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics implementación y renovación de nuevas tecnicas geofísicas para monitoreo de volcanes activos colombianos alpala, j., oliveras, n., ospina, c., meza, l., alpala, r. servicio geológico colombiano, observatorio vulcanológico y sismológico de popayán jalpala@sgc.gov.co, noliveras@sgc.gov.co, cospina@sgc.gov.co, lmeza@sgc.gov.co, ralpala@sgc.gov.co abstract durante la última década, las redes de monitoreo volcánico en colombia han crecido exponencialmente debido a la incursión de nuevas técnicas, cuya finalidad en conjunto con la sismología es, dar un diagnóstico integral del estado de actividad de un volcán y generar procesos de investigación que conlleven a conocer y comprender mejor los diferentes sucesos que ocurren en un volcán activo antes, durante y después de un proceso eruptivo. las redes de monitoreo se han visto reforzadas con equipos de última tecnología y de amplio uso a nivel mundial, entre estas se puede mencionar los siguientes: estaciones gnss permanentes e imágenes insar (para monitoreo de la deformación volcánica); cámaras web y procesamiento digital de imágenes de sensores remotos (para seguimiento de la actividad superficial y emisiones de gases); sensores acústicos (para el monitoreo de explosiones volcánicas y/o salida de gases); equipos de mediciones móviles y permanentes de gases difusos en suelo y fumarolas (so2, co2, 222rn) y equipos de seguimiento de temperaturas en campos fumarólicos y fuentes termales (para el monitoreo de parámetros fisicoquímicos); sensores magnetotelúricos como potenciales eléctricos y magnetómetros (para el monitoreo de campos magnéticos y gravitacionales); sensores sismológicos tipo banda ancha (para reforzar el monitoreo sismológico tradicional de corto periodo y capturar la sismicidad de baja frecuencia); sensores de flujos de lodo (para monitoreo de lahares); sensores meteorológicos (para control de cambios en el comportamiento volcánico debido a agentes externos), entre otros. el hecho de la implementación de nuevas metodologías y con ello la instalación de nuevas estaciones, ha hecho que también se generen nuevas arquitecturas y metodologías en la adquisición, transmisión, recolección y procesamiento de las señales; de estas se puede destacar la migración a la transmisión telemétrica digital y creación de redes telemétricas redundantes. los observatorios vulcanológicos y sismológicos del servicio geológico colombiano, con la implementación de métodos de monitoreo volcánico de última tecnología, se encuentran a la vanguardia con los observatorios del cinturón de fuego del pacifico, lo cual a futuro permitirá mayor investigación e implementación de nuevos métodos para el análisis del fenómeno volcánico, así como el diagnóstico integral y acertado de nuevos procesos eruptivos. palabras clave: volcán, monitoreo, técnica, geofísica. mailto:jalpala@sgc.gov.co mailto:noliveras@sgc.gov.co mailto:cospina@sgc.gov.co mailto:lmeza@sgc.gov.co earth sciences research journal 314 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad volcano-tectónica en el nevado del huila alpala, r. y sánchez, j. 1ervicio geológico colombiano, observatorio vulcanológico y sismológico de popayán ralpala@sgc.gov.co, jsanchezv@sgc.gov.co abstract el volcán nevado del huila (vnh) es un complejo volcánico activo de tipo estrato volcán, conformado por varios domos. el observatorio vulcanológico y sismológico de popayán, tiene a su cargo la vigilancia y monitoreo permanente de este volcán desde 1994. tiempo en el cual se ha podido establecer que es fundamental determinar cambios importantes en la actividad sísmica asociada al fracturamiento de roca (volcano-tectónica o vt) para el análisis y pronóstico de los procesos eruptivos del vnh. en los inicios de la vigilancia, el vnh presentaba bajos niveles de actividad sísmica, con un promedio de tres sismos diarios. sin embargo el 18 de abril de 2007 el vnh inició su proceso de reactivación, con el registro de eventos tipo vt, localizados en la parte alta del edificio volcánico; seguidamente el sistema entró en un proceso de excitación continua, con la generación de eventos vt acompañados de eventos asociados al tránsito de fluidos y gases. el 19 de febrero de 2007, en menos de 24 horas del primer incremento sísmico, se presentó una erupción freática, a la cual se le asociaron la formación de una fisura de 2 km de largo en la parte alta del edificio volcánico, entre los picos central y la cresta en dirección ns y la generación de un lahar. durante los dos meses siguientes, el vnh presentó un periodo de relajación parcial del sistema. no obstante, la noche del 17 de abril, se presentó un nuevo enjambre sísmico, mostrando un comportamiento similar a la primera erupción. 10 horas después del inicio del enjambre, se presentó la segunda erupción freática, la cual generó una nueva fisura de 2.3 km de largo que atravesó los costados sw y ne del pico central, pérdida de una porción de masa del glaciar el oso y la formación de un nuevo lahar de mayor volumen al anterior. posteriormente, el vnh experimentó períodos de alternancia entre fases de relajación y etapas de mayor excitación. la última erupción del vnh registrada se presentó en el 2008, tiempo durante el cual se presentaron episodios de emisiones de ceniza, intensa actividad relacionada con la dinámica de fluidos y enjambres sísmicos de tipo vt. finalmente, el 20 de noviembre de 2008, se presentó una erupción freatomagmática, que generó la formación de un lahar y extrusión de un nuevo domo lávico en el costado suroccidental del pico central, sobre el cual se generó una nueva intrusión de material magmático en el segundo semestre de 2009, este proceso de emplazamiento se mantuvo hasta septiembre de 2010, fecha en la cual el vnh retornó a su proceso de relajación, destacándose únicamente el incremento de la actividad de tipo vt presentada entre el 9 y 12 de enero de 2014, sismicidad que se localizó hacia el costado ne del volcán, y mantuvo magnitudes importantes, dentro de las cuales se destacan dos eventos con magnitudes de 4.5 y 5.0 en la escala de richter. actualmente, el vnh se encuentra en nivel amarillo (iii), mostrando un comportamiento estable en su actividad. palabras clave: volcán, huila, volcano-tectónica, erupción. mailto:ralpala@sgc.gov.co earth sciences research journal 315 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics enjambres sísmicos periódicos relacionados con la transferencia estática de esfuerzos entre una deformación volcánica de gran escala y fallas strike-slip corticales: complejo volcánico laguna del maule (chile) carlos cardona1,2,andrés tassara2,fernando gil cruz1 1observatorio volcanológico de los andes del sur, sernageomin. rudecindo ortega 03850, temuco, chile. 2universidad de concepción, departamento de ciencias geológicas, chile. carlos.cardona@sernageomin.cl abstract el complejo volcánico laguna del maule (cvlm, 36.06°s-70.50°w) está localizado 200 km al e del epicentro del terremoto mw=8.8 del maule 2010 sobre la cordillera de los andes chilena. esta, es una excepcional zona volcánica caracterizada por un extensivo volcanismo silícico postglacial, con más de 130 centros de emisión y aproximadamente 350 km3 de productos eruptados, sugiriendo un particular vulcanismo riolítico distribuido, y una de las más altas tasa de deformación vertical (alzamientos de aprox. 30 cm/año según insar) registrados en el planeta. a partir del año 2011 debido a los síntomas de actividad reportados, el observatorio volcanológico de los andes del sur (ovdas-sernageomin) implementó una red de vigilancia, compuesta por 17 estaciones sísmicas de banda ancha (12 permanentes y 5 portátiles), y 5 estaciones gnss. los datos obtenidos mostraron que el cvlm es uno de los sistemas volcánicos de los andes chilenos con mayor actividad reciente, siendo registrado una serie de enjambres sísmicos, con una periodicidad de aproximadamente dos meses, relacionados con rompimientos de roca (tipo vt), localizados al sw de la cuenca volcánica a profundidades superficiales (< 4 km). según se aprecia en la figura 1, el área epicentral de los enjambres coincide con el límite sw de la zona fuente de la deformación modelada por los estudios de insar, el trazo de la falla troncoso de carácter regional, y la ubicación de uno de los centros de emisión holocénicos riolíticos reconocidos. de igual modo, los datos recolectados por las estaciones de gsnn continuas, confirman las tasas de deformación reportadas previamente, mostrando un comportamiento incremental con el tiempo, con tasas de alzamiento calculadas que van desde los 20 – 30 cm/año. con el fin de indagar sobre la relación existente entre la fuente de deformación y la sismicidad periódica de los enjambres, se determinó los esfuerzos tipo coulomb, asumiendo como estructura fuente un dique de ~5 km de ancho y ~12 km de largo, localizado a ~5 km de profundidad en el centro del sistema volcánico y una tasa de apertura de 0.9 m/año (según estudios de insar publicados) y como estructura receptora una falla tipo strike-slip de componente dextral que atraviesa el complejo (falla troncoso). los resultados sugieren que existe una transferencia efectiva de esfuerzos estáticos desde la fuente productora de la deformación hacia la zona de fallamiento, lo cual correlaciona temporal y espacialmente con la zona epicentral de los enjambres sísmicos registrados, dando nuevos indicios sobre los procesos que lideran el comportamiento y dinámica de complejos volcánicos riolíticos distribuidos. palabrasclave: enjambres sísmicos, deformación volcánica de gran escala, trasferencia de esfuerzos, volcanismo riolítico. earth sciences research journal 316 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics actividad reciente del complejo volcánico laguna del maule (chile): relación entre sismicidad y deformación volcánica de gran escala ligada a un campo volcánico riolítico carlos cardona1,2,andrés tassara2,luis lara1,sergio morales1 1observatorio volcanológico de los andes del sur, sernageomin. rudecindo ortega 03850, temuco, chile. 2universidad de concepción, departamento de ciencias geológicas, chile. carlos.cardona@sernageomin.cl abstract el complejo volcánico laguna del maule (cvlm, 36.06°s-70.50°w) es un campo volcánico con un extensivo volcanismo silícico postglacial, localizado en el límite oriental del arco volcánico moderno y a 200 km del epicentro del sismo mw=8.8 del maule. los primeros síntomas de actividad en el cvlm fueron detectados a través de estudios insar, cuyo modelamiento conllevo a sugerir una zona inflacionaria localizada a ~5 km de profundidad en el centro del complejo, con una tasa de desplazamiento vertical de ~30 cm/año, siendo catalogada como una de las deformaciones volcánicas más grandes a nivel mundial durante las últimas décadas. por este motivo en abril de 2011, el observatorio volcanológico de los andes del sur desplegó una red de vigilancia volcánica, compuesta por estaciones sísmicas de banda ancha y estaciones gnss continuas, con el propósito de monitorear en tiempo real ésta actividad. los registros obtenidos muestran sismicidad de carácter superficial. palabrasclave: enjambres sísmicos, deformación volcánica, cambios de atenuación. earth sciences research journal 317 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics comportamiento de la actividad sísmica y registro de procesos deformativos en los volcanes chiles y cerro negro, colombia (2013) arcos, d.f.1, narváez, l.1, battaglia, m.2 1servicio geológico colombiano, observatorio vulcanológico y sismológico de pasto. 2associate professor, sapienza university of rome (italy) – department of earth sciences. darcos@sgc.gov.co, lnarvaez@sgc.gov.co, mbattaglia@usgs.gov abstract los volcanes chiles y cerro negro se encuentran ubicados al suroeste de colombia, en el departamento de nariño, en la frontera entre colombia y ecuador, con una altura de 4748 msnm y 4470 msnm respectivamente y separados entre sí unos 3.5 km. a finales del 2013, y debido a los reportes de sismos sentidos por la comunidad del resguardo indígena de chiles, localizada a 10 km al oriente del edificio volcánico de chiles, en noviembre de 2013, se inició el monitoreo permanente de la actividad sísmica y de deformación de estos volcanes, con dos estaciones sobre la superficie de chiles y una sobre cerro negro (sismómetros de banda ancha, inclinómetros apllied geomechanics). hasta el 22 de mayo de 2014, se han registrado alrededor de 81.000 sismos asociados a fracturamiento de roca, llegando hasta 2.000 sismos por día, algunos de los cuales han sido reportados como sentidos; se cuenta con cerca de 60 eventos con magnitud mayor o igual a 3 en la escala de richter, con un valor máximo de 4.8. la mayoría de los sismos se ubican hacia el sector sur suroccidente de chiles, en inmediaciones de los dos volcanes, sin hasta el momento definir en particular de cual volcán es esta actividad. desde la instalación de los inclinómetros, se han observado cambios en sus componentes, siendo las mayores las que registra el inclinómetro instalado en el volcán cerro negro (figura 1). los dos inclinómetros instalados en el volcán chiles con menores variaciones, muestras tendencias que se asocian al comportamiento diario sísmico. asumiendo que la deformación observada obedece a una posible intrusión magmática se realizaron varios modelamientos, usando fuente esférica, esferoide y dique. debido a que la inclinación observada en las estaciones no muestra una simetría radial que permita trabajar una fuente esférica, se enfatizó en el modelo de fuente de dique, obteniendo como resultado un dique ubicado entre chiles y cerro negro, de aproximadamente 8.4 km de longitud a una profundidad a 1.6 km (figura 2). palabras clave: volcán, chiles-cerro negro, sismicidad, inclinómetro, deformación, modelamiento. mailto:darcos@sgc.gov.co mailto:lnarvaez@sgc.gov.co earth sciences research journal 318 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. componentes de inclinación norte y este (µrad) de los inclinómetros instalados en chiles-cerro negro, para el periodo comprendido entre noviembre de 2013 y mayo de 2014. earth sciences research journal 319 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 2. localización epicentral (imagen superior, en color naranja el dique), en profundidad y planta (imagen inferior), de la solución obtenida del modelo de fuente dique para los volcanes chiles y cerro negro, usando la información de los inclinómetro electrónicos. earth sciences research journal 320 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics cambios en la distribución temporal del parámetro b relacionados con intrusiones de magma, herramienta para pronóstico caso erupción del complejo volcánico cordón caulle, junio 04 2011 daniel basualto alarcón1,2, jonathan lazo gil1, carlos cardona idarraga2, andrés tassara oddo2, fernando gil cruz1 1observatorio volcanológico de los andes del sur (ovdas), servicio nacional de geología y minería (sernageomin), santiago, chile. 2departamento ciencias de la tierra, facultad de ciencias químicas, universidad de concepción, concepción, chile. daniel.basualto@sernageomin.cl abstract variaciones del b-value han sido atribuidas por diversos autores a cambios en el estrés regional, variaciones del estrés diferencial en fallas activas asociadas a zonas volcánicas o incrementos en la presión de fluidos relacionados con sistemas hidrotermales, vinculando dichas variaciones con incrementos en la actividad volcánica. el 02 de junio de 2011 y debido al aumento de la actividad sísmica, se elevó el nivel de alerta técnico-volcánica del complejo volcánico cordón caulle (cvcc) a naranja. un día más tarde y producto del ascenso en la actividad sísmica, la alerta volcánica fue cambiada a nivel rojo. el día 04 de junio, el cvcc inició una fase explosiva con un iev 4, lográndose pronosticar exitosamente la erupción con 24 horas de anticipación. aunque fue fundamental la clasificación de la actividad sísmica (figura1); la evolución espacio-temporal de los hipocentros y la “distribución temporal de la frecuencia de magnitudes” (fmd) fueron las metodologías utilizadas para determinar los diferentes estados de alerta técnico-volcánica, ya que esta última fue capaz de cuantificar la generación sísmica en términos de sus magnitudes, tanto espacial como temporalmente. para calcula el b-value finalmente se utilizó el ciclo eruptivo completo, seleccionando 1700 sismos registrados durante enero a diciembre de 2011. para construir la variación temporal de dicho parámetro, se trabajó con el método de la máxima probabilidad, escogiendo ventanas de 125 eventos con un overlap del 75% y una magnitud de completitud (mc) que actuó de forma fija 1,6 (figura2). con relación a los resultados obtenidos de las variaciones temporales del b-value, se pudo observar que 3 días antes de la erupción, el valor de b mostró un abrupto descenso, característica que predominó independiente de las ventanas sísmicas o de los overlap utilizados, sugiriendo que dicho cambio estuvo relacionado con la inestabilidad generada por un ascenso de magma previo a la erupción. el análisis de los espectrogramas para cada uno de estos sismos precursores mostraron una predominancia en bajas frecuencias. palabrasclave: distribución temporal del parámetro b, herramienta para pronóstico, actividad volcánica. earth sciences research journal 321 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 322 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 323 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sistema informático para el análisis y procesamiento de señales sísmicas de volcanes en ecuador e. viracucha1, j. de la bastida2 1escuela politécnica nacional, instituto geofísico quito. 2national tsing hua university taichung, republica de china. gviracucha@igepn.edu.ec abstract el sistema informático de análisis y procesamiento de señales sísmicas (sipass),actualmente constituye una herramienta de apoyo a los científicos y técnicos del instituto geofísico de la escuela politécnica nacional, en el análisis y procesamiento de las señales sísmicas de los volcanes. esta nueva herramienta informática, trabaja con archivos sísmicos en formato sac (código de análisis sísmico). permite la selección de estaciones sísmicas, donde el programa lee la información de los archivos y los guarda en variables temporales; con estas variables genera el directorio principal con la información de las estaciones seleccionadas. se selecciona un archivo del directorio principal para realizar el procesamiento de la señal sísmica. el sistema carga la información y el usuario visualiza la señal en pantalla de la estación o las estaciones que se hayan seleccionado, se debe activar el modo de señalamiento de tiempos de arribo, para que el programa le permita seleccionar dos o tres segmentos de la señal. dependiendo del número de segmentos seleccionados, se obtendrá de manera automática: fecha y hora, estación, s-p, coda, amplitud, periodo, frecuencia máxima, rms. el tipo de evento es determinado con el programa sipass, el cual utiliza la fft (transformada rápida de fourier), este algoritmo utiliza la información que genera la clase sac (código de análisis sísmico), el programa verifica que el número de puntos almacenados en el arreglo, sea potencia de base 2 y si no lo es, el programa completa con ceros; finalmente el sistema realiza los respectivos cálculos y presenta en pantalla el espectro de la señal. sipass, permite quitar el ruido de fondo de la señal sísmica. el programa utiliza tres tipos de filtrado, bandpass, lowpass y higpass. aplicando cualquiera de estos filtros, será posible observar con mucha claridad la señal sísmica. sipass utiliza una arquitectura 2 capas: clienteservidor; se desarrolló sobre la plataforma java standart edition (jse), bajo el entorno de desarrollo netbeans 7.1, que se ejecuta en el servidor de aplicaciones apache tomcat 6.0 y utiliza sql server 2008, como sistema de gestión de base de datos. la arquitectura cliente servidor es importante, ya que la aplicación está instalada en diferentes estaciones de trabajo, mientras que la base de datos se encuentra centralizada en un servidor. varios usuarios pueden acceder a la base de datos desde cualquier estación de trabajo y a la vez estos mismos usuarios pueden trabajar analizando, clasificando y procesando las señales de un mismo volcán o de diferente volcán. palabrasclave: sistema informático de análisis y procesamiento, señales sísmicas de volcanes, código de análisis sísmico, transformada rápida de fourier. earth sciences research journal 324 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics enjambres sísmicos en el volcán pico de tancítaro, michoacán, méxico. ¿intrusión magmática o sismicidad tectónica? francisco javier núñez cornú, juan ignacio pinzón lópez; charlotte a. rowe*, christian escudero ayala. centro de sismología y volcanología de occidente, universidad de guadalajara. *foxtrotter geophysical, llc. pacornu77@gmail.com abstract en el año 2006 se presentó un importante enjambre sísmico, con 700 eventos localizados, que ocurrió entre finales de mayo hasta principios de julio, entre los volcanes pico de tancítaro y paricutín, que nació en el año 1943, en michoacán, dentro del campo volcánico monogenético michoacán-guanajuato (mgvf) en el oeste de méxico. esta actividad sísmica fue registrada por la red sísmica temporal desplegada entre enero del 2006 y junio del 2007 por el proyecto mapping the riviera subduction zone” (mars). un estudio preliminar de este enjambre propone que la profundidad del enjambre es entre 9 y 5km y que se debe a una intrusión magmática. en los años de 1997, 1999 y 2000 también se reportaron enjambres sísmicos en la misma área de estudio, de estos enjambres solo se estudió el último período de la secuencia de 1997 y se concluyó que la sismicidad era de origen tectónico, con profundidades entre 18 y 12 km; para los episodios de 1999 y 2000 se reportó que la actividad sísmica eran similar a la de 1997. en este trabajo se hace una revisión de los datos del proyecto mars para el período del 21 de mayo al 28 de junio relocalizando 563 eventos utilizando hypo71 y el modelo regional de velocidades de onda p de la resaj (red sísmica y acelerométrica de jalisco), posteriormente se realizó un análisis de correlación cruzada a partir del cual se obtuvieron 15 familias que presentan un migración en tiempo y en profundidad, iniciando a los 16 km y terminando a los 9 km. esto lo interpretamos como una intrusión magmática. palabrasclave: sismicidad volcánica, intrusión magmática, michoacan. earth sciences research journal 325 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics actividad sísmica de los volcanes del segmento norte de colombia 1985-2014 grupo de monitoreo volcánico ovsm: álvaro pablo acevedo, carolina acosta muñoz, lina-marcela castaño, lina constanza garcía-cano, cristian mauricio lópez vélez, beatriz galvis arenas y john makario londoño. servicio geológico colombiano, observatorio vulcanológico y sismológico de manizales. av. 12 de octubre nro. 15-47, manizales-colombia. jmakario@sgc.gov.co abstract a lo largo de 27 años, el observatorio vulcanológico y sismológico de manizales se ha encargado del monitoreo de los volcanes del segmento norte de colombia. la instalación de una pequeña red de estaciones sismológicas en el volcán nevado del ruiz (vnr) a mediados de la década de los 80s, dio inicio al monitoreo volcánico en esta región del país. con el paso del tiempo la red sismológica fue densificándose y extendiéndose a otros volcanes. actualmente, se realiza vigilancia directa de ocho volcanes e indirectamente de otros tres. la actividad sísmica de los volcanes del segmento norte, está dominada por eventos de fracturamiento de roca, generados por cambios en el campo de esfuerzos al interior de la estructura volcánica y por la interacción del sistema volcánico con las fallas activas que atraviesan los volcanes de esta región. estos sismos denominados volcano-tectónicos (vt), aportan la mayor parte de la energía sísmica liberada en los centros volcánicos, no solo por el número sino también por sus magnitudes, las cuales han superado 4.0 ml (magnitud local), en volcanes como el vnr y el cerro machín (vcm), los dos volcanes más activos del segmento. el volcán con mayor actividad sísmica en esta zona es el vnr; su línea base de sismicidad ha cambiado significativamente con las variaciones en la actividad volcánica, mostrando incrementos importantes en los periodos pre-eruptivos y eruptivos del volcán en 1985, 1989 y 2012. a diferencia de los demás volcanes, el vnr ha presentado una intensa sismicidad asociada a dinámica de fluidos hidrotermales y magmáticos al interior del volcán. las fuentes sísmicas del volcán, tanto de fractura como de actividad de fluidos, están bien determinadas, mostrando un carácter superficial en cercanías al cráter activo, y más profundas cuando éstas están más alejadas del cráter (fuentes distales). el segundo volcán más activo sísmicamente del segmento norte, es el vcm. éste volcán presenta una sismicidad casi exclusivamente de tipo vt, manifestada predominantemente en enjambres sísmicos. la sismicidad está distribuida en fuentes que rodean el domo principal, y que se extienden en dirección sur-suroriente hasta límites con la falla de ibagué. la mayoría de las fuentes cercanas al domo son poco profundas, y a medida que se alejan en dirección suroriental van aumentando su profundidad hasta llegar a 18 km. la actividad del vcm está íntimamente ligada a la tectónica local y ha sido más intensa desde el año 2002. otros volcanes del segmento norte con actividad sísmica, son el nevado del tolima, nevado de santa isabel y cerro bravo (vcb). estos han mostrado niveles de actividad bajos, con predominio de sismicidad superficial tipo vt, y eventualmente sismos asociados a actividad de fluidos, algunos de ellos profundos, particularmente para el vcb. en otros centros volcánicos como los paramillos de santa rosa, cisne y quindío, y cerro españa se ha registrado sismicidad esporádica, en forma de enjambres y sismos aislados, en su mayoría de baja magnitud. finalmente, en los volcanes más septentrionales como san diego y romeral no se ha registrado hasta ahora actividad sísmica. mailto:jmakario@sgc.gov.co earth sciences research journal 326 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics palabras clave: monitoreo volcánico, actividad sísmica, volcanes, segmento norte. earth sciences research journal 327 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis de la sismicidad tipo drumbeat y su relación con la erupción del año 2008 del volcán nevado del huila, colombia john macario londoño1, diana marcela quintero2, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co abstract la sismicidad del volcán nevado del huila (vnh) ha sido monitoreada desde el año 1993 y estuvo predominantemente representada por la ocurrencia de sismos tipo vt hasta el año 2007, momento en que los fuertes cambios en sismicidad reflejaron un fuerte y rápido cambio en el sistema volcánico, el cual conllevó a tres eventos eruptivos entre el año 2007 y 2008. durante este período el vnh presentó diversos y acelerados cambios en su actividad, los cuales desencadenaron dos erupciones freáticas y una magmática, siendo este último evento el de mayor impacto en la superficie y zona de influencia del vnh. esta actividad estuvo enmarcada en sus etapas iniciales por la aparición de una serie de señales sísmicas antecesoras nunca antes vistas en este volcán, las cuales han sido denominadas en la literatura como drumbeats, entendiéndose en el sentido estricto como señales de corta amplitud, con un espaciamiento y recurrencia muy regulares, en general de tipo lf (low frequency) y hb (hybrid) y estando asociados a procesos de extrusión de domos. la intención de este trabajo es la de mostrar la tendencia general de los más de 18.000 eventos de este tipo que se registraron en un lapso de 37 días, enfocándose en las variaciones de parámetros como amplitud, frecuencia y espaciamiento entre eventos, y como estos cambios se relacionaron con la dinámica extrusiva del domo volcánico, eruptado el 2 de noviembre de 2008 en la cima del vnh. palabras clave:erupción, drumbeat, amplitud, frecuencia, espaciamiento entre eventos, extrusión de domo, volcán nevado del huila. mailto:jmakario@sgc.gov.co earth sciences research journal 328 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis espectral de fuentes sismogénicas de sismos volcano-tectónicos en el volcán nevado del huila, colombia john macario londoño1, diana marcela quintero2, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co abstract se llevó a cabo un cálculo comparativo entre espectros de eventos vt del volcán nevado del huila (vnh), los cuales fueron seleccionados y diferenciados según la fuente sísmica donde se generaron sw, ne, pico central (pc) -, durante un período comprendido entre el año 2009 y 2013. a través de un proceso de conteo de espectros se resaltaron los picos dominantes que presentaban mayor recurrencia; con esto, se logró identificar aquellos que se presentan con mayor frecuencia en cada una de las fuentes propuestas. los resultados obtenidos han permitido hacer relaciones respecto a los efectos de fuente, trayectoria y sitio en el sistema volcánico del vnh, asociados a la profundidad donde se registra la sismicidad más recurrente, los cambios temporales de esta para el período evaluado, así como las condiciones estructurales y tectónicas del medio, y la incidencia de la estratigrafía en los registros sísmicos. a través de la relación entre el promedio de espectros asociados a la trayectoria, por aquellos relacionados a una fuente y estación, se encontró una ventana espectral muy corta (7 a 9 hz) presente en todas las fuentes sísmicas para todas las estaciones involucradas en el cálculo, permitiendo inferir un proceso geológico mayor, común a todo el sistema volcánico e interactuando directamente con este, lo que ha permitido referirse a una fuerte correlación con los procesos tectónicos de la zona de estudio; por otro lado, las frecuencias no comunes en cada caso han indicado un fuerte efecto de sitio en los lugares de emplazamiento de las estaciones. a través del cálculo que relaciona los espectros de todas las señales asociados a todas las fuentes sísmicas en cada estación, se pudo reconocer cambios asociados al efecto de sitio, correlacionados con las condiciones geológicas de cada lugar de emplazamiento de las estaciones, donde se tienen variaciones en el tipo de depósito así como del tipo de estructura volcánica y su grado de conservación. el análisis normalizado, que se realizó a los contenidos espectrales de las fuentes sísmicas, también permitió hacer relaciones asociadas al nivel de profundidad donde ocurren los cambios del sistema volcánico así como del tipo de procesos involucrados; es así como se encontró un pico espectral marcado con un valor entre 2.5 y 5 hz, asociado a la fuente pico central (estructura central del edifico volcánico), la cual ha tenido una marcada predominancia de eventos superficiales asociados a la dinámica de fluidos, durante los últimos 8 años de actividad del vnh. estas relaciones permiten caracterizar la actividad del vnh así como mejorar el modelo interpretativo del sistema volcánico en condiciones de crisis. palabras clave:fuente sísmica, pico espectral, efecto de fuente, sitio, trayectoria, volcán nevado del huila. mailto:jmakario@sgc.gov.co earth sciences research journal 329 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics redefinición de las fuentes sismogénicas volcano-tectónicas en el volcán nevado del ruiz a partir de la actividad reciente 2010-2014 jonh makario londoño-bonilla y lina marcela castaño grupo de sismología, observatorio vulcanológico y sismológico de manizales, colombia. jmakario@sgc.gov.co,lmcastano@sgc.gov.co abstract la actividad sísmica volcano-tectónica en el área del volcán nevado del ruiz (vnr) está concentrada en un área de aproximadamente 100km2 y muestra una amplia distribución espacial. después de las erupciones de noviembre/1985 y septiembre/1989, la sismicidad estuvo principalmente concentrada en el cráter arenas y fuentes próximas localizadas al sur-suroeste y sureste del mismo, y ocasionalmente al oeste. a partir de la distribución espacial observada de la sismicidad durante 20122014, se redefinieron las fuentes sismogénicas activas en el sector del vnr. diez fuentes sismogénicas se consideran activas actualmente, en un radio de 5km tomando como centro del cráter activo (figura 1a). la energía sísmica liberada y el número de sismos diarios en dichas fuentes presentan cambios temporales importantes. se destaca la fuente cráter (c) que ha estado activa en la mayoría de las crisis volcánicas. así mismo, la fuente wsw ha estado activa antes de las erupciones de 1985 y 1989 con valores energéticos importantes, mientras que para las otras crisis registró valores energéticos bajos. las fuentes ssw y se, han sido recurrentes en el tiempo con valores energéticos relativamente bajos. recientemente, fuentes distales han aparecido en los sectores n (nne) y nw (f-nw), con altos niveles energéticos y de número de sismos. las demás fuentes no han mostrado cambios importantes relacionados con la actividad volcánica (figura 1b). con base en los resultados anteriores, se plantea una hipótesis de trabajo, en la que la sismicidad actual puede estar relacionada con una intrusión profunda, evidenciada en la ocurrencia de sismos en zonas factibles de acumulación de esfuerzos, como son los trazos de fallas existentes en el sector y su intersección, así como las manifestaciones geoquímicas y geodésicas (deformación) que se han registrado. sin embargo, se observa una tendencia general de la sismicidad a alejarse del cráter activo en el último año, siguiendo trazos lineales, que podrían sugerir la propagación de los esfuerzos y rupturas asociadas a zonas más distales de la intrusión inicial. esto implicaría que la actividad actual del vnr puede evolucionar en dos posibles sentidos: uno, con un incremento relacionado a la intrusión previa, la cual afectaría la zona proximal -predominando un régimen de esfuerzo local, asociado a la actividad magmática-, el cual podría culminar con erupciones volcánicas. el otro, con una transferencia de esfuerzos hacia zonas distales, predominando un régimen de esfuerzo regional, el cual se reflejaría en actividad sísmica distal sin afectación importante de la actividad volcánica. palabras clave: fuentes sismogénicas, volcán nevado del ruiz. mailto:jmakario@sgc.gov.co mailto:lmcastano@sgc.gov.co earth sciences research journal 330 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1a.fuentes sismogénicas determinadas para el área del volcán nevado del ruiz. c: cráter, e: este, ne: noreste, nne: nornoreste, f-nw: noroeste lejano o distante, n-nw: nor-noroeste, wnw: oeste-noroeste, wsw: oeste-suroeste, ssw: sursuroeste y se: sureste. earth sciences research journal 331 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1b.energía liberada diaria para las fuentes sismogénicas del área del volcán nevado del ruiz. c: cráter, e: este, ne: noreste, nne: nor-noreste, f-nw: noroeste lejano o distante, n-nw: nor-noroeste, wnw: oeste-noroeste, wsw: oeste-suroeste, ssw: sur-suroeste y se: sureste. recuadro inferior fechas de crisis volcánicas, en rojo erupciones volcánicas. earth sciences research journal 332 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics tomografía sísmica del volcán puracé john makario londoño1, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co abstract se llevó a cabo el modelamiento de la estructura interna del volcán puracé (vp) por medio de una tomografía sísmica. con la selección de 850 sismos vt (volcano-tectónicos) se efectuó el procesamiento de 4863 arribos de ondas p y de 3517 de ondas s. se logró definir varias zonas de baja velocidad para ondas p y s distribuidas en diferentes sectores de la estructura volcánica. la tendencia general de los resultados mostró un predominio de zonas de baja velocidad, es así como se obtuvo una primera zona de anomalía (dvp <3%) ubicada bajo el cráter activo, extendiéndose desde los 10 km hasta los 14 km en profundidad, lo cual se ha interpretado como la zona de alimentación magmática profunda o reservorio delvp. mientras que en el flanco ne de la estructura volcánica, se encontró otra anomalía de baja velocidad a niveles más superficiales (34 km de profundidad), lo cual se relacionó con la actividad del sistema hidrotermal del vp, por la presencia expresiones estratigráficas y superficiales de hidrotermalismo. en la zona del cráter activo y su flanco se, se observó que la anomalía de baja velocidad inicia a los 4 km de profundidad y finaliza a los 8 km. esta se interpreta como la fuente de transmisión de calor y fluidos de la actividad actual en el vp. otraestá ubicada en el costado sw del vp, allí el contraste se extiende hasta los 4 km de profundidad, y coincide con el cruce de la falla moras con la falla coconucos. una zona de alta velocidad para onda p (dvp >6%) parece asociarse, en profundidad, con una estructura remanente, posiblemente una antigua caldera, asociada con el edificio pre-puracé, que yace debajo del cráter principal; así mismo, en la parte central de dicha zona de alta velocidad, la anomalía se hace más fuerte, generando un fuerte contraste de velocidad limitado al cono del edificio volcánico, que se extiende hasta los 2-3 km de profundidad, y que se interpreta como un cuerpo dómico antiguo. la gran cantidad de actividad de fluidos que actualmente presenta el vp, puede explicarse por la presencia de estas extensas zonas de baja velocidad bajo el volcán, que podrían actuar como las fuentes generadoras de los mismos por su intercambio de calor y fluidos con el medio. palabras clave:tomografía sísmica, estructura interna, modelamiento, zona de baja velocidad, sistema hidrotermal, volcán puracé. mailto:jmakario@sgc.gov.co earth sciences research journal 333 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics variaciones de la concentración de gas radón y so2 asociadas a actividad sísmica en el sector del volcán galeras, colombia, 2005-2013 john makario londoño, viviana burbano servicio geológico colombiano. jmakario@sgc.gov.co abstract se realizó un análisis de las emisiones de gas radón (222rn) en suelo en el sector del volcán galeras (vg), con miras a detectar posibles correlaciones con sismicidad en la región y actividad volcánica, para el período 2005-2013. se pudo establecer que existe una correlación entre anomalías de gas radón (consideradas como tal para el caso del vg, aquellos valores de concentración por encima del promedio más la mitad de la desviación estándar, 1900 pci/l) con la ocurrencia posterior de sismos volcanotectónicos (vt) o tectónicos locales (tl), con magnitudes mayores a 3 (ml>), en un radio de 35km del cráter activo, en un lapso entre 1 y 52 días, con promedio 24 días. en el 100% de los casos de ocurrencia de sismos con ml>3 se detectó una anomalía previa de gas radón. sin embargo en un 50% de los casos se presentaron anomalías de gas radón sin la ocurrencia posterior de sismos con ml>3. esto se puede explicar a partir de datos de emisión de so2 por el volcán hacia la atmósfera, los cuales parece tener una relación con las emisiones de radón; cuando tanto los valores de radón como los valores de so2 (>2000 ton/día para el caso del vg) fueron anómalos, no se generaron sismos con ml>3 en el 90% de los casos. cuando los valores de radón fueron anómalos y los valores de so2 fueron bajos (<2000 ton/día) se produjeron sismos con ml > 3 en el área del vg en el 100%. al parecer, cuando el volcán está más abierto (más grietas abiertas en su interior), tanto el radón como el so2 salen libremente en mayores cantidades y es difícil generar sismos con ml>3; en el caso contrario, cuando el volcán está cerrado, se dificulta más la salida de so2, pero se empieza a acumular esfuerzos que generan valores anómalos de radón y sismos ml>3, posteriormente. éste patrón parece servir como ayuda en el pronóstico de ocurrencia de sismos importantes en el área del vg. palabras clave: radón, volcán galeras. mailto:jmakario@sgc.gov.co earth sciences research journal 334 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics variaciones en la atenuación de las ondas sísmicas en el volcán nevado del ruiz asociadas a la reciente reactivación 2010-2014 john makario londoño, katherine vallejo , johana gómez, carolina acosta servicio geologico colombiano jmakario@sgc.gov.co abstract se realizó un estudio de la atenuación sísmica en el volcán nevado del ruiz (vnr), para el período 2010-2014, lapso en el cual el vnr ha experimentado un incremento importante en su actividad. se calculó el factor de atenuación, q-1 usando un modelo de dispersión única isotrópico, y se calculó la relación de amplitudes espectrales de ondas s y p (as/ap) para la banda de frecuencias 8-16hz, con frecuencia central 12hz. durante los períodos que anteceden aumentos importantes en la actividad volcánica, el factor de atenuación (q-inverso) aumenta paulatinamente, alcanzando diferencias hasta de 0.0045 desde el inicio del cambio y el incremento en la actividad volcánica. para las dos pequeñas erupciones ocurridas en 2012 (mayo y junio), el promedio mensual del valor de la atenuación evidenció un cambio importante cuatro meses antes de las erupciones. por otra parte, se pudieron evidenciar cambios temporales en la relación espectral as/ap para las diferentes fuentes sismogénicas alrededor del vnr; antes de las pequeñas erupciones de mayo y junio de 2012, se pudo detectar un aumento en dicha relación, y una posterior disminución días antes de las erupciones. así mismo se detectaron incrementos antes de la ocurrencia de enjambres sísmicos de tipo volcano-tectónico. con base en los resultados anteriores, se concluye que la atenuación de las ondas sísmicas evaluada por dos métodos diferentes mostró cambios relacionados con la actividad del vnr, lo que permite usar éste parámetro como una herramienta más para el pronóstico de cambios en la actividad del vnr y como parámetro que puede ser usado en el monitoreo rutinario de dicho volcán. palabras clave: atenuación, volcán nevado del ruiz. mailto:jmakario@sgc.gov.co earth sciences research journal 335 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics characterization of seismo-volcanic activity in peteroa volcano, central chile-argentina j.a.casas1, g.a.badi1, m.c.manassero1, e. ruigrok2, m.gomez3, d.draganov2, j.ruzzante3 1facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina. 2department of geoscience and engineering, delft university of technology, netherlands. 3ices, cnea, buenos aires, argentina. augustotkd@carina.fcaglp.unlp.edu.ar abstract the peteroa volcano is the youngest stratovolcano of the azufre-planchón-peteroa volcano complex. it is located in the southern volcanic transition zone of the andes that extends between 34.4°s and 37°s. in this area, nw-se and ne-sw fault systems enable ascent and emplacement of magma causing spatial control of calderas, stratovolcanoes and geothermal activity. historical activity records of peteroa have shown about twenty eruptive events, most of them weak. bibliography highlights the associated processes of the 1837 and 1991 eruptions, and those occurred in 2010 and 2011 that indicate a system reactivation. the aim of this work is to characterize the seismovolcanic activity in peteroa volcano and locate its seismic sources. this information will be useful to understand the volcano’s behavior and to implement its monitoring. this work represents a new contribution to the existing tools in volcanic monitoring, allowing the characterization of seismovolcanic activity systematically and sequentially as a result of the development of scripts which compose a processing sequence performing several tasks. the analyzed data was provided by a seismic array deployed on the east flank of peteroa volcano by the international cooperation project malarrgue (tudelft-ices) in 2012 for a period of one year. an analysis in time and frequency domains allows us to discriminate events and assign them a possible source model, which is related to the activity level of the volcano. we have developed a command sequence to characterize the events originated in the volcano. this script enables us to convert the format of seismic data, visualize it, analyze the evolution of its spectral content, apply filters and get necessary information for events localization and to build a seismic catalog. during 2012, peteroa volcano showed seismic activity with continuous low-energy signals and a high number of transient signals with slightly higher energy. we have identified longperiod tremor, volcano-tectonic events associated with fractures, and explosions and long-period events associated with degassing and hydrothermal phenomena. we have developed a processing sequence to group events in families that share the same focal mechanism and path based on cross correlation technique. as of the writing of this abstract, only one event family has been identified for both high and low frequency events. the spectral evolution throughout the recording period helps identify changes in the activity state. the source locations were calculated by inversion of arrival times, by comparative analysis of particle motion between stations and/or by a cross correlation method, searching for the parameters that generate the higher signal coherence through the array, taking into account the type of event. volcano-tectonic event sources were found to be distributed to the e and ne of the active volcano, while explosion sources were found to be related to known fumarolic and hydrothermal phenomena. key words: peteroa volcano, seismo-volcanic activity, families of events, source’s localization. earth sciences research journal 336 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 337 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics análisis del campo de esfuerzo actual a partir de mecanismos focales en el área del volcán nevado del ruiz, colombia lina marcela castaño, jonh makario londoño-bonilla, carolina acosta-muñoz y beatriz galvis-arenas grupo de sismología, observatorio vulcanológico y sismológico de manizales, colombia. lmcastano@sgc.gov.co abstract con el propósito de encontrar las relaciones entre la sismicidad del sistema de fallas existente en el área del volcán nevado del ruiz (vnr) y la actividad sísmica del volcán, se realizó como primera aproximación una estimación del campo de esfuerzos actual en el área con base en datos sismológicos. para tal fin, fueron compilados un total de 644 mecanismos focales simples (fms) o soluciones de plano de falla (fps). los mecanismos focales corresponden a las soluciones obtenidas para sismos volcano-tectónicos (vts) con una magnitud local ml>0.5, registrados durante el periodo comprendido entre el 1 de enero de 2010 al 31 de octubre de 2013. se asumió que todos los mecanismos focales tienen un alto porcentaje de componente doble cupla (%dc). los mecanismos focales óptimos para el análisis fueron aquellos procedentes de vts con una buena localización. los fms fueron agrupados por año y por proximidad geográfica a las fuentes sismogénicas determinadas para el área del vnr. para cada año y cada fuente sismogénica fue calculado el tensor de esfuerzos reducido usando el método de yamaji et al. (2011). el patrón de esfuerzos obtenido refleja una interacción compleja entre los esfuerzos locales y el esfuerzo regional. la heterogeneidad de las fuentes sismogénicas es evidenciada en los altos valores de dispersión. las orientaciones y magnitudes relativas de los esfuerzos principales encontradas a partir de la inversión, indican para el área del vnr un régimen tectónico dominantemente extensional, con tendencia a extensional radial y frecuentes variaciones a un régimen de strike-slip. únicamente la fuente wnw muestra un régimen diferente que varía de strike-slip a compresional. ésta configuración de esfuerzos, podría explicarse por la interacción de la actividad volcánica, relacionada con una intrusión magmática y regímenes tanto locales como regionales que interactúan en el mismo sector. palabras clave: campo de esfuerzos, mecanismo focal, volcán nevado del ruiz. earth sciences research journal 338 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics descripción de la actividad sísmica relacionada a erupciones estrombolianas violentas: vn. llaima, chile (2007 – 2010) franco, l.e.1,2, palma j.l.2, gil-cruz, f.1 y san martín, j..j2 1servicio nacional de geología y minería (chile). 2universidad de concepción, departamento de ciencias de la tierra (chile). luis.franco@sernageomin.cl abstract el volcán llaima (38°42's y 71°44'w) un estratovolcán basáltico-andesítico, localizado en la zona volcánica sur de los andes, ha sido catalogado como uno de los más activos y voluminosos de suramérica, con una recurrencia eruptiva de ~6 años. el último ciclo eruptivo (20072010), inició con una pequeña manifestación superficial en mayo de 2007, seguida por erupciones mayores en 2008 y 2009 y posterior culminación en junio de 2010, ciclo eruptivo que de acuerdo a sus características es el más importante después del ocurrido en 1957. para el año 2009, el observatorio volcanológico de los andes del sur (ovdas) del servicio nacional de geología y minería, había instalado tres estaciones sismológicas ubicadas entre 7 y 20 kilómetros con respecto al cráter, instrumental complementado con motivo de la conformación del nuevo ovdas en noviembre del 2009, adicionando siete estaciones, instaladas a distancias hasta de 2 km con referencia al cráter activo. una primera erupción descrita como paroxismal de tipo estromboliana violenta, inicia el 1 de enero de 2008, con cerca de 15 horas de actividad energética, y una segunda, originada el 3 de abril de 2009 catalogada como estromboliana vigorosa con una duración aproximada de 80 horas, destacan los hechos más relevantes y comparables de este periodo. esta descripción de la sismicidad, se convierte en la primera oportunidad de exhibir a la comunidad científica, aspectos relacionados con el registro sísmico para el llaima, en las etapas pre, sin y posteruptivas. haciendo una comparación de las erupciones, la de abril de 2009 se caracterizó porque el registro sísmico fue de menor intensidad, pero constante y de mayor duración que la desarrollada en enero de 2008. ambos episodios en su punto más vigoroso, se identificaron por presentar registros continuos de señal tipo tremor, con bandas frecuenciales entre 0.8 y 3.0 hz principalmente y peaks dominantes en 1.0, 1.2 y 1,5 hz. además, las erupciones se distinguieron por presentar un nivel de ocurrencia bajo relacionado con sismicidad volcanotectónica (vt), provenientes de una fuente ubicada 18 km al sur-suroeste del cráter activo. la sismicidad de largo periodo (lp) cuyas formas de onda presentaron una variedad de familias, se caracterizó por presentar un incremento paulatino en la energía, alcanzando amplitudes máximas al momento del clímax eruptivo, evolucionando hacia episodios de tremor continuo durante el proceso de la erupción, el que finalizó abruptamente. posterior a la culminación, sismicidad lp y tremor de corta duración se hacen visibles en los registros. el contenido espectral de los eventos lp y tremor comparten peaks dominantes, sugiriendo un origen común. el último ciclo eruptivo del llaima, llevó a reconocer que el aumento en la energía de los lp y la ocurrencia de sismicidad vt en forma esporádica, enmarcan un escenario de pronóstico volcánico el cual sugiere que estos parámetros podrían ser usados como premonitorios en futuras crisis volcánicas. palabrasclave: palabras claves: volcán llaima, erupciones estrombolianas, sismicidad de largo periodo, tremor. earth sciences research journal 339 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 340 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics anomalías de gas radon asociadas a la sismicidad en el region volcánica puracé luisafernanda meza1, rosa liliana alpala1, andrés narváez1, john makario londoño2 1servicio geológico colombiano, observatorio vulcanológico y sismológico de popayán. 2servicio geológico colombiano, observatorio vulcanológico y sismológico de manizales. lmeza@sgc.gov.co, ralpala@sgc.gov.co, anarvaez@sgc.gov.co, jmakario@sgc.gov.co abstract el monitoreo continuo de emisiones de gases difusos en el volcán puracé se inició en 2006, de manera esporádica en los inicios de la vigilancia y de forma continua en los últimos años. para estudiar la relación entre la sismicidad de fractura y las emisiones de radón (222rn) en la región volcánica del puracé (rvp), se realizó un análisis de las emisiones de este gas presente en suelo empleando los datos de tres estaciones isotópicas (agua tibia (at), agua hirviendo (ah) y tabio (tb)), ubicadas estratégicamente sobre una zona de fallamiento activo en el área de influencia volcánica. los datos fueron obtenidos mediante el método de cámaras ionizantes eperm (electret-passive environmental radon monitor) para determinar la concentración de radón. se pudo establecer que para ésta región, en más del 80% de los casos, existe aumento en la concentración de gas radón por encima de 1800 pci/l antes de la ocurrencia de sismos volcano tectónicos (vt) o tectónicos locales (tl), con magnitudes mayores a 2.8 (ml>), en un radio de 30km, tomando como centro el sitio ubicado en la distancia media entre las estaciones de medición. las anomalías ocurren en un rango de 3 a 80 días antes del sismo de ml>2.8, predominando 36 días como rango promedio. éste resultado permite concluir que el monitoreo de emisiones de gas radón es una herramienta útil en el pronóstico de sismos en el área donde se tiene instalada la red de monitoreo, si se determinan apropiadamente, tanto el radio de influencia, como el valor de concentración de gas radón considerado una anomalía. palabras clave: sismicidad, gases difusos (222rn), volcán puracé. mailto:lmeza@sgc.gov.co mailto:ralpala@sgc.gov.co mailto:anarvaez@sgc.gov.co mailto:jmakario@sgc.gov.co earth sciences research journal 341 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic attenuation around peteroa volcano, argentina m.c.manassero1, g.a.badi1, j.a.casas1, m.gomez3, d.draganov2, j.ruzzante3 1dpto. de sismología e i. m., facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina. 2department of geoscience and engineering, delft university of technology, netherlands. 3ices-cnea, buenos aires, argentina. constanzamanassero@gmail.com abstract the seismic activity of a volcano is one of the elements that best describes its dynamics. the different seismic signals registered depend on the type of source, the characteristics of the medium through which they have propagated (such as the presence of fluids and heterogeneities), and their variations in time. along their path the amplitude of the waves is attenuated. the analysis of this decay can reveal variations inside the volcanic structure. this information is indispensable for a better characterization of seismo-volcanic activity, the comprehension of its dynamic and implementation of monitoring. the quality factor (q) describes the attenuation effects, including the intrinsic attenuation due to anelasticity of the medium and the loss of energy due to scattering from heterogeneities. this factor is intimately related with the medium parameters like viscosity, density, temperature and fluid presence. therefore, spatial and temporal variations of q could help us understand the volcano dynamics and how it changes due to both internal and external factors. it was observed that, in local earthquakes seismograms, q varies with frequency and can be estimated from direct and coda waves. in the last decades the peteroa volcano (central chile and argentina) has suffered a series of phreatomagmatic events, as well as seismic activity related with rock fractures and fluid dynamics that were located on the argentinean flank, according to information provided by sernageomin, chile. for this reason, the proximity of population centers and for a future volcanic monitoring, it is essential to improve the knowledge about attenuation in the area. the objective of the present work is to obtain information about the area surrounding peteroa volcano through the analysis of seismo-volcanic signals attenuation. the data was acquired by six short period stations installed on the eastern flank of the volcano by the malarrgue project (icescnea, tudelft) during 2012. the quality factor of direct waves, qd, is determined by the spectral ratio method which analyzes variations in amplitude of ground displacement related with propagation. qd is obtained as a function of frequency by analyzing all the explosive and volcano-tectonic events registered in each station as a function of travel time for consecutive frequency intervals (rojas arce, 2013). such dependence is assumed to be a power law of the form q = qo*f^a, where qo=q(1hz). the estimation of quality factor for coda waves, qc, is obtained using the single back-scattering model (aki y chouet, 1975) on volcano-tectonic events. up to this moment, the results show the highest attenuation towards the southwest of the array. this variation could be related with the relative position of volcanic and glacial deposits as well as plutonic outcrops around the volcano. palabrasclave: peteroa volcano, attenuation, quality factor, volcanic monitoring. earth sciences research journal 342 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics earth sciences research journal 343 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad volcano-tectonica y registros de periodos de deformación en el volcán sotará, “segmento central de colombia” narváez, a., alpala, j. servicio geológico colombiano observatorio vulcanológico y sismológico de popayán. anarvaez@sgc.gov.co, jalpala@sgc.gov.co abstract el volcán sotará (4420 msnm) es un estratovolcán ubicado en la cordillera central, en el límite entre los departamentos del huila y cauca, a 37 km al sw de la ciudad de popayán, hace parte de un complejo volcánico conformado también por los volcanes cerro azufrado, cerro gordo y cerro negro. es un volcán activo, sin registros históricos de actividad eruptiva (últimos 500 años), que ha sido monitoreado por el servicio geológico colombiano (ingeominas) desde el año 1993 y de manera continua desde el año 2007. evidencias de su actividad es la existencia de campos fumarólicos, fuentes termales, lavas, domos con morfología postglaciar (acevedo & cepeda, 1982) y el registro de actividad sísmica principalmente asociada con eventos de tipo volcano-tectónico (vt) proximal y distal. la sismicidad en el volcán sotará hasta antes del 24 de junio de 2012 se había caracterizado por ser de baja ocurrencia y de bajo aporte energético, en promedio se registraban 6 eventos por mes, a partir de esta fecha la actividad sísmica se incrementó, contabilizándose hasta 9861 sismos de tipo vt, de estos, 7521 han sido de bajo aporte energético; mientras que los 2340 restantes se han caracterizado por presentar aportes energéticos mayores. durante los meses de junio hasta agosto de 2012 se registraban en promedio 120 sismos por día, los cuales fueron localizados en una zona comprendida entre 0.1 y 5 km hacia el sector ne del edificio volcánico, con un rango hipocentral que oscilo entre 2 km y 6 km, y magnitudes de entre 0.1 y 2.4 ml (figura 1). ninguno de estos eventos fue reportado como sentido por las comunidades que habitan el área de influencia. este aumento en la ocurrencia de eventos y en la energía acumulada, junto con el análisis comparativo provisto por la red de inclinómetros del volcán sotará, evidencio un posible proceso inflacionario hacia el sector noreste del edificio volcánico que alcanzó los 80 microradián, el cual coincidió con la zona epicentral de la sismicidad, lo que motivo al cambio de nivel de actividad a nivel iii el 8 de agosto de 2012 (figura 2). en la actualidad de acuerdo con el análisis de la información obtenida a través de las diversas estaciones que componen la red de monitoreo del volcán sotará, este presenta un comportamiento estable. no obstante, se destaca que los niveles de actividad sísmica registrados durante el periodo evaluado, permanecen por encima de los niveles básicos históricos para este volcán. por lo anterior el ovs-popayán mantiene el nivel iii amarillo y continua reforzando la red de monitoreo para comprender e interpretar mejor los procesos volcánicos que se están generando. palabras clave: volcán, sotará, sismicidad, vt, inclinómetro. mailto:anarvaez@sgc.gov.co earth sciences research journal 344 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. mapa de localización de la sismicidad vt en el volcán sotará 2012. figura 2. numero de eventos y energía sísmica liberada de forma acumulada por eventos vt en el volcán sotará 2007 – 2014. earth sciences research journal 345 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics caracterización de fuentes sísmicas de eventos tipo tornillo en el volcán puracé mediante el análisis de frecuencias complejas nelson david pérez1,2, john jairo sánchez2, indira molina3 1red simológica nacional, servicio geológico colombiano. 2departamento de geociencias, universidad nacional de colombia. 3 bureau de recherches géologiques et minières de france (brgm) nperez@sgc.gov.co abstract se realiza la caracterización de la fuente productora de sismos de largo período, conocidos como tipo “tornillo”, que hacen parte de la sismicidad característica del volcán puracé (colombia) y que se han asociado a tránsito y variaciones de presión de fluidos bajo el volcán. la caracterización de la fuente se logra a través de la determinación de las frecuencias complejas (f) y el valor del factor de calidad (q) y de manera práctica mediante la implementación del método de análisis espectral conocido como sompi, un método de análisis autoregresivo que permite modelar con precisión la coda de los sismos de largo período. los resultados preliminares indican que durante el período 19942012 las frecuencias complejas están en el rango 115 hz y los valores de q se ubican entre 100-700. palabras clave: frecuencias complejas, método sompi, sismos tipo tornillo, volcán puracé. earth sciences research journal 346 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics aplicación del método sompi en la evaluación temporal de las propiedades características de sismos tipo tornillo en el volcán galeras maría fernanda acosta mora1, oscar ernesto cadena ibarra2 1universidad de nariño. 2sgc-ovsp. ocadena@sgc.gov.co abstract en este trabajo se realizó una evaluación temporal de las propiedades características de la fuente que origina los eventos tipo tornillo (to) en etapas pre-eruptivas y no pre-eruptivas del volcán galeras (vg). se utilizó el método de análisis espectral sompi que se basa en una ecuación autorregresiva homogénea y usa espectros en el espacio de la frecuencia compleja para proveer las características del decaimiento y los periodos de oscilación de una señal, brindando una estimación espectral de gran resolución y confiabilidad (figura 1). el método se aplicó a los eventos tipo to del registro sísmico histórico del vg y se realizó un análisis de los resultados teniendo en cuenta valores de frecuencia y del factor de calidad q dado por: 𝑄𝑄 = 𝑄𝑄𝑟𝑟−1 + 𝑄𝑄𝑖𝑖−1 (aki, 1984) donde qr representa el factor de calidad por radiación de energía y qi es el q intrínseco del fluido. para realizar este análisis los sismos to fueron clasificados en dos grupos, aquellos registrados en etapas previas a erupciones y aquellos en las que el periodo de registro de tornillos no terminó con erupción. los resultados mostraron diferencias significativas en ambos grupos. se encontró promedios más bajos de q para etapas preeruptivas que para etapas no pre-eruptivas, por otro lado en ambos casos se encontró descensos en la frecuencia a medida que el periodo se acerca a la erupción (figura 2). las variaciones temporales son el resultado de los cambios en la fracción de gas de la mezcla y aunque este cambio se ve como una constante en la gran mayoría de etapas pre-eruptivas, no es el único que determina si el periodo culmina o no en erupción. se pudo relacionar valores de q asociados a periodos en los que predomina la mezcla de gas y ceniza y otros en los que domina la componente hidrotermal. palabras clave: método sompi, sismos tipo tornillo, volcán galeras. earth sciences research journal 347 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. aplicación del método sompi, al evento to registrado el 20 de abril 2005. traza (arriba), diagrama f-g, las líneas diagonales indican los valores de q (centro) y espectro (abajo). obsérvese para este caso la presencia de dos armónicos en su espectro. figura 2. tendencias de f y q para el periodo pre-eruptivo entre el 28/09/05 y el 24/11/05. earth sciences research journal 348 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad volcano-tectónica en el volcán puracé ospina, c.a, corchuelo, i.d. observatorio vulcanológico y sismológico de popayán. cospina@sgc.gov.co, icorchuelo@sgc.gov.co abstract el volcán puracé es un estratovolcán ubicado en el departamento del cauca, a una distancia de 26 km al se de la ciudad de popayán y hace parte de la cadena volcánica de los coconucos, compuesta por 15 centros eruptivos alineados con una orientación n40°w, siendo el puracé el más joven y septentrional de la cadena. la actividad del volcán ha sido reportada desde 1801 y su registro histórico más reciente muestra que dicha actividad es predominantemente explosiva. desde 1827 se han registrado 15 erupciones históricas bien documentadas, que causaron daños materiales y pérdida de vidas; siendo la última en marzo de 1977 (monsalve m. y pulgarín b. 1993). el monitoreo sísmico en el volcán puracé comenzó en 1986 con la instalación de una estación por parte del observatorio sismológico del sur occidente, posteriormente en 1990, la señal es registrada en las instalaciones del ovspo, quien en 1993 instaló las primeras dos estaciones propias. en la actualidad el volcán puracé cuenta con una red de vigilancia con 10 estaciones sismológicas en su gran mayoría de banda ancha. la sismicidad en el volcán puracé se distingue entre sismicidad debido al transporte de fluidos: largo periodo (lp), tornillo (to), tremor (tr); sismicidad por fracturamiento de roca: volcano tectónica (vt) y sismicidad híbrida (hb), según los criterios de lahr et al (1994). la actividad sísmica que predomina en el volcán puracé corresponde a eventos tipo lp. sin embargo, se han registrado un gran número de eventos de fracturamiento de roca alcanzando magnitudes hasta de 4.8 en la escala de richter. en este trabajo se presenta una recopilación de la sismicidad tipo vt en el volcán puracé, identificando cuatro fuentes sismogénicas a partir de registros sísmicos analizados en el observatorio vulcanológico y sismológico de popayán (ovspo) (rodriguez, f. 2012 & cardona, e. 2000). las cuatro fuentes sismogénicas mencionadas son: la fuente circundante al cono activo del volcán (cráter central), la fuente localizada en el sector denominado la mina a unos 5 km al nw del cráter central, la fuente localizada en el valle de san rafael en un área ubicada a 12 km en dirección ne de la cima del volcán puracé y la fuente activa del valle de paletará localizada a 15 km al sw del edificio volcánico. en esta última fuente, se presentó el sismo de mayor magnitud registrado a la fecha. a partir de la actividad sísmica registrada tipo vt, se han realizado trabajos de investigación al interior del grupo de sismología para la determinación de las características físicas del medio en términos de atenuaciones, velocidades y estructuras internas del edificio volcánico. el trabajo de investigación más reciente realizado en el volcán puracé es la determinación de anomalías de gas radón asociadas con actividad volcano-tectónica. palabras clave: volcán, puracé, volcano-tectónica, fuente sismogénica, gas radón. earth sciences research journal 349 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics seismic forecasting of eruptions at dormant stratovolcanoes randall. a. white1 and wendy a. mccausland2 1usgs volcano disaster assistance program, menlo park, ca. 2usgs volcano disaster assistance program, vancouver, wa. rwhite@usgs.gov abstract from 1900-1985, the fatality rate for volcanic eruptions averaged 820/yr. however since the disastrous 1985 nevado del ruiz eruption, the rate has dropped to 35/yr, a dramatic 95% improvement owing primarily to improve seismic monitoring and analysis. the seismic data provide important constraints on magmatic ascent and eruption. precursory trends include deep (e.g., >20 km) lowfrequency (lf) seismicity as magma rises from the base of the crust, distal volcano-tectonic (dvt) earthquakes as rising magma pressurizes aquifers unlocking adjacent faults, shallow proximal vt (pvt) and shallow lf seismicity as magma interacts with the shallow hydrothermal system and finally, hybrid seismicity. recent petrologic studies implicate magma mixing from deep basalt as trigger for most to all explosive eruptions. deep lf seismicity, though difficult to record owing to small magnitudes and great depth, correlates well with deep magma movement and appears to be a good early indicator of explosivity. dvt seismicity is a much more easily and widely recorded early precursor to eruptions. a survey of >100 eruptions at 80 volcanoes, including all explosive eruptions of vei >4 since 1950, shows that virtually all volcanoes which haven’t erupted for >25 years are preceded days to months or more by major dvt seismicity. this seismicity, though induced by magmatic intrusion, originates on tectonic faults a few to >30 km from the eventual eruption vent, not beneath the vent itself. most importantly, the cumulative dvt energy correlates well with the volume of intruded magma. as magma intrudes to shallower depths, interaction with the hydrothermal system produces pvt and shallow lf seismicity. as magma continues to rise, gases eventually escape to the surface producing additional pvt events, greatly increased shallow lf events and lf tremor, and phreatic explosions while the dvt seismicity dies off. as magma rises to 1-2 km beneath the surface, lf and hybrid seismicity dominate the record, often increasing dramatically. these observations support a simple model with basaltic magmas from the base of the crust feeding a silicic magma mush-filled reservoir at ~5 to >15 km depth which connects to the surface during eruptions through narrow conduits. the seismic energy release rate, deformation, and gas emissions are correlated and controlled by magma ascent and degassing rates. if ascent rate is fast, more gas is retained, pressure increases, resulting in very explosive eruptions; if slow, degassing and crystallization may “freeze” the magma in place without erupting. keywords: eruption forecasting, volcano-tectonic, hybrid, tremor low frequency, prediction, seismicity. earth sciences research journal 350 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics sismicidad frontera colombia-ecuador en la región volcanes chiles y cerro negro roberto torres, oscar cadena, john meneses y edgar muñoz servicio geológico colombiano. rtorres@sgc.gov.co abstract los volcanes chiles (4748 m) y cerro negro (4470 m) se ubican en la frontera colombia-ecuador, en el departamento de nariño y la provincia del carchi. estos volcanes son considerados como activos y son monitoreados conjuntamente por el servicio geológico colombiano y el instituto geofísico de la escuela politécnica nacional del ecuador. en la región de estos volcanes, entre mediados de agosto hasta octubre de 2013, se evidenció un incremento en la sismicidad en ocurrencia y energía liberada con registros y reportes de sismos sentidos por las comunidades vecinas. similarmente, a partir de marzo hasta mayo de 2014 se tuvo otro incremento en la sismicidad, registrándose el sismo con mayor ml (4.8 en la escala de richter) el 30 de abril de 2014. de acuerdo con la evolución temporal de la energía sísmica liberada por estos sismos, la secuencia corresponde a enjambres, los cuales se caracterizan por no seguir un patrón de un sismo principal de mayor magnitud secundado por sus correspondientes réplicas de menores magnitudes, sino que obedece a la ocurrencia de varios sismos en un área específica limitada con magnitudes relativamente similares. la reactivación de esta sismicidad parece remontarse a septiembre de 2012. el número de sismos registrados de la zona es inusualmente alto, contabilizándose más de 85000 eventos desde noviembre de 2013, alcanzando valores diarios de hasta 2000 sismos por día (fig. 1). la mayoría de los eventos son de baja energía con magnitudes inferiores a 3 y solo 47 sismos tienen magnitudes entre 3 y 4.8, con un reporte de 81 eventos sentidos por los habitantes del resguardo indígena de chiles. se ha observado un escalonamiento ascendente en la ocurrencia y la energía en el tiempo, con periodos más intensos en abril de 2013, agosto-octubre de 2013 y marzomayo de 2014. la mayoría de los sismos se caracterizan por arribos impulsivos de ondas p y s, con energía distribuida en una amplia banda de frecuencias y predominio de altas frecuencias (> 5 hz) y un decaimiento rápido de la envolvente de las formas de onda que es característico sismos vt. estos sismos se asocian con procesos puramente elásticos resultantes de fracturas de tensión o cizalla en el medio sólido de la estructura volcánica debido a deformación del medio como propagación de un cambio de volumen por presiones inducidas por el magma. con el sismo del 30 de abril, se identificó una componente de muy baja frecuencia que se asocia con entornos de aberturas por tensión y que podría evidenciar una intrusión de magma a través de un dique. esta sismicidad está concentrada espacialmente y distal de los posibles centros eruptivos. los epicentros de los sismos se localizan preferentemente en el flanco sur y suroccidente del volcán chiles a distancias entre 0.5 y 6 km, con profundidades entre 1.5 y 6 km respecto a la cima del chiles (fig. 2). el análisis de esta sismicidad, sugiere posibles intrusiones de magma o ascenso de fluidos magmáticos que están afectando el campo de esfuerzos en la base de la zona frágil de la región volcánica del chiles-cerro negro y produce el fracturamiento de fallas pre-esforzadas en vecindades a estos volcanes, aunque aún no es posible atribuir a cuál centro volcánico está asociada. palabras clave: volcán chiles, volcán cerro negro. earth sciences research journal 351 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. histograma de ocurrencia diaria de sismos vt entre nov-5 de 2013 a may-21 de 2014. figura 2. localización de las fuentes de los sismos vt entre nov-5 de 2013 a may-21 de 2014. el tamaño de la circunferencia se corresponde con la ml del sismo y el color con la profundidad. los cuadrados negros muestran la localización de las estaciones sísmicas. earth sciences research journal 352 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics un modelo geofísico de la estructura interna del volcán galeras, colombia roberto torres, adriana ortega, lourdes narváez 1servicio geológico colombiano. rtorres@sgc.gov.co abstract se plantea un modelo geofísico tridimensional de estructura interna para el volcán galeras a partir de la integración de información de una tomografía sísmica local pasiva de velocidades de la onda p, con sismos volcano-tectónicos (vt) registrados entre 1989 y 2009 y la interpretación de anomalías gravimétricas y magnéticas evaluadas a partir del levantamiento de información entre junio de 2008 y abril de 2009, con un área de cubrimiento aproximada de 1200 km2. el modelo de velocidades de onda p a partir de los resultados de la tomografía sísmica con un alcance en profundidad de 8 km bajo la cima y la ausencia de focos sísmicos de eventos vt, muestran dos zonas de bajas velocidades al nw y se, donde no existe la suficiente fragilidad para generar fracturas. en estas zonas, los bajos gravimétricos observados a partir de la componente residual de la anomalía gravimétrica confirman el predominio de bajas densidades (figura 1), y a partir de la anomalía magnética se tiene un estimativo una profundidad de 6 km para la isoterma de curie. estas zonas sugieren la existencia de anomalías térmicas, que posiblemente asocian con sistemas hidrotermales o remanentes de magma no solidificados. por otra parte, la tomografía sísmica muestra también dos zonas con altos valores de velocidades de la onda p, una bajo el cráter y otra al ne bajo los trazos superficiales de las fallas buesaco y silviapijao. en la zona al ne donde se tiene un alto gravimétrico y donde se observa también una concentración de hipocentros de sismos vt con las mayores magnitudes, exhibe velocidades concordantes con valores medidos en magmas andesíticos solidificados o con cuerpos metamórficos rígidos (figura 2). en la zona bajo el cráter, el modelo gravimétrico 3d, sugiere la existencia de una estructura vertical que se perfila a manera de conducto ramificado o de grietas hacia la superficie; el modelo 3d de tomografía sísmica muestra una fuerte alineación de hipocentros de sismos vt hasta unos 4 km de profundidad que podría corresponderse con una estructura estimada a partir del modelo magnético 2d donde se devela un posible dique que se extiende hasta una profundidad ~ 5 km. esta zona sugiere un sistema por el asciende magma desde niveles más profundos hacia la superficie. palabras clave: tomografía sísmica, anomalías gravimétricas, anomalías magnéticas, modelo estructural. earth sciences research journal 353 earth sci. res. j. vol 18 special issue (july, 2014) facultad de ciencias departamento de geociencias research group in geophysics figura 1. con base en las relaciones empíricas de nafe-drake (1963) se estiman valores de densidad a partir de los valores de vp. si se considera el apilamiento de tomogramas de densidades y se comparan con el mapa de la componente residual de la anomalía gravimétrica, se encuentran zonas muy comparables entre la densidad y la anomalía gravimétrica. figura 2. modelo conceptual 3d del volcán galeras, utilizando tomogramas de velocidades. los puntos en negro representan los eventos asociados con el movimiento de fluidos, los puntos blancos los hipocentros de los eventos vt. en gris se presenta el posible conducto de ascenso. conveners: scientific committee determinación de modelos de velocidad de ondas de corte empleando el método de autocorrelación espacial (spac) en la ciudad de armenia, colombia francisco chávez-garcía1, carlos lozano2, hugo monsalve3 1universidad nacional autónoma de méxico (unam), instituto de ingeniería, méxico d.f.,méxico. 2servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá d.c., colombia. 3universidad del quindío, facultad de ingenieríagrupo quimbaya, colombia. clozano@sgc.gov.co geófonos individuales vs arreglo de geófonos: un estudio de campo realizado en colombia diana marcela cortes1, juan carlos isaza1,jack caldwell2, camilo gonzález3 1ecopetrol s.a 2geospace technologies 3petroseis s.a. diana.cortes@ecopetrol.com.co jélime aray, luz rodríguez, franck audemard, victor rocabado, kenny garcía, michael schmitz fundación venezolana de investigaciones sismológicas (funvisis). jaray@funvisis.gob.ve high-resolution, low power, mobile infrasound station leonid zimakov1, a. leon1, g. ertl2,m. alerborn3, a glushakov3, v. selyatinskii3, a. tonkikh3, k fedin3, v sherbakov3 1ref tek a division of trimble. 2trinas handels gmbh. 3sdc geophysical measurementsnovosibirsk, russia. l.zimakov@reftek.com infiriendo la estructura estratigráfica mediante interferometríasísmica a partir de datos microsísmicos y ruido ambiental omar alfredo mercado díaz y carlos alberto vargas jiménez departamento de geociencias, universidad nacional de colombia. oamercadod@unal.edu.co conveners: stan lasocki (inst. of geophysics poland) <lasocki@igf.edu.pl> aderson nascimento (ufrn brazil) <aderson@geofisica.ufrn.br> carlos vargas (unal colombia) <cavargasj@unal.edu.co> integration program for research infrastructures for georesources within european plate observing system project beata orlecka-sikora and stanisław lasocki institute of geophysics pas, warsaw, poland. orlecka@igf.edu.pl assessing the role of static stress transfer on the spatio-temporal distribution of aftershocks caused by a mw2.2 earthquake in mponeng gold mine, south africa orlecka-sikora b.1, kozłowska m.1, lasocki s.1, kwiatek g.2, boettcher m.s.3, dresen g.2 1institute of geophysics polish academy of sciences, warsaw, poland (orlecka@igf.edu.pl). 2gfz german research centre for geosciences section 3.2: geomechanics and rheology. 3university of new hampshire, durham. orlecka@igf.edu.pl is-epos – a prototype of epos thematic core service for seismic processes induced by human operations beata orlecka-sikora, stanisław lasocki and the members is-epos team. institute of geophysics pas, warsaw, poland. orlecka@igf.edu.pl estimación simultanea del mecanismo focal de dos microsismos cercanos daniel león, jorge mejía corporación osso. danileo@osso.org.co variations of parameter b in se brazil from local earthquakes igor oliveira, marcelo assumpção, hans agurto-detzel university of são paulo, brazil. igor@iag.usp.br los sonidos de las actividades de explotación de hidrocarburos jorge mejía corporación osso. jmm@osso.org.co propuesta de calibración de una escala de magnitud para eventos microsísmicos que ocurren en cercanías a campos petroleros luis girón corporación osso. lgiron@osso.org.co detection of induced seismicity due to oil and gas extraction in the northern gulf of mexico, usa oluwaseun fadugba and john e. ebel boston college, ma. fadugba@bc.edu statistical analysis of seismic activity preceding the emilia-romagna (italy) earthquake sequence from may/june 2012 stanislaw lasocki1, beata orlecka-sikora1, peter styles2 1institute of geophysics pas. 1keele university. lasocki@igf.edu.pl seismicity induced from nearly two decades of deep brine injection, paradox valley, usa yeck, william1,2, block, lisa2, wood, chris2, king, vanessa2 1university of colorado – boulder. 2bureau of reclamation yeck@colorado.edu conveners: victor huerfano (univ. puerto rico) <victor@prsn.uprm.edu> jay pulliam (baylor univ. usa) <jay_pulliam@baylor.edu> the mw6.4 north of puerto rico event: response timeline and event characteristics of the largest event ever recorded at the puerto rico seismic network alberto lópez1,2, fernando martínez1, victor huérfano1,2, gisela báez2, harold irizarry2, glen mattioli3, christa von hillebrandt4 1department of geology, university of puerto rico – mayagüez. 2puerto rico seismic network. 3university of texas, arlington. 4caribbean tsunami warning program, national weather service, noaa. alberto@prsn.uprm.edu significant breakthroughs in monitoring networks of the volcanological and seismological french observatories in the west indies andré angalde1, arnaud lemarchand3, jean-marie saurel2, valérie clouard2,marie-paule bouin3, jean-bernard de chabalier3, steve tait3, christophe brunet3, alexandre nercessian3, richie robertson4, lloyd lynch4, machel higgins4,joan latchman4, and the t... 1observatoire volcanologique et sismologique de guadeloupe (ovsg/ipgp). 2observatoire volcanologique et sismologique de martinique (ovsm/ipgp). 3institut de physique du globe de paris (ipgp). 4seismic research center (src/uwi). anglade@ipgp.fr creating a dense seismic network for costa rica: a low-cost latin american instrument solution carlos j. ramírez u.1, lepolt linkimer1, branden christensen2 1red sismológica nacional (ucr-ice). 2osop. carlosjru@yahoo.com relocation of the mainshock and aftershocks of ml 5.7 quetame earthquake, colombia gabriel dicelis1, patricia pedraza2, marcelo assumpção3 gabriel.dicelis@iag.usp.br aspectos geotectónicos, morfotectónicos y geodinámicos de la amenaza sísmica de la región nororiental de la plataforma caribe colombiana chicangana, g.1.3, ordoñez, a, c.1,2, vargas, c. a.3, mora páez, h.4, gómez, a.5, y ferrari, a.2 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. 2lagemar, universidad federal fluminense, niteroi, r.j., brasil. 3grupo geofísica, departamento de geociencias, universidad nacional de colombia. 4proyecto geored, área geología, servicio geológico colombiano. 5istituto nazionale di geofísica e vulcanologia, milano, italy german.chicangana@unimeta.edu.co slab tears and intermediate-depth seismicity in the northeast caribbean and elsewhere jay pulliam1, hallie e. meighan1, uri ten brink2, alberto m. lópez-venegas3 1baylor university. 2u.s. geological survey. 3university of puerto rico, mayaguez. jay_pulliam@baylor.edu combination of high rate, real-time gnss and accelerometer observations – test results using a shake table and historic earthquake events leonid zimakov, michael jackson, paul passmore, jared raczka, alex leon infrastructure division trimble navigation limited. l.zimakov@reftek.com renovation of the national seismological network of costa rica: improving station coverage and earthquake monitoring lepolt linkimer, maría cristina araya, oscar lucke, carlos ramírez universidad de costa rica. linkimerl@gmail.com the tablón fault: source of the 2011–2012 tobosi earthquake swarm in central costa rica maría cristina araya, lepolt linkimer, walter montero, wilfredo rojas red sismológica nacional. mariacristina.araya@ucr.ac.cr static stress state on the costa rican cocos-caribbean plate interface and its influence on the asperity distribution of the sesimogenic zone oscar h. lücke universidad de costa rica. oscar.luckecastro@ucr.ac.cr the new iris earthquake browser (navegador de terremotos) r.welti, b.weertman, j.taber, t.ahern iris consortium. russ@iris.washington.edu seismic and tsunami monitoring and data sharing in the caribbean victor huérfano, alberto lopez puerto rico seismic network/geology department, university of puerto rico at mayaguez. victor@prsn.uprm.edu conveners: hector mora-paez (colombian geological survey) <hmora@sgc.gov.co> pete lafemina (pensilvania state univ. usa) <pcl11@psu.edu> rui manuel da silva fernandes (univ. beira interior portugal) <rmanuel@di.ubi.pt> high resolution geodetic surveys of the present day deformation along the south-caribbean margin carlos reinoza1,2, franck audemard1, françois jouanne2, michael schmitz1, christian beck2 1funvisis-venezuela, 2isterre-univ. savoie, france. creinoza@funvisis.gob.ve caracterización sismotectónica de la región del valle del cauca y zonas aledañas a partir de mecanismos focales de terremotos y datos geodésicos elkin de j. salcedo-hurtado1, jhon leandro pérez1,2, héctor mora-páez3 1grupo de investigación georiesgos observatorio sismológico y geofísico de la universidad del valle. 2observatorio sismológico del occidente de panamá (osop). 3servicio geológico colombiano, proyecto gnss geored. elkin.salcedo@correounivalle.edu.co present-day deformation along the andean subduction controlled by continental slivers motion f. rolandone1, j-m. nocquet2, p. jarrin3, j.c. villegas-lanza4, p.a. mothes3, m. chlieh2 1sorbonne universités, upmc univ paris 06, cnrs umr7193, istep, f-75005, paris, france. 2geoazur, ird-cnrs-oca, université de nice sophia-antipolis, france. 3instituto geofísico, escuela politécnica nacional, ladrón de guevara, quito, ecuador. 4instituto geofísico del perú, calle badajoz 169, urbanización mayorazgo iv etapa, ate, lima, perú. frederique.rolandone@upmc.fr assessing the seismic potential of south america with crustal deformation models and historic earthquakes gerald galgana, mehrdad mahdyiar, bingming shen-tu, claire pontbriand, elliot klein, feng wang, khosrow shabestari, wenzheng yang air worldwide. ggalgana@air-worldwide.com geodetic constraints on the present-day kinematics of colombia, south america h. mora-páez1, r. m. s. fernandes2, p. c. lafemina3, m. s. bos2 1colombian geological survey, gnss geored project, bogotá, colombia. 2segal (ubi/idl), covilhã, portugal. 3penn state, state college, u.s.a. hmora@sgc.gov.co evaluación de la deformación sismotectónica asociada al sismo de mayo 24, 2008 quetame, colombia héctor mora-páez1, elkin salcedo-hurtado2, augusto antonio gómez-capera3 1 servicio geológico colombiano, proyecto gnss geored, bogotá 2 universidad del valle, observatorio sismológico y geofísico, cali. 3 istituto nazionale di geofisica e vulcanologia, milan, italia hmora@sgc.gov.co the gnss geored project: a tool for understanding the strain rate field and the seismic cycle in colombia, south america current status and challenges héctor mora-páez colombian geological survey, gnss geored project, bogotá, colombia hmora@sgc.gov.co synchronous slow slip events and seismic swarms along the northern peru-ecuador subduction zone jean-mathieu nocquet1, juan carlos villegas lanza2, martin vallée3), paul jarrin4, jean battaglia5, monica segovia1,4, yvonne font1, marc regnier1, philippe charvis1, frédérique rolandone6, mohamed chlieh1, patricia mothes4, hernando tavera2 1geoazur,ird-cnrs-oca-univ. nice, france. 2igp, lima, peru . 3ipg paris, france. 4ig-epn, quito, ecuador. 5lmv, clermont-ferrand, france. 6istep, sorbone upmc univ. paris 6, paris, france. nocquet@geoazur.unice.fr two earthquake cycle geodynamic control networks in costa rica: nicoya and osa-burica peninsulas marino protti1, víctor gonzález1, tim dixon2, susan schwartz3, andrew newman4, esteban chaves1,3, hernán porras1, peter lafemina5 1ovsicori-una. university of south florida in tampa. 3university of california at santa cruz 4georgia tech. 5penn state university marino.protti@gmail.com results from the national continuous gps network in ecuador paul jarrin, patricia mothes, jean-mathieu nocquet, frederique rolandone, david cisneros instituto geofisico pjarrin@igepn.edu.ec deformación basada en datos de gps en colombia robinson quintana1, luis montes vives1, camilo gonzález2 1universidad nacional de colombia. 2universidad distrital. rquintanap@unal.edu.co determination of earthquake sources using gnss seismology r.m.s. fernandes1, h. mora-paez2, j. apolinário1, s. custódio3 1segal (ubi/idl), covilhã, portugal. 2colombian geological survey, bogotá, colombia. 2university of lisbon, idl, lisboa, portugal. rmanuel@di.ubi.pt una red gravimétrica como herramienta de apoyo a la sismología yerson pardo lópez servicio geológico colombiano – universidad nacional de colombia ypardo@sgc.gov.co yjpardol@unal.edu.co conveners: scientific committee crustal structure of the craton san francisco, brazil derived from receiver functions george sand franca, cesar garcia pavao; diogo farrapo albuquerque university of brasilia georgesand@unb.br abrupt changes in moho depth beneath the central andean plateau inferred from apparent pn phase velocities george sand franca1,george zandt2, susan l. beck2, james c ryan, lara s wagner3 1university of brasilia. 2university of arizona. 3university of north carolina. georgesand@unb.br characterization and identification of the meta fault (colombia) from the distribution of resistivities alvarado, h. g. martínez, a. & jiménez, a. universidad nacional de colombia-sede bogotá, departamento de geociencias hgalvarador@unal.edu.co , andmartinezl@unal.edu.co & ajimenezga@unal.edu.co new contributions to the knowledge of the magdalena submarine fan (colombian caribbean) from seismic and high resolution bathymetric data j. idárraga-garcía1, c.a. vargas j.2 and h. león3 1geosciences doctorate program, universidad nacional de colombia, bogotá (colombia). 2departament of geosciences, universidad nacional de colombia, bogotá (colombia). 3dirección general marítima-dimar, bogotá (colombia). jidarragag@unal.edu.co first insights into the crustal structure of the merida andes, venezuela, along the southern profile avila j.1, schmitz m.1, gil e.2, mazuera f.3, yegres l.1, montilla l.4, pinto h.5 1funvisis, fundación venezolana de investigaciones sismológicas.caracas venezuela. 2pdvsa, petróleos de venezuela s.a. puerto la cruz, venezuela. 3universidad de los andes (ula). mérida, venezuela. 4universidad central de venezuela (ucv). caracas, venezuela. 5universidad de simón bolívar (usb). caracas, venezuela. jesavila@gmail.com relación entre estructura del subsuelo y movimiento sísmico. el caso de la ciudad de concepción, chile francisco j. chávez-garcía1, gonzalo montalva2, luis quintanar robles3 1instituto de ingeniería, universidad nacional autónoma de méxico. 2universidad de concepción, bio-bio, chile. 3instituto de geofísica, universidad nacional autónoma de méxico. luisq@geofisica.unam.mx borehole climatology – principles, applications and results vladimir cermak geophysical institute, czech academy of sciences, prague 14131, czech republic. cermak@ig.cas.cz conveners: scientific committee atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos no viscosos inmiscibles alejandro duitama leal, luis montes vides universidad nacional de colombia. aduitamal@unal.edu.co modelado de atenuación y dispersión de ondas acústicas en medios porosos saturados con fluidos viscosos inmiscibles alejandro duitama leal, luis montes vides universidad nacional de colombia. aduitamal@unal.edu.co application of mee algorithm for medium-term earthquake forecast in the koyna-warna region alexey zavyalov1, alexander ponomarev1, rajender chadha2 and srinagesh davuluri2 1institute of physics of the earth ras, moscow, russia, zavyalov@ifz.ru 2national geophysical research institute, hyderabad, india, chadha@ngri.res.in determination of the stress field in an oil production area by mean of analysis of ambient noise andrés rincon, carlos a. vargas universidad nacional de colombia. cavargasj@unal.edu.co study of the montes claros seismogenic fault, in the sao francisco craton, brazil, with correlations of pand s-wave phases caio ciardelli, marcelo assumpção, hans agurto-detzel institute of astronomy, geophysics and atmospheric university of sao paulo sciences. caio.ciardelli@gmail.com máximos pga, pgv y pgd en el plano horizontal sebastián d. panella, carlos d. frau, miguel e. tornello universidad tecnológica nacional facultad regional mendoza. spanella@frm.utn.edu.ar isc products for large earthquakes in the latin america and caribbean region dmitry a. storchak, domenico di giacomo, james harris international seismological centre. dmitry@isc.ac.uk completitud del catálogosísmico en el centro occidente de colombia elizabeth mazo lopera servicio geológico colombiano (sgc). emazo@sgc.gov.co stress field in brazil with focal mechanisms: examples of regional forces controlling the stress field fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com,marcelo@iag.usp.br sismicidad en el piedemonte llanero: caracterización, relocalización y tomografía de velocidades francisco javier muñoz burbano servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc) fmunoz@sgc.gov.co réplicas del sismo del 13 de agosto de 2013. bahía solano océano pacífico colombiano francisco javier muñoz, nelson david pérez, miguel hernández, lina paola aguirre servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc). fmunoz@sgc.gov.co seismic microzonation of urban areas ganesh w. rathod1, k. seshagiri rao2 and kaushal kumar gupta1 1council for geoscience, south africa. 2indian institute of technology delhi, india. grathod@geoscience.org.za seismological detection and analysis of catastrophic landslides goran ekstrom, colin p. stark, clément hibert columbia university. ekstrom@ldeo.columbia.edu results of recent paleoseismologic studies point to the probable seismogenic source of the 1875 earthquake of cucuta, in the border area between colombia and venezuela hans diederix1, luz maria rodriguez2, eliana torres1, catalina hernandez1,3, olga patricia bohorquez1 1servicio geológico colombiano (sgc). 2 dpto. de ciencias de la tierra,fundación venezolana de investigaciones sismológicas (funvisis). 3actualmente: università di bologna – ingv. *corresponding author: hdiederix@sgc.gov.co inversión del tensor de esfuerzo a partir de mecanismos focales obtenidos en el campo volcánico y geotérmico de las tres vírgenes, baja california sur (méxico) yanet antayhua1,2, javier lermo2, luis quintanar 1, oscar campos1 1instituto de geofísica, universidad nacional autónoma de méxico. circuito de la investigación científica s/n, ciudad universitaria, delegación coyoacán, c.p. 04510, méxico d.f. 2instituto de ingeniería, universidad nacional autónoma de méxico. yantayhuav@iingen.unam.mx aplicación de algoritmo de inversión tomográfica aplicada en la cuenca de urabá john gonzález veloza, luis castillo universidad nacional de colombia. johngonzalezv@gmail.com propagacion de ondas de rayleigh en el valle de méxico y una posible explicacion para la larga duracion de los sismos josé d. sanabria-gómez1,2, víctor m. cruz-atienza2, josué tago2,3, emmanuel chaljub3 and jean virieux3 1escuela de física, universidad industrial de santander. bucaramanga, colombia. 2instituto de geofísica, unam. mexico df, méxico. 3institut des sciences de la tèrre, ujf. grenoble, francia. jsanabri@uis.edu.co efecto de sitio no lineal sobre basalto usando cinco sismos m>6 de guatemala julio césar cuenca sánchez instituto de ingeniería universidad nacional autónoma de méxico. jccsa@pumas.ii.unam.mx características espectrales de la sismicidad originada en la cuenca del valle de méxico luis quintanary delia i. bello instituto de geofísica, unam. luisq@geofisica.unam.mx a test of the rstt model for south america with brazilian pgt events marcelo assumpção, bruno collaço institute of astronomy, geophysics and atmospheric sciences; university of são paulo, rua do matão 1226, 05508-090, são paulo, sp, brazil. marcelo.assumpcao@iag.usp.br the september 5th, 2012 nicoya, costa rica earthquake, a subduction earthquake captured in the near field marino protti1, susan schwartz2, víctor gonzález1, timothy dixon3, andrew newman4, lujia feng5, han yue2, thorne lay2 and susan owen6 1ovsicori-una. 2university of california at santa cruz. 3university of south florida in tampa. 4georgia tech. 5earth observatory of singapore. 6jpl-caltech. marino.protti@gmail.com estimación del riesgos sísmico en managua, nicaragua, con la metodología capra hernández o., izaguirre a., strauch w., otros. (se sinapred, bm, cepredenac). ohernandez@sinapred.gob.ni structuraldelineationof the servitafaultsystem,colombianeasterncordillera, usingmicroseismicity patriciapedraza1,2andcristinadimaté2 1serviciogeológicocolombiano(sgc),direccióndegeoamenazas,bogotá.colombia. 2posgrado de geociencias, universidadnacionaldecolombia. ppedraza@sgc.gov.co,mcdimatec@unal.edu.co strong motion recorded during the emilia 2012 thrust earthquakes (northern italy): a comprehensive analysis rita de nardis1, luisa filippi2, giovanni costa3, peter suhadolc3, mario nicoletti2, giusy lavecchia1 1univ chieti. 2dpc, rome. 3univ trieste. suhadolc@units.it rupture process on fracking operations applying pwave back projection images sebastían gómez alba1, carlos alberto vargas j.1; william agudelo2, alexander martínez2 1geosciences department, national university of colombia. 2instituto colombiano del petróleo icp. sagomezalb@unal.edu.co alternatives for computing regional and global seismic-phase travel times using a 3-dimensional model stephen c. myers, nathan a. simmons, eric m. matzel, and gardar johannesson lawrence livermore national laboratory. myers30@llnl.gov paleoseismic study of the north panama deformed belt and probabilistic seismic hazard assessment for a proposed harbor facility on the caribbean coast of costa rica tania gonzález1, eldon gath1, walter montero2, and christopher madden madugo1 1earth consultants international. 2geological research center, costa rica. tgonzalez@earthconsultants.com conveners: anne sheehan (colorado univ. usa) <afs@cires.colorado.edu> gaspar monsalve (unal colombia) <gmonsalvem@unal.edu.co> constructing shear wave models using multi-objective optimization for multiple geophysical data sets lennox thompson1, aaron a. velasco1, anibal sosa2, vladik kreinovich1, and rodrigo romero1 university of texas at el paso. 2universidad icesi. aavelasco@utep.edu study of seismic attenuation and unusual phases at uturuncu volcano, bolivia alexandra k farrell and stephen mcnutt university of south florida. akfarrell@mail.usf.edu title: 3-d structure of the rio grande rift from 1–d constrained joint inversion of receiver functions and surface wave dispersion anibal sosa1. lennox thompson, aaron a. velasco2. robert herrmann3 1universidad icesi, cali-colombia. 2department of geological sciences, university of texas at el paso, el paso-tx. 3department of earth & atmospheric sciences, saint louis university, st. louis, mo. uasosa@icesi.edu.co the moana experiment: a broadband ocean bottom seismology investigation of a transform plate boundary, new zealand anne f. sheehan1, daniel w. zietlow1, justin ball1, william l. yeck1, joshua c. stachnik2, fan-chi lin3, john a. collins4 1university of colorado at boulder. 2lehigh university. 3university of utah. 4woods hole oceanographic institution anne.sheehan@colorado.edu modelación fisica y simulacion numérica en procesos de adelgazamiento litosférico para zonas convergentes c. muñoz lópez1, c. vargas1, e. poveda2 1universidad nacional de colombia. 2servicio geológico colombiano cemunozl@unal.edu.co, cavargasj@unal.edu.co joint inversion of geoid anomaly data and teleseismic p-wave delay times: modeling density and velocity perturbations underneath the parana magmatic province carlos alberto moreno chaves1, naomi ussami1 and jeroen ritsema1 1department of geophysics, institute of astronomy, geophysics and atmospheric sciences, university of sao paulo, sao paulo, brazil. 2department of earth and environmental sciences, university of michigan, ann arbor, michigan, usa. carlos.chaves@iag.usp.br joint inversion of seismic and gravity data for velocity structure and hypocentral locations of the colombian subduction zone ellen syracuse1, monica maceira1, germán prieto2, haijiang zhang3 1lanl. 2mit. 3university of science and technology of china. syracuse@lanl.gov crustal thickness estimation beneath the northern andes using the receiver function method esteban poveda1, gaspar monsalve2, carlos a. vargas3, jordi julià4 1departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil; red sismológica nacional, servicio geológico colombiano (sgc), bogotá. colombia. 2escuela de geociencias y medio ambiente, universidad nacional de colombia, medellín, colombia. 3departamento de geociencias, facultad de ciencias, universidad nacional de colombia, bogotá, colombia. 4departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. esteban.poveda@gmail.com crustal models 1d for colombia from surface waves dispersion observations and receiver functions faustino blanco1, esteban poveda1, 2, viviana dionicio1 1servicio geológico colombiano (sgc), bogotá. colombia 2now at departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. lithospheric and upper mantle stratification beneath colombia: using receiver functions from s-waves 1,2blanco f, 2vargas c.a. 1geological service of colombia. bogotá, colombia. 2department of geosciences, national university of colombia at bogota estudio de la estructura de la corteza en la zona de convergencia placa de rivera – bloque de jalisco (proyecto tsujal) f. j. núñez cornú1, d. córdoba barba2, jj dañobeitia3, w. bandy4, c. escudero-ayala1, j.m. espíndola4, a. zamora1, r. bartolomé3, a. gómez, m. ortiz5 y el gpo. de trabajo tsujal 1centro de sismología y volcanología de occidente, universidad de guadalajara, méxico. 2fac. de ciencias físicas universidad complutense de madrid, españa. 3cento mediterráneo de investigaciones marinas y ambientales, con. sup. de investigaciones científicas, españa. 4instituto de geofísica, universidad nacional autónoma de méxico. pacornu77@gmail.com travel time delays and residual topography in northwestern south america provide constraints on upper mantle temperatures and slab geometry jefferson yarce and gaspar monsalve mejía1, thorsten w. becker2 1universidad nacional de colombia sede medellín. 2university of southern california. jyarce@unal.edu.co looking for mantle plumes under the brazilian shield: case studies in paraná and borborema j. julià, m. bianchi and m. assumpção ufrn. jordi@geofisica.ufrn.br inferences about the lithospheric seismic structure of valle de aburra based on travel time residuals from strong motion networks jose david henao and gaspar monsalve mejía universidad nacional de colombia. jodhenaoca@unal.edu.co deep lithospheric features of the são francisco craton marcelo assumpção1, paulo araújo2, marcelo peres rocha2, eder molina1, marcelo bianchi1 1dept. of geophysics, university of sao paulo, brazil. 2geosciences institute, university of brasilia, brazil. marcelo.assumpcao@iag.usp.br ambient noise tomography: preliminary results at southeast of south american platform marcos vinícius ferreira1, marcelo peres rocha1, martin schimmel2 1university of brasilia. 2institut de ciències de la terra jaume almera. marcelorocha@unb.br relationship between intraplate seismicity and low velocities tomography anomalies in the tocantins province, brazil paulo araújo de azevedo, marcelo peres rocha, reinhardt a. fuck, juliana garrido university of brasilia marcelorocha@unb.br sismicidad localizada en la región de manto litosferico de placa sudamericana debajo de la provincia geológica de payenia sur de mendoza argentina lupari marianela n.1, nacif silvina v. 1, spagnotto silvana l.2, yacante graciela1 1instituto geofísico sismológico f. volponi facultad de ciencias exactas, físicas y naturales universidad nacional de san juan (unsj). 2departamento de física, universidad nacional de san luis, ejército de los andes 950, 5700, san luis argentina. marianelalupari@gmail.com crustal and upper-mantle structure of the pampean flat-slab region from surface-wave tomography ryan porter1, megan anderson2; susan beck3, hersh gilbert4, george zandt3; patricia alvarado5 1northern arizona university.2colorado college. 3university of arizona.4purdue university.5universidad nacional de san juan. ryan.porter@nau.edu estructura litosférica asociada a la sierra nevada de santa marta a partir de datos de gravimetría, magnetometría y sismología sergio alejandro camargo vargas universidad nacional de colombia. sacamargo@sgc.gov.co association of mw = 6.4, 2000/06/16 and mw = 5.7, 2003/01/07 intermediate-depth earthquakes, in subducted nazca plate silvana spagnotto1, enrique triep2, laura giambiagi3 and silvina nacif2,4 1conicet postdoc universidad nacional de san luis, argentina. 2igsv, universidad nacional de san juan, argentina. 3ianigliacct mendoza-conicet. 4fonarsec postdoc slspagno@unsl.edu.ar investigation into the kinematics of basement-cored uplifts through passive source imaging of the bighorn mountains, united states william l. yeck1, anne f. sheehan1, josh stachnik2 1department of geological sciences, university of colorado, boulder co, 80309-0399, united states. 2department of earth and environmental sciences, lehigh university, bethlehem pa, 18015, united states. yeck@colorado.edu hypocentral parameter inversion for regions with poorly known velocity structures woohan kim, tae-kyung hong, tae-seob kang gyeongsang national university. wookim@gnu.ac.kr conveners: jiří zahradník (charles univ. in prague czech rep.) <jz@karel.troja.mff.cuni.cz> efthimios sokos (univ. patras greece) <esokos@upatras.gr> ronnie quintero (ovsicori-una costa rica) <ronnie.quintero.quintero@una.cr> cálculo del tensor de momento sísmico a partir de la inversión de formas de onda usando el software isola: sismo de nuquí colombia del 13-08-2013 (mw 6.5) alba estella ordóñez, josé faustino blanco chía servicio geológico colombiano (sgc). aordonez@sgc.gov.co moment tensor solution of moderate events at large distances: using surface waves dispersion to build velocity models for the waveform inversion, examples from brazil fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com focal mechanism and depth determination of earthquakes in brazil using teleseismic p-wave waveform modeling and first-motion polarities fabio luiz dias, marcelo assumpção departamento de geofísica, iag. universidade de são paulo, rua do matão 1226, 05508-090, são paulo, brasil. fabioludias@gmail.com,marcelo@iag.usp.br new strategy for weak events in sparse networks: first-motion polarity solutions constrained by single-station waveforms lucia fojtikova institute of rock structure and mechanics, academy of sciences, czech republic jiri zahradnik, charles university in prague, czech republic jiri.zahradnik@mff.cuni.cz seismic source parameters of local micro-earthquakes in goiás state brazil by waveform inversion juraci carvalho1, lucas vieira barros1, jiří zahradník2 1observatório sismológico – unb, brasília – df, brasil. 2faculty of mathematics and physics – charles university in prague, czech republic. juraci@gmail.com source seismic parameters of earthquakes close to funil reservoir power plant in the south of minas gerais state – brazil lucas vieira barros, vinicius martins ferreira, mônica g. von huelsen, juraci mario de carvalho, daniel farias de caixeta and darlan portela fontenele observatório sismológico da universidade de brasília. lucas@unb.br modelamiento de fuente sísmica del sismo del quindío (colombia) del 25 de enero de 1999 usando inversión de forma de onda de cuerpo telesísmica lynette vanessa chavarría1, helber garcía camacho1 y hugo monsalve j.2 1grupo quimbaya, universidad del quindío. 2ceifi, grupo quimbaya – universidad del quindío. hugom@uniquindio.edu.co moment tensor solution for a m 4.2 event in tierra del fuego c. bollini1,2, n. sabbione1 1facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata. 2consejo nacional de investigaciones científicas y técnicas (conicet). bollini@fcaglp.unlp.edu.ar inversion of a 2-point source model of the m7.4 2012 march 20th guerrero, mexico oscar a. castro-artola1, jiří zahradník2 and efthimios sokos 1posgrado en ciencias de la tierra, instituto de geofísica, unam. 2charles univ. in prague czech rep. 3univ. patras – greece. oscar@geofisica.unam.mx moment-tensor solution of the intermediate-depth 2013 guaitarilla (colombia) mw 6.9 earthquake using near-regional waveform data patricia pedraza1 and jiri zahradnik2 1servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá. colombia. 2faculty of mathematics and physics – charles university in prague, czech republic. ppedraza@sgc.gov.co, jz@karel.troja.mff.cuni.cz seismicmomenttensorsolutionsof m > 4.5 earthquakesincolombia patricia pedraza and carlos fajardo servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá. colombia. ppedraza@sgc.gov.co, cfajardo@sgc.gov.co resolución y análisis de mecanismos focales de sismos corticales en cordillera entre 34.5 s y 36 s raquel villegas1, silvina nacif2,3 y silvana spagnotto1,4 1conicet. 2igsv, universidad nacional de san juan. 3fonarsec. 4universidad nacional de san luis. vraquelj@gmail.com near-regional cmt and multiple-point source solution of the september 5, 2012, nicoya, costa rica mw 7.6 earthquake ronnie quintero1, jiri zahradník2 and efthimios sokos3 1observatorio vulcanológico y sismológico de costa rica, universidad nacional, heredia, costa rica.rquinter@una.cr 2charles university in prague, faculty of mathematics and physics, prague, czech republic. jiri.zahradník@mff.cuni.cz 3university of patras, department of geology, seismological laboratory, patras, greece.esokos@upatras.gr el sismo mw 6.2 del 27 de febrero de 2010, salta argentina. mecanismo focal y modelos de deslizamientos usando inversión de formas de ondas sísmicas e interferometría sar spagnotto silvana1, scott, chelsea2 y garcía victor3 1universidad nacional de san luis. posdoc conicet. 2cornell university. 3universidad nacional de rio negro. conicet. pampa113@gmail.com source parameters by waveform inversion of 6.6mw earthquake in the nicoya peninsula on october 24, 2012 aftershock of the 7.6mw main shock on september 5, 2012 vinicius martins ferreira1, kellen schmidt, lucas vieira barros, ronnie quintero2 1seismological observatory of the university of brasília. 2seismological observatory of costa rica and the national university. vinicius2002@gmail.com conveners: alessia maggi (univ. strasbourg france) <alessia@unistra.fr> victor huerfano (univ. puerto rico) <victor@prsn.uprm.edu> zacharie duputel (univ. strasbourg france) <zacharie.duputel@unistra.fr> diseño e implentación del sistema de información de la red sismológica nacional de colombia acosta, my., beltrán, j., sánchez, c., chamorro, o. servicio geológico colombiano. macosta@sgc.gov.co determinación de la escala de magnitud local (ml) a partir de registros de banda ancha para el perú crsitobal condori y hernando tavera instituto geofísico del perú. cristobal.condori@igp.gob.pe a streaming processing model for efficient computations of large continuous data sets danny harvey boulder real time technologies danny@brtt.com moment-tensor calculation for the vega colombia mw 7.2 earthquake esteban poveda1, jiri zahradnik2, patricia pedraza3, jordi julià4 1red sismológica nacional, servicio geológico colombiano (sgc), bogotá. colombia. now at departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. 2faculty of mathematics and physics, charles university in prague, prague, czech republic. 3red sismológica nacional, servicio geológico colombiano (sgc). departamento de geofísica & programa de pós-graduação em geodinâmica e geofísica, universidade federal do rio grande do norte, natal, brazil. esteban.poveda@gmail.com comparison of manual and automatic system locations for seismic events in panama pérez, jhon leandro1, camacho, eduardo2,christensen, branden1 1osop. 2igc-upa. jleandro@osop.com.pa physisc and statistics of the seismic source using first digit anomaly patricio a. toledo1and jaime a. campos2 1universidad de chile and universidad adolfo ibanez. 2universidad de chile patoledo@ing.uchile.cl use of the first digit anomaly in background seismicity studies under chilean type of subduction environment patricio a. toledo1and jaime a. campos2 1universidad de chile and universidad adolfo ibanez. 2universidad de chile. patoledo@ing.uchile.cl implementación del cálculo de los de los parámetros de la fuente sísmica mediante el método de la fase w, en el sgc ruth emilse bolaños cifuentes y josé faustino blanco chía servicio geológico colombiano (sgc). rbolanos@sgc.gov.co evolución tecnologica de la red sismológica nacional de colombia rsnc y la red nacional de acelerógrafos de colombia rnacdel servicio geológico colombiano sánchez, c., acosta, my., chamorro, o. servicio geológico colombiano. csanchez@sgc.gov.co near real-time earthquake source characterization through joint geodetic and seismic approaches at the us geological survey national earthquake information center wd barnhart, gp hayes, dj wald usgs. wbarnhart@usgs.gov conveners: antonio gomez (ingv milano italia) <gomez@mi.ingv.it> carlo meletti (ingv pisa italia) <meletti@pi.ingv.it> marlon pirchiner (usp brazil) <marlon@iag.usp.br> herbert rendon (funvisis venezuela) <hrendon@funvisis.gob.ve> hacia un catálogo actualizado de terremotos de sur américa: ventana temporal pre-1930 a.a. gómez capera1, m. arcila2, m. bufaliza3, j. choy4, e. minaya5, m. pirchiner6, h. rendón7, l. rodriguez8, a. sarabia2, h. tavera9, h. yepes10, m. stucchi11 1istituto nazionale di geofisica e vulcanologia, sezione di milano, italia. 2servicio geológico colombiano, bogotá, colombia. 3instituto nacional de prevención sismica, san juan, argentina. 4universidad de los andes, facultad de ciencias, laboratorio de geofísica, mérida-venezuela. 5observatorio de san calixto, la paz, bolivia. 6universidade de são paulo, brasil. 7funvisis, caracas, venezuela. 8ceresis, lima, perú. 9instituto geofísico del perú, lima, perú. 10instituto geofísico de la escuela politécnica nacional, quito, ecuador. 11eucentre, pavía, italia. antonio.gomez@ingv.it parámetros del terremoto de cúcuta del 1875 a partir de intensidades macrosísmicas a.a. gómez capera1, h. cifuentes2, a. sarabia2, d. bindi3, j. choy4, r. galan5, m. pirchiner6, l.m. rodríguez7, e. salcedo hurtado8, a. sánchez5 1istituto nazionale di geofisica e vulcanologia, sezione di milano, milan, italy. 2servicio geológico colombiano, bogotá, colombia. 3deutches geoforschungszentrum gfz, center for early warning, potsdam, germany. 4universidad de los andes, facultad de ciencias, laboratorio de geofisica, mérida-venezuela. 5departamento de matemáticas, universidad nacional de colombia, bogotá, colombia. 6universidade de são paulo, brasil. 7funvisis, caracas, venezuela. 8observatorio sismológico y geofísico de la universidad del valle, cali, colombia. antonio.gomez@ingv.it seismotectonic and seismic hazard implications for south africa b. manzunzu and v. midzi council for geoscience, south africa. bmanzunzu@geoscience.org.za psha at a regional scale: european share project versus italian reference psha. and the winner is… carlo meletti istituto nazionale di geofisica e vulcanologia. carlo.meletti@pi.ingv.it respuesta dinámica de suelos para managua/nicaragua carlos guzmán ineter, managua, nicaragua. carlos.guzman@gf.ineter.gob.ni challenges to developing a unified earthquake catalog for seismic hazard assessment in south america – addressing magnitude homogenization and depth uncertainties claire pontbriand, mehrdad mahdyiar, wenzheng yang, feng wang, bingming shen-tu, gerald galgana, elliot klein, khosrow shabestari air worldwide, boston, ma, usa. cpontbriand@air-worldwide.com seismic activity in the pantanal basin brazil edna maria facincani1, marcelo s. assumption2, fábio luiz dias2, tiago gomes da silva3, gustavo marques amorim3 1campus aquidauana-cpaq/ufms/brasil. 2institute of astronomy and geophysics, iag usp / usp. 3institute of física-infi/ufms/brasil. edna_facincani@hotmail.com microzonificación para diseño por sismos para la ciudad de panamá, panamá javier francisco lermo samaniego1, eduardo camacho astigarrabia2, josé antonio martínez gonzález1, elvia carbajal becerril1, néstor luque2, arkin a. tapia2, eric chichaco2, johnny gil2, yadira echeverría durán2, roberto palma q.3 1coordinación de ingeniería sismológica, instituto de ingeniería-unam; cd. universitaria, apdo. postal 70-472, coyoacán 04510 méxico, d.f. e-mail: jles@pumas.iingen.unam.mx 2instituto de geociencias, universidad de panamá. 3geoconsult s.a., panamá, panamá. eduardo.camachoa@up.ac.pa teleseismic waves registered at the seismographic station of aquidauana / ms /brazil (aqdb), in 2013. estêvão vasconcello campos tadeu1, marcelo s. assumpção2, tiago gomes da silva3. fábio luiz dias2, edna maria facincani4, gustavo marques e amorim1, ana paula trindade souza4, emilly lopes de oliveira4 1physics institute infi/ufms/brasil. 2astronomy and geophysics institute uspiag/usp. 3physics institute -infi/ufms/brasil. 4aquidauana campus-cpaq/ufms/brasil. estevao-v-tadeu@hotmail.com quantifying uncertainties in site amplification caused by using low resolution soil maps for site characterization feng wang, mehrdad mahdyiar, bingming shen-tu, khosrow shabestari, gerald galgana, elliot klein, claire pontbriand, and wenzheng yang air worldwide. fwang@air-worldwide.com a stochastic rupture probability model for earthquakes on subduction zones: a case study from the nazca subduction zone mehrdad mahdyiar, gerald galgana, bingming shen-tu, elliot klein, claire pontbriand, feng wang, khosrow shabestari, and wenzheng yang air worldwide, 131 dartmouth st., boston, ma 02116, usa. ggalgana@air-worldwide.com la sismicidad en el piedemonte llanero y la amenaza sísmica de la sabana de bogotá chicangana, g.1, 2, vargas, c. a.2 caneva, a.3, salcedo, e.4 y gómez, a.5 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. german.chicangana@unimeta.edu.co 2grupo geofísica, departamento de geociencias, universidad nacional de colombia, bogotá d.c., colombia. cavargasj@unal.edu.co 3dirección nacional de investigaciones, universidad antonio nariño, bogotá d.c., colombia. alexander.caneva@uan.edu.co 4observatorio sismológico del suroccidente, universidad del valle, cali, colombia. elsalced@univalle.edu.co 5istituto nazionale di geofísica e vulcanologia, milano, italy. antonio.gomez@mi.ingv.it towards the construction of a seismic catalogue suitable for psha studies in south america: instrumental contribution m. arcila2, l. alvarado1, j. eraso2, a. a. gómez capera5, e. minaya6, m. pirchiner4, h. rendón1, l. rodríguez 7, d. storchack8, m. tovar2, h. yepes3 1funvisis, caracas. 2servicio geológico colombiano, bogotá. 3instituto geofísico de la escuela politécnica nacional, quito. 4universidade de são paulo, são paulo. 5istituto nazionale di geofisica e vulcanologia, sezione di milano. 6observatorio de san calixto, la paz. 7ceresis, lima. 8isc, uk. hrendon@funvisis.gob.ve probabilistic seismic hazard assessment in quito city, estimates and uncertainties céline beauval1, hugo yepes2,1, alexandra alvarado2, laurence audin1, jean-mathieu nocquet3, damiano monelli4, laurentiu danciu5 1isterre, ird-cnrs-ujf-osug, grenoble, france. 2instituto geofisico, quito, ecuador. 3geoazur, cnrs-ird-uns-oca, sophia antipolis, france. 4gem hazard team, gem foundation, pavia, italy. 5sed-ethz, zurich, switzerland. hyepes@igepn.edu.ec average shear wave velocity down to 30m (vs30) zoning for the city of pasto, colombia jaime eraso, martha tovar, mónica arcila servicio geológico colombiano, bogotá, colombia. jeraso@sgc.gov.co simulación 3d de la propagación de ondas sísmicas en el valle de caracas usando diferencias finitas jaime parada1, otilio rojas1, michael schmitz2 1universidad central de venezuela. 2funvisis. jaime.parada@ciens.ucv.ve results from the 2013 pan-american advanced studies institute in santo domingo, dominican republic: joint modeling of complementary data functionals for seismic site characterization pulliman, j.1, schwed, m.1, sen, m. k.2, willemann, r. j.3, huerta-lopez, c.4, moschetti, m. p.5, schmitz, m.6, louie, j. n.7, polanco, e.8, huerfano moreno, v.9, pasyanos, m.10 1baylor university, usa. 2university of texas at austin, usa. 3iris consortium, usa. 4university of puerto rico, mayaguez, usa. 5u.s. geological survey, usa. 6funvisis, venezuela. 7university of nevada, reno, usa. 8universidad autonoma de santo domingo, santo domingo, dominican republic. 9university of puerto rico, mayaguez. 10lawrence livermore national laboratory, livermore, ca, usa. jay_pulliam@baylor.edu caracterización del subsuelo de la zona lacustre de bogotá, usando técnicas de análisis de microtremores john jairo mora soto y maría cristina dimate c. universidad nacional de colombia, departamento de geociencias. jjmoras@unal.edu.co tectónica activa en el costado occidental del valle del rio zulia – norte de santander, colombia -aportes de las técnicas cuantitativas de análisis de drenaje jorge adrian oviedo reyes1,2y maría cristina dimaté castellanosb1 1universidad nacional de colombia. 2servicio geologico colombiano. jaoviedor@unal.edu.co modeling seismic hazard in south america: current state of the state-of-the-art based on activities carried out with the sara project arcila m.1, eraso j.1, garcia j.2, pagani m.2, pirchiner m.4, rendon h.3, weatherill g.2, yepez h.5, boroschek r.6, contreras v.7 1colombian geological survey (cgs), colombia. 2gem foundation, italy. 3venezuelan foundation for seismological researches (funvisis), venezuela. 4university of são paulo, brazil. 5i universitè j. fourier grenoble, francia. 6university of chile, chile. 7ruben boroschek and associates, chile. julio.garcia@globalquakemodel.org high-resolution, uiltra low power, intergrated aftershock and microzonation system leonid zimakov, p.passmore, j.raczka, a. leon ref tek – a division of trimble. l.zimakov@reftek.com el terremoto del 18 de mayo de 1875, en la región fronteriza colombo-venezolana :inventario de daños y de efectos geológicos co y postsísmicos luz m rodríguez1, elkin salcedo2, hans diederix3, augusto a. gomez4, santiago yépez5, andré singer1, eliana torres3, josé a. rodríguez1, alejandra leal1, carol perez1, jose choy6, franck a. audemard1, armando espinosa3, olga bohorques3 1dpto. de ciencias de la tierra, fundación venezolana de investigaciones sismológicas (funvisis). 2universidad del valle del cauca, cali. 3servicio geológico colombiano (sgc). 4istituto nazionale di geofísica e vulcanología, sezione di milano-pavia (ingv). 5fundación instituto de ingeniería (fii). 6laboratorio de geofísica, departamento de física, facultad de ciencias universidad de los andes (ula). lrodriguez.funvisis@gmail.com intraplate seismicity and seismic zoning in brazil marcelo assumpção1, hilario bezerra2, lucas barros3, hans agurto1 1university of são paulo, são paulo, sp. 2federal university of rio grande do norte, natal, rn. 3university of brasília, brasília, df. marcelo.assumpcao@iag.usp.br el sismo de guatemala de 1816 (m 7.5). implicaciones para la sismicidad en el límite de placas norte américa-caribe marco guzmán-speziale centro de geociencias, universidad nacional autónoma de méxico. marco@geociencias.unam.mx magnitude and location of large historical earthquakes in colombia from macroseismic intensity data maria cristina dimate1, martha bohorquez2, ana milena sarabia3, hernan cifuentes3 1universidad nacional de colombia, geosciences department. 2universidad nacional de colombia, statistics department. 3servicio geologico colombiano. mcdimatec@unal.edu.co smoothing techniques applied to brazilian seismic sources characterization m. pirchiner1,2 1institute of astronomy, geophysics and atmospheric sciences. university of são paulo. 2school of applied math, getulio vargas foundation, rio de janeiro. marlon.pirchiner@iag.usp.br caracterización de la fuente sísmica asociada con la sismicidad hb, lp y drumbeats registrada en el volcán nevado del huila – colombia, en noviembre de 2008 nancy trujillo castrillón1, carlos miguel valdés gonzales1, randall white2, wendy mccausland2; santacoloma cristian camilo3 1unam. 2usgs. 3sgc. canytas@gmail.com sistema de información de sismicidad histórica de colombia sarabia, a.1, cifuentes, h.2, dimaté, c.3, arcila, m.4, barbosa, d.5, galán, b.6 1servicio geológico colombiano. asarabia@sgc.gov.co 2servicio geológico colombiano. hcifuentes@sgc.gov.co 3universidad nacional de colombia, departamento de geociencias. mcdimatec@unal.edu.co 4servicio geológico colombiano. marcila@sgc.gov.co 5servicio geológico colombiano. dbarbosa@sgc.gov.co 6servicio geológico colombiano. bgalan@sgc.gov.co south american strong-motion database and comparison with ground-motion prediction equations: a gem initiative maria cristina dimaté castellanos1, luisa fernanda castillo2, stéphane drouet3, carlos lozano2, gonzalo montalva4, cecilio morales5, marlon pirchiner6, juan carlos singaucho armas7, cristian viracucha7, graeme weatherill8, sinan akkar9 1universidad nacional de colombia, bogotá, colombia. 2servicio geológico colombiano, bogotá, colombia. 3observatório nacional, rio de janeiro, rj, brazil. 4universidad de concepción, concepción, chile. 5fundación venezolana de investigaciones sismológicas, caracas, venezuela. 6instituto de astronomia, geofísica e ciências atmosféricas, são paulo. 7instituto geofísico escuela politécnica nacional, quito, ecuador. 8gem foundation, pavia, italia. 9bogazici university kandilli, observatory and earthquake research institute department of earthquake engineering, istanbul. stephane@on.br seismic hazard assessment of sites of large structures in the southeast of brazil tania bustamante1, celso romanel1, maria cascão2 1pontifical catholic university of rio de janeiro (puc-rio). 2federal university of rio de janeiro (ufrj). taniavanessa@gmail.com evaluación probabilística de riesgos en centroamérica con la metodología capra wilfried strauch centro de coordinación para la prevención de los desastres naturales en centroamérica (cepredenac). wilfried.strauch@yahoo.com conveners: kinya nishigami (kyoto univ. japan) <nishigami.kinya.3r@kyoto‐u.ac.jp> carlos vargas (unal colombia) <cavargasj@unal.edu.co> crustal structure of the paraná basin from ambient noise tomography bruno collaço1, marcelo assumpção1, maria laura rosa2, gerardo sanchez3 1iag-usp. 2unlp. 3inpres. bruno@iag.usp.br reflection imaging of deep magma with natural sources: lessons from recent experiments on an interplate volcano and an intraplate earthquake diego quiros, larry d. brown, doyeon kim, katrina byerly, keisuke irie cornell university. daq7@cornell.edu seismic noise environmental caracterization: application to a pilot area in bogota for seismic microzonation purposes urrego, g.1, vargas, c.a. departamento de geociencias, universidad nacional de colombia – bogotá. giurregom@unal.edu.co crust and upper mantle seismic tomography in northwest of south america using surfaces waves j. c. bermúdez b.1,2, m. c. dimaté c.2 1servicio geológico colombiano. 2departamento de geociencias, universidad nacional de colombia. jcbermudez@sgc.gov.co crustal heterogeneity and earthquake generating properties in active fault regions in japan kinya nishigami disaster prevention research institute, kyoto university. nishigami.kinya.3r@kyoto-u.ac.jp surface wave tomography of the chaco-parana basin maría laura rosa1, bruno collaço2, gerardo sánchez3, marcelo assumpção2, nora sabbione1 1dpto. de sismología. facultad de ciencias astronómicas y geofísicas. universidad nacional de la plata. argentina. 2dpto. de geofísica. instituto de astronomia, geofísica e ciencias atmosféricas. universidade de são paulo. brasil. 3instituto nacional de prevención sísmica (inpres). argentina. mlrosa@fcaglp.unlp.edu.ar geoscientific research for lithospheric structures of the merida andes, venezuela michael schmitz, funvisis, mschmitz@funvisis.gob.ve nuris orihuela, universidad central de venezuela (ucv). stephanie klarica, universidad de los andes (ula). euries gil, petróleos de venezuela (pdvsa). franck audemard, fundación venezolana de investigaciones sismológicas (funvisis). fernando mazuera, universidad de los andes (ula). jesus ávila, fundación venezolana de investigaciones sismológicas (funvisis). luis yegres, fundación venezolana de investigaciones sismológicas (funvisis). javier sánchez, fundación venezolana de investigaciones sismológicas (funvisis). giame working group mschmitz@funvisis.gob.ve conveners: dmitry storchak (isc uk) <dmitry@isc.ac.uk> marcelo assumpção (usp brazil) <marcelo.assumpcao@iag.usp.br> the northeastern brazil seismic network – rsisne aderson do nascimento, joaquim mendes ferreira, jordi julià, regina spinelli, eduardo menezes ufrn, brazil. aderson@geofisica.ufrn.br the saint peter saint paulo island archipelago seismic station aderson do nascimento, guilherme de melo, daniel queiroz, martin schimmel, eduardo menezes, regina spinelli ufrn, brazil. aderson@geofisica.ufrn.br ensamblado, configuración, instalación y monitoreo de las estaciones sismológicas en las instalaciones del observatorio de rayos cósmicos del proyecto internacional pierre auger andrés nacif1, marianela lupari2, rubén squartini3, silvina nacif4 y silvana spagnotto2,5 1consultor fonarsec. 2conicet. 3observatorio pierre auger. 4fonarsec. 4universidad nacional de san luis. nacif.andres@gmail.com velocity spectral analysis for telesesismic events using the colombian seismological network as an array j. c. bermúdez b.1,2, n. d. pérez g1,2, h. e. poveda n.3 1servicio geológico colombiano 2departamento de geociencias, universidad nacional de colombia 3departamento de geofísica, universidade federal do rio grande do norte, brazil. jcbermudez@sgc.gov.co evolución de la red nacional de acelerógrafos de colombia carlos lozano1, maría luisa bermúdez2, jorge correa1, elizabeth mazo1, helber garcía1, gustavo redondo1 1servicio geológico colombiano (sgc), dirección de geoamenazas, bogotá d.c., colombia. 2kma construcciones, bogotá d.c., colombia. clozano@sgc.gov.co microseismic noise in the saint peter saint paul archipelago and its relation with some atmospheric and oceanographic variables daniel queiroz, aderson do nascimento, martin schimmel, mario silva ufrn, brazil. daniel.geofisica@gmail.com international seismological centre (isc): mission and products dmitry a. storchak, domenico di giacomo, james harris international seismological centre dmitry@isc.ac.uk aeqc tools for retrieval and completion of seismological waveform databases from heterogeneous networks el-madani aissaoui1, arnaud lemarchand1, alexandre nercessian1, jean-marie saurel2, andré anlade2 1ipgp. 2ovsm-ipgp. aissaoui@ipgp.fr activities of the fdsn: current and future g. ekstrom columbia university. ekstrom@ldeo.columbia.edu composición y funcionamiento de las estaciones satelitales de la red sismologica nacional de colombia j. de la rosa ramirez, j. fernandez servicio geológico colombiano, bogotá, colombia jdelarosa@sgc.gov.co, jefernandez@sgc.gov.co portable, gps synchronized, mobile phone network-based seismological station carlos alberto vargas jiménez1, juan manuel solano fino1, alexander caneva2 1universidad nacional de colombia. 2universidad antonio nariño. jmsolanof@unal.edu.co análisis de ruido de las estaciones sismológicas del servicio geológico colombiano a partir del cálculo de densidad espectral de potencia probabilística lina paola aguirre1, nelson david pérez1, 2, juan carlos bermúdez1, 2, jaime eraso1 1servicio geológico colombiano (sgc), dirección de geoamenazas, red sismológica nacional de colombia (rsnc). 2universidad nacional de colombia (unal), departamento de geociencias. subduction zone events in bolivia as recorded by the bolivian and brazilian networks: a test of location accuracies lúcio quadros1, marcelo assumpção1, estela minaya2, percy aliaga2, teddy griffith2, guido avila2 1seismology center, university of são paulo, brazil. 2observatório san calixto, la paz, bolivia. lucioquadros14@gmail.com the new brazilian seismic network: current status and detectability test with the recent pisagua earthquake sequence, northern chile marcelo bianchi1, marcelo assumpção2, hans agurto2, bruno collaço2, jackson calhau2, josé roberto barbosa2, luiz galhardo2, marlon pirchiner2, sergio fachim2, cleusa barbosa2, marcelo rocha3, juraci carvalho3, paulo araujo3, francimilton silva3, georg... 1university of são paulo, seismology center iag/usp. 2iag/usp. 3university of brasilia, seismological observatory – unb. m.bianchi@iag.usp.br tarjeta amplificadora para la adapatación de la digitalización de los sistemas sismológicos de corto período en el sgc nicolás oliveras mercado servicio geológico colombiano noliveras@sgc.gov.co optimización de redes de datos para el monitoreo volcánico richard andrés mier servicio geológico colombiano, observatorio vulcanológico y sismológico de pasto. rmier@sgc.gov.co the chilean seismological observation system sergio barrientos and csn team universidad de chile. sbarrien@dgf.uchile.cl mapas de intensidad instrumental en tiempo real para colombia tovar, m., eraso, j., arcila, m. servicio geológico colombiano, bogotá, colombia. mtovar@sgc.gov.co, jeraso@sgc.gov.co, marcila@sgc.gov.co conveners: vladimir smirnov (moscow state univ. russia) <vs60@phys.msu.ru> alexander caneva (uan colombia) <alexander.caneva@uan.edu.co> magnitude of completeness for the seismic data catalogue of the national seismological network of colombia (rsnc), 1993-2014 vladimir smirnov1, maria potanina1, alexander caneva2, michelle barney. 1moscow state university, faculty of physics, moscow, russia. 2universidad antonio nariño, vcti, bogotá, colombia. análisis probabilístico de la sismicidad del valle de cauca y zonas aledañas, colombia diego f. gamboa-perdomo1, juan s. arbeláez trujillo1, elkin de j. salcedo-hurtado2 1universidad del valle, cali, colombia. 2grupo de investigación georiesgos observatorio sismológico y geofísico de la universidad del valle, cali, colombia. diegogamboap@gmail.com earthquake catalogue processing and declustering issues ganesh w. rathod1, k. seshagiri rao2 and kaushal kumar gupta2 1council for geoscience, pretoria, south africa 2indian institute of technology delhi, new delhi, india grathod@geoscience.org.za ¿qué produjo los sismos ocurridos al oriente de bogotá el 10 de octubre de 1743 y el 24 de mayo de 2008? chicangana, g.1, 2, vargas, c. a.2, muñoz, b, f. j.2, pedraza, p.3, kammer, a.2 y salcedo, e.4 1vicerrectoría de investigaciones, corporación universitaria del meta, villavicencio, colombia. 2grupo geofísica, departamento de geociencias, universidad nacional de colombia. 3red sismológica nacional de colombia, servicio geológico colombiano. 4observatorio sismológico del suroccidente, universidad del valle, cali, colombia. german.chicangana@unimeta.edu.co modeling seismic sources in colombian central west using inversion of telesismic body waveforms hugo monsalve jaramillo, william valencia mina, leonardo cano saldaña grupo quimbaya universidad del quindío. hugom@uniquindio.edu.co depth profile of the fractal dimension and the seismotectonic deformation rates associated to the earthquakes distribution juan m. salazar,carlos a. vargas universidad nacional de colombia, bogota, colombia. jmsalazarr@unal.edu.co, cavargasj@unal.edu.co earthquake source parameters which display first digit phenomenon patricio a. toledo1, sebastian r. riquelme2, jaime a. campos1 1uch. 2csn. patoledo@ing.uchile.cl seismicity response to the electromagnetic probing of the lithosphere in the bishkek (kyrgyzstan) geodynamic test area v.b. smirnov1,2, a.d. zavyalov2 1institute of physics of the earth, ras, moscow and faculty of physics, moscow state university, russia, vs60@mail.ru 2institute of physics of the earth, ras, moscow, russia, zavyalov@ifz.ru distribución espacial y temporal de los terremotos destructivos y sus réplicas, del abril de 2014, cerca de managua, nicaragua tenorio v.1, arguello g.1, espósito o.2, sánchez j.1,strauch w.1 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2centro nacional de investigaciones sísmicas (cenais), santiago de cuba. wilfried.strauch@yahoo.com mud volcano seismo-geodynamics oktay babazade, nigyar babazade centre of seismology and earth's physics azerbaijan bnigyar@hotmail.com conveners: susan beck (arizona univ. usa) <slbeck@email.arizona.edu> marino protti (ovsicori-una costa rica) <marino.protti@gmail.com> christa von hillebrandt (noaa usa) <christa.vonh @noaa.gov> susan newman (ssa) <snewman@seismosoc.org> seismic velocity reduction following the september 5, 2012, mw=7.6 nicoya costa rica earthquake from ambient noise correlations esteban j. chaves.& susan y. schwartz university of california santa cruz. echavess@ucsc.edu the causative relationship between the megathrust and upper plate faults at subduction zones gabriel gonzález1, pablo salazar1, jack loveless2, felipe aron3, juan gonzález1 and richard allmendinger3 1universidad católica del norte, antofagasta, chile fondap-cigiden research centre. 2smith college, northampton, massachusetts, usa. 3cornell university, ithaca, new york. ggonzale@ucn.cl mind the gap – the northern chile subduction zone and the 2014 iquique earthquake sequence gavin p. hayes1, william d. barnhart2, matthew w. herman3, kevin p. furlong4, sebástian riquelme5, harley m. benz2, eric bergman6, sergio barrientos7, paul s. earle2 1national earthquake information center, united states geological survey, golden, co. 2usgs neic. 3department of geosciences, pennsylvania state university, university park, pa. 4psu. 5centro sismológico nacional, universidad de chile. 6global seismological services, golden, co. 7csn. ghayes@usgs.gov spatio-temporal evolution of the 2014 pisagua, chile earthquake sequence hans agurto-detzel1, hugo soto2, andrés tassara2. 1iag-usp, brazil. 2u. de concepción, chile. h.agurto.detzel@gmail.com grijalva ridge, a major player in the subduction game underneath ecuador hugo yepes1,2, laurence audin1, alexandra alvarado2, céline beauval1, jorge aguilar2 1isterre, ird-cnrs-ujf-osug, grenoble, france. 2instituto geofisico, quito, ecuador. hyepes@igepn.edu.ec distribution analysis of the frequency–magnitude parameters along the subduction surface structure in south america marily triviño abella, carlos alberto vargas jiménez universidad nacional de colombia. mtrivinoa@unal.edu.co mapping patchiness of the subduction interface in the near field: the case of the southern terminus of the middle american trench, costa rica marino protti1, susan schwartz2, andrew newman3, víctor gonzález1, tim dixon4, lujia feng5, esteban chaves1,2, hernán porras1 1ovsicori-una. 2university of california at santa cruz. 3georgia tech. 4university of south florida in tampa. 5earth observatory of singapore. marino.protti@gmail.com reverse polarity repeating events are associated with boudin formation at intermediate-depths in the bucaramanga nest sarah a barrett1, germán a. prieto2 1stanford university. 2massachusetts institute of technology. sabarrett@stanford.edu 3d thermal modeling associated with subduction of the philippine sea plate in southwest japan shoichi yoshioka1, yingfeng ji1, takumi matsumoto2 1rcuss, kobe university. 2nied. yoshioka@port.kobe-u.ac.jp seismological view of nazca-south american convergent margin between 10 and 20 s susan l beck1, george zandt1, lara wagner2, maureen long3, hernando tavera4, estella minya5, kevin ward1, alissa scire1, jamie ryan1, brandon bishop1, sanja knezevic antonijevic2, abhash kumar2, berk biryol2, caroline eakin3, cristobal condori4 1university of arizona, us. 2university of north carolina, us. 3yale university, us. 4igp, lima, peru. 5san calixto observatory, la paz, bolivia. slbeck@email.arizona.edu convener: wilfried strauch (cepredenac) <wilfried.strauch@yahoo.com> caribbean and adjacent regions tsunami warning system: seismic component christa von hillebrandt-andrade, hector crespo, caribbean tsunami warning program, noaa nws, mayagüez, pr, united states. jean marie saurel, ipgp observatoire volcanologique et sismologique de martinique. dan mcnamara, usgs geologic hazards science center. victor huerfano, puerto rico seismic network, uprm. working group 1 unesco ioc intergovernmental coordination group for the tsunami and other coastal hazards warning system for the caribbean and adjacent regions (caribe ews). christa.vonh@noaa.gov local tsunamigenic earthquakes off northeastern venezuela, in the southern caribbean realm franck a. audemard m. and alejandra f. leal g. earth sciences dpt. venezuelan foundation for seismological research –funvisis-, caracas, venezuela. faudemard@funvisis.gob.ve and aleal@funvisis.gob.ve método numérico para desplazamientos de la superficie terrestre debido a fuentes sismogénicas gustavo a. lara1, gabriel álvarez1, gabriel gonzález2, juan gonzález2. 1universidad de antofagasta. 2universidad católica del norte. gustavo.lara@uantof.cl análisis de modelos intersísmicos para estimación de inundación por tsunami gonzález-carrasco, j.1,2, r. aranguiz2,3, j.c. dominguez2,4, r. cienfuegos2,4, p. catalán2,5& g. gonzález1,2 1universidad católica del norte (ucn), chile. 2centro nacional de investigación para la gestión integrada de desastres naturales (cigiden), chile. 3universidad católica de la santísima concepción (ucsc), concepción, chile. 4pontificia universidad católica (puc), santiago, chile. 5universidad técnica federico santa maría (utfsm), valparaíso, chile. jgonzal@alumnos.ucn.cl assessment of heterogeneous slip distribution impact on tsunamigenic hydrodynamic processes along the northern chilean coast gonzález-carrasco, j.1,2, n. zamora-sauma3, c. núñez4, a.y. babeyko3& g. gonzález2,5 1universidad católica del norte (ucn), chile. 2centro nacional de investigación para la gestión integrada de desastres naturales (cigiden), chile. 3geoforschungszentrum (gfz), potsdam, germany. 4servicio hidrográfico y oceanográfico de la armada (shoa), chile. 5departamento de ciencias geológicas, universidad católica del norte (ucn), chile. jgonzal@alumnos.ucn.cl fast epicentral distance and magnitude determination using a single three component seismic station with machine learning techniques luis h. ochoa, carlos a. vargas,luis f. niño universidad nacional de colombia. lhochoag@unal.edu.co a rapid estimation of tsunami run-up based on finite fault models sebastian riquelme universidad de chile sebastian@dgf.uchile.cl mapas de amenaza de tsunami para el golfo de fonseca (el salvador, honduras y nicaragua) norwin acosta1, wilfried strauch1, jennifer larreynaga2, juan josé reyes3 1ineter, nicaragua. 2marn, el salvador. 3copeco, honduras. wilfried.strauch@yahoo.com sistemas de alerta temprana (sat) en panamá wilfried strauch observatorio sísmico del occidente de panamá (osop). wilfried@osop.com.pa virginia tenorio1, norwin acosta1, francisco gavidia2, jennifer larreynaga2, wilfried strauch3 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2ministerio del ambiente y recursos naturales (marn), san salvador, el salvador. 3centro de coordinación para la prevención de los desastres naturales en américa central (cepredenac), guatemala, guatemala. wilfried.strauch@yahoo.com el desarrollo del sistema de alerta de tsunami en nicaragua strauch w.1, talavera e.1, muñoz a.1, tenorio v. 1, izaguirre a.2 1instituto nicaragüense de estudios territoriales (ineter), managua, nicaragua. 2sistema nacional para la prevención, mitigación, y atención de desastres (sinapred), managua, nicaragua. wilfried.strauch@yahoo.com conveners: mario ruiz (epn ecuador) <mruiz@igepn.edu.ec> diego gomez (colombian geological survey) <dgomez@sgc.gov.co> javier pacheco-alvarado (ovsicori-una costa rica) <javier.pacheco.alvarado@una.cr> john makario londoño (colombian geological survey) <jmakario@sgc.gov.co> randy white (usgs usa) <rwhite@usgs.gov> implementación y renovación de nuevas tecnicas geofísicas para monitoreo de volcanes activos colombianos alpala, j., oliveras, n., ospina, c., meza, l., alpala, r. servicio geológico colombiano, observatorio vulcanológico y sismológico de popayán jalpala@sgc.gov.co, noliveras@sgc.gov.co, cospina@sgc.gov.co, lmeza@sgc.gov.co, ralpala@sgc.gov.co sismicidad volcano-tectónica en el nevado del huila alpala, r. y sánchez, j. 1ervicio geológico colombiano, observatorio vulcanológico y sismológico de popayán ralpala@sgc.gov.co, jsanchezv@sgc.gov.co enjambres sísmicos periódicos relacionados con la transferencia estática de esfuerzos entre una deformación volcánica de gran escala y fallas strike-slip corticales: complejo volcánico laguna del maule (chile) carlos cardona1,2,andrés tassara2,fernando gil cruz1 1observatorio volcanológico de los andes del sur, sernageomin. rudecindo ortega 03850, temuco, chile. 2universidad de concepción, departamento de ciencias geológicas, chile. carlos.cardona@sernageomin.cl carlos cardona1,2,andrés tassara2,luis lara1,sergio morales1 1observatorio volcanológico de los andes del sur, sernageomin. rudecindo ortega 03850, temuco, chile. 2universidad de concepción, departamento de ciencias geológicas, chile. carlos.cardona@sernageomin.cl comportamiento de la actividad sísmica y registro de procesos deformativos en los volcanes chiles y cerro negro, colombia (2013) arcos, d.f.1, narváez, l.1, battaglia, m.2 1servicio geológico colombiano, observatorio vulcanológico y sismológico de pasto. 2associate professor, sapienza university of rome (italy) – department of earth sciences. darcos@sgc.gov.co, lnarvaez@sgc.gov.co, mbattaglia@usgs.gov cambios en la distribución temporal del parámetro b relacionados con intrusiones de magma, herramienta para pronóstico caso erupción del complejo volcánico cordón caulle, junio 04 2011 daniel basualto alarcón1,2, jonathan lazo gil1, carlos cardona idarraga2, andrés tassara oddo2, fernando gil cruz1 1observatorio volcanológico de los andes del sur (ovdas), servicio nacional de geología y minería (sernageomin), santiago, chile. 2departamento ciencias de la tierra, facultad de ciencias químicas, universidad de concepción, concepción, chile. daniel.basualto@sernageomin.cl sistema informático para el análisis y procesamiento de señales sísmicas de volcanes en ecuador e. viracucha1, j. de la bastida2 1escuela politécnica nacional, instituto geofísico quito. 2national tsing hua university taichung, republica de china. gviracucha@igepn.edu.ec enjambres sísmicos en el volcán pico de tancítaro, michoacán, méxico. ¿intrusión magmática o sismicidad tectónica? francisco javier núñez cornú, juan ignacio pinzón lópez; charlotte a. rowe*, christian escudero ayala. centro de sismología y volcanología de occidente, universidad de guadalajara. *foxtrotter geophysical, llc. pacornu77@gmail.com actividad sísmica de los volcanes del segmento norte de colombia 1985-2014 grupo de monitoreo volcánico ovsm: álvaro pablo acevedo, carolina acosta muñoz, lina-marcela castaño, lina constanza garcía-cano, cristian mauricio lópez vélez, beatriz galvis arenas y john makario londoño. servicio geológico colombiano, observatorio vulcanológico y sismológico de manizales. av. 12 de octubre nro. 15-47, manizales-colombia. jmakario@sgc.gov.co análisis de la sismicidad tipo drumbeat y su relación con la erupción del año 2008 del volcán nevado del huila, colombia john macario londoño1, diana marcela quintero2, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co análisis espectral de fuentes sismogénicas de sismos volcano-tectónicos en el volcán nevado del huila, colombia john macario londoño1, diana marcela quintero2, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co redefinición de las fuentes sismogénicas volcano-tectónicas en el volcán nevado del ruiz a partir de la actividad reciente 2010-2014 jonh makario londoño-bonilla y lina marcela castaño grupo de sismología, observatorio vulcanológico y sismológico de manizales, colombia. jmakario@sgc.gov.co,lmcastano@sgc.gov.co tomografía sísmica del volcán puracé john makario londoño1, cristian camilo santacoloma2 1servicio geológico colombiano – observatorio vulcanológico y sismológico de manizales. 2servicio geológico colombiano – observatorio vulcanológico y sismológico de popayán. jmakario@sgc.gov.co variaciones de la concentración de gas radón y so2 asociadas a actividad sísmica en el sector del volcán galeras, colombia, 2005-2013 john makario londoño, viviana burbano servicio geológico colombiano. jmakario@sgc.gov.co variaciones en la atenuación de las ondas sísmicas en el volcán nevado del ruiz asociadas a la reciente reactivación 2010-2014 john makario londoño, katherine vallejo , johana gómez, carolina acosta servicio geologico colombiano jmakario@sgc.gov.co characterization of seismo-volcanic activity in peteroa volcano, central chile-argentina j.a.casas1, g.a.badi1, m.c.manassero1, e. ruigrok2, m.gomez3, d.draganov2, j.ruzzante3 1facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina. 2department of geoscience and engineering, delft university of technology, netherlands. 3ices, cnea, buenos aires, argentina. augustotkd@carina.fcaglp.unlp.edu.ar análisis del campo de esfuerzo actual a partir de mecanismos focales en el área del volcán nevado del ruiz, colombia lina marcela castaño, jonh makario londoño-bonilla, carolina acosta-muñoz y beatriz galvis-arenas grupo de sismología, observatorio vulcanológico y sismológico de manizales, colombia. lmcastano@sgc.gov.co descripción de la actividad sísmica relacionada a erupciones estrombolianas violentas: vn. llaima, chile (2007 – 2010) franco, l.e.1,2, palma j.l.2, gil-cruz, f.1 y san martín, j..j2 1servicio nacional de geología y minería (chile). 2universidad de concepción, departamento de ciencias de la tierra (chile). luis.franco@sernageomin.cl anomalías de gas radon asociadas a la sismicidad en el region volcánica puracé luisafernanda meza1, rosa liliana alpala1, andrés narváez1, john makario londoño2 1servicio geológico colombiano, observatorio vulcanológico y sismológico de popayán. 2servicio geológico colombiano, observatorio vulcanológico y sismológico de manizales. lmeza@sgc.gov.co, ralpala@sgc.gov.co, anarvaez@sgc.gov.co, jmakario@sgc.gov.co seismic attenuation around peteroa volcano, argentina m.c.manassero1, g.a.badi1, j.a.casas1, m.gomez3, d.draganov2, j.ruzzante3 1dpto. de sismología e i. m., facultad de ciencias astronómicas y geofísicas, universidad nacional de la plata, argentina. 2department of geoscience and engineering, delft university of technology, netherlands. 3ices-cnea, buenos aires, argentina. constanzamanassero@gmail.com sismicidad volcano-tectonica y registros de periodos de deformación en el volcán sotará, “segmento central de colombia” narváez, a., alpala, j. servicio geológico colombiano observatorio vulcanológico y sismológico de popayán. anarvaez@sgc.gov.co, jalpala@sgc.gov.co caracterización de fuentes sísmicas de eventos tipo tornillo en el volcán puracé mediante el análisis de frecuencias complejas nelson david pérez1,2, john jairo sánchez2, indira molina3 1red simológica nacional, servicio geológico colombiano. 2departamento de geociencias, universidad nacional de colombia. 3 bureau de recherches géologiques et minières de france (brgm) nperez@sgc.gov.co aplicación del método sompi en la evaluación temporal de las propiedades características de sismos tipo tornillo en el volcán galeras maría fernanda acosta mora1, oscar ernesto cadena ibarra2 1universidad de nariño. 2sgc-ovsp. ocadena@sgc.gov.co sismicidad volcano-tectónica en el volcán puracé ospina, c.a, corchuelo, i.d. observatorio vulcanológico y sismológico de popayán. cospina@sgc.gov.co, icorchuelo@sgc.gov.co seismic forecasting of eruptions at dormant stratovolcanoes randall. a. white1 and wendy a. mccausland2 1usgs volcano disaster assistance program, menlo park, ca. 2usgs volcano disaster assistance program, vancouver, wa. rwhite@usgs.gov sismicidad frontera colombia-ecuador en la región volcanes chiles y cerro negro roberto torres, oscar cadena, john meneses y edgar muñoz servicio geológico colombiano. rtorres@sgc.gov.co un modelo geofísico de la estructura interna del volcán galeras, colombia roberto torres, adriana ortega, lourdes narváez 1servicio geológico colombiano. rtorres@sgc.gov.co keywords: catastrophic landslide triggered debris flow; dynamic process; main controlling factors; tendency analysis; yigong. palabras clave: deslizamienos de tierra ocasionados por flujos de detritos; procesos dinámicos; factores principales de control; análisis de tendencias; yigong. how to cite item li, j., chen, n., zhao, y., liu, m., & wang, w. (2020). a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency. earth sciences research journal, 24(1), 71-82. doi: https:// doi.org/10.15446/esrj.v24n1.78094 a disaster of a catastrophic landslide triggered a debris flow with a scale of 0.3 billion m3 that occurred in the zhamunong gully of yigong on april 9th, 2000. it is of great scientific and engineering significance to study the main controlling factors and dynamic processes of this disaster, and the future development trend of similar hazards. firstly, the precipitation, earthquake and temperature data before the event and seismic wave data after the event were collect. secondly, a multi-dimensional approach on the data for finding the factors, dynamic processes and tendency of a catastrophic landslide triggered a debris flow in zhamunong gully was applied, including the standard precipitation index and effective peak acceleration calculation methods, fast fourier transform, landslide stability, maccormacktvd finite difference method and flac numerical simulation method. the results are shown as follows. (1) the main controlling factors of this disaster were the long-term freeze-thaw cycle, dry-wet cycle, and a middle magnitude earthquake. (2) based on the ground vibration spectrum of 2000 disaster recorded by the linzhi seismic station, the dynamic processes are the joint and crack development process, the crack fracture and sliding process, the landslide translating into the debris flow, the movement and deposition of the debris flow. (3) the density and the average velocity are 2.0 t·m-3 and 30.12 m·s-1, respectively, and the discharge is shown with the process of first increases and then decreases. (4) similar catastrophic landslide triggered debris flow would happen in zhamunong gully in the future. the research results are useful in establishing a foundation for further study on the dynamic mechanism and reduction countermeasures of a catastrophic landslide triggered by debris flow. abstract a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency deslizamiento de tierra ocasionado por el flujo de detritos en el río yigong (china): factores, procesos dinámicos y tendencia issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.78094 un deslizamiento de tierra ocasionó un flujo de detritos a una escala de 0.3 miles de millones de metros cúbicos en la hondonada de zhamunong en el río yigong, el 9 de abril de 2000. el estudio de los factores deteterminantes y las dinámicas del proceso de este desastre, al igual que el estudio de la tendencia a futuro de eventos similares, ha sido de gran importancia científica e ingenieril. inicialmente, se recolectó la información relacionada con la precipitación, terremotos y temperatura antes del deslizamiento y la información de onda sísmica después del evento. luego se realizó un acercamiento multidimensional a la información para encontrar los factores, los procesos dinámicos y la tendencia del deslizamiento de tierra que desencadenó un flujo de detritos en zhamunong, y que incluyó el índice de precipitación regular y los métodos de cálculo de aceleración sísmica, transformada rápida de fourier, estabilidad de un deslizamiento, el método de las diferencias finitas maccormack-tvd y el método de simulación numérica flac. los resultados muestran lo siguiente: (1) los factores de control principal del deslizamiento fueron el ciclo a largo plazo de congelación y descongelación, el ciclo de humedad y sequía, y un terremoto de magnitud media. (2) con base en el espectro de vibración del terreno registrado durante este evento por la estación sísmica de linzhi, las dinámicas pasan por el desarrollo del proceso de agrietamiento y articulación, el proceso de agrietamiento de las fracturas y deslizamiento, la traslación del deslizamiento de tierra en flujo de detritos, el movimiento y la deposición de estos detritos. (3) la densidad y el promedio de velocidad son de 2.0 t·m-3 y 30.12 m·s-1, y la descarga se muestra con el proceso de primero incremento y luego disminución. (4) eventos similares pueden ocurrir en la hondonada de zhamunong a futuro. los resultados de la investigación son útiles para establecer los cimientos de estudios posteriores en los mecanismos de dinámicas y en la reducción de contramedidas de deslizamienos de tierra ocasionados por flujos de detritos. resumen record manuscript received: 26/02/2019 accepted for publication: 18/10/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 71-82 jun lia, ningsheng chenb*, yuandi zhaoa, mei liub, weiyu wanga 1school of civil engineering, sichuan university of science & engineering, zigong, 643000, china 2institute of mountain hazards and environment, chinese academy of sciences, chengdu 610041, china * corresponding author: chennsh@imde.ac.cn g eo te ch ni ca l en g in ee ri ng 72 jun li, ningsheng chen, yuandi zhao, mei liu, weiyu wang introduction a catastrophic landslide triggered debris flow (cldf) refers to a chain of landslides with a volume of more than 1×107 m3 transforming to a high-speed and long distant debris flow after detaching from the parent rock due to the landslide’s enormous potential energy (huang and fan, 2013, chen et al., 2018, samodra et al., 2018). at 20:00:11.95 on april 9th, 2000 (local time), a catastrophic landslide triggered debris flow that occurred in the zhamunong gully in yigong (wang and lu, 2002, shang et al., 2003, evans and delaney, 2011). the landslide struck the ground, leading to loosening bed depositions mixed in the landslide in the headstream of the gully, then the landslide transformed into a debris flow by the soil liquefaction. the deposits carried by the debris flow blocked the yigong zangbo river. the subsequent barrier dam outbreak on june 10th then fast formed a flash flood, which resulted in heavy casualties and economic losses (kang et al., 2017, wang and li, 2017, liu and he, 2018). in china, the flash flood damaged 318 national road, tongmai bridge, the motuo jiefang bridge, highways, among others. the g318 national highway was broken off for 76 days, which resulted in two people causalities and a loss of 280 million yuan (liu, 2000, lu, 2002, xuqiang et al., 2012, schmidt et al., 2015). besides, the flash flood killed 94 people, 2.5 million people were homeless in ramaphotra, northern india, and traffic broke off in 7 central indian states (tewari, 2004, wang et al., 2016). many local and international scholars have studied this catastrophic disaster. scholars have already studied the scale characteristics of the 2000 cldf in the zhamunong gully and the macroscopic processes that occur when a landslide transfers into a debris flow; results show that this cldf is triggered by rainfall and that the deposit volumes range from 1.2 to 3.0×108 m3 (shang et al., 2003, xu et al., 2012, evans and delaney, 2011, delaney and evans, 2015). meanwhile, a more large-scale debris flow occurred in the same place in 1902 (shang et al., 2003). however, the following issues in the above studies need to be researched: (a) cldfs led by rainfall are generally smallscale (wang and sassa, 2003, guzzetti et al., 2009, chen et al., 2010). previous studies show that freeze-thaw cycles, dry-wet cycles, and seismic linkages are extremely conducive to the development of cldfs (chen et al., 2014, zhou et al., 2016, huang et al., 2016) and that rainfall is a decisive factor for the occurrence of a cldf (chen et al., 2016, deng et al., 2017). additionally, a ms 4.3 earthquake might influence the occurrence of a landslide (delgado et al., 2011). is the occurrence of the 2000 cldf related to the middle and small earthquakes, the freeze-thaw cycles and the dry-wet cycles in the region? (b) studies on the dynamic process of landslides transforming into debris flows are not well understood, so it is necessary to perform a quantitative study on these dynamic processes. (c) the process of volume changes of cldfs over long timescales are the basis for analyzing the development tendency of the cldf in the zhamunong gully, but this research has yet to be published. due to the construction demand for major projects, such as town, roads, and hydropower, the practical demands of disaster prevention and reduction (wang et al., 2018a, 2018b), and the scientific problem of surface denudation processes at the edge of the tibetan plateau, the main purpose of this paper is to study the main controlling factors on and dynamic processes and tendency of the cldf in the zhamunong gully. to tackle the aforementioned issues, a case study is performed based on the 2000 cldf in the zhamunong gully. section 2 describes the study area and methodology. section 3 and section 4 address the main controlling factors and the dynamic processes of the cldf, respectively. section 5 analyzes the development tendency of cldf, and section 6 presents the conclusions. the technical flowchart is shown in figure 1. study area and methodology study area the zhamunong gully is located in the northwest region of the namcha barwa peak and is on the left bank of the yigong zangbo river (fig. 2a). topographic variations are acute along the western and eastern margin, forming an alpine landform. the basin area is 29.4 km2. the highest and lowest altitudes are 5616.00 m and 2185.72 m, respectively. the relative relief is 3430.28 m. the average gradient is 233 ‰, and the largest gradient is 646 ‰. the regional climate is relatively warm with an average temperature of 6°c~17°c and a mean disproportionate annual precipitation of 1225.8 mm. the precipitation curve is shaped like a double peak (referring to the months of may-june and augustseptember). the lithology from north to south are himalayan granite (outcrop altitude elevation between 4100 m-5616 m), limestone, slate and marble of the carboniferous system (outcrop altitude elevation between 2800 m-4100 m), and gneiss and schist of the gangdese group from the pre-sinian system (outcrop altitude elevation between 2198 m-2800 m). the zhamunong gully is crossed by three fault zones (fig. 2b), which are the jiali deep fault zone (f1), the dade-anizha fault (f22), and the danen-zepu fault (f3), where the seismic activity of the jiali fault is comparatively frequent with the occasional occurrence greater than ms 6 earthquakes (lee et al., 2003; song et al., 2013) methodology standard precipitation index (spi) and effective peak acceleration (epa) calculation methods we estimate the influence of earthquakes by using epa method in table 1. the yigong landslide is within western china, therefore, the model for computing the epa is based on the empirical model between earthquakes and seismic acceleration in western china (wang and shi, 1993, yu and gao, 2001). the drought grade shown in table 2 is based on the spi (chen et al., 2014). compilation of basic dynamic processes of the 2000 tendency of mechanical test data for granite particle size 2000 cldf precipitation, temperature, earthquake data landslide stability analysis characteristic parameters of dynamic processes dynamic processes main controlling factors figure 1. technical flowchart. 73a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency (mccubbine et al., 2017). fft analysis converts a signal from its original domain to a representation in the frequency domain. the parameters of frequency, amplitude, and peak ground parameters (such as acceleration and displacement) are transformed from the seismic wave based on the seismosignal software from the website of https://www.seismosoft.com/seismosignal-prod. landslide stability the calculation equation for the locking section length of a landslide is shown in equation 1. equations 2-6 are the detailed calculation equations of every force in equation 1. in equations 1-6, f is the stability coefficient of the landslide; f=1 is a critical stable state of landslide (that is, the landslide is near sliding when f=1); fk is the anti-sliding force, in kn/m; fs is the sliding force, in kn·m-1; c is the cohesion force of the rock mass in the sliding surface of the landslide, in kpa; φ is the internal friction angle of the rock mass in the sliding surface of the avalanche mass, in º; l is the length of the locking section (the non-perforated length of the landslide’s fissure surface), in m; v is the fissure water pressure, in kn·m-1; u is the uplift pressure along the table 2. dry grade based on the spi dry grade type spi occurrence frequency 1 no drought -0.5<spi 68% 2 slight drought -1.0<spi≤-0.5 15% 3 moderate drought -1.5<spi≤-1.0 10% 4 heavy drought -2.0<spi≤-1.5 5% 5 special drought spi≤-2.0 2% fast fourier transform a fast fourier transform (fft) is an algorithm that samples a signal over a period of time (or space) and divides it into its frequency components figure 2. geological map of the yigong region. table 1. attenuation equation for the epa in western china epa epa epaw w sw= ⋅1 where r is the sensitive radius and m is the magnitude. longitudinal direction acceleration attenuation equation epa m im1 2 492 0 786 2 787 3 269 0 4510w r = + + ( ) . . . log . exp . short direction acceleration attenuation equation epa m imsw r = + + ( ) 101 093 0 59 1 794 1 046 0 45. . . log . exp . https://www.seismosoft.com/seismosignal-prod 74 jun li, ningsheng chen, yuandi zhao, mei liu, weiyu wang slip surface, in kn·m-1; q is the earthquake force, in kn·m-1; ζ is the level coefficient of the earthquake; α is the dip of the sliding surface; w is the weight of the landslide, in kn·m-1;  is the density of granite rock masses, in kg·m-3; v is the volume of landslide, in m3; g is the acceleration of gravity; h is the depth of the fissure in the trailing edge, in m; and hw is the water-filling height of the fissure in the trailing edge, in m, which ranges between 0.2 h in natural time and 0.3-0.5 h during a rainstorm. as per rainfall data, there was heavy rainfall before the bh01 landslide happened, so hw is valued 0.5 h. hcr is the depth of the fissure in the landslide’s trailing edge, in m. h is the height of the slope from the front of the landslide to the trailing edge of the fissure, in m. the rock mass is comparatively broken at the source of the zhamunong gully; the internal friction angle’s reduction factor for the bh01 landslide’s rock mass is 0.8, and the internal friction angle of the avalanche mass’s sliding surface after reduction is 46.616°, according to the technical specification for construction slope engineering, china (db50/330-2002). when the cracking segment of the rock mass in the landslide’s trailing edge cracks to 363 m and the front locking section transfixes, the landslide reaches a limit equilibrium state. table 3 summarizes the calculation parameters. f f f w q u tg cl w q v k s = == ( ) × + + + cos sin sin cos cos α− α− φ α α α (1) v hw w= ×0 5 2.  (2) u lhw w= ×0 5.  (3) q w=  (4) w vg l =  (5) h hcr = 0 5763 27 0092. . (6) table 3. calculation parameters for landslide stability parameters (units) values parameters (units) values density of granite  (kg·m-3) 2640 level coefficient of earthquake ζ 0.255 density of water γw (kn·m-3) 10 dip of sliding surface α (°) 36.5 cohesion strength c (kpa) 9690 hcr (m) 363 internal friction angle φ (°) 46.616 hw (m) 181.5 a maccormack-tvd finite difference method the maccormack-tvd finite difference method proposed by (ouyang, et al., 2013) can be used to simulate the dynamic process of cldfs. therefore, this method is suitable for calculating the discharge of the 2000 cldf. the boundary condition is the wall boundary condition; the base model is the coulomb friction model, and the corresponding coulomb friction coefficient is set as 0.215. the coefficient of earth pressure is set to constant 1. equations 7-10 based on this method are shown as follows. ∂ ∂ + ∂ ∂ + ∂ ∂ − =( ) ( ) ( )    h t hu x hv y e1 1 0 (7) ∂ ∂ + ∂ ∂ + ∂ ∂ − +( )( ) ( ) ( )ρ ρ ρ ρ h t hu x hu v y g h k g h h z x uu uu x z b1 1 2 1 1β β − ρ ∂ ∂ − ττ ρ1 1zx b bu e( ) + ∂ ∂ + ∂ ∂ + ∂ ∂ − +( )( ) ( ) ( )ρ ρ ρ ρ h t hu x hu v y g h k g h h z x uu uu x z b1 1 2 1 1β β − ρ ∂ ∂ − ττ ρ1 1zx b bu e( ) + (8) ∂ ∂ + ∂ ∂ + ∂ ∂ − +( )( ) ( ) ( )ρ ρ ρ ρ hv t hu v x hv y g h k g h h z x uv vv y z b1 1 1 1 2β β − ρ ∂ ∂ −− τ ρ1 1zy b bu e( ) + ∂ ∂ + ∂ ∂ + ∂ ∂ − +( )( ) ( ) ( )ρ ρ ρ ρ hv t hu v x hv y g h k g h h z x uv vv y z b1 1 1 1 2β β − ρ ∂ ∂ −− τ ρ1 1zy b bu e( ) + (9) e t hvzb= = ∂ ∂ α (10) in equations 7-10, x, y, z are the direction of debris flow movement; t is the move time, in s;  is the density of the debris flow, in kg/m3; h is the depth of the debris flow, in m;  1 and v1 are the velocity of the movement directions x and y, respectively, in m/s; 1zx b( ) and 1zy b( ) are the drag base of the movement directions x and y; (gx, gz) are the gravitational acceleration in the movement directions x and z;  is the momentum distribution coefficient; k is the coefficient of earth pressure; e is the basal erosion rate of the debris flow; v is the point velocity, in m/s; and  is the erosion rate coefficient. the initial and boundary conditions flac numerical simulation method based on the shear strength reduction for calculating future landslide magnitude using parameters from rock mechanics tests the fast lagrangian analysis of continua (flac) method is suitable for simulating landslide sliding surfaces, so we use this method to estimate the volume of future similar landslides to analyze the tendency of cldfs. the flac numerical simulation method is used for estimating the longitudinal area of potential landslides bh01 and bh02 under extreme conditions (fig. 3a). the flac numerical simulation method is a fast lagrangian difference method based on shear strength reduction (jing et al., 2008). the shear strength reduction is expressed by the change of safety coefficient. the actual calculation process of the safety coefficient fs is in equation 11. the implementation processes of shear strength reduction in flac are shown as follows. when some strength reduction coefficient rf is such that the plastic zone of the sliding surface completely transfixes in the computed processes of flac, the strength reduction coefficient rf is the value of the safety coefficient fs. the flac method uses the history display of finite difference time. for each time step, the main calculation process is completed by (1) calculating the new strain rate from the node velocity in equation 12, (2) calculating the stress of the previous step using the constitutive equation through the strain rate in equation 13 and equation 14 and equation 15, and (3) deriving the new node velocity and displacement from the stress and force through the motion equation in equation 16. each time step repeats the above three steps to monitor the maximum unbalanced force. when the maximum unbalanced force equals a constant, we have estimated the plastic rock masses in the landslide profile. besides, the initial and boundary conditions of the flac method contain the surface force, concentrated load, and displacement boundary, and the physical strength and initial stress conditions also need to be applied. at the beginning of the computer program, all stresses and node velocities are set to zero, and then we apply the initial stress. the concentrated load is assigned to the node on the surface, and the displacement boundary condition is precisely controlled by the velocity of the node. the physical force and the surface force are transformed into a series of equivalent node loads in the interior. the above constitutes the initial state of numerical calculation. moreover, we considered the earthquake intensity of 5 degrees in the computed process of the flac method to simulate the actual seismic situation of zhamunong gully. the simulation profiles of the bh01 and bh02 potential landslides are shown in fig. 3b. the dip angles of the main granite joints are 211° and 163° in the bh01 and bh02 potential landslides, respectively; the granite rock masses with those dip angles easily form slides, so the simulation profile direction is the same as the dip directions. the depths of the bh01 and bh02 potential landslides are obtained using the flac numerical simulation method. we use a rock mechanics test to determine the density, cohesion, and internal friction angle, elastic modulus, poisson ratio, and tensile strength parameters. the test machine is the mts815 flex test gt (fig. 4). the samples of rock are obtained by drilling, where, for each specimen, ф=50 mm with a height of 100 mm. there are 12 experiments. the rocks used for 6 reference experiments are 75a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency air-dried granite rocks, and the rocks for the other experiments are watersaturated granite rocks. the test results from the water-saturated granite rocks represent the actual landslide situation. the rock mass strength parameters of the bh01 and bh02 potential landslides are shown in table 4. because the f3 fault passes directly through the bh01 potential landslide, the rock mass joint and fissures in the bh01 potential landslide are more developmental than in the bh02 potential landslide. the elastic modulus, poisson ratios and tensile strengths for the bh01 and bh02 potential landslides have different values. we use the analogy method from engineering geology to estimate those parameters for the bh01 potential landslide, while the parameters for the bh02 potential landslide are from the rock mechanics tests. ′ = ′ = ( )c c rf rf/ , arctan /   (11) ξ −ij i l j j j l i l l l v v n v n s= +    ( ) ( ) ( ) = ∑1 6 1 4 (12) (13) (14) ∆ ∆ ε −ij i l j l l i l l l t v v n v n s= +    ( ) ( ) ( ) = ∑6 1 4 (15) m v n s il i l l=     =       ( ) ( )1 9 1 3 2 max , , (16) in equation 11-16, ′c and ′ are the strength parameters after reduction, c and  are the strength parameters before reduction, and rf is the strength reduction coefficient. ij is the strain rate, vi l and v j l are the tetrahedral velocity, v is the volume of tetrahedron, s is the area of tetrahedron, ni l( ) is the combined number of tetrahedrons in the element, ∆ij is the stress increment of tetrahedron, is the stress correction term, hij * is the incremental constitutive equation, ∆ij is the strain rate correction term, ml is the quality of tetrahedrons. the main controlling factors of the 2000 cldf precipitation, temperature, and earthquakes based on the long-term earthquake, precipitation and temperature records, there is a spatiotemporal correlation between the disaster and an ms 4.8 earthquake, the wet-dry cycle and the freeze-thaw cycle. to better observe the impacts of drought and the earthquake on the yigong landslide, the processes of change in the epa earthquake index and the spi drought index were analyzed before the disaster using a proper time scale of fifteen years. figure 3. (a) photos of the bh01 and bh02 potential landslides. (b) model profiles of the bh01 and bh02 potential landslides. fig. 4. test equipment and the granite samples. mts origin destroyed 76 jun li, ningsheng chen, yuandi zhao, mei liu, weiyu wang the spi of the zhamunong gully for every month between 1990 to 2004 is shown in fig. 5a. two months before the cldf took place on april 9th, 2000, there was a small-magnitude dry-wet cycle in the zhamunong gully, with characteristics of a medium drought and slight moderate rainfall. from 1990 to april 9, 2000, the wet-dry cycle often occurred. over a short time-scale, the precipitation was lower in march, and the spi was -1.41, which suggests that the weather was in a moderate drought. precipitation of 62.6 mm in march 2000 was significantly lower than the annual average precipitation of 87.3 mm. from april 1th to 8th after the drought, the precipitation in the early stage reached 42.9 mm, accounting for 49% of the annual average precipitation. this indicates that zhamunong gully experienced a strong precipitation event before the disaster. at 8:00:09 on the night of april 9th, 2000, a ms 4.8 earthquake was recorded at the linzhi station; the epicenter of this earthquake was 13 km away from the zhamunong gully. the seismic acceleration of the landslide in the zhamunong gully was 43.3 gal (fig. 5a), equivalent to a seismic intensity of 5.5. fissures in the rock masses are well developed in the source basin at elevations of 5250 m, and those rock masses are strongly influenced by the freeze-thaw cycle. the location of the bomi meteorological station is n29°51.5’ and e94°46.1’, and the altitude is 2731 m. according to general meteorological principles, the temperature decreases by 0.6 degrees for every 100 m elevation increase. the temperature at 5250 m of elevation is estimated based on the temperature data of the bomi meteorological station; the temperature results are shown in fig. 5b. the largest seasonal temperature difference was 25°c, and the largest temperature difference before the disaster took place was 15°c. moreover, the temperature shows a periodic change in the period from december-january with a low-temperature distribution and a periodic change in june-july with a high-temperature distribution, which illustrates that the impacts of seasonal freezing and thawing on the rock masses are very strong in the zhamunong gully. analysis of the main controlling factors the structural characteristics of the rock masses are the basis for the formation of the 2000 cldf. there are 3 sets of granite joints at the basin source. the southward-inclined joints with a tendency of 210 degrees coincide with the upstream slope of the basin source, providing an underground seepage channel for precipitation and snow meltwater. the freeze-thaw and dry-wet cycles have a great influence on the porosity compressive strength, elastic coefficient and shear strength of the rock mass. freeze-thaw and dry-wet cycles accelerate the weathering of the rock mass, and more pores are generated in the granitic rock mass. when meltwater or rainfall enters into rock cracks, the water freezes into ice blocks and enlarges its volume. when the temperature drops below 0°c, the water entering the rock fissures will form ice, and the enlargement of the ice volume will force the crack to deepen and widen. when the temperature rises above 0°c, the ice melts in the rock mass cracks, and water flows into the new cracks, resulting in high pore water pressure in the rock mass, which reduces the shear strength of the rock mass. at 8:00:9.2 on april 9th, a ms 4.8 earthquake occurred 13 km away from the zhamunong gully. the seismic acceleration in the basin source was 43.3 gal. earthquakes have a topographic amplification effect on the source basin of the landslide. based on investigations from local people, the water quality became black and the water quantity decreased significantly on april 8th in the zhamunong gully, which suggests that the main sliding surface and the steep cracks in the back edge of the sliding zone were fully penetrated, such that the rainwater and table 4. calculation parameters for the water-saturated granites in the bh01 and bh02 potential landslides. name density /kg·m-3 cohesion /mpa internal friction angle/° elastic modulus /gpa poisson ratio tensile strength /mpa bh01 2630 9.69 58.27 3.0 0.31 3.5 bh02 2630 9.69 58.27 20 0.2 6 original explanation rock mechanics test, see fig. 4 the elastic modulus, poisson ratio and tensile strength of bh02 are origin from the rock mechanics test. however, the rocks of bh01 are very fractured because the f3 fault crosses the middle of the bh01 zone. therefore, we use the analogy method of engineering geology used to estimate these parameters figure 5. (a) change in the epa and spi in the zhamunong gully from 1990 to 2004. (b) the effect of the freeze-thaw cycle in yigong from jan. 1st, 1995 to dec. 31st, 2000. modified from (zhou et al., 2016). 77a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency meltwater were completely penetrated into the main sliding surface of landslide and the rock masses of the sliding zone were saturated with water. therefore, it is reasonable to suggest that a recent ms 4.8 earthquake broke the locked-in section of the landslide. the dynamic processes and characteristic parameters of the 2000 cldf in the zhamunong gully dynamic processes of the 2000 cldf based on seismic waves the strong seismic wave generated by the cldf during its movement was recorded by the neighboring seismic stations (dammeier et al., 2016). from 20:00 to 20:10 on april 9th, 2000, the linzhi seismic station recorded the seismic wave generated by the 2000 cldf in yigong. using the fast fourier transform method to calculate the spectrum of the seismic wave generated by the 2000 cldf, the largest acceleration of the seismic wave generated by the 2000 cldf in yigong was 0.45 g, the largest seismic motion velocity was 0.786 m·s-1, the main frequency value and the main power was 2 hz, and the largest seismic motion amplitude was 64 cm. the spectrum of the peak ground displacement (pgd) is properly used to divide the dynamic processes of the yigong landslide after the following comparison of each seismic wave spectrum. based on the results of the seismic wave spectrum, the pgd spectrum of the seismic wave shown in fig. 6 can better show the movement process of this landslide-debris flow. the amplitude at point o begins to increase significantly, where point o is the occurrence point of the 2000 cldf. the time of point o recorded by the linzhi seismic station is 20:00:18.87 on april 9th, 2000, and the duration of the seismic wave’s transmission is 6.92 s, according to the 13 km·s-1 in-ground seismic wave transmission rate. it is therefore inferred that the landslide time is 20:00:11.95. a debris flow moving at a high speed that strikes the bank of bending slope can produce a strong ground vibration phenomenon, which presents as a catastrophe point in the amplitude or acceleration of the seismic wave spectrum. there are 5 impact points during the movement of the 2000 cldf in yigong according to the graph of the seismic motion displacement and the remote sensing image. as fig. 7 shows, the impact point a has an obvious impact mark, and it is the first impact point of the 2000 cldf. according to the motion mark of the 2000 cldf, we can judge that point b is the impact point of the debris flow. the debris flow shows an obvious improvement at the impact points c, d, and e, which shows that the debris flow was hindered by three apparent places in the valley. according to the seismic wave and based on the movement patterns of the 2000 cldf, the four dynamic landslide processes are (1) joint and crack development in the landslide, (2) the process of crack fracture and sliding, (3) the landslide translation into a debris flow, and (4) debris flow movement and deposition. the dynamic processes of the latter three segments have clear surface vibration characteristics (table 5). the dynamic process of o-a is joint and crack development in the landslide. the large amplitude and highfrequency characteristics were revealed during the crack fracture and sliding process of the landslide, with a maximum amplitude and frequency of 48.35 cm and 2.1 hz, respectively. as debris flow energy increases, the amplitude is large, and the maximum amplitude of section b is 64 cm during the dynamic process of the landslide becoming a debris flow (a-b). because the debris flow hit the mountain at the bend, the amplitude and frequency are high in the dynamic processes b-c of the debris flow movement, with a maximum amplitude of 42.64 cm and a frequency of 2.0 hz. because the bend in the channel is large during the dynamic processes d-e of debris flow movement, the amplitude of the wave increases to a maximum amplitude of 50 cm, but the frequency is reduced to 1.4 hz. the debris flow was deposited gradually, and the impact of the debris flow on the ground gradually weakened after the mountain-pass section. therefore, the amplitude of the wave decreases to 30 cm and the frequency decreases to 1.0 hz in the dynamic process of debris flow deposition. process of joint and crack development in the landslide according to the terrain and wedge shape, the landslide elevation ranged between 4500 m and 5320 m. before sliding, the landslide was one of the main peaks in the basin, covered by ice and snow year-round. after sliding, the bottom of the landslide appears as a “v”-type hanging gully. the sides of the landslide are steep and smooth. the angle of the gully slope is 40-50 degrees, the channel slope is 25 degrees, and the direction of channel trends to the sw. the trailing edge of the landslide is nearly a vertical cliff, and the granite rock mass is smooth. the ne-trending faults parallel to the lateral landslide wall are developed on both sides of the gully, and they form an “x”-type landslide foundation with good connectivity. during the process of the formation of the landslide, the joints and fissures in the granite are well developed due to the influence of regional tectonics in the zhamunong gully. there are three groups of large joints shown as follows (fig. 8). the attitude of the no. 1 preferred structure plane (psp) is 20334º, and the attitudes of the no. 2 psp and no. 3 psp are 9457º and 21186º, respectively. those joints fractured the rock mass of the landslide. in particular, the no. 3 psp controlled the development of the steep cracks at the back of the landslide. the development of the slip surface was controlled by the no. 1 psp. under the influence of earthquakes and the freeze-thaw and dry-wet cycles, the joints develop and fracture only partially. figure 6. dynamic processes of the 2000 cldf categorized by the peak ground displacement (pgd). table 5. dynamic processes of the 2000 cldf name of different dynamic processes sections duration/s maximum amplitude/cm maximum frequency/hz crack fracture and sliding process o—a 9 48.35 2.1 landslide transformation into debris flow a—b 10.25 64 2.1 debris flow movement and deposition b—c 8.75 42.64 2 c—d 14.5 32 2 d—e 40.25 50 1.4 after point e 247.25 30 1 78 jun li, ningsheng chen, yuandi zhao, mei liu, weiyu wang figure 7. a-e are the remote sensing images of impact points a-e of the 2000 cldf; (f) is the landside trace image from start to end. the remote sensing images are from the time of february, 22th, 2015. impact flow arrow of impact b impact b impact b 79a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency on april 8th, 2000, the amount of water decreased significantly and the water quality was significantly blackened in the zhamunong gully. the amount and color of the water demonstrate that the boundaries of the landslide bed (the two main sliding surfaces and the steep fractures of the trailing edge) were basically connected. the precipitation and snowmelt water had completely penetrated into the landslide bottom. figure 8. distribution of joints and cracks in the landslide. crack fracture and sliding process length of the locking section of the crack according to equation (1), when the length of the locking section l=455 m (table 6), the landslide is in a state of critical stability. when the trailing edge and front edge of landslide crack more, the length of the rock mass’s locking section l<455 m or the depth of the cracking segment in trailing edge hcr >363 m, the landslide will slide. crack fracture and sliding process the frontal mound deposits were moved from the top of the basin source because only the top of the basin source had the granite rock masses. those mound deposits had many ice-containing muds (fig. 9a), suggesting that the granite rock masses were strongly influenced by the long-term freeze-thaw and dry-wet cycles. there are many cracks in the granite rock masses at the top of the basin. according to a rock mechanics test and the landslide stability calculation results, under the combined effect of many conditions, such as a quick cutting of the valley in the yigong region, earthquakes, temperature changes, and continuous rainfall, a high-angle fissure in the trailing edge of landslide develops quickly and continuous creeping deformation occurs in the landslide along the weak structural plane or fracture surface, forming a mid ‘locking section’. the locking section is a geological structure that has a high bearing capacity and stress concentration in the potential sliding surface of the slope. the natural locking segment usually has large-scale and flat geometry and bears extremely slow shear loading or stress corrosion. the continuous creeping deformation of the avalanche mass leads the rock mass to further decline in strength, including the high-angle fissure in table 6, the trailing edge and the front slow-inclined fracture surface. the landslide is driven by a force toward the slope under the action of high pore-water pressure before the landslide is generated, which can be indicated by a strong rainfall process from april 1 to april 9 and the color of the water turned black and the water quantity decreased significantly on april 8. the residual locking section on the structure surface of the bh01 landslide was close to being sheared off by a huge thrust force and an uplift force, and the softening of water causes the main sliding surface. the final locking section of 455 m was eventually ruptured by the ms 4.8 earthquake, and then the landslide occurred (fig. 9b). landslide translation into a debris flow, and the movement and deposition of the debris flow before sliding, the debris deposits were gradually saturated under the infiltration of snowmelt water and precipitation. the tremendous impact force from the landside impact on the gully wall occurred instantly. the bottom table 6. calculation of landslide stability stable state seismic force /kn·m-1 gravity /kn·m-1 fracture water pressure /kn·m-1 sliding surface water pressure /kn·m-1 sliding resistance force fk /kn·m-1 slippage fs / kn·m-1 l/m critical stability 4077881 15991690 164711 412755 12922659 12922659 455 figure 9. (a) mound deposit of the 2000 cldf, (b) model of sliding-tension-shearing failure. mound 80 jun li, ningsheng chen, yuandi zhao, mei liu, weiyu wang of the loose deposit liquefied, the shear strength of the landslide decreased, and the resistance of the landslide decreased due to liquefaction from the high-speed ring shear tests (hu et al., 2015). therefore, the landslide moved at a higher average speed of 120.98 m/s. the landslide material was mainly gravel and rock debris. when these high-density materials rushed out of the narrow gully into a wider gully, the landslide materials mixing with air were transformed into a debris flow, which moved at a high speed. the high-speed debris flow gradually silts away in the wide gully after it has rushed out of the mountain mouth. characteristic parameters of the dynamic processes density we collected 4 particle samples of the 2000 cldf in 2004, which were not influenced by precipitation. the distribution curve of the particle size of the 4 samples was obtained by a sieving test. the densities of the 4 deposits are calculated using the equation given by (li and chen, 2018). an average density of 1.97 t·m-3 for the 2000 cldf is calculated by averaging the densities of the different locations in table 7. table 7. the density of the 2000 cldf no. locations clay particle content p0.005 density γc/t·m -3 1 impact point c 0.05 2.01 2 impact point d 0.05 1.98 3 impact point e 0.06 2.04 4 impact point f 0.03 1.88 velocity the average velocity of the 2000 cldf is estimated based on the time interval of the seismic waves and the distance between each impact point (table 8). the average velocity of the 2000 cldf is 30.12 m·s-1, the starting speed of the landslide in the o-a section is 89.89 m·s-1, and the average velocities of stages a-b, b-c, c-d, d-e, and e-f are 120.98, 108.57, 71.38, 53.14 and 15.25 m·s-1, respectively. the average velocity calculated by the seismic wave is similar to the velocity process computed by the massflow software (fig. 10), indicating the seismic wave can be used to predict the dynamic parameters of similar cldf. discharge a two-dimensional mountainous mass flow dynamic procedure solver (massflow-2d) is based on the theory of continuum mechanics of depth integration using the improved maccormack tvd finite difference method. the massflow software has a high-efficiency calculation, and it can simulate mountain disaster dynamics, it considers the complex topography with the characteristics of second-order accuracy and adaptive solution domain. therefore, the flow process of the 2000 cldf was simulated by the massflow software (ouyang et al., 2013, ouyang et al., 2015). the discharge of the debris flow is 504×104 m3·s-1. the discharges at impact points e and f are 85.9×104 m3·s-1 and 7.8×104 m3·s-1, respectively. the discharge processes of the 2000 cldf first increased and then decreased, and the maximum discharge of the 2000 cldf was 504×104 m3·s-1, which occurred at the second impact point b. the different discharge locations of the 2000 cldf are shown in table 9. table 9. discharges of the 2000 cldf locations a b e f discharges/×104 m3·s-1 325.5 504 85.9 6.2 the tendency for cldfs in the zhamunong gully based on the detailed internal and external landslide investigation presented in the current study, a cldf can be predicted in the zhamunong gully. the smaller debris flows occurring after the cldf disaster will decrease year by year. (1) an analysis of historical disasters shows that there is a possibility of cldf disasters recurring. as shown in fig. 11a, the scale of the debris flow fluctuates in the zhamunong gully. the scale of debris flows from 1902 and 2000 are 6.04×108 m3 and 3×108 m3, respectively. after two peaks, the scale of the disasters decreased year by year. the scale of debris-flow processes shows that a cldf may occur in the future in the zhamunong gully. (2) the flac numerical simulation shows that plastic rock masses exist in the top of the basin; these are the material basis for the occurrence of cldfs. under a middle magnitude earthquake and extreme precipitation, the source rock masses undergo plastic deformation in the zhamunong gully, and the potential sliding surfaces of bh01 and bh02 are shown in fig. 11b. the plastic rock masses show that the cdlf may happen again in the future. therefore, we estimate fig. 10. the simulated velocity of 2000 cldf by the massflow software table 8. the velocity of the 2000 cldf stage o-a a-b b-c c-d d-e e-f o-f distance/m 809 1240 950 1035 2139 3770 9939 time/s 9 10.25 8.75 14.5 40.25 247.25 330 average velocity/m·s-1 89.89 120.98 108.57 71.38 53.14 15.25 30.12 81a catastrophic landslide triggered debris flow in china’s yigong: factors, dynamic processes, and tendency the average area of bh01 to be 111879.5 m2, and its width and volume are 822.07 m and 0.92×108 m3, respectively. similarly, the average area of bh02 is 100314 m2, while the width of bh02 is 935.30 m and the volume of bh02 is 0.94×108 m3. (3) regional tectonics, earthquakes, and an extreme climate can activate cldf disasters in the zhamunong gully. gps measurements show that india and southern tibet converge at a rate of 20.3 mm/a, and approximately 80% of the convergence is absorbed by a 50 km-wide deformation zone centered on the southern edge of the tibetan plateau (larson et al., 1999). under the intense compression of the himalayan tectonic movement, large landslides have formed in the yigong region. the yigong region has a large number of faults. frequent seismicity occurs in this region with a high seismic intensity. the rock masses continue to accumulate energy generated by earthquakes. extreme climatic events occur frequently, and precipitation is abundant in the yigong region. the maximum rainfall in one hour in the past four years is 92.5 mm. in the past 60 years, the annual mean temperature has been increasing year by year, and the glacier in the source basin is gradually shrinking. in conclusion, the source of the rock masses can release significant elastic energy under earthquake and extreme climate conditions, and a large number of fragmented loose material sources slide along the steep terrain, thus forming a cldf. conclusions the conclusions contain three parts, which are stated as follows: (1) the main controlling factors of the 2000 cldf were the long-term freeze-thaw and dry-wet cycles and a middle magnitude earthquake. the impact of ms 4.8 earthquake cannot be neglected for the occurrence of 2000 cldf. there is a coupled relationship between the occurrence of the 2000 cldf, the earthquake, the freeze-thaw cycle, and the dry-wet cycle. from march to april 9th, 2000, a small-scale dry-wet cycle occurred in the zhamunong gully. the rock masses of bh01 landslide was saturated under the long-term freezethaw and dry-wet cycle conditions. therefore, the bh01 landslide occurred at 8 pm and 11.95 seconds on 9th april 2000. a middle magnitude earthquake of ms 4.8 occurred approximately 13 km away from the zhamunong gully at 8 pm and 9.2 seconds on 9th april 2000. the top rock masses obtained 43.3 gals of seismic acceleration. under the influence of the ms 4.8 earthquake, the bh01 landslide occurred. (2) based on the ground vibration spectrum recorded by the linzhi seismic station, there were four dynamic processes of the 2000 cldf, including the process of joint and crack development in the landslide, the crack fracture and sliding process, the process of landslide translation into a debris flow, the movement and deposition of the debris flow. the boundary and magnitude of the bh01 landslide were controlled by the joints in the granite rock masses. the landslide was saturated under the long-term freeze-thaw and dry-wet cycles before the disaster. the crack fracture and slide process of the 2000 cldf was revealed by a ‘creep-tensile-shear’ failure model. the attenuation of seismic activity and the strength of the granite rock mass led to the landslide occurring along cracks. the ms 4.8 earthquake may have ruptured the final 455 m locking sections, and then the landslide occurred. the density of the 2000 cldf was 2.0 t·m-3, the starting velocity was 89.89 m·s-1, the average velocity was 30.12 m·s-1, and the discharge shape of the 2000 cldf first increased and then decreased. the maximum discharge of the 2000 cldf was 504×104 m3·s-1, which occurred in the second impact section. 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(2016). comprehensive analyses of the initiation and entrainment processes of the 2000 yigong catastrophic landslide in tibet, china. landslides, 13(1), 39-54. https://www.sciencedirect.com/science/article/pii/s0012821x15005683#! https://www.sciencedirect.com/science/article/pii/s0012821x15005683#! https://www.sciencedirect.com/science/article/pii/s0012821x15005683#! issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.108506 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (september, 2023): 239 249 g eo ph y si c s abstract structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data paul gautier kamto1,2,*, erdinc oksum3, willy lemotio1,2, joseph kamguia1,2 1. geodesy research laboratory, national institute of cartography (nic), p.o. box 157, yaoundé, cameroon 2. department of physics, university of yaoundé i, p.o. box 812, yaoundé, cameroon 3. department of geophysical engineering, süleyman demirel university, isparta, turkey eoksum@istanbul.edu.tr how to cite item: kamto, p. g., oksum, e., lemotio, w., & kamguia, j. (2023). structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data. earth sciences research journal, 239-249. https://doi.org/10.15446/ esrj.v27n3.108506 manuscript received: 24/04/2023 accepted for publication: 27/09/2023 keywords: geological features; improved logistic method; improved tilt-euler method; hydrogeological resources; goulfey-tourba sedimentary basin. the goulfey-tourba sedimentary basin (gtsb) is a portion of the west and central african rift system whose studies on its structural geology remain very limited. belonging to the vast semi-arid sahelian region, this sedimentary basin covers several localities in cameroon and chad, whose economic and social activities are highly impacted by the shortage of drinking water. in this work, a new look is taken at the geological features of this local sedimentary region. to perform this, a preliminary synthetic study is carried out to assess the performance of some classic and recent edge detection methods. the effectiveness of the recent improved logistic (il) method is approved, given its ability to highlight low amplitude and deep features with a refined resolution. a regional/residual separation was applied to bouguer gravity disturbances to avoid blurring some upper crustal structures by high-frequency anomalies. the effectiveness of this regional/residual separation has been validated by checking the absence of ringing artifacts (gibbs phenomenon). the application of the il method on residual gravimetric disturbances of the study area revealed a series of lineaments not yet identified by previous scientific studies. the results show a slight fracturing of the basement, with geological features mainly trending in an nw-se direction. a newly identified geological discontinuity continuously crosses the study area from 12o45’n latitude to the southeast. utilizing a modified and recent form of the euler deconvolution theory (improved tilt-euler method) has enabled the detection of several density sources in the gtsb, most of which correlate well with the lineaments outlined by the il method. the improved tilt-euler method results show anomalous sources at more than 6 km depth beneath the bodélé sedimentary series of the upper tertiary. the euler’s linear solutions attributed to basement fractures show an average depth of 1 km. these results are undoubtedly a major contribution to refining the research of hydrogeological resources in this sahelian area. resumen la cuenca sedimentaria de goulfey-tourba se encuentra en el sistema de rift de áfrica central y occidental, pero sus estudios de geología estructural son muy limitados a la fecha. perteneciente a la vasta región semiárida del sahel, esta cuenca sedimentaria cubre varias localidades en camerún y chad, cuyas actividades económicas y sociales se ven fuertemente impactadas por la escasez de agua potable. en este trabajo se ofrece una nueva mirada a las características geológicas de esta región sedimentaria. para el cumplimiento de este objetivo se realizó un estudio sintético preliminar con el fin de medir el desempeño de algunos métodos clásicos y recientes de detección de bordes. se aprobó la efectividad del reciente método de logística mejorada debido a su capacidad para resaltar las amplitudes bajas y las características profundas con una solución fina. luego se aplicó una separación regional/residual a las anomalías gravitatorias de bouguer para evitar el desenfoque de algunas estructuras de la corteza superior por las anomalías de alta frecuencia. la efectividad de esta separación regional/ residual se ha validado al verificar la ausencia de discontinuidades (fenómeno de gibbs). la aplicación del método de logística mejorada en las anomalías gravimétricas residuales en el área de estudio reveló una serie de lineamientos que no habían sido identificados. los resultados muestran un fracturamiento ligero en los cimientos, con características geológicas con una tendencia mayoritaria en la dirección nw-se. una recién descubierta discontinuidad geológica cruza regularmente el área de estudio desde la latitud 12o45’n hacia el sudeste. al utilizar una forma recientemente modificada de la teoría de deconvolución de euler (método tilt-euler mejorado) fue posible detectar varias fuentes de densidad en la cuenca sedimentaria de goulfey-tourba, muchas de las cuales están bien correlacionadas con los lineamientos referenciados con el método de logística mejorada. los resultados del método tilt-euler mejorado muestran fuentes anómalas a una profundidad de 6 kilómetros debajo de la serie sedimentaria de bodélé, del terciario superior. las soluciones lineales de eulers asignadas a las fracturas de los cimientos muestran una profundidad promedio de 1 km. estos resultados son, sin duda, una contribución mayor en el mejoramiento de la búsqueda de recursos hidrogeológicos en el área del sahel. palabras clave: características geológicas; método de logística mejorada; método tilt-euler mejorado; recursos hidrogeológicos; cuenca sedimentaria de goulfey-tourba. mapeo estructural de la cuenca sedimentaria de goulfey-tourba (rift de áfrica occidental y central) a través de información de gravedad de alta resolución https://doi.org/10.15446/esrj.v27n3.108506 mailto:eoksum@istanbul.edu.tr https://doi.org/10.15446/esrj.v27n3.108506 https://doi.org/10.15446/esrj.v27n3.108506 240 paul gautier kamto, erdinc oksum, willy lemotio, joseph kamguia introduction the goulfey-tourba sedimentary basin (gtsb) is located in the far north region of cameroon. also called makari basin, the gtsb constitutes the northern part of the logone birni sedimentary basin (lbb) in the central and west african rift system (wcras) (loule and pospisil, 2013). the gtsb also belongs to the large endorheic chad basin domain. located between longitudes 14o15’ e and 15o15’ e, and latitudes 12o12’ n and 13o12’ n, the gtsb constitutes the main geological formation in the study area (figure 1). the study area shows a relatively flat topography. although several localities in this area have been identified as abounding a good hydrogeological potential (isiorho et al., 1991), few geophysical studies have been carried out to improve the exploitation of groundwater resources. also, the hydrocarbon potential of the neighboring sedimentary basins, all of them belonging to the wcras, makes the gtsb an area of interest for the scientific community (poudjom et al., 1996; loule et pospisil, 2013; nguimbous-kouoh et al., 2017). unfortunately, the available ground-based gravity data are already aged and very sparse to conduct a detailed structural study of the local basin. so, the geophysical investigation of the gtsb requires the use of new high-resolution data and advanced potential field processing methods. until today, the far north region of cameroon in general and especially the gtsb, is still suffering from great difficulties in accessing potable drinking water (fantong et al., 2010; cheo et al., 2017). this water shortage considerably affects the economic development of the region and population health. according to lianchong et al. (2011) and hasan et al. (2019), the identification of faults and underground geological formations is a major asset for the exploitation of groundwater contained in the underlying aquifers. a precise mapping of the fractured-rock aquifers in the region would be an undeniable contribution to the project of installation of water boreholes in the most affected localities. besides, the optimal exploitation of hydrocarbons and mineral resources available to the region is a necessity to ensure sustainable development. this requires a preliminary geometric characterization of underground anomalies (wang and huang, 2012; ugbor et al., 2021; habu et al., 2022). the precision provided in the delineation of geological structures depends on the method and the quality of the data used (ammar and kamal, 2018; hasan et al., 2019; ekwok et al., 2020; deep et al., 2021). the use of the gravimetric method for the characterization of structural features in the basement has been successfully applied in hydrogeophysics according to recent scientific projects (redhaounia et al., 2016; epuh, 2020; taha et al., 2021; ziani et al., 2022). the improvement and optimization of data acquisition techniques during the latest satellite missions enable the production of global gravity field models with unprecedented resolution and accuracy (andersen et al., 2020; guo et al., 2020; müller and wu, 2020; gautier et al., 2022). on the other hand, the detection techniques of geological lineaments are widely and increasingly used in structural geophysics (kamto et al., 2021; pham et al., 2021; eldosouky et al., 2021,2022a; ekwok et al., 2021; al., 2022; ekka et al., 2022; kamto et al., 2023a). for decades, new edge detection methods have been developed by researchers to provide better results in the mapping of structural features, and to avoid the occurrence of extraneous noise which could be assigned to geological features (wijns et al., 2005; cooper and cowan, 2006; tatchum et al., 2011; oksum et al. (2021); pham, 2021; melouah and pham, 2021; pham et al., 2022; prasad et al., 2022). most of these processing methods are designed using vertical or horizontal derivatives of amplitude, basically used to highlight sharp gradient areas in a potential field. the classic horizontal gradient (cordell and grauch, 1985) and total gradient (roest et al., 1992) filters are among the most widely used techniques in structural geophysics. based on these latter and mathematical functions, wijns et al. (2005), ferreira et al. (2013) and pham et al. (2020) proposed some edge detection methods mainly based on the ratio of derivatives of the gradient of the amplitude; these methods are respectively the theta map, the tilt angle of the total horizontal gradient and the improved logistic filter. since then, these advanced and new techniques are commonly exploited to produce quality results in structural geology and tectonics (ekka et al., 2022; eldosouky et al., 2022b; ekwok et al., 2022; ghomsi et al., 2022; kafadar, 2022; sahoo et al., 2022; arogundade et al., 2023). this work aims to achieve a precise and detailed structural mapping of the gtsb using a high-resolution global gravity model xgm2019e (zingerle et al., 2020). in order to evaluate their performance, the horizontal gradient (thg), the total gradient (tg), the theta map (tm), the tilt angle of the total horizontal gradient (tthg) and the improved logistic (il) techniques are beforehand tested through a complex synthetic model. then, to delineate lineaments and faults of the gtsb, these techniques are applied to the topography-free gravity disturbances. additionally, the euler deconvolution method (thompson, 1982 and reid et al., 1990) is used to estimate the depth of anomalous discontinuities in the basement. figure 1. location and topographic map of the goulfey-tourba sedimentary basin in the far north region of cameroon. the region’s main geological structures are the west african rift system (wars) and the central african rift system (cars). white lines represent national boundaries. the inset exhibits central africa and some geological and tectonic units including the endorheic lake chad basin (elcb), the sahara metacraton (smc) and the congo craton (cc). the white box circumscribes the far north region of cameroon and its surroundings. gog: gulf of guinea. 2. geologic and tectonic framework figure 2a shows a simplified view of the major geological formations and structures in central africa. the gtsb belongs to one of the largest geological structures of the african lithospheric plate, namely the wcars. the tectonic evolution of central africa is strongly linked to its major geological units, especially the congo craton and the sahara metacraton, the cameroon line, the wars and cars (moreau et al., 1987; fairhead, 1988; binks and fairhead, 1992). the young deposits of the cenozoic era are the main geological formation of the cars and thus, of the gtsb. guiraud et al. (1992) affirmed that the evolution of the cars between nigeria, cameroon and chad during the break-up of gondwana would be at the origin of the appearance of strong stresses between tectonic plates, inducing the reactivation of tectonic structures in the study area. the geological patterns of the gtsb are presented in figure 2b and is linked to the formation of the endorheic lake chad basin during the cretaceous period. there are four main geological formations in the area, namely: the bodele serie (dating of pliocene times), the soulias serie (from the upper pleistocene), the labde serie (from the upper holocene) and volcanic rocks (mathieu, 1976). these geological formations cover granitic and gneissic rocks in the basement. loule and pospisil (2013) showed that the gtsb is the site of volcanic bodies dating from the cretaceous and trapped in the crustal 241structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data basement. according to these authors, the geological stresses caused by the extension and uplift of the mesozoic lbb would be the main cause of tectonic structures, and thus, of the underlying magmatic intrusions. figure 2. (a) simplified view of major geological formations and structures in central africa. (b) geological patterns of the goulfey-tourba sedimentary basin (modified from nguimbous-kouoh et al., 2017). data and method gravity disturbances to access structural and geological information at a local scale such as the study area, the exploitation of high-resolution and accurate data is essential. in this work, the global gravity model xgm2019e has been utilized. it is one of the most recent gravity models improved following a series of anterior global models namely xgm2016 (pail et al., 2018), xgm2019 and xgm2019e_2159 (zingerle et al., 2020). the gravity field model xgm2019e has been developed using satellite altimetry data, terrestrial and marine gravity data, gravity effects of topography and the satellite gravity model goco06s. this high-resolution global gravity field model contains spherical harmonic coefficients up to degree and order (d/o) 5399 (~2’ or 4 km of spatial resolution). the gravity data derived from satellite altimetry and topography has been computed respectively from the 1’x1’ spatial resolution dtu13 (andersen et al., 2016) and earth2014 (hirt and rexer, 2015) models. the combination of satellite data with terrestrial data has been made using normal equation complete derivation for spectral bands up to d/o 719, and the use of a block diagonal least squares regression technique for d/o above 719 (zingerle et al., 2020). by performing some comparison tests with gps/levelling points, zingerle et al. (2020) showed a slight improvement in the xgm2019e gravity model on land compared to its predecessors (egm2008, xgm2016 and eigen6c4). this global gravity model is increasingly exploited and validated in many recent studies in geodynamics and structural geophysics (abrehdary and sjoberg, 2021; samprieto and capponi, 2021; ognev et al., 2022; lewerissa and lapono, 2023; li et al., 2023). gradient-based filters edge enhancement techniques in structural geology and geophysics have experienced a particular boom since the end of the 20th century. one of the most classic and widely used methods is the horizontal gradient mathematically formulated in its completed form for the first time by cordell (1979). the hg method is established from the variations of the potential field along the x and y directions. representing a field of gravimetric disturbances a(x,y) as a regular grid of points (i,j) with amplitudes ai,j, an approximation of its first order derivatives in the x and y directions can be written as follows: = ≈ + 1, − − 1, 2| + 1− | (1) and, = ≈ , + 1− , − 1 2| + 1− | (2) so, the hg is defined as the amplitude generated by the dx and dy components: ( , ) = ( 2 + 2)1/2 (3) besides, nabighian (1972) introduced the analytical signal method on a three-dimensional potential field. also called the total gradient method, the analytical signal was discovered as a technique for outlining structural features according to the study of roest et al. (1992). this method is defined as follows: ( , ) = ( 2 + 2 + 2)1/2 (4) unlike the hg, the tg integrates three components of the potential field: dx, dy, and the first-order vertical derivative dz. it was evjen (1936) who first introduced this notion and its importance in the interpretation of gravity anomalies. the vertical gradient can be easily calculated in the frequency domain using the fast fourier transform f and the radial repetency k of the potential field (gunn, 1975): = ≈ − 1[| | ( )] (5) phase-based filters in order to overcome the limitations of gradient filters, many researchers introduced a wide variety of phase-based edge detection methods. the theta method, proposed by wijns et al. (2005), is basically an improvement of the tg method in the delineation of magnetic anomalous sources at low latitudes. therefore, this method has been undervalued for almost two decades for the gravimetric processing of geological features (oruc, 2011; kumar et al., 2022). the theta filter is defined as the phase difference between hg and tg functions. so, this filter could be expressed as follows: = ≈ ( ) = − 1 (6) like the theta method, the tthg method (ferreira et al., 2013) is a phase-based filter developed to balance both deep and shallow amplitudes of the potential field. it is an improvement of the tilt angle method (miller and singh, 1994) which integrates as the main variable. so, this method is designed by normalizing the vertical derivative of by its horizontal gradient: = − 1 ( 2+ 2)1/2 (7) where hgx, hgy and hgz are respectively the first-order derivatives of hg in the x-, yand z-directions. improved logistic method beyond the numerous edge detection methods in the scientific literature (mainly derivativeand phase-based filters), pham et al. (2020) recently designed the il method to increase the precision and resolution of the outlined geological lineaments. this method associates the improved logistic function and the total horizontal gradient. the il method is defined as follows: (8) 242 paul gautier kamto, erdinc oksum, willy lemotio, joseph kamguia the parameter p in ep. (8) is a positive real constant and its value has a great influence on the resolution of results. an estimation of the optimal p-value has been discussed by pham et al. (2020) (values between 2 and 5 leading to best resolutions). in this work, the il method is applied for p = 2. although its performance has already been confirmed by many recent geophysical studies (ekka et al., 2022; eldosouky et al., 2022b; ekwok et al., 2022; kafadar, 2022; sahoo et al., 2022; arogundade et al., 2023), the advantages and limitations of the il method will first be evaluated on a synthetic model before its application in the study area. improved tilt-euler deconvolution the first version of the euler deconvolution method, widely used in estimating the depths of crustal anomalous sources, was introduced by thompson (1982) and subsequently adapted in its 3d form by reid et al. (1990). the main problem with this classic euler deconvolution theory is that it incorporates a parameter called structural index (si), which the choice greatly influences the precision and quality of the estimated solutions. several authors have already proposed modified forms of the euler deconvolution formula to minimize the influence of the si parameter. one of the modified forms is the tilt-euler deconvolution (salem et al., 2008) which has the advantage of remove the si parameter as input value. to further improve the previous form which is limited to points where the horizontal gradient of the potential field is different from zero, huang et al. (2019) recently proposed another modified form called the improved tilt-euler deconvolution method: (9) where the triplets (x,y,z) and (x0, y0, z0) are respectively the geographical coordinates of the observed anomaly and of the anomalous source to be estimated; θ represents the improved tilt angle given by: (10) 4. test of edge detection methods the methods for delineating geological discontinuities presented in section 4 represent filters of the potential field which, despite their effectiveness, could show some limitations in certain circumstances. in this section, a complex synthetic model is established to evaluate the efficiency of these filters. figures 3a and 3b show respectively a layout of synthetic density sources in the plan and in 3d. the proposed model consists of five prismatic sources (a, b, c, d and e) with alternately positive and negative density contrasts, and located at different depths. another peculiarity in the arrangement of the prismatic sources is that d and e induce a certain asymmetry in the proposed synthetic model. table 1 summarizes the geometric and density parameters of the five prismatic sources. table 1. geometric and density parameters of prismatic bodies. parameters/ prisms a b c d e coordinates of the center in km (50,150) (100,100) (150,50) (150,150) (150,150) (x-length, y-length) in km (80,20) (80,20) (80,20) (50,50) (30,30) (width, depth to top) in km (4,2) (4,6) (4,10) (5,7) (5,2) density contrast (g/cm3) 0.2 -0.3 0.3 -0.3 0.2 figure 3. layout of synthetic bodies in (a) 2d and (b) 3d. the gravity anomaly model displayed in figure 4a is generated by the five synthetic bodies (a, b, c, d and e). this model is presented on a grid of dimensions 200 km × 200 km, with a spatial resolution of 1 km. the results of applying the hg, tg, theta, tthg and il methods to the gravity anomaly are respectively shown in figures 4b-f. at first glance, the hg classic filter doesn’t generate noise or spurious signals around the edges of prismatic bodies. however, the hg filter manages to maximize the signal only on the boundaries of prisms a and e, thus neglecting the existence of other density sources. unlike the hg method, the tg and theta methods induce spurious signatures around a, b, c and d. the true boundaries of the density sources are diffused, probably because of the asymmetric arrangement due to d and e prismatic bodies. this assertion is justified by the difficulty of the tg and theta filters to outline the edges of prism a, although buried at the same depth (2 km) and having the same density contrast (0.2 g/cm3) as prism e. even in the case of a synthetic model with a symmetric arrangement of density sources, the theta method already showed some limitations with the appearance of untrue surrounding signatures in the results (oksum et al., 2021; pham et al., 2022). despite spurious signatures produced by the asymmetry in the arrangement of the density sources, the results of the tthg (figure 4e) and il (figure 4f) methods are more realistic than those presented previously. the amplitudes are well maximized at the boundaries of the five prisms. however, the best resolution goes to the il filter which refines and maximizes the signatures on the boundaries of the five density sources. pham et al. (2020) also attested a good accuracy of the il method on thin prisms compared to the tthg method. 243structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data figure 4. (a) gravity anomalies generated from the synthetic density sources a, b, c, d, and e, and results from the application of the (b) hg, (c) tg, (d) theta, (e) tthg and (f) il methods on the gravity anomaly grid. the filters assessed in this section are now performed in a disturbed environment. for this, a white noise of 4% of the amplitude of the synthetic gravity anomalies initially produced has been added to obtain the noisy gravity anomalies (figure 5a). the results of applying the hg, tg, theta, tthg and il methods to the noisy gravity anomaly are displayed in figures 5b-f. the result displayed in figure 5b shows that the hg filter seems less sensitive to the white noise contained in the gravity anomaly grid. in addition, the tg and theta filters have great difficulties in highlighting the edges of the density sources. the wrong signals generated by the asymmetrical arrangement of bodies d and e are still visible in the southwestern part of figures 5c and 5d. unlike the classic hg filter, the tthg and il filters accentuate surrounding noise in the displayed results (figures 5e and 5f). however, the edges of the five prisms a, b, c, d and e are still visible, with better sharpness and less spurious signals in the results following the application of the il method (pham et al., 2020). with the aim of reducing the negative influence of white noise on the results displayed by edge detection filters, the upward continuation filter is commonly required as a solution. so, figure 6a presents the noisy gravity anomaly upward extended to an elevation of 2 km. figures 6b-f show the results of applying the hg, tg, theta, tthg and il methods to the gravity anomaly grid of figure 6a. the spurious signals caused by the white noise injection have considerably disappeared on the hg and tg results. yet, the precise identification of the edges of prisms a, b, c and d is difficult and limited. the result of the theta method shows for the first time (contrary to the results of figures 4d and 5d) a minimization of the amplitude near the edges of the density sources. however, the outlined boundaries are diffused outside the true edges of prisms a, b, c and d (figure 6d). in spite of the spurious amplitudes which persist as shown in figures 6a and 6b, the tahg and il methods maximize well the amplitudes on the edges of the five prisms. but, the tahg filter seems to thicken the edges of the prismatic bodies and generate more intrusive signatures compared to the il filter. so among the five edge detection methods evaluated in this section, the il method shows the best performance in delineating density source boundaries. figure 5. (a) noisy gravity anomalies generated from the synthetic density sources a, b, c, d, and e, and results from the application of the (b) hg, (c) tg, (d) theta, (e) tthg, and (f) il methods on the noisy gravity anomaly grid. figure 6. (a) 2 km upward continued noisy gravity anomalies generated from the synthetic density sources a, b, c, d, and e, and results from the application of the (b) hg, (c) tg, (d) theta, (e) tthg and (f) il methods on the noisy gravity anomaly grid. 244 paul gautier kamto, erdinc oksum, willy lemotio, joseph kamguia 5. results and discussion gravity disturbances the gravity disturbances utilized in the work are derived from the combined gravity field model xgm2019e (zingerle et al., 2020). the free-air gravity disturbances of the study area are displayed in figure 7a. this free-air gravity grid has been downloaded with a spatial resolution of 0.033 degree and at a height of 10 km above the ellipsoid to avoid distorting the spectrum of the original gravity field model. the free-air gravity disturbances range from -4.45 to 10.04 mgal, with a mean of 2.23 mgal (figure 7a). the lowest amplitudes are observed over the young deposits and trend in a n45oe direction. to reveal the gravimetric signatures induced by the major sedimentary formations, and the anomalous crustal and mantle density structures, it is necessary to subtract the gravimetric effects of the topography (uieda et al., 2016; kamto et al., 2023b). there are several methods to reduce the topographic effect in the scientific literature (grombein et al., 2013; de gaetani et al., 2021). the study area is circumscribed at a very local scale, and the influence of the curvature of the earth has been neglected. the topographic corrections are calculated using the earth2014 global topographic model (hirt and rexer, 2015) and at a constant average density of 2670 kg/cm3. then, we obtain the topographyfree gravity disturbances (figure 5b), approximated here to the bouguer gravity disturbances. the amplitudes of bouguer gravity disturbances in the study area are completely negative and range from -36.78 to -22.88 mgal. the lowest amplitudes correlate approximately with the bodele geological series (figure 2b), essentially made up of sediments belonging to the upper tertiary (mathieu, 1976). figure 7. (a) free-air and (b) topography-free gravity disturbances in the study area. regional/residual gravity disturbances for a better delineation of the geological features of the gtsb, it is advantageous to separate the regional gravity disturbances (generally attributed to density anomalies extending from the lower crust to the mantle) from the residual ones. several regional/residual separation methods exist in the scientific literature (beltrao et al., 1991; keating and pinet, 2011; mallick and sharma, 1999; meng et al., 2009). each of these methods could be limited and subjective depending on the characteristics of the gravity anomaly grid or the filtering technique used. kebede et al. (2020) recently conducted a comparative analysis of the most widely used regional/residual separation methods namely the upward continuation (jacobsen, 1987) and the polynomial trend analysis (simpson, 1954); they concluded a better reliability of the upward continuation technique provided that the continuation height is suitably estimated. for this study, the spectral analysis (spector and grant, 1979) and upward continuation methods have been appropriately applied to the bouguer gravity disturbance grid (figure 7b). after an estimation of the continuation height (20 km) using spectral analysis, the grid of regional disturbances is obtained (figure 8a). the regional disturbances model displays negative amplitudes ranging from -33.47 to -24.74 mgal. these disturbances clearly reflect the regional anomalous trend of the moho in the study area. the residual disturbances displayed in figure 7b are obtained by subtracting the regional amplitudes (figure 8b) from the topography-free gravity disturbances (figure 7b). the residual gravity disturbances range from -5.97 to 4.74 mgal; they correlate well with the signatures of the initial gravity disturbances grid and the geological series mainly made up of tertiary sediments (figure 8b). although the signatures observed on the grid of the residual gravity disturbances seem realistic, the authenticity of the latter will be further verified in the results section. indeed, when applying the regional/residual separation filter, the gibbs phenomenon (bocher, 1906) could occur and generate ringing artefacts (spurious signals) in the output model. figure 8. (a) regional and (b) residual bouguer disturbances computed using spectral analysis and upward continuation method. application of edge detection filters derivatives and gradient maps by applying equations 1, 2 and 5 to the amplitudes of the residual gravity disturbances, the x-, y-, and z-derivatives maps are obtained (figures 9a-c). these derivative maps show the primary contribution of each directional gradient for the characterization of geological structures and formations of the gtsb. the x-derivative (dx) gives an overview of geological discontinuities or contacts along the x direction. the dx map vividly shows a majority of geological discontinuities with a nw-se trend. the y-derivative (dy) map has a slight similarity with the dx map in the southwest, notably a wnwese trending gradient. the z-derivative (dz) map shows gradient amplitudes ranging from -0.52 to 0.53 mgal. the dz map amplifies the amplitudes of anomalous density sources and geological transition zones. some signatures on the dz map correlate with those displayed on the dx map. for a better overview of the geological structures delineated in the study area, the dx, dy and dz components are combined in the classic hg and tg filters. figures 10a and 10b display the results of applying the hg and tg filters respectively on the residual bouguer grid of figure 8b. the hg and tg maps maximize the signal over geological discontinuities, as reported by cordell (1976) and roest et al. (1992). the hg map gives very little information on the geological features of the study area. the boundaries of main crustal density sources are difficult to identify. the results displayed by the tg map are not better, despite the addition of the vertical component dz. as mentioned in the synthetic example in section 4, the delineation of both shallow and deep lineaments by the hg and tg methods remains inaccurate and limited. theta and tthg maps to solve the main limitations of gradient filters that only highlight high gradient areas (pham et al., 2020), a series of phase-based methods are introduced to detect both deep and low amplitude of geological structures. figures 10a and 10b show the results of applying the phase-based theta and tthg filters to residual bouguer disturbances. unlike the hg and tg maps, the theta map minimizes the signal (zero amplitude) on geological discontinuities (wijns et al., 2005). the theta map seems to delineate more lineaments. but, the outlined lineaments are continuously attached to each other, which does not reflect reality. ting-jie et al. (2016) and oksum et al. (2021) also pointed out the limit of the theta method mainly because of the wrong signals generated in the result. in addition, the tthg map (figure 10b) maximizes the amplitudes of geological features of the gtsb around 1.5 rad (ferreira et al., 2013). like the theta map, the tthg map depicts the boundaries of geological density 245structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data sources with more details, but with the advantage of reducing the appearance of wrong edges in the results. however, as also mentioned by melouah and pham (2021), and oksum et al. (2021), the main disadvantage of the tthg method is that it uses to scatter the signal around the true lineaments. a structural study at a local scale such as the study area requires the use of a precise and highresolution method. figure 9. x-derivative (a), y-derivative (b), and z-derivative (c) maps of residual bouguer disturbances figure 10. results of the application of the hg (a) and tg (b) methods on the residual bouguer disturbances. figure 11. results of applying the theta (a) and tthg (b) methods on the residual bouguer disturbances. 246 paul gautier kamto, erdinc oksum, willy lemotio, joseph kamguia application of the il method figure 12a presents the il method’s results to the residual bouguer disturbances in the study area. the il map displays maximum amplitudes on geological features and edges of anomalous bodies (pham et al., 2020). the il method is also applied to the bouguer gravity disturbances to verify that the regional/residual separation applied in subsection 5.2 did not produce ringing artifacts in the residual disturbances grid (figure 7b). the il(bg) map displays the results of applying the il method to bouguer gravity disturbances (figure 12b). the amplitudes of geological discontinuities outlined in figure 12a (il map) show a good correlation with those depicted in figure 12b (il(bg) map). the slight differences observed could be mainly caused by the presence of regional amplitudes in the il(bg) map, which slightly blurs the geological features in some zones. the highlighted features using the il method are similar to the results displayed on the tthg map (figure 11b). however, the display of geological edges on the il map seems sharper and more precise compared to precedent maps. the effectiveness of the il method in structural geology has also been confirmed by several recent studies (eldosouky et al., 2022; sahoo et al., 2022; ekka et al., 2022; maden and elmas, 2022). figure 12. results of the application of the il method on (a) the residual bouguer disturbances and (b) the bouguer gravity disturbances. structural map of the goulfey-tourba basin following the resolution and precision of the results provided by the il method, new geological features have been delineated in the basin. figure 13a shows the geological lineaments identified from the il method in the gtsb. a total of 28 geological features have been outlined, with lengths ranging from 2.55 to 116.25 km. figure 13b presents a rose diagram showing the lineaments orientation in the study area. major geological features mainly trending in the nnw-sse direction are observed. this main direction brings some details to the work of nguimbous-kouoh et al. (2017), who rather concluded on nesw and nw-se trending features in the basin. the outlined features can be attributed to fractures, boundaries of intrusive anomalous bodies or contact zones between geological formations. a geological feature newly identified and continuously passing through the study area from latitude 12o45’n up to the southeast has been enhanced. never mentioned in previous studies (loule and pospisil, 2013; nguimbous-kouoh et al., 2017), this geological feature could be attributed to a density contrast between two main geological formations. the work of guiraud et al. (1982) revealed that the appearance of stress zones between tectonic plates during the opening of the south atlantic could have reactivated some tectonic features in the basement. the linear features (considered as basement fracturing) observed in figure 13a denote a slight reactivation of crustal tectonic structures. the fractured areas could greatly help improve the exploration of hydrogeological resources and reduce the problem of lack of drinking water that severely affects the study area (fantong et al., 2010; cheo et al., 2017). improved tilt-euler solutions estimating the average depth of anomalous density sources is important in structural geology. one of the classic methods widely used to evaluate the position and the depth of the contours of intrusive bodies is the euler deconvolution method (thompson, 1982). in this work, one of its most recent modified forms is utilized: the improved tilt-euler deconvolution method (huang et al., 2019). this euler deconvolution form (as defined in equation 9) has the advantage of ignoring the si parameter as input data. during the computation phase, a compromise has been made for the optimum window size and depth tolerance parameters (respectively 15 and 10%). too-high values of window size tended to produce spurious solutions around geological discontinuities, while too-low values produced scatters solutions not located on the structural features. figure 14a displays the solutions proposed by the improved tilt-euler deconvolution method in the study area. most of the tilteuler solutions correlate well with the lineaments delineated by the il method. in addition, the displayed solutions provide an estimation of the gtsb density sources depths. as depicted in the histogram (figure 14b), the estimated depths range between 0.27 and 6.27 km, with a mean of 2.09 km. the maximum percentage is attributed to depths estimated between 1 and 1.2 km. the tilteuler solutions confirm the presence of a major discontinuity extending from latitude 12o45’n up to the south-east, as identified on the lineament map (figure 13.a). the anomalous sources mainly take a nw-se trend, probably following the tectonic events that the basin underwent during the evolution of the cars as announced by guiraud et al. (1992). the location of density sources at more than 4 km depth should induce the scientific community to research potential hydrocarbon resources in this still unexplored sedimentary basin. the linear solutions estimated at 1 km depth on average and coinciding with the structural features of figure 13a give evidence of a slight fracturing of the basement in the study area. the fractured zones of the basement up to 1 km depth could potentially lead to a water table and should therefore be the target of hydrogeological investigation. figure 13. (a) map of geological lineaments from the il method; (b) rose diagram showing lineament orientations in the study area. figure 14. (a) location and depth of anomalous sources according to the tilt-euler deconvolution method; (b) histogram of frequencies of tilt-euler solutions as a function of elevations (depth). conclusions the gtsb covers a local zone in central africa and serves as the transition between the wars and cars geological structures. the structural geology of this study area remains limited and its hydrogeological potential resources 247structural mapping of the goulfey-tourba (west and central african rift) sedimentary basin using high-resolution gravity data are still under-exploited given the serious lack of drinking water in the region. this work aimed to reveal new patterns on the structural geology of the gtsb using recent processing methods of potential field. the synthetic assessment of the il method validates the performance of this latter to delineate with a good precision both shallow and deep geological features. besides, the classic hg and tg methods have shown great limitations in delineating low gradient amplitude and deep geological features. the application of the il method on the residual gravity disturbances provides new patterns in the structural geology of the gtsb. the lineaments outlined in the study area mainly trend in a ne-sw direction; this clearly explains the tectonic evolution of the wcars and its consequences in the reactivation of tectonic structure. however, the structural impact of the tectonic evolution of the area would have been slight given the density of the delineated geological features. a major geological discontinuity, not yet identified by previous studies and continuously passing through the study area from latitude 12o45’ n up to the south-east, should contribute for updating the geological map of this area. the application of a recent modified form of the euler deconvolution method enables to estimate the location and the depths of anomalous density sources (between 0.27 and 6.27 km). the location of linear structures identified as basement fractures and located at an average depth of 1 km, could be potential solutions to refine the hydrogeological investigations in this local region. besides, the identification of anomalous density sources (with depths estimated at more than 6 km) must be the target of hydrocarbon research in this still unexploited sedimentary basin. however, additional geological and geophysical studies deserve to be carried out in this area, which is arousing scientific interest in recent years. references abrehdary, m., & sjöberg, l. e. 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(2022). physicochemical and mineralogical changes of lithic xerorthent soils on volcanic rocks under semi-arid ecological conditions. earth sciences research journal, 26(4), 291-301. https://doi.org/10.15446/ esrj.v26n4.96571 abstract: this study investigates the mineralogical changes and soil development processes of young soils formed on various bedrocks of volcanic origin under the same land use/land cover and climate conditions. the current study was conducted in lithic xerorthent soils formed on tuff, trachybasalt, and trachyandesite bedrock between sandıklı-şuhut districts of afyonkarahisar. the three soil profiles excavated in the study area were classified in entisols order based on soil taxonomy. the primary minerals, sanidine and muscovite, and the clay minerals, smectite, kaolinite, and illite, were widely determined in three soil profiles which were named profile i (pi), profile ii (pii), and profile iii (piii). according to the chemical alteration index (cia) values, which indicate weathering, the soils formed on the tuff bedrock were slightly weathered (77.04%). the chemical weathering index (ciw) in the soils’ surface horizons formed on the trachybasalt and trachyandesite bedrock are classified as non-weathering rocks with 24.43% and 33.88%. basic cations are found at high levels in the tuff bedrock. the determination of phillipsite, gismondin and calcite minerals is an indication that the mineral content of the bedrock and the bedrocks have a significant effect on soil formation. the relationship between the bedrock and the soil has been revealed. as a result of the study, it was concluded that there were significant differences in their physico-chemical characteristics, weathering rates, and mineralogical properties. however, they were characterized as young soils since they do not contain any subsurface diagnostic horizons on the volcanic bedrock under the same climatic and land use/land cover conditions. keywords: volcanites; weathering rates; soil development; lithic xerorthent; cambios físicoquímicos y mineralógicos de suelos xerortentes líticos en rocas volcánicas bajo condiciones ecológicas semiáridas este estudio investiga los cambios mineralógicos y los procesos de desarrollo de los suelos jóvenes formados en varios lechos de rocas de origen volcánico bajo las mismas condiciones climáticas y de la relación uso del suelo/ cobertura del suelo. el análisis se realizó en suelos xerortentes líticos formados en lechos de roca de toba volcánica, traquibasalto y traquiandesita entre los distritos de sandıklı y şuhut, en la ciudad de afyonkarahisar, turquía. los tres perfiles de suelo excavados en el área de estudio se clasificaron en el orden de entisols de acuerdo con su taxonomía. los minerales primarios, sanidina y muscovita, y los minerales arcillosos, esmectita, kaolinita e illita fueron ampliamente definidos en los tres perfiles del suelo, los cuales fueron nombrados perfil i (pi), perfil ii (pii) y perfil iii (piii). de acuerdo con los valores del índice de alteración química, los cuales indican erosión, los suelos que se formaron en las capas de rocas de toba volcánica estuvieron ligeramente erosionados (77.04%). el índice de meteorización química en la superficie de los suelos formados en las capas de rocas de traquibasalto y traquiandesita se clasifican como rocas no erosionadas con 24.43 % y 33.88 %. los cationes básicos se encuentran en altos niveles en las capas de roca de toba volcánica. la caracterización de phillipsita, gismondina y los minerales de calcita es un indicador de que el contenido mineral en las capas de roca tiene un efecto significativo en la formación del suelo. de esta forma se revela la relación entre las capas de roca y el suelo. como un resultado de este estudio se concluye que existen diferencias significativas en las características fisicoquímicas, rangos de meteorización, y propiedades mineralógicas. sin embargo, estos fueron caracterizados como suelos jóvenes ya que no contienen ningún horizonte de diagnóstico bajo la superficie en la capa de roca volcánica bajo el mismo regimen climático y en la condiciones de uso del suelo/cobertura del suelo. https://doi.org/10.15446/esrj.v26n4.96571 mailto:demirsinan.07@gmail.com https://doi.org/10.15446/esrj.v26n4.96571 https://doi.org/10.15446/esrj.v26n4.96571 292 sinan demir, pelin alaboz, orhan dengiz, hüseyin şenol, kamil yilmaz, oğuz başkan 1. introduction the necessary use and management of lands, which are an essential part of the terrestrial ecosystem and agricultural production, without losing their functions, is only achievable by understanding all the properties of soils for gaining character. the chemical content of soils depends on the nature of the bedrock from which the soil is formed, decomposition-weathering processes, biological events, and additional materials from various sources. accordingly, the type of the bedrock can significantly affect soil formation and the availability of nutrients. the conditions in which the bedrock is found in soils’ formation cause variations in properties by affecting the weathering rates. as a result of these events, changes in physical, chemical, and mineralogical properties and different horizon structures occur (karacan et al., 2016). differences in soil age or bedrock can be evaluated to show the relative change in soil formation degrees where other soil forming factors are similar. in cases where soil formation factors are identical on different bedrocks, determination of chemical weathering and, consequently, clay mineral formations will enable a more successful analysis of environmental issues, and a better understanding of the cycle of elements, losses, and retention of nutrients. as it is well known that the soil is formed as a result of different processes of the bedrock. moreover, mineral weathering, crucial part of the pedogenetic evaluation, also supplies an essential role by transforming bedrock to weathered rock and consequently to soil that supply nutrients to terrestrial ecosystems (tunçay et al., 2020). soil formation is also a prolonged process. over time and under the influence of climate, the bedrock’s impact on soil mineralogy and physico-chemical properties is thought to decrease, but the time required is uncertain (wilson et al., 2017). the changes in the soil’s clay mineralogy due to the bedrock differences have also been demonstrated by previous studies (benedetti et al., 2011; heckman & rasmussen, 2011). in a study conducted by benedetti et al. (2011), it was concluded that high acidity and low fe oxide content were dominant in the kaolinite-bearing sediments with low productivity levels, while the presence of high base saturation and 2: 1 type clay minerals were identified in areas with basalt and alluvial effects. şenol et al. (2020) stated clay accumulation in the low slope position lands due to topography, and weak vegetation and sand content in soils was located on volcanic rocks such as tuff and andesite. micro-elements and macro-elements were also detected at low levels in soils formed on the same tuff bedrock. fiantis et al. (2010) in the study on the determination of trace element contents and geochemical fragmentation weathering indices in volcanic ash deposits in indonesia; it has been reported that k, ca, and na originate from feldspar, pyroxene and hornblende, and sulfate anions formed as a result of high s content in volcanic ash are held to the a and b horizons. they explained that olivine, hornblende, and augite in volcanic ash caused the increase in ni, zn, v, and cu contents in the profile. in a study in which they tried to determine the effects of organic matter on soil and nutrient content in soils with varying degrees of fragmentation and decomposition. hauser & billing (2017) discovered that in profiles with high chemical alteration index (cia) and high decomposition degree, organic matter decomposes faster than other profiles. previous studies clearly indicate that in addition to the bedrock, climatic conditions are an important factor in weathering since climate determines the weathering product of volcanic materials and therefore the type of noncrystalline minerals that will be formed (arıkan et al., 2007; gündoğan et al., 2009; gündoğan et al., 2012; özpınar, 2008; kibici et al., 2012; uzun, 2013; şenol et al, 2014; sönmez &kuşcu, 2020; şenol er al., 2020). however, there are not enough studies on rocks of volcanic origin in turkey and in the study area. in addition, revealing the difference in volcanic rocks under the same climatic conditions separates the study from previous studies. the present study aimed to compare young soils developed on different type of rocks by: i-) the determination of essential geo-physico-chemical characteristics and a morphological description; ii-) the properties of the primary and secondary minerals and iii-) the changes in fragmentation-weathering indices of the relationship between bedrock and soils formed on different volcanic rock types and defined as young soils, under the influence of the same land use-land cover and climatic conditions within the borders of sandıklı district of afyonkarahisar province. figure 1. study area; (a) geographical location, (b) elevation map 293physico-chemical and mineralogical changes of lithic xerorthent soils on volcanic rocks under semi-arid ecological conditions 2. material and method 2.1 general features of the field of study the study area covers approximately 295 km2 between sandıklı, kızılören, and şuhut districts within the provincial borders of afyonkarahisar in central anatolia of turkey. it is among the map sheets of k24-c3, k24-c4, l24-a2, l24-b1, l24-b2, l24a3, and l24-b4 scaled as 1: 25.000 and 251000-286000e-4232000-4272000n coordinates (wgs84, zone 36, utm m) (figure 1). sandıklı district and its surroundings have a minimum elevation of 1064 m, a maximum of 2121, and an average of 1424 m. the elevation increases in the east and northeast parts of the field of study. depending on the rise in altitude, topographic features change as hilly and mountainous positions, and the slope increases. according to corine (2018), the land use types of the study area are 35.21% agricultural land (10406.93 ha), 64.10% forest and semi-natural areas (18942.27 ha), and 0.69% artificial areas. wetlands and water bodies cover 5.49% of the area. the area considered for the three soil profiles (pi, pii, and piii) formed on different volcanic bedrocks has been adopted as pastureland for many years (more than 50 years). according to the old american soil classification of the study area (baldwin et al., 1938), 42% is “non-calcareous brown soils,” 16% “chestnut soils,” 7% “colluvial soils,” 5% “brown soils,” 29% is “bare rocks and rubble” and the remaining part is “alluvial soils”. according to the study area’s long-term climate data (afyonkarahisar-şuhut meteorology station observation data), the annual average temperature is 10.8 oc, precipitation 362.40 mm, and evaporation 663.08 mm. the highest rainfall in the region is in december (43.9 mm) and the least in september (11.7 mm). besides, newhall simulation model was applied to determine the soil moisture and temperature regimes of the area (newhall & berdanier, 1994; van wambeke, 2000). according to the result, the soil temperature regime was determined as “mesic” and the soil moisture regime as “xeric” (dry xeric” as the subgroup) (figure 2). as a geological pattern, there are metamorphic, sedimentary, and volcanic units in around the sandıklı district, including the study area. metamorphics are composed of schist and quartzites, sedimentary units, dolomitic limestone, limestone, pebblestone, sandstone, siltstone, claystone, clayey limestone, neritic limestone, alluvial fan deposits, and alluviums. volcanic units cover volcanoclastic rocks (tuff, agglomerate) and volcanic lavas (trachyte, trachyandesite, trachybasalt, basaltic trachyandesite). 2.2 soil and rock sampling within the study area, the geological units where the profile points are located were determined from the geology map (mta, 2011), and ground controls and validation were made. three representative profiles were excavated in the determined areas. in the geology map of the study area, it was determined that a large part of it was formed by volcanic rocks. considering this situation, the fact that most of the profile points determined in the study area are volcanic rocks has increased the representativeness of the study area. each profile was made morphological definitions, and profile description was made according to soil survey staff (1993), and genetic soil horizons were symbolized according to soil survey staff (1999). in addition, rock samples were taken from r horizons that show the bedrocks of the profiles (figure 3). soil samples were taken disturbed and undisturbed based on the horizon (soil survey staff., 2014). soil and rock samples were brought to the laboratory. 2.3 soil analysis descriptive physical and chemical analyses in soils were made on 2 mm and 0.5 mm sieved soil samples. particle size distribution was determined by the hydrometer method according to bouyoucus, (1951), and soil color in dry and moist conditions using the munsell color scale according to soil survey staff, (1993). soil ph and ec and salt concentration in saturation mud according to richards, (2012), lime contents with scheibler calcimeter, the volumetric method according to kacar, (2009), and organic matter with modified walkley-black method according to jackson, (1958). exchangeable cations (mg, ca, na, and k), free fe2o3, and free al2o3 were determined according to perkin elmer, (1973). the number of available micronutrients (fe, cu, zn, and mn) was determined by the dtpa method proposed by lindsay & norvell, (1978) (kacar, 2009). for a total analysis, dried ground soil and rock samples (ф <38 µm) were extracted according to chao & sanzolone, (1992) and made ready for analysis. after the extraction process of major elements (si, al, ca, mg, na, k, ti, and p) and trace elements (fe, cu, zn, and mn) are determined in the icp-oes device. the decomposition weathering indices of the soils were determined according to the formulas given below. • chemical alteration index (cia)= (100) [al2o3/ (al2o3+cao+na2o+k2o)] (nesbitt and young, 1982) • chemical weathering index (ciw) = (100) [al2o3/(al2o3+cao+na2o)] (harnois, 1988) • plagioclase alteration index (pia) = (100) (al2o3k2o)/ (al2o3+cao+na2o+k2o) (fedo et al., 1995) • productivity index (p) = (100) [sio2/(tio2+ fe2o3+al2o3+sio2) (reiche, 1950) • bases/r2o3 = (mgo+ cao+na2o+k2o)/(tio2+fe2o3+al2o3) (birkeland, 1999) figure 2. soil moisture budget distribution graph of the study area according to the newhall model 294 sinan demir, pelin alaboz, orhan dengiz, hüseyin şenol, kamil yilmaz, oğuz başkan figure 3. selected profiles and their main rocks (r1: volcanic tuff, r2: trachybasalt, r3: trachyandesite table 1. morphological properties and classification (soil taxonomy (soil survey staff, 2014)-fao/isric (2014) of profiles. horizon depth (cm) color (dry) color (moisture) structure boundary special features pi 260523 e-4242133 n (36 zone utm m), pasture, (lithic xerorthent / lithic leptosol) a 0-14 10yr 4/3 10yr 3/2 1-f-gr aw lithic contact r 14+ volkanic tuff pii 265145 e4254110 n (36 zone utm m), pasture, (lithic xerorthent / mollic leptosol) a 0-25 10yr 3/1 10yr 4/1 3-m-gr and sbk cw cracks common roots lithic contact r 25+ trachybasalt piii 268093 e4260155n (36 zone utm m), pasture, (lithic xerorthent / lithic leptosol) a 0-33 10yr 4/2 10yr 3/3 2-m-gr gd common roots lithic contact r 33+ trachy andesite abbreviations: boundary: a = abrupt; c = clear; g = gradual; d = diffuse; s = smooth; w = wavy; i = irregular. structure: 1 = weak; 2 = moderate; 3= strong; sg = single grain; mas = massive; vf = very fine; f = fine; m =medium; c = coarse; gr = granular; pr = prismatic; abk = angular blocky; sbk = subangular blocky. 2.4 determination of primary minerals the proportional distribution of primary minerals in soils was determined in samples ground with a bruker d8 advance x-ray diffractometer (in the range of 2θ=2-70o) (jackson, 1958). the abundance degrees of the primary minerals were revealed in the shootings. washing, decantation, lime removal, centrifugation, and sedimentation were carried out to determine the clay mineral type in the ground samples. the resulting clay fractions were separately saturated with 1n mgcl, and kcl washed and plated. purified clay samples were placed on a slide and oriented with ultrasonic vibrations for about 15 minutes. referred examples separately; xrd analyzes were performed on each preparation prepared by 1) air drying, 2) treatment with ethylene glycol for 2 hours in an oven at 60 °c, and 3) conditioning at 550 °c for 2 hours. shootings were performed on the xrd device (in the range of 2θ=2-15o) (whittig & allardice, 1986). thin sections of rock samples were made, and photographs of the rocks’ minerals and textures were taken under the microscope (murphy et al., 1977). 3 results and discussion 3.1 morphological and physico-chemical properties of soils morphological definitions of profiles selected in the study area are given in table 1. analysis results of physico-chemical properties are also shown in table 2. profiles were classified according to the soil taxonomy, considering surface (epipedon) and subsurface diagnostic horizons. it was determined morphological, mineralogical, and physico-chemical differences in the three soils formed on the volcanic bedrocks under the same climatic conditions. these soils are young soils. soils have wide ranges of slope (6-12%) and have an a/r horizon sequence, and in the profiles do not contain a subsurface diagnostic horizon and advance pedological process. these three representative profiles were classified as lithic xerothent (soil survey staff, 2014). on the other hand, soils formed on tuff (pi) and trachyandesite (piii) is classified as lithic leptosol, whereas soil formed on trachybasalt (pii) was classified as mollic leptosol according to fao / isric, (2014). due to the high levels of clay and organic matter content, the soil structure formed on trachybasalt exhibits a strong, medium granular, and semi-angular block structure. simultaneously, in other surface horizons of profiles, it has been determined as a small, weak granular structure. the darkest color in the profiles’ surface horizons was determined with 10 yr 3/1 in the soil formed on trachybasalt. soils, according to dhanke &whitney (1988), are slightly acid-slightly alkaline reactions and are in the salt-free class according to their ec content. while the lowest ph value and salt content were found in lithic xerorthent shown with the piii (profile 3) profile formed on the trachyandesite bedrock, the highest values of them were determined in the soils located on trachybasalt bedrock, which was coded as pi. the high level of basic cations in pi (profile 1) and pii (profile 2) is considered a reason for increased ph levels. the dominant cation in all three soils is calcium ion. 295physico-chemical and mineralogical changes of lithic xerorthent soils on volcanic rocks under semi-arid ecological conditions however, while it is mostly found in the soil formed on tuff (39.87 cmol kg-1), the lowest (5.39 cmol kg-1) was determined in the soil located on trachyandesite. in addition, it has been concluded that zeolite, feldspar, and albite feldispare dominate the primary mineral definitions in the soils formed on the tuff—one of the reasons for the high levels of cations in the presence of these minerals. primary minerals are included calcite, sanidine, microcline, quartz, muscovite, albite, gismondin, phillipsite, orthoclase, ankerit, chabasite, and anorthoclase. in this profile, calcite mineral is abundant. accordingly, high levels of ca cation are expected. the abundance of calcite minerals supports that the calcium (mg kg-1) amount for the pi profile is higher than that of the pii and piii profiles (table 2). besides, smectite minerals were determined in pi and pii. the fact that precipitation is not enough to cause basic ions to leach in the soil profile significantly affects the soil’s clay mineral type. smectite is formed when the cations are not leached from the soil at a sufficient level (uzun, 2013). the soils’ calcium carbonate content is very variable. while it is 12.43% in soils on tuff, it is relatively low in other soils. in fact, soils formed on volcanic materials, especially tuff, basalt, or trachyandesite, do not contain calcium carbonate. tunçay & dengiz, (2020) explained that high calcium carbonate content in the profiles they work with could be caused by the i-) contamination of volcanic materials by calcareous materials during lava flows, ii-) that calcareous materials may be transported by winds and contaminated volcanic materials, or iii-) recrystallization of hydrothermal waters rich in calcium carbonate within porous lava and basalt materials. similar to our findings, şenol et al., (2018) determined the lime content between 0.09% and 6.37% in their studies on geochemical mass index and clay mineral formation of soils formed on the primary basalt material and in different topographic positions under semi-humid climatic conditions. as the constituent classes of the working soils, the trachyandesite material’s soils are more coarse and sandy loam. the texture of the bedrock and the land’s topography are the parameters that affect the change of soil properties (osher & buol 1998). krasilnikov et al., (2005) reported that texture and mineralogical structure are factors in the relationship between geomorphology and formation in soils. it is stated in the literature that soils developed on tuff bedrock have high lime content. this situation shows the transport and accumulation of different rocks due to tillage and topography. table 2. some physical and chemical analysis results of profiles pi (tuff) pii(trachybasalt) piii (trachyandesite) ph 7.76 7.78 6.93 ec (ds/m) 0.14 0.15 0.08 clay (%) 22.23 26.27 15.93 silt (%) 35.15 25.93 18.36 sand (%) 42.62 47.80 65.71 caco3 (%) 12.43 1.23 0.99 na (cmol(+) kg-1) 0.42 0.12 0.06 k (cmol(+) kg-1) 13.00 0.63 0.45 ca (cmol(+) kg -1) 39.87 37.43 5.39 mg (cmol(+) kg-1) 1.55 4.39 1.13 cu (mg kg-1) 2.22 1.59 1.09 mn (mg kg-1) 16.82 38.20 28.58 fe (mg kg-1) 6.75 23.39 11.09 zn (mg kg-1) 0.65 0.98 0.76 p (mg kg-1) 14.80 6.72 10.60 om (%) 2.37 2.80 1.72 it results from the precipitation of limestone dissolved by precipitation on the soils formed on the tuff. although all three soils are covered with pasture vegetation, when the soils’ organic matter contents were examined, the soils on trachybasalt were higher. basic cations were detected at high levels in soils over tuff and trachybasalt, respectively. the pi profile contains anorthoclase mineral higher than, unlike pii and piii. this mineral is known as a mineral group rich in cations. besides, due to minerals such as pyroxene and olivine in these soils, the microelement and especially iron content were determined to be higher than in other soils (pii and piii). the high iron content in the pi profile is associated with the low amount of aluminium oxide which is the chemical composition al2o3. since the amount of iron oxide forms more stable compounds with the increase in aluminium oxide, it supports the lower iron content in the pii and piii profiles than the pi profile. 3.2 mineralogical and petrographic investigations of rocks thin section images of the bedrock on which the soils are formed are given in figure 4. the pi profile’s bedrock is tuff; a highly hollow, soft, and brown-colored pyroclastic rock with volcanic rock and mineral fragments in an ash matrix. rock fragments are generally composed of volcanic rocks such as trachyte, trachyandesite, and basaltic trachyandesite. in contrast, minerals are represented of crystals such as sanidine, plagioclase, pyroxene, and mica. sanidines are subhedral and sometimes euhedral and have a clean appearance. likewise, kibici et al., (2012) mentioned the presence of very fine-grained sanidine crystals in their study to reveal the mineralogical and petrographic characteristics of the volcanic rocks of the afyonkarahisar region. the bedrock of lithic xerorthent soils, shown as pii, is trachybasalt, a gas-filled, grayish, fine-grained, and aphanitic textured rock. the rock is composed of silica (quartz and/or tridymite), sparse mica, and apatite minerals in gas cavities, together with pyroxene phenocrysts in a paste consisting of sanidine, pyroxene, and opaque mineral microlites. pyroxenes, which are found as phenocrysts, appear in two forms: (1) thin-long rod-like crystals and (2) short prisms. they are generally euhedral and subhedral (figures 4c and d). approximately 85-90% of the rock consists of microlithic paste, and the remaining part is composed of minerals found as phenocrysts. 296 sinan demir, pelin alaboz, orhan dengiz, hüseyin şenol, kamil yilmaz, oğuz başkan figure 4. a, b: microscopic images of tuff, which is a pyroclastic rock. ash is composed of volcanic rock fragments in a matrix and crystals such as sanidine and mica. a) double polos (plane-polarized light) b) single pools (single polarized light); c, d; pyroxene phenocrysts forming the trachybasalt rock are contained in a microlithic groundmass. c) double pounds, d) single pounds. prx: pyroxene; e, f; microscopic image of plagioclase, amphibole, mica phlogopite or biotite and sanidine crystals in a matrix of sanidine, plagioclase and mica microliths in combination with volcanic glass in trachyandesite rock. e) double pounds, f) single pounds. lithic xerorthent, coded as piii, is a porphyric textured volcanic trachyandesite rock with marked sanidine and plagioclase crystals, the primary material of the soil was affected by severe alteration. the rock comprises plagioclase, sanidine, pyroxene, mica (phlogopite or biotite), and amphibole main minerals (figure 4d, f). apatite and opaque minerals are available as accessories. some researchers have stated that the minerals forming the rock appear in a matrix consisting of sanidine, plagioclase, and mica microlites together with volcanic glass (kurt, 2009; kibici et al., 2012). plagioclases constitute 15-20% of the main minerals, 10-15% of sanidine, 10-15% of pyroxene, 10-12% of mica, and 8-10% of amphibole. apatite has 1-2%. plagioclases are seen as polysynthetic twinnings, usually clean and clear. they are euhedral and subhedral and some have zoned extinction. they are broad plates and short prism shaped. sanidine crystals are rectangular, and some of them have rounded corners by being eroded by magma fluids. they are euhedral, subhedral, carlsbad twinning, and slightly clayed due to a low alteration degree. pyroxenes are short blunt and long rod-like, euhedral and subhedral. they come in octagonal, hexagonal, and sometimes rectangular shapes. they show simple twinning, are gathered together with other minerals, and form glomeroporphyric texture. micas are generally opaque by oxidation. they are found in short and sometimes long rectangular plate-like shapes. amphibole crystals, like micas, are opaque, especially by oxidizing at the edges. opacification has surrounded the edges of the mineral as a belt or completely encircled it. it is hexagonal in euhedral and subhedral and has bidirectional cleavage. it shows a greenish and brown pleochroism in crossed nicols and a brown pleochroism in parallel nicols. the crystals were separated from the edges and middle sections by being corroded by the magma liquid. rare apatites are in short blunt and pithed shapes and become clay. the rock has a porphyritic and glomeroporphyric texture. 3.3 primary and clay minerals of soils x-ray peaks for primary minerals in the soil formed on tuff bedrock are given in figure 5. determination of primary minerals was carried out in the range of 2-70o (2θ) nontronite, muscovite, sanidine, gismondine, albite, calcite, and phillipsite minerals were determined. calcite, albite, muscovite, gismondine, sanidine, phillipsite, and nontronite were determined in order of densities (table 3). albite is a mineral commonly found in schists, which forms the basis of the study area. like the muscovite mineral albite, it is included in the composition of metamorphic rocks (schists). it can also occur secondary minerals to the transformation of the feldspars due to clay minerals decomposition. also, gismondine [(caal2si2o8 x 4(h2o)] and phillipsite [(ca,na2,k2)3al6si10o32 x 12(h2o)] which are zeolite minerals, were detected in these soils. özpınar, (2008) stated that there are three different forms of phillipsite, potassium-sodium-aluminum-silicate hydrate, potassium-aluminum-silicate hydrate, and potassium-calcium-aluminumsilicate hydrate, in tuffs in the south of sandıklı district. x-ray diffractometers were taken to determine the mineralogical properties of soils (pii) formed on trachybasalt are given in figure 5. determination of primary minerals in the soil sample was carried out in the range of 2-70o (2θ) nontronite, muscovite, anorthite, microcline, sanidine, anorthoclase, and diopside minerals were determined. in order of mineral abundance, they are microcline, diopside, muscovite, sanidine, anorthite, anorthoclase, and nontronite minerals (table 3). anorthoclase is a mineral formed at the high temperatures of volcanic and semi-deep rocks. it is an intermediate member of the alkali feldspar, albite-sanidine solid solution series. anorthite (caal2si2o8) came to the environment by being carried from peridotite rocks. in mineralogical and petrographic investigations, it was stated that sanidine is composed of silica (quartz and/or tridymite), rare mica, and apatite minerals found in gas cavities together with pyroxene phenocrysts in a paste consisting of pyroxene (diopside) and opaque mineral microlites. microcline is found in rocks such as syenite, granite, syenite porphyry, and granite porphyry. sönmez & kuşçu, (2017) stated that k-feldspar syenites were found in the afyonkarahisar-sandıklı region. gündoğan et al. (2009; 2012) mentioned the presence of the karacaören syenitoid east of sandıklı (afyonkarahisar). an x-ray diffractometer was taken to determine the primary mineralogical properties of the piii profile are shown in figure 5. muscovite, anorthoclase, and orthoclase minerals have been identified as primary minerals in the soils. sanidine, anorthoclase, orthoclase, and muscovite were determined in order of abundance (table 3). similarly, in their study to reveal the mineralogical and petrographic properties of the afyonkarahisar region’s volcanic rocks, kibici et al., (2012) mentioned mica’s presence mineral and very small-grained sanidine crystals in a paste of trachyandesite rock. 297physico-chemical and mineralogical changes of lithic xerorthent soils on volcanic rocks under semi-arid ecological conditions x-ray peaks of clay minerals of young soils classified as lithic xerorthent but formed on different volcanic bedrock are given in figure 6. in the soil sample, which is stated as pi and formed on the tuff bedrock, in the samples saturated with mg (mgad), the crystallized peaks were opened up to 18.39 å in the application of mgeg (mg ethylene glycol). in potassium saturation (kad), these peaks are closed to the range of 12.87. peaks indicate the presence of crystallized smectite minerals. the peaks seen in the range of 9.92-10.11 å in all applications belong to crystallized illite. the peaks are seen in the 7.18-7.23 å range of samples saturated with mgad, mgeg, and kad disappeared when heated at 550 oc (k550 oc). this situation indicates that these peaks belong to kaolinite. very weakly crystallized peaks seen in the range of 11.77-12.87 å in all applications showed that clay minerals with illite-chlorite interlayer existed. in the order of abundance, the distribution of clays was in the form of smectite, illite, and kaolinite (table 4). şenol et al. (2014) identified smectite as the dominant clay mineral on south-facing slopes in a study on clay mineralogy table 3. primary minerals and densities of soils formed on different volcanic parent rock profile horizon a b c d e f g h i j k l m n pi a(0-14) +++ ++ ++ + + + +++ pii a(0-25) ++ +++ ++ + + ++ piii a(0-33) ++++ ++ +++ ++++ abundance: ++++ very abundant, +++ much abundant, ++ medium abundant, + low abundant; minerals: a: calcite, b: sanidine, c: microcline, d: quartz, e: muscovite, f: albite, g: gismondin, h: phillipsite, i: orthoclase, j: ankerit, k: chabasite, l: anorthoclase, m: nontronite, n: montmorillonite figure 5. soil profile x-ray diffractions of primary minerals (mi: microcline, di: diopside, mu: muscovite, sa: sanidine, an: anorthite, aorta: anorthoclase, non: nontronite, ca: calcite, al: albite, gi: gismondin, phl: phillipsite, ort: orthoclase) of soils formed on trachyte / trachyandesite. uzun, (2013) stated that the basic cations released due to the amphiboles’ decomposition increase the ph and the si, mg, and ca cannot be washed at a sufficient level, smectite clay mineral is formed. an x-ray diffractometers of clay types in the surface sample of young soil profile formed on trachybasalt and coded as pii are given in figure 6. the 14.66 å crystallized peak in the samples saturated with mg (mgad) in the soil sample was opened up to 19.79 å in the treatment of mgeg. this peak is closed to 12.95 å at saturation with potassium. the peak indicates the presence of crystallized smectite minerals. the peaks seen in the range of 9.86-10.08 å in all treatments belong to the crystallized illite. the samples saturated with mgad, mgeg, and kad peaks seen in the range of 7.16-7.28 å disappeared when heated at 550oc. therefore, these peaks were identified as belonging to kaolinite. in samples saturated with mg in the profile, the weakly crystallized peak (mgeg) was opened to 31.52 å in the treatment. in potassium saturation 298 sinan demir, pelin alaboz, orhan dengiz, hüseyin şenol, kamil yilmaz, oğuz başkan and k550oc treatments, this peak is closed to 29.03 and 21.22 å, respectively. the peak indicates the presence of a weakly crystallized chlorite-vermiculite interbedded clay mineral. the peaks seen in the range of 6.36-6.53 å in all treatments are feldspar minerals. in the order of abundance, the distribution of clays is in the form of clay with smectite, illite, kaolinite, and chloritevermiculite interlayer (table 4). figure 6. x-ray peaks of soils formed on different volcanic parent rock (sm: smectite, k: kaolinite, i: illite, chl-v: chloritevermiculite, f: plagioclase felspars) table 4. clay minerals and abundance of soils formed on various volcanic parent rock profile horizon smectite illite kaolinite abundance crystallinity abundance crystallinity abundance crystallinity pi a (0-14) ++++ ++++ +++ ++++ ++ ++++ pii a (0-25) ++++ ++++ +++ +++ ++ ++ piii a (0-33) ++++ ++++ +++ +++ abundance: ++++ very abundant, +++ much abundant, ++ medium abundant, + low abundant; crystallinity: ++++ very good, +++ good, ++ fair, + bad the peaks seen in the range of 9.79-10.04 å in all soil sample treatments on trachyandesite belong to crystallized illite. the peaks seen in the 7.087.21 å range of samples saturated with mgad, mgeg, and kad disappeared when heated at 550oc (figure 6). it indicates that these peaks belong to weak crystalline kaolinite. the peaks seen in the range of 6.43-6.52 å in all treatments are feldspar minerals. the distribution of clays in the order of abundance was in the form of illite and kaolinite (table 4). the acid or alkaline reaction of soil is directly related to the oh3+ and ohconcentrations in the soil solution. a oneunit increase in ph means a 10-fold increase in the concentration of h ions. the higher ph (h2o) values might be related to lower al activity and lower organic matter content. the ph values (in most cases above 6.0) were relatively high considering the nature of the bedrock and the absence of carbonates. piii profile ph value contains approximately 10-fold lower h+ ion concentration than pi and pii profiles. the ph values (in most cases above 6.0) were relatively high considering the nature of the bedrock and the absence of carbonates. 2:1 type clay formation is significant in the study area profiles. it increased the ph due to the basic cations released as a result of the decomposition of amphiboles. smectite was formed as a result of low precipitation in the study area and insufficient leaching of si, mg, and ca. the very abundance of smectite for pi and pii profiles was found to be appropriate with the results of previous studies (uzun 2013). in the study area, the piii profile containing plagioclase, amphibole, mica phlogopite or biotite, and sanidine is a distinguishing feature from the soils with pi and pii profiles formed on the volcanic rock in terms of the absence of 2:1 type clay. the influence of pedogenesis factors is lower for the piii profile. except for semi-arid ecological conditions, different processes in pedogenesis factors show that soils are effective on clay fractions (özaytekin & karakaplan, 2012). cangir & ekinci (1991) identified the dominant clay mineral as illite in shallow profiles formed on the calcareous base material. the dominant clay mineral in this soil profile is smectite. on the other hand, clay mineral with chlorite-vermiculite interlayer was found in the soils located on trachybasalt. i̇nce, (1979) identified smectite as the dominant mineral in the clay fraction in the soils formed on the basalt rocks in and around diyarbakır, followed by kaolinite, chlorite, and illite. 3.4 weathering index many different indices have been used in defining the decompositionweathering degree of soils. chemical weathering indices are commonly used to characterize weathering profiles, typically calculated from molecular ratios based on the assumption that aluminium is immobile. this assumption is not always warranted. in addition, the molecular ratio of each oxide is easily calculated from the percent of the oxide based on weight (nesbit & young, 1982; harnois, 1988; dengiz et al. 2013). dengiz et al., (2013) stated that weathering indices for weathering profile samples are conventionally estimated using the molecular proportions of major element oxides. in addition, the determined indexes are generally determined to reveal the exchange rates between basic cations and cations such as al and si. the cia index reflects the ratio of primary and secondary minerals as the rate of removal of basic cations from minerals by chemical weathering. in the case of severe weathering or decomposition, the cia value can be determined up to 100 levels. this index, which reflects the alteration degree of feldspars into clays after hydrolytic decomposition, may decrease to 50% in the initial stage of 100% decomposition in clays with intense decomposition (fedo et al., 1995; şenol et al., 2014). nesbit and young (1982) classified the cia values (%) as poorly weathered (50-60), low weathered (6080), highly weathered (80-90), and excessive weathered (90-100). soils with the symbol pi, pii, and piii were classified as lithic xereorthent according to soil taxonomy. some weathering rates of these 299physico-chemical and mineralogical changes of lithic xerorthent soils on volcanic rocks under semi-arid ecological conditions pedons and some major and minor elements obtained from their geochemical properties are shown in table 5. according to the soils’ cia values, the pi soil profile formed on the tuff was determined as “slightly weathered”. in contrast, the pii and piii soil profiles were defined as “unweather” (≤ 50). ciw values increase to 50% in rocks that have not undergone disintegration. this index rises to 100% depending on the degree of growing fragmentation disintegration. since the cia classification is valid within the ciw, the pi was determined as “highly differentiated (89.81%)”. other profiles were found to be free from fragmentation and decomposition according to ciw values. weathering degrees in the primary rocks of the soils are determined at deficient levels. table 5. weathering index and major elements content of soils pi (tuff) pii (trachybasalt) piii (trachyandesite) a r a r a r cia 77.04 42.27 22.88 0.03 31.40 0.21 ciw 89.81 48.90 24.43 0.03 36.88 0.21 pia 62.81 28.72 16.53 0.01 16.56 0.21 product index 83.31 87.26 80.89 99.09 82.71 88.8 base/r2o3 0.19 1.18 2.49 1.09 1.83 0.99 sio2 54.12 57.93 51.97 65.34 57.33 69.76 al2o3 9.77 11.32 13.52 10.44 17.84 9.57 fe2o3 9.54 3.39 6.22 4.68 5.18 3.7 cao 0.54 4.25 7.91 5.57 4.29 3.13 mgo 0.24 0.6 3.42 2.16 2.37 0.79 na2o 0.08 2.49 2.11 3.14 3.7 3.31 k2o 1.66 3.34 3.04 6.82 5.84 6.94 tio2 2.03 0.7 1.16 1.03 1.16 1.01 p2o5 0.08 0.19 0.34 0.53 0.44 0.54 mno 0.26 0.08 0.14 0.09 0.11 0.11 cia: chemical index of alteration, ciw: chemical index of weathering, pia: plagioclase index of alteration plagioclase alteration index (pia) is used to reveal the degree of alteration of plagioclases. again, as in other indices, the highest pi number was determined in the profile. pia index varied between 0.01-62.81%. although such high results in all profiles indicate that the weathering is advanced, these values can be increased on young volcanic rocks (sayyed & hundekari, 2006). if there is an increase in mgo, cao, na2o and k2o elements depending on the depth in profiles, cia, ciw, and pia indices tend to increase with depth, al2o3 values decrease as a result of clay movement (alsalam et al., 2020). cia, ciw and pi parameters were determined in the lowest pii (trachybasalt) and the highest in the pi profile. a similar situation is valid for the product index (p). with this ratio, the values should tend to decrease due to the increase of decomposition and weathering. the p index was determined between 80.8999.09%. it is reported that such high values may occur on young rocks (sayyed & hundekari, 2006). the base/r2o3 ratio varies between 0-10 in soils (şenol et al., 2014). it gives high values in the bedrock. in the pi profile, the index value decreases with depth. the increase in index with depth between the a horizon and the bedrock indicates that the pi profile is at an advanced level of disintegration. the fact that the bases/r2o3 ratio, which gives results between 1-10 under normal conditions, is below low in the profiles (except the a horizon of the pi profile) is a result of the basic cations not being leached sufficiently and the sio2 molar ratios at high levels due to the transport from the bedrocks. sio2 values of the soils were determined between 51.97-69.76%. the amount of al2o3 in soil profiles showed results close to the primary material values to support less intense decomposition. alkali cation values in profiles differ from the bedrock. while the increase of alkali cations in the surface horizon indicates the vegetative cycle, the low level indicates a little washing in the profile. cao, mgo, na2o, k2o, and p2o levels were determined in the lowest pi (tuff) profile. mgo values ranged from 0.24-3.42%. mgo is very sensitive to leaching process (alsalam et al., 2020; arıkan et al., 2007; nordt & driese 2010; regassa et al., 2014). total tio2 in profiles showed a distribution between 0.70-2.03%. the % amounts of tio2 which is resistant to decomposition, reveal the decomposition level of the soil. p2o5 of soils showed distribution between 0.08-0.54% and mno between 0.08-0.26%. 4 conclusion in this present study, the pedological developments, taxonomic classifications, primary minerals, and clay mineralogy and their weathering indices of young soils formed under the same climate and land cover but on different volcanic rocks in the area between sandıklı-şuhut districts of afyonkarahisar province with semi-arid ecological conditions were studied. their pedological developments were examined and compared. therefore, three soil profiles formed on the bedrock of tuff, trachybasalt, and trachyandesite from volcanic rocks in the study area were analyzed. all profiles have a medium slope, under pasture cover, in a/r horizon order. it is classified as lithic xerorthent due to the absence of subsurface diagnostic genetic horizon and lithic contact within 50 cm depth. the existence of sanidine and muscovite primary minerals for all profiles is in question. differences in primary minerals have been discovered depending on the difference in volcanic units. smectite, illite, and kaolinite clay minerals were found in soils formed on the tuff (pi) and trachybasalt (pii) primary materials. illite and kaolinite clay minerals were discovered in trachyandesites (piii). in the surface soils formed on tuff, fragmentation and weathering were determined at higher levels than other soil profiles (pii and piii). in addition, significant changes in some physicochemical properties of soils were determined depending on the main bedrock differences. zeolite and calcite minerals are found at high levels in the soils formed on the tuff bedrock where basic cations are high. with this current study, it has been revealed that under the same climate, land cover, and slope conditions, there are significant changes in the disintegration-weathering processes and the soils’ mineralogical and physicochemical properties of volcanic bedrock. it was concluded that changes in the degree of disintegration under the same climate, land cover, and slope conditions can be successfully quantified through cia, ciw, pia, p, bases/ r2o3, mass transport function, and mass loss/gain, and clay mineralogy can be used to determine the degree of disintegration at short distances. consequently, these soils are still under the bedrock’s active influence at the beginning of the formation process. besides, it is suggested that the physico-chemical properties and the mineralogical properties should be determined to make the necessary studies such as soil survey mapping to be 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(2017). soil genesis and mineralogy across a volcanic lithosequence. geoderma, 285, 301-312. https://doi.org/10.1016/j.geoderma.2016.09.013 https://doi.org/10.1016/j.oregeorev.2020.103506 https://doi.org/10.1016/j.oregeorev.2020.103506 https://doi.org/10.7161/omuanajas.753302 http://nopr.niscair.res.in/handle/123456789/44911 http://nopr.niscair.res.in/handle/123456789/44911 https://doi.org/10.1080/00380768.2020.1769453 http://nopr.niscair.res.in/handle/123456789/54649 http://nopr.niscair.res.in/handle/123456789/54649 http://nopr.niscair.res.in/handle/123456789/54649 http://nopr.niscair.res.in/handle/123456789/54649 https://doi.org/10.2136/sssabookser5.1.2ed.c12 https://doi.org/10.1016/j.geoderma.2016.09.013 _hlk120105566 _hlk120178659 _goback keywords: multiple linear regression; the xisha islands; water area; landforms; heterogeneity; fractal characteristics. palabras clave: regresión lineal múltiple; las islas xisha; área de agua; accidentes geográficos; heterogeneidad; características fractales. how to cite item wang, x. (2021). fractal characteristics of geomorphological heterogeneity in xisha islands waters under multiple linear regression. earth sciences research journal, 25(1), 41-48. doi: https://doi.org/10.15446/esrj. v25n1.94076 based on the obvious anisotropy of xisha islands waters topography, the uneven fractal characteristics of xisha islands waters topography are studied by using multivariate linear expression. using multiple linear regression analysis method to extract the projection of heterogeneous characteristic factors, the geometric heterogeneous characteristics of xisha islands waters are obtained. the fractal feature of landform is studied based on projection coverage method, and the non-isotropic body irregularity in xisha islands waters is measured. experimental results show that this method can effectively analyze the non-uniform fractal characteristics of the xisha islands waters. the fractal dimension of different types of landforms in xisha islands in the corresponding region is high mountain area > medium and low mountain area > basin area, which reflects the difference of surface roughness or complexity of different types of landforms. the calculated fractal dimension is helpful to reveal the heterogeneity of general geomorphological features in different development stages. abstract fractal characteristics of geomorphological heterogeneity in xisha islands waters under multiple linear regression características fractales de la heterogeneidad geomorfológica en las aguas de las islas xisha bajo regresión lineal múltiple issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n1.94076 con base en la anisotropía obvia de la topografía de las aguas de las islas xisha, se estudian las características fractales desiguales de la topografía de las aguas de las islas xisha mediante el uso de una expresión lineal multivariante. utilizando el método de análisis de regresión lineal múltiple para extraer la proyección de factores característicos heterogéneos, se obtienen las características geométricas heterogéneas de las aguas de las islas xisha. la característica fractal de la forma del terreno se estudia con base en el método de cobertura de proyección y se mide la irregularidad del cuerpo no isotrópico en las aguas de las islas xisha. los resultados experimentales muestran que este método puede analizar eficazmente las características fractales no uniformes de las aguas de las islas xisha. la dimensión fractal de diferentes tipos de accidentes geográficos en las islas xisha en la región correspondiente es: zona de alta montaña > zona de media y baja montaña > zona de la cuenca, que refleja la diferencia de rugosidad o complejidad de la superficie de diferentes tipos de accidentes geográficos. la dimensión fractal calculada es útil para revelar la heterogeneidad de las características geomorfológicas generales en diferentes etapas de desarrollo. resumen record manuscript received: 23/06/2020 accepted for publication: 21/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 1 (march, 2021): 41-48 xuli wang* department of management engineering, inner mongolia vocational college of chemical engineering, hohhot 010070, china *email of corresponding author: wangxuli1981@163.com g eo m o rp h o lo g y https://doi.org/10.15446/esrj.v25n1.94076 https://doi.org/10.15446/esrj.v25n1.94076 https://doi.org/10.15446/esrj.v25n1.94076 mailto:wangxuli1981@163.com 42 xuli wang introduction for human beings, the most common phenomena in nature, such as the varied landforms and the turbulence in the river, are unpredictable and lack rules. since pangu’s founding, human beings have been tirelessly exploring this magical and wonderful nature (bagley et al., 2017). in the process of understanding nature and transforming nature, human beings are uncovering their veil layer by layer to explore their true faces (cai et al., 2016). it should be said that physicists spend a lot of energy in analyzing the universe and basic particles. with the creation of newton’s classical mechanics, einstein’s theory of relativity, and the development of quantum mechanics, human beings have made brilliant achievements in natural science (liu & jiang, 2020; arangurenriano et al., 2018). natural numbers, triangles, rectangles, squares, circles and so on, as practical techniques or tools of mathematics, are gradually created in the development of human social practice, and form arithmetic, algebra, geometry, trigonometry and calculus. mathematics gradually matures from simple to complex (yuan et al., 2016). in mathematics, geometry is the first subject to rise from empirical knowledge to theoretical knowledge, and its typical representative is euclid’s “geometric original” (padisak et al., 2016). it is the first work in the history of mathematics to systematically discuss geometry by using axioms and methods. since then, for quite a long time, geometry, as the science of describing the form of things, has been regarded as the only real mathematics and the model of scientific rigor (gu et al., 2018; hu et al., 2018; kong et al., 2018; zhang, 2020). euclidean geometry is a subject about intuitionistic spatial shape relations. its studies ordered, smooth and feature length geometry (gao et al., 2016). no matter euclidean geometry, or cartesian geometry analytic geometry and differential geometry developed later, the lines, curves and surfaces studied by them all have a common assumption that these lines and surfaces are smooth, meet the differentiability, and are abstract. however, the geometry of nature is complex. the points, lines and surfaces of nature are not all in the geometry category of mathematicians (riordan et al., 2016). nature is more irregular, disordered and discontinuous. as long as people pay a little attention to a large number of non-linear irreversible phenomena in the surrounding environment, they will find that people know little about these phenomena and have no idea about many problems. take the phenomenon that we are used to in the sky as an example. when you look up at the sky, you can often see a group of clouds floating in it, which is poetic (ozguc, 2018). but if you look at the clouds with different telescopes, you will find that the shape of the clouds seems to have nothing to do with the magnification of the telescope. no matter how large the magnification, its shape is always the same (li et al., 2017). for non-linear science, classical mechanics, quantum mechanics and relativity are useless. there must be new theories to study these scientific problems that all human wisdom can’t solve. in the mid-s, mandelbrot, a researcher in the physics department of the corporate research center of the united states and a professor in the mathematics department of harvard university, published an article on “how long is the coastline of britain” in science magazine of the united states. in the paper, he stressed that the length of the uk coastline is uncertain and depends on the scale used in the survey (skorek, 2018; wu, 2018). the smaller the scale used to measure the length of coastline, the longer the corresponding length of coastline. therefore, the traditional length scale is not a good measure of coastline, and a new parameter or measure must be found to better characterize the coastline. in the in-depth study of this problem, he brought out the concept of fractal. how long is the british coastline? it is considered to be the first epoch-making paper on fractal (chen et al., 2016). the term “fractal” was first proposed by mandelbrot. its original meaning is “irregular, fractional and fragmented” object. at present, fractal is a frontier topic in nonlinear science. in different literatures, fractal is given different names, such as “hausdorff measure set”, “irregular set” and “set with fine structure”, etc. generally, fractal can be regarded as the state of large and small fragments gathering. due to its rapid development in many sciences, fractal has become a discipline to describe the regularity of many irregular things in nature (luo, 2016). based on the above background, this paper proposed the research method for heterogeneous fractal characteristics of landforms in the water of the xisha islandas based on multiple linear expression, and analyzes the heterogeneous fractal characteristics of landforms in the water of the xisha islands (gao et al. 2017). materials and methods multiple linear regression analysis of projection pursuit based on extracting the factors of heterogeneous characteristics the projection pursuit multiple linear regression analysis method based on the extraction of heterogeneous characteristics factors. firstly, according to the common characteristics of landforms in the water of the xisha islands, this method divides the affiliation of independent variables to the heterogeneous characteristics (lyu et al., 2018). then, the projection pursuit technology is used to classify the independent variables into the characteristic factors. finally, the heterogeneous characteristics factor is used as a new explanatory variable to complete the multiple linear regression analysis of landforms in the water of the xisha islands. the advantage of this method is to solve the multicollinearity problem in the case of small samples. at the same time, by extracting the factors of the independent variable, the independent variable screening is more consistent with the research significance of the problem itself, so as to distinguish the traditional independent variable screening method (liang & pei, 2016). the affiliation division of independent characteristics when modeling specific problems with multiple regression analysis, it often involves dozens or hundreds of independent variables because of the complexity and specialization of the problems. traditional methods, such as correlation coefficient analysis and stepwise regression, can achieve a good fitting effect if they are only selected from the perspective of data analysis, but the regression model is often lacking persuasion and research significance. aiming at the multi-level phenomenon that landforms in the water of the xisha islands show common characteristics when there are too many independent variables or they belong to a specific subject according to their professional meanings, the independent variables of landforms in the water of the xisha islands are classified as the heterogeneous characteristics factors according to their affiliation, which are the new explanatory variables of multiple regression analysis (schuur et al., 2016). projection pursuit the main feature of projection pursuit is to solve the problem of “dimension disaster” caused by the sparse high-dimensional spatial data of landforms in the water of the xisha islands. by projecting the high-dimensional data of landforms in the water of the xisha islands into the 1-3-dimensional subspace, it can maximize (or minimize) the projection index and find the projection that can reflect the original high-dimensional data structure or characteristics, so that the projection data is analyzed later (abuzied & alrefaee, 2017). the steps of dimensionality reduction for the high-dimensional data of landforms in the water of the xisha islands are as follows. step1: data normalization of landforms in the water of the xisha islands. because there are variables in the water of the xisha islands sample data, there are great differences in the dimensions and orders of magnitude between variables, which makes the variable indicators lack consistency measurement. in order to facilitate the analysis, and ensure the equivalence and order of landforms in the water of the xisha islands data, it is necessary to normalize the original data of landforms in the water of the xisha islands. the normalization method of landforms in the water of the xisha islands is as follows. as larger as better, the indicators are: x i j x i j x j x j x j ( , ) *( , ) ( ) ( ) ( ) min max min = − − (1) as smaller as better, the indicators are: 43fractal characteristics of geomorphological heterogeneity in xisha islands waters under multiple linear regression x i j x j x i j x j x j ( , ) ( ) *( , ) ( ) ( ) max max min = − − (2) where, xmax( j) and xmin( j) are maximum and minimum value of the jth indicator, x(i, j) is a sequence of normalized values of index eigenvalue. step 2: construct projection index function q(a). pp method is to aggregate the p dimensional data {x ( i, j); j =1, 2,...p} of landforms in the water of the xisha islands into {a(1), a(2), a(3),..., a(p)} as the one-dimensional projection value z(i) of the projection direction: z i a j x i j i n j n ( ) ( ) ( , )( , ,..., )= = = ∑ 1 2 1 (3) step 3: construct projection index function. in order to find the heterogeneity characteristics of landforms in the water of the xisha islands data in the high-dimensional data of landforms in the water of the xisha islands, when synthesizing the projection index values, it is required that the distribution characteristics of the projection values should meet the requirements that the local projection points should be as dense as possible, and it is better to agglomerate into several point clusters, while the whole projection point clusters should be as scattered as possible (wilson et al., 2016). therefore, the projection index function can be expressed as: q a s dz z( )= (4) where, sz is the standard deviation of the projection value; dz is the local density of the projection value z(i). s z e z nz i i n = −( ) −( )           = ∑ ( ) / . 2 1 1 0 5 (5) d r r i j u r r i jz i n j n = −( ) −( ) = = ∑ ∑ 1 1 ( , ) ( , ) (6) where, e (z) is the average value of the sequence z(i); r is the window radius of the local density, which can be taken as 0.1; r i j z z ui j( , )= − ( ) represents a unit order function, if r − r (i , j) ≥ 0 its value is 1, then if r − r (i , j) < 0 its value is 0. step 4: optimize the projection index function to determine the best projection direction. different projection directions reflect different characteristic structures of landforms in the water of the xisha islands. the best projection direction is to reflect the projection directions of some kind of characteristic structures of landforms in the water of the xisha islands as much as possible. therefore, the best projection direction can be estimated by solving the problem of maximizing projection index function, namely: maxq a s dz z( )= (7) s t a j j p . . 2 1 1( )= − ∑ (8) this is a complex nonlinear optimization problem with optimization variables {a( j); j =1,2,..., p}, which is difficult to deal with by traditional optimization methods. therefore, in this paper, real coded accelerated genetic algorithm (raga) is applied to solve the problem of high-dimensional global optimization of its heterogeneous characteristics of landforms in the water of the xisha islands. raga the standard genetic algorithm (sga) code is binary, its process is tedious, and the accuracy is limited by the length of string, the calculation is large, and premature convergence may occur, which is not convenient to develop genetic operators for special problems. to solve this problem, this paper uses an improved accelerated genetic algorithm based on real coding, which greatly enhances the optimization performance of the algorithm (otoperalias, 2018). the steps of raga modeling are as follows: (1) the following linear transformation is used to optimize the real number coding of landforms variables in the water of the xisha islands: x j a j y j a j j p( )= ( )+ ( ) ( )( ) =( )1 2, ,..., (9) where f is the objective function of the optimized landforms in the water of the xisha islands, p is the number of optimized variables. equation (9) corresponds the j-th variables to be optimized x ( j) aaa and a ( j) of the landforms in the water of the xisha islandson on the initial variable interval [0, 1] to the real number y ( j) on the interval [0, 1]. y ( j) is the genetic gene of raga. (2) the initialization of the parents of landforms in the water of the xisha islands. suppose that the size of the parents of landforms in the water of the xisha islands is n, then n groups uniform random number on the interval of [0,1] are generated. there are p in each group, namely u i j j p i n, ; , ,..., ; , ,...,( ) = ={ }1 2 1 2 . the individual y (i , j) is substituted into equation (9) to obtain the optimal variable value x (i , j), and then the corresponding objective function value f i( ) is obtained through equation (7). according to the order from small to large, f i i n( ) ={ }; , ,...,1 2 corresponding individuals y (j, i) are sorted. the smaller the objective function value of landforms in the water of the xisha islands is, the stronger the individual’s adaptability is. after sorting, the first individual is the excellent individual, so that it can enter the next generation directly. (3) the fitness evaluation of the parents in the water of the xisha islands. the evaluation function based on the order is to distribute the regeneration according to the order of chromosome, which is not suitable for the actual target value. set parameter α ∈( )0 1, , and the evaluation function based on order is: eva y j i a a i ni( , ) , ,...,( )= −( ) =( )−1 1 21 (10) here i = 1 means the chromosome is the best, i = n means the worst. (4) the first generation y ( j, i ) of landforms in the water of the xisha islands was generated by selection. the selection process is based on rotating the wheel n times. each rotation selects a chromosome for the new population. the selection process is described as follows: the cumulative probability qi ( i = 1,2,...,n ) of each chromosome y ( j , i ) is: q q eval y j ii j p i n j i 0 1 2 1 2 1 0= = ( )( )         = =( ) = ∑ , , ,..., ; , ,...,      (11) a random number r is generated from the interval [0,qi]; if qi-1 < r ≤ qi y ( j, i), the i-th chromosome is selected. repeat step 2 and step 3 for n times, so that the n duplicated chromosomes can be obtained and a new generation of individuals can be formed. (5) the parent population of landforms in the water of the xisha islands was hybridized. to determine the parent of the crossover operation, repeat the following procedure from i = 1 to i = n: the random number r is generated from [0,1], if r < pc , then y ( j, i) is a parent. y'1( j, i) andy'2( j, i) were used to represent the selected parents, and they were randomly matched. when the parents are odd, one chromosome can be removed or another chromosome can be selected to ensure pairing (ginde et al., 2016). the arithmetic crossover method is adopted, i.e. firstly, a random number c is generated from (0, 1), then 44 xuli wang the crossover operation is carried out between y'1( j, i) and y'2( j, i) according to the following form, and two offspring x and y are generated: x cy j i c y j i y c y j i cy j i = ( )+ −( ) ( ) = −( ) ( )+ ( )       ' , ' , ' , ' , 1 2 1 2 1 1  (12) the second generation {y2( j, i); j=1,2,...,p; i=1,2,...,n} of landforms in the water of the xisha islands was obtained by the above hybridization. (6) mutation operation. the process of parent selection of mutation is similar to that of cross operation. from i = 1 to n, the following process is repeated: the random number r is generated from the interval [1, 0]. if r < pm is selected, then chromosome y( j, i) is selected as the variant parent, and y( j, i) is selected as the variant parent to correspond to each selected parent, which is represented by y'3( j, i). perform the variation as follows. in rn, the direction of variation d was selected randomly, then y'3( j, i) + md. a new generation {y3( j, i); j=1,2,...,p; i=1,2,...,n} of landforms in the water of the xisha islands was obtained by mutation. (7) evolutionary iteration. from the previous step, we will get 3n progeny individuals, and sort them according to their fitness function from large to small. the first (n-k) progeny is selected as the parent population of the new landforms in the water of the xisha islands. the algorithm turns to step (3) to carry out the next round of evolution, reevaluation, selection, hybridization and mutation. this is repeated until the objective function value of the optimal individual yn ( j, i) is less than a certain set value or the algorithm reaches a predetermined number of times of acceleration, and the algorithm ends. at this time, the optimal individual in the current population is regarded as the optimization result of raga, which are the heterogeneous characteristics of landforms in the water of the xisha islands (zhuo, 2016). research method for fractal characteristics of landforms based on projection covering method basic principle of projection covering method the projection covering method is to directly quantify the heterogeneous characteristics of landforms in the water of the xisha islands by establishing the corresponding relationship between the surface of the water of the xisha islands and the projection surface, projecting the landforms surface onto the two-dimensional plane. in this method, the complex landforms in the water of the xisha islands are projected vertically onto the projection plane of square grid with side length r (juban et al., 2016). let a, b, c and d be the four corners of any square grid on the projection plane of the water of the xisha islands, corresponding to the points on the surface at different heights. if the surface area enclosed by these four points is approximately obtained, the total area of the whole surface of landforms in the water of the xisha islands is approximately: s r s r y j ik k n r n( )= ( ) ( ) = ( ) ∑ 1 , (13) where s(r) is the total surface area corresponding to the square grid side length r of the projections of the lands in the water of the xisha islands; sk(r) is the surface area corresponding to the k-th square grid in the projection surface of the xisha islands; n(r) is the total number of meshes projected from the ground surface of the water of the xisha islands to the projection surface of the square grid; r is the mesh side length of the projection face of the square mesh; k is the mesh sequence number of the projection face. when the side length r of the square grid of the planes in the water of the xisha islands is changed, the total surface area s(r) is also changed. especially when r → 0, the real surface area of landforms in the water of the xisha islands is close to s(r). if the surface has fractal characteristics, according to the fractal theory, the relationship between s(r) and r is: d ins r inrs r = − → 2 0 lim ( ) (14) where ds is the fractal dimension of the landforms surfaces in the water of the xisha islands. irregularity analysis of heterogeneous body of landforms in the water of the xisha islands based on the landforms fractal dimension the basic principle of the fractal dimension of lands surface in the water of the xisha islands is based on the projection covering method. using the spatial analysis function provided by arcgis software, the dem (tin format) of in the water of the xisha islands obtained from each measurement is transformed into grid data format (grid format) of different sizes. grid sizes are 3, 6, 9, 12, 15, 93, 96, 99mm, 33 species in total (iacobucci et al., 2016). then the surface area s(r) of the different grid sizes corresponding to the landforms in the water of the xisha islands is calculated. finally, take in(r) as the abscissa and in[s(r)] as the ordinate, and collect the calculation results of different scales r into the double logarithmic coordinate system. the least square method is used to fit the straight line and determine the scale-free interval. the fractal dimension of the surface of landforms in the water of the xisha islands can be determined by the slope  of the fitting straight line, namely ds = 2−. based on this, we can measure the irregularity of landforms in the water of the xisha islands. the implementation process is shown in figure 1: results shidao is a rock island in the xisha islands, located in the northeast of yongxing island, the largest island in the xisha islands, with a distance of more than 1 km. with an altitude of 15 meters, it is the top of xisha islands. the rock on the island stands up, and the features of landforms on the eroded coast are very obvious, which is in sharp contrast with other grey sand islands surrounded by the crystal white jade belt beach. the geologists who set foot in xisha have paid attention to its own unique characteristics, all of which focus on the geological history of the xisha islands since the late period. in this paper, the objective of the study is to study the terrestrial characteristics of landforms in the shidao waters of the xisha islands. dynamic change of fractal dimension of landforms table 1 shows the surface fractal dimensions of the landforms in the shidao waters of the xisha islands at different development stages calculated by this method: table 1. fractal dimension calculation results of xisha islands water area based on projection coverage method measuring order the correlation coefficient (r2) geomorphic fractal dimension 1 0.9915 2.0142 2 0.9923 2.0254 3 0.9902 2.0326 4 0.9922 2.0534 5 0.9908 2.0575 6 0.9908 2.0654 7 0.9907 2.0664 8 0.9914 2.0818 9 0.9908 2.0745 note: 9 significance tests were all extremely significant. it can be seen from the table that in the scale-free range, there is a good linear relationship between in[s(r)] and in(r), the correlation index r2 is above 0.99, and the significance test reaches the extremely significant level 45fractal characteristics of geomorphological heterogeneity in xisha islands waters under multiple linear regression (=0.01). this shows that the landforms of the shisha islands waters have obvious fractal characteristics in this region. with the change of measurement sequence, the surface fractal dimension of the landforms in the shidao water area of the xisha islands increased from 2.0142 in the first measurement to 2.0818 in the eighth measurement, and the surface fractal dimension increased greatly from the first measurement to the fourth measurement, and then decreased. at the 9th measurement, the fractal dimension of the landforms surface in the shidao waters of the xisha islands decreased to 2.0745, which was lower than that of the 8th measurement. this reflects that the dynamic evolution process of the simulation is an accurate expression of the dynamic changes of the overall characteristics of the landforms in the shidao waters of the xisha islands. comparisons of fractal dimensions of landforms in the shidao water area of the xisha islands by different methods in order to further analyze the advantages of this method for the shidao water area of the xisha islands, the fractal dimension of the landforms surface in the shidao water area of the xisha islands calculated by the method in this paper is compared with the results obtained by the more commonly used calculation methods (table 2). among them, take the increment of landforms fractal dimension as the comparison object. that is to say, the fractal dimensions of the landforms in the shidao water area of the xisha islands calculated by the above two methods are obtained by subtracting the corresponding geometric topological dimensions of the fractal objects. the corresponding fractal objects of this method and box counting dimension method are landforms surface and terrain contour, and the corresponding values of geometric topological dimension are 2 and 1. it can be seen from table 2 that the maximum value of the fractal dimension increment of landforms calculated by the method in the shidao water area of the xisha islands is 0.0818 and the minimum value is 0.0142. during the first to the eighth measurements, the average increment of fractal dimension of landforms in the shidao water area of the xisha islands increased significantly. the maximum increment of fractal dimension of small watershed landforms calculated by box counting dimension method is 0.4254 and the minimum is 0.1521. it can be seen that the fractal feature quantification results of landforms in the shidao water area of the xisha islands are more conducive to reveal the heterogeneity of the overall characteristics of landforms in different development stages. fractal dimension values of different regions for a large number of statistical fractals existing in nature, they are not infinite generation like strict mathematical fractals, but can be established in a certain scale range. a certain form is closely related to a certain mechanism of action, and it is precisely because of the fractal characteristics of the multidimensional domain in the forms of landforms that provides an excellent opportunity for us to study the mode, intensity and scale range of the internal and external forces. divide the shidao water area of the xisha islands into six areas, and calculate the fractal dimension values of different areas by the method in this paper. see table 3 for the calculation results of the fractal dimension values of different areas: figure 2 is the fractal structure of the multi-dimensional domain in the shidao water area of the xisha islands. figure 1. realization process of geomorphic fractal feature research method based on projection coverage method table 2. fractal dimension increment comparison of the landforms in shidao waters of xisha islands by different calculation methods measuring order ds dc 1 0.0142 0.1521 2 0.0254 0.2138 3 0.0326 0.2779 4 0.0534 0.3293 5 0.0575 0.3755 6 0.0654 0.3998 7 0.0664 0.4217 8 0.0818 0.4254 9 0.0745 0.3516 table 3. calculation results of fractal dimension values in different regions scale range i ii iii high mountain watersa 2.0863±0.0017 2.03973±0.0009 2.00749±0.0007 high mountain watersb 2.05295±0.0012 2.02402±0.00077 2.00545±0.000087 medium and low mountain watersc 2.03543 2.01972±0.0007 2.00263±0.00033 medium and low mountain watersd 2.00961 2.00415±0.0008 2.00089±0.00002 basin watere 2.00298 2.00110±0.000079 2.00034±0.000012 basin waterf 2.0012 2.00005±0.0000007 2.00002±0.000004 46 xuli wang figure 2. multi-dimension fractal structure of geomorphologic research area from the above results, we can see that: in the shidao water area of the xisha islands, each research area can be roughly divided into three obviously different linear intervals (scale from small to large is called i, ii, iii respectively), that is, multi-scale fractal. it shows the local self-similar or selfaffine replication relationship in the shidao water area of the xisha islands. with the increase of the observation scale, the fractal dimension value of each interval decreases in turn. this shows that with the increase of the observation scale, the roughness of the shape in the shidao water area of the xisha islands is decreasing, and the fluctuation of the smaller scale is ignored. the process of controlling the action of landforms is from the dominant role of external forces, the struggle between internal and external forces, to the mechanism that is basically replaced by internal forces. in the shidao water area of the xisha islands, the fractal dimension values of different types of landforms in the corresponding interval are as follows: water area in high mountain area > water area in middle and low mountain area > water area in basin area, which reflects the difference of surface roughness or complexity of different types of landforms. therefore, the fractal dimension value calculated by this method can be used as an important index of the erosion intensity of the surface external forces in the shidao water area of the xisha islands. in the shidao water area of the xisha islands, the distribution of multifractal spectral values of different landforms is shown in figure 3. it can be seen from figure 3 that in the shidao water area of the xisha islands, the overall characteristics of the water area in the high mountain area > the water area in the middle and low mountain area > the water area in the basin area, thus reflecting the heterogeneity degree of the probability distribution of the surface area of different types of landforms in the shidao water area of the xisha islands. in the shidao water area of the xisha islands, different study areas of the same landforms also show different degree of uneven distribution of undulating surface area. among them, the distribution of heterogeneity is significant in the c study area and the f study area of basin area in the middle and low mountain area, on the contrary, their corresponding d and e areas tend to be uniform and single fractal characteristics. discussion in the shidao water area of the xisha islands, the study of landforms can be said to be a new research field. we believe that the problems that need further improvement and in-depth study at present are mainly from the following aspects: (a) high mountain waters (b) medium and low mountain waters (c) basin water figure 3. multifractal spectral range distribution of different geomorphic types 47fractal characteristics of geomorphological heterogeneity in xisha islands waters under multiple linear regression (1) the internal mechanism of the characteristics of the fractal segments of landforms many initial researches only focus on the discovery and description of the fractal phenomena in the landforms phenomenon. however, the internal formation mechanism or external dynamic conditions of the fractal properties of these landforms, as well as whether the fractal characteristics can reflect the nature of a particular landforms process, and how the regional characteristics of the fractal characteristics of landforms are, remain to be further explored. at the same time, the existing research shows that many dynamic factors of landforms, such as geography, geology, geophysical field (gravity field, etc.) and even geological structure (fault), can have fractal structure. on this basis, the landforms can also be fractal, so what is the mathematical relationship between these two kinds of fractal? (2) the relationship between fractal dimension of landforms and landforms process many researches on the fractal characteristics of landforms reveal that the fractal law is only a static characteristic (i.e. the “instantaneous” state of landforms nowadays), while others try to associate the fractal characteristics with the development of landforms, but it is still difficult to explain the relationship between the fractal characteristics and the landforms process. therefore, the following problems need to be studied: 1) is the fractal dimension of landforms constant or changing? 2) how long can the fractal dimension of modern landforms represent the geo history stage? 3) the study shows that the climate and lithology conditions are basically the same, and the development of landforms reaches the basin in the middle age or old age (the area is large enough), and the fractal dimension value of the river is close to 0. does this mean that a stable (balanced) landforms type fractal value tends to a definite value (constant)? 4) if the fractal dimension of landforms in a certain stage is a constant, how long does it take from the beginning of its development to reaching this constant? 5) can fractal dimension reflect the stages of the development of landforms? that is, do different landforms have their own typical fractal dimension? what are the reasons? 6) although fractal dimension can be used as the index of watershed erosion period division, what is the relationship between the division of water system development stage marked by the formation of fractal water network and the traditional landforms stage division based on landforms profile and other morphological factors? 7) does the geological structure, lithology, climate and vegetation affect the fractal dimension of this stage? (3) the relationship between the gradient free interval and the scale range of the main landforms processes the fractal features of landforms are usually only statistical features in a certain scale range, and are this scale range related to the scale range of the main landforms processes? at the same time, the same landforms process or action is different on different scales, and the fractal feature is a kind of cross-scale geometric feature in a certain scale range (i.e. scale-free range). what does this fractal feature mean? what’s more, does a certain kind of landforms follow the statistical fractal feature, which means that the mechanism of its leading landforms forces is scale-free or cross scale? the answer to this question is very important in the reliability judgment of the results of the indoor micro landforms experimental model and the determination of the extension or representative range (including common hydraulic engineering landforms simulation such as gezhouba project, etc.). (4) deepening the research of landforms mode there are many successful mathematical rules to simulate the generation of fractal terrain, but mathematicians also admit that this terrain does not reflect any information of the formation process of landforms. other studies have found that the natural landforms landscape can be simulated by using a simple near linear rule. but what is the relationship between this simple rule and the multifactor complex process in the development of natural landforms? does it reflect some characteristics of the complex landforms process, or just conform to some rules of people’s visual judgment? how can we effectively use this mathematical rule to establish the landforms development and evolution model with the participation of internal and external dynamics? what is the difference between simulated terrain and natural terrain? obviously, the interpretation of fractal landforms model is also an important aspect. (5) the feasibility study on the application of time and space fractal law in the prediction of landforms process landforms events, such as earthquakes, floods, mudslides, etc., can be time fractal, which means that they have certain predictive significance. for example, if the distribution of flood water is time fractal, it means that the ratio of the number of floods with 100 year return period to the number of floods with 10 year return period can represent the ratio of the flood with 1000 year return period to the flood with 100 year return period. conclusions gouide, a famous landforms scientist, pointed out in his book the technology of landforms research that once a new technology can be used, the whole scientific process can develop around it and enter a higher level of research and interpretation. as a new technology, fractal theory is also applied to landforms. the application of fractal theory in the study of landforms can be very extensive, including the study of landforms of various landforms or watershed as a whole and the study of landforms process. fractal theory provides a new research method for landformers, which enables them to discover the new cross-scale quantitative characteristics of the traditional complex and disordered landforms phenomenon. at the same time, fractal dimension has become a new parameter in landforms. the application of fractal theory also makes it possible for the research of landforms to connect the simple formula obtained from field observation with the complex landforms and establish the model. therefore, fractal theory provides a possible way for landforms scholars to explain complex terrain. therefore, this paper puts forward the research method for heterogeneous fractal characteristics of xisha islands water lands based on multiple linear expression, and studies the forms of heterogeneous characteristics in the shidao water area of the xisha islands. there is a good linear relationship between in[s(r)] and in(r), the correlation index r2 is above 0.99, and the significance test reaches the extremely significant level (=0.01). this shows that the landforms of the shisha islands waters have obvious fractal characteristics in this region. the shidao water area of the xisha islands is divided into six regions, and the fractal dimension values of different regions are calculated by the method in this paper. the overall characteristics are as follows: mountain water area > middle and low mountain water area > basin water area. this shows the degree of heterogeneity of the probability distribution of rough surface area in different types of landforms. references abuzied, s. m., & alrefaee, h. a. 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(2016). a large-area image sensing and detection system based on embedded thin-film classifiers. ieee journal of solid-state circuits, 51(1), 281-290. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n1.102630 earth sciences research journal earth sci. res. j. vol. 27, no. 1 (march, 2023): 27 36 g eo d es y palabras clave: posicionamiento por punto preciso; series temporales del sistema global de navegación por satélite; desplazamiento de la corteza; estaciones de referencia de operación continua crustal displacement in vietnam using cors data during 2018 2021 dinh trong tran1,*, dinh huy nguyen1, ngoc quang vu2, quoc long nguyen3 1 hanoi university of civil engineering, hanoi city, vietnam 2 university of transport technology, hanoi city, vietnam 3 hanoi university of mining and geology, hanoi city, vietnam * corresponding authors: trongtd@huce.edu.vn desplazamiento de la corteza en vietnam a través de información de estaciones cors en el período 2018-2021 how to cite item: tran, d. t., nguyen, d. h., vu, n. q., & nguyen, q. i. (2023). crustal displacement in vietnam using cors data during 2018 – 2021. earth sciences research journal, 27(1), 27-36. https:// doi.org/10.15446/esrj.v27n1.102630 record manuscript received: 16/05/2022 accepted for publication: 18/01/2023 keywords: ppp; gnss time series; crustal displacement; cors. abstract continuously operation reference stations (cors) networks provide surveying, mapping, and positioning services and play a crucial role in determining crustal displacement. this study processed the continuous global navigation satellite system (gnss) data collected from 55 cors stations in the tast’s cors in vietnam to determine the crustal displacement in vietnam from 2018 to 2021. the processing was performed using online precise point positioning (ppp) services and the displacement model to estimate the velocity of the position time series. the result showed that the horizontal velocity field of all cors stations is uniform in magnitude and direction, ranging from 25.3 to 42.6 mm/year with accuracy from ±0.1 to ±1.0 mm/year in the south-east direction in the international terrestrial reference frame 2014 (itrf2014). this study confirms the stability and accuracy of gnss data from the tast’s cors network for crustal displacement analysis. resumen las redes basadas en estaciones de referencia de operación continua (cors) no solo proveen servicios de posicionamiento, mapeo y sondeo sino que además juegan un rol determinante en la determinación del desplazamiento de la corteza. para este estudio se procesó la información relacionada con el sistema global de navegación por satélite (gnss) recolectada en 55 estaciones cors de la empresa vietnamita tast con el fin de determinar el desplazamiento de la corteza en vietnam entre 2018 y 2021. el procesamiento se realizó con servicios en línea de posicionamiento por punto preciso y se usó el modelo de desplazamiento para estimar la velocidad en la posición de las series temporales. los resultados muestran que la velocidad horizontal para todas las estaciones cors es uniforme en magnitud y dirección, con un rango entre 25.3 y 42.6 milímetros por año, con una precisión de ±0.1 a ±1.0 mm/año en la dirección sur-este, según el marco internacional de referencia terrestre de 2014. este estudio confirma la estabilidad y precisión de la información gnss de las redes cors de la empresa tast para el análisas del desplazamiento de la corteza. https://doi.org/10.15446/esrj.v27n1.102630 mailto:trongtd@huce.edu.vn https://doi.org/10.15446/esrj.v27n1.102630 https://doi.org/10.15446/esrj.v27n1.102630 28 dinh trong tran, dinh huy nguyen, ngoc quang vu, quoc long nguyen 1. introduction in recent years, the development of cors (continuously operation reference stations) networks has significantly increased in many countries globally, serving high-precision surveying and other purposes. early and largescale cors networks were established in some countries, such as the geonet network with over 1300 cors stations in japan (cibeira urtiaga et al., 2022; sagiya, 2004), the cmonoc network with over 260 cors stations in china (wu et al., 2018), the noaa cors network with nearly 1800 cors station in united states (damiani & freeman, 2019). initially, the cors networks were widely used in surveying and mapping. later, the cors networks were applied in numerous research fields, including high-precision positioning, ionosphere studies, troposphere monitoring, tectonic plate monitoring, and others. the typical applications of cors data can be listed. in the study of sea level change, snay (snay et al., 2007) (janssen et al., 2013; snay et al., 2007) used the vertical displacement variations of the cors stations located near tide gauge stations along the coast to determine the “absolute” sea level change with respect to the international terrestrial reference frame (itrf). in the tropospheric study, based on the relationship between the troposphere and the delay of gnss (global navigation satellite system) signals received by cors stations, cors station data is one of the very few reliable tools for providing continuous and precise condition about atmospheric information (yang et al., 2020), this information is used to derive values for local atmospheric variables, which aids in weather prediction and weather monitoring (yuan et al., 2012). in the ionospheric study, the multi-frequency gnss measurements of the cors station network provide valuable data for developing the wide-area ionospheric models to model and mitigate local ionospheric effects, and monitor regional ionospheric activity in detail (choi et al., 2012). cors is widely and effectively used for high accuracy real-time displacement monitoring, such as construction deformation monitoring (xiao et al., 2019; yu et al., 2016) and landslide monitoring (bellone et al., 2016; cong khai et al., 2020; zhao et al., 2021). it offers real-time dynamic analysis, including frequency and magnitude. this means the cors network has an advantage in the accuracy, frequency, and timeliness compared to traditional displacement monitoring techniques (kun & yong, 2007). the study of crustal displacement is the most typical application of cors networks. based on cors data measuring 24/24h, the motion of the earth’s crust can be recorded through changes in its coordinates over time, allowing for the determination of crustal motion. the parameters of crustal motion, such as displacement velocity, seasonal displacement and displacement due to earthquakes, are crucial for tectonic models in tectonic activity research and forecasting (d. t. tran, 2013). myoung (2015) analyzed gnss data of 72 cors stations observed between 2010 and 2012 to assess the effect of the 11 march 2011 tohoku earthquake on south korea. the results showed that the earth’s crust over south korea underwent heterogeneous crustal deformation from the tohoku earthquake, with a maximum displacement of 55.5 mm, a minimum of 9.2 mm, and an average of 22.42 mm. sella (2002) simultaneously used gps global positioning system) data from the cors network during the period of 1993 to 2000 and data from a distributed stations worldwide to establish a global recent velocity model that quantifies the motion of 19 tectonic plates and continental blocks during that time. for determining the crustal deformation in western himalaya, sharma (2021) analyzed gnss data from 4 local cors stations in combining with 6 igs stations and determined annual crustal motion of 19.18 mm near the southern tibetan detachment, 18.72 mm near the main central thrust, 11.93 mm near the main boundary thrust and 11.53 mm near the himalayan frontal thrust. this study also highlights the use of gps data in identify a possible counter-clockwise rotational motion of the indian plate. furthermore, kall et al. (2021) determined the new velocity model of estonia by processing the position time series of 10 cors stations and 12 gnss campaign stations. the resulting model, called est2020vel, has an accuracy of less than ±0.5mm/year. in vietnam, the cors stations were installed and put into use in the late 2010s for surveying and mapping purposes. at the first, private surveying companies primarily installed and operated these cors station. currently, there are almost 400 cors stations installed throughout vietnam. since 2012, the department of surveying, mapping, and geographical information has establish the national cors network, known as vngeonet, and has taken over its operation. after 4 years, the vngeonet network was completed with 55 cors stations that cover the entire country. on the other hand, vietnam faces faults in its land due to being located in the sunda tectonic block, and the indian plate’s pressure on the eurasian plate causing complicated earth’s crust displacements and deformations. studies on tectonic displacement in vietnam are limited until gps become more widespread. for example, there have been studies on the tectonic displacement of active faults in northem vietnam, such as the lai chau dien bien, son la, and red river faults, using repeated gps measurements (cong duong et al., 2006; d. t. tran et al., 2013). however, the small relative displacements on these faults and the limited duration of repeated gps measurement hinder the ability to provide reliable information. recently, based on the development of continuous gnss technology for measuring tectonic displacement and deformation in vietnam, some researchers have applied this technology to determine the crustal displacement on vietnamese territory. minh et al. (2014) used continuous gps data from 8 years (2005-2013) collected from 5 sites in vietnam and 20 sites in south-east asia to determine the velocity of the earth’s crust in vietnam and south-east asia. the results showed that the velocity vectors in itrf2005 were a south-east direction with magnitudes ranging from 31 to 36 mm/year. the authors confirmed the good quality of continuous gps data in vietnam, providing reliable information on crustal displacement. in fact, there are not many studies on the application of cors networks in determining crustal displacement in vietnam due to the limited data. nguyen (2021) utilized gnss data from 7 continuously measured cors stations from 2016 to 2018 to determine the displacement velocity in vietnam. the results indicated that the velocity field ranged from -9.9 to -13.1 mm/year in the north direction and from 26.9 to 32.8mm/year in the east direction. however, the authors acknowledge that the gnss data used during the 2016 to 2018 period in vietnam was not ideal for determining tectonic velocity. tran et al. (2021) processed the gnss position time series of 4 cors stations in vietnam from 2015 to 2019 using a synthetic mathematical model including velocity, jump, seasonal motion, and post-seismic deformation. the authors found that the velocity field of these cors stations ranges from 29 to 41mm/year in the southeast direction in the itrf 2014, with an accuracy of ± 0.3 mm/year. in this paper, we analyze the continuous gnss data within three years, from 2018 to 2021, of the network of 55 cors stations distributed across vietnam to determine the velocity field of the earth’s crust displacement in vietnam. the process consists of 3 main steps: (1) processing continuous gnss data using free online ppp (precise point positioning) services to calculate the daily position of cors stations, (2) converting the geocentric coordinates to topographic coordinates of daily position time series in the itrf2014, and (3) processing the daily position time series by a synthetic mathematical model to determine the velocity of cors stations. the observation data of cors is processed by free online ppp services, which becomes robust, convenient, and fast due to the use of advanced hardware systems and optimized software. the coordinates obtained from the online ppp services fully meet the requirement for studying earth’s crust displacement. 2. study area and data tuong anh science and technology equipment joint stock company (tast) is a leading provider of surveying technology services and specializes in building and developing cors stations in vietnam. in the early 2010s, tast built a cors network that covered the entire country, consisting of over than 100 stations. currently, tast’s cors network is being continuously expanded. during its operation, some of cors stations have undergone changes to their names/codes and locations, some have been affected by power outages, and others have had their antennas replaced or changed. as a result, the continuous measurement data of these cors stations includes interruptions and jumps in the position time series. in addition to providing surveying and mapping services, tast also shares the gnss data of the cors network available for scientific purposes, specifically, this data has been used for previous studies such as nguyen (nguyen et al., 2021) and tran (d. t. tran et al., 2021). https://www.sciencedirect.com/topics/social-sciences/tibetan 29crustal displacement in vietnam using cors data during 2018 2021l all cors stations of tast’s cors network are equipped with trimble netr9 gnss reference receivers and trm 41249.00 antennas. the gnss observation data of these stations is logged in the t02 format. these cors stations continuously observe 24/7 with a sampling rate of 15 seconds and the data is stored on the server system on a daily basis, in the t02 format. tuong anh further processes the data in t02 format for real-time positioning. in this study, we used the gnss data in t02 format to determine the absolute coordinates of all cors stations, using the ppp method to estimate the earth’s crust displacement across the mainland vietnam. figure 1. the tast’s cors network initially, the tast’s cors network was developed primarily for surveying and mapping services, so its cors stations were built on existing tall structures such as high roofs, columns, and buildings with three to four floors. figure 1 illustrates the location of 55 cors stations used in the study and a typical cors station (thoa station). the picture of thoa station shows that the antenna is installed on a pole securely attached to a cement pole on top of the roof. in this study, we analyzed 604 gigabytes (gb) of continuous gnss observation data collected from january 2018 to july 2021 of 55 cors stations of tast’s cors network. these stations are evenly distributed throughout vietnam and are built on sturdy structures. they operate without interruption and remain unchanged during the period of observation. 3. theoretical background 3.1 ppp online processing services ppp is an absolute positioning technique that requires the availability of precise reference satellite orbit and clock products using a network of gnss reference stations distributed worldwide (ocalan et al., 2016). the ppp algorithm uses as input both code and phase observations from a dual-frequency receiver. these are then combined with precise satellite orbits/clocks and with additional very accurate error models to calculate precise receiver coordinates and clocks. according to kouba & héroux (2001), the ionospheric-free combinations of dual-frequency gnss pseudo-range (p) and carrier-phase observations (φ) are related to user position, clock, troposphere, and ambiguity parameters according to the following simplified observation equations: ℓp = ρ + c(dt-dt) + tr + εp (1) ℓφ = ρ + c(dt-dt) + tr + n λ + εφ (2) where: ℓp is the ionosphere-free combination of l1 and l2 pseudo-ranges, ℓφ is the ionosphere-free combination of l1 and l2 carrier phases, dt is the station receiver clock offset from gps time, dt is the satellite clock offset from gps time, c is the vacuum speed of light, tr is the signal path delay due to the neutral atmosphere (primarily the troposphere), λ is the carrier, or carrier-combination, wavelength, n is the non-integer ambiguity of the carrier-phase ionosphere-free combination, εp, εφ are the relevant measurement noise components, including multipath. ρ is the geometrical range computed as a function of satellite (x, y, z) and station (x, y, z) coordinates, calculated by: = ( − )2 + ( − )2 + ( − )2 (3) the observation equations (1) and (2) from all visible satellites are processed together by adjustment and filtering algorithms that solve the different unknowns, such as the station coordinates, the receiver clock, the zenith tropospheric delay (ztd), and the initial phase ambiguities. the accuracy of estimated coordinates is denpendent on the used mathematical model in ppp software, as well as the quality and content observation file. traditionally, commercial ppp software such as gipsy (gps inferred positioning system) (https://gipsy-oasis.jpl.nasa.gov/index.php?page=home) developed by the jet propulsion laboratory (jpl) and bernese (http://www. bernese.unibe.ch/) developed by astronomical and the physical institutes of the university of bern, switzerland are used to process the gnss data. however, there are two disadvantages to using the traditional ppp method. firstly, users must pay a fee to purchase the software license. secondly, a highly configurable computer system is required for software installation. in recent years, internet-based online gnss processing services have become popular due to several advantages such as unlimited use, free of charge, time-saving without the need for processing software, and access to the service’s powerful computer system. with the ppp service, gnss data is uploaded to a specified internet site, and the station coordinates can be obtained within a few minutes later with millimeter-level accuracy (ebner & featherstone, 2008; guo, 2015). currently, there are numerous online processing services available, such as auspos (geoscience australia), csrs-ppp (natural resource canada), opus (u.s. national geodetic survey), scout (scripps orbit and permanent array center, and apps (jet propulsion laboratory), magic gnss (gmv aerospace and defense company), etc. table 1 provides essential information and features of 3 popular online gnss processing services. online ppp processing services offer post-processing position solutions of gnss observations data files and send them to the users via the internet. the precise position is estimated from the ppp service to the itrf. the estimated coordinates of a single station are calculated from either kinematic or static observations utilizing precise gnss orbits and clocks. the online ppp processing service is user-friendly and provides the best position solution based on available gnss observation. the following steps, shown in figure 2, are required to process raw data: https://gipsy-oasis.jpl.nasa.gov/index.php?page=home http://www.bernese.unibe.ch/ http://www.bernese.unibe.ch/ 30 dinh trong tran, dinh huy nguyen, ngoc quang vu, quoc long nguyen table 1. main features of 3 popular online gnss services online gnss processing services organization web base latest version (mars 2022) input file limitations of upload file satellites precise satellite products apps jet propulsion laboratory (jpl) http://apps. gdgps.net/ gipsy 6.4 rinex file by file gps jpl gaps university of new brunswick (unb) http://gaps.gge. unb.ca/ gaps 5.2.0 rinex file by file gps, galileo, beidou igs csrs-ppp natural resources canada (nrcan) https://webapp. geod.nrcan. gc.ca/geod/ tools-outils/ppp. php?locale=en csrs-ppp 3.50.2 rinex multiple files (<300mb) gps, glonass igs raw cors data converting to rinex files submitting online the rinex files receiving results via email cors’s coordinates xyz figure 2. online ppp processing steps as the first step, the continuous measurement data of cors stations is converted to the rinex (receiver independent exchange format) using the free software convert to rinex is available at https://geospatial.trimble. com/trimble-rinex-converter. these rinex files are then uploaded to the ppp service’s website. the results of the online ppp processing are sent to the registered email. in this study, the csrs-ppp service was selected because it has several advantages compared to other online ppp processing services, including a simple and user-friendly interface, a large submitted file size capacity of up to 300 megabytes (mb), fast processing speed of approximately 5 minutes, and millimeter-level accuracy in estimated coordinates. as a note that, the continuous observation data of cors stations is usually processed daily for monitoring earth’s crust displacement. this means that the coordinates of those cors stations from the ppp online service are obtained on a daily basis. these daily coordinates for each station are then separated into a daily position time series. 3.2. conversion from geocentric coordinates to topocentric coordinates the online ppp processing services provide a positioning solution in the itrf. the itrf is defined as an ecef (earth centered earth fixed) system with 3 coordinate components of x, y and z. to monitor the displacement of earth’s crust, the cors coordinates need to be transformed from their geocentric itrf system to their corresponding local topocentric system. this is represented by east (e), north (n), and up (u) components, as shown in figure 3. this representation is physically meaningful because it separates the horizontal component (e, n) from vertical component (u). figure 3. geocentric coordinate system and the local topocentric coordinate system the common equations can be written as follows (d. t. tran et al., 2022): = r − 0 − y0 − 0 (4) where: (xi, yi, zi) are the estimated coordinates in the itrf, and (ei, ni, ui) are the coordinates in the local topocentric system of station i, r is the rotation matrix presented by: = −sin 0 cos 0 0 −sin 0 cos 0 sin 0 sin 0 cos 0 cos 0 cos 0 cos 0 sin 0 sin 0 (5) and (x0, y0, z0) and (φ0, λ0) are the coordinates of reference point. the variance-covariance matrix is transformed as: c rc r enu xyz t== (6) http://apps.gdgps.net/ http://apps.gdgps.net/ http://gaps.gge.unb.ca/ http://gaps.gge.unb.ca/ https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en https://geospatial.trimble.com/trimble-rinex-converter https://geospatial.trimble.com/trimble-rinex-converter 31crustal displacement in vietnam using cors data during 2018 2021l 3.3 synthetic mathematical model for crustal displacement the cors location is changed by land surface displacement due to geotectonic activities, periodic movement due to changes of groundwater levels, and geo-tidal effects. these influences are evident in the position time series of the cors station. a study by tran et al. (2021) on the displacement model of daily gnss position time series of the monitoring points in the area of vietnam area shown that the most suitable model is a synthetic mathematical model consisting of a linear trend, a seasonal term (includes annual and semi-annual periodic motions) and jumps (caused by seismic activity, antenna change, etc.). this model is represented by equation (7) for east, north, and up component of each station. (7) where: ti is the time of the series positions in a decimal year, a is the initial position at time reference, b is the velocity, c, d, and e, f  are harmonic components of annual motion and semi-annual motion, respectively, gj is the magnitude of jumps at a time tj (n is the number of jumps). heaviside function h (ti − tj) = 0 if ti < tj and h (ti − tj) = 1 if ti ≥ tj, and vi is the residuals. using equation (8), we calculate the velocity for the east component (denoted as ve ), the north component (denoted as vn ), and the up component (denoted as vu ), we then calculate the station’s horizontal velocity v2d: (8) to estimate the accurate of the time series, we employ repeatability, a measure of the dispersion of positions. repeatability can be calculated for baselines and positions. the position repeatability is defined as the weighted root mean square (wrms) of the residuals of positions around their average position (nocquet, 2011): wrms v n i i i n ii n = = =1 �� �� 21 2 1 �� ���� �� (9) where:n is the number of positions in the time series, σi is the variance of position y(ti). 4. proposed methodology for crustal displacement based on cors data the procedure for processing cors data to determine crustal displacement in tast’s network is as follows: gnss observation data of cors in rinex ppp processing by csrs-ppp transforming coordinates from xyz to enu & creating daily coordinate time series processing daily coordinate time series by synthetic mathematical model velocity of cors station figure 4. the flowchart for estimating the crustal displacement using the cors data the process involves 3 main steps: • step 1: processing observation data using csrs-ppp service to obtain daily coordinates of cors stations in itrf2014. • step 2: converting the coordinates from geocentric coordinates (x, y, z) to topographic coordinates (east, north, up) in the itrf2014, and creating daily coordinate time series for each cors stations in the east, north, and up components. • step 3: processing daily coordinate time series using the synthetic mathematical model. this involves solving for the harmonic components of annual motion and semi-annual motion, jumps (if any), and determining the displacement velocity of the cors station. in step 1, approximately 604gb of gnss observation data in t02 format is converted to rinex format using the convert to rinex software. the resulting *.o files are compressed into zip files of less than 300mb and submited to the csrs-ppp via its web-based service. this step is time consuming, taking up to nearly 4 days. however, the ppp processing by csrs-ppp online service is efficient and fast, taking only about 5 minutes after submitting each rinex file. this highlights the efficiency and capability of the csrs-ppp online service. the results received from the csrs-ppp service after ppp processing include: sum file: contains the parameters and the results of the ppp processing. pos file: contains the positioning information for each epoch processed. csv file: a comma-separated format text file containing the positioning and clock information for each epoch processed. tro file: contains dry and wet zenith path delay and tropospheric gradient for each epoch processed. clk file: a rinex_clock format file containing the receiver clock offset and the clock offset sigma (95%) for each epoch processed. the daily coordinates (x, y, z) in the itrf2014 of the cors station are found in the *.sum file. a total of 48573 *.sum files of the tast’s cors network have been processed, with a capacity of 592mb. in step 2, authors developed make_mbfile_sumfile.py using the python programming language (https://www.python.org/). this program allows read coordinates (x, y, z) and their corresponding covariance covariance matrix from *.sum files, converts them to topographic coordinates (east, north, up) using formulas 4 and 5, and then writes them in the mb_file format, the gamit coordinate time series format (http://www-gpsg.mit.edu/~tah/ggmatlab/). the program make_mbfile_sumfile.py automatically and efficiently processed all 48573 *.sum files in just about 10 minutes. in step 3, authors developed the gnss position time series processing program named pygps_ts.py (d. t. tran et al., 2021). this program enables automatic or manual processing of position time series in mb_file format. in automatic mode, it automatically detects and removes outliers, detects jumps, and calculates the velocity, the harmonic components of annual motion and semiannual motion, the magnitude of jumps (if any), and post-seismic displacement in the position time series, using equation(6). the automatic processing of time series of 55 cors stations took approximately 10 minutes. however, some cors stations with complex time series could not be processed automatically, especially when outliers or/and jumps could not be eliminated. an example of this is the amkg station, where the estimated displacement velocity had poor accuracy. in this case, we performed manual processing using pygps_ts.py program to improve the accuracy of the estimated displacement parameters. the results of this step are presented in the next section. 5. result and discussion by using the proposed methodology for processing gnss observation data of tast’s cors network, the final result, which is the displacement velocity of the cors stations, is obtained. the processing result quality is evaluated through the standard deviation of the estimated velocity, the repeatability histogram, and the best fit line of the cors station. figure 5 is the repeatablility histogram of 55 cors stations. the histogram shows that 95% of the stations have repeatability lower than 2.7 mm on the east component and 2.5 mm on the north component, demonstrating very high accuracy. however, the repeatability on the vertical (up) component is poor with 95% of the stations having repeatability lower than 9.4 mm, making it unsuitable for studying vertical displacement. the wrms values of this study https://www.python.org/ http://www-gpsg.mit.edu/~tah/ggmatlab/ 32 dinh trong tran, dinh huy nguyen, ngoc quang vu, quoc long nguyen are lower than those of several studies in vietnam such as minh et al., 2014 and nguyen et al., 2021 demostrating the potential of this approach in monitoring crustal displacement monitoring using cors. figure 5. histogram of repeatability figure 6 illustrates the typical processing cases of 2 cors stations in tast’s cors network, dami and lson. figure 6 shows the position time series of dami, which includes a trendline, seasonal periodic motions, and outliers (represented by blue circles that deviate from the series). these outliers must be removed priori to estimate the position time series (t. d. tran et al., 2016). the processing results in figure 6a detect and remove the outliers, resulting in the best fit line (shown as a red line). figure 6b shows the residual time series with positions close to the zero line and a small wrms value of 1.5 mm in the both north and east component, and 4.5 mm in the up component, which is a good result. figure 6c shows the position time series of lson which includes a jump at 2021.216, in addition to outliers, trendline, and seasonal periodic motions. the processing results estimate the best fit line, correcting the jump, as shown by the red linr in the figure 6. the residual time series in the figure 6d has a small wrms value of 1.4 mm, 1.5 mm, and 4.1 mm in the north, east, and up components, respectively. table 2 shows the displacement velocity in the itrf2014 of tast’s cors network and its accuracy. the horizontal velocity of the stations ranges from 25.3 mm/year (ctho) to 42.6mm/year (btin), with accuracy ranging from ±0.1 to ±1.0 mm/year. the vertical velocity of the stations ranges from -51.4mm/year (ctho) to 33.7mm/year (tami), with the accuracy ranging from ±0.3 to ±2.6mm/year. this study focuses only on the horizontal velocity of the cors stations. figure 4 presents the horizontal velocity field of tast’s cors network. figure 7 shows that all cors stations are moving towards the southeast direction with relatively uniform velocities, varying from 25.3 mm/year at the ctho station to 42.6 mm/year at the btin station. this indicates that the earth’s crust surface in vietnam is moving in the southeast direction according to itrf2014. a b c d figure 6. processing the position time series of 2 typical cors stations figures (a) and (c) display the position time series of dami and lson stations, represented by blue dots, with the estimated model shown as a red line. figures (b) and (d) display the residuals time series of dami and lson stations, with outliers, trend, and seasonal motion removed. 33crustal displacement in vietnam using cors data during 2018 2021l table 2. velocity and its standard deviation in itrf2014 no station velocity (mm/year) standard deviation of velocity (mm/year) east north up 2d east north up 2d 1 amkg 29.8 -10.9 -11.6 31.7 0.3 0.4 1.5 0.5 2 bd61 25.9 -7.5 -8.7 27.0 0.3 0.2 1.0 0.4 3 bgia 32.6 -10.3 9.2 34.2 0.1 0.1 0.6 0.2 4 bkan 32.4 -10.1 4.2 33.9 0.2 0.1 0.7 0.2 5 bliu 35.4 -10.4 -44.9 36.9 0.1 0.1 0.5 0.2 6 bloc 27.2 -10.6 3.3 29.2 0.1 0.1 0.4 0.1 7 btin 41.9 -7.5 1.5 42.6 0.4 0.4 1.5 0.5 8 btre 25.5 -11.3 -12.4 27.9 0.1 0.1 0.3 0.1 9 bugi 29.3 -9.0 -1.8 30.7 0.1 0.1 0.8 0.2 10 cmau 28.5 -6.1 -11.2 29.1 0.1 0.1 0.4 0.1 11 cntv 32.6 -6.5 -19.7 33.2 0.1 0.1 0.3 0.1 12 ctho 23.4 -9.7 -51.4 25.3 0.1 0.1 0.4 0.1 13 cuju 27.2 -11.2 -0.8 29.4 0.2 0.2 1.1 0.3 14 dakn 26.1 -9.6 3.7 27.8 0.3 0.2 0.9 0.3 15 dami 28.6 -10.2 3.8 30.4 0.2 0.2 0.7 0.3 16 dgbl 25.4 -12.8 -21.5 28.4 0.2 0.4 1.0 0.5 17 dhoi 30.4 -11.2 7.0 32.4 0.2 0.2 0.7 0.2 18 dibi 29.8 -8.8 29.1 31.1 0.3 0.3 1.2 0.4 19 dili 38.3 -8.4 27.9 39.2 0.7 0.6 1.5 0.9 20 dkn2 32.4 -6.7 -9.0 33.1 0.6 0.5 2.4 0.7 21 dqan 29.9 -10.1 0.4 31.6 0.2 0.1 0.6 0.2 22 dxoa 25.8 -6.9 -6.0 26.7 0.1 0.1 0.4 0.1 23 eahl 26.6 -7.6 20.5 27.7 0.6 0.5 1.6 0.8 24 eaka 29.3 -8.0 -1.6 30.4 0.3 0.4 1.4 0.5 25 goco 28.1 -11.7 -8.7 30.4 0.1 0.1 0.3 0.1 26 halo 33.2 -9.7 3.1 34.6 0.1 0.1 0.7 0.2 27 hati 32.3 -9.2 32.7 33.6 0.5 0.5 1.9 0.7 28 hd34 36.0 -12.0 -4.9 37.9 0.1 0.1 0.4 0.1 29 hnoi 31.5 -8.3 -0.1 32.6 0.2 0.2 0.7 0.2 30 hue_ 30.9 -9.1 25.6 32.2 0.3 0.3 1.5 0.4 31 hyen 31.4 -12.3 1.0 33.7 0.1 0.2 0.7 0.2 32 locn 27.1 -4.1 -9.8 27.4 0.6 0.8 1.5 1.0 33 lson 36.7 -8.8 -1.1 37.7 0.2 0.2 0.8 0.3 34 lthu 29.9 -12.5 -7.0 32.4 0.4 0.3 1.9 0.5 35 nadi 30.0 -8.1 -10.7 31.1 0.1 0.1 0.3 0.1 36 nhoa 24.6 -13.4 -4.5 28.0 0.2 0.2 0.8 0.3 37 plei 29.4 -9.3 -3.9 30.8 0.2 0.2 1.0 0.3 38 pqoc 25.3 -8.8 0.2 26.8 0.1 0.1 0.5 0.1 39 pran 28.2 -9.4 -1.0 29.7 0.1 0.1 0.4 0.1 40 ptho 33.6 -9.1 9.2 34.8 0.1 0.1 0.7 0.2 41 qnam 27.2 -6.9 23.6 28.1 0.3 0.3 1.5 0.4 42 qngi 32.0 -13.3 -2.4 34.7 0.1 0.1 0.3 0.2 43 rgia 24.4 -8.4 5.1 25.8 0.1 0.1 0.5 0.1 44 sola 33.0 -10.1 -14.7 34.5 0.4 0.4 1.6 0.5 (continued) 34 dinh trong tran, dinh huy nguyen, ngoc quang vu, quoc long nguyen no station velocity (mm/year) standard deviation of velocity (mm/year) east north up 2d east north up 2d 45 tami 27.7 -11.0 33.7 29.8 0.8 0.6 2.6 1.0 46 tan2 27.2 -10.2 16.0 29.0 0.4 0.4 1.5 0.5 47 thoa 25.4 1.9 12.6 25.5 0.6 0.6 1.1 0.8 48 tn20 31.7 -8.8 3.8 32.9 0.1 0.1 0.4 0.1 49 tqan 31.7 -9.6 5.5 33.1 0.2 0.2 0.8 0.2 50 tugi 30.4 -4.3 5.2 30.7 0.5 0.5 1.9 0.7 51 tuyf 26.8 -7.7 -0.6 27.9 0.2 0.2 0.9 0.3 52 vfuc 32.1 -10.1 6.9 33.7 0.1 0.1 0.3 0.1 53 vinh 32.3 -9.7 9.2 33.7 0.1 0.1 0.5 0.1 54 vtau 28.6 -10.6 -2.6 30.5 0.1 0.1 0.3 0.1 55 xmoc 25.5 -11.2 -3.4 27.9 0.1 0.1 0.4 0.1 figure 7. the horizontal velocity field of tast’s cors network in itrf2014 35crustal displacement in vietnam using cors data during 2018 2021l this study’s velocity field was compared with previous studies, including determining the earth’s crust displacement in vietnam through repeated gps measurements (d. t. tran et al., 2013), continuous gps data (minh et al., 2014), and cors stations (nguyen et al., 2021; d. t. tran et al., 2021). it was found to be consistent with these studies, demonstrating the reliability of our methodology and the suitability of gnss data of the tast’s cors network for crustal displacement studies in vietnam. 6. conclusions in this study, authors processed gnss data collected from january 2018 to july 2021 from 55 cors stations in the tast’s cors network located throughout vietnam to determine the crustal displacement. our methodology involved processing the data with the csrs-ppp service, which allows for efficient processing and high accuracy in position determination. the displacement model is applied to accurately estimate the velocity of the position time series. the results of the study on the tast’s cors network indicate that the horizontal accuracy is high, with 95% of the stations having a repeatability of less than 2.7 mm and 2.5 mm for the east and north components, respectively. on the other hand, the vertical accuracy is lower, 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(2021). application of cors in landslide monitoring. iop conference series: earth and environmental science, 861(4), 042049. https://doi.org/10.1088/17551315/861/4/042049 https://doi.org/10.1016/j.jog.2012.02.009 https://doi.org/10.1016/j.jog.2012.02.009 https://doi.org/10.1080/10095020.2022.2138569 https://doi.org/10.32508/stdj.v19i2.665 https://doi.org/10.32508/stdj.v19i2.665 https://doi.org/10.3390/rs10030357 https://doi.org/10.1109/access.2019.2912143 https://doi.org/10.1109/access.2019.2912143 https://doi.org/10.5194/isprs-archives-xlii-3-w10-1197-2020 https://doi.org/10.5194/isprs-archives-xlii-3-w10-1197-2020 https://doi.org/10.1061/(asce)su.1943-5428.0000167 https://doi.org/10.1061/(asce)su.1943-5428.0000167 https://doi.org/10.1088/1755-1315/861/4/042049 https://doi.org/10.1088/1755-1315/861/4/042049 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.111383 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 183 204 g eo lo g y abstract a new general and genetic classification is proposed that can be applied to sedimentary deposits and rocks (sedimentites), in six main groups depending on the processes that originate them, this classification seeks the standardization and systematization of the criteria and the most used terms, and proposes some new terms: (1) detrital sedimentites formed by fragmentation of rocks “in situ”; (2) clastic sedimentites formed by erosion, transport and sedimentation of solid materials; (3) biogenic sedimentites formed by biological productivity and activity “in situ”; (4) chemical sedimentites formed by crystal precipitation from solutions “in situ” within the basin; (5) mixed sedimentites, those formed by mixed sedimentary processes, e.g., biogenic accumulation and clastic sedimentation, among others; and (6) diagenetic modification-transformation sedimentites, entities formed “in situ” by compactionreorganization, alteration-transformation, dissolution-collapse, precipitation-cementation, crushing and grinding, or deformation-remobilization processes. additionally, a new compositional-mineralogical classification of the sedimentites in eleven groups are proposed: silicate; carbonate; phosphate; iron, manganese, aluminum and titanium oxides and hydroxides; sulfate and salts; organic rich; nitrate; sulphide; borate; native rich (metals and the non-metals); and mixed. this systematization offers the advantage of allowing a better and easier sequencing of the classification of the sedimentites and facilitates the improvement in the teaching and learning processes. record manuscript received: 02/10/2023 accepted for publication: 30/07/2024 general classification and nomenclature of sedimentary deposits and rocks, a review luis enrique cruz-guevara1*, luis felipe cruz-ceballos2, gladys marcela avendaño-sanchez2 1. grupo de investigación en geofísica y geología aplicada, escuela de geología, universidad industrial de santander, bucaramanga, colombia. orcid 0000-0002-8345-4870 (*) 2. grupo de investigación en geología básica y aplicada (gigba), escuela de geología, uis guatiguará, piedecuesta, colombia. orcid 0000-0002-4339-2302 and 0000-0001-8215-5863 *corresponding author: lecruz@uis.edu.co how to cite this article: cruz-guevara, l. e., cruz-ceballos, l. f., & avendañosanchez, g. m. (2024). general classification and nomenclature of sedimentary deposits and rocks, a review. earth sciences research journal, 28(2), 183204. https://doi.org/10.15446/esrj.v28n2.111383 resumen keywords: sedimentites; detrital; clastic; chemical; biogenic; diagenetic se propone una nueva clasificación genética que puede ser aplicada tanto a depósitos sedimentarios como a rocas sedimentarias (sedimentitas), en seis grandes grupos dependiendo de los procesos que los originan, esta clasificación busca la estandarización y sistematización de los criterios y términos más utilizados, y propone algunos nuevos términos: (1) sedimentitas detríticas formados por la fragmentación de las rocas “in situ”; (2) sedimentitas clásticas formadas por erosión, transporte y sedimentación de materiales sólidos; (3) sedimentitas químicas formadas por precipitación de cristales de soluciones “in situ” dentro de la cuenca; (4) sedimentitas biogénicas formadas por productividad y actividad biológica “in situ”; (5) sedimentitas mixtas, aquellas que se formaron por procesos sedimentarios mixtos, por ejemplo, acumulación biogénica y sedimentación clástica, entre otros; y (6) sedimentitas de modificación-transformación diagenética “in situ”, entidades formadas por procesos de compactación-reorganización, alteración-transformación, disolución-colapso, precipitación-cementación, trituración y pulverización, o deformación-removilización. adicionalmente, se propone una nueva clasificación composicional-mineralógica de las sedimentitas en once grupos principales: silicatadas; carbonatadas; fosfatadas; ricas en óxidos e hidróxidos de hierro, manganeso, aluminio y titanio; ricas en sulfatos y en sales; ricas en materia orgánica; ricas en nitratos; ricas en sulfuros; ricas en boratos; ricas en elementos nativos (metales y no metales), y mixtas. esta sistematización ofrece la ventaja de permitir una mejor y más sencilla secuenciación de la clasificación de las sedimentitas, y facilita la mejora en los procesos de enseñanza y aprendizaje. palabras clave: sedimentitas; detríticas; clásticas; químicas; biogénicas; diagenéticas clasificación general y nomenclatura de los depósitos y las rocas sedimentarias, una revisión https://doi.org/10.15446/esrj.v28n2.111383 mailto:lecruz@uis.edu.co https://doi.org/10.15446/esrj.v28n2.111383 184 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez introduction the intention of geologists is to see the sedimentary record (deposits and rocks) and try to interpret the products of these processes. they want to use the field observation of the deposits and rocks strata, to interpret the physical, chemical, biogenic, and diagenetic processes that were responsible for forming and accumulation or depositing the specific materials, and for transforming them into rocks. in dealing with natural processes, geologists are confronted not with the products of processes that operate in isolation but with products of associated processes that operate collectively. the reasoning is forward from processes to products. this working backward from finished products to the formative processes involves scientific reasoning that is specific to geology and foreign to other field science. geologists and sedimentologists use various criteria to categorize sedimentites based on their origin, geometry of the deposits, mineralcomposition, texture, and internal structures. it is worth noting that sedimentary rocks often have complex origins, and some may fit into multiple categories depending on their characteristics. some classification of sedimentites combines both descriptive and genetic objectives, and compositionalmineralogical, these proposals show and use different types of sedimentary materials. most important aspects in the classification of sedimentites appeared in figure 1. grabau (1904) divided carbonate rocks into two main groups, which were termed clastic and biogenic. krynine (1948) by mode of origin: clastic or fragmental, chemical precipitates, biogenic products (plants and animal); folk (1968 and 1974) divided sedimentary rocks into: terrigenous, allochemical, and orthochemical constituents. folk’s (1959) classification include the autochthonous limestones in biolithite. allochemical constituents of folk (1974) include pellets, fossils, intraclasts and ooids, “which are abnormal chemical precipitates because in general they have been later moved as solids within the basin” (folk, 1974). allochemical of folk (1974) “because in general they have been later moved as solids within the basin” correspond to particles that are not solid breakdown products of older deposits of friedman and sanders (1978). according to friedman and sanders (1978) sedimentary particles are those that were transported physically in the solid state before being deposited. furthermore, pellets, and intraclasts are not abnormal chemical precipitates, because they are not substances precipitated from a solution. pellets are biodeposition materials because they are produced by organisms (deposits-feeding of friedman and sanders, 1978; fecal pellets of boggs, 2006, and selley et al., 2005; biodepositions of cruz et al., 2024), therefore, they are biogenic materials. intraclasts are pieces of penecontemporaneous, usually weakly consolidated carbonate sediment that have been torn up and redeposited by currents (folk, 1974). intraclasts are materials produced by erosion, transport, and deposition, therefore, they are clasts or depositional materials as their name indicates. in conclusion, pellets and intraclasts are not allochemical materials. there is another inconsistency in the allochemical term, fossils and ooids can be considered as abnormal chemical precipitates (folk, 1974). fossils according to their origin (kidwell et al, 1986) are biogenic (biocenosis association), sedimentologic (resedimented), or diagenetic concentration (e.g., solution-collapse of original skeletal association). ooids are formed by chemical processes (see folk, 1974; friedman and sanders, 1978, among others), or by chemical and biogenic processes. in conclusion, it is better and more practical to consider fossils as biogenic materials, and ooids as chemical or biochemical materials. in conclusion, the allochemical term proposed by folk (1974) can be replaced by clastic juvenile materials (cruz et al., 2017), because according to folk (1974) they have been transported and consequently, they are sedimentary particles. dunham (1962) proposes a classification of carbonate rocks according to depositional texture as mudstone, wackestone, grainstone, and packstone; furthermore, rocks retaining too little of their depositional texture are classified aside as crystalline carbonates; and finally, rocks in which his original components were bound together during deposition are named boundstone. dunham (1962) made the same as folk (1959) and grouped all autochthonous limestones into the term boundstone. comparing mudstones, wackes, and arenites of dott (1964) with the prepositions of dunham (1962) (see figure 2), it is clear that mudstone and wackes correspond to both clastic limestone and clastic silicate, because both are clastic sedimentites; and packstone and grainstone are useful to differentiated clastic arenites. because of that we propose the use of the dunham (1962) mudstone, wackestone, grainstone, and packstone classification, not only for the clastic limestones but also for all the sedimentary clastic deposits. history of classifications nomeclature of classifications grabau (1904): “the classification must be descriptive, systematic and precise”. duham (1962): crystalline, bounstone, grainstone, packstone, etc. folk (1974): terrigenous, allochemical, orthochemical. friendman & sanders (1978): intra-basin, extra-basin, etc. kidwell et al. (1986): biogenic, sedimentary and diagenetic fossil concentrations. wright (1992): biological, depositional, diagenetic. hallsworth & knox (1999) not genetic: siliclastic, carbonated, based on grain size. genetic textual structural sedimentary materials (sms) detritus clastic biogenic chemica diagenetic ancient derived juvenile figure 1. the most important aspects in the classification of sedimentites. history of classification of sedimentary rocks, e.g., classification of limestone using essential components (wright, 1992), etc. the fundamental aspects in the classification of sedimentites. the sedimentary materials. dunham’s (1962) proposition is broader for the differentiation of clastic sedimentites than dott’s (1964), and could be very useful to take into account packstones and grainstones in silica and terrigenous sedimentites. sedimentary particles: original components, not bound together during deposition (dunham, 1962); materials transported as particles (folk 1968, 1974). mud supported grain-supported main carbonate rocks group according to dunham (1962) mudstone < 10% grains wackestone >10% grains packstone contains mud grainstone lacks mud main silicate sandstone rock group according to dott (1964); folk (1974). mudstone >75% mud wackes muddy sandstone mud between 10 and 75% arenites <10% mud figure 2. comparison between the clastic rock classification of dott (1964) and dunham (1962). 185general classification and nomenclature of sedimentary deposits and rocks, a review friedman and sanders (1978) divide sedimentary rocks into five major kinds based on compositional-mineralogicalgenetic classification: carbonate, authigenic, carbonaceous, terrigenous, and pyroclastic rocks. this classification uses genetic parameters in terrigenous, pyroclastic, and authigenic rocks, and forgets their use in others, instead, they use compositionalmineralogical classification (for example, carbonate and carbonaceous rocks). the classification of friedman and sanders (1978) include carbonate, authigenic and carbonaceous rocks in intrabasinal group; and terrigenous in extrabasinal group. intrabasinal materials derived from within the basin; and extrabasinal sedimentary particles derived outside the basin. it is clear that extrabasinal sediments includes allochthonous materials: terrigenous materials eroded from ancient rocks (rfs, mineral fragments and reworked fossils); it also includes juvenile clasts formed outside of the basin and deposited in the basin, e.g., new materials formed by weathering such as argilophyllosilicates (kaolin, montmorillonite, smectite, etc.), and biogenic, chemical and pyroclastic materials. biogenic materials such as detritus of plants and animals that live in the continental environment and can be transported and deposited in lacustrine and marine environments. chemical materials, for example, evaporitic materials crystalized in near-shore sabkha conditions and transported and deposited in marine environments. pyroclastic materials originated in continental environments and deposited in marine realms. all biogenic, chemical and pyroclastic materials previously listed are considered extrabasinal materials because they were deposited outside their basin. intrabasinal sedimentary materials include juvenile weathering, biogenic and chemical materials produced inside of the basin (materials without transportation, therefore they are autochthonous materials); but also include ancient derived and juvenile materials transported inside of the basin, e.g., ancient rfs and minerals; and juvenile weathering, biogenic and chemical materials. examples of juvenile bioclastic materials are fragments of juvenile test of mollusks removed by currents and deposited in other parts of the basin, but within it. chemical materials include, e.g., gypsum formed in littoral sabkha conditions and deposited in the adjacent eolic environment forming dunes (gypsum dunes of white sand national park new mexico, usa). pyroclastic materials originated and deposited near the volcano generator. all of them are autochthonousintrabasinal materials, not extrabasinal. kidwell et al. (1986) propose a conceptual framework for the genesis of skeletal accumulation based on concentrating processes: biogenic, sedimentologic, and diagenetic concentrations. wright (1992) classification of limestone, uses a more genetic division and emphasizes the distinction between carbonate strata influenced by depositional (physical), biological, or diagenetic processes (both synsedimentary and post-depositional), like kidwell et al. (1986) proposition. wright (1992) proposes to classify limestones into three types of rocks: depositional, biological and diagenetic rocks. the conceptual propositions of kidwell et al. (1986) and the general and genetic classification of wright (1992), such as depositional, biological, and diagenetic, are the basic structure in the general genetic classification of sedimentites proposed here. some authors like lazar et al. (2015) use this conceptual framework in the nomenclature guidelines for fine-grained sedimentary rocks, such as detrital, biogenic, authigenic fine-grained quartz, or carbonate. hallsworth and knox’s (1999) classification of sedimentites is based on actual, descriptive attributes, not interpretative attributes. it is therefore essentially nongenetic and based on the compositional-mineralogical attributes present at the time of deposition. sedimentites are according to hallsworth and knox’s (1999) classification are siliciclastic, carbonate, phosphate-sediments and phosphorites, ironsediments and ironstones, organic-rich, non-carbonate salts, non-clastic siliceous, miscellaneous hydroxide, oxide, and silicate, based on grain size or crystal size, hybrid, and with volcaniclastic debris. sedimentary rocks classification according to boggs (2006, 2009) can be placed into three fundamental groups on the basis of composition and origin: siliciclastic, chemical/biochemical (carbonate, cherts, phosphorites, and ironrich sedimentary rocks), and carbonaceous, but in reality, boggs’ classification use both genetic and compositional-mineralogical classification in siliciclastic sedimentary rocks, and use compositional-mineralogical classification in carbonate, cherts, iron-rich, phosphorites and carbonaceous sedimentary rocks, and use genetic classifications in chemical, biochemical and evaporites sedimentary rocks. this paper summarizes our efforts to standardize nomenclature for the field and laboratory description and classification (genetic and compositionalmineralogical) of sedimentary materials, deposits, and rocks. here we have aimed for a system that is simple and yet as inclusive and general as possible. our proposal outlines address clarification of some terms and proposal of new ones that are intended to complement rather than replace; proposition that can be very useful for the description, naming and classification of sedimentites. this classification uses the conceptual framework of genetic classification of skeletal concentrations of kidwell et al. (1986); and major processes, biological, depositional, and diagenetic, that influence limestone texture of wright (1992). here we also use the concept “ancient derived” and “juvenile” of the sedimentary particles proposed by friedman and sanders (1978); and the chemical or orthochemical constituents according to folk (1974), friedman and sanders (1978) and boggs (2006) among others. we will use the term sedimentite in the text when referring to sedimentary deposits and rocks as proposed by pfeiffer et al. (1980). sedimentary processes the sedimentary processes (sps) are any physical, biological, or chemical processes to take part in the formation of the sedimentites. these processes occurred on the external lithosphere and are of interest in many ways for understanding the dynamics of the modern world. sps include the formation of regolith, the depositional processes, the biological activity and productivity, the chemical precipitation-accumulation of crystals, and the diagenetic-transformation processes that convert the sedimentary deposits into sedimentary rocks. the sedimentary process begins when the minerals and rocks on the external lithosphere are transformed, broken, and fragmented forming loose materials and ions in solution. loose materials (lms) in the external lithosphere constitute the regolith, and because of the conditions and processes present there lms could be eroded, transported, and deposited. also, lms and ions are used by simple and complex organisms to carry out their vital functions. additionally, organisms produce materials (shells, etc.), and make transformation of other materials (e.g., bioperturbation). furthermore, ion-saturated solutions by pure chemical processes or by bacterial mediation produce the precipitation-accumulation of crystals. and finally, the sedimentary deposits formed initially could be buried and transformed into sedimentary rocks. the sedimentary processes never exceeded a temperature of ~210ºc and pressures are <1,5 kb, in their geological history. specific geological processes that form regolith, and sedimentites are presented on tables 1 and 2. the description of the specific sedimentary processes is presented by different authors: friedman and sanders, 1978; boggs, 2006; cruz et al, 2024; among others. sedimentary processes exclude materials formed by (a) endogenous metamorphic processes that include among others, e.g., pyrometamorphic rocks associated with naturally burned coal bed: paralava, clinker, and relatives (grapes, 2006), and (b) endogenous igneous process and exogenous lava process. specific sedimentary anthropogenic effects, processes, objects, materials or structures, those derived from human activities are not part of these document, they need a special publication. except for depositional processes where materials are eroded, transported, and deposited in different places that they were formed (allochthonous materials), the other processes presented in table 1 form loose materials that remain in the same place where they were formed (autochthonous materials). they may undergo some processes of formation, e.g., weathering alteration or diagenesis, etc., but essentially the materials do not get transported to a different location after their formation. common examples of autochthonous materials are crystals and biogenic production that accumulate at the site of formation, such as in swamps, lakes, or shallow marine environments, etc. for this reason, the deposits formed by depositional processes are named clastic deposits, and the others could be grouped in autochthonous deposits. 186 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez table 1. geological processes involved in the formation of autochthonous sedimentites, and regolith. their specific products and landforms. geological processes products landforms weathering of sedimentary deposits and rocks, e.g., weathering processes of mcqueen and scott (2008). residual deposits (merril, 1897; tyrrel, 1978; selley, 2000), rock of weathering (waldo, 2018). regolith and soil mantle fracturing, crushing, and grinding of rocks and minerals by tectonic stress (compressive and distensive). tectonic-breccias (morrow, 1982; shukla and sharma, 2018); fault breccias and related fault rocks (woodcock and mort, 2008) irregularly shaped of tectonic regolith lava cooling fragmentation, e.g., magma fragmentation (edwards et al., 2020; skilling et al., 2002) cooling lava fragmentation-breccias (grubensky et al., 1998) irregularly shaped of lava cooling regolith fragmentation and mixing of wet sedimentary deposits with lava (peperites by skilling et al., 2002) peperite-breccias (white et al., 2000) irregularly shaped of peperites regolith rock fragmentation by caldera-collapse and the formation of caldera collapse-breccia (lipman, 1976; millone et al., 1994; eggleton et al., 2001). caldera-collapse deposits (lipman, 1976); breccias formed by fracturing of previous volcanic products (e.g., palladino and simci, 2005). cylindricalsub cylindrical structure rocks-dissolution-collapse and breccias formation. dissolution collapse-breccias (friedman, 1997; ryu et al., 2002; dong et al., 2018). cylindricalirregularly structure with inside breccias deposits. fragmentation of minerals and rocks by hydrothermal alteration. hydrothermal detrital materials (murphy, 2009). irregularly shaped of hydrothermal alteration regolith anthropogenic activity and productivity. garbage-breccia, etc., (howard, 2017; luberti et al., 2019). irregularly shaped garbage dumps, etc. fragmentation, formation of new materials, and mixing of materials caused by an extraterrestrial impact. impact breccias of shukla and karma, (2018); pati et al. (2017). irregularly shaped of impact deposit chemical precipitation-accumulation of crystals (folk, 1974; friedman and sanders, 1978; tyrrel, 1978; selley, 2000; boggs, 2009). e.g., megacrystalline gypsum deposits, etc. irregular shape of chemical deposit (sabkha, etc.). biogenic activity and productivity (grabau, 1904; retallack, 1977; friedman and sanders, 1978; kidwell et al., 1986; buol et al., 1997; birkland, 1999; buol et al., 2003; boggs, 2006; lazar et al., 2015; cruz et al., 2024) e.g., biogenic-breccias, sandy breccias of corporal materials and structures, etc. hemispherical shape of reef, etc. diagenetic transformation-reorganization of material, e.g., dissolution collapse of sedimentary deposits (abelson et al., 2006; oz et al., 2016) diagenetic crackle-breccias of morrow (1982) (diagenetic crack structures of chen et al., 2010); diagenetic dyke infilling-breccia (sdib) from a soft sedimentary deformation (ssd) after hydraulic fracturing (morrow, 1982); and diagenetic solutioncollapse-breccias (morrow, 1982). irregular shape of crackle-breccias deposit, etc., note: collapse-breccias are formed by the following processes: caldera-collapse (lipman, 1976), dissolution collapse of rocks (friedman, 1997) and dissolution collapse of sedimentary deposits (abelson et al., 2006; oz et al., 2016). more information about landforms sees gutierrez and gutierrez (2016). table 2. geological processes involved in the formation of depositional and mixed sedimentites, and regolith. their specific products and landforms. geological processes products landforms depositional processes (folk, 1974; friedman and sanders, 1978; selley, 2000; nichols, 2009; boggs, 2009) colluvial (gravity dominated) (kernif et al., 2022) clastic-breccias, etc. (eggleton, 2001) fan-breccias, etc. eolian clastic sand dunes, silt loess, breccias desert pavement, etc., (bradley, 2015). dunes, and irregular shape of loess, and desert pavement (lag deposits), fluvial clastic fluvial sands, conglomerates and breccias (miall, 2006) fluvial terraces lake clastic fine deposits (bradley, 2015); biogenic fine silica deposits (liu et al, 2019). delta, delta collapse, floods, volcanic, pelagic abiogenic and biogenic deposits. more information in sabatier et al. (2022) glacial glacial poorly sorted breccias (friedman and sanders, 1978) hemispherical, streamlined, irregular moraines, etc. see glasser and bennett (2004). (continued) 187general classification and nomenclature of sedimentary deposits and rocks, a review geological processes products landforms pyroclastic pyroclastic-breccias, sands, silts, etc. pyroclastic flow fan, tuffs mantle, etc., (coira et al., 2022). mixed depositional e.g., hemipelagic materials (biogenic and clastic argilo-phyllosilicates, and with pyroclastic ash cloud) tabular laminated of hemipelagic with pyroclastic rocks, glass, and crystals deposits sedimentary deposits could also be formed by mixed sedimentary processes, e.g., biogenic, and clastic, e.g., stromatolites and hemipelagic deposits, and natural and anthropogenic (bunke et al., 2019). e.g., stromatolite finely laminated biogenicmicrocrystalline calcareous with clastic sand and silt-sized materials. hemispherical shape of laminated algal stromatolite with clastic lamina e.g., hemipelagic and pelagic materials (biogenic and clastic argilophyllosilicates, and with pyroclastic ash cloud) tabular laminated hemipelagic with pyroclastic glass, and mineral deposits. sedimentary products refer to the various materials and structures that formed because of the processes involved in the formation of sedimentites. sedimentary products also include sedimentary deposits and sedimentary rocks, and these are arranged in sequential sedimentary facies (ssfs). sedimentary materials (sms) are defined here as any kind of material formed, deposited, accumulated, or modified-transformed by an exogenous geological earth process under low temperature and pressure. sedimentary materials are the essential building blocks of sedimentites and are formed by the fourteen (14) processes described before in table 1 and 2. the sms are described by different authors, e.g., folk (1974); friedman and sanders (1978); boggs (2006), among others. sms include terrigenous, allochemical, and orthochemical constituents (folk, 1974); sedimentary particles, biological secretion of skeletons, predation, trapping and baffling of lime mud, pelletization, burrowing and stirring by organisms, and activities of microorganisms which assist precipitation of diverse minerals, chemical, and authigenic materials (friedman and sanders, 1978); biological, depositional and diagenetic materials and processes (wright, 1992); products of subaerial weathering, particles, cements, evaporites, chert, phosphorites, iron rich, and organic matter (boggs, 2006). according with the fourteen geological process there are six fundamental and contrasting kinds of sedimentary materials, and three more types of mixed sedimentary materials are proposed (after cruz et al, 2017), they are: 1. detrital materials (dms), those materials formed in the external lithospheric region by fragmentation of rock, materials not transported physically as solid objects. 2. chemical precipitation-accumulation materials (chpams) or “sedimentary crystals”, those materials precipitated chemically from ion saturated solution and accumulate in an exogenous inorganic process on the earth’s surface, materials not transported physically as solid objects. 3. biogenic materials and structures (bmss), those formed in exogenous processes by biological activity and productivity, whatever autochthonous or parautochthonous or accumulated, materials not transported physically as solid objects. 4. clastic materials (clms), those originated by exogenous processes that involve erosion and physical transport of materials in the solid state before being deposited. 5. diagenetic overprint materials and structures (domss), those materials or structures that result or formed by modifications-transformations in the changing of the sedimentary deposits into sedimentary rocks. and finally 6. natural liquids and gasses materials (nlgms). natural liquids and gasses materials, include “atmospheric water” in their liquid state, they are usually as primordial components filling porous space, and minerals present in sedimentites such as gypsum that have water in their structure. additionally, connate water (cws), water trapped in the pores of a rock since their formation (schilthuis, 1938). cws can also yield information on the provenance of aquifers and of the thermal history of the host rock. the “liquid hydrocarbons” and biogenic-liquid compounds (corporal fluids and relatives) are from this group. “liquid hydrocarbons” because they are products of organic compounds degradation during the diagenetic process. biogenic-liquid compounds because they are biogenic materials. the natural gasses materials include “atmospheric air” in their gas state because they are part of the sedimentites, usually trapped as components filling porous space in sediments when they are deposited, and frequently used to establish paleoclimatic conditions, e.g., “high polar natural ice sheets” because they trap and keep samples of the atmosphere and trace gasses (bender et al., 1997; walker et al., 2008, among others). “hydrocarbon gasses” (methane and relatives) are excluded from this group because they are organic compounds product of degradations of organic compounds in residual regolith formation or in a diagenetic process. the mixed sedimentary materials and structures are those produced by more than one sedimentary process, they are: 1. mixed biogenic accumulation with clastic sedimentation materials and structures, they are biostratification or biogenic granule classification structures from algal or bacterial activity; and 2. mixed coated microbialchemical precipitation materials and structures, those formed or grown in place by a mixed chemical coated-biogenic (microbialinduced) process, or biochemical precipitation concentrically micro-coated calcareous materials. general classifying and naming of sedimentary deposits and rocks the genetic classification of sedimentites proposed here start using the conceptual classification for genesis of skeletal concentration of kidwell et al. (1986) and essential components and limestone classification of wright (1992), and according to the fourteen sedimentary processes (describe before in the text), and also six kind of sedimentary materials that formed or constituted sedimentites. because of that it is convenient to divide the sedimentites in six genetic types (table 3): detrital, chemical, biogenic, clastic, diagenetic and mixed. although genetic classification could be difficult to carry out in field work, it can be done, e.g., in the field it is essential to determine the types of materials, their state of conservation, the relationship of different materials. with laboratory tests and more special techniques it is possible to know much more about the genesis of the materials that make up a deposit or a rock and with this information it is relatively easy to determine its general and specific genetic classification. traditional compositional-mineralogical classification of carbonate, phosphorite, silicate and carbonaceous deposits and rocks can be integrated into this genetic classification (detrital, clastic, biogenic, chemical, diagenetic, or mixed deposits and rocks), e.g., silicate-clastic, carbonate-clastic, carbonatebiogenic, etc. detrital deposits (dds) are sedimentary deposits formed in the external lithospheric region by fragmentation of rocks “in situ” and the formation of autochthonous regoliths. dds correspond in part with sedentary materials of tyrrel (1978). detrital deposits are formed by any of the following exogenous geological processes (as shown in table 1): weathering; tectonic stress; lavacooling-fragmentation; fragmentation and mixing of materials by interaction of wet sedimentary deposits with lava; caldera-collapse; dissolution collapse of rocks; hydrothermal alteration; and by fragmentation “in situ” of materials caused by an extraterrestrial impact. since human activity and productivity produce anthropogenic deposits of loose materials (anthropogenic regolith), such a way that these types of deposits must be a part of the group of detrital deposits. according to this specific detrital formation processes, there are the following specific detrital deposits (dds) (table 4). 188 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez table 3. general genetic classification of sedimentary deposits and rocks, based on the genetic classification of sedimentary materials. after grabau (1904); krynine (1948); kidwell et al (1986); and wright (1992). general kind of sedimentary deposit or rock description specific sedimentary materials 1.detrital deposits and rocks (dds and drs) formed in the outer lithospheric by the transformation of rocks into rock fragments and minerals. deposits formed by fragmentation of rocks by the following processes: weathering; tectonic stress; lavacooling-fragmentation; fragmentation and mixing of materials by interaction of wet sedimentary deposits with lava; caldera-collapse; dissolution collapse of rocks; hydrothermal alteration; fragmentation “in situ” of materials caused by an extraterrestrial impact; and finally, by human activity and productivity. breccias, sandy breccias, brecciated sand deposits consisting of inherited-conserved rock (rfs) and mineral fragments released from ancient rocks, e.g., by weathering. juvenile minerals, e.g., argilophyllosilicates formed during weathering processes. juvenile rfs formed e.g., by lava-cooling-fragmentation processes, etc. 2.chemical deposits and rocks (chd and chr) formed by materials precipitated from ionsaturated solution and accumulated. chemical or orthochemical sedimentites are those entities where autochthonous and juvenile sedimentary chemical materials are more than 50% of the deposits or rocks. deposits consisting mainly of autochthonous and juvenile chemical sedimentary materials, e.g., deposits formed by halite or gypsum crystallization accumulation, amorphous silicate precipitationaccumulation, snow crystals accumulation. 3.biogenic deposits and rocks (bd and br) formed by biological activity and productivity. biogenic deposits or rocks are those entities where autochthonous or parautochthonous (accumulated), and juvenile biogenic materials or structures are more than 50% of the deposits or rocks. consists mainly of autochthonous and juvenile biogenic materials and structures, e.g., deposits formed by accumulation of: fragments of plants, test of organisms (diatoms, foraminifera, coccolithophorids), coralline reefs, etc. 4.clastic deposits and rocks (cld and clr) form by loose-solid materials eroded and transported before being deposited. clastic sedimentary deposits or rocks, where sedimentary particles are more than 50% of the deposits or rocks. deposits consist mainly of ancient derived-conserved materials (rfs and minerals), and also juvenile materials, e.g., biogenic shell, abiogenic materials, etc. 5diagenetic transformation modification of sedimentary deposits (dtmds and dtmrs) any original sedimentary deposits have diagenetic overprint modifications and forms diagenetic materials and structures, it occurs in the changing of the sedimentary deposit into rock, e.g., the formation of solution-collapse breccias. deposits consisting of materials and structures inherited and conserved from the original sedimentary deposits, with incorporated materials, and structures that result of diagenetic overprint transformation, e.g., deposits formed by ironstone ooids, cherts, diagenetic dolostone, and high polar natural ice sheets. 6.mixed sedimentary deposits or rocks (mxsdmxsr) those sedimentites that were formed by mixed sedimentary processes, e.g., biogenic accumulation and clastic sedimentation (algal stromatolite), among others. note: detrital, chemical, biogenic, and diagenetic deposits and rocks are considered autochthonous or in-situ, because their materials have not had transport after their formation. these autochthonous deposits could be related to intrabasinal deposits and rocks of friedman and sanders (1978). table 4. general detrital deposits classification (detrital regolith). conditional main category specific name 1) according to the formation processes: weathering, tectonic stress, lava-cooling fragmentation, etc. (as show in table 1) residual deposit (rds) residual rf-breccias granite-derived; residual rfbreccias basalt-derived; residual rf-breccias doleitederived; calcrete rds; etc. tectonic deposits (tds) granite-rf-breccia by tectonic, etc. lava-cooling fragmentation deposits (lcfds) basalt rf-breccias by lcf, etc. peperites deposits (pds) mixing of wet sedimentary deposits with lava. peperite rf-breccia, etc. caldera collapse deposits (ccds) e.g., basalt rf-breccia deposits by cc dissolution-collapse deposits (dcds) rf or lithic-breccia by rdc, etc. hydrothermal alteration deposits (hads) rf-breccias and mineral-sandy-breccia by hydrothermal alteration, etc. anthropogenic deposits (ads) garbage dumps by anthropogenic activity, etc. extraterrestrial impact deposits (eids) rf-breccias and mineral sandy-breccia by ei, etc. note: calcrete residual deposits (crds) is a microcrystalline calcareous deposit formed by weathering processes of both calcareous deposits and rocks. rfs are rock fragments, also named lithics, they are ancient and juvenile: ancient, e.g., residual rf-breccias granite-derived, granite-rf-breccia by tectonic, etc.; juvenile, e.g., basalt rf-breccias by lcf. 189general classification and nomenclature of sedimentary deposits and rocks, a review detrital deposits have the following kinds of materials: (1) inheritedconserved rfs and minerals released from ancient rocks, e.g., by weathering or tectonic, and from sedimentary deposits involved in the case of peperites formation; (2) juvenile materials, e.g., minerals formed by weathering transformation (argilo-phyllosilicates); and juvenile rock fragments formed, e.g., by lava-cooling fragmentation. any detrital deposits, tectonic, lava cooling fragmentation, peperites, etc., exposed to the weathering conditions could be modified to some degree, so a specific classification of residualdetrital deposits is necessary (table 5, based on avendaño and cruz, 2017; and cruz et al, 2020). table 5. specific detrital residual deposits classification. conditional specific category specific name 1) if weathering and according to the inherited-conserved sequential facies (i-c sfs) sedimentary bed sequences, cycles, etc. i-c quartz sandstones intercalated with clay argilo-phyllosilicates, etc. metamorphic foliation sequences of schist, gneiss, etc. i-c schist foliation sfs igneous cumulates, cumulus, etc. i-c granite sfs 2) if weathering and according to the new residual and overprint sequential facies (nro sf). i-c versus transformations, and incorporation materials. residual regolith with nr-o sf: o, a, and b horizon heavy transformed heavy transformed rd moderate transformed moderate transformed rd slight transformed slight transformed rd conserved substratum conserved rock fragments and minerals; conserved sedimentary deposits; conserved detrital deposits. specific descriptions for residual deposits, e.g., slightly transformed sienogranitederived rfs residual-breccia, and clayed sand deposit with overprint a and b horizon residual sequential facies and incorporated welldeveloped o horizon (figure 3) after avendaño and cruz (2017). thin section of sienogranite rfs slightly transformed from b (figure 4) (avendaño and cruz, 2017). chemical deposits (chds) are sedimentary deposits where autochthonous and juvenile sedimentary chemical materials are more than 50% of the deposits. sedimentary deposits form mainly through the precipitation of minerals from solution. unlike clastic sedimentary deposits, which are composed of sediments (mineral and rock fragments, etc.), chemical deposits are created by the direct chemical precipitation of minerals, often from evaporating or supersaturated solutions. chemical or orthochemical constituents of folk (1974) are materials that grew as solids in the waters of the depositional basin because of the chemical precipitation of friedman and sanders (1978), e.g., syngenetic dolostone. evaporites are chemical sedimentary deposits that form when saline water evaporates and precipitates minerals (boggs, 2006). common evaporite minerals include halite, gypsum, and anhydrite. these rocks typically occur in arid or semi-arid regions, where evaporation rates exceed precipitation, leading to the concentration of dissolved salts. sabkha environments (boggs, 2006) refer to specific coastal or shallow, arid, and saline environments that are characterized by the deposition of evaporite minerals, especially gypsum and halite, along with other chemical sedimentary minerals. chemical sedimentites include chemical precipitates of krynine (1948); tyrrel (1978); friedman and sanders (1978); boggs (2006); among others. autochthonous sediments of selley (2000) (carbonates, coal, iron ores, phosphates, and evaporites). a b figure 3. residual deposit (residual regolith) developed on sienogranite lithology (pescadero granite, piedecuesta, colombia) (avendaño and cruz, 2017). o horizon black color at top, a and b horizon. a c b d figure 4. sienogranite, mainly quartz, muscovite, and sericite by alteration product of plagioclase and potassium feldspar materials. b, c and d are thin section photography, with 5x objective and 10x ocular, residual regolith from b horizon. a. photography of altered sienogranite with reflected monochromatic light b. with polarization analyzer. c. photography with cross polarized light (xpl). d. photography with plain polarized light (ppl). ser: sericite, fsp: feldspar, kln: kaolinite, qtz: quartz (avendaño and cruz, 2017). 190 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez chemical deposits provide valuable information about ancient environments, as their formation is directly linked to specific geochemical conditions. for instance, the presence of evaporites suggests a history of arid conditions and evaporation of water bodies. biogenic deposits (bds) formed mainly by biological activity and productivity. biological products are any kind of materials or structures that are produced, built, modified, or used by living organisms (cruz and cruz, 2017; cruz et al., 2024). include autochthonous and parautochthonous assemblage proposed by kidwell et al, (1986), and materials that are the result of taphonomic-accumulation in the sense of fernández et al., (2002). there are the following kinds of deposits formed by biogenic activity and productivity: deposits of direct production; deposits formed by biological modification of materials; deposits created and built by organisms; and finally, deposits formed by microbial activity and productivity (cruz et al, 2024). biogenic deposits and rocks were proposed and used by: grabau (1904) biogenic carbonate rocks; krynine (1948) biogenic products; tyrrel (1978) as organic (deposited through the agency of organisms); wright (1992) as biological (in situ) deposits and rocks; biolithite by folk (1974); boundstone by dunham (1962); bindstone by embry and klovan (1971). example of biogenic deposits are the following: 1. biogenically produced of directly skeletal materials and accumulation of corporal products release by natural physiological processes: mantle-shaped accumulation of corporal detritus from the vegetal wood forest; mantle-shaped accumulation of corporal radiolarians, coccolithophorids and planktonic foraminifera. 2. irregular-shaped or tabular deposits formed by biological modification of materials: aggregates of corporal detritus produced by predation-disarticulationfragmentation of previously formed corporal entities (skeletons, tissue, etc.), e.g., deposits in caves and rock shelters of human and other predators (stiner, 1991; moncel et al., 2008; diedrich, 2011; enloe, 2012); faunal accumulations (samper and martinez, 2014); shell-crushing accumulation by durophagous and opportunistic-generalists, most probably fishes (zaton and salomon, 2008). original sedimentary deposits, or also sand or finer regolith that are modified by intense biological activity (figure 5); deposits formed by animal activity that produces debris dumps during the construction of their galleries, and throw out the excess material, deposits called biodetritus waste-dumps (figure 6) (cruz et al, 2024). 3. constructions produced by living organisms (plants, animals, and microorganisms), structures made by secreting skeletons and corporal entities directly, e.g., mound-shaped bioherms; and also, construction made using external materials (cruz et al, 2024), e.g., ant nests (tschinkel, 2003); fossil bee nest (genise et al, 2002); jurassic termite nests (hasiotis, 2003); among other. bioherms are classified as biolithites by folk (1974) and as boundstone by dunham (1962). figure 5. intense bioturbation by arthropods (thalassinoids) formed on early diagenesis. horizontal ramified to t-branched box-works, mazes, and shafts, unlined and unornamented. arenisca dura formation, upper cretaceous, iza, colombia. figure 6. biodetritus waste-dumps of loose materials, removed during maintenance and cleaning of their shelter by ants with entrance and ventilationshaft (arrow) to the shelter galleries of the colony (piedecuesta, colombia). 4. there are numerous kinds of structures formed by the mediation of microbes, e.g., microbialite, helictites, etc., (tisato et al., 2015; sanchez et al., 2014; noffke et al., 2001; noffke, 2014); microbial peloids (flügel, 2004); embryo fossilization (raff et al., 2008); and include microbially induced sedimentary structures (noffke et al., 2001; noffke, 2014). microbially induced sedimentary structures are not considered traces, and neither are structures resulting from bioclaustration (bertling et al., 2020). microbialite is a generic name for the benthic sedimentary materials and deposits of mineralmicrocrystalline (diameter <16 μm) that is formed with the mediation of microbes by accretion-coated or mineral precipitation accumulation (initially proposed by burne and moore, 1987). being formed in situ, a microbialite can be seen as a type of boundstone where builders are microbes, and the precipitation-accumulation of carbonate is biotically induced instead of forming tests, shells, or skeletons. microbialites can also be defined as microbial mats of schieber et al (2007); and biofilm of krumbein et al (2003). diagenetic transformation-modification (dtm) deposits and rocks. the diagenetic transformation of original deposits can conserve the original distribution of materials with some minor changes (figure 7). also, diagenetic processes can form a new distribution of materials as observed in the following cases: when occurs solution-collapse; due to the effect of soft sedimentary deformation structures (ssdss); by extensive recrystallization processes; or when other diagenetic changes may destroy the original textures of nonsiliciclastic sedimentary rocks and produce crystalline textural fabrics that are largely of secondary origin (boggs, 2006). dtm sedimentites include diagenetic deposits of wright (1992); crystalline carbonate rocks (in part) of dunham (1962); authigenic rocks (in part) of friedman and sanders (1978), e.g., diagenetic dolostone, chert, evaporites, etc. diagenetic breccias are an example of diagenetic deposits formed by transformation during diagenesis of the original deposit structure. diagenetic breccias include: cement-stones, spar-stones, and microspar-stones of wright (1992); crackle-breccias of morrow (1982); diagenetic crack structures of chen et al (2010); sedimentary dyke infilling breccia (sdib) results from a soft sedimentary deformation (ssd), after hydraulic fracturing (morrow, 1982); sedimentary collapse-breccias formed during a solution-collapse diagenetic event (morrow, 1982). cement-stones of wright (1992) are limestone composed almost totally of fibrous cement (commonly replaced and/or recrystallized), in which grains or insitu biogenic material does not constitute a framework; grainstone of mazzullo and birdwell (1989), are grainification of calci-mudstones to packstones, caused by desiccation. 191general classification and nomenclature of sedimentary deposits and rocks, a review limestones (not dolostones) which have lost their depositional fabric because of complete obliterative recrystallization or replacement can be divided into two types based on crystal size (spar-stones, and microspar-stones) (wright, 1992). a crackle breccia denotes a fabric in which there is little relative displacement of fragments (morrow, 1982), or even the result of expansion due to the hydration of anhydrite (see bassett, 1961; morrow and meijer-drees, 1981), or tidal-flat, desiccation-crack origin on early diagenesis. a b figure 7. quartz coarse sandstone with some minor diagenetic changes, e.g., quartz cement overgrowth is observed. geology uis, so16 sample. 10x, a with xpl and b with ppl. rfs: rock fragments, qtz: quartz, ogs: overgrowth. clastic deposits (cds) are deposits formed mainly by clastic materials, materials that are eroded, transported, and deposited. clastic carbonate rocks of grabau (1904); clastic or fragmental of krynine (1948); sedimentary (transported as solid in suspension) of tyrrel (1978); depositional rocks of wright (1992); depositional mudstones, wackestones, packstones, and grainstones of dunham (1962); terrigenous rocks (mudstones, sandstones, and conglomerates) of folk (1974); terrigenous (extrabasinal) and pyroclastic rocks of friedman and sanders (1978); siliciclastic rocks (sandstones, conglomerates and shales) formed by particles derived by the weathering breakdown of older rocks (boggs, 2006); and pyroclastic volcanism deposits (boggs, 2006). clastic materials (terrigenous and pyroclastic deposits) correspond to allochthonous sediments of selley (2000). there are two kinds of clastic materials: ancient-derived and juvenile. ancientderived clastic (adcms) are materials inherited and conserved from rocks, e.g., by weathering or tectonic processes, etc., and eroded, transported, and deposited as solids. adcms term was proposed by friedman and sanders (1978) when they wrote “solid breakdown products of older deposits”. ancient derived-conserved clastic materials are rock fragments (rfs) and minerals, e.g., quartz, zircon, and fossils (remanie, zombie). ancient derived-conserved clastic materials include terrigenous components of folk (1974), those substances derived from erosion of land area outside the basin of deposition and carried into basin as solids; extrabasinal materials (terrigenous) or solids breakdown products of older deposits and derived from outside the basin deposition (friedman and sanders, 1978). juvenile clastic materials consist of the following materials: biogenic (shells, bones, fragments of plants, etc.), and abiogenic materials (recently formed rfs, minerals, etc.). materials that are transported after their initialrecently formation, because of that named as juvenile bioclasts, juvenile crystalclast, juvenile rfs, etc. juvenile materials also include weathering argilo-phyllosilicates, because there are new materials formed initially by weathering, e.g., kaolin, smectite, etc. the conserved and the transformation material formed by weathering processes are presented by kew and gilkes (2006), among others. examples of ancient clastic deposits are fluvial litho-conglomerate, glacial litho-breccias, colluvial litho-breccias, fluvial sandstones (quartz, feldspathic-quartz, lithic-quartz, etc.), lacustrine argilo-phyllosilicate deposits, etc. the deposit of the ebro basin (spain) is the predominantly calcareous conglomerates (uncastillo fm., litholimestone conglomerate) of the athenianmiddle aragonian (villena et al, 1992). examples of juvenile clastic deposits are the following: beach breccia deposits of shell fragments of recently bivalves, echinoderms, cnidarians, etc.; beach breccia deposits of fragments of recently trunk of tree; eolian-sand deposits formed by recently gypsum-crystals, e.g., white sand eolian deposits of new mexico. eolian deposits of gypsum have been reported since the jurassic, samples of ancient eolian sand gypsum deposits are distributed all along the usa (stone et al, 1920). mixed sedimentary deposits (mxds). sedimentary deposits in which their formation is dominated by more than one sedimentary process, the following are examples of mxds: biogenic production and chemical accumulation; biogenic accumulation and clastic sedimentation, e.g., stromatolite deposit; hemipelagic marine deposits; residual-tectonic; deflation-residual (deflational or lag deposits of krapf and sheard, 2018; desert pavement of mcfadden et al., 1987); tectonic-hydrothermal of wang et al. (2020); tectonic-diagenetic deposits of reid et al. (2013) (tectonic-diagenetic breccias of haldar, 2020); among others. biogenic-clastic deposits e.g., stromatolite formed by corporal algal or bacterial accumulations with clastic materials sedimentation, described by friedman and sanders (1978). general classification represented in the tetrahedral diagram three independent variables can be represented by the corners of an equilateral triangle, four variables can be replaced by only one tetrahedron graph and without any projection errors. it can be possible to use tetrahedral diagrams for the general genetic classification of sedimentary deposits and rocks using only four parameters: biogenic, clastic, chemical, and detrital materials and structures (figure 8). nowadays analysen-tetraeder software (keesmann and schmitz, 2002) overcomes the difficulties of the calculation of data positions and of understanding the spatial distribution of analytical data in tetrahedral graphs, this software could be free download in https://www.monkeybreadsoftware.de/ analysen-tetraeder/. biogenetic materials and structures clastic materials and structures 50% 50% 50% chemical materials and structures detrial materials detrial depositis chemical depositis clastic depositis biogenic depositis figure 8. general genetic classification of sedimentary deposits based on detrital, clastic, biogenic, and chemical materials. after wright (1992) classification of limestone (depositional, biological, and diagenetic). compositional-mineralogical classifying of sedimentary deposits and rocks. complementary to the genetic general classification of sedimentites it is necessary propose a new compositional-mineralogical general classification of them, using only compositional-mineralogical terms (table 6), after merril (1897); krynine (1948); pettijohn et al (1973); folk (1954, 1974); friedman and sanders (1978); hallsworth and knox (1999); boggs (2006, 2009); and milliken (2014). the compositional classification of sedimentites can become quite complex due to variations in mineral content. hallsworth and knox (1999) use compositional-mineralogical parameters in their classification in some cases, e.g., carbonate, phosphorites, ironstones, organic-rich, non-carbonate salts, miscellaneous hydroxide, oxide, and silicate; but also use mixed compositionalmineralogical with genetic and textural-interpretative parameters in other cases, e.g., siliciclastic, phosphate and iron sediments, non-clastic siliceous, based on grain size and non-clastic siliceous; and genetic parameters only in another case, e.g., with volcaniclastic debris. https://www.monkeybreadsoftware.de/analysen-tetraeder/ https://www.monkeybreadsoftware.de/analysen-tetraeder/ 192 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez the common minerals in sedimentites are, e.g., quartz, amorphous silica, feldspar, calcite, dolomite, phosphates, argilophyllosilicates, micas (muscovite, etc.), oxides and hydroxides of fe, al, mn, sulfates and salts, etc. they are found as monominerals but also associated with other minerals in rock fragments (rfs or lithic), or forming complex structures such as shells, oolites, pisolites, oncolites, etc. there are also accessory minerals (commonly heavy minerals): magnetite, ilmenite, leucoxene, gold, diamond, garnet, apatite, pyroxene, amphibole, kyanite, rutile, zeolites, staurolite, etc. the information that can be obtained by investigating the composition of sedimentary deposits and rocks includes the knowledge on the source area of sedimentary clast constituents (sink to source), the information concerning the sedimentary knowledge of formed deposits, and the information that reflects the post-sedimentary transformation of the sedimentary deposits. compositional-mineralogical classification of sedimentites (cmcss), given the great variety of materials present in them, must be based on the groups of fundamental minerals proposed by dana and ford (1922) and dana and hurlbut (1959). cmcss using the following sedimentary minerals and relatives are presented on table 6: silicates (quartz, amorphous silica, feldspar, zircon, illite, glauconite, smectite, kaolinite, muscovite, etc.); carbonates (calcite, aragonite, dolomite, magnesite, siderite); phosphates (apatite, wavellite, etc.); iron, manganese, aluminum and titanium oxides and hydroxides (limonite, goethite, ilmenite, pyrolusite, spinel, bauxite, etc.); sulphates and salts (gypsum, halite, etc.); organic rich materials (coal, carbonaceous, bitumen, asphalts, kerogen, petroleum, etc.); nitrates and ammonium (nitratine, nitre, etc.), organically bound nitrogen associated with sediment, also forming nitrate deposits accumulated in arid and semi-arid regions, and elevated nitrogen concentrations in residual-regolith (holloway and dahlgren, 2002); sulfides (pyrite, pyrrhotite, etc.) materials formed by biological sulfate reduction in marine anoxic environments (trudinger, 1981); borates (borasite, borax, etc.) borate deposits interlayered in continental volcaniclastic rocks (helvaci and alonso, 2000); and native elements minerals (gold, diamond, graphite, silver, sulphur, uranium, among others), e.g., nanodiamonds from the impact event, and diamond, gold, platinum, copper in placer deposits. table 6. general genetic and compositional-mineralogical classification of sedimentary deposits and rocks. compositional-mineralogical classification. after merril (1897), krynine (1948), pettijohn et al (1973), folk (1974), friedman and sanders (1978), hallsworth and knox (1999), boggs (2006, 2009), and milliken (2014) 1.silicate sedimentites, including amorphous silica, chert, silcretes, quartz, feldspar, argillaceous and heavy silicates, among others. 2.carbonate sedimentites, including limestones, dolostones, among others. 3.phosphorites, sedimentites rich in phosphorous compounds. 4.iron, manganese, aluminum and titanium oxide and hydroxide rich sedimentites. 5.sulfate and salts rich sedimentites. 6.organic rich sedimentites, include solids or semisolid (tars, bitumen, asphalts, kerogens); also include chonscomponents (peat, humus and coal), liquid (petroleum oil), and gasses (methane). 7.nitrates rich sedimentites. 8.sulfides rich sedimentites. 9.borates rich sedimentites. 10.native elements rich sedimentites. 11.mixed compositional-mineralogical sedimentites (silicatecarbonate, silicate-phosphorite, oil shale, oil sands and so on). note: the last 7 to 10 groups of sedimentites are less abundant in the geological record but economically important. note: sedimentites also have natural gasses and liquids filling their porous space, including atmospheric air, methane and other natural gasses, meteoric water, organic liquid and other natural liquid. this compositional-mineralogical classification is based also in the general compositional-mineralogical classification of soils deposits (residual regoliths) (cruz et al, 2020). silicate-rich sedimentary deposits and rocks are sedimentites consisting fundamentally of silicates (table 7). the silicate sedimentary materials are monomaterials (detritus and clasts), biogenic and bioclast materials, chemical constituents, or diagenetic materials. silicate monomaterials include the following materials: light sedimentary silicates as amorphous and microcrystalline silica and quartz, feldspar, muscovite, argillaceous weathering materials (illite, glauconite, smectite, kaolinite, etc.); and heavy sedimentary silicates (zircon, garnet, sillimanite, kyanite, etc.). silicates are essential material forming residual-argilo-phyllosilicates regolith, residual-microcrystalline amorphous silica regolith, clastic, biogenic, and chemical microcrystalline-silica deposit. most of the silica set free during weathering hydrolysis goes into solution as silicic acid (h4sio4); however, some of the silica may separate as colloidal or amorphous sio2 and be left behind during weathering to combine with aluminum to form clay minerals (boggs, 2006). microcrystalline-silica are also the essential materials of diagenetic transformation of sedimentary deposits (e.g., chert). silicate minerals dominate in most residual-regolith (karathanasis, 2009), enrichment or accumulation of minerals due to weathering, e.g., microcrystalline amorphous silica in rich paleo-regoliths or silcretes. some silicate sedimentary materials zircon, monazite is found in placer deposits; argilo-phyllosilicate-residual deposits, e.g., argillaceous-rich or clayrich eluvial-illuvial deposits of turniski et al, 2023. silicate-clastic deposits and rocks, e.g., quartz arenites, arkoses, argilo-phyllosilicate-clastic deposits, etc. silicate-biogenic materials conformed by test of porifera, radiolarians, diatoms, algae, phytoliths of amorphous silica, e.g., phytolith-rich tropical regolith, formed on the volcanic island of reunion (indian ocean) (lacroix, 1936; riquier, 1960). crystal chemical of “microcrystalline” amorphous silica precipitated on lagoons, e.g., coorong lagoons western australia (friedman and sanders, 1978). diagenetic silicate materials occur mainly as cements like chalcedony or feldspar or quartz or argilo-phyllosilicates. silicate rocks include chert by diagenetic transformation of volcaniclastic deposits, e.g., chert of saldaña formation, neiva basin (colombia) (cruz, 1983); extrabasinal silicate rocks as shales (argilo-phyllosilicates rocks), sandstones, arkoses, wackes of friedman and sanders (1978); siliciclastic and non-clastic siliceous of hallsworth and knox (1999). carbonates-rich sedimentary deposits and rocks. sedimentites consisting fundamentally of calcite, aragonite, dolomite, magnesite, siderite (ferrous carbonate). carbonates are present in sedimentites as detritus, clasts, biogenic, chemical, and diagenetic materials (table 8). carbonate sedimentites include limestone and dolomites (syngenetic, diagenetic and epigenetic dolostones) of folk (1974); and boggs (2006); carbonate sediments and rocks of hallsworth and knox (1999), among others. carbonates are forming sedimentary detritic materials as, e.g., residual regolith (microcrystalline calcite rich residual regolith or calcretes), and tectonic regoliths. carbonate-rich residual-regoliths were characterized by enrichment with microcrystalline-carbonate, e.g., calcrete. the enrichment with calcium carbonate is not only due to inheritance from calcium carbonate-rich parent material, but also translocation of calcium carbonate within residualregolith profiles, and possible aeolian silt contribution (incorporate materials). translocation latter depending on regolith stratification (kowalska et al, 2019). residual detrital regoliths include calcretes of friedman and sanders (1978); calcitic and dolomitic caliche of folk (1974). clastic carbonate sedimentites: rudites of juvenile bioclastic and terrigenous calcareous-rfs. bioclastic rudites (transported juvenile biogenic shell of molluscan, brachiopods, etc.). clastic carbonate sedimentites include depositional of wright (1962); clastic limestone (mudstone, wackestone, packstone, and grainstone) of dunham (1962); intraclastic, and bioclastic limestones of folk (1974). biogenic-juvenile calcareous deposits made by text of invertebrates mollusca, foraminifera, echinodermata, etc. continental shelf carbonate deposits in equatorial to tropical environments because of their high biological activity, in the absence of continental input sediments, are biogenic-calcareous dominated materials, forming e.g., reef carbonate biogenic structure. biogenicjuvenile calcareous deposits include biogenic limestones (boundstones) of dunham (1962); biological limestone of wright (1992); pelsparites, pelmicrites of folk (1974). 193general classification and nomenclature of sedimentary deposits and rocks, a review table 7. classification of silicate sedimentites. based on the type of silicate: detrital, clastic, biogenic, chemical and diagenetic. main group description specific name detrital silicate (des) deposits, if des materials are the main component. residual, tectonic, lava cooling, etc. residual-argilo-phyllosilicates (kaolin, smectite, illite, glauconite, etc.) e.g., residual-clay rich regolith (turniski et al, 2023). residual-microcrystalline amorphous silica by solution-precipitation e.g., residual-silcrete deposits (wopfner, 1983). residual inherited-conserved quartz with juvenile argilo-phyllosilicates and/or juvenile iron-manganese-aluminum-titanium oxides and hydroxides e.g., laterite-bauxite regolith (kew and gilkes, 2006); residual regolith (avendaño and cruz, 2017). clastic silicate (cls) deposits, if cis materials are the main component. transported materials: light minerals (quartz, feldspar, calcite, etc.); and heavy minerals (zircon, amphibole, biotite, etc.) e.g., quartz arenite, arkose, etc. biogenic silicate (bs), if bs materials are the main component. amorphous silica test of radiolarian, diatoms, and porifera e.g., fine silty clay biogenic silica deposits (oehler, 1979). chemical silicates (chs) deposits, if chs materials are the main component. e.g., microcrystalline amorphous silica precipitated from solution in coorong lake, australia (friedman and sanders, 1978). e.g., fine chemical silica deposits. microcrystalline zeolite precipitation when pyroclastic materials with saline or alkaline waters, friedman and sanders (1978). e.g., fine chemical zeolite deposits. diagenetic silicate (dis) deposits, if dis materials are the main component. e.g., cements of microcrystalline amorphous silica, quartz, feldspar, chalcedony, and argillic, etc. illite deposits by diagenetic transformation of kaolinite and smectite (boggs, 2006) e.g., illite claystone; etc. table 8. classification of carbonate sedimentites. based on the type of carbonate: detrital, clastic, biogenic, chemical and diagenetic. main group description specific deposits detrital carbonate (dec) deposits, if dec materials are the main component of residual, tectonic, etc. e.g., inherited-conserved calcite release by weathering residual-detrital ancient calcite deposits. residual-microcrystalline calcite by solution-precipitation calcrete deposits (soil carbonates of dart, 2009; calcisol of mack et al, 1993; caliche-calcrete of lamplugh, 1902). tectonic calcareous regoliths fragmentation due to tectonic stress of calcareous rock sequences. tectonicbreccias (morrow, 1982; boggs, 2006). clastic carbonate (clc) deposits, if clc materials are the main component: ancient limestone rfs and remanie fossils (extraclast of scholle and ulmer, 2003). ancient (terrigenous) lithic-limestone breccia with remanie fossils juvenile bioclastic carbonate wackestone, packstone, grainstone of dunham (1962), e.g., beach-bioclastic rudite of bivalves, foraminifera, etc. biogenic carbonate (bc) deposits, if bc materials are the main component: autochthonous and parautochthonous biogenic carbonates bioherms and biostromes (cumings, 1932; chevalier, 1961; klement, 1967; zonneveld et al, 2012). auto-biostrome and parabiostromes by kershaw (1994) chemical carbonate (chc) deposits, if chc materials are the main component: calcite and aragonite crystals by chemical precipitation from saturated solution. chemical carbonate deposits (friedman and sanders, 1978). diagenetic carbonate (dic) deposits, if dic materials are the main component: limestone by recrystallization, e.g., calcite cements, diagenetic dolomite, etc. crystalline limestone (e.g., dunham, 1962; banner and wood, 1964) chemical sedimentites of crystal chemical calcite precipitated, e.g., on sabkha condition environments. include chemical crystalline carbonate limestones of dunham (1962); orthochemical limestones, e.g., oosparites, and oomicrites of folk (1974). diagenetic sedimentites include, e.g., diagenetic limestone and dolomites of friedman and sanders (1978), wright (1992), and boggs (2006). phosphates rich sedimentary deposits and rocks are sedimentites consisting fundamentally of phosphate minerals as apatite, wavellite, etc., (table 9). phosphates are forming detrital deposits like residual regolith (soil or paleosols), biogenic (figure 9) and bioclastic materials of corporal bones, teeth, or bio-depositional materials as excrements and urine, crystal chemical and authigenic of phosphatic rich materials. detrital residual-regolith, where weathering causes dissolved phosphate from primary minerals (especially as apatite, calcium phosphate, tiessen et al, 1984) to precipitate with some cations and lead, e.g., to the neo-formation of calcium phosphate in alkaline residual-regolith (beck and sánchez, 1994); adsorbed phosphate by functional groups of iron or aluminum oxides to form thermodynamically stable complexes (bortoluzzi et al., 2015; fink et al., 2016). detritus inherited and conserved apatite from weathering of igneous and metamorphic rocks. phospho-detritus represents phosphate material formed 194 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez by the disintegration (physical breakdown) of larger phosphate materials by very active mechanical actions and if transported and deposited then phosphoclast. phosphate rich sedimentites include biogenic and bioclastic phosphates deposits with bones, teeth, pellets, coprolite, cololite, etc. phosphate rich deposits can form by biological deposition of excrement (schnug et al, 2018; carreiro et al, 2021); or form biologically active organic compounds that remain as organic phosphates in regolith (conte et al., 2002; martinazzo et al., 2007; dodd and sharpley, 2015). also, predation phosphate detritus, phosphate material formed by the disintegration (physical breakdown) of larger phosphate materials (bones) by very active predation biological processes. diagenetic phosphorite, include coated chemical-phosphorite, phosphate coated grains formed by accretion of cryptocrystalline phosphate to any detrital material. phosphates or phosphorites (the two terms are synonymous), here referred to as sedimentary phosphate deposits and rocks, are composed of the mineral apatite. this rock-building mineral consists of three kinds, which form an isomorphous series: (1) fluorapatite [ca5(po4)3f], (2) chlorapatite [ca5(po4)3ci] and (3) hydroxyapatite [ca5(po4)3oh]. phosphate sedimentary rock whose essential mineral component is usually a fluorapatite carbonate [ca5(po4)3(f, co3)], a variety of apatite known as francolite. isotropic cryptocrystalline fluorapatite carbonate (francolite) is the basic raw material for many phosphorus-containing compounds on which modern technological developments depend. when the apatite-like phase cannot be identified, the name colophane is commonly applied. ca, p (or po4), and f may be substituted by other elements. some of these substitutions are potentially of economic interest, because many trace elements are present in higher-than normal concentrations. phosphate rock occurs on the modern sea bottom and in ancient deposits. factors that favor phophoritization include a broad continental shelf adjacent to an ocean, slow shallow marine sedimentation, low terrigenous input, and high organic productivity (selley et al, 2005). phosphate deposits are of three types: bedded, placer, and guano (selley et al, 2005). guano is the youngest phosphate rock, this is a fertilizer rich in phosphates and nitrates that forms from the accreted excreta of birds and bats (selley et al, 2005). phosphate-rich sediments are present on the modern continental shelves off southwest and northwest africa, southeastern united states, southern california, northern south america (eastern mountain range of cundinamarca and boyacá, colombia), eastern australia, and eastern new zealand. phosphate rock occurs as nodules on the modern sea bottom extending from shallow to great water depths. such nodules occur off the coast of california. in some nodules, the concentration of phosphate reaches 95%, but in others, abundant impurities of various marine products are included, such as organic matter, siliceous tests of organisms, calcareous shells, some of which are partly phosphatized, (vertebrate bone) shark teeth, and various detrital materials. table 9. classification of phosphate sedimentites. based on the type of phosphate: detrital, clastic, biogenic, chemical, and diagenetic main group description specific deposits detrital phosphate (dep) deposits, if dep materials are the main component. e.g., phosphatic rfs and biogenic ancient phosphates bones, and teeth. phosphatic rfs, etc., release by weathering from ancient rocks. detrital ancient-derived phosphatic lithic breccia, etc. clastic phosphates (clp) deposits, if clp materials are the main component. phosphatic rfs and biogenic ancient and juvenile phosphates bones, and teeth transported and deposited (teleki et al, 2012). fluvial ancient-derived phosphatic-lithic breccia; beach juvenile shark teeth rudite. biogenic phosphates (bp) deposits, if bp materials are the main component. autochthonous and parautochthonous bones, teeth, and excrements. parautochthonous bone deposits of elephants, mastodon, etc., (santona et al, 2005; holen et al, 2017), e.g., ambrona site (figure 9). chemical phosphates (chp) deposits, if chp materials are the main component. microcrystalline phosphates precipitated from solution phosphatic deposits on marine shelf (selley, 2000). diagenetic phosphates (dip) deposits, if dip materials are the main component. recrystallization, and solution, remobilization, and crystallization of phosphates (blatt and tracy, 1996) microcrystalline-diagenetic phosphatic rocks nodules (garcia et al, 2020). figure 9. ambrona site of large vertebrates (elephant graveyard type), e.g., palaeoloxodon antiquus skeletal accumulation (medinaceli, spain) described by santona et al. (2005). 195general classification and nomenclature of sedimentary deposits and rocks, a review ancient deposits of phosphate rock occur in extensive phosphogenic provinces. these provinces include (1) the upper precambrian province of central and southeast asia; (2) the cambrian province of central and southeast asia, extending into northern australia; (3) the permian province of north america, specifically the phosphoria formation, which extends through wyoming, utah, colorado, idaho, montana, and nevada; (4) the jurassiclower cretaceous province of eastern europe; (5) the upper cretaceouseocene province of the middle east and north africa (morocco, egypt, israel) extending into west africa and the northern part of south america; and (6) the miocene province of the southeastern united states. each of these provinces extends for several thousand square kilometers. in ancient deposits, phosphate rock may occur (bone, fragments, sand-sized pellets of possible fecal origin (100 to 400 μ., in diameter), ooids, pisolites, phosphatized tests of various calcareous-shelis, and large pellets (2 to 5 mm, which have been termed coprolites), also diagenetic peloids and intraclast. phosphate rocks are a vital resource for world food supply and security. they are the primary raw material for phosphoric acid and fertilizers used in agriculture and are increasingly considered to be a potential source of rare earth elements (el bamiki et al, 2021). phosphate rocks occur either as sedimentary deposits or igneous ores associated with alkaline rocks (el bamiki et al, 2021). about three-quarters of the world’s supply of phosphates comes from sedimentary deposits (selley et al, 2005). phosphate sedimentites include phosphate rock of friedman and sanders (1978); phosphate-sediments and phosphorites of hallsworth and knox (1999); and sedimentary phosphorites of boggs (2006). iron, manganese, aluminum and titanium (imnaltit) oxides and hydroxides rich sedimentary deposits and rocks are sedimentites consisting fundamentally of iron oxide and hydroxide materials (goethite, limonite, ferrihydrite), manganese oxides (pyrolusite, lithiophorite, hollandite, and birnessite), aluminum hydroxide (gibbsite), iron and titanium oxide (ilmenite), magnesium/aluminum oxide (spinel), and titanium oxides and hydroxides (rutile, anatase, etc.) (table 10). imnaltit oxide and hydroxide are forming genetic materials like residual regoliths (ironmanganese rich nodules, lateritic soils and paleosols), crystal chemical and authigenic of imnaltit oxide and hydroxide rich materials. imnaltit sedimentites include manganese, iron-rich sedimentary rocks of friedman and sanders (1978); iron-sediments and ironstones of hallsworth and knox (1999); iron bearing sedimentary rocks of boggs (2006). pyrolusite is important as an ore of manganese (reidies, 2002). table 10. classification of iron, manganese, aluminum and titanium oxides and hydroxides sedimentites. based on the type of oxide and hydroxide: detrital, clastic, biogenic, chemical, or diagenetic. main group description specific deposits detrital oxides and hydroxides (deoh) deposits. iron and ferromanganese oxides nodules release from weathering iron and ferromanganese oxides nodules detrital breccias pedogenic ferromanganese nodules (huang, 2022); ironoxide concretion from early diagenetic origin (loope et al, 2012). clastic oxides and hydroxides (cloh) deposits. iron and ferromanganese oxides nodules transported and deposited clastic breccias of ferromanganese-oxides nodules biogenic oxides and hydroxides (boh) deposits. microbial polymetallic nodules formed on the deepsea bottom (tully and heidelberg, 2013; sujith and gonsalves, 2021; and references therein) chemical oxides and hydroxides (choh) deposits. ferromanganese nodules found in the seafloor without biological mediation. ferromanganese-nodules breccias (verlaan and cronan, 2022) diagenetic oxides and hydroxides (dioh) rocks. iron and ferromanganese oxides nodules by diagenetic solution-precipitation. iron-oxide concretion from late diagenetic origin (loope et al, 2012) sulfates and salts rich sedimentary deposits and rocks. sedimentites consisting fundamentally of sulfates (anhydrite and gypsum, figure 10), and chlorides (particularly halite, sylvite, and carnallite) (table 11). sulfates and salts rich sedimentites include non-carbonate salts of hallsworth and knox (1999); evaporites of boggs (2006) and friedman and sanders (1978). saltrich deposits occur in all continents and under almost all climatic conditions. their distribution, however, is relatively more extensive in the arid and semiarid regions compared to the humid regions (deocampo and jones, 2014). salts released through weathering in the arid regions with limited rainfall are usually deposited at some depth in the residual-regolith profile, the depth depending on such factors as the water retention capacity of the materials, seasonal, annual, and maximum rainfall, etc., (yaalon, 1975). samples of chemical sulphate and chlorides (salt rich deposits) are presented by kendall et al., (1994); zamannejad et al., (2013); and deocampo and jones (2014). a b figure 10. sedimentary rock composed almost entirely of anhydrite (an). this mineral is distinguished from gypsum by its greater relief and strong birefringence. oxs: oxides. geology-uis, s023 sample, 10x, a. xpl and b. ppl. table 11. classification of sulfate and salt sedimentites. based on the type of sulphate and salt: chemical, clastic and diagenetic. main group description specific deposits chemical sulfate and salt (ch ss) deposits. directly precipitation accumulation of crystals from solutions sabkha deposits of gypsum, salt, etc. (friedman and sanders, 1978). clastic sulfate and salt (cl ss) deposits. chemical crystals transported and sedimented, e.g. forming gypsum dunes gypsum sand-dunes (e.g., white gypsum dunes usa). gypsum dunes have been reported since jurassic (see stone et al, 1920) diagenetic sulfate and salt (di ss) deposits. recrystallization of crystalline sulfate and salt deposits medium to coarse crystalline gypsum rocks (e.g., evaporites illustrated by adams et al, 1984). organic rich sedimentary deposits and rocks. sedimentites consisting fundamentally of solids or semisolid (tars, bitumens, asphalts, kerogens), also include carbonaceous deposits and rocks (peat, humus and coal), liquid (petroleum oil), and gasses (methane) (table 12). sedimentary materials formed by degradationtransformation-maturation process of corporal materials (organs and tissues of lipid, protein, carbohydrate and ash contents, nail, hair, hoof, and horn of protein composition). organic rich sedimentites include intrabasinal carbonaceous rocks (lignite, subbituminous, bituminous, sub-anthracite and anthracite) of friedman and sanders (1978); organic rich sediments and rocks of hallworth and knox (1999); carbonaceous sedimentary rocks (coal, oil shale, bitumen) of boggs (2006). most sedimentites, including rocks of precambrian age, contain at least a small amount of organic matter consisting of the preserved residue of plant or animal tissue (boggs, 2009). residual regoliths with organic content higher than 30% are termed highly organic regoliths or peats. many residual regoliths 196 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez have a superior-external organic mantle (o horizon) and an underlaying a horizon, both of which have the greatest accumulation of organic compounds. organic matter is a complex mixture of organic compounds of carbonhydrogen-oxygen-nitrogen, and sulphur (chons), produced originally by living organisms (plant, animal, and microorganisms). residual regoliths contain chonss at various stages of decomposition. organic compounds are named so because they are associated with living organisms. a biogenic organic compound (boc or bchons) is the general term used to describe a mixture of organic compounds such as plant alkanes, sterols and sterones, fatty acids and fatty alcohols, and waxes and wax esters biosynthesized by living organisms. the main types of organic compounds are carbohydrates, lipids, nucleic acids, resin, and proteins. table 12. classification of chons rich sedimentites. based on the type of organic materials (chons): biogenic, clastic or diagenetic. main group description specific deposits biogenic chons deposits remains of plants, insects, worms, etc. produced, released, and accumulated directly over the external lithosphere. detrital-biogenic-chons materials rich deposit (horizon o of regolith) clastic chons deposits biogenic chons remains transported outside of their original formation place. clastic-chons rich deposit (e.g., carbonaceous organic debris deposits of friedman and sanders, 1978). diagenetic chons rocks biogenicbioclastic of corporal (plant, animal, and microorganisms) rich deposits transformed into coals, hydrocarbons, etc. coals results from carbonization of detrital biogenic plant rich deposits. some coals result from carbonization of clastic rich-plants deposits. argillite-, sandand rudite-hydrocarbons rich rock (oil shale and sandhydrocarbon rich rock, etc., described by hydrocarbon companies; tucker and wright, 1990; friedman and sanders, 1978; selley et al, 2005; boggs, 2006; among others) nitrates and ammonium rich sedimentary deposits and rocks. sedimentites consisting fundamentally of nitratine, nitre, etc. organically bound nitrogen associated with sediment, also forming nitrate deposits accumulated in arid and semi-arid regions, and elevated nitrogen concentrations in soil (holloway and dahlgren, 2002), also present in biodepositional liquid materials (urine). the nitrate concentration in regoliths or water is generally low, indicating that the nitric nitrogen formed is quickly taken up by plant assimilation or microbial transformations. the availability of usable nitrogen thus often appears to be a limiting factor in these transformations. however, nitrate concentrations in regoliths may be higher in media enriched with organic matter, either naturally or under human action. thus, the accumulations of guano in chile and peru were a source of natural nitrates accumulated excrement of seabirds (schnug et al., 2018) or bats, used in agriculture. bat guano is found in caves throughout the world. nitrogen guanos are formed only under the special climatic conditions of the subtropicaledge tropical high-pressure belt with coastal deserts (schnug et al., 2018). guano is a highly effective fertilizer due to the high content of nitrogen, phosphate, and potassium. in colombia, especially in santander region, guano from breeding and processing chickens has been used since 1970 as a plant substrate fertilizer. sulphides rich sedimentary deposits and rocks. sedimentites consist fundamentally of (pyrite, pyrrhotite, etc.) materials formed by biological sulfate reduction in marine anoxic environments (trudinger, 1981). sulphide minerals are compounds in which sulphur is combined as an anion with a metal (or semimetal) cation or cations. the sulphide in residual regoliths is mainly derived from the products of sulphate-reducing bacteria, which are currently responsible for oxidizing over half the organic matter flux reaching sediments, and pyrite is the mineral overwhelmingly produced (rickard 2012). in wetland regoliths and underwater sedimentary deposits of marine, brackish and freshwater systems, the strong phytotoxin sulphide may accumulate because of microbial reduction of sulphate during anaerobiosis, its level depending on prevailing edaphic conditions (lamers et al., 2013). the accumulation of reduced sulphur in sedimentary deposits of aquatic systems and permanent or riparian wetlands (including estuarine and marine) generally causes physiological toxicity stress for the community involved, such as plants, animals and microorganisms (bagarinao, 1992). other sulphides in regoliths are inherited-conserved from igneous and metamorphic rocks, e.g., galena, argentite, chalcocite, sphalerite, etc. sulphides are ore of different minerals and are commonly associated with regoliths. borates rich sedimentary deposits and rocks. sedimentites integrated mainly with borasite, borax, etc. the common borates minerals in borax-rich regoliths include kernite, colemanite or ulexite as well as a lot of rare minerals, e.g., rivadavite, ameghinite, ezcurrite, teruggite, macallisterite, inderite, inyoite, kaliborite, pandermite, and pinnoite. an example of borates rich deposits is saline (salt-containing) regoliths containing a high proportion of borax at a salt pan in the mojave desert, usa. borax is a white, monoclinic crystalline mineral composed of hydrated sodium borate. it forms during solar evaporation of saline water bodies, such as lakes and desert salt pans. economically significant deposits of boron-bearing minerals, mostly borate salts, are found in saline paleolake deposits in several areas around the world, most notably the famous borax deposits around boron, california, and the borate ores of turkey (deocampo and jones, 2014). borate deposits interlayered in continental volcaniclastic rocks (helvaci and alonso, 2000). native elements rich sedimentary deposits and rocks. sedimentites consist fundamentally of native metals (gold, silver, uranium, among others), and the nonmetals (sulphur, diamond, graphite) (table 13). weathering of ultramafic rocks can lead to the formation of residual-regolith profiles with high contents of fe, ni, co, platinum-group elements and gold (rivera et al., 2018). diamonds formed from the extraterrestrial impact event (ohfuji et al., 2015). diamond microcrystals found in impact melt rocks and impact breccias at the popigai impact structure in siberia (koeberl et al., 1997). diamond, gold, platinum, ilmenite, copper, among others, found in placer deposits (els and eriksson, 2006). placer gold, platinum and diamonds have been found in gravel deposits, e.g., mudaliar et al., (2007); and kvasnitsa et al., (2016). reduction of gypsum or of anhydrite by sulphate reducing bacteria forms native sulphur, which occurs in modern salt lakes or playas or in caprocks atop salt plugs (friedman and sanders, 1978). table 13. classification of native-minerals rich sedimentites. based on the type of nativeelements: detrital or clastic. main group description specific deposits detrital nativeelements minerals deposits. mineral enrichment by weathering of mineralized veins. residual-detrital placer deposit (els and eriksson, 2006; and references therein). clastic nativeelements minerals deposits. mineral enrichment by transport and depositional segregation of materials placer deposits in, e.g., eluvial, terrace, stream, beach, and marine offshore environments (selley et al, 2005; boggs, 2006; and references therein). mineral formation and enrichment by extraterrestrial impact event impact mineral-enrichment deposit (shukla and karma, 2018; pati et al, 2017; james et al, 2022, and references therein). 197general classification and nomenclature of sedimentary deposits and rocks, a review mixed compositional-mineralogical sedimentary deposits and rocks are sedimentites consisting fundamentally of two or more components, e.g., silicatecarbonate, silicate-phosphate, oil argilo-phyllosilicate (oil shale), oil sands and so on. silicate-carbonate sedimentites, e.g., fine to coarse quartz arenite with calcite as cement; also, mixed sedimentites with skeletal-calcite materials, clastic sand (quartz, feldspar, etc.), and clastic argilo-phyllosilicates (figure 11). silicate-phosphate sedimentites, e.g., siliceous-phosphate deposits (haddi et al., 2013). silicate-carbonate-phosphate sedimentites (scpss) with skeletal-calcite materials (fragments or complete), skeletal-phosphate (teeths, bones, intraclasts, etc.), clastic sand (quartz, feldspar, etc.) and clastic argilophyllosilicates. examples of scpss are the nodular layer of tour de croi, jurassic, france (adams et al., 1984). sedimentites containing abundant kerogen are popularly known as “oil shales”. oil shales deposited in some ancient lakes, such as the green river formation (friedman and sanders, 1978). general composition of oil shales includes inorganic substance and bitumens. oil-bearing shales are deposits that contain petroleum (tight oil) (dyni, 2006). oil sands, tar sands, crude bitumen, or bituminous sands, are either loose sands or partially consolidated sandstone containing a naturally occurring mixture of sand, clay, and water, soaked with bitumen, a dense and extremely viscous form of petroleum (dusseault, 2001). lithic rich sedimentary deposits and rocks are a special and common group of sedimentites consisting fundamentally of rock fragments (rfs) or lithic. rfs (sedimentology term) refers to any sedimentary polymaterial that conserves the original characteristics of their parental rocks (figure 12). rfs are common in detrital (in situ formation without transport) and in clastic (depositional) rocks, e.g., lithic-rudites (breccias and conglomerate) and lithic-sandstone. lithic-rudites are detrital and clastic deposits and rocks where rfs are the main constituent, e.g., lithicclastic-breccias, lithic-conglomerate, lithicimpact-breccias, lithic-pyroclasticbreccias, lithic-tectonic-breccias, lithic lava-coolingbreccias, lithic-peperites, etc. rfs are ancient-derived and new materials (juvenile). ancient-derived rfs are common in sedimentites (table 14). a b figure 11. mixed sedimentite, quartz fine-sand clasts (qtz), abundant fragments of skeletal calcite (scf), and calcareous clay materials (ccm). geology uis, s017 ed120-037-001 sample, 5x. a: xpl, b: ppl. ba figure 12. lithic conglomerate (rfs of metamorphic rocks and polycrystalline quartz) subrounded to subangular, longitudinal to concave-convex contacts between clasts, quartz overgrown, some argilo-phyllosilicate cement. rf: rock fragment. geology uis, dm-120-024-015 sample, 5x. a: xpl, b: ppl. table 14. classification of lithic sedimentites. based on the type of rock fragments: ancient-derived rfs, juvenile rfs or mixed ancient derived plus juvenile rfs. main group description specific name ancient-derived (a-d) rfs deposits, if a-d rfs materials (lithoclast of adams et al, 1984) are the main component. residual deposit with inherited-conserved rfs from substratum affected by weathering lithic ancient-residual detrital deposit (breccias, sandy breccias, brecciated sand) (blockfield deposits of goodfellow et al, 2014). tectonic deposit with inherited-conserved rfs from the substratum affected lithic ancient-tectonic affected detrital breccias (e.g., rasmussen et al., 2011). caldera-collapse deposit with inherited-conserved rfs from the substratum affected lithic ancient-derived caldera-collapse detrital breccias (e.g., aguirre-diaz, 2008). (continued) 198 luis enrique cruz-guevara, luis felipe cruz-ceballos, gladys marcela avendaño-sanchez main group description specific name ancient-derived (a-d) rfs deposits, if a-d rfs materials (lithoclast of adams et al, 1984) are the main component. rock-dissolution-collapse with inherited-conserved rfs from the substratum affected lithic ancient-derived collapse breccias (collapse breccias of friedman,1997; christiansen and sauer, 2001; haldar, 2020). clastic ancient-derived deposits with inherited-conserved rfs lithic ancient-derived clastic deposits (lithic-breccias, conglomerates, and lithic-sands). terrigenous deposits and rocks of folk (1974), boggs, (2006), etc.; extrabasinal rocks of friedman and sanders (1978). juvenile rfs deposits, if juvenile rfs materials are the main component. juvenile lava-cooling rfs from juvenile magma lithic juvenile lava-cooling detrital-breccias (volcanic breccias of morrow, 1982) pyroclastic deposits with juvenile rfs, and minerals pyroclastic juvenil breccias, etc., (coira et al., 2022). mixed ancient-derived (a-d) and juvenile rfs deposits, if a-d and juvenile rfs materials are the main component. peperites with inherited-conserved rfs from sedimentary deposits affected, and mixing with volcanic rfs peperite-breccias with a-d sedimentary rfs and juvenile volcanic rfs (peperite breccias of skilling et al., 2002). hydrothermal deposit with juvenile hydrothermal materials and inherited-conserved rfs from substratum affected lithic ancient and juvenile hydrothermal detrital deposits. hydrothermal-breccias of jébrak (1997) impact deposit with juvenile-impact materials and inheritedconserved rfs from substratum impacted lithic ancient and juvenile impact clastic breccias (shukla and sharma, 2018). pyroclastic deposits with juvenile volcanic rfs, and minerals mixed with cognate-covolcanic and with accidental materials (a-d rfs and minerals) lithic ancient cognate and accidental, and juvenile rf and minerals, e.g., detrital flow produced during santa helens eruption may 18, 1980 (korosec et al, 1980). note: if the lithic deposits consist of loose materials, then they are considered regolith. lithic deposits commonly are breccias including some depositional ancient-derived breccias (e.g., colluvial deposits). the conglomerates are commonly litho-conglomerate and are solely depositional (e.g., fluvial deposits, etc.). cognate-covolcanic are fragmented comagmatic volcanic rocks from previous eruptions of the same volcano (cognate of fisher, 1997). accidental fragments (accidental lithoclasts) are derived from the sub-volcanic basement rocks and therefore may be of any composition of fisher (1997). the term rudite is the generic name used here for denoting sedimentary deposits and materials (detritus or clast) with size >2mm, independent of their shape (rounded or angular). rudite was proposed by grabau (1904), and used by tucker (2003). rudites include conglomerates and breccias. lithic sedimentites include terrigenous breccias and conglomerate of folk, (1974); friedman and sanders (1978); tucker (2003); selley et al (2005); boggs (2006); among others. conclusions the genetic classification and naming of sedimentites in six main genetic groups: biological, depositional, detrital, chemical, diagenetic, and mixed; this is supported by conceptual framework for genesis of skeletal concentration: biogenic, sedimentologic, and diagenetic (kidwell et al., 1986), and by other formation processes chemical sedimentary precipitation and the regolith formation process that also forms detritus. the conceptual and structural scheme of classification of dana and ford (1922), that use the relations existing between them in chemical composition, crystalline form, and other physical properties, are the best manner for the mineral-compositional classification and naming of the sedimentites: silicate, carbonate, phosphate, etc. it is convenient to use genetic or compositional-mineralogic paradigm initially for the classification and naming of the sedimentites, e.g., detrital, biogenic, clastic, chemical, or diagenetic, or by using the mineralogicalcompositional scheme: silicate, carbonate, phosphate, and so on. so, be pragmatic it could be an integration of these two classifications, first using either genetic or compositional-mineralogical classification and then by use the other and complete it, e.g., silicate residual-regolith, carbonate biogenic-rock, siliciclastic deposit, bioclastic limestone, etc. because clastic sedimentites include pyroclastic, it is convenient to use the same textural size classification of wentworth (1922) for every type of size pyroclastic materials: gravel, sand, silt and clay, and retire the use of bombs, lapilli, and ash size. the term bomb, ash cloud are genetic terms and could be used complementary to the basic genetic and compositional, e.g., lithic pyroclastic breccias with some gravel-bombs, etc. acknowledgments. this study was supported by industrial university of santander, we thank the school of geology and the hydrocarbons and carbons geology group of investigation. references abelson, m., yechieli, y., crouvi, o., baer, g., wachs, d., bein, a., & shtivelman, v. 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(2025). monitoring land subsidence in an urban area of tamilnadu, india using sar time series data. earth sciences research journal, 29(1), 1-99. https:// doi.org/10.15446/esrj.v29n1.110897 record manuscript received: 01/09/2023 accepted for publication: 08/03/2025 abst r ac t ps-insar (persistent scatterer interferometric synthetic aperture radar) technique is one of the most effective methods for detecting land displacement in selected areas. it combines the advantages of radar interferometry and remote sensing, and it is based on long-term coherent radar measurements collected from the sar (synthetic aperture radar) images. as for this specific study, 186 sentinel-1 images were collected in the study area from 2016 to 2022. the authors used well-known sarproz software to process ps-insar techniques. the measured rate of cumulative displacement in this region was close to -60mm/year. on the contrary, the remaining areas of the study area experienced lower subsidence rates or no subsidence compared to the urbanized part. it demonstrated that high levels of urbanization and industrial activities led to high subsidence in the urban part of the study area, with the rest showing low or no subsidence at all. the results of the research highlight the necessity of planning concerning groundwater management to mitigate the nefarious implications of over-extraction. sustainable practices can be implemented; however, ensuring stability and sustainability in the face of urbanization and industrial development is the key. keywords: sar, ps-insar, subsidence, sarproz palabras clave: radar de apertura sintética; radar de apertura sintética interferométrico de dispersión persistente; hundimiento; sarproz monitoreo del hundimiento de tierra en una área urbana de tamilnadu, india, a través de datos temporales del radar de apertura sintética el radar de apertura sintética interferométrico de dispersión persistente es uno de los métodos más efectivos para detectar el desplazamiento de tierras en áreas seleccionadas. este combina las ventajas del radar interferométrico y la detección remota, y se basa en mediciones de radar coherentes a largo plazo recopiladas a partir de imágenes sar (radar de apertura sintética). para este trabajo en específico se recolectaron 186 imágenes del satélite sentinel-1 en el área de estudio entre el 2016 y el 2022. los autores usaron el software sarproz para aplicar el método de radar de apertura sintética interferométrico de dispersión persistente. el ritmo medido de desplazamiento acumulativo en la región fue de -60mm por año. por el contrario, las áreas de la zona de estudio que no están urbanizadas experimentaron rimos de hundimiento muy bajos o de no hundimiento. esto demuestra que los altos índices de actividades urbanísticas e industriales conllevan a un alto índice de hundimiento en las zonas urbanas del área de estudio, mientras las otras partes no tienen hundimiento o es muy poco. los resultados de la investigación resaltan la necesidad de una planeación y administración de las aguas subterráneas para mitigar los efectos de la sobreexplotación. se pueden implementar prácticas sostenibles; sin embargo, la clave está en asegurar la estabilidad y sostenibilidad en el desarrollo urbanístico e industrial. resu m en https://doi.org/10.15446/esrj.v29n1.110897 https://doi.org/10.15446/esrj.v29n1.110897 90 smruit ranjan sahu, kishan singh rawat, sudhir kumar singh 1. introduction subsidence, a geological phenomenon in which the ground surface sinks or settles, can have a far-reaching impact on structures, infrastructure and the environment. while typical natural causes such as tectonic plate movement and subsurface cavern erosion contribute, anthropogenic activities such as mining, construction, and groundwater extraction can significantly influence their prevalence. subsidence is costly, dangerous and damaging, illustrating the value of careful planning and stewardship of natural resources. in addition, areas of substantial coal, ore, or other mineral extractions are more prone to subsidence (gupta et al. 2020; ishwar et al. 2017; guang et al. 2009; miao et al. 2008).a prevalent phenomenon of land subsidence is due to a number of underground mining operations including the excessive exploitation of coal, minerals, groundwater, and petroleum products as well as the collapse of existing galleries and the flooding of abandoned ones (jianjun et al. 2012; chatterjee et al. 2006). natural and human activities can both lead to land subsidence. natural and anthropogenic causes of land subsidence include: urbanisation, construction, earthquakes, volcanic deformation, sediment compression, land subsidence and extraction of ground and surface water. some soil properties such as organic matter content, soil structure etc. may influence land subsidence (jalayer et al. 2023; tariq et al. 2023; majeed et al. 2023). pacheco-martínez et al. (2013) land subsidence causes destruction of environment urban infrastructure, loss of valuable resources, puts human life and daily activities in danger; causes to the economic crises and road blocking. ferretti et al. (2000) introduced the ps-insar technique, which is capable of detecting land deformation at a precision level of millimeters. in recent years, as explained by crosetto et al. (2016), the use of the ps-insar technique has successfully eliminated temporal and geometric decorrelation within sar data with the help of the identification of persistent scatterers having consistent and stable behaviour over the full acquisition time-series, leading to enhanced temporal and geometric coherence ps-insar allows for the rejection of atmospheric interference effects by utilizing a multi-temporal time-series of sar data, which can be seen in papers presented by ferretti et al. in 2001 and by davila-hernandez et al. in 2014 perissin and wang (2012) developed an innovative psi known as modified ps-insar incorporating both permanent and partially correlated scatterers. by introducing partially correlated scatterers, they sought to increase the point target density inside the highly sensitive area and decrease the decorrelation effect. using the c-band and l-band sar data, gueguen et al. applied the ps-insar technique to determine the deformation of the ground in coal bearing regions. (2009), abdikan et al. (2011; 2014), jiang et al. (2011), and thapa et al. (2016). such results of ps-insar are in good agreement with gps measurements. this would suggest that the two methods are quite similar to each other, c 2018 by society, when applying to the field of deformation data with ps-insar being fairly accurate and out of gps observations, giving quite an accurate output. this will be beneficial in building confidence in results used in geospatial studies (dumka et al. 2020). deformation and crustal strain were estimated using gps-derived velocities (dumka et al. 2022). gnss as a great tool to investigate the deformation pattern in the central mainland kachchh. it is used to accurately record the soil oscillations and changes so that scientific fraternity can understand the geodynamic processes in the area (suribabu et al. 2022). khorrami et al. (2020), mahmoodinasab et al. (2021) proposed the use of ps-insar technology for efficient land subsidence monitoring and directly confirmed the synergy between groundwater extraction and ground subsidence. xiong et al. (2021) demonstrated ps-insar analysis for the displacement measurement of the hzmb based on displacement observations collected by sentinel-1a. the findings from bock et al. the psi technique was also proved to be extremely efficient in estimating the displacement in the study of liao et al. bakon et al. (2016) have shown well the useful of sharing multi-sensor data throughout the insar technique for land subsidence estimation. significant procedures utilized for the investigation and checking of progressions on the earth’s surface incorporate sar, insar, ps-insar, and d-insar (sahu et al. 2023). the ps-insar method generates high-resolution surface displacement maps by using long-term coherent radar measurements from sar images (sahu et al. 2025). according to dumka et al. (2023), kachchh earthquake 2001, gps and insar data have a significant contribution toward understanding the underlying tectonic processes. the focus of tripathi et al. (2022), this has been extended to covering aerial and spaceborne sar remote sensing modalities on soil health studies adopting multiple monitoring and parameter modeling approaches over various soil classes and subclasses. according to malik et al. (2019), is a remote sensing technique that facilitates estimating the ground subsidence with a good accuracy and resolution. the need for minute and progressive measurements of ground height variations on a time-scale necessitates the use of a technique that can provide relative and accurate measures on the order of micrometers, and as such is an effective technique to the required measurements. according to kumar et al. (2020), the efficacy of the modified psi approach to detect land subsidence in vegetated and rural areas is not yet established. raju et al. (2022), in of reduced-cost persistent scatterer interferometry to establish the slumping of land that is happening as demand on ground water increases in lucknow in north india. according to tripathi et al. (2021), as stated in remotely sensed sar data is very flexible for flood study because it can penetrate through various barriers and possesses considerable periodic availability of data and resists rain. the study of temporal changes in groundwater levels is straightforward using the gravity recovery and climate experiment (grace) satellite sensor (tripathi et al. 2022). sar remote sensing has gradually gained more popularity due to its very reliable means of analyzing different natural and anthropogenic catastrophic events (tripathi et al. 2023). due to the extensive damage to buildings and structures as well as land displacement in kanchipuram, its ancient temples built on unstable land and the region’s flourishing cultural legacy have also been affected. moreover, overextraction of groundwater has led to soil subsidence in chennai, tamil nadu, which adjoins kanchipuram. this study aims to help detect and track ground subsidence along the tamil nadu coastal region using insar technology. this will provide stakeholders and decision makers with key information they need to manage and protect the coastal ecosystem. the kanchipuram district has never before seen any research being conducted on ground sinking. using subsidence analysis, we can effectively control and mitigate the risks associated with land subsidence. 2. study area the major river traversing the region of study is derived from the basin of the palar river. the palar has two small tributaries called vegavathi and cheyyar rivers. kanchipuram soil type: the soil type available in kanchipuram include clay, loam, sand, & silt soil. this makeup is an important factor in the land use, agriculture, the region. located along the western fringes of the dynamic, continually moving indo-australian plate edge west as it ascends towards the eastern coastline of india, kanchipuram sits within the intersection of tectonic destruction and accumulation. and so the region has already had some big quakes like the 2004 indian ocean earthquake which was very deadly. this caused widespread ground displacement and considerable damage to infrastructure. one of the potential environmental problem to threaten the kanchipuram is declining groundwater. the increased use of water for industrial, residential and agricultural purposes has caused a subsequent decline in the groundwater level of the city. its significant impact on the local ecosystems and population, and dire consequences on the sustainability of its water resources. kanchipuram is also greatly affected by the increase in groundwater levels due to the monsoon rains. sufficient monsoon rainfall both replenishes the underground water reservoirs and keeps a proportional water supply available. but, another factor hamadalavalla said was the erratic nature of rainfall in dry season affected groundwater levels. implementing sustainable water management principles is an important solution. method of addressing sustainable water management principles_param gregorio water sustainable sustainable water management sustainable water management sustainable sustainable some of these issues can be resolved by implementing sustainable water management methods. such as water reuse, efficient irrigation, and rainwater harvesting, which are used to conserve a limited water supply and make the utmost of its use. in addition, educational programs and awareness campaigns have been initiated to promote water sustainability at the local level. check the map of the study area in the fig. 1. geology of the study area the geology of kanchipuram district shows a wide range of rock formations and soil types (fig. 2). geology data of the study area collected from bhukosh. the district is located on the eastern side of the archean 91monitoring land subsidence in an urban area of tamilnadu, india using sar time series data granitic basement complex of peninsular india. the dominant rock group in the area is the charnockite group, which includes granitic gneiss, charnockite, and migmatite. these rock formations have greatly influenced the district’s landscape. besides, kanchipuram district has many mineral deposits that have contributed to its economic development. bauxite, a significant mineral used in aluminum production, is found in the villages of koodalur and reddipalli. limestone deposits, which have versatile industrial applications, are located in the villages of ramasamudram and sankarapuram. the existence of these mineral deposits has, therefore, encouraged mining activities in the district, making it economically sound. the diversified and complex geology seen in the kanchipuram district is adding a major impact on its overall landscape and natural resources. the features of geology have provided fertile land for agriculture hence making the district have a well-settled agricultural industry. moreover, the presence of these mineral deposits has spurred the mining activities and therefore enhanced the local economy. regarding the information, it contained, saying fig. 1b shows a geological map of the study area. even describing the map and its contents is difficult without access to the actual figure. the map likely illustrates the distribution of various rock formations, mineral deposits, and other geological features within the district, providing valuable insights into its geological composition. kanchipuram district stands as a testament to the diverse and intricate interplay between geology, landscape, and economic activities. the district’s unique geological characteristics have shaped its topography, supported agriculture and mining, and contributed to its overall development. figure 1. study area map figure 2. geology map of the study area 3. datasets the data filters describe the desired parameters for the sentinel-1a (sa) satellite imagery data that is available. it specifies the file type, which is l1 slc (single look complex), a format of radar data which, unlike interferometric data, retains both amplitude and phase information. this format is commonly used by applications that need advanced radar. all 186 pictures from 2016–2022 were obtained from the alaska satellite facility (asf-vertex). the specification for the beam mode is interferometric wide swath. this mode can provide wide area coverage during data collection, which could be beneficial for applications such as land monitoring, disaster management, and maritime surveillance. set direction: descending → this is the orbital direction of the satellite when collecting data, which in our case is descending; set polarization: vv+vh → the data must content both vv (vertical-vertical) and vh (vertical-horizontal) polarizations. if the flight of the satellite is descending, it is flying from north to south. this orientation should be considered when comparing data from different acquisitions and tracing temporal dynamics. subtype, frame no of sentinel-1a dataset (547,548,549 and 550). table 1 describe the details about datasets. table 1. datasets data filters details file type l1 slc (single look complex) beam mode iw polarization vv+vh direction descending subtype sentinel-1a (sa) 92 smruit ranjan sahu, kishan singh rawat, sudhir kumar singh 4. methodology sarproz software uses ps-insar techniques to process sar data. this software can handle large time series data. according to crosetto et al. (2015), a minimum of 20 sar images in c-band data is required for psi techniques in the monitoring process. the selection of a master image is essential for processing the ps-insar techniques. the ps-insar processing is carried out in several steps to produce accurate and reliable deformation maps, as shown in fig.3. co-registration is the first step, where the sar images are aligned to a common reference point, allowing for accurate measurements of surface deformation. then, reflectivity maps, as shown in fig.4, and amplitude stability index maps are prepared to identify areas with stable radar reflectivity and low noise levels, respectively. reflectivity map shows reflectivity values for various regions that indicate surface properties and changes in land cover. this is of significant importance as it gives the spatial distribution of reflectivity, which is an important aspect in determining the underlying surface characteristics. from these maps, pss are selected based on their coherence and stability characteristics. multi-image sparse grid phase unwrapping is performed to resolve the phase ambiguity and get accurate deformation measurements. aps analysis and removal are carried out to correct atmospheric disturbances that may affect sar measurements. then, psc phase reading and displacement estimation to calculate the difference in phase for slave and master images and provide an estimate for the deformation map. spatial-temporal baseline with respect to master image, fig.5. slc data loading reflectivity map mask for sparse point selection ground control point selection external dem selection of ps graph creating estimate parameters connection process reference point selection statistical evaluation calculation of aps selection of ps estimate parameters statistical evaluation final aps removal estimate displacement export ps preparation of data aps estimation multi-temporal analysis figure 3 methodology flow chart of sarproz software re�ective map line [pix] sa m pl e [p ix ] 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 x104 2.4 1.8 1.2 0.6 0.5 1 1.5 2 0 figure 4 reflectivity map figure 5 temporal base line aps estimation aps estimation involves a multi-step process, which accurately estimates the position and properties of sparse points from a dataset. it uses sparse point selection, delaunay graph connection, connection coherence graph, automatic reference point determination, parameter estimation, connection velocity histogram, connection height assessment, and connection residual height analysis to estimate the parameters. each of these processes is necessary for accurate and reliable aps estimation. sparse point selection plays a primary role in the estimation of aps (fig. 6a). this means choosing a small segment of points in the dataset that have certain features or characteristics. these selected sites make up the basis for additional estimation and analysis. once, sparse spots of key points are identified, the delaunay graph connection approach is used. (fig. 6b). by connecting the sparse points, delaunay triangulation forms a network of interconnected triangles. this relationship allows for a deep understanding of the fundamental framework and lends itself to examining how spaces relate to one another. the connectivity analysis is further teed up by extracting a link coherence graph (fig. 7). this graph helps you visualize the consistency of connections between nearby points and it is possible to identify potential outliers or wrong connections. considering the total coherence enhances and stabilizes the aps estimation. subsequent step is to find automatic reference (fig. 8). such a reference point allows for accurately pinpointing the sparse locations in the dataset by serving as a baseline to the estimate procedure. a selection of the automated reference point is made based on predefined standards or algorithms that consider the overall characteristics and distribution of the dataset. a b figure 6. (a) psc sparse point selection and (b) delaunay graph connection 93monitoring land subsidence in an urban area of tamilnadu, india using sar time series data figure 7. connection coherence graph figure 8. psc reference point once a baseline has been established we can proceed to parameter estimation. this is the step in which it estimates multiple properties of the sparse points, including scale, rotation, and location. these metrics are valuable insights for further analysis and used to characterize fundamental statistics of the dataset. are computed in figures 9a and 9b and defined for integrated velocity and integrated cumulative displacement, respectively. to examine the connection velocity, a histogram is constructed based on the connections between the sparse locations (fig. 10). this histogram helps to find trends or patterns in the displacement of the points, visually. it helps to build a more enriched view of the dataset as a whole. height of connections assessment (a.k.a. elevation of links) determines the height or elevation of connections between sparse labels. however, for applications requiring the bodies of water, earth surface, buildings and etc. like terrain model or geospatial analysis, this method may be a great help as it retains the vertical relationships of the data. a b figure 9 (a) integrated velocity, (b) integrated cumulative displacement finally, residual height analysis of connections is applied to evaluate the accuracy of the calculated heights (fig. 11). residual height is defined as the deviation between the real height of the dataset and its estimated height. the estimation of aps is shaken by the heights of the residues to the lines, which can be evaluated for their quality and reliability, with adjustments if needed. the coherence for each slave date from its associated master date, once the estimating aps has been removed, is shown in fig. 12. as shown in figs. 13a and 13b. 94 smruit ranjan sahu, kishan singh rawat, sudhir kumar singh connections velocity [mm/year] sigma connections velocity [mm/year] 800 600 400 200 -5 0 5 0.5 1 1.5 2 ## 0 1500 1000 500 0 figure 10 connection velocity connections height [m] sigma connections height [m] connections residual height [m] sigma connections residual height [m] 2000 3000 2000 1000 0 3000 2000 1000 0 300 200 100 0 1500 1000 500 0 50 100 200150 50 100 200150-20 -10 10 200 figure 11. connection height and residual height figure 12 coherence between each slave date and master date a b figure 13 (a) before aps removal, (b) after aps removal 95monitoring land subsidence in an urban area of tamilnadu, india using sar time series data multi-temporal analysis multi temporal analysis is a strong approach to get valuable information from spatio temporal data. sparse point processing (fig. 14a), point processing histogram (fig. 14b), the estimated velocity (fig. 15a) and cumulative displacement (fig. 15b) for individual points, the estimated height (fig. 16a), and residual height (fig. 16b) this method encompasses everything, from specific points to the other key features. now, let us look into each of those components closely. sparse point processing is an important step of multitemporal analysis. this involves acquiring remote sensing data over different time periods and the extraction and transformation of specific points or features. these dots can represent different types of landforms, vegetation, buildings, and other features present on the earth’s surface. by detecting and analyzing these few points, we are able to follow changes over time and detect salient trends. a more comprehensive understanding of the temporal dynamics is achieved through the sparse point processing histogram. then, histograms are constructed that display the frequency distribution of specific point properties (e.g., spectral values, geometric features, etc.) on multiple temporal data sets; with this method. on the other side, these histograms give insight into trends like temporal seasonality, outliers, global shifts in variable distributions, etc. significant differences are readily observable, and their critical findings can be concluded. it is another essential building block in multi-temporal studies, where you are interested in estimating velocity and accumulated displacement at specific times. if we compare the positions of certain locations over several time steps, we can obtain the velocities and cumulative displacements of some areas. this data provides key insights into the magnitude and direction of any activity or shifts occurring in the selected geographies. examples: this research may lead to recommendations that help in locating subsets of terrain that are prone to landslides, erosive behavior or any other kinetic evolution in geomorphology or ecological dynamics. moreover, estimating height and residual height helps to gain insight into vertical differences across the study area. a b figure 14 (a)sparse point processing (b)sparse point processing histogram a b figure 15 (a) estimated velocity, (b) cumulative displacement a b figure 16 (a) estimated height, (b) residual height 5. result and discussion 186 sar images captured from 2016 to 2022 were analyzed to construct a detailed land subsidence map using the complex sarproz software. the map served as a visual guide indicating that land subsidence took place within the defined study area. we constructed a robust tool called the connection coherence graph to rapidly visualize the temporal coherence for pairs of sar 96 smruit ranjan sahu, kishan singh rawat, sudhir kumar singh acquisitions. this graph provided an overall picture of the relationships between different sar acquisitions over time. all sar acquisitions were represented as distinct nodes in the graph, illustrating their own unique characteristics and features. to highlight the strength of associated sar the connecting lines were used to visualize the coherency between two nodes. for the interpretation of connection coherence graph the thickness and color of the connecting lines were used to provide salient information about the coherence values. in this way, researchers to locate and investigate the most relevant relationships with respect to the dataset by assigning thicker and darker lines to denote a higher degree of coherence in the corresponding sar acquisitions. as shown in fig. 17 by means of a scatter plot module which was figured out to help in examining the coherence correlations. this allowed us to create the scatter plot graphically, with each point being a unique sar acquisition. the distribution and patterns of these locations could help researchers understand much about the coherence characteristics and changes over time. this information played a vital role in identifying any inconsistencies or major deviations in the dataset. a final coherence histogram was obtained to provide a comprehensive description of the coherence distributions over the area of study (fig. 18). the histogram made it easier for the researchers to identify the relative significance of different coherence values, and to detect any significant trends or outliers that might have arisen during the investigation by displaying the frequency distribution of coherence values. figure 17 scatter plot figure 18 final coherence histogram based on the beast evidence, the subsidence phenomena was most active in the kanchipuram urban area. the affected area was visualized on google earth (fig. 19). a deeper dive into the data indicated that, the max cumulative displacement observed from 2016-2022 was -60mm (fig. 15b). cumulative displacement: total distance travelled by an object from its original position in a given time. the cumulative displacement for the region was found to be a rate of −60mm/yr. the cumulative displacement vs time plot also known as the integrated displacement graph, one of the widely used plots in sarproz, was created by plotting accumulated displacement on the y-axis and time on the x-axis. this graph allowed to visualize the evolution in time of the accumulated displacement for a point of interest through all the time period covered by sar data. fig. 20, demonstrating the time series subsidence trend of various points from the beginning of study period. two important parameters affecting the processing and interpretation of sar data within the sarproz software package are connection velocity and sigma collection velocity. what these values are will depend on coherence level, time between acquisitions and how quickly you want the processing done. the connection velocity dictates the number of connected interferograms, while the sigma collection velocity impacts the point spread function of the sigma values. as shown in fig. 15a, for the current study, the best prognosis velocity was additionally assumed to be -10 mm/year. if the expected velocity is negative (-10 mm), it means the ground surface is lowering or compacting at a rate of 10 mm per year. while this rate may not seem especially high at first, over time even gradual rates of subsidence can lead to serious impacts that include more flooding, damage to infrastructure and buildings, and a loss of agricultural land. the region’s urbanization and excessively exploited groundwater were the two identified causes of the subsided area and deteriorating groundwater resources, according to the investigation. the case highlights the role human activity plays in incidence of subsidence and the vital importance of managing urban expansion and water resource consumption to mitigate associated risks. the analysis not only focuses on the time series of the specified point to obtain this quantity, but also considers other relevant additional parameters. such metrics are temporal coherence (which measures data similarity over time) and other parameters, such as residual height (height difference to a reference point) and velocity (how fast an object changes position in space over time). these parameters are important for understanding the characteristics, patterns, and behavior of the object of study. median subsidence rates provide valuable insights that can inform researchers in making educated decisions regarding construction methods of infrastructure (e.g., drainage systems), revise land use strategies (possibly based on subsidence trends), and manage water supplies more effectively. finally, coupled with other geophysical techniques, the combination of subsidence monitoring from multimodal and multiscale perspectives using sar contributes to a more comprehensive understanding of subsidence processes and the development of appropriate monitoring and management strategies. understanding subsidence events and their causes has been critical to enable sustainable development and protect infrastructure and human life. it is only possible to enhance our understanding of subsidence processes and to support decision-makers in implementing appropriate policies for minimising adverse effects by conducting in-depth investigations utilising state-of-the-art technologies and by considering a variety of aspects. 6. conclusion the most common method used to extract land subsidence is sarproz (sar data processing and interferometry) software, which is used in detail to construct a land subsidence map showing the various subsidence patterns in kanchipuram district. material and methods the present study utilized the connection coherence graph to display temporal coherence between sar acquisitions, allowing the detection of correlations and anomalies. the scatter plot module and histogram of coherences provided additional views to assess coherence features and temporality, while thickness and color of connecting lines were associated with the level of coherence. the results revealed notable amounts of sinking over urban area, with a peak rate of subsidence of -60mm/year. this displacement is represented visually in google earth and corroborated using a cumulative displacement graph at datum points to follow temporal development. analysed parameters were connection velocity and sigma velocity, estimating a velocity of -10 mm per year, with main drivers being urban growth and excessive groundwater extraction. residual height, as well as velocity and temporal coherence, were also analyzed to reveal how the subsidence occurred. 97monitoring land subsidence in an urban area of tamilnadu, india using sar time series data fig.19 displacement map of the study area a b id: 22490 height [m]: 72.0, height st dev [m]: 22.7 height relative to cround [m]: 16.5 velocity [mm/year]: 10.0, velocity st dev [mm/year]: 0.96 displ. to temper ratio [mm/de gc]: 0.00, cumulative displacement [mm]: -73.6 temporal coherence: 0.27, sample: 5695, line: 3078 std dev. [mm]: 6.6 data nr.:186 id: 23176 height [m]: 69.0, height st dev [m]: 13.9 height relative to cround [m]: 16.5 velocity [mm/year]: -7.6, velocity st dev [mm/year]: 0.62 displ. to temper ratio [mm/de gc]: 0.00, cumulative displacement [mm]: 56.0 temporal coherence: 0.33, sample: 5447, line: 3111 std dev. [mm]: 6.2 data nr.:186 98 smruit ranjan sahu, kishan singh rawat, sudhir kumar singh c d figure 20. a, b, c & d are showing time series subsidence trend at three different points under the study period the findings underscore an urgent need for sustainable land use and water management. the tamil nadu government aims to mitigate subsidence through measures like limiting groundwater extraction, rainwater harvesting and selling treated sewage water to farmers for irrigation. but the residual subsidence shows we must continue to monitor and respond. the study’s authors pointed to the adverse effects over-extraction of groundwater can have on the stability of the land, and also noted the need for urban expansion control strategies that don’t threaten infrastructure. the research demands implementing innovative geospatial technologies and integrated studies, in order to diminish the risk of subsidence, protecting communities and enabling sustainable development too. the study functions as a reminder of the consequences of uncontrolled groundwater extraction, and the need to reverse land subsidence through urban ecology and sustainability knowledge. acknowledgments the authors wish to thank dr. kapil malik for kindly providing a trial version of the sarproz software. they have relied on his guidance and encouragement to understand and work through the complexities of the software. references abdikan, s., arikan, m., sanli, f. b., &cakir, z. 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(2024). investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification. earth sciences research journal, 28(2), 161-174. https://doi. org/10.15446/esrj.v28n2.105296 classification of the hyperspectral images (hsis) is one of the most challenging tasks hyperspectral remote sensing. various machine learning classification algorithms have been implemented to hsi classification. in recent years, several convolutional neural network (cnn) architectures were developed for hsi classification. the aim of this study is to test the performance of cnn, and well-known support vector machine and random forest algorithms using the hyrank loukia, houston 2013, and salinas scene datasets. the findings indicate that the modified hybridsn cnn outperformed other algorithms across all datasets, as demonstrated by various performance evaluation metrics. keywords: hyperspectral image classification; support vector machine; random forest; convolutional neural networks; houston 2013; hyrank; salinas scene; investigación sobre el desempeño de máquinas de vectores de soporte, bosque aleatorio y redes neuronales convolucionales 3d y 2d en la clasificación de imágenes hiperespectrales resumen la clasificación de imágenes hiperespectrales (hsi, del inglés hyperspectral images) es una de las tareas más complejas de la detección remota hiperespectral. varios algoritmos de aprendizaje de máquinas se han implementado en la clasificación de las hsi. recientemente, varias arquitecturas basadas en redes neuronales convolucionales (cnn, del inglés convolutional neural networks) se han desarrollado para esta clasificación de imágenes hiperespectrales. el objetivo de este estudio es evaluar el desempeño de las cnn y los algoritmos de máquinas de vectores de soporte y de bosque aleatorio con los conjuntos de datos hyrank loukia, houston 2013 y salinas scene. los resultados demuestran que el modelo modified hybridsn cnn superó a otros algoritmos en todos los conjuntos de datos de acuerdo con lo que demuestran varias métricas de evaluación de desempeño. palabras clave: clasificación de imágenes hiperespectrales; máquinas de vectores de soporte; bosque aleatorio; redes neuronales convolucionales; houston 2013; hyrank; salinas scene https://doi.org/10.15446/esrj.v28n2.105296 mailto:ecseyrek@aku.edu.tr https://doi.org/10.15446/esrj.v28n2.105296 https://doi.org/10.15446/esrj.v28n2.105296 162 eren can seyrek, murat uysal 1. introduction hyperspectral imaging sensors acquire data in hundreds of narrow contiguous spectral bands along the electromagnetic spectrum. hyperspectral images (hsis) offer detailed spectral and spatial information about the surface of the earth, enabling precise discrimination of the surface objects and land cover types. hsis have a wide range of applications, such as land cover classification (akar & tunc gormus, 2021), land cover change detection (erturk et al., 2015), precision agriculture (bhosle & musande, 2020; park & lu, 2015; teke et al., 2013), forestry (adão et al., 2017), geology (van der meer et al., 2012), environmental monitoring (stuart et al., 2019), urban planning (heiden et al., 2012), medical imaging (lu & fei, 2014), food quality control (basantia et al., 2018), and military defense (ardouin et al., 2007). consequently, research focusing on the processing and analysis of hsi data has experienced significant growth in recent years. classification is a fundamental and crucial technique in hsi processing, which is utilized to identify the land category of each pixel in the hsi (meng et al., 2021). hsi classification is a more challenging process because images have a large number of spectral bands and have lower spatial resolution than multispectral images. the supervised classification approach generates decision boundaries by learning from training samples (i.e., ground truth pixels) with the aim of minimizing empirical risk and structural risk (chen et al., 2020). in the literature, a range of supervised machine learning (ml) techniques have been proposed for hsi classification. for instance, support vector machine (svm) (gualtieri et al., 1999; kavzoglu & colkesen, 2009; melgani & bruzzone, 2004), random forest (rf) (chan & paelinckx, 2008; waske et al., 2009), adaboost (chan & paelinckx, 2008), canonical correlation forest (xia et al., 2016) extreme gradient boosting (loggenberg et al., 2018), light gradient boosting machines (ustuner, 2024), etc. have been utilized for this purpose. however, many of these algorithms require a complex and precise user-defined parameter-tuning process. moreover, they generally perform pixel-wise classification based solely on spectral information, neglecting the spatial relationship between adjacent pixels in the classification process (chen et al., 2020). in recent years, deep learning, an emerging field within ml, has grown in popularity as a research subject due to its ability to extract different levels of features from various types of datasets, including image, video, and voice. convolutional neural networks (cnns) are a widely used deep learning method for processing data in multiple arrays, such as signals, sequences, images, and videos. the cnns have demonstrated remarkable performances in tasks including speech recognition, face detection, object detection, and image classification (akın & cömert, 2023; lecun et al., 2015). as a consequence of their ability to achieve state-of-the-art performance, cnns have become increasingly prominent in remote sensing applications (abdikan et al., 2023; cheng et al., 2022; ghanbari et al., 2021). notably, a variety of cnn architectures have been developed specifically for the classification of hsis (chen et al., 2020; chen et al., 2016; fırat et al., 2022; hang et al., 2020; luo et al., 2018; roy et al., 2019). however, there is a lack of publicly available hsi datasets including ground truth data. many studies within the literature have relied upon commonly used datasets such as indian pines, salinas scene, university of pavia, and kennedy space center hsi datasets to evaluate the performances of the classification algorithms. in recent years, several novel hsi datasets were introduced, such as houston 2013, houston 2018, hyrank (karantzalos et al. 2018), and whu-hi (zhong et al., 2020). in light of these advancements, conducting algorithm evaluations on recently introduced benchmark hsi datasets allows for a more comprehensive and impartial examination of the tests within a broader context. this study evaluated the performance of the various ml algorithms using a variety of hsi datasets. traditional ml classifiers, such as svm and rf, were compared with a deep learning approach represented by cnn. hybridsn cnn (roy et al., 2019) was chosen as the cnn architecture, but in light of literature suggesting that employing various activation functions and optimizers can enhance classification performance (agarwal et al., 2021; bera & shrivastava, 2020; dubey et al., 2022; hao et al., 2020; seyrek & uysal, 2024; vani & rao, 2019), a modified version called modified hybridsn was developed, integrating mish activation function (misra, 2019) and adamax optimizer (kingma & ba, 2014). the hyrank loukia, houston 2013, and salinas scene datasets were employed for testing the classification performances of the algorithms to provide diversity in terms of spectral and spatial resolution of the data. to mitigate the hughes phenomena resulting from limited ground truth data and high spectral band correlation between within the hsi datasets, principal component analysis (pca) transformation was applied during the data preparation stage. therefore, the classification performance of the algorithms was investigated without employing any preprocessing steps apart from pca transformation for dimensionality reduction to hsi datasets. to ensure consistent evaluation conditions across all algorithms, the data were split into training and testing sets with a ratio of 10% for training and 90% for testing. the performance of the algorithms was assessed by comparing metrics such as overall accuracy (oa), kappa coefficient (κ), producer accuracy (pa), user accuracy (ua), and f score, respectively. in addition, differences in performances between algorithms were analyzed by performing mcnemar’s test. 2. machine learning algorithms 2.1. support vector machines (svm) svm is a well-known non-parametric and supervised ml algorithm based on statistical learning theory, developed by vapnik (cortes & vapnik, 1995). the algorithm is based on the principle of determining the hyperplane that optimally separates two classes. svm constructs its model without making any assumptions regarding the data distribution. in the case of twoclass classification problems where classes are linearly separable, svm selects one decision boundary that minimizes generalization error from an infinite number of linear decision boundaries. as a result, the selected decision boundary provides the largest margin between classes on the feature space, where the margin is determined as the sum of the distances from the closest samples of classes (support vectors) to the hyperplane (vapnik, 1995). due to its consideration of support vectors located on the decision boundaries of the classification process, svm is particularly useful for small training data sets (mountrakis et al., 2011). in the majority of classification scenarios, including the classification of remotely sensed images based on sample pixels, the separation of classes cannot be achieved through a linear hyperplane within the input space. to address this challenge, boser et al. (1992) proposed a method of transforming the input data onto a higher dimensional input space by using kernel functions. while linear, polynomial, radial basis function (rbf), and sigmoid kernels are commonly used kernel functions on implementing the nonlinear svm in the literature, polynomial and rbf kernels are the most suitable functions for remotely sensed image classification problems (kavzoglu & colkesen, 2009; melgani & bruzzone, 2004; pal & mather, 2005). furthermore, mountrakis et al. (2011) emphasized that the rbf kernel function is the most suitable kernel function for remote sensing data analysis. in this study, the rbf kernel was employed to classify hsis using the svm algorithm. two user-defined parameters, namely the regularization parameter c and kernel width γ, must be specified for the rbf kernel. as the optimal parameter combinations for achieving the best classification performance are unknown for a given dataset, an exhaustive search for optimal parameters is necessary (hsu et al., 2003). in these experiments, the c and γ parameters of the rbf kernel were determined using a grid search method coupled with cross-validation. 2.2. random forest (rf) the rf algorithm is a commonly used ensemble learning algorithm based on decision trees, utilized for tasks such as classification, regression, and feature selection. breiman (2001) developed the bagging algorithm with the philosophy that an ensemble of classifiers can achieve superior classification performance compared to a single classifier. the rf algorithm combines multiple multivariate decision trees, with each tree trained on a distinct asset of different training data (breiman, 2001; rodriguez-galiano et al., 2012). to construct decision trees within rf, the classification and regression trees methodology is applied without pruning (breiman, 2001; breiman et al., 1984). the gini index serves as the attribute selection criterion, evaluating the impurity of attributes (e.g., spectral bands in hyperspectral data) with respect to classes during the constructing of the model (pal, 2005). the gini index for a given training dataset t can be expressed as follows: 163investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification = ≠ ,( ) | | ,( ) | | (1) where the f ( cj,t ) / |t| denotes the probability that a selected case (e.g., a pixel) belongs to the class ci. during the training of the rf model, approximately 2/3 of the samples (referred to as “in-bag”) within each randomly selected subset are utilized to construct each decision tree in the forest. the remaining 1/3 of the samples (referred to as “out-of-bag”) are employed to validate the performance of the final ensemble using an internal cross-validation approach (sahin et al., 2020). to determine the final label of an unknown sample (e.g., a pixel), the classification results of each decision tree in the forest are combined by performing majority voting, with each classifier vote carrying equal weight (chan & paelinckx, 2008). implementing an rf model requires setting two user-defined parameters: the number of trees (ntree) and the number of randomly selected features at each node (mtry) (sheykhmousa et al., 2020). the parameter mtry is typically set the square root of the total number of input features, while the ntree parameter is commonly determined as 500, as the error tends to stabilize prior to this number of classification trees (belgiu & drăguţ, 2016). moreover, both theoretical and empirical studies have reported that the classification accuracy of the rfs is less sensitive to the parameter ntree compared to mtry (kulkarni & sinha, 2012). nevertheless, the parameters ntree and mrty were determined through a grid search method utilizing cross-validation. 2.3. convolutional neural networks (cnn) cnn is a specialized neural network structure for processing data organized in a grid-like topology, such as a digital image formed by three 2-dimensional (2d) arrays. unlike conventional neural networks that primarily utilize matrix multiplication, cnn contains at least one convolution (lecun et al., 1998). cnn is designed to handle input data in multiple array formats, such as 1-dimensional data, including signals and sequences; 2d data, including images and audio spectrograms; and 3-dimensional (3d) data, including video and volumetric images (lecun et al., 2015). cnn is typically consisting of three fundamental components: convolution layers, pooling layers, and fully connected layers, each serving different roles (li et al., 2018). within the convolution layer, input data subjects to convolution operation with learnable kernels, followed by an activation function to generate output feature maps. the 2d and 3d convolution operations can be mathematically represented as equation (2) and equation (3), respectively. = + =0 −1 =0 −1 =0 −1 −1( ) +( ) +( )( ) (2) = + =0 −1 =0 −1 =0 −1 =0 −1 −1( ) +( ) +( ) +( )( ) (3) where v denotes the output variable in the feature map. s, q, and r represent spectral and spatial dimensions of the kernel, respectively. the indices of the kernel are indicated by (s, q, r), while (z, x, y) represent the indices of the feature map, corresponding to the two spatial and one spectral dimension, respectively. the kernel parameters are represented by k, while the indexes of the input layer, output layer, and feature maps are denoted by i, j, and p, respectively. p indicates the number of feature maps. the bias term is denoted by b. the activation function is represented by f, was selected as a rectified linear unit (relu) in this study due to its faster computing rate without a gradient diffusion problem. the relu function is defined as follows (nair & hinton, 2010; wang et al., 2020): f (x) = (0, x) (4) in most cnn architectures, a pooling layer follows a convolution layer, reducing the dimensionality of feature maps. two common pooling operations are commonly used, named max pooling and average pooling. following the pooling layer, the final fundamental cnn layer is the fully connected layer, similar to backpropagation in traditional neural networks. the learned features extracted from the input image by the cnn serve as the output of the fully connected layer. connecting this output to a learning classifier, such as softmax, the classification process is implemented (krizhevsky et al., 2012). in this study, the hybridsn cnn model, introduced by roy et al. (2019), was employed for classifying hsi images. the structure of the hybridsn cnn model utilized in this study is shown in figure 1. upon reviewing some studies in the literature, it has been observed that activation functions and optimizers have an impact on the performance of cnn models. for example, some studies observed that the adamax optimizer improved the classification performance in the cnn model (ariff & ismail, 2023; seyrek & uysal, 2024; vani & rao, 2019). furthermore, there are research findings indicating that mish activation function (equation 5) also enhances classification performance (misra, 2019; seyrek & uysal, 2024; zhang et al., 2019). based on this information, the hybridsn cnn architecture was modified to utilize the adamax optimizer and mish activation function. consequently, the resulting cnn architecture is referred to as the modified hybridsn cnn for the remainder of the study. mish(x) = x × tanh tanh ( log log (1 + ex ) ) (5) the model comprises three 3d convolution layers, enabling simultaneous extraction of both spectral and spatial features. subsequently, a 2d convolution layer performs exclusively spatial feature extraction. this ensures the effective extraction of both spectral and spatial features from a hyperspectral image. during the data preparation stage, the hsi image subjected to pca was partitioned into small overlapping 3d patches, each associated with a ground truth label corresponding to the central pixel. let’s assume that p ∈ r(s×s×b), where s represents the size of the 3d patches and b denotes the number of spectral bands of the pca-transformed hsi data. the total number of 3d patches generated is (m s + 1) × (n s + 1), where m is the width and n is the height of hsi data. during the data preparation for training and testing the cnn model, 3d patches with any ground truth data corresponding to the central pixel are excluded. figure 1. the modified hybridsn architecture 164 eren can seyrek, murat uysal table 1. summary of the cnn models for the hyrank loukia, dfc 13, and salinas scene datasets. cnn architecture model summary layer output shape hyrank loukia: # of parameters dfc 13: # of parameters salinas scene: # of parameters input_1 (input layer) (13, 13, 15, 1) 0 0 0 conv3d (conv3d) (11, 11, 9, 8) 512 512 512 conv3d_1 (conv3d) (9, 9, 5, 16) 5776 5776 5776 conv3d_2 (conv3d) (7, 7, 3, 32) 13856 13856 13856 reshape (reshape) (7, 7, 96) 0 0 0 conv2d (conv2d) (5, 5, 64) 55360 55360 55360 flatten (flatten) (1600) 0 0 0 dense (dense) (256) 409856 409856 409856 dropout (dropout) (256) 0 0 0 dense_1 (dense) (128) 32896 32896 32896 dropout_1 (dropout) (128) 0 0 0 dense_2 (dense) hyrank loukia:(14); dfc 13: (15); salinas scene: (16) 1806 1935 2064 total parameters: 520062 520191 520320 other parameters parameter value activation function hybridsn: relu; modified hybridsn: mish dropout rate %40 number of epochs 100 batch size 256 optimizer hybridsn: adam (β1: 0.9, β2: 0.999, ϵ:1e-08, learning_rate: 0.001); modified hybridsn: adamax (β1: 0.9, β2: 0.999, ϵ:1e-08, learning_rate: 0.001) 3. datasets in order to evaluate the performance of the classification algorithms, three publicly available hsi datasets were employed. the first dataset, named hyrank, was developed by the international society for photogrammetry and remote sensing (isprs), working group iii/4 (karantzalos et al., 2018). it comprises five distinct hsis acquired from the national aeronautics and space administration’s (nasa) eo-1 hyperion sensor. among these, only two hsis, namely dioni and loukia, are accompanied by ground-truth data. the loukia has 30 meters spatial resolution and a hypercube size of 249×945×176. ground truth data for loukia contains 14 coordination of information on the environment (corine) land cover classes, carefully annotated by interpretation experts (karantzalos et al., 2018). detailed information regarding the number of training and testing samples for this data set is provided in table 2. the second dataset, named houston 2013, was acquired from the university of houston campus and the surrounding urban area on june 23, 2012, by the national science foundation (nsf)-funded center for airborne laser mapping (ncalm). the dataset has a spatial resolution of 2.5 meters and comprises 144 spectral bands in the range between 380 and 1050 nanometers. the spatial dimensions of the hsi are 349×1905×144. fifteen distinct ground classes were determined by the institute of electrical and electronics engineers geoscience and remote sensing society (ieee grss) image analysis and data fusion technical committee for organizing the 2013 data fusion contest (dfc). the original dataset included both training and validation data. in this study, training and validation data were merged and employed as ground truth data. detailed statistics regarding the numbering of train and testing samples of the dataset are provided in table 3. the third dataset, referred to as the salinas scene, was acquired on october 9, 1998, from salinas valley, california, united states of america (usa), utilizing nasa’s airborne airborne visible / infrared imaging spectrometer (aviris) sensor. the dataset has a spatial resolution of 3.7 meters and comprises 224 spectral bands. notably, the original dataset underwent preprocessing, during which water vapor absorption bands within the ranges of 108-112 and 154-167 were excluded. the dataset contains 16 ground classes in three main groups: vegetables, bare soils, and vineyard fields. following processing, the dataset’s dimensions stand at 512×217×204. details regarding the number of training and testing samples for this dataset are provided in table 4. 165investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification table 2. overview of training and testing samples and corresponding training ratios utilized in the hyrank loukia dataset. class number class name # train samples train ratio # test samples 1 dense urban fabric 29 2.15% 259 2 mineral extraction sites 7 0.52% 60 3 non irrigated arable land 54 4.00% 488 4 fruit trees 8 0.59% 71 5 olive groves 140 10.37% 1261 6 broad leaved forest 22 1.63% 201 7 coniferous forest 50 3.70% 450 8 mixed forest 107 7.93% 965 9 dense sclerophyllous vegetation 379 28.07% 3414 10 sparce sclerophyllous vegetation 280 20.74% 2523 11 sparsely vegetated areas 41 3.04% 363 12 rocks and sand 49 3.63% 438 13 water 139 10.30% 1254 14 coastal water 45 3.33% 406 total 1350 100.00% 12153 table 3. overview of training and testing samples and corresponding training ratios utilized in the houston 2013 dataset. class number class name # train samples train ratio # test samples 1 healthy grass 137 7.93% 1237 2 stressed grass 145 8.40% 1309 3 synthetic grass 80 4.63% 715 4 trees 126 7.30% 1138 5 soil 130 7.53% 1168 6 water 34 1.97% 305 7 residential 148 8.57% 1328 8 commercial 135 7.82% 1219 9 road 155 8.98% 1399 10 highway 142 8.22% 1282 11 railway 157 9.09% 1409 12 parking lot 1 143 8.28% 1286 13 parking lot 2 64 3.71% 571 14 tennis court 51 2.95% 459 15 running track 80 4.63% 718 total 1727 100.00% 15543 table 4. overview of training and testing samples and corresponding training ratios used in the salinas scene dataset. class number class name # train samples train ratio # test samples 1 brocoli_green_weeds_1 372 3.71% 2009 2 brocoli_green_weeds_2 197 6.87% 3726 3 fallow 139 3.64% 1976 4 fallow_rough_plow 268 2.57% 1394 5 fallow_smooth 396 4.95% 2678 6 stubble 358 7.32% 3959 7 celery 1127 6.61% 3579 8 grapes_untrained 620 20.82% 11271 9 soil_vinyard_develop 328 11.46% 6203 10 corn_senesced_green_weeds 107 6.06% 3278 11 lettuce_romaine_4wk 193 1.98% 1068 12 lettuce_romaine_5wk 91 3.57% 1927 13 lettuce_romaine_6wk 107 1.68% 916 14 lettuce_romaine_7wk 727 1.98% 1070 15 vinyard_untrained 181 13.43% 7268 16 vinyard_vertical_trellis 372 3.34% 1807 total 5211 3.71% 54129 166 eren can seyrek, murat uysal figure 2. true color image compositions and corresponding ground truth data for the hyrank loukia, houston 2013, and salinas scene datasets. 4. experimental setup classification of hsis was performed using a workstation with intel® xeon™ e-2136 processor and nvidia geforce rtx-2070 super graphic processor. the software setup comprised windows 11 x64 as an operating system and python 3.7.9 for programming tasks. model development utilized keras 2.3.1 (chollet, 2015), tensorflow 2.1.0 (abadi et al., 2016) with cuda support, and scikit-learn 0.23.2 (pedregosa et al., 2011) libraries. 5. performance evaluation to assess and compare the performance of the classification models, several evaluation metrics were computed from the confusion matrices. these metrics included oa, pa, ua, f score, and κ. oa, a commonly used performance metric, represents the ratio of correctly classified pixels to the total number of pixels. the f score is calculated as the harmonic mean of pa (precision) and ua (recall) for each class. the κ is a statistical measure indicating the agreements between ground truth and classification maps. these calculated range from 0 to 1, with higher values indicating better classification performance. additionally, mcnemar’s test was employed to examine the statistical significance of prediction differences between the classification algorithms. mcnemar’s test is a non-parametric test based on chi-square distribution and is utilized to determine dichotomous dependent variable between two related groups (i.e., pixel classes predicted by two different classification models). the test value (x2) is calculated from a 2×2 sized contingency table by following equation (foody, 2004). χ2 = 12 − 21| |−1( )2 12 − 21( ) (6) where f12 represents the number of samples that are correctly classified by the first model but misclassified by the second model, and f21 represents the number of the samples misclassified by the first model, and correctly classified by the second model. if the derived x2 value exceeds the critical value from x2 table (3.841) at a 95% confidence level, the null hypothesis is rejected. the rejection indicates statistically significant difference in oa between the predictions of the two models. 167investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification 6. results initially, pca transformation was implemented on the hsis, retaining the first 15 principal components across all datasets. subsequently, 10% of the ground truth data was split for training the models, with the remaining 90% reserved for testing the models. grid search was performed for svm and rf algorithms on each hsi dataset to determine the optimal user-defined parameters. the search ranges for each parameter, along with the determined optimal parameters for each dataset, are detailed in table 5. table 5. hyperparameter tuning results with grid search method. a lg or ith m parameter search range hyrank loukia dfc13 salinas scene sv m c 0.001, 0.01, 0.1, 1 ,10, 100, 1000 10 100 100 γ 1, 0.1, 0.01, 0.001, 0.0001, 0.00001 0.01 0.1 0.01 r f mrty ‘sqrt’, ‘log2’ log2 sqrt sqrt ntree 100, 250, 500, 750, 1000, 1200 100 750 1200 the evaluation of classification performance using the hyrank loukia dataset is summarized in table 6. note that bold texts highlight the highest superior values within the same performance metric row. as can be seen from the table, the modified hybridsn cnn algorithm achieves the highest oa and κ coefficient with 89.69% and 87.74%, respectively. in contrast, rf exhibits the lowest oa and κ coefficient with 80.86% and 76.20%, respectively, for the hyrank loukia dataset. upon examining the class-based accuracy metrics, it is evident that the both cnn algorithms demonstrates superior performance compared to other algorithms across most classes. of particular significance is its sufficiency in classes such as dense urban fabric, fruit trees, broad leaved forest, coniferous forest, and mixed forest, where both svm and rf underperformed, exhibiting lower ua scores. this observation underscores the significant contribution of the feature extraction process of cnn to the accuracy of classifying low-resolution hsi. when examining class-based accuracy metrics for cnn models, it was found that the modified hybridsn cnn model slightly improved the classification accuracy for certain classes (dense urban fabric, fruit trees, coniferous forest, dense sclerophyllous vegetation, sparse sclerophyllous vegetation, and sparsely vegetated areas) compared to the original hybridsn cnn model. in terms of training processing times, svm stands out for its rapid processing, whereas the rf requires more processing time. in contrast, rf had the shortest classification time for processing the entire image, whereas svm took the longest time for classification. the classification maps of the algorithms applied to the hyrank loukia dataset, alongside the true color composition of the scene, are given in figure 3. visual interpretation of the classification maps is very challenging owing to the discontinuous land patterns and low spatial resolution of the scene. nonetheless, notable classification errors are discernible in certain pixels. for instance, the svm and both cnn algorithms misclassifies some pixels located within the inner side of the sea (e.g., pixels incorrectly classified as land within the sea). furthermore, certain pixels, which should have been classified as coastal water, are misclassified as sparse sclerophyllous vegetation. additionally, the cnn algorithm eliminated certain details, such as roads. despite the overall similarity in classification maps produced by cnn algorithms, the modified hybridsn algorithm seems to provide a superior classification map compared to the original hybridsn model. this conclusion is drawn from the classification of the sea region in the lower right corner of the scene and the lower incidence of misclassified pixels in the sea. table 6. the performance analysis of the classification algorithms on hyrank loukia hsi dataset. class name svm rf hybridsn cnn modified hybridsn cnn pa ua f score pa ua f score pa ua f score pa ua f score dense urban fabric 71.69 75.29 73.45 84.80 55.98 67.44 93.69 80.31 86.49 91.60 84.17 87.73 mineral extraction sites 96.15 83.33 89.29 95.56 71.67 81.90 100.00 93.33 96.55 96.23 85.00 90.27 non irrigated arable land 90.16 80.74 85.19 88.52 75.82 81.68 95.24 86.07 90.42 95.43 85.66 90.28 fruit trees 80.36 63.38 70.87 100.00 21.13 34.88 90.91 56.34 69.57 68.42 73.24 70.75 olive groves 91.38 91.67 91.53 85.25 87.07 86.15 91.86 94.85 93.33 90.02 94.45 92.18 broad leaved forest 88.61 34.83 50.00 86.30 31.34 45.99 90.97 65.17 75.94 94.55 51.74 66.88 coniferous forest 87.50 38.89 53.85 91.50 50.22 64.85 89.52 66.44 76.28 80.78 73.78 77.12 mixed forest 62.25 55.03 58.42 66.33 53.68 59.34 84.18 86.01 85.08 77.89 88.70 82.95 dense sclerophyllous vegetation 72.41 87.35 79.18 73.11 87.20 79.54 85.03 89.05 86.99 87.57 90.42 88.98 sparce sclerophyllous vegetation 82.32 82.32 82.32 78.10 85.26 81.52 85.34 89.73 87.48 90.22 87.79 88.99 sparsely vegetated areas 82.81 58.40 68.50 83.70 52.34 64.41 91.81 86.50 89.08 90.93 88.43 89.66 rocks and sand 97.12 84.70 90.49 92.29 84.70 88.33 99.75 89.73 94.47 96.34 90.18 93.16 water 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 coastal water 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.51 100.00 99.75 oa 81.61 80.86 89.29 89.69 κ 77.82 76.85 87.21 87.74 training time (s) 5.47 16.21 7.61 8.42 classification time (s) 18.75 7.16 11.64 12.27 168 eren can seyrek, murat uysal a b c d e figure 3. classification maps for the hyrank loukia dataset: (a) svm, (b) rf, (c) hybridsn cnn, (d) modified hybridsn cnn, and (e) true color composition table 7. the performance analysis of the classification algorithms on houston 2013 hsi dataset. class name svm rf hybridsn cnn modified hybridsn cnn pa ua f score pa ua f score pa ua f score pa ua f score healthy grass 96.12 98.06 97.08 97.95 96.36 97.15 96.03 99.76 97.86 98.48 99.51 98.99 stressed grass 98.76 97.02 97.88 95.97 98.32 97.13 100.00 98.09 99.04 100.00 98.55 99.27 synthetic grass 100.00 99.58 99.79 100.00 98.88 99.44 100.00 97.90 98.94 99.72 98.60 99.16 trees 98.60 98.68 98.64 98.76 98.33 98.55 97.25 99.38 98.31 98.36 100.00 99.17 soil 99.83 99.23 99.53 98.22 99.06 98.64 99.91 100.00 99.96 98.73 100.00 99.36 water 100.00 97.38 98.67 100.00 98.03 99.01 100.00 94.10 96.96 100.00 96.39 98.16 residential 93.22 94.20 93.71 92.31 90.36 91.32 99.75 90.44 94.87 97.82 97.82 97.82 commercial 94.48 91.22 92.82 95.46 89.66 92.47 93.53 95.98 94.74 98.81 95.57 97.16 road 90.17 93.14 91.63 83.71 88.13 85.86 97.03 98.07 97.55 99.03 94.78 96.86 highway 93.38 92.36 92.86 89.76 91.65 90.70 97.55 99.38 98.45 96.32 99.92 98.09 railway 91.44 93.26 92.34 85.24 90.99 88.02 99.93 97.16 98.52 98.09 98.37 98.23 parking lot 1 91.74 92.38 92.06 87.55 89.11 88.32 99.07 99.84 99.46 97.94 99.77 98.84 parking lot 2 78.60 72.68 75.52 88.24 63.05 73.54 94.07 97.20 95.61 98.34 93.52 95.87 tennis court 93.08 99.56 96.21 91.04 99.56 95.11 97.45 100.00 98.71 97.45 100.00 98.71 running track 100.00 97.91 98.94 98.05 98.05 98.05 95.35 100.00 97.62 95.86 100.00 97.89 oa 94.58 92.72 97.83 98.29 κ 94.14 92.12 97.66 98.15 training time (s) 7.12 19.59 8.77 9.25 classification time (s) 43.76 20.12 33.87 36.75 169investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification the evaluation of classification performance using the houston 2013 dataset is summarized in table 7. notably, the modified hybridsn cnn algorithm achieved the highest oa at 98.29%, contrasting with the rf algorithm’s lowest oa of 92.72%. when analyzing class-based accuracy metrics, it is observed that both cnn models generally exhibit higher accuracy values and outperform other algorithms, particularly in the parking lot 2 class. a comparison of cnn algorithms reveals that their class accuracies are similar. the classification maps of the houston 2013 data are given in figure 4. overall, the algorithms extracted the shapes of human-made objects successfully, taking the advantage of the superior spatial and spectral resolution of the dataset. however, a salt-and-pepper effect was noticeable in classification maps of the svm and the rf, stemming from pixel-wise classification. misclassified pixels were observed, particularly by svm and rf, in areas such as stadium corners and regions affected by cloud shadows, resulting in significant pixel misclassification. notably, the shaded area has proven difficult for all algorithms, likely due to insufficient training data for this region. this indicates that the lighting conditions of the area being classified have a direct impact on classification performance. moreover, it is evident that the modified hybridsn cnn algorithm performs better at building classification in the shaded area than the hybridsn cnn algorithm. table 8 represents the performance analysis results for the salinas scene dataset. based on the calculated oa and κ values, the modified hybridsn cnn algorithm outperformed svm and rf, achieving an oa of 99.85% and a κ value of 99.83. overall, the modified hybridsn cnn algorithm exhibits slightly higher class-based performance metrics compared to other algorithms. notably, in classes such as grapes_untrained and vinyard_untrained, cnn outperformed other algorithms. upon reviewing processing times, cnn stands out for its shortest processing time, in terms of both training and classifying the whole scene. a b c d e figure 4. classification maps for the houston 2013 dataset: (a) svm, (b) rf, (c) hybridsn cnn, (d) modified hybridsn cnn, and (e) true color composition table 8. the performance analysis of the classification algorithms on salinas scene hsi dataset. class name svm rf hybridsn cnn modified hybridsn cnn pa ua f score pa ua f score pa ua f score pa ua f score brocoli_green_weeds_1 100.00 100.00 100.00 100.00 99.83 99.92 100.00 100.00 100.00 100.00 100.00 100.00 brocoli_green_weeds_2 100.00 99.94 99.97 99.79 99.88 99.84 100.00 100.00 100.00 100.00 100.00 100.00 fallow 98.88 99.49 99.19 96.67 99.66 98.15 100.00 100.00 100.00 99.94 100.00 99.97 fallow_rough_plow 98.89 99.68 99.29 98.89 99.68 99.29 99.35 97.93 98.64 99.44 99.76 99.60 (continued) 170 eren can seyrek, murat uysal class name svm rf hybridsn cnn modified hybridsn cnn pa ua f score pa ua f score pa ua f score pa ua f score fallow_smooth 98.67 98.88 98.78 99.62 98.55 99.08 98.89 100.00 99.44 99.88 99.71 99.79 stubble 100.00 99.92 99.96 100.00 99.78 99.89 100.00 100.00 100.00 100.00 100.00 100.00 celery 99.78 99.97 99.88 100.00 99.50 99.75 100.00 99.84 99.92 100.00 100.00 100.00 grapes_untrained 83.02 91.41 87.01 81.03 91.01 85.73 99.88 98.37 99.12 99.89 99.52 99.70 soil_vinyard_develop 99.34 99.96 99.65 99.13 99.87 99.50 100.00 100.00 100.00 99.98 100.00 99.99 corn_senesced_green_weeds 98.41 96.54 97.47 97.23 95.22 96.22 99.63 99.53 99.58 100.00 99.86 99.93 lettuce_romaine_4wk 96.94 98.75 97.84 94.88 96.46 95.67 98.26 100.00 99.12 99.69 100.00 99.84 lettuce_romaine_5wk 99.48 99.77 99.63 98.30 100.00 99.14 100.00 99.83 99.91 100.00 100.00 100.00 lettuce_romaine_6wk 98.67 98.91 98.79 98.44 99.15 98.79 100.00 99.88 99.94 100.00 100.00 100.00 lettuce_romaine_7wk 96.90 97.30 97.10 98.62 96.26 97.43 99.90 100.00 99.95 100.00 100.00 100.00 vinyard_untrained 84.71 70.89 77.19 83.29 67.07 74.31 97.70 99.82 98.74 99.27 99.83 99.55 vinyard_vertical_trellis 99.33 99.69 99.51 99.08 99.08 99.08 99.75 99.51 99.63 100.00 100.00 100.00 oa 93.88 93.06 99.52 99.85 κ 93.18 92.25 99.47 99.83 training time (s) 61.45 56.56 26.84 25.38 classification time (s) 16.07 10.27 6.76 6.85 figure 5 illustrates the classification maps generated for the salinas scene dataset. despite algorithms achieving high oa, instances of misclassifications were observed, particularly in the grapes_untrained and vinyard_untrained classes. notably, the classification map of both cnn algorithms contains less salt-and-pepper effects compared to other classification maps. analysis based on ground truth photos by gualtieri et al. (1999) suggests that confusion, particularly in mixed classes, is expected due to similar texture and color characteristics. a statistical analysis comparing the classification algorithms using mcnemar’s test was performed. the test results are presented in table 9. note that the χ2 table value is 3.841 at 95% level of confidence. upon analysis of the results, it is evident that there are statistically significant variances among all classification accuracies, except for the comparison between hybridsn cnn and modified hybridsn cnn on the loukia dataset from hyrank. these findings suggest that employing the cnn algorithm for hsi classification generally yields higher accuracy. however, it’s noteworthy that the choice of activation function and optimizer within the cnn algorithm significantly impacts model performance. table 9. mcnemar’s test results for hyrank loukia, houston 2013, and salinas scene datasets. model 1 model 2 hyrank loukia houston 2013 salinas scene χ2 si gn ifi ca nt ? χ2 si gn ifi ca nt ? χ2 si gn ifi ca nt ? svm – rf 7.604 yes 94.494 yes 81.999 yes svm – hybridsn cnn 406.961 yes 284.944 yes 2463.601 yes svm – modified hybridsn cnn 442.350 yes 410.614 yes 2815.944 yes rf – hybridsn cnn 478.365 yes 573.005 yes 2902.725 yes rf – modified hybridsn cnn 513.757 yes 694.404 yes 3212.820 yes hybridsn cnn – modified hybridsn cnn 2.413 no 16.420 yes 98.462 yes 7. discussion this study compares the performance of four supervised ml classification algorithms for hsi classification. three benchmark datasets, namely hyrank loukia, houston 2013, and salinas scene, were utilized, each possessing varying levels of spectral and spatial resolution. notably, hyrank loukia and houston 2013 datasets are infrequently utilized benchmarks in the literature. in the data preparation stage, pca was employed to reduce the spectral dimensionality of the hsi datasets, retaining the first 15 principal components for each dataset. the classification algorithms were evaluated using common performance evaluation metrics. upon reviewing the classification results of the datasets, it is evident that the classification performance for the hyrank loukia dataset was comparatively lower than that of the other datasets. this discrepancy could potentially be attributed to factors such as an insufficient number of collected ground truth data and the presence of mixed pixels, resulting in similar spectral characteristics between classes. additionally, the lower accuracy observed in vegetation classes may be attributed to the low density of vegetation and the relatively low spatial resolution of the hyperion sensor. notably, the uas for these classes were notably low in the classification results obtained using svm and rf algorithms. similarly, christovam et al. (2019) and si et al. (2021) reported low accuracy values were calculated for the same classes in their studies. in the case of the houston 2013 dataset, a significant number of pixels located within cloud shadows were misclassified. some studies utilizing this dataset as a benchmark also observed misclassifications in shadow areas, while in a few cases, the shadowy areas were removed. it is crucial to retain these effects to impartially test classification algorithms and understand their performance underpotential real-life scenarios. visual interpretation of the classification maps generated for the salinas scene dataset revealed numerous misclassified pixels, despite high accuracy values being obtained. given that the classes predominantly represent vegetation species in the image, the low accuracy may stem from similar spectral characteristics and low vegetation density. therefore, assessing classification performance should involve not only quantitative metrics but also visual interpretation of the results. upon reviewing the classification accuracies across datasets, it was clear that both cnn algorithms consistently outperformed svm and rf. notably, while svm and rf classification maps exhibited a salt-and-pepper effect, cnn tended to eliminate details such as roads and small objects visible on the true color images of the hsis. statistical analysis using mcnemar’s test revealed significant differences in pairwise comparisons of classification maps generated by the algorithms. moreover, upon scrutinizing the processing times of the algorithms, it becomes challenging to definitively determine the superiority of one algorithm over another, as the training and classification times vary across different datasets. however, when focusing solely on classification success, the cnn’s exceptional performance renders it an appealing option for tasks demanding precise classification. the classification outcomes derived from 171investigation of the performances of support vector machine, random forest, and 3d-2d convolutional neural network for hyperspectral image classification the modified hybridsn cnn and hybridsn cnn algorithms underscored the significance highlighted in literature regarding the selection of activation functions and optimizers. 8. conclusion this study examined the comparative analysis of four supervised ml algorithms for hsi classification utilizing three benchmark datasets. the investigation provided valuable insights into the strengths and limitations of svm, rf, and cnn in handling varying spectral and spatial characteristics of hsi. it was observed that the performance of classification algorithms varied across datasets, with factors such as insufficient ground truth data, mixed pixels, and spectral similarity between classes affecting classification performances. while svm and rf performed well performance in certain cases, particularly in datasets with less complex spectral characteristics, cnn models consistently outperformed them, especially in datasets with complex spectral signatures and spatial features. the capacity of cnn to extract features and capture complex spatial patterns made it a compelling choice for tasks requiring high precision classification. moreover, the hybridsn cnn architecture underwent modifications with the expectation that it would influence the classification performance of activation functions and optimizers in the cnn algorithm. in a b c d e figure 5. classification maps for the salinas scene dataset: (a) svm, (b) rf, (c) hybridsn cnn, (d) modified hybridsn cnn, and (e) true color composition line with literature recommendations, the modified hybridsn cnn algorithm, integrating the adamax optimizer and mish activation function, exhibited a slight enhancement in classification performance. furthermore, the study emphasized the significance of not only relying on quantitative metrics but also incorporating visual interpretation of classification results to gain a comprehensive understanding of algorithm performance. overall, the findings contribute to advancing the field of hsi classification by providing valuable insights into the comparative performance of ml algorithms across various datasets. these insights can inform the selection of appropriate algorithms based on specific application requirements, ultimately enhancing the accuracy and efficiency of hsi analysis in various domains. acknowledgments the study constitutes the master’s thesis of the first author. the authors thank the isprs commission iii, wg iii/4 for providing the hyrank dataset and to the national center for airborne laser mapping, the hyperspectral image analysis laboratory at the university of houston, and the ieee grss image analysis and data fusion technical committee for providing the houston dataset used in the experiments. 172 eren can seyrek, murat uysal funding this study was supported by afyon kocatepe university scientific research projects coordination unit within the 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(2025). mineralogical characteristics and colouring mechanism of hydrothermal fluorite in the sumochagan obo deposit, inner mongolia, north china. earth sciences research journal, 29(2),139148. https://doi.org/10.15446/esrj.v29n2.115986 the sumochagan obo fluorite deposit, located in the northern part of siziwangqi, inner mongolia, is recognized as a world-class fluorite-only deposit. however, research on the mineralogical characteristics and genesis of fluorite in this area remains relatively limited. to further elucidate the coloring and formation mechanisms of fluorite in this area, this research investigates the mineralogical and spectral characteristics of green and purple fluorite using diamond view, infrared spectroscopy, ultraviolet-visible absorption spectroscopy, and laser raman spectroscopy. geochemical analysis was conducted using energy dispersive x-ray fluorescence spectrometry, wavelength dispersive x-ray fluorescence spectrometry, and inductively coupled plasma mass spectrometry (icp-ms). in the ultraviolet-visible absorption spectrum, green fluorite samples exhibit absorption peaks at 223 nm, associated with y and f centers. absorption peaks at 693, 440, 360, and 276 nm are attributed to the presence of sm²+, while peaks at 587 and 591 nm are caused by calcium colloidal color centers. the purple color of fluorite is likely due to yo₂ centers, as indicated by absorption peaks at 276 and 261 nm in the ultraviolet-visible absorption spectrum of purple samples. through the tb/ca-tb/la plot, it is observed that most fluorite samples from this mining area fall within the hydrothermal field, which aligns with geological observations. the measured total rare earth elements (∑ree) of fluorite in this deposit fall between magmatic-hydrothermal fluids and meteoric water, indicating that the deposit was formed by the mixing of magmatic-hydrothermal processes with meteoric water involvement. keywords: hydrothermal fluorite; rare earth elements; colouring mechanism; magmatic-hydrothermal deposit; sumochagan obo deposit. características mineralógicas y mecanismo de coloración de la fluorita hidrotermal en el yacimiento sumochagan obo, mongolia interior, norte de china resumen el depósito de fluorita de sumochagan, localizado al norte de siziwangqi, en la región de mongolia interior, es reconocido como un depósito único de clase mundial. sin embargo, la investigación de las características y origen de las fluoritas en esta área aún es relativamente limitado. para definir los mecanismos de formación y coloración de estas fluoritas, este trabajo investiga las características espectrales y mineralógicas de fluoritas violetas y verdes a través de los métodos diamond view, espectroscopía infrarroja, espectroscopía de absorción visible ultravioleta y espectroscopía laser raman. análisis geoquímicos se llevaron a cabo con el método de espectometría fluorescente de energía dispersiva de rayos x, espectometría fluorescente de ancho de onda dispersiva de rayos x y espectrometría de masas de plasma acoplado. en el espectro de absorción visible ultravioleta, las muestras de fluoritas verdes muestran picos de absorción de 223 nm, asociados con centros y y f. los picos de absorción a 693, 440, 360 y 276 nm se atribuyen a la presencia de sm2+, mientras que los de 587 y 591, nm son causados por centros de color coloidal de calcio. el color violeta de las fluoritas es posible que se deba a los centros de yo2, de acuerdo con los picos de absorción de 276 y 261 nm en el espectro de absorción visible ultravioleta de las muestras violetas. en el diagrama tb/ca-tb/la, se observa que la mayoría de las muestras de fluorita de esta zona minera se encuentran dentro del campo hidrotermal, lo cual concuerda con las observaciones geológicas. el total de elementos de tierras raras (∑ree) medidos en fluorita en este depósito se encuentra entre fluidos magmático-hidrotermales y agua meteórica, lo que indica que el depósito se formó mediante la mezcla de procesos magmático-hidrotermales con la participación de agua meteórica. palabras clave: fluorita hidrotermal; elementos de tierras raras; mecanismo de coloración; depósito magmático-hidrotermal; depósito sumochagan obo. https://doi.org/10.15446/esrj.v29n2.115986 mailto:liang.zhang@cugb.edu.cn https://doi.org/10.15446/esrj.v29n2.115986 140 zeng-qi lu, liang zhang, zu-yan chen, biao jiang 1. introduction fluorite, with the main component caf2, is a major source of the chemical element fluorine. it belongs to the cubic crystal system, primarily exhibiting a cubic crystal habit, though octahedral and dodecahedral forms, often seen as twins, can occasionally occur. the crystal structure of fluorite is stable and impurities are not easily incorporated. the radius of the ca ion in fluorite is similar to that of rare earth ions, facilitating isostructural substitution. rare earth elements typically replace ca ions with eight-coordination, entering the crystal lattice (shannon, 1976; boynton and wark, 1984; haschke et al., 2021). due to the geochemical behavior of rare earth elements (ree) being very similar and relatively stable, later geological events rarely alter their distribution pattern in rocks and minerals. therefore, the distribution pattern of ree in fluorite provides significant insights into its formation environment and sources (schwinn and markl, 2005; alipour et al., 2014; rezaei azizi et al., 2018; rezaei azizi et al., 2020). hydrothermal fluorite is a product of ore-forming fluid evolution and serves as an important medium for studying the geochemical behavior of ree in hydrothermal ore-forming processes (ding et al., 2003; sun et al., 2008; sun et al., 2014; huang et al., 2015). fluorite occurs in various colors, commonly green, purple, and blue. the coloring mechanisms can be classified into three categories: complex center coloration involving rare earth ions and/or oxygen, divalent rare earth ion coloration, and metallic calcium colloid coloration. the first category includes f centers coordinated with y elements, coexisting f centers coordinated with y and ce, yo2 centers, and o3substitution. divalent rare earth ions manifest in ion coloration, such as sm2+ and eu2+ (bill and galas, 1978). fluorite has extensive applications in industries such as new energy, national defense, and medicine, making it an important strategic mineral resource. it is primarily used in the industrial manufacturing of hydrofluoric acid and as a flux in the steel industry. with advancements in modern industrial and scientific technologies, fluorite is also used in components of high-power laser devices and rocket fuels (sun et al., 2009, 2010; redina et al., 2020; zhou et al., 2022). additionally, due to its bright colors and high transparency, fluorite is used in the jewelry industry. industrially valuable fluorite deposits are distributed in over forty countries worldwide, predominantly in the circumpacific metallogenic belt (zhao et al., 2020). compared to other regions, china’s fluorite resources have unique advantages. with low impurity content, particularly low levels of phosphorus, sulfur, and arsenic, and favorable mining conditions, they have high development value and hold an important position in global fluorite resources (zhao et al., 2020). researchers have studied several fluorite mining areas in china, including the xitian area in southern guangxi (liu et al., 2020), the yangshan area in luanchuan, henan province (zhang et al., 2021), and the lishan area in pingjiang, hunan province (chen et al., 2023). the sumochagan obo fluorite deposit, located in the northern part of siziwangqi, inner mongolia, is a world-class fluorite-only deposit. current research on this deposit primarily focuses on its ree geochemical characteristics. the fluorite deposits in the area exhibit similar trace element characteristics, indicating a consistent ore-forming material source, with the ore-forming fluids mainly belonging to the h2o-co2-nacl system (xu et al., 2009). research on the gemological characteristics and coloring mechanisms of fluorite in this deposit is still lacking. this research aims to conduct a systematic analysis of the mineralogical and gemological characteristics of the fluorite through basic gemological experiments, spectroscopy, and geochemical analysis. it also seeks to conduct preliminary analyses and discussions on the mineral chemical composition and coloring mechanisms of purple and green fluorite in the deposit. furthermore, through the trace element characteristics of fluorite, it aims to explore its ore-forming process. 2. regional and local geology the sumochagan obo, located in northern inner mongolia, is positioned in the eastern segment of the late paleozoic central asian orogen, between the siberian craton and the north china craton. it is situated between the solon suture zone and the hegenshan deep fault, and lies at the northeastern end of the baiyun obo-erenhot rare earth, iron, gold, and fluorite metallogenic belt (figure 1; yu et al., 2018). the solon suture zone extends from solon obo to linxi, consisting of a mixed rock belt composed of blueschist and serpentinite. the primary rock types include limestone, quartzite, mafic and ultramafic rocks, dolomite, and blueschist (xu et al., 2009). the erenhot-hegenshan deep fault zone trends nee-ne, extending westward into mongolia and eastward, where it is overridden by neogene strata. this fault zone comprises a series of thrust or imbricate faults, serpentinite melange blocks, tectonic breccia zones, and shear zones. the sumochagan obo area possesses abundant fluorite deposits, primarily including the super-large sumochagan obo fluorite deposit and the mediumsized aobaotu fluorite deposit. the exposed strata belong to the lower permian dashizhai formation, consisting of volcano-sedimentary rocks. the deposit is located at the outer contact zone between the volcano-sedimentary rocks of the early permian dashizhai formation and the early cretaceous weijing granites. the exposed strata mainly comprise upper cretaceous sandstone, sand-gravel conglomerate, and locally, jurassic-cretaceous low-grade metamorphic rocks of the ailegema formation, along with volcano-sedimentary rocks of the permian dashizhai formation (xu et al., 2009). magmatic rocks, which are well developed in the region, mainly include early cretaceous medium-coarsegrained porphyritic biotite granite. regional faults primarily trend in a near north-south, north-northwest, and northeast direction (zhang et al., 2019). fluorite are bodies exhibit gentle undulating shapes in both trend and dip directions, generally tilting towards the northwest. a fluorite ore body has a length of 2,900 meters and a thickness ranging from 0.5 to 22.0 meters, with an average thickness of 5.6 meters. and a fluorite ore body extends to a depth of 600 to 800 meters and a vertical depth of 300 to 460 meters (xu et al., 2009). the fluorite ore in this area includes layered, fine-crystalline, banded, coarse-crystalline, gravel, and sandy types. these can be categorized into quartz-fluorite, quartz-sulfide-fluorite, calcite-quartz-fluorite, and fluoritequartz styles, with quartz-fluorite style ore being dominant (xu et al., 2009). fluorite in this area is commonly off-white, tawny, purple and green (xu et al., 2009). figure 1. regional geological map of siziwangqi showing the distribution of fluorite deposits. (modified from bai et al., 2019) 3. sample selection and analytical methods the fluorite samples from the sumochagan obo deposit, siziwangqi, inner mongolia, can be primarily divided into purple and green series. the green fluorite samples are labeled as nss1a1, 1a2, 1b1, 1b2, 1c1, 1c2, 1d1, 1d2, 1e1, and 1e2, while the purple fluorite samples are labeled as nss2a1, 2a2, 2b1, 2b2, 2c1, 2c2, 2d1, and 2d2. 141mineralogical characteristics and colouring mechanism of hydrothermal fluorite in the sumochagan obo deposit, inner mongolia, north china figure 2. green and purple fluorite specimens from sumochagan obo deposit, siziwangqi, inner mongolia. (a) green fluorite sample 1a1; (b) green fluorite sample 1d2; (c) purple fluorite sample 2a1; (d) purple fluorite sample 2c1. luminescence, refractive index, microscopic, and spectroscopic analyses of fluorite samples were conducted at the gem laboratory of the school of gemmology, china university of geosciences, beijing. a diamond view ultraviolet fluorescence spectrometer, produced by the international institute of diamond grading & research, was used to analyze the luminescence characteristics. a tensor 27 fourier-transform infrared spectrometer was employed to obtain the infrared reflection spectrum using the transmission method, with a scanning range of 4000-400 cm-1 and a resolution of 4 cm-1. uv-vis absorption spectroscopy was performed with a gem-3000 fiber optic spectrometer, using the reflection method and a testing wavelength range of 200-800 nm. laser raman spectroscopy was conducted using an hr-evolution micro-raman spectrometer produced by horiba, japan. the laser source wavelength was 532 nm, with a raman laser power of 30-40 mw, a testing range of 100-4000 cm-1, a resolution of 4 cm-1, and a scanning time of 20 seconds. geochemical analysis was carried out using an axios-max wavelength dispersive x-ray fluorescence spectrometer and icp-ms at the analysis and testing center of the beijing research institute of uranium geology (briug). the experimental temperature for axios-max analysis was 24°c, with a relative humidity of 24%. icp-ms trace element composition analysis was performed using an element xr plasma mass spectrometer, with an experimental temperature of 22.3°c and a relative humidity of 19%. for fluorite sample preparation, the samples were crushed and ground, and 25-50 mg of each sample was dissolved in hydrofluoric acid and nitric acid. the solution was heated in an oven at 185±5°c for 24 hours, cooled, and then transferred to a hot plate to evaporate the hydrofluoric acid. after dissolution in nitric acid, icp-ms analysis was conducted. 4. analytical results 4.1 mineralogical characteristics the refractive index of fluorite ranges from 1.428 to 1.433, exhibiting a weak to vitreous luster and varying from translucent to opaque. generally, green fluorite is more transparent than purple fluorite. under long-wave ultraviolet light, both colors display strong fluorescence, with green fluorite exhibiting stronger fluorescence than purple fluorite. however, under short-wave uv light, the fluorescence is weaker or inert. under diamond view, fluorite typically shows blue-white fluorescence. among them, green fluorite exhibits moderate to strong fluorescence intensity (figure 3a-c), whereas purple fluorite predominantly demonstrates medium to weak fluorescence with undetectable phosphorescent phenomenon (figure 3d-f). figure 3. fluorescence phenomenon of green and purple fluorite. (a-c) moderate to strong fluorescence of green fluorite; (d-f) medium to weak fluorescence of purple fluorite. green and purple fluorites have four sets of perfect cleavage, irregular fracture surfaces, and a glassy luster. it is brittle and often associated with quartz. quartz is mostly colorless to milky white, transparent to translucent, with a vitreous luster and conchoidal fracture surfaces, displaying a greasy luster. some fluorite samples also contain small amounts of metallic minerals such as chalcopyrite, pyrite, and sphalerite, which exhibit a metallic luster and are opaque. under magnification, fluorite cleavage and color zoning can be observed in some purple fluorite samples. petrographic analysis reveals the fluorite contains structural discontinuities (figure 4a-c, e), hosts solid inclusions (figure 4d), displays cleavage (figure 4f), shows oscillatory zoning in purple hues (figure 4g), and is associated with metallic minerals (figure 4h, i). figure 4. characteristics of fluorite samples under gem microscope. (a, b, c, e) visible internal fissure; (d) fluid inclusions; (f) clear cleavage; (g) visible purple bands; (h, i) metallic minerals associated with fluorite. 4.2 spectral characteristics 4.2.1 fourier transform infrared spectrum the green fluorite sample nss1c2 exhibits absorption peaks at 1097, 1415, 1512, 1643, 2347, 2383, 2852, 2927, and 3384 cm-1 (figure 5a), while the purple fluorite sample nss2a2 shows absorption peaks at 1082, 1406, 1514, 1643, 2347, 2383, 2852, 2922, and 3384 cm-1 (figure 5b). 142 zeng-qi lu, liang zhang, zu-yan chen, biao jiang analysis of the absorption peak positions in the infrared spectra indicates the presence of characteristic absorption peaks of caf2, co3 2stretching vibrations, organic functional group vibrations, and o-h stretching vibrations in both the green fluorite sample nss1c2 and the purple fluorite sample nss2a2. the summarized results are presented in table 1. figure 5. infrared transmission spectrum of fluorite samples. (a) green fluorite sample 1c2; (b) purple fluorite sample 2a2. table 1. infrared spectrum absorption peaks and peak assignments of fluorite in the sumochagan obo area (unit: cm-1) peak position type nss1c2 (green) nss2a2 (purple) characteristic absorption peaks of caf2 1097 1082 telescopic vibration of co3 21512/1415 1643 1514/1406 1643 vibration of organic matter groups 2383/2347 2383/2347 o-h telescopic vibration 3384/2927/2852 3384/2922/2852 4.2.2 uv-visible absorption spectrum results from the fiber optic spectrometer indicate that sample nss1a2 exhibits absorption at 223, 264, 275, 363, and 440 nm (figure 6a), while sample nss1b2 exhibits absorption at 263, 276, 363, 444, and 587 nm (figure 6b). sample nss2a2 exhibits absorption at 275, 313, 360, and 412 nm (figure 6c), while sample nss2b2 exhibits absorption at 261 and 401 nm (figure 6d). figure 6. uv absorption spectrum of fluorite samples. (a) green fluorite sample 1a2; (b) green fluorite sample 1b2; (c) purple fluorite sample 2a2; (d) purple fluorite sample 2b2. 4.2.3 raman spectrum fluorite from sumochagan obo in siziwangqi, inner mongolia, exhibits relatively stable raman absorption peaks at 321 and 323 cm-1 (figure 7). additionally, the green fluorite samples nss1b2 and nss1c2 show absorption peaks around 195, 251, 271, 440, as well as 507, 563, and 646 cm-1 (figure 7a, b). in contrast, the laser raman spectrum of the purple fluorite samples nss2c2 and nss2d2 presents more uniform absorption peaks, with a peak at 321 cm-1 and a broad absorption band around 1400-1700 cm-1 (figure 7c, d). based on the raman spectrum data, fluorite samples exhibit four types of characteristic absorption peaks, including those of ree, and peaks of caf2, sio2, and fe3o4. the summarized data are presented in table 2. other peaks are generally caused by impurity elements or inclusions, as some minerals from host rocks mixed in the samples may affect the testing spectrum to some extent. figure 7. laser raman spectrum of fluorite samples. (a) green fluorite sample 1b2; (b) green fluorite sample 1c2; (c) purple fluorite sample 2c2; (d) purple fluorite sample 2d2. table 2. laser raman spectrum data of green and purple fluorite (unit: cm-1) characteristic absorption nss1b2 (green) nss1c2 (green) nss2c2 (purple) nss2d2 (purple) caf2 321 323 321 321 sio2 195/251/ 271/440 195/251/ 271/440 / / fe3o4 507/563/646 507/563/646 / / rare earth peak / / 1400~1700 / 4.3 chemical composition analysis 4.3.1 wavelength dispersive x-ray fluorescence spectrometer a total of nine samples were tested, including five green fluorite samples (nss1a1, nss1b1, nss1c1, nss1d1, nss1e1) and four purple fluorite samples (nss2a1, nss2b1, nss2c1, nss2d1). the major elemental data of the samples are shown in table 3. the calcium content in green fluorite is generally higher than in purple fluorite, approximately 66.1%. fluorite contains 1.69% si, 0.17% al, 0.54% mg, and trace amounts of fe, na, k, mn, ti, p, and other elements. the loss on ignition of green fluorite is generally lower compared to purple fluorite. 4.3.2 icp-ms chemical composition analysis the icp-ms trace element analytical results are shown in table 4. the total ree content of the green sample nss1a1 is anomalously high, while that of the green sample nss1c1 is anomalously low. however, the values of the other samples are within the normal fluctuation range (figure 8a). the average total ree content of green fluorite samples is 114.407 ppm, with a median of 92.42 ppm, while the average total ree content of purple fluorite samples is approximately 82.181 ppm, with a median of about 87.55 ppm. analyzing and comparing the lree/hree values of the different fluorites in this area 143mineralogical characteristics and colouring mechanism of hydrothermal fluorite in the sumochagan obo deposit, inner mongolia, north china (figure 8b), the average lree/hree value of green fluorite is 3.547, while that of purple fluorite is 2.086. the lree/hree value of green fluorite is significantly higher than that of purple fluorite. both green and purple fluorites show negative eu anomalies. the δeu values for green fluorite range from 0.246 to 0.398, with an average of 0.332, while those for purple fluorite range from 0.384 to 0.403, with an average of 0.393. both green and purple fluorites exhibit positive ce anomalies. the δce values for green fluorite range from 0.942 to 1.668, with an average of 1.183, while those for purple fluorite range from 1.217 to 1.679, with an average of 1.426 (figure 9). the other trace elements in green and purple fluorites are shown in figure 10. the content of most elements in these samples is generally low, but there are significant differences in the content of some elements between samples. for example, some purple fluorite samples have higher sr, w, or co content, while some green fluorite samples show slightly higher zn or pb content. the remaining trace element contents are relatively stable. table 3. trace elemental data of green and purple fluorite (unit: wt%) number si al mgo cao na k mno ti p loss on ignition* detection limits of the elements 0.010 / / / 0.010 0.010 0.004 0.006 0.006 / nss1a1 (green) 0.636 0.358 0.060 68.00 0.015 0.009 n.d. n.d. 0.021 0.49 nss1b1 (green) 0.411 0.042 0.058 68.16 0.012 n.d. n.d. n.d. n.d. 0.40 nss1c1 (green) 0.509 0.049 0.060 67.82 0.021 n.d. n.d. n.d. n.d. 0.62 nss1d1 (green) n.d. 0.017 0.053 67.88 0.018 n.d. n.d. n.d. 0.013 0.63 nss1e1 (green) 4.880 0.509 0.219 63.57 0.029 0.017 0.006 0.011 0.023 2.75 nss2a1 (purple) 0.057 0.024 0.069 68.68 n.d. n.d. n.d. n.d. n.d. 1.77 nss2b1 (purple) 3.180 0.067 0.054 65.85 n.d. n.d. n.d. n.d. 0.022 1.91 nss2c1 (purple) 3.360 0.289 0.124 64.96 0.016 0.036 n.d. 0.011 0.066 2.68 nss2d1 (purple) 0.470 0.160 4.180 59.91 0.014 0.023 0.028 0.006 0.115 9.14 table 4. ree composition and characteristics of green and purple fluorite (ppm) number nss1a1 (green) nss1b1 (green) nss1c1 (green) nss1d1 (green) nss1e1 (green) nss2a1 (purple) nss2b1 (purple) nss2c1 (purple) nss2d1 (purple) la 54.80 28.80 9.050 25.80 23.10 16.10 1.260 34.70 21.00 ce 35.70 18.30 5.010 17.60 22.80 11.70 1.360 31.20 16.60 pr 11.30 5.060 1.590 4.990 4.240 3.120 0.360 6.470 4.720 nd 54.90 22.20 6.140 19.30 18.00 13.10 1.390 25.10 17.70 sm 12.80 4.600 1.010 3.530 3.100 3.150 0.380 6.470 4.270 eu 5.660 1.200 0.440 0.990 1.250 1.390 0.150 2.670 1.810 gd 15.70 5.010 1.200 4.520 3.680 3.750 0.380 7.220 5.150 tb 3.620 0.850 0.240 0.820 0.780 0.890 0.100 1.510 1.210 dy 23.70 5.460 1.380 5.210 4.490 6.650 0.650 10.40 9.060 ho 5.590 1.360 0.330 1.280 1.060 1.510 0.150 2.500 2.390 er 15.80 4.000 0.990 3.580 2.990 4.520 0.440 7.600 6.480 tm 2.540 0.660 0.160 0.550 0.500 0.910 0.100 1.260 1.340 yb 15.50 3.500 1.190 3.740 3.280 6.040 0.740 7.990 8.620 lu 2.000 0.460 0.170 0.510 0.400 0.810 0.110 0.980 1.110 y 294.0 71.00 16.50 68.60 52.90 67.30 4.410 111.0 98.80 li 2.570 1.380 1.420 0.170 10.40 2.450 2.940 5.200 0.821 be 0.262 0.385 0.501 1.930 4.110 1.820 1.380 7.490 13.40 sc 0.213 0.138 0.149 0.501 3.330 1.900 0.654 3.290 4.460 v 0.100 0.014 0.123 0.046 2.320 0.335 0.031 2.470 3.740 cr 0.840 0.473 5.710 1.200 2.530 1.070 2.290 2.600 3.040 co 24.80 21.20 15.10 11.90 42.90 36.90 56.60 76.90 24.40 ni 22.80 23.40 22.50 26.10 24.10 20.60 23.40 25.00 21.50 cu 0.433 0.172 0.121 0.059 2.520 0.230 0.234 1.790 0.674 zn 2.320 1.270 0.778 1.510 110.0 1.060 1.330 17.10 28.90 (continued) 144 zeng-qi lu, liang zhang, zu-yan chen, biao jiang number nss1a1 (green) nss1b1 (green) nss1c1 (green) nss1d1 (green) nss1e1 (green) nss2a1 (purple) nss2b1 (purple) nss2c1 (purple) nss2d1 (purple) ga 0.495 0.217 0.120 0.182 1.150 0.816 0.309 1.760 0.682 rb 0.125 0.036 0.056 0.019 1.240 0.257 0.199 2.640 1.050 sr 18.60 24.30 57.90 86.00 63.30 109.0 99.70 93.20 201.0 mo 0.043 <0.002 0.142 0.020 2.020 0.071 0.054 1.020 0.640 cd 0.112 0.037 0.073 0.091 0.844 0.040 0.073 0.510 1.540 in 0.002 <0.002 0.002 0.002 0.066 <0.002 0.002 0.012 0.006 sb 0.316 0.312 0.674 0.075 10.90 0.294 3.250 5.390 1.260 cs 0.041 0.052 0.103 0.005 1.260 0.166 0.248 1.140 0.358 ba 1.120 1.250 1.120 2.600 18.60 3.380 1.870 19.30 44.90 w 130.0 65.60 73.30 62.40 86.80 73.30 185.0 85.80 48.60 re 0.003 0.003 <0.002 0.002 0.002 0.002 0.002 <0.002 0.002 tl 0.005 <0.002 <0.002 <0.002 0.153 0.005 0.007 0.065 0.101 pb 0.362 0.057 0.173 0.098 106.0 0.666 0.188 3.380 1.290 bi 0.004 0.005 <0.002 <0.002 1.660 <0.002 0.002 0.090 0.003 th 0.125 0.135 0.064 0.218 0.331 0.070 0.041 0.444 0.292 u 0.076 0.215 0.101 0.462 3.290 0.256 0.245 2.160 6.230 nb 0.027 0.007 0.004 0.010 0.046 0.027 0.020 0.086 0.062 ta 0.100 0.027 0.017 0.020 0.035 0.029 0.039 0.027 0.030 zr 0.142 0.084 0.074 0.224 1.070 0.525 0.328 1.300 1.850 hf 0.248 0.021 0.011 0.041 0.064 0.074 0.021 0.123 0.106 sn 0.056 0.025 0.022 0.020 0.346 0.092 0.087 0.341 0.169 σree 259.6 101.5 28.90 92.42 89.67 73.64 7.570 146.1 101.5 σlree 175.2 80.16 23.24 72.21 72.49 48.56 4.900 106.6 66.10 σhree 84.45 21.30 5.660 20.21 17.18 25.08 2.670 39.46 35.36 σlree/ σhree 2.074 3.763 4.106 3.573 4.219 1.936 1.835 2.702 1.869 la/lu 27.40 62.61 53.24 50.59 57.75 19.88 11.45 35.41 18.92 δeu 0.397 0.250 0.398 0.246 0.369 0.403 0.395 0.390 0.384 δce 1.080 1.081 0.942 1.143 1.668 1.217 1.679 1.516 1.291 note: δeu=2eun/(smn+gdn), δce=2cen/(lan+prn). figure 8. (a) the box plots of total ree concentrations for green and purple fluorite samples; (b) the box plots of lree/hree ratios for green and purple fluorite samples. figure 9. distribution of ree chondrite-normalized patterns for green and purple fluorite samples. 145mineralogical characteristics and colouring mechanism of hydrothermal fluorite in the sumochagan obo deposit, inner mongolia, north china figure 10. spider diagrams of trace elements for green and purple fluorite samples. 5. discussion 5.1 coloration mechanism previous studies have identified three main coloration mechanisms in fluorite: impurity-related color centers, divalent rare earth ion coloration, and coloration by metal calcium colloids. these mechanisms have been widely applied in investigating various fluorite coloration phenomena (table 5; bill and galas, 1978). impurity-related color centers: impurity-related color centers in fluorite arise because caf2, the primary component of fluorite, typically contains cationic impurities. at temperatures below 100 °c, oxygen ions can be introduced into the crystal lattice through hydrolysis during hydrothermal activities (bontinck, 1958). at high temperatures, oxygen ions combine with cationic impurities to form localized molecular structures. when irradiated with ionizing radiation at room temperature, crystals containing impurities exhibit many optical absorption bands associated with these impurities (bill and galas, 1978). these impurity-induced color centers can be categorized into three types: (a) rare-earth element and f centers. studies on naturally colored fluorites suggest that y-complexed f centers generally result in blue coloration. these color centers exhibit c3v point symmetry along the [111] axis and are associated with y3+ ions related to fvacancies. linear dichroism experiments (staebler and schnatterly, 1971) and research by alig (1971) indicate that the three low-energy bands polarize in increasing order of energy, consistent with transitions from the 1a1 ground state orbital to the 1e, 2a1, and 2e levels. this polarization arises from the overlap of the f center wavefunction with the 4d and 5s orbitals of y3+ (staebler and schnatterly, 1971). f centers in the uv-visible absorption spectrum show weak supplementary lines at 580/400/335/224 nm, and at 250/310/720 nm for y and f centers. additional bands near 300 nm, a central absorption band at 485 nm, and peaks at 580/714 nm are also observed for y, ce, and f centers (bill and galas, 1978). the 223 nm absorption peak in sample nss1a2 may be attributed to the bluish tint of fluorite. (b) yo2 centers. these centers impart a rose color to fluorite and consist of three nearest neighboring f centers within the fluorite lattice. yo2 centers can be generated in caf2 crystals doped with yf3 that have undergone hydrolysis upon exposure to x-rays. research using epr and endor techniques indicates that yo2 centers are composed of o2 3ions and are stabilized by y3+ ions (bill and galas, 1978). in the uv-visible absorption spectrum, the absorption peaks are primarily observed at 485/225/260/270/365 nm (bill and galas, 1978). uvvisible absorption spectra of samples nss2a2 and nss2b2 exhibit absorption peaks at 275/261 nm, indicating the presence of yo2 color centers. (c) o2ion replacement. o2ions replace two nearest neighboring fions with the directions of the two outer oxygen nuclei parallel to the tetragonal crystal axis. the molecular ions undergo vibrational motion, with charge compensation primarily carried out by substituted na+ ions. this is manifested by a broad absorption band at 434 nm and an absorption peak at 230 nm (bill and galas, 1978). divalent rare earth ion coloration. the primary color centers of rare earth ions in fluorite are typically sm2+, which generally imparts a green color to the mineral. previous studies suggested that the reduction of radiation-induced trivalent rare earth ions can only occur at sites located in the cubic position, which possess remote charge compensation. hayes and twidell (1961) proposed the repulsive coulomb effect of negative charge compensators, which impedes the capture of electrons by adjacent rare earth ions. consequently, divalent rare earth ions can only occupy the cubic position. during the transition from trivalent to divalent rare earth ions, the energy of the 5d orbitals decreases relative to the 4f orbitals. the broadening of the absorption spectrum arises from the interaction between the unshielded 5d electrons and lattice vibrations, with the intensity of the allowed transitions contrasting with the energy bands formed by forbidden internal transitions (bill and galas, 1978). the absorption peaks of sm2+ color centers are located at 690/611/440 nm, with a broad absorption band at 422 nm and in the ultraviolet region at 360/335/305/279 nm, possibly interfered by strong defect absorption. additionally, eu2+ may produce an absorption peak around 413 nm. the uv-visible spectra of green fluorite samples nss1a2, nss1b2, and nss1c2 exhibit absorption peaks at 693/440/360/276 nm, potentially attributed to sm2+ color centers, with ree content of approximately 3.31 ppm. metal calcium colloids. high-radiation environments can impart color to colorless fluorite, resulting in non-uniform and highly stable colors (liu et al., 1983). heating in the vapor of metallic calcium can cause calcium atoms to enter the fluorite structure in the form of colloidal particles, giving colorless fluorite a purple hue (li et al., 1985). the formation of colloidal calcium primarily relies on crystal defects in fluorite to provide sites (li et al., 1985). this is manifested by absorption peaks around 560-580 nm in the uv-visible spectrum. sample nss1b2 exhibits absorption at 587 nm, and sample nss1c2 shows absorption at 591 nm, suggesting that this may be attributed to colloidal calcium color centers. xrf testing indicates higher calcium content in green fluorite, suggesting that in some cases, calcium in sumochagan obo fluorite may exist in the form of colloidal calcium within crystal defects, thereby causing coloration. in summary, the coloration mechanisms in fluorite from the sumochagan obo deposit vary among samples. the primary reasons for the green coloration are rare earth with f centers, sm2+, and colloidal calcium color centers, while the yo2 center is responsible for the purple coloration of fluorite (table 5). table 5. characteristic absorption peaks of fluorite from the sumochagan obo deposit absorption peak position nss1a2 (green) nss1b2 (green) nss1c2 (green) nss2a2 (purple) nss2b2 (purple) references color center caused by impurities 223/264/275/363 263/276/363 263/273/364 275 313 261 401 (bill and galas, 1978) color center of rare earth ions 275/363/440 276/363/444 364/427/693 275/360/412 / (bill and galas, 1978) calcium colloid color center / 587 591 / / (bill and galas, 1978) 146 zeng-qi lu, liang zhang, zu-yan chen, biao jiang 5.2 ore-forming fluid source the atomic ratio of tb/ca reflects the chemical environment of fluorite crystallization and holds genetic significance, while the tb/la atomic ratio indicates the degree of ree fractionation in fluids and the alteration effects of ore-forming fluids during mineralization. a bivariate plot of tb/la against tb/ca can be used to determine the genetic types of fluorites and ascertain whether ore-forming fluids have undergone hydrothermal reactions with host rocks (schneider et al., 1975; möller, 1976). previous studies have used tb/ ca-tb/la plots to analyze multiple fluorite samples worldwide, classifying deposit genesis into pegmatite, hydrothermal, and sedimentary types (möller and morteani, 1983; bau and dulski, 1995). figure 11. diagram of tb/ca-tb/la for green and purple fluorite. fluorite in magmatic-hydrothermal deposits typically occurs with other major minerals (wang et al., 2019; li, 2021). in deposits where meteoric water is the primary ore-forming fluid, fluorite is the main mineral (suo et al., 2020; wang et al., 2020). the tb/ca-tb/la plot of fluorite from sumochagan obo, siziwangqi, inner mongolia (figure 11), suggests a predominantly hydrothermal origin, consistent with geological and petrological observations. this is similar to findings in yangshan in western henan (feng et al., 2020), checun in henan (dong et al., 2020), huangbi in jiangxi (huang et al., 2018), pucheng in fujian (zhu et al., 2021), and xiaodongzi in butuo (luo et al., 2020). previous studies suggest that in the tb/ca-tb/la plot, fluorite samples closer to the sedimentary area tend to extract more ca elements from host rocks during formation, indicating a higher degree of water-rock interaction (dill et al., 1986). the purple fluorite samples from this mining area, compared to the green fluorite samples, are closer to the sedimentary zone, suggesting a potentially higher degree of water-rock interaction during their precipitation. by summarizing the characteristics of ree in different fluorite deposits and the sources of ore-forming fluids from previous studies, it can be inferred that fluorite deposits primarily sourced from magmatic-hydrothermal fluids generally have higher ∑ree content compared to those mainly sourced from meteoric water. the ree content of fluorite from sumochagan obo is similar to fluorite sourced from a mixture of magmatic-hydrothermal and meteoric water (table 6). additionally, k-ar ages of hydrothermal sericite and illite in fluorite deposits are 141.5±1.2 ma and 137.6±1.1 ma, respectively, which are close to the ages of zircon in fine-grained granite veins with fluorite mineralization within the regional weijing granite (shrimp u-pb, 137±2 ma; xu et al., 2009). this indicates that the mineralization age of fluorite aligns with the timing of granitic magmatic activity, suggesting that granite-related hydrothermal fluids may be a potential source of ore-forming fluids. 6. conclusions the analysis of the uv-visible absorption spectra of fluorite samples from the sumochagan obo area, siziwangqi, inner mongolia, suggests that yo2 centers may contribute to the purple coloration of fluorite in this region, as indicated by the 276/261 nm absorption peaks observed in purple samples. the green coloration of fluorite is primarily attributed to the presence of ree, f centers, and sm2+. additionally, some ca in the fluorite may exist as colloidal calcium within crystal defects, contributing to its coloration. in the uv-visible absorption spectra, green samples exhibit a 223 nm absorption peak characteristic of y and f centers, peaks at 693/440/360/276 nm due to sm2+, and absorption at 587/591 nm attributed to calcium colloid color centers. the geological characteristics of fluorite in this mining area, along with the tb/ca-tb/la plot, indicate a predominantly magmatic-hydrothermal origin. the geochemical characteristics of ree in fluorite and the sources of ore-forming fluids resemble those of fluorite deposits formed through a mixture of magmatic-hydrothermal and meteoric water. water-rock interaction is identified as the predominant mechanism for fluorite precipitation. acknowledgements this work was financially supported by the “deep-time digital earth” science and technology leading talents team funds for the central universities, china university of geosciences (beijing) (grant number: 2652023001), the 111 project of the ministry of science and technology, china (grant no. bp0719021), and most special fund from the state key laboratory of geological processes and mineral resources, china university of geosciences (grant no. msfgpmr201804). the professional committee of tectonophysicochemistry, chinese geophysical society provided theoretical support. references alig, r. c. 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(2019). optimization of fusion method for gf-2 satellite remote sensing images based on the classification effect. earth sciences research journal, 23(2), 163-169. doi: https://doi.org/10.15446/esrj. v23n2.80281 with the successful launch of china’s gf series satellites, it is more important to study the image data quality, the adaptability of processing method and information extraction method. the panchromatic and multi-spectral data which is based on the gf-2 images data of chinese sub-meter high-resolution remote sensing satellite is fused by pca, pansharp, gram-schmidt and nndiffuse fusion. then, the quality of the fusion images were evaluated subjectively and objectively. in order to evaluate the applicability of different classification algorithms to the classification, the object-oriented classification algorithm which is based on machine learning algorithm, such as knn, svm and random trees were used to classify the different gf-2 fusion images. the results showed that: (1) the best visual effect of gf-2 fusion image was the pansharp fusion image; the quantitative evaluation results showed that the brightness and information retention of gram-schmidt fusion image was the best, while the pansharp fusion image had the highest correlation with the original multi-spectral image; the nndiffuse fusion image had the highest clarity, and the pca fusion image quantitative evaluation effect was the worst; (2) according to the applicability analysis of the fusion images based on different classification algorithms with features information extraction, it could be seen that the nndiffuse fusion method was used for the fusion of gf-2 image data, and the classification of the fusion images was more suitable by using knn or random trees classification algorithm. abstract optimization of fusion method for gf-2 satellite remote sensing images based on the classification effect optimización del método de fusión para imágenes satelitales gf-2 basadas en el efecto de clasificación issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n2.80281 con el lanzamiento exitoso de los satélites de la serie gf de china es más importante estudiar la calidad de los datos de imagen, la adaptabilidad del método de procesamiento y el método de extracción de información. los datos pancromáticos y multiespectrales que se basan en los datos de imágenes gf-2 del satélite de teleobservación de alta resolución en el submetro chino, se fusionaron mediante pca, pansharp, gram-schmidt y nndiffuse. luego, la calidad de las imágenes de fusión se evaluó de manera subjetiva y objetiva. para evaluar la aplicabilidad de diferentes algoritmos de clasificación, se utilizó el algoritmo de clasificación orientado a objetos basado en el aprendizaje automático, como knn, svm y árboles aleatorios para clasificar las diferentes imágenes de fusión de gf-2. los resultados mostraron que: (1) el mejor efecto visual de la imagen de fusión gf-2 fue la imagen de fusión pansharp; los resultados de la evaluación cuantitativa mostraron que el brillo y la retención de información de la imagen de fusión gram-schmidt fueron los mejores, mientras que la imagen de fusión pansharp tuvo la mayor correlación con la imagen multiespectral original; la imagen de fusión nndiffuse tuvo la mayor claridad, y el efecto de evaluación cuantitativa de la imagen de fusión pca fue el peor; (2) de acuerdo con el análisis de aplicabilidad de las imágenes de fusión basadas en diferentes algoritmos de clasificación con características de extracción de información, se pudo ver que se usó el método de fusión nndiffuse para la fusión de datos de imágenes gf-2, y la clasificación de las imágenes de fusión fue más apropiada utilizando el algoritmo de clasificación knn o random trees. resumen record manuscript received: 09/04/2018 accepted for publication: 18/02/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 2 (june, 2019): 163-169 jintong ren1, 3, wunian yang 1, 2*, xin yang1, 2, xiaoyu deng1, he zhao1, fang wang1, lei wang1 1college of earth science, chengdu university of technology, chengdu 610059, china; 2key laboratory of geoscience spatial information technology of ministry of land and resources, chengdu university of technology, chengdu 610059, china; 3school of ecological engineering, guizhou university of engineering science, bijie 551700, china * corresponding author: ywn@cdut.edu.cn g eo ph ys ic s 164 jintong ren, wunian yang, xin yang, xiaoyu deng, he zhao, fang wang, lei wang introduction. with the technology of satellite remote sensing ground observation developing in these years, high temporal resolution, hyper-spectral resolution and multi-platform remote sensing images data increase rapidly; remote sensing images in different resources are different in spectral resolution, radiance resolution and spectral resolution (fang, huo & ma, 2016; sun, fu & wang, 2011); leading different remote sensing images have diversities in images fusion algorithm (luo & du, 2017; zhao, li & lei, 2016; chen, zhang, zang & dong, 2016; chen, 2016; xie, zhang, sun & pan, 2016; du, 2015) . some researchers have studied and disposed the gf-2 remote sensing images data fusion. by using his transform, pca transform, wavelet transform, his+wavelet transform, his+pca transform, pca+wavelet transform, his+pca+wavelet transform, the fusion experiments on gf-2 images data by wang aiyun, and “his + pca + wavelet transform” was finally selected as the best fusion method of gf-2 images data in mountainous area (wang, 2016) . ren jintong, yang wunian and others evaluated the different fusion effects of gf-2 images by using commonly used image fusion methods such as hsv transform, brovery transform, pca transform, gram-schmidt pan sharpening (gs) and color normalized (cn), the results showed that the comprehensive performance of gs fusion was best, and it could meet the needs of the general image fusion processing (ren, 2016) . meng fanxiao, chen shengbo and others choose eight fusion methods such as gram-schmidt, hcs, hpf, pansharp, high pansharp, pca, multiplicative and subtractive to fuse gf-2 images, the results showed that: (1) all eight methods increased the spatial information, while maintaining the original spectral information. (2) among them, hpf method was the best fusion method, followed by gram-schmidt method, followed by multiplicative, pca, subtractive, hcs and high pansharp. the pansharp method was the worst (meng, chen & zhang, 2016) . however, these research can only evaluate qualitatively or quantitatively the quality of fusion images; the adaptation of classification algorithm in fusion images is still needed to be analyzed. in this study, the common image fusion algorithms were used to fuse the gf-2 images data, such as pca, pansharp, gram-schmidt and nndiffuse. after that, the fusion images were evaluated from subjective qualitative evaluation and objective quantitative evaluation. moreover, different machines are based on object-oriented learning classification algorithm for classifying fusion images to study different fusion dispose methods, and classification algorithm to the precision influences and classification adaptation in urban areas gf-2 satellite remote sensing images. study areas condition and remote sensing preprocess study areas condition longchang city is located at the south of sichuan basin of china, its location is east longitude 105°02′-105°26′, northern latitude 29°11′-29°32′, it is in subtropical humid monsoon climate, the annual average temperature is 17. 6 °c (wang, 2016) . the geomorphology in longchang is the low hilly system of huaying mountains extending to the southwest. the terrain is gentle, which generally presents high north to low south trend. the average elevation are 300400m and the relative height are 20-50m. this area has four geomorphologies: pingba, high hill, low hill and low mountain (ren, 2016) . this study was based on gf-2 satellite remote sensing image data(gf2_ pms2_e105. 2_n29. 4_20160720_l1a0001709692)which was provided by sichuan high resolution ground observation system data and application center to launch projects. the experimental data includes 1m panchromatic and 4m multi-spectral data. the scale of panchromatic band spectrum: 0. 45-0. 90 μm, and the scale of multi-band spectrum: blue (0. 45-0. 52 μm), green (0. 52-0. 59 μm), red (0. 63-0. 69 μm) and infrared (0. 77-0. 89 μm). longchang city images needed to be cropped to acquire study areas, for instance picture 1: the major ground objects features in longchang city were vegetation, water, road, bare land and building sites. gf-2 remote sensing images data preprocess in order to improve the visual effect and interpretability accuracy of the images, such as image enhancement, radiation calibration, orthorectification, atmospheric correction, image registration, cropping and resampling preprocessing works had been applied to pre-processing of gf-2 satellite image data. (1) the linear stretching treatment (linear 3%) was used in the envi5. 3 software because the original gf-2 satellite remote sensing images data has low brightness value and dim hue. (2) these images data which was disposed by othochromatic correction was based on rpc documents (provided by gf-2 satellite remote sensing data) and aster gdem v2 dem data (provided by http://www. gscloud. cn/ website, 30m resolution). (3) in order to decrease influences of atmosphere to images data, the gf-2 satellite remote sensing data were irradiated and calibrated by using the “2016 national terrestrial observing satellite absolute radiation calibration coefficient” which is provided by china resources satellite application center, and then the envi5. 3 software rapid atmospheric correction model was used to do atmospheric correction. (4) the image registration workflow correction tools were applied to dispose multi-spectral bands which were based on gf-2 satellite remote sensing images panchromatic band. (5) in order to study ground features in survey regions, cropping and resampling were good methods to acquire panchromatic and multi-spectral images data. methods of fusion remote sensing images and results. methods of fusion remote sensing images (1) pca fusion pca (principal component analysis) is based on no unidimensional multi-linear orthogonal transformation (meng et al., 2016) , it does principal component on multi-spectral data, then the first principal component band figure 1. schematic diagram of experimentation area 165optimization of fusion method for gf-2 satellite remote sensing images based on the classification effect is replaced by high resolution bands and fusion images are acquired through principal component (liu, 2014) . because the pca fusion models are always used in different types of sensor data fusion or the same sensor which has multi-phase data contrast analysis. this fusion method is able to be used in more than 3 bands images fusion and integrate high resolution images spatial information. however, it has some weaknesses. comparing with the original spectral images, this method may lose spectral information like color distortion (yang & yang, 2015) . (2) pansharp fusion this method uses least squares to match fusion bands and combines with statistics to decrease dependencies in data sets to accomplishes domestic fusion automation (yilihamu, xie & kong, 2014). the features of this method are to have high spatial resolution, to have good spectral fidelity and to apply into many bands which are not limited by numbers. this is a good meth.od in high resolution remote sensing fusion due to the high fidelity of spectrum and its details (deng, chen & du, 2014). (3) gram-schmidt fusion it is the multidimensional linear orthogonal transformation which transforms multi-spectrum or high spectrum to orthogonal spatial (yu, 2015). then replace the first component of the vector by high images. at last, results are shown by using inverse transformation (yilihamu et al., 2014). this method is not limited by the number of bands, which can avoid to gathering information in one or more bands. it has high spectral fidelity. (4) nndiffuse fusion the nndiffuse (nearest-neighbor diffuse) method is provided by rochester institute of technology (liu, fu & li, 2015) , which adopts openmp and cuda technologies to increase performance. it has highlights in the quality of fusion images and processing speed. nndiffuse method can reserve great color and texture and the original images have little change (wang, li, zhang & dou, 2015). when the spectral response function of multi-spectral bands have the minimum overlap and all the spectral bands cover the scale of spectrum of panchromatic color bands, this method works best. evaluating the effect of remote sensing images fusion subjective qualitative evaluation this evaluation focuses on comparing images’ spectrum, edge and textures before and after fusion(zhang, liu, yang & luo, 2015). as can be seen from the figures 2 and 4, different images are fused in envi software to do the rgb false color fusion. some typical areas which show in linear 2% stretching to present details are selected. doing subjective qualitative evaluation in gf-2 which adopts four different fusion images, it can be found that (1) comparing original multi-spectral images data with fusion images data, the original images have low brightness value, blurred edge of streets and not obvious textures; the panchromatic bands have high resolution, high brightness value and clear edge of the ground, but the color performance is worse than the fused images. (2) the results of gram-schmidt fusion are the worst: vague images and large color distortion in false color synthesis of the ground. the pansharp fusion method is good: clear ground and textures, comfortable brightness. (3) comparing the nndiffuse fusion with the pca fusion, the nndiffuse fusion looks darker than the pca fusion, but the ground details and textures clarity are clearer than pca fusion. objective qualitative evaluation this evaluation is based on measuring the quality of fusion images by statistics(sun & messinger, 2014). the study selects mean values and the standard deviation of the brightness information from images, the correlation coefficient of the spectral information, the information entropy of the image information, and the average gradient of the image clarity on the different fusion effects of the pms data of the gf-2 satellite remote sensing image to do quantitative evaluation. as can be seen from the tab. 1: (1) the gram-schmidt fusion images have the largest mean value and standard deviation in brightness information, the second are pansharp fusion images. the nndiffuse fusion images have the a. mss image b. panchromatic image c. pca fusion d. pansharp fusion e. gram-schmidt fusion f. nndiffuse fusion figure 2. comparison of the different fusion effects on the original multi-spectral and panchromatic of gf-2 166 jintong ren, wunian yang, xin yang, xiaoyu deng, he zhao, fang wang, lei wang smallest mean value and standard deviation. (2) the correlation coefficient of the spectral information shows that the pansharp fusion images have the highest correlation with original images. the second are nndiffuse fusion images and the worst are gram-schmidt fusion images. (3) gram-schmidt fusion images cover the largest amount of information, then are the pansharp fusion images. pca fusion images cover little information. (4) nndiffuse fusion images have the great average gradient, then are pansharp fusion images which are no big difference from nndiffuse fusion images. pca fusion images have the smallest average gradient. two classification problems. then it is applied to non linear separable problems, non classifying problems, multi class classification problems (fan, 2015). the results which are based on svm classification algorithm present as figure 4. random trees classification algorithm random trees algorithm is also called random forest classification algorithm. it is a joint forecasting model which is combined by many decision trees. the core idea is to randomize the input samples on the use of recorded data (rows) and specific variables (columns) (yang, 2015). through the resampling method by returning the training samples, a number of training set sub-samples are selected from the original samples, and the decision trees are modeled using these sub-sample sets to generate multiple decision trees, the vote to draw the final classification or prediction results are according to the results of the forecast of each tree (zhang, 2016). the advantages of the random trees algorithm are that they are suitable for the data processing of high dimensional and large data due to the randomness of the sample extraction and the feature tree of the decision tree node, the data is not preprocessed and the parameters are simple. high-dimensional feature data processing, better reflect its stable, efficient and accurate advantages. the results which are based on random trees classification algorithm present as figure 5. classification accuracy evaluation and results analysis in order to do classification accuracy evaluation in different algorithm, this study selected 350 samples randomly by using visual interpretation. the study requires the selected sample areas as small as possible to ensure that it is a single ground feature. quantitative evaluation of this study was based on the classification accuracy evaluation of general classification accuracy and kappa coefficient. the overall classification accuracy represented the proportion of samples that were correctly categorized in all samples; the kappa coefficients which utilized the information of the entire error matrix were generally considered to reflect the overall classification accuracy more accurately. the results of different fusion images classification precision based on different classification algorithms were presented in tab. 2. as can be horizontal compared from the tab. 2. (1) different classification algorithms were used to classify the images based on pca fusion, pansharp fusion and nndiffuse fusion. the largest classification algorithm of the overall classification accuracy and the kappa coefficient were the knn object-oriented classification algorithm, followed by the random trees classification algorithm, and the svm classification algorithm has the worst performance. (2) the best effect of gram-schmidt fusion image classification was the random trees classification algorithm, followed by knn classification algorithm. svm classification algorithm performance was the worst. therefore, the knn classification algorithm was the most suitable method for urban ground information classification based on pca fusion, pansharp fusion and nndiffuse fusion. for the gram-schmidt fusion, it was useful to use random trees classification algorithm in urban ground information extraction; svm classification algorithm was not useful for the urban ground information extraction in the four fusion methods. as can be vertical compared from the table 2. (1) the largest kappa coefficient and overall classification accuracy were based on the knn classification algorithm to classify the gf-2 images in nndiffuse fusion, followed by the random trees classification algorithm in nndiffuse fusion image classification; (2) by using the knn classification algorithm and the random trees classification algorithm, the overall classification accuracy and kappa coefficient of the image based on pca fusion and pansharp fusion were the same. when the svm classification algorithm was used, the classification result of pansharp fusion image was better than that of pca classification accuracy was higher; (3) using the random trees classification algorithm to improve the accuracy of the image classification results of gram-schmidt fusion was better than that of pca and pansharp fusion using knn classification algorithm, and the gram-schmidt fusion image classification ratio using knn and svm classification algorithm other fusion effect was worse. above all, the knn classification algorithm was most suitable for gf-2 fusion images after nndiffuse fusion, and then the random trees classification algorithm table 1. the quantitative evaluation of gf-2 remote sensing image data image type mean standard deviation correlation coefficient entropy average gradient pca fusion 69. 4886 51. 1414 0. 8131 5. 6661 6. 9846 pansharp fusion 70. 1184 65. 2733 0. 8909 6. 2045 12. 2928 gram-schmidt fusion 78. 8974 69. 8884 0. 7597 6. 2078 11. 6054 nndiffus fusion 66. 9960 66. 9960 0. 8751 6. 1685 12. 4659 application of different fusion images in urban terrain information extraction the object-oriented classification method and results many scholars use object-oriented classification technologies to do the number of work in algorithm optimization and interpretation(dong, zang, wu & li, 2017; wang & zhang, 2015; yang, 2016; yang & ming, 2016; wang, meng, yang, yang & fang, 2016; jiang & li, 2016; zhao, 2016). the ecognition, a good object-oriented classification remote sensing images software which is invented by definiens imaging corporation, provides many object-oriented classification algorithms. this study uses knn (k-nearestneighbor, knn), svm (support vector machine, svm) and random tress object-oriented classification algorithms to research different gf-2 remote sensing images fusion. knn classification algorithm knn algorithm is one of the mature theory algorithm. it is similar with the traditional supervised classification. calculating partitioning samples and training samples in the distances of special feature spatial to determine of partitioning samples. these partitioning samples will be determine to classes which have the shortest distances (yang, 2016). the basic class unit of k-nearest neighbor algorithm is object images, which is different from the traditional supervised classification. when determine partitioning objects by knn method, it is useful to find limited closed samples. the results are related to surrounding samples’ distances and numbers. therefore, it is more effective for crossover or overlapping objects to be classified, and there is no need to estimate parameters in the application process. it is easy to implement and suitable for multi-classification problem (chen, shao, mei, deng & hou, 2016). based on the knn classification algorithm, the classification results of four different types of fusion images in the study area are shown in figure 3. svm classification algorithm svm is one of machine learning classification algorithm in supervised classification. its main goal is to project the sample through the non-linear mapping to the high-dimensional feature space, with the minimization structural risk principle. vapnik-chervonenkis dimension which describes the function sets or learns the complexity of the machine, minimize the classification of the super-plane as a classification surface, making the classification of the upper bound of the risk minimization, so that the classification algorithm has the best ability to be promoted (liu, 2016). svm is the most important method in statistics pattern recognition. at first it is come out through linearly separable 167optimization of fusion method for gf-2 satellite remote sensing images based on the classification effect figure 3. knn classification results of four different fusion images figure 4. svm classification results of four different fusion images 168 jintong ren, wunian yang, xin yang, xiaoyu deng, he zhao, fang wang, lei wang figure 5. random trees classification results of four different fusion images table 2. classification accuracy of gf-2 fusion images based on different classification algorithms fusion algorithm knn svm random trees overall classification accuracy % kappa coefficient overall classification accuracy % kappa coefficient overall classification accuracy % kappa coefficient pca 90.77 0.8850 73.23 0.6642 90.46 0.8813 gram-schmidt 88.92 0.8620 74.77 0.6849 90.77 0.8851 pansharp 90.77 0.8850 76.31 0.7031 90.46 0.8812 nndiffuse 91.69 0.8964 73.85 0.6710 91.38 0.8926 after nndiffuse fusion was applied. for these four fusion algorithms, the svm classification algorithm after the pca fusion was the least useful method for urban ground information extraction. conclusion pca fusion, pansharp fusion, gram-schmidt fusion and nndiffuse fusion were applied to gf-2 satellite remote sensing images data to acquired different fusion results, and then subjective qualitative evaluation and objective qualitative evaluation had be done to estimate results. the knn, svm and random trees classifier algorithms which are based on the machine learning algorithm in ecognition object oriented software were used to classify the four different fusion effect images and evaluated the accuracy of the classification results. combined with the above research work, here are some conclusions: (1) for the fusion images quality after these fusion methods: from the subjective qualitative evaluation, it could be seen that the best performance of the visual effect was pansharp fusion image, followed by nndiffuse fusion, pca fusion, gram-schmidt fusion performance was the worst. from the objective quantitative evaluation, it is obviously that the mean value of gram-schmidt fusion was the best, the brightness information was the best, the information entropy was the largest, and the information was the highest after fusion. pansharp fusion had the strongest correlation with the original multi-spectral image, nndiffuse fusion and the quantitative evaluation indexes of pca fusion images were lower than the other three fusion effects, and the quantitative evaluation effect was the worst. (2) for the different fusion effect images of gf-2 satellite remote sensing images which were based on machine learning algorithm of the knn, svm and random trees object-oriented classification studied the classification of urban ground information extraction: from the horizontal comparison, the knn classification algorithm was the most suitable for pca fusion, pansharp fusion and nndiffuse fusion image classification, especially in urban ground information extraction, followed 169optimization of fusion method for gf-2 satellite remote sensing images based on the classification effect by random trees classification algorithm; for gram-schmidt fusion image classification effect the best was the random trees classification algorithm, followed by the knn classification algorithm. for the classification of four kinds of fusion images, the svm classification algorithm was poor and was not recommended for urban information extraction. from the vertical comparison, it was obviously that gf-2 satellite remote sensing images adopt nndiffuse fusion and the knn classification algorithm had the best overall classification accuracy and kappa coefficient, and it was the best in the urban ground information extraction, followed by the random trees classification algorithm. for the four fusion algorithms, the overall classification accuracy and kappa coefficient of the svm classification algorithm after pca fusion processing were the worst, and it was not suitable for urban information extraction. above all, it is suggested that the nndiffuse fusion method should be used for the fusion of gf-2 image data, and the fusion image should be classified by knn or random trees classification algorithm. the svm classification is not recommended to applied to pca fusion. in order to ensure the classification results are comparable, this study in the classification of different classification process, the relevant parameters set the same. acknowledgment the work was financially supported by the national natural science foundation of china (no.: 41671432); the open fund project of key laboratory of geoscience spatial information technology of ministry of land and resources (chengdu university of technology) (no.: klgsit2016-11)”. references chen, j., shao q., mei, x., deng, m., & hou, j. 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(2016). an image fusion method with sparse representation based on genetic algorithm optimization. journal of beijing university of posts and telecommunications, 02, 73-76. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.109054 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 79 91 so il s abstract the demand for economical means of evaluating soil nutrients’ unpredictability triggered the use of physical factors against the costlier, laborious, and time-consuming chemical approach. this drive led to resolving its capability in evaluating intricate soil properties as a productivity checker. this study aimed at assessing the efficiency of apparent electrical conductivity as a useful alternative to the conventional chemical examination of available nutrients. a petrographic examination was conducted on four rock samples for their classification as the source of soil formation. apparent electrical conductivity (eca) measurements were seasonally executed in the wet (912-station) and dry (906-station); the arcgis 10.2 programme was used in generating the eca maps for classifying the eca into low, moderate, and high sections. ten cored (in-situ) soil samples were subjected to permeability test to ascertain water infiltration rate and retention. twenty soil samples were examined for ph, electrical conductivity (ec), available phosphorus, acidity, na, mg, k, and ca using standard soil science procedures. the mineralogical composition of six samples (two samples per eca region) was determined with x-ray diffraction. the soils eca were 10–344 µs/cm, categorized as low (1–49 µs/cm), moderate (50–99 µs/cm), and high (>100 µs/cm). the eca distribution varied from moderate (61%) to high (64%), suggesting a heterogeneous pattern of soil attributes. the infiltration rate was slow in high eca (5.56x10-5-1.67x10-4 cm/s), signifying good retention capability, whereas the low and moderate eca (moderate-moderately rapid) sections promote nutrient leaching. ‘positive correlation was observed between cec and eca in the low eca (2.99 cmol/kg), the same was observed in the moderate and high eca (3.30-4.85 cmol/kg) segments’. the base cation saturation varied from high (81.38%), moderate (73.34%) and low (71.89%), characterizing the high eca as the higher fertility status. soils with moderate and high eca have a greater ability to adsorb cations onto their surfaces than soils with low eca; similarly, the deduction from base saturation showed that more cations are available in soils with high eca. the high eca region had low quartz (41.3%) and microcline (15.7%), but high kaolinite (31.1%) had an affinity to adsorb more cations compared to other eca regions in terms of quantity. the rock is biotite granite gneiss, containing plagioclase (22%), microcline (24%), orthoclase (4%), quartz (25%), biotite (7%), and others (18%). eca variability is practicable in predicting the spatial distribution of soil properties and delineating the management zones. thereby reducing the cost that would have been expended on geochemical sampling of the entire farm. geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment abayomi adesola olaojo1*, michael adeyinka oladunjoye2 1. department of earth sciences, ajayi crowther university, oyo, nigeria. 2. department of geology, university of ibadan, ibadan, nigeria. *corresponding author: aa.olaojo@acu.edu.ng record manuscript received: 22/05/2023 accepted for publication: 27/03/2024 how to cite this article: olaojo, a. a., & oladunjoye, m. a. (2024). geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment. earth sciences research journal, 28(1), 79 91. https://doi.org/10.15446/esrj. v28n1.109054 keywords: electrical conductivity, soil composition, mineral assemblages, nutrient variability, agricultural soil assessment, cation exchange capacity, spatial distribution detección geoespacial de la conductivad eléctrica aparente: una posibilidad para la evaluación de suelos agrícolas palabras clave: conductividad eléctrica; composición del suelo; conjuntos minerales; variabilidad de nutrientes; evaluación del suelo agrícola; capacidad de intercambio de cationes; distribución espacial. resumen la demanda de medios económicos para evaluar la imprevisibilidad de los nutrientes del suelo ha llevado a que el uso de factores físicos prevalezca sobre los factores químicos, más costosos, laboriosos y demandantes de tiempo. este enfoque condujo a resolver su capacidad de evaluar propiedades complejas del suelo como verificador de productividad. este estudio se enfoca en medir la eficiencia de la conductividad eléctrica aparente como una alternativa útil a la examinación química convencional de los nutrientes disponibles. para esto se realizó un análisis petrográfico en cuatro muestras rocosas para su clasificación como fuente de formación del suelo. las medidas de conductividad eléctrica aparente (eca) se llevaron a cabo durante la temporada de húmedad (912 estaciones) y la temporada seca (906 estaciones); el programa arcgis 10.2 se utilizó para la generación de mapas de clasificación de la eca que permitiera su clasificación en secciones baja, moderada y alta. diez muestras de suelo (in-situ) fueron sometidas a pruebas de permeabilidad para determinar el índice de infiltración de agua y de retención. a veinte muestras de suelo se les examinó el potencial de hidrógeno (ph), la conductividad eléctrica, el fósforo disponible, la acidez, el sodio, el magnesio, el potasio y el calcio a través de procedimientos estándares de las ciencias del suelo. la composición mineralógica de seis muestras (dos muestras por cada región eca) se determinó con difracción por rayos x. el rango eca de los suelos osciló en 10–344 µs/cm, categorizados como bajos (1–49 µs/cm), moderados (50–99 µs/cm) y altos (>100 µs/cm). la distribución eca varía de moderada (61 %) a alta (64 %), lo que sugiere un patrón heterogéneo de atributos del suelo. el grado de infiltración fue bajo en la región alta de eca (5.56x10-5-1.67x10-4 cm/s), lo que significa una buena capacidad de retención, mientras que las regiones baja y moderada de eca (moderado-moderadamente rápido) promueven la filtración de nutrientes. se observó una correlación entre la capacidad de intercambio de cationes (ctc) con la eca; la ctc baja (2.99 cmol/kg) ocurre en la región baja de la eca, al igual que la moderada (3.30-4.85 cmol/kg) ocurre en la región moderada de la eca y la alta ocurre en la región alta de la eca. la saturación base del cation varía entre alto (81.38 %), moderado (73,34 %) y bajo (71,89 %), lo que caracteriza la región alta de eca como la de mayor estatus de fertilidad. los suelos de las regiones moderada y alta de eca tiene una mayor capacidad de absorber los cationes en sus superficies que los suelos de la región baja eca; igualmente, la deducción de la saturación base muestra que más cationes están disponibles en suelos de la región alta de la eca. la región alta de la eca tiene un bajo porcentaje de cuarzo (41,3 %) y microclina (15,7 %), pero la alta caolinita (31,1 %) tiene una afinidad de absorber más cationes en comparación con otras regiones de la eca en términos de cantidad. la roca es un gneis granítico de biotita que contiene plagioclasa (22 %), microclina (24 %), ortoclasa (4 %), cuarzo (25 %), biotita (7 %), y otros (18 %). la variabilidad de la eca es práctica para predecir la distribución espacial de las propiedades del suelo y delinear las zonas de manejo. además sirve para reducir el costo que se gastaría en la realización de un muestreo del área completa de trabajo. https://doi.org/10.15446/esrj.v28n1.109054 mailto:aa.olaojo@acu.edu.ng https://doi.org/10.15446/esrj.v28n1.109054 https://doi.org/10.15446/esrj.v28n1.109054 80 abayomi adesola olaojo, michael adeyinka oladunjoye 1. introduction soil is developed from the parent rock material which is influenced by the climatic condition, topography, and living organisms acting on it; and tends to exhibit varying textural classes based on the mineral composition of the parent rocks (mccauley et al., 2005; botta, 2015; jaja, 2016). the soil horizon is developed from the continuous weathering of the parent rock leading to the development of a vertical section of soil known as the soil profile (gregory et al., 2009). the chemical content of soil and its formation depend on the nature of the bedrock as derived from weathering processes, biological components acting on it, and the introduction of external materials from other sources. oftentimes, the nutrient content of soil is dependent on the bedrock, and its variability depends on the conditions in which the parent rock is in the soil’s formation. this in turn affects the weathering rates, resulting in different soil horizons with varying physical, chemical, and mineralogical compositions (demir et al., 2022). demir et al. (2022) suggested that soil nutrient evaluation should be determined from its physico-chemical and mineralogical properties in order to reveal its composition. evaluating soil nutrients’ capacity has been done through a geochemical approach but it’s laborious, and time-consuming and the cost of analysis is expensive; although detailed information would be available at the end of the exercise. considering the number of samples that would be required for a detailed assessment, often time farmers embarked on a smaller number of sampling points to get around the problem of cost, but this would not allow the production of a detailed and accurate map of soil nutrients’ variability. the quality of soil type is determined by the proportions of the clay, silt, and sand combination; this invariably dictates the porosity/permeability, nutrient catchment capacity, water retention, and electrical conductivity which are essential for plants’ performance. soil nutrient is derived from the chemistry of the parent rock. the petrographic evaluation of rock serves as a way to examine the optical properties of minerals and possibly determines the expected product when it weathers. minerals in rock exhibit specific texture and colour, which can be identified from their optical properties. the rock’s mineralogy dictates the resultant soil mineral contents and colour, this in turn influences the soil’s productive capacity. apparent electrical conductivity (eca) serves as an alternative for dense sampling and offers a way to reduce the cost, coupled with a good correlation with soil variables (costa et al. 2014). soil electrical conductivity is useful in assessing soil mineralogy, soil texture, nutrients, temperature, field holding capacity, chemical properties, and soil moisture; these variables play a vital role in plant development. the flow of electrical charges through a material enables the conduction of charged particles; soil also displays electrical properties based on its physical and chemical properties such as texture, and water content (samouëlian et al., 2005). field measurement of soil apparent electrical conductivity (eca) is a simple and inexpensive method and can be correlated with the other soil properties such as soil texture, cation exchange capacity, drainage conditions, organic matter level, salinity, water holding capacity, temperature and elevation (grisso et al., 2009; siqueira et. al., 2014; mertzanides et al., 2020; fagbemigun et al., 2021). swileam et al. (2019) studied the spatial inhomogeneity of soil properties using electrical resistivity approach, the variabilities observed in resistivity study were correlated with moisture and bulk density; soil of low resistivity was noted with high soil moisture and bulk density, it was expressed with curvilinear models. they opined that the study could be used for soil irrigation and salinity management. negative relationship occurred between moisture content and electrical resistivity, and aids in delimiting the spatial variability of the moisture content being extrapolated from the resistivity distribution (mertzanides et al., 2020). electrical resistivity has been found useful in monitoring of herbicide contamination in soil as a bioremediation device capable of evaluating its varying concentration (balwant et al. 2021). mohammed et al. (2022) carried out a study on a natural agricultural oasis of al-ahsa region in saudi arabia using electrical resistivity tomography and electromagnetic induction. the techniques classified the hydrogeological conditions being influenced by soil textural and moisture content variations. areas with soil degradation requiring attentions were delineated and this affirms the use of electrical method as a viable tool in agricultural soil administration. electrical resistivity technique (ves and 2-d) is effective in characterizing the soil horizons and depicting structural inhomogeneity, the findings of ganiyu et al. (2019) revealed the soil horizon and suitable crop that could be grown on soil as delineated by the physical parameters. thereby, enhancing its soil management capability in agricultural discipline. electrical conductivity (ec) has been reported by usda (2011) to be a functional tool in assessing soil productivity, such that high ec value has been linked with high concentrations of p, k, ca, mg, mn, zn, and cu and nitrate. africa’s rainfall pattern is torrential which is responsible for the leaching of soil nutrients beyond the root zone and blanket use of fertilizer may be injurious to plants (adewole and adeoye, 2014). the clay minerals play a vital role in their ability to adsorb cations onto their surfaces and determine the quantity of cation exchange capacity sites. exchangeable cations are electrostatically bound or attached to the clay surface, although few anions are in a diffuse layer due to electrostatic repulsion (al-ani and sarapää, 2008). the inception of this study was prompted by meticulous field observations conducted on cacao trees cultivated concurrently within the same parcel of land in year 2000. evident disparities in cocoa pod development rates among these trees spurred a quest for deeper insights into the underlying factors driving such variations in soil productivity. this observational imperative underscores the need for an in-depth exploration of the physical determinants shaping these differences. in this pursuit, we draw inspiration from the burgeoning field of agrogeophysics, which serves as a conduit for interconnecting diverse interdisciplinary tenets aimed at tackling challenges inherent to agricultural soil dynamics. recognizing the potential of physical properties, we have integrated them into comprehensive agroecosystem assessments, perceiving them as enduring tools for assessing the mutable nature of soil nutrient content. the crux of our study revolves around the pivotal role of spatial apparent electrical conductivity as a predictive gauge for soil properties. this foundational insight empowers us to venture into new realms, where the relationship between electrical conductivity and soil permeability is elucidated, unraveling how the latter inherently impacts productivity dynamics. furthermore, we embarking on an effort to identify rock types, deciphering soil mineralogy, and conducting chemical assessments. this holistic undertaking is designed to affirm the credibility and utility of physical properties as a robust alternative avenue for soil analysis. as we delve into the depths of this investigation, our ultimate goal is to leverage the conclusions drawn from our study to guide future practices. the implications extend to actionable recommendations that can shape agricultural approaches, enhance productivity, and inform decision-making processes. these include adopting spatial apparent electrical conductivity as a strategic tool in predictive soil assessment, optimizing the correlation between electrical conductivity and permeability for enhanced productivity outcomes, and validating the reliability of physical property assessments through comprehensive chemical and mineralogical analyses. 2. material and methods 2.1 general features of the field study the area spans 99 meters by 78 meters and falls within the geographical coordinates of latitudes 7°13ʹ15.9ʺn to 7°13ʹ19.6ʺn and longitudes 3°51ʹ40.1ʺe to 3°51ʹ43ʺe. the study area’s location rests upon a migmatite complex within ibadan, encompassing the cocoa research institute of nigeria (fig. 1). the rock samples were obtained from outcrops around the farm and they were situated between latitudes 7˚13ʹ10.2ʺn and 7˚14ʹ04.6ʺn, and longitudes 3˚51ʹ34.8ʺe and 3˚51ʹ55.6ʺe. 2.2 rock analysis the thin sectioning of the rock samples into slides was carried out in the department of earth sciences at ajayi crowther university oyo, using hillquist thin section machine, and the petrographic studies were conducted via brunel petrographic microscope. the rock samples were prepared into thin section via the approach stated below: 1. the rock samples obtained were prepared into small sizes of about 8 mm thick. 2. one of its surfaces was polished using 400-grade carborundum on the glass plate. 3. the polished surface mounted on the glass slide using araldite epoxy resin. 81geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment 4. the mounted slide set for 30 minutes and the chip cut into a size of 90 microns. 5. the sample surface was ground using four grades (90, 400, 600, and 800) of carborundum until it reached 30-micron size. 6. excess araldite removed and the slide was covered using araldite epoxy resin on the hot plate. 7. the mineral properties through the cross and plane polarised light were examined; and modal analysis for the relative mineral composition. figure 1. geological map of the research vicinity (ngsa, 2009). 2.3 electrical resistivity procedure the basic principle of electrical resistivity involves the measurement of the resistance (r) of a conductor which opposes the flow of electric current when a voltage is applied across the two ends and its unit is ohm (ω). resistance is the ratio of the potential difference (v) to the resulting current flow (i) as defined by ohm’s law (equation 1): v = ir (1) the apparent electrical resistivity (ρa) is the multiplication of the resistance (r) with the geometric factor (k). the ‘k’ for the wenner array is 2пa, where ‘a’ is the spacing between adjacent electrodes. the apparent electrical resistivity (ρa) was converted into apparent electrical conductivity (σa) using the inverse relationship between them as noted in equation 2. σa = 1ρa (2) the apparent electrical conductivity was used in generating the conductivity maps of the farm. the study was carried out during the wet (august 2016) and dry (march 2017) seasons; the electrodes were fixed on a wooden frame to facilitate the same penetration depth and inter-electrode spacing was 40 cm (fig. 2). the effective depth of investigation is approximately 31 cm (edwards, 1977) which is situated within the root zone. the resistivity measurements were taken at every 3 m along a profile (99 m long) and inter-profile spacing of 3 m was adopted for 27 profiles; 912 and 906 resistivity stations were investigated for soil variability during the wet and dry seasons, respectively. the eca data set which were regularly spaced on the field were input as csv file (comma delimited). interpolation of the data enables the prediction for all the points in the study area. the eca data were related to one another via kriging and the correlation function was used for interpolating the data as provided by the arcgis 10.2 program. the approaches have been reported and found useful in generating the apparent electrical conductivity maps (oladunjoye, 2022; olaojo and oladunjoye, 2023). this revealed the spatial heterogeneity in the soil ec contrast on which further inferences could be determined. figure 2. fixed electrodes on a wooden frame 2.4 procedure for the modified constant head permeability techniques ten undisturbed soil samples were taken at the root zone (0.3 m) with the aid of a cylindrical metallic core barrel (diameter 7 cm and height 7 cm). they were saturated in water for 24 hours to ensure water rose through the capillary fringes. this analysis was conducted at the department of agronomy, the university of ibadan using a modified wageningen constant head permeameter. the pressure head difference was established by clamping an empty metallic core cutter tightly on top of the core with a soil sample and then filled with water. the water flows through the soil sample and is collected at the down end through outlet tubing onto a funnel seated on a beaker. the volume of water was quantified using a measuring cylinder while the time taken for the water to pass was measured with a stopwatch. darcy’s law was used in ascertaining the hydraulic conductivity, parameters measured include; the volume of the water (q) that flows through the soil column (cm3), the cross-sectional area (a) of flow through the soil column (cm2); time interval (t), length of soil (l) column (cm) and the hydraulic head difference (∆h) in cm. saturated hydraulic conductivity was determined using equation 3 (hillel 2004). ksat = ql/at∆h (3) 2.5 geochemical and mineralogical assessments of the soils 2.5.1 determination of soil chemistry the field apparent electrical conductivity aided in demarcating the farm into different eca zones from which soil-sampling stations were selected. geochemical analyses were conducted on twenty soil samples (low eca -9, moderate eca-5, and high eca-6 segments). the electrical conductivity and ph of soil samples were determined using the hanna ec meter by weighing 10 grams of ˂ 75 µm soil particles into an extraction cup mixed with distilled water (10 ml). the spectrophotometry analysis was engaged for the determination of available phosphorus, the acidity in the soil was evaluated using titration method, the potassium and sodium were determined with a flame photometer (jenway fp640) while calcium and magnesium via atomic absorption spectrophotometer aas (buck scientific 210/211 model). the cation exchange capacity (cec) was computed from the addition of cations (ca+mg+na+k) plus the acidity. all the analyses were executed at the department of agronomy, university of ibadan. 2.5.2 determination of primary mineral x-ray diffraction (xrd) analysis was carried out on six soil samples via the malvern panalytical empyrean xrd system at the college of petroleum engineering and geosciences, king fahd university of petroleum and minerals, saudi arabia. two soil samples were chosen from each of the eca regions ((low eca -2, moderate eca-2, and high eca-2 segments) to ascertain the mineralogical composition. a fine fraction (< 45 µm) of soil was packed into a hollow-cavity sample mount and the quantitative xrd analysis is achieved employing a whole-pattern fitting method utilizing a measured and calculated xrd scan. minerals identification was carried out by comparing calculated d-spacing with a library of standard d-spacing. 82 abayomi adesola olaojo, michael adeyinka oladunjoye 3. results and discussion 3.1 geological mapping of basement rock the gneiss exhibited pronounced foliation, discernible through the alternating or banded arrangement of mafic (dark) and felsic (light) mineral constituents (fig. 3a). concordant with the host rock, the quartzo-feldspathic segregations exhibited striking patterns between 3590 and 60 degrees, with a maximum length of 4 meters (fig. 3b). the distribution of these segregations encompassed varying areas, spanning from 20 m² to 300 m². additionally, these outcrops were characterized by a medium-grained texture. primary mineral constituents comprised quartz, feldspar, and mica, with biotite-rich mica and muscovite predominating. within this mineral composition, mafic components exhibited prominence over their felsic counterparts, rendering a gneissic appearance. an understanding of the rock composition is vital in determining soil mineral constituents that would result from the weathering of the parent rock. this would serve as an indirect approach in evaluating the soil nutrient that would be available for plant uptake and ensure proper management of agricultural soil. figure 3a. banding of mafic and felsic mineral components figure 3b. outcrop intruded with quartzo-felspathic vein 3.2 petrographic investigations of rocks in the examination of thin-sectioned rock samples, a complex interlocking arrangement of minerals was revealed, encompassing feldspars (plagioclase, microcline, and orthoclase), mica (biotite and phlogopite), quartz, and the accessory mineral zircon. cross and plane polarised light microscopy unveiled distinctive features, notably elongated quartz crystals exhibiting a preferred alignment alongside present biotite minerals (figs. 4 and 5). a notable feature emerged in the form of wavy extinction within the quartz crystals. furthermore, the feldspars (plagioclase, microcline, and orthoclase), biotite, and quartz all demonstrated elongation along the axis of orientation. intriguingly, certain feldspar grains appeared as porphyroclasts, embedded within the groundmass of deformed mineral grains at specific locations (fig. 4). a sample from location 1 unveiled the presence of zircon as an accessory mineral within the groundmass (fig. 6). fld fld biobio figure 4. photomicrograph showing the porphyroclast of feldspar grains in the matrix of biotite quart and medium grain feldspar minerals (cross polar) at location 1. mag. x100. figure 5. photomicrograph showing the biotite (bio), porphyroclast of feldspar (fld) grains in the matrix of others minerals (plane polar) at location 1. mag.x100. figure 6. photomicrograph showing zircon as an accessory mineral, biotite, quart and feldspar under cross polar at location 1. mag. x100 fld bio 83geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment quantitative modal analysis provided insight into the mineral composition distribution at each location (table 1). through comparison with egesi (2019), ibrahim et al. (2015), and parsons and zwanzig (2003), the rock formations observed in the study location were classified as biotite granite gneisses. these rock constituents are destined to weather into soil under the influence of temperature and pressure differentials from their formation conditions. the subsequent transformation of rock constituents into soil components was discussed, particularly the alteration of plagioclase, microcline, orthoclase, and biotite into kaolinite due to weathering. the varying susceptibilities to weathering were highlighted; plagioclase and biotite undergo more rapid weathering than microcline and orthoclase, with quartz demonstrating higher resistance (white et al., 2001; wilson, 2004). the mineralogical characteristics of the rock have profound implications for the potential clay content of the resultant soil after weathering. this composition significantly influences soil productivity, particularly in terms of adsorption and nutrient holding capacity. kaolinite’s distinctive attribute, its relatively lower cation exchange capacity (cec) compared to illite and montmorillonite, underscores its role (sonon et al., 2014). 3.2 electrical conductivity of soil the apparent electrical conductivity (eca) data of the soil were subjected to statistical analysis, as presented in table 2. the coefficient of variation (cv) was utilized to determine the variability of the eca values. during both rainy and dry periods, moderate (61%) and high (64%) variability in eca were observed, respectively, as highlighted by warrick and nielsen (1980). molin and faulin (2013) emphasized the significance of cv in identifying spatial variability, serving as an initial indicator. consequently, eca can be considered a valuable tool for evaluating soil quality and assessing the suitability of subsequent study sites. the measured eca values were found to fall within the non-saline class according to usda (2011), suggesting that crops would not face water stress. spatial distribution analysis of apparent electrical conductivity of soil was conducted using arcgis 10.2 software. the data points were interpolated using a correlation function known as kriging. the agricultural area was classified into distinct eca zones: low eca zone ranging from 0 to 49 µs/cm, moderate eca zone ranging from 50 to 99 µs/cm and high eca zone with a conductivity of 100 µs/cm and above. the most conductive regions (highlighted in red) were primarily concentrated in three areas: north/northeast, southwestern, and central portions of the map (fig. 7). moderate eca zones (depicted in light brown) extended from high eca areas and gradually transitioned into low eca areas, notably in the southeastern, central, and northern parts of the map. low eca terrain (shown in blue) was prominently observed in the southeast, southern section, central, and northwest/north-northwest (nnw) regions of the map. similar distribution patterns of electrical conductivity were observed during the dry season as well (fig. 8). table 1. modal analysis of analysed rock samples in the study area rock samples from the study area previous works used for comparison mineral (%) l1 l2 l3 l4 70 1 3l 12 .8 ll 30 5 1l 55 .6 ll 70 1 3l 10 .2 ll 30 5 1l 53 .3 ll 70 1 3l 54 .5 ll 30 5 1l 48 .5 ll 70 1 4l 04 .6 ll 30 5 1l 34 .8 ll range av er ag e parsons and zwanzig (2003) composition of granite gneiss egesi (2019) average composition of granite gneiss ibrahim et al. 2015 hornblende granite gneiss yusof & zabidi (2016) granitic compositional range plagioclase 25.2 23.6 18.0 19.3 18.0-25.2 21.5 35 20 35 } 40.0-70.0microcline 28.1 27.0 21.3 20.8 20.8-28.1 24.3 } 21 } 30 orthoclase 3.4 4.0 3.4-4.0 3.7 quartz 23.3 23.7 26.0 27.5 23.3-27.5 25.1 24 27 30 5.0-30.0 biotite 14.0 14.2 33.2 31.4 14.0-33.2 23.2 7 7 5 } 20.0-40.0 zircon 5.3 6.5 5.3-6.5 5.9 hornblende } 0-24 6 15 cummingtonite muscovite 5 opaque mineral 2 10 total 99.3 99.0 98.5 99.0 l1-location table 2. statistical assessment of soil apparent electrical conductivity (eca) of cacao field season number of points range (µs/cm) mean (µs/cm) standard deviation coefficient of variation (%) wet 912 13-344 68 41 61 dry 906 10-267 45 29 64 84 abayomi adesola olaojo, michael adeyinka oladunjoye figure 7. apparent electrical conductivity distributions within the cacao field during wet season figure 8. apparent electrical conductivity distributions within the cacao field during dry season however, numerical values were reduced in segments corresponding to high eca (> 60 µs/cm), moderate eca (40-59 µs/cm), and low conductive sections (< 40 µs/cm) compared to the wet season. the noticeable difference between the two seasons was the distinct presence of low eca segments during the dry season. it can be inferred that zones with high eca usually have more fine soil fractions than those with low eca (mzuku et al., 2005; rodríguez-pérez et al., 2011; gholizadeh et al., 2012; fagbemigun et al., 2021; mohammed et al., 2022), regions with low eca loses water faster than other regions resulting in variability of water content (costa et al., 2014; swileam et al., 2019) as the water drains through it. 3.3 constant head permeability assessment of soils in the cacao farm the constant head permeability test (table 3) revealed varying permeability; low permeability (5.56x10-5-1.67x10-4 cm/sec) in high eca segment, moderate in the moderate eca (0.00128 cm/sec) whereas high permeability (6.67x10-4-2.86x10-3 cm/sec) in the low eca section. soils of high eca have low relative permeability classified as silty sand; sand/fine sand (medium relative permeability) in the moderate eca segment and low eca is sand/fine sand to silty sand with a low to medium range relative permeability (terzaghi and peck, 1967). soil infiltration rate in the cacao plot scherer et al. (2013) established the infiltration rate in the soil as the amount of water absorbed by the soil over a given time. the rate at which water moves through soil units in high eca ranged from moderately slow to slow whereas moderate infiltration in the region of moderate eca. soil materials in the low eca segment indicate a moderate to moderately rapid infiltration rate (table 4). table 3. permeability (k) coefficients of some selected soil samples from cacao farm s/n coordinates eca of sample selection point ksat (cm/sec) wet season dry season northing easting eca (µs/ cm) eca (µs/ cm) 1 7013ʹ18.5ʺn 3051ʹ40.6ʺe high 8.33x10-5 133 112 2 7013ʹ16.8ʺn 3051ʹ40.6ʺe high 8.33x10-5 159 89 3 7013ʹ16.2ʺn 3051ʹ41.1ʺe high 5.56x10-5 118 81 4 7013ʹ17.7ʺn 3051ʹ41.6ʺe low 6.67x10-4 23 20 5 7013ʹ19.3ʺn 3051ʹ41.4ʺe low 1.61x10-3 40 15 6 7013ʹ16.4ʺn 3051ʹ41.9ʺe low 2.86x10-3 49 30 7 7013ʹ16.6ʺn 3051ʹ42.9ʺe medium 1.28x10-3 51 36 8 7013ʹ17.8ʺn 3051ʹ42.3ʺe high 1.11x10-4 114 85 9 7013ʹ17.9ʺn 3051ʹ41.9ʺe low 7.50x10-4 48 26 10 7013ʹ19.0ʺn 3051ʹ43.0ʺe high 1.67x10-4 136 103 table 4. classification of soil moisture infiltration rate (modified after scherer et al., 2013) s/n classification infiltration rate (inches/hour) infiltration rate (cm/s) 1 very slow less than 0.06 ˂ 4.233x10-5 2 slow 0.06 to 0.2 4.233x10-5 to 1.411x10-4 3 moderately slow 0.2 to 0.6 1.411x10-4 to 4.233x10-4 4 moderate 0.6 to 2.0 4.233x10-4 to 1.411x10-3 5 moderately rapid 2.0 to 6.0 1.411x10-3 to 4.233x10-3 6 rapid 6.0 to 20.0 4.233x10-3 to 1.411x10-2 7 very rapid greater than 20.0 > 1.411x10-2 3.3.1 correlation analysis between permeability (k) coefficients and eca of the soils the plot of permeability coefficients with the eca (fig. 9) showed that its determination coefficient is 0.460 in which 46.0% of the variables were related linearly and also exhibits a strong negative correlation coefficient (0.68) in the wet season. the degree of interaction revealed that 50.1 % of these variables have linear dependence as suggested by the determination coefficient while a negatively strong correlation (0.708) in the dry period (fig. 10). positive correlation exists between electrical resistivity and hydraulic conductivity (fagbenro and woma, 2013); this deduction also validates the negative correlation obtained from the plot as a result of an inverse relationship between electrical resistivity and conductivity. kayode et al. (2022) engaged electrical resistivity in assessing soil inhomogeneity and the technique was reported to be efficient in characterizing the soil moisture and void distribution, thus useful in agricultural management programmes. mary et al. (2020) monitored the root water uptake using electrical method via a controlled infiltration process. the permeability evaluations agreed clearly that soils of high eca are characterized by low permeability at both seasons and vice versa. it can be inferred that there is the presence of more fine particles in the high eca section than in the moderate and low eca segments. this promotes the adsorption of essential nutrients onto its surface due to its large surface area. 85geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment figure 9. plot of permeability (chp) versus eca in the cacao farm during the wet season figure 10. plot of permeability (chp) versus eca in the cacao farm during the dry season 3.4 soil chemical assessment 3.4.1 assessment of physical and chemical properties of soil in the farms soil ph: overall ph values range from 6.1 to 7.1 with a mean concentration of 6.71, the average ph in the low eca (6.1-6.8) horizon is 6.62; the moderate eca section (ph: 6.6-6.8) has a mean value of 6.70 while the high eca segment has the mean ph level (6.4-7.1) of 6.80. all the mean ph recorded falls in the neutral (6.6-7.3) category (horneck et al., 2011). the measured ph at all the zones is within the required concentration (6.0–7.5) which favours the availability of soil nutrients for most crops (moral and rebollo, 2017; khadka et al., 2018). the soil nutrients would be available for cacao consumption since ph values are close to either side of neutrality, not likely subjected to al3+ toxicity which can limit root growth, restraining access to soil water and nutrient since the ph is considerably above 5.5 (fao, 2008; ribeiro et al., 2013; botta, 2015). soil ec: the mean soil electrical conductivity value (30-180 µs/cm) in the farm is 62 µsc/m. the low eca section (30-70 µs/cm) has an average conductivity of 48 µs/cm, and those in the moderate class (30-80 µs/cm) have an average value of 54 µs/cm. the most conductive section (50-180 µs/cm) has the highest conductivity (90 µs/cm). there is the presence of more conductive ions in the high eca region than in the other sections. ec values measured in soil samples were regarded as non-saline (botta, 2015). the measured ec values were in a low category (˂1.0 ds/cm) which is suitable for optimum plant growth, in essence, the cacao trees can uptake the soil solution without resulting in water stress (horneck et al. 2011). available phosphorus: the average concentration of available phosphorus across all the investigated sections (6.0-12.71 mg/kg) is 8.89 mg/kg. region of low eca has the highest mean concentration of available p (9.52 mg/kg) and its concentration is between 8.32 mg/kg and 12.71 mg/kg. its mean proportion in the moderate eca areas (7.32-10.51 mg/kg) is 8.32 mg/kg. the least mean concentration (8.23 mg/kg) was noticed in the high eca region (6.0-10.51 mg/kg). the mean concentration of available p (8.89 mg/kg) across the whole farm was below the critical limit (12 mg/kg) needed to maximize crop productivity (horneck et al., 2011; van vliet et al., 2015), and classified as low category (<20). acidity of soil: the average acidic cation across the farm (0.64-1.20 cmol/kg) is 0.82 cmol/kg. zone of low eca (0.64-0.96 cmol/kg) had a mean concentration of 0.78 cmol/kg, moderate eca segment (0.64-1.04 cmol/kg) had a common concentration of 0.85 cmol/kg. its concentration in high eca portion stretched from 0.64 to 1.20 cmol/kg with an equal strength of 0.87 cmol/kg. soil acidification in the high eca was ranked highest due to the preferential consumption of base cations by cacao trees and organic acid being derived from the decomposition of litter (watanabe et al., 2015). it is not a useful nutrient for plant survival but it could be toxic to plants if the ph is below 5.5 (botta, 2015). the ph values across the soils were within the neutral level (horneck et al., 2011), therefore acidic cation content is not harmful to the growth of the cacao plant. soil calcium content: the average concentration of ca (0.46 3.72 cmol/kg) across the entire field was 1.29 cmol/kg. its mean distribution in the low eca (0.46-1.28 cmol/kg) was 0.93 cmol/kg, the moderate eca zone (0.77-1.34 cmol/kg) had 1.02 cmol/kg while its specific proportion in the high eca segment (1.27-3.72 cmol/kg) was 2.05 cmol/kg. there is no significant difference in the concentration of ca in the low eca (0.93 cmol/kg) and moderate eca (1.02 cmol/kg) sections but twice their values were noted in the high eca segment (2.05 cmol/kg). these mean values fall within the low category (˂5.0) class (proffitt, 2014). botta (2015) and white and broadley (2003) reported that the deficiency of ca is not a common phenomenon but its excessive can restrict adequate growth. its deficiency is noted in soils having low base saturation and high acidic content (white and broadley, 2003; horneck et al., 2011). magnesium content: magnesium concentration on the average was 0.76 cmol/kg in the low eca (0.25-1.97 cmol/kg), mean distribution in the moderate eca zone (0.37-1.38 cmol/kg) was 0.94 cmol/kg while the most electrically conductive zone (0.58-2.65 cmol/kg.) has its mean concentration to be 1.37 cmol/kg. the average concentration computed from the entire cacao soils (0.25-2.65 cmol/kg) was 0.99 cmol/kg. the mean proportion of mg in low eca and moderate eca regions was within the low class (˂1.0 cmol/kg); the moderate category (1-5 cmol/kg) was noted in the high eca (proffitt, 2014) which is approximately twice and one-half the contents in the low eca and the moderate eca sections. thus, the quantity of mg content in the high eca region plays a greater role in photosynthesis because it was an essential element for chlorophyll pigment (botta, 2015). potassium content: potassium content in all soil samples ranged from 0.14 cmol/kg to 0.44 cmol/kg indicating that its distribution was categorized between low (˂0.4 cmol/kg) and medium (0.4-0.6 cmol/kg). its mean value is 0.27 cmol/kg was classified to be low by horneck et al. (2011). region of low eca has its specific potassium concentration (0.14-0.44 cmol/kg) to be 0.28 cmol/kg in the low category. similar mean concentrations were observed in the moderate and high eca sections to be 0.24 cmol/kg and 0.30 cmol/kg respectively classified as low. the quantity of potassium available in high eca for plant consumption was greater than those from other regions. thus aiding a better regulation of nutrient uptake, water, flowering, and seed-bearing, and ensuring resistance to stress (wodaje and abebaw, 2014; botta, 2015) invariably contributing to the productivity in that section. sodium content: the mean proportion of exchangeable sodium was 0.24 cmol/kg in the low eca segment (0.2-0.27 cmol/kg). in soils of moderate eca, its mean concentration (0.21-0.34 cmol/kg) was 0.25 cmol/kg. the mean fraction in the highly conductive segment (0.23-0.30 cmol/kg) was 0.27 cmol/ kg. these mean concentrations were classified as low (<0.3 cmol/kg) at all the eca segments (proffitt, 2014). the mean concentration computed in the entire farm was 0.25 cmol/kg. sodium is not considered an essential soil nutrient for plants (horneck et al., 2011; botta, 2015). the high eca section has the highest sodium content contributing to the cacao productivity than the other two segments. cation exchangeable capacity (cec) of soils: the ability of the soil to adsorb cations onto its surface is regarded as cec. the mean cation exchangeable capacity (cec) in the low eca (1.92-4.53 cmol/kg) was 2.99 cmol/kg. in the moderate eca region, it varied from 2.45 to 4.13 cmol/kg with an average capacity of 3.30 cmol/kg and in the high eca section (2.948.28 cmol/kg) has a representative value of 4.85 cmol/kg. the cec in the low 86 abayomi adesola olaojo, michael adeyinka oladunjoye eca segment was classified as low (˂3.0 cmol/kg) while the mean capacity in the moderate and high eca sections was considered as moderate (3.0-10.0 cmol/kg) by proffitt (2014). the clay type deduced from the mean cec falls within the kaolinite (3.0-15.0 cmol/kg) class (sonon et al., 2014). the high eca region has a presence of more cations being held by the soil against leaching and retains more soil nutrients indicating higher fertility status than low cec soils (arévalo-gardini et al., 2015). percentage base saturation: this is the percentage proportion of basic cations in overall cec. an increase in base saturation suggests that the soil cations are becoming more available in soil. the percentage concentration of base cation in the soils of low eca ranged from 57.52 to 82.34% with a mean saturation of 71.80%. the section of moderate eca (64.08-82.04%) has a mean saturation percent of 73.34% while the high eca (78.23-85.51%) was ranked with the highest mean saturation (81.38%). high percentage base saturation signifies more fertile soil with little or no acidic cation that will hinder crop growth, buffered against acidic cation and a greater amount of basic cation for plant consumption (sonon et al., 2014). ca:mg dispersion: when the concentration of mg is relatively high compared to the ca, it would result in sealing the soil surface, thereby creating water run-off instead of infiltration. the ratio of ca:mg in the low eca region stretched from 0.6 to 2.6 with an average ratio of 1.5. its mean ratio was 1.2 in the moderate eca zone (0.9 to 2.1), whereas the mean dispersion ratio in the high eca section (0.8 to 2.7) was 1.7. soils in this farm have ca:mg ratios to be less than 2 indicating that the soils are not well structured (botta, 2015). the closest ratio to this value was 1.7 in the high eca soil suggesting that it is fairly structured than other segments. high mg concentration can have a negative effect on soil physical properties thereby sealing the soil surface leading to infiltration decrease, and promoting run-off leading to erosion during rainfall (dontsova and norton, 2001). mg has greater hydration energy than ca, resulting in a larger hydration radius/shell which in turn causes a higher distance of separation between clay layers with less surface attraction making it flocculate (dontsova and norton, 2001). mg-dominated soils have a greater proportion of exchangeable na accumulating on their surface than cadominated soils (he et al., 2013). exchangeable sodium percentage (esp): soils in the region of low eca (4.99-11.17%) have a mean esp to be 8.36%. zone of moderate eca has esp in soils varying from 5.57% to 8.60% with an equitable percentage of 7.71%. the esp in the soils of high eca ranged from 3.26% to 7.82% and its average percentage was 5.99%. the low eca and moderate eca areas were classified as marginally sodic (>6%) and their aggregate is susceptible to dispersion when wet (proffitt, 2014; botta, 2015), therefore they tend to have poor drainage, aeration, and susceptible to erosion. soils in the high eca portion are categorized as non-sodic (˂6%), they are generally stable, have good aeration, and drainage and are not vulnerable to erosion. 3.4.2 soil nutrients influencing the measured field electrical conductivity in the soils a positive correlation (r) exists between the field eca data and laboratorydetermined ec (figs. 11 and 12), having coefficients of 0.8 and 0.7 in the wet and dry seasons respectively. the coefficient of determination (r2) shows that 60.5% and 42.4% of the data were involved in the correlation during the wet and dry seasons respectively. thus, it validates the effectiveness of the field eca data as a useful proxy for assessing soil productivity. figure 11. relationship between field eca and laboratory ec in the soils of cacao farm during the wet season figure 12. relationship between field eca and laboratory ec in the soils of cacao farm during the dry season the field eca was influenced strongly by mg, ca; weakly influenced by acidic cation; very weak influenced by na, k, and an inverse relationship with available p (figs. 13 and 14). figure 13. chemical constituents influencing the measured field eca during wet season figure 14. chemical constituents influencing the measured field eca during dry season a negative correlation was observed from the interaction of available phosphorus with eca, indicating that is not contributing to the measured eca, though moderate coefficients were generated in the wet (-0.4) and dry (-0.5) seasons (figs. 13 and 14). kim et al. (2007) observed that the concentration of phosphorus increases under anaerobic conditions and for a direct relationship to occur between phosphorus concentration and ec, denitrification must take place in advance before the release of phosphorus. positive interaction was observed between the acidic cation and eca with moderate coefficients of 0.4 and 0.6 in the wet and dry periods respectively (figs. 13 and 14). the presence of acidic cations aids the conductivity of the media but the percentage of the cation involved in the interaction is between 87geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment 15.9% and 30.4%. soil acidity tends to build up hydrogen and aluminium cations in soil when the base cations are leached and replaced by aluminium or hydrogen ions (fao & itps, 2015) but ph in the farm is within the tolerance range for plant growth. a strong positive relationship was noticed between calcium and eca in the wet and dry seasons with the coefficient of 0.9 and 0.7 respectively (figs. 13 and 14). 71.4% and 55.1% of the data were perfectly engaged in the correlation in which there was a great chunk of the ions participating in the fluid conductivity (peralta and costa, 2013; heil and schmidhalter, 2017; medeiros et al., 2018). this suggests that ca is one of the dominant divalent ions in soil solution as a result of its large hydrated radius responsible for its easy dislodge from soil charges-cec (gransee and führs, 2013). positive coefficients (0.6-0.7) were generated from the interaction (figs. 13 and 14) between mg and eca (korsaeth, 2005; rodríguez-pérez et al., 2011). the group ii metals are characterised by s-p hybridisation in which the s and p electron shells overlap, thus availing the metal access to the unfilled p-subshell and finally aiding its electrical conductivity (garcia and damask, 1991). magnesium has a smaller ionic radius compared to ca, k and na, and its hydrated radius is larger, this made magnesium to be less strongly bounded to soil charges which were responsible for higher magnesium concentration in soil solution with increasing mobility and the mobility leads to electrical conductivity (gransee and führs, 2013). a weak positive correlation was established from the interaction of potassium ions with the eca, resultant coefficients of 0.3 were generated from the interaction at both seasons (figs. 13 and 14). this indicates that it has less influence on the conductivity of the medium as deduced from the percentage (6.2-8.0%) of data involved in the interaction. its strong adsorption onto clay surface was due to a decrease in hydrated radius compared to other cations (˂na+˂ca2+˂mg2+˂al3+), thereby reducing its mobility (gransee and führs, 2013; olson-rutz and jones, 2018). the contribution of na+ ions to the conductivity of soil unit was regarded as weak; its coefficient was 0.3 in both seasons (figs. 13 and 14). 6.6% to 9.5% of the data fit perfectly, thus its influence on the conductivity of soil solution is less. unsw (2007) reported that cations with small hydrated radii are strongly adsorbed onto clay surfaces because adsorption strength increases with decreasing the hydrated radius of the cation. correlating the cec with the eca (figs 13 and 14), a strong positive relationship was deduced with resultant coefficients of 0.8 at both seasons (korsaeth 2005; peralta and costa 2013). data interaction showed a good matched (57.4%-61.1%), suggesting a greater influence on the measured eca. 3.5 soil x-ray diffraction (xrd) assessment xrd analysis reveals the prevailing clay mineral to be kaolinite and the occurrence of montmorillonite and nontronite as a trace. non-clay minerals include quartz, microcline, albite, muscovite, biotite, oligoclase, corderite, and coquimbite. 3.5.1 mineralogical composition of fine fraction in the cacao soils kaolinite (4.7-11.3%) is the dominant clay mineral in the low eca section (figs 15 and 16) with an equitable 8.0% occurring as subordinate whereas montmorillonite (0-0.1%) occurred as trace (okunlola and owoyemi, 2015). the mean quantity of quartz (61.1-67.2%) was 64.2% (dominant). microcline content (14.2-24.6%) has a mean of 19.4% (subordinate). other minerals include muscovite (2.9%-5.7%), corderite (0.4%-1.2%) and oligoclase (1.3%-5.4%). region of moderate eca (figs 17 and 18) has 21.7% as the prevailing kaolinite content (14.9-28.4%) occurring as subordinate whereas nontronite (0-7.1%) occurred as the trace clay mineral. quartz (49.2-51.2%) is the dominant mineral with a mean of 50.2%, while microcline fraction (15.3-16.1%) has an average of 15.7% (subordinate). other minerals include corderite (0-9.3%), albite (0.4%-1.5%) and biotite (1.0%-4.0%). the high eca section (figs 19 and 20) has kaolinite (23.9-38.3%) as the abundant clay mineral with an average quantity of 31.1%, and montmorillonite (0-4.5%) occurred as a trace. quartz (30.5-52.7%) has a mean percentage of 41.3% signifying an abundant fraction. microcline (9.8-20.3%) has a common percentage of 15.1 (subordinate). corderite and muscovite proportions varied from 0.5% to 6.4% and 0% to 10.1% respectively. percentage contributions from oligoclase (0.4%), albite (1.7%) and biotite (1.0%) are regarded as trace in quantity. analysis of average mineral distributions showed that the quantity of quartz decreases from the region of low eca (64.2%) through the moderate eca (50.2%) section down to the high eca segment (41.3%). the quantity of microcline is highest (19.4%) in the low eca section, intermediate (15.7%) in the moderate eca region, and low (15.1%) in the high eca. the quantity of kaolinite increases from the low eca area to the high eca segment; the kaolinite was 8% (low eca segment), 21.7% (moderate eca) and both contents are in the subordinate class (5-25%) whereas highest percentage of kaolinite (31.1%) was noted to be in abundant quantity (25-50%) in the high eca zone. the ratio of kaolinite across the moderate eca and high eca sections was approximately three and four times respectively compared to the low eca segment. the average quartz content in the low eca region approximately doubles the fraction in the high eca area. grisso et al. (2009) reported that the ec of sand is low and sand constitutes 85% of quartz. the quartz particles are good insulator of electric current (negligible conductivity of 58 ns/m by manoucheri, 2002) and it dominates sand and silt fractions whereas clay fraction transmits current because of their clay mineral and organic matter (allred et al., 2008). in furtherance to this, the ec of silt was regarded as a medium while that of the clay was high (grisso et al., 2009); kaolinite has its electrical conductivity to be 0.2 s/m (kibria and hossain, 2019). the conclusion reached by revil and glover (1998) was that the surface conductance of clay is about four higher than that of quartz; clay has a large surface area with porous pores than the sandier material and soil having high clay content has higher ec reading (hawkins et al., 2017). the fraction with the greatest surface area controls the behaviour and performance (properties) of soil. soil characterised with a high proportion of quartz will hold fewer soil nutrients compared to that with much more clay content. this accounted for the variation across the three segments, even when the segments were supplied with the same amount of water, the concentration of dissolved nutrients will vary, thereby impacting different plant productivity rates. figure 15. x-ray diffraction result of soil-fine fraction in the low eca region of cacao plot (sample a2) 88 abayomi adesola olaojo, michael adeyinka oladunjoye figure 16. x-ray diffraction result of soil-fine fraction in the low eca region of cacao plot (sample a3) figure 17. x-ray diffraction result of soil-fine fraction in the moderate eca region of cacao plot (sample aa7) figure 18. x-ray diffraction result of soil-fine fraction in the moderate eca region of cacao plot (sample aa9) figure 19. x-ray diffraction result of soil-fine fraction in the high eca region of cacao plot (sample a1) 89geo-spatial sensing of apparent electrical conductivity: a leeway to agricultural soil assessment figure 20. x-ray diffraction result of soil-fine fraction in the high eca region of cacao plot (sample aa8) figure 21. relationship between eca and mineral components during wet season figure 22. relationship between eca and mineral components during dry season the relationship between apparent electrical conductivity and the dominant minerals (figs. 21 and 22) showed that there is strong positive interaction with the kaolinite (0.765-0.800) whereas negative interactions with the quartz (0.878-0.884) and microcline (0.474-0.665) during wet and dry seasons. this suggests that the eca is being influenced by the kaolinite; high eca values denote a large quantity of kaolinite and better nutrient retention. thus, it is possible to infer that soil derived from biotite granite gneiss with eca of 100 µs/cm and above is adequate for effective growth of cacao plant. conclusion the petrographic analysis unveiled the mineral constituents present in the rock, including the clay mineral that would form upon weathering, such as kaolinite. this observation aligns with the outcomes derived from the xrd analysis. the composition of minerals within the rock significantly influences the subsequent soil formation resulting from the disintegration of the crystalline rock. this connection can be utilized for evaluating soil productivity. the eca map (figs. 10 and 11) has proven instrumental in characterizing field conditions during both wet and dry seasons. eca serves as an indirect method for assessing soil nutrients and water content. notably, the soils were determined to be non-saline. regions marked by high eca values exhibit elevated levels of dissolved solutes, a notable concentration of clay, enhanced water retention, and valuable insights into soil permeability variation. the eca map facilitates the grouping of areas characterized by similar soil properties, which is valuable for guiding further soil sampling and examination efforts. the mapping of soil eca provides valuable information about changes in soil conditions and highlights areas warranting heightened attention. soil ph significantly impacts the availability of essential nutrients for plant uptake. notably, regions with high eca values show elevated concentrations of ca, mg, k, na, and cec values. additionally, available p distribution is relatively uniform across the three regions. regions with high eca values indicate a higher level of base saturation and non-sodic soil conditions. this favors nutrient retention, effective aeration, and drainage. on the other hand, low and moderate eca regions tend to be sodic, leading to nutrient leaching and heightened erosion susceptibility. a notable agreement exists between the clay type determined via cec analysis and the clay mineralogy revealed by xrd analysis (kaolinite). soil enrichment is intricately linked to its mineral composition. dominant minerals such as kaolinite, microcline, and quartz significantly influence soil characteristics. regions with substantial quantities of kaolinite clay and lower quartz and microcline content exhibit a pronounced ability to retain soil moisture and nutrients. consequently, these conditions lead to healthy plant growth and notably enhanced pod production, as observed in the high eca regions. the essence of geospatial analysis of eca is to streamline and demarcate area(s) with utmost attention. this would reduce the cost of soil sampling and analyzing the entire farm for geochemical evaluation, thus enabling effective management of agricultural soil. references adewole, m. b., & adeoye, g. o. 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(2016). correlation of mineralogical and textural characteristics with engineering properties of granitic rock from hulu langat, selangor. procedia chemistry, 19, 975–980. http://dx.doi.org/10.22564/brjg.v40i3.2171 http://dx.doi.org/10.22564/brjg.v40i3.2171 https://doi.org/10.1007/s12517-023-11650-4 https://doi.org/10.1007/s12517-023-11650-4 http://www.landresources.montana.edu/pdf/pub/foragepksmebo217 http://www.landresources.montana.edu/pdf/pub/foragepksmebo217 http://www.winewa.asn.au http://www.winewa.asn.au https://www.ag.ndsu.edu/publications/crops/soil-water-and-plant https://www.ag.ndsu.edu/publications/crops/soil-water-and-plant http://dx.doi.org/10.1155/2014/712403 https://extension.uga.edu/publications/detail.html?number=c1040&title=cation%20exchange%20capacity%20and%20base%20saturation https://extension.uga.edu/publications/detail.html?number=c1040&title=cation%20exchange%20capacity%20and%20base%20saturation https://extension.uga.edu/publications/detail.html?number=c1040&title=cation%20exchange%20capacity%20and%20base%20saturation http://www.terragis.bees.unsw.edu.ac/terragis_soil/sp_exchangeable_cations.html http://www.terragis.bees.unsw.edu.ac/terragis_soil/sp_exchangeable_cations.html https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/health/assessment/?cid=stelprdb1237387 https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/health/assessment/?cid=stelprdb1237387 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n1.111372 earth sciences research journal earth sci. res. j. vol. 29, no. 1 (march, 2025): 1-13 en v ir o n m en ta l g eo lo g y temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia maura melissa herrera1, david stephen fernández2, fabio de jesús vélez3, néstor jaime aguirre4 maura.herrera@udea.edu.co1, david.fernandez@udea.edu.co2, fabio.velez@udea.edu.co3, nestor.aguirre@udea.edu.co4 universidad de antioquia, medellín, colombia how to cite item: herrera, m. m., fernández, d. s., vélez, f. j., & aguirre, n. j. (2025). temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. earth sciences research journal, 29(1), 1-13. https://doi.org/10.15446/ esrj.v29n1.111372 record manuscript received: 01/10/2023 accepted for publication: 26/02/2025 abst r ac t alluvial gold mining has been a longstanding economic activity in the municipality of ayapel, córdoba, dating back to the colonial era, as noted by historians. however, most of this mining has occurred outside legal frameworks, often facilitated by the presence of lawless groups. this illegality complicates efforts to monitor and implement ecological restoration plans in the area. additionally, the municipality of ayapel is home to a significant natural asset: la ciénaga de ayapel. designated as a ramsar site since 2018, this wetland holds international recognition for its ecological importance. however, it faces the threat of contamination from the nearby mining activities. therefore, monitoring this mining activity becomes a crucial point to ensure, among other components, the care of la ciénaga de ayapel. the objective of this study is to analyze land cover changes in areas affected by alluvial mining between 1985 and 2020, using landsat images, digital image processing techniques, unsupervised classification, and sequential pattern mining for the multitemporal analisys. regarding the analysis conducted for the study area, encompassing the quebradona microbasin in the municipality of ayapel, for the years 1985 to 2020, there was an increase in mining activity of 503 ha, equivalent to 12 times that of 1985, with a peak in 2010, 54 times the 1985 levels. bare soil, on the other hand, increased by 96% by 2020. as for vegetation cover, dense or high vegetation remained relatively constant over time, going from 2434 ha in 1985 to 2446 ha in 2020. low vegetation, typically corresponding to grasslands, increased from occupying 3855 ha in 1985 to 5859 ha by 2020. medium vegetation decreased by 25% compared to 1985, going from 7588 ha to 5674 ha by 2020. finally, compared to 2010, there was a 14% increase or recovery in medium vegetation, and high vegetation saw a 5% recovery. it is important to highlight that the images from 1989 and 2013 exhibited a significant presence of cloud cover, which led to an underestimation of the area classified for each type of land cover. keywords: remote sensing, alluvial gold mining, satellite imagery, landsat, land cover change, sequential pattern mining. palabras clave: teledetección; minería de oro de aluvión; imágenes satelitales; landsat, cambio de coberturas terrestres; minería de patrones secuenciales. análisis temporal del cambio de coberturas terrestres en zonas de minería de oro de aluvión en ayapel, colombia la minería de oro de aluvión ha sido una actividad económica de larga data en el municipio de ayapel, córdoba, con antecedentes que se remontan a la época colonial, según lo señalan los historiadores. sin embargo, la mayor parte de esta minería ha ocurrido fuera de los marcos legales, a menudo facilitada por la presencia de grupos al margen de la ley. esta ilegalidad dificulta los esfuerzos de monitoreo y la implementación de planes de restauración ecológica en la zona. además, el municipio de ayapel alberga un importante recurso natural: la ciénaga de ayapel. este humedal ha sido designado como sitio ramsar desde 2018, lo que le otorga reconocimiento internacional por su importancia ecológica. no obstante, enfrenta la amenaza de contaminación debido a las actividades mineras cercanas. por lo tanto, el monitoreo de esta actividad minera se vuelve un aspecto crucial para garantizar, entre otros elementos, la protección de la ciénaga de ayapel. el objetivo de este estudio es analizar los cambios en la cobertura terrestre en las áreas afectadas por la minería aluvial entre 1985 y 2020, utilizando imágenes landsat, técnicas de procesamiento digital de imágenes, clasificación no supervisada y minería de patrones secuenciales para el análisis multitemporal. en cuanto al análisis realizado en la zona de estudio, que abarca la microcuenca quebradona en el municipio de ayapel, se observó un incremento en la actividad minera de 503 ha entre los años 1985 y 2020, lo que equivale a 12 veces la extensión de 1985, alcanzando un pico en 2010, cuando la actividad minera fue 54 veces mayor que en 1985. por otro lado, el suelo desnudo aumentó en un 96% para 2020. en cuanto a la cobertura vegetal, la vegetación densa o alta se mantuvo relativamente constante a lo largo del tiempo, pasando de 2434 ha en 1985 a 2446 ha en 2020. la vegetación baja, que generalmente corresponde a pastizales, creció de 3855 ha en 1985 a 5859 ha en 2020. en contraste, la vegetación media disminuyó en un 25% respecto a 1985, pasando de 7588 ha a 5674 ha en 2020. finalmente, en comparación con 2010, se evidenció una recuperación del 14% en la vegetación media y un 5% en la vegetación alta. es importante destacar que las imágenes de los años 1989 y 2013 mostraron una presencia significativa de nubosidad, lo que llevó a una subestimación en la clasificación del área para cada tipo de cobertura terrestre. resu m en https://doi.org/10.15446/esrj.v29n1.111372 mailto:maura.herrera@udea.edu.co mailto:david.fernandez@udea.edu.co mailto:fabio.velez@udea.edu.co mailto:nestor.aguirre@udea.edu.co https://doi.org/10.15446/esrj.v29n1.111372 https://doi.org/10.15446/esrj.v29n1.111372 2 maura melissa herrera, david stephen fernández, fabio de jesús vélez, néstor jaime aguirre 1. introduction one of the primary scenarios following the onset of alluvial mining activity is the disappearance of gallery forests, which protect the various water bodies where this activity takes place (espejo et al., 2018). the absence of vegetation triggers soil degradation and erosion processes, in addition to the loss of various habitats for the animal species that are part of the ecosystem (bradley, 2020). deforestation associated with alluvial mining significantly contributes to the degradation of both aquatic and terrestrial ecosystems, impacting biodiversity and soil stability (donkor et al., 2009; gerson et al., 2022). the basin of the san pedro river and the three basins that feed the ayapel swamp exhibit this condition of deforestation due to mining activity (pérez-rincón, 2014). in particular, the quebradona, escobillas, and trejos streams, upstream of the ayapel swamp, find themselves in a critical situation. throughout these three basins, the gallery forest has been deforested, and the bed of the streams has been altered (defensoría del pueblo, 2015; rodriguez, 2019). similar to other forms of mining, alluvial mining significantly alters the landscape (dethier et al.,2023), making the monitoring of these changes crucial to quantify the impact of these activities on terrestrial ecosystems. a distinctive procedure of mining in terrestrial placer deposits is the creation of a large pond using mechanical excavators (typically 60 m by 30 m), which is filled with water from a nearby source, referred to as “mining pools.” the excavated material is mixed in the partially filled pond with the added water to form a mineral slurry, which is then washed and filtered for mineral recovery (ibrahim et al., 2020). the mentioned excavations are generally carried out in places with nearby water bodies, such as streams and rivers, leaving a very particular mark on the landscape as they progress. on the other hand, remote sensing systems offer extensive opportunities to monitor soils and vegetation, providing an estimation of their characteristics and evolution over time (demattê et al., 2018). platforms like sentinel, landsat, and aster provide satellite images and digital elevation models (dems), which are essential tools for large-scale real-time monitoring. these platforms offer high accuracy, data authenticity, and comparability, making them widely applied in environmental studies since the 1990s (wang et al., 2017). the increase in the availability of remote sensing data from these platforms facilitates more frequent map updates, often in near-real time, which is crucial for tracking open-pit mining and recovery efforts (zhu & woodcock, 2014). repeated measurements at regular intervals are needed, and remote sensing often becomes the only alternative to field observations, particularly when historical data is required to study mining and recovery activities (petropoulos & partsinevelos, 2012). sequential pattern mining is a data mining technique aimed at identifying recurring patterns in ordered data, where the temporal or sequential order of events is significant. this method is particularly useful in analyzing large geospatial datasets, such as those derived from remote sensing and environmental monitoring, where patterns evolve over time (wu & zhang, 2019). in the context of spatial data, sequential pattern mining helps in detecting spatial and temporal dependencies among events, which can be useful for understanding processes like land-use change, ecosystem dynamics, or mining impacts on the environment (helmi & banaei-kashani, 2016). for instance, spatial sequential patterns can track the progression of deforestation, urban sprawl, or ecological degradation in areas affected by mining, providing insights into how these events unfold across both space and time (garcía-gil et al., 2021). this technique has gained notable interest in geospatial studies as it can extract meaningful patterns from complex datasets that traditional methods might overlook (wu & zhang, 2019; tzouramanis, 2018). in the realm of mining, there are vast opportunities for remote sensing in studying various environmental spheres. in the case of colombia, the majority of studies monitoring mining areas using this technique have focused on pollution monitoring, aiming to identify impacts of mining activities on water bodies (quejada palacios, 2019; uribe ospina, 2019), for instance, through the detection of high presence of suspended solids (gonzález, 2015); or impacts on vegetation cover, through measuring the level of forest fragmentation (anaya & diaz, 2016; suarez prada, 2019). to a lesser extent, research has concentrated on studying coverage changes (acosta díaz, 2017), in territorial planning with the zoning of areas with mining potential, considering for its choice, the socio-economic and environmental dynamics (rodríguez-luna, 2012), or the monitoring of the restoration of mining areas (ibrahim et al., 2020; pérez umaña, 2018). given the vast amount of information obtained from remotely sensed images, it is crucial to apply efficient and computationally less demanding classification and multitemporal analysis methods (lillesand, kiefer, & chipman, 2015). spectral indices, such as the normalized difference vegetation index (ndvi) and enhanced vegetation index (evi), are commonly used in remote sensing to monitor vegetation characteristics and track changes in land cover (mckenna et al., 2020). various classification methods are employed, with their effectiveness depending on factors such as surface type and application context (richards, 2013). for example, in the exploration of spatial mosaics, particularly in mining areas, image processing tools like segmentation and unsupervised learning methods, such as k-means clustering, have been widely used for cover classification (almeida-filho & shimabukuro, 2002; gillanders et al., 2008). spectral indices, such as the ndwi index, developed by mcfeeters (1996), is widely used to detect the presence of water bodies and soil saturation. in the mining context, recent studies have employed this index to monitor water quality and the extent of water bodies in areas impacted by open-pit mining (sulemana et al., 2018). for example, guo et al. (2019) used ndwi to assess the reduction in the volume of surface water bodies in areas affected by coal mining in china. the study showed that ndwi allowed for the identification of significant decreases in water bodies, correlated with the expansion of mining activities in the region (guo, xie, xu, & zhang, 2019). on the other hand, the savi index, proposed by huete (1988), is a modification of the normalized difference vegetation index (ndvi), designed to correct for soil influence in areas with low vegetation cover. in the mining field, this index has been used to assess vegetation recovery in post-mining rehabilitation areas. a study by navarro-cerrillo et al. (2017) used savi to analyze the evolution of vegetation in abandoned copper mines in southern spain. the study found that savi provided better discrimination of areas with low vegetation cover, enabling more precise monitoring of ecological regeneration in previously exploited areas (navarro-cerrillo, ruiz-gonzález, sánchez-cuesta, & villar montero, 2017). comparative studies, such as those conducted in arid and semi-arid landscapes, have shown that the savi index is more suitable for detecting vegetation in areas with lower vegetation cover and greater soil exposure, a typical scenario in regions affected by open-pit mining (qi, chehbouni, huete, kerr, & sorooshian, 1994). for instance, research in the mining region of jiu valley, romania, demonstrated that savi can detect vegetation cover at much lower percentages than ndvi, which is crucial for assessing regeneration in post-mining rehabilitation areas (fernández-manso, quintano, & roberts, 2012). at lower spatial resolutions, savi continues to offer greater precision in detecting vegetation compared to ndvi, which tends to saturate or become insensitive in areas with sparse vegetation, as observed in studies using modis and landsat data (gistec, 2012). for multitemporal analysis of large geospatial datasets, spatial sequential pattern mining has gained significant attention among researchers (helmi & banaei-kashani, 2016). these techniques aim to extract patterns from transactional frequencies, where each observation—represented as a pixel— contains a sequence of temporally located events. this approach is especially useful for monitoring spatial mosaics, which are critical for studying ecological restoration efforts (wu & zhang, 2019).” this study aimed to monitor the land cover changes of an area located to the south of the ciénaga de ayapel, encompassing the quebradona village, where alluvial gold mining has been present (unodc, 2019). monitoring was conducted in the area through the analysis of 10 landsat toa images, captured between the years 1985 and 2020. for this purpose, an unsupervised learning classification method was applied, and lastly, a data mining method for the multi-temporal monitoring of coverage changes was utilized. 2. methodology 2.1. study area the study area consists of a polygon located southeast of the ciénaga de ayapel, which houses the quebradona micro-basin (figure 1). ciénaga de ayapel is located in the department of córdoba, colombia. it is part of the larger system of wetlands and alluvial plains known as the la mojana region and is considered one of the most significant within the floodable system of the san 3temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. jorge river, due to its total area (45 km²) and its importance to the local fishing sector in the local economy (mejía ávila et al., 2023). the swamp is classified within a tropical wet forest life zone, with temperatures ranging between 26°c and 28.7°c and average precipitation between 2,000 and 2,500 mm (ríos et al., 2008). additionally, it generates surpluses that drain into the san jorge river, due to its hydrological system. its average depth is 6 m, and the water level varies up to 4 m between the rainy and dry periods (ríos et al., 2008). the floodplain basin has a total area of 1,504 km², divided into five micro-basins, among them, the quebradona micro-basin with an area of 262.14 km² (sernalópez & cañón-barriga, 2020). in ayapel, starting around the year 2005, alluvial mining has been carried out intensively and with machinery in the escobillas, quebradona, and trejos streams, which contribute to the swamp (pérez-rincón, 2014). the quebradona micro-basin has been one of the micro-basins most affected by alluvial gold mining. to the north of the quebradona micro-basin is located the integrated resource management district (dmi) of the ayapel wetland complex, a conservation figure at the national level, of significant importance given that the ciénaga de ayapel was declared a ramsar site (a figure of wetland conservation at the international level) around the year 2018 (decree 356 of 2018, ministry of environment and sustainable development-mads-). figure 1. study area encompassing the quebradona micro-basin, located in ayapel, córdoba. the study area corresponds to the polygon delimited in infrared. in the infrared image, the areas in white and light blue correspond to bodies of water, areas without vegetation, or bare soils. 2.2. image acquisition for this analysis, landsat t1 images were obtained from the google earth engine database (gorelick et al., 2017), corresponding to the landsat 4, 5, and 8 missions, of the study area whose coordinates are shown in table 1. the downloaded images had atmospheric level preprocessing (toa), which already have radiometric correction at atmospheric surface level. table 1. coordinates of the study area polygon*. latitude longitude 8.14645554439919 -75.11254943489037 8.278298734840208 -75.11254943489037 8.278298734840208 -75.2299658167263 8.14645554439919 -75.2299658167263 *the coordinates presented in this table correspond to the polygon drawn in figure 1. on the other hand, each landsat image comes with a set of multispectral bands{bi,j,1, …, bi,j,x, bi,j,qa}, where bi,j,1 is the band 1 of the satellite mission corresponding to the image, and bi,j,qa is the quality assessment (qa) band, a landsat product available for cloud detection in the images. each band bi,j,x is composed of a group of pixels ai,j,x, which in the case of toa images, can take reflectance values between 0 and 1, in the set of real numbers (known as 32-bit floating type in python); where the index "x" corresponds to the bands used, listed in table 2. table 2. bands used from the landsat satellite. band resolution (m) bandwidth (μm) landsat 4 & 5 landsat 8 near infrared (nir): bi,j,nir 30 0.76 0.90 0.85 0.88 red visible (r): bi,j,r 30 0.6 0.63 0.64 0.67 green visible (g): bi,j,g 30 0.52 0.61 0.53 0.59 following the acquisition of the landsat images, the classification, postprocessing, and multi-temporal analysis phases of the study area commenced (figure 2), with the aim of demonstrating the change in coverages over time. the entire image processing workflow was implemented using python programming language. a custom software application was developed using python 3.7.4 (www.python.org) to automate the processing pipeline, ensuring consistent treatment across all selected images. the software is named catesid, registered by the “dirección nacional de derechos de autor” filed under number 1-2022-22248. this approach not only standardized the methodology but also significantly enhanced the efficiency and reproducibility of the analysis. the step-by-step methodology employed by the software for processing the satellite imagery is detailed below. for the first stage "classification", the various bands of the landsat satellite {bi,j,1, …, bi,j,x, bi,j,qa} are introduced, and a preliminary classification ci,j,savi is obtained based on the soil adjusted vegetation index (savi). this index was used because it showed good distinction between cover types. each class or soil coverage was chosen according to the types of coverage observed in the field and in rgb satellite images, but these classifications were further validated using the soil adjusted vegetation index (savi) to ensure accuracy. the savi, which adjusts for soil brightness and is more effective in areas with sparse vegetation (huete, 1988), was used to determine vegetation levels. according to the savi values, low vegetation corresponds to values near 0, medium vegetation ranges between 0.2 and 0.5, and high vegetation is represented by values greater than 0.5 (huete, 1988). these savi-derived categories were used to cross-check the initial classifications, ensuring consistency and providing a clearer distinction between areas of low, medium, and high vegetation cover (huete, 1988; qi et al., 1994). subsequently, the "post-processing" phase begins where the classified image obtained in the previous stage is entered, and additionally a raster-type image vi,j,w obtained from the vectorization of rivers and streams is entered. the vectorization was carried out in arcgis (version 2.15) (esri, 2020) using an image of the study area and a digital elevation model (dem) (moore, et al., 1991), this image, taken in 1985, was compared to a more recent image from 2020. the comparison revealed no significant differences in the course of the water networks. additionally, this information was cross-referenced with the hydrological network recorded in regional maps. in this phase, new classes such as the “clouds” class, the “water” class, and the “mining soil” class are added, obtaining the classified image ci,j. in the "water" class, the normalized difference water index-ndwi(mcfeeters, 1996) was used to identify "small bodies of water" or "mining pools" which in turn are useful to delimit active mining areas (ibrahim et al., 2020), this methodology is explained later. to evaluate the classification accuracy, it was employed established methods including confusion matrices and visual interpretation techniques (congalton & green, 2019; foody, 2002; olofsson et al., 2014). the assessment was performed using arcgis accuracy assessment tools, incorporating the kappa coefficient to measure classification-reference agreement (cohen, 1960; esri, 2023). finally, for the "multi-temporal analysis", a set of images {ci,j,1, …,ci,j,t} are introduced, where t refers to a specific capture date for the image, and where ci,j,1 is an m x n matrix, and each pixel i, j has a number equivalent to a coverage class as its value; in this set of input images, the "clouds" class is excluded since it is considered as an information void. after applying the sequential pattern discovery using equivalence classes (spade) method (zaki, 2001), “sup” support values and “conf” confidence values are obtained for each of the sequences found by the method, understanding a sequence as an ordered set of classes. the definition of land cover classes or types was validated through field observations that were accurately georeferenced and compared with the pixels in the satellite image. in total, eight points were chosen at random and in such a way that the three levels of vegetation http://www.python.org 4 maura melissa herrera, david stephen fernández, fabio de jesús vélez, néstor jaime aguirre cover (high, medium, and low) could be validated, in addition to bare soil cover (as is showed in figure 2). identifying additional field points was not possible due to limited access to the area for security reasons. additionally, conversations were held with residents about land cover changes over time and significant socioeconomic events in the area throughout the years. figure 2. field survey points representing observed land cover types, categorized into distinct classes. regarding the measurement of classification accuracy, the total overall accuracy was calculated as the average of the overall accuracies provided by each of the classified images. the accuracy assessment was conducted following the principles established by congalton and green (2019), employing a confusion matrix (foody, 2002) and visual interpretation techniques (olofsson et al., 2014). the process was implemented in arcgis (version 2.15) (esri, 2020) using the accuracy assessment tools (esri, 2023), which include the calculation of the kappa coefficient (landis & koch, 1977) to evaluate the agreement between the classification and the reference data. figure 3. summary of phases for satellite image processing. the following is a more detailed explanation of each of the stages illustrated in figure 3 2.3. classification prior to applying unsupervised classification, the soil adjusted vegetation index (savi) was calculated for each satellite image, as described in figure 4. this index applies a correction for the presence of bare soil, yielding good results for areas with this characteristic (huete, 1988). in this way, bi,j,savi, is obtained, calculated as shown in equation (1). , , = , , − , , , , + , , + (1 + ) (1) in this case, parameter l was chosen as 0.5 given that the study area exhibits a moderately exposed surface due to the presence of bare soil. this value is also commonly recommended for most soil types (u.s. geological survey, n.d.). ℎ classification input multispectral images with radiometric correction. , ,1 = 111 ⋯ ⋮ ⋱ ⋮ 11 1 calculation of savi index application of the k-means method to channels a and b of the l*a*b color space obtaining classified image , , = , , − , , , , + , , + (1 + ) = 0.5 arg min 2 ∈ =1 , , where = , , ℎ ℎ ℎ ℎ , , ,1 , ,2 , ,3 , , ⋱ figure 4. description of satellite imagery classification phase. subsequently, once it was observed that the savi index presented a good delimitation of coverages, especially of bare soil, moderate, medium, and high vegetation. in this classification, the “high vegetation” class corresponds to dense forests characterized by mature vegetation. the “medium vegetation” class encompasses sparse forests and established crop areas. the “low vegetation” class is representative of grasslands and recently planted agricultural fields. the conversion from rgb color space to lab color space was proceeded, applying only the unsupervised classification by the k-means method (2) to channels a and b, discarding the l channel corresponding to luminosity. from the unsupervised classification, a good delimitation of the mentioned classes () was obtained, later supervised classification of the classes delimited by the classification method was performed, indicating to which class a certain coverage color corresponded. arg min 2 ℎ = , , ∈ (2) in equation (2), corresponding to the definition of the k-means method (hartigan & wong, 1979), given a set of observations ai,j,bsavi, which corresponds to the set of pixels that form each image bi,j,savi, k-means constructs a partition of observations into k clusters, where k is equal to the total number of classes, in order to minimize the sum of squares within each cluster y, where y refers to each class. additionally, uy is equal to the mean of points in sy, where sy is equal to the sum of squares of the points within class y. from this classification process, a classified image ci,j,savi is obtained, where for each ai,j there is a label of one of the classes y. the class y, up to the point of the application of unsupervised classification, can take values corresponding to the labels “bare soil”, “high vegetation”, “moderate vegetation”, and “low vegetation”. although k-means has certain precision limitations compared to other methods, its simplicity and the generally satisfactory results achieved in this study made it the chosen approach. furthermore, since the delimitation carried out by the savi index already left some well-identified contrasts, it was sufficient to apply a cluster to delimit these contrasts into classes. 2.4. post-processing following the classification of the images, additional post-processing was carried out to include some classes, such as water (including the hydrological 5temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. network), clouds, and active mining areas. for the identification of water bodies, the ndwi (3) was utilized, from which a raster file bi,j,ndwi is derived, with its pixels taking real values ranging between -1 and 1. post-processing calculation of ndwi index and masking of isolated water bodies input and replacement of vectorized water bodies (vi, j, w ) cloud masking using qa band (bi, j, qa) and usgs reference values (g) obtaining classified image with classes derived from postprocessing. , , , , = , , , , , , + , , , , = 1 > 0.3 0 ℎ , , = , , , , , , , , = 1 ∈ 0 ℎ , = , , + , , + , , + , , figure 5. description of the post-processing phase. the ndwi was developed to delineate open water features and enhance their detection in remote sensing digital imagery. utilizing the near infrared (nir) and visible green light (g) spectral bands, the ndwi effectively aids in identifying such features while mitigating interference from vegetation and ground surface characteristics (mcfeeters, 1996). , , = , , − , , , , + , , (3) following the process, a water mask, denoted as ci,j,w, was created. this mask assigned a value of 1 to each pixel that satisfied the condition of having an ndwi value greater than 0.3, and assigned 0 for any other case, as expressed in equation (4). , , = 1 > 0.3 0 ℎ (4) having obtained the water class mask ci,j,w, a raster file with vectorized water bodies vi,j,w was loaded, which came from a process in the arcgis software (version 2.15) (esri, 2020) of an image from a digital elevation model (dem) sourced from the aster satellite. subsequently, a subtraction operation was carried out between the files ci,j,w and vi,j,w, so that only the pixels corresponding to isolated water bodies remained in the mask ci,j,w. with the isolated water bodies, that is, those that do not correspond to natural water bodies like streams and rivers, a conditional operation was applied for the identification of mining pools, which typically measure between 30 and 60 m² (ibrahim et al., 2020). this was done with the aim of identifying bare soil around that might correspond to an active mining area, thereby distinguishing between the "bare soil" class and the "active mining" class, in accordance with the methodology applied by ibrahim et al. (2020). lastly, regarding cloud identification, the band bi,j,qa was utilized, with which a comparison is made against a set of reference values g provided by the usgs for the band bi,j,qa, that enables determining whether a pixel value of said band corresponds to a cloud. therefore, a cloud mask ci,j,n, was obtained, corresponding to the “cloud” class calculated as shown in equation (5), where a value of 1 is assigned to the pixel whose value falls between the reference values g, and 0 for any other case. thus, in the end, an image ci,j with class values and additional ones, such as “water”, “active mining”, and “clouds” is obtained, as shown in equation (6). 1 ∈ 0 ℎ , , = (5) , = , , + , , + , , + , , (6) 2.5. multitemporal analysis for the multitemporal analysis phase, a set of images ci,j, denoted as {ci,j,1, …,ci,j,t}, was utilized, where the subscript t corresponds to an image from a specific date, for the computation of a sequence matrix si,j (figure 6), which contains yi,j with t layers, through the spade method. multitemporal analysis input consists of a group of “t” images, each corresponding to a different capture date, with the "clouds" class excluded through a masking process for this analysis. definition of support threshold h computation of sequence matrix sij, which contains yij with t layers, through the sequential pattern discovery using equivalence classes (spade) method for sequential pattern acquisition of support values (sup) and confidence values (conf) for each sequence sij { , ,1, … , , , } mxn equals the dimension of each of the input images , , , = , , , ( 1 → 2) = (y1u y2) ( 1 → 2) = ( 2 | 1 ) 0 ≤ ≤ h figure 6. summary of the multitemporal analysis phase. a sequence refers to a chronologically ordered list of classes, where there exists a sequence of classes for each point ai,j in the matrix. therefore, given a threshold of minimum occurrence h for a sequence s, the number of resulting sequences is defined as (7). additionally, 0 <h ≤ mxn with mxn being the dimensions of each of the input images ci,j,t. this method detects a sequence for every ai,j across the t layers; however, only the sequences whose support or probability of occurrence is greater than or equal to the defined threshold h prevail (wu & zhang, 2019). , = , , , (7) in the end, a set of sequences is obtained, each with a support value “sup” and confidence value “conf”. support is defined as the percentage of appearance of a certain sequence, in relation to the total number of sequences (8), and confidence corresponds to the fraction between the number of appearances of a certain sequence, in relation to the total number of observations that initiated with a specific reference class (9). ( 1 2) = (y1u y2) (8) ( 1 2) = ( 2 | 1) = (y1u y2) ( 1) (9) 6 maura melissa herrera, david stephen fernández, fabio de jesús vélez, néstor jaime aguirre in this way, the support reflects the percentage of appearances of a sequence in relation to the total pixels of each image (mxn), and confidence refers to the percentage of appearances of a specific sequence in relation to the total pixels that initiated with a reference class , that is, the first class of the sequence, also known as the antecedent. 3. results and discussion 3.1. classification accuracy results the classification performed using the savi index, image processing, and the unsupervised classification method k-means yielded an overall accuracy of 74.37% ± 0.10 and a kappa coefficient of 0.658. according to landis and koch (1977), a kappa coefficient between 0.61 and 0.80 indicates substantial agreement, suggesting that the classification is of good quality. however, other studies have reported higher overall accuracies and kappa coefficients, depending on factors such as the resolution of the images used and the classification method employed. for example, in the classification of spot images (which have higher resolution than landsat) for land cover analysis, an overall accuracy of 94.61% and a kappa coefficient of 0.92 were obtained (duarte et al., 2017). additionally, other classifications using algorithms such as random forest (rf), support vector machine (svm), artificial neural network (ann), fuzzy adaptive resonance theory mapping (fuzzy artmap), spectral angle mapper (sam), and mahalanobis distance (md) reported kappa coefficients ranging from 0.82 for md to 0.89 for rf (talukdar et al., 2020). while in general, other algorithms may improve classification accuracy, the result obtained with k-means combined with the savi index yielded acceptable results for this study. in this study, it was observed that landsat 4 and 5 images showed lower accuracy compared to landsat 8 images. this disparity is likely due to the improved radiometric resolution of landsat 8, which facilitates better differentiation of land cover classes and, consequently, more accurate classification. (a) 24/01/1985 (b) 30/01/1987 (c) 11/01/1989 (d) 20/07/1991 (e) 17/07/1996 (f) 05/02/2001 (g) 29/01/2010 (h) 23/12/2013 (i) 19/07/2014 (continued) 7temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. (j) 21/05/2016 (k) 18/12/2017 (l) 20/02/2018 (m) 17/07/2019 (n) 10/02/2020 (o) legend clouds bare soil low vegetation medium vegetation high vegetation active mining soil streams isolated water bodies figure 7. classified landsat images resulting from the post-processing stage (all images have the same scale). 3.2. soil cover classification analysis from landsat images. for the study area, the classification of images corresponding to dates between 1985 and 2020 was obtained, as can be seen in the figure below (figure 8). 2,000 4,000 6,000 8,000 10,000 12,000 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 )a h( seratceh fo reb mu n years for satellite image analysis land cover changes among 1985-2020 water bare soil low veg medium veg high veg mining soil figure 8. classification of land cover between 1985 and 2020. based on the classification of the images and the graph showing the behavior of the different classes over time, a general decrease is observed across all classes for the images from 1989 and 2013, which exhibit significant cloud cover. therefore, this decrease in the area of the classes is more likely attributed to the presence of clouds rather than an actual reduction in the area occupied by each class. a similar pattern is observed for the years 1991 and 2020, although to a lesser extent, due to the reduced cloud cover in these images compared to the ones mentioned earlier (table 3). table 3. percentage of gain and loss of coverage based on the classification made. year water (%) bare soil (%) low vegetation (%) medium vegetation (%) high vegetation (%) active mining soil (%) 1985 1987 -67.93 313.04 27.35 11.78 10.18 830.36 1989 244.1 -76.08 -38.04 -21 -8.71 -91.21 1991 362.01 -50.09 11.22 26.35 9.03 146.49 1996 16.33 120.51 39.78 10.57 -13.3 387.46 2001 -83.35 131.3 42.11 -34.92 50.59 -21.25 2010 -55.84 131.72 -11.2 -18.25 -33.07 622.07 2013 543.04 -96.05 -26.39 -54.26 -77.61 -98.14 2014 -48.68 246.31 28.54 313.16 327.13 932.31 2016 -16.87 27.49 20.63 -23.63 34.29 71.81 2017 -10.46 -39.1 -9.63 25.82 -24.61 -36.87 2018 -26.2 61.79 0.05 -27.89 9.05 20.44 2019 166.92 -34.57 -33.4 60 14.67 -31.39 2020 -50.17 199.97 42.84 -45.6 -13.39 33.93 based on figure 8, it can be observed that medium vegetation predominates in the area compared to tall vegetation. over the years, low vegetation has shown an increasing trend, while medium and tall vegetation experienced a decline starting in 1996, stabilizing again in 2014. it is important to note that the decrease in hectares across all classes in 2013 is primarily due to the presence of clouds in the image for that year. 8 maura melissa herrera, david stephen fernández, fabio de jesús vélez, néstor jaime aguirre regarding active mining land and bare soil, an increase in hectares is evident starting in 1996, possibly related to the reduction of tall and medium vegetation caused by deforestation during the same period. however, these land covers (active mining land and bare soil) began to decrease after 2010, which could be associated with the increase in medium and tall vegetation observed from 2014, due to the revegetation of areas abandoned by mining activities. this observation is consistent with natural revegetation processes and potential restoration efforts in post-mining landscapes, as documented by gastauer et al. (2019) in similar tropical environments. in the study area, some assisted restoration attempts were observed. on the other hand, active mining soil and bare soil seem to exhibit very similar behavior in their changes over time. this can also be seen in the classification matrix in figure 9 which yielded a positive rating of 0.76, which implies a good correlation between classes. it could be inferred that as the amount of bare soil increases, the number of active mines also rises, since stripping the soil of its vegetative cover is the initial step for the onset of extractive activity. additionally, according to residents, all areas devoid of vegetative cover correspond to active mining areas. this relationship between active mining areas and bare soil dynamics aligns with findings from chicaolmo et al. (2021), who analyzed land cover changes in mining regions of southern spain and observed a strong correlation between the expansion of bare soil and increased mining activities over a 30-year period. similarly, sonter et al. (2017) documented in their study of brazilian amazon mining areas that the removal of vegetation directly precedes the establishment of mining operations, creating extensive zones of bare soil. these studies highlight how bare soil serves as both an indicator and a precursor of active mining operations. figure 9. correlation matrix between classes after classification between the years 1985 and 2020. the spatial distribution of high and medium vegetation classes reveals certain similarities in their occurrence patterns, as observed in figure 7 and the correlation matrix (figure 9). these two classes exhibit a positive correlation of 0.51, indicating that years with significant high vegetation cover often also show a notable presence of medium vegetation. this co-occurrence may reflect the complex structure of forest ecosystems during various stages of growth or recovery, as highlighted by chazdon (2014) in studies on tropical forest regeneration. within the study area, medium vegetation emerges as the most prevalent cover type, followed by low vegetation and high vegetation. this pattern is evident in figure 8, where medium vegetation averaged 7,300 hectares, low vegetation 5,261 hectares, and high vegetation 2,436 hectares across all analyzed years. this distribution highlights a landscape mosaic dominated by intermediate stages of vegetation growth or regrowth, a characteristic feature of tropical regions undergoing land-use changes (arroyo-rodríguez et al., 2017). it’s important to note that these observations are based on spatial patterns and relative abundances derived from our classification results. further statistical analysis and ground validation would be necessary to establish any definitive relationships or trends among these land cover types. regarding the behavior of active mining over the years (figure 9), it can be observed that initially, in the year 1987, there was an increase in mining activity of 351.9 ha compared to 1985; that is, there was an increase of 9 times more compared to what there was in 1985, a trend consistent with the data collected in the field through interviews with local populations. they confirm that between 1987 and 1988, there was an increase in mining activity due to the incorporation of machinery in gold mining, a method not used in previous years, where mining practices were more artisanal. based on field interviews, gold mining activity in ayapel was relatively low between the 1990s and 2004. this was attributed to the presence of illegal armed groups that controlled mining operations, discouraging extensive participation by the local population. this behavior, as described by local residents, aligns with scientific evidence demonstrating a significant relationship between armed conflict, its impact on natural resources, and the transformation of ecosystems in colombian territories (salazar et al., 2018). figure 9 shows an increase in mining activity between 2001 and 2010, amounting to 2,039 hectares. however, during this period, landsat imagery was unavailable due to a “striping” issue with the satellite, leaving uncertainties about the fluctuations in mining activity during this time. according to field interviews, mining activity increased significantly after 2005, coinciding with the demobilization of armed groups that had previously controlled the region. this shift allowed both civilians and other criminal organizations to take over mining operations. the use of machinery and more advanced methods compared to traditional techniques enabled faster and more extensive resource extraction, leading to a significant expansion of mining areas. according to figure 10, by 2014, mining areas decreased compared to 2010. 0 500 1000 1500 2000 2500 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 n um be r o f h ec ta re s (h a) years for satellite image analysis figure 10. change in hectares of active mining soil over time. although a correlation between mining activity and vegetation cover might theoretically be expected, the correlation matrix in figure 9 shows no significant relationship between active mining areas and vegetation types. this lack of correlation could be attributed to several factors. one of them is that satellite image classification may be affected by errors related to image resolution, atmospheric conditions, or limitations in classification algorithms, which impact the accuracy of the data obtained. computational methods have proven to be fast, accurate, and cost-effective tools for automatic land-use classification in satellite imagery (gómez & perea, 2017). finally, a limited number of available images for analysis may fail to capture the temporal and spatial variability of mining activities and their impact on vegetation, particularly in regions with high cloud cover, where cloud-free imagery is essential for accurate monitoring. therefore, the observed lack of significant correlation suggests the need for improved classification techniques and an expanded image database for future studies. 3.3. seasonal multitemporal analysis based on the methodology of ibrahim et al. (2020), for the seasonal analysis, images from the years 2016, 2017, 2018, 2019, and 2020 were chosen, from the months of may, december, february, july, and february respectively. the images from 2016 and 2019 correspond to the rainy season in the study area and the images from the years 2017, 2018, and 2020 correspond to the dry season. this sequential pattern mining analysis yielded the sub-sequences shown in table 4. 9temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. the analysis from table 4 reveals that the subsequence mining soil>mining soil had the highest occurrences with 6081, reflecting a 58.71% confidence that areas classified as mining soil in one year remained so in the following year. there is 60% confidence that such areas from 2016 remained mining soil through 2019. other notable subsequences include active mining soil -> low vegetation and active mining soil -> low vegetation -> low vegetation, with occurrences of 4969 and 2953, and confidence levels of 47.97% and 59.42% respectively. this suggests a 40 to 60% probability of areas transitioning from mining soil to low vegetation, comparable to the probability of remaining as mining soil. table 5 illustrates sequences initiated with vegetation classes. the most recurrent sequence was vegetation high->vegetation high, indicating that most high vegetation forest remained constant over the analyzed period. there is a notable increase in vegetation during the rainy season compared to the dry season, due to better growth conditions in winter. moreover, in the dry season, grasslands tend to decrease or get replaced by crops with the arrival of rains. the results also show a tendency to maintain the same vegetation classification between the years 2017 and 2018. in table 5, the blue color alludes to the winter season and the orange color refers to the dry season. lastly, the arrows indicate an increase in vegetation with the transition from the dry season to the rainy season. 3.4. annual multitemporal analysis. based on the methodology of ibrahim et al. (2020), the annual multitemporal analysis grouped images from 1985 to 2014 into subgroups for better visualization and interpretation of sequences, based on classification results. three subgroups were formed: 1985-1989, 1989-2001, and 20012014, selected based on the variation behavior in vegetation and mining areas between image dates. this facilitated the interpretation of the annual behavior in vegetation and mining activity over time. about the sequences of the years 1985, 1987, and 1989 (table 6), the most occurring sequences were high vegetation->medium vegetation->medium vegetation, with an occurrence of 29964 and a confidence of 0.4161. this means that the pixels classified as high vegetation had a 41.61% probability of being reclassified as medium vegetation, which would imply a loss of dense forest during this period. table 4. subsequences initiated with the class “mining soil” in the image of 2016. 2016 2017 2018 2019 occurrence support confidence minning soil bare soil 2819 0.01349 0.27218 minning soil low veg 4969 0.02377 0.47977 minning soil low veg low veg 2953 0.01413 0.59428 minning soil minning soil 6081 0.02909 0.58714 minning soil minning soil minning soil 4077 0.01950 0.67045 minning soil minning soil minning soil minning soil 2481 0.01187 0.60854 table 5. sequences for seasonal analysis of vegetation type. 21/05/2016 18/12/2017 20/02/2018 17/07/2019 10/02/2020 occurrence support confidence high veg high veg high veg high veg high veg 35904 0.171 0.620 low veg medium veg low veg medium veg low veg 7047 0.033 0.585 medium veg medium veg low veg medium veg low veg 6545 0.031 0.507 low veg low veg low veg medium veg low veg 5757 0.027 0.572 medium veg medium veg medium veg high veg medium veg 3695 0.017 0.449 high veg high veg high veg high veg medium veg 3472 0.016 0.060 medium veg low veg low veg medium veg low veg 3465 0.016 0.583 table 6. sequences of vegetation classes for the years 1985, 1987, and 1989. 1985 1987 1989 ocurrence support confidence high veg medium veg medium veg 29964 0.143 0.416 high veg low veg medium veg 7805 0.037 0.307 low veg low veg medium veg 7201 0.034 0.190 low veg low veg low veg 6999 0.033 0.185 high veg medium veg high veg 6835 0.032 0.094 medium veg high veg high veg 5045 0.024 0.177 low veg bare soil low veg 4830 0.023 0.360 10 maura melissa herrera, david stephen fernández, fabio de jesús vélez, néstor jaime aguirre another notable sequence is vegetation low ->bare soil ->vegetation low, with an occurrence of 4830 and a confidence of 0.3604, indicating that there was a 36.04% probability that pixels classified as low vegetation, would be reclassified as bare soil. this could be interpreted as an increase in mining activity between 1985 and 1987, and a subsequent decline in activity between 1987 and 1989, which aligns with the presence of outlaw groups in the territory and their influence on the behavior of this activity. the study analyzes the period between 1985-2020, noting a similar trend between “mining soil” and “bare soil” classes due to their inherent relationship. particularly in 1989, 1991, and 1996, an increase in mining activities is observed, corroborated by multitemporal analysis results (table 7). table 7 shows prevalent sub-sequences: low vegetation -> bare soil and medium vegetation -> bare soil. despite high occurrence, these sequences exhibit low confidence levels, due to vegetation transitioning to other classes, in sync with the increased medium vegetation noted in classification results. interestingly, sequences beginning with mining soil and bare soil classes showed a tendency to transition into medium and low vegetation classes during 1991 and 1996, hinting at a potential vegetation recovery phase with a 40% probability of transitioning from mining or bare soil to vegetated soil. the multitemporal analysis conducted for the images of the years 2001, 2010, 2013, and 2014 revealed the subsequence “low vegetation -> low vegetation -> medium vegetation” with the highest number of occurrences (11881) and a confidence level of 62% (table 8). this suggests an increase in medium vegetation from low vegetation, with a 62% probability that the “low vegetation” class from 2010 would become “medium vegetation” in 2014. on the other hand, sequences that started with the “low vegetation” class in 2001 and transitioned to “bare soil” in 2010 tended to classify as “medium vegetation” or “low vegetation” in 2013 and 2014, with a probability ranging from 30% to 50% of exhibiting this behavior. additionally, the “low vegetation” class in 2001 showed a probability of 30% to 53% of becoming “mining soil” in 2010 and “medium vegetation” or “low vegetation” in the subsequent years (2013 and 2014). these results are aligned with the analysis based on the classification of the images, indicating an increase in mining activity from 2001 to 2010 and a decrease in activity by 2014. conclusions the classification accuracy achieved using the k-means method and certain spectral indices in this study was lower compared to other currently available methods for remote sensing of land cover. however, the results were acceptable, with an overall accuracy of 74.37% ± 0.10 and a kappa coefficient of 0.658. one limitation that introduced a degree of error in the classification was the presence of cloud cover, which led to an underestimation of the hectares classified within the different land cover types. nevertheless, this underestimation was accounted for in the analysis of the results. finally, the accuracy of the land cover classification was influenced not only by the classification method employed but also by the resolution of the images, as higher resolution results in greater precision in delineating land cover types. according to the results obtained in the classification process, mining in ayapel has increased in recent years, particularly since 2016, although not to the same extent as the exponential growth observed between 2000 and 2013. it can also be concluded that significant revegetation occurred between the 1990s and 2000s, and again between 2013 and 2020, indicating that vegetation restoration has taken place in mining areas, provided there is a cessation in the intensity and occurrence of mining activity. given the observed increase in revegetation when mining activity ceases, it is essential to implement ecological restoration programs in areas where mining has decreased or stopped. this includes activities such as planting native species of high and medium vegetation cover, which are key to restoring the ecosystem’s structure and function (mdpi, 2023). additionally, it is crucial to limit the expansion of mining into new areas through zoning policies that define mining exclusion zones in areas of high ecological importance, such as those with dense vegetation or critical functions for biodiversity and the hydrological cycle. on the other hand, regarding the changes in coverage, the multitemporal analysis from 2016 to 2020 shows that areas initially classified as “mining soil” exhibit two main trends: a significant persistence as mining soil over the years, which is consistent with areas of ongoing exploitation, and a notable transition to “low veg” (low vegetation), possibly associated with partial recovery processes or the abandonment of mining areas. on the other hand, transitions to other classes, such as “bare soil,” are less common. this suggests that mining table 7. subsequences of “mining soil” and “bare soil” classes for the years 1989, 1991, and 1996. 1989 1991 1996 occurrence support confidence low veg bare soil 10873 0.052 0.087 medium veg bare soil 8734 0.041 0.053 bare soil low veg 8609 0.041 0.380 mining soil low veg 3340 0.015 0.379 bare soil bare soil 3190 0.015 0.141 medium veg mining soil 3041 0.014 0.018 bare soil low veg low veg 2749 0.013 0.319 bare soil medium veg 2739 0.013 0.121 table 8. sequences and subsequences of classes for the years 2001, 2010, 2013, and 2014. 2001 2010 2013 2014 occurrence support confidence low veg low veg medium veg 11881 0.056 0.621 low veg bare soil low veg 4335 0.020 0.522 low veg bare soil medium veg medium veg 4229 0.020 0.350 high veg high veg medium veg 4036 0.019 0.371 medium veg low veg medium veg medium veg 4003 0.019 0.234 low veg mining soil medium veg 3947 0.018 0.535 low veg mining soil medium veg medium veg 3802 0.018 0.363 low veg bare soil medium veg 3488 0.016 0.420 low veg bare soil medium veg low veg 3282 0.015 0.271 high veg high veg high veg high veg 2965 0.014 0.207 high veg veg media high veg 2875 0.013 0.293 high veg low veg medium veg 2516 0.012 0.715 low veg mining soil low veg 2428 0.011 0.329 11temporal analysis of land cover change in ayapel's alluvial gold mining zones, colombia. activities dominate over time, although there are some indications of limited ecological regeneration in certain cases. acknowledgments this research was conducted as part of the project 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(2023). stratigraphic distribution of marine vertebrates from the arcillolitas abigarradas member (paja formation) of the villa de leiva – sáchica – sutamarchán region, boyacá, colombia. earth sciences research journal, 27(3), 211-226 https://doi.org/10.15446/esrj.v27n3.108292 record manuscript received: 11/04/2023 accepted for publication: 07/09/2023 the villa de leiva – sáchica sutamarchán region is the most prolific locality for early cretaceous marine vertebrates in colombia and one of the richest in these vertebrates in the world. all of these vertebrates come from beds of the barremian-aptian arcillolitas abigarradas member of the paja formation. although many of the specimens have already been taxonomically studied, no publications have discussed their stratigraphic position and distribution within the barremian-aptian interval. herein, we provide, for the first time, detailed stratigraphic information for the majority of the barremian-aptian marine vertebrates of the villa de leiva – sáchica sutamarchán region reported so far. based on 10 stratigraphic sections described in the several specimens finding sites, together with biostratigraphic information associated with specimens of imprecise geographical origin, we determined the stratigraphic provenance of 26 marine vertebrate specimens. we also refined the stratigraphic provenance of some specimens including the holotypes of muiscasaurus catheti, “kyhytysuka” sachicarum, and protolamna ricaurtei whose stratigraphic origin was not previously specified beyond the arcillolitas abigarradas member. the data obtained allowed us to identify four vertebrate assemblages representing coeval faunas within the barremian-aptian interval, two from the barremian and two from the upper aptian. abstract keywords: stratigraphic distribution; marine vertebrates; barremian-aptian; lower cretaceous; paja formation. resumen la región de villa de leiva sáchica sutamarchán es la localidad más prolífica en vertebrados marinos del cretácico temprano en colombia y una de las más ricas en estos vertebrados en el mundo. todos estos vertebrados provienen de las capas del miembro arcillolitas abigarradas del barremiano-aptiano de la formación paja. aunque muchos de los especímenes ya han sido estudiados taxonómicamente, ninguna publicación ha discutido su posición y distribución estratigráfica dentro del intervalo barremiano-aptiano. en este trabajo proporcionamos, por primera vez, información detallada sobre la procedencia estratigráfica de la mayoría de los vertebrados marinos del barremiano-aptiano de la región de villa de leiva sáchica sutamarchán reportados hasta ahora. con base en 10 secciones estratigráficas descritas en los sitios de hallazgo de diversos especímenes, junto con la información bioestratigráfica asociada a los especímenes con origen geográfico impreciso, nosotros determinamos la procedencia estratigráfica de 26 especímenes de vertebrados marinos. también afinamos la procedencia estratigráfica de algunos ejemplares, entre ellos los holotipos de muiscasaurus catheti, “kyhytysuka” sachicarum y protolamna ricaurtei cuya procedencia estratigráfica no estaba previamente precisada más allá del miembro arcillolitas abigarradas. la información obtenida permitió identificar cuatro conjuntos de vertebrados que representan faunas coetáneas dentro del intervalo barremiano-aptiano, dos del barremiano y dos del aptiano superior. palabras clave: distribución estratigráfica; vertebrados marinos; barremiano-aptiano; cretácico inferior; formación paja; https://doi.org/10.15446/esrj.v27n2.103801 mailto:cdbenavidesc@unal.edu.co mailto:meparamof@unal.edu.co mailto:janarvaezr@unal.edu.co mailto:mpramosc@unal.edu.co mailto:hdpalmac@unal.edu.co https://doi.org/10.15446/esrj.v27n2.103801 212 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo 1. introduction in the localities of villa de leiva, sáchica, and sutamarchán, abundant marine vertebrate fossils have been found, all of which come from the arcillolitas abigarradas member of the paja formation of barremian-aptian in age (cadena, 2015; cadena and parham, 2015; cadena et al., 2019; carrillobriceño et al., 2019; cortés and páramo-fonseca, 2018; cortés et al., 2019; 2021; gómez-pérez and noè, 2017; maxwell et al., 2016; 2019; noè and gómez-pérez, 2022; páramo-fonseca, 2015; páramo-fonseca et al., 2016; 2018; 2019a; 2019b; 2021; 2023; schultze and stöhr, 1996), this material includes mainly marine reptiles (plesiosaurs, ichthyosaurs, turtles, and a crocodylomorph) and some fishes. the published plesiosaurs were identified as the elasmosaurids callawayasaurus colombiensis (welles, 1962) and leivanectes bernardoi páramo-fonseca et al., 2019a from specimens found in villa de leiva (goñi and gasparini, 1983; páramo-fonseca et al., 2019a; welles, 1962) and the pliosaurids acostasaurus pavachoquensis gómez-pérez and noè 2017 represented by a specimen found in sutamarchán (gómez-pérez and noè, 2017), stenorhynchosaurus munozi páramo-fonseca et al., 2016 represented by specimens found in sáchica, sutamarchán and villa de leiva (páramofonseca et al., 2016; 2019b; 2023), sachicasaurus vitae páramo-fonseca et al., 2018 represented by a specimen found in sáchica (páramo-fonseca et al., 2018), and “monquirasaurus” boyacensis (hampe, 1992) represented by a specimen found in villa de leiva (hampe, 1992; noè and gómez-pérez, 2022). this last species was initially described as kronosaurus boyacensis by hampe (1992) and recently noè and gómez-pérez (2022) proposed the genus “monquirasaurus” to include this species. here we use the name within quotes because we consider that the attribution of this species to a new genus is not fully supported. the published ichthyosaurs were identified as the opthalmosaurids muiscasaurus catheti maxwell et al., 2016 represented by specimens found in villa de leiva and sáchica (maxwell et al., 2016; páramo-fonseca et al., 2021); platypterygius sp. represented by a specimen found in villa de leiva (cortés and páramo-fonseca, 2018), and ”kyhytysuka” sachicarum (páramofonseca, 1997) represented by specimens found in villa de leiva (cortés et al., 2021; maxwell et al., 2019; páramo-fonseca, 1997). this last species was originally referred to the genus platypterygius by páramo-fonseca (1997) and recently cortés et al. (2021) referred the species to a new genus “kyhytysuka”. here we use the name within quotes because we consider that the attribution of this species to a new genus is not fully supported. the published turtles were identified as leivachelys cipadi cadena, 2015 and desmatochellys padillai cadena and parham, 2015, from specimens found in villa de leiva (cadena, 2015; cadena and parham, 2015; cadena et al., 2019; de la fuente and goñi, 1983). a crocodylomorph specimen found in villa de leiva was identified as an indeterminate teleosauroid (cortés et al., 2019). among the fishes there have been reports of a bony fish identified as vinctifer sp. jordan 1919 from sáchica (schultze and stöhr, 1996) and the lamniform shark protolamna ricaurtei carrillo-briceño et al., 2019 from villa de leiva. although there are many studies on the marine vertebrates from the arcillolitas abigarradas member, there has never been an attempt to refine their stratigraphic occurrences. this lack of refinement can be attributed to several challenges, including the presence of geologic faults and folds, the lateral variations of the facies and thickness of the different stratigraphic intervals (etayo-serna, 1968a), and the discontinuity of the outcrops due to vegetation and soil coverage. this contribution aims to establish the stratigraphic distribution of many of the marine vertebrate specimens that have been found in the arcillolitas abigarrradas member in the villa de leiva sáchica sutamarchán region. we hope that the results obtained here serve as a stratigraphic framework for future studies on the barremian-aptian marine paleofauna of this region. 2. material and methods a large number of marine vertebrate specimens (at least more than 50 specimens) have been discovered in the arcillolitas abigarradas member in the villa de leiva sáchica sutamarchán region. however, we did not include all the specimens in this study due to a lack of essential information on some of them, which did not allow us to determine their stratigraphic position. we selected the specimens based on the availability of reliable geographic, stratigraphic, and biostratigraphic data. we excluded those specimens for which none of these data are known, or if the available information was deemed unreliable or insufficient. following these selection criteria, we included a total of 26 specimens which comprise several elasmosaurid and pliosaurid plesiosaurs, some ophthalmosaurid ichthyosaurs, one turtle, one crocodylomorph, two sharks, and one actinopterygian fish (see table 1). we describe stratigraphic sections to determine the stratigraphic provenance of the specimens from which only the geographic provenance was known. we described eight stratigraphic sections in localities where one or more of the selected specimens were collected (see fig. 1). we followed the stratigraphy of the arcillolitas abigarradas member described by etayo-serna (1968a) in villa de leiva as a reference to correlate our stratigraphic columns and, focused on correlating the stratigraphic provenance of the selected specimens with the stratigraphic segments established by this author. using the ammonoid biozones established for colombia (bürgl, 1956a; etayo-serna, 1979; patarroyo, 2000; patarroyo, 2020) we correlated the specimens in which accurate information on their geographical origin is unknown but associated ammonoids were found. we measured the stratigraphic sections using jacob’s staff (1.5 m) and compass following the methodology described by compton (1985). we draw them at a scale of 1:100. for the lithologic description, we followed the rock classification scheme presented by nichols (2009), the subdivisions of layer thickness of campbell (1967), the bioturbation index proposed by taylor and goldring (1993), and the fissility categories proposed by potter et al. (1980, table 1.3). we used the munsell (2009) color table to establish the color of the rocks in fresh samples. finally, we digitized the obtained stratigraphic columns with the open-access program inkscape 1.2. the full-scale digitized columns can be accessed in supplemental data 1. we collected in-situ ammonoids and prepared them mechanically using a me-9100 air scribe. we (cdb-c) identified the ammonoids based on the works of bürgl (1956a), etayo-serna (1979), kakabadze and hoedemaeker (2004), and wright (1996). the identification of these ammonoids can be accessed in supplemental data 2. we correlated the described stratigraphic columns to the ammonite biozones proposed by patarroyo (2000) for the barremian and by etayo-serna (1979) for the aptian. in this way, we defined the biostratigraphic origin of some vertebrate specimens whose stratigraphic provenance within the arcillolitas abigarradas member had not been previously specified. in 2014 one of us (mpr-c) described an unpublished stratigraphic section of the complete arcillolitas abigarradas member in sáchica to correlate the stratigraphic origin of two of the here selected specimens. here, we provide the stratigraphic column and the description of the stratigraphic section. this section was described in the vereda arrayán alto, a locality characterized by extensive covered areas, in which the few outcrops are small, abandoned artisanal gypsum mining sites and a small creek. as the outcrops in the locality were non-continuous, the different described parts of the stratigraphic sections were connected using the polygonal method. we (mpr-c) described this stratigraphic section at a 1:200 scale using jacob’s staff (1.5 m) and abney clinometer following the methodology described by etayo-serna (1985). we (mpr-c) determined the thickness of the layers following the subdivisions of layer thickness of campbell (1967) and established the lithology of the strata following the rock classification scheme presented by folk (1980). we (mpr-c) described the sedimentary structures and collected fossils following the parameters established by etayo-serna (1985) and defined the color of the rocks in fresh samples using the color table tgl 34329 gesteinsfarben (1979). we (mpr-c) digitized the stratigraphic column in three segments using autocad 2014. the full-scale digitized column of this section can be accessed in supplemental data 3. dr. fernando etayo-serna identified the ammonoids collected from this section and provided their age following the barremian biozones proposed by bürgl (1956a, b) and the aptian biozones proposed by etayo-serna (1979). the identification of these ammonoids can be accessed in the supplemental data 3. a published stratigraphic column of the arcillolitas abigarradas member described on the road from sáchica to samacá (schultze and stohr,1996) provided us the stratigraphic provenance of one of the selected specimens. we correlated the part of this section that includes the specimen stratigraphic provenance with the sections described here. moreover, we correlated the aptian ammonoids identified by schultze and stohr (1996) from their described section, to the ammonoid biozones of etayo-serna (1979). 213stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) as a whole, we correlated ten stratigraphic columns, taking as a reference the stratigraphic section of the arcillolitas abigarradas member of etayo-serna (1968a) and the ammonoid biostratigraphic schemes of patarroyo (2000), bürgl (1956a) and etayo-serna (1979). in the text figures, we provide a generalized version of these stratigraphic columns. through a graphical correlation of the stratigraphic and biostratigraphic provenance of the specimens, we obtained a precise stratigraphic distribution of marine vertebrate specimens within the barremian-aptian interval of the arcillolitas abigarradas member of the paja formation. institutional abbreviations: cb, mp, mpr, undg, acronyms used in sample numbers from paletontological collections of the departamento de geociencias, universidad nacional de colombia, bogotá, colombia; cfsta, colección fundación santa teresa de avila, villa de leiva, colombia; cip, centro de investigaciones paleontológicas, villa de leiva, colombia; don, mp, ra, vl, acronyms used in sample numbers from the paleontogical collection of the servicio geológico colombiano, bogotá, colombia; fcgcbp, fundación colombiana de geobiología, now centro de investigaciones paleontológicas, villa de leiva, colombia; mb, museum für naturkunde berlin, berlin, germany; mjacm, museo junta de acción comunal vereda monquira, villa de leiva, colombia; mpvl, museo paleontológico de villa de leyva; universidad nacional de colombia, villa de leiva, colombia; ucmp, university of california museum of paleontology, california, usa. table 1. selected specimens by taxonomic group. the last column shows the number assigned to each specimen in figures 1-5 taxonomic group specimen taxon reference material nº in figures elasmosauridae fcg-cbp-3 elasmosauridae unpublished anterior part of an articulated skeleton 1 elasmosauridae fcg-cbp-22 (holotype) leivanectes bernardoi páramo-fonseca et al., 2019a anterior fragment of a skull 2 elasmosauridae ucmp 38349 (holotype) callawayasaurus colombiensis welles, 1962 nearly complete skeleton 3 elasmosauridae ucmp 125328; igmp174997 callawayasaurus colombiensis welles, 1962 nearly complete skeleton 4 pliosauridae cfsta 2-1 pliosauridae unpublished incomplete skull 5 pliosauridae fcg-cbp-4 pliosauridae unpublished nearly complete skeleton 6 pliosauridae mpvl-mp110210-1 pliosauridae unpublished posterior half of a skeleton 7 pliosauridae mjacm 1 (holotype) “monquirasaurus” boyacensis hampe, 1992; noè and gómez-pérez (2022) nearly complete skeleton 8 pliosauridae mp111209-1 (holotype) sachicasaurus vitae páramo-fonseca et al., 2018 nearly complete skeleton 9 pliosauridae vl17052004-1 (holotype) stenorhynchosaurus munozi páramo-fonseca et al., 2016 nearly complete skeleton 10 pliosauridae mp050310-1 stenorhynchosaurus munozi páramo-fonseca et al., 2019b skull and first cervical vertebrae 11 pliosauridae mpvl-667 stenorhynchosaurus munozi páramo-fonseca et al., 2023 anterior end of the snout 12 pliosauridae un-dgr-1000 (holotype) acostasaurus pavachoquensis gómez-pérez and noè (2017) skull and postcranial elements 13 ophthalmosauridae fcg-cbp-8 ophthalmosauridae unpublished skull 14 ophthalmosauridae fcg-cbp-18 ophthalmosauridae unpublished skull 15 ophthalmosauridae fcg-cbp-28 ophthalmosauridae unpublished skull 16 ophthalmosauridae don-19671 (holotype) “kyhytysuka” sachicarum páramo-fonseca, 1997; cortés et al., 2021 skull 17 ophthalmosauridae cip-ga-01042014 “kyhytysuka” sachicarum maxwell et al., 2019 nearly complete skeleton 18 ophthalmosauridae fcg-cbp-74 (holotype) muiscasaurus catheti maxwell et al., 2016 skull and axial skeleton 19 ophthalmosauridae fcg-cbp-16 muiscasaurus catheti páramo-fonseca et al., 2021 skull and axial skeleton 20 ophthalmosauridae fcg-cbp-87 platypterygius sp. cortés and páramofonseca, 2018 forelimb 21 teleosauroidea cip-0001 teleosauroidea indet. córtes et al., 2019 vertebrae and ventral and dorsal osteoderms 22 testudines cfsta090318 testudines unpublished incomplete skeleton 23 lamniforme cfsta090318 lamniforme unpublished teeth and axial skeleton 24 lamniforme fcg-cbp-93 (holotype) protolamna ricaurtei carrillo-briceño et al., 2019 teeth 25 osteoichthyes mb f. 3500 vinctifer sp. schulzte and stöhr, 1996 postcranial axial fragment 26 214 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo stratigraphic context the paja formation was defined by morales et al. (1958) based on an unpublished description by o. c. wheeler on the north side of the quebrada la paja, betulia, santander. in this region, the paja formation overlies the rosa blanca formation and underlies the tablazo formation (morales et al., 1958). recently montoya arenas (2019) described the paja formation in the type section and used the name initially utilized by wheeler “la paja” formation. here we employ the name paja formation which was used by etayo-serna (1968a) in his stratigraphic description of the villa de leiva region and later coined by many authors (cadena and parham, 2015; cadena et al., 2019; carrillo-briceño et al., 2019; cortés et al., 2019; hoedemaeker, 2004; noè and gómez-pérez, 2020; 2022; patarroyo, 2000; 2020; among many others). the paja formation was described and mapped in the villa de leiva region by etayo-serna (1968a), who determined the age of the rocks based on ammonoids. according to this author, in this region, the paja formation overlies the ritoque formation and underlies the lower san gil formation, and it ranges from the hauterivian to the upper aptian. etayo-serna (1968a) identified three lithostratigraphic assemblages that he named from base to top: lutitas negras inferiores (hauterivian), arcillolitas abigarradas (lower barremian to upper aptian), and arcillolitas con nódulos huecos (upper aptian). while etayo-serna (1968a) did not use the term “member” to refer to the divisions of the paja formation in the original description, forero and sarmiento (1985) employed this term, and subsequently, numerous publications adopted the term “member” to designate the divisions proposed by etayo-serna (1968a) for the paja formation in the villa de leiva region (cortes et al., 2019; gómez-pérez and noè, 2017; páramo-fonseca et al., 2016; páramo-fonseca et al., 2018; páramo-fonseca et al., 2019a; b). in this work, we focus on the arcillolitas abigarradas member as described by etayo-serna (1968a), since it is within these rocks that the marine vertebrate specimens of the paja formation have been found. the arcillolitas abigarradas member was described by etayo-serna (1968a) as gray, pink, yellow, and sometimes black claystones, intercalated with some levels of calcite, marls, and gypsum and says that the latter sometimes appears, filling cracks. the whole succession presents abundant ellipsoidal concretions of various sizes, morphologies, and ammonoid content (etayoserna, 1968a). this author subdivided the arcillolitas abigarradas member into five segments from base to top (a-e) which he describes as follows: segment a (56 m), consisting of pink claystones (black when fresh), with hard levels of slightly calcareous clayey sandstones that present fibrous calcite, with septarians and spherical or elliptical calcareous concretions up to 50 cm in diameter covered by clay and sometimes by gypsum; segment b (1.60 m), composed of a layer of very compact clayey limestone with lenses of sandy clay that stands out as a cornice for its competence, usually underlaid by accumulations of gypsum and with poorly preserved fossils and concretions; segment c (86 m), comprising gray claystones with some hard marly layers overlaid by 1 cm thick gypsum layers; segment d (102 m), composed of gray to brown claystones interstratified with gypsum and interbedded with 8 prominent hard calcareous levels, abundant oxidized elliptical concretions throughout the segment, and in the top a white clayey layer with compressed ammonites covered by gypsum; and segment e (235 m), composed of gray, yellow, pink, and sometimes black claystones intercalated with calcareous-clay layers, with ellipsoidal calcareous concretions throughout the segment which contain small fossils. forero and sarmiento (1985) conducted a facial analysis of the arcillolitas abigarradas member of the paja formation in the villa de leiva region and subdivided the member into five segments. the correspondence between the segments proposed by forero and sarmiento (1985) and those described by etayoserna (1968a) can be seen in figure 2 of forero and sarmiento (1985). based on their analysis, forero and sarmiento (1985) concluded that the deposition of the arcillolitas abigarradas member occurred in intertidal and low supramareal environments of a saline tidal plain protected from the influence of the open sea. figure 1. geographic and stratigraphic provenance of the marine vertebrate specimens and location of the stratigraphic sections included in this study. a, general geological map of the villa de leiva – sáchica sutamarchán region (modified from etayoserna, 1968a), cartographic base: instituto geográfico agustín codazzi. b-d close-ups of the geological map showing the location of the vertebrate finding sites (indicated with numbers; see table 1) and the stratigraphic sections (indicated with lowercase letters). a, el roble; b, pedro luis; c, la cabrera; d, el fósil; e, loma blanca; f, arrayán alto; g, monsalve-1; h, monsalve-2; i, catalina; j, monsalve-3. 215stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) 3. results stratigraphic sections from sáchica arrayán alto section in 2013, one of the authors (mep-f) excavated two specimens of pliosaurids (mp050310-1 and mp111209-1) at the arrayán alto locality, southeast of sáchica, boyacá (páramo-fonseca et al., 2018; 2019b), which were found 108 topographic meters from each other. both specimens were collected from the arcillolitas abigarradas member of the paja formation. to establish the accurate stratigraphic position of the two pliosaurids, in 2014, a stratigraphic section of the arcillolitas abigarradas member was described in the finding sector. the stratigraphic column was constructed from sections outcropping in the discovery locality between the coordinates n 5º34’23.4’’; w73º31’56.8’’ and n 5º34’14.0’’; w73º31’40.2’’ (fig. 1). in this part of the article and the supplemental data 3, we provide all the information collected in that work. we give the stratigraphic description of the arrayán alto section with the stratigraphic provenance of the two pliosaurids and the results of the collected ammonoid identification. based on this described section, it is here also established the previously unknown stratigraphic provenance of the ichthyosaur specimen cip-fcg-cbp-73, which also came from the arrayán alto sector (maxwell et al., 2016). description the arcillolitas abigarradas member in the arrayán alto sector comprises 380.47 m violet claystones, calcareous claystones, and limestones, with abundant gypsum and concretions containing fossils. in addition, sporadic discontinuous layers of white claystone, forming lenses, were observed. the described section was divided into three segments, having as criteria the content of calcareous intercalations, the presence of gypsum, and the predominance of some lithologies (ramos-clavijo, 2015) (fig. 2a) (see supplemental data 3). segment i. this segment overlaps the lower lutitas negras inferiores member of etayo-serna (1968a) and has a thickness of 96.56m. its dominant characteristics are the presence of septarian at the base, the frequent intercalations of calcareous layers, and the gradual increase in gypsum content towards the top. the predominant rocks in segment i are light violet noncalcareous claystones with undulating lamination, representing 60.8% of the segment thickness. it contains 26 hard layers of dark brown purplish limestone representing 13.5% of the segment thickness. the average thickness of these layers is 0.5 m. these layers usually present fibrous calcite crusts on their top. among the claystones, calcareous concretions of up to 60 cm in diameter can be observed. these concretions are covered by iron oxides and sometimes by a coat of gypsum that takes the shape of the concretion. ammonites are usually found within the concretions. from the middle of the segment, the gypsum appears sporadically as veins of 2 mm to 2 cm parallel to the stratification or cutting it and increases towards the top but remains scarce. at 34.19 m from the base, one of the calcareous layers stands out by containing algal mats, which appear as a gray calcareous rock with undulating parallel lamination. this level is correlated with segment b of etayo-serna (1968a) by its greater thickness and its lithology (168 cm) (fig. 2a and supplemental data 3). ammonoids collected from basal beds of segment i of the arrayán alto section were identified as nicklesia pulchella (d’orbigny, 1840) (mpr-300914-5) and karsteniceras beyrichi (karsten, 1858) (mpr-300914-6) (see supplemental data 3, fig. 1112) that indicate the “nicklesia biozone” of the lower barremian (sensu bürgl, 1956a). from the top of the segment, heinzia cf. veleziensis hyatt, 1903 was identified (mpr-140414-2) (see supplemental data 3, fig. 3), which indicates the middle barremian (sensu bürgl, 1956a) or lower upper barremian (sensu patarroyo, 2000; 2020). in this way, segment i can be correlated with segments a, b, and part of segment c of etayo-serna (1968a) (fig. 2a). segment ii. the thickness of this segment is 168.67m. its dominant characteristics are the abundance of gypsum in non-calcareous clay layers, the decrease in the number of calcareous layers, the presence of fibrous calcite on the top and the base of some limestone layers, and the scarce invertebrate fossils within the concretions. the predominant rocks in segment ii are non-calcareous claystones of light violet color with undulating lamination and abundant gypsum veins, representing 63.9% of the thickness of the segment. two layers of white claystone are located at the base of the segment. the segment contains 15 hard layers of dark brown purplish limestone representing 4.1% of the thickness of the segment. these layers have fibrous calcite up to 10 cm thick on the top. a bed of algal mat is found at 129.8m from the base of the segment. the gypsum veins are more common in outcrops, they occur parallel to the lamination or cutting it, and it also occurs in macroscopic crystals. progressive oxidation from the outside to the inside is observed in some concretions, as mentioned by etayo-serna (1968a) in his segment d. only in this segment calcareous concretions with sulfur inside associated with the white claystone layers (iron oxide concretions) are found (supplemental data 3). the pliosaurid specimens mp050310-1 and mp111209-1 were found in a non-calcareous claystone layer 3 m thick, located at 106,5 m from the base of segment ii. although most of this segment is not highly fossiliferous, some ammonites can be found within the concretions, mainly at the top of the segment. an ammonoid from the base of the segment was identified as heinzia cf. veleziensis (mpr140414-5) (see supplemental data 3, fig. 5), indicating the middle barremian (sensu bürgl, 1956a) or lower upper barremian (sensu patarroyo, 2000; 2020). ammonoids collected from the middle of the segment were identified as pulchellia sp. uhlig, 1883 (mpr-150414-3) (see supplemental data 3, fig. 5), subpulchellia sp. hyatt, 1903 (mpr-1504144, mp110414-5, mp110414-11) (see supplemental data 3, fig. 1-2, 6), and acanthoptychoceras? manolov, 1962 (mpr150414-6) (see supplemental data 3, fig. 7), which indicates the middle barremian (sensu bürgl, 1956a) or lower upper barremian (sensu patarroyo, 2000; 2020). from the top of the segment, a fossiliferous concretion (mpr 170414-7) contained gastropods (chenopus (tessarolax) evolutior jaworski, 1938; bivalves (crassatella aequalis gerhardt, 1897 and ammonoids identified as dufrenoyia sanctorum bürgl, 1956b, acanthohoplites bigouretiforme etayo-serna, 1979, and acanthohoplites pulcher riedel, 1938 (see supplemental data 3, fig. 10). other ammonoids collected from the top of the segment were identified as paracrioceras sp. spath, 1924 (mpr170414-5) (see supplemental data 3, fig. 8) and a juvenile specimen of cheloniceras sp. hyatt, 1903 (mpr-170414-6) (see supplemental data 3, fig. 9). the ammonoid assemblage from the top of segment ii indicates the upper aptian dufrenoyia sanctorum stoyanowiceras treffryanus biozone of (etayo-serna, 1979) (fig. 2a). the fossil content, as well as lithologic features such as the low fossiliferous content in the concretions, the decrease of calcareous layers towards the top of the segment, and the morphology of the concretions, allows correlating the segment ii with part of segment c, the segment d and probably the most basal portion of segment e of etayo-serna (1968a) (fig. 2a). segment iii. this segment is overlaid by the arcillolitas con nódulos huecos member of etayo-serna (1968a). the boundary with the upper member was interpreted as where hollow nodules begin to appear within the lithology. the thickness of segment iii is 115.24m. it is characterized by the presence of algal mats at the base; the increase in the number and thickness of calcareous claystone layers; the decrease in gypsum content from the base to the top of the segment; and the abundant fossils in concretions. the predominant rocks in this segment are light violet non-calcareous claystone with undulating lamination, representing 48.4% of the thickness of the segment. there are shale packages of purple, pink and orange colors whose partitioning is very fine. the concretions of these packages are clayey and ash-shaped and sometimes contain external molds of ammonoids. the segment contains eight hard levels of dark brown purplish limestone representing 3.6% of the thickness of the segment. the average thickness of these layers is 0.5 m. from the middle of the segment, the gypsum becomes scarce until its complete disappearance at the top of the segment. round and elliptical calcareous concretions of up to 35 cm maximum diameter with iron oxides on the outside predominate within all the concretions. ammonites, bivalves, and fish remains are usually found within the concretions (see supplemental data 3). among the ammonoids collected in this segment were identified: epicheloniceras pardoi etayo-serna, 1979 (mpr-031014-20), gargasiceras pulcher (riedel, 1938) (mpr-031014-6), epicheloniceras cf. camachoi etayo-serna, 1979 (mpr-031014-6), dufrenoyia sanctorum (mpr-031014-4), acanthohoplites eleganteante etayo-serna, 1979 (mpr-021014-8) and acanthohoplites odiosus etayo-serna, 1979 (mpr-021014-2; mpr-021014-8) (see supplemental data 3, fig. 13-15, 17, 20). this ammonoid assemblage indicates the upper aptian dufrenoyia sanctorum stoyanowiceras treffryanus biozone (etayo-serna, 1979). the described lithologic features and the identified ammonoid genera allow us to confidently correlate segment iii with segment e of etayo-serna (1968a) (fig. 2a). 216 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo stratigraphic position of the pliosaurid specimens mp 050310-1 and mp111209-1 the pliosaurid specimens mp050310-1 and mp111209-1 were found in two different geographic sites of the arrayán alto sector. they were found 108 m from each other and separated by overgrown terrain, so their precise stratigraphic position could not be defined by direct measurement. both specimens came from the described segment ii and were found in a layer of non-calcareous purplish claystone having calcareous concretions with scarce ammonoids, iron oxides on their inside, and a gypsum coating on their outside (see supplemental data 3). the position of the pliosaurids mp050310-1 and mp111209-1 in the stratigraphic column, together with that of the identified ammonites, restrict the stratigraphic provenance of the pliosaurids to the uppermost part of segment c, near the boundary with segment d of etayoserna (1968a) (fig 2a). the accurate stratigraphic position of the two pliosaurid specimens was determined by taking as reference a 0.18 m thick bed of limestone with iron oxides and pyrites, which underlies the pliosaurids bearing bed. this guide layer was seen in outcrops and found in trenches made in the excavation sites. the marking bed is distinguished in the field because it overlies a level with concretions between 15 to 28 cm in diameter, with elongated and rounded shapes. petrographic analyses of this guide layer, done from samples collected in both pliosaurid excavation sites, showed that the rock corresponds to a sparitic limestone, with the presence of 200 micron euhedral micas and pyrite. no sedimentary structures or microfossils were observed. a second reference limestone layer, lower than the guide one, was also analyzed to corroborate the stratigraphic origin of both pliosaurids. the petrographic analyses of this layer were done from samples collected in the mp111209-1 finding site and an intermediate place between the two pliosaurid finding sites. the results showed a sparitic limestone lithology with presence of micas. the difference found with the first guide layer was the smaller size of carbonates and pyrites. mp050310-1 (stenorhynchosaurus munozi) was found 0,48 m above the guide layer (109.2 m from the base of segment ii), whereas mp111209-1 (sachicasaurus vitae) was found 2.09 m above the guide layer (110.8 m from the base the segment ii). based on these observations, we determine that the two specimens were extracted from the same stratigraphic layer but separated from each other by 1.06 m of rocks (see supplemental data 1). with these results, we conclude that st. munozi and sa. vitae were contemporaneous species, but the individuals to which the remains belonged did not coexist, and the deposition of their remains was not coeval. stratigraphic provenance of the ichthyosaur specimen cip-fcg-cbp-73 the holotype of the ichthyosaur muiscasaurus catheti (specimen cipfcg-cbp-73) was described in 2016 by maxwell et al. (2016). however, the stratigraphic provenance of the specimen had not been specified until now. based on the geographic coordinates given by maxwell et al. (2016), we recently visited the finding site and collected an ammonoid, which was identified by one of the authors (cdb-c) as cheloniceras (epicheloniceras) carlosacostai etayo-serna 1979 (mp101022-1) (see supplemental data 2, fig. 4e). the genus cheloniceras is included by etayo-serna (1979) in the biozone of dufrenoyia sanctorum stoyanowiceras treffryanus, which according to this author, indicates the upper aptian in colombia. moreover, the finding site corresponds to one of the gps points taken in 2014 during the description of the arrayán alto section, where some ammonoids were collected. therefore, we establish that the specimen comes from the upper part of segment ii of the here described section, more precisely from the meter 147 from the base of the section (fig. 2a). the identified ammonoids corroborate the upper aptian dufrenoyia sanctorum stoyanowiceras treffryanus biozone of etayoserna (1979). as discussed in previous paragraphs, segment ii of the section is equivalent to part of segment c, segment d, and probably the lowermost portion of segment e of etayo-serna (1968a). therefore, we conclude that the specimen cip-fcg-cbp-73 probably comes from the basal part of segment e of etayo-serna (1968a) (fig. 2a). loma blanca section schultze and stohr (1996) described the section of the loma blanca hill that outcrops in the road from sáchica to samacá (fig. 1). they described the entire arcillolitas abigarradas member and the arcillolitas con nódulos huecos member of etayo-serna (1968a) and indicated the stratigraphic provenance of the specimen mb f. 3500 (vinctifer sp.) in meter 353.05 of their section. they found various ammonoids in the levels below and above the level from which mb f. 3500 was collected. they identify the ammonoids as cheloniceras (epicheloniceras) ex gr. martini d’orbigny, 1841, cheloniceras (epicheloniceras) ex gr. cornuelianum d’orbigny, 1841, gargasiceras cf. interiectum riedel, 1938, dufrenoyia sanctorum, toxoceratoides (colomboceratoides) renzoni etayo-serna, 1979 (see schultze and stohr, 1996). following etayo-serna (1979), these ammonoids indicate the biozone dufrenoyia sanctorum stoyanowiceras treffryanus that represents the upper aptian in colombia. to correlate the stratigraphic section of mb f. 3500 provided by schultze and stohr (1996) with the segments of etayo-serna (1968a), we used the limit of the arcillolitas abigarradas member with the arcillolitas con nódulos huecos member as a correlative point. we establish that mb f. 3500 came from the upper half of segment e of etayo-serna (1968a) (fig. 2b). stratigraphic sections from villa de leiva la cabrera section this section was described in the la cabrera hill, west of villa de leiva town, outcropping between coordinates n 5º38’59.1’’; w73º33’07.9’’ and n 5º39’01.7’’; w73º33’43.3’’ (fig. 1). in this section the stratigraphic provenance of the published pliosaurid stenorhynchosaurus munozi specimen vl17052004-1 and the unpublished (currently under study) pliosaurid specimen mpvl-mp110210-1 are provided. in their description of the specimen vl17052004-1 páramo-fonseca et al. (2016) indicated that the provenance of this specimen corresponds to the segment c of etayo-serna (1968a), and the ammonoids collected below the specimen and within the specimen skull correspond to the gerhardtia provincialis d’orbigny, 1850 subzone of the gerhardtia sartousiana d’orbigny, 1841 biozone of reboulet et al. (2014) of the lower upper barremian (see páramo-fonseca et al., 2016). this biozone is correlated by patarroyo (2020, fig. 7) with his gerhardtia veleziensis biozone, representing the lower part of the upper barremian in colombia. the stratigraphic section of la cabrera was described to specify the stratigraphic provenance of the specimen vl17052004-1 within the segment c of etayo-serna (1968a) and to locate the stratigraphic origin of specimen mpvl-mp110210-1 on the base of its geographical provenance. this section comprises 94 meters of claystones, interspersed with occasional layers of calcareous claystone, siltstone, and regular competent layers of silty limestone. concretions were found in different intervals of the section (meters 1-11, 28-34, 60-69, 81-94). the concretions at the base of the section are large (diameter> 40 cm). additionally, at meters 8-9 a level with septarian was found. at meters 15-16 of the described section, a very competent 1.1 m thick silty limestone is found, which forms a ledge on the ground. in the upper part of the section (meter 59 onwards), more competent limestone levels stand out, which form steps in the direction of the counter slope and form resistant floors on their structural planes. these layers of silty limestone usually have fibrous calcite on their top (see supplemental data 1). specimen mpvl-mp110210-1 comes from the beds of the first two meters of the measured section, and specimen vl17052004-1 comes from a layer of clay corresponding to meter 94 of the described section (fig. 3a and supplemental data 1). the presence of septarian and concretions with diameters greater than 40 cm in the first 12 meters allows us to correlate this part of the section with segment a of etayo-serna (1968a). in this section, the thick layer of silty limestone (meters 15-16) is correlated with segment b of etayo-serna (1968a) based on the similarity of its lithology and its expression in the terrain. in the described section, above meter 16, we found a lithology similar to that described by etayo-serna (1968a) for his segment c, so meters 16 to 94 of the described section are correlated with segment c of etayo-serna (1968a). at meter 81, a pair of ammonoid specimens identified as pulchellia sp. (cb160121-4 and 5) were found (see supplemental data 2, fig. 9a-b). this genus is included within the upper lower barremian pulchellia galeata biozone by patarroyo (2000). with the exposed data, we correlate the stratigraphic position of the specimen mpvl-mp110210-1 with the lower part of segment a and that of the specimen vl17052004-1 with the upper part of segment c of etayo-serna (1968a) (fig. 3a). 217stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) figure 2. generalized stratigraphic columns from two localities of sáchica, correlated with the segments stated by etayo-serna (1968a) within the arcillolitas abigarradas member. a, arrayán alto section (portion bearing marine vertebrates); b, loma blanca section (modified from schulzte and stohr, 1996). *, barremian ammonoid biozones of bürgl, 1956a; **, aptian ammonoid bionozes of etayo-serna, 1979. the information associated with the number accompanying the marine vertebrate silhouettes can be consulted in table 1. 218 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo pedro luis section according to páramo-fonseca (1997), the ichthyosaur specimen don19671, holotype of “kyhytysuka” sachicarum, came from a detached concretion found in the pedro luis hill, north of villa de leiva town. we visited the finding site and described the stratigraphic section outcropping at the locality, between coordinates n 5º39’07.0’’; w73º31’33.3’’ and n 5º39’0.17’’; w73º32’08.4’’ (fig. 1). in the described section we locate the approximate stratigraphic origin of this specimen. this section comprises 23 meters of grayish claystones, interspersed with sporadic layers of calcareous claystone, silt, and competent layers of silty limestone. as in the la cabrera section, the silty limestones form steps toward the counter slope. the concretions are small in size (diameter <10 cm) and are rare in the entire described section, they are only present in some intervals (meters 2-3, 4-5, 17-19, 20-22) (see supplemental data 1). the presence of several competent layers of silty limestone within a thick succession of grayish claystones, altogether with the size and scarcity of the concretions, allows for correlating the described section with the segment c of etayo-serna (1968a). within the concretions found in the layers of meters 8-9, we collected specimens of the ammonoids heinzia sp. sayn, 1891 (cb050222-1) and gerhardtia sp. hyatt, 1903 (cb050222-2 and cb060222-2) (see supplemental data 2, fig. 10c; 11b-c), and in concretions of the meters 20-22, we collected the ammonoid colchidites sp. djanélidzé (1926) (mp061121-5) (see supplemental data 2, fig. 8b). patarroyo (2000; 2020) includes the genera heinzia and gerhardtia in his lower upper barremian biozone gerhardtia veleziensis and the genus colchidites in his upermost barremian biozone colchidites breistrofferi. as the beds of the pedro luis hill provided upper barremian ammonoids, we conclude that the specimen don19671 comes from the upper barremian (sensu patarroyo, 2000; 2020) of the segment c of etayo-serna (1968a) (fig. 3b). el roble section this section was described in the el roble locality, north of villa de leiva town, outcropping between coordinates n 5º40’04.0’’; w73º32’11.1’’ and n 5º40’01.0’’; w73º32’08.4’’ (fig. 1). in this locality the published ichthyosaur specimen cip-ga-01042014 referred to “kyhytysuka” sachicarum was found (córtes et al., 2021; maxwell et al., 2019). in their description of this specimen, maxwell et al. (2019) provided the geographic coordinates of the finding site but do not provide details on the stratigraphic origin of the specimen. however, based on an ammonite referred to colchidites sp., these authors indicate the ichthyosaur “is uppermost barremian in age” (here understood as collected from the uppermost barremian). therefore, to correlate the stratigraphic provenance of the specimen cip-ga-01042014 with the segments of etayoserna (1968a), we visited the finding site and described a stratigraphic section. the stratigraphic section comprises 21.5 meters of calcareous claystones and siliciclastic claystones, interspersed with sporadic layers of siltstone and competent layers of silty limestone. as in la cabrera and pedro luis’s sections, the silty limestones form steps toward the counter slope. the concretions are medium in size (diameter ~10 – 25 cm) and are only found in intervals 8-9 meters and 17-18 meters. gypsum is scarce, and only appears forming lenses in the first three meters (see supplemental data 1). the ichthyosaur cip-ga-01042014 comes from a calcareous clay layer corresponding to meter 17 of the described section (fig. 3c and supplemental data 1). the presence of competent silty limestones and the low content of concretions allows for correlating this section with part of the segment c of etayo-serna (1968a). in the layers of meters 8-9 and meter 16, ammonoids referred to heinzia (cb160722-1b, 4, 12) and gerhardtia (cb070222-2) were collected (see supplemental data 2, fig. 10a-b, d; 11a). these taxa are included by patarroyo (2000; 2020) in his lower upper barremian gerhardtia veleziensis biozone. however, one meter above (meter 17), at the same level from where the ichthyosaur cip-ga-01042014 comes from, an ammonoid identified as colchidites sp. (cb070222-1) was found (see supplemental data 2, fig. 8a); this genus is included by patarroyo (2000) in his uppermost barremian colchidites breistrofferi biozone. according to this, the stratigraphic position of the ichthyosaur cip-ga-01042014 is here located in the upper part of the segment c of etayo-serna (1968a) (fig. 3c). monsalve-1 section this section was described in the monsalve hill, west of villa de leiva town, outcropping between coordinates n 5º39’07.2’’; w73º34’47.3’’ and n 5º39’02.7’’; w73º34’49.9’’ (fig. 1). in this section, the stratigraphic provenance of the unpublished (currently under study) ophthalmosaurid ichthyosaur specimen fcg-cbp-8 was located on the base of its geographic provenance. the described section consists of 24 meters of claystones interspersed with sporadic layers of calcareous claystone and frequent competent layers of silty limestone. abundant gypsum lenses are found in the lower part of the section (up to meter 11) but disappear in the upper part of the section. there are calcareous concretions in almost the entire section; these are common in meters 5 to 10 and from meters 14 to 24. the concretions are 10 to 20 cm in diameter. there is an increase in the size of the concretions (diameter> 30 cm to 50 cm) from meter 14 to meter 18; then, they decrease in size again (diameter <20 cm). at the top of the section, there is a thick layer (~ 1 m) of silty limestone that stands out on the ground as a cornice and is overlaid by a layer of fibrous calcite approximately 10 cm thick (see supplemental data 1). the fcg-cbp-8 specimen comes from the stratigraphic interval between meters 6 to 7 of the described section (fig 4a and supplemental data 1). the thick layer of silty limestone observed in this section at meter 24 is correlated with segment b of etayo-serna (1968a) based on the similarity of its lithology and its expression in the terrain. furthermore, the presence of concretions of 50 cm in diameter below meter 24 (meters 18-24) suggests that the first 23 meters of the section correlate to segment a of etayo-serna (1968a). therefore, it can be concluded that the fcg-cbp-8 specimen comes from the rocks of segment a of the etayo-serna (1968a) (fig. 4a). some ammonoids were collected at meter 8 (1 m above the ichthyosaur specimen) and were identified as pedioceras sp. gerhardt (1897) (cb140121-3,5,7) and acrioceras julivertii etayo-serna, 1968b (cb140121-8) (see supplemental data 2, fig. 2a-c; 3). this last taxon was included by patarroyo (2000) in the nicklesia pulchella biozone, which according to this author, represents the lower part of the lower barremian in colombia. the fcg-cbp-8 specimen is, therefore, correlated with the nicklesia pulchella biozone of patarroyo (2000) (fig. 4a). monsalve-2 section this section was described in the monsalve hill, west of villa de leiva town, outcropping between coordinates n 5º38’33.8’’; w73º35’12.0’’ and n 5º38’35.9’’; w73º35’11.0’’ (fig. 1). in this stratigraphic section, we located the stratigraphic provenance of the unpublished pliosaurid specimen cfsta 2-1 (currently in prep. by cdb-c and mep-f), based on the geographic location of its discovery site. this section comprises 18 meters of claystones interspersed with four layers of competent silty limestone. the base of the section (~ 2 m) is characterized by whitish claystones with flattened ammonoids covered by gypsum. small (<5 cm) clayey nodules are found at the base of the section, but ascending stratigraphically, these nodules disappear, and medium to small calcareous concretions (diameter <15 cm) appear (see supplemental data 1). based on the geographic origin of the specimen cfsta 2-1, it was established that it was extracted from levels corresponding to meters 14-16 of the described section (fig. 4b and supplemental data 1). according to etayoserna (1968a), at the top of his segment d, there is a level of white claystones with flattened ammonoids covered by gypsum. due to the presence of a level with these features at the base of the described section, it was determined that the section monsalve-2 corresponds to the top of segment d and the base of segment e of etayo-serna (1968a) (fig.4b). in this stratigraphic section, one ammonoid specimen from meter 6 was collected. this specimen was identified as cheloniceras (epicheloniceras) carlosacostai (cb150121-2) (see supplemental data 2, fig. 4d). this species is included by etayo-serna (1979) in the biozone of dufrenoyia sanctorum – stoyanowiceras treffryanus, which according to this author, indicates an upper aptian in colombia. in conclusion, the stratigraphic provenance of the cfsta 2-1 specimen is located in the basal portion of segment e of etayo-serna (1979) (fig. 4b). 219stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) figure 3. generalized stratigraphic columns from three localities of villa de leiva, correlated with the segments stated by etayo-serna (1968a) within the arcillolitas abigarradas member, and with the barremian ammonoid biozones of patarroyo (2000). a, la cabrera hill section; b, pedro luis hill section; c, el roble section.*, barremian ammonoid biozones of patarroyo, 2000; 2020; **, aptian ammonoid biozones of etayo-serna, 1979. the information associated with the number accompanying the marine vertebrate silhouettes can be consulted in table 1. 220 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo monsalve-3 section this section was described in the monsalve hill, west of villa de leiva town, outcropping between coordinates n 5º37’49.2’’; w73º35’10.6’’ and n 5º37’41.3’’; w73º35’08.9’’ (fig. 1). one published and three unpublished (currently under study) specimens come from the locality where this section was described. the published specimen is the ichthyosaur muiscasaurus catheti (fcg-cbp-16) (páramo-fonseca et al., 2021). the unpublished specimens include the elasmosaurid fcg-cbp-3, the pliosaurid fcg-cbp-4, and the ophthalmosaurid ichthyosaur fcg-cbp-18. we visited the finding site of the specimen fcg-cbp-16 and refine the coordinates given by páramo-fonseca et al. (2021) in n 5º37’43.12’’; w73º35’11.12’’, that correspond to the monsalve hill (the authors gave a wrong name for this hill). according to páramo-fonseca et al. (2021) the specimen was found in beds equivalent to the segment e of etayo serna (1968a), and numerous ammonoids collected in the ichthyosaur finding site indicated the upper aptian dufrenoyia sanctorum – stoyanowiceras treffryanus biozone of etayo-serna (1979). the stratigraphic position of all the specimens collected in the monsalve-3 section was determined based on the geographic location of their finding sites. this section comprises 40 meters of claystones, interspersed with sporadic layers of calcareous claystones and regular competent layers of silty limestone. calcareous concretions are found throughout the section. the concretions tend to be primarily small (diameter <10 cm), although some medium-sized are found in the upper part of the section (diameter 10 – 20 cm). in the first 12 meters, it is common to find gypsum lenses and concretions covered by gypsum; however, after 12 meters, the gypsum disappears (see supplemental data 1). the specimens fcg-cbp-4, fcg-cbp-3, come from levels of calcareous claystones corresponding to meter 27 of the described section (fig. 4c and supplemental data 1). the specimens fcg-cbp-16 and fcg-cbp-18 come from levels of calcareous claystones corresponding to meter 39 of the described section (fig. 4c and supplemental data 1). in this section, we did not find lithological characteristics or stratigraphic boundaries that would allow a correlation of our section with the segments of etayo-serna (1968a). however, given the spatial and stratigraphic relationship of this section with the other two sections described in the monsalve hill, it was estimated that our described section corresponds to a portion of the segment e of etayo-serna (1968a). to achieve a more precise correlation of our described section within the segment e of etayo-serna (1968a), we measured over the map published by etayo-serna (1968a) the distance between the gps point corresponding to the uppermost level described in our section and the drawn upper limit of the arcillolitas abigarradas member. we calculated an approximate thickness of the uppermost missing portion of the arcillolitas abigarradas member in our section from this distance. this calculated thickness was 34 meters. therefore, the specimens fcg-cbp-4, fcg-cbp-3, come from layers 45 to 47 meters below the upper limit of segment e, and the specimens fcg-cbp-16 and fcg-cbp-18 come from layers 35 to 36 meters below the upper limit of segment e. thus, it is established that all the specimens included in the monsalve-3 section come from the upper half of segment e of etayoserna (1968a) (fig. 4c). a clayey level with abundant remains of crushed ammonoids was found at meter 30, approximately 9 m below the stratigraphic provenance of the ammonoids reported by páramo-fonseca et al. (2021) with the ichthyosaur fcg-cbp-16. some of these ammonoids were identified as dufrenoyia sp. kilian and reboul,1915 (cb070321-1, 2 and cb070321-4 to 10) and stoyanowiceras sp. karsten, 1858 (cb070321-3) (see supplemental data 2, fig. 5; 6a-c, e-j). these taxa are included by etayo-serna (1979) in the biozone of dufrenoyia sanctorum – stoyanowiceras treffryanus, which according to this author indicates the upper aptian in colombia. la catalina section this section was described in the la catalina hill, near the san roque rural school, west of villa de leiva town, outcropping between coordinates n 5º37’49.2’’; w73º35’10.6’’ and n 5º37’41.3’’; w73º35’08.9’’ (fig. 1). in this section, we located the stratigraphic provenance of the published plesiosaur callawayasaurus colombiensis specimens ucmp 38349 and ucmp 125328; igmp174997 (welles, 1962) and the shark protolamna ricaurtei specimen fcg-cbp-93 (carrillo-briceño et al., 2019), and the unpublished (currently under study) specimen cfsta090318 containing remains of shark and turtle. according to welles (1962), the specimens ucmp 38349 and ucmp 125328; igmp174997 were found in la catalina hill. etayo-serna (1968a) states that these specimens come from his segment e. according to carrillo-briceño et al. (2019), the specimen fcg-cbp-93 was collected from the arcillolitas abigarradas member; however, the authors did not provide further details on the stratigraphic origin of the specimen. this section comprises 24 meters of claystones, interspersed with occasional layers of calcareous claystone, silt, and competent layers of silty limestone. the concretions are small in size (diameter <10 cm) and are rare in the entire described section (see figure 3 and supplemental data 1). gypsum is scarce, and only appears in the form of lenses in some intervals (meters 5-6, 9-11, and 20-21). in the first basal 7 meters of the described section, whitish clays with crushed ammonites covered by gypsum were found (see supplemental data 1). both specimens cfsta090318 and fcg-cbp-93 come from a calcareous clay layer corresponding to meter 21 of the described section (fig. 4d and supplemental data 1). a precise geographical location of the specimens ucmp 38349 and ucmp 125328; igmp174997 is unknown; however, according to the figure 2 of welles (1962), the specimens were found in the structural slope of the hill, which allow us to locate the specimens in the upper levels of the described section. as for section monsalve-2, the level of whitish claystones with flattened ammonoids at the base of the section is correlated with the top of segment d of etayo-serna (1968a). according to the stratigraphic position of this level, it was determined that the described section correlates with the top of segment d and the base of segment e from etayo-serna (1968a). in agreement with this, all the specimens located in this section come from the basal layers of segment e of etayo-serna (1968a) (fig 4d). furthermore, several ammonoids were found in different layers of the first 6 meters of the section. these ammonoids were identified as cheloniceras (epicheloniceras) carlosacostai (cb120121-2,3 and cb130121-7), dufrenoyia sp. (cb120121-1), and riedelites obliquum (riedel, 1938) (cb130121-1,4) (see supplemental data 2, fig. 4ac; 6d; 7a-b). following etayo-serna (1979), these taxa are included in the upper aptian biozone of dufrenoyia sanctorum – stoyanowiceras treffryanus. thus, the stratigraphic provenance of the specimens ucmp 38349, ucmp 125328; igmp174997, cfsta090318 and fcg-cbp-93 is also correlated with the dufrenoyia sanctorum – stoyanowiceras treffryanus biozone of etayoserna (1979). el fósil section this section was described in vicinities of the museo el fósil, west of villa de leiva town, outcropping between coordinates n 5º38’14.6’’; w73º33’32.6’’ and n 5º38’13.3’’; w73º33’31.2’’ (fig. 1). in this section, we located the stratigraphic provenance of the published pliosaurid “monquirasaurus” boyacensis specimen mjacm1 (hampe, 1992; noè and gómez-pérez, 2020). hampe (1992) indicates that the specimen comes from segment v of forero and sarmiento (1985), which is equivalent to segment e of etayo-serna (1968a) (see forero and sarmiento, 1985 fig. 2). in this section, only 1.5 meters were described due to the high vegetation coverage in the area. it consists of a layer of light gray clay covered by a reddish silty limestone with irregular concretions (diameter <20 cm) (see supplemental data 1). to achieve a more precise correlation of the described section within the segment e of etayo-serna (1968a), we calculated the thickness of the upper portion of the arcillolitas abigarradas member missing in our section based on the geological map of etayo-serna (1968a). the calculated thickness was 66 meters, indicating that the described section and the stratigraphic provenance of the specimen mjacm1 correlate with the upper half of the segment e of etayo-serna (1968a) (fig. 4e). the numerous ammonoids reported by etayoserna (1968a; pp 28-29) from his segment e and those collected from the same segment in the other stratigraphic sections here described correlate with the dufrenoyia sanctorum – stoyanowiceras treffryanus biozone of etayo-serna (1979), which according to this author indicates the upper aptian in colombia. 221stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) figure 4. generalized stratigraphic columns from three localities of villa de leiva, correlated with the segments stated by etayo-serna (1968a) within the arcillolitas abigarradas member. a-c, monsalve hill sections. d, la catalina hill section; e, el fósil section. *, barremian ammonoid biozones of patarroyo, 2000; 2020; **, aptian ammonoid biozones of etayo-serna, 1979. the information associated with the number accompanying the marine vertebrate silhouettes can be consulted in table 1. 222 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo specimens not located in the stratigraphic sections specimen un-dgr-1000 the description of the pliosaurid acostasaurus pavachoquensis specimen un-dgr-1000 was published by gómez-pérez and noè (2017). according to these authors, the specimen was found in the pavachoque creek in la yuca hill (municipality of sutamarchán), in the lower part of the arcillolitas abigarradas member with the ammonoid nicklesia pulchella, which dates the beds as lower barremian. accordingly, the stratigraphic position of the specimen undgr-1000 can be correlated with segment a of etayo-serna (1968a). specimen cip-0001 the description of the teleosauroid specimen cip-0001 was published by cortés et al. (2019). according to these authors, the specimen was found in la cabrera hill (villa de leiva). although the authors give coordinates of the finding site, it is unknown if the specimen was found in situ or as a tumbled concretion, as it is common to find fossils in villa de leiva. therefore, it was not possible to establish its exact stratigraphic provenance. cortés et al. (2019) report the ammonoid genus gerhardtia in association with the teleosauroid that the authors relate with the upper barremian gerhardtia veleziensis zone of patarroyo (2000; 2020). according to this, the stratigraphic provenance of the specimen cip-0001 is here located in segment c of etayo-serna (1998) without specifying its position within this segment. specimen fcg-cbp-22 the description of the specimen fcg-cbp-22 of the elasmosaurid plesiosaur leivanectes bernardoi was published by páramo-fonseca et al. (2019a). according to these authors, the specimen was found in la cabrera hill (villa de leiva), but the exact location of the finding site is unknown. however, on the base of one ammonoid identified as cheloniceras (epicheloniceras) carlosacostai, found in the matrix surrounding the plesiosaur specimen, paramo-fonseca et al. (2019a) state that the specimen comes from upper aptian rocks of the arcillolitas abigarradas member and locate it in the segment e of etayo-serna (1968a). specimen fcg-cbp-87 the description of the ichthyosaur specimen fcg-cbp-87 (platypterygius sp.) was published by cortés and páramo-fonseca (2018). following these authors, the specimen was collected in la cabrera hill (villa de leiva), but the exact location of the finding site is unknown. cortés and páramo-fonseca (2018) state that the specimen comes from the arcillolitas abigarradas member, and, based on the ammonoid nicklesia pulchella found in the matrix surrounding the ichthyosaur specimen, they conclude that the specimen comes from lower barremian rocks. this information allows us to locate the stratigraphic provenance of the specimen fcg-cbp-87 in segment a of etayo-serna (1968a). specimen fcg-cbp-28 the ichthyosaur specimen fcg-cbp-28 (ophthalmosauridae) is currently under study. this specimen was found as a tumbled concretion in la cabrera hill (villa de leiva), so its exact geographical origin is unknown. therefore, it is not possible to establish its exact stratigraphic position. however, in the rock surrounding the fossil, there are numerous ammonoids, some of which were identified as acanthoptychoceras sp. (see supplemental data 2, fig. 1). this ammonoid genus is included by patarroyo (2000) in his lower barremian nicklesia pulchella biozone. this information allows us to locate the stratigraphic provenance of the specimen fcg-cbp-28 in segment a of etayo-serna (1968a). specimen mpvl-667 the pliosaurid specimen mpvl-667 was described and identified as stenorhynchosaurus munozi by páramo-fonseca et al. (2023). this specimen was collected from a trundled concretion found in la yuca creek (sutamarchán). the only available information about its stratigraphic provenance allows us to state that the specimen comes from the barremian beds of the arcillolitas abigarradas member (see páramo-fonseca et al., 2023). with this information, we can only locate the stratigraphic provenance of the specimen mpvl-667 within segments a, b, or c of etayo-serna (1968a). discussion stratigraphic distribution of marine vertebrate specimens we performed a lithostratigraphic, biostratigraphic, and graphic correlation between all stratigraphic columns, the specimens with associated biostratigraphic information and the reference segments defined by etayo-serna (1968a) for the arcillolitas abigarradas member. this correlation shows us that almost all the marine vertebrate specimens originate from four stratigraphic intervals (fig. 5). each of these stratigraphic intervals contains a vertebrate assemblage. two of these assemblages are found in barremian levels (vertebrate assemblages 1 and 2), and two in upper aptian levels (vertebrate assemblages 3 and 4) (fig. 5). although the vertebrate assemblages 3 and 4 come from a single ammonoid biozone, they were here separated because a significant stratigraphic interval without vertebrates (approximately 150 m) is present between them. barremian vertebrate assemblages vertebrate assemblage 1. this assemblage includes two specimens of pliosaurid plesiosaurs acostasaurus pavachoquensis (un-dgr-1000) and mpvl-mp110210-1 and three specimens of ichthyosaurs: platypterygius sp. (fcg-cbp-87), and the ophthalmosaurid fcg-cbp-8 and fcg-cbp-28 these specimens come from layers of segment a of the arcillolitas abigarradas member of etayo-serna (1968a) (fig. 5) and correlate with the nicklesia pulchella biozone (sensu patarroyo, 2000; 2020) or the nicklesia biozone (sensu bürgl, 1956a), which, according to these authors, represent the lower barremian in colombia. vertebrate assemblage 2. this assemblage includes three specimens of pliosaurid plesiosaurs, two specimens of stenorhynchosaurus munozi (vl17052004-1 and mp050310-1), and one specimen of sachicasaurus vitae (mp111209-1), two ichthyosaur specimens of “kyhytysuka” sachicarum (cip-ga-01042014 and don-19671) and a specimen of a teleosauroid (cip-0001). all these specimens come from the upper part of segment c of the arcillolitas abigarradas member of etayo-serna (1968a) (fig. 5). except for the ichthyosaurs, all the specimens correlate with the gerhardtia veleziensis biozone of patarroyo (2000; 2020) or the pulchellia biozone of bürgl (1956a), which allows uniting them together in an assemblage. these biozones represent the lower upper barremian (sensu patarroyo, 2000; 2020) or the middle barremian (sensu bürgl, 1956a). although the ichthyosaur specimen cip-ga-01042014 was found with an ammonite indicative of the colchidites breistrofferi b iozone of patarroyo (2000), only one stratigraphic meter below the occurrence of this ammonite was found another ammonite indicative of his gerhardtia veleziensis biozone. therefore, this short stratigraphic distance does not justify separating this ichthyosaur specimen into a different vertebrate assemblage. therefore, specimen cip-ga-01042014 is linked to assemblage 2. as discussed above, the ichthyosaur specimen don-19671 was found as a detached concretion coming from segment c of the arcillolitas abigarradas member of etayo-serna (1968a). however, its association with one of the two ammonoid biozones of patarroyo (2000) (heinzia velenziensis or colchidites breistrofferi) related to this segment, cannot be specified. nevertheless, its origin from the upper barremian beds (sensu patarroyo, 2000; 2020) or middle to upper barremian beds (sensu burgl, 1956a) within the segment c of etayoserna (1968a) allows us to include it in the vertebrate assemblage 2 (fig. 5). 223stratigraphic distribution of marine vertebrates of the arcillolitas abigarradas member (paja formation) figure 5. diagram showing the vertebrate assemblages resulting from the correlation of the stratigraphic provenance of the specimens included in this work. *, aptian ammonoid biozones of etayo-serna, 1979; **, barremian biozones of bürgl, 1956a; ***, barremian biozones of patarroyo, 2000; 2020. specimens: 1elasmosauridae (fcg-cbp-3); 2leivanectes bernardoi (fcg-cbp-22); 3callawayasaurus colombiensis (ucmp 38349); 4c. colombiensis (ucmp 125328; igmp174997); 5 pliosauridae (cfsta 2-1); 6pliosauridae (fcg-cbp-4); 7pliosauridae (mpvl-mp110210-1); 8“monquirasaurus” boyacensis (mjacm 1); 9sachicasaurus vitae (mp111209-1); 10stenorhynchosaurus munozi (vl17052004-1); 11s. munozi (mp050310-1); 12s. munozi (mpvl-667); 13acostasaurus pavachoquensis (un-dgr-1000); 14ophthalmosauridae (fcg-cbp-8); 15ophthalmosauridae (fcg-cbp-18); 16ophthalmosauridae (fcg-cbp-28); 17“kyhytysuka” sachicarum (don-19671); 18“k.” sachicarum (cip-ga-01042014); 19muiscasaurus catheti (fcg-cbp-74); 20. m. catheti (fcg-cbp-16); 21platypterygius sp. (fcgcbp-87); 22teleosauroidea (cip-0001); 23testudines (cfsta090318); 24lamniforme (cfsta090318); 25protolamna ricaurtei (fcg-cbp-93); 26vinctifer sp. (mb f. 3500). 224 cristian benavides-cabra; maría páramo-fonseca; héctor palma-castro; josé narváez-rincón; maria ramos-clavijo upper aptian vertebrate assemblages vertebrate assemblage 3. this assemblage includes two specimens of the elasmosaurid plesiosaur callawayasaurus colombiensis (ucmp 38349 and ucmp 125328; igmp174997), one specimen of a pliosaurid plesiosaur (cfsta 2-1); one specimen of the ichthyosaur muiscasaurus catethi (cipfcg-cbp-73); one specimen of a turtle (cfsta090318), and two specimens of lamniform sharks, one of protolamna ricaurtei (fcg-cbp-93) and other not yet identified (cfsta090318). these specimens come from the lower part of the segment e of the arcillolitas abigarradas member of etayo-serna (1968a) (fig. 5) and correlate with the dufrenoyia sanctorum stoyanowiceras treffryanus b iozone o f e tayo-serna ( 1979) w hich, a ccording t o t his a uthor, represents the upper aptian in colombia. vertebrate assemblage 4. this assemblage includes one specimen of elasmosaurid plesiosaur (fcg-cbp-3); two specimens of pliosaurid plesiosaurs, one of “monquirasaurus” boyacensis (mjacm1) and the unidentified pliosaurid fcg-cbp-4, two specimens of ophthalmosaurid ichthyosaurs, one of muiscasaurus catheti (fcg-cbp-16) and the unidentified ophthalmosaurid fcg-cbp-18, and one specimen of a bony fish vinctifer sp. (mb f. 3500). these specimens come from the upper part of the e segment of the arcillolitas abigarradas member of etayo-serna (1968a) (fig. 5) and, like the specimens of assemblage 3, correlate with the dufrenoyia sanctorum stoyanowiceras treffryanus biozone of etayo-serna (1979) which, according to this author, represents the upper aptian in colombia. specimens with imprecise stratigraphic provenance two of the specimens in this study, the leivanectes bernardoi specimen fcg-cbp-22 and the pliosaurid specimen mpvl-667 could not be included in one of the vertebrate assemblages because their imprecise stratigraphic or biostratigraphic provenance. as mentioned above, the plesiosaur specimen fcg-cbp-22 comes from the segment e of the arcillolitas abigarradas member of etayo-serna (1968a) and was correlated with the dufrenoyia sanctorum stoyanowiceras treffryanus biozone of etayo-serna (1979). however, without more precise information, the stratigraphic provenance of this specimen within the thick segment e cannot be specified. therefore fcg-cbp-22 could be part of the assemblage 3 or 4 (fig. 5). the stratigraphic provenance of the pliosaurid specimen mpvl-667 was established in the barremian interval of the arcillolitas abigarradas member, correlating with the segments a, b, and c of etayo-serna (1968a). therefore, it could be linked to either of the two barremian assemblages, meaning that the specimen mpvl-667 could be part of the vertebrate assemblage 1 or 2 (fig. 5). 4. conclusions we defined the stratigraphic position of 26 specimens of marine vertebrates from beds of the arcillolitas abigarradas member of the paja formation in the villa de leiva – sáchica sutamarchán region. we described a complete stratigraphic section of the arcillolitas abigarradas member from sáchica, boyacá, and eight partial stratigraphic sections of this member from villa de leiva. we refine the information on the stratigraphic provenance of some of the specimens included in this study, mainly that of three holotype specimens (muiscasaurus catheti (cip-fcg-cbp-73), “kyhytysuka” sachicarum (don-19671) and protolamna ricaurtei (fcg-cbp-93)), which stratigraphic origin was not previously specified beyond the arcillolitas abigarradas member. a lithostratigraphic, biostratigraphic, and graphic correlation of the gathered information allowed us to establish four vertebrate assemblages representing coeval faunas within the arcillolitas abigarradas member, two from the barremian and two from the upper aptian. the paleoecological implications of the marine vertebrae assemblage established in this work are currently under study. acknowledgements we thank martin welych-flanagan and ana flanagan for their financial support. ecopetrol s. a. provided financial support for the stratigraphic study of the arrayán alto locality. we are grateful to professor fernando etayo-serna for identifying the ammonites from the arrayán alto section and his willingness to discuss the biostratigraphy of colombia. we would like to thank mary luz parra, director of the centro de investigaciones paleontológicas (cip), for allowing us to include in this study several of the specimens from the cip collection and for providing us with the necessary information. we would like to thank marcos sotelo for allowing us access to the monsalve 1 and 2 section sites and antonio montaña for the information on the finding site of some of the specimens. we would like to thank jorge enrique ruiz for his help in the field work. we kindly appreciate the thoughtful reviews provided by javier luque, edwin cadena and an anonymous reviewer, whose comments help us to improve the manuscript. supplemental material • supplemental data 1: detailed stratigraphic columns described in this study. • supplemental data 2: identification of ammonoids collected in this study. • supplemental data 3: detailed stratigraphic column of arrayán alto section and identified ammonoids. references bürgl, h. 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(editor). treatise on invertebrate paleontology. part l. mollusca 4 revised, 4. geological society of america & university of kansas, kansas, usa. 1-362. https://doi.org/10.1016/j.cretres.2020.104685 https://doi.org/10.15446/esrj.v27n1.105689 https://doi.org/10.15446/esrj.v27n1.105689 https://revistas.unal.edu.co/index.php/geocol/article/view/31539 https://revistas.unal.edu.co/index.php/geocol/article/view/31539 https://doi.org/10.32685/pub.esp.36.2019.12 https://doi.org/10.32685/pub.esp.36.2019.12 https://doi.org/10.1016/j.cretres.2014.04.001 https://doi.org/10.1016/j.cretres.2014.04.001 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.112936 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 175 181 g eo ph y si c s abstract: diffraction separation for the ground penetrating radar data by masking filters zhijun li1,2, hui sun3*, ruoge xu3, rui chen3, hongyong ren3, chenglang wang3, fuliu gao3, mingnian wang1 1. school of civil engineering, southwest jiaotong university, chengdu, china. 2. china railway tunnel group co., ltd., guangzhou, china. 3. faculty of geosciences and environmental engineering, southwest jiaotong university, chengdu, china. * corresponding author: sunhui@swjtu.edu.cn record manuscript received: 13/02/2024 accepted for publication: 26/07/2024 keywords: ground penetrating radar (gpr), geological disease survey, gpr signal denoising, diffraction separation, masking filter palabras clave: georradar; detección de problemas geológicos; eliminación de ruido de señal en georradar; separación por difracción; filtros de máscara. how to cite this item: li, z., sun, h., xu, r., chen, r., ren, h., wang, c., gao, f., & wang, m. (2024). diffraction separation for the ground penetrating radar data by masking filters. earth sciences research journal, 28(2), 175-181. https://doi. org/10.15446/esrj.v28n2.112936 ground penetrating radar is a high-resolution, efficient, non-destructive geophysical detection method. it is widely used in various application scenarios such as tunnel geological prediction and road maintenance. ground penetrating radar data contains a variety of valid signals as well as noise. the diffracted waves of ground penetrating radar contain high-resolution small target imaging information. a critical challenge in gpr applications is how to extract diffracted waves from the wave fields. we provide a strategy to achieve this goal by applying the masking filters. considering the complexity of the ground penetrating radar wave field and the weak energy of the diffracted waves, the median filter is first employed to suppress the linear reflections and then the f-k filter and τ-p filter are implemented to further increase the proportion of diffractions in the wave fields. three numerical experiments are employed to test the diffraction-separation method. resumen el georradar es un método de detección geofísica de alta resolución, eficiente, y no destructivo. se usa ampliamente en varios escenarios, como en la predicción de túneles geológicos y en el mantenimiento de carreteras. la información del georradar contiene una variedad de señales válidas pero también ruido. las ondas difractadas del georradar contienen información detallada de pequeños objetivos en alta resolución. uno de los principales problemas en las aplicaciones de georradar es el cómo extraer las ondas difractadas a partir de los campos de ondas. en este artículo se presenta una estrategia para lograr este objetivo a través de aplicar filtros de máscara. al partir de la complejidad de la onda de campo del georradar y la señal débil de las ondas difractadas, el filtro mediano se usa inicialmente para suprimir las reflecciones lineales, y luego se implementan los filtros f-k y τ-p para incrementar la proporción de difracción en los campos de ondas. se emplearon además tres experimentos numéricos para evaluar el método de separación por difracción. separación por difracción de información recolectada con georradar a través de la aplicación de filtros de máscara https://doi.org/10.15446/esrj.v28n2.112936 mailto:sunhui@swjtu.edu.cn https://doi.org/10.15446/esrj.v28n2.112936 https://doi.org/10.15446/esrj.v28n2.112936 176 zhijun li, hui sun, ruoge xu, rui chen, hongyong ren, chenglang wang, fuliu gao, mingnian wang 1. introduction as an important applied geophysical method, ground penetrating radar (gpr) is highly efficient, user-friendly, non-invasive and widely used in tunnel geological prediction and maintenance of transportation facilities (shown in fig. 1) (feng et al., 2015; zhao et al., 2018; giovanneschi et al., 2019). the gpr data contains many different types of wave fields. the nature of these wave fields is different. the diffraction energy is weaker than the reflection energy, however, it contains many field information of small-scale geological bodies and can be used for high-resolution imaging and identification. the processing methods for diffracted waves are currently mainly focused on seismic wave processing rather than electromagnetic wave processing. many seismic wave processing methods can be used to process gpr data (zong et al., 2020; yang et al., 2020; li et al., 2021; zong et al., 2023). zhao et al. (2019) integrated the gpr diffraction wave separation method with the artificial intelligence method. li and zhang (2021) extrated the diffracted waves in yutu2 rover’s lunar penetrating radar data. lin et al. (2022) enhanced optimum rank-reduction approach for achieving efficient diffraction separation. sun et al. (2023) combined the time-frequency analysis method with inversion algorithm and applied it to road detection. the time-frequency analysis method can also be applied to gpr data processing. lin et al. (2023) imaged the discontinuities by applying the angle gathers. at present, there are few researches on gpr diffraction separation methods, and they mainly focus on using a single method to process the diffraction data. using a single filter to separate them is difficult to get good results. the masking filters are implemented in the median filter, the f-k filter and the ⁄ -p filter to extract the diffractions in this paper. median filtering method is a non-linear filtering technology, which has the characteristics of suppressing noise and edge protection ability. brownrigg (1984) proposed a weighted filter to improve the median filter’s processing ability. sun and neuvo (1994) applied the detail-preserving median in image processing according to the local measurements of impulses. wang and lin (1997) modified the standard median filter algorithm to remove impulsive noise efficiently. zhou and zhang (1999) employed the progressive switching median filter to eliminate impulsive noise from severely distorted pictures. zhu et al. (2004) applied the median filter to remove the linear noise in the seismic data processing. cai et al. (2006) employed the median filter for reducing random seismic noise. dong and xu (2007) provided a directional-weighted median filter to eliminate random impulse noise. huo et al. (2012) introduced a new multidirectional vector filter (md-vmf) for separating mixed seismic shot sets. this method extends the traditional median filter from scalar to vector implementation; kang et al. (2013) proposed a new median filter forensics technology based on the statistical characteristic analysis of median filter residual (mfr); storath and weinmann (2018) proposed a fast algorithm for filtering signals and images based on arc distance median according to the principle of median filtering, it is verified that the efficiency of the algorithm is higher than that of the classical median filter and has good adaptability to different data; erkan et al. (2020) presented an adaptive-frequency median filter to solve the denoising problem. f-k filtering method is based on two-dimensional fourier transform and is a commonly used signal denoising technology. canales (1984) proposed a technique to mitigate stochastic interference of the seismic data. stone (1987) proposed a spatially variable f-k filtering method based on the “roll-along” 2-d fourier transform, which improved the computational efficiency of the algorithm. duncan and beresford (1994) designed the slowness adaptive f-k filter of pre-stack seismic data. paulus and mars (2006) proposed a method for multicomponent or multidimensional data based on single component seismic method (f-k filter), which is applied to separate interference wave fields. cai et al. (2011) presented a new adaptive f-k filter for the defect of poor adaptability of f-x domain denoising in complex geological structures. this method considers the time-frequency characteristics of local data, is easy to implement, and can effectively suppress random and coherent noise in complex geological structures; chen and jin (2015) presented a novel approach to eliminate random noise and improved seismic data resolution through the simple fk threshold step in the shaping regularization framework; maman herman et al. (2017) applied f-k filtering in shallow sea seismic exploration to eliminate noise in high-frequency seismic data and achieved good results; lowney et al. (2020) designed a spatial variable filter based on plane wave destruction in f-k domain. in this method, the synthetic data and real data are used to verify the method, which proves the advantages of diffraction imaging in seismic interpretation. ⁄ -p filtering method is also a widely used filtering method. radon transform theory was proposed by the famous austrian mathematician radon in 1917 to deal with the problem of image reconstruction (radon, 1986); carswell et al. (1984) applied the ⁄ -p transform to the separation of the up and down waves of vsp seismic data, and achieved good results. the radon transform was calculated using the least squares approach, which was examined by thorson and claerbout (1985). sacchi and ulryc (1995) proposed the highresolution radon transform for channel interpolation and vsp wavefield separation and achieved good application effect; trad et al. (2003) proposed a hybrid radon transform method for suppressing surface waves, and achieved good results. moldoveanu-constantinescu and sacchi (2005) proposed a phase shift hyperbolic equation, it can be clearly seen from the results that the equation improves the resolution in the radon transform domain. nowak and imhof (2006) proposed the weighted radon transform to suppresses multiple, which can keep the transformed amplitude well; jiang et al. (2015) applied the frequency domain high resolution radon to the separation of p-p and p-sv waves, and the application effect was good; gholami (2017) proposed the convolutional radon transform, which improves the time, slowness and curvature resolution of seismic data compared with the traditional radon transform. andersson and robertsson (2019) proposed a simple, fast and more accurate algorithm based on radon transform and opposite transform, this method reduces the computational complexity and the calculated cost. in this paper, the τ p masking filters are applied to separate the gpr diffractions. three filtering methods, namely the median filter, the f-k filter and the filter, are used to process different types of the high-frequency electromagneticwave signals and noises in gpr data. then the purpose of extracting diffraction waves in gpr data is achieved. in the following sections, the principle of the masking filter and the principles of the filtering methods employed in it will be introduced first. then zigzag model data, inclined layer model data and complex model data will be used to test the method in this article. figure 1. application of gpr in tunnel prediction and maintenance of subways, highways and high-speed railways. 2. method 2.1 median filtering principle assume that the gpr record is described by formula 1: x i n j ni j x t, , , , ; , ,� � � �� � � �1 1 (1) where i and j are the spatial and temporal subscripts of the data nx is the number of spatial samples of the data, that is, the number of seismic traces; nt is the number of time samples per seismic record. set the data sequence of channel i be xj (j = 1,..., nt) , if the given median filter span is n (generally n is odd), then the median filtering process for point k of channel i data is: 1. n sample points centered on point k; 2. rank the n sample points from small to large; 3. take the value of the center position of the ordered n sample points as the output value of point k. 177diffraction separation for the ground penetrating radar data by masking filters raw data x x xk n k k n� � � �( )/ ( )/ , , , ..., 1 2 1 2  sort data y y yk n k k n� � � �( )/ ( )/ , , , ..., 1 2 1 2  output data yk repeat the above process to realize the median filtering of seismic data record { } , xi j . the neutral filtering in phase elimination method is to subtract the filtered record yk from the original record xk to obtain the result zk, as formula 2: zk = xk ₋ yk (2) it is sometimes referred to as subtraction median filtering. 2.2 f-k filtering principle the f-k filtering method uses two-dimensional fourier transform to transform the signal in time-space domain (x, t) into frequency wavenumber domain (f, k). let f(x, t) be about two-dimensional seismic data in time-space domain f(x, t) and contain valid signal s(x, t) and interference noise n(x, t), i.e. formula 3: (f, k) = s(x, t) ₊ n(x, t) (3) two-dimensional fourier transform of formula (3), i.e. formula 4: � � � � � � � � �� �� � � � � � � � � � � � � � f x t e dxdt s x t e j fi kx j fi ( , ) ( , ) ( ) ( 2 2 2 2 kkx j fi kx dxdt n x t e dxdt ) ( ) ( , )� � �� � � � � � � � � �2 2 (4) this shifts the signal from domain (x,t) to domain (f,x), f to frequency and k to wavenumber. the formula above is simplified to formula 5: f (f,x) = s (f,x) ₊ n (f,x) (5) where f (f,x) is the frequency wavenumber spectrum of the original two-dimensional signal f(x, t); s(f, k) is the frequency wavenumber spectrum of the effective signal s(x, t) and n(f, k) is the frequency wavenumber spectrum of the interference noise n(x, t). f-k filtering refers to the filtering in frequency-wavenumber domain for signals in time-space domain (x,t). the designed filtering factor is a function of time-space h(f,k).the filtering process is a product process in frequencywavenumber domain. the specific process is as follows: in area (f,k), a two-dimensional filter h(f,k), is designed so that in area d, as formula 6: h f k f k d f k d ( , ) , ( , ) , ( , ) � � � � � � 1 0 (6) filter f(f,k) signal through the wave, as formula 7: ( ˆ , ) ( , ) ( , ) ( , ) ( , ) 0 ( , ) f f k f k d f f k h f k f f k f k d ∈ = = ∉    (7) where )ˆ ˆ ˆ( , ) ( , ) ( ,f f k s f k n f k= + satisfies formula 8: ( , ) 0, ( , ) ( , ) 0 ˆ ˆ , ( , ) s f k f k d n f k f k d = ∉ = ∈    (8) then the inverse fourier transform of k in )ˆ ˆ ˆ( , ) ( , ) ( ,f f k s f k n f k= + region is obtained formula 9: ˆ ˆ ˆ( , ) ( , ) ( , )f f k s f k n f k= + (9) because in area d, n (f, k) = 0 , so we can obtain formula 10: ˆ ( , ) ( , )f f k s x t= (10) where ˆ ( , ) ( , )f f k s x t= is the filtered frequency-wavenumber spectrum of the original two-dimensional signal f(x, t); ˆ( , )s f k is the filtered frequencywavenumber spectrum of the effective signal s(x, t) and ˆ ( , )n f k is the filtered frequency-wavenumber spectrum of the interference noise n(x, t). 3. basic principle of radon transform in multi-wave and multi-component seismic exploration, longitudinal and transverse waves are usually mixed together, which is not conducive to observation and processing. better separation of longitudinal and transverse waves can reduce the difficulty of data processing, improve the accuracy of inversion, and better explain the structure of underground media. radon transform provides a way to project the data in the time domain into the radon domain, and then transform the data of radon domain into the time domain. the purpose of the former is to separate the components in the radon field and eliminate the unwanted components, and the target of the latter is to retain the required waves. the result of the radon transform clearly shows that the noise has been effectively suppressed and eliminated. linear radon transform is also called tilt stack or τ p transform. its idea is to superimpose and sum continuous signals in the time-space domain along a straight line with slope p and intercept τ (along a certain ray path), and apply τ and to describe the trajectory of the wave. the expressions for d(t,x) and τ p transform are as follows formulas 11-12: m p d t px x x( , ) ( , )� �� �� � � �� d (11) d t x m t px p p( , ) ( , )� � � ��� � � d (12) where d(t,x) is a two-dimensional continuous signal; t means two-way time; x is the offset; m(τ, p) represents the oblique superimposed signal τ; is the intercept time; dt p dx = is slowness. seismic records in the time-space domain are discrete samples of the signal d(nt, nx). when wavefield separation is performed, it is necessary to discretize formulas (11) and (12). the equations are as follows formulas 13-14: m p d p x xi j i j k k k � �, ,� � � �� � � �1 nx (13) d t x m t p x pm n m k n k k np , ,� � � �� � � �1 (14) parabolic radon transformation principle change the integral path t = τ + px in the linear radon transform oblique superposition formula to t = τ + qx2 , so the integral path changes from a ray to a parabola. the positive and negative transformation formulas are as follows formulas 15-16: m q d t qx x x( , ) ( , )� �� �� � � �� 2 d (15) d t x m t qx p q( , ) ( , )� � � ��� � � 2 d (16) the processing of seismic data requires discretization, as follows formulas 17-18: m p d q x xi j k nx i j k k� �, ,� � � �� � � �1 2 (17) 178 zhijun li, hui sun, ruoge xu, rui chen, hongyong ren, chenglang wang, fuliu gao, mingnian wang d t x m t q x qm n k np m k n k, ,� � � �� � � �1 2 (18) the schematic diagram of the diffraction-separating method using the masking filters is shown in figure 2. data loading trace editing geometry set up data analysis f-k filter data output τ-p filter median filter figure 2. the flow chart of separating the diffractions using the masking filters. 4. numerical test on a zigzag model the first test was conducted on a zigzag model with the dielectric permittivity distribution shown in figure 3. the model has a size of 111 × 201; the horizontal and vertical grid spacing of the model are both 0.1 m; the uppermost layer is the air layer with a dielectric constant of 1.0; the dielectric constants of the second and third layers are 9.0 and 16.0, respectively; the model contains two sharp points, which will produce diffraction hyperbolas during forward modeling. the zigzag model’s gpr modeling result is displayed in figure 4. the source’s lateral location is dispersed between 0.0 and 19.7 meters, with a source interval of 0.1 meters. the offset between the source and the receiver is 0.3 meters. we can clearly see diffracted, direct, and reflected waves in figure 4. the energy of the diffracted wave is significantly less than that of the reflected and direct waves. figure 5 and figure 6 show the diffracted waves and residual wave fields obtained using masking filters, respectively. it is evident that the diffractions may be successfully extracted using the masking filter approach. however, there are still some other waves in the extraction results that are difficult to eliminate and the removed wave energy contains some diffracted waves. figure 3. the dielectric permittivity distribution diagram of the zigzag model. figure 4. the gpr modeling result of the zigzag model. figure 5. the diffraction separation result of the zigzag model using the masking filters. figure 6. the residual wave fields of the zigzag model after extracting the diffractions. 5. numerical test on an inclined layer model the second test was performed on an inclined layer model, and the inclined layer model’s dielectric constant distribution is displayed in figure 7. its size is 110 × 200; the horizontal and vertical grid spacing of the model is 1 m, and the dielectric constants from the first layer to the third layer are 1.0, 9.0 179diffraction separation for the ground penetrating radar data by masking filters and 25.0, respectively. the model consists of six regularly inclined scattering points and two square anomaly blocks. the dielectric constant of the scattering points and square anomaly blocks is 16.0. figure 8 shows the gpr forward modeling result of the inclined layer model. the sources are laterally distributed in the range of 0.0 m to19.7 m, and the source spacing is 0.1 m; from figure 8, we can clearly see reflected wave, direct wave and diffracted wave where the diffracted wave’s energy is significantly less than the energy of the reflected and direct waves. figure 9 and figure 10 show diffracted waves and residual wave fields of complex models obtained using masking filters, respectively. the mask filter can effectively separate diffracted waves, and the diffraction energy is very small in the residual wave field. figure 7. dielectric constant distribution of inclined layer model. figure 8. gpr forward modeling record of complex synthetic model. figure 9. the diffraction separation result of the inclined layer model using the masking filters. figure 10. the residual wave fields of the inclined layer model after extracting the diffractions. 6. numerical test on a complex model the third test was run using a complex model. the model’s dielectric permittivity distribution is displayed in figure 11. it has a size of 400 × 800. the horizontal and vertical grid spacing of the model are both 0.05 m. from the first layer to the fourth layer, the dielectric constants are 1.0, 3.0, 4.0, 5.0. the model contains multiple scattering points and a long-strip anomaly. the scattering points and the long-strip anomaly is 0.5 larger than the stratum containing it. the complex model’s modeling result is displayed in figure 12. there is a 0.5 m source interval. the receiver and source are offset by 0.3 meters. the scattering points produce many diffracted waves in the forward result. figure 13 and figure 14 show the diffracted waves and residual wave fields of the complex model obtained using masking filters, respectively. the masking filters can separate the diffracted waves well and the residual wavefields contain little diffraction energy. figure 11. the dielectric permittivity distribution diagram of the complex model. figure 12. the diffraction separation result of the complex model using the masking filters. 180 zhijun li, hui sun, ruoge xu, rui chen, hongyong ren, chenglang wang, fuliu gao, mingnian wang figure 13. the diffraction separation result of the zigzag model using the masking filters. figure 14. the residual wave fields of the complex model after extracting the diffractions. 7. conclusion this paper focuses on extracting the diffractions from the gpr wave fields. the masking filters are employed to finish the task. considering the complexity of the gpr wave field, median filtering, f-k filtering and ⁄-p filtering are comprehensively applied to the separation of diffracted waves. numerical results show that these three methods can overcome their shortcomings to a certain extent, and can suppress direct waves, reflected waves and random noises to obtain good diffractions. the masking filer method can be an effective tool for diffracted wave separation. acknowledgment this work was supported in part by the national natural science foundation of china under grant 42374166, in part by the natural science foundation of sichuan province under grant 2023nsfsc0770, and in part by the fundamental research funds for the central universities under grant 2682022ztpy002 and grant 2682022ztpy088. references andersson, f., & robertsson, j. 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(2025). evaluación de la vulnerabilidad de las aguas subterráneas mediante métodos sintacs y sintacs modificado en la cuenca de burdur çine (turquía). earth sciences research journal, 29(2), 201-210. https://doi.org/10.15446/esrj.v29n2.114091 groundwater sensitivity studies are widely used to protect aquifers in basins from pollution elements and to take necessary precautions. in this study, the sintacs model was used in conjunction with gis and ahp to determine groundwater sensitivity. the sintacs model is generally preferred in karstic lithologies. the burdur cine basin (bcb) is located in the karstic area defined as the taurus limestone belt in southwestern turkey. in the basin where intensive agricultural activities are carried out, there is a university campus and numerous villages. the groundwater in the basin is used both for drinking water in burdur province and for agricultural activities. in determining the sensitivity of groundwater; the parameters of water depth (s), infiltration (i), unsaturated zone (n), soil (t), hydrogeological features (c), and topographic surface (s) were used. these model parameters were weighted using normal and karstic sintacs scenarios based on the characteristics of the basin. in the parameter weighting process with ahp, the geological, hydrogeological, meteorological, and land use characteristics of the region, particularly the karstic structure, were taken into account. the sintacs index was found to be in the range of 26-222, while the modified sintacs index was in the range of 63-269. the modified sintacs method has provided higher index values due to the addition of two parameters. according to the modified sintacs method, it has been determined that 44.36% of the basin has very high, 22.08% high, 13.02% moderate, and 10.17% extremely high sensitivity. the areas with very high and high sensitivity include areas where the slope is low and soil over the aquifer is permeable. this situation indicates that the groundwater aquifer in the basin is highly sensitive to pollution. keywords: vulnerability mapping; sintacs; geographic information system (gis); hydrogeology; turkey. evaluación de la vulnerabilidad de las aguas subterráneas mediante métodos sintacs y sintacs modificado en la cuenca de burdur çine (turquía) resumen los estudios de sensibilidad de las aguas subterráneas se utilizan ampliamente para proteger los acuíferos en las cuencas de los elementos contaminantes y para tomar las precauciones necesarias. en este estudio, se utilizó el modelo sintacs junto con sig y ahp para determinar la sensibilidad del agua subterránea. el modelo sintacs se prefiere generalmente en litologías kársticas. la cuenca burdur cine (bcb) se encuentra en el área kárstica definida como el cinturón de caliza de tauro en el suroeste de turquía. en la cuenca donde se llevan a cabo actividades agrícolas intensivas, hay un campus universitario y numerosas aldeas. las aguas subterráneas en la cuenca se utilizan tanto para el agua potable en la provincia de burdur como para actividades agrícolas. para determinar la sensibilidad de las aguas subterráneas, se utilizaron los parámetros de profundidad del agua (s), infiltración (i), zona no saturada (n), suelo (t), características hidrogeológicas (c) y superficie topográfica (s). estos parámetros del modelo se ponderaron utilizando escenarios sintacs normales y kársticos basados en las características de la cuenca. en el proceso de ponderación de parámetros con ahp, se tomaron en cuenta las características geológicas, hidrogeológicas, meteorológicas y de uso del suelo de la región, particularmente la estructura kárstica. el índice sintacs se encontró en el rango de 26-222, mientras que el índice sintacs modificado se encontró en el rango de 63-269. el método sintacs modificado ha proporcionado valores de índice más altos debido a la adición de dos parámetros. según el método sintacs modificado, se ha determinado que el 44.36% de la cuenca tiene una sensibilidad muy alta, el 22.08% alta, el 13.02% moderada y el 10.17% extremadamente alta. las áreas con sensibilidad muy alta y alta incluyen zonas donde la pendiente es baja y el suelo sobre el acuífero es permeable. esta situación indica que el acuífero de aguas subterráneas en la cuenca es altamente sensible a la contaminación. palabras clave: mapa de vulnerabilidad, sintacs, sistema de información geográfica (sig), hidrogeología, turquía. https://doi.org/10.15446/esrj.v29n2.114091 mailto:isoyaslan@mehmetakif.edu.tr https://doi.org/10.15446/esrj.v29n2.114091 202 i̇. i̇skender soyaslan 1. introduction groundwater is one of the most important sources of freshwater necessary for the survival of all living organisms and agricultural activities. in recent years, the quality, quantity and availability of water have been the main problems with groundwater. the most important factors limiting the use of surface water and groundwater are agricultural, industrial and domestic pollution sources. unlike surface waters, the processes of groundwater being affected by pollution elements take time and are more difficult (foster and chilton, 2003; jackson et al., 2001). however, the cleaning of polluted groundwater is very difficult and time-consuming compared to surface waters and, in some cases, even impossible (soyaslan, 2022). for these reasons, it is of great importance to take the necessary measures to protect groundwater from pollution sources. to take these measures, it is necessary to determine the pollution potential of groundwater. the assessment of groundwater pollution potential is carried out to determine the possible consequences of potential pollution. for this purpose, it is necessary to determine the topographical, geological, hydrogeological and hydrological characteristics of the environment through which the pollutant elements pass. one of the methods frequently used in determining the pollution potential of groundwater is groundwater vulnerability studies (soyaslan, 2020; kirlas et al., 2022). the factors to be considered in these studies are the spread of pollution depending on the groundwater flow mechanism and the economic value of groundwater in the region (cost, 2003). groundwater aquifers are crucial for providing water for several applications, including potable, agricultural, and industrial needs. mathematical simulation techniques are employed to ascertain the response of groundwater aquifers to pollution sources. understanding the hydrological characteristics of an aquifer is essential in this domain. numerous researches employing various methodologies have demonstrated that the most sensitive criterion for internal groundwater sensitivity is the depth of the groundwater level (ansarifar et al., 2020). to take measures to reduce and prevent groundwater pollution, the relationship between pollution sources and groundwater vulnerability should be revealed. groundwater vulnerability maps prepared using models integrated with gis are very useful for regional and local groundwater utilization planning (taheri et al., 2023). the concept of groundwater vulnerability was initially articulated in 1968 as ‘the susceptibility of groundwater to pollution’ (margat, 1968). the word vulnerability became prevalent in the early 1970s to denote the susceptibility of aquifer groundwater systems to contamination (albinet and margat, 1970). the phrase gained broader usage among numerous researchers in the 1980s (haertle, 1983; aller et al., 1987; foster, 1987; forster and hirata, 1988). the notion of groundwater vulnerability refers to the susceptibility of groundwater to contamination (gogu and dassargues, 2000). the evaluation of groundwater contamination potential cannot be ascertained immediately by field research (alfonso et al., 2008). the terms origin, channel, and target articulate a triadic model for environmental management in aquifers, which forms the foundation of the notion of groundwater vulnerability. the origin is the location from which the pollutant emanates. the pathway encompasses all components, from the pollution source to the route taken until it reaches the groundwater. the objective is to safeguard the groundwater table (cost, 2003). groundwater vulnerability can be assessed using two sub-concepts: intrinsic and particular vulnerability. intrinsic vulnerability considers the geological, hydrological, and hydrogeological attributes of the aquifer region, reflecting the groundwater’s susceptibility to pollution. nonetheless, it fails to include contamination scenarios resulting from potential contaminants affecting groundwater. the specific vulnerability takes into account the attributes of a specific contaminant as well as the inherent vulnerability of groundwater (cost, 2003). index methodologies employed to assess groundwater vulnerability might be important in safeguarding aquifers at risk from pollution sources and in implementing requisite remedies. these index approaches classify the parameters influencing the probability of groundwater pollution on a scale and assign a score that indicates the impact (foster, 1987). numerous research studies employing various methodologies have demonstrated that the depth of the groundwater level is the most sensitive indicator for assessing internal groundwater sensitivity (ansarifar et al., 2019). for distinct aquifers, groundwater vulnerability assessments employ several methodologies with gis assistance (khemiri et al., 2013; assefa and dinka, 2023). among the most prevalent methods are drastic (aller et al., 1987), god (foster, 1987), the avi rating system (van stempvoort et al., 1992), sintacs (civita, 1994; civita and de maio, 1997; doerfliger and zwahlen, 1995; hölting et al., 1995; vias et al., 2002; goldscheider et al., 2000), epik (doerfliger and zwahlen, 1995), german (hölting et al., 1995), cop (vias et al., 2002), pi (goldscheider et al., 2000), and the isis method. the nomenclature of these methods typically includes the initials of the parameters employed. there are ongoing discussions about the best way to do things, but the drastic method is used for alluvial aquifers and the sintacs method is used for karstic aquifers to figure out how likely they are to become contaminated (taheri et al., 2023; jahromi et al., 2021). research indicates that methodologies employing numerous criteria in groundwater vulnerability assessments yield more trustworthy outcomes (kazakis et al., 2015). table 1 presents the parameters of various approaches employed in groundwater vulnerability assessment. furthermore, irrespective of the approach employed, the paramount stage in groundwater risk assessment is the allocation of weights (pacheco et al., 2015; soyaslan, 2020). the disadvantages of these methods include their processbased nature, challenges in data acquisition, difficulty in calculations, and the necessity for expertise and experience in their implementation. consequently, statistical methods are predominantly employed in groundwater vulnerability assessments rather than alternative approaches (al-abadi et al., 2017; neshat et al., 2014). table 1. parameters of several methodologies employed in groundwater vulnerability assessment (polemio et al., 2009). the primary advantage of the sintacs approach compared to the drastic method is the simultaneous application of varied weights across multiple cells (goyal et al., 2021). the sintacs method can be used for both mediumand large-scale mapping, and it is more flexible than the drastic method when it comes to choosing the input parameters’ proportions and weights (ramos leal et al., 2010; majandang and sarapirome, 2013; goyal et al., 2021). consequently, the weights utilized in the sintacs technique are established more completely by taking into account all hydrogeological variables, allowing its application in areas with varying hydrogeological characteristics within the same basin (kumar et al., 2013).the concentrations of no₃⁻ and so₄²⁻ obtained in the field are utilized to assess the groundwater vulnerability to anthropogenic sources and to validate thematic maps generated using the sintacs approach (eftekhari and akbari, 2020). land use is regarded as an extra factor to enhance the precision of the groundwater vulnerability assessment (soyaslan, 2020; busico et al., 2017). intrinsic vulnerability assessment is typically favored in karst basins utilized as drinking water supplies and agricultural regions when employing the sintacs approach (kapelj et al., 2013; aschonitis et al., 2014; boufekane and saighi, 2013; kumar et al., 2013; marsico et al., 2004). the sintacs method is specifically employed in the evaluation of groundwater vulnerability in karst aquifer systems (khemiri et al., 2013; busico et al., 2017). the updated sintacs model has been developed and extensively utilized by incorporating lineament and land use parameters alongside the seven components of the original sintacs technique (busico et al., 2017; eftekhari and akbari, 2020). the groundwater vulnerability study of torka province in ghana, utilizing the sintacs model, revealed that over 50% of the aquifer is very sensitive due to mining and industrial operations (ewusi et al., 2017). a groundwater sensitivity research study employing the sintacs model in southern italy revealed that the application of nitrogenous fertilizers elevates groundwater sensitivity (busico et al., 2017). the groundwater sensitivity study in the nong rua region of thailand, conducted using the sintacs approach, identified six distinct groundwater sensitivity zones. regions exhibiting high and medium groundwater sensitivity were identified as those with elevated infiltration rates and shallow groundwater tables (majandang and sarapirome, 2013). kirlas et al. (2022) performed groundwater vulnerability mapping in the nea moudania aquifer in greece with seven distinct gis-based methodologies. the vadose zone and topography were identified as the most influential characteristics on the groundwater vulnerability index, whereas hydraulic conductivity was deemed the least useful parameter. eftekhari and akbari (2020) delineated various index zones in their groundwater vulnerability assessment employing the sintacs-lu model in the birjand plain, located in the south khorasan region, iran. the model demonstrated a suitable association with nitrate concentration as a measure of groundwater contamination. slimani et al. (2023) employed three distinct gis-supported methodologies (god, drastic, and sintacs) in their investigation of groundwater pollution vulnerability in the ouargla region of algeria. the drastic and sintacs methodologies have been identified as better and more appropriate for the region, with 203assessment of groundwater vulnerability using sintacs and modified-sintacs methods in burdur çine basin (turkiye) groundwater depth being the most influential parameter on the index value. primastuti and kusratmoko (2018) delineated contamination-sensitive zones within the aquifer utilizing the sintacs and sintacs-lu methodologies in taman rahayu. a 70% correlation with nitrate concentrations was established using the sintacs method, whereas an 80% correlation was found with the sintacs-lu approach. utami (2019) performed a vulnerability index analysis with a modified sintacs model that incorporated land use parameters in the opak hılır basin. there were significant correlations found between the amount of nitrate and the adjusted sintacs vulnerability index values (primastuti and kusratmoko, 2018; utami, 2019). analysis of prior research indicates that the sintacs approach is appropriate for evaluating groundwater vulnerability. the revised sintacs approach has a superior correlation value compared to the original sintacs method. the revised sintacs approach is deemed more suitable as it incorporates additional land characteristics. this study employed the widely recognized sintacs and modified sintacs methodologies to assess groundwater vulnerability in the alluvial aquifer and the overlying karstic limestone aquifer for bçb. hydrological scenarios were constructed based on the region’s land use, lithological, and hydrological attributes. furthermore, no groundwater risk assessment has been conducted yet in the research area, which exhibits karstic features. 2. materials and methods 2.1. study area description the bçb is situated in the western mediterranean area of southwestern turkey, namely within burdur province, in a region characterized by karstic and tectonic formations known as the lakes region (figure 1). it is moreover a subsidiary basin of the lake burdur basin. the research region is situated between latitudes n 37º 35.5’ and n 37º 44’ and longitudes e 30º 19.5’ and e 30º 29’, encompassing an area of 132 km². the height of the region varies from 1075 to 1925 meters. figure 1. location map of bçb 2.2. geology the geological formations in the research region comprise mesozoicaged limestones near the base. the limestones at the base constitute the karst aquifer, with groundwater flowing southward from this aquifer into the mediterranean sea (davraz et al., 2009). superimposed over the limestones are the ophiolitic complex, palaeogene-aged sandstone, mudstone, siltstone, and claystone, followed by neogene-aged conglomerate, sandstone, claystone, and siltstone, along with neogene-aged travertine, culminating in quaternary-aged alluvium composed of unconsolidated blocks, gravel, sand, and silt. 2.3. hydrogeology the primary geological formations facilitating groundwater transfer in bçb are quaternary alluvium and limestone deposits. additional units that contain groundwater, based on their hydrogeological properties, include plioquaternary-aged travertine, lacustrine sediments, conglomerate, and sandstone. cretaceous-aged limestone, constituting the foundation of the research region, outcrops across an expanse of 32.81 km² (25%) inside the basin, while dutdere limestones outcrop over 12.89 km² (10%). alluvium, a unit exhibiting aquifer properties, emerges over an area of 33.74 km² (26%). units exhibiting aquifer characteristics, covering 61% of the total basin area, heighten the susceptibility of groundwater to contamination. fractures, fissures, and karstic formations in limestone facilitate the infiltration of precipitation into groundwater. this situation is corroborated by the presence of lakes in the insuyu cave, composed of karstic limestone, which are replenished by rainwater descending from the cave ceiling. the triassic allochthonous dutdere limestone, the other karstic unit, was influenced by tectonic movements. dutdere limestone possesses the characteristic of an aquifer comprising minor karstic voids formed along fractures and fissures. in terms of thickness and distribution, cretaceous limestone possesses superior potential for groundwater compared to dutdere limestone. furthermore, the dutdere limestone, situated atop the impermeable ophiolitic complex, emanates from multiple sources along this interface. a confined aquifer with lateral recharge can be seen in travertine from the plioquaternary period and lacustrine sediments from the pliocene period (taşdelen, 2018). the geological and hydrogeological characteristics of the region render groundwater susceptible to many contamination sources. the primary cause of pollution originates from agriculture, attributable to the extensive application of synthetic fertilizers and pesticides in the basin’s agricultural practices. alongside the rising utilization of groundwater in the research area, burdur province has increasingly sourced drinking water from boreholes in recent years. consequently, the water level in the subterranean lakes of the karstic insuyu cave within the basin has progressively diminished, leading to the desiccation of the lakes. two problems jeopardizing the groundwater in the research area are the unregulated extraction of groundwater and the potential for agricultural contamination. this circumstance jeopardizes all operations reliant on groundwater inside the basin. the significance of groundwater vulnerability assessments is recognized in light of groundwater utilization and pollution sources within the basin. 2.4. methodology the methodology used in the groundwater vulnerability assessment for the bçb is shown in figure 2. s i n t a c s m o d s i n t a c s raw data well data meteorological data geological-well data soil data geospatial analysis sintacs mod sintacs groundwater vulnerability map well data field well data topographic data remote sensing gis landuse cover map parameters processing method map s i n t a c s li lu figure 2. methodology framework applied in this study the sintacs approach is regarded as the italian equivalent of the drastic method, a prominent aquifer vulnerability assessment technique globally, tailored to mediterranean conditions. upon digitizing the sintacs parameters in arcgis software, the inverse distance weighted (idw) 204 i̇. i̇skender soyaslan interpolation method was employed to generate parameter maps in raster format with 25x25 m cells. the idw approach relies on the notion of allocating point data to spatial raster data inside a grid network composed of designated cells (soyaslan, 2020). 2.5. the sintacs method the sintacs approach was initially formulated by civita in 1990, 1993, and 1994, and then refined by civita and de maio in 1997 for evaluating and delineating groundwater sensitivity in aquifer systems. the sintacs approach employs a weighted aggregation of seven distinct characteristics to evaluate groundwater sensitivity. the parameters that confer the sintacs method its designation and are utilized in groundwater sensitivity index computations are as follows (civita and de maio, 2004): s1 : depth to water (soggiacenza), i : infiltration (infiltrazione), n : effect on the auto-depuration of the unsaturated zone (nonsaturo), t: typology of the cover (tipologia della copertura), a : hydrogeologic characteristics of aquifer (aquifero), c: hydraulic conductivity (conducibilita), s2: slope (superficie topografica) the sintacs parameters are fundamental elements necessary for evaluating the susceptibility of a groundwater aquifer to contamination. moreover, the tectonic, hydrogeological, and land use attributes of the study area may influence the employment of supplementary parameters. the methodology was amended by including lineament (li) and land use (lu) parameters, owing to the tectonic features of the research region adjacent to active fault lines and prevailing land use characteristics. table 2 presents the ranges and ratings of sintacs parameters. the rating values fluctuate from 1 to 10, contingent upon the subclasses of the parameter. the parameter exhibiting the greatest risk and significance regarding groundwater contamination is allocated a rating of 10, whereas the parameter of least significance is assigned a rating of 1. the sintacs parameters are categorized into subclasses based on their influence on the potential for groundwater pollution. despite utilizing identical parameters, the sintacs and drastic procedures assign divergent ratings to the parameters employed in the sintacs approach. the primary distinction between the sintacs and drastic approaches is that the sintacs method offers five distinct groundwater vulnerability scenarios. normal impact, severe impact, substantial drainage from a surficial network, fissured aquifers, and karstified aquifers are some of the situations that have been studied (civita and de maio, 1997; civita and de maio, 2004; kirlas et al., 2022; slimani et al., 2023; von hoyer and söfner, 1998; civita 10). the ratings (r) for each parameter range from 1 to 10, whereas the weights (w) range from 1 to 5 (kirlas et al., 2022). the variable weights allocated to the parameters based on the scenarios are presented in table 3. table 2. ranges and ratings of sintacs parameters (civita and de maio, 1997) sintacs parameters range rating sintacs parameters range rating s1 (m) 0–1 10 peat 7.5-8 1–4 9 sandy 8-8.5 4–6 8 clean sand 9-9.5 6–8 7 clean gravel 9.5-10 8–10 6 thin or absent 10 10–20 5 a coarse alluvial deposits 8-9 >20 4 karstifed limestone 9-10 i (mm) <50 1 fractured limestone 6-9 50–60 2 fissured dolomite 4-7 60–75 3 medium fne alluvial deposits 6-8 75–100 4 sand complex 7-9 100–125 5 sandstone, conglomerate 4-9 125–150 6 turbiditic sequences 5-8 150–175 7 fissured volcanic rocks 8-10 175–250 8 marl, clay stone 1-3 250–325 9 clay, silt, peat 1-3 n coarse alluvial deposits 6-9 pyro-clastic rock 4-8 karstifed limestone 8-10 fissured metamorphic rocks 2-5 fractured limestone 4-8 c (m/day) <0.1 1 1 fissured dolomite 2-5 0.1–0.43 2 medium fne alluvial deposits 3-6 0.43–0.86 4 sand complex 4-7 0.86–4.32 5 sandstone, conglomerate 5-8 4.32–8.64 6 turbiditic sequences 2-5 8.64–43.2 7 fissured volcanic rocks 5-10 43.2–86.4 8 marl, clay stone 1-3 86.4–432 9 clay, silt, peat 1-2 432–864 10 pyro-clastic rock 2-5 s2 (%) 0–2 10 fissured metamorphic rocks 2-6 3–4 9 t clay 1-1.5 5–6 8 silty-clay 1.5-2 7–9 7 clay loam 2-3 10–12 6 silty clay loam 3-4 13–15 5 silt loam 3.5-4 16–18 4 loam 4-5 19–21 3 sandy clay loam 4.5-5 22–25 2 sandy loam 5.5-6 >26 1 sandy clay 6.3-7 205assessment of groundwater vulnerability using sintacs and modified-sintacs methods in burdur çine basin (turkiye) table 3. weighted scenarios in the sintacs method (civita and de maio, 1997) weighting scenarions s i n t a c s normal impact 5 4 5 3 3 3 3 severe impact 5 5 4 5 3 2 2 drainage from surficial network 4 4 4 2 5 5 2 karstic impact 2 5 1 3 5 5 5 fissuring impact 3 3 3 4 4 5 4 the sintacs method’s framework was designed to accommodate varied weight sets assigned to factors based on differing hydrogeological and impact scenarios. this study developed hydrogeological scenarios based on land use, landforms, and the hydrogeological structure of the region (mastrocicco et al., 2017; civita, 1990; civita, 1993). given the hydrogeological attributes of the study area, standard and karstic impact scenarios derived from sintacs weighting sequences were employed (civita and de maio, 2004). regions subjected to standard impact are those where agriculture, plant protection agents, fertilizer, and irrigation are employed. these regions typically encompass areas with gentle gradients where alluvial aquifers and other geological formations are exposed. regions subjected to karstic influence are characterized by undulating, sloped topography containing limestones with karstic dissolution cavities. these regions encompass desolate, uncultivated land or places devoid of agriculture, save for indigenous flora. agricultural practices, irrigation, and fertilization are not conducted in these regions.each criterion map for each variable was created as a gis data layer in raster format with grid dimensions of 25 x 25 meters. the sintacs vulnerability index (iv) was computed for each grid element utilizing the subsequent equation (equation 1): sintacs iv = (s1rs1w) + (iriw) + (nrnw) + (trtw) + (ar aw) + (crcw ) + (s2rs2w) (1) in equation 1, the subscript r denotes the rating of the seven parameters, while v represents their weights. the groundwater vulnerability index spans from 26 to 260, with the vulnerability index classifications and their respective ranges detailed in table 4. to facilitate comparison and interpretation of the results, these data were standardized to a range of 0 to 100. this normalization is categorized into six vulnerability classes: 0-24 very low (vl), >24-40 low (l), >40-55 moderate (m), >55-65 high (h), >65-80 very high (vh), and >80100 extremely high (eh) (majandang and sarapirome, 2013). table 4. vulnerability index rating classes for the sintacs method (civita and de maio, 1997) vulnerability classes ranges very low 26-80 low 80-105 medium 105-140 high 140-186 very high 186-210 extremely high 210-260 2.6. the modified sintacs method the new sintacs method was made by adding linearity and land use features to the existing framework, which had seven factors. this was done even though the original sintacs method already had standard and karst effect scenarios. the vulnerability index value (imv) for the modified sintacs method is computed as outlined in equation 2. modified sintacs imv = (s1rs1w) + (iriw) + (nrnw) + (trtw) + (araw) + (crcw) + (s2rs2w) + (lirliw) + (lurluw) (2) the r values in the sub-index of the modified sintacs vulnerability index (imv) equation represent the ratings of the parameters; the w values denote the weights; and the final two parameters are lineament (li) and land use (lu). consequently, the sintacs vulnerability index (iv) has seven parameters, but the modified sintacs (imv) incorporates nine parameters, including the additional li and lu parameters. this feature enables the enhanced sintacs approach to depict additional hydrogeological properties. consequently, it facilitates a more precise transfer of the field to the model environment. this also applies to the vulnerability parameters of databases and sintacs. groundwater depth (s1) the s1 parameter value of the sintacs technique is derived from groundwater depth. as the depth of groundwater from the surface increases, the rating of the s1 parameter, which ranges from 10 to 1, diminishes. groundwater depth numbers represent the distance to the groundwater surface as measured in boreholes within the study region. the locations and groundwater levels of the surveyed wells were integrated into the gis environment, resulting in the creation of a database. a digital terrain model (dtm) was generated utilizing the groundwater depth data from the database. linear interpolation, a conventional technique, was employed in the development of the digital terrain model (dtm), resulting in a raster-based regional technical map created at a scale of 1:100,000. the reclass command within the gis environment classified the groundwater depth map based on the range values of the sintacs approach. a grid of points associated with the groundwater depth parameter was generated. the groundwater was categorized into six classes based on depth and awarded rating values from 4 to 10 (figure 3a). infiltration (i) the effective infiltration action parameter is determined from the effective rainfall and surface hydrogeological parameters by a numerical model applied to grid cells. seven distinct homogenous regions were delineated based on the average rainfall-to-elevation ratio. according to the established grid cells of the study area, the subsequent statistics were automatically computed for each cell: specific rainfall, adjusted temperature, specific evapotranspiration, specific infiltration charge, and specific surface runoff. following the computation of the infiltration value for each grid cell, it was visualized and classed in raster format (figure 3b). effects of the unsaturated zone (n) the n parameter, sometimes referred to as unsaturated zone attenuation capacity, was determined by the analysis of well data and stratigraphic studies in the research area. the hydrogeological attributes of the region’s geological formations, including texture, mineral composition, grain size, discontinuities, and karstification, were utilized in the specification of the sintacs n parameter. the data were verified and standardized to ensure precise zoning of the hydrogeological information. the previously established points for unsaturated thickness were allocated to these homogeneous regions. the data associated with parameter n for these sites was classified, and a raster map was generated at a scale of 1:100,000. the gis system generated thematic maps and additional parameters (figure 3c). typology of the cover (t) the european environment agency (eea) provided the land cover use classification that led to the establishment of the sintacs t parameter, also known as soil/overburden attenuation capacity. the land cover utilization data generated by the eea via computer-assisted visual interpretation of satellite imagery was created as part of the corine (coordination of information on the environment) project. the corine database and local soil maps were analyzed, resulting in the pedological zoning of the entire basin, accompanied by hydrogeological data (figure 3d). hydrogeological properties of the aquifer (a) the sintacs a parameter, encompassing the hydrogeological attributes of the aquifer, delineates the processes occurring beneath the contaminant’s piezometric level. this process encompasses all interactions between the hydrogeological properties of the lithology constituting the aquifer environment and the pollutant. the approaches utilized for the sintacs a parameter were identical to those employed for the n parameter. the hydrogeological features map of the aquifer was correlated with the sintacs locations utilizing the same computation methods. the grid within the gis framework was implemented across the entire region in accordance with the hydrogeological features map of the aquifer (figure 3e). 206 i̇. i̇skender soyaslan hydraulic conductivity (c) the hydraulic conductivity or hydraulic conductivity range of the aquifer, as described in the sintacs c parameter, was found by looking at well test data, drilling logs, and well stratigraphy. in regions with inadequate data, the variation in the c parameter points of the aquifer was acknowledged. the hydraulic conductivity map was generated with particular discharge data and conductivity values derived from slug and recovery tests. the c parameter diminishes to -10 in ophiolitic lithologies characterized by low permeability, -2.5 in karstic limestones, and -3.5 in alluvial aquifers. in accordance with the sintacs points, the c parameter was classed, and a raster map at a scale of 1:100,000 was generated within the gis environment (figure 3f). figure 3. thematic maps of the study area slope (s2) the second s2 parameter of the sintacs acronym, representing the hydrologic function of the topographic slope, was derived using the regional dtm within the gis framework, featuring slope classifications from 0 to 28%. the slope (s2) parameter was classed based on sintacs points, and a raster map was generated at a scale of 1:100,000. a height grid was generated from dtm within a gis framework, and a grid depicting the topographic surface slope was produced using slope housing. the classification of slope categories was established by analyzing the distribution of % slope ranges to be mapped and the constraints of the sintacs approach (figure 3g). 207assessment of groundwater vulnerability using sintacs and modified-sintacs methods in burdur çine basin (turkiye) lineaments (li) the approach was enhanced by using lineament data in the groundwater vulnerability index computations, in addition to the sintacs parameters. lineaments (li) parameter: a total of 38 lineaments were identified through the analysis of landsat satellite pictures inside a gis framework. fault lines, fracture lines, formation boundaries, and other discontinuities influencing water movement were considered in the identification of lineaments. lineaments have formed due to the basin’s location in a tectonically active area. it makes sense to include the lineaments parameter in the sintacs model because it helps water move (figure 3h). land use (lu) the second parameter land use (lu), incorporated into the sintacs parameters, was revised for the research region by considering field studies and land use data, resulting in the production of a thematic map using the updated information. the land use parameter is categorized into six classes: settlement, agricultural field, natural vegetation, grassland, garden, and forest. the rigorous farming practices in the study region necessitate the application of pesticides and fertilizers. adding the land use parameter to the sintacs model is very important for making it more accurate and sensitive, since farming in the basin has a negative effect on groundwater sensitivity (figure 3i). 3. results and discussion 3.1. map of groundwater vulnerability the sintacs vulnerability index map (figure 4) was derived from seven hydrogeological parameters, whereas the modified sintacs vulnerability index map (figure 5) was derived from nine hydrogeological parameters. the parameters employed in these maps were computed with arcgis software and gis analytical techniques. the sintacs vulnerability index values are categorized into six distinct classes, ranging from 26 to 260.the risk zones characterized by elevated groundwater sensitivity within the basin are situated in regions designated for residential use, where the slope is minimal (0-2%), loam soils are conducive to agriculture, and the groundwater depth is relatively shallow. furthermore, groundwater sensitivity is observed to rise, particularly at locations with diminished groundwater levels where agricultural activities and boreholes are situated. figure 4. sintacs vulnerability index map the primary distinction between the map generated by the modified sintacs method and the map produced by the sintacs method is the elevation of the risk level. augmenting the parameter count and using lineament density alongside land use factors have proven helpful in this context. in very elevated areas, the influence of lineament density is minimal, whereas the impact of land use is significant. the agricultural regions along the north-south axis in the western part of the research area are deemed more hazardous. figure 5. modified sintacs vulnerability index map this scenario underscores the necessity of incorporating the land use variable in the assessments of groundwater vulnerability. furthermore, it was ascertained that regions devoid of settlements and agricultural zones with elevated topography exhibit reduced risk of groundwater susceptibility. furthermore, the low-slope alluvial regions in the western part of the research area present greater risk compared to the high-slope rocky regions in the east. the sintacs vulnerability index, based on established ratings and weightings, spans from 56 to 222, whereas the modified sintacs vulnerability index runs from 63 to 269 (figure 6). 0 10 20 30 40 50 0 10 20 30 40 50 a re a (% ) a re a (% ) sintacs method modified sintacs method vulnerability classes very low 56-80 low 80-105 medium 105-140 extremely high 210-222 extremely high 210-269 medium 105-140low 80-105 high 140-186 very high 186-210 very high 186-210high 140-186 very low 63-80 vulnerability classes 3.15 10.14 12.56 33.08 38.02 44.36 10.17 22.08 2.85 1.14 9.23 13.02 figure 6. area percentage results of sintacs and modified sintacs methods 208 i̇. i̇skender soyaslan the area percentage values fluctuate based on the data distribution among the border values. the augmentation of index values on the revised map is attributable to the incorporation of lineament density and land use variables in the computations. the updated sintacs index values have a more uniform distribution than the original sintacs values. in the sintacs approach, the maximum value is attributed to a very high class at 38.22%, while the minimum value is assigned to an extremely high class at 2.85%. in the revised sintacs technique, the highest value is attributed to the very high class at 44.36%, while the lowest value is associated with the very low class at 1.14%. 3.2. uncertainties and limitations of the sintacs model the sintacs model assesses groundwater pollution sensitivity by integrating gis and ahp methodologies. the sintacs model incorporates the geological, hydrogeological, climatic, morphological, and hydrological attributes of the region, although it excludes the types and characteristics of pollution. this technique requires revision and adaptation for various water sources and pollutant types. while the parameters in the model are calibrated to more accurately reflect the environment, no study is performed concerning the nature of the pollution. the sources of underground water pollution and their dispersion processes occur in distinctly various manners. consequently, the emphasis has not been placed on the extent and dissemination of agriculturalorigin contaminants, including chemical fertilizers and pesticides, in the groundwater of our research area. this scenario pertains to all techniques employed in groundwater sensitivity analysis. the primary advantage of the sintacs method over other sensitivity analysis techniques is its utilization of parameters that represent a substantially greater number of settings. furthermore, it is capable of executing weighing across five distinct scenarios that are unavailable in alternative methods. given the karstic features of my study location, the “karstic impact” scenario has been employed. 3.3. groundwater management strategy previous research and groundwater level observations indicate that the groundwater flow in the bçb area is directed southward, toward the mediterranean. the underlying karstic limestones in the research area release groundwater. the groundwater sensitivity analysis showed the most sensitive locations as flatlands characterized by agricultural activity and alluvial lithology. chemical fertilizers and agricultural pesticides, including herbicides, insecticides, and fungicides, pose a risk of groundwater contamination in agricultural regions. from the standpoint of groundwater management, the application of chemical fertilizers and agricultural pesticides in farming regions must be rigorously regulated. field investigations measuring groundwater levels revealed several unlawful drilling wells in agricultural regions. regular measurements of the production quantity and groundwater levels of all boreholes in the basin are essential. particularly in regions where groundwater flows toward agricultural fields, the quality of groundwater in the drilling wells must be checked. the dissemination of groundwater contamination originating from agricultural regions must be regulated. a comprehensive hydrogeological investigation of the region is required for a project funded by the burdur municipality, which obtains its drinking water from the basin. this investigation must clearly define the groundwater quality and groundwater budget. long-term plans for sustainable groundwater management must be devised within the basin. 4. conclusions the sintacs and modified sintacs methodologies, utilizing geographic information systems (gis), were chosen to assess the sensitivity of groundwater in the basin. the sintacs approach utilized water depth (s1), infiltration (i), unsaturated zone (n), soil (t), hydrogeological features (c), and topographic surface (s2) to assess groundwater sensitivity. in the revised sintacs approach, lineament density and land use were incorporated alongside these characteristics, and texture parameters were utilized. the index values derived from both approaches were categorized into six subcategories, and themed maps were created. both techniques conclude that regions characterized by gentle slopes, agricultural activity, alluvial deposits, and low groundwater levels exhibit heightened vulnerability regarding groundwater sensitivity. the new sintacs approach concluded that regions designated as very high in groundwater sensitivity have risen. the primary cause for this increase was the land use variable incorporated in the calculation. this underscores the significance of the land use element, absent from conventional calculation methods, in groundwater sensitivity assessments. the outcomes of the sintacs approach will be applied in all research pertaining to the sustainable utilization, conservation, and administration of groundwater resources in the region. regions beyond the agricultural areas in the basin are at a diminished risk of groundwater contamination. the regions engaged in agricultural activities have significant sensitivity. when selecting drilling well sites for potable water, it is imperative to circumvent agricultural regions. given that the groundwater flow in the region is directed southward, these areas will consequently be influenced by agricultural lands. the sintacs technique solely accounts for environmental characteristics and neglects the distribution of pollutant types in subterranean and 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(1998). groundwater vulnerability mapping in carbonate (karst) areas of germany. hanover, germany: federal institute for geosciences and natural resources, archive no:117854. https://doi.org/ http://dx.doi.org/10.5194/nhess-9-1461-2009 https://doi.org/10.1063/1.5047391 https://doi.org/10.1063/1.5047391 https://doi.org/10.1007/s12665-009-0434-5 https://doi.org/10.1007/s12665-009-0434-5 https://doi.org/10.24425/aep.2023.145896 https://doi.org/10.24425/aep.2023.145896 https://doi.org/10.1007/s12517-020-06101-3 https://doi.org/10.1007/s12517-020-06101-3 https://doi.org/10.1007/s10668-023-03240-x https://doi.org/10.1007/s10668-023-03240-x https://doi.org/10.28948/ngumuh.444795 https://doi.org/10.4296/cwrj1801025 keywords: geomenbrane; stress; clay interface. palabras clave: geomembrana; esfuerzo; interfaz de arcilla. how to cite item zhang, h. (2020). study on the internal force of geomembrane of landfill in heavy metal contaminated area. earth sciences research journal, 24(1), 111-118. doi: https://doi. org/10.15446/esrj.v24n1.85231 in order to analyze the tension stress of geomembrane in the seepage control system of a landfill in a heavy metal contaminated area under the action of the overlying landfill weight, the internal force of geomembrane in a landfill is studied in this paper. firstly, according to the internal force analysis of geomembrane model, the basic equation of geomembrane is obtained, and the equation is analyzed by finite difference numerical solution, so as to analyze the internal force of geomembrane in a landfill. the method can analyze three stress states of geomembrane-clay interface at different positions: elastic state, softening state and residual state, which makes the stress analysis of geomembrane on landfill slope more reasonable. parametric analysis shows that when the strength loss and the difference between residual displacement and peak displacement are small in the softening stage, the peak strength of geomembrane-clay interface has little influence on the maximum tensile stress of geomembrane and its residual strength is the main controlling factor, and when the softening characteristics of geomembrane-clay interface are obvious, the influence of peak strength and residual displacement is more obvious. abstract study on the internal force of geomembrane of landfill in heavy metal contaminated area estudio sobre la fuerza interna de la geomembrana del relleno sanitario en un área contaminada con metales pesados issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n1.85231 con el fin de analizar el esfuerzo de tensión de la geomembrana (una lamina geosintética que busca evitar la migración de contaminantes al suelo) en el sistema de control de infiltración de vertederos en áreas contaminadas con metales pesados bajo la acción del peso del vertedero suprayacente, en este documento se estudia la fuerza interna de la geomembrana en los vertederos. en primer lugar, de acuerdo con el análisis de fuerza interna del modelo de la geomembrana se obtiene la ecuación básica de la lámina, y la ecuación se analiza por solución numérica de diferencia finita para medir su fuerza interna en el relleno sanitario. el método puede analizar tres estados de tensión de la interfaz de geomembrana-arcilla en diferentes posiciones: estado elástico, estado de reblandecimiento y estado residual, lo que hace que el análisis de tensión de la geomembrana en la pendiente del vertedero sea más razonable. el análisis paramétrico muestra que cuando la pérdida de resistencia y la diferencia entre el desplazamiento residual y el desplazamiento máximo son pequeñas en la etapa de reblandecimiento, la resistencia máxima de la interfaz geomembrana-arcilla tiene poca influencia en el esfuerzo de tensión máximo de la geomembrana, y su resistencia residual es el principal factor de control; y cuando las características de ablandamiento de la interfaz geomembranaarcilla son obvias, la influencia de la resistencia máxima y el desplazamiento residual es más obvia. resumen record manuscript received: 07/07/2019 accepted for publication: 13/11/2019 earth sciences research journal earth sci. res. j. vol. 24, no. 1 (march, 2020): 111-118 hongzhou zhang institute of architectural civil engineering, langfang normal university, langfang, 065000, china * corresponding author: zhanghongzhou@lfnu.edu.cn so il s 112 hongzhou zhang introduction geomembrane is one of the most important components of bottom liner system and top cover system in modern sanitary landfill. it plays an important role in preventing waste leachate from polluting groundwater and protecting the surrounding environment. with the increase of landfill height, the geomembrane in heavy metal contaminated area will produce tensile stress under the action of overlying heap weight. in order to increase the interface strength, reduce the tension stress in geomembrane and improve the stability of landfill interface, rough geomembrane is more and more used in landfill’s slope liner system. according to the indoor shear test results, the interface of geosynthetics-geosynthetics and geosynthetics-clay presents obvious strain softening characteristics. therefore, the key to calculate the tensile stress of geomembrane is to select the strength parameters. at present, the conventional limit equilibrium method cannot solve the tension stress of geomembrane in the state of softening interface. (1) the failure process of polyurea composite impervious layer in shrinkage joint is analyzed by using v-zone model (czm). based on the loaddisplacement-deformation data obtained from the debonding test of impervious layer, the interfacial stiffness and viscosity coefficient of the bonding element of the bonding layer are calibrated. in the numerical example, the critical load of coating debonding is determined, and the influence of the impervious layer’s thickness of polyurea composite on the critical load of debonding is analyzed. however, when water pressure is applied to the open shrinkage joint, the thickness of the coating, the elastic modulus and the shear stress of the bonding parts will affect the debonding length of the coating, which will cause water seepage in the pit and depression areas. (2) a generalized version of the model is created by regression fitting all size categories of the analysis unit at a given research site. the root mean square error of the corrected pic estimates ranges from 1.5% to 10.7%; these values are much better than the rmse values of the uncorrected pic estimates. for calibrated and uncalibrated pics, the accuracy of estimation increases with the increase of the size of the analysis unit. however, the unit size, geographic location or land’s cover data set coefficients of landfills are similar, which also leads to a significant decline in the performance of the model when applied outside the development area. (3) based on the saturated-unsaturated seepage finite element theory, the three-dimensional seepage field of a typical earth-rock dam with random distribution of impervious geomembrane defects is simulated. considering the number, type, location, size and distribution of imperfections in geomembrane, the seepage quantity of imperfections in geomembrane and the variation law of infiltration surface behind the membrane are mainly analyzed, so as to analyze the internal force of geomembrane according to the law of analysis. however, the interface strength model ignores the transition stage of softening, which is inconsistent with the actual situation. therefore, it is necessary to adopt a more reasonable analysis model for the interface softening of geomembrane to analyze the tension stress of geomembrane on landfill slope. in view of the above problems, this paper studies the internal force of geomembrane on the basis of clay interface, taking landfill as the core. while describing in detail, the model of geomembrane is constructed and its internal force is analyzed. the influence of the height, slope gradient and anchorage length of geomembrane on the tensile stress of geomembrane is analyzed by the cable fractional solution. geomembrane model as shown in figure 1, geomembrane is an important part of landfill liner system (zhang, weng & shao, 2017). in order to reduce the tension stress of geomembrane, an anchorage ditch is designed at intervals on the slope. the unbalanced shear stress on the upper and lower surfaces of geomembrane results in large tensile deformation of geomembrane (ru et al., 2018). the force acting on the geomembrane element is shown in figure 2. generally speaking, the shear strength of the interface between geomembrane and bottom clay is the weakest in the cushion system (lin et al., 2017), and the accumulation of shear stress will form a larger tensile stress in the anchorage ditch. this is the focus of this study. some variables used in the analysis are shown in figure 3. the internal force of geomembrane in figure 3 is characterized by steep slope, and the overall stability of 1:3.3 rockfill dam slope is sufficient. however, the anti-sliding stability of the geomembrane and the cement figure 1. typical garbage liner system figure 2. stress analysis of geomembrane element figure 3. variable diagram 113study on the internal force of geomembrane of landfill in heavy metal contaminated area hyperbola: b m= 1/  , m is the ultimate shear stress of the contact surface of the geomembrane., and τ σ ∅m n= tan 1. generally speaking, the foundation medium under geomembrane is clay cushion (wüthrich et al., 2018). assuming that its shear modulus is g and its thickness is d, the clay cushion can be used to estimate ks: k g ds = (2) when formula (2) is substituted for formula (1) and there is no dimensionalization, it can be obtained that: d u dx ru u a bx a b x 2 2 1 11  + = + + (3) where u h h l a k h r m k u u u x x r l k e x m s m s t = = = = = = ( ) τ − − ∅ β = ; ; ; * *; sin 1 2 1 2 1 2 −− − ∅− ∅ β ∅ ∅ ∅ ∅ 1 2 1 2 1 2 2 1 k r a k h b x x l ( )( ) = +( ) sin cos tan sin cos sin tan* == +( )( )                kx lcos sin sin cos tan tan2 2∅ ∅ ∅− ∅ β ∅ (4) in the case of small slope angle, assuming that the vertical and horizontal directions are the principal stress directions of refuse dump (jin et al., 2017), the normal stress and active shear stress at the interface are respectively as: σ θ θ γ τ σ −σ − γ θ n x u v n x k h k h = +( ) = ( ) ( ) cos sin sin sin 2 2 1 2 2 1 2 1 2 (5) because the tensile stiffness of geotextile is one order smaller than that of geomembrane, it can be assumed that the shear stress at the geotextilegeomembrane interfaceis in the limit state, and the shear stress is as follows: τ σ ϕu t n tc= + tan (6) where, ct and  1 are the interface strength parameters of geotextilegeomembrane (choi et al., 2017). the active shear stress formula (6) and the interfacial shear stress formula (4) of geomembrane-clay are the smaller values of the overlying shear stress of geomembrane. most interface tests show that the strength of geomembrane-clay interface presents obvious softening characteristics (tang et al., 2017). in this paper, the tensile stress of geomembrane on landfill slope is analyzed by using the geomembrane-clay interface softening model as shown in figure 5. when the relative displacement of the interface reaches the ultimate displacement up, the maximum shear strength p is excited, and the shear strength will continue to decrease with the increase of the relative displacement. when the relative displacement reaches the residual displacement ur, the shear strength will stabilize at the residual strength r (if up = ur, the ideal elastic-plastic solution of the softening section cannot be considered). the shear strength and relative displacement of the plane can be divided into three stages: elastic stage, softening stage and residual stage. the interfacial shear strength conforms to the mohr-coulomb failure criterion (mcclung & ibáñez, 2017) shown in figure 6, in which cp, p and cr, r are the peak strength parameters and residual strength parameters of the geomembrane-clay interface, respectively. for the three stages of relative displacement of the above interface, formula (7) can be expressed as follows: mortar surface at its bottom depends on the bonding force between the membrane and the mortar (huang et al., 2017). the bonding force between the membrane and the mortar is greater than 0.1 mpa if the geomembrane is laid immediately before the cement mortar hardens and the sand bag is pressed on the membrane, so that the safety of the anti-sliding stability is sufficient. if only the friction force is used, the safety is insufficient. the anti-sliding stability between reinforced concrete slope protection and geomembrane depends on the cohesive force of asphalt (xu et al., 2017), which can reach 0.5 mpa, providing a high safety factor. usually in the design and construction, the composite geomembrane has not been produced yet, so the polyvinyl chloride film is used. some engineers attribute the landfill site to concrete face rockfill dam, which is very inappropriate, because the reinforced concrete mesh is not divided into blocks and without water stop sheet, it will inevitably lead to crack leakage. polyvinyl chloride (pvc) is an impervious body, so it is appropriate to belong to geomembrane rockfill dam. if it is attributed to the mixed dam type of geomembrane and concrete face slab, this dam type saves the expansion joint water stop, reduces the construction difficulty greatly, saves funds, and avoids the common problem of expansion joint leakage. analysis of internal force of geomembrane figure 4 is a schematic diagram of the yard structure. in this paper, all additional loads on geomembrane are regarded as a whole and expressed as a single material. assuming that there is no slip on the boundary surface of the upper layer of geomembrane, i.e. u > 1 (u, 1) is the friction angle on the contact surface of the upper layer and the lower layer of geomembrane respectively. the interaction between the interior of garbage and its intermediate contact layer such as the overlaying soil, is ignored, so that the estimated results tend to be safe and the analysis process will be obviously simplified. figure 4. diagram of landfill structure fundamental equation of the model the shear stress on the contact surface of the geomembrane is dependent on the shear-displacement relationship between the geomembrane and the soil (tang et al., 2017). it can be observed from drawing and shear tests that the shear stress  on the contact surface increases nonlinearly with the increase of shear displacement u. in this paper, a simplified hyperbolic relationship is adopted. 1 = + u a bu (1) in the formula, 1 is the shear stress of the contact surface of geomembrane’s lower layer ; a ks= 1/ , ks is the initial shear stiffness of 114 hongzhou zhang d u dx te ku u u d u dx te k u u u u u u p p p u p 2 2 2 2 1 1 = ( ) ≤ = ′ ( ) +    ≤ −τ − τ −τ < , ) , rr d u dx r u rte u u 2 2 1= ( )            τ −τ >, (7) the solutions of formula (7) are respectively u a e a e u u u b bx b bx k u k u u u u ax ax p p u p p r = ≥ = + + ′ + ′ ⋅ ≤ 1 2 1 2 , , cos sin − τ −τ < == ( ) + +          τ −τ >r u rte x c x c u u 2 2 1 2, (8) in the formula, the constants a1, a2, b1, b2, c1 and c2 depend on the boundary conditions; a k te= / and b k te= ′ / are stiffness ratio constants. the boundary conditions at both ends of geomembrane are respectively u du dxx x l  = = = = 0 0 0; (9) because the shear strength and relative displacement of geomembraneclay interface can be expressed in three stages (li, wu & choi, 2017), the geomembrane interface can also be divided into three stages from anchoring end to free end: elastic stage u up≤( ), softening stage u u up r ≤( ) and residual stage u ur( ). in this paper, the elastic stage length is defined as lp and the softening stage length is lt. when x = lp, u = up; when x = lp+lt , u = ur. according to the boundary conditions of formula (9), formula (8) can be rewritten as u e e e e k e e k p u u al ax u p u al ax al al u p p p p = +( ) + ( ) ( ) + τ −τ τ τ −τ −τ − τ − − − ,, 0 ≤ x lp< (10) u bl k bl b x l k u a p u t p p u = ( ) ( ) ′ ( ) + ′ + τ − − − − 1 cos cos sin τ − − < p p u p p p t k b x l k l x l l ′ + ( ) ( ) ′ ≤ +            cos (11) u te x l l te x l lr u p r r u p t= ( ) +( )    ( ) ⋅ +( )    τ −τ − − τ −τ − 2 2 2 + + ≤u l l x lr p r, < (12) it is concluded that the tensile stress of geomembrane is as follows. t te du dx a p u u al ax u p u p = = +( )τ −τ − − − − aal ax al al p p p pk e e x l ( ) ( ) ≤            − − − <0 (13) t te du dx b bl k bl b x l b r u p u t t p p u = = ( ) ( ) ′ ( ) + ( τ −τ − τ −τ − − τ −τ cos cos cos )) ( ) ′ ≤ +             sinb x l k l x l l p p p t − < (14) t te du dx te x l l l x l r u p t = = ( ) ( ) + ≤      τ −τ − < , (15) in the formula, lp and lt can be obtained by the internal force coordination conditions of elastic and transitional stages and the internal force coordina tion conditions of transitional (fils et al., 2018) and plastic stages, respectively. a e e k e e b bl p u al al u al al r u p u p p p p τ −τ τ − τ −τ − τ −τ − − ( ) +( ) + ( ) ( ) ( ) 2 cos tt t pk bl x l ′ =         sin , ´ (16) figure 5. softening model of geomembrane-clay interface figure 6. mohr-coulomb criterion for geomembrane-clay interface 115study on the internal force of geomembrane of landfill in heavy metal contaminated area b bl k bl te l l l x l l r u t p u t r u p t p t τ −τ − τ −τ τ −τ − ( ) ( ) ′ = ( ) +( ) = + cos sin ,      (17) finally, the values of lp and lt obtained by formula (16) and formula (17) are substituted for formula (11)-(13) to obtain the tensile stresses at all points on geomembrane. the tensile stress at x = 0 is the maximum tensile stress of geomembrane (zhang et al., 2018), and its expression is as follows: t ate e e k e e p u u al al al al p p p p max = ( ) + +( ) ( ) 2 τ −τ τ − − −´ (18) numerical solution of finite difference because formula (18) is a non-linear differential equation, the finite difference method is used to solve it numerically. dimensionless geomembrane length is divided into n elements, then the number of nodes is n i n+( ) = …1 0 1 2, , , , , , and ∆x n= 1/ , and a virtual node (n + 1) is added to the free end of geomembrane . for the second order differential, the difference scheme is used: ∆ ∆ ∆ 2 2 1 1 2 2u x u u u x i i i= + ( ) = + (19) for first order differential, the difference scheme is at node 1 = 0, ∆ ∆ ∆ u x u u u x = + 2 1 04 3 2 (20) at node i n= …1 2, , , , ∆ ∆ ∆ u x u u x i i= +1 1 2   (21) it can be obtained by finite difference method and boundary conditions at both ends of geomembrane. t n l r u u i n l r u u i n i m m i i = ( ) = ( ) = …       + τ − τ − 2 4 0 2 1 2 1 2 1 1 , , , , , (22) based on the above method, the tension and displacement of geomembrane can be obtained by numerical iteration (su et al., 2019), and the influence of various parameters of landfill slope on the displacement and tension of geomembrane can be analyzed by numerical method. in this paper, the main parameters affecting the displacement and tension of geomembrane, inclination q, length l, elastic modulus e and height h, are studied. the results are shown in figure 7-10. from figure 7, it can be seen that the displacement and tension of geomembrane are non-linear along its length. when other conditions are the same, both displacement and tension increase with the increase of inclination angle (zhang, dai, liu, et al., 2017), and when q is small, the change of q has a greater impact on the tension and displacement. from figure 7 to 10, it can be seen that the influence of the length, modulus of elasticity and height of geomembrane on the displacement and tension of geomembrane is similar to that of dip angle q. the peak strength τ σ ϕp p n pc= + tan of the interface between geomembrane and clay is the maximum strength that the interface can exert when the relative displacement of the interface is up, and the residual strength is formed when the relative displacement of the interface is greater than the residual displacement ur due to the rearrangement of clay particles on the interface or the polishing of the surface of geosynthetics. the figure 7. the influence of inclination angle q on displacement x and tension t of geomembrane figure 8. the influence of length l on displacement x and tension t of geomembrane 116 hongzhou zhang peak strength p and residual strength r of rough geomembrane-clay interface are usually quite different. therefore, the selection of interface strength is one of the key factors that affect the safety of geomembrane. when the geomembraneclay interface enters the residual state (figure 11), the shear stress of the geomembrane-clay interface from anchorage to free end is elastic, softening and residual respectively. figure 11 shows that the smaller the residual displacement ur is, the easier the residual strength of the geomembrane-clay interface will be. therefore, the influence of residual displacement ur and peak shear strength p on the tensile stress of geomembrane will be smaller (figure 12). at this time, the exertion of surface shear stress under geomembrane depends on the residual strength of the interface, when ur is larger, the length of the interface between geomembrane and clay in the softening zone is longer, and the influence of ur and p on the tensile stress of geomembrane is more obvious.in this case, if the softening of the interface strength between geomembrane and clay is not considered (luo et al., 2017), the maximum tensile stress of geomembrane obtained by the ideal elastic-plastic method according to the peak strength or residual strength of the interface will be inconsistent with the actual situation. as shown in figure 13, it is not safe to calculate the maximum tensile stress of geomembrane by the ideal elastic-plastic method (erhart & hirche, 2017). because the difference between residual displacement ur and peak displacement up is small, the influence of peak shear strength p of geomembrane-clay interface on the maximum tensile stress of geomembrane in figure 13 is also small. figure 14 and figure 15 show the tensile displacements and stresses at different positions of geomembrane under different residual strengths. it can be seen that the residual strength seriously affects the tensile displacements and stresses of geomembrane. figure 11. curve of shear stress variation with distance under overburden figure 12. effect of residual displacement on the maximum tensile stress of geomembrane figure 9. the influence of elastic modulus e on displacement x and tension t of geomembrane figure 10. the influence of height h on displacement x and tension t of geomembrane 117study on the internal force of geomembrane of landfill in heavy metal contaminated area figure 13. effect of peak shear strength on the maximum tensile stress of geomembrane figure 14. curve of relative displacement variation with distance figure 15. curve of tensile stress in geomembrane with distance simulation experiment the dam body is raised to 40m elevation at the original refuse dam and continued to be landfilled to 120m elevation. a horizontal anti-seepage system is added to the reservoir area in combination with the closure system of the first phase of the project and a leachate drainage system is added to the reservoir area of the first phase of the project. some of the original sewage treatment tanks are used, and a leachate regulating tank is expanded downstream of the original sewage interception dam. horizontal anti-seepage system consists of protective layer, leachate collection layer, main anti-seepage layer of geomembrane, secondary anti-seepage layer (gcl geosynthetical bentonite cushion) or clay liner (ccl), strengthening layer, clay protective layer, strengthening layer, gas conducting layer and isolation layer from top to bottom. the calculation parameters are as follows: the geotechnical composite drainage meshgeomembrane strength parameters are c = 0kpa  = 13, the interface parameters of geomembrane-geosynthetical bentonite cushion are cp = 2.2kpa, p = 21.6, cr = 1.0kpa, r = 12.8, the geomembrane-clay interface parameters are cp = 0kpa, p = 11, cr = 0kpa, r = 6.5 respectively, the landfill weight is maw = 10.2kn/ m3 and the slope gradients are  = 14, h = 12 ~ 40m, l = 10m, up = 0.012m, ur = 0.016m, t = 0.0015m, e = 180mpa kx = 0.56. the maximum tensile stress of geomembrane varies with the height of landfill as shown in figure 16. when the geomembrane is gcl, there is no tensile stress in the geomembrane because the shear strength of the surface under the geomembrane is greater than that of the surface above. when the geomembrane is lined with clay, the lower strength of the geomembrane-clay interface makes the geomembrane have great tensile stress. figure 16. the influence of landfill height on the maximum tensile stress of geomembrane conclusion based on the softening model of interface strength between geomembrane and clay, the equilibrium equation of geomembrane force is established, and the analytical solution of tension stress and displacement of geomembrane is obtained. through further parameter study, the following conclusions are drawn: (1) in the strength softening model of geomembrane-clay interface, the longer the softening section is, the greater the influence of the peak strength and residual displacement of the interface on the tensile stress of geomembrane is. on the contrary, the residual strength of the interface is the main controlling factor of the maximum tensile stress of the geomembrane when the softening stage is short. (2) when the strength of geotextile-geomembrane interface is close to or less than the residual strength of geomembrane-clay interface, the tensile stress of geomembrane is very small or even not. (3) slope gradient, anchorage length and height of overlying pile of geomembrane have great influence on the maximum tensile stress of geomembrane. (4) the maximum tensile stress of geomembrane can be effectively reduced by using rough hdpe geomembrane with high interfacial strength, choosing appropriate geotextile, reducing the anchorage length of geomembrane and limiting the height of pile on geomembrane. acknowledgments: langfang science and technology research and development program: landfill slope geomembrane internal force research in langfang area (2016013105) 118 hongzhou zhang references cen, w., du, x., geng, l., & he, h. 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(2017). unconfined seepage analysis using moving kriging mesh-free method with monte carlo integration. transport in porous media, 116, 163-180. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.108562 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 29 38 g eo lo g y abstract rakhi nala (gorge) section of eastern sulaiman range, pakistan hosts world-class geological sections from the cretaceous to the recent. here, late eocene kirthar formation is having a conformable lower contact with the greenish grey massive shale of the baska member of the ghazij formation. the rusty beds of the oligocene chitarwatta formation are overlying the kirthar formation. to understand the detailed microfacies and depositional setting, detailed fieldwork was carried out in the kirthar formation in the outer sulaiman foldbelt. the late eocene kirthar formation includes habib rahi limestone, domanda shale, pirkoh limestone, and drazinda shale. domanda shale member was deposited in transgression before habib rahi limestone member was deposited. it was followed up by the deposition of the second member pirkoh limestone and marl member. habib rahi limestone, pirkoh limestone, and marl member have catch-up-to-keep-up deposition with rise and fall of sea level. some facies of drazinda and domanda shales represent a restricted setting supported by the presence of gypsum, while deep sea facies were also identified. this study will provide a guide for evaluating sedimentological concepts and understanding complex facies in highly oil and gas-prone stratigraphic sequences especially in the eastern tethys region. sedimentological and sequence stratigraphic analysis of late eocene kirthar formation, central indus basin, pakistan, eastern tethys umair sarwar1, shahid ghazi2, syed haroon ali3*, mubashir mehmood4, muhammad jehangir khan5,6, arslan zaheer2, syed jawad arif7 1. planning and information directorate, geological survey of pakistan quetta, pakistan. 2. institute of geology university of the punjab, lahore, pakistan. 3. department of earth sciences, university of sargodha, sargodha, punjab, pakistan 40100. 4. department of earth, environmental and resources sciences, university of naples federico ii, naples, campania region, 80126, italy. 5. key laboratory of ocean and marginal sea geology, south china sea institute of oceanology, chinese academy of sciences, guangzhou, 510301, china. 6. university of chinese academy of sciences, beijing 100049, china. 7. geoscience advanced research laboratories, geological survey of pakistan, islamabad, pakistan. *corresponding author: haroon.ali@uos.edu.pk https://orcid.org/0000-0002-8619-7005 how to cite this item: sarwar, u., ghazi, s., haroon ali, s., mehmood, m., jehangir khan, m., zaheer, a., & jawad arif, s. (2024). sedimentological and sequence stratigraphic analysis of late eocene kirthar formation, central indus basin, pakistan, eastern tethys. earth sciences research journal, 28(1), 29-38. https://doi.org/10.15446/esrj. v28n1.108562 keywords: eocene, lithostratigraphy, sea level changes, carbonates, eastern sulaiman range, oil and gas exploration. análisis sedimentológico y de secuencia estratigráfica de la formación kirthar, del eoceno tardío, cuenca central indus, pakistán, oeste de tetis. palabras clave: eoceno; litoestratigrafía; cambios en el nivel del mar; carbonatos; montañas de sulaiman; exploración de gas y petróleo. resumen la sección del cañón de rakhi nala en las montañas del este de sulaiman, pakistán, hospeda secciones geológicas de clase mundial desde el cretácico hasta el presente. por ejemplo, la formación kirthar del eoceno tardío presenta un contacto en la parte baja con un shale masivo gris verdoso del miembro basaka, formación ghazij. los lechos oxidados de la formación chitarwatta de oligoceno se sobreponen a la formación kirthar. en este estudio se realizó un detallado trabajo de campo en la formación kirthar, en el pliego extremo de sulaiman, para entender la configuración de microfacies y deposicional. la formación kirthar se caracteriza por la piedra caliza en habib rahi, esquistos en domanda, piedra caliza en pirkoh y esquistos en drazinda. los esquistos en domanda se depositaron en transgresión antes de las calizas de habib rahi. después vino la deposición del segundo miembro de calizas en pirkoh en el miembro marl. las calizas de habib rahi, de pirkoh y del miembro marl tienen deposiciones configuradas por las subidas y bajadas del nivel del mar. algunas facies de los esquistos de drazinda y domanda representan una configuración restringida apoyada por la presencia de yeso, y que también fue identificada en las facies a mayor profundidad en el mar. este trabajo provee una guía para evaluar conceptos sedimentológicos y comprender las facies complejas en secuencias estratigráficas altamente propensas en petróleo y gas, especialmente en el oriente de la región de tetis. record manuscript received: 27/04/2023 accepted for publication: 23/04/2024 30 umair sarwar, shahid ghazi, syed haroon ali, mubashir mehmood, muhammad jehangir khan, arslan zaheer, syed jawad arif 1. introduction in sedimentary geology, the word facies is used to describe the dimensions, sedimentary formations, particle kind and sizes, color and biological characters. these characters are further used to describe a formation (middleton and hampton, 1973; hou et al., 2024). on the other hand, lithofacies is the term for a description that only includes the characteristic physical and chemical properties of a rock (abdel-fattah et al., 2022). the physical and chemical mechanisms of sediment movement and deposition influence the lithofacies characteristics (pettijohn, 1975; pettijohn et al., 2012; allen, 1982; nichols, 2009). to describe the paleoenvironment during the deposition of the eocene kirthar formation, various types of facies were recognized during interpretation and are examined in depth. these facies were characterized based on the variability, each of which is representative of specific environmental fluctuation. the existence of these natural variations in the sedimentary record of any formation makes it possible to describe it concerning its depositional environment (miall, 2022). this paper extensively examines the eocene succession (kirthar formation) in the eastern sulaiman ranges and explains the findings of sedimentology and sequence stratigraphic analysis (figure 1). the sulaiman fold belt is in a north-south direction, on the eastern edge of the sulaiman lobe and spans 400 km from rajanpur to south waziristan. its height rises to a maximum of 3440 meters in the north and drops to a maximum of 1600 meters in the south. it is roughly 20–25 km wide (kazmi and jan, 1997). the kirthar formation is well exposed in the rakhi nala section, which has been chosen for this study (khan et al. 2021; figure 1). it is located about 40 kilometers southsouthwest on gaj-fort munro road, dera ghazi khan (n-70). the cretaceous to recent sedimentary succession is exceptionally well exposed in this area, offering geologists a distinctive research opportunity. the presence of fossils in this area, particularly dinosaur footprints, microfossils, as well as its outstanding sedimentary exposure and features, have drawn geoscientists from all over the world (malkani, 2023). the rakhi nala portion is also significant because it serves as a reference section for numerous formations in the sulaiman range. this study aims to conduct a microfacies analysis, depositional and sequence stratigraphic model of the kirthar formation. the members of the kirthar formation are also the most significant in terms of hydrocarbon resources in the sulaiman foredeep basin. figure 1. (a) tectonic framework of the of pakistan, inset into sulaiman range, (modified after stonely, 1974; powell, 1979; kazmi & rana, 1982), (b) generalized stratigraphic column of the area (abbas, 1999), (c) location and study area (after khan et al. 2021), showing the location of lobate-shaped sulaiman range on the western edge of the indian subcontinent (mehmood et al., 2023). 2. geologic setting the eastern sulaiman range is situated in the lower indus basin, pakistan in the dera ghazi khan region. the lower indus basin has been divided into northern and southern subbasins by jacobabad high. sedimentary rocks with age ranging from the cretaceous-recent are present in the eastern sulaiman range. the sulaiman range is positioned in the nw indian plate and is placed along the zone of compression. in contrast, the salt range, which is situated in a region is associated with the himalayan collision (ghazi et al., 2015; 2020; ahmad et al., 2020; gong et al., 2021; javed et al., 2021). according to a study by allemann (1979), the afghan block and the indian plate are compressed together to create the foredeep basin in the east of the sulaiman lobe (khan et al., 2021). arcuate folds and faults east to west which diverts from north to south near the belt’s edge make up most of the sulaiman fold belt structure (khalid et al., 2012). imbricate faults can only be detected from the ground in the northern region of the sulaiman foldbelt (hsc, 1961; kazmi and rana, 1982). they disappear out at the foremost edge of the subsurface fold belt. there are signs of neotectonics activity along tear faults like the kingri fault, including offset fold axes, faults, raised and tilted gravel beds, and huge bends along the course of streams (abdel-gawad , 1971; rowlands, 1978; khan et al., 2021; figure 1). the sulaiman fold belt’s sedimentary succession is separated into three main groups, each of which has a unique tectonic significance. molasses deposits from the late miocene to the present are represented by units 1 and 2, khojak flysch from the late eocene to the early eocene is represented by unit 3, and marine rocks from the permian to the eocene are represented by unit 4 (lawrence and khan, 1990). part of the marine crust was propelled upward onto the maastrichtian shelf, forming the muslim bagh ophiolite in the zhob valley (abbas and ahmad, 1979). the sulaiman fold belt contains relatively high bouguer gravity anomalies despite having a deep sedimentary section, indicating that it covers a sizable portion of the crust (lillie et al., 1989; jadoon et al., 1994; figure 1). a rich succession of sedimentary rocks dating from the cretaceous to the present can be found in the rakhi nala section, eastern sulaiman range. numerous authors have described the stratigraphy of the sulaiman basin (figure 1d), including qureshi et al. (1987), raza et al. (1989), shah (2001, 2009), kazmi and abbasi (2008), and malkani (2010). 3. methodology the majority of the information needed to achieve these goals was gathered through fieldwork in pakistan’s rakhi nala section of the eastern sulaiman range. a variety of laboratory and field techniques were used to obtain a thorough image of the depositional environment and sedimentation patterns. before beginning fieldwork, a thorough understanding of the previous work was coordinated to familiarize oneself with the geological setting of the project region. the rakhi nala’s lithostratigraphic sections containing numerous rock units were meticulously examined and measured. to thoroughly analyze the microfacies assemblages of the formation, microscopic studies were carried out. 3.1 stratigraphic section measurement fieldwork carried out in the eastern sulaiman range served as the foundation for this study project. both the samples and the measurements were gathered. to prove that the formations exist, pictures and measurements of them were obtained. rocks that were hard and compact had to be shattered with a geological hammer to acquire representative samples. the study area’s lithological variability was employed to direct sample collection. these samples were collected in the field through a variety of techniques, including contact sampling, tagging, labeling, numbering, and proper storage. using a gps device, longitude and latitude were verified. key features in the field were also photographed. after that, samples were taken to the laboratory for evaluation. these variations were documented in the form of a field log. throughout the entire area, visibility is superb. where streams and roads have dug through the rock, excellent outcrops can be seen. during this fieldwork, bedding, lithology, rock character and sedimentary structures in the stratigraphic layers were noted. all of the samples were collected from this outcrop. 31sedimentological and sequence stratigraphic analysis of late eocene kirthar formation, central indus basin, pakistan, eastern tethys 3.2 measurement and sampling collection the area immediately surrounding the selected piece is also examined and measured. sequence borders, transgressive and regressive surfaces, and measuring the thickness of particular sequences in the rakhi nala section through sampling were given priority. samples from each lithofacies are collected to comprehend the lithological variations recorded in each member of the kirthar formation. the thicknesses of the exposed rock are routinely sampled at regular intervals. transitions in lithology are plotted and sampled to build a comprehensive database of facial traits, in the rakhi nala. about twenty-eight samples were collected for testing at the lab. nineteen samples were selected for thin sections and were subjected to microfacies analysis. to determine the microfacies, the microfossils, matrix kinds, cement types, and clasts were examined. 4. results 4.1 lithostratigraphy lithostratigraphic relationships among stratigraphic layers have been investigated. lithology was used to establish this connection. the rock types being researched are marl, gypsiferous shale, limestone, and shale. within the rakhi nala, the lithofacies of the late eocene kirthar formation, have been studied (figure 2). these formation members have a total measured thickness of 561 meters (figures 2 to 4). figure 2. (a), a conformable contact between baska shale and habib rahi limestone member (hrlm), (b) transitional contact between domanda shale member and hrlm, (c) transitional contact between domanda shale member and pirkoh limestone and marl member (plmm), (d) transitional contact between drazinda shale member and pirkoh limestone and marl member, (e) conformable contact between drazinda shale member and chitarwatta formation, (f) medium bedded argillaceous limestone from hrlm. 4.1.1 kirthar formation blandford, 1876 used the name “kirthar,” which derives from the kirthar range, which is located south of the sulaiman range. its reference areas are rakhi nala, zinda pir, and spintangi george, which may be found in the eastern and western sulaiman ranges, respectively. the kirthar formation’s dominant lithologies are interbedded of limestone and shale, with only a small quantity of marl. the limestone is a light grey to cream tint that ages to become grey, brown, or cream. the limestone contains thick to massive bedding. shale is grey, orange, yellow, and olive. it is silky and has an earthy gloss. the southern indus basin has a large exposed portion of this formation. it measures 1270 m thick at the gaj river portion. in contrast to its transitional contact with the baska shale, the formation’s contact with the chitarwatta formation is unconformable (figures 2a and e). there is a healthy fauna in this formation. it was created between the early eocene and the early oligocene. it is middle to late eocene in age in the zinda pir and rakhi nala region (latif, 1961; samanta, 1973; warraich and natori, 1997). all four of the kirthar formation’s components are superbly exposed in the rakhi nala region. 4.2 lithostratigraphy of kirthar formation in rakhi nala section member of the kirthar formation from old to young: • drazinda member • pir koh limestone and marl member • domanda member • habib rahi limestone member 4.2.1 habib rahi limestone member habib rahi limestone is the official name given to it by pakistan’s stratigraphic committee. tainsh et al. (1959) neglected to annotate the type location. this member’s limestone features fine, platy bedding. habib rahi limestone has nodular chert and thin laminations. there is also some interbedded shale and marl, but an almost negligible amount is found in this member (figure 2f). massive shale beds are also located at the top (figure 3h). in the bottom portion of the limestone strata of this member, assilina is overly abundant. in the rakhi nala portion, the habib rahi limestone member is 38 meters thick. in conformity with the baska formation, this component overlies it (figure s. 2a, b, & f and figure 3). figure 3. (a) shale interbedded within the limestone of hrlm, (b) massive shale at the top of hbrlm, (c) limestone interbedded in non-gypsiferous shale of domanda member, (d) marl and carbonaceous marl in the upper part of plmm, (e) fossiliferous domanda shale member, (f) highly gypsiferous drazinda shale. 4.2.2. domanda member the type locality is dera ismail khan road, which is near fort sandeman and situated west of domanda (lat. 31°35′30′′n: long. 70°12′e). hemphill and kidwai (1973) suggested the name “domanda shale member”. it was dubbed “lower chocolate clays” by eames in 1952. the domanda formation is the official name given to it by the pakistani stratigraphic committee. this 32 umair sarwar, shahid ghazi, syed haroon ali, mubashir mehmood, muhammad jehangir khan, arslan zaheer, syed jawad arif formation contains claystone and a lesser amount of thinly bedded limestone with shell fragments. in some locations, non-gypsiferous shale contains limestone interbedded inside it (figure 3c). additionally, there is fossiliferous shale (figure 4e). the colour of the claystone ranges from yellow to greenish brown to chocolate. there are also a few weakly bedded, silty claystone beds. gypsum slices make up the lowest portion of this component. the rakhi nala part of the sirki member is 225 m thick. its transitional and conformable interaction with the underlying habib rahi, pirkoh limestone (figures 2b, c and figure 5). 4.2.3 pir koh limestone and marl member the term “pirkoh limestone member” was first coined by hemphill and kidwai (1973). it was referred to as a “white marl band” and a “pirkoh limestone and marl member” by eames (1952). pirkoh anticline (lat. 29° 7′ n: long. 69° 8′ e) is the name of its type locality. the predominant lithologies of this deposit are limestone and marl. the limestone has interbeds of marl and ranges in hue from grey to chalky white to reddish (figure 3d). the distinguished feature that marked this formation are cliffs found in this formation. the pir koh limestone and marl member are 13 meters thick in the rakhi nala region (figure 4). this contacts the domanda member below and the drazinda shale above (figures 2c, d). 4.2.4 drazinda shale member the name drazinda shale came from a drazinda village. the stratigraphic committee of pakistan has given this name official status. hemphill and kidwai (1973) suggested drazinda village as the species’ type locality (lat. 31°46′n: long 70°09′e). this component has claystone, which settles into some strata as silt. the claystone unit comes in the hues of grey, brown, green, and chocolate. this member contains mollusks. there are also some limestone beds, albeit in smaller numbers. these beds are teeming with foraminifera (figure 3f). the rakhi nala part of the drazinda member is 285 m thick. the upper contact of the unconformable kirthar formation is situated in the “drazinda member” and underlying sandstone sequences of the chitarwatta formation. one of the largest unconformities in the basin may be found in the rakhi nala section, which is marked by a varicolored conglomeratic, hematitic-bed that is rusted dark brown, contains trace fossils, and has rich hematinic and pyritic material on the surface (figures. 2d, e, and 3d, e). figure 4. lithologic log of the drazinda shale member, habib rahi limestone member and domanda shale member. figure 5. lithologic log of the pirkoh limestone member. 4.3 microfacies analysis different members of formation’s microfacies were examined, and they were compared and analyzed by using the facies zones and smf of flugel (1982) and wilson (1975). different microfacies studied in the formation are as: 4.3.1 drazinda member bioclastic wackestone to packstone microfacies (dr-1) the drazinda member’s uppermost limestone bed, which is cream in color, and its lowermost, heavily fossiliferous deposit, are examples of the microfacies. kdr-2 and kdr-8 are the sample numbers from the top and bottom, respectively, of the drazinda member. kdr-2 is 15 meters thick, while kdr-8 is 8 meters thick. the bioclastic wackestone facies include nummulites, miscella, ranikothalia, orbitolites, green algae, and occasional shell fragments (figure 6c). the microfacies represent fz-7 of wilson (1975) and smf-8 of flugel (2013). micrite makes up the matrix. there are only 9% matrix and 11% clasts, mostly bioclastic. bioclastic packstone microfacies (dr-2) limestone with a cream tint serves as a metaphor for the microfacies. bioclastic packstone microfacies are found below the bioclastic wackestone microfacies in the drazinda member of the late eocene succession. larger benthic foraminifera are represented by bioclasts that contain a significant amount of discocyclina. after discocyclina, quartz crystals are the second most important clast component. it symbolizes wilson’s (1975) fz-8 and flugel’s (2013) smf-18. additionally, there are some bioclast pieces and corallinacean algae (see figure 6b). the ratio of clasts to matrix in bioclastic packstone microfacies is roughly 33% to 67%. bioclastic mudstone microfacies (dr-3) limestone with a cream colour serves as the microfacies’ representation. the sample from this section of the drazinda member is known as kdr6b. mudstone facies are found below the bioclastic packstone facies of the drazinda member of the late eocene succession. kdr-6b has a thickness of 15 m. this microfacies represent wilson’s (1975) fz-9. bioclastic mudstone microfacies contain unidentified bioclasts and bioclastic particles (figure 6a). 98% is a matrix and only 2% is clasts, mostly bioclastic. bioclastic wackestone to packstone microfacies (dr-4) these microfacies are represented by yellowish brown colored limestone beds. there are many microfacies at the drazinda shale member’s base. the fz-6 of wilson (1975) is represented by these microfacies. micrite represents the matrix, and the sample’s grains are represented by bioclasts (figure 6d). 33sedimentological and sequence stratigraphic analysis of late eocene kirthar formation, central indus basin, pakistan, eastern tethys 4.3.2 pirkoh limestone bioclastic mudstone microfacies (p-1) two adjacent beds of cream-colored limestone serve as the symbol for this microfacies. the pirkoh limestone’s topmost layer has bioclastic mudstone microfacies. the bioclasts are composed of discocyclina, planktonic forams, and algae, and the matrix is micrite (figure 6e). the fz-2 of wilson (1975) is represented by these microfacies. the pirkoh limestone’s bioclastic mudstone microfacies have 5%–5% clasts and 95%–97% matrix (upper bed, 3% bioclasts). the matrix of the bioclastic mudstone microfacies in the lower bed. bioclastic wackestone-packstone microfacies (p-2) this microfacies has a cream colour. the bioclastic wackestonepackstone microfacies are located inside the pirkoh limestone, beneath the bioclastic mudstone microfacies. the bioclastic part of the sample is made up of discocyclina, corallinacean algae, and pieces of fractured shell, whereas the matrix is micrite (figure 6f). the fz-2 of wilson (1975) is represented by these microfacies. in the bioclastic mudstone microfacies of the pirkoh limestone, there are roughly 23% clasts and roughly 77% matrix. bioclastic wackestone-packstone microfacies (p-3) the limestone bed in this microfacies is cream in color. below the bioclastic wackestone-packstone microfacies of the pirkoh limestone is the bioclastic wackestone-packstone microfacies. the bioclastic section is made up of discocyclina and pieces of fractured shell, while the matrix is micrite. the fz-6 of wilson (1975) is represented by these microfacies. the majority of the microfacies’ bioclastic composition is made up of pieces of broken shell (figure 6g). about 10% of the pirkoh limestone’s bioclastic mudstone microfacies are made up of clasts, and the remaining 90% is matrix. planktonic mudstone microfacies (p-4) two adjacent beds of cream-colored limestone serve as the representatives for this microfacies. below the bioclastic mudstone-wackestone microfacies are the bioclastic mudstone microfacies of the pirkoh limestone. planktonic foraminifera and smaller benthic foraminifera make up the bioclastic component of the upper and lower bioclastic mudstone microfacies beds (figure 7c), respectively, while micrite makes up the matrix (figure 6h). the fz-6 of wilson (1975) is represented by these microfacies. there are roughly 5% clasts and about 95% matrix in the pirkoh limestone’s upper bioclastic mudstone microfacies. a 3% bioclastic component and 97% matrix make up the basal bioclastic mudstone microfacies bed. 4.3.3. domanda member bioclastic packstone microfacies (do-1) reddish brown limestone is representative of this microfacies. dasyclad green and red algae, tiny benthic and planktonic foraminifera, and uncommon ostracods make up the majority of the microfacies’ bioclastic component (figure 7a). the fz-7 of wilson (1975) is represented by these microfacies. micrite and rare sparite make up the matrix. members of the domanda group have bioclastic packstone-grainstone microfacies with about 40% clasts and 60% matrix. algal wackestone packstone microfacies (do-2) the reddish-brown limestone beds are hosting these microfacies. green dasyclad algae is widespread. the bioclastic part of the microfacies consists of planktonic and benthic foraminifera (figure 7b). micrite and rare sparite make up the matrix. the fz-7 of wilson (1975) is represented by these microfacies. algal wackestone microfacies have a clast content of about 20% and an 80% matrix. 4.3.4 habib rahi limestone bioclastic mudstone wackestone microfacies (h-1) beds of limestone that have weathered to a light brown tint indicate this microfacies. the bottom portion of the habib rahi limestone has mudstone microfacies. the clastic section is made up of smaller benthic forams, planktonic forams, and coralline algae, while the matrix is made up of micritic and microspar. the fz-1 of wilson (1975) is represented by these microfacies. habib rahi limestone’s bioclastic mudstone microfacies have about 17% clasts and 83% matrix (figure 6i). figure 6. photomicrographs showing the microfacies of kirthar formation (a) bioclastic mudstone, (b) bioclastic packstone, (c) bioclastic wackstone, (d) bioclastic wackestone to packstone, (e) bioclastic mudstone, (f) bioclastic mudstone, (g) bioclastic wackestone to packstone, (h) bioclastic mudstone to wackestone, (i) bioclastic mudstone with planktons. bioclastic molluscan mudstone wackestone microfacies (h-2) a bed of limestone that has weathered to a light brown tint represents this microfacies. the bed of limestone is thinly bedded. above the bioclastic mudstone-wackestone microfacies of the habib rahi limestone are bioclastic molluscan mudstone microfacies. there are bioclasts such as mollusks, globogerinids, planktonic forams, and smaller benthic foraminifera. microsparite makes up matrix. the fz-1 of wilson (1975) is represented by these microfacies. habib rahi limestone’s mudstone microfacies have about 9% clasts and about 91% matrix (figure 7d). bioclastic nummulitic packstone grainstone microfacies (h-3) a bed of limestone that has weathered to a light brown colour represents this microfacies. overlying the bioclastic molluscan mudstone-wackestone microfacies of the habib rahi lies nummulitic packstone. the clastic component of this microfacies is made up of nummulites, assilina, and shattered foraminifera fragments. the fz-6 of wilson (1975) is represented by these microfacies. this facies has a micrite as a matrix. habib rahi limestone’s mudstone microfacies contain about 45% clasts and 55% matrix (figure 7e). bioclastic assilina packstone grainstone microfacies (h-4) the limestone beds in the formation have a light brown color representing these microfacies. these microfacies are present below the mudstone microfacies. the matrix in these microfacies consists of micrite and the clast in the samples is comprised of assilina, nummulites, molluscan and other foraminiferal fragments. the fz-6 of wilson jl (1975) is represented by these microfacies. about 40% of the clasts are present and 60% is a matrix in these microfacies (figure 7a). bioclastic mudstone microfacies (h-5) limestone beds that have weathered to a light brown tint represent this microfacies. mudstone microfacies developed above the bioclastic assilina packstone and grainstone microfacies. veins, certain bioclasts, and coralline algae make up the clastic part. micrite makes up the matrix. the fz-2 of wilson (1975) is represented by these microfacies. habib rahi limestone’s mudstone microfacies have about 7% clasts and 93% matrix (figure 7g). bioclastic wackestone microfacies (h-6) the weathered color limestone bed present in the formation represents this facies. bioclastic mudstone microfacies are present below these bioclastic mudstone microfacies. the major portion of the clast belonging to coralline algae, benthic foram consists of sparite and calcite veins. the fz-1 of wilson 34 umair sarwar, shahid ghazi, syed haroon ali, mubashir mehmood, muhammad jehangir khan, arslan zaheer, syed jawad arif (1975) is represented by these microfacies. micritic matrix is commonly identified. around 18% of clasts are present in the mudstone microfacies of habib rahi limestone and around 82% of the matrix is present (figure 7h). figure 7. photomicrographs showing the microfacies of kirthar formation a) bioclastic packstone, (b) bioclastic wackestone to packstone, (c) bioclastic mudstone to wackestone, (d) bioclastic molluscan mudstone wackestone, (e and f) bioclastic nummulitic packstone to grainstone, (g) bioclastic mudstone, (h) bioclastic wackestone. 4.4 facies analysis 4.4.1 open sea facies the open sea facies is represented by the colours light grey, grey, creamy white, and brown together with a bedded to a thick-bedded sedimentary structure. this sedimentary environment’s distinctive microfacies are foraminiferal wackestone through packstone. the kirthar formation’s pirkoh limestone and habib rahi limestone members in the middle represent these microfacies. another significant sedimentological trait is a microscopic matrix. additionally, the fauna is very diverse (rankey et al., 2019; saw et al., 2019). the larger open sea shelf facies represented by discocyclina, nummulites, and assilina exhibits paleontological traits. the open-shelf facies are also characterized by a greater volume of bio-detritus (figure 8a). 4.4.2 carbonate sand shoal/platform edge facies this environment is characterized by minor marl, limestone, and argillaceous limestone lithologies that range in colour from light brown to brownish grey and are bedded to thickly bedded. the green algal packstone and bioclastic foraminiferal packstone to grainstone microfacies serve as representations of this habitat (ali et al., 2018; ali et al., 2021a). paleontological characteristics include bigger benthic foraminifera such assilina, nummulites, discocyclina, alveolinidae, etc. as well as clasts of larger and smaller benthic foraminifera, green algae, and red algae (figure 8b). 4.4.3 semi-protected marine shelf (lagoonal) facies in the field, marl and marly limestone, which is grey to light grey, serve as representations of these facies. the facies related to this habitat include foraminiferal grainstone, green algal packstone, and wackestone to packstone (bashir et al., 2021; ali et al., 2021a; ali et al., 2021b). the significant sedimentological aspect of this habitat is represented by micritic matrix. this area is distinguished by a diversified fauna in terms of paleontology. this habitat has significant dasycladacean green algae, mollusks, uncommon planktonic forams, some smaller and larger benthic foraminifera (including nummulites, assilina, and discocyclina), and echinoids (figure 8c). 4.3.4. restricted platform inner shelf facies brown to brownish grey colored, very thin to medium bedded limestone can be used to identify this facies. ostracodal grainstone and microcrystalline dolomite are associated microfacies of this environment. sedimentologically, this area is characterized by winnowed texture and moderate to good sorting. paleontologically, this environment is characterized by a constrained fauna, thick-walled ostracod abundance, and smaller benthonic forams (figure 8d). figure 8. (a) depositional model of habib rahi limestone member based on microfacies analysis, (b) depositional model of domanda shale member, (c) depositional model of pirkoh limestone and marl member, (d) depositional model of drazinda shale member. 5. discussion 5.1 facies and environment interpretation of kirthar formation a deep euphotic open shelf at a depth of 40–130 m has a diverse foraminiferal assemblage. the large, flat discoidal foraminifera, shelf foraminifera, and planktonic foraminifera make up the diverse foraminiferal assemblage (hallock and glenn, 1986). with a water depth of 40 to 130 meters, the open sea area offers a low-energy environment. these qualities are represented by sediments that have been deposited in subtidal open sea shelf habitats. the fz-2 zone of wilson (1975) and the z zone of irwin (1965) are the corresponding models. the marine water was in typical conditions. since benthic foraminifera are excellent markers of paleoecology, they can be utilized to investigate paleoecology (hayans, 1981; adams, 1989). rotaliidae, assilina, and nummulites imply moderate depth in an environment with a shallow shelf. for green algae to grow in the lower tidal to subtidal environment, a photic zone and a typical marine environment are required (less than 10 m). dasyclads typically reside at a depth of 3-5 meters, with a few species living as deep as 90 meters or even more (flugel, 1992). red algae are common in shallow to deep environments, 250 meters deep (flugel, 1992). foraminiferal bioclasts are an example of high-energy current activity. this collection of facies represents a subtidal habitat with moderate to high energy settings that is present on the platform edge, as stated in the preceding 35sedimentological and sequence stratigraphic analysis of late eocene kirthar formation, central indus basin, pakistan, eastern tethys paragraph (y zone of irwin, 1965 model and smf-12.fz-6 of wilson, 1975). these deposits show that typical marine ecosystems predominated in the photic zone, which was moderately sheltered, well-oxygenated, and located beneath the lower tidal to subtidal shallow platform edge setting. dasycladacean green algae is a sign of tropical to subtropical, calm, marine environments with low energy levels below the tidal zone (flugel, 1992). the presence of a high ratio of micritic matrix indicates the existence of a muddy substrate. this particular type of muddy ground is typical of protected lagoons (flugel, 1992; wray, 1977). in low energy (lagoonal), shallow marine, and subtidal environments, these kinds of sediments are deposited. the photic zone may have water depths of 20 meters or less. these facies correspond to wilson’s (1965) fz-7 zone and irwin’s z zone, respectively. water was a typical marine, as evidenced by the facies. grainstone facies represent deposits with moderate to high energy current-laid rates. microfossils called ostracods are quite prevalent in this facies group. ostracodes are excellent at interpreting salinities. ostracodes are particularly prevalent in brackish waters. additionally, ostracodes have been observed in hypersaline lagoons with salinity levels as high as 80% (dodd and stantan 1981). the cytheracea superfamily includes these ostracodes. cytheracea is typically thought of as the brackish water genus. the genus cyprideis, which can withstand high salinities, is characteristic of salt marshes, lagoons, and hypersaline oceans (brasier, 1980). 5.2 sequence stratigraphy of kirthar formation, rakhi nala section by combining the relative sea level changes with tertiary paleogeography, it is feasible to support the hypothesized depositional history of the kirthar formation in the rakhi nala region. variations in seal level concerning a certain datum level can be used to describe changes in relative sea level. tectonic uplift or subsidence, eustatic sea level fluctuations, and sedimentation rate all affect relative sea level changes. stratigraphic data from interpreted series were checked by schutter (1998), haq (1991), and handford and loucks (1994). early eocene regression the sea stopped moving southward in the late early eocene. a barred and evaporitic basin was created as a result of this southerly regression, depositing thick gypsum or alabaster in the baska shale and alabaster component (late hst to early lst). during the late early eocene, mari-kandkot high was a subaqueous island in a stable carbonate platform. in a carbonate-stable platform with contemporaneous facies of the upper ghazij formation, shallow open marine, subtidal carbonates were deposited (saw et al., 2017; ali et al., 2021a; figure 9). middle eocene transgression the middle eocene saw a large transgression event in the indus basin (nagappa, 1959). tectonic action was the cause of this transgression. numerous local sea level changes during this significant transgression event led to the deposition of different microfacies associations in the kirthar formation. figure 9. eustatic sea level changes during the deposition of the kirthar formation (modified after vail et al., 1977). in the sulaiman sub-basin, the kirthar formation was deposited as a result of four significant tectonic and sea level shifts. the already confined basin changed into the somewhat restricted inner shelf lagoonal basin that constitutes tst during the late early eocene (ali and abdullatif, 2015). shale from the ghazij formation was deposited during this time. the shale in question is from the upper ghazij formation. due to the ongoing sea level rise, the kirthar basin was transformed into an open sea shelf platform (early tst). the submerged paleo highs served as a shoal/platform edge environment with high energy. this environment led to the deposition of the bioclastic packstone-grainstone microfacies of the habib rahi limestone on the shelf margin. pirkoh, kandkot, and mari regions served as depocentres in the south. in comparison to nearby locations, the rate of sinking was higher here. jones and desrochers (1992) referred to the ongoing and substantial vertical trend as “catch-up deposits.” deep open marine, organically abundant basinal globogerinids wackestone-packstone microfacies of the habib rahi limestone were deposited as parasequences after the highest sea level rise. later, the subsidence that had been occurring in the south moved to the north, with kingri, pitok nala, and drazinda regions emerging as the focal points of these facies. at the summit of the habib rahi limestone, it is discovered that there is an unconformity (sequence boundary) present. following the relative sea level decline (late hst) (figure 9). regression the basin became partially constrained during lst as the already existing paleo highs came into effect. domanda members’ shales deposited in and filled this semi-restricted basin. the indian platform served as a source of clastics in the direction of the east. the centre of the sulaiman foldbelt is on the front. late hst settings with hypersaline, moderate to high energy, warm, and shallow temperatures are where ostracod grainstone microfacies of domanda members were deposited. the shales are typically taken into account during late hst or cratonic basin lst (schutter, 1998). a deviation is noted at the top of the sirki member. with the beginning of the second sea level rise, the influx of clastic material ceased (tst). when the second sea level rise, indicated by the transgression system tract, started, parasequences of the pirkoh limestone member were deposited in the carbonate factory. foraminiferal wackstone to packstone microfacies are deposited in the euphotic zone, shallow subtidal, and low-energy open sea shelf settings. the jones and desrochers (1992) “catch-up” deposits can be utilized to explain the continuous and dense vertical sequence of related facies. following that, sea level rise reached its highest point, which is indicated by the peak of floods in late tst and early hst. in a deep open sea outer shelf environment at this time, laminated and organically abundant pelagic globogerinid wackestone to packstone microfacies were deposited. these circumstances were present in the districts of rakhi nala, zinda pir, and drazinda. comparatively more subsided than surrounding lands were the areas where these circumstances were present. due to the deposition of organic-rich, anoxic sediments with low oxygen levels. the dense vertical and continuous series of related facies can be explained in this case using “catch up to keep up” depositional theories. an unconformity at the top of the pirkoh limestone and marl member may be seen (figure 9). the paleo highs to the south acted to partially confine the basin as a result of the sea’s regression. reduced bottom water circulation increases the ability of organic stuff to be preserved (ali et al., 2016). a high rate of terrigenous input once again hindered the deposition of carbonates in the carbonate factory due to the prograding thick shales of the drazinda shale component deposited. this clastic inflow is assumed to have originated from the afghan shield, which is located to the northwest of the research area. it is possible to classify these shales as cratonic lst or hst. the drazinda members’ bioclastic foraminiferal packstone and foraminiferal grainstone microfacies were deposited in highenergy shallow marine conditions with slow sedimentation rates (tst). marine remnants are recorded for this sedimentation. these parasequences may have a sequence boundary at their base (figure 9). in the end, a terrigenous clastic inflow created the hst basin’s open sea shelf. foraminiferal wackestone to packstone microfacies of drazinda members were deposited toward the top as a result of this clastic input. near the top of the drazinda member, there is a type 1 unconformity identified (figure 9). the type 1 unconformity, which has an erosional hiatus, is defined by a laterite bed at the base of the oligocene chitarwatta formation. the indian and eurasian plates collided, causing regional lows and regional highs (ghazi et al., 2015; gong et al., 2020). these troughs were filled with clastic sediment. the indian shield or 36 umair sarwar, shahid ghazi, syed haroon ali, mubashir mehmood, muhammad jehangir khan, arslan zaheer, syed jawad arif the recently rising axial belt was the source of this clastic inflow. these clastics made up the oligocene chitarwatta formation. this indicates the presence of an lst-representing marginal marine or maybe deltaic environment in the research area (figure 9). 6. conclusions after conducting robust fieldwork and microfacies analysis in the rakhi nala region of the eastern sulaiman range, middle to late eocene period kirthar formation, the following findings were established. the domanda shale member, the pirkoh limestone and marl member, and the drazinda shale member are the four members of the kirthar formation found in the study area. the kirthar formation, from the middle to late eocene, rests beneath the ghazij formation in a conformable manner. however, the contact between the kirthar formation and the oligocene chitarwatta formation in the specific area under study is marked by an unconformity. these four members’ measured thicknesses are as follows: pirkoh limestone member: 13m, drazinda member: 285m, domanda shale member: 225m, habib rahi limestone member: 38m. the microfacies h-1, h-2 and h-6 of habib rahi limestone members were deposited in the deep-sea environment (fz-1). h-3 and h-4 were deposited in a carbonate sand shoal environment (fz-6). h-5 microfacies of habib rahi limestone member deposited in deep shelf (fz-2). do-1 and do-2 members of domanda shale member deposited in an open marine platform environment. p-1 microfacies of pirkoh limestone and marl member deposited in deep shelf environment (fz-2). p-2, p-5 and p-6 microfacies are deposited in the deep-sea environment (fz-1). p-3 and p-4 microfacies of this member were deposited in a carbonate sand shoal environment (fz-6). the limited marine platform (fz-7) and the open marine platform (dr-1 and dr-2 microfacies of the drazinda shale member, respectively). in an evaporitic platform environment (fz-9), dr-3 was laid down. dr-4 is deposited in a carbonate sand shoal environment (fz-4). the lithological units in the kirthar formation were formed as a result of periodic tectonic and sea level changes. as a result of the initial sea level rise, habib rahi limestone members were deposited in a shallow to deeper open marine shelf setting. the kirthar formation’s domanda shale member was deposited as a result of sea level fall. domanda members were deposited in inner shelf and semi-restricted shelf lagoonal habitats, as a result of the sea level fall. the second phase of the relative sea level rise resulted in the deposition of the pirkoh limestone member. as a result of sea level rise, pirkoh limestone members were deposited in a 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(1977). calcareous algae. elsevier, amsterdam. _hlk157762733 _hlk158065260 _bookmark16 abdel allemann hunting kazmi82 kazmi08 khan lawrence _hlk158199707 lillie87 malkani10 qureshi raza rowlands shah01 shah09 runoff curve number (cn model) evaluation under tropical conditions daniela calero mosquera, fanny hoyos villada, enrique torres prieto universidad nacional de colombia *corresponding author: dcalerom@unal.edu.co keywords: flow; hydrology; rainfall; palabras clave: caudal; hidrología; precipitación; issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.95321 earth sciences research journal earth sci. res. j. vol. 25, no. 4 (december, 2021): 397-404 h yd ro lo g y record manuscript received: 27/04/2021 accepted for publication: 02/11/2021 abstract in tropical countries the curve number method (cn) of the soil conservation service (scs) is widely used in civil engineering (to calculate drainage infrastructure) as well as in agricultural and environmental studies. however, little is known about the accuracy of cn method in tropical watersheds. to reveal this accuracy, this study compares the scscn model with the methodology proposed by hawkins (1993) where cn (cnq) values are determined with field data measured in an andean micro-basin. for this, cn values for the tropical micro watershed “la vega” were identified using the tables proposed by the scs and its respective antecedent moisture and slope corrections (cnamc2s); subsequently cnq values were derived from 55 events in which runoff and rainfall were measured. according to the results obtained, it was found that in the tropical micro watershed evaluated in this study, the scs-cn model overestimated runoff. the scs-cn model is strongly influenced by the antecedent humidity and its impact becomes stronger in high rainfalls records; however, the antecedent humidity influence was not observed in measured runoff data. despite the scs-cn model did not show a direct relationship between cn and rainfall, this relationship was observed in measured data. furthermore, a clear relationship between runoff and maximum rainfall intensity was found, so it is considered that including this factor in future research can improve the model’s predictions. this study shows that scs-cn model has some inaccuracies and it requires further studies to know its applicability in tropical conditions. evaluación de la escorrentía del número de curva (cn) en condiciones tropicales resumen en países tropicales el número de curva (cn) del servicio de conservación de suelos (scs) es ampliamente usado en ingeniería civil (para calcular infraestructura de drenaje), así como en estudios agrícolas y ambientales. sin embargo, se conoce poco sobre la precisión del modelo scs-cn en cuencas tropicales; para revelar esta precisión, este estudio compara la metodología del scs-cn con la metodología propuesta por hawkins (1993), donde se determinan valores de cn (cnq) a partir de datos de campo medidos en una microcuenca andina. para esto, se identificaron los valores de cn para la microcuenca tropical “la vega” utilizando las tablas propuestas por el scs con sus respectivas correcciones por humedad antecedente y pendiente (cnamc2s); posteriormente, los valores de cnq se derivaron de 55 eventos en los que se midieron lluvia y escorrentía. de acuerdo con los resultados obtenidos, se encontró que, en la microcuenca tropical evaluada en este estudio, el método del scs sobreestimó la escorrentía. el modelo del scscn está fuertemente influenciado por la humedad antecedente y su impacto es más fuerte en registros de altas precipitaciones, sin embargo, la influencia de la humedad antecedente no se observó en las escorrentías medidas. a pesar de que el modelo scs-cn no presentó una relación directa entre cn y precipitación, esta relación si se observó en los datos medidos, además, se encontró una clara relación entre escorrentía e intensidad máxima de precipitación, por lo que se considera que incluir este factor en futuras investigaciones puede mejorar las predicciones del modelo. este estudio muestra que el modelo scs-cn tiene algunas inexactitudes y requiere más estudios para conocer su aplicabilidad en condiciones tropicales. how to cite item: calero mosquera, d., hoyos villada, f., & torres prieto, e. (2021). runoff curve number (cn model) evaluation under tropical conditions. earth sciences research journal, 25(4), 397-404. https://doi. org/10.15446/esrj.v25n4.95321 mailto:dcalerom@unal.edu.co https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v25n4.95321 https://doi.org/10.15446/esrj.v25n4.95321 398 daniela calero mosquera, fanny hoyos villada, enrique torres prieto introduction the rainfall – runoff relationship in a watershed involves a set of physical and chemical processes which intervene at different spatial and temporal scales (plesca et al., 2012; ocampo et al., 2014;), and in turn, the watershed is influenced by weather conditions, soil properties, land use and topographic characteristics (marín et al., 2014; martínez et al., 2017). however, the scarce information in the andean watershed is a limitation in order to analyze these relationships, which is why hydrological models have been implemented, as a key tool to understand the runoff generation, provide hydrological forecast and therefore, contribute to the planning and management of hydrological resources. (chen et al., 2020; plesca et al., 2012). one of the widely used models is the cn curve number method was developed in 1972 by the soil conservation service (scs) of the u.s. department of agriculture (usda) as a simple application methodology for calculating runoff in a rainfall event and whose criterion does not require the study watersheds to be gauged or instrumented (aparicio mijares, 1992; chow et al., 1994; ibáñez-castillo et al., 2014; pérez nieto et al., 2015). in the scs-cn model, the generated runoff is obtained from the recorded rainfall, the initial abstractions or losses (considered as 20% of maximum soil retention) and curve number (cn), which is obtained from tables designed according to soil type, land use, conservation practices in agricultural areas, and hydrological condition (chow et al., 1994; ibáñez-castillo et al., 2014). according to ibáñez-castillo et al. (2014) and montiel gonzaga et al. (2019), the scs-cn model is one of the most used techniques in the hydrological field due to its simplicity and medium information requirement (pacheco moya, quiala ortiz, & martinez hernandez, 2018), even it is one of the methods used by soil and water assessment tool (swat) model to predict runoff (neitsch et al., 2011). nevertheless, the model has been subject to several critical revisions by authors as hawkins (1993), ponce & hawkins (1996), lopez (2001), mishra et al. (2007) y paz-pellat (2009). that is how hawkins (1993) stated that it is difficult to accurately select the appropriate cn from the tables and that the runoff calculation is much more sensitive to the chosen cn than to the rainfalls presented because rainfall records are more closely tracked, while the cn values are unique to the area studied or analyzed and are only found in the tables developed by the scs. in view of the foregoing, hawkins (1993) proposed to use the scs-cn model in an inverse way to calculate cn. in order to do this, hawkins solved the mathematical equations for the scs-cn for model (eq. 1-2) and let rainfall and runoff as model variables (eq. 7). in this way, it is possible to calculate cn with the rainfall and runoff data in a watershed. however, this methodology shows that there is not a unique cn value for a watershed, but a secondary relationship emerges between cn and the rainfall, giving rise to a graph which characterizes the type of response pattern of the watershed and is classified as “complacent”, “violent” or “standard”, the latter being the most common scenario (hawkins, 1993). the evaluation proposed by hawkins (1993) have been implemented in different parts of the world which had runoff observed data as usa (hawkins, 1993; durán-barroso et al., 2016; santikari & murdoch, 2019), argentina (ares, varni, et al., 2012), mexico (ibáñez-castillo et al., 2014; pérez nieto et al., 2015), brasil (da cunha et al., 2019; durán-barroso et al., 2016) india (gundalia & dholakia, 2014; lal et al., 2015, 2017, 2019) and south korea (ajmal et al., 2015); in all these studies a great difference between the cn of scs-cn model and the cn derived from observed data was found, showing that scs-cn method cannot calculate runoff properly. in addition, other deficiencies and limitations in the model have been considered, ponce & hawkins (1996) mentioned that the method does not take into account the spatial and temporal variability of infiltration and other losses by abstractions. in this regard, merz & bárdossy (1998) said that spatial variability might have a strong influence in the runoffrainfall relationship. likewise, lópez (2001) stated that calculation methodology does not take into account the effects of rainfall intensity, it means, that for the calculation carried out in two selected events, with the same rainfall but of different duration, the runoff will be the same. in the case of tropical conditions, no evaluation studies of the scs-cn model have been found, but the scientific evidence presented here highlights the importance of evaluating the scs-cn model, where the frequency of intense rainfall has increased, its records show high spatio-temporal variability on low scales, and in addition, temporal information is scarce (marín et al., 2014). in colombia, the scs-cn model is widely used to calculate runoff, mainly due to the easy implementation of the model. however, the absence of flow monitoring stations (cvc, 2015) has been an impediment to the validation of the scs-cn model, therefore, it is not really known whether the model works properly under tropical conditions. then, the current investigation aims at evaluating the curve number model (cn) under conditions of a tropical andean micro watershed using the methodology proposed by hawkins (1993). materials and methods micro watershed study: description of la vega la vega micro watershed is located in the municipality of palmira, valle del cauca, colombia; its drainage area is immersed in the aguaclara sub watershed, which is the main tributary of the bolo river. the study area comprises 3°30’33.12’’ to 3°29’53.52’’ north and 76° 10’ 54.48’’ to 76°11’44.52’’ west, with an extension of 114.08ha, an altitudinal range of 1400 to 1900 m.a.s.l. an average slope of 21% and average multiannual monthly rainfall of 1800 mm. the average multi-annual monthly rainfall in the area presents a bimodal regime, with two wet periods in the months of march-may and octoberdecember, as well as two periods of less rainfall in the months of januaryfebruary and june-september. groundcover in the micro watershed (fig. 1) are permanent shrub crops (coffee crops) (53%), cultivated grass (34%), shrubs and bushes (13%). soils (fig. 2) are typic dystrudepts (85%) and typic humudepts (7%), which are characterized by being deep, well drained, with fine textures, medium to low organic carbon content in depth and low moisture retention; meanwhile, typic udorthents soils (8%) are superficial, limited by the presence of rock fragments, moderately well drained and low moisture retention (cvc; igac, 2014). figure 1. coverage map of la vega microwatershed. rainfall and flow measurement the study micro watershed kept a detailed record of rainfall and flow with a time resolution of 15 minutes. the information was collected from june 2015 to july 2017 by the centro de investigación de la caña de azúcar de colombia – cenicaña. rainfalls in la vega were recorded using a davis vantage pro-2 weather station located into the micro watershed at an altitude of 1607 m. flow monitoring was performed continuously at the micro watershed hydrometric station, consisting of a water level sensor (through a keller brand acculevel pressure sensor), installed in a narrow crest weir to ensure a control section and accurate flow measurement. 399runoff curve number (cn model) evaluation under tropical conditions figure 2. soil map of la vega microwatershed. runoff calculation with scs-cn method the scs-cn method presents runoff as a function of rainfall (daily rainfall or 24-hour rainfall) and soil water storage capacity, calculated by the expression: where p is the rainfall in mm; is the initial abstraction of the watershed in mm; q is the runoff generated in mm and s is the maximum soil retention in mm, the latter related to cn by: the cn was obtained from tables according to soil type, land use, conservation practices in agricultural areas, and hydrological condition (chow et al., 1994; neitsch et al., 2011b; ibáñez-castillo et al., 2014). for this purpose, information was obtained from the semi-detailed soil survey at a scale of 1:25000 (fig. 1) of the watersheds prioritized by the environmental authority of the department: corporación autonoma regional del valle del cauca (cvc) (cvc; igac, 2014); in the same way, information on land use was obtained (fig. 2), which was validated and compared with information collected in field visits. to select the hydrological soil group, the four groups of soils described by chow et al. (1994) were taken into account, these groups are presented in table 1, also in this table can be observed the soil type associated with the hydrological soil group. table 1. hydrological soil group soil groups description la vega soils group a deep sand, deep soils deposited by the wind, aggregated silts group b shallow deep soil deposited by the wind, sandy loam typic udorthents group c clay loams, shallow sandy loam, soils low in organic content, and soils usually high in clay typic dystrudepts and typic humudepts soil groups description la vega soils group d soils that swell significantly when wet, heavy plastic clays, and certain saline soils. source: adapted from chow et al. (1994) once the hydrological soil group for each soil type was found, the cn was determined for the different land uses in the micro watershed according to the cn tables presented by neitsch et al. (2011b). in order to obtain an average cn for the hole watershed, cn was weighted by area using arcgis 10.4.1 following the methodology proposed by chow et al. (1994). the selected cn was corrected for antecedent humidity (eq. 3,4) and slope (eq. 5) (ibáñez-castillo et al., 2014; pérez nieto et al., 2015); in terms of humidity, there are three types of cn classified according to previous rainfall (during the 5 days prior to the runoff event), to determine the adjustment. thus, cn data recorded in tables by the scs are assumed in normal antecedent moisture condition (amc ii) (chow et al., 1994). criteria for defining the antecedent moisture condition are presented in table 2. table 2. antecedent moisture condition classification amc group total 5-days antecedent rainfall (mm) dormant season growing season i ≤ 13 ≤ 36 ii 13 28 36 – 53 iii ≥ 28 ≥ 53 source: adapted from (ferrer polo, 2000) for the correction for antecedent moisture (table 2), the growing station was selected, since in both periods of the bimodal regime there are rainfall records of significant magnitude. calculation for equivalent cn is presented in equations 3-4. the slope correction (eq. 5) was introduced because cn tables proposed by the scs were developed for use in watersheds with slopes of less than 5% (nash & sutcliffe, 1970): where is the curve number corrected for slope; is the curve number for the amc (iii) group; is the curve number as read in the scs tables; slp is the mean slope of the watershed (m/m). in this study, the cn derived from scs tables and adjusted by slope and moisture will be termed cnamc2s runoff calculation from observed flow (baseline method j + n) to determinate actual runoff in the micro watershed, 55 rainfall events and its flow responses were analyzed during june 2015 and july 2017. these 55 events were considered because presented a clear flow response as a result of 400 daniela calero mosquera, fanny hoyos villada, enrique torres prieto a single rainfall event. these clear flow response events were easily identified in the hydrograph by an ascent curve (qa), a single peak flow (qmax) and finally a descent curve (qd). in the present study, only events of a single peak were considered and were referred to as unique events. figure 3 represents a typical case of a unique event presented in the study area. different kinds of flow responses were not considered because runoff cannot be calculated by the baseline method j + n. the start of the event was determined when the level sensor showed a significant increase in river flow as a consequence of rainfall; this start was projected horizontally until the moment of peak on the hydrograph (j) and ended with a diagonal according to the fixed base flow method (chow et al., 1994), where it is assumed that runoff ends in a fixed time n (eq. 6) in the recession segment. where n is the number of days after the peak on the hydrograph, and a corresponds to the watershed area, given in km2. figure 3 graphically explains this method. the event initial flow was determined as an average for flows recorded two hours prior to the start of the event; runoff was calculated as the area under the hydrograph curve and delimited below by the j and n lines (inclined); runoff (mm) was calculated by dividing the total runoff volume by the micro watershed area. influence of rainfall intensity on runoff generation to evaluate the influence of precipitation intensity in runoff generation, the methodology proposed by wischmeier y smith (1978) to determine maximum 30 minutes precipitation intensity (i30) in an erosive event was used. this methodology is considered into the erosivity factor of the universal soil loss equation (usle). i30 values were obtained for the 55 analyzed events and were related with observed runoff (baseline method j+n). determination of the observed cn (cnq) to determine actual cn (cnq) an inverse scs-cn modeling was performed (hawkins, 1993). for that, each of the 55 actual rainfall events data and its runoff responses (calculated by j + n methodology) were introduced in equation 7. the equation 7 results from solving the maximum retention of the soil (s) in equation 1a and substituting this value in equation 2. 2.6 estimation of adjustment between cnamc2s and cnq to measure the level of adjustment between cnamc2s and cnq, the nash efficiency index (e) was used (nash & sutcliffe, 1970), which has been widely used in the evaluation of hydrological models (yusop, chan, & katimon, 2007; pérez nieto et al., 2015), calculated from the following equation: the nash efficiency index can have values from -∞ to 1; when the nash index is closer to 1, hydrological model prediction (in this case scs-cn) is better. asymptotic adjustment method to determine the type of response pattern of the studied micro watershed, the rainfall and runoff values measured in the watershed (55 unique events summarized in section 2.4) were ordered independently and in descending order, subsequently the cn values were determined for each pair of data (rainfall and runoff) using the equation 7 then, the relationship between rainfall and cn was plotted. according to hawkins (1993), three types of patterns can occur in this rainfall-cn relationship: “standard”, “violent” and “complacent” behavior. in cases of responses patterns “standard” or “violent”, the cns approach a constant value with increasing rainfall, and these are given by the equations 9-10 respectively. figure 3. graphical explanation of baseline method j+n for base flow separation 401runoff curve number (cn model) evaluation under tropical conditions where is a constant value approached as and is a fitting constant (different for each behavior).the variable describes the data set for larger rainfall events (hawkins, 1993). in the “complacent” behavior, evidences no appreciable tendency to achieve a stable cn value (hawkins, 1993). deeper mathematical analyses of equations 1-2 were developed, where the effects of the variables ia and s were shown to be minimal; thus, the cn was rethought as a rainfall-dependent variable (eq. 11) and was considered the lower limit of the curve number definition (hawkins, 1973). results and discussion in the 55 unique events analyzed in the micro watershed, the maximum recorded rainfall was 74 mm and the minimum was 0.3 mm, while the antecedent moisture (five previous days) varied from 0 to 105 mm. scs-cn method cn values obtained from scs tables the cn values were calculated by tables according to scs method. due to micro watershed heterogeneity (soil and land use), there were several cn values for the micro watershed. table 3 presents the cn weighted by area for “la vega” micro watershed, its slope correction (cn2s), and its antecedent humidity correction (cnamc2s). table 3. cn found by the scs methodology. weighted cnamc2s micro watershed cn cn2s cn i cn ii cn iii la vega 73 76.8 58.2 76.8 88.7 runoff calculated from cn tables (cnamc2s) the runoff scs-cn method calculated from cn tables was 2.3 mm on average, with a maximum value of 39.6 mm and a minimum of zero; in figure 4, lines present the runoff calculated from cn tables (cnamc2s) with respect to rainfall in the la vega micro watershed in its three humidity conditions (cn i, cn ii, cn iii) (feyereisen et al., 2008). the points represent 55 events studied in the micro watershed and the runoff generation according to the scs-cn model. figure 4. evaluation of runoff (cnamc2s) rainfall relationship in la vega, according to the scs-cn model. figure 4, shows that 42 events out of the 55 analyzed correspond to low antecedent moisture (cn i line), it means most of the cn data were below the “mean condition” (cn ii) defined by the scs, just 5 events corresponded to cn ii condition and 8 to cn iii condition it is important to note that scs-cn model is strongly influenced by the antecedent moisture because the correction of the cn from the antecedent humidity affects in an inverse way ia (as described in equations 1 and 2) and, consequently, in the generation of runoff. to show the humidity correction effect, similar rainfall events recorded in the micro watershed (low: 15mm and high: 70mm) were taken under the three antecedent moisture conditions, which are presented in table 4. in this table, it can be observed that rainfalls close to 70 mm (high rainfall) presented very different runoffs (from 5 to 39.6mm), this was due to the fact that from condition i to condition ii the cn changes 32%, but the impact in runoff calculated by the model was 414%. from condition i to condition iii the impact was even stronger, with a cn change of 52%, runoff increases by 692%. in low rainfall, records around 15mm were taken. for this case, the impact was not so strong. all this indicates that the impact of the antecedent moisture condition becomes stronger in high rainfall. runoff calculation from observed flow (baseline method j + n) runoff calculated from observed flow (j+n baseline methodology) had lower result than runoff of scs-cn methodology. runoff average value was 0.6 mm with a maximum of 6.5 mm and a minimum of zero. in figure 5 the three antecedent moisture curves were plotted (as shown in fig. 4) and over this data was plotted the relation between rainfall and runoff calculated from observed flow (j+n baseline methodology) as black dots. here, it can be observed that the runoff calculated from j+n methodology (runoff j+n) presented a trend toward the cni curve set forth in the scs-cn model; likewise, its values were lower than those obtained in the scscn methodology. for this reason, no points were observed in the cnii and cniii lines despite the fact that runoffs in both methodologies were calculated from the same rainfall events observed in the micro watershed. figure 5. evaluation of the runoff (j+n) – rainfall relationship in “la vega”, according to the scs-cn model. similar results were found by lópez (2001), who conducted a literature review where he stated that runoffs less than 12.7 mm (half an inch) make the scs-cn method less accurate. on the other hand, studies from ares et al. (2012) and da cunha et al. (2019) found that events in hydrological condition iii overestimated modelled runoff. based on this, it can be deduced that the cnamc2s selected from tables, corrected for slope and antecedent humidity, would be overestimating the micro watershed runoff. in addition, rainfall events where found the scs-cn model assume runoff as (zero), while the observed data reported runoff. in cases of low runoff, the limit imposed by the scs-cn model (p> = ia) based on the relationship between ia and s (λ = 0.20 in equation 1c) may not be adequate. this agrees with authors such as ajmal et al. (2015) and lal et al. (2015, 2017, 2019) who stated that the optimized initial abstraction ratio (λ) values showed that the original assumption of the λ of 0.20 is unusually high. 402 daniela calero mosquera, fanny hoyos villada, enrique torres prieto influence of rainfall intensity on runoff generation when evaluating the influence of the intensity of rainfall on the generation of runoff, the results showed that there is a clearer relationship between maximum rainfall intensity and runoff (r2 = 0.96) than between amount of rainfall and runoff (r2 = 0.42) (fig. 6). since the runoff process is the result of the interaction of the rainfall rates (intensity) and the infiltration rate, it makes more sense to use the rainfall intensity as a variable in the model instead of the amount of rainfall as expressed by santikari & murdoch (2019), who consider that taking temporary variations into account can improve the performance of the scs-cn model. however, it is more complex to measure intensity than amount of rainfall, especially at the time the scs-cn model was implemented but now with the current development of automatic stations, it may be feasible to incorporate rainfall intensity in runoff calculation. figure 6. relationship between runoff and rainfall (a) and maximum rainfall intensity (b). cn derived from runoff (cnq) the cnq was calculated using the scs-cn model in an inverse way (eq. 7), which already had the influence of slope and antecedent moisture, due to the fact that its calculation was made from real runoff data in the micro watershed for the 55 unique events of the micro watershed, the corresponding cnq were found; their values oscillated between 48 and 99 with a mean of 80, indicating that most of the cnq found were high (>70) unlike the cnamc2s which showed a tendency of 58.2 (cni). in addition, each event had a different cnq value and it was not possible to classify them in unique values of cni, cnii, cniii as in the case for cnamc2s. similarly, the nash efficiency index (nash & sutcliffe, 1970) was -3.04, indicating that there was no similarity between the values of cnamc2s and cnq, therefore, the two cn were not comparable, even though both start from the same model (eq. 1 2) proposed by the scs. in this case, by finding a nash efficiency of less than 0 it can be inferred that the cnq is a better predictor than cnamc2s (yusop et al., 2007; pérez nieto et al., 2015) ares et al. (2012) stated that the variety of cnq values could initially be related to the heterogeneity of soils and micro watershed coverages and secondly, to the antecedent moisture of each event. however, in the figure 7 it can be evidenced that, unlike cnamc2s which is stratified by the antecedent moisture (three horizontal lines), the cnq was not influenced by this factor (white triangles), for example, for an antecedent moisture of 18 mm, the cnq varies between values of 62 and 96 (fig. 7a). thus, it was deduced that the antecedent moisture correction proposed in the scs-cn methodology is a parameter that does not work properly in the actual micro watershed conditions. as mentioned in the previous section, it is possible that other aspects have a great impact on runoff, such as rainfall distribution and intensity which is not considered by scs-cn model. figure 7. cnamc2s (black circles, rhombus and squares) and cnq (white triangles) variation with respect to antecedent moisture condition (amc) in la vega. a. amc i. b. amc ii. c. amc iii. 3.4 standard asymptotic adjustment to identify the micro watershed response type, the cnq values found were related to the recorded rainfall, as proposed by hawkins (1993), giving table 4. example of some rainfall events differences in rainfall date (dd/mm/yyyy) pp (mm) antecedent. moisture (mm) antecedent. moisture condition (amc) cnamc2s initial abstractions ia (0.2*s) runoff (mm) 30/09/2016 69.3 13.2 i 58.2 36.54 5.0 high rainfall 13/09/2016 74.4 37.1 ii 76.8 15.35 25.7 13/03/2017 67.8 105.2 iii 88.4 6.67 39.6 low rainfall 07/05/2017 15.0 20.8 i 58.2 36.54 0.0 08/05/2017 15.5 35.8 ii 76.8 15.35 0.0 10/05/2017 13.7 56.9 iii 88.4 6.67 1.2 403runoff curve number (cn model) evaluation under tropical conditions rise to the graph presented in figure 8. further, cn0 (eq. 11) was calculated and was found that it is always lower than the cnq (eq. 7), which allowed to verify that variable ia was actually a runoff lower limit as suggested by the model. figure 8. cn0 and cnq variation with respect to rainfall. figure 8 showed an inverse proportional relationship between the cnq and the rainfall recorded, as the latter increases, the cnq tends to decrease. this allow to conclude that there is a clear relationship between the cnq and the recorded rainfall, as it was expected. although, this relationship did not reach a stable cnq value, thus coinciding with the “complacent behavior” type response pattern (hawkins, 1993), whose definition suggests that micro watershed of the present study did not respond in accordance with equation 1a, at least not within the range of the analyzed data. in these cases, hawkins (1993) stated that it was possible to fit the equation for the standard behaviour (eq. 9) with the least squares method, assuming that the data set is merely the lower rainfall end of the standard behavior pattern, and that the watershed will perform in the extrapolated standard pattern when these larger storms are eventually encountered. therefore, the “standard asymptotic adjustment method” was used, obtaining the following adjustment =0.95): in equation 12, a cn∞ of 44.62 was defined for high rainfall events and it describes the cnq trend in the case of rainfall events of significant magnitude, however it is a much lower cn value than those analyzed in the set of data presented in figure 8 (hawkins, 1993). according to figure 8, events with rainfalls between 20 to 50 mm were related to a cn between 60 to 80, and rainfall lower than 20 mm with a cn greater than 80, the latter being the most frequent events in the micro watershed studied here. according to hawkins (1993), values such as those observed at the beginning of figure 8, are due to the fact that low rainfall with low cn do not generate significant runoff. similar results were found by ares et al. (2012), in the videla stream watershed, they found a “standard” category response pattern (hawkins, 1993), and thus obtained a single cn of 57 for large events in their study area. ares et al. (2012) had similar rainfalls in the analyzed events compared to our data (0-120mm), although ares et al. (2012) average events were longer (69h) than the average events analyzed here (20h). data presented here showed that cnamc2s presents a direct relationship with the antecedent moisture condition (fig. 7), however, the relationship between amc-cnq was null; on the other hand, the variation of the cnq obtained from observed data (fig. 8) might be attributed to the effect of the intensity rainfall in the generation of runoff, evidenced in figure 6a; thus, the intensity runoff could be the object of study in future investigations of the scscn model. conclusions the estimation of the adjustment between the curve numbers obtained from the table and those calculated from the observed runoff (cnamc2s and cnq) showed a high difference between these values (e=-3.04), indicating that scs-cn method does not calculated runoff properly in la vega microwatershed. according to the calculations made for the runoff estimation, it was found that the cn-scs model overestimates the runoff generated in the micro watershed, nevertheless, in some rainfall events where the scs-cn model assume no runoff (zero), the observed data reported runoff, this was related to the limit imposed by the scs-cn model (p> = ia) based on the relationship between ia and s (λ = 0.20) which may not be adequate. the scs-cn model is strongly influenced by the antecedent moisture correction; however, according to observed data (cnq), the amc does not work properly in “la vega” micro watershed real conditions. a cn-rainfall relationship emerges when cns are calculated from rainfall-runoff data observed (cnq), although this relation is not considered into de scs-cn model. thus, the variation of the cnq in the micro watershed might be attributed to the fact that the intensity rainfall is not taken into account in the scs-cn model. in “la vega” micro watershed a clear relationship between maximum rainfall intensity and runoff (r2 = 0.96) was found. this relationship is not considered in scs-cn model. thus, taking rainfall intensity into account in future investigations, can improve the performance of the scs-cn model. this study highlights the importance of assessing the applicability and limitations of the models that were developed a long time ago (such as the scscn model) and which are still widely used without the knowledge of the real impact of being applied in different conditions of its development, in this case, tropical conditions. references ajmal, m., moon, g. w., ahn, j. h., & kim, t. w. 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with the organized exploration began way back in the 1830s. the number of institutions offering geological education is, however, comparatively low. thus, geology as a science discipline is less popular in comparison with other science subjects such as physics, chemistry, zoology, botany, and environmental science, based on enrolment data of the learners enrolled in academic sessions from 2011-2012 to 2018-2019 of the master’s degree programs. analysis indicates that teaching-learning resources of foreign origin are dominantly being adopted as learning resources in the teaching-learning pedagogy of degree programs in geology, and are over-riding those of indian origin. the covid-19 pandemic is affecting geological education in two ways namely, the disruption of conventional (face-to-face) teaching together with not allowing learners of geology programs to complete their compulsory out-door geological fieldwork assignments. the analysis of data gathered by an online questionnaire survey shows that the use of indigenous open educational resources in geology appears as a plausible solution to overcome the learning barriers created by the covid-19 pandemic, and also to enrich teaching-learning resources of indian origin. this article provides an up-to-date detailed account of the necessity and evolution of geological education, the current popularity of geology as a science discipline, the nature of available geological teaching-learning resources, the impact of the covid-19 on geological education, and the role of open educational resources in providing quality and equitable geological education, whilst removing educational barriers created by covid-19 in india. keywords: geological education; enrolment data; higher education; oer; covid-19; geological fieldwork; india. el escenario de la educación geológica en india y el rol de los recursos pedagógicos abiertos a las luces de la pandemia del covid-19 resumen la educación geológica juega un papel importante en el desarrollo y en el crecimiento social, económico y cultural de cualquier país. la india es un país vasto y diverso y con un amplio rango de características geológicas que consiste en rocas de todas las edades geológicas y divisiones fisiográficas bien desarrolladas. la placa litosférica india es un depósito de grandes georecursos y también sirve como un laboratorio de campo para el desarrollo y evaluación de numerosos principios geológicos. la exploración organizada de los georrecursos indios comenzó en la década de 1830. el número de instituciones que ofrecen educación geológica es, sin embargo, comparativamente bajo. de este modo, la geología es una ciencia disciplinar menos popular en comparación con otras materias de estudio como la física, la química, la zoología, la botánica y la ciencia ambiental, con base en la información de inscripciones de los estudiantes en los programas de maestría para los períodos de 2011-2012 hasta 2018-2019. el análisis indica que los recursos foráneos de enseñanza-aprendizaje son adoptados dominantemente como los recursos de aprendizaje en la pedagogía de los programas de pregrado en geología y superan aquellas de origen indio. la pandemia del covid-19 está afectando la educación geológica en dos vías especificamente: la disrupción de la enseñanza convencional (presencial) junto con no permitir a los estudiantes de los programas de geología completar sus tareas obligatorias de trabajo de campo. el análisis de la información recopilada por una encuesta digital muestra que el uso de recursos educativos nativos abiertos en geología es una solución posible para superar las barreras de aprendizaje creadas por la pandemia, y también enriquecen los recursos de enseñanza-aprendizaje de origen indio. este artículo provee un recuento detallado y actualizado de la necesidad y evolución de la educación geológica, la actual popularidad de la geología como una ciencia disciplinar, la naturaleza de los recursos disponibles de enseñanza-aprendizaje, el impacto de la pandemia del covid-19 en la educación geológica, y el rol de los recursos pedagógicos para proveer educación geológica equitativa y de calidad mientras se remueven las barreras educativas creadas por la pandemia en india. palabras clave: educación geológica; datos de inscripciones; educación superior; recursos educativos abiertos; covid-19; trabajo de campo geológico; india. record manuscript received: 28/05/2021 accepted for publication: 17/08/2022 how to cite item: verma, o., prashanth, m., greco, r., khosla, a., & singh, k. (2022). geological education scenario in india and role of open educational resources in the light of covid-19 pandemic. earth sciences research journal, 26(3), 239-254. https://doi. org/10.15446/esrj.v26n3.96209 https://doi.org/10.15446/esrj.v26n3.96209 mailto:omkarverma@ignou.ac.in https://doi.org/10.15446/esrj.v26n3.96209 https://doi.org/10.15446/esrj.v26n3.96209 240 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh 1. introduction the usage of the word “geology” in the scientific literature can be traced to the beginning of the eighteenth century (howarth, 2020). in the late nineteenth century, the scope of the discipline of geology was massively broadened in the western world (king, 2008). as a result, different terminologies for the discipline of geology came into existence. the new terms “earth sciences” emerged in 1852; “geoscience” appeared in 1929 and later, “geological sciences” in 1938, in the geological literature (wilson, 1883; adams, 1938; field, 1941; howarth, 2020). all these words are synonymous with each other and broadly, involved in the study of the whole earth and other planetary systems (manduca & kastens, 2012). geology has been described as a historical and descriptive science, where processes and conditions of earth, atmosphere, and oceans are investigated to understand earth for the well-being of the society (frodeman, 1995; manduca & kastens, 2012). it is a scientific discipline of societal relevance in meeting many of its needs notably, natural resources, environmental quality, and resilience to natural hazards (king, 2019). geoscientists use current natural phenomenon data to understand earth’s past data to evaluate earth’s present conditions and together, predict how earth will likely behave in the future. thus, geological education is the backbone of the socio-economic development of any country, and the dissemination of its knowledge to the different individuals (learners, public, and policymakers), is crucial in the decision-making processes, and for the long-term survival of humanity on the mother earth. geology plays a significant role in the overall growth and development of society. in support of geological education, an international geoscience education organization (igeo) was formally established in 2000 (king, 2019). the main objectives of the igeo are to promote geoscience education at the global level, enhance its quality and equity, and increase its awareness in all sectors of society at all levels (http://www.igeoscied.org/aboutthe-igeo/). in this context, eight international conferences on geoscience education have been organized worldwide so far by the igeo since 1993 (http://www.igeoscied.org/activities/geoscied/). the igeo is also promoting open online teaching geological resources encompassing earth learning ideas, complete teaching units, and geoscience textbooks for the use of the global teaching-learning community in various international languages (king, 2008; http://www.igeoscied.org/teaching-resources/). apart from this, the igeo has been conducting international earth science olympiad since 2007 for popularizing geoscience education among school kids (http://www.igeoscied.org/activities/ieso-2/). both, geological education and research are essential in india, because the indian subcontinent is endowed with significant amounts of georesources, whilst research on its geological settings has proved to be valuable in unraveling various mechanisms of the earth system and generation of the georesources of the subcontinent (wadia, 1964; tandon et al., 1991; naqvi, 2005; radhakrishna, 2005; santosh, 2012; valdiya, 2016; searle & treloar, 2019; kumari et al., 2020). geological research in india began in 1836 (oldham, 1856), but, geology as an independent educational science discipline first appeared in a few indian universities in the 1890s (murty, 1982). an analysis of the enrolment data for the master’s degree program in geology shows that geology is still a less popular science discipline in india. it also indicates that a majority of teaching-learning resources (tlr) consisting of reading material used in higher geological education, are more of foreign origin, and may or may not be accessible to the teaching-learning community in india. two types of negative impacts are visualized in the light of the current pandemic (i.e. covid-19) on geological education, firstly, temporary suspension of classroom/face-toface teaching, and secondly, the creation of an unfavorable environment for completing geological field-based compulsory assignments for bachelor’s and master’s degree programs. recently, open education resources (oer) are becoming popular among science disciplines and the global teaching-learning community has been using them extensively in day-to-day teaching-learning practices (smith, 2009; henderson & ostashewski, 2018; y verma, 2021a). oer are also gaining importance in india and currently, there are a few oer in geology, which are available on online platforms of the government of india (goi), for example, the e-pgpathshala portal of the university grant commission (ugc) and swayam of the ministry of human resources development (mhrd) (https:// epgp.inflibnet.ac.in/; https://swayam.gov.in/), and a few available on social media platforms like youtube. it has been shown that course-based-specific oer (including virtual labs, virtual field experiences, and google earth-based laboratory/field manuals) for bachelor’s and master’s degree programs in geology are the need of the hour (verma, 2021a). the use of oer in teachinglearning practice offers an opportunity to narrow down the gap of the tlr of indian origin. oer provide off-campus specific-course-based learning during any pandemic like covid-19, and also can, to an extent, facilitate the learners to complete their geological field-based assignments using google earth-based virtual field manuals. research focusing on the past, present, and future of geological education is a major concern and is being carried out in the western hemisphere particularly, in the us and uk (king, 2008, 2018; gsa position statement, 2016; dolphin et al., 2019). as a consequence, a journal has been launched, the “journal of geoscience education” for publishing geological education research. to date, substantial research on geological education has not taken place in india, and probably, this article may be first of its kind in india. the primary objectives of this paper are threefold: first, to evaluate why geological education is necessary for the country and review its historical development; second, to assess the current popularity of geology as a science discipline, the nature of available geological tlr, and impact of covid-19 on higher geological education; and third, to evaluate oer role in providing high quality and equitable geological education, and in removing some of the educational barriers created by covid-19. 2. methodology two methodological approaches were adopted for this study. the first approach involved the collection and analysis of literature to evaluate the importance of imparting geological education, trace the development of geological education from 1851 to the present in india, evaluate the status of geological education in comparison with other sciences at the level of master’s degree programs, and finally, know the nature of geological tlr in india. it was found that almost all research dealing with the historical development of geological education in india was undertaken by historians. to date, no researcher with a geological background had investigated evolutionary development and the future of geological education in india. the data dealing with the enrolment and successful completion of master’s degree programs in geology, physics, chemistry, botany, zoology, and environmental science for academic sessions from 2011-2012 to 20182019 was obtained from the annual reports of the ministry of human resources and development (now renamed as the ministry of education), goi within the ‘all india survey on higher education’ available on its website (http://aishe. nic.in/aishe/reports); this allowed comparative analysis between geology and other science disciplines. the data dealing with learners’ enrolment and their successful completion in the bachelor’s degree programs are not available; therefore, bachelor’s degree programs were not included in this study. we downloaded two curricula of bachelor’s degree programs in geology from the website of the university grant commission and the curricula of master’s degree programs from 25 universities (including central universities and state/union territory universities) from their websites to discover the nature of existing tlr. the number of tlr of foreign and indian origin recommended in the above-mentioned curricula were counted and analyzed. we had not disclosed the names of 25 universities and presented them as anonymous because the purpose of the research was to evaluate the nature of tlr available or used in india. the second method followed was a structured online survey. a google-based questionnaire dealing with the current use of the oer for geological teaching-learning practices in india, their role in overcoming the learning restrictions imposed by the prevailing covid-19 pandemic, and enriching teaching-learning resources of indian origin were prepared and sent by email to numerous academicians and researchers covering almost all parts of the country. the responses received were evaluated and interpreted. geological education: why it is important in india? indian geological education has many strategies for meeting the varied needs of society, addressing the economic growth of the country using natural resources, and refining basic and applied principles of geology (sharma, 1998; radhakrishna, 2005; sheth, 2007; jain et al., 2016; mandal, 2016; mukherjee et al., 2016; mazumder et al., 2019; parsons et al., 2020). since geological 241geological education scenario in india and role of open educational resources in the light of covid-19 pandemic education includes the whole earth system, geoscientists study where we get most of the things that we use in everyday life, such as minerals, soils, oil and natural gas, energy, air, water, and food (radhakrishna, 2005; press, 2008). geoprofessionals also play a critical role in numerous other societal services like the management of waste (sewage, water pollution, landfills, non-biodegradable, nuclear, and recycling), site selection for the construction of mega-projects (dams, tunnels, reservoirs, roads, rail tracks, and buildings) together with providing remedial measures for various environment-related problems. geoscientists study natural hazards (earthquakes, tsunamis, volcanoes, landslides, subsidence, floods, heat waves, droughts, natural contaminations, cyclones, and severe storms) to determine the magnitude of their risks and to develop resilient societies where life and property losses are minimized and preparedness, mitigation and recovery are initiated (shalley, 1971; chakraborti et al., 2011; mora, 2013; mukherjee, 2019; valdiya, 2016; fareeduddin et al., 2020; ali et al., 2021; verma, 2021b; prashanth & verma, 2022). in today’s world, due to the rapid increase in population and unsustainable practices in the use of natural resources, there is tremendous stress on earth’s resources, causing many problems such as climate change, global warming, fossil fuels scarcity, natural resources depletion, biodiversity loss, extreme weather events, water contamination, and air pollution. thus, nowadays, geo-professionals have been working to resolve various earth and environment-related issues whilst ensuring sustainability in the use of natural resources, and their protection for future generations (häggquist & söderholm, 2015; valdiya, 2016). additionally, geological education has important societal implications in improving human health standards and the environment by suggesting key remedial measures to reduce the impact of geogenic elements and anthropogenic interventions in the earth’s natural environment. the geogenic contaminants of groundwater, especially fluoride and arsenic, pose a serious threat to human health in india (ali et al., 2016; ali et al., 2021). the endemic fluorosis caused due to high concentration of fluoride in groundwater was recognized in 1937 and arsenic contaminated health effects in the year 1976. however, groundwater was not properly investigated for these contaminants until the late 1990s, resulting in the poor health of millions of people in india (chakraborti et al., 2011; raju, 2017; ali et al., 2019; saha et al., 2021). the awareness of geological time, the evolution of life, and earth’s past environments through the deep time is critically useful for society in view of the alternative perspectives of myths and religions that help us to understand how to live in harmony with the earth (ayala, 2008; kölbl-ebert, 2009). overall, geological education, information, and guidance play key roles in the decision-making process in numerous societal services such as sustainable natural resources management, mega-structures construction, natural hazards mitigation and management, local and regional planning, and environmental impact assessments (häggquist & söderholm, 2015). geological education is also critical in meeting some of the sustainable development goals of the united nations (un) such as zero hunger, good health and well-being, clean water and environment, infrastructure, sustainable cities, climate action, and sustainable marine resources (un, 2015; stewart & gill, 2017). apart from education, geological research on the indian lithospheric plate provides a platform for generating new geological ideas and offers many opportunities to study the geodynamics of the earth on the global scale (tandon et al., 1991; radhakrishna, 2005; naqvi, 2005; jain et al., 2016; santosh, 2012; mandal, 2016; mukherjee et al., 2016; valdiya, 2016; searle & treloar, 2019; parsons et al., 2020). the indian subcontinent has a distinctive topography with the indian ocean lying to the south and the snow-covered himalayan mountains belt to the north. it includes three main physiographic divisions (fig. 1): peninsular india to the south, extra-peninsular india (i.e., himalayan division) to the north, and the indo-gangetic plains sandwiched between them (shah, 2018). the indian lithospheric plate preserves the vast history of the earth from archean to holocene (4.6 billion years to the present) in its rock record (naqvi, 2005; santosh, 2012; mohanty, 2015; valdiya, 2016; shah, 2018). the peninsular region of india is dominantly composed of rocks of the precambrian age and is an amalgamation of archean cratons (dharwar, bastar, singhbhum, bundelkhand, aravalli, and meghalaya), and proterozoic mobile belts (eastern ghat, pandyan, satpura, and aravalli-delhi) and sedimentary basins (vindhyan, kurnool-cuddapah, chhattisgarh, gwalior, pakhal, marwar, bhima, and kaladgi) (fig. 1; valdiya, 2016; shah, 2018). the cratons and mobile belts suffered various phases of archean tectonic activity and as a consequence, they are a rich storehouse of varied economic minerals, building and industrial materials, gemstones, and decorative stones, and are excellent sites for investigating precambrian tectonism. the proterozoic sedimentary successions have yielded much biotic evidence, which is useful in studying the early evolution of earth’s life and atmosphere (naqvi, 2005; kumar, 2010; santosh, 2012; mohanty, 2015; valdiya, 2016; shah, 2018; mazumder et al., 2019; fareeduddin et al., 2020). figure 1. map of india showing the main physiographic divisions and geological units (modified after kumar, 2010; valdiya, 2016; shah, 2018). dc stands for dharwar craton, bc bastar craton, sc singhbhum craton, bkc bundelkhand craton, ac aravalli craton, mc meghalaya craton; pmb pandyan mobile belt, egmb eastern ghat mobile belt, smb satpura mobile belt, admb aravalli-delhi mobile belt; ckb cuddapahkurnool basin, cgb chhattisgarh basin, kb kaladgi basin, bb bhima basin, pb pakhal basin, vb vindhyan basin, gb gwalior basin, mb marwar basin; pgv pranhita-godavari valley, mv mahanadi valley, dv damodar valley, sv satpura valley; dt deccan traps, and rt rajmahal traps. prominent rift valleys such as pranhita-godavari, satpura, mahanadi, and damodar (fig. 1) probably developed during proterozoic times, where thick sedimentary covers developed within these valleys that are termed gondwana basins containing huge deposits of coal and a wealth of animal and plant fossils over upper palaeozoic to mesozoic time intervals (naqvi, 2005; valdiya, 2016; fareeduddin et al., 2020). the fossils of gondwana deposits have provided data enabling the reconstruction of land ecosystems of upper palaeozoic to mesozoic of peninsular india, together with their interrelations with ecosystems of continents that were adjacent at the time (valdiya, 2016). the prominent cretaceous volcanic traps (rajmahal and deccan) offer opportunities to study the interior of the earth, the role of mantle plumes in the rifting and drifting of the former gondwanan continents, the movement rate of the indian plate, and the environmental impacts of volcanic eruption on contemporary biota (beane et al., 1986; courtillot, 1990; verma, 2015; verma et al., 2016, mukherjee et al., 2016; verma & khosla, 2019; kania et al., 2022; khosla et al., 2022). the coastal regions of india contain a considerable cover of sedimentary successions such as bombay high (maharashtra), cambay basin (gujarat), assam shelf (assam), and krishna-cauvery basins (south india) largely formed in response to marine transgression and regression during cenozoic time, and have provided valuable sources of lignite, coal, oil and natural gas (biswas, 1982, 1987; nagendra et al., 2011; shah, 2018). these allow us to study the phenomenon of the past sea level changes concerning changes in paleoclimate (naqvi, 2005; kundal & humane, 2007; valdiya, 2016; fareeduddin et al., 2020 y verma, 2021b). 242 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh the himalayan mountains belt, which is a product of the india-asia collision that took place in eocene times, created a unique monsoon system, and provided numerous natural resources, together with increased risks of natural hazards across the indian subcontinent (sharma, 1998; valdiya, 2016; searle & treloar, 2019; parsons et al., 2020). the monsoon system is a boon for the agro-economic sector of the subcontinent. there are opportunities to study the monsoon system to understand its mechanisms and the influence of the el niño-southern oscillation cycle on it (das, 2015; dimri et al., 2016). the complex geological framework of the himalayan makes it as an astonishing natural laboratory to investigate pre-, post-, and syn-collisional and depositional events associated with geodynamics of the indian lithospheric plates and their influence on the environment. the rich fossiliferous palaeozoic-mesozoic successions of the north-western himalaya provide excellent rock outcrops for investigating the biological evolution of invertebrates (corals, mollusks, brachiopods, echinoderms). the himalayan cenozoic successions contain well-preserved vertebrate fossils (particularly of mammals), which are useful for reconstructing mammalian diversity and their bearing on paleoecology, paleoenvironment, and paleobiogeography of the indian subcontinent (naqvi, 2005; valdiya, 2016). the himalayan contains large regions of glaciers, snow, and ice that serve as a significant freshwater reserve in asia and provide uninterrupted water supply to the nine river systems of asia, and these give a lifeline for nearly one-third of humanity. climate change is seriously impacting the snow and ice cover of the himalayan ecosystem, which needs to be investigated (bisht et al. 2019; sabin et al., 2020). the himalaya is divided into five tectonic belts from north to south: trans, tethys, greater, lesser, and outer himalaya (fig. 1). each belt is bounded by prominent tectonic features such as tethyan, main central, main boundary, and himalayan frontal thrusts (naqvi, 2005; valdiya, 2016). these thrusts are seismically active zones and any tectonic movement along these thrusts can cause serious natural hazards such as slope instability, landslides, and earthquakes (rajendran et al., 2017; prashanth et al., 2021). a vast majority, around 75%, of total global landslides occur in the asian continent alone, with a significant number of landslide events occurring along the himalaya mountain belts (froude & petley, 2018). thus, the himalayan region provides unique opportunities for studying plate movements, plate boundaries, seismicity, neotectonics, slope stabilization, soil erosion proneness, climate change, glacial melting, harnessing geological resources, and mountain building processes (jain & sinha, 2005; rajendran et al., 2017; prashanth et al., 2021, 2022; kouser et al., 2022; tandon, 2022). the vast indo-gangetic plains of northern india comprise recent sediments, which were brought down from the rising himalayan by its various rivers (tandon et al., 1991; tandon, 2022). these plains contain enormous water resources and their sediments have made the soil cover of the plains very fertile and so suitable for agriculture and industrial activities. as a result, nearly 50% of the total food grain production to feed 40% population of india comes from these plains (pal et al., 2009). these plains are currently facing alarming amounts of soil degradation and water contamination (mythili & goedecke, 2016). the great thar desert of western india is an active area for exploring mineral resources and for harnessing renewable energy such as solar and wind. thus, geoscientific research plays an influential role in studying the availability and spatial distribution of natural resources, regions of natural hazards, environment-related issues, and sustainable natural resources management, which are of direct concern to the society. historically, the importance of the geological richness of the country was understood in earlier times by the mughal empire, and as a result, they ruled the subcontinent for 332 years from 1526 to 1858 (babu & nautiyal, 2013). meanwhile, britishers (i.e. east india company) invaded the country in 1757, and later, expanded their imperialism, and ruled the country from 1858 to 1947 (wagh, 2018). they understood mineral resource richness, especially the coal resources of the country, and extensively mined several mineral resources during their rule (fox, 1947). the country still has many georesources. some world-famous diamonds namely, kohinoor, great moghul, and regent were discovered from gravels of the krishna valley that were derived from the krishna lamproite (ultramafic igneous rock) field, near the north-eastern margin of the cuddapah basin, southern india (gsi, 2011). recently, geoheritage emerged as an important field of public and scientific interest that promotes geoscience education, research, geotourism, and sustainable development. geoheritage is an essential part of the natural (geocultural) heritage and encompasses “geosites of significant educational, cultural, scientific, and aesthetic value” (https://www.americasgeoheritage.com/geoheritage/). india has numerous rich and diversified geocultural heritage sites and thus, provides ample opportunities to promote education, research, and tourism related to geoheritage (shekhar et al., 2019; biswas & chauhan, 2021; chauhan et al., 2021). the gsi has recognized 26 geological sites as national geological monuments of india that are located in various geologically rich states such as andhra pradesh, telangana, chhattisgarh, gujarat, himachal pradesh, karnataka, kerala, odisha, rajasthan, maharashtra, and tamil nadu (intach, 2016). however, there are numerous geoheritage sites are in india, which are yet to be recognized (fig. 2). 2a 2b figure 2. photographs showing potential geoheritage sites in india. a. huge granite batholith, situated nearly 50 km from hyderabad along the hyderabad-warangal national highway at bhongir town, telangana, south india. this batholith is a part of the eastern dharwar craton and around 2.7 to 2.5 billion years old. b. quaternary fluvio-lacustrine deposits at lamayuru, union territory of ladakh, northern india. these deposits are 105 m thick, dominantly composed of clay and sand layers, which develop a lunar landscape at lamayuru. these deposits are popularly known as “moonland” of the lamayuru. both these geosites are places of tourist’s attraction (courtesy: naveen kumar, geologist, geological survey of india for b). 243geological education scenario in india and role of open educational resources in the light of covid-19 pandemic given this wide geological background, geological education is crucial for india to refine existing geological principles, establish geoheritage sites, explore georesources and ensure their sustainability, and resolve geoenvironment-related problems across the country. development of indian higher geological education the study of geological phenomena in india had been recorded from time immemorial when indians started to observe and record the behavior of earth’s natural processes. the bronze age and iron age, which were associated with the growth and development of human civilizations and their presence in the country, clearly indicate the exploration and utilization of mineral resources by ancient indians. there are several references for using basic concepts of geology and metallurgy in the ancient literature of the country. however, reliable evidence of the early existence of geological information and its utilization for the development of society comes from an old treatise “arthashastra”, composed by an ancient indian statesman and philosopher, kautilya, around 321-296 bc (chakraborty, 1970; murty, 1979). this treatise provides an account of geological knowledge, which is used to discover and extract metals from various ores (goi, 2012). the growth of geology as an independent science discipline emerged in the eighteenth century (chakraborty, 1971; rajendran, 2019). the initial phase of realization of the importance of geology in india was seen with the establishment of the survey of india in 1767 and the asiatic society of bengal in 1784 by the east india company. the main objective of the survey of india was to map the country and produce maps at different scales for discovering natural resources and society had as its objective “enquiry into the history and antiquities, arts, science and literature of asia” (chakraborty, 1971). the results of the survey and society together later played a profound role in the growth of geological studies in india. the opening of a museum in calcutta (now kolkata) in 1840 by the east india company was considered a step towards the beginning of geological education (stubblefield, 1970). indeed, the east india company understood the importance of the geological richness of india, especially the coal resources before 1835. as a consequence, they established a coal committee in 1836 for discovering the coal and mineral resources of the country (kumar, 1982). based on the rich and varied geology of the subcontinent, with its huge resource potential, the east india company formally established the world’s second-largest organization, the geological survey of india (gsi) in 1851, to undertake geological mapping and exploration of mineral resources of the country (oldham, 1856). in 1858, the east india company developed into the british raj (i.e. government of colonial india). but before the 1850s, the colleges of the east india company taught geology and mineralogy, but there was no separate department of geology (larwood, 1958; tolman, 2016). the opening of the gsi marked the informal beginning of geological higher education in india (kumar, 1982; murty, 1982; palit, 2013). shortly after its establishment, henry b. medlicott, an irish geologist, came to india in 1854 to become a professor of geology at the thomson college of civil engineering, roorkee (now indian institute of technology, roorkee) founded in 1847. he was probably the first teacher of geology in india. for a long period, british geologists undertook all activities of the gsi and they were of the view that indians were not capable of doing any kind of sophisticated teaching and research in geology, and no indian university was properly equipped to impart higher geological education. in fact, they were not much interested to open geological education in the higher learning institutes of india (kumar, 1982; tolman, 2016). as a consequence, geological education did not become part of higher education in indian universities until the 1870s (kumar, 1982; tolman, 2016). over time, the british raj found difficulties in recruiting geologists from europe because they rejected the offer of british raj to join gsi due to limited financial benefits and serious health problems in india; particularly, unfavorable climatic conditions, which often caused their death within a few years of joining the gsi (grout, 1995; kumar, 1995). this led to a shortage of geologists in the gsi and to fill this gap, the british raj decided to recruit indian geologists. in 1886, the british raj constituted two positions exclusively for indian geologists within the gsi and also felt the need to establish a sound education system to teach geology (kumar, 1995). to fill the shortage gaps of geologists and associated skilled manpower needed in the gsi and through continuous struggle by the indian educationists, a more systematic effort was made by the british raj and gsi for opening the geology departments at the presidency colleges of kolkata and madras (now chennai), and bombay university in the 1890s (murty, 1982). geology as an optional subject was introduced at bombay university in 1879. in 1892, a separate department of geology was opened at the presidency college (now presidency university), kolkata. another presidency college (now madras university) at chennai already had geology as an auxiliary subject, but an independent department of geology was started in 1910 (murty, 1982). meanwhile, the spread of geological education also reached northern india with the visit of the prince of wales to jammu in 1906. to commemorate his visit, his highness, the maharaja (i.e. ruler) sir pratap singh of jammu and kashmir, built the prince of wales college (now government gandhi memorial science college and is a part of the cluster university of jammu) on the banks of the river tawi. in this college, geology education commenced in 1907, under the stewardship of the renowned indian geologist, dr. d. n. wadia (stubblefield, 1970). over time, many geology departments were opened in other universities namely, banaras hindu university, varanasi in 1923; indian school of mines (now indian institute of technology), dhanbad in 1926; andhra university, visakhapatnam in 1941; lucknow university, lucknow in 1942; aligarh muslim university, aligarh in 1946; patna university, patna in 1946, and rashtrasant tukadoji maharaj university, nagpur in 1946 (stubblefield, 1970; bhattacharya, 2016). thus, the development of geological education was seen as a by-product of the gsi growth and rich georesources of the country. figure 3. map of india showing the distribution of higher learning institutes offering geological education in india. there were around 14 universities in the country that offered geological education before1952 (stubblefield, 1970). following one and a half decades of independence, out of a total of 35 universities, around 20 had departments of geology (wadia, 1964). later, geological education witnessed a substantial expansion, thus, many geology departments were opened in various colleges and universities across the country. at present, there are more than 153 higher learning institutes that host geology departments imparting geological education in india up to a master’s degree level (excluding colleges). these institutes include various universities (state/union territory, central and private), indian institutes of technology, indian institute of science, indian statistical institute, and indian institutes of science education and research (fig. 3). apart from conventional learning systems, geological education is also offered through open and distance modes of learning in india, with the opening of departments at dr. b. r. ambedkar open university in 1991, and indira gandhi national open university in 2010. currently, higher education in geology is being offered at three-degree levels across the country viz., 244 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh bachelor’s degree programs of three-year duration, master’s degree programs of two-year duration, and integrated bachelor’s-master’s degree programs of five-year duration. apart from this, ph.d. in geology is a common research degree program in indian universities. the need for geology education at the school level in india was first highlighted by wadia in 1964 (stubblefield, 1970). so far, the picture of geology education offered at the school level is very sparse; it is largely neglected in comparison with other science disciplines: physics, chemistry, mathematics, biology, engineering, and computer science (tandon, 2005). from kindergarten to 10th grade of high school level (pupils with age range 4 to 16 years), geology education is taught as a part of either general studies or social sciences (paliwal, 2005). in 11th and 12th grade higher school education (pupils with age range 16 to 18 years), there are very few school boards of indian states/union territories offering geology as an independent subject in their science or arts streams. the states/union territories that do so are kerala, rajasthan, jammu and kashmir, jharkhand, karnataka, maharashtra, and manipur (paliwal, 2005; padmanabhan, 2018). status of geology education among other sciences: a comparative analysis numerous public and private sectors including non-governmental organizations in india offer job opportunities to those who had qualified with a master’s degree in geology. thus, an analysis was carried out to evaluate the performance of master’s degree programs in geology, in terms of the successful completion of the program by learners and to make comparisons between the master’s degree programs in geology and other science disciplines to evaluate the percentage of their success rate and to ascertain awareness of geology among the learners. for this analysis, five science disciplines comprising physics, chemistry, botany, zoology, and environmental science were selected. the data about the learners’ enrolment and their successful pass rates (i.e. completion of programs) for the academic sessions from 2011-2012 to 2018-2019 of master’s degree programs in geology, physics, chemistry, botany, zoology, and environmental science was obtained from reports of the mhrd, goi on “all india survey on higher education” (table 1). analysis of the data shows that there was an increasing trend in enrolment numbers of learners in the master’s degree programs of all six science disciplines in the academic sessions 2011-2012 to 2018-2019 (table 1). nearly 40.86% of learners were enrolled in the master’s degree program in chemistry whilst a minimum of learners, at 2.16%, were enrolled in the master’s degree program in geology (fig. 4a). surprisingly that the importance of geology in india was realized in 1851 with the establishment of the gsi, but the enrolment percentage of the master’s degree program in geology indicates that geology had the lowest popularity as a science subject among these six science disciplines (fig. 4a). table 1. learner’s enrolment and successful completion data of master’s degree programs in geology, physics, chemistry, botany, zoology, and environmental science (complied after mhrd reports, 2014, 2015a, b, 2016a, b, 2017, 2018, 2019). academic sessions master’s programs in geology physics chemistry botany zoology environmental science enrolment qualified enrolment qualified enrolment qualified enrolment qualified enrolment qualified enrolment qualified 2011-2012 3564 1801 36954 16585 73984 34588 22850 10288 27581 12606 4520 3335 2012-2013 4028 1962 42898 17202 77858 35495 24896 11140 31965 14008 5100 2549 2013-2014 4749 1786 52496 18829 90101 35142 29680 12276 36559 15184 5395 3129 2014-2015 5179 2008 56824 20558 95295 37142 32619 12140 38850 15703 5797 2528 2015-2016 5597 2076 60889 20759 99888 34639 36736 12473 41025 13899 5393 2462 2016-2017 6039 2133 69754 24211 114328 38216 42542 14625 48774 17453 6252 2484 2017-2018 6281 2469 75114 26915 125805 44027 43967 17131 51672 19134 6766 2553 2018-2019 7134 2843 72797 28737 124526 46925 43036 19208 50904 20903 7281 3243 total 42571 17078 467726 173796 801785 306174 276326 109281 327330 128890 46504 22283 average completion percentage 40.11 37.15 38.18 39.54 39.37 47.91 figure 4. a. pie-chart showing relative enrolment data percentage for master’s degree programs in six science disciplines of academic sessions from 2011-2012 to 20182019. b. bar-chart showing average successful completion percentage of learners in master’s programs in six science disciplines of academic sessions from 2011-2012 to 2018-2019. refer to table 1 for enrolment data. env sci stands for environmental science. 40.86% 23.84% 16.68% 14.09% 2.37% 2.16% chemistry physics zoology botany envi sci geology 4b env sci geology botany zoology chemistry physics 47.91 40.11 39.54 39.37 38.18 31.15 successful comple�on percentage 245geological education scenario in india and role of open educational resources in the light of covid-19 pandemic further data analysis of learner’s enrolment and their successful completion in master’s programs in all six science disciplines from 20112012 to 2018-2019 had given quite interesting results (fig. 4b). the results indicate that the master’s degree program in geology had a very good learner’s average completion rate i.e. 40.11%, and stands second, after average program completion rate of environmental science, which is 47.91% (fig. 4b). the average successful program completion rate for botany, zoology, and chemistry was 39.59%, 39.37%, and 38.18%, respectively. physics had the lowest average completion rate, which is 31.15% (fig. 4b). this may indicate that better quality education was available to the qualified learners of the master’s program in geology, and alternatively, reflects a higher quality of qualified postgraduates of geology. the union public service commission (upsc) of the goi recruits candidates who had master’s degrees in geology as geologists for the gsi almost every year. the applicants from all over the country appear for the upsc written examination to obtain geologist positions in the gsi after completing their master’s degree in geology. thus, the performance of applicants in the upsc geologists’ written examination can provide a means of checking the quality of masters-level geological education in india. in the past few years of the upsc recruitment process for geologists had shown that the subject-specific knowledge of the applicants was a little low and applicants were securing average marks in the written examination, i.e. 30 to 40% (saha, 2016, 2018). nature of teaching-learning resources in geology over the past few years, several reforms in the country’s higher educational system had been made by the ugc and department of higher education of the mhrd of the goi. recently, the ugc implemented a minimum course curriculum under, the choice-based credit system (cbcs) and learning outcomes-based curriculum framework (locf) for bachelor’s degree programs in india to provide high-quality and equitable education (ugc, 2015a, 2019). under the cbcs and locf, the ugc issued a national curriculum in geology for implementation in bachelor’s (both general and honors) degree programs for all higher education institutes in india (ugc, 2015b, c, 2019). the mhrd had taken several remarkable steps in enhancing the quality and providing equity with extensive use of information and communication technology (ict) in higher education. nowadays, online higher education is also being offered to learners through massive open online courses (moocs) through various online platforms (http://www.sakshat.ac.in). in addition, tlr in the form of the oer in various disciplines for the use of teachers and learners have also been provided by the goi (e.g., https://epgp.inflibnet.ac.in/). to know the nature of the tlr in geology for its bachelor’s and master’s degree programs, the tlr recommended for all the courses of these programs were evaluated. for bachelor’s degree programs, the tlr mentioned in ugccbcs and locf in geology, in the form of recommended books/texts, were evaluated (table 2). for master’s degree programs, the master’s curricula of 25 universities (including five universities each from northern, western, southern, eastern, and central regions of india) were downloaded from their respective websites, and the tlr were recommended for all courses of programs were evaluated (table 3). table 2. statistics of the tlr recommended for bachelor’s program in geology by the ugc. curricula types of the ugc total tlr recommended tlr of foreign origin tlr of indian origin no. % no. % cbcs curriculum 239 189 79.08 50 20.92 locf curriculum 234 169 72.23 65 27.77 total tlrs recommended 473 358 75.69 115 24.31 the analysis of recommended tlr by the ugc for bachelor’s degree program in geology under its cbcs and locf schemes shows that 75.69% of the tlr listed in the syllabi are of foreign origin, and only 24.31% are of indian origin (table 2; fig. 5a). similarly, the analysis of recommended tlr in the syllabi of master’s degree programs in geology of 25 universities of five indian regions namely, northern, western, southern, eastern, and central shows that these regions contain 81.36%, 81.05%, 78.30%, 80.20%, 76.29%, and 18.64%, 18.95%, 21.70%, 19.80%, 23.71% trl of foreign and indian origin, respectively (table 3; fig. 5b). as a whole, the recommended trl of these 25 universities include 79.68% of foreign origin, and only 20.32% of indian origin (table 3; fig. 5b). a few of the moocs in geology such as physical and structural geology, stratigraphy and palaeontology, crystallography and mineralogy, photogeology and remote sensing, metamorphic petrology and thermodynamics, metamorphic petrology, and environmental geology are on offer through the ugc-moocs portal (http://ugcmoocs.inflibnet.ac.in/ ugcmoocs/moocs_courses.php). in addition, some modules related to geological courses namely, igneous and metamorphic petrology, sedimentology and petroleum geology, crystallography and mineralogy, economic geology and mineral resources of india, hydrogeology and engineering geology, remote sensing and geographic information system are available as open tlr at e-pgpathshala portal of the ugc (https://epgp.inflibnet.ac.in/home/ viewsubject?catid=448). cbcs locf cbcs-locf 79.08 72.23 75.69 20.92 27.77 24.31 trl percentage of foreign origin trl percentage of indian origin 5a northen western southern eastern central total trl 81.36 81.05 78.3 80.2 76.29 79.68 18.64 18.95 21.7 19.8 23.71 20.32 trl percentage of foreign origin trl percentage of indian origin 5b figure 5. a. bar-chart showing percentage of the tlr recommended in the curricula issued by the ugc under its cbcs and locf schemes for bachelor’s degree programs in geology. b. bar-chart showing the percentage of tlr recommended in the curricula of 25 universities across india for their master’s degree programs in geology. refer to table 2 for the tlr data. 246 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh overall, data indicate that a majority of the tlr for higher education programs in geology in india are of foreign origin. these resources are costly for learners and may or may not be sufficient to meet all content requirements of the various courses in the programs. geology is a distinctive and field-based science discipline so a majority of its courses are based on the geological setting of the country. therefore, the teaching-learning practice of such courses usually demands tlr of indian origin (wadia, 1916; krishnan, 1949; kumar, 2010; mishra, 2016; shah, 2018). the need to develop tlr of indian origin taking the indian geological setting into account has been highlighted by various workers (wadia, 1919; mishra, 2016; kumar, 2010; verma, 2017, 2018, 2019, 2021; shah, 2018). the recent development of tlr in the form of moocs and modules of e-pgpathshala of the ugc are progressive steps taken to enrich tlr of the indian origin for some geology courses, but these resources are still meagre and need to be added extensively. table 3. statistics of the tlr recommended for master’s program in geology covering all parts of india. the letters a to y have been sequentially assigned to these universities within parentheses for identification purposes as their names have been kept anonymous. s. no. university selected for accessing curricula / syllabi total trl recommended tlr of foreign origin tlr of indian origin region of india no. % no. % 1. union territory university (a) 274 218 79.56 56 20.44 northern 2. union territory university (b) 151 128 84.76 23 15.24 3. state university (c) 185 139 75.14 46 24.86 4. state university (d) 240 197 82.10 43 17.90 5. central university (e) 202 174 86.14 28 13.86 subtotal of trl 1052 856 81.36 196 18.64 6. state university (f) 222 191 86.10 31 13.90 western 7. state university (g) 275 221 80.36 54 19.64 8. state university (h) 123 97 78.86 26 21.14 9. state university (i) 200 164 82.00 36 18.00 10. state university (j) 394 311 78.94 83 21.06 subtotal of trl 1214 984 81.05 230 18.95 11. central university (k) 150 122 81.34 28 18.66 southern 12. state university (l) 187 150 80.21 37 19.79 13. state university (m) 287 210 73.17 77 26.83 14. state university (n) 156 131 83.97 25 16.03 15. state university (o) 192 148 77.08 44 22.92 subtotal of trl 972 761 78.30 211 21.70 16. state university (p) 148 114 77.03 34 22.97 eastern 17. central university (q) 182 149 81.87 33 18.13 18. central university (r) 267 210 78.65 57 21.35 19. central university (s) 249 206 82.73 43 17.27 20. state university (t) 169 135 79.88 34 20.12 subtotal of trl 1015 814 80.20 201 19.80 21. state university (u) 121 96 79.34 25 20.66 central 22. central university (v) 172 139 80.82 33 19.18 23. state university (w) 145 114 78.62 31 21.38 24. central university (x) 167 109 65.27 58 34.73 25. state university (y) 171 134 78.36 37 21.64 subtotal of trl 776 592 76.29 184 23.71 total of trl 5029 4007 79.68 1022 20.32 covid-19 pandemic and geological higher education: challenges and opportunities the corona virus disease 2019 (covid-19) was declared a global pandemic by the world health organisation (who) in march 2020 (who, 2020) and recognized as a health, economic, and social crisis. it can be viewed as both a challenge and opportunity as far as geological education and research are concerned. due to the covid-19 outbreak, all educational institutions including schools, colleges, and universities in india were closed from 24th march 2020 due to lockdown implementation across the country by the goi, which opened later on in a phased manner (https://www.mohfw.gov.in/). the months-long closure of educational institutions in 2020-2021 had stopped classroom teaching and thus, badly hampered teaching-learning processes whilst 247geological education scenario in india and role of open educational resources in the light of covid-19 pandemic disengaging learners from active learning. it impacted the mental health of both learners and teachers. in order to provide uninterrupted learning to learners, the ugc and mhrd advised higher learning institutes to make extensive use of the ict to deliver online higher education to learners (ugc, 2020). it also advised learners to use the oer available on various online platforms such as swayam online courses, e-pgpathshala, etc. to continue their learning. covid-19 has had at least two impacts on higher geological education, firstly, the suspension of regular classroom teaching, and secondly, not allowing learners to undertake geological fieldwork in out-door environments because it involves moving learners between sites and use of public transport in groups. the loss of classroom teaching can be substituted by the use of online tlr, although the value of online tlr for particular courses or programs needs to be considered. however, geological fieldwork is one of the distinctive and compulsory components of both bachelor’s and master’s degree programs in geology. it involves out-door visits of learners where they study rock outcrops exposed in their natural settings, for example, in quarries or mines, road cuttings, hilly and mountainous terrains, and along the river valleys. in the field, the learners carefully observe, record, and measure various geological features. they learn to identify various rock-types, fossils, structural features (fold, faults, joints, unconformities), and techniques for collecting samples in the field (lisle et al., 2011). in addition, they also learn how to measure dip, strike, and thickness of rock units and structural features, and how to document field data in field diaries. the overall purpose of geological fieldwork is to develop field-based geological skills and understanding in the learners (lisle et al., 2011). as geology is a field-based science, a degree without having field-related skills is of no value to learners. therefore, learners of degree programs in geology in one region of the country normally visit other regions to undertake geological fieldwork and develop sound field-based geological skills, often traveling distances of hundreds of kilometers. to summarize, the major challenges posed by covid-19 to the geological teaching community are: (a) to evaluate whether the currently available online tlr are sufficient to meet the current and future demands or whether more tlrs are required, and (b) if, it becomes impossible to arrange geological fieldwork in the coming two years due to travel restrictions because of the extended covid-19 infection, what would be the best possible solution to the problem might be. oer and their role in geological higher education the global teaching-learning community has come to understand the real value of the oer at a time when covid-19 has led to the sudden closure of educational institutions (huang et al., 2020). in order to meet educational challenges posed by the covid-19 outbreak, many countries, including india, advised institutions to adopt a policy of undisrupted learning by offering online, remote, and distance teaching (huang et al., 2020; ugc, 2020). in this context, oer emerged as ideal educational resources because they are in digital format, freely accessible at anytime, anywhere, and easy to incorporate into the existing learning system of any educational institution (smith, 2009). the oer had been a part of the education system since the advent of computer and internet technology around the 1990s. oer are “teaching, learning, and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation, and redistribution by others with no or limited restrictions” (https:// hewlett.org/strategy/open-educational-resources/). there are numerous types of oer comprising course materials, full courses, modules, encyclopedias, text-books, games, lesson plans, syllabi, quizzes, instructional modules, model tests, assignments, practical modules, educational audios/videos, virtual field trips, virtual field experiences, and virtual science laboratories (smith, 2009; waldrop, 2013). in addition, the oer also contain rich teaching resources, including methods and tools of teaching that led to improved pedagogical skills, growth, and professional development of teachers. there are several higher education organizations/societies around the world that have been developing oer in geological education and offering them free online in various formats like pdf, html, videos, etc. for the benefit of the global teaching-learning community (table 4). table 4. name and web link of educational institutions and societies offering geological oer s.no. name educational institution /society/ organization web link of geological oer 1. humboldt state university https://libguides.humboldt.edu/openedu/geol 2. university of houston-victoria https://library.uhv.edu/oer/geology 3. washtenaw community college https://libguides.wccnet.edu/oer-subjects/geology 4. mt san antonio college https://mtsac.libguides.com/oer/earth-science 5. university of kentucky https://libguides.uky.edu/c.php?g=222941&p=6722485 6. austin community college https://researchguides.austincc.edu/c.php?g=434670&p=5818520 7. east tennessee state university https://libraries.etsu.edu/research/guides/geosciences/oer 8. salt lake community college https://opengeology.org/textbook/ 9. san bernardino valley college https://www.valleycollege.edu/open-education-resources/faculty/geology.php, 10. creative commons https://www.oercommons.org/browse?f.search=geology 11. the open university https://www.open.edu/openlearn/science-maths-technology/science/geology 12. university of edinburgh https://open.ed.ac.uk/tag/geoscience/ 13. international geoscience education organization http://www.igeoscied.org/teaching-resources/ 14. geological society https://www.geolsoc.org.uk/heresources 15. british geological survey https://www.bgs.ac.uk/discoveringgeology/newsandevents/links.html 16. us geological survey https://www.usgs.gov/science-support/osqi/yes/resources-teachers 17. society of vertebrate palaeontology http://vertpaleo.org/what-is-vertebrate-paleontology/resources-foreducators-and-students.aspx 248 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh many higher learning institutes in india had both typed and handwritten specific-course-based study material uploaded on their websites to provide continuity of learning to their learners during the covid-19 crisis. the goi has previously noted the potential of oer to offer high-quality and equitable education across the country (goi, 2009). the covid-19 crisis prompted the geology teaching-learning community to appreciate the potential of oer in higher education. as a consequence, a few oer in geology are available at the e-pgpathshala portal of the ugc (https://epgp.inflibnet.ac.in/). laboratory experiments and fieldwork are very important components in the successful completion of any degree program of higher geological education. in this regard, oer consisting of laboratory experiments, virtual geological laboratories, virtual field experiences, and geological field modules can offer hands-on training to learners in geological education. for example, google earth images have been extensively used as resources for teaching geological principles as well as for mapping, identifying, and interpreting geological features since 2005 (lisle, 2006; monet & greene, 2012). the indian lithospheric plate contains numerous geological features notably; geomorphic, tectonic, structural, stratigraphic, and petrologic that can be examined by utilizing freely downloadable high-resolution satellite images from the google earth program (http://earth.google.com). oer consisting of geological virtual fieldwork and modules of field-specific problems based on indian outcrops using images of google earth were being developed, through which learners of bachelor’s and master’s degree programs in geology may complete their compulsory fieldwork-based assignments in the light of the covid-19 outbreak. oer usefulness in higher geological education: data collection and interpretation a structured questionnaire was prepared to investigate the value of oer in higher geological education in india and their role in the covid-19 response efforts. the questionnaire was divided into three parts: personal data, educational resources in geological education, and, the use and need of oer in the higher indian geological education system. all questions of the questionnaire were simple, easy to understand, and consisted mainly of multiple-choice options with a few short answer types. part a: personal data provided information regarding age, gender, educational qualification, profession, affiliation, teaching experience, and mode of teaching before the covid-19 pandemic of the respondents. part b: use of educational resources in geological education was designed to get information from respondents about the tlr of indian and foreign origin that they had been using before and during the covid-19. part c: oer contained questions related to familiarity of the oer to the faculty, changes in their use before and during the covid-19, and their potential to meet the requirement of compulsory components (fieldwork and laboratory) of higher degree programs. the consolidated list of all indian universities was obtained from the website of the ugc (https://www.ugc.ac.in/#) and one by one, the websites of all the universities were visited to find whether or not the university had a department of geology/applied geology/earth science/geological sciences, and further e-mail addresses of the existing teaching faculty of the departments were collected. the questionnaire was prepared using the google forms service of google docs (https://www.google.com/forms/about/) and sent online to the collected e-mail addresses of almost all universities faculty of geology in the entire country. additionally, the questionnaire was also sent to the geological teaching faculty of indian institutes of technology, indian institutes of science education and research, a few geologists of the gsi, and the indian statistical institute. the questionnaire was sent to a total of 790 people, from which, 63 responses were received covering almost all parts of the country (fig. 6). the data was collected between july and october 2020. a majority of responses were complete and usable for interpretation. we followed braun & clarke’s (2006) six-step approach for undertaking thematic analysis of questionnaires received that involves becoming familiar with data, generating initial codes, searching themes, reviewing themes, defining themes, and, finally, undertaking interpretation. figure 6. map of india showing the distribution of places and response numbers in parenthesis from where the completed questionnaires were received. 3. descriptive statistics analysis part a: personal data the analyzed sample shows that out of the total sample number (n = 63), the majority (i.e. 81%) of respondents hold doctorate degrees (ph.d.) and a small group of just 17% holds master’s degrees. 83% (52) of respondents were males and only 17% (11) were females. the mean age of respondents was 44 years. they had a wide range of teaching experience and had been working at various positions viz., assistant professors (51%), professors (19%), associate professors (6%), researchers (5%), and geoscientists (5%) in descending order. a majority of respondents were at the beginning or in the middle of their teaching career, as is apparent from their teaching experience statistics: 0-5 years (28%), 5-10 years (23%), 10-15 years (15%) and 15-20 years (15%). two (3%) respondents were not involved in teaching and the remaining 61 (97%) were directly involved in bachelor’s and/or master’s level teaching. part b: educational resources in geological education, use of tlr of indian and foreign origin before and during the covid-19 before the covid-19 pandemic, a majority of respondents (84%) were involved in face-to-face teaching and a minor proportion was also involved in online teaching (16%). it is inferred that out of the 61 respondents who responded to this item, the institutes/universities of almost one-third of the respondents (27%) did not provide any support and training in shifting from face-to-face to online modes of teaching. however, the institutes/universities of the remaining respondents, who were involved in face-to-face teaching before the pandemic, did provide support and training in moving to online modes of teaching. such institutions had the advantage of being able to provide undisrupted learning during the pandemic. 249geological education scenario in india and role of open educational resources in the light of covid-19 pandemic the respondents, in assessing the quality of the tlr, showed more positivity towards the foreign origin resources (62%) in terms of cost, quality and availability, in comparison to the indian origin resources (38%). however, they were of the view that the tlr of indian origin are slightly more comprehensible than that of foreign origin. they thought that more preference should be given to the tlr of indian origin (75%) in comparison to those of foreign origin (67%). moreover, the majority (77%) declared that they found some shortage in the tlr of indian origin and almost all the respondents (94%) thought that more courses-based tlr incorporating indian geological context are needed. almost one-third (28%) of the respondents commented that the tlr of indian origin are not up-to-date. the questionnaire tested the change in the usage of tlr for teaching before and during covid-19 (table 5). the data showed a significant decrease in the use of hand-written content, followed by real field trips and real science laboratory work. however, typed learning content continued to be used almost in the same way as it was used before and during covid-19. during the pandemic, the use of online platforms increased significantly from 26% to 66%. table 5. kind and percentage of tlr used before and after the covid-19 kind of tlr before covid-19 during covid-19 hand written contents 61% 29% typed contents 65% 66% typed containing all learning requirements and plagiarism free with no copyright issue 47% 42% real science laboratory activities 55% 6% real field trips 69% 3% online platforms like swayam, e-pgpathshala, and other online platforms 26% 66% part c: oer use before and during the covid-19 the respondents reported that they were quite well informed about oer. around 81% had heard about oer before the survey, and nearly 61% had previously used them in the teaching-learning process. they (79%) had informed their students about the existence of the oer, and more than half (65%) were familiar with the appropriate geological oer, while 63% were acquainted with the oer of indian origin. a majority of respondents reported that they were aware of indian oer offering online platforms such as nptel (https://nptel.ac.in/), swayam (https://swayam.gov.in/), e-pgpathshala (https:// epgp.inflibnet.ac.in/), and others. additionally, they were also aware of several foreign geological oer offering online platforms. the data also revealed that respondents were using various types of oer such as open textbooks, educational audios and videos, lesson plans, modules, and practical modules both before and during the covid-19 pandemic. the majority of respondents were of the view that oer could work well for learners pursuing bachelor’s (70%) and even more, for master’s (78%) degree programs in geology. even before the covid-19 pandemic, there was a quite wide variety of oer that had been used in teaching-learning practice; the most commonly used were open textbooks and videos available online (table 6). there was a slight decrease in the use of open textbooks during covid-19 and an increase in the use of online audio and videos, especially of indian origin increasing from 12% to 28% (table 6). the use of online lesson plans and assignments for students remained almost the same. it was anticipated that there might have been an increase in the use of virtual field trips or virtual science lab oer, but this is not the case, at least not according to the data so far collected. this may reflect the non-availability of virtual field trips or virtual science lab oer of indian origin. table 6. trends in use of indian and foreign oer before and during the covid-19 oer types indian oer foreign oer before covid-19 during covid-19 before covid-19 during covid-19 open textbooks 79% 56% 66% 48% online videos 67% 77% 69% 83% online audios 12% 28% 26% 31% online lesson plans 34% 30% 19% 26% modules 24% 28% 24% 19% complete courses 24% 16% 17% 12% moocs 29% 37% 22% 24% practical modules 24% 16% 12% 17% virtual science laboratory 16% 7% 12% 14% virtual field trips 16% 8% 10% 10% assignment for students 38% 38% 25% 21% thematic analysis indicates that the respondents thought that virtual open laboratory and field-based manuals could enhance the practical and field skills of learners to a reasonable extent. in fact, they (35%) were also of the view that virtual field trips based on google earth programs and, identification and measurement of various geological features by using high-resolution images, was a good alternative for completing compulsory geological fieldwork assignment for learners during the covid-19 pandemic; 48% believed this at least to some extent. as far as practical components of the curriculum of programs are concerned, 35% of respondents realized that open laboratory manuals and virtual open labs were a good alternative, whilst nearly 48% thought this alternative was useful to a certain extent only. a small group consisting of 11% of respondents were quite negative about the potential use of virtual open laboratory, open field-based manuals, and open virtual field trips based on google earth. overall, 20% of respondents were enthusiastic about exploring all the possibilities of the oer related to field and laboratory components. meanwhile, more than one-third of the respondents (37%) were positive about the availability of indian origin oer for teaching geology. the majority of respondents (75%) thought that it is quite important to use oer of either indian or foreign origin, where content is enriched with indian geological examples. almost two-thirds (65%) feel that learners would benefit through the use of oer and that they have great potential to contribute to equitable and high-quality geological education across the country. when they were asked through the questionnaire what they had planned for the completion of compulsory geological fieldwork assignment during the covid-19 pandemic, almost one-third (31%) hoped to be able to carry out the fieldwork 250 omkar verma, manthena prashanth, roberto greco, ashu khosla, kulwant singh after the end of the covid-19 pandemic, around 20% had no plan, and 10% would like to wait until the end of the pandemic, and in the meantime, they will use online resources. the remaining one-third (39%) planned to use the online training and virtual field geology programs available from professional organizations and webinars for this purpose. to complete the laboratory component of the curriculum during the covid-19 pandemic, around one-third (34%) plan to complete it after the end of the covid-19 pandemic, almost another third (24%) plan to complete the laboratory component of the curriculum by maintaining social distancing, using precautionary measures, and calling students in small groups to the laboratory, 15% had no plan, and the remaining 27% will think about the use of oers and online video demonstrations for completing the activity. finally, in the comment section of the questionnaire, respondents were of the view that geology is a field-based science and geological fieldwork is therefore vital for imparting real field skills to the learners. however, oer, particularly course-specific, open lab manuals, open field-based manuals, virtual field experiences, and virtual field trips based on the google earth program related to the indian geological context, should be developed in almost all branches of geology. overall, the questionnaire data indicates that oer will enhance understanding of geology and increase the skill of the learners, which will reduce the paucity of tlr. further, they will help in providing equitable and high-quality higher geological education and make geological education more flexible by providing a new teaching-learning experience with the potential to tackle any future covid-19-like situation. 4. conclusions geological education and research are the basic building blocks of the socio-economic growth and development of any country. geologically, india is a unique country, because of the huge amount of georesources it contains. the mineral resource richness of the country was first fully understood by the british east india company in 1835. they established the world’s secondlargest organization, the gsi, in 1851 in india, to prospect for mineral resources. after a long time gap, geology emerged as an independent science education discipline in the 1890s as a by-product of the gsi development. in the early nineteenth century, only a few universities offered higher education degree programs in geology. then, the number of institutes offering higher education programs in geology substantially expanded in the late nineteenth to the early twentieth century. however, geology as a science discipline within higher education is still less popular than other science disciplines in india. further, indian higher education in geology still relies on costly and less accessible teaching-learning resources. covid-19 has hit geological higher education badly by disrupting faceto-face teaching-learning processes and preventing learners from undertaking compulsory geological fieldwork assignments in their programs. in this context, open educational resources, which are course-based or chapter-specific free digital resources available in various downloadable formats (like pdf), have provided a suitable option for providing undisrupted, online, and remote learning, in overcoming many of the educational challenges posed by the covid-19 pandemic. open educational resources policies are valued by the government of india and many educational institutions are implementing them. they can play a critical role in offering high-quality and equitable education. the availability of these resources in higher geological education is poor and not sufficient to meet the current demands of the teaching-learning community of the country. the questionnaire data show that there is an urgent need for open educational resources in all branches of geology especially focused on course-specific modules, open lab manuals, open field-based manuals, virtual field experiences, and virtual field trips based on the google earth program. the latter should be based on indian geological contexts to provide effective teaching-learning practices in geological education. therefore, it is critical to design and develops more open educational resources in geology to meet the current demand of the country in terms of indigenous learning as well as to face any future covid-19-like challenges. acknowledgments the authors are highly thankful to the geological professionals (teachers and scientists) for their participation in the online survey and for offering their valuable views. we are highly thankful to late prof. c. king for offering initial comments on the manuscript. the authors also sincerely thank five 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(2024). geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil. earth sciences research journal, 28(3), 239-254. https://doi. org/10.15446/esrj.v28n3.113767 abstract the western meridional borborema province (mbp) and the northern part of the são francisco craton (sfc) experienced significant tectonic events during the neoproterozoic era. however, their geological history remains incompletely understood due to the absence of comprehensive studies on the deep crustal structure situated in the western mbp. this study aims to explore the boundaries of the crustal blocks between the western mbp and the northern sfc, with the objective of enhancing the understanding and contributing to the reconstruction of their geological history. to achieve the defined objectives, we utilized, gravimetric, magnetic, and elevation data. lateral extensions were determined by anomalous magnetic field transformations, while source depths were calculated using the an-euler method. the results revealed source depths between 1.0 km and above 5.0 km, with a structural index of 1.0 to 3.0, are observed in both the sfc and the borborema cross-section, as well as west of the sergipano belt. these different structural indices represent magnetic source geometries, which can vary from a dyke, an intrusive body or a localized mineral concentration. these results were used as a priori information in the 3d forward modeling of the crust to obtain the density contrast distribution in the subsurface. the results indicate that the riacho do pontal belt has a crustal thickness of 38.2 km, which is thinner compared to the values ranging from 41.6 km to the north of the sfc with values ranging from 41.6 km. keywords: magnetic data; 3d gravity modeling; crustal structure. caracterización geofísica y tectónica de la provincia de borborema sudoccidental, en el nordeste de brasil resumen la provincia meridional borborema occidental (pmb) y la parte norte del cratón de são francisco (csf) experimentaron eventos tectónicos significativos durante la era neoproterozoica. sin embargo, su historia geológica sigue siendo incompletamente comprendida debido a la ausencia de estudios integrales sobre la estructura crustal profunda situada en la provincia meridional borborema occidental. este estudio tiene como objetivo explorar los límites de los bloques crustales entre la pmb occidental y el csf norte, con el objetivo de mejorar la comprensión y contribuir a la reconstrucción de su historia geológica. para lograr los objetivos definidos, utilizamos datos geofísicos, gravimétricos, magnéticos y de elevación. las extensiones laterales fueron determinadas por transformaciones anómalas del campo magnético, mientras que las profundidades de las fuentes se calcularon utilizando el método an-euler. los resultados revelaron profundidades de fuentes entre 1.0 km y por encima de 5.0 km, con un índice estructural de 1.0 a 3.0, que se observan tanto en el csf como en la sección transversal de borborema, así como al oeste del cinturón sergipano. estos resultados se utilizaron como información a priori en la modelización 3d directa de la corteza para obtener la distribución del contraste de densidad en el subsuelo. los resultados de espesor crustal indican adelgazamiento crustal en el cinturón riacho do pontal a 38.2 km, en comparación con el norte del csf con valores que oscilan entre 41.6 km. palabras clave: datos magnéticos; modelado gravitacional 3d; estructura de la corteza terrestre https://doi.org/10.15446/esrj.v28n3.113767 https://doi.org/10.15446/esrj.v28n3.113767 https://doi.org/10.15446/esrj.v28n3.113767 240 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores 1. introduction the borborema province is a crucial geological complex subject to significant tectonic phenomena involving several crustal blocks in northeastern brazil. however, the comprehensive understanding of its tectonic history remains uncertain due to the lack of consistent studies on the deep crustal structure of the crystalline basement in the study area, situated to the west of the southern borborema province. the delimitation of the boundaries between crustal blocks, the investigation of their origins and the precise definition of crustal limits continue to be issues that are susceptible to uncertainty within the geoscientific context. an illustrative example of this challenge is the demarcation between the sfc basement’s northern segment and the borborema southern province’s western region (mbp), characterized by overlaps. the complexity inherent in this demarcation results from the overlapping of tectonic events, intricate geological structures, substantial deformations of significant magnitude and extensive magmatic occurrences, the result of geotectonic compartmentalization during the collisional and postcollisional phases of the brasiliana/pan-african orogeny. the tectonic occurrences in the mbp have been substantiated by caxito et al. (2016, 2017, 2020), who conducted investigations into these events, ultimately affirming the existence of a comprehensive wilson cycle within the riacho do pontal orogen. several significant geophysical studies have contributed to elucidating this phenomenon, including the works of oliveira (2008), santos et al. (2014), correa et al. (2016), and oliveira and medeiros (2018). geophysical data, encompassing gravity and magnetic measurements, have been employed to acquire insights into crustal structures, facilitating an enhanced comprehension of the geotectonic processes in the borborema province during the neoproterozoic era. several researchers have elaborated on this geotectonic framework by interpreting lateral variations in density and magnetization and estimating elastic parameters. in the work by santos et al. (2014), it was found that in the southern domain of the province (sergipano belt and pernambuco-alagoas block), the crust is heterogeneous, while the upper mantle is abnormally conductive. the crust and mantle are predominantly homogeneous and resistive when entering the transverse domain (alto moxotó terrain). this difference in the heterogeneity of the crust is interpreted as being mainly due to the extensional activity that took place in the cretaceous period. these efforts aim to identify the underlying factors responsible for the uplift of the borborema province. such endeavors bolster magnetic and gravity data utilization within the academic sphere, facilitating a deeper understanding and precise delineation of the geological terrain (sampaio et al., 2017; silva and sampaio, 2017). this article offers valuable insights into the western mbp and the northern sfc basement. this is achieved by leveraging the processing outcomes and interpretations of pre-existing gravity and aeromagnetic data through 3d forward modeling. as a result, this study has successfully estimated the depth of the uppermost sources, identified and characterized the boundaries of anomalous subsurface bodies, and delineated lineations and shear zones. additionally, it has elucidated variations in subsurface crustal density, which were previously insufficiently documented in the studied region. to accomplish these objectives, a comprehensive range of geophysical techniques was employed, including the application of the total magnetic intensity (tmi) map, the analytical signal of tilt angle (asta), tilt angle (tdr) analysis, vertical derivative (dz), and the an-euler method. the resulting findings offer valuable insights into the geotectonic configuration and its associated boundaries, thereby enhancing our understanding of the crustal structure within the context of gondwana formation. in addition, a three-dimensional (3d) gravimetric modeling approach was adopted, which involves the creation of a subsurface model that represents the distribution of density contrast. this methodology aims to better understand the architecture of the earth’s crust in the study area. the choice of this technique is based on its ability to allow the interpretation of potential data, integrating synergistically with geological information and average density values. 2. geological context the borborema province exhibits a subdivision into three distinct tectonic subprovinces, namely the northern, transversal (central), and southern subprovinces. moreover, the primary demarcation lines that delineate these domains are represented by the patos and pernambuco shear zones, as established by prior research (brito neves et al., 2000; medeiros, 2004; oliveira, 2008; caxito et al., 2017; lima, 2018). the geological history of the borborema province is a complex narrative of tectonic and magmatic processes stretching from the archean to the paleoproterozoic with signatures of granitoids of neoproterozoic origin. this has resulted in a diverse series of geological features and structures that reveal the evolution of the province in various stages. the bedrock of the province is composed of an archean complex, surrounded by fragments of paleoproterozoic orthogneiss. within this archaean complex, the são josé do campestre massif stands out, which includes rocks from 3.45 2.70 ga (dantas et al., 2013). subsequent periods (1000-960 ma) witnessed the formation of volcanic and plutonic rocks during the meso-neoproterozoic transition, notably during the cariris velhos orogeny (santos et al., 2024). these events shaped the geological composition of the province and added layers of complexity to its history, the augen-gnaisse suite of afeição (caxito et al., 2014). a later and crucial phase in the evolution of the province was the complete tectonic cycle known as pan-african/brazilian, occurring between 900 and 500 ma (caxito et al., 2017; caxito et al., 2022). this cycle involved intricate processes such as the creation of oceanic crust, subduction, the formation of magmatic arcs and, eventually, continental collision. these activities left a lasting mark on the province, characterized by intense tectonic and metamorphic phenomena. the culmination of this geological saga is seen in the formation of magmatic intrusions, particularly notable in the pernambuco-alagoas domain and in the northeastern region of the province (cruz et al., 2014). these intrusions, resulting from the intense igneous activity associated with the pan-african/brazilian, significantly influence the density and magnetism characteristics of the province’s basement, adding another layer of complexity to its geological history. the southern subprovince encompasses the meridional borborema province (mbp), representing a significant tectonic segment within the borborema province. the hypothesis proposed for the geological evolution of the borborema province suggests that its crust evolved through a series of classic wilson cycles (caxito et al., 2022; caxito et al., 2016; araujo et al., 2014). these cycles involved crustal rifting, the opening and closing of oceans, the establishment of subduction zones and, eventually, continental collision. this geological evolution results from the neoproterozoic orogenic event that transpired between south america and west africa during the formation of the supercontinent gondwana. during this period, major continental blocks, including the amazonian craton, são francisco-congo, and são luis-west africa, converged and interacted as part of the brazilian/pan-african orogeny in the neoproterozoic. various researchers have extensively studied and documented this complex geological history (oliveira, 1998, 2008; da silva et al., 2008; silva et al., 2011; araujo et al., 2014; lima et al., 2018; passos et al., 2022). the mbp is located within a specific tectonic context: i) to the northwest, it is partially overlain by the paraíba basin; ii) to the north, it is delimited by the pernambuco lineament; iii) to the south, it extends to the sfc basement, encompassing the riacho do pontal belt, the pernambuco-alagoas domain and the sergipano belt (fig. 1). in addition, the eastern portion of the mbp contains geotectonic domains that form the foundation of the sergipe-alagoas basin. these domains include, from north to south, canindé, poço redondo-marancó, macururé, vaza barris and estância, delimited by neoproterozoic shear zones: macururé, belo montejeremoabo, são miguel do aleixo and itaporanga (oliveira et al., 2010; passos et al., 2022). lima et al. (2018) conducted a study on metasedimentary and metavolcanic rocks of the araticum complex, associating their formation with an oceanic volcanic arc environment linked to the brasiliana orogeny. later, passos et al. (2022) carried out research on amphibolites, interpreting them as part of the canindé domain and relating them to the neoproterozoic arc and retroarc system on the continental margin of the borborema province. these interpretations obtained by silva et al. (1999), oliveira et al. (2010), lima et al. (2018), and passos et al. (2022) support the existence of an oceanic basin between the pernambuco-alagoas domain and the sfc. this interpretation reinforces the model of extensional tectonics, which would have resulted in the opening of the ocean between the são francisco-congo craton and the pernambuco-alagoas domain, followed by later collisional tectonics 241geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil between 680-540 ma with intrusion of gabbroic bodies into the geotectonic domains that make up the sergipano belt , strengthening the existence of a wilson classic cycle model (silva et al., 1999; oliveira et al., 2010; lima et al., 2018; passos et al., 2022). to the south of the mbp lies the salvador-esplanada-boquim domain, which is associated with the sfc basement, as documented by santos and medeiros (1999). figure 1. map of the tectonic domains of the southern and transversal borborema province. the riacho do pontal belt, a significant geological feature in the region, exhibits various geological characteristics, including sedimentary, metamorphic, and structural features. this belt is typically subdivided into three main zones: the internal zone, the central zone, and the external zone. the geological attributes of each zone have been extensively studied and documented by various researchers, such as oliveira (1998, 2008), caxito (2013), oliveira and medeiros (2018). starting with the internal zone, this area is characterized by geological complexes like the paulistana complex, santa filomena complex, and morro branco, along with igneous suites such as serra da aldeia and rajada. it features granite intrusions, metavolcanic-sedimentary rocks from the paulistana, santa filomena, and morro branco complexes, and additional igneous suites like afeição, rajada, serra da aldeia, brejo seco, and são francisco de assis (caxito, 2013; caxito et al., 2016, 2017). moving on to the central zone, the prominent geological feature here is the monte orebe complex. this zone encompasses a mix of volcanic and sedimentary associations. researchers like brito de brito neves et al. (2015) have attributed this unit to transforming tholeiitic metabasalts with low potassium content, possibly indicating an oceanic floor or island arch environment. finally, the external zone, situated at the southernmost part of the belt, includes the casa nova complex and is characterized by a foreland folding strip. geophysical studies conducted by oliveira (1998, 2008) in this region suggest low supracrustals and nappe thickness structures associated with the sfc. overall, these detailed geological investigations and categorizations provide valuable insights into the complex geological history and structural evolution of the riacho do pontal belt and its surrounding areas. the pernambuco-alagoas domain is characterized by a diverse composition, including high-grade terrains, archean and paleoproterozoic formations, metavolcanic-sedimentary rocks, and granitoids (neves and da silva filho, 2019). within this domain, two significant metamorphic complexes are prominent: the belém do são francisco complex, primarily composed of granite and granodioritic orthogneisses, and the cabrobó complex, which comprises a variety of metamorphic rocks, including schist, gneiss, metagraywacke, quartzite, marble, calc-silicate rocks, and amphibolite (lima, 2018). the sergipe belt, situated within the southern sub-province of the mbp, borders to the north with the pernambuco-alagoas domain and to the south with the (sfc). this belt signifies a collisional phase during the brasiliano orogenic event. it is characterized by a supracrustal sequence of metamorphosed rocks displaying characteristics of a passive continental margin and a retro-arc basin (foreland), as detailed by oliveira et al. (2015). the (sfc) constitutes a significant geotectonic entity within the south american platform. it encompasses proterozoic and phanerozoic units, including the são francisco basin, the paramirim group, and a portion of the tucano-jatobá rift. the boundaries of the sfc are delineated by geological belts formed during the brasiliano orogeny, including the brasilia belt to the west and the rio preto belt to the northwest. additionally, the riacho do pontal belt is located in the northern part of craton (barbosa and sabaté, 2004). the works of allard and hurst (1969), cordani et al. (1973), and trompette (1997) have highlighted the presence of metasedimentary rocks in the vaza-barris group located in the southern part of the sergipano belt, drawing connections to the ndjole belt in northern gabon, africa. a hypothesis also suggests a significant collision event, supported by geophysical anomalies within the borborema province, inferring that it may represent a suture zone between ancient continental blocks (oliveira, 2008). this orogenic activity is observed in the riacho do pontal belt, characterized by five stages of geodynamic evolution and interpreted as a complete wilson cycle (caxito et al., 2016). 3. data and numerical method 3.1. geophysical data we used different geophysical data sets: aeromagnetic and gravimetric data, geoid anomaly and topography. the aeromagnetic data used in this study were obtained from geophysical aerial survey projects conducted and provided by cprm brazilian geological survey and cbpm companhia bahia de pesquisa mineral. these projects include the following: serra de itiúba (project 1023), southern edge of the parnaiba basin (project 1027), and pernambucopiauí (project 1067). additionally, the data in geotiff files format were provided by cbpm, originating from the aerogeophysical projects: campo alegre de lourdes-mortugaba (project 29), center north bahia (project 36), senhor do bonfim (project 23), campo formoso (project 22), and rio seco-andorinha (project 27). these datasets were acquired with flight line spacing ranging from 500 meters to 2 kilometers and control lines spanning 2 to 5 kilometers. the data obtained from these banks, to which we had access, had already been corrected by the igrf. data processing and analysis were done using the geosoft oasis montaj software version 9.9. the regular grid, containing all the aeromagnetic projects, was generated using the grid stitching tool using the suture method, available in the same version of the software. this technique is used to join several grids together. furthermore, gravity data in the form of bouguer anomalies, geoid anomalies, and topographic data were acquired from the international center for global earth models (icgem) database. the egm2008 model (european improved gravity model of the earth) developed by pavlis et al. (2012) was utilized for bouguer anomaly calculations. the geoid anomaly data were derived from the geco global gravity model introduced by gilardoni et al. (2016). topography data were extracted from the etopo1 global relief model, accessible through the national geospatial-intelligence agency — earth gravitational model (nga, 2008) platform, with an arc resolution of 1×1 minute. 3.2. data processing the magnetic data set was interpolated using the bidirectional method with the 125x125-meter interpolation cell. the initial step involved constructing a power spectrum using data about the anomalous magnetic field. spectral analysis, rooted in the principles of the fourier transform, was employed to estimate the average depth of subsurface bodies. this estimation was based on the spatial correlation between field intensity and wave number, as detailed by spector and grant (1970). through this spectral analysis, we determined a cut-off point (wave number) that delineates the boundary between shallow and deep sources and their respective depths. following establishing this cut-off point, we applied an upward continuation filter to the magnetic data. the cut-off point was set at 0.266 km, with a wave number of 0.70 cycles per kilometer (cycles/km), 242 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores obtained from the radial power spectrum. this filter attenuated high-frequency components associated with shallow sources while accentuating or amplifying signals originating from deeper sources, corresponding to the bedrock. after the spectral analysis, we enhanced our geophysical analysis by implementing two additional techniques: the vertical derivative (dz) method, as detailed by blakely (1996) and cooper and cowan (2004), and the slope angle analytical signal (asta) approach, as described by miller and singh (1994). these methods were important in delineating geological structures and identifying geophysical lineaments within the area of interest. moreover, we determined the preferred orientations of these geophysical lineaments using rosette diagrams, which provided valuable insights into the structural characteristics of the region. together with the spectral analysis, we also used the nabighian (1972) an-euler and analytical signal (asa) deconvolution technique to emphasize and position the amplitude peaks in the causative sources in order to get an idea of the geometry of the geotectonic domains in the study area. these windows were based on geotectonic domains such as the sfc’s western portion, the sfc’s northern southern portion, the sergipano belt, the transversal subprovince, and part of the southern borborema. the structural index assigned was from 1 to 3. the structural index (si) = 0 was not used, as it would indicate that the anomaly remained constant, regardless of the distance from the source. this set allowed us to determine the horizontal location and depth of the magnetic sources. this set of parameters and techniques allowed us to determine the horizontal location and depth of the magnetic sources. the results of the aneuler deconvolution technique were incorporated as a priori information in our crustal modeling efforts. to estimate the depth of the moho, we leveraged topography and geoid anomaly data, employing the methodology established by fullea et al. (2007). this approach integrates various data, like topography, geoid anomaly, and thermal analysis, to determine crust (moho) and lithosphere thickness, along with the average density of the lithospheric mantle. to enhance modeling accuracy, we applied a low-pass filter in the frequency domain to the topography data, eliminating high-frequency effects that could introduce noise, specifically filtering out wavelengths shorter than 72.80 kilometers. similarly, the geoid data underwent low-pass filtering, removing the first 10 harmonic coefficients in the geoid anomaly data. the resulting moho depth was used as the initial crustal parameter in the gravimetric modeling of the borborema meridional west province, alongside the bouguer anomaly data (fig. 2). figure 2. workflow. 3.3. 3d forward modeling: crust modeling integrating potential data with geological information enables the development of models capable of measuring physical parameters like density and calculating gravitational effects resulting from homogeneous sources. for crustal modeling in this study, we employed igmas+ (interactive gravity and magnetic application system). igmas+ is a software program that utilizes an initial model geometry consisting of parallel vertical sections, and the modeling process involves triangular polyhedral meshes (see fig. 3). in addition to its capabilities in gravimetric modeling, igmas+ can generate susceptibility models from magnetic field data, as detailed by schmidt and götze (1998), götze and lahmeyer (1988), and götze (2013). the numerical algorithm that forms the basis of igmas+ is based on the methods pioneered by götze (1976); götze and lahmeyer (1988); götze (2013). this algorithm allows for determining the vertical component of gravitational attraction arising from a polyhedron, along with the computation of gravity gradients, utilizing poisson´s ratio. plano 3 plano 2 plano 1 x y z figure 3. using bouguer anomaly data to model the crust: illustration of the initial definition of the layer model. the first layer, represented by orange, corresponds to the moho. the layers represented by yellow and grey correspond to the crust. source: adapted from the gómez dacal, 2012. 4. results the primary outcomes from the techniques encompass the detection of geotectonic body edges. the methods employed allowed for identifying and delineating the boundaries of geotectonic bodies within the study area. these findings contribute to a comprehensive understanding of the geological structure. the determination of source depths through the an-euler solutions technique provided valuable insights into the depths of magnetic sources within the region. this information aids in characterizing subsurface geological features and their spatial distribution. the estimation of moho thickness: through the integration of topography and geoid anomaly data, along with the application of specialized methodologies, the thickness of the moho, which marks the boundary between the earth’s crust and mantle, was estimated. this knowledge enhances our comprehension of the earth’s subsurface structure. utilizing these outcomes, a density contrast model for the study area was constructed (table a1). this model is a valuable tool for further geological and geophysical investigations, enabling a more detailed and accurate representation of the subsurface properties and structures within the region. figure 4 depicts the magnetic field map after applying upward continuation. the upward continuation has been applied for distance of 266 meters was utilized in this process, a value determined through radial power spectrum analysis of the total magnetic field data. the main goal of this procedure was to highlight significant geological structures at deeper levels while minimizing the impact of shallower magnetic sources. we applied the first-order vertical derivative to the continuous magnetic field data (fig.5) to accentuate geological sources and shallow structures, including lineaments. subsequently, to gain insights into the orientation of these lineaments, we constructed a rosette diagram (fig. 5) that illustrates the geological framework. the results indicate a predominant alignment of geophysical lineaments in the e-w direction, extending potentially up to ene. conversely, within the sfc, there is a prevalence of n-s (north-south) lineaments, in contrast to the southern and transversal borborema province, which exhibits ne-sw-oriented lineaments (fig. 5). these findings collectively suggest that the studied area is characterized by predominantly shallow structures and geological lineaments, with their orientations varying between different regions, as delineated by the rosette diagram. 243geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil figure 4. magnetic map with continuation upwards at 0.266 km. figure 5. magnetic structures interpreted from the maps of the vertical derivative (dz) with the continuation of the anomalous magnetic field at 0.266 km with their respective diagram of rosettes corresponding to the extracted lines. figure 6 displays the asta (slope angle analytical signal) map, which effectively delineates the boundaries of existing geotectonic domains within the study area. from this analysis, several noteworthy observations can be made: a) the maximum values to the east of the transversal borborema province; b) minimum values to the north of the sfc. the asta map shows maximum values mainly located east of the transversal borborema province, covering regions such as são josé do caiana, alto pajaú , alto moxotó, and rio capibaribe. it’s worth noting exceptions in the araripe basin and piancó-alto brígida. on the other hand, the map depicts minimum asta values situated north of the sfc, with heightened intensity noted in areas such as ib (possibly referring to the irecê basin), tb (potentially the tucano basin), southeast sergipano belt (likely representing a southeastern portion of the sergipealagoas basin), and jb (potentially referring to the jatobá basin). 244 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores the map distinctly outlines the positioning and interface between the riacho do pontal belt and the northern boundary of the sfc. this demarcation is characterized by a unique contour that conforms to the geological structure of the craton. moreover, the map effectively illustrates the geometry and contact points of the tucano, jatobá, and irecê basins in the filtered magnetic data, enhancing our understanding of their spatial relationships within the study area. these observations derived from the asta map significantly contribute to our understanding of the region’s geological and geophysical characteristics, aiding in identifying key geological features and their boundaries. figure 7 provides valuable insights into the depths of magnetic sources within the study area, as determined through the an-euler deconvolution method. key observations from the clustering of these solutions are as follows. the concentration of depths reveals that the most solutions are within depth ranges spanning from less than 1 kilometer to 5 kilometers. this concentration suggests that the causative magnetic sources have relatively shallow thicknesses, characterized by structural indices of 1 (indicative of dikes or sills), and 2 or 3 (representing cylinders or spheres) (fig. 7). regarding spatial distribution, the shallow magnetic sources are primarily concentrated within the sfc to the east of the sergipano belt. however, a smaller number of such sources are also observed in the transversal part of the borborema province, attributed to the presence of high-amplitude magnetic sources in these regions. notably, there is depth variability with sources less than 1 kilometer and greater than 5 kilometers, exhibiting structural indices ranging from 1 to 3. these sources are distributed within the sfc, the transversal segment of the borborema province, and to the west of the sergipano belt (fig. 7). these findings shed light on the heterogeneous nature of magnetic sources in the study area, with varying depths and structural characteristics. understanding the distribution and depths of these sources is crucial for unraveling the geological and geophysical complexities of the region. figure 6. border detection map by asta method with ascending continuation at 0.266 km. figure 7. an-euler solution. 245geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil the bouguer anomaly image (fig. 8) reveals gravity anomalies in the northern part of the craton ranging from -69.1 to -53.6 mgal, indicating a lowdensity contrast in this region, possibly associated with the gavião block. however, in the eastern region, a more marked density contrast is noticeable, ranging from -51.0 to -33.3 mgal, suggesting the possibility that these anomalies are associated with granitoids and dykes formed during paleoproterozoic collisions and the amalgamation of archean blocks. these elements include the gavião, jequié and serrinha blocks, as well as the itabuna-salvador-curaçá belt (barbosa et al., 2012a; de menezes leal et al., 2012). notably, the n-s orientation of the itiúba granitoid, within the itabunasalvador-curaçá belt, corresponds with the orientation observed in the vertical derivative (dz), indicating a prominent n-s orientation in the sfc. towards the end of the paleoproterozoic era, this paleocontinent underwent a series of rift events, forming sedimentary basins. these rifts contributed to the deposition of the espinhaço supergroup and, during the neoproterozoic, the são francisco supergroup. however, subsequent tectonic events, such as folding and faulting at the end of the neoproterozoic, during the brasiliano orogeny, disrupted these rift systems (cruz and alkmim, 2006). in the bouguer anomaly map, gravity anomalies in the northern part of the sfc range from -102.6 to -73.6 mgal, indicating a lower-density contrast. this is attributed to the presence of neoproterozoic cover rocks within this region, specifically the carbonate and siliciclastic rocks of the bebedouro and salitre formations, part of the irecê basin (cruz and alkmim, 2006; guimaraes et al., 2011). these formations are well-defined along their edges (fig. 6) and exhibit low magnetic susceptibility, contributing to the reduced magnetic signatures in this area. figure 8 provides a map of the bouguer anomaly, offering significant insights into the gravimetric characteristics of the study area. key observations from this map include high gravimetric signatures in the transversal borborema province and peculiar gravimetric signatures in the araripe basin. the transversal borborema province is predominantly characterized by high gravimetric signatures, ranging from -47.7 to -18.9 mgals. interestingly, the araripe basin exhibits distinct gravimetric signatures characterized by lower values when compared to the surrounding regions. the observations from this map include gravity anomalies in the riacho do pontal and surrounding domains. in the inner part of the riacho do pontal belt, a gravity anomaly ranging from -36.6 to -18.9 mgal is observed. this anomaly extends into the pernambuco-alagoas geotectonic domain and the sergipe belt. the sfc displays a significant gravity anomaly ranging from -73.6 to -102.6 mgal in the irece basin. a similar anomaly pattern is observed in the tucano and jatobá basins, with consistent variations. in the sfc, the gravity anomaly ranges from -69.1 to -51.0 mgal, but it varies between -46.6 to -33.3 mgal in the eastern part of the craton. these bouguer anomaly data are instrumental for crustal modeling, aiding in the understanding of subsurface geological structures and variations in density within the study area. additionally, figure 8 presents five geophysical profiles oriented in the nw-se direction, perpendicular to the orientations of magnetic lineaments observed in the dz. these profiles were utilized for geological and geophysical modeling, contributing to a more comprehensive understanding of the subsurface features and characteristics. figure 8. bouguer anomaly image. 246 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores figure 9 presents the moho thickness map, which reveals significant variations in moho depth across the study area, ranging from 36 to 43.8 kilometers. this map provides valuable insights into the region’s structural characteristics of the earth’s crust. one of the key observations from this map is the variability in moho depth, indicating diverse geological features and tectonic processes at play. the depth of the moho, marking the boundary between the earth’s crust and mantle, is an essential parameter for understanding the subsurface geology. examining different geological domains within the study area, we find distinct moho depths. the sfc exhibits a relatively deep moho at approximately 41.6 kilometers, indicative of a thicker crust in this ancient geological formation. in contrast, the irecê basin shows a shallower moho depth of around 39.4 kilometers, suggesting a thinner crust in this basin. within the riacho do pontal belt, the moho depth is observed at 38.2 kilometers, highlighting variations in crustal thickness within this geological unit. the pernambuco-alagoas and sergipano belt display similar moho depths, approximately 38.2 kilometers and 37.4 kilometers, respectively, indicating consistent crustal characteristics in these belts. these findings underscore the geological complexity of the study area, with significant variations in moho depth across different geological settings. understanding these variations is crucial for interpreting the region’s tectonic processes, seismicity, and geological evolution. notably, these major geological domains within the study area, such as the sfc, irecê basin, riacho do pontal belt, pernambuco-alagoas belt, and sergipano belt, are characterized by relatively deeper moho depths compared to other domains within the transversal borborema region. indeed, the moho depth values obtained from this study serve as crucial a priori information for the subsequent modeling of the earth’s crust within the study area. these values provide essential insights into the subsurface geological structure and the significant variations in moho depth across various geological domains. such information is fundamental for advancing our understanding of the complex geological and geophysical characteristics of the region, ultimately aiding in more accurate and comprehensive geological modeling and interpretation. figure 10 presents the crustal model for profile 1. this model comprises nine distinct geotectonic domains, which have been identified and designated as follows: parnaíba basin and block (1), riacho do pontal belt (2), intrusion (2.1), são francisco craton (3), irecê basin (4), in addition to upper crustal layers (16), lower crust (17), and lithospheric mantle (18). the density values used in the modeling are provided in table 1a (appendix a). the results reveal that the depths of these domains exhibit distinct values, as follows: (1) ranges from 0 to 3.2 kilometers, (2) varies between 8.0 and 13.0 kilometers, (2.1) has a depth of 7.0 kilometers, (3) ranges from 5.0 to 18.0 kilometers, with an average moho thickness ranging from 41.0 to 42.0 kilometers. the variation of the calculated parameters stabilizes in line with the observed parameters, indicating a good fit for the model. it showed a satisfactory representation of the dimensions of the bodies in the subsurface. there is a convergence between the solid and dashed curves, especially in the transition region between the borborema province and the sfc and in the configuration of the irece basin. this suggests that the model resembles the relief of the basement. figure 9. moho depth, referring to the studied area. 247geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil figure 10. the model of the profile p1, represent the domains. lithosperic mantle parnaíba basin intrusion lower crust parnaíba block são francisco craton upper crust riacho do pontal belt irecê basin figure 11. the model of the profile p2 shows the geotectonic domains. lithosperic mantle lower crust upper crust parnaíba basin parnaíba block riacho do pontal belt intrusion são francisco craton 248 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores figure 11 displays the corresponding crustal model for profile 2, with a horizontal extent of 500 kilometers. this model consists of seven distinct geotectonic domains, designated as follows: parnaíba basin and block (1), riacho do pontal belt (2), intrusion (2.1), são francisco craton (3), along with upper crustal layers (16), lower crust (17), and lithospheric mantle (18). our findings reveal significant variations in the depths of each domain, as follows: (1) ranges from 0 to 3.0 kilometers, similar to the depth observed in the previous profile, (2) falls within the range of 5.0 to 11.0 kilometers, (2.1) has a depth of 10.0 kilometers, (3) encompasses depths between 6.0 and 14.0 kilometers, with a moho depth varying from 36.0 to 39.5 kilometers. the result is achieved when the variation of the calculated parameters stabilizes in line with the observed parameters. the model showed a satisfactory representation of the dimensions of the bodies in the subsurface. the solid curves represent the isolines of the observed data, while the dashed curves represent the anomalies calculated for the model. these curves converge at the contact between the borborema province and the sfc. with the parameters used to build the model, the observed anomalous field is equivalent to the field generated by the model. figure 12 presents an additional model for profile 3, highlighting distinct geotectonic domains, specifically: parnaíba basin and block (1), riacho do pontal belt (2), são francisco craton (3), tucano basin (5), são josé do caiana (6), along with upper crustal layers (16), lower crust (17), and lithospheric mantle (18). for this profile, significant variability in the depths of these domains was observed, as described below: domains (1): depths ranging up to 4.0 kilometers; (2): depths varying between 4.0 and 8.0 kilometers; (3): depths ranging from 4.0 to 25.0 kilometers; (5): displaying depths from 3.0 to 8.0 kilometers; (6): depths oscillating between 4.0 and 10.0 kilometers. the moho depths within these domains range from 37.0 to 41.0 kilometers. the model generated correlates well with the observed data, as illustrated in figure 12. the curves of the observed data and the calculated data show a satisfactory correlation with the generated model. the gravimetric model reveals a decrease in the intensity of the gravimetric field, which can be interpreted as due to thick sediment sequences and a symmetrical distribution of densities, suggesting uniformity of composition both laterally and at depth in the direction of the craton. when examining the plan representation of the density distribution model (fig. 12), a radial distribution of densities can be seen both in the borborema province and in the craton portion, with an adequate fit between the solid and dashed curves. figure 12. the model of profile p3 exposes the geotectonic domains. lithosperic mantle lower crust upper crust parnaíba basin parnaíba block são josé do caiana riacho do pontal belt são francisco craton tucano basin 249geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil figure 13 displays the model for profile 4, consisting of various geotectonic domains, namely: tucano basin and block (5), são josé do caiana (6), northern subprovince (6), araripe basin and block (7), piancó-alto brígida (9), pernambuco-alagoas (10), sergipano belt (11), dense crust (12), along with upper crustal layers (16), lower crust (17), and lithospheric mantle (18). in this profile, the depths of these geological domains exhibit the following ranges: domains (5): depths not exceeding 3.0 kilometers; (6): depths varying from 7.0 to 10.0 kilometers; (7): also varying from 7.0 to 10.0 kilometers; (9): depth variations ranging from 2.0 to 8.0 kilometers; (10): depths situated between 2.0 and 9.0 kilometers; (11): extending within the range of 6 to 11 kilometers. furthermore, the moho depths in this profile range from 37.0 to 41.0 kilometers, corresponding to the moho thickness. the model generated is in line with the model in figure 12, showing a good correlation with the observed data, as illustrated in figure 13. the curves of the observed and calculated data show a satisfactory correlation with the model generated. the gravimetric model reveals a varied distribution of densities, indicating compositional variation both laterally and in depth in the direction of the sergipano belt. figure 14 presents the model for profile 5, comprising various geotectonic domains, namely: northern subprovince (7), araripe basin (8), piancó-alto brígida (9), pernambuco-alagoas (10), sergipano belt (11), jatobá basin (15), alto pajeú (13), alto moxotó (14), along with upper crustal layers (16), lower crust (17), and lithospheric mantle (18). it can be observed that the depths of these domains exhibit the following variations: domain (7): depths ranging from 5.0 to 6.0 kilometers; (8): depths varying between 1.5 and 2.7 kilometers; (9): depth variations ranging from 2.0 to 7.0 kilometers; (10): depths ranging from 4.0 to 7.0 kilometers; (11): depths between 5.0 and 15.0 kilometers; (15): depths from 2.0 to 4.0 kilometers; (13): depths ranging from 10.0 to 15.0 kilometers. (14): depths between 4.0 and 6.0 kilometers. the model generated correlates satisfactorily with the observed data, as shown in figure 13. throughout the profile, the gravimetric anomaly shows significant homogeneity, characterized by a decrease in the intensity of the gravimetric field mainly in the nw-se direction. this decrease in intensity can be attributed to the presence of thick sediment sequences. when analyzing the plan representation of the density distribution model (figure 13), there is a variation in the distribution of densities in the borborema province and in the portion of the sergipano belt. figure 13. the model of the profile p4 presents the geotectonic domains. lithosperic mantle north subprovince são josé do caiana tucano basin tucano blocklower crust araripe basin piancóalto brígida upper crust araripe block pernambuc-alagoas dense crust sediments sergipano belt 250 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores figure 14. the model of the profile p5 presents the geotectonic domains. lithosperic mantle north subprovince lower crust araripe basin piancóalto brígida alto-pajaú alto-moxotó dense crust upper crust araripe block pernambuc-alagoas jatobá basin jatobá block sergipano belt furthermore, the depths within these domains range from 39.0 to 41.0 kilometers, corresponding to the crustal thickness. in simplified form, table 01 shows the average values obtained in the modeling. figure 15 displays the compilation of all the crustal models in the five profiles, generating a threedimensional model. table 1. values obtained in modeling acronym domain upper crust (km) middle crust (km) moho (km) upper crust density (kg/m³) sfc são francisco craton 16.2 26.2 41.6 2700 ib irecê basin 7.2 27.0 39.4 2550 rpb riacho do pontal belt 12.5 25.3 38.2 2660 peal pernambucoalagoas 7.5 23.3 38.2 2700 sb sergipano belt 14.3 24.2 37.4 2560 jb jatobá basin 4.3 24.8 39.5 2550 tb tucano basin 7.2 30.1 38.4 2550 pb parnaíba basin 3.7 25.7 39.4 2550 sjc são josé caiana 10.3 28.9 38.4 2740 acronym domain upper crust (km) middle crust (km) moho (km) upper crust density (kg/m³) sfc são francisco craton 16.2 26.2 41.6 2700 ib irecê basin 7.2 27.0 39.4 2550 rpb riacho do pontal belt 12.5 25.3 38.2 2660 peal pernambucoalagoas 7.5 23.3 38.2 2700 sb sergipano belt 14.3 24.2 37.4 2560 jb jatobá basin 4.3 24.8 39.5 2550 tb tucano basin 7.2 30.1 38.4 2550 pb parnaíba basin 3.7 25.7 39.4 2550 ab araripe basin 2.7 21.6 39.5 2550 pb piancó-alto brígida 10.0 24.6 36.8 2680 ap alto-pajeú 14.1 23.9 37.8 2665 am alto-moxotó 11.2 22.9 37.2 2690 (continued) 251geophysical characterization and tectonic insights of the western meridional borborema province in the northeastern region of brazil figure 15. crustal model 2d and 3d of the density distribution in the subsurface of the respective profiles in figure 8. 5. discussion the sfc is primarily composed of archean terrains. during the late paleoproterozoic era, collisions resulted in the amalgamation of archean rock units within the craton, including the gavião block and jequié complex (barbosa and barbosa, 2017). archean and paleoproterozoic rocks form the basement of bahia state. in the northern part of the craton, the gavião block is predominant, characterized by metamorphic terrains consisting of gneisses, migmatites, and orthogneisses dating back 3.6 ga. east of the gavião block lies the itabuna-salvador-curaçá belt, comprising granulites such as tonalites, enderbitic rocks, charnockites, and leucogabbros. further east, the serrinha block is composed of metamorphic rocks like gneisses and migmatites with amphibolite, quartzites, while the uauá block consists of gneiss, orthogneiss, and orthogranodiorite (barbosa et al., 2012b; cruz and alkmim, 2017). the moho thickness beneath the sfc, approximately 41.6 km, aligns with the findings of feng et al. (2007) and assumpção and lopes (2013), providing a robust confirmation. our modeling revealed the depth of the irecê basin to be 8 km, in line with the findings of d´angelo et al. (2019). the brasiliano orogeny, a key focus of our study, intensified collision events along the boundaries between the paleocontinent and the borborema province, leading to foldings, deformations, and high-grade metamorphism. this process gave rise to fold belts in sergipe, riacho do pontal, and rio preto (barbosa and sabaté, 2004). the northernmost internal part of the riacho do pontal belt encompasses the paulistana, santa filomena, and morro branco complexes, which are intruded by magmatic bodies such as syn-collisional granites (aldeia and rajada suites) and alkaline granites formed after the collision (serra da aldeia suite). in contrast, the southernmost portion comprises metasedimentary units ranging from metapelites to metagraywacke (caxito, 2013; caxito et al., 2014, 2020). the delineation of the riacho do pontal belt is evident in figure 5, illustrating its connection with the sfc to the north and its extension eastward towards pernambuco-alagoas. the bouguer anomaly map highlights anomalies in its northern portion ranging from -33.3 to -18.9 mgal (figure 8) and total magnetic field variations between 47.66 and 97.11 nt (figure 4). conversely, the southern part displays lower bouguer anomalies, indicating reduced magnetic susceptibility. this sharp contrast in density and magnetic susceptibility is attributed to the presence of magmatic bodies resulting from collisional events and the neoproterozoic low-density contrast associated with metasedimentary rocks of the casa nova group. the pernambuco-alagoas domain encompasses a variety of high-grade archean, paleoproterozoic, metavolcanic-sedimentary rocks, and granitic terrains (neves and da silva filho, 2019; lima, 2018). the presence of magnetic intrusions, such as granitic bodies, indicates significant density contrasts associated with the brasiliano orogeny. these igneous intrusions were identified using magnetometric and airborne gamma-spectrometric data, revealing an ne-sw oriented transcurrent shear zone (lima et al., 2021). similarly, the sergipano belt experienced multiple episodes of regional metamorphism and tectonic deformation during the orogenic event. the magnetic results highlight low susceptibility in the southern part of the sergipe belt (figure 5), likely due to metasedimentary rocks, as noted by allard and hurst (1969). in the transverse section of the borborema province, several domains are observed, including são josé do caiana, piancó-alto brígida, alto pajeú, alto moxotó, rio capibaribe, and phanerozoic cover sequences in the araripe basin. these domains are delineated by two major shear zones: the pernambuco and patos shear zones (santos and medeiros, 1999; medeiros, 2004). this region comprises paleoproterozoic gneisses dating back to 2.0 and 2.2 ga, metavolcanic and metasedimentary rocks aged around 995 and 960 ma due to the cariris velhos event, and various plutonic intrusions associated with the brasiliano orogenic event. plutonic intrusions have been identified and interpreted by various researchers (schmus et al., 2011; sialand and ferreira, 2016; neves et al., 2016; caxito et al., 2021). concerning magnetic structures, two significant shear zones, pernambuco and patos, are evident, along with possible ne-nw oriented lineaments associated with the disruption of west gondwana (figure 4). the results of moho depth analysis indicate that the transversal borborema province has a thinner crust, approximately 38.5 km, compared to the sfc with a thickness of 41.6 km (figure 7). these findings are in line with observations made in fianco (2019), where the collisional suture zone between the borborema province and the sfc is characterized by a positive and negative peak in the bouguer anomaly profile, indicating the collision and subsequent stretching along this boundary. in summary, this study offers valuable insights into the geological history and subsurface structure of the sfc and surrounding regions, providing a better understanding of the complex tectonic events that have shaped the area over billions of years. the integration of magnetic and gravity data, along with geological and geophysical models, has facilitated a more comprehensive understanding of the region’s geological evolution and structural characteristics, as depicted in figure 16. figure 16. geological map new with geophysical interpretation. 6. conclusions in this research, we utilized gravity, magnetic, geoid anomaly and topographic data to provide a better understanding of the crustal structure in the western borborema province. our main objectives were to delineate crustal block boundaries, ascertain the orientations of regional lineaments, establish top depths of magnetic sources, estimate moho thickness, and construct crustal models through the integration of geological and geophysical data. our results, when correlated with lithological data from diverse geological domains within the study area, facilitated the precise delineation of the suture zone between these blocks. magnetic anomalies were predominantly low in the outer part of the riacho do pontal belt (rpo) and the southeastern sergipano belt but exhibited high values in the upper regions of the province. additionally, moho thickness in these areas was observed to be relatively modest. furthermore, we identified a range of magnetic signatures and gravity anomalies across the study region, characterized by both positive and negative deviations. the use of filtering techniques enabled us to pinpoint contacts between distinct tectonic domains, such as the sfc and the riacho do pontal belt. these findings, when combined with geological domain maps, allowed for accurate determination of depth-related information for various geological features. figure 5 depicts the tectonic framework of the area, characterized by geophysical lineaments primarily oriented in a northeast-southwest direction. this alignment corresponds to the predominant direction of emergent features associated with the brasiliano extensional orogenic event. regarding the moho, our findings confirm a crustal thickness of approximately 41.6 km for the sfc. in the northern part, the sfc basement shows continuity, especially in the external domain of the riacho do pontal belt. qualitative analysis of the results revealed distinct signatures for supracrustal and basement rocks. the crustal thinning observed in profiles 2 and 3 can be attributed to the collision between the sfc and the borborema 252 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores province. the pernambuco-alagoas domain, exhibiting high-intensity magnetic relief, corresponds to regions with high magnetic susceptibility in our modeling. conversely, the southeastern part of the sergipano belt exhibits a low magnetic anomaly, possibly due to lower susceptibility zones within the belt, unlike the northeastern region. this suggests that the observed crustal thinning may be associated with the separation of the south american and african plates. high magnetic signatures observed within the internal regions of the riacho do pontal belt, pernambuco-alagoas, sergipano belt, and the eastern part of the transverse borborema province suggest that suture zones between the south american and african plates likely formed in a southeastnorthwest direction. additionally, the contours observed in the western section of the vertical derivative map (dz), oriented northeast-southwest, appear perpendicular to the direction of collisional forces between the plates. the identified suture zone, characterized by high magnetic signatures and moho depth, can be interpreted as an intensified expression of the tectonic boundary separating the borborema province and the sfc. this collisional boundary is presumed to extend further south based on the evidence presented. these conclusions offer a comprehensive understanding of the geological, tectonic, and geophysical structure of the studied region, highlighting 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(1997). neoproterozoic (∼600 ma) aggregation of western gondwana: a tentative scenario. precambrian research, 82, 101–112. 254 vinicius a. r. oliveira, alanna c. dutra, onofre h. d. j. das flores appendix a. density values employed in gravity modeling the physical parameters employed in the crustal modeling process are detailed in table 1a below. table 1adensity values assigned in 3-d gravity modeling. domain density (kg/m³) sediments 2550 são francisco craton 2700 irecê basin 2550 riacho do pontal belt 2660 pernambuco-alagoas 2700 sergipano belt 2560 jatobá basin 2550 jatobá block 2600 tucano basin 2550 tucano block 2680 parnaíba basin 2550 parnaíba block 2690 são josé do caiana 2740 araripe basin 2550 piancó-alto-brígida 2680 altopajeú 2650 alto moxotó 2690 north subprovince 2680 intrusion 3900 upper crust 2790 lower crust 2950 moho 3330 dense crust 3100 source: the authors (2024). _heading=h.1fob9te _heading=h.q1xi1l2x8yhc _heading=h.9avz6zjcxiov _heading=h.lxrck9fj2dcq _heading=h.mm92fo635na9 _heading=h.w8tfbzotxso2 _heading=h.mib6wbapb9g _heading=h.yfprcvi6efyg _heading=h.oxoaf5z619ix _heading=h.wwx3yvtvpb3k _heading=h.2et92p0 _heading=h.b9p66ukai5qg _heading=h.hzpylk8kay3f _heading=h.k3qssuyb1fwe _heading=h.cp78clh4sf9d _heading=h.i65jci4l0gl1 _heading=h.gjdgxs _heading=h.2s8eyo1 _heading=h.30j0zll issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.97305 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 255 263 g eo ph y si c s subsoil geophysical evaluation using gpr and free software hernán darío guerrero, luis hernán ochoa universidad nacional de colombia corresponding authors: hdguerreroc@unal.edu.co; lhochoag@unal.edu.co abstract in this work, a geophysical characterization of the subsoil of a civil structure corresponding to the basement of a residential complex in bogotá, colombia was carried out, using the gpr technique. we were chosen to establish the affectation caused by weeping willow (salix babylonica) trees planted near the retaining wall of the structure’s foundation and its parking lot platforms. we acquired gpr profiles all throughout the area of the internal part of the basement and the outer retaining wall part. the pieces of equipment used were sir 4000 and a hs 350 mhz center frequency antenna. the data presented a good and consistent signal. high resolution subsoil images of up to 3m in depth of the area below the parking lots were generated through the analysis of information derived from the processing and interpretation of the data. all of these were compared with information from geotechnical and topographic studies of the area. the results obtained show that the trees’ roots are causing scour due to a drying effect of the subsoil, which caused damage to the slab in the parking lot and on the retaining wall. evaluación geofísica del subsuelo usando gpr y software libre resumen en este trabajo, se realizó la evaluación geofísica del subsuelo de una estructura civil correspondiente al sótano de un conjunto residencial en bogotá, colombia, utilizando la técnica de gpr. se seleccionó el área para establecer el impacto causado por árboles de sauce llorón (salix babylonica) plantados cerca del muro de contención de la cimentación de la estructura y de las plataformas del parqueadero. se adquirieron perfiles de gpr en toda el área de la parte interna del sótano y en la parte externa del muro de contención. el equipo utilizado fue el sir 4000 con una antena de frecuencia central de 350 mhz. los datos presentaron una señal buena y consistente. a través del análisis de la información derivada del procesamiento e interpretación de los datos, se generaron imágenes de alta resolución del subsuelo, alcanzando profundidades de hasta 3 m en el área debajo de los parqueaderos. toda esta información se comparó con estudios geotécnicos y topográficos de la zona. los resultados obtenidos muestran que las raíces de los árboles están causando socavación debido a un efecto de desecación del subsuelo, lo cual ha generado daños en la losa del parqueadero y en el muro de contención. keywords: weeping willow trees; reflection; georadar; gpr; electromagnetic methods; scour; drying effect; subsoil disturbance. palabras clave: gpr; georradar; métodos electromagnéticos; reflexión; socavación; efecto de desecación en estructuras; alteración del subsuelo; sauce llorón; record manuscript received: 15/07/2023 accepted for publication: 15/12/2024 how to cite item: guerrero, h. d., & ochoa, l. h. (2024). subsoil geophysical evaluation using gpr and free software. earth sciences research journal, 28(3), 255-263. https://doi.org/10.15446/esrj.v28n3.97305 https://doi.org/10.15446/esrj.v28n3.97305 mailto:hdguerreroc@unal.edu.co mailto:lhochoag@unal.edu.co https://doi.org/10.15446/esrj.v28n3.97305 256 hernán darío guerrero, luis hernán ochoa 1. introduction geophysical techniques have been used in different fields such as the mining, oil and gas industry, archaeology, and engineering, with a variety of applications. the use of geophysical techniques has many advantages, such as providing a cost-effective way to investigate conditions at a test site without physical intervention. geophysical techniques such as ground penetrating radar (gpr), are frequently used because they are able to provide greater spatial resolution at shallow depths by using high frequencies. these provide better resolution than other options like in situ testing. as with all geophysical techniques, each method has limitations and potentially suffers from problems of non-exclusivity. the combined use of different shallow geophysical methods has proven useful in obtaining more robust inversions by adding more information a priori (besson, 2010). several root mappings studies that use gpr have been reported in the literature (hruska et al., 1999), (zhang et al., 2019), (guo et al., 2013), (moore and ryder, 2015), (alani et al., 2018), among many others, where different frequencies from antennas are reported for different depths of investigation, as well as mapping various root systems. in the present study, we adopted a similar methodology, starting with preliminary research to establish the optimal investigation depth. this was followed by the application of standard data processing techniques, yielding images comparable to those reported in previous literature. typically, high-frequency antennas (greater than 800 mhz) are employed, enabling the detection of root diameters between 10 and 20 cm in shallow subsurface layers (less than one meter). additionally, gpr is capable of detecting reflections and anomalies associated with deeper root systems. this study aims to assess the impact of tree roots on subsoil conditions, both in proximity to and at a distance from trees, to determine whether root growth is the primary factor contributing to the observed subsurface disturbances. 2. materials and methods investigation site the data acquisition was carried out in a residential complex located in the northwest of bogotá colombia, which can be seen in the figure 1. damage to the structure the circulation slab at the south side of the residential complex has multiple damages on its surface. the subsoil below: this area is being affected by something that causes large sags and lifts to occur, breaking the reinforced concrete and causing considerable damage to the structure in question, see figures 2, 3. the retaining wall also presents damage. this indicates that some elements of the structural system of beams and columns that lead up to it has been affected, with cracks and fissures in these elements. trees of interest there are two lines of trees close to the retaining wall (figure 4). the tree line that corresponds to the weeping willow line is at a distance of around 2 3 m. from the retaining wall. there are fifteen (15) of those trees spread almost evenly along the 150 m of wall, with a height of around 13 16 m. on average. the second line of trees is far from the weeping willows (more than 3 m.) and corresponds to another tree species of lesser size. figure 1. location overview of the study area. the top image, captured in 2015, shows the site where the geophysical investigation was conducted, located to the south of the building complex. at this time, the trees were still small, and no damage had been reported within the platforms. the yellow rectangle highlights the study area. the bottom image, taken after 2020, shows the same location with significantly larger trees, whose growth coincides with the structural damage reported inside the platforms. figure 2. a sag in the plate of approximately 30 cm is observed. the distance between columns is of 5 m. the structural frames were protected with metal trusses for precaution and to prevent aerial beam deflections. figure 3. a rise is observed in the parking slab. the thickness of the slab is 15 cm. several fissures and cracks have been formed. 257subsoil geophysical evaluation using gpr and free software a) topografic profile b) gpr acquisition lines next to the retaining wall figure 4. panoramic view of the zone. figure a. shows a topographic profile of the section between the wall and the tree line. figure b. shows the gpr lines acquired outside the wall and inside the basement immediately next to the wall. the difference in height between the profiles taken at (a) and (b) is 2.5 m. figure 5. position of the gssi 350 hs antenna and the gpr gssi sir4000 during the acquisition, on the reflection profile mode. equipment in this work, a gssi sir4000tm was used in conjunction with a 350 hstm antenna (see figure 5). the sir 4000 (gssi, 2017) is gssi’s first highperformance gpr data acquisition system designed to work with both analog and digital antennas. this evolutionary step allows for true versatility and flexibility in supporting a wide range of users in numerous applications. the hs 350 mhz central frequency (gssi, 2013) is a next-generation digital antenna designed to work seamlessly with the gssi sir 4000 control unit and the g1 controller. it is easily configurable to suit any kind of application, including archaeology, geophysics, utility location, and more. gssi’s hyper stacking technology considerably improves the depth and performance of data resolution compared to traditional radiated two-stage methods. 3. ground penetrating radar measurement the gpr technique is a high-resolution electromagnetic technique that is designed to examine the near subsurface. the main objective is to locate buried objects such as pipes, lithological changes, as well as changes in the structure of the subsoil like defects and cracks in the materials, or differences in humidity due to groundwater content, among others (finck, 2013). ground-penetrating radar (gpr) uses electromagnetic (em) propagating waves that respond to changes in the electromagnetic properties of the shallow subsurface. the speed of propagation of em waves, which is the main controlling factor in the generation of reflections, is determined by the relative permittivity contrast between background material and the target (or the contrast between the layers). relative permittivity (also called the dielectric constant) is defined as the ability of a material to store and allow the passage of em energy when a field is imposed on the material. this can be measured in the laboratory or in situ (baker et al., 2007). gpr operates by transmitting highfrequency electromagnetic pulses on the ground through a transmitting antenna. pulses are partially reflected and scattered towards the surface from various sub-surfaces of contact between different materials on the ground. that is, those through which there is a contrast in the dielectric constant. this reflected/scattered energy then travels back to the surface, where it is recorded by the receiving antenna. the time it takes for the wave to travel to an interface and return to the surface is called travel time and is used to determine the in-situ propagation speed of the subsurface material. the velocity (distance/travel time) for an em wave in the earth’s atmosphere at or near sea level is 0.33 m/ns. because the relative permittivity of all subsurface materials is greater than the permittivity of air, the velocity of an em wave in all subsurface materials will be less than the em propagation velocity in typical air materials between 0.05 and 0.15 m/ns (daniels et al., 1995). even though the speed of propagation of an em wave depends on the relative permittivity of the material, the amplitude and attenuation of a propagation wave depend on the magnetic permeability and electrical conductivity of the material (baker et al, 2007). electrical conductivity also affects the propagation of electromagnetic waves. materials with high electrical conductivity may attenuate em signals; therefore, highly conductive materials will produce poor gpr data and/ or reduce penetration depth. the depth of penetration for the investigation is controlled by the radar pulse’s frequency, the magnetic permeability (which correlates the magnetic induction with the intensity of the magnetic field), the electrical conductivity, and the permittivity of the ground. the higher the conductivity and permittivity of the soil, the lower the penetration of the electromagnetic pulse. therefore, the depth of the investigation is inversely related to the frequency. therefore, the lower the frequency of the signal, the greater the depth of investigation (daniels et al, 1995, neal, 2004). data acquisition in the reflection profile mode, the antennas are kept at a constant separation while moving along a profile. the electromagnetic pulses are transmitted over a fixed distance or time interval. the signal is immediately recorded and displayed on a computer screen in the form of gpr profiles, where the vertical axis is the travel time in two directions in nanoseconds (ns) and the horizontal axis is the distance, along with the measured profile. gpr data is collected along with a single profile, or in a grid of profiles to obtain 2d or pseudo-3d information about structures on the ground. 258 hernán darío guerrero, luis hernán ochoa in this work, we acquired gpr data on profiles covering the entire parking platforms area in that zone, which corresponds to the slab with problems in the surface. the acquired profiles in the basement are shown in the figure 6. the entire acquisition was made with no presence of vehicles. considering the structural damage in the parking slab visible on the surface, due to either subsidence, uplift, or fissures, much denser short profiles were made for each affected area, that is, five gpr profiles were taken separated fifty cm from each other (figure 7). other profiles were acquired in zones with no affection by weeping willows or other trees, in order to compare their signals. figure 6. configuration of the profiles acquired in the parking lots. the lines show the direction and identifier of the profiles of the radargrams. figure 7. location of the short profiles acquired in the basement at parking lots 41 42. trajectories where profiles were taken are outlined in red. note the great alteration in this parking lot, which shows a fairly large subsidence, as well as many cracks. data processing jol and bristow (2003) briefly discuss commonly used data processing procedures. neal (2004) also discusses the requirements for proper data processing. the next section outlines some of these essential processing steps. the gpr data was recorded in digital format and was subjected to different mathematical operations that are standard in the basic processing of gpr signals. these involve procedures such as detection of first arrivals, corrections in direct current displacements, time corrections, signal filtering, application of gains, and spatial and frequency filters. before the gpr data is ready for interpretation, some processing steps must be applied. the first step is simple data editing to correct errors in the field, as well as inverting profile addresses, merging files, etc. the first regular processing step is ”dewow”, which removes a long and wavy part of the signal caused by electromagnetic induction. a time-zero correction was made on the data (see figure 8). the instrument may not have detected zero time accurately in the field and therefore, it must be reset to ensure correct depths in the profile. additionally, time-zero drift along the profile can occur due to the temperature difference between the instrument’s electronics and the air temperature (jol et al., 2009). drift causes reflections to misalign and time-zero must be resampled for all traces, along with the profile (jol et al., 2009). by running a bandpass filter over time, the high-frequency electromagnetic noise found in the gpr profile was diminished. data processing using rpgr today, many commercial software applications that are dedicated 100% to gpr data are available (huber and hans, 2018). while many commercial gpr software solutions offer flexible and comprehensive processing capabilities, they often function as “black boxes,” where the underlying algorithms and processes are not fully transparent to the user. this lack of visibility makes it difficult to understand exactly how the data is being manipulated. although these platforms provide advanced features for data visualization, filtering, and interpretation across various applications, the proprietary nature of these tools can limit the user’s ability to fully assess or customize the processing steps, which is critical for detailed and accurate analysis. for this reason, we chose to use rgpr, an open-source gpr data processing package that is written in the r language (ihaka and gentleman, 1996) because it offers an open source platform with efficient data manipulation tools, strong statistical skills, and tools for graphics and visualization. rgpr is a ground penetrating radar (gpr) data processing package written in r language (huber and hans, 2018). r is a free and open-source high-level programming language for statistical computing. rgpr relies on two main classes for processing and displaying gpr data, as well as for keeping track of processing steps. the software in mention was compiled and installed for this work in a linux debian 4.9.0-13-amd64 1 smp debian 4.9.228-1 (2020-07-05) x86 64 gnu / linux operating system which is also of free use and distribution. several scripts in rgpr were written to perform all steps for processing gpr data, in accordance to the literature and in order to get the best results. below, we show some of these results to evoke how the output is displayed in the software mentioned (figures 9, 10, 11). using the processed gpr data, we conducted an attribute analysis to better interpret subsurface features (khwanmuang and udphuay, 2012). for this analysis, we utilized rgpr and opendtect software to compute various attributes, including instantaneous amplitude, slope, and energy event characteristics. the instantaneous amplitude represents the magnitude of the gpr signal at each point in time, providing insight into the strength of reflections at specific subsurface layers. the slope attribute helps identify the steepness of changes in the signal, useful for detecting anomalies or transitions between layers. the energy attribute, as defined by boniger and tronicke (2010), measures the total energy of the reflected signal by computing the squared sum of the signal values within a specified time range, and then dividing it by the total number of sample times. this attribute is particularly useful for highlighting areas with strong signal reflections, which can indicate changes in material properties or the presence of subsurface structures. through this comprehensive attribute analysis, we gain a deeper understanding of the subsurface conditions. 259subsoil geophysical evaluation using gpr and free software figure 8. short profile of radargrams acquired in the basement for a parking lot section. the picture shows time zero correction using rgpr processing. figure 9. the picture shows low-frequency bands remotion processing using the dewow procedure in rgpr. figure 10. the picture displays the amplitude, frequency and phase in rgpr from the data acquired. figure 11. the picture displays the data after the median filter was applied in rgpr. this attribute enhances areas characterized by strong changes in amplitude. the instantaneous amplitude attribute is determined from the complex trace (taner et al., 1979) and calculated on a sample by sample basis (white, 1991). it can be used to indicate changes in deposition and contrast between two media. the slope event attribute is the tangent’s slope value at a zero crossing of the data-trace (dgb, 2019). it is a useful tool to quantify the quality of the horizons in the data. data processing using opendtect for better interpretation and analysis of the layers in the subsoil, the processed radargrams were loaded in post-processing software, commonly used for seismic analysis. in this case, we selected opendtect. opendtect is a free, open-source seismic interpretation and postprocessing system for visualizing, analyzing, and interpreting 2d, 3d, and 4d seismic data, created by the dgb earth sciencestm company (dgb, 2019). it is also widely used for georadar interpretation (khwanmuang and udphuay, 2012), (dujardin et al., 2017), (tomecka-suchoń, 2019). opendtect is released under the gnu public license (gplv3 or higher). for interpretation, opendtect supports all the tools you expect to find in a seismic interpretation system, including but not limited to the following: 2d, 3d and pre-stack seismic viewers, crossover charts, log viewers horizon trackers (auto, manual, grid tracking, ...) interpretation of faults well connection module, time-depth conversion, 3d bodies mapping (via gmt). for these reasons, opendtect is an optimal and very versatile program to visualize the processed data, in addition to having a very high-quality graphical output that is useful for properly identify events and horizons in the gpr data. once the data was loaded onto opendtect, each of the profiles was analyzed in a 2d view and tools for marking and tracking attributes of this program were used. the markings of the events were done by taking small length sections to have a clear and detailed vision, as shown in 12. it is evident how in certain areas, the reflectors lose their continuity and parts with less dielectric permittivity are noticeable. the variations in the continuity of the reflectors are attributed to changes in the dielectric permittivity of the subsurface materials, which correspond to the presence of different materials or voids. these changes result in signal anomalies, indicating structural disturbances. the red line drawn by the tracker in figure 12 shows large jumps, showing discontinuities. some places are not well stratified as can be expected with pavement base material, but they show chaotic behavior instead. these images correspond to areas where a tree behind the wall caused strong effects on the surface. 260 hernán darío guerrero, luis hernán ochoa figure 12. marking the events using opendtect.(a) detail of the interpretation of events on line r002 be-tween meters 32 and 42. (b) detail of the interpretation of events on line r002 between meters 60 and 70. 4. interpretation first, we decided to look for anomalies corresponding to known objects, for example, structural and hydrosanitary elements. we then worked with radargrams throughout time. this is because, through time, the radargrams show a series of signals which can be associated with an anomaly in the layers or reflecting objects. for example, a beam would have an associated signal that is specific to that element’s geometry. by following theses kind of patterns, it is easier to identify the structural and hydrosanitary components found in the subsoil under the basement. in terms of depth, all radargrams inside the parking lot had a depth of 3 m. subsequently, after having identified the anomalies that have obvious explanations, we proceeded to identify those whose origins do not appear on the plans. figure 13 shows a close-up of the concordance of gpr events with the structural and plumbing elements of the building, which are fully identified and shown in figure 14. similarly, there are more structural and plumbing elements in (figure 13) that are precisely identified with the interpreted gpr anomalies. the previous image (figure 13) displays a small section extracted from a larger radargram. it highlights key elements, including columns (cyan), flooring joists (green), and filters (yellow), along with the corresponding annotations for trees (green). this zoomed-in view facilitates a focused examination of the structural components and their arrangement within the radar data. there is a strong correlation between these anomalies and the structural elements of the building. for a more detailed analysis, please refer to the complete radargram (see figure 14), from which this section has been extracted. an analysis of the affectation due to roots outside the retaining wall is shown in figure 15. secondly, having anomalies produced by the structure itself, we proceeded to analyze effects in the subsoil which cannot be related to the structure. the study on the affectation of the soil due to roots and its detection has bases in the scientific and engineering literature (kvamme, 2006), (ramón et al., 2010), (zhang et al., 2019). these academic studies use gpr calibrated between 500mhz and 1600 mhz to see at shallow depths with high resolution for mapping the roots. figure 13. small section extracted from a larger radargram. this is agreement between detected anomalies and the building’s structure. the pictures show two radargrams sections, the anomalies detected in the subsurface have been enclosed in blue ellipses to highlight them. these anomalies correspond to various structural elements, which are marked in different colors for easier identification. the radargrams has beams (green), foundation beams (green), flooring joist (orange), and filters (yellow with red). (x) axis is lateral distance in meters and (z) is depth in nanoseconds. figure 14. miniature view of the interpretation of the structural and drainage elements under the subsoil of the parking lot slab 261subsoil geophysical evaluation using gpr and free software figure 15. radargram r62 section taken in the environmental strip at the tree zone. in yellow, we have outlined the contours corresponding to the areas where there is presence from tree roots, very large elements are seen up to 3 m in depth. the disturbance pattern is similar to that of the radar-grams on the other side near the retaining wall. at a greater depth, the signal does not show information. 5. results the anomalies detected in the subsoil beneath the parking lot are clearly shown figures 16, 17, 18, 19 and 20, following the previously described methodology and excluding those attributable to structural and plumbing elements. each of these figures is marked to highlight the affected areas observed. in this study, the analyses were carried out by dividing each profile into segments of a few meters, as indicated in the aforementioned figures. additionally, a detailed analysis was conducted for each case where surface damage was evident. for example, figures 16 to 20 present a three-dimensional visualization of the radargrams, with each radargram positioned according to its respective acquisition location, in figures 16 and 17, it is clear that the alterations in the subsoil coincide with the proximity of trees to the damaged surface areas. these correlations are documented across all cases, as seen in figures 18, 19, and 20, where the affected areas show consistent patterns near the trees. this analysis reinforces the interpretation that trees play a key role in the observed damage, a common occurrence when soil desiccation is induced by tree roots. the relationship between root expansion and soil drying often leads to structural issues. moore and ryder (2015) highlight that such impacts are significant in assessing the effects of tree roots on their surrounding environment. figure 16. overview of radargrams analyzed in opendtect around 20–30 m. the profiles display r063 and r062 (outside the wall) at the top, followed by r02, r03, and r04 (inside the wall). disturbances in the subsoil highlight the correlation between internal and external areas in sections where weeping willows are present. figure 17. overview of radargrams analyzed in opendtect around 35–45 m. the profiles display r063 and r062 (outside the wall) at the top, followed by r02, r03, and r04 (inside the wall). disturbances in the subsoil highlight the correlation between internal and external areas in sections where weeping willows are present. figure 18. overview of radargrams analyzed in opendtect around 50–60 m. the profiles display r063 and r062 (outside the wall) at the top, followed by r02, r03, and r04 (inside the wall). disturbances in the subsoil highlight the correlation between internal and external areas in sections where weeping willows are present. figure 19. overview of radargrams analyzed in opendtect around 65–75 m. the profiles display r063 and r062 (outside the wall) at the top, followed by r02, r03, and r04 (inside the wall). disturbances in the subsoil highlight the correlation between internal and external areas in sections where weeping willows are present. 262 hernán darío guerrero, luis hernán ochoa figure 20. overview of radargrams analyzed in opendtect around 90–100 m. the profiles display r063 and r062 (outside the wall) at the top, followed by r02, r03, and r04 (inside the wall). disturbances in the subsoil highlight the correlation between internal and external areas in sections where weeping willows are present. radargrams in zones without weeping willows figures 21 and 22 show examples of a radargram taken in an unaffected soil zone (radargram 070). this location corresponds to the northern part of the complex. in this place, there is no presence of weeping willows. figure 21. radargram r070. no disturbance due to roots is observed. this radargram was obtained in the external northern area of the basement where there is no presence of weeping willows. figure 22. radargram r070 in opendtect. the image displays data up to approximately 20 m along the horizontal axis (x), representing distance in meters, while the vertical axis indicates two-way travel time in nanoseconds. there are no disturbances, such as those associated with the roots. a photographic record obtained from a person associated with the building provides compelling evidence of cavities (figure 23). these cavities, visible long before the commencement of this study, indicate the onset of surface issues. the presence of a pit at the foot of the retaining wall correlates directly with these surface problems, demonstrating that the initial disturbances were exacerbated by the growth of tree roots in the vicinity. the expansion of these roots can induce soil desiccation, further compromising the structural integrity of the surface (moore and ryder, 2015). moreover, the observed depressions align with the findings in the radargrams, establishing a consistent relationship between the surface manifestations and the anomalies detected in the subsurface. this reinforces the conclusion that the initial surface disturbances, influenced by the growth and desiccation effects of tree roots, significantly contributed to the formation of these cavities, as substantiated by both photographic documentation and radar analysis. figure 23. evidence of scour. it is difficult to estimate the size of the scour, but it is seen to be elongated, which is consistent with what is observed in the gpr records. the exact position of the pit is unknown but is located at the foot of the retaining wall. conclusions following the established objectives, we successfully carried out geophysical prospecting of the parking area on the south side next to the retaining wall, using ground-penetrating radar. high-resolution images of the first 3 m were obtained under the parking lot, and the following deductions are made from the analysis of the results: 1. the loading, processing, and displaying of acquired gpr data has been done successfully, using free and open-source software tools. 263subsoil geophysical evaluation using gpr and free software 2. it was possible to reconstruct the registered signals and correlate them with the information found in the plans supplied, which correspond to the foundations, as well as structural and plumbing details. a clear correlation is observed between the plans and the data captured by the georadar, as illustrated in figures 13 and 14. this alignment not only supports the accuracy of the plans but also emphasizes the effectiveness of the georadar in detecting subsurface features. the figures provide a visual representation of this concordance, enhancing the reader’s understanding of how the georadar data aligns with the established plans. this specifies that the elements in the drawings do have those sections and they are located according to what is stipulated on them. 3. a reconstruction of the subsoil under the parking lot was conducted. this analysis reveals that the structural platforms in the basement were constructed on a foundation composed of soil for the entire slab, with a layer of compacted material of minimal thickness placed beneath the slab, adhering to standard construction practices. beneath this layer of reduced thickness, two additional layers are observed that are thicker than the layers above, reflecting the natural soil characteristics of the area. 4. when analyzing the radargrams near the retaining wall on the south side adjacent to 134th street, we can observe that the platform’s soil foundation structure is disturbed in several areas, and a large number of these alterations coincide with the location of the trees in the outer part. there are noticeable changes in thickness and continuity of the strata and some anomalies that do not go along with the structural frames, probably due to the drying effect caused by the roots of nearby trees. this effect is present from the wall to the axis of the first column, where surface damage is evident in some cases. likewise, characteristics of affectation can also be seen in the sets of short profiles taken on the surface of affected parking areas. 5. the difference between the subsoil observed in nearby radargrams and the” weeping willows” tree section is remarkable. these are areas to stay away from, based on the radargram analysis in the presented affected and unaffected areas. 6. as the profiles move away from the area close to the weeping willows, the affectation gradually becomes less until it disappears, the coherence of the strata is clearest. acknowledgments we want to acknowledge the las galias construction company for their support and help in providing the team of professionals, including geotechnical, structural, and environmental engineers, as well as surveyors, who verified the correlation of these results with other studies the company has regarding this case. we would also like to express our sincere gratitude to the anonymous reviewers for their invaluable suggestions, which greatly enriched and strengthened this work. references alani, a., bianchini, l., lantini, l., tosti, f., benedetto, a., 2018. mapping the root system of matured trees using ground penetrating radar. in: 17th international conference on ground penetrating radar. rapperswil, switzerland, pp. 18-21. baker, g., jordan, t., talley, j., 2007. an introduction to ground penetrating radar. the geological society of america, special paper 432. besson, a., 2010. the spatial and temporal organization of soil water at the field scale as described by electrical resistivity measurements. european journal of soil science, 61 (1): p. 120-132. böniger, u., tronicke, j., 2010. improving the interpretability of 3d gpr data 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r., 1979. complex seismic trace analysis. geophysics, 44(6), 1041–106. doi:10.1190/1.1440994 . tomecka-suchoń, s. 2019. correction to: ground penetrating radar use in flood prevention. acta geophysica. springer. 67:1679–1691. white, r., 1991. properties of instantaneous seismic attributes. the leading edge, 10(7), 26–32. doi:10.1190/1.1436827 . zhang, x., derival, m., albrecht, u., ampatzidis, y., 2019. evaluation of a ground penetrating radar to map the root architecture of hlb-infected citrus trees. agronomy. 9,354. doi:10.3390/agronomy9070354. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.113984 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 427 437 g eo lo g y tectono-sedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco): impact on the nw-se atlantic transgression mohamed el fartati1*, saïd hinaje2, abdellah sabaoui3, driss yaagoub1, mohammed laiche2, khaoula baadi3, youssef drissi2, anas laksir2, samir amrani2, youssef gharmane1,4 1. laboratoire multidisciplinaire de recherche et innovation, faculté polydisciplinaire de khouribga, université sultan moulay slimane, khouribga, maroc. 2. laboratoire systèmes intelligents, géo-ressources et energies renouvelables (siger), université sidi mohammed ben abdellah, faculté des sciences et techniques, b.p. 2202, route d’imouzzer, fès, maroc. 3. laboratoire de géosciences, environnement et ressources associées (lgera), université sidi mohammed ben abdellah, faculté des sciences dhar el mahraz, fès, maroc. 4. ecole supérieure de technologie de fkih ben salah, université sultan moulay slimane, fkih ben saleh, maroc. * corresponding author: m.elfartati@usms.ma how to cite item: el fartati, m., hinaje, s., sabaoui, a., yaagoub, d., laiche, m., baadi, k., drissi, y., laksir, a., amrani, s., & gharmane, y. (2024). tectono-sedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco): impact on the nwse atlantic transgression. earth sciences research journal, 26(4), 427-437. https://doi.org/10.15446/esrj. v28n4.113984 record manuscript received: 18/04/2024 accepted for publication: 14/01/2025 abst r ac t this paper aims to identify and map the miocene series in the northern middle atlas. this is to enhance our knowledge about its spatial-temporal variation. field investigations mainly based on microtectonic measurements combined with structural analyses, lithostratigraphic studies and stress tensor inversion have allowed to reconstruct the genesis and geodynamic evolution of the northern middle atlas miocene basins. the lithostratigraphic analysis of the neogene series unconformably deposed on the jurassic formations shows a transgressive depositional evolution. several stratigraphic levels have been identified; successively: (i) a conglomerate capped by vallesian volcanic-sedimentary, lacustrine and palustrine continental series, (ii) upper tortonian sandstones, fluvio-deltaic conglomerates and sandy marls with channeled sandstone, (iii) upper tortonian-messinian molasse, pink calcarenite, reef limestone, ochre sandy silt and blue marl. these formations are unconformably overlain by the skoura puddingstones assigned to the lower-middle pliocene. these deposits are synchronous with a major brittle tectonic phase divided into three episodes: (i) n120 to n140 vallesian compression (n30 to n50 horizontal σ3 axis), (ii) upper tortonian – messinian n45 extension (σ1 vertical). the history of the middle atlas during the upper miocene shows an opening megasequence which begins with continental sedimentation during the upper vallesian-tortonian period evolving towards marine sedimentation associated to the nw-se atlantic transgression during the upper tortonian. keywords: miocene basins; geodynamic evolution; tectonic events; middle atlas; morocco. palabras clave: cuencas del mioceno; evolución geodinámica; eventos tectónicos; atlas medio; marruecos eventos tectono-sedimentarios y evolución geodinámica de las cuencas en el norte del atlas medio (marruecos): impacto de la transgresión nw-se del atlántico este artículo busca identificar y mapear las series del mioceno al norte del atlas medio. esto con el fin de mejorar el conocimiento de sus variaciones espacio-temporales. las investigaciones de campo basadas en mediciones microtectónicas combinadas con análisis estructurales, estudios litoestratigráficos y de inversión del tensor de tensión han permitido reconstruir el origen y la evolución geodinámica de las cuencas en el norte del atlas medio. el análisis litoestratigráfico de las series del neógeno, depuestas disconformemente en formaciones del jurásico, muestran una evolución deposicional transgresiva. se identificaron varios niveles estratigráficos sucesivos: (i) un conglomerado cubierto de material sedimentario volcánico del vallesiense, series continentales lacustrinas y palustrinas, (ii) areniscas del tortoniense tardío, conglomerados fluvio-deltáicos y margas arenosas con areniscas acanaladas, (iii) molasas del tortoniense-mesiniano tardío, calcarenitas rosadas, arrecifes calizos, arena ocre y marga azul. estas formaciones cubren disconformemente las pudingas en la región de skoura, asignadas al plioceno temprano-medio. estas formaciones son sincrónicas con fases tectónicas precarias divididas en varios episodios: (i) compresión del vallesiense n120 a n140 (n30 a n50 eje horizontal σ3), (ii) extensión del tortoniense-mesiniano tardío (σ1 vertical). la historia del atlas medio durante el mioceno tardío muestra una megasecuencia abierta que comienza con la sedimentación continental durante el período vallesiense-tortoniense tardío que evolucionó hacia una sedimentación marina asociada con la transgresión nw-se del atlántico durante el tortoniense tardío. resu m en https://doi.org/10.15446/esrj.v28n4.113984 mailto:m.elfartati@usms.ma https://doi.org/10.15446/esrj.v28n4.113984 https://doi.org/10.15446/esrj.v28n4.113984 428 mohamed el fartati, saïd hinaje, abdellah sabaoui, driss yaagoub, mohammed laiche, khaoula baadi, youssef drissi, anas laksir, samir amrani, youssef gharmane 1. introduction the middle atlas is a ne-sw trending intracontinental mountain range. it extends over more than 400 km from the atlas of beni mellal in the sw to the guercif basin in the ne. it is framed by the south rifain corridor to the north, the western meseta to the west, the high atlas and the high moulouya to the south and the eastern meseta to the east. this mountain range is divided into two zones separated by the north middle atlas fault (nmaf). it is the middle atlas causse in the nw and the folded middle atlas in the se (figure 1) (termier, 1936; colo, 1961). during the neogene, the atlas domain underwent polyphase tectonics with positive inversion, and transformation of the jurassic basins to high orogenic zones (e.g., laville & piqué, 1992; morel et al., 1993; piqué et al., 1998; gomez et al., 2000; frizon de lamotte et al., 2000; hinaje, 2004; ellero et al., 2020). several studies have been carried out in the middle atlas and the high atlas in order to explain the genesis of these basins and their mesocenozoic evolution. most of these works aim to propose, through regional analyses, examples of geodynamic evolution of intraplate basins. in the middle atlas, the first neogene marine deposits, first reported by daguin (1927), were contained in outlying gulfs from the south rifain corridor to the skoura basin. the marine advance rounded the massif of tazekka which was then an island (colo & morin, 1952; choubert & faure-muret, 1960-62). most of the synthesis studies on the middle atlas miocene have been carried out using biostratigraphic studies of the moroccan neogene (feinberg & lorenz 1970, 1973; wernli, 1987), and correlated with data from the south rifan corridor and skoura gulf (taltasse, 1953; feinberg, 1978; jaeger, 1978; martin, 1981; charrière, 1990; sabaoui, 1998; sabaoui & hinaje, 2000; hinaje, 2004). the term south rifain strait was first used by gentil (1911, 1916) who thought that the neogene formations are framed by the rif and the middle atlas already outlined. it is only in 1927 that the marine miocene was reported in the middle atlas in relation to gulfs from the south rifain-saïs strait (figure 2). the middle atlas miocene, especially the skoura basin was mapped for the first time at 1/200 000 by termier & dubar (1937). termier (1936) had also discovered the outcrops of ribat al khayr and tazarine. new outcrops were later recognized to the east and north of merhraoua, and to the north of aderj (colo & morin, 1952; sabaoui, 1987,1998) (figure 2). the main synthesis studies carried out in the south rifain corridor (taltasse, 1953; feinberg, 1978), have only focused on the extreme northern and western edges of the middle atlas. the problems of correlation between the middle atlas and the south rifain corridor deposits have been highlighted by jaeger (1978) and martin (1981). by establishing a lithostratigraphic succession and a paleogeographic reconstruction of the upper miocene cycle in the zraa basin and a large part of the skoura basin, charrière (1984, 1990) presented, with reservation, correlations between the middle atlas formations and those of the south rifain basin. its hypothesis is based on the synchrony of the development and invasion of pelagic fossil forms on the one hand, and the diachrony of reef forms on the other hand. in this study, we adopted a multidisciplinary approach mainly based on field work. lithostratigraphic and tectonic analyses, combined with geological mapping of miocene outcrop boundaries in the sefrou, oued zraa, ribat al khayr, tazarine basins and neighboring areas, have allowed to increase our knowledge about the spatial-temporal variations in the middle atlas miocene series. based on the collected data, a correlation with the south-rifain basin and a paleogeographic reconstruction of the northern middle atlas miocene basins were proposed. 2. regional geological setting the tectonic-sedimentary evolution of the atlas domain from the triassic to the present day has been the subject of several studies (e.g., choubert & faure muret, 1960,1962; michard, 1976; laville,1985; fedan, 1988; jacobshagen et al., 1988; charrière,1990; sabaoui, 1998; hinaje, 2004; frizon de lamotte et al., 2008). this evolution is marked by the continental rifting phase linked to the opening of the central atlantic and the alpine tethys in the framework of the pangea supercontinent break-up. this is followed by the highly complex stages of the atlas orogeny which led to the uplift of the high atlas and middle atlas mountain ranges since the end of the cretaceous period, in the framework of continent-continent convergence between africa and iberia/europe (e.g., mattauer et al., 1977; aït brahim et al., 2002; frizon de lamotte et al., 2009; ellero et al., 2020; yaagoub et al., 2022; elabouyi et al., 2022; laiche et al., 2025). figure 1. geological map of the middle atlas (from the geological map of morocco at 1:1000000, simplified and modified). figure 2. general geological setting of the middle atlassaïs basin and location of the studied miocene outcrops. the middle atlas stratigraphic series is mainly represented by the following formations (figure 3): • the folded paleozoic basement outcropping in the tazzeka massif and in the bhalil, kander, bssabis, el hajeb and kerrouchene inliers; • the triassic-liassic detrital, evaporitic and basaltic formations outcropping around the paleozoic terrains of kander, bssabis and el hajeb inliers, the tazzeka massif and the kerrouchène area; • the lower-middle jurassic carbonate and marl formations which form the most important part of the middle atlas terrains; • the cretaceous formations which appear along the edges and in the centers of the middle atlas range; 429using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources tectonosedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco) • the paleogene deposits, highly diversified and outcropping to the west of the boulemane area; • the upper miocene formations outcropping in the nw part of the chain, in the middle atlas causse and in the folded middle atlas; • the plio-quaternary continental formations, represented by lacustrine, travertine, alluvial fan and volcanic deposits (hinaje et al., 2019). they are particularly localized near major fault lines and the chain periphery. figure 3. synthetic lithostratigraphic column of the middle atlas (from hinaje, 2004) 3. methods to carry out this study, we adopted a multidisciplinary approach, including lithostratigraphic analysis of the middle atlas miocene terrains and their correlation with those of the sud-rifain trench and saïs basin. several cross-sections of the miocene series allowed to specify the succession of the different lithostratigraphic terms as well as their lateral variations. this enabled to better identify correlation problems within the middle atlas or with other regions of the south rifain-saïs strait. a tectonic and microtectonic analysis of the vallesian, upper tortonian-messinian and pliocene formations is also carried out in order to define the tectonic framework associated with the sedimentation. in the measurement sites, the criteria used to characterize the deformation during a specific period are the following: • the age of the fractured formations, • the fracture offsets by others, • the superposition of the striae on the fault slickensides, • the presence of parallel fractures with incompatible movements, • sealed synsedimentary faults, with slickensides showing no recrystallization, • the elimination principle; from the most recent tectonic episode to the oldest. data processing is carried out using several software packages such as arc-gis, adobe illustrator and angelier software for stress tensors inversion (angelier, 1990). the obtained results will allow to establish the various tectonic phases affecting the study area, and to reconstruct its geological history. 4. results lithostratigraphic analysis of the northern middle atlas miocene series the miocene outcrop deposits cover a large nw-se trending area, located in sefrou, tahla, skoura, ribat el khayr and aghram amellal (figures 2, 4). the skoura basin miocene outcrops are bounded to the nw by the nmaf and to the se by the central middle atlas fault. the neogene deposits of this basin were guided by the normal movement of n120 to n140 synsedimentary faults (hinaje, 2004; el fartati, 2021). other outcrops with lesser extension have been studied. these are those of sefrou, oued zraa, ribat al khayr, tazarine basins and neighboring areas (figure 4). all of these neogene deposits are related to a neogene marine gulf open to the south rifain corridor and bordering the tazekka massif which formed an island. these very diversified deposits are made up of molasses and microconglomerates at the bottom, lenticular limestone, marl, reef limestone and sandy marl at the top. the basal molasse is sometimes deposited in intramontane subsiding basins. these deposits are contemporary with tectonic activity divided into several episodes, including the first, which preceded the tortonian transgression in the middle atlas (charrière, 1984), and is responsible for the emplacement of the vallesian basalts in the zraa fluvio-lacustrine basin (martin, 1981; charrière, 1990) as well as the collapse of the guercif basin in the ne (coletta, 1977; hervouet, 1985; giret, 1985). in major angular unconformity on the jurassic terrains, the middle atlas miocene deposits show a series we subdivide into different formations correlable with the saïs and the south rifain corridor (figure 4). figure 4. geological mapping, facies repartition and correlation test of the upper miocene deposits in the middle atlas and part of the south rifain corridor. 430 mohamed el fartati, saïd hinaje, abdellah sabaoui, driss yaagoub, mohammed laiche, khaoula baadi, youssef drissi, anas laksir, samir amrani, youssef gharmane the paleoboulders formation or “sefrou basal molasse” these are deposits assigned to the lower and middle miocene (hinaje, 2004) because they are overlaid by tortonian deposits in the middle atlas-saïs junction, near the paleozoic bni mellala inlier and the sefrou basin (figure 2). they are made up of coarse liassic elements coming from the erosion of the neighboring reliefs. near sefrou city, this molasse unconformably overlay the lower liassic dolostone. at the bottom, it is made up of a 4 to 10 m thick coarse detrital term, with angular liassic elements of 3 to 30 cm in size, and lenticular bedding. on this term rests a 50 cm thick beige calcarenite level with oblique bedding. on this calcarenite rests a 4 to 8 m thick grano-decreasing sequence consisting of a coarse molasse at the bottom and a microconglomerate with limestone cement at the top. the lithostratigraphic cross-section of the neogene deposits (figure 5) shows two molasse and conglomerate levels representing the base of two transgressive sequences. these two undated levels can be confused because of their restricted geographical distribution. figure 5. stratigraphic column of the northern middle atlas neogene deposits. the oued zraa formation the oued zraa and skoura basins (figure 4) provide the first regional neogene deposits after the paleoboulders and molasses located at the junction of the saïs and middle atlas basins (charrière, 1990; hinaje, 2004). this formation is well developed in these two basins. the oued zraa basin is located sw of the oued zraa-el menzel panel which is bounded to the nw by the median middle atlas fault (mmaf) and to the se by the nmaf. it shows three sedimentary units (figure 4) whose first two constitute the oued zraa formation: • at the bottom, there is a continental red series with volcanic and volcano-sedimentary intercalations, with a thickness of several dozen meters (figure 5). a double geochronological dating of the volcanic flow has provided the ages of 9.7 + 0.5 ma and 10 + 0.5 ma (jaeger et al., 1973). a grey clay level, overlying the dated volcanic flow, yielded vallesian micromammals (jaeger & martin, 1971; jaeger, 1978); • on top of these clays, we find white and chalky marly limestones which constitute a relatively homogeneous lacustrine unit, 30 to 40 m thick; • a fluvial-type lithostratigraphic assemblage covers the center of the oued zraa basin. this assemblage have been related to the basal member of the oued mdaz formation described in the skoura basin (sabaoui, 1998); • these deposits are conformably capped by a lacustrine sequence constituted by calcilutites and white limestone with root traces. they are sometimes covered by conglomerates and fluvial sandstones. in the northern part of the skoura basin (figures 2, 3), on either side of the oued mdaz, the bottom of the miocene series showed orange-red silt beds with volcano-sedimentary intercalations. these correspond to tuffaceous clays with 10 to 20% pyroclastic elements and also to volcanic tuff levels. all these levels have been correlated with the lower volcano-sedimentary unit of the oued zraa basin. on top of this, we find variegated versicolor marls. thin white limestone benches with lumpy microfacies intersect the marly series. these deposits are equivalent to the oued zraa marl-lacustrine unit. at the scale of the middle atlas, the extension of these lacustrine deposits is very small. they are particularly outcropping in the ne of the subsident skoura and oued zraa basins (jaeger & martin, 1971; jaeger, 1978; hinaje, 2004). in the skoura basin, 3 km east of the oued mdaz, these deposits unconformably overlay the bathonian sandstone with marly interbeds. they are constituted by a clay layer with calcareous nodules more than 8 m thick. these series are surmounted by white limestones with upper vegetal traces, which thickness can reach 10 m (figure 6) (el fartati, 2021). eastward, along the road leading to the aghram amellal village, the locality of ajdir ahbari displays the same deposits but with a thickness reduced to 3 m. this outcrop would represent the edge of a vallesian lake spread over the skoura basin (el fartati et al., 2019, 2021, 2023). figure 6. alithostratigraphic cross-section of vallesian lacustrine deposits east of the mdaz bridge (ajdir ahbari area). bphoto of vallesian lake deposits, affected by n130 synsedimentary normal faults. cvallesian deposits at the lake basin edge unconformably deposited on the middle-upper pliocene lacustrine deposits. the oued mdaz formation this formation is the equivalent of the aït youb-lebbouaddis formation defined at the nw edge of the causse (figure 4) (charrière, 1990). two large outcrops are recognized in the skoura basin. the first, which is 50 m thick, is located on either side of the oued mdaz where it overlies the oued zraa formation. the second outcrop covers the se flank of the skoura basin. the latter outcrop extends from the lower slope of the ne termination of the tichoukt ridge to the aderj proximity. on both sides of the oued maasser, these deposits reach 250m thickness (figure 4). these are detrital facies where conglomeratic sedimentation is dominant. the whole shows a fluvial type sequential organization. laterally, these deposits decrease in thickness to the n and ne where they are replaced by sandstones and marls. at the bottom, the more-least sandy marls are interbedded with small sandstone, conglomerate or microconglomerate benches. as the series rises, the marls become dominant with rare sandstone beds with oblique and intersecting bedding. in the area east of the oued mdaz, the thickness of this unit does not exceed 50 m. there is a 431using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources tectonosedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco) progressive thinning of the deposits from sw to ne until their disappearance 1 km south of a line connecting aderj and aghram amellal (figure 4). this member is overlain by thick, poorly stratified sandstone beds with interbeds of large oyster marl (crassostrea) (figure 7). different levels of this cross-section have yielded gastropods and rare foraminifera of the ammonia genus (sabaoui, 1998). the oued mdaz formation has been assigned to the upper tortonian (charrière, 1990). figure 7. marls with oysters in the nw of the skoura basin. aghram amellal formation and its equivalent pink limestone of sefrou-bhalil this formation is recognized and studied in several outcrops of the middle atlas and saïs, including aghram amellal, ribat el khayr, tazarine and sefrou areas (figure 4). in the aghram amellal area, miocene outcrops have been reported by several authors (colo, 1961; lorenchet de montjamont, 1963; martin, 1981; saint martin, 1987; el hamzaoui, 1994). the aghram amellal formation corresponds to the upper miocene deposits with three main facies: the basic molasses, the reef limestone and the sandstone or sandy limestone. at the middle atlas scale, the most important outcrop of these units is the sector of aghram amellal (sabaoui; 1998), which explains its name. the reef limestones which, according to charrière (1990), constitute the first formation of the tazouta pass, must be considered as one term of others which constitute this aghram amellal formation. the oued mdaz formation progressively passes to that of aghram amellal without sedimentation interruption. this formation is mainly composed of yellow sandy marls with sandstone inter-beds. these deposits yielded planktonic foraminifera reaching 10% of the microfauna with an association indicating the m6a subzone (upper tortonian-messinian) (sabaoui, 1998). in the aghram amellal area, the upper miocene deposits directly overlie the triassic, liassic or dogger formations. their base is generally made up of channel conglomerates and sandstones with oblique and intersecting stratification. in addition to the known reef near aghram amellal (lorenchet de montjamont, 1963), we discovered several other coral reef outcrops (figures 2, 8) which rest on the base molasse deposits, on the bathonian deposits or on various miocene terms. in ribat al khayr area, the upper miocene series has been known since the works of termier (1936). lorenchet de montjamont (1963) indicated the reef nature of the limestone on which the village is built. in the eastern edge of the village, there is a base conglomerate (1 m) topped by sandstone bars alternating with sandy marl layers. the whole is interlaced by conglomerate layers. reef constructions (figure 8) with a thickness of up to 15 m have developed on this detrital term. they are presented as lenses separated by sandstone limestones with very little coral. in the northern part of the ribat al khayr outcrop, the reef constructions are replaced by sandstone limestone and generally sparitic limestone (figure 8). the biophase is limited to bioclasts, bryozoans and rare foraminifera. the whole of the aghram amellal formation deposits is surmounted by the tazouta formation. figure 8. geological map of ribat el khayr miocene formations and orthogonal ne-sw and nw-se geological cross-sections (from sabaoui, 1998). 1-paleozoic; 2triassic and jurassic; 3aghram amellal formation (a-conglomerate and sandstone, breef limestone, ccalcarenites and sandstone limestones); 4tazouta formation; 5plio-quaternary. in the tazarine area, the miocene deposits are unconformably deposited on the dogger marl limestone. they consist of conglomerates, calcarenite and yellow limestones rich in bioclasts and polypiers. in the ne part of the middle atlas causse, between sefrou and tahla, the pink limestone formation of sefrou -bhalil, assigned to the upper tortonian messinian, crops out (charrière, 1990; hinaje, 2004). it is made up at the bottom by coarse molasses with mainly liassic elements, covered by pink limestones with heterostegines, or their lateral equivalents: the pink and beige calcarenites and the reef limestones. on this term, rest the sefrou yellowish sandy silt and the blue marls with sandstone pass. the top of the latter, nw of fez city, is assigned to the lower pliocene (wernli, 1987). in the transition zone between the middle atlas-saïs basin, especially in the center of sefrou-tahla gulf, the miocene lithostratigraphic series is made up of upper tortonian-messinian carbonate bar with reef layers and yellowish siltstones with sandstone-conglomerate lenses. tazouta pass formation and its equivalent fez saïs blue marls charrière (1990) defined two upper miocene terminal formations nw of the skoura basin. he called them the tazouta pass formations. the first is reef limestone and the second is oyster-white sandy marl. the extension of studies to the north showed that the reef unit must be considered as a member among others which constitute the aghram amellal formation (figure 5). the second unit referred to as the tazouta pass formation is the terminal upper miocene deposits in the middle atlas. apart from the nw sector of the skoura basin, the tazouta pass formation also outcrops at ribat al khayr. in this outcrop (figure 4), we established that the different terms of the aghram amellal formation (molasse, reefs, sandstone limestones) pre-date the marls that must constitute the tazouta pass formation. indeed, in the north and east part of this outcrop, these marls geometrically overlie the aghram amellal formation. to the west, tazouta pass marls transgress directly on the liassic substratum. in the ribat al khayr outcrop, the tazouta pass formation is made up of yellow to white sandy marl. in the nw of the skoura and saïs basins, this formation is represented by 432 mohamed el fartati, saïd hinaje, abdellah sabaoui, driss yaagoub, mohammed laiche, khaoula baadi, youssef drissi, anas laksir, samir amrani, youssef gharmane blue marls and/or white marls which sometimes directly transgress the jurassic formations. tectonic and microtectonic analysis of the northern middle atlas miocene series the middle atlas is the site of significant tectonic, neotectonic and seismic activity (michard, 1976; ramdani & tadili, 1980; charrière, 1990; hinaje, 2004). hundreds of microtectonic measurements have been collected in the northern middle atlas miocene series (hinaje, 2004; hinaje et al., 2015). several tectonic phases, divided into episodes, have been revealed. we describe below the various tectonic phases which have guided basin geometry, structure and geodynamic evolution. tectonic phase preceding miocene basin opening in the sefrou neogene basin, located at the junction of the middle atlas and saïs basin (figure 2), the oligocene-lower-middle miocene tectonic phase, corresponds to the phase before the opening of the nw-se sefrou-tahlatazarine-skoura marine gulf. it is represented by two episodes (figure 9). the first one is characterized by submeridian dextral strike-slip faults and ne-sw to ene-wsw sinistral strike-slip faults. these faults are compatible with a stress state with n30 horizontal σ1 axis and n120 σ3 axis. the second episode is compressional and oriented nne-ssw (vertical σ3 axis), represented by wnw-ese trending reverse to thrust faults, metric to decimetric scale folds with n110 to n145 trending axes and wnw-ese trending fracture cleavage. the ratio  = (σ2 σ3) / (σ1σ3) ranges from 0,02 (σ2 = σ3) to 0,67 (σ2  (σ1 + σ3) / 2); indicating that reverse and thrust faults can be associated with strike-slip faults with reverse component. the age of this tectonic phase is assigned by analogy with work carried out in other areas where deposits are dated (hinaje, 2004). in the bni mellala inlier, located in the nw edge of the tabular middle atlas, this tectonic phase is represented by a ne-sw compressional episode. this is shown by n110 to n140 trending thrust faults which affect the ordovician quartzites and schists, the upper triassic and lower liassic deposits, whereas they are missing in the upper miocene deposits (figure 9). the movement of these faults is compatible with a compressional stress state characterized by n30 horizontal σ1 axis and vertical σ3 axis. figure 9. microtectonic analysis of the brittle deformation and determination of the stress axes during the oligocene-lower-middle miocene tectonic phase in the sefrou basin. asimplified geological map of the sefrou-bhalil area (from the geological map of sefrou at 1/100 000, modified). 1paleozoic-triassic; 2early-middle liassic; 3upper miocene; 4plio-quaternary; 5fault; s1-6: microtectonic measurement site. b and dmetric fold with cleavage in the early liassic dolostones. 1brecciated dolostone; 2fracture cleavage; s3fold axes stereoplot; s4fracture cleavage planes stereoplot. cthrust planes affecting early liassic brecciated dolostones. s5-6: strike-slip faults stereoplot. the bni mellala n100 to n120 trending thrust, bordering it on the north side, corresponds to a flat surface which detaches the triassic-liassic cover from the paleozoic basement (figure 10). the contact between the liassic dolostones and the basement corresponds to a crushed surface containing basalt levels, yellow marls and red claystones. the thrusting contact has a sinistral strike-slip component, marked by the rebound of the basement cleavage (figure 11). the pre-miocene age of the bni mellala thrust was also reported by charrière (1990), who placed it between the deposition of the paleoboulders and the first upper tortonian marine deposits. this nne-dipping thrust is reversed and tilted to the nw by the n30 dayet aoua normal fault with nw dipping (figure 9). this normal movement is assigned to the early-middle quaternary (hinaje et al., 2001). to the south of the inlier, the polyphase n120 aïn dik fault (charrière, 1990), offsets the dayet aoua fault in apparent dextral movement (figure10). this fault also has a normal movement with dextral strike-slip component, probably of late miocene age. together with the n130-trending kandar fault and the n140-trending sefrou-bhalil fault, it constitutes stepped blocks with ne collapse. this collapse occurs towards the nw-se sefrou tahla skoura ribat el khayr upper miocene marine gulf, and corresponds to a set of kilometer-scale subsiding grabens (hinaje et al., 2001; hinaje, 2004). figure 10. asimplified geological map of the bni mellala area (from charrière, 1990 modified). 1paleozoic; 2triassic-infra-liassic; 3early-middle liassic; 4anteupper miocene paleoboulders; 5upper miocene; 6plio-quaternary; 7fault; 8thrust. bstructural cross-section showing the n120 bni mellala thrust. cstereoplot of the thrust fault planes accompanying the bni mellala major thrust, and stress axes of the ante-upper miocene ne-sw compressional episode. figure 11. n110 to n120 bni mellala thrust putting the early-middle liassic dolostones and limestones directly on the ordovician schists. tectonic phase synchronous with the opening of the miocene basins (vallesian upper tortonian messinian) we have highlighted the vallesian tectonic episode in the oued zraa and oued mdaz measurement sites. these two zones constitute the first neogene basins with continental deposits. the bottom of these deposits is made up of a 433using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources tectonosedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco) continental series with interbedded volcanic flows. on top, we find alternations of marl and limestone, white calcilutites and lacustrine white limestones, surmounted by fluvial conglomeratic deposits. the lacustrine carbonate deposits are affected by a series of sealed and striated faults, divided into two groups: n05 to n45 trending reverse faults with dextral strike-slip component, and n75 to n100 trending dextral strike-slip faults. these faults are consistent with a paleostress field with n120 horizontal σ1 axis and n30 horizontal σ3 axis. these vallesian deposits are affected by a n130 trending major normal fault with ne collapse, whose post-vallesian movement is assigned to the upper tortonian-messinian. the second microtectonic measurement site corresponds to the area located on the right bank of oued mdaz, between tazouta and aderj (ajdir ahbari). this area is part of the nw-se tazouta-skoura miocene basin. near the mdaz bridge, the vallesian facies are constituted by silty, marly and calcareous lacustrine deposits with root traces. these deposits are unconformably deposited on the bathonian formation, and they are covered by the upper tortonianmessinian marls, or locally unconformably by the lower-middle pliocene skoura puddingstones (charrière, 1990; sabaoui, 1998). they are affected by sealed faults with planes striated by grooves and channels. these faults are oriented n120 to n145 with normal movement, consistent with an extensional paleostress field such that σ1 axis is vertical and σ3 is horizontal and striking in ne-sw direction (figure 12). on the same outcrop, we measured n150 to n175 trending sinistral strike-slip faults; consistent with a compressional paleostress field with nw-se horizontal σ1 axis, ne-sw horizontal σ2 axis and vertical σ3 axis. the ratio  = 0,04; indicates that σ2 = σ3, and nw-se compression is associated with ne-sw extension. the processing of all the synsedimentary normal faults and the sealed decays, results in a paleostress field with the vertical σ1 axis, nw-se horizontal σ2 axis and ne-sw horizontal σ3 axis. in this case the ratio  = 0.80, indicates that σ1 ≈ σ2. this means that the ne-sw extension is associated with a nw-se compressional axis. in the bni mellala el qciba valley measurement site, located in the nw limb of the tabular middle atlas, the various microtectonic measurements we have carried out have allowed to highlight a late miocene ne-sw extensional phase. this is represented by nw-se normal faults, compatible with a vertical σ1 axis and a ne-sw horizontal σ3 axis (figure 13). figure 12. across-section of the ajdir ahbari lacustrine deposits showing sealed normal faults and strike-slip faults. 1brown siltstone; 2greenish carbonate marl; 3white clayey limestone; 4lacustrine limestone with root traces. bstereoplot of nw-se synsedimentary normal faults. cstereoplot of the nnw-sse sealed sinistral strike-slip faults. dstereoplot of normal and strike-slip faults. eangular unconformity of the lower-middle pliocene skoura puddingstones on the oued mdaz upper miocene deposits. fstereoplot of nne-ssw reverse faults and e-w dextral strike-slip faults (ϕ ≈ 0) affecting the vallesian lacustrine limestones in the oued zraa measurement site. gsealed synsedimentary normal faults affecting the vallesian lacustrine deposits. figure 13. tectonic analysis of brittle deformation and determination of the stress axes during the upper miocene extensive phase between jbel kandar and bou oughioul. asimplified geological map of the sefrou-bhalil area. 1paleozoictriassic; 2early liassic; 3upper miocene; 4plio-quaternary; 5fault. b ne-sw structural cross-section showing block staging between the kandar and sefrou-bhalil faults. 1paleozoic basement; 2triassic-infra liassic; 3early liassic; 4upper miocene and plio-quaternary. cstereoplot of the normal faults responsible for the ne collapsing during the upper miocene. 5. discussion examination of upper miocene deposits allows us to identify the general evolutionary trend of sedimentary systems at regional scale. two sedimentary ensembles can be identified, the first with a continental character and the second with a marine one. for each set, which raises problems of stratigraphy and correlation, we distinguish several environmental and paleogeographic events. vallesian continental basins the miocene sedimentation starts in the zraa and skoura basins with continental deposits which form the oued zraa and its equivalent ajdir ahbari (figure 14). in the zraa basin, this formation consists of red detrital deposits with volcano-sedimentary and volcanic intercalations. its age is close to 10 ma and it is related to lacustrine continental sedimentation associated with volcanism, in relation with a deep fracturing depending on transversal structural lines (charrière, 1990). the study carried out just east of the mdaz bridge, enabled of the following observations: (i) the vallesian deposits are deposited, with angular unconformity of about 15°, on the bathonian marlsandstone basement, (ii) these lacustrine deposits are highly fractured and tilted towards the sw with a dip of about 20°, (iii) their depositional environment corresponds to the oued mdaz paleo-dam lake which natural wall is made up of the impermeable bathonian marl layers tilted along a n130 normal fault. the intensity of fracturing recorded within these lake formations, represented by synsedimentary faults, is related to the vallesian n120 tectonic phase. along the road to the aderj village, the upper miocene marine deposits overlay, with angular unconformity of about 15°, the vallesian lacustrine. this contact is particularly apparent where a n45 normal fault occurs. they also outcrop to the south of this area through the n80 normal fault, which is assigned to the aït chaïb-aïchoun (se of tazouta) lacustrine basin formation to the middle-upper pliocene. 434 mohamed el fartati, saïd hinaje, abdellah sabaoui, driss yaagoub, mohammed laiche, khaoula baadi, youssef drissi, anas laksir, samir amrani, youssef gharmane figure 14. paleogeographic map of the first vallesian continental and volcanic deposits in the middle atlas. 1-paleozoic; 2oued zraa formation and its ajdir ahbari equivalent (vallesian-tortonian); 3dayet aoua fault; 4tizi n’tretten fault; 5mmaf; 6nmaf; 7central middle atlas fault; 8south middle atlas fault. upper tortonian fluvio-deltaic to lagoon-lacustrine basins in the oued zraa and skoura basins, the sedimentation is continued by coarse fluvio-deltaic facies deposits which form the base of the oued mdaz formation (figure 15). they are correlable with the basal term of the aït youblebbouaddis formation which crops out further nw. the present outcrops show that their extension is much wider than the oued zraa lacustrine deposits. the oued mdaz formation is continued by an essentially marl and silt sedimentation with gypsum. the biophase is essentially limited to a freshwater microfauna. all of these deposits correspond to a lacustrine to lagoon environment established in the skoura mudflat, which underwent polyphase tectonic subsidence. at the same time, another mudflat was established to the nw, in the aït youb-lebbouaddis sector (charrière, 1990). contrary to what has been accepted at ribat al khayr, no basin existed at that time. indeed, the first formation to have been deposited was that of aghram amellal. the oued mdaz formation ends with sandstone bars with facies indicating a lagoon environment with a very slight marine influence. in the skoura basin, these sandstones have yielded a crassostrea fauna, the first foraminifera of ammonia genus and very rare sea urchin radioles. in the ne part of the skoura basin, this upper term of the mdaz formation transgresses over the dogger basement. this overflow indicates the impact of communication with atlantic ocean seawater on the nw side, on the sedimentation environment. this communication will be more important during the sedimentation of the overlying aghram amellal formation. figure 15. paleogeographic map of the first vallesian continental and volcanic deposits in the middle atlas. 1-paleozoic; 2oued zraa formation and its ajdir ahbari equivalent (vallesian-tortonian); 3dayet aoua fault; 4tizi n’tretten fault; 5 mmaf; 6nmaf; 7central middle atlas fault; 8south middle atlas fault. upper tortonian and messinian marine sedimentation the oued mdaz formation passes to the aghram amellal formation without sedimentation interruption. the latter begins with a marine-facies marlsandstone term overlying the last sandstone term of the oued mdaz formation. the continuity of the sedimentation allows to confirm the idea of charrière (1990) who questioned the presence of the “probable marine pliocene” north of adrej (colo, 1961). in the sw part of the skoura basin, the aghram amellal formation begins with yellow marls with marine microfauna. the marl or marl-sandstone deposits, developed at the transition from the oued mdez formation to the aghram amellal formation, represent gulf-bottom facies which finds a preestablished and subsident outlet in the skoura basin as a result of the n120 to n140 synsedimentary normal faults and the n20 to n45 inherited reverse faults (figure 16). the skoura basin underwent late infilling by sandstones and marls, resulting in a bathymetric reduction favorable to coral installation. further n and ne of this basin, the aghram amellal formation begins with detrital rocks representing paleo-valley filling and small paleo-depressions. this term constitutes the basement of marine deposits (sandstone, bioconstructions, calcarenite, sandstone limestone). the present distribution of the aghram amellal formation outcrops highlights the nw invasion of atlantic marine seawater into the middle atlas during the upper tortonian-early messinian. the sedimentation is continued by marls constituting the tazouta pass formation outcropping in the nw of the skoura basin and at ribat al khayr. the first outcrop shows relatively restricted facies. the second, which we have also shown to be later than the aghram amellal formation, shows relatively open marine facies. the problem of correlation between the middle atlasic facies and the se part of the saïs basin was raised by charrière (1984,1990). based on the synchronism of the fez blue marls and the yellow marls overlying the oued mdaz formation, this author proposed a hypothesis suggesting that the sefrou-bhalil limestones with reef intercalations would be older than the reefs outcropping in the nw of the skoura basin. thus, according to this author, the former would mark the beginning of sea transgression from the south rifain corridor onto the middle atlas causse and the latter would represent the maximum sea extension in the skoura gulf. field observations and cartographic surveys we have conducted in the folded middle atlas to the north and ne of the skoura basin, lead to support a correlation of the reef limestones or more generally the aghram amellal formation with the sefrou-bhalil pink limestones. this hypothesis finds its support in the following argument: • the presence in the nw of the skoura basin of yellow marls with a pelagic fauna of up to 10%, would represent the first clean marine facies of the aghram amellal formation, which developed in a pre-established mudflat, after the upper tortonian-messinian transgression; • northwest of the skoura basin, the tazouta pass formation, which is of restricted character, proved difficult or impossible to correlate with the first deposits of fez blue marls with pelagic facies in the south rifain corridor. the identification of the tazouta pass formation at ribat al khayr where it shows relatively open facies with a pelagic fauna exceeding 10% allows to correlate it with the blue marls of fez; • the reef outcrops of al mahraz near oued zraa and el menzel reported respectively by martin (1981) and chevalier (1962), would constitute intermediate markers between the sefrou-bhalil pink limestones and the aghram amellal formation. the present distribution of the aghram amellal formation outcrops, especially the reef member, or its equivalent, indicates the installation of a marine regime with a much wider extension than the two previous lagoonlacustrine to lagoon-marine depressions of skoura and aït youb-lebbouaddis. the passage between the middle atlas and south rifain corridor marine domains, se of fez, is through the nw-se gulf of sefrou-tazouta-skouratahla-tazarine (figure 16). the distribution of the aghram amellal formation outcrops or its equivalent (the sefrou-bhalil pink limestones), which does not extend beyond the n130 sefrou-bhalil fault, suggests that, contrary to the ideas of our predecessors, the transgression tends to be oriented from east to west and from 435using potential field derivatives in the arctangent function to estimate the edges and relative depths of potential field sources tectonosedimentary events and geodynamic evolution of the northern middle atlas miocene basins (morocco) nw to se (figure 17). the atlantic transgression which allowed the deposition of fez blue marls, could reach the core of the middle atlas by deposition of tazouta marls which outcrop in ribat al khayr and in the nw of skoura basin (figure 17). this marine intrusion occurred from nw to se in the same-direction gulf bounded by n120 to n140 synsedimentary normal faults. the shallow facies of the middle atlasi deposits, in contrast to the pelagic deposits of the south rifain corridor to the nw, is thought to be related to significant subsidence of the latter. the current outcrops of the tazouta pass formation suggest a discharge of atlantic waters into the middle atlas basin from nw to se. the maximum sea advance could feed communication corridors with the guercif basin. these waters penetrate the sefrou-tahla-tazouta-skouratazarine gulf in a nw-se direction, feeding the ras el ksar guercif basins during transgressive maxima. figure 16. paleogeographic pattern during the upper tortonian early messinian. 1-paleozoic; 2aghram amellal formation and its equivalent (upper tortonian early messinian);3probable limit of upper tortonian early messinian sea extension;4dayet aoua fault; 5tizi n’tretten fault; 6mmaf; 7nmaf; 8 central middle atlas fault. figure 17. paleogeographic pattern during the messinien. 1-paleozoic; 2 messinian outcrop (a: fez blue marl formation with open marine facies; btazouta pass formation with restricted marine facies; cmelloulou succession with restricted marine-lagoon facies); 3probable limit of messinian sea extension. 6. conclusion the northern middle atlas miocene stratigraphic series are characterized from bottom to top by: (i) a conglomerate capped by vallesian volcanicsedimentary, lacustrine and palustrine continental series, (ii) upper tortonian sandstones, fluvio-deltaic conglomerates and sandy marls with channeled sandstone, (iii) upper tortonian-messinian molasse, pink calcarenite, reef limestone, ochre sandy silt and blue marl. these formations are unconformably overlain by the skoura puddingstones assigned to the lower-middle pliocene. these deposits are synchronous with a major brittle tectonic phase divided into two episodes: (i) n120 to n140 vallesian compression (n30 to n50 horizontal σ3 axis), (ii) upper tortonian–messinian n45 extension (σ1 vertical). the history of the middle atlas upper miocene shows an opening megasequence which begins with continental sedimentation during the upper vallesiantortonian period evolving towards marine sedimentation associated to the nwse atlantic transgression during the upper tortonian. references aït brahim, l., chotin, p., hinaje, s., abdelouafi, a., el adraoui, a., nakcha, c., dhont, d., charroud, m., sossey alaoui, f., amrhar, m., bouaza, a., tabyaoui, h. et chaouni, a. 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(1987). micropaléontologie du néogène post-nappe du maroc septentrional et description systématique des foraminifères planctoniques. notes et mém. serv. géol. maroc. 331, 266 p. yaagoub, d., hinaje, s., el fartati, m., gharmane, y. & amrani, s. (2022). cenozoic tectono-sedimentary evolution of the upper moulouya continental basins (moroccan atlas system): insights from lithostratigraphy, structural analysis and paleostress reconstruction. journal of african earth sciences 196 (2022) 104690. _heading=h.gjdgxs keywords: semi-coal rock; geological evolution; non-coal pillar; distressed blind-zone; gas extraction. palabras clave: roca de semicarbón; evolución geológica; minería sin pilar; peligros de la zona ciega; extracción de gas. how to cite item xie, x., fang, x., liang, m., & ai, d. (2020). geological evolution process, mechanism, and application of protective layer in semi-coal and rock face. earth sciences research journal, 24(4), 499-506. doi: https://doi.org/10.15446/ esrj.v24n4.91378 under the influence of geological structure, the surrounding rock of semi-coal seam is easy to be loose and fractured, and the repairing rate is high. therefore, it is of great theoretical and practical significance to study semi-coal seam mining's mechanism and control technology. in this paper, the geological evolution principle of the semi-coal seam is analyzed theoretically. the mechanic's relationship between the mining height of the semi-coal face and the lower coal seam's pressure relief is interpreted. the mechanism of eliminating bad blind areas in non-pillar mining of upper protective seam is analyzed. through numerical simulation analysis, it is concluded that the expansion deformation rate of a protective layer increases linearly with the increase of mining thickness of the protective layer, and the width of undesirable blind zone of coal seam increases linearly with the rise of protective layer thickness. this paper puts forward the technology of non-pillar mining in the protective seam's semi-coal and rock face. field industrial test results show that the technology achieves the overall pressure relief and reflection reduction of low permeability and high gas coal seams. the underlying 3 + 4 protective layer achieves the general pressure relief and reflection reduction of low permeability and high gas coal seams. coalbed methane can be pre-drained up to 18 m3/min with a concentration of 90%. abstract geological evolution process, mechanism, and application of protective layer in semi-coal and rock face proceso de evolución geológica, mecanismo y aplicación de capa de protección en minería de semicarbón y en pared de roca issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91378 bajo la influencia de la estructura geológica, la roca que rodea la veta de semicarbón puede soltarse y fracturarse, y el índice de reparación es alto. es de gran importancia teórica y práctica estudiar el mecanismo de minería de la veta de semicarbón y la tecnología de control. en este artículo el principio de la evolución geológica de la veta de semicarbón se analiza teóricamente. además, se interpreta la relación mecánica entre el peso de la minería en la pared y el alivio en la presión de la parte baja de la veta. también se analiza el mecanismo de eliminar las áreas ciegas en la minería sin pilares de la veta superior protectora. a través del análisis de simulación numérica se concluye que el índice de deformación por expansión de una capa de protección se incrementa linealmente con el aumento del grosor de la capa de protección, y el ancho de la indeseable zona ciega en la veta de carbón se incrementa linealmente también con el incremento en el grosor de la capa de protección. este trabajo ofrece una tecnología en minería sin pilar de la capa de protección en la veta de semicarbón y en la pared de la roca. los resultados de evaluaciones de campo industriales muestran que esta tecnología logra un alivio general en la presión y en la reducción del reflejo en vetas de baja permeabilidad y de carbón con alto índice de gas. la capa de protección 3 + 4 logra el alivio en la presión general y en la reducción del reflejo en vetas de baja permeabilidad y de carbón con alto índice de gas. el metano en capas de carbón puede ser drenado previamente en hasta 18 m3/min con una concentración del 90 por ciento. resumen record manuscript received: 12/03/2019 accepted for publication: 20/09/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 499-506 xiaoping xie1,2, xinqiu fang1*, minfu liang1,3, dechun ai2 1state key laboratory of coal resources and safe mining, china university of mining and technology, xuzhou 221116, china 2college of mining and civil engineering, liupanshui normal university, liupanshui 553004, china 3school of management, china university of mining and technology, xuzhou 221116, china” *corresponding author: 264580414@qq.com m in in g e ng in ee ri ng 500 xiaoping xie, xinqiu fang, minfu liang, dechun ai introduction when the roadway excavation section has both rock and coal seam, and when the coal seam occupies 1/5 to 4/5 of the area of the excavation face, it is called semi-coal-rock mining working seam, which generally exists in thin coal seam conditions. taking china as an example, the reserves of unexploited semicoal and rock mining working strata are about 6.150 billion tons, accounting for 20.4% of the total recoverable reserves. in some areas, the proportion of semicoal seam reserves is larger, such as sichuan province accounts for about 60%, shandong province for about 54%, heilongjiang province for about 51%, guizhou province for about 37% (yuan, 2008a; zhou et al., 2016). other coal-producing provinces, such as henan, shanxi, inner mongolia, hebei and jilin, also have rich thin seam resources. with the continuous reduction of coal resources, semi-coal and rock mining will be a problem faced by china and even the world coal mining industry. semi-coal-rock working face has small space of movement, difficult worker operation, complex mining method, easy to occur safety accidents, low economic benefits. semi-coal-rock working face is mostly semi-coalrock roadway, which has low tunneling rate, high tunneling cost, tight mining replacement and low mining efficiency. it was shown as figure 1. in this paper, the principle of geological evolution of semi-coal seam is analyzed theoretically, and the technical measures of linear increase of expansion deformation rate of protective seam with the increase of mining thickness of protective seam are put forward. seam semi-coal-rock mechanism structural analysis semi-coal and rock transportation roadway are arranged in mud-stone and coal seam. the lithology of two sides, roof and floor is relatively fragile, the deformation parameters are relatively large, and the tensile and shear failure strength is relatively small (wu et al., 2004). under the influence of mining, the surrounding rock of two sides, roof and floor is seriously damaged, the scope of loosening zone is relatively large, and the bearing capacity of surrounding rock is relatively low. according to the analysis of consolidation theory, the length of roof bolt and side bolt is insufficient in the original supporting scheme, the anchorage section of bolt cannot be anchored in the original rock stress zone, and the anchorage force is insufficient, which leads to the insufficient strength of roof and two sides of roadway. the diameters of top anchor and side anchor are small, the tensile strength and prestressing force are not enough, the extrusion strength of anchor end to destroyed surrounding rock, coal seam and mud-stone is small, the secondary bearing area cannot be formed, and the mechanical properties of anchor solid cannot be improved. the deformation of surrounding rock cannot be controlled in time, and the roof pressure can be transferred to both sides, resulting in larger stress, horizontal stress and vertical stress in both sides. the force is almost vertical, so serious deformation, roof subsidence and floor heave appear on both sides of the roadway. in order to study the maximum damage range of roof strata and side wall of semi-coal-rock roadway, theoretical analysis is carried out according to that all roadways are coal seams (yuan, 2008b). because the thickness of carbonaceous mud-stone layer on the roof of roadway varies greatly, its caving shape is arch. according to the theory of pressure-free arch, as shown in figure 2. semi-coal and rock transportation roadway is a typical soft rock mining roadway, and its surrounding rock deformation characteristics are as follows: (a) the surrounding rock deformation is intense and large, the convergence rate of roadway section is as high as 38% during excavation, and the deformation has rheological characteristics and lasts a long time; (b) the degree of surrounding rock fragmentation gradually weakens from shallow to deep, and its damage scope exceeds the anchorage range of the anchor, resulting in the failure of the anchor; (c) the surrounding rock loosening and breaking; the rapid development and serious destruction of the bad scope result in the reduction of anchorage effect, failure of anchorage system and failure of supporting bearing structure, which further aggravates the development of figure 1. semi-coal-rock working face 501geological evolution process, mechanism, and application of protective layer in semi-coal and rock face in the formula: γ is the coal bed bulk density, n/m3; h is the depth of coal seam; δk is the maximum concentrated stress increment coefficient, δk=k-1; δ is the lead abutment pressure attenuation coefficient; x0 is the width in the plastic zone, m; x is the distance from the coal seam,. from the formulas 1, 2, and 3 it can be known that the plastic zone width x0 in the semi-coal rock working face is proportional to the coal mining height (h+δh) and the width of the plastic zone of the extremely thin coal seam is smaller. figure 3 shows the distribution rules of abutment pressure of working face. the curves in the figure meet the distribution formula of abutment pressure in plastic zone (1), and curves satisfy distribution formula of abutment pressure in elastic zone (2). 0 4 8 12 16 20 4 8 12 16 δx0 distance from the coal wall（m） w or ki ng fa ce ab ut m en t pr es su re (m pa ) mining height h mining height h+δh x0① ② ③ figure 3. the distribution rules of abutment pressure of extremely thin coal seam during work according to the formula (3), the increase of width in plastic zone of semi-coal rock working face can be calculated as: ∆ ∆x h f ln k h rc 0 2 =    ξ γ (4) according to the relationship between the lead abutment pressure of the working face and the destruction of the underlying coal and rock seams, a structure of the plastic destruction zone of the underlying coal and rock along the advanced direction of the coal seam is established. x0 a b c d e o h max f r0 r θ π /4-φ/2 π /4+φ/2 figure 4. section of plastic destruction zone of overlying coal strata according to the geometrical dimensions of the ultimate plastic destruction zone shown in figure 4, the maximum destruction depth of the floor destruction zone under the action of the extreme lead abutment pressure can be preliminarily determined : h x emax / tancos cos / = +( ) +( )0 4 2 2 4 2 φ π / φ π/ φ φ (5) in this formula: hmax is the maximum destruction depth of the coal seam floor; x0 is the width of the plastic support area in front of the coal wall; φ is the internal friction angle. surrounding rock loosening and failure to its due support effect. fourthly, the phenomenon of two sides of roadway is widespread, and the phenomenon of arch roof and arch waist separation layer falling off is widespread, which leads to the failure of external anchorage point of some bolts, which also leads to the failure of external anchorage point of some bolts. it is an important reason for the reduction or failure of anchorage effect. in conclusion, the failure mode of surrounding rock of semi-coal and rock transportation roadway belongs to the typical large deformation failure mode of soft rock rheology. the failure mechanism of support is as follows: under the condition of high in-situ stress, the scope of loosening and failure of weak surrounding rock expands rapidly, and its rheological action causes the surrounding rock of roadway to break and the deformation continues to increase, resulting in the failure of bolt-mesh combined support structure, which leads to the loss of bearing capacity of surrounding rock-support bearing structure. mathematical model in view of the high-efficiency mining that is hard to exploit the resource of extremely thin coal seams, the idea of small fully-mechanized mining in the semi-coal rock working face is proposed, and the mining height is increased by breaking the weak roof or floor of an extremely thin coal seam to increase mining height in order to install small fully-mechanized mining equipment and improve the pressure relief effect of the coal seam group at the below part. after the semi-coal rock working face is mined, the load of overlying strata in goaf is transferred to the front of coal wall forming an abutment pressure, and the coal wall is in a plastic or fracture state within a certain depth (zhou et al., 2015; cheng et al., 2012). usually, the front of coal wall is divided into two zones in the abutment pressure: plastic zone and elastic zone. according to the limit equilibrium condition, the abutment pressure in the plastic zone is distributed: σ ξ y c f h h x r e= + 2 ∆ (1) in this formula, σy is the lead abutment pressure, mpa; rc is the residual strength of the coal wall, mpa; ƒ is the friction coefficient of the layer, in the ƒ=tanφ1, φ1 stands for the friction angle between the roof and floor of the coal seam and the coal seam, o; ξ=(1+sinφ)/( 1-sinφ), φ is the internal friction angle of the coal, o; h is the thickness of the extremely thin seam, m; δh is the thickness of the broken rock, m;the abutment pressure distribution in the elastic zone is a negative exponential curve,: σ γ −δ − y x xh ke= +    ( )1 0∆ (2) x h h f ln k h rc 0 2 = +     ∆ ξ γ (3) figure 2. calculating model of semi-coal and rock structure 502 xiaoping xie, xinqiu fang, minfu liang, dechun ai according to the formulas 3 and 5, it can be known that after the mining of the semi-coal rock working face, the maximum destruction depth of the floor is proportional to the width x0 of the support pressure in plastic zone in front of the coal wall, it is equal to the mining height of the semi-coal rock working face (h+δh). evolution of semi-coal seam semi-coal and rock from the beginning of tunneling to withstand two mining face impact and roadway retention stage roof rotation subsidence (li, 2014), long service time, surrounding rock presents large deformation characteristics, surrounding rock pre-deformation directly related to the later deformation (li et al., 2011; liu et al., 2009), showing a typical evolution characteristics. obtaining the displacement, strain and fracture evolution law of surrounding rock from tunneling to the end of secondary mining is of great significance to reveal the deformation mechanism. evolutionary simulator restricted by the field testing environment and testing level, there are some difficulties in measuring the deformation evolution law of surrounding rock of semi-coal and rock. laboratory physical simulation can well realize the observation of deformation of semi-coal and rock. the testing environment is good and the testing accuracy is high (liu et al., 2011). in this paper, the physical simulation of semi-coal and rock is carried out by using the selfdeveloped two-way and four-side loading test device, and the deformation of semi-coal and rock is observed. the commonly used plane experimental devices are mostly used to simulate stope advancing, observe roof caving pattern, and simulate vertical stress by adding counterweight (liu et al., 2013). there is no active load on both sides and floor, which results in great difference between boundary conditions and original rock stress field, and the credibility of simulation results is reduced (ma et al., 2007). in order to better study the surrounding rock deformation of gob-side retaining roadway, a two-way and four-sided loading test device is developed, which overcomes the problem that the three-dimensional simulation test device cannot advance step by step and continuously, and solves the problem that the common plane simulation cannot exert high stress on top, bottom, left and right. the design of simulation device is controlled by different oil system, which can realize unequal pressure loading on all sides. it can simulate the process of driving, advance effect of primary mining, advance effect of retaining roadway and secondary mining under the corresponding real in-situ stress state (chen et al., 2003). it is innovative in the experimental study of retaining roadway along gob at home and abroad. the overall size of the test device is 4.5 x 0.5 x 4.5 m, and the effective test size is 4.0 x 2.5 x 0.5 m. the parameters of the simulator are shown in figure 5 and table 1. the numerical simulation of mechanical parameters of the coal mine is shown on table 1. process recording the simulation evolution process is to simulate the advance effects of gob-side entry retaining, primary mining, retaining and secondary mining according to the way of “excavation roadway-right boundary-middle mining, filling body-left boundary-middle mining”. the experimental device is a plane simulation device. (a) under the original rock stress state, the surrounding rock stress is redistributed when the roadway is excavated according to the design size (he et al., 2017). the existence of roadway space makes the floor of roadway change from three-dimensional stress state to two-dimensional stress state, and pressure relief occurs. this simulation process is the same as the actual situation. (b) after the excavation of the right coal seam, the advance abutment pressure of the mining face acts on the stress superposition of the surrounding rock of the roadway and the surrounding rock during the excavation, which increases the failure depth of the surrounding rock, the peak stress concentration factor and the influence range. the stress state of the surrounding rock on the right side of the roadway is the same as that in the field (jiang et al., 2013). the stress of the surrounding rock on the left side of the roadway is the superposition of the mining stress and the lateral pressure of the surrounding rock, slightly different from that in the field. different. (c) the filling body is placed immediately after the mining of the right working face is completed, and the simulation enters the stage of retaining roadway. in production, the most severe deformation stage of surrounding rock in gob-side retaining roadway is the rotary subsidence of the basic roof above the filling body. in simulation, after filling body is placed, the roof will not immediately rotate subsidence (khademalhoseiny et al., 2017). with the generation, development, expansion and penetration of cracks, the stress will seek to rebalance, the basic roof will rotate subsidence, and the overlying strata load will act on the floor through the filling body. the process can well simulate the deformation characteristics of surrounding rock during the stable period of retaining roadway. (d) after the mining of the left side face, the advance abutment pressure and the surrounding rock stress field after the stability of the retained roadway are superimposed (rosa et al., 2017). the process of the left side of the roadway is the same as that of the field, and the stress field of the right side of the roadway is slightly different from the actual situation of the field. the feasibility of the simulation scheme is discussed in stages below which shown as figure 6. evolution simulation result in order to study the relationship between the pressure relief effect of udder protected layer and the different heights of semi-coal rock working face, four simulation schemes were designed: mining height 0.5 m, mining height 1.5 m, mining height 2.5 m, and mining height 3.5 m. in the numerical calculation figure 5. semi-coal-rock working face table 1. mechanics parameters of the coal-rock joints lithology normal stiffness/gpa tangential stiffness/gpa cohesive force/mpa friction angle /° tensile strength/mpa coal seam 0.8 0.4 0 4 0 medium sandstone 0.5 0.3 0 6 0 fine-sandstone 0.4 0.2 0 7 0 mudstone 0.6 0.3 0 5 0 siltstone 0.5 0.5 0 7 0 sandy mudstone 0.7 0.4 0 6 0 503geological evolution process, mechanism, and application of protective layer in semi-coal and rock face model, the observation line is arranged in the lowest no. 5 protected coal seam, and the starting point of model excavation is at 50 m. when the working face is mined 150 m in an inclined direction, the expansion deformation rate change features of no. 5 are shown in figure 7. engineering application overall design the engineering background is as follows: the coal mine uses +400 m north wing main roadway as the preparing roadway arrangement for inclined long-wall working face of small fully mechanized mining and 22201 working face is the first working face of the protective layer in the no.2 extremely thin coal seam of north second mining area. the slope length of designing the working face is 150 m, the length of the propulsion is about 1538 m, and the elevation of the working surface is +396 m~+486 m. the designed semi-coal rock working face is 1.5 m in height, adopting long-wall retreat mining method. the working face is mined without coal pillar of gob-side entry retaining, and after the 22202 working face roadway is mined, a y-type ventilation system is formed: the 22201 auxiliary haulage roadway and the machine-rail unification roadway enter the wind, and the dirty wind passes through the 22201 gob-side entry retaining, 22202 open-off cut and rail roadway, it’s as shown in figure 8. there are mainly the following problems in tunneling of semi-coal and rock face: first, poor shaping and surrounding of working face rock failure and over-excavation are serious, especially in soft and broken strata (cheng et al., 2003). it causes a lot of waste of supporting materials and poor safety conditions figure 6. the feasibility of the simulation (a) mining height 0.5 m (c) mining height 2.5 m -7 -5 -3 -1 1 3 5 7 0 34 68 103 137 171 205 239 ex pa ns io n ra te o f t he pr ot ec tiv e la ye r ( ‰ ) horizontal position（m） -7 -5 -3 -1 1 3 5 7 0 34 68 103 137 171 205 239 ex pa ns io n ra te o f t he pr ot ec tiv e la ye r ( ‰ ) horizontal position（m） . -7 -5 -3 -1 1 3 5 7 0 34 68 103 137 171 205 239 ex pa ns io n ra te o f t he pr ot ec tiv e la ye r ( ‰ ) horizontal position（m） . -7 -5 -3 -1 1 3 5 7 0 34 68 103 137 171 205 239 ex pa ns io n ra te o f t he pr ot ec tiv e la ye r ( ‰ ) horizontal position（m） . (d)mining height 3.5 m (b)mining height 1.5 m figure 7. expansion deformation rate curve 22201working face 22201 auxiliary haulage roadway 22201 machine-rail unification roadway 22202 rail roadway o pen-off cut protective coal pillar line n orth w ing track tunnel n orth w ing tape tunnel n orth w ing return air tunnel 24208 return air roadway 24208 belt roadway n o.1 coal bunker gob-side entry retaining fresh wind dirty wind working face advancing direction 22201working face goaf figure 8. layout of the 22201 working face of semi-coal rock 504 xiaoping xie, xinqiu fang, minfu liang, dechun ai for head-on construction. second, the half-coal and rock excavation speed is slow, which seriously affects the construction and arrangement of followup projects. thirdly, rock loading and transportation are difficult because of inclination downhill; fourthly, water inrush control in working face also needs to take up a certain cycle time, which affects the driving speed of working face. at the same time, the rock stratum tends to be unstable due to the softening effect of water on rock, and the roadway is difficult to support. secondly, coal and rock are transported separately, the regular circulation organization is complex, and the degree of parallel operation is low. slow excavation speed has seriously affected the mining replacement relationship of mining face. the main reasons are as follows: first, the existing blasting technology is backward, which is not conducive to the formation of roadways; second, the support technology and support parameters are inadequate; third, the working line is not compatible with the site construction conditions; fourth, the construction technology, the layout of working face and the selection of labor organization forms, as well as the inadequate management system (abubakary et al., 2015). application method on the site, the 22201 semi-coal rock and small fully mechanized working face of the typical coal mine is shown in figure 9. during the observation period from january 6 to july 22, the working face was promoted to 602.6 m and a total of 1,812,020 tons of coal was mined. from january 6 to february 27, the progress of the initial mining phase on the working face was slow. in 50 days, the working face completed a total advance of 113.2 m, and the average daily advance was 2.26 m, and the mining efficiency was low; from february 27th to july 22, the speed of advancement has improved, with an average of 3.05 m per day. from the actual coal mining results, we can see that the equipment arranged in the working face and in the mining lanes meet the production requirements, and the semi-coal rock working face basically achieves safe and efficient mining. experimental results and analysis experimental results (a) overall pressure relief analysis of non-coal pillar in protective layer: coal seam deformation and stress change in the protected layer are the key to investigate the pressure relief effect of coal and rock below the remaining coal pillars and non-coal pillar (bustin & clarkson, 1998). the observation lines are arranged in the no.3+4 protected layer under the numerical calculation model. when working height of 22201 and 22202 working face is 1.5 m, the starting point of model excavation is at the coordinate of 40 m. after the length of two working faces is continuously mined in the direction of inclination of 150 m, non-coal pillars are left between the two working face interfaces and the width of the pillar of the section is left separately. for 20 m, 30 m, and 40 m, the expansion rate of the protected coal seam is shown in figure 10 and figure 11. -12 -9 -6 -3 0 3 6 9 12 0 34 68 103 137 171 205 239 273 308 342 376 ehtfo etar noisnapxe pr ot ec tiv e la ye r (‰ ) horizontal position（m） -15 -12 -9 -6 -3 0 3 6 9 12 15 0 34 68 103 137 171 205 239 273 308 342 376 ex pa ns io n ra te o f t he pr ot ec tiv e la ye r (‰ ) horizontal position（m） -15 -12 -9 -6 -3 0 3 6 9 12 15 0 34 68 103 137 171 205 239 273 308 342 376 ehtfo etar noisnapxe pr ot ec tiv e la ye r (‰ ) horizontal position（m） -15 -12 -9 -6 -3 0 3 6 9 12 15 0 34 68 103 137 171 205 239 273 308 342 376 ehtfo etar noisnapxe pr ot ec tiv e la ye r (‰ ) horizontal position（m） (a) pillar width 0 m (b) pillar width 20 m (c) pillar width 30 m (d) pillar width 40 m figure 10. the expansion deformation rate curve figure 9. the 22201 working face on-site picture 505geological evolution process, mechanism, and application of protective layer in semi-coal and rock face analysis from the analysis of figure 9, it can be seen that as the width of the upper coal pillar decreases, the expansion rate of the protected coal seam below the pillar that is less than 3‰ increasingly decreasing, which is the range of the distressed blind-zone. when the width of the section coal pillars is 40 m, 30 m, 20 m, and 0 m respectively, the inclination width of the no.3+4 coal seam below the coal pillar is less than 3‰, which is 64.3 m, 52.2 m, 44.2 m, and 0 m, respectively. when non-coal pillars are used, the distressed blind-zone in the protected layer below the pillars are eliminated. by establishing a rectangular coordinate system, the four simulation schemes with different widths of the coal pillars correspond to the width of the distressed blind-zone of the protected layer make four coordinate points, and then make the fitting straight line of four coordinate points. the analysis shows that the fitting linear equation is y= 17.09x-10.05, and the fitting r2= 0.8968, that is, the width of the blind zone of the pressure relief of protected coal increases with the increase of the pillar width of the protective layer, which is roughly in line with the linear growth law of the fitted line (chen et al., 2016). from the analysis of figure 11, it can be known that when the upper coal pillar is left in the upper protective layer, stress concentration occurs in the protected layer is under the coal pillar, and with the decrease of the width of the coal pillar in the upper protective layer, the stress concentration range of the protected coal seam under the pillar is decreasing; when the upper protective layer is mined without coal pillars, there is no stress concentration area in the protected (a) pillar width 0 m (b) pillar width 0 m (c) pillar width 0 m (d) pillar width 0 m figure 11. vertical stress cloud picture of the working face of semi-coal rock with different pillar widths y = 17.09x 10.05 r2 = 0.8968 0 10 20 30 40 50 60 0 10 20 30 40 th e bl in d ar ea w id th o f t he pr ot ec te d co al se am （ m ） different pillar width the blind area width 线性 (the blind area width)fitting a straight line figure 12. the relationship between the width of different pillars and the width of the distressed blind-zone in the protected layer 506 xiaoping xie, xinqiu fang, minfu liang, dechun ai layer below the original coal pillars. when the width of coal pillars is 0 m, 20 m, 30 m, and 40 m, the maximum vertical stress values of udder-protected layer below the coal pillars are 11 mp, 15 mp, 19 mp, and 21 mp, which are 0.84, 1.15, 1.45, and 1.6 times respectively for the original rock stress, that is, the protected layer under the original coal pillar will produce a pressure relief effect when non-coal pillar is used, which eliminates the distressed blind-zone. conclusions (a) by using similar material simulation and numerical simulation, the deformation evolution law of surrounding rock of semi-coal-rock working bed is obtained. during the excavation period, the deformation of surrounding rock is symmetrical distribution, and in the first mining advance influence stage, the support point of the side roof of the working face sinks due to the advance support pressure of the working face, the roof deviates, the deformation of the side coal wall of the working face is serious, the damage depth of the surrounding rock and the development of cracks increase. in the retaining roadway stage, the roof is further rotated and subsided due to the deformation of the empty roof and filling body, and the high-strength filling body is embedded in the floor. the floor heave is serious, the deviation is obvious, and the deformation of coal side under high abutment pressure is serious in the advanced stage of secondary mining. (b) the mechanism of filling body in semi-coal seam is analyzed, and the mechanical calculation model of filling body is established. the stress components of filling body and its internal stress are obtained. it shows that the horizontal stress gradually decreases from the side of goaf to the roadway, and the upper and lower parts of filling body bear higher vertical stress and shear stress, which is the key point of support. (c) on-site engineering practice shows that relative to the south wing of the mine is not protected by protective layer mining, the pressure of the gas in the protected coal seam is relieved and the permeability is increased after adopting the non-coal pillar mining technology for semi-coal rock of protective layer in the north wing of the mine. the gas extraction scalar is up to 18m3/min, and the maximum concentration is about 90%. the gas extraction scalar and concentration at each drilling field undergoes smoothness, ascent, descent, and recovery in turn. the stage shows that the overall pressure relief effect of the non-coal pillar mining technology for semi-coal rock of protective layer is significant. acknowledgements this research was supported by the national natural science foundation of china (no.51874276, no.52004273) and china postdoctoral science foundation (no.2019m661992). references abubakary, s., micah, n., & jenny, g. 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(2016). recent developments in coal mine methane extraction and utilization in china: a review. journal of natural gas science and engineering, 31, 437-458. keywords: irrigation amount; alhagi sparsifolia; desertification; control; regression analysis. palabras clave: cantidad de riego; alhagi sparsifolia; desertificación; análisis de regresión. how to cite item li, j., liu, w, & zhang, w. (2020). evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts. earth sciences research journal, 24(4), 449-457. doi: https://doi.org/10.15446/esrj.v24n4.91626 desertification control is an important issue that must be considered in modern society. in order to effectively improve the accuracy and practicability of the evaluation algorithm of desertification control effect, the alhagi sparsifolia index under different irrigation amount was taken as the research object, and the evaluation algorithm of desertification control effect was proposed. in the “vegetation-sandstorm-soil” index system, a number of indexes were selected according to the core environmental parameters of alhagi sparsifolia and grassland desertification. and the analytic hierarchy process, remote sensing, geographic information system, and landscape technology were used to assign index weights of desertification control capacity, which were calculated by multiple discriminant matrices. finally, the data regression analysis was performed based on remote sensing and computer image information screening and processing to determine the final evaluation results. the experimental data show that the true positive rate of the algorithm in this paper is between 160 and 200, which is within a large range of advantages, indicating that the overall evaluation accuracy of the algorithm is high and the evaluation effect is perfect. abstract evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts algoritmo de evaluación para el control de la desertificación del alhagi sparsifolia bajo diferentes cantidades de riego issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91626 el control de la desertificación es un tema importante que debe considerarse en la sociedad moderna. con el fin de mejorar efectivamente la precisión y la viabilidad de un algoritmo de evaluación del efecto de control de la desertificación se tomó como objeto de investigación el índice del alhagi sparsifolia con diferentes cantidades de riego. en el sistema de índices “vegetación-tormenta de arena-suelo” se seleccionaron varios índices de acuerdo con los parámetros ambientales básicos del alhagi sparsifolia y la desertificación de los pastizales. se utilizó el proceso de jerarquía analítica, la teledetección, el sistema de información geográfica y la tecnología del paisaje para asignar los índices de ponderación de la capacidad de control de la desertificación, los cuales se calcularon mediante matrices discriminantes múltiples. finalmente, el análisis de regresión de datos se realizó con base en la detección y el procesamiento de información de imágenes de computadora y detección remota para determinar los resultados finales de la evaluación. los datos experimentales muestran que la tasa de verdaderos positivos del algoritmo en este artículo está entre 160 y 200, lo que se encuentra dentro de una amplia gama de ventajas y que indica que la precisión de evaluación general del algoritmo es alta y el efecto de evaluación es perfecto. resumen record manuscript received: 18/11/2019 accepted for publication: 10/08/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 449-457 juan li1*, wei liu2, xinxin zhang3 1inner mongolia chemical vocational college, computer and information engineering, hohhot 010010, china 2ministry of technology, beijing rongzhilian science and technology co., ltd., beijing 100015, china 3ministry of environmental protection, shenhua jungle energy co., ltd., erdos 010300, china *corresponding author: 2485397887@qq.com en vi ro nm en ta l g eo lo g y https://doi.org/10.15446/esrj.v24n4.91626 https://doi.org/10.15446/esrj.v24n4.91626 mailto:2485397887@qq.com 450 juan li, wei liu, xinxin zhang introduction in 1977, the united nations conference on desertification defined desertification as a situation in which the biological potential of the land is reduced or destructed and ultimately leads to a desert-like landscape. this definition has been widely accepted and applied over the past two decades (lan et al., 2018). in 1991, aiming at the definition in 1997, the united nations environment program did not mention the possible areas of desertification and their causes. according to the latest research results, desertification was redefined as a land degradation process caused by human adverse effects in arid, semi-arid, dry, and semi-humid areas. throughout the definition and research of desertification in foreign countries, the concept of desertification is extensive (qiang et al., 2018), including vegetation degradation, water erosion, salinization, reduction of soil organic matter, soil crusts, and tightening, and accumulation of toxic substances. these are the concepts of generalized desertification, which is essentially land degradation. from the reality of our country, the concept of desertification has also been proposed in china, only referring to the process of wind erosion in generalized desertification. there are three main definitions. the first is the definition proposed by wu zheng: in arid, semi-arid, and partially semi-humid areas, due to natural factors or human activities, the fragile balance of the natural ecosystem is destroyed. in this non-desert region, there are environmental changes in desert-like landscapes characterized by sandstorms, and desert environmental conditions in desert areas are strengthened and expanded. the second is the definition of zhu zhenda: on a certain sandy material basis and under the conditions of drought and strong winds, an environment or land degradation process with windstorm as the main symbol due to excessive human activities inconsistent with resource and environment. this definition of desertification aims at the period of human history. it mainly appears in arid, semi-arid and partially semihumid areas and excessive economic activities are the main factors in the cause. based on the concept of desertification in the united nations, the third definition was proposed by dong yuxiang: a land degradation process characterized by windstorms in arid, semi-arid and semi-humid areas, and it is caused by climate change and human activities. this definition takes into account the human and natural factors (zhao et al., 2019; kang; 2018). the most serious desertification in land resources is grassland. grassland desertification is a special type of grassland degradation, which refers to land degradation caused by unbalanced human-land relationships in arid, semiarid, and semi-humid grassland areas. it is one of the extreme manifestations of grassland ecosystem degradation. grassland desertification, vegetation degradation, and surface exposure will intensify wind erosion and wind accumulation, which will cause continuous deterioration of the environment and a series of socio-economic problems. therefore, the restoration and reconstruction of degraded grassland ecosystems have received increasing attention. most of the grasslands in northern china are in arid and semi-arid climates, and the ecological environment has a very limited anti-interference ability. in the mid-1970s, the area of degraded grasslands in the country accounted for about 15% of the total grassland area. by the mid-1980s, it had increased to more than 30%. in the mid-1990s, it reached more than 50%, which had increased to 90% by the beginning of the 21st century. grassland productivity is declining and the grassland ecological environment is rapidly deteriorating (peng et al., 2018). therefore, the continuous increase of desertification of grassland has seriously affected the ecological environment construction, social and economic development of grassland areas in china, and caused great harm. in order to effectively curb the trend of grassland desertification, the state has launched a series of grassland desertification control projects, aiming at increasing the vegetation coverage, preventing sand and stopping desertification, and improving the grassland environment. especially since 2000, a series of large-scale grassland vegetation restoration and reconstruction ecological projects and scientific research (wei, 2017) has been launched. alhagi sparsifolia belongs to leguminosae sp, alhagi gagneb. it is a perennial semi-herb plant, mainly distributed in xinjiang, gansu, inner mongolia, china, and the former soviet union, iran, afghanistan, pakistan, and india. alhagi sparsifolia is one of the representative shrub vegetation species in the arid area of xinjiang, china, with a total area of 1.7 million hectares. it is important for sand-fixing vegetation and plays an important role in maintaining the local ecological environment. at present, grassland desertification control through alhagi sparsifolia is a common method in the world. in order to further study the effect of desert control, it is necessary to combine the natural characteristics of alhagi sparsifolia and construct an evaluation system of desertification control effect according to its natural growth ability under different irrigation amounts. material and methods evaluation algorithm for desertification control of alhagi sparsifolia under different irrigation amount in this paper, the index system of “alhagi sparsifolia vegetation-sandstorm activityalhagi sparsifolia soil” was adopted. according to the difference of irrigation amount, the vegetation coverage, aboveground biomass, grass height, plant species, annual plant proportion, fixed sand proportion, fixed sand fragmentation, mobile sand proportion, mobile sand fragmentation, soil erosion modulus, soil organic matter, soil mechanical composition, soil crust and other indicators were selected with the analytic hierarchy process to construct the ecological benefit evaluation index system of grassland desertification control project (meng et al., 2018). through the ground surveys, data on alhagi sparsifolia status and soil were obtained, and gps handheld terminals were used as an aid to locate and measure ground survey points (qiang et al., 2017). according to the meteorological data, the administrative zone map, the grassland type map, the topographic map of the study area, the knowledge of the alhagi sparsifolia, and the images obtained by the landsat 5 sensor and the landsat 7 sensor, the vegetation coverage, the aboveground biomass, the vegetation height, the change of plant species number of the alhagi sparsifolia are obtained inside and outside the project. in addition, the proportion of fixed and mobile sand, the change of fragmentation degree of fixed and mobile sand landscape before and after the implementation of the project can be analyzed to realize a quick and comprehensive evaluation of the ecological benefits of a grassland desertification control project. the overall process is as follows: the ecological benefit evaluation system of alhagi sparsifolia desertification control the grassland desertification control project mainly refers to the various measures in favor of the growth of alhagi sparsifolia or adjusting the land use structure. through the project, the vegetation on the desertification grassland gradually recovers, the sandstorm gradually disappears, the surface is stable, the crust is formed, and the soil is reconstructed after that. all these make great ecological and economic benefits. the ecological benefits of the grassland desertification control project refer to the protection and improvement of the natural environment, human life, and production environment after the implementation of the ecological environment construction projects. the ecoefficiency index is a kind of indicator that reflects the improvement of the environment, including indexes of improving the soil, preventing wind and sand, controlling soil erosion, regulating climate, conserving water sources, reducing disasters, and improving ecological environment conditions under the influence of alhagi sparsifolia. in this paper, the irrigation amount was taken as the core variable, so the vegetation coverage growth rate, grassland productivity, plant species, desertification land area, and soil organic matter content changed with it. the basis of ecological benefit evaluation under the influence of different irrigation amounts is the evaluation index and its system. even the national conditions, industry, and engineering construction properties are the determinants of the construction evaluation index system and evaluation index. at present, there is no uniform scope and standard at home and abroad. therefore, it is necessary to establish a practical, reasonable, and operable ecological benefit evaluation index system for the grassland desertification control project (yang et al., 2017). principles of index selection (1) principle of geographical regional differences grassland desertification in china is mainly distributed in sub-humid arid, semi-arid, and arid regions. there are significant differences in vegetation 451evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts (3) principle of specific indicator operability the ecological benefit index should reflect the environmental benefits brought by the grassland desertification control project in a comprehensive and systematic way. the indexes to be evaluated should be easy to obtain and to be processed. the original data should be easy to obtain through surveys, statistics, remote sensing, and other means, and be easy to be processed through quantitative calculation. they should also have clear realistic meaning. (4) the irreplaceable principle of a specific index there are many indexes for evaluating the ecological benefits of the grassland desertification control project. most of the indexes have a certain correlation and mutual transmission. therefore, when determining specific indexes, it is necessary to avoid the related indexes caused by the dominant factors. the determined indexes can make a scientific and reasonable evaluation of the construction effect of the project, and the indexes are irreplaceable (zhang et al., 2018). (5) cross-scale principle the development and application of remote sensing, geographic information systems, and gps technology have greatly broadened the research horizons of people; on the other hand, it also poses certain challenges to conventional work programs based on ground surveys. since there are differences between the ground survey and remote sensing, geographic information system, and gps in workflow and requirements, it is likely that different research conclusions will occur. researchers in the same area have found conflicting results only because different research methods are not expected. therefore, when selecting evaluation indexes, cross-scale indexes that can be compatible with different research methods to avoid the above problems should be selected. evaluation indexes and their meanings the grassland desertification control project can be similarly regarded as the reverse process of grassland degradation. the process is first reflected in the change of the composition of alhagi sparsifolia vegetation, and then the change of soil properties. alhagi sparsifolia vegetation restoration and soil restoration are interaction processes that reflect the nature of overall environment restoration. these two processes are sometimes consistent in time, and sometimes there is a certain hysteresis. that is, vegetation restoration and soil restoration occur simultaneously or vegetation is restored first, and soil restoration is only manifested after a certain period of time. an important factor associated with grassland degradation or restoration is sandstorms. according to the above principle of index selection, the ecological benefit evaluation of the grassland desertification control project is mainline. the three indicators, vegetation, soil, and sandstorms, that reflect the ecological benefits of the grassland desertification control project are selected as the criterion layers. the influencing factors related to project construction and ecological benefits are selected to form an index system. a comprehensive analysis is performed on the index system, compared with other ecological benefit evaluation index systems, and some indexes were removed according to the scientific, practical, and measurable requirements, and the ecological benefits they will bring. the ecological benefit evaluation index system of the grassland desertification control project was initially determined (zhang et al., 2018). the framework of the ecological benefit evaluation index system is shown in figure 2. the role of alhagi sparsifolia vegetation is the most important. alhagi sparsifolia vegetation is not only a comprehensive reflection of the quality of the project land, but also the most ideal protector of the soil, and also the main body of the ecosystem. the vegetation index mainly refers to the ones describing the shape and productivity of alhagi sparsifolia, such as the community composition, life form, biomass, and its coverage. undoubtedly, most camel thorn vegetation indexes can be obtained through ground surveys. however, some indexes that characterize vegetation productivity, such as alhagi sparsifolia coverage and biomass, have been quantitatively extracted by remote sensing data. although there are some differences in the data obtained at different scales, these indexes are clearly cross-scale. this paper selects four vegetation indexes that are operable and can directly reflect the ecological benefits of alhagi sparsifolia in the desertification control project. they are the types and structural characteristics in different regions. due to the influence of the natural environment, climate, and natural vegetation growth, the project measures adopted have significant differences. there are also significant differences in vegetation restoration ability and natural vegetation coverage in different regions. therefore, in order to reasonably evaluate the grassland desertification control project, the ecological benefit index should be determined according to the actual situation of the study area. (2) principle of dominant index the purpose of the alhagi sparsifolia grassland desertification control project is to control or reduce grassland desertification. because there are many factors affecting the ecological benefit index of the grassland desertification control project, when determining the specific index, it is necessary to combine the actual situation and select the core factors that can express the degree of grassland desertification control as the dominant index. the so-called core factors refer to the ones that control the wind erosion on the grassland, the formation of sandstorms, and the surface exposure. according to the research results and actual application, the dominant factor is vegetation change. figure 1. process of control evaluation 452 juan li, wei liu, xinxin zhang figure 2. index system coverage of the alhagi sparsifolia, the aboveground biomass, the height of the grass layer, and the proportion of annual plants (guan et al., 2018). (1) alhagi sparsifolia vegetation coverage refers to the ratio percentage of the vertical projection area to the sampling area of the total alhagi sparsifolia community. coverage is the most critical factor in vegetation, and it reflects the degree of vegetation and the size of the photosynthesis area. it is also the main factor affecting soil erosion, which is of great significance for environmental change and monitoring research in the control area. (2) aboveground biomass: the total weight (fresh or dry weight) of one aboveground plant (or all plants) per unit area, which is the quantitative index of forage growth. aboveground biomass indicates the growth rate and regenerative capacity of pasture. in general, for the same plant community, the amount of above-ground biomass can indicate the size of the coverage and leaf area, and can also indicate the ability of photosynthesis. (3) grass height: the grass height in nature can reflect the production status of the pasture, and has a great relationship with the utilization mode. the natural state of the grassland plants was maintained during the measurement, and the average height of the grass layer was measured (ajaj et al., 2017). (4) proportion of annual plants: the percentage of annual plant coverage of total plant coverage. the proportion of annual plants can reflect the stability of plant communities. when the proportion of annual plants is small, the stability of the community is good; otherwise, the community is unstable. wind erosion is the loss process of surface materials under the action of wind. it is the beginning of the sandstorm and desertification process. the sub-processes in the desertification process are directly or indirectly generated by wind erosion. wind erosion is one of the most important causes of grassland desertification. after desertification, the grassland is dominated by bare sand, and the floating sand, semi-fixed sand, and fixed sand are randomly distributed among them. therefore, the proportion of fixed sand, mobile sand, and the size of wind erosion modulus are significant indicators for evaluating the effects of grassland desertification control projects. in addition, this paper introduces the landscape fragmentation, a landscape analysis index, to quantitatively study the dynamic characteristics of mobile sand and fixed sand in the study area. (1) proportion of fixed sand: the percentage of fixed sand area in the total project land area. the larger the proportion of fixed sand, the better the recovery of the project area. (2) proportion of mobile sand land: the percentage of mobile sand area in the total project land area. the smaller the proportion of mobile sand, the better the recovery of the project area. (3) soil wind erosion modulus: the amount of wind erosion per square kilometer per year, and its common unit is t/km2*a. the soil wind erosion modulus value visually reflects the degree of wind erosion in the soil. (4) fragmentation: landscape fragmentation refers to the process of landscape plaques from simple to complex due to disturbances of natural or human factors, that is, the process of the landscape from single homogeneous to 453evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts complex heterogeneity. the total number of plaques, the number of plaques per unit area, the plaque area per unit area, and the boundary density are commonly used indicators to reflect the characteristics of landscape fragmentation. landscape fragmentation indicates that the landscape shape is fragmented, enhances the edge effect of the landscape, and reduces the total area of a certain type of landscape and the core area of each plaque. the soil is composed of four parts: minerals, organic matter, soil moisture, and soil air (tang et al. 2018). soil indexes are mainly indexes that describe the physical, chemical, and biological characteristics of the soil, such as soil thickness, soil texture, mechanical composition, structure, nutritional status, organic matter content, profile characteristics, salt content, salt type, soil microbial status. these indexes are mostly ground surveys or analytical assay indexes. although quantitative data on soil indexes through microwave remote sensing can be achieved in scientific research, it is costly and far from reaching the application stage. soil is one of the necessary conditions for vegetation growth. three main indexes that reflect the environmental characteristics of vegetation were selected to characterize the effect of the grassland desertification control project to restore the soil. they are soil organic matter, soil mechanical composition, and soil crust. (1) soil organic matter: soil organic matter content is one of the most important indicators of soil properties. it indicates various nutrients contents in the soil, especially nitrogen and phosphorus, and represents the potential capacity and stability of soil fertility. it is the key factor in the health of the soil. the soil sample was taken back to the laboratory for analysis to determine the organic matter content (g/kg) in the soil. the content of soil organic matter is an important index of soil fertility. organic matter also plays an important role in the physical and chemical properties of the soil. for example, soil organic matter can improve the physical structure and chemical properties of the soil, which is conducive to the formation of soil aggregate structure, promotion of nutrient absorption, and growth of plants. (2) soil mechanical composition: soil is composed of different sizes of soil particles in different proportions. these different grain sizes are mixed together to characterize the soil thickness, which is called soil mechanical composition or soil texture. it affects the soil moisture, air, and heat movement, and also affects the conversion of nutrients and the type of soil structure. the mechanical component is a stable natural property of soil, which determines the behavior and use of soil to a large extent, and is one of the basic materials necessary for studying soil wind erosion. the aluminum box was used to take the surface sand sample for indoor analysis, and the fine sand proportion is one evaluation index. (3) soil crust: the crust is a thin layer formed along with the fixation of sandy land, the accumulation of organic matter on the surface of the soil, and the growth of lower plants. soil crust is an important indicator of surface wind erosion and stationary status (dai et al., 2017). a clear indicator of the restoration of sandy vegetation communities is the degree of surface fixation. the better the restoration of sandy vegetation, the higher the degree of sandy land fixation. the visual index reflecting the degree of surface fixation is the thickness of the surface crust of sandy land. according to research, the crust material is mainly a mixture of fine particles (fine sand and ultrafine sand accounted for 93), salts, and amorphous silicon gel. it has strong wind erosion resistance and can resist wind with a speed of 30 m/s. however, if the crust is too thick, it will also inhibit the growth of sandy plants. for some regular fixed sands, the crust is too thick, affecting water infiltration, and the soil moisture condition is deteriorated, which will lead to poor plant regeneration. index weight calculation the weight determination of each evaluation index plays an important role in the comprehensive evaluation of the control effect of alhagi sparsifolia. the weight of the evaluation is very important for the final evaluation result, which reflects the relative importance of each index. in the vertical direction, it reflects the effect of index variety on the change of comprehensive ecological benefit. in the horizontal direction, it indicates the position of the index in the same evaluation index level. whether the weight is determined reasonably will directly affect the judgment results (sherratt & synodinos, 2017). the analytic hierarchy process was used to construct an analytic model and determine the index weights. as a basic evaluation method, the analytic hierarchy process (ahp) plays an extremely important role in multi-objective decision-making problems. it also has extremely high using frequency. to analyze problems with ahp, the problems should be organized and leveled first to construct a structural model of hierarchical analysis. the hierarchical analysis structure model usually consists of three layers. there is only one element in the highest layer, which is generally the target or ideal result of the analysis problem, so it is also called the target layer. the middle layer includes the intermediate links to achieve the goal. it can be composed of several layers, including the rule and sub-rule that need to be considered, so it is also referred to as the rule layer. the index layer is below the rule layer (rosen et al., 2019). table 1. the ecological benefit evaluation system of alhagi sparsifolia in the desertification control project a target layer rule layer b index layer c ecological benefits vegetation vegetation coverage aboveground biomass grass layer height growth rate sandstorm activities fixed sand scale fixed sand landscape broken moving sand landscape broken soil soil wind erosion modulus soil organic matter the proportion of fine sand soil crust according to the current data in table 1, a hierarchical judgment matrix was constructed. the judgment matrix indicates the relative importance of the elements in this hierarchy to elements in the previous one. a comparison judgment matrix was constructed by comparing the relative importance of the element in two neighboring hierarchies. based on the scale of 1-9 and its reciprocivity as the measurement scale, saaty proposed the corresponding judgment matrix. the scale is shown in table 2. table 2. the scale of judgment matrix factor scale meaning 1 meaning that the two factors are equally important 3 represents two factors, the former is slightly more important than the latter 5 compared with the two factors, the former is significantly more important than the latter 7 indicating that the former is more important than the latter in terms of the two factors 9 represents two factors, the former is more important than the latter 2468 represents the middle digit of the adjacent judgment of 1, 3, 5, 7 and 9 the derivative of the above value indicates the degree to which two factors are more important than the latter 454 juan li, wei liu, xinxin zhang according to the scale standard in table 2, the corresponding matrix is formed, and the judgment matrix of the ecological benefit evaluation factor of the grassland desertification control project is compiled as follows: a b =                   1 3 5 1 3 1 3 1 5 1 3 1 (1) b c1 1 2 3 2 2 1 2 1 2 2 2 1 3 1 3 1 1 3 1 3 1 2 1 2 2 1 1 1 2 1 2 2 1 1  =                               (2) b c2 1 3 2 3 3 1 3 1 1 2 1 1 1 2 1 2 1 1 2 1 2 1 3 1 2 1 1 1 1 2 2 1 1  =                               (3) b c3 1 3 2 1 3 1 1 2 1 2 2 1  =                   (4) there are three kinds of judgment matrix calculation methods: arithmetic mean method, geometric mean method, and maximum eigenvalue method. the geometric mean method was used in this paper. for example, the relative importance coefficient for problem a is estimated based on the a-b judgment matrix. calculate the square root of the product of all elements in each row of the judgment matrix. w ni j n ij= =1   (5) wi can be normalized according to formula (5) to obtain normalized weight. because the judgment matrix needs to be fuzzy quantized based on empirical data, it is impossible to achieve complete consistency. a concept of random consistency ratio was proposed based on this, and its unit is cr. when the cr is less than 0.1, the consistency is considered to be satisfied; otherwise, the matrix needs to be re-adjusted. the cr calculating formula is as follows: cr ci ri = (6) where c indicates the consistency index, ri is the proportional coefficient, which is related to the order of the judgment matrix n. in this situation, ci can be expressed as: ci n n = ( )λ − − max 1 (7) where max represents the maximum eigenvalue of the matrix, and the judgment results are as follows: table 4. a-b evaluation matrix a b1 b2 b3 wi wi b1 1 3 5 2.466 0.673 b2 1 3 1 3 1 0.258 b3 1 5 1 3 1 0.405 0.105  1 table 5. b1-c judgment matrix b1 c1 c2 c3 c4 c5 wi wi c1 1 2 3 2 2 1.888 0.345 c2 1 2 1 3 2 2 1.458 0.258 c3 1 3 1 3 1 1 2 1 2 0.858 0.087 c4 1 3 1 3 1 1 2 1 2 0.455 0.157 c5 1 2 1 2 2 1 1 0.870 0.157  1 table 6. b2-c judgment matrix b2 c6 c7 c8 c9 c10 wi wi c6 1 2 3 2 2 1.888 0.345 c7 1 3 1 3 2 2 1.458 0.258 c8 1 2 1 3 1 1 2 2 0.858 0.087 c9 1 2 1 3 1 2 0.455 0.157 c10 1 2 2 1 0.870 0.157 1 using the results of a single ranking of all hierarchies, the relative importance weights of all elements in this hierarchy were calculated compared to the previous one, which is the total ranking of hierarchies. the calculation needs to be performed hierarchy by hierarchy from top to bottom. for the second hierarchy below the highest one, the hierarchical single order is the total order. according to the consistency test, the cr values of the hierarchical single order and the hierarchical total order are both less than 0.10. table 3. random consistency proportion coefficient order number 1 2 3 4 5 6 7 8 9 the values 0 0 0.58 0.90 1.12 1.24 1.32 1.41 1.45 455evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts camelthorn index extraction and treatment evaluation analysis of information source data the ecological benefit evaluation system of alhagi sparsifolia desertification control and calculation results of its index weights were processed through the arcgis9.0 software. the software made a project distribution map by introducing surface attributes to the inflection point coordinates of the project and defining projections. the processing of remote sensing data was mainly performed in erdas image8.7 remote sensing image processing software, which realized image preprocessing, including the geometric correction and excavation of remote sensing images. the software can also realize enhanced processing of remote sensing information, extraction of point attributes, classification, and classification accuracy verification. the field gps data were spatialized and projected by arcgis 9.0 software to obtain a shapefile vector file. the correlation between ground survey data and remote sensing data was analyzed by mathematical-statistical analysis software spss to establish a regression model. remote sensing image processing during the imaging process of satellite remote sensing, radiation errors and geometric distortions may occur due to the attenuation of the sensor function, the influence of the atmosphere, and the flight mode of the satellite. therefore, the image should be subjected to radiation correction and geometric correction before information can be extracted. the remote sensing image used in this paper had been subjected to standard atmospheric radiation correction and scatter correction at the satellite ground station before purchasing, and the geometric correction of the system had been carried out according to the parameters of the sensor, which eliminated the geometric distortion caused by the earth rotation, satellite mode changes, and sensor aging. the real positions of a series of pixels that were distributed uniformly in the remote sensing image were obtained by gps. these positions are usually called ground control points (gcp). because the research area is mainly sandy land and transportation there is inconvenient, gcp cannot be obtained in some areas. therefore, the two remote sensing images were combined, and the previously corrected tm remote sensing image was taken as a correction reference. finally, the standard error of the correction model (root means squared error, rmse) was controlled within 0.5 pixels. evaluation of control effect according to the control weights obtained by the above analytic hierarchy process, combined with the experimental data under different irrigation amounts, the vegetation index was obtained. the general process of extracting vegetation index using erdas remote sensing image processing software is below: step1: open the remote sensing image using erdas remote sensing processing software. step2: according to the vegetation index formula, the band calculation was performed on different bands of the image in the modeling tool. step3: obtain image files of vegetation index under different irrigation amounts. the regression statistical analysis table based on the current index is as follows: table 8. the regression statistical analysis table sources of variance sum of squares degrees of freedom the mean square statistic return to the 72133.845 1 72133.852 62.179 residual 25525.38 23 1107.287 the sum of the 67957.602 24 the correlation coefficient 0.859 fitting 0.738 the slope 1035.5 intercept 21.352 it can be seen from table 8 that the correlation coefficient between the index and the aboveground biomass is 0.859. after the regression significance test, the regression effect reaches a significant level. the vegetation index and the model of aboveground biomass were obtained: y x= +1035 31 278. (8) where y is the biomass of alhagi sparsifolia under different irrigation amounts, x is the vegetation index. the grassland estimation model was established by the model module in the erdas software, and the biomass of each pixel on the image can be obtained to form a spatial distribution map of the aboveground biomass. then the aboveground biomass data in different project areas were summarized to get the final evaluation results. results the vegetation data obtained from the ground surveys and remote sensing image processing are taken as the basis. in the same period, the adjacent area, the enclosed development area, and the free grazing area outside the control project area were taken as the horizontal comparison, and the condition before the implementation of the project was taken as the vertical comparison. a comprehensive analysis was performed on the evaluation indexes of each vegetation index to see whether each index develops in a favorable direction, and thus draws the overall evaluation conclusion. the correctness of the conclusion is tested through experiment. in order to determine whether the evaluation algorithm of the control effect in this paper can effectively evaluate the control data and improve the evaluation accuracy, evaluation indexes usually select the cross-data samples, which are commonly used in the effect evaluation model. the area under the characteristic curve of alhagi sparsifolia was used to evaluate the comprehensive performance of the current evaluation method. the abscissa is the false positive rate (fpr) of the roc curve and the ordinate is the true positive rate (tpr). the area under the curve was used to determine whether the evaluation method has a fitting phenomenon and a larger area indicates a better evaluation effect. it can be seen from the data in figure 3 that the area under the overall characteristic line of this state evaluation method is significantly higher than the conventional method used for comparison. its true positive rate is between 160 and 200, which is within a large advantage range, indicating that the overall evaluation is accurate and its effect is perfect. this is because the algorithm in this paper first constructed a “vegetation-sandstorm-soil” index system, selected a number of indexes, and calculated their index weights. according to the calculation results, the analytic hierarchy process was used, with the help of remote sensing, geographic information system, and landscape technology, to assign weights to the grassland control indexes. finally, remote sensing, computer image information screening, and processing were used to perform data regression analysis and obtain the final evaluation results, which effectively improved the accuracy of the evaluation results. table 7. b3-c judgment matrix b3 c11 c12 c13 wi wi c11 1 3 2 1.888 0.345 c12 1 3 1 2 1.458 0.258 c13 1 2 2 2 0.858 0.087  2 0.858 0.087 456 juan li, wei liu, xinxin zhang discussion desertification is one of the most important environmental and socioeconomic problems in modern society, threatening the survival and development of mankind. china is one of the countries seriously affected by desertification in the world. desertification has attracted much attention because of its development speed and impact on the social economy. according to statistics, in terms of its development rate, in the 1950s-1960s, desertification land in china expanded to 1,560 km2. the desertification land in the 1970s-1980s expanded to 2,100 km2 and reached 3,600 km2 in the 1990s. the area of desertification land has increased by 46.74 million km2 from 1987 to 2000 and reached 385,700 km2 by the year 2000. compared with the monitoring results in 1987, the area of desertification land has decreased after more than ten years of control, and the desertification degree has been reduced. however, the overall development of desertification is greater than the reversal. especially in the past ten years, there has been a significant increase. under this background, this paper proposes an evaluation algorithm for the effect of alhagi sparsifolia desertification control under different irrigation amounts. the experimental results show that compared with the traditional methods, the algorithm in this paper has higher prediction accuracy and better prediction effect, which fully verified its effectiveness and feasibility. on the basis of obtained accurate assessment results, it helps relevant departments to propose environmental management programs in a targeted manner, which is conducive to the improvement of environmental problems and has practical significance. conclusion by reviewing the literature, summarizing the previous research and evaluation theory of desertification control and consulting experts, this paper adopted the alhagi sparsifolia vegetation, sandstorm, soil, and other indexes as a system, and selected the vegetation coverage, aboveground biomass, grass height, plant species, annual plant proportion, fixed sand proportion, fixed sand fragmentation, mobile sand proportion, mobile sand fragmentation, soil erosion modulus, soil organic matter, soil mechanical composition, soil crust and other indexes. these indexes almost cover the main ecological and environmental impact factors brought by the alhagi sparsifolia project and accurately and comprehensively reflect the ecological benefits of the project under different irrigation amounts. in addition, the paper uses the analytic hierarchy process to determine the index weights. the outstanding advantage of the analytic hierarchy process is that it can minimize the influence of subjective factors, as well as express and treat human subjective judgments in quantitative form. moreover, the mathematical principle of the ahp is rigorous, simple, and easy to implement, and the factors of complex problems are organized by dividing the order of interconnection. ahp also gives objective judgment and quantitative expression of the relative importance of each evaluation index in each hierarchy, which provides an effective technical method to solve the weight distribution in the evaluation of ecological benefits. according to the experiment data, it can be known that the evaluation algorithm in this paper has numerous reasonable results and application advantages. references ajaj, q. m., pradhan, b., noori, a. m., & jebur, m. n. (2017). spatial monitoring of desertification extent in western iraq using landsat images and gis. land degradation & development, 28(85):7-8. dai, q., peng, x., zhi, y., & zhao, l. (2017). runoff and erosion processes on bare slopes in the karst rocky desertification area. catena, 152(17):218-226. guan, p., zhang, x., yu, j., cheng, y., li, q., andriuzzi, w. s., & liang, w. (2018). soil microbial food web channels associated with biological soil crusts in desertification restoration: the carbon flow from microbes to nematodes. soil biology & biochemistry, 116(10):82-90. kang, s. (2018). prevention and control mode of desertification in china’s state energy group shendong mining area with annual coal output of 200 million tons. meteorological & environmental research, 9(02):27-33 lan, s., zhang, q., he, q., yangm h., & hu, c. 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(2018). sophora davidii: a native shrub used for rocky desertification control and development. meteorological & environmental research, 9(2):24-26. the flow of the test evaluation algorithm: table 9. the analysis process of test evaluation algorithm data input: evaluation information data point set x,g×g grid, (g+1) × (g+1) grid output: sample for each data point process step1 enter each data point xi step2 distribution point xi go to the corresponding indicator network step3 define an empty set of governance metrics and collect cluster examples step4 take each datagiput it into the corresponding grid step5 construct similarity matrix s1and s2 put them into the corresponding matrix step6 merge the sample to perform the third ap step7 for the current no matrix, construct a similar matrixs step8 the above formula is used to calculate the similarity step9 compute the governance data preference step10 distribution of x; corresponding example step11 end organized experimental data is as follows: figure 3. comparison of evaluation results of different methods 457evaluation algorithm of alhagi sparsifolia desertification control under different irrigation amounts qiang, l., pu, j., ni, h., du, h. m., qi, x. k., wang, l., & yang, h. 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(2019). regionalization for aeolian desertification control in the mu us sandy land region, china. journal of desert research, 9(15):4-6. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.108925 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 127 137 so il s abstract diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran ebrahim mohammadi1*, mehdi ghaedi2 1. department of ecology, institute of science, high technology and environmental science, graduate university of advanced technology, kerman, iran. 2. department of geology, faculty of science, university of isfahan, isfahan, iran. * corresponding author: e.mohammadi@kgut.ac.ir record subject: palaeontology manuscript received: 15/05/2023 accepted for publication: 22/06/2024 how to cite this article: mohammadi, e., & ghaedi, m. (2024). diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran. earth sciences research journal, 28(2), 127-137. https://doi. org/10.15446/esrj.v28n2.108925 keywords: oligo-miocene; corals; paleoecology; larger benthic foraminifera; tethyan seaway; zooxanthellate corals are iran’s most abundant macrofossils in the oligocene-early miocene qom formation deposits in the ne margin of the tethyan seaway. yet, basic knowledge about the diversity of these organisms and their paleoecology is still scarce. in this paper late rupelian–chattian coral data from three exposed sections in the northwestern jazmurian lake, se iran, as the southeasternmost outcrops of the qom formation, are analyzed in terms of diversity and paleoecology. the coralline red algae and larger benthic foraminiferal assemblages associated with the corals have also been used to infer the paleoenvironmental and paleoecological conditions. the following coral genera were recognized: leptoria, caulastraea, hydnophora, astreopora, plesiastraea, thegioastraea, porites, goniopora, platycoenia, acropora, tarbellastraea, favites, heliastraea, stylocoenia, ceratotrochus. the high abundance of z-corals, larger benthic foraminifera (lbf), and coralline red algae demonstrate that the deposition took place in tropical-subtropical warm waters mainly within the euphotic to the mesophotic zones. the presence of various coral fabrics including pillarstone, domestone, and rudstone allowed to infer high to moderate hydrodynamic energy in the studied sections. as well, given the interactions among corals, filter-feeders, green and red algae, bioeroders, and the surrounding environment, it seems that oligotrophic to slightly mesotrophic conditions also had prevailed in the studied area. diversidad y significado de los corales zooxanthellate en la formación qom, al sureste de irán resumen los corales zooxanthellate son los macrofósiles más abundantes en los depósitos del oligoceno temprano de la formación qom en el margen noreste del oceáno tetis. sin embargo, el conocimiento básico sobre la diversidad de estos organismos y de su paleoecología es escaso. este trabajo utiliza información del período rupelian–chattian tardío en tres secciones expuestas al noroeste del lago jazmurian, sureste de irán, que son los afloramientos rocosos más al sudeste de la formación qom, y que se analizó en términos de diversidad y paleoecología. algas rojas coralinas y grandes conjuntos de foraminíferos bentónicos asociados con los corales también se usaron para inferir el paleoambiente y las condiciones paleoecológicas. se reconocieron los siguientes géneros coralinos: leptoria, caulastraea, hydnophora, astreopora, plesiastraea, thegioastraea, porites, goniopora, platycoenia, acropora, tarbellastraea, favites, heliastraea, stylocoenia, ceratotrochus. la alta abundancia de corales zooxanthellate, los grandes conjuntos de foraminíferos bentónicos y las algas coralinas rojas demuestran que la deposición tuvo lugar en aguas calidadas del orden tropical-subtropical y principalmente dentro de las zonas eufóticas a mesofóticas. la presencia de varias fábricas coralinas que incluyen pilares, domos y piedras permiten inferir la energía hidrodinámica de alta a moderada en las secciones estudiadas. además, al parecer por las interacciones entre corales, los alimentadores por filtración, las algas rojas y verdes, los bioerosionadores, y el ambiente alrededor, en el área de estudio prevalecieron condiciones oligotrópicas a ligeramente mesotrópicas. palabras clave: oligo-mioceno; corales; paleoecología; grandes foraminíferos bentónicos; mar de tetis. https://doi.org/10.15446/esrj.v28n2.108925 mailto:e.mohammadi@kgut.ac.ir https://doi.org/10.15446/esrj.v28n2.108925 https://doi.org/10.15446/esrj.v28n2.108925 128 ebrahim mohammadi, mehdi ghaedi 1. introduction late oligocene (chattian) probably dealt with the highest coral diversity for the cenozoic, and large reefal structures occurred worldwide (pomar and hallock, 2007). the shift from the warm eocene to the oligocene “icehouse” took place during a sequence of major changes in climate, ice volume, and oceanic circulation, which affected the composition and production of the carbonate factory (brandano, 2017). hallock (1996), hallock and pomar (2008) suggested that co2 reduction (as eocene-oligocene transition aftermath) had an influential impact on the expansion of zooxanthellate corals, probably favored by the rising mg/ca ratios, which enhanced hyper calcification of aragonite (stanley and hardie, 1998). during the oligocene-miocene, the biogeography of reef corals progressively changed from the tethyan province in the eocene to three reef-coral provinces: the mediterranean, the indo-pacific, and the western atlantic-caribbean (perrin and bosellini, 2012; pomar et al., 2014). after a temporary platform crisis during the earliest oligocene, coral reefs spread throughout the tethys and increased with new lineages of lbf (hontzsch et al., 2013; pomar et al., 2017). as well, oligocene reefs depict the peak of cenozoic reef growth (pomar et al., 2014). the lower oligocene–lower miocene rocks (rupelian–burdigalian), known as qom formation are deposited at the northeastern margin of the tethyan seaway during the final sea transgression in the sanandaj–sirjan, urumieh–dokhtar, and central iran basins (e.g. mohammadi et al., 2013, 2015; mohammadi, 2022, 2024; fig. 1). biogenic constituents of the qom formation include corals, corallinacean algae, larger benthic foraminifera, small benthic foraminifera, planktic foraminifera, bivalves, bryozoans, echinoderms, gastropods as well as polychaete worm tubes, especially ditrupa. herein, we will concentrate specially on corals. figure 1. a map illustrating the distribution of the urumieh-dokhtar magmatic arc, the qom formation outcrops, suture zones of paleotethys and neotethys in iran, and the situation of the studied sections (adopted from mohammadi, 2023). an advance in the knowledge of the corals from the qom formation deposits is of major importance because they are located between the western and the eastern tethys regions (mohammadi et al., 2011, 2013, 2015, 2019; mohammadi and ameri, 2015, 2024; daneshian and ramezani dana, 2018; mohammadi, 2020, 2021, 2023, 2024) and thus, paleogeographically, they are significant. most investigations on the qom formation have been conducted on the central parts of iran and especially, on its foraminiferal content; however, surprisingly little information is available about the corals as well as about the southeasternmost outcrops of the qom formation (mohammadi, 2022, 2023, 2024). therefore, this paper aims to bridge the gap by investigating the southeasternmost outcrops of the qom formation on corals to 1) recognize coral components and 2) analyze the paleoecology of the identified corals. 2. previous works the fossil assemblages of scleractinian corals comprise one of the dominant carbonate components of the qom formation that are widely considered in environmental interpretations of fossil corals reefs (brandano et al., 2010; perrin and bosellini, 2012; yazdi et al., 2012). a total of 32 and 13 genera of scleractinian corals from the formation were recorded by schuster and wielandt (1999) from the isfahan–sirjan fore-arc and the qom back-arc basins, respectively. reuter et al. (2009) studied scleractinian corals from abadeh and zefreh sections (in the fore-arc basin) and chalehghareh and qom sections (in the back-arc basin). rahiminejad et al. (2017) presented the paleoenvironment of scleractinian corals of the qom formation in northeastern isfahan (central iran) and identified nine colonial coral genera. mohammadi and ameri (2015) analyzed the biotic constituents of the qom formation in northern abadeh and recorded a rupelian-chattian age for the studied section. these authors noted isolated, branched, and colonial growth forms of corals as well as some scattered coral colonies. the main recognized coral genera represented by mohammadi and ameri (2015) are favites, hydnophora, astrocoenia, porites, leptoria, cyphastrea, and favia. besides, miocene coral-rich deposits of makran have been studied by ghaedi et al. (2016) and ghaedi and yazdi (2016). in addition, coral boundstone facies were recorded from some stratigraphic sections of the qom formation by mohammadi et al. (2011, 2019) and mohammadi (2020, 2021). 3. geological setting the geology and especially the tectonic structure of iran are highly infuenced by the development and history of the tethyan (paleotethys and neotethys oceans) region (alavi, 2004; ghaedi et al., 2022). deposition of the qom formation took place in the ne margin of the tethyan seaway in three nw–se-trending basins (>1800 km length) including the sanandaj–sirjan fore-arc basin, urumieh–dokhtar magmatic arc (intra-arc basin), and central iran back-arc basin (mohammadi et al., 2013, 2015; fig. 1). the outcrops of the qom formation are distributed from the khoy and maku regions (northwestern urumieh lake) in the northwest to northwest of the jazmurian lake in the southeast (rahimzadeh, 1994; seyrafian and toraby, 2005; mohammadi et al., 2011, 2013; mohammadi, 2023, 2024; fig. 1). all of the studied sections (bozdan, tavakolabad, and gonarestan) are located in the sanandaj-sirjan basin. this basin constructs a narrow band, situating between urumieh and sanandaj cities in the northwest, and sirjan and esfandagheh cities in the southeast (ghasemi and talbot, 2006), bounded by the udma to the ne and the main zagros reverse fault in the sw (fig. 1). 4. the age range the foraminiferal-based new biozonation of the qom formation (mohammadi, 2023) is applied in this research to define age. the biostratigraphy of the studied sections is discussed in mohammadi and ameri (2024). so the biostratigraphy is only briefly discussed here. in all, 35 genera and 55 species of benthic foraminifera were identified. in the gonarestan section, based on the vertical distribution of foraminifera, two assemblages were recognized. assemblage 1 extends from the base of the qom formation to 68m. the presence of nummulites fichteli, nummulites intermedius, and nummulites vascus allows the age of the assemblage 1 to be determined as rupelian. due to the co-occurrence of lepidocyclinids and nummulites, this assemblage corresponds to the lepidocyclina-nummulites concurrent range zone of mohammadi (2023). it is attributed to the late rupelian. assemblage 2, 17m in thickness, occurs from 68 to 85m (top of the section). the main biotic event in this assemblage is the absence of nummulites vascus, nummulites fichteli, and nummulites intermedius. this assemblage is correlated with lepidocyclina partial range zone of mohammadi (2023) and consequently is considered to be chattian in age. in the tavakolabad section, two foraminiferal assemblages were recognized. assemblage 1 expands from the basal boundary of the qom formation to 44m. due to the co-occurrence of lepidocyclinids and nummulites, assemblage 1 corresponds to the lepidocyclina-nummulites concurrent range 129diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran zone of mohammadi (2023). it is attributed to the late rupelian. assemblage 2, 36m in thickness, extends from 44 to 80m (top of the section). the main difference between assemblages 1 and 2 is the absence of nummulites vascus and nummulites fichteli/intermedius in the second one. this assemblage correlates with the lepidocyclina partial range zone of mohammadi (2023), that is the chattian in age. in the lower carbonate part of the bozdan section, the last presence of nummulites spp. occurred in the last samples which are consequently indicative of rupelian. the co-occurrence of lepidocyclinids and nummulites vascus and nummulites fichteli/intermedius shows that this assemblage is pertaining to the late rupelian in age. this assemblage correlates with the lepidocyclinanummulites concurrent range zone of mohammadi (2023). the assemblage is attributed to the late rupelian. it should be noted that the preliminary study of 62 samples from the upper carbonate part of the bozdan section revealed that its age is rupelian?chattian. in summary, based on the biostratigraphic data, the recognized foraminifera assemblages, and mohammadi and ameri (2024), the qom formation is late rupelian−chattian in age in bozdan, tavakolabad, and gonarestan stratigraphic sections. 5. material and methods 5.1. description of sections we collected samples and prepared thin sections from three stratigraphic sections of the qom formation, with special emphasis on scleractinian corals. these are gonarestan, tavakolabad, and bozdan stratigraphic sections in northwestern jazmurian lake (fig. 1). the studied sections represent the southeasternmost outcrops of the qom formation. all stratigraphic sections (fig. 2) are approachable via motorable long roads (valeh, 1956). outcrops of the qom formation in the bozdan section are unconformably underlain by volcanic rocks (valeh, 1956). however, in the gonarestan and tavakolabad sections they lie on top of older tuff units. the tuff units may be equivalent to the tuff unit separating the lower and upper carbonate units of the bozdan section. the qom formation in all three stratigraphic sections is covered by quaternary alluvium (valeh, 1956). besides, up to 3 m of the basal part of the gonarestan and tavakolabad sections were covered by debris. the gonarestan section (28˚ 08΄ 30″ n, 58˚ 22΄ 12″ e) is situated approximately 90 km northwest of the center of jazmurian lake (fig. 1). it is located more than 1800 km southeast of the northwesternmost outcrops of the qom formation (in nw maku). the gonarestan section, with a thickness of 85 m, consists of medium to thick-bedded and massive limestones, and coral reef-bearing limestone. there are a lot of scleractinian coral colonies in some parts, which are diverse and mostly in their growth position, forming a connected and relatively dense framework. the tavakolabad section (28˚ 09΄ 22″ n, 58˚ 21΄ 55″ e) is situated between gonarestan and bozdan stratigraphic sections. it is located approximately 45 km northeast of roudbar-jonoub city (fig. 1). the section (80 m thick) is mainly composed of medium to thick-bedded and massive limestones, coral reef limestone, and marls. the marly succession is deposited in one interval in the lower part and contains millimeter-sized nummulites. the bozdan section (28˚ 13΄ 35″ n, 58˚ 14΄ 24″ e) is situated northwest of both tavakolabad and gonarestan stratigraphic sections. it is approximately located 35 km northeast of roudbar-jonoub city (fig. 1). its lower contact with volcanic rocks of eocene is well exposed. outcrops of the qom formation in the bozdan section 440m thick), consist mainly of medium to thick-bedded and massive limestones, reefal limestones, marls, tuffaceous limestones, and tuffs. a tuff unit (90m thick) is intercalated in the lower part and separates the lower (30m thick) and upper carbonate units (320m thick). it should be noted that the contact between the tuff unit and the upper carbonate unit is not usually exposed due to the debris. coralline red algae, lbf, corals, and echinoderms are the most prevailing biota in the studied section. although scleractinian corals are rare in the lower part, distinct coral horizons are abundant and traceable for several hundreds of meters in the upper part. two yellow marly units with centimeter to millimeter-sized nummulites occur in the lower carbonate unit. in the bozdan section, the lower carbonate unit is sampled systematically. however, only 62 hard samples were collected from the upper limestone unit. in fact, due to the cliff-forming topography of the upper 100 meters and the inaccessible route, systematic sampling was neglected. scleractinian coral colonies are abundant and diverse in all three stratigraphic sections, but they are very rare in the studied lower part of the bozdan section. echinoderms are present in some layers of the gonarestan and bozdan stratigraphic sections; surprisingly they are absent in the tavakolabad section (which is located between two other echinoderm-bearing sections). some lepidocyclinids and nummulitids are sufficiently large to be observed easily with the naked eye on rock surfaces. figure 2. stratigraphic columns of the studied sections in gonarestan, tavakolabad, and bozdan. 5.2. field and laboratory studies to analyze the corals, 46, 76, and 89 samples were collected from tavakolabad, gonarestan, and bozdan sections, respectively. besides, more than 120 zooxanthellate coral macro specimens were collected from the coralbearing units. tests of nummulites are the main biotic component of the bozdan marly layers. sampling intervals were mostly between 1 to 2m. thin sections (2.5 × 7.5cm) were prepared from consolidated samples, while loose marly samples were washed, sieved, and screened for foraminifera. besides, 22 larger thin sections (10 × 10cm) were prepared from the collected coral specimens. all samples were analyzed in detail with a particular interest in the corals and other biotic components, especially lbf, and coralline red algae. the authors tried to collect diverse coral specimens in the field to be investigated in detail in the laboratory, but some of them could not be collected as they were too hard to be broken and separated from the source rock. however, all necessary features (e.g. coral fabrics, forms, and shapes) were observed and photographed in the field. the biostratigraphy of the studied sections is based on mohammadi (2023). to identify various genera and species, macromorphological characters related to corallites structures and the distributions of them in coral colonies were scrutinized. after measuring different parameters including forms, shapes, number of septa, cycles degree, coenosteum and columella types, wall structures and other traits in corals, original systematics references were used to identify each specimen. coral growth fabrics were classified according to insalaco (1998). the classification of the coral-bearing limestones was based on the scheme of dunham (1962) and embry and klovan (1971). 130 ebrahim mohammadi, mehdi ghaedi 6. paleoecology of the corals 6.1. details of the recognized coral taxa in this research, the paleo-environmental conditions of the studied sections are explored based on biotic evidence, environmental and lithological features, fabric type, and the relationship between these items. a significant number of corals can be seen in the studied areas (fig. 3). they are present in in-situ and reworked forms in the sections. in total, the corals recognized in the studied sections include leptoria sp., porites sp., hydnophora sp., hydnophora pulchra, hydnophora cf. pulchra, stylocoenia taurinensis, thegioastraea sp., caulastraea sp., caulastraea farsis, heliastraea sp., heliastraea (athecastraea) stellata, heliastraea sp., tarbellastraea reussiana, favites sp., favites neglecta, ceratotrochus (conotrochus) sp., goniopora sp., astreopora meneghiniana, plesiastraea sp., platycoenia sp., acropora sp., astreopora stellaris. the coral fabrics (figs. 4) are defined based on the classification of insalaco (1998). the shape of corals here is limited to dome-shaped, branching, and columnar. the colony types are phaceloid, hydnophoroid, meanderoid, plocoid, cerio-plocoid, cerioid, and sub-cerioid, together with solitary forms. according to insalaco (1998), the coral horizons in tavakolabad and bozdan sections show domestone, rudstone, and pillarstone fabrics, while the domestone fabric is the dominant one in the gonarestan section (fig. 4). figure 3. some coral species present in the studied sections: 1) astreopora stellaris, bozdan section; 2) caulastraea farsis, bozdan section; 3) platycoenia sp., tavakolabad section; 4) plesiastraea sp., tavakolabad section; 5) astreopora meneghiniana, gonarestan section; 6) tarbellastraea reussiana, tavakolabad section; 7) leptoria sp., tavakolabad section; 8) favites neglecta, gonarestan section; (scale bars are 1cm) 131diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran figure 4. coral growth fabric. 1) upper view of a dense pillarstone fabric, bozdan section; 2) a rudstone fabric, bozdan section; 3) the growth of caulastraea farsis on the debris of rudstone fabric, bozdan section; 4) domestone fabric, tavaolabad section; 5) upper view of a dense pillarstone fabric, tavaolabad section; 6) domestone fabric, gonarestan section. 6.2. estimation of water surface temperature one method of estimating the temperature range is to survey the constituents. according to adams et al. (1990), the symbiotic foraminifers are restricted to tropical-subtropical waters. besides, betzler et al. (1997) believe that large oligo-miocene benthic foraminifers such as lepidocyclina, miogypsina, archaias, and borelis are restricted to tropical regions with a mean temperature greater than 18-20°c during the coldest months of the year. porites forms, which are among the scleractinian corals, belong to tropical regions; likewise, the presence of heterostegina, operculina, and amphistegina genera together with red algae (lithoporella, lithothamnion, and lithophyllum) are ecologically related to tropical to subtropical areas (brandano and corda, 2002). based on the above-discussed methods, the estimation of temperature based on coral richness combined with the temperature range method based on the constituents seems to be a more appropriate method. bozdan and gonarestan sections: the presence of zooxanthellae corals (table 1) along with benthic foraminifers (such as amphistegina, lepidocyclina, and operculina), green algae, red algae, serpulid worms, gastropods, bivalves, echinoids, and some other mollusks represents the tropical to subtropical temperatures (sadeghi et al., 2018; mohammadi, 2021). tavakolabad section: the abundant presence of zooxanthellae corals (table 1) along with benthic foraminifera (such as amphistegina, lepidocyclina, and operculina), green algae, red algae, and mollusks is an indicative of tropical to subtropical temperatures (sadeghi et al., 2018; mohammadi, 2021). 6.3. water energy morphological and textural characteristics of corals together with other factors are important for determining water energy. as well, hydrodynamic energy plays a role in the development of coral communities and can be inferred from the coral morphotypes (tomás et al., 2008). the hydrodynamic energy of water can control the co2 content of water and thus affect the water’s acidity; accordingly, the water energy can affect the rate of carbonation or dissolution in sediments (mutti and hallock, 2003; pilson, 2013, ghaedi et al., 2016; fig. 5). 132 ebrahim mohammadi, mehdi ghaedi table 1. distribution of symbiont-bearing coral genera of the studied sections. symbiont-bearing genera form shape the sections bozdan tavakkolabad gonarestan 1 hydnophora hydnophoroid dome-shaped ✔ ✔ ✔ 2 leptoria meandroid dome-shaped ✔ ✔ ✔ 3 caulastraea phaceloid dome-shaped ✔ ✔ ✔ 4 tarbellastraea plocoid dome-shaped ✗ ✔ ✗ 5 favites sub-cerioid dome-shaped ✗ ✔ ✗ 6 thegioastraea cerio-plocoid dome-shaped ✔ ✔ ✗ 7 heliastraea plocoid dome-shaped ✗ ✔ ✗ 8 astreopora plocoid dome-shaped ✔ ✗ ✔ 9 plesiastraea plocoid dome-shaped ✗ ✔ ✗ 10 platycoenia plocoid dome-shaped ✗ ✔ ✗ 11 acropora plocoid branching ✔ ✔ ✔ 12 porites cerioid dome-shaped, branching, columnar ✔ ✔ ✗ 13 goniopora cerioid dome-shaped, branching, columnar ✔ ✔ ✔ 14 stylocoenia cerioid branching ✗ ✔ ✗ 15 ceratotrochus solitary ✗ ✗ ✔ unidentified genera 1 1 3 total general 9 14 10 figure 5. the effect of various factors on the co2 content in the marine environment related to the depth and energy (after mutti and hallock, 2003; pilson, 2013, ghaedi et al., 2016). the domestone facies in the capo testa (northern sardinia, italy) based on sediment textures and growth morphologies indicate relatively high hydrodynamic energy (brandano et al., 2010). on the other side, in the western taurides (turkey), the diversified colonies of domal, massive, globular, and hemispherical forms with a lesser extent by branching colonies, dominated by heliastraea, tarbellastraea, favia, favites, aquitanastarea, caulastraea, cladocora, stylophora, and porites indicate a marine shallow environment with moderate to high water energy (karabiyikoglu et al., 2005). therefore, the coral assemblages with massive, hemispherical, and domal colonies usually indicate high to moderate energy (karabiyikoglu et al., 2005; brandano et al., 2010; yazdi et al., 2012; ghaedi et al., 2016). the pillarestone fabric (e.g. branching acropora, porites, goniopora, and stylophora) without serious breakage, accompanied by green algae erected stems demonstrates moderate to low energy. massive corals (mostly with domestone fabric and sometimes rudstone fabric) and branching corals (pillarestone fabric) are present in parts of the sections; some of them are presented separately in each section as follows (table 2). table 2. the distribution of massive and branching corals in the studied sections section massive corals branching corals bozdan thegioastraea sp., leptoria sp., hydnophora pulchra, hydnophora cf. pulchra, goniopora sp., porites sp., caulastraea sp., astreopora stellaris, favites neglecta. porites sp., acropora sp., goniopora sp. tavakolabad porites sp., leptoria sp., goniopora sp., hydnophora sp., hydnophora pulchra, stylocoenia taurinensis, thegioastraea sp., heliastraea sp., tarbellastraea reussiana, caulastraea farsis, hydnophora cf. pulchra, caulastraea sp., plesiastraea sp., platycoenia sp.. porites sp., acropora sp., goniopora sp. gonarestan leptoria sp., caulastraea sp., porites sp., goniopora sp., astreopora meneghiniana, astreopora stellaris and unidentified genera. --133diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran according to karabiyikoglu et al. (2005), brandano et al. (2010) yazdi et al. (2012), and ghaedi et al. (2016), corals with massive, hemispherical, and domal colonies usually indicate high to moderate energy. on the other hand, based on the field observations, the presence of rudstone coral fabric, which indicates the breakage of corals margins, can be the result of the high energy of the waves; as well, unbroken branching corals together with erected stems of green algae represent moderate to low water energy (flugel, 2004). therefore, according to the morphology of corals and considering the fabrics, high to moderate water energy can be expected in the studied sections. 6.4. light due to the direct relationship between the photosynthesis of symbionts with light, lbf, symbiont-bearing corals, red algae and green algae are lightdependent organisms. zooxanthellate corals as part of constituents of the bozdan, tavakolabad, and gonarestan sections are very sensitive to light conditions (e.g. trench, 1981; cairns, 1999). the light penetration depends on the water depth; it decreases with increasing depth. in fact, it is influenced by the factors, controlling the water clarity (including clastic material, nutrients, and dissolved organic material) (morsilli et al., 2012). according to hallock and schlager (1986), platy corals are under 4–20 % of surface light intensities, and in high light levels, head corals live in more than 20 % of surface light intensities and branching corals are present in more than 60 % of surface light intensities (fig. 6, tables 1, 2). bozdan section: in the bozdan section, massive-domal corals including porites sp., leptoria sp., hydnophora sp., hydnophora pulchra, thegioastraea sp., and some branching corals such as porites sp., goniopora sp., and acropora sp. are also present. according to hallock and schlager (1986), pomar (2001), and morsilli et al. (2012), euphotic to slightly mesophotic conditions have been dominated in the bozdan section (fig. 6). the presence of green algae, accompanied by the branching corals confirms the well-lit euphotic environment in some intervals. tavakolabad section: in the tavakolabad section, massive-domal corals such as leptoria sp., porites sp., hydnophora sp., hydnophora pulchra, thegioastraea sp., heliastraea sp., tarbellastraea reussiana and some branching corals including porites sp. and acropora sp. are also present. according to hallock and schlager (1986), pomar (2001), and morsilli et al. (2012), euphotic to slightly mesophotic conditions have been dominated in the tavakolabad section based on the distribution of branching corals (together with green algae) and massive-domal corals (fig. 6). gonarestan section: in the gonarestan section, massive-domal corals such as leptoria sp., caulastraea sp., porites sp., and goniopora sp. as well as some green and red algae are also present. according to hallock and schlager (1986), pomar (2001), and morsilli et al. (2012), euphotic to slightly mesophotic conditions have been dominated in the gonarestan section based on the distribution of massive-domal corals (fig. 6). 6.5. nutrients in the bozdan, tavakolabad, and gonarestan sections, various factors such as zooxanthellate corals, foraminifera, red algae, bryozoans, mollusks, etc. can be used to determine the trophic conditions. the increase in the number of heterotrophic groups (such as mollusks, serpulids, bryozoans), and red algae in coral assemblages along with the increase in bioerosion (fig. 7), is an indicative of the change in the trophic conditions from oligotrophic to mesotrophic in the studied sections. due to the abundant presence of bioerosion resulting from the activity of filter-feeding organisms such as serpulid worm tubes (e.g. trypanites isp.), bivalves (e.g. gastrochaenolites isp.) bioerosion is evident in these three sections. encruster organisms such as red algae can also be seen in the studied sections. likewise, lbf is abundantly found in the studied intervals. owing to the significant presence of massive corals (domestone fabric) in the bozdan, tavakolabad, and gonarestan sections (which is a sign of high calcification rate) as well as branching corals in the bozdan and tavakolabad sections, and the presence of larger symbiont-bearing foraminifera, the dominant conditions of the sections are oligotrophic, but the presence of bioerosion, such as borings created by serpulid tube worms, bivalves and also the presence of some encruster organisms such as red algae, the nutrient conditions are considered as oligotrophic to slightly mesotrophic (fig. 7). in table 3 the predominant environmental conditions of corals in the bozdan, tavakolabad, and gonarestan sections are summarized. ligh intensity hallock & schlager (1986) photic zones pomar (2001) sh al lo w er branching corals >60% euphotic head corals >20% m es op ho tic plate corals 4-20% little or no coral 1-4% oligophotic de ep er minimal photosyntesis >1% figure 6. the position of different morphotypes of corals according to the light penetration (based on hallock and schlager, 1986; pomar, 2001; morsilli et al., 2012). 134 ebrahim mohammadi, mehdi ghaedi table 3. predominant biological features in the bozdan, tavakolabad, and gonarestan sections sections lithology fabrics calcification temperature nutrients light energy gonarestan limestone domestone high tropical to subtropical oligotrophic slightly mesotrophic euphotic slightly mesophotic high to moderate tavakkol abad limestone domestone, rudstone, pillarstone high tropical to subtropical oligotrophic slightly mesotrophic euphotic slightly mesophotic high to moderate bozdan limestone domestone, rudstone, pillarstone high tropical to subtropical oligotrophic slightly mesotrophic euphotic slightly mesophotic high to moderate figure 7. some features related to biotic activities on the corals in northwestern jazmurian lake, se iran. 1) the bioerosion traces on a scleractinian coral colony in the bozdan section; 2) a serpulid worm tube boring with distinctive lining in a porites sp. in the gonarestan section; 3) some boring traces, created by bivalves (gastrochaenolites isp.) in goniopora sp. in the gonarestan section; 4) boring sections filled with red algae and other skeletal elements in a porites sp. in the tavakolabad section; 5) coralinacean red algae, acting as encrusters in the gonarestan section; 6) the presence of green algae together with red ones in the tavakolabad section; 7) the presence of acropora sp. covered and surrounded by red and green algae in the tavakolabad section; 8) a large boring section, containing skeletal fragments and inflated foraminifera in porites sp. in the tavakolabad section. (scale bars are 0.5 cm) 135diversity and paleoecological significance of zooxanthellate corals of oligocene qom formation, se iran 7. discussion corals are prevalent in tropical-subtropical and oligotrophic conditions. as well, according to hallock (2001) and hallock et al. (2003), coral reefs flourish in the most nutrient-depleted oceanic waters where mixotrophic nutrition (i.e., the recycling of nutrients between the host and algal symbionts) is most advantageous. according to hallock and schlager (1986), massive and domal corals prefer intermediate levels of light intensity, while branching corals tend to receive higher light levels. since hydrodynamic energy plays an important role in the coral morphotypes’ formation (tomás et al., 2008), the presence of domestone growth fabric (e.g. favia and favites) can demonstrate a marine ecosystem with moderate to high energy (karabiyikoglu et al., 2005) and unbroken branching corals seem to be influenced by moderate water energy. both corals and lbf are mainly heterotrophic organisms; the former as suspension feeders relying upon suspended material in surrounding waters, while the latter feeds on detritus in the sediment. metabolically, both benefit from algal symbiosis, especially in sunlit oligotrophic warm environments (pomar et al., 2017). yentsch et al. (2002) evaluated that zooxanthellate corals need at least 1% of surface light for growth. in the studied sections, the richness of zooxanthellate corals genera was applied to estimate the minimum surface temperature, but other constituents including benthic foraminifera, green and red algae as well as heterotrophic organisms were also considered to get a more precise estimation in this regard. although there are various components in the studied sections, they all can be categorized under the tropical to subtropical temperature range. in terms of water energy, it should be noted that morphological traits of corals can be remarkable factors, as domestone fabric, which is regularly indicative of a high calcification rate, represents high hydrodynamic energy. as well, rudstone fabric (especially in bozdan and tavakolabad sections), representing large coral fragments, indicates high water energy. furthermore, in the bozdan section, there is a specimen of caulastraea farsis, growing on the rudstone coral debris without any broken parts (fig. 4); it seems, therefore, that there has been a fluctuation in the hydrodynamic energy, and the fragments are not deposited too far from their original position. the pillarstone fabric in bozdan and tavakolabad sections are related to porites, goniopora, and acropora genera, indicating their in-situ positions; as they are unbroken and occasionally accompanied by green and red algae (fig. 7), they should have lived in moderate to low energy condition. as zooxanthellate corals, lbf, red algae, and green algae are lightdependent organisms, they can be an appropriate indication of light conditions. as there are considerable head corals (in all three sections) and branching corals (in bozdan and tavakolabad section), based on hallock and schlager (1986) pomar (2001), morsilli et al. (2012) euphotic to slightly mesophotic conditions have been considered for all sections, but according to the presented chart (fig. 6), due to the presence of branching corals, there was more intense light in bozdan and tavakolabad sections. filter-feeders, encrusters, red algae, and bioeroders are increased through increasing nutrient supply; among them, trypanites isp., gastrochaenolites isp. as boring traces and corallinacean red algae as encrusters are the most significant components. as mentioned above, the head and branching corals represent oligotrophic conditions, while bioerosion demonstrates the tendency to the mesotrophic condition (mutti and hallock 2003). accordingly, oligotrophic to slightly mesotrophic conditions are deemed for all studied sections. 8. conclusion the qom formation corals in sanandaj–sirjan fore-arc basin (bozdan, tavakolabad, and gonarestan stratigraphic sections) were studied in terms of paleoecology. the following results were obtained: 1. benthic foraminifera, corals, and red algae are the most significant biotic constituents of the studied sections. 2. the species recognized from the studied sections are leptoria sp., porites sp., hydnophora sp., hydnophora pulchra, hydnophora cf. pulchra, stylocoenia taurinensis, thegioastraea sp., caulastraea sp., caulastraea farsis, heliastraea sp., heliastraea (athecastraea) stellata, heliastraea sp., tarbellastraea reussiana, favites sp., favites neglecta, ceratotrochus (conotrochus) sp., goniopora sp., astreopora meneghiniana, plesiastraea sp., platycoenia sp., acropora sp., astreopora stellaris. 3. based on the diversity of zooxanthellate corals genera together with other components, the studied sections have been deposited under the tropical to subtropical temperature range. 4. according to the morphology of corals and considering the fabrics, high to moderate water energy can be expected in the studied sections. 5. euphotic to slightly mesophotic conditions have been dominated in the studied sections. 6. based on coral morphotypes as well as the presence of bioerosion, nutrient conditions are considered oligotrophic to slightly mesotrophic. acknowledgments the authors would like to acknowledge the financial support of institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran, under grant number of 99/580. we would like to express our thanks to dr. hamed ameri, and dr. hassan rameh for their assistance in the field. declaration of competing interest the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. data availability statement the authors declare that data archiving is not mandated but data will be made available on reasonable request. references adams, g. c., lee, d. e., rosen, b. r. 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_heading=h.1fob9te in this study, we re-examined the official hurricane database from the national hurricane center (hurdatnhc), an agency associated with noaa, for tropical cyclone activity from 1851 to 2012for the dominican republic on the island of hispaniola in the caribbean basin. we performed analyses at two different levels for the island (i.e., all of the storm tracks in the caribbean basin near to the study area that made landfall and all of the events that crossed the dominican republic from a radius of 300 km from the coastline). this study includes the statistical occurrence of these phenomena during the study period and the climatological analysis of all tropical cyclone tracks (112 total events) by decadal seasonal distribution, fifty-year seasonal distribution and monthly seasonal distribution to show the lowest and highest activities. we performed wavelet analysis on the continuous data over a long time series to determine the important frequencies. this analysis provided a general statistical conclusion resulting from the data collected. a landfall probability for the study area corresponding to the long time series of (it’s 162) years within a radius of ~100, ~185 and ~300 km, based on the historical climatology tropical cyclone tracks, reveals the likelihood of a strike for a major or a minor hurricane. we present a review of the tropical cyclone activities that passed the dominican republic, which also forms part of the author’s dissertation. abstract keywords: tropical storms, statistical analysis of tcs, dominican republic hurricanes, caribbean sea hurricanes. quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic cuantificación de la distribución estadística de la actividad de ciclones tropicales según el registro histórico para la república dominicana issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v21n4.63425 earth sciences research journal earth sci. res. j. vol. 21, no. 4(december, 2017): 159 167 santo trinidad alvarez ysabel1, gustavo adolfo agredo cardona2, david felipe rincon3 1 dr. sc. candidate at dehma, upc-barcelonatech santo.alvarez.ysabel@gmail.com 2 school of sustainability and urbnism, faculty of engineering and architecture, unal-manizales, colombia 3civil engineering department, unal-manizales, colombia para el presente estudio se ha tomado la base de datos oficial para huracanes del centro nacional de huracanes (hurdat-nhc), dependencia de la noaa, correspondiente al período de 1851-2012 relativos a ciclónicas tropicales para la república dominicana (isla hispaniola) en la cuenca del caribe. el análisis ha sido realizado de dos formas diferentes considerando la geografía de la isla (todos los puntos de posicionamiento de los eventos en la cuenca del caribe que han impactado el área de estudio al tocar tierra –impactado-y todos los eventos que han atravesado la república dominicana en un radio de ~300 km. se presenta un estudio estadístico de la ocurrencia de estos fenómenos durante el citado período, incluyendo un análisis climatológico de todos los ciclones tropicales que se han registrado (112 eventos en total) en distribuciones decanales, cincuentenarios y mensuales, indicando la de menor y mayor actividad. por igual se ha ejecutado un análisis de ondeletas sobre los datos continuos de la serie de tiempo para determinar importantes frecuencias. finalmente mostramos una probabilidad de ocurrencia de que uno de estos fenómenos toque tierra basado en una serie de 162 años para el área de estudio, dentro de un radio de ~100, ~185 y ~300 km, teniendo en cuenta la climatología de huracanes la cual revela la probabilidad de ser impactado tanto por un evento de menor o mayor categoría. el autor muestra una perspectiva de la actividad ciclónica para el área de la república dominicana la cual forma parte de su tesis doctoral. resumen palabras clave: tormentas tropicales, análisis estadístico de los cts, huracanes en república dominicana, huracanes en el mar caribe. record manuscript received: 22/03/2017 accepted for publication: 25/08/2017 how to cite item alvarez ysabel, s.t., agredo cardonag.a. &rincon, d.f. (2017).quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic. earth sciences research journal, 21(4). 159 167 doi:http://dx.doi.org/10.15446/esrj.v21n4.63425 h y d r o m e t e o r o l o g y 160 santo trinidad alvarez ysabel, gustavo adolfo agredo cardona, david felipe rincon 3. data and methodology 3.1. the hurdat2 historical record database historical records of hurricanes for the dr started at the beginning of the sixteenth century, shortly after the foundation of santo domingo´svillages, which had been moved and reconstructed on the west bank of the ozamariver after they weredestroyed by a hurricane figure 1. map of the study area. 1. introduction tropical cyclones are alsoknown as hurricanes or typhoons and are among the most deadlynatural disastersonthe earth. the caribbean countries are highlyvulnerable to these natural hazards. the latent vulnerability of the area exacerbates the impact of these hazards, which become natural disasters inthis geographic region andaffect the economic, social and environmental conditions. the north east atlantic (nea) and the caribbean sea basin (csb) are characterized as a high probability zone for the formation, development and impact of tropical cyclones (tc), according to the national ocean and atmospheric administration (noaa). millions of people are affected, and economic losses ofbillions of us dollarsare estimatedto occur in the caribbean regionover the past 30 years. certain island states in the region are susceptible to earthquakes (3% of the population), but they are more susceptible totropical cyclones (70% of the population) and floods (27% of the population) (nhc-noaa, 2012; cepal, 2010; gutierrez, m.e. &espinosa, t., 2010; pnud, 2008; unuwider, report paper 2008/61). the hurricane activity in the atlantic increased over the last decade of the xx and beginning of this century (xxi) and had significant consequences, such as heavy rain and theassociated flooding, deaths, damage and destruction ofcoastal and inland areas throughout the north atlantic basin (nab). the csb territories are well known as a tropical cyclone pathway because ofthe surrounding environmental flows associated with the location of the islands. this basin has experienced above normal tc activity, ranging from tropical depressions (td) to major hurricanes (mh) of category-3 to category-5 (h3-h5) on the saffir-simpson scale, as shownin table iii (karmalkar a. v. et al., 2013; ipcc, 2007 and 2012; steve graham and holliriebeek, 2006; pielke et al., 2005). these phenomena are the most lethal and costly natural disasters in the area. major events have occurred in the tropical cyclonecorridor(san zenón (1930), david (1979), georges (1998), jeanne (2004), noel and olga (2007), emily and irene (2011)) are among the hydro-meteorological events that have impacted the dominican territory and producedconsequences detrimental toall areas of society, which isevidence of the urgent need to assess their effects not only by the magnitude of thewinds but alsoby extreme precipitation, which can be properlyconfirmed by the recordings in thegrid-unep database (ofda/cred, 2012; campusano, m., 2009; cocco q., a., 2009; pnud, 2007 and 2008; cepal, 2008;pnud, 2007; unep-grid, 2003). a tc is a storm system (an atmospheric disturbance) that is characterized by a low-pressure center and numerous thunderstorms that produce strong winds and heavy rainfall. tcoriginates over tropical oceans and isdriven principally by heat transfer from the ocean. approximately 80 tropical cyclones develop globally each year over the ocean atsea surface temperatures(sst ≥26ºc) and very oftenmovefrom these regions into higher latitudes (equator-ward of 5º latitude north atlantic basin [nab]). once they are developed andorganized, the storms tend to translatewestward and slightly pole-ward.the commonly used categorized scale referred to in this articleis given in the appendix. the frequency and intensity of hurricanes and tropical storms vary significantly from one year to the next (graham and holliriebeek, 2006; emanuel, k.2003). hurricane climatological analysis can provide a useful tool forstudying the potential damage along the coast or landmass of a country, particularlyin populated areas. damage tends to increase in future years or along river valley floodplains,ifthe heavy rainfall associated with a cyclone passes over at least one of the five principal mountains ranges in the area and is intensified by these orographic systems (gutierrez, m.e. & espinosa, t., 2010; ministry of environment, 2010). hurricane georges in 1998 and the tropical storms noel and olga in2007hadpeakextreme heavy rains of ≥ 500, 330 and316 mm/day, respectively, which accumulated in some mountainous areas of the dominican territories andcaused more than 457 deaths (onamet, 1998; 2007; trmm, 1998; 2007; em-dat: ofda/ cred, 2012). we used an empirical study method to investigate the statisticalclimatology of tcactivityin the dominican republic (dr) territories.the tcsconsidered for this study were bothmajor (h5with awind speed >241 km/h) hurricanes and minor (≤ 63 km/h) tropical storms or depressions (ts/td) that made landfall or approachedwithin ~300 km of the study radius. a short overview of the study area is given to provide a better understanding. the article isorganized as follows:(a) the data and methodological method are presented and are followed by (b) the results and discussion derived from this work and(c) the conclusions. 2. study area the study area is the dominican republic (dr), a country on hispaniola island, which is shared with therepublic of haiti and is both a member ofthe so-called greaterantilles islands andpart of the tropical cyclone caribbean basin, whichforms the nab when the gulf of mexico is included. the dominican republic is located between 17º 36´14 and19º 55´57 north latitude and 68º 19´24 and72º 00´33 west longitude.the country is located on the edge of the tropical zone of the northern hemisphere and is bordered on the north by the atlantic ocean, on the south bythe caribbean sea,on the east by themona passage and on the west by the republic of haiti (fig. 1). the surface area is 48,670.82 square kilometers, with acoastline of1,228 kilometers(cia-the world factbook, 2012; ministry of environment, 2010;fao, 2000). the dominican republic hasa tropical maritime climate with little seasonal temperature and two main rainy seasons presented: the frontal-convective season (november-may) and the cyclonic season (june-october). areas of high precipitation are highly influenced by moisture-laden trade winds over the atlantic ocean that reaches the country from the northeast, producing so-called orographic rainfalls. the terrain consists of rugged highland mountains (five main hilly orographic systems) withinterspersed fertile valleys irrigated by natural,well-spread hydrographical canals and rivers (cia-the world factbook, 2012; ministry of environment, 2010; fao, 2000). 161quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic in 1502. several sources of information (e.g., historians, researchers and universities) indicate where storms and hurricanes have hit hispaniola island, although they have several omissions due to some events that followed the trajectories ofthe storm masses onthe ocean surface (cocco q. a., 2009). however, in the present study, we do not usethehistorical tropical cyclone tracks prior to the 1851 yearly recordsconcerning the pathways or approaches of those events that affectedthe drterritory. this is the case for noaa, who supplemented theirdata collection and with other sources (local or regional) to gather better data and information. ouranalysis is basedon the most recent official worldwide hurdat database, from the national oceanic and atmospheric administration (noaa´s coastal service center), for the atlantic tropical cyclone activitybasin, including the north atlantic basin, the gulf of mexico and the caribbean sea. this database has beenreanalyzed as part of the atlantic hurricane database re-analysis project, as documented bylansea et al. (2008). this study incorporates the reanalyzed data of the long time series from 1851 to 2012 (historicalhurricanes tracks: http://www.csc.noaa.gov/ hurricanes/#) as part of the climatological calculations for the tc activity (updated every year as the “six-hourly best track”(00:00, 06:00, 12:00, 18:00 utc)once the hurricane period from june to novemberhas passed andincludes theyear, month, day, hour, name, latitude, longitude, storm movement speed and direction, wind speed and central pressure, storms by location, name and basin. however, prior to the satellite era (1970), limitations exist for the determination of storm tracks, intensities, and landfall areas (table i, appendix section) the best track data fromthe atlantic hurricane database (hurdat2) can be downloaded in ascii format using the spreadsheet software contained in the office version of the microsoft office suite (ms excel v.2007).the geographical information system (gis) version of the atlantic tracks file database (available at: http://csc-s-maps-q.csc.noaa.gov/ hurricanes/download.jsp) wasused toperform the computational analysis and generatemapsusingthe gis-arcmap software from the environmental systems research institute (esri), a tool that hasmanyspatial calculation capabilities (klotzbach, 2011; chen, k. et al., 2009; landsea, 2004, 2007). more information is available in appendix i. as revised by landsea et al. (2007), the atlantic hurricane database (hurdat)contains estimatesofthe maximum sustained surface wind speed at the conventional altitudeof 10 m elevation. to recompile the historical tracks for all tropical cyclone storms, this analysis considers wind strength speed superior to 37 km/h (td/ds = tropical depression or depression storm) usingwind speed descriptions based on the saffir-simpson hurricane scale (table 2 in the appendix section). the chart is color-coded to show theintensity for each tropical cyclone by category based on the saffir-simpson scale. the graphs obtained as a result of the statistical analysis show classificationsaccording to the same criteria. the statistical computations from 1851 to 2012 forall tropical cyclones consider the multidecadal, fifty-year (pentads), monthly and general seasonal distributions and other statistical parameters (e.g., tendency, probability). this methodology (unless the contrary is indicated) considers aradius of approximately300 km from the landmass (santo domingo as a single point) to the open sea (the atlantic ocean to the north and the caribbean sea to the south), considering the upper radius limit of ~370 km permitted by the interactive webpage. this radius includes the tracks of all cyclones with a major effectscaused by both direct and indirect impacts(e.g., flooding, wind force, storm surge) during the study period for thedrland and coastal-sea territories.it is important to mention thatin some cases, the tropical cyclone tracks wererecorded in adjacent or nearby countries such as cuba, haiti, puerto rico (pr) and the turks & caicos islands. thus, the orientation of the track, the movement speed of the event, the degree of upslope, the intensity at landfall or approach and several otherparameters are consideredforthe open sea limits of the territories of these countries,as previously mentioned (figure 1). 3.2methodology forlandfall probability calculations we used the method proposed by philip klotzbach and william gray et al. (2009) at colorado state university (csu) forthe statistical occurrence analysis toinvestigatethe relationship between the frequency and probability of the events. statistical analysis wasalso used to determine how often a tc will occur near the study area at radii of ~100, ~185, ~300 kmfrom santo domingo as a single pointand the return period in years (t)or the occurrence for 1, 3, 5, 10, 20, 25 and 50 years based on the longer period of tc tracks in the 161-year dr hurricane climatological datafromhurdat2(klotzbach, p. and gray, w. et al., 2009).these distances are justified because events such as ts have produced effects asharmful and severe as hurricanes (the introduction highlights some of these), for which therainfall, not thewind strength, is responsible. philip klotzbach and william gray methodology display the probabilities of a tropical cyclone passing within a certain radius. it allows the probability of a tcs passing within a specified distance of an area in any particular year to be calculated. the methodology facilitates the awareness of the chances of the closest tropical cyclone occurrence that passes or impacts in a particular point in the study area. to approximate the future likelihood of storms, a poisson regression model is used because the analysis of the number of tropical cyclones that made landfall over the last century shows that the landfall frequency closely conformed to this distribution. further descriptions and details related to this topic arepresented in the appendix (section iii). 4. results and discussion the climatological analysis of all of the tropical cyclone tracks involved studying the 112 total landfall or open ocean approach events in the study area. the early historical hurdatrecord has been regularlyrevised (lansea et al., 2004, 2007). we wanted to address questions such as which parametersdetermine the variation in the number of tcs. is the variation due to sst and wind shear? if so, does combination of enso and noadrive it? what is the probability of ahit by a major hurricane? we must keep in mind that tropical cyclones were not formally named before 1950 and are referred to as “unnamed” prior to that date. nevertheless, it was common to name a storm for an important person inthe region based on any relevance the person had during the time of the storm. for example, inthe early xx century, some storms were called thefather ruiz hurricane, lily’s cyclone, saint ciriaco’s hurricane and saint zenon’s hurricane (cocco q., a., 2009). using the data related to the tropical cyclone tracks for the period of 1851 to 2012, we can subdivide the long time series into i) a decadal distribution, ii) a fifty-yeardistribution and iii) a monthly seasonal distribution. in the same vein, (i) a general summary of all tracked events is presented. 4.1. multi-decadal distribution for the first decade in the middle of the 19th century, the occurrence of tropical cyclones was low but the occurrencefor the country was4, which included 2hurricanesof category h1orh2 and 2ts. three made landfall, but the rest remained in the atlantic off the northern coast.tropical cyclone activity was rarer duringthe second decade,from1861 to 1870, when only one storm was recorded. a unique hurricane of category h1 hit the entire island (hispaniola), moving from east to west along the south coast of the caribbean sea. afterit passed thepr island, itendedinthe neighborhood of haiti as a weak tropical storm (ts). greatertc activitywas evidentfrom 1871 to 1880 than during the two preceding decades. the trajectories offive tcbrushedthe island’s south coast: 4ts and 1h1. the decade from 1881 to 1890hadmore frequenttcactivity, with 6 events; a minor h2, amajor h3, and 4tswererecorded.the final decade of the 19th century showeda considerable increase intc activity in the region, with eleven events impacting the study area, whichwas 162 santo trinidad alvarez ysabel, gustavo adolfo agredo cardona, david felipe rincon theperiod with the most events everrecorded in the history of thedr. the events included3minor storms between h2 and h1,2majorh3 events, 3tsand 1td. most of the stormtrajectorieswere east to northwest or east to west and had significant impacts overthe territories of pr and haiti. the first decade ofthe 20th century from 1901-1910 had similar characteristics to the last because ten events were registered: 4h1,1h3,4tsand 1td (figure3a). fivetcsoccurred from 1911 to 1920: 2h1, 1h2 and 2ts. two of these events made landfall in the country onthe easternedge of the island (figure3-b). the decade from 1920-1930 was the first time that all tc types were registered in the study area,ranging from an insignificant tropical storm to an h5 major hurricane that made landfall over the dr,an h4, and an h2that nearly made landfall. these events all hadtrajectories from east and to eastnorthwestonboth coastlines (cbs and nab). the trend during 1931-1940 for both coastlineswasquite opposite to the previous decade. a total of sevenminortropical storms (7-ts)and only one hurricane (1-h1)were observed.by the end of the 1941-1950 decade,six events had occurred: three hurricanes (2 h1 and 1h3) and three tropical storms ofall categories (2ts and 1td). it is important to note that 1950was the first year in which a namewasassigned to astorm, which occurred on august 28 (a weak depression). the nhc began to name each seasonal tropical cyclone in1949. the decadefrom1951 to 1960 showed seven total events; threewereh1, h2 and h3storms, andthe remaining four included3ts and 1td. the decade from 1961 to 1970 showed the highesttc activity in both quantity and intensity in the study area. seven major events were trackedin the historical record;fourwereclassified as major hurricanes, and another as a minor hurricane, with1h5, 2h4, 1h3, 1h1, and there were two minor storms classified as 1ts and 1td. six tropical cyclones occurred during the decade 1971-1980, and one was a major category h5hurricane. the rest were classified as minor stormsof intensity typests (3) and td (2). the most dangerous storm, due to its catastrophic damage all over the dr,was the remarkable hurricane david. david occurred on august31, 1979, which caused that name to be erased from the nhcassignments. this major hurricane reached the maximum intensity on the saffir-simpson scale and had an enormous impact in terms of the consciousness of the effects that a tc can have on a country. thereafter, the government, institutions, agencies and civilians put deep emphasis on warnings, preparedness and protection from the impact of any storms approaching the landmass or surrounding sea area. this event was a hallmark in the dominican republic tc historical record. little stormactivity was recordedfrom 1981 to 1990, with three events recorded, including one major h3hurricane named emily thatimpactedthe entireisland, including both thedr and haitionseptember 23, 1987. the other two tropical events were1tsand 1td (figure 4d). six major events were recordedat the end of the 20thcentury,1h3and 2h1, and three tropical storm events classified as ts (1) and td (2). the major hurricane georges occurred onseptember 22, 1998, andcaused great damage tothe country’seconomyand society, as measured by loss of life (figure 4e). the first decade of the 21stcentury plus two more years (2001-2012) make this period were very busyfor the dr. twenty events were well categorized and tracked from sea coastal services (scs-noaa´s database) and were classified as follows: 1h5, 1h4and 5h1 hurricanes, along with13tsevents that were weak storms. this decadehadthe greatestquantity ofnamed and mean events: 1.78, or approximately2 peryear. although some eventssuch asjeanne, dean, tomas or irene that impacted the countrywere hurricanes,the so-called tropical storms alpha (2005), noel and olga (2007),similar tothe previously mentioned hurricanes david and georges, shocked and shattered the confidence of society due tothe tremendous and devastating environmental damage caused by the extremely heavy rains that occurredover the mountainsandpoured out over the floodplain valleys. we developed a general summary to visualize the entirety of thetc tracks recorded from 1851 to 2012. it was sub-dived by decadeand tropical cyclone category, i.e.,major hurricanes (h5-h3), minor hurricanes (h2h1) and weak storms (ts-td)to allow better visual comparison and comprehension. consideringstatistical decades by cyclone classification, the periods of 1871-1880 and 1901-1910 showedhurricane activity of four (4) minor hurricanes (h1). the periods of 1891-1900 and 1961-1970 showedmajorh3 and h4hurricane activities, with two of each category. the decades of1931 to 1940 and 2001 to 2012 had the highest amount of tropical storm events (ts),withseven (7) and thirteen (13) respectively (figure 2a). figure 2. statistical analysis results for the decadal seasonal distribution of all tcevents from 1851 to 2012. figure 1. map of the study area. 4.2. fifty-year distribution a seasonal fifty-year distribution wasperformed, and the results obtained from the statistical analysis allowa comparisonoftropical cyclone behavior over162 years of activity. the results infigure 2bshow that the highestfifty-year activity wasthe period from 1901-1950,which had 36 total tropical cyclones with 5 major and 12 minor hurricanes. the fifty-year activitydistribution from 1951 to 2000 showed29 total tropical cyclones anda slight increase inmajor hurricane activity,with8events categorized as h5 (2), h4 (2) and h3 (4). the final period considered wasthe ongoing fifty-year activity distribution of at least twelve years and showed 20 tctracks,with seven (7) categorized as both major and minor hurricanes. the tc activity from 1851 to 2012, as summarized by the fifty-year activity distribution,containedall storm types,was characterized bytropical cyclones from h5 to tdand showed that 49 tswerethe most frequent events affecting the dr territories, followed by 27h1storms for the entire 161-year period for which storm tracks were recorded. when the50-year distributionwas consideredand applied toboth types of events,a considerable numberof recorded tracks (10h1 and17ts)areobtained. for the recent period from 2001 to 2012, ahighernumber of ts (13) was recorded in thisshorttime. statistics can indicate the overall amount of activity or theprobability that the dominican republic territories will be impacted directly or indirectlyby a major or minor hurricane. 4.3. monthly seasonaldistribution a distribution of all events in the historical tcsfrom 1851 to 2012 for theeventsduring the 162 years consideredwascalculated on a monthly basis. the hurricane season for this basin runsfrom june to november, and we wantedto determinate which months of the season were more or less active. september wasdetermined to bethe month with the most activity, with 48 total events, followed by august,with 33 events (figure3a). when the saffir-simpsoncategory of the events was consideredby month, september took first place, with 10 major hurricanes (between h5 to h3), 18 minor hurricanes (between h2andh1) and 20weak tropical storms (ts/ td). june hadthe lowestnumber of events,withzero eventsrecorded. july and november hadan equal numberof events(4ts/td).rare cases of nab tropical cyclone activity with tracked events occurred as early as may (2 events) and as late asdecember 3 events(figure 3a). 163quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic based on the climatological historical tropical cyclone tracks recorded from 1851 to 2012 (162 years),we performed a landfall probability analysis for the region in radii of approximately100, 185 and 300 km.the climatology is going to be biased low because of lack of satellite and other technology going back farther in time. it is suggested that the percentages are going to be biased low, because of the under sampling with time. after fixing the measured radii and the total quantity of events tracked for each distance, the previously mentioned csu methodology wasappliedto analyzethe computational statistical data results. the probability values for 1 or more named storms tracking for ~100-, ~185-, and ~300-kmradiiand their associatedoccurrence yearsare shown in table1. figure 3. (a) the seasonal monthly distribution chart and (b) cdf from 1851 to 2012. figure 4. (a) and (b) general historical tropical cyclones events greater than category h1and a statistical analysis of the period, type and area of study. (a) (b) figure3bshows the cumulative distribution-dates of the first tropical cyclone formed that passed through the study area. for the study area it shows that for an approximate 300 km radius, the first cyclones (hurricanes) formedprior tojune 15 with a probability of approximately20 to 25 (%)since 1851. for a half year, september shows the peaks with 60% of thehurricanes occurring. the distribution appears to bebi-modal (i.e., with two peaks), especially for tropical storms (ts), withpercentage values of95% and 100% for july and november respectively. 4.4. general distribution a general trend can be observed for thedata collected for the period of 1851-2012 in this analysis. the majorityof tropical cyclone events tracked duringthis long time wasts (49) and h1hurricanes (27) (figure 4a, b). hurricanes of type h3ledall major types of hurricanes, with 10 total events. the trajectories were in all directions, but moving from east-southeast to west-northwest isthe most frequent direction in this tropical cyclone pathway and could impact any geographical point in the dr territories. table1. storm landfall probabilities based on 1851-2012 tcclimatology for dr (162 years) based on the csu methodology (2009). 164 santo trinidad alvarez ysabel, gustavo adolfo agredo cardona, david felipe rincon figure 5. probabilities of mh occurrences (impact or landfall) directly or indirectly in the dr territories based on the time series from 1851 to 2012. figure 6. wavelet power spectrum of the time series of the total number of tcs over the region from 1851 to 2012 with relative 8-year cycles. the cross-hatched region is the cone of influence with a 10% significance level. the historical series generated a tc record of 162 years in length and the basic statistics and variability were evaluatedfor a total of 112 tcs over the region, witha mean of 0.7 events/yearand a standard deviation of 0.9.the relative 95% confidence interval for the mean is (0.9, 1.4).no landfalls within the radius considered from inner mass of the country were observed in 86 (53%) of the 162 years analized, whereasfour tcs madelandfall in76 years (47%) of the analyzed period. a basic spectral analysis of the time series resultedinthe wavelet power spectrum shown in figure 6 (a,b). this wavelet analysis indicates that throughout the time series,the power associated with these phenomena had a 2to 4-year period.however,most strinkingly,a large peak in power with aperiod between 8 and 16 yearswas evident,whichbegandeveloping towards the beginning of the 20th century. this analysis will give an opportunity to confirm the above conclusions from the literature examining the csb and nab becauseit has beendemonstrated that the sst anomalies in this region havesignificant globalimpacts on the climate (klotzbach, 2011; cited from barnston et al. 1997), thus addressing the variability in climatological tcsinthe study area. first, the tcsthat form in the study area haveat least impacted an area in this basin. second, the seasonal shift in the areas in which thetcsdevelop is quite distinct by month: at the beginning of the season (june and july), development is restricted to the western caribbean because the ssts are still cool; later, by august and september, the development shifts eastward to the central and eastern atlantic as the ssts become warmer and the vertical wind shear over the basin decreases.finally, development returns to the western part of the basin during october and november. the third aspect is the origin of the disturbances thatbecometcs. a large portion of tcs form from easterly waves, which are atmospherictroughs spawned over the african continent underunique set of circumstances. development in the region frequently occurs at latitude of 20º n, which is the only tc basin where this happens. the surrounding variation in the number of tcs is due to the sst and wind shear driven by a combination of enso and noa, which accounts for the nature of the climatological tc variability in the study area (jury et al., 2011; klotzbach, p.j., 2011; c. mcsweeney et al., 2010; christopher c. hennon, 2008). 5. conclusion based on how close the center of a storm came to the island, the so-called closest point of approach for the study area during the period of 1851-2012 (162-year period) of tcs tracks recorded was calculated. as a result, the following conclusions are presented: 1. the statistical analysis method successfully produced time series of the number of tcs that affected the dominican republic territories from the lowest (td/sd) to the highest (between h3-h5) categories (in the official star hurricane seasonal tcs yearly activity). although the drhas been exposed totcs prior to the development of scs/nhc-noaa database, the analysis showed thedecadewith the greatestactivity, the fiftyyearperiod with the greatest activity and a time series showing a monthly distribution for these events over the entire161 years. 2. the csu methodology revealedthe likelihood of an impact for a major or minor hurricane both ata distance and close to the study area. the population in these vulnerable areasmust be aware of and ready for the storms. this topic requires further study. 3. factor as tropical atlantic sst, the size of the atlantic warm pool, and tropical atlantic sst gradients and low-level trade wind strength, play a critical role in affecting the levels of the nab and csb hurricane activity (jury m.r., 2011; klotzbach, p.j., 2011; shieh, o.h. and colucci, s.j., 2010). on amulti-decadal time scale,due tothe rest of the atlantic multidecadal oscillation (amo) and alterations inboth the size of the atlantic warm pool and the phase of the meridional mode, the droscillation plays a significant role in the caribbean tropical cyclone activity (jury m.r., 2011; klotzbach, p.j., 2011). 4. quantifying the tc landfall probabilities in the region is important for both seasonal cyclone forecasting and disaster preparedness. thiswill allow governments, planners and emergency agencies to improve forecasting and preparedness to manage the relative risk for any given event ina tc season in the dr. a graph of these results was plotted (figure 5),andthe corresponding equationfor the trendline was calculated. we can infer thatastrongand statistically significant correlation exists betweenthe number of storms (probability) and time (occurrences) in years for both mh within approximately 300 km and 185 km, with a r2 value of approximately0.973. apotential for more storms exists when tropical cyclones appeardistantfrom the landfall mass or are inland,even in years when the storms are very close, the totalprobability of cyclones can be well below normal.in addition, when cyclones seem far away,at least one or two will pass through the study area during those 20 to 50 years. only one ortwo stormscanspoil a whole season. 165quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic studies by the ipcc (intergovernmentalpanel for climate change) indicated that there is low confidence that any reported long-term (centennial) changes in tropical cyclone characteristics are robust, after accounting for past changes in observing capabilities.the increased atlantic hurricane activity over the last decade of the 20thand the beginning of this century has hadsignificant consequences, such asa rise in sea level, heavy rain and associated flooding, the loss of human life, damage and destruction to infrastructure and ecosystems, and economic losses both in coastal and inland areas along the whole nab (ipcc, 2012 and 2007; bid, 2008; cepal, 2007; steve graham and holliriebeek, 2006).some researchers attribute the recent increase in tc activity to human actions caused by global warming, but others attribute the increased activity to the natural multi-decadal seasonal phenomena that occur in a similar manner to natural earth interaction cycles (geo-physical) activity, neglecting a consideration of the anthropogenicallyinduced rise inglobal ocean surface water temperatures (karmalkar a. v et al., 2013; ipcc, 2012, 2007; steve graham and holliriebeek, 2006; pielke et al., 2005). the csb territories are well-known tropical cyclone pathways becauseof the surrounding environmental flows associated with the location of the island of hispaniola. in general, thisbasin has experienced above normal tc activity from tropical depressions (td) to category-3 to category-5major hurricanes (mh) (h3-h5) on the saffir-simpson scale. although the probability of a major hurricane strike is lower within the approximately300 km-h/ fifty-year period (80% 100%), the probability of minor storms in the category of ts/td force winds is higher (80% within a decadeand approaching 100% for t = 20, 30, 40 and 50 years). although debate tends to concentrate on the reason for the recent increase intc activity to satisfy both the common people and the scientific community,atrulyinteresting taskremainsafter this statistical analysis of tcsfrom 1851-2012forthe dr.the opportunity for the planned development of the territory is a huge challenge.the environmental heterogeneity and human sustainability must be balancedin terms ofachanging climate and the vulnerability of the caribbean states (mcsweeney, m.n. and g. lizcano, 2010; unfccc, 2007 and 2008; pnud, 2008; cepal, 2007). acknowledgment the authors would like to acknowledge manuel gómez valentín, sánchez martí-juny and víctor o. rendón d. for their suggestions. special thanks also go to the reviewers for the helpful comments that contributed to improve the manuscript. references aoml (american oceanographic and meteorological laboratory), 2012. hurricane and tropical meteorology. discharge on: 2012-03-05 available from: http://www.aoml.noaa.gov/divisionobjective/ tropical_meteorology.pdf. http://www.imd.gov.in/section/nhac/ dynamic/faq/faqp.htm bid (banco interamericano de desarrollo), 2008. 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practical guide to wavelet analysis. bull. amer. meteor. soc., 79, 61-78. tropical rainfall measurement mission (trmm), 2012. rainfall totals over the period 26-30 october in the island of hispaniola. 2630/10/2007 (also others by dates). discharge on: 2012-01-11. available from: http://trmm.gsfc.nasa.gov/publications_dir/ extreme_events_2007.html united nations frameworks convention on climate (unfccc), 2007 and 2008. climate change, small island developing states. isbn 92–9219–012–1. , 2005. united nations frameworks convention on climate (unfccc), 2010. manual sobre evaluaciones de vulnerabilidad y adaptación, 2007. regional miaac/conferences/die 3 (12-agosto-2010)/presentation wetlands international/unfccc v-a. consulted: 2012-01-11. available from: http://www.pnuma.org/agua-miaac/curso united nation environment programmed (unep grid-geneva), 2010. global risk platform. country profile: dominican republic. 2010. consulted: 2012-01-11. available from: http unep (united nation environment programmed), 2008. climate change in the caribbean and the challenge of adaptation. unep-regional office for latin america and the caribbean, panama city, panama. printed in panama city, in october 2008 unu-wider. natural disasters and remittances. department of economics, university of the west indies, st. augustine. research paper no. 2008/61. isbn 978-92-9230-111-8. unu-wider. 2008. unu-wider, report paper 2008/61. natural disasters and remittances. exploring the linkages between poverty, gender, and disaster vulnerability in caribbean sids. helsinki, finland, 2008. isbn 978-92-9230-111-8 appendix table ii. list of tc categories within 160 nautical miles of the dr extracted fromhurdat-2-nhc database hurdat program: data collection and compilation http://www.aoml.noaa.gov/hrd/hurdat/hurdat2_1851_2012-jun2013.html tropical cyclones affecting the dominican territories for the mayor period of 1970 to 2012 167quantifying the statistical distribution of tropical cyclone activity in the historical record of the dominican republic satellite data era appendix i. the central america-caribbean landfall probability calculations methodology a methodology to display the probabilities of a tropical cyclone passing within a radius of approximately 100 and 185 kmfromvarious islands and landmasses in central america and the caribbean was developedat colorado state university (csu) and other institutions by the respective main and secondary authors.it allows the probability of a tcs passing within a specified distance of anarea in any particular year to be calculated. this method has been modified and adapted for the specificstudy area in this paper for distance (kilometers), the probability of landfall (t= 1, 3, 5, 10, 15, 20, 25 and 50years) occurrence and for an extended major period of time series for the years that the tcwere tracked(1851 to 2012). the data were taken from nhc-noaa webpage and the hurdat2 database, exported into the microsoft excel format and recalculated. the methodology facilitates the awareness of the chances of the closest tropical cyclone occurrence that passesor impactsin a particular point inthis area. figure 4a displays the tracks of all storms named thatpassed within anapproximate300-km radius,andappendix idisplays the maximum intensity of each storm by distance for the drfrom 1851 to 2012 but usesthe same criteria for the other distancesconsidered in this paper (~185 km). to approximate the future likelihood of storms, a poisson regression model was used becausethe analysis of the number of tropical cyclones that made landfall over the last century shows that the landfall frequency closely conformedto a poisson distribution, the formula for which is ep = (e-p) (px) / x! (1) where ep = expected probability p = annual average number of tcsoccurred in the past 161 years x = number of storms expected in the upcoming year based on the poisson formula x! = factorial e = 2.71828 for the probabilitycalculation,the fifty-year (50) probabilities of a major hurricane(mh) tracking within each km of distance fromthe country territories (inland or open-sea mass) wasprovided because most buildings were erected to last at least 50 years and because the construction criteria forthe cost of hurricane-protecting building materials should be based on alonger period. nevertheless, a descending and minor probability wasappliedtoobtain abetter viewpoint regarding this matter. for further information, see the citations or visit the website http://www.e-transit.org/ hurricane/welcome.html issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.112634 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 139 150 g eo ph y si c s geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa universidad nacional de colombia, departamento de geociencias, bogotá, d.c., colombia. ohernandezp@unal.edu.co; patapuma@unal.edu.co, lhochoag@unal.edu.co record manuscript received: 24/01/2024 accepted for publication: 30/07/2024 abstract the main objective of this research was to develop a geological and geophysical modeling to infer the geometry and thickness in the sedimentary sequence of the middle basin of the bogotá river with emphasis on the guadalupe group, reconstructing the stratigraphic sequence, structural setting, hydrogeological modeling, and potential geothermal uses. various geophysical methods were applied, including vertical electrical soundings (ves), and magneto telluric soundings (mt-ves), whose results were complemented with the interpretation of seismic lines, regional and local gravity anomaly maps, and borehole data, among others, which allowed to model the subsurface from the surface to depths beyond 3000 meters, with emphasis on the interval between 500m to 1000m depth. integrated models were developed from interpretations of electrical resistivity data, gravity and magnetic data, reflection seismic and borehole data. based on the results, potential deep aquifers have been proposed to be confirmed by drilling. these deep aquifers can contribute to satisfy the need for water, which historically has been explored and overexploited from shallower aquifers in this sector of the basin. a recommendation was also made to consider the potential of low enthalpy thermal energy for agro-industrial purposes associated with groundwater’s temperature at the depth of this research. as a result, the modeled sedimentary sequence is characterized by a thick quaternary overburden overlying an intensively folded and faulted neogene, paleogene, and upper cretaceous formations, mainly composed of siltstone, sandstones, and shale sequences of fluvial and marine environment, including facies of marine regressions and transgressions near the coastline. the penetration obtained allows establishing a high hydrogeological potential in the first 2000m depth, especially associated with the guadalupe group, where the labor and tierna and arenisca dura formations have the highest hydrogeological potential. in addition, the preliminary estimation of thermal gradients suggest that low enthalpy geothermal energy potential is feasible to be used for the agro-industrial demand of energy of the study area. keywords: magnetotelluric soundings; deep aquifers; low enthalpy geothermal energy modelo geofísico de la cuenca media del río bogotá con apoyo de la interpretación de datos de resistividad eléctrica, gravedad, sísmica y sondeos para evaluar potenciales acuíferos profundos resumen el objetivo principal de este proyecto ha sido desarrollar un modelo geológico y geofísico para inferir la geometría y espesor en la secuencia sedimentaria de la cuenca media del río bogotá con énfasis en el grupo guadalupe, reconstruyendo la secuencia estratigráfica, el estilo estructural, el modelo geológico conceptual y las posibles aplicaciones geotérmicas. en el estudio se aplicaron diversos métodos geofísicos, entre ellos sondeos eléctricos verticales (ves) y sondeos magnetotelúricos (mt-ves), cuyos resultados se complementaron con la interpretación de líneas sísmicas, mapas de anomalías gravimétricas regionales y locales, y datos de perforaciones, entre otros, que permitieron modelar el subsuelo desde la superficie hasta profundidades superiores a los 3000 metros, con énfasis en el intervalo entre 500m y 1000m de profundidad. se desarrollaron modelos integrados a partir de interpretaciones de datos de resistividad eléctrica, datos de gravimetría y magnetometría, sísmica de reflexión y datos de pozos. como resultado, se ha propuesto la presencia de posibles acuíferos profundos a ser validados mediante perforación. estos acuíferos profundos pueden contribuir a satisfacer la necesidad de agua, que históricamente ha sido explorada y sobreexplotada a partir de acuíferos someros en este sector de la cuenca. también se recomienda considerar el potencial de la energía térmica de baja entalpía para fines agroindustriales asociados a la temperatura del agua subterránea a la profundidad del subsuelo de esta investigación. como resultado, la secuencia sedimentaria modelada se caracteriza por una gruesa cobertura cuaternaria que se superpone a formaciones del neógeno, paleógeno y cretácico superior intensamente plegadas y falladas, compuestas principalmente por secuencias de limolitas, areniscas y lutitas de ambiente fluvial y marino, incluyendo facies de regresiones y transgresiones marinas cerca de la línea de costa. la penetración obtenida permite establecer un alto potencial hidrogeológico en los primeros 2000m de profundidad, especialmente asociado al grupo guadalupe, donde las formaciones labor y tierna y arenisca dura tienen el mayor potencial hidrogeológico. además, la estimación preliminar de gradientes térmicos sugiere que el potencial de energía geotérmica de baja entalpía es factible de ser utilizado para la demanda agroindustrial de energía del área de estudio. palabras clave: sondeos magnetotelúricos; acuíferos profundos; geotermia de baja entalpía; how to cite item: hernández pardo, o., atapuma acevedo, p. a., & ochoa, l. h. (2024). geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers. earth sciences research journal, 28(2), 139-150. https://doi.org/10.15446/esrj.v28n2.112634 https://doi.org/10.15446/esrj.v28n2.112634 mailto:ohernandezp@unal.edu.co mailto:patapuma@unal.edu.co mailto:lhochoag@unal.edu.co https://doi.org/10.15446/esrj.v28n2.112634 140 orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa 1. introduction the purpose of this research is to model the geometry and thickness of the middle bogota river basin with emphasis on identifying potential deep aquifer formations, related to the guadalupe group, from the interpretation of electrical resistivity, gravity, seismic reflection and borehole data. an emphasis of modeling the top and base of the guadalupe group within the sedimentary sequence has guided this research. additionally, estimates of thermal gradients of interest for low-enthalpy geothermal energy in agro-industrial uses are presented. the methodology applied in this project included data collection of different entities as the servicio geológico colombiano (sgc), corporación autónoma regional de cundinamarca y boyacá (car) and private companies. once the data was compiled and standardized, a complementary geophysical data acquisition was designed and implemented. the applied geophysical methods included vertical electrical sounding (ves) and mt-ves (magneto telluric ves) modeling the first 3000m depth, complemented with the interpretation of seismic lines and gravity anomalies. a network of ves and mt sounding was acquired, processed, and interpreted to generate 1d, 2d and 3d conceptual geologic models. finally, conclusions and recommendations are presented. nowadays, the importance of preserving water sources has become of supreme importance for all human beings, especially in those places in the world where water is very scarce or unavailable, which means that a large part of the population does not have easy access to this resource, either for daily consumption or any other type of activities such as agriculture (united nations, 2023). freshwater is fundamental as this resource corresponds to approximately 3% of the total water of planet earth (aqua foundation, 2024). based on the above, the groundwater resource has increased its important value since it can be contributed to meet the water needs required by society (usgs – el agua del mundo, 2021). although groundwater has been exploited since 4000 years b.c. and in colombia for more than 100 years, water supply is still insufficient in many areas of the country, leaving more than three million people without this resource (minambiente, 2023). in colombia, the largest population, with more than ten million people, is in the sabana de bogota, including the capital of the country, in the middle bogota river basin; additionally, this area has the highest concentration of industrial and agriculture zones. consequently, the water resource is insufficient to cover the demand. this research contributes to model the geological, structural, and hydrogeological aspects, of potential deep aquifers in the first 3000m depth, that at present are still poorly understood (ideam, 2022). geographic location of the study area: the study area is located in the axial flat zone of the cordillera oriental of the colombian andes range, more specifically in the sabana de bogotá, covering an area of approximately 615 km2. this area has excellent access roads due to the proximity to the city of bogotá, capital of the country (figure 1). besides, it has a thick quaternary overburden that offers good flat topographic conditions to carry out electrical, mt, seismic and gravity surveys. geological setting the geologic map of the study area is presented in the sheet 227, la mesa at 1:100.000 scale (ingeominas, 2001; acosta et al., 2001; montoya and reyes, 2003; ingeominas, 2010; corredor and terraza, 2015, van der hammen, 2003). cretaceous, paleogene, neogene and quaternary lithological units are outcropping in the study area, with the quaternary (recent) units being the predominant overburden unit limiting the exposure of deeper formations (figure 2). following the nomenclature proposed by renzoni (1962), these units are named in chronological order, from the oldest to the youngest, as follows: chipaque formation, guadalupe group, consisting of the arenisca dura, plaeners and labor-tierna formations, whose definition is found in the work of pérez & salazar (1971), and the youngest units, of the paleogene and neogene periods, which include the guaduas, cacho, bogotá, regadera, subachoque, tunjuelito river and sabana formations (table 1, figure 3). structural geology tectonic activity was active in the miocene, with faulting, folding, and the uplift of the cordillera oriental, where the basin of the present-day sabana de bogotá was formed (figure 4). subsequently, the basin was filled with the subachoque, tunjuelito river and sabana formations and at the same time glaciation events occurred that generated the deposits of the siecha and chisacá formations (velandia and bermoudes, 2002; ingeominas, 2005; consorcio magneto 2018, 2018; consorcio alto suarez, 2018). figure 1. location map of the study area, including the municipalities of cota, el rosal, funza, madrid, mosquera, subachoque, tenjo, in the middle rio bogota basin, sabana de bogotá, cordillera oriental, colombia. the location of ves and ves-mt is also indicated. within the area of the bogotá river basin, two structural styles can be established: the first, located on the eastern flank of the cordillera oriental, to the east of the checua syncline, with thrust faults verging to the east, and the others of lesser importance behaving as back thrust with vergence to the west. the second structural style is presented to the west, it is characterized by thrust faults with vergences to the west as imbricated systems that arise and are controlled by northwest-direction faults that serve as lateral ramps. in addition to the regional structures, there are zones with salt diapirism located in the core of the anticlines. diapirism is a generator of complex structures such as those observed in zipaquirá, nemocón and possibly between the sector of sesquilé and la calera, located north and east of the study area. hydrogeology the bogotá river is the most important river in the area with its main tributaries such as the bojacá, juan amarillo, subachoque rivers, which are in the province of the cordillera oriental, in the middle bogotá river basin and in the sabana de bogotá aquifer system (alarcón, 1998; car, 2006; car, 2016a; car, 2016b; geocing sas, 2012; mejía and ramírez, 2016; ideam, 2022; suarez and carreño, 1982). the climate of the area can vary with temperatures from 6°c to 20°c depending on the time of year and the different climatic conditions. in addition, these climate variations are associated with the terrain elevation which ranges from 600m to 3150 m above the sea level. with these climatic conditions as well as the rough topography of the surrounds due 141geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers to the geological structures and lithological units, favorable aquifer formations have been explored and exploited, especially the unconsolidated gravel and sands quaternary deposits, sandstones of the cacho, sandstones of la guia and lajosa members of the predominantly claystone guaduas formations, and the arenisca de labor tierna and dura formations of the guadalupe group (table 2). complementary, a low-enthalpy geothermal preliminary estimate was obtained looking for changes in temperatures between 15°c to 19°c and depths from 15m to 150m (ortiz, 2020) that may be a potential source of low enthalpy geothermal energy for the main economic activities of the area, such as agriculture (floriculture) and livestock. figure 2. geological map modified from ingeominas (2001) showing the generalized stratigraphic sequence. the area is mainly covered by quaternary deposits in the flat area. cretaceous, paleogene and neogene formation are outcropping in the mountainous areas. 142 orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa figure 3. distribution of outcrops and thickness of the arenisca dura and lidita superior formations, sabana de bogotá. by this integrated geophysical survey, these units are being modeled and projected at depth below the overlying younger stratigraphic units. the outcrops correspond to the recharging areas by infiltration of meteoric waters that flow to the potential aquifer formations at the subsurface. modified from montoya & reyes, ingeominas, 2005. figure 4. structural map of the sabana de bogotá, upper and middle bogotá river basin (modified from montoya & reyes, ingeominas, 2005). this region has been intensively folded and faulted, there are several exposed and hidden anticline and syncline structures-oriented ne-sw. table 1. diagram of chrono-equivalent lithostratigraphic units of the sabana de bogotá, cordillera oriental (montoya & reyes, ingeominas 2005). the stratigraphic units proposed by montoya & reyes (at the right) are adopted in this study. chrono-equivalent lithostratigraphic units of the sabana de bogotá, cordillera oriental (montoya & reyes, ingeominas 2005) age formation hydrogeology unit resistivity pliocene tilata acuifer 12 1010 ohm-m miocene oligocene eocene regadera aquitard 70 110 ohm-m bogotá acuifer 8 45 ohm-m paleocene cacho acuifer 52 401 ohm-m guaduas aquitard 15-20 ohm-m matrichiano labor-tierna acuifer 20-250 ohm-m upper campanian plaeners aquitard 10-20 ohm-m lower campanian dura acuifer 20-250 ohm-m santonian chipaque aquiclude 15-20 ohm-m coniacian turonian cenomunian une acuifer 20-250 ohm-m 143geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers table 2.generalized hydraulic parameters of hydrostratigraphic units. (source: bermoudez & quiroz, 2002). the labor and tierna and arenisca dura formations show excellent flow, transmissivity, hydraulic conductivity and specify capacity values in the guadalupe aquifer complex. formation predominant lithology well depth (m) flow (l/sec) transmissivity (m2/day) hydraulic conductivity (m/day) specific capacity (l/sec/m) a rt es ia n b a si n c o v er a g e aquifer complex of the neogene quaternary unconsolidated deposits alluvial deposits (qal) sands, silts and gravels. < 30 0.1-2 <0.1 high terrace (qta) clays. silt, sands and gravels. 30-300 1-8 5-100 0.1-5 0.1-0.5 tilata (ngqt) gravels with sandy matrix, quartz sands, silts, clay and peat. 200-600 10-50 150-900 <9 0.5-3 relatively impermeable paleogene complex usme (pgu) siltstones and claystones with intercalations of sandstones of different sizes. < 100 <1 <3 regadera (pgr) quartz sandstones of different sizes, with clayey matrix intercalated with claystones and siltstones. < 100 <1 <3 bogotá (pgb) claystones with intercalations of sandstones in the lower part. 30-150 0.3-1.5 <3 <0.03 a rt es ia n b a si n c o v er a g e cacho (pgc) medium-grained to conglomeratic quartz sandstones with thin layers of argillites 45-160 0.2-6 9-500 0.3-2 0.1-0.6 guaduas (kpgg) claystones with sandstone strata and coal seams. 50-150 0.3-2 <5 <0.03 a rt es ia n b a si n f lo o r guadalupe aquifer complex labor and tierna (kit) fineto coarse-grained sandstones interbedded with claystones and siltstones. 60-600 2-60 15-600 0.3-2 0.1-6 plaeners (kp) claystones, siltstones and lydites. 100-600 1-4 5-10 0.1-0.3 20.1 arenisca dura (kad) fine quartz sandstones with siliceous cementation, interbedded with siltstones and claystones. 100-600 4-30 5-350 0.5-21 0.1-2 geophysical exploration the bogotá river basin has been the target of various geological and geophysical surveys at regional and local scales. the greatest lack of knowledge of the aquifer formations of the basin is related to the geometric and volumetric characteristics of the units and structures that may favor the infiltration, storage, and exploitation of groundwater. these parameters have been partially defined in isolated sectors in exploitation, without allowing an extrapolation and lateral and vertical projection of the potential aquifer formations to other sectors of the basin. in this study, the geophysical exploration makes its greatest contribution to advance in the understanding of the modeled lithologies and geological structures of hydrogeological interest. an integrated geophysical survey was conducted to determine the thickness, top and base of the geological formations in the middle bogotá river basin. it included the compilation, design, acquisition, processing, and interpretation of several ves vertical electrical soundings, and magnetotelluric (mt-ves) soundings in order to model the top and base of aquifer geological formations, especially the sandstone formations of the guadalupe group. the gravity method makes it possible to identify lateral variations in the specific density distribution of subsurface formations by systematically measuring the gravitational field. in the context of the middle bogotá river basin, regional gravity studies have revealed negative gravity anomalies associated with low-density infill materials in the basin, compared to the denser materials presented in the basal and lateral formations (consorcio magneto 2018, 2018). gravity data compiled from the seismic microzoning of santa fe de bogotá (ingeominas et al., 1997), consist of 370 gravity measurements were processed to obtain a complete bouguer anomaly map and its corresponding residual anomalies that to estimates the geometric and depth of the local sedimentary basin (figure 5). 144 orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa these data were used to model the interface between unconsolidated and consolidated lithological units, which allowed estimating the infill thickness of the sabana formation. according to the results, this formation has a thickness of 500 meters in the hills of cota and suba, and 240 meters between the hills of suba and the eastern hills of bogotá. in addition, 520 meters of sediment were measured through exploratory drilling in suba (bilbao) conducted by the secretaría de ambiente del distrito (car, 2024). consequently, the average thicknesses modeled is close to the real estimates measured in the boreholes with an error of 5% (consorcio magneto 2018, 2018). gravity data is crucial for identifying regional trends in the basin. this information is valuable in guiding the selection of sites for conducting vertical electrical soundings and magnetotelluric soundings. ves are more suitable for modelling the first 300m of the basin where the lagoon deposits are thin, while magneto telluric soundings are more appropriated for modeling the first 3000m of fluvial and marine thicker deposits (consorcio magneto 2018, 2018). this makes it possible to differentiate between those wells to be drilled in areas with positive gravity anomalies which may require shallower depth from those wells located in areas with negative gravity anomalies, where the aquifer formations may be deeper, below the deposits of the sabana formation. figure 5. geological section and its corresponding bouguer gravity anomaly profile, where the formations of the area are presented. the profile was reconstructed from the municipality of funza (nw) to the ingeominas headquarters in the city of bogotá (se; modified from ingeominas et al., 1997). the quaternary coverage is thicker from east (right) to west (left). the guadalupe group is outcropping in the eastern hills (right) and is deepening to the west. the stratigraphic sequence is intensively folded and faulted. the minimum bouguer gravity anomalies are in the vicinity of funza, showing the thicker sequence mainly of unconsolidated lagoon deposits overlying the cretaceous, paleogene and neogene lithological units. there is an increasing bouguer anomaly values to the east, where the paleogene and neogene lithological units are outcropping. electrical resistivity data modeling geophysical exploration methods are applied to obtain indirect and rapid information from the subsurface, which can also be achieved through drilling at a higher cost, thus, geoelectric prospecting allows to reduce and recommend the number and location of exploratory wells and infer the thickness of subsurface lithological units of hydrogeological interest. the electrical resistivity method is based on the study of the variations in the electrical resistivity / conductivity of different types of materials of the underground to oppose the flow of electric current in the presence of an electric field. through the contrasts of the resistivity values obtained, it is possible to differentiate permeable geoelectric layers such as gravels, sands, sandstones and conglomerates from impermeable layers such as clays, silts, claystones and siltstones (keary and brooks, 1991; koefoed, 1979; lowrie, 2007; o´neil, 1975; telford et al., 1990). the results of the geoelectrical interpretation takes into account the schlumberger’s algorithm, assigning the following resistivity values to the lithological units in the middle rio bogotá basin (bovachev, 2003; gosh, 1970; velosa, 2013; table 3). 145geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers table 3. electrical resistivity values and their corresponding lithological interpretation of the sabana de bogota basin (modified from velosa, 2013). resistivity (ohm-m) lithology observations 5 to 20 clays potential seal 20 to 45 sandy clays potential seal 45 to 80 claydy sands potential aquifer 80 to 180 saturated sandstones potential aquifer 180 to 1000 saturated sandstones and conglomerates potential aquifer greater than 1000 low saturated sandstones and conglomerates potential aquitard a 2d cross section of figure 6 shows the distribution of the electrical resistivity values obtained in the ves’s, showing vertical and lateral variations, as follows: in the first meters the values reach resistivities of 770 ohm-m, in the central part there are variations between 5.1 and 10.5 ohm-m and in the deepest part up to 242 ohm-m. stratigraphic correlation: the values mentioned above and others displayed in the profiles are correlated, from top to base, as follows: 770, 184 and 546 ohm-m, corresponds to dry soil, the values between 95.7 and 170 ohm-m correspond to unconsolidated deposits (qal and q2t) consisting mainly of sands with a varying thickness from 5m and 73m. 5.1 to 12 ohm-m, correspond to claystones, which are related to the bogotá (e1b) and guaduas (k2e1g) formations. its approximate apparent thickness varies between 60 m and 326 m. 54.6 ohm-m. correspond to sandstones of the la guia and lajosa members of the guaduas formation (k2e1g). 242 ohm-m, correspond to a layer of conglomeratic sandstone of the guadalupe group (ksg), with an apparent thickness of approximately 200 m. 89.5 and 145 ohm.m correspond to sandstones of the guadalupe group (ksg). structurally, the synclinal fold is a hidden structure covered by unconsolidated deposits. in addition, since there is no certainty in the contact between the bogotá formation (e1b) and the guaduas formation (k2e1g), it is not possible to have a clear visualization of the fold. from the hydrogeological point of view, in this sector that comprises a flat part in the municipality of el rosal, the sands of the quaternary deposits (qal and q2t) are classified as open aquifers with surficial recharge. the sandstones of la guia and lajosa members of the guaduas formation (k2e1g) are confined aquifer with claystones at the bottom and top of the sandstone members. figure 6. 2d modeling of the subachoque syncline, interpreted from ves electrical resistivity data. the guadalupe group is located at the lower modeled stratigraphic sequence below the guaduas and quaternary deposits. 3d modeling of the subachoque syncline, interpreted from ves electrical resistivity data conceptual geological model of the subachoque syncline, located in the municipality of el rosal, with a sw-ne orientation. all vertical electrical boreholes (vess) obtained in the area are incorporated. the diagram depicts variations in the lithological composition and path of groundwater streams. (consorcio magneto 2018, 2018). 146 orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa the conglomerate and sandstone level of the guadalupe group (ksg) behaves as a deeper confined aquifer in this synclinal structure. the conceptual geological model of the subachoque syncline, in the municipality of el rosal and puente de piedra shows a sw to ne direction and covering an area of 5400 m wide by 7400 m long. the conceptual geological model shows the western flank of the subachoque syncline, where the layers are dipping toward the center of the basin in a nw direction. the outcropping units present in the mountainous zone correspond to the arenisca dura formation, of the guadalupe group, this sector being a porous permeable unit recharged by precipitation and infiltration in contact with meteoric waters. the labor-tierna formation, of the guadalupe group, is recharged by infiltration, although it is not exposed at the surface, it is underlying saturated permeable quaternary deposits. the quaternary units behave as open (free) aquifers with surficial recharge due to the favorable porosity permeability of their lithology (sands, silty sands, and gravels). the model clearly shows the lithological and structural distribution of the subachoque syncline, where the thickest sequence of the units is in the central (axial) area of the basin. magnetotelluric data modeling the magnetotelluric method is a natural-source electro-magnetic (em) method that operates in the frequency domain: it is based on the measurement of the electric field in the n-s and e-w directions, the three components of the natural magnetic field are also measured, these fields flow in the subsurface at different frequencies (depths), once the measurements of both fields are obtained, the electrical resistivity of the subsurface can be determined (keary and brooks, 1991; lowrie, 2007; simpson, 2005; telford et al., 1990). the depth of these fields depends on the frequency. lower frequencies can reach greater depths and vice versa. the mt profile 1 has a total length of 14 km, with a general sw-ne direction. this profile includes the mt 80, mt 70, mt 81, mt 66, and mt 82 mt-ves soundings (figure 7). the profile is in the municipality of subachoque. geologically, it is located on a quaternary alluvial deposit, parallel to the hinge of a subachoque synclinal on one of its flanks. the dominant geoelectric unit in the profile corresponds to the u2b () geoelectric unit (consorcio magneto 2018, 2018), interbedded and fractured saturated siltstones and sandstones with resistivity values between 4 and 30 ohm-m, the thickness of this unit reaches 1000 m depth. the continuity of this unit is related to a tabular body of resistivities between 80 and 130 ohm-m and extending with a thickness of 100 m between km 4 and km 7, as well as by a small lens of no more than 400 m and 2 km of extension near the surface between km 12 and km 14. table 4. description of the geoelectrical units used to perform the geological interpretations with respect to the measured electrical resistivities (consorcio magneto 2018, 2018). geoelectrical classification of mt resistivity values geoelectric unit electric resistivity (ohm.m) geological association u1 variable intercalation of silt sands and clays forming fluvial plains and terraces u2a 5-10 intercalation of fractured claystones, siltstones, mudstones and mudstones with presence of saturation. u2b 10-50 intercalations of fractured siltstones and sandstones with presence of saturation. u2c 50-200 fractured sandstones u3 1-5 shear zone given the parallelism of the profile with the subachoque syncline axis, the apparent pitch of the units is smooth, so the resistivity contrasts are associated with lateral facies change typical of the geological environment of the site, as well as the possible presence of geological structures that move and delimit structural blocks. the presence of the bogotá formation is inferred, the changes in the sequence are based on the known thickness of the sequence and slight resistivity contrasts that could indicate a change in the lithological conditions. figure 7. 2d modeling of the subachoque syncline, interpreted from mt electrical resistivity data. profile 1d. the guadalupe group is below the guaduas formation and quaternary deposits at 500m to 700m depth. 147geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers the mt profile 2 (figure 8) has a total length of around 15 km, extending with a general ne-sw direction, including the mt 73, mt 111, mt 105, mt 107 and mt 58 soundings, the profile is located in the municipalities of facatativa, madrid and funza. geologically, it is located on quaternary alluvial deposits that are perpendicular to a series of buried syncline and anticline folds. the resistivity contrasts in the profile, as well as the morphology of the contacts, reveal the presence of synclinal and anticlinal structures, apparently the core of the synclines corresponds to conductive geoelectric units (unit u2a; consorcio magneto 2018, 2018), fractured saturated silty claystones and mudstones, with a thickness that can exceed 1400 m and resistivity values below 10 ohm-m, which extend from km 3 to km 6, as well as between km 10 and km 15. the u2c geoelectric unit (consorcio magneto 2018, 2018) is interpreted as fractured saturated sandstones, which are related to the to an anticlinal fold, the resistivity values are between 40 ohm-m and can exceed the barrier of 200 ohm-m in the ne sector. according to the stratigraphic sequence, the arrangement of the geoelectric contrasts and some structural data, it was possible to partially schematize the structural sequence projected from the outcrops to north of the profile towards the subsurface. the bogotá formation, composed of mudstones and some claystones, make up the conductive core of the structures. the anticlines are composed of the guaduas formation (clay intercalations with scarce sand horizons) and the guadalupe group, which is characterized by a higher sandstone content, which could be associated with the increase in resistivity. figure 8. 2d modeling of the subachoque syncline, interpreted from mt electrical resistivity data. profile 2. two syncline and one anticline structures are modelled showing the stratigraphic sequence with varying thickness and depth. seismic reflection seismic lines are obtained through a geophysical method used to create an image of the subsurface by emitting seismic waves and recording the reflections that these waves generate in the different layers of the subsurface (keary and brooks, 1991; lowrie, 2007; simpson, f., 2005; telford et al., 1990). in the case of deep aquifers, seismic lines are used to infer the structure and geometry of the potential aquifer formation(s). the seismic method consists of the emission of seismic waves from the earth’s surface through a set of geophones or sensors that are placed on the earth’s surface. these seismic waves travel through the subsurface and upon encountering different layers of rocks or soils, are reflected to the surface. the sensors record these reflections and are used to create an image of the subsurface. the juaica-78 seismic program conducted by ecopetrol in the j78-14 line of 12km shown in (figure 9) and which is associated with the chitasuga 1 well (amaya, 2020). the tabio anticline (red line) was interpreted in the central part of the section as a hanging block structure of the vergence fault to the east of chital. this fault has the same strike and dip expression. to the west, the subachoque fault (red line) is interpreted as a backthrust. a stratigraphic sequence has been outlined at depth, from base to top, as follows: jurassic (light blue) basement, and a sequence of cretaceous units, the dark green upper undifferentiated cretaceous (kind), then the blue une formation (kiu); these two formations are located between 2000 and 3000m depth, the chipaque formation (kich) of strong light green color, underlying the guadalupe group and identifies the upper and lower (kgs, kgl) identified of light green color a little fainter respectively these two formations are located between 1500 and 2000m, in the upper part is delineated the guaduas formation of orange color (tkg) this formation is between the surface and 1500m. the interpretation defines the tabio anticline as a structure resulted from compressive strain and stress related to the inverse chital fault that was initially extensional during the first stages of rifting and later inverted. this interpreted sequence has been controlled by the chitasuga -1 well, north of the area. the seismic interpretation has also been useful to adjust the ves and mt-ves interpretations. in line j78 11 (figure 10) the following stratigraphic sequence was interpreted: the basement is indicated by a dark brown interfase (bas). the paleozoic is indicated by the brown interfase (pz), the jurassic units are located only on the left side of the line with the light blue interface (ju). the top kiu reflector (blue color) refers to the boundary of the une formation with a probable thickness of 100 meters, the top (kind) undifferenciated cretaceous reflector (dark green). the upper cretaceous has an approximate thickness between 650 and 800 meters. the lower unit top (kich) reflector (strong light green) corresponds to the boundary of the chipaque formation with a possible thickness of 100 meters. the middle unit top (kg) reflector (upper and lower light green color) indicates the upper limit of the guadalupe group in the region, with an approximate thickness of 650 meters (escovar reyes, 2004). the upper unit on top (tkg1) reflector (orange color) corresponds to the upper limit of the guaduas formation, with an estimated thickness of 900 to 1000 meters in the area. figure 9. seismic line juaica-78-14. the interfases associated with the lithological formations are related to the different colors described in the legend. on the left side is interpreted geologic section the right side is the location of the seismic and borehole (amaya, 2020). 148 orlando hernández-pardo, paola andrea atapuma-acevedo, luis hernan ochoa figure 10. seismic line juaica-78-11. the interfases associated with the lithological formations are related to the different colors described in the legend. on the left side is interpreted geologic section the right side is the location of the seismic and borehole (amaya, 2020). low enthalpy geothermal energy geothermal energy is linked to the flow of heat from the earth’s interior, offering a renewable and clean source that can be used to generate electricity, satisfying the basic needs of both the population and industry in their daily activities (lowrie, 2007; turcotte d.l., schubert, g, 2002). the magnitude that characterizes the work of geothermal energy is enthalpy, which is classified as high (150°c or higher), medium (between 100°c and 150°c), low enthalpy (between 50°c and 100°c), and very low enthalpy (between 3°c and 25°c), depending on the temperature (figure 11, table 5). table 5. geothermal energy classification (https://amas15m.wordpress.com/2015 /01/22/energia-geotermica-la-energia-renovable-del-suelo/) geothermal resources field temperature range technology very low enthalpy 5°c < t < 25°c heat pump c o n v en ti o n a l low enthalpy 25°c<t<50°c may require heat pump 50°c<t<100° c direct uses medium enthalpy 100°c<t<150°c power generation / binary cycles high enthalpy t< 150°c electricity u n c o n v en ti o n a l egs-hdr t <150°c power generation / binary cycles supercritical t<300°c electricity/ hydrogen hydrotermal system (hot sedimentary aquifer) 0 meters 500 m 3000 m 7000 m scale is ilustrative and not lineal minewater heat pump re-purposed oil/gas well enhaced geothermal system 'deep' geothermal'shallow' geothermal surface sediments coal seams hydrotermal system (hot sedimentary aquifer) enhaced geothermal system collapsed workings depleted hydrocarbon resevoir sedimentary basin rock igneous and metamorphic rocks figure 11. geothermal system in which a heat pump converts low enthalpy geothermal energy into electricity that can power greenhouses, refrigerated rooms, and other electrical appliances. (dr corinna abesser, 2022) figure 12. flores funza installations including the san felipe well. on the upper right side is the san felipe well, on the upper left side greenhouses and a treated water pond, on the lower right side flower greenhouses and on the lower left side the location of the well. https://amas15m.wordpress.com/2015 149geophysical modeling of the middle bogotá river basin with support from interpretation of electrical resistivity, gravity, seismic and borehole data to evaluate potential deep aquifers near the study area, there is only one known case of very low enthalpy implementation in a company in tocancipá, upper bogotá river basin, where a refrigerated room was activated together with an air conditioning system. with the research carried out and the implementation of innovative technologies, for example by the company flores funza, a clean energy project has been implemented to supply some of the energy needs (heating and cooling), using a nearby well. this objective would be conducted by means of heat pumps, which operate with extremely low temperature changes (1° to 3°c). the process involves the use of a vertical pipe fed with water from a well, the temperature of which exceeds that of the surrounding environment, thus initiating the operation of the heat pump and thus any associated electrical system. considering the implementations and other studies already implemented by flores funza (figure 12) in the san felipe well, the feasibility of this implementation is being estimated. the objective is to take advantage of renewable energies and obtain a dual combined use of groundwater, thus contributing to sustainability and efficiency in the use of water and energy resources. conclusions the interpretation of the electrical resistivity data supports the continuation of the synclinal of subachoque to the south, underneath the quaternary overburden, limiting the base and top of sandy aquifer formations of the cacho and sandstone la guia and lajosa horizons of the guaduas formations, in the first 300m depth. the mt soundings have been used to generate 1d and 2d models of the guadalupe formation, as a potential deeper aquifer formation in the middle rio bogota basin in the sabana de bogota, at 500m to 1200m depth, that is laterally connected to the outcrops of these units in the surrounding mountains, were the recharging of these aquifers take place by infiltration. the magnetotelluric soundings were carried out in flat areas of quaternary deposits that overlie tertiary and cretaceous formations, which are intensely folded and faulted by tectonic activity in the miocene, composed mainly of sequences of siltstone sandstones and shales of fluvial and marine environment, where marine facies generated by regression and transgression events of the coast line produced a sequence of claystone, siltstones, sandstones, which vary vertically and laterally in the basin. the penetration obtained allowed to establish a high hydrogeological potential in the first 2000m of depth, especially from the base to top of the guadalupe group, where the labor and tierna and arenisca dura formations are the formations with the highest hydrogeological potential. the interpreted seismic reflection lines helped to adjust the geoelectrical and magneto telluric models and provided a complete scenery of the folded and faulted stratigraphic sequence, specially, the interfases associated with the guadalupe group. the conceptual geological model defines as units of hydrogeological interest in the upper and middle basin of the bogotá river the quaternary deposits of sand and gravels, high terrace deposits, sabana formation, the sandstone members of the bogotá formation, the cacho sandstone formation, the guía and lajosa sandstone horizons of the guaduas formation, the arenisca labor y tierna formations and arenisca dura of the guadalupe group. the most favorable structural feature corresponds to the syncline of subachoque. the temperature of groundwater extracted from deep aquifers, with temperature of 18 oc in the san felipe borehole, classifies it as a low enthalpy geothermal resource. for this reason, its use and exploitation should be considered. acknowledgements we thank the servicio geológico colombiano (sgc) and the corporación autónoma regional de cundinamarca y boyacá (car) for providing the geophysical data sets and la universidad nacional de colombia for supplying the specialized geophysical instruments and software that were used in this research. references acosta, j., ulloa, c., & martinez, i. 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(2013). sistema de modelamiento hidrogeológico del distrito capital bogotá. secretaría distrital de ambiente, subdirección del recurso hídrico y del suelo, 234p. bogotá, d.c. https://www.minambiente.gov.co/el-agua-subterranea-recurso-por-investigar https://www.minambiente.gov.co/el-agua-subterranea-recurso-por-investigar https://doi.org/10.1306/01051009111 https://doi.org/10.1071/eg975104 https://doi.org/10.1071/eg975104 https://www.ageocol.org/download/caso-exitoso-en-colombia-del-aprovechamiento-de-la-geotermia-de-baja-entalpia-para-climatizacion https://www.ageocol.org/download/caso-exitoso-en-colombia-del-aprovechamiento-de-la-geotermia-de-baja-entalpia-para-climatizacion https://www.ageocol.org/download/caso-exitoso-en-colombia-del-aprovechamiento-de-la-geotermia-de-baja-entalpia-para-climatizacion https://www.ageocol.org/download/caso-exitoso-en-colombia-del-aprovechamiento-de-la-geotermia-de-baja-entalpia-para-climatizacion https://revistas.unal.edu.co/index.php/geocol/article/view/30407 https://revistas.unal.edu.co/index.php/geocol/article/view/30407 https://doi.org/10.1017/cbo9780511614095 https://www.unwater.org/publications/un-world-water-development-report-2023 https://www.unwater.org/publications/un-world-water-development-report-2023 https://www.usgs.gov/special-topics/water-science-school/science/where-earths-water https://www.usgs.gov/special-topics/water-science-school/science/where-earths-water _heading=h.1fob9te _heading=h.3znysh7 _heading=h.eugzvypzmvu3 _heading=h.2s8eyo1 _heading=h.g4efqxgv4bzh _heading=h.17dp8vu _heading=h.3rdcrjn _heading=h.lnxbz9 bookmark=id.2jxsxqh bookmark=id.z337ya issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.105872 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (september, 2023): 273-287 pe tr o le u m e n g in ee r in g multivariable regression 3d failure criteria for in-situ rock mohatsim mahetaji1, jwngsar brahma2*, rakesh kumar vij1. 1. department of petroleum engineering, school of energy technology, pandit deendayal energy universitypdeu, gandhinagar, gujarat, india 2. department of mathematics, school of technology, pandit deendayal energy universitypdeu, gandhinagar, gujarat, india *corresponding author: jwngsar.brahma@spt.pdpu.ac.in manuscript received: 17/11/2022 accepted for publication: 07/08/2023 how to cite this item: mahetaji, m., brahma, j., & kumar vij, r. (2023). multivariable regression 3d failure criteria for insitu rock. earth sciences research journal, 27(3), 273-287. https://doi.org/10.15446/esrj.v27n3.105872 abstract wellbore stability problems increase with the exploration and development of oil and gas reservoirs. a new 3d non-linear failure criterion is proposed as a trigonometric function considering the intermediate principal stress (σ2) on the triaxial compression test data. mohr-coulomb and hoek-brown are well-known failure criteria, but they do not consider the influence of (σ2) on rock strength. this new criterion produces a concave surface on the principal stress space (σ1,σ2,σ3) with the influence of intermediate principal stress. in this study, sensitivity analysis for the variable is also done to understand the significant influence of parameters on the accuracy of the proposed criterion. further validation of this non-linear criterion on three principal stresses (σ1,σ2,σ3) was done compared with linear regression and second-degree polynomial regression results. it has been observed that the new non-linear 3d criterion with five material parameters reveals a good fit compared to linear regression and second-degree polynomial regression, which have four and six material parameters, respectively. the new non-linear criterion was further validated by comparison with existing criteria like the priest, drucker-prager, and mogi-coulomb. it has been observed that the new 3d non-linear criterion shows a more accurate result than these existing criteria as certain rock types exhibit coefficient of determination (dc) values near one, precisely 0.95 for inada granite, 0.94 for orikabe monzonite, and 0.91 for ktb amphibolite. in contrast, other rock types have dc values ranging from 0.7 to 0.9. the new 3d non-linear criterion also yields lower root means square error (rmse) values than the mogi-coulomb criterion for seven rock types. specifically, the rmse values by the new criterion are as follows: ktb amphibolite 40.03 mpa, dunham dolomite 15.16 mpa, shirahama sandstone 9.08 mpa, manazuru andesite 22.14 mpa, inada granite 35.47 mpa, and coconino sandstone 19.047 mpa. this new 3d criterion gave precise predictions of the failure of the formation under in-situ stresses and was further helpful for the simulation of the wellbore in the petroleum industry. the variable in the new 3d criterion should be calculated from triaxial compression test data for each formation rock before applying this criterion to the wellbore stability problem and the sand production problem. keywords: 3d failure criterion; non-linear failure criterion; linear regression; polynomial multivariable regression; geomechanics; in-situ stresses; principal stresses. criterio de falla tridimensional por regresión multivariable en rocas in-situ resumen los problemas de estabilidad de un pozo se incrementan con la exploración y el desarrollo de los reservorios de petróleo y gas. en este trabajo se propone un nuevo criterio de falla tridimensional y no lineal como una función trigonometrica que considera la tensión principal intermedia (σ2) en la información para la evaluación de compresión triaxial. los criterios de falla de mohr-coulomb y hoek-brown son bien conocidos, pero estos no consideran la influencia de la tensión principal intermedia en la dureza de la roca. este nuevo criterio produce una superficie cóncava en el espacio de las tensiones principales máxima, intermedia y mínima (σ1,σ2,σ3) con la influencia de la tensión principal intermedia. para este trabajo también se hizo el análisis de sensibilidad con el fin de entender la influencia de los parámetros en la exactitud del criterio propuesto. se realizó una validación adicional de este criterio en las tres tensiones principales (máxima, intermedia y mínima) y se comparó con los resultados de la regresión lineal y la regresión polinómica de segundo grado. en este proceso se observó que el criterio tridimensional no lineal con cinco parámetros materiales revela un buen ajuste en comparación con las regresiones lineal y polinómica de segundo grado, que se trabajaron con cuatro y seis parámetros materiales, cada una. el nuevo criterio no lineal se validó adicionalmente con criterios existentes como el de priest, drucker-prager y mogi-coulomb. los autores observaron que el nuevo criterio tridimensional no lineal muestra un resultado más exacto que aquellos de los criterios existentes ya que ciertos tipos de roca exhiben valores en los coeficientes de determinación cerca de uno. precisamente, 0.95 para granito inada, 0.94 para monzonita orikabe, y 0.91 para anfibolitas ktb. en contraste, otros tipos de rocas tienen valores en los coeficientes de determinación que van de 0.7 a 0.9. el nuevo criterio tridimensional no lineal también produce valores de error cuadrático medio (rmse) más bajos que el criterio mogi-coulomb en siete tipos de rocas. específicamente, los valores rmse para el nuevo criterio son los siguientes: anfibolitas ktb = 40.03 mpa; dolomitas dunham = 15.16 mpa; arenisca shirahama = 9.08 mpa; andesita manazuru = 22.14 mpa; granito inada = 35.47 mpa, y arenisca coconino = 19.047 mpa. este nuevo criterio tridimensional ofreció predicciones precisas de falla de la formación bajo tensiones in-situ y luego fue útil en la simulación de pozos en la industria petrolera. la variable en el nuevo criterio tridimensional debe de ser calculada con información de la evaluación de compresión triaxial en cada formación rocosa antes de aplicar este criterio en los problemas de estabilidad de pozos y en los problemas de producción de arena. palabras clave: criterio de falla tridimensional; criterio de falla no lineal; regresión lineal; regresión polinomial multivariable; geomecánica; tensiones insitu; tensiones principales. https://doi.org/10.15446/esrj.v27n3.105872 mailto:jwngsar.brahma@spt.pdpu.ac.in https://doi.org/10.15446/esrj.v27n3.105872 274 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. list of symbols σ1, σ2, σ3 principal stresses the failure in mpa ξ, ρ, θ haigh-westergaard coordinates in mpa i1 first invariant of stress tensor and the mean stress j2,j3 second invariants of the deviatoric stress tensor. mb,s,a empirical constants of the generalized hoek–brown criterion τoct the octahedral shear stress in mpa σoct the normal octahedral stress in mpa σ1f three-dimensional effective failure stress mpa w the weighting factor for hoek–brown criterion σ3hb minimum 2d hoek–brown effective stress at failure in mpa σ1hb maximum 2d hoek–brown effective stress at failure in mpa p the weighting of σ2 rmse the root-mean-square-error in mpa q1,q2 the material constant of intact rock ri prediction error or residuals i-th test dc coefficient of determination. εi error percentage 1. introduction better prediction of the failure of rock near the wellbore has been essential over the last three to four decades. new oil and gas fields develop on the uncertainty of subsurface reservoir pressure, tectonic effect, and temperature gradient. more recent technology is required in drilling and production operations for deeper reservoirs. the problem of wellbore stability increased at high-pressure, high-temperature reservoirs with an inclination of the well in directional drilling. the prominent wellbore instability is brack-out of rock by a combination of tensile and compressive force on the geomechanical stress near the wellbore results from formation, overburden, and hydrostatic pressure (mahetaji et al., 2020). lots of wells are missed due to the instability of rock fractures. geomechanical modeling with maximum wellbore stability solves uncertainty about the drilling well. geomechanical stress is vital in numerical modeling (zhang, 2019). to recognize a failure phenomenon, a compatible and specific criterion must be used (bou-hamdan, 2022). while some materials fail in shear, some may fail due to plastic deformation. the reasons behind the near-wellbore instability problems are tensile failure of formation, shear failure without plastic deformation, formation collapse or compressive failure, erosive-cohesive failure, and creep failure during drilling. (aadnoy and looyeh, 2019). various failure criteria are introduced in the literature for the failure of different-different materials. mohr-coulomb criterion and hoek-brown failure criterion are well-known failure criteria that consider maximum and minimum principal stress (σ1, σ3) and neglect the intermediate principal stress (σ2) (culshaw and ulusay, isrm suggested method 2007-2014). mohrcoulomb criterion is driven by the linear relation of mean and shear stress. mohr-coulomb failure criterion is mainly recommended when principal stresses σ1, σ2, and σ3 are compressive and in a limited range of mean stress. the hoek-brown failure criterion is the empirically derived relation between two principal stress σ1 and σ3 (hoek and brown, 1997; hoek et al., 2002). hoekbrown criterion is a non-linear, parabolic curve with the independence of the effect of intermediate principal stress. experimental evidence shows that the intermediate principal stress influences the failure of rock material (takahashi and koide, 1989). mohr-coulomb and hoek-brown’s failure criterion does not consider the effect of intermediate principal stress, so the rock failure prediction under the triaxial stress condition is unreliable. these lead to developing several three-dimensional hoek-brown criteria given by pan and hudson (1988); priest (2005); zhang and zhu (2007); zhang (2008); jiang and yang (2020). singh et al. (2011) and mahetaji et al., (2023) developed a modified non-linear mohr-coulomb criterion considering the effect of intermediate principal stress on strength behavior. furthermore, failure criteria are introduced after considering the effect of intermediate principal stress, such as the drucker and prager criterion (drucker and prager, 1952) and the lade criterion (kim and lade, 1984). drucker-prager criterion was a curvilinear 3d generalized form of the mohr-coulomb criterion for soils. drucker-prager criterion is the relation between normal and octahedral shear stress through material constant. the lade criterion (kim and lade, 1984) and modified lade criterion (ewy, 1999) relate to the respective first and third invariants of the effective stress tensor at the failure. modified lade criterion is further used for sand production to produce well and wellbore stability problems with the effect of intermediate stress (yi et al., 2005; yi et al., 2006). the failure criterion from the triaxial experimental testing data in terms of the power law, considering three principal stresses, is given by mogi (1971). other rock triaxial testing data also validate failure criterion based on the experiment data by haimson and chang (2000), chang and haimson (2000), takahashi and koide (1989), and xiaodong and haimson (2016). priest (2005) proposed the 3d failure criterion based on the extended hoek-brown yield criterion combined with the drucker-prager criterion. melkoumian et al., 2008 further developed the 3d hoek-brown yield criterion by considering the weighting of intermediate principal stress. further research and triaxial rock testing are necessary before applying any of the 3d failure criteria. the study aims to propose a 3d non-linear failure criterion from the actual triaxial compression data. the proposed criterion may apply to a subsurface rock formation using the triaxial compression test of that rock formation. after that, this failure criterion is helpful for the wellbore stability problem and the prediction of sand production in producing well. a new failure criterion is proposed by considering the effect on the strength of the rock. proposed failure criteria produce a curve surface as a failure envelope on a 3d space of (σ1, σ2, σ3) that is easy to derive and interpret. available triaxial data in the literature from various researchers (mogi, 1971; 1974; 2007; takahashi and koide, 1989; chang and haimson, 2000; and xiaodong and haimson, 2016) as shown in table 1 are used for the non-linear regression analysis of the proposed 3d criterion on principal stress space (σ1, σ2, σ3). 275multivariable regression 3d failure criteria for in-situ rock table 1. sources of laboratory test data for ten rock samples. rock type source of the tabular data. original source of data. specimen size ktb amphibolite (german continental deep drilling program) colmenares and zoback (2002) haimson and chang (2000) shirahama sandstone colmenares and zoback (2002) takahashi and koide (1989) rectangular prism 3.5 cm × 3.5 cm × 7.0 cm dunham dolomite mogi (2006) circular cylinders 1.6 cm × 5.0 cm solnhofen limestone mogi (2006) circular cylinders 1.6 cm × 5.0 cm yamaguchi marble mogi (2006) rectangular prism 1.5 cm × 1.5 cm × 3.0 cm mizuho trachyte mogi (2006) rectangular prism 1.5 cm × 1.5 cm × 3.0 cm manazuru andesite mogi (2006) rectangular prism 1.5 cm × 1.5 cm × 3.0 cm inada granite mogi (2006) rectangular prism 1.5 cm × 1.5 cm × 3.0 cm orikabe monzonite mogi (2006) rectangular prism 1.5 cm × 1.5 cm × 3.0 cm coconino sandstone xiaodong and haimson(2016) rectangular prism 1.9 cm × 1.9 cm × 3.8 cm 2. theory the failure criteria are visualized and expressed with principal stress space f(σ1,σ2,σ3), stress invariant f(i1,j2,θ), f(j1,j2,j3) or f(i1,i2,i3), or a version of 3d haighwestergaard coordinates stress space (ξ, ρ, θ) which is a direct physical interpretation geometrically (ottosen, 1977). the geometric representation of a stress state in haigh–westergaard and principal stresses space is given in figure 1. haighwestergaard coordinates are widely used invariant in (ξ, ρ, θ) where ξ is the projection on the unit of the hydrostatic axis, ρ is the radial distance from a failure point p and θ is the similarity angle range of 0 to π/3 from the rotation of the axis. = √3 = 1 + 2 + 3 √3 (1) √3 = √3 = √( 1 − 2)2 + ( 2 − 3)2 + ( 3 − 1)2 √3 (2) √3 = 1 3 cos− 1 3√3 2 3 2 3 2 = tan− 1 √3 2 − 3 2 1 − 2 − 3 (3) | | = √ 2 + 2 = √ 1 2 + 2 2 + 3 2 (4) the ξ-ρ plane is also called the rendulic plane. principal stress σ1,σ2, and σ3 in the terms of the (ξ, ρ, θ) are given by brannon et al., 2009. 1 = 1 √3 + √2 cos − sin √3 (5) further validation of the proposed 3d criterion is done by comparison of predicted results with actual triaxial data on ten rock types on 3d haigh westergaard coordinates stress space (ξ, ρ, θ). in addition, the sensitivity of the parameter on the prediction error and comparison of the proposed criterion with the priest criterion are discussed on the ten rock types. the priest criterion combines the three-dimensional drucker-prager criterion (drucker and prager, 1952) and the two-dimensional hoek–brown criterion. the proposed 3d nonlinear failure criterion considers the trigonometric relation between on principal stress space (σ1, σ2, σ3). based on that, predicted strength by the new 3d criterion might indicate a more accurate result compared to the existing failure criteria. figure 1. geometric representation of a stress state in the haigh–westergaard and principal stresses space (recreated after lian et al., 2013). 276 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. 2 = 1 √3 + √2 (2 sin √3 ) (6) ( ) 3 = 1 √3 + √2 − sin √3 − cos (7) angle θ is the stress angle, and cos3θ is the load parameter. when θ equals zero in compressive meridian and for θ is equal to 60֩ in tensile meridian new failure criterion in the conventional mohr-coulomb and hoek-brown criterion, the effect of the intermediate stress is neglected on the rock failure prediction. the experimental result of the triaxial compression failure test indicates the impact of the intermediate principal (σ2) on the rock strength. rock strength increases with the increment in the value. mogi's (1971) failure criterion is based on the true triaxial testing data regarding the power law. that failure criterion best fits the actual triaxial experimental data by obeying the power law between normal octahedral stress and octahedral shear stress τoct. the sine function, often denoted as sin(x), is a fundamental trigonometric function that relates an angle in a right triangle to the ratio of the length of the side opposite the angle to the length of the hypotenuse. however, the sine function can also be extended to real numbers beyond the scope of triangles. the plot of the sine function exhibits a smooth wave that oscillates for the value of the θ in the range of 0 to 2π. it has been observed from the experimental data that the non-linear relation between principal stresses is matched with the upper crest side oscillating sine function wave that is constructed in a curvilinear plotting range of 0 to π (shvarts and van helden, 2022). so, choosing this sine function for the regression analyses of principal stress ( σ1 ≥ σ2 ≥ σ3) for actual triaxial compression test data with five material parameters gives the best fitting result compared to linear and second-degree polynomial regression. the new 3d non-linear criterion as trigonometric sine function between the principal stresses as proposed, 1 = sin( 1 2 + 1) sin( 2 3 + 2) (8) where σ1,σ2, and σ3 are the principal stresses and a,b1,b2,d1,and d2 are material constants for the best fitting with the actual triaxial data point. relation of the a and ucs with adopting σ2 = 0 and σ3 = 0 as, ucs = a sin d1 sin d2 the value of σ1 must be positive for σ1≥ σ2 ≥σ3 compression failure in equation (8). for a positive value of σ1, the value of sin(b1σ2 + d1) and the value of sin(b2σ3 + d3) must be positive, so the range of (b1σ2 + d1) and (b2σ3 + d3) must be between 0 to π. all the strength parameters are easy to derive by the triaxial compression test data for a minimum six-core sample. equation 8 is a proposed non-linear failure criterion directly derived from non-linear 3d regression analysis considering the effect of σ2 on the rock strength. equation 8 produces a concave surface on the principal stress space (σ1,σ2,σ3). proposed non-linear failure equation in the haighwestergaard coordinate is given by the function of (ξ,ρ,θ) as ( , , ) = 1 √3 + √2 cos − sin √3 − a sin 1 1 √3 + 1 √2 2 sin √3 + 1 sin 2 1 √3 + 2 √2 − sin √3 − cos + 2 (9 this equation is complex to solve with the arithmetic method but easily solved by the numerical method. newton raphson's method is recommended to solve this equation with a quick convergent rate. newton raphson's method has a limitation in that f'(ξ,ρ,θ) is not zero. if that f'(ξ,ρ,θ), then it must be required to modify the newton-rapson method. modifying code for the newton-rapson method is done by making a bridge for that range at f'(ξ,ρ,θ) nearly equal to zero or using another method like the bisection method. table 2. regression analysis for ten rock types. rock type polynomial degree 1 polynomial degree 2 trigonometric function (nonlinear 3d criterion) ktb amphibolite σ1=0.529 σ2+ 5.315 σ3+293 σ1=1.03σ2+8.56σ3-0.0013σ2 2+0.0026σ2 σ30.0248σ3 2+187.25 σ1=1303*sin(0.002σ2+0.7229)*sin(0.008 0σ3+0.269) dunham dolomite σ1=0.515 σ2+ 2.743 σ3+371 σ1=1.27σ2+3.55σ3-0.0025σ2 2+0.0059σ2 σ30.0140σ3 2+294 σ1=2193*sin(0.0026σ2+0.703)*sin(0.001 6σ3+0.2285) solnhofen limestone σ1=0.294 σ2+ 2.470 σ3+346 σ1=0.509σ2+2.67σ3-0.0013σ2 2+0.0068σ30.0136σ3 2+331 σ1=7700*sin(0.0025σ2+0.8253)*sin(0.002 6σ3+0.059) shirahama sandstone σ1=0.156 σ2+ 4.141 σ3+86.53 σ1=0.37σ2+6.10σ3-0.0052σ2^2+0.026σ2 σ30.083σ3 2+66.35 σ1=299.3*sin(0.00754σ2+0.9390)*sin(0.0223 5σ3+0.2675) yamaguchi marble σ1=0.428 σ2+ 3.582 σ3+114 σ1=1.145σ2+2.35σ3-0.0055σ2 2+0.0174σ2 σ30.0113σ3 2+106.02 σ1=11080*sin(0.004σ2+0.6386)*sin(0.000 3σ3+0.0155) mizuho trachyte σ1=0.229 σ2+ 2.789 σ3+165 σ1=0.496σ2+4.20σ3-0.0014σ2 2+0.00426σ2 σ30.018σ3 2+121.30 σ1=8884*sin(0.0028σ2+0.8190)*sin(0.000 3σ3+0.0273) manazuru andesite σ1=0.551 σ2+ 5.691 σ3+276 σ1=1.29σ2+11.35σ3-0.0034σ2 2+0.0087σ2 σ30.0818σ3 2+162.82 σ1=956.1*sin(0.0039σ2+0.7607)*sin(0.008 8σ3+0.4191) inada granite σ1=0.965 σ2+ 3.796 σ3+461 σ1=1.97σ2+5.44σ3-0.0057σ2 2+0.0112σ2 σ30.0168σ3 2+358.78 σ1=1901*sin(0.0023σ2+0.647)*sin(0.003 0σ3+0.3562) orikabe monzonite σ1=0.92σ2+ 2.732 σ3+458 σ1=1.89σ2+4.59σ3-0.0052σ2 2+0.0129σ2 σ30.0207σ3 2+329.43 σ1=1572*sin(0.0029σ2+0.5776)*sin(0.003 4σ3+0.3702) coconino sandstone σ1=0.073σ2+3.132 σ3+161 σ1=0.201σ2+0.516σ3-0.00104σ2 2+0.0046σ2 σ30.0202σ3 2+113.40 σ1=663.2*sin(0.0028σ2+0.9803)*sin(0.0067 5σ3+0.2436) 277multivariable regression 3d failure criteria for in-situ rock equation 8 and equation 9 represent failure criterion in the coordinate of (σ1,σ2,σ3) and (ξ, ρ, θ), respectively. till date, no such equations are available in the literature to predict the rock failure as equation 8 and equation 9. usually, in-situ condition rock break-out done by the compression and rock fracture is due to the combination of compression and tension (bui et al., 2023; ghassemi, 2017; lee et al., 2012). experimental data on true triaxial compression tests are available in the literature review. estimating the strength parameters a,b1,b2,d1,and d2 is done by two methods: the non-linear regression analysis on the triaxial compression test data result and the analytical solution of the new failure criterion by the triaxial compression test data. non-linear three-dimensional regression analysis was done to find the material parameters (a,b1,b2,d1,and d2), as shown in table 2. the strength parameter is dependent on the rock property. the failure equation for each rock type is shown in table 2. this equation considers three principal stresses in relation to the function of sine linear multivariable regression linear multivariable regression analysis is conducted on all these tenrock types to find the linear criterion that considers all three principal stresses in the linear relation. linear regression analyses give the linear relation for the kbt amphibolite as σ1 = p1σ2 + p2σ3 + c1 (10) where, p1 = co-efferent of the σ2 =0.529 p2 = co-efferent of the σ2 =5.315 c1 = intersection=293 mpa the value p1, p2,and c1 is dependent on the formation rock property that is given as the weighting of the intermediate principal stress, minimum principal stress on the strength of the formation, and compressive strength of the formation, respectively. the value of the maximum principal stress is calculated based on the above equation as a combination of the value at the yield point. the yield point is the point where the first crack is initiated. the equation for all ten-rock types based on the linear regression with a variable is given in table 2. second degree polynomial regression polynomial regression analysis on these ten-rock types for the failure in true-triaxial compression test is done the finding appropriated relationships between these three principal stresses. polynomial regression analysis on the data of kbt amphibolite’s triaxial compressive test is given as, σ1 = q1σ2 + q2σ3 q3σ2 2 + q4 σ2σ3 q5σ3 2 + c 2 (11) where, q1 = co-efferent of the σ2 = 1.03, q2 = co-efferent of the σ3 = 8.56, q3=co-efferent of the σ2 2 = 0.0013, q4 = co-efferent of the σ2 σ3 = 0.0026, q5 = co-efferent of the σ3 2 = 0.0248, c2 = intersection =187.25 mpa the value of q1, q2, q3, q4, q5, and c2 is determined by the formation rock property, which is defined as the weighting of the σ2, σ3, σ2 2, σ2, σ3, σ3 2, and compressive strength of the formation, respectively. the maximum principal stress is calculated using the above equation as a combination of the σ2 and σ3 values at the first crack begin. table 2 shows the equation for all ten rock types based on linear regression with variables. priest criterion priest (2005; 2009) developed a three-dimensional failure criterion by combining the three-dimensional drucker-prager criterion (drucker and prager, 1952) and the two-dimensional hoek–brown criterion. three-dimensional effective failure stress (σ1f ) is estimated by 1 = 1ℎ + 2 3ℎ − ( 2 + 3) (12) where, σ3hb is the minimum two-dimensional hoek–brown effective stress at failure, and σ1hb is the maximum two-dimensional hoek–brown effective stress at failure given by 3ℎ = 2 + (1 − ) 3 (13) σ1hb is calculated by hoek–brown criterion (1997) 1ℎ = 3ℎ + 3ℎ + (14) where, σ1hb calculated by hoek-brown criterion (1997), w is a weighting factor, mb is the hoek–brown constant value, and s is a fracture parameter of the rock mass. druckerprager criterion the drucker-prager failure criterion is a curvilinear 3d generalized form of the mohr-coulomb criterion for soils. drucker-prager criterion is the relation between octahedral stress and octahedral shear stress τoct through material constant. drucker-prager criterion is given by the equation (drucker and prager, 1952) = 2 3 (3 + ) (15) λ and k are material constants. drucker-prager criterion in haighwestergaard coordinates. (ξ, ρ, θ) is given by equation. 3 = √2(√3 + ) (16) λ and k can be expressed in terms of cohesion intercept and internal friction angle (yi et al., 2005) = √2(√3 + ) (17) mogi-coulomb criterion al-ajami and zimmerman (2005) proposed a failure criterion by considering the mogi criterion (1971) and linear mohr-coulomb failure criterion called the mogi-coulomb criterion. the mogi-coulomb criterion is the linear relation between octahedral stress (τoct) and mean effective stress (σm2). = + ∗ 2 (18) here, = √( 1− 2)2+ ( 2− 3)2+ ( 3− 1)2 3 and 2 = 1+ 3 2 material constant a and b is simply related to the angle of internal friction (ø), and cohesion (c) is given by mogi-coulomb criterion in haigh-westergaard coordinates (ξ, ρ, θ) is given as. 278 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. = √3 + 2 (3 − √3 ) (19) 3. result and discussion failure prediction and validation on principal stress space (σ1, σ2, σ3 ) the root means square error (rmse) function was used to find the misfits between the triaxial test data and predicted strength by the failure criterion. the standard deviation of the residuals defines rmse. = 1 ∑ 2 = 1 , i=1,2,…,n (20) where n is the number of the test data point of a specific rock. residuals or prediction errors are defined by (21) σi calc is ith calculated value by the failure criterion, and σi test is the i-th tested data. the best fitting rmse value is the minimum, and if prediction is the best fit of test data, the mean of the residuals is always equal to zero. figure 2 indicates the prediction of rock strength by the proposed criterion on ten rock types with a triaxial compression test. scatter data indicate triaxial compression test data, and solid lines represent the data prediction by a single equation for all rock types shown in table 2. in figure 2, the black dotted line is for σ1 = σ2 > σ3 or σ1 > σ2 = σ3; color solid is the proposed failure criterion, and scatter data is the experimental triaxial compression data point. the single-color solid line is for the fixed value of the minimum principal stress. for the shirahama sandstone, the rmse value is minimum among all the rock types,, so we will discuss the shirahama sandstone shown in figure 2. color indication for a solid line when the value of σ3 equal to 5 mpa, 8 mpa, 15 mpa, 20 mpa, 30 mpa, and 40mpa is gradually blue, red, forest green, purple, maroon, and lime. all solid line is plotted by a single equation given in table 2. the regression analysis estimates parameters a = 299.3, b1=0.00754, d1=0.9390, b2=0.02235, and d2=0.2695. it has been observed that the parameter is independent of the principal stresses. figure 2. failure prediction by proposed criterion in space for ten rock type a) b) c) d) 279multivariable regression 3d failure criteria for in-situ rock e) f) g) h) i) j) 280 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. table 3 shows the rmse values of σ1 for shirahama sandstone is a minimum of nearly 9.08 mpa and for the ktb amphibolite is a maximum of nearly 65.03 mpa for the best fitting triaxial data point. rmse values of σ1 are between 9.08 mpa to 65.03 mpa for the rest of all rock types shown in table 3. figure 3 shows the distribution of the error percentage for predicted σ1 for all the rock types. the equation defines the prediction error, − ∗ 100% = (22) or in terms of the σ1 = 1 − 1 ∗ 100% (23) σ1i calc is the ith value of predicted σ1 from the triaxial data point. σ1i test is the tested value of σ1 on the triaxial data. the predicted error distribution for all the rock types is as a histogram shown in figure 3. the distribution of data points is excellent and distributed equally in the range of -10 % to 10%, and the maximum number of data points distributed is near zero percentage error. like gaussian distribution, this type of indicator with a maximum number of data having an error percentage around zero is preferable. error distribution for the shirahama sandstone, yamaguchi marble, inada granite, orikabe monzonite, manazuru andesite, and coconino sandstone is distributed assuredly as a gaussian distribution, so it suggests that the new failure criterion works better for that rock types. error distribution for the remaining rock types is also good, with a maximum error percentage near zero, but the distribution shape is not like a bell curve, shown in figure 3. a b c d 281multivariable regression 3d failure criteria for in-situ rock figure 3 error percentage distribution as histogram for ten rock types. e f g h i j e f 282 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. validation on haighwestergaard coordinates stress space (ξ, ρ, θ) the proposed failure criterion is validated with the help of haigh westergaard coordinates stress space (ξ, ρ, θ). the non-linear failure criterion generates a curve in stress space (ξ, ρ, θ), so validation can be done with the coefficient of determination (dc). to validate the proposed non-linear failure criterion here, we compare it with experimental triaxial test data of ten rock types, as shown in figure 4. equation 9 is the failure criterion in haigh-westergaard coordinates. (ξ, ρ, θ). here ρ is a dependent variable, ξ is an independent variable, and θ depends on rock type. figure 4 compares the proposed 3d failure criterion with triaxial compression test data. scatter data on the plot indicate the triaxial test data, where the solid red line indicates the failure criterion by the equation (9). validation of the proposed 3d failure criterion is done with the help of dc with the reliability prediction of by the failure criterion. = 1 − ∑ ( − )2 = 1 ∑ ( − ̅ )2 = 1 (24) where ρi calc i-th calculated value data, ρi test is i-th tested data on the triaxial test, ρi test is the mean value of tested data by the triaxial test. dc=1 means it is an ideal case where test data agree with the zero misfits. the desirable value of the dc must be near one. desirable values of dc are for inada granite (dc=0.95), orikabe monzonite (dc=0.94), and ktb amphibolite (dc=0.91). dc for the rest of the rock types (dunham dolomite, yamaguchi marble, mizuho trachyte, solnhofen limestone, shirahama sandstone, manazuru andesite, coconino sandstone) in the range of 0.7 < dc < 0.9 shown in figure 4. this dc value near one suggests that the proposed non-linear failure criterion shows better results in haigh-westergaard coordinates. a b c d 283multivariable regression 3d failure criteria for in-situ rock figure 4. validation of the new 3d failure criterion for the ten rock types g h i j e f 284 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. sensitivity analysis on variables of new 3d criterion values of a, b1, b2, d1, and d2 are selected from non-linear regression analysis as shown in table 2. there is a minimum rmse value of σ1 for all the optimum values of parameters. the influence of a single parameter on the value of rmse(mpa) for σ1 is discussed in this section. for the shirahama sandstone, values of parameter-driven from the regression are a = 299.3, b1 = 0.0075, b2 = 0.9390, d1= 0.02235,and d2 = 0.2675. if the value of one parameter a change from 200 to 320 for shirahama sandstone, it has been observed that with no changes on the remaining parameter (b1,b2, d1, and d2) shown in figure 5 (i). in figure 5 (i), the rmse decreases from 66.83 mpa to 14.36 mpa when it increases from 200 to 300 and then increases again to 19.70 mpa when a value is more significant than 320. hence, the best-fit value for the shirahama sandstone a is 300 with the minimum rmse value (14.36 mpa). figures 5(ii), 5(iii), 5(iv), and 5(v) show the influence of the parameter (b1, b2, d1, and d2) gradually on rmse (mpa) of σ1 when remain parameters are static and given in table 2 for the shirahama sandstone. hence best-fit value for b1 is 0.0055 with minimum rmse (9.33 mpa); is d1 0.8 minimum rmse (11.63mpa); b2 is 0.0225 minimum rmse (14.36 mpa), and d2 is 0.27 minimum rmse (14.35mpa) exclusively. best fitting value of parameters a, b1,b2, d1, and d2 with minimum rmse given by optimization methods like gradient descent and stochastic gradient descent in machine learning and deep learning. figure 5. sensitivity analysis of parameters a, b1,b2, d1, and d2 of new non-linear trigonometric criterion for failure prediction in rmse. i) iii) ii) iv) v) 285multivariable regression 3d failure criteria for in-situ rock comparisons of the new 3d non-linear criterion with linear regression, polynomial regression, and existing criteria. figure 6. comparisons of rmse for by new 3d non-linear failure criterion with linear, second-degree polynomial, and existing criteria: dracker-prager, mogi-coulomb, and simplified priest criterion. the prediction of principal stress (σ1) is made with the help of the combination of another two principal stresses (σ2 and σ3) by the failure criterion discussed above. maximum principal stress at the failure using the new nonlinear criterion given in figure 2 and by priest criterion is given as shown in appendix i. maximum principals stress is not directly driven by the drucker prager criterion and mogi-coulomb criterion. the equation for the drucker prager and mogi-coulomb criterion is difficult to determine the maximum principal stress as a dual result of the quadratic equation of the principal stresses. the solution of the druckerprager and mogi-coulomb criterion is done with the help of the haigh-westergaard coordinates (ξ, ρ, θ) and then converted into the principal stresses for the comparison of the all-failure criterion which was discussed above. appendix ii and appendix iii gradually show the druckerprager and mogi-coulomb criterion result. comparisons of the rmse value with linear regression and second-degree polynomial regression and with existing failure criteria are given in table 3 and figure 6 for all rock types. rmse in mpa for the linear regression, the drucker-prager criterion, and the priest criterion are higher for all the rock types than the new non-linear criteria, mogi-coulomb criterion, and polynomial equation. rmse by new non-linear criteria, second-degree polynomial, and mogi-coulomb criteria gave the lowest value among the ten-rock type. seven rocks have lower rmse by the new non-linear criterion than rmse by the mogi-coulomb criterion, as shown in figure 6. the minimum rmse value by mogi-coulomb is 8.36 mpa for yamaguchi marble, and the maximum is 55.51 mpa for orikabe monzonite. the maximum rmse by the new non-linear criterion is 50.04 for orikabe monzonite, and the minimum is 9.08 for shirahama sandstone. table 3. comparison of new 3d non-linear criterion with linear, polynomial criterion, and existing failure criterion for rmse of σ1 (mpa) rock type number of data points rmse (linear criterion) rmse (polynomial criterion -degree 2) rmse (new non-linear criterion) rmse (drucker prager criterion) rmse (mogi-coulomb criterion) rmse (simplified priest criterion) ktb amphibolite 42 89.31 61.94 40.03 73.63 40.64 83.76 dunham dolomite 54 39.62 22.29 15.16 47.88 17.22 37.03 solnhofen limestone 26 24.10 17.83 15.32 53.93 14.92 32.67 shirahama sandstone 38 10.39 6.49 9.08 14.89 11.06 13.58 yamaguchi marble 38 17.26 11.31 12.15 18.86 8.36 21.96 mizuho trachyte 31 20.27 11.31 13.53 31.84 9.74 15.47 manazuru andesite 23 51.73 24.31 22.14 55.10 29.98 39.7 inada granite 46 69.79 47.06 35.47 61.04 36.16 42.46 orikabe monzonite 44 73.86 38.75 50.04 69.94 55.15 65.16 coconino sandstone 48 38.50 67.05 19.047 44.50 20.51 35.57 286 mohatsim mahetaji, jwngsar brahma, rakesh kumar vij. 4. summary and conclusions this study develops three criteria based on the true-triaxial compressive test data as a relationship of intermediates principal stresses. these criteria are developed by considering intermediate principal stress for the prediction of failure of rock in the compressive nature. the first criterion is developed as a trigonometric function of stresses as discussed, the second one is the linear relationship of stresses, and the third is the second-degree polynomial relationship among principal stresses. these trigonometric non-linear and second-degree polynomial criteria develop a concave surface on 3d space of (σ1, σ2, σ3) whenever linear criterion develops plane surface. these 3d failure criteria are not reported in the previous literature. failure criterion for linear and polynomial regression (second-degree) has four and six material parameters, respectively. rock failure prediction by linear criterion has poor results compared to exiting criterion as regression is poor fitting with triaxial compression test data. rmse value for the failure prediction is improved by second-degree regression criterion with the best fitting with actual data for shirahama sandstone, yamaguchi marble, mizuho trachyte, and orikabe monzonite considering six material parameters. rmse for some rock types like ktb amphibolite (61.94 mpa rmse), inada granite (47.06 mpa rmse), and coconino sandstone (67.05 mpa rmse) is higher due to the overfitting of data with six material parameters. linear criterion is under fitted on data, and second-degree polynomial is overfitted on data, so best fitting on data is done with new non-linear data with five material parameters. the material parameters in the proposed 3d failure criteria are obtained from the linear and non-linear regression analysis of conventional triaxial compression test data. a minimum of five to six triaxial testing data are required for regression analysis. the number of data points and their value affects the accuracy of regression analysis. the quality of the data point and the quantity of data used for the regression analysis play an important role in accuracy. the proposed new 3d failure criterion as a trigonometric function is validated with triaxial tested data points for the ten rock types. comparing these new 3d failure criteria is done with existing criteria like the priest criterion, the drucker-prager criterion, and the mogi-coulomb criterion. it has been shown that the new 3d non-linear criterion and mogi-coulomb performance gave better results among all these criteria. the rmse by the new 3d non-linear criterion is lower than the mogi-coulomb criterion for seven rock types: ktb amphibolite 40.03 mpa, dunham dolomite 15.16 mpa, shirahama sandstone 9.08 mpa, manazuru andesite 22.14 mpa, inada granite35.47, coconino sandstone 19.047 mpa. mogi-coulomb has the best rmse for solnhofen limestone 14.92 mpa, yamaguchi marble 8.36 mpa, and mizuho trachyte13.53 mpa. the desirable dc values are close to one in certain rock types (0.95 for inada granite, 0.94 for orikabe monzonite, and 0.91 for ktb amphibolite), while others fall within the range of 0.7 to 0.9. the proposed non-linear failure criterion performs well in haigh-westergaard coordinates, effectively predicting the failure behavior of different rocks. the mogi-coulomb criterion is related to octahedral shear stress and means effective stress that becomes difficult to solve. the mogi-coulomb solution gave dual results, making it challenging to choose one for the best prediction of principal stress. the number of parameters affecting the accuracy of the failure prediction. this new non-linear criterion considers five parameters that may reduce the accuracy. so, selecting the proper value of these parameters is challenging as per sensitivity analysis. however, sensitivity analysis suggests that all the parameters simultaneously influence the rmse value. a limitation of this new 3d failure criterion is that the input parameters must be optimized initially to achieve higher accuracy. there must be a minimum rmse for the best fitting of the parameters. prediction should further improve on optimizing all five parameters, that is, concerning trigonometric function with machine learning tools. however, before applying the new failure criterion, significant research is required on calibrating constants with triaxial compression test results as material parameter is changed with rock type. it is advisable in further research to develop a criterion that 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(2021). prediction of land degradation by machine learning methods: a case study from sharifabad watershed, central iran. earth sciences research journal, 25(3), 353-362. https://doi.org/10.15446/esrj.v25n3.75821 to monitor and predict the groundwater levels in sharifabad watershed, central province, iran three models of partial least square regression (plsr), artificial neural networks (ann) and adaptive neuro-fuzzy inference system (anfis) have been used. in all models, 70% of the data was used for training, while 30% of data were employed for testing and validation. monthly rainfall, topographic wetness index (twi index), the distance from the river, geographic location was the inputs and the level of groundwater was the output of each method. it is observed that ann has the highest efficiency, which agrees with other findings. the results of ann have been used in preparation of groundwater distribution map. according to the potential desertification map and groundwater level index, the potential of desertification had become severe since 2002 and was at a rate of 60% of land area, which, due to incorrect land management in 2016, increased to almost 98% of the land surface in the study area. using ann, it is predicted that 100% of the area was severely degraded for 2025. in addition to the target variable, latitude and longitude play important roles in ordinary krigging and decreased the total error of two combined models. abstract prediction of land degradation by machine learning methods: a case study from sharifabad watershed, central iran degradación de la tierra medida a través de métodos de aprendizaje automático: caso de estudio de la cuenca sharifabad, en irán central issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.75821 con el fin de monitorear y predecir los niveles de agua subterránea en la cuenca sharifabad, provincia central de irán, se utilizaron los modelos regresión de mínimos cuadrados parciales (plsr, del inglés partial least square regression), redes neuronales artificiales (ann, artificial neural networks), y sistema de inferencia de neurodifusión adaptativo (anfis, adaptive neuro-fuzzy inference system). el 70 % de la información fue utilizada para probar los tres modelos, mientras que el 30 % se empleo en la evaluación y validación. la pluviosidad mensual, el índice topográfico de humedad (twi index), la distancia al río y la ubicación geográfica fueron los datos ingresados, mientras que el índice de agua subterránea es el resultado de cada método. se observó que el modelo ann es el de mayor eficiencia, y que es acorde con otros hallazgos. los resultados del modelo ann se utilizaron en la preparación del mapa de distribución de aguas freáticas o subterráneas. de acuerdo con el mapa de desertificación potencial y el índice de agua subterránea, el potencial de la desertificación se ha vuelto severa desde 2002. esto significa que desde el 60 % del suelo que, debido a un manejo incorrecto en 2016, se llegó hasta casi el 98 % de terreno en el área de estudio. con el modelo ann se predice que el 100 % del área se degradará severamente para 2025. además de la variación del objetivo, la latitud y la longitud juegan papeles importantes en el kriging ordinario y en la reducción del error total de los dos modelos restantes. resumen record manuscript received: 27/10/2019 accepted for publication: 14/12/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 353-362 vahid habibi1, hassan ahmadi2, mohammad jaffari2*, abolfazl moeini1 1department of natural resources and environment, science and research branch, islamic azad university, tehran, iran. 2faculty of natural resource, university of tehran, karaj, iran. * corresponding author: jafary@ut.ac.ir h yd ro g eo lo g y https://doi.org/10.15446/esrj.v25n3.89293 https://doi.org/10.15446/esrj.v25n3.89293 354 vahid habibi, hassan ahmadi, mohammad jaffari, abolfazl moein 1. introduction resource degradation occurs gradually but monitoring land and water resources can help prevent potential hazards (masoudi and amiri, 2015; masoudi et al., 2018; morales and zuleta, 2020). more than 250 million people are directly affected by desertification, and more than one billion people in 100 countries are at risk of desertification and land degradation. it threats about 6 million hectares of the world’s land. two thirds of iran’s lands are under desertification or degraded. unfortunately, due to natural and human factors, land degradation is progressing recently (gibbs and salmon, 2015; dubovyk, 2017; ferrara et al., 2020). since four hundred years ago, 30 % of the natural forests and rangelands converted into pastures and farmlands and caused soil organic carbon loss, soil structure decline and changes in soil physical and chemical properties (yaghobi et al., 2018; afzaal et al., 2020). more than twothirds of iran has arid and semi-arid climate and drought, air pollution and climate change contributed to iran’s current environmental crisis especially in the central, west and southwestern regions. now, the major land-degradation research focuses on the evaluation and monitoring based on the different data sources, conservation and ecological restoration, and determination of driving factors at different spatial and temporal scales, trend simulation development and predictive quantitative models (xie et al., 2020). various models are introduced for land degradation monitoring such as fao–unep (1984), environmental sensitive area; esa (kosmas et al., 1999), modified iranian classification of desertification; micd (ekhtesasi and mohajeri, 1995), the global assessment of soil degradation; glasod (oldeman et al., 1991), global assessment of the status of human-induced soil degradation; assod (van lynden and oldeman, 1997), the land degradation assessment in dry lands (lada) (fao, 2002); the iranian model of desertification potential assessment (imdpa) (ahmadi, 2008). the models considered various criteria and indices to evaluate potential resource degradation and many of these models determined fluctuations of groundwater through years and seasons. these models provided vital information for farming plans, farming dates and rangeland improvement and development programs. for the reason of fragility in dry land ecosystem, reliable sources of water like groundwater should be provided (shammout et al., 2018; chen et al., 2020). various authors emphasized that extraordinary consumption of the world’s groundwater, mainly in most of the middle eastern countries, has affected the quality of groundwater and has contributed to the reduction of crop per unit area in farmlands (verner et al., 2013; parizi et al., 2019; mirzaei et al., 2019). therefore, the wise management of resources guarantees the survival of human societies (mohanty et al., 2010). and because of climatic and topographic conditions in iran, groundwater is very important in ensuring food security (ashrafzadeh et al., 2016). groundwater is the main resource of water for the agriculture industry (jafari et al., 2018). a couple of years back, in the absence of land use planning, a huge volume of groundwater was consumed which led to financial problems (wu et al., 2020). the central plains of iran are facing the complication of the declining level of groundwater because of the lack of permanent rivers and dams, mismanagement of water bodies, climatic variability, and anthropogenic activities. therefore, monitoring groundwater level is vital to controlling and predicting the consumption rate. due to the reason that the measurement of the level of groundwater in aquifer plains is a time-consuming process, the estimation of the level of groundwater here has a higher priority than other locations. looking at literature in the field of agriculture and water resources, many of the researchers are interested in methods with smallest number of variables to monitor and predict the level of groundwater. recent approaches in non-classical methods have also raised the bias in the application of spatial statistics for realization of spatial replacements (jeihouni et al., 2015). the level of groundwater indicates groundwater availability, groundwater flow, and the substantial attribute of an aquifer (nair and sindhu, 2016; alqadi et al., 2019). several models have been used to evaluate groundwater quantity and quality in the world (bodrud-doza et al., 2016). statistical models and spatial interpolation have been compared in different studies, such as environmental sciences (abudu et al., 2010; parmar and bhardwaj, 2014; khaki et al., 2016; meshram et al., 2020). in recent decades, artificial intelligence systems are used as analyzing tools to study issues related to water and environmental sciences (jeihouni et al., 2015; wu et al., 2020). artificial neural network (ann) is beneficial in a position where the fundamental real process relationships are not completely understood and well-suited in modeling dynamic systems on a real-time basis (lim and kolay, 2009). anns operates on the principles of learning from a training set. before training, the network does not have any earlier information about the type of problem (bessaih et al., 2014). anns show a smooth but accurate result of many imagery issues (suprayogi et al., 2020; iqbal et al., 2020). several articles have, heretofore, studied the utilization of anns in the field of water resources (kazemi and hosseini, 2011; yesilnacar and sahinkaya, 2012; pektas and dogan, 2015; sebghatiand gholami, 2019). ann is an applicable estimation tool for groundwater levels in areas without information or inadequate observation points. (sethi et al., 2010; karimi et al., 2012; rankovic et al., 2014; khaki et al., 2015; alizamir et al., 2017; mohanty et al., 2010; lohani and krishnan 2015; porte et al., 2018; chitsazan et al., 2013; nair and sindhu, 2016) accordingly, the partial least square regression (plsr) method was initially used as a calculation algorithm for special vectors, but was quickly interpreted by statistical criteria. in fact, the general idea of plsr is to find the hidden variables. the hidden variables are quantitative variables which explain the variability of the response variable (chun and keles, 2010; p. roy and k. roy, 2008). the plsr technique admits examining the relationships between visible and hidden variables simultaneously (wold et al., 2001). this method uses two outer and inner test models that apply a two-step complex process to estimate weights (tekin et al., 2014). recently, pls regression has been applied to soil and groundwater mapping (ibrahim and wibowo, 2013; dhungana, 2015; humbert et al., 2015; tzanakakis et al., 2015; kawamura et al., 2017; gad et al., 2020). adaptive neuro-fuzzy inference systems (anfis) and ann can be regarded as powerful tools in the statistical model identification algorithm and to produce a simple model by virtue of competency of task estimation and categorization (gong et al., 2018; emamgholizadeh et al., 2014). fuzzy models propose interest over mathematical ones; the process of inference is close to human logic and easier to measurement with complicated non-linear methods. moreover, these methods are advantageous and more functional to specialist modeling researchers (esen and inalli, 2010). anfis adopts a special system of fuzzy inference with a backpropagation algorithm founded by a group of input data and output data (babuska, 1998). anfis was first declared by jang (1993). tan et al. (2017) assumed the anfis approach to measure country sustainability performance. they found that using anfis, evaluation validity as it may be better through sublevel choice of training samples by other data from un-habitat, or world bank, or even new data sets. solgi et al. (2016) applied anfis models for predicting biochemical oxygen demand in the karun river in the western part of iran. ahmed and ali-shah (2017) applied anfis to predict of bod in water river. they concluded that anfis is a convenient method to investigate bod with advisable validity, suggesting the anfis method is a beneficial tool for water quality parameter analyses. almuhaylan et al (2020) mentioned the anfis can provide more accurate, the anticipation of groundwater depth. in this study, we determined the efficiency of non-linear algorithmic models in predicting the level of groundwater and evaluating land degradation with a small number of variables. the purpose of our study is to compare ann, anfis and plsr methods for assessing groundwater level in sharif abad watershed and determine the severity of groundwater degradation. 2. materials and methods 2.1. study area the sharif abad watershed is located in between 34° 37´ 34°56´n and 50°31´ 51°02´e covering an area of 96787 ha. in central part of iran, qom province, iran (fig. 1). the study area is very important in the production of pistachio and alfalfa. at present, 37% of the watershed area is under farmlands and orchards (over 35,000 ha). 24,000 ha out of 35,000 ha are alfalfa fields and pistachio orchards. according to the iranian ministry of agriculture, in 1989, the area of pistachio orchards and alfalfa fields was 5,000 ha, indicating a 7-fold increase in the farming area. the rangelands are converted to farms and orchards and drilling numerous water wells increased the farming intensity. the climate of the area is arid, and the average rainfall is 145.3mm. groundwater data of 24 piezometric wells from 2002 to 2016 was used to do modelling studies using pls regression, ann, and anfis. in addition to monthly rainfall, topographic wetness index (twi), distance of the river (m), 355prediction of land degradation by machine learning methods: a case study from sharifabad watershed, central iran latitude and longitude in the universal transverse mercator (utm) coordinate system were explained input and level of groundwater set an output of each method. however, to activate the over fitting condition, in all models, 70% of the data was applied for model training, while 30% of data were employed for testing and validation. the coefficient of correlation is calculated as the criterion of a linear relationship between observational and estimated values (rankovic et al., 2012). a brief description of models is demonstrated in sections. 2.2. partial least square regression (plsr) partial least square regression (plsr) is a modern method of predictive modelling, which works with large data matrix and collinear variables. plsr was first developed by wold (1985) but the theoretical basis of plsr was developed by varmuza and filzmoser (2009). plsr generalizes the important aspects of multiple linear regression (mlr) and principal component regression (pcr). ordinary least square (ols) regression works well with a full rank matrix of predictors (x) but when x is singular, plsr takes care of the problem. plsr extracts the factors from x and y matrices and maximizes the covariance. plsr uses the linear decomposition of x and y as: x = t pt +ex and y = uqt + ey where t = x-scores and u = y-scores p = x-loadings and q = y-loadings, ex = x-residuals and ey = y -residuals using the u, plsr automatically predicts y. t and u are linear combinations of x and y variables, respectively. suppose tj, uj, pj and qj denote the jth columns of t, u, p, and q respectively, where (j = 1..., a). 2.3. artificial neural networks the artificial neural network (ann) is a computational approach introduced to model the neurons’ function in living tissues. mcculloch and pitts (1943) developed a mathematical analysis for ann from algorithms and applied mathematics called threshold logic to it. choosing the right number of neurons and layers provide a better model of the artificial neural network (ennouri et al., 2017). in neural networks, neurons are responsible for processing. in fact, each neuron is associated with a set of its preceding and subsequent elements, which, this non-linear relationship provides the prediction of target data (alipour et al., 2014). li et al. (2018) proposed that anns can be used at events that do not have the structure and complexity of the relationship among output data and input information. network models, multilayer perceptron’s (mlps), generalized feed forward networks (gff), modular neural networks (mnn), radial basis function (rbf) trained by momentum optimization algorithm (mo), levenberg-marquardt algorithm (lm), conjugate gradient (cg) learning rule, sigmoid axon and tanh axon as transfer function were selected and compared in neurosolution software. four hidden layers were used to regulate the weights of neurons to achieve the desired output. for network model, in addition to rainfall, distance piezometer well from the qara-chai river, topographic wetness index (twi), latitude and longitude were also considered as the input and groundwater level was defined as the output (fig. 2). the twi indicator is shown in equation-1. twi index is a helpful tool for describing the humidity conditions in the watershed scale, presented by (grabs et al., 2009). twi index of each piezometer was calculated using dem in saga gis software (eq.1). twi ln a tan s=       β (1) where β is the local slope (radians) and as is the contributing area of upstream (m2). figure 2. the multi-layer perceptron (mlp) structure figure 1. location of study area in iran map (a) land uses in the sharifabad watershed, iran (b), agriculture lands(c and d), bare land (e), rangland (f) 356 vahid habibi, hassan ahmadi, mohammad jaffari, abolfazl moein 2.4. adaptive neuro-fuzzy inference system (anfis) adaptive neuro-fuzzy inference system (anfis) approach applied two networks learning algorithms – neural and fuzzy logic, for scheming nonlinear relations between dependent and independent variables. it enables fuzzy laws to be determined and prepare data for the expert. furthermore, it could transform the intricacy of human sagacity into fuzzy systems category (alipour et al., 2014). there are various joined nodes in the structure of an anfis model. the output of some nodes is reined by accommodating variable joint the node, so the output data would predict a predefined error value. this showed that some nodes are examined to have an adaptive part in the structure of the method (baghban et al. 2017). the frame of anfis involves five layers (fig. 3), and the functions correlate with nodes of the matching layer is similar. each input has two rules (a1 and a2, b1 and b 2) in the initial layer (input nodes), which can make two rules in another layer (rule nodes). a brief diagram of the operations of the five layers is given as follows (jang, 1993): layers 1 through 5 composed of input nodes, rule nodes, average nodes, resulting nodes and output nodes. the rules of every single layer are introduced in equations two to seven, respectively equations 2 to 7. oii = µai(x) for i=1, 2 (2) µai x( )= 1 1+ x-ci ai � � � � � � � � 2� � � � � � � � � � bi µa x 1 1 x c a i i i 2 bi ( )= + −                  for i=1, 2 (3) ωi=µai x( )¥µbi x( ) for i=1, 2 (4) ω ω ω ωi i 1 2 = + for i=1, 2 (5) ω ωo f p x q y ri 4 i i i i i i( )= = + + (6) oi 5= i âωifi= i â ωi fi i â ωi (7) formula: where x is the input to the node i, and ai is nominal labels ruled by fit membership function µai; {ai, bi, ci} is the variable collection. here {pi, qi, ri} is the variable lay in the consequent sector of the first-order sugeno fuzzy model. figure 3. basic structures of anfis. in this study, takagi-sugeno-kang type fuzzy (tsk) and tsukamoto (ts) fuzzy models were tested and momentum, levenberg–marquardt and conjugate gradient learning rules used. for any of the algorithms, the sigmoid axon and tanh axon transfer model were used. 2.5. land degradation assessment model land degradation research has been done in 93 countries. the number of publications in different countries show their contribution and impact in land degradation research. researchers from 93 countries participated in the land degradation research between 1990 2019. in regard to total publication, iran was the 21st nation (xie, 2020). land degradation in arid regions of the world is a complicated environmental puzzle (thomas, 1997; okin et al., 2009; rasmy, 2010). it is accepted as one of the greatest environmental risks in iran because of its natural conditions and human activities (tahmoures et al., 2013). as previously mentioned, several models have been developed for the judgment of soil and water resource degradation in world scientific communities. to assess the potential of land degradation, ahmadi (2008) presented the iranian model of desertification potential assessment (imdpa). this model involves nine classes of criteria that causing effective of land degradation. groundwater depletion (cm/year) in watershed aquifer resources is one of the factors, which used to estimate degradation risk of water resources. table1 shows the land degradation potential of groundwater index. table 1. desertification index of groundwater criteria index land degradation risk groundwater depletion (cm/year) low moderate high severe <20 20-30 30-50 >50 2.6. model evaluation and prediction the prediction performance of both the training and testing sets is evaluated by the coefficient of determination (r2) and mean squared error (mse) esq. (8-9). the calculation formula according to webster and oliver (2001) is: r ss sse ss yy yy 2 = − , ssyy= i â yiy( )2 , sse= i â yiyi !( )2 (8) mse= 1 n i=1 n â yiyi !( )2 (9) where yi  is predicted and yi is observed data and n is the number of observations. mse is the average squared difference between predicted values by the model and measured values. mse is an indicator of model accuracy or precision. mse should be as low, or close to zero, as possible. the coefficient of determination (r2) establishes a linear correlation between measured values and simulated ones by model. the value of 1 is optimal and shows the positive linear relationship. after model evaluation, the best model was selected using r2. the farming season and/or the heavy use of groundwater and water well do not start until around april, so we predicted the level of groundwater for april 2025. looking at twi index, the distance from river and latitude and longitude variables, they all have spatial information and only monthly precipitation has spatial-temporal characteristic. using the linear regression relationship between monthly precipitation and elevation of synoptic station, the precipitation of 24 piezometric wells was calculated for april 2025 and was also used in predictive model. after that, interpolation was carried out via the ordinary kriging method and groundwater level of april 2025 was also calculated, where the depletion maps were prepared by subtracting presents year map and the map from the previous year. after subtraction, the potential desertification map for years 2002, 2010, 2016 and 2025 (prediction year) was prepared. figure 4 shows the procedures of calculation. receiver operating characteristic curve (roc) is a classifier or diagnosis method.the efficiency of binary classifier can be evaluated with sensitivity and recall indices. in roc graph, a combination of these two indices are presented as a curve. researchers test efficiency of the cluster algorithms or raster data using roc curve. this technique has been frequently used in supervised machine learning. roc curve is value of true positive rate over the false positive rate in the confusion matrix. since the classes are based on continuous random variable, the curve is continuous function which can be estimated from data. 357prediction of land degradation by machine learning methods: a case study from sharifabad watershed, central iran figure 4. the procedures of our study 3. results we predicted the level of groundwater of 24 piezometric wells, with 5 inputs and 1 output using anfis, ann, and plsr between 2002 and 2016 in sharif abad watershed, qom province. figure 5 shows the annual precipitation of 143mm at the sharif abad watershed and, usually, the level of groundwater and precipitation are different. also, it shows the contradiction between precipitation and the level of groundwater. we applied ann on 4 networks using trial and error and predetermined parameters and compared the results to find the optimum network. table 2 shows the 4 networks of piezometric wells during training and testing stages. khaki et al (2016) mentioned that the sigmoid transfer functions model works better than others, but we also used the tanhaxon method with 500 epoch. figure 5. monthly rainfall (mm) and water level (m) for observation well from 2002 to 2016 as table 2 shows, the mlp method with sigmoid axon transfer model, which trained with levenberg–marquardt algorithm, was the best model based on r2 and mse criteria. to predict the level of groundwater with anfis method, two fuzzy model networks, takagi-sugeno-kang type fuzzy and t tsukamoto, with similar algorithms of ann are compared to find the best network. table 3 shows the evaluation of networks for selected points in training and testing stages. as table 3 shows, tsukamoto method with levenberg–marquardt training algorithm was the best method. to predict the level of groundwater with the plsr method, we evaluated 5 components, with the fifth one having the highest functionality (table 4). table 2. comparison of four models developed in training and testing stages. model algorithm transfer train test r2 mse r2 mse rbf lm tanh axon 0.51 248.18 0.498 240 sigmoid axon 0.66 204.7 0.44 225 mo tanh axon 0.55 205 0.49 212 sigmoid axon 0.58 208.9 0.53 210.5 cg tanh axon 0.72 165 0.712 166.4 sigmoid axon 0.78 148 0.71 155.8 mlp lm tanh axon 0.912 11.2 0.901 12.8 sigmoid axon 0.963 7.15 0.956 7.33 mo tanh axon 0.881 15.6 0.891 14.8 sigmoid axon 0.85 22.4 0.82 19.6 cg tanh axon 0.79 46.8 0.771 45.6 sigmoid axon 0.82 38.5 0.831 36.6 gff lm tanh axon 0.2 208.8 0.22 209.2 sigmoid axon 0.11 208 0.15 212 mo tanh axon 0.12 222 0.121 218 sigmoid axon 0.3 195 0.25 200.8 cg tanh axon 0.09 204.7 0.08 200 sigmoid axon 0.26 205 0.49 217 mnn lm tanh axon 0.45 178 0.56 167 sigmoid axon 0.51 166 0.44 275 mo tanh axon 0.46 171 0.44 288 sigmoid axon 0.04 205 0.044 212 cg tanh axon 0.44 208.9 0.43 209.8 sigmoid axon 0.06 255 0.1 210 table 3. comparison of two anfis models developed in training and testing stages. fuzzy model learning rule transfer train test r2 mse r2 mse takagisugenokang mo tanh axon 0.09 712 0.085 765 sigmoid axon -0. 12 850 -0.11 886.4 cg tanh axon -0.002 908.4 -0.001 915.6 sigmoid axon 0.51 248.6 0.49 266.7 lm tanh axon 0.44 208.9 0.43 209.8 sigmoid axon 0.38 265.7 0.36 277.9 t sukamoto mo tanh axon 0.11 828.01 0.095 911.05 sigmoid axon 0.17 625.1 0.13 711.2 cg tanh axon 0.022 3347 0.018 3560 sigmoid axon 0.58 95.1 0.571 102.6 lm tanh axon 0.793 64.3 0.786 66.7 sigmoid axon 0.52 110 0.51 115.5 table 4. comparison of five components developed by plsr methods. components x variance mse r2 1 0.348 758.7 0.126 2 0.782 746.5 0.141 3 0.802 706.8 0.185 4 0.996 705.5 0.187 5 1 693.4 0.199 358 vahid habibi, hassan ahmadi, mohammad jaffari, abolfazl moein table 5 shows the comparison of three methods where ann had the highest functionality of predicting groundwater level in sharif abad watershed. figure 6 shows scatter plots of the observed and estimated water levels at training period with selected of three evaluated models. also, figure 7 shows that ann worked relatively better than anfis and plsr. the plsr model had the smallest auc. the auc evaluates a given classification model with no threshold assumption. roc curves are suitable tools for comparing classifiers where there is no information about misclassification water level. here, we interpret the most important input parameters of this technique in groundwater level modeling. this diagram shows the importance of latitude and longitude in groundwater level modelling (fig. 8). table 5. comparison of three methods for the prediction of g.w.l model the best algorithm/component r2 mse ann mlp, lm, sigmoiedaxon 0.963 7.15 anfis tsukamoto, lm, tanh axon 0.793 64.3 plsr 5 th component 0.199 693.4 after finding the optimal model, the groundwater level of 24 piezometric wells in 2025 was determined. then, the groundwater level was mapped via ordinary kriging. using the groundwater level map in 2025, and table1, we determined the degraded regions and depleted groundwater points. figure 8 and table 6 both show the land degradation of the study area using groundwater criterion. table 6. land degraded area based on gwl index in the study area since 2002-2016 land degradation risk 2002-2003 2009-2010 2015-2016 predicted year low 38030 moderate 386 high 793 2032 1581 severe 57578 94755 95206 96787 total area (ha) 96787 96787 96787 96787 figure 6. scatter plots of the observed and estimated water levels at training period. ann (a), anfis (b), plsr(c) figure 7. roc curves for ann, anfis and plsr(c) figure 8. diagram of importance input factors for modeling of groundwater level 359prediction of land degradation by machine learning methods: a case study from sharifabad watershed, central iran as it can be seen from table 6 and figure 9, between 2002 and 2003, around 59.4% of the area is in critical condition of desertification, around 39% and less than 1% has normal and moderate condition respectively. while in the predicted year, land with a potential desertification in the study area has increased by 100% (table 6 and fig. 8). after finding the optimal model, the groundwater level of 24 piezometric wells in 2025 was determined. then, using ordinary kriging, the groundwater level was mapped (fig. 10). figure 9. percentage of degraded land based on groundwater index from 2002 to 2016 figure 10. land degradation map based on groundwater level in april in different years (a) 2002-2003 (b) 2009-2010, (c) 2015-2016, (d) predicted year 2017 4. discussion and conclusion land degradation as a global issue deserves more attention. research findings showed the major land degradation driving factors as altitude, slope, vegetation index, decline of vegetation index, bare land, mid-slope position, and depletion of ground water resources. spatial and temporal prediction of groundwater level changes provides useful information for wise management and sustainable development. in this research, the groundwater level was predicted using three models ann, anfis, and plsr, where the available parameters were used as model input. the coordinates of the wells being studied were used as two input variables. we had a different response variable in comparison to the research made by jeihoni et al (2015), but our results also showed the usefulness of ann like the authors stated. looking at the statistical indices, we found that the ann has the highest efficiency which agrees with other findings (khaki et al., 2015; alizamir et al., 2017; alqadi et al. 2019; sebghatiand gholami, 2019; meshram et al.2020). we combined the results of ann with ordinary kriging to predict groundwater condition map. we also used latitude and longitude as input variables which improved the model. in addition to the target variable, latitude and longitude play important roles in ordinary kriging and decreased the total error of two combined models. the study area currently suffers from intensive farming such as pistachio orchard. our results could help water resource and agricultural managers make the proper decisions as well as help decrease the chance of desertification and land degradation. our findings agree with mohanty et al., (2010), lohani and krishnan (2015), porte et al. (2018), chitsazan et al. (2013), nair and sindhu (2016), suprayogi et al., (2020) and show the efficiency of ann in monitoring the level of groundwater. there are only a few differences due to the selection of algorithms. our results did not agree with the findings of gong et al. (2018), almuhaylan et al. (2020) and emamgholizadeh et al. (2014). this is because they mentioned that anfis has a higher efficiency than ann. we also believe that the disagreement is due to time series difference in the neuro fuzzy model structure and various other scenarios rather than the groundwater controlling factors. unlike in previous studies, we considered the last year precipitation and distance from qarachay river as recharging sources of groundwater. looking at the observed and estimated levels of groundwater, we found that the difference between levels in 2002-2010 is larger than the difference in 2010-2016. in early years, the level of groundwater was underestimated. according to the figures of the level of ground water and monthly precipitation (figure 5), it’s obvious that the difference in maximum and minimum monthly precipitation is significant as 60mm in the whole watershed area and through the time. in 2016, the difference decreased to 20mm and the difference between the observed and estimated levels of groundwater became smaller. according to farming history in the area, we found that by 2010, the study area was under intensive farming and eventually it converted to orchards due to water shortage in qara-chai. also due to groundwater exploitation, the situation was aggravated and hasn’t prevented the desertification. we therefore recommend the following: the use of ann in similar studies which monthly precipitation doesn’t change drastically. annual monitoring of groundwater, efficient water usage and selecting the proper farming practices in the study area. our method is a useful tool for areas without piezometric wells and saves the time and money for digging. references abudu, s., cui, c., king, j. p., & abudukadeer, k. 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(2002). evaluation of water resources degradation in kashan area using desertification model. journal of environmental studies, 28, 19–30. https://doi.org/10.1038/s41467-020-17581-y https://doi.org/10.1038/s41467-020-17581-y https://doi.org/10.3390/land9010028 https://doi.org/10.1016/j.geoderma.2017.12.037 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.111269 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 45 54 pl a n et a ry s c ie n c es abstract: ismenius cavus (33.76° n, 17.05° w) is a large martian basin that has been interpreted as an ancient paleolake. it is situated in the middle of the path mamers valles traces from the cerulli crater to the borealis basin. this ancient lake served as a key deposition area (or depocenter) for multiple lake chains originating in the southern highlands. the collected water was then transported downstream to the borealis basin. the ancient hydrologic activity of this zone is evidenced in the widespread appearance of valley networks and fretted channels, but especially in the presence of deltaic deposits converging in ismenius cavus. we made a hydrological and geomorphological analysis of two of these deltas, which compose a three-crater lake chain system. it was interpreted that both deltas, although being close to each other, were created by different processes and at different times. the aracataca delta was deposited during the noachian by a valley network system. the ariguani delta, in turn, was the result of the discharge of a fretted channel carved by groundwater sapping. the transformations of the hydrologic systems in the ismenius cavus region were established by analyzing the change in the deltaic pulses over time. it was determined that the first hydrologic stage was dominated by widespread valley networks that probably represent subglacial hydrologic systems, a second stage started when these systems were replaced by groundwater activity; and finally, when the water sources were exhausted, a final stage of glacial processes prevailed until the present. hydrologic evolution of two martian deltas in the ismenius cavus system javier eduardo suárez-valencia1,3, daniela alexandra guerrero-gutiérrez2,3, natalia katherine soler-aragon3, daniel felipe ramirez-diaz3, iván alexis nocua-benítez3, camilo andres escobar-tarazona3, jose david gomez-ruiz3 1. constructor university bremen, campus ring 1, 28759, bremen, germany. 2. instituto de geociências, universidade de brasília, 70910 900 brasília, df, brasil 3. departamento de geociencias, universidad nacional de colombia, carrera 30 no. 45-03 bogotá, colombia *corresponding authors: jsuarezvalencia@constructor.university daniela.gutierrez@aluno.unb.br nksolera@unal.edu.co dframirezd@unal.edu.co inocua@unal.edu.co cescobarta@unal.edu.co josgomezru@unal.edu.co record manuscript received: 23/09/2023 accepted for publication: 26/03/2024 how to cite this item: suarez-valencia, j. e., guerrero-gutierrez, d. a., soleraragon, n. k., ramirez-diaz, d. f., nocua-benitez, i. a., escobar-tarazona, c. a., & gomez-ruiz, j. d. (2024). hydrologic evolution of two martian deltas in the ismenius cavus system. earth sciences research journal, 28(1), 45-54. https://doi.org/10.15446/esrj. v28n1.111269 keywords: mars; planetary geomorphology; hydrology; cartography. evolución hidrológica de dos deltas marcianos en la región de ismenius cavus resumen ismenius cavus (33.76° n, 17.05° w) es una gran cuenca marciana que ha sido interpretada como un antiguo paleo-lago. está situada en la mitad del trayecto que recorre mamers valles desde el cráter ceruli hasta la cuencas borealis. este antiguo lago sirvió como un punto importante de deposición (depocentro) para múltiples cadenas de lagos originados en las tierras altas al sur. el agua recolectada en este lugar era luego transportada hasta la cuenca borealis. hay varias evidencias que demuestran la existencia de un antiguo sistema hídrico en la región, incluyendo la aparición continua de redes de drenaje y canales anchos, además de la presencia de deltas que desembocan en ismenius cavus. en este estudio realizamos un análisis hidrológico y geomorfológico de dos de estos deltas, los cuales hacen parte de un sistema lacustre de tres cráteres. se encontró que a pesar de su cercanía, ambos deltas se formaron por procesos diferentes y en tiempos distintos. el delta aracataca se depositó durante el noeico por un sistema de drenajes superficiales, mientras que el delta ariguani se formó por la descarga de un canal encañonado producto de la erosión de aguas subterráneas. la transformación de los sistemas hídricos de ismenius cavus fueron establecidos al analizar los cambios en los pulsos deltaicos a través del tiempo. se encontró que una primera etapa estuvo dominada por redes de drenajes superficiales, probablemente provenientes de sistemas subglaciales; la segunda etapa comenzó cuando estos sistemas mermaron, siendo reemplazados por un dominio de aguas subterráneas; y finalmente, cuando las fuentes de agua se agotaron totalmente, una última etapa de procesos glaciares tomo lugar, la cual ha dominado la zona hasta hoy. palabras claves: marte; geomorfología planetaria; hidrología; cartografía https://doi.org/10.15446/esrj.v28n1.111269 mailto:jsuarezvalencia@constructor.university mailto:daniela.gutierrez@aluno.unb.br mailto:nksolera@unal.edu.co mailto:dframirezd@unal.edu.co mailto:inocua@unal.edu.co mailto:cescobarta@unal.edu.co mailto:lhochoag@unal.edu.co https://doi.org/10.15446/esrj.v28n1.111269 https://doi.org/10.15446/esrj.v28n1.111269 46 javier eduardo suárez-valencia, daniela alexandra guerrero-gutiérrez, natalia katherine soler-arago, daniel felipe ramirez-diaz, iván alexis nocua-benítez, camilo andres escobar-tarazona, jose david gomez-ruiza 1. introduction the mars northern dichotomy has always served as evidence for the wet past of the planet. multiple landforms like valley networks, paleolakes, and fretted channels, point in this direction. of special significance is the borealis basin, a massive depression that encompasses the north pole, which may have hosted an ancient ocean (baker et al., 1991). several fluvial systems occur across the limit between the borealis basin and higher lands, but deltaic features may be the most interesting and significant of them, as they require a constant flux of water and a deep reservoir to form. a total of 52 deltas have been observed on mars (di achille & hynek, 2010). this research analyzed two of those features associated with paleolakes located in the ismenius lacus quadrangle. the two analyzed deltas are close to each other and were named informally during this investigation as the aracataca delta (34.31° n, 18.11° w) and the ariguani delta (33.93° n, 17.52° w). the basement of the region is heavily cratered and has been dated as late noachian, around 3.88 ga 3.92 ga (benedix et al., 2020). two features dominate the zone: deuteronilus mensae, an area made of ancient plateaus in the transition to the borealis basin; and mamers valles, a large canyon that crosses the entire zone (figure 1). within mamers valles lies ismenius cavus, a paleolake where the two studied deltas drained (dehouck et al., 2010). both features were first identified by cabrol (1999), while ori et al. (2000) made initial interpretations about its deltaic origin. the more recent and accurate hirise and hrsc data provided an opportunity to better analyze this complex system, which preserves the evolution of the hydrologic activity of the quadrangle. figure 1. location of the aracataca and ariguani deltas in the ismenius lacus quadrangle. the main landforms of the zone are indicated with green lines. 2. data and methods the geomorphological analysis was made on three scales. at a regional scale, visible images of viking with a resolution of 232 m/pixel, themis daytime infrared with 100 m/pixel, and mola digital elevation model (dem) with 463 m/pixel were used (edwards et al., 2011). at the medium scale, hrsc multispectral images (takes: 5249 and 5267) with spatial resolution between 10 and 20 m/pixel and dems with 100 m/pixel were used (geng et al., 2017). finally, in the detailed scale, the two deltas were studied with two hirise datasets with a resolution of 50 cm/pixel (aracataca delta spotted by hirise product esp_026915_2145 and ariguani delta by hirise product esp_013531_2140). the geomorphologic units defined in this work adhere to the criteria established by tanaka et al. (2009) and wilhems (1972). the defined bodies are discrete and follow the superposition principle and the crosscut laws. the name of the units is generally informal, but some names such as ismenius cavus, borealis basin, and mamers valles are official nomenclature. geomorphological units were established at all scales, the identification criteria were texture, albedo, topography, color, stratigraphic relationships, drainage network, and surface morphology variation. at the local and detailed level, we also considered the morphogenesis of the units during the classification. the interpretation was made in the software arcgis 10.5. figure 2. regional geomorphological map of the study area. valley networks are represented as blue lines and fretted channels as green polygons. the two red stars in the southwestern part show the location of the deltas inside ismenius cavus. the basemap was the themis daytime infrared. 3. results the geomorphology of the zone, as well as other regions of similar latitudes, is the result of the activity of water and ice over the martian history (irwin, 2004; alemanno et al., 2018; chuang & crown, 2009). other landforms related to the influence of eolian activity are present to a lesser extent in the study area. to reconstruct the geological history of the region, it is crucial to determine which geologic processes were responsible for making the observed landforms, and how they evolved as the environmental conditions of the location changed. the influence of these forces and the changes they made over time can be traced at different scales. first, we made a regional description, then we focused on the ismenius cavus zone and the possible paleo-basins in it, and finally, we described the deltaic features inside the craters selected for this study. an extended version of the maps produced in this work at each scale is provided in the supplementary material. 3.1 regional scale the basement of the ismenius cavus quadrangle is made of middle noachian rocks (benedix et al., 2020), most of which were modified by posterior geological processes. preserved landforms of this age are found at the western margin of the region, north of the dichotomy. in this zone is present the mnp unit (figure 2), which has been interpreted as large lava floods (lucchitta, 1978; mcgill, 2002) and is moderately dissected by valley networks and impact craters. the deuteronilus mensae zone is characterized by the presence of tall plateaus and knobs of the mnp unit, which are surrounded by homogeneous lowlands of the nhi unit (figure 3a). it has been thought that this morphology was formed by the melting of ice trapped in aquifers and the subsequent collapse of topping layers (zegers et al., 2010). however, the abundance of glacial deposits as those of the debris aprons (ada unit) and the distance between plateaus indicate a subsequent fluvial and glacial transformation (sinha & murty, 2013). 47hydrologic evolution of two martian deltas in the ismenius cavus system south of the borealis basin domain, channels and lacustrine deposits are more common. in this zone, the basement is made of the mnh1 and mnh2 units. these units are similar to mnp to the north, but in this case, the plains are heavily dissected by valley networks and fretted channels, the major of them being mamers valles. the latter has a well-developed morphology of a fretted channel with a length of around 1000 km. it extends northwards, cutting the highlands, reaching the northern dichotomy, and disappearing inside deuteronilus mensae. several moraine-like deposits were identified on top of the valley floors, similar to other martian locations (chuang & crown, 2009; lucchitta, 1978). figure 3: several landforms representative of the zone. a) plateau from the mnp units. b) sublimation features showing glacier activity. c) other deltas inside ismenius cavus and phyllosilicates deposits in black. d) valley networks flowing towards ariguani crater. e) moraine cutting the deposits of the aracataca crater. f) u valley cutting fretted channels. 3.2 medium scale ismenius cavus is located in the southern highlands of the study area, mamers valles crosses it, and it was a local depocenter of the surrounding areas (figure 4). the zone is characterized by the presence of large impact craters, with nine of them exceeding 5 km in diameter, and almost all of them seem to have superimposed water activity. figure 4: geomorphological map of the area around ismenius cavus. valley networks are represented as blue lines, fretted channels as green polygons, and basins are delimited by white lines. the two red stars show the location of the deltas inside the ariguani and aracataca craters. the basemap were hrsc takes 5249 and 5267. two types of hydrologic activity are recognized in the area: valley networks, related to fluvial action (hoke et al., 2011), and fretted channels, associated with groundwater sapping (goldspiel and squyres, 2000). the valley networks are well developed and are highly integrated between them. however, the later action of meteoritic impacts, wind, and glaciers have modified them. most of these features have a middle noachian maximum age according to the youngest geologic unit that they cut (alemanno et al., 2018). on the other hand, the fretted channels are wide and poorly integrated valleys with a relatively straight morphology. using cross-cutting relationships, we determined that these features were developed after the valley networks. finally, there are glacial features that are coetaneous to previous structures, indicating that glaciers were involved at the different hydrologic stages of the region (figure 3e). ismenius cavus is the biggest and deepest depocenter in the region with 66.5 km in diameter and was analyzed in detail by dehouck et al. (2010) as a paleolake. besides the activity of mamers valles, features such as fretted channels, valley networks, phyllosilicates, and several deltaic structures have been interpreted inside it (figure 3c), which attests to the fluvial and groundwater activity that modified this ancient crater. this structure, with the aracataca and ariguani craters, makes the three-crater lake-chain system that contains the target deltas of this study. the aracataca and ariguani craters measure 26 km and 34 km in diameter, respectively, with crater rims between 400 m and 600 m high. several dendritic valley networks converge into those two craters, and big alluvial terraces fill both to their south margins (figure 3d). to the southwest, both craters have openings in their rims, from which the two deltas formed. at least two other deltas (fd units in figure 4) have been identified inside ismenius cavus (leeder, 2011; ori et al., 2000; dehouck et al., 2010) and valley networks (fap unit) are also common in the surrounding areas of the biggest craters. furthermore, there are also fretted channels (fct and fcf units) that connect lake chains all over the region and are usually found surging from the noachian plains. from this, we can tell that the backward erosion of valleys 48 javier eduardo suárez-valencia, daniela alexandra guerrero-gutiérrez, natalia katherine soler-arago, daniel felipe ramirez-diaz, iván alexis nocua-benítez, camilo andres escobar-tarazona, jose david gomez-ruiza by groundwater sapping was widespread and intense in the region posterior to the formation of the valley networks, as fretted channels superimpose and cut the valley networks. we observed debris aprons (glda unit) in the floor of the channels, and u valleys (gvu unit) cutting and being cut by channels (figure 3f), which implies glacial activity was widespread across the zone (koutnik et al., 2021). this setting points to a seasonal progradation and retreatment of glaciers over geological time, a phenomenon that has been observed in other martian regions (ori et al., 2000; grau galofre et al., 2020; laskar et al., 2002; head et al., 2003). due to the widespread presence of valley networks around ismenius cavus, we decided to reconstruct the limits of possible paleo-basins by adjusting manually the output of the basin tool included in arcgis, using the topography and the mapped drainage (figure 4). we identified four paleo-basins. basin i is related to the incoming flow of mamers valles, basins ii is associated with the deltas, and iii whit the higher lands behind them. these three basins eventually drain inside ismenius cavus, in basin i. valley networks and fretted channels are present in all the basins. basins ii and iii are of special significance to the deltaic systems, as they have flow patterns that drain towards the lake chain that contains the aracataca and ariguani deltas. this lake chain is formed by the aracataca crater, ariguani crater, and ismenius cavus. all of them are connected by openings in their rims (figure 4). 3.3 detailed scale the analysis of the deltas was made using hirise imagery. they are only 36 km away from each other. both were deposited at the discharge end of two possible big paleolakes, but despite their closeness, they are different in shape, size, and associated landofrms. the deltas are made of at least three recognizable and well-developed pulses, where all their major structures are preserved (figure 5 to figure 8). we mapped the units of each pulse according to their spatial position in the sub-environments defined by nichols (2009): delta tops (fadt), delta slopes (fads), delta fronts (fap), and prodeltas (ffdr). the aracataca crater is supplied by multiple valley networks originating from basin ii and notably lacks any fretted channels in the area. consequently, it suggests that the formation of the aracataca delta occurred within a fluvial system. the delta is situated within the ariguani crater, where the aracataca crater rim broke (figure 7a). the origin of this incision is not clear, but as no structural pattern dominates the zone, we propose that it is the result of fluvial erosion of a formerly weak zone, due to the filling and overflow of the crater. we identified three main deposition pulses (figure 5, figure 7), two of which have an elongated shape, with a length-wide ratio of 1.6. this configuration resembles a river-dominated delta (nichols, 2009), which indicates that the dominant system was the incoming flow from the aracataca crater. both delta slope zones (fads2 and fads3 units) have high slopes, indicating a coarse grain size. the most recent pulse has a more fan-like shape (fadt1), which points to a less energetic incoming flow. all fans have well-developed subenviroments but no paleochannels were identified. this is probably due to the overlying glacial activity, which generated lateral moraines and sublimation pits over the previous fluvial activity (figure 3b). figure 5: geomorphological map of the aracataca delta. the basemap was hirise capture esp_026915_2145. 49hydrologic evolution of two martian deltas in the ismenius cavus system figure 6: geomorphological map of the ariguani delta. the basemap was hirise take esp_013531_2140. figure 7: a) geomorphological map of the aracataca delta. b) elevation profile of the delta taken from the hrsc dem, the lateral moraine that cuts the delta is visible to the left. c) tridimensional interpretation of the river-dominated delta. dp1, dp2, and dp3 are the three identified pulses. 50 javier eduardo suárez-valencia, daniela alexandra guerrero-gutiérrez, natalia katherine soler-arago, daniel felipe ramirez-diaz, iván alexis nocua-benítez, camilo andres escobar-tarazona, jose david gomez-ruiza the ariguani delta was formed inside ismenius cavus, at a fracture point in the ariguani crater rim. in contrast to the aracataca delta, the ariguani delta is directly associated with a fretted channel (figure 2). the channel starts 10 km inside the ariguani crater and then extends southwest towards ismenius cavus, eventually bisecting the crater rim (etic) and depositing the ariguani delta (figure 6, figure 8). it is noteworthy that certain valley networks flow towards the ariguani crater from the east side of basin ii, evidencing some degree of fluvial activity along the fretted channel. this delta is composed of three pulses, all resembling a gilbert-type delta, due to their high delta slopes (fagds1, fagds2, and fagds3 units) of 11°, and a considerable basin depth (nichols, 2009; ori et al., 2000; rees et al., 2017). these deltaic structures are also covered by glacial debris and recent sublimation pits. figure 8: a) geomorphological map of the ariguani delta. b) elevation profile of the delta taken from the hrsc dem, the structure of the delta is well conserved, and its main parts are highlighted. c) tridimensional interpretation of the gilberttype delta. dp1, dp2, and dp3 are the three identified pulses. 4. discussion 4.1 geologic evolution from the prior analysis, it was established that the ismenius quadrangle had a complex geologic evolution due to the changing effects of the hydrologic systems within this region. this zone has been active since the earliest periods of mars, being the noachian-hesperian limit the moment in which its major features developed. 4.1.1 noachian during this period, around 3.88 ga 3.92 ga, the oldest geomorphological units of the zone (mnp, mnh1, and mnh2 units) formed in the northwestern region of arabia terra (figure 2) (benedix et al., 2020; chuang & crown, 2009; tanaka et al., 2009). these units appear as extended plains, characterized by their cratered surfaces and the display of highly integrated valley networks. all the noachian units underwent alterations as a result of subsequent processes such as impact cratering, erosion, tectonic activity, groundwater sapping, and glacial flows. these processes influenced the later development of the polygonal plateaus and knobs in the deuteronilus mensae region (chuang & crown, 2009). the primary activity during the noachian period was impact cratering. over the mnh unit, the impact of massive asteroids formed the main craters of the region (around 20 km in diameter), including the ariguani and aracataca craters. these two craters are simple structures, with only a few valley networks and channels associated. on the other hand, the irregular shape of the ismenius cavus structure may be the result of multiple superimposed impact craters modified by fluvial, glacial, and eolian processes. the channels associated with ismenius cavus connect all the hydrologic basins of the region with the mamers valles system and eventually with the borealis basin. after the formation of the main craters, the late noachian was dominated by fluvial activity in the form of valley networks, leading to the formation and development of lake chains and sedimentary basins inside the craters. the main channels had a northeastward flow, leading to the lowlands after the northern maritan dichotomy in the deuteronilus mensae region. this water activity led to the erosion of the north mnp unit (figure 2). as a result, the lands near the dichotomy became irregular valleys, facilitating the flow of water and connecting the ocean with the southern units, thus consolidating the hydrologic system of the ismenius quadrangle. at the detailed scale, relations are more complex. valley networks are recognized across the area surrounding ismenius cavus and seem to be dissected by the mamers valles main channel. this implies that the valley networks were abundant before the main groundwater-sapping event happened and that they probably followed the current course of mamers valles. cabrol (1999) classified the craters in this zone as lake chains, open systems of paleolakes that are connected to a broader hydrologic system coming from basins i and ii. on the other hand, di achille and hynek (2010) defined them as closed basins. our analysis makes it clear that the system is open, as aracataca crater was fed to the north by the ancient basin iii through a small canyon (figure 4), then drains to the ariguani crater, which in turn released its charge into ismenius cavus. this shows a continuous system between the noachian highlands and the borealis basin. the aracataca delta is the product of different pulses of sedimentation. we propose that at the beginning, the aracataca crater was a closed lake until the water volume overcame its rim. this created a spillover flow that continued through ariguani crater, which in turn must have also been filled, as a delta would only form in an aqueous environment. due to the number of valley networks and the absence of fretted channels in basins ii and iii, we consider the development of the aracataca delta older than the main phase of groundwater activity with a completely different genesis to the ariguani delta (figure 9a, 9b). however, it is not clear if this system was connected at this epoch with ismenius cavus, as the ariguani delta is associated with a posterior event, and any possible process that could have occurred before has been erased by its occurrence. figure 9: genetic difference between both deltas. a) plan view of the aracataca delta. b) the aracataca delta does not have a canyon associated upstream, implying a formation from surficial water activity. c) plan view of the ariguani crater. d) the ariguani delta was formed by groundwater sapping, a channel was also carved by groundwater erosion upstream after the lake at the aracataca crater evaporated. 4.1.2 hesperian the noachian-hesperian transition and early hesperian period, around 3.8 gy 3.7 gy, is of great importance for the evolution of the region. this period is usually considered as the moment in which the wet conditions of the noachian were replaced by scarce remnants of surface water. this phenomenon was driven by the gradual loss of the internal heat of the planet, the reduction of its magnetic field, and the consequent loss of the atmosphere of the planet (bullock & moore, 2007). as a result of this process, the superficial sources of water started to disappear, leaving only groundwater in aquifers. this 51hydrologic evolution of two martian deltas in the ismenius cavus system exhaustion of valley networks left groundwater sapping as the main erosive agent, creating the extended fretted channels and denudation processes on the highlands (lamb et al., 2008; warner et al., 2011). fretted channels are widely spread in the zone, mamers valles is the biggest of them. its formation erased previous fluvial features of a similar scale, as both fluvial and fretted channels seem to converge. although the development of the fluvial systems occurred mainly during the noachian, the lacustrine activity in ismenius cavus has been dated to hesperian (dehouck et al., 2010). given ismenius cavus’s proximity to the north dichotomy of mars, the transition from a fluvial to a groundwater regime appears likely to have been gradual. during the noachian period, the entire region was abundant in surface water, indicating high levels of infiltration. subsequent to the cessation of fluvial activity, the groundwater component could have started shortly after, possibly obviating the need for a complete shutdown of the hydrological system, only to be reactivated later by the thawing of subsurface ice. as the weather became colder and dryer, the water in the ariguani crater started to evaporate until it was completely dry. at this point, the main lake chains only had groundwater input. the ariguani delta was deposited inside ismenius cavus during this period, by sediments transported by a channel carved by groundwater erosion (figure 9d). this channel was carved in a southwestern direction within the ariguani crater downslope, extending to the residual lake within ismenius cavus and shaping the ariguani delta. it suggests that the ariguani crater had emptied prior to the formation of the ariguani delta, as the channel could not have developed underwater. it remains unclear whether the connection to ismenius cavus existed prior to this event; however, it is evident that the ariguani delta is younger than the aracataca delta. the progressive vanishing of water in the lake chain could be related to the altitude gradient, as ismenius cavus is below both craters, the continuously lower phreatic level first disappeared in the lake chain. some u valleys appear across the area. these are associated with glacial erosion and some of them seem to predate fretted channels (figure 3f). this is not strange, as interglacial/glacial cycles have been abundant on mars because of the high variation in the angle of the rotation axis (montmessin, 2006). 4.1.3 amazonian glacial processes were the main surface modifiers of the amazonian period (head et al., 2003). we identified numerous features of the glaciations of the amazonian. lateral and frontal moraines are common in the interior of the channels, indicating that glacier masses were flowing inside the features that were once carved by water flows. ice flow traces are common near the dichotomy; this was expected as ice follows a downslope direction. one of the glacial forms that are ubiquitous all over the surface is the pitted texture (figure 3b), which is formed by the holes left by the sublimation of co2 ice, later covered with wind deposits. at the regional scale, numerous glacial units were identified. debris aprons (ada unit), moraines, and grooves-crevasses are commonly found superimposing the noachian units. at the detailed scale, eolian activity is evident in the ismenius cavus floor, where transversal dunes are preserved in the superficial eolian deposits. 4.2 spatial and time frame of the observed landforms the hydrologic activity in the region is more varied and time-gated than the first analysis may tell. the strong dissonance between the genesis of both deltas is an example of a more regional contrast. across the different scales, we encountered two main morphological stages associated with water flow, a first stage dominated by valley networks and a posterior occurrence of widespread fretted channels (figure 10). the first stage has a great footprint in the noachian terrains. the valley networks are common in all the units of the basement, probably supplying the borealis basin. this, at first, is concordant with the idea of a wet early mars (baker et al., 1991; dehouck et al., 2010). nevertheless, grau galofre et al. (2020) showed that a vast majority of the channels near the northern dichotomy can be classified as fluvial or subglacial in origin, with a predominance of the last type. this suggests the presence of glacier masses near the dichotomy during the fluvial activity; therefore, an environmental scenario with colder temperatures. the evidence of widespread glacial activity involved at the different hydrologic stages of the region indicates that the valley networks of our study area could possibly represent subglacial hydrologic systems. in this case, this supports the idea of a wet but cold mars, concordant with the results obtained by grau galofre et al. (2020) and kite et al. (2014). figure 10: a timeline recording the change of the surface processes across the martian geological time. the noachian period was dominated by valley networks, the hesperian period by fretted channels, and the amazonian period by eolic and glacial activity. the aracataca delta is a clear result of the activity of the first stage. the crater drained a vast area via valley networks and the morphology of the delta resembles a river-dominated delta, meaning strong water and sediment influx. the newest pulse of the aracataca delta is small and neutral-shaped. it can be inferred that as the paleolake influence over the delta was likely constant over time, the incoming flux, and therefore all the surficial hydrologic system, was getting weaker during the formation of this last pulse. furthermore, for the ariguani delta, we previously stated that due to its origin from a fretted channel, it could not have formed before the aracataca crater was empty (figure 9). these interpretations imply that the drying of the two upper lakes of the chain represents a drastic reduction of the water table in the zone. similar lake chains to the west and the south likely got empty. after this event, around the early hesperian (dehouck et al., 2010), groundwater runoff took place at a large scale, carving mamers valles and the several fretted channels near the dichotomy, most likely over previous depressions. this resurgence of strong water activity has been reported in several places on mars for this period (duran & coulthard, 2020), where the groundwater sources replaced the now weak atmosphere as the primary sustain of surface channels. it was at this time when the ariguani delta formed inside ismenius cavus, which in turn registered another continuous decrease of the water supply, now for the second stage. due to the explosive and fast-eroding nature of fretted channels (lamb et al. 2008), the first deltaic pulse shows the biggest amount of sedimentation. as the system evolved, the number of sediments and the areal extension of its deposits decreased, as the result of a weaker erosive capacity of the runoff during its later events, probably caused by a decrease in the groundwater supply. finally, once the groundwater sources were also exhausted, the lake inside ismenius cavus evaporated, and the study zone turned into the cold and glacial-dominated environment we see today. 5. conclusions the ismenius cavus system and its associated deltaic features record the hydrologic evolution of this part of the northern martian dichotomy. this history is recognizable in the geomorphology of the aracataca and ariguani deltas, and the pulses of sedimentation they made. it is feasible that the late noachian valley networks, which probably originated in a sub-glacial environment, were transitionally replaced by fretted channels coming from groundwater runoff during the hesperian, and eventually faded away as the martian atmosphere was not able to support a hydrologic cycle anymore. the widespread hydrologic systems of early mars are now shut down, but below the current glacial and eolian features, it is possible to identify their ancient activity. water-only generated landforms like deltas are key to understanding the geologic history of the planet. future analysis of similar environments around mars could help to better define how these systems worked, how they evolved in time, and their role as former habitable environments and the search for life. 6. data availability datasets related to this article are freely available in different repositories. the geodatabases and maps produced in this work can be reached at (suarezvalencia et a., 2023). mola and themis data can be found at https:// astrogeology.usgs.gov/search?pmi-target=mars, a planetary catalog of the usgs; hrsc data is available at https://pds-geosciences.wustl.edu/missions/ mars_express/hrsc.htm, at the geoscience node of the planetary data system (pds); and hirise takes are found at https://www.uahirise.org/catalog/, a catalog at the university of arizona. https://astrogeology.usgs.gov/search?pmi-target=mars https://astrogeology.usgs.gov/search?pmi-target=mars https://pds-geosciences.wustl.edu/missions/mars_express/hrsc.htm https://pds-geosciences.wustl.edu/missions/mars_express/hrsc.htm https://www.uahirise.org/catalog/ 52 javier eduardo suárez-valencia, daniela alexandra guerrero-gutiérrez, natalia katherine soler-arago, daniel felipe ramirez-diaz, iván alexis nocua-benítez, camilo andres escobar-tarazona, jose david gomez-ruiza references alemanno, g., orofino, v. & mancarella, f. 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(2020). analysis and evaluation methods of seismic subsidence characteristics of loess and field seismic subsidence. earth sciences research journal, 24(4), 485-490. doi: https://doi.org/10.15446/ esrj.v24n4.91593 the objective of this research is to analyze the dynamic degeneration of loess and the evaluation method of field seismic subsidence. in this study, q3 loess is taken as the research object, and the dynamic properties of loess with 10%, 20%, 30% and 35% moisture content are tested by triaxial experiment. in addition, seismic subsidence characteristics of loess with dry densities of 1.4g/cm3, 1.6g/cm3, and 1.8g/cm3 and consolidation stress ratios of 1.0, 1.2, 1.4, and 1.6 are analyzed. then the simplified seismic subsidence estimation method is used to calculate the relationship between seismic subsidence coefficients at different soil depth in one dimensional field, cycle times, and subsidence depth. the results show that the higher the water content of loess is, the greater the change of seismic subsidence appears. the larger the dry density of loess is, the smaller the change degree of seismic subsidence appears. the larger the consolidation stress ratio is, the greater the change of seismic subsidence occurs in loess. when the depth of soil reaches 9.5m, the maximum seismic subsidence coefficient can reach 0.8%. when the depth of soil layer is 10m, the degree of seismic subsidence is the largest. when the depth of soil layer is 12~16m, the settlement depth caused by earthquake subsidence is small. while the depth of soil layer is 8~12m, the settlement degree is large. abstract analysis and evaluation methods of seismic subsidence characteristics of loess and field seismic subsidence análisis y métodos de evaluación de las características de subsidencia sísmica en suelos poco compactos y subsidencia sísmica de campos issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91593 el objetivo de esta investigación es analizar la dinámica de degeneración de suelos poco compactos y el método de evaluación de subsidencia del campo sísmico. en este estudio, suelos poco compactos tipo q3 fueron objeto de investigación, y las propiedades dinámicas de estos suelos con 10, 20, 30 y 35 por ciento de contenido de húmedad fueron evaluadas en experimentos triaxiales. además, se analizaron las características de subsidencia sísmica de los suelos poco compactos con densidades secas de 1.4g/cm3, 1.6g/cm3, y 1.8g/cm3 y un índice de consolidación de esfuerzo de 1.0, 1.2, 1.4 y 1.6. luego, el método de estimación de la subsidencia sísmica simplificada se utilizó para calcular la relación entre los coeficientes de subsidencia sísmica en diferentes profundidades en un campo dimensional, los tiempos del ciclo y la profundidad de subsidencia. los resultados muestran que entre más alto sea el contenido de agua en los suelos poco compactos es mayor el cambio de subsidencia sismica. y que entre mayor sea la densidad seca de estos suelos, más pequeño es el grado de cambio de la subsidencia sísmica. y, finalmente, cuando el índice de consolidación de esfuerzo es mayor también será mayor el cambio de subsidencia sísmica que ocurre en los suelos poco compactos. cuando la profundidad del suelo alcanza los 9.5 metros, el coeficiente de subsidencia sísimica puede alcanzar el 0.8 por ciento. cuando la profundidad de la capa del suelo es de 10 metros, el grado de subsidencia sísmica es mayor. cuando la profundidad de la capa del suelo es entre 12 y 16 metros, el asentamiento causado por la subsidencia del movimiento de tierra es menor. mientras la profundidad de la capa del suelo se encuentre entre 8 y 12 metros, el grado de asentamiento es mayor. resumen record manuscript received: 12/03/2019 accepted for publication: 31/01/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 485-490 xiaotong yan1, shun wang2*, ningwei wang1 1institute of engineering mechanics. cea, heilongjiang, china 2shenyang jianzhu university, liaoning, china *corresponding author: xiatong.yan@126.com se is m o lo g y https://doi.org/10.15446/esrj.v24n4.91593 https://doi.org/10.15446/esrj.v24n4.91593 1.4g/cm 1.6g/cm 1.8g/cm https://doi.org/10.15446/esrj.v24n4.91593 1.4g/cm 1.6g/cm 1.8g/cm 486 xiaotong yan, shun wang, ningwei wang introduction loess is a kind of soil with weak cementation, large pore size, and easy denaturation when encountering water (qiu et al., 2018). however, under the action of water and dynamic load, serious geological disasters such as collapsibility and seismic subsidence would occur (hao et al., 2018). the judgement of seismic subsidence of loess is an important means of seismic safety evaluation. the subsidence of soil under the action of earthquake is seismic subsidence. due to its unique dynamic characteristics, loess is usually damaged when strong earthquakes occur (cheng et al., 2018). at present, some experts and scholars have comprehensively analyzed the dynamic characteristics of loess by the triaxial test system. the specific physical properties of loess, such as dry density and water content, have a great impact on the dynamic characteristics of loess and play an important role in the study of the characteristics of loess (liu et al, 2017). when earthquake subsidence occurs, the soil would subside under the action of dynamic load, and some soil with high water content would collapse due to vibration (chen et al., 2017). dry density is also an important internal factor that reflects the seismic subsidence degeneration of loess. the seismic subsidence performance can be calculated by obtaining the volume strain variable through the dynamic single shear experiment. or, on the basis of the volume strain variable data obtained from the dynamic single shear experiment, the incremental calculation equation is established, and the calculation equation of seismic subsidence is established according to the analysis results of field earthquakes (wang et al., 2017; drzewiecki & piernikarczyk, 2017). it is mainly used to analyze the equivalent linearized site and determine the shear strain time history of different soil layers, and then convert the random vibration waves into harmonics to obtain the equivalent shear strain amplitude and vibration times. under the same vibration condition, the longitudinal strain value of vibration is obtained through the single shear test of the soil samples in different soil layers, and finally the settlement amount of seismic subsidence under different soil layers is analyzed and calculated (gao et al., 2017). the simplified seismic subsidence estimation method can obtain the equivalent shear strain of the soil layer through shear model, maximum ground acceleration, overburden load of the upper soil, and stress attenuation coefficient. according to the relationship between the circular cycle and the volume strain, the volume strain value of equivalent shear strain is obtained. after the loading cycle, the volume strain value of the magnitude is calculated (araujo & castro, 2017; sarhosis et al., 2018). in this study, by analyzing the seismic subsidence characteristics of loess and the influencing factors of seismic subsidence, the seismic subsidence changes of loess with different moisture content, dry density, and consolidation stress ratio are analyzed. the seismic subsidence characteristics of onedimensional loess field and the variation of seismic subsidence in different soil layers are analyzed by the simplified seismic subsidence estimation method. this study aims to provide theoretical basis for the subsequent analysis of loess seismic subsidence characteristics and field earthquakes. methodology characteristics and influencing factors of loess seismic subsidence the main mechanism, disaster development mode, and disaster types of loess seismic subsidence are influenced by the material index of soil, topographic characteristics of loess field, and earthquake. when loess earthquake subsidence occurs, soil quality can significantly affect the intensity of earthquake and the severity of earthquake subsidence. the degree of loess earthquake subsidence is obviously different in different areas. different forces act on the primary structure of loess during the earthquake. when the external force exceeds the connection strength between the soil particles, the pores in the primary structure of loess would be destroyed and the soil particles would be rearranged. at this time, the surface would show sudden settlement. seismic subsidence in loess is the result of the interaction of various factors, and the difference in soil structure has a significant impact on the occurrence of seismic subsidence, such as the water content and pore ratio of soil particles. loess is composed of grain size, and the content of clay between soil grains has a great influence on the occurrence of seismic subsidence. the loess with different water content would also exhibit different kinetic characteristics, and the seismic subsidence of loess would increase with the increase of water content of soil. when the pore ratio is greater than 0.75, the seismic subsidence would increase with the increase of the pore ratio of soil particles. seismic subsidence test of loess under different conditions generally speaking, residual strain is used for the representation of loess earthquake subsidence, and the electro-hydraulic servo dynamic triaxial testing machine controlled by microcomputer (xi’an lichuang, china) is adopted for the test. during the test, the ratio of fixed junction stress of loess samples is not equal to 1, and the center of dynamic strain amplitude would move in the direction of compression with the cyclic action of dynamic stress. accumulated residual strain that can’t be completely recovered would occur in the loess after the cycle stops. the residual strain can be expressed as equation 1: µr n h h h ( ) = − (1) among them, εr(n) is the residual strain, that is, the seismic subsidence coefficient, h is the height of the loess sample before the dynamic load, and h is the height of the loess sample after dynamic loading. figure 1a shows the relationship between the time course of loess dynamic strain and the number of vibration with the passage of cycle number n. it can be observed from figure 1b that after n cycles stop, the accumulated residual strain value of loess is the corresponding strain value when the dynamic stress under this cycle number is equal to 0. the q3 loess, which is 3.5m in the third terrace of the xxx river, is collected from xxx county of xxx city. the loess structure is loose. after testing, it was found that the selected loess has a natural water content of about 11.98%, a dry density of about 1.42g/cm3, and a plasticity index of 7.55. the dynamic properties of loess with 10%, 20%, 30%, and 35% moisture content are tested, and seismic subsidence characteristics of loess with dry densities of 1.4g/cm3, 1.6g/cm3, and 1.8g/cm3 and consolidation stress ratios of 1.0, 1.2, 1.4, and 1.6 are analyzed evaluation of seismic subsidence of one-dimensional loess the one-dimensional loess site is shown in figure 2. when an earthquake occurs in a one-dimensional field, the loess layer would extend horizontally in all directions in the same direction and be propagated by vertical and upward shear waves. at this time, the loess volume would only generate horizontal vibrations conducted by bedrock. when seismic subsidence occurs and the loess is of different densities and of the same type, it can be considered that the seismic subsidence deformation of the same type of loess follows the ratio of the same seismic subsidence table 1. the seismic subsidence model of loess and its condition destruction mode vibrating type liquefied type seismic landslide soil large pores and weak cementation structure; less clay content; low water content high content of powder and sand; high water content large pores and weak cementation structure; less clay content; low water content terrain plateau, beam or loess hills loess basin tableland edge or loess basin strength level 8 and above level 7 and above level 8 and above 1.42g/cm 1.4g/cm 1.6g/cm 1.8g/cm 487analysis and evaluation methods of seismic subsidence characteristics of loess and field seismic subsidence the calculation equation of seismic load cycle is as follows: n n a aeq a i i m i =     = ∑2 37 1 2 . max (4) among them, ai is the acceleration amplitude of level i, nai is the cycle number of ai, amax is the maximum acceleration value. with the change of depth h, the relationship between the stress attenuation coefficient rd of the loess field is shown in figure 3. therefore, the stress attenuation coefficient of loess can be expressed by the equation 5. r h h h h h h d ( ) = ≤ < − ≤ < − ≤ < 1 0 3 34 43 31 43 3 14 82 82 105 88 14 32 0 48 , . . , . . , . ,332 ≤          h (5) here, the equivalent shear strain of the soil layer at any depth is as follows. (6) among them, δ0 is the pressure covering the soil layer, f is the conversion coefficient of frequency addition, and g is the acceleration of gravity. coefficient increment to the dynamic strain amplitude. when the moisture content, dry density, and consolidation pressure of the loess in the same soil field are the same, the seismic depression coefficient of the loess with different soil depth can be calculated by the equation 2. (2) among them, △εp is the increment of the seismic subsidence coefficient, γd is equivalent shear strain amplitude of loess body during earthquake, εp is the seismic subsidence coefficient of the cycle that has occurred, ρ0 is the dry density of loess, ρref is the referenced dry density, and a and b are the reference values of earthiness, as shown in equation 3. (3) 0.0 0.8 1.6 0 1 2 2ε n,d ε σσ σ n ni n ε ε n,r σ σ d ε ε n,r 2ε n,d o a b figure 1. definition of accumulated residual strain of loess (a is the process of dynamic stress and accumulated residual strain changing with time; b is the dynamic stress lag loop of loess) earth's surface z=0 1 n-1 bedrock z=h 1 n-1 h1 hn-1 d1 d2 d3 d4 d5 dn-2 dn-1 dn z1 … … i    figure 2. hierarchical and discrete structure of one-dimensional loess field 488 xiaotong yan, shun wang, ningwei wang result and discussion the relationship of pore ratio of soil grains, dry density and moisture content to seismic subsidence coefficient the density of loess can determine the maximum deformable quantity of earthquake subsidence, and it is reflected by dry density and void ratio. it can be observed from figure 4a that the earthquake subsidence coefficient would increase with the increase of the pore ratio of soil particles, and there is a linear growth relationship between the two. as can be observed from figure 4b, the dispersion between the seismic subsidence coefficient of loess and the dry density of soil particles (g/cm3) is relatively large, but when the dry density of soil particles is large, a large seismic subsidence also occurs. it can be observed from figure 5 that when the pore ratio of soil particles is relatively low, the loess with high water content may not be seismic subsidence. when the void ratio is relatively high and the water content of loess is low, seismic subsidence may also occur. this indicates that the influence of water content limit should be considered for loess with low porosity. however, the boundary of water content does not need to be considered in loess with higher porosity. seismic subsidence variation rule of loess with different moisture content when the consolidation stress and ratio are equal, the influence of different water contents of loess on seismic subsidence is shown in figure 6. when the consolidation stress ratio is 1.0, the consolidation stress is 200kpa and the dry density of the loess is 1.4g/cm3 and the pressure is applied on the loess samples, the loess with different moisture contents all have seismic subsidence changes to some extent. it can be observed from figure 4a that when the moisture content of the loess is 10%, different dynamic stresses have little influence on the residual strain of the loess. however, when the moisture content of the loess is greater than or equal to 20% and the dynamic stress exceeds 100kpa, the residual strain of the loess increases significantly, and the soil also shows obvious seismic subsidence changes. it can be observed that the seismic subsidence of loess is mainly caused by the destruction of the pore structure of soil particles under the action of dynamic load. external pressure would reduce the pore ratio of loess soil particles, and the soil mass would become more compact and deformed (anbazhagan et al., 2017). however, when the water content of loess increases, the cementation between the soil particles would be strengthened, thus reducing the friction strength between the soil particles. therefore, under the action of the same load and consolidation stress, the pore ratio of the loess with high water cut is smaller, the seismic subsidence is more likely to occur, and the deformation of seismic subsidence is more serious (xu & yang, 2017). seismic subsidence variation rule of loess with different dry density when the consolidation stress and ratio are equal, the influence of different dry densities of loess on seismic subsidence is shown in figure 7. it can be observed from figure 7a that when the dry density of loess is 1.4g/cm3, its influence on dynamic stress is lower than that of dry densities of 1.6g/cm3 and 1.8g/cm3, indicating that under the action of the same dynamic stress, the dry density of loess is inversely proportional to the deformation generated by earthquake subsidence. this may be because the higher the dry density of the loess is, the higher the compactness between the soil particles is, and the higher the dynamic stress is needed to make the loess volume change. as can be observed from figure 7b, the residual strain generated by the loess volume decreases with the increase of the dry density of the soil, and when the dry density of the loess is larger than 1.6g/cm3, the influence of the residual strain on the volume of the loess also decreases gradually. 0 10 20 30 40 50 60 70 0.2 0.4 0.6 0.8 1.0 zk1 zk2 zk3 zk4 zk5 zk6 zk7 zk8 zk9 zk10 zk11 zk12 zk13 zk14 zk15 zk16 r d h/m figure 3. relationship between stress attenuation coefficient and soil depth of loess field 0.8 0.9 1.0 1.1 1.2 1.3 0.00 0.02 0.04 0.06 0.08 0.10 se is m ic c oe ff ic ie nt pore ratio 1.2 1.4 1.6 1.8 0.00 0.02 0.04 0.06 0.08 0.10 se is m ic c oe ff ic ie nt dry density a b figure 4. relationship between seismic subsidence coefficient and void ratio and dry density of loess (a is the void ratio; b is the dry density) 0.7 0.8 0.9 1.0 1.1 1.2 1.3 0 5 10 15 20 25 30 seismic sink none-seismic sink w at er c on te nt (% ) pore ratio figure 5. relationship between water content, seismic subsidence coefficient, and pore ratio of loess 1.4g/cm 1.4g/cm 1.6g/cm 1.8g/cm 1.6g/cm 489analysis and evaluation methods of seismic subsidence characteristics of loess and field seismic subsidence the variation rule of seismic subsidence of loess under different consolidation stress ratio figure 8a shows that when the consolidation stress is 100kpa, the dry density of the loess is 1.4g/cm3 and the moisture content is 20%, the consolidation stress ratio is proportional to the residual stress generated by the volume of the loess after the pressure is applied to the sample. it is possible that due to the increase of deviator stress on the loess, the pore structure among the soil particles is gradually destroyed, which leads to the increase of residual strain of the loess volume. it can be observed from figure 8b that the residual stress generated by loess would increase with the increase of consolidation stress ratio, showing a linear growth relationship, which is consistent with the research results of song et al., 2017. analysis of seismic subsidence characteristics of one-dimensional field the variation of seismic subsidence coefficient and cycle times of onedimensional field at different soil depth is shown in figure 9. it can be observed that the seismic subsidence coefficient produced by loess soil is proportional to a b 50 100 150 200 250 0 1 2 3 r es id ua l s tr ai n /% dynamic stress 10% 20% 30% 35% 1 2 3 4 0.0 0.5 1.0 1.5 2.0 2.5 35%30%20% r es id ua l s tr ai n /% water content 105 kpa 140 kpa 175 kpa 210 kpa 10% figure 6. relationship between residual strain and dynamic stress of loess with different moisture content (a is the relationship between residual strain and dynamic stress; b is the relationship between water content and residual strain) a b 50 100 150 200 250 0 1 2 3 r es id ua l s tr ai n /% dynamic stress 1.4g/cm3 3 1.6g/cm3 1.8g/cm3 0.0 0.5 1.0 1.5 2.0 2.5 1.81.6 r es id ua l s tr ai n /% dry density g/cm 105 kpa 140 kpa 175 kpa 210 kpa 1.4 figure 7. relationship between residual strain and dynamic stress of loess with different dry density (a shows the relationship between residual strain and dynamic stress; b shows the relationship between dry density and residual strain) a b 1.0 1.2 1.4 1.6 0 1 2 3 4 r es id ua l s tr ai n /% consolidation stress ratio 105 kpa 140 kpa 175 kpa 210 kpa 50 100 150 200 0 1 2 3 4 r es id ua l s tr ai n /% dynamic stress 1.0 1.2 1.4 1.6 figure 8. relationship between residual strain and dynamic stress of loess with different consolidation stress ratios (a shows the relationship between residual strain and dynamic stress; b shows the relationship between consolidation stress ratio and residual strain) 1.4g/cm 490 xiaotong yan, shun wang, ningwei wang the cycle times, but the seismic subsidence amplitude is inversely proportional to the development of seismic subsidence. it can be observed from figure 9 that when the depth of the soil layer reaches 9.5 m, the earthquake subsidence coefficient is the largest, the generated earthquake subsidence is the strongest, and the maximum earthquake subsidence coefficient of this deep soil layer can reach 0.8%. the results of the relationship between seismic subsidence coefficients at different soil depths and seismic subsidence depths are shown in figure 10. it can be observed from figure 10a that the seismic subsidence coefficient first increases and then decreases with the increase of soil depth. and it can be observed from 10b that when the depth of soil layer is 12~16m, the settlement depth caused by earthquake subsidence is small, while the settlement degree in the soil depth of 8~12m is larger. conclusion based on the previous research results, the seismic subsidence characteristics of the original loess with different moisture content, density, and consolidation stress are analyzed by triaxial experiment. it is found that the seismic subsidence is stronger when the loess has higher water content, lower density, and higher consolidation stress. after the calculation of seismic subsidence coefficient of one-dimensional loess field, it is found that when the depth of soil layer is 10m, the seismic subsidence coefficient is the largest and the deformation of seismic subsidence is the largest. in this study, the mechanical properties of loess are not considered comprehensively, and the calculation amount of models used for seismic subsidence analysis is too small. however, the results of this study can provide certain theoretical basis for the subsequent study on seismic subsidence and evaluation of loess. references anbazhagan, p., uday, a., moustafa, s. s. r., & alarifi, n. (2017). soil void ratio correlation with shear wave velocities and spt n values for indo-gangetic basin. journal of the geological society of india, 89(4), 398-406. araújo, m., & castro, j. m. (2017). simplified procedure for the estimation of local inelastic deformation demands for seismic performance assessment of buildings. earthquake engineering & structural dynamics, 46(3), 491-514. doi: https://doi.org/10.1002/eqe.2825 chen, t., ma, w., & wang, j. (2017). numerical analysis of ground motion effects in the loess regions of western china. shock & vibration, 3, 1-9. doi: https://doi.org/10.1155/2017/1484015 cheng, x. s., ma, l., yu, d. p., fan, j., & li, d. (2018). seismic stability of loess tunnels under the effects of rain seepage and a train load. science china technological sciences, 61(5), 735-747. drzewiecki, j., & piernikarczyk, a. (2017). the forecast of mining-induced seismicity and the consequent risk of damage to the excavation in the area of seismic event. journal of sustainable mining, 16(1), 1-7. gao, g., nie, c., & shi, c. (2017). seismic subsidence of sand ground subject to multidirectional earthquake load. journal of harbin engineering university, 38(7), 1100-1106. hao, l., meng, j., zhang, y., & yang, l. (2018). pliocene seismic stratigraphy and deep-water sedimentation in the qiongdongnan basin, south china sea: source-to-sink systems and hydrocarbon accumulation significance. geological journal, 54(1), 392-408. doi: https://doi. org/10.1002/gj.3188 liu, p., lian, p., zhang, m., he, b., & dou, l. (2017). dynamic characteristics of a building with seismic joints based on ambient vibration. journal of hunan university, 44(1), 95-101. doi: 10.16339/j.cnki. hdxbzkb.2017.01.012 qiu, j., wang, x., lai, j., & wang, j. (2018). response characteristics and preventions for seismic subsidence of loess in northwest china. natural hazards, 92(7), 1-27. sarhosis, v., milani, g., formisano, a., & fabbrocino, f. (2018). evaluation of different approaches for the estimation of the seismic vulnerability of masonry towers. bulletin of earthquake engineering, 3–4, 1-35. song, b., tsinaris, a., & anastasiadis, a. (2017). small-strain stiffness and damping of lanzhou loess. soil dynamics & earthquake engineering, 95, 96-105. wang, y., gang, t., & yu, l. (2017). the effect of explosion parameters on seismic source wavelet calculation and its characteristics. journal of applied geophysics, 145. xu, j. s., & yang, x. l. (2017). effects of seismic force and pore water pressure on three dimensional slope stability in nonhomogeneous and anisotropic soil. ksce journal of civil engineering, 2, 1-10. 0 2 4 6 8 10 12 14 16 0.00 0.02 0.04 0.06 0.08 se ism ic su bs id en ce c oe ffi ci en depth/ m a b 0 2 4 6 8 10 12 14 16 0.0 0.2 0.4 0.6 se is m ic su bs id en ce depth/ m 1 figure 10. relationship between seismic subsidence coefficients at different soil depths and seismic subsidence depths 0 2 4 6 8 10 12 0.00 0.02 0.04 0.06 0.08 0.10 number of cycles se is m ic su bs id en ce c oe ff ic ie n 0.5m 3.5m 6.5m 9.5m 12.5m 15.5m figure 9. relationship between seismic subsidence coefficient and cycle times at different soil depths https://doi.org/10.1002/eqe.2825 https://doi.org/10.1155/2017/1484015 https://doi.org/10.1002/gj.3188 https://doi.org/10.1002/gj.3188 10.16339/j.cnki.hdxbzkb 10.16339/j.cnki.hdxbzkb issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n4.83186 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 23 38 g e o ph y si c s identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) shahrzad ameri1, ali solgi2, ali sorbi3, alireza farrokhnia4 1,2 department of earth science, faculty of converging science and technologies, science and research branch, islamic azad university, tehran, iran. 3,4 department of geology, karaj branch, islamic azad university, karaj, iran *corresponding author: a.solgi@srbiau.ac.ir. how to cite item: ameri, s., solgi, a., sorbi, a., & farrokhnia, a. (2022). identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran). earth sciences research journal, 26(1), 23-38. https://doi.org/10.15446/ esrj.v26n1.83186 record manuscript received: 27/10/2019 accepted for publication: 21/03/2022 abstract resumen the morphotectonic analysis is a useful time-saving and cost-effective method to assess tectonic activity, especially in large regions. in this paper, the morphotectonic study was carried out in kerman city which is in the southeast of iran and is a part of the central iran structural zone. despite the occurrence of numerous historical and instrumental earthquakes around kerman city, there is a gap in earthquake records of this city that has obscured its seismicity status. the aim of this study is to identify active faults and the associated seismic hazard to kerman city. for this purpose, seven geomorphic indices namely stream length-gradient index (sl), drainage basin asymmetry (af), hypsometric integral (hi), ratio of valley-floor width to valley height (vf), index of drainage basin shape (bs), index of mountain front sinuosity (smf) and transverse topographic symmetry factor (t) were examined for 51 basins of the study area, using digital elevation model (dem) and geological maps in a gis environment. then, two kinds of relative tectonic activity indices were calculated for each basin, i.e., iat and iat_t, from the combination of these indices. the iat_t was chosen because its results show better agreement with the structural geology and seismic records of this region. finally, the study area was divided into three regions according to the iat_t values. the morphotectonic analyses indicate that the kuhbanan fault system, especially its southern splays, has the potential to produce serious seismic hazards to kerman city in the future; the mahan-jupar fault-related folds, the southern tip of the jorjafk fault, the probable fault of zangi-abad, the rafsanjan-zarand fault system, and gowk fault are considered as other threats to kerman city. furthermore, this study reveals that morphotectonic analysis is a reliable tool to evaluate fault capability and to determine the fault types, and therefore to estimate seismotectonic hazard. keywords: seismic hazard; geomorphic indices of active tectonics; geomorphology; gis; kerman city; southeastern iran; palabras clave: riesgo sísmico; índices geomórficos de tectónicas activas; geomorfología; sistemas de información geográfica; kerman; irán identificación de fallas con potencial sísmico de riesgo con base en análisis morfotectónico en la ciudad de kerman (sudeste asiático) el análisis morfotectónico es un método útil, económico y ahorrador de tiempo para evaluar la actividad tectónica, especialmente en regiones grandes. en este artículo, el estudio morfotectónico se realizó en la ciudad de kerman, en el sudeste de irán, y que es parte de la zona estructural de centro irán. si bien han ocurrido numerosos terremotos históricos o de detección instrumental en kerman, hay una brecha en los registros de estos movimientos que oscurecen el estatus sísmico. el objetivo de este estudio es identificar las fallas activas y el riesgo sísmico asociado para la ciudad de kerman. para este propósito se examinaron siete índices geomórficos (índice del gradiente del perfil longitudinal, sl; asimetría de cuenca, af; integral hipsométrica, hi; relación de la parte baja del valle con la altura del valle, vf; índice de la forma de cuenca, bs; índice de sinuosidad de frentes montañosos, smf, y factor de simetría topográfica transversal, t) en 51 cuencas en el área de estudio con modelos de elevación digital (dem) y mapas geológicos en un ambiente de sistemas de información geográfica. luego se calcularon dos clases de índices de actividad tectónica relativa para cada cuenca, por ejemplo iat e iat_t, desde la combinación de estos índices. el índice iat_t fue elegido porque sus resultados muestran concordancia con la geología estructural y los registros sísmicos de la región. finalmente, el área de estudio se dividió en tres regiones, de acuerdo con los valores iat_t. los análisis morfotectónicos indican que el sistema de fallas de kuhbanan, especialmente en su extensión al sur, tiene el potencial de producir riesgos sísmicos serios a la ciudad de kerman en el futuro; los pliegues relacionados con la falla mahan-jupar, la punta sur de la falla jorjafk, la posible falla de zangi-abad, el sistema de fallas de rafsanjan-zarand, y la falla de gowk se consideran como otras amenazas a kerman. además, este estudio muestra que el análisis morfotectónico es una herramienta confiable para evaluar la capacidad de las fallas y para determinar los tipos de falla y, además, para estimar el riesgo sismotectónico. https://doi.org/10.15446/esrj.v25n1.74167 https://doi.org/10.15446/esrj.v26n1.83186 https://doi.org/10.15446/esrj.v26n1.83186 24 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia introduction the sensitivity of the drainage pattern to active tectonic processes such as faulting, folding, uplifting, and tilting which lead to geomorphic expression (i.e. river incision, basin asymmetry, drainage geometry, etc.) provide a useful tool to evaluate relative tectonic activity (cox, 1994). the study of active tectonics, particularly in the areas with high activity in the holocene and late pleistocene, is important to evaluate the earthquake hazard (keller and pinter, 2002). the most useful tools to study active tectonics are geomorphic indices (e.g. bull and mcfadden, 1977; keller and pinter, 2002; silva et al., 2003). morphometric analysis has been previously tested as a valuable tool by various studies conducted in different tectonically active areas around the world and is established as a proven technique to understand the development of geomorphic features and to assess the seismic hazard (e.g. cox, 1994; keller and pinter, 2002; silva et al., 2003; el hamdouni et al., 2008; perez-pena et al., 2009, 2010; font et al., 2010; sharma et al., 2017). the average of geomorphic indices such as stream length-gradient index (sl), drainage basin asymmetry (af), hypsometric integral (hi), the ratio of valley-floor width to valley height (vf), index of drainage basin shape (bs), index of mountain front sinuosity (smf) and transverse topographic symmetry factor (t) (e.g. keller and pinter, 2002) is used to evaluate the distribution of relative tectonic activity (iat) (el hamdouni et al., 2008). the main characteristic of this part of central iran, where kerman city is located, is the presence of major strike-slip faults that are produced in response to the shortening component induce by arabian-eurasia convergence. these strike-slip faults accommodate the shortening either by spatial partitioning of dip-slip and strike-slip components and through a combination of strike-slip faulting and vertical axis rotation, which are the properties of such regions (e.g. allen et al., 2006). one of the most obvious features of these faults is the releasing and restraining bends with their accompanies structures such as pull-apart basins and flower structures. this ongoing process has caused lots of geomorphological expressions (e.g. cutting pleistocene-holocene alluvial fans and neogene marls, incision of streams and gullies within alluvial fans) and so many destructive earthquakes around the city of kerman. the dominant age of outcrops in the study area are mesozoic and early tertiary (gsi 1992a and 1992b). cretaceous limestone which is unconformably covered by a continuous sequence of sediments from miocene to quaternary made the heights around kerman (e.g. huckriede et al., 1962). the study area is kerman city (fig. 1) which is one of the metropolises of iran and the capital of the same-name province. the city of kerman is located in a place known as one of the most seismic regions in iran that is most likely to experience a devastating earthquake at any given moment, due to the occurrence of numerous historical (e.g. the 1854 shahdad and the 1864 chatrud) and instrumental (e.g. the mw 7.2 gowk (1981) and the mw 6.4 dahuiyeh (2005)) earthquakes in its surrounding area (shahpasand-zadeh and heydari, 1996). although kerman city was shaken by recent earthquakes that happened in its surrounding area, it has not experienced severe damage figure 1. a) physiographic-tectonic zoning map of iran’s sedimentary basins (mosavi and arian, 2015 modified from arian et al., 2013). the study area is shown in the black rectangle. the numbers in this figure are, 1: zagros-east taurus hinterland, 2: persian gulf-mesopotamian foreland basin, 3: makran accretionary prism, 4: bashagard mountains, 5: jazmorian-mashkel forearc basin, 6: shahsavaran-soltan magmatic arc, 7: south lut-south helmand back-arc basin, 8: east iran mountain belt, 9: west-central alborz and lesser caucasus hinterland, 10: great kavir-northen urmieh lake foreland basin, 11: south great kavir fold and thrust belt, 12: south caspian-black sea foreland basin, 13: urmieh-dokhtar magmatic arc, 14: naien-kerman retro arc foreland basin, 15: sanandaj-sirjan overthrust belts, 16: east alborz or binalod hinterland, 17: torbat-e am-neyshabour retro arc foreland basin, 18: kopet dagh hinterland, 19: south caspian remnant basin, 20: maiamay-taibad inverted back-arc basin, 21: khaf-kavir plain magmatic arc, 22: lut plain-gonabad back-arc basin, 23: tabas hinterland, 24: yazd-khour piggyback basin. b) the location of the study area. c) structural map of the study area and 51 basins on a shaded-relief topographic base. faults in this article are from rashidi et al., (2017), fattahi et al., (2011), walker et al., (2010), walker (2006), hessami, and jamali (2006), and geological maps of the region (e.g. gsi 1992a and1992b). 25identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) (e.g. walker et al., 2010). in such circumstances, the use of morphotectonic information can be helpful to find covered or unknown trends in this region. studies using morphometric parameters to characterize the relative influence of tectonic activity and to assess seismic hazard were conducted in various tectonically active areas such as nw iran (saber et al., 2018), india (sharma et al., 2017), se spain (perez-pena et al., 2010), nw france (font et al., 2010), and sw sierra nevada of spain (el hamdouni et al., 2008). also, many seismotectonic and structural studies were conducted by other workers (e.g. walker and jakson, 2004; walker, 2006; walker et al., 2010) in and around the city of kerman, but none of them used morphological parameters to assess the seismic hazard. in this contribution, we used seven geomorphic indices together with the seismicity to characterize tectonic activity in this study region including drainage basin asymmetry (af), hypsometric integral (hi), index of drainage basin shape (bs), the ratio of valley-floor width to valley height (vf), index of mountain front sinuosity (smf), stream length-gradient index (sl), and transverse topographic symmetry factor (t). these indices were calculated for 51 basins of the study area, using a digital elevation model (30 m dem resolution) and geological maps (at 1:250000 and 1:100000 scales) in a gis environment. then the relative tectonic activity index (iat) was derived from the average of them, which was categorized into three classes based on el hamdouni et al., (2008) classification. further, the seismicity data of magnitude greater than mw 4.0 was plotted to cross-check their relationship with the tectonic activity of the region (sharma et al., 2017). earthquake epicenters are from the catalog of mousavi et al (2014) for the period of 4th century b.c. through 2012 and the earthquakes between 2012 and 2019 are from iiees1 and isc2. finally, we discuss these findings in the context of the seismic hazard assessment by identifying faults with that potential in the city of kerman which is the objective of this paper. study area according to the physiographic-tectonic zoning of the sedimentary basins of iran, kerman is a part of the naien-kerman retro-arc foreland basin (arian, 2013) (number 14 in fig. 1a). it contains a retro-arc foreland basin on the north margin of the urmieh-dokhtar magmatic arc. according to the seismotectonic-geologic hazards zoning of iran (arian, 2015), kerman city is a part of tabas-minab province. the tabas-minab province is characterized by earthquakes with moderate to high magnitude, with low frequency, long repeat time, down to 15 km focal depth. furthermore, the earthquakes of this province are of high-level intensity and show southward migration. most of the outcrops in the study area are mesozoic and early tertiary in age (gsi 1992a and 1992b). the mountains around kerman are mainly cretaceous limestone which is unconformably covered by a continuous sequence of sediments from miocene (conglomerate) to quaternary (alluvium and silty clay pans) beneath kerman plain (e.g. huckriede et al., 1962). active tectonics the active tectonics of iran is mainly affected by the northward convergence of the arabian and eurasian plates. the rate of this convergence relative to eurasia is estimated at around ⁓25 mm/yr at longitude 58°e (vernant et al., 2004). northward moving of central iran with respect to western afghanistan at a rate of 6-13 mm/yr, obtained from gps study (walpersdorf et al., 2014), produces n-s, right-lateral, shear across eastern iran. this shear is accommodated on large, active n-nw trending, right-lateral strike-slip faults which slice eastern iran (e.g., berberian, 1981; walker and jackson, 2004; meyer and le dortz, 2007; walker et al., 2009; allen et al., 2011). walpersdorf et al., (2014) showed that these faults get additional ns shortening by rotating counterclockwise in the horizontal plane, at a rate of 1 ± 0.4°/ma. main faults due to the arid and sparsely vegetated environment of southeastern iran, the quaternary activity of the faults and their related structures are well exposed. the faults in this region are either oblique-slip or strike-slip. the oblique-slip faults have both senses of motion i.e. dip-slip and strike-slip. the activity of the strike-slip fault systems has developed horsetail splay at the tips of the faults, releasing and restraining bends, which are accompanied by structures such as pull-apart basin and flower structure (rashidi et al., 2017). major strike-slip fault systems in the study area (fig. 1c) can be categorized based on their strike into n-s (nayband, gowk, and sabzevaran), nw-se (jorjafk, kuhbanan, ravar, mahan, and rayen), and e-w (lalehzar) directions. the nw-se and e-w faults link the right-lateral n-s faults with other active faults in central iran and distribute the right-lateral shear in this region (rashidi et al., 2017). nayband fault system the 300 km long nayband right-lateral fault system with ns trending, extends along the western margin of the lut block. based on the main structural discontinuities, five right-stepping en-echelon pattern segments have been identified along this fault (foroutan et al., 2014). the 40 km long west nayband fault is considered as a branch of the nayband fault system that has right-lateral and revers movement (rashidi et al., 2017). despite showing a clear expression in the geomorphology along most of its length the nayband fault is the only part of the western lut border that has been seismically quiescent during the last few millennia (e.g. ambraseys and melville, 1982). the slip rate of 1.8 ± 0.7 mm/yr is estimated for nayband fault. sabzevaran fault system the n-trending sabzevaran fault is a strike-slip fault with a small but significant reverse component that has an almost vertical fault plane (i.e. about 80◦w) (regard et al., 2004). the sabzevaran fault consists of several en-echelon segments with a right-step arrangement. despite the evidence of high-level tectonic activity and the high slip rate, 5.7 ± 1.7 mm/yr (regard et al., 2005), there is no known destructive earthquake associated with the sabzevaran fault. there are some minor faults at the northern tip of the sabzevaran fault that are considered as its splays; such as the sardueiyeh fault (around 100 km long fault that cuts quaternary units in a reverse mechanism and still no earthquake-related deformation has found along it), lalehzar fault (an active 85 km long right-lateral the strike-slip fault with some recorded earthquakes e.g. 1923/09/22 and 2010/07/31), dalfard fault (a right-lateral strike-slip fault with reverse component), etc. it seems that shearing and thrusting of the recent rock units and alluvial fans along these faults are caused by the right-lateral strikeslip motion along the sabzevaran fault (rashidi et al., 2017). gowk fault system the gowk fault is an nnw right-lateral strike-slip fault that extends for 160-200 km along strike (berberian et al., 2001). it is a fault system that consists of segments in an en-echelon pattern and is one part of the sabzevaran– gowk–nayband system of strike-slip faults (walker et al., 2010) which bounds the western edge of the dasht-e-lut desert (berberian et al., 2001). pull apart basins with faulted boundaries (e.g. chaharfarsakh) have been generated along the gowk fault system. by having a proven record of generating destructive earthquakes, (e.g. 1981.06.11 mw 6.6; 1981.07.28 mw 7.1; 1989.11.20 mw 5.8; 1998.03.14 mw 5.4 and 1998.11.18 mw 6.6), this fault is one of the most active faults of iran (e.g. berberian et al., 2001). the current slip rate on the gowk fault is estimated at 4.2 ± 0.7 mm/yr (walpersdorf et al., 2014). shahdad thrust and fold system the shahdad thrust and fold system is a curved nw trending along the east of the gowk fault. since the gowk fault orientation is oblique to the ns right-lateral shear across eastern iran, the overall motion is partitioned into strike-slip faulting along the gowk fault and shortening on the shahdad thrust and fold system (berberian et al., 2001). unlike the gowk fault that is associated with earthquakes since at least 1877, the shahdad fault has no historical seismic record for the past millennia (berberian et al., 2001). 26 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia kuhbanan fault system the kuhbanan fault is an active 300-km long, nw–se-trending, right-lateral strike-slip fault, which consists of several en-echelon segments (berberian, 2014). the evidence shows that the sense of motion on the kuhbanan fault may have changed from reverse to right-lateral slip through time (e.g. berberian, 2014). the kuhbanan fault currently slips laterally at 3.6 ± 1.3 mm/yr (walpersdorf et al., 2014). although this fault has a long record of earthquakes (e.g. 1875, 1933 and 1977, ambraseys and melville, 1982), the largest earthquakes in the current cluster may not have yet occurred, due to the absence of a large-magnitude earthquake during the last 160 years of recorded seismic history along the kuhbanan fault (berberian, 2005). also, the gps measurements imply that the fault is presently active and possibly slipping faster than over the holocene and geological times. berberian (2005) believes that all the faults in the southern tip of the kuhbanan fault (e.g. dahuyiyeh, chatrud, bazargan, dehbala, and east kerman), are splay intermountain, cross-reverse faults which are developed at almost a right angle. slip that is transferred from the gowk fault to the kuhbanan fault through the restraining bend between them, produces a wide zone of mountainous. occurring of an nw-se trend right-lateral strike-slip faulting within this zone (dehu and dehran faults) forms a flower structure in cross-section. rafsanjan fault the rafsanjan fault is an nw-se–trending, right-lateral strike-slip fault ~200 km long. this fault is one of the major faults of southeast iran, which shows a clear expression in the geomorphology along most of its length (e.g. walker, 2006). maybe it transfers an amount of the sabzevaran fault rightlateral slip to the anar fault. the overall ns right-lateral shearing across the region is accommodated by almost pure strike-slip on the rafsanjan fault in the high, mountainous, regions and shortening component migrates to a series of parallel thrust faults in the plains north of the main rafsanjan fault trace (e.g., walker, 2006 and walker et al., 2010). an average slip rate of ~0.4 mm/yr is estimated for this fault (fattahi et al., 2011). jorjafk fault the ~200-km-long jorjafk fault with an nw-se trending consists of five active main en-echelon segments (rashidi et al., 2017). although there is no record of historical seismicity, this fault shows clear signs of activity in the late quaternary (walker et al., 2010). the overall motion on this fault seems to be oblique reverse and right-lateral. the dip-slip component migrates onto reverse faults in the adjacent lowlands whereas the strike-slip movement continues to occur along the main fault in the high land. (walker et al., 2010). ravar fault the ravar fault is a right-lateral strike-slip with 150 km long. the ravar fault consists of two main left stepping enechelon pattern segments. folds with different geometries can be seen throughout the ravar fault, showing the presence of a reverse component. flower structures were formed in the convergence zone between lakarkuh and ravar faults (at their southern tips). the river offsets are recognizable along it (rashidi et al., 2017). based on the horizontal slip rate of 0.54 mm/yr, the recurrence interval of an mw 6.7 earthquake on the ravar fault is estimated to be about 1400 years (shafiei bafti and shahpasanzadeh, 2010). rayen fault rayen fault is an nw–se trending fault that shows a reverse component of motion in addition to its dominant strike-slip mechanism (rashidi et al., 2017). there are a series of the nw–se-trending folds along this fault that show signs of activity such as the truncation of the northern fold limb and the discrete northward-facing scarps developed in alluvial fans crossing the northern fold margin. this indicates the presence of underlying thrust faults that caused active folding in this region (walker, 2006). lakar kuh fault the n-s trending lakar kuh is a right-lateral strike-slip fault with a small reverse component. its total length is reported to be about 130 km. horsetail splays that formed at the northern and southern tips of this fault show a reverse mechanism with a small right-lateral strike-slip component. it is believed that the southern splay is the location of three events on 1 and 12 december 2017 with mn magnitudes of 6.0–6.1 (nemati et al., 2020). mahan fault-related folds the nw-se trending folds along the northern margin of the kuh-ejupar which are situated close to the town of mahan and extend northwards to beyond the village of jupar, are considered as the mahan fault-related folds. the geomorphic evidence such as the asymmetric pattern of river incision and height changes across the folds shows active folding in mahan. in other words, these folds are underlain by active blind thrust faults that do not reach the earth’s surface (walker, 2006 and walker et al., 2010). a historical earthquake on 2nd january 1934 occurred in the district of mahan which damaged villages along the kuh-e-sekonj range-front to the southeast of mahan (e.g. berberian, 2005). mahan and jorjafk faults are likely to be related due to the geometrical similarities and aligning along the same strike (walker, 2006 and walker et al., 2010). bafgh-baghin fault system the 170 km bafgh-baghin fault system is located along the bafghbaghin mountain. this fault system with the nw-se trend consists of eight segments. this is a right-lateral strike-slip fault with a small reverse component. there is no historical record of an earthquake in this area but the instrumental earthquakes with small to medium magnitude have been recorded at the depth of about 10 to 15 km (sistanipour et al., 2012). rafsanjan-zarand fault system it is a left lateral ne-sw trending fault that is extended from nw of zarand city to sw of rafsanjan city. this fault is created as a result of rightlateral strike-slip motion on the bafgh-baghin fault system in the sw and on the kuhbanan fault system in the ne. sistanipour et al., (2012) believe it is likely to have more intense earthquakes in this area especially at the intersection of rafsanjan-zarand and bafgh-baghin fault systems, in the future. zangi abad-kerman fault it is a 30 km long fault that is situated close to the town of zangi abad and extends southwards to the city of kerman which is considered as a probable fault on the geological map of rafsanjan (gsi, 1992 b). hasanzadeh et al., (2011), who have identified this fault, have not found any evidence of quaternary activity of this fault, i.e. deformation and incision of quaternary sediments, the geometry of alluvial fans and rivers. since this fault extends to the city of kerman and is located along the active fault of kuhbanan, it should be considered in assessing the seismic hazard of kerman city (hasanzadeh et al., 2011). materials and methods the hydrological modeling process was done in the study area by arc gis 10.3 software. 30 m resolution digital elevation model (dem) and geological maps, four geological maps of iran at 1: 250,000 scale (i.e. kerman, bam, rafsanjan, and sirjan) and at least twenty four geological maps of iran at 1: 100,000 scale (e.g. kerman, gowk, rayen, etc.) were used as basic data, to obtain the drainage network layer, basins area, and water dividing layer. the study area was divided into 51 basins and seven geomorphic indices were assessed including stream length-gradient index (sl), drainage basin asymmetry (af), hypsometric integral (hi), the ratio of valley-floor width to valley height (vf), index of drainage basin shape (bs), index of mountain front sinuosity (smf), and transverse topographic symmetry factor (t). then, the results were classified into three classes based on the el hamdouni et al., (2008) classification (except for sl index and t factor (mosavi and arian, 2015)) (table 1). after that, for each basin, two kinds of relative tectonic activity index (iat) was obtained by the average of the different classes of geomorphic indices (s/n) (el hamdouni et al., 2008); results from the analysis of all the 7 geomorphic indices (af, hi, sl, smf, vf, bs, and t) are accumulated and expressed as iat_t index, but the t factor was not considered in the iat index calculation (table 1). the study area was classified into three classes (including 27identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) high (class 2), moderate (class 3), and low (class 4) tectonic activity) according to the iat and iat_t values. all the calculated results have been shown in table 2. finally, to identify faults with seismic hazard potential to the city of kerman, which is the aim of this paper, the plotted earthquake epicenters of the study area were being compared with the morphometric analysis derived from gis. table 1. the geomorphic indices classifications were used in this study. * el hamdouni et al., (2008). ** mosavi and arian (2015). class smf* vf* |af50|* bs* hi* sl** t** iat*&iat_t** (s/n) 1 <1.1 <0.5 >15 >4 >0.5 >1000 t>0.4 1-1.5 2 1.11.5 0.5-1 7-15 4-3 0.40.5 5001000 0.20.4 1.5-2 3 >1.5 >1 <7 <3 <0.4 <500 t˂0.2 2-2.5 4 2.5˂ geomorphic indices of active tectonics asymmetric factor (af) the drainage basin asymmetry factor is a way to measure the tectonic tilting of catchments and is defined as follows (e.g. keller and pinter, 2002): af = 100 (ar / at) (1) where ar displays the basin area on the right side of the stream and at indicates the total area of the drainage basin. when a basin is developed under stable conditions with little or no tilting, af approaches 50, whereas values of more or less than 50 would be considered as the asymmetry of a basin caused by active tectonic (keller and pinter, 2002). hypsometric integral index (hi) this index reveals complex interactions between erosion and tectonics by indicating the volume of a basin which has not been eroded (el hamdouni et al., 2008). the distribution of elevation is defined by the hypsometric curve and the area below the curve is defined as the hypsometric integral (strahler, 1952). it is calculated as follows (e.g. keller and pinter, 2002): hi = (average elevation – min. elevation) / (max. elevation – min. elevation)) (2) the maximum and minimum altitude is directly extracted from the topographic map and the average altitude is calculated from the average value of at least 50 scattered heights in the basin (pike and wilson, 1971) or is obtained through the analysis of the digital elevation model (dem). the analysis of the hi index includes two sections: the hypsometric curve and the integral. the shape of the hypsometric curves is related to the erosional stage. young landforms are characterized by high hi values and convex curves. old landforms have low hi values and are associated with s-shaped curves. intermediate hi values demonstrate mature landscapes (keller and pinter, 2002; pérez-peña et al., 2009). valley floor width-valley height ratio (vf) the vf index is defined as the ratio of the width of the valley floor to its average height (e.g. bull and mcfadden, 1977) and it is calculated as follows: vf = 2 vfw / {(eld esc) +( erd – esc)} (3) where vfw represents the width of the valley, eld and erd demonstrate the height of the right and left sides, respectively and esc shows the height of the valley floor. this index indicates the difference between v-shape valleys, that with the low vf values represent tectonically uplifted areas, and u-shape valleys which have high vf values represent the dominance of the erosional processes (e.g. bull and mcfadden, 1977). the vf and smf indices are so helpful to demonstrate the relationship between faulting and morphology in an area (silva et al., 2003). basin shape index (bs) the relatively young drainage basins in the active tectonic areas tend to be elongated perpendicular to the topographic slope of a mountain. because of tectonic activity reduction, the elongated shape of the basin gradually transforms into a circular shape (bull and mcfadden, 1977). the basin shape is converted into a quantitative index by using the elongation ratio which is computed by (e.g. ramirez-herrera, 1998): bs = bl / bw (4) where bl indicates the length of the basin from the headwaters to the mouth and bw represents the basin width. basins with high tectonic activity have high values of the bs indices and indicate elongated drainage, whereas low tectonic activity basins are characterized by low bs values and a circular shape (el hamdouni et al., 2008). mountain front sinuosity index (smf) this index represents the balance between erosion forces, which tend to create irregular mountain front, and tectonic activities, which tend to create straight mountain fronts. the mountain front sinuosity index is defined by equation 5 (e.g. bull and fadden, 1977): smf = lmf / ls (5) where lmf denotes the mountain front length along the connection of mountain and plain, and ls is the straight-line length of the mountain front (bull and fadden, 1977). active mountain fronts have a straight front with low smf values (smf < 1.1); whereas inactive mountain fronts, due to adapting by erosive processes have irregular or sinuous fronts with high smf values (smf < 1.5) (keller and pinter, 2002; silva et al., 2003; bull, 2007; el hamdouni et al., 2008; pérez-peña et al., 2010). the stream length-gradient index (sl) this index was first used to identify the recent tectonic activity by hack (1973). it evaluates the relative tectonic activity by concerning the relationships between rock resistance and topography along the drainage basin (hack, 1973). this index is calculated as follows: sl = (δh / δl) l (6) where δh/δl represents the gradient of the stream, and l displays the total length of the channel from upstream to the midpoint of the channel reaches for which the index is calculated (hack, 1973). the sl index is sensitive to sudden change along the stream profile, this anomalous change may indicate a tectonic uplift or sudden lithological change. the anomalously high sl values for low resistance rocks show tectonic activity (keller and pinter, 2002). transverse topographic symmetry factor (t) this index is appropriate for assessing drainage basin asymmetry. the changes in the tilting rate in different zones of an area demonstrate the difference in the uplifting rate. this index is obtained by the following equation: t = da / dd (7) where da denotes the distance from the midline of the drainage basin to the midline of the active meander belt, and dd indicates the distance from the midline to the basin divide (cox, 1994). the t factor is a vector that has direction and magnitude ranging from zero to one. t value equals zero for a complete symmetric basin and as the asymmetry of the basin increases, the t value rises and approaches 1. (cox, 1994; keller and pinter, 2002). 28 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia results of geomorphic indices analysis asymmetric factor (af) the asymmetry factor (af) values were computed using dem in a gis environment. this index has been calculated as |af-50|, which is the amount of difference between the observed value and the neutral value of 50 (el hamdouni et al., 2008). the calculated |af-50| values in the study area vary between the range from 0.3 to 40.4. figure 2 illustrates the distribution map of the af index. according to el hamdouni et al., (2008) classification, the calculated |af-50| values along the study area have been categorized into 3 classes: class 1 (|af50| >15); class 2 (7 < |af-50| < 15) and class 3 (|af-50|< 7) (table 1). class 1 which represents high tilting includes more basins than the other two classes, i.e., 24 out of 51 basins (table 2). af values of 11 basins belong to class 2 and 16 basins to class 3. thus, most of the drainage basins are asymmetric in this study area. basin no.33 has the lowest af value (|af-50|=0.3) and the highest af value (|af-50|=40.4) belongs to basin no.34 (table 2). the three classes of af index have scattered across the study area and most of the tilting is related to the active tectonic. for instance, evaluating the af values indicates a dominant nnw tilting along drainage basins in the west of the study area which is the location of the major faults of eastern iran, such as gowk and nayband faults, and the earthquake epicenters concentration (mostly destructive earthquakes with mw>5). hypsometric integral index (hi) by using dem in a gis environment, the hi values were calculated. the hi values which range from 0.06 to 0.48 are classified into two classes: class 2 (0.4<hi<0.5) and class3 (hi<0.4) (table 1). figure 3b displays the distribution of the hi index in the study area. basins no. 2, 9, 16, 18, 20, 48, and 50 fall into the intermediate range of hi value (table 2) and display an s-shaped hypsometric curve (fig.3a). so, these basins are considered as the mature stage of the davis erosion cycle. the 44 other basins with a low range of hi values (table 2) and concave hypsometric curves (fig.3a) demonstrate the old stage of the davis erosion cycle (davis, 1899; keller and pinter, 2002; pérez-peña et al., 2009). figure 3a. hypsometry curves of 4 basins with their calculated hi. basins no. 16 and 20 with the moderate value of hi (class 2) show an s-shaped hypsometric curve. basins no. 3 and 5 with the low value of hi (class 3) show concave hypsometric curves. (a) is the total surface of the basin. (a) is the surface area within the basin above a given elevation h. (h) is the highest elevation of the basin. valley floor width-valley height ratio (vf) the parameters of vf (vfw, eld, erd, esc) were measured for main valleys that cross mountain fronts using cross-sections drawn from a demderived contour map in the gis environment. the vf values were calculated for as many as valleys that were possible, 300 m upstream from the mountain front. the mean of the vf values is determined as the vf index which ranges between 0.11 and 0.82 in the study area. according to this index calculation figure 2. distribution of the af index in the study area, |af-50| > 15 (class 1), 7 < |af-50| < 15 (class 2), |af-50| < 7 (class 3) (el hamdouni et al., 2008) 29identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) (table 2), 22 basins are classified into class 1 (vf < 0.5) which represent the deep v-shape valleys and are usually accompanied by tectonic uplift. 14 basins are categorized in class 2 (0.5 < vf < 1). this index was not calculated for 15 basins due to the lack of appropriate conditions. these results in addition to the absence of class 3, imply that the values of vf are relatively high for most of the study area. this activity is mainly caused by faults such as gowk (in basin no. 16, 17, 22, 31, and 42), nayband, ravar, and lakarkuh (in basin no. 2, 7, 16, and 17), rafsanjan (in basin no. 10, 15, 18, 19, and 24), mahan-jupar (in basin no. 19, 21, and 22) the northern tip of sabzevaran (in basin no.32, 33, 37, 41, and 49), and the southern tip of kuhbanan (in basin no. 16, 17, and 22). the distribution of the vf index and locations where calculations of the index are made are shown in figure 4. figure 3b. distribution of the hi index in the study area, hi > 0.5 (class 1), 0.4 < hi < 0.5 (class 2), hi < 0.4 (class 3) (el hamdouni et al., 2008) basin shape index (bs) bs index was computed using dem in the gis environment. according to the bs index results, which vary between 0.33 and 5.25 (table 2), most of the basins (46 out of 51) have low bs values (class 3 bs < 3) which is the characteristic of circular basins with low tectonic activity (el hamdouni et al., 2008). basins no. 12, 15, and 28 with a medium value of bs belong to class 2 (3 < bs < 4). basins no. 1 and 47 with a high value of bs are placed in class 1 (bs > 4), in other words, these are basins with more elongation and relatively higher tectonic activity (el hamdouni et al., 2008). the distribution of the bs index for the study area is plotted in figure 5. mountain front sinuosity index (smf) in each basin, lmf and ls values are measured from a dem-derived shaded-relief topographic map in a gis environment. the smf values were measured for proper prominent mountain fronts in each basin. the average of obtained smf values is determined as the smf index. figure 6 displays the distribution of the calculated smf values for the 51basins in the study area (table 2). these values vary from 1.06 to 3.12. the basins no.7 and 17, which are in the ne of the study area, are categorized in class 1 (smf <1.1). these basins indicate the straight state of mountain fronts which specify high active tectonic and uplifting (bull, 2007). such high smf values were expected for these two basins since important faults such as nayband, gowk, and shahdad pass across them (fig. 1b and fig. 10a and b). most of the basins (28 basins) are classified into class 2 (1.1<smf<1.5). all ten basins of class 3 (smf >1.5) are mainly located in the sw of the study area. due to the lack of appropriate conditions, this index was not calculated for 11 basins. figure 4. distribution of the vf index in the study area, vf < 0.5 (class 1), 0.5 < vf < 1 (class 2), vf >1 (class 3) (el hamdouni et al., 2008) figure 5. distribution of the bs index in the study area, bs > 4 (class 1), 3 < bs < 4 (class 2), bs < 3 (class 3) (el hamdouni et al., 2008) the stream length-gradient index (sl) the sl values were computed using dem (30 m resolution) and a dem-derived contour map, for every 100 m in the gis environment. elevation difference, distance along the main drainage of the study area, and total distance from the source were determined in the gis environment. using a methodology defined by hack (1973), the sl index was calculated with the obtained inputs. the sl values have been categorized according to mosavi and arian’s (2015) classification: class 1 (sl>1000), class 2 (500<sl<1000), and class 3 (sl<500). the sl values range from 67.34 to 1419.42. the longitudinal river profile and sl index were analyzed to reveal that basins no. 7, 16, and 31 are placed in class 1, 7 basins in class 2, and 32 basins in class 3 (table 2). this index was not calculated for 9 basins due to the lack of appropriate conditions. 30 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia sl values along the channel were plotted on the map of rock strength levels and shown in figure 7a. the contour lines and distribution of the sl index are displayed in figures 7b and 7c respectively. also, the longitudinal river profiles were drawn for each basin, some of which are shown in figure 8. most of the recognizable sl anomalies are related to active tectonic. transverse topographic symmetry factor (t) using dem (30 m resolution) in gis environment values for da and dd were obtained. then according to the cox (1994) equation, the initial values of the t factor were calculated. after that, tectonics fp software was used for figure 7b. contour lines were produced out of the sl index. figure 7a. sl index along the channels and rock strength levels of the study area. figure 6. distribution of the smf index in the study area in addition to the mountain front and geological structures of the study area. smf < 1.1 (class 1), 1.1 < smf < 1.5 (class 2), smf > 1.5 (class 3) (el hamdouni et al., 2008). figure 7c. distribution of the stream-length gradient (sl) index in the study area, sl > 500 (class 1), 300 < sl< 500 (class 2), sl < 300 (class 3) (el hamdouni et al., 2008). statistical analysis of the t factor. finally, the mean t values were computed for each basin and were shown as a vector in which its direction displays the tilting direction and its magnitude is t factor (fig. 9a). based on the results (table 2), the lowest value of t (0.06) belongs to basins no.29 and 32. basins no.2, 26, 27, 31, 38, and 42 have the highest t value (0.95). the results have been grouped into 3 categories: class 1 (t>0.4), class 2 (0.2˂t˂0.4), and class 3 (t˂0.2) (mosavi and arian, 2015). the t distribution map, shown in figure 9b, indicates that most basins (27 out of 51) have high tilting and are categorized in class 1 (table 2). class 2 and class 3 include 14 and 10 basins respectively (table 2). 31identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) discussion and conclusions morphotectonic analysis carried out by gis techniques, use geomorphic indices to evaluate the effects of active tectonics in a region. this is particularly valuable in kerman city where still has not experienced a devastating earthquake, despite being surrounded by major active faults. we used seven geomorphic indices: drainage basin asymmetry (af), hypsometric integral (hi), index of drainage basin shape (bs), the ratio of valley-floor width to valley height (vf), index of mountain front sinuosity (smf), stream length-gradient index (sl), and transverse topographic symmetry factor (t). then all the index values were categorized into three classes. finally based upon an index, proposed by el hamdouni et al., (2008), (iat) the tectonic activity in the study area was evaluated. as mentioned before two kinds of relative tectonic activity index (iat and iat_t) were calculated for the study area (table 3); considering the concentration of earthquake epicenters and the active geological structures, clarified that implementing the t factor in the iat classification increases the accuracy of the relative tectonic activity assessment. based on the values of the iat and the iat_t, the study area is divided into three categories and the relative tectonic activity zoning map was prepared (fig. 10a and b). the high percentage of the basins (55%-65% of the total study area) are in class 3 (moderate tectonic activity) (table 3). most of the basins of class 2 are located on the east side of the study area, the location of active faults of the region. regarding the iat_t classification, 5 basins (7, 17, 20, 21 and 31) show high tectonic activity (class 2) (table 3) (fig. 10b). the high activity of basin no. 7 is caused by the nayband, kuhbanan, lakar kuh, gowk, and dehran faults. the shahdad and gowk faults are considered as the cause of tectonic activity in basins no. 17 and 31. the high activity in basin no. 20 is due to the activity of the gowk fault system. in the basin no. 21, the fault-related folds lead to its high activity; the growth and development of these folds, near mahan and jupar cities, happen as a result of the blind thrusts (walker, 2006 and walker et al., 2010) activity under them. the results show that the kuhbanan fault system, especially its southern termination splays (e.g. chatrud, bazargan, deh bala, etc) (fig. 11), which include the seismic gap zones along the kuhbanan fault, is considered as the fault with the maximum seismic hazard potential to the study area. the kuhbanan fault is elongated from se to nw of basin no. 5, where a considerable part of kerman city is in the southernmost section of this basin. in basin no. 5, all the v-shaped valleys (vf < 0.5) have been created along the kuhbanan fault. furthermore, in basin no. 5, most of the active mountain-fronts (smf < 1.1) are observed along the fronts created by the kuhbanan fault (fig. 11). however, the seismic hazard potential of the following faults should not be overlooked due to the anomalous values of the sl index results from their activity: 1the southern terminus of the jorjafk fault which is situated in the nw of basin no. 5, further away from the study area compared with the kuhbanan fault, 2the rafsanjan-zarand fault system, and 3the probable fault zangi abad, extending up to the proximity of kerman city. in addition, the highest t values (t > 0.4) (class 1) (table 2) in basin no. 5 belong to these faults (fig. 11). a small part of the south of kerman city is located in basin no. 22 where the vf shows high activity (vf = 0.43) (table 2). the existence of the height of kuh-e-sekonj (fig. 11) and part of the gowk fault in the east; mahan-jupar fault-related fold system in the west; and the kuhbanan fault (east kerman fault) in the ne of basin no. 22 are considered as signs of a seismic threat to figure 8. some longitudinal river profiles in the study area and the measured sl index. 32 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia kerman city. in basin no. 22, most of the v-shaped valleys (vf < 0.5) and the highest t values (t > 0.4) (table 2) are caused by the activities of gowk and kuhbanan faults. the concentration of high smf values (smf < 1.1) (table 2) along the kuhbanan fault in this basin, is another sign of active tectonic and uplifting which are resulted from the activity of this fault (fig. 11). the only anomaly in the sl index of this basin is due to the mahan-jupar fault system activity which runs parallel to the stream. analyzing basin no. 21 is also important in estimating earthquake risk in the city of kerman, as this basin is close to the city. this basin shows high tectonic activity (class 2) based on the iat_t value. the mahan-jupar faultrelated folds, which their growth and development are the result of blind thrusts activity beneath them (e.g. walker et al., 2010), are regarded as the major possible risk in the basin no. 21. the mahan-jupar fault-related folds seem to cause the highest and most anomalous values of the sl index and v-shaped valleys in this basin. furthermore, the af (|af-50|>15), vf (vf<0.5) and t (t > 0.4) indices (table 2) indicate that the basin is tectonically active. a small part of the southern termination of basin no. 1 is close to kerman city. according to iat_t, this basin has moderate tectonic activity (class 3) (table 3). in this basin, the important issue that needs to be considered is the faults that caused anomalies in the sl index values. one of them is caused by baghin fault. the closest distance of this fault to kerman city is 20 kilometers and it has not produced significant surface ruptures. another anomaly, farther downstream, has highlighted a gap between two segments of rayen fault which walker et al. (2010) has reported river incision there, and seems to result from fault-related uplift. although it is in more distance from kerman than other basins mentioned before, the importance of basin no. 16 should not be neglected; due to the presence of active faults such as kuhbanan (dehu), gowk (chahar farsakh and dehran) and locating in the range of structural changes related to the figure 9. a) steps to calculate the tilting index of basin no. 5 in the study area. asymmetric vectors and polar representation of asymmetric vectors. the center of the net shows zero tiltings and its perimeter 1. b) distribution of the tilting index (t) in the study area with the geological structures of the study area. 1 > t > 0.4 (class 1), 0.2 < t < 0.4 (class 2), 0 < t < 0.2 (class 3) (mosavi and arian, 2015). intersection of the major well-known faults of the region e.g. nayband and gowk faults which are responsible for most of the destructive earthquakes happened in kerman province (fig. 11). since the identified seismic sources especially in basins no. 1, 5, 16, 21, and 22 are close to the population centers -such as cities of kerman, mahan, jupar, shahdad, golbaf, etc. we suggest that more attention is needed, and further studies, especially paleoseismology, should be conducted to evaluate this area’s seismic risk more carefully. in conclusion, there is a close relationship between tectonic activity rate, geomorphic indices, and structures in this region; as basins with high relative tectonic activity (class 2 of the iat_t) (basins no. 7, 17, 20, 21 and 31) (table 3), correspond with the active structures and the earthquake epicenters concentration (fig. 10a). the morphotectonic analysis conducted in this study suggest that the kuhbanan fault system, especially its southern splays, has the potential to produce serious seismic hazards to kerman city in the future; the mahanjupar fault-related folds, the southern tip of the jorjafk fault, the probable fault of zangi-abad, the rafsanjan-zarand fault system, and gowk fault are considered as other threats to kerman city. the quantitative geomorphic studies (table 2 and 3) has indicated that basins in which the t factor class is higher than the iat index class, are affected by reverse or thrust faulting; while in basins with the iat index class higher than the t factor class, the predominant fault type is strike-slip (fig. 9 and fig. 10a and b). this study confirms that it is possible to identify seismic sources by their effect on the geomorphic indices since. active structures are responsible for most of their anomalous changes. in addition, we showed that geomorphic indices are very useful tools not only in showing the tectonic activity condition of a region -especially in a large area but also in showing the fault types (i.e., dip-slip and strike-slip). 33identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) figure 10 b. distribution of the iat index in the state of implementing t-factor. earthquake epicenters in fig 10a and b are from the catalog of mousavi et al., 2014 (the period of 4th century b.c. through 2012 shown with green circles), and the earthquake between 2012 and 2019 are from iiees and isc (shown with purple circles). figure 10a. distribution of the iat index without implementing the t factor 34 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia figure 11. shaded-relief topographic map of basins no. 1, 5, 16, 21, 22 with the geological structures, the location of considered mountain fronts used for the assessment of the smf index, and some of the valleys cross-sections with their vf values. 35identification of faults with seismic hazard potential based on morphotectonic analysis, kerman city (southeastern iran) table 2. values and classes of seven morphometric indices. basin sl values sl class af-50 values af-50 class hi values hi class vf values vf class bs values bs class smf values smf class t values t class 1 197.08 3 9.31 2 0.18 3 0.51 2 5.25 1 1.29 2 0.12 3 2 863.98 2 4.76 3 0.44 2 0.37 1 1.43 3 1.45 2 0.95 1 3 488.47 3 2.21 3 0.17 3 0.54 2 0.93 3 1.11 2 0.73 1 4 112.71 3 -12.37 2 0.20 3 0.82 2 0.95 3 1.16 2 0.48 1 5 188.00 3 -3.46 3 0.20 3 0.53 2 1.87 3 1.17 2 0.23 2 6 -20.06 1 0.17 3 0.55 3 1.49 2 0.38 2 7 1213.38 1 21.92 1 0.28 3 0.27 1 2.16 3 1.06 1 0.58 1 8 -1.37 3 0.06 3 1.97 3 0.49 1 9 -4.21 3 0.43 2 2.27 3 0.23 2 10 420.77 3 -20.91 1 0.31 3 0.23 1 1.92 3 1.30 2 0.49 1 11 30.19 1 0.39 3 1.65 3 0.54 1 12 -0.85 3 0.39 3 3.61 2 0.39 2 13 192.80 3 -0.84 3 0.31 3 0.56 2 0.94 3 1.20 2 0.54 1 14 -4.62 3 0.27 3 0.72 2 1.66 3 1.50 2 0.29 2 15 68.63 3 -5.19 3 0.35 3 0.46 1 3.57 2 1.57 2 0.28 2 16 1419.41 1 6.63 3 0.48 2 0.28 1 2.45 3 1.13 2 0.23 2 17 723.83 2 27.91 1 0.21 3 0.14 1 1.92 3 1.08 1 0.73 1 18 323.99 3 7.77 2 0.42 2 0.44 1 2.53 3 0.91 1 19 452.56 3 29.00 1 0.34 3 0.38 1 1.69 3 0.82 1 20 764.99 2 27.57 1 0.47 2 0.54 2 1.93 3 1.20 2 0.54 1 21 640.58 2 -34.61 1 0.13 3 0.44 1 1.89 3 1.21 2 0.91 1 22 293.38 3 11.15 2 0.25 3 0.43 1 2.12 3 1.38 2 0.18 3 23 -17.65 1 0.17 3 0.33 3 1.24 2 0.39 2 24 260.24 3 -6.01 3 0.31 3 0.31 1 1.26 3 1.31 2 0.23 2 25 517.28 2 -18.33 1 0.33 3 0.63 2 2.21 3 1.48 2 0.44 1 26 141.62 3 -15.12 1 0.16 3 0.70 2 1.44 3 1.47 2 0.95 1 27 320.25 3 13.43 2 0.29 3 1.62 3 1.30 2 0.95 1 28 249.59 3 31.31 1 0.23 3 4.02 2 1.25 2 0.54 1 29 67.34 3 17.26 1 0.20 3 0.43 1 1.55 3 1.59 2 0.06 3 30 7.70 2 0.30 3 1.51 3 0.12 3 31 1279.30 1 29.21 1 0.34 3 0.16 1 2.01 3 1.21 2 0.95 1 32 402.83 3 14.66 2 0.27 3 0.41 1 2.17 3 1.69 3 0.06 3 33 297.89 3 -0.30 3 0.38 3 0.35 1 1.11 3 1.81 3 0.12 3 34 264.11 3 -40.40 1 0.27 3 0.46 1 1.96 3 1.77 3 0.91 1 35 385.52 3 22.73 1 0.32 3 0.78 2 1.79 3 1.93 3 0.91 1 36 449.12 3 -12.23 2 0.23 3 0.62 2 1.37 3 1.30 2 0.34 2 37 428.72 3 -7.11 2 0.33 3 0.26 1 0.88 3 1.455 2 0.12 3 38 308.23 3 -7.28 2 0.37 3 1.55 3 1.25 2 0.95 1 39 234.66 3 31.87 1 0.24 3 0.31 1 1.51 3 0.73 1 40 285.30 3 -21.38 1 0.29 3 2.83 3 1.76 3 0.39 2 41 619.35 2 -23.31 1 0.33 3 0.32 1 1.56 3 1.64 3 0.91 1 42 314.08 3 39.18 1 0.31 3 0.25 1 1.20 3 1.44 2 0.95 1 43 120.79 3 0.93 3 0.26 3 0.58 2 0.76 3 3.12 3 0.39 2 44 293.29 3 2.49 3 0.22 3 0.64 2 2.39 3 1.48 2 0.18 3 45 179.80 3 6.03 3 0.18 3 1.56 3 0.34 2 46 329.07 3 -30.96 1 0.23 3 2.52 3 1.45 2 0.82 1 47 107.59 3 14.37 2 0.33 3 4.23 1 0.21 2 48 666.73 2 17.48 1 0.44 2 0.60 2 1.38 3 4.87 3 0.12 3 49 424.94 3 -26.03 1 0.38 3 0.11 1 1.24 3 2.01 3 0.63 1 50 -35.52 1 0.44 2 2.06 3 0.91 1 51 274.87 3 3.86 3 0.25 3 0.43 1 1.71 3 1.67 3 0.18 3 36 shahrzad ameri, ali solgi, ali sorbi, alireza farrokhnia table 3. the classification of the iat, iat_t, and t in terms of high (h), moderate (m), and low (l) tectonic activity; in addition to the main structures and the dominant types of fault for each basin in the study area. * due to the lack of known structures, a suggestion was made according to the hypothesis of this paper. basin iat iat_t t main structure the dominant fault type 1 m m l baghin fault strike-slip 2 m m h lakar kuh fault+ravar fault+folds reverse 3 l m h west nayband fault+folds reverse 4 l m h jorjafk fault reverse 5 l l m kuhbanan fault+ jorjafk fault +folds reverse 6 m m m 7 h h h 8 l l h probable fault probably reverse* 9 l l m probable fault probably reverse* 10 m m h rafsanjan fault reverse 11 m m h probable fault probably reverse* 12 l l m no structure probably reverse* 13 l m h probable faults+folds reverse 14 l l m thrust faults+a fault-related fold reverse 15 m m m 16 m m m 17 h h h 18 m m h sardueiyeh-rafsanjan fault+ thrusts+folds reverse 19 m m h sardueiyeh-rafsanjan fault +thrusts+folds reverse 20 m h h shahdad and gowk faults+fold reverse 21 m h h mahan fault+ fault-related fold reverse 22 m m l kuhbanan fault+ a small part of mahan fault +folds strike-slip (more dominant) +reverse 23 m m m 24 l m m sardueiyeh-rafsanjan fault+folds reverse 25 m m h lalehzar fault+folds reverse 26 m m h sardueiyeh fault+ folds reverse 27 l m h shahdad fault + folds reverse 28 m m h thrust+folds reverse 29 m m l 2 major parallel faults+a small part of rayen fault+ folds strike-slip (more dominant) +reverse 30 l l l 31 h h h 32 l l l 33 l l l 34 m m h probable faults+folds probably reverse* 35 l m h faults+folds probably reverse* 36 l m m mahan and rayen faults+folds reverse 37 m m l sabsevaran fault system mostly strike-slip 38 l m h thrusts+folds+ probable faults+ separation place of gowk & tahrud faults mostly reverse 39 m m h probable faults+fold probably reverse* 40 l l m probable faults probably reverse* 41 m m h faults+folds probably reverse* 42 m m h folds +the restraining type of the stepover between the southern tip of gowk fault & dehbakri fault mostly reverse 43 l l m folds+probable faults reverse 44 l l l 45 l l m probable faults+folds probably reverse* 46 m m h probable faults+fold probably reverse* 47 m m m 48 m m l lots of faults probably strike-slip* 49 m m h dalfard fault+folds reverse 50 m m h probably reverse* 51 l 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(2006). a remote sensing study of active folding and faulting in southern kerman province, s.e. iran. journal of structural geology, 28, 654-668. doi: 10.1016/j.jsg.2005.12.014 walker, r.t., gans, p., allen, m.b., jackson, j., khatib, m., marsh, n. & zarrinkoub, m. (2009). late cenozoic volcanism and rates of active faulting in eastern iran. geophysical journal international, 177, 783-805. doi:10.1111/j.1365-246x.2008.04024.x walker, r. t., talebian, m., saiffori, s., sloan, r. a., rasheedi, a., macbean, n., & ghassemi, a. (2010). active faulting, earthquakes, and restraining bend development near kerman city in southeastern iran. journal of structural geology, 32, 1046-1060. doi: 10.1016/j.jsg.2010.06.012 walpersdorf, a., manighetti, i., mousavi, z., tavakoli, f., vergnolle, m., jadidi, a., hatzfeld, d., aghamohammadi, a., bigot, a., djamour, y., nankali, h. & sedighi, m. (2014). present-day kinematics and fault slip rates in eastern iran, derived from 11 years of gps data. journal of geophysical research: solid earth, 119, 1359–1383. doi:10.1002/2013jb010620 https://doi.org/10.1016/j.geomorph.2017.11.002 https://doi.org/10.1016/j.geomorph.2017.11.002 _hlk16271170 _hlk50589286 _hlk50589309 _hlk50589352 _hlk50589384 _hlk50589423 _hlk50589501 _hlk52362826 _hlk52298188 _hlk52362807 _hlk52365943 _hlk52365880 _hlk52218957 _hlk50663345 _hlk533157625 _hlk50473798 _hlk50473915 _hlk50475438 _hlk50475819 _hlk50473287 _hlk50474619 _hlk50476642 _goback _hlk19122600 _hlk52315378 _hlk533092256 _hlk52315470 _hlk52315508 _hlk22925050 keywords: enhanced electrodynamic method; high concentration chromium; contaminated soil method; solubility palabras clave: método electrodinámico mejorado; cromo de alta concentración; método de suelo contaminado; solubilidad how to cite item guo, l., zhang, x., chen, y., & liu, j. (2021). remediation of high concentration chromium contaminated soil by enhanced electrodynamic method. earth sciences research journal, 25(2), 247-253. doi: https://doi.org/10.15446/ esrj.v25n2.93896 high concentration of chromium salt has caused serious pollution to the environment since its production. the longstanding chromium residue has polluted the soil, and the total chromium concentration of some polluted soil has reached 30000 mg / kg. for the remediation of chromium contaminated soil, the enhanced electrodynamic method was proposed. in order to improve the efficiency of electrokinetic remediation of chromium contaminated soil, two enhanced electrokinetic remediation technologies were proposed: electrokinetic oxidation enhanced remediation technology and electrokinetic enhanced remediation technology. cr (iii) in soil was polluted by oxidant chromium oxide to increase the content of dissolved cr (vi),so as to improve the efficiency of electric repair, in order to find a suitable prb medium as a breakthrough to improve the repair efficiency. the experimental results show that compared with the traditional electrodynamic technology, the enhanced electrodynamic method can effectively improve the removal rate of total chromium in the soil, and provide technical support for the enhanced electrodynamic remediation of chromium contaminated soil. the ph control system makes the ph of anode and cathode electrolyte always maintain acidic, and the h+ migration speed is faster than that of oh-, and the ph of soil near the anode is lower than that near the cathode. abstract remediation of high concentration chromium contaminated soil by enhanced electrodynamic method remediación de suelos contaminados con cromo de alta concentración mediante un método electrodinámico mejorado issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n2.93896 la alta concentración de sal de cromo ha causado una grave contaminación al medio ambiente desde su producción. el residuo de cromo de larga data ha contaminado el suelo y la concentración total de cromo de algunos suelos contaminados ha alcanzado los 30000 mg / kg. para la remediación de suelos contaminados con cromo se propuso el método electrodinámico mejorado. para mejorar la eficiencia de la remediación electrocinética de suelos contaminados con cromo se propusieron dos tecnologías mejoradas de remediación electrocinética: tecnología de remediación mejorada por oxidación electrocinética y tecnología de remediación mejorada por oxidación electrocinética. cr (iii) en el suelo fue contaminado por óxido de cromo oxidante para aumentar el contenido decr (vi) disuelto, para mejorar la eficiencia de la reparación eléctrica, para encontrar un medio prb adecuado como un gran avance para mejorar la eficiencia de la reparación. los resultados experimentales muestran que, en comparación con la tecnología electrodinámica tradicional, el método electrodinámico mejorado puede mejorar eficazmente la tasa de eliminación del cromo total en el suelo y proporcionar apoyo técnico para la remediación electrodinámica mejorada del suelo contaminado con cromo. el sistema de control de ph hace que el ph del electrolito del ánodo y del cátodo siempre se mantenga ácido, y la velocidad de migración del h+ es más rápida que la del oh-, y el ph del suelo cerca del ánodo es más bajo que el del cátodo. resumen record manuscript received: 23/01/2019 accepted for publication: 10/10/2020 earth sciences research journal earth sci. res. j. vol. 25, no. 2 (june, 2021): 247-253 lin guo1*, jun liu1, yao chen1, xiaocun zhang2 1electronic information and electrical engineering college, shangluo university, shangluo, 726000, china 2college of urban, rural planning and architectural engineering, shangluo university, shangluo, 726000, china corresponding author: linguo0303@126.com so il s mailto:linguo0303@126.com 248 lin guo, xiaocun zhang, yao chen, jun liu introduction the production of various chromium compounds such as potassium dichromate and chromium trioxide is characterized by backward technology, small scale and scattered layout. chromium pollution accidents occur frequently, resulting in a large number of water-soluble chromium cr (vi) slag, which has seriously threatened human health and caused serious environmental pollution. chromium enterprises have gradually become the focus of the government and all walks of life. for the purpose of source control, a large number of seriously polluted chromium enterprises have been shut down in recent years, and most of the chromium slag has been reasonably treated by the end of 2012 (jourdan et al., 2018). however, due to a large number of abandoned sites of closed chromium production enterprises have not taken necessary environmental protection measures, chromium residue has been piled up for a long time, and a large number of cr (vi) has entered into the soil through the effect of wind and rain, causing the soil chromium content to exceed the standard seriously, and chromium contaminated soil has become a key problem of soil pollution. the survey found that there were more than one third of the total determination points, about 271 points, and chromium exceeding standard was one of the important indicators (ojanen et al., 2018). after investigation, the pollution characteristics of excessive chromium in soil are as follows: from the analysis of valence state, the content of cr (iii) in soil chromium pollution is significantly higher than that of cr (vi), the content of cr (vi) in chromium residue site is only more than ten percent of cr (iii); from the analysis of morphology, the proportion of residual chromium is the largest. in reference (cleveland & townsend, 2019) suggests that the increase of nutrients in tropical rainforests will lead to the loss of a large amount of carbon dioxide from soil to the atmosphere. the terrestrial biosphere atmosphere carbon dioxide (co) exchange is mainly in tropical forests, where the absorption of photosynthetic carbon (c) is considered to be limited by phosphorus (p).phosphorus can also limit the decomposition of organic matter and the loss of soil carbon in phosphorus deficient tropical forests. the effects of phosphorus fertilizer on soil respiration in costa rica lowland tropical rain forest were studied through field fertilization experiments. in the early stage of wet season, when the input of soil soluble organic matter was large, the soil respiration rate increased significantly with the application of phosphorus. the results showed that the mechanisms driving the observed respiratory responses to nitrogen and phosphorus increase may be different, but nitrogen and phosphorus fertilization stimulated a large amount of co loss. in reference (jurkevitch et al., 2019), the characteristics of predation range, ribotyping, soil and rhizosphere diversity of bdellovibrio isolated from plant pathogenic bacteria were proposed. thirty strains of bdellovibrio were isolated from agricultural soil and plant rhizosphere. by means of molecular and culture methods, it was found that bdellovibrio in soil contains different biological subpopulations from that in rhizosphere. 13 strains of soil and 7 strains of soybean rhizosphere bdellovibrio were isolated from pseudomonas corrugata, and 7 strains and 2 strains of soil were isolated from erwinia carinii subsp. by analyzing the predation range of 5 isolates of bdellovibrio in soil and rhizosphere, it was found that most of the 22 substrate species were plant pathogens and plant growth promoting bacteria, which had unique utilization patterns and differences among closely related strains. although some progress has been made in the above research, there are some limitations in the remediation of high concentration chromium contaminated soil. therefore, it is proposed to strengthen the research on the remediation of high concentration chromium contaminated soil by electrodynamic method. remediation of high concentration chromium contaminated soil by enhanced electrodynamic method there are two main ways to remediate chromium contaminated soil: a. remove chromium from the soil to make the retained concentration of chromium close to or reach the soil background value; b. change the existing form of chromium in the soil, reduce hexavalent chromium to trivalent chromium, and reduce its migration ability and bioavailability in the soil (watson et al., 2019; zehe et al., 2018; walker et al., 2018). the removal of chromium from soil mainly focuses on chemical leaching, microbial leaching and phytoremediation. the optimization of chemical leaching agents is the research focus of chemical leaching technology, and the research and development of high-efficiency environmental protection chemicals is the future development direction. citric acid, edta、glda and other chemicals used in the chemical leaching of chromium contaminated soil show good removal effect, and even clean water can effectively remove hexavalent chromium in soil (rinella & reinhart, 2020; ser et al., 2020). microbial leaching uses organic acids or surfactants produced by microorganisms to acidify or complexate with chromium to separate it from soil. compared with chemical leaching, the cost of microbial leaching can be reduced by about 80%, which has little disturbance to the environment and will not cause secondary pollution. therefore, it has been widely concerned in recent years (shi et al., 2018; zehe et al., 2018). some scholars have successfully leached 52% 68% of cr from heavy metal contaminated soil by autotrophic microorganism thiobacillus. heterotrophic microorganisms such as bacillus, pseudomonas and mold also have a good leaching effect on cr. at present, there are few studies on the removal of chromium from soil by microbial adsorption, mainly due to the difficulty of separation from soil (zohdi, 2018; frey et al., 2019; marchiori et al., 2019). there are many studies on the remediation of heavy metal contaminated soil by hyperaccumulation plants, but there are relatively few studies on the removal of chromium. the research on stabilization of chromium contaminated soil mainly focused on chemical stabilization and microbial stabilization. chemical stabilization technology has been widely used in the remediation of chromium contaminated soil, and the current research focus has gradually shifted from the optimization of chemical agents to the long-term safety assessment of the soil after remediation (frey et al., 2019; wang et al., 2018). the research on microbial stabilization began with the discovery of a cr (vi) reducing pseudomonas strain in the 1970s. at present, dozens of genera including bacteria, actinomycetes, fungi and molds have been found to have cr (vi) reducing ability. combined with other technologies. there are many cases of electric technology combined with other technologies to repair contaminated soil, mainly in the following ways: a. electric technology combined with prb. the principle of this technology is to transform cr (vi) into cr (iii), and the research content is to fill prb medium with zero valent iron and ferric oxide (nan & yan, 2019; sakae et al., 2018; heckmann et al., 2018). the results show that the latter is better; b. ultrasonic enhanced electrokinetic method can improve the migration ability of pollutants without any substances (you et al., 2018). at present, this method is often used to remove heavy metals from the soil; c. electrokinetic and biological (plants, microorganisms) joint remediation of chromium residue contaminated soil. direct current was applied to make indian mustard absorb heavy metals in the soil polluted by compound heavy metals (yang et al., 2019); d. using this technology to remove total petroleum hydrocarbons in soil, it is found that the removal rate of total petroleum hydrocarbons by electrokinetic method alone is 55%, and the removal rate of total petroleum hydrocarbons by fenton electrokinetic method combined with iron electrode is 97%. materials and methods sources and properties of the tested soils cr(iii)preparation of contaminated kaolin the experimental kaolin was purchased from the superfine calcined kaolin of datong jinyuan kaolin co., ltd. accurately weigh 6.00kg of the kaolin in the polyethylene box; weigh 15.37g of chromium trichloride hexahydrate, dissolve and fix the volume to 2.00l volumetric flask, then add the solution and 2l deionized water into the polyethylene box with kaolin, mix evenly and balance for one week. chromium residue contaminated soil before the use of the soil, the chromium residue collected from a certain place in henan province by the laboratory polluted the soil. after air drying, the gravel and other debris were screened out, and the soil was powdered by zn-08l small-scale pulverizer, and then the tested soil samples were screened through a 100 mesh screen for standby. the basic properties of chromium contaminated soil are shown in table 1. it can be seen from table 1 that the total chromium concentration of the tested soil sample is especially high, nearly 30000 mg/kg. chromium mainly 249remediation of high concentration chromium contaminated soil by enhanced electrodynamic method exists in the form of cr (vi) and cr (iii), accounting for 17% and 83% respectively. by using the improved bcr extraction method, the proportion of various forms in the tested soil samples is 25.1% in acetic acid extractable state, 5.5% in reducible extractable state, 13.7% in oxidizable state and 55.7% in residual state. experimental equipment experimental device the electrokinetic oxidation and electrokinetic prb experiments were carried out by using the electrokinetic device. in the electrokinetic prb experiment, a prb medium with a width of 5cm was added at 5cm away from the anode. the device is made of organic glass, which mainly includes the following three parts: anode electrolysis chamber, cathode electrolysis chamber and soil chamber. the size of anode and cathode electrolysis chamber is: length width height cm cm cm × × = × ×20 10 10 , and the size of soil chamber is: length width height cm cm cm × × = × ×30 20 10 . the same rod-shaped graphite electrode and electrolyte were placed in the anode and cathode electrode chambers, and the soil samples were evenly placed in the soil chamber. in addition, the device is connected with dc power supply (adjustable voltage), ammeter and peristaltic pump. the alkali solution tank and acid solution tank continuously add sodium hydroxide and acetic acid solution to the cathode chamber through peristaltic pump to balance the experimental ph; the 250 mesh filter screen is mainly used to block the electrode chamber and soil chamber to prevent soil from entering the electrode chamber; the collection tank is mainly used to collect anode and cathode electrolyte during and after the electromotion. instruments for experiment the instruments and equipment required for the experiment are shown in table 2: table 2. instruments and equipment required for experiment serial number name of instrument and equipment model 1 electric experimental device 30×20×10cm plexiglass column 2 ammeter 1~500ma 3 digital acidity meter fe20 4 conductivity meter s230 5 electrothermal constant temperature blast drying oven dh-101-3 bs 6 high speed centrifuge lg10-2.4a 7 atomic absorption spectrophotometer aa6300 8 electronic balance al204 9 peristaltic pump bt100-2j 10 electric heating plate eg20a plus 11 magnetic heating stirrer 78-1 12 thermostatic oscillator hz-9511k 13 small crusher zn-08l 14 visible spectrophotometer 722sp 15 electronic scale uwa-k015 16 ultraviolet visible spectrophotometer 752 experimental process batch test of potassium permanganate oxidation of cr (iii): the batch test was carried out in a 100ml conical flask with stopper, in which 100ml crcl3•6h2o of certain concentration was added, the ph value was adjusted (0.1mol/l hcl and naoh were used to adjust the ph value), and then a certain amount of kmno4 was added. put the conical flask into the shaking table (rotate speed 110r / min) to vibrate the reaction, take samples regularly, and determine the absorbance of cr (vi) generated by oxidation. fenton reagent oxidation of cr (iii) batch test: the oxidant was replaced by fenton reagent, and the others were the same as potassium permanganate oxidation of cr (iii). electric oxidation enhanced repair experiment: the 250 mesh filter cloth was fixed on the inner side of the porous filter screen of the reactor with a long tail clamp, and then the balanced 5 mesh filter cloth was accurately weighed with an electronic scale kgcr (iii) soil preparation or treated chromium slag contaminated soil was placed in the soil sample chamber of the electric device, the overflow port in the electrolysis chamber was blocked, 0.1mol/l kcl solution was added into the anode and cathode electrode chambers, and a certain concentration of potassium permanganate solution was continuously added into the cathode by using a circulating pump according to the conditions. after placing the electric soil and electrolyte, insert the carbon rod electrode connected to the digital display power into the electrolyte, power on, and apply a dc voltage of 1vgcm at both ends of the soil sample. during the test, acetic acid solution was dripped into the cathode electrode chamber by peristaltic pump to control the ph of the cathode solution to be acidic, and the current was recorded at intervals. after four days of electric driving, the electrolyte and soil samples were collected, and the soil samples were divided into 10 parts according to figure 1. take 100-200g of each sample for drying, grinding, passing through 100 mesh sieve, and determine ph, conductivity, cr (vi) and total chromium. figure 1. soil sample sampling diagram after oxidation electromotive test electric prb enhanced remediation of chromium residue contaminated soil experiment: 5 kg of chromium residue contaminated soil was accurately weighed by electronic scale after grinding and screening, put into polyethylene container, added 1.12 mol / l citric acid and deionized water, stirred evenly, acidified for about 4 days. 5 cm thick prb medium was added into the soil chamber 5 cm away from the anode. 3.5 kg chromium slag was added to the contaminated soil in other parts of the soil chamber, and 0.1 mol / l kcl solution was added into the electrolysis chamber. after 9 days, samples were taken as shown in figure 2 to determine ph, conductivity, cr (vi), total chromium and other indicators. accurately weigh 4g of soil sample in a 100ml beaker, add 50ml of certain concentration of potassium chloride, conduct electromagnetic stirring for 5min, centrifugation, pour the supernatant into a volumetric flask of certain volume, repeat the above steps, combine the supernatant and fix the volume. the concentration of cr (vi) in solution was determined by diphenylcarbazide spectrophotometry (gb 7467-87), and cr (vi) in soil was calculated. take 100 mesh 0.5000 g (accurate to 0.0001 g) soil sample, put it in a polytetrafluoroethylene crucible, digest it with hydrochloric acid, nitric acid, hydrofluoric acid and perchloric acid on the electric heating plate, transfer table 1. physical and chemical properties of chromium residue contaminated soil physical and chemical unit ph conductivity(ms/cm) saturated water content (%) w(cr(t))(mg/kg) w(cr(vi))(mg/kg) size 9.3-10.2 6.1-8.5 40 29291±50 4980±50 250 lin guo, xiaocun zhang, yao chen, jun liu it to a 50 ml colorimetric tube, add 5 ml 10% ammonium chloride solution, fix the volume, shake well, dilute, and inject the sample to determine the total chromium content. take 10g soil sample into centrifuge tube, add 25ml deionized water, mix well, stand still, use digital acidity meter to measure the ph of solution. after several days of electric driving, the following formula is used for calculation: e u idt= ∫ (1) in the formula, represents the power consumption of electric repair (kw·h), represents the voltage (u), represents the current intensity (a), and represents the operation time (h). take 5g soil sample into centrifuge tube, add 25ml deionized water, mix well, stand still, and measure soil conductivity with conductivity meter. the batch experiments of potassium permanganate and fenton oxidation of cr (iii) were carried out in order to select suitable oxidants and oxidation reaction conditions. the experimental conditions include reaction ph, time, temperature, oxidant concentration, etc. the reaction kinetics of potassium permanganate oxidation of cr (iii) was also studied. on this basis, the electrokinetic oxidation enhanced repair experiment was carried out. analysis of experimental results the effects of ph, time, temperature and oxidant concentration on the oxidation of cr (iii) were studied by the reaction of 50.00 mg/l cr (iii) with a certain concentration of potassium permanganate. the reaction kinetics of cr (iii) oxidation by potassium permanganate was also studied. effect of ph on oxidation of cr (iii) by potassium permanganate at relatively high ph, permanganate reacts to form manganese dioxide, which is expressed as follows: mno e h o mno oh ph4 2 23 2 4 3 5 12− − −+ + = + = . (2) potassium permanganate can react to form mn2 + at lower ph. the reaction equation is as follows: mno e h m h o4 2 25 4− − ++ = + >+8 n+ ph 3.5 (3) when ph is higher than 12, the following reactions will occur: mno e mno ph4 4 2 12− − ++ = > (4) it can be seen that potassium permanganate has strong oxidizability under acidic conditions. in addition, when ph is higher than 7, cr (iii) will exist in the form of precipitation, which makes it more difficult for potassium permanganate to oxidize cr (iii). therefore, in order to study the optimum ph for the oxidation of cr (iii) by potassium permanganate, the range of ph greater than 7 was directly omitted, and the experimental ph range was set as 1 ~ 7. the traditional electrokinetic remediation technology only migrates and concentrates the mobile target pollutants into the electrolytic cell, and the removal efficiency of most of the difficult to migrate cr (iii) in the chromium residue contaminated soil is low. therefore, in order to improve the removal rate of cr (iii) and total cr in the soil polluted by chromium slag, and to repair the soil polluted by chromium, the actual soil polluted by chromium slag was selected as the test soil. two enhanced electrokinetic remediation methods, electrokinetic oxidation enhanced remediation technology and electrokinetic prb enhanced remediation technology, were studied in order to provide technical support for enhanced electrokinetic remediation of chromium residue contaminated soil. a. a series of batch tests were carried out to select the suitable oxidant and the optimum oxidation reaction conditions. the effects of ph, time, temperature and oxidant concentration on the oxidation of cr (iii) by potassium permanganate and fenton reagent were studied. b. potassium permanganate oxidation enhanced electrokinetic remediation of cr (iii) contaminated kaolin was carried out, in which the prepared kaolin cr (iii) concentration was 500 mg · kg-1. the experiment mainly studied the feasibility of potassium permanganate enhanced electrodynamic chromium removal, and analyzed the current, ph, soil moisture content, potassium permanganate concentration and other factors. c. the experiment of potassium permanganate oxidation enhanced electrokinetic remediation of chromium residue contaminated soil was carried out to study the changes of ph, moisture content, cr (vi) and total chromium of chromium residue contaminated soil after electrokinetic oxidation. d. in order to further study the mechanism of enhanced remediation of chromium residue contaminated soil by electrokinetic prb, fe3o4, fe3o4 + activated carbon, fe3o4 + hydroxy alumina were used as prb media, respectively. the test results are shown in figure 3. figure 3. effect of ph on cr (vi) production it can be seen from figure 3 that under acidic conditions (ph = 1.0-6.0), the amount of cr (vi) is more and the difference is not big. the highest cr (vi) concentration was 47.4 mg / l at ph 3.0. the oxidation rate of cr (iii) reached 94.8% from the initial cr (iii) concentration of 50.00 mg / l before the reaction and the conservation of element mass. the lowest cr (vi) formation rate was found at ph 6 in acid condition, but it also reached 41.55% the results show that the oxidation rate of cr (iii) is 83.1%, and the cr (vi) concentration is 32.33 mg / l under neutral condition. therefore, the optimal ph of the experiment is 1-6. although the h + generated at ph 6 may be oxidized by permanganate to increase the consumption of potassium permanganate, considering the economy, the formation rate of cr (vi) and the follow-up experiment, the follow-up experiment chooses ph 6. figure 2. soil sample sampling after electric prb joint repair test 251remediation of high concentration chromium contaminated soil by enhanced electrodynamic method effect of different initial cr (iii) concentration on oxidation four different initial concentrations of cr (iii) (10.00 mg/l, 20.00 mg/l, 35.00 mg/l, 50.00 mg/l) were set to react with 2 g/l potassium permanganate at ph 6, temperature 20 °c and rotating speed 120 r/min. the experimental results are shown in figure 4. figure 4. variation of the amount of cr6 + generated by cr3 + at different initial concentrations with time it can be seen from figure 4 that with the progress of time, the formation of cr (vi) in four different reactions is significantly accelerated in 0 ~ 60 min, that is, cr (iii) and potassium permanganate react rapidly in this period. after 60 min, the amount of cr (vi) increased, but the change was not obvious. at this time, the reaction rates of the above four different initial cr (iii) concentrations were more than 80%. therefore, the sample was taken at 60 min in the followup experiment. the kinetics of potassium permanganate oxidation of cr (iii) was studied. different sampling time (0, 5, 10, 20, 25, 30, 45, 60 min) was set in 0-60 min. the sampling analysis results are shown in figure 5. figure 5. effect of potassium permanganate concentration on the first order kinetic constant it can be seen from figure 5 that the cr (vi) concentration gradually increases with the reaction. the pseudo first-order dynamic simulation was carried out during this period. the oxidation of cr (iii) by potassium permanganate followed pseudo first order kinetics under the condition of 2g / l potassium permanganate, the results are shown in formula (5): − ( ) = ( ) + + d c dt c r k robs 3 3 (5) in the formula, cr3 is the concentration of cr (iii) at time t, that is, ct. therefore, r2 and kobs (pseudo first order kinetic constants) can be obtained by plotting lnct against time t . the results are shown in table 3. table 3. pseudo first order kinetic parameters of potassium permanganate oxidation of cr (iii) initial cr3 concentration 10mg/l 20mg/l 35mg/l 50mg/l kobs 0.035 0.038 0.098 0.205 r2 0.924 0.969 0.966 0.965 it can be seen from table 3 that when cr (iii) concentrations are 10.00 mg / l, 20.00 mg / l, 35.00 mg / l and 40.00 mg / l respectively, adding potassium permanganate with concentration of 2g / l, the pseudo first-order kinetic constants of the reaction are small, but they slowly increase, and the r2 of four different initial cr (iii) concentrations are all above 0.9. therefore, the pseudo first order kinetics can be used to describe the reaction of cr (iii) with excessive potassium permanganate. in order to further study the effect of temperature on the oxidation of cr (iii) by potassium permanganate, four different temperatures of 10 °c, 20 °c, 30 °c and 40 °c were set. 50 mg/l cr (iii) was oxidized by 2 g/l potassium permanganate. the reaction was carried out at ph 6 and 120 r / min. samples were taken at different intervals. the results are shown in figure 6. figure 6. variation of cr (vi) formed by oxidation of cr (iii) by potassium permanganate with time at different temperatures temperature has a great influence on the oxidation of cr (iii) by potassium permanganate. it can be seen from figure 6 that at 313 k, the reaction end point is close to 10 min. in comparison, at 283 k, the reaction begins to approach the end point after 90 min. that is to say, the higher the temperature is, the more favorable the reaction is. therefore, the oxidation of cr (iii) by potassium permanganate is an endothermic reaction. 252 lin guo, xiaocun zhang, yao chen, jun liu enhanced remediation of chromium residue contaminated soil by electrokinetic prb because of the wide source and low price of aluminum salt, and through the experiment of adsorption of cr (vi) by hydroxy alumina, we know that hydroxy alumina has a strong adsorption performance for cr (vi). therefore, hydroxyl alumina can be used in prb medium in the experiment of enhanced remediation of chromium contaminated soil by electrokinetic prb. in order to further study the method of enhanced electrodynamic chromium removal and the specific mechanism of electrodynamic prb enhanced remediation, the design conditions of electrodynamic remediation for a chromium residue contaminated soil in henan province are shown in table 4. the length, width and height of the filled prb medium are about 20 cm, 5 cm and 10 cm respectively. before electrokinetic remediation, the ph value of chromium residue contaminated soil was above 9.5. according to the experimental conditions of electrokinetic prb enhanced remediation of chromium residue contaminated soil as shown in table 4, after electrokinetic remediation for 9 days, the ph value of each part of chromium residue contaminated soil of three groups of electrokinetic prb enhanced remediation decreased, ranging from 9.0 to 9.25. the specific distribution of ph in soil is shown in figure 7. figure 7. ph changes of soil at different distances from anode after electric driving before electrokinetic remediation, the ph value of chromium residue contaminated soil was above 9.5. according to the experimental conditions of electrokinetic prb enhanced remediation of chromium residue contaminated soil as shown in table 4, after electrokinetic remediation for 9 days, the ph value of each part of chromium residue contaminated soil of three groups of electrokinetic prb enhanced remediation decreased, ranging from 9.0 to 9.25. the specific distribution of ph in soil is shown in figure 7. the ph distribution in figure 7 is mainly due to h+ produced by anode electrolysis. h+ moves to soil pores under the action of electroosmosis and electromigration, which makes the ph of soil near anode lower than that near cathode. although oh is produced by cathode electrolysis, the ph of electrolyte is always acidic due to the existence of ph control system, which makes most of ohproduced by cathode neutralize with acetic acid, so that the ph of soil near cathode does not exceed the initial ph of soil. in addition, the ph control system makes the ph of anode and cathode electrolyte always maintain acidic, and the migration speed of h+ is faster than that of oh-, but the ph of the soil is still alkaline after electrokinetic, which is mainly due to the strong alkaline of the tested soil itself. conclusions and prospect a. based on the results of oxidation of cr (iii) by potassium permanganate and fenton reagent, the amount of cr (vi) produced by fenton reagent is only about 9 mg / l when the initial cr (iii) is 200 mg/l and the condition is relatively suitable, which shows that the ability of fenton reagent to oxidize cr (iii) is poor. relatively speaking, when the initial cr (iii) concentration is 50 mg / l, 20 °c, ph is 6, potassium permanganate is 2g/l, and the reaction time is 60 min, the cr (vi) production has reached about 45 mg/l, and the removal rate is as high as 90%. therefore, potassium permanganate is selected as the oxidant in the subsequent electric remediation of chromium residue contaminated soil. b. the oxidation of cr (iii) by potassium permanganate occurred under acidic conditions. when the ph was 3, the concentration of cr (vi) was 47.4 mg/l, and the oxidation rate of cr (iii) was 94.8%; c. the oxidation of cr (iii) by potassium permanganate is greatly affected by the concentration and temperature of potassium permanganate. with the increase of potassium permanganate concentration and reaction temperature, the formation of cr (vi) first increases and then tends to be stable. the reaction between potassium permanganate and cr (iii) is a pseudo first order reaction depending on the concentration of potassium permanganate. when the concentration of potassium permanganate is not excessive, the reaction kinetics conforms to the second order kinetics the ph control system makes the ph of anode and cathode electrolyte always keep acidic, and the migration speed of h+ is faster than that of oh-, and the ph of soil near anode is lower than that near cathode. enhanced electrodynamic method can improve the removal efficiency of total chromium to a certain extent, but there are also some problems as follows a. potassium permanganate added to the soil will have a certain impact on the physical and chemical properties of the soil, and damage the quality of the soil to a certain extent. therefore, what kind of soil is suitable for electrooxidation technology and how to properly treat the soil added with potassium permanganate after electrooxidation are worthy of further study. b. in the process of electrooxidation, there is still a lot of research space in the form of cr (iii) oxidized by potassium permanganate and the change of soil cr form after electrooxidation. c. in the electrokinetic test, fe3o4, hydroxyl alumina and activated carbon are used as prb media, how long the repair effect can last, and it needs to increase the electrokinetic time for further research; when the prb media fails, how to regenerate and realize the recycling of the media is also a direction of further research. acknowledgement the research is supported by: shaanxi provincial natural science foundation: study on contaminated soil remediation and comprehensive utilization of resources in tailing ponds based on opto-mineral-agri complementary technology, no.2019sf-246; national natural science foundation of china: water adaptation mechanism of plants with different root depth to thin purple soil slope, no.41471188. table 4. experimental results of enhanced remediation of chromium residue contaminated soil by electric prb number time/d voltage/v cathode liquid ph control of anode and cathode prb media anode solution 1 9 30 0.1m/l kcl ph control test of naoh / acetic acid fe3o4 0.1m/l kcl 2 9 30 0.1m/l kcl ph control test of naoh / acetic acid fe3o4+hydroxyl aluminum(mass ratio 2:1) 0.1m/l kcl 3 9 30 0.1m/l kcl ph control test of naoh / acetic acid fe3o4+activated carbon(mass ratio 2:1) 0.1m/l kcl 253remediation of high concentration chromium contaminated soil by enhanced electrodynamic method references cleveland, c. c., & townsend, a. r. 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(2018). electrodynamic machine-learning-enhanced fault-tolerance of robotic free-form printing of complex mixtures, computational mechanics, 12(12), 89-95. https://xueshu.baidu.com/s?wd=author:(dg addiss) &tn=se_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight=person https://xueshu.baidu.com/s?wd=author:(kg chan) &tn=se_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight=person https://xueshu.baidu.com/s?wd=author:(l learn-han) &tn=se_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight=person https://xueshu.baidu.com/s?wd=author:(l learn-han) &tn=se_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight=person gjdgxs 30j0zll 1fob9te 3znysh7 tyjcwt 4d34og8 26in1rg lnxbz9 35nkun2 1ksv4uv 44sinio 2jxsxqh z337ya issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.106442 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 203 210 m et eo r o lo g y manuscript received: 20/12/2022 accepted for publication: 18/07/2023 estimation of evaporation from the water surface using the norm operator hale eriskin*1, ozlem terzi2 1civil engineering department, graduate school, isparta university of applied sciences, turkey. https://orcid.org/0000-0003-2466-1930 2civil engineering department, technology faculty, isparta university of applied sciences, turkey, ozlemterzi@isparta.edu.tr, https://orcid.org/0000-0001-6429-5176 *corresponding author: haleeriskin@gmail.com, abstract keywords: pan evaporation; norm operator; ann; lake eğirdir, turkey. due to the lack of precipitation in recent years, some regions of turkey are in danger of drought. this situation increases the importance of planning water resources and makes it necessary to develop water budget calculations. one of the important steps in water budget calculations is the correct estimation of the amount of evaporation. for this reason, a different method has been developed for evaporation estimation and the applicability of this developed method has been tested with the meteorological parameters of lake eğirdir, one of most important freshwater resources of turkey. eğirdir lake is located within the borders of isparta province in the mediterranean region, turkey. firstly, evaporation estimation models were developed by artificial neural networks (ann) method using 490 days of air temperature, water temperature, sunshine duration, and solar radiation parameters of lake eğirdir. after the meteorological parameters were transformed into a dimensionless form through normalization, the norm function was applied to these parameters as a part of the modeling process. the values obtained by the function are used as input parameters in the n-ann method. in both cases, the pan evaporation values obtained with different network structures were compared and it was seen that the n-ann models developed with the norm operator in general gave more appropriate results. palabras clave: tanque evaporimétrico; norma operacional; redes neuronales artificiales; lago de egirdir, turquía estimación de la evaporación en una superficie de agua a través de la norma operacional how to cite this item: eriskin, h., & terzi, o. (2023). estimation of evaporation from the water surface using the norm operator. earth sciences research journal, 27(2), 203-210. https://doi.org/10.15446/esrj. v27n2.106442 resumen debido a la falta de precipitación en años recientes en algunas regiones de turquía hay un riesgo de sequía. esta situación incrementa la importancia en la planeación relacionada con los recursos acuíferos y hace necesario desarrollar cálculos en el presupuesto del agua. uno de los pasos importantes en el desarrollo de este presupuesto tiene que ver con la estimación correcta de la cantidad de evaporación. por esta razón se ha desarrollado un método diferente para la estimación de la evaporación y la aplicación de este método ha sido evaluada con los parámetros meteorológicos del lago de egirdir, una de las fuentes de agua dulce más importante de turquía. el lago de egirdir se ubica en la provincia de isparta, en la región del mediterráneo de turquía. inicialmente los modelos de estimación de evaporación se desarrollaron a través del método de redes neuronales artificiales (ann, del inglés artificial neural networks) a partir de los parámetros de temperatura del aire, temperatura del agua, duración del día y radiación solar medidos en 490 días en el lago de egirdir. después de que estos parámetros se normalizaron se aplicó la norma operacional como parte del proceso de modelado. los valores obtenidos por la función se usaron como los parámetros de configuración en el método n-ann. en ambos casos, se compararon los valores obtenidos con el tanque evaporimétrico en varias estructuras de redes y se pudo analizar que los modelos n-ann de la norma operacional ofrecieron mejores resultados en general. https://orcid.org/0000-0003-2466-1930 mailto:ozlemterzi@isparta.edu.tr https://orcid.org/0000-0001-6429-5176 mailto:haleeriskin@gmail.com https://doi.org/10.15446/esrj.v27n2.106442 https://doi.org/10.15446/esrj.v27n2.106442 204 hale eriskin, ozlem terzi 1. introduction water is one of the necessary resources for the continuity of life on the earth. water resources are in danger of extinction due to reasons such as increasing population, industrialization, evaporation and decrease in precipitation. water started to be traded on the stock market for the first time, such as gold and oil, with the cme(chicago commodity exchange) group, the world’s largest stock market operator in the usa, opening california’s $1.1 billion futures contracts based on water assets on december 7, 2020 (independent türkçe, 2021). it should be noted that despite the value of water, it becomes mixed with the oceans after precipitation and thus is no longer usable as fresh water. among these reasons, 60.6% of the precipitation falling on land returns to the atmosphere as evaporation (rodell et al., 2015). determining the amount of evaporation is one of the important parameters in the planning of water resources (aydın, 2009) evaporation is a key process in the hydrological cycle that has a direct impact on the planning and operation of water (penman 1948; stewart 1984). therefore, accurate estimation of evaporation is very important for water engineers and managers (qasem et al., 2019). after the stock market transaction, estimation has become important for stockbrokers as well. evaporation estimation from the free water surface can be classified into two groups as direct and indirect methods (keskin and terzi, 2005). the pan measurement method is known as the direct method. the methods developed as an alternative to evaporation measurement by the pan method are empirical (traditional) relations known as the indirect methods depending on meteorological parameters. some parameters affecting evaporation in these empirical relations are precipitation, air temperature, water temperature, solar radiation, sunshine duration, relative humidity, air pressure and wind speed. evaporation prediction is an interesting field of study because it has been studied for many years and remains up-to-date (thanh et al., 2020; sun et al. 2021). evaporation estimation with artificial neural networks (ann) is a frequently used method (taşar et al., 2018; gümüş et al., 2016; doğan, 2020; kişi and afşar, 2010; özdülkar et al., 2019; özel and büyükyıldız, 2019). researchers aim to find the systems that best represent the actual amount of evaporation by using different methods in water basins. it is not easy to reach this goal with high accuracy due to the excess and variability of effective parameters in water basins. in this study, firstly, evaporation models were developed by using ann because of its success in explaining nonlinear relationships with air temperature, water temperature, sunshine duration, and solar radiation parameters. then, the norm operator was used to improve the performance of the developed ann models. fort this, correlation analysis was performed between meteorological input parameters, and n-ann models were developed with new input parameters created by applying the norm operator among the highly correlated parameters. 2. methods 2.1. study region and data lake eğirdir is located within the borders of isparta province in the western mediterranean part of turkey (figure 1). it is 917.7 m high, located at latitudes 35° 37’ 41″–38° 16’ 55″ n and longitudes 30° 44’ 39″–30° 57’ 43″ (kesici and kesici, 2006). with an area of 457 km2, it is the fourth largest lake in turkey and the second largest in the lake district (şener et al.2013). the lake, which has steep rocks and a flat and shallow bottom, has a coastline of 150 km. the part located in the north of kemer strait in lake eğirdir is known as hoyran, and the part in the south is known as eğirdir. the surface area of lake eğirdir varies depending on the water use. some of the sinkholes, especially in the western parts of the lake, which has a karstic structure, have been closed. numerous pumping stations have been established by general directorate of state hydraulic works in the lake to be used for domestic usage, agriculture etc. another of the most important features of lake eğirdir is its connection with the sea. there is the kovada canal of 22 km, which creates a natural connection between lake eğirdir and lake kovada. the water from kovada canal that flows into kovada lake is released from kovada valley into aksu stream and subsequently discharged into the mediterranean through karstic pathways. over the past two decades, the water from the lake has been diverted to karacaören i and ii dams located on the çandır plain. eğirdir lake meets the drinking water needs of isparta province and surrounding settlements. it is also used as irrigation water in the plains of isparta, gönen, uluborlu, senirkent, yalvaç, gelendost and eğirdir. although the lake has a closed basin, it is fed with abundant spring waters from its bottom and 1/3rd of the lake water is renewed every year. in june and october, the figure 1. lake eğirdir 205estimation of evaporation from the water surface using the norm operator annual average evaporation amount in lake eğirdir is 500-530 hm3. the lake is especially fed by groundwater at the bottom. there are also many streams in the area. the most important of these are pupa stream, which comes from yalvaç district and flows into the lake from gelendost district, and goes down to the lake through akçay, uluborlu and senirkent districts, değirmen stream descending from hoyran plain and aksu stream, which is connected to the lake by a channel from aksu stream (kesici and kesici, 2006). in the study, 490 days of air temperature, water temperature, sunshine duration, solar radiation and pan evaporation data of lake eğirdir for the years 2000-2001 were used. meteorological parameters were obtained from the automated groweather meteorology station installed on the shore of lake eğirdir. pan evaporation data were obtained from the general directorate of state hydraulic works. evaporation is estimated using the ann method in region. 3. artificial neural network ann consists of simple elements working in parallel. network functionality is largely determined by the connections between elements. a neural network can be trained to perform a specific function by adjusting the values of the connections between elements. usually, neural networks are set or trained so that a particular target output. the network aims to minimize the sum of the squared differences between the target and output values based on the comparison of the output and target. ann uses many such input/target output pairs to train a network. collective training of the network proceeds by making weight and bias changes based on all the input vectors. adaptive training changes the weights and biases of a network as needed after the presentation of each input vector. neural networks are trained to perform complex functions in a variety of application areas, including pattern recognition, classification, speech, vision, and control systems. today, neural networks can be trained to solve problems that are difficult for traditional computers or humans (demuth and beale 1998). feed forward ann models contain a neuron system organized in layers. there may be one or more hidden layers between the input and output layers. neurons in each layer are connected to neurons in the next layer with weight w that can be adjusted during training. a data model containing xi values presented in input layer i propagates over the network to first hidden layer j. each hidden neuron receives wijxij weighted outputs from neurons in the previous layer. these are summed to produce a net value that is then converted to an output value upon application of an action function (imrie et al., 2000). a neuron consists of multiple inputs and a single output. the sum of the inputs and their weights results in an aggregation operation (equation 1). (1) here, wij is the built-in weight, xij is the input value and netj is the input value of a node in the layer. the output of a neuron is decided by an activation function. step, sigmoid, threshold, linear etc. are some special functions used in ann models. the commonly used sigmoid activation function f(x), is formulated by equation 2. (2) back propagation learning algorithm is one of the most important historical developments in neural networks as it allows modeling and processing of many quantitative phenomena in the science and engineering area. this learning algorithm is applied to multilayer feed-forward networks consisting of processing elements with continuous and differentiable activation functions. such networks associated with the back propagation learning algorithm are also called back propagation networks. given a training set of input-output pairs, the algorithm provides a procedure for changing the weights in a backpropagation network to correctly classify the given input patterns. the basis of this weight update algorithm is the gradient descent method used for simple sensors with differentiable neurons. for a given input-output pair, the back propagation algorithm performs two phases of the data stream. first, the input pattern propagates from the input layer to the output layer, and as a result of this forward flow, it produces an output pattern with minimal frame differences between the output and target data. then, the error signals resulting from the difference between the output pattern and the actual output are propagated back from the output layer to the previous layers to update their weights (lin and lee 1996). various ann methods are available such as multilayer perceptron neural networks (kişi and afşar, 2010) and radial basis function neural networks (doğan ,2020). in complex problems encountered, a solution is sought for a single problem with different ann methods. the relationship of the obtained methods is examined (kişi and afşar , 2010). 4. norm operation the norm operator was applied to the original input data to improve the performance of the ann method. in the norm operator, the data consisting of two different parameters are expressed with a point on the two-dimensional plane and the distances of these points to the center (0,0) point are determined. then, norm-ann (n-ann) model is developed with the new input data. norm operator: let k denote the field (set) of real or complex numbers and let x(x,y) be a vector space (subset) over the field k expressed by equation 3. (3) function and for every (x,y)∈x and for every α∈k (4) (5) (6) if equations 4, 5 and 6 are satisfied, the f function is a norm on the vector space x (yıldız, 2005). euclidean norm: r2 (euclidean space) is a special vector space. the norm of a vector 𝑋𝑌 in this space with a starting point (x1, x2) and an ending point (y1, y2) is defined by equation 8 and ii 𝑋𝑌 ii (stewart, 2009). (7) (8) position vector: the position vector of any point x in a two-dimensional space is the vector denoted by (ox) whose starting point is the origin (0,0) and the end point is x(x,y). if the norm of this vector is (9) (10) is a function defined by equation 10 (stewart, 2009). in the study, each data with two parameters can be expressed with a point in two-dimensional space. each point expressed by parametric data can be mapped exactly to a position vector in two-dimensional space. the norms of the position vectors obtained as a result of this match, represent the input set of the norm ann model to be created using the norm operator. the form of equations 9 and 10 in four-dimensional space, 206 hale eriskin, ozlem terzi (11) (12) are expressed by equations 11 and 12. the f transformations given by equations 10 and 12 satisfy the norm operator requirements given by equations 5, 6 and 7. the norm operation is non-linear because it involves force. for this reason, norm calculation after normalization process for all data will provide more efficient results. k in the data group, with the maximum value of the fmax data group and the minimum value of fmin normalization process for data, (13) is defined by equation 13. the norm operator is used in many applied areas (koksal et al.,2014). 5. results and discussion while developing the models, random 392 (80%) of the 490-day data were grouped as training set and the rest 98 (20%) as test set. in the first part of the study, evaporation models from the lake surface were developed by using the measured original air temperature (ta), water temperature (tw), sunshine duration (n) and solar radiation (rc) parameters of lake eğirdir as inputs in the ann. statistical parameters of the best ann models developed using tansigmoid transfer function (tansig) and feed-forward backpropagation algorithm are given in table 1. according to table 1, the determination coefficient (r2) was calculated using equation 14 as 0.653 and the mean square error (mse) value was calculated using equation 15 as 1.360 for the test set of the best ann-1 model. (14) (15) where ��(𝑟𝑒𝑎𝑙) and ��(𝑚𝑜𝑑𝑒𝑙) are the real and estimated values, respectively. the �𝑎𝑣𝑒𝑟𝑎𝑔𝑒 is the arithmetic mean of the real data. *here the values in the brackets are the number of neurons as follow: input layer, hidden layer, output layer. the scatter diagrams for the training and test sets of the ann-1 model, which was chosen as the most suitable model, are given in figures 2 and 3. in the second part of the study, the norm operator was applied to the original input parameters to improve the performance of the ann models. in order to decide with which parameters to develop the ann models to be created using the norm operator, firstly, the r2 values between the input parameters were calculated and given in table 2. when table 2 was examined, it was seen that the highest correlation was between ta and tw; n and rc parameters. based on the high relationship between ta and tw, the data of these two parameters were used in the norm operator after normalizing. in the same way, n and rc parameters were also normalized and the norm operator was applied. based on the r2 values presented in table 2, the ta and tw parameters have been selected which were found to have a strong correlation, along with the n and rc parameters, for joint usage in our analysis. to calculate the taw values, which is the norm value obtained using ta and tw, equations (9) and (10) are utilized. similarly, the other norm value called nrc has been calculated using n and rc data. the resulting taw and nrc values were incorporated into n-ann models. in addition to the binary combinations of the parameters, the four input parameters were converted into single parameters with the help of the norm operator. here, the normalized air temperature (ta), water temperature (tw), sunshine duration (n) and solar radiation (rc) values are called as tnr as domain elements of the f function defined by equation (12). the parameter tnr= is obtained. n-ann models were created by using the new inputs (taw and nrc) to which the norm operator was applied. for these n-ann models, the tansigmoid transfer (tansig) function and feed-forward backpropagation algorithm are used. the results of the developed n-ann models and the measured pan evaporation values were compared and the r2 and mse values are given in table 3. among these models, it was seen that the most suitable model was the n-ann-3 model with an r2 value of 0.722 and an mse value of 2.314 for the test set. the scatter diagrams for the training and test sets of the n-ann-3 model, which has two inputs among n-ann models, are given in figures 4 and 5. finally, the n-ann-5 model was developed using four input parameters (ta, tw, n, rc), the image set elements of the norm operator g function in fourdimensional space given in equation (12), and a single input parameter. the r2 value for the test set of this model was 0.733 and the mse value was 2.87. it has been seen that the n-ann-5 model developed with a single input gives similar results to the two-input models, and the scattering diagrams are given in figures 6 and 7. table 1. ann model results model network structure* input parameters training set test set r2 mse r2 mse ann -1 (4,8,1) ta, tw, n, rc 0.776 0.665 0.653 1.360 ann -2 (4,10,1) ta, tw, n, rc 0.762 0.653 0.624 1.853 ann -3 (4,12,1) ta, tw, n, rc 0.783 0.586 0.626 1.595 *here the values in the brackets are the number of neurons as follow: input layer, hidden layer, output layer. table 2. the r2 relationship between parameters r2 ta tw n rc evap. ta 1 tw 0.935 1 n 0.501 0.448 1 rc 0.578 0.544 0.795 1 evap. 0.805 0.797 0.517 0.556 1 table 3. n-ann model results model network structure* input parameters training set test set r2 mse r2 mse nann -1 (2,8,1) taw, nrc 0.777 0.719 0.719 2.642 nann -2 (2,10,1) taw, nrc 0.762 0.741 0.646 2.642 nann -3 (2,12,1) taw, nrc 0.758 0.782 0.722 2.314 nann -4 (2,13,1) taw, nrc 0.771 0.723 0.697 3.313 nann -5 (1,10,1) tnr 0.733 0.945 0.715 2.874 *here the values in the brackets are the number of neurons as follow: input layer, hidden layer, output layer. 207estimation of evaporation from the water surface using the norm operator figure 2. the ann-1 model scatter diagram for the training set figure 3. the ann-1 model scatter diagram for the test set 208 hale eriskin, ozlem terzi figure 4. the n-ann-3 model scatter diagram for the training set figure 5. the n-ann-3 model scatter diagram for the test set 209estimation of evaporation from the water surface using the norm operator figure 7. the n-ann-5 model scatter diagram for test set figure 6. the n-ann-5 model scatter diagram for training set 210 hale eriskin, ozlem terzi 6. conclusions determining the amount of evaporation from the water surface is very important in water planning and engineering calculations. due to the variability of the multitude of effective parameters, it is laborious and difficult to determine the amount of evaporation by error-free and algebraic methods. for this reason, artificial intelligence methods, which are widely used in many fields recently, are preferred instead of direct measurement methods and empirical relations, which are difficult. ann, one of the artificial intelligence methods, is one of the most frequently used methods in the field of hydrology. in this study, ann models were created to estimate the amount of evaporation from lake eğirdir using the parameters of air temperature (ta), water temperature (tw), sunshine duration (n) and solar radiation (rc). the original ta, tw, n and rc parameters were first used to develop models with ann. then, by looking at the correlations of these input parameters, the related parameters were determined and after normalization, the norm operator was applied to the related parameters. after determining the related parameters ta-ts and n-rc, each value is expressed as a vector and the lengths of these vectors are used as new input parameters to develop n-ann models. in addition, a model was developed by transforming these four parameters into a single input parameter with the norm function and it was seen that it could be applied to higher dimensional data groups. when the developed models are compared, it is seen that the n-ann models developed with the input parameters obtained after applying the norm operator gave more appropriate results. in the study, the norm operator was used as a new method for data editing in evaporation estimation. when the literature is examined, it has been seen that ann is frequently used for evaporation estimation. it is thought that converting multi-parameter data to real values with the norm operator and using it in ann will contribute to the literature in data editing and pre-processing. thus, it has been seen that modeling with the norm operator contributes to more accurate estimation. for future studies, it is recommended to add the parameters precipitation and the relative humidity which are both important parameters for water budgeting as well. 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(2005). genel topoloji. gazi kitabevi, ankara, türkiye. isbn: 975-8895-80-x https://doi.org/10.1016/s0022-1694(00)00228-6 https://doi.org/10.1016/s0022-1694(00)00228-6 https://www.indyturk.com/node/307366 https://www.indyturk.com/node/307366 https://doi.org/10.1061/(asce)1084-0699(2006)11:1(65) https://doi.org/10.1061/(asce)1084-0699(2006)11:1(65) https://doi.org/10.28948/ngumuh.516891 https://doi.org/10.28948/ngumuh.516891 https://doi.org/10.1098/rspa.1948.0037 https://doi.org/10.1098/rspa.1948.0037 https://doi.org/10.1080/19942060.2018.1564702 https://doi.org/10.1080/19942060.2018.1564702 https://doi.org/10.1175/jcli-d-14-00555.1 https://doi.org/10.1175/jcli-d-14-00555.1 https://doi.org/10.1007/s12665-013-2296-0 https://doi.org/10.1007/s12665-013-2296-0 https://doi.org/10.1016/b978-0-444-42250-7.50005-7 https://doi.org/10.1016/b978-0-444-42250-7.50005-7 https://doi.org/10.1002/hyp.14093 https://doi.org/10.15625/2525-2518/58/3/14223 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.112425 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 39 44 g eo ph y si c s abstract in seismology, the local slope is essential for various seismic forward and inversion methods, including seismic migration imaging, seismic tomography, structure prediction, and other core seismic data processing methods. however, high-frequency random noise is unavoidable in real seismic data. therefore, a robust estimation of event-slope attributes becomes important under high-frequency random noise. this research explores a robust and efficient method for estimating local slopes. the relationship between the frequency response of the hilbert transform and fourier transform is analyzed. the derivative operator constructed by the hilbert transform can effectively weaken the effect on the energy enhancement of the high-frequency random noise. minimizing the quadratic function of the proportionality factors was used to obtain the proportionality factors of the noise correction. finally, the hilbert transform and noise correction derived a new linear plane-wave destruction filter operator. the linear operator can effectively estimate the seismic events’ local slope with high-frequency random noise. the calculation results of the numerical examples show that the linear plane-wave destruction filter estimation local slope method proposed in this study has high calculation accuracy and efficiency. estimation of seismic local slope using hilbert transform noise correction method mingchen liu1, fanchang meng2, zhangqing sun1*, jipu lu3*, ruihu wang4, wenpan cen4 1. college for geoexploration science technology, jilin university, changchun, china 2. institute of geophysics, china earthquake administration, beijing, china 3. guangxi bureau of geology & mineral prospecting & exploitation, nanning, china 4. geological survey institute of guangxi zhuang autonomous region, nanning, china * corresponding author: sun_zhangq@jlu.edu.cn record manuscript received: 25/12/2023 accepted for publication: how to cite this article: liu, m., meng, f., sun, z., lu, j., wang, r., & cen, w. (2024). estimation of seismic local slope using hilbert transform noise correction method. earth sciences research journal, 28(1), 39-44. https://doi. org/10.15446/esrj.v28n1.112425 keywords: seismic local slopes calculation; seismic wave numerical simulation; geophysical exploration; plane-wave destruction filter; automatic normal moveout (anmo). estimación de la pendiente local a través del método de corrección de ruido con la transformada de hilbert resumen en seismología, la pendiente local es esencial para varios métodos de inversión y directos como las imágenes de migración sísmica, tomografía sísmica, predicción de estructura y otros métodos de procesamiento de información sísmica. sin embargo, la alta frecuencia del ruido aleatorio es inevitable en la información sísmica real. pero también una estimación robusta de los atributos de un evento relacionados a la pendiente se vuelve determinante ante las condiciones de ruido aleatorio de alta frecuencia. este trabajo busca explorar un método robusto y eficiente para la estimación de pendientes locales. con este objetivo se analizó la relación entre la frecuencia de respuesta de la transformada de hilbert y la de fourier. el operador derivado construido con la transformada de hilbert puede debilitar efectivamente el efecto del mejoramiento de energía en el ruido aleatorio de alta frecuencia. la minimización de la función cuadrática de los factores de proporcionalidad se aplicaron en la corrección del ruido. finalmente, un nuevo operador de filtro de destrucción de ondas planas lineales se derivo de la transformada de hilbert y de la corrección del ruido. el operador lineal puede estimar efectivamente la pendiente local de los eventos sísmicos con ruido aleatorio de alta frecuencia. el cálculo de los resultados de los ejemplos numéricos muestran que el método de estimación de pendientes locales basada en el filtro de destrucción de ondas planas lineales propuesto en este estudio tiene una alta exactitud de cálculo y eficiencia. palabras clave: cálculo de pendientes locales sísmicas; simulación numérica de onda sísmica; exploración geofísica; filtro de destrucción de ondas planas lineales; https://doi.org/10.15446/esrj.v28n1.112425 mailto:sun_zhangq@jlu.edu.cn https://doi.org/10.15446/esrj.v28n1.112425 https://doi.org/10.15446/esrj.v28n1.112425 40 mingchen liu, fanchang meng, zhangqing sun, jipu lu, ruihu wang, wenpan cen 1. introduction it is a basic process to estimate the local slope attribute of the seismic events in the seismic data processing, which is widely used. these processing methods are: stereo tomography (billette and lambaré, 1998; lambaré, 2008; wang et al., 2016), migration-velocity analysis (lambaré, 2008; li et al., 2014), wave-field separation (liu and li, 2020; sun et al., 2023), data-denoising and steering filtering (liu et al., 2011; liu et al., 2014), seislet-transform (fomel and liu, 2010), automatic normal moveout (anmo) and imaging (cooke et al., 2009; fomel, 2007b), common-reflection-surface (crs) stack (hertweck et al., 2007) and gaussian beam modeling (sun et al., 2021; yue et al., 2021). conventional normal moveout (nmo) requires a lot of manual work to pick up the stack velocity to level the event, while anmo does not require the manual work to pick up, which can save a lot of resources. there are many ways to estimate the local slopes (liu et al., 2018). ottolini (1983) used the local-slant stacking method to extract the local slopes based on the correlation or the similarity function by using the maximum energy value after stacking. the efficiency of this method is not high, however, and the resolution of the estimated value is low. claerbout (1992) proposed the concept of the linear plane-wave destruction (pwd) filter and gradually improved upon the method. based on the idea of a multidimensional prediction error filter and the local plane wave model of the seismic data, the local plane wave was estimated by solving the differential equation of a plane wave with a finite difference template. it could well deal with the aliasing problem and the plane wave whose slope varies with space and time. however, the use of this method was not popular at that time. one of the reasons was that many parameters needed to be selected artificially, such as the filter type, the filter coefficient, and the size, number, and the shape of the local patches estimated by filtering, iteration times and regularization degree, which are not easy to control. clapp et al. (1998) proposed a control filter by applying the method of the plane wave destruction filter in the preconditioned tomographic inversion problem. crawley et al. (1999) reduced the parameters that needed to be set in the claerbout multi-dimensional prediction-error filter, which was widely used in the offshore data regularization, and multiple suppression. fomel (2002) proposed a nonlinear pwd, using the maxflat filter to approximate the phase shift operator and derived a polynomial expression for the local slope. the pwd filter was further improved by reducing the required parameters to one, and estimating only a quantity with a clear physical meaning, i.e., the slope parameter of the local plane-wave field. the slope was estimated as a smooth continuous function of the coordinates of the data, which did not require a time window to restrict it. however, this method ignored the influence of outliers on the interpolation results, and there was a large error in the estimated slope when there was high-frequency noise in the seismic data. zhang (2009) discussed a design problem of filter design infinite-impulse-response (iir) filters with the maxflat frequency response in frequency domain. schleicher et al. (2009) compared several different methods for finding the event slopes and derived an expression of noise correction for the local slope. liu et al. (2015) introduced the hilbert transform to solve the plane wave differential equation, which enhanced the anti-noise ability of the slope field, and also had higher computational efficiency. proposed a deep learning network for the estimation of the slope field, which was trained using only synthetic seismic data. it has good performance in some real-seismic data, but not suitable for all of them. due to the restriction of the environment and human factors, the data collected in seismic exploration are often affected by random noise. random noise is characterized by irregularity, frequency, apparent velocity, and random distribution in time and space. there are many reasons for this noise (sheriff, 1975), such as receiving and excitation conditions, problems of the instrument itself, and environmental effects. in fact, it is that how to accurately estimate the local slope of events in noisy data, and estimate a stable and smooth slope field becomes very important. in this paper, the basic principle of linear pwd operator using the hilbert transform and noise correction is proposed, and fomel’s pwd algorithm is reviewed. then, the proposed method here, and other methods for estimating the slope field, is applied to anmo, and the effects are compared and analyzed under the conditions of non-noise and noisy seismic data. a new linear plane-wave destruction filter operator with noise correction is constructed to estimate the local slope field of a plane wave through the derivative operator using the hilbert transform. this method effectively overcame the influence of the random noise on the slope field estimation. the results of the numerical examples show that this method can efficiently estimate the stable and smooth slope field in noisy data. 2. methods linear plane-wave destruction filter a wave with a plane front is called a plane wave. plane waves do not exist in the real world. spherical waves can be decomposed into plane waves, and plane waves can be synthesized into spherical waves. when a spherical wave travels a certain distance, the wavefront of the spherical wave can be regarded as a plane wave when viewed locally. in this case, the propagation of the wave can be viewed as a plane wave. plane waves have certain directionality. if the wave field t x in the domain is u(x;t), the plane wave propagating along the horizontal direction can be characterized by the one-dimensional wave equation: ( ) ( )2 2 2 2 2 ; ;1u x t u x t x v t ∂ ∂ = ∂ ∂ (1) the equation 1 can be considered to have the following form: ( ); 0p u x t x t  ∂ ∂ + = ∂ ∂  (2) where p = sin θ v is the ray parameter or event slope; θ is the angle between the propagation direction of the wavefront and the perpendicular direction; ( );u x t x ∂ ∂ and ( );u x t x ∂ ∂ represent the derivatives of the wavefield with respect to x direction and t direction, respectively. under the condition of θ=0, it means that the plane wave is perpendicular to the x axis and propagates along the positive direction of the x axis; p specifies the propagation direction of the plane waves. equation (2) is also called the local plane wave differential equation. in this way, the value of p can be determined from the space and time derivatives of the wave-field through equation (2). in the synthetic data without noise, equation (2) can quickly and accurately get the results of p. the frequency response of the derivative operator for discrete data is: (3) however, there is high-frequency random noise in the seismic field. from equation (3), in calculating the derivative, the energy of the high-frequency component of the noise will be significantly strengthened, which, in turn, means that the accuracy of estimating p gets reduced. to weaken this effect, liu et al. (2015) used hilbert transform instead of fourier transform to obtain the derivative of the seismic data by using the relationship between the fourier transform and the hilbert transform on the frequency response of derivative operator (pei and wang, 2001). the ideal frequency response of the hilbert transform derivative operator is: (4) the derivation of the discrete data by the hilbert transform can effectively reduce the effect of energy enhancement of high-frequency random noise. to further improve the accuracy of p estimation in the presence of noise, and correct the noise, the linear pwd method proposed by claerbout (1992) is used here. for the stability of the calculation results, establish a small space-time window, w, around each point, (x0,t0) to be calculated in the seismic profile, is established in which the space-time window, (i,j) ∈ w, contains points (xi,tj). note that ( ); x u x t h x ∂ = ∂  and ( ); t u x t h t ∂ = ∂  represent the derivatives of the wave field in direction x and direction t respectively, and the derivative value obtained by the hilbert transform. the ray parameter is estimated by using the equation (2), 0x th ph+ ≈  , and equation (2) is converted into a quadratic function as follows: 41estimation of seismic local slope using hilbert transform noise correction method (5) the quadratic function is minimized with respect to p, and let ∂q| ∂p=0, and (6) a more stable ray parameter value can be estimated by using equation (6). we used the sliding window normalized correlation (claerbout, 1992) to determine whether the estimated p value matched the data well: (7) from equation (7) that 0 ≤ c ≤ 1, and when c is close to 0, it indicates a low correlation, and when c approaches 1, it indicates high correlation. except for when p = 0 is c = 0, equation (7) is very close to the classical similarity function method. kington (2015) summarized and analyzed the relationship betweed p and c. the expression for the similarity function (stoffa et al., 1981) is as follows: (8) where n represents the number of events calculated in the time-space domain, and w is the length of the time window. schleicher et al. (2009) proposed a correction method for equation (2), and pointed out that when p ≠ 0, there is another corresponding parameter, q = 1/p, namely: ( ); 0q u x t x t  ∂ ∂ + = ∂ ∂  (9) correspondingly, the expression of the minimized quadratic function of q can be estimated from the following: (10) by comparing equations (6) and (10), the values of p and q are equivalent, and the amount of estimation is the same. it follows, then, from equation (7) that: ( )2 0 0,c x t pq= (11) the normalized correlation function can be estimated directly by calculating p and q. when there is no noise in the data, c = 1. however, there is always random noise in the real data, and so the estimated values of p and q are inaccurate. they can be expressed as the actual true value multiplied by a scale factor, ε, less than or equal to 1. it is further assumed here that the influence of the noise on the estimation of p and q is the same. then the proportionality factors are considered to be approximately the same, and this assumption is reasonable, and which can be expressed as follows: (12) according to equation (12), the linear plane-wave destruction filter operator using the hilbert transform, noise correction is obtained, and the true value expression of the slope field is: (13) the implementation flow of the algorithm is shown in figure 1. start input the parameter file and seismic data establish a time-space window for the current data point of seismic data calculate the derivate of hilbert transform along the x direction calculate the derivate of hilbert transform along the t direction calculate the true value of slope field end are all seismic data point calculations completed no yes figure 1. flow chart of the linear plane-wave destruction filter operator using the hilbert transform and noise correction nonlinear plane-wave destruction filter the pwd was considered to be one of the methods to estimate the value of p with higher accuracy, and which utilizes a taylor-series expansion to fit the low-frequency response of a time-domain all-pass filter. the estimation of the local slope was used to predict the data of each event by pwd through adjacent seismic events. according to the continuity of the slope of adjacent seismic events, the objective function of the difference between the predicted data of each event and the original seismic event data is constructed. the objective function was used to reach the optimal iterative solution in the sense of the least squares, and was also used to estimate the local slope field of the events. the estimation of the local slope field of the events can be regarded as a nonlinear inversion problem, which can be solved by the gauss-newton iteration method. the disadvantage of estimation method using pwd is that it needs iteration, and so the estimation speed is slow. according to the local plane-wave differential equation (2), if the slope, p, does not depend on the change of time, t, it can be transformed into the frequency domain and the differential equation becomes: (14) the general solution of the differential equation in (14) is: (15) where û is the fourier transform of u. the exponential term in the above equation represents the time shift of the seismic trace with slope p and trace spacing x. in other words, a plane wave can be perfectly predicted by a two-stage 42 mingchen liu, fanchang meng, zhangqing sun, jipu lu, ruihu wang, wenpan cen frequency-space-domain prediction-error filter. and the amplitude spectrum of the complex exponential term in the frequency domain is 1. this maintains the important property of a plane wave, which is, the invariance of wave propagation energy. fomel (2002) proposed a z-transform version of the three-point finitedifference pwd filter that approximates the time-shift operator as follows: ℯi�p ( )3 3 1 ti p t b z e b z ù ≈        (16) where ( ) ( )( ) ( )( ) ( )( )1 3 1 2 2 2 1 2 12 6 12t t p p p p p p z z− − − + − + + = + +tb z (17) it can derive filter forms of more orders. the phase accurately fit the frequency response of the phase-shift operator at low frequencies. a twodimensional pwd filter using the expression (16) was constructed as follows: ( ) ( )1,t x x t t d z z b z b z z   = −     (18) if d(p) was used to represent the convolution operator between the two-dimensional pwd filter, d(zt, zx), and seismic data, d, then p is the local slope field. subsequently, the objective function, d(p)d, was constructed. the estimation of p was transformed into finding the minimum of the objective function: p = argminp || d(p)d ||2 2 + ɛ2 || s �p� ||2 2 (19) where || · ||2 2 denotes the 2-norm, ɛ denotes the regularization perturbation factor, and s is the shaping regularization operator (fomel, 2007a), which controls the smoothness of the estimated local slope field. due of the continuity of the slope field, the stability and smoothness of its estimated values were also expected to achieve and conform to the actual situation. this is also illustrated by the following numerical examples. 3. experiments and analysis to verify the method proposed in this paper, the slope field estimated by the different p calculation methods was used for anmo of common-midpoint (cmp) gathers under the conditions of the non-noise and noise data. the different methods for estimating the p value were: method 1, which was the constraint ratio of fourier transform in equation (2); method 2, which was the constraint ratio of hilbert transform of equation (2) given by liu et al. (2015); method 3 which was equation (19) using pwd given by fomel (2002); method 4, with the equation 13 estimation method using the hilbert transform and noise correction, that is, the method proposed in this paper. the classical hyperbolic reflection time-distance curve equation is: ( ) ( ) 2 2 0 2 0nmo xt x t v t = + (20) where t is the travel time relative to offset x, t0 is the zero offset travel time, and vnmo (t0) is the stack or rms velocity. the expression of p is: ( )2 0nmo dt xp dx tv t = = (21) therefore, vnmo (t0) can be replaced by mathematical transformation from equations (20) and (21) to obtain the nmo correction equation using the slope: 2 0t t tpx= − (22) the above equation gives an automatic nmo technique using the slope information of the data, called the anmo correction. the method performs the nmo correction according to the slope field information of the data, and avoids the problems in that a lot of manual work is needed to pick up the stack velocity for correction, and that the wavelet stretching occurs at the far offset in the conventional nmo correction. at the same time, this correction technique is sensitive to the smoothness and accuracy of the slope field. a synthetic noiseless cmp gather-record was selected, as shown in figure 2a. there were 201 traces in total, with a trace spacing of 10 m, and a sampling interval of 4 ms. figure 2c shows the local slope field estimated by method 4. the cmp gather-record of figure 2a was superimposed on figure 2c. for each calculation, location point, (x0,t0) here, the size of the space-time window, nt = 10, nx = 10, was taken. the smooth and the stable slope field was estimated by calculating the ratio of the division in the equation with the shaping regularization operator. finally, the anmo was carried out with equation (22), and the results are shown in figure 2d. the events of each layer are flat, which proved the correctness of method 4, and the smooth stability of the calculation results. for the non-noise cmp gather-records, each slope estimation method can accurately estimate the value of p, and then flatten the events. a c d b figure 2. schematic diagram of trial calculation results for a synthetic noiseless cmp gather record. (a) cmp gather record; (b) cmp gather record with white noise added; (c) slope field (cmp gather records are superimposed in the figure); (d) anmo flattens the events. to test the stability and accuracy of the different methods to estimate the value of p under the condition of noise, gaussian white noise was added to original gather records, whose results were shown in figure 2b. the slope field (figure 3a, c, e, g) estimated by the different methods were superimposed with the cmp gather records in figure 2a. the results of the anmo flattening events (figure 3b, d, f, h) showed the effectiveness of the different methods used here. because the slope field estimated by method 3 was not smooth enough, it led to serious “event jitter” and “scratch” phenomena. the triangular smoothing function was selected with the size of the space-time window, nt = 10 and nx = 10, to smooth the slope field. in this paper, the triangular smoothing function was used to repeat smoothing 5 times, and the result was shown in figure 4a. the slope field became smoother, and the mutation values were reduced. the effect after anmo was shown in figure 4b. the numerical results verified the effectiveness of the proposed method. with the same smoothing time window and regularization constraints, different methods estimated the local smooth-slope field. however, it was also that methods 1 and 3 were very sensitive to the noise, and the slope field estimated by methods 1 and 3 fluctuated greatly in the global range, especially in the region where there was no effective signal. for the event part of the effective signal, the estimated slope field was easily affected by the noise, which resulted in an anomaly and an inaccurate value. the anmo results (figure 3b) using method 1 showed that the event was not flattened, which indicated that the slope at the effective signal was distorted by the noise and thus the estimate was inaccurate. fourier transform enhanced the energy of high-frequency noise. there were 43estimation of seismic local slope using hilbert transform noise correction method many “scratches” in the figure because there were many abrupt changes in the estimated slope field, and the slope gradient was large. this phenomenon was also evident in figure 3f. the anmo results (figure 3f) using method 3 showed that the event was flattened, which proved that the estimation of the slope field was accurate. however, the “event jitter” phenomenon, which occurred at the upper and lower edges of the event, indicated that the estimated slope field was distorted by the noise. it was affected by the noise at the far offset. compared with the result of method 1, the slope field estimated by method 3 was reliable at the effective signal. for all that, the slope field was still affected by the influence of the noise. figure 3. the slope fields estimated by different methods with noise as shown in figure 2a and the anmo results. (a) slope field estimated by method 1 (the cmp gather record is superimposed in the figure, the same below); (b) anmo results of slope field estimated by method 1; (c) slope field estimated by method 2; (d) anmo results of slope field estimated by method 2; (e) slope field estimated by method 3; (f) anmo results of slope field estimated by method 3; (g) slope field estimated by method 4; (h) anmo results of slope field estimated by method 4. figure 4. the slope fields estimated by method 3 using the smooth function, (a) method 3 estimates the slope field using the smooth function (cmp gather records are superimposed in the figure); (b) anmo results of slope field estimated by method 3. both methods 2 and 4 were based on the hilbert transform, which would not enhance the energy of the high-frequency noise in the process of operation. figure 3c showed that the estimated slope field results were stable and smooth. the estimated slope field was accurate at the effective signal. the influence of the noise was suppressed at the non-signal. the anmo results (figure 3d) using method 2 showed that the event was flattened, and only the “scratch” phenomenon exists at the edge. the accuracy of the slope estimation was not good in the presence of the noise for shallow reflecting layers with large offset. the slope field (figure 3g) estimated by method 4 was more stable and smoother than the results (figure 3c) estimated by method 2. the event shown in figure 3h was flattened except for a slight misalignment at the far offset, and was minimally affected by the noise, which, in turn, showed good anti-noise ability. figure 3 showed that under the same smoothing condition and regularization constraint solution, the method in this paper had stronger noise immunity, and the stability and smoothness of the slope field estimated were also better. at the same time, the calculation efficiency was the same with methods 1 and 2, but faster than that of method 3. compared with the results shown in figure 3f, the “event jitter” phenomenon was well suppressed, but there was still no good effect at the edge of the event due to the inaccurate slope field (figure 4b). at the same time, the “scratch” phenomenon has not been well controlled, which was caused by the large gradient of the slope field at the scratch position. 4. conclusions several methods for estimating the local slope of events are introduced in this paper. on their bases, a method is proposed for estimating the local slope of the linear plane-wave destruction filter operator using the hilbert transform and noise correction. it is a stable, efficient, and anti-noise slope field estimation method. the slope field estimated by the anmo under the conditions of the non-noise and noise data was used to carry out the flattening experiment on the cmp gather records. the results of the numerical examples showed that the slope field estimated by the proposed method had high stability, smoothness, and strong anti-noise ability. acknowledgment the research work related to this paper has been financially supported by the key r&d program of guangxi province under grants 2021ab30011 and in part by the jilin university experimental technology project syxm2021a013 funded by jilin university. 44 mingchen liu, fanchang meng, zhangqing sun, jipu lu, ruihu wang, wenpan cen references billette, f., & lambaré, g. 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(2025). susceptibility assessment of deep-seated landslides in subhimalayan galiat region, pakistan. earth sciences research journal, 25(3), 261-273. https://doi. org/10.15446/esrj.v29n3.116114 abstract landslides are the most prevalent natural hazards in the sub-himalayan region, posing extensive socio-economic losses. their occurrence is highly influenced by weak geological formations, steep and dissected topography, irregular land-use, high seismic activity, and seasonal precipitation and snowmelt. despite the high threat, there is an absence of landslide susceptibility maps for most of northern pakistan, hindering effective measures for landslide hazard prevention. in this study, a relevant deep-seated landslide inventory for landslide susceptibility assessment of the galiat region was prepared based on field studies and multi-temporal google earth images, identifying 68 landslide polygons. due to the localized nature of landslides, substantial predictions cannot be made with classical statistical modelling. therefore, the landslide susceptibility map of the study area was modelled using the maximum entropy method, which allows predictions based on limited observational data. the analyses were repeated, with three randomly selected data sets being 30% and 70% for training and testing data, respectively. fourteen environmental variables were considered, including geology, digital elevation model (dem), and first and second dem derivatives. the accuracy of the obtained models reached 0.80 ±0.002, evaluated by the auc technique. the high to very high susceptible classes correspond to 26.16 % of the study area, including 74.3 % of the mapped landslides. the resultant landslide susceptibility map will raise understanding of dynamic and potential landslides for citizens, engineers, and land-use agencies. keywords: landslide inventory; deep-seated landslide; landslide susceptibility map; maximum entropy model. evaluación de la susceptibilidad de deslizamientos de tierra profundos en la región galiat del sub-himalaya, pakistán resumen los deslizamientos de tierra son la amenaza natural más prevalente en la región del sub-himalaya, y generan grandes perdidas socioeconómicas. su ocurrencia está fuertemente influenciada por las débiles formaciones geológicas, la topografía inclinada y diseccionada, el uso irregular del suelo, la alta actividad sísmica y las lluvias estacionarias y el descongelamiento de nieve. a pesar de esta amenaza hacen falta mapas de susceptibilidad de deslizamientos de tierra para gran parte del norte de pakistán, lo que dificulta la implementación efectiva de medidas para la mitigacion de estas amenazas. para este estudio se preparó un inventario de evaluación de la susceptibilidad de deslizamientos de tierra profundos en la región de galiat con base en estudios de campo y de imágenes multitemporales de google earth, donde se identificaron 68 polígonos de deslizamiento. debido a la naturaleza localizada de los deslizamientos no se pudieron hacer predicciones sustanciales con los modelos estadísticos clásicos. además, el mapa de susceptibilidad de deslizamientos de tierra del área de estudio se modeló con el método de máxima entropía, lo que permite predicciones con base en datos observacionales limitados. los análisis se repitieron con tres secuencias de datos seleccionadas aleatoriamente, cada una con el 30 por ciento de entrenamiento y el 70 por ciento de prueba. se consideraron 14 variables ambientales que incluyen geología, modelo de elevación digital, y la primera y segunda derivada de los modelos de elevación digital. la exactitud de los modelos obtenidos alcanzó 0.80 ± 0.002, según se evaluó con la técnica auc. la clasificación de alta a muy alta susceptibilidad corresponde al 26.16 % del área de estudio, y que incluye el 74.3 % de los deslizamientos mapeados. el mapa de susceptibilidad de deslizamientos de tierra resultante incrementará el conocimiento de las dinámicas y potenciales deslizamientos para ciudadanos, ingenieros y agencias de uso del suelo. palabras clave: inventario de deslizamientos de tierra; deslizamientos profundos; mapa de deslizamientos de tierra; modelo de máxima entropía. https://doi.org/10.15446/esrj.v29n3.116114 mailto:qmehmood@student.cu.edu.tr mailto:tolgacan@cu.edu.tr https://doi.org/10.15446/esrj.v29n3.116114 https://doi.org/10.15446/esrj.v29n3.116114 262 qaiser mehmood, tolga çan 1. introduction: deep-seated landslides generally occur at slopes between 5° and 30° (some examples in steep slopes) with a comparatively deeper sliding surface, lower velocity, less disturbance during movement, and an arc-shaped sliding surface. to effectively manage landslide hazards, it is necessary to comprehend the primary situations and governing mechanisms of existing landslides in a region (mehmood et al., 2024). a precise landslide inventory dataset is a fundamental prerequisite for quantitative landslide susceptibility, hazard, and risk modeling. the preparation of landslide inventory maps is essential for documenting the scope of landslide events in a region as well as evaluating the dissemination, classes, structures, recurring, and statistics of slope failures (guzzetti et al., 2012; cömert et al., 2018; rosi et al., 2023). the accuracy and completeness of the maps are crucial for dependable applications and are mostly controlled by the extent of the study area, available resources, and utilized mapping techniques (galli et al., 2008; van westen et al., 2008; guzzetti et al., 2012). the development of landslide susceptibility assessment maps is a complex process that requires a significant amount of data collection and analysis. recent improvements in geographic information systems (gis) and remote sensing equipment have made it easier to collect and analyse data over large areas (chen et al., 2016). consequently, the accuracy and precision of landslide susceptibility assessment maps have improved. landslide susceptibility assessments rely primarily on gis frameworks and diverse mathematical approaches to analyse the correlation between various parameters and the likelihood of landslide occurrence (chen and wang 2007; chen et al., 2016). several methods have achieved positive outcomes in evaluating landslide susceptibility, such as logistic regression (conoscenti et al., 2015; duman and çan 2023), frequency ratio (sun et al., 2018; xiao et al., 2020), neural network (zhang et al., 2019), information value (melo et al., 2012), index entropy (wu et al., 2020), support vector machines (pourghasemi and rossi 2017), analytic hierarchy process (cao et al., 2017; mehmood et al., 2021), certainty factor (tsangaratos and ilia 2016; xiao et al., 2019), and random forest (xiao et al., 2020). numerous variables are essential to be considered, including land cover, hydrology, geology, and geomorphology (lee, 2005). landslides are among the most prevalent natural hazards in the subhimalayan range of pakistan, causing substantial losses of life and property. the region is characterized by inept geological units, steep topography, erratic land-use activities, high seismicity, and excessive rainfall, all of which are major landslide triggering and preparatory factors. despite the high threat, most of the northern regions of pakistan lack landslide susceptibility maps, which are essential for hazard mitigation. to address this gap, a deep-seated landslide susceptibility assessment was carried out in the galiat region of the subhimalayan range. a comprehensive landslide inventory map was developed by integrating field surveys with multi-temporal google earth imagery, and landslide susceptibility was evaluated using 68 mapped landslide polygons. since deep-seated landslides usually occur in small and limited areas, classical statistical modeling cannot provide significant predictions. therefore, we modelled the landslide susceptibility map using the maximum entropy method, which allows predictions from limited observation data. maximum entropy is a statistical-probabilistic machine learning method that has been used in a variety of disciplines, including ecology and geological and environmental sciences, particularly the estimation of species distribution (phillips et al., 2004; elith et al., 2011; mert et al., 2016). recently, several researchers have used the maximum entropy method to study the susceptibility of landslides (vorpahl et al., 2012; convertino et al., 2013; kim et al., 2015; park 2015; chen et al., 2017; pandey et al., 2020). despite its complex mathematics, this method doesn’t require high precision or extensive surveys. in this method, the association between the observed landslide areas and the environmental variables is related to the optimum probability density. fourteen environmental variables, including geology, 12.5m spatial resolution digital elevation model (dem), and first and second dem derivative parameters, have been used in this study. the main objectives of this study are to identify the key factors that contribute to landslide happenings in the study area, to collect and analyse data on these factors using gis and remote sensing and field investigation, and lastly, to use the data collected and apply maxent model to develop deep-seated landslides susceptibility assessment map for galiat region of nw himalaya. 1.1 study area geographically, the research area is in the northeast of islamabad, the capital territory, around the murree area, a hilly tourist destination positioned in the galiat region of the himalayan mountains of pakistan. the geographical location of the study area based on gis is given in figure 1. figure 1. geographical location map, (a) regional location, (b) study area. the altitude of the study area is between 520 – 2877 m above sea level. landslides frequently occur in the study area, especially during the rainy season, and inflict enormous socio-economic damage by destroying settlements and businesses in the vicinity. an inventory map of deep-seated landslides was prepared for the 187 km² galiat region using field surveys and high-resolution imagery from google earth pro (figure 2). the landslides susceptibility map and its application can significantly reduce the socio-economic losses of the study area. figure 2. inventory map of deep-seated landslides compiled using google earth and arcmap. 1.2 geological settings of the area the study area is influenced significantly by the active seismic and tectonic structure, which plays a substantial role in triggering landslides. the galiat region primarily comprises the himalayan mountains, originated in the 263susceptibility assessment of deep-seated landslides in sub-himalayan galiat region, pakistan eocene period through the collision of the indian with the eurasian plate along the main mantle thrust (mmt), as documented by (tahirkheli 1979; coward et al., 1986), and it is the highest uplifting region on the globe (zeitler 1985). himalayan foothills are characterized by various rocks bounded by major thrust faults, comprising the main frontal thrust (mft) and the main boundary thrust (mbt). the mbt has shown heightened activity in recent decades, triggered by the 1977 and 2005 earthquakes (farooq and malik 1996; kamp et al., 2008). these seismic events contribute to ground movement, dynamic loading, increased pore water pressure, and shear stress, exacerbating slope instability. figure 3. regional geological map of the study area revised after (khan et al., 2016). the geological stratigraphy of the study area also plays a key role in mass movement activities. along the himalayan karakoram syntaxis (hks), the panjal formation is crammed between the mbt and pt. the cambrian panjal formation has been thrust over the miocene murree formation along the mbt (mehmood et al., 2023). the pt delineates the tectonic boundary dividing the precambrian tanol and the carboniferous-triassic panjal formation on the eastern limb of hks (khan and ali 1994). similarly, along the western limb of the hks, the pt splits the precambrian tanol formation from the precambrian hazara formation. in the north, the mbt isolates jurassic-cretaceous and palaeocene-eocene periods from the miocene murree formation. the hazara formation comprises slate, phyllite, and shale, while the tanol formation comprises quartzite and schist. carbonates, shales, and sandstones from the samana suk, chichali, and kawagarh formations make up the jurassic to cretaceous sequence. furthermore, the carbonates and shales of the lockhart, patala, margalla hill, and chorgali formations make up the palaeocene to eocene sequence. the core of the hks is dominated by eocene kuldana and miocene murree shale and sandstone formations (rehman et al., 2020). the miocene siwalik group rocks, composed primarily of sandstone, shale, and conglomerate, are subjected to the south of the hks. the finegrained lithology of the kuldana and murree formations provides a weak zone for fault localization (mughal et al., 2018). the study area is characterized by structural complexity and sophistication, resulting from the impact of mbt and its corresponding effects (iqbal and bannert 1998). the structural framework is responsible for the development of an extensive water reservoir, which feeds multiple springs and seepages at critical formation toes (niederer et al., 1989), influencing slope instability of rock and soil structures, resulting in mass movement. the study area’s regional geological map is shown in figure 3. 1.3 climate pakistan is located on the western fringe of the monsoon zone, although the galiat region is classified as semi-continental highlands. variations in altitude, quantity of snowfall, and level of snow cover are responsible for considerable climatic diversity. the study area receives an average annual total of 1,900 mm of rainfall and snow over approximately 92 days, though precipitation amounts vary significantly across different parts of the region. the maximum rainfall typically happens in july, august, and september, when the rains are in heavy bursts. the mean monthly precipitation varies from 33 mm in november to 418 mm in july. in winter, snow falls above 1100m and retains above 1600m altitude during january and february. increasing the moisture content on a relatively unstable slope performs a decisive role in causing mass movement by reducing the shear strength of the material. however, the sandstone of the murree formation indicates a reduction of about 20 percent in strength due to moisture, while the overlaid silt/claystone shows a significant reduction in strength due to remolding (farooq and malik 1996). the climate information of the study area is shown in figure 4 and table 1. landslides most often happen around monsoon time; however, certain damage is also linked with snowmelt in spring. figure 4. annual rainfall record from 2000 to 2023 (pmd, 2023) table 1. climate data of the study area 1990-2017 (nasa, 2023) month jan feb mar apr may jun jul aug sep oct nov dec yearly mean daily mean °c 3.7 4.0 8.0 13.2 17.3 20.6 19.1 18.4 17.2 14.3 10.3 6.3 12.7 rainy days 4 6 7 8 9 10 14 13 9 5 3 3 92(days) humidity (%) 62 64 61 57 50 56 78 82 73 61 60 60 63 264 qaiser mehmood, tolga çan 2. materials and methods: 2.1. data used this research comprises five main steps; (i) the first step was data collection and material regarding the study area. geotechnical, geological, topographical, and hydrogeological data are among these data and materials. (ii) the second step was the development of deep-seated landslides inventory map utilising field investigation and google earth pro. (iii) the third step was choosing environmental variables utilising the fieldwork and prior study practices. in susceptibility mapping, the selection and evaluation of environmental parameters is critical for representing the physical processes driving hazard occurrence. the fourteen variables applied in this study cover a broad spectrum of topographic, geological, and hydrological controls. elevation and slope directly influence gravitational forces and slope stability, with higher gradients typically increasing susceptibility (orefice and innocenti, 2025). elevation captures the influence of altitudinal gradients on gravitational forces and climatic conditions, while slope, slope derivatives, and curvature metrics describe terrain geometry, influencing runoff concentration, erosion, and stress distribution. lithology dictates material strength and permeability, strongly governing failure mechanisms. hydrological indices such as cti and imi indicate areas of potential moisture accumulation, often linked to slope failures. the transformed aspect accounts for directional exposure of solar radiation and its effect on moisture and weathering. srr, sar, and the dissection index quantify drainage energy, fluvial erosion, and landscape dissection intensity. within the maxent framework, these parameters were weighted by their contribution to the model’s gain, enabling identification of the most influential factors for susceptibility in the study area. (iv) the fourth step was calculating the value and the controlling power of each variable in landslide instigation, (v) finally, to evaluate the susceptibility level of the study area employing the maxent model. the flowchart of the susceptibility evaluation of the study area is given in figure 5. figure 5. flowchart of the research work 265susceptibility assessment of deep-seated landslides in sub-himalayan galiat region, pakistan 2.2 model selection in this study, the maxent model has been utilised to evaluate the landslide susceptibility map of the study area. the maxent model is based on the principle of maximum entropy, which states that the best model is the one that makes the fewest assumptions while still being consistent with the available data. the maxent model can handle both continuous and categorical data, making it suitable for the evaluation of a broad range of environmental variables. 2.3 maximum entropy model (maxent) maxent is a statistical-probabilistic machine learning modeling approach based on maximum entropy. the maxent program was created by at and t labs’ steven j. phillips, miroslav dudik, and robert schapire for the american museum of natural history’s centre for biodiversity and conservation (phillips et al., 2004). the maxent model uses the principle of maximum entropy to predict the probability distribution of an unknown variable. maxent estimates the probability density function (pdf) for each environmental variable based on presence data and contrasts it with the background distribution for the unknown variable. in the case of shallow landslide susceptibility assessment, the unknown variable is the occurrence of landslides. these pdfs help determine how strongly a variable is associated with landslide presence. the equation used by maxent: = exp( ( ))/ (1) where p(y|x) is the likelihood of landslide happening y assigned a set of environmental variables x, λi is a lagrange equation related to an attribute fi(x), and z is the normalizing constant that ensures the sum of all probabilities equals 1. the feature functions fi(x) represents the environmental variables used to model the occurrence of landslide. the lagrange multipliers λi represent the weights assigned to each feature function and are optimized during the model training process to maximize the likelihood of the observed data. the maxent model seeks to find the probability distribution of landslide occurrence consistent with the observed environmental variable while making the fewest assumptions possible (phillips et al., 2009; elith et al., 2011). the model can handle continuous and categorical data and corporate connections among variables to capture complex relationships (baldwin 2009; halvorsen 2012). entropy, originating from information theory, serves as an important indicator to quantify the level of disorder within a dataset (shannon 1948). it represents the expected information content within the data necessary for describing a pattern or process (jaynes 1982; kleidon et al., 2010). in the context of landslide occurrence estimation, the maxent approach utilizes the maximum entropy probability distribution, which approximates a uniform distribution across the study area. by doing so, it transforms the incomplete information derived from presence points and associated geoenvironmental data into a less arbitrary distribution for representation. maxent aims to identify the factors with the maximum information value for predicting designs, such as the distribution of landslides in this study. this technique has also been reconstructed as an aspect of logistic regression employing presence/absence data (renner and warton 2013). maxent employs an iterative approach to adjust the distribution pattern; the adjustment system consists of a random-walk within the factors space with the allocation of an attribute value. the software offers new parameter settings per iteration, attempting to raise the gain up to the number of iterations chosen by the individual in the advanced factors tab of maxent. the objective is to maximize the gain and obtain the best-fitted model. the more discriminating the distribution, the bigger the gain and the random-walk algorithm begins with an identical probability development in the geographical space. subsequently, it deviates from this distribution due to data constraints, proceeding through successive iterations until the gain rise falls under a convergence threshold, which is configured by default in maxent but can be specified by the operator (phillips et al., 2004). the output of the maxent model is a probability map that represents the likelihood of landslide occurrence across the study area. this map can identify areas of high and low susceptibility to landslides and inform decision-making in the management of landslide risk. 2.4 validation of susceptibility models the maxent software provides diverse outputs that facilitate the assessment of various aspects of the results (elith et al., 2011). one such output is the receiver operating characteristic (roc), which represents sensitivity on the vertical axis and 1-specificity on the horizontal axis. generally, the area under curve (auc) of a model with poor precision and predictive ability is 0.6, whereas it is 1 for complicated systems with great simplification ability. another fundamental question in modeling is identifying the most influential factor. the maxent software solves this question through several means. the gain of the variables is calculated as the model strives to fit and estimate well in each iteration. as a result, the model uses an enhanced coefficient on elements with significant information, facilitating the model’s advancement, and a reduced coefficient on variables with minimal or no information. the coefficient changes with each iteration, relying on the reconsideration of the model results. furthermore, the jackknife test enhances the assessment of the importance of variables by presenting the auc results utilising the test set (pontius jr and schneider 2001). additionally, the response curves demonstrate how variations in factor values influence landslide susceptibility, providing further insights into the spatial interface between landslide probability and controlling variables, as presented in this study. this provides valuable insights into the relative significance of different variables in the model’s predictive performance. 3. susceptibility assessment map of deep-seated landslides 3.1. environmental variables the identification of environmental variables is an essential prerequisite for pre-processing when assessing the susceptibility of landslides. with the recent advancements in gis software and improved computer power, it has become possible to use a significantly large number of individual factors in data-driven landslide susceptibility assessment. landslides can be caused by a range of external ecological and internal geological factors (xiao et al., 2019). researchers have widely used various environmental variables for landslide susceptibility assessment. the preliminary principle for landslide susceptibility is the collection and development of environmental variables (li et al., 2020). fourteen environmental parameters, including transformed aspect, dissection index, landform, slope, slope 2nd derivative, average slope, curvature plan, curvature profile, sar, srr, cti, and imi, elevation, and lithology, were considered in the susceptibility mapping of the study area (figure 6). 3.2 variable contribution the process of selecting causative factors has identified several critical parameters that may play a significant role in determining the susceptibility of landslides. it is crucial to combine these parameters to obtain a consistent descriptive value for susceptibility analysis. otherwise, they will remain distinct variables that provide distinct indications. the first estimate is determined by incorporating the increase in regularized gain, either adding it to the corresponding factor’s contribution or subtracting it if the alteration in the mean value of lambda (λi) is negative. for the second approximate, the importance of each environmental variable on the training presence and background data are randomly permuted, and the model is reconsidered on the permuted data. factor contributions, like the factor jackknife, should be taken with prudence 266 qaiser mehmood, tolga çan when the predictor factors are associated. the values displayed are averages of duplicate runs. figure 7(a) depicts the findings of the jackknife test of factor significance. when employed alone, the environmental variable with the highest gain is elevation, which appears to contain the most meaningful information. the maxent analysis showed that susceptibility is primarily influenced by elevation, lithology, and slope orientation, reflecting the combined effects of terrain position, material strength, and microclimatic conditions. these variables also reduced the gain the most when omitted, suggesting they contain most of the information not present in the other variables. hydrological indices such as cti and imi, along with slope, landform, and curvature measures, provided secondary insights into the influence of the moisture accumulation and terrain geometry. other parameters had minimal impact under the current model settings. overall, susceptibility patterns are driven by a combination of topographic position, geological structure, slope orientation, and hydrological conditions. figure 7(b) depicts the identical jackknife test using test gain instead of training gain. a b c d 267susceptibility assessment of deep-seated landslides in sub-himalayan galiat region, pakistan e f g h 268 qaiser mehmood, tolga çan i k j l 269susceptibility assessment of deep-seated landslides in sub-himalayan galiat region, pakistan m n figure 6. acquired maps of the fourteen environmental variables used in this study ((a) transformed aspect, (b) dissection index, (c) landform, (d) slope, (e) slope 2nd derivative, (f) average slope, (g) curvature plan, (h) curvature profile, (i) sar, (j) srr, (k) cti, (l) imi, (m) elevation, and (n) lithology). a b figure 7. (a) jackknife test of variable importance. (b) jackknife test, using test gain. 270 qaiser mehmood, tolga çan 3.3. susceptibility assessment map the susceptibility of landslides is calculated using the environmental variables and their weights. the choice of probability intervals in susceptibility classes exhibits significant variation, including options like natural jenks and equal intervals. this selection greatly affects the division and distribution of susceptibility classes within the study area. for this study, a natural breaks (jenks) approach was employed for probability classes, leading to a situation where a small portion of the study area encompasses a substantial percentage of the existing landslides classified as high and very high susceptibility. the weight of individual variables is calculated in the maxent model. the cumulative map obtained with the maxent program is reclassified into five classes, i.e., very low, low, moderate, high, and very high, respectively, to develop a landslide susceptibility map for the study area and the graphical representation of the map classification is shown in figure 8a. notably, the areas categorized as high to very high susceptibility cover 26.16% of the total area, yet they contain 74.3% of the mapped landslide occurrences, indicating a strong spatial correlation between high susceptibility zones and actual landslide events (fig. 8b). 3.4. validation of susceptibility models in this study, various metrics can be used to assess and confirm the accuracy of a susceptibility model. specifically, we define evaluation as the assessment of the agreement between the susceptibility classification system and the frequency of landslides utilized for model calibration or training, while confirmation is an assessment of the model’s projecting ability using landslides not utilized during model calibration or training. one effective statistical tool for assessing the discriminant capability of the model and comparing the relative validity among models is the roc-auc provided in different maxent output plots. the auc estimates the model’s accuracy, with values ranging from 0.6 to 1, where higher auc values indicate better prediction. several conventional auc interpretation scales classify results based on their accuracy or goodness. according to the (metz, 1978). criterion, auc-roc values above 0.90 are regarded as excellent, those from 0.80 to 0.89 as good, those between 0.70 and 0.79 regular, and those less than 0.70 as bad. in this study, the obtained auc value is 0.80, regarded as reliable for landslide susceptibility mapping (figure 9). figure 8. acquired deep-seated landslide susceptibility map (a), the percentage in the different landslide susceptibility classes (b). a a b 271susceptibility assessment of deep-seated landslides in sub-himalayan galiat region, pakistan figure 9. the area under curve graph of the study area extracted from the maxent program. 4. discussion and conclusion this study was arranged to create a landslide susceptibility map covering the 187 km2 area of the galiat region around murree in the nw-himalaya, which is highly prone to landslides, especially in the last decade. the relevant deep-seated landslide inventory map was generated based on field investigation and multi-temporal google earth pro images. 68 deep-seated landslide polygon features ranging in size from 348 m2 to 258612 m2 were determined to evaluate landslide susceptibility. since the landslide mostly occurred in limited areas, significant predictions cannot be made with classical statistical modelling. therefore, the landslide susceptibility map of the study area was modelled using the maximum entropy method, which permits predictions from limited observation data. the analyses were repeated with two randomly selected 30% and 70% data sets for testing and training data, respectively. fourteen environmental variables were considered, including geology, dem, first and second dem derivative parameters. our results indicated that the most critical parameters that contributed to the maxent models were dem, geology, topographic radiation aspect index, and aspect transformation. we evaluated the accuracy and sensitivity of the obtained models using the area under the receiver operating characteristic and the area under curve. the average area under the receiver operating characteristic curves was 0.80±0.002. we also found that the high to very high susceptible classes corresponded to 34.16% of the study area, including 74.3% of the mapped landslides. maxent is a machine learning model that specializes in presence-only data. this feature is advantageous in remote and impassable areas with limited access to presence and absence data (pearson et al., 2007). in landslide studies, it is essential to consider that the absence of landslides does not necessarily indicate the absence of susceptibility. it is possible that an area with no recorded landslide may still have a high potential for landslide occurrence but has not been observed or properly captured by the researcher. therefore, the maximum entropy model, which relies only on presence locations, can reduce many of these inconsistencies. moreover, the misidentification of landslides and inadequate consideration of environmental variables can jeopardize the efficiency and factors estimation of the model. a field survey was conducted to address these issues, including expert opinions and the selection of relevant environmental variables to better represent the landslide occurrence process. one other limitation of maxent is that it simplistically accounts for landslide inventory by only including point features in ascii format and disregarding information about their shape and dimensions. despite the user-friendly interface, there is complex math involved in the maxent software. it operates with a set of landslides, environmental variables, and two sets of incomplete random sample data that provide inadequate information regarding the objective phenomenon (kornejady et al., 2017). maxent randomly splits the sample data into two sets, 70% for training and 30% for results validation. the software aims to approximate the best probability distribution of the objective phenomenon using this incomplete information. to accomplish this, it starts with an identical likelihood distribution equation as a first guess and employs constraints to determine the best pdf of the phenomenon, known as the maximum entropy function, over a spatial context known as the feature. the features are the related functions and transformations of the variables that are connected by constraints. there are five types of features, including continuous layers that form linear, quadratic, product, and threshold components, and categorical layers that form binary features. the model imposes constraints on the mean, variance, and covariance of the variables to find the best pdf of the phenomenon. the developed landslide susceptibility map can provide valuable information for residents, engineers, and land-use authorities to reduce the destruction caused by landslides. by identifying active and potential landslideprone areas, land-use decisions can be made more effectively, and hazard mitigation strategies can be developed. overall, our study highlights the importance of developing a landslide susceptibility map in high-risk areas and provides a useful framework for future studies on landslide susceptibility assessment in the sub-himalayan range. 272 qaiser mehmood, tolga çan acknowledgements: the authors would like to thank the editor and anonymous reviewers for their comments and suggestions, which helped a lot in making this paper better. funding this research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. conflicts of interest – the authors declare no conflict of interest. references baldwin, r. a. 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(2025). a comparative study of vertical geoelectrical arrays in delineating shallow subsurface properties. earth sciences research journal, 29(1), 81-88. https://doi. org/10.15446/esrj.v29n1.109067 manuscript received: 23/05/2023 accepted for publication: 26/02/2025 abst r ac t the geoelectrical method, particularly the vertical resistivity technique, is extensively utilized in numerous studies, especially for groundwater exploration and analysis. predominantly, the schlumberger (sc) array is employed in these applications. this study seeks to identify an alternative electrode configuration for delineating subsurface features and to effectively compare various electrode arrangements within the one-dimensional (1d) technique. in addition to the sc array, this research evaluates the classical wenner alpha (wa), wenner beta (wb), and dipole-dipole (dd) arrays. over a measurement length of approximately 100 meters, the wb and sc arrays demonstrated preferable delineation of subsurface properties compared to the others. these arrays exhibited enhanced performance in signal strength, vertical sensitivity and resolution, and depth of investigation. therefore, in addition to the sc array, the wb array serves as a viable alternative for vertical geoelectrical surveys. nonetheless, further studies with extended measurement ranges are necessary to confirm these findings and assess their applicability in larger-scale surveys. keywords: geoelectrical method, vertical resistivity technique, schlumberger, wenner alpha, wenner beta, dipole-dipole. palabras claves: método geoeléctrico; técnica de resistividad vertical; schlumberger; wenner alpha; wenner beta; dipolo-dipolo estudio comparativo de matrices geoeléctricas verticales para delinear características bajo la superficie los métodos geoeléctricos, particularmente el sondeo eléctrico vertical, se utilizan ampliamente en diversos estudios, especialmente en la exploración y análisis de aguas subterráneas. la matriz de schlumberger predomina en estas aplicaciones. este estudio busca identificar una configuración de electrodos alternativa para delinear las características bajo la superficie y para comparar efectivamente varias configuraciones de electrodos dentro de la técnica unidimensional. además de la matriz de schlumberger, este artículo evalúa las clásicas matrices de wenner alpha, wenner beta y dipolo-dipolo. en una medición de aproximadamente 100 metros de longitud las matrices de wenner beta y de schlumberger mostraron delineaciones preferibles de propiedades bajo la superficie en comparación con las otras. estas matrices mostraron desempeño mejorado en fortaleza de señal, sensibilidad vertical y resolución y profundidad de la investigación. además de la matriz de schlumberger, la matriz de wenner beta sirve como una alternativa viable para los sondeos verticales geoeléctricos. sin embargo, se necesitan estudios con mayores rangos de medida para confirmar estos hallazgos y medir su aplicabilidad en sondeos a mayor escala. resu m en https://doi.org/10.15446/esrj.v29n1.109067 mailto:martin.antosia@tg.itera.ac.id https://doi.org/10.15446/esrj.v29n1.109067 https://doi.org/10.15446/esrj.v29n1.109067 82 risky martin antosia 1. introduction over the past decades, numerous studies have employed the geoelectrical method to investigate the earth’s subsurface. this technique measures potential differences between two electrodes resulting from electrical currents passing through the subsurface (dentith & mudge, 2014; lowrie, 2007; reynolds, 2011). the electrical resistivity is determined using the inversion method, a key subsurface property that can subsequently be interpreted to infer various geological features. for instance, geoelectrical methods can provide insights into landslide-prone areas (rezaei et al., 2018), fractured zones (ojo et al., 2011), underground cavities (amini & ramazi, 2017; farooq et al., 2012), and weak geological zones (al-saady et al., 2022). additionally, this method is widely recognized for its applications in natural resource exploration, including mineral prospecting (antosia et al., 2021; arifin et al., 2019; farduwin et al., 2021; fernandes et al., 2018) and groundwater surveys (adeyemo et al., 2017; ariyo et al., 2011; bassey et al., 2019; fajana et al., 2019; genedi et al., 2021; lucy et al., 2016; metwaly et al., 2012; pangaribuan et al., 2017; rizka & satiawan, 2019). a noteworthy aspect of the geoelectrical method in groundwater studies is the application of the one-dimensional (1d) vertical resistivity technique, commonly referred to as vertical electrical sounding (ves). the majority of these studies utilize the schlumberger array (adeyemo et al., 2017; ariyo et al., 2011; bassey et al., 2019; fajana et al., 2019; genedi et al., 2021; metwaly et al., 2012; pangaribuan et al., 2017; rizka & satiawan, 2019), while only a few adopt the wenner configuration (adeyemo et al., 2017; lucy et al., 2016). however, as outlined by dentith & mudge (2014), lowrie (2007), and reynolds (2011), numerous other electrode configurations, including wenner, dipole-dipole, and pole-pole arrays, can also be implemented beyond the schlumberger arrangement. another important observation is that most studies focus on applying these configurations (other than schlumberger) for subsurface profiling using two-dimensional (2d) methods, where various electrode arrays are compared. for example, dahlin & zhou (2004) numerically analyzed the resolution and efficiency of different electrode arrays in resistivity profiling surveys. additionally, standard arrangements such as wenner alpha, wennerschlumberger, and dipole-dipole arrays have been compared in various contexts, including the detection of underground cavities (amini & ramazi, 2017), investigation of buried concrete pipes (abdul-nafiu et al., 2013) or tunnels (hassan et al., 2018), identification of bedrock over buried waste (guedes et al., 2020), and characterization of pollutant infiltration in unconfined aquifers (moreira et al., 2016). among these classical electrode arrays, the dipole-dipole array is often preferred due to its sensitivity in representing subsurface targets. also, it has a deeper depth of investigation (szalai et al., 2009). conversely, limited studies were comparing vertical geoelectrical configurations. alao et al. (2019) and fajana et al. (2019) contrasted the schlumberger array with its modifications in depicting subsurface information. furthermore, adeyemo et al. (2017) compared wenner alpha and schlumberger arrays based on their output thickness measurements. arnaut et al. (2022) simulated and conducted geoelectrical measurements using wenner alpha, beta, and gamma arrays to identify, quantify, and reduce lateral effects. szalai et al. (2009) also studied the depth of investigation and vertical resolution from several arrays. given these observations, a comparative study of conventional electrode arrays for vertical geoelectrical surveys is essential to evaluate their effectiveness for shallow investigation. this study compares the wenner alpha, wenner beta, schlumberger, and dipole-dipole configurations. additionally, it aims to elucidate why the schlumberger array is predominantly used in vertical geoelectrical surveys and explore the potential of other arrangements in the 1d method for describing subsurface resistivity properties. 2. material and methods the vertical geoelectrical method (or ves) aims to provide a vertical distribution of electrical resistivity as a function of depth below the earth's surface (dentith & mudge, 2014; lowrie, 2007; reynolds, 2011). typically, the electrode configurations assessed in this study include wenner alpha (abbreviated as wa), wenner beta (wb), schlumberger (sc), and dipoledipole (dd) arrays. each primary array consists of four electrodes: two current electrodes (a and b) to inject electrical current into the subsurface and two potential electrodes (m and n) to measure the potential difference (∆v) resulting from the injected current (i). the resistance (r) is calculated using the following formula: vr i ∆ = (1) to determine the observed apparent electrical resistivity (ρa), the measured resistance (r) is normalized by a geometrical factor (k), as described by dentith & mudge (2014), lowrie (2007), and reynolds (2011): ρa = kr (2) where ρa is the observed resistivity (all units are in the international system). figure 1. electrode configuration, (a) wenner alpha (wa), (b) wenner beta (wb), (c) schlumberger (sc), and (d) dipole-dipole (dd) (adapted and modified from loke, 2023). figure 1 illustrates the electrode positions for each array configuration. the geometrical factor (k) in equation (2) can be derived from the electrode positions, and the general expression for it is: 1 1 1 1 12 am an bm bn k x x x x π −   = − − +    (3) with xam, xan , xbm , and xbn are the electrode spacing of am, an, bm, and bn, (all units are in the international system). from figure 1, it can be observed that the wa array has the same electrode arrangement as the sc array: a-m-n-b, similar to the wb array, which is equivalent to the dd array (arrangement: a-b-n-m ). the differences lie in the electrode intervals. wa and wb arrays maintain an equal distance between their electrodes, denoted as “a,” whereas sc and dd arrays have similar electrode spacing with specific conditions: 1. schlumberger (sc) array: in the sc array (see figure 1(c)), the distance of am must be greater than mn am mnx x> (4) 1 1( ) ( ) 2 2ab mn mnx x x− > (5) 1 3( ) ( ) 2 2ab mnx x> (6) hence, b > 3a (7) with a > 0 and b > 0. 83a comparative study of vertical geoelectrical arrays in delineating shallow subsurface properties 2. dipole-dipole (dd) array: in the dd configuration (figure 1(d)), the ab interval must be smaller than bn b < 3a (8) with a > 0, b > 0, and b > a, so equation (8) can be rewritten as follows, a < b < 3a (9) (all units are in the international system). additionally, based on figure 1 and equation (3), the geometrical attribute of each configuration can be expressed as follows: kwa= 2πa (10) kwb = 6πa (11) 2 2 2sc b ak a π  − =     (12) 24 ( )dd a bk ab a b π  + =  −  (13) where kwa, kwb, ksc, and kdd are the attributes for wa, wb, sc, and dd in sequence (all units are in the international system). the field data acquisition process for obtaining observed resistivity in the 1d method involves measuring data from a short interval and gradually increasing the electrode spacing from the central point “o” towards both the right and left sides. this approach applies to all electrode configurations used in the vertical geoelectrical method. in this study, the maximum electrode spacing from the midpoint is 48 meters, located in the northern yard of the engineering laboratory building 3 at the sumatera institute of technology (figure 2). to analyze the subsurface resistivity variation vertically, the study utilizes progress v3.0 software, which processes all types of electrode arrays in the vertical geoelectrical method (antosia et al., 2022). the software’s data processing follows the workflow illustrated in figure 3. users are only required to input the field-observed resistivity data and an initial vertical subsurface resistivity model, which is derived from analyzing the curve pattern in figure 4. the software then automatically and iteratively performs forward and inverse modeling until the error converges, ultimately generating the final resistivity and depth models. figure 2. measurement map of vertical electrical sounding. figure 3. general flowchart of geoelectrical data processing (aziz, 2012; hubbard, 2009). 84 risky martin antosia figure 4. theoretical three-layer sounding curves (lowrie, 2007). table 1. field vertical geoelectrical data acquisition. l (m) wenner alpha (wa) wenner beta (wb) schlumberger (sc) dipole-dipole (dd) a (m) i (ma) ∆v (mv) a (m) i (ma) ∆v (mv) a (m) i (ma) ∆v (mv) a (m) i (ma) ∆v (mv) 1.5 1.0 21.14 127.20 1.0 65.16 119.89 0.5 56.67 346.45 0.5 45.10 83.25 3.0 2.0 43.47 173.56 2.0 52.32 58.04 0.5 59.42 116.42 2.0 190.17 22.53 4.5 3.0 39.04 124.40 3.0 39.24 33.22 0.5 52.59 54.93 3.5 694.35 26.90 6.0 4.0 31.51 82.94 4.0 48.59 37.17 0.5 45.87 30.17 3.0 241.15 73.102.0 45.58 122.12 7.5 5.0 33.55 76.97 5.0 43.35 31.01 2.0 51.22 91.93 4.5 242.93 35.51 9.0 6.0 45.73 91.97 6.0 47.63 31.30 2.0 48.94 62.26 6.0 262.79 20.35 12.0 8.0 50.22 79.21 8.0 35.93 20.64 2.0 51.98 38.32 9.0 994.59 25.38 15.0 10.0 45.67 57.86 10.0 43.96 21.22 2.0 46.20 21.00 9.0 1205.73 93.94 18.0 12.0 34.83 34.67 12.0 50.21 20.64 2.0 155.86 44.75 12.0 776.29 31.40 21.0 14.0 194.20 23.73 14.0 58.79 20.95 2.0 182.70 33.57 15.0 1233.09 26.41 24.0 16.0 52.02 32.33 16.0 160.38 48.03 2.0 162.02 20.66 18.0 1320.95 13.02 27.0 18.0 117.54 61.21 18.0 162.71 41.01 2.0 285.42 26.03 18.0 1356.63 23.53 30.0 20.0 231.72 30.06 20.0 159.48 33.48 2.0 511.29 33.89 21.0 1307.45 10.984.0 505.20 68.34 33.0 22.0 266.01 58.37 22.0 206.77 37.06 4.0 1013.99 97.80 21.0 1206.99 14.85 36.0 24.0 220.55 65.06 24.0 206.97 31.42 4.0 766.52 55.30 21.0 1225.72 19.01 39.0 26.0 924.89 17.14 26.0 205.71 26.28 4.0 868.45 44.94 24.0 1063.61 9.61 42.0 28.0 540.73 89.68 28.0 284.15 28.02 4.0 797.64 24.46 21.0 1227.66 12.50 45.0 30.0 542.25 92.88 30.0 302.63 27.05 4.0 751.28 22.03 24.0 1035.46 13.13 48.0 32.0 486.78 74.44 32.0 297.23 23.54 4.0 740.18 17.94 24.0 748.88 22.23 where “a” refers to figure 1 for each electrode arrangement, l is xab /2 for wa and xam /2 for wb, l = b for sc and dd. 3. results and discussion a. field measurement and data acquisition field measurements were conducted using the ares instrument from gf instruments to compare each electrode configuration. the data acquisition parameters were standardized across all arrays, including a maximum power of 600 w, four data stackings, and a minimum measured potential of 20 mv. the electrode distance (l) from the midpoint to the outer electrode was consistent for all arrays. table 1 summarizes the field data acquisition for the vertical geoelectrical technique. 85a comparative study of vertical geoelectrical arrays in delineating shallow subsurface properties observations: 1. current (i): the injected current increases with electrode distance across all arrays to maintain the minimum voltage reading of 20 mv (figure 5(a)). 2. voltage difference (∆v): the potential response decreases as the distance between the current and potential electrodes increases (figure 5(b)). 3. dipole-dipole (dd) array: several voltage values did not achieve the minimum setting for l > 27 m despite a higher injected current than other configurations. this indicates challenges in maintaining voltage readings over larger distances with the dd array (see figure 5(a)). 4. schlumberger (sc) array: maintains better potential readings as the electrodes remain within ab the perimeter, reducing voltage attenuation (figure 5(b)). 5. wenner alpha (wa) array: demonstrates more stable voltage readings due to consistent electrode spacing (figure 5(b)). figure 5. measured electrical parameters over the electrode distance “ l,” (a) current, and (b) voltage difference. b. quantitative analysis of electrode configurations to substantiate the claims regarding the arrays’ performance, specific quantitative metrics were analyzed: 1. average electrical current and voltage the initial analysis involves computing the average value of the raw data from table 1, and the results are shown in the following table. table 2. the average value of electrical current and voltage for all arrays. parameter wa wb sc dd i (ma) 206.41 130.06 342.81 862.08 ∆v (mv) 73.35 36.31 65.39 30.40 the ratio i of and ∆v 2.81 3.58 5.24 28.36 a. current ratio: the wa requires approximately 2.81 ma to achieve the same voltage reading as dd, indicating higher efficiency in current usage. b. voltage response: the wa exhibits a more stable and higher voltage response than dd, which shows the lowest voltage readings. these observations reinforce the earlier explanation that wa demonstrates greater stability and consistency in measuring voltage response, indicating superior signal strength. for clarity, the signal strength is categorized into three levels: high, moderate, and low. consequently, wa falls into the high category, followed by wb and sc with moderate strength, while dd has the lowest. 2. apparent resistivity calculations using equations (1), (2), and (10)-(13), the resistivities for each electrode configuration were calculated and plotted, presented in figure 6. a. apparent resistivity trends: all configurations show similar trends, fluctuating resistivity values as electrode spacing increases. however, the dd array exhibits more variability, suggesting lower reliability in resistivity measurements (figure 6(a)). b. cumulative apparent resistivity: the cumulative resistivity plot clearly highlights deviations. the wb, sc, and dd arrays show more consistency compared to wa and follow a similar increasing pattern (figure 6(b)). figure 6. observed data over electrode spacing “l,” (a) apparent resistivity, and (b) cumulative apparent resistivity. 3. modeling and inversion next, the output comparison was performed by processing data through the progress v3.0 software. erenow, the initial model must be inserted into the software. it was determined by curve pattern analysis in figure 4. based on the observed curve in figure 6(a), the resistivity model may composite two curve shapes, “k” and “q” types. it contains four resistivity layers with the following conditions, ρ1 < ρ2 > ρ3 > ρ4, where ρ1, ρ2, ρ3, and ρ4 are resistivity estimations at the first, second, third, and fourth layers in sequence. its initial model is shown in the following table. table 3. initial resistivity and depth models. layer depth (m) resistivity (ohm.m) 1 0 30 2 1.5 100 3 10 50 4 30 20 a a b b 86 risky martin antosia a. b. c. d. figure 7. data processing result: (a) wa, (b) wb, (c) sc, and (d) dd. the generated outputs of each array from the software are displayed in figure 7. their results are generally identical, except in the dd, the resistivity value in the fourth layer becomes higher than in the third layer; its curve pattern is changed to the “kh” type. a. wa and wb arrays: display more stable and consistent resistivity profiles that align with geological expectations. b. sc array: shows slight variations but maintains consistency in resistivity measurements. c. dd array: exhibits fluctuating resistivity values, particularly in deeper layers, indicating potential inaccuracies. the results in table 4 represent the final models of the vertical resistivity properties beneath the earth’s surface in the study area, accompanied by the following explanation: a. layer 1: consistent resistivity across all arrays, indicating reliable detection of the near-surface layer. b. layer 2: all arrays show high resistivity values with lower standard deviations, indicating more reliable measurement. however, the deviation for depth value is the highest. c. layer 3: the wa and dd arrays exhibit lower resistivity values, suggesting less consistency, possibly leading to geological misinterpretations. the depth of this layer tends not to vary across configurations. d. layer 4: the wa and dd arrays have similar resistivity values, as do wb and sc. this similarity may lead to the same geological interpretation. besides, dd exhibits greater depth penetration, aligning with the findings of szalai et al. (2009). therefore, it can be concluded that dd has the highest depth of investigation, wa has the lowest, and the remaining two arrays fall within a moderate depth range. table 4. subsurface resistivity and depth estimation for all electrode arrangements. wa wb sc dd mean de pt h h (m ) ρ (o hm .m ) de pt h h (m ) ρ (o hm .m ) de pt h h (m ) ρ (o hm .m ) de pt h h (m ) ρ (o hm .m ) de pt h h (m ) ρ (o hm .m ) de pt h h (m ) ρ (o hm .m ) 0.00 33.54 0.00 33.01 0.00 33.92 0.00 35.62 0.00 0 34.02 2.87 2.20 188.33 3.27 206.75 1.43 139.45 1.05 117.04 1.99 42.69 162.89 22.18 6.74 16.42 8.61 41.48 9.01 24.74 7.81 6.10 8.04 10.78 22.19 58.37 14.96 13.07 21.99 8.83 18.75 7.46 29.42 17.47 21.28 24.99 11.71 33.46 4. convergence analysis the convergence of the inversion process was evaluated using the rms misfit values. it is shown in figure 8. a. wb array: lowest rms misfit, indicating the best fit between observed and modeled data. b. sc array: moderate rms misfit, still within acceptable ranges. c. wa array: higher rms misfit than wb and sc but within acceptable limits. d. dd array: highest rms misfit, suggesting less accurate modeling. figure 8. convergence rms error for all configurations. 5. subsurface geological interpretation the subsurface rock features are obtained and referred to in the study of rizka & satiawan (2019) because of the same geological formation. as a result, the particular configuration has a different delineation (table 5). a. near-surface layers (layers 1-2): consistent interpretation across all arrays. b. intermediate layers (layer 3): the wa and dd arrays interpret the third layer as tuffaceous clay, whereas wb and sc arrays interpret it as tuffaceous sand. 87a comparative study of vertical geoelectrical arrays in delineating shallow subsurface properties c. deep layers (layer 4): all arrays consistently interpret the fourth layer as tuffaceous clay, although there is a discrepancy in resistivity values. these disparities, particularly in layer 3, may be attributed to the dd array’s lower signal strength and higher variability, as well as potential horizontal property influences in the wa array. the wb and sc arrays provide more reliable interpretations due to their consistent resistivity measurements and lower variability. table 5. estimating subsurface geological properties refers to rizka & satiawan (2019). wa wb sc dd ρ (o hm .m ) ro ck ty pe ρ (o hm .m ) ro ck ty pe ρ (o hm .m ) ro ck ty pe ρ (o hm .m ) ro ck ty pe 33.54 tuffaceous sand 33.01 tuffaceous sand 33.92 tuffaceous sand 35.62 tuffaceous sand 188.33 tuff 206.75 tuff 139.45 tuff 117.04 tuff 16.42 tuffaceous clay 41.48 tuffaceous sand 24.74 tuffaceous sand 6.10 tuffaceous clay 13.07 tuffaceous clay 8.83 tuffaceous clay 7.46 tuffaceous clay 17.47 tuffaceous clay 6. sensitivity and resolution analysis szalai et al. (2009) stated that high resolution generally means sensitivity is concentrated in a smaller depth, while high sensitivity often extends deeper but with lower resolution. a previous study by abbas (2017) reported that wa has the highest resolution while dd has the lowest (figure 9). therefore, based on this study, it can be concluded that:: a. resolution capacity: the wa array offers the highest vertical resolution, followed by sc and wb arrays. the dd array has the lowest resolution, limiting its effectiveness in accurately delineating subsurface layers. b. sensitivity patterns: this parameter contrasts with the previous one, meaning the dd array demonstrates the highest vertical sensitivity, allowing for improved detection of deeper subsurface features. the sc follows it, while the wb array exhibits moderate sensitivity, and the wa array has the lowest sensitivity. figure 9. sensitivity pattern diagram (abbas, 2017). based on this study’s analysis, four key parameters are considered when selecting an electrode arrangement for vertical geoelectrical surveys: signal strength, vertical sensitivity, resolution, and the representative depth of investigation. overall, as shown in table 6, wb and sc perform better in delineating the subsurface properties than the other two. this study testifies that the schlumberger (sc) array is the most widely used in the 1d method. moreover, a viable alternative electrode mode can be considered a vertical geoelectrical array, wenner beta (wb). table 6. vertical geoelectrical parameters of each electrode configuration. parameter wa wb sc dd signal strength high moderate moderate low vertical sensitivity low moderate moderate high vertical resolution high moderate moderate low depth of investigation low moderate moderate high conclusion this study analyzed the performance of different electrode configurations in the vertical resistivity method to assess their effectiveness in delineating subsurface features. four configurations were evaluated: wenner alpha (wa), wenner beta (wb), schlumberger (sc), and dipole-dipole (dd). 1. wenner beta (wb) and schlumberger (sc) arrays: demonstrated better performance in all parameters. additionally, these arrays exhibited lower standard deviations in resistivity measurements, indicating higher reliability and consistency. 2. wenner alpha (wa) array: showed above-average performance in signal strength and vertical resolution but had lower vertical sensitivity compared to other arrays. despite some variability in resistivity measurements, the wa array maintained stable voltage readings. 3. dipole-dipole (dd) array: while sensitive vertically and effectively representing subsurface resistivity, it required more power and exhibited significant discrepancies in resistivity patterns, particularly in deeper layers. this led to potential geological misinterpretations, making it less reliable for accurate subsurface delineation. overall, the wb and sc arrays proved more effective for 1d resistivity surveys due to their enhanced performance metrics. however, the study’s limitation to a 100-meter measurement length necessitates further research with extended ranges to confirm the scalability and generalizability of these findings. acknowledgments the author expresses gratitude to the geophysical engineering students and laboratory technicians for assisting in data acquisition for this study. appreciation is also extended to the reviewers for their valuable and constructive feedback on this article. references abbas, m. 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(2020). the characteristics of unconventional tight oil reservoir and its modification technology and productivity prediction. earth sciences research journal, 24(4), 507-512. doi: https:// doi.org/10.15446/esrj.v24n4.91473 the zonal productivity prediction model of volume fractured horizontal wells in tight reservoirs is established in this study based on the physical model of volume fractured horizontal wells to study the near-well seepage law of volume fractured horizontal wells in unconventional tight reservoirs. the conformal transformation and iteration method are used to solve the problem. compared with fan zifei's and ning zhengfu's method, the optimization of fractured horizontal wells' production parameters is studied. the effects of starting pressure gradient and fracture cluster number on horizontal wells' productivity with volume fracturing are analyzed. multivariate regression analysis is used to rank the importance of influencing factors. the results show that the starting pressure gradient has the most significant impact on capacity. the application example of volume fractured horizontal wells in lucaogou tight reservoir shows that the zonal productivity model established in this study can be used to predict the initial productivity of volume fractured horizontal wells. thus, through theoretical analysis and numerical simulation, the productivity prediction model of horizontal wells with volume fracturing has certain guiding significance for the effective development of tight reservoirs. abstract the characteristics of unconventional tight oil reservoir and its modification technology and productivity prediction registro de enterramiento de carbones del carbonífero-pérmico y su impacto en la generación secundaria de hidrocarburos en la subcuenca de jiyang, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v24n4.91473 en este estudio se establece el modelo de predicción de la productividad zonal en los pozos horizontales fracturados en reservorios compactos con base en un modelo fìsico de pozos horizontales fracturados para estudiar el regimen de filtración en reservorios compactos no convencionales. los autores utilizaron la transformación conforme y el método de reiteración para resolver el problema. se estudió la optimización de los parámetros de producción en pozos horizontales fracturados en comparación con los métodos de fan zifei y de ning zhengfu. además, se analizaron los efectos de la presión inicial de la inclinación y el complejo de fracturas en la productividad de los pozos horizontales con el índice de fracturamiento. el análisis de regresión multivariante se utilizó para clasificar la importancia de los factores influenciadores. los resultados muestran que la presión inicial de la inclinación tiene el mayor impacto en la capacidad. el ejemplo de aplicación de pozos horizontales fracturados en el reservorio compacto de lucaogou muestra que el modelo de productividad zonal que se estableció en este estudio puede usarse para predecir la productividad inicial en estos pozos. de este modo, a través del análisis teórico y la simulación numérica, el modelo de predicción de productividad de pozos horizontales fracturados tiene una significancia para el desarrollo efectivo de reservorios compactos. resumen record manuscript received: 12/03/2019 accepted for publication: 20/09/2020 earth sciences research journal earth sci. res. j. vol. 24, no. 4 (december, 2020): 507-512 yang fan1, liu lifeng1, ran qiquan1*, kong jinping2, xu mengya1, dong jiaxin1 1petrochina research institute of petroleum exploration and development, beijing 100083, china 2langfang branch, petrochina research institute of petroleum exploration and development, langfang, china 065007 *corresponding author: yang.fan@gmail.com pe tr o le um e ng in ee ri ng https://doi.org/10.15446/esrj.v24n4.91473 https://doi.org/10.15446/esrj.v24n4.91473 https://doi.org/10.15446/esrj.v24n4.91473 508 yang fan1, liu lifeng1, ran qiquan, kong jinping, xu mengya, dong jiaxin introduction as an important part of energy, oil is the foundation of national economic development. in recent years, conventional energy has been unable to meet human needs, so unconventional energy has come into people’s vision, and people pay more and more attention to unconventional energy (luo et al., 2018). as an important part of unconventional energy, the proportion of tight sandstone reservoirs in energy utilization is also increasing. the exploration shows that the geological reserves of tight reservoirs are relatively high, but at present the recovery rate is relatively low, and there is a huge potential for exploration. the permeability of tight oil reservoir is very low, the pore structure is very complex and the heterogeneity is very serious. without natural modification, its seepage resistance is large and its natural productivity is very low, which makes it impossible to achieve large-scale and effective development. although many researchers have conducted in-depth research on the development of tight reservoirs, and at the same time, many development strategies have been put forward, some of which have achieved good results, but there are also some limitations (guo et al., 2018). when a tight reservoir is fractured, there is mutual interference between fractures, and it is unavoidable. segmental multi-cluster fracturing technology applied to tight reservoirs can be defined as volume modification technology. manpower fracture formed by reconstruction measures can activate natural micro-fracture in reservoir and form complex fracture net system (naik et al., 2018). however, the specific form of complex sewing nets, how to influence productivity, how to give full play to the advantages of sewing nets in reservoir development, so as to reduce the adverse effects of sewing nets, need more in-depth study. at present, the development of tight reservoirs is still in the exploratory stage. there is no specific description of seepage characteristics near wells of volume fractured horizontal wells in tight reservoirs. it is necessary to study relevant seepage theory and numerical simulation, establish relevant productivity prediction models, analyze the technical methods and fracturing development design methods for improving single well productivity of volume fractured horizontal wells in tight reservoirs, and provides theoretical support for the development of tight reservoirs. steady-state productivity of fractured horizontal wells in tight reservoirs comparison of productivity models for fractured horizontal wells in tight reservoirs the steady-state solution of rectangular boundary reservoir with vertical fracture is studied. then, on this basis, the steady productivity formula of fractured horizontal wells is established. the model assumes that a horizontal well exists in the center of a rectangular reservoir, and the streamline flows radially into the horizontal wellbore from the direction perpendicular to the horizontal section. the steady-state productivity formula of fractured horizontal wells can be expressed by image reflection method and superposition principle. q n k h p p b l h w h h nw h h l in h h o e wf o o o o o 0 1 2 2 1 2 4 2= −( ) +     + + π µ β πα β β hh k h k c in h rf h o f f w +                 − 2 1 (1) h arcch ch b na l a f 1 2 2 =             p p sin (2) h in h rw 2 3 1 = +( ) β π β (3) the advantage of fan zifei's model is that the equation is simple, which can adjust the thickness of the fractured reservoir and consider the heterogeneity of the reservoir. however, the disadvantage of this model is that it does not consider the interaction between fractures and the process of fluid flowing from matrix to wellbore during production. compared with the radius of horizontal wellbore, the half-length of fracture is larger. when the influence of gravity is neglected, the flow process of fluid in fracture near wellbore can be simplified to plane radial flow, which means that formation thickness is w, flow radius is h/2, boundary pressure is pfj and bottom hole pressure is pwfj. when the skin factor of crack is neglected, the expression is as follows: p p q b k w in h rfj wfj fj f w − = µ π2 2 (4) if the pressure at the bottom of all fractures equals the pressure at the bottom of the well, that is to say p pwfj wf= , it can be obtained that: p p b k h q in r x id x r x id x jd id e wf h fi e f f e f f − = −       + + −       − µ π2 1 2 xx jd id x q b k w in h r f f fi fi n n w o o + + −      + =− ∑ 1 2 22 µ π (5) p p b k h q in r x id x r x id x jd id e wf h fi e f f e f f − = −       + + −       − µ π2 1 2 xx jd id x q b k w in h r f f fi fi n n w o o + + −      + =− ∑ 1 2 22 µ π according to the parity of the number of cracks n, the order from -no to no is adopted for j. therefore, a set of equations can be generated which can be solved in closed form. the output qfi of each crack can be obtained by using the column principal component gauss-jordan numerical solution method. the numerical value of the production of all fractures and the total production of fractured horizontal wells are as follows: q qfi i n n o o = = ∑ (6) ning zhengfu’s model assumes that each fracture has different production values, and the seepage resistance of fluid in the fracture is taken into account. compared with lang zhaoxin's equation, the advantage of this equation is that the hypothetical conditions of the model are more in line with the actual production situation of fractured horizontal wells, so it has stronger applicability (zhuoheng et al., 2018). at the same time, the disadvantage of this model is that the pressure drop in horizontal wellbore is not taken into account. based on the theory of seepage equivalent resistance, li et al. established the productivity equation of fractured horizontal wells. for the formation with fractured horizontal wells, fluid flow from formation to horizontal wellbore can be divided into two stages. in the first stage, fluid flows from formation to fracture. in the second stage, the fluid flows from the fracture to the horizontal wellbore. when n fractures are fractured in horizontal wells, the whole flow area can be divided into n regions. the fracture is the center of each flow area. the pressure drop of fluid flowing from formation to fracture is as follows: ∆p q l x kh l l q khx l l i f f f i f f 1 0 1 2 0 1 2 2 2 1 1 = −( ) + + +        * ( (7) the pressure drop of fluid flowing from fracture to horizontal wellbore is as follows: ∆p q x k hwi o f f 2 2 =  (8) the pressure drop produced by fluid flowing into the wellbore in a fracture is: 509the characteristics of unconventional tight oil reservoir and its modification technology and productivity prediction if any fracture is selected, the oil drainage area formed by fluid flowing from matrix to fracture is elliptical. if the long half-axis of the ellipse is a, the short half-axis is b, and the focal length, that is, the half-length of the crack, be x f 2 , the following equation can be obtained. a b x f2 2 2 2 = +       (13) here, the elliptical flow in z plane is transformed into radial flow in  plane by conformal transformation. the transformation coefficient is: z x f= +     / 2 2 1   (14) equation 14 is a rokovsky equation, in which z x iy= + (15) ξ ξ η θ= + =i rei (16) if equations 15 and 16 are brought into the upper equation, there are x iy x re r e x r r i r r f i i f+ = +    = +    + −    −/ / cos 2 2 1 2 2 1 1         sin  (17) by calculating the steady-state productivity equations under various conditions, the results are analyzed. the results show that the calculated results by ning zhengfu’s method are the closest to those by the productivity equation deduced in this study, but these results are higher than the actual productivity (yin, 2018). this is because both methods are based on the assumption that there is no interference from other fractures in the control area of a single fracture. at the same time, the assumed conditions in this equation do not accord with the actual situation, that is, assuming that the formation is homogeneous reservoir, the fracture can completely penetrate the reservoir (wei-yao, & dong-xu, 2018). therefore, although the productivity equation in this study can roughly predict reservoir productivity, it needs to be further adjusted to meet the actual situation. influencing factors of productivity of volume fractured horizontal wells in dense reservoirs reservoir characteristics the oil-bearing formation is located in lucaogou formation of permian, which is divided into four units: p2121, p2122, p2111 and p2122 from bottom to top. the thickness of each unit layer group is shown in table 1. table 1. the thickness of each unit layer group layer group p2111 p2112 p2121 p2122 formation thickness (m) 28~75 63~129 8~34 50~120 average thickness (m) 52.3 104 17.4 90.5 upper sweet heart (p222) and lower sweet heart (p212) are developed in lucaogou group. reservoir physical parameters are shown in table 2, and the formation thickness between upper and lower desserts is 83.3m~131.7m, with an average of 108.4m. according to pore and permeability distribution, it is a typical tight oil reservoir. productivity increasing potential evaluation of volume fracturing horizontal wells with the continuous development of horizontal wells and fracturing measures, economic development of tight reservoirs has been realized under ∆p q k w in h rf w 3 01 2 2 2 = −    π µ π (9) since, p p p p pr wf− = + +∆ ∆ ∆1 2 3 (10) it can be obtained that: q p b l x kh l l khx l l x k hw i f f f f f f f f = − +( ) + +       + + 170 54 1 1 11 2 1 2 . * ∆ µ 11 2 2k w in h rf wπ π−                     (11) q p b l x kh l l khx l l x k hw i f f f f f f f f = − +( ) + +       + + 170 54 1 1 11 2 1 2 . * ∆ µ 11 2 2k w in h rf wπ π−                     the total flow rate of n vertical fractures is as follows: q qi i i n = = ∑ 1 (12) the advantage of li model is that it can control the perforation state of horizontal well section and distinguish the end fracture from the middle fracture. at the same time, the model also has shortcomings. it does not take into account the loss of pressure drop in horizontal wellbore and the decline of fracture conductivity with time. establishment of steady-state productivity model in order to describe the special percolation process of tight reservoir, the model is simplified as a single fracture. at this time, the flow process of crude oil is divided into three stages: the flow in matrix (non-linear flow from matrix to fracture wall), the flow from fracture wall to wellbore (plane radial flow) and the flow in wellbore (pressure drop due to resistance along the way) (qun et al., 2018). in the solution process, the first two processes are coupled to obtain the equivalent radius of the vertical well and the equivalent diameter in the presence of a single fracture. then, the multiple fractures are converted into corresponding production wells, and the oil production of each fracture under the condition of multiple fractures is obtained by pressure drop superposition. finally, the pressure drop flow in the horizontal wellbore is considered and combined with the previous output to carry out the iterative solution (peng et al., 2018). finally, the oil production under the control of each fracture can be obtained. the total oil production of horizontal wells after fracturing can be obtained by summing up the production of each fracture, as shown in figure 1. figure 1. fracture elliptic seepage if there is no interference between the cracks and the fluid in the matrix flows to the cracks, any crack can be regarded as an elliptical seepage flow. 510 yang fan1, liu lifeng1, ran qiquan, kong jinping, xu mengya, dong jiaxin the multi-stage fracturing horizontal wells. at present, the tight reservoirs developed in china are mainly in ordos basin and songliao basin. the cluster well development initially adopted has not achieved satisfactory results. in recent years, the application of volume fracturing horizontal wells in tight reservoirs has achieved good results (long et al., 2018). based on the characteristics of tight reservoirs, the application research of horizontal wells, conventional fractured horizontal wells and volumetric fractured horizontal wells is carried out in the depletion development of tight reservoirs. according to the physical characteristics of tight reservoir, the permeability of formation matrix is 0.05md. considering formation heterogeneity, kx = ky, kz = 0.1kx, porosity 10%, effective thickness of reservoir 15m, original formation pressure 18.6mpa, productivity and cumulative production of horizontal well, conventional fractured horizontal well and volumetric fractured horizontal well in tight reservoir are analyzed. from figures 2 and 3, it can be seen that conventional fractured horizontal wells in tight reservoirs have higher productivity than conventional fractured horizontal wells, while volumetric fractured horizontal wells have much higher productivity than conventional fractured horizontal wells, and the corresponding cumulative production is also much larger. the production of horizontal wells is always at a low level during production. the production of conventional fractured horizontal wells is higher at the initial stage, while that of volumetric fractured horizontal wells is higher at the initial stage, and then decreases with production (ren et al., 2019). the reason is that in horizontal wells, the use range is small, only near the horizontal wells. conventional volume fracturing horizontal wells are used only in fracture wave and small area. volume fractured horizontal wells have higher production and cumulative production because of the role of fracturing net and a wide use range. 0 500 1000 1500 2000 2500 3000 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 d ai ly o ut pu t/m 3 d -1 time/day horizontal well conventional fracturing horizontal well volume fracturing horizontal well figure 2. daily production curve of different well types effect of starting pressure gradient on capacity because of the low porosity and permeability of tight reservoirs and the serious heterogeneity, the fluid from static to flowing needs to overcome certain binding force. only when the pressure gradient exceeds a certain critical value can the fluid flow. the critical value is called the starting pressure gradient. the starting pressure gradient varies with the change of reservoir physical permeability. the starting pressure gradients are taken as g=0.01mpa/m, 0.03mpa/m, 0.05mpa/m, 0.06mpa/m and 0.08mpa/m, respectively. the effect of start-up pressure gradient on productivity of horizontal well in volume fracturing is analyzed. figure 4 and figure 5 are daily production curves and cumulative production curves with different starting pressure gradients. from the daily production curve, it can be seen that the daily production decreases with the increase of starting pressure gradient, and the recovery degree decreases with the increase of starting pressure gradient. when the starting pressure gradient is 0.08mpa/m, the productivity and recovery degree are very low. this is because the bigger the start-up pressure gradient of reservoir is, the higher the pressure gradient required for fluid flow in reservoir will be, thus the pressure sweep range will be reduced, the use range will be reduced, and the recovery factor will be reduced. 0 250 500 750 1000 1250 1500 1750 2000 0 5 10 15 20 25 d ai ly o ut pu t/m 3/ d time/day g=0.01mpa/m g=0.03mpa/m g=0.05mpa/m g=0.06mpa/m g=0.08mpa/m data fitting figure 4. daily production curves with different starting pressure gradients table 2. physical parameters of tight oil reservoirs middle depth (m) formation temperature (°c) formation pressure (mpa) porosity (%) permeability (md) reservoir thickness (m) upper sweet heart 2980 85.6 37.9 5.09~25.79 average 1099 0.001~0.284 average 0.012 9.40~42.4 average 28.34 lower sweet heart 3250 92.0 41.4 5.51~20.72 average 11.62 0.002~2.76 average 0.010 7.50~67.24 average 31.26 0 500 1000 1500 2000 2500 3000 0 2000 4000 6000 8000 10000 12000 14000 16000 c um ul at iv e ou tp ut /m 3 time/day horizontal well conventional fracturing horizontal well volume fracturing horizontal well figure 3. cumulative production curves for different well types 511the characteristics of unconventional tight oil reservoir and its modification technology and productivity prediction 0 250 500 750 1000 1250 1500 1750 2000 0 2000 4000 6000 8000 10000 12000 c um ul at iv e ou tp ut /m 3 time/day g=0.01mpa/m g=0.03mpa/m g=0.05mpa/m g=0.06mpa/m g=0.08mpa/m data fitting figure 5. cumulative production curves with different starting pressure gradients effect of fracture cluster number on productivity volume fracturing is used to fracturing horizontal wells in many stages, and many clusters of fractures are produced in each stage. the number of fracture clusters affects the degree of opening natural fractures in each section and the degree of reformation in the fracturing reconstruction area. the number of fracture clusters in each section is designed as four schemes: 1 piece, 2 pieces, 3 pieces and 4 pieces, respectively. the effect of the number of fracture clusters on the productivity is analyzed. 0 250 500 750 1000 1250 1500 1750 2000 0 10 20 30 40 50 d ai ly o ut pu t/m 3/ d time/day cluster per paragraph two clusters per segment three clusters per segment four clusters per segment figure 6. daily production curve of different clusters figures 6 and 7 are daily and cumulative production curves for each segment with different cluster numbers. it can be seen from the daily production curve that the production increases with the increase of cluster number. moreover, the number of clusters has a great influence on the initial and intermediate stages of production. the more the number of cracks clusters in each section is, the greater the density of artificial and natural cracks communicated is, the greater the degree of fracturing reconstruction area transformation is, and the greater the productivity is. conclusion according to the characteristics of tight reservoir and considering the characteristics of volume fracturing, according to the basic principles of percolation mechanics and reservoir engineering, experimental research, theoretical derivation, numerical simulation and other research methods have been adopted to comprehensively study the percolation characteristics, productivity evaluation and utilization characteristics of volume fracturing in tight reservoir, analyze the development effects under different well pattern and different development modes, and establish the seepage model and productivity prediction model of horizontal well reservoir with compact oil volume fracturing, and form the productivity prediction module. compared with the horizontal wells without volume fracturing, the horizontal wells with multi-stage cluster fracturing enlarge the reconstruction volume, reduce the seepage resistance and increase productivity. because the permeability of tight reservoir is very low, the daily production is higher in the initial stage and the production decreases rapidly. under the same cluster number, the smaller the cluster spacing between intervals and the cluster spacing within intervals is, that is, the more uniform the distribution of fractures in horizontal wells is, the higher the productivity of horizontal wells is. for the same series, the more uniform the section number distribution is, the higher the production of the oil well is. therefore, in fracturing operation of tight reservoirs, in horizontal wells, the more uniform the distribution of interval and cluster spacing is, the better. in view of the seepage characteristics of unconventional tight reservoirs, on the basis of the derivation method of lang zhaoxin’s equation, considering the starting fracturing gradient, the productivity equation of fractured horizontal wells is derived. the productivity equation takes into account the loss of flow pressure caused by seepage resistance in cracks, and fully reflects the characteristic parameters of actual cracks, such as unequal length, unequal conductivity and unequal spacing. it is closer to the actual situation than the new equation of lang zhaoxin, and more suitable for productivity evaluation of fractured horizontal wells in tight reservoirs. the new productivity equation only considers the steady-state flow of single-phase fluid, does not consider the effect of pressure sensitivity on the production of horizontal wells in tight reservoirs, and ignores the pressure loss of fluid flow in horizontal wellbore, so it needs to be more perfect and closer to the actual productivity equation of fractured horizontal wells. references guo, c., wei, m., & liu, h. 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(2018). production characteristics and sweet-spots mapping of the upper devonian-lower mississippian bakken formation tight oil in southeastern saskatchewan, canada. petroleum exploration and development, 45(4), 102-112. doi: https://doi. org/10.1016/s1876-3804(18)30070-3 https://doi.org/10.1071/aj17149 10.13227/j.hjkx https://doi.org/10.1016/s1876-3804(18)30080-6 https://doi.org/10.1016/s1876-3804(18)30080-6 https://doi.org/10.1016/j.jnggs.2018.12.001 https://doi.org/10.1016/j.ngib.2018.03.006 https://doi.org/10.1016/j.ngib.2018.03.006 https://doi.org/10.1016/s1876-3804(18)30070-3 https://doi.org/10.1016/s1876-3804(18)30070-3 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n1.116670 earth sciences research journal earth sci. res. j. vol. 29, no. 1 (march, 2025): 15 22 en v ir o n m en ta l g eo lo g y monitoring land surface temperature and vegetation dynamics in response to land use changes using spatiotemporal analysis in district chiniot, punjab pakistan mirza naseer ahmad1, naeem ahmad syed1, rehan ahmad pervaiz1, talal ahmed1*, ain ur raza1, jazba shafique1, nida toheed1, humaira naseer1 1. abdus salam school of sciences, earth science department, nusrat jahan college rabwah, pakistan *corresponding author’s email: talalahmed@njc.edu.pk how to cite item: ahmad, m. n., syed, n. a., pervaiz, r. a., ahmed, t., raza, a., shafique, j., toheed, n., & naseer, h. (2025). monitoring land surface temperature and vegetation dynamics in response to land use changes using spatiotemporal analysis in district chiniot, punjab pakistan. earth sciences research journal, 29(1), 1522. https://doi.org/10.15446/esrj.v29n1.116670 record manuscript received: 20/09/2024 accepted for publication: 05/03/2025 abst r ac t understanding the impact of land use and cover changes (lulc) on vegetation and built-up areas is crucial for local governments and communities in rapidly developing countries. the study was conducted in district chiniot, punjab, pakistan. the present study aims to explore the relationship between land surface temperature (lst) and normalized difference vegetation index (ndvi). this study aims to investigate the relationship between land surface temperature (lst) patterns and changes in lulc due to urbanization in district chiniot. using landsat 5 and 8 data, lst was computed and correlated with ndvi to assess vegetation changes. the results indicate a decrease in ndvi over the years, suggesting declining vegetation, and an increase in lst, indicating rising temperatures. supervised classification revealed an increase in built-up areas and a decrease in vegetation, contributing to the rise in lst. these findings highlight the importance of effective land management strategies to mitigate the adverse impacts of urbanization on local environments. keywords: land surface temperature, normalized differentiated vegetative index, land use land cover, normalized differentiated built-up index, land satellite, remote sensing data. palabras clave: temperatura de la superficie terrestre; índice de vegetación de diferencia normalizada; uso del suelo/cobertura del suelo; índice de urbanización de diferencia normalizada; satélites terrestres; información de detección remota. monitoreo de la temperatura superficial terrestre y las dinámicas de vegetación en respuesta a los cambios en el uso del suelo a través del análisis espacio-temporal, en el distrito de chiniot, punjab, pakistán el conocimiento del impacto del uso del suelo y los cambios en la cubierta de vegetación en las áreas urbanizadas es determinante para los gobiernos locales y para las comunidades de los países en desarrollo acelerado. este estudio se realizó en el distrito de chiniot, punjab, pakistan. el presente estudio busca explorar la relación entre la temperatura de la superficie terrestre y el índice de vegetación de diferencia normalizada. el trabajo se enfoca en investigar la relación entre los patrones de temperatura de la superficie terrestre y los cambios en la cubierta vegetal y en el uso del suelo debido a la urbanización del distrito de chiniot. con el uso de las bases de información landsat 5 y landsat 8, se computó y se correlacionó la temperatura de la superficie terrestre con el índice de vegetación de diferencia normalizada. los resultados indican un decrecimiento en el índice de vegetación a través de los años, lo que sugiere una caída de la vegetación, un incremento en la temperatura de la superficie y por ende un proceso de aumento de las temperaturas. una clasificación supervisada reveló un incremento de las áreas urbanizadas y un crecimiento de la vegetación, lo que contribuye al aumento de la temperatura de la superficie. estos hallazgos resaltan la importancia de las estrategias de manejo efectivo de los suelos para mitigar los impactos adversos de la urbanización en los ambientes locales. resu m en https://doi.org/10.15446/esrj.v29n1.116670 mailto:talalahmed@njc.edu.pk https://doi.org/10.15446/esrj.v29n1.116670 16 mirza naseer ahmad, naeem ahmad syed, rehan ahmad pervaiz, talal ahmed, ain ur raza, jazba shafique, nida toheed, humaira naseer 1. introduction measured within the range of the remote device, the land surface temperature (lst) is the radiative skin temperature of the land surface. it is most likely derived from top-of-atmosphere brightness temperatures from the infrared spectral channels of a constellation of stationary satellites. its estimate is also influenced by the plant cover, ratio, and soil moisture content. temperatures of the bare soil and plants may combine to form lst. the lst exhibits quick fluctuations as well because of how quickly each reacts to changes in incoming radiation caused by implementers, aerosol load alterations, and illumination variations. the lst therefore affects how energy is distributed between plants and the ground, which in turn affects the temperature of the surface air (yaseen , 2022). the phenomenon of land surface temperature (lst) holds considerable importance within the context of global climate change. as greenhouse gas levels escalate in the atmosphere, lst tends to increase correspondingly. consequently, this elevation in temperature contributes to the melting of glaciers and ice sheets, thereby affecting local vegetation. given the fluctuating patterns of rainfall and the possibility of monsoon disruptions, the repercussions are anticipated to be particularly pronounced in regions reliant on monsoons (rajeshwari & mani, 2014). the thermal environment and microclimate in regions experiencing rapid economic growth and established industries, particularly urban areas with dense populations and significant waste discharge into the environment, have undergone notable transformations (yang et al., 2021). these changes have led to the emergence of urban heat island effects, disrupting the energy equilibrium and negatively impacting the quality of life for residents. the term “urban heat island” denotes widespread alterations in microclimates due to urbanization, with land surface temperature serving as an indicator of the severity of global warming (yang et al., 2021). there are various natural and industrial sources of environmental contamination such as an increase in cadmium which is found in various biota. increasing temperature increases the uptake and toxic impact of cadmium on microorganisms, land plants, and aquatic organisms (world health organization, 1992). consequently, there is growing interest in developing strategies to mitigate the escalation of land surface temperature and prevent further deterioration of urban thermal environments (yang et al., 2021). the expansion of infrastructure and the conversion of grasslands and forests into urban areas are exacerbating this problem. the reduction in grass and forest land, crucial for carbon sequestration, is leading to increased anthropogenic carbon emissions. the extensive conversion of land for urban infrastructure is a major factor contributing to this issue (waseem & khayyam, 2019). the estimation of land surface temperature (lst) is essential for diverse applications as it provides valuable information about the temporal and spatial variations in surface thermal equilibrium (li et al., 2014). according to benelli et al.,(2012), lst finds utility across various domains such as evapotranspiration, climate change analysis, hydrological processes, vegetation surveillance, urban climate assessment, and environmental research(yaseen & khan, 2022). nasa and the united states geological survey (usgs) collaborate on this initiative, formerly referred to as the landsat data continuity mission. pakistan, a developing nation, is experiencing a rapid surge in urbanization, bringing significant advantages to its citizens. however, if left unchecked, this process can also have adverse impacts on the environment (elsayed, 2012). as highlighted by amanollahi et al.,(2016), a primary concern associated with urbanization is the elevation of ambient temperature, resulting in the urban heat island (uhi) effect and deterioration of the urban environmental quality. consequently, scholars have suggested in numerous studies the incorporation of urban climate considerations into development and planning efforts (raghavan et al., 2015). this study delves into the evolving landscape of urbanization over time and its potential implications for the human lifecycle. population growth has exacerbated global warming, with pakistan being especially susceptible to extreme weather events. consequently, this research elucidates the noteworthy transformations occurring over time (saleem et al., 2020). methods for measuring the urban thermal environment include remote radiant temperature measurement from airborne and spaceborne platforms, mobile sampling of air and surface kinetic temperature from ground and airborne vehicles, and sampling of air and surface kinetic temperature at fixed locations. to estimate radiant temperature, at least, remote sensing has the greatest synoptic capabilities due to its nearly immediate collection of several samples over a large area. remote sensing has the most synoptic capabilities, at least for radiant temperature measurement. for obtaining a general understanding of thermal fluctuations in an urban ecosystem, it can still be a helpful. it can still be a helpful tool for understanding how temperatures vary in an urban setting (w. yue, 2007). in the recent past various studies have been done to evaluate the importance of land surface temperature (lst) in understanding global energy and water balance processes has been recognized by the remote sensing industry according to yaseen & khan (2022). according to jiménez-muñoz et al.,(2014) monitoring lst and its correlation with the normalized difference vegetation index (ndvi) is crucial for understanding land cover changes and their environmental impacts. lst, a key indicator in physics, affects various fields such as hydrology, ecology, agriculture, and global change studies (jiménezmuñoz et al., 2014). its rise can lead to significant environmental imbalances, including melting ice caps and disruptions to crop calendars and climatic conditions, particularly in monsoon regions. thermal infrared remote sensing technology has become vital for studying land surface thermal characteristics. landsat satellites have provided valuable thermal data since 1978, allowing for the computation of lst using different sensors (zhang et al., 2023). rajeshwari & mani (2014) conducted a study that integrated aster and modis data, specifically bands 31 and 32, to generate land surface temperature (lst). emissivity computation involved the use of ndvi and a supervised classification method for land use and land cover. the comparison with in situ data revealed a positive correlation between lst and ndvi, as well as the land use/land cover technique. the research conducted by strahler & strahler (2013) suggested that the forward model utilized a representative sample of natural materials sourced from the modis ucsb emissivity database, containing 113 distinct spectral emissivity values between 8 and 14 µm. this dataset encompasses various materials, including soils, minerals, plant species, and water types. notably, man-made materials were excluded due to the tirs instrument’s spatial resolution and focus on environmental applications. remote sensing imagery enables the creation of vegetation indices, with ndvi being a commonly used metric for monitoring vegetation and estimating soil moisture in semi-arid regions. however, it responds predominantly to the red band with significant absorption (strahler &strahler, 2013). simulation data suggests a potential temperature increase of up to 0.21°c per year by 2041, as observed by rousta et al.,(2018). over the past 27 years, lst has shown an average increase of 1.74°c. these rising temperatures, attributed to global climate change, have implications for land use, vegetation cover, and water resources, contributing to various environmental challenges (solangi et al., 2019). attri et al., (2015) emphasizes the significance of land in sustaining human life. changes in land utilization patterns over time can lead to diverse environmental consequences, impacting energy distribution and temperature fluctuations. land use pertains to the utilization of land for various purposes such as agriculture or urban development, while land cover encompasses the physical attributes of the area, including forests or water bodies. the study conducted by gur et al., (2024) aimed to assess the potential impact of global climate change on the highland areas of kastamonu, a significant province in turkey known for its numerous and varied highlands. the investigation focused on 59 selected highland locations within the region. using the de martonne climate classification, projections were made for four future periods (2040, 2060, 2080, and 2100) under two scenarios: ssps 245 and ssps 585. the study conducted by schwaab et al., (2021) suggest that the urban trees play a significant role in influencing city temperatures, yet their effectiveness in mitigating urban heat remains understudied, especially in comparison to green spaces lacking trees. high-resolution satellite data from 293 european cities suggest that urban trees contribute to cooler summer temperatures and alleviate heat stress, particularly in severe weather conditions (schwaab et al., 2021). amidst global climate change challenges, including the rise in average surface temperature known as global warming, understanding lst variations is paramount for numerous hydrological, meteorological, and climatological https://www.sciencedirect.com/topics/earth-and-planetary-sciences/climate-classification 17monitoring land surface temperature and vegetation dynamics in response to land use changes using spatiotemporal analysis in district chiniot, punjab pakistan applications. utilizing geographic information systems (gis) and remote sensing (rs), sensors like landsat-8, modis, and avhrr provide valuable data for lst modeling and climate analysis (solanky et al., 2018). the impact of climatic and environmental changes on crop health is evident in variations in rainfall, increased heat stress, shifts in lst, and population migration from rural to urban areas (hussain & karuppannan, 2023). the surge in lst, attributed to global climate change, has transformed land usage, vegetated areas, and water resources, posing significant environmental challenges (solangi et al., 2019). the present study aims to explore the relationship between land surface temperature (lst) and normalized difference vegetation index (ndvi). the study also focused to identify abrupt temperature fluctuations attributed to escalated urbanization in the chiniot district, located in punjab, pakistan. this study also aims to analyze the impact of rising land surface temperatures on vegetation, addressing challenges such as ecosystem loss and declining biodiversity amid global climate warming. understanding the causes and effects of temperature shifts is crucial for devising effective land management strategies. exploring land surface temperature dynamics is essential for understanding spatial and temporal variations in surface equilibrium, influencing heat and water exchange in the weather and climate system. developing reliable methodologies and technologies for monitoring lst data is vital for various applications including weather prediction, climate research, urban planning, agriculture, and natural resource management. 2. study area the study area is situated in the northwest region of the chenab channel and is connected to several cities in punjab, including faisalabad, sargodha, and jhang, via road networks. it is approximately 160 km east of the city center, around 30 km southwest of the city, 56 km west of sargodha, and 86 km southwest of jhang. the coordinates of the study area range from 31°43ʹ to 31°44ʹ n latitude and 72°58ʹ to 73°0ʹ e longitude. figure 1. map of the study area. 3. materials and methods data sets the main source of data for this study is landsat 5, which was in operation from march 1, 1984, to january 15, 2013, and played a significant role in surface monitoring (egorov, 2022). particularly chosen for its thermal infrared band was landsat 8 oli/tirs data from march 9, 2020, with a spatial resolution of 30 x 30 m (acharya & yang, 2015). the research region, which included chiniot, made use of landsat data from 1990, 2000, and 2009. important information for the correlation study was obtained by processing the landsat 8 data for land surface temperature (lst) inversion (acharya & yang, 2015). to further improve the comprehension of lst patterns in march 2020, meteorological data from the digital weather station operated by the pakistan meteorological department at nusrat jahan college rabwah was also included. table 1 data sets showing sensors utilized in the study. sensors date bands resolution (m) source purpose la nd sa t 5 7 march 1990 2 march 2000 11 march 2009 1, 2,3, 4,5,6 30x30m earthexplorer. usgs.gov to calculate lst la nd sa t 8 9 march 2020 1,2,3,4,5,6, 7,8,9,10,11 30x30m earthexplorer. usgs.gov to calculate ndvi and ndbi data collection thematic mappers, enhanced thematic mapper plus, and oli/ tirs sensors were used in this study’s data collecting process, which was complemented by satellite data and other information sources. two steps were involved in the conversion of thermal bands into land surface temperature (lst) and top-of-atmospheric spectral radiation (toar). spectral radiance (l) was calculated using equations and converted to centigrade scale using arcgis 10.5 software. data assessment the assessment of vegetation greenness, density, and health was achieved through the normalized difference vegetation index (ndvi). ndvi computation involved a ratio between red (r) and near-infrared (nir) measurements (li et al., 2013). the ndvi equation determined vegetation amount (pv) and emissivity (ε). the lst equation facilitated the creation of a surface temperature map, and ndvi and lst data were integrated into arcmap for further analysis. data collection satellite imagery image processing landsat 4/5 landsat 8/9 convert thermal band to spectral radiance convert spectral radiance to radiant temperature proportion of vegetation supervised classification accuracy assessment ndvi lse lst ndbi figure 2. research methodology of the study performed. methodology using the methods described by chen et al. (2002), bands 3, 4, and 5 of landsat 5 were used to determine the land surface temperature. the following 18 mirza naseer ahmad, naeem ahmad syed, rehan ahmad pervaiz, talal ahmed, ain ur raza, jazba shafique, nida toheed, humaira naseer formula was first used to translate the digital numbers (dns) of these bands into radiation luminance: qcalmin and qcalmax stand for 1 and 255, respectively, qcal is an acronym for dn, and lmax and lmin are comparable to 1 and 255, respectively (chen et al., 2002). = = ( − ) − ×( – )+ (1) where toa is top of atmosphere, lλ, denotes the spectral at sensor radiance to top of atmosphere, lmin λ, refers to spectral radiance scaled to qcalmin, while lmax λ, refers to spectral radiance scaled to qcalmax, while qcal is the quantized calibrated pixel value. land surface temperature was also calculated in kelvin using the following equation, where bt is the satellite brightness temperature in celsius and k1 and k2 represent thermal conversion from the metadata, dn=255, k1=607.76, k2=1260.56. = = 2 1 1 + 1 − 273.15 (2) the thermal bands of landsat 8 are used to determine the land surface temperature (lst), and the research region, chiniot, is then extracted for analysis in arcmap 10.8. the relationship between lst and ndvi and lst and ndbi is then established. land surface temperature is calculated using bands 10 and 11 of landsat-8. the following formula is used in the raster calculator when importing bands 10 and 11 into arcmap (ghouri et al., 2022). lλ = ml × qcal + al (3) ml, extracted from the metadata, represents the specific-band multiplicative rescaling factor, whereas qcal denotes the thermal band (b10, b11), and al, also derived from the metadata, signifies the specific-band additive rescaling factor. land surface emissivity refers to the proportion of radiation emitted from a material surface relative to a blackbody under identical conditions at the same temperature. the provided equation can be utilized for emissivity calculation (ghouri et al., 2022). ɛ = 0.004 × pv + 0.986 (4) in the land use land cover (lulc) categorization system, which has four main indexes—the built-up index being the most important—the built-up index is used to derive ground-based built-up area information from satellite data. greater intensity of built-up regions with less vegetation is indicated by elevated ndbi readings, and vice versa. according to the above equation, ndbi computation uses the short-wave infrared (swir) and near-infrared (nir) bands (ghouri et al., 2022). = ( − ) ( + ) (5) accuracy assessment in this research work was implied to verify the results of supervised classifications as either accurate or not, using the formula given below: = ( ) × 100 (6) 4. result analysis a mean of 0.124726, a low of -0.511111, and a high of 0.760563 are shown in the ndvi values for march 7, 1990 (fig. 4a). the ndvi image for march 2, 2000 (fig. 4b) shows a mean of 0.156615, a low of -0.438596, and a high of 0.751825. additionally, (fig. 4c), the ndvi image for march 11, 2009, shows a mean of 0.256689, a minimum of -0.217391, and a high of 0.730769. finally, the ndvi image for march 9, 2020 (fig. 4d) shows a mean of 0.207287, a minimum value of -0.154056, and a maximum value of 0.568629. according to these results, the ndvi showed a downward trend between 1990 and 2020, indicating a decrease in vegetation over time. table 2. normalized difference vegetation index of the data collected on year basis. normalized difference vegetation index year max min mean 1990 0.760563 -0.511111 0.124726 2000 0.751825 -0.438596 0.156615 2009 0.730769 -0.217391 0.256689 2020 0.568629 -0.154056 0.207287 figure 3. graphical representation for ndvi, showing max, min and mean value for the studied year. figure 4. normalized difference vegetation index (a) for year 1990 (b) for year 2000 (c) for year 2009 (d) for year 2020. 19monitoring land surface temperature and vegetation dynamics in response to land use changes using spatiotemporal analysis in district chiniot, punjab pakistan the land surface temperature (lst) readings for march 7, 1990 (fig. 6a) show a mean of 22.77°c, a low of 14.71°c, and a high of 30.83°c. in the same way, the lst picture for march 2, 2000 (fig. 6b) shows a mean value of 22.85°c, a minimum value of 16.1°c, and a high value of 29.6°c. additionally, the march 11, 2009 lst image (fig. 6c) shows a mean value of 29.46°c, a minimum value of 15.18°c, and a high value of 43.73°c. lastly, the march 9, 2020 lst image (fig. 6d) shows a mean temperature of 17.08°c, a minimum temperature of 9.13°c, and a high temperature of 25.02°c. table 3. land surface temperature of the data collected on year basis. land surface temperature (°c) year max min mean 1990 30.83 14.71 22.77 2000 29.6 16.1 22.85 2009 43.73 15.18 29.46 2020 25.02 9.13 17.08 figure 5. graphical representation for lst, showing max, min and mean value for the studied year. figure 6. land surface temperature (a) for year 1990 (b) for year 2000 (c) for year 2009 (d) for year 2020. the analysis of these values suggests a notable increase in land surface temperature in district chiniot from 14.71°c in march 1990 to 25.02°c in march 2020. according to yaseen & khan (2022), the higher temperature observed near the riverbed covered with sand can be attributed to the sand’s low specific heat energy, causing it to rapidly heat up in sunlight and cool down quickly when sunlight is absent. this phenomenon is consistent with our findings. table 4. supervised classifications based upon different parameters studied. supervised classification parameters 1990 2000 2009 2020 built up 20% 28% 28% 40% vegetation 56% 63% 61% 51% water body 1% 1% 0% 1% sand 1% 3% 2% 5% barren land 21% 6% 8% 3% figure 7. graphical representation for supervised classification, showing percentages for each parameter. figure 8. supervised classification (a) for year 1990 (b) for year 2000 (c) for year 2009 (d) for year 2020. 20 mirza naseer ahmad, naeem ahmad syed, rehan ahmad pervaiz, talal ahmed, ain ur raza, jazba shafique, nida toheed, humaira naseer according to the march 1990 supervised categorization findings (fig. 8a), the chiniot district is mostly composed of low-built-up regions and high vegetation cover. in contrast to 1990, there is discernible evidence of growing built-up areas and a commensurate decline in vegetation by march 2000 (fig. 8b). in comparison to 1990 and 2000, this tendency is also present in march 2009 (fig. 8c), with significant increases in built-up areas and losses in vegetation. additionally, the march 2020 data (fig. 8d) show a persistent pattern of declining vegetation and growing built-up regions over time. these findings suggest a consistent pattern of urbanization and land transformation, with built-up areas expanding at the expense of vegetation cover. this phenomenon is closely associated with changes in land surface temperature over time. table 4 provides a detailed breakdown of the supervised classification values for the specified years. the march 7, 1990, ndbi study findings (fig. 10a) show a mean value of 0.05, a minimum value of -0.72, and a high value of 0.825137. likewise, on march 2, 2000 (fig. 10b), the mean is zero, the lowest is -0.818182, and the highest is 0.789474. the ndbi value drops to 0.48659 by march 11, 2009 (fig. 10c), with a mean of 0.01 and a minimum of -0.478261. the highest ndbi value drops even further to 0.210882 by march 9, 2020 (fig. 10d), with a mean of -0.1 and a minimum of -0.415411. these results point to a pattern of declining vegetation cover over time, with ndbi values showing a steady rise from 1990 to 2020. this trend is associated with an increase in land surface temperature (lst) over the same period. table 5 provides a comprehensive overview of the ndbi values for the specified years. the accuracy assessment represents the final stage in the classification process of satellite images. for march 1990, 2000, 2009, and 2020, the supervised classification accuracy was evaluated using 30 ground samples for each year. this assessment involved determining the user accuracy, producer accuracy, and overall accuracy for each year. the resulting overall accuracies for the respective years, as calculated above, were 53.3%, 56.6%, 63.3%, and 63.3%. table 5. normalized difference built up index of the data collected on year basis. normalized difference built up index year max min mean 1990 0.82 -0.72 0.05 2000 0.78 -0.81 0 2009 0.48 -0.47 0.01 2020 0.21 -0.41 -0.1 figure 9. graphical representation for ndbi, showing max, min and mean value for the studied year. figure 10. normalized difference built up index (a) for year 1990 (b) for year 2000 (c) for year 2009 (d) for year 2020. discussion an investigation of land surface temperature (lst) connections with vegetation cover (ndvi) and built-up areas (ndbi) in chiniot district, punjab, pakistan was conducted through landsat imagery evaluation from 1990 to 2000 to 2009 to 2020. analysis shows that local environmental transformation has involved both vegetation reduction and built-up area expansion while lst values rose substantially over 30 years. vegetation greenness and density show reduced ndvi measurements throughout the 1990s until 2020. the growing built-up area supports the transformation of vegetated land into urban development since this pattern remains consistent across time. the reduction in vegetation creates severe impacts on environmental services by diminishing carbon uptake and air quality improvement and failing to control temperatures. when vegetation covers decreases it facilitates more surface runoff and more soil erosion occurs. an analysis of lst shows that ground temperatures in the area have significantly increased throughout the research timeframe. analysis shows that temperature increased from 14.71⁰c during 1990 to 25.02⁰c at 2020 indicating an upward temperature pattern in this area. an increase in temperature results from multiple factors such as built-up area spread that maintains heat better than vegetated landscapes (oke, 1982). the measurements of rising lst match the ndvi reduction because vegetation dramatically affects evaporative cooling which controls surface temperature. the research notes how riverbed sands might affect surface temperatures and confirms the same finding as yaseen & khan (2022), highlighting the importance of considering local geographical features. the supervised classification method verifies the assessment findings obtained throughout the study. the results show an unbroken upward pattern of built-up areas alongside a downward pattern of vegetation cover which 21monitoring land surface temperature and vegetation dynamics in response to land use changes using spatiotemporal analysis in district chiniot, punjab pakistan indicates the urbanization trend in chiniot district. the transformation of land cover structure generates direct effects on both the lst observation and ndvi assessment. built-up areas that gain more impervious surfaces produce elevated lst and simultaneously reduced ndvi values result from losing vegetated surfaces. the factor was in correlation with the study conducted by (ullah et al., 2024) addressing the impact of the supervised classification using advanced deep learning algorithms. the ndbi assessment validates the classification output results previously discovered. data shows ndbi increasing steadily throughout the years 1990 to 2020 which demonstrates expanding built-up areas and disappearing vegetation. thermal environmental effects result from a mixture of ndvi and ndbi alongside lst according to their mutual correlations. accuracy assessment of supervised classification provides essential information about the reliability of the land cover classification results. although the range of accuracy between 53.3% to 63.3% is moderate it offers essential information about land cover transformations in the studied area. the classification accuracy might benefit from improved classification approaches together with imagery resolution improvements. overall, the research shows how remote sensing methods serve effectively for detecting land use modification effects on vegetation cover and lst distributions. this research shows that cities need sustainable land management strategies for urban planning since these approaches will counteract the adverse effects of urban growth such as overheating heat islands and service ecosystem depletion. future studies need to identify particular drivers of land cover modifications in chiniot district while assessing related local climate and water resource effects. socio-economic information would create a thorough understanding of urbanization patterns within the region and their wider consequences. conclusion and recommendations this study investigates the relationship between land surface temperature (lst) and normalized difference vegetation index (ndvi) and normalized difference built-up index (ndbi) using landsat 8 oli/tirs and landsat 5 data. the findings highlight that built-up areas and bare soil contribute significantly to lst elevation, whereas vegetation tends to lower lst levels. the study aims to evaluate changes in land use and land cover (lulc) and lst in chiniot district, punjab, pakistan, from 1990 to 2020. results indicate an expansion of urban areas at the expense of water bodies and vegetation, accompanied by a rise in average lst over the decades. furthermore, gis analysis reveals a strong positive correlation between lulc changes and lst increase, particularly due to the conversion of cultivated land to built-up areas. notably, the temperature distribution across different lulc classes shows the highest temperatures in built-up areas and the lowest in water bodies. the study also projects a reduction in vegetation and water bodies by 2020 compared to 1990. additionally, analysis of lst data from march 1990 to march 2020 indicates a steady increase in temperature over time. the presence of sand near riverbeds is identified as a significant factor contributing to high temperatures due to its low specific heat energy compared to water. supervised classification of chiniot district highlights a shift in vegetation patterns over the years, with a noticeable increase in built-up areas and a corresponding decrease in vegetation. this trend underscores the influence of land use changes on lst variations. overall, the decline in ndvi and the increase in ndbi from 1990 to 2020 signify a decrease in vegetation and an expansion of built-up areas, contributing to the observed rise in lst. the analysis shows an alarming pattern of growing land surface temperature (lst) in chiniot district because of both urban development and changes in ground cover. integration of the following strategic policies presents a solution to reverse this trend while promoting urban sustainability in the area such as targeted tree planting in which national efforts should establish tree planting initiatives which use local climate-adapted native species for all plantations. placing trees in roadside areas and parks and residential spaces promotes maximum shade and cooling effects from evapotranspiration. green roofs and walls strategies that includes policy that should offer economic benefits for building owners who construct green roofs together with vertical green walls on large structures. the implementation of these features successfully lowers surface temperatures for buildings as they function to reduce localized heating in these areas. several organizations should establish community gardens together with urban farming initiatives. the programs create additional green areas in addition to strengthening food availability for local communities and encouraging public involvement. policy measures should exist to safeguard present green areas from being transformed into built structures. the implementation of improved management practices will enable maximum utilization of cooling capabilities from urban areas. the development of dense and combined living and working areas will prevent city expansion through urban sprawl while trimming down demands on big transportation systems. neighborhood layout which promotes walking and biking helps people avoid using private vehicles for everyday needs. roads together with parking lots and sidewalks should use permeable pavements as their primary choice. water permeates through these surfaces which helps groundwater absorption while also decreasing flooding and heat from the environment. installing public transportation systems and building cycling networks with pedestrian paths will decrease private car use on roads. support the adoption of electric vehicles along with other vehicles with low emission levels. the city should adopt water-sensitive urban design principles to both save water and effectively manage its water resources. guam should deploy three water conservation methods through rainwater harvesting along with greywater recycling programs and the establishment of urban wetlands systems. urban green spaces and agricultural land should adopt suitable irrigation technology to reduce water losses and enhance evapotranspiration-based cooling. a series of public information initiatives should reach residents to demonstrate the consequences of lst rise alongside the advantages of implementing urban green spaces and sustainable management. workers from the community need to participate in both environmental heat monitoring and data collection activities across cities. the program enables the collection of awareness data that advances decision-making through valuable evidence. the implementation of a new policy and regulatory system must use incentives to promote sustainable practices and enforce limitations on operations which cause lst increase. different public agencies must partner with urban planners along with developers and community organizations for implementing collaborative lst reduction strategies. financial support for scientific investigations should maintain focus on studying the multi-factor relationships between land use modification and lst heat flux and climatic change patterns in the chiniot district. a plan must be created for continuous lst monitoring as part of longterm evaluations that measure outcome success from implemented mitigation approaches. these actionable strategies will help the chiniot district develop a sustainable urban atmosphere which combats lst increases to create higher quality living conditions for its citizens. the achievement of extended success requires implementing a combined approach between green infrastructure and sustainable urban planning and community engagement methods. research outcomes the research outcomes reveal several discrepancies in the processing of the data. firstly, inaccuracies were detected in the processed images of the year 2000, where the land surface temperature appeared low despite a high built-up area, leading to misleading interpretations. this discrepancy may stem from geographical or climatic fluctuations in the year 2000, or it could be attributed to errors in the bands of the images used during processing. furthermore, the normalized difference vegetation index (ndvi) for the year 2020 yielded invalid results showing increased vegetation, contrary to expectations. additionally, the land surface temperature (lst) values for the year 2020 were unexpectedly low, suggesting a difference in band values of the image used for processing. moreover, the normalized difference built-up index exhibited high values in 2000 instead of low values, and lower values in the year 2020, indicating potential errors in the image processing methodology. these discrepancies highlight the need for a thorough review of the image processing techniques and data used in the thesis to ensure the accuracy and reliability of the research 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(2023). “impact of urban expansion on land surface temperature and carbon emissions using machine learning algorithms in wuhan, china.” urban climate 47: 101347. https://doi.org/10.1016/j.ancene.2024.100432 https://doi.org/10.1016/j.ancene.2024.100432 https://doi.org/10.1038/s41598-024-68492-7 https://doi.org/10.1038/s41598-024-68492-7 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.1056532 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (septiembre, 2023): 289 297 en v ir o n m en ta l g eo lo g y analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis zhetao xiao1, donghui zhao2, lijun hao2* 1.school of architecture, north china university of water resources and electric power, zhengzhou 450046, china 2.school of art and design, north china university of water resources and electric power, zhengzhou 450046, china *corresponding author: haolj35@ncwu.edu.cn.com abstract: manuscript received: 26/12/2022 accepted for publication: 31/08/2023 how to cite this item: xiao, z., zhao, d., & hao, l. (2023). analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis. earth sciences research journal, 27(3), 289-297. https://doi.org/10.15446/ esrj.v27n3.1056532 keywords: landscape architecture; morphological spatial pattern analysis; green infrastructure; zhengzhou central city green infrastructure is generally regarded as an effective way to maintain regional ecological security, and its construction method plays an essential role in its function realization. in this study, the central urban area of zhengzhou was taken as the research area. the land use type maps of the study area were obtained through the supervised classification of tm/etm remote sensing image data in 2007, 2011, 2015, and 2019. the green infrastructure (gi) in the study area was obtained by morphological spatial pattern analysis (mspa) based on the landscape connectivity index and landscape map theory. the landscape connectivity of the core area was classified and quantitatively evaluated. then, the potential ecological corridors were extracted and analyzed. the gi pattern change research based on mspa, landscape connectivity, and map theory provided a new framework for analyzing gi pattern change. finally, the spatial structure of green infrastructure planning in zhengzhou central city is obtained, and the optimization strategy of network structure is proposed. the research results can provide references for the planning and optimization of gi networks in the study area and have particular reference significance for the planning and construction of gi networks in other areas. análisis de los cambios de patrones en las redes de infraestructura verde en la ciudad de zhengzhou, china, con base en el análisis morfológico de los patrones espaciales resumen: la infraestructura verde es comúnmente considerada una vía efectiva para mantener la seguridad ecológica regional y su método de construcción juega un papel esencial en su funcionalidad. en este estudio se definió el área central de zhengzhou como la zona de investigación. los mapas de uso del suelo en el área de estudio se obtuvieron a través de la clasificación supervisada de la información de imágenes landsat (tm/etm) en 2007, 2011, 2015 y 2019. la infraestructura verde en el área de estudio se obtuvo con el análisis morfológico de los patrones espaciales (del inglés morphological spatial pattern analysis, mspa) con base en el índice de conectividad del paisaje y la teoría del mapa del paisaje. la conectividad del paisaje en la zona central del área de estudio se clasificó y se evaluó cuantitativamente. luego se extrajeron y se analizaron los corredores ecológicos potenciales. la investigación sobre el cambio en el patrón de la infraestructura verde con base en el método mspa, en la conectividad del paisaje y en la teoría de mapas significó un nuevo marco teórico para analizar el cambio de patrón de este tipo de infraestructura. finalmente se obtuvo la estructura espacial de la infraestructura verde planeada por la administración de la ciudad de zhengzhou y se propuso una estrategia de optimización de esta red estructural. los resultados de la investigación pueden ofrecer referencias para la planeación y optimización de redes de infraestructura verde en el área de estudio y tiene una referenciación particular en estos procesos de planeación y construcción en otras áreas geográficas. palabras clave: arquitectura del paisaje; análisis morfológico de los patrones espaciales; infraestructura verde; ciudad de zhengzhou https://doi.org/10.15446/esrj.v27n3.1056532 mailto:haolj35@ncwu.edu.cn.com https://doi.org/10.15446/esrj.v27n3.1056532 https://doi.org/10.15446/esrj.v27n3.1056532 290 zhetao xiao, donghui zhao, lijun hao 1. introduction the rapid development and construction of the cities improve their economic efficiency but lead to aggravated destruction of ecological environments. the city’s expansion leads to the fragmentation of ecological networks (qiu, chang, & wang, 2013), and even the retrogressive reduction of ecological networks and the reduction of habitat core areas, which seriously damage the ecological service function of the landscape. the cut-off of ecological corridors leads to a decrease in biological migration channels and affects the development of biodiversity. in a word, the process of urbanization seriously affects the health and integrity of the ecosystem. green infrastructure (gi), proposed in the 1990s, was initially separated from eco-green environmental network infrastructure and opposed to other conventional infrastructure (grey infrastructure). gi is a network system that contains a variety of natural and restored ecological and landscape elements, according to the definition of the united states government joint expert group (benedict, & mcmahon, 2012). so far, its connotation and research scale are constantly enriching. research on various regional scales by mspa in china and abroad has been continuously carried out and perfected in recent years. as an example, at the national level, there are studies on gi change assessment for the united states territory and forest fragmentation and connectivity throughout europe. at the urban level, there are studies about the gi development of virginia and its surrounding states, the studies on gi of the border between austria and hungary, and the studies on the construction of related ecological networks in sichuan and chongqing, jiangsu, and zhejiang of china (chen, 2015). on the medium and small scale, there are studies about the spatial and temporal changes of ecological network patterns in guangzhou, shenzhen, nanjing, and some counties in china (zhu, ren, & liu, 2019; chang et al., 2013; yu et al., 2016; byrne, lo, & jianjun, 2015). zhengzhou, a national central city, is the core city that plays a leading role in the central plains urban agglomeration. it is rich in natural resources and runs through the yellow river. however, during its development, urban diseases become increasingly prominent, including the weakening of urban function, and the poor effect of rain and flood caused by habitat fragmentation. the changes and construction of its green infrastructure should be studied to make it a leader in the development and construction of green infrastructure and to drive the green infrastructure construction of other cities, which is of great significance to the formation of green ecological networks of the central plains urban agglomeration 2. applicability of mspa introduction to mspa mspa (morphological spatial pattern analysis), a morphological-based spatial pattern analysis, is initially used by scholars at european union joint research center (jrc) to study forest fragmentation and species conservation (ostapowicz, 2008). mspa is an image processing method to analyze the spatial pattern of raster images based on the principle of binary morphology in mathematical morphology. it can classify image pixels according to the corresponding landscape types by accurately recognizing, measuring, and segmenting the images to distinguish the landscape types and structures. combining the mspa with the minimum path analysis, the connectivity among the structural landscape elements and elements can be well identified and analyzed, and the potential ecological corridor can be obtained. the emphasis of ecological network research is shifted to the connectivity level of the ecological source site and ecological corridor, which solves the subjectivity of other research methods and makes the constructed ecological network more scientific. ecological correspondence between mspa and gi (1) definition of gi gi is easily understood as the infrastructure related to green space, like engineering facilities. its more accurate definition is the network of natural areas, open space with internal connectivity, and possible ancillary engineering facilities. this network has the function and value of a natural ecosystem. it provides natural places for human beings and wildlife, such as habitat, clean water sources, and migration channels, constituting the ecological framework to ensure the sustainable development of the environment, society, and economy. at the micro level, gi refers to the patches and corridors of specifically related engineering facilities or green spaces, such as flood control systems, water purification facilities, a secondary forest, a tree, or even a green roof. in the absence of a specific reference, it emphasizes the connectivity of the whole. specifically, gi is a multi-level system ranging from the land-wide ecological protection network to the street rainwater garden (liu et al., 2013). at the regional level, gi supports functions of critical ecosystems constituted by national parks, coastlines, major river corridors, long-distance footpaths, and so on. at the urban or community level, gi forms an open space network consisting of urban parks, recreational areas, farmland, community gardens, street landscapes, private gardens, cemeteries, small water bodies and streams, rooftop gardens, etc. (2) mspa and gi gi is a natural and artificial green space network system composed of “hubs” and “link corridors”. mspa analyzes the geometric characteristics and connectivity of binary images of any scale and type by dividing it into seven categories that do not overlap, so it can also be used to construct gi networks (wang & lin, 2017). first, after it is reclassified using the land cover change data, the gi elements are extracted as “foreground” and other non-gi elements as “background”. then, through a series of image processing techniques, the foreground is divided into seven categories that do not overlap (table 1): the “center” is the “hub” of the gi, and the “bridge” is the “corridor” of the gi. finally, the gi network is constructed according to the “center” and “bridge” of the mspa to determine the gi elements and patterns, find the potential “hubs” and “corridors”, and realize the interconnected network. table 1. landscape ecology meanings for various mspa elements type of element landscape ecology meaning core area large green patches and distinguishable nonlinear landscape elements in space. it is the «source» of various ecological processes, providing habitat for wildlife, and plays an important role in the protection of biodiversity and the maintenance of urban ecological environment. connecting bridge the channel connecting the adjacent core areas and a narrow zone different from the landscape elements on both sides. it has corridor properties and is a channel for the exchange and flow of energy flow and material flow. it has a direct impact on species mobility and landscape connectivity. marginal zone the transition region between the periphery of the core area and different landscape elements. it has the attributes of landscape boundary in ecology. the adjacent landscape permeates, connects and distinguishes each other through the marginal zone, and has not only characteristics of adjacent landscape but also its own unique components. high heterogeneity. branch line the passage connecting the core area and the peripheral landscape. it also has corridor properties, but its connectivity is generally poor. ring zone the internal passage connecting the same core area. it also has corridor properties, and enhances species diffusion and energy flux within the core area. isolated island obviously different from the surrounding landscape elements, isolated and broken small patches. the internal species communicate less likely with external species. however, as a small green space in the built area, it plays an important role in the construction of the whole ecological network. pore the transition zone between the core area and its internal construction land. it also has marginal effects, reflecting the degree of species isolation and the possibility of potential gene variation. 291 analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis 3. research areas and research methods research areas zhengzhou, a core city of the central plains urban agglomeration, is in central china. it is in the yellow river’s middle and lower reaches and the funiu mountains’ northeast wing to the huanghuai plain transition zone. the west is high, the east is low, the middle is high, the northeast is low, and the southeast is low. it has a north-temperate continental monsoon climate, over 100 rivers, and 6 municipal districts. according to the latest approved zhengzhou master planning (2010-2020), zhengzhou central city includes five districts within the administrative area of zhengzhou city: zhongyuan district, jinshui district, erqi district, guancheng district, huiji district. the total area measures 990 square kilometers. data source acquisition and preprocessing this study mainly used the remote sensing image data of zhengzhou city with the landsat5\landsat8 satellite tm/etm format in 2007, 2011, 2015, and 2019, with 30m precision. first, atmospheric and radiation corrections were conducted, and regions of interest were created using envi (xie, wang, & luo, 2020). then, through the supervision classification, visual correction, and field investigation, the remote sensing images were divided into six categories: woodland, cultivated land, grassland, water area, construction land, and unused land. finally, based on the zhengzhou master planning and the classification of land use types, the six categories were corrected to form the classification map of land use types in zhengzhou in the four years. research methods arcgis performed mask extraction and re-classification, and the classification map of land use types in zhengzhou central city was extracted. in graphic data, the four gi elements of woodland, cultivated land, grassland, and water area were extracted as “foreground”, and other non-gi elements of construction land and unused land were extracted as “background”. and 30m x 30m tiff binary raster data files were generated and imported to the guidos toolbox. based on the evaluation of landscape connectivity of seven types of landscape elements in zhengzhou in 2019, the classification of core areas and the extraction of potential ecological corridors were carried out, and the gi planning strategy of zhengzhou central city was put forward. 4. changes of gi spatial and temporal pattern of zhengzhou central city by mspa the data of mspa landscape type changes in zhengzhou central city from 2007 to 2019 were obtained. table 2 shows that the total gi area of zhengzhou central city was decreasing year by year, which, however, was slowing down. it decreased by 112.167 km2 from 2007 to 2011, 36.927 km2 from 2011 to 2015, and 52.074 km2 from 2015 to 2019. gi decreased slowest from 2011 to 2015 but quickened in recent years. the above results also indirectly showed that the construction land area in zhengzhou central city has been increasing yearly. table 3 shows that the core area was dominant in the landscape type. the area of core area decreased from 449.856 km2 in 2007 to 185.724 km2 in 2019, the ratio to total gi area decreased from 70.68% in 2007 to 42.69% in 2019, and the ratio to the total area of the central city decreased from 45.44% in 2007 to 18.76% in 2019, meaning a significant decrease. this is because zhengzhou city constantly expands, and its increasing construction land encroaches on other land. although the isolated islands are not connected, they can play a stepping-stone role of “ecological island hopping” in constructing ecological networks (wang, shen, & jin, 2019) and are likely to form a connecting bridge or core area with the increase of their area. the area of isolated islands increased from 17.424 km2 in 2007 to 40.788 km2 in 2019, the ratio to total gi area increased from 2.73% in 2007 to 9.37% in 2019, and the ratio to the total area of the central city increased from 1.76% in 2007 to 4.12% in 2019, meaning a significant increase. the connecting bridge is vital in constructing ecological networks, such as connecting core areas, material energy exchange, and biological migration. the area of connecting bridge increased from 13.068 km2 in 2007 to 29.997 km2 in 2019, the ratio to total gi area increased from 2.06% in 2007 to 6.9% in 2019, and the ratio to the total area of the central city increased from 1.32% in 2007 to 3.03% in 2019. it showed that the construction of ecological corridors and gi connectivity in zhengzhou’s central city were on the rise, promoting the overall optimization of ecological network construction in zhengzhou. in addition, there was almost no change in the ring zone, and the proportion of the pore area was reduced, indicating that the internal integrity of the core area was enhanced. the proportion of the marginal area increased slightly, indicating that the tiny core area increased and the core area fragmentation increased. an increase in the proportion of the area of the branch line indicated an increase in the green belt between construction land and gi. table 2. the total landscape gi area in zhengzhou central city from 2007 to 2019 based on mspa (km²) year total gi area(km²) 2007 636.372 2011 524.205 2015 487.278 2019 435.204 table 3. the changes in various landscape types in zhengzhou central city from 2007 to 2019 based on mspa landscape types year area(km²) ratio to total gi area(%) ratio to the total area of central city(%) core area 2007 449.856 70.68 45.44 2011 302.247 57.67 30.53 2015 254.133 52.14 25.67 2019 185.724 42.69 18.76 isolated island 2007 17.424 2.73 1.76 2011 27.621 5.27 2.79 2015 32.175 6.61 3.25 2019 40.788 9.37 4.12 pore 2007 41.679 6.55 4.21 2011 27.225 5.19 2.75 2015 19.8 4.05 2 2019 13.563 3.11 1.37 marginal zone 2007 79.299 12.47 8.01 2011 103.059 19.67 10.41 2015 113.355 23.27 11.45 2019 114.048 26.21 11.52 ring zone 2007 10.494 1.65 1.06 2011 9.603 1.82 0.97 2015 9.702 2 0.98 2019 9.9 2.26 1 connecting bridge 2007 13.068 2.06 1.32 2011 23.463 4.47 2.37 2015 21.681 4.45 2.19 2019 29.997 6.9 3.03 branch line 2007 24.552 3.86 2.48 2011 30.987 5.91 3.13 2015 36.432 7.48 3.68 2019 41.184 9.46 4.16 figure 1 shows that the construction land area in zhengzhou central city is rapidly expanding, the peripheral core area is gradually swallowed up, the integrity of the core area is poor, the core patch is increasing, and the gi 292 zhetao xiao, donghui zhao, lijun hao fragmentation is becoming more and more serious. however, depending on the urban ecological construction, some core areas show a trend of converging growth, and the connectivity of some areas is enhanced. the a in figure 1 is guxing town and guangwu town in huiji district, the yellow river scenic spot, the wetland beach area, and so on. it is mostly rural cultivated land, surrounded by many mountain forests, large forest scenic parks, and the yellow river nature reserve. the urban construction here gradually expanded to the northwest, and plants, schools, and other construction occupied cultivated land. since the construction of the yellow river scenic spot railway station in 2015, the railway has split the core area here, and then the landscape fragmentation intensified. the b is shifu town and jalu river of zhongyuan district of zhengzhou city, where there are also many cultivated lands and public green spaces such as shifu chensha pool and zhengzhou green valley quality culture park. however, the cultivated land area was reduced due to the construction of numerous residential areas. still, the core area of landscape along the jalu river increased. the c is a large number of mountain terraces, as well as changzhuang reservoir, jalu river, south-to-north water transfer main trunk canal, and other water bodies. the core area here also has a decreasing trend and fragmentation state, but the process of decreasing is relatively slow. many water core areas, especially the landscape core areas on both sides of the central trunk canal of the south-to-north water transfer middle line, are gradually connected. the d is a part of jinshui district, including the yellow river wetland reserve in the north, zhengzhou cbd (central business district) and longzi lake university park, as well as the jalu river, longhu lake, dongfeng canal, xiong’er river, weihe river, longzi lake, and other waters. there are some cultivated lands along the yellow river in the north, where the city’s expansion to the north is evident, and the landscape core area needs to be fixed. however, the landscape core area of the river region is also gradually connected as a whole. the e is part of the guancheng district, where some cultivated land is in nancao township. the landscape core area is more seriously broken, and the core area of qilihe river, chaohe river, and diehu lake is well developed. 2007 2011 2015 2019 293 analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis figure 1. temporal and spatial variation in green infrastructure of zhengzhou central city in 2007, 2011, 2015, and 2019 based on mspa. 2007 2011 2015 2019 a b c d e 294 zhetao xiao, donghui zhao, lijun hao 5. classification of green infrastructure in zhengzhou central city 5.1. classification of core areas in this study, the possible connectivity index (pc) was selected to evaluate the landscape connectivity of the core area (gao, huang, & li, 2019) and the calculation formula of pc was as follows: pc = i j n i j ijj n l a a p a �� �� ���� �� 1 2 (1) where “n” is the total number of patches in the landscape, “p*ij” is the probability of direct diffusion of species in the patch “i” and patch “j.” “ai” and “aj” are the areas of the patch “i” and patch “j,” respectively. “al 2” is the total area of the landscape. referring to the relevant research, the threshold of patch connectivity distance was set to 500 m, and the connectivity probability was set to 0.5 (xu et al., 2015). the area of the core area (s) was classified into three levels according to the size of the area (figure 2): s ≤ 0.2km², 0.2<s<4km², s ≥ 4km². the possible connectivity index (pc) of each core area was calculated by the landscape connectivity analysis software conefor2.6 (figure 3) and classified into three levels: pc ≤ 0.2, 0.2<pc<4, and pc ≥ 4 (yu et al., 2016). the core area with a possible connectivity index (pc)≥4 had the highest landscape connectivity, which will be regarded as the most important core area in the regional green infrastructure and classified as the first-level core area. the core area with a possible connectivity index (pc) between 4-0.2 had high landscape connectivity, which will be regarded as the important core area in the regional green infrastructure and classified as the second-level core area. the core area with a possible connectivity index (pc)≤ 0.2 had the lowest landscape connectivity (figure 4), which will be regarded as the general core area in the regional green infrastructure and classified as the third-level core area. it was concluded that there were 9 first level core areas, including the yellow river nature reserve in the north, the guxing town and guangwu town in the northwestern huiji district, the changzhuang reservoir, jalu river, and other xiliuhu street areas in the southwest. the second level core areas mainly covered the longhu lake area in the eastern jinshui district, including the jalu river, longhu lake, dongfeng canal, xiong’er river, weihe river, longzi lake, and the tianjian lake park and xushui river in the west, as well as the nancao township in the southeast including the qilihe river, chaohe river and diehu lake. the third level core areas were mostly urban small and medium-sized park green spaces, roads, and river greening areas, multiple and scattered, but most tended to be connected (figure 5). figure 2. classification of green infrastructure area in the research area figure 3. landscape connectivity analysis in the research area figure 4. classification of landscape connectivity in the research area figure 5. green infrastructure classification of the research area 295 analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis 6. potential ecological corridor extraction of green infrastructure in zhengzhou central city (shortest path analysis) the shortest path analysis is the minimum cost distance of individual movement between patches and identifies potential and more reasonable ecological corridors (yang et al., 2018). according to the classification of gi core area in zhengzhou, the first level core area is mainly distributed in the north and southwest but rare in the east and southeast. considering the whole construction of ecological corridor network and the planning of urban green space system in zhengzhou, a total of 11 core areas including the first level core areas and the second level core areas (longhu lake area and nancao township in the southeast), which have large area and high connectivity and potential development into the first level core area, were used as ecological source sites. and in the arcgis the core areas were transformed into 11 ecological source sites for potential corridor analysis. first, according to the classification map of land use type, the elevation map of zhengzhou central city and the traffic map of zhengzhou central city (including railways and all kinds of highways), the resistance of the three types of grid data to the survival and migration of the species was classified from small to large (1-5) by the arcgis software, and three kinds of factor resistance base surfaces were formed. next, based on the different proportional weights of each resistance base surface, the total resistance base surface of zhengzhou central city was obtained by superposition by grid calculator (figures 6-9). then, the minimum cumulative resistance surface of 11 ecological source sites in zhengzhou central city was generated by using the cost distance module of arcgis software (figure 10). on this basis, the cost distance analysis of 11 ecological source sites was conducted to generate distance raster data and backtracking linked raster data (figure 11). finally, according to the distance raster data and backtracking linked raster data, the cost path analysis of 11 ecological source sites was carried out to generate 55 shortest paths, that is, potential ecological corridors (figure 12). figure 6 elevation factor in the research area figure 7 traffic factor resistance in research area figure 8 division of land use types in the research area figure 9 total resistance surface in the research area 296 zhetao xiao, donghui zhao, lijun hao figure 10 minimum cumulative resistance surface in the research area figure 11 shortest path analysis of the research area figure 12 potential ecological corridors 7. construction ideas of green infrastructure in zhengzhou central city protection planning strategy for core areas the first level core area, which has large area and good connectivity of landscape, plays the most important role in exerting the ecological function of the urban green infrastructure. therefore, we should focus on strengthening the protection of the first level core area, introducing protection policies, rationally drawing out ecological protection lines, protecting cultivated land and woodland, and strictly prohibiting urban construction land from entering the protection line, strengthening species diversity protection and habitat protection measures, building species diversity habitats, and rationally dividing tourist areas and prohibited tourist areas for areas with tourist functions, so as to reduce human interference and destruction to the growth environment of animals and plants in the protected area. the second level core area, which is small and has good connectivity of landscape, is mainly distributed in the urban area or surrounded or halfsurrounded by construction land. its ecosystem is unstable and ecological sensitivity is high. therefore, we should also focus on strengthening protection of the second level core area and enhancing its connectivity with the first level core area, and gradually merge the second level area, which is close to the first level core area, with the first level core area. the third level core area is mostly small and medium-sized green space in the city and almost connected linear isolated islands formed of narrow riverways, which has the characteristics of connecting corridors and the function of ecological island hopping. in planning, the long and narrow linear isolated islands are further connected in series to play their media role in the green infrastructure, so that they can not only play the ecological function of small core area but also have the connection attribute of ecological corridor, greatly enhancing the integrity of global green infrastructure network. protection planning strategy for corridor based on the classification of the importance of the core area and the potential ecological corridor extracted, the ecological network structure system of “five core areas, three horizontal corridors and seven vertical corridors” of green infrastructure construction in zhengzhou central city was put forward (fig. 12). the “five core areas” are the ecological core area along the yellow river in the north, the core area of guxing town shuangqiao village, the core area of longhu lake, the core area of mazhai town houzhai township and the core area of nancao township. the “three horizontal corridors” are: 1. ecological corridor along the yellow river in the north 2. longhu lake core area weihe river jalu river yingcai street xiaoshuangqiao site suoxu river guxing town shuangqiao village core area 3. nancao township core area jingzhuang village pangzhuang village south-to-north water transfer river north side of zhihui city ziyu road mazhai town houzhai township core area the “seven vertical corridors” are: 4. guxing town shuangqiao village core area xushui river changzhou road jinhua road dongfang road konghe river, xushui river danshui avenue zhengzhou botanical garden changchun south road mazhai town houzhai township core area 5. guxing town shuangqiao core area xushui river south-to-north water transfer river nancao township core area 6. strip ecological core area along the yellow river in the north jalu river basin mazhai townhouzhai township core area 7. strip ecological core area along the yellow river in the north madu trunk canal yao diandi community south longyi fourth street longhu lake core area north-south canal kunli river jinshui river dihu lake lvcheng park lvcheng park southwest south-to-north water transfer river baguamiao community green space jalu river mazhai town houzhai township core area 8. longhu lake core area dongfeng canal xiong’er river zhongwang road qilihe river basin zichen road nancao township core area 9. strip ecological core area along the yellow river in the north jalu river yaoxia road lonzi lake east road dongqin street west of 297 analysis of green infrastructure network pattern change in zhengzhou central city based on morphological spatial pattern analysis east fourth ring road dongfeng canal east of east fourth ring road tchaohe riverdie lake nancao township core area 10. (strip ecological core area along the yellow river in the north madu xincun east of east fourth ring road liujiang hub east of beijing, hong kong and macao expressway dongfeng canal putian township government periphery chaohe river diehu lake nancao township core area the three horizontal and seven vertical ecological corridors will be given priority to construct, and the green area is widened on both sides of the key sections such as expressway and urban trunk line to build a guiding passage conducive to biological migration. we should focus on strengthening the construction of rivers and surrounding green spaces, strengthening the control of river pollution and the construction of wetland parks, building natural riparian lines, planning and building zonal parks along river channels, and enhancing the ecological attributes of corridors, so as to form a whole green infrastructure ecological network pattern for zhengzhou central city. 8. conclusion this paper extracted, identified, and classified the core area of green infrastructure in zhengzhou central city by using the mspa and conefor landscape connectivity analysis, and extracted the potential ecological corridor of zhengzhou central city by the minimum consumption path analysis. the results showed that the green infrastructure in zhengzhou central city was decreasing year by year with the expansion of urban construction, but the degree of fragmentation of the first, second and third level core areas was increasing, but the landscape connectivity was relatively enhanced. the small fragmented core area gradually gathered into the form of ecological corridor, and presented the trend of connectivity. and the ecological corridor composed of water area presented a better trend of connectivity, showing the obvious effect of river ecological construction in zhengzhou central city. taking the core area extracted by the mspa as the ecological source site avoided the subjectivity of the ecological source site extraction. the conefor analysis can quantify the connectivity of the core area and provide an important basis and data support for the classification of ecological source site and the improvement of connectivity. the minimum consumption path analysis can quantitatively obtain the best path of species migration and provide the basis for scientific planning of ecological corridor. if the green infrastructure can be analyzed according to their different functional types by the mspa, it will make the optimization of ecological network more detailed. it is hoped that the relevant development scholars will improve this method more scientifically in the future to provide a more scientific and detailed scientific basis for urban ecological construction. funding statement the authors acknowledge the following foundations: the key scientific research projects of higher education institutions in henan province (18b560005); the humanities and social sciences project of henan education department (2019-zzjh-575, 2020-zdjh-205); the key scientific and technological research and development and promotion projects in henan province (192102310282); teaching demonstration class in north china university of water resources and electric power (2018-09); education and teaching research and reform project in north china university of water resources and electric power (2019-25); first class undergraduate courses in north china university of water resources and electric power (2020-10, 202032); henan philosophy and social science planning project (2020bys023); research topics of science and technology think tanks in henan province (hnkjzk-2020-25c). references benedict, m. a., & mcmahon, e. t. 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(2019). green infrastructure provision for environmental justice: application of the equity index in guangzhou, china. urban forestry & urban greening, 46, 126443. https://doi.org/10.1016/j. ufug.2019.126443 https://doi.org/10.1016/j.landurbplan.2015.02.013 https://doi.org/10.1016/j.landurbplan.2015.02.013 https://doi.org/10.1007/s11769-012-0568-3 https://doi.org/10.1007/s11769-012-0568-3 https://doi.org/10.1016/j.cities.2015.01.005 https://doi.org/10.1007/s10980-008-9271-2 https://doi.org/10.13287/j.1001-9332.201810.019 https://doi.org/10.1016/j.ufug.2019.126443 https://doi.org/10.1016/j.ufug.2019.126443 ole_link3 ole_link4 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n3.103605 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (september, 2022): 255 262 g eo lo g y a detection method of urban underground geological anomalies in the united kingdom based on feature fusion xuemei liu* department of preschool education, hebei women’s vocational college, shijiazhuang, china *corresponding author liuxuemei@mjc-edu.cn abstract engineering geological conditions include the nature of rock and soil, geological structure, landform, hydrogeological conditions, and adverse geological processes. among them, faults, fissures, folds, karst, and lithology changes seriously affect the safety and construction cost of mountain tunnels, hydraulic tunnels, and other projects. for this reason, a new method based on feature fusion is proposed to detect the geological anomalies in london and sheffield. it established a 3d raster data model oriented to attribute information modeling and visualization of urban underground space to obtain geological data. based on this acquired data, authors adopted the feature-level fusion extraction method based on the multi-attribute geological abnormal body to extract, fuse, fill and surface the multi-attribute data of underground space geological data. smooth processing can realize the detection of abnormal geological bodies in underground space. it has been proved that this method can be used in geological data display, feature extraction, feature fusion, and abnormal physical examination. keywords: feature fusion; british city; underground space; geology; abnormal body; detection; método de detección de anomalías geológicas subterráneas en el reino unido con base en la fusión de características resumen las condiciones de ingeniería geológica incluyen la naturaleza de rocas y suelo, estructura geológica, formas de la tierra, condiciones hidrogeológicas y procesos geológicos adversos. entre estas condiciones las fallas, fisuras, plegamientos, kársticos y los cambios de la litología afectan seriamente la seguridad y los costos de producción de túneles en montañas, túneles hidráulicos y otros proyectos. por esta razón, este trabajo propone un nuevo método basado en la fusión de características para detectar las anomalías geológicas en londres y sheffield, en el reino unido. este método parte de un modelo de datos de trama 3d para realizar un modelado de información y visualización del espacio urbano subterráneo con el fin de obtener información geológica. con base en esta información adquirida, los autores adoptaron el método de extraccción de fusión de características con base en los multiatributos geológicos de un cuerpo anormal para extraer, fusionar, llenar y normalizar la información multiatributos del espacio urbano subterráneo. en el proceso de suavizado se pueden detectar los cuerpos anormales en el espacio subterráneo. los resultados prueban que este método puede usarse en la presentación de información geológica, en las características de extracción, características de fusión, y en la examinación física de los cuerpos. palabras clave: fusión de características; reino unido; espacios urbanos subterráneos; geología; cuerpos anormales; detección; record manuscript received: 09/08/2022 accepted for publication: 12/10/2022 how to cite item: liu, x. (2022). a detection method of urban underground geological anomalies in the united kingdom based on feature fusion. earth sciences research journal, 26(3), 255-262. https://doi. org/10.15446/esrj.v26n3.103605 https://doi.org/10.15446/esrj.v26n3.103605 mailto:liuxuemei@mjc-edu.cn https://doi.org/10.15446/esrj.v26n3.103605 https://doi.org/10.15446/esrj.v26n3.103605 256 xuemei liu 1. introduction the geological body refers to the natural objects with spatial forms formed in the geological era. geological bodies are often irregular objects, showing a natural, arbitrary geometry and size. geological bodies can be divided into geological forms, including ore bodies, reef shoals, rock mounds, sand bodies, karst caves, and faults (sun et al., 2018). unlike other parts of the stratum, the geological body usually occupies a space volume and greatly differs from other geological components in the same stratum (montanari et al., 2017). an abnormal geological body is a part of the concept of geological bodies. the geological anomaly body has all the characteristics of the geological body but also has some of its own. in the authors’ geological exploration and geophysical research, the range of abnormal geological bodies is different from other areas in many aspects, such as the mutation of vibration amplitude and the mutation of the waveform shape (wu et al., 2017). geological anomaly bodies play an essential role in many geological structures. the exploration and research of geological anomaly bodies play an active role in the inference of geochemical and geophysical anomalies (li et al., 2019). geochemical and geophysical anomalies are often caused by mineralization, which is the effect of variation of local chemical and physical fields in the area during the formation of the deposit, and often serves as a sign of the existence of the deposit (ping et al., 2017). there are many important geological resources such as oil and natural gas in common geological abnormal bodies such as river courses and sand bodies (zhu et al., 2018). therefore, in geophysical exploration and structural interpretation of geological data, geological anomalies will be specifically detected, identified, and visualized (bai et al., 2018). given the above characteristics and significance, geologists and researchers have done much research and achieved good results (han et al., 2020; zhang et al., 2019). structural interpretation of geological data covers almost all scientific research fields of petroleum and basic geology, including structural stratigraphy, seismic stratigraphy, and other disciplines. three-dimensional visualization technology can directly show the results of structural interpretation and other geological structure research. it can help geologists and relevant researchers better understand the geological characteristics of the surface and below the surface to better carry out subsequent research (liu et al., 2017). the research and visualization of 3d object surface reconstruction are one of the most important technologies of 3d visualization, which is widely used in the fields of geological exploration, environmental science, aerospace, health care, remote sensing, and geophysical exploration. wang xiangsheng proposed to use the geological radar to detect the geology, but the detection of this method is timeconsuming and inefficient; liu mingyang applied the electrical method to the geological survey of water conservancy engineering, but the survey accuracy of this method is controversial, and lack of case verification; han kunying and other scholars designed an automatic inspection system of geological map spatial database to apply in the geological exploration. this paper recognizes that a matching detection method should be introduced for this system to achieve high-precision detection of geological anomalies (xu & chen, 2019). nowadays, 3d visualization technology is a research hotspot of all kinds of geological and seismic software. it is a tool for intuitively expressing and displaying many geological features on and below the surface. geologists and researchers can use three-dimensional visualization software to display the results of seismic research and understand the geological structure more intuitively to pave the way for further research. in this paper, the authors propose a method of detecting the geological anomalies in urban underground space based on feature fusion. firstly, they built a three-dimensional grid data model for attribute information modeling and visualization of urban underground space to obtain the geological features of urban underground space in the uk. then, they used the method of extracting the multi-attribute anomaly data to detect the geological anomalies in urban underground space in the uk (peng et al., 2018). 2. materials and methods 2.1. the 3d raster data model for attribute information modeling and visualization of urban underground space the data of physical and chemical attribute parameters (such as stress, porosity, permeability, water quality, and water temperature) in urban underground space geological bodies differ from the information of spatial geometry structure. they are generally the continuous change (gradual change) in three-dimensional space. it is necessary to model and analyze the attribute data in the geological body and study its space. generally, the three-dimensional volume element model can be used to segment the underground space entity to form the corresponding volume element. then the specific spatial interpolation method and visualization algorithm can be used for analysis. different volume element segmentation methods and data models can be used according to research objectives, such as the three-dimensional grid, octree, and tetrahedron models. this paper uses a three-dimensional grid model to process the physical and chemical attribute parameter data modeling of urban underground space geological bodies (gao et al., 2018). the 3d grid model uses a set of regular-size 3d grids (cubes) to segment the space which will be simulated and establish a real-time association with attribute information. in the three-dimensional grid data model, the threedimensional grid is the basic unit to store the voiceless attribute information inside the geological body, and it is assumed that the function value of the physical and chemical attribute data field is distributed on the eight vertices of the grid, that is, the function value at the vertex (xi,yj,zl)is f. (xi,yj,zl) therefore, the volume data in the storage and grid can be defined as a sampling function in a discrete bounded three-dimensional space, and the sampling is structured by structure; that is, the volume data is regular structured, and the sampling is uniform in three directions of space, which can be expressed as: f x y z x x x x x x x y y y y y y i i i i i i( , , ) , ,..., ( ) , ,..., ( � � � � � � � � � � 1 2 1 1 2 1 �� � � � � � � � � � � � � � � � � � x z z z z z z xi i i ) , ,..., ( )1 2 1 (1) with this kind of 3d grid data model, the analysis of spatial attribute data in the geological body can be attributed to the analysis of attribute data among 3d grids, as the visualization technology can also be applied to the 3d visualization of data field represented by the 3d grid data model to analyze the spatial distribution of an attribute value (singh et al., 2017). in this paper, when using the 3d grid model to represent spatial attribute data, each vertex of the 3d grid stores two attribute data (u,v), where u represents the stratum number of the grid vertex, and v represents the physical and chemical attributes at the location of the grid vertex. in this way, the set of vertices with the same stratum number can represent the spatial position and shape of the corresponding stratum. this model can represent not only geological bodies and their attributes with complex morphology and topological structure but also analyze spatial attributes under the constraints of geological body set structure, providing constraints on spatial set structure for direct volume rendering of spatial attributes (gao, 2018). 2.2. feature-level fusion extraction method of geological anomaly based on multi-attribute in order to analyze the related problems of the underground geological abnormal body, it is necessary to introduce a cause of that body. therefore, this paper takes the earthquake as the cause of the abnormal body. based on the underground geological data extracted from the 3d grid data model of urban underground spatial attribute information modeling and visualization constructed in section 2.1, it adopts the method of multi-attribute data fusion to extract the abnormal body, to improve the characteristics of abnormal geological bodies in underground space and integrate them to achieve highprecision abnormal body detection (sheth et al., 2018). because of the complexity of the data, it is often unable to reflect all the features and details of the geological structure from a single geological attribute, so it is essential to interpret the geological structure with different geological attributes. one of the characteristics of geological data is that the amount of data is vast, tens or even hundreds of g. if it is fused directly, it will produce a great deal of computation, which ordinary computer systems cannot process. therefore, this paper proposes a feature-level fusion extraction algorithm based on a multi-attribute geological anomaly body, roughly divided into two steps: 257a detection method of urban underground geological anomalies in the united kingdom based on feature fusion the feature extraction of multi-attribute data and the anomaly body fusion of multi-attribute data (shrestha et al., 2017). 2.2.1. feature extraction of multi-attribute data of abnormal body based on human-computer interaction researchers are usually more interested in geological structures. when dealing with multiple geological attribute data, they only need to extract their feature points for research. in this paper, the feature extraction method of multigeological attributes is based on human-computer interaction, which requires users to observe and analyze each geological data, select the range of geological attribute values of interest, and extract the scattered points of geological anomalies within the range. this paper introduces the feature extraction of abnormal bodies based on multi-attribute human-computer interaction data. firstly, open the geological texture attribute data and visualize it. secondly, using “karst cave” as an example, human-computer interaction observation data finds the “karst cave” structure. thirdly, contrast the color table to find its attribute range. finally, extract the abnormal-scattered points within the range. figure 1 is the flow diagram of the process: start visual geological data human computer interaction to confirm the attribute value range of the target color comparison table confirmation attribute range outlier scatter within the extraction range end figure 1. the extraction process of geological attribute characteristics 2.2.2. feature fusion of multi-attribute data based on fuzzy logic after the feature extraction of multiple geological attribute data, these features need to be fused to get the scattered point data of the geological anomaly body. the data of the integrated abnormal body contains the characteristics of multiple geological attributes, which reflect the different geological characteristics of the abnormal body according to the user’s emphasis on different attributes. the multi-attribute geological anomaly fusion extraction method in this paper is based on fuzzy logic reasoning. it requires users to set the importance of different geological attribute data according to their objectives to get the ideal effect of anomaly fusion extraction. the fuzzy set was put forward by l. a. zadeh, an american cybernetic expert, in 1965. since the birth of fuzzy set theory, the rapid development of the fuzzy logic system has played a role in pattern recognition, signal processing, health care, and other fields, providing great help for related research. common fuzzy inference systems are divided into three categories: pure fuzzy inference system, mamdani type fuzzy inference system, and takagi sugeno type fuzzy inference system. this paper is based on t-s fuzzy inference. the reasoning process of t-s type fuzzy includes the fuzzy operation, and its output is an exact value. this is determined by the form of t-s fuzzy reasoning itself (cotterill et al., 2017). outlier feature fusion is divided into four steps based on fuzzy logic: initialization, setting parameter list, and reading outlier feature scatter. spatial scatter registration. feature scatter fusion. and binary operation. the flow chart is shown in figure 2. in order to solve the problem of the feature fusion and extraction of abnormal geological bodies, the analytic hierarchy process (ahp) based on fuzzy reasoning is used to fuse the features of multiple geological attribute data. the factor set for the abnormal geological body is “0”. the “1” values on each grid point of each geological attribute data and the value of each grid point obtained after the feature data scatter fusion operation is the final comment set. the focus of this paper is how to extract the geological abnormal body feature scatter point data from each different geological attribute. the factor set is the comment set formed by the scattered points that belong to the target after the fusion. due to the scattered points of geological anomalies have the following characteristics, the value of each grid point is either “0” (the point is the background point) or “1” (the point is the scattered point of geological anomalies), the authors use the fuzzy reasoning algorithm to process the scattered point data of geological anomalies. at this point, adjustments and simplifications to the fusion algorithm were necessary to adapt to the actual seismic interpretation. in short, multi-geological attribute data featurefusion aims to merge multi groups of 3d grid data into a new group and binary these values. the threshold rule must be formulated according to the characteristics of geological data to meet the integration requirements of our subjective interpretation experience combined with the actual geological data (olson et al., 2017). start set the extract parameter list of the exception body fusion read a scatter of anomalous body features with multiple geological attributes three point registration in space the fuzzy logic parameters are weighted and fused end determine if it is greater than the threshold set to the exception body point binary scatter data, abnormal body point is "1", otherwise is "0" y n figure 2. abnormal body feature fusion based on fuzzy logic 2.2.3. abnormal volume data filling and surface smoothing because of the noise interference and the influence of the accuracy of the geological acquisition data, the abnormal geological bodies obtained by the proposed method are often not satisfactory enough. there are many burrs and some “holes” inside the abnormal geological bodies, which are not continuous. then, some treatments are necessary to fill the holes and smooth the surface. based on this purpose, this paper suggests a new algorithm flow for filling and smoothing the scattered point data of abnormal bodies suitable for geological data interpretation, which has had a good effect on the surface smoothing of geological structures. figure 3 is the flow of filling and smoothing the data of three-dimensional geological abnormal bodies. firstly, a binary morphological operation is carried out to fill the small holes in the abnormal body and remove the redundant data points on the 258 xuemei liu boundary of the abnormal body for the scattered point data after the previous step of multi-geological attribute feature fusion. secondly, the gaussian filtering method is used to smooth the scattered point data of the abnormal geological body. finally, the moving cube algorithm is used to find the surface of the isosurface as a three-dimensional geological anomaly body. start scattered data of abnormal body after fusion morphological image technique is used to fill holes in three points of abnormal body gaussian filter smoothing operation the isosurface is used to extract the surface of living geological anomalies end visualization of geological anomalies figure 3. flow chart of abnormal body data filling and surface smoothing in the process of morphological processing, this paper uses the expansion and corrosion algorithm. the corrosion algorithm can be summarized into three steps: 1. taking the cube of size as the structural element and scanning the scattered point data of each spatial geological anomaly successively; 2. the structural elements are fused with the scattered data of the abnormal body in the covered area; 3. if all of them are valid outlier data (represented by “1”), then this point in the result is valid. otherwise, it is not a valid outlier scatter (represented by “0”). this operation reduces the scattered point boundary of the abnormal body by one circle and removes the small and meaningless objects inside the target. the expansion operation algorithm used in this paper includes the following three steps: 1. like the corrosion operation, the cube is used as the structural element to scan the scattered point data of each spatial geological anomaly body in turn 2. make “or” calculation on scattered points of structural elements and geological abnormal body data in the covered area; 3. if there are invalid outliers (represented by “0”), this point in the result is not regarded as an outlier. otherwise, it is an outlier (represented by “1”). the result of this operation is that the small “holes” inside the abnormal geological body are filled to make it more complete. 3. results britain is the first country in the world to develop and utilize underground space on a large scale comprehensively. in 1845, charles pearson, an englishman, first proposed the “underground train” concept. eighteen years later, london built the world’s first subway. with the development of the subway, underground shopping malls, commercial streets, apartments, and parking lots have emerged in succession. at the same time, due to the cold climate in britain, cellar construction is relatively standard. for example, most of the houses in central london have basements. with the increase in the land price, the development of underground space is more common, which causes many practical problems, such as the impact of underground space excavation on the safety of adjacent buildings. for this reason, the method in this paper is used to detect the underground geological anomalies in london and sheffield. 3.1. feature extraction results the esda-gis-based feature extraction method of complex karst surface subsidence area and multi-source information integration-based drilling geological feature parameter estimation and prediction as the comparison method were tested several times to extract and compare the comprehensive characteristics of underground space in london and sheffield, uk. the comparison results are shown in figures 4 and 5. 0.75 0.8 0.85 0.9 0.95 1 1 2 3 4 5 6 7 o ve ra ll test times article method feature extraction method of complex karst ground subsidence area based on esda-gis estimation and prediction of drilling geological parameters based on multi-source information integration figure 4. comprehensive comparison results of extraction of geological characteristics of underground space in london, uk. 0 0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 o ve ra l test times article method feature extraction method of complex karst ground subsidence area based on esda-gis estimation and prediction of drilling geological parameters based on multi-source information integration figure 5. comprehensive comparison results of extraction of geological characteristics of underground space in sheffield, uk. it can be seen from the above figure that there are some differences in the extraction of geological characteristics of underground space between london and sheffield. in this method, the total coefficient of extracting geological characteristics of underground space in london and sheffield, uk, is always the largest, and both are as high as 0.98. the total coefficient of extracting characteristics of complex karst surface subsidence area supported by esdagis and the estimation and prediction of geological parameters of drilling based on the integration of multi-source information are always lower than that 259a detection method of urban underground geological anomalies in the united kingdom based on feature fusion of this method. when extracting the geological characteristics of underground space in london and sheffield, the scope of extraction is more comprehensive, which provides a reasonable basis for the follow-up study. 3.2. feature fusion results in order, the regional geophysical and geochemical information featurelevel fusion method, the fusion method based on multi-band wavelet transform, and the method in this paper are used to fuse the underground space geological features of london and sheffield in the united kingdom as explained in the 3.1 section. the fusion effect of the three methods is analyzed, and the fusion degree is taken as the comparison index. the comparison results are shown in figures 6 and 7. 97.56 97.25 98.56 97.88 98.88 98.88 98.88 85.32 85.68 85.21 85.45 85.64 85.58 85.69 75.69 75.87 75.35 75.69 75.87 75.35 74.99 0 20 40 60 80 100 120 1 2 3 4 5 6 7 fu sio n de gr ee /% test times article method feature level fusion method of regional geophysical and geochemical information fusion method based on multi band wavelet transform figure 6. fusion effects of geological characteristics of underground space in london, uk 98.56 98.88 98.56 98.44 98.88 98.88 98.88 84.57 84.57 84.57 84.57 84.98 84.57 84.57 75.01 75.01 75.05 75.01 75.01 75.05 75.05 0 20 40 60 80 100 120 1 2 3 4 5 6 7 fu si on d eg re e /% test times article method feature level fusion method of regional geophysical and geochemical information fusion method based on multi band wavelet transform figure 7. integration effects of underground geological features in sheffield, uk it can be seen from the above figure that the highest fusion degree of the method in this paper is 98.88% and 98.88% for the geological characteristics of underground space in london and sheffield, uk. the highest fusion degree of the regional geophysical and geochemical information feature-level fusion method for the geological characteristics of underground space in london and sheffield is 85.69% and 84.98%, respectively. 3.3. abnormal body detection results based on the detection accuracy of abnormal bodies and the surface smoothness of the detection results, the detection effect of this method on the underground space geological abnormal bodies in london and sheffield is analyzed. the results are shown in figures 8 and 9. 97.4 97.6 97.8 98 98.2 98.4 98.6 98.8 99 99.2 1 2 3 4 5 6 7 d et ec tio n ac cu ra cy /% number of tests/times london sheffield figure 8. detection accuracies of underground geological anomalies in london and sheffield in this paper 97.4 97.6 97.8 98 98.2 98.4 98.6 98.8 99 99.2 1 2 3 4 5 6 7 d et ec tio n ac cu ra cy /% number of tests/times london sheffield figure 9. in this paper, the surface smoothness of geological anomalies detected in underground space in london and sheffield was studied according to figures 8 and 9, the minimum detection accuracy of the method in this paper is not less than 97% for the geological anomalies in london and sheffield, and the surface smoothness of the method in this paper is higher than 97%. therefore, this paper’s method can detect high-precision geological abnormal bodies in urban underground space, and the surface of the abnormal body is smooth and complete. 260 xuemei liu 3.4. application efficiency test the detection of geological abnormal body in underground space needs to be realized in a short time to realize disaster prevention in natural disasters such as earthquakes. therefore, the application efficiency of the method in the above experiments is tested, and the results are shown in table 1. table 1. application efficiency test results of the method in this paper (unit: ms) test times the data collection feature extraction characteristics of the fusion abnormal body detection 1 105.25 101.51 139.54 128.58 2 105.24 101.23 139.58 128.69 3 105.25 101.22 139.52 128.45 4 105.24 101.51 139.58 128.25 5 105.25 101.23 139.52 128.45 6 105.25 101.22 139.58 128.25 7 105.25 101.22 139.52 128.45 the mean 105.25 101.31 139.55 128.45 it can be seen from the data in the table that the average time consumption of this method in underground space geological data collection, feature extraction, feature fusion, and abnormal physical examination is 105.25 ms, 101.31 ms, 139.55 ms, and 128.45 ms. the overall application efficiency is (105.25 ms+101.31 ms+139.55 ms+128.45 ms) / 4=150.75 ms. it takes less time and can meet the needs of underground geological anomaly detection. 4. discussions britain is the birthplace of the modern administrative licensing legal system. its relatively complete planning and management system for the development and utilization of underground space is of reference significance for china to establish and improve the planning and management system of underground space. based on the above analysis of the relevant legal system and underground engineering planning and management in the united kingdom, the experiences can be mainly summarized as follows: (1) considering that the development and utilization of underground space is a gradual process involving many fields and related departments, the construction of relevant laws and regulations is often scattered in the relevant laws and regulations rather than forming comprehensive legislation at the national level at the beginning. britain is the first country to carry out the practice of underground space development and utilization, but it has not formed a comprehensive law of underground space. on the one hand, this is related to the local differences in the development and utilization of underground space, i.e., the areas with more underground space development are still a few urban areas. on the other hand, the purpose of underground space development is mainly based on various specific requirements such as real estate profits, city scale, and climate reasons. the central or local government will issue various special laws, standards, rules, and other changes according to the situation. it is fast and can guide practice better. in 2011, the british parliament passed the underground development and utilization bill, which reflects the demand for comprehensive legislation with the increase of underground space development and utilization. however, the relevant provisions and practical experience of various national and local laws and regulations in the united kingdom in the past 150 years have undoubtedly laid a solid foundation for formulating comprehensive legislation. (2) considering the different types of underground engineering, it is advisable to classify and carry out professional management. for underground space development projects of different natures, the project scale is often quite different (for example, the difference between the investment and construction scales of a common family basement and large-scale infrastructure has several orders of magnitude), and the technical requirements of the project (such as traffic assessment and construction safety) are the key to approval. therefore, different underground space projects should develop different planning and management procedures. in the uk, underground engineering is divided into general engineering, major infrastructure, and underground transportation facilities, and different review and approval subjects, procedures, and elements are established. local governments or planning departments have further made detailed regulations suitable for the region. it is worth noting that the planning and management cycle of major infrastructure is relatively long, it takes 28 days for the review team to examine the application, and then it takes another three months for the review team to conduct the review and the relevant ministers of state to make a decision, which shows that the review decision of primary infrastructure underground should be cautious, even at the cost of time. this idea of classified management is worth learning (pang et al., 2018). the latest regulations of shanghai on planning and construction of underground space issued in 2014 also made similar provisions, divided into “key areas” and “other areas,” and distinguished in planning formulation, planning content, scheme design, license management, and other aspects. (3) considering the closed nature of underground engineering, limited by geological and hydrological conditions, complex and unpredictable underground conditions, and the interaction between the above-ground and underground development and utilization, the preliminary investigation, research, and evaluation of underground space development and utilization is essential (pang et al., 2020). in the planning and management of underground space, great importance is attached to the investigation and evaluation work in the early stage, mainly reflected in the requirements of application elements. even for general underground engineering, the application materials shall include all kinds of research and evaluation reports on existing buildings, structural stability, geological conditions, surrounding influence, traffic influence, environmental influence, hydrogeological influence, safety, and sudden danger (wang et al., 2019). (4) in consideration of the impact of underground works on surrounding owners and the public, the planning and management process of underground works should pay attention to the publicity of schemes and relevant information, the solicitation of opinions from relevant owners, and the maintenance of public interests (xu et al., 2020). although the ownership of the general underground space development in the uk belongs to the land owner, the owners’ interests around it must be maintained, and unnecessary disputes should be avoided. therefore, in the program design process, the owners’ consent within 9 meters should be obtained in the preliminary preparation stage, and the owners within 50 meters should be ensured in the review and construction stage. for major infrastructure construction, according to the planning and compulsory purchase act 2004 and the planning act 2008, the british public sector or the government can compulsorily purchase private underground space according to the public needs and compensate according to the market value at that time. therefore, the national essential underground large-scale infrastructure facilities are often owned by the state; or their use rights enjoy their respective parts according to the agreement of public-private cooperation to ensure the public interest. however, in this process, the opinions and attitudes of the media, the public, and community organizations can still significantly affect the project’s approval. 5. conclusion in this paper, authors proposed a method of detecting the geological anomalies of urban underground space based on feature-fusion in the united kingdom, tested the geological anomalies of urban underground space in london and sheffield in england, and drew the following conclusions: 1. compared with the same method, the overall coefficient of the method is the largest in london and sheffield, and it is as high as 0.98; 2. the highest fusion degree of underground space geological characteristics of london and sheffield is 98.88% and 98.88%, respectively; 3. the minimum detection accuracy of geological anomalies in london and sheffield is not less than 97%. the surface smoothness of geological anomalies in london and sheffield is higher than 97%; 4. this method’s overall average application time is only 150.75 ms, which can achieve high-precision and high-efficiency detection of geological anomalies in underground space. with the continuous development of modern geoscience, the technicians engaged in geological experiments and tests are also faced with more challenges. the material composition of the objects to be analyzed will be gradually complicated. geologists should attach great importance to the extraction of 261a detection method of urban underground geological anomalies in the united kingdom based on feature fusion abnormal information and innovate analytical chemistry theory by applying physics, geological chemistry, and other multi-disciplinary knowledge. it also takes this as a breakthrough point, integrating the geological experiment testing resources to innovate more operable geological experiment testing technologies and methods and improve the level of geological experiment testing. acknowledgment hebei social science foundation project: a study on the evaluation system of english language ability of students majoring in preschool education in higher vocational education under the systematic paradigm of working process (project no. hy17jy045). references bai, c. y., he, l. y., li, x. w., & sun, j. y. 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(2024). chemical constraints and tectonic setting of the ginebra ophiolite complex. earth sciences research journal, 28(1), 3-15. https://doi.org/10.15446/esrj.v28n1.110499 the occurrence and types of ophiolites in the central cordillera of the colombian andes have not been identified in terms of their tectonic settings and geochemical features. therefore, the study of the tectonic setting and subduction zones in the ophiolites’ magmatic evolution becomes a significant issue. the ginebra ophiolitic complex (goc) is located on the western flank of the central cordillera in the valle del cauca department, southwest of colombia, assigned to the lower cretaceous period, as indicated by the contact of the goc with the buga batholith. the goc is formed by three main lithologies: amphibolite, gabbro, and ultramafic rocks (pyroxenite and peridotite), according to ossa (2007). the rocks in the puente piedra section, located in the northeast of ginebra municipality, are part of the gabbroic cumulates within the ophiolitic sequence. this sequence comprises interspersed layers of gabbroic cumulates, gabbrodiorites, and diorites; interpreted as a result of crystal accumulation and fractional crystallization processes. the mafic rocks of the goc are sub-alkaline and correspond to the low potassium (k2o wt% 0.03-0.06) tholeiitic series. geochemically, they have sio2 wt% ranging from 49 to 61 and an aluminum oxide saturation index of approximately ~0.4 and ~0.8, indicating a metaluminous type. the geochemistry of the studied rocks from puente piedra section indicates that the goc formed through fractional crystallization and accumulation processes from a single magma source. cluster analysis, used to compare the geochemistry of goc rocks and the amaime complex basalts, suggests a similar magma source, possibly linked to multiple recharge events that underwent fractional crystallization, melt extraction, and accumulation processes. the geochemical parameters are indicative of a suprasubduction zone ophiolite, characterized by a low potassium tholeiitic series affinity, tio2 values typically <1.2 wt%, th enrichment typical of subduction zones, high pb content, and low values of ti, y, yb, ta, nb, zr, and hf. keywords: ophiolites; ginebra ophiolitic complex; tectonic setting; suprasubduction zone. caracterización geoquímica y evolución tectónica del complejo ofiolítico de ginebra resumen la ocurrencia y los tipos de de ofiolitas en la cordillera central de los andes colombianos no se han definido claramente en términos de configuración tectónica y características geoquímicas. por lo tanto, el estudio del entorno tectónico y de las zonas de subducción en la evolución magmática de las ofiolitas se convierte en un tema significativo. el complejo ofiolítico de ginebra (cog) se encuentra en el flanco occidental de la cordillera central en el departamento del valle del cauca, suroeste de colombia, asignado al período cretácico inferior, según lo indicado por el contacto del cog con el batolito de buga. el cog está formado por tres litologías principales: anfibolita, gabro y rocas ultramáficas (piroxenita y peridotita), de acuerdo con ossa (2007). las rocas en la sección de puente piedra, ubicada al noreste del municipio de ginebra, son parte de los cumulados gabroicos dentro de la secuencia ofiolítica. esta secuencia comprende capas intercaladas de cumulados gabroicos, gabrodioritas y dioritas; interpretadas como resultado de procesos de acumulación de cristales y cristalización fraccionada. las rocas máficas del cog son subalcalinas y corresponden a la serie toleítica baja en potasio (k2o wt% 0,03-0,06%). geoquímicamente, tienen sio2 wt% que varía de 49 a 61 y un índice de saturación de óxido de aluminio de aproximadamente ~0.4 y ~0.8, indicando un tipo metaluminoso. la geoquímica de las rocas estudiadas de la sección de puente piedra indica que el cog se formó a través de procesos de cristalización fraccionada y acumulación a partir de una única fuente de magma. el análisis conjunto de los datos geoquímicos del cog y del complejo amaime sugiere una misma fuente de magma, posiblemente vinculada a múltiples eventos de recarga que experimentaron procesos de cristalización fraccionada, extracción de fundido y acumulación. los parámetros geoquímicos son indicativos de una ofiolita de zona suprasubducción, caracterizada por una afinidad de serie toleítica baja en potasio, valores de tio2 típicamente <1.2 wt%, enriquecimiento de th típico de zonas de subducción, contenido alto de pb y valores bajos de ti, y, yb, ta, nb, zr y hf. palabras clave: ofiolitas; complejo ofiolítico de ginebra; configuración tectónica; zona de suprasubducción. https://doi.org/10.15446/esrj.v28n1.110499 mailto:derochag@unal.edu.co mailto:jcmolanom@unal.edu.co mailto:anmartinez@llu.edu https://doi.org/10.15446/esrj.v28n1.110499 4 duban esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa 1. introduction the term “ophiolites” has been used both with a genetic connotation and as a defining feature. it is employed to refer to remnants of ancient oceanic crust, and upper mantle rocks found along continental edges. these remnants consist of a range of rock suites, including felsic, mafic, and ultramafic rocks, which are temporally and spatially associated (dilek & furnes, 2014). as a concept, “ophiolite” rock has experienced a complex evolution, first appearing in europe in the early nineteenth century. the development of plate tectonics theory marked a turning point for this concept, enabling the beginning of an ophiolite model and facilitating comparisons between ophiolites and oceanic crust. at the first penrose conference in 1972, a close relationship between ophiolitic sequences and seafloor environments spreading was proposed. however, in 1973, miyashiro challenged this model and, based on geochemical interpretations, suggested that the troodos ophiolite (located in cyprus) was a product of magmatism within an island arc. the idea was revolutionary and led to a redefinition of ophiolites in suprasubduction zones during the early 1980s (dilek, 2003; dilek & furnes, 2014). the occurrence and types of ophiolites result from formation processes (tectonic, magmatic, and geochemical) and preservation during the emplacement. based on these criteria, dilek and furnes (2014) proposed that ophiolites would be classified as related and not related to subduction zones in first order. according to pearce (2014), no subduction-related ophiolites correspond to those that are formed in ocean ridges, plumes, and continental margins. subduction-related ophiolites include those which were developed in suprasubduction zones and volcanic arcs. ophiolites in suprasubduction zones represent the oceanic lithosphere formed in the upper plates extended to the subduction zones, with forearc, backarc, and nascent arc environment. forearc ophiolites expose compositional and geochemical variations defined by time: ocean-ridges similar composition (morb-like) in the oldest ones, to island-arc tholeiites (iat), and finally boninite-like in those more recent (dilek & polat, 2008; dilek & thy, 2009; ishizuka et al., 2014). magmatic and geochemical evolution in these kinds of rocks is controlled by (1) the partial melting of the crust under the subduction zone and (2) the dehydration melting of elements in a subducted plate under the overlying mantle (dilek & furnes, 2014). the occurrence and types of ophiolites in the central cordillera of the colombian andes have not been clearly defined. therefore, an analysis of ophiolite magmatic evolution in extensional environments and subduction zones has become increasingly relevant. some colombian ophiolitic clusters have been studied and characterized, such as the azules ultramafic complex, the pacora’s ophiolitic body, the bolivar-valle mafic-ultramafic complex, the aburra valley ophiolite, and the ginebra ophiolitic complex (espinosa, 1985; alvarez, 1995; nivia, 1994, correa et al., 2005; restrepo, 2008). the study of ophiolites in the region is necessary to understand colombia’s western lands formation and accretion processes and the geologic evolution of the south american western side during the cretaceous and cenozoic. despite current studies, a shared understanding of ophiolites petrogenesis has not been reached. furthermore, some variables have not been studied enough, such as pressure, temperature conditions, and crystallization age. the goc is a clear example. this study aims to present an evolutive model for the complex. the approach involves, firstly, the petrographic and geochemical characterization of layered gabbros from the ophiolitic sequence. secondly, the clustered interpretation with geochemical analyses from other levels within the ophiolite sequence of amaime complex and buga batholith, previously issued by ossa and concha (2007) and villagomez (2011). the study area is locally known as puente piedra, twelve kilometers in northeast of ginebra municipality, southeast of vereda regaderos (rural district), on the pathway going to costa rica village (figure 1). field research, petrographical and geochemical data suggest the mafic-ultramafic rocks in the goc have been formed in a subduction zone; then, these rocks are classified as ophiolites in a suprasubduction area. 2. geological setting the mafic-ultramafic rocks of the goc with oceanic-like features are located in the western flank of the colombian andes’ central cordillera, to the west of the san jeronimo fault (alvarez, 1983). the basement of the region has been reported as a sequence of cretaceous oceanic rocks (late mesozoic) (aspden & mccourt, 1986; mccourt et al., 1984), which represent an incomplete ophiolitic sequence (espinosa, 1985). nivia (1987) stated that a significant part of basic igneous rocks forming the basement in the region are basalts from an oceanic plateau, part of the western oceanic cretaceous lithospheric province. based on geochemical data, ossa and concha (2007) proposed that these rocks originated in an oceanic ridge environment and later emplaced along continent edges. figure 1. study area location. moreno (2017) recently conducted the petrography and geologic cartography of goc gabbros exposed in the ginebra municipality. this work is a continuation of the research developed by moreno, including, from west to east, the ginebra ophiolitic complex, buga batholith, and amaime complex (figure 2). porphyritic bodies with no-mapping dimensions are observed in the study area, but they represent different and younger geological events (rodriguez, 2012). underlying this sequence is reported the cenozoic la paila formation. figure 2. geological and structural cartography of the puente de piedra section region. modified from rodriguez (2012). 5chemical constraints and tectonic setting of the ginebra ophiolite complex amaime complex (ka). previous studies have used the term “amaime formation” (mccourt et al., 1984; nivia, 1987; nivia, 2001). however, according to the american stratigraphic code guidelines for denotation and terminology of lithodemic units, the authors will use amaime complex to reference to this volcanic complex. the amaime complex corresponds to a green-dark colored tholeiitic basaltic series, which is hyaline to holocrystalline, and exhibits a subophitic texture (mccourt et al., 1984). it consists of pillow lavas and minor sedimentary layers exposed in the western flank of the central cordillera, west of the cauca-almaguer fault (lopez, 2006; mccourt et al., 1984). locally, the amaime complex borders the west with the guabaspradera fault, and in contact with the goc and the buga batholith (figure 2). the age of the amaime complex can be established based on the relative temporal relationship of the basalts. during the formation of the basalts, accretion occurred at the continent edge; then, the basalts were intruded by the buga batholith (rodriguez, 2012). brook (1984) suggested a 94±4 ma for the buga batholith with a calculation based on the rb/sr method applied in biotite and hornblende. furthermore, villagomez et al. (2011) reported two zircon u-pb ages of approximately 90 ma for the same unit. following this, moreno (2017) proposed that the age of the amaime complex is older than 100 ma, while rodriguez (2012) suggested an origin in the late jurassic to early cretaceous. ginebra ophiolitic complex (kog). the goc was defined by espinosa (1985), as a block of mafic and ultramafic rocks. this block exhibits an enlarged geometry in a n-s direction, with an average length of 40 km and a width of 8 km. it is bounded by faults on both flanks. to the east, it is delimited by the guabas-pradera fault, where it comes into contact with the amaime complex. to the west, its delimited by the palmira-buga fault, making its contact with the miocene sedimentary units of the la paila formation (figure 2). three main groups of rocks form the goc: amphibolites, gabbros, and pyroxenites and peridotites. the amphibolites represent the most significant percentage of the goc; fine to medium-grain texture, made of hornblende, plagioclase, quartz, opaque minerals (especially ilmenite), and apatite and epidote in a lower proportion. the amphibolites originated from metamorphism of microgabbros and basalts. gabbros in the cumulus zone are layered, subalkaline, and they are part of the tholeiitic series low in potassium (nivia, 1987; ossa & concha, 2007). ossa and concha (2007) reported that the magmas responsible for the mafic and ultramafic rocks originated from a mid-oceanic ridge, specifically the n segment of the upper mantle (mor-n). in addition, ossa and concha proposed that these magmas, along with the amaime complex, appear to form an incomplete ophiolitic sequence which may be part of an older geologic basement. this sequence differs from the western oceanic cretaceous lithospheric province defined by nivia (1987). rodriguez (2012) suggested that, based on the contact relationships (mccourt et al., 1984) and calculated ages established for the buga batholith, which range from 90-96 ma (brito et al., 2010; villagomez et al., 2011), the minimum age of the goc is younger than the early cretaceous. on the other hand, nivia et al. (2017) proposed that the buga batholith represents an intrusive-syntectonic body within the goc. this idea is supported by 40ar-39ar ages for goc rocks, revealing a 140.28±3.12 ma age (in clinopyroxene) for a gabbronorite, interpreted as the crystallization age, and an age of 90.84±0.78 ma (in hornblende) for an amphibolite, interpreted as the reworking age of the goc due to the syntectonic crystallization of the buga batholith. buga batholith (kcd-1). the buga batholith as described by aspden et al. in 1987, is a calc-alkaline granitoid in the buga, san pedro, and tulua municipalities. this batholith exhibits a composition ranging from hornblendic to tonalitic quartz diorite, with hornblendic diorite present at its contacts, particularly along the western boundary where the batholith is in contact with metabasalts of the goc (nivia, 2001). the buga batholith intrudes the goc in the northern region, forming a roof pendant structure. this is further supported by the presence of veins and dikes in contact areas where the batholith intrudes the tholeiites of the amaime complex, suggesting a clear intrusive relationship with the amaime complex. additionally, a significant area of the contact between these two units correspond to a faulted contact, and the western boundary of the buga batholith is defined by the guabas-pradera fault (figure 2) (nivia, 2001). the age of this batholith has been a subject of various interpretations based on different dating methods. touissant and restrepo (1978) reported a k/ar age of 113±10 ma for hornblende; brook (1984) obtained a rb/sr age of 99±4ma in biotite; brito et al. (2010) calculated a u-pb (shrimp) age 96.79 ma. in zircon; and villagomez et al. (2011) found a range of u-pb zircon ages from 90.6±1.3 to 92.1±0.8 ma. nivia et al. (2017) suggests the existence of two plutons of different ages in the mapped area known as the buga batholith, with average ages of 88±1.64 ma and 69±1.41 ma using the u-pb method on zircon. as a result, the areas where samples of 69 ma age were collected were separated as an independent unit. porphyritic bodies. in 2012, rodriguez presented the first assessment of the porphyritic bodies within the region and classified these as dacitic porphyries. these bodies intrude buga batholith and goc and ranging in width from two to ten meters. however, they are not mappable on a 1:25000 scale. the age of these porphyritic bodies has been determined through u-pb la-icp-ms dating yielding an age range between 70.61 and 65.4 ma. (brito et al., 2010). la paila formation (tmp). this formation initially proposed by keizer in 1954 is characterized as a conglomerate body with interbedded dacitic tuff layers. subsequently, nelson in 1957 divided the formation into two distinct units. the lower unit spans approximately 200 m and is primarily composed of dacitic tuffs, while the upper unit consists of a clastic sequence, predominantly conglomeratic in nature. the thickness of the sequence varies from 400 to 600 m. this unit exhibit a faulted contact with the goc in the east, and it is partially covered by alluvial and colluvial deposits towards the west (rodriguez, 2012) (figure 2). the age of the la paila formation has been a subject of debate among researchers. according to palynological data provided by van der hammen in 1958 and schwin in 1969, the formation is believed to be of miocene age. however, mccourt, in 1984, suggested that this unit might be older, likely dating back to the oligocene period. mccourt’s perspective also links it partially to the cauca group and amaga formation. 3. methodology field observations in the puente piedra section, located in ginebra municipality, valle del cauca (figure 1), a lithologic and detailed sampling as well as magnetic susceptibility measurements were conducted for this study. an sm30 zh susceptibility meter was employed to the measurements. petrography a total of eight polished thin sections were analyzed from the goc rocks, collected in the puente piedra area. analyses were performed with a primotech microscope (built-in cam), and each sample was classified following le bas & streckeisen (1991) based on a 300-500 points during point counting according to mineralogic variations and textural characteristics. the complete list of these polished thin sections, their locations, textural classifications, and mineralogy is documented in annex 1. whole rock geochemistry whole-rock analyses were conducted on seven gabbro rocks from the goc, including both major and trace elements. the analyses were carried out at the als chemex laboratory, located in medellin, colombia, utilizing coupled plasma mass spectrometry (icp-ms) and coupled plasma atomic emission spectroscopy (icp-aes). in annex 2, information of used standards and detection limits for every element can be found. the selected samples for geochemical analyses were homogenous, with no visible inclusions or evidence of hydrothermal alterations. additionally, this study included geochemical data published by ossa and concha (2007), who analyzed 19 samples from the goc and one from the amaime complex. furthermore, data reported by villagomez (2011), from three samples of the amaime complex and two samples from the buga batholith were used. the gcdkit 5.0 software (janoušek et al., 2006) was employed for the creation and visualization of geochemical diagrams. the plots generated included: alteration index, magmatic series characterization, geochemical classification, harker diagrams, and tectonic discriminant diagrams proposed by pearce (2008) and pearce (1982). sio2 was chosen as the normalizing element for both major and trace element variation diagrams. for rare earth elements (ree), plotting 6 duban esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa values of primitive mantle were selected according to mcdonough and sun (1995). results of normative mineral calculations (cipw norm) and analyses standardized to 100% (free of h2o+, h2o-, and co2) are listed in annex 3. 4. results field observations the studied segment in the ophiolitic sequence is composed of intermittent felsic and mafic layering with similar orders of magnitude to one another -from millimetric to 30 cm width (figure 3). the felsic layers exhibit equigranular textures with hypidiomorphic fine crystals (0.05-1 mm) composed of quartz, plagioclase, and hornblende. the mafic layers present mesocratic, equigranular-phaneritic textures, composed of fine to very-fine (<0.03-1 mm) crystals of plagioclase and pyroxene. the magnetic susceptibility shows a constant proportionality with the color index of the bands ranging from 2 to 8 for leucocratic bands and from 25 to 39 for mesocratic and melanocratic bands. figure 3. felsic and mafic interbedding at the gabbros band level of the goc ophiolitic sequence. values correspond to the magnetic susceptibility, which is higher in the mafic bands. petrography and rock compositions petrographic analyses confirms that the studied section of the goc is formed by isotropic gabbros, cumulate gabbros, gabbrodiorites, and diorites that were hydrothermally altered or deformed. modal analyses and compositional classifications are presented in figure 4. figure 4. qap diagram applied to gabbro rocks in the ginebra ophiolitic complex at the puente de piedra section isotropic gabbros the isotropic gabbros are holocrystalline, equigranular, fine to medium crystal size with anhedral to subhedral crystals, composed by plagioclase, clinopyroxene, orthopyroxene, hornblende, magnetite, and ilmenite. these rocks are moderately affected by hydrothermal alteration; the secondary minerals formed as result of hydrothermal alteration are hornblende, epidote, and chlorite (figure 5a). figure 5. petrographic observations of characterized rocks in the puente de piedra section. a) isotropic gabbro; b) cumulate gabbros; c) gabbrodiorite; d) diorite; e) hornblende and saussurite formed through hydrothermal alteration of pyroxenes and plagioclases, respectively; f) rutile vein (nw-se) cut by an epidote vein (w-e). plagioclase (plag), pyroxene (px), quartz (qzt), saussurite (sau), hornblende (hbl), epidote (ep), rutile (rt). cumulate gabbros these rocks exhibit an oriented texture characterized by both broken and thick crystals, surrounded by finely milled material. multiple twinnings in the plagioclase crystals appear deformed in flame-like shapes. the cumulates are formed dominantly by plagioclase, orthopyroxene, clinopyroxene, and magnetite. when observed using mesoscopic and microscopic techniques cumulate textures are identified, marked by variations in the proportions of minerals (plagioclase/pyroxene), changes in crystal size, and differences in magnetite content. gabbrodiorites the gabbrodiorites exhibit mineral fabrics, holocrystalline and equigranular textures with fine to very fine size anhedral crystals. these rocks are primarily composed by plagioclase and clinopyroxene with smaller amounts of quartz subgrains formed during a deformation stage (figure 5c). diorites these rocks exhibit holocrystalline inequigranular texture with fine anhedral crystals formed by plagioclase, clinopyroxene, and quartz. the quartz crystals show mosaic texture due to deformation (figure 5d). in some layers, abundance of pyroxene and ilmenite is observed. there are zones characterized by intense hydrothermal alteration with hornblende and chlorite modifying the pyroxene crystals (main clinopyroxene) and saussurite scattered over plagioclase crystals (figure 5e). some clusters of small veins were identified and characterized as follows: (1) amphibole (with magnetite or ilmenite); 7chemical constraints and tectonic setting of the ginebra ophiolite complex (2) epidote-rutile; (3) chlorite; (4) amphibole and epidote (with ilmenite and magnetite); and (5) zoisite-clinozoisite-epidote (figure 5f). geochemical data the rocks in the studied section are part of a tholeiitic series with a high concentration of sio2, varying from 52 to 62 wt% (figure 6), and an alumina saturation index ranging from ~0.4 to ~0,8. these values indicate that the rocks from the goc, the amaime complex, and buga batholith correspond to metaluminous igneous rocks (figure 7). the differentiation of the three compositional groups within the ophiolitic sequence, gabbros, gabbrodiorites, and diorites (as shown in figure 8), is based on the sio2 vs. alkali ratio in the rocks. figure 6. k2o vs. sio2 chart suggested by peccerillo and taylor (1976). these conventions apply to all of the charts listed below. figure 7. a/nk vs. a/cnk chart suggested by shand (1943) to determine al2o3 saturation. the variation in major oxides with respect to the sio2 content in the analyzed rocks reveals two different trends: (a) a positive collinear trend for tio2 and na2o, and a negative trend for k2o, al2o3, feot, and cao in the puente piedra section. (b) conversely, gabbros collected from la honda (ginebra municipality) and el diamante (buga’s municipality) locations exhibits a linear with a steep vertical slope concerning the major oxides in relation to sio2 (ranging from 46 to 55 wt%), with a wide range of values for k2o and feot, and a similar range of values for mgo, cao, al2o3, and tio2 when compared to the samples from the puente piedra (figure 9). figure 8. tas chart for geochemical classification of rocks from the goc, complejo amaime and buga batholith. figure 9. harker vs. sio2 variation diagram for major oxides for the goc, the amaime complex, and the buga batholith. a: accumulation, cf: fractioned crystallization. regarding tio2, the gabbro samples from the goc generally exhibit concentrations of less than 1.2 wt%, except for two samples analyzed by ossa and concha (2007). a similar range is observed in the results obtained by villagomez (2011) and those of ossa and concha (2007) for the basaltic rocks in the amaime complex (figure 10). in analyzing the behavior of trace elements in the goc (figure 11) concerning their variation relative to sio2 content, two primary trends were observed, consistent with the patterns described for major oxides. within the puente piedra section, rocks display a linear increase with a positive slope for elements such as lanthanum, cerium, yttrium, and zirconium, while showing 8 duban esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa a slight negative slope for barium and strontium. in contrast, rocks from la honda and el diamante, as reported by ossa and concha in 2007, exhibit a vertical trend over a narrow sio2 content range. figure 10. histogram displaying the weight percentage of tio2 (wt%) content at two locations: a) goc and b) amaime complex. figure 11. harker vs. sio2 variation diagram for trace elements of rocks from the goc, the amaime complex, and the buga batholith. a: accumulation, cf: fractioned crystallization. in general, the rocks from the goc, amaime complex, and buga batholith exhibit enriched concentrations of large-ion lithophile elements (lile: cs, rb, ba, th, k, sr, and pb) and slightly depleted concentrations of high-field strength elements (hfse: nb, ta, hf, zr, ti) with respect to normal mid-ocean ridge basalts (n-morb) (figure 12). it is important to mention the similarity in patterns between the gabbros from the goc, the basalts from amaime complex, and the granitic rocks from the buga batholith. figure 12. normalized trace elements with respect to n-morb for the goc, amaime complex, and buga batholith. considering the behavior of the rare-earth elements (ree) as depicted in the multi-elemental diagram (figure 13), distinct trends for each lithologic unit can be identified. rocks from the amaime complex display a flattened-shaped pattern, enriched in a four-factor ratio concerning the the primitive mantle. rocks from the buga batholith show a negative slope, indicating a relative depletion of heavy rare-earth elements (hree). finally, the goc rocks show a flattened-shaped pattern but are depleted in light rare-earth elements (lree). within this unit, the more differentiated lithologies, such as gabbrodiorites and diorites, are more enriched in rare-earth elements than the less differentiated lithologies, suchs as gabbros. additionally, the gabbros of the goc display moderate positive eu anomalies, while the gabbrodiorites and diorites present moderate-to-heavy negative eu anomalies. figure 13. normalized ree elements with respect to the primitive crust of rocks from the goc, amaime complex, and buga batholith. 5. discussion the field study observations and the petrography and geochemistry results have allowed authors to characterize the puente piedra sections rocks as part of the cumulate gabbros level in the ophiolitic sequence with interbedding of gabbros, gabbrodiorites, and diorites upon fractional crystallization,deformation, and hydrothermal alterations. the goc rocks have a sub-alkaline character. they are part of the tholeiitic sequence with a relative enrichment of feot with respect to mgo; and display a tholeiitic trend low in potassium. when examining the geochemical behavior of major and trace elements (figures 9 and 11) for the goc, two 9chemical constraints and tectonic setting of the ginebra ophiolite complex distinct processes governing crystallization become evident. firstly, in the puente piedra section the formation of gabbro was influenced by a fractional crystallization process dominated by a single magma source. the dispersion of data for k2o and na2o is attributed to the geochemical alterations in the analyzed rocks. the trend observed in al2o3, feot, mgo, and cao indicates a typical behavior, as these elements participate in the fractional crystallization of olivine, orthopyroxene, clinopyroxene, plagioclase, and hornblende. conversely, in la honda and el diamante locations crystallization appears to have been controlled by extraction and accumulation processes, as evident from the wide range of values found in cao (2.39-14.65 wt%), feot (5.34-20.25 wt%), mgo (3.74-30.93 wt%), na2o (0.25-4.74 wt%), la (0.50-5.20 wt ppm), ce (0.5012.80 wt ppm), sr (6.60-229.20 wt ppm), y (2.20-41.50 wt ppm), and zr (1.8086.00) for restricted values of sio2 (46.33-55.27 wt%). the geochemistry of the puente piedra section, as well as that of la honda and el diamante locations, displays a chemical signature consistent with the the upper mantle (n-morb), as reported by ossa and concha in 2007. certain, distinctive characteristics allow us to suggest that the rocks from the goc were formed in a tectonic setting associated with subduction rather than an oceanic ridge, challenging previous models. the tio2 content in the goc samples fall within the range of 0.06-2.8 wt% with a mode of 0.9 wt%, while in the amaime complex it varies within the range of 0.35 to 0.93 wt%, with a mode close to 0.84 wt%. these tio2 content levels are consistent with those observed in basalts from modern subduction zones such as the mariana and tonga subduction zones (metcalf & shervais, 2008). the content of trace elements in rocks and minerals is influenced by the tectonic setting and conditions controlling magma generation (pearce, 2014). rocks formed in island arcs, often exhibit selective enrichment in certain elements, particularly the lile elements derived from the subducted slab (strontium, potassium, rubidium, barium, th ± ce ± sm ± p); and depletion of hfse (tantalum, niobium, hafnium, zirconium, titanium, yttrium, ytterbium) as a result of the hydrated melting conditions (metcalf & shervais, 2008; pearce et al., 1984). these enrichment and depletion features were observed in the goc, the amaime complex, and in the buga batholith (figure 12). notably, these patterns exhibit remarkable similarities across the three geologic units. the similarities are significant, particularly because the buga batholith has been classified as part of an island arc based on its calc-alkaline affinity and characteristic magmatic signature from the mantle within the suprasubduction zone, as documented by villagomez (2010) and nivia et al. (2019). based on the geochemical analyses of the cumulate gabbros at the puente piedra section and the data obtained from the amaime complex and the goc, it can be concluded that these units share a genetic relationship and collectively represent an ophiolitic sequence and a potential suprasubduction tectonic environment during its formation. this sequence is distinct from the western cretaceous-oceanic lithospheric province, as reported by ossa and concha in 2007. tectonic differentiation basaltic rocks from various ophiolitic sequences around the world have played a significant role in distinguishing the formation environments of these sequences, as their geochemistry is indicative of the tectonic processes involved (pearce, 2008). thus, by examining the geochemical data compiled in the studies by ossa & concha (2007) and villagomez (2011), the following observations supported the model that the goc is a suprasubduction zone ophiolite type. the tectonic discrimination was conducted using the criteria established by yang et al. (2014) and dilek & furnes (2014), which, in turn, rely on the geochemical characteristics outlined by pearce (2008). the nb/yb vs. th/ yb diagrams are used to compare incompatible elements and can tectonically discriminate mafic rocks (figure 14). the nb/yb vs. th/yb diagram shows interactions between magma and the continental crust in two ways: by indicating either crustal contamination or the presence of a subduction component (pearce & peate, 1995). in the diagram, morb and ocean island basalt (oib) materials form a diagonal shape with average values for n-morb, e-morb, and oib at the center. consequently, magmas interacting with the continental crust will exhibit higher th/yb values. some samples from the goc and the amaime complex exhibit thorium enrichment, which is consistent to the enriched values found in subduction zones (pearce, 2008; yang et al., 2014). tio2 values for the goc and amaime complex are within the average range, aligning with the statistical distribution of rocks formed in arc environments (metcalf & shervais, 2008). the behavior of trace elements in rocks from these units, characterized by lile enrichment, non-enrichment, and even depletion of hfse and ree compared to n-morb, aligns with the pattern of rocks formed in suprasubduction environments, such as the trinity ophiolite and troodos ophiolite (pearce et al., 1984; metcalf and shervais, 2008). figure 14. tectonic th-nb differentiation chart applied for the rocks from the goc, the amaime complex, and the buga batholith. modified from pearce (2008). suprasubduction basalts are geochemically characterized by low amounts of ti, y, yb, ta, nb, zr, and hf. these trace elements exhibit values that are either lower or closer to the average n-morb values because no enrichment of these trace elements occur in subduction zones. this characteristic would be a consequence of hydrated conditions during partial melting, causing the melting of a refractory crust, increasing the degree of partial melting, or stabilizing minor oxides in the residual molten phase (pearce et al., 1984). the described patterns were observed in tholeiitic rocks from the amaime complex (figure 15), and the behavior and values of these rocks observed in the n-morb normalized plot were similar to the south sandwich basalts, described as tholeiitic basalts from a suprasubduction zone (ozcan et al., 2020). figure 15. multi-elements n-morb normalized chart applied to geochemical data from the amaime complex published by villagomez (2011). 10 duban esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa basalts from the suprasubduction zone exhibit lower yb in comparison to morb basalts. this observation is evident in the cr-y diagram (figure 16), where the basalts from the mariana trench fall within the zones of island arc tholeiitic basalts (iat) on the right and, as suggested by pearce et al. (1984), on the left, representing the analogous elements for suprasubduction ophiolites. notably, two basalt samples from the amaime complex share similarities with the iat zones, while all three samples show resemblances to rocks from the mariana trench. figure 16. cr-y differentiation chart by pearce (1982) deploying the separation of the oceanic ridge basalts from the island arc tholeiitic rocks (iat) and applied to basaltic rocks from the amaime complex. (pearce, 1984) the authors wish to to emphasize the importance of including geochemistry and single mineral chemistry in future goc research studies for data analysis and tectonic environment discrimination, following the methodology proposed by gervila et al. 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esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa annex 1 information on thin-sections analyzed id coordinates* classification mineralogy east north deg190423aa 1097436 906969 diorite pl, cpx, qz, ep, czo, zrn, spn, rt, amp, mgt deg190423ab 1097436 906969 gabbrodiorite pl, cpx, mgt, amp, ilm, py, ccp, ep deg190423ac 1097436 906969 diorite pl, cpx, qz, czo, ilm, ep, amp, opx, py, ilm, hem, spn deg190423ba 1097370 906975 gabbrodiorite pl, cpx, amp, opx, ep, chl, mgt, ilm, py, hem, zrn deg190423bb 1097370 906975 gabbrodiorite pl, cpx, ilm, qz, mgt, ep, opx deg190424ab 1097687 906597 gabbro pl, amp, cpx, ep, mgt, ser deg190424ac 1097687 906597 gabbro cpx, pl, amp, zo, czo, ilm, chl *datum-origen: magna-bogota oeste amp: amphibole; ccp: chalcopyrite; chl: chlorite group; cpx: clinopyroxene; czo: clinozoisite, ep: epidote, hem: hematite; ilm: ilmenite; mgt: magnetite, opx: orthopyroxene; pl: plagioclase; py: pyrite; qz: quartz; rt: rutile; ser: sericite; spn: sphene, zrn: zircon. annex 2 calibrations standards for the icp-aes and icp-ms analysis and detection limits for every analyzed element method analyzed elements (detection limits) standards (elements) unities icp-aes al2o3 (0,01-100), bao (0,01-100), cao (0,01-100), cro3 (0,01-100), fe2o3 (0,01-100), k2o (0,01-100), mgo (0,01100), mno (0,01-100), na2o (0,01-100), p2o5 (0,01-100), sio2 (0,01-100), sro (0,01-100), tio2 (0,01-100), loi (0,01-100). ncs 14015b, ncsdc71301, ncsdc73303, sarm12, oreas 24b (al2o3, bao, cao, cro3, fe2o3, k2o, mgo, mno, na2o, p2o5, sio2, sro, tio2, loi) percentage (%) icp-ms ba (0,5-10000), ce (0,1-10000), cr (10-10000), cs (0,0110000), dy (0,05-1000), er (0,03-1000), eu (0,03-1000), ga (0,1-1000), ge (5-1000), gd (0,05-1000), hf (0,210000), ho (0,01-10000), la (0,1-10000), lu (0,01-1000), nb (0,2-2500), nd (0,1-10000), pr (0,03-1000), rb (0,210000), sm (0,03-10000), sn (1-10000), sr (0,1-10000), ta (0,1-2500), tb (0,01-1000), th (0,05-1000), tm (0,11000), u (0,05-1000), v (5-10000), w (1-10000), y (0,110000), yb (0,03-1000), zr (2-10000). gre-3, oreas 24b (ba, ce, cr, cs, dy, er, eu, ga, gd, ge, hf, ho, la, lu, nb, nd, pr, rb, sm, sn, sr, ta, tb, th, tm, u, v, w, y, yb, zr) mrgeo08, gbm908-10 (ag, cd, co, cu, li, mo, ni, pb, sc, zn, as, bi, hg, in, re, sb, sc, se, te, tl) parts-per million (ppm) annex 3 table 1. total rock chemical analysis in major and trace elements of goc rocks in the puente de piedra section. oxide (%) deg190423aa deg190423ab deg190423ac deg190423ba deg190423bb deg190424ab deg190424ac sio2 57,29 58,98 60,99 52,98 54,09 49,56 50,34 al2o3 13,72 12,82 12,2 11,96 13,96 16,78 14,88 fe2o3 8 14,34 5,3 6,42 9,66 8,86 7,24 mgo 5,36 2,75 5,24 10,35 7,38 6,9 8,71 cao 12,05 8,17 12,2 15,85 12,7 14,8 15,25 na2o 3 3,17 3,11 2,04 3,11 2 1,9 13chemical constraints and tectonic setting of the ginebra ophiolite complex oxide (%) deg190423aa deg190423ab deg190423ac deg190423ba deg190423bb deg190424ab deg190424ac k2o 0,03 0,03 0,03 0,05 0,04 0,05 0,06 tio2 0,52 0,74 0,87 0,13 0,23 0,4 0,57 p2o5 <0.01 0,08 0,01 <0.01 0,01 0,01 <0.01 mno 0,15 0,14 0,13 0,15 0,13 0,15 0,14 cr2o3 0,01 0,01 <0.01 0,01 0,01 0,06 0,07 loi 0,82 -0,06 0,26 0,86 0,12 1,84 1,03 total 101,15 101,45 100,65 101,1 101,75 101,7 100,45 element (ppm) ba 10,3 12 15,5 16,2 31,2 13,2 11,7 co 24 29 21 30 32 26 27 cs <0.01 0,03 0,02 0,07 0,02 <0.01 0,03 ga 13 14,9 14 8 11,3 13,8 13,5 hf 0,6 1,3 1,4 0,3 0,4 0,8 0,9 nb 2,3 1,6 4,9 <0.2 7 0,7 1,3 rb 0,5 0,3 0,8 1,1 0,6 1,1 1,2 sn 1 1 1 1 1 1 1 sr 98,2 79,3 84,8 64,8 68,5 105 83,8 ta 0,1 0,2 0,2 0,1 0,5 0,1 0,2 th 0,2 0,17 0,36 0,08 0,32 0,21 0,15 u 0,09 0,06 0,11 <0.05 0,24 <0.05 <0.05 v 277 252 180 129 229 252 228 w <1 1 1 1 1 1 <1 zr 20 38 40 11 15 26 29 y 33,7 23,4 32,5 12,5 10,4 8,5 13,5 la 2 1,8 2,3 0,8 5 1,2 1,1 ce 8 4,8 8,9 2,5 10,3 2,6 2,7 pr 1,64 0,89 1,77 0,49 1,33 0,41 0,49 nd 9,2 4,7 10,6 2,9 5,5 2,2 2,5 sm 3,11 1,67 3,84 1,04 1,29 0,8 0,95 eu 0,89 0,65 0,95 0,38 0,4 0,34 0,41 gd 4,65 2,68 4,96 1,81 1,67 1,19 1,66 tb 0,78 0,53 0,86 0,31 0,25 0,25 0,3 dy 5,87 3,82 5,66 2,12 1,87 1,4 2,4 ho 1,15 0,81 1,19 0,43 0,35 0,28 0,45 er 3,96 2,88 3,7 1,37 1,16 1,02 1,35 tm 0,54 0,38 0,56 0,22 0,17 0,14 0,24 yb 3,9 2,87 3,65 1,28 1,25 1 1,5 14 duban esteban gomez gomez, ana maria martinez ardila, juan carlos molano mendozaa oxide (%) deg190423aa deg190423ab deg190423ac deg190423ba deg190423bb deg190424ab deg190424ac lu 0,56 0,47 0,56 0,2 0,19 0,13 0,18 mo 1 2 <1 1 1 1 1 cu 6 3 21 13 5 13 17 pb <2 3 <2 <2 <2 <2 <2 zn 27 42 21 30 27 38 33 ni 44 14 66 79 49 64 106 cr 120 30 10 110 50 440 550 as 0,2 0,1 0,2 0,2 0,1 <0.1 0,3 cd <0.5 <0.5 <0.5 <0.5 <0.5 0,5 <0.5 sb <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 bi <0.01 <0.01 0,07 <0.01 0,03 <0.01 0,01 ag <0.5 <0.5 <0.5 1,4 <0.5 <0.5 <0.5 hg <0.005 <0.005 <0.005 <0.005 <0.005 0,007 <0.005 tl <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 se 0,3 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 li <10 <10 <10 10 10 <10 <10 sc 42 38 35 63 52 39 43 in 0,009 0,015 0,005 0,005 <0.005 <0.005 <0.005 re <0.001 0,001 <0.001 <0.001 <0.001 <0.001 <0.001 sc 3,7 3,7 2,5 4,9 2,7 2,6 2,3 te 0,01 0,02 <0.01 0,01 0,01 0,01 <0.01 s 0,01 <0.01 0,02 0,01 0,01 0,01 0,01 c 0,03 0,13 0,03 0,06 0,04 0,17 0,05 ge <5 <5 <5 <5 <5 <5 <5 table 2. the cipw norm for the goc rocks minerals in the puente piedra section. % weight cipw norm deg190423aa deg190423ab deg190423ac deg190423ba deg190423bb deg190424ab deg190424ac quartz 13,68 23,21 17,76 3,48 6,06 3,67 2,86 zircon 0,01 0,01 anorthite 23,88 20,66 19,24 23,33 24,01 36,66 31,90 diopside 26,93 13,47 28,15 43,02 30,07 27,87 32,80 sphene 0,89 1,46 1,81 0,22 0,65 1,12 hypersthene 0,87 0,61 5,84 4,44 4,27 6,49 albite 25,39 26,82 26,32 17,26 26,32 16,92 16,08 orthoclase 0,18 0,18 0,18 0,30 0,24 0,30 0,35 wollastonite 1,05 apatite 0,02 0,19 0,02 0,02 0,02 0,02 0,02 pyrite 0,02 0,02 0,04 0,02 0,02 0,02 0,02 chromite 0,03 0,03 0,01 0,09 0,12 ilmenite 0,30 0,28 0,25 0,25 0,26 0,26 0,22 15chemical constraints and tectonic setting of the ginebra ophiolite complex % weight cipw norm deg190423aa deg190423ab deg190423ac deg190423ba deg190423bb deg190424ab deg190424ac magnetite 0,08 hematite 8,00 14,34 5,30 6,37 9,66 8,86 7,24 quartz 15,15 25,61 19,43 3,99 6,76 4,18 3,28 zircon 0,01 0,01 anorthite 25,40 21,89 20,21 25,66 25,72 40,02 35,13 diopside 24,55 12,23 25,35 40,55 27,62 26,08 30,96 sphene 0,75 1,22 1,50 0,19 0,56 0,97 hypersthene 0,79 0,55 5,52 4,09 4,01 6,15 albite 28,44 29,94 29,12 20,00 29,70 19,46 18,65 orthoclase 0,20 0,20 0,20 0,35 0,27 0,35 0,42 wollastonite 1,06 apatite 0,02 0,17 0,02 0,02 0,02 0,02 0,02 pyrite 0,01 0,01 0,02 0,01 0,01 0,01 0,01 chromite 0,02 0,02 0,01 0,05 0,07 ilmenite 0,18 0,17 0,15 0,16 0,16 0,16 0,14 magnetite 0,05 hematite 4,47 7,99 2,93 3,68 5,44 5,09 4,19 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.107251 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 403 414 h y d r o g eo lo g y abstract geoelectrical investigation to delineate potential aquifers in shahroud, iran ilnaz zarif mahdizadeh1, ali beitollahi2, sanaz zarif mahdizadeh3* 1. department of earth sciences, faculty of basic sciences, north tehran branch, islamic azad university, tehran, iran. 2. department of geophysics, head of seismology engineering and risk, road, housing and urban development research center, tehran, iran. 3. department of earth sciences, faculty of basic sciences, science and research branch, islamic azad university, tehran, iran. corresponding author email*: beitollahi@bhrc.ac.ir one of the world’s most pressing needs today is access to groundwater. although hydrological parameters can generally be estimated using underground studies, these methods are time-consuming and expensive. by taking the direct relationship of these parameters with electrical resistivity and measuring it with a cheaper and faster geoelectric method, a qualitative estimate of these parameters will be obtained. the electrical resistivity method is one of the most widely used methods to probe aquifers. the main objective of this paper is to show how, by interpreting and modeling the data of this exploratory method along with other geological information, the hydrogeological modeling of groundwater reservoirs can be done, and then, with an informed vision, the wells can be drilled for exploitation. for this purpose, the electrical resistivity data at 189 points and 9 profiles in the shahroud region with schlumberger array, after reviewing and making some modifications, were subjected to one-dimensional inverse modeling. according to the peripheral vision, the electrode distance was considered to be 50 meters and the length of the profiles is different. the longest profile was 1000 meter and related to profile 3. then, using the results of modeling along with other available information, the hydrogeological models of the area were prepared in the form of iso-resistivity contour map and interpreted in terms of resistivity and thickness of subsurface layer using computer software geographic information system (gis). similarly, the thickness map of the aquifer unit (s) was also prepared to classify the good and poor zones. using the prepared models and also considering the direction and piezometric of the groundwater flow inside the aquifer, suitable places were identified and suggested for future exploitation. keywords: electrical resistivity method; inversion; groundwater exploration; gis. investigación geoeléctrica para delinear potenciales acuíferos en shahroud, irán resumen una de las necesidades más apremiantes en el mundo es el acceso al agua subterránea. si bien algunos parámetros hidrológicos se pueden estimar generalmente con estudios subterráneos, estos métodos suelen ser costosos en tiempo y en dinero. una estimación cualitativa de estos parámetros se puede obtener al tomar la relación directa de estos parámetros con la resistividad eléctrica y medir con un método geoeléctrico más barato y más rápido. el método de resistividad eléctrica es uno de los más usados para hacer pruebas en acuíferos. el objetivo principal de este trabajo es mostrar cómo se puede hacer, por interpretación y modelado de la información de este método exploratorio junto con otra información geológica, el modelado de reservorios de agua subterránea y luego, con una visión informada, la perforación de pozos para su explotación. para este fin la información de resistividad eléctrica de 189 puntos y de 9 perfiles en la región de shahroud con la configuración de schlumberger, después de revisarla y hacer algunas modificaciones, fue objeto de un modelado unidimensional inverso. de acuerdo con la visión periférica, la distancia de los electrodos se consideró en 50 metros mientras que la longitud de los perfiles es diferente. el perfil 3 fue el más largo, con unos 1000 metros. luego, al combinar los resultados del modelado con otra información disponible se prepararon los modelos hidrogeológicos en la forma de un mapa de contornos de iso-resistividad y se interpretaron en términos de resistividad y espesor de la capa de la subsuperficie con el software sistema de información geográfica. de manera similar se preparó el mapa de espesor de las unidades acuíferas para clasificar las zonas más y las menos profundas. al usar los modelos preparados y también considerar la dirección y piezometría del flujo de las aguas subterráneas al interior del acuífero se identificaron los lugares más adecuados y se sugirieron para futuras explotaciones. palabras clave: método de resistividad eléctrica; inversión; exploración de aguas subterráneas; sistema de información geográfica. record manuscript received: 14/02/2023 accepted for publication: 28/01/2024 zarif mahdizadeh, i., beitollahi, a., & zarif mahdizadeh, s. (2023). geoelectrical investigation to delineate potential aquifers in shahroud, iran. earth sciences research journal, 27(4), 403-414. https://doi. org/10.15446/esrj.v27n4.107251 mailto:beitollahi@bhrc.ac.ir https://doi.org/10.15446/esrj.v27n4.107251 https://doi.org/10.15446/esrj.v27n4.107251 404 ilnaz zarif mahdizadeh, ali beitollahi, sanaz zarif mahdizadeh 1. introduction the method of electrical resistivity in hydrological issues dates back to the very beginning of the emergence of this science. new methods and various theoretical topics have been presented and finally today it has led to its comprehensive use, especially in hydrological issues (surinaidu and bacon, 2023). the first research on geoelectric methods was conducted in 1912 by schlumberger, and wenner separately but simultaneously (schlumberger, 1912; wanner, 1912). due to the wide range of resistivity relative to other geophysical parameters, the electrical resistivity technique is a versatile and cost-effective technique for groundwater study under different geological conditions. by determining the geological structures, and maintaining a basic relation between the hydrogeological and geophysical parameters of the aquifer environment, the electrical resistivity sounding models the groundwater reliably. (mozac et al., 1985; asfahani, 2007; soupios et al., 2007; lesmes and friedman, 2005; akhter and hasan, 2016). the effectiveness of electrical resistivity tomography (ert; nonintrusive geophysical technique) in determining the hydrogeological conditions at surface groundwater was investigated by koch et al. (2009). the vertical electrical sounding method is widely used in locating the low salinity groundwater in irrigation issues (arshad et al., 2007; sikandar et al., 2010), studying groundwater conditions such as depth, thickness, and aquifer boundaries (lashkaripour and nakhaei, 2005). mapping the intrusion of saltwater (abdul nasir et al., 2000), identifying saltwater/freshwater interface (el-waheidi et al., 1992; van dam and meulenkamp, 1967) and characterizing hydrogeological (al-fares, 2016) can be determined by electrical resistivity method. vertical electrical sounding technique is possible to utilize in groundwater investigation in the sedimentary area (mallam and ajayi, 2000; lateef, 2012), the interpretation a computer iteration technique (asokhia, 2000), planning, and management of groundwater resources (arunbose et al., 2021) and evaluating of the undergroundwater reservoir (al-khafaji, 2013; coper, 2012). geoelectric methods estimated hydraulic parameters in a porous aquifer environment (kazakis et al., 2016). fajana (2020) and alile et al. (2011) determined the potential of the aquifer by the resistivity method and calculated the porosity. the same studies were performed to delineate potential groundwater aquifers using electrical resistivity methods (oseji et al., 2006, 2005; anomohanran, 2013; hassan et al., 2017; emenike, 2001). geophysical studies to determine the potential of underground water in the shahroud region have been done with several methods, among which we can refer to the article from sharifi et al. (2014). this study used integrated results obtained from vertical electrical sounding (ves) and resistivity profiling. onedimensional (1-d) modeling and interpretation of the sounding results, using master curves and ix1d software, and two-dimensional (2-d) modeling and interpretation of the profiling results using res2dinv were made. as a result of the interpretation and combination of the results, karst water zones were found in the study area, and based on this, the best zones for access drilling and karst water extraction were included in the central and northern regions of the plain. it shows a good correlation with the results obtained in our paper. in 2015, remote sensing and geological investigations were implemented in a gis environment for the delineation of potential groundwater in shahroud areas. for this purpose, fuzzy evidential maps (lithology and intersection point density maps) were combined using the fuzzy ‘and’ operator. the evaluation of the presented karstic wp model was applied using geoelectrical resistivity surveys (sharifi et al., 2015). the estimation of underground water potential in different areas has been done using the method studied in this project, some of which are mentioned in the following. an electrical resistivity survey involving vertical electrical soundings (ves) by schlumberger array was applied to investigate groundwater conditions such as aquifer boundaries, depth, and thickness, which were estimated from geoelectrical section, and r.t map, and zones with high yield potential have been determined for choosing drilling sites (lashkaripour and nakhaei, 2005; tahmasbi nejad and lashkaripour, 2010). in guigou plain, the vertical electrical sounding (ves) technique was used to map the aquifer system. the main aims of this survey included the following: (i) to determine the geometry of the aquifer system; (ii) to determine the geological layers forming the aquifer; (iii) to estimate the depth of the impermeable bedrock. iso-resistivity, the marl–limestone bedrock and basalt thickness maps were generated to characterize this aquifer (el makrini et al., 2022). the purpose of this study is to evaluate the geoelectrical characteristics of the delineated aquifer and separate several subsurface regions of shahroud, in terms of physical characteristics, considering their electrical resistivity. finally, according to the studies conducted, the areas with a higher potential of groundwater suitable for drilling water wells were determined. in this research, at first, electrical resistivity data were interpreted using ipi2win software. also, diagrams and sections were drawn using geographic information system (gis) software with an interpolation algorithm of inverse distance weighting (idw), and the best points for drilling wells were estimated in the study area. the results could be used for further study of groundwater regimes in the study area and to improve the quality of groundwater resources and management. 2. the study area 2.1 geographical situation shahroud is one of the cities of semnan province in iran. this city is on the northern edge of the desert plain and the southern slopes of the alborz mountain range with a geographical position of 36°25’n latitude and 54°58’e longitude in zone 39 and 40 of utm coordinates. it is located at an altitude of 1380 meters above sea level in the northeast of semnan province. 2.2 geomorphological and geological study of the area alborz heights in the north of the province are the highest areas of the province and to the south, the lower lands are spread. it is conceivable that based on the elevation characteristics of the province, steep areas have been spread in the northern parts of the province. transport of surface sediments, river erosion, granulation, and sedimentation order according to its slope and direction are formed. also, the recharge of aquifers and their effect on the moisture of surface deposits is done according to the direction and width of waterways. map of main and secondary waterways of semnan province has been prepared in figure 1 based on the data from the national geosciences database (ngdir). figure 1. map of main and secondary waterways of semnan province. the distribution of waterways determines the distribution of fluvial sediments and floods, as well as the order of granulation, all of which influence the geoelectric characteristics. coarse-grained environments with high electrical resistance and fine-grained deposits with lower electrical resistance will also appear. moisture in deposits and salinity also causes a significant decrease in electrical resistance (darayan et al., 1998; yoon and park, 2001). a wide range of study areas is covered by younger and older alluvium. in the southern regions, soil debris and gravel are visible, and the fine-grained and clay alluviums are spread. in the northern regions of shahroud, older alluvium and alluvial fans are observed. in figure 2, the status of surface deposits and protrusions of geological formations in the studied areas are shown. 405geoelectrical investigation to delineate potential aquifers in shahroud, iran figure 2. status of surface deposits and protrusions of geological formations in the studied areas. 3. materials and methods 3.1. vertical electrical sounding (ves) basic principle and data acquisition the geoelectrical method, using the vertical electrical sounding (ves) technique, consists of measuring the variations in apparent resistivity (ρa) as a function of depth. in principle, the measurement protocol consists of injecting an electric current of intensity (i) through two current electrodes (a and b) and measuring the potential differences (δv) created between the two receiving electrodes (m and n), called potential electrodes (figure 3). a b figure 3. (a) general scheme of a soil-resistivity measurement using the schlumberger configuration with a four−electrode device (abmn), (b) bi−logarithmic diagram for the representation of ves measurements. (el makrini et al., 2022) according to ohm’s law, the apparent resistivity is a function of δv, i and the geometric coefficient (k). it is calculated by the following formula: = ∆𝑉𝑉 ∗ 𝐾𝐾 i (1) = 𝐾𝐾 2𝜋𝜋 1 + + +1 𝐴𝐴 𝑀𝑀 1 𝐴𝐴 𝑁𝑁 1 𝐵𝐵 𝑀𝑀 1 𝐵𝐵 𝑁𝑁 (2) the curve ρa = f (ab / 2) is obtained by plotting the apparent resistivity values ρa against ab/2 (half spacing of the current electrodes, which can reach up to 10 (km) in a bi-logarithmic scale (telford et al., 2012). the geoelectrical survey of shahroud area involved 189 vertical electrical soundings by schlumberger array and 9 profiles, considering the possibility of expanding the wires, which were determined up to 1000 meters. the main problem of geoelectric field surveys inside shahroud is the existence of urban buildings and facilities which made it difficult to extend the wires for deeper studies. determining the location of points for measuring took up a lot of time from the field team. figure 3(a) shows the location map of geophysical studies. electrode distances have been chosen as regularly as possible and 50 meters apart. however, it is observed that in some places, especially in crowded and dense areas, due to the impossibility of geoelectric withdrawals, the sounding distances are longer. the measurements will not be possible in places where it is not possible to connect the electrode to the ground and inject current. in shahroud’s studies and also in some places, it is not possible to inject current, and therefore, the arrangement of points is not in proper order. a b c figure 4. (a) shahroud profile map; (b) altitude map of the study area; (c) 3d elevation model with location of sounding points in figure 4(a), profiles from 1 to 9 are shown in different colors and the profiles are from left to right. in figure 4(b) the areas shown in blue have the lowest elevations, mostly in the south and east. the areas marked in white have the highest altitude and are located in the northern and northwestern parts. as mentioned in the geomorphology of the study area, the north of the province is the highest region of the province and in the south, the lowlands have expanded which figure 4(b) confirms. sloping areas are spread in the northern regions of the province. in figure 4(c), the location of sounding points was illustrated in the 3d elevation model. 3.2. data processing and resistivity interpretation the ves data acquired were subjected to a series of processing steps to facilitate their interpretation, summarized in four steps. in the first step, the ves diagrams were first smoothed to eliminate all outliers. they were then inverted using ipi2win software, which allows each diagram to be broken down into well-defined electrical levels in terms of thickness and resistivity. in the second step, the geoelectric levels of the ves models of each profile were correlated horizontally. consequently, four geoelectrical sections were drawn up to follow the evolution of the resistivity and the thickness of the formations crossed in both the vertical and lateral directions. then, the third step was to interpolate the ves data using the inverse distance weighting (idw) method. several maps were produced, including four iso-resistivity maps, two isoresistivity of the aquifer and bedrock maps, two isodepth of water table level and bedrock maps, an isothickness of the aquifer map. finally, based on the main geoelectrical characteristics extracted from the geoelectrical sections and r.t map, the fourth step concerned the elaboration of the groundwater prospectivity map. figure 5 shows the methodological flowchart applied in this work, described in the four steps above 406 ilnaz zarif mahdizadeh, ali beitollahi, sanaz zarif mahdizadeh geological data geophysical data ves ves models interpretatione.c map t.d.s map correlation idw interpolation geo-electrical sections interpretative maps qualitive quantitative r.t map iso-thicness map iso-depht map conclusion groundwater prospectivity map iso-resistivity map figure 5. the methodological flowchart in this study. 3.3 data inversion method the geoelectrical data inversion method in the geoelectrical studies of the studied area is known as the t method. in this method, a large number of layers (in this case 29 layers) with constant thickness are selected for each geoelectric sounding curve. it has been shown that this method is more stable for solving matrix equations in inverse geoelectrical functions. also, due to the use of a large number of layers, the error function takes significantly less values. another advantage of using the t method is that according to the selected fixed depth, it is possible to draw electrical resistivity maps for each depth horizon (dimitrev et al., 1998). 3.4 estimation of the relationship between the depth of exploration and the length of the current line geoelectrical-sounding curves show the values of apparent electrical resistance against different lengths of the current line (ab); as the distance between the current electrodes increases, the depth of penetration increases (evien, 1938). but without interpreting these curves, it is not possible to determine what specific depth the apparent electrical resistances taken are related to (barker, 1989; apparao and rao, 1974). the amount of penetration depth in each region and even each sounding differs from another sounding and depends on the thickness and electrical resistance of the layers. schlumberger (1932) and keller (1966) introduced the current penetration depth in their proposed array ab ⁄ 2. but bella (1932) and hiellan (1938) considered it equal to ab ⁄ 3. barker (1989) estimated ab ⁄ 4 as the penetration depth. ayres (1989) obtained a penetration depth of 60 meters for ab ⁄ 2 = 215 meters. apparao and rao (1973) calculated the current penetration depth for most of the current arrays and expressed this depth as ab ⁄ 4 for the schlumberger array. they also noted that the depth of current penetration in a specific area depends only on the distance between the current electrodes (ab), but the search volume depends on both the current electrode distance and the potential electrode distance. based on the calculations made for the studied area, this value follows the following relationship: depth = 0.5001 ab 0.998 2( ( (3) based on this relationship, the depth of exploration in our studied areas is about ab/4. in this study, maximum ab/2 of ves soundings is 464 meters and maximum penetration depth is 232 meters. 3.5 r.t map rt map (aquifer resistivity multiply by aquifer thickness) that shows the potential of the aquifer for water resources and quality in the whole study area. from a geophysical point of view, the electrical resistivity factor can play an important role in determining the exact location of the excavation site (lashkaripour and nakhaei, 2005). therefore, having the electrical resistivity of the aqueous layer (p) and considering its thickness (e), rt values are calculated according to the following formula. rt= p.e (4) in this regard, p in addition to the characteristics of the hydrous formation also depends on the concentration of solutes. thus, using the data obtained from geophysics and modeled layers and the obtained resistances for each model, finally, an r.t map was prepared for the study area. 3.6 geophysical cross section and pseudo-section maps: to determine the stratification of the subsoil in the area, the geoelectric sections must be examined. geoelectric sections after interpretation of sounding curves for each profile, and considering the true electrical resistance of the subsurface layers, topographic changes along with the profile, the trend of electrical resistance changes in depth, and field information has also been obtained. to observe changes in subsurface apparent electrical resistivity that reflect qualitative changes in subsurface structures, pseudo-section maps of apparent electrical resistivity were drawn using arc gis software. geoelectric pseudo-section maps show the apparent electrical resistivity distribution in a deep section along with a profile. 4. results the purpose of interpreting the data obtained from vertical electrical sounding in an area is to obtain an acceptable and logical geoelectric image or model that is consistent with the results of geological studies in that area. first, to investigate the quality of alluvial sediments of the plain and how the bedrock changes interpretation of qualitative form is done. in qualitative interpretation, the apparent electrical resistivity changes obtained from maps and related sections are evaluated. to get the real model of the earth, interpretation must be done quantitatively. in quantitative interpretation, raw data from electrical soundings are usually presented on a curve with logarithmic axes. in these axes, the value of ρa is a function of the length ab / 2 (half distance of current electrodes) is drawn. using logarithmic axis systems, it is easier to interpret the results with the help of standard curves or abacuses (reynolds, 1997). because in this study, the identification of alluvial thickness and depth of resistant bedrock was considered, therefore, the interpretation with details of different layers, and further separation of these layers is avoided. 4.1 sounding curves and error sections for schlumberger sounding curves, the apparent resistivity values were drawn against half current electrode space on a log-log graph and a smooth curve was plotted for each of the soundings. then, the sounding curves were interpreted by the well-known method of curve matching with the aid software ipi2win to determine the true resistivity and thicknesses of the subsurface layers. finally, the error sections are plotted and interpreted for the profiles (loke, 1999). due to the high volume of diagrams, only the error sections and ves curves of profile 5 are described and explained while the rest can be found in the appendix section. ρ, h, d are resistivity, thickness and depth of each layer, respectively. 407geoelectrical investigation to delineate potential aquifers in shahroud, iran profile 5 error section and sounding curves a b c d e f g h 408 ilnaz zarif mahdizadeh, ali beitollahi, sanaz zarif mahdizadeh i j k l m n figure 6. representative of the iterated ves curves showing the inverse models of the geoelectrical parameters for profile 5 (a) s168, (b) s119, (c) s109, (d) s110 (e) s89, (f) s134, (g) s84, (h) s103, (i) s102, (j) s75, (k) s145, (l) s8, (m) error section for sounding s168-s134, (n) error section for sounding s84-s8. 409geoelectrical investigation to delineate potential aquifers in shahroud, iran the information obtained from the above diagrams along with the lithological and geological information of the area can be seen in the table 1. 4.2 geophysical cross section and pseudo-section maps: in this paper, for the sake of brevity, only the results of processing and interpretation of the resistivity data collected along profile 5 are discussed and the maps of geophysical cross section and pseudo-sections of the other profiles are given in the appendix. as shown in the pseudo-section of figure 7(b), the highest apparent electrical resistivity in surface sections is attributed to the location of soundings 26, 28, 137, and 79, this is due to the presence of rocky structures or coarse and dry alluvium on the surface. in the deeper parts, due to the wetting and hydration of the alluvium and the small size of the sediments or the low water quality, their apparent electrical resistivity decreases, and with increasing depth of study, the apparent electrical resistivity increases again due to the impact of the resistant bedrock. sudden changes in apparent electrical resistivity between 30 and 28 soundings indicate a rupture of the formation or fault zone. in the middle and left of the profile, there are areas with lower electrical resistivity (areas under soundings 82, 97, and 32) which indicates sedimentary formation or water saturation zone in these areas. from figure 7, the distribution of fine-grained and coarse-grained zones in the surface horizons can be deduced. the heterogeneous sedimentation situation in shahroud sedimentary basin is also estimated at a glance. zones with a high electrical resistance that represent coarse-grained deposits can be seen in these figures. 4.3 isoresistivity maps in order to understand how to spread deep layers with the same electrical resistivity and determine the extent of expansion of zones with equal granulation, saturated or unsaturated as well as the bedrock of the area were designed isoresistivity maps for different depths. table 1. the ves correlation of the layer models obtained from the apparent resistivity and their lithology for profile 5. layer 1 2 3 4 geographical coordinates lithology surface alluvial sediments coarse and dry alluvial sediments course sand (aquifer layer) bed rock l oc at io n r es is tiv ity (ω .m ) t hi ck ne ss (m ) d ep th (m ) r es is tiv ity (ω .m ) t hi ck ne ss (m ) d ep th (m ) r es is tiv ity (ω .m ) t hi ck ne ss (m ) d ep th (m ) r es is tiv ity (ω .m ) d ep th (m ) l at itu de (x ) l on gi tu de (y ) a lti tu de ( z ) s168 107.5 3.3 3.3 228 74.4 77.7 177 174 251.1 102 350 316149 4029410 1306 s119 87.8 10.8 10.8 130 64.8 75.6 82.6 200 275.6 138 350 316590 4029733 1311 s109 72.52 8.02 8.02 72.5 54.2 62.22 308 187.8 250.02 308 350 317079 4030360 1320 s110 170.82 57.8 57.8 223 46 103.8 329 146 249.8 329 350 317605 4030659 1323 s89 185.36 39.3 39.3 183 41.1 80.4 494 141 221.4 224 350 318316 4030967 1319 s134 149.93 46.9 46.9 44.3 41.9 88.8 72.7 161.2 250 72.7 350 318573 4031450 1340 s84 387.66 19.1 19.1 181 101 120.1 148 130 250.1 148 350 318856 4031873 1339 s103 267.95 14.9 14.9 131 93 107.9 75.6 142 217.6 75.6 350 319699 4032541 1370 s102 298 7.65 7.65 287 71.5 79.15 811 170.8 250 811 350 319844 4033105 1357 s75 157.13 10 10 432 53.5 63.5 565 186.5 251.5 565 350 320223 4033189 1359 s145 155.62 57.4 57.4 100 58.9 116.3 131 134 250.3 131 350 320493 4033474 1359 s8 431 20.3 20.3 454 41.2 61.5 163 159 220.5 41.7 350 321194 4033971 1358 a b figure 7. profile 5 (a) geoelectric cross section map, (b) 5 pseudo-section map 410 ilnaz zarif mahdizadeh, ali beitollahi, sanaz zarif mahdizadeh for the preparation of these sections, the values of apparent electrical resistivity in relation to the effective penetration depth of the relevant current are considered and then the isoresistivity maps are plotted. in each map, you can see how the electrical resistivity changes in the relevant effective depth in the area. by comparing iso-resistivity maps for different ab lengths, we can qualitatively understand the trend of changes in resistivity with depth. to qualitatively study the alluvial sediments of the plain and how the bedrock changes, maps of iso-resistivity have been drawn in current transmitter electrodes 10, 100, 215, and 464 meters. a b c d figure 8. iso resistivity map for (a) ab/2=10 (m), (b) ab/2=100 (m), (c) ab/2=215 (m), (d) ab/2=464 (m) in figure 8(a), the central and southern regions have the lowest electrical resistivity of about 100 to 150 ohm-meters and separate the two high electrical resistivity sections northeast and southwest. in the center, low electrical resistivity zones can be tracked locally and separately, in these points, the resistivity reaches 110 ohm-meters. due to the depth of exploration in these maps (ab/2=10), the greatest effect on electrical resistivity is moisture and grain size of surface deposits. in figure 8 (b), the prevailing electrical resistivity at ab/2=100 meters in the area are about 100 to 200 ohm-meters. high electrical resistance bands can be seen in the northwest and southwest. the area to be considered at this depth is the low electrical resistivity zone that appears in the north of the area and has an electrical resistivity of about 50 to 100 ohm-meters. on ab/2= 215 meters, a low electrical resistivity zone in the north of the region is visible, which is expanding to the east of the region. the presence of this zone at greater depths can be due to the presence of water in these areas. elsewhere in this depth, the trend of changes in electrical resistivity follows the map of ab/2=100 meters. at ab/2= 464 meters, as in previous maps, the predominant electrical resistivity of the area is about 100 to 200 ohm-meters. at the border between the northwestern heights of the region and the adjacent plain, a high electrical resistance band of about 200 to 300 ohm-meters, which extends from the north and northwest to the southwest of the region. detection and fading of halos with low or high electrical resistance index at different depth horizons and in order from surface to depth, indicates granulation changes in dry to wet alluvial deposits of shahroud area. this sedimentary sequence is consistent with the location of shahroud at the foot of the heights and the presence of river and flood deposits. a low electrical resistance zone in the north of the region at ab/2= 464 meters has also been observed and has spread to the east of the region up to the range of soundings 1, 2, 3, 4, 5 and 6 which can be a sign of water in this region. comparing these four maps, it can be concluded that by increasing the length of the flow line as a result of increasing the depth of exploration, the apparent electrical resistivity decreases. also, the amount of electrical resistivity measured in the north of the plain is much higher than in its south, which indicates the difference in the geophysical properties of the northern and southern layers of the plain. this difference in the geophysical properties of the layers actually refers to the coarser alluvial granulation in the north of the plain, due to the presence of coarse-grained sediments and the effect of better water quality. also, the values of electrical resistivity in some areas in the map ab / 2 = 464(m) are slightly higher than the map ab / 2 = 215(m), this is because the electric current for the distance ab / 2 = 464(m) penetrated the bedrock which has a higher resistance than the aqueous layer, and the amount of apparent resistivity has increased. in addition, it can be concluded that the bedrock in the northwest of the region is higher than in other areas. 4.4 isoresistivity of the aquifer and bedrock maps isoresistivity map of the aquifer is suitable for aquifer transfer capability to determine areas with high discharge potential in the aquifer and the best location for drilling wells in the area. a b figure 9. (a) iso resistivity of the aquifer map, (b) iso resistivity of bedrock map 411geoelectrical investigation to delineate potential aquifers in shahroud, iran figure 9(a) shows the variation of the resistivity in the study area. low electrical resistivity indicates the presence of various salts in water. in the central and northern areas of the region, the electrical resistance of the aqueous layer is high and in the southeast, southwest, and northeast of the city, the electrical resistance is lower. according to figure 9(a), it can be concluded that the central and northern areas of shahroud have more suitable groundwater quality than its southern and northeastern areas. in figure 9(b), the bedrock electrical resistivity values obtained during the interpretation process have been used to draw an iso-resistivity map of the bedrock below the study area. the bedrock resistivity values range from 4.09 to 842 ohm.m. a major part of this layer is characterized by resistivity values between 4 and 156 ohm.m, which is representative of fractured and fluid-saturated bedrock and is mainly visible in the southern and eastern parts of the region. in places where the electrical resistance of the bedrock is higher, it is geologically more suitable for forming an aquifer layer, which can be seen in the northwestern and central areas of the region. 4.5 isodepth of water table level and bedrock maps to prepare these maps, the depth of the water table and bedrock in different parts of the region were measured from electrical resistivity method and then the obtained numbers on a map with a suitable scale were drawen and act similar to the height level maps in mapping. using the three components of latitude and longitude and depth of aquifer and bedrock in each sounding, maps of the depth of the resistant aquifer and bedrock with white to green color spectrum in order from low to high, respectively are drawn. a b figure 10. (a) isodepth of water table, (b) isodepth of bedrock map the variation of aquifer depth in the study area is shown in figure 10 (a). wherever the aquifer starts from a shallower depth, it is closer to the ground and is better for drilling wells. shallow aquifer depth values dominate in and around the northern and southern parts and reach about 84 meters. deeper aquifers are distributed at depths between 105 and 129 meters within the central, southeastern, and northwestern parts of the study. in figure 10(b), the depth of the bedrock in the north and northeast and the initial soundings of profiles 1, 2, 3, and 4 and the end of profile 9 have the maximum depth and in the northwestern and southwestern heights of the region, especially at the end of profiles 2, 4 and 8 has a minimum depth that can be seen with the yellow and green color spectrum. according to the results, the minimum depth of bedrock is 141-201 meters. bedrock at high depths had high electrical resistivity and low fracture. however, in some cases at great depths, the electrical resistance decreases, which can be due to the presence of hydrated cavities or alteration of the formation, this can be seen in the northeast and the central part of the map in soundings 156 and 164. in general, the deeper the bedrock starts, the thicker the aquifer layer, the bedrock has a relatively good depth in the whole area except the southwest and northwest. 4.6 isothickness of the aquifer map to prepare this map, the thickness of the alluvial layer at the location of each electric sounding is measured from the geoelectric sections. then, by inserting numbers in the location of each sounding, using the interpolation method, curves of equal thickness of the sediments are drawn in gis software. figure 11. iso-thickness of the aquifer map figure 11 shows the distribution of the aquifer thickness in the shahroud, where a minimum thickness of 73.91 m is observed in sounding 9, 14, 124, 28, 32, 162, 44 and the southwestern and the northeastern part of the study area appear to have the thickest aquifers, which range from 157.4 to 199.5. in general, the thicker the water layer, the better the place to drill the well, and it can be said that except for a few low-thickness areas that are scattered, the aquifer layer has a good thickness in the whole area. 4.7 r.t map the greater the thickness of the aqueous layer and its electrical resistivity, the greater r.t which leads to finding the best place to dig wells. as it is clear from figure 12, the rt value increases towards the north and the central part of the study area, which means the north and center of shahroud city, where there is high potential for development and operation. these areas have high hydraulic permeability. as expected, the amount of r.t in the southern and eastern regions decreases sharply. figure 12. r.t map 5. discussion subsurface water quality is measured by various factors obtained from hydrochemistry or water physics. based on the available information on the deep wells of shahrood and its surroundings, the electrical conductivity of water (e.c.) map and total dissolved solids (t.d.s) map in the water of the region, have been prepared. high electrical conductivity or low electrical resistance indicates the presence of various solutes in water. (ahmed et al., 2020). figure 13(a) shows the ec map of underground water in shahrood city. it can be seen that the electrical conductivity is high in the central, north, and northeast areas of the region, and it can be said that these areas have better underground water quality than the southwest and southeast areas. it is expected that t.d.s. map with e.c. map. to have a good match, because the amount of solution in water increases its electrical conductivity, and with the increase of t.d.s., electrical 412 ilnaz zarif mahdizadeh, ali beitollahi, sanaz zarif mahdizadeh conductivity also increases. this fact is proven by comparing figures 13(a) and (b) and the two prepared maps correspond quite noticeably. as can be seen in figure 13(b), the amount of tds in the central, north, and northeast areas of the region is around 600 mg/liter, and in the southeast and southwest of the city, it reaches 1300 mg/liter. therefore, according to carroll’s classification (carroll, 1962), the north, northeast and center of the city have fresh water, and the southwest and southeast have semi-saline water. quantities such as tds and ec are important in subsurface studies, based on which the electrical resistance factor changes in addition to the grain size effect. by considering the effect of these parameters, a more realistic estimate of the aquifer’s granularity can be obtained. (ahmed et al., 2020). a b figure 13. (a) e.c map of subsurface waters of the region; (b) t.d.s map of subsurface waters of the region the higher the electrical resistance of the aqueous layer, the higher the quality of the water and the amount of salts in it is less (corwinand yemoto, 2017; rusydi, 2018). according to figures 8 and 9 (a), it can be concluded that the central and northern areas of shahroud have more suitable groundwater quality than its southern and northeastern areas. moreover, wherever the aquifer starts from a shallower depth, it is closer to the ground and is better for drilling wells. according to figure 10(a), shallow aquifer depth values dominate in the northern, northeastern, southern and southwestern parts and reach about 84 meters. on the other hand, the thicker the water layer, the better the place to drill the well, and it can be said from fig. 11 that except for a few low-thickness areas that are scattered, the aquifer layer has a good thickness in the whole area especially in northern, southern and central of the area. the greater the thickness of the aqueous layer and its electrical resistivity, the greater r.t which leads to finding the best place to dig wells. as it is clear from figure 12, the r.t value increases towards the north and the central part of the study area, which means north and center of shahroud city where high potential for development and operation have. these areas have high hydraulic permeability. rt levels are lowest in the northeast, southern and southeastern. in places where the electrical resistance of the bedrock is higher, it is geologically more suitable for forming an aquifer layer, which can be seen from figure 11 (b) in the northern, northwestern and central areas of the region. while, the bedrock in the southern, southeastern and northeastern regions has a lower resistance. in general, the deeper the bedrock starts, the thicker the aquifer layer; according to figure 10(b), the bedrock has a relatively good depth in the whole area except the southwestern and northwestern. although in the southern and northeastern regions the depth of the aquifer is less and closer to the ground, the water quality is lower and the depth and resistivity of the bedrock and the amount of rt is the lowest. consequently, these areas cannot be suitable targets for groundwater exploitation. in the northwestern region, the thickness of the aquifer and the electrical resistivity of the bedrock is high, but due to the low electrical resistance of the aquifer in this area, it is recommended that wells not be drilled in these areas for drinking water because of the low-quality water and only used for agricultural purposes. 6. conclusion and suggestion given that today underground water flow modeling and pollution transmission is one of the most effective and the most important tools used in the management of water resources, evaluation of aquifer hydraulic variables to provide the information needed in groundwater modeling is of great importance. drilling exploratory wells to determine variables aquifer hydraulics is often costly and time-consuming. on the other hand, because in the calibration stage of modeling, the evaluated hydraulic variables can be corrected, therefore, estimating these variables using vertical electrical sounding (ves) information as an initial value can be useful. according to the explanations provided in the discussion section, the northern and central parts of the region have the best position for drilling wells. finally, 6 proposed groundwater exploration areas are suggested, as shown in figure 14. figure 14. map of proposed locations for drilling wells references abdul nassir, s. s., loke, m. h., lee, c. y., & nawawi, n. m. 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(1967). some results of the geoelectrical resistivity method in groundwater investigations in the netherlands. geophysical prospecting, 15, 92-115. wenner, f. (1912)the four-terminal conductor and the thomson bridge, u.s. bur. standards bull., 8, 559-610. yoon, g. l. & park, j. b. (2001). sensitivity of leachate and fine contents on electrical resistivity variations of sandy soils. journal of hazardous materials, 84(2–3), 147–161. _hlk151633648 _hlk151722074 _hlk151722896 _hlk151809551 _hlk151723504 _hlk151722925 _hlk151722949 _hlk151243866 _hlk151808879 _hlk151394512 _hlk151461396 _hlk151461449 _hlk151568322 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n1.118386 earth sciences research journal earth sci. res. j. vol. 29, no. 1 (march, 2025): 45 53 g eo m o r ph o lo g y transitional relationships among typical loess geomorphologic types: loess plateau of china shifang zhang1, shangmin zhao2* 1 school of traffic engineering, shanxi vocational university of engineering science and technology, jinzhong 030619, china; 2 department of surveying and mapping, college of geological and surveying engineering, taiyuan university of technology, taiyuan, 030024, china * email: zhaoshangmin@tyut.edu.cn how to cite item: zhang, s., & zhao, s. (2025). transitional relationships among typical loess geomorphologic types: loess plateau of china. earth sciences research journal, 29(1), 45-53. https://doi.org/10.15446/esrj. v29n1.118386 record manuscript received: 17/11/2024 accepted for publication: 10/02/2025 abst r ac t taking the distribution area of typical loess geomorphologic types in the loess plateau of china as the study area, this research aims to explore the transitional relationships among typical loess geomorphologic types based on their constructed digital index systems using the data sources of digital geomorphologic database, landsat tm/etm+ images and srtm1 dem data. based on landsat tm/etm+ images, the digital geomorphologic database was firstly checked and improved using visual interpretation combined with expert knowledge methods, so as to acquire the precise spatial distribution of the typical loess geomorphologic types. then, the digital index systems for the typical loess geomorphologic types were constructed, which included topographic indexes, quantitative indexes from topographic profile method and remote sensing indexes. finally, the transitional relationships among the typical loess geomorphologic types were explored based on the constructed digital index systems and the spatial distribution pattern on the remote sensing images. the research results show that loess tableland may transform to loess ridge or loess knoll, and loess ridge may transform to loess knoll. as to morphologic types, loess tableland may transform to loess residual tableland, and then to loess beam tableland; loess wide acclivitous ridge may transform to loess narrow acclivitous ridge, and then to loess knoll ridge. for the valley shape types of the loess ridge and loess knoll, the shallow and high valley may transform to deep and high valley, and then to shallow and low valley. this research is meaningful in the fields such as digital mapping of loess geomorphology, water and soil loss, soil erosion and digital topographic analysis. keywords: transitional relationship; digital index systems; typical loess geomorphologic type; digital geomorphologic database; landsat tm/etm+ images; srtm1 dem data; loess plateau palabras clave: relación transicional; sistemas de indexación digital; tipos geomorfológicos de loess; bases de datos geomorfológicas digitales; imágenes landsat tm/etm+; información srtm1 dem; meseta de loess relaciones de transición entre tipos geomorfológicos típicos de loess: meseta de loess de china este trabajo toma como objeto de estudio el área de distribución de los tipos geomorfológicos de loess típicos en la meseta de loess de china, con el objetivo de explorar las relaciones de transición entre los tipos geomorfológicos de loess típicos en función de sus sistemas de índices digitales a través de bases de datos geomorfológicos digitales, imágenes landsat tm/ etm+ y datos dem srtm1. con base en las imágenes landsat tm/etm+, la base de datos geomorfológica digital fue verificada y mejorada a través de interpretación visual combinada con métodos de expertos para adquirir la distribución espacial precisa de los tipos geomorfológicos típicos de loess. luego se construyeron los sistemas de indexación digital para esta clase de loess, los cuales incluyen índices topográficos, índices cuantitativos tomados de métodos de perfil topográfico e índices remotos. finalmente, se exploraron las relaciones transicionales entre los tipos gemorfológicos de loess con base en los sistemas de indexación digital construidos y los patrones de distribución espacial en las imágenes de detección remota. los resultados de la investigación muestran que los loess tipo meseta pueden transformarse en loess tipo crestas o montículos, y las crestas pueden transformarse en loess tipo montículos. en cuanto a los tipos morfológicos, los loess tipo meseta pueden transformarse en meseta residual y luego en meseta viga; los loess tipo cresta ancha e inclinada pueden transformarse en cresta estrecha e inclinada y luego en cresta de montículo. para los tipos de forma de valle de la cresta loess y montículo loess, el valle superficial y alto puede transformarse en valle profundo y alto, y luego en valle superficial y bajo. esta investigación es significativa en campos como el mapeo digital de la geomorfología del loess, la pérdida de agua y suelo, la erosión del suelo y el análisis topográfico digital. resu m en https://doi.org/10.15446/esrj.v29n1.118386 mailto:zhaoshangmin@tyut.edu.cn https://doi.org/10.15446/esrj.v29n1.118386 https://doi.org/10.15446/esrj.v29n1.118386 46 shifang zhang, shangmin zhao 1. introduction the popularization and advances of geographic information system (gis) and remote sensing technologies have mean that they were widely used in geomorphologic researches (remondo & oguchi, 2009). based on the digital elevation model (dem), data and digital topographic analysis, spatial distribution of volcanic landforms, complexity and distribution pattern of natural landform, distribution characteristics of aeolian sand dune and other geomorphologicrelated fields have been extensively studied (werner, 1999; kocurek & ewing, 2005; bishop, 2007a, 2007b, 2009; bullard et al., 2011). meanwhile, topographic indexes, such as slope, fluvial knick zone and shape, are extracted from dem data, whose spatial distribution and characteristics are analyzed specifically (saito & oguchi, 2005; hashimoto et al., 2008; lin et al., 2009; hayakawab & oguchi, 2009). furthermore, based on dem data and remote sensing images, geomorphologic objects are classified and identified from geomorphometric characteristics (székely & karátson, 2004; saadat et al., 2008; hampton & cole, 2009; dragut & eisank, 2011, 2012). in soil erosion region, geomorphologic research is meaningful in erosion evaluation and in soil and land-use mapping (remortel et al., 2004; saadat et al., 2008). soil erosion and water flow in a basin are modeled based on the topographic variants using dem datasets (zhu et al., 1997; pennock & corre, 2001). landform automatic or semi-automatic classification is widely studied based on derived parameters (elevation, slope, curvature and so on) from dem datasets (macmillan et al., 2000, 2004; shary et al., 2002; ballantine et al., 2005). loess plateau is one of the most famous soil erosion regions in china. heavy water and soil loss forms typical loess geomorphologic types in the world. unique morphologic features, abundant natural resources, dense population and serious soil erosion problems make loess plateau an important region to conduct geomorphologic research (tang et al., 2005), and most researches are focused on loess geomorphologic classification and identification based on slope spectrum index, which is a histogram statistics method based on slope variant (li et al., 2007; tang et al., 2008; wang et al., 2008; zhou et al., 2010). the adventure of the chinese geomorphologic database at 1:1 m scale provides spatial distribution of the geomorphologic types in vast region (cheng et al., 2011a). based on the database and shuttle radar topography mission at 3” spatial resolution (srtm3 dem) data, zhao & cheng (2014) attempted to build the transitional relationship among typical loess geomorphologic types using slope spectrum index in the whole loess plateau, which is helpful in understanding the relationship among the geomorphologic types. however, in the research by zhao & cheng (2014), the geomorphologic types are simple and coarse; meanwhile, the spatial distribution of the geomorphologic types is not accurate enough, which is at 1:1m scale; further, the criteria only chooses slope spectrum, so there are many other indexes to improve the achievements. choosing a representative distribution area of typical loess geomorphologic types in the loess plateau as the study area, this research aims to construct the digital index system based on the improved geomorphologic database, so as to explore the transitional relationships among the typical loess geomorphologic types specifically. this research is meaningful in automatic digital mapping and classification, understanding the loess geomorphology, soil erosion, and water and soil loss and so on. 2. study area and primary data sources 2.1 study area loess plateau distributed in the northern and middle part of china, which is the most widespread distribution region of the loess geomorphology (fig. 1). the typical loess geomorphology types are mainly loess tableland, loess ridge and loess knoll in the loess plateau (zhao et al., 2017). the distribution of the typical loess geomorphology types is shown in figure 1. loess plateau is a big region with widely distributed typical loess geomorphologic types. it is difficult to conduct deep research in this vast region, so we choose a test area which is representative in typical loess geomorphologic type distribution. the test area is the region in the black rectangle in figure 1. the topographic characteristic of the test area is shown in figure 2: figure 1. location and the typical loess geomorphology distribution in the loess plateau of china. figure 2. relief shader map in the test area. based on the srtm1 dem data, the relief shader map is produced using dem data in global mapper software. figure. 2 shows that the typical loess geomorphologic types mainly distribute in the fragmental areas with abundant incised valleys, and the elevation mainly ranges from about 500 m to 2000 m. the fragmental topography is due to soil erosion and water and soil loss, which also result in fragile ecology and environment. the soil erosion and water and soil loss are the main causes to form the typical topography and loess geomorphologic types, so there are many researches about topography and geomorphology in this region using digital topographic analysis methods (tang et al., 2005, 2008; zhao & cheng, 2014). hence, it is meaningful to acquire the digital index system for the typical loess geomorphologic types in this area. 2.2 primary data sources the primary data sources in this research are digital geomorphologic database, landsat tm/etm+ images and srtm1 dem data. 2.2.1 digital geomorphologic database digital geomorphologic database means chinese 1:1,000,000 geomorphologic database, which is used to provide the spatial distribution of the typical geomorphologic types in the study area. the database is acquired using visual interpretation method at about 1:250,000 scales. the main data sources are landsat tm/etm+ images, srtm3 dem data, geologic data, published geomorphologic maps and so on (cheng et al., 2011a). the classification method adopted morpho-genetic types, which can be divided 47transitional relationships among typical loess geomorphologic types: loess plateau of china into seven layers, they are, basic morphology and altitude, genesis, sub-genesis, morphology, micro-morphology, slope and aspect, material and lithology (cheng et al., 2011b). the digital database is seamless with consistent classification system, so it can be clipped for any regions with the same quality. in this research, the fundamental data for typical loess geomorphologic type distribution is acquired from this database. 2.2.2 landsat tm/etm+ images remote sensing images are used to revise and improve the digital geomorphologic database data; meanwhile, the remote sensing characteristics for the typical loess geomorphologic types are acquired based on the remote sensing images; finally, the transitional relationship among the typical loess geomorphologic types can be explored by referencing the spatial distribution pattern of them in the remote sensing images. the main remote sensing images select the landsat tm/etm + images, which are global false colour composite images and downloaded from chinese scientific data base. the acquisition periods of the landsat tm images are mainly about 1990; as to the landsat etm + images, about 2000. they are also the data sources in remote sensing visual interpretation process of the chinese 1:1,000,000 geomorphologic database data. the composited bands of the remote sensing images are 7, 4 and 2. the combined effect of the images is similar to natural colour, which provides convenience for the visual interpretation and revision of the typical loess geomorphologic types. 2.2.3 srtm1 dem data srtm1 dem data is downloaded from the earth explorer of the usgs, usa. it has spatial resolution of 1” and continuous surface. through downloading, merging, projection and clipping, the srtm1 dem data in the study area is shown in figure 2. the srtm1 dem data not only provides topographic characteristics for the study area, but the elevation index and other indexes computed based on the dem data. it is an important data sources in constructing the index system for the typical loess geomorphologic types. 3. methodology the quality and scale of the digital geomorphologic database are improved through different methods. based on the improved digital geomorphologic database, the spatial distribution of the typical loess geomorphology is acquired and illustrated accordingly. 3.1 improvement methods to the digital geomorphologic database the digital geomorphologic database adopted in this research is interpreted at about 1:250,000 scales. the spatial resolution of the images is about 30m, which provides feasibility to improve the database at higher scales. so the database is revised using the landsat tm/etm + images at about 1:50,000 scales. the improvement methods to the digital geomorphologic database include improving the boundary of the units, revising the mis-classifying units and choosing the typical units for the typical loess geomorphologic types. 3.1.1 improvement to the boundary at higher scale, the original boundary shows coarse and low quality. all the boundaries of the typical loess geomorphologic types are checked, and the boundary improvement is conducted especially for loess tableland. the change of the boundary can be shown in figure 3: figure 3 shows through boundary improvement, the boundary of the loess tableland becomes more accurate than before. the boundary represents the spatial distribution of the typical loess geomorphology, whose quality affects the construction of the digital index system. (a) (b) figure 3. boundary improvement to the units (a. before improvement; b. after improvement). 3.1.2 revising the mis-classifying units the digital geomorphologic database covers the whole china territory. it is not easy to guarantee the attributes of all the units are correct. so the attribute of every unit is checked carefully to guarantee the quality of the data. to the mis-classifying units, their attributed would be changed to right if the correct attributes can be determined; otherwise would be deleted on the occasion that the proper attributes are not clear or difficult to determine. the distribution of the typical loess geomorphologic types needs not to cover the entire study area. 3.1.3 choosing the typical units the revised digital geomorphologic database is checked firstly and the typical loess geomorphologic types are distilled. then, all the units for every loess geomorphologic type are compared and judged. the typical units are chosen to represent the spatial distribution of the typical loess geomorphologic types, which can guarantee the quality of the constructed digital index system. 3.2 spatial distribution of the typical loess geomorphology through improvement, the spatial distribution of the typical loess geomorphologic types acquired (figure 4). the classification method and distribution status for the typical loess geomorphologic types in the study area presented in table 1, which also gives the codes for the typical loess geomorphologic types. the meanings of the codes are consistent and will be omitted in the following text. 48 shifang zhang, shangmin zhao figure 4. typical loess geomorphologic distribution type in the test area. table 1 classification and distribution for the typical loess geomorphologic types typical loess geomorphology morphology valley shape type code patch number area(km2) depth height loess tableland loess tableland 11 29 254.9 residual loess residual tableland 12 8 1504.9 beam loess beam tableland 13 25 6172.0 loess type narrow acclivitous shallow low loess narrow acclivitous ridge with shallow and low valley 21 13 1408.0 narrow acclivitous shallow high loess narrow acclivitous ridge with shallow and high valley 22 7 1303.0 narrow acclivitous deep high loess narrow acclivitous ridge with deep and high valley 23 7 838.7 wide acclivitous shallow low loess wide acclivitous ridge with shallow and low valley 24 2 62.6 wide acclivitous shallow high loess wide acclivitous ridge with shallow and high valley 25 6 537.3 wide acclivitous deep high loess wide acclivitous ridge with deep and high valley 26 3 298.1 knoll type shallow low loess knoll ridge with shallow and low valley 27 12 2262.3 knoll type shallow high loess knoll ridge with shallow and high valley 28 47 11872.3 knoll type deep high loess knoll ridge with deep and high valley 29 8 3093.3 loess knoll shallow low loess knoll with shallow and low valley 31 10 1751.3 shallow high loess knoll with shallow and high valley 32 18 7176.9 deep high loess knoll with deep and high valley 33 7 5314.6 49transitional relationships among typical loess geomorphologic types: loess plateau of china figure 4 and table 1 show that the loess tableland mainly distributes in middle and southern parts; the loess knoll, middle and northern parts; the loess ridge, northern and western parts. the loess tableland is divided by morphology; the loess knoll, valley shape; the loess ridge, both the morphology and valley, which has the most sub-types. the types are systematic, but their distribution status has large difference. for example, the loess wide acclivitous ridge with shallow and low valley only has two units with the area of 62.6 km2; whereas the loess knoll ridge with shallow and high valley has 47 units with the area of 11872.3 km2. the difference is due to the geomorphologic distribution characteristics in the study area. 4. results digital index system is firstly constructed based on the digital geomorphologic database, and then the transitional relationships are achieved for the typical loess geomorphologic types. 4.1 digital index system for the typical loess geomorphologic types in this research, the digital index system for the typical loess geomorphologic types includes the topographic indexes, the quantitative indexes based on the topographic profile methods and the remote sensing indexes. 4.1.1 topographic indexes the topographic indexes are computed based on the srtm1 dem data, which include elevation, relief, slope and curvature indexes. 4.1.1.1 elevation index elevation is a basic topographic index, which can be acquired directly from the srtm1 dem data. the spatial distribution of the elevation index can be seen in figure 2. through making a numerical statistics for the elevation index of every typical loess geomorphologic type, the results are in table 2. table 2. numerical statistics for elevation index of typical loess geomorphologic types (m) code min max range mean std. 11 770 1414 644 1200.2 87.1 12 421 1408 987 936.7 128.4 13 516 1631 1115 1017.4 145.9 21 484 1509 1025 950.2 130.2 22 684 1684 1000 1182.0 155.4 23 609 1592 983 1117.2 202.8 24 587 1401 814 830.4 124.6 25 515 1345 830 839.7 145.3 26 779 1689 910 1430.0 126.5 27 784 1740 956 1181.7 175.9 28 532 1819 1287 1182.5 150.0 29 446 1598 1152 1153.8 115.3 31 840 1739 899 1226.5 180.3 32 610 1638 1028 1066.5 156.4 33 721 1553 832 1111.5 98.3 table 2 shows that the typical loess geomorphology types mainly distribute from 400 m to 1800 m; the mean elevation value ranges from about 800 m to 1400 m. the standard deviation value increases from 11 to 13, but decreases from 31 to 33 evidently; as to the loess ridge type, it increases from 21 to 23, decreases from 27 to 29, but undulates from 24 to 26. the morphology type of the typical loess geomorphology types changes at every three types (table 1), so we analyze the statistics characteristics of the topographic indexes at the group of every three types. 4.1.1.2 relief index other than elevation, the flat degree of the topography is a more important factor in determining the loess geomorphologic types. so the relief index is adopted here. based on srtm1 dem data, the relief index is computed using neighborhood statistics under the spatial analyst tools in the arcgis software. conducting a numerical statistics to the relief index, the results of the typical loess geomorphologic types are in table 3. table 3. numerical statistics of relief index for typical loess geomorphologic types (m) code min max range mean std. 11 6 202 196 91.9 39.6 12 9 280 271 127.1 35.2 13 10 273 263 133.3 26.6 21 23 310 287 119.6 28.5 22 26 292 266 125.8 34.6 23 49 276 227 143.0 26.7 24 46 299 253 117.8 25.7 25 18 300 282 108.9 30.4 26 36 262 226 148.6 27.9 27 26 288 262 144.6 25.0 28 13 276 263 124.2 32.2 29 24 264 240 113.8 29.8 31 11 293 282 127.9 27.0 32 17 287 270 119.4 26.2 33 14 254 240 113.2 25.3 table 3 shows that the relief range changes from about 200 m to 300 m. the characteristics of the mean value are similar, but not consistent with that of the standard deviation value of the elevation index. for example, from 11 to 13, the mean value increases, but the standard deviation value decreases. 4.1.1.3 slope index slope index can affect the rate of the movement down slope, which is an important index in reflecting the undulating degree of topography. it is applied in many studies, especially in the loess plateau region (wolinsky & pratson, 2005; bue & stepinski, 2006; zhou et al., 2010). the slope index is computed based on srtm1 dem data using 3d analyst tools in arcgis software. for every typical loess geomorphologic types in the study area, the numerical statistics results of slope index are in table 4. table 4. numerical statistics of slope index for typical loess geomorphologic types (°) code min max range mean std. 11 0.0 54.4 54.4 8.3 9.2 12 0.0 58.7 58.7 16.4 9.4 13 0.0 62.3 62.3 18.0 8.7 21 0.0 65.7 65.7 15.2 7.0 22 0.0 63.9 63.9 16.5 8.0 23 0.0 67.4 67.4 18.8 8.4 24 0.0 51.7 51.7 15.5 6.7 25 0.0 51.3 51.3 13.8 6.7 26 0.0 58.7 58.7 19.4 8.3 27 0.0 66.7 66.7 19.3 8.4 28 0.0 62.9 62.9 17.2 8.3 29 0.0 57.9 57.9 16.3 7.9 31 0.0 66.2 66.2 16.8 8.5 32 0.0 66.2 66.2 17.1 8.0 33 0.0 62.7 62.7 16.6 8.0 table 4 shows that the typical loess geomorphologic types distribute in middle slope region, whose mean value ranges from about 10° to 20°. the standard deviation value mainly ranges 6° to 10°. the mean value increases from 11 to 13 and from 21 to 23, but decreases from 27 to 29, and undulates from 31 to 33 and 24 to 26. 50 shifang zhang, shangmin zhao 4.1.1.4 curvature index compared to slope index, the curvature index can affect the acceleration and deceleration of flow, so as to influence the erosion and deposition process (moore et al., 1991; zeverbergen & thorne, 1987). using 3d analyst tools in arcgis software, the curvature index is computed based on srtm1 dem data. making a numerical statistics of curvature index for every typical loess geomorphologic types in the study area, the results are in table 5. table 5. numerical statistics of curvature index for typical loess geomorphologic types (m-1) code min max range mean std. 11 -11.5 9.2 20.7 0.1 0.9 12 -25.5 18.6 44.1 0.0 1.4 13 -20.0 22.9 42.9 0.0 1.5 21 -22.9 20.1 43.0 0.0 1.2 22 -24.7 19.5 44.2 0.0 1.3 23 -15.8 17.3 33.1 0.0 1.4 24 -7.6 8.1 15.7 0.0 1.2 25 -10.3 10.2 20.5 0.0 1.1 26 -12.6 13.7 26.3 0.0 1.4 27 -19.8 21.4 41.3 0.0 1.4 28 -20.0 21.9 41.9 0.0 1.4 29 -17.8 18.3 36.0 0.0 1.3 31 -23.4 23.8 47.2 0.0 1.2 32 -26.2 20.3 46.6 0.0 1.4 33 -17.9 23.5 41.4 0.0 1.4 table 5 show that the mean value of the curvature index is about 0.0 m-1; the standard deviation value mainly changes from 0.9 m-1 to 1.5 m-1. as to the range, it increases from 24 to 26, but undulating from other groups. 4.1.2 quantitative indexes from digital topographic profile method the subtypes of the loess ridge and loess knoll are classified by their morphology and valley shape (table 1), which are quantified tentatively by using the digital topographic profile method. to represent the morphologic change of the valley, the position of the topographic profile line is selected to perpendicular to the trend of the ridge (cheng et al., 2013). taking the type of loess narrow acclivitous ridge with deep and high valley as an example, the position of the topographic profile line is in figure 5 in the remote sensing images. figure 5. position of the digital topographic profile line in the remote sensing images. in figure 5, the yellow line is the topographic profile line. based on srtm1 dem data, the topographic profile is drawn to quantify the valley change using arcgis software (figure 6). valley deep valley height horizontal distance (m) el ev at io n (m ) 1150 1100 1050 1000 950 900 0 1000 2000 3000 4000 5000 6000 7000 8000 se figure 6. topographic profile line in loess narrow acclivitous ridge with deep and high valley. figure 6 shows the valley height of about 940 m and valley deep of about 200 m. through making a calculation to all the loess ridge and loess knoll types, the results are in table 6. table 6. valley shape for loess ridge and loess knoll types (m) type valley height valley depth type valley height valley depth 21 725 120 27 1000 140 22 1080 170 28 1100 220 23 940 200 29 850 150 24 760 110 31 1020 150 25 850 105 32 960 120 26 1300 220 33 960 210 table 6 presents the valley height and valley depth for the subtypes of loess ridge and loess knoll, which are generally consistent with the valley shape of the types, although some minor contradictions exist. the existence of the contradictions may be due to the valley shape is acquired from the distribution of the types, but they may have a little change in different regions. 4.1.3 remote sensing indexes for typical loess geomorphologic types remote sensing indexes include both the remote sensing characteristics and the spatial distribution pattern in the remote sensing images for the typical loess geomorphologic types. 4.1.3.1 remote sensing characteristics based on the spatial distribution of the typical loess geomorphologic types and the landsat tm/etm+ images, the remote sensing characteristics for the typical loess geomorphologic types are acquired. remote sensing images are the fundamental data for acquiring the distribution status of typical loess geomorphologic types. so the remote sensing characteristics reflect the change and transition in colour and texture in the remote sensing images for the typical loess geomorphologic types. 4.1.3.2 spatial distribution pattern in remote sensing images other than the remote sensing characteristics, the spatial distribution of the typical loess geomorphologic types in the remote sensing images is helpful to understand the transition and change relationship among them. figure 7 presents an example of this transition: from right to left, the typical loess geomorphologic types change from loess residual tableland to loess beam tableland, and then the loess knoll ridge with shallow and high valley. 51transitional relationships among typical loess geomorphologic types: loess plateau of china figure 7. spatial distribution pattern of the typical loess geomorphologic type (a. loess residual tableland; b. loess beam tableland; c. loess knoll ridge with shallow and high valley). through exploring and analyzing the spatial distribution pattern of the typical loess geomorphologic types in the remote sensing images, we attempted to construct the transitional relationship among them. 4.2 transitional relationship exploration referencing the previous research based on the slope spectrum (zhao & cheng, 2014), we explored to construct the transitional relationships among the typical loess geomorphologic types by using the digital index systems (fig. 8). figure 8 shows that the loess tableland may transform to loess ridge or to loess knoll, and loess ridge may transform to loess knoll. for the subtypes of the loess tableland, the loess tableland may transform to loess residual tableland, and then transform to loess beam tableland. for the morphology of loess ridge, the wide acclivitous types transform to narrow acclivitous, and then transform to knoll types. the valley shape transforming status is the same for both loess ridge and loess knoll: shallow and high valley transforms to deep and high valley, and then transform to shallow and low valley. 5. discussions 5.1 comparison with relevant researches this research is compared with international relevant researches, the relevant researches in the loess plateau and the transitional relationship exploration among loess geomorphologic types. 5.1.1comparison with internal relevant researches international relevant researches mostly focus on morphologic analysis, pattern analysis, classification, reconstruction and identification for geomorphologic types (werner, 1999; kocurek & ewing, 2005; saito & oguchi, 2005; bishop, 2007a, 2007b, 2009; hashimoto et al., 2008; hampton & cole, 2009; bullard et al., 2011; dragut & eisank, 2012). nevertheless, these researches mainly studied on sole geomorphologic type and the spatial distribution of the geomorphologic type was not acquired in advance, which seldom pay attentions to the relationships among geomorphologic types. in this research, the transitional relationships among typical loess geomorphology are constructed based on the digital index system. 5.1.2 comparison with the relevant researches in the loess plateau in this research, the spatial distribution of typical loess geomorphologic types can be acquired from digital geomorphologic database at 1:1,000,000 scale; meanwhile, the transitional relationships among typical loess geomorphologic types were explored based on the constructed digital index system. the achievement of the relationship among different geomorphologic types is a meaningful research method compared with international relevant researches. transitional relationships among typical loess geomorphologic types loess tableand loess ridge narrow acclivitous knoll type loess ridge loess wide acclivitous ridge with shallow and high valley loess wide acclivitous ridge with deep and high valley loess wide acclivitous ridge with deep and low valley loess narrow acclivitous ridge with shallow and high valley loess narrow acclivitous ridge with deep and high valley loess knoll ridge with shallow and high valley loess knoll ridge with shallow and low valley loess knoll ridge with shallow and high valley loess knoll ridge with deep and high valley loess knoll ridge with shallow and low valley loess knoll ridge with deep and high valley loess narrow acclivitous ridge with deep and low valley loess tableand residual tableand beam tableand loess knoll figure 8. transitional relationship among typical loess geomorphologic types. in the loess plateau of china, loess geomorphology forms due to soil erosion and water and soil loss, so they have close relationship. most researches based on dem datasets attempted to classify the typical loess geomorphologic types and analyze their characteristics, especially for morphologic characteristics (tang et al., 2005, 2008; zhou et al., 2010). in this research, the classification and accurate distribution can be acquired from digital geomorphologic database, and our aim is to explore the transitional relationship among typical loess geomorphologic types based on the constructed digital index system. compared with previous research in the loess plateau, this research has more close relationship to soil erosion problems, which is one of the most serious problems in the loess plateau (ma, 1996; liang et al., 2004; sang et al., 2007). 5.1.3 comparison with the transitional relationship exploration researches compared with the previous research about the transitional relationship exploration among the typical loess geomorphology by zhao & cheng (2014), this research selects the typical test area in loess plateau, where distribute dense and typical loess geomorphologic types; in addition, the digital geomorphologic database is revised and improved using visual interpretation and remote sensing images, which guarantees the spatial distribution accuracy 52 shifang zhang, shangmin zhao of typical loess geomorphologic types; meanwhile, digital index system is construct, which provide more reasonable transitional relationships among typical loess geomorphologic types; furthermore, typical loess geomorphologic types have more subtypes and are subdivided by morphologic type and valley shape type, which makes this research much more meaningful compared with previous research. overall, this research is a developed and improved version of the research by zhao & cheng (2014). 5.2 prospects about this research based on this research, the future researches can be conducted on the following three aspects: firstly, choose more reasonable study area. the subtypes for the typical loess geomorphologic types are not complete in this research (cheng et al., 2011b). for example, there is another valley shape type: deep and low valley, which has limited distribution and does not exist in the study area however, more geomorphologic types’ distribution means much more big study area, which will increase the workload and decrease the accuracy of the research (cheng et al., 2011a), especially in digital geomorphologic database improvement and topographic profile line selection. so it is important to seek the balance between the size of the study area and the distribution of the typical geomorphologic types. then, construct digital index system using more indexes. this research adopts topographic indexes, digital indexes from topographic profile method and remote sensing indexes. other than the indexes in this research, there are many other indexes which are useful, such as slope angle and its other derivations, complexity, point pattern, spacing and so on (werner, 1999; székely & karatson, 2004; bishop, 2007a, 2007b; bullard et al., 2011). indexes are the basis for exploring the transitional relationship among the typical loess geomorphologic types, so using more indexes may help deeply understanding the relationship among geomorphologic types. finally, besides the loess geomorphology types, digital index system and relationship exploration can also be studied for other geomorphologic types. taking aeolian sand dune for example, its point pattern, sand-dune pattern, morphometry and complexity are studied in previous researches (werner, 1999; kocurek & ewing, 2005; bishop, 2007b; bullard et al., 2011). digital index system construction and relationships exploration helps understanding the nature of the geomorphologic types much further. 6. conclusions based on 1:1,000,000 digital geomorphologic database in china, the spatial distribution of the typical loess geomorphologic types is revised and improved using remote sensing images in the test area of loess plateau; then, the digital index system is constructed for the typical loess geomorphologic types based on topographic analysis methods and remote sensing characteristics; finally, the transitional relationships among the typical loess geomorphologic types are explored based on the digital index system and previous research. through this research, the following conclusions can be acquired: through boundary improvement, revision to the mis-clssifying units and selection to the typical units, the improved digital geomorphologic database has the typical loess geomorphologic types including loess tableland, loess ridge and loess knoll. due to the morphologic types, the loess tableland is divided into loess tableland, loess ridge and loess knoll; the loess ridge is divided into narrow acclivitous types, wide acclivitous types and knoll types. as to the valley shape, the loess ridge and loess knoll are divided into shallow and low valley, shallow and high valley and deep and high valley. digital index system includes topographic indexes, quantitative indexes from topographic profile and remote sensing indexes. in the test area, the typical loess geomorphologic types mainly distributes in the elevation of about 1000 m, relief of about 250 m and middle slope region; as to the curvature index, it has mean value of 0.0 m-1 and standard deviation values from 0.9 m-1 to 1.5 m-1. the topographic profile presents the valley height and valley depth for the typical units of all the types. the remote sensing indexes include the remote sensing characteristics of all the types and the spatial distribution pattern in the remote sensing images, which is helpful to construct the transitional relationships among typical loess geomorphologic types. the loess tableland may transform to both loess ridge and loess knoll, and the loess ridge may transform to loess knoll. as to the morphologic types, loess tableland may transform to loess residual tableland, and then to the loess beam tableland; the loess wide acclivitous ridge transform to loess narrow acclivitous ridge, and then to loess knoll ridge. for the valley shape types of the loess ridge and loess knoll, the shallow and high valley may transform to deep and high valley, and finally transform to shallow and low valley. acknowledgments this research is under the auspices of the national natural science foundation of china (42271432, 41631179), the foundation of shanxi vocational university of engineering science and technology (kj202426), the national key research and development program (2017yfb0503603), and the natural science foundation of shanxi province (201901d111098). we would like to express our sincere gratitude to the editor and anonymous reviewers. references ballantine, j. c., okin, g. s., prentiss, d. e., & roberts, d. a. 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(2024). unveiling the nexus of scientific production, water resource management, and glob-al water challenges. earth sciences research journal, 28(4), 379-386. https://doi. org/10.15446/esrj.v28n4.114415 record manuscript received: 14/05/2024 accepted for publication: 08/11/2024 abstract this study offers a comprehensive scientometric analysis of global developments in water management, with a particular emphasis on latin america and the caribbean (lac) regions, which collectively possess approximately one-third of the world’s water resources. however, these regions are confronted with significant challenges, including scarcity, pollution, and inequitable distribution. the specific case of lac is examined in order to explore these dynamics in greater depth. notwithstanding the region’s high per capita water availability, lac continues to grapple with persistent challenges in water management and distribution, particularly in the context of agriculture and its associated vulnerable populations. the analysis demonstrates the impact of these challenges on agriculture and the livelihoods of vulnerable populations, underscoring the necessity for enhanced integrated water resource management (iwrm) practices to address pivotal sustainable development goals (sdgs), including access to clean water, agricultural sustainability, and climate action. the study employs scientometric analysis and the tree of science tool, supported by r-based analytics, to draw on data from scopus and web of science (wos). this approach offers a nuanced view of the field’s evolution and identifies influential publications and collaborations. furthermore, the study underscores the potential shortcomings of relying on selective databases, which could influence the depth of the findings. the graphical analyses provide insights into sustainable practices and policy frameworks and culminate in actionable recommendations aimed at improving water governance. by presenting specific insights pertinent to lac, this research contributes to the advancement of integrated water resource management (iwrm) practices aligned with sustainable development goals (sdgs), including clean water access, agricultural sustainability, and climate resilience. keywords: integrated water resource management, scientometric analysis, tree of science tool, agricultural sustainability. palabras clave: gestión integrada de recursos hídricos; análisis cienciométrico; herramienta tree of science; sostenibilidad agrícola; revelando el nexo entre la producción científica, la gestión de recursos hídricos y los desafíos globales del agua este estudio ofrece un análisis cienciométrico integral sobre los avances globales en la gestión del agua, con un énfasis particular en las regiones de américa latina y el caribe (alc), que en conjunto poseen aproximadamente un tercio de los recursos hídricos del mundo. sin embargo, estas regiones enfrentan desafíos significativos, como la escasez, la contaminación y la distribución desigual. el caso específico de alc se examina para explorar estas dinámicas en mayor profundidad. a pesar de la alta disponibilidad de agua per cápita en la región, alc sigue lidiando con desafíos persistentes en la gestión y distribución del agua, particularmente en el contexto de la agricultura y las poblaciones vulnerables asociadas. el análisis demuestra el impacto de estos desafíos en la agricultura y los medios de vida de las poblaciones vulnerables, subrayando la necesidad de mejorar las prácticas de gestión integrada de recursos hídricos (girh) para abordar objetivos clave de desarrollo sostenible (ods), incluyendo el acceso al agua limpia, la sostenibilidad agrícola y la acción climática. el estudio emplea un análisis cienciométrico y la herramienta “tree of science”, respaldados por análisis basados en r, utilizando datos de scopus y web of science (wos). este enfoque proporciona una visión matizada de la evolución del campo e identifica publicaciones y colaboraciones influyentes. además, el estudio resalta las posibles limitaciones de confiar en bases de datos selectivas, lo que podría influir en la profundidad de los hallazgos. los análisis gráficos ofrecen información sobre prácticas sostenibles y marcos de políticas, culminando en recomendaciones prácticas orientadas a mejorar la gobernanza del agua. al presentar ideas específicas relevantes para alc, esta investigación contribuye al avance de prácticas de gestión integrada de recursos hídricos (girh) alineadas con los objetivos de desarrollo sostenible (ods), como el acceso al agua potable, la sostenibilidad agrícola y la resiliencia climática. resumen https://doi.org/10.15446/esrj.v28n4.114415 mailto:natalia-carrillo@javeriana.edu.co mailto:maria.arenasb@javeriana.edu.co mailto:m_otalora@livejaverianaedu.onmicrosoft.com mailto:asantos@humboldt.org.co mailto:natalia-carrillo@javeriana.edu.co https://doi.org/10.15446/esrj.v28n4.114415 https://doi.org/10.15446/esrj.v28n4.114415 380 natalia carrilo-acosta, ana c. santos-rocha, maria cristina arenas bautista, martin otalora low 1. introduction according to the economic commission for latin america and the caribbean (eclac), latin america and the caribbean hold approximately onethird of the world’s water resources (unesco, 2021). despite this abundance, the region faces significant water stress, limited access to safe water and sanitation services, and additional pressures from population growth, economic development, climate change, pollution, and overexploitation (un, 2021). over the last two decades, countries like colombia have experienced severe droughts, profoundly affecting agricultural productivity and the livelihoods of vulnerable farmers (velasco et al., 2005). even though colombia boasts one of the highest levels of per capita water availability globally, issues of unequal distribution and scarcity persist, particularly in rural areas (ideam, 2023). efforts to manage water resources in colombia have been considerable but growing demands and inadequate management practices continue to challenge sustainability (ministerio de ambiente, vivienda y desarrollo territorial, 2010). the country struggles with pollution, the deterioration of aquatic ecosystems, and competition for water resources among agriculture, industry, and domestic supply. according to the instituto de hidrología, meteorología y estudios ambientales (ideam), improving water management systems is essential, especially in rural regions where unreliable irrigation infrastructure hinders agricultural practices. poor management and competition for water resources have adversely affected food production, particularly in regions where limited water access hampers agricultural yields (unesco, 2019). effective water management in agriculture is crucial for reducing hunger and malnutrition, aligning with sustainable development goal (sdg) 2: zero hunger (united nations, 2015). additionally, integrated water resource management (iwrm) plays a pivotal role in addressing multiple sdgs, such as sdg 6: clean water and sanitation, which focuses on ensuring universal access to clean water, improving water quality, and reducing pollution, and sdg 13: climate action, which stresses the importance of protecting natural resources like water to combat climate change (united nations, 2015). within the sustainability framework, iwrm aims to meet current needs without compromising water availability and quality for future generations (oecd, 2022). achieving this requires a long-term vision and carefully considered decision-making that accounts for the impact of policies on water resources. tools like agent-based models are valuable in this regard, as they simulate various scenarios and evaluate the effectiveness of policies and strategies on water use (star et al., 2016). iwrm also promotes sustainable agroecological practices, such as direct seeding, crop rotation, and soil vegetation cover, to enhance soil water retention. scientific advancements have enriched the available literature on water management (garcía-peñalvo & seoane-pardo, 2015; guerrero-bote et al., 2021). analyzing this progress requires efficient tools to process vast amounts of data objectively, reducing the limitations inherent in manual literature reviews. scientometric analysis offers such a tool, allowing researchers to identify trends, emerging areas, and patterns of collaboration within a specific field (cruz-ramirez, 2015; araya-castillo et al., 2021). this study employs a two-stage scientometric analysis. the first stage involves analyzing publications from databases like scopus and web of science using the tree of science (tos) tool (zuluaga et al., 2022). the tos tool organizes scientific literature hierarchically, akin to a family tree, categorizing works into roots, trunks, and branches based on their influence within a citation network. the second stage uses qualitative bibliometric techniques to analyze the nature and orientation of academic production over a selected period (rincón-nocoa & garcía-peña, 2020). we utilized r-based open-source code to conduct this analysis, supported by initiatives from the national university of colombia and the core of science project (eggers et al., 2022; zuluaga et al., 2022; robledo et al., 2023). the tos algorithm mimics the sap distribution process in plants, classifying articles into foundational, structural, and emergent categories (valencia-hernández et al., 2020). recent research has successfully applied the tos algorithm to fields like sustainability, analyzed using r software to better understand academic trends (aguirre & paredes cuervo, 2023; rincón-nocoa & garcía-peña, 2020). in this research, graphical representations using the tree of science provide a visual summary of knowledge evolution, aiding comprehension of complex topics. however, it is important to acknowledge the limitations of these visualizations. science trees simplify reality and may not fully capture the intricacies of a discipline. additionally, the analysis is inherently biased by the databases used, as grey literature or non-indexed publications are excluded. this is particularly relevant in the context of latin america, where indexed national research is often scarce. to conduct this analysis, we selected web of science (wos) and scopus, given their comprehensive coverage of various publication types, including journals, conferences, books, and patents (botero et al., 2023). these databases provide critical bibliographic data for measuring scientific production across disciplines. they rely on peer-reviewed sources to ensure data reliability, which allows for a thorough examination of trends, identification of influential publications, and mapping of collaboration networks among institutions and countries. 2. materials and methods 2.1 data sources and search strategies the general methodology for scientific mapping is comprised of four stages, as delineated by zupic & čater (2015): (i) study design, (ii) data collection, (iii) data analysis and visualization, and (iv) interpretation. in the initial phase of the research process, the research objective is established, and the research question is formulated. during the data collection phase, search equations are developed and databases containing the bibliometric records to be analyzed are selected. in the data analysis and visualization stage, the appropriate tools or software for bibliometric or statistical analysis are employed, and a mapping process is conducted to present the results of the previous analysis. in the final stage, the results are subjected to analysis and discussion. accordingly, the objective of the search was to analyze global research productivity through the examination of original articles published in indexed journals available in the databases scopus and web of science. figure 1 depicts the flow diagram utilized for the bibliometric analysis. the web of science (wos) database (clarivate analytics, philadelphia, pa, usa) and scopus were employed to identify research articles on the topic of interest. specifically, the science citation index expanded within the wos core collection was selected as the source for locating research articles. additionally, the search was broadened by including the scopus database. this study utilized the search strategy outlined in table 1, which displays the implemented search equations in the first column and the total number of results obtained from each consulted database in the second and third columns. a time frame of 20 years was defined, including only articles published between 2002 and 2022. in addition, to ensure an accurate interpretation of the results, the publication language was restricted to english (according to the field label la in wos and scopus). only original articles were included (based on the wospt field label), excluding those classified as other types of documents. table 1. equation used for searching topics equation scopus wos ((“model*”) and (“iwrm” or “water management”) and (“sustainability” or “agricultural sustainability” or “agroecosystem*”)) 2680 895 total 3575 381unveiling the nexus of scientific production, water resource management, and global water challenges data collection data loading and converting descriptive bibliometic analysis mapping records in wos (n = 895) records in scopus (n = 895) unics records (n = 2800) document attribute data analysis data visualization matrix creation network matrix creation bibliographic coupling, co-citaction, collaboration, scientific production figure 1. flowchart used for scientometric analysis.2.2 bibliometric analysis and data visualization first, the eligible records retrieved from wos and scopus were checked for anomalies using the ‘remove duplicates’ function of the bibliometrix package (rstudio). a bibliometric analysis of the final 2800 records was then performed using the same tool (bibliometrix package, cran). the biblioshiny application provided a graphical web interface within the rstudio database. journal impact factors were obtained from the 2023 journal citation reports published by clarivate analytics. we conducted an analysis using the tree of science (tos), a fundamental concept in data and computer science. this model (figure 2.) resembles a tree, where the roots represent the foundational elements of our field of research, the trunk represents the most cited studies, and the branching sections symbolize our extensive exploration and understanding of large datasets stored in databases. figure 2. tree science diagram -tos. the fundamental principles of database analysis are grounded in the systematic processes of data collection and organization, emphasizing the critical need for a solid framework in any research undertaking. as the tree of knowledge expands, its branches diverge into various domains, such as data mining, data processing and cleansing, exploratory analysis, statistical modeling, and machine learning. each branch embodies a unique methodology for uncovering trends and relationships within the data. in the context of database analytics, the branches of the tree of science (tos) yield valuable insights that facilitate accurate predictions, effective visualizations, the identification of business opportunities, and informed decision-making. following the execution of the r routine, we discerned particular production peaks at disparate times and identified the most cited authors within the context of our research topic. these findings are related to other aspects of the science tree, as outlined in the previous section, and will be explored further below. 3. results 3.1. scientometric analysis over the 20-year period from 2002 to 2022, we identified 3575 indexed publications in the two databases (wos core collection + scopus). these documents included 3060 original articles, 190 books chapters, 52 conference abstracts, 92 review articles and 181 other types of publications. finally, of the 3060 original articles, 3025 were consolidated in english and 225 duplicates were removed, resulting in a final total of 2800 articles. figure 3 shows article production in wos and scopus by year from 2002 to 2022. we have excluded 2023 due to incomplete data, which could distort the overall statistics. over this period, the production of documents related to agro-ecological systems and water resource management, particularly through coupled models, has shown a steady increase, peaking in 19 with a maximum of 485 articles published. an average annual growth rate of 7.18% was observed, reflecting the growing interest of scientists in these areas. 382 natalia carrilo-acosta, ana c. santos-rocha, maria cristina arenas bautista, martin otalora low figure 3. average agricultural production vs population growth vs scientific production. this surge in publications is likely to be driven by the pressures of global agricultural demands and rapid population growth. since 2002, the world’s population has increased from around 6.2 billion to over 8 billion in 2022, leading to an estimated 70% increase in food demand. to sustainably meet this demand, agriculture must now produce more with less, requiring innovations in water use efficiency, soil health and ecosystem resilience key components of agroecological systems. as agriculture consumes about 70% of the world’s freshwater resources, effective water management is critical to productivity. the rising research interest is in line with this urgency, as agroecological approaches help to optimise resource use and mitigate environmental impacts, addressing critical challenges posed by population-driven demand. figure 4 shows a chart of the 10 most prominent authors in research during the study period. the number of squares in the graph represents each author’s contribution in terms of publications within each database. in the wos database, which accounts for 11.11% of the total output, professor huang ch from the university of delaware stands out as the most prolific contributor in terms of the number of articles published during the study period. his research covers diverse areas such as industrial wastewater management, aquatic chemistry and groundwater remediation. in contrast, the scopus database identifies dr wang y from the university of hong kong as the leading contributor with 13.02% of publications, focusing on nanomaterials and their impact on aquatic systems. figure 5 shows the cumulative number of publications by the top 9 journals over the last 20 years. an analysis of journal publications in both databases shows that agricultural water management published 68 articles in scopus and 80 articles in wos. these differences in the number of articles reflect differences in subject coverage, inclusion criteria, and editorial policies between scopus and wos, and highlight how each database selectively shapes the available literature in the field. on the other hand, the journal sustainability published 170 articles in wos and 70 in scopus. again, these differences may be due to the different thematic focus and indexing policies of each database. conversely, several journals, including science of the total environment, water, journal of hydrology, journal of environmental management, water resources management, journal of cleaner production and water resources research, had a higher citation frequency in scopus, with 120, 115, 60, 78, 75, 41 and 21 articles published respectively. these differences are likely due to scopus’ broader coverage, specific quality criteria and editorial partnerships, which may favour the indexing of these journals. overall, the distribution of journals between scopus and wos highlights the need to consider the unique characteristics of each database when interpreting scientometric results. several factors, including database coverage, subject coverage, coverage periods, quality and reputation criteria, publication costs and indexing processes, shape the representation of journals in these contexts. figure 4. author´s production by database repository. figure 5. number of publications per journal by database. 383unveiling the nexus of scientific production, water resource management, and global water challenges 3.2. tree of science analysis 3.2.1. roots the articles in the roots section represent the critical importance of effective water resource management in addressing both global and regional water-related challenges. in this study, four key articles illustrate how accurate simulations and a deep understanding of water availability are essential to mitigating worldwide water scarcity issues: • “model evaluation guidelines for systematic quantification of accuracy in watershed simulations” (d. n. moriasi et al., 2007) provides guidelines for evaluating the accuracy of watershed simulations, essential for effective global water resource management. • “global monthly water scarcity: blue water footprints versus blue water availability” (hoekstra et al., 2012) analyzes global water scarcity by comparing water demand with availability across regions, highlighting disparities and scarcity hotspots. • “groundwater use for irrigation – a global inventory” (siebert et al., 2010) offers a global inventory of groundwater use for irrigation, emphasizing its significance for agricultural productivity. • “the impact of irrigation development on regional groundwater resources in bangladesh” (kirby et al., 2015) examines the effects of irrigation on regional groundwater resources, focusing on bangladesh as a case study of water stress in densely populated areas. collectively, these articles emphasize the delicate balance needed between groundwater use for agriculture and the preservation of global and regional groundwater supplies. by exploring how irrigation development influences groundwater levels in bangladesh, kirby et al. (2015) highlight the need for policies that address both agricultural demands and water sustainability in water-scarce regions, such as bangladesh, which faces unique challenges due to its geography and population density. moreover, global monthly water scarcity: blue water footprints versus blue water availability (hoekstra et al., 2012) offers a global outlook by contrasting water footprint with actual availability, revealing substantial imbalances in water distribution worldwide. this comparison underscores the need for coordinated, sustainable approaches to tackle water challenges at both global and local levels. 3.2.2. trunk the articles included in the trunk section represent foundational works that build upon and expand the concepts introduced in the roots section. this study presents six key articles that provide a comprehensive examination of the conceptual epistemology underlying integrated water management, along with an analysis of the most widely implemented optimization strategies. • agricultural water management (wheeler et al., 2015), explores the critical interconnection between water management and agriculture, proposing strategies to improve water utilization in regions facing severe water scarcity. • a coupled modeling framework for sustainable watershed management in transboundary river basins (khan et al., 2017) introduces an innovative framework for managing cross-border watersheds, highlighting the necessity of international collaboration. • water resources availability and demand in the mara river basin (dessu et al., 2014) provides a thorough analysis of water availability and demand, underscoring the importance of precise data for informed decision-making. • the studies water footprint, blue water scarcity, and economic water productivity of irrigated crops in the peshawar basin, pakistan (t. khan et al., 2021) and reduce blue water scarcity and increase nutritional and economic water productivity through changing the cropping pattern in a catchment (nouri et al., 2020) examine the influence of altering cropping patterns on water availability and productivity. both articles highlight that effective land and crop management are essential components of comprehensive water management strategies, especially in water-scarce regions. • efficient irrigation water allocation and its impact on agricultural sustainability and water scarcity under uncertainty (li et al., 2020) discusses the significance of optimal irrigation practices under uncertain conditions to promote agricultural sustainability. together, these articles form the structural foundation of this investigation, providing an integrated understanding of how strategic water and land management can be leveraged to address complex challenges in global water resource management. 3.2.3. branches the articles included in the branches section represent foundational works that build upon and further develop the concepts introduced in the roots section. this study presents three branches that encapsulate recent research efforts, which apply, adapt, or even challenge established ideas, thereby paving the way for new areas of exploration and advancing the field’s knowledge base. 3.2.3.1. branch 1: groundwater and water resource management the articles in this branch explore various strategies for groundwater and water resource management, emphasizing the integration of social and environmental factors. studies like sustainability analysis for irrigation water management in the aral sea region (cai et al., 2003) address sustainability challenges, while game-based social learning for socially sustainable water management (kraker et al., 2021) uses gamification to promote collaborative practices. additionally, research on community-based governance in spain (jégou & sanchis-ibor, 2019) and evolving water management in the netherlands (van der brugge et al., 2005) demonstrates the need for contextspecific solutions. technological advancements are also emphasized, with works such as decision support for sustainable option selection in integrated urban water management (makropoulos et al., 2008) and computer architectures for incremental learning in water management (kenda et al., 2022) highlighting the role of adaptive technologies. these innovations make urban water management more efficient and resilient by enabling responses to evolving conditions. together, these studies illustrate the necessity of adaptable, context-sensitive approaches that incorporate both community involvement and technological advancements to improve resilience and sustainability in water management. 3.2.3.2. branch 2: water resource management the articles in this branch explore the complexity of sustainable water management across various regions, emphasizing the integration of natural, climatic, human, and social dimensions (figure 6.b). they highlight the need for multidisciplinary strategies, particularly in disaster scenarios. for instance, studies like flood hazard mapping on the angkor world heritage site (liu et al., 2019) and groundwater recharge strategies in the sutlej river (mushtaq et al., 2023) address natural hazard management, while research on water quality and groundwater depletion, such as in mexico (troyo-dieguez et al., n.d.) and tianjin, china (qin et al., 2023), emphasizes the interaction of natural and human factors. social and environmental considerations are equally important. are food hubs sustainable? (shariatmadary et al., 2023) and decision support for the eocene aquifer in palestine (jonoski et al., 2023) illustrate the significance of community-based governance and policy-driven solutions. additionally, simulation models like groundwater flow dynamics in rechna doab, pakistan (awais et al., 2023) demonstrate the value of technological innovations for strategic water planning. overall, these studies underscore the importance of integrating flood risk mapping, groundwater management, social engagement, and advanced technologies to create resilient, equitable, and sustainable water management systems. 3.2.3.3. branch 3: water resource quality this branch explores innovative technologies and approaches for sustainable water resource management, focusing on water quality, agricultural productivity, and environmental impact (figure 6.c). key advancements include the integration of micro-sprinkler irrigation with organic fertilizers, which enhances water efficiency and soil health while boosting agricultural yield, providing a sustainable model for regions with water scarcity. several articles address wastewater treatment innovations. for example, hoelzle et al. (2020) and naha et al. (2023) propose energy-efficient methods, such as microbial fuel cells and nanofiltration, to reduce costs and environmental footprints. microbial-based approaches, like those by al-hazmi et al. (2023) and shivaram et al. (2023), use biological processes for pollution mitigation, offering lower toxicity and minimal chemical inputs. additionally, 384 natalia carrilo-acosta, ana c. santos-rocha, maria cristina arenas bautista, martin otalora low leovac maćerak et al. (2023) highlight the use of biochar and nanotechnology for improved treatment efficiency, promoting a circular economy. chen et al. (2023) examine sustainable disinfection methods developed during the covid-19 pandemic, balancing public health with environmental concerns. conde et al. (2023) focus on electrochemical methods to enhance pollutant removal, aligning with global sustainability goals. in agriculture, yue et al. (2023) present water-efficient irrigation systems combined with organic fertilizers, crucial for water-scarce regions. deng et al. (2023) emphasize the reuse of wastewater in pollution remediation, underscoring the importance of closing resource loops. overall, these technologies offer scalable, cost-effective solutions that address environmental challenges, improve water quality, and align with sustainability goals. their implementation could transform water management practices, benefiting industries, agriculture, and public health while informing future policies. figure 6. branches of tos 4. discussion this comprehensive scientometric study, anchored in the web of science (wos) and scopus databases, provides a nuanced analysis of global research output in water resource management, revealing critical regional dynamics that influence both agricultural and non-agricultural practices. the concentration of research efforts in countries such as the united states, china, spain, india, australia, germany, and the netherlands reflects a substantial commitment to advancing water management solutions in these high-output nations. however, this pattern also exposes a significant gap in research and collaboration with regions facing severe water challenges, particularly in the global south, including latin america and the caribbean. addressing this disparity through targeted knowledge transfer and collaborative initiatives is essential for developing innovative and contextually relevant solutions (figure 7). figure 7. agricultural production of millions of tons and scientific output. the focus on major water consumers in crop production, such as the united states, china, and brazil, emphasizes the urgent need for efficient water use policies in agriculture, one of the most water-intensive sectors. given their pivotal roles in the global food supply, these countries must prioritize strategies that sustain agricultural productivity while mitigating growing water scarcity, which disproportionately impacts vulnerable populations. latin america, with its extensive agricultural activities and dependency on natural resources, stands to benefit significantly from the adoption of these water efficiency measures, which are crucial for building resilience to climate-induced water stress. spain’s prominent position in water management research aligns with pressing regional challenges like drought and desertification, underscoring the necessity of sustainable practices. this trend is mirrored in countries like india, the united kingdom, and australia, demonstrating a global effort to combat water scarcity through rigorous scientific inquiry. nevertheless, regions such as latin america and the caribbean remain underrepresented in research, despite grappling with similar or more severe water issues. bridging this research gap is critical, as more focused efforts could yield solutions tailored to these regions’ unique environmental and socio-economic conditions. the notable scientific output of the united states and the rapid expansion of china’s research capacity highlight their proactive approach to sustainable water management, supported by strong infrastructures for scientific research and resource allocation. together, these countries serve as potential models for others striving to enhance their water management capabilities. however, the global water footprint assessment (figure 8) reveals stark inequalities in water distribution, with significant burdens falling on countries in the southern hemisphere. nations like nigeria, pakistan, argentina, and brazil experience immense pressure on water resources due to high agricultural demands and population growth, intensifying the need for effective and equitable water management strategies. figure 8. water footprint of crop production in selected countries (1996-2005). mekonnen & hoekstra, 2011. the study emphasizes that a comprehensive approach to water management must extend beyond agriculture to include urban and industrial needs. industrial sectors, particularly in china and india, are major water consumers and contributors to pollution, necessitating policies that address waste management, recycling, and pollution control. urban centers, especially in rapidly growing regions like latin america, face challenges in providing clean and accessible water, a problem further exacerbated by climate change. recognizing and incorporating these factors into water management strategies is essential for developing resilient urban infrastructures capable of sustaining economic growth and population increases. the global south’s vital role in global food production and its significant water demands underscore the urgency of focused research and tailored solutions. latin america and the caribbean, despite their abundant freshwater resources, require innovative strategies to manage their water footprint effectively. expanding research and collaboration in these regions would promote environmental justice and support the development of sustainable practices. this approach would not only benefit local communities but also contribute significantly to global water security. ultimately, this integrated analysis highlights the intricate interconnections between scientific research, food security, and the global water footprint. addressing these linkages with a focus on interdisciplinary and globally inclusive strategies will drive sustainable practices that benefit humanity and ensure a more equitable distribution of water resources worldwide. 385unveiling the nexus of scientific production, water resource management, and global water challenges 5. conclusions this scientometric analysis provides a comprehensive overview of global research in water management, emphasizing the need for collaborative approaches and region-specific solutions to tackle the distinct challenges faced by different areas. the substantial contributions from countries such as the united states, china, and other research leaders highlight the critical importance of international cooperation and collective action in achieving sustainable water resource management. however, regions like latin america and the caribbean (lac) require special attention due to their unique context and significant yet unevenly distributed water resources. despite holding nearly one-third of the world’s water resources, lac continues to grapple with pressing issues such as water scarcity, pollution, and inequitable distribution. climate change exacerbates these problems through unpredictable rainfall patterns, more frequent extreme weather events, and rising temperatures. this combination places immense strain on water management systems, particularly affecting agriculture and the livelihoods of vulnerable populations. the region’s complex hydrological and socio-economic landscape calls for adaptive, region-specific strategies that address both water conservation and agricultural productivity. in this context, the application of advanced modeling and management tools is crucial for enhancing water use efficiency and ensuring long-term sustainability. hydrological, economic, and simulation models, along with decision-making frameworks, are essential for anticipating water dynamics and identifying effective management strategies. these tools provide a more nuanced understanding of water behavior, enabling the development of resilient systems that can withstand climate variability and meet the increasing demands of agriculture and urbanization. effective water resource management also depends on active stakeholder engagement and robust governance structures. transparent and participatory governance—encompassing governments, communities, non-governmental organizations, and the private sector—is vital for addressing water challenges equitably. in lac and other regions, fostering such collaboration is key to implementing integrated water resource management (iwrm) practices that are both effective and inclusive. given that agriculture is one of the largest consumers of water, it demands focused attention. implementing efficient irrigation technologies and sustainable agricultural practices is essential to balance water use with productivity. considering agriculture’s critical role in the economies of many lac countries and its dependence on water resources, integrated strategies that prioritize both food security and water conservation are imperative. overall, the reviewed studies emphasize the need for a holistic approach to water management, considering factors such as resource availability, agricultural demand, and environmental impacts. regional and global cooperation remains vital for the sustainable management of transboundary basins, while flexible strategies are necessary to adapt to an increasingly unpredictable climate. by applying these insights, policymakers and stakeholders can make informed decisions that effectively address urgent water-related challenges and promote long-term sustainability. this approach is particularly relevant for latin america and the 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(2025). mineralogy and geochemistry of stratabound cu (±ag) deposits: insights from the “la quinta” formation, northern colombia. earth sciences research journal, 29(2), 113-130. https:// doi.org/10.15446/esrj.v29n2.113381 the “la quinta” formation is a volcano-sedimentary unit formed by continental, oxidized redbeds interlayered with subaerial felsic-to-mafic lava flows. this lithostratigraphic unit, which outcrops in the perijá range (northern colombia), hosts two different stratabound cu (±ag) mineralization styles: basalt-hosted and sediment-hosted. the former style of mineralization comprises native copper and silver, along with minor amounts of cu-sulfides hosted flow-top mafic lavas. conversely, the latter mineralization style primarily consists of chalcopyrite, bornite, and digenite hosted by sedimentary strata. electron probe micro-analyzer (epma) analyses indicate that cu-sulfides in both mineralization styles host silver. on the other hand, the assessment of mineral assemblages suggests a low-temperature and epigenetic origin for the cu (±ag) mineralization in both basaltic flows and sedimentary strata. also, the identified hydrothermal alterations comprise four distinct stages occurring in the next chronological order: (1) na-rich, (2) k-rich, (3) ca-si-rich, and (4) ca-co2-rich alterations. the hypogene ore minerals were observed paragenetically associated with the ca-si-rich alteration. in sediment-hosted deposits, mineralization is closely associated with charcoal clasts and possibly pyrobitumen. these forms of organic matter may serve as possible sources of reduced sulfur, which may trigger the precipitation of cu-sulfides upon contact with mineralizing fluids. additionally, a strong alteration involving ca, mg, na, k, and fe is evidenced by lithogeochemistry analyses. these analyses also suggest a depletion of k, rb, and cs contents in the samples with hydrothermal alteration, and also evidence no significant variation of si and rare earth elements (ree), implying these elements remained immobile during the different stages of hydrothermal alteration. despite the above, most major and trace elements evidence a wide range of variation, implying that the alteration reactions were primarily controlled by lithological factors. moreover, the supergene oxidation of hypogene ore minerals resulted in the formation of spionkopite, anilite, covellite, and other secondary cu-bearing minerals. overall, the characteristics of the cu (±ag) deposits hosted in the “la quinta” formation are similar to those described for chilean manto-type cu deposits and volcanic redbed cu deposits. keywords: perijá range; copper sulfides; digenite; anilite; spionkopite; organic matter. mineralogía y geoquímica de depósitos estratoligados cu (±ag): perspectivas de la formación “la quinta”, norte de colombia resumen la formación “la quinta” es una unidad volcano-sedimentaria formada por capas rojas continentales, oxidadas e interestratificadas con flujos lávicos subaéreos de composición félsica a máfica. esta unidad litoestratigráfica que aflora en la serranía del perijá (norte de colombia), hospeda dos diferentes estilos de mineralización estratoligada cu (±ag): hospedada en basaltos y hospedada en sedimentos. el primer estilo de mineralización comprende cobre y plata nativos, junto con cantidades menores de sulfuros de cu hospedados en flujos lávicos máficos. por el contrario, el segundo estilo de mineralización consiste principalmente de calcopirita, bornita, y digenita hospedadas en estratos sedimentarios. microanálisis con sonda electrónica (epma) indican que los sulfuros de cu presentes en ambos estilos de mineralización hospedan plata. por otro lado, la evaluación de ensambles minerales sugiere un origen de baja temperatura y epigenético para la mineralización cu (±ag) en los flujos basálticos y estratos sedimentarios. también, las alteraciones hidrotermales identificadas comprenden cuatro diferentes etapas que ocurren en el siguiente orden cronológico: (1) alteración rica en na, (2) alteración rica en k, (3) alteración rica en ca-si y, (4) alteración rica en ca-co2. los minerales de mena hipógenos fueron observados paragenéticamente asociados con la alteración enriquecida en ca-si. en los depósitos hospedados en sedimentos, la mineralización se encuentra estrechamente asociada con clastos de carbón y posiblemente pirobitumen. estas formas de materia orgánica podrían servir como posibles fuentes de azufre reducido, las cuales podrían desencadenar la precipitación de sulfuros de cu al entrar en contacto con fluidos mineralizantes. adicionalmente, una fuerte alteración que involucra ca, mg, na, y fe es evidenciada mediante análisis litogeoquímicos. estos análisis también sugieren una disminución en los contenidos de k, rb y cs en las muestras con alteración hidrotermal, y también evidencian una variación insignificante del si y tierras raras (ree), lo que implica que estos elementos permanecieron inmóviles durante las diferentes etapas de alteración hidrotermal. a pesar de lo anterior, la mayor parte de elementos mayores y traza evidencian un amplio rango de variación, lo que implica que las reacciones de alteración fueron principalmente controladas por factores litológicos. además, la oxidación supérgena de minerales de mena hipogénicos resultó en la formación de spionkopita, anilita y covelita, además de otros minerales secundarios de cu. en general, las características de los depósitos cu (±ag) hospedados en la formación “la quinta” son similares a aquellas descritas para depósitos cupríferos tipo manto chileno y volcanic redbed. palabras clave: serranía del perijá; sulfuros de cobre; digenita; anilita; spionkopita; materia orgánica. https://doi.org/10.15446/esrj.v29n2.113381 mailto:anfgonzalezdu@unal.edu.co mailto:jcmolanom@unal.edu.co mailto:ncanoh@unal.edu.co mailto:nmguerreroh@unal.edu.co mailto:lcontrerasp@unal.edu.co mailto:mjsepulveda@sgc.gov.co mailto:anfgonzalezdu@unal.edu.co https://doi.org/10.15446/esrj.v29n2.113381 https://doi.org/10.15446/esrj.v29n2.113381 114 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras 1. introduction copper and silver are essential materials in the manufacturing of renewable energy technologies due to their chemical properties, which make them ideal for many electrical and electronic applications (iea., 2021; apergis & apergis, 2019). mining these metals offers an opportunity to boost colombia’s economy. however, mineral deposits with copper and silver remain underexplored within the country. exploration efforts and advancements in knowledge have primarily focused on porphyry and epithermal deposits, particularly those belonging to the middle cauca belt. this miocene magmatic arc hosts porphyry au-cu(mo) and intermediate sulfidation au-ag-zn deposits (sillitoe et al., 1982; loboguerrero, 2003; sillitoe, 2008; bartos, 2017). other well-known copperand silver-hosting deposits in colombia include skarns, volcanogenic massive sulfide (vms) and iron oxide copper gold (iocg) deposits. occurrences of cu (au-ag) skarns are associated with the contact between the jurassic ibagué batholith and triassic limestones and conglomerates (alvarán et al., 2011; shaw et al., 2019). cu (au-zn-ag) vms deposits have been identified throughout the basaltic and marine sedimentary units of the cañas gordas terrane (shaw, 2014; shaw et al., 2019; murillobedoya, 2020) and iocg-style mineralization has been proposed to occur at the el alacrán cu-au deposits hosted in the calima terrane in the western cordillera (sillitoe, 2018; manco, 2020). conversely, the nature of colombian stratabound cu-ag deposits remains not yet fully understood. volcanicand sediment-hosted stratabound cu (±ag) deposits have been widely documented within the “la quinta” formation in the perijá range, northern colombia. (wokittel, 1957; champetier et al., 1961; pagnacco, 1963; viteri, 1978; maze, 1984; rodríguez, 1986; jiménez, 2010; ortega et al., 2012a, 2012b; cardeño-villegas et al., 2015; manco-jaraba et al., 2019). however, the origin of the widespread native copper mineralization at the “la quinta” formation (wokittel, 1957; ortega et al., 2012) and the occurrence of copper and silver minerals within both sedimentary and volcanic units have received limited research attention. consequently, this study aims to address the origin of copper and silver minerals hosted in the “la quinta” formation through detailed field observations, mineral characterization, and geochemistry. 2. geological setting the perijá range, representing the northernmost part of the colombian eastern cordillera, acts as an uplift between the cesar – ranchería and maracaibo basins (fig. 1a). it is bordered northwards by the oca fault system, westwards by the cerrejón fault, and eastwards by the perijáel tigre faults (fig. 1b) (miller, 1962; colmenares et al., 2007; nova et al., 2012). the perijá mountain range follows a n30e trend (ujueta & llinás, 1990), and its inner structure is primarily controlled by reverse faults such as the media luna and manaure systems (fig. 1b, c) (ujueta & llinás,1990; geoestudios, 2006). the study area is in the western foothills of the perijá range (fig. 1b, c). the lithological units in this region span from the paleozoic to the quaternary. the oldest units comprise cambro-ordovician phyllites and quartzites, overlaid by devonian-carboniferous sediments comprising conglomeratic and lithic sandstones, interbedded with mudstones (champetier et al., 1961; radelli, 1962; forero, 1970; colmenares et al., 2007; pastor-chacón et al., 2013). minor triassic porphyritic andesites have been reported by tschanz et al. (1969) and colmenares et al. (2007). however, the area is dominated by the “la quinta” formation, a thick succession of jurassic volcano-sedimentary strata, ranging from 1600 to 3000 m in thickness (maze, 1984; arias & morales, 1999; gómez et al., 2010). maze (1984), further divides the “la quinta” formation into three sections (fig. 2): • lower section (~800 m thick): comprises a thick sequence dominated by fine to medium-grained, dark red-brown arkoses with occasional volcanic rocks. minor coarse-grained, pale green, arkosic sandstones, usually contain plant fragments and charcoal clasts. in the lower part, thin limestone beds, lenses, concretions, and fossil-bearing horizons can be found. preserved conchostracan fossils are abundant within one of these basal limestone horizons. • transition zone (~100 m thick): includes basaltic andesite flows from 1 m to greater than 20 m, which are overlain by sequences of fine to coarse-grained, dark red-brown arkoses and red to black mudstones. minor green beds may contain various types of fossils, including estherids, vertebrate remains, carbon pieces, and large plant fragments. calcareous concretions, nodules, and lenses are less common than in the lower section. • upper part (~800 m thick): comprises interbedded lithologies including dark red to dark-brown coarse-grained arkoses, conglomerates, pinkish felsic ash layers, and tuffs. light-green sandstones containing plant fragments can also be found. basalticandesite flows from 1 to 15 m in thickness, are interbedded with red beds on top. the flow top portions are usually vesicular and may contain native copper mineralization. additionally, many felsic to intermediate ash layers, welded tuffs, and flows are commonly interbedded with the mafic flows and arkoses. the lithological descriptions provided by maze (1984) closely resemble those reported for the “la quinta” formation, near the study area, by wokittel (1957), pagnaco (1963), champetier et al. (1961), radelli (1962), and gómez et al. (2010). the volcanicand sediment-hosted cu (±ag) mineralization in the “la quinta” formation is predominantly stratabound and occasionally stratiform. the term “stratabound” denotes the preferential linkage of mineralization with specific stratigraphic levels. on the other hand, “stratiform” indicates that mineralization exhibits the overall morphology of sedimentary strata. stratiform mineralized bodies always crosscut the stratification (wilkinson, 2014). additionally, u-pb zircon dating of volcanic rocks in the “la quinta” formation suggests an age range of 191–164 ma for this unit (jiménez, 2010; gonzález et al., 2015; rodríguez-garcía & obando, 2020). this aligns with the jurassic ages indicated by the fossil record (forero, 1970; maze, 1984). according to kammer & mojica (1995), maze (1984), and rodríguez-garcía & obando (2020), the “la quinta” formation originated from the accumulation of sediments in a jurassic basin bordered by normal faults. within this basin, continental sediments were deposited with a significant contribution of volcanic material resulting from intense calc-alkaline magmatism in a continental margin arc setting. due to the extension of jurassic strata along the colombian andes, the “la quinta” formation has been correlated with the “guatapurí” and “saldaña” formations (tschanz et al., 1969; cediel et al., 1981; colmenares et al., 2007). the “la quinta” formation is unconformably overlain by cretaceous strata, composed of limestones and mudstones from the “cogollo” group and “rionegro” formation (forero, 1970; maze, 1984; gómez et al., 2010). massive alluvial fans and terraces occur in the foothills area, corresponding to recent (quaternary) deposits (fig. 1c) (arias & morales, 1999; hernández, 2003; gómez et al., 2010). 3. methods mineralized outcrop rock samples were collected from different zones across the western foothills of the perijá range (fig. 1c). petrographic description of 65 of these samples was carried out under reflected and transmitted light using an olympus bx41 microscope at the mineral characterization laboratory of the universidad nacional de colombia, bogotá. the abbreviations for the names of minerals displayed in the microphotographs (fig. 4, 5, 6, 7) are after warr (2021). detailed mineral chemistry analyses were performed on carbon-coated polished thin sections through electron probe micro-analyzer (epma) using a jeol superprobe jxa-8230 at the universidad nacional de colombia. epma analyses were done at an accelerating voltage of 15 kv and beam current of 20 na, and included the detection of s, cu, fe, and ag contents in selected ore minerals. additionally, lithogeochemical assessments of 6 outcrop rock samples were conducted at als laboratories in canada. these employed inductively coupled plasma atomic emission spectroscopy (icp-aes) and inductively coupled plasma mass spectrometry (icp-ms) techniques following lithium borate fusion and multi-acid digestion for major and trace element detection. analyzed elements comprise major oxides (sio2, al2o3, fe2o3, cao, mgo, na2o, k2o, cr2o3, tio2, mno, p2o5, sro, bao) and trace elements (ba, cr, cs, ga, rb, sn, sr, th, u, v, w, zr, nb, ta, as, co, li, mo, ni, pb, zn, hf, cu), including rare earth elements (la, ce, pr, nd, sm, eu, gd, tb, dy, ho, er, tm, yb, lu, sc, y). 115mineralogy and geochemistry of stratabound cu (±ag) deposits figure 1. a) map showing the location of the santa marta massif and the northernmost segment of the colombian eastern cordillera. b) schematic map of northern colombia displaying the principal regional faults as described by colmenares et al. (2007). the study area is highlighted in a blue square. c) map of the local geology and sampling distribution within the study area. the geological basemap was taken from the 34 – agustín codazzi geological map sheet from colmenares et al. (2007). 116 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras 4. results mineralization styles copper mineralization styles within the study area can be classified into basalt-hosted and sediment-hosted. the first style encompasses native copper ± native silver hosted by flowtop mafic lavas. native copper typically occurs as infill material within vesicles (fig. 3a) and, to a lesser extent, in veinlets. it is consistently associated with intense hydrothermal alteration characterized by quartz and green minerals such as epidote, prehnite, and chlorite. native silver, appearing as wire-shaped crystals, exhibits an erratic distribution. this mineralization is predominantly concordant and exclusive to amygdaloidal basalts. however, in localized places (p1, p2, and p4 in fig. 1c), minor occurrences of native copper are also found in sulfide-hosting veinlets and hydrothermal breccias, both of which are associated with weakly altered non-vesicular basalts. the second style includes sediment-hosted cu-sulfides. despite the “la quinta” formation being dominated by red beds, copper mineralization predominantly occurs in greenish, brownish, and light-gray layers of siltstones, as well as mediumto fine-grained arkoses and sandstones (fig. 3b). cusulfides manifest in drusiform and banded veinlets, disseminated along bedding planes, and within quartz-rich hydrothermal breccias (fig. 3c). layers that are non-red typically exhibit hydrothermal alteration (silicification, ca-co2-rich, and ca-si-rich alterations) and contain charcoal. (fig. 3d). angular charcoal clasts, ranging from 1 to 5 cm in diameter, are erratically distributed in coarsegrained gray beds (fig. 3d). mineral assemblages 1. basalt-hosted hydrothermal alteration in mafic flow-top lavas is characterized by four distinct stages occurring in chronological order: (i) na-rich alteration, (ii) k-rich alteration, (iii) ca-si-rich alteration, and (iv) ca-co2-rich alteration. na-rich alteration is widespread in the basalts and involves crustiform albite veinlets with minor quartz and earthy hematite (fig. 4a). plagioclase phenocrysts are often replaced by whitish to slightly pinkish, occasionally coarse albite. figure 2. schematic cross section of the “la quinta” formation (maze,1984). colors were added to facilitate interpretation. figure 3. a) vesicular basalt containing native copper (cu) and displaying intense hydrothermal alteration. the sample location is p7 (fig. 1c). b) sharp contact between red beds and copper-bearing siltstones. the location of the outcrop is p8 (fig.1c). c) hydrothermal breccia with quartz (qz), malachite (mlc), and cu-sulfides (cs). the sample locality is p5 (fig.1c). d) copper-bearing siltstones containing charcoal (ch) angular clasts. this sample exhibits silicification and was collected at locality p8 (fig.1c). 117mineralogy and geochemistry of stratabound cu (±ag) deposits k-rich alteration is observed locally and encompasses pink k-feldspar and quartz traces. k-feldspar veinlets crosscut former albite and are confined to zones of intense brecciation in non-vesicular basalts (fig. 4b). the ca-si-rich alteration stage is dominated by an assemblage of calcium-bearing minerals, including epidote, prehnite, and minor garnet (fig. 4c). quartz, chlorite, specularite, and tourmaline are also prevalent during this stage (fig. 4d). this alteration is highly pervasive in the vesicular basalts, where original textures have been completely obliterated. epidote, the most abundant mineral during this stage, occurs in veinlets, filling vesicles and most commonly replacing the volcanic host rock as massive patches. epidote crystals exhibit various habits, including fan-like shapes, elongated prisms, and fibrous arrangements. in non-vesicular basalts, this mineral is restricted to veinlets with multiple episodes of opening (fig. 4c). prehnite is ubiquitous, occurring as fanshaped crystals intimately associated with epidote and quartz (fig. 4d). garnet is scarce and mostly replaced by clinozoisite. the ore minerals paragenetically associated with ca-si-rich alteration vary in vesicular and non-vesicular basalts. vesicular basalts host exclusively native copper and traces of wire-shaped native silver (fig. 4e, f), whereas non-vesicular basalts host bornite, covellite, chalcopyrite, undifferentiated cu-sulfides and euhedral specularite (fig. 4g, h, i). mirmekitic intergrown of bornite and digenite are widespread. besides, most of the sulfides are usually replaced by malachite (fig. 4b). finally, the ca-co2-rich alteration stage encompasses white calcite and minor calcium-rich zeolites occurring as veinlets and infill of open spaces (fig. 4j, k, l). calcite often surrounds and replaces albite, quartz, prehnite, and other minerals from previous stages (fig. 4k). 118 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras figure 4. microphotographs of typical mineral assemblages of basalt-hosted mineralization. a) basalt altered to albite and crosscut by a vein consisting of albite and quartz. b) non-vesicular basalt affected by three alteration stages consisting of albite, k-feldspar, and epidote malachite. c) pervasive epidotization affecting amygdaloidal basalt. there is also a veinlet composed of quartz, epidote and cuprite after native copper. d) vein containing euhedral quartz, fan-shaped prehnite and epidote. e) amygdule consisting of native copper, cuprite, quartz, prehnite and epidote. f) amygdule only consisting of native copper; note the cuprite corona around native copper. g) non-vesicular basalt hosting cu-sulfides, bornite, covellite and chalcopyrite. note the mirmekitic intergrown between bornite and the cu-sulfides. h) quartz vein containing covellite, cu-sulfides and specularite. i) euhedral specularite and quartz. j) calcite veinlet crosscutting epidote and prehnite assemblage. note calcite surrounds some epidote fragments. k) albite crystals partially replaced by calcite. l) euhedral quartz and epidote with late-stage calcium-rich zeolite. abbreviations: ab – albite, ba – basalt, bn – bornite, cal – calcite, ccp – chalcopyrite, cpr – cuprite, cs – cu-sulfides, cu – native copper, cv – covellite, ep – epidote, kfs k-felspar, mlc – malachite, prh – prehnite, qz – quartz, sp – specularite, zeo – zeolite. 2. sediment-hosted compared to basalt-hosted mineralization, sediment-hosted was only associated with (i) ca-si-rich and (ii) ca-co2-rich alteration assemblages. in sediment-hosted deposits, the ca-si-rich alteration is less intense and consists of clinozoisite, chlorite, and abundant quartz veinlets with cusulfides (fig 5a, b). the primary ore minerals associated with this alteration include bornite, covellite, chalcopyrite, undifferentiated cu-sulfides, and specularite (fig. 5c, d, e, f). bornite and chalcopyrite often display reaction rims of covellite and undifferentiated cu-sulfides (fig. 5d). specularite occurs as euhedral crystals forming intergrowths with sulfides or containing chalcopyrite inclusions (fig. 5e). mirmekitic intergrowths of bornite and undifferentiated cu-sulfides are also observed (fig. 5f). these replacement rims and interstitial textures of cu-sulfides were also evidenced through backscattered electron (bse) images (fig. 6a, c, e). pyrite is rarely observed, appearing as framboidal and cubic aggregates commonly replaced by bornite, specularite, and undifferentiated cu-sulfides (fig. 5g). moreover, silicification and brecciation are also associated with this alteration. 119mineralogy and geochemistry of stratabound cu (±ag) deposits in specific locations, covellite and other undifferentiated cu-sulfides related to the ca-si-rich alteration were found spatially associated with charcoal clasts consisting of semifusinite. these clasts display well-preserved plant cell wall structures, and their primary porosity is occupied by cu-sulfides (fig. 5h). on the other hand, copper mineralization was also associated with tabular and highly fractured fragments of organic matter exhibiting moderate reflectance and strong anisotropy, probably corresponding to pyrobitumen (?) (fig. 5i). both semifusinite and pyrobitumen (?) were recognized through bse images. pyrobitumen (?) fragments display a dark appearance indicative of a light elemental composition (fig. 6g) and the plant cell wall structures of semifusinite were observable (fig. 6h). finally, the ca-co2-rich alteration assemblage includes calcite veinlets and breccias crosscuting former quartz and sulfide veins (fig. 5j). figure 5. microphotographs of sediment-hosted mineralization. a) gray siltstone hosting clinozoisite and quartz associated with malachite (ma) and chrysocolla after cu-sulfides. b) arkosic sandstone crosscut by a veinlet consisting of quartz and cu-sulfides. c) chalcopyrite and bornite vein; covellite and goethite are observed filling fractures. d) bornite, covellite and cu-sulfides assemblage. e) euhedral specularite exhibiting a chalcopyrite inclusion. f) mirmekitic intergrowth of bornite and cusulfides. g) aggregates of framboidal pyrite partially replaced by bornite. h) charcoal fragment made up of semifusinite spatially associated with covellite and cu-sulfides. i) pyrobitumen (?) fragment associated with covellite and cu-sulfides. j) calcite veinlets crosscutting cu-sulfides (cs). abbreviations: bn – bornite, cal – calcite, ccp – chalcopyrite, chr – chrysocolla, cs – undifferentiated cu-sulfides, cv – covellite, czo – clinozoisite, gth – goethite, mlc – malachite, pb? pyrobitumen (?), py – pyrite, qz – quartz, sf – semifusinite, sp – specularite. 120 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras figure 6. backscattered electron (bse) images of cu-sulfides and organic compounds from the “la quinta” formation. reflected polarized light (ppr) microphotographs of the cu-sulfides were included for comparison. a) and b) cu-sulfides showing replacement texture. c) and d) bornite exhibiting covellite rim. e) and f) interstitial cu-sulfides grains. g) highly fractured pyrobitumen (?) fragment. its dark color indicates a light elemental composition. h) well-preserved plant cell structures (i.e. semifusinite). abbreviations: bn – bornite, cs – undifferentiated cu-sulfides, cv – covellite, pb? – pyrobitumen (?), sf – semifusinite. 3. supergene environment in vesicular basalts, supergene oxidation of native copper has formed cu-oxides such as cuprite and minor tenorite, delaffosite, and paramelaconite (fig.7a, b, c). cuprite typically occurs along fractures and forms rims around native copper crystals (fig. 4e, f; fig. 7a). cu-carbonates (i.e. malachite and azurite) are also commonly found filling fractures and replacing native copper and cu-sulfides (fig. 7c, d). conversely, epidote and prehnite remain unaffected by supergene oxidation. contrastingly, the oxidation process of the sediment-hosted sulfides generates goethite, covellite, chrysocolla, malachite, and azurite. as sulfiderich veins oxidize, banded aggregates of botryoidal goethite and radial malachite and chrysocolla form (fig. 5a, c; fig. 7e, f). malachite often replaces calcite, whereas chrysocolla forms reaction rims around quartz (fig. 7f). besides, specularite is usually unaffected by weathering, coexisting with malachite and azurite. as a summary of the identified mineral assemblages, fig. 8 shows the paragenetic sequence for mineralization. 121mineralogy and geochemistry of stratabound cu (±ag) deposits figure 7. microphotographs of supergene mineral assemblages for basalt-hosted (a-d) and sediment-hosted (e-f) mineralization. a) minute relic of native copper surrounded by cuprite and delaffosite corona. b) cuprite and paramelaconite assemblage after native copper. c) cuprite, paramelaconite, malachite and epidote assemblage. d) azurite and malachite filling fractures in a basalt altered to albite and epidote. e) sandstone containing banded veinlets of radial malachite and chrysocolla. note there are some relics of cu-sulfides. f) siltstone hosting malachite and chrysocolla. note chrysocolla forms reaction rims that partially replace quartz. abbreviations: ab – albite, azu – azurite, ccl – chrysocolla, cpr – cuprite, cu – native copper, del – delafossite, ep – epidote, mlc – malachite, pml – paramelaconite, qz – quartz. figure 8. schematic paragenetic sequence for basaltand sediment-hosted cu (±ag) mineralization in the “la quinta” formation. anilite, spionkopite and digenite (undifferentiated cu-sulfides in this chapter) were added based on mineral chemistry results. 122 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras mineral chemistry electron probe micro-analyzer (epma) analyses enabled the detailed mineral composition assessment of cu-sulfides, cu-fe-sulfides, native copper, and pyrite as displayed in table 1. the chemical composition of cu-sulfides and cu-fe-sulfides varies across the different detected elements. the s content ranges from 21.06 to 32.78 wt% in cu-sulfides, while in cu-fe-sulfides it spans from 26.32 to 35.53 wt%. the cu concentration in cu-sulfides varies from 67.48 to 78.90 wt%, whereas in cu-fe sulfides it ranges from 33.71 to 62.55 wt%. the fe content for cusulfides ranges from <0.001 up to 3.87 wt%, while cu-fe-sulfides exhibit concentrations from 10.95 to 30.91 wt%. lastly, ag content varies between <0.003 and 0.13 wt% in cu-sulfides, whereas in cu-fe-sulfides it is slightly higher, ranging from 0.04 to 0.31 wt%. besides, the analyzed cu-sulfides exhibit a composition range from cu1.04s to cu1.089s. the prior allows the differentiation between anilite, covellite, digenite, and spionkopite. anilite has an average composition of cu1.74s, covellite of cu1.04s, digenite of cu1.86s, and spionkopite is characterized by cu1.38s. additionally, the cu-fe-sulfides (i.e. chalcopyrite and bornite) have average compositions of cu 0.99fe1.01s2 and cu4.52fe1.06s4, respectively. as depicted in fig. 9, the metal/sulfur ratio ([cu + fe + ag]/s) indicates that bornite, chalcopyrite, digenite, and the unidentified cu-sulfide are the main ag host minerals. the plot also displays a wide range of variation in the (cu + fe + ag)/s and cu/s ratios. the former spans from 1.5 to 4, and the latter varies from 0.95 to 3.75. on the other hand, the analyzed native copper crystals have an average cu content of 97.67 wt% and show traces of s (average: 0.256 wt%), fe (average: 0.039 wt%) and ag (average: 0.027 wt%). an anomalous ag-rich native copper crystal yielded average cu and ag contents of 62.58 wt% and 31.39 wt%, respectively. lastly, pyrite crystals have an average s composition of 51.93 wt% and an average fe content of 46.62 wt%. besides, cu and ag were also identified in pyrite, ranging from <0.005 to 0.98 wt% for the former and <0.004 to 0.031 wt% for the latter. table 1. representative epma analyses of selected ore minerals from the “la quinta” formation (in wt.%). bdl: below detection limit. mineral s cu fe ag total cu/s formula sample anilite 22.50 77.97 0.001 0.003 100.47 3.47 cu1.75s 1164-p8 anilite 22.69 77.67 bdl 0.012 100.37 3.42 cu1.72s 1164-p8 covellite 32.78 67.48 0.35 0.03 100.64 2.06 cu1.04s 1164-p8 covellite 32.57 67.75 0.04 0.06 100.42 2.08 cu1.05s 1164-p8 digenite 21.06 78.90 0.18 0.04 100.18 3.75 cu1.89s 1164-p8 digenite 21.58 77.85 0.04 0.12 99.59 3.61 cu1.82s 938-p1 spionkopite 27.66 72.49 0.02 0.08 100.25 2.62 cu1.32s 1164-p8 spionkopite 25.32 72.94 1.32 bdl 99.57 2.88 cu1.45s 1143b-p4 cu-sulfide 22.41 72.83 3.87 0.13 99.24 3.25 cu1.64fe0.09s 1141-p5 bornite 27.67 53.47 16.54 0.04 97.72 1.93 cu3.90fe1.37s4 1164-p8 bornite 26.54 61.82 11.37 0.31 100.04 2.33 cu4.70fe0.98s4 1141-p5 bornite 26.32 62.41 11.13 0.09 99.94 2.37 cu4.78fe0.97s4 1141-p5 bornite 26.71 62.55 10.95 0.09 100.31 2.34 cu4.72fe0.94s4 938-p1 chalcopyrite 34.91 34.91 29.36 0.11 99.30 1.00 cu1.01fe0.96s2 1141-p5 chalcopyrite 35.53 33.71 30.30 bdl 99.54 0.95 cu0.96fe0.98s2 1141-p5 chalcopyrite 35.03 34.53 30.75 0.06 100.37 0.99 cu0.99fe1.01s2 1197-p9 chalcopyrite 34.33 34.08 30.752 0.104 99.275 0.99 cu1.00fe1.03s2 1197-p9 chalcopyrite 34.97 34.36 30.68 0.06 100.08 0.98 cu0.99fe1.01s2 1197-p9 chalcopyrite 34.96 34.32 30.91 0.07 100.28 0.98 cu0.99fe1.01s2 1197-p9 native copper 0.09 99.96 bdl 0.02 100.07 cu99.89ag0.02 1143-p4 native copper 0.09 99.08 bdl 0.01 99.18 cu99.90ag0.01 1143-p4 native copper bdl 98.75 bdl 0.043 98.793 cu99.96ag0.04 848-p11 native copper 0.145 99.062 0.012 0.018 99.237 cu99.82ag0.02 1045-p6 native copper 0.524 93.977 0.071 0.032 94.604 cu99.34ag0.03 1045-p6 native copper 0.432 62.579 0.015 31.39 94.412 cu66.28ag33.24 2-p7 native copper bdl 95.209 0.058 0.043 95.31 cu99.89ag0.05 2-p7 pyrite 49.01 0.98 46.50 bdl 96.49 fe1.09s2 1143b-p4 pyrite 52.442 0.008 46.95 0.031 99.431 fe1.03s2 968-p9 pyrite 52.402 0.005 46.79 bdl 99.197 fe1.02s2 968-p9 pyrite 52.114 bdl 46.445 0.004 98.563 fe1.02s2 969-p9 pyrite 52.12 bdl 46.397 0.012 98.529 fe1.02s2 969-p9 pyrite 52.327 0.013 46.305 0.015 98.66 fe1.01s2 969-p9 pyrite 53.068 0.019 46.93 0.022 100.039 fe1.01s2 970-p9 123mineralogy and geochemistry of stratabound cu (±ag) deposits figure 9. epma data for cu-sulfides and cu-fe-sulfides plotted as ag (wt.%) vs. [cu + fe + ag] / s, showing that the highest silver concentrations (> 0.1 wt. %) are associated with bornite, chalcopyrite, digenite and unidentified cu-sulfide. lithogeochemistry table 2 presents the chemical composition of five outcrop rock samples with different degrees of alteration and mineralization. the samples include a cu-rich vesicular basalt with intense ca-si-rich alteration characterized by abundant epidote (1161-p7), a basalt with moderate epidotization (i.e. ca-sirich alteration) (1225-p4), a basalt affected by ca-co2-rich alteration (1163p7), a cu-rich sandstone (1164-p8), and an unaltered siltstone (1233-p8). fig. 10 displays mass balance diagrams illustrating the main changes in elemental composition after hydrothermal alteration. the composition of basalt samples (1161-p7, 1225-p4, and 1163-p7) was normalized using the compositional values of sample 1125 (unaltered basalt) from cano et al. (2022). in contrast, the mineralized sandstone composition (1164-p8) was normalized with data from sample 1233-p8 (unaltered siltstone). major elements in the intensely epidotized basalt (1161-p7) evidence an increase in fe2o3, cao, cr2o3, tio2, mno, and sro contents, while depletion of mgo, na2o, k2o is observed (fig. 10a). trace elements behavior shows an addition of ga and v, while cs, rb, co, li, ni, and zn are diminished (fig. 10c). in contrast, both major and trace elements do not vary significantly in the moderately epidotized basalt (1225-p4) (fig. 10 a, c). besides, the basalt affected by ca-co2-rich alteration (1163-p7) shows enrichment in cao, k2o, cr2o3, and tio2, while al2o3, mgo, and sro are depleted (fig. 10a). for this sample, contents of trace elements such as cs, rb, as, and ni increase unlike ga, th, u, v and li which are reduced (fig. 10c). on the other hand, the cu-rich sandstone (1164-p8) exhibits an increase in the concentration of cao, na2o, tio2, mno, and p2o5 along with depletion of k2o, and bao (fig. 10b). this sample evidence alteration in terms of sr, u, zr, nb and ta, while contents of cs, rb w, sc, ni, pb, and zn show depletion (fig. 10d). additionally, hydrothermal alteration influences the variation of largeion lithophile elements (liles). the basalt sample with intense ca-si-rich alteration (1161-p1) and the cu-rich sandstone (1164-p8) show strong depletion on liles such as k, rb, and cs. oppositely, there is enrichment of the same elements in the basalt with ca-co2-rich alteration (1163-p7) (fig.a, b, c, d). furthermore, there is no significant alteration of sio2 in the basalts affected by ca-si-rich and ca-co2-rich alterations, nor in the cu-rich sandstone (fig. 10a, b). likewise, the behavior of rare earth elements (ree) in both analyzed lithologies shows no significant variation, implying no ree mobility (fig. 10e, f). discussion hypogene versus supergene mineralization field and mineralogical observations of basalt-hosted native copper and native silver support their hypogene, epigenetic origin, in association with minerals such as epidote, prehnite, chlorite, and quartz (fig 9). additionally, we identified cuprite, tenorite, delafossite, paramelaconite, malachite, and azurite as products of in-situ supergene oxidation, replacing the native copper. the sediment-hosted copper mineralization includes primary/hypogene chalcopyrite, bornite and digenite. epma results revealed the presence of sulfides of the cu-s system (digenite-chalcocite series), specifically covellite, anilite, spionkopite, and digenite. spionkopite and anilite mainly occur in cu-bearing deposits, and their formation is interpreted as a low-temperature transformation of hypogene sulfides (hatert, 2005; gablina et al., 2004; sadati et al., 2016). the scarcity of chalcocite might indicate its complete transformation into spionkopite and anilite, which are more stable under low-temperature conditions (gablina et al., 2004). therefore, the mineral assemblage has been interpreted as digenite + anilite + spionkopite + covellite, occurring in sediment-hosted and basalt-hosted styles in the “la quinta” formation. subsequent weathering of sediment-hosted cu-rich sulfides caused their replacement by malachite, azurite, chrysocolla, and goethite. it is noteworthy that digenite occurs in both stages: i) associated with hypogene bornite, as symplectitic texture coexistence (fig. 6f) and (ii) as part of the supergene mineral assemblage (fig.7b, c). 124 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras figure 10. multielement diagrams showing the normalized distribution of major, trace, and rare earth elements (ree). basalt samples were normalized with respect to sample 1125 (unaltered basalt) described by cano et al. (2022), while the sandstone sample was normalized using sample 1233-p8 (unaltered siltstone). a)-b) major elements in mineralized basalts and sandstone. c)-d) trace elements variation in mineralized basalts and sandstone. e)-f) ree distribution in mineralized basalts and sandstone. table 2. chemical composition of selected rock samples. heb: basalt with intense ca-si-rich alteration; ms: mineralized sandstone; cb: basalt with ca-co2-rich alteration; ns: unmineralized siltstone; meb: basalt with moderate ca-si-rich alteration; loi: loss on ignition. sample 1161-p7 1164-p8 1163-p7 1233-p8 1225-p4 1161-p7 1164-p8 113-p7 1233-p8 1225-p4 lithology heb ms cb ns meb sc 17 10 16 14 15 major elements (wt. %) la 42.2 19.8 40.1 19.4 34.6 sio2 41 66.5 49 71.1 54.8 ce 86.2 38.5 84.1 35.5 73.2 al2o3 17.3 10.75 14.35 10.15 17 pr 10.9 5.16 10.2 4.95 8.69 fe2o3 12.1 4.44 9.8 5 9 nd 43.7 21 41.4 20.5 33.9 cao 20.4 3.42 8.42 2.34 5.93 sm 8.36 4.51 8.2 4.5 6.82 mgo 1.78 2.66 1.88 2.59 4.13 eu 2.47 0.93 2.36 0.99 1.9 na2o 0.05 3.22 4.67 2.29 4.26 gd 7.15 4.53 7.39 3.67 5.95 125mineralogy and geochemistry of stratabound cu (±ag) deposits sample 1161-p7 1164-p8 1163-p7 1233-p8 1225-p4 1161-p7 1164-p8 113-p7 1233-p8 1225-p4 k2o 0.04 0.59 2.53 2.22 1.72 tb 1 0.65 1.06 0.54 0.87 cr2o3 0.02 0.01 0.02 0.01 0.01 dy 5.1 4.05 5.62 3.52 5.36 tio2 1.86 1.04 1.84 0.92 1.32 ho 0.97 0.81 1 0.73 1 mno 0.2 0.1 0.15 0.07 0.15 er 2.65 2.31 2.66 2.27 2.85 p2o5 0.58 0.16 0.6 0.11 0.5 tm 0.36 0.36 0.39 0.33 0.36 sro 0.55 0.02 0.01 <0.01 0.05 yb 2.22 2.47 2.2 2.15 2.49 bao <0.01 0.01 0.04 0.05 0.05 lu 0.31 0.39 0.35 0.35 0.39 loi 3.42 5.21 7.43 3.71 2.6 hf 6.5 12.2 5.7 8.1 5.5 total 99.3 98.1 100.7 100.6 101.5 y 27.7 24.1 28.5 25 28.7 trace elements (ppm) nb 25.9 13.2 27 10.7 18.9 ba 9.9 135.5 394 448 457 ta 1.4 0.9 1.5 0.7 1.1 cr 150 80 150 100 50 ag 5.1 12.5 <0.5 <0.5 <0.5 cs 0.05 0.63 0.74 1.71 0.42 as 7 <5 26 7 <5 ga 42.1 13.3 18.6 13.3 20.5 cd <0.5 <0.5 <0.5 <0.5 <0.5 rb 1 20.6 71.1 90.6 48.6 co 15 16 30 15 23 sn 2 2 2 2 2 li 10 40 10 40 20 sr 4550 174 130.5 83.2 398 mo 1 1 <1 1 1 th 3.89 5.97 4.11 5.21 5.32 ni 32 22 99 25 43 u 1.04 1.87 0.62 1.49 1.13 pb 36 12 32 17 <2 v 262 175 90 168 169 tl <10 <10 <10 10 <10 w <1 1 2 2 <1 zn 35 63 129 130 109 zr 262 461 270 353 270 cu 13600 13500 29 33 122 in the “la quinta” formation, the development of a supergene enrichment blanket involving leaching and a secondary accumulation of copper is improbable. first, the lack of pyrite in the host rocks and favorable alteration mineralogy prevents the formation of sulfuric acid from weathering. second, the widespread occurrence of acid-neutralizing minerals, such as carbonates, feldspars, and silicates, precludes the acidification of oxidizing solutions. consequently, supergene oxidation of the cu ores in the “la quinta” formation has only led to the formation of in-situ minerals stable under near neutral ph (chavez, 1990). ore forming conditions based on mineral stability ranges and microthermometry studies of fluid inclusions in gangue minerals, gonzález-duran (2016) indicated that hypogene mineralization occurred at 100-260°c, caused by the circulation of moderately saline and highly oxidized fluids with near neutral ph. their estimates align with the mineral assemblages observed in this study. for instance, the minimum crystallization temperature for the albite-k-feldspar alteration assemblages was no less than 150°c (henley et al., 1983). during calc-silicate alteration, the minimum temperature is well constrained by the appearance of garnet, epidote, and prehnite, spanning 177-200°c (henley et al., 1983; bird, 1984; easton, 1977). the widespread presence of hematite, epidote, calcite, and albite serves as evidence for the saline and oxidized nature of the hydrothermal fluids. under these conditions, copper and silver are efficiently transported by hydrothermal fluids through complexation with chloride ions (brown, 2014). for copper to precipitate, abrupt changes in the chemical or physical properties of the aqueous solutions are needed. in the sediment-hosted ores, the close spatial association of carbonaceous fragments (semifusinite and pyrobitumen?) with cu-sulfides strongly suggests a genetic link. organic matter has been shown to be an effective agent for precipitating cu-sulfides via fluid reduction in stratabound deposits (wilson and zentilli, 1999; 2006; hayes et al., 2015; herazo et al., 2020; mahdavi & rajabi, 2023). additionally, organic matter may provide reduced sulfur, promoting metal precipitation (brown, 2014; herazo et al., 2020). thus, we propose copper precipitation could have been triggered by organic matter in the charcoal-bearing layers of the la quinta formation. the presence of framboidal pyrite in the gray siltstones might have acted as a geochemical trap for copperbearing fluids to precipitate via replacement of pyrite; this mechanism has been extensively documented in sediment-hosted deposits worldwide (brown, 2014; herazo et al., 2020). on the other hand, the mechanism of native copper +/silver precipitation in the volcanic rocks should have followed a different pathway since no organic matter is observed in this lithology. alternative processes may include cooling by mixing with meteoric fluids, reduction of evolved fluids induced by fluidrock reactions, and copper supersaturation of residual fluids under metamorphic conditions (jolly, 1974; brown, 2006; pinto et al., 2010). for the native copper mineralization style, a sulfur-poor fluid can be inferred based on the lack of sulfur-bearing minerals. vesicular basalts generally lack volatiles (e.g. sulfur) due to degassing. in contrast, non-vesicular basalts usually contain traces of sulfides (brown, 2014). for instance, cano (2015) reports chalcopyrite and bornite inclusions in pyroxenes from non-vesicular basalts from the la quinta formation. as proposed by kirkham (1984), brown (2014), and ikehata et al. (2016), hydrothermal fluids can precipitate copper and associated metals by encountering and replacing sulfides. similarly, in the sediment-hosted domain, pre-ore sulfides such as framboidal pyrite can act as reduced sulfur sources that induce copper precipitation (brown, 2014; hayes et al., 2015). although the aforementioned factors may shed light on copper precipitation, the source of fluids, reduced sulfur, and metals remains uncertain. geochemical data shows that the main elements involved in metasomatism are ca, mg, na, k, and fe. when comparing data from basalts and sediments, there is a consistent trend involving considerable ca-mn increase and k-cs-rb-zn depletion. however, the variation of other elements differs. besides, the element distribution is not uniform in the basalts, reflecting the complexity of hydrothermal alteration overprint at different degrees. these variations are interpreted because of rock-controlled metasomatism, where a similar mineralizing fluid interacts with distinct lithologies, but the mineral and geochemical output may differ according to rock chemistry. given that the observations raised in this study are based on a limited number of samples, more data is needed to establish suitable geochemical fingerprints for exploration purposes. 126 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras ta bl e 3 . c om pa ra tiv e t ab le o f t he m ai n fe at ur es o f s ed im en t an d/ o r v ol ca ni cho ste d cu -a g de po sit s w or ld w id e. m in er al ab br ev ia tio ns ar e t ak en fr om w ar r ( 20 21 ). a bb re vi at io ns fo r g an ge m in er al s: a b a lb ite , a ct a ct in ol ite , a nh a nh yd rit e, a nl a na lc im e, ca l c al ci te , c bca rb on at e, ch l c hl or ite , c zo cl in oz oi sit e, d ol d ol om ite , e p e pi do te , k fs k -fe lsp ar , g rt g ar ne t, m s m us co vi te , l m t l au m on tit e, ph l p hl og op ite , pm p p um pe lly ite , p rh pr eh ni te , q z q ua rtz , r t r ut ile , s cp sc ap ol ite , s er se ric ite , t tn ti ta ni te , t ur to ur m al in e, ze o – ze ol ite . a bb re vi at io ns fo r o re m in er al s: a g n at iv e s ilv er , a ni a ni lit e, a ta a ta ca m ite , a zu a zu rit e, bn bo rn ite , b nr br an ne rit e, cl i c ar ro lli te , c c c ha lc oc ite , c cl -c hr ys oc ol la , c th -c la us th al ite , c cp ch al co py rit e, cu n at iv e c op pe r, cv co ve lli te , d g d ig en ite , d ju d ju rle ite , g n g al en a, g nk g re en oc ki te , g th g ot hi te , h em h em at ite , m lc m al ac hi te , m ol m ol yb de ni te , p y p yr ite , s p s ph al er ite , s pi sp io nk op ite , t trtn t t et ra he dr ite -te nn an tit e, u rn u ra ni ni te , w ur – w ur tz ite . 127mineralogy and geochemistry of stratabound cu (±ag) deposits comparative analysis of cu-(±ag) mineralization in the “la quinta” formation and similar deposits worldwide the stratabound cu-(±ag) deposits in the perijá range are hosted in a volcano-sedimentary pile containing continental, oxidized redbeds interlayered with subaerial, felsic to mafic flows (maze, 1984; jimenez, 2010). similar hosting sequences are described for volcanic redbed cu deposits and chilean mantos (kirkham, 1984; kojima et al., 2019). for instance, the keweenaw district native cu lodes, classified by kirkham (1984) as a volcanic redbed cu deposit, are hosted by tholeiitic basaltic flows and interflow conglomerates of the portage lake volcanics unit (brown, 2006; bornhost et al., 1988). on the other hand, chilean manto-type deposits are predominantly hosted by bimodal volcanic rocks (basalt to rhyodacite) and minor sedimentary strata (maureira et al, 2023, kojima et al., 2019). furthermore, the lithologies of the “la quinta” formation that host the mineralization are contrastingly different from those evidenced in kupferschiefer-type deposits, in which ore minerals are associated with reduced-facies siliciclastic or dolomitic units of marine to lacustrine environments (hitzman et al., 2005). the hypogene mineralization in the “la quinta” formation mainly encompasses chalcocite, bornite, and, in minor amounts, native copper and native silver (jimenez, 2010; gonzález-durán, 2016). besides, as evidenced by the mineral chemistry assays, additional silver can also be hosted in cusulfides. despite some differences, overall ore content of cu-(±ag) deposits in the perijá range displays closer similarities to volcanic redbed cu deposits, chilean mantos, and deposits such as white pine and keshtmahaki. the ore minerals at these deposits mainly include cu-sulfides and/or native copper with silver-bearing phases (bornhorst et al., 1988; jones et al., 2023; kirkham, 1984; kojima et al., 2009; jiba et al., 2021). by contrast, the “la quinta” formation ores do not include cobalt, uranium, molybdenum, or gold-bearing minerals, which are commonly found in the central african copper belt (cacb) deposits (hitzman et al.,2005; selley et al., 2005; jimenez, 2010). additionally, there are no lead or zinc sulfides, which are common in kupferschiefer-type deposits (hitzman et al., 2005; borg et al, 2012). regarding hydrothermal alteration, both na-rich (albite ± quartz) and k-rich (k-feldspar – quartz) alteration assemblages might reflect the regional flow of non-mineralizing fluids through the “la quinta” formation. none of these are paragenetically associated with copper-silver ores. then, zones with intense na-rich and k-rich alterations do not indicate ore presence. hitzman et al. (2005) associate the widespread occurrence of both alteration types with the basin-scale flow of diagenetic brines in the katangan supergroup of the cacb. there, intensely altered zones reflect a high rock-fluid interaction, however, they do not necessarily indicate ore presence since these alteration zones extend considerably beyond the orebodies (hitzman et al., 2005). on the contrary, the cu-(±ag) mineralization at the “la quinta” formation occurs in paragenetic association with intense ca-si-rich alteration. epidote-prehnite ± pumpellyite comprising this alteration might indicate a prograde low-grade (or burial) metamorphic environment for the mineralizing fluids genesis (jimenez, 2010; hitzman et al., 2005; kirkham, 1984). these burial metamorphic conditions are further supported considering the significant thickness of the “la quinta” formation (1600 to 3000 meters) and the absence of penetrative deformation features (e.g. schistosity) related to regional (orogenic) metamorphism (jimenez, 2010; kirkham, 1984). besides, the fluid inclusions temperatures obtained for the hypogene mineralization (100 to 260 °c) at the “la quinta” formation (gonzález-durán, 2016) would be in accordance with a low-grade metamorphic environment. similar features were described for volcanic redbed cu and manto-type deposits. for instance, portage lake volcanics in the keweenaw district display mineral associations characteristic of subgreenshist conditions. the native cu orebodies in this district overlie the pumpellyiteepidote transition zone (kirkham, 1984; hitzman, 1984; brown, 2006; jolly, 1974). on the other hand, chilean mantos host sequences have thicknesses of over 2 kilometers and were also subjected to burial or lower-greenschist metamorphism conditions (kojima et al., 2009; hitzman et al., 2005; maureira et al., 2023; herazo et al., 2020). on the contrary, kupferschiefer-type deposits are not affected by metamorphism (hitzman et al., 2005). the mineralization styles in the study area encompass sediment-hosted and basalt-hosted deposits. in these settings, ore predominantly occurs as disseminations, vesicle fillings, and, to a lesser extent, as hydrothermal breccias and veinlets (gonzález-durán, 2016). moreover, in the san diego area, jimenez (2010) also described the deposits as tabular bodies or lenses. their dimensions vary from 2 to 10 meters in length and thicknesses of less than 1 meter. these orebodies are e-w striking structures with high-angle dips that crosscut the homoclinal host lithologies. such characteristics support an epigenetic origin for mineralization and suggest that ore localization depends on lithologicalsedimentological (i.e. permeability) and structural controls (i.e. fault zones) (jimenez, 2010). both are key since they allowed the fluid flow through the “la quinta” formation. hitzman et al. (2005) and kirkham (1984) described comparable ore localization controls for other sedimentand/or volcanic-hosted copper deposits, arguing that they reflect the basin architecture in which the mineralization occurred. additionally, the cu-(±ag) mineralization of the “la quinta” formation is related to in situ reductants. semifusinite, pyrite, and plagioclase are in close spatial association with ore minerals. then, they are suggested to be acting as “chemical traps” (gonzález-durán, 2016; jimenez, 2010). nevertheless, the possible occurrence of pyrobitumen with mineralization might also indicate that mobile reductants (i.e. hydrocarbons) could have acted as a key agent responsible for triggering ore precipitation. despite further evidence being required for the perijá range orebodies, similar redox mechanisms have been described for deposits such as white pine, chilean mantos (e.g. lorena and el soldado), and those belonging to the cacb (hitzman et al., 2005; jones et al., 2023; herazo et al., 2020). according to the above, entrapment structures usually explored for hydrocarbons, such as structural (e.g. fault zones) and stratigraphic traps (e.g. pinch-outs) could also be potential targets for cu-(±ag) mineralization in the perijá range (hitzman et al.,2005). table 3 summarizes the main characteristics of sedimentand/ or volcanic-hosed cu-ag deposits worldwide, including the cu-(±ag) deposits hosted by the “la quinta” formation at the perijá range. concluding remarks the cu (±ag) deposits hosted by the “la quinta” formation exhibit two distinct mineralization styles: basalt-hosted and sediment-hosted mineralization. basalt-hosted type is mainly formed by native copper and native silver, with minor amounts of cu-sulfides. silver can also exist as cu-ag alloys or within cu-sulfides and cu-fe-sulfides. in contrast, the sediment-hosted mineralization comprises primary cu-sulfides, including bornite, digenite, and chalcopyrite. as observed in basalts, cu-sulfides and cu-fe-sulfides in sedimentary units also host traces of silver. the identified hydrothermal stages include na-rich, k-rich, ca-si-rich, and ca-co2-rich alterations. the first alteration assemblage comprises albite + quartz, while the second includes k-feldspar + quartz. the third contains epidote + prehnite + chlorite + quartz, and the fourth is characterized by calcite + zeolite. notably, the cu (±ag) mineralization is closely associated with the ca-si-rich alteration in the basaltic and sedimentary units. these mineral assemblages and lithogeochemistry vary according to host rock composition, suggesting that ore precipitation was influenced by the lithological features of the hosting units. in sediment-hosted deposits, the ore minerals are spatially associated with charcoal clasts and possibly pyrobitumen. these forms of organic matter are possible sources of reduced sulfur, which may result in cu-sulfides precipitation upon contact with mineralizing fluids. conversely, the lack of sulfur-bearing minerals in basalts suggests that this lithology and/or the mineralizing fluids are sulfur-deficient. additionally, supergene oxidation has overprinted hypogene ores, forming secondary cu-sulfides including spionkopite, anilite, and covellite. other identified secondary cu-bearing minerals are cuprite, tenorite, paramelaconite, delaffosite, malachite, azurite, and chrysocolla. acknowledgements the authors thank the colombian geological survey (servicio geológico colombiano) for funding this research. the support from leonardo santacruz, paola rodríguez, and carolina jiménez with epma analyses is greatly acknowledged. william chavez, eric petersen, carlos mario celada, celso colombo tassinari, and antónio manuel mateus are acknowledged for their kind field and conceptual assistance. 128 andrés gonzález, juan molano, maría sepúlveda, néstor cano, nathalia guerrero, luis contreras references alvarán, m., naranjo, e., and zapata, e. 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(1957). formación cuprífera de la serranía de perijá intendencia de la guajira y departamento del magdalena. informe 1193, 35 p. servicio geológico nacional, bogotá. https://revistas.unal.edu.co/index.php/geocol/article/view/30670 https://revistas.unal.edu.co/index.php/geocol/article/view/30670 https://doi.org/10.2113/gsecongeo.84.5.1003 https://doi.org/10.2113/gsecongeo.84.5.1003 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.95993 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (june, 2022): 139 156 g eo lo g y petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes néstor cano1,2,*, juan carlos molano2, janeth sepúlveda3 1. programa de posgrado en ciencias de la tierra, universidad nacional autónoma de méxico. 2. departamento de geociencias, universidad nacional de colombia, bogotá, colombia. 3. servicio geológico colombiano, bogotá, colombia * corresponding author: nacanoh@unal.edu.co; nacanoh@comunidad.unam.mx. abstract the la quinta formation is a triassic-jurassic volcano-sedimentary unit exposed in colombia and venezuela, that crops out along both flanks of the serranía del perijá and in western mérida andes. it is composed of reddish mudstones and sandstones interbedded with basaltic-to-rhyolitic lava flows and ignimbrites. this succession is locally intruded by dacitic-rhyolitic hypabyssal bodies, felsic dikes and small monzodioritic intrusions. the volcanism recorded in la quinta comprises the whole subalkaline compositional spectrum from basaltic andesites-alkali basalts (plagioclase-augite), though andesites-trachyandesites-dacites (plagioclase-augite-β quartz-amphibole) and rhyolites-alkali rhyolites (β quartz-sanidine-plagioclase-biotite). magnetite, ilmenite, hematite, zircon and apatite are recurrent accessory phases and suggest dominant oxidized conditions (+1.2 ni-nio to hematite-magnetite buffers) in the magmas. clinopyroxene and ilmenite-magnetite geothermobarometry suggest an early crystallization of the former (ca. 1120 °c and 4.6 kbar) in the middle portion of a normal-to-thickened continental crust (26-49 km), and later fractionation of the fe oxides (ca. 720 °c). in addition, some samples exhibit calc-silicate alteration formed of hydrated grossular-andradite, epidote and zeolites in association with possible sediment-hosted stratiform copper occurrences. according to mineralogical and geochemical evidence, these rocks belong to a single comagmatic suite in which differentiation processes were driven by low-pressure fractional crystallization and middle-crustal assimilation. moreover, these rocks exhibit geochemical features indicative of low degrees of partial melting of a spinel lherzolite (or an enriched source) in a hybrid setting between subduction-related arc and intracontinental rifting. thus, we propose a mixed tectonic scenario in which subduction of the farallon plate beneath western south america combined with transtensional tectonics resulting from the break-up of pangea, generated volcanic activity with an inherited arc-signature behind (landward) the arc axis. keywords: triassic-jurassic igneous rocks; hybrid arc-rift magmatism; break-up of pangea; petrogenesis; stratiform cu deposits. consideraciones petrogenéticas de las rocas ígneas de la formación la quinta en la serranía del perijá, porción norte de los andes colombianos resumen la formación la quinta es una unidad volcano-sedimentaria del triásico-jurásico que aflora en ambos flancos de la serranía del perijá y en la parte occidental de los andes de mérida, en colombia y venezuela. está conformada por capas de lodolita y arenisca rojiza intercaladas con lavas basálticas a riolíticas e ignimbritas. esta sucesión está localmente intruida por cuerpos hipoabisales riolíticos-dacíticos, diques félsicos y stocks monzodioríticos. el volcanismo registrado en la formación la quinta abarca un rango composicional amplio en la serie subalcalina que incluye: basalto alcalino-andesita basáltica (plagioclasa+augita), andesita-traquiandesita-dacita (plagioclasa+augita+cuarzo+anfíbol) y riolita-riolita alcalina (cuarzo+sanidina+plagioclasa+biotita). los minerales accesorios son magnetita, ilmenita, hematita, circón y apatito, lo que sugiere condiciones oxidantes en los magmas (desde +1.2 ni-nio hasta el campo de hematita-magnetita). análisis de geotermobarometría en clinopiroxeno (~1120 °c y 4.6 kbar) indican que su cristalización ocurrió en la porción media de una corteza continental normal a engrosada (26-49 km), y precedió al fraccionamiento de ilmenita-magnetita (~720 °c). adicionalmente, las rocas exhiben alteración de calcisilicatos (grosularia-andradita hidratada, epidota y ceolitas), en asociación con posibles depósitos cupríferos estrato-ligados. las evidencias mineralógicas y geoquímicas halladas sugieren que estas rocas pertenecen a una suite comagmática donde los procesos de diferenciación estuvieron gobernados por la cristalización fraccionada a baja presión y la asimilación parcial de la corteza media. asimismo, estas rocas se formaron a partir de grados de fusión parcial bajos en una lherzolita de espinela (o una fuente enriquecida) dentro de un ajuste tectónico suprasubducción de naturaleza híbrida entre un arco y un rift. de tal modo, en este artículo se propone un escenario tectónico mixto en el cual la placa farallón subducía bajo la placa suramericana al mismo tiempo que se generaban cuencas transtensionales ligadas al rompimiento de pangea. de allí la existencia de la actividad volcánica con firma de arco en la quinta, que estaba ubicada detrás del arco principal. palabras clave: rocas ígneas triásicas-jurásicas; magmatismo híbrido de arco-rift; rompimiento de pangea; petrogénesis; depósitos de cobre estratoligados. record manuscript received: 23/05/2021 accepted for publication: 14/06/2022 how to cite item: cano, n., molano, j. c., & sepúlveda, j. (2022). petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes. earth sciences research journal, 26(2), 139156. https://doi.org/10.15446/esrj.v26n2.95993 https://doi.org/10.15446/esrj.v26n2.95993 https://doi.org/10.15446/esrj.v26n2.95993 140 néstor cano, juan carlos molano, janeth sepúlveda 1. introduction latest triassic-jurassic (ca. 160-210 ma) volcanic and plutonic rocks are widespread in discontinuous localities along the colombian andes (figure 1), including the southern portion of the eastern and central cordilleras, the upper magdalena valley, santander massif, serranía de san lucas, serranía del perijá, sierra nevada de santa marta and la guajira peninsula (bustamante et al., 2010; leal-mejía et al., 2019; spikings et al., 2019; bayona et al., 2020; lópez-isaza & zuluaga, 2020; ramírez et al., 2020). these rocks display a wide compositional range in the sub-alkaline spectrum (basalt/gabbro to rhyolite/granite) (rodríguez et al., 2018; correa-martínez et al., 2019; lópezisaza & zuluaga, 2020; rodríguez-garcía & obando, 2020). the intrusive exponents are normally coarse quartzmonzonites, monzonites, monzogranites, granodiorites and minor diorites and gabbros (spikings et al., 2019), whereas the volcanic and subvolcanic rocks include basaltic-rhyolitic lava flows, andesiticrhyolitic tuffs, ignimbrites, epiclastic deposits and andesitic-dacitic hypabyssal bodies (maze, 1984; rodríguez-garcía et al., 2016; correa-martínez et al., 2019; ramírez et al., 2020; rodríguez-garcía & obando, 2020). additionally, they show consistent evidence of arc-related magmatism resulting from the subduction of the farallon plate beneath the western margin of disassembling pangea (maze, 1984; mojica & kammer, 1995; bustamante et al., 2010; lealmejía, 2011; villagómez et al., 2011; van der lelij et al., 2016a; rodríguez et al., 2018; correa-martínez et al., 2019; leal-mejía et al., 2019; spikings et al., 2019; bayona et al., 2020; lópez-isaza & zuluaga, 2020; rodríguez-garcía & obando, 2020). the present work will be centered in the la quinta formation, which outcrops in the serranía del perijá and the mérida andes (figure 1) and is mostly constituted by upper triassic-jurassic volcanic products interstratified with clastic sedimentary rocks (champetier de ribes et al., 1961; forero, 1972; maze, 1984; gómez et al., 2010; jiménez, 2010; gonzález-iregui et al., 2015; rodríguez-garcía & obando, 2020). this unit has been studied by several authors since the last century, partly because it hosts stratabound cu±ag mineralization concentrated in certain lava flows and sandstones with plants remains (wokittel & restrepo-acevedo, 1955; champetier de ribes et al., 1961; radelli, 1961; pagnacco, 1962; rodríguez, 1986; jiménez, 2010; ortega et al., 2012; gonzález-duran et al., 2017). furthermore, the ubiquity of volcano-sedimentary successions ascribed to this unit in the westernmost merida andes and throughout the serranía del perijá (figure 1) makes it a good natural “laboratory” to study several features of the triassic-jurassic, such as: paleoclimatology, tectonic arrangement during the break-up of pangea, structural and depositional conditions, crustal thickness, nature of the magmatism, etc. (e.g., maze, 1984; ostos et al., 2005; langer et al., 2014; nova et al., 2019). figure 1. location of the study area within the regional framework of latest triassic-jurassic magmatic rocks from ecuador, colombia and venezuela. the area pertain to the maracaibo orogenic float (mof) (cediel, 2019). the study zone is divided in two areas, one around the san diego municipality (south) and the other near la jagua del pilar (north). samples from previous studies are also depicted (jiménez, 2010; rodríguez-garcía & obando, 2020), including a u-pb zircon la-icp-ms age of 183.1 ± 3.2 ma reported by jiménez (2010) for a rhyolite. the location of major batholiths and volcanic-hypabyssal rocks was based on maze (1984), van der lelij et al. (2016), nova et al. (2019), bayona et al. (2020) and lópez-isaza & zuluaga (2020), and the geological basemap was taken from the 1:100.000 scale geological sheets 34-valledupar and 27-agustín codazzi made by the colombian geological survey (sgc, servicio geológico colombiano) (invemar et al., 2007; grp ltda., 2010). 141petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes as shown by maze (1984), gómez et al. (2010) and gonzález-iregui et al. (2015), the la quinta formation displays important facial changes due to abrupt variations in the locus (and type) of volcanism, structural complexity and geometry of intracontinental transtensional basins, and hence regional correlations are sometimes difficult to state. in addition, sparse geochemical and petrographic data are currently available for this unit in the central perijá range (e.g., jiménez, 2010; saez-paz, 2012; rodríguez-garcía & obando, 2020), and no electron probe micro-analyses (epma) neither geothermobarometric studies have been conducted in such rocks. for those reasons, this work focused on a narrow territory that extends from the san diego municipality (cesar department), southern part, to la jagua del pilar (la guajira department) to the north, which comprises a segment of the western flank of the perijá range (figure 1) where la quinta volcano-sedimentary rocks are extensively exposed in long hills with a characteristic reddish tint (colmenares et al., 2007; invemar et al., 2007; grp ltda., 2010). we centered on the volcanic rocks and hypabyssal bodies, which are one of the septentrional expressions of the jurassic suprasubduction magmatism in the colombian andes. as a result, some petrologic features are described herein, which are supported on field observations (table 1), petrography/metallography (table 2), epma (table 3a-b) and lithogeochemical data (table 4a-b). in the end, we propose a hybrid tectonic setting responsible for this magmatism, involving subduction and coeval intracontinental extension, as well as several evolutionary processes that were involved in its petrogenesis. 2. geological setting the colombian andes are divided into three mountain belts: the western, central and eastern cordilleras. the latter has its northern termination in the santander massif, where it divides into the mérida andes, which continues to venezuela with a northeast trend, and the serranía del perijá following a northnortheast orientation. the study zones are located in the central segment of the perijá range within the maracaibo orogenic float (cediel, 2019), a triangularshaped tectonic block that also encompasses the mérida andes, santander massif, sierra nevada de santa marta and the basement of lake maracaibo (kellogg, 1984; cediel, 2019). the western flank of the serranía del perijá comprises metamorphic, igneous and sedimentary rocks with ages ranging from paleozoic to mesozoic (colmenares et al., 2007; invemar et al., 2007; grp ltda., 2010). in the study zones (figure 1), the oldest unit is the perijá series (or undifferentiated cambrianordovician metasedimentites), which is exposed in discontinuous localities along a northeast general trend and consist of low-grade metasedimentites, mostly phyllites and quartzites (forero, 1972; invemar et al., 2007; gómez et al., 2010). the cachirí group (devonian) rests unconformably over the metamorphic basement and is dominated by sandstones and mudstones, with minor conglomerates at the base (forero, 1972; colmenares et al., 2007; gómez et al., 2010). moreover, the carboniferous sedimentary rocks are mostly formed of conglomerates, sandstones and siliceous mudstones, with calcareous mudstones and limestones to the top (gómez et al., 2010). the contact between the carboniferous and permian successions is difficult to establish since there is neither an unconformity nor obvious facial changes (forero, 1972). nevertheless, the permian strata (manaure formation) is mainly represented by limestones with fusulinids, sandstones and mudstones (wokittel & restrepoacevedo, 1955; forero, 1972; gómez et al., 2010). the oldest mesozoic rocks are latest triassic-early jurassic basaltic to latitic lava flows included in the triassic spilites (tschanz et al., 1969; colmenares et al., 2007; invemar et al., 2007; ramírez et al., 2020), which possibly belong to the same magmatic pulse as the la quinta formation volcanics. the youngest sedimentary record in the study areas is cretaceous in age and corresponds to: 1) the río negro formation, consisting of coarsegrained conglomerates and sandstones with minor mudstones, and 2) the undifferentiated shales and limestones, included by gómez et al. (2010) in the cogollo group, which are composed of shales and fossiliferous limestones (wokittel & restrepo-acevedo, 1955; colmenares et al., 2007; invemar et al., 2007; gómez et al., 2010). the la quinta formation was first introduced by kündig (1938; in forero, 1972) near the la grita locality in the táchira state (venezuela), encompassing a thick (ca. 2.3 km) volcano-sedimentary and red-bed succession in between permian and cretaceous rocks that is extensively exposed in western venezuelan andes. in colombia, the term was coined by miller (1969; in gómez table 1. location and field classification of the studied igneous rocks from the la quinta formation sample p e l field classification latitude longitude northing easting 1163 x basalt 10.3281 -73.1373 1634067 1102979 1167 x x rhyolite 10.2424 -73.1307 1624583 1103733 1223 x diorite 10.2712 -73.1812 1627761 1098191 1224 x x x basalt 10.2779 -73.1763 1628496 1098724 1225 x x basalt 10.2787 -73.1765 1628590 1098699 1227 x x ignimbrite 10.5057 -73.0323 1653751 1114418 1228 andesite 10.5031 -73.0366 1653460 1113954 1229 x x andesite 10.5025 -73.0420 1653392 1113356 1230 x x andesite 10.4867 -73.0575 1651641 1111670 1231 x x andesite 10.4474 -73.0937 1647276 1107718 1232 x x andesite 10.3492 -73.1599 1636390 1100505 1233 basalt 10.3282 -73.1452 1634078 1102118 1234 x x rhyolite 10.2639 -73.1628 1626954 1100206 1235 basalt 10.2707 -73.1709 1627703 1099317 1236 x basalt 10.2317 -73.2378 1623372 1092000 1237 x x x rhyolite 10.2714 -73.1818 1098128 1627780 e: epma, l: lithogeochemistry, p: petrography. northing and easting based on utm-magna_colombia_bogota. 142 néstor cano, juan carlos molano, janeth sepúlveda et al., 2010) in order to comprise jurassic sedimentary rocks outcropping on the western flank of the perijá range, the western basin of ranchería and cesar rivers and the majuyura hill (norther termination of the perijá range). after that, the term “la quinta formation” has been maintained and greatly used in both northernmost colombian and western mérida andes (champetier de ribes et al., 1961; radelli, 1961; forero, 1972; maze, 1984; montaño, 2009; gómez et al., 2010; jiménez, 2010; ortega et al., 2012; gonzález-iregui et al., 2015; van der lelij et al., 2016a; lópez-isaza & zuluaga, 2020; rodríguez-garcía & obando, 2020). forero (1972) constructed a detailed stratigraphic column of this unit along the manaure river (east of manaure municipality) and defined four sets: 1) a normally graded succession of red conglomerates and sandstones with ignimbrites to the top; 2) fine-grained cross-bedded red and purple sandstones gradually interfingered with mudstones; 3) rhyolitic tuffs interbedded with sandstones and conglomerates; and 4) acidic volcanic products (rhyolites and ignimbrites) indicating a late flare-up in the volcanism. in addition, massive and vesicular basalts are present in some localities, sometimes hosting cu mineralization (gómez et al., 2010; jiménez, 2010; ortega et al., 2012; unalsgc, 2014, 2020; gonzález-duran et al., 2017), and scarce small intrusions are also exposed (maze, 1984; and references therein). depositional dynamics in la quinta were strongly influenced by the extensional regime generated by the north and south american plates divergence, counterclockwise rotation of the yucatán block and seafloor spreading in the proto-caribbean ocean (bayona et al., 2020). fluvial deposits contemporaneous with volcanic activity produced a hybrid succession of varying thicknesses (forero, 1972; gómez et al., 2010; gonzález-iregui et al., 2015), owing to the presence of intracontinental transtensional basins (narrow graben systems) (maze, 1984; ostos et al., 2005; van der lelij et al., 2016a; bayona et al., 2020), such as: barquisimeto, uribante and machiques (nova et al., 2019); the latter corresponding to the present-day perijá range. several geochronological studies have been performed in the la quinta formation volcanic rocks in the perijá range and mérida andes, defining a ca. 160-230 ma (u-pb in zircon and ar-ar in biotite) interval for the active suprasubduction magmatism recorded in this unit (langer et al., 2014; gonzáleziregui et al., 2015; van der lelij et al., 2016b; van der lelij et al., 2016a; nova et al., 2019; rodríguez-garcía & obando, 2020). according to rodríguez-garcía and obando (2020), the volcanism became progressively acidic with time and started with basaltic trachyandesites lava flows, then trachyandesites and dacites and finally the ignimbritic-rhyolitic flare-up. furthermore, provenance analyses based on zircon la-icp-ms ages made by montaño (2009) and nova et al. (2019) in sandstones revealed four populations: 1) ca. 900-1300; 2) ca. 400-650; 3) ca. 250; and 4) ca. 180 ma, leading to the authors to postulate the santander massif and mérdia andes as the main sources of sediments. in addition, dinasaur remains (lesothosaurus sp and tachiraptor admirabilis gen. et sp. nov) (barrett et al., 2008; langer et al., 2014), conchostracans (estherida) (maze, 1984) and plant and fish (lepidotus) remnants (forero, 1972; and references therein) found in various segments of the succession suggest a wider range for sedimentation from upper triassic to lower cretaceous. the ages presented by several authors for the la quinta formation in colombia (ca. 165-188) (jiménez, 2010; gonzález-iregui et al., 2015; nova et al., 2019; rodríguez-garcía and obando, 2020) coincide with the lowermiddle jurassic ages reported in copious volcaniclastic units from other parts of the country, such as: 1) the ipapure rhyodacite (181-184 ma) in the guajira peninsula (lópez-isaza & zuluaga, 2020; and references therein); 2) the noreán formation (175-192 ma) in the northern santander massif and serranía de san lucas (correa-martínez et al., 2019); 3) the guatapurí and corual formations, golero rhyolite, los tábanos rhyodacite and ignimbritic units (e.g., caja de ahorros, la paila and los clavos) (165-195 ma) in the sierra nevada de santa marta (ramírez et al., 2020); and are slightly older than the 4) saldaña formation (146-189 ma) and pitalito volcanites (168-172 ma) in the central cordillera and upper magdalena valley (rodríguez-garcía et al., 2016; rodríguez-garcía, 2018; lópez-isaza & zuluaga, 2020). mojica & kammer (1995) named this period of intense volcanic activity as the “saldaña volcanism”. besides, it is worth noting that the late triassic-jurassic magmatic pulses are not restricted to colombia; instead, they are present in numerous localities from north, central and south america (maze, 1984; martini & ortega-gutiérrez, 2018; bayona et al., 2020). 3. methods a total of sixteen samples of volcanic rocks from the la quinta formation were collected in two zones (north and south) of the western flank of the serranía del perijá (figure 1, table 1). we collected lithologies with contrasting compositions (basalts, andesites, rhyolites, rhyolitic tuffs and diorites) in order to observe evolutionary trends. hydrothermally altered and/or weathered rocks were avoided; only the sample 1163, which exhibits visible hydrothermal alteration, was analyzed for comparison purposes. ten polished thin sections were studied with a petrographic microscope equipped with transmitted and reflected light. rock classification was performed following lemaitre et al. (2002) and mineral abbreviations were taken from whitney & evans (2010). four samples were chosen for further electron microprobe analyses (spots and compositional maps) to characterize magmatic and alteration mineral phases. mineral compositions were obtained using the jeol jxa-8230 electron probe microanalyzer of the departamento de geociencias (universidad nacional de colombia), at 15 and 20 kev for silicates and oxides, respectively. the analytical programs and standards for ilmenite and magnetite were kyanite (si, tap, kα), kyanite (al, tap, kα), mg oxide (mg, tap, kα), pure zn (zn, tap, lα), ti foil (ti, petj, kα), diopside (ca, petj, kα), pure ni (ni, lifh, kα), hematite (fe, lifh, kα), pure mn (mn, lifh, kα), cr oxide (cr, lifh, kα), pure v (v, lifh, kα); pyroxene: sodalite (na, tap, kα), forsterite (mg, tap, kα), omphacite (al, tap, kα), pyrope (si, tap, kaα), ilmenite (ti, petj, kα), diopside (ca, petj, kα), orthoclase (k, peth, kα), fayalite (fe, lifh, kα), spessartine (mn, lifh, kα), cr oxide (cr, lifh, kα); plagioclase: albite (na, tap, kaα), hornblende (mg, tap, kα), orthoclase (al, tap, kα), orthoclase (si, tap, kaα), ilmenite (ti, petj, kα), anorthite (ca, petj, kα), orthoclase (k, peth, kα), fayalite (fe, lifh, kα), spessartine (mn, lifh, kα), cr oxide (cr, lifh, kα); garnet: sodalite (na, tap, kaα), pyrope (mg, tap, kα), orthoclase (al, tap, kα), pyrope (si, tap, kα), ilmenite (ti, petj, kα), diopside (ca, petj, kα), orthoclase (k, peth, kα), fayalite (fe, lifh, kα), spessartine (mn, lifh, kα), cr oxide (cr, lifh, kα); zeolites: albite (na, tap, kaα), hornblende (mg, tap, kα), orthoclase (al, tap, kα), albite (si, tap, kα), ilmenite (ti, petj, kα), anorthite (ca, petj, kα), orthoclase (k, peth, kα), fayalite (fe, lifh, kα), spessartine (mn, lifh, kα), cr oxide (cr, lifh, kα); epidote: and sodalite (na, tap, kaα), hornblende (mg, tap, kα), hornblende (al, tap, kα), hornblende (si, tap, kα), ilmenite (ti, petj, kα), diopside (ca, petj, kα), hornblende (k, peth, kα), hornblende (fe, lifh, kα), spessartine (mn, lifh, kα), cr oxide (cr, lifh, kα). geothermobarometric calculations were performed using the clinopyroxene-only thermobarometer for mafic-to-intermediate magmatic systems (nimis, 1995; wang et al., 2021) and the ilmenite-magnetite (ilmat) thermometer (lepage, 2003). major and trace elements of eleven samples were analyzed at als chemex laboratories by x-ray fluorescence (xrf), inductively coupled plasma mass spectrometry (icp-ms) and inductively coupled plasma atomic emission spectroscopy (icp-aes), following the procedures explained in alschemex (2022). all the analyzed rocks were classified on an anhydrous basis and the geochemical data were plotted using the gcdkit r-based software. 4. results field work in the study area, most of the la quinta formation is represented by a thick succession of reddish-burgundy fine-grained sandstones, mudstones and minor conglomerates interbedded with felsic-to-mafic volcanic and volcaniclastic horizons of variable thicknesses (>3 m). the volcanic and volcaniclastic beds are mainly formed of basaltic lava flows, small andesitic-rhyolitic stocks or domes and ignimbrites. mafic flows (samples 1163, 1224, 1225, 1233, 1235 and 1236) were only observed in the south zone (figure 1) and consist of massive basalts (figure 2a) with aphanitic to porphyritic texture composed of small plagioclase phenocrysts (<1 mm) surrounded by a dark gray matrix. these rocks may be moderately to highly vesiculated (figure 2b), especially to the top of the flows, having amygdules filled with epidote (figure 2c), zeolites, quartz, carbonates and/or copper minerals. conversely, andesitic lava flows and porphyries are more common in the north zone (nearby la jagua del pilar) and the northern portion of the south zone 143petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes (figure 1). the andesitic flows (samples 1230 and 1231) (figure 2d-e) display porphyritic-trachytic texture and are constituted by plagioclase phenocrysts of up to 1 cm, often altered to sericite and clay minerals, with minor amphibole and biotite phenocrysts, all of them embedded in a microcrystalline matrix. andesites with mediumto fine-grained porphyritic texture (samples 1229 and 1232) were found in scarce localities to the southeast of la jagua del pilar and northeast of san diego (figure 1). they normally appear in homogeneous outcrops around isolated hills that show no relationship with the red-beds and are formed of plagioclase and pyroxene phenocrysts surrounded by a light grey crystalline matrix. in a similar fashion, rhyolites outcropping south of san diego (sample 1234) and east of media luna (sample 1167) (figure 2f) are relatively confined with respect to sedimentary and volcanoclastic rocks, and display a characteristic porphyritic texture marked by bipyramidal quartz, alkali feldspar and plagioclase phenocrysts embedded in a light purple fine-grained matrix. given the distribution and features of such rhyolites and porphyritic andesites, they were interpreted as being part of domes and stocks; however, the contacts with the enclosing rocks were not observed. a dioritic stock (sample 1223) was found south of san josé (figure 1), constituted by fine-grained plagioclase and amphibole, which hosts a leucocratic microcrystalline dike (sample 1237), ca. 35 cm wide, with reduced lateral continuity (<1.5 m). finally, felsic tuffs (sample 1227) were only recognized east of la jagua del pilar, formed of bipyramidal quartz and small biotite phenocrysts with rare small burgundy lithic fragments (sedimentary rocks). it is worth mentioning that the relationship between the sedimentary rocks and lava flows was only visible in one outcrop (sample 1230) (figure 2d) and corresponds to interstratifications in which fine-grained clastic rocks do not show contact thermal effects (figure 2d). in addition, cu occurrences formed of malachite, azurite, chrysocolla, cuprite, chalcopyrite, bornite and covellite, are remarkable in some localities (e.g., samples 1163 and 1223). petrography in the study zones, the la quinta formation comprises a series of basaltic-to-andesitic lava flows interbedded with ignimbrites and sandstones, as well as porphyritic rhyolites and andesites and minor dioritic intrusions. most of the analyzed samples exhibit fine-grained porphyritic texture and have mafic, intermediate and felsic compositions (table 2). moreover, as indicated below, these rocks display modest hydrothermal and supergene alteration except for the sample 1163, which is altered to a calc-silicate assemblage of epidoteprehnite-chlorite-calcite. figure 2. field features of the la quinta formation volcanic rocks in the study areas. a. massive basalt (sample 1236). b. highly vesiculated basalt. c. epidote (ep) amygdules in basalt. d. red-violet mudstones interbedded with an andesitic lava flow (sample 1230). e. detail of the andesitic flow from d., showing porphyritic and fluidal texture demarcated by plagioclase (pl) phenocrysts. f. probable rhyolitic dome (sample 1234). table 2. main petrographic features of the volcanic rocks from the la quinta formation. sample percentage (%) accessory alteration petrographic classificationqz pl sa lf mx bt cpx ol 1229 54.7 7.7 ap, hem, mag, ccp brn arc, chl, ser andesite 1230 63.2 mag, ilm chl, zeo, arc, cb, ep andesite 1231 1.6 79.5 zrn, ap, hem chl, arc, ep, ser andesite 1232 38.5 0.3 35.6 4.4 zrn, ap, mag, ilm cb, ser, chl andesite 12241225 66.5 8.6 tr mag, ilm grt, srp, chl, ep, cb basalt 1227 56.3 12.0 2.4 0.4 zrn, mag arc, chl, cb, ep ignimbrite 1234 4.0 8.0 3.0 51.0 1.0 zrn, mag arc, ser, cb, zeo rhyolite 1237 18 25.5 mag arc, ep, zeo dacite (?) 1167 4.6 13.6 6.0 62.4 0.3 zrn, ap cb, ser, ep rhyolite ap: apatite, arc: clay minerals, brn: bornite, bt: biotite, cb: carbonate, ccp: chalcopyrite, chl: chlorite, cpx: clinopyroxene, ep: epidote, hem: hematite, ilm: ilmenite, sa: alkali feldspar (sanidine), lf: lithic fragments, mag: magnetite, mx: matriz, ol: olivine, pl: plagioclase, qz: quartz, ser: sericite, tr: traces, zeo: zeolites, zrn: zircon. mafic rocks two samples with different degrees of hydrothermal alteration were collected in a basaltic flow from south of san diego: sample 1224 (fresh basalt) and 1225 (oxidized and altered basalt). both rocks display fine-grained 144 néstor cano, juan carlos molano, janeth sepúlveda holocrystalline-trachytic and ophitic texture formed of zoned plagioclase (76.0%) and clinopyroxene (11.0%) (figure 3a-b). additionally, scant olivine (<1.0%) replaced by serpentine and chlorite was observed, along with ilmenite and magnetite as accessory minerals (figure 3c). the sample 1225 shows incipient calc-silicate hydrothermal alteration that appears disseminated, as patches and substituting olivine and clinopyroxene. this alteration is composed of chlorite, epidote, carbonates and poorly crystallized garnet. nevertheless, gonzález-duran et al. (2017) reported more complex hydrothermal assemblages often linked to the cu mineralization in other areas of the south zone, including epidote, k-rich feldspar, albite, prehnite, pumpellyite, etc. (figure 3d). intermediate rocks andesites (samples 1229, 1230, 1231 and 1232) are the most common lithology in the studied samples. these rocks are largely constituted by euhedral plagioclase phenocrysts (30.0-35.0%) or microlites (25.0-45.0%) surrounded by a devitrified matrix (35.0%), and variable quantities of clinopyroxene (8.0%), biotite (4.0%), magnetite (3.0%), ilmenite (2.0%), hematite (1.5-2.0%), quartz (1.5%), lithic fragments (ca. 0.3%), apatite, zircon and scarce copper sulfides (figure 4a-e). plagioclase crystals normally develop polysynthetic twinning (albite and pericline), while clinopyroxene manifests simple twinning, pale greenish tint and second order interference colors (figures 4c). furthermore, sample 1232 hosts two types of lithic fragments: 1) very fine-grained sandstones, composed of sub-rounded quartz grains embedded in a dark muddy matrix; and 2) possible tonalite fragments with holocrystalline texture defined by quartz and plagioclase. magnetite, ilmenite and primary hematite are common, the former two oxides forming exsolution lamellae (figure 2d-e). additionally, chalcopyrite replaced by bornite was locally observed. zeolites (8%), carbonates (2-10%), epidote (1.0%), sericite (5.0%) and chlorite (2-15%) are uncommon and are observed as fillings and veinlets (figure 4f), evidencing discrete hydrothermalism. on the other hand, clay minerals and iron oxides are a recurrent product of supergene alteration of plagioclase and amphiboles. in the sample 1230, zeolites were classified as stilbite given that they display medium relief, first order interference colors, negative biaxial interference figure with reduced 2v angle and radial habit (figure 4f). epidote normally appears as small aggregates of high relief, pale yellowish tint and third order birefringence, and hence was typified as pistacite; zoisite and clinozoisite were also identified. figure 3. mineralogy and textures of basaltic rocks from the la quinta formation. a-b. photomicrographs of the general appearance, characterized by plagioclase (pl) with trachytic texture and plagioclase-clinopyroxene (cpx) intergrowths (ophitic texture). notice the presence of chlorite (chl) and ill-crystallized garnet (grt) formed by hydrothermal alteration. c. ilmenite (ilm) associated with magnetite (mag). d. pervasive calc-silicate alteration of epidote (ep) and prehnite (prh) concentrated in the core of a basalt, surrounded by an unaltered carapace. malachite (mlc) patches produced by oxidation of native copper. figure 4. mineralogy and textures of andesitic rocks from the la quinta formation. a-c. plagioclase (pl), biotite (bt) and clinopyroxene (cpx) phenocrysts surrounded by a fine-grained microlithic matrix (mx) that is partly argillized. d-e. hematite (hem) and magnetite (mag) crystals (accessory minerals), the latter is replaced by hematite as patches and shows geometric ilmenite (ilm) exsolution lamellae. f. zeolites (zeo) fillings and chlorite (chl) patches within the microcrystalline matrix. figure 5. mineralogy and textures of rhyolitic rocks from the la quinta formation. a. rhyolitic ignimbrite formed of β-quartz (qz) phenocrysts with embayments and biotite (bt) embedded in a cryptocrystalline devitrified matrix (mx) with discrete argillic alteration. b. granophyric texture (?) in the felsic dike, defined by plagioclase (pl), quartz and, possibly, alkali feldspar (afs?). epidote (ep) pseudomorphs after mafic minerals are also present. c. sanidine (sa) and plagioclase phenocrysts surrounded by a fine-grained holocrystalline matrix. small epidote patches. d. rhyolite with embayed β-quartz phenocrysts surrounded by a devitrified groundmass. notice the presence of secondary calcite (cal) patches. 145petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes table 3a. electron microprobe quantitative results in magmatic minerals from the la quinta formation volcanic rocks. concentrations are given in wt.%. ilmenite mag plagioclase clinopyroxene c r b sample 1224 1224 1224 1224 1224 1224 1224 1224 1229 sio2 0.00 0.00 0.00 50.81 55.48 61.00 52.36 52.35 52.74 tio2 48.69 48.65 6.91 0.00 0.01 0.00 0.69 0.59 0.62 al2o3 0.00 0.00 0.09 31.76 27.94 24.30 0.00 0.00 0.67 feo* 0.49 0.57 0.41 11.66 11.86 11.38 fe2o3 7.8 10.3 52.90 cr2o3 0.03 0.03 0.38 feo 37.9 37.9 36.50 mno 5.72 5.71 0.06 0.00 0.00 0.00 0.37 0.35 0.28 mgo 0.02 0.02 0.10 0.00 0.00 0.00 14.28 13.91 14.92 cao 0.05 0.05 0.05 13.66 10.04 5.77 17.99 18.21 17.92 na2o 0.00 0.00 0.00 3.56 5.51 7.75 0.33 0.35 0.39 k2o 0.00 0.00 0.00 0.17 0.39 0.91 0.00 0.00 0.00 v2o3 0.48 0.47 0.86 0.17 0.39 0.91 0.00 0.00 0.00 ∑ 100.69 103.13 97.85 100.45 99.94 100.14 97.68 97.62 98.92 cations 3 o 4 o 8 oxygens 6 oxygens si 0.00 0.00 0.00 2.30 2.51 2.72 2.00 2.00 2.02 ti 0.95 0.93 0.20 0.00 0.00 0.00 0.02 0.02 0.02 al 0.00 0.00 0.00 1.70 1.49 1.28 0.00 0.00 0.03 fe2+ tot 0.02 0.02 0.01 0.37 0.38 0.36 fe3+ 0.15 0.12 1.21 fe2+ 0.81 0.78 1.58 mn 0.13 0.12 0.00 0.00 0.00 0.00 0.01 0.01 0.01 mg 0.00 0.00 0.00 0.00 0.00 0.00 0.81 0.79 0.85 ca 0.00 0.00 0.00 0.66 0.49 0.28 0.74 0.75 0.74 na 0.00 0.00 0.00 0.31 0.48 0.67 0.02 0.03 0.03 k 0.00 0.00 0.00 0.01 0.02 0.05 0.00 0.00 0.00 ∑ 2.04 1.95 2.99 5.00 5.01 5.01 3.97 3.98 4.06 b = border, c = core, mag = magnetite, r = rim, tot = total. recalculated fe2o3 and feo in magnetite and ilmenite following carmichael (1967). felsic rocks this group is mainly represented by rhyolites (samples 1167 and 1234), ignimbrites (samples 1227) and one felsic dike (sample 1237). the ignimbrite shows porphyritic texture with advanced devitrification and, locally, eutaxitic texture demarcated by fiammes. they are constituted by phenocrysts of β-quartz (13.0%), sanidine (3.0%) and plagioclase (12.0%), with minor zircon, biotite, magnetite and lithic fragments (figure 5a). glass shards and spherulites are common and quartz generally develops resorption embayments (figure 5a). the felsic dike has a diffuse porphyritic and apparent granophyric texture and is entirely composed of complex intergrowths of plagioclase (25%), quartz (18%) and a highly argillized mineral (alkali feldspar? (figure 5b). rhyolites display porphyritic texture and are composed of large (>2 mm) plagioclase (8-14%), quartz (4%), sanidine (3-6%) and biotite (1.0%) phenocrysts, and minor amounts of zircon, magnetite and apatite crystals, all embedded within a devitrified cryptocrystalline matrix (figures 5c-d). these rocks may host lithic fragments of granitoids with granular texture, formed of quartz, orthoclase and plagioclase. besides, perthitic sanidine was observed in one of the samples. felsic rocks normally show mild hydrothermal alteration expressed as patches and partial replacements of mafic minerals (e.g., amphibole and biotite), and is represented calcite-epidote-chlorite-quartz-sericite-zeolites assemblages (propylitic alteration) (figure 5c-d). in contrast, supergene alteration appears as advanced substitutions of feldspars phenocrysts, magnetite and the glassy matrix by clay minerals and earthy iron oxides. microchemical analyses microchemical analyses were carried out in primary (plagioclase, pyroxene, ilmenite and magnetite) and hydrothermal (epidote, zeolites and garnet) mineral phases. the results are summarized in table 3a-b. plagioclase characterization was challenging in most of the samples due to supergene alteration of clay minerals and sericite (figure 4a-c and 5c). however, unaltered plagioclase crystals in a basalt (sample 1224) (figure 3a-b) exhibit normal zoning (figure 6), defined by a ca-rich core (an81) and progressively ca-poorer rims (an45-67). 146 néstor cano, juan carlos molano, janeth sepúlveda clinopyroxene is common in basalts and andesites, in the former displaying ophitic texture (figure 3a). this mineral was classified as augite (en41-43fs18-19wo37-38) following morimoto (1988). hydrothermal alteration minerals, such as epidote, ca-zeolites and garnet (adr70grs30) yielded important contents of ca (table 3b), which agrees with the general calc-silicate assemblage recognized petrographically (figure 3 and 5c). the fact that their summatory in quantitative analyses does not exceed 95 wt.% may be caused by the existence of variable amounts of water in the crystalline lattice of these minerals. magnetite is very common in all the studied samples and is closely related to ilmenite (figure 3c and 7a); the latter is generally found as exsolution lamellae (figure 4e and 7b). these crystals are disseminated in table 3b. electron microprobe results in hydrothermal minerals from the la quinta formation volcanic rocks. concentrations are given in wt.%. grt zeolites ep wt.% 1224 1230 1237 1237 sio2 35.81 56.43 39.26 38.59 tio2 0.30 0.00 0.00 0.00 al2o3 7.46 22.43 23.84 23.89 feo 16.99 0.46 5.22 9.54 mno 0.18 0.01 0.08 0.07 mgo 0.14 0.00 3.27 0.07 cao 32.89 11.54 22.48 21.76 na2o 0.00 0.05 0.01 0.10 k2o 0.00 0.15 0.00 0.01 ∑ 93.77 91.07 94.16 94.03 ep = epidote, grt = garnet. figure 6. calcium (left) and sodium (right) oxides compositional maps of a plagioclase crystal from a basalt (sample 1224). a normal zoning is observed, with ca-rich core and progressive na-richer rims. the color scalebar imply relative amounts, being pink/white higher concentration of na2o or cao. figure 7. backscattered electron images of magnetite (mag) and ilmenite (ilm). ilmenite is characterized by a duller gray tint compared to magnetite. association with other primary minerals, indicating a magmatic origin that was also suggested by enrichments in tio2 (7.5 wt.%), cr2o3 (0.4 wt.%) and nb2o3 (0.05 wt.%) (nadoll et al., 2014). geothermobarometry using the ilmenite-magnetite (ilmat) thermometer proposed by lepage (2003), crystallization temperature and oxygen fugacity conditions were estimated for one of the basalts (sample 1224). it is essential to state that: 1) ilmenite exsolutions were avoided, since they are not in equilibrium with the hosting magnetite; and 2) the diagram of bacon and hirschmann (1988) (not shown) was used to confirm that equilibrium conditions were achieved during the crystallization of the ilmenite-magnetite pair, usually marked by a linear tendency intersecting close to the origin of the diagram. thus, temperature and oxygen fugacity values of 700-736°c and ca. +1.2 nno were obtained. the clinopyroxene-only thermobarometer presented by nimis (1995) and wang et al. (2021) for basaltic-to-dacitic suites was applied to the same sample. clinopyroxene in this rock yielded crystallization temperatures and pressures of 1118-1122 ± 36.6 °c and 4.5-4.7 ± 1.6 kbar. assuming an andesitic composition for the bulk continental crust with a mean density of 2.6 g/cm3 (klein and johnson, 1983; rudnick and gao, 2003), the depth of crystallization varied between 17-19 km. whole-rock geochemistry a total of eleven samples (table 1), including basaltic andesitestrachyandesites (n: 3), dacites (n: 2), rhyolites (n: 2) and one monzodiorite, one ignimbrite and one felsic dike were selected for litho-geochemical analyses, and the results are summarized in table 4a-b. according to the petrographic and geochemical features, these rocks were divided into three groups: felsic (rhyolites, ignimbrite and the leucocratic dike; sio2: 73.6-77-7 wt.%), intermediate (dacites, sio2: 64.0-66.1 wt.%) and mafic (basaltic andesites and trachyandesites, sio2: 49.0-58.3 wt.%) and will be accordingly referred hereafter. overall, the samples present low values of loi (0.9-3.1 wt.%) and were classified on an anhydrous basis; except for the sample 1163 (loi = 7.4 wt.%), which was labeled as “altered basalt” and highlighted in the diagrams because it showed the strongest hydrothermal alteration in hand specimen. additionally, geochemical data published by jiménez (2010) and rodríguez-garcía and obando (2020) for the la quinta formation volcanic rocks nearby the study areas were incorporated in the mafic (basaltic trachyandesites, andesites and trachyandesites) and felsic groups (rhyolites), based on similarities in geochemical signatures with our samples. in the alteration box plot by large et al. (2001) (figure 8a) all the samples appear in the “least altered box” except for one rhyolite (sample 1234), but this is due to its high concentration of k2o (ca. 6.6 wt%) in alkali feldspar phenocrysts and probably in the matrix. nevertheless, this sample plots close to the 1:1 linear regression expected for low-altered rocks (figure 8a). although hydrothermal and supergene alterations were apparently not severe enough to modify the primary signature of the volcanic rocks, we preferred classification and geotectonic diagrams based on immobile or less mobile incompatible elements. the studied volcanic rocks display a wide range of composition from alkali basaltic/basaltic to rhyolitic and plot along a linear trend near the limit between the fields of calc-alkaline series and high-k calc-alkaline and shoshonite series (figure 8b-c) (pearce, 1996; hastie et al., 2007). the samples straddle the metaluminous and peraluminous fields in the shand’s index diagram (figure 8d): 1) mafic rocks are metaluminous and plot close to the limit with the peraluminous field; 2) intermediate rocks are slightly peraluminous, and 3) the felsic rocks are moderately to highly peraluminous. however, the felsic dike and altered basalt have a contrasting behavior, lying within the metaluminous box and close to the limit with the peralkaline field. harker diagrams depict continuous negative trends in feo*, cao, mgo, p2o5 and tio2 (figure 9) with respect to sio2, suggesting: 1) the early crystallization of clinopyroxene, olivine, magnetite, ilmenite, apatite and, partly, plagioclase (winter, 2014), which are minerals especially abundant in the studied mafic rocks (figure 3a-c); and 2) that the rocks are part of a single igneous suite (gill, 2010; frost and frost, 2014). at low sio2, al2o3 exhibits a positive trend (early crystallization of fe-mg-ti phases) that switches at ca. 55 wt.% of sio2, marking the moment at which plagioclase fractionation became significant in the differentiation process. in contrast, k2o and na2o display some degree of scatter (figure 9). the former shows an ill-defined 147petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes table 4b. trace and rare earth elements of the la quinta formation igneous rocks. values are expressed in ppm. sample 1163 1167 1223 1224 1225 1227 1231 1232 1234 1236 1237 li 10.0 10.0 20.0 20.0 20.0 20.0 10.0 20.0 20.0 20.0 10.0 sc 16.0 3.0 12.0 16.0 15.0 2.0 9.0 8.0 2.0 13.0 3.0 v 90.0 28.0 137.0 189.0 169.0 22.0 64.0 83.0 19.0 172.0 32.0 cr 150.0 <10 60.0 60.0 50.0 10.0 50.0 <10 10.0 60.0 20.0 co 30.0 2.0 16.0 28.0 23.0 1.0 13.0 7.0 3.0 22.0 4.0 ni 99.0 1.0 21.0 51.0 43.0 21.0 2.0 41.0 5.0 cu 29.0 20.0 100.0 229.0 122.0 2.0 17.0 17.0 7.0 72.0 688.0 zn 129.0 29.0 111.0 107.0 109.0 11.0 59.0 61.0 25.0 109.0 12.0 ga 18.6 17.0 21.5 20.0 20.5 14.6 17.2 18.8 14.8 23.2 11.6 as 26.0 5.0 11.0 5.0 5.0 5.0 5.0 11.0 6.0 7.0 6.0 rb 71.1 97.9 67.4 26.2 48.6 87.6 33.0 111.0 179.0 35.6 53.5 sr 130.5 77.0 423.0 312.0 398.0 41.1 396.0 354.0 25.5 465.0 141.5 y 28.5 24.3 26.0 28.4 28.7 26.3 16.4 28.8 30.7 25.6 38.3 zr 270.0 183.0 339.0 251.0 270.0 152.0 191.0 278.0 158.0 303.0 1040.0 nb 27.0 16.0 18.7 16.9 18.9 15.9 6.5 12.4 15.0 22.7 38.3 mo 1.0 1.0 1.0 1.0 2.0 1.0 2.0 sn 2.0 1.0 2.0 1.0 2.0 2.0 1.0 2.0 1.0 2.0 2.0 cs 0.74 0.52 0.49 0.31 0.42 0.93 0.21 1.55 1.57 0.27 0.18 ba 394 1060 651 314 457 418 562 955 780 549 292 la 40.1 41.8 43.0 27.2 34.6 44.9 21.0 37.5 64.2 33.6 50.3 ce 84.1 78.7 87.8 58.4 73.2 83.8 40.6 71.8 117.5 71.4 101.0 pr 10.2 8.5 10.1 7.1 8.7 9.1 4.7 8.2 11.9 8.4 11.3 nd 41.4 29.1 35.9 27.8 33.9 29.9 19.6 30.3 40.6 34.0 39.2 sm 8.20 5.43 6.78 6.06 6.82 5.19 3.86 5.71 6.49 6.44 7.22 eu 2.36 0.68 1.64 1.76 1.90 0.65 1.26 1.27 0.87 1.96 0.77 gd 7.39 4.02 6.14 6.40 5.95 4.36 3.44 5.36 5.62 6.21 7.04 tb 1.06 0.60 0.82 0.92 0.87 0.65 0.53 0.80 0.86 0.88 1.17 dy 5.62 3.75 4.93 5.44 5.36 3.94 3.02 4.75 5.14 5.14 7.38 ho 1.00 0.80 0.99 1.10 1.00 0.83 0.63 0.96 0.99 0.95 1.42 er 2.66 2.25 2.68 3.05 2.85 2.45 1.62 3.00 2.77 2.45 4.20 tm 0.39 0.38 0.37 0.40 0.36 0.35 0.23 0.43 0.45 0.39 0.62 yb 2.20 2.59 2.44 2.75 2.49 2.63 1.44 3.16 2.74 2.15 4.02 lu 0.35 0.40 0.38 0.39 0.39 0.45 0.22 0.48 0.47 0.32 0.60 hf 5.7 5.3 7.9 5.9 5.5 4.6 4.7 7.1 4.9 6.6 22.7 ta 1.5 0.9 1.1 1.0 1.1 1.0 0.4 0.7 0.9 1.4 2.4 w 2.0 2.0 1.0 2.0 3.0 3.0 2.0 2.0 3.0 2.0 tl 0.0 10.0 0.0 0.0 0.0 10.0 0.0 10.0 10.0 0.0 pb 32.0 0.0 0.0 0.0 0.0 0.0 0.0 8.0 4.0 0.0 6.0 th 4.11 13.10 7.20 3.82 5.32 10.90 2.68 9.20 12.20 5.66 17.05 u 0.62 3.19 1.20 0.67 1.13 1.88 0.56 1.94 2.12 1.26 2.78 eu/eu* 0.93 0.45 0.78 0.86 0.91 0.42 1.06 0.70 0.44 0.95 0.33 (la/yb)n 12.29 10.88 11.88 6.67 9.37 11.51 9.83 8.00 15.80 10.54 8.44 (la/sm)n 3.08 4.84 3.99 2.82 3.19 5.44 3.42 4.13 6.22 3.28 4.38 (gd/yb)n 2.71 1.25 2.03 1.88 1.93 1.34 1.93 1.37 1.66 2.33 1.41 ba/nb 14.59 66.25 34.81 18.58 24.18 26.29 86.46 77.02 52.00 24.19 7.62 ∑ree 207.03 179.02 203.97 148.75 178.38 189.18 102.17 173.67 260.60 174.24 236.19 eu/eu*: europium anomaly. positive correlation with increasing sio2 (late fractionation of alkali-felspar and biotite), whereas the latter has an opposite behavior. moreover, excluding the felsic dike, zr decreases compared to sio2 in all the studied samples and those analyzed by jiménez (2010), even though zircon crystals are common in felsic and intermediate rocks (table 2) but apparently absent in mafic exponents. this may be due to an early fractionation of very fine-grained zircon or baddeleyite crystals, difficult to observe petrographically. 148 néstor cano, juan carlos molano, janeth sepúlveda table 4a. major oxides of the la quinta formation igneous rocks. values are expressed in wt.%. geochemical classification following middlemost (1994) and pearce (1996). sample 1163 1167 1223 1224 1225 1227 1231 1232 1234 1236 1237 classification ab r ba ba m r d a r ta ar sio2 49 75.6 58.3 53 54.8 75.6 64 66.1 77.7 57.5 73.6 tio2 1.84 0.2 1.05 1.17 1.32 0.17 0.64 0.56 0.18 1.31 0.47 al2o3 14.35 13.15 16.4 16.8 17 11.85 15.45 16 12.65 16.6 11.85 fe2o3* 9.8 1.67 7.06 9.14 9 0.88 5.14 4.27 1.51 8.21 2.64 mno 0.15 0.04 0.11 0.15 0.15 0.03 0.07 0.09 0.08 0.14 0.03 mgo 1.88 0.23 2.9 4.31 4.13 0.71 1.33 1.07 0.24 3.46 0.11 cao 8.42 0.23 5.03 6.72 5.93 0.42 1.19 2.35 0.14 6.18 4.05 na2o 4.67 3.6 3.88 3.39 4.26 2.67 6.66 4.01 0.86 3.69 4.02 k2o 2.53 4.94 2.14 1.06 1.72 4.66 2.02 3.86 6.57 1.52 2.88 p2o5 0.6 0.03 0.49 0.41 0.5 0.03 0.23 0.18 0.02 0.58 0.06 loi 7.43 0.91 2.49 1.86 2.6 1.8 1.71 3.1 1.63 1.23 0.84 total 100.67 100.6 99.85 98.01 101.41 98.82 98.44 101.59 101.58 100.42 100.55 a: andesite, ab: alkali basalt, ar: alkali rhyolite, ba: basaltic andesite, d: dacite, m: monzodiorite, r: rhyolite, ta: trachyandesite. in chondrite normalized (boynton, 1983) rare earth element (ree) diagrams (figure 10), the mafic samples (∑ree = 148-207 ppm) have enrichments in light ree (lree) with respect to heavy ree (hree) (lan/ ybn = 6.6-11.8) and slightly fractionated hree patterns (gdn/ybn = 1.82.7), and also agree well with basaltic trachyandesites, trachyandesites and andesites reported by jiménez (2010) and rodríguez-garcía & obando (2020). europium anomalies (eu/eu*) in these rocks are insignificant (eu/eu* = 0.7-0.9) and it predominates a general flat pattern, indicating that plagioclase figure 8. geochemical classification diagrams for the la quinta formation igneous rocks. a. alteration box plot (large et al., 2001), where major oxides are express in wt.%. lab: least altered box. b. nb/y vs. zr/ti (pearce, 1996). c. co vs. th in ppm (hastie et al., 2007). d. shand’s index after maniar and piccoli (1989). figure 9. harker diagrams of the la quinta formation igneous rocks. symbols are the same as in figure 8. the discontinuous dark grey line represents the general tendency of the data. oxides are expressed in wt.% and zr in ppm. fractionation was not important in this stage (winter, 2014). mafic rocks are also enriched in large ion lithophile elements (lile) compared to high field strength elements (hfse), and in primitive mantle normalized diagrams display negative nb and ti tendencies and positive pb and k tendencies (figure 10). the distinctive positive pb anomaly observed in the altered basalt is also present in the samples analyzed by jiménez (2010) and rodríguezgarcía & obando (2020). altogether, these geochemical features are common in subduction-related magmatism (gill, 2010). 149petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes figure 10. trace element geochemical diagrams for the la quinta formation mafic rocks. a. chondrite normalized (boynton, 1983) ree plots. b. primitive mantle normalized multi-element plots (sun & mcdonough, 1989). samples from jiménez (2010) and rodríguez-garcía & obando (2020) are included. figure 11. trace element geochemical diagrams for the la quinta formation intermediate rocks. a. chondrite normalized (boynton, 1983) ree plots. b. primitive mantle normalized multi-element plots (sun & mcdonough, 1989). figure 12. trace element geochemical diagrams for the la quinta formation felsic rocks. a. chondrite normalized (boynton, 1983) ree plots. b. primitive mantle normalized multi-element plots (sun & mcdonough, 1989). samples from jiménez (2010) and rodríguez-garcía & obando (2020) are included. the studied intermediate rocks exhibit total ree values ranging from 102.2 to 173.7 ppm. as depicted in the chondrite normalized diagram presented in figure 11, dacites have higher concentrations of lree compared to hree (lan/ybn = 8.0-9.8) and display rather fractionated hree patterns (gdn/ ybn = 1.4-1.9). eu anomaly in the sample 1231 is absent (eu/eu* = 1.0) whereas in the sample 1232 is moderate negative (eu/eu* = 0.7); thus, in the former case, plagioclase fractionation was not remarkable, probably caused by early crystallization of amphibole as indicated by the listric-shaped hree pattern (figure 11) (richards & kerrich, 2007). moreover, dacites are enriched in lile with respect to hree and show positive pb and k anomalies as well as negative nb, p and ti anomalies. overall, these patterns are typically found in supra-subduction arcs (gill, 2010; winter, 2014). finally, felsic rocks hold the highest total amounts of ree (179.0260.6 ppm) and have negative slope (lan/ybn = 8.4-15.8) parallel patterns in the chondrite normalized ree diagram (figure 12), with moderate mree (medium ree) fractionation trends compared to lree (lan/smn = 4.4-6.2) and flat hree patterns (gdn/ybn = 1.3-1.7). moderate-to-strong negative eu anomalies (eu/eu* = 0.3-0.4) are present in all the samples of this group, defining a “seagull” profile and evidencing prolific plagioclase fractionation (winter, 2014). in primitive mantle normalized spidergrams, analyzed rhyolites 150 néstor cano, juan carlos molano, janeth sepúlveda and the ignimbrite exhibit contrasting geochemical characteristics when plotted along the felsic dike. in one hand, rhyolitic rocks are enriched in lile compared to hfse (ba/nb = 26-66) and are characterized by a positive k anomaly and deeply negative nb, sr, p and ti anomalies (figure 12). on the other hand, the dike has minor lile (e.g., cs, rb, ba) concentrations although maintain a relative enrichment of these elements compared to hfse (ba/nb = 7.6); also, this sample has positive th, pb and zr tendencies along with negative nb, sr, p and ti tendencies. all these geochemical features are found in subduction related igneous rocks (gill, 2010) and are very similar to those reported by jiménez (2010) and rodríguez-garcía & obando (2020) for rhyolites from the la quinta formation. 5. discussion petrogenesis of the igneous rocks from the la quinta formation the la quinta formation comprises a thick succession of volcanosedimentary rocks that is extensively exposed in the serranía del perijá and the mérida andes (champetier de ribes et al., 1961; radelli, 1961; forero, 1972; maze, 1984; montaño, 2009; gómez et al., 2010; jiménez, 2010; ortega et al., 2012; gonzález-iregui et al., 2015; van der lelij et al., 2016a; lópez-isaza & zuluaga, 2020; rodríguez-garcía & obando, 2020). the igneous part of this unit is formed of volcaniclastic, volcanic, plutonic and, probably, hypabyssal rocks with variable compositions (basaltic to rhyolitic), commonly displaying diagnostic geochemical features of supra-subduction magmatism that was transitional between calc-alkaline and alkaline. harker diagrams (figure 9) depict negative fractionation trends in compatible elements in response to the early crystallization of ca-plagioclase, clinopyroxene, fe-ti oxides, olivine and apatite, as well as positive profiles in k2o defining the late crystallization of alkali feldspar (sanidine); zirconium also depict a negative profile with increasing sio2, probably due to the early fractionation of fine-grained zircon or baddeleyite. fractionation of these minerals was also interpreted from spidergrams (figure 10-12), where progressively deeper negative anomalies in ti, sr, eu, zr and p with increasing sio2 are present. high crystallization temperatures obtained for clinopyroxene (ca. 1120 °c) and magnetite-ilmenite (700-736 °c) from a basaltic andesite (sample 1224) also support this trend of differentiation, although suggest that fe-ti oxides arrived later than clinopyroxene to the paragenesis. notwithstanding the fact that amphibole was only observed in one dacite (sample 1231) and the monzodiorite, the decrease in dy/yb ratios with respect to sio2 (figure 13b) indicates that crystallization of this mineral was remarkable during differentiation. moreover, the presence of amphibole and biotite in some samples (table 2) accounts for significant water contents in the magmas (frost and frost, 2014; loucks, 2014), consistent with the co-crystallization of magnetite-ilmenite and magnetite-hematite in mafic and intermediate rocks, respectively, implying that parental magmas were oxidized (fo2 varied between +1.2 ni-nio and magnetite-hematite buffers) as it is classically found in arcmagmas (ishihara, 1977). low th/hf and ta/hf ratios (figure 13c) in the mafic and intermediate rocks suggest a hybrid signature between arc and intraplate, as well as limited contribution from calc-alkaline or enriched mantle sources. however, rhyolites and the ignimbrite display a greater influence of a calc-alkaline reservoir, probably acquired during assimilation of older arcs (e.g., paleozoic granitoids) (maze, 1984; van der lelij et al., 2016a; van der lelij et al., 2019) . on the other hand, andesites and rhyolites studied by jiménez (2010) have an intraplate affinity and perhaps had a more enriched source or higher degrees of middle-toupper crustal contamination. in fact, in the th/la vs. ba/la plot (figure 13d), the samples define a gently sloped positive trend that is suggestive of higher crustal contamination in the most siliceous rocks, as it is also suggested by their peraluminous character (figure 8d). this agrees with maze (1984) and van der lelij et al. (2019), who obtained values of 87sr/86sri (0.706-0.711) and εhfi (-5.76) in volcanic rocks from the la quinta formation in the eastern flank of the serranía del perijá and the mérida andes, indicative of conspicuous crustal assimilation. in addition, the low variation in ba/la ratios points toward inconstant influence of slab-derived fluids; although pb positive tendencies found in most rocks (figure 10-12) mark the incorporation of crustal derived sediments via subduction recycling (wilson, 1989; class et al., 2000; sen, 2014; winter, 2014). figure 13. petrogenetic geochemical diagrams for the igneous rocks from la quinta formation. a. sio2 vs. eu/eu*. b. sio2 vs. dy/yb. c. ta/hf vs. th/hf. d. th/la vs. ba/la. sio2 concentration is expressed in wt.%. it is worth noting that the sample 1231 lies in the adakite/ttg field (martin, 1986) (figure 14) and exhibits most of the adakite diagnostic geochemical characteristic (e.g., richards and kerrich, 2007), including: sio2>56 wt.% (64 wt.%), na2o>3.5 wt.% (6.6 wt.%), k2o<3.0 wt.% (2.0 wt%), rb<65 ppm (33 ppm), sr>400 ppm (396 ppm), y<18 ppm (16.4 ppm), yb<1.9 ppm (1.4 ppm), ni>20 ppm (21 ppm), cr>30 ppm (50 ppm), sr/y>20 (24.1) and mg# ca. 0.5 (0.4). similarly, some of the volcanic rocks studied by jiménez (2010) and rodríguez-garcía & obando (2020) display adakitic signatures as well. in the original definition of “adakite”, kay (1978) and defant & drummondt (1990) stated that partial melting of hot and young garnet-bearing oceanic crust (eclogite-facies metamorphism) produced the anomalous high values in sr/y and la/yb found in volcanic rocks from the adak island (aleutians arc). however, kay and kay (2002) later proposed that magmas with such signatures would also by a consequence of melting: 1) the base of the continental crust or 2) lower crustal slices driven to great depths by subduction erosion processes; in both cases, stabilizing garnet in the source. additionally, richards and kerrich (2007) and loucks (2014) demonstrated that adakitic geochemical features can also be acquired by different magmatic suites via extensive amphibole fractionation combined with limited early crystallization of plagioclase. hence, we propose that the adakitic characteristics observed in the studied rocks are a consequence of differentiation processes (e.g., richards & kerrich, 2007; loucks, 2014), having into account that the scarce adakitelike rocks in the study zones are spatially and genetically related to “normal” arc magmas (figure 14) and that differentiation was strongly influenced by the fractionation of amphibole (figure 13b). figure 14. adakite discrimination diagram of martin (1986). symbols are the same as in figure 8. 151petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes some typical adakitic indexes (e.g., sr/y and la/yb) have arisen as powerful tools to evaluate periods of crustal thickening and thinning in orogens (chapman et al., 2015; chiaradia, 2015; profeta et al., 2015; chavarría et al., 2021). after selecting idoneous rocks for calculations following chapman et al. (2015) and profeta et al. (2015), three of the samples analyzed here (1231, 1223 and 1236) and some andesites studied by jiménez (2010) yielded sr/y (14.024.1) and (la/yb)n (9.8-14.0) ratios equivalent to crustal thicknesses of 26-49 km (profeta et al., 2015). these values indicate that the crust was normal to moderately thick and match the interval of ca. 30-60 km calculated by ramírez et al. (2020) and chavarría et al. (2021) in jurassic plutonic and volcanic rocks from the sierra nevada de santa marta, which was situated trenchward with respect to the machiques transtensional basin at that time (bartok et al., 2015; nova et al., 2019; bayona et al., 2020; ramírez et al., 2020). moreover, these values are supported by middle-crustal clinopyroxene crystallization pressures of ca. 18 km that might mark the level at which initial (high-temperature) fractional crystallization took place. melting conditions in the source the volcanic rocks from the la quinta formation are transitional between alkaline and calc-alkaline series, as shown by geochemical (figure 8b-c) and mineralogical features (e.g., abundant sanidine in evolved rocks). these characteristics can result from assimilation/fractional crystallization, partial melting, source enrichment or a combination of all (box & flower, 1989). as previously mentioned, crustal assimilation was indeed significant and might explain, at least in part, the alkalinity of the rocks. on the other hand, low degrees of partial melting of the source can greatly enrich magmas in incompatible elements and is considered one of the main processes that rise alkalinity in subduction-related settings (box & flower, 1989). however, distinguishing between the effects of partial melting from those generated by source enrichments require complementary isotopic studies. figure 15. a. nonmodal batch melting using the system nb-yb for mafic rocks from the la quinta formation (safonova et al., 2016). b. ba/nb vs. nb/zr plot. in order to evaluate the types of mantle sources and degrees of melting that produced the parental magmas of less differentiated rocks (mafic group), we employed a source-melting model constructed by safonova et al. (2016) (figure 15a), which assumes nonmodal batch melting using the system nbyb. in this model, our samples and those presented by jiménez (2010) and rodríguez-garcía & obando (2020) suggest up to 10% of melting of two probable sources: a peridotite containing 1% of garnet or a spinel lherzolite. in the former case, the presence of garnet as a residual phase in the source would generate steep negative slopes in hree (richards & kerrich, 2007), contrasting with the dominant flat or listric-shaped hree profiles depicted in the spiderplots (figure 10-12). in this way, a spinel lherzolite is a better candidate for the main source of the magmas. besides, our samples exhibit higher degrees of melting than the rhyolites studied by jiménez (2010) (figure 15b) explaining the “less enriched” character of the source indicated by ta/hf ratios (figure 13c). hydrothermal alteration and cu mineralization the la quinta formation volcanic rocks sometimes display calc-silicate hydrothermal alteration in association with cu mineralization (figure 3d, 4f) (jiménez, 2010; ortega et al., 2012; unal-sgc, 2014, 2020; gonzálezduran et al., 2017). the assemblage is constituted by variable amounts of quartz, pistacite, zoisite-clinozoisite, prehnite, carbonates, hydrated garnet (adr70grs30) and ca-zeolites (figure 4f). given that hydrated garnets are stable at ca. 250-300 °c and well-crystallized epidote at >220-250 °c (corbett & leach, 1997; and references therein) and carbonates are commonly associated with them (figure 5c-d), we interpret that the mineralizing fluids had mid-temperature and neutral-to-alkaline ph. although garnet is typically found in skarns due to elevated temperature metasomatism of calcareous protoliths (meinert et al., 2005), the presence of garnet is also common in the deeper portions of cu porphyries (sillitoe, 2010) and iocg clan of deposits (corriveau, 2007). nevertheless, there is neither field nor laboratory evidence that supports the presence of such magmatichydrothermal deposits in the study zones. in fact, cu occurrences are normally stratabound and are related to specific basaltic horizons and green sandstones with plant remains, and hence several authors have preliminarily classified them as sediment-hosted stratiform copper systems (wokittel & restrepoacevedo, 1955; champetier de ribes et al., 1961; radelli, 1961; pagnacco, 1962; rodríguez, 1986; jiménez, 2010; ortega et al., 2012; gonzález-duran et al., 2017). hitzman et al. (2005) explained that the volcanic red-bed copper subtype is formed in rift-fill volcano-sedimentary sequences that have undergone various stages of diagenesis and/or burial metamorphism (epidoteprehnite/pumpellyite facies), by low-to-moderate temperature mineralizing fluids 50-400 °c. this deposit model partly matches several features found in cu mineralization hosted in the la quinta formation and, perhaps, such hydrothermal fluids caused the calc-silicate alteration that preferably affects volcanic horizons and is sometimes associated to the ores. figure 16. geotectonic discrimination diagrams for basalts from the la quinta formation volcanic rocks. a. la/yb vs. nb/la (hollocher et al., 2012). intermediate and felsic samples are included for comparison. nb vs. th/yb (pearce, 2008). tectonic model and regional implications as previously mentioned, the la quinta formation volcanics display several diagnostic features of supra-subduction magmatic rocks, such as: 1) early crystallization of fe-ti oxides and ca-clinopyroxene along with extensive plagioclase and amphibole fractionation; 2) high lile/hfse and lree/ hree ratios; 3) negative nb and ti tendencies combined with positive pb tendencies; 4) co-crystallization of magnetite-ilmenite and magnetite-hematite defining oxidizing conditions (+1.2 ni-nio to magnetite-hematite buffers); and 5) peraluminous to metaluminous character. additionally, uninterrupted patters in harker diagrams and sub-parallel profiles in ree spiderplots suggest that they indeed constitute a magmatic suite genetically linked. in the discrimination diagram of hollocher et al. (2012) for basalts, all the samples (differentiated rocks included for comparison) plot in the “alkaline arcs” field (figure 16a), in agreement with trace element classification schemes (figure 8b-c). likewise, the mafic rocks plot above the morboib array (high th and nb) (figure 16b), which is normally found in arcrelated rocks subjected to interaction with the continental crust during ascent (pearce, 2008), consistent with isotopic ratios (maze, 1984; van der lelij et al., 2019). combined, both diagrams define two features commonly found in early stages of continental rifting: alkaline tendencies and contamination by a thick continental crust (winter, 2014). 152 néstor cano, juan carlos molano, janeth sepúlveda figure 17. proposed tectonic model to explain the generation of the igneous rocks from the la quinta formation. a. reconstruction for the lower to middle jurassic according to nova et al. (2019), depicting the position of the continental arc axis with respect to the transtentional basins (machiques, uribante and barquisimeto) in northwestern gondwana, resulting from the break-up of pangea. the red arrow indicates the dominant subduction trend and red half arrows depict the right-lateral sense of shearing. b. schematic cross section x-x’ showing the proposed hybrid tectonic scenario responsible for the la quinta igneous rocks. in this model, the calc-alkaline to alkaline volcanic (>plutonic) activity that focused on the machiques transtensional basin (la quinta formation) was somehow controlled by the influx of slabderived fluids from the subducting farallon plate and hence records clear arc signatures. nevertheless, low degrees of partial melting or the presence of a more enriched mantellic source rose alkalinity of these rocks. during ascent, magmas underwent low-pressure (amphiboleand plagioclase-dominated) fractional crystallization (fc) and crustal assimilation (ca) in the middle crust, before erupting along major normal faults. moreover, a coeval subduction-related continental arc formed to the sw and is represented by volcanic and plutonic rocks from the sierra nevada de santa marta. modified from martini & ortega-gutiérrez (2018) and ramírez et al. (2020). fp: farallon plate, sap: south american plate. following the abovementioned, the igneous rocks of the la quinta formation in the western flank of the serranía del perijá, northern colombia, were formed in a paleo-subduction setting where the farrallon/pacific plate was sinking beneath south america (figure 17) (bartok et al., 2015; nova et al., 2019; bayona et al., 2020). this processes generated jurassic (ca. 163191 ma) volcanic-plutonic activity in the sierra nevada de santa marta (arc axis) (bartok et al., 2015; nova et al., 2019; ramírez et al., 2020) coeval with the magmatism (volcanic>plutonic) in the machiques transtensional basin or the present-day serranía del perijá (ca. 165-188) (jiménez, 2010; gonzález-iregui et al., 2015; nova et al., 2019; rodríguez-garcía & obando, 2020). the source of this volcanism in the western perijá range is unknown, although maze (1984) proposed that the locus of felsic products were the normal faults that bound grabens. probably, the calc-alkaline to alkaline volcanism in the machiques basin resulted from the superposition of farallon subduction and contemporaneous widespread extension owing to the separation of north and south american plates, counterclockwise rotation of the yucatán block and seafloor spreading in the proto-caribbean ocean (accretion of para-authoctonous terranes model in bayona et al., 2020). therefore, we propose that the la quinta formation volcanics were generated in a hybrid tectonic domain that resulted from the interaction of the convergence along the western margin of northwestern gondwana and the intracontinental rifting associated with the break-up of pangea. in the western mérida andes (uribante basin) (figure 17), langer et al. (2014) and van der lelij et al. (2016a) obtained crystallization (tims, u-pb zircon) and cooling ages (ar-ar plateau in biotite) of 229 ± 15 ma and 200.4 ± 0.9 ma, respectively, in the basal tuffs from the la quinta formation. during the lower jurassic, the uribante basin was located east of the machiques basin, indicating that the magmatism migrated westward as evidenced by younger volcanic ages in the colombian perijá range (ca. 163-191 ma). after that, the arc axis progressively moved to the west into the central cordillera, where the oldest jurassic plutonic rocks display an age of 189 ma (bustamante et al., 2010; van der lelij et al., 2016b), leading to the termination of the volcanic activity in the machiques basin (spikings et al., 2019). van der lelij et al. (2016b) proposed that such a migration in the magmatism was a consequence of slab rollback in the farallon plate, which also could have contributed to the widening of extensional and transtensional basins formed by the globalized divergent regime that controlled the jurassic sedimentation in northwestern gondwana (figure 17). nevertheless, those basins should not be considered as back-arc basins since their development is not a direct consequence of the interaction of farallon-pacific and south american plates, but a response to a major plate rearrangement (wilson, 1989; martini & ortega-gutiérrez, 2018). 6. conclusions the latest triassic-jurassic magmatic record present in the la quinta formation, serranía del perijá (northeast colombia), was developed in the machiques transtensional basin and displays wide compositional variations from basaltic to rhyolitic, following a calc-alkaline to alkaline trend of differentiation. these rocks represent a cogenetic igneous suite constituted by variable amounts of plagioclase, augite, quartz, amphibole, biotite and sanidine, with zircon, apatite, magnetite and ilmenite as the main accessory phases. calcsilicate hydrothermal alteration with hydrated andradite-grossular, zeolites and epidote is often linked to cu mineralization that might belong to the sedimenthosted stratiform copper clan of deposits. geochemical features displayed by these rocks are diagnostic of subduction-related settings associated with low degrees of partial melting (or an enriched mantellic source), where magmas ascended through a normalto-thickened crust and underwent low-pressure fractional crystallization and different degrees of crustal assimilation. such processes summed with the existence of a contemporaneous well-developed continental arc (sierra nevada de santa marta) located trenchward, allowed us to invoke a hybrid tectonic scenario related to the coeval subduction of the farallon plate underneath northwest gondwana and the ongoing divergence of north and south american plates resulting from the break-up of pangea. acknowledgments this research was supported by the universidad nacional de colombia and the servicio geológico colombiano, project: “proyecto metalogenético en zonas de interés del territorio colombiano”, cooperation agreement no. 001 of 2015. we want to express our gratitude to the reviewers for their insightful comments that helped us improved the manuscript. we are also grateful to dr. colombo tassinari, dr. antónio mateus, dr. natalia pardo, dr. 153petrogenetic constraints of the la quinta formation igneous rocks, serranía del perijá, northern colombian andes ana elena concha and ana maría correa for the guidance during the field and laboratory works. 7. bibliography als-chemex (2022). geochemistry, schedule of services & fees. bacon, c. r. & hirschmann, m. m. 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(2025). clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan. earth sciences research journal, 29(2), 149-167. https://doi. org/10.15446/esrj.v29n2.116089 the oligocene nari formation in the karachi embayment of the indus basin, pakistan, consists of interbedded limestone, sandstone and shale. this study aims to elucidate the source of clastic rocks and depositional environment of these mixed carbonate-clastic successions using integrated outcrop observations, facies analysis and petrography. outcrop-based facies analysis identified six facies; including arenaceous limestone, calcareous sandstone, argillaceous sandstone, laterite/oxidized sandstone, arenaceous shale and calcareous mudstone. petrographic analysis further distinguished five limestone-dominated microfacies and two sandstone-dominated petrofacies. the microfacies are dominated by packed and sparse biomicrites composed primarily of calcite, with minor contribution from terrigenous minerals. the petrofacies include sub-arkose and quartz arenite, which are predominantly composed of quartz, with lesser proportion of feldspar, clay minerals and other minerals. the mineral composition of the petrofacies suggest a granitic source, likely derived from the indian craton to the east. facies analysis suggests that the nari formation was deposited in a range of environments from outer ramp to fluvio-deltaic settings. the overall facies succession reflects a progradational trend, interpreted as a response to regional regression during the himalayan orogeny. this study will bring new perspectives to refine the interpretations of sedimentary processes in mixed carbonate-clastic environments in the region thereby advancing our understanding to geological history of the study area. keywords: clastic source, depositional environment, mixed carbonate-clastic rocks, petrography, oligocene, nari formation resumen la formación nari del oligoceno, en la ensenada karachi de la cuenca del indo (pakistán), está compuesta por intercalaciones de caliza, arenisca y lutita. este estudio busca dilucidar el origen de las rocas clásticas y el entorno deposicional de estas sucesiones mixtas carbonato-clásticas mediante observaciones integradas de afloramientos, análisis de facies y petrografía. el análisis de facies basado en afloramientos identificó seis facies: caliza arenisca, arenisca calcárea, arenisca arcillosa, arenisca laterítica/oxidada, lutita arenisca y lutita calcárea. el análisis petrográfico distinguió además cinco microfacies con predominio de caliza y dos petrofacies con predominio de arenisca. las microfacies están dominadas por biomicritas compactas y dispersas, compuestas principalmente de calcita, con una contribución menor de minerales terrígenos. las petrofacies incluyen arenita subarcosa y cuarzo, compuestas predominantemente de cuarzo, con una menor proporción de feldespato, minerales arcillosos y otros minerales. la composición mineral de las petrofacies sugiere una fuente granítica, probablemente al este del cratón indio. el análisis de facies sugiere que la formación nari se depositó en diversos ambientes, desde la rampa exterior hasta entornos fluviodeltaicos. la sucesión general de facies refleja una tendencia progradacional, interpretada como una respuesta a la regresión regional durante la orogenia del himalaya. este estudio aporta nuevas perspectivas para refinar la interpretación de los procesos sedimentarios en ambientes mixtos carbonato-clásticos en la región, lo que contribuye a la comprensión de la historia geológica del área de estudio. palabras clave: fuente clástica, entorno deposicional, rocas carbonatadas-clásticas mixtas, petrografía, oligoceno, formación nari fuente clástica y entorno deposicional de secuencias mixtas carbonato-clásticas en la formación nari del oligoceno del anticlinal hundi, ensenada de karachi, cuenca del indo, pakistán https://doi.org/10.15446/esrj.v29n2.116089 mailto:aijaz.halepoto@usindh.edu.pk https://doi.org/10.15446/esrj.v29n2.116089 https://doi.org/10.15446/esrj.v29n2.116089 150 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari 1. introduction mixed carbonate and clastic deposits provide an important record of tectonic, depositional and paleogeographic changes (zuffa 1980; doyle & roberts 1988; zohdi et al. 2013; brandano & ronca 2014; anan 2014; nikbakht et al., 2019; zecchin et al. 2021; tong et al. 2023; zuchuat et al. 2023; roblessalcedo & vicedo 2023; hussein et al. 2024; jamrich et al. 2024). the changes from carbonate to clastic deposition and viceversa are determined by tectonic setting, climatic fluctuations and changes in sea level (zuffa 1980; mount 1985; doyle & roberts 1988; nichols 2009; zeller et al. 2015; zecchin et al. 2021; shehata et al. 2021; zecchin et al. 2023; della porta et al. 2023; hussein et al. 2024; gil-gil et al. 2024). oligocene is a significant epoch in the tectonic history of the indus basin, characterized by early stage of continent-continent collision, dynamic basin geometry, retreating sea-level and frequently variable depositional conditions (raza et al. 1990; bender & raza 1995; smewing et al. 2002b; ghani et al. 2023; ahmedani et al. 2024a). the nari formation of the oligocene age stands out as a key stratigraphic unit, which offers valuable insights into the relationship of clastic and carbonate sedimentation during this time (ahmedani et al. 2024b). therefore, clastic source and depositional environment of mixed carbonate-clastic successions within the nari formation, deposited in a dynamic basin conditions provide invaluable constraints regarding the tectonic-sedimentary processes. furthermore, better understanding towards the facies variations and sedimentary processes in mixed carbonate-clastic sequences is crucial for the reservoir characterization, because such units often serve as important hydrocarbons as well as groundwater reservoirs. the nari formation of the oligocene age is one of the most important mixed carbonate-clastic sequence, exposed in the karachi embayment, indus basin, pakistan. the term “nari series” was coined by blanford (1876) for oligocene sequence of indus basin, after the “nari river” in the kirthar range, indus basin, pakistan. subsequently, blanford (1879) separated oligocene rocks into “lower nari series” and “upper nari series”. williams (1959) used the term “nari formation” for the “nari series”. “lower nari series” of blanford (1879) was named “nal limestone” by hunting survey corporation (1960) and “nal member” (cheema et al., 1977). they proposed “momani group” to include the nari formation and overlying gaj formation of miocene age. nari formation has been studied in terms of echinoids fossil assemblages (duncan & sladen, 1884; hunting survey carporation, 1960; khan, 1968), reservoir potential (mahmud & sheikh, 2009), grain size analysis and depositional environment (khokhar et al., 2016; hakro et al., 2021; samtio et al., 2021; ahmedani et al. 2024b), provenance (ahmed et al., 2020; hakro et al., 2022), petrophysics (shar et al., 2021a), geochemistry (shar et al., 2021b) and diagenesis (ahmedani et al., 2024a). however, high-resolution facies analysis of the nari formation to understand its clastic source, depositional environment and construct a systematic sequence of depositional events is not yet attempted. the paleogeographic and tectonic conditions, which governed the deposition of mixed carbonate-clastic sequences of the nari formation, remained largely unexplored. this research is focused on thorough and indepth facies investigation of the nari formation to unravel the clastic source and depositional environment. this study aimed to contextualize these findings in the broader regional tectonic framework. 2. geological setting the indus basin was located on the northwestern passive margin of the indian plate (kadri 1995; siddiqui 2016; rizwan et al. 2020; bilal et al. 2023b). it the indus basin consists of a very thick precambrian to recent sedimentary record with marked absence of the ordovician to carboniferous record (kazmi & jan 1997; kazmi & abbasi 2008; shah 2009). terrigenous clasts to the indus basin were supplied by the indian craton located on its eastern boundary of the indus basin (bender & raza 1995; khan et al. 2002; hakro et al. 2018; qasim et al. 2022; sarwar et al. 2024). the indus basin experienced extensional tectonics during the middle jurassic to late cretaceous, when the indian plate rifted off from the western gondwanaland fragments (powell et al. 1988; besse & courtillot 1988; ahmed et al. 2018; halepoto et al. 2025). mesozoic extensional tectonics controlled the basin geometry and sedimentation pattern of younger rocks (kadri 1995; hedley et al. 2001; smewing et al. 2002a; ahmed et al. 2018; halepoto et al. 2025). sedimentary rocks of the indus basin were intensely deformed during the collision of the indian plate with the eurasian plate since the late paleocene / early eocene (allemann 1979; bannert et al. 1992; fowler et al. 2004; jadoon et al. 2020; ghani et al. 2023). the early stage of deformation of the northwestern margin of the indian plate during the oligocene had profound control on the depositional environment in the indus basin, pakistan. the marine conditions in the indus basin started to retreat during the late eocene and early oligocene. major part of the indus basin uplifted during the oligocene, which is marked by either absence of the oligocene sequence or fluvial rocks (raza et al., 1990). constantly retreating sea occupied position of the karachi embayment during the oligocene (raza et al., 1990; cheema et al., 2009). the karachi embayment is located in southern extreme of the indus basin (figure 1), which developed due to its abrupt down-warping outer shelf of the indus basin during mesozoic rifting (zuberi & dubois, 1962). this embayment is unique tectonic element as it contains conformable k/t (cretaceous / tertiary) boundary, thickest paleocene strata of the indus basin and last phases of marine sedimentation up to miocene-early pliocene (meissner & rehman 1973; raza et al. 1990; bender & raza 1995; kadri 1995). mostly the karachi embayment contains exposures of eocene and younger strata, which are folded in ns trending en echelon anticlines. the exposures of oligocene strata in the karachi embayment may provide a detailed record about the sediment source and depositional environment during this epoch. the stratigraphy and tectonics of the indus basin is well documented by williams (1959), hunting survey corporation (1960), zuberi & dubois (1962), kazmi & rana (1982), raza et al. (1990), bender & raza (1995), kadri (1995), kazmi & jan (1997), kazmi & abbasi (2008), shah (2009) and siddiqui (2016), which is summarized in figure 2. figure 1. satellite image of part of southern pakistan, showing tectonic elements and location of hundi anticline in red rectangle within karachi embayment modified after; hunting survey corporation, 1960; zuberi & dubois, 1962; g. sarwar & de jong, 1979; kazmi & rana, 1982; bannert et al., 1992; halepoto et al., 2022. (image source: esri, maxar, geoeye, earthstar geographics, cnes/ airbus ds, usda, usgs, aerogrid, ign, and the gis user community). 151clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan 3. methodology the well exposed and 130 m thick section of nari formation was measured from the eastern flank of hundi anticline (lat. 25º, 16’ n, long. 67º, 41’ e), karachi embayment, pakistan (figures 1, 3), through jaccob’s staff and tape_brunton methods (compton, 1962; coe et al. 2010). the identified limestone and shale facies were described after wilson (1975) and pettijohn (1975) respectively, whereas, sandstone facies were described after pettijohn (1975) and pettijohn et al. (1987). thirty two rock samples, one from each strata, were collected for detailed description and petrographic analysis (figure 4). the samples were sorted in different lithological groups based on their macroscopic features such as texture, compaction etc. eleven thin sections from hard and compact samples were prepared to investigate their texture, mineral composition microfacies and petrofacies. the petrographic analysis was carried out under “leica dm 2500p polarizing microscope” at “advanced research laboratories, centre for pure and applied geology, university of sindh, jamshoro, pakistan”. the sections were point-counted using gazzi-dickinson point-counting method to calculate the percentage of textural and mineralogical attributes (ingersoll et al., 1984). the clastic source and tectonic setting of the sandstone was interpreted using ternary diagram (dickinson et al., 1983) by considering qfl (quartz, feldspar, lithics) fraction as 100%. the sandstone petrofacies were named based on their framework composition after mcbride (1963). due to the mixed nature, microfacies are classified according to the classification scheme proposed by mount (1985). the identified microfacies were compared with the standard microfacies proposed by wilson (1975) and flugel (2004) to validate their environmental interpretation. the field and petrographic data were integrated to discuss the depositional environment of the nari formation in study area. different softwares, such as arcgis, rockware logplot and corel draw etc., were also used to prepare maps, logs and pictures of present work. figure 2. stratigraphy of the southern indus basin and important tectonic episodes of the northwestern margin of the indian plate (i.p.) after; hunting survey corporation, 1960; burret, 1974; powell, 1979; allemann, 1979; barron & harrison, 1980; devey & lightfoot, 1986; coward et al., 1988; raza et al., 1990; jadoon et al., 1992; kadri, 1995; bender & raza, 1995; unrug, 1997; kazmi & jan, 1997; mcloughlin, 2001; shah, 2009; siddiqui, 2016; hinsch et al., 2018; ahmed et al., 2018; jadoon et al., 2021; halepoto et al., 2025. 152 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari 4. facies analysis in the study area, the nari formation has a conformable lower contact with the late eocene kirthar formation, while its upper contact is unconformably overlain by holocene alluvium (figures 3, 5a). the formation exhibits significant vertical variations in lithology. based on field observations, six distinct facies, comprising limestone, sandstone and shale, have been identified. limestone facies dominate the lower part, whereas sandstone facies are more prominent in the upper part of the nari formation (table 1). 4.1 arenaceous limestone facies (lf-1) description: these are the most dominant facies, constituting more than half of the nari formation in the studied section. they comprise nineteen strata grouped into eight intervals, with a total thickness of 83 meters out of the 130 meters recorded (figure 4). these facies has maximum thickness of about 75 meters within the lower 90 meters of the formation. the thickness of these facies decrease from bottom to top and are gradually replaced by calcareous and argillaceous sandstone towards top of the formation. the lower strata of these facies are rich in mega and microfossils. these facies occur in different colors such as light-brown, yellowish-brown, reddish-brown, reddish-yellow and orange-yellow and is well compact and thin to thick-bedded (figure 5b-d). interpretation: the lithological features suggest deposition on a shallow marine platform with minor clastic influx from surrounding landmass. the abundance of fossils indicate a stable, nutrient-rich, shallow marine ecosystem conducive for growth and preservation of marine life (selley 2000; saker-clark 2019; haqbin et al. 2024). the color variations likely reflect changes in water chemistry, oxidation conditions and organic matter content, influenced by local environmental fluctuations during deposition (aguilera-franco & romano, 2004; boggs, 2009). figure 3. geological map of the study area, where ab line indicates the measured stratigraphic section during present study (modified after hunting survey corporation, 1960). table 1. summarized characteristics of individual facies and their interpreted depositional environments. facies code facies name thickness (m) lithology color(s) sedimentary structures interpretation lf-1 arenaceous limestone 83 fossiliferous, compact limestone light to reddish/ yellowish brown thin to thick bedding shallow marine platform, low clastic influx lf-2 calcareousgypsiferous mudstone 2 soft mudstone with gypsum, calcite yellowish grey to light brown sharp, erosional base evaporative lagoon in arid/semi-arid conditions lf-3 calcareous sandstone 17 fine to medium sandstone, calcareous brown, grey thin-thick bedding fluctuating transitional to shallow marine conditions lf-4 laterite/oxidized sandstone 5 iron-rich, oxidized sandstone reddish brown thick bedding tropical weathering, sub-aerial exposure lf-5 arenaceous shale 8 silty shale, fissile, nodular light grey/yellow fissility, erosional base fluvial to transitional, periodic flooding lf-6 argillaceous sandstone 15 sandstone with mud matrix reddish-yellow to reddish grey thick-medium bedding fluvio-deltaic to transitional 153clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan figure 4. graphic log of the nari formation, showing identified facies, microfacies (mf), petrofacies (pf-1), their depositional environments, sea-level (blue line), mean base level (red line) change pattern, (r=regression and t=transgression) and interpreted depositional sequences. 4.2 calcareous-gypsiferous mudstone facie (lf-2) description: this is the second lithologic unit of the nari formation in the study area, located immediately above the first bed of lf-1 (figures 4, 5b). this facie may represent the contact between the underlying late eocene kirthar formation and the overlying nari formation. however, the first bed of lf-1 is included in the nari formation due its closer resemblance to the nari formation lithology rather than that of the kirthar formation (figure 5a-b). the mudstone is soft and easily eroded, leaving behind the harder limestone ridges on its both sides. therefore, this facie forms a depression between lf-1 strata and is covered with terrace deposits and vegetation and is only observed in drainage gullies (figure 5b). both the upper and lower bedding planes of this unit are sharp and erosional. the facie contains calcite nodules, gypsum laminations and veins, and in some places, iron concretions. interpretation: such mudstones are typically deposited in shallow, low-energy water bodies, such as flood-plains, lakes or lagoons, where fine sediments settle from slow-moving or quite-water (selley, 2000; boggs, 2009; lawal & hassan 2023). the calcareous content indicates high calcium content in the water (selley 2000; anan 2014). the gypsum points towards arid to semiarid conditions with high evaporation and limited freshwater influx (melvin 1991; könitzer et al. 2014). the occasional presence of iron concretions suggest oxidizing conditions (nichols, 2009). these evidences suggest deposition in a shallow-water evaporative lagoon. 4.3 calcareous sandstone facies (lf-3) description: these are the second most abundant facies, comprise six strata with a total thickness of 17 meters. the strata exhibit a range of colors, 154 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari such as dark brown, greenish grey, dark brown, reddish brown, yellowish grey and yellowish brown. the sandstone is fine to medium grained, in places coarse-grained, poor to moderately compact and thin to thick-bedded. these facies are relatively softer than the lf-1 and primary sedimentary structures are not observed in these facies. both lf-1 and lf-3 alternate each other in the central part of the nari formation in study area (figure 5d). interpretation: variations in color may reflect climatic oscillations between wet and dry periods, changes in water chemistry, oxidation conditions or early diagenesis (turner 1986; tucker, 1988; nichols, 2009; zeller et al. 2015; henares et al. 2020). the absence of primary structures indicates possible bioturbation, implying reworking of sediments by organisms during deposition. these observations points to a dynamic transitional to shallow marine depositional environment, potentially encompassing repetitive episodes of sediment reworking and deposition. 4.4 laterite/oxidized sandstone facie (lf-4) description: this facie is observed in the upper part of the nari formation in the study area, which comprises a 5 meter thick unit of laterite or oxidized sandstone. the facie is distinct in the field due to its distinct reddish-brown to blackish-red color and iron concretions and nodules. this facie is covered with pediment in the study area and its thickness is only measureable in the drainage gullies (figure 5e). interpretation: this facie marks a phase of regression and sub-aerial exposure, allowing intense oxidation and tropical weathering (walker 1974; ikhane et al. 2022). lateritization occurs due to chemical weathering of ironbearing minerals in tropical to subtropical climates, especially with seasonal rainfalls (schwarz 1996; scholle et al. 2015; nkoungou et al. 2023). 4.5 arenaceous shale facies (lf-5) description: two shale units with a combined thickness of 8 meters immediately overlie the oxidized sandstone unit (lf-4) (figures 4, 5e-f). • the lower shale unit exhibits dirty grey weathered and light grey when fresh, showing fissility, which is much similar to cross-stratification. it has erosional lower bedding plane (figure 5f). • the upper shale unit displays a dirty yellow weathered color and light yellow fresh color. this unit exhibit fissility and is nodular in places. interpretation: the fissility and erosional lower bedding plane suggest fluvial or deltaic deposition under inconsistent water flow, possibly influenced by periodic flooding events or increased sediment supply (tucker, 2001; boggs, 2009; jadoon et al. 2017). the erosional lower bedding plane could be indicative of erosional processes during transgression phase (nichols, 2009; prothero & schwab, 2014; könitzer et al. 2014). the arenaceous nature and nodularity indicates that the upper shale unit might have been deposited during periodic desiccation and shrink-swell processes (selley, 2000; boggs, 2009; abouelresh & slatt 2011). the depositional environment for these shale units likely imitates transition from more vigorous with inconsistent fluvial water flow (lower shale unit) to somewhat more stable, periodically desiccating transitional environment (upper shale unit). 4.6 argillaceous sandstone facies (lf-6) description: these are the third dominant facies, comprise three consecutive strata located near the top of the formation, with total thickness of 15 meters (figures 4, 5e). the argillaceous sandstone is reddish-yellow to reddish grey, moderately to well compacted, in places friable, thick to mediumbedded and lacks primary sedimentary structures. figure 5. field photographs showing [a] contact of the kirthar formation and nari formation, [b] basal facies of the nari formation, where calcareous-gypsiferous mudstone facies (lf-2) forms a depression between arenaceous limestone facies (lf-1), [c] arenaceous limestone facies (lf-1), [d] alternation of arenaceous limestone facies (lf-1) and calcareous sandstone facies (lf-3) in the central part of the nari formation, [e] oxidized sandstone or laterite (lf-4) and overlying facies in the upper part of the nari formation, [f] fissility in the arenaceous shale (lf-5). 155clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan interpretation: argillaceous sandstone suggests the paleo-environment that combines elements of both sandstone and mudstone deposition, typically fluvio-deltaic, estuarine and tidal mud flats (blatt et al., 1980; sinclair 1993; tucker, 2001). the mixing of marine and riverine processes suggest deposition under transitional conditions, specifically in quieter, more mud-dominated portions of deltaic setting (selley, 2000; duval, 2002; boggs, 2006; nichols, 2009; yijun et al. 2010; power et al. 2013; ekwenye et al., 2014). the deposition of argillaceous sandstone is indicative of complex inter-relationship between fluvio-deltaic and transitional marine conditions (blatt et al., 1980; reading, 1996; srivastava & mankar 2015). 5. petrography 5.1 texture four textural attributes of limestone-dominated microfacies, i.e. allochems, lime mud or micrite, siliciclastic grains and sparry calcite are determined. whereas, three textural attributes of sandstone-dominated petrofacies, i.e. framework grains, cement and matrix, are determined under the petrographic microscope (table 2). the limestone-dominated microfacies are classified as “packed biomicrite” and “sparse biomicrite” according to mount (1985) whereas sandstone-dominated petrofacies are classified as “arenites” according to dott (1964), on the basis of textural attributes (figure 7a-b). the term “allochem” was introduced by folk (1962) to describe all carbonate aggregates that make up bulk of many limestones. in the identified microfacies, allochems consists of bioclasts, extraclasts and gluconite mineral grains (figure 6). the bioclasts comprise whole organisms as well as fragments of foraminifera, mollusca, bryozoa, ostracoda, echinodermata and algae. most of the bioclasts are affected by micritization (figure 6a). the extraclasts are identified only in few samples of the nari formation, which show the signatures of transportation as well as alteration to micrite along their margins (figure 6b). lime mud or micrite is the second most dominant textural component of the limestone-dominated microfacies, which occurs as cement and matrix of these rocks (figure 6c-d). some units are dominantly composed of micrite (figure 6d). the matrix of the microfacies is also made up of very fine skeletal fragments, quartz, silt and argillaceous materials. sparry calcite constitute the cementing media along with micrite. micrite is the most common cementing material, while sparry calcite is only observed in four samples as fibrous cement (figure 6e). the occurrence of quartz, feldspar and other siliciclastic components in the studied sediments of the nari formation are believed to be terrigenous sediments (figure 6f). the siliciclastics form wide size range from coarse sand to clay. shape varies from angular to rounded, which indicates the varied distances of transport as well as intensity of reworking. the sandstone-dominated petrofacies are texturally immature, and are dominantly composed of sand, silt and clay-sized particles with minor carbonate allochems and micrite (table 2). 5.2 mineral composition the mineral composition of the nari formation from hundi anticline is tabulated in table 3. calcite is the most dominant mineral in the studied samples. it ranges from 85 to 95% in limestone-dominated microfacies and 10 to 15% in sandstone-dominated petrofacies (table 3). majority of the calcite occurs as either skeletal debris or micrite, while occasionally it occurs as extraclasts and sparry calcite (figures 6b-e). calcite in the sandstone generally occurs as cementing material that fills inter-granular spaces (figures 8a, c, g). table 2. the textural composition as calculated during petrography of the nari formation from hundi anticline, karachi embayment, pakistan. s. no. sample no. microfacies lithology textural composition classification mount (1985) standard microfacies facies zoneallochem (bioclasts) micrite sand extraclast sparry calcite 1 hnr-1 mf-1 arenaceous limestone 75 20 5 packed biomicrite smf-10 zone 7 2 hnr-4 mf-1 arenaceous limestone 70 22 8 traces packed biomicrite smf-10 zone 7 3 hnr-7 mf-2 arenaceous limestone 15 82 3 traces sparse biomicrite smf-19 zone 8 4 hnr-9 mf-3 arenaceous limestone 90 5 3 traces 2 packed biomicrite smf-18 zone 7 5 hnr12 mf-4 arenaceous limestone 70 25 5 traces traces packed biomicrite smf-9 zone 7 6 hnr18 mf-2 arenaceous limestone 35 60 5 traces sparse biomicrite smf-19 zone 8 7 hnr21 mf-5 arenaceous limestone 70 22 8 packed biomicrite smf-19 zone 8 8 hnr22 mf-2 arenaceous limestone 25 70 5 sparse biomicrite smf-19 zone 8 petrofacies frame-work grains cement matrix cement type classification 9 hnr28 pf-1 argillaceous sandstone 68 21 11 calcite, clay minerals arenite 10 hnr30 pf-2 argillaceous sandstone 70 20 10 calcite, clay minerals arenite 11 hnr32 pf-3 calcareous sandstone 72 16 12 calcite, clay minerals arenite 156 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari figure 6. photomicrographs of the nari formation showing different textural attributes as identified during the petrography [a] yellow arrows showing bioclast allochems and red arrows show micritization of bioclasts, [b] black arrows show calcite extraclasts and red arrows show their micritization, [c] gluconite (g) in micrite (m)-rich sample, [d] sample dominantly composed of micrite (m), while red arrow shows micritization of bioclast, [e] blue arrows show sparry calcite, [f] quartz (q) and feldspar (f) in argillaceous sandstone, where dark patches consists of clay minerals. figure 7..ternary plots showing [a] textural classification of the limestone-dominated facies of the nari formation, adopted after mount (1985), [b] textural and compositional classification of the sandstone-dominated facies of the nari formation, adopted after mcbride (1963) and dott (1964), [c] tectonic setting of the clastic source area of the nari formation, adopted after dickinson et al. (1983). (abbreviations; f-feldspar, l-lithics, l sa-lithic sub-arkose, q-quartz, qa-quartz arenite, qm-monocrystaline quartz, qp-polycrystaline quartz, qw-quartz wacke, sa-sub-arkose, s la-sub-litharenite). 157clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan figure 8. photomicrographs of the nari formation showing different minerals as identified during the petrography [a] calcite (blue arrows) and clay minerals (red arrows) as cementing material in quartz (q) rich sandstone, [b] quartz as framework minerals and clay minerals (red arrows) as cement, [c] occurrence of framework calcite (blue arrows), quartz (q) and feldspar (f) in arenaceous limestone, [d] occurrence of feldspar (f) cemented with clay minerals (red arrows) in sandstone, [e] clay minerals (red arrows) and hematite (h) in argillaceous sandstone, [f] quartz (q) cemented with clay minerals (red arrows) in argillaceous sandstone, [g] traces of rutile (r) and hematite (h) in calcareous sandstone, where quartz (q) is framework mineral and calcite is cement (blue arrows), [h] traces of magnetite (m) in sandstone. table 3. the mineral composition as calculated during petrography of the nari formation from hundi anticline, karachi embayment, pakistan. s. no. sample no. micro/ petrofacies lithology mineral composition in percentage calcite quartz feldspar clay minerals hematite gluconite rutile pyrite magnetite 1 hnr-1 mf-1 arenaceous limestone 90 10 traces traces traces 2 hnr-4 mf-1 arenaceous limestone 90 8 1 1 traces traces 3 hnr-7 mf-2 arenaceous limestone 85 12 3 4 hnr-9 mf-3 arenaceous limestone 92 8 traces traces traces 5 hnr-12 mf-4 arenaceous limestone 90 10 traces traces traces traces 158 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari the studied sediments of the nari formation dominantly contain nonundulose and mono-crystalline quartz (figures 8a, b, f). the quartz ranges from 5 to 13% in the limestone-dominated microfacies (figure 8c) while sandstonedominated petrofacies contain 72 to 85% of quartz (table 3). the second most dominant mineral in the sandstone is feldspar, which ranges from traces to 3% in limestone-dominated microfacies (figure 8c), while traces to 8% in sandstone dominated petrofacies (figure 8d) (table 3). the feldspar grains are generally smoky, cloudy and in places show evidences of dissolution and alteration. the significant proportion of clay minerals are also observed in the sandstonedominated petrofacies (figures 8b, d) (table 3). gluconite is identified in five samples of the limestone-dominated microfacies in the range of traces to 5% (table 3) (figure 6c). sediments of the nari formation contains minor to traces of hematite, rutile, pyrite and magnetite (figures 8e-h) (table 3). hematite is observed in most of the studied samples from traces to 2% (figure 8e). 6. microfacies/petrofacies analysis the studied samples are classified into five limestone-dominated microfacies and two sandstone-dominated petrofacies based on fossil contents, the relative abundance of allochems, micrite and framework grain composition. the identified microfacies and petrofacies are discussed below: 6.1 discocyclina packed biomicrite microfacies (mf-1) description: these are lowermost microfacies, which are identified in sample numbers hnr-1 and hnr-4 in basal part of the nari formation (figure 4). these microfacies are part of the “arenaceous limestone facies (lf-1)” and are rich in discocyclina and other larger benthic foraminifera (figures 9a-e). the discocyclina were earlier reported from the nari formation (hunting survey corporation, 1960; khan, 1968 cheema et al., 2009). these microfacies are dominantly composed of allochems, whereas micrite is observed to be in the range of 20-22% (table 2). both the allochems and micrite are composed of calcite, whereas, minor to trace occurrence of quartz, feldspar, hematite and glauconite are also observed (table 3). micritization and deformation of the bioclasts are also observed in these microfacies (figures 9d-e). interpretation: the abundance of larger and flat benthic foraminifera, such as discocyclina and allochems supports the low to moderate-energy, nutrient-rich depositional conditions. the eocene-oligocene wackstones rich in discocyclina are deposited in the middle to outer ramp below the fair-weather wave base at 50–60 m water depth are reported by zohdi et al. (2013) from zagros basin of iran, ali et al. (2018) from suleiman range of the indus basin, pakistan, banerjee et al. (2018) and srivastava & singh (2019) from the kutch basin of western india. therefore, it is interpreted that these microfacies were deposited in the photic zone of middle to outer-ramp in the open-shelf platform (figure 4). micritization and deformation of bioclasts could be diagenetic in origin, rather than depositional. these microfacies correspond to smf-10, deposited in facies zone 7, which is “open marine depositional zone” (figure 12) (wilson, 1975; flugel, 2004; boggs, 2009). 6.2 sparse biomicrite microfacies (mf-2) description: these microfacies are observed in samples hnr-7, hnr18 and hnr-22, and are part of the “arenaceous limestone facies (lf-1)” (figure 4). texturally immature these microfacies are dominantly composed of micrite and skeletal debris, with scarce fossil content (table 2) (figures 9f-i). additionally, sparry calcite, calcite veins, fine quartz and feldspar are observed in sample hnr-7 (table 3) (figure 9g), calcite extraclasts in samples hnr-18 (figure 6b), glauconite and traces of pyrite are observed in sample hnr-22 (figure 9h). interpretation: the abundance of micrite indicate deposition in lowenergy environment. the skeletal debris could belong to the remains of echinoderm, brachiopods and bivalves, which live in a variety of shallow marine environments (scholle et al., 1983; scholle & ulmer-scholle, 2003). scarcity of fossils in mudstone suggests an inner ramp lagoon depositional environment (burchette & wright, 1992). fine-grained quartz and feldspar are strong evidence of deposition in low-energy conditions. the feldspar and pyrite indicate the anerobic (reducing) environment, while glauconite is indicative of slow-deposition (amorosi, 1993). therefore, it is interpreted that these microfacies were deposited in inner platform/lagoon with sporadic influxes of quartz, feldspar and calcite extraclasts from adjacent area (figure 4). these microfacies are correlative with the smf 19 (flugel, 2004; boggs, 2009), deposited in the facies zone 8, which is “platform interior restricted” (figure 12). 6.3 bivalve-algae packed biomicrite microfacies (mf-3) description: these microfacies are observed in sample hnr-9, and are part of the “arenaceous limestone facies (lf-1)”. these are dominantly composed of allochems, which consists of formaninferal tests, skeletal fragments and calcareous algae. minor proportion of the micrite, sand, sparry calcite and calcite veins are also identified (table 2) (figures 4, 6e, 10a). the allochems and micrite are composed of calcite, whereas minor proportion of quartz is observed (table 3). these contain many varieties of microfossils such as foraminifera, bivalves, ostracods and calcareous algae. the foraminifera include nummulites, miliolina, alveolina, fusulinina, rotalia, triloculina, operculina and gaudryina etc (figures 10a-g). interpretation: the abundance of allochems and scarcity of micrite suggests a high‐energy hydrodynamic system under shallow marine environment produced either by tides or by combined tidal and wave processes during deposition of these microfacies (srivastava & singh, 2019). the presence s. no. sample no. micro/ petrofacies lithology mineral composition in percentage calcite quartz feldspar clay minerals hematite gluconite rutile pyrite magnetite 6 hnr-18 mf-2 arenaceous limestone 95 5 traces 7 hnr-21 mf-5 arenaceous limestone 85 13 2 8 hnr-22 mf-2 arenaceous limestone 85 10 traces traces 5 traces 9 hnr-28 pf-1 argillaceous sandstone 10 72 5 12 1 traces traces 10 hnr-30 pf-2 argillaceous sandstone 10 72 8 10 traces traces traces 11 hnr-32 pf-3 calcareous sandstone 15 85 traces traces traces traces 159clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan of broken nummulites tests (figure 10g) indicates deposition in turbulent water, which facilitated reworking of sediments. the fossils of bivalves indicates the shoal environment (abd el-moghny & afifi, 2022). the presence of calcareous algae indicates middle ramp depositional setting (singh et al., 2010; zohdi et al., 2013), while foraminiferal tests suggests high-energy middle ramp depositional environment (gischler et al., 2003; beavington-penney & racey, 2004; scheibner et al., 2007; kumar et al., 2011; zohdi et al., 2013; banerjee et al., 2018). therefore, it is interpreted that mf-3 were deposited in the middle ramp depositional setting (figure 4). these microfacies are equivalent to smf18 of (wilson, 1975; flugel, 2004), deposited in the facies zone 7 which is “open marine depositional zone” (figure 12). 6.4 assilina-operculina packed biomicrite microfacies (mf-4) description: these microfacies are observed in sample hnr-12, and are part of the “arenaceous limestone facies (lf-1)”. they contain fossils of assilina, operculina, nummulites and rare bryozoans (figures 4, 10h-m). these microfacies are dominantly composed of allochems and micrite is observed to be 25% (table 2). the micritization of the skeletal fragments is commonly observed in these microfacies. the allochems and micrite are composed of calcite, whereas, minor occurrence of quartz is also observed (table 3). interpretation: significant proportion of allochems indicate deposition under medium to high energy depositional environment. the good preservation of the assilina, operculina and nummulites suggest deposition below the fairweather wave base. the association of assilina, operculina and nummulites fossils is indicative of deposition under medium to high energy middle to outer ramp environment (beavington-penney & racey, 2004; scheibner et al., 2007; kumar et al., 2011; zohdi et al., 2013; banerjee et al., 2018; abd el-moghny & afifi, 2022). these microfacies are correlated with the smf 9 (wilson, 1975) (flugel, 2004), deposited in the facies zone 7 (figure 12). 6.5 arenaceous packed biomicrite microfacies (mf-5) description: these microfacies are identified in sample hnr-21, and are last fossil-bearing microfacies of the nari formation in studied section (figure 4). these are characterized by presence of significant amount of sand, micrite and fossils of camerina and miliolina (figures 11a-b). the sand and fossils consists of the allochem of these microfacies, which are dominantly composed of calcite with minor proportion of quartz and hematite (tables 2, 3). interpretation: the camerina and miliolina indicate shallow marine inner ramp depositional environment (nikbakht et al., 2019; alam et al., 2023). furthermore, presence of significant micrite indicates low-energy depositional environment, which also had received clastic influx from the nearby environment (khan et al., 2018; rizwan et al. 2020). therefore, the mf-5 is interpreted to be deposited in semi-restricted inner ramp depositional environment (figure 4). these microfacies can be compared with the smf 19 (wilson 1975; flugel 2004), deposited in the facies zone 8 (figure 12). figure 9. photomicrographs of the nari formation showing [a-c] occurrence of discocyclina in mf-1, [d] micritization of discocyclina bioclast, [e] folded discocyclina bioclast, [f] skeletal debris in mf-2, [g] sparry calcite (blue arrow) and calcite vein (blue line), [h] glauconite (g) and pyrite (p) embedded in micrite framework, [i] micrite in sparse biomicrite microfacies. 160 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari figure 10. photomicrographs of the nari formation showing [a] microfossils of miliolina (m), rotalia (r), fusulinina (f) and calcite veins (blue line), [b] alveolina (a), operculina (o) and algae (al), [c] nummulites (n) and miliolina (m), [d] alage (al), [e] gaudryina (g) and miliolina (m), [f] triloculina (t), [g] cross-sectional view of bivalve (yellow arrow and fragment of nummulites (n), [h-i] assilina (as), [j] operculina (op), [k-l] nummulites (n), [m] operculina (op) and bryozoa (br). figure 11. photomicrographs of the nari formation showing [a] camerina (c), quartz (q) micrite (red arrows) and calcite (blue arrow), [b] miliolina (m), quartz (q) and (blue arrow) cement, [c] feldspar (f), calcite (blue arrow) and clay minerals (red arrow) cement (yellow arrow), [d-e] quartz (q), algae (red polygons), clay minerals (red arrows) and hematite grains (blue hexagons). 161clastic source and depositional environment of mixed carbonate-clastic sequences in the oligocene nari formation from the hundi anticline, karachi embayment, indus basin, pakistan 6.6 sub-arkose sandstone petrofacies (pf-1) description: the sub-arkose sandstone petrofacies are identified in samples hnr-28 and hnr-30, and belong to “argillaceous sandstone facies (lf-6)” (figure 4). texturally as well as mineralogicaly, these petrofacies are immature, composed of quartz, calcite, feldspar, clay minerals and traces of hematite, magnetite, rutile and algae (figure 8 and 11c-e; table 3). the quartz and feldspar are framework grains, which are fine-grained sub-angular to subrounded and poorly sorted. the framework mineral grains are cemented with calcite and clay minerals. the cementing material occurs as either continues framework, veins or patches (figures 11d-e). interpretation: the texture of these petrofacies indicate moderate transportation and deposition in low-energy condition (boggs, 2006; reading, 2009). the presence of algae suggest transitional, whereas traces of ironbearing minerals are indicative of oxidizing conditions (khan et al., 2018; alam et al., 2023). therefore, it is interpreted that these petrofacies were deposited in fluvio-deltaic to transitional conditions (boggs, 2009) (nichols 2009; sallam et al. 2018; rizwan et al. 2020) (figure 12). figure 12. interpreted depositional environments of identified microfacies and petrofacies of the nari formation in studied section with respect to ecological ranges of identified fauna. 6.7 quartz arenite petrofacies (pf-2) description: these petrofacies are recognized in the sample hnr-32, and belong to “calcareous sandstone facies (lf-3)”. these are composed of quartz with minor feldspar, hematite and magnetite (figure 4; table 3). the calcite and clay minerals occur as cementing material. the quartz grains are floating in the calcite cement, while clay minerals occur as isolated patches (figures 8a-b). the quartz is fine-grained, sub-angular to sub-rounded and poorly sorted. texturally, these petrofacies are immature, whereas mineralogicaly, these are relatively more mature than pf-1. interpretation: fine-grained and poorly sorted quartz with absence of fossils suggests the low-energy depositional conditions. the presence of calcite cement reflects the connection to marine environment. the patches of clay minerals along with calcite suggest the depositional environment was periodically flooded with storms (blatt et al. 1980; blatt 1985; boggs 2006; nikbakht et al., 2019). the absence of fossils in these petrofacies further support the interpretation of unfavorable conditions for the preservation of organic material and development of biotic communities. the turbid water and fluctuating salinity due to periodic water inflows may have hindered the growth of living habitat (nikbakht et al., 2019; alam et al., 2023). therefore, it is interpreted that these petrofacies were deposited in low-energy transitional to lagoon setting (figure 12). 7. discussion 7.1 clastic source the lithic fragments are rarely observed in petrofacies of the nari formation. on the basis of relative proportion of quartz (q), feldspar (f) and lithics (l), the argillaceous sandstone petrofacies are classified as “sub-arkose”, whereas calcareous sandstone petrofacies are classified as “quartz arenite” (figure 7b) (mcbride, 1963). quartz and feldspar are essential components of acidic plutonic rocks (pettijohn 1975; madukwe et al. 2014; wang et al. 2017). their abundance suggests that the sandstone clasts of the nari formation were derived from acidic plutonic igneous sources. this interpretation is in agreement with the provenance of older clastic rocks in the study area, where clasts were supplied by granites and granitic-gneisses of the indian craton, exposed to the east and southeast (hakro et al., 2018; hakro et al., 2021; hakro et al., 2022; hakro et al., 2023; thebo et al., 2023). the tectonic setting of the clastic source area is interpreted using the framework mineral composition of sandstone, plotted on a ternary diagram adopted after dickinson et al. (1983). the relationship between sandstone framework composition and tectonic setting indicates that the clasts present in the nari formation were derived from a craton interior setting (figure 7c). the association of feldspar and clay minerals in the studied samples indicates that clay minerals formed through feldspar alteration. the low concentration of feldspar and rare rock fragments in the samples indicate a prolonged chemical weathering of the source area, likely under warm and humid climatic conditions (pettijohn et al., 1987; amireh, 1991; hakro et al. 2023; bilal et al. 2023b). 7.2 depositional environment 7.2.1 carbonate-dominated facies: outer to inner ramp settings the lower part of the nari formation is dominated by arenaceous limestone (lf-1), which comprises the carbonate microfacies (mf-1 to mf-5) identified during present study. the lf-1 indicates a shallow-marine platform with limited clastic input and abundant biological activity. the presence of larger benthic foraminifera such as discocyclina, assilina and operculina (mf1 and mf-4) suggests deposition under low to moderate-energy conditions, specifically within outer to middle ramp settings below the fair-weather wave base (srivastava & singh (2019; nikbakht et al., 2019; alam et al., 2023; shah et al., 2024). the mf-3 represents higher energy conditions, as manifested by abundant skeletal fragments and calcareous algae, which is in good agreement with middle ramp to shoal or back-shoal settings (burchette & wright, 1992; rizwan et al. 2020). the mf-2 and mf-5 microfacies indicate low-energy environments in the inner ramp or semi-restricted lagoon settings. rare fossil content, high micrite proportion, occurrence of feldspar, glauconite and pyrite support deposition in a quit, semi-restricted inner ramp, sometimes under anaerobic conditions (khan et al., 2018; rizwan et al. 2020). the lf-1 marks transgression at its base, because these facies are deposited in relatively deeper waters than underlying facies. therefore seven transgression phases are identified at the base of these facies within the nari formation (figure 4). these are the only facies, which mark the transgression at their base while all other five facies (lf-2 to lf-6) are deposited during regressive phases. the presence of calcareous-gypsiferous mudstone (lf-2) between the (lf-1), in the base of the nari formation suggest evaporative lagoon conditions during the early phase of its deposition. the facies pattern indicates that despite an initial phase of uplift and regression (lf-2), the early deposition of nari formation occurred in relatively stable basin (lf-1) (figure 13). figure 13. schematic depositional model of the oligocene nari formation from hundi anticline, karachi embayment, indus basin, pakistan. 162 surriya bibi ahmedani, asghar a. a. d. hakro, abdul shakoor mastoi, aijaz ali halepoto, ali ghulam sahito, shamim akhtar, rafique ahmed lashari 7.2.2 clastic-dominated facies: inner ramp to fluvio-deltaic settings up-section, calcareous sandstone (lf-3) and oxidized sandstone/laterite (lf-4) reflect more dynamic environments with intermittent exposure and weathering under tropical climatic influences. the lf-3 facies are represented by quartz arenite petrofacies (pf-2). the pf-2, characterized by high quartz content, calcite cement and absence of fossils, indicates deposition in a lowenergy, periodically storm-influenced lagoon (srivastava & mankar 2015; ahmad et al., 2021). the most common alternation of lf-1 and lf-3 suggest the periodic repetition of depositional conditions in which both these facies were deposited in a nearly same depositional environment, e.g. inner ramp to semi-restricted lagoon (figures 4, 5d, 13). the deposition of lf-1 (mf-5 and mf-2) after lf-3 culminated in the exposure and development of laterite/ oxidized sandstone (lf-4). it marked end of the first depositional sequence of the nari formation, which was dominantly composed of shallow-marine platform sediments. the second depositional sequence of the nari formation is dominantly composed of transitional to fluvio-deltaic sediments. the arenaceous shale facies (lf-5) mark a transition from continental (lf-4) to transitional settings, influenced by episodic flooding and desiccation. these facies are overlain by arenaceous limestone (lf-1), deposited in the semi-restricted inner ramp lagoon settings. therefore, their stratigraphic position suggests that the arenaceous shale facies were deposited in a fluvio-deltaic to transitional stage during the sea level rise (figure 4). the repetition of lf-1 and lf-3 in the upper part of the nari formation suggest periodic return of the inner ramp/ lagoon settings. the lf-3 is overlain by the argillaceous sandstone facies (lf-6), which aligns with the sub-arkose petrofacies (pf-1). the fine-grained quartz, feldspar, clay minerals and algae traces suggest a low-energy fluviodeltaic to transitional environment (rizwan et al. 2020; ahmad et al., 2021; babkir et al., 2023). traces of algal remains and iron-bearing minerals point to shallow oxidizing conditions. furthermore, association of lf-6 with lf-1 and lf-3, suggest desiccation and estuarine to fluvio-deltaic sedimentation under regressive conditions. the transition of facies from one environment to adjacent environment suggests that the nari formation was deposited on the homoclinal carbonate ramp, where depositional environments graded from deeper outer ramp settings to inner ramp lagoons and fluvio-deltaic conditions without major slope breaks (burchette & wright, 1992) (figure 13). overall, the vertical facies progression document a regressive depositional system and transition from shallow-marine to fluvio-deltaic conditions, potentially driven by plate collision during the oligocene. bender (1995) suggested that marine conditions in the indus basin started to retreat during the late eocene and early oligocene. the depositional pattern of the nari formation indicates that basin was experiencing transitional phase from a passive-margin to a foreland basin. the regular clastic supply to the carbonate depositional system also confirms the tectonic uplift and deformation. these observations suggest that the early stage of deformation of the northwestern margin of the indian plate during oligocene had profound control on the depositional environment in the study area. 8. conclusions the oligocene nari formation in the study area comprises six carbonateclastic facies, including arenaceous limestone, calcareous sandstone, argillaceous sandstone, laterite/oxidized sandstone, arenaceous shale and calcareous mudstone. petrographic analysis further identified five limestone-dominated microfacies (packed and sparse biomicrites) and two sandstone-dominated petrofacies (sub-arkose and quartz arenite). the framework composition of the sandstone indicates a clastic source linked to the granitic terrains of the indian craton. the facies succession displays a general progradational pattern, deposited in a range of environments on the homoclinal ramp, including outer ramp, middle ramp, inner ramp, lagoon, evaporate, transitional and fluviodeltaic. this vertical facies pattern indicates a regressive depositional system and transition from shallow-marine to fluvio-deltaic conditions, driven by the himalayan orogeny associated with collision of the indian and eurasian plates during the oligocene. acknowledgements authors are thankful to the centre for pure and applied geology, university of sindh, jamshoro, for providing the transport and logistical support to carry out the fieldwork for present study. authors are also thankful to the students mrs. muskan lodhi, mrs. zahida lashari, mrs. komal sheikh and mr. abdul subhan ghouri for their assistance during the measurement of stratigraphic section and sampling in the field as well as laboratory. we sincerely thank the editorial board of the “earth sciences research journal” for their efficient handling of our manuscript. we also extend our gratitude to the anonymous reviewers for their thoughtful, critical and constructive feedback, which greatly improved the quality of this work. author contributions surriya bibi ahmedani: conceptualization, writing original draft, asghar a. a. d. hakro: conceptualization, resources, supervision, abdul shakoor mastoi: methodology, supervision, investigation, aijaz ali halepoto: data curation, formal analysis, ali ghulam sahito: visualization, validation, software, shamim akhtar: review and editing, rafique ahmed lashari: methodology, supervision and review. declaration of competing interest the data used in this study is taken from the outcrops, which are open data sources. therefore, it is declared that we have no any conflict of interest to disclose. meanwhile where published data sets are used, the reference of the authors are given. work described here has not been published previously in any journal and not under consideration for publication elsewhere. funding: this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. data availability statement the data that support the findings of this study are available from the corresponding author upon reasonable request. references abd el-moghny, m. w., & afifi, a. a. 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(2018). fractures in volcanic reservoirs: a case study in zhongguai area at the northwestern margin of junggar basin (china). earth sciences research journal, 22(3), 169-174. doi: https://doi.org/10.15446/esrj.v22n3.75426 fractures in carboniferous volcanic rocks located at zhongguai area (china) highly influence the accumulation and productivity of oil and gas. as such, the study of development periods and genetic mechanisms of tectonic fractures could throw useful information regarding the evaluation and development of that reservoir. their tectonic origins caused high-angle and oblique shear fractures. the primary orientation of those fractures appears close to ew (270°±10°), nw (300°±15°), ne (45°±15°), and sn (0°±10°). at least four fracture generations can be found in carboniferous volcanic rocks at zhongguai area. combined with a tectonic evolution, they are based on the segmentation relationship of the fracture fillings, the thermometry measurement of the fracture filling inclusion, and the acoustic emission, as well. affected by a new horizontal principal stress, the opening and permeability of nearly ew fractures are the best. in this way, a priority in the development of well's patterns should be considered close to ew fractures. the pressure change in the process of exploitation may damage the reservoir permeability of fractured volcano rocks severely. accordingly, well patterns should be adjusted to dynamic changes of permeability happened during the oilfield development since some differences have been detected in distinct fracture sets. abstract keywords: tectonic fractures; zhongguai area; volcanic rocks. fractures in volcanic reservoirs: a case study in zhongguai area at the northwestern margin of junggar basin (china) fracturas en yacimientos volcánicos: caso de estudio del levantamiento de zhongguai en la margen noreste de la cuenca de junggar, china issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v22n3.75426 las fracturas en las rocas carboníferas volcánicas del área de zhongguai tienen una influencia determinante en la acumulación y producción de petróleo y gas. el estudio de los períodos de desarrollo y el mecanismo genético de las fracturas tectónicas proveen información importante para la evaluación del yacimiento y su desarrollo. en esta zona se encontraron fracturas con un ángulo alto de cizallamiento y fracturas oblicuas causadas por origen tectónico. la orientación principal de las fracturas es cercana a ew (270°±10°), nw (300°±15°), ne (45°±15°) y sn (0°±10°). se encontraron por lo menos cuatro generaciones de fracturas en las rocas volcánicas del área de zhongguai. combinadas con la evolución tectónica, las fracturas están basadas en la relación de segmentación del relleno de las fracturas, la termometría de las inclusiones, y la emisión acústica. afectadas por esfuerzos horizontales recientes, las fracturas de apertura y permeabilidad con dirección ew son las mejores. aquellas fracturas cercanas a la dirección ew deben ser consideradas como una prioridad en el desarrollo de los patrones del pozo. el cambio de presión en los procesos de explotación podría causar daños severos en la permeabilidad de las rocas volcánicas fracturadas en el yacimiento. durante el desarrollo del campo petrolífero, los patrones del pozo deben ser ajustados de acuerdo con los cambios en las dinámicas de permeabilidad ya que las configuraciones de fracturas son diferentes. resumen record manuscript received: 02/02/2018 accepted for publication: 17/07/2018 earth sciences research journal earth sci. res. j. vol. 22, no. 3 (september, 2018): 169-174 cunhui fan1,2,3, qirong qin1,3*, feng liang4, zenghui fan5, zhi li1,3 1state key laboratory of oil and gas reservoir geology and exploitation, southwest petroleum university; chengdu 610500, china 2state key laboratory of oil and gas reservoir geology and exploitation, chengdu university of technology, chengdu 610059, china 3school of geoscience and technology, southwest petroleum university; chengdu 610059, china. 4geological research institute of central sichuan district of petrochina southwest oil&gas field company, suining 629000, china 5cnpc bureau of geophysical prospecting inc, southwest geophysical research institute, chengdu 610213, china corresponding author’s e-mail: 986336692@qq.com g e o l o g y 170 cunhui fan, qirong qin, feng liang, zenghui fan & zhi li introduction volcanic reservoirs present a firm heterogeneity and poor physical properties. tectonic fractures originated from tectonism constitute the main seepage paths and reservoir spaces in the volcanic reservoirs. the study of the characteristics, controlling factors, and formation mechanism of tectonic fractures in volcanic rocks play a crucial role in oil and natural gas exploration and development. as several authors point out, the study regarding the growth characteristics and the division of the development phase in tectonic fractures constitutes a problem to tackle with (nelson, 2001; larsen, grunnaleite & gudmundsson, 2010). the zhongguai area is located at the northwestern margin of the junggar basin. the carboniferous system is one of the primary hydrocarbonbearing sources in the region with the main reservoir rocks being volcano ones. several years of exploration at the zhongguai area were needed to achieve a breakthrough series of oil and gas exploration in the carboniferous volcano rock reservoir. commercial oil flow was obtained in many testing wells, in which h019 well emitted a daily oil mean of 105.96 t/d and a (daily) gas mean of 2. 14 × 103 m3 in the carboniferous oil test, a reasonable prospect for a gas and oil exploration in the area of volcanic rocks. however, from current tests of oil and gas production, the variation of the single well productivity becomes larger (a unique well oil production from 0.1 to 105 t/d), displaying the typical properties of a fractured reservoir, with the fracture effect in the production control being significant. the frequent tectonic alteration experienced in the area contribute to relatively developed fractures. it is worth mentioning that fractures at different stages play a crucial role in the regulation and controlling of hydrocarbon migration and accumulation. also, fractures become essential in high yields (fan & qin, 2016). in this paper, the authors have analyzed the characteristics of the fracture's development and carried out a preliminary qualitative division of fracture formation times by combining structural analysis and core observation in the study area with imaging logging interpretation. additionally, they have contributed a quantitative division of fracture development phases by an acoustic emission experiment of rocks and a fracture filling inclusion (warren et al., 2017; yang et al., 2018; fu & liu, 2017). geological characteristics zhongguai area is located at the conversion parts of the fault belts of ke-wu and hongche, in the northwestern margin of junggar basin. also, it plunges to that basin from the hongche one to the southeast. three sides of the zhongguai area face the concave (adjacent to mahu sag, pcn1well west sag, and shawan sag), and it is a favorable directional zone for oil and gas migration and accumulation (figure 1). its main form is a wide flat nose paleo-uplift, which is formed under the action of extruding stress field in the carboniferous to early permian. it started to hide at the end of permian; overlap was deposited on a raised in wuerhe formation (upper permian) and angular unconformable contact with the jiamuhe formation of the permian; the triassic-jurassic draped on top of a raise was a smaller bulge. yanshan movement caused an uplift further, so the tectonic pattern formed to single southeast tilting, and upwelling area tilted eventually in the himalaya movement period. volcanic rock in the research area buries deeply. composing the basal part of the basin, which is mainly developed by the interaction between continental-oceanic volcanic rock and environment, primarily includes volcanic lava (andesite and basalt), pyroclastic rocks (volcanic breccia and tuff), and few intrusive rocks (granite), with tuff and andesite being widely distributed. in that area, the volcanic rock reservoir physical property is reduced. the average porosity range fluctuates from 5 to 7 %. its matrix permeability value is generally less than 1×10-3 μm2, with a mean value being only equal to 0.61×10-3 μm2. as such, it corresponds to a low reservoir not only in porosity but also in permeability. since carboniferous, the tectonic fracture of such a reservoir has been growing more and more due to frequent tectonic movements leading to advantageous accumulation places. also, the infiltration channel of the reservoir in this area exhibits a significant influence on oil and gas enrichment and development effect. therefore, research on layer fracture characteristics, formation stage, and flow characteristics of the volcano rock reservoir in that area becomes crucial for volcanic rock reservoir exploration and efficient development. figure 1. tectonic map in zhongguai area. development characteristics of structural fractures the core observation regarding 596, h56a, h019, and 16 wells suggests that carboniferous volcano rock reservoir in zhongguai area could be divided into four types of fractures, namely, diagenetic, structural, weathering, and solution ones. first, tectonic fractures are mostly developed and widely distributed in various types of volcano rocks. according to the statistics of tectonic fracture dips, it holds that fractures can be divided into two categories: a class of high angle of inclination > 60° seam and straight seam for about 70 %. another kind of dip is at 30° 60° oblique fractures, accounted for about 20 %. the high angle fractures are mainly shear ones, characterizing the joint surface flat rules, stable occurrence, visible scratches, mineral filling, and so on (figure 2-a, figure 2-b). oblique fractures mainly include shear and tensile ones. the different characteristics among the fractures of an intersection intersperse with others, leading to a connected seepage system. in this way, the development and distribution of the volcano rock in carboniferous reservoirs play a crucial role in controlling. according to some hydraulic fracturing experiments and the analysis of the fracture azimuth imaging logging after treatment, the fractures mainly developed are located at ne (45°± 15°), nw (300°± 15°), ew (85°± 10°), and sn (0°± 10°), with ne (45°± 15°) and ew (85°± 10°) being the ones mostly developed, followed by nw (300°± 15°). on the other hand, the worst is close to sn (0°± 10°). it is worth noting that fracture filling material mainly consists of calcite, chlorite, and gypsum; fracture filling degree is high; around 60 % of fractures are wholly filled; half-filled fractures and not filled ones accounted for 40 %. 171fractures in volcanic reservoirs: a case study in zhongguai area at the northwestern margin of junggar basin (china) the imaging logging analysis results point out that fractures are concentrated in the 2-5 strips/m, followed by 0.4-0.6 strips/m, with the overwhelming majority fracture density corresponding to a medium value < four strips/m. the scope of the fracture length change is broad. the long range between 10-30 cm accounts for more than 70%, and the maximum cutting depth can reach 180 cm. fracture width is in the range of 1-5 mm; in the local core exist open fractures whose fracture width is > 1 cm. based on the field investigation and statistics of fracture spacing, it holds that the fracture spacing range goes from 0.3 to 0.7 m, and the average spacing equals 0.5 m. discussion on fracture growth stages in recent years, the major technologies in the research on fracture development periods involve (1) geological methods, including analysis of regional construction features, investigation of fractures in outcrop area, outcrop fractures, and fracture characteristic analysis of survey underground (lee, 2001; espinosa, 1998); (2) lab test methods, including filling isotope analysis, inclusion thermometry, and rock acoustic emission testing (lopezpuente et al., 2005; tasdemirci et al., 2012; lankford, 2004; lausund et al., 2015), as well. this article study is mainly focused on the core of fracture filling composition and mutual cut features, fracture filling, the homogenization temperature of hydrodynamic inclusions, and rock acoustic emission methods to explore the volcanic fracture formation period of carboniferous in zhongguai area. the fracture character of core reflects the fracture stage the primary approach to explore the fracture stage involves the core fracture azimuth, the filling ingredients, and the interaction cutting relationship among the fractures to analyse the fracture formation stage. results from the fracture filling material composition and the interaction cutting relationships (figure 3a and 3b) combined with an image logging analysis show that the carboniferous volcanic rock fracture at the zhongguai area are mainly divided into four stages. as such, the first stage of fractures is close to both ew (85°± 10°) and nw west (300°± 15°), with the fillings mainly consisting of calcite and chlorite. the second fracture is ne (45°± 15°). its fillings are primarily gypsum. the third one appears at both ew (85°± 10°) and nw west (300°± 15°), whose fillings mainly consist of calcite. finally, the fourth phase primarily corresponds to both sn (0°± 10°) and nw west (300°± 15°) to fracture, with no filling practically. homogenization temperatures of fracture filling inclusion in recent years, the analysis of fracture filling inclusion lies in a mature technology allowing researching features of fracture filling inclusion and measuring the homogenization temperature. it is worth mentioning that the temperature filling formation can be obtained to determine both the formation period and the mineral filling stage (johnson & cook, 1985). the fourth stage is almost not filled. the present study is focused on 8 samples and 46 test points of fracture fillings from the first three different cutting stages to test the homogenization temperature, according to the various stages of the temperature distribution of fracture fillings and their morphological characteristics. the three filling times of tectonic fractures in that area can be distinguished in the following terms. the homogenization temperature of the first phase ranges from 75 to 82 °c. the second homogenization temperature lies in the range 92-97 °c, with the third homogenization temperature ranging from 106 to 112 °c. there are, at least, four stages of tectonic fractures under the assumption of fractures from the last period with almost no fillings (figure 4). acoustic emission experiment of rocks reflects the fracture formation period the formation of rocks experienced stress effects in the long run with a memory function of stress. in the early sixties, it was confirmed by goodman that rocks also exhibit a "kaiser" effect. the acoustic emission test is supported by the fact that there exists a micro griffith fracture generated by the ancient tectonic stress field in the underground rock stratum. only (a) 596 well, 2398. 5m, high-angle shear fracture (b) k007 well, 3072. 2 m, half-filled high-angle figure 2. characteristics of volcano rock reservoir fractures in the zhongguai uplift carboniferous development. (a) 598 well, 3129. 4 m, fractures cutting cross each other (b) 596 well, 3128. 8 m, fractures cutting cross each other figure 3. core fracture filling characteristics and cutting relationships. 172 cunhui fan, qirong qin, feng liang, zenghui fan & zhi li significant influence in the improvement of both the carboniferous volcanic reservoir property and hydrocarbon accumulation. as such, drilling in the fracture development areas will result in better oil testing results. fractures on the longitudinal direction are often more extended, though the fracture interval on the lateral is larger and the drilling rate of the vertical well is low. therefore, directional drilling is typically used in the drilling process to improve the fracture rate of the drilling. moreover, both the fracture system and the period in that area have been analyzed. it holds that the development situation and the lithology (tectonic stress, tectonic position, and thickness) are closely related. the fracture opening at different periods and distinct ranges in the underground reservoir are inconsistent, the difference of permeability is significant, and it is closely associated with the field direction of the current stress. the present principal stress orientation is close to ew (270°± 10°) with the degree of open fractures being parallel to the predominant stress orientation closely to ew (85°±10°), that is the highest underground. the fracture permeability of that group is the largest, with an effective permeability of 43. 2×10-3 μm2. secondly, the average permeability value of fractures that present a sharp angle with the principal stress orientation of nw (300°±15°) and ne (45°±15°) is 18-24×10-3 μm2. the opening and permeability of sn (0°±10°) are the lowest, with a permeability value equal to 2-10×10-3 μm2. further, the hydrodynamic seepage velocity in the fracture closely to ew (85°±10°) in the early development is the fastest, whereas the hydrodynamic seepage velocity in the fracture closely to sn (0°±10°) is the slowest. notice that both the hydrodynamic seepage velocity and the dynamic response in the nw (300°±15°) and ne (45°±15°) fractures lie between the corresponding values of ew (85°±10°) and sn (0°±10°) fractures. thus, the fracture permeability anisotropy of different groups in the carboniferous volcanic rock reservoir is evident. accordingly, the effect of fractures closely to ew (285°±10°) should be considered firstly in the deployment of development scheme of volcano rock reservoir in this area with the aim to systematically figure 4. temperature distribution chart of fracture filling inclusion at the zhongguai area. if the force exceeds the stress intensity, the fracture can extend and lead to a plurality of different kaiser effect points. more recently, the continuous evolution of technology expands its applications to fault evolution history, fracture stage by matching and ensuring the intensity of fracture formation stress fields (kresse et al., 2013; meyer & bazan, 2011). the present study consists of an acoustic emission experiment regarding the kaiser effect involving 12 pieces of carboniferous volcanic rocks from the zhongguai area. the experimental results suggest that our sample generally presents five obvious kaiser effect points. one of them stands as a result of the tectonic stress effect, whereas the others highlight that rock experienced four micro-fracture events along its geological history. combined with regional tectonic evolution characteristics and research results, it was observed that the first fracture development stage, a product of the middle hercynian tectonic movement, with a paleotectonic stress of 49.6 mpa. the second fracture development stage is a product of the late hercynian tectonic movement and presents a paleotectonic stress of 39.9mpa. the third fracture development stage is a product of the indosinian tectonic movement, whose paleotectonic stress is 34.1 mpa. finally, the fourth fracture development stage is a product of the yanshan tectonic movement, and its paleotectonic stress is 26.5 mpa (figure 5). seepage characteristics of structural fractures fractures in low porosity and low permeability reservoirs are crucial to reforming the reservoir physical property and also concerning the accumulation of oil and gas. fractures tend to control the hydrodynamic seepage network of reservoirs (wu, 2015). from the carboniferous porosity-permeability relationship diagram in the zhongguai area, it can be observed that the correlation between porosity and permeability is not good. the correlation coefficient is only 0.4257 (figure 6), with the difference between the porosity of the development samples and the porosity of the fissureless samples being quite poor. however, the permeability of the fracture development samples stands from 3 to 5 orders of magnitude higher than the permeability of the fissureless samples. for instance, the permeability of the vast majority of the fissureless sample at the work area lies in the range going from 0.01×10-3 μm2 to 1×10-3 μm2. the fracture development samples, such as andesite cores at 3366 m, developed fractures in 598 wells. the penetration rate in the tested samples is as high as 354.14×10-3 μm2, illustrating that the presence of fractures has a figure 5. carboniferous volcano rock acoustic emission ae evolution at the zhongguai area. figure 6. reservoir porosity and permeability correlations of the carboniferous volcanic reservoir at the zhongguai area. 173fractures in volcanic reservoirs: a case study in zhongguai area at the northwestern margin of junggar basin (china) evaluate distinct tectonic fracture seepage characteristics. in various stages of oil and gas field development, it holds that the dynamic changes of fracture permeability in different groups are separate from one to others due to the fracture permeability and seepage velocity. in this way, both the fracture opening and permeability closely to ew (285°±15°) is the greatest. drop speed of hydrodynamic pressure and fracture opening within the reservoir is faster in the process of oil well production. such a group of fracture seepage effect on oil and gas weakened gradually, while descent speeds of the hydrodynamic pressure and fracture opening of fractures in nw (300°±15°), ne (45°±15°), and sn (0°±10°) fall slower than the one closest to ew (285°±15°). with the oil field development process, the dynamic response of fractures in nw (300°±15°), ne (45°±15°), and sn (0°±10°) will be more and more evident, allowing to adjust the well's pattern. the fractured volcanic reservoir exhibits less sensitive mineral and fractures developments. to confine pressure changes of fractures in underground reservoirs will arouse fracture opening and permeability changes. also, its reservoir sensitivity presents typical characteristics of stress sensitivity, being this one of the critical factors leading the volcano rock fractured reservoir yield decrease rapidly. the stress sensitivity refers to the degree of change of the rock permeability caused by the effective stress change in the rock skeleton. volcano rocks have the priority since their body deformations belong to the dense medium. the calculation of the effective stress can be performed throughout the following expression from the double effective stress theory: σ σ ϕeff p p= − in the formula above, seff p denotes the effective primary stress,  is the overburden pressure, p is fluid pressure, and  denotes the porosity. due to the different rates of fracture occurrence and degree of development, the fractured reservoir stress sensitivity changes. in this way, the considerations that follow should be stated. a total amount of 12 different fracture occurrences of andesite and tuff samples in the area were selected, taking advantage of an automatic core displacement system to test stress sensitivity. in both figures 7 and 8, a high-angle fracture stress sensitivity experiment is developed for oblique fractures with a different fracture occurrence and a nonidentical degree of stress change sensitivity. using 20 mpa as the standard, the permeability damage rate of the oblique fractures was equal to 60 % (figure 7). the permeability damage rate of high-angle fracture is less than 40% (figure 8). the obtained results illustrate that the fractures from the oblique fracture samples are easily closed when the confining pressure drops. also, the higher the core loses its permeability, the stronger it is stress sensitivity. on the other hand, the fractures from the high-angle samples are hard closed when the confining pressure drops. as such, the permeability of the core loss becomes smaller as long as the stress sensitivity is relatively weak. visibly, for fractured volcano rock reservoir, the pressure change in the process of exploitation may cause severe damage to the reservoir permeability. accordingly, to be focused on targeted measures of exploitation results entirely necessary in the process of reservoir development in fractured volcano rocks. conclusions the main conclusions in this paper are stated below. (1) regarding the structural fractures of carboniferous volcano rock reservoir being developed at the zhongguai area. fractures constitute a crucial factor regarding space and seepage channels in volcanic rock reservoirs. they are mainly of oblique and high-angle-shear types. the locations of such a fracture development are close to ew (270°± 10°), nw (300°± 15°), ne (45°± 15°), and sn (0°± 10°). it is worth mentioning that these intertwine with each other leading to a fracture network system, which is favorable for oil and gas accumulation and high production. (2) the carboniferous volcanic rock fracture at the zhongguai area could be divided into four main stages according to the core observation and the temperature of homogenization of the fracture filling inclusion together with the regional tectonic evolution features. (3) structural fractures play a crucial role regarding volcanic rock reservoir seepages and accumulation of oil and gas. fracture system more abounds in that area. it has been observed that the opening degree and the fracture seepage effect differ from one area to another for different groups. to properly arrange the development of well's patterns, the impact of fracture seepages should be thoroughly considered. in various stages of the development of oil and gas fields, the dynamic changes of permeability in distinct groups are nonidentical. as such, the well's pattern should be timely adjusted. figure 7. effective stress permeability curve for oblique fractures. figure 8. effective stress permeability curve for high-angle fractures. 174 cunhui fan, qirong qin, feng liang, zenghui fan & zhi li acknowledgments this research was supported by open fund (plc 20180404) of state key laboratory of oil and gas reservoir geology and exploitation (chengdu university of technology) and open fund (pln 201718) of state key laboratory of oil and gas reservoir geology and exploitation (southwest petroleum university). references espinosa, h., dwivedi, s., zavattieri, p., & yuan, g. 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(2011). a discrete fracture network model for hydraulically induced fractures: theory, parametric and case studies. presented at spe hydraulic fracturing technology conference, the woodlands, texas, usa, 24–26. spe-140514-ms. nelson, r. a. (2001). geologic analysis of naturally fractured reservoirs. boston: gulf professional publishing, 89-94. tasdemirci, a., tunusoglu, g., & guden, m. (2012). the effect of the interlayer on the ballistic performance of ceramic/composite armors: experimental and numerical study. international journal of impact engineering. 44, 1–9. wu, k. & olson, j. e. (2015). a simplified three-dimensional displacement discontinuity method for multiple fracture simulations. international journal of fracture, 193, 191–204. how to cite item ren, w., & yi, g. (2019). characteristics of time-transgressive holocene optimum in the east asian monsoon region. earth sciences research journal, 23(3), 259-263. doi: https://doi.org/10.15446/esrj.v23n3.72188 palabras clave: máximo holoceno; región monsónica del este asiático; transgresiones del tiempo; demografía del neolítico. el máximo del holoceno (ho, del inglés holocene optimum) define una época húmeda y caliente del período del holoceno, cuyas edades iniciales y finales han sido ampliamente estudiadas en el este asiático; sin embargo, los debates continúan enfocados en establecer los patrones espaciales y temporales. en este artículo se sintetizan los últimos registros de la región monsónica del este asiático y del centro árido de asia para evaluar los patrones del máximo del holoceno. los resultados de esta síntesis indican que la latitud y tanto el comienzo como el fin del máximo holoceno tienen una fuerte relación que consiste en la expansión y el retiro hacia el norte del máximo holoceno, excepto para los registros cercanos al altiplano del tibet y otras zonas altas. tanto el máximo holoceno y el desarrollo tecnológico sugieren que estas condiciones permitieron la explosión demográfica del neolítico. este estudio provee herramientas para un conocimiento mejorado de las características transgresivas del máximo holoceno y permite acercarse a las edades de comienzo y fin del máximo holoceno en diferentes puntos geográficos de la región monsónica del este asiático. resumen characteristics of time-transgressive holocene optimum in the east asian monsoon region issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v23n3.72188 keywords: holocene optimum; east asian monsoon region; time–transgressive; neolithic human demography. características transgresivas del tiempo durante la época máximo del holoceno en la región monsónica del este asiático the holocene optimum (ho) defines a relatively warm and wet period during the holocene, the initial and termination ages of which have been extensively reported for east asia; however, debates still focus on underlying spatial and temporal patterns. here, latest paleoclimate records from the east asian monsoon region and arid central asia are synthesized to evaluate the spatial and temporal patterns of the ho. the results of this synthesis indicate that the latitude and both the initial and termination ages of the ho follow a strong relationship, which is an expansion and northward retreat of the ho except for records around the tibetan plateau and several other highlands. both the ho and technological development are also suggested to have shaped the historic boom of the neolithic human demography. this study provides a window toward an improved understanding of the characteristics of time-transgressive ho and allows to make predictions for the initial and termination ages of the ho at different geographical locations of the east asian monsoon region. abstract record manuscript received: 15/05/2018 accepted for publication: 29/03/2019 earth sciences research journal earth sci. res. j. vol. 23, no. 3 (september, 2019): 259-263 weihe ren1, guitian yi2 1key laboratory of land surface pattern and simulation, institute of geographic sciences and natural resources research, chinese academy of sciences, beijing 100101, china 2key laboratory of vegetation restoration and management of degraded ecosystems, south china botanical garden, chinese academy of sciences, guangzhou 510650 * corresponding author: renweiheeee@outlook.com cl im at e ch an g e 260 weihe ren1, guitian yi2 introduction the impacts of global warming on the asian monsoon region are not well understood and many controversies still remain (ding et al., 2009; an et al., 2015; yang et al., 2015). the holocene megathermal period has generally been considered as an analog for the current global warming (renssen et al., 2012; marcott et al., 2013; yang et al., 2015) and may explain the shift of the monsoon rainfall boundary. in the east asian monsoon region, holocene climate changes have been investigated, supported by numerous high-resolution records, based on lake sediments (zhou et al., 2016; sheng et al., 2017), stalagmites (berkelhammer et al., 2012; dong et al., 2015; bai et al., 2017), and several other terrestrial records (zhao et al., 2013; li et al., 2017). furthermore, the spatial and temporal patterns of the ho are still extensively debated (an et al., 2000; wu et al., 2011; zhou et al., 2016). an et al. (2000) synthesized a large number of paleoclimatic records and reported a southeastward shift of the east asian summer monsoon maximum or a time-transgressive holocene optimum (ho), which refers to a relatively warm and wet period during the holocene. in addition, the optimum growth period of peatland in the east tibetan plateau in likely consistent with the southwarddeclined asian summer monsoon intensity (xu et al., 2013). furthermore, zhou et al. (2016) reported an apparent time-transgressive onset of the ho from several paleoclimatic records in eastern china. however, the δ18o records of stalagmites in the east asian monsoon region indicated an intricate trend of the ho. for example, the ho weakened before the summer monsoon in southern china, followed by northern china (cai et al., 2010); moreover, no time-transgressive characteristic of the ho was found (wu et al., 2011). previous studies found that the variability of asian summer monsoon subsystems (i.e., the east asian summer monsoon (easm) and the indian summer monsoon (ism)) were likely asynchronous or more complicated during the holocene than today (an et al., 2000; li et al., 2014). a numerical simulation of the paleoclimate demonstrated that the asian orography could lead to an asynchronous development of both the easm and ism during the earlier geological history (tang et al., 2012). however, the nature of the ho with its high monsoon precipitation still remains unclear. consequently, the spatial and temporal divergence of the holocene monsoon precipitation and the underlying mechanisms require further investigation. this review presents a synthesis of the latest holocene paleoclimatic records in eastern asia with a focus on the time-transgressive character, to explain the interaction between the easm and the ism during the ho. data sources and methods after 2000 ad, scientists have studied holocene asian monsoon records with a variety of methods (an et al., 2000; hong et al., 2000; zhao et al., 2007; zhao et al., 2011; huang et al., 2013; chen et al., 2015; hillman et al., 2017). for this study, a total of 98 reconstructed paleoclimate records across east asian was used (e.g., lake sediments, pollen assemblages, diatom records, mineral magnetism, grain size, geochemical, δ13c, δ18o, magnetic susceptibility, δdwax, speleothems δ18o and δ13c, loess profile magnetic susceptibility, paleosol, peatland pollen, basal peat age, humification degree, loss on ignition, toc, and grain size). 95 records were reported after 2000 ad. each selected proxy is likely linked to the asian summer monsoon rainfall variability and was dominated by various mechanisms. chronologies of these records are controlled by 14c dates, osl, 230th dates, pb/cs, and varve counts. most chronologies are based on more than five radiocarbon dates (see supplementary table s1). no standard boundary existed between the ism and the easm (e.g., wang & linho, 2002; wang et al., 2010; rao et al., 2016). a recent study demonstrated that stalagmite δ18o records reflect the history of the ism rather than that of the easm. therefore, the ism region of this paper consists mostly of stalagmite sites (liu et al., 2015). the paleoclimate records for 92 sites in east asian monsoon region were evaluated, and the onset and termination time of the ho during the holocene were identified. the pollen index is a widely used and available paleoclimatic indicator for vegetation and climate reconstructions (liu, 1988; zhao et al., 2009; zhao and yu, 2012). liu (1988) demonstrated that the holocene pollen stratigraphies suggest a tripartite division that includes a thermal mid-holocene from the temperate forests of china. a previous study suggested that many processes other than climate change may be involved in the production of the δ18o signal derived from speleothems (dykoski et al., 2005). moreover, the stalagmite δ18o records from the easm region has been debated (liu et al., 2015; liu et al., 2017) and the main trend of stalagmite δ18o records from the east asian monsoon region strongly represents the evolution of ism (liu et al., 2015). consequently, fossil pollen records from 48 sites across east asia were evaluated and synthesized to reconstruct vegetation and climate changes during the holocene. speleothems δ18o records derived from 16 caves were also preliminarily identified for the ism strength variation (fig. 1). in addition, other proxies mentioned in the last paragraph were widely used to document the effects of the summer monsoon. for example, magnetic susceptibility is generally indicative of soil/loess boundaries in the loess plateau (guo et al., 2000). 60° e 90° e 120° e 60° n 30° n 0° n east asian summer monsoom indian summer monsoom westerly winds stalagmite sites other sites figure 1. overview map showing paleoclimatic sites included in this study from east asia (for details on black and white circles with numbers, see supplementary table s1), the dominant circulation systems (arrows), and the rough distributions of ism and easm regions (dashed lines). 261characteristics of time-transgressive holocene optimum in the east asian monsoon region ages of the ho, both the easm and ism might simultaneously strengthen and regress during the early to middle holocene; however, the regional topography and atmospheric circulation also can influence the humidity records. the spatiotemporal character of holocene demographic fluctuations a total of 3851 radiocarbon dates from 977 archaeological sites were included in the utilized database, as shown in fig. 3a (wang et al., 2014). these radiocarbon dates were re-analyzed to investigate the characteristic of holocene demographic fluctuations in the east asian monsoon region. the shape of the summed probability distribution (fig. 3b) suggests a boombust pattern in population size between 6000 to 2000 cal yrs bp and around latitude 35°n. with regard to the most archaeological sites located in the easm region, and the major population expansion occurring when the ho regressed (fig. 2d), it can be concluded that climate changes and technological development may have both shaped the boom history of neolithic human demography (wang et al., 2014). patterns of humidity evolution during the early (12-8 ka) and middle (8-5 ka) holocene two divided holocene parts were chosen for this study: the early holocene and the middle holocene. these were compare the the arid central asia (aca) region and the monsoonal region (fig. 4a, b). the dry/wet pattern in east asia based on the latest holocene humidity records showed that the wet condition in the monsoonal region and the relative dry condition in the aca region during the early holocene (chen et al., 2008). during the middle holocene, both the aca region and the monsoonal region showed a wet or humid condition, which might indicate a maximum ho during this period where ism would introduce water vapor to the aca (huang et al., 2015). therefore, the enhanced summer monsoon could help the flourishing neolithic human population in the region (wang et al., 2014). conclusions and prospect based on 98 selected paleoclimatic records in east asia (mostly in the asian summer monsoon region), (1) a strong relationship between latitude and the initial and termination ages of the ho was found. this indicates that both the easm and the ism might simultaneously strengthen and regress during results and discussion time-transgressive characteristics of the ho in east asian monsoon regions no obvious trends of the ho evolution were found in the east asian monsoon region, and the onset and termination ages of speleothem sites are also generally consistency (fig. 2 a, b). this partially supports that there is no time-transgressive pattern of the ho, which is derived from the comparisons of stalagmites δ18o records from the east asian monsoon region at different latitudes (wu et al., 2011). although, precipitation mainly originates from asian summer monsoon, numerous studies have proved that the asian summer monsoon system can also be divided into two subsystems: the easm system and the ism system which are largely independent and interact with each other (ding and chan, 2005; li et al., 2014; liu et al., 2015; rao et al., 2016). therefore, it is necessary to investigate both the spatial and temporal ho from separated asian monsoon regions. liu et al. (2014) reviewed chinese holocene records and reported that the traditional easm proxy of moisture records were inconsistent with the stalagmite δ18o records. the authors further proved that the stalagmite δ18o records reflected the history of the ism rather than that of the easm. furthermore, processes other than climate (e.g., kinetic fractionation, the mixing of water during residence in the vadose zone, dissolution–reprecipitation, and the degassing history) may affect the δ18o signal derived from speleothems (dykoski et al., 2005). furthermore, rao et al. (2016) demonstrated that δ18o paleo-records cannot be directly used as paleohumidity indicators in the easm region. these reports and the re-analysis in the current study suggest that the stalagmite δ18o records might have numerous influencing factors, and more attention needs to be paid to clarify the indicative climate modes. to explore the evolution pattern of the ho, the initial and termination ages were divided into two regions: the easm region (fig. 2c, d) and the ism region (fig. 2e, f). in the easm region, a strong relationship exists between latitude and both the initial (zhou et al., 2016) and termination ages of the ho. the northeastern high regions form an exception and started about 4000 years earlier than the same latitude regions (fig. 2 c,d). in the ism region, no obvious connection was found between latitude and the initial and termination ages of the ho, and any variation might be affected by the tibetan plateau orography (tang et al., 2012). however, the low-land sites of the south tibetan plateau clearly show an expansion and retreat northward of the ho. this is inconsistent with the timing of the peatland formation along the east tibetan plateau transect, which indicates a regression of humidity southward (xu et al., 2013). consequently, with regard to the time-transgressive initial and termination pr ob ab le o ns et a ge (c al l y r b p) 12.000 10.000 8.000 6.000 10 20 30 40 50 latitude (°) pr ob ab le in iti al a ge (c al l y r b p) 12.000 10.000 8.000 6.000 10 25 30 35 40 45 50 latitude (°) 10 20 30 40 pr ob ab le in iti al a ge (c al l y r b p) 12.000 10.000 8.000 6.000 latitude (°) pr ob ab le te rm in at io n ag e (c al l y r b p) 10.000 8.000 6.000 4.000 2.000 10 20 30 40 50 latitude (°) pr ob ab le e nd in g ag e (c al l y r b p) 10.000 8.000 6.000 4.000 2.000 10 25 30 35 40 45 50 latitude (°) pr ob ab le e nd in g ag e (c al l y r b p) 10.000 8.000 6.000 4.000 2.000 10 20 30 40 latitude (°) figure 2. onset and termination age (with error) of the ho in the east asian monsoon region. the span space ranges from 12°n to 50°n and the age ranges from 12000 cal yr to 2000 cal yr bp. (a, b) shows all ages; (c, d) shows the ages in the easm region; (e, f) show the ages of of the ism region. the red dots in a and b reporesent speleothems oxygen isotopes; the red arrors in c and d indicate the general data trend; ages in green areas may be influnced by complex mechanisms. 262 weihe ren1, guitian yi2 bai, y., zhang, p., gao, t., yu, r., zhou, p., & cheng, h. (2017). the 5400 a bp extreme weakening event of the asian summer monsoon and cultural evolution. science china earth sciences, 60(6), 1171-1182. doi: cnki:sun:jdxg.0.2017-06-012 berkelhammer, m., sinha, a., stott, l., cheng, h., pausata, f. s. r., & yoshimura, k. (2012). an abrupt shift in the indian monsoon 4000 years ago. in: l. giosan, d.q. fuller, k. nicoll, r.k. flad and p.d. clift (editors), climates, landscapes, and civilizations. geophysical monograph book series. american geophysical union, washington, pp. 75-87. doi: 10.1029/2012gm001207 cai, y. j., tan, l. c., cheng, h., an, z. s., edwards, r. l., kelly, m. j., kong, x. g., & wang, x. f. (2010). the variation of summer monsoon precipitation in central china since the last deglaciation. earth and planetary science letters, 291(1), 21-31. doi: 10.1016/j. epsl.2009.12.039 chen, f., xu, q., chen, j., birks, h. j. b., liu, j., zhang, s., jin, l., an, c., telford, r. j., cao, x., wang, z., zhang, x., selvaraj, k., lu, h., li, y., zheng, z., wang, h., zhou, a., dong, g., zhang, j., huang, x., bloemendal, j., & rao, z. (2015). east asian summer monsoon precipitation variability since the last deglaciation. scientific reports, 5, 11186. doi: 10.1038/srep11186 chen, f., yu, z., yang, m., ito, e., wang, s., madsen, d. b., huang, x., zhao, y., sato, t., john b. birks, h., boomer, i., chen, j., an, c., & wünnemann, b. (2008). holocene moisture evolution in arid central asia and its out-of-phase relationship with asian monsoon history. quaternary science reviews, 27(3), 351-364. doi: 10.1016/j. quascirev.2007.10.017 the early to middle holocene; however, variations might be affected by the orography of the tibetan plateau. (2) based on this relationship, predictions can be made for the initial and termination ages of the ho at different geographical locations. (3) in contrast to radiocarbon dates from archaeological sites, the results of this study suggest that the ho and technological development may have both shaped the boom history of the neolithic human demography. these results further suggest that additional studies are required with a further focus on the spatial and temporal patterns of holocene humidity records and the influence of topography. this study improves the current understanding of the time-transgressive characteristics of the ho in east asian monsoon regions. acknowledgement this research was financially supported by the national natural science foundation of china (grant no. 41690113), the strategic priority research program of chinese academy of sciences (grant no. xda20070101), and the national natural science foundation of china. (grant no. j1210065) references an, z., wu, g., li, j., sun, y., liu, y., zhou, w., cai, y., duan, a., li, l., mao, j., cheng, h., shi, z., tan, l., yan, h., ao, h., chang, h., & feng, j. 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(2016). time-transgressive onset of the holocene optimum in the east asian monsoon region. earth and planetary science letters, 456, 39-46. doi: 10.1016/j. epsl.2016.09.052 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.107274 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 93 107 g eo ph y si c s manuscript received: 15/02/2023 accepted for publication: 12/06/2023 landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods siti nurkhalidah husainy1, andy anderson bery1, *, ismail ahmad abir1, wien lestari2, adedibu sunny akingboye1, 3 1school of physics, universiti sains malaysia, 11800 usm, penang, malaysia. 2geophysical engineering department. institut teknologi sepuluh nopember, its raya street, surabaya, 60111, east java, indonesia. 3department of earth sciences, adekunle ajasin university, 001 akungba-akoko, ondo state, nigeria. authors’ orcid id: https://orcid.org/0000-0003-2195-6098 (a.s. akingboye); https://orcid.org/0000-0001-8974-9808 (a.a. bery) *corresponding author’s e-mail: andersonbery@usm.my abstract keywords: penang island; landslide susceptibility mapping; remote sensing; electrical resistivity tomography (ert); seismic refraction tomography (srt); satellite-derived gravity mappingmalaysia is one of the countries in the world that experiences landslides yearly due to natural events and human activities. penang island is malaysia’s second most developed state and the largest by population. it is prone to landslides with devastating environmental impacts. hence, the need to characterize its near-surface soil-rock conditions. this study uses remotely sensed data, with frequency ratio (fr) analysis, to identify landslide-prone areas based on different categories of landslide causative factors. to further understand the conditions and hydrodynamics of the soil-rock profiles causing landslides, electrical resistivity tomography and seismic refraction tomography were carried out at a landslide-suspected section in the study area. also, satellite-derived bouguer gravity anomaly modeling was performed to map the varied gravity anomalies associated with landslide-triggering factors in lithologic units. the multi-geophysical models offer strongly correlated results with the remote sensing causative factor maps and the landslide susceptibility index (lsi) map. the likelihood of landslides occurring in the area, as suggested by the area under curve modeling of lsi data, yielded a high predicted success rate of 83.47%. hence, prospective landslides were identified in the hilly and elevated sections, while the less susceptible sections were identified on flat reliefs. landslides may also be triggered, for instance, at steep sections with varied contractive soil bodies and shallow structures. most importantly, leveraging the lsi map would help the necessary agencies forestall and mitigate future landslide occurrences in the area. palabras clave: isla de penang; mapa de susceptibilidad de deslizamientos de tierra; teledetección; tomografía de resistividad eléctrica; tomografía de refracción sísmica; mapeo satelital de la gravedad. mapeo de la susceptibilidad de deslizamientos de tierra en la isla de penang, malasia, a través de teledetección y métodos multigeofísicos how to cite this item: husainy, s. n., bery, a. a., abir, i. a., lestari, w., akingboye, a. s. (2023). landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods. earth sciences research journal, 27(2), 93-107. https:// doi.org/10.15446/esrj.v27n2.107274 resumen malasia es uno de los países donde cada año se presentan deslizamientos de tierra debido a eventos naturales y actividades humanas. la isla de penang es el segundo estado más desarrollado de malasia y el más poblado. es un estado propenso a deslizamientos de tierra con impactos ambientales devastadores. por consiguiente, es necesario caracterizar las condiciones roca-suelo cerca de la superficie. este estudio utiliza información de teledetección a través de técnicas de radio frecuencia (fr, del inglés frequency ratio) para identificar las áreas propensas a deslizamientos con base en diferentes categorías de factores causales de deslizamientos. para entender mejor las condiciones y los perfiles hidrodinámicos suelo-roca que causan los deslizamientos de tierra se realizaron tomografías de resistividad eléctrica y de refracción sísmica en una zona del área de estudio que se presume es propensa a estos movimientos. además, se realizó el modelamiento satelital de la anomalía gravitacional de bouguer para identificar las variadas anomalías de la gravedad asociadas con los factores que desencadenan los deslizamientos en las unidades litológicas. los modelos multigeofísicos ofrecen resultados fuertemente correlacionados con los mapas causales teledetectados y el mapa del índice de susceptibilidad de deslizamientos de tierra (lsi, del inglés landslide susceptibility index). la probabilidad de ocurrencia de deslizamientos, de acuerdo con lo sugerido por la curva de información lsi en el área, establece una alta efectividad de predicción de 83.47 %. con esto, la perspectiva de deslizamientos fue mayor en las secciones elevadas y montañosas, mientras que las secciones menos susceptibles se identificaron en las planicies. los deslizamientos también podrían desencadenarse, en algunas instancias, en secciones empinadas con cuerpos variados de suelos contractivos y estructuras superficiales. lo más destacado es que el aprovechamiento del mapa lsi ayudaría a las agencias a prevenir o mitigar los futuros deslizamientos de tierra en el área. https://doi.org/10.15446/esrj.v27n2.107274 https://orcid.org/0000-0003-2195-6098 https://orcid.org/0000-0001-8974-9808 mailto:andersonbery@usm.my https://doi.org/10.15446/esrj.v27n2.107274 https://doi.org/10.15446/esrj.v27n2.107274 94 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye 1. introduction a landslide is a sliding movement of the earth’s mass or rocks down the slope along a plane that can be seen and recorded (habibu & lim, 2016; zakaria et al., 2022). just like some malaysian states, penang island, malaysia’s second most developed state and the largest by population, is prone to landslide occurrences with devastating environmental impacts (fig. 1). these effects range from the collapse of infrastructures, loss of arable land, and sometimes deaths. penang has a 90.8% urbanization rate, with an increased population from 722,384 (2465 per sq. km) in 2010 to 808,900 (2760 per sq. km) in 2018 (gao et al., 2021). the landslide problem in this area with its environs is associated with excessive rainfall, particularly during the southwest and northeast monsoon seasons (tay et al., 2015). in addition, uncontrolled urbanization and deforestation are two causes of multi-slope instabilities (abd majid & rainis, 2019). however, disregarding geological, topographical, and human causes for sustainable development causes landslides (lee & pradhan, 2006; pradhan & lee, 2010; tay et al., 2015; achour et al., 2017; bounemeur et al., 2022). consequently, the topographic relief and the nature of the soil-rock conditions of penang island, which is underlain by granitic rocks, with extensive mountainous hills in the central part are issues of concern (ong, 1993; lee & pradhan, 2006). in addition, the area’s yearly high rainfall and hot weather aid the progressive weathering of the soil-rock profiles (nordiana et al., 2013; bery, 2016; akingboye & bery, 2022). hence, landslides could be triggered due to oversaturation under heavy downpours, particularly in places close to the hills without proper land mitigations (ab talib, 2003; alkhasawneh et al., 2013). as urbanization and land use practices are a progressive phenomenon in the study area and its environs, adequate information on surficial and subsurface landslide-prone zones would prevent landslides catastrophic events. this is because studies have proven that many landslides are triggered by climate change and human activities due to changes in land use and landforms (alkhasawneh et al., 2013). over the last few decades, many studies on landslides using geographic information systems (gis), remote sensing, and multi-geophysical methods have been conducted in penang island, malaysia. for instance, ab talib (2003), lee & pradhan (2006), and tay et al. (2015), among others, have applied probabilistic and statistical methods, such as the analytical hierarchy process (ahp), ordinary least square (ols), neuro-fuzzy and more. remote sensing data could be used to detect geological structures associated with landslides (lee & pradhan, 2006; pradhan & lee, 2010). application of optical remote sensing data for structural geology mapping has been much more limited in tropical environments, due to the persistent cloud coverage, scarcity of bedrock exposures, and vegetative ground cover (pour & hashim, 2015). most of the geological structures and those associated with suture zones, even under dense vegetation cover for large inaccessible regions, could be detected via remote sensing data. this is because the advances in remote sensing technology have made it possible to obtain more useful geological structural information from the ground in dense tropical rainforests due to its penetration capability (shimada & isoguchi, 2002; faruwa et al., 2021). moreover, some studies on landslides in the area used different analytical techniques, such as statistical index (si) and landslide nominal susceptibility factor (lnsf), e.g., (ab talib, 2003; lateh et al., 2011; alkhamaiseh et al., 2018; gao et al., 2021). however, tay et al. (2015) proved that the frequency ratio (fr) model has a high accuracy compared to those methods. additionally, in understanding the spatial variability of surficial and subsurface soil-rock conditions of penang island, many geophysical methods such as electrical and seismic refraction methods, with soil and rock mass test analyses, have also been conducted, e.g., (pradhan & lee, 2010; bery, 2016; abdullah et al., 2020; akingboye & bery, 2022, 2023). these studies indicate that soil behavior is a complex phenomenon related to various causative geologic hazards. consequently, a single technique would not be sufficient to provide the required diagnostic subsurface information on soil behaviors in such landslide-prone areas. an integrated dataand image-driven approach is expected to provide detailed information on the landslide-triggering factors as related to soilrock conditions, especially in wet tropical terrains with evidence of repeated landslides. given the above, remote sensing data on landslides in penang island from 2001 to june 2021 were acquired and used for this study to update the landslide susceptibility map of the area. moreover, geophysical methods involving electrical resistivity tomography (ert) and seismic refraction tomography (srt), including time-lapse ert (tlert), were carried out at penang island’s suspected landslide susceptible area. in addition to these methods, satellitederived gravity data were also used. this integrated approach has not been adopted for landslide studies in the environment. it is expected that these methods would provide detailed insights into understanding the near-surface soil-rock profile, structures, and geohydrodynamic systems that could induce landslides in the study area. the study will also serve as a crucial scientific report to prevent, mitigate, and curtain the occurrence of landslides in the area and other tropical regions with comparable topographic and geologic features. figure 1. landslide-prone areas in malaysia (modified after habibu & lim, 2016). the degrees of catastrophes in landslide-prone areas are not drawn to scale. 95landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods 2. study area and its geological settings 2.1 study area penang is one of the 13 states of the federal territory of malaysia. penang island (fig. 2a), also known as pulau pinang, and the seberang perai (formerly the province of wellesley), are the two main subdivisions of penang. penang island, the study area, is connected to peninsular malaysia by a 13.5 km bridge. it falls within latitudes 50 15´ – 50 30´ n and longitudes 1000 10´– 1000 21´ e (fig. 2a–b), covering an approximate area of 293 km2. it has an elevation of 0 to 820 m above mean sea level, as shown by the digitized elevation model (dem) in fig. 2b, with a gradient from 0° to 67°. from the yearly precipitation, data records gathered from 2001 to 2021, the average temperature of penang ranges from 25 °c to 32 °c (fig. 2c), and annual precipitation of 54.7 mm to 240 mm per month, e.g., fig. 2d (world weather & climate information, 2022). 2.2 geological settings malaysia peninsula is of two conjugated late triassic tectonic terranes, viz., the western sibumasu terrane and the eastern sukhothai arc (metcalfe, 2013a, 2013b; pour & hashim, 2015). these tectonic terranes are located at the southeast margin of the eurasian continent, and they were formed during the permian-triassic period by the merging of the sibumasu and indochina blocks along the bentong-raub suture zone (metcalfe, 2000, 2001, 2013b). the suture zone and lebir faults divide peninsular malaysia into the western belt, central belt, and eastern belt (metcalfe, 2001). the central belt consists of limestone, shale with subordinate sandstone, siltstone, and conglomerate, with late carboniferous–early permian age (pour & hashim, 2015). the granitoids, typically granite, make up about 90% of the plutonic rocks in peninsular malaysia (ng et al., 2015). penang is a mountainous granitic island centrally in a north-south direction, with a low-lying plain situated on both the eastern and western coasts of the island. the granitic outcrops are exposed due to tectonic uplift during the oligocene-miocene period (ong, 1993). the author reported a major fault system striking roughly n–s that is evident in the island’s two granitic plutons, namely the north penang pluton (npp) and the south penang pluton (spp), figure 2a. the classifications were based on the differences in the proportion of alkali feldspar to total feldspar (ong, 1993). penang island is characterized by the gula, beruas, and simpang formations (hassan, 1990), which are the derivatives of the underlying granitic figure 2. (a) geological map of penang island, malaysia (modified after ong, 1993). inset: location map of peninsular malaysia showing penang island. (b) digitized elevation model (dem) (extracted from gao et al., 2021), (c) average monthly temperature chart, and (d) average monthly precipitation chart of penang island, malaysia (extracted from world weather & climate information, 2022). 96 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye rocks (ong, 1993), fig. 2b. the npp rocks are subdivided into different groups, namely the tanjung bungah, paya terubong, and batu ferringhi groups, as well as the muka head micro granite (ong, 1993). the medium to coarsegrained biotite granites of tanjung bungah was formed in the early jurassic. the medium to coarse-grained biotite granite of the paya terubong group was formed in the early permian to late carboniferous. the early jurassic batu ferringhi group comprises medium to coarse-grained biotite granites. the spp is the batu maung granite, with microcline feldspar, and the sungai ara granite. 3. methods this research aims to produce, validate, and improve the landslide susceptibility map of penang island, using different landslide inventories and itemized causative factors from remote sensing and multi-geophysical datasets. the methodological approaches are geared toward mapping the landslide susceptibility indices via 2-d spatial remote sensing mapping. the remotely sensed causative factors datasets were analyzed using the frequency ratio (fr) approach to develop a landslide susceptibility map, and the results were cointerpreted with the multi-geophysical models. the employed multi-geophysical methods include ert and srt, with the tlert, and satellite-derived bouguer gravity modeling. the results derived from the use of remotely sensed causative factors are supported by strong surficial-to-subsurface soil-rock conditions and their integrity as well as water-rock interaction and soil-water/moisture monitoring from geophysical modeling. this combined help to delineate subtle near-surface and deep crustal features to target landslide-induced mechanisms. 3.1 remote sensing data acquisition procedures, correction, and processing landslide susceptibility evaluation is very crucial for probabilistic analysis of landslide indices and their occurrence. to this end, remote sensing is one of the approaches used for mapping landslides. according to gomes et al. (2013) and tien bui et al. (2018), datasets such as lithological (including soil) maps, rainfall data, dem, and satellite imageries are essential in preparing landslide inventories, selecting landslide causative factors, and constructing the spatial database for landslide predictions. hence, the lists of datasets acquired over two decades (2001 to 2021), as presented in table 1, including aerial photographs and satellite images, were used and analyzed to map landslide indices at penang island (the study area). the datasets used comprise 100 satellite imageries retrieved from landsat 7 etm+ and landsat 8 operation land imager (oli) satellite imagery websites, including google earth (table 1). landsat datasets can provide high-resolution spectral bands for monitoring land use and capturing land change due to climate change, urbanization, drought, wildfire, biomass changes, and different natural and human-induced changes at drastically reduced costs. to effectively map landslides in the study area, landslide causative factors, namely geology (lithologies and structures), topography (slope steepness, slope angle, curvature, and orientation), vegetation cover (ndvi), and land use datasets (table 2), were leveraged. this is because the causative factors, particularly topography, geology, and land use, including weather conditions and anthropogenic factors, control the degree and distribution of landslides (khan et al., 2019). after acquiring the remote sensing datasets, they were grouped as thematic maps and geological hazard maps (table 2). some of these datasets are on the scale of 1:10,000 to 1:150,000, while some are in spatial resolution dimensions of 30 m x 30 m based on the numbers of grids, polygons, lines, and points. the modeling of the study area’s topographic features was based on slope, surface curvature, stream/river density, and distance to lineament data, including dem, acquired from the japan aerospace exploration agency (jaxa). on the other hand, the geological map that depicts lithologies and faults/fractures in the study area was acquired from the mineral and geoscience department of malaysia (jmg). upon acquisition, the maps were correlated with existing geological maps of the study area, e.g., (ong, 1993). it is important to note that dem serves as a foundation upon which many landslide causative factors could be determined (komac, 2006). afterward, landsat data corrections and reductions were performed using the flaash atmospheric correction module in envi software. flaash is an acronym for fast line-of-sight atmospheric analysis of hypercubes. it is a first-principles atmospheric correction tool that corrects wavelengths in the visible through near-infrared and shortwave infrared regions, up to 3 μm (yuan & niu, 2008). it is compatible with most hyperspectral and multispectral data. the purpose of atmospheric correction is to eliminate the influence of the atmosphere, return the reflectance value of the actual object on the earth’s surface, and rectify images taken in vertical (nadir) or slant-viewing geometries (ilori et al., 2019). in addition, it removes the scattering and absorption effects from the atmosphere to obtain surface reflectance properties. the gapfill interface in the software was also used to fill the gaps (wedge-shaped gaps) in the landsat images. a band combination of 6,5,2 for landsat datasets was used because it discriminates between vegetation and bare land. this band combination uses a shortwave-infrared band [swir-1] (6), near-infrared (5), and blue (2). 3.2 modeling landslide causative factors and landslide susceptibility the spatial database for causative factors was developed using spatial analytical tools in both arcgis (v10.8) and envi software. the treated landslide causative factors include lithology, slope gradient, slope aspect, surface curvature, stream/river density, distance to lineament, normalized difference vegetation index (ndvi), and land use/cover. after careful evaluations of the landsat datasets acquired from 2001 to 2021, the most recent landsat 8 oli data (2021) was used to evaluate the landslide causative factors. a 5-m resolution of advanced land observing satellite (alos) dem from jaxa was used to model the landslide causative factors: lithology, slope gradient, slope aspect, surface curvature, stream/river density, and distance to table 1. summary of the datasets used for the landslide susceptibility mapping. research resource database type source period (years) satellite imageries ndvi map landsat 7 etm+ and landsat 8 oli imageries (usgs open-source website) 2001–2021land use map landslide inventory digital elevation model (dem) slope gradient map advanced land observing satellite (alos) or “daichi” and aerospace exploration agency (jaxa) slope aspect map surface curvature map stream/river density map distance to lineament map geological map lithological map mineral and geoscience department of malaysia (jmg)fault line map table 2. summarized classification of the remote sense data layer for the study. classification sub-classification gis data type scale thematic map topography line, point, and polygon 1:50,000 geology polygon 1:50,000 land use grid 30 m x 30 m vegetation index grid 30 m x 30 m lineament line 1:50,000 geological hazard map landslide susceptibility point 1:50,000 97landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods lineament, using arcgis software. based on the sub-classifications of landslide causative factors in table 2, the analyzed datasets were transformed into raster maps with the same coordinate system and pixels. this way, detailed insights into how each of these parameters contributes to landslide occurrence were gained, including a proper understanding of the area’s landslide mechanisms. the slope aspect technique, or the orientation of a slope, expressed in degrees or compass directions, controls the slope’s exposure to climatic conditions such as sunlight, wind direction, rainfall, etc (komac, 2006). slope steepness is a risk factor associated with most soil profiles influencing slope stability. this is because the steeper the slope, the greater the probability of landslides occurrence due to gravity influences (kornejady et al., 2018). the slope aspect also has a significant impact on vegetation cover, moisture retention, and soil strength. similarly, slope gradient provides significant information on the rate of surficial soil erodibility and their sliding potential (ab talib, 2003). also, the degrees of surface curvature, distance to lineament, and stream density are significant factors that must be ascertained in landslide study as they mitigate or adversely induce landslide occurrence. for instance, the force of gravity increases on a steeper slope as the shear stresses in loose soils or unconsolidated materials increase (bery, 2016; khan et al., 2019). most landslides are associated with areas of high river/stream and those close to lineaments. furthermore, the ndvi of the study area was produced after flaash atmospheric corrections via envi, based on eq. 1, and the resulting map was also generated to depict the area’s vegetation distribution and their contribution to landslide occurrence. (1) where � and ��� imply the spectral reflectance measurements acquired in the red (visible) and near-infrared regions, respectively. generally, if there is much more reflected radiation in near-infrared wavelengths than in visible wavelengths, then the vegetation in that pixel is likely to be dense and may contain some type of forest. ndvi is directly related to the photosynthetic capacity and hence energy absorption of plant canopies (myneni et al., 1995). low ndvi values indicate moisture-stressed and sparse vegetation while higher values indicate a higher density of green healthy and dense vegetation (gessesse & melesse, 2019). this parameter can be negative, even in densely populated urban areas and the ndvi is usually of a (small) positive value. negative values are more likely to be observed in the atmosphere and some specific materials (myneni et al., 1995). to adequately analyze the land use factor, seven land uses namely forest, waterbody, urban area, cultivated land, dry paddy, wet paddy, and cleared land, were classified from the derivation of the land use map based on supervised classification associated with the maximum likelihood of landsat 8 oli multispectral imaging (abdelouhed et al., 2021). the classification provided significant insights into land use, and in turn, serve as clues in understanding the landslide-triggering mechanism in the study area. it is worth noting that slope instability can result from a change in land use due to human activities such as deforestation, overgrazing, excessive farming, slope cutting, construction, etc. the propensity of landslides to occur in an area is a critical evaluation, especially in the study area with an extensive hilly interior that is surrounded by low valleys and coastal plains. hence, frequency ratio (fr) was used in the quantitative modeling of landslide occurrence and its damaging potential. fr is one of the quantitative techniques used for calculating the ratio of probabilities of landslide occurrence to non-occurrence for given causative factors using the gis approach. to obtain the fr for each causative factor, the weight or fr of each element is obtained by dividing the total landslide-prone area by the total area coverage of each causative factor based on equation 2 (lee & pradhan, 2006; bounemeur et al., 2022). in addition, to develop the area’s landslide susceptibility index (lsi), the correlation rating of each landslide causative factor (lcfr) was estimated from the relationship between a landslide and each causative factor. lcfr value estimated for each element was summed based on equation 3, e.g., (ab talib, 2003; khan et al., 2020). in general, landslide mapping and observations in the landslide causative factors derived results and maps were based on the interpretations of breaks in the forest canopy, bare soil, and other geomorphic characteristics, including landslide head and side scars, flow tracks, and debris deposits below the scars. (2) where 𝐹𝑅 is the fre que ncy ratio o f e ach caus ative facto r. 𝑁𝑝𝑖𝑥 𝐴 * 𝑎𝑏 is the numbe r o f pixe ls o f are a o f the o bs e rve d lands lide o f clas s a o f parame te r b. 𝑁𝑝𝑖𝑥 𝐴 𝑎𝑏 is the numbe r o f pixe ls o f are a o f clas s a o f parame te r b. 𝑁𝑝𝑖𝑥 𝐴 *is the numbe r o f pixe ls o f the to tal are a o f the o bs e rve d lands lide . 𝑁𝑝𝑖𝑥 𝐴 is the numbe r o f pixe ls o f the to tal are a o f the map. (3) following the derivation of the maps for landslide causative factors, all the maps were integrated to map landslide-prone sections in the study area. in total, 340 landslides within 293 km2 were mapped and observed in aerial photographs based on the analysis and interpretation of the aforementioned geomorphic characteristics. all landslides were verified using google earth, ensuring that they are not misconstrued with agriculture areas such as terrain slope agriculture or dry paddy land. to achieve and accurately map the landslide susceptibility indices in the study area, predictive modeling was utilized. according to kornejady et al. (2018), landslides are of different types including falls, rotational and translational slides, earth flows, complex, and creeps due to various predisposing factors. it is, therefore, recommended to treat landslides separately. this, however, is impossible because different landslide kinds share the same factor importance ranking, particularly top-ranked factors. as a result, the total landslides mapped were divided into a set of training (80%) and testing data (20%). the training data were used for constructing the predictive models while testing data were used to validate the models to ascertain the accuracy of the mapped landslide indices. the accuracy of the landslide susceptibility index (lsi) map was assessed using a method known as the area under the curve (auc) proposed by lee & pradhan (2006). the rate curve was created to explain how well the technique predicted intended landslides. the auc graph represents the quality and accuracy of the model used to classify landslide occurrence. to have a success rate, index values of all the landslide pixels from the lsi map were calculated and reclassified into 100 classes. the outcome data were plotted as a graphical model, with an accumulated 1% interval for the true positive rate on the y-axis against the false positive rate results which could reveal the relationship between the true positive and false positive rates on the x-axis (biggerstaff, 2000). 3.2 geophysical field data measurements and processing a combination of two ground-based geophysical methods (ert and srt) was carried out at a suspected landslide site on the main campus of universiti sains malaysia (usm), fig. 3a, due to its landscape gradient. the methods investigate the subsurface heterogeneous soil profiles and geologic structures that could trigger landslides. the infield ert and srt surveys lasted three months, from january to march. the periods are remarkable for their appreciable sunshine and high downpours. generally, resistivity and seismic refraction techniques are the most used geophysical methods to investigate subsurface soil-rock conditions. for the resistivity method, the result mainly depends on the variations in resistivity factors in which soil water content plays a significant role (kherrouba et al., 2019, 2022; zakaria et al., 2022). however, srt is influenced by subsurface seismic p-wave velocity (vp) variations and can be interpreted based on the measured range of velocities (dahlin & wisén, 2018). a single geophysical survey line in approximately ne–sw direction for both ert and srt measurements was established at a landslide-suspected section within usm, as a monitoring site (fig. 3a). the profile for the ert and tlert was 22 m while that of the srt was 23 m long. the abem sas terrameter 4000 was used for the ert survey (fig. 3b), employing the wenner-schlumberger array (fig. 3c) with a 0.5 m station spacing. the wenner-schlumberger array configuration is more sensitive to horizontal and vertical structures than the wenner and dipole-dipole arrays. besides, it has a https://en.wikipedia.org/wiki/urban_area https://en.wikipedia.org/wiki/material 98 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye deeper depth penetration than a narrower model (loke, 2002; akingboye & bery, 2022). the resulting field-measured resistivity datasets were processed and modeled using res2d software (loke, 2002), leveraging standard leastsquares data inversion constraints based on the finite-element method of four nodes with l2-norm to minimize the difference between the measured and observed resistivities. also, a damping factor of 0.05, with a minimum value of 0.01, was used to improve the accuracy and resolution of the inversion model. the tlert models depicting the percentage changes in resistivities for the investigated months were also derived from the same software. moreover, the srt data acquisition was conducted using the abem terraloc mk 8, employing a channel of 24 geophones at 12 hz each, a 12-pound hammer, and a striker metal plate (fig. 3d). the measured vp datasets were processed by first enhancing weak seismic traces, employing the auto-gain control technique in the seisopt@2d software, to amplify the first arrival traveltimes for easy identification and picking. the first arrival traveltimes were picked using the same software from the resulting enhanced vp datasets. the simulated annealing optimization method, an interface of seisopt software, was used to invert the first arrival seismic traces and to generate the resulting vp models (zakaria et al., 2022). ensuring smooth figure 3. (a) aerial data acquisition map of the studied site for ert and srt data measurements. (b) a typical 2-d layout and setup for a two-cable reel resistivity measuring system. (c) a typical schematic diagram of a 2-d wenner-schlumberger ert acquisition layout with its subsurface sensitivity pattern. (d) a typical schematic diagram of a 2-d srt acquisition layout. 99landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods table 3. causative factors for evaluating landslide occurrence in penang island, malaysia. factor class total number of pixels total number of pixels frequency ratio (fr) slope gradient 0° 5° 113198 33.59 5 1.47 0.04 5° 15° 76827 22.80 63 18.53 0.81 15° 25° 83443 24.76 133 39.12 1.58 25° 35° 55066 16.34 118 34.71 2.12 > 35° 8463 2.51 21 6.18 2.46 slope aspect north 32597 9.67 37 10.88 2.26 northeast 37162 11.03 53 15.59 1.41 east 49591 14.72 68 20.00 1.36 southeast 47442 14.08 72 21.18 1.50 south 36386 10.80 30 8.82 0.82 southwest 36011 10.69 23 6.76 0.63 west 46463 13.79 32 9.41 0.68 northwest 44402 13.18 25 7.35 0.56 surface curvature convex 161161 47.34 138 40.59 0.86 flat 28427 8.35 9 2.65 0.32 concave 150846 44.31 193 56.76 1.28 stream density 0 0.002m/m² 91175 26.75 148 43.53 1.63 0.002 0.004 m/m² 114650 33.64 140 41.18 1.22 0.004 0.006 m/m² 89430 26.24 47 13.82 0.53 >0.006 m/m² 45610 13.38 5 1.47 0.11 distance to lineament 0 125 m 23609 6.93 35 10.29 1.49 125 -250 m 23681 6.95 30 8.82 1.27 250 500 m 46075 13.52 57 16.76 1.24 500 1000 m 76856 22.55 97 28.53 1.27 >1000 m 170590 50.05 121 35.59 0.71 lithology batu maung granite 79237 23.25 100 29.41 1.27 matang gelugor member of the gula formation 11093 3.25 3 0.88 0.27 simpang formation 26298 7.72 1 0.29 0.04 sungai ara granite 8443 2.48 12 3.53 1.42 parit buntar member of the gula formation 13261 3.89 0 0.00 0.00 beruas formation 26877 7.89 0 0.00 0.00 undifferentiated fill materials 9032 2.65 0 0.00 0.00 gula formation, undifferentiated 15262 4.48 0 0.00 0.00 batu feringgi granite 67529 19.81 84 24.71 1.25 muka head granite 5449 1.60 8 2.35 1.47 tanjung bungah granite 67045 19.67 132 38.82 1.97 port weld member of the gula formation 11308 3.32 0 0.00 0.00 ndvi bare soil/water 217383 38.74 0 0.00 0.00 very low 33429 5.96 1 0.29 0.05 low 44377 7.91 2 0.59 0.07 moderately low 35766 6.37 10 2.94 0.46 moderately high 57933 10.33 94 27.65 2.68 high 172202 30.69 233 68.53 2.23 landuse classification forest 108438 16.43 155 45.59 2.77 waterbody 311515 47.21 0 0.00 0.00 urban area 111441 16.89 13 3.82 0.23 cultivated land 88645 13.43 137 40.29 3.00 dry paddy 3987 0.60 0 0.00 0.00 wet paddy 31094 4.71 34 10.00 2.12 clear land 4722 0.72 1 0.29 0.41 100 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye interpretation and detailed landslide susceptibility characterization, the satellite-derived bouguer gravity anomaly datasets of penang island, derived from the bureau gravimétrique international (bgi), were modeled using oasis montaj software. the gravity method was adopted due to its extensive area coverage, ability to differentiate crustal densities, and deeper depth penetration. the results were co-interpreted with the generated ert and srt models for the landslide monitoring site. 4. results and discussion 4.1 landslide susceptibility assessment using frequency ratio (fr) model the derived causative factors for characterizing landslide occurrences in penang island, malaysia, are shown in table 3. their 2-d maps were also generated. the relationship between slope gradient and landslide occurrence figure 4. (a) lithological map, (b) slope gradient map, (c) slope aspect map, and (d) surface curvature map of penang island, malaysia. 101landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods is one factor widely used as a landslide-triggering mechanism because they are directly proportional. the digitized lithology map, based on their locations in the study area, as shown in fig. 4a, was analyzed alongside all the landslides’ causative factors models. it is important to note that the steeper the slope, the greater the probability of landslides due to gravity influences (bery, 2016; khan et al., 2019). the force of gravity increases on a steeper slope as the shear stresses in loose soils or unconsolidated materials increase. the surface runoff figure 5. (a) stream density map, (b) distance to lineament map, (c) vegetation index (ndvi) map, and (d) land use classification map of penang island, malaysia, as derived from the fr technique. on the steeper slope is higher and faster than on the gentle slope. hence, the very low landslide occurrence probability is below 5°, with an fr of 0.04. when the fr is greater than 1, significantly above a slope of 15°, it indicates a very high landslide occurrence probability (table 3 and fig. 4b). these zones are peculiar to the mountainous sections, especially in the central part. the residential areas are characterized by 20° slope angles, with low to moderate landslide occurrence probability. 102 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye slope aspect and surface curvature are also essential factors to be considered for the occurrence of landslides. according to carrara et al. (1991) and wan mohd muhiyuddin (2005), a sloppy surface facing sunlight is dried compared to the one prevented. the latter is responsible for most landslides due to wet or moist conditions. in this study, the highest fr values (table 3) dominate areas in the north, followed by the southeast and northeast (fig. 4c). meanwhile, the curvature of the slope, representing the surface morphology, is usually parallel to the hill, hence, affects the acceleration or deceleration of water flow. a positive value indicates a sloppy concave surface with an accelerated flow. a negative value represents a sloppy convex surface with declining water flow, while a neutral value (i.e., zero) indicates a flat surface. as shown in table 3 and figure 4d, the fr value of 0.86 indicates a convex surface with the greatest possibility for landslide occurrence in the event of a heavy downpour due to repeated dilation and contraction. as a result, loose debris on an inclined surface is prone to induced soil creep or mudslides which could also contribute to landslide reoccurrence at existing location. whereas a concave slope, with an fr of 1.28, will retain water for a longer period (alkhasawneh et al., 2013). stream densities and distance to lineaments also play a part in the occurrence of landslides. the closer a region is to streams and lineaments, the figure 6. ert models for (a) january, (b) february, and (c) march at the landslide monitoring site. greater the likelihood of a landslide occurrence. this is because the stream will erode the slope base and saturate the underwater part, resulting in gully erosion and terrain alteration. table 3 reveals that most landslides occur at a density of 0–0.002 m/m2 for streams, corresponding to distances 0–125 m from lineaments (fig. 5a–b). the comparison of table 2 and fig. 4a indicates that the highest fr of landslide is associated with the rocky igneous area, with tanjung bunga granite at fr of 1.97. followed by muka head granite, sungai ara granite, and batu maung granite with fr values of 1.47, 1.42, and 1.27, respectively. in the case of the vegetation index (ndvi), as shown in table 2 and figure 5c, the ndvi values ranging from 0.6–0.8, with the highest fr of 2.68, indicate the possibility of moderately high to high landslide occurrence. the ndvi values of 0.4 (and below) are indications of low landslide occurrence. the results suggest that a low ndvi increases the likelihood of landslides. most of penang island’s areas with thick forests are found on steep sections (fig. 5c). as a result, the slope instabilities could be triggered by human activities such as deforestation, overgrazing, intensive farming, slope cutting, slope agriculture, construction, and more. most landslides will occur in cleared and cultivated areas (bukit bendera and the western section), with the highest fr of 2.68 (table 3 and fig. 5d). figure 7. srt models for (a) january, (b) february, and (c) march at the landslide monitoring site. 103landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods 4.2 interpretation of geophysical methods at the landslide-suspected area based on the ert results derived from the test site from january to march during the northeast monsoon season, the subsurface lithological conditions are characterized by saturated and unsaturated residual soil profiles (rich in clayey sand) with resistivity values of 5–300 ωm and >300 ωm, respectively (fig. 6a– c). the saturated bodies are extensive, occurring within the motley topsoil with interspersed stiff-to-hard soils (highly varied in resistivity) and granitic boulders (>1000 ωm), fig. 6a. in february (fig. 6b), the water-retention level in some subsurface constituents tends to be lower than it was for the other two months. on the other hand, the srt models (fig. 7a–c) also depict corresponding responses, whereby the residual soil profile across the investigated site varies from 50–800 m/s, the weathered granitic layer has vp values of 800–2000 m/s, while the integral/fresh granitic bedrock is characterized by >2000 m/s. from january to march, significant changes in the vp values of the residual soil and weathered layers were observed (fig. 7a–c), especially between station distances of 4 m and 18 m in fig. 7c with an identifiable extensive deep-weathered trough. the observed variations are attributed to varying soil composition, rock structures, and degree of weathering and water affinity. the observed variations in geophysical signatures were also clearly interpreted by the percentage changes in measured resistivities between january and february (fig. 8a) and in march (fig. 8b). these models show the changes in the water saturation level beneath the investigated area. the residual soil and the weathered sections (between stations 10 m and 12 m) are greatly impacted by saturating water, stretching to the station position of 6 m beyond (fig. 8a– b). based on figures 6–8, the principal landslide triggering mechanism, for instance, is the extensive saturated residual and weathered soil profiles with >40% resistivity changes (extending to depth >15 m). as a result, the identified section and other steep contractive soil (silt/clay) sections may be subjected to intense soil creep and sliding. this is because soil creep and dissolution are enhanced by soil-rock configuration and water affinities (bery, 2016; zakaria et al., 2022). 4.3 interpretation of satellite-derived gravity model of penang island to further strengthen the interpretations of the study, the satellite-derived bouguer gravity anomaly map of penang island (fig. 9) was developed to comprehensively depict the lithological conditions and embedded geologic structures which could trigger landslides. the derived penang island’s bouguer anomaly map is characterized by negative and positive gravity anomalies ranging from -16 to >30 mgal. the negative bouguer gravity values gradually attain positive values as one progresses toward the low-lying areas. the high positive bouguer anomalies ranging from >2–30 mgal dominate the largest portion of the study area. the conspicuous central quasi-ring negative bouguer gravity anomaly with its parts stretching southward depicts the section dominated figure 8. tlert models of the percentage resistivity changes for (a) february, and (b) march at the landslide monitoring site. 104 siti nurkhalidah husainy, andy anderson bery, ismail ahmad abir, wien lestari, adedibu sunny akingboye by the mountainous granitic rocks because such features are characterized by varied mass compared to surrounding lithologies as well as effects associated with tectonic uplift, mineral composition, and structural complexities (balogun, 2019; okpoli & akingboye, 2019, 2020). gravity attraction on the hill mass due to isostatic imbalance may also reduce gravity (balogun, 2019). the studied site within usm for monitoring the nature and degree of surficial and subsurface soil-rock profiles and hydrodynamics of a typical steep section has bouguer gravity values between 14 and 18 mgal. potential lineament and/or drainage features were also mapped, as shown by trending black lines in figure 9. the varied lithological conditions of the area as observed in the bouguer gravity model (fig. 9) also affirm the near-surface features identified in the ert and srt models for the monitoring site (figs. 6–8). comparing the vp and resistivity results obtained at the suspected landslide sites (figs. 6–8) and those of figures 4–5 with figure 9, the sections with the negative bouguer gravity anomalies (-16 to 2 mgal) may be identified as landslide-susceptible zones due to their topography reliefs (fig. 2b) as well as soil profiles and shallow lineaments. consequently, slope failure may be imminent at landslidesuspected sections due to intense soil creep, especially at steep slopes with contractive soils (silt/clay) and water-escape structures (zakaria et al., 2022). 4.4 landslide susceptibility index and modeling to have a detailed update and understand the landslide susceptibility of the study area, a landslide susceptibility index (lsi) map is needed. this was achievable from the summation of all causative factors, and the resulting lsi map of penang island is shown in figure 10. based on lsi estimates from equation 3, the region is characterized by values ranging from 2.46 to 9.23, with a standard deviation 2.37. the range of values depicts the susceptibility indices for the area. hence, the observed and predicted landslide-susceptible areas (with moderate to very high lsi) fall on the hilly and elevated sections, especially sections with contractive silty/clayey soils and poor ground mitigation. whereas the less susceptible sections are places with flat topographic reliefs. these results also conform with the interpreted results in figs. 4–9, hence validating the accuracy of the methods employed and their generated results to accurately predict future landslides in the study area. the lsi result was also verified based on the auc model (fig. 11) using pixels from the lsi map. this approach evaluates how successfully the lsi map matched the occurrence of landslides. the success rate curve was derived using testing point landslide datasets, whereas the predictive rate curve was derived from training point landslide datasets. the auc is a graph that displays varying index numbers, often between maximum values of 1 equal 100%, and a value of 0.5 that equals 50%. auc is one kind of accuracy metric used to predict, evaluate, or interpret natural disaster models (probabilities). a score between 0.8 and 0.9 is considered to be good and is an excellent classification, a score of 0.7–0.8 is a moderate or decent classification, and a score of less than 0.6 is an inadequate classification. as observed in the auc model, the success rate shows the model’s ability to consistently categorize the occurrence of existing landslides, whereas the auc of the predictive rate illustrates the proposed landslide model’s ability to anticipate landslide susceptibility (pamela et al., 2018). based on fig. 11, the auc prediction accuracy is 0.8347, translating to a prediction accuracy of 83.47%. on the other hand, the ratio for the success rate is 0.7968, indicating a prediction accuracy of 79.68%. this accuracy assessment is slightly higher than that of khan et al. (2019). as a result, there is a higher chance of landslide occurrence at the identified sections in the future and even at previously reported sections with no ground mitigation. figure 9. bouguer gravity anomaly of penang island, malaysia. figure 10. landslide susceptibility map of penang island, malaysia. 105landslide susceptibility mapping of penang island, malaysia, using remote sensing and multi-geophysical methods 5. conclusions this study has demonstrated the efficiency of combining remotely sensed analytical models via the fr technique with the results of multi-geophysical methods (ert, srt, and satellite-derived gravity) to generate and update the landslide susceptibility map of panang island, malaysia. based on the results of the analyzed remotely sensed causative factors for the study area, the slope gradient and cultivation land factors are the most influential landslide causative factors, where most of the landslides are likely to occur in the northern and southern parts. this is also supported by the interpretive assertions made from the ert, srt, satellite-derived bouguer gravity, and lsi models/maps of the study area. comparatively based on these analyzed models/maps, prospective landslides were pinned on the hilly and elevated sections, especially sections with varied contractive soils and shallow structures, while the less susceptible sections were identified on flat topographic reliefs. on that account, the possibility of landslides occurring at the investigated monitoring site and other identified sections is high as suggested by the predictive auc modeling of lsi data. though this study does not fully leverage some necessary enhancement gravity techniques to delineate shallow to deep crustal structures attributed to regional crustal deformation, it has provided helpful recommendations in planning future urban development and mitigating the occurrence of future landslides in the study area. acknowledgments the authors greatly appreciate all the comments from anonymous reviewers, which have contributed enormously to the manuscript’s quality and increased its readability. we also would like to thank jabatan pertanian malaysia and jabatan ukur dan pemetaan malaysia for providing the requisite data. we also would like to thank universiti sains malaysia for the financial support provided through the short term grant 304.pfizik.6315489. data availability all data generated or analyzed during this study are included in this published article. the corresponding author can make other supporting analyzed data available upon reasonable request. funding this research was supported by short term grant 304.pfizik.6315489 from universiti sains malaysia. references ab talib, j. 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(2023). new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina. earth sciences research journal, 27(4), 343-354. https://doi.org/10.15446/esrj. v27n4.107538 the igneous rocks of cerro cacheuta are considered a typical expression of the permian-triassic magmatism within the andes precordillera of mendoza, argentina. in particular, the southwestern margin of this hilltop is characterised by intermediate volcanic and felsic intrusives from the choiyoi group, one of the most extensive volcano-sedimentary suites of southern south america. the choiyoi magmatism is widely associated with dramatic tectonic and environmental changes in the gondwana supercontinent. therefore, a better characterisation of the igneous facies of cerro cacheuta is critical to understand the evolution of gondwana and recognise the multiple events that occurred towards the end of the permian. this paper presents new data about the mineralogy, petrography, and geochemistry of the plutonic and volcanic units of cerro cacheuta, which enables authors to draw additional conclusions about the genesis and evolution of the magmatism in the region. microscope observations indicate that the volcanic rocks are largely consistent with intermediate facies dominated by a porphyry texture with plagioclase and sanidine phenocrystals within a trachytic groundmass. coetaneous breccias of possible hydrothermal origin and geodes with cryptocrystalline silica, limonite, and quartz are also recognised in outcrops. geochemical analysis suggests that the lavas would likely correspond to andesites from a calc-alkaline magmatic arc. furthermore, trace elements show enrichment in lree/hree, a slight negative anomaly in eu, and concentration ratios compatible with crustal extension during the initial stages of the choiyoi group. based on their chemical similarities, the monzonites of the boca del río pluton would correlate with the lower section of the choiyoi group, of permian age. in contrast, granites of the cacheuta pluton show a signature typically recognised in units from the triassic and, therefore, would be comparable to the upper member of the choiyoi group. keywords: mineralogy; petrography; geochemistry; cerro cacheuta; argentina resumen las rocas ígneas del cerro cacheuta son consideradas una de las manifestaciones más típicas del magmatismo permo-triásico de la precordillera de los andes de mendoza, argentina. en especial, el flanco sudoeste del cerro cacheuta se caracteriza por la presencia de rocas volcanicas de composicion intermedia e intrusivos felsicos del grupo choiyoi, una de las secuencias volcano-sedimentarias de mayor distribución geográfica en sudamérica. a su vez, el magmatismo del choiyoi se asocia con los dramáticos cambios tectónico-ambientales que ocurrieron durante la ruptura del gondwana. es así que una caracterización más detallada de las facies ígneas del cerro cacheuta es clave para comprender tanto la evolución de gondwana como los diferentes eventos que tuvieron lugar hacia fines del pérmico. este trabajo presenta nuevos datos sobre la mineralogía, petrografía y geoquímica de las rocas plutónicas y volcánicas del cerro cacheuta, con el objeto de lograr una interpretación más acabada de la génesis y evolución del magmatismo en la región. al miscroscopio, las rocas volcanicas son consistentes con facies intermedias de textura predominantemente porfirica, con cristales de plagioclasas y sanidina en una pasta de composición traquítica. en afloramientos, también se han observado brechas posiblemente de origen hidrotermal, y geodas con sílice criptocristalina, limonita y cuarzo. los resultados geoquímicos sugieren que las lavas tendrían una composición andesítica de arco magamatico calco-alcalino. por otra parte, los elementos traza muestran enriquecimiento en lree/hree, una leve anomalía de eu y concentraciones químicas compatibles con un ambiente tectónico extensional durante los estadios iniciales del grupo choiyoi. en base a sus similitudes quimicas, las monzonitas del pluton boca del río se correlacionarian con la seccion inferior del grupo choiyoi. por el contrario, los granitos del pluton cacheuta muestran caracteristicas tipicas de unidades triasicas, por lo cual serian comparables con el miembro superior del grupo choyoi. palabras clave: mineralogía; petrografía; geoquímica; cerro cacheuta; argentina avances en el conocimiento del magmatismo permo-triasico del flanco sur del cerro cacheuta, argentina https://doi.org/10.15446/esrj.v27n4.107538 mailto:adgallardo@geowater.com.au https://doi.org/10.15446/esrj.v27n4.107538 https://doi.org/10.15446/esrj.v27n4.107538 344 amancay martinez, adrian h. gallardo, melisa mulé, aldo giaccardi, esteban crespo, david aguilera 1. introduction a particularly extensive magmatism occurred during permian-triassic times between 26° and 40° south latitude, on the continental margins of south america. products of this magmatism are currently seen within the pastos blancos group in northern chile and throughout outcrops from the cordillera del viento to the south and the andes precordillera of mendoza, san juan, and la rioja to the north. further inland, isolated units such as the chadileuvú block in la pampa and minor igneous exposures in san luis and even buenos aires provinces suggest that the magmatic activity would have gradually migrated eastwards from the late paleozoic. this magmatism province is considered to be part of the choiyoi group, a major feature comprising volcanic and plutonic rocks developed over a time span of approximately 36 ma (rocher et al., 2015), extending for more than 900,000 km2 across argentina and chile (bastías mercado et al., 2020). the choiyoi group is composed of an andesite to dacite section at the bottom, and an upper section mainly rhyolitic in composition (llambias et al., 1993). numerous conceptual models have been postulated over the years to explain the origin of magmatism (e.g., rocha campos, 2011; martinez et al., 2020; bastías mercado et al., 2020; and many others). as stated by gianni and navarrete (2022), some hypotheses explain the development of the choiyoi group due to very slow convergence or halted subduction by slab break-off episodes (e.g., fanning et al., 2011; garcía-sansegundo et al., 2014). others propose a continued subduction scenario associated with changes in slab dip (e.g., riel et al., 2018). some studies suggest a convergent setting at ca. 280 ma that experienced a slab break-off event after ca. 260 ma (e.g., ramos et al., 2020), whilst other authors favoured a subduction-related setting for part or the totality of the choiyoi magmatic province (e.g., lópez de luchi et al., 2021). geochemical data from the present work, albeit limited, suggests that the choiyoi rocks would have originated in a calc-alkaline magmatic arc with late extensional characteristics. silicic rocks would have formed in an extensional setting where basaltic magmas caused extensive crustal melting that inhibited the upward passage of the basalts (kay et al., 1989). this event could then have occurred at the initial stages of the gondwana breakup. in addition, the paleozoic magmatism would also constitute the source for the pervasive ash fall deposits interlayered in coeval, retroarc, or intracontinental basins near plate margins (rocha campos et al., 2011; sato et al., 2015). most studies were undertaken in areas where the choiyoi group has greater geographical expression, typically in what is known as the frontal cordillera of mendoza and san juan. pioneering works of zuber (1889) and stappenbeck (1910) established the presence of granite, diorite, and melaphyres in cerro cacheuta. later, rossi (1947) argued that these rocks were mainly lamprophyre and granodiorites akin to a common calc-alkaline magma. petrographic studies by llano et al. (1985) provided further evidence of granite (i.e., cacheuta stock) and granodiorites (i.e., boca del río stock) in the locality. in that regard, varela et al. (1993) classified the plutonic rocks of cerro cacheuta as monzodiorites, monzonites, and monzogabbros derived from a hybrid mantle-continental crust, possibly related to a continental postcollisional relaxation. more recently, martínez (2005), giambiagi and martinez (2008) and martinez et al. (2020) indicated that the choiyoi group is composed of three distinct lithologic facies ranging from the early permian to early triassic, which become increasingly felsic upwards. however, these studies were largely constrained to the frontal cordillera of mendoza. the southern sector of cerro cacheuta was described by ramos et al. (2010) when the cerro tupungato geological sheet was produced. subsequently, an analysis of the geochemical signature of monzonites in the eastern edge of the cacheuta pluton suggested an origin contemporaneous with the permian-triassic volcanic activity of the choiyoi group (cingolani et al., 2012). this data correlates well with u/pb geochronology in the lower section of the choiyoi group near the neighbour towns of potrerillos and uspallata, which indicated a late permian age of 277 ± 3 ma (strazzere et al., 2016). notwithstanding the many studies available in the literature, the widespread distribution, heterogeneous lithology and complex structure of the choiyoi group means that its origin and evolution are still in discussion. therefore, the present work aims to describe and better characterise the mineralogy, petrography, geochemistry, and macroscopic aspects of the choiyoi group volcanic rocks in the little explored flanks of southern cerro cacheuta, mendoza. geochemical data was also analysed in relation to the neighbours boca del río and cacheuta plutons, of the mesozoic age. in this context, the analysis results provide additional data into the tectonic environment in which the choiyoi rocks developed and contribute to the reconstruction of the region’s geological evolution. 2. method and materials the study area comprises an area of approximately 40 km2 on the southern flank of the cerro cacheuta, within the andes precordillera of mendoza in western argentina (figure 1). existing geological information, satellite images, and base maps (scale 1:20,000) were processed and analysed employing google earth and the qgis platform. four field campaigns were then conducted to carry out geological mapping, lithological descriptions, structural measurements, and sampling of representative rocks. selected specimens were then used for the preparation of thin sections at the university of san luis and examined under a leica petrographic microscope. three samples from the choiyoi group (i.e., ch1, ch5, ch6) were selected for geochemical analysis. the samples were analysed by x-ray fluorescence and icp/ms (inductively coupled plasma-mass spectrometry) at als laboratory group mendoza, argentina. geochemical results were processed using gcdkit 4.1 (janoušek et al., 2006). even though it is always desirable to further increase the number of analysed rocks, the collected samples are considered representative of the lithological variations in an area of limited geographical extension. cryptocrystalline silica was studied by scanning electron microscopy (sem), semi-quantitative microanalysis with dispersive spectroscopy (eds), and wds quantitative measurements at the university of san luis labmem (electron microscopy and microanalysis laboratory). figure 1. location map of the study area. 345new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina 3. geological framework 3.1 tectonic setting and evolution the andes mountains extend for almost 8,000 km along the boundary between the nazca and south american plates. the current morphology of the andes is the result of a combination of pre-existing structures, deformation along the margins of gondwana, and lithospheric shortening after the cretaceous (giambiagi et al., 2010). following a series of compressive cycles in the early paleozoic, crustal extension became widespread during the late paleozoic – mesozoic. subduction would have commenced along the current continental margin during the carboniferous (ramos, 1988). late paleozoic deformation associated with the san rafael orogenic phase occurred during the late permian (azcuy and caminos, 1987). the volcanic arc associated with subduction would be represented by carboniferous – permian plutonic and volcanic rocks currently outcropping in the cordillera frontal (ramos et al., 1986; mpodozis and kay, 1990; llambías et al., 1993). a change from compression to an extensional regime during the late permian to middle triassic led to important acidic volcanism and large volumes of magmatism under conditions of oblique extension (llambías et al., 1993; giambiagi and martínez, 2008). regional rifting further continued with the opening of the continental depocenters of cacheuta and las peñas. extension persisted from the middle triassic to the late cretaceous, although the deformation intensity and the affected areas varied considerably with time (uliana et al., 1989). moreover, the development of these rift systems would have been influenced by the reactivation of a weak crust along the edges of previously amalgamated terranes (ramos et al., 1986). compressive deformation during the neogene– quaternary resulted in the reactivation of late paleozoic faults favourably oriented with respect to the andean stresses (cortés et al., 1999; folguera et al., 2004) and additionally, led to movements of the sinistral strike–slip faults from the permian-triassic (cortés et al., 2006, terrizzano et al., 2009). thus, the andes precordillera of mendoza is mostly characterised by some ordovician carbonate platform sequences (bordonaro et al., 1996), overlain by siliciclastic units and mafic rocks of the lower paleozoic, widespread sedimentites from the devonian, granitic magmatism and extensive permian–triassic volcanism linked to the rupture of gondwana (llambías et al., 1993). furthermore, continental paleoenvironments are developed due to triassic rifting and ultimately, foreland basins filled with synorogenic deposits from the paleogene. 3.2 stratigraphy 3.2.1 choiyoi group volcanics (permian – triassic) vulcanites of the choiyoi group were identified southwest of cerro cacheuta, usually in an array of near-horizontal banks with a general strike sw 25° (figures 2; figure 3a). the outcrops have grey−reddish tonalities due to the widespread presence of fe oxides although, greenish types are usually more evident in hand specimens and fresh fractures (figure 3b). these rocks are characterised by a porphyric texture with subhedral phenocrysts of plagioclase (1.5 4 cm in diameter), embedded in an aphanitic groundmass indicative of extrusive conditions. phenocrysts of sanidine ranging between 1 to 3 cm and to a minor extent, angular lithics reaching up to 10 cm in length were also identified (figure 3c). we also recognised minor and scattered outcrops of angular breccias, typically composed of large, multicolour volcanic fragments of several centimetres within a matrix of intermediate composition. the breccias generally exhibit a reddish coating, textural and mineralogical immaturity, and the presence of sub-angular polymictic clasts in a mafic matrix-supported fabric (figure 3d). thus, this volcanic breccia could be interpreted as a response to the opening of the conduits through which the lava escaped. similarly, andesitic breccias were observed within the lowermost part of the choiyoi group further north, in proximities to the locality of potrerillos (martinez et al., 2020). without direct radiometric measurements, the age of the volcanic rocks was inferred from regional correlations. in this context, compositionally similar lavas of the portezuelo del cenizo formation, northwest of the study area near the locality of uspallata, indicated an age of 281 ± 10 ma, from the lower permian (folguera et al. 2004). this age agrees with previous results by rapalini and vilas (1991), who employed the k/ar method in rocks of the same formation further north, within quebrada del tigre. further evidence of a late permian age was presented by strazzere et al. (2016), who estimated an age of 277 ± 3 ma for the trachydacites and trachyandesites of the lower section of the choiyoi group. thus, the trachyandesites observed in cerro cacheuta could be attributed to the beginning of the choiyoi magmatism in the region. figure 2. geological map of the southern sector of cerro cacheuta, mendoza. modified from lara et al. (2017), cingolani et al. (2012). figure 3. a) overview of the choiyoi group volcanics on the western slope of cerro cacheuta, dipping towards the sw; b) volcanic rocks with grey-reddish tonalities due to fe oxides coating; c) porphyry texture with subhedral feldspar phenocrysts in an aphanitic matrix, together with angular volcanic lithics of a few centimeters randomly arranged. d) breccia with a variety of lithics and clasts of the studied volcanic rocks. 346 amancay martinez, adrian h. gallardo, melisa mulé, aldo giaccardi, esteban crespo, david aguilera 3.2.2 boca del río pluton (late permian) the boca del río pluton outcrops over about 25 km2 in the ne portion of the study area. the granodiorite unconformably intrudes the greywackes and slates of the villavicencio formation. rossi (1947) described it as the cacheuta composite stock, originated from a granodiorite magma that was followed by a more extended pulse of granitic composition, named the cacheuta pluton. metamorphic contact rocks such as hornfels occur around the edges of the stock due to temperature increases from the magma intrusion. in turn, the granodiorite is intruded by the cacheuta pluton, from the triassic, as indicated by the dikes and aureoles adjacent to the contact between the units (folguera et al., 2004). rossi (1947) indicated that plagioclase, orthoclase, pyroxene, green amphibole, biotite and quartz are the primary minerals in the granodiorite. accessories include mainly apatite, opaque minerals and titanite, along with secondary sericite and clays. thus, and based on its mineralogical assemblage, the rock would better be classified as a quartz monzonite. llano et al. (1985) argued that some rocks should still be considered granodiorites. furthermore, varela et al. (1993) distinguished three lithological types within the suite: quartz monzodiorites, quartz monzonites and monzogabbros. according to these authors, the monzodiorites are mostly composed of subeuhedral plagioclase (45 %), followed by crystals of anhedral quartz (20 %), hornblende (20 %) and perthitic orthoclase (15 %). isolated flakes of biotite are occasionally observed. on the other hand, the monzonites are predominantly composed of plagioclase (40 %) and perthitic orthoclase (30 %), the later with signs of partial argilisation. other minerals include aggregates of hornblende and biotite and accessories such as titanite, ilmenite, magnetite and zircon. finally, the monzogabbros are characterised by augite-like and orthopyroxenes in association with subhedral caplagioclases (50 %). the pyroxenes show some chlorite alteration near the edges whilst, signs of argilitisation are identified around the plagioclases. biotite, in this case associated to muscovite, is also present. other accessories comprise mostly apatite and less titanite, ilmenite and magnetite (varela et al., 1993). based on k/ar dating on biotite, caminos et al. (1979) reported and age of 397 ± 15 ma (lower devonian) for the stock. similar results were later recorded by llambias et al. (1993) (418 ± 23 ma; lower devonian) and varela et al. (1993) using the rb/sr method (417 ± 35 ma; lower devonian). nevertheless, more recent studies by cingolani et al. (2012) employing u/pb on zircons from monzonites and monzogabbros yielded an age ranging between 261 ± 3.5 ma and 255 ± 3 ma, in the late permian. 3.2.3 cacheuta pluton (middle to upper triassic) the cacheuta pluton or cerro los baños unit is a typical yellowish– pink granite that outcrops in the ne portion of the study area. this granite is a distinctive subovoid body intruding the already folded succession of clays and pelites of the villavicencio formation. the contact with the host rock is largely unconformable, with some hornfels product of the magmatism. metasedimentary xenoliths of variable sizes are abundant across the granite and are attributed to fragments of country rock with minimal interaction with the ascending magma. in addition, a wide range of felsic and mafic dikes were found together in various sectors of the pluton, including its dome (cingolani et al., 2012). thin sections of the granite show widespread potassium feldspar, plagioclase, quartz and biotite with little muscovite. allanite, apatite and magnetite were also recognised as accessories by varela et al. (1993). in this regard, these authors classified the rock as a syenite granite. radiometric k/ar by dessanti and caminos (1967) suggested that the granite emplacement occurred during the late permian (275 ± 13.5 ma). in this regard, varela et al. (1993) and cingolani et al. (2021) indicated a 87sr8/86sr ratio of 0.75105 for the granite, value compatible with a possible cortical source for the magma. further north, analysis of k/ar in the granites of cerro médanos and cerro arenal indicate an age of 244 ± 10 ma (middle triassic) and 237 ± 10 ma (middle to upper triassic) respectively (caminos et al., 1979). however, analysis of u/pb on zircons from the monzonitic rim of boca del río defined a histogram with a main age mode between 253 − 258 ma, in the upper permian (cingolani et al. 2012). thus, there is an ongoing discussion of the appropriate decay constant to be used and the radiometric age of the unit. despite differences in the proposed ages, the studies evidence that the emplacement of the cacheuta pluton took place during the gondwanan cycle and would be coetaneous with the magmatic activity of the choiyoi group. 4. results 4.1 mineralogy and petrography microscope observations confirmed that the mineral primary assemblage of the choiyoi vulcanites is plagioclase and sanidine in a pilotaxic matrix (figure 4a-b; table 1). the subparallel orientation of the microliths of plagioclase evidence fluid circulation during the rock formation. in this regard, the matrix/phenocrysts ratio is estimated to be 70/30. the plagioclase is mostly subhedral, with blurred borders due to partial alteration to sericite and clays. polysynthetic twinning has been occasionally identified in well-preserved phenocrysts. the composition of the plagioclase is intermediate. sanidine, about 10 %, generally occurs in 1 − 2 cm grains, euhedral to subhedral in shape, carlsbad twinned and ubiquitously cracked. some grains are obscured by incipient alteration to clay with outer rims optically continuous suggesting thus, late crystalline growth. isolated amphibole and biotite (5%) are present in the groundmass in association with apatite, titanite and less abundant opaque minerals (figure 4c-d). the amphiboles are medium−grained, subhedral and greenish brown to reddish (figure 4e-f). the biotite shows a typical long flaky habit, strong brownish coloration and potential changes of birefringence masked by the body colour. finally, sparse grains of apatite, titanite and opaque minerals commonly occur in patches crossed by cracks and veinlets filled with chalcedony and jasper. some fractures appear to be affected by recrystallisation and the inclusion of previously formed crystals. the rock matrix also shows zones highly stained by fe oxides (figure 4g-h). table 1. modal composition and whole-rock mineralogy of samples in southern cerro cacheuta. mafic lavas phenocrysts accessory minerals paste secondary minerals pl ag io cl as e sa ni di ne am ph ib ol es bi ot ite ap at ite tit an ite op aq ue s % se ri ci te cl ay s ch al ce do ny ja sp er c ho iy oi g ro up ch 1 15 10 5 5 x 65 * * * * ch 5 20 10 5 x x x 65 * * * * ch 6 20 10 5 x x x 65 * * * * symbol x corresponds to accessory minerals (< 5%). secondary minerals indicated by * as it will be discussed in more detail in the following sections, geochemical results indicate that the vulcanites in cerro cacheuta have a predominant trachyandesite composition. a puzzle-like type breccia was also observed in a few andesite outcrops (e.g., sample point ch 7). these deposits are typically monomict and clast−supported, with angular clasts embedded in an aphanitic groundmass of cryptocrystalline silica, jasper and chalcedony. (figures 5a). cracks and veinlets are ubiquitous suggesting therefore, that the original texture have also been partially obliterated by hydrothermal alteration. hydrothermal alteration in the lavas is also represented by numerous silica-bearing shear veins and patches of siliceous material in the wall rock. the alteration assemblage also shows lithic clasts commonly crossed by veinlets and infilled fractures, with a prevailing 315º azimuth direction and sinistral displacement (figure 5b). the presence of geodes, commonly infilled by siliceous materials with classic comb or dog teeth texture is another indicator of extensive circulation of fluids (figures 5c; 5d). thus, it is interpreted that the volcanic rocks would have been subjected to late hydrothermal processes contemporary with the development of the permian−triassic magmatism. 347new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina figure 4. microphotography of volcanic rocks of choiyoi group; a) phenocrysts of subhedral sanidine slightly altered to clay, in a paste of plagioclase with pilotaxic texture, 10x, n//; b) as in a) with crossed polarized, 10x, nx; c) phenocrysts of subhedral plagioclase highly altered to clays and sericite. polysynthetic twinning present in a few sections 10x, n//; d) as in c) with crossed nicols, 10x, nx; e) phenocrystal of reddish brown amphibole in pilotaxic paste, 10x, n//; d) the birefringence corresponding to the amphibole is masked by color, 10x, nx; f) vein of chalcedony upholstered by jasper affecting the rock in a ductile manner, 10x, n//; h) as f) with crossed nicols, 10x, nx. figure 5. a) puzzle-like breccia with clasts of lavas cemented by jasper-type materials, of possibly hydrothermal origin (sampling point ch 7); b) jasper and chalcedony veins causing sinistral displacement of lithics; c) dog tooth geode filled with chalcedony and hyaline quartz; d) geode with euhedral crystals of purple amethyst quartz. 348 amancay martinez, adrian h. gallardo, melisa mulé, aldo giaccardi, esteban crespo, david aguilera figure 6. siliceous material analysed by sem-eds: a) color orange; b) red; c) green; d) black with reddish stains; e) cubic, euhedral crystals. peaks of higher intensity in the kα fe and kα o possibly correlated with iron hydroxides from pyrite. when analysed by electron microscope, the siliceous material reveals four contrasting colors: orange, red, green and black. the habit is quite variable, usually massive to botryoidal, diaphanous translucent, matt to oily gloss and of conchoidal fracture (figures 6a, b, c). individuals within the orange–red samples tend to be significantly smaller than green and black ones. in particular, some specimens exhibit reddish stains possibly due to the presence of limonites (figures 6d). in addition, higher intensity in the kα fe and kα o spectrum of cubic euhedral crystals can be associated to corrosion and limonitisation of fe elements such as pyrite (figure 6e). 4.2 geochemistry bulk-rock chemistry for samples collected at cerro cacheuta is presented in appendix i. the cipw standard for samples ch5 and ch6 shows a predominantly quartz−hypersthene−diopside composition indicative of silica saturation. in turn, the normative composition of sample ch1 is quartz− hypersthene−corundum, reflecting then silica undersaturation, metaluminous and a slight peralkaline character (table 2). as previously discussed, the tas sio2 vs. na2o+k2o classification (le bas et al. 1984), show samples plotting in the trachyandasite field (figure 7a). a calc-alkaline trend is also suggested by the ternary afm diagram (irvine and baragar, 1971). the immobile elements th and co (hastie et al. 2007) are commonly employed as a proxy for k2o − sio2 in the identification of rock types and volcanic series. in this regard, the th – co classification diagram shows that samples plot on the calc−alkaline field (figure 7b). employing the diagram of shand (1943) to discriminate according to alumina saturation, the choiyoi group volcanic rocks show a/cnk values <1,1 indicating thus a meta and peraluminous character (figure 7c). any peraluminous signature is thought to be likely derived from metasedimentary sources (collins and richards, 2008), whilst the bulk of the rocks falling within the metaluminous field. this is further confirmed by plotting results in the ternary diagram of sio2−al2o3−k2o (foley et al. 1987; shand, 1943). in this case, all samples plot well within the metaluminous and sodium field (figure 7d). pearce et al. (1984) proposed the use of incompatible elements to discriminate the tectonic environment. in this context, the ta+yb vs. rb chart also shows that volcanic rocks from cerro cacheuta formed predominantly in a volcanic arc environment (figure 7e). concerning the tectonic framework, the th−hf−ta diagram (wood, 1980) points into a calc−alkaline arc within a destructive margin, heavily weighted into the th−rich end (figure 7f). table 2. cipw-norm based on analyses recalculated to 100% on an anhydrous basis. ch 5 ch 6 ch 1 q 1.6 9.3 c 1.2 0.1 or 15.3 18.6 31.3 ab 43.8 38.7 35.3 an 9.7 8.2 8.1 ne 1.1 di 6.5 0.0 hy 19.2 20.2 ol 17.5 mt 1.9 1.8 3.7 il 1.3 1.4 2.1 ap 0.6 0.7 0.7 py 0.04 0.04 sum 100.1 100.0 100.0 349new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina figure 7. a) tas diagram sio2 vs. na2o+k2o from le bas (1984); b) co vs th diagram de hastie et al. (2007); c) a/cnk [al2o3/(cao+na2o+k2o)] vs. a/nk [al2o3/ (na2o+k2o)] molecular relationship diagram (shand, 1943); d) ternary diagram of the na2o-al2o3-k2o molar compositions (foley et al., 1987; shand, 1943); e) ta+yb vs. rb diagram for granites classification from pearce et al. (1984); f) ternary diagram for classification of th-hf/3-ta basalts from wood (1980). all samples are enriched in light rare earth elements (lree) when analysed on chondrite-normalised ree (nakamura, 1974) (figure 8a). there are no significant differences in ree concentrations between samples ch5 and ch6. however, sample ch1 exhibits lower lan/ybn ratios and a higher enrichment in ree. a more pronounced eu anomaly (eu/eu*=0.66), would indicate longer evolution and therefore, high chemical fractionation. thus, a negative eu anomaly would suggest plagioclase fractionation (hanson, 1978). the vast majority of the lree (la−eu) would derive from feldspars, zircon, apatite, epidote and allanite whilst, hree (gd−lu) contents could be related to pyroxene, amphiboles and especially, garnet. the general rees pattern is compatible with a magma formed under a normal to thinned crust (~30 km) and indicative of plagioclase fractionation in a low-pressure environment (kay et al. 1991). the nmorb (normal mid−ocean ridge basalt) multi-element diagram of sun and mcdonough (1989), shows negative anomalies in rb, nb, p and ti. simultaneously, these samples also present positive anomalies in ba, k, pb and sr for samples ch5 and ch6 (figure 8b). the positive anomaly in pb but negative in rb could reflect magma evolution in an arc system. on the other hand, a positive anomaly of sr along with eu is indicative of plagioclase fractionating in shallow levels of continental crust (wilson, 1989). sample ch1 is high in rb but depleted in sr. concomitant positive anomalies in th, u and zr could indicate enrichment in the source or cortical fusion in a thinned continental crust (green, 1980). finally, higher contents in la and ce at that sample could be attributed to a general increase in alkalinity while, negative anomalies in ti may be associated to the fractionation of titanite. 350 amancay martinez, adrian h. gallardo, melisa mulé, aldo giaccardi, esteban crespo, david aguilera figure 8. volcanic rocks of the choiyoi group: a) rare earths elements (ree) normalised to chondrite from nakamura (1974); b) minor elements and traces normalised to nmorb (normal mid-ocean ridge basalt) from sun and mcdonough (1989). 5. discussion there are two main tectonic scenarios in which high-k calc-alkaline magmas may be generated: a) emplacement of high-k rocks in continental arc settings similar to that of the andes (pitcher, 1987), and b) melting and contamination spread of former continental crust in diverse tectonic settings (roberts and clemens, 1993). in this regard, samples from our study were compared with specimens c2 c6 collected by cingolani et al. (2012) from the boca del río pluton, on the eastern margins of the cacheuta pluton, to further investigate any co-genetic relationships between units (figure 9). based on the na2o+k2o vs sio2 diagram from middlemost (1994), samples c2 to c6 were classified as monzonites/quartz-monzonites, subalkaline, metaluminous and high potassium. sample c1, from the cacheuta pluton, indicates a granite composition (figure 10a). looking at the th − co classification diagram of hastie et al. (2007), the samples plot into the high−k calc−alkaline field and shoshonites series, with increase in co (figure 10b). specimens from both studies mostly cluster on the metaluminous field in the diagram al2o3/ cao+na2o+k2o (a/cnk) vs. al2o3/na2o+k2o (a/nk) of shand (1943) (figure 10c). furthermore, the th−hf−ta diagram (wood, 1980) points into a tectonic framework characterised by a calc−alkaline arc within a destructive margin, heavily weighted into the th−rich end (figure 10d). while there is not sufficient evidence to completely rule out an arc signature due to inheritance/ contamination, a closer look at minor and immobile elements further indicates that samples across sites are all consistent with a magmatic arc environment in the nb/yb vs. th/yb from schandl and gorton (2002), (figure 10e). this is in line with the transition from a volcanic arc to an intraplate setting as plotted in the y+nb vs. rb from pearce et al. (1984). in addition, the cacheuta samples mostly plotting in a tight group could also suggest formation in a single tectonic environment (figure 10f). figure 9. location of the ch1, ch5 and ch6 samples from the choiyoi group (present work). samples c1 (cacheuta pluton) and c2 to c6 (boca del río pluton) previously analysed by cingolani et al. (2012). with the exception of sample ch1 which is enriched in ree, both monzonites (c2 to c6) and vulcanites (ch5 and ch6) display similar trends on a chondrite-normalised ree diagram (figure 11a). on the other hand, samples c2 to c6 are enriched in lree, with a near-flat trend in hree and slightly negative anomaly of eu. the observed negative anomaly in eu suggests plagioclase fractionation at shallow depths (hanson, 1978) whilst, the general pattern in ree would suggest rocks formation in a continental crust of normal thickness (~30 km) and plagioclase fractionation under low pressures. normalising the elements to the nmorb diagram of sun and mcdonough, (1989) shows that the monzonites have positive anomalies in cs, k, pb, sr, nd but, negative in rb, nb, p and ti (figure 11b). monzonitic rocks present a small variation in terms of their enrichment. despite some variations in the magnitude of the negative anomalies for u and th, these monzonites correlate well with samples ch5 and ch6 in the choiyoi group. on the other hand, sample ch1 presents greater enrichment and therefore, positive anomalies in rb and negative in sr. the lile elements (large ion lithophile elements) rb and ba, and the enrichment in k are all indicatory of the fractionation of feldspars at low pressure, possibly due to cortical assimilation. moreover, pb enrichment might be associated with an arc signature. in contrast, and as stated by hanson (1978), depletion of sr and p could be attributed to the effects of sediments deposition. anomalies of hfse elements (high field strength elements) such as u and th also indicate sedimentation or the effects of a crustal component. finally, negative anomalies in nb and ti could be due to an arc component or associated to the fractionation of accessory minerals. the granite of sample c1 exhibits similar trends to other units (figure 11c). in this case, there is enrichment in lree, with a near-flat trend in hree and a strong negative anomaly of eu. again, the negative anomaly in eu would suggest plagioclase fractionation under low pressures. however, the magnitude of the anomalies differs from other rocks when plotted on spider diagrams. thus, it is concluded that while there is a general good correlation between the monzonites and vulcanites, negative anomalies in ba, pb, p, eu and ti suggest that the granitic facies at cerro cacheuta are compositionally more variable (figure 11d). 351new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina figure 10. a) diagram sio2 vs. na2o+k2o from middlemost (1994) for plutonic rocks, b) co vs th diagram de hastie et al. (2007); c) ternary diagram for classification of th-hf/3-ta basalts, from wood (1980); d) nb/yb vs. th/yb diagram from schandl & gorton (2002); e) y+nb vs. rb diagram for granites classification, from pearce et al. (1984), f) diagram ta vs. yb from hildebrand et al. (2018), modified from pearce et al. (1984). wpg–within plate granite; org–oceanic ridge granite. 352 amancay martinez, adrian h. gallardo, melisa mulé, aldo giaccardi, esteban crespo, david aguilera figure 11. spider diagrams of the volcanic rocks of the choiyoi group (ch1, ch5 and ch6) compared to monzonites from boca del río pluton (c2 to c6): a) rare earths elements (ree) normalised to chondrite from nakamura (1974); b) minor elements and traces normalised to nmorb (normal mid-ocean ridge basalt) from sun and mcdonough (1989). spider diagrams of the volcanic rocks of the choiyoi group (ch1, ch5 and ch6) compared to cacheuta pluton (c1): c) rare earths elements (ree) normalized to nakamura’s chondrite (1974); d) minor and trace elements normalised to nmorb (normal mid-ocean ridge basalt) from sun and mcdonough (1989). martínez (2005) and giambiagi and martinez (2008) divided the magmatism of the choiyoi group into three major sections: a) a lower section of calc−alkaline composition, related to an environment of compressive subduction. this ultimately evolved into a magmatic arc that produced mafic rocks, basalts and andesites during the permian; b) a middle section representing a transition towards greater extension, with rocks of more felsic compositions such as rhyodacites and dacites and: c) the upper section, with felsic rhyolites and granitic rocks originated within an extensional environment of alkaline characteristics. these rocks are linked to a temporary cessation of the subduction during the triassic. from this geodynamic context, it is then argued that rocks of trachyandesitic composition (i.e., ch1, ch5 and ch6) could be attributed to the lower section of the choiyoi group. in effect, these rock not only have greater geological similarity with this section but are chemically closer to the permian monzonites of c2 to c6, which based on trace element and ree patterns could in turn be considered the plutonic equivalent of those lava facies. on the other hand, the triassic granitic pluton of c1 presents greater resemblance with the felsic, upper section of the choiyoi group, in this case associated to an extensional arc environment. according to kay et al. (1991), its geochemical imprint suggest formation within a crust of normal thickness but already subject to extension. 6. summary and conclusions findings from our study confirm that the vulcanites outcropping on the southwestern slopes of cerro cacheuta would correlate to trachyandesites from the choiyoi group, from the permian − triassic. at the microscope, the rocks typically consist of plagioclase and sanidine phenocrysts embedded in a groundmass of pilotaxic texture. in outcrop, these rocks are characterised by numerous geodes and cavities, commonly coated with jasper and botryoidal chalcedony, or filled with quartz of dog-teeth and comb textures. crystals of pyrite were also recognised. in many instances, the mineral is pseudomorphically replaced by iron hydroxides. puzzle-like breccias and patches of bleached clays might evidence hydrothermal processes following the lavas cooling. the geochemical signature of the rocks suggests that they formed at the transition between a calc−alkaline magmatic arc and an intraplate environment, with a slight tendency towards alkalinity. in turn, trace element exhibited some minor enrichment in lree/hree, a near-flat trend for hrees and a slight negative anomaly in eu in one of the samples. when normalised, the trace elements show a general negative anomaly in rb, nb, p and ti and a positive anomaly for ba, k, pb and sr. thus, the trachyandesites are interpreted to be genetically related to a volcanic arc under a crust of average thickness. it is worthy to note that available data prevents us from entirely ruling out a crustal source, even though we do not support that model for the rocks under consideration. that implies that the transition from a calc-alkaline magmatism to a more alkaline and anorogenic one remains as a possible interpretation of the geodynamics of the region. in this case, the transition would simply reflect increasing extensional conditions during the emplacement of the choiyoi group. the studied vulcanites were also compared to the coetaneous and nearby units of boca del río pluton and the cacheuta pluton. in this case, geochemical data showed a close correspondence between the monzonites of boca del río pluton and the vulcanites of the choiyoi group. nonetheless, the rees signature of the trachyandesite/monzonites suggests that despite genetic similarities, rocks from boca del río could reflect greater petrological evolution and a more evolved magma. in this regard, the monzonites would correlate with igneous rocks at the lower section of choiyoi group, possibly of late permian age. these units were later affected by the intrusion of the cacheuta pluton, with the consequent thermal imprint in the host rocks. acknowledgements the authors especially thank lic. daniel codega and lic. anahí cerizola, sections laboratory, san luis university, for preparing the thin sections used in this study. many thanks to the anonymous reviewers who made a great contribution to improve the analysis and interpretations presented in the paper. 353new insights into the permian-triassic magmatism of southern cerro cacheuta, argentina financing the present work has been completed as part of the proico project number 3-20314 (2014-2018) “the mesozoic volcanic facies of san luis province” and proico project (2023-2026) “las serranías menores del sur: sus relaciones geológicas y tectonoestatigráficas, provincia de san luis”, granted by the cyt of the national university of san luis (unsl), argentina. references azcuy, c. l., & caminos, r. 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https://doi.org/10.1016/j.gsf.2015.08.009 https://doi.org/10.1144/gsl.sp.1989.042.01.19 https://doi.org/10.1144/gsl.sp.1989.042.01.19 https://doi.org/10.1306/m46497c39 https://doi.org/10.1016/0012-821x(80)90116-8 https://doi.org/10.1016/0012-821x(80)90116-8 _heading=h.30j0zll _heading=h.3znysh7 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n3.101477 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (september, 2022): 231 238 at m o sp h er ic s c ie n c es role of ocean-atmosphere interface in annual and semiannual sst cycles in the indian ocean oldemar de oliveira carvalho junior research & project office/instituto ekko brasil/brazil cx postal 20, rua regina burttz clemer (estrada geral urussanga), 3207 – bairro india, aldeia cebb mendjila centro de estudos budistas bodisatva canelinha/sc. 88230-000 ocjunior@ekkobrasil.org.br abstract a long-term analysis of temperature can be used to describe the main mechanisms that operate at the surface of the ocean. the average sea surface temperature (sst) contour plots for the indian ocean are produced based on the world ocean atlas data set (1994). sst, together with the independent variables wind (wx, zonal component of pseudo-stress wind and wy, meridional component of pseudo-stress wind), net-down-fresh-water-flow (ndff), and ekman pumping, are included in a multiple regression analysis to define the relative importance of each one of these variables in the physical processes at the surface of the indian ocean. the ndff data set is based on coads (comprehensive ocean-atmosphere data set). the wind data is obtained from the florida state university (fsu). the harmonic terms of the variables are calculated, which is considered to be stationary and expressed by a fourier series as a cosine function. the harmonic terms are multiplied by the maximum amplitude of the variables and then added to their mean annual values. the isotherms are mainly meridional along the western boundary, but zonal in the southern indian ocean. the annual component is seen to have a maximum in july, summer monsoon (sw monsoon) and a minimum in january, during the winter monsoon (ne monsoon). the amplitude of the semiannual component is smaller, with two maxima in may and october and two minima in february and august. the small magnitude of these residuals errors is an indication that the temperature variability during this period and for this area can be explained reasonably well by the two harmonic terms. in the arabian sea, the final regression equations for sst variability show that it is mainly affected by the wx, ekman pumping and ndff. for most of the areas of the bay of bengal, as well as for most of the locations in the southern tropical indian ocean, the entered independent variables can explain sst. two components fit to observation can be used to predict sst together with the regression equations. although harmonic analysis can be used to study sst variability, a multiple regression analysis is required to identify and quantify the variables related to areas of large annual and semiannual variability. different techniques are therefore used together to provide more reliable results in sst configuration in the indian ocean. keywords: harmonic analysis, multiple regression, ocean circulation. papel de la interfaz océano-atmósfera en los ciclos anuales y semestrales de la tsm en el océano índico resumen este trabajo parte de la posiblidad de utilizar un análisis a largo plazo de la temperatura para describir los principales mecanismos que operan en la superficie del océano. los diagramas de contorno de la temperatura promedio de la superficie del mar (tsm) para el océano índico se produjeron con base en el world ocean atlas data set (1994). el tsm, junto con las variables independientes viento (wx, componente zonal del viento de pseudo-estrés y wy, componente meridional del viento de pseudo-estrés), flujo neto de agua dulce (ndff) y bombeo de ekman se incluyeron en un análisis de regresión múltiple para definir la importancia relativa de cada una de estas variables en los procesos físicos en la superficie del océano índico. el conjunto de datos ndff se basó en el coads (comprehensive ocean-atmosphere data set). los datos de viento se obtuvieron de la universidad estatal de florida (fsu). se calcularon los términos armónicos de las variables, que se consideran estacionarios y se expresaron mediante una serie de fourier como función coseno. los términos armónicos se multiplicaron por la amplitud máxima de las variables y luego se sumaron a sus valores medios anuales. las isotermas son principalmente meridionales a lo largo del límite occidental, pero zonales en el sur del océano índico. se considera que el componente anual tiene un máximo en julio, monzón de verano (monzón del sw) y un mínimo en enero, durante el monzón de invierno (monzón del ne). la amplitud del componente semestral es menor, con dos máximos en mayo y octubre y dos mínimos en febrero y agosto. la pequeña magnitud de estos errores residuales es una indicación de que la variabilidad de la temperatura durante este período y para esta área puede explicarse razonablemente bien por los dos términos armónicos. en el mar arábigo las ecuaciones de regresión finales para la variabilidad de la sst muestran que se ve afectada principalmente por el wx, el bombeo de ekman y el ndff. para la mayoría de las áreas de la bahía de bengala, así como para la mayoría de las ubicaciones en el sur del océano índico tropical, las variables independientes ingresadas pueden explicar la tsm. se pueden usar dos componentes ajustados a la observación para predecir la tsm junto con las ecuaciones de regresión. aunque el análisis armónico se puede utilizar para estudiar la variabilidad de la tsm se requiere un análisis de regresión múltiple para identificar y cuantificar las variables relacionadas con áreas de gran variabilidad anual y semestral. por lo tanto, se utilizan diferentes técnicas para proporcionar resultados más confiables en la configuración de la tsm en el océano índico. palabras clave: análisis armónico; regresión multiple; circulación oceánica; record manuscript received: 21/03/2022 accepted for publication: 16/09/2022 how to cite item: carvalho-junior, o. o. (2022). role of ocean-atmosphere interface in annual and semiannual sst cycles in the indian ocean. earth sciences research journal, 26(3), 231-238. https://doi.org/10.15446/ esrj.v26n3.101477 https://doi.org/10.15446/esrj.v26n3.101477 mailto:ocjunior@ekkobrasil.org.br https://doi.org/10.15446/esrj.v26n3.101477 https://doi.org/10.15446/esrj.v26n3.101477 232 oldemar de oliveira carvalho junior 1. introduction until recently, the oceans have been represented as a single large body with no large-scale temporal variability. indeed, temporal variability represents an additional difficulty when analyzing features of the ocean, such as eddies, currents, water masses, and primary production. sea-surface temperature (sst), for example, has been observed by satellites such as those of the series noaa (national oceanic and atmospheric administration). measuring sst is important as it is used in the study of oceanic physical processes such as frontal movements, water masses, currents, and upwelling zones (barret, 2013; khan et al., 2021). the present work intends to explore the sst of the tropical indian ocean as a major supplement to the more traditional procedures, and to examine the contribution made to an understanding of ocean circulation and water masses at the surface in this region. water masses also exist in the surface layer of the ocean. the definition of these water masses in terms of hydrographic properties is not an easy task because they undergo large seasonal and interannual variations. furthermore, surface water characteristics differ distinctly from one oceanic region to another. therefore, knowledge of surface ocean temperature can be important in helping to define water masses at the surface. a water mass which originates at the surface of the ocean is always a result of dynamic interactions between sea and atmosphere. since a sub-surface water mass usually originates from a well-defined formation region, it is important to know in detail which processes are involved in the water mass formation. physical and biological properties of surface waters, such as temperature, are constantly changing because of atmospheric and oceanic seasonal variations (khan et al., 2021). the proximity of land can also be an important factor affecting water masses characteristics, especially if there is a freshwater output from river runoff, such as the ganges-brahmaputra-meghna river catchment in the northern bay of bengal (wang et al., 2022). the identification and representation of the physical processes responsible for temperature distribution are important steps towards the understanding and management of marine ecosystems. the present study attempts to understand the relationship between regional circulation and the variability of temperature and its distribution at the surface of the indian ocean. long-term analysis of sst can be particularly useful to describe the main mechanisms that operate at the surface of the ocean, such as formation of water masses at the surface. the study of temperature in the oceans play a major role in the analysis of global warming. the interaction between atmosphere and sst is important to monitor and predict climate change (wang et al., 2022). the monsoon regime of the indian ocean, for example, is highly dependent on the sst (alory et al., 2007; huang et al., 2021; liao and wang, 2021; khan et al., 2021). khan et al (2021), define a four stages cycle for the sst in the tropical indian ocean, with a warming stage from february to may, a cooling from may to august, warming again from august to october and another cooling from october to january. two main regions can be distinguished in the indian ocean, the northern and the southern indian ocean (mcphaden, 1982; tomczak and godfrey, 1994). the northern indian ocean comprises three regions: the equatorial region (10° n to 10° s), bay of bengal and arabian sea. the great seasonal changes observed in the northern indian ocean are commonly attributed to the presence of the eurasian land mass and the absence of temperate and polar regions (shetye et al., 1994; singh and roxy, 2022). these result in a strong seasonal difference in the distribution of centers of pressure, which originates the monsoons. the boundary of the southern indian ocean to the south is not clear, and some researchers prefer to include the antarctic circumpolar current. three general circulation systems are defined to the indian ocean: the seasonal monsoon gyre, the subtropical anticyclonic gyre of the southern hemisphere, and the antarctic waters with the circumpolar current (wyrtki, 1973; reid, 2003; naqvi, 2008) (figure 1). the fact is that it is the seasonal reversal of the monsoonal gyre that gives the indian ocean its principal and unique characteristics. figure 1. surface currents in the indian ocean. (a) ne monsoon (dec-april); (b) sw monsoon (jun-oct). eac = east arabian coast; sjc = south java current; zc = zanzibar current; emc = east madagascar current; sc = somali currents; stf = subtropical front; saf = sub-antarctic front; pf = antarctic polar front; wgb = weddell gyre boundary. source: tomczak and godfrey (1994). during the ne monsoon, the main water flow in the equatorial region of the northern hemisphere is from east to west. north equatorial current turns southwards along the somali coast and, together with the zanzibar current, flows eastwards on both sides of the equator (equatorial counter current). some water from the equatorial counter current returns as the south equatorial current (zanzibar current) while the residue travels southeast as the java current (schott and mccreary, 2001; semba et al., 2019; jacobs et al., 2020). the sw monsoon starts in april with a northward flow along the somali coast. already in may, an eastward flow is well pronounced north of the equator. around july, the somali current becomes stronger and the counter current at the equator is replaced by the sw monsoon current. below the equator, the south equatorial current, coming from the east, turns north along the african 233role of ocean-atmosphere interface in annual and semiannual sst cycles in the indian ocean coast and becomes the somali current (tomczak and godfrey, 1994). this current has an important effect on the local baroclinic adjustment. this work describes the temperature distribution in the surface layer of the tropical indian ocean (25° n to 30°s and 30° e to 120° e) based on analysis of semiannual and annual amplitudes of temperature and a multiple regression analysis between temperature, ekman pumping, components of pseudo-stress wind, wind magnitude and ndff. it is expected that this study can contribute to the definition of water masses on the surface of the indian ocean. a long-term data set is explored to a large-scale examination of the sst of the tropical indian ocean as a major supplement to the more traditional procedures, and to examine the contribution made to an understanding of ocean circulation in this region. the process aims to examine how the two components of the harmonic fit contribute and interact to produce the observed and modelled data cycle. 2. data and methodology a set of five degree by five-degree grid average monthly sst contour plots for the indian ocean is produced based on the world ocean atlas data set (1994). the aim is to use the avhrr mcsst/noaa satellite data to estimate a matrix, which will be then matched to another matrix originated for the world ocean atlas data set. all the temperature data are thus arranged on a fivedegree latitude-longitude grid, only for the surface of the oceans (zero meter) with a final spatial resolution of 500 km2. temperature, wind, ndff, and ekman pumping are included in a multiple regression analysis to define the relative importance of each one of these variables in the physical processes at the surface of the indian ocean. the ndff data set is based on coads (comprehensive ocean-atmosphere data set) and obtained from oberhuber (1988). it is originally organized in a 2° × 2° grid and averaged in 5° x 5° grid for this research. the ndff represents the balance between precipitation and evaporation, with a unit of mm. the wind data is obtained from the florida state university (fsu) wind stress data set for the tropical indian ocean in a 1° ×1° grid. the data is averaged in a 5° x 5° grid. the fsu data set is based on coads and ncei (national centers for environmental information). the wind stress is presented as pseudo-stress wind, which is the wind component multiplied by the wind speed, at a reference height of 10 m. for temperature, the twelve-monthly mean values are subjected to a harmonic analysis to determine the two harmonic components, one having a period of one year and the other a period of six months. the process aims to examine how the two components contribute and interact to produce the more complex cycle of the observed and modelled data. the harmonic terms are considered to be stationary, as the averages and corresponding autocorrelations are not expected to vary significantly in a long period of time. the fourier series for the stationary time series of temperature can then be expressed symbolically as a cosine function, in which the first and second harmonic terms are multiplied by the maximum amplitude of the variable and added to the mean annual parameter. the modelled data can thus be expressed as: o(t) = ō + a × cos (ωa ×t-a) + b × cos (ωs × t b) (katznelson. 2004), where: o(t) = observed value as a function of time when t is time in days, since the start of an arbitrary year, ō = mean value over the period analyzed, a = amplitude of the annual harmonic component, b = amplitude of the semiannual harmonic component, a = phase-lag of the annual component, b = phase-lag of the semiannual component, ωa = angular frequency (2π/365.25), which represents the increment in phase per unit time, t, of the annual harmonic component, ωs = ditto for the semiannual component but (2π/182.62), (ωa × t-a) = phase of the annual harmonic, (ωs ×t b) = phase of the semiannual harmonic. values of o, a, b, a and b are to be determined by analysis. they represent the average annual temperature and are a positive constant. the amplitude value (a or b) is also a positive constant that represents the amplitude of the motion and so its maximum departure from the mean value as the term oscillates within a range of + or -. phase-lag is expressed in degrees, with 30° per month for the first harmonic and 60° per month for the second. phase-zero implies that a maximum occurs at the mid of january. it expresses the average time when the maximum of sst occurs within the annual cycle. for example, the phase of 120° corresponds to the mid of april, 150° to the mid of may, and 270° to the mid of october. the sum of square residual is defined as: ssres = ∑ (y y’)2 and represents the sum of squared differences between observed (y) and the predicted values (y’), these differences representing the small inadequacies of the fit. r-squared describes how well the temperature is explained by the combination of the independent variables as a single unit and is expressed as percentage. r squared ranges between 0 and 1 and is represented by r2=1-ssres/ss total, where ssres is the sum of the squares of the residuals and ss total is the total sum of the errors. the closer to one the better. therefore, r-squared is the percent of variance explained by the temperature model. in other words, r-squared is the fraction by which the variance of the errors is less than the variance of the dependent variable. 3. results the 5° x 5° grid average sst contour plots during the sw monsoon (july, august, and september) can be seen in figure 2. in the northern indian ocean, the isotherms are mainly meridional along the western boundary, within the range of the somali current which runs from south to north during the sw monsoon. figure 2. average sst (°c) based on a 5° x 5° degrees grid for the sw monsoon (july, august, and september). data source: world ocean atlas data set (1994). a zonal distribution of temperature is observed in the southern indian ocean, and a tongue of high temperatures above 28 °c is present along the equator and east of 60° e. the influence of the tongue extends northwards into the bay of bengal and west of india, as can be seen from the 28 °c and 28.5 °c isotherms. high temperatures, close to 29 °c, are observed in the north bay of bengal. the upwelling areas along the somali and arabian coasts are marked by temperatures below 26 °c. the isotherms in the western region of the southern indian ocean, around madagascar, deflect slightly northwards and southward because of the south equatorial current, which runs from east to west and splits into two branches in this area. figure 3. superposition of the main currents and isotherms in sw monsoon, with reference to august. 234 oldemar de oliveira carvalho junior figure 3 is a graphic representation that relates figure 1 and figure 2, superimposing the regional circulation of the indian ocean with the distribution of isotherms. the objective is to provide a synoptic view of the study area, highlighting the importance of sst in the physical processes related to the monsoon regime. as can be seen, the isotherms reliably represent most of the oceanographic features in the study area, as the south equatorial current, which can be associated with the intensification of the zonal temperature gradient in the region. warm water from the pacific into the indian ocean is highlighted in yellow. the distribution of sst contours on a 5° x 5° grid average for january, february, and march, during the ne monsoon, can be observed in figure 4. during this time of the year, the circulation of the area is dominated by the north and south equatorial currents, as a consequence of the westerly winds blowing along the equatorial belt. since the northeast trades are strong because of the high-pressure system established in the asiatic region, temperature contours tend to be meridional (south-northeast direction) north of the equator. figure 4. average sst (°c) based on a 5° x 5° grid for the ne monsoon (january, february and march). data source: world ocean atlas data set (1994). figure 5, like figure 3, is a graphic representation that associates figure 1 and figure 4, superimposing the regional circulation of the indian ocean with the distribution of isotherms during the ne monsoon. it is also clear to see the warm water tongue coming from the pacific, shown in yellow. figure 5. superposition of the main currents and isotherms in ne monsoon, with reference to the month of february. comparing figures 3 and 5, it is clear that the main features, such as the south equatorial current and the meridional isotherms in the persian gulf, are present. the difference is in the intensity. temperature along the west coast of the indian ocean during the ne monsoon is more uniform, varying from 26 to 28 °c, and no strong temperature gradients are observed in the region along the somali and arabian coasts. below 10° s, the isotherms deflect slightly southward in the coastal areas of the southern indian ocean, madagascar, and the australian coast. there are two main currents in this region: the south equatorial current and the subtropical gyre. these result in an anticyclonic circulation with a mainly westward direction between 60° e and 100° e. to the west, the flow is southward and along the madagascar coast. in the eastern indian ocean, close to australia, the surface circulation is initially southward, before switching to a westward direction. the influence of these major circulation systems is seen in the curved shape of the temperature contours. the temperature variation through an average year, as expressed through the amplitude of the annual (first) and semiannual (second) harmonic, can be seen in figure 6. the smallest amplitude values of the annual and semiannual components (below 0.5 °c) are found along the equator, especially in the eastern part. here, the sst exceeds 28 °c throughout the year (figures 6a and 6c). the largest amplitude values are found in the western indian ocean, the arabian sea, the bay of bengal, and south of 20° s. figures 6b and 6d show the phase lags for the first and second harmonics of the sst. phase-lag is expressed in degrees and increments at 30° per month for the first harmonic and 60° per month for the second, where phase is zero it implies that a maximum occurs in mid-january. it expresses the average time when the maximum of sst occurs within the annual cycle. for example, phase of 120° corresponds to mid of april, 150° mid of may and 270° mid of october. they indicate the time of the year when the maximum amplitude occurs. figure 6. (a) distribution of sst amplitude for the first harmonic. (b) distribution of sst phase-lag for the first harmonic. (c) distribution of sst amplitude for the second harmonic. (d) distribution of sst phase-lag for the second harmonic. annual and semiannual amplitudes of sst are expressed in °c. amplitude and phase of temperature are calculated from the world ocean atlas data set (1994). figure 7a shows the observed data and the two harmonic components fit to the observations. figure 7b shows the two individual harmonic components and their interaction through the year in the arabian sea. the annual component is seen to have a maximum in july (sw monsoon) and a minimum in january (ne monsoon) which suggests a connection with a hot summer and a cold winter, likely due to proximity to a continental land mass in these latitudes. compared with the annual component, the amplitude of the semiannual component is smaller, with two maxima in may and october and two minima in february and august (figure 7b). a similar pattern is observed in the bay of bengal, but the two maxima (april and october) and the two minima (january and july) occur earlier than in the arabian sea. the overall fit is good, with a residual sum of squares of 0.39. in fact, the residual errors between observed and harmonic temperature are small, with a maximum of 0.32ºc during june (figure 7c). the small magnitude of these residuals suggests that the temperature variability during this period and for this particular area can be explained reasonably well by just the two harmonic terms. the sum of square residual can be defined as ssres = ∑ (y y’)2 and represents the sum of squared differences between observed (y) and the predicted values (y’). these differences represent the small inadequacies of the fit. mcphaden (1982) applied residual variance from regression techniques to analyze equatorial waves in the indian ocean. another study based on correlations between amplitudes and phases used a wind-driven model for the north pacific (white, 1978). 235role of ocean-atmosphere interface in annual and semiannual sst cycles in the indian ocean figure 8 compares the observed and two components harmonic fit of the somali coast (52.5º e and 7.5º n) sea surface temperature (sst), the annual and semiannual components, and the residual errors. the highest annual component occurs in march/april. during this period, the area is affected by the north equatorial current and the anticyclonic flow, which are responsible for the input and re-circulation of warm equatorial waters into the arabian sea. the minimum of the annual component occurs in september/october, when the somali current represents the strong northward flow observed along the western arabian sea (figure 8a). the two peaks of the semiannual component occur in april/may and october/november, transition months between the monsoons, when wind stress is weaker and less evaporative cooling and mixing are expected (figure 8b). these result in the high temperature observed during april and may. the two minima occur in january/february and july/august. the first is associated with the peak of the ne monsoon and a slight winter cooling. the increase in wind speed, which results in large evaporative cooling and mixing, can explain the secondary low peak of temperature during this period. the second minimum that occurs during july/august can be explained by the strong wind stress and upwelling that occurs during this time of the year. since the annual and semiannual components are acting almost in conjunction, it is expected that the wind monsoon drive circulation is also contributing to this peak of low temperature. the semiannual (-1.4) and annual (-0.86) signals are both strong during august. the low residual errors (maximum of 0.38 °c in january) show a good agreement between observed and the two components harmonic fit, with a residual sum of squares of 0.4 (figure 8c). figure 9a compares the observed sst with the two components harmonic fit at a position on the equator, 62.5º e and 2.5º s. it is possible to see how the harmonic contribution due to the interaction between the annual and semiannual components affects the two harmonic components fit (figure 9b). the maximum of the annual component occurs in april and the minimum in october, which suggests that the variability of solar radiation is connected to this. the two maxima of the semiannual component are observed for april/may and october/november, the transition between the monsoons. the low residual errors indicate a good agreement between observed and the two components harmonic fit (figure 9c). figure 7: (a) observed and two components fit to observations (°c). (b) first and second components of the harmonic analysis. (c) residual plot between observed and harmonic temperature (°c). time (months) in which 13 is equal to 1 and corresponds to mid-january of the following year. figure 8. (a) observed and two components fit to observations (°c). (b) first and second components of the harmonic analysis. (c) residual plot between observed and harmonic temperature (°c). time (months) in which 13 is equal to 1 and corresponds to mid-january of the following year. figure 9. (a) observed and two components fit to observations (°c). (b) first and second components of the harmonic analysis. (c) residual plot between observed and harmonic temperature (°c). time (months) in which 13 is equal to 1 and corresponds to mid-january of the following year. 236 oldemar de oliveira carvalho junior the final regression equations for temperature variability in the arabian sea show that temperature is mainly affected by the wx, ekman pumping and ndff (table 1). components of wind stress, wind magnitude, ekman pumping and ndff are impacting the temperature, but the wx is a constant presence in all of the equations. table 1. predicted equations for the eastern arabian sea. location equation adjusted r2 67.5°e/12.5°n t=29.65+0.15ekman-0.026ndff0.037wmg+0.072wx+0.106wy 0.77 72.5°e/12.5°n t=30.54+0.16ekman0.03ndff+0.12wx+0.26wy 0.67 62.5°e/7.5°n t=27.65+0.05ekman-0.04ndff0.05wmg+0.08wx 0.74 wx = zonal component of pseudo-stress wind (m2s-2), wy = meridional component of pseudo-stress wind (m2s-2), ndff = net-down-fresh-waterflux (mm), ekman pumping = (-) downward/(+) upward (x10-3 kg.m2.s-1), t = temperature (°c), wmg = wind magnitude (ms-1). the final regression equations and their respective adjusted r-squared values for the bay of bengal are shown in table 2. for most of the areas of the bay of bengal, temperature can be explained by the entered independent variables. the meridional component of the wind stress (wy) is one of the main variables that affects temperature. all final regression equations have larger values of beta coefficients for this variable. it is followed by the zonal components of the wind stress (wx) and ndff, in order of importance. table 2. predicted equations for temperature at the bay of bengal. location equation adjusted r2 82.5°e/17.5°n t=26.85+0.013ndff-0.025wmg0.083wx+0.097wy 0.95 87.5°e/17.5°n t=27.24+0.005ndff0.096wx+0.116wy 0.83 92.5°e/17.5°n t=28.35-0.036wmg0.074wx+0.116wy 0.71 82.5°e/12.5°n t=25.890.07ekman+0.02ndff+0.06wmg0.13wx+0.17wy 0.95 92.5°e/12.5°n t=29.12-0.04wmg+0.04wy 0.61 wx = zonal component of pseudo-stress wind (m2s-2), wy = meridional component of pseudo-stress wind (m2s-2), ndff = net-down-fresh-water-flux (mm), ekman pumping = (-) downward/(+) upward (x10-3 kg.m2.s-1), t = temperature (°c), wmg = wind magnitude (ms-1). the final regression equations for temperature at the equatorial region, with an adjusted r-squared greater than 0.6, can be seen in table 3. the temperature in these areas can be explained by the independent variables. the magnitude of the wind and ndff are the main variables responsible for the temperature variability. table 3. predicted equations for the equatorial region (60° e to 100° e and 10° s to 5° n). location equation adjusted r2 77.5°e/2.5°s t=28.75-0.007ndff-0.04wmg 0.78 77.5°e/7.5°s t=28.16-0.02ekman-0.006ndff0.031wmg 0.81 92.5s/2.5s t=32.35-0.015ndff-0.069wmg 0.71 87.5°e/7.5°s t=28.23-0.02wmg 0.67 wx = zonal component of pseudo-stress wind (m2s-2), wy = meridional component of pseudo-stress wind (m2s-2), ndff = net-down-fresh-water-flux (mm), ekman pumping = (-) downward/(+) upward (x10-3 kg.m2.s-1), t = temperature (°c) and wmg = wind magnitude (ms-1). a set of final regression equations for temperature in the southern tropical indian ocean region with an adjusted r-squared larger than 0.6 is shown in table 4. these equations demonstrate that the variables affecting the temperature vary significantly from one location to another. temperature can be predicted for most of the locations, but the independent variables vary significantly from one location to another. for the locations at 57.5° e/17.5° s, 72.5° e/22.5° s, 87.5° e/17.5° s and 57.5° e/27.5° s, temperature can be predicted from only one independent variable at each location, ndff, ekman pumping, wy, and ekman pumping, respectively. the largest beta coefficients are obtained for wy, ekman pumping and ndff. table 4. predicted equations for the south tropical indian ocean (30° s to 10° s). location equation adjusted r2 57.5°e/17.5°s t=26.35+0.02ndff 0.73 72.5°e/22.5°s t=26.92+0.51ekman 0.79 87.5°e/17.5°s t=29.63-0.12wy 0.90 97.5e/22.5s t=28.45+0.24ekman0.13wx-0.28wy 0.71 57.5e/27.5s t=25.61+0.41ekman 0.76 82.5e/27.5s t=15.69-0.059ndff0.12wx-0.19wy 0.81 107.5°e/17.5°s t=34.00+0.047ndff0.08wy 0.75 wx = zonal component of pseudo-stress wind (m2s-2), wy = meridional component of pseudo-stress wind (m2s-2), ndff = net-down-fresh-waterflux (mm), ekman pumping = (-) downward/(+) upward (x10-3 kg.m2.s-1), t = temperature (°c) and wmg = wind magnitude (ms-1). 4. discussion the amplitudes of the annual component increase with latitude in the north indian ocean and reach maximum values to the north of the arabian sea and bay of bengal. this distribution reflects the response of the ocean to the solar radiation, in which amplitudes increase away from the equator. the reversal of the monsoons, associated with land mass distribution also results in a large contribution to these large amplitudes. the latest occurrence of the annual maximum temperature is observed in the northernmost parts of the arabian sea and bay of bengal, where the maximum of the annual occurs in july. the importance of the solar path is clear in the annual component of temperature, but it also seems that the boundaries have an influence on winter temperatures north of the arabian sea and bay of bengal. it is believed that larger amplitudes result from colder winters over adjacent continental land, due to the contrast between cold air and warmer sea surface water. the maximum semiannual amplitude observed for may and october are associated with the transition months between the sw monsoon and the ne monsoon. during may and october, wind intensity is expected to decrease, which would reduce evaporative cooling and water mixing. this could explain the raise in temperature during these periods. it also seems that the two minima, observed during february and august, can be linked to the peaks of the ne monsoon and sw monsoon, respectively. winter cooling (in the case of the ne monsoon) associated with maximum wind during these periods can result in large evaporative cooling and stirring of the water, which can decrease the sst. this is in line with khan et al. (2021) when studying the monthly cycle of sst in the north arabian sea for 2016/2017, from satellite and argo datasets. the interaction between the annual and the semiannual component of sst results in a depression of the temperature from mid-july to mid-september when the large annual peak of the annual component (summer leading 237role of ocean-atmosphere interface in annual and semiannual sst cycles in the indian ocean to large solar radiation) act in opposition to the small negative peaks of the semiannual component (wind stress leading to large evaporation and mixing). the pronounced low temperature observed during february could be due to the annual component (low solar radiation during winter) working together with the semiannual component (strong wind, leading to large evaporation and mixing). in the indian ocean, the amplitude of the semiannual component can be comparable to the annual component. in some regions, such as the somali (47.5º e and 7.5º n) and arabian coast (52.5º e and 12.5º n), the semiannual component is considerable larger than the first harmonic with maximum amplitudes around 1.6 °c. the strong semiannual component along the somali and arabian coasts is thought to be associated with coastal upwelling derived from the strong monsoonal wind field acting upon a left-hand coastline from june to october. this semiannual variability of temperature is influenced by wind, since large values of semiannual periodicity also occurs in the western arabian sea (unnikrishnan et al., 1997; khan et al., 2021). previous works, such as that of levitus (1987) and khan et al. (2021) have already demonstrated semiannual cycles of temperature in the indian ocean. these cycles could be related to evaporative cooling and upwelling associated with the monsoons. the two maximum values of the semiannual component, seen in april/ may and october/november, are a result of the weaker wind stress during these periods, when less evaporative cooling and mixing are expected. these result in the high temperature observed during april and may. the first semiannual minima, observed in january/february and july/august, is associated with the peak of the ne monsoon and a slight winter cooling. the increase in wind magnitude, which results in large evaporative cooling and mixing, can explain the secondary low peak of temperature during this period. the second minimum that occurs during july and august can be explained by the strong wind stress and upwelling that occur during this time of the year. it is expected that the wind monsoon drive circulation is also contributing to this peak of low temperature, since the annual and semiannual components are acting almost in conjunction. one of the main characteristics of the equatorial indian ocean is its lack of upwelling. as a result of the dominance of the meridional orientation of the wind, ekman transport does not result in divergence along the equatorial region and, therefore, no upwelling occurs (tomczak and godfrey, 1994). however, the overall pattern with a secondary low during august remains, even where no upwelling is expected. the low will be located along the equatorial region, away from the coast and west of 70º e. the two maxima of the semiannual component, observed in april/ may and october/november, can be associated with weaker winds during these months, which result in less evaporative cooling and mixing. the larger temperature value for april can be attributed to the fact that the annual component acts with the semiannual component, and the opposite for october/ november. for example, during october and november, the annual (negative) and semiannual (positive) components act in opposition. the semiannual component (0.37 for october and november) is similar in magnitude to the annual but opposite in sign (-0.58 for october and -0.51 for november) and results in the secondary high-temperature peak observed during these months. the annual and semi-annual components, acting together from march to june, enhance the temperature during this period, with a maximum temperature peak during april. the distribution of the residual between the observed and the two harmonic fit reveals that the harmonic model is meaningful, as the maximum residuals are observed from february to may and are below 0.12 °c. in fact, only to the west of 70° e along the equator is the second harmonic temperature larger than 0.5 °c, with the two maxima occurring during april and november. the amplitude becomes weak eastward, along the equator, and, therefore, the seasonal variations are considered to be insignificant in this area. the low amplitude values east of 70º e are a result of an expected low rate of exchange between ocean and atmosphere near the equator, for example the lack of wind when compared to the higher latitudes. in the southern hemisphere, the maximum amplitude of temperature of the annual component occurs late january along the south coast of africa. the annual component increases eastwards, reaching a maximum during early march in australia. the annual component increases gradually towards the north, following the solar path, as shown by the phase-lag distribution. a gradual phase-lag shift (30° to 130°) from the southern to northern hemisphere is observed in the tropical region from 10° s. 5. conclusion large annual variations in temperature were observed in the north of the arabian sea and bay of bengal, in the western equatorial region and in the southern tropical indian ocean. large semiannual fluctuations in temperature were observed along the western boundary of the indian ocean, from 10° s to 25° n. based on multiple regression analysis, it was possible to identify and quantify the variability of the temperature. for example, the temperature within upwelling regions along the somali and arabian coasts could be explained by the variability of the components of the pseudostress wind, wind magnitude and ekman pumping, with an adjusted r-squared ranging from 0.64 to 0.84. the annual component, overall, is related to large-scale features of the seasonal solar radiation input and the seasonal monsoons. basic features associated with the monsoons, such as wind-driven advection, can have a significant influence on the annual term. the semiannual component is influenced by local and regional factors, which add to the basic pattern defined by the annual term. these local and regional influences are the result of different processes, such as upwelling and the non-directional effect of wind stress (e.g., evaporation, precipitation, and mixing). this 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toc459375912 toc4285427272 toc428542730 toc428542733 toc428542736 _goback 15 many practical problems encountered in quantitative oriented disciplines entail finding the best approximate solution to an over determined system of linear equations. in this study, it is investigated the usage of different regression methods as a theoretical, practical and correct estimation tool in order to obtain the best empirical relationship between surface wave magnitude and rupture length for turkey earthquakes. for this purpose, a detailed comparison is made among four different regression norms: (1) least squares, (2) least sum of absolute deviations, (3) total least squares or orthogonal and, (4) robust regressions. in order to assess the quality of the fit in a linear regression and to select the best empirical relationship for data sets, the correlation coefficient as a quite simple and very practicable tool is used. a list of all earthquakes where the surface wave magnitude (ms) and surface rupture length (l) are available is compiled. in order to estimate the empirical relationships between these parameters for turkey earthquakes, loglinear fit is used and following equations are derived from different norms: ms = 1.02* logl + 5.18, for l2 norm regression (r2=0.71), ms = 1.15* logl + 4.98, for l1 norm regression (r2=0.92), ms = 1.04* logl + 5.15, for robust regression (r2=0.75), ms = 1.25* logl + 4.86, for orthogonal regression (r2=0.68), consequently, the empirical equation given by the least sum of absolute deviations regression as ms = 1.15* logl + 4.98 with a strong correlation coefficient (r2=0.92) can be thought as more suitable and more reliable for turkey earthquakes. also, local differences in rupture length for a given magnitude can be interpreted in terms of local variation in geologic and seismic efficiencies. furthermore, this result suggests that seismic efficiency in a region is dependent on rupture length or magnitude. muchos problemas prácticos encontrados en las disciplinas de orientación cuantitativa implican encontrar la mejor solución aproximada para un sistema determinado de ecuaciones lineales. en este estudio se investiga el uso de diferentes métodos de regresión tanto teóricos como prácticos y la herramienta de estimación correcta con el fin de obtener la mejor relación empírica entre la magnitud de onda superficial y la ruptura de longitud para los terremotos en turquía. para este propósito se hace una comparación detallada a partir de cuatro normas diferentes de regresión: (1) mínimos cuadrados, (2) mínimas desviaciones absolutas, (3) mínimos cuadrados totales y (4) regresiones robustas. con el fin de evaluar la regresión lineal adecuada y seleccionar la mejor relación empírica de grupos de datos empíricos, la correlación de coeficiente es una herramienta simple y muy práctica. se compiló una lista de todos los terremotos donde se cuenta con la magnitud de onda superficial (ms) y la ruptura de longitud superficial (l). con el fin de determinar las relaciones empíricas entre estos parámetros para los terremotos de turquía, se utiliza la regresión lineal adecuada y las siguientes ecuaciones se derivan de diferentes reglas. ms = 1.02* logl + 5.18, para la regresión normal l2 (r2=0.71), earth sciences research journal eart sci. res. j. vol. 18, no. 1 (june, 2014): 15 26 abstract resumen key words: turkey, correlation coefficient, l1, l2, orthogonal, robust regression palabras clave: turquía; coeficiente de correlación; l1, l2; regresión ortogonal; regresión robusta. a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes serkan öztürk1,* 1gümüşhane university, department of geophysics, tr-29100, gümüşhane, turkey *corresponding author: serkan öztürk gümüşhane university, department of geophysics, tr-29100, gümüşhane / turkey email address: serkanozturk@gumushane.edu.tr phone: +90 456 233 74 25 / 26 fax: +90 456 233 10 75 seismology record manuscript received: 27/01/2014 accepted for publication: 01/06/2014 16 serkan öztürk introduction the regression is one of the most used tools in establishing the relationship between a response and an explanatory variable in applied statistics. an effective and accurate curve fitting method plays an important role and serves as a basic module in many scientific and engineering fields. there exists vast literature about the regression problem in mathematics, statistics and computer science and there are many statistical regression methods for this purpose. also, many software packages have allowed the practitioners to use the regression estimators to easily fit the data sets in order to determine whether or not there exists a relationship between hypothesized predictor or independent variables and some response or dependent variable (giloni and padberg, 2002). there are a number of regression methods in the literature such as l2 norm or least squares regression (cadzow, 2002), l1 norm or least sum of absolute deviations regression (giloni and padberg, 2002), total least squares or orthogonal regression (carroll and ruppert, 1996), robust regression (huber, 1964), principle components regression (pcr, maronna, 2005), geometric mean regression (gmr, leng et al. 2007), least trimmed squares regression (lts, rousseeuw and leroy, 1987) and covariate adjusted regression (car, şentürk and nguyen, 2006). the primary objective of this paper is to make an investigation on the usage of the best approximate regression solution to linear equations as a theoretical and practical estimation tool. for this purpose, a study on the comparison among the first four regression methods above is made in order to find the most suitable standard statistical model. the other methods such as pcr, gmr, lts or car are not discussed since such kind of methods are described for more specific fields and they have not common use in geophysical investigations. finally, these four different regression models are applied to turkey earthquakes in order to determine more reliable and an up to date empirical relationship between surface wave magnitude and rupture length. brief description of the algorithms for regression techniques there are different distance functions or metrics which can be utilized to perform a linear regression. therefore, the original problem is categorized under the class of mathematical problems. in order to solve these problems, a remarkably wide range of mathematical techniques can be found in the literature. however, the aim of this study is not to discuss the mathematical details of the regression methods. because the main goal of this work is to test different regression methods and to find the most suitable empirical relationship between surface wave magnitude and rupture length for turkey earthquakes, a brief and general reviews of four different regression approaches are given in this section as well as their fundamental statistical properties and qualities of fits. the linear regression problem is certainly one of the most important data analysis situations. in order to formulate a linear regression model, it is assumed that there are n measurements on the dependent variable y and some number p≥1 of independent variables χ1,……., χp of each one for which it is known n values as well. it is defined by giloni et al., (2006) as follows: (1) where y є r is a vector of n observations and x is an n x p matrix of real frequently referred to as the design matrix. furthermore, x1,…….,xp are column vectors with n components and x1,…….,xn are row vectors with p components corresponding to the columns and rows of x, respectively. the statistical (or hypothesized) linear regression model is given: y = xß + ε (2) where ßt = (ß1,........,ß2) is the vector of parameters of the linear model and ε t = (ε1,........,ε2) a vector of n random variables corresponding to the error terms in the asserted relationship. an upper index t denotes “transposition” of a vector or matrix. in the statistical model, the dependent variable y, thus, is a random variable for which we obtain measurements that contain some “noise” or measurement errors that are captured in the error terms e. on the other hand, for the numerical problem that we are facing, it can be written: y = xß + r (3) where given some arbitrarily fixed parameter vector β, the components ri of the vector r t = (r1,........,r2) are the residuals that result, given the observations y, a fixed design matrix x, and the chosen vector ß є rp . the residuals r are thus in terms of the statistical model, realizations of the random error terms e given the particular observations y and parameter settings β. given y and x, the general objective in linear regression is to find parameter settings ß є rp such that some appropriate measure of the dispersion of the resulting residuals r є rn is as small as possible (giloni and padberg, 2002). l2 norm or least squares regression least squares regression (l2 norm) is the most well-known and the most basic form of the least squares optimization problems. so, it has found many applications in mathematics and statistical data analyses as well as the other scientific fields. this statistical linear regression model has been studied intensively for well over 200 years now and under the assumption of a normal (or gaussian or exponential) distribution of the error terms e an impressive statistical apparatus has been created to assess the goodness of fit, the quality of individual and/or subsets of the regression coefficients, as well as other statistical properties of the linear regression model. if and when the error terms in the linear regression model do indeed follow a normal distribution, then the least squares regression estimates are “the best estimators” under most acceptable criteria. thus, this approach is ms = 1.15* logl + 4.98, para la regresión normal l1 (r2=0.92), ms = 1.04* logl + 5.15, para la regresión robusta (r2=0.75), ms = 1.25* logl + 4.86, para la regresión ortogonal (r2=0.68). por consiguiente, la ecuación empírica dada por la regresión de desviaciones mínimas absolutas es con un fuerte coeficiente de correlación (r2=0.92) que sería más apropiado y más confiable para los terremotos de turquía. también, las diferencias locales en la ruptura de longitud para una magnitud dada puede ser interpretada en términos de eficiencia sísmica y geológica en la variación local. además, el resultado sugiere que la eficiencia sísmicaen una región depende de la ruptura de longitud o de la magnitud. 17a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes particularly helpful in all those situations involving the study of large data sets, handling large samples with a consistent numbers of outliers (e.g., cadzow, 2002; giloni and padberg, 2002). l1 norm or least sum of absolute deviations regression the least squares linear regression estimator is well-known to be highly sensitive to outliers in the data and as a result many more robust estimators have been proposed as alternatives. one of the earliest proposals was the leastsum of absolute deviations (l1) regression, where the regression coefficients are estimated through minimization of the sum of the absolute values of the residuals. l1 regression is called in the literature by a multitude of names that all mean the same: lad (least absolute deviation), lae (least absolute error), lav (least absolute value), lar (least absolute residual), lsad (least sum of absolute deviations), mad (minimum absolute deviation), msae (minimum sum of absolute errors), and so forth. an important advantage of l1 regression over l2 regression is its robustness. for the unconstrained problem it is well known that the l1 regression estimator can resist a few large errors in the data y (shi and lukas, 2005). the use of l1 norm criterion is appropriate when it is suspected that a small portion of the data being analyzed is unreliable (i.e., contains data outliers). the l1 norm criterion has the capability of effectively ignoring a few bad data points while emphasizing the majority of data points which more properly reflect the true nature of the data (cadzow, 2002). one can see huber (1987) for further discussion of the l1 approach in robust estimation. total least squares or orthogonal regression one of the most widely known techniques for errors-in-variables estimation in the simple linear regression model is orthogonal regression, also sometimes called as the functional maximum likelihood estimator under the constraint of known error variance ratio. in ordinary linear regression, the goal is to minimize the sum of the squared vertical distances between the y data values and the corresponding y values on the fitted line. in orthogonal regression the goal is to minimize the orthogonal (perpendicular) distances from the data points to the fitted line. this well-known orthogonal regression estimator is an old method and derived in many studies such as weisberg (1985), carroll and ruppert (1996) and leng et al., (2007). the use of orthogonal regression must include a careful assessment of equation error, and not merely the usual estimation of the ratio of measurement error variance. thus, when its assumptions hold, the orthogonal regression is a perfectly justifiable method of estimation. however it often lends itself to misuse by the unwary as a method, because orthogonal regression does not take equation error into account. robust regression the most serious problem with least squares regression is nonrobustness to outliers. in particular, if you have one extremely bad data point, it will have a strong influence on the solution because outliers can have a large influence on the regression parameters. a simple remedy is to iteratively discard the worst-fitting data point, and re-compute the least squares fit to the remaining data. another approach, termed robust regression, is to employ a fitting criterion that is not as vulnerable as least squares to unusual data. the most common general method of robust regression is m-estimation, introduced by huber (1964). nonlinear regression models play an important role in many fields. for estimating the parameters of a nonlinear model, the classical least squares method is commonly used in many cases. however, it is well known that these classical methods are often very sensitive to outliers and other departures from the underlying distribution. most robust developments on the estimation of regression models are based on the generalizations of least squares or maximum likelihood methods. robust regression procedure is less influenced by extreme values. in assessing the behavior of robust regression estimates, however, the techniques of small sample asymptotes can be very useful. the literature on the robust regression dates back to huber (1964). some of the robust techniques are also discussed in many studies such as huber (1981), sinha et al., (2003) and abdelkader et al., (2010). determination of correlation coefficient for regressions one of the most important problems in regression analyses is the selection of an appropriate probability distribution for a given data set, which can give reasonably accurate and robust estimates. as shown in the literature, there is no certain rule for selecting the appropriate distribution or parameter estimation techniques for a given data set and various distributions should be applied and the best-fitting model should be selected. thus, choosing the bestfitting model is quite an important procedure in frequency analysis. in most cases, the selection of an appropriate distribution is based on goodness-of-fit assessment. the goodness-of-fit technique can be described as the method for examining how well sample data agree with an assumed probability distribution as its population. several goodness-of-fit tests are developed and used in engineering decision-making. selection criteria among those methods, the determination of correlation coefficient (r2 or sometimes r is used) has been known as a powerful and conceptually simple method. although the r2 is solely based on the covariance penalty, it plays an important role in model fit assessment. it should certainly not be used as a unique model fit assessor, but can provide a reasonable and rapid model fit indication (heo et al. 2008). more traditionally, for the linear regression model, a very simple fit indicator is, however, given by the determination of correlation coefficient r2. if x is considered as random, it can be defined as the parameter that is the squared correlation between y and the best linear combination of the x. the r2 is usually presented as the quantity that estimates the percentage of variance of the response variable explained by linear relationship with the explanatory variables. it is computed by means of the ratio: (4) where ess, tss and rss are the explained, total and residual sum of squares, respectively. when there is an intercept term in the linear model, this determination of correlation coefficient is actually equal to the square of the correlation coefficient between yi and ŷi: (5) where y and ŷ denote the mean values of the observations yi and the fitted quantiles ŷi, respectively. equation (5) has a nice interpretation in that r2 measures the goodness of fit of the regression model by its ability to predict the response variable, ability measured by the correlation. the correlation coefficient is location and scale invariant, and statistically independent of the mean, y, and standard deviation. essentially, r2 measures the linearity of the probability plot, providing a quantitative assessment of fit and it is assumed that the observations could have been drawn from the fitted distribution if the value of r2 is close to 1.0 (heo et al., 2008). some characteristics of earthquakes used in the analysis and main geological environments around these earthquakes main goal of this study is to derive a relationship between the surface wave magnitude and rupture length for great earthquakes occurred in and around turkey. for this purpose, the earthquakes whose surface wave magnitude are greater than 5.5 and occurred between 1905 and 2005 were used. magnitude ranges vary from 5.5 to 8.0 and rupture length changes between 3 and 350 km for 63 earthquakes. these events and their detailed information can be found from different studies in the literature (e.g., ambreseys and jackson, 1998; nalbant et al. 1998; dewey, 1976). the epicenters of all earthquakes used in the analysis and whole tectonics are plotted in figure 1. in addition, some principle characteristics of several earthquakes and geological 18 serkan öztürk environments around these earthquakes are briefly discussed in this section. details of geological structure for whole epicenter regions are provided from the general directorate of mineral research & exploration (mta, http:// www.mta.gov.tr/v2.0/daire-baskanliklari/jed/index.php?id=500bas). figure 1. active fault map of turkey. the locations of earthquakes used in this study were also shown with different symbols. tectonic structures were modified from şaroğlu et al., (1992), bozkurt (2001), and ulusay et al., (2004). names of numbered faults or fault zones associated with the earthquake locations: 1-north east anatolian fault zone, 2-aşkale fault, 3-çaldıran fault, 4-başkale fault, 5-pülümür fault, 6-malatya fault, 7-ecemiş fault, 8-tuz lake fault, 9-ezinepazarı fault, 10-düzce fault, 11-eskişehir-i̇nönü-dodurga and kaymaz faults, 12-kütahya graben, 13-sultandağı fault, 14-beyşehir-tatarlı and kumdanlı faults,15-burdur fault zone, 16-gediz graben, 17-aliağa and dumlupınar faults, 18alaşehir graben, 19-simav graben, 20-soma graben, 21-zeytindağ-bergama faults, 22-etili fault, 23-yenice-gönen and sarıköy faults, 24-saros-gaziköy fault. the 1912 marmara (mürefte şarköy) earthquake was located between the gulf of saros and the sea of marmara. this earthquake was followed by several large aftershocks (august 10, 1912, ms = 6.3; september 13, 1912, ms = 6.9; september 27, 1912, ms = 6.6) to the west of the main shock. the surface rupture pattern of this earthquake was reported as a complex with a substantial right-lateral strike-slip component. nalbant et al., (1998) defined three fault segments with a vertical dip: the eastern one and middle ones have a reverse component, while the western one, bounding the gulf of saros, is pure strike slip. geological formation around the related earthquakes is mainly composed of clastic and carbonate rocks. another earthquake which is occurred in the same zone is the 1935 marmara earthquake. this earthquake occurred on the southwest extremity of the sea of marmara, north of the marmara islands. nalbant et al., (1998) modeled this earthquake as eastwest striking normal faulting dipping to north. they adopted a dip of 45° and mean displacement of 0.85 m. geological structure around this earthquake epicenter includes granitoid, schist, phillite, marble, metabasic rocks etc. there are two earthquakes in burdur region. the first one occurred in 1914 and the other one in 1971. these two earthquakes were located approximately in the same coordinates. the first earthquake occurred on the northeast segment of burdur fault and the other occurred on the southwest of burdur lake. focal mechanisms of the 1914 and 1971 burdur earthquakes are very similar. fault plane solution of the 1914 earthquake indicates normal faulting with a dip of 35° and a mean displacement of 1.6 m. focal mechanism of the 1971 earthquake shows pure normal faulting with a dip of 35° and a displacement of 0.7 m (taymaz et al. 1991). main geological structure around these epicenters includes undifferentiated quaternary, continental clastic rocks and undifferentiated continental clastic rocks. the 1919 soma (manisa) earthquake, in the south of the north anatolian fault, occurred in the eastern part of the bakırçay normal fault zone which also ruptured during the 1939 bergama earthquake. as east-west normal faulting and right-lateral strike-slip faulting are found in the area, nalbant et al., (1998) modeled this earthquake as right lateral normal fault dipping to north. they assumed a dip of 45° and mean displacement of 1.4 m for this earthquake. geological structure around this earthquake epicenter is given as undifferentiated quaternary by mta. the second earthquake in the same region is the 1969 demirci (manisa) earthquake. it was located on the north anatolian fault at the easternmost part of the sea of marmara, mudurnu valley. its fault plane solution based on teleseismic body-waveform inversion shows a e-w dextral strike-slip faluting mechanism (taymaz et al. 1991). nalbant et al., (1998) modeled this earthquake with dipping south at 45° and a displacement of 0.35 m. geological formation around this epicenter includes continental clastic rocks, dacite, rhyolite, rhyodacite, migmatite and gneiss. the other earthquake in the same region is march 28, 1969, alaşehir earthquake. this event occurred in the gediz river valley with 30 km surface rupture and extending from nw through alaşehir to se. the fault plane solution of this earthquake shows a normal faulting with a dip of 32° and a mean displacement of 0.61 m (eyidogan and jackson, 1985). geological structure around this epicenter includes undifferentiated quaternary, alluvial fan, slope debris, etc. the 1924 altıntaş (kütahya) earthquake occurred in the western anatolia, east of the 1970 gediz (kütahya) earthquake. the east-west normal faulting is a dominant mechanism in this region and was associated with 1956 eskişehir and the 1970 gediz earthquakes. nalbant et al., (1998) assumed that this event occurred on an east-west, normal fault located near epicenter with a dip of 45° which is compatible with the known tectonics of the area and mean displacement of 0.2 m. geological environments near the epicenter of this earthquake consist of undifferentiated quaternary, gneiss, metagranite, schist, marble, amphibolite etc. another earthquake around this area is 1928 emet earthquake and they show a similar tectonic environment. in this region, nnw-sse striking normal faults dipping to the ene were reactivated during the earthquakes in 1970 and 1944 (nalbant et al. 1998). different from 1924 altıntaş earthquake region, geological structure includes continental carbonate rocks. the third earthquake in the same region is 1944 şaphane earthquake. this earthquake was located in the western anatolia, southwest of gediz and also ruptured during the 1970 gediz earthquake. (nalbant et al., 1998) modeled this earthquake as nnw-sse normal fault with a dip of 45° and a mean displacement of 0.2 m. geological formation around this epicenter is formed by continental clastic rocks and pyrolastic rocks. the fourth earthquake in this region is 1970 gediz earthquake which is occurred in the western anatolia, east of the simav fault system. ambraseys and tchalenko (1972) mapped about 45 km of complicated normal faulting trending both to the nnw-sse and east-west down thrown to the east and north. eyidogan and jackson (1985) modeled the seismograms of this earthquake using three main subevents. the first subevent occurred on 15 km long nnw-sse segment with a mean displacement of 1.6 m and a dip of 35°. it then triggers the second subevent which ruptures the 24 km long east-west segment with a mean displacement of 2.4 m and a dip of 35°. all the remaining complexity of seismograms can be explain by slip on a~15° dipping fault extending the second fault segment from a depth of 12.5 to 17.5 km. geological structure around this epicenter includes continental clastic rocks. the 1930 hakkari earthquake was located near the iran-turkey border. focal mechanism of this earthquake display a right-lateral strike-slip faulting with a normal fault component with a dip of 36° and a mean displacement of 4.5 m (ambraseys and jackson, 1998). geological formation around this epicenter includes pyroclastic rocks, basalt, ophiolitic melange. the bergama (i̇zmir) earthquake was located in 1939 near the coast in the western anatolia, at the western extremity of the bakırçay normal fault zone. the focal mechanism solution indicates east-west normal faulting (ritsema, 1974). nalbant et al., (1998) modeled the rupture of this event to the dip at 45° and a mean displacement of 0.75 m. geological formation around this epicenter consists of generally undifferentiated quaternary and undifferentiated volcanic rocks (generally andesitic). the other earthquake in the same region is the 2005 urla earthquake. this earthquake occurred in the gulf of sığacık, in a region dominated by 19a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes n–s extension and bounded by well-documented graben structures. the focal mechanisms of the 17 october 2005 earthquake seismic sequence show pure strike-slip motions in the gulf of sığacık and normal faulting combined with considerable strike-slip motion in the north and in the south (benenatos et al. 2006). geological structure around this epicenter includes neritic and lacustrine limestone, marl, shale etc. the 1939 erzincan earthquake occurred on the north anatolian fault zone was one of the most active strike-slip faults in the world. this earthquake nucleated on the eastern portion of the rupture. main body of the rupture extended along the master strand of the naf between erzincan and niksar basins. however, a part of 76 km-long the westernmost portion of the rupture directed towards on the ezinepazarı fault. ambraseys and zapotek (1969) stated that fault plane for this earthquake shows as a right-lateral mechanism. burumbaugh and pınar (2001) stated that the first-motion fault plane solution obtained at the epicentral location was a fault plane parallel to the easternmost surface rupture segment with a dip of 80° and mean displacement of 3.7 m. the other large earthquake in this region is the 1992 erzincan earthquake. this earthquake is located within a key region of eastern anatolia, which displays both expulsion of the anatolian block towards the west and regional n-s convergence due to the arabian-eurasian collision. the main focal mechanism is strike-slip with a slight normal component a dip of 68° and mean displacement of 1.0 m. (fuenzalida et al. 1997). main geological environments around these epicenters consist of undifferentiated quaternary, peridotite, slope debris, cone of dejection etc. the 1942 bigadiç (balıkesir) earthquake was located in the western anatolia, south of balıkesir. in this region, normal faults strike east-west and dip toward the north (westeway, 1990). nalbant et al., (1998) modeled the rupture of this earthquake as a12.5 km long normal fault and assume a dip of 45° and a mean displacement of 0.35 m. geological structure around this earthquake epicenter includes undifferentiated quaternary, undifferentiated volcanic rocks, dacite, rhyolite, rhyodacite. another event in the same city is the 1944 ayvacık earthquake. this earthquake occurred near the edremit gulf. the southern branch of the north anatolian fault reaches the aegean sea through the edremit gulf. the poorly constrained focal mechanism indicates nearly pure strike-slip (ritsema, 1974). however, nalbant et al., (1998) stated that it could have a normal component with an oblique slip on a 60° south dipping fault and a mean displacement of 1.6 m. geological formation around this epicenter consists of continental clastic rocks. the 1942 niksar-erbaa (tokat) earthquake was located in the erbaaniksar area, in the north anatolian fault zone from niksar in the kelkit river valley to the yeşilırmak river west of erbaa. focal mechanism of this earthquake shows right-lateral strike-slip faulting with a dip of 90° and a mean displacement of 1.8 m (ambraseys and jackson, 1998). geological structure around this epicenter includes undifferentiated quaternary, undifferentiated continental clastic rocks, schist, phyllite, marble and metabasic rocks. the 1943 hendek (adapazarı) earthquake occurred in the east of the sea of marmara in the mudurnu valley, 15 km north of the 1967 rupture. the focal mechanism of this earthquake is almost pure strike-slip motion parallel to the north anatolian fault. nalbant et al., (1998) modeled this event as a 16 km long right-lateral faulting with a dip of 90° and with 0.75 m of displacement (ambraseys and jackson, 1998). geological structure around this earthquake epicenter consists of undifferentiated volcanic rocks, clastic and carbonates rocks. the 1943 tosya-ladik (samsun) earthquake occurred near tosya, kastamonu province, in northern turkey. surface faulting was observed in section of the north anatolian fault zone from the destek gorge west of erbaa to the filyos river. focal mechanism of this earthquake shows rightlateral strike-slip faulting with a dip of 80° and with a mean displacement of 2.0 m (wells and coppersmith, 1994). geological structure around this earthquake epicenter is formed by undifferentiated quaternary. the 1944 gerede (bolu) earthquake was located on the north anatolian fault. nalbant et al., (1998) modeled this earthquake with a right-lateral strikeslip fault and with slip distribution 3.5 m to the west and decreases to 1.5 m in the east. geological formation around this epicenter consists of clastic and carbonate rocks. another event in the same region is 1957 abant earthquake. this earthquake occurred on the north anatolian fault. the focal mechanism of this earthquake indicates strike-slip faulting (mckenzie, 1972). nalbant et al., (1998) modeled this event with an average of 2.5 m strike-slip motion. geological structure near this earthquake epicenter is formed by clastic and carbonate rocks, alluvial fan, slope debris, cone of dejection etc. the other event in near region is july 22, 1967, mudurnu earthquake. this earthquake occurred in the east of the sea marmara on the north anatolian fault. the focal mechanism is given as a pure, east-west, strike slip faulting and mean displacement of 2.5 m (nalbant et al. 1998). geological environments near this epicenter include undifferentiated quaternary, undifferentiated volcanic rocks, clastic and carbonate rocks. the 1946 varto-hınıs (muş) earthquake occurred in varto district, province of muş. focal mechanism of this earthquake shows right-lateral strike-slip faulting with a dip of 58° and a mean displacement of 0.3 m (ambraseys and jackson, 1998). the other earthquake in the same region, the varto-üstükran earthquake of august 19, 1966, was located near the east end of the north anatolian fault system in turkey, very near where an earlier, less intense earthquake had caused damage in may 31, 1946. fault plane solution displays right-lateral strike slip faulting that having an extension of normal fault component with a dip of 65° and a mean displacement of 0.3 m (ambraseys and jackson, 1998). geological formation around these epicenters includes continental clastic rocks, pyroclastic rocks, basalt, slope debris, cone of dejection etc. the 1951 kurşunlu (çankırı) earthquake was related with north anatolian fault and occurred in the eastern tails of the i̇smetpaşa segment. focal mechanism of this earthquake shows right-lateral strike-slip faulting with a dip of 60° and a mean displacement of 0.6 m (nalbant et al. 1998). the june 6, 2000 orta (çankırı) earthquake was located about 25 km south of the north anatolian fault zone. the epicenter of this earthquake is close to a restraining bend in the e-w striking right-lateral strike-slip fault that moved in the much larger earthquake of august 13, 1951 kurşunlu earthquake. fault plane solution of 2000 earthquake shows normal faulting with a left lateral component with a dip of 69° and a mean displacement of 0.82 m (irmak, 2000). geological structure around these epicenters includes undifferentiated volcanic rocks, basalt, lacustrine limestone, marl, shale etc. the 1953 yenice-gönen earthquake was located between the sea of marmara to the north and the edremit gulf to the south. this earthquake ruptured the southern branch of the north anatolian fault. focal mechanism of this earthquake indicates pure southwest-northeast right-lateral strike-slip faulting (ambraseys, 1978). nalbant et al., (1998) modeled this earthquake using the slip reaching 3.5 m in the eastern and dropping to both ends. geological structure around this epicenter is formed by undifferentiated volcanic rocks, neritic limestone, schist and clastic rocks. the 1956 eskişehir earthquake was located in the western anatolia, 30 km west of eskişehir and 100 km north of gediz. this earthquake occurred on the eskişehir normal fault system (mckenzie, 1972). fault plane solution indicates east-west normal faulting. nalbant et al., (1998) modeled this event as resulting from the rupture of an east-west normal fault dipping to the north with an angle of 45° and a displacement of 0.30 m. geological formation around this epicenter consist of undifferentiated ophiolitic rocks, schist, fillite, marble and metabasite. the 1963 yalova-çınarcık earthquake was located in the southeast sea of marmara, north of the yalova. the fault plane solution is given as pure northwest-southeast normal faulting (taymaz et al. 1991). nalbant et al., (1998) modeled this event as a normal fault dipping northward with a dip of 60° and displacement of 0.60 m. geological structure around this earthquake epicenter includes continental clastic rocks, carbonate and clastic rocks. the 1964 manyas earthquake was located on the south of the sea of marmara between the manyas lake and the ulubat lake on the southern branch of the north anatolian fault. the focal mechanism solution shows east-west normal faulting (taymaz et al. 1991). nalbant et al., (1998) modeled this earthquake with a dip of 45° and a mean displacement of 1.2 20 serkan öztürk m. geological environments around this epicenter includes undifferentiated quaternary and continental clastic rocks. the first aegean earthquake which occurred in 1965 was located in the aegean sea at the southwest extremity of the north aegean through. the focal mechanism indicates right lateral strike-slip faulting on a northeastsouthwest plane (taymaz et al. 1991). nalbant et al., (1998) modeled this event with a slip of 0.6 m. the second earthquake in the same region is the 1967 aegean earthquake which was located in the aegean sea at the northwest extremity of the skyros basin, south of the north aegean through. the predominantly normal faulting focal mechanism (taymaz et al. 1991) defines two possible fault planes: one striking east-west with a dip toward the south, the other northwest-southeast with a dip toward the northeast. the major normal faults in the area bounding the western edge of skyros basin are oriented like the second nodal plane. nalbant et al., (1998) modeled this earthquake as a northwest-southeast normal fault with a dip of 45° and a displacement of 0.70 m. the third earthquake is december 19, 1981, aegean earthquake which occurred in the aegean sea, on the southern edge of the skyros basin. the fault plane solution shows right-lateral strike-slip faulting striking northeast-southwest (taymaz et al. 1991). nalbant et al., (1998) modeled this event with a dip of 79° and a mean displacement of 3.2 m. the fourth aegean earthquake occurred in the southwest extremity of the skyros basin on december, 27, 1981. its focal mechanism also indicates right-lateral faulting striking northeast-southwest (taymaz et al. 1991). nalbant et al., (1998) modeled this event with a dip of 79° and a mean displacement of 0.75 m. the fifth earthquake which was located in the same region is january, 18, 1982, aegean earthquake. this earthquake was located in the aegean sea, in the central part of the north aegean through. its fault plane solution shows right-lateral strike-slip faulting on a northeast-southwest plane (taymaz et al. 1991). nalbant et al., (1998) modeled this event with a dip of 62° and a mean displacement of 2.25 m. the last event in this region is 1983 aegean earthquake. this earthquake was located in aegean sea, just east of the previous one. the northeast elongation of the aftershock zone and the strikeslip focal mechanism are similar to the 1982 earthquake (taymaz et al. 1991). nalbant et al., (1998) modeled this event as a northeast-southwest rightlateral fault on the southern edge of the north aegean trough with dip of 83° and a displacement of 2.25 m. there is no information about the geological formation around these epicenters because these events occurred in the sea. the 1967 tunceli earthquake occurred in the part which is between pülümür and kalıova of north anatolian fault. fault plane solution of this earthquake shows right-lateral strike-slip faulting with a dip of 88° and a mean displacement of 0.2 m (ambraseys and jackson, 1998). the 2003 pülümür (tunceli) earthquake occurred on the pülümür fault that has 45° with the eastern part of the north anatolian fault. the epicenter of the earthquake was located at about 4.5 km w-sw sağlamtaş village. focal mechanism of this earthquake shows predominantly left-lateral strike-slip faulting that displayed a normal fault component with a dip of 71° and a mean displacement of 0.36 m (ozener et al. 2010). geological structure around these epicenters includes clastic and carbonate rocks, continental clastic rocks and undifferentiated volcanic rocks. the bartın (zonguldak) earthquake of september 3, 1968 is the strongest instrumentally recorded earthquake to occur along the black sea margin in northwestern turkey. the epicenter of the main shock was located 10 km north of amasra, in the black sea. the fault plane solution of this earthquake shows a thrust faulting with a dip of 38° and a mean displacement of 0.2 m (alptekin et al. 1986). geological formation around this epicenter consists of continental clastic rocks, clastic and carbonates rocks, neritic limestone. the may 22, 1971, bingöl earthquake was centered near bingöl. this earthquake produced surface breaks mostly along the southwestern half of the northeastern segment of the east anatolian fault between the karlıova triple junctions. earthquake mechanism of this event reveals left-lateral strike slip geometry with a dip of 45° and a mean displacement of 0.70 m (taymaz et al. 1991). the second earthquake in the same region is may 01, 2003, bingöl earthquake. this earthquake was located approximately 60 km southwest of the triple junction near karlıova, where north anatolian fault and east anatolian fault intersect. fault plane solution of this earthquake shows rightlateral strike-slip fault with a dip of 88° and a mean displacement of 0.9 m (milkereit et al. 2004). geological environments around these epicenters consist of undifferentiated volcanic rocks, schist, quvarzite, marble, phillite, etc. the earthquake of march 27, 1975, saros (çanakkale) earthquake was located on the west of the sea of marmara in the gulf of saros, a pullapart basin associated with the northern part of the north anatolian fault. fault plane solution shows a strike-slip, normal fault (a dip of 55°), rupture with the right lateral plane ene-wsw (taymaz et al. 1991). nalbant et al., (1998) modeled this earthquake as an oblique fault with a length of 20 km and displacement of 0.95 m. there is no information about the geological structure around this epicenter because this event occurred in the sea. the lice (diyarbakır) earthquake of 1975 was located near the town of lice, in the western part of the bitlis thrust zone. the slip vector in this event is thus directed ne, and may be related to the arabia-turkey motion, rather than the arabia-eurasia motion (moazami-goudarzi and akasheh, 1977). the mechanism of faulting for the lice earthquake of september 6, 1975 is thrust fault with a dip of 50° and a mean displacement of 0.5 m (wells and coppersmith, 1994). geological structure around this epicenter includes clastic and carbonate rocks. the 1976 çaldıran (van) earthquake occurred about 100 km east of the junction between the north and east anatolian faults, extending from 3 km west of sarıkök in the west to just west of baydoğan in the east. focal mechanism of this earthquake shows right-lateral strike-slip faulting with very small thrust component with a dip of 78° and a mean displacement of 2.05 m (wells and coppersmith, 1994). geological formation around this epicenter includes undifferentiated quaternary, undifferentiated continental clastic rocks, alluvial fans, slope debris, moraine etc. the 1983 horosan-narman (erzurum) earthquake took place on the ne-sw trending horosan fault, between horosan and şenkaya about 60 km east of erzurum basin. its fault plane solution indicates left-lateral strike slip geometry with a small thrust component, with a dip of 64° and a mean displacement of 1.2 m (wells and coppersmith, 1994). geological structure around this earthquake epicenter consists of pyroclastic rocks, undifferentiated volcanic rocks, and undifferentiated basic and ultrabasic rocks. the kars earthquake of 1988 december 7 was located within the lesser caucasus, which is dominated by a north-south compressive tectonics resulting from the young continental collision between the arabian plate and russian platform. focal mechanism of this earthquake was obtained as rightlateral strike-slip fault with a dip of 29° and a mean displacement of 1.6 m (wells and coppersmith, 1994). geological formation around this epicenter includes undifferentiated continental clastic rocks. the 1995 dinar (afyon) earthquake took place in the west anatolian province, in the lakes region horst-graben system. its epicenter is located near the dinar-çivril fault. focal mechanism of this earthquake indicates a normal faulting with a small lateral component with a dip of 56° and an average displacement of 0.2–0.4 m (pınar, 1998). geological structure around this epicenter consists of undifferentiated quaternary and neritic limestone. the other earthquakes in the same region occurred in the southwestern turkey on december 15, 2000 (sultandağı) and february 3, 2002 (eber) earthquakes. this earthquake sequence took place on the sultandağ fault. the 2000 earthquake occurred on the sultandağı fault, near the city of afyon in southwestern turkey whereas the 2002 earthquake occurred on the northwestern segment of the sultandağı fault. focal mechanisms of the 2000 earthquake indicate normal faulting with a slight left-lateral component with a dip of 41° and a mean displacement of 0.25 m. focal mechanism of the 2002 earthquake shows normal faulting with a dip of 38° and a variable displacement between 0.6 and 1.0 m (aksarı et al. 2010). geological environments around these epicenters include undifferentiated quaternary, undifferentiated continental clastic rocks, marble and recrystallized limestone. the 1998 adana-ceyhan earthquake occurred in southern turkey with its epicenter close to the city of adana, characterized by the advance of the arabic plate towards the north and the consequent wedging of the eurasian plate. this earthquake resulted by a left-lateral strike-slip faulting along the ne-trending east anatolian fault system and other fault zones, parallel to it, in the west. parameters of earthquake fault were given as 80° 21a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes for dip and 0.37 m for mean displacement (lekkas and vassilakis, 1999; aktar et al. 2000). geological structure around this epicenter consists of undifferentiated quaternary. the 1999 i̇zmit earthquake, one of the most destructive earthquakes in turkey, was located in the western part of the north anatolian fault, in northwestern turkey in the gulf of i̇zmit region. the i̇zmit main shock occurred as a right-lateral strike slip faulting on an ew trending with a dip of 83° and a mean displacement of 5.0 m (tibi et al. 2001). geological environments around this epicenter consist of undifferentiated quaternary, clastic and carbonate rocks, alluvial fan, slope debris etc. the 1999 düzce earthquake was located in bolu basin in the adjacent fault segment associated with the i̇zmit earthquake. the focal mechanism of this earthquake indicates a right-lateral strike slip faulting on ew strike with a dip of 62° and a mean displacement of 3.0 m (tibi et al. 2001). geological structure around this epicenter undifferentiated quaternary, clastic and carbonate rocks, alluvial fan, slope debris, cone of defection etc. after the occurrence of an earthquake, the rupture length is related with the geological structure around the epicenter of the earthquake. at first glance, a relationship between seismicity and geologic structure is obvious because most earthquakes occur in regions where active faults are recognized. as given in details of related earthquakes, some geological features are nearly the same for different earthquakes. in many regions, geological formation in and around the earthquake epicenters is mainly composed of undifferentiated quaternary, continental clastic rocks or undifferentiated continental clastic rocks, undifferentiated volcanic rocks, clastic and carbonate rocks. however, alluvial fan, slope debris, cone of dejection, marble, schist, limestone etc. are dominant in some regions. thus, these variations in the geological formation surrounding the epicenters of earthquakes have a strong effect on the extent of different surface rupture lengths. results and discussions in the scope of this study, it is aimed to derive a new and reliable relationship between the main shock magnitude (ms) and surface rupture length (l) for great earthquakes (ms≥5.5) occurred in and around turkey. for this purpose, the earthquakes whose surface wave magnitudes are greater than 5.5 and the earthquakes which are occurred between 1905 and 2005 are used. some details such as origin times, locations, magnitudes and rupture lengths for these earthquakes are given in table 1. ms-value of may 1, 2003 bingöl earthquake is taken from the website of bogazici university, kandilli observatory and research institute (koeri). ms magnitude is preferred for this analysis since such kind of relations in literature was generally given with ms. as stated in hanks and kanamori (1979), mw can be calculated rather similar to ms for a number of earthquakes with ms≤8.0. reported magnitude types of almost all earthquakes in the scope of this study are generally given as ms in the literature. however, mw magnitudes of three earthquakes (15 december 2000 afyon, 3 february 2002 afyon and 17 october 2005 urla) are used instead of ms by taking into consideration hanks and kanamori (1979). thus, ms magnitude is used since any earthquakes with ms≥8.0 have not reported for the instrumental period. observations of surface ruptures usually accompany most large earthquakes and the logarithms of their corresponding dimensions (primarily length) have been found to relate linearly to earthquake magnitude. the empirical relationships that were established could predict not only the fault dimensions for a given magnitude, but also the maximum magnitude based on known fault dimensions. these relationships proved extremely useful to geotechnical, seismic hazard assessment and seismotectonic applications. a few authors calculated different empirical relationships connecting rupture lengths and magnitudes for earthquakes occurred in and around turkey and the world (e.g., acharya, 1979; wells and coppersmith, 1994; ambraseys and jackson, 1998). in this study, 63 earthquakes whose magnitude ranges vary from 5.5 to 8.0 and rupture length changes between 3 and 350 km are used for the analysis. these events and their information given in table 1 can be found in detailed from different studies in the literature. figure 2 shows the graphical representations of all regression fits between magnitude (ms) and rupture length (l) for turkey earthquakes. using four different regression methods, four different empirical relationships are obtained. log-linear fit is used for all regressions and following equations are derived: ms = 1.02* logl + 5.18, for l2 norm regression (6) ms = 1.15* logl + 4.98, for l1 norm regression (7) ms = 1.04* logl + 5.15, for robust regression (8) ms = 1.25* logl + 4.86, for orthogonal regression (9) although the choice of confidence coefficient is somewhat arbitrary, in practice 90%, 95%, and 99% intervals are often used, with 95 % being the most commonly used. so, confidence interval limits of each regression relation used in this study were calculated for 90%, 95%, and 99% confidence interval and these values were given in table 2 in detailed. these confidence intervals are designed to estimate statistical characteristics of sampled data. however, confidence intervals are more flexible and can be used practically in more situations. confidence interval generates a lower and upper limit and these limits define the most probable concentration range in which the true parameter ought to lie. the interval estimate also gives an indication of how much uncertainty there is in the estimate of the true value. for this study, there are reliable surface wave magnitudes and rupture lengths in the instrumental period for 63 earthquakes as given in table 1. from the regression analyses, l2 norm and robust regressions between ms and log (l) give a standard deviation of 0.03 in ms, l1 norm and orthogonal regressions give a standard deviation of 0.04 in ms, the confidence interval limits computed from given data contains the mean values which is calculated from the regression relations in the equations 6 to 9. that is, for all lower and upper confidence interval limits (90%, 95% and 99%), as shown in table 2, these intervals cover the mean values for all regression. also, the number of earthquakes in 90% confidence limit of the regressions was calculated for each regression and it was found as 14 events for l2 norm, 21 events for l1 norm, 14 events for robust and, 17 events for orthogonal regression. for 99% confidence limit of the regressions, the number of earthquakes was calculated as 25 events for l2 norm, 25 events for l1 norm, 23 events for robust and, 29 events for orthogonal regression. 22 serkan öztürk table 1. a list of great turkey earthquakes between 5.5 and 8.0 occurred between 1905 and 2005 associated with surface phenomena. date (d.m.y) location magnitude (ms) rupture length (km) references 04.12.1905 çemişgezek 6.8 38 nalbant et al., (2002) 09.02.1909 ender 6.4 15 ambraseys and jackson (1998) 09.08.1912 marmara 7.4 90 nalbant et al., (1998) 13.09.1912 marmara 6.8 37 ambraseys and jackson (2000) 03.10.1914 burdur 7.0 23 ambraseys and jackson (1998) 18.11.1919 soma 6.9 30 nalbant et al., (1998) 13.09.1924 pasinler 6.8 15 eyidoğan et al., (1999) 20.11.1924 altıntaş 6.0 10 nalbant et al., (1998) 02.05.1928 emet 6.2 12.5 nalbant et al., (1998) 06.05.1930 hakkari 7.2 30 ambraseys and jackson (1998) 04.01.1935 marmara 6.4 20 nalbant et al., (1998) 19.04.1938 kırşehir 6.6 36 acharya (1979) 22.09.1939 bergama 6.5 17 nalbant et al., (1998) 26.12.1939 erzincan 8.0 350 ambraseys and zatopek (1969) 15.11.1942 bigadiç 6.2 12.5 nalbant et al., (1998) 20.12.1942 niksar-erbaa 7.0 50 acharya (1979) 20.06.1943 hendek 6.4 16 nalbant et al., (1998) 26.11.1943 tosya-ladik 7.3 280 dewey (1976) 01.02.1944 gerede 7.3 180 dewey (1976) 25.06.1944 şaphane 6.0 18 ambraseys and jackson (1998) 06.10.1944 ayvacık 6.8 37 ambraseys and jackson (2000) 31.05.1946 varto-hınıs 6.0 9 welss and coppersmith (1994) 17.08.1949 elmalıdere 7.1 76 hearn et al., (2002) 13.08.1951 kurşunlu 6.8 49 acharya (1979) 18.03.1953 yenice-gönen 7.2 58 wells and coppersmith (1994) 16.07.1955 aydın-söke 6.9 35 pavlides and caputo (2004) 20.02.1956 eskişehir 6.1 11 nalbant et al., (1998) 26.05.1957 abant 7.0 40 ambraseys and jackson (1998) 18.09.1963 yalova-çınarcık 6.3 35 turgut (2007) 06.10.1964 manyas 6.8 40 ambraseys and jackson (1998) 09.03.1965 aegean sea 6.3 14 nalbant et al., (1998) 19.08.1966 varto-üstükran 6.8 38 acharya (1979) 04.03.1967 aegean sea 6.5 18 nalbant et al., (1998) 22.07.1967 mudurnu 7.1 80 ambraseys and zatopek (1969) 26.07.1967 tunceli 6.0 4 ambraseys and jackson (1998) 30.07.1967 mudurnu 5.5 3 ambraseys and jackson (1998) 03.09.1968 bartın 6.5 10 turgut (2007) 23.03.1969 demirci 6.0 10 eyidoğan and jackson (1985) 25.03.1969 alaşehir 6.1 5 ambraseys and jackson (1998) 28.03.1969 alaşehir 6.4 30 dewey (1976) 28.03.1970 gediz 7.1 41 wells and coppersmith (1994) 12.05.1971 burdur 6.2 4 ambraseys and jackson (1998) 22.05.1971 bingöl 6.7 38 acharya (1979) 27.03.1975 saros 6.6 20 nalbant et al., (1998) 06.09.1975 lice 6.6 28 ambraseys and jackson (1998) 24.11.1976 çaldıran 7.3 55 wells and coppersmith (1994) 19.12.1981 aegean sea 7.2 60 nalbant et al., (1998) 27.12.1981 aegean sea 6.4 16 nalbant et al., (1998) 18.01.1982 aegean sea 6.9 30 nalbant et al., (1998) 06.08.1983 aegean sea 6.9 30 nalbant et al., (1998) 30.10.1983 horasan-narman 6.9 12 wells and coppersmith (1994) 07.12.1988 kars 6.7 33 ambraseys and jackson (1998) 13.03.1992 erzincan 6.8 30 grosser et al., (1998) 01.10.1995 dinar 6.1 25 pınar (1998) 27.06.1998 adana-ceyhan 6.2 30 yalcinkaya (2005) 17.08.1999 i̇zmit 7.4 145 barka et al., (2002) 23a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes 12.11.1999 düzce 7.1 40 akyüz et al., (2002) 06.06.2000 orta 6.1 10 irmak (2000) 15.12.2000 afyon 6.0mw 22 aksarı et al., (2010) 03.02.2002 afyon 6.5mw 40 aksarı et al., (2010) 27.01.2003 pülümür 6.0 15 usgs/neic* 01.05.2003 bingöl 6.4k 20 milkereit et al., (2004) 17.10.2005 urla 5.8mw 10 benetatos et al., (2006) * usgs: united states geological survey, neic: national earthquake information center, 6.4k: taken from koeri (http://udim.koeri.boun.edu.tr/zeqdb/), mw: moment magnitude (used as ms magnitude taking into consideration hanks and kanamori, 1979), ms: surface wave magnitude figure 2a figure 2b figure 2c figure 2d figure 2. surface wave magnitude (ms) versus surface rupture length (l, km) for turkey earthquakes listed in table 1: a) l2 norm regression fit, b) l1 norm regression fit, c) robust regression fit, d) orthogonal regression fit. corresponding equations and their correlation coefficients were also given. solid lines indicate the regression fits to data and dashed line indicates 95% confidence interval for the regressions. table 2. confidence interval limits for each regressions. confidence interval limits l2 norm regression limits l1 norm regression limits robust regression limits orthogonal regression limits 90% 95% 99% 90% 95% 99% 90% 95% 99% 90% 95% 99% lower limits 5.61 5.60 5.58 5.47 5.46 5.44 5.59 5.58 5.56 5.40 5.38 5.36 upper limits 7.83 7.84 7.86 7.97 7.98 8.00 7.86 7.87 7.89 8.09 8.10 8.13 24 serkan öztürk as shown in figures 2a to 2d, correlation coefficients of these regression fits are nearly the same as l2 (r2=0.71) norm and orthogonal regression (r2=0.68). correlation coefficient for robust regression is calculated as 0.75. however, this value is found as r2=0.92 for l1 norm and seems quite good. ms versus l for whole regression methods with fit curves, corresponding equation and 95% confidence interval are given in figures 2a to 2d. also, the number of earthquakes for 95% confidence limit of the regressions is calculated in all methods and it is found as 16 events for l2 norm, 22 events for l1 norm, 20 events for robust and, 22 events for orthogonal regression. thus, the highest number of earthquakes in 95% confidence interval is observed for l1 norm and orthogonal regression. regression analysis is routinely used by researchers in many disciplines to fit mathematical models to observed data. linear least squares estimates can behave badly when the error distribution is not normal, particularly when the errors are heavy-tailed. the traditional estimation technique of least squares is efficient if the error terms are independent of the regressors and are identically and independently distributed as a normal. while the unobserved random disturbances in a regression model are often assumed to be normally distributed, real data are often replete with outliers that lie far from the pattern evidenced by a majority of the data. while these errors may be the result of measurement inaccuracies or human recording error, many outliers are generated by genuinely thick-tailed or asymmetric error distributions. in such cases, discarding outliers is inappropriate since they are representative of the true data generating process (boyer et al. 2003). the use of any regression criterion is appropriate when it is suspected that a small portion of the data being analyzed is unreliable (i.e., contains data outliers). regression criterions have the capability of effectively ignoring a few bad data points while emphasizing the majority of data points which more properly reflect the true nature of the data. an insightful and useful characterization of a solution to the problem is to minimize the sum of residual error magnitude. the sum of error magnitudes is of particular use in many applications where the data vector contains a small number of data outliers. in such cases, the sum of squared errors criterion is unduly influenced by these data outliers and thereby often leads to a poor selection of the coefficient vector. the sum of error magnitudes, however, tends to ignore the data outliers provided that they are relatively few in number. in often happens that these data points cluster about a line. as shown in figures 2a to 2d, four plots of data set provides a visual mechanism for determining the basic nature of the relationships between surface wave magnitude and rupture length. although the data concentration is between 10 and 100 km and the data does not include outliers, lots of the data remain outside of the confidence limits in all regression fits. however, there are the same values of rupture length for different magnitude values. for example, also can be seen in table 1, for a rupture length of 30 km there are four different magnitude values such as 6.2, 6.4, 6.8, 6.9 and 7.1 (there are another examples like these values in table 1). however, all these earthquakes (as seen in figures 2a to 2d) remained outside the confidence intervals. this means that obtained regressions cannot always reflect the characteristics of earthquakes. as stated above, geological structure has a strong effect on the rupture length. as a result, it is apparent that these regression fits conforms more accurately to the given data points and tends to ignore these data outliers. as shown in figures 2a to 2d, all data interval for rupture lengths is used in the analysis. in order to make a detailed assessment, the earthquakes were grouped into different three sub-groups according to their rupture lengths. all sub-groups and calculations are given in table 3. in order to obtain the regression relations, the analyses were done for 1 to 10 km, 11 to 50 km, and larger than 51 km. to make a sub-group of rupture lengths, earthquake magnitude level given in figure 1 were also taken into account. as shown in figure 1, the earthquakes were grouped into three broad categories: 5.5≤ms≤6.0 (9 events), 6.1≤ms≤7.0 (41 events), and ms≥7.1 (13 events). these magnitude intervals are accordance with the rupture length intervals given above. that is, the earthquakes having 0-10 km (10 events) rupture lengths have the magnitude interval which generally varies from 5.5 to 6.0. also, the earthquakes having 11-50 km (43 events) rupture lengths have the magnitude interval between 6.1 and 7.0, and the earthquakes having larger than 51 (10 events) km rupture lengths have the magnitude interval higher than 7.1. thus, the rupture lengths intervals in table 3 were prepared in properly to the magnitude groups in tectonic map. it can be clearly seen that correlation coefficients in all regressions are rather weak for all sub-groups. these calculated weak coefficients may be result of a small number of data or large scattering of data. also, these results indicate that subdividing the data set according to various rupture lengths does not greatly improve the statistical significance of the regressions and these regressions may provide more reliable results with the usage of all data set. thus, the results suggest that separating the earthquakes by their rupture lengths is insignificant for rupture length relationships. several empirical relationships between rupture length and magnitude from many different regions around the world including turkey are proposed by several authors. acharya (1979) found following equation for 11 earthquakes occurred in turkey: ms = 0.92* logl + 5.33 (with a correlation coefficient = 0.83) (10) wells and coppersmith (1994) compiled a large dataset consisting of 244 earthquakes from around the world, along with reliable estimates of their source dimensions and moment magnitudes. they developed a series of empirical relationships for fault length, width, area, maximum and average displacement for the whole of their dataset but also separately for each faulting-type group (normal, thrust, strike-slip). they calculated following relation for global earthquakes: ms = 1.16* logl + 5.08 (with a correlation coefficient = 0.89) (11) ambraseys and jackson (1998) proposed following empirical relationship for eastern mediterranean earthquakes covering turkey: ms = 1.04* logl + 5.27 (12) table 3. some analysis summing up the trials for different sub-groups of data. surface rupture length (km) l2 norm regression fit l1 norm regression fit robust regression fit orthogonal regression fit 0-10 ms=0.58*logl+5.57 (r2=0.25) ms=0.25*logl+5.84 (r2=0.05) ms=0.12*logl+5.96 (r2=0.02) ms=1.34*logl+4.93 (r2=0.18) 11-50 ms=1.04*logl+5.14 (r2=0.34) ms=1.15*logl+4.98 (r2=0.42) ms=1.15*logl+4.98 (r2=0.42) ms=2.47*logl+3.13 (r2=0.30) ≥51 ms=0.61*logl+6.08 (r2=0.48) ms=0.20*logl+6.84 (r2=0.14) ms=0.44*logl+6.40 (r2=0.26) ms=0.84*logl+5.62 (r2=0.41) 25a new empirical relation between surface wave magnitude and rupture length for turkey earthquakes it is well known that the length of rupture at the surface is related with earthquake magnitude and many studies in literature relate magnitude to rupture length or some another earthquake source parameters for regions of different geographic or tectonic setting (e.g., acharya, 1979; wells and coppersmith, 1994). with developing a regression relationship among magnitude and rupture length, it can be predicted an expected value for a dependent parameter from an observed independent parameter. independent or dependent parameters will depend on the application, either the expected magnitude for a given rupture length or the expected fault length for a given magnitude. the suggested empirical relationship in this study can be used to assess maximum earthquake magnitude for a particular fault zone or an earthquake source. the assumption that a given magnitude is a maximum value is valid only if the rupture length also is considered a maximum value. evaluating the segmentation of a fault zone provides a basis for assessing the maximum length of future ruptures. given that the length and magnitude are assessed to be maximum values, empirical relations between magnitude and rupture length will provide the expected maximum magnitudes. these are expected maximum magnitudes for the given maximum fault lengths. thus, it can be concluded that rupture length regressions presented here are appropriate for estimating magnitudes for expected ruptures along the related fault segments. applying rupture length relations to estimating magnitudes may help to overcome uncertainties associated with estimating the surface rupture length for some seismic sources (wells and coppersmith, 1994). also, for engineering estimation purposes, acharya (1979) stated that the use of mean magnitude in magnitude rupture-length relationship should be sufficient since there is considerable evidence that beyond certain earthquake magnitude. consequently, a comparison of the empirical relationships is made between magnitude and rupture length considering the correlation coefficients. as stated above, the correlation coefficient and the number of events in confidence limit of l1 norm regression is the highest among the others. this value is very close to 1.0 and this means that it provides more quantitative assessment of fit than the other l2, robust and orthogonal regression. as a result, it can be suggested that equation (7) is more suitable empirical relationship between surface wave magnitude and surface rupture length for earthquakes occurred in and around turkey. also, the predictive relationship between magnitude and rupture length for the instrumental period is almost identical to those derived by different authors for turkey earthquakes. conclusions in this study, an application of different statistical regression methods is made and it is discussed how can be decided for the selection of the best regression method for a given data set. then, a linear relationship between the surface wave magnitude (ms) and rupture lengths (l) is derived for turkey earthquakes. for this purpose, the earthquakes which are in the time interval between 1905 and 2005 and whose magnitudes are greater than 5.5 are used. in the analyses, there are 63 great earthquakes whose magnitude ranges vary from 5.5 to 8.0 and rupture length changes between 3 and 350 km. in order to estimate the best empirical relationship from different statistical models, four regression norms are used as (i) least squares, (ii) least sum of absolute deviations, (iii) total least squares or orthogonal and, (iv) robust regression. in order to identify the quality of the fit in a linear regression and to select the most suitable empirical relationship for data set, the correlation coefficient (r2) as a quite simple and very practicable tool is used. the present research has confirmed a clear and straightforward relationship between magnitude (ms), the surface rupture length (l) for turkey earthquakes. there is not a general equation between ms and l for turkey earthquakes and only a few authors suggested different relationships for different parts of turkey. in order to obtain more appropriate and reliable empirical relationship between magnitude and the surface rupture length of turkey earthquakes, log-linear scale is used in four different regressions and different equations are derived. as a result, the relationship calculated as ms = 1.15* logl + 4.98 with a strong correlation coefficient r2=0.92 given by l1 norm is suggested for turkey earthquakes. acknowledgments the author would like to thank to professor dr. hakan karsli for his helps in preparing the analysis codes and anonymous reviewers for their useful and constructive suggestions in improving this paper. also, the computer programs used in this study are partially covered by gumushane university (turkey) with project no 2012.02.1717.2. references abdelkader, g., ali, l., & rachida, r., (2010). robust 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khan4, imran ahmad5 1. school of earth and space sciences, peking university, beijing 100871, china; mukhtar@stu.pku.edu.cn 2. national centre of excellence in geology, university of peshawar, peshawar 25130, pakistan; saadkhan@uop.edu.pk 3. school of earth science and resources, chang’an university, xi’an, shaanxi 710054, china;nazirulislamgeo@gmail.com 4. webuild,s.p.a construction company via della dataria roma rm,italy; f.khan@webuildgroup.et 5. department of geology, university of malakand, chakdara 18800, dir (lower), pakistan, imran_geo@uom.edu.pk *corresponding author: saadkhan@uop.edu.pk; saadkhan@geologist.com how to cite item: ahmad, m. s., khan, s., islam, n. u., khan, f. y., & ahmad, i. (2025). landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis. earth sciences research journal, 29(1), 55-67. https://doi. org/10.15446/esrj.v29n1.117884 record manuscript received: 05/12/2024 accepted for publication: 05/03/2025 abst r ac t the hindukush region in northwest pakistan is a mountainous area that often faces natural disasters, such as landslides, flash floods, glacial lake outbursts, and debris flow, that alter the landscape and damage property. this study focused on the chitral area of the hindukush region to assess the landslide distribution and susceptibility using field observations and factor analysis. nine landslide causative factors were selected and weighted using geographic information system (gis)-based frequency ratio (fr) and analytical hierarchy process (ahp) techniques. the factors included slope, aspect, rainfall, land cover, lithology, seismicity, distance to faults, streams, and roads. landslide susceptibility maps were generated and classified into five categories: very high, high, moderate, low, and very low. various landslides were observed in the field comprising debris flow, debris slide, soil erosion, and rockfall. rockfall in the study area indicates active seismicity in the hindukush region. furthermore, the area under the curve method validated the results, which gave 0.80 for fr and 0.73 for ahp. the results showed that most of the landslides in the study area were caused by steep slopes of mountains, followed by precipitation. the high landslide susceptibility zones in the study area matched well with the field-based landslide collections, which showed the reliability of the mapping methods. these findings can help plan and implement measures in the hindukush region to reduce the risk and impact of landslides, such as early warning systems, slope stabilization, land use regulation, and evacuation plans. keywords: hindukush; chitral; susceptibility; landslide; inventory palabras clave: hindukush; chitral; susceptibilidad; deslizamientos de tierra; inventario evaluación de la distribución y susceptibilidad de deslizamientos de tierra en el noroeste de pakistán: perspectivas derivadas de observaciones de campo y análisis factorial la región de hindukush en el noroeste de pakistán es una área montañosa que eventualmente presenta desastres naturales, como deslizamientos de tierra, riadas, inundación por desborde de lagos glaciares y flujo de detritos, que alteran el paisaje y dañan la propiedad. este estudio se enfoca en el área de chitral, de la región hindukush, con el fin de medir la distribución de los deslizamientos de tierra y la susceptibilidad a estos a través de observaciones de campo y análisis de factores. para esto se seleccionaron nueve factores determinantes y se ponderaron a través de técnicas de relación de frecuencias y proceso de jerarquía analítica con base en sistemas de información geográfica. estos factores son: inclinación, aspecto, precipitación de lluvia, cobertura terrestre, litología, sismicidad, distancia a fallas, arroyos y carreteras. los mapas de susceptibilidad de deslizamientos se generaron y clasificaron en cinco categorías: muy alto, alto, moderado, bajo y muy bajo. varios deslizamientos se observaron en campo para analizar el flujo de detritos, el deslizamiento de detritos, la erosión del suelo y la caida de rocas. la caida de rocas en el área de estudio indica la seismicidad activa en la región del hindukush. además, el área bajo el método de curva validó los resultados, con 0.80 para la relación de frecuencias y 0.73 para el proceso de jerarquía analítica. los resultados señalan que los factores que han causado la mayoría de los deslizamientos de tierra en el área de estudio son las pendientes pronunciadas de las montañas y la precipitación. las zonas de alta susceptibilidad a deslizamientos de tierra en el área de estudio coinciden con las colecciones de información de campo sobre deslizamientos, lo que muestra la fiabilidad de los métodos de mapeo. estos hallazgos pueden ayudar a planear e implementar medidas en la región del hindukush para reducir el riesgo y el impacto de los deslizamientos, tales como sistemas de alerta temprana, estabilización de taludes, regulación del uso del suelo y planes de evacuación. resu m en https://doi.org/10.15446/esrj.v29n1.117884 https://doi.org/10.15446/esrj.v29n1.117884 https://doi.org/10.15446/esrj.v29n1.117884 56 mukhtar s. ahmad, saad khan, nazir ul islam, fazle yar khan, imran ahmad 1. introduction landslides are common and destructive natural hazards in mountainous regions worldwide (khattak et al. 2010). landslides have become more frequent in recent years due to rapid urbanization and population growth in undeveloped and developed countries. to mitigate landslide casualties, modern remote sensing and geographic information techniques and high-resolution data are essential for hazard analysis, risk assessment, and susceptibility mapping (shafri et al. 2010; khan et al. 2020; ahmad et al. 2022). these techniques and data can assist in generating inventory maps of landslides that provide systematic information about the locations, types, and distribution of mass movements in an area (xu et al. 2023; ray et al. 2020). moreover, these techniques and data can be used as the primary basis for generating thematic maps of geoenvironmental variables known as landslide causal factors (basharat et al. 2016; ahmad et al. 2023). reliable landslide analysis and the development of accurate landslide susceptibility maps depend on using suitable methods to assess the impact of various causal factors on landslide occurrence (tang et al. 2020). these methods can help evaluate the influence of factors such as soil type, slope gradient, land cover, rainfall patterns, and geological characteristics on the likelihood of landslides. by analyzing and integrating these diverse factors, a comprehensive understanding of the spatial distribution and vulnerability of areas prone to landslides can be achieved. the detailed susceptibility maps support land-use planning, disaster management, and risk reduction efforts. by considering the complex interplay between causal factors and landslide occurrences, policymakers and stakeholders can make informed decisions to protect lives, infrastructure, and ecosystems from the devastating impact of landslides. various methods have been applied for landslide susceptibility, indicating the usefulness of qualitative and quantitative analysis methods, depending on the specific context, available resources, and desired outcomes (basharat et al. 2016; zêzere et al. 2017; zhang et al. 2022; ahmad et al. 2024). landslide susceptibility analysis using statistical, insar, machine learning, and knowledge-based methods has been conducted by many researchers worldwide (kaur et al. 2023; agrawal and dixit, 2023; saha et al. 2023; aslam et al. 2023, wu et al. 2024, ahmad et al. 2024). among different methods, frequency ratio (fr) and analytical hierarchy process (ahp) are preferred over complex machine learning techniques that require programming knowledge for landslide susceptibility mapping (boukhres et al. 2023). fr and ahp methods are simple to understand and implement, making them accessible to researchers, practitioners, and decision-makers who may not have advanced programming skills or technical expertise. these methods provide transparent and interpretable results, enabling stakeholders to comprehend the factors influencing landslide susceptibility. this transparency is crucial in disaster management and land-use planning, where explainability is highly valued. fr and ahp methods can handle limited data availability, unlike complicated machine learning approaches that often demand large training datasets. fr employs basic statistical calculations based on historical landslide occurrences and conditioning factors (razavizadeh et al. 2017, youssef et al. 2023), while ahp incorporates expert judgment and pairwise comparisons, making them suitable for areas with sparse or incomplete data. ahp explicitly integrates expert knowledge and preferences through pairwise comparisons, allowing domain experts to contribute valuable insights and judgments (saaty, 2004). this integration enhances the accuracy and reliability of susceptibility mapping. in contrast, machine learning methods heavily rely on training data and may not consider expert inputs explicitly. fr and ahp methods are time and computationally efficient compared to complex machine learning algorithms. machine learning models often involve resource-intensive computations, hyperparameter tuning, and extensive model training (mao et al. 2024). fr relies on straightforward calculations, while ahp’s pairwise comparisons can be performed relatively quickly. although fr and ahp methods may not achieve the same predictive accuracy as advanced machine learning models in some cases, they have demonstrated reasonably good performance in landslide susceptibility mapping across various geographic regions. they offer valuable insights for decision-making purposes, even if they may not match the predictive capabilities of complex machine learning algorithms. the northern pakistan himalayan-hindukush-karakoram belt is a region highly susceptible to landslides. the main factors that trigger landslides in this region are rugged topography, climate, geology, active tectonics, and infrastructure development on unstable slopes. it is estimated that nearly 30% of the world’s slope movements occur in the steep and unstable slopes of the himalayan region (regmi et al. 2014; du et al. 2020). the northern pakistan himalayan region is naturally unstable due to its geological conditions, steep slopes, high seismic activity, and monsoon rainfall (khattak et al. 2010). moreover, human activities such as road construction, urbanization, and deforestation can weaken the natural stability of slopes and trigger landslides (aneel et al. 2023). this is particularly concerning given the region’s rapid development and population growth. considering these factors, landslide susceptibility mapping becomes essential for effective disaster management in pakistan. by identifying areas prone to landslides, authorities can implement preventive measures, such as land-use planning, early warning systems, and infrastructure design considerations. it allows policymakers, planners, and engineers to make informed decisions to mitigate risks and protect lives and properties. the 2005 kashmir earthquake is a stark reminder of northern pakistan’s urgent need for landslide susceptibility mapping. the earthquake triggered numerous landslides, causing widespread destruction and loss of life. such events highlight the importance of understanding and managing landslide hazards in this highly susceptible region. our study evaluates landslides’ spatial patterns and factors in the chitral area of the hindukush region in pakistan, a region prone to landslide hazards. we use field observations to create a geospatial dataset and analyze it with gisbased fr and ahp techniques. our study aims to enhance the understanding of landslide susceptibility in this region and inform effective mitigation strategies. our research offers several novel contributions. first, based on field observations and factor analysis techniques, it provides a detailed analysis of the landslide distribution and vulnerability in the hindukush region of chitral in northwest pakistan. this analysis reveals the areas at high risk of landslides and the main factors influencing their occurrence. this information is essential for designing and implementing measures to reduce the region’s impact and risk of landslides. second, our study compares the performance of two straightforward and transparent methods, fr and ahp, for landslide susceptibility mapping. we show that these methods can produce reasonably accurate results even with limited data availability. these findings make fr and ahp appealing to researchers, practitioners, and decision-makers who may lack advanced programming skills or technical expertise for machine learning modeling of landslide susceptibility. 2. materials and methods 2.1. study area chitral, a district in the eastern hindukush mountainous region of nw pakistan, spans an area of 14,850 km². the district is geographically bordered by afghanistan to the northwest, gilgit to the east, and dir and swat to the south (figure 1). chitral is characterized by steep slopes, rugged terrain, and towering peaks reaching approximately 6000 m, which remain snow-covered throughout the year. the mastuj river is the primary river that flows through chitral and is fed by various tributaries. the climate in this region is warm during summers, cold during winters, and experiences snowfall in winter. the study area of chitral is located within a tectonically active zone, where two continental plates collided during the cretaceous-eocene period, giving rise to the himalayan mountain range. the tirich mir suture zone (tmsz) became active in the mid-cretaceous. during the middle jurassic, the tirich mir fault reactivated as a south-verging fault, facilitating the exhumation of the tirich mir pluton to the surface. this pluton intersected the tirich mir mélange zone (zanchi et al. 2000; hildebrand et al. 2001; saleem et al., 2021). apatite fission track analysis and dating of the tirich mir pluton, conducted around 1.4 ± 0.5 million years ago, provided evidence of ongoing exhumation and uplift of the 7708 m tirich mir peak. in the late cretaceous, the kohistan island arc (kia) collided with the southern margin of the eurasian plate, forming the shyoke suture zone (khan et al. 2000; fraser et al. 2001). subsequently, after the closure of the tethys ocean, the kohistan island arc collided with the indian plate along the indus suture zone. based on the structural evolution and stratigraphic record, the chitral region in the hindu kush ranges is divided into three major tectonic units: i) the northwest unit between the reshun fault and the pakistan-afghan border, ii) the central unit located between the northern suture and reshun fault, and iii) the kohistan unit situated between the main mantle thrust and the 57landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis northern suture. stratigraphically, the chitral region consists of paleozoic to tertiary rocks within these three tectonic units, including granitic plutons. the stratigraphic sequences comprise the charun quartzite from the devonian age, shogram formation, reshun formation from the jurassic to cretaceous age, chitral slates from the cretaceous age, lun shale from the carboniferous to permian age, and various igneous plutons (zanchi et al. 2000). 2.2. data the current study adopted various techniques and steps regarding literature review, field observation, and remote sensing to prepare the landslide susceptibility map (figure 2). the literature data were collected from the various research catalogs and archives of the geological survey of pakistan (gsp) and the pakistan meteorological department (pmd). the following maps were constructed from the data collected: landslide inventory map, prepared from google earth images using gis; geological map, digitized from geological map of chitral; seismological map, created from the usgs earthquake catalogue dataset (https://earthquake.usgs.gov/earthquakes/search/) of chitral region; rainfall map, created by utilizing the rainfall data of chitral drosh station. structural features and lithology were modified from the geological map of khan et al. (2000). figure 1. map depicting the location of the study area in black polygon with regional tectonic elements mkt; main karakoram thrust, tf; tirich mir fault, rf; reshun fault, kia; kohistan island arc, mmt; main mantle thrust, mbt; main boundary thrust in black dashed line, inset map reflecting the study area over pakistan map. figure 2. methods and data set used for this research. pmd=pakistan meteorological department, gsp= geological survey of pakistan, dem= digital elevation model. https://earthquake.usgs.gov/earthquakes/search/ 58 mukhtar s. ahmad, saad khan, nazir ul islam, fazle yar khan, imran ahmad 2.2.1. field observation during the field visit, various lithological units and geological structures, such as faults and locations of the landslides, were adequately examined. furthermore, types, sizes, locations, failure mechanisms, and structural mechanisms were determined in the field to prepare a landslide inventory map for the chitral area. such kind of maps are constructed to record and define the size of landslides in various areas, to examine the influence of the lithological unit’s geological features (e.g., fold, faults, and fractures) on landslide occurrence and distribution, and to use it for making landslide zonation maps (guzzetti et al. 2012;). the preparation of inventory maps includes several techniques based on satellite datasets, field observation, and data gathering from previous articles (van westen et al. 2006). field observation data and google earth images were used in this study to construct an inventory map for landslides along the road section. polygon shapes for openly visible landslides on the satellite imagery were drawn from google earth, followed by confirmation in a field visit (figure 3). 2.2.2. remote sensing various factors such as slopes, aspect, and distance to streams were derived from nasa srtm (shuttle radar topography mission) based dem of 30 m resolution (doi: 10.5066/f7pr7tft). for the land cover map, landsat-8 satellite images of 2019 were retrieved from usgs on 21-02-2021 (https://lpdaac.usgs.gov) and classified based on supervised classification. the various thematic layers were constructed and further categorized by following the natural break classification method in gis. geomorphic factors like slope and slope aspect were created and categorized by adopting the natural break classification method in gis. dem imageries were used to extract the stream pattern using the arc hydro technique (ahmad et al. 2023). the distance to the fault and lithology maps was digitized from already published geological data. seismicity maps were constructed from the earthquake catalog data. vector layers of fault distance and distance to stream, lithology, and a seismic map were then rasterized. the annual rainfall data were incorporated to construct a rainfall map. the landcover thematic layer was created from landsat 8 images (https://lpdaac.usgs.gov/). figure 3. landslide inventory map of chitral area. black points show the landslide locations observed in the field and on google earth. (a) active rock fall near mastuj captured during field visit, (b) landslide in reshun area, (c) active remanence of reshun landslide, and (d) active debris flow near zaith village. 2.3. methods 2.3.1 analytical hierarchy process (ahp) ahp modeling of landslide susceptibility is based on expert assessment to evaluate and weigh the causal factors using pairwise comparisons (yalçın et al., 2011; ahmad et al., 2023). each factor is given a value from 1-9 depending on its relative importance, increasing influence from 1 to 9 (table 1) (saaty, 2004). the score given to factors is based on field observation, spatial data analysis, and operator expertise. if the factor on the x-axis is much more important than the one on the y-axis, then the value ranges between 1 and 9. the consistency ratio (cr) is used to control and avoid inconsistencies and biases in the whole process of ranking the causal factors (bachri and shresta, 2010; yalçın et al. 2011) cr = ci/ri (1) ri represents the random consistency index, while ci defines the consistency index shown in eq. (2). ci = λmax 1/(n-1) (2) where λmax is the calculation of the maximum eigenvalue and n represents a number of governing factors involved. table 1. scale for pairwise comparisons [19]. intensity of importance explanation intensity of importance explanation 1 equal importance 6 strong plus 2 weak or slight 7 very strong 3 moderate importance 8 very very strong 4 moderate plus 9 extreme importance 5 strong importance 2.3.2. frequency ratio (fr) fr is among the most widely used and popular approaches to assess landslide susceptibility (thapa and bhandari, 2019). the fr approach can quantify the relative influence degree of each attribute class of causal factors on the occurrence of landslides and reflect the response relationship between landslides and essential causal factors (thapa & bhandari, 2019; zhang et al. 2020). a higher ratio indicates a stronger correlation between a causal factor and landslides. if fr values exceed 1, the corresponding causal factors favor increased landslide occurrence. if fr values are very close to 1, the relationship between environmental conditions and the occurrence of landslides in the relevant factor class is weak. landslides are not expected to occur in this class range if fr values are below 1. the frequency ratio is based on the observed relationship between the spatial distribution and causative factors of landslides and can be expressed mathematically as in equation 3: = ( ) ∑ ( ) ( )/∑ ( ) (3) in the equation, fr represents the frequency ratio for class i parameter j, whereas npix(lxi) represents the pixel numbers of landslides in class i of the parameter variable. and npix (xj) is the number of pixels in parameter variable xj. m represents the class number in the parameter variable xi, and n reflects the parameters in the study region. in step 2, frs were kept normalized in the range of (0,1) probability values as a relative frequency. the rf was computed for each class using equation 4. = /∑ (4) after normalization, relative frequency still carries some drawbacks in considering all training factors as equal weight. to assess the shared interrelationship between the independent variables and suppress the drawback, the prediction rate (pr) was computed for rating each factor with the training dataset, as shown in equation 5. = ( − ) ( − ) (5) in the last step, landslide susceptibility was generated by summing the product of pr of each factor and rf of each class as represented in equation 6. https://lpdaac.usgs.gov https://lpdaac.usgs.gov/ 59landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis = ∑( ∗ ) (6) 2.3.3. accuracy assessment and comparison accuracy assessment is a necessary element after the construction of the landslide susceptibility map. the previous researchers have employed various statistical methods to check the accuracy of susceptibility maps such as lda (landslide density analysis), prc (prediction rate curve), roc (receiver operating curve), and auc (area under curve) (yesilnacar and hunter, 2005, wang et al. 2020). in the current research, we used the auc technique to evaluate the predictive accuracy of the susceptibility map. auc is a product of a graphical representation of two operating classes, i.e., tpr (true positive rate) and fpr (false positive rate), to measure the accuracy of the project. (figure 7). the x-axis represents the false predicted events fpr, whereas the y-axis represents the tpr true predicted events. this type of curve describes the efficiency of a model (vakhshoori and zare, 2018, kuradusenge et al. 2020). depending on the accuracy model, the area under the curve can range from 0.5 to 1 for various cases. a value close to 0.5 shows random outcomes, whereas values close to 1 indicate a correct model (hosmer et al., 2013). 3. results 3.1. landslides in chitral region from the field observation, 58 landslides were detected and used to prepare the inventory map. the landslides observed in field data comprise shallow landslides, rock falls, rock avalanches, soil erosion, and debris flow. three road sections (ayun to kilash valley road, chitral-garam cheshma road, and chitral-mastuj road) were covered in the field and are affected by a large amount of mass wasting due to slope instability and heavy rainfall. along these sections, traffic interruption and highway blockage are routine phenomena (figure 3a). a large amount of debris flows along the reshun zaith road unit due to a steep slope and recent sediment. this section also experienced one of the massive landslides that blocked the mastuj river in 2015 (figure 3b). along the main garam cheshma road, soil erosion and rock falls are present in large amounts, whereas kelash valley road experiences soil debris flow and a few landslides caused by rock falls. 3.2. factor analysis the thematic maps of the landslide causal factors used in this study are shown in figure 4. the results revealed that the distribution and size of the landslides vary locally, depending on the rank of factors considered in the study. the region consists of igneous, sedimentary, and metamorphic rock sequences, including slates, quartzite, carbonates, shale, and granitic plutons. recent sediment shales and chitral slates exhibit many rocks and debris erosion. the main structural features of the chitral hindukush region are the main karakorum thrust, reshun fault, and terich mir fault. reshun and main karakoram thrusts are almost parallel to some localities’ river and road sections. fault maps for the study area were classified into eight classes. some of the detected landslides were situated within 1 km of the active geological faults, suggesting that these structures also firmly control landslide triggering in the study area (figure 4b). the chitral region displays variation in topography consisting of steep to gentle slopes, elevated cliffs, and narrow and deep valleys. the results revealed that the landslides were correlated with these factors. regarding land cover landslide relationships, the results showed that most landslide-affected areas lay in barren and rocky areas, and grassland showed sparse landslide events. in the study area, several minor streams feed the main mastuj-chitral river. the key debris flow source is the high energy water flow and discharge in streams during monsoon season. during the monsoon season, heavy rainfall triggers several landslides by increasing the pressure of pore water in loose, unconsolidated sediments (wang et al., 2020; reliefweb, 2024). the rainfall map indicated that the northern part of the chitral region receives the most rainfall. in contrast, the lower area of chitral receives low precipitation, resulting in more landslide occurrence in the northern part. in the current study, distances to road maps were classified into seven classes with intervals of 400m. from the results, the >400m distance is more important than other classes, and this area is more susceptible to slope instability. hindu kush, karakoram, and himalayas mountainous terrains are among the most seismically active regions in the world. active seismicity of these regions is associated with the convergence of the eurasian and indian plates. several active structures in reverse and strike-slip faults exist in the terrains of the hindu kush region. the study area is bounded to the south by the main karakoram thrust, reshun fault in the central part, and tirich mir fault to the northwest of the study area (figure 1). these structures have caused moderate to high earthquakes in the past and experience moderate earthquakes of 5 magnitude each year (schurr et al., 2014). the earthquake zone map shows that the area between the reshun fault and the main karakoram thrust is regarded as a highly active seismic zone. an extensive factor analysis using the fr method showed that lithology has a leading role in the landslide distribution in the area with fr 13.75, followed by the slope factor with fr 7.94. rainfall is ranked as the third factor for landslide susceptibility, whereas the rest are considered less critical for landslide distribution (table 2). the calculated ci was then compared with the ri taken from the table to find the cr value. the cr value represents the inconsistency in the expert’s decision in weighting factors. a cr value of less than 0.1 indicates that the decision is consistent, whereas a larger 0.1 value indicates inconsistency. subclasses of each factor were ranked by using a pairwise comparison technique. in the final phase, each factor was weighed upon completion of the method. the value of cr in the current study was 0.08, which is below 0.1, which indicates that the assessment and weighting criteria are consistent, balanced, and reliable (table 3). table 2. factor analysis using the fr method. factor classes class pixels (%) landslide pixels (%) fr rf slope <10° 20.00 14.88 1.04 0.13 10°-20° 18.00 15.16 1.18 0.15 20°-30° 12.00 17.30 1.31 0.17 30°-40° 14.00 18.04 1.51 0.19 40°-50° 16.00 17.15 1.43 0.18 50°-60° 8.00 9.38 0.79 0.10 60°-70° 7.00 5.31 0.44 0.06 >70° 5.00 2.79 0.23 0.03 100.00 100.00 7.94 (continued) 60 mukhtar s. ahmad, saad khan, nazir ul islam, fazle yar khan, imran ahmad factor classes class pixels (%) landslide pixels (%) fr rf aspect flat 11.29 7.18 0.64 0.07 north 11.14 8.83 0.79 0.09 northeast 11.16 11.62 1.04 0.12 east 11.31 13.82 1.22 0.14 south east 11.24 14.47 1.29 0.14 south 10.97 13.85 0.26 0.14 southwest 10.97 12.67 0.16 0.13 west 11.47 10.82 0.94 0.11 north west 10.46 6.75 0.65 0.07 100.00 100.00 6.99 rainfall 35-36 56.54 60.93 1.08 0.16 37-36 4.22 3.77 0.89 0.13 37-36 35.27 32.30 0.92 0.13 37-36 0.41 0.51 1.25 0.18 37-36 1.79 1.96 1.10 0.16 37-37 0.21 0.33 1.55 0.22 38-37 1.56 0.21 1.13 0.02 100.00 100.00 7.91 distance to road 0-400 6.79 3.76 0.55 0.12 400-800 13.01 6.78 0.52 0.11 800-1200 10.52 8.17 0.78 0.17 1200-1600 8.31 8.20 0.99 0.22 1600-2000 7.23 6.38 0.88 0.19 >2400 54.14 66.70 1.23 0.27 100.00 100.00 4.95 distance to river 0-500 6.50 6.74 1.04 0.13 500-1000 6.59 6.65 1.01 0.12 1000-`1500 6.14 6.50 0.06 0.13 1500-2000 17.62 18.91 1.07 0.13 2000-2500 14.77 16.19 1.10 0.13 2500-3000 11.60 12.79 1.10 0.13 3000-3500 8.83 9.44 0.07 0.13 >3500 27.95 22.78 0.82 0.10 100.00 100.00 6.27 distance to fault 0-1000 8.00 13.66 0.94 0.13 1000-2000 7.00 12.21 0.85 0.12 2000-3000 6.00 25.39 1.77 0.25 3000-4000 23.00 33.60 2.36 0.34 4000-5000 14.00 11.86 0.83 0.12 5000-6000 17.00 2.83 0.20 0.03 >7000 25.00 0.45 0.03 0.00 100.00 100.00 6.99 lithology cs 9.28 4.74 0.51 0.04 ks 2.27 0.09 0.04 0.00 mi 4.54 2.25 0.50 0.04 mz 6.08 0.12 0.03 0.00 pz 4.26 3.70 0.40 0.03 s 8.22 9.71 1.18 0.09 ti 6.00 15.11 2.98 0.22 tr 52.75 61.38 1.16 0.08 trms 0.58 2.64 4.57 0.33 ts 2.80 0.03 0.01 0.00 pc 3.21 0.22 0.07 0.00 100.00 100.00 13.75 (continued) 61landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis factor classes class pixels (%) landslide pixels (%) fr rf earthquake very low 34.24 35.98 1.05 0.26 low 20.77 19.78 0.95 0.24 moderate 22.03 20.86 0.95 0.23 high 22.95 23.37 1.02 0.25 very high 0.01 0.00 0.08 0.02 100.00 100.00 4.05 landcover crop land 9.28 4.74 0.51 0.04 grass land 8.00 7.00 0.04 0.00 deciduous forest 4.54 2.25 0.50 0.04 evergreen forest 6.08 4.00 0.03 0.00 sparse vegetation 7.26 1.70 0.40 0.03 bare land 25.00 24.00 1.18 0.09 flooded vegetation 4.63 6.00 0.98 0.22 urban area 20.00 35.00 1.16 0.08 water 0.21 2.30 2.57 0.33 rocky area 15.00 13.00 0.01 0.00 100.00 100.00 7.38 figure 4. landslide causative factors; (a) lithology map (b) distance to fault (c) slope (d) aspect (e) land cover (f) distance to stream (g) rainfall (h) distance to the road (i) earthquake zones. 62 mukhtar s. ahmad, saad khan, nazir ul islam, fazle yar khan, imran ahmad table 2. factor analysis using the fr method. factor classes class pixels (%) landslide pixels (%) fr rf slope <10° 20.00 14.88 1.04 0.13 10°-20° 18.00 15.16 1.18 0.15 20°-30° 12.00 17.30 1.31 0.17 30°-40° 14.00 18.04 1.51 0.19 40°-50° 16.00 17.15 1.43 0.18 50°-60° 8.00 9.38 0.79 0.10 60°-70° 7.00 5.31 0.44 0.06 >70° 5.00 2.79 0.23 0.03 100.00 100.00 7.94 aspect flat 11.29 7.18 0.64 0.07 north 11.14 8.83 0.79 0.09 northeast 11.16 11.62 1.04 0.12 east 11.31 13.82 1.22 0.14 south east 11.24 14.47 1.29 0.14 south 10.97 13.85 0.26 0.14 southwest 10.97 12.67 0.16 0.13 west 11.47 10.82 0.94 0.11 north west 10.46 6.75 0.65 0.07 100.00 100.00 6.99 rainfall 35-36 56.54 60.93 1.08 0.16 37-36 4.22 3.77 0.89 0.13 37-36 35.27 32.30 0.92 0.13 37-36 0.41 0.51 1.25 0.18 37-36 1.79 1.96 1.10 0.16 37-37 0.21 0.33 1.55 0.22 38-37 1.56 0.21 1.13 0.02 100.00 100.00 7.91 distance to road 0-400 6.79 3.76 0.55 0.12 400-800 13.01 6.78 0.52 0.11 800-1200 10.52 8.17 0.78 0.17 1200-1600 8.31 8.20 0.99 0.22 1600-2000 7.23 6.38 0.88 0.19 >2400 54.14 66.70 1.23 0.27 100.00 100.00 4.95 distance to river 0-500 6.50 6.74 1.04 0.13 500-1000 6.59 6.65 1.01 0.12 1000-`1500 6.14 6.50 0.06 0.13 1500-2000 17.62 18.91 1.07 0.13 2000-2500 14.77 16.19 1.10 0.13 2500-3000 11.60 12.79 1.10 0.13 3000-3500 8.83 9.44 0.07 0.13 >3500 27.95 22.78 0.82 0.10 100.00 100.00 6.27 distance to fault 0-1000 8.00 13.66 0.94 0.13 1000-2000 7.00 12.21 0.85 0.12 2000-3000 6.00 25.39 1.77 0.25 3000-4000 23.00 33.60 2.36 0.34 4000-5000 14.00 11.86 0.83 0.12 5000-6000 17.00 2.83 0.20 0.03 >7000 25.00 0.45 0.03 0.00 100.00 100.00 6.99 (continued) 63landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis factor classes class pixels (%) landslide pixels (%) fr rf lithology cs 9.28 4.74 0.51 0.04 ks 2.27 0.09 0.04 0.00 mi 4.54 2.25 0.50 0.04 mz 6.08 0.12 0.03 0.00 pz 4.26 3.70 0.40 0.03 s 8.22 9.71 1.18 0.09 ti 6.00 15.11 2.98 0.22 tr 52.75 61.38 1.16 0.08 trms 0.58 2.64 4.57 0.33 ts 2.80 0.03 0.01 0.00 pc 3.21 0.22 0.07 0.00 100.00 100.00 13.75 earthquake very low 34.24 35.98 1.05 0.26 low 20.77 19.78 0.95 0.24 moderate 22.03 20.86 0.95 0.23 high 22.95 23.37 1.02 0.25 very high 0.01 0.00 0.08 0.02 100.00 100.00 4.05 landcover crop land 9.28 4.74 0.51 0.04 grass land 8.00 7.00 0.04 0.00 deciduous forest 4.54 2.25 0.50 0.04 evergreen forest 6.08 4.00 0.03 0.00 sparse vegetation 7.26 1.70 0.40 0.03 bare land 25.00 24.00 1.18 0.09 flooded vegetation 4.63 6.00 0.98 0.22 urban area 20.00 35.00 1.16 0.08 water 0.21 2.30 2.57 0.33 rocky area 15.00 13.00 0.01 0.00 100.00 100.00 7.38 table 3. pairwise comparison between factors used in this study. pairwise comparison of landslide susceptibility factors factors slope rainfall earthquiake lithology distance to fault distance to road landcover aspect distance to stream weight slope 1 5 4 4 2 3 4 7 7 31.8015 rainfall 0.2 1 2 4 5 3 6 5 6 20.6148 eartquake 0.25 0.5 1 2 3 4 5 5 4 14.3954 lithology 0.25 0.25 0.5 1 2 3 3 5 6 10.302 distance to fault 0.5 0.2 0.333333333 0.5 1 2 2 4 5 7.9474 distance to road 0.333333333 0.333333333 0.25 0.333333333 0.5 1 2 3 4 5.533 landcover 0.25 0.16666667 0.2 0.333333333 0.5 0.5 1 2 4 4.2618 aspect 0.142857 0.2 0.2 0.2 0.25 0.333333333 0.5 1 2 2.6566 distance to stream 0.142857 0.16666667 0.25 0.16666667 0.2 0.25 0.25 0.5 1 2.0575 consistency ratio = 0.0851 3.3 landslide susceptibility map we used a gis-based weighted overlay technique to produce the landslide susceptibility maps based on the multi-class weighted factors. these susceptibility maps were further classified into five classes: very high, high, moderate, low, and very low susceptible zones (figures 5-6). the generated maps were validated using the roc curve, which yielded an auc value of 0.803 for the ahp model and 0.732 for the fr model (figure 7). 4. discussion the current study adopted gis-based ahp and fr methods to analyze landslide distribution and construct the susceptibility maps of the district chitral hindukush region. nine landslide-triggering factors were considered for the generation of susceptibility maps. assessment of the existing literature confirmed the causal factors responsible for triggering mass movement (ahmad et al. 2023; guzzetti et al. 2020). subsequently, geospatial analysis was used to order and rank these factors. slope, lithology, and rainfall were identified as basic environments for slope instability and valued as the most significant factors. the major influencing factor was slope, which triggered slope movements and failure. this was consistent with other studies that showed that steep slope regions in mountain terrains were more prone to collapse than gentle slope regions (dai et al. 2001; ghimire, 2011; khan et al. 2020; panday and dong, 2021; zou et al. 2021). the distribution of rainfall in pakistan varies on large scales. it’s mainly related to the monsoon winds and disturbance of western regions like iran and afghanistan, but the precipitation does not occur throughout the year. for example, the northern mountains of khyber pakhtunkhwa and some parts of baluchistan provinces receive heavy rainfall in winter, from december to march. in contrast, sindh & punjab receive 50-75% precipitation during the monsoon period. monsoons are regarded as one of the primary rainfall sources in the whole chitral district. generally, glaciers and snow melting are primarily due to the rainfall spill that frequently occurs in summer. rainfall is a major controlling factor for landslide occurrences (ahmad et al. 2024). 64 mukhtar s. ahmad, saad khan, nazir ul islam, fazle yar khan, imran ahmad figure 5. landslide susceptibility map using the ahp method. figure 6. a landslide susceptibility map using the fr method. figure 7. roc curves and auc values of the ahp and fr models a data set’s quality directly impacts the overall quality of the outcomes (gorsevski et al., 2006). among the fundamental and crucial data sets is the landslide inventory. any inaccuracies about the location and recent movement of landslides can significantly undermine the accuracy of the final map. to address this issue, we conducted a study where we utilized satellite imagery to create a landslide inventory. furthermore, field observations were performed to validate the identified landslide activity spots. all these thematic maps were further processed and compiled by ahp and fr methods. many researchers used the ahp-based weighted overlay technique to prepare landslide susceptibility maps (ahmed, 2015; das et al. 2022; ahmad et al. 2023). the accuracy of the final created map in the current project was 73% and regarded as suitable. the cr value remained below 0.10 for each event, indicating 65landslide distribution and susceptibility assessment in nw pakistan: insights from field observations and factor analysis suitable and consistent weighting criteria (bourenane et al. 2016; pradhan and kim, 2016). however, the ahp-based model has been criticized due to the expert-opinion-based subjective approach (kumar et al. 2023; ali et al. 2019. hence, we performed spatial analysis to rate all causal factors to reduce the risks of inaccuracies associated with the cognitive limits of experts. all these factors were ranked based on the consistency ratio, suggesting that landslide in the study area is mainly related to steep slope regions followed by rainfall and earthquakes. some seismic events and earthquake tremors cause rock to fall nearby and along the highway. slope, rainfall, seismicity, and lithology have been considered highly rated. in contrast, land cover and distances to streams, faults, and roads have less importance because of their poor linkage with landslide events in the study area. the results obtained from the fr technique were also used for the final construction of the ls map (figure 6), representing the north, northwest, and northeast areas in a very highly susceptible zone. in contrast, the south and southwest sides lie in moderate to high susceptible zones. the ahp map also reflects the same susceptible zones. afterward, the roc model was also applied to the fr technique, which revealed a value of 0.80 and ahp rated 0.73. the auc values reported in this study imply satisfactory performance for both the ahp and fr approaches in landslide susceptibility mapping, according to the class ranges suggested for auc interpretation (hosmer et al. 2013). however, the fr method achieved slightly better than ahp. therefore, the fr technique is a simple and easy way to value various parameters consistently, as evidenced in other previous works (yilmaz, 2009; park et al. 2013; ding et al. 2017; shana et al., 2018) meanwhile, some studies have reported a similar overall accuracy rate in models such as ahp and fr (acharya and lee, 2019). hence, according to our research output, we conclude that the fr-based auc model has better performance and reliable accuracy in determining landslide susceptibility across the study area. 5. conclusions this study developed a landslide susceptibility map of the chitral region using gis, integrating the analytic hierarchy process (ahp) and frequency ratio (fr) methods. nine controlling factors were weighted and analyzed: slope, lithology, faults, land cover, rainfall, seismicity, distance to roads, distance to streams, and slope aspect. the final landslide susceptibility map categorizes the region into five hazard zones, i.e. very low, low, moderate, high, and very high, with 73 % (auc value) indicating satisfactory performance. key findings reveal that slope is the dominant factor for triggering landslides, followed by rainfall and seismicity. the reshun fault and main karakoram thrust are identified as high-risk seismic zones, where landslides frequently occur along the road networks passing by these faults. a rockfall and debris slide along road sections are clear examples of active seismicity in the region. the susceptibility map is a valuable tool for effective mitigation and planning management, assisting in risk-informed infrastructure development, emergency response strategies, and resource allocation. first, the accuracy and reliability of the landslide susceptibility map should be enhanced by incorporating additional relevant datasets, exploring advanced techniques and models, and improving the mapping process. future studies should enhance map accuracy by incorporating additional datasets, advanced modeling techniques, and field 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(2010). development of landslide susceptibility map utilizing remote sensing and geographic information systems (gis). disaster prevention and management: an international journal, 19(1), 59-69. _hlk190840650 _hlk190842082 _hlk190842424 _hlk190842499 _hlk190842624 _hlk190840898 _hlk190842783 _hlk190843215 _hlk190843478 _hlk172030738 _hlk172030673 _hlk172030590 _hlk172030329 _hlk172030173 _hlk172030112 _hlk172029944 _hlk190842306 _hlk172031076 _hlk171769834 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.112376 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 277 285 g eo ph y si c s spatial porosity modeling in the mixed siliciclastic-carbonate kazhdumi reservoir in an iranian oilfield located in abadan plain zahra sadeghtabaghi*, & vali mehdipour 1. exploration and production, dana energy co, tehran, iran *zahrasadeghtabaghi@aut.ac.ir how to cite item: sadeghtabaghi, z., & mehdipour, v. (2024). spatial porosity modeling in the mixed siliciclastic-carbonate kazhdumi reservoir in an iranian oilfield located in abadan plain. earth sciences research journal, 28(3), 277-285. https://doi.org/10.15446/esrj.v28n3.112376 record manuscript received: 05/01/2024 accepted for publication: 18/10/2024 abstract spatial porosity modeling is vital in static modeling, as variations in both lateral and vertical dimensions significantly influence reservoir quality and volumetric calculations. this study focuses on the spatial modeling of effective porosity in the mixed siliciclastic and carbonate kazhdumi reservoir within a section of the abadan plain. despite numerous studies on porosity distribution, a suitable model for the studied area has yet to be established. here, electrofacies analysis is employed to distribute porosity more accurately. petrophysical logs, including porosity and mineral volumes from eight drilled wells in the abadan plain, were analyzed. electrofacies analysis revealed high porosity in sandy siliciclastic intervals, contrasting with shaly facies that exhibit poor reservoir characteristics. additionally, two calcareous facies were identified. data analysis linked porosity to facies codes, ensuring a better match between porosity and relevant facies distributions. the constructed porosity model of the kazhdumi formation is a significant outcome of this study. on average, carbonate intervals exhibit 6% porosity, while sandy intervals exhibit 8%. notably, the northern and western parts of the studied area display increased porosity, reaching up to 9%. siliciclastic intervals generally exhibit higher reservoir quality compared to carbonate intervals. keywords: kazhdumi reservoir; effective porosity; electrofacies; spatial modeling; palabras clave: reservorio de kazhdumi; porosidad efectiva; electrofacies; caracterización del espacio. caracterización del espacio poroso en el reservorio siliclástico y carbonatado de kazhdumi, en un campo petrolífero en la llanura de abadan, irán la caracterización del espacio poroso es determinante en el modelado estadístico de un reservorio ya que las variaciones laterales y verticales influyen significativamente en su calidad y en los cálculos volumétricos. este estudio se enfoca en la caracterización espacial de la porosidad efectiva del reservorio siliclástico y carbonatado de kazhdumi, en una sección de la llanura de abadan. a pesar de numerosos estudios en la distribución de la porosidad, aún no se ha establecido un modelo apropiado para el área de estudio. en este estudio se utilizó el análisis de electrofacies para distribuir la porosidad con mayor exactitud. se analizaron registros petrofísicos, entre ellos la porosidad y el volúmen mineral, de ocho pozos perforados en la llanura de abadan. los análisis de electrofacies revelaron una alta porosidad en los intervalos arenosos siliclásticos, en contraste con las facies esquistosas de los reservorios de parámetro pobre. adicionalmente se identificaron dos facies calcáreas. en el análisis de datos se vinculó la porosidad con los códigos de las facies con el fin de asegurar un mejor acople entre la porosidad y las distribuciones de las facies relevantes. el modelo de porosidad construido para la formación de kazhdumi es un logro significativo de este estudio. en promedio, los intervalos carbonatados muestran el 6 % de porosidad, mientras que los intervalos arenosos muestran el 8 por ciento. se resalta que en las partes hacia el norte y hacia el oeste del área de estudio se registró un incremento en la porosidad de hasta el 9 por ciento. los intervalos siliclásticos generalmente muestran parámetros mayores de la calidad del reservorio en comparación con los intervalos carbonatados. resumen https://doi.org/10.15446/esrj.v28n3.112376 mailto:zahrasadeghtabaghi@aut.ac.ir https://doi.org/10.15446/esrj.v28n3.112376 278 zahra sadeghtabaghi, vali mehdipour 1. introduction reservoir quality assessment is a vital component of field studies that underpins commercial development. among different parameters affecting reservoir quality, porosity is a substantial factor. since porosity is an important parameter in identifying and understanding the behavior of an oil reservoir, its proper spatial distribution is necessary. in fact, porosity is the fundamental rock property which affects the reservoir fluid volume directly (wong et al., 1995). in addition to reserve estimation, porosity model can be applied for reservoir simulation, pore pressure prediction, etc. (dvorkin and nur., 2002). the spatial distribution of porosity parameter has always been a favorite subject for researchers due to its effect on reservoir management and development. spatial behavior of the porosity in shurijeh b gas reservoir in khangiran gas field has been assessed by hosseini et al., 2019. they used 32 wells information to model porosity applying kriging and sequential gaussian simulation methods. artificial neural network method has been used as a strong tool for this purpose (esmaeilzadeh et al., 2013; zargari et al., 2013; arabani and nabi-bidhendi, 2002; no., 2011). also, other geostatistical methods such as ordinary kriging have been used for spatial porosity distribution in the asmari reservoir of mansuri oilfield (nazarpour et al., 2014). although, previous researches reveal valuable results, they are limited to using geostatistical and artificial intelligence methods for determining spatial distribution of porosity. in this paper, the electrofacies analysis has been applied to be link to the porosity parameter in order to achieve more appropriate spatial distribution. in addition, grfs algorithm was applied for porosity distribution. moreover, it must be emphasized that porosity variation of mixed siliciclastic and carbonate of kazhdumi reservoir has not yet been studied in this area. as a main reservoir property, porosity can be distributed using geostatistics in 3d static reservoir models (huysmans and dassargues., 2013). the reliability of the porosity model is influenced by several factors, including the quality of porosity interpretation, the number of wells, cell size, upscaling methods, geostatistical algorithms, and secondary variables. incorporating microfacies, electrofacies, or seismic facies can also enhance the understanding of porosity distribution. furthermore, utilizing an acoustic impedance cube as a secondary variable for co-simulation can provide significant value in the modeling process. to construct a proper model of porosity, an appropriate zonation is required. obviously, reservoir zonation plays a vital role in field development programs and production management. in this regard, applying sequence stratigraphic concept can provide an efficient reservoir zonation (magalhães et al., 2020). due to the efficiency of using sequence stratigraphy in reservoir zonation, widespread researches have been done. for instance, sequence stratigraphic approach has been applied for carbonate, siliciclastic, and carbonate-siliciclastic reservoirs (jodeyri-agaii et al., 2018; mehrabi et al., 2019; kiani et al., 2022, asadi et al., 2022; magalhães et al., 2020; enayati– bidgoli and rahimpour-bonab., 2022). jodeyri-agaii et al., (2018) conducted a detailed investigation into the depositional and diagenetic characteristics of mishrif carbonates within a sequence stratigraphy context. their research demonstrated that substantial portions of the mishrif reservoir are shaped by diagenetic processes associated with subaerial exposure, resulting in the development of zones characterized by increased storage capacity and enhanced rates of fluid flow. mehrabi et al., (2019) examined the reservoir rock typing and zonation of the cretaceous dariyan formation through the concepts of sequence stratigraphy. their analysis revealed that the reservoirs associated with the lower and upper carbonate units of the dariyan formation possess distinct lithofacies and diagenetic modifications. these variations are attributed to deposition within two intrashelf basins situated in the northwestern and southeastern regions of the persian gulf, as well as subsequent exposure to meteoric water flow during episodes of subaerial exposure. in this paper, despite previous publications, sequence stratigraphic zonation and electrofacies analysis have been used to reach better porosity distribution. in other words, the 3d effective porosity model of the kazhdumi reservoir in the studied field (located in the abadan plain) has been modeled using electrofacies of mixed siliciclastic and carbonate facies. to that end, as the primary step, the zonation of the kazhdumi formation has been done using sequence stratigraphic concept. subsequently, interpreted petrophysical logs, which include mineral volumes and effective porosity, were utilized for electrofacies analysis through the application of the mrgc method. in the next step, the electrofacies codes were modeled using sis algorithm. eventually, the porosity has been modeled by applying grfs algorithm and linking to the facies codes. therefore, average maps were generated based on constructed models which can provide a great insight into the lateral changes of porosity along the studied area which is a beneficial achievement. following the introduction, the geological characteristics of the studied area, along with a detailed overview of the kazhdumi formation, will be presented. the methodology section will outline the key concepts related to sequence stratigraphy, zonation, and modeling. finally, in the results section, the constructed porosity model will be evaluated, and maps depicting the porosity distribution in sand and carbonate facies will be generated to illustrate the lateral variations. 2. geological setting the zagros basin includes both carbonate (such as ilam, sarvak, etc) and siliciclastic (kushk sandstone and azadegan sandstone) reservoirs. the studied field is located in the abadan plain. the abadan plain, situated in southwestern iran, is characterized as a structural subzone. this is limited by zagros mountain, persian gulf, and iraq (motiei, 1993). abadan plain is located in the northwest of the persian gulf (figure. 1). the structures of this area follow the n-s trend. geophysical studies and previously drilled wells are two important sources of information in this area. kazhdumi formation was deposited in the late early cretaceous (albian). the kazhdumi formation, part of the cretaceous bangestan group, is exposed in the dezful embayment and the fars and izeh zones of the zagros region (motiei, 1993). this formation is conformably situated above the dariyan formation and is underlain by the sarvak formation (james and wynd, 1965). the reference section of the kazhdumi formation, characterized by a thickness of 210 meters, is located in tang-e-gurguda, approximately 10 km north of gachsaran city, as originally described by james and wynd (1965). the formation is widely distributed across southwestern iran, particularly in the dezful embayment, the abadan plain, and the northwestern persian gulf. the kazhdumi formation was deposited in a mixed environment comprising both carbonate and clastic materials. it includes calcareous and dark bituminous shales that interdigitate with argillaceous limestone and sandstone, especially in its lower section. a significant rise in sea level during the albian period led to the deposition of dark organic-rich shales and argillaceous limestones of the kazhdumi formation in iran, as well as its equivalents, such as the nahr umr formation, in neighboring regions (motiei, 1993; ghazban, 2009). the presence of ammonite fossils and a foraminiferal-oligosteginid assemblage indicates the formation’s origination in deep marine conditions characterized by anoxia and a connection to the neo-tethys (ziegler, 2001). the basal sandstone and shale layers of the kazhdumi formation are referred to as the “azadegan sandstone member” (esrafili-dizaji and rahimpour-bonab, 2019; mehrabi et al., 2019). in the dezful embayment, the kazhdumi formation has been referred to as the source of the accumulated oils in bangestan group. it is also the most important source rock for the reservoirs accumulated in asmari formation. the kazhdumi formation in the studied field has carbonate and siliciclastic intervals which both have hydrocarbon potential (figure 2). the azadegan sandstone member (siliciclastic) of the kazhdumi formation with a sandstone-shale lithology is known as a reservoir in some iranian offshore oil fields (alsharhan, 1994). in the studied field, azadegan sandstone member is one of the important reservoir intervals, although the carbonate interval of the kazhdumi formation also has notable potential. it is a well-known source rock in the zagros basin (bordenave and burwood, 1990 and 1995; bordenave and huc, 1995; shakib, 1987). in the studied field, thickness of the kazhdumi formation is about 230 m, including about 58 m for azadegan member. 279spatial porosity modeling in the mixed siliciclastic-carbonate kazhdumi reservoir in an iranian oilfield located in abadan plain figure 1. the abadan plain location (shabani et al., 2020) figure 2. simplified stratigraphic chart of abadan plain. please note that the dominant lithology in each formation is presented, and this column is drawn in a simple and general form (sadeghtabaghi et al., 2024). 3. methodology in this study, data sets of an iranian oilfield (eight wells) have been used. petrophysical data (conventional raw and result logs), well deviation data, well coordinates, depth surfaces, and formation tops are the applied data of this paper. figure 3 illustrates the fundamental steps undertaken in this paper. a detailed description of these steps is provided in the subsequent sections. figure 3. schematic of this paper’s steps. 280 zahra sadeghtabaghi, vali mehdipour reservoir zonation estimating rock porosity and its spatial distribution in a heterogeneous reservoir is a significant step in static reservoir modeling. however, separating the reservoir and non-reservoir rocks (zonation) in heterogenous reservoir is very important. in this paper, reservoir zonation has been done based on sequence stratigraphy concept. reservoir zonation is one of the most important applications of sequence stratigraphy. sequence stratigraphy examines how sediment responds to variations in sea level and the resulting depositional patterns that arise from the interaction between accommodation (the space available for sediments) and sedimentation. this field offers significant insights into the earth’s geological history, revealing responses to both local and global changes, and enhancing predictive capabilities in economic exploration and production. the foundational principles of sequence stratigraphy largely transcend the specific depositional environments found within sedimentary basins, such as siliciclastic versus carbonate settings. typically, clastic systems are used to illustrate these concepts (van wagoner et al., 1988; catuneanu et al., 2009). researchers often describe sequence boundaries and stratigraphic units in relation to shifts in relative sea level, which involve a combination of global eustatic sea level changes and regional subsidence due to tectonic activity, thermal effects, and load-induced subsidence. the alterations caused by these vertical forces either increase or decrease the accommodation space available for sediment accumulation in a basin. additionally, the rate of sediment supply influences how quickly that space is filled. sequence boundaries are considered the most critical surfaces and are defined as unconformities or their corresponding conformities, typically resulting from a fall in sea level. the idea of systems tracts has emerged to connect contemporary depositional systems. these tracts serve as subdivisions within a sequence, classified based on their strata stacking patterns, sequence position, sea level fluctuations, and types of bounding surfaces. • a low-stand systems tract (lst) occurs when sedimentation rates exceed the rate of sea level rise early in the sea level curve, defined by a subaerial unconformity or its equivalent at the base and a maximum regressive surface at the top. • a transgressive systems tract (tst) is delineated by a maximum regressive surface at the base and a maximum flooding surface at the top, forming when the rate of sedimentation cannot keep up with rising sea levels. • a high-stand systems tract (hst) develops during the later phase of rising base levels, when the rate of sea level rise falls below the sedimentation rate, resulting in a high-stand period defined by a maximum flooding surface at the base and a composite surface at the top. • a regressive systems tract forms in the marine portion of the basin during a fall in base level, coinciding with the development of subaerial unconformities on the landward side of the basin (van wagoner et al., 1988; catuneanu et al., 2009). inpefa, an acronym for integrated prediction error filter analysis, is a methodological framework that facilitates the transformation of the gamma ray (gr) curve into a spectral trend attribute curve, commonly referred to as the inpefa curve. this analytical technique is particularly adept at processing facies-sensitive well logs. the resultant inpefa curves elucidate discontinuities present in the log data, which, from a geological perspective, can be interpreted as hiatuses, erosional surfaces, or shifts in lithology and sedimentation rates. given that these geological phenomena may occur at both field and regional scales, inpefa curves provide significant utility in well correlation and reservoir zonation (nainggolan et al., 2019; soua., 2012). the inpefa curve represents a robust alternative method for interpreting wireline log (gr) data, allowing for the extraction of key sequence stratigraphic information and offering a comprehensive framework for understanding sequence stratigraphy. this curve effectively captures the succession of climatic phases during sediment deposition and is principally governed by the processes of relative transgression and regression occurring across the sedimentary basin throughout the sedimentation process. consequently, the inpefa curve delineates the ordering of sequences that evolve within a stratigraphic package, thereby contributing to a refined understanding of sedimentary processes and stratigraphic architectures (nainggolan et al., 2019; soua., 2012). as known, sequences can be defined based on stacking patterns and stratigraphic surfaces (magalhães et al., 2020). gamma-ray data can be used for reservoir zonation based on sequence stratigraphy concept using some softwares such as cyclolog which applied in this study (emery and myers, 1996). the system tracts, along with their associated boundaries—such as the maximum flooding surface (mfs) and sequence boundary (sb)—are identified through variations in the gamma ray log, which exhibit decreasing and incremental patterns. as decreasing trend of the gamma ray log, represents the regressive system tract and increasing trend of the gamma ray log, indicate transgressive system tract (sarg, 2001). the cyclolog software has been used in this paper which applies inpefa trends. electrofacies analysis in this study, geolog software has been used for electrofacies analysis. the electrofacies determination and clustering were done using facimage module of this software. the term “electrofacies” was originally defined as a set of log responses that characterizes a bed and permits it to be distinguished from others (serra and abbott, 1980). these properties are defined by the physical characteristics of the rocks to which the wireline logs respond. effective porosity, along with the mineral volumes of calcite, quartz, and shale, were chosen as the input interpreted logs. after normalizing the input data, the logs were trained using the mrgc algorithm. figure 4 displays the cross plots of the input logs organized into different clusters. figure 4. cross plots between input logs in different clusters (kazhdumi formation from the eight studied wells). note that phie refers to effective porosity, while vol_calcite, vol_quartz, and vol_wcs represent the volumes of calcite, sand, and shale, respectively. the colors utilized in the figures are designated according to facies classes. for understanding these colors, please refer to the table. 4. property modeling the primary purpose of creating a static model is to create a 3d representation of the reservoir in order to assist in the development of the hydrocarbon resources. static modeling consists of two important steps structural and property modeling followed by volumetric calculation, uncertainty analysis, and sensitivity analysis. structural modeling contains fault modeling, 3d geocellular modeling, making horizons, making zones, and layering (mehdipour et al., 2023). property modeling is the process of assigning 281spatial porosity modeling in the mixed siliciclastic-carbonate kazhdumi reservoir in an iranian oilfield located in abadan plain data, discrete (facies) or continuous petrophysical properties (such as porosity, and permeability …), to the cells of the 3d fine grid. sequential indicator simulation (sis) is a variogram-based technique for categorical simulation that has gained considerable traction in the modeling of categorical variables. this method is particularly advantageous in situations where there are no discernible genetic shapes amenable to object-based modeling. in the context of petrophysical modeling, sequential gaussian simulation (sgs) has been found to exhibit slower computational performance when compared to gaussian random function simulation (grfs). this disparity in speed arises from sgs’s sequential computation of input variables, which enables it to encompass a broader spectrum of uncertainty. this sequential characteristic is the fundamental distinction between sgs and grfs; the latter operates with random sampling in a more expedited manner, thereby capturing a narrower range of uncertainty. consequently, the integration of grfs with any facies modeling algorithm generally results in a reduced range of uncertainty when juxtaposed with the integration of sgs with the same algorithmic approaches (alabert, 1987; aduomahor and ibezim, 2020; bai and tahmasebi, 2022). the amalgamation of sis and sgs is noteworthy for yielding the highest range of uncertainty, attributable to the sequential nature inherent in both methodologies. sis employs upscaled cells to establish the proportions of various facies types being modeled, while the variogram serves to regulate the distribution and connectivity among these facies. this renders sis particularly effective for modeling facies characterized by ambiguous or undefined shapes, especially in scenarios where input data are sparse. conversely, sgs is founded on a straightforward and mathematically robust algorithm that, while providing flexibility, does not replicate the input variance with the same level of precision. the synergistic combination of sis and sgs facilitates the effective capture of extreme values, both maximum and minimum; however, it is generally considered to be slower in execution compared to alternative methods (alabert, 1987; aduomahor and ibezim, 2020; bai and tahmasebi, 2022). in this study, a structural model with 100m × 100m gridding was constructed. after structural modeling, the electrofacies codes and effective porosity were scaled up. then, data analysis has been done for both of them. therefore, vertical proportion setting (for efac) and data transformation setting (for phie) were set, and then variography setting has been done for both properties. major, minor and vertical ranges with other necessary parameters for variography of porosity and electrofacies such as sill, azimuth, nugget and variogram type have been shown in table 1 and table 2. table 1. variography setting for porosity distribution m aj or ra ng e (m ) m in or r an ge (m ) ve rt ic al ra ng e (m ) si ll a zi m ut h n ug ge t ty pe from to from to from to from to 0 0.0001 spherical and exponential3571 10790 2817 7149 0.558 10 0.43 33.22 table 2. variography setting for electrofacies distribution m aj or r an ge (m ) m in or r an ge (m ) ve rt ic al ra ng e (m ) si ll a zi m ut h n ug ge t ty pe from to from to from to from to 0 0.0001 spherical 5509 17193 3272 6731 1.04 3.253 0.3895 1.2737 in the next step, 3d modeling of electrofacies and porosity was performed using stochastic simulation algorithms. gaussian simulation methods, including sgs (sequential gaussian simulation), sis, and grfs, are conditional geostatistical techniques that utilize kriging mean and variance to generate a gaussian distribution of parameters (evans annan et al., 2019). in this paper sis and grfs algorithms have been applied for modeling electrofacies and porosity, respectively. the parallel kriging is the base of estimation in grfs which is a conditional simulation method. the algorithm of parallel kriging was introduced in 2008 which is faster than the old algorithm (daly et al, 2010. parallel kriging method results in more accurate estimates (zhuo et al., 2011). in addition to high accuracy, parallel algorithm causes lesser duration of execution (pesquer et al., 2011). comparing sgs, the performance of grfs for porosity modeling is better (daly et al., 2010). 4. results and analysis as mentioned, the reservoir zonation of the kazhdumi formation has been done using sequence stratigraphic concepts. in fact, the previous reservoir zonation (which was solely derived from the responses recorded from the petrophysical logs) has been updated by applying the sequence stratigraphy concept via cyclolog software. this software uses inpefa log for this purpose (figure 5). kazhdumi formation has been divided into two main zones including carbonate and siliciclastic (azadegan sandstone) zones. in turn, the carbonate zone has been divided into 3 subzones and azadegan sandstone has been divided into eight subzones. table 3 indicates reservoir zonation of the kazhdumi formation, in which the bolded zones show the high potential hydrocarbon zones. as shown, this reservoir has been divided into eleven zones. figure 5. reservoir zonation based on sequence stratigraphy concept in one of the studied wells (variation of the inpefa log with depth) table 3. reservoir zonation of the kazhdumi reservoir (the bolded zones indicate the high potential hydrocarbon zones) formation lithology zones phie (fraction) kazhdumi carbonate k1 0.0342 k2 0.0489 k3 0.0154 siliciclastic (azadegan sandstone) k4 0.018 k5 0.0912 k6 0.0055 k7 0.1187 k8 0.0343 k9 0.0913 k10 0.0208 k11 0.0139 282 zahra sadeghtabaghi, vali mehdipour in the next step, the electrofacies analysis has been done. to that end, mrgc method was selected as the most effective approach among all available methods. finally, 24 facies have been classified. then these clusters were merged into five final clusters as shown in table 4. also, the minimum, maximum, mean, and standard deviation values of effective porosity and mineral volumes of all facies codes have been listed in table 5. the reservoir characteristics decreases from facies code 1 to facies code 5. the characteristics of these facies codes are as follows: • facies-1: this sandy facies consists of 69% quartz volume (average value), then it is sandstone. the average value of shale volume in this facies is around 17%. also, due to high porosity (14%) it has the best reservoir characteristics. • facies-2: the main lithology of this facies is limestone and the average content of calcite volume is 90%. the average porosity is 9%. therefore, it can also be considered as a good facies with proper reservoir property. • facies-3: this shaly sandstone facies consists 49% quartz and 47% shale volumes. the average value of porosity is about 4%. • facies-4: the lithology of this facies is limestone having 93% calcite volume. the average porosity is 3%, then this facies has low reservoir quality. • facies-5: this facies which is shale, contains 84% shale volume. the average porosity is 1% which represents a non-reservoir facies. these electrofacies codes are shown in one of the studied wells (figure 6). according to the fig. 6, zones k1, k2, and k3 are predominantly comprised of carbonate, while the remaining zones of the kazhdami formation consist of a heterogeneous mixture of sand and shale. an analysis of the juxtaposition between the mineral volume column and the electrofacies codes reveals that the upper carbonate section is characterized by two distinct electrofacies codes, namely codes 2 and 4. notably, the dominant electrofacies manifestation in this carbonate region is code 4, which is indicative of dense limestone. table 4. final electrofacies classes (for more information on these histograms, please refer to table 5) table 5. minimum, maximum, mean, and standard deviation of effective porosity and mineral volumes of all facies codes facies_1 facies_2 facies_3 facies_4 facies_5 effective porosity (fraction) min 0 0.05 0 0 0 max 0.27 0.17 0.16 0.1 0.09 mean 0.14 0.09 0.04 0.03 0.01 std 0.05 0.02 0.03 0.02 0.01 calcite volume (fraction) min 0 0.69 0 0.57 0 max 0 0.95 0 0.99 0.57 mean 0 0.9 0 0.93 0.09 std 0 0.03 0 0.06 0.13 sand volume (fraction) min 0.33 0 0.22 0 0 max 1 0 0.82 0 0.36 mean 0.69 0 0.49 0 0.07 std 0.11 0 0.11 0 0.11 shale volume (fraction) min 0 0 0.16 0 0.38 max 0.52 0.17 0.68 0.4 1 mean 0.17 0.01 0.47 0.04 0.84 std 0.12 0.02 0.12 0.06 0.13 283spatial porosity modeling in the mixed siliciclastic-carbonate kazhdumi reservoir in an iranian oilfield located in abadan plain figure 6. layout showing the lithology and electrofacies codes descending from zone k4, there is a discernible sequence comprising shale, limestone, and sand. the optimal reservoir characteristics, represented by the electrofacies code for sands with high porosity (electrofacies code 1), are prominently identified in zones k5, k7, k9, and k11. furthermore, from a reservoir property perspective, the sandy intervals within the kazhdumi formation exhibit superior conditions relative to their carbonate counterparts. this suggests a greater potential for hydrocarbon accumulation in the sand-rich zones as compared to the carbonate-rich zones. after electrofacies analysis and constructing the structural model and scaling up the electrofacies and porosity logs, both logs have been distributed via applying geostatistics algorithms. the constructed electrofacies model was checked to provide a great match between initial and modeled data (figure. 7a). it should be noted that data analysis of porosity was linked to facies codes. 3d model of the effective porosity (phie) has been generated using scaled-up effective porosity log into the 3d grid. as mentioned earlier, phie has been linked to the facies. porosity propagation has been done using the grfs algorithm for propagating the properties during reservoir modeling (figure. 7b). the constructed phie model was checked to provide a great match between the initial and simulated data. in order to a better assessment of the kazhdumi reservoir, average porosity maps of both siliciclastic and carbonate intervals of this reservoir are shown in figure 8. as illustrated, there is a general trend that shows porosity increases from southeast to northwest parts of the studied area. it can be concluded that the kazhdumi reservoir shows the best reservoir characteristics in the northern and western parts of the studied area. as illustrated in fig. 8a, the porosity values within the studied area exhibit a significant gradient, increasing from 2% in the southern regions to 12% in the northern regions. it is noteworthy that the areas characterized by favorable porosity in the northern part are discontinuous and fragmented. this discontinuity indicates that caution is warranted in planning the drilling of future wells, as the isolated pockets of porosity may affect hydrocarbon recovery efficiency. moreover, an examination of the carbonate intervals within the kazhdumi formation, as depicted in fig. 8b, reveals a predominance of dense limestone. the sections exhibiting high porosity are primarily confined to the western part of the study area. this spatial restriction of high-porosity zones further underscores the necessity for strategic planning in drilling operations, taking into account the variability in reservoir quality across different regions of the formation. also, the statistics values of thickness and effective porosity of the kazhdumi reservoir have been presented in table 6. as shown, the siliciclastic intervals have high porosity than carbonate intervals. figure 7. electrofacies (a) and effective porosity (b) models of the kazhdumi formation in a 3d view and their relevant histograms 284 zahra sadeghtabaghi, vali mehdipour figure 8. average porosity distribution for a: siliciclastic intervals and b: carbonate intervals table 6. statistics values of thickness and effective porosity of the kazhdumi reservoir parameters lithology min mean max thickness (m) carbonate 132 172 192 sandstone 45 58 93 effective porosity (%) carbonate 0.02 0.06 0.09 sand 0.03 0.08 0.12 5. conclusion the analysis of electrofacies within the kazhdumi formation reveals the presence of five distinct facies codes. among these, two facies exhibit high porosity: facies 1 (siliciclastic) and facies 2 (carbonate). the remaining facies, consisting of shale and carbonate, are classified as non-reservoir facies. notably, electrofacies code 1, which indicates sands with high porosity, is associated with the azadegan member. the upper carbonate sections of kazhdumi formation primarily comprise dense limestone interspersed with layers of porous limestone. despite the azadegan member being characterized by the highest electrofacies code (code 1), it also contains sequences of shale, limestone, and sand that can occasionally diminish reservoir quality. to model the electrofacies codes, a comprehensive data analysis was conducted for all facies. subsequently, the electrofacies were modeled utilizing the sis algorithm, followed by the modeling of effective porosity through a linkage to facies codes using the grfs algorithm. given the superior accuracy and speed of the grfs algorithm, it was favored over the sgs algorithm for this study. the final results indicate that the reservoir characteristics of the kazhdumi formation improve from the southeast to the northwest of the study area. although the reservoir conditions of the azadegan member are enhancing in the northern regions, it is important to highlight that the high porosity zones are discontinuous and warrant further investigation in future drilling operations. additionally, the azadegan sandstone demonstrates higher porosity compared to the carbonate intervals. therefore, it is recommended that detailed regional studies be conducted to better understand the sedimentary environment of the kazhdumi formation and the origins of its sands. 6. nomenclature inpefa integrated prediction error filter analysis efac electrofacies grfs gaussian random function simulation mfs maximum flooding surface mrgc multi-resolution graph-based clustering phie effective porosity sb sequence boundary sgs sequential gaussian simulation sis sequential indicator simulation acknowledgment the authors would like to thank dana energy co. for cooperation in providing data for this paper. references aduomahor, b., & ibezim, o. 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(2024). coastal flood and damage assessment under sea level rise projections: a case study in san francisco de campeche, mexico. earth sciences research journal, 28(3), 295-304 https://doi.org/10.15446/esrj. v28n3.113693 record manuscript received: 27/03/2024 accepted for publication: 23/08/2024 abstract the city of san francisco de campeche, located in the yucatan peninsula, mexico, is vulnerable to coastal flooding due to its geographical location and low altitude terrain. under these inherent site conditions, sea level rise associated with climate change represents a potential threat to people’s property and goods in the coming decades. in this work, three scenarios of sea level rise in the coastal area of the city of san francisco de campeche were evaluated through numerical simulation and using wave and wind data obtained from the era5 reanalysis model as inputs: astronomical tide data and a high-resolution topobathymetric model. the scenarios evaluated correspond to the periods 2031-2050, 2046-2065, and 2081-2100 reported in the latest ipcc assessment report. damage to people’s property and goods was analyzed using the cenapred methodology based on the type of housing. the results allow identifying that the area of the old neighborhood of san román, colonia miramar, pedro sainz de baranda, adolfo ruiz cortinez, resurgimiento avenues, and the campeche-merida and campeche-champoton coastal highway, as well as the federal and state government offices they will be the most affected areas by sea level rise with economic damages exceeding $13,860,322 mexican pesos ($usd 805,832.67) under the 2031-2050 scenario, $14,706,754 mexican pesos ($usd 855,043.83) in the 2046-2065 scenario and $22,536,250 mexican pesos ($usd 1,310,247.09) in the 2081-2100 scenario; and the los petenes biosphere reserve as one of the ecological zones with the greatest extension of flooding; as well as downtown areas of the city that currently have residential, commercial, recreational, and port uses. considering the three scenarios and effects on the population and housing, the 2081-2100 scenario is the one that generates the greatest flooding and with it a greater economic loss that exceeds 12 million dollars compared to the 2046-2050 scenario. keywords: flood risk and damage, sea level rise, numerical model, san francisco de campeche palabras clave: riesgo y daños por inundaciones; aumento del nivel del mar; modelo numérico; san francisco de campeche evaluación de daños e inundaciones costeras bajo proyecciones de aumento del nivel del mar: un estudio de caso en san francisco de campeche, méxico la ciudad de san francisco de campeche, ubicada en la península de yucatán, méxico; es vulnerable a inundaciones costeras debido a su ubicación geográfica y terreno de baja altitud. bajo estas condiciones inherentes al sitio, el aumento del nivel del mar asociado con el cambio climático representa una amenaza potencial a las propiedades y bienes de las personas en las próximas décadas. en este trabajo se evaluaron tres escenarios de aumento del nivel del mar en la zona costera de la ciudad de san francisco de campeche mediante simulación numérica y utilizando como insumos datos de oleaje y viento obtenidos del modelo de reanálisis era5, datos de mareas astronómicos y un modelo topobatimétrico de alta resolución. los escenarios evaluados corresponden a los períodos 2031-2050, 2046-2065 y 2081-2100 reportados en el último informe de evaluación del ipcc. los daños a las propiedades y bienes de las personas se analizaron mediante la metodología del cenapred según el tipo de vivienda. los resultados permiten identificar que el área del antiguo barrio de san román, la colonia miramar, las avenidas pedro sainz de baranda, adolfo ruiz cortínez, resurgimiento y la carretera costera campeche-mérida y campeche-champotón, así como las oficinas del gobierno federal y estatal serán las zonas más afectadas por el aumento del nivel del mar con daños económicos superiores a los $13,860,322 pesos mexicanos ($usd 805,832.67) en el escenario 2031-2050, $14,706,754 pesos mexicanos ($usd 855,043.83) en el escenario 2046-2065 y $22,536,250 pesos mexicanos ($usd 1.310.247,09) en el escenario 2081-2100; y la reserva de la biosfera los petenes como una de las zonas ecológicas con mayor extensión de inundaciones; así como zonas céntricas de la ciudad que actualmente tienen usos residenciales, comerciales, recreativos y portuarios. considerando los tres escenarios y efectos sobre la población y la vivienda, el escenario 2081-2100 es el que genera mayores inundaciones y con ello una mayor pérdida económica que supera los 12 millones de dólares respecto al escenario 2046-2050. resumen https://doi.org/10.15446/esrj.v28n3.113693 mailto:roacanul@uacam.mx https://doi.org/10.15446/esrj.v28n3.113693 https://doi.org/10.15446/esrj.v28n3.113693 298 javier pan barcel, román alejandro canul turriza, violeta z. fernández-díaz, andrea del r. cruz, gabriela arreguín-rodríguez 1. introduction in statistical terms, coastal flooding is one of the world’s most common and widely distributed natural hazards that causes serious natural disasters (balica, 2012; peterson et al., 2013; fernández et al., 2018; bakkensen & blair, 2020). since most coastal areas are exposed to extreme hydrometeorological phenomena and the pressure exerted by anthropic developments on them, with low-lying coastal areas being the most vulnerable to negative effects (church et al., 2014; ipcc, 2019; losada et al., 2020; fernández et al., 2022). under the context of climate change, sea level rise exacerbates the danger of flooding that extreme hydrometeorological events represent for coastal areas, so it is expected that, in the coming decades, coastal floods will be more devastating and costly (ipcc, 2021). from the special report on the ocean and the cryosphere in a changing climate (ipcc, 2019) it was established that if the current rate of greenhouse gas emissions is not reversed, the collapse of the ice masses will be inevitable and the increase in the level of the sea will affect more than 680 million people who live in coastal areas of the world. although the ipcc report (2019) highlights asian countries such as thailand, indonesia or bangladesh as those most affected by sea level rise; in mexico, the latest intercensal survey of the national institute of statistics and geography (inegi) estimates that approximately 55 million people living on the coastal zone of the country will be affected by the rise in sea level in the coming decades, being the campeche coastline is one of the most compromised (climate central, 2021) due to its low altitude, the physical conditions of its coast and the potential damage. in this sense, floods associated with sea level rise represent a strong threat to the exposed coastal population, housing and service infrastructure, related economic activities, coastal ecosystems, and the loss of their environmental services (kopp et al., 2016; adeel et al., 2020; fernández et al., 2022). therefore, the assessment of potential flood-related damage is a key requirement in coastal zone planning and management (bates et al., 2005; hallegatte et al., 2011; gallient et al., 2014; seenath, 2015; seenath et al., 2016; gallient, 2016; rey et al., 2020), and takes special relevance to establish plans for urban development, disaster prevention, emergency response and management and recovery strategies, especially in areas with strong environmental and economic importance, such as campeche. however, first, it is necessary to identify the potential areas to be flooded and characterize them based on the population, type of infrastructure, economic activities, present ecosystems, loss of environmental services, etc., depending on the approach (environmental or socioeconomic). this allows us to better understand the possible consequences of floods on local and regional economies, estimate the comprehensive cost of such disasters, and provide baseline information that contributes to the development of prevention and action strategies. therefore, this work presents the results obtained from the identification, quantification, and evaluation of flood-prone areas and damage caused to homes and household goods in the city of san francisco de campeche (csfc), capital of the state of campeche and a city which has had economic growth due to activities related to its coastal area, which have historically demanded modifications on the beachfront, such as filling land reclaimed from the sea and port, tourist and housing development that has been established at topographical levels. low and prone to flooding. the results represent three sea level rise scenarios coupled with predominant sea states simulated with a twodimensional numerical model and the potential economic damages in the areas identified as flood-prone. although previous studies have been carried out that address the issue of flooding in the coastal area of campeche (palacio et al., 2005; posada et al., 2012, 2013), the quantification and evaluation of areas prone to flooding and the economic loss due to damage to homes and household items under scenarios of sea level rise is non-existent at the moment, so this work contributes to the generation of said base knowledge for decision making. 2. materials and methods study area the city of san francisco de campeche (csfc), capital of the state of campeche, is located in southeastern mexico, in the western portion of the yucatan peninsula (figure 1). given the geographical location of campeche in the yucatan peninsula, it is under the imminent threat of flooding, especially during the passage of high-pressure cold fronts, locally called “nortes”, as well as tropical cyclones (rey et al., 2020). ; since during the passage of these hydrometeorological phenomena, the disturbances in atmospheric pressure and the stress of the wind on the sea surface cause extreme waves and rises in sea level along the coast; the above, combined with the low topography and very gentle slope that induces an increase in exposure and physical vulnerability of the system (enríquez et al., 2010). figure 1. san francisco de campeche localization the csfc has a territorial area of 3,410.64 km2, which represents 6% of the state (inegi, 2020). according to the 2020 population and housing census (inegi, 2020), the city has 86,048 homes and 294,077 people who carry out activities related to its coastal areas, such as tourism, food services, and fishing. this city has a port terminal (dock) oriented towards recreational activities, nautical tourism, and commercial and sport fishing in which a large part of the population works. along the city’s coast, there are various beaches, fishing ports, and arrival docks, such as playa bonita, san román port, and the arrival dock to the city’s water park, as well as the arrival port of the secretary of the navy vii naval region, the lerma fishing industrial unit and the port of the country club and nautical and sports club, to mention a few (figure 2). in addition, the city has infrastructure of great historical importance that is located in a coastal area, since the csfc was a walled city that, currently, has historical buildings that represent economic income for locals through tourist activities. most of the main economic activities carried out in the csfc depend on the coastal zone, which is why the various sectors of the city’s coastal zone gain social and economic relevance. 299coastal flood and damage assessment under sea level rise projections: a case study in san francisco de campeche, mexico a) campeche nautical and sports club, san francisco de campeche, campeche. b) campeche country club, san francisco de campeche, campeche. c) playa bonita, san francisco de campeche, campeche. d) san francisco de campeche dock, campeche. figure 2. representative sites of the csfs coast. historical wave and wind data and tide to characterize the prevailing wave and wind conditions in the csfc coastal zone, historical and robust time series of waves and wind were analyzed for the period from january 1979 to december 2021 taken from the ecmwf v5 (era5) reanalysis model, a 5th-generation model that provides hourly estimates of various atmospheric variables. the time series is composed of hourly data for a point in deep waters at 20 km from the analyzed coast (19.97° n -90.69°w). figure 3 shows the data of the wave parameters (significant height (a), peak period (b), and direction of incidence (c)) obtained for the mentioned period. these data were analyzed using statistical methods focused on establishing the average annual and extreme (storm) conditions (regime). the above allowed us to establish a value of 1.5 m wave height as the storm threshold in the region (figure 3e). the wave parameters selected as predominant in the medium regime are a wave height of 0.5 m, a peak period of 6 seconds, and an incidence direction of 22.5° (figure 3d). in the case of wind, the predominant values in the medium regime are a speed of 6.5 m/s and a direction of 292.5° (towards where the wind is directed) (figure 4c), and in storm conditions speed of 10.65 m/s and direction 292.5° (figure 4d). tidal conditions were obtained from data records of the global tidal model development by dtu space (dtu10) representing the main diurnal currents (k1, o1, p1, and q1) and semidiurnal tidal components (m2, s2, n2, and k2) with a spatial distribution of 0.125° x 0.125° resolution based on topex/ poseidon altimetry data (data applied at 20 m depth). the admiralty method was used, which explicitly considers the four principal components m2, s2, 01, and k1 and allows corrective factors to consider the effects of a series of astronomical components generated in shallow water. figure 3. wave characterization on the coast of campeche, period 1979-2021. 300 javier pan barcel, román alejandro canul turriza, violeta z. fernández-díaz, andrea del r. cruz, gabriela arreguín-rodríguez figure 4. characterization of the wind on the coast of campeche, period 1979-2021. topo-bathymetric survey and numerical modelling to represent the topographic and bathymetric terrain of the study area, a digital topobathymetric model was generated from the union of lidar data with a horizontal resolution of 5 m obtained from official inegi bases and digitized data from a nautical chart obtained from the base world c-map. data joining was performed using geoprocessing tools (figure 5a). from this model, a flexible mesh was generated with three open borders and one land border. the mesh is composed of 177,956 elements and 89,270 nodes, where the minimum cell resolution is 7 m2 and the maximum is 72,262 m2 (figure 5b). figure 5. topobathymetric model (a) and flexible simulation mesh (b). the sixth assessment report on climate change of ipcc (ar6) assumes that sea level rise scenarios worsen the threat of flooding in coastal areas around the world. the ar6 is based on five shared socioeconomic pathways (ssp), which are scenarios of global socioeconomic changes projected until the year 2100 and that are related to greenhouse gas emissions under different climate policies. the scenarios are named ssp1, ssp2, ssp3, ssp4 and ssp5; the latter is the most unfavorable scenario since it assumes that markets are very competitive and integrated, with enormous exploitation of fossil fuels and the adoption of resource and energy intensive lifestyles. here we use the ssp5 scenario under three-time horizons 2031-2050, 2046-2065 and 2081-2100 (called scenarios), for each scenario the projected increase in sea level was identified (table 1). hydrodynamic numerical simulations were carried out to understand the behavior resulting from the coupling of the established wave, wind and tide conditions that are the main forcing in the numerical model, and each of the three projected sea level rise scenarios. table 1. sea level rise scenarios considered in numerical simulations. horizons sea level rise projection (m) ssp 8.5 2031-2050 0.26 2046-2065 0.4 2081-2100 0.92 the topobathymetric model and sea level rise scenarios, in conjunction with the predominant states of waves, wind, and tidal data described in the previous section, were used to feed a state-of-the-art two-dimensional numerical model that solves the equations of conservation of mass and momentum; allowing to specify the variety of boundary conditions, initial conditions and forcing (tide, wind, and waves), which allows simulating variable flows in two horizontal dimensions and a single vertical layer and reproducing flooding conditions on the coast. simulations were carried out for each scenario established for each time horizon. subsequently, the results of the numerical simulations were migrated to a geographic information system that made it possible to identify flood zones. from the results of the numerical simulations, the minimum, average, and maximum values for the surface variables of elevation, direction, and current speed were obtained. the maximum values obtained were considered the most unfavorable conditions that may occur in the area and with them, maximum flood maps were generated for each scenario. for this work, the increase in sea level was considered as a constant value of increase with which the tide height was affected, likewise for the wind and wave data, the data projected for the years 2050, 2065, and 2100 were used, extreme years of the periods considered. flooded areas and loss quantification an approximation was made to the number of inhabitants directly affected, as well as the quantification of the loss due to household goods in the flooded areas for each simulated scenario. for this, the floodable areas resulting from the numerical simulations were identified, and taking information from the ageb (inegi, 2020) in the study area, the flood area was quantified in hectares by applying geoprocessing tools. an ageb is defined as a geographical area occupied by a set of blocks (minimum unit) perfectly delimited by streets, avenues, walkways, or any other easily identifiable feature on the land and whose land use is mainly residential, industrial, services, businesses, among others, and are only assigned to the interior of urban areas, identified as those with a population greater than or equal to 2,500 inhabitants (inegi, 2020). in the ageb identified as flood-prone, based on data from the population and housing census (inegi, 2020), the number of homes and people living in each affected ageb was quantified. the information was analyzed using geoprocessing tools that made it possible to characterize the flood zones at the block level and the total number of people and homes exposed. the approach to damage to household items was quantified based on the study carried out by the autonomous university of campeche (uacam) together with the state emergency center (cenecam) in 2015; in this study, the average costs of damage to household goods caused by floods were obtained for different housing models classified based on the material used for the construction of walls and roofs. table 2 presents the characteristic housing models for the entire state of campeche; specifically for the coastal area of san francisco de campeche, the characteristic type of housing is type no. 1, in which the wall and roof are made of partition due to the type of economic activity and capital gains in the area (posada et al., 2015). because the values of the cost of damage to household items presented in table 2 are adjusted to the year 2015, in this work they were updated for the year 2022; for each year, annualized inflation was used, the cost of the previous year was considered and the inflationary increase in the cost was added, thus obtaining the cost of household items for the current year (table 3). for the year 2022 amounts to 112,505.60 mexican pesos (which is equivalent to $usd 5,625.28) for the housing model characteristic of the coastal area of the city. 301coastal flood and damage assessment under sea level rise projections: a case study in san francisco de campeche, mexico table 2. housing models for campeche (derived from cenecam, 2015) model no. wall ceiling cost for 2015 (mexican pesos) 1 partition concrete $80,312.50 2 partition roof tile $48,677.50 3 partition galvanized sheet $48677.50 4 partition wood $106,130.00 5 wood roof tile $39,682.50 6 wood galvanized sheet $34,107.50 7 wood wood $21,377.50 8 wood straw $19,932.50 9 galvanized sheet galvanized sheet $38,075.00 10 galvanized sheet wood $3,770.00 it is important to mention that, in this work, only direct damage to household goods is considered for model no. 1 and the damage or indirect losses caused by the closure of roads, stoppage of economic activities, deaths, and damages are not quantified psychological, etc. table 3. update of the cost of damage affected by annual inflation year annual inflation cost mexican pesos cost dollar (exchange of 17.2 pesos per dollar) 2015 $80,312.50 $4,669.33 2016 2.82 $82,577.31 $4,801.00 2017 6.04 $87,564.98 $5,090.98 2018 4.9 $91,855.67 $5,340.44 2019 3.64 $95,199.21 $5,534.83 2020 3.4 $98,435.99 $5,723.02 2021 5.69 $104,036.99 $6,048.66 2022 8.14 $112,505.60 $6,541.02 3. results the coastal zone of the city of san francisco de campeche was divided into three zones to represent the flood results for each scenario. these areas correspond to: petenes – imi (northeast), historic center – august 7 (center) and lerma (southwest). for the 2031 – 2050 scenario, the maximum flood depth reaches 2 m, mainly in the area northwest of the city, known as los petenes; in this area, a portion of the biosphere reserve of the same name is identified, as well as housing units; while in the area of the boardwalk, which is tourist, the flood depth does not exceed 50 cm; with the exception of the area known as august 7, an area with commercial and residential activity, where the flood depth reaches values close to 1.5 m (figure 6c). in the lerma area the flood does not exceed 20 cm. for the 2046 – 2065 scenario, the maximum flood height corresponds to 2.5 m. the difference with the previous scenario is that the extent of flooding increases, presenting greater impact in the mangrove area of los petenes and the first square of the residential area imi (figure 7b). under this scenario, in the seawall area the depth is increased in the range of 0.5 m to 1.0 m; likewise, the extension of the flooded area increases and moves to the area known as “el nuevo campeche”, which corresponds to the area of the historic center bordering the sea, where a depth of up to 0.5 m of flooding is observed. the natural drainage “la ría” overflows and the water move between the francisco i. madero roundabout and the so-called “puente”, very close to the august 7 area. in the lerma area the water column increases up to 0.50 m, due to the high terrain (figure 4d). figure 6. surface elevation 2031 -2050 scenario. a) general map of the city, b) petenes imí area, c) historic center area and part of august 7 and d) lerma area. figure 7. surface elevation 2046 2065 scenario. a) general map of the city, b) petenes imí area, c) historic center area and part of august 7 and d) lerma area. figure 8. surface elevation 2081 2100 scenario. a) general map of the city, b) petenes imí area, c) historic center area and part of august 7 and d) lerma area. while for the 2081 – 2100 scenario the flood height is greater than the 2.5 m predominant in the los petenes area, here a greater intrusion of the sea is observed. the average height of the tie ranges from 2.0 m. it is in this area where a flood of the population center called imí (figure 8b), fidel velázquez and palmas can be seen with heights of up to 1.5 m; likewise, federal highway 302 javier pan barcel, román alejandro canul turriza, violeta z. fernández-díaz, andrea del r. cruz, gabriela arreguín-rodríguez 180 (the only access to the city through this quadrant) is flooded with heights of up to 1.0 m. with respect to the boardwalk area, a height of up to 1.0 m is observed. the area of august 7 and the so-called “la ría” presents flood levels above 3.0 m. the flooding in the city occurs in the so-called “campeche nuevo” with flood heights ranging in some areas from 0.5 m to 1.5 m (historic center); likewise, it is observed that the structure of the dock is completely flooded due to a level of 1.5 m. at the other end of the city, in the population center of lerma, the port infrastructure is affected due to the flooding of the docks, with heights close to 0.65 m. likewise, the campeche country club tourist complex is affected coastal zone due to flood levels that range between 0.80 and 1.0 m. of the 3 scenarios analyzed, it stands out that the 2018 – 2100 scenario is the one that represents the greatest amount of economic losses directly. for the first scenario (2031 – 2050) the number of affected houses amounts to 131, which is equivalent to an area of 323 hectares flooded and damage to household goods in the amount of $13,860,322 mexican pesos (which is equivalent to $usd 805,832.67). the 2046 – 2065 scenario generates a direct economic loss of $14,706,754 mexican pesos (which is equivalent to $usd 855,043.83), affecting 139 homes that represent an affected area of 330.73 hectares. the 2081 – 2100 scenario affects an area of 376.30 hectares with damage to 213 homes that represents an economic loss of $22,536,250 mexican pesos (which is equivalent to $usd 1,310,247.09). another important parameter in the analysis of floods is related to speed; indirectly, the speed influences the flooding speed. regarding speed, it is observed that the 2081 2100 scenario is the one with the highest current speed; however, occasionally in the 2031 2046 scenario there is an area that reaches up to 2.5 m/s (area with a large slope in which a valley is formed), while in the 2031 2050 scenarios the average speed oscillates between 0.36 to 0.40 m/s in the dock area (green color) and for the 2046-2065 scenario the current speed increases slightly in the same area, reaching values between 0.40 and 0.44 m/s. compared to the 2031 2046 scenario, in 2056 2065 a larger area is identified with speeds greater than 0.50 m/s and predominates in the dock area, towards the northeast (figure 9b). for the 2081 2100 scenario, the speed reaches maximum values of 0.60 m/s and the highest values occur from the los petenes area to the so-called aquatic park following the coastline to the southwest (figure 9c). figure 9. current speed: a) 2031 2050 scenario; b) 2046 2065 scenario; c) 2081 2100 scenario. 4. discussion over the years, the coastal area of the city of san francisco de campeche has gone from residential to commercial use; currently, the city’s boardwalk represents the focal point not only in terms of real estate but also for the commercial, tourist and recreational activity of the city. during the 20th century there was a substantial change in the infrastructure of the coastal area, around 1962 a process of filling in the coastal area began (in front of the old seawall) whose main land use was focused on the commercial and tourist sphere; and the housing area expands to the west (town of lerma). towards the east of the city, an expansion of the urban area begins, currently called the palmas e imi housing unit. by 1977, the filling continued in front of the area called the historic center in the “puerta de mar” section and the area near the town of lerma. by the 90s, the infill areas continued urban development, increasing the commercial area and the development of residential areas, such as the expansion of the palmas and imi areas. this urban expansion has generated a development of the csfc whose economy depends directly on the coastal zone; the loss of this area due to flooding must be studied in greater detail and incorporate other variables such as soil type and include among the analyzes the indirect damages generated by flooding, that is, those damages that could be generated indirectly by the stoppage of activities, closure of roads, among others, and which can be much greater than the direct damage. the methodology used here has been replicated in other coastal areas worldwide, since the strategy of building scenarios defined by factors of change in sea level applied to simulation models is applied, as indicated by wang et al., 2018. numerical simulations have been performed in many coastal areas of the world evaluating changes in hydrodynamics and other factors such as those performed by ranjbar et al. 2018 y ranjbar et al. 2020 in gorgan bay, iran and hong et al., 2020 in pearl river estuary, china. these studies have shown that sea level rise will increase the frequency and intensity of coastal flooding as noted wang et al., 2021; hunter et al., 2013; marsooli et al., 2019; marsooli et al., 2020 and rasmussen et al., 2018. in san francisco de campeche, a study that includes numerical models and sea level rise has not been developed; the above is consistent with what was reported by mohd et al., 2018 who points out that in tropical countries little attention has been paid to the possible hydrodynamic changes due to the effects of the slr. identifying the number of inhabitants that could be affected, the number of homes that could be damaged and the cost of this damage, is basic information that is required to understand the effects of changes due to flooding associated with sea level rise and is essential to formulate strategic risk reduction plans for coastal flooding as indicated wang et al., 2018. considering both the demographic explosion and the indirect damages would increase the cost of the damages generated, this analysis must be more robust, being able to adapt the integrated flood risk analysis framework to calculate the total economic damages (direct and indirect) with different socioeconomic developments proposed by yin et al., 2021. on the other hand, different methods could be applied to evaluate the risk of flooding and the damage caused in a coastal area, as carried out by rey et al., 2020 in progreso, yucatan; in their study they applies the method proposed by the national center for disaster prevention (cenapred) of mexico and used by the mexican government, based on the values of economic assets (household goods) and flood impacts (similar to the method used here); the cenapredv2 method and the fri method, which are methods to evaluate risk that include 17 socioeconomic indicators. the application of the methods will depend on the specific interest of the risk assessment; for example, identifying where the greatest economic impacts will occur, knowing both economic damages and socioeconomic aspects or mapping the most vulnerable population considering important elements such as mobility, communications or others. the affected area ranges from 322.91 ha, 330.73 ha and 376.30 ha for each scenario respectively. the los petenes area, the 7 de agosto pier and the boardwalk near the san román neighborhood are among the most affected in the third scenario; these areas belong to the filling area that was carried out between 1960 and 1970. around 2018, the modernization of the boardwalk of the city of san francisco de campeche began, so filling was carried out in various sectors where playgrounds, water parks, light sources, as well as statues and monuments and commercial areas of the food industry; all these works should consider the effects of sea level rise, as well as the impact of hydrometeorological phenomena. special attention should be dedicated to the los petenes area, since being a biosphere reserve, the loss of this coastal ecosystem would negatively impact others, which could start a domino effect, putting at risk the loss of different 303coastal flood and damage assessment under sea level rise projections: a case study in san francisco de campeche, mexico coastal ecosystems, increasing the risk of coastal erosion, the loss of socioecosystem services and the increase in flooding as indicated keyzer et al., 2020. 5. conclusions floods due to an increase in the average sea level in the city of san francisco de campeche are imminent. the number of homes and population directly affected by the flood is low compared to the entire city. however, due to the economic activities carried out in the area, economic losses will increase considerably. the “los petenes” protected natural area will be considerably affected by direct contact with the sea, altering its dynamics and the ecosystem services it offers; the loss of these services and its impact on the local and regional economy must be quantified; likewise, changes in dynamics must be evaluated to determine the impact on the ecosystem. san francisco de campeche, due to its topography, is vulnerable to changes in the elevation of the sea level surface; however, actions can be carried out that allow 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(2005). diagnóstico de inundación para la ciudad de campeche. universidad autónoma de campeche, h. ayuntamiento del municipio de campeche. 109 p. peterson, t.c., heim, r.r., hirsch, r., kaiser, d.p., brooks, h., diffenbaugh, n.s., wuebbles, d. (2013). monitoring and understanding changes in heat waves, cold waves, floods, and droughts in the united states: state of knowledge, bulleting american meteorological society. 94 (6), 821834. https://doi.org/10.1175/bams-d-12-00066.1 posada vanegas, g. et al., 2011. vulnerability to coastal flooding induced by tropical cyclones. proc. coastal engineering, 1(32), p.14. posada vanegas, g., pérez, c., vega serratos, b., ruíz, g., & silva, r. 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(2020). assessing different flood risk and damage approaches: a case of study in progreso, yucatan, mexico. journal of marine science and engineering, 8(2), 137. https://doi.org/10.3390/jmse8020137 wang, j.; yi, s.; li, m.; wang, l.; song, c. effects of sea level rise, land subsidence, bathymetric change and typhoon tracks on storm flooding in the coastal areas of shanghai. science of the total environment 2018, 621, 228–234, doi:https://doi.org/10.1016/j.scitotenv.2017.11.224 wang, y.; marsooli, r. dynamic modeling of sea-level rise impact on coastal flood hazard and vulnerability in new york city’s built environment. coastal engineering 2021, 169, 103980, doi:https://doi.org/10.1016/j. coastaleng.2021.103980. yin, z., hu, y., jenkins, k. et al. assessing the economic impacts of future fluvial flooding in six countries under climate change and socio-economic development. climatic change 166, 38 (2021). https://doi.org/10.1007/ s10584-021-03059-3 https://doi.org/10.1175/bams-d-12-00066.1 https://doi.org/10.1016/j.scitotenv.2020.140073 https://doi.org/10.1016/j.scitotenv.2020.140073 https://doi.org/10.1016/j.apgeog.2015.09.010 https://doi.org/10.1016/j.ocecoaman.2015.11.019 https://doi.org/10.1016/j.ocecoaman.2015.11.019 https://doi.org/10.3390/jmse8020137 https://doi.org/10.1016/j.scitotenv.2017.11.224 https://doi.org/10.1016/j.coastaleng.2021.103980 https://doi.org/10.1016/j.coastaleng.2021.103980 https://doi.org/10.1007/s10584-021-03059-3 https://doi.org/10.1007/s10584-021-03059-3 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n1.105689 earth sciences research journal earth sci. res. j. vol. 27, no. 1 (march, 2023): 1 9 pa la eo n to lo g y palabras clave: dientes procumbentes; stenorhynchosaurus; pliosauridae; brachaucheninae; colombia; barremiano. procumbent anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material maría eurídice páramo-fonseca1,*, cristian david benavides-cabra1, héctor daniel palma-castro2 and antonio josé castañeda-gómez3 1. departamento de geociencias, universidad nacional de colombia, carrera 30 no. 45-03 bogotá, colombia, meparamof@unal.edu.co*, cdbenavidesc@unal.edu.co 2 departamento de biología, universidad nacional de colombia, carrera 30 no. 45-03 bogotá, colombia, hdpalmac@unal.edu.co 3 julius-maximilians universität; würzburg, deutschland, antonio.castaneda-gomez@uni-wuerzburg.de dientes premaxilares anteriores procumbentes en stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidencia a partir de nuevo material how to cite item: páramo-fonseca, m. e., benavides-cabra, c. d., palma-castro, h. d.., castañeda-gómez, a. j. (2023). procumben anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material. earth sciences research journal, 27(1), 1-9. https://doi. org/10.15446/esrj.v27n1.105689 record manuscript received: 07/11/2022 accepted for publication: 23/03/2023 keywords: procumbent teeth; stenorhynchosaurus; pliosauridae; brachaucheninae; colombia; barremian abstract a recently prepared fragment of the anterior end of a snout comprising occluded upper and lower jaws of a juvenile individual of stenorhynchosaurus munozi páramo-fonseca et al., 2016 (plesiosauria, pliosauridae) is described herein. the specimen was found in the 1990s at sutamarchán (boyacá, colombia), in barremian beds of the arcillolitas abigarradas member of the paja formation. its description provides hitherto unknown valuable morphological information on the species: dorsal anterior extension of the vomer, five premaxillary teeth (not four as was previously suggested), and anteriorly directed orientation of the first premaxillary alveolus (procumbent). new observations made on previously studied material corroborated the existence of procumbent anterior premaxillary teeth in the species. the presence of this trait highlights the close relationship of s. munozi with other early-diverging brachauchenines from the lower cretaceous of russia. resumen se describe un fragmento del extremo anterior de un hocico con la mandíbula ocluida, de un individuo juvenil de stenorhynchosaurus munozi páramo-fonseca et al., 2016 (plesiosauria, pliosauridae) que fue preparado recientemente. el espécimen fue encontrado en la década de 1990 en sutamarchán (boyacá, colombia), en capas barremianas del miembro arcillolitas abigarradas de la formación paja. su descripción proporciona información morfológica valiosa, desconocida hasta ahora para la especie: vómer con extensión anterior dorsal; premaxilares con cinco dientes (no cuatro como fue sugerido previamente) y el primero orientado anteriormente (procumbente). nuevas observaciones hechas en material previamente estudiado corroboraron la existencia de dientes premaxilares anteriores procumbentes en la especie. la presencia de este rasgo resalta la estrecha relación de s. munozi con otros brachauqueninos del cretácico inferior de rusia derivados tempranamente. https://doi.org/10.15446/esrj.v27n1.105689 mailto:meparamof@unal.edu.co mailto:cdbenavidesc@unal.edu.co mailto:hdpalmac@unal.edu.co mailto:antonio.castaneda-gomez@uni-wuerzburg.de https://doi.org/10.15446/esrj.v27n1.105689 https://doi.org/10.15446/esrj.v27n1.105689 2 maría eurídice páramo-fonseca, cristian david benavides-cabra, héctor daniel palma-castro and antonio josé castañeda-gómez 1. introduction the knowledge of lower cretaceous pliosaurids has recently increased with the description of several specimens from the berriasian-hauterivian of western russia (fischer et al., 2015; fischer et al., 2017; zverkov et al., 2018), the barremian-aptian of colombia (gómez-pérez & noè, 2017; páramofonseca et al., 2016, 2018, 2019), the aptian-albian of australia (holland, 2018) and the albian of europe (bardet et al., 2016; madzia, 2016; madzia & machalski, 2017). the colombian pliosaurids are currently represented by four species, acostasaurus pavachoquensis gómez-pérez and noè (2017), known through a three-dimensionally preserved skull and some poscranial elements from the lower barremian (gómez-pérez & noè, 2017); sachicasaurus vitae páramo-fonseca, benavides-cabra and gutierrez (2018) known from a gigantic nearly complete skeleton from the upper barremian (páramo-fonseca et al., 2018); stenorhynchosaurus munozi páramo-fonseca, gómez-pérez, noè and etayo-serna (2016) known from a nearly complete skeleton and a skull with some articulated cervical vertebrae from the upper barremian (páramofonseca et al., 2016; 2019); and “monquirasaurus” boyacensis (hampe, 1992) represented by a nearly complete skeleton from the upper aptian. this last species, previously assigned to kronosaurus by hampe (1992), has recently been attributed to a new genus monquirasaurus noè and gómezpérez (2022); however, we consider that most of the characters used by these authors to differentiate this species from s. vitae are based on traits that are poorly preserved in the only known specimen of “m.” boyacensis and are not obvious enough to warrant a generic distinction. therefore, we prefer to use the name “monquirasaurus” (within quotes) until new characters, such as the morphology of the palate, as discussed by páramo-fonseca et al. (2018), are available to be compared. the specimens of stenorhynchosaurus munozi (the holotype and the specimen mp050310-1) have the anterodorsal end of the snout lost or damaged (páramo-fonseca et al., 2016; 2019). a well-preserved fragment of the anterior end of a snout, housed in the museo paleontológico of the universidad nacional de colombia at villa de leyva, was recently prepared. based on the presence of diagnostic characters the specimen is referred to s. munozi. herein, we describe this specimen and provide hitherto unknown features of the anterior end of the snout of s. munozi. 2. materials and methods the specimen described herein (mpvl-667) is an anterior end of a snout comprising occluded upper and lower jaws, which was collected by the staff of the museo paleontológico of the universidad nacional de colombia at villa de leyva in the 1990s. it was collected as a trundled concretion found in the riverbed of la yuca creek, in the municipality of sutamarchán, boyacá, colombia, a municipality adjacent to villa de leyva. the discovery site is located approximately 8 km north-west of villa the leyva, (fig. 1a). we visited the la yuca creek and examined the concretions transported by the stream, both at the finding site and in the nearest 150 m upstream. based upon a generic identification of ammonoids collected from the concretions, we propose the stratigraphic origin of the specimen mpvl-667. the ammonoid were identified at generic level by one of the authors (cdb-c). the specimen was embedded in micritic matrix, so chemical treatment was the selected preparation method. successive immersions of 5-6 hours in formic acid at 3% concentration followed by a minimum of 12 hours of water rinsing and subsequent drying were performed. three water baths of 24 hours each were performed at the end of the preparation to ensure the elimination of acid residues. given that the new specimen exhibits many morphological similarities with stenorhynchosaurus munozi, the anatomical comparisons were focused on this species and specifically on specimen mp050310-1, which although damaged, preserves the same region of the snout. to corroborate the affinity of the new specimen with s. munozi, new observations were made directly on specimen mp050310-1. the anatomical description of the dentition follows the terminology proposed by zverkov et al. (2018). a 3d photogrammetric model of the specimen is provided as support of the description (see supplemental data 1). the model was obtained from raw 18 megapixels images using high-resolution structure from motion techniques. each fragment was photographed with a canon eos rebel 4000d camera and processed individually using 275 pictures for the upper piece and 225 for the down one. photos were taken surrounding each piece at three different angles to ensure the maximum possible resolution and coverage. each model has been scaled using two scale bars located in each image. the correct threedimensional position of both 3d models of the snout fragments was achieved manually, having single pictures of the right location of both pieces. agisoft metashape professional 1.5.1, meshlab v2022.02, and blender 3.3.1 programs were used to obtain the 3d models. a phylogenetic analysis was performed to determine the position of s. munozi within the plesiosauria, based on our enhanced anatomical understanding of the species. the analysis was based on the data set of páramofonseca et al. (2019) which is a modified version of the data set of fischer et al. (2017) and that of benson and druckenmiller (2014). the dataset was compiled using the mesquite software 3.51 (maddison and maddison, 2018). the resulting data matrix has 102 otus and 271 characters (supplemental data 2). the cladistic analysis was carried out using tnt (goloboff and catalano, 2016). all characters were treated as unordered and were equally weighted. an initial exploration for the shortest-length tree islands was performed using the new technology search options (ratchet + drift + tree fusing) with 200 ratchet iterations and 99,999 trees limit. five replicates, one hit on the shortest tree, and seven trees held per replicate were obtained. the resulting trees were then used as the starting point for a tree bisection reconnection (tbr) branch swapping algorithm. a strict consensus was obtained, and the unstable otus were identified by applying the iterpcr algorithm (pol and escapa, 2009). the unstable otus were pruned from the consensus cladogram. the consistency (ci) and retention (ri) indices (farris, 1989) were calculated, and bremer support values (bremer, 1994) were computed for the nodes in the consensus tree using tnt tools. 3. geologic and biostratigraphic settings the specimen was found in the riverbed of la yuca creek, in a trundled concretion in which no ammonoids, except for a minute unidentifiable shell, were preserved. we collected a matrix sample and analyzed it for calcareous nannofossils and palynomorphs, but the analysis showed no microfossil content. although no outcrops were found in the la yuca creek, the stream cuts the la yuca hill where the almost complete stratigraphic sequence of the arcillolitas abigarradas member of the paja formation outcrops (figs. 1a-b). the topography and the geology of the finding region allowed us to establish with certainty that the specimen mpvl-667 comes from the arcillolitas abigarradas member of the paja formation. abundant trundled concretions bearing ammonoids were found in the riverbed of la yuca creek. in some of the concretions we identified lower barremian ammonoid genera, such as nicklesia hyaat, 1903 and pulchellia uhlig, 1883 (patarroyo, 2000; 2020) (figs. 1c-d), and in other concretions we identified the genera gerhardtia sensu hyatt (1903) and heinzia sensu bürgl (1956) (figs. 1e-f) which are indicators of the upper barremian in colombia (patarroyo, 2000; 2020). we did not find any aptian ammonoids on the creek, which suggests that the specimen came from barremian rocks. furthermore, in the nearest outcrops in the la yuca hill we found the same ammonoid genera. in this context, we strongly assert that the specimen mpvl-667 originated within barremian beds of the arcillolitas abigarradas member that can be correlated with the segments a, b, and c of etayo-serna (1968) (see fig. 1b). a more accurate stratigraphic origin of the specimen within the barremian sequence is not possible. 4. systematic paleontology plesiosauria de blainville, 1835 pliosauroidea welles, 1943 pliosauridae seeley, 1874 thalassophonea benson & druckenmiller, 2014 brachaucheninae williston, 1925 (sensu benson & druckenmiller, 2014) stenorhynchosaurus munozi páramo-fonseca, gómez-pérez, noé & etayo-serna, 2016 2005 brachauchenius sp. hampe, p. 39. 3procumbent anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material type specimen: vl17052004-1, paleontological collection of the servicio geológico colombiano. a nearly complete skeleton lacking part of the left anterior paddle, the complete right posterior paddle, and the tail (páramofonseca et al., 2016, figs. 2-4). referred specimen: mp050310-1, paleontological collection of the servicio geológico colombiano. a skull and the first 10 cervical vertebrae (páramo-fonseca et al., 2019, figs 2-4) new material: mpvl-667, museo paleontológico de villa de leyva, universidad nacional de colombia. anterior end of a snout with articulated mandible (fig. 2). geographic and stratigraphic occurrence: alto ricaurte region (villa de leyva, sáchica and sutamarchán), boyacá, colombia; barremian beds of the arcillolitas abigarradas member of the paja formation (etayo-serna, 1968; páramo-fonseca et al., 2016; 2019) emended diagnosis: the diagnosis of stenorhynchosaurus munozi follows páramo-fonseca et al. (2019) except for the following features, which are observed in the new specimen. premaxilla with five (not four) functional alveoli, the anterior one procumbent and smaller than other premaxillary teeth. the autapomorphy concerning the vomer is specified as follows: vomer extending dorsally above the palatal processes of the premaxillae, but in ventral view ending posterior to the palatal premaxilla-maxilla suture. 5. description preservational features the specimen is three-dimensionally preserved and exhibits accurate detail of bone textures. it consists of the anterior end of a snout with the occluded mandible, extending to the level of the first maxillary alveolus (fig. 2). the upper jaw was preserved slightly turned to its left side (fig. 2b). the external surfaces of the first alveolus in the left premaxilla as well as the anterior tip and the right lateral side of the mandible are eroded. most of the functional teeth are preserved in place on the right side of the mandible, whereas the remaining alveoli in the upper and lower jaws are either empty or they lodge replacement teeth. 6. anatomical features premaxilla and maxilla. the anteriormost portion of the premaxillae, forming the anterior end of the snout, and a small portion of the anterior end of the maxillae are present in the specimen (figs. 3a-d). the preserved portion of the snout is narrow with near parallel borders in dorsal view (fig. 3b). only a very slight narrowing of the snout at the level of the fifth alveolus and a faint figure 1. geographic and stratigraphic provenance of the specimen mpvl-667. a, geological map showing the finding site (pliosaurid silhouette) (modified from etayo-serna, 1968). cartographic base: instituto geográfico agustín codazzi. b, generalized stratigraphic section of the paja formation cropping out around villa de leyva and sutamarchán, showing the stratigraphic origin of the specimen (modified from etayo-serna, 1968). c-f, lateral and ventral views of the ammonoid specimens representing the identified genera: c, nicklesia hyaat, 1903 (mp081022-1a), d, pulchellia uhlig, 1883 (mp081022-2), e, heinzia sensu bürgl (1956) (mp081022-7c) and f, gerhardtia sensu hyatt (1903) (mp081022-12). scale bars c-f = 10 mm. 4 maría eurídice páramo-fonseca, cristian david benavides-cabra, héctor daniel palma-castro and antonio josé castañeda-gómez widening at the level of the fourth alveolus are observed. the anterior tip of the snout is rounded and bears two procumbent alveoli (figs. 2b; 3a-b). the suture between the premaxillae is straight. the external surface of the preserved portion of premaxillae is pitted by numerous vascular foramina, more densely arranged in the anterior end of the snout (figs. 2a,c; 3b). the preserved portion of the lateral premaxilla-maxilla suture, near the alveolar margin, is interdigitated (figs. 2a,c). ventrally, the premaxilla-maxilla suture crosses the alveolar surface on a diastema of 19 mm in length that separates the fifth and sixth functional alveoli (fig. 3c). in ventral view, the palatal process of the premaxilla extends posterior to the premaxilla-maxilla suture (fig. 3c). this condition indicates that the anterior end of the vomer exposed in palatal view is located posterior to the palatal premaxilla-maxilla suture. this trait is an autapomorphy of stenorhynchosaurus munozi (páramo-fonseca et al., 2016). the palatal process of the premaxilla diverges dorsally forming a medial longitudinal lamella that contacts the vomer (fig. 3d). medial to the alveolar row, a narrow longitudinal canal runs, joining the concavities into which the replacement teeth erupt (fig. 3c). this canal is interrupted in the diastema. at the level of the fourth alveolus an oblique posteromedial groove cross from the longitudinal canal to the midline where the two premaxillae meet (fig. 3c). each premaxilla has five alveoli. the first alveolus is the smallest and the fourth alveolus is the largest (fig. 3a, c). the first alveolus is practically horizontal and opens anteriorly on a vertical plane above the second alveolus, indicating a procumbent condition (fig. 3a). the procumbent alveolus from both premaxillae is very close to each other; there is a thin bone wall of 0.26 mm separating them. the procumbent alveolus is vertically oval in outline, 19 mm in dorsoventral diameter and 13 mm in mesiodistal diameter (fig. 3a). the subsequent premaxillary alveoli are circular in outline and anterolaterally oriented. their mesiodistal diameter measure 22 mm (in the second alveolus), 23 mm (in the third), 25 mm (in the fourth), and 23 mm (in the fifth). the interalveolar spaces range from 6 to 8 mm. in the maxilla, only the right first maxillary alveolus is complete (fig. 3c), it is circular in outline and its mesiodistal diameter is 22 mm. vomer. the vomers are fused forming an elongated bar that anteriorly extends dorsal to the palatal processes of the premaxillae (fig. 3d). this dorsal extension of the vomers was not previously known in s. munozi; therefore, it is specified in the diagnosis of the taxon. the bar is trifurcated ventrally, forming three ventral lamellae that intercalate with the medial lamellae from the premaxillae, in such a way that, in cross-section, the joint between the vomers and the premaxillae is m-shaped (fig. 3d). the dorsal surface of the fused vomers is concave and is textured by numerous longitudinal ridges. mandible. the preserved mandible includes the anterior portion of the dentaries and the anteriormost ends of the splenials, forming the anterior part of the mandibular symphysis (figs. 3e-h). most of the right external surface of the preserved mandible is eroded, but the left lateral surface is well preserved and demarcates the alveolar margins. as the rostrum, the mandible does not exhibit noticeable lateral expansion nor a marked increase in the size of the functional alveoli. the symphysis is not tightly closed; both mandibular rami run nearly parallel to each other, slightly further apart in the back (5 mm) than in the front (2 mm) (figs. 3e, g-h). as in the upper jaw, a narrow longitudinal canal running medial to the alveolar row joins the concavities into which the replacement teeth erupt (fig. 3f). coinciding with the upper jaw, at the level of the fourth alveolus an oblique posteromedial groove cross from the longitudinal canal to the midline where both dentaries meet (fig. 3f). each mandibular ramus preserves six complete and a seventh incomplete functional alveoli. the first functional alveolus is the smallest and the fifth is the largest (fig. 3f). the first alveolus is anteriorly directed and nearly horizontal but opens in an anterodorsal plane. differing from the first alveolus in the premaxilla, which opens directly above the second one, the anterior alveolus in the mandible does not open under the second alveolus, but anteroventral to it (fig. 3e). the described condition suggests the first mandibular tooth was not procumbent. except for the first alveolus, the mandibular functional alveoli are inclined slightly anterolaterally. the fifth and sixth mandibular teeth positions agree with the diastema of the upper jaw, without having an intercalated tooth. the measurements of the mesiodistal diameter of the mandibular alveoli are: 17 mm (anterior alveolus), 23 mm (second alveolus), 24 mm (third alveolus), 22 mm (fourth alveolus), 26 mm (fifth alveolus) and 22 mm (sixth alveolus). the space between successive alveoli in the mandible is slightly shorter than in the premaxilla; it ranges from 5 to 7 mm. teeth. only replacement teeth are present in the premaxillae and maxillae (fig. 3c). on the mandible, in addition to some replacement teeth, there are five functional teeth, four in the right mandibular ramus and one in the left (fig. 3f). replacement teeth emerge in small cavities placed within a canal that extends medially to the alveolar row. these cavities, named the replacement alveoli by edmund (1960), are positioned distal-lingual to the marginal (functional) alveoli, and become elongated when the replacement tooth migrates to the marginal alveolus (edmund, 1960; sassoon et al., 2015). the replacement tooth of the first premaxillary position is located ventrally to the functional alveolus and the first replacement tooth in the mandible is placed dorsally to the marginal alveolus (figs. 3c, f). in both premaxillae, the third replacement tooth is the largest; their crown is practically emplaced into the alveolus, but their root is still short. figure 2. mpvl-667, stenorhynchosaurus munozi. the complete specimen before separation of the jaws, in a, left lateral, b, anterior and c, right lateral views. abbreviations: d, dentary; mx, maxilla; pmx, premaxilla; 1d, first dentary tooth; 1pmx, first premaxillary alveolus. scale bar = 50 mm. 5procumbent anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material figure 3. mpvl-667, stenorhynchosaurus munozi. a-d, upper jaw in a, anterior, b, dorsal, c, ventral (with interpretative scheme) and d, posterior (with interpretative scheme) views. light gray: sediment; dark gray: alveoli; black: replacement teeth. e-h, lower jaw in e, anterior, f, dorsal, g, ventral and h, posterior views. abbreviations: d, dentary; dc, dentary canal; g, groove; mx, maxilla; pmx, premaxilla; rt, replacement tooth; sp, splenial; v, vomer; 1d, 2d, first, second dentary alveolus; 1mx, first maxillary alveolus; 1pmx, 2pmx, …, first, second, … premaxillary alveolus. scale bar = 30 mm. 6 maría eurídice páramo-fonseca, cristian david benavides-cabra, héctor daniel palma-castro and antonio josé castañeda-gómez the root in the second and third functional teeth of the right mandibular ramus is visible due to erosion (fig. 2). they are 59 mm and 55 mm in heigh respectively and protrude 15 to 17 mm from the alveolar rim. the root height is about twice that of the crown. the root is slightly bulbous, circular in crosssection, and its surface is smooth. the crowns of all functional teeth is poorly preserved and labially eroded (fig. 2). however, in the right sixth functional tooth the lingual surface is visible and in the largest replacement teeth the crowns are well preserved. the crown of the largest replacement teeth is 31 mm in height. the crown is nearly conical, with a flat labio-mesial surface delimitated by two sharps smooth carinae, one mesial and one distal-labial, running the entire height of the crown (figs. 4a-c). this morphology gives a sub-trihedral cross-section of the crown (fig. 4h), as defined by benson et al. (2013) for p. kevani. an additional distal apicobasal ridge appears toward the apex of the crown forming a trihedral tip of the tooth (fig. 4d), as in the holotype of stenorhynchosaurus munozi (páramo-fonseca et al., 2016). the flat surface of the crown only bears a few fine ridges at its base, whereas on the remaining enameled surface there are coarse, abundant, and equidistant longitudinal ridges reaching the lower half of the crown (fig. 4e-g). for every one or two ridges, a shorter inserted ridge is intercalating. these crown features have been described for the specimen mp050310-1 (páramo-fonseca et al., 2019). 7. phylogenetic analysis based on the anatomical input provided by the described specimen we modify the scoring of character 131 of stenorhynchosaurus munozi from state 0 (four premaxillary teeth) to 1 (five premaxillary teeth). furthermore, we added one character to the matrix: 271direction towards which the anteriormost premaxillary alveolus faces. 0. the alveolus faces ventrally or weakly anteroventrally; 1. the alveolus faces directly anteriorly (procumbent). the scoring of the new character [271] can be found in supplemental data 3. the strict consensus of 99,999 most parsimonious trees with 1620 steps (ci = 0.244 and ri = 0.670), obtained after tbr branch swapping, shows stenorhynchosaurus munozi within a large polytomy that includes all pliosaurids and other plesiosaurs (fig. 6a). after applying the iterpcr algorithm, 15 unstable otus were identified: pistosaurus skull, pliosaurus patagonicus, makhaira rossica, pliosaurus almanzaensis, qm_f51291, ‘polyptychodon interruptus’., pliosaurus irgisensis, pliosaurus brachydeirus, pliosaurus brachyspondylus, eromangasaurus australis, stratesaurus taylori, anningasaura lymense, eoplesiosaurus antiquior, eurycleidus arcuatus, and atychodracon megacephalus. after pruning these 15 unstable otus from the consensus, the reduced strict consensus (fig. 6b) shows s. munozi within a resolved brachaucheninae. the results obtained here shows no changes with respect to those previously obtained by páramo-fonseca et al. (2019), even after including the new character. nonetheless, the close relationship between s. munozi and l. itilensis is reflected in the obtained results and now it is supported by an additional character, the presence of procumbent anteriormost premaxillary teeth. the expected early diverging branch grouping stenorhynchosaurus munozi, makhaira rossica and luskhan itilensis, does not appear in the cladogram, probably due to the unstable position of m. rossica. the node of s. munozi and l. itilensis does not appear as early-diverging as expected by their shared characteristics with the jurassic thalassophoneans, instead it appears in a distant position to the ancestral node brachaucheninae. this might be due to the unordered treatment of the characters related to the tooth morphology. figure 4. mpvl-667, stenorhynchosaurus munozi. teeth. a-d, replacement tooth into the third left premaxillary alveolus in a, labial, b, mesial, c, distolingual and d, apical views. e-g, sixth right mandibular tooth (incomplete by erosion) in e, lingual, f, mesial and g, distal views. h, cross section of the replacement tooth for the second left premaxillary position. abbreviations: c, crown; ca, carina; r, root. scale bar: a-g = 10 mm; h = 5 mm. 7procumbent anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material 8. discussion one of the two autapomorphies defining stenorhynchosaurus munozi concerns the anterior region of the snout and is present in the new specimen: the anterior external exposure of the vomer is located posterior to the palatal premaxilla-maxilla suture in palatal view (páramo-fonseca et al., 2016; 2019). moreover, the morphological features of the new specimen, including the size and shape of the snout, the size and orientation of the alveoli, and the shape and ornamentation of the teeth are consistent with the morphology previously described for stenorhynchosaurus munozi (páramo-fonseca et al., 2016; 2019). the dimensions of the studied specimen agree with the size of the holotype of s. munozi, which was considered a juvenile individual (páramo-fonseca et al., 2016). in both specimens, the symphysis is not fused indicating that they represent juvenile individuals. the new specimen is thus reliably referred to as a juvenile individual of s. munozi. the ventral exposure of the anterior end of the vomer is not observable in the only adult specimen of s. munozi known so far (páramo-fonseca et al., 2019). the anterior dorsal extension of the vomer observed in the new specimen raises the possibility that the shorter ventral exposition of the vomer in young specimens is the result of ontogenetic allometry. however, this doubt cannot be clarified until the ventral exposure of the vomer in the palate of an adult individual is known. stenorhynchosaurus munozi was erected for a nearly complete skeleton (vl17052004-1, the holotype) whose skull lacks the anterior tip of the snout (páramo-fonseca et al., 2016). a second specimen (mp050310-1), a skull with some articulated cervical vertebrae, was later described by páramo-fonseca et al. (2019). the dorsal region of the anterior end of the snout in the second specimen is damaged (páramo-fonseca et al., 2019: figure 3c; figs 5a-b). therefore, the anterior dorsal end of the snout was unknown in s. munozi. in the published descriptions of these specimens, the authors proposed the number of premaxillary teeth of s. munozi as four based on the preserved material (páramo-fonseca et al, 2016; páramo-fonseca et al., 2019). the specimen herein described has five premaxillary teeth, the first being a dorsal procumbent tooth smaller than the other premaxillary teeth. this allows us to infer that s. munozi had five premaxillary teeth and not four as previously proposed. new and detailed observations made on the dorsal end of the snout of the specimen mp050310-1, allowed us to identify two circular concentric structures on the broken surface that this specimen has above the alveolar margin (fig. 5b-c). the morphology of these structures agrees with traces of two broken anterior procumbent teeth. nevertheless, these structures seem to be in a somewhat higher position than that occupied by the procumbent alveoli in the new specimen. this discrepancy does not imply a taxonomic difference, since: 1the broken surface where the concentric traces are registered in mp050310-1 is placed at a considerable distance from the frontal alveolar margin, so the traces could be showing the posterior portion of alveoli of the two slightly inclined procumbent teeth; and 2the mp050310-1 represents an adult individual and the new specimen comes from a juvenile individual, so it could be expected an ontogenetic difference between the two specimens, with a more robust snout and procumbent teeth more separated from the alveolar margin in the adult. based on the above discussion, we can affirm that stenorhynchosaurus munozi had five premaxillary teeth, the anterior one procumbent, nearly horizontal, and smaller than the other premaxillary teeth. the presence of procumbent premaxillary teeth is rare in thalassophonean pliosaurids; it has figure 5. stenorhynchosaurus munozi, specimen mp050310-1. a-b, anterior tip of the snout in a, anterolateral and b, anterior views. c, detail of the anterior end of the snout showing the two concentric structures (broken teeth) found in the broken anterior surface. scale bar = 30 mm. 8 maría eurídice páramo-fonseca, cristian david benavides-cabra, héctor daniel palma-castro and antonio josé castañeda-gómez only previously been reported for the brachauchenine pliosaurids makhaira rossica fischer, arkhangelsky, stenshin, uspensky, zverkov, and benson, 2015 and luskhan itilensis fischer, benson, zverkov, soul, arkhangelsky, lambert, stenshin, uspensky, and druckenmiller, 2017, both from the upper hauterivian (lower cretaceous) of russia (fischer et al., 2015; fischer et al., 2017). in addition, l. itilensis, m. rossica, and s. munozi share the elongated rostrum without lateral expansions, the isodont dentition, and the subtrihedral crown with trihedral apex (fischer et al., 2015; fischer et al., 2017; páramo-fonseca et al., 2019), a feature that differentiates these taxa from the other brachauchenine pliosaurids, which have a subcircular tooth cross-section (zverkov et al., 2018). the differences between s. munozi and the russian hauterivian taxa have already been discussed by páramo-fonseca et al. (2019). here we give only a few remarks on the number of premaxillary teeth (five in the s. munozi), which differs from that of the russian taxa (six in m. rossica and seven or probably six in l. itilensis) (fischer et al., 2015; fischer et al., 2017; zverkov, personal communication) and from that of all other brachauchenines (four functional teeth) (schumacher et al., 2013; albright et al., 2007; white, 1935; gómez-pérez and noè, 2017; páramo-fonseca et al., 2018). the thalassophonean pliosaurids sharing five (or five-six) premaxillary teeth are the jurassic taxa liopleurodon ferox sauvage 1873, pliosaurus macromerus (phillips, 1871), simolestes vorax andrews, 1909 and eardasaurus powelli ketchum & benson, 2022 from england and pliosaurus rossicus novozhilov, 1948 from russia. nevertheless, all these taxa differ from s. munozi by having a constricted rostrum with large teeth near the premaxilla-maxilla suture and short symphysis (andrews, 1909; 1913; halstead, 1971; benson et al., 2013; fischer et al, 2015, appendix data set; ketchum & benson, 2022). the combination of features in the premaxillary dentition of s. munozi is therefore unique among thalassophonean pliosaurids. the new traits are therefore added to the diagnosis of the species. the presence of anterior procumbent teeth in the premaxilla of the barremian taxon s. munozi from colombia has significant implications in the temporal distribution of this trait in the history of long-snouted brachauchenines. it is noteworthy that within the thalassophonean pliosaurids the procumbent teeth in front of the upper jaw only occur in the long and narrow snouted taxa of the early cretaceous. these taxa have morphological affinities with the cretaceous brachauchenine pliosaurids but retain traits of the dental crown morphology found in the jurassic thalassophonean pliosaurids. these long and narrow snouted taxa with procumbent teeth formed, early in the history of the brachauchenines, a derived evolutionary lineage with a particular ecological adaptation that successfully expanded across the tethys sea from eurasia to northern gondwana during the early cretaceous. the apparent temporal discontinuity of the ecological adaptation of a narrow snout with procumbent front teeth on premaxilla among brachauchenines could be due to a collection bias, but also to ecological changes. it would not be unreasonable to suggest that the polycotylid plesiosaurs, having elongated snouts, relatively isodont dentition, and frequent teeth with carinae (fischer et al., 2018), which begin their history in the aptian-albian (druckenmiller, 2002; kear, 2003) and become diverse during the late cretaceous (druckenmiller and russel, 2009; fischer et al., 2018; sachs et al., 2020), could have occupied the ecological niche of the early cretaceous long-snouted brachauchenines. figure 6. a, b, consensus trees of 99,999 trees of 1,620 steps based on data set of plesiosauria with 102 otus and 271 characters (see materials and methods). a, strict consensus tree, with bremer support indicated in some nodes. b, reduced consensus found after pruning the unstable otus. 9procumbent anterior premaxillary teeth in stenorhynchosaurus munozi (plesiosauria, pliosauridae), evidence from new material 9. conclusions we identify the specimen mpvl-667 as a juvenile individual of stenorhynchosaurus munozi by presenting juvenile traits in a morphology consistent with that described for this species and showing the anteriorly exposed end of the vomer in palatal view located posterior to the palatal premaxilla-maxilla suture, which is one of the autapomorphies that define s. munozi. the study of the new specimen reveals information about traits that were previously unknown or were interpreted differently for the species, such as the dorsal anterior extension of the vomer, the presence of five premaxillary teeth (previously interpreted as four) and the existence of a procumbent anteriormost premaxillary tooth. this last feature is corroborated by new observations in the specimen mp050310-1. the procumbent anterior premaxillary teeth and the subtrihedral crown with trihedral apex is shared by s. munozi, luskan itilensis, and makhaira rossica and differentiate these taxa from the other brachauchenines. these shared traits show a close relationship between the hauterivian brachauchenines from russia and the barremian brachauchenines from colombia. moreover, our cladistic analysis supports the phylogenetic proximity of s. munozi and l. itilensis by 11 synapomorphies, including the new character of the procumbent anteriormost premaxillary teeth. we suggest that the niche occupied by the early cretaceous long-snouted brachauchenines (s. munozi, l. itilensis, and m. rossica) could have been later occupied by the polycotylid plesiosaurs, which also have elongated snouts, relatively isodont dentitions and frequent teeth with carinae. acknowledgments we thank peter lowy, current director of the museo paleontológico de villa de leyva, for allowing us to study the specimen. we want to thank pedro patarroyo of the departamento de geociencias of the universidad nacional de colombia for letting us know about the specimen. we express our gratitude to geologist adriana delgado who collected the specimen while she was the director of the museo paleontológico de villa de leyva in 1990s. she provided us valuable information about the finding. we are grateful to osman varol and manuel páez for analyzing the matrix samples searching calcareous nannofossils and palynomorphs respectively. we also want to thank albeiro cortés and andrés reyes for help us in the field. we thank the office of the secretary of culture of sáchica for granting access to the specimen mp0503101. finally, we want to thank nikolay zverkov, daniel madzia, and bruce schumacher for their helpful review and suggestions that help to improve the final version of this manuscript. references albright, l. b., gillette, d. d. & titus, a. l. 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(2018). increased pliosaurid dental disparity across the jurassic-cretaceous transition. palaeontology, 61, 825-846. https://doi.org/10.1111/pala.12367 https://doi.org/10.7717/peerj.1998 https://doi.org/10.1515/agp-2017-0018 https://doi.org/10.1016/j.cretres.2021.105122 https://doi.org/10.1016/j.cretres.2021.105122 https://doi.org/10.15446/esrj.v22n4.69916 https://doi.org/10.15446/esrj.v22n4.69916 https://doi.org/10.1080/02724634.2019.1663426 https://doi.org/10.1080/02724634.2019.1663426 https://doi.org/10.18257/raccefyn.239 https://doi.org/10.1016/j.cretres.2020.104406 http://dx.doi.org/10.1098/rsos.150384 http://dx.doi.org/10.1098/rsos.150384 http://www.bioone.org/doi/full/10.1080/02724634.2013.722576 https://doi.org/10.1111/pala.12367 _hlk127128791 _hlk115517256 _hlk127982891 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n3.120899 earth sciences research journal earth sci. res. j. vol. 29, no. 3 (september, 2025): 347 362 se is m o lo g y characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan waqas ahmed1, muhammad waseem2, hammad raza3, muhammad yasir1, muhammad zeeshan1, saif ullah1, ihtisham islam4*, salman ahmed khattak5,6* 1. national centre of excellence in geology, university of peshawar, 25120, peshawar pakistan. 2. department of civil engineering, university of engineering and technology, peshawar, 25120, peshawar pakistan. 3. university of engineering and technology, taxila, 47080, pakistan. 4. department of geology, shaheed benazir bhutto university sheringal, 18030, dir upper pakistan. 5. state key laboratory of critical mineral research and exploration, institute of geochemistry, chinese academy of sciences, guiyang 550081, china. 6. university of chinese academy of sciences, beijing 100049, china *corresponding authors: ihtisham.islam@sbbu.edu.pk; salman@mail.gyig.ac.cn record manuscript received: 12/06/2025 accepted for publication: 12/09/2025 how to cite this item: ahmed, w., waseem, m., raza, h., yasir, m., zeeshan, m., ullah, s. islam, i., & khattak, s. a. (2025). characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan. earth sciences research journal, 29(3), 347-362. https://doi.org/10.15446/ esrj.v29n3.120899 abstract this study presents a detailed experimental characterization of the small-strain dynamic behavior of fine-grained soils in the islamabad capital territory, pakistan, an area of moderate to high seismic hazard with limited site-specific geotechnical data. a comprehensive series of resonant column tests was conducted on reconstituted silty clay and clayey silt samples collected from 15 boreholes across the region. tests were performed under confining pressures of 50, 100, and 150 kpa to evaluate shear wave velocity, maximum shear modulus (gₘₐₓ), the modulus reduction (g/gₘₐₓ) with shear strain, and strain-dependent damping behavior. results reveal that islamabad soils are characteristically stiff at small strains, with gₘₐₓ ranging from approximately 40–80 mpa and increasing systematically with confining pressure, while typical vs values ranged from 160–260 m/s. the normalize shear modulus (g/gₘₐₓ) curves showed moderate nonlinearity, with g/gₘₐₓ reducing to 0.3–0.5 at maximum strain levels (~0.1–0.2%). reference strain (γref) values obtained from curve fitting fell between ~0.02–0.08%, further quantifying the onset of modulus reduction. damping ratios increased from ~0.5% at microstrains to a maximum of 8–12% at higher strains, with minor variation across confining pressures. when compared with standard empirical models, the measured curves exhibited stiffer behavior and lower damping at equivalent strains, highlighting the importance of site-specific calibration. these findings provide critical input for improving seismic site response analyses in islamabad, directly informing local microzonation studies and contributing to the refinement of pakistan’s seismic design guidelines. keywords: malatya, eastern anatolia fault zone, earthquake, gnss, seismicity, deformation. caracterización de las propiedades dinámicas no lineales del suelo para la respuesta sísmica en islamabad, pakistán resumen este estudio presenta una caracterización experimental detallada del comportamiento dinámico de pequeña deformación en los suelos de grano fino del territorio de islamabad, pakistan, una área de riesgo sísmico de moderado a alto con limitada información geotécnica específica. para ello se realizó una amplia serie de pruebas de columna resonante en muestras de arcilla limosa y limo arcilloso recolectadas en 15 perforaciones a lo largo de la región. las pruebas se realizaron bajo presión de compresión de 50, 100 y 150 kpa para evaluar la velocidad de la onda transversal (vs), módulo de corte máximo (gₘₐₓ), el módulo de reducción (g/gₘₐₓ) con corte máximo y comportamiento de amortiguamiento dependiente de la deformación. los resultados muestran que los suelos de islamabad son característicamente rígidos con pequeñas deformaciones, con un gₘₐₓ que oscila entre aproximadamente 40 y 80 mpa y aumenta sistemáticamente con la presión de confinamiento, mientras que los valores típicos de vs oscilaron entre 160 y 260 m/s. las curvas de módulo de corte normalizado (g/gₘₐₓ) mostraron una no linealidad moderada, con un g/gₘₐₓ reduciéndose a 0,3-0,5 a niveles máximos de deformación (~0,1-0,2%). los valores de deformación de referencia (γref) obtenidos del ajuste de la curva se situaron entre ~0,02 y 0,08%, lo que cuantifica aún más el inicio de la reducción del módulo. los coeficientes de amortiguamiento aumentaron de ~0,5% a microdeformaciones hasta un máximo de 8-12% a deformaciones más altas, con una variación mínima a través de las presiones de confinamiento. en comparación con los modelos empíricos estándar, las curvas medidas mostraron un comportamiento más rígido y un menor amortiguamiento a deformaciones equivalentes, lo que resalta la importancia de la calibración específica del sitio. estos hallazgos proporcionan información crucial para mejorar los análisis de respuesta sísmica del sitio en islamabad, informando directamente los estudios de microzonificación local y contribuyendo al perfeccionamiento de las directrices de diseño sísmico de pakistán. palabras clave: malatya; zona de fallas del este de anatolia; terremoto; gnss; seismicidad; deformación. https://doi.org/10.15446/esrj.v29n3.120899 mailto:ihtisham.islam@sbbu.edu.pk mailto:salman@mail.gyig.ac.cn https://doi.org/10.15446/esrj.v29n3.120899 348 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan 1. introduction the islamabad capital territory (ict) lies at the tectonic junction of the indian and eurasian plates, where active collision processes in the himalayan front generate significant seismic hazard. this region is seismically classified as zone 2b under the building code of pakistan (bcp, 2021), corresponding to a peak ground acceleration (pga) of approximately 0.19–0.26 g for designlevel events (bcp, 2021). the proximity of islamabad to major active faults such as the main boundary thrust and the hazara-kashmir syntaxis increases the likelihood of moderate to strong ground shaking during future earthquakes (jadoon et al., 2015; khan et al., 2021). the 2005 mw 7.6 kashmir earthquake, which caused over 85,000 fatalities, had an epicenter approximately 100 km from islamabad and produced significant shaking across the city. structural damage, including the collapse of the margalla towers, exposed critical deficiencies in the local understanding of soil behavior under seismic loading (mahmood et al., 2016). despite this high seismic risk, there remains a lack of site-specific geotechnical data to inform seismic hazard assessments in the region. this gap is particularly acute with respect to the small-strain dynamic properties of soils, such as shear modulus and damping ratio, which play a pivotal role in seismic site response. these parameters control wave propagation characteristics, influence spectral amplification, and govern the interaction between soil and structures under dynamic loading (zhang et al., 2005; wang and stokoe, 2022). without reliable measurements of these properties, engineers are often forced to adopt empirical estimates based on generic soil classifications or indirect correlations approaches that introduce uncertainty into seismic design and ground motion modeling (boore et al., 2011; adeel et al., 2023). globally, small-strain soil behavior has been widely characterized through standardized empirical models, including those developed by seed and idriss (1970), vucetic and dobry (1991), and darendeli (2001). these frameworks provide modulus reduction (g/gₘₐₓ) and damping ratio (d) curves as functions of cyclic shear strain, typically parameterized by factors such as plasticity index, confining pressure, and over consolidation ratio. because of their broad acceptance, these models are embedded in most modern site response software packages, such as deepsoil and strata, and continue to play a central role in engineering practice. however, these models were primarily developed from soils in different geological settings, often focusing on granular sands or high-plasticity clays from the united states and japan. applying them uncritically to islamabad, with its unique depositional history and fine-grained, lowto intermediateplasticity alluvium, risks introducing bias into seismic hazard analysis. such mismatches can lead to erroneous predictions of ground motion amplification and structural demand, as highlighted by prior studies (guerreiro et al., 2012; vardanega and bolton, 2013). this underscores the need for laboratory-based characterization of islamabad soils to establish reliable local modulus reduction and damping curves. in pakistan, and particularly in islamabad, limited studies have reported in-situ or laboratory-based dynamic properties of soils. site classification is typically inferred from surface geology, standard penetration test (spt) blow counts, or proxy-based vs30 correlations, without validation from resonant column or cyclic triaxial testing (khan and khan, 2016; aaqib et al., 2024). previous investigations by khan et al. (2021) and adeel et al. (2023) attempted to map vs30 across the islamabad–rawalpindi region using empirical correlations, but these estimates remain unverified due to the absence of measured shear wave velocities and modulus reduction curves for local soils. recent microzonation efforts have also highlighted this data gap, calling for the acquisition of site-specific dynamic soil properties to support more robust seismic hazard models (khan and khan, 2017; raza et al., 2025). among the laboratory methods available for evaluating the small-strain stiffness and damping behavior of soils, the resonant column test remains one of the most effective and widely accepted approaches. resonant column testing offers a controlled environment for evaluating shear wave velocity (vs), shear modulus (g), and damping ratio over strain levels typically ranging from 10⁻⁴% to 10⁻²%, which corresponds to the range experienced during moderate earthquakes in near-surface soils (hardin and drnevich, 1972; cha et al., 2014). furthermore, the ability to vary confining pressure allows simulation of in-situ stress conditions across different soil depths, enhancing the representativeness of the results for site response applications. dry-state resonant column testing is particularly suitable for low-plasticity fine-grained soils, where small-strain behavior is not significantly influenced by pore water pressure (blaker and degroot, 2020; mog and anbazhagan, 2022). islamabad’s near-surface geology consists predominantly of quaternary alluvial and colluvial deposits, including silty clays, clayey silts, and occasional sandy layers overlying weathered bedrock (sheikh et al., 2008; khan and khan, 2016). these fine-grained soils often exhibit low to moderate plasticity, relatively stiff consistency, and complex microstructure due to mixed depositional and diagenetic processes. as such, they are unlikely to conform to standard empirical modulus reduction models developed for either high-plasticity clays or granular soils. understanding the actual nonlinear stress-strain response of these materials under cyclic loading is essential for realistic site response predictions and the development of locally calibrated design spectra. to date, no laboratory-based modulus reduction (g/gₘₐₓ) or damping ratio (d) curves have been reported for islamabad soils. existing site response studies have relied on generic empirical models or indirect correlations, which introduces significant uncertainty into seismic hazard evaluations. this absence of laboratory-derived curves represents a critical research gap that limits reliable microzonation and seismic design in the region. to address this critical knowledge gap, the present study undertakes a comprehensive laboratory investigation of the small-strain dynamic properties of silty clay and clayey silt soils in the islamabad region. reconstituted specimens from 15 representative borehole locations were tested in a resonant column apparatus under isotropic confining pressures of 50 kpa, 100 kpa, and 150 kpa. these pressure levels were selected to simulate shallow to intermediate depths typically relevant for seismic site classification and response modelling. the study focuses on: (1) to measure shear wave velocity (vs) and maximum shear modulus (gₘₐₓ) of islamabad soils under varying confining pressures; (2) to characterize the modulus reduction (g/gₘₐₓ) and damping ratio (d) behaviour across a representative strain range; and (3) to evaluate the applicability of standard empirical models (e.g., darendeli, and vucetic & dobry) for predicting the nonlinear behaviour of these soils. the results provide the first high-resolution laboratory dataset of g/gₘₐₓ and d–γ curves for islamabad’s native fine-grained soils and offer insight into how these materials respond under cyclic loading conditions. by comparing the measured behavior against well-established models, the study also assesses the extent to which existing empirical frameworks are suitable for use in this region. ultimately, the findings support the development of improved input parameters for seismic site response analyses and contribute to a more reliable foundation for performance-based earthquake engineering and seismic code development in pakistan. 2. materials and methods 2.1 site and soil sampling soil samples were collected from multiple sites within the islamabad capital territory, covering a range of typical local soil conditions (fig. 1). the area’s geology consists of quaternary alluvial deposits (valley-fill silts, clays, and sands) overlying denser pleistocene sediments and weathered rock (limestone) toward the margalla hills. these quaternary deposits, composed primarily of low-plasticity silts and clays, play a critical role in seismic response, as their stiffness and damping properties control the amplification of ground motions within the city. this study therefore focused on fine-grained soils that are common in the top 20 m across the city. undisturbed samples were obtained using thin-walled shelby tubes, which were pushed in boreholes at depths of 5–15 m. in total, 25 tube samples were collected from 15 locations. each sample was immediately sealed and transported to the laboratory for testing. the borehole sites were strategically selected to represent the major geomorphological units of islamabad, including alluvial plains, colluvial deposits, and transitional zones near the margalla hills. this ensured coverage of the dominant fine-grained soils (silty clays and clayey silts) that govern seismic response. sites were also distributed across areas of ongoing urban development and seismic microzonation interest, so that the dataset would be both scientifically representative and practically relevant for hazard assessment. 349waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak figure 1. geological map of islamabad showing borehole (bh) locations and major rock formations. structural features such as thrust faults and synclines are also depicted for regional context 2.2 resonant column (rc) testing accurate evaluation of dynamic soil properties requires careful specimen preparation, particularly in resonant column (rc) testing. in this study, tests were performed on reconstituted specimens of silty clay with a plasticity index (pi) < 9, indicative of low-plasticity, silt-dominated behavior. although reconstitution may alter natural soil fabric and cementation, previous studies have shown that for low-plasticity silts and silty clays, the small-strain stiffness and damping behavior of intact and reconstituted specimens are broadly comparable (blaker & degroot, 2020; mog & anbazhagan, 2022). specimens were tested in the dry state, consistent with the current configuration of the gcts resonant column apparatus (fig. 2), which does not support back-pressure saturation or pore water pressure measurements. dry testing is commonly used in similar studies and is considered reliable for smallstrain behavior in low-plasticity soils. blaker and degroot (2020) compared dry and saturated tests on low-plasticity silty clays and found that the shear modulus and damping ratios at small strains (γ < 0.001%) were essentially unchanged by saturation. mog and anbazhagan (2022) also demonstrated that for silty clays, dry-state testing captured the small-strain stiffness plateau reliably, noting only minor differences in damping ratio at larger strains (>0.1%) due to the absence of pore pressure effects. similarly, bozyigit and altun (2023) reported negligible differences in small-strain stiffness for dry and saturated tests on silty sand–clay mixtures, supporting the appropriateness of dry-state testing in resonant column studies of low-pi soils. for small-strain behavior where interparticle water movement is minimal dry testing provides representative and reliable dynamic properties for engineering applications. all tested specimens were prepared by reconstituting air-dried soil from each sampling location. the soil was oven-dried at 105 °c, gently pulverized, and sieved through a no. 40 mesh to remove oversized particles. the reconstituted material was statically compacted in five uniform layers inside a cylindrical mold. this procedure achieved target dry densities of 1.65–1.80 g/cm³, which approximate the in-situ conditions observed in borehole logs and confirmed by laboratory moisture–density relationships. each specimen was trimmed to a standard size (height: 100 mm, diameter: 50 mm), encased in a latex membrane, and mounted in the resonant column cell between beryllium copper end caps. a vacuum of approximately 20 kpa was applied to ensure membrane conformity and eliminate air voids between the sample and confining chamber. the top platen was connected to the drive system and displacement transducers, while the bottom platen remained fixed. isotropic confining pressures of 50, 100, and 150 kpa were applied using compressed air, representing stress levels typical of depths up to 10–15 m in islamabad soils. before each stage, the specimen was reconsolidated under the new confining pressure and allowed to equilibrate for at least 30 minutes, ensuring stable conditions and eliminating residual strain effects from prior loading. this procedure minimized the risk of bias in stiffness and damping measurements across different stress level. figure 2. (a–d) setup and operation of the gcts resonant column test system for soil samples, including sample preparation, cell assembly, and testing phases; (e) complete resonant column test system the sample was excited at low torsional amplitudes to induce shear strains of ~0.0001%–0.001%. the system swept across excitation frequencies 350 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan (typically 50–150 hz) to identify the fundamental mode. shear wave velocity (vs) was calculated using: = (2 ) ⋅ and the maximum shear modulus (gmax) was obtained from: gmax = ρ ⋅ vs 2 where ρ is the specimen’s mass density, h is specimen height, and η = 0.75 is the mode shape factor. drive voltages were progressively increased to generate strain levels up to ~0.2%. at each strain level, the updated resonant frequency and free vibration decay were measured. damping was evaluated using the free vibration decay (fvd) (logarithmic decrement) method: = ln 1 2 = 2 4 2 + 2 where: a1 and a2 are successive peak amplitudes of fvd. for validation, the half-power bandwidth method was also applied under steady-state vibration (ssv) conditions, consistent with astm d4015 procedures: = 2 1 2 where: • f1 and f2 are the half-power frequencies, and • fr is the resonant frequency. both methods yielded consistent results in the small-strain range. for nonlinear behavior at higher strains (>0.01%), the fvd method provided more stable and representative damping values due to the inherent asymmetry of frequency response in soil. determination of dynamic soil properties in the gcts resonant column apparatus requires precise calibration of the drive mechanism to account for its mass moment of inertia (i₀). this calibration was conducted using an aluminum rod (ρ = 2700 kg/m³) as the calibration specimen. natural frequencies were measured in two states: without added mass (ω₁) and with a known additional mass (ω₂). the mass moment of inertia was then computed using: = ( cal + mass)( 2 2 1 2) 1 2 2 2 where: • ical: mass moment of inertia of the calibration rod • imass: mass moment of inertia of the added mass • ω₁, ω₂: angular frequencies without and with the added mass, respectively this approach, based on standard practices, assumes linear elastic behavior and negligible damping during calibration. incorporating the calibrated i₀ into the gcts cats software ensures accurate measurements of shear wave velocity and shear modulus. all calibration and testing procedures were performed in accordance with astm d4015 guidelines for resonant column testing of soils. at each strain level, the shear modulus (g) was computed from the updated resonant frequency, and damping ratio (d) was determined. the outcome of each test was a set of shear modulus vs. strain (g–γ) and damping ratio vs. strain (d–γ) data, typically normalized by gmax for analysis and comparison. 2.3 data processing and interpretation the raw output from the laboratory tests consisted of resonant frequency and damping measurements at various strain levels for each test (rc data). these were processed to compute engineering parameters. the shear modulus values from rc were normalized as g/gmax to facilitate comparison of modulus reduction behavior between samples. damping ratio values were plotted versus log shear strain to examine material damping characteristics. empirical curves were fitted to the g–γ data using a modified hyperbolic model (darendeli, 2001), empirical curves were fitted to the g–γ data using a modified hyperbolic model (darendeli, 2001). the best-fit parameters, reference strain (γref) and curvature exponent (a), were extracted for each confining pressure. additionally, the effect of confining pressure on gmax was evaluated by fitting a power-law relationship , where σ’m is mean effective confining stress. this allowed to infer a pressure exponent for the soil stiffness, and to adjust measured moduli to a common reference stress for comparison across samples. all test results were tabulated and plotted, and key values were extracted for each soil for discussion. 3. results 3.1 soil properties and test sample characteristics table 1 summarizes the index properties of representative soil samples from each borehole, including natural moisture content, specific gravity, atterberg limits, void ratio, and uscs classification. all tested specimens were fine-grained alluvium from the islamabad area, generally characterized by low to intermediate plasticity and an average void ratio of ~0.8. natural moisture contents ranged from 9–13%, while atterberg limits indicated plasticity indices (pi) between 1 and 9, typical of low-plasticity silts (ml) and borderline silty clays (cl-ml). two samples (bh-04 and bh-15) with slightly higher plasticity (pi = 9) were classified as lean clay (cl). unconfined compressive strengths measured using a pocket penetrometer generally ranged from 100–200 kpa, indicating stiff consistency. 3.2 sem and xrd scanning electron microscopy (sem) images of the tested silty clay (dry, powdered form) reveal a heterogeneous microstructure composed of angular, plate-like particles with loosely packed aggregates and significant microporosity. at low magnification, the images highlight the aggregated structure and heterogeneity of the sample matrix. higher magnification sem images further reveal edge-to-face and edge-to-edge particle contacts, typical of silty clay mineral platelets (fig. 3). energy-dispersive x-ray spectroscopy (eds) analysis complements the sem observations by confirming the presence of major elements typical of silty clay mineralogy: oxygen (61%), silicon (8%), aluminum (3.5%), and iron (0.7%), alongside minor components (k, mg, ca) (fig. 3). these data support the interpretation of a predominantly silicate matrix (illite/kaolinite) with minor carbonate or iron oxide phases. the lack of visible pore water films and the observed microstructural features indicate the suitability of dry testing for small-strain dynamic behavior characterization in low-pi silty clays (as per astm d4015). figure 4 shows the x-ray diffraction (xrd) analysis conducted on the dry powdered silty clay sample. the diffraction pattern exhibits distinct peaks, with a prominent reflection at 2θ ≈ 26.75° corresponding to quartz (sio₂) the dominant crystalline phase, comprising approximately 75% of the sample. additional peaks in the 20°–30° range confirm the presence of clay minerals such as kaolinite and illite (13%), typical of fine-grained alluvial soils. peaks corresponding to muscovite and biotite (11%) identify the mica 351waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak group, while minor reflections from calcite (1%) are also observed. this mineralogical composition supports the silty clay classification and the low plasticity measured in index tests. quartz-rich, non-expansive mineralogy and observed plate-like fabrics are consistent with high small-strain stiffness and comparatively low damping. these features explain why the soils maintain stiffness at microstrain levels but show progressive modulus reduction as particle contacts are mobilized at higher strains. the limited proportion of swelling clays further reduces hysteretic damping, directly linking mineralogy to the nonlinear seismic response documented in this study (see discussion). in particular, the dominance of quartz (~75%) provides a rigid particle skeleton, while the limited clay fraction (~13% illite/kaolinite) explains the low pi values (≤9) and moderate modulus reduction. together with mica (~11%), this assemblage accounts for the consistently high stiffness and low damping observed in mechanical tests (next section). the edge-to-face and edge-to-edge contacts among clay platelets observed in sem images likely enhance stiffness at very small strains, but as strain increases these contacts are progressively mobilized, producing the nonlinear modulus reduction trends documented in this study. similarly, the limited clay fraction and absence of expansive minerals reduce interparticle sliding hysteresis, explaining the relatively low damping ratios observed in resonant column tests. table 1. representative index properties, moisture content ranges, and uscs group classification of borehole samples bore hole specific gravity (gs) moisture content (%) plastic limit (%) liquid limit (%) pi void ratio (e) group name group symbol bh-01 2.67 10–13 26.73 21.98 4 0.84 silty clay (low plasticity) cl-ml bh-02 2.65 9–12 26.67 24.95 2 0.81 silt (low plasticity) ml bh-03 2.68 10–14 19.82 25.56 6 0.87 silty clay (low plasticity) cl-ml bh-04 2.69 8–10 9.62 18.43 9 0.9 lean clay cl bh-05 2.67 10–13 25.39 29.4 4 0.82 silty clay (low plasticity) cl-ml bh-06 2.65 9–12 20.46 22.93 2 0.78 silt (low plasticity) ml bh-07 2.65 9–12 21.18 23.78 3 0.79 silt (low plasticity) ml bh-08 2.66 9–12 19.29 22.23 3 0.71 silt (low plasticity) ml bh-09 2.66 9–12 17.91 21.18 3 0.71 silt (low plasticity) ml bh-10 2.65 8–11 20.46 21.49 1 0.72 silt (low plasticity) ml bh-11 2.66 9–12 20.28 23.53 3 0.75 silt (low plasticity) ml bh-12 2.66 9–12 21.53 24.85 3 0.81 silt (low plasticity) ml bh-13 2.67 9–12 18.29 21.29 3 0.81 silt (low plasticity) ml bh-14 2.65 9–12 np 0.78 silt (low plasticity) ml bh-15 2.69 10–14 23.67 32.48 9 0.88 lean clay cl gs = specific gravity of soil solids; w = moisture content; pl = plastic limit; ll = liquid limit; pi = plasticity index; np = non plastic; e = void ratio; group name and group symbol per uscs classification 352 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan figure 3. sem images show platy, angular morphology of silty clay; eds confirms a silicate-rich matrix dominated by o, si, al, and fe, characteristic of low-pi clay minerals figure 4. xrd pattern of the tested silty clay identifying dominant mineral phases 353waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak 3.3 small-strain shear wave velocity and shear modulus figures 5(a)–(c) present the comprehensive variation of shear wave velocity (vs) with shear strain across 15 boreholes for confining pressures of 50 kpa, 100 kpa, and 150 kpa. at very small shear strains (< 0.001%), shear wave velocity values remain near their maximum, highlighting the intact, elastic soil structure. repeatability tests on duplicate specimens from two boreholes (not shown) indicated variations of less than ±3% in vs and gmax and ±5% in damping ratios across the measured strain range. this level of agreement is consistent with typical resonant column variability reported in the literature, supporting the robustness of the dataset. across all 15 boreholes, typical small-strain vs values range from approximately 160~180 m/s for 50 kpa, 180~250 m/s for 100 kpa, and 200–260 m/s for 150 kpa. these values reflect the small-strain stiffness of the silty clay matrix and align well with the expected range reported for similar soil types (vucetic and dobry, 1991; darendeli, 2001). as shear strain increases beyond ~0.001%, a gradual decrease in vs is observed across all boreholes, indicating the onset of nonlinear soil behaviour and progressive softening of the soil skeleton. at strain levels of approximately 0.01%, vs values typically decrease to 140–160 m/s, and at 0.05–0.1% strain, they further reduce to 120–150 m/s, consistent with strain-induced rearrangement and reduced inter-particle stiffness. the effect of confining pressure is evident in the higher small-strain vs values observed at increased confining pressures, confirming its well-known role in enhancing small-strain stiffness. at small strains, vs values cluste tightly across all boreholes, aligning with the 150–250 m/s range reported for low-plasticity cohesive soils (l’heureux & long, 2017; wang et al., 2017). as strain increases beyond ~0.001%, vs progressively decreases converging to ~90–120 m/s by 0.1% strain, consistent with trends observed by mog and anbazhagan (2022) in similar silty clays. while data scatter increases slightly at strains >0.03%, it remains consistent with natural variability and experimental noise, reinforcing the robust pattern of nonlinear vs softening across different confining pressures and borehole sites. fitting a power-la relationship ( yielded an exponent n ≈ 0.18, lower than typical values (~0.3–0.5**) for granular soils and many clays (hardin & drnevich, 1972; vucetic & dobry, 1991; darendeli, 2001). this reduced stress dependence aligns with the moderate plasticity possible over consolidation or aging effects of the tested silty clays, which limit the sensitivity of stiffness to confining pressure changes. similar behavior has been reported for low-pi or lightly bonded clays, suggesting a more gradual stiffness increase with depth in these islamabad soils compared to typical sands or highly plastic clays. absolute values of small-strain stiffness measured in islamabad soils place them within nehrp site class d (stiff soil) and approaching site class c in certain zones, aligning well with prior indirect estimates for the region, for instance, previous spt-based approximations near the margalla tower site suggested vs in the range of 175–350 m/s (mahmood et al., 2016; khan and khan, 2017; aaqib et al., 2024). the direct laboratory measurements confirm that the surficial islamabad lays and silts are generally stiff (but not hard or cemented), comparable to other alluvial deposits in tectonically active regions (anastasiadis et al., 2001; govindaraju and bhattacharya, 2012; kanlı et al., 2006; kockar et al., 2010) . shear modulus reduction with strain figures 6(a)–(c) illustrate the variation of shear modulus (g) with shear strain for all 15 boreholes tested at investigated confining pressures. these plots collectively capture the essential strain-softening behavior of the soils and emphasize the pronounced effect of confining pressure on small-strain stiffness and modulus reduction. at very small strain amplitudes (γ < 0.0015%), the shear modulus remained nearly constant, with gmax values across all boreholes approximately: 50 kpa: 45–65 mpa, 100 kpa: 40–70 mpa, and 150 kpa: 50–80 mpa. this clear increase in gmax with confining pressure aligns with the established relationship between effective stress and small-strain stiffness (vucetic & dobry, 1991; darendeli, 2001; theland et al., 2021; cha et al., 2014), and confirms that even in a heterogeneous site like islamabad, confining pressure systematically enhances soil particle contacts and shear stiffness. as shear strain exceeded ~0.0015%, modulus reduction became more pronounced across all boreholes, reflecting strain-softening due to inter-particle slippage and structural rearrangement. similarly, modulus reduction behavior did not differ noticeably between soils with pi near 9 and non-plastic silts, with all curves falling within the ±15% variability envelope. figure 5. shear wave velocity versus shear strain for 15 boreholes at 50 kpa, 100 kpa, and 150 kpa, illustrating confining-pressure-dependent nonlinear stiffness reduction 354 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan figure 6. shear modulus variation with shear strain across 15 boreholes at confining pressures of 50 kpa, 100 kpa, and 150 kpa figures 7(a)–(c) present the variation of normalized shear modulus (g/ gmax i th shear strain for the 15 boreholes. across all boreholes, the initial stiffness plateau (g/gmax ~1) was observed consistently below 0.001% strain, while as strain increased, a gradual yet clear softening trend developed. this trend was particularly evident in comparing the overall rate of modulus reduction across different confining pressures: at 0.01% strain, g/gmax values for the 15 boreholes clustered higher for 150 kpa (0.85–0.95) compared to 50 kpa (0.80–0.90). at 0.1% strain, g/gmax values for 150 kpa grouped closer to 0.5, reflecting how higher confining pressure mitigates stiffness reduction and delays the onset of significant modulus reduction trends noted by darendeli (2001) and vucetic and dobry (1991). the maximum strains tested in this study were approximately 0.18% at 50 kpa, 0.12% at 100 kpa, and 0.11% at 150 kpa. at these maximum strains, the normalized shear modulus (g/gmax) decreased to about 0.32, 0.41, and 0.52, respectively demonstrating a consistent moderate nonlinearity. this behavior reinforces that while these silty clays retain significant stiffness at small strains, nonlinear site response analysis is essential for large earthquake strains due to appreciable softening starting at ~0.01% strain. these consistent results across 15 boreholes reinforce confidence in the experimental data and emphasize the importance of incorporating pressuredependent modulus reduction curves in nonlinear site response models. minor variability at larger strains highlights the site-specific nature of soil microstructure and stress history, while the tight grouping of curves within ±15% confirms the overall reliability of the modulus reduction trends. the observed increase in small-strain moduli with confining pressure suggests higher wave propagation speeds and impedance contrasts in deeper, denser layers, which will reduce nonlinear effects during strong shaking and enhance the accuracy of seismic site response analyses and microzonation in islamabad. figure 7. normalized shear modulus (g/gₘₐₓ) versus shear strain for soils from 15 borehole specimens at confining pressures of 50, 100, and 150 kpa. data points represent the complete dataset, with best-fit curves and ±15% confidence bands illustrating variability across all specimens 355waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak figure 8 compiles the normalized shear modulus (g/gmax) versus shear strain (γ) data from all 15 borehole samples tested at confining pressures of 50 kpa, 100 kpa, and 150 kpa. data points are represented as blue circles (50 kpa), green squares (100 kpa), and red triangles (150 kpa), while solid lines represent best-fit curves for 50 kpa (blue) and 150 kpa (red), developed using the darendeli (2001) modified hyperbolic model. the darendeli model expresses the normalized shear modulus reduction as: max = 1 1 + ref where γ is the shear strain, γref is the reference shear strain (at which g/gmax=0.5), and a is a fitting exponent that governs the curvature of the modulus reduction trend. for each pressure condition (50 kpa and 150 kpa), best-fit values of γref and a were determined by minimizing the sum of squared errors between measured and predicted g/gmax values. this approach ensures the model accurately captures both the initial stiffness plateau and the progressive softening at larger strains. the resulting fitted curves closely align with the experimental data, with deviations generally within ±15% up to ~0.05% strain, showing the reliability of the fits and the experimental trends. at small strains (<0.001%), g/gmax remains close to 1.0 across all pressures, reflecting the linear elastic response of the soil matrix. as strain increases, modulus reduction initiates gradually. at 0.01% strain, g/gmax values typically range from 0.88–0.95 (150 kpa), 0.85–0.9 (100 kpa), and 0.80–0.88 (50 kpa). at 0.1% strain, these values reduce to 0.50–0.58 (150 kpa), 0.45–0.53 (100 kpa), and 0.38–0.48 (50 kpa). the maximum shear strains tested were approximately 0.18% (50 kpa), 0.12% (100 kpa), and 0.11% (150 kpa), with corresponding g/gmax values of about 0.32, 0.41, and 0.52, respectively. the 100 kpa data consistently fall within the 50 kpa–150 kpa fitted envelope, confirming smooth transitional behaviour with confining pressure. overall, this figure underscores the pronounced confining pressure dependence of modulus reduction in islamabad’s silty clay and clayey silt soils. the close alignment between experimental data and the darendeli model fits highlights the robustness of the dataset and the necessity of incorporating pressure-specific g/gmax curves in nonlinear site response analyses for accurate seismic hazard assessment in islamabad. table 2 presents the fitted γref and exponent a values for the tested soils at 50, 100, and 150 kpa, compared against widely cited empirical models (darendeli, vucetic & dobry, vardanega & bolton, zhang et al.). the fitted γref values increase systematically with confining pressure (0.065–0.082%), while the curvature exponent a remains stable (~0.87–0.91). these values fall within or slightly above published ranges for low-plasticity silts and clays, confirming the consistency and reliability of the experimental dataset. figure 8. normalized shear modulus (g/gₘₐₓ) curves for soils tested at confining pressures of 50, 100, and 150 kpa. data points represent results from all 15 borehole specimens, and best-fit curves were developed using the darendeli (2001) modified hyperbolic model figure 9 illustrates the correlation between the measured normalized shear modulus (g/gmax and the calculated values obtained using the modified hyperbolic model across three confining pressures: 50 kpa, 100 kpa, and 150 kpa. across all three pressure levels, the calculated g/gmax values exhibit a strong linear correlation with the measured data, as indicated by consistently high r² values of 0.95 and low rmse values ranging from 0.045 to 0.051. the alignment of the data along the 1:1 line confirms that the adopted model captures the strain-dependent modulus reduction on with high accuracy. notably, the spread of points slightly increases at lower normalized modulus values (g/gmax < 0.5), reflecting the inherent variability in soil behavior at larger shear strains. nevertheless, the close clustering of data around the diagonal in all cases underscores the model’s ability to reliably represent nonlinear stiffness behavior across a range of confining pressures. table 2. best-fit modulus reduction parameters (modified hyperbolic model) and comparison to common models curve / model reference strain, γref (% strain) exponent, a (–) notes 50 kpa (fit) 0.065 0.87 this study 100 kpa (fit) 0.071 0.87 this study 150 kpa (fit) 0.082 0.91 this study darendeli (2001) ~0.07 ~0.90 typical for low-pi soils; pidependent vucetic & dobry (1991) 0.04–0.08 0.8–1.0 plasticity-dependent families vardanega & bolton (2013) 0.05–0.08 ~0.85 meta-analysis (pi, confining stress trends) zhang et al. (2005) 0.06–0.09 ~0.90 low-to-moderate pi silts/clays model form / max = [1 + ( / ref) ]−1 356 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan figure 9. comparison of measured and calculated normalized shear modulus (g/gmax) around the 1:1 line at confining pressures: (a) 50 kpa; (b) 100 kpa; (c) 150 kpa 3.5 material damping characteristics the material damping ratio (d) of the islamabad silty clay soils (pi ≈ 7) was measured using both the steady-state vibration (ssv) and free vibration decay (fvd) methods in resonant column tests across a broad strain range (~0.0001%~0.1%) (fig. 5). at very small strains (γ ≈ 0.001%), both methods yielded consistent and low damping ratios (0.5–1.5%), reflecting the quasielastic limit where the soil skeleton dissipates minimal energy. across all tests, the minimum damping ratio (dmin) was consistently ~0.5–1% at microstrain levels, while the maximum damping ratio (dmax) reached ~10–12% at shear strains of 0.1%. this low small-strain damping is typical of dense, lowplasticity silty clays and implies limited attenuation of minor vibrations—an important consideration for seismic site response. as shear strain increased into the 0.001–0.01% range, damping ratios rose gradually. by γ ≈ 0.01%, damping ratios reached ~3–5%. in the 0.01–0.1% strain interval, damping increased more rapidly, reaching maximum values of ~10–12%. at very small strains (<0.005%), the fvd and ssv damping estimates align closely, confirming the reliability of either approach in the linear viscoelastic range. however, at medium and large strain amplitudes (>0.005%), the fvd-derived damping ratios consistently increased more rapidly with strain, especially under higher confining pressures. this divergence reflects the fvd method’s superior ability to capture nonlinear, strain-dependent energy dissipation, as it is not constrained by frequency response symmetry assumptions that limit the ssv approach. to accurately model the strain-dependent damping behaviour of the soil samples across different confining pressures, the experimental damping ratio data (for both fvd and ssv) were fitted using a modified hyperbolic equation, inspired by the frameworks of hardin and drnevich (1972). the equation is expressed as: ( ) = min + ( max − min ) + where λ(γ) is the damping ratio at shear strain γ, λmin and λmax represent the minimum and maximum damping ratios respectively, γr is the reference strain capturing the transition zone, and a is a shape parameter dictating the curve’s steepness. in this study, λmin was fixed to the minimum observed experimental damping ratio for each confining pressure to ensure consistency with smallstrain behaviour. the other parameters (λmax, γr , and a) were determined by nonlinear least-squares fitting to the experimental data, enabling site-specific calibration. to substantiate the use of the adopted damping ratio model in this study, it is important to highlight that several contemporary and classical works have employed or supported similarly structured equations involving reference strain and curve-shaping parameters. for instance, (wichtmann et al., 2015) proposed empirical expressions for damping ratio as a function of shear strain, where parameters analogous to γr and α controlled the curvature and transition slope. similarly, araei and ghodrati (2017) applied nonlinear regression to predict damping behaviour using confining pressure, strain amplitude, and frequency factors intrinsically linked to the parameters of the current model. conti et al. (2020) also formulated constitutive models where damping evolution followed strain-sensitive transitions governed by steepness and shift parameters. these precedents validate the theoretical basis and practical applicability of the chosen model, particularly for low-plasticity soils under dry testing conditions, as in this study. figures 10-11 compare ssv and fvd damping ratio measurements at confining pressures of 50 kpa, 100 kpa, and 150 kpa. at 50 kpa, the data are closely grouped along the 1:1 line, with most points falling within the ±15% bands, reflecting excellent agreement between methods. the ±15% bands shown in the plots represent an engineering tolerance envelope that encompasses the scatter of measured data across all borehole samples. as confining pressure increases to 100 kpa, the scatter widens slightly, with some data points extending beyond the ±15% bands but still remaining within ±30%, indicating moderate variability. at 150 kpa, the scatter is more pronounced, and a few data points fall outside the ±30% bands. however, this variability appears to be random, rather than a systematic underestimation by the ssv method. overall, these results highlight the generally strong agreement between ssv and fvd methods across confining pressures, with most data within ±30% of the 1:1 line. the increased scatter at higher pressures likely reflects the nonlinear soil behavior and measurement sensitivity under these conditions, rather than a consistent measurement bias. this aligns with findings by mog and anbazhagan (2022) and vucetic and dobry (1991), who also noted that ssv and fvd methods typically agree well at small to moderate strains, while slight divergence at larger strains is expected due to measurement complexities. the data also revealed that confining pressure had a minor but consistent effect on damping behavior. at equivalent strain levels, higher confining pressures slightly reduced damping ratios likely because increased confinement constrains the soil skeleton, reducing microslip and hysteresis at inter-particle contacts. however, this effect was modest and not pronounced enough to necessitate pressure-specific damping curves for engineering analysis. 357waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak the minimum damping ratios (dmin) extrapolated as γ → 0, were about 0.5–1%, slightly lower than typical published values (often 1–2%) for similar low-pi silty clays (mog & anbazhagan, 2022; wang & stokoe, 2022). from an earthquake engineering perspective, this low small-strain damping implies that small earthquake tremors or high-frequency shaking components may be less attenuated, potentially transmitting more vibrational energy to structures—a caution also highlighted by (rahman et al., 2021). at larger strain levels during strong earthquakes, however, the observed increase in damping (up to 10–15%) would help limit extreme amplification. figure 10. damping ratio versus shear strain for fvd and ssv measurements at 50 kpa, 100 kpa, and 150 kpa, with experimental data and fitted hyperbolic curves figure 11. comparison of damping ratios (ssv vs. fvd) with ±15% and ±30% confidence bands around the 1:1 line at confining pressures: (a) 50 kpa; (b) 100 kpa; (c) 150 kpa figure 12 compares the measured and calculated damping ratios obtained from the modified hyperbolic model across three confining pressures: 50 kpa, 100 kpa, and 150 kpa, for both fvd (frequency domain) and ssv (single sinusoid) methods. at 50 kpa, the ssv method yields a relatively strong correlation (r² = 0.86, rmse = 0.76), outperforming fvd (r² = 0.75, rmse = 1.17), which shows greater scatter and underprediction at higher damping levels. at 100 kpa, both methods demonstrate slightly reduced performance (r² ≈ 0.72–0.74), with comparable rmse values around 1.07%, suggesting similar predictive capacity but modest divergence from the 1:1 line. the best fit is observed at 150 kpa, particularly for ssv (r² = 0.87, rmse = 0.80), indicating that damping behaviour becomes more reliably captured in stiffer soils under higher confining stress. across all pressures, the model shows a consistent tendency to underpredict damping ratios, especially in the mid-tohigh damping range. the fvd method exhibits greater dispersion, particularly at lower confining pressures, while the ssv method offers better alignment at 50 and 150 kpa. these results affirm the validity of the adopted damping model while emphasizing the influence of estimation method ad confining pressure on fitting accuracy. figure 12. comparison of measured and calculated damping around the 1:1 line at confining pressures: (a) 50 kpa; (b) 100 kpa; (c) 150 kpa figure 13 presents the variation of damping ratio with normalized shear modulus (g/gmax) for the tested silty clay at confining pressures of 50 kpa, 100 kpa, and 150 kpa. the data consistently show that damping ratio increases as g/gmax decreases, reflecting increased energy dissipation with progressive modulus reduction. at small shear strains (g/gmax ≈ 1), the damping ratio is 358 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan low and similar across all confining pressures. as strain increases, the damping ratio rises more sharply, particularly at higher confining pressures (100 kpa and 150 kpa), indicating enhanced energy dissipation with higher confinement. these results indicate the nonlinear and pressure-dependent damping behaviour of the low-plasticity silty clay (vucetic & dobry, 1991). figure 13. variation of damping ratio with normalized shear modulus at confining pressures of 50 kpa, 100 kpa, and 150 kpa, with experimental data and fitted polynomial curves. 4. discussion figure 14 compares the experimentally fitted curves for the tested silty clay soil at confining pressures of 50 kpa, 100 kpa, and 150 kpa against wellestablished standard models (vucetic & dobry, 1991; darendeli, 2001; zhang et al., 2005). figure 14a compares the measured normalized modulus reduction (g/gmax) curves for the tested silty clay soil at 50, 100, and 150 kpa confining pressures with established empirical models for pi=15. at small shear strains (γ < 0.001%), all models and the tested data exhibit near-unity stiffness, confirming the elastic response of the soil matrix. as strain increases, the tested data show a more pronounced reduction in g/gmax compared to the predictions by vucetic and dobry (1991) and vardanega and bolton (2013), particularly at large strains (γ > 0.01%). this observation is consistent with the findings of guerreiro et al. (2012) and kishida (2017), who reported that low-pi silty clays exhibit accelerated modulus reduction at larger strains and higher confining pressures. the closer alignment of the tested data with the darendeli (2001) and zhang et al. (2005) models particularly at intermediate strain levels reflects the importance of confining pressure and pi on modulus reduction in low-plasticity silty clays. these comparative trends underscore that while generalized models provide valuable upper and lower bounds for engineering design, site-specific calibration remains critical for accurately capturing the nonlinear behaviour of silty clays subjected to dynamic loading. figure 14b presents the variation of normalized shear modulus (g/gmax) with shear strain (γ) for the tested silty clay at 50, 100, and 150 kpa confining pressures, overlaid with predictions from standard models. at low strains (γ < 0.001%), the tested soil data show near-unity g/gmax across confining pressures, consistent with the intact elastic response described by vucetic and dobry (1991) and darendeli (2001). as strain amplitude increases, the modulus reduction in the tested data aligns more closely with the darendeli (2001) and zhang et al. (2005) predictions, particularly for the pi=15 lower-bound trends reported in studies like guerreiro et al. (2012) and kishida (2017). however, notable deviations appear at higher strains (γ > 0.01%), where the tested data consistently degrade faster than vucetic and dobry (1991) and vardanega and bolton (2013) models predict highlighting the limitations of these models for low-pi, low ocr silty clays under higher confining pressures. this observation resonates with findings kishida (2017), who identified that the vucetic and dobry (1991) model tends to overestimate stiffness at large strains for low-pi soils, unlike the darendeli and zhang models which better capture this nonlinear modulus reduction trend. the separation between the standard models’ upper and lower bounds (pi=100 vs. pi=15) underscores the role of plasticity in modulating straindependent modulus reduction. the fact that the tested data consistently fall within or below the lower bound (pi=15) trends suggests that the tested soil’s low plasticity (pi < 7) is a key factor governing its pronounced modulus reduction. the mineralogical composition helps explain these discrepancies. the dominance of quartz (~75%) and mica (~11%) creates a rigid particle framework that increases small-strain stiffness and limits energy dissipation, while the limited clay fraction (~13% illite/kaolinite) reduces plasticity and moderates modulus degradation. in contrast, the vucetic & dobry (1991) and darendeli (2001) models were developed from soils with higher clay content and plasticity, which naturally display greater softening and higher damping. this difference highlights the importance of incorporating site-specific mineralogical context when interpreting dynamic soil properties and selecting appropriate ground response models. figure 14. comparison of normalize shear modulus (g/gₘₐₓ) versus shear strain: (a) fitted curve at 100 kpa confining pressure compared with standard modulus reduction models for pi = 15 and pi = 100; (b) experimental curves at 50, 100, and 150 kpa compared with standard modulus reduction models figure 15 presents a comparative evaluation of the experimentally derived damping ratio curves from this study using both frequency domain (fvd) and time domain (ssv) methods against several benchmark empirical models for cohesive soils. the fvd and ssv curves demonstrate consistently lower damping across the full strain range compared to standard models, especially beyond shear strain levels of γ = 0.01%. the ssv curve remains below all reference models up to γ ≈ 0.03%, after which it closely parallels the slope of the (epri, 1993c) average curve but remains consistently lower in magnitude. the fvd curve initially aligns with the darendeli (2001) model for pi = 15 at 100 kpa, diverging downward beyond γ = 0.03%, and eventually falling below all empirical curves except 359waqas ahmed, muhammad waseem, hammad raza, muhammad yasir, muhammad zeeshan1 saif ullah1, ihtisham islam, salman ahmed khattak the epri and ssv lines. this trend highlights a systematic underestimation of damping for moderate to large strains (γ > 0.01%), with the fvd and ssv curves peaking at ~8% and ~6%, respectively, at 0.2% strain, compared to >15–20% for darendeli’s high-pi models. however, the deviation remains within ±15% of the measured values, which is considered acceptable for most engineering ground response analyses. the current model therefore provides sufficiently reliable input for design purposes, though future refinements may explore pressure-sensitive adjustments to improve mid-strain accuracy. these discrepancies are likely attributed to the soil’s low plasticity index (pi < 7) and dry testing conditions, which inherently limit hysteretic damping and pore-fluid-related energy dissipation. this behaviour is in agreement with studies such as guerreiro et al. (2012) and mog and anbazhagan (2022), who reported similarly subdued damping in dry, low-pi silty clays. furthermore, classic experimental curves by ishihara et al. (1975), taylor et al. (1975), and kokusho et al. (2005) developed for higher-plasticity cohesive soils show notably higher damping (e.g., ~18–21% at 0.01–0.1% strain), emphasizing the divergence for low-pi materials. the vucetic and dobry (1991) model, formulated for pi ≈ 15, overlaps significantly with darendeli’s predictions and further contrasts the more restrained damping observed in this study. all curves reflect the expected nonlinear increase in damping with strain, the fitted curves from this study offer a more conservative, site-specific estimate of energy dissipation suitable for islamabad’s stiff, over consolidated silty clays. these findings underscore the critical role of pi-sensitive and site-calibrated models in ground response analysis, particularly in tectonically active regions with dominantly low-plasticity soils. figure 15. comparison of fitted damping ratio curves from this study (fvd and ssv methods) with established empirical models for cohesive soils for engineering practice, the measured curves provide more accurate, sitespecific input than generic models, particularly for site class d soils prevalent in islamabad. these curves can directly inform nonlinear site response analyses and soil-structure interaction modeling, addressing gaps in existing building codes and moving beyond reliance on proxy-based vs estimations. previous site response and microzonation studies in islamabad (khan & khan, 2017; adeel et al., 2023; raza et al., 2025) have reported amplification factors of approximately 2–4× for mid-period ground motions. the laboratory-derived modulus reduction and damping characteristics presented in this study align with those findings and provide direct experimental support for the reported amplification trends. consequently, specialized ground improvement for soft soil response may not be broadly necessary in islamabad’s fine-grained deposits. the fitted γref and a parameters (table 1) confirm that islamabad soils exhibit delayed nonlinearity compared to standard low-pi models, particularly under higher confining pressures. the extracted parameters align closely with darendeli (2001) and zhang et al. (2005), while diverging from vucetic & dobry (1991) at larger strains, reflecting the mineralogical controls of quartzrich, low-clay matrices. this dataset already has important implications for microzonation and ground motion prediction, offering a more realistic basis for site-specific seismic demand. while these fine-grained soils appear competent and non-liquefiable, further study should evaluate potential sandy layers. extending this testing approach to other major cities (karachi, lahore, peshawar) will help develop a national database of dynamic soil properties, complemented by in-situ geophysical measurements to validate laboratoryderived curves and ensure their field relevance. overall, the results enhance the seismic design toolkit, supporting more reliable, site-adapted hazard assessments and structural performance predictions. 5. conclusions this study presents the first laboratory-based evaluation of the smallstrain dynamic properties of islamabad soils using resonant column tests. silty clay and clayey silt samples from 15 boreholes exhibited high initial stiffness, with maximum shear modulus (gmax) increasing systematically with confining pressure (45–80 mpa across 50–150 kpa). normalized modulus reduction (g/ gmax) curves showed gradual softening, with values decreasing to 0.3–0.5 at ~0.1–0.2% strain. damping ratios ranged from ~0.5–1% at microstrains to ~10–12% at higher strains. these values were consistent across boreholes, with repeatability within ±3–5%. best-fit γref values increased from 0.065% at 50 kpa to 0.082% at 150 kpa, with exponent a stable at ~0.87–0.91. these parameters confirm pressure-dependent nonlinearity consistent with international reference models. comparison with standard empirical models (vucetic & dobry, 1991; darendeli, 2001) demonstrated that islamabad soils retain higher stiffness and lower damping than predicted for pi = 15–100 clays. the dominance of quartz (~75%) and mica (~11%) in the mineralogy explains the high stiffness and low damping, while the limited illite/kaolinite fraction (~13%) accounts for the low pi values (≤9) and moderate modulus reduction. this mineralogical fabric, characterized by edge-to-face contacts, constrains energy dissipation at small strains but permits progressive modulus degradation at larger strains. together, these findings highlight the need for site-specific calibration of nonlinear soil behavior in the region. replacing generic empirical curves with the measured site-specific dataset has direct consequences for seismic hazard assessment in islamabad. generic models overestimate damping and stiffness, which can underestimate amplification and lead to unconservative designs. our curves indicate amplification levels up to 20–30% higher in the 0.1–1.0 s period range, aligning with prior microzonation studies that reported ~2–4× amplification. the developed modulus reduction and damping curves can be directly integrated into one-dimensional ground response analyses and used to refine bcp-2021 site classification and design spectra, ensuring more reliable seismic design. incorporating these curves into microzonation and seismic code development will yield more realistic design spectra, improve hazard mapping, and support performance-based earthquake engineering for pakistan’s capital region. acknowledgements we are thankful to national centre of excellence in geology, university of peshawar, pakistan for providing facilitates for carrying out this research. author contributions the authors confirm contribution to the paper as follows: study conception and design: waqas ahmed; muhammad waseem; data collection: muhammad zeeshan; salman ahmed khattak; analysis and interpretation of results: muhammad waseem, hammad raza; draft manuscript preparation: 360 characterization of nonlinear dynamic soil properties for seismic site response in islamabad, pakistan waqas ahmed, muhammad yasir, ihtisham islam. all authors reviewed the results and approved the final version of the manuscript. funding this study is supported by the higher education commission of pakistan ref no. 20-17192/nrpu/r&d/hec/2021. data availability all data are included in the manuscript. conflict of interest the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. references aaqib, m., nguyen, v.-q., javaid, o., khan, a. h., ashraf, m. a., & bhusal, b. 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(2005). normalized shear modulus and material damping ratio relationships. journal of geotechnical and geoenvironmental engineering, 131(4), 453-464. _hlk199774496 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n4.103275 earth sciences research journal earth sci. res. j. vol. 26, no. 4 (december, 2022): 271 278 so il s manuscript received: 21/06/2022 accepted for publication: 24/11/2022 estimation of soil losses due to water erosion in the dagua river basin, colombia carolina martínez lópez, juan carlos menjivar flóres y daniela calero mosquera universidad nacional de colombia camartinezlo@unal.edu.co abstract keywords: basin; erodibility; soil conservation; soil degradation; usle; the dagua river basin, in colombia, is the most important source of water for the valle del cauca ecosystem, however, due to poor agricultural practices, it has been affected by water erosion. this study aimed at estimating soil erosion in the dagua river basin, using the universal soil loss equation (usle). the results show that most of the area presents erosivities that are between 1000-5000 mj.mm.ha-1, corresponding to low and very low categories. on the other hand, erodibility ranged from 0.143 to 0.842 t. ha.h. mj-1 mm-1 ha-1, which is framed in the categories from weak to extremely erodable, where the low to medium category predominates. regarding soil losses due to erosion, it was found that more than 20% of each of the municipalities of dagua, restrepo, la cumbre, and vijes, showed high and very high erosion, particularly in the areas with bare soils and crops such as pineapple, contribute strongly, sometimes reaching over 1000 t ha-1 yr-1. therefore, it is important to promote practices such as contour or contour planting, integrated crop cover management, land uses that integrate trees, and in more critical cases to consider ecological restoration processes. resumen la cuenca del río dagua, colombia, es la fuente de agua más importante para el ecosistema del valle del cauca, sin embargo, debido a las malas prácticas agrícolas, se ha visto afectada por la erosión hídrica. el objetivo de este estudio fue estimar la erosión del suelo en la cuenca del río dagua, utilizando la ecuación universal de pérdida de suelo (usle). los resultados muestran que la mayor parte del área presenta erosividades que se encuentran entre 1000-5000 mj.mm.ha-1, correspondientes a categorías bajas y muy bajas. por otro lado, la erodabilidad osciló entre 0,143 y 0,842 t. ha.h. mj-1 mm-1 ha-1, lo que se enmarca en las categorías de débil a extremadamente erodable, donde predominó la categoría baja a media. en cuanto a las pérdidas de suelo por erosión, se encontró que más del 20% de cada uno de los municipios de dagua, restrepo, la cumbre y vijes, presentaron erosión alta y muy alta, y en particular las áreas con suelos desnudos y cultivos agrícolas como la piña, contribuyeron fuertemente, alcanzando en ocasiones más de 1000 t ha-1 año-1. por lo tanto, es importante promover prácticas como la siembra en contorno o contorno, el manejo integrado de la cobertura de cultivos, los usos de la tierra que integren árboles y, en casos más críticos, considerar procesos de restauración ecológica. palabras clave: cuenca; conservación de suelos; degradación de suelos; erodabilidad; usle. estimación de perdidas de suelo causadas por la erosión del agua en la cuenca del río dagua, colombia how to cite this item: martinez-lópez, c, menjivar-flóres j. c., & mosquera-calero d. (2022). estimation of soil losses due to water erosion in the dagua river basin, colombia. earth sciences research journal, 26(4), 271-278. https://doi. org/10.15446/esrj.v26n4.103275 https://doi.org/10.15446/esrj.v26n4.103275 mailto:camartinezlo@unal.edu.co http://mj.mm.ha http://mj.mm.ha https://doi.org/10.15446/esrj.v26n4.103275 https://doi.org/10.15446/esrj.v26n4.103275 272 carolina martínez lópez, juan carlos menjivar flóres y daniela calero mosquera 1. introduction soil degradation by erosion is one of the most important problems affecting both natural ecosystems and agroecosystems worldwide (panagos et al., 2018; borrelli et al., 2020; sharda et al., 2021). this process consists of the gradual loss of the surface layer of the soil due to the action of wind or water (borrelli et al., 2017; ma et al., 2021), which has led to the reduction of its productive potential, generating negative effects not only environmental but also socioeconomic (pandey et al., 2021). erosion is a natural process that is caused by the combination of several factors such as steep slopes, climate, soil characteristics, type, and state of general cover. in turn, it is a process that is accelerated by the influence of anthropic activities (clean crops, planting in favor of the slope, extensive livestock, absence of conservation practices) (amundson et al., 2015; abdulkareem et al., 2019), and that is of great importance if one takes into account that the soil is a non-renewable natural resource thus, world food production is threatened by soil erosion, a figure that reaches average values of 30 t ha1 year 1, which leaves around 10 million hectares of unproductive farmland each year (gachene et al., 2020). it has been pointed out that this phenomenon can cause annual socioeconomic losses of approximately 40 billion dollars worldwide (crosson, 1995; oldeman et al., 2017), and in environmental terms, likely, more than 90% of the soil of the planet earth deteriorates in 2050 (bouma and montanarella, 2016; keesstra et al., 2016). on the other hand, approximately 48% of soils have already been degraded in colombia (ideam and udca, 2015) according to the above, over the last decades, research has been carried out through experimentation and modeling of erosion, which allows for counteracting its effects (ma et al., 2021). in most experimental methods, soil erosion is calculated either in runoff plots under natural rainfall or under simulated conditions (el kateb et al., 2013; anache et al., 2017). however, erosion predictions can be carried out by taking advantage of mathematical models that express the efficiency of sediment transport from a hillside and a network of channels (kirkby and morgan, 1980; németová et al., 2020). the prediction of erosion has a long history of more than seven decades (bennett, 1939; alewell et al., 2019), where one of the most used models is the usle, taking into account its high degree of flexibility, data accessibility, and comparability of results that allow adapting the model to almost all types of conditions in the world (alewell et al., 2019). this equation was developed in the 1950s, by the united states department of agriculture and the soil conservation service, and is one of the most important tools in conservation planning, helping to predict soil loss (meinen and robinson, 2021), even at the basin level when integrated with geographic information systems (gis) (mitasova et al., 1996, iroumé et al., 2011). in colombia, the hydrographic basin of the dagua river is the most important source of water for the valle del cauca ecosystem (aguirre et al., 2017). this region has more than eight life zones, which represents an enormous environmental value (loaiza et al., 2014). in this territory, the dominant covers are agricultural and silvopastoral, and only 2,867 ha (0.36%) are covered by native forests (iavh, 2015) (reina-rodríguez et al., 2016). despite the above, due to poor agricultural practices, ecosystems have been deteriorated (daza et al., 2012; cardona et al., 2014), since more than half of the territory presents processes of water erosion (loaiza et al., 2012). in this way, mass movements and the annual dumping of 250,000,000 kg t of sediments into buenaventura bay have been reported, causing enormous socioeconomic and environmental losses (reyes et al., 2010; loaiza et al., 2014). the objective of the research was to estimate the loss of soil due to water erosion in the dagua river basin, colombia, using the empirical model of soil erosion (usle), to obtain information for decision-making regarding the conservation strategies and resource management for the study area. 2. materials and methods 2.1 location of the study area the hydrographic basin of the dagua river is located on the western flank of the western mountain range, on the valle del cauca pacific (gonzáles et al., 2016) and is made up of the municipalities of dagua, buenaventura, la cumbre, restrepo, vijes, yotoco, and calima darien. its coordinates include 3 ° 53’13.2 ‘’ to 3 ° 27’39.6 ‘’ of north latitude and 76 ° 57 ‘21.6’ ‘to 76 ° 40’29.99’ ‘of west longitude. it has a total area of 1,422 km2, of which about 98.2% correspond to the hillside area and 1.8% to the marine plain (cvc, 2007). the average annual rainfall in the upper part of the basin is 900-1600 mm, while in the lower part it can reach 8000 mm (aguirre et al., 2017). figure 1. location of the study area and the distribution of pluviometric, pluviographic and climatological stations. 273estimation of soil losses due to water erosion in the dagua river basin, colombia 2.2 climatic data to obtain climate information, records of eighteen climatological, pluviometric, and pluviographic stations belonging to the autonomous regional corporation of valle del cauca (cvc), available for the study area, were analyzed, of which five are conventional (daily records) and thirteen are automatic (logs every 10 minutes); the above for a period of five years (20152019). thus, figure 1 presents this area, as well as the location of the stations used in the research. 2.3 soil information based on the review of the information provided by the cvc advanced geographic viewer for the year 2021 and the document called “semi-detailed soil survey scale 1: 25000 of the basins prioritized by the cvc” from 2014, it was possible to obtain the cartographic information and identify the main soil orders present in the study area, which belong to the inceptisols, entisols, histosols, alfisols, mollisols, and andisols. likewise, the properties required for the study were: texture, organic matter, permeability, and structure. 2.4 the digital elevation model (dem) for the elaboration of the slope map, a digital elevation model supplied by the cvc with a resolution of 30 m was taken as input, with radiometric correction, from which it was possible to obtain the slope map. it was worked on a file geodatabase using magna sirgas coordinate system for colombia. 2.5 cover layer and land use these layers were provided by the cvc (2021), which presents updated information for 2018. a total of 94 types of coverage were identified. the main covers correspond to the dense mixed highland forest, cultivated pasture, and secondary or transition vegetation, while the main established agricultural crop is pineapple (ananas comosus (l.) merr). 2.6 erosion models 2.6.1 universal soil loss equation-usle: the empirical usle model was used, which has been disseminated and used worldwide for more than four decades (alewell et al., 2019). this model allows for estimating the long-term average erosion, and it is found in function of six factors, as presented in equation 1 (wischmeier and smith, 1978). 6 1n a rklscp = =∑ (1) where: a is the soil loss calculated per unit area (t. ha. yr-1), which is the result of the product of the six factors, r is the erosivity of rain, a factor that reflects its aggressiveness and runoff (mj mm ha-1 h-1 yr-1), k is the erodability, which represents the susceptibility to detachment of soil particles (t. ha. h. ha-1. mj-1. mm-1), l is a slope length factor (dimensionless), s is slope steepness (dimensionless), c is a plant cover factor (unitless), and p is a management practice factor. r factor to determine the r factor, precipitation events with a record greater than or equal to 12.7 mm, and separated by at least six hours (erosive events) were analyzed. this factor was calculated with the methodology proposed in the usle by wischmeier and smith (1978); however, taking into account that in this case, the records of the pluviographic band are not available, but rather the accumulated precipitation in 10 minutes, it was proposed to use this interval to determine the average precipitation intensity (im) required in the calculation of the kinetic energy (e) for each mm of rain, based on equation 2: e = 0.119 + 0.0873 ∗ logim (2) once the total energy was obtained in each 10-minute interval, the value of e was multiplied by the total precipitation recorded in said interval (eq. 3): ei = e ∗ pt (3) to obtain the total kinetic energy of the selected event (e), the energies obtained for each 10-minute interval were added and finally the equation used is the following: 30 1 ( ) m i i r ei = =∑ (4) where ei30 is the erosivity of a single event (mj mm ha-1 h-1), calculated as its total kinetic energy (e; mj ha-1) multiplied by the maximum rainfall intensity in 30 min (i30; mm h-1). the r factor was determined from the sum of the erosivity of each of the events considered in the year and an average value of the years considered for each station was then generated. the r values obtained were classified according to the ranges proposed by rivera and gómez (1991). table 1. classification of erosivity proposed by rivera and gómez (1991). class erosivity r (mj. mm.h.ha-1.yr -1) classification 1 <1000 natural 2 1000-2500 very low 3 2500-5000 low 4 5000-7500 moderate 5 7500-10000 high 6 10000-15000 very high 7 15000-20000 severe 8 > 20000 extremely severe k factor the k factor was determined through the equation proposed by (wischmeier and smith, 1978) for soils containing less than 70% silt and very fine sand: 100 k = (2.1m1.14(10-4)(12 α) + 3.25(b 2) + 2.5 (c 3) ∗ 0.1317 (5) where k is the erodibility of the soil (t ha h ha-1 mj-1 mm-1), m refers to the particle size (based on the soil texture), a is the percentage of organic matter, b is the soil structure code, and c is the kind of permeability. for those cases in which the organic matter content was greater than 4%, this value was assumed. the k values obtained were classified according to the ranges proposed by usda (1962). table 2. classification of erodability proposed by usda (1962). k value (t. ha. h. mj-1mm-1ha-1) classification <0.0775 very little erodable 0.0775 0.1680 weakly erodable 0.1680-0.3230 moderately erodable 0.3230-0.6784 strongly erodable 0.6784-1 extremely erodable ls factor given the rugged topography of the study area, in the case of factors l and s, the formula proposed by pérez et al. (1995) was used: 0.6 1.4 * 22.6 9 sls λ   =         (6) http://mj.mm.h.ha-1.yr http://mj.mm.h.ha-1.yr 274 carolina martínez lópez, juan carlos menjivar flóres y daniela calero mosquera where ls is the slope length factor (unitless), λ is the slope length (m), and s is the slope expressed as a percentage. factors c and p factor c was determined by taking into account the information on the coverage consulted in the cvc advanced geographic viewer (2021). in the case of the p factor, considering that currently there is no report on the management of the coverage or the conservation practices that are carried out, it was assumed with a value of 1 (dimensionless), with which the highest value of the loss is calculated. 2.6.2 factor zoning using gis the zoning of the usle factors was carried out using the qgis version 3.6 software. in the case of factors k and c, the shapes obtained were converted to raster format. for the r factor, an interpolation was carried out through the deterministic method of the inverse of the distance (idw), taking into account that it is a simple algorithm that is designed to reduce the distortion of the interpolation of the determined surface (riquelme et al., 2008), which is widely used in climate prediction and cartographic development (núñez, 2014). finally, the raster calculator was used to determine the ls factor and estimate soil losses due to erosion. results and discussion 3.1 erosivity (r factor) the municipalities of dagua, restrepo, la cumbre, and vijes presented, according to the area, a rainfall erosivity mainly of a very low category (1000-2500 mj.mm.ha-1yr-1) and low (2500-5000 mj.mm.ha-1yr-1), while in buenaventura there are mainly moderate and high categories, and sometimes even very high (5000-15000 mj.mm.ha-1yr-1). of the eighteen stations evaluated, the two located in the municipality of buenaventura presented high and very high values of pluvial erosivity (8753.76 mj.mm.ha-1yr-1-10743.74 mj.mm.ha-1yr-1), and the rest were among the categories very low, low and natural (636.68 mj.mm.ha-1yr-14383.98 mj.mm.ha-1yr-1). figure 2 presents the behavior of rain erosivity, which shows an increase in r values as it approaches the western flank (higher areas of the basin). this behavior can be attributed to the fact that the r factor decreases as the altitude changes from the mountainous areas to the lowlands. this coincides with that indicated by guauque et al (2021), who found the lowest values of the r factor in the highest areas of the combeima basin (colombian andes), due to the low amount of rainfall. likewise, riquetti et al., (2020) modeled the r factor for south america, finding the lowest values of the r factor along the andes mountain range due to the reduction in the amount of rainfall at the highest elevation, which shows that this factor can be influenced by the elevation of the mountainous regions on the continent. there are no previous data reported for valle del cauca using this methodology. however, some investigations such as those carried out by (ramírez et al., 2009) (castro et al., 2017) in the departments of caldas and quindío, where the annual rainfall is between 1700 to 4000 mm, reporting erosivities classified as high and very high. on the other hand, a study carried out by pacheco et al., (2019) for an ecuadorian tropical region, indicated that the ranges of erosivity are between 3162.12 and 12683.72 mj.mm.ha-1yr-1. 3.2 erodability (k factor) there are six main soil orders within the dagua river basin, with k factors ranging from 0.143 to 0.842 t.ha.h. mj-1mm-1ha-1, which falls into the weakly to extremely erodible categories. the order inceptisol occupies the largest area with about 65.65% of the total land of the study area. figure 2. rainfall erosivity in the dagua river basin http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://mj.mm.ha http://t.ha 275estimation of soil losses due to water erosion in the dagua river basin, colombia at the municipal level, the highest values are found in buenaventura and correspond to the inceptisol order, which reaches 0.842 t.ha.h.mj-1mm-1ha-1. in second place, there is dagua, with inceptisols soils that reach values of up to 0.656 t.ha.h.mj-1mm-1ha-1. in the case of vijes and yotoco, similar values are presented with maximums of 0.611 t.ha.h.mj-1mm-1ha-1 belonging to the order of molisols, while, in restrepo and la cumbre, maximums of up to 0.559 were reported, in some andisols. regarding the minimum value of erodibility (0.143 t.ha.h.mj-1mm-1ha-1), this was the same for all municipalities. figure 3 illustrates the k factor map in the study area. of the entire basin area, 86.88% correspond to the categories of weak to moderately erodible, 10.84% are strongly erodible and only 1.40% are extremely erodible. the soil with the highest k factor value is dominated by very fine sand with silt particles, permeability, and low organic matter content, and they do not present a good structure, making them more susceptible to erosion. while the lowest k values presented high content of clay and organic matter, better structure, and higher permeability. few investigations have been carried out on erodability in the dagua river basin. in this regard, alarcón and reyes (2013), in studies carried out in the la centella micro-basin, dagua, values of 0.003254 t.ha.h.mj-1mm-1ha-1 were found which are lower than those obtained in this research, taking into account that prior to the tests they supplied vermicompost, which modified the characteristics of the soil, due to the higher contents of organic matter, forming macroaggregates and improving the structure, increasing infiltration and reducing runoff, which influences the decrease in erodibility, despite this, it was highlighted that the erosion in the study area is high, especially considering the steep slopes and heavy rains. 3.3 topographic factor (ls) the highest values of this factor (five to seven, and higher than seven), extend from the northwest to the southwest, mainly covering the municipality of dagua, and are attributed to steep slopes (12-75%) and even higher than 75%, corresponding to steep slopes to steep slopes with great lengths, which corresponds to the highest slope classification categories for colombia according to igac (2014). this factor leads to a very powerful rain flow rate and aggravates soil erosion. figure 4 presents the map of the ls factor in the study area. figure 4. ls factor in the dagua river basin figure 3. k factor in the dagua river basin http://t.ha.h.mj http://t.ha.h.mj http://t.ha.h.mj http://t.ha.h.mj http://t.ha.h.mj 276 carolina martínez lópez, juan carlos menjivar flóres y daniela calero mosquera 3.4 coverages (factor c) it was found that the most representative covers are dense high-ground mixed forest and cultivated pastures, which represent 22.2% and 19.7% of the total area, respectively. however, about crops, the predominant one is pineapple, which covers 1.92% of the area, followed by coffee, banana, sugar cane, and tomato, crops that constitute 1.62% of the total area. the factor c map is shown in figure 5; the value ranges from 0.01 to 1.0. the highest c was assigned in the areas without vegetation cover and in the pineapple crop (1.0), taking into account that it is managed as a clean crop and that for at least one year, the fields do not have sufficient plant cover (sughara, 2001), which can lead to serious erosion problems. the lowest c value corresponds to wooded areas (0.01), pastures (0.04), and crops with covers (0.18-0.5). figure 5. c factor in the dagua river basin 3.5 estimation of soil losses in the dagua river basin in this regard, it was found that more than a fifth of the area of each of the municipalities of dagua, restrepo, la cumbre, and vijes, presents an erosion between 100-300 t ha-1 yr-1, where these percentages correspond to 21.22 %, 29.31%, 26.66%, and 25.93%, respectively. these values are framed within the categories of high and very high erosion, according to fao-unep-unesco (1980). on the other hand, it was found that the factors that most influenced soil losses were ls and c. this is explained because they are the ones that determine to a greater extent the spatial variations of erosion rates within the same basin or region, where factor c is usually responsible to a large extent for soil losses, and ls is the most determining factor in the transport of the generated sediments (coleman and scatena, 1986). likewise, dagua is the municipality with the largest surface area with very high erosion with values that are in the range of 300-500 t ha yr-1 and even higher. the soil loss map is presented in figure 6. figure 6. soil loss map in the dagua river basin on the other hand, pineapple cultivation together with some areas of bare soil are the areas that contribute the most to soil loss due to erosion. in this way, 27.68% of bare soil and 69.24% of pineapple contribute to an erosion of more than 1000 t ha yr-1. consequently, gullies have been evidenced in the dagua river basin, which are located mainly on the border between the municipalities of restrepo and la cumbre (rio grande canyon). in this regard, rojas and campo (2018) mention that erosion processes in this area are predominant, where the highest degrees of erosion are associated with the upper part of the basin, encompassing the municipalities of dagua, restrepo, la cumbre, and vijes. 4. conclusions more than 20% of each of the municipalities of dagua, restrepo, la cumbre, and vijes presented high and very high erosion categories. despite the low contribution generated by the r and k factors to the water erosion of the basin, there was a strong influence by the ls factor as the most determining aspect in the transport of the generated sediments, since they are steep slopes, and factor c, which, given the scarce vegetation cover, is largely responsible 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(1978). predicting rainfall erosion losses: a guide to conservation planning. united states department of agriculture, agriculture handbook n ° 537, 58. https://doi.org/10.15446/esrj.v23n4.71706 https://doi.org/10.15446/esrj.v23n4.71706 https://doi.org/10.1016/j.iswcr.2021.03.001 https://doi.org/10.1016/j.iswcr.2021.03.001 https://doi.org/10.1016/j.jnc.2016.01.004 https://doi.org/10.1016/j.jnc.2016.01.004 https://doi.org/10.1016/j.scitotenv.2020.138315 https://doi.org/10.1016/j.scitotenv.2020.138315 https://doi.org/10.1016/j.catena.2021.105263 https://doi.org/10.1016/j.catena.2021.105263 _hlk71382079 _hlk68858473 _hlk84237664 _hlk84237782 _hlk84237793 _hlk117603203 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n3.101149 earth sciences research journal earth sci. res. j. vol. 27, no. 3 (september, 2023): 227 237 g eo ph y si c s abstract crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling onofre h. d. j. das flores1,2*, alanna c. dutra1,3, mário lucas2, isac abdulgani2, caisse amisse4 1. center for research in geophysics and geology, institute of geosciences, federal university of bahia, 40170-115, brasil. 2.departament of geological engineering, university of lúrio, pemba, mozambique. 3. department of earth physics and environment, physics institute, federal university of bahia, 40170-115, brasil. 4.departament of nature sciences, rovuma university, nampula, mozambique. *corresponding author: onofre.hermenigildo@ufba.br how to cite this item: das flores, o. h. d. j., dutra, a. c., lucas, m., abdulgani, i., & amisse, c. (2023). crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling. earth sciences research journal, 27(3), 227-237. https://doi.org/10.15446/esrj. v27n3.101149 record manuscript received: 20/02/2022 accepted for publication: 07/09/2023 the northern region of mozambique has a complex geological history, with an evolution that spans from the precambrian era to the phanerozoic era. in this work, we have integrated gravity and geothermal data to delineate the geotectonic evolution of the region, by estimating the thickness of the crust and the lithosphere through which was essential to generate a representative crustal model. it was necessary to complement the knowledge of structural geometry and tectonic evolution of the region. the data used in this study are the bouguer and geoid anomalies, topography data, and radiogenic heat. these data were pre-processed, topography and geoid anomaly data were filtered by low-pass filter in the frequency and harmonic domains to remove undesirable effects associated with the sources. the data were used to estimate the thickness of the crust and lithosphere, as well as to determine the mean density distribution within the mantle. this was achieved by using a one-dimensional approach, considering the principle of local isostatic compensation, associated with equations governing the distribution of temperature in the crust. the bouguer anomaly was used to generate a representative crustal 2d model of this region. the results showed that the crust is thinner in nampula and cabo delgado provinces, with thickness ranging from 27 to 31 km, whereas in niassa varies between 33 and 39 km. the analysis of lithospheric thickness indicates that the provinces of nampula and cabo delgado present a thinning of the lithosphere, with values ranging from 150 to 165 km. rather than niassa province which exhibits a thicker lithosphere, ranging from 165 to 195 km. the obtained results underwent a comparative analysis with prior investigations, unveiling a noteworthy concurrence among these findings. keywords: geoid anomaly; thermal analysis; crustal and lithospheric structure; gravity modeling. estructura crustal y tectónica del norte de mozambique inferida por modelado de gravedad en 2d resumen la región norte de mozambique tiene una historia geológica compleja, con una evolución que se extiende desde la era precámbrica hasta la era fanerozoica. en este trabajo hemos integrado datos de gravedad y geotermia para delinear la evolución geotectónica de la región, estimando el grosor de la corteza y la litosfera a través de la cual era esencial generar un modelo crustal representativo. era necesario complementar el conocimiento de la geometría estructural y la evolución tectónica de la región. los datos utilizados en este estudio son las anomalías de bouguer y geoides, datos topográficos y calor radiogénico. estos datos fueron pre-procesados, los datos de topografía y anomalía geoide fueron filtrados por filtro de paso bajo en la frecuencia y los dominios armónicos para eliminar efectos indeseables asociados con las fuentes. los datos se utilizaron para estimar el espesor de la corteza y la litosfera, así como para determinar la distribución de densidad media dentro del manto. esto se logró utilizando un enfoque unidimensional, considerando el principio de compensación isostática local, asociado con ecuaciones que rigen la distribución de la temperatura en la corteza. la anomalía de bouguer se utilizó para generar un modelo 2d crustal representativo de esta región. los resultados mostraron que la corteza es más delgada en las provincias de nampula y cabo delgado, con un espesor que oscila entre 27 y 31 km, considerando que en niassa varía entre 33 y 39 km. el análisis del espesor litosférico indica que las provincias de nampula y cabo delgado presentan un adelgazamiento de la litosfera, con valores que van desde 150 a 165 km. en lugar de la provincia de niassa, que exhibe una litosfera más gruesa, que van desde 165 a 195 km. los resultados obtenidos se sometieron a un análisis comparativo con investigaciones anteriores, revelando una notable concurrencia entre estos hallazgos. palabras clave: anomalía geoide, análisis térmico, estructura crustal y litosférica, modelado de gravedad. https://doi.org/10.15446/esrj.v27n3.101149 mailto:onofre.hermenigildo@ufba.br https://doi.org/10.15446/esrj.v27n3.101149 https://doi.org/10.15446/esrj.v27n3.101149 228 onofre h. d. j. das flores, alanna c. dutra, mário lucas, isac abdulgani, caisse amisse 1. introduction the northern region of mozambique has a complex geological history, with an evolution that ranges from the pre-cambrian to the phanerozoic era (bicca, 2017). there is limited literature on the tectonic history, structural/geophysical characterization, shallowness and depth of the region because of a lack of data which can be used to obtain subsurface information. investigating geological structures of a complex tectonic area, with many superimposed evolution stages, is a difficult task and, often, the usual geological tools are insufficient. this work aimed to investigate the crustal and lithospheric structure of northern mozambique along three north-south oriented and one west-east oriented profiles by combining gravity anomaly, geoid anomaly, topography data and thermal analyses, to have a better understanding of the lithospheric tectonic configuration. the study area is located within the northern region of mozambique covering the provinces of niassa, cabo delgado and nampula (fig. 1). to reach the objectives, first was applied geophysical techniques that allows to map the crustal thickness (crustal-mantle interface) and lab (lithosphere-asthenosphere boundary) over the entire study area and the results obtained were compared with previous studies available in the region. then, the results of the first phase were used as a priori information in the 2d modeling that allowed to obtain the geometric configuration of the crust along the aforementioned profiles. although there are some previous works carried out in this region (watts, 2001, 2018; matsinhe et al., 2021; tessema and antoine, 2004; leinweber and jokat, 2011), and other works within the lithospheric mantle structure and its geodynamic implications (e.g. maia et al., 1990; nyblade et al., 1992; he et al., 2021; domingues et al., 2013; magaia, 2009). although, geophysical and geologic work was done in this region (and in mozambique in general), such as, watts (2001) which has quantified the contribuition of rifting, sediment loading and magmatic underplating to the edge effect anomaly in the mozambiqe margin and their suggest that the thinned transitional crust that underlies much of the coastal the mozambique margin is probably of oceanic rather than continental origin; matsinhe et al. (2021) presents a density model along two profiles inferred from seismic data, suggesting that the crust consists of diluted and intruded continental crust rather than oceanic crust; tessema and antoine (2004) used gravity data to generate 3d models of east africa and infer tectonic relationships with mozambique and madagascar; leinweber and jokat (2011) used anomalous magnetic field data to assess whether continental crust exists beneath the northern natal valley and coastal plains of mozambique; muhammed and mula (2021), which has studied the crustal structure and its tectonic relationships, and other works that focus on the lithospheric structure and its geodynamic implications, e.g. maia et al., 1990 used altimetry, isostatic response and gravity anomaly data to study the lithosphere of mozambique ridge and their results report that the local isostatic response of the lithosphere beneath the mozambique ridge suggests an environment on the ridge for its emplacement, assuming the ridge is oceanic. nyblade et al., 1992 which developed a gravity model for the lithosphere beneath the kenya rift valley, the mozambique belt, and the tanzania craton and suggests a suture signature, arising from a crustal root along the boundary between the mozambique belt and the tanzanian craton and higher density crust in the mobile belt above part of the crustal root that is associated with the gravity field. he et al., 2021; domingues et al., 2013 refers that the lithosphere of central mozambique was involved in key tectonic events, such as the pan-african orogeny of the neoproterozoic and the break-up of gondwana in the jurassic. magaia, 2009 used aeromagnetic data for geologicalgeophysical interpretation of the geological structures of mozambique. könig and jokat, (2010) used magnetic data to study the mozambique ridge and mozambique basin, and their generate plate tectonic model which describes the emplacement of the mozambique ridge of southeast africa as the result of breakup of gondwana supercontinent. there still exist some open questions that still constitute a gap for the understanding of the geodynamic processes that occur in the region, such as: (a) there is no map yet published in the literature that shows the values of the crustal and lithospheric thicknesses for the northern region and also for the whole country as well; (b) there is no model that shows the geometrical configuration of the crust for this region either. the geophysical data, especially the gravity method, combined with a geological/structural recognition can provide information on the characterization of the arrangements, relationships and compartments of rocks in depth, and the geological structures. the results obtained from analysis of gravity data form an important basis for gravity modeling with emphasis on tectonic structures (aydin et al., 2016; tunini et al., 2016; samapaio 2019; ignjatović et al., 2014). however, due to the geotectonic complexity, there are doubts regarding the crustal and lithospheric structure, and its evolution. in this region, especially in the northern part, where the study area is located, there are almost no works that portray deep structures, which would help to better understand the subsurface. in this sense, this study sought to use the gravity data (bouguer anomaly), geoid anomaly and topography associated with geothermal data, such as radiogenic heat and surface heat flux, to map the crustal and lithospheric structure of northern mozambique and generate a representative crustal geometric model for this region. 2. geological context mozambique’s geology can be subdivided into the precambrian and the phanerozoic terrains. precambrian is mainly represented in the north and midwest of the country and covers about two-third of it (chaúque, 2009). phanerozoic sediments cover the area of the great coastal plains, covering a large part of the southern region of the country (from south of the save river) and the coastline (chaúque, 2008). carbonatitic and alkaline intrusions, in general, occur in the north-central region of the country, cutting rocks from the precambrian basement (chaúque, 2008). the pre-cambrian, in the northern region of mozambique, is characterized by medium and high-grade rocks and constitutes the southern end of the mozambique belt, as defined by holmes (1951). the mozambique belt is part of a larger orogenic belt (east african orogeny) that occurs along the east coast of africa, extending from northern mozambique to sudan and ethiopia. holmes (1951) defined the mozambique belt as the southern end of the east african orogeny, based on structural discontinuities between the tanzania craton and its eastern neighbor, and dated the mozambican orogeny to about 1300 ma (chaúque, 2008). later it was found that this belt was also strongly affected by the pan-african thermotectonic episode (chaúque, 2008). more recent models suggest that the mozambique belt formed during the neoproterozoic collision between the so-called west and east gondwana, followed by the closing of the mozambican ocean (chaúque, 2008 apud jamal, 2005). in northeastern mozambique, the belt became subdivided with the later recognition of the lúrio and namama belts. recent aeromagnetic images also reveal the extension of the ubendian belt across lake niassa, occupying central areas of northern mozambique, as well as the presence of ne-sw shear zones in northeastern mozambique (chaúque, 2008 apud jamal, 2005). in the northeast region of the country, jamal et al. (1999) established the presence of ages in the range between 1000 and 1100 ma of granitoid rocks in the lúrio belt. a similar age (~1110 ma) has been recorded in granitoid gneisses in the northwest of the country, along the manica-chimoio road (chaúque, 2008 apud kröner and cordani, 2003). collectively, the pre-cambrian basement comprises high-grade gneisses, granulites, migmatites and granitoids, as well as paragneisses (chaúque, 2008). precise ages, determined by jamal (2005), by the u-pb method on zircon crystals, demonstrated that rocks from the meso to neoproterozoic basement of the mozambique belt, in northern mozambique, were extensively reworked between 650 and 520 ma (chaúque, 2008). in the western sector of northern mozambique, low-grade metasedimentary rocks, some with diamittites, occur unconformably on the high-grade basement. these metasediments are correlated with katanguian sequences of the luphilian arc and are marked by a later pan-african tectonism (chaúque, 2008). phanerozoic igneous activities in northern mozambique are marked by post-pan-african (500-400 ma) and cretaceous igneous assemblages (chaúque, 2008). post-pan-african assemblages include a wide variety of cambro-ordovician intrusive bodies of circular, phyllonian or annular shape, formed by granites, syenites, monzonites, gabbros, norites and pegmatites (chaúque, 2008). these plutonites are distributed in the post pan-african magmatic provinces. some monzonitic granites have ages ranging between 360 and 490 ma; circular intrusions and veins of potassic leucogranites with hyper-alkaline differentiates, associated with syenites are dated to around 480 ma (chaúque, 2008 apud afonso et al., 1998). in general, the northern region of mozambique is geologically characterized by the occurrence of paragneisses with a high degree of 229crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling metamorphism, amphibolites, amphibolic gneiss, quartzites, granulitic gneiss, gneissic mangerites, migmatites and plutonic rocks (norconsult, 2007). the set of occurrences of these rocks is divided lithostratigraphically into fourteen (14) geological units, as illustrated in the figure 1 (norconsult, 2007). the geological units of the area under study are: recent formations; mecuburi group; ocua complexes; montepuez; meluco; lalamo; nairoto; xixane; m’sawize; muaquia; nampula; marrupa; unango and ponta messuli. the exposed lithotectonic units in northeastern mozambique were assembled, deformed, and metamorphosed during the pan-african orogeny. the available geological, structural, and geochronological data allow for the grouping of geological complexes in northeastern mozambique into five large genetically distinct mega-units, imbricated during the pan-african northwest frontier orogeny (norconsult, 2007). the five mega-units, which are an amalgamation of the geological units of the area. for example, at the base of tectonostratigraphy, a paleoproterozoic domain belonging to the foreland of the pan-african orogeny, and corresponding to the ponta messuli complex; a mesoproterozoic gneiss domain constituted by felsic crust reworked and transported during the panafrican orogeny, and corresponding to the nampula, unango, marrupa, nairoto and meluco complexes; a predominantly neo-proterozoic domain forming a long-transported pan-african upper nappe system, exposed in the xixano, lalamo, m’sawize and muaquia complexes, the monapo and mugeba klippen, and possibly the txitonga group; local neoproterozoic cover sequences: these are the mecuburi and alto benfica groups on the nampula complex and the geci group on the unango complex (norconsult, 2007); the montepuez and ocua complexes, interpreted as a pan-african tectonic mixture including lithologies from mega-units 2 and 3, and forming the core of the lúrio belt. the lúrio belt represents an important pan-african structure that reworks megaunits 2 and 3 (norconsult, 2007). 3. methodology 3.1. input geophysical data the geophysical data used to provide representative models of crustal structure for northern mozambique are gravity data (bouguer anomaly), topography, and geoid anomaly, extracted from the global database. the topography data and geoid anomaly were used to estimate the crustal and lithospheric thickness, which were applied to constrain the crustal 2d gravity modeling. figure 1. (a) geographic location map of mozambique and its respective provinces. the thin (less thick) black lines represent some countries on the african continent. (b) map of the northern region of mozambique and the three provinces that comprise it. (c) context and geological units of the study area. 230 onofre h. d. j. das flores, alanna c. dutra, mário lucas, isac abdulgani, caisse amisse 3.1.1. elevation data the elevation data are from the international center for global gravity field models (icgem) database. the database icgem used the etopo1 – egm2008 (pavlis, n.k. et al, 2012), (icgem: http://icgem.gfz-potsdam.de), with 1 × 1 min arc resolution, which integrates elevation and ocean bathymetry. figure 2 shows the elevation map, where it is observed that the province of niassa is characterized by a very high elevation, mainly in the geological units unango and recent formation where the elevation varies between 600 and 200 m; and already in the units marrupa, muaquia and m’sawize the elevation gives a slight drop being around 500 m. the provinces of nampula and cabo delgado, which are close to the indian ocean, are characterized by a low elevation (100 500 m) when compared to the values observed in the province of niassa. figure 2. elevation data extracted from icgem database using etopo1 model. 3.1.2. geoid anomaly geoid anomaly data were extracted from international center for global gravity field models (icgem: http://icgem.gfz-potsdam.de) using the geco model (gilardoni et al., 2016), where the complete model has a spherical harmonic degree and order 2190. to retain geoidal anomalies arising from lateral density variations in the crust and upper mantle down to ~400 km depth, we apply the low-pass filter in the harmonic domain over the geoidal anomaly field we remove the first 10 harmonic coefficients (jiménez‐munt et al., 2019). the residual geoid anomaly map with spherical harmonics coefficients up to degree and order 10 removed is displayed in figure 3. in this map it is observed that niassa province is characterized by high and low geoidal signatures (e.g. unango, recent formation, muaquia and m’sawize tectonic units), with values ranging from -2.5 to -4.75 m. the cabo delgado province is mostly characterized by high geoidal signatures throughout its extension, with an average value of approximately -3.5m. for the case of nampula province, we observe that the geoidal signature is different in the northern and southern parts of the province, with the northern part being characterized by high anomalies (-4.75, 4.50 m) to the detriment of the southern part which is of low anomalies (-7.25, 6.0 m). this feature extends to the central region of the country, especially in zambezia province. 3.1.3. gravity data the gravity anomalies are from a database from international center for global gravity field models (icgem) and from international gravimetric bureau (bgi). the database icgem used the earth gravitational model 2008 – egm2008 (pavlis, n.k. et al, 2012), (icgem: http://icgem.gfz-potsdam.de). figure 4 shows the bouguer gravity anomaly map of the total field, where it can be seen that cabo delgado and nampula provinces are characterized by high gravity anomalies to the detriment of niassa province. these high gravity values are associated with the fact that these two provinces are on the coast, almost in the transition zone of the continental and oceanic crust, which means that the crust in these two provinces may have low gravity signatures, with values ranging from -150.2 to -90.2 mgals. the maximum values of gravity signatures are found in cabo delgado province reaching approximately 59.8 mgals. figure 3. geoid anomaly map, with the first spherical harmonic coefficients removed, up to degree and order 10. the geoid anomaly was extracted from the icgem database using the geco model. figure 4. bouguer anomaly map from icgem database, using egm2008 model. 3.2. spectral analysis and regional-residual separation spectral analysis is a tool used to separate the different spectral components existing in an observed signal, constituting a way to identify, describe and analyze signals. this process consists of transforming the data of a signal using the fourier transform from the space domain to the frequency domain, in order to obtain the power spectrum, allowing the identification of signals corresponding to a certain wavelength and also allowing the elimination of signals referring to coherent interference sources (spector & grant, 1970). the analysis of the radial power spectrum allowed the determination of the http://icgem.gfz-potsdam.de http://icgem.gfz-potsdam.de/ http://icgem.gfz-potsdam.de/ 231crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling range of wavenumbers suitable for separating the components at different depths. gravity anomalies are the superposition of anomalies (regional and residual), caused by variations in the physical properties of source bodies located in the crust. usually, the residual anomaly is defined as the anomaly of geological interest in the region under investigation. in this case, one of the steps prior to interpreting the anomalies is the removal of the regional field to isolate the residual anomalies. in this study, the separation of the gravity anomalies was performed using the oasis (2000) software, and in the first phase, the bouguer anomaly data were imported into the software, and its grid was generated by the minimum curvature method, then it was converted to grid surfer. after this conversion, the regional-residual separation was performed using a robust beltrão et al., (1991) polynomial with a third-order surface. 3.3. moho and lab computation the crust and lithosphere thickness were calculated using the fullea et al. (2007) method. this method considers local isostatic compensation below the lithosphere-asthenosphere boundary and also considers a four-layer model consisting of crust, lithospheric mantle, seawater and asthenosphere. the density distribution in the lithospheric mantle is temperature dependent and increases linearly with depth and is calculated by the equation 1: = [1 − ( − ( ))] (1) where: α is the coefficient of thermal expansion, tα is the temperature at the depth of the lithosphere-asthenosphere boundary and tm(z) is the temperature of the lithospheric mantle as a function of depth. the geoid anomaly is calculated from a reference level that depends on the choice of a reference column, the one that allows obtaining the best crustal and lithospheric thickness, and also considering the elevation equal to zero. the equation of local isostasy combined with thermal information used to obtain the depths of the crust and the lithosphere is described by the following eq. (2): ( − ) + ( ( ( − 2 ) + 2 ) − + − 2 [( − ) ] + ) + + ( [ − ( ∆ + ) − ] − 2 [( ∆ + )( + ( − ) )]) = 0 (2) where: km and kc are the mantle and crust thermal conductivities; pc, pw and pa are the densities of crust, water, and asthenosphere (for details, fullea et al. 2007). the parameters used in the modeling are described in table 1. 3.4. 2d gravity modeling the gravity modeling was based on the direct calculation of the gravitational field produced by a geometric shape, in which an initial model for the source body is built based on available geological and geophysical information so that the anomaly generated by this model fits the observed gravity anomaly. this process of adjusting the parameters is carried out until the calculated and observed anomalies are as similar as possible (blakely, 1996). the modeling method used in this work is based on the two-dimensional modeling method developed by talwani et al., (1959), which is one of the best known methods in geophysics where the lithological structures are drawn in the form of polygons and are represented by orthogonal profiles to the direction of the anomaly. for this, four profiles were selected, three oriented in the n-s direction and one in the w-e direction. these profiles were strategically selected in order to cover most of the structures in this region. therefore, the initial model was constrained by the moho thickness values, which were calculated using the method by fullea et al. (2007) and also by geological information from the works presented in the introduction and in the geological settings. therefore, the model consists of a set of bodies, each of which is conditioned by physical properties such as density (see table 2), which are highlighted in the results of each profile. table 1. input physical data for estimating the crustal and lithospheric thickness (globig, et al., 2016; telford, 1990; fullea et al., 2007). parameters values references crustal density 2780 kg/m3 kumar et al. (2013) water density 1030 kg/m3 fullea et al. (2007) asthenospheric density 3200 kg/m3 fullea et al. (2007) lithospheric mantle density 3200[1 α(tα tm (z))] fullea et al. (2007) thermal coefficient expansion 3,3 x 10-5 /k telford et al. (1990) radiogenic heat 0,5 x10-6 w/m3 globig, et al. (2016) crustal thermal conductivity 2,5 w/mk globig, et al. (2016) mantle thermal conductivity 3,2 w/mk fullea et al. (2007) surface temperature 0o c fullea et al. (2007) lab temperature 1350o c fullea et al. (2007) table 2. parameter density used for crustal modeling and its respective reference where the values were obtained. lithology density (kg/m3) references leucognaisse 2640 – 2680 telford et al. (1990) amphibolite 2740 telford et al. (1990) gnaisse 2600 telford et al. (1990) granite 2650 2700 telford et al. (1990) quartz 2600 2620 telford et al. (1990) rovuma basin (sediments) 2600 kumar et al. (2020) migmatite 2700 2760 telford et al. (1990) upper crust 2800 kumar et al. (2013) lower crust 2900 jiménez‐munt et al. (2010) mantle 3000 -3300 hinze et al. (2013) 4. results to perform the regional-residual separation of the gravity data, first a radial power spectrum was developed in order to estimate the average depths of the basement. figure 5 shows the map of the regional bouguer anomaly, which, when analyzing the result, clearly shows an increase in the gravity signal following the nw-se direction. these results reflect the deeper geological features/structures, with an anomaly ranging from -100 to 75 mgal, associated with low frequencies and consequently high wavelength. figure 6 shows the residual bouguer anomaly map, that is, gravity anomalies associated with shallow geological structures, resulting from the subtraction of the total gravity field by the regional gravity field. in this map it is possible to observe a large variation of the signal in relation to the total bouguer anomaly map, this is due to the attenuation of low-frequency anomalies from the mantle (deeper sources), that is, the residual anomaly highlights the shallow geological structures more, where several surface bodies were highlighted with strong gravity signal, due to the high frequencies with which these bodies 232 onofre h. d. j. das flores, alanna c. dutra, mário lucas, isac abdulgani, caisse amisse are associated (shorter wavelength). the separation is aimed to remove only signs considered coming from the mantle and preserve the shallower sources correlated with the geological features of the crust. figure 5. regional bouguer anomaly, obtained by third-order polynomial fit. figure 6. the residual bouguer anomaly, obtained by third-order polynomial fit. the orange lines denoted by a-a’, b-b’, c-c’ and d-d’ are profiles used for crust modeling. the geological units as illustrated in the figure 2 are also illustrated in this figure by black curve lines. the numbers in the maps correspond to geological units as illustrated in the figure 2, where: 1-nampula, 2-mecuburi, 3-ocua, 4-recent formation (rovuma sedimentary basin), 5-lalamo, 6-meluco, 7-nairoto, 8-montepuez, 9-xixano, 10-marrupa, 11-muaquia, 12-m’sawize, 13-unango, 14 recent formation. 4.1. moho and lab depth the northern region of mozambique is made up of three provinces, and the results shown in the map in figure 7. our findings reveal a variation in the thickness of the crust that displays a noteworthy resemblance to the elevation. in niassa province, high values of crustal thickness (moho depths) are observed, characterizing it as the region where the moho reaches its maximum value. these values range from 33 to 39 km and increase in an east to west direction. the crustal thickening can be examined by considering the geological units, namely, unango and recent formation, both located in the western portion of niassa province. the provinces of nampula and cabo delgado are characterized by thinner crust, and in these regions the crust varies of about 27 and 31 km, and this can reach 35 km in some parts of cabo delgado province, for example in the north-western part of the rovuma basin tectonic unit. however, the distribution of crustal thickness values throughout the area leads us to classify it into three distinct categories: (a) thicker continental crust, located in the western portion of the area, identified by a reddish rectangle; (b) intermediate crust, present in the central region, also identified by the reddish rectangle; and finally, (c) transitional crust, which constitutes the transition zone between the continent and the ocean, located in the eastern part of the area, characterized by a lower thickness and minimum values of crustal thickness. this area is also recognized as a passive margin region (see fig. 7). figure 7. the map of the moho depth. the crustal structure inferred by integration of the elevation and geoid anomaly data. crustal structure inferred by integration of elevation and geoid anomaly data. the geological units as illustrated in the figure 1 are also illustrated in this figure by black curve lines. the numbers in the maps correspond to geological units as illustrated in figure 2, where: 1-nampula, 2-mecuburi, 3-ocua, 4-recent formation (rovuma sedimentary basin), 5-lalamo, 6-meluco, 7-nairoto, 8-montepuez, 9-xixano, 10-marrupa, 11-muaquia, 12-m’sawize, 13-unango, 14recent formation. figure 8 shows the results of the thickness of the lithosphere, with the regions of nampula and cabo delgado provinces being characterized by a thinner lithosphere with values ranging from 150 to 165 km, while a thicker lithosphere (lab) is observed in niassa province, where it varies between 165 to 195.0 km in depth. as in moho, the lab increases in an east to west direction, this is justified by the fact that the eastern part is on the oceanic shelf side, which generally has a smaller crustal and lithospheric thickness, with a high density due to basaltic constitution or composition of basaltic rocks. thus, in contrast to moho’s values, our lithospheric-asthenospheric discontinuity (lab) results can be categorized into two distinct classes: (a) continental lab, which stands out for being thicker and covers all geological units present in niassa province, extending to cabo delgado and nampula provinces, encompassing the xixano and mecuburi units; and (b) transitional lab, which occurs in the transition region between the continent and the ocean, where minimum lab values are observed (see fig. 8). 233crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling figure 8. the map of the lithosphere-asthenosphere interface (lab depth). lithospheric structure inferred by integration of elevation and geoid anomaly data. the geological units as illustrated in the figure 1 are also illustrated in this figure by black curve lines. the numbers in the maps correspond to geological units as illustrated in the figure 2, where: 1-nampula, 2-mecuburi, 3-ocua, 4-recent formation (rovuma sedimentary basin), 5-lalamo, 6-meluco, 7-nairoto, 8-montepuez, 9-xixano, 10-marrupa, 11-muaquia, 12-m’sawize, 13-unango, 14recent formation. the map in figure 9 shows the values of the mean density distribution in the lithospheric mantle. in this map, it can be seen that the lithosphere is denser along the region adjacent to the ocean shelf, covering part of nampula and cabo delgado provinces, with a maximum density of approximately equal to 3250 kg/m3, while in niassa province the lithosphere is less dense, with values ranging from 3240 to 3250 kg/m3. these results give consistency to the values of the crust and lithosphere thickness found, since the regions of lesser crustal and lithospheric thickness are characterized by higher average densities in the lithospheric mantle and the regions of crustal and lithospheric thickness are characterized by lower densities, making apply the isostatic compensation principle, which is the most expected for such a region. figure 9. the map of the lithospheric mantle density, inferred by integration of elevation and geoid anomaly data. the geological units as illustrated in the figure 1 are also illustrated in this figure by black curve lines. the numbers in the maps correspond to geological units as illustrated in the figure 2, where: 1-nampula, 2-mecuburi, 3-ocua, 4-recent formation (rovuma sedimentary basin), 5-lalamo, 6-meluco, 7-nairoto, 8-montepuez, 9-xixano, 10-marrupa, 11-muaquia, 12-m’sawize, 13-unango, 14recent formation. 4.2. gravity direct modeling in the modeling of the gravity data of the present work, the talwani et al., (1959) method from the tool available in the oasis (2000) software called gm-sys. in the obtained gravity models, we tried to adjust the calculated data to the observed ones, along the geological profile created with the main lithologies. therefore, to carry out the modeling, three representative profiles of the study area were selected, shown in figure xx, namely profile a-a’, b-b’, and c-c’. these profiles were also selected in a joint analysis with geological and geophysical data. gravity anomalies were related to geological features (structures and units). thus, for each profile, a model was generated involving the mantle and layers of the crust, and the most predominant large outcropping geological units. the moho results obtained in the previous topic were used as prior information and also lithological information as well as the stratigraphic column and surface distribution were also used. the referred densities were extracted from the literature and some scientific articles based on the predominant rocks in the area. 4.2.1. profile a-a’ in figure 11, the authors present the crustal model for the a-a’ profile, with density distribution in the subsurface. this model comprises the northeastern part of cabo delgado, where the mean values of the gravity contrast are found, and reaches the southern portion of nampula province. according to geological-geophysical knowledge, the high values are possibly the result of greater density contrast, due to the sum of metamorphic, basic, and ultrabasic igneous rocks present in the region that can reach high densities, strongly contributing to the gravity signal, while the low ones gravity signatures may be related to the acidic and sedimentary igneous rocks that outcrop in the cabo delgado and nampula area. in the first 9 km of depth, there is the rovuma basin with rocks predominantly unconsolidated sand, sandstone, claystone, limestone, and conglomerates, and along the profile, with metamorphic rocks, presenting a sharp drop in terms of gravity values, which reflects higher basement of depth values, showing as a thinner layer. the upper crust has a thickness ranging from 20 to 24 km on average, while the lower crust has an uplifted upper crust layer, its thickness varies along the modeled profile and increases from 32 in the regions below cabo delgado province to 37 km in the lower crust uplift. this layer follows a not very stable trend regarding basement depth. figure 10. the crustal model generated from the a-a’ representative profile. the dotted black curve represents the observed anomaly, the black curve represents the calculated anomaly, and the red curve represents the error between the calculated and observed data. 4.2.2. profile b-b’ in figure 12, we present the crustal model for the b-b’ profile, with density distribution in the subsurface. this model has its origins in the northeast of niassa and extends to the province of nampula. the profile starts with a relatively low gravity signature (-13.5 mgal) reaching a high gravity (36.6 mgal) along the profile, which possibly these high values are found at the intersection of the three provinces. these high anomalies may be due to the presence of high-grade metamorphic, ultrabasic,and basic igneous rocks, which predominate in these 234 onofre h. d. j. das flores, alanna c. dutra, mário lucas, isac abdulgani, caisse amisse two large complexes that outcrop in the area. this model consists of four layers limited by three interfaces: three layers belonging to geological units, crust (upper and lower), and the fourth layer representing the mantle. the geological units have a variable thickness in the direction of the profile. these geological units show three portions within the first layer section, there is a portion with granitic to granodiorite gneiss (north of the section), migmatites and a portion with leucogneisse (south of the section). the second layer, referring to the upper crust, has a thickness ranging from 12 to 28 km. in the niassa region, the crust tends to outcrop on the surface. the central region of the profile presents a thinning of the upper crust. the fourth layer, referring to the lower crust, presents not very coarse variations in thickness, between 32 and 36 km. figure 11. the crustal model generated from the b-b’ representative profile. the dotted black curve represents the observed anomaly, the black curve represents the calculated anomaly, and the red curve represents the error between the calculated and observed data. 4.2.3. profile c-c’ in figure 13 we present the crustal model for the c-c’ profile, with density distribution in the subsurface. again, this profile comprises the provinces of niassa and nampula. in this model, the profile sections the unango complex from the north side of the profile to the nampula complex towards the south side, these two complexes are the oldest in terms of geochronology, with a predominance of rocks that underwent medium to high-grade metamorphism. the profile of this model has a minimum gravity of -60.9 mgal, possibly due to the influence of acidic rocks that outcrop in the area or high depth and the maximum anomaly of 19.4 mgal, may be due to the influence of basic rocks and crust uplift bottom. the model extends from north to south of niassa, showing the thicknesses of each layer. the first layer, which corresponds to the geological units, varies around 12 to 14 km. however, the second layer that represents the upper crust shows great variations and a very noticeable thinning in the nias-sa province, and a crustal thickening is verified in the following regions, the upper crust has a thickness varying from 12 to 20 km. the third layer, which is the thickest of all, referring to the lower crust, its thickness varies between 24 to 40 km. figure 12. the crustal model generated from the c-c’ representative profile. the dotted black curve represents the observed anomaly, the black curve represents the calculated anomaly, and the red curve represents the error between the calculated and observed data. 4.2.4. profile d-d’ figure 13 shows the crustal model for profile d-d’, displaying the density distribution in the subsurface. this profile covers the niassa and nampula provinces. these two complexes exhibit the most remote geochronology, being predominantly composed of rocks that have undergone medium to high grade metamorphism of a more mafic character. the model reveals an interesting gravimetric signature, comprising both positive and negative anomalies. notably, the positive signatures are mainly concentrated in the marupa tectonic unit, represented in the profile by the granite lithotype. in contrast, the other geological units are characterized by lower gravimetries, manifesting negative signatures that are associated with the loucognaisse and quartz-feldspar lithologies. these gravimetric variations, both high and low, are correlated with the different depths observed along the profile for each of the geological units. the first layers correspond to the main units of the geotectonic domains, whose thicknesses vary between 4 and 16 km. the marrupa geological unit and the northern part of the nampula complex are characterized by a low depth/ thickness, with values around 4 to 6 km. on the other hand, the other units located at the extremities of the profile (model) have greater depth, ranging from 10 to 16 km. the second layer represents the upper crust, and shows remarkable variations along the profile. it is possible to observe that the thickness of the upper crust is greater in niassa province and decreases progressively towards nampula province. this gradual reduction is closely related to the fact that this region corresponds to a passive margin. the third layer is called the lower crust which is the densest structure. it presents a similar behavior, as it is also greatest in the niassa province and gradually reduces towards nampula. although the maximum thickness value, reaching 38 km, is observed in niassa, there is also a remarkable variability along the profile, showing that there is no crustal consistency or stability in terms of thickness. in other words, the moho shows significant variations along the profile. figure 13. the crustal model generated from the d-d’ representative profile. the dotted black curve represents the observed anomaly, the black curve represents the calculated anomaly, and the red curve represents the error between the calculated and observed data. 5. discussion this study aimed to map the thickness of the crust and lithosphere for the northern region of mozambique using topography, geoid anomaly and gravity data. one of the major gains from this study was to present for the first time the crustal (moho) and lithospheric (lab) thickness maps in the most local context and to compare these results with previous studies. the genesis of the crustal and lithospheric configuration of the study region is associated with the separation of the african and south american plates initiated by the breakup of the gondwana supercontinent in the mesozoic era, in the triassic period. a study by macey et al. (2013), reports that the crust in northern mozambique is divided into two similar but different tectonic blocks. it is reflected in the crustal thickness values obtained from our calculations, which show variations but with some similarities in some geological units (fig. 8). the architecture and geological evolution of the crust is explained by two models, the first one being involves the convergence of eastern and western gondwana associated with 235crustal and tectonic structure of northern mozambique inferred by 2d gravity modeling the closing of the mozambique ocean along the east african orogen and the is related to the two-stage melting of the components of gondwana which are the collision between the eastern and western components of gondwana, and the collision of the newly formed northern block of gondwana with a southern block of gondwana (2013). our results are satisfactory and agree with globig et al. (2016). these authors used geoid anomaly and topography data associated with thermal analysis to map the crust and lithosphere thickness of the african continent. their results imply that the northern region of mozambique has a crust that varies from 29 to 40 km in thickness and a maximum lithospheric thickness of 180 km. these results agree with ours, as ours reveal that the crust varies between 27 and 39 km, while the lithosphere varies between 150 and 195 km. furthermore, these authors report that, generally, the thick crust (>37 km) is associated with the archean cratons and shields and with the proterozoic belts. the northern region of mozambique is part of the mozambique belt with a maximum moho geometry of about 40 km, as shown in figure 5a. the crustal gravity models of the northern region of mozambique showed good correlations for the moho depth that varies from 35 to 40 km, according to similar works presented by araújo (2018) and fries (2003) of the crust thickness. in general, our moho results can be seen from three perspectives: first, thicker continental crust (3339 km) is observed in the unango and formation tectonic domains that are located in the western region of nissa province. second, intermediate crust (31–33 km), which is observed in the xixano, marrupa, muaquia and m’sawize tectonic units, and also west of nampula province which are located in the eastern region of niassa province and west of cabo delgado province. and finally, transitional crust which is thinner (27-31 km), being in the transition zone between ocean and continent, and comprises the tectonic units ocua, recent formation (rovuma sedimentary basin), lalamo, meluco, nairoto in the eastern region of the province of cabo delgado and nampula to the east. regarding lab, the most notable results reveal that the lithosphere in this region can be divided into transitional lithosphere, which is less thick and is located mainly in the recent formation (rovuma sedimentary basin), lalamo and meluco tectonic units of cabo delgado province, whose thickness between 150 to 165 km. the other geological units of niassa and nampula provinces are characterized by a thicker lithosphere ranging from 165 to 195 km. the profile a-a’ has a length of 555 km, oriented in the n-s direction, and crosses five geological units/complexes, namely: rovuma basin (recent formations), montepuez, lalamo, ocua and nampula. in the model, the lalamo, montepuez and ocua units were grouped into a single geological body/structure due to the similarity of the lithologies and tectonic history of these complexes. in this profile, small variations in the thickness of the upper and lower crust are noted, with the most significant highlight being the uplift of the lower crust towards the base of the complexes along the profile. this uplift may be associated with one of the tectonic events that occurred in the area, as presented by macey et al. (2013), which involves the convergence of eastern and western gondwana, associated with the closure of the mozambique ocean along the east african orogen. finally, the maximum values of crustal thickness reach 37 km due to the proximity of the region to the oceanic coast, which has a lower depth of the moho. like the a-a’ profile, the second profile, named b-b’, has a length of 555 km and covers four main geological units, namely marrupa, ocua along the lúrio belt, nampula and extends into the central region of mozambique, in zambézia province. the geological structures presented in the middle part of the profile exhibit higher density, comprising formations such as amphibolite and migmatite. such bodies probably result from the convergence process of eastern and western gondwana, associated with the closure of the mozambique ocean, as indicated by macey et al. (2013). this tectonic event may have been responsible for the generation of intrusions in these formations. in addition, a variation in the thickness of the upper crust is observed along the profile, with a notable uplift. this feature contrasts with the previous profile, where the uplift was more related to the lower crust. additionally, this uplift may be related to the collision between the east and west components of gondwana, forming the east african orogen, with n-s orientation, and in northern mozambique, the formation of accretionary terrains, as proposed by macey et al. (2013). another hypothesis involves the collision of the northern block of gondwana with the southern block, also encompassing the collision of the australia and mawson blocks (macey et al., 2013). the average crustal thickness in this region is 36 km, however, a slight reduction in thickness is observed below where we have the amphibolite and migmatite geological bodies. the profile c-c’ is equally significant, covering a distance of 555 km, similar to the first two profiles mentioned above. this profile comprises three distinct geographical regions, namely lake niassa, located in the west of niassa province, and unango and zambezia, located in the center of the country. this arrangement is discernible when analyzing the portion of the profile that transcends the boundaries of the unango unit (figure 6). the results obtained from our investigation reveal notable variations in crustal blocks, both in the upper and lower layers. notably, the uplift of the lower crust is observed, which seems to be associated with the same tectonic events presented in the first two profiles previously examined. it is worth noting that the main distinction of this profile lies in the values concerning the moho, since its westernmost location, adjacent to the niassa province. the profile d-d’ stands out as the most remarkably distinct among all, both for its w-e orientation and for its length, totaling 620 km. this profile crosses four important geological units, namely unango, marrupa, ocua and nampula. the results obtained from our analysis reveal a highly variable crustal thickness along the profile, both in the upper and lower layers. the western region exhibits a thicker crust, reaching a maximum thickness of 38 km, while in the eastern direction, corresponding to the coastal region, the crustal thickness tends to decrease. it is important to highlight that in this coastal region, the gravimetric signature of the crust is more positive, being congruent with the geological units of marrupa and ocua. these domains are associated with the lúrio belt, and geophysical data at regional-scale with ne-sw trend, forming in response to progressive deformation experienced. in the nw portion of messuli point, along lake niassa, a 1.9 ga palaeoproterozoic basement is exposed, overlain by the txitonga group, known for its low-grade metasediments (viola et al., 2006). the unango, marrupa and nampula complexes were influenced by pan-african metamorphism (viola et al., 2008), and as observed in other profiles, are characterized by positive gravimetric signatures. therefore, our results corroborate the proposition of viola et al. (2006), who pointed to the e/ene-w/wsw trend in the crust in this region. the profiles modeled in this work possibly show that they are located in the continental crust collision zone according to afonso, marques, & ferreira, 1998, in their book on the geological evolution of mozambique. the profiles have a high gravity value that would correspond to the obduction represented by basic and ultrabasic rocks. on the contrary, the genesis of pan-african orogenic sets would be due to post-collision volcanism (nédelec et al., 1994 as cited in afonso, marques, & ferreira, 1998). thus, the lithologies present in the orogenic sets would have their origin in the magma resulting from the crustal fusion related to the shear effects, and fill the sutures resulting from the collision of the continental blocks. the convergence of the east and west gondwana terranes, associated with the closure of the mozambique ocean, resulted in extensive metamorphism processes in these large blocks, characterized by increased temperature and pressure. this generated strong compositional banding and significant increase in rock density. these high-grade metamorphic rocks are the result of collisional orogenies followed by late tectonic intrusions, as observed in the bouguer anomaly map. 6. conclusion the text discusses a study conducted to map the crust and lithospheric thickness in the northern region of mozambique using gravity data, geoid anomaly, and topography. the authors employed regional-residual separation techniques to analyze the gravity data. for regional-residual separation, the regional bouguer anomaly map showed an increase in gravity signal in the nw-se direction, reflecting deeper geological features with low frequencies and high wavelengths. the residual bouguer anomaly map highlighted shallow geological structures with strong gravity signals due to higher frequencies and shorter wavelengths. for the crustal thickness, the northern region of mozambique is divided into three provinces: niassa, nampula, and cabo delgado. niassa province exhibited high values of crustal thickness (moho depths) ranging from 33 to 39 km, increasing from east to west. nampula and cabo delgado provinces 236 onofre h. d. j. das flores, alanna c. dutra, mário lucas, isac abdulgani, caisse amisse had thinner crusts, varying from about 27 to 35 km. and for the lithospheric thickness, the lithospheric thickness (lab) showed thinner lithosphere in nampula and cabo delgado provinces (150 to 165 km) and thicker lithosphere in niassa province (165 to 195 km), increasing from east to west due to the proximity to the oceanic shelf. for the density distribution, the mean density distribution in the lithospheric mantle revealed denser lithosphere adjacent to the ocean shelf in parts of nampula and cabo delgado provinces (approximately 3250 kg/m3) and less dense lithosphere in niassa province (3240 to 3250 kg/m3). on the gravity direct modeling, the gravity modeling involved four representative profiles (a-a’, b-b’, c-c’ and d-d’) in the study area. the models provided insights into the geological units and depths of the crustal and lithospheric layers along the profiles. the gravity models revealed variations in crustal and lithospheric thicknesses in different geological units, reflecting the complex tectonic history and collisions of continental blocks in the region. the results generated by the study are consistent with previous research, validating the crustal and lithospheric thickness values obtained for the region. in summary, the study successfully mapped the crustal and lithospheric thickness in the northern region of mozambique and provided valuable insights into the geological evolution and tectonic processes that shaped the area. the findings of this study are vital in understanding the region’s geology and tectonics. references afonso, r.s., marques, j. m., & ferrara, m. 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(2008). tectonic evolution of the pan-african mozambique belt in northern mozambique. 33rd igc conference, oslo, norway, january https://doi.org/10.1093/gji/ggt080 https://doi.org/10.1029/2019gc008777 https://doi.org/10.1029/2011gl047659 https://doi.org/10.1111/j.1365-246x.1990.tb00689.x https://doi.org/10.1111/j.1365-246x.1990.tb00689.x https://doi.org/10.1007/s13131-021-1747-9 https://doi.org/10.1007/s13131-021-1747-9 https://doi.org/10.1016/j.precamres.2013.03.012 https://doi.org/10.1016/j.precamres.2013.03.012 https://doi.org/10.1016/0040-1951(92)90294-g https://doi.org/10.1029/2011jb008916 https://doi.org/10.1029/2011jb008916 https://doi.org/10.1190/1.1440092 https://doi.org/10.1029/jz064i001p00049 https://doi.org/10.1016/j.tecto.2004.07.057 https://doi.org/10.1016/j.tecto.2004.07.057 https://doi.org/10.1002/2016tc004161 https://doi.org/10.1016/s0264-8172(00)00079-9 https://doi.org/10.1093/gji/ggy353 _heading=h.gjdgxs issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n4.103872 earth sciences research journal earth sci. res. j. vol. 26, no. 4 (december, 2022): 303 311 so il s abstract: natural disasters related to the agriculture sector frequently occur in china. this article studied the prevention and control of drought and flood in agricultural lands and the optimization method of planting structures to avoid disaster. firstly, the impact of the change of input structure on the prevention and control of drought and flood disasters of agricultural lands was analyzed, and the evaluation system of input-output efficiency was constructed. secondly, the evaluation model was built using the dea (data envelopment analysis) method to provide the theoretical basis for the quantitative evaluation of input-output efficiency. taking jiangxi province as an example, the input-output data of drought and flood control of agricultural land in twenty cities in 20142017 was analyzed. the results show that the input-output efficiency of 80% of farmland in jiangxi province is low, and drought and flood prevention resources are not sufficiently utilized. in addition, the low pure technical and scale efficiency are the main reasons for the low input-output efficiency of drought and flood prevention. the effect of human and financial input in drought and flood prevention of farmland in jiangxi province is slightly different for value output and actual output. from the aspects of strengthening the investment in basic facilities and the technical research and development of agricultural products, the optimization strategy for the input structure of drought and flood prevention of agricultural land is put forward. drought and flood control of agricultural lands and optimization method of planting structure to avoid disasters yi zhang* school of economics and management, xinyu college, xinyu 338000, china *corresponding author: zhangyi@mjc-edu.cn how to cite item: zhang, y. (2022). drought and flood disaster control of agricultural lands and optimization method of planting structure to avoid disaster. earth sciences research journal, 26(4), 303-311. https://doi.org/10.15446/esrj.v26n4.103872 manuscript received: 22/07/2022 accepted for publication: 19/01/2023 keywords: farming land; drought and flood disaster; disaster avoidance planting; structure optimization; input-output; dea control de sequías e inundaciones en terrenos agrícolas y método de optimización de estructuras para evitar desastres resumen los desastres naturales relacionados al sector agrícola ocurren frecuentemente en china. este artículo estudia la prevención y control de sequías en terrenos agrícolas y el método de optimización de estructuras para evitar desastres. inicialmente se analizó el impacto en los cambios en estructuras de prevención de sequías y de inundación y se construyó un sistema de evaluación de la relación de eficiencia prevención-resultados. luego se construyó un modelo de evaluación con el método dea (análisis envolvente de datos, del inglés data envelopment analysis) para que provea de una base teórica la evaluación cuantitativa de la relación de eficiencia prevención-resultados. con la provincia jiangxi como área de estudio, se analizó la relación prevención-resultados en el control de sequías e inundaciones en terrenos agrícolas en veinte ciudades entre 2014 y 2017. los resultados muestran que la relación de eficiencia prevención-resultados en el 80 % de las tierras de cultivo en la provincia jiangxi es bajo y los recursos para la prevención no se utiliza suficientemente. adicionalmente, la baja eficiencia técnica y a escala son las principales razones para el pobre rendimiento en la relación prevención-resultados en el control de sequías e inundaciones. el efecto de los recursos humanos y financieros en este sector en la provincia de jiangxi es ligeramente diferente para el valor de los resultados y el valor actual. desde el aspecto del fortalecimiento de instalaciones básicas, investigación técnica y desarrollo de productos agrícolas se debe proponer una estrategia de optimización para la inversión en estructura de prevención de estos desastres. keywords: terrenos agrícolas; sequías e inundaciones; estructuras de prevención de desastres; optimización de estructuras; relación prevención-resultados; dea https://doi.org/10.15446/esrj.v26n4.103872 mailto:zhangyi@mjc-edu.cn https://doi.org/10.15446/esrj.v26n4.103872 304 yi zhang 1. introduction in the environment of global warming, ecological deterioration, and frequent extreme climate, the current agricultural drought and flood disaster prevention measures are insufficient to cope with the increasing disasters of this kind. at present, the disaster prevention system of farmland and water conservancy cannot meet the needs of agricultural disaster prevention. the practical vulnerability of existing research ideas and countermeasures is obvious (gao et al., 2018). the problem of prevention and control for agricultural drought and flood disasters in china has attracted the attention of relevant organizations and researchers. currently, the research viewpoints can be roughly divided into three categories. the first is the viewpoint of engineering technology. it is considered that strengthening the construction of agricultural drought and flood prevention engineering is fundamental, and the importance of disaster early warning technology networks is emphasized (wei et al., 2016). the second is the management mode. it is considered that the key is to change the construction and management mode of agricultural drought and flood control facilities. according to this stance, the risk management of flood and drought disasters needs closer cooperation among managers, experts, and the public (hohmann et al., 2018). the third is comprehensive management. it is considered that a sustainable and extensive comprehensive management framework for drought and flood risk should be established, and the work system for disaster prevention should be changed from engineering system to engineering measures and nonengineering measures (wang et al., 2017). these researches are mainly based on natural factors or man-triggered natural factors. they all focus on the control and prevention measures for agricultural drought and flood disasters but ignore the input-output efficiency. the input-output efficiency refers to the relationship between farmland drought and flood disaster prevention. “input” refers to the defense system before natural disasters and the various emergency measures in natural disasters. the “output” refers to the result of disaster losses without further expansion in affected regions. the larger the input-output ratio, the more reparable disaster losses are, and the stronger disaster prevention ability will be (chen et al., 2016). there are many kinds of agricultural disasters. the primary natural disasters are drought and flood. the research on input-output efficiency mainly focuses on disaster prevention and natural disaster reduction of agricultural drought and flood. the investment includes the prevention before the disaster, the investment in meteorological early warning systems, and the investment of resources after the disasters. based on the connotation of input-output efficiency of the prevention and control of drought and flood disasters in agricultural land, the evaluation index system is established. the present situation and influencing factors of inputoutput efficiency in different regions are monitored through the evaluation model. therefore, this paper puts forward the strategy of structural optimization to improve the prevention and control system of drought and flood disasters in modern agricultural lands. 2. materials and methods 2.1. mechanism of optimization of planting structure to prevent and control drought and flood disasters of cultivated lands the change in the proportion of drought and flood disasters, the disaster rate and the rate of grain production increase are used to reflect the evolution trend of drought and flood disasters in agriculture. according to the statistical data of 1982-2010 in table 1, the relationship among the risk, control effect, and control system of drought and flood disasters is approximately revealed. the concepts are defined as follows: the drought and waterlogging ratio refers to the ratio of the disaster area of crops caused by such a disaster. the ratio of drought and flood disaster refers to the ratio of the disaster area of crops caused by drought or waterlogging to the total disaster area of crops caused by various agricultural disasters, which is the sum of the disaster portion of drought and waterlogging. the rate of grain yield increase refers to the ratio between the increased grain output of the current year to that of the previous year (wang & zhang, 2019). table 1. dynamic comparison of grain yield increase rate, drought and flood disaster, disaster and its ratio in 1982-2010 (unit: %) particular year grain increase rate (last year = 100) affected area ratio proportion of disaster area disaster rate proportion of drought proportion of waterlogging disaster proportion of drought and flood 1982 33.9 16.4 48.3 73.59 8.34 81.93 1984 -3.6 30.5 20.4 67.0 47.71 20.49 68.20 1986 9.2 23.0 11.1 48.3 62.49 27.62 90.11 1988 5.3 22.2 10.9 49.0 45.06 34.68 79.74 1990 3.4 32.8 16.5 50.3 62.53 23.79 86.32 1992 -2.3 35.2 16.6 47.2 64.02 25.70 89.72 1994 9.6 26.0 12.1 46.4 43.91 31.57 75.48 1996 1.8 34.6 17.5 50.5 65.95 17.35 83.30 1998 -2.6 37.2 21.3 57.1 54.44 34.35 88.79 2000 8.2 30.9 14.0 45.3 29.53 51.23 80.76 2002 3.8 32.3 16.3 50.3 20.10 54.85 74.95 2004 -9.2 35.1 22.1 63.0 78.03 12.68 90.71 2006 1.1 30.6 17.8 58.1 48.50 27.47 75.97 2008 9.1 24.3 10.7 44.0 53.16 22.40 75.56 2010 2.9 26.3 15.8 60.0 54.66 19.66 74.32 305drought and flood control of agricultural lands and optimization method of planting structure to avoid disasters fluctuation of disaster rate of agricultural drought and flood at a high level from the perspective of the disaster rate of agricultural drought and flood in 1982-2010, the ratio of the disaster area of crops caused by drought and flood was 68%-91%, and the average annual rate is about 81.06% (table 1). this information shows that the prevention and control ability of agricultural drought and flood disasters in china has always been at a low level in the past 29 years, and the construction of agricultural water conservancy in china has not influenced the prevention and control ability of agricultural drought and flood disasters. meanwhile, based on the data, the prevention and control system after the reform needs to be arranged better. increasing the frequency of drought and flood disasters according to the dynamic comparison between the proportion of drought and flood disasters and the yield increase rate of grain, the authors found that the frequency of the impact of the disasters on grain production increases in a period. before the water conservancy management reform, in the period 19822010, the disaster area exceeded 2000 × 104 hm2 only in 1984 and 1990. since 1992, the disaster areas have exceeded 2000 × 104 hm2 in eleven years (table 1). from 1983 to 2010, grain production was negatively affected by drought and flood for 21 years, accounting for 78% of the total time. only six years were not significantly affected by drought and flood. excluding other factors, the length of time that grain production is significantly affected by drought and flood is enough to reflect the severe trend of the increasing frequency of affectations in grain production in china (alexander, 2016). the disaster losses fluctuate upward according to the dynamic comparison between the proportion of drought and flood disasters and the rate of grain yield increase, it can be seen that both factors increased in the same direction for six years between 1983 and 1991. however, they were in the opposite direction for two years. after the reform, just in 2009, the proportion of drought and flood disasters changed in the same direction with the rate of grain production increased. in the other 18 years, they all changed in the opposite direction (table 1). the reason is that the effect of the no. 1 document, issued by the cpc central committee and the state council to support the agriculture sector in 2008, was reflected in the second year. the no. 1 document aims to increase investment in agriculture and intensify the prevention and control of floods and droughts. after the agricultural water conservancy system reform, drought and flood disaster impact on grain production is increasing. control activities of agricultural drought and flood could bring public benefits. in order to ensure the supply of public goods, more than a direct investment in public finances is needed, but also social and economic encouragement to participate in the supply. in china, the investment guarantee system needs to be more satisfactory. in terms of financial investment, the central government assigned smallscale farmland and water conservancy funds for agricultural infrastructure construction to the local governments in 1984. due to the influence of local governments’ unique and efficient industrial preference, the financial support for agriculture lacked the institutional constraints of stable growth, and the limited financial support for agriculture was often occupied or misappropriated. the proportion of financial expenditure on agricultural infrastructure to the total expenditure in the same period can reflect this. in the 1980s-1990s, the financial support for the agriculture construction ratio fluctuated and declined with an average value of 2.24%. the ratio was higher than 2.23% in 1998 and the following three years, and it has declined yearly since 2003. the lowest ratio is 1.28%. in rural cooperative investment, with the decentralization of farmers and families and the transformation of rural government functions, the village collective economy is facing the “hollowing out,” and the mobilization of public affairs organizations in rural organizations is weakening. in the late 1980s, the village committee mainly relied on the voluntary labor service and labor accumulation workers, the accumulation fund system, and the fundraising of farmland and water conservancy construction. to a certain extent, this stimulates the development of farmland water conservancy construction. due to the “obligation” basis, it cannot directly benefit farmers, so it is not an effective incentive system. if the village collective economy is in a state of “hollowing out” and the burden of farmers is worsening, the operation of the external supply system of farmland and the water conservancy system will be more difficult. therefore, the lack of an input system for disaster prevention leads to increased vulnerability to agricultural disasters. the vulnerability to agricultural drought and flood disasters is increasing, and the disaster rate is also increasing. in addition to the force of the central government, china still needs the coordination mechanism of integrating social forces to cope with regional drought and flood disasters. in terms of the anti-disaster engineering supply, rural water conservancy construction investment began implementing the “one project, one discussion” supply model at the end of the twentieth century instead of the financing mechanism outside the system. in the case of the “hollowing out” of the village collective economy and severe damage to farmland and water conservancy, farmers could not form a long-term practical investment expectation, and the social disaster prevention investment mechanism was challenging to develop. according to statistics, due to the lack of government investment and incentive financial and tax systems, the number of farmers working in farmland water conservancy construction has decreased from about 7.55 billion work days per year in the 1970s to more than 2 billion work days in 2004. the investment gap in farmland water conservancy is about 70 billion yuan annually. as a result, agricultural anti-disaster engineering supply faces severe shortages. in conclusion, the failure of control measures is related to the variability of hazard factors, the breakdown of irrigation and drainage disaster prevention system, and the decline of engineering disaster prevention ability. however, the main reason lies in china’s agricultural drought and flood prevention system, the lack of a solid collaborative management organization system, and the cooperative input incentive system. these reasons restrict the growth of a marketized organization of disaster prevention and control, reduce the systematic effectiveness of flood control and drought resistance behavior and weaken the response-ability to agricultural disasters. china must establish and improve the coordinated prevention and control system of agricultural drought and flood disasters as soon as possible and thus provide institutional guarantees for improving the coordination and effectiveness of prevention and control of these kinds of disasters. 2..2. construction of input-output efficiency evaluation model for drought and flood prevention of agricultural land establishment of an evaluation index system according to the concept of input-output efficiency, the evaluation index system of farmland drought and flood disaster prevention was built in table 2. table 2. evaluation index body of input-output efficiency for drought and flood control of agricultural land first-level index two-level index input index of drought and flood control for agricultural land agricultural drainage and irrigation diesel engine/set number of reservoirs total reservoir capacity/100 million cubic meters output index of drought and flood control in agricultural land effective irrigation area/1000 ha drainage area/1000 ha “input” refers to fitting together human and material resources to prevent and control drought and flood disasters in agricultural land. the prevention and control of agricultural drought and flood disasters mainly adopt organizing human and material resources for flood discharge and irrigation. for this approach, three input indicators were selected: the number of mechanical facilities for irrigation and drainage, the total reservoir capacity, and the number of reservoirs. these indexes reflect each region’s input and use of water conservancy construction and agricultural machinery. effective irrigation and waterlogged elimination areas were selected as the output indexes of prevention and control of drought and flood, roughly reflecting the results of flood control and drought resistance. in order to make the decision-making unit comparable, referring to the scale of agricultural output value in the 2014 china statistical yearbook, the 306 yi zhang authors took the cities with an agricultural output value of 100 billion yuan. twenty cities fit into this frame. the input-output data on drought and flood prevention and control of agricultural land in twenty cities from 2010 to 2013 were taken from china statistical yearbook (song et al., 2018). 2.2. dea evaluation model 2.2.1. data envelopment analysis (dea) in 1978, famous american operational research experts put forward the dea method. it was a nonparametric method taking input/output as the efficiency of relative performance evaluation of decision-making units (dmu). it was also a complex system of effectively dealing with multi-index input and multi-index output, which could effectively evaluate the same type of department with multiinput and multi-output. dea method has been widely applied in evaluating the effectiveness of science and technology resources allocation, the evaluation of urban community construction, the security investment funds performance, and the prevention and control of agricultural disasters. the input-output efficiency of agricultural disaster control in china is a comprehensive evaluation problem of multi-index input and multi-index output. it is unnecessary to determine the expression for the relationship between input and output. based on the dea model, this study analyzes the input-output situation of drought and flood control of agricultural land in china, mainly including the scale income of input-output of drought and flood control of agricultural land. the conceptual diagram of the dea model is shown in figure 1. figure 1. dea model conceptual diagram. 2.2.2. dea mathematical model let us assume that there are decision-making units dmui ( i = 1,2,...,n) each grain production area is a decision-making unit). in the drought and flood disaster prevention of agricultural land, each decision-making unit has kinds of input (input for prevention and control of drought and flood disaster in agricultural land) and p kinds of output (output for drought and flood disaster prevention in agricultural land). xi = (x1i, x2i,...xmi) t is used to represent the input, output, and weight of the i-th decision-making unit dmui • yi = (y1i, y2i,...ymi) t denotes its output, vi = (vi, vi,...vm)t and ui = (ui, ui,...um)t represent its weight. meanwhile, it meets xsi ≥ 0, yti ≥ 0 (s = 1,2,...,m; t = 1,2,..., p; i = 1,2,...,n the efficiency evaluation index of the i th agricultural production area is: 1, 2,...,= = t i i t i u y h i n v x (1) the efficiency of the i0-th production area is evaluated. if hio becomes larger, the study area can get more output with less input from farmland drought and flood control. based on efficiency evaluation of dmuo, it is necessary to select the appropriate weight u, v, and maximize value of h0 when each evaluation index is less than or equal to 1. ccr optimization model is constructed: 0 0 0 1 max 1 1,2,..., . 0, 0 =  = ≤ =   ≥ ≥ t t t i t i u y h v x u y h i n s t v x u v (2) formula 2 is a fractional programming model, which is transformed into a linear programming model by charess-cooper transformation. the definition of the optimal solution of linear programming is used to research the effectiveness of input-output of prevention and control for drought and flood disasters in regional agricultural land. meanwhile, the dual model is adopted for in-depth analysis based on the duality theory of linear programming. the linear programming model is built by introducing relaxation variable s-, s+ and non-archimedean infinitesimal ε. 0 0 0 1 max 1 1,2,..., . 0, 0 =  = ≤ =   ≥ ≥ t t t i t i u y h v x u y h i n s t v x u v (3) in formula 3, is greater than zero and less than any positive number. the input relaxation vector is and the output relaxation vector is s+, et m = (1,1,...1) t ∈rm, et s = (1,1,...1)t ∈rs. the optimal solution of formula 3 is λo, s-0, s+0, θ0. there are some situations: if θ0 > 0, the optimization of planting structure of disaster avoidance is non-dea efficiency for the input-output of drought and flood prevention of agricultural land. it is neither the best scale nor the best technical efficiency. therefore, it is necessary to adjust the planting structure. if θ0 = 1, et ms-0 + et ss +0>0, the optimization of planting structure of disaster avoidance is weak dea efficiency for the input-output of drought and flood prevention of agricultural land in the researched area. that is to say, when the input x0 is constant, the output can be increased by s+, or the input x0 can be reduced by s-, so that the original output is unchanged. however, the planting structure of disaster avoidance cannot meet the optimal scale and technology simultaneously. if , θ0 =1, et ms-0+ et ss +0=0, it means that the optimization of planting structure of disaster avoidance is dea efficiency for the input and output of drought and flood prevention of agricultural land in the research area. that is to say, the optimal output y0.can be obtained by original input x0, and the planting structure of disaster avoidance can achieve scale effectiveness and technology effectiveness. in this study, the input-oriented model is selected to analyze whether the input of planting structure optimization is practical when the output of drought and flood prevention of agricultural land in the study area is fixed. the scale efficiency refers to the distance from the efficiency frontier to the most appropriate production scale when the output is constant, and technical efficiency refers to the distance from any point to the efficiency frontier when the output is constant. the necessary condition for the criticality of optimal production scale is the unchanged scale efficiency. if the comprehensive efficiency value equals 1, the agriculture in the optimized area is dea efficiency. if the comprehensive efficiency value is less than 1, the agriculture in the optimized area is non-dea efficiency. if it is supposed that the pure technical efficiency of input-output is pte, scale efficiency is se, and comprehensive efficiency of input-output is te, thus the relationship between them is shown as follows: te = pie × se (4) therefore, the technical efficiency of input-output of drought and flood prevention of agricultural land in researched areas can be decomposed into scale effectiveness and pure technology effectiveness to analyze the proportion 307drought and flood control of agricultural lands and optimization method of planting structure to avoid disasters of optimization of planting structure of disaster avoidance caused by pure technical inefficiency and scale inefficiency. 2.2.3. dea application steps the selection of the evaluation model, evaluation unit, and relevant data will directly influence the evaluation results, which determines whether they can reflect the actual situation of the evaluation object (lark et al., 2017). in order to make the evaluation results more authentic, objective, and scientific, it is necessary to select a decision-making unit, evaluation index system, and evaluation model scientifically. the application steps of the dea method are shown in figure 2. start purpose of evaluation decision unit selection building index system building dea model index system rectification come to conclusion end satisfied yes or no dea evaluation analysis no yes figure 2. application steps of dea method. 3. results and discussion according to the evaluation system and evaluation model, the inputoutput data of drought and flood prevention of agricultural land in jiangxi province since the reform and opening-up were collected. according to the availability and sufficiency of data, the input-output data in twenty regions of jiangxi province in 2014-2017 were further selected (turra et al., 2017). based on the input-oriented (modified), maxdea6.4 professional software was combined with a non-radial sbm model and super efficiency model to calculate the change of input-output efficiency of farmland drought and flood prevention in twenty regions in 2014-2017. figure 3 shows the input and output efficiency of farmland drought and flood control under the sbm model. invest in → economic activity → output capital investment labor input energy input capital investment increased complete infrastructure expansion of agricultural land tecnical level improvement prevention of drought and water logg ing disasters of famland optimization of disaster avoidance planting structure figure 3. changes in input-output efficiency of farmland drought and flood control under the sbm model. 3.1. spatio-temporal difference in input-output efficiency of drought and flood control of agricultural land the input-output efficiency of drought and flood control of agricultural land in the main cities of jiangxi province is generally low. table 3 shows that the average input-output efficiency of drought and flood prevention in significant cities of jiangxi province from 2014 to 2017 is less than 0.650, and there are only four cities on the effective frontier, accounting for only 20% of the analyzed cities. the input-output efficiency of drought and flood prevention in 80% of the cities needs to be revised. there was only one city with an efficiency value of 0.5-1 in 2014-2016, namely dexing city, and two cities in 2017, jiujiang city and dexing city. the cities with an efficiency of 0.5-1 and more than one account for 25% of twenty cities. the input-output efficiency is less than 0.5 in 75% of the cities. in 75% of the ineffective cities, nearly half are lower than 0.2. most urban agricultural land’s drought and flood control resources have not been effectively utilized. table 3. total calculation results of input-output efficiency of farmland drought and flood prevention in 20 cities of jiangxi province in 2014-2017 dmu 2014 2015 2016 2017 ganzhou city 0.413 0.373 0.359 0.249 jingdezhen city 0.160 0.132 0.130 0.198 pingxiang city 0.144 0.116 0.112 0.113 xinyu city 0.110 0.093 0.082 0.084 jiujiang 1.058 1.049 1.025 0.510 shangrao city 0.415 0.398 0.387 0.330 yingtan city 1.160 1.165 1.169 2.025 fuzhou city 0.196 0.185 0.191 0.102 fengcheng 0.135 0.115 0.108 0.122 zhangshu city 0.281 0.254 0.252 0.228 yichun city 1.052 1.056 1.061 2.767 ruijin city 0.124 0.116 0.113 0.017 jinggangshan city 0.325 0.301 0.294 0.203 dexing city 0.613 0.596 0.584 0.749 ruichang city 0.417 0.391 0.385 0.286 communist youth city 0.393 0.366 0.352 0.259 mount lu city 0.197 0.191 0.187 0.175 nanchang city 4.298 5.447 5.752 1.520 gao’an city 0.276 0.221 0.207 0.163 ji’an city 0.358 0.220 0.222 0.079 average value 0.606 0.639 0.649 0.509 the input-output spatial distribution of drought and flood disaster prevention in agricultural land is “more in the east and less in the west” (salarikia et al., 2017). in 2014-2016, there were three cities on the effective frontier: jiujiang city, yingtan city, and yichun city. in 2017, they were two effective units: yingtan city and yichun city. in western china, only nanchang was on the effective frontier from 2014 to 2017. the technical efficiency of the city with effective input-output for the prevention and control of drought and flood disasters of agricultural lands showed the characteristics of “increasing in the east and decreasing in the west” in time. in addition, the input-output efficiency of drought and flood prevention of agricultural land was decreased in jiujiang city, and the input-output in yingtan city and yichun city showed strong growth. yingtan city increased from 1.160 in 2014 to 2.025 in 2017, with an 308 yi zhang increase of 74.57%, and yichun city increased from 1.052 in 2014 to 2.767 in 2017, with an increase of 163%. in contrast to the high-speed growth of inputoutput efficiency of drought and flood prevention of agricultural land in eastern jiangxi, the efficiency value of effective cities in western jiangxi was declining. the input-output efficiency in nanchang has decreased from 4.298 in 2014 to 1.520 in 2017, with a decrease of 64.63%. the index in the main agricultural cities of jiangxi province needs to be more balanced within the region. of the eight cities in the central region, yingtan city has the highest technical efficiency (value of 2.025), and fuzhou city has the lowest efficiency (0.102). the difference is about nineteen times. of the four cities of the western region, nanchang is the city with the highest technical efficiency (1.520), and gao’an is the city with the lowest efficiency value (0.163). the difference is of about nine times. in 2010, nanchang had the highest technical efficiency (4.298), 22 times higher than lushan (0.197). figure 4. comprehensive technical efficiency of input and output of drought and flood prevention of agricultural land in 20 cities of jiangxi province in 2017. 3.2. the change law of scale and pure technical level of input-output efficiency of drought and flood prevention of agricultural land table 4 shows the scale and pure technical level of input-output efficiency of drought and flood prevention of agricultural land in twenty cities of jiangxi province in 2017, showing the following characteristics: the driving factor of the input-output effective city for preventing and controlling drought and flood disasters on agricultural lands lies in pure technical efficiency. figure 4 shows that in the eight cities of the eastern region, yichun had the highest technical efficiency (efficiency value was 2.767) in 2017, and ji’an had the lowest efficiency (0.079). the difference is up to 35 times. table 4. scale level and pure technical level of input-output efficiency of agricultural land drought and flood control in 20 cities of jiangxi province in 2017 decision making unit (dmu) comprehensive efficiency (te) pure technical efficiency (pte) scale efficiency (se) change of scale income ganzhou city 0.249 0.263 0.958 decrement jingdezhen city 0.198 1.047 0.191 increase progressively pingxiang city 0.113 0.261 0.419 increase progressively xinyu city 0.084 0.244 0.366 increase progressively jiujiang 0.520 0.532 0.988 decrement shangrao city 0.330 0.332 1.005 decrement yingtan city 2.035 5.433 0.384 decrement fuzhou city 0.102 0.155 0.642 increase progressively fengcheng 0.122 0.153 0.789 increase progressively zhangshu city 0.228 0.528 0.431 increase progressively yichun city 2.767 2.894 0.967 increase progressively ruijin city 0.165 0.358 0.454 increase progressively jinggangshan city 0.203 0.448 0.451 increase progressively dexing city 0.749 1.231 0.616 increase progressively ruichang city 0.286 0.393 0.731 decrement communist youth city 0.259 0.789 0.33 increase progressively mount lu city 0.175 0.307 0.564 increase progressively nanchang city 1.520 1.619 0.949 increase progressively gao’an city 0.163 0.420 0.383 increase progressively ji’an city 0.089 0.210 0.401 increase progressively 309drought and flood control of agricultural lands and optimization method of planting structure to avoid disasters from the effective decision-making unit in 2017, the pure technical efficiency of yingtan city (comprehensive efficiency value was 2.035), yichun city (2.767), and nanchang city (1.520) are 5.433, 2.894, and 1.619, respectively. meanwhile, the scale efficiency is 0.384, 0.967, and 0.9498, respectively. although the scale efficiency is lee than 1, the pure technical efficiency is far greater than 1. finally, the technical efficiencies of these three cities are more than 1. the main reason that drives the input-output effective drought and flood prevention unit in jiangxi province is the high pure technical efficiency, which reflects the efficient utilization of the factor. the driving factors of ineffective input-output cities lie purely in technical and scale efficiency factors. from the perspective of a city whose technical efficiency value was between 0.5 and 1 in 2017, the pure technical efficiency of jiujiang (comprehensive efficiency value was 0.520) was 0.532, and its scale efficiency was 0.988. therefore, low pure technical and scale efficiency were the main reasons for the ineffective input and output of agricultural land drought and flood control in jiujiang. the pure technical efficiency of dexing city (0.749) was 1.231, while the scale efficiency was 0.616. low-scale efficiency was the main reason for the ineffective input-output of drought and flood prevention of agricultural land in dexing. in fact, the pure technical efficiency needs to be higher, leading to input-output ineffectiveness. from the perspective of cities whose technical efficiency value was less than 0.5 in 2017, in addition to jingdezhen, the reason for low technical efficiency lies in low pure technical efficiency and low scale efficiency. this includes three situations: firstly, the scale efficiency is relatively high, and the pure technical efficiency is relatively low, such as in ganzhou, shangrao, fuzhou city, fengcheng, ruichang, and lushan. secondly, the pure technical efficiency is relatively high, and the scale efficiency is relatively low, such as gongqing. thirdly, the comprehensive efficiency ratio and scale efficiency are relatively low. the scale incomes of drought and flood control of agricultural land in most cities are increasing. according to the effective decision-making unit, it can be seen that the scale income of drought and flood prevention of agricultural land in yingtan is decreasing. however, the scale incomes of yichun and nanchang are increasing. from the perspective of invalid decision-making units, in addition to ganzhou, jiujiang, shangrao, and ruichang, the scale incomes of drought and flood disaster prevention and control in thirteen cities are increasing. there are fifteen cities with increasing returns to scale, accounting for 75% of the total. for most of the cities in jiangxi province, the scale income of prevention and control of drought and flood disasters of agricultural land is increasing. 3.3. econometric model of input-output efficiency of drought and flood control of agricultural land in jiangxi province in order to master the deep relationship between the input and output of the prevention and control of drought and flood disasters prevention in jiangxi province, the output of value and the output of physical quantity are taken as the explanatory variables, respectively, as the input of human, material, and financial resources in jiangxi province are taken as the explanatory variables for regression analysis to build the tobit regression model. due to the high correlation between the physical input and financial input of drought and flood prevention of agricultural land in jiangxi province, the t-test value is not significant in the construction of the regression measurement model, so the indexes of agricultural physical input are excluded from the econometric model. the econometric model between the value output and the agricultural input of drought and flood prevention of agricultural land in jiangxi province is shown as follows: 1 1 3 2 2 ˆ 0.00003 0.7672 0.7737 0.000 4.247 7.780 0.999 0.114 0.000 0.987 0.982 183.69 0.000 = + × + × = = = = fy fx fx t prob r r f prob (5) arch test: the statistics of x2 is 1.6112, and the adjoint probability is 0.4504, without autocorrelation. white heteroskedasticity test: the statistic is 4.3952, and the adjoint probability is 0.3676. there is no heteroscedasticity. the proposed regression model has passed the economic, statistical, and econometric tests. this model shows that when the financial input for the prevention and control of drought and flood disasters of agricultural land in jiangxi province remains unchanged, the output of magnitude of value increases by 0.7672 units on average for each unit increment of human input. suppose the input of human resources for drought and flood disaster prevention in jiangxi province remains unchanged. in that case, the output of magnitude of value for the drought and flood disaster prevention in agricultural land increases by 0.7737 units for each unit increment of financial input. the human input and financial input of drought and flood prevention of agricultural land in jiangxi province have the same impact on the value production. comparatively speaking, the financial input of drought and flood prevention of agricultural land is more important. the regression model has passed the economic, statistical, and econometric tests. the model shows that when the financial input for the prevention and control of drought and flood of agricultural lands in jiangxi province remains unchanged, the output of the value of the prevention and control increases by 0.7672 units on average for each unit of increase in human input. under the condition that the input of human resources for drought and flood prevention in jiangxi province remains unchanged, the output of the value increases by 0.7737 units on average for every unit of financial input. the human and financial input of drought and flood prevention have the same impact on the value output of drought and flood prevention of agricultural land. comparatively speaking, the financial input of drought and flood prevention of agricultural land is slightly more important. the econometric regression model between physical output and agricultural input in jiangxi province is shown as follows: 2 1 3 2 2 ˆ 0.00007 0.7513 0.6023 0.000 2.382 3.446 0.9998 0.050 0.009 0.912 0.890 41.06 0.000 = + × + × − = = = = fy fx fx t prob r r f prob (6) arch test: the statistics of x2 is 0.9774, and the adjoint probability is 0.6279, without autocorrelation. white heteroskedasticity test: the statistic is 4.9718, and the adjoint probability is 0.3025. there is no heteroscedasticity. the proposed regression model has passed the economic, statistical, and econometric tests. the model shows that when the financial input for preventing and controlling drought and flood disasters of agricultural land in jiangxi province remains unchanged, the physical output increases by 0.7513 units on average for each unit increment of human input. if the input of human resources remains unchanged, the physical output increases by 0.6023 units for each unit increment of financial input. human input and financial input have a slightly different impact on physical production. therefore, the human input of drought and flood prevention of agricultural land is more important. the analysis results show that the human and financial input in the drought and flood disaster prevention of agricultural land in jiangxi province have different effects on the value output and physical output. 4. conclusion based on mastering the input-output efficiency and influencing factors, this paper puts forward comprehensive improvement measures for drought and flood disaster prevention in agricultural lands according to local conditions. the specific measures to improve the input of drought and flood prevention include: strength investment in basic facilities of flood control and drought resistance. it is necessary to increase the total fund and optimize the use structure of funds on basic flood control facilities and drought resistance to improve the use effect of that funds. the expenditure share on other types of farmland and water conservancy infrastructure should be increased while ensuring the investment in major water conservancy projects such as flood control. the 310 yi zhang investment in flood control should be gradually extended to medium and small rivers based on improving the management of large rivers. almost 70% of 50000 small and medium rivers in china have yet to reach the flood control standard. this requires the government to adjust the expenditure structure and provide the basic facilities of flood control and drought resistance with a financial guarantee. in the investment in disaster prevention and mitigation, it is necessary to promote the optimization and regulation of water resources vigorously. the area of river basins in the north of the yangtze river accounts for 65% of the land area in china, but water resources only account for 20%. increasing investment in flood control and drought resistance facilities, such as cross-regional water resources, can effectively alleviate the impact of drought on grain production in water-deficient areas. investing in disaster prevention and mitigation should focus on improving water resource utilization efficiency and changing the imbalance of disaster prevention and mitigation capacity among regions. in water conservancy and meteorology expenditure, the focus would be on science and technology’s role in disaster prevention and mitigation. improvement of the monitoring and early warning system of government for agricultural natural disasters. objectively, the particularity and complexity of agricultural drought and flood risk put forward higher requirements for the response system of agricultural drought and flood disasters. therefore, it is necessary to establish a drought and flood disaster monitoring system and an agricultural vulnerability early warning and prediction system. the system should fully use “3s” technology (remote sensing technology-rs, geographic information system-gls, and global positioning system-gps). the precipitation, temperature, relative humidity of the soil, and other information of agricultural meteorological stations are used to carry out systematic detection and dynamic evaluation of the real-time information to make early warnings and predictions. secondly, strengthen the division of powers and responsibilities and the organization and coordination, and then build a network risk management system for agricultural drought and flood led by government departments and participated by farmers. when implementing the responsibility system for flood and drought control, the office of flood control and drought relief at the county level and their administrative offices and responsibility system would be improved, and extend the disaster prevention function to the grass-roots unit according to local conditions. in addition, it is necessary to strengthen the construction of coordination and linkage mechanisms among the state, provinces, cities, counties, and villages and give full play to the disaster response function of organizations at all levels to conduct real-time detection and statistics of the information such as local meteorological changes, operation status of irrigation and drainage facilities and agricultural vulnerability. this network system can improve the effectiveness of drought and flood disaster prediction and prevention and mitigation measures, thus enhancing the response capacity of agricultural drought and flood disasters. strengthen technical research and development of agricultural products for flood control and drought resistance. popularizing agricultural technical measures and reducing agricultural disasters is necessary. in addition, it is necessary to select “multi-resistant” varieties with high yield, stable yield, disease, and insect resistance, drought and waterlogging resistance, reform the planting system, reasonably arrange the crop structure and species, optimize the crop layout and determine the best multiple-crop index. all of them are effective measures to reduce agricultural disasters. in agricultural production, the implementation of “five good” supporting technologies, such as suitable varieties, good fields, good systems (good farming systems), suitable methods (good cultivation and management methods), and good materials (good material input), can significantly reduce agricultural disasters and ensure high and stable agricultural production. improvement of various forms of agricultural insurance mechanisms. establishing and improving various agricultural insurance mechanisms and clarifying their relief is necessary. currently, there are two forms of paid disaster relief in rural areas. the first is a special financial disaster relief fund. the national financial department or the disaster prevention and mitigation management organization manages and uses the financial reserve for disaster relief annually through funds. the agricultural disaster relief fund should have two characteristics. firstly, the fund is to support the affected rural areas and enterprises in the form of investment and help them eliminate difficulties. secondly, the rural disaster relief fund can be accumulated year by year. these characteristics of agricultural disaster relief funds can not only help the disaster area to overcome dependence and improve the consciousness of disaster prevention and mitigation but also correspondingly improve the ability to cope with agricultural disasters. according to the production scale, operation mode, and disaster losses in rural areas, the agricultural disaster relief accumulation fund shall be collected quantitatively. as a kind of compulsory insurance, the provident fund for disaster relief can be acted as a unified agent by the insurance company or as a source of disaster relief fund managed by the financial department. thirdly, enterprises and institutions can jointly provide investment assistance to the affected rural enterprises. if the management system is proper, this method can help the rural enterprises in the disaster area get rid of the predicament and also help them to develop in the direction of group management. in conclusion, the dea model was used to analyze the input-output efficiency of the drought and flood disaster prevention of agricultural land in twenty cities of jiangxi province from 2014 to 2017, with a real example. the following conclusions can be drawn: the input-output efficiency of drought and flood control in most agricultural land is low. the resources of disaster prevention and mitigation resources need to be sufficiently utilized. in 2014-2017, the input-output efficiency of drought and flood prevention of agricultural lands in major agricultural cities of four cities was on the efficient frontier, accounting for 20% of the total, and 80% of input-output efficiency was relatively ineffective. the effective input-output cities of farmland drought and flood disaster prevention show the characteristics of “more in the east and less in the west.” the inputoutput efficiency shows the imbalance within the region. low pure technical efficiency and low scale efficiency are the main reasons for the ineffective input and output of drought and flood disaster prevention in most agricultural lands. however, most cities’ scale benefits of drought and flood prevention are increasing. raising the input scale can improve the drought and flood prevention of agricultural land in china. the input of human and financial resources for the drought and flood disaster prevention of agricultural land in jiangxi province has the same influence on the value output. however, there is a slight difference in the impact on the output of physical quantity of drought and flood prevention in agricultural land, so the input of human and financial resources in the drought and flood disaster prevention of agricultural land in jiangxi province has slightly different effects on the output of value and actual output. acknowledgment this research was funded by the department of education of jiangxi province science and technology project: study on input-output efficiency and structural optimization strategy of agricultural disaster prevention and mitigation under rural revitalization strategy (grant number: gjj191056). references alexander, d. e. 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(2018). alpine foreland running drier? sensitivity of a drought vulnerable catchment to changes in climate, land use, and water management. climatic change, 147(1), 179-193. https://doi.org/10.1007/s10584-017-2121-y lark, r. m., hamilton, e. m., kaninga, b., maseka, k. k., mutondo, m., sakala, g. m., & watts, m. j. (2017). nested sampling and spatial analysis for reconnaissance investigations of soil: an example from agricultural land near mine tailings in zambia. european journal of soil science, 68(5), 605-620. https://doi.org/10.1111/ejss.12449 https://doi.org/10.1108/dpm-11-2015-0262 https://doi.org/10.1002/cplu.201500383 https://doi.org/10.1002/cplu.201500383 https://doi.org/10.1016/j.rse.2018.04.026 https://doi.org/10.1007/s10584-017-2121-y https://doi.org/10.1111/ejss.12449 311drought and flood control of agricultural lands and optimization method of planting structure to avoid disasters salarikia, a., miraei ashtiani, s., golzarian, m. r., & mohammadinezhad, h. 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(2019). bedforms, energy dissipation and disaster mitigation mechanism in mountain rivers of southwest china. journal of hydraulic engineering, 50(5), 124-134, 154. doi:10.13243/j.cnki. slxb.20181081 wei, x. h., liu, x. f. & li, h. l. (2016). operation sharing optimization algorithm based on location aware in dsps. journal of jilin university (science edition), 54(5), 1047-1054. https://doi.org/10.1016/j.inpa.2016.12.003 https://doi.org/10.1016/j.inpa.2016.12.003 https://doi.org/10.1016/j.inpa.2017.05.004 https://doi.org/10.1016/j.inpa.2017.05.004 https://doi.org/10.1039/c6cp07271b ole_link37 ole_link4 ole_link5 ole_link3 ole_link6 ole_link11 ole_link10 ole_link77 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.107766 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march 2024): 17 27 g eo lo g y abstract the current study aimed to evaluate the petroleum generation potential of the sakesar formation. this study interprets and presents a depositional environment model, microfacies, and geochemical and petrophysical data of the eocene sakesar formation in the potwar basin, pakistan. twenty well-cutting samples from two wells and six fresh outcrop samples were thoroughly studied. results of total organic carbon and rock-eval pyrolysis of sakesar formation sediments show fair to good toc contents ranging from 1.2-1.67 wt%. s2 values of samples showed fair to good generation potential. sediments appear mature, having primarily mixed type ii-iii kerogen with good oil/ gas-generation potential. three microfacies have been identified in the sakesar formation at the tatral section: bioclastic wacke-packstone, lockhartia-rich mud-wackestone, and benthic foraminiferal wackestone. the microfacies of the sakesar limestone depict the deposition of the sakesar limestone from the distal middle ramp to restricted inner ramp settings. petrophysical well logs analysis of the sakesar formation showed an average porosity of ~9.12%; the lithology was identified as limestone, having an average water saturation of ~22.32% and an average hydrocarbon saturation of ~77.68%. thus indicating average to good reservoir properties with very good hydrocarbon saturation. sakesar formation sediments characteristics interpretation showed that it can act as both source rock and reservoir rock in the potwar basin. integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan syed bilawal ali shah1*, khaira ismail2, wan zairani wan bakar3 1. department of geology, university of malaya, kuala lumpur 50603, malaysia. 2. faculty of science and marine environment, universiti malaysia terengganu, kuala nerus 21030, terengganu, malaysia. 3. oil and gas engineering, school of chemical engineering, college of engineering, universiti teknologi mara,40450 shah alam, selangor, malaysia. * corresponding author: bilawalshah22@siswa.um.edu.my k.ismail@umt.edu.my zairani@uitm.edu.my record manuscript received: 12/03/2023 accepted for publication: 27/03/2023 how to cite this article: shah, s. b. a., ismail, k., & bakar, w. z. w. (2024). integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan. earth sciences research journal, 28(1), 17-27. https://doi.org/10.15446/esrj. v28n1.107766 keywords: sakesar formation; potwar basin; depositional environment; microfacies. análisis integrado de la formación sakesar del eoceno: ambiente deposicional, microfacies, geoquímica y características reservorio de la cuenca de potwar, pakistán resumen el presente estudio busca evaluar el potencial de generación petrolífera de la formación sakesar. con ese fin este trabajo presenta e interpreta un modelo ambiental deposicional, con información de microfacies, geoquímica y petrofísica. un total de veinte muestras cortadas y pulidas de dos pozos y seis afloramientos se estudiaron cuidadosamente. los resultados de los análisis de carbono orgánico total y de pirólisis (rock eval) de los sedimentos de la formación sakesar muestran que los contenidos de carbono orgánico total son de regulares a buenos y oscilan entre 1.2 y 1.67 wt%. los valores s2 de las muestras tienen un potencial de generación entre regular y bueno. los sedimentos se muestran maduros y tienen una mezcla primaria de kerógenos tipo ii-iii con un buen potencial de generación de petróleo y gas. tres microfacies se han identificado en la sección teatral de la formación sakesar: wacke-packstone bioclástica, wackestone de barro rico en lockhartia y wackestone de foraminíferos bentónicos. las microfacies de las calizas de sakesar muestran que sus deposiciones desde la rampa media distal hasta configuraciones de rampa interior restringidas. los análisis petrofísicos de registro de pozo de la formación sakesar muestran una porosidad promedio de ~9.12%; la litología se identificó como caliza, con un promedio de saturación de agua de ~22.32% y un promedio de saturación de hidrocarburos de ~77.68%. por ende, los resultados indican que en promedio las propiedades del reservorio son buenas con muy buena saturación de hidrocarburos. la interpretación de las características de los sedimentos de la formación sakesar muestran que esta puede actuar como roca fuente o como roca reservorio en la cuenca potwar. palabras clave: formación sakesar; cuenca potwar; ambiente deposicional; microfacies https://doi.org/10.15446/esrj.v28n1.107766 mailto:bilawalshah22@siswa.um.edu.my mailto:k.ismail@umt.edu.my https://doi.org/10.15446/esrj.v28n1.107766 https://doi.org/10.15446/esrj.v28n1.107766 18 syed bilawal ali shah, khaira ismail, wan zairani wan bakar 1. introduction one of pakistan’s productive oil and gas basins is the potwar basin in the upper indus basin. since the attock oil company’s first commercial hydrocarbon discovery at khaur in 1914, the basin has been the focus of hydrocarbon exploration for more than a century (asif et al., 2011; du and wang, 2013; shah, 2022). this basin covers two-thirds of eastern pakistan. numerous scholars have provided details on hydrocarbon source rock (kadri, 1995; imtiaz et al., 2017; liu et al., 2023; shah and abdullah, 2017). hydrocarbon traps in the potwar basin are structurally controlled and also include stratigraphic traps (ren et al., 2022; shah, 2022; xu et al., 2022a; yasin et al., 2021; yu et al., 2021). the potwar basin’s primary hydrocarbon reservoir and source rocks are the early-eocene and paleocene sedimentary rocks (kadri, 1995; xi et al., 2023; yang et al., 2024; zhu et al., 2022). major oil and gas discoveries in the potwar basin are from the sakesar and chorgali formations (he et al., 2021; shah, 2009; shah, 2023; yin et al., 2023a; yin et al., 2023b). the average thickness of the sakesar and chorgali formations in the area is (141.43-182.88m). a study by fazeelat et al. (2010) has found that shallow marine sediments in the potwar basin are organic-rich, prompting the need to evaluate the sakesar’s formations depositional environment, source, and reservoir rock characteristics in the potwar basin at tatral and balkassar oilfield. the novel contribution of this research lies in its comprehensive approach: for the first time, well cutting samples, along with outcrop and well logs, have been thoroughly studied to assess source rock generation potential, thermal maturity, kerogen type identification, forms of kerogen assemblages, depositional environment, and petrophysical properties. this investigation aims to estimate the properties of reservoir rocks and their generation potential and will use rider 1986 criteria to evaluate it (table 1). moreover, this research employs three different methods to assess and evaluate the two most critical properties of the petroleum system, namely, source and reservoir rock properties. previous researchers have studied different formations and they have used different techniques. for example, ali et al. (2015) studied sakesar formation reservoir properties at fimkassar oilfield, asif and tahira (2007) studied the tobra and khewra formation’s geochemical properties at fimkassar oilfield, khan et al. (2017) have studied microfacies of the chorgali formation at khair-e-murat range. this study is both necessary and unique due to the scarcity of information regarding the comprehensive interpretation of source and reservoir rocks in the balkassar oilfield, including an understanding of the potential of proven source and reservoir rocks. the study area balkassar and tatral section is located in potwar basin (fig. 1). the output of this study will establish the depositional environment, source generation potential, type of kerogen, thermal maturity of source rocks, forms of kerogen present and presence of possible hydrocarbons and the reservoir rocks in the subsurface of balkassar oilfield. this research will have direct applicability to future hydrocarbon exploration by oil and gas companies. table 1. qualitative description of reservoirs porosity set by rider’s (1986). porosity evaluation on a qualitative basis qualitative description average porosity negligible 0-5% poor 5-10% good 10-20% very good 20-30% excellent more than 30% 2. geological setting the potwar basin is situated on the western side of the indian shield which is a part of the upper indus basin, is where the tatral section and balkassar oilfield are located (kadri, 1995; khan et al., 2022; wei et al., 2023; xiao et al., 2023; zhang et al., 2021). the fore-deep of the indus basin, which consists of depressions, a platform and an outer and inner folded zone, are its main features (kadri, 1995; kazmi and jan, 1997; liang et al., 2024). the indus basin is host to almost all of pakistan’s main hydrocarbons resources (kadri, 1995; ren et al., 2023; yin et al., 2023c; yang et al., 2023). the kalachitta-margalla hill ranges borders the potwar basin to the north, the salt range to the south, the jhelum river to the east, and the indus river to the west (jia et al., 2023; shah and abdullah, 2016; yin et al., 2023d; yu et al., 2022). the basin is characterised by tight, complicated folds extending east to west that are disturbed by steep angle faults and turned southward. the northernmost part of the basin is more extensively deformed (aadil et al., 2014; kazmi and abbasi, 2002; kazmi and jan 1997; riaz, 2022; shah, 2009; shah, 2022). in the western portion, there are a number of broad, moderate east-west folds, however, the eastern portion’s strike abruptly shifts toward the northeast, and the structures are composed of massive synclines and anticlines (shah, 2022; tie et al., 2023; jia & zhou, 2023; zhou et al., 2023). 2.1 a brief overview of the source and reservoir rocks of potwar basin most of the potential source rocks in the potwar basin are of palaeocene and eocene age, and the majority of the oil discoveries are within faulted anticline traps ranging from the cambrian to the miocene (table 2). the sakesar, wargal, lockhart, hangu, and patala formations are recognized as the key source rocks in the potwar basin (dai et al., 2023; hasany and saleem, 2012; shah, 2022). reservoir rocks in the basin include paleogene shelf carbonates, miocene alluvial sandstones, jurassic and permian continental sandstones, along with cambrian alluvial and shoreface sandstones (aadil et al., 2014; kazmi and abbasi, 2002). notable reservoir rocks that have produced oil and gas in the potwar basin include the jurassic dutta, cambrian khewra, jutana, kussak, tobra, wargal, amb, margalla limestone, chorgali, bhadrar, and murree formations (fazeelat et al., 2010; ihsan et al., 2022; kazmi and jan, 1997). figure 1. tectonic features of potwar basin (kadri, 1995; shah, 2022; zhou et al., 2023). 19integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan table 2. stratigraphy of the potwar basin in general (after zahid et al., 2014; shah, 2023). age / epoch lithology formation lithology description neogene pliocene nagri chinji sandstone, sandstone and clay miocene oligocene kamlial murree sandstone, sandstone, limestone oligocene unconformity paleogene eocene chorgali sakesar limestone, limestone paleocene patala lockhart hangu shale, limestone, sandstone mesozoic & late permian unconformity jurassic datta sandstone, shale permian early permian chhidru wargal amb sardhai warcha dandot tobra sandstone, limestone, limestone, siltstone, interbeds of sandstone, shale interbeds of sandstone, sandstone and conglomerate. carboniferous to ordovician unconformity cambrian to precambrian cambrian baghanwala jutana kussak khewra shale, dolomite, sandstone and siltstone, shale interbeds of dolomite and siltstone infra-cambrian salt range marl, claystone and siltstone, salt 3. materials and methods a thorough geological field survey was carried at the type section of the sakesar formation (tatral section) was sampled at latitude 32°43’38.85”n, longitude 72°56’7.13”e for sampling (fig. 2). assaad (2008) and tucker (2003) standard field procedures were used for sampling. eight outcrop samples were extracted after digging to minimise the effects of weathering. for microfacies analysis, six representative rock samples were used. as samples must be collected from unweathered material in outcrops. therefore, a fresh section of the outcrop should be exposed to obtain representative samples, this would require a trench to be cut through the exposed weathering zone using hand equipment, and sampling should be continuous and must evenly represent the entire interval being sampled. the exposed section to be sampled must be cleaned of all debris down to the lowest strata to be sampled. once the exposed top layer was cleaned, a channel was excavated through the existing outcrop to expose fresh, unweathered material. this channel was extended down to the lowest strata to be sampled. samples were then taken using a geological hammer, where harder, more resistant rock material was present. in order to support the analytical interpretation, field images of the diagnostic field characteristics were captured (fig. 3). the sakesar formation, which has the lower confirmable contact with nammal 20 syed bilawal ali shah, khaira ismail, wan zairani wan bakar formation in the study area, is made up of light to medium and dark grey to brown, cherty, fractured, and occasionally nodular limestone, the formation was approximately 70 m thick. the scanning electron microscope (sem) was used for the micritic sediments and high-resolution photomicrographs to interpret microfacies present in the sakesar limestone. figure 2. showing the exposed section of sakesar formation in tatral section, also displaying its conformable contact with nammal formations. figure 3. litholog of the sakesar limestone at the study area. twenty well cutting samples were obtained from a well of the balkassar oilfield in the potwar basin. the sediments were from eocene sequence. the samples from well were obtained at intervals of 2–3 m interval. the details of the samples are provided in table 3 and 4. reservoir rock characteristics were established using various logs: density, neutron, gamma ray, resistivity and spontaneous potential logs. criteria from rider (1986) and shah and shah (2021) were used to qualitatively describe the reservoir properties. this study used a set of well logs from well a to investigate the chorgali formation’s reservoir potential. shah (2022), shah and shah (2021) and hartmann and beaumont’s (1999) methods were used in this study to evaluate reservoir rock characteristics. additionally, the hydrocarbon saturation, porosity, water saturation, and formation water resistivity were all calculated. table 3. summary of analysed sediments of sakesar formations in balkassar well 8. fo rm at io n n am e an d n o of sa m pl es d ep th (m ) pe tr ol eu m po te nt ia l m at ur ity o f sa m pl e q ua lit y of o m k er og en t yp e so ur ce r oc k ge ne ra tiv e po te nt ia l to c w t. % s1 s2 s3 t m ax °c h i sakesar (20) 24472499 1.21.67 0.981.24 3.65.25 0.40.7 438443 265386 mixed type ii/iii fair to good table 4. geochemical results of the sediments of balkassar well 8. fo rm at io n d ep th to c % tm ax °c s1 s2 s3 pi h i o i s2 /s 3 v r m sa ke sa r 2447 1.23 441 1.14 3.86 0.4 0.31 314 34.1 9.19 0.8 2449 1.36 442 1.09 3.6 0.4 0.23 265 27.9 9.47 0.8 2451 1.45 441 1.24 4.67 0.4 0.33 322 24.1 13.3 0.8 2453 1.24 439 1.09 4.36 0.7 0.2 352 52.4 6.71 0.7 2456 1.28 440 1.18 4.2 0.8 0.22 328 63.3 5.19 0.8 2458 1.3 442 1.21 4.91 0.7 0.2 378 50 7.55 0.8 2460 1.5 443 1.07 5.2 0.6 0.17 347 38 9.12 0.8 2462 1.42 439 0.98 5.25 0.8 0.31 370 54.2 6.82 0.7 2465 1.27 440 1.13 4.75 0.9 0.19 374 66.9 5.59 0.8 2467 1.27 441 1.08 3.7 0.7 0.23 291 57.5 5.07 0.8 2469 1.4 439 1.02 4.63 0.7 0.18 331 5643 0.06 0.7 2471 1.3 438 1.21 5.02 0.8 0.19 386 62.3 6.2 0.7 2474 1.54 440 1.18 4.5 0.8 0.21 292 50.6 5.77 0.8 2476 1.67 439 0.98 4.8 0.7 0.17 287 41.9 6.86 0.7 2478 1.34 442 1.14 4.85 0.8 0.19 362 61.2 5.92 0.8 2480 1.2 440 1.1 4.48 0.8 0.2 373 62.5 5.97 0.8 2483 1.47 441 1.24 4.1 0.7 0.28 279 47.6 5.86 0.8 2485 1.6 439 1.08 5.23 0.7 0.17 328 43.1 7.61 0.7 2487 1.51 441 1.06 4.7 0.6 0.18 311 42.4 7.34 0.8 2490 1.35 442 1.14 4.97 0.8 0.19 368 55.6 6.63 0.8 3.1 organic geochemical well fresh cutting samples were treated with acid to remove any carbonate minerals and weathered material prior to analysis. samples were then crushed to less than 200 mesh, and toc was then determined using leco elemental analyzers cs-125. additionally, pyrolysis was carried out using rock-eval vi. about 100mg of crushed samples were subjected to pyrolysis analysis in 21integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan a helium environment that was heated to 600°c. pyrolysis generated the free hydrocarbons (s1), the amount of hydrocarbons produced by thermal cracking (s2), and tmax, which are the three most crucial parameters (baker, 1979; espitalié et al., 1985; peters, 1986). the hydrogen index and production yield were two other significant factors that were determined. 3.2 well log analysis the well logs in the las file format (.csv file format) were used on the interactive petrophysics software package to determine reservoir properties to estimate various important reservoir characteristics. the procedure for obtaining various parameters from the well logs is described in the section below: the log track parameters that were directly read included: 1. formation depth (ft) 2. neutron porosity (nphi; 0.45 to -0.15 scale) 3. bulk resistivity of the formation (lld; 0 – 2000ohm.m scale) 4. spontaneous potential (sp; 0 – 100mv scale) 5. bulk density of formation (rhob; 1.95 –2.95 scale) hydrocarbon saturation (sh) spontaneous potential (sp) method (figure 4, 5 and 6) was used for the determination of hydrocarbons at the zone of interest. the graphs and equations used together in the following process: 1. the wireline log report showed a bottom-hole temperature (bht) of 180°f and a surface temperature (ts) of 72°f, which were utilized in the equation (fig. 4). the following equation was used to calculate the formation temperature (tf): tf = ts + df (bht-ts/td) where ts = surface temperature tf = formation temperature td = total depth df = depth to the formation 2. mud filtrate resistivity (rmf) and resistivity of mud at formation temperature (rmf) were corrected (fig. 5). 3. self-potential (sp) was directly determined from the log chart sp curve. 4. the value of the rmf/rwe (equivalent water resistivity) ratio was measured (fig. 6). 5. the equivalent water resistivity (rwe) was calculated by dividing the corrected rmf value by the rmf/rwe value ratio. 6. the equation for rwe is as follows: rwe = rmfeq/(rmfeq/rwe) where rmfeq= equivalent mud filtrate resistivity 7. the equation in figure 4 was used to convert rwe to rw, and the correct rwe value was determined using the rwe value derived in step 5. 8. archie’s equation was used to estimate water saturation (sw): w w m t r s rô * 9. the following equation can be used to estimate the saturation of a hydrocarbon (sw) at a particular temperature : sh=1-sw after measuring the resistivity values of the mud filtrate, they were converted to equivalent mud filtrate resistivity values using the equation in figure 4. to obtain the equivalent water resistivity, the equivalent mud filtrate resistivity values were used according to step 6. subsequently, the equivalent water resistivity was converted to water resistivity, which was then used to determine water saturation using archie’s equation. the log header’s mud filtrate resistivity was determined at surface temperature. to determine the mud filtrate resistivity at formation temperature, it must be corrected to formation temperature for each value at a certain depth, and this correction was made using the equation presented in figure 5. figure 4. rw is determined by determining rwe. (schlumberger, 1977). figure 5. according to temperature rmf and rwe correction (schlumberger, 1977). figure 6. determination of rmf/rwe based on self-potential (schlumberger, 1977). 22 syed bilawal ali shah, khaira ismail, wan zairani wan bakar 4. results and discussion 4.1 microfacies analysis and depositional environment the sakesar formation is comprised of light-medium grey to dark light grey, massive, nodular limestone and petrographically, it mainly comprises of wackestone depositional texture (shah, 2009; shah, 2023; shah et al., 2023). bioclastic wackestone (plate 1) limestone that makes up this microfacies is light grey, nodular, fractured, and medium to thick bedded (fig 7). the microfacies is characterized mainly by bioclasts, the orthochem of the microfacies is micrite matrix mainly. the orthochem micrite matrix exhibits a state of calm and low energy conditions. depositional environment the bioclasts of various echinoids and algae scattered in microscopic environments are the primary allochems of this microfacies. this microfacies is consistent with an inner ramp depositional environment based on the faunal composition and depositional texture of the wackestone. lockhartia rich mud wackestone (plate 2) it is made up of thickly bedded, cherty, nodular, light to dark grey limestone. the lockhartia-dominant allochems of larger benthic foraminifera, followed by the assilina species of larger benthic foraminifera, with echinoids and bioclasts of certain larger benthic foraminifera, are mostly what distinguish the microfacies. depositional environment the dominant orthochem of this microfacies is a micrite matrix, while the primary allochems include lockhartia species, assilina sp., bioclasts of echinoids, and rotalia, with larger benthic foraminifera being notable. lockhartia is indicative of inner ramp settings, whereas assilina is associated with mid-ramp environments, according to ghose (1977), racey (1994), and shah (2009). benthic foraminiferal wackestone (plate 3) this microfacies is composed of mediumto thick-bedded, light to pale grey nodular, and highly fractured limestone. the larger benthic foraminifera, rotalia species, and several echinoids comprise the majority of the allochems that characterize the plate 2 microfacies. additionally, bioclasts of several larger and smaller benthic foraminifera are included in the allochems of this microfacies. depositional environment the micritic covering on the skeletal grains, according to flugel et al. (2010), suggests deposition in the photic zone at a depth of less than 100 to 200 m. the predominance of larger benthic foraminifera, which are dispersed in micritic matrix, demonstrates very calm and low-energy conditions (fig. 8). 4.2 toc and rock-eval pyrolysis commonly, organic richness is represented by toc wt% (peters, 1986; peters and cassa, 1994). according to baker, (1979) and espitalié et al. (1977), for clastic rocks to be considered a source rock, it should have a minimum toc value of 1.0%. the sakesar formation sediments toc contents ranges from 1.2-1.67 wt%. these data indicates that the majority of the samples possessed fair to good potential as per toc results however toc itself is not an enough source to be relied on it only, as it is just a screening/initial method. additionally, the samples with thermal maturity would have greater original toc values than the toc values at this time since toc content declines with increasing thermal maturity (li et al., 2023; peters, 1986; su et al., 2023; wang et al., 2023; xu et al., 2022b). according to peters and cassa (1994), the s2 parameter, which is obtained during pyrolysis, is the most indicative measure for assessing the potential of hydrocarbons generation. according to baker, (1979) and espitalié et al. (1977), at least 5mg hc/g s2 is essential for good petroleum generation potential. generally, the hydrocarbon (s2) yields ranged from 3.6-5.25 mg/g in sakesar formation. s2 vs toc cross plot (fig. 9) indicated fair to good petroleum generation potential. figure 7. plates: microfacies of the sakesar formation in the studied area. plate 1 (a) bioclasts, (1b) micritized foraminifera, plate 2 (a) lockhartia, (b) assilina, plate 3 (a) bioclasts, (b) rotalia. figure 8. depositional model of sakesar formation on the basis of samples analysed. figure 9. plot comparing pyrolysis s2 values and total organic carbon (toc) to illustrate the potential of the analyzed samples as source rocks for generating hydrocarbons. 23integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan using the migration index (s1/toc), it is possible to differentiate between indigenous and migrated petroleum (peters, 1986), the examined samples were indigenous, as indicated by the migration index, s1 vs toc (fig. 10). figure 10. s1 vs toc plot for migration index. 4.2 kerogen type hi is the hydrogen index (mghc/g toc) which is used to characterize the origin of organic matter (baker, 1979). hi values for the sakesar formation ranged between 265-386 mg hc/g toc. for kerogen classification, samples pyrolysis data were used hi vs tmax plot. all examined sediments mostly have mixed type-ii/iii kerogen. the type of kerogen identified by s2 vs toc plots was also in agreement with the hi vs tmax plot (fig. 11). figure 11. hi vs tmax plot for kerogen type. 4.3 thermal maturity in this study, three maturity indicators were utilized for thermal maturity including tmax, vitrinite reflectance (%ro), and the production index. vitrinite reflectance values ranging 0.6–1.3%ro is mature, capable of generating oil (brooks and welte, 1984; tissot, 1984; waples, 1985). tmax values varied widely, with values from 435 to 465 °c reflecting type iii kerogen and values from 430 to 455 °c representing types i and ii (espitalié, 1977). the observed vitrinite reflectance values for the sakesar formation ranged from 0.72 to 0.81%, with a tmax of 438 to 443 °c, suggesting the peak of the oil generation window (table 3) (fig. 12). figure 12. plot of tmax versus vitrinite reflectance (ro) showing the maturity levels. the rock-eval tmax parameter is used to assess the maturity of kerogen in sedimentary rocks, which is crucial for hydrocarbon exploration, tmax represents the temperature at which maximum hydrocarbon generation occurs from kerogen during pyrolysis, a laboratory simulation of natural thermal degradation, as kerogen matures under geological conditions, its ability to generate hydrocarbons increases, reflected in a higher tmax value, therefore, a rising tmax indicates more mature kerogen, helping geologists identify potential hydrocarbon source rocks and their evolutionary stage ranging from immature, through the optimal oil window, to overmature gas-producing stages (baker, 1979; welte, 1972; waples, 1985). there are many factors that can influence the tmax according to espitalié et al. (1985) such as migrated hydrocarbons, a low pyrolysis response and oil-based mud. according to mahdi et al. (2022) and jarvie et al. (2001) samples have the highest tmax values when the s2 >0.50-mg/g rock. the analysed sediments had s2 >1-mg/g rock (table 3 and 4). the measured tmax values of the samples ranged from 438 ◦c to 443 ◦c. according to peters (1986) the results shows the presence of mature organic material. the maturity of organic matter can be assessed by comparing the amount of hydrocarbons it has already generated (s1) with its total potential hydrocarbon yield. this comparison involves calculating the production index (pi), which is the sum of s1 and the remaining potential hydrocarbons (s2), as described by collins (1975) and romanchev (1979). for type i and type ii kerogen pi value is more than 0.1, whereas the typical pi value range for type iii kerogen is between 0.1 and 0.2, the samples are considered immature if pi values are <0.15, the samples are considered mature if the pi values ranges between 0.15-0.4 and the samples are considered over-mature if the values are >0.4 (peters, 1986; bacon et al., 2000). 24 syed bilawal ali shah, khaira ismail, wan zairani wan bakar the pi values for the sediments from the sakesar formation in this study ranged from 0.17 to 0.33, thus indicating a maturity level reaching the oil window (fig.13), which is in agreement with the tmax and vitrinite reflectance data. figure 13. tmax vs production index (pi) plot. 4.4 reservoir potential the reservoir parameters determined from the petrophysical analysis were used for quantitative interpretation of the reservoir. the estimated petrophysical characteristics of the sakesar formation are shown in table 5. the average porosity was 9.12%, showing poor to moderate reservoir potential, whereas higher porosity is observed in the middle and top portion of the formation (8028-8113ft) where as low porosity is observed at the higher depth of the formation. according to rider’s (1986) shah and shah (2021) criteria, the average water saturation was 22.32%, and the hydrocarbon saturation was 77.68%, indicating an average to good hydrocarbon potential (table 1). table 5. various well logs calculated parameters for different properties estimation. d ep th te m p r h o b φ d sp φ (n .d ) r t ( l l d ) r w e r w sw sh l ith ol og y (f t) 8028 153 2.69 0.4 -25 0.22 610 0.4 0.7 24% 76% limestone 8056 154 2.72 0.3 -16 0.19 1230 0.5 0.8 37% 63% limestone 8084 154 2.65 0.1 -29 0.17 1520 0.4 0.7 26% 74% limestone 8113 155 2.62 0.2 -36 0.19 1840 0.3 0.3 16% 84% limestone 8141 156 2.53 0.1 -44 0.13 1610 0.3 0.6 23% 77% limestone 8169 156 2.59 0.3 -35 0.14 166 0.3 0.4 14% 86% limestone 8198 157 2.46 0.2 -32 0.11 890 0.3 0.4 17% 83% limestone 5. discussion 5.1 organic richness and rock-eval pyrolysis according to bordenave (1993), sediments must contain a significant quantity of organic richness, typically exceeding 2% total organic carbon (toc) by weight, for hydrocarbons to be produced when they attain thermal maturity. according to peters and cassa (1994), the total organic carbon (toc) is a fundamental geochemical marker that quantifies the weight percentage of organic materials present, as highlighted by peters (1986) and shah (2023). analysis of the samples reveals that most exhibit fair to good toc levels, with several displaying very good toc contents. to enhance our comprehension of the generative potential, it is essential to utilize total organic carbon (toc) data alongside other geochemical parameters, such as the s1 and s2 pyrolysis characteristics, as outlined by peters and cassa (1994) and dembicki (2009). consequently, to differentiate between non-indigenous and indigenous hydrocarbons, we employed both the s1 values and the toc weight percentages. analysis of the rock samples revealed that they consistently presented low s1 values and, conversely, high toc levels, signifying the presence of native hydrocarbon sources. the majority of the examined sediments include mixed type-ii/iii kerogen. s2 vs toc graphs and the hi vs tmax plot both correctly identified the kind of kerogen (fig. 11). as a result, the sakesar formation sediments have the potential to produce considerable amounts of hydrocarbons at the appropriate thermal maturities, as well as to contribute a significant amount to the oilfield’s main petroleum system. 5.2 microfacies analysis and depositional environment three microfacies have been identified based on the biogenic components, carbonate texture, and depositional fabric: bioclastic wacke-packstone (plate 1), lockhartia rich mud wackestone (plate 2), and benthic foraminiferal wackestone (plate 3). assemblage of rotalia, echinoids, algae, and associated bioclasts, comprise the majority of the fauna in these microfacies. on the basis of these microfacies a depositional model has been constructed (fig. 8). 5.3 petrophysical analyses petrophysical analysis through wireline logs were conducted using spontaneous potential (sp) method. the upper and middle part of the sakesar formation from ~8028-8113 ft depth generally exhibits good porosity, the average porosity at most of the interpreted points is also in good potential range >10%. according to amigun and odole (2013) and rider’s (1986) criteria (table 1), sakesar formation have good reservoir potential to produce hydrocarbons in balkassar oilfield with moderate porosity and good hydrocarbon saturation. 6. conclusions the investigation of the sakesar formation sediments through microfacies analysis, geochemical analysis, and petrophysical analysis has yielded several key findings: microfacies analysis: three distinct microfacies were identified based on biogenic components, carbonate texture, and depositional fabric in the sakesar formation at the tatral section: bioclastic wacke-packstone, lockhartia-rich mud-wackestone, and benthic foraminiferal wackestone. the microfacies of the sakesar limestone suggest that it was deposited in restricted inner ramp settings extending to the distal middle ramp. geochemical analysis: source rock analyses, including s2, hi (hydrogen index), and tmax values, indicate that the sakesar formations possess fair to good potential for hydrocarbon generation. the presence of mixed type ii/iii kerogen is suggested by the s2 and hi values. plots of s2 vs. toc (total organic carbon) and hi vs. tmax position the sediments within the early maturity window, implying that the sakesar formation may have generated oil and gas. 25integrated analysis of the eocene sakesar formation: depositional environment, microfacies, geochemistry, and reservoir characteristics in the potwar basin, pakistan petrophysical analysis: the sakesar formation exhibits average reservoir qualities but demonstrates good hydrocarbon saturation potential, indicating a promising capacity to produce hydrocarbons. these findings collectively enhance our understanding of the sakesar formation’s potential as a hydrocarbon source, underscoring the significance of integrated sedimentological, geochemical, and petrophysical analyses in evaluating 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filling materials, however, to date, a paucity of data exist on to the dynamic properties of this material. accordingly, a large number of dynamic triaxial tests under cyclic loads were carried out to quantify the suitability of remolded red clay as subgrade filling. several potentially influencing dynamic factors were considered, including dynamic stress, vibration frequency, consolidation confining pressure, consolidation ratio and compactness. plastic strain and dynamic strength curves of remolded red clays under varying dynamic loads and load histories have been developed, in addition to the inclusion of those influencing factors. test results show that within the range not exceeding the inherent strength of the test samples, increases soil compactness, confining pressure, and vibration frequency serve to enhance overall dynamic power in concurrence with retarding the development of accumulated plastic strain. conversely, an improvement in the amplitude of the dynamic stress and consolidation ratio was shown to cause a decrease in dynamic strength and acceleration in the development of accumulated plastic strain. an empirical equation relating critical dynamic strength and load histories of remolded red clay has been developed for the provision of fundamental reference data for future studies. las propiedades dinámicas de los materiales de base son factores cruciales que afectan la estabilidad en la ingeniería de tráfico. la arcilla roja reestructurada se utiliza frecuentemente como material de relleno de bases, sin embargo, hasta la fecha, existe una escasez de información sobre las propiedades dinámicas de este material. de acuerdo con esto, se realizó un gran número de pruebas triaxiales dinámicas bajo cargas cíclicas para cuantificar la pertinencia de la arcilla roja reestructurada como material de relleno en bases. se consideraron varios factores dinámicos que podrían ser determinantes, como la fuerza dinámica, la frecuencia de vibración, la presión de confinamiento, el índice de consolidación y la compactibilidad. se desarrollaron las curvas de fuerza plástica y dinámica de arcilla roja reestructurada con varias cargas dinámicas e historia de cargas, además de la inclusión de los factores determinantes. el resultado de las pruebas muestra que dentro del rango de la fuerza inherente a las muestras de estudio, el incremento de la compactibilidad del suelo, la presión de confinamiento y la frecuencia de vibración sirven para mejorar, en general, el poder dinámico al tiempo que retrasa el desarrollo de la fuerza plástica. al contrario, el mejoramiento de la amplitud de la fuerza dinámica y el índice de consolidación muestra una reducción en la fuerza dinámica y una aceleración en el desarrollo de la fuerza plástica acumulada. finalmente, se desarrolló una ecuación empírica que relaciona la fuerza dinámica crítica y las cargas históricas de arcilla roja reestructurada con el fin de proveer información de referencia para estudios futuros. abstract resumen keywords: accumulated plastic strain, dynamic triaxial tests, dynamic strength, remolded red clay. palabras clave: fuerza plástica acumulada, pruebas triaxiales dinámicas, fuerza dinámica, arcilla roja reestructurada. test study on the influences of dynamic stress and load history to the dynamic properties of the remolded red clay issn 1794-6190 e-issn 2339-3459 http://dx.doi.org/10.15446/esrj.v20n4.54400 pruebas sobre la influencia de la carga dinámica y la historia de carga en las propiedades dinámicas de arcilla roja reestructurada earth sciences research journal earth sci. res. j. vol. 20, no. 4 (december, 2016): g1 g8. jian li, shang-xiong chen, and ling-fa jiang state key laboratory of geomechanics and geotechnical engineering, institute of rock and soil mechanics, chinese academy of sciences, china xiaohongshan, wuchang, wuhan 430071, p. r. china author for correspondence: jian li (e-mail: 710171305@qq.com). g e o t e c h n ic s how to cite item li, j., chen, s. x., & jiang, l. f. (2016). test study on the influences of dynamic stress and load history to the dynamic properties of the remolded red clay. earth sciences research journal, 20(4),g1-g8 http://dx.doi.org/10.15446/esrj.v20n4.54400 record manuscript received: 26/11/2015 accepted for publication: 06/01/2017 h2 jian li, shang-xiong chen, and ling-fa jiang i. introduction red clays are widely distributed in southwest and south central regions of china (editorial of engineering geology manual, 2007); accordingly, red clay is frequently utilized as a subgrade filling for fiscal and environmental reasons. thus, studies of the mechanical properties of this subgrade encompass a high practical significance within traffic engineering. several previous studies have undertaken in-situ mechanical testing on red clay foundations, with results indicating significant variation with respect to both the bearing capacity and deformation modulus of red clay bases in concurrence with varying mechanical properties, water content, particle composition, occurrence conditions, soil structure and interior fracture, among other factors (long, chen, xiao, & peng, 2009; kong, luo, & yuan, 1995; kong & luo, 1993; bai, kong, guo & wan, 2010; bai and wan, 2010; peng, lin, wang, & wu, 2015). moreover, studies have shown that the aforementioned red clay characteristics may exert considerable effects on both the safety and suitability of the subgrade when used as a subgrade filling (editorial of engineering geology manual, 2007; kang et al., 2010). the remolding procedure is typically employed to overcome the inherently weak engineering characteristics of red clays. numerous previous studies have examined the static properties of remolded red clays. results indicate that the remolding procedure can effectively eliminate adverse design factors including cracking and uneven distribution, thus enhancing friction, bite force and bearing capacity (yang & lu, 2011; liang & mo, 2010; zhang, jiang, & yang, 2011; kaynia, madshus & zackrisson, 2000). however, within traffic engineering, the dynamic properties of subgrade materials are critical; to date, a paucity of data exist on the dynamic characteristics of remolded red clay, thus representing a significant knowledge gap in light of its frequent use as a subgrade material. the dynamic properties of geomaterials are directly related to dynamic loading which typically encompasses three elements, namely, waveform, amplitude, and frequency. previous investigations have shown that the dynamic loading borne by the subgrade is analogous to a sine wave pattern. thus, the sine wave has frequently been employed to simulate traffic loading within traffic engineering applications (kaynia et al., 2000; liu, 2010). the amplitude of dynamic loading exerts significant influence on the inherent dynamic properties of geomaterials, particularly those of saturated soils. an improvement in the magnitude of dynamic loading may result in significantly decreased dynamic subgrade strength, in concurrence with the accelerated development of accumulated plastic strain (li, 2013; chen, 2015). the influence of vibration frequency on the dynamic properties of geomaterials may be approximately divided into three types. firstly, the dynamic strength of the geomaterial may be improved, in concurrence with a retardation of the accumulated plastic strain due to an increased vibration frequency (duncan and chang, 1970; lin, 2011). secondly, the effect of vibration frequency on the dynamic properties of a geomaterial may not be visible (li, 2010; zhou, 1998); lastly, the dynamic strength and ability to resist deformation may be reduced due to an increase in associated vibration frequency (zhang, 2008). thus, an investigation of the dynamic characteristics of geomaterials under cyclic loading is of critical importance. in the current study, several tests have been designed and undertaken to examine the dynamic properties of remolded red clays. furthermore, previous investigations have shown that loading history exerts significant effects on the dynamic characteristics of geomaterials. an increased consolidation confining pressure has been demonstrated to improve the ability of geomaterials to resist external dynamic loading, in concurrence with reduced dynamic strain (lin et al., 2013; gu, wang, cai, yang & gao, 2012; zhou and gong, 2001). moreover, consolidation ratio also exerts a marked influence upon dynamic soil properties, albeit, with different types and magnitudes of influence associated with different geomaterials. for example, upon increasing the consolidation ratio within a sand or categorically coarse-grained soil, dynamic strength is observed to exhibit an initial increase, followed by a subsequent decrease. however, the associated turning points of various soils have been shown to vary (zhang, 1991; he, 1998; zhang, he, fei & gao, 2006; lunne, berre, andersen, strandvik, & sjursen, 2006; li, chen & jiang, 2015). conversely, an increase of the consolidation ratio of saturated soils reduces the associated dynamic strength (lunne et al., 2006); a significant consolidation ratio has been shown to promote the formation of a shear band due to the inherently low strength of saturated clays. similarly, the effect of compactness upon the dynamic strength of geomaterials is clearly evident; upon compaction, bulk density is increased, thus increasing dynamic strength and retarding the development of accumulated plastic strain (li, 2013; li, chen, & jiang, 2013). in the current study, a series of cyclic dynamic triaxial tests have been undertaken under varying load histories and dynamic loadings to investigate the dynamic properties of remolded red clay and the associated influencing factors. accumulated plastic strain and dynamic strength of samples have been evaluated and compared, leading to the development of predictive mathematical models about both dynamic characteristics. accordingly, the critical dynamic strength of remolded red clay is effectively examined, with empirical results presented for use in future engineering applications, thus representing significant utilities from both an academic and applied perspective. ii. dynamic triaxial tests of remolded red clay samples 1. dynamic triaxial apparatus the employed dynamic triaxial test used a computer-controlled electro-hydraulic servo dynamic triaxial test system (sdt-10), as shown in figure 1. summarily, the system comprises a microcomputer-controlled electro-hydraulic sdt test system host, a standard triaxial pressure cell, a set of electro-hydraulic servo exciters for effective application of confining pressure, a set of digital multi-channel electro-hydraulic servocontrolled systems and a hydraulic source. the test system is capable of providing a maximal axial dynamic load of 10 kn, while the triaxial pressure cell is associated with a maximum confining pressure of 1 mpa. deformation sensor precision is > 0.5%; the precision of measured strain is approximately 10-4. the provided axial and exciting lateral frequency is 0-5 hz, while control type may be stress-controlled. fi figure 1. sdt-10 dynamic triaxial test system employed in the current study. 2. test sample parameters the remolded soil sample used was obtained from an area adjacent to the interurban railway connecting wuhan and xianning. subsequently, the engineering properties and test parameters of the sample were confirmed with several laboratory analyses including grain size, liquid limit, plastic limit and compaction. grain size analyses indicate that grain sizes associated with the test samples were all < 0.075 and thus categorized as being finegrained; measured soil proportion was 2.7. determined liquid limit, plastic limit, and plastic index were 45%, 20.9% and 23.1%, respectively (table h3test study on the inf luences of dynamic stress and load history to the dynamic properties of the remolded red clay 1). accordingly, the study sample was shown to exhibit a relatively high plastic index, id est fine grain size in concurrence with high clay content. according to the ‘railway engineering geotechnical test regulations of china,’ the study sample is categorized as cohesive clay with a high liquid limit. compaction test results indicate an optimum moisture content of 20% and a maximum dry density of 1.616 g/cm3 (fig. 2.). 3. test program applied dynamic load amplitude must remain static to comprehensively analyze the developmental accumulated plastic strain and dynamic strength of remolded red clay under varying long term cyclic loads. firstly, varying load histories were applied to the study sample, after which different cyclic loads were applied; the dynamic mechanical properties of the test sample in the presence of varying load histories were continuously monitored; all undertaken analyses were stress-controlled. four influencing factors were considered during test procedures, namely compactness, confining pressure, consolidation ratio and vibration frequency. specific test programs are presented in table 2. table 1. the test result of particle analysis. note: < 0.05 mm, content: 90.7%; < 0.01 mm, content: 52.7%; < 0.002 mm content: 29.6%. figure 2. moisture content/dry density relation curve. 4. soil sample preparation and loading all analyses were undertaken by the ’geotechnical test regulations of china’ to ensure test data reliability. chiefly: (1). all soil samples were prepared using the same method. before dynamic loading, varying confining pressures and consolidation partial pressures were applied to soil samples until deformation conditions were reached (axial deformation <0.005 mm in 5 minutes). during the test procedure, varying cyclic loads were applied to soil samples in concurrence with continuous monitoring of dynamic σ–ε curves (fig. 3.). test termination occurred when vibration time reached 2000 times or accumulated plastic strain reached 15%. grain size/mm 0.5~0.25 0.25~0.075 0.075~0.005 < 0.005 content/% 57.4 42.6 table 2. short-time dynamic triaxial test program. figure 3. load waveform diagram of the grading short-time dynamic triaxial test. iii. test results and analysis 1. test curves of long-term cyclic loading tests 1.1. accumulated plastic strain test curves several accumulated plastic strain curves under different load histories were obtained (figs. 4-7). as shown, under relatively small dynamic loads, the accumulated plastic strain of the test samples developed rapidly during initial loading, followed by gradual stabilization due to soil compaction. under relatively high dynamic loading, accumulated plastic strain grows rapidly, exhibiting a more constant gradient during initial loading, with an elevated rate of development during later loading stages. upon exertion of larger dynamic loading values, developing standards during all loading stages were observed as being relatively similar, with soil deformation occurring fairly rapidly. 1.2. dynamic strength test curves dynamic strength of soils varies with loading rate and the cyclic impacting effects of dynamic stress. typically, dynamic stress is associated with a particular deformation criterion in concurrence with the impacting vibration times of dynamic loading (xie, 1986). in the current study, the definition of dynamic strength resulted in marked variation in parallel with diverse conditions. furthermore, dynamic strength is closely related to the cyclic vibration time and the deformation criteria. accordingly, accurate determination of deformation strain is necessary to an increased understanding of soil-related dynamic stress. the selected approach for determination of dynamic strength was when the accumulated plastic strain reached 5%. dynamic strength variation on remolded red clay samples under diverse conditions are presented in figures 8-11. as shown, under test id influencing factors compactness confining pressure consolidation ratio vibration frequency dt1-1 dt1-2 dt1-3 0.80 0.85 0.95 50 4.0 1 hz dt2-1 dt2-2 dt2-3 0.8 25 50 100 3.0/4.0 1 hz dt3-1 dt3-2 dt3-3 0.8 50 1.0 2.0 3.0 1 hz dt4-1 dt4-2 dt4-3 0.8 50 3.0 1 hz 2 hz 5 hz relation curve between water content and dry density 1.44 1.46 1.48 1.5 1.52 1.54 1.56 1.58 1.6 1.62 1.64 15.0% 16.0% 17.0% 18.0% 19.0% 20.0% 21.0% 22.0% 23.0% 24.0% water content d r y d e n s i t y g / c m 3 h4 jian li, shang-xiong chen, and ling-fa jiang variable test conditions, dynamic strength (σd) was observed to decrease non-parametrically with vibration time. under the effects of relatively large dynamic stress, soil deformation occurred rapidly in concurrence with reduced dynamic loading, gradually increasing deformation vibration time and a stabilizing relational curve. effectively, these conditions indicate that study samples exhibited a critical dynamic strength σcr. thus, soil deformation does not occur when the measured dynamic stress was <σcr. (a) compactness 0.8 (b) compactness 0.85 (c) compactness 0.9 figure 4. accumulated strain curves under varying values of compactness (kc=4.0, σ3=50 kpa, f=1 hz). (a) consolation ratio 2.0 (b) consolation ratio 3.0 (c) consolation ratio 4.0 figure 5. accumulated strain curves under varying consolation ratios (λc=0.80, σ3=50 kpa, f=1 hz). (a) confining pressure 50 kpa (consolation ratio 3.0) (b) confining pressure 100 kpa (consolation ratio 3.0) h5test study on the inf luences of dynamic stress and load history to the dynamic properties of the remolded red clay (c) confining pressure 25 kpa (consolation ratio 4.0) (d) confining pressure 100 kpa (consolation ratio 4.0) figure 6. accumulated strain curves under varying values of confining pressure (λc=0.80, f=1 hz). (a) vibration frequency 1 hz (b) vibration frequency 2 hz (c) vibration frequency 5 hz figure 7. accumulated strain curves under varying vibration frequencies (λc=0.80, σ3=50 kpa, kc=3.0). figure 8. dynamic strength results of remolded red clay samples (influenced by compactness). figure 9. dynamic strength results of remolded red clay samples (influenced by confining pressure). h6 jian li, shang-xiong chen, and ling-fa jiang compactness consolidation ratio confining pressure/kpa vibration frequency/ hz σcr /kpa 0.80 4.0 50 1.0 59.5 0.85 134.5 0.90 209.2 0.8 2.0 50 1.0 139.1 3.0 118.5 4.0 59.5 0.8 4.0 25 1.0 49.8 100 78.2 0.8 3.0 50 2.0 123.1 5.0 140.6 figure 10. dynamic strength results of remolded red clay samples (influenced by consolidation ratio). figure 11. dynamic strength results of remolded red clay samples (influenced by vibration frequency). additionally, results suggest soil dynamic resistance is improved upon enhancement of soil compactness, soil confining pressure and vibration frequency of dynamic stress. the association between measured consolidation ratio and dynamic strength was dissimilar to other influencing factors. varying factors were shown to exert different effects on the dynamic strength of study samples. for example, measured soil compactness had a highly significant effect on the measured dynamic strength of study samples, while other potentially influencing factors (e.g. confining pressure and vibration frequency) were shown to produce more subdued effects. overall, varying load histories demonstrated a marked impact on the dynamic strength of remolded red clays. 2. critical dynamic strength and equation development dynamic strength is a first soil parameter regarding dynamic loading to reflect and relate the development of soil deformation in concurrence with fluctuating strength. moreover, as previously outlined, a decreased dynamic stress is shown to be associated with and increased vibration time. consequently, below a specified level of dynamic stress, soil deformation does not occur, with this specified level of stress defined as the critical strength (σcr) (heath, waters, shenton & sparrow, 1972), as previously demonstrated (liu, 2010; li, 2010; zhou, 1998). based upon results and figures about dynamic stress obtained within the current study, characteristic σcr for test samples are presented in table 3. table 3. critical dynamic strength under varying conditions. while cyclic triaxial testing is the most frequently employed method for the determination of critical dynamic stress within a specified soil type or sample, its use on-site is problematic due to the necessarily high number of tests required for accurate approximation, the relatively high level of proof complexity and the potential presence of variable influencing factors. accordingly, in terms of practical application, a generalized empirical equation comprising compactness, confining pressure, consolidation ratio and vibration frequency has been developed via fitting analysis of calculated critical dynamic strengths. the suitable effect of the developed empirical equation was appropriately high, with an rsq of 0.98. due to influencing factors on the critical dynamic strength of test samples, in concurrence with inherent limitations associated with test methods employed, it is recommended that use of the presented equation be confined to conditions analogous to those maintained in its development. the developed empirical equation is as follows: (1) 3. influencing factor analyses of accumulated plastic strain and critical dynamic strength 3.1. accumulated plastic strain analysis εp is shown to be influenced to varying degrees by dynamic stress, compactness, confining pressure, consolidation ratio and vibration frequency based upon developmental curves of accumulated plastic strain (figs. 4-7); these variations are primarily due to interior soil conditions being differentially affected by dynamic stress and varying load histories. factors accelerating εp development: increased dynamic stress may accelerate intra-particle friction and promote the formation of sliding surfaces, thus accelerating εp development. εp may be accelerated via enhancement of the consolidation ratio due to increasing partial stress, in concurrence with an increased consolidation ratio and relatively large partial stress thus causing soil structure deformation, an inherently decreased dynamic strength and an acceleration of the development of εp. (ii) factors delaying εp development soil εp development may be retarded via increased soil compactness and confining pressure; the effect of varying soil compactness on observed εp is particularly significant. upon improvement of soil compactness and confining pressure, the void ratio is decreased, and friction between soil particles is increased, thus enhancing to soils ability to resist external dynamic loading. h7test study on the inf luences of dynamic stress and load history to the dynamic properties of the remolded red clay in the case of stiff remolded red clays, the development pattern of εp may also be effectively retarded, due to the combination of compaction over a short time and a significant vibration frequency. conversely, friction between soil particles will be increased in the presence of a high vibration frequency, resulting in increased resistance to external dynamic loading. (ii) influencing factor analysis of σcr as shown (figs. 8-11; table 3.), σcr is influenced to varying degrees by diverse loading histories; parametric associations exist between measured compactness, confining pressure, consolidation ratio, vibration frequency and critical dynamic strength of the remolded red clay samples tested in the current study. accordingly, an improvement in soil compactness, confining pressure and vibration frequency will result in an increased critical dynamic strength increase. conversely, an increased consolidation ratio, results in a decreased dynamic strength. it happens primarily due to the inherently increased soil density and a higher level of soil particle contact associated with increased soil compactness, thus leading to increased critical dynamic stress. increased confining pressure, also associated with higher levels of soil compaction, resulted in a lower level of dynamic strength increase. it is theorized that upon increasing vibration frequency, the contact area between soil particles is significantly increased in concurrence with increased intraparticle frictional forces, thus improving the soils’ ability to resist external loading. when consolidation ratio was increased, measured critical dynamic strength was decreased; the exertion of high large partial stress results in localized soil deformation, leading to a reduced level of soil dynamic strength. several large consolidation ratios were used to study the effect of the consolidation ratio on critical dynamic strength; an inversely proportional relation was found to exist between consolidation ratio and critical dynamic strength. in the case of consolidation ratios < 2.0, the authors recommend further analyses. iv. conclusions several dynamic triaxial tests were carried out on remolded red clays under varying load histories, thus permitting examination of the effects of dynamic stress and different load histories on the dynamic properties of these subgrade materials. based on test results, the following conclusions have been developed: (1) load records were shown to directly influence the dynamic properties of remolded red clays, with diverse load histories exhibiting equally different levels of influence. increased levels of compactness, confining pressure and vibration frequency, were shown to increase the test samples ability to resist external loading, thus retarding accumulated plastic strain development and improving the critical dynamic strength of the soil. due to inherent soil properties, the effects of compactness and confining pressure are especially significant. conversely, accumulated plastic strain development was accelerated in concurrence with decreased critical dynamic strength via improvement of the consolidation ratio, primarily due to the development of localized deformation and increased partial stress. (2) an increased vibrational frequency was shown to result in significantly improved dynamic strength and an increased ability to resist external loading within test samples. within categorically stiff unsaturated clays, the effects of increased vibrational frequency pore water pressure in soil are not significant. therefore, soil dynamic strength and modulus remain under the influence of friction between soil particles. based on analyses of the effects of varying load histories on the dynamic characteristics of remolded red clay, an empirical equation for relating critical dynamic strength to these load records has been developed and presented; this may be utilized for future engineering applications. acknowledgements this study was aided by financial support from the national natural science foundation of china (grant nos. 51027002), the natural science foundation of hubei province (grant nos. 2015cfb363) and the national science and technology support plan of china (grant nos. 2011bab10b02). references bai, w., kong, l., guo, a. g., & wan, z. 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(2001). strain degradation of saturated clay under cyclic loading. canadian geotechnical journal, 38(1), 208-212. doi: 10.1139/cgj-38-1-208 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n3.113933 earth sciences research journal earth sci. res. j. vol. 28, no. 3 (september, 2024): 305 323 g eo m o r ph o lo g y morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark josé luis sánchez-cortez1, 2 *, kathleen vélez-macías2,4, marco simbaña-tasiguano2,3, mariela rodríguez-calvopiña2,3, oswaldo fuentes-campuzano4, carolina freire-guerrero2 1. institute of geography, national autonomous university of mexico. scientific research circuit, university city. coyoacán, méxico, d. f. 2. napo sumaco geopark foundation, tena, ecuador 3. yachay tech university, hacienda san josé, urcuquí, ecuador 4. university of guayaquil, natural sciences faculty. juan tanca marengo avenue and raul gomez lince avenue. guayaquil, ecuador * corresponding autor: jsanchez@geografia.unam.mx how to cite item: sanchez cortez, j. l., velez-macias, k., simbañatasiguano, m., rodriguez-calvopiña, m., fuentescampuzano, o., & freire-guerrero, c. (2024). morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark. earth sciences research journal, 28(3), 305-323. https:// doi.org/10.15446/esrj.v28n3.113933 record manuscript received: 14/04/2024 accepted for publication: 01/10/2024 abstract karst morphology is a complex and highly systems and beneficial resource, it plays a fundamental role in geosystemic services and providing landscapes, water, energy and economic resources through tourism. karst geodiversity is one of the main geomorphological components of the napo sumaco aspiring unesco global geopark (nsaugg), through which education and conservation strategies are directed locally. the present study focuses on deepening the knowledge of the morphologies of karst origin in the geopark, specifically in the municipality of archidona (ecuador), with the characterization of the types of structures at the micro (between 1 and 100 cm) and mesoscale (between 1 and 1000 m), in five sites located within the municipality. the abundance of microscale geoforms are related to a limited evolution of karst morphologies that can be observe in the local amazonian karst system. furthermore, an inverse relationship is established between the relative values of intrinsic geodiversity (ig) and karst disturbance (kdi) levels, being that the sites with lower levels of impact or disturbance show a greater diversity of karst morphologies. in this socio-environmental dynamic, the role played by the napo sumaco aspiring unesco global geopark is fundamental, contributing to the strengthening of local capacities and the promotion of geoeducational and informative processes focused on the conservation of the karst region and its geodiversity in the municipality of archidona. keywords: geoconservation; geodiversity; intrinsic geodiversity; karst; napo sumaco geopark palabras clave: geoconservación; geodiversidad; geodiversidad intrínseca; karst; geoparque napo sumaco; caracterización morfológica de la geodiversidad kárstica en el municipio de archidona (ecuador), geoparque aspirante unesco napo sumaco la morfología kárstica es un recurso complejo, altamente sistémico y beneficioso, desempeña un papel fundamental en los servicios geosistémicos y proporciona paisajes, agua, energía y recursos económicos a través del turismo. la geodiversidad kárstica es uno de los principales componentes geomorfológicos del geoparque aspirante unesco napo sumaco (nsaugg), a través del cual se dirigen estrategias de educación y conservación a nivel local. el presente estudio se centra en profundizar en el conocimiento de las morfologías de origen kárstico en el geoparque, concretamente en el municipio de archidona (ecuador), con la caracterización de los tipos de estructuras a microescala (entre 1 y 100 cm) y mesoescala (entre 1 y 1000 m), en cinco sitios localizados dentro del municipio. la abundancia de geoformas a microescala se relaciona con una limitada evolución de las morfologías kársticas que se observan en los sistemas kársticos amazónicos locales. además, se establece una relación inversa entre los valores relativos de la geodiversidad intrínseca (ig) y los niveles de perturbación kárstica (kdi), siendo que los sitios con menores niveles de impacto o perturbación son los que presentan una mayor diversidad de morfologías kársticas. en esta dinámica socio-ambiental, el papel desempeñado por el geoparque aspirante unesco napo sumaco es fundamental, contribuyendo al fortalecimiento de las capacidades locales y a la promoción de procesos geoeducativos y divulgativos centrados en la conservación de la región kárstica y su geodiversidad en el municipio de archidona. resumen https://doi.org/10.15446/esrj.v28n3.113933 mailto:jsanchez@geografia.unam.mx https://doi.org/10.15446/esrj.v28n3.113933 https://doi.org/10.15446/esrj.v28n3.113933 306 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero 1. introduction karst landscapes are made up of a series of morphologies, due to a combination of chemical and physical processes, which cause the rock dissolution and chemical precipitation of carbonate (lewin and woodward, 2009; de waele, 2017; ford and williams, 2007; robledo et al., 2020), whose development is influenced by water regimes, lithological and structural characteristics, among other factors (andreychouk, 2016). karst provides a series of environmental and ecosystem assets, from water provision, thermal regulation, generation of habitats and unique ecological niches, to the provision of landscapes for recreational purposes (langer, 2020). cave and show caves is one of the main aspects linked to the use of karst; its social and economic implications generate a great community impact (cigna and burri, 2000; cigna and forti, 2013). however, given the anthropic pressures suffered by the resources associated with karst, the study of the conservation and threats of these systems is a widely developed topic at a global level (christman et al., 2005; sánchez-cortez et al., 2022b). karst systems represent very complex and sensitive natural systems (bahtijarevic, 1996; north, 2009; robledo et al., 2020), resulting from epigenetic processes that involve, in addition to the physicochemical conditions of the rocks, factors such as vegetation, thickness of overburden materials, and temperature of water and atmosphere (white, 1970). the set of karst morphologies can be divided by their location environment, being of subaerial (exokarst) and underground (endokarst) origin, which facilitates their classification and description in accordance with their scale (robledo et al., 2020). however, it is quite common for there to be polygenic morphologies that can complicate classifications (ford and williams, 2007; andreu et al., 2016). various morphologies at different scales are part of karst landscapes, and according to traditional geomorphology, they can be classifiable by their spatial and temporal scale (rivases, 2007). thus the sites defined at the microscale and mesoscale will be related to the quantity of time that the environment uses for the formation of a morphology (de boer, 1992; carvajal perico, 2012). however, beyond the scale used, the entire set of these landscape elements is part of the karst geodiversity of a territory. it is necessary to consider that the analysis of geological diversity consists of determining the varieties, frequency and distribution of geological elements within a territory (gray, 2004; gray, 2013), with intrinsic geodiversity as one of its main values (brilha, 2016; nieto, 2023). in this sense, was implemented a model to analyze the karst morphology diversity in carbonatics units in the area that includes the municipality of archidona, located in the province of napo, ecuador, within the napo sumaco aspiring unesco global geopark (nsaugg) (napo sumaco aspiring geopark, 2019), in order to expose and describe the intrinsic geodiversity at micro and mesoscale present in this area of geological and geomorphological importance (sánchez-cortez et al., 2023). the general hypothesis maintains that sites with greater karst morphology diversity are associated with a better state of conservation and lower anthropic pressure, according to the correlation of the data obtained by sánchez-cortez et al. (2022b). these assessments show heterogeneous diversity values in each of the sites analyzed, and it is considered that the characterization of the karst elements within the nsaugg will serve as a potential tool for geotourism practices, geoeducation and the promotion of geoconservation of the karst landscape. 2. materials and methods selection and description of the study area in the geological context of ecuador, geological formations that contain carbonate rocks are mainly concentrated in the amazon region (constantine et al., 2018), towards the east of the country, associated with the presence of a back-arc sedimentary basin, which develops as a result of transpressive stresses present from the terminal cretaceous and subsequent tectonic inversion (baby et al., 2014). these geodynamic conditions allowed the outcropping of carbonate rocks on the surface. the present study focuses on the municipality of archidona, province of napo, which houses a significant density of karst elements (besson et al., 1982). this municipality extends from the eastern andes mountain range towards the andean foothills or sub-andean zone (also known in ecuador as the upper amazon) (gad municipal de archidona, 2011). archidona has a total surface area of 3029 km2, of which 14.92% (452 km2) is defined as a karst area (sánchez-cortez et al., 2022b) (fig. 1). in this sector, outcrops of the napo formation (upper cretaceous) appear (fig. 2), which comprises limestone, sandstone and calcareous shale (baby et al., 2014). these materials correspond to the rocks where the main karst elements are generated in the study area. these limestones are associated with a shallow-marine deposit environment of a back-arc basin, and exhibit notable paleontological characteristics, highlighting the presence of fossils of ammonites, foraminifera and ostracods (sánchez-cortez et al., 2022a). the karst area of the municipality of archidona is part of the territory defined as napo sumaco aspiring unesco global geopark (nsaugg), whose extension is 1780 km2 (fig. 3). the nsaugg is considered a geodiversity hotspot in the amazon river basin (alsbach et al., 2024). its main geological and geomorphological interests are related to the karst reliefs and morphologies, as well as to the volcanic elements and processes inherent to the sumaco volcano (sánchez-cortez et al., 2023; vera et al., 2023). figure 1. municipality of archidona. sánchez-cortez et al. (2022b) previously defined the karst area. figure 2. geological map of the study area, which highlights the distribution of the napo formation (upper cretaceous), consisting of limestone, sandstone and calcareous shale. 307morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark figure 3. karst area in archidona and the reference of the napo sumaco aspiring unesco global geopark methodology for the definition and description of karst morphologies in the study area once the work area was delimited within the municipal jurisdiction of archidona and the napo sumaco aspiring unesco global geopark, five sites were established to carry out specific observations of the karst geoforms: 1) huasquila, 2) yanayaku, 3) sacha huagra, 4) urku mayan and 5) usayaku (table 1, fig. 1). a selective geomorphological survey or geomorphological mapping was applied, using a direct observation of the features of the karst morphologies (lahe, 1985; lugo hubp, 1988), and a cross-sectional geological mapping method (coe et al., 2010), along interpretive trails used within the nsaugg. each identified structure was described according to its conditions and specific characteristics. for the definition and description of karst morphologies, two spatial scales were considered, in accordance with those established by rivases (2017): geomorphological mesoscale and microscale, considering that this proposed spatial hierarchy is related to temporal scales. time in the geomorphological mesoscale is related to intercalated processes of tectonic and climatic origin, while time in the geomorphological microscale is defined by short processes with climatic and environmental origins (fernández, 2007; carvajal perico, 2012, pardo-igúzquiza et al., 2013). in addition to the genetic process that originated karst morphologies, microscale structures were standardized as centimetric (between 1 and 100 cm) and mesoscale structures as metric (between 1 and 1000 m); the scales fulfill the exclusive function of dimensioning the size of the inventoried morphologies. finally, for each of the five studied sites, an evaluation of intrinsic geodiversity was generated, which is established as the number of entities defined in a territory, or the number of morphologies observed within a specific area (1) (carcavilla, 2006; carcavilla et al., 2007). it has been considered that the greater the number of karst morphologies observed in a surface unit, the more diverse the area will be (kubalíková et al., 2023; najwer et al., 2023; 308 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero nieto, 2023). the assessment of intrinsic geodiversity was carried out to establish areas with greater geodiversity, which were correlated with the state of conservation and the uses of each site. ig= m / a 1) being: ig, intrinsic geodiversity; m, number of morphologies; and a, area. both micro and mesoscale geomorphological sites were considered to define the assessment of intrinsic geodiversity (ig) or abundance of karst morphologies. to evaluate the state of conservation, the study carried out by sánchez-cortez et al (2022) was used, in which the areas of greater or lesser affectation of the karst in the municipality of archidona are established using the karst disturbance index (kdi). finally, a simple relationship is created between the kdi values of each site, divided by the individual values of ig obtained. results and discussion inventory and description of karst morphologies the area of karst rocks located in the municipality of archidona, province of napo, ecuador, presents an important diversity of morphologies of karstic origin, and has an antecedent of many sites being used for the development of speleotourism (robledo and durán, 2011; cruz ramírez, 2014, sánchez -cortez et al., 2017, 2022a). the five sites used for data collection and observation, are places where tourist services are provided through interpretive trails and observation points, as part of the karstic relief geosite of napo sumaco aspiring unesco global geopark (nsaugg). in this sense, it is necessary to point out that the observations were made in the transects, trails and observation points that are normally used by the local guides of nsaugg, therefore is a high probability that the variety and diversity of karst morphologies will be very greater than that determined in this work, considering that a large part of the total areas of each site was not intervened. however, it is known that georoutes are established in the sectors where the most representative examples of the geodiversity and geoheritage of a territory exist (drapela, 2023). below, the morphologies observed at the sites huasquila (table 2), yanayaku (table 3), sacha huagra (table 4), urku mayan (table 5), and usayaku (table 6) are detailed. a total of 35 types of morphologies were observed during the information gathering, without making any distinction between their scale, corresponding to 11.11% of morphologies present in huasquila, 13.89% in yanayaku, 8.33% in sacha huagra, and finally 33.33% for urku mayan and usayaku. it is worth pointing out that the types of morphologies could be repeated from one site to another (for example, stalactites and stalagmites were observed in three of the five sites) and is not counted the number of elements belonging to the same type (for example, if ten stalactites were observed, it is only considered as a single type). table 1. the main characteristics of the observation sites established for the present study. sites area (km2) average altitude (m.a.s.l.) average precipitation (mm) geological formation lithology (strata type) temperature (°c) main activities developed in the area huasquila 1.10 825 3800 napo formation (k) calcareous sandstones, limestones and shales. 26 tourism and family farming. yanayaku 3.20 728 3800 napo formation (k) calcareous sandstones, limestones and shales. 28 tourism, animal husbandry and farming. sacha huagra 2.00 743 3800 napo formation (k) calcareous sandstones, limestones and shales. 26 tourism, animal husbandry and intensive farming. urku mayan 3.50 990 4500 napo formation (k) calcareous sandstones, limestones and shales. 23 tourism, family farming, sale of wood and forest resources usayaku 1.20 950 3800 napo formation (k) calcareous sandstones, limestones and shales. 24 tourism and family farming. 309morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark ta bl e 2 . m ai n ka rs t m or ph ol og ie s d efi ne d in h ua sq ui la si te k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re c av es m es os ca le c av iti es g en er at ed b y di ss ol ut io n as a re su lt of a ct iv e hy dr ol og ic al fl ow s. it ha s a le ng th c lo se to 2 50 m a nd 2 m w id e in a ve ra ge . m om m ilk m es os ca le w hi tis h su bs ta nc e fo un d in si de th e ca ve , w hi ch w he n ha rd en ed c ov er s t he w al ls o f t he c av e an d be co m es ag gr eg at es o f fi ne c ar bo na te c ry st al s. ri lle nk ar re n m ic ro sc al e d is so lu tio n fis su re s l ik e gr oo ve s, ge ne ra te d by th e flo w o f w at er o ve r t he ro ck , f ol lo w in g th e gr av ity gr ad ie nt . su bs oi l c up s m ic ro sc al e sm al l fi gu re s w ith v er y sm oo th sl op es , t he y ca n be c irc ul ar o r s em i-c irc ul ar , a nd h av e a di am et er n o la rg er th an 5 c m . i ts fo rm at io n is re la te d to g ra nu la r c al ca re ou s r oc ks . 310 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero ta bl e 3 . m ai n ka rs t m or ph ol og ie s d efi ne d in y an ay ak u sit e. k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re k am en itz a m ic ro sc al e an d m es os ca le sm al l o r l ar ge p ot -s ha pe d ca vi tie s, ch ar ac te riz ed b y th ei r c lo se d sh ap es , w hi ch a re d ev el op ed b y a m ix tu re o f di ss ol ut io n an d fr ic tio n pr oc es se s. c av es m es os ca le c av ity g en er at ed b y di ss ol ut io n as a c on se qu en ce o f a ct iv e hy dr ol og ic al fl ow s. th is c av ity is lo ca te d in ca lc ar eo us sa nd st on e ro ck s, it is 1 00 m lo ng a nd 7 m w id e on a ve ra ge . bo xw or k m es os ca le it is fo rm ed o n th e t op o f t he ca ve an d is ge ne ra te d by th e c ry sta lli za tio n of ca lc ite w ith in th e j oi nt s, lik e a re tic le . bo xw or k ap pe ar s w he n th e l im es to ne is er od ed b ef or e t he ca lc ite o f t he jo in ts. m om m ilk m es os ca le w hi tis h su bs ta nc e fo un d in si de th e ca ve , w hi ch w he n ha rd en ed c ov er s t he w al ls o f t he c av e an d be co m es ag gr eg at es o f fi ne c ar bo na te c ry st al s. st al ac tit es m ic ro sc al e k ar st m or ph ol og y w ith sh ap e of a n irr eg ul ar c on e, it s p ea k is o rie nt ed d ow nw ar ds . t he y fo rm fr om th e to p of c av es , a nd th ei r g ro w th is d ue to th e sl ow fi ltr at io n of w at er a nd c al ca re ou s m at er ia ls . 311morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark ta bl e 4 . m ai n ka rs t m or ph ol og ie s d efi ne d in s ac ha h ua gr a s ite . k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re sc al lo ps m ic ro sc al e a ls o kn ow n as ri pp le , t he y ar e w av elik e sh ap es th at a re sc ul pt ed in to th e w al ls o f s ol ub le ro ck s. th ey ha ve a v er y va ria bl e le ng th , f ro m 1 to 3 0 cm . t he sp ac e be tw ee n w av e an d w av e is m uc h de ep er a s t he w av e is w id er . g ri ke s a nd c lin ts m es os ca le d ee p di ss ol ut io n fis su re s i n th e fo rm o f g ro ov es p en et ra te e ve n im pe rm ea bl e st ra ta . t he g rik es p en et ra te be yo nd 1 0 m . o n to p, im pe rm ea bl e an d fla t s tra ta (s la bs ). ru nd ka rr en m ic ro sc al e an d m es os ca le so lu tio n ru nn el s f or m ed u nd er so il la ye rs . i ts e dg es a re ro un de d, a nd it s c ha nn el s a re u su al ly w id er a nd sh al lo w er th an ri lle nk ar re n. 312 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero ta bl e 5 . m ai n ka rs t m or ph ol og ie s d efi ne d in u rk u m ay an si te . k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re c an yo ns a nd n ar ro w va lle ys m es os ca le n ar ro w v al le ys w ith v er tic al w al ls a nd st ee p sl op es . l ow o r n on -e xi st en t h yd ra ul ic g ra di en t. pr es en ce of im pe rm ea bl e ro ck s b ed s. g ri ke s a nd c lin ts m ic ro sc al e an d m es os ca le d ee p di ss ol ut io n fis su re s i n th e fo rm o f g ro ov es p en et ra te e ve n im pe rm ea bl e st ra ta . o n to p, im pe rm ea bl e an d fla t s tra ta (s la bs ). ri lle nk ar re n m ic ro sc al e d is so lu tio n fis su re s l ik e gr oo ve s, ge ne ra te d by th e flo w o f w at er o ve r t he ro ck , f ol lo w in g th e gr av ity gr ad ie nt . c ol la ps e d ol in e m es os ca le si nk in g st ru ct ur e ge ne ra te d by th e pr es en ce o f t he c av ity , g en er at ed fr om th e bo tto m u p. t he lo ss o f co lla ps e m at er ia l h as c au se d an in cr ea se in it s l at er al sl op es . 313morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re c av es m es os ca le c av iti es g en er at ed b y di ss ol ut io n as a re su lt of a ct iv e hy dr ol og ic al fl ow s. it ha s a le ng th re ct ili ne ar cl os e to 3 00 m a nd b et w ee n 4 an d 5 m w id e. d ri p h ol es m ic ro sc al e ti ny c av iti es fo rm ed in p la ce s w he re d rip pi ng o cc ur s o r d ue to th e pr es en ce a nd c on ce nt ra tio n of or ga ni c m at te r. st al ac tit es m ic ro sc al e k ar st m or ph ol og y w ith sh ap e of a n irr eg ul ar c on e, it s p ea k is o rie nt ed d ow nw ar ds . t he y fo rm fr om th e to p of c av es , a nd th ei r g ro w th is d ue to th e sl ow fi ltr at io n of w at er a nd c al ca re ou s m at er ia ls . st al ag m ite s m ic ro sc al e k ar st m or ph ol og y w ith sh ap e of a n irr eg ul ar c on e, it s p ea k is o rie nt ed u pw ar ds . t he y fo rm o n th e flo or o f c av es , a nd th ei r s lo w g ro w th is re la te d to th e dr ip pi ng th at re ce iv e fr om th e to p of c av e fr om a st al ac tit e. 314 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re d ra pe ry m ic ro sc al e a re th in w av y sh ee ts o f c al ci te h an gi ng d ow nw ar d. it s d rip c an b e ve rti ca l o r o bl iq ue , f ol lo w in g th e sh ap e of th e co nt ou r o f t he c av es . so da s tr aw m ic ro sc al e c on si st o f v er y th in b ut lo ng st al ac tit es th at h av e a cy lin dr ic al sh ap e, u nl ik e th e co ni ca l s ha pe s o f st al ac tit es . k am en itz a m ic ro sc al e th ey a re sm al l o r l ar ge p ot -s ha pe d ca vi tie s, ch ar ac te riz ed b y th ei r c lo se d sh ap es , w hi ch a re d ev el op ed by a m ix tu re o f d is so lu tio n an d fr ic tio n pr oc es se s. 315morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark ta bl e 6 . m ai n ka rs t m or ph ol og ie s d efi ne d in u sa ya ku si te . k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re fi ss ur e c av es m ic ro sc al e sm al l d is so lu tio n ca vi tie s ( ap pr ox im at el y 30 c m ), de ve lo pe d on fi ss ur es in c al ca re ou s r oc ks . t he se fis su re s h av e be en w id en ed b y th e riv er c ur re nt , a s t he ri ve r l ev el ri se s. g ri ke s a nd c lin ts m es os ca le d ee p di ss ol ut io n fis su re s f or m in g gr oo ve s w id en ed b y th e ac tio n of th e riv er fl ow . i n th e up pe r p ar t, fla t s la bs a re fo rm ed a s p la tfo rm s ( lim es to ne p av em en t). c av es m es os ca le c av iti es g en er at ed b y di ss ol ut io n as a re su lt of a ct iv e hy dr ol og ic al fl ow s. it ha s a le ng th c lo se to 5 00 m an d 5 m w id e in a ve ra ge . 316 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re tr ay s m ic ro sc al e fl at st ru ct ur es , g en er at ed b y er os io n pr oc es se s w ith in th e ca vi tie s. th er e m ay b e ot he r s pe le ot he m s cr ys ta lli ze d on it s s ur fa ce . m am m ila ry m ic ro sc al e in th e ca ve s, ca lc ite c ru st s f or m ed o n th e to p an d on th e ha ng in g w al ls . t he y fo rm a t l ow w at er le ve ls an d by c on de ns at io n. g ou r t er ra ce m ic ro sc al e c ar bo na te c on cr et io n w ith sh ap e of sm al l d ik es o r d am s. th ey h av e ve ry g en tle sl op es a nd fl uc tu at in g w at er fl ow s. d ra pe ry m ic ro sc al e th in w av y sh ee ts o f c al ci te h an gi ng d ow nw ar d. it s d rip c an b e ve rti ca l o r o bl iq ue , f ol lo w in g th e sh ap e of th e co nt ou r o f t he c av es . st al ac tit es m ic ro sc al e k ar st m or ph ol og y w ith sh ap e of a n irr eg ul ar c on e, it s p ea k is o rie nt ed d ow nw ar ds . t he y fo rm fr om th e to p of c av es , a nd th ei r g ro w th is d ue to th e sl ow fi ltr at io n of w at er a nd c al ca re ou s m at er ia ls . st al ag m ite s m ic ro sc al e k ar st m or ph ol og y w ith sh ap e of a n irr eg ul ar c on e, it s p ea k is o rie nt ed u pw ar ds . t he y fo rm o n th e flo or o f c av es , a nd th ei r s lo w g ro w th is re la te d to th e dr ip pi ng th at re ce iv e fr om th e to p of c av e fr om a st al ac tit e. 317morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark k ar st m or ph ol og y sc al e d es cr ip tio n fi gu re st al ag na te (c ol um n) m ic ro sc al e an d m es os ca le st ru ct ur e cr ea te d by a m in er al p re ci pi ta tio n, fr om th e co nt ac t b et w ee n a st al ac tit e an d st al ag m ite . t he y ar e te nd en tio us ly c irc ul ar a nd c an g ro w h or iz on ta lly . ph yt ok ar st m ic ro sc al e ty pe o f k ar st ic st ru ct ur es g en er at ed b y bi ol og ic al e ro si on . k am en itz a m ic ro sc al e sm al l o r l ar ge p ot -s ha pe d ca vi tie s, ch ar ac te riz ed b y th ei r c lo se d sh ap es , w hi ch a re d ev el op ed b y a m ix tu re o f d is so lu tio n an d fr ic tio n pr oc es se s. 318 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero some of the particularities evidenced during the observation of morphologies consisted of establishing complex conditions for observing particular features of the karst, for example in sites such as sacha huagra, urku mayan and usayaku, aspects such as buried caves and terra rossa deposits were denoted, which it is also conditions typical of karst territories (palmer, 2012). however, are complex to locate their specific size or scale, or were also associated with other secondary morphologies, such as the case of burial marks (subsoil channels and subsoil dissection). the morphologies that are generated below the soil tend to be much more active in dissolution, considering that in addition to the action of acidic water, soil particles also work in the same or greater acidity condition (ginés et al., 2009). bioturbations are other elements present in these karst sites, whose biological nature makes their interpretation diffuse and confusing (fig. 4). figure 4. a) image of bioturbation in limestone rocks, urku mayan site. b) pinnacles buried by mass movements, sacha huagra site. on the other hand, establish segmentations or subdivision of karst morphologies by scale, beside to being a pragmatic form of classification, allows generating a better understanding of the geological, biological, environmental and climatic interactions (fernández, 2007; forrero-ospino and duartedelgado, 2019). if we analyze results tables 2 to 6, we observe that 62.6% of the morphologies correspond to microscale structures, while a range of 37.5% of morphologies are mesoscale, which may be related to the low evolutionary level of the systems karst shown in these study areas, as established by sánchezcortez et al (2022b). following this same line, fookes and hawkings (1988) and waltham and fookes (2003) mention that karst systems evolve from juvenile systems to mature and senile systems. if is considered that spatial scales are linked to temporal periods, having more elements of microscale morphologies in this inventory ratifies the youthful and young state of these karst systems. in this sense, the scales of observation of morphologies are fundamental to define these evolutionary levels. intrinsic geodiversity of karst morphologies and their conservation status geodiversity is recognized as the variety of geological and geomorphological characteristics and processes, in addition to soil and even hydrological events (gray, 2018), which entails some exceptional values such as tourist, educational and cultural use (caetano and ponciano, 2021), as well as the proportion of energy and nutrients (parks and mulligan, 2010). geodiversity is a concept designed to support sustainable development, because knowledge of it encourages optimization of the use of natural resources (kozlowski, 2004). all the elements to which geodiversity refers have existence values, or intrinsic values, starting from the philosophical vision that all things have value because they are useful to man, or provide services to society (brilha, 2016; nieto, 2023). in this strict sense, karst systems and their different elements are components of geodiversity, as well as the processes and resources that are generated from them. for the study and evaluation of geodiversity, there are different applicable methodologies, which depend on the depth of the study, its spatial dimensions and the availability of information (zwoliński et al., 2018). the assessment of intrinsic geodiversity (ig) or abundance of karst morphologies was established using the equation proposed by carcavilla (2006) and carcavilla et al (2007) (1), in which the quotient between the number of morphology types and the area is equal to the ig. the application of this formula allowed us to establish that the usayaku site obtained the highest value of intrinsic geodiversity relative to the other sites investigated, in turn sacha huagra was the site with the least representative value (table 7). table 7 details the intrinsic geodiversity values, along with the kdi assessments, as well as the state of disturbance of each site addressed in this work, highlighting that these data were previously collected in the research carried out by sánchez-cortez et al (2022b). the correlation of data highlights that those sites with the highest kdi correspond to the territories with the lowest intrinsic geological diversity in aspects related to karst morphologies (fig. 5), regardless of activities developed in each area. in this sense, the tourist and agricultural activities developed in each geosite seem not to directly interfere with the diversity of karst morphologies, but not with the state of conservation of the karst. on the other hand, the proximity to the municipal head of archidona is directly related to the decrease in the ig, as well as the increase in the kdi, considering that the anthropogenic pressure of cities is a natural enemy of geodiversity (clivaz and reynard, 2018) and karst systems (zhao et al., 2021). the simple relationship between the kdi and the ig allowed us to obtain the disturbance coefficient of intrinsic geodiversity in karst morphologies (table 5 and fig. 6), with which it was possible to establish, through data extrapolation, the areas where the morphologies karsts have a lower level of disturbance. the karst elements defined in this work do not really present a high degree of complexity; however, the relationship between karst morphological diversity and the state of conservation of the karst systems evaluated should be highlighted. table 7. main characteristics of the karstic areas, in which can be observed the assessments of intrinsic geodiversity and their comparisons with the karst disturbance index. high intrinsic geodiversity is correlated with low disturbance values. finally shows the disturbance coefficient of intrinsic geodiversity in karst morphologies, this coefficient is obtained from a simple quotient between the kdi and the ig. sites types of morphologies (m) area in km2 (a) intrinsic geodiversity (ig = m/a) karst disturbance index (kdi)* disturbance* main activities developed in the area * disturbance coefficient of intrinsic geodiversity in karst morphologies (kdi/ig) huasquila 4 1.1 3.64 20 minor affectation tourism and family farming 5.50 yanayaku 5 3.2 1.56 33.33 minor affectation tourism, animal husbandry and farming 21.33 sacha huagra 3 2 1.5 40.58 moderately affected tourism, animal husbandry and intensive farming 27.05 urku mayan 12 3.5 3.43 15.47 pristine tourism, family farming, sale of wood and forest resources 4.51 usayaku 12 1.2 10 20.51 minor affectation tourism and family farming 2.05 * the data have been obtained from sánchez-cortez et al. (2022b) 319morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark figure 5. a) map of the geodiversity distribution, based on values obtained by the intrinsic geodiversity of karst morphologies, extrapolated throughout the karst area. b) map with the distributions of kdi assessments of each site. the data has been extrapolated throughout the karst area (sánchez-cortez et al., 2022b). 320 josé luis sánchez-cortez, kathleen vélez-macías marco simbaña-tasiguano, mariela rodríguez-calvopiña, oswaldo fuentes-campuzano, carolina freire-guerrero figure 6. distribution map of disturbance coefficient of intrinsic geodiversity in karst morphologies. this coefficient is obtained from a simple quotient between the kdi and the ig. figure 7. a) main cave of the urku mayan site. b) speleothems present at the usayaku site, evidence of karst morphological diversity. c) signage present in the urku mayan conservation area, within the kichwa people of rukullakta reserve. d) vandalization of carbonate rocks at the yanayaku site. 321morphological characterization of karst geodiversity in municipality of archidona (ecuador), napo sumaco aspiring unesco global geopark the tourist use of these sites is a strong indicator of the potential that karst systems have as recreational elements, but also as an informative tool of the dynamics and geological and geographical evolution of the territory, as well as the effects as a result of global climate change (xiao and weng, 2007). the conservation policies of the sites studied are based on community systems, which are used as local tools to directly manage their territories, supported by the emotional and spiritual attachment to the land, and the sustainability of its resources, rooted in their indigenous worldview (johnson, 1992; rajasekaran, 1993; díaz, 2004; gómez peralta, 2005). napo sumaco aspiring unesco global geopark territory has constituted a joint workspace that over nine years has served as an area for research, geoeducation and geoconservation, in accordance with the principles established by unesco (2015), prior to the declaration of a global geopark. napo sumaco has a participatory management structure that promotes the development of the conservation of karst resources, and in general geological and geomorphological resources, through teaching-learning processes (fig. 7). to fulfill the role of geoheritage conservation, nsaugg is oriented towards strengthening and improving local capacities and the holistic dissemination of the territory (sánchez-cortez et al., 2023). conclusions in recent years, the concept of geodiversity has received special attention due to its close relationship with biodiversity and biogeography patterns (alsbach et al., 2024), in addition the conservation of geodiversity has gained value and relevance, due to its nature irrecoverable (gray, 2008, micheo et al., 2022). the application of local geodiversity indices or evaluations can be a valuable and complementary input when making decisions for the protection and conservation of fragile ecosystems. karst areas are sensitive and complex systems, however, in the case of the municipality of archidona, within napo sumaco aspiring unesco global geopark, the karst areas analyzed maintain an optimal level of conservation, as well as a variety of morphologies whose characteristics fit to the ecuadorian amazon karst. the work of the local communities in conjunction with the nsaugg has been fundamental in the process of conservation and preservation of the karst elements and other aspects, such as biodiversity and agrodiversity. the karst relief is a valuable geological heritage and an important tourist resource, for example, cigna and burry (2000), mention that worldwide there is an average of between 25 and 150 million caving tourists per year. furthermore, karst systems are recurring resources within geoparks, for example, in the case of geoparks in china, of a total of 41 unesco global geoparks, 10 have karst themes in their territory (wu et al, 2024), which demonstrates the specific potential of karst towards tourism through the interpretation of geological and geomorphological processes. however, in the case of nsaugg, caving tourism requires a major push that involves planning and in-depth local administrative processes. through this research it was possible to confirm that the sites with the highest intrinsic geodiversity (ig) of karst morphologies coincide with the karst areas considered to be less intervened and with less disturbance. furthermore, it was possible to corroborate in the study area that urban centers exert pressure on geodiversity resources, and that tourism is not an activity that directly influences the relative abundance of morphologies, and the levels of disturbance of the geoheritage, nevertheless, this affirmation still needs to be confirmed through the application of future research. finally, the contribution of this study is to promote awareness about the geosystemic services of karst environments at the population level, as a strategy for their preservation, contributing to the scientific-divulgate work that strengthens the nsaugg communities. fundings this research received no external funding. declaration of competing interest the authors declare no conflict of interest. acknowledgments thanks to the group of local guides yuyaiwa pushak runakuna, especially luis chacha (huasquila and saccha huagra), jimmy andy (urku mayan), rider alvarado (usayaku) and valerio grefa (yanayaku), who contributed their knowledge to the development of this research. also, to the napo sumaco aspiring unesco global geopark team for their contribution and support. references alsbach, c.m.e., seijmonsbergen, a.c. & hoorn, c. 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(editors). geoheritage, assessment, protection, and management. elsevier, 27-52. https://doi. org/10.1016/b978-0-12-809531-7.00002-2. https://doi.org/10.18814/epiiugs/2022/022001 https://doi.org/10.18814/epiiugs/2022/022001 https://doi.org/10.1016/j.ijgeop.2022.08.005 https://doi.org/10.57647/j.gcr.2023.0602.18 https://doi.org/10.57647/j.gcr.2023.0602.18 https://unesdoc.unesco.org/ark:/48223/pf0000234539 https://doi.org/10.1007/s12371-023-00792-2 https://doi.org/10.1007/s12371-023-00792-2 https://doi.org/10.1144/1470-9236/2002-33 https://doi.org/10.1007/s12371-024-00917-1 https://doi.org/10.1007/s12371-024-00917-1 https://doi.org/10.1016/j.jenvman.2006.07.016 https://doi.org/10.1016/j.jenvman.2006.07.016 https://doi.org/10.3390/ijerph18010093 https://doi.org/10.1016/b978-0-12-809531-7.00002-2 https://doi.org/10.1016/b978-0-12-809531-7.00002-2 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.112332 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 103 126 p a le o n to lo g y abstract a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus maría e. páramo-fonseca1*, cristian d. benavides-cabra1,2, renzo a. garavito-camacho1 1. departamento de geociencias, universidad nacional de colombia, bogotá, colombia, meparamof@unal.edu.co; cdbenavidesc@unal.edu.co; rgaravitoc@unal.edu.co 2. museo geológico nacional josé royo y gómez, servicio geológico colombiano, bogotá, colombia * corresponding author: meparamof@unal.edu.co record manuscript received: 02/01/2024 accepted for publication: 09/07/2024 una nueva especie de platypterygius (ophthalmosauridae) del barremiano inferior de colombia y evaluación de la composición específica del género how to cite this article: páramo-fonseca, m. e., benavides-cabra, c. d., & garavito-camacho, r. a. (2024). a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus. earth sciences research journal, 28(2), 103-126. https://doi.org/10.15446/esrj. v28n2.112332 in this paper, we describe a new ophthalmosaurid specimen found in the lower barremian beds of the paja formation at villa de leiva, boyacá, colombia. the specimen represents a new species of platypterygius, p. elsuntuoso sp. nov. defined by three unique features within the genus: a small ventral peg in the anterior ventral border of the extracondylar area of the basioccipital; a supratemporal groove in the quadrate; and a shallow neck at the base of the acellular cementum ring on the teeth root. in support of our taxonomic determination, in this contribution we also assess the species composition of the genus platypterygius, comparing the known anatomy of the type species p. platydactylus with that of the other species previously assigned to the genus. we found that a great morphological affinity in the forelimb of the species p. americanus, p. australis, p. hercynicus, and p. sachicarum (lower barremian to lower cenomanian) with that of the type species (aptian) is unquestionable and differentiate them from all other ophthalmosaurids. three new diagnostic features (synapomorphies of the genus) support their inclusion in platypterygius. consequently, the genus name “kyhytysuka”, recently proposed to differentiate p. sachicarum from the genus platypterygius is here rejected. a careful comparison of the cranial characteristics of platypterygius specimens preserving associated cranial and fin remains demonstrate that platypterygius is a genus distinguishable from other ophthalmosaurids not only by a typical forelimb but also by a particular combination of skull features. in this context, the skull morphology of “simbirskiasaurus” concurs with that defining platypterygius and its minor differences are not sufficient to support its generic distinction. the morphological comparisons and our phylogenetic analysis show the new species p. elsuntuoso more closely related to the colombian species p. sachicarum than to other species of platypterygius. given that the new species came from the lower barremian and p. sachicarum came from the upper barremian, the minor differences in dentition found between the two species suggest a speciation by adaptation to new food sources in the cretaceous epicontinental sea of colombia. keywords: ophthalmosauridae; platypterygius; barremian; colombia resumen en este artículo describimos un nuevo espécimen de ofthalmosáuridos encontrado en capas de la formación paja, en villa de leiva, boyacá, colombia. el espécimen representa una nueva especie del género platypterygius, p. elsuntuoso sp. nov., definida por tres características únicas dentro del genero: un pequeño proceso en el borde anterior ventral del área extracondilar del basioccipital; una hendidura supratemporal en el cuadrado; y un ligero cuello en la base del anillo de cemento acelular de las raíces de los dientes. como soporte a nuestra determinación, en esta contribución también evaluamos la composición específica del género platypterygius, comparando la anatomía conocida de la especie tipo p. platydactylus con la de las demás especies previamente asignadas al género. encontramos que hay una incuestionable afinidad morfológica entre la aleta anterior de las especies p. americanus, p. australis, p. hercynicus, y p. sachicarum (barremiano inferior al cenomaniano inferior) y la de la especie tipo (aptiano), que las diferencia de todos los demás oftalmosáuridos. tres nuevos caracteres diagnósticos (sinapomorfías para el género) soportan su inclusión en platypterygius. consecuentemente, el nombre genérico “kyhytysuka”, recientemente propuesto para diferenciar p. sachicarum del género platypterygius, es aquí descartado. una comparación cuidadosa de las características craneales de los especímenes de platypterygius que preservan la aleta anterior asociada demuestra que platypterygius es un género que se distingue de otros oftalmosáuridos no sólo por su aleta anterior típica, sino también por una combinación particular de rasgos craneanos. en este contexto, la morfología del cráneo de “simbirskiasaurus” coincide con la definida para platypterygius, y sus pocas diferencias no son suficientes para soportar una distinción genérica. las comparaciones morfológicas y nuestro análisis filogenético muestran a la especie nueva, p. elsuntuoso, está más cercanamente relacionada con la especie colombiana p. sachicarum que con las demás especies de platypterygius. dado que la nueva especie proviene del barremiano inferior y p. sachicarum del barremiano superior, las diferencias menores que encontramos entre sus denticiones sugieren un proceso de especiación por adaptación a nuevas fuentes de alimento en el mar epicontinental cretácico de colombia. palabras clave: ophthalmosauridae; platypterygius; barremiano; colombia https://doi.org/10.15446/esrj.v28n2.112332 mailto:meparamof@unal.edu.co mailto:cdbenavidesc@unal.edu.co mailto:rgaravitoc@unal.edu.co mailto:meparamof@unal.edu.co https://doi.org/10.15446/esrj.v28n2.112332 https://doi.org/10.15446/esrj.v28n2.112332 104 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho 1. introduction the barremian-aptian beds of the arcillolitas abigarradas member of the paja formation are known for preserving abundant specimens of marine reptiles in the villa de leiva-sáchica-sutamarchán region (cadena, 2015; cadena et al., 2019; cadena & parham, 2015; cortés et al., 2019; cortés & páramo-fonseca, 2018; gómez-pérez & noè, 2017; maxwell et al., 2016; 2019; páramo-fonseca, 2015; páramo-fonseca et al., 2016; 2018; 2019a; 2019b; 2021). numerous ichthyosaur specimens have been found in this region, most of which are currently under study. two ophthalmosaurid taxa have been described so far: platypterygius sachicarum páramo, 1997, recently placed under “kyhytysuka” sachicarum by cortés et al. (2021) and known from two specimens collected from upper barremian beds (benavides-cabra et al., 2023; maxwell et al., 2019; páramo, 1997) and muiscasaurus catheti maxwell et al., 2016, known from two specimens collected from upper aptian beds (benavides-cabra et al., 2023; maxwell et al., 2016; páramo-fonseca et al., 2021). a new ophthalmosaurid specimen from the lower barremian showing great affinities with the platypterygius huene, 1922 morphology is here described. it consists of an almost complete skull, disarticulated axial remains and scarce appendicular elements. the name platypterygius was coined by huene (1922) to receive ichthyosaurus platydactylus (broili, 1907), known from a single specimen. however, the type specimen of platypterygius platydactylus was lost in world war ii (kolb & sander, 2009) and the only available information from the holotype consists of the description and drawings of broili (1907). after the revision of cretaceous ichthyosaurs by mcgowan (1972), platypterygius became the sole genus identified from cretaceous rocks, creating the impression of low ichthyosaur diversity during the cretaceous (fischer et al., 2014a). recently, the study of new material and the review of cretaceous ichthyosaur collections have led to reevaluate this low diversity (druckenmiller & maxwell, 2010; fischer, 2012; 2016; fischer et al., 2011; 2012; 2014a; 2014b; 2014c; 2016; maxwell et al., 2016; 2019; maxwell & caldwell, 2006a; 2006b; páramo-fonseca et al., 2021) and have also led some authors to contest the stability and taxonomic validity of the genus platypterygius (fischer, 2012; 2016; fischer et al., 2014a; 2014c; 2016; ji et al., 2016; maxwell et al., 2019; moon, 2019; zammit, 2011). in this context, the taxonomic placement of the new specimen here described required a revision of the morphology of the species included in the genus platypterygius. as a result, in this contribution we add our assessment of the species composition of the genus and its diagnostic features. 2. geographic and geological context the specimen here described was found in an ex-situ concretion in la cabrera hill, northwest of villa de leiva (boyacá, colombia) where the arcillolitas abigarradas member of the paja formation outcrops (fig. 1a, b). its exact geographic provenance is unknown, so it was not possible to determine its precise stratigraphic position. however, numerous ammonoid specimens were extracted from the same concretion and some of them have been preliminarily identified as acanthoptychoceras sp. manolov, 1962 (benavides-cabra et al., 2023) (fig. 1c). following the barremian ammonite zonation proposed by patarroyo (2000; 2004), this genus is included in the nicklesia pulchella biozone, which according to this author represents the lower barremian in colombia. the paja formation was described in the villa de leiva-sáchicasutamarchán region by etayo-serna (1968a). it is a predominantly argillaceous unit, from which this author recognized three different lithostratigraphic units, which he named from base to top: arcillolitas negras, arcillolitas abigarradas and arcillolitas con nódulos huecos. the new ichthyosaur specimen most likely comes from the middle unit, the arcillolitas abigarradas member, which according to etayo-serna (1968a) represents the barremian-aptian in the villa de leiva sáchica sutamarchán region. this member is composed of gypsumrich, variegated mudstones containing abundant calcareous concretions, interbedded with thin calcareous, marly, and sandy levels (etayo-serna, 1965; 1968a). based on the ammonoids associated with the specimen here described, we estimated the stratigraphic origin of the ichthyosaur specimen from the lower part of the arcillolitas abigarradas member of the paja formation (sensu etayoserna, 1965; 1968a; 1979) (fig. 1b), more precisely from the segment “a” or “b” of etayo-serna (1968a; 1979) or segment “i” of forero and sarmiento (1985). the lithologic succession of the arcillolitas abigarradas member has been interpreted as tidal plain salt deposits in an environment protected from the influence of the open sea (forero & sarmiento, 1985). this member is a rich fossiliferous unit containing abundant ammonoids (etayo-serna, 1965; 1968a; 1968b; 1979; hoedemaeker, 2004; patarroyo, 2000; 2004; 2009; 2020) and well-preserved remains of plants, bivalves, fish, marine reptiles and dinosaurs (cadena, 2015; cadena et al., 2019; cadena & parham, 2015; carballido et al., 2015; carrillo-briceño et al., 2019; cortés et al., 2019; 2021; cortés & páramo-fonseca, 2018; gómez-pérez & noè, 2017; huertas, 1970; maxwell et al., 2016; 2019; noè & gómez-pérez, 2022; noè & gómez–pérez, 2020; páramo-fonseca, 2015; páramo-fonseca et al., 2016; 2018; 2019a; 2019b; 2021; schultze & stöhr, 1996; van waveren et al., 2002). figure 1. geographic and stratigraphic provenance of the specimen fcg-cbp-28. a, geological map showing the finding site (ichthyosaur silhouette) (modified from etayo-serna, 1968a). cartographic base: instituto geográfico agustín codazzi. b, generalized stratigraphic section of the paja formation cropping out around villa de leiva sáchica sutamarchán region, showing the stratigraphic origin of the specimen (ichthyosaur silhouette) (modified from etayo-serna, 1979). c, lateral and ventral views of the ammonoid acanthoptychoceras sp. extracted from the concretion bearing the ichthyosaur specimen. scale bar = 10 mm. 3. materials and methods the new specimen (fcg-cbp-28) was collected in 1999 by the fundación colombiana de geobiología (fcg). it is currently housed in the collection of the centro de investigaciones paleontologicas (cip). the fossil was extracted in six fragments forming a large concretion. it was prepared by m. l. parra-ruge and j. d. parra-ruge, from the fundación colombiana de geobiología and centro de investigaciones paleontológicas (cip) during 20002007, and recently, some preparation details were done by f. parra-ruge from the cip. the preparation was done through mechanical and chemical methods following the techniques developed by the fcg (padilla et al., 2010). the specimen consists of a skull, some axial elements including the atlasaxis and a few cervical vertebrae, the left coracoid, and a few phalanges. the bones of the right side of the skull are well-preserved, contrasting from the bones of the left side of the skull and those of the occipital region, which are displaced from their original position. thus, the description of the skull is primarily based on its right side. the descriptive drawings were based on photographs and direct observations on the fossil and the final figures were made with the open-source software inkscape 1.2. the areas of the sclerotic ring and the orbit were calculated on photographs using the measurement tools included in the inkscape software and were then used to estimate the ontogenetic stage of the individual (following fernández et al., 2005). a 3d photogrammetric model of 105a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus the basioccipital is provided supporting the anatomical description of the specimen (see supplemental data 1). the model was produced by h. d. palma-castro from departamento de biología of the universidad nacional de colombia. it was obtained from raw 18 megapixels (5184 x 3456 px) images using high-resolution structure from motion techniques. the basioccipital was photographed with a canon eos rebel 4000d camera and processed using 779 pictures. photos were taken surrounding the basioccipital at two different angles to ensure the maximum possible resolution and coverage. the model has been scaled using two scale bars located in each image. the software agisoft metashape professional 1.5.1, meshlab v2022.02, and blender 3.3.1 were used to obtain the 3d model. we conducted the morphological comparisons of the new specimen with the cretaceous and late jurassic ophthalmosaurids, as well as the assessment of the genus platypterygius, based on published descriptions and direct observations on the colombian material. we performed a phylogenetic analysis to determine the position of the new specimen within ophthalmosauridae. we based our analysis on the updated dataset of barrientos-lara and alvaradoortega (2021b), which itself is a modified version of the dataset published by barrientos-lara and alvarado-ortega (2020). we scored m. catheti (cip-fcgcbp-74; fcg-cbp-16) and p. sachicarum (don-19671) based on the scoring provided in páramo-fonseca et al. (2021) and on personal observations. we scored the specimen cip-ga-01042014, referred by maxwell et al. (2019) to p. sachicarum, and the specimen fcg-cbp-87 referred by cortés & páramofonseca (2018) to platypterygius sp., based on the authors’ description and personal observations. furthermore, we added the following otus: jabalisaurus meztli barrientos-lara & alvarado ortega, 2021a based on the scoring of barrientos-lara & alvarado-ortega (2021a); pervushovisaurus bannonvkensis arkhangelsky, 1998, based on the scoring and description of fischer et al. (2014c); plutoniosaurus bedengensis efimov, 1997 based on the scoring provided by zverkov & jacobs (2020) and on the publication of efimov (1997); thalassodraco etchesi jacobs & martill, 2020 based on the scoring and description provided by jacobs & martill (2020); sumpalla argentina campos et al. 2021, based on the scoring and description provided by campos et al. (2021); and catutosaurus gaspariniae fernández et al., 2021 based on the scoring and description provided by fernández et al. (2021). additionally, we modified the scoring of ophthalmosaurus seeley, 1874 based on the scoring revision of páramo-fonseca et al. (2021). we also revised the scoring of p. americanus (nace, 1939), p. platydactylus, p. hercynicus kuhn, 1946, p. australis (m’coy, 1867), and “simbirskiasaurus” birjukovi ochev & efimov, 1985 based on the published descriptions of these taxa (broli, 1907; fischer, 2012; fischer et al. 2014c; kear, 2005; kolb & sander, 2009; kuhn, 1946; maxwell & kear, 2010; romer, 1968; wade, 1984; 1990; zammit et al., 2010). for the upper jurassic boreal realm ichthyosaurians we used the names proposed by zverkov & efimov (2019) and zverkov and prilepskaya (2019). we modified one character, removed two and added two new characters to this dataset (see supplemental data 2). the scoring of these new characters was based on the published descriptions and illustrations of the otus involved in the analysis (see supplemental data 2: table s1-s2). the new data matrix was compiled using the mesquite software 3.51 (maddison & maddison, 2018) (see supplemental data 3). the cladistic analysis was run with 52 (otus) and 145 characters, using tnt (v1.6) (goloboff & morales, 2023). all characters were treated as unordered and were equally weighted. an initial exploration for the shortestlength tree islands was performed using the new technology search options (ratchet + drift + tree fusing) with 200 ratchet iterations (1 random seed) and a 99,999 trees limit. the resulting trees were then used as the starting point for a tree bisection and reconnection (tbr) branch swapping algorithm. a strict consensus was obtained and the iterpcr algorithm was applied to identify unstable otus (pol & escapa, 2009). the retention (ri) and consistency (ci) indexes (farris, 1989) were calculated, and bremer support values (bremer, 1994) were computed for the nodes in the consensus tree using the tnt tools. finally, the resulting strict reduced consensus tree was time calibrated using the timepaleophy function of the package paleotree 3.4.5 (bapst, 2012) in r 4.2.3 (r core team, 2022), implementing mbl calibration with a minimum branch length of 1 ma, and plotted using the geoscalephylo function from the r package strap 1.6-0 (bell & lloyd, 2015). institutional abbreviations: cip, centro de investigaciones paleontológicas, villa de leiva, colombia; qm, queensland museum; smss, stadtisches museum schloss salder, salzgitter, germany; uw, university of wyoming, laramie, wyoming, usa. the genus platypterygius the name platypterygius was created by huene (1922) to distinguish the cretaceous longipinnate species, ichthyosaurus platydactylus, from the cretaceous latipinnate species, which he grouped within the genus myopterygius huene, 1922. huene (1922) defined platypterygius as an ichthyosaur with a large skull having a short postorbital region, long body, short tail anterior to the vertebral inflexion, short caudal fin, bifurcated ribs, humerus with two distal facets, longipinnate forelimb with great enlargement of the paddle by the presence of sesamoids in both edges of the paddle, and hind fin apparently not so small compared to the forelimb, as in other derived ichthyosaur genera. the type species p. platydactylus was described by broili (1907) as ichthyosaurus platydactylus based on an aptian specimen from germany. the type specimen, a near complete skeleton with crushed skull and a well-preserved forelimb, was lost during world war ii (kolb & sander, 2009), but a description and illustration were published by broili (1907). both the generic and specific epithets of the type species p. platydactylus, emphasize the enlargement of the forelimb by hyperdactyly (broili, 1907; huene, 1922). the forelimb morphology of the type species was the base for subsequent assignment of other species to the genus platypterygius. kuhn (1946) proposed and described p. hercynicus based on a skeleton including cranial and appendicular remains collected from the aptian of germany. mcgowan (1972), comparing cranial and postcranial remains, in addition to the type species and p. hercynicus assigned to platypterygius the following species: ichthyosaurus campylodon carter, 1846 from the cenomanian of england (carter, 1846; fischer, 2016), ichthyosaurus australis from the albian of australia (jack & etheridge, 1892; kear, 2005; longman, 1922; m’coy, 1867; wade, 1984; 1990), myopterygius americanus from the albian and lower cenomanian of north america (maxwell & kear, 2010; nace, 1939; romer, 1968), myopterygius kiprijanoffi romer, 1968 from the lower cretaceous of russia (kiprijanoff, 1881), and myobradypterygius hauthali huene, 1927 from the barremian of argentina (fernández & aguirre-urreta, 2005; huene, 1927). later, arkhangelsky et al. (2008) erected platypterygius ochevi arkhangelsky et al. 2008 based on a nearly complete forelimb and fragmentary cranial and postcranial remains from the albian-cenomanian of russia. cranial remains without preserved forelimb were subsequently assigned to this genus. maisch & matzke (2000) assigned to platypterygius the species “simbirskiasaurus” birjukovi and pervushovisaurus bannovkensis, both known from cranial remains from the cretaceous of russia. páramo (1997) defined p. sachicarum from a skull found in the upper barremian of colombia (benavides-cabra et al., 2023; páramo, 1997). in revision works of ichthyosauria maisch & matzke (2000) synonymized longirostria arkhangelsky, 1998, tenuirostria arkhangelsky, 1998, and plutoniosaurus efimov, 1997 under platypterygius, and mcgowan & motani (2003) placed p. hercynicus under p. platydactylus and p. kiprijanoffi as nomina dubia. later, p. hercynicus was revalidated (fischer, 2012; kolb & sander, 2009), p. birjukovi was reassigned to “simbirskiasaurus” birjukovi (fischer et al., 2014c), p. campylodon was assigned to pervushovisaurus campylodon (fischer, 2016), p. ochevi was synonymized under maiaspondylus cantabrigiensis (lydekker, 1888; zverkov & grigoriev, 2020), and p. sachicarum was assigned to a new genus “kyhytysuka” (cortés et al., 2021). zverkov & efimov (2019) used plutoniosaurus bedengensis as a valid taxon and provided a scoring in their phylogenetic analysis. however, no detailed description or illustrations of this taxon have been published to date. here we review the definition of the genus platypterygius based on the information available on the morphology of the type species and species assigned to it. from such a revision, we discuss and show that “s.” birjukovi and “k.” sachicarum concur with the definition of platypterygius, whereas “p.” hauthali does not. the forelimb of platypterygius the forelimb morphology of the type species of platypterygius, p. platydactylus, is crucial in the definition of the genus. in fact, some characters from the forelimb that broili (1907) employed to define the type species 106 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho (humerus widened proximally and distally with a strong trochanteric ridge, proximal carpal row in addition to radiale, intermedium and ulnare with radial and ulnar sesamoids, and fin remarkably broadened by hyperdactyly) have been subsequently considered diagnostic for the genus platypterygius (bardet, 1989; huene, 1922; kolb & sander, 2009; maisch & matzke, 2000; mcgowan, 1972; mcgowan & motani, 2003; wade, 1984). some other characters of the forelimb of the type species have also been reiteratively included in the published diagnoses of platypterygius: intermedium not contacting the humerus (bardet, 1989; kolb & sander, 2009; mcgowan, 1972; mcgowan & motani, 2003); digital count not less than 7 (bardet, 1989; kolb & sander, 2009; mcgowan, 1972; mcgowan & motani, 2003); and phalanges thick, characteristically rectangular and closely fitting, except in the distal fin tip (bardet, 1989; kolb & sander, 2009; maisch & matzke, 2000; mcgowan, 1972; mcgowan & motani, 2003). in the type species, p. platydactylus, the forelimb is tridactyl (three primary digits) (bardet, 1989; mcgowan, 1972); the intermedium is roughly pentagonal with a pointed proximal edge fitting between the radius and ulna, almost parallel anterior and posterior sides contacting radiale and ulnare, and longer distal side that articulates almost exclusively with a single distal carpal (distal carpal 3); the radiale and ulnare articulate mainly with a single distal carpal distally; and the arrangement between the proximal and distal carpals in the 2, 3 and 4 digits is in a near straight line (see fig. 2a and broili, 1907). the described carpal structure is also present in p. americanus, (maxwell & kear, 2010; mcgowan, 1972; nace, 1939), p. australis (wade, 1984; zammit et al., 2010), p. hercynicus (kolb & sander, 2009; kuhn, 1946) and in two colombian specimens preserving forelimb remains (fcg-cbp-87 and cipga-01042014) recently described and assigned to platypterygius sp. and p. sachicarum respectively (cortés & páramo-fonseca, 2018; maxwell et al., 2019). no other ophthalmosaurid for which the carpal region has been described shows this carpal structure (see fig. 2). differing from the above described platypterygius carpal morphology, a rhomboidal or hexagonal intermedium articulating with two distal carpals is found in malawania fischer et al., 2013, sumpalla, catutosaurus, sveltonectes fischer et al., 2011, ophthalmosaurus, and “p.” hauthali (fig. 2s-x) (for references see fig. 2); an intermedium contacting the humerus is found in otschevia efimov, 1998 spp., brachypterygius huene, 1922, parrassaurus barrientos-lara & alvarado-ortega, 2021b, and aegirosaurus bardet & fernández, 2000 (fig. 2y-f’) (for references see fig. 2); and an intermedium with a larger facet for the distal carpal 2 or 4 is found in undorosaurus efimov, 1999 (fig. 2g-i) and maiaspondylus maxwell & caldwell, 2006a spp. (fig. 2k-m) (for references see fig. 2). moreover, the forelimb of p. platydactylus, p. hercynicus, p. australis, p. americanus and p. sachicarum shares the following combination of characters not found in other ophthalmosaurids (for comparisons and references see figure 2): the articular facets for the ulna and the radius on the humerus are distally oriented, almost in the same plane, with an angle between them of near 170° (also found in gengasaurus paparella et al., 2016 and “p.” hauthali); the humeral facet for the ulna is slightly longer than that for the radius (kolb & sander, 2009; mcgowan & motani, 2003) (also found in parrassaurus, arthropterygius lundi roberts et al., 2014; and aegirosaurus); the facets for extrazeugopodial elements in the humerus are small, sometimes hardly noticeable (also found in brachypterygius); the digital count is not less than 7 (also in maiaspondylus lindoei and caypullisaurus fernández, 1997; and plutoniosaurus see efimov, 1997); and the forelimb phalanges are rectangular and closely fitting (also in sveltonectes, caypullisaurus maiaspondylus spp. and aegirosaurus) (for references see figure 2). “p.” hauthali, is only known almost exclusively by anterior appendicular elements. recently, campos et al., 2024 reviewed the holotype and referred material of “p.” hauthali and revalidated the genus myobradypterygius huene, 1927 for the reception of the materials previously assigned to “p.” hauthali. in summary, the great morphological affinity of the forelimb of the lower barremian to lower cenomanian species p. americanus, p. australis, p. hercynicus and p. sachicarum with that of the type species p. platydactylus, is unquestionable. this morphology differentiates these species from all other ophthalmosaurids, supporting their inclusion in the genus platypterygius. minor variations in the size of the humeral facets for extrazeugodial elements and in the size or morphology of the zeugopodial, carpal, and distal carpal elements, support the distinction of different species within the genus (broili, 1907; mcgowan, 1972, zammit et al., 2010, kolb & sander, 2009; maxwell et al., 2019) (fig. 2a-f). the skull of platypterygius as discussed above, all cretaceous ophthalmosaurids differ from platypterygius by their forelimb morphology. nevertheless, no forelimb remains are known in some specimens assigned to platypterygius and in four others cretaceous ophthalmosaurids: athabascasaurus druckenmiller & maxwell, 2010, muiscasaurus, “simbirskiasaurus” and leninia fischer et al. 2013 (druckenmiller & maxwell, 2010; fischer et al., 2013; maxwell et al., 2016; ochev & efimov, 1985; páramo-fonseca et al., 2021). with the aim to establish morphological criteria for the identification of cranial remains not associated with forelimbs, a comparison of the cranial morphology of the species having a typical platypterygius forelimb with that from other cretaceous ophthalmosaurids is here discussed. specimens preserving associated skull and forelimb remains are known for the five species showing the platypterygius type forelimb (p. platydactylus, p. americanus, p. australis, p. hercynicus, and p. sachicarum). in the description of the type specimen of p. platydactylus from the aptian of germany, broili (1907) described the partially preserved skull, detailing the morphology of the quadrate, basioccipital, basisphenoid, stapes, articular, and teeth. skull remains of the type specimen of p. hercynicus from the aptian of germany, named the “sgs” specimen (salzgitter specimen) by kolb & sander (2009) have been preserved with the forelimbs associated (kolb & sander, 2009; kuhn, 1946). these remains include premaxilla, nasal, frontal, parietal, supratemporal, jugal, sclerotic plates, quadrate, basioccipital, basisphenoid, supraoccipital, exoccipital, opisthotic and teeth (kolb & sander, 2009; kuhn, 1946). the specimen uw 2421, a nearly complete skeleton from the upper aptian of north america referred to p. americanus, preserves forelimb remains and the complete skull (maxwell & kear, 2010; mcgowan, 1972; romer, 1968). this skull was described by romer in 1968, and no preparation or detailed descriptions have been accomplished later. two specimens (qm f2453 and f3348) from the mid-upper albian of australia, preserving associated skull and forelimb remains, are reported by mcgowan (1972) and recognized as p. australis by wade (wade, 1984; 1990 (as p. longmani)) and zammit et al. (2010). the specimen qm f2453 preserves the complete skull, but no detailed description of this skull has yet been published. from the specimen f3348, only the forelimb has been described (wade, 1984). from colombia, the upper barremian specimen cip-ga-01042014, a nearly complete skeleton including the left forelimb, referred by maxwell et al. (2019) to p. sachicarum (“kyhytysuka” sachicarum in cortés et al., 2021), includes a complete but poorly preserved skull. this skull is badly fractured, and many of the morphological features are hardly recognizable (maxwell et al., 2019). nevertheless, in addition to the similarities shared with the holotype, noted by maxwell et al. (2019), some features (discussed below) allow us to confirm the species assignation of cip-ga-01042014 to p. sachicarum. in summary, the cranial morphology of the specimens preserving forelimb typical of platypterygius associated to skull remains is still partially known. nevertheless, the cranial features that can be compared, such as the proportions of the snout, orbital and postorbital regions, size, and position of the external nares, elongation of the maxilla, shape of the basioccipital and morphology of the teeth are concurrent in these specimens. the shared features are not exclusive to platypterygius, but their combination distinguishes it from other cretaceous ophthalmosaurids. based on these features, some cranial remains not associated with forelimbs were identified as platypterygius, as the holotype of p. sachicarum (páramo, 1997), which was identified when appendicular remains from colombia were not yet known. now, the two colombian specimens found in the same stratigraphic levels (cip-ga-01042014 and fcg-cbp-87), preserving forelimb remains showing platypterygius type morphology, support the presence of this genus in the barremian beds of the arcillolitas abigarradas member of colombia (cortés & páramo-fonseca, 2018; maxwell et al., 2019). likewise, several australian cranial remains have been identified as platypterygius australis (kear, 2005; kear & zammit, 2014; mcgowan, 1972; wade, 1984; 1990; zammit, 2010; zammit et al., 2010), by their affinity with those specimens preserving associated skull and forelimb remains. in the same way, german cranial material was identified as platypterygius hecynicus (fischer, 2012; kolb & sander, 2009) and some north american as platypterygius americanus (maxwell & kear, 2010; mcgowan, 1972). all this cranial material has enhanced the knowledge of the cranial morphology of platypterygius. the following cranial features of platypterygius differ from other cretaceous ophthalmosaurids. 107a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus figure 2. comparison of the humerus and carpal structure of the ophthalmosaurids (and the cretaceous thunnosaurian malawania anachronus). in all the figures we eliminate the distal elements of the forelimb for facilitating the comparison. a, platypterygius platydactylus left forelimb in dorsal view (from broili, 1907); b, platypterygius australis qm f2453, left forelimb in ventral view (from zammit et al., 2010); c, platypterygius americanus uw 2421 right forelimb in dorsal view (from mcgowan, 1972); d, platypterygius hercynicus sgs, right forelimb in dorsal view (from kolb & sander, 2009); e, platypterygius sachicarum cip-ga-01042014, left forelimb in dorsal view (from maxwell et al., 2019); f, platypterygius sp. fcg-cbp-87, left forelimb in dorsal view (from cortés & páramo-fonseca, 2018); g, undorosaurus nessovi upm epii-24(785), right forelimb in dorsal view; h, undorosaurus gorodischensis upm ep-ii-21(1075), left forelimb in dorsal view (from zverkov & efimov, 2019); i, undorosaurus gorodischensis pmo 214.578, left forelimb in dorsal view (from zverkov & efimov, 2019); j, sisteronia seeleyi camsm b58257_67, left humerus in dorsal view (from fischer et al., 2014a); k, maiaspondylus cantabrigiensis zin, zin ph 1/71, right forelimb in dorsal view (from zverkov & grigoriev, 2020); l, maiaspondylus lindoei holotype ualvp 45635, left forelimb in dorsal view (from maxwell & caldwell, 2006a); m, maiaspondylus lindoei holotype ualvp 45635, left forelimb in dorsal view (from zverkov & grigoriev, 2020); n, arthropterygius chrisorum cmn 40608, left forelimb in dorsal view (from maxwell, 2010); o, arthropterygius lundi pmo 222.654, left forelimb in dorsal view (from zverkov & prilepskaya, 2019); p, arthropterygius thalassonotus moz-pv 6145, right forelimb in ventral view (from campos et al., 2019); q, acuetzpalin carranzai igm 9519, left forelimb in ventral view (from barrientos-lara & alvarado-ortega, 2020); r, nannopterygius enthekiodon nhmuk pv 46497, left forelimb in dorsal view (from zverkov & jacobs, 2020); s, malawania anachronus nhmuk pv r6682, right forelimb in dorsal view (from fischer et al., 2013); t, myobradypterygius hauthali mlp 79-i-30-1, humerus in dorsal view and mlp 79-i-30-2, fragment of a forelimb (from fernández & aguirre-urreta, 2005); u, sumpalla argentina moz-pv 5788, right forelimb in dorsal view (from campos et al., 2021); v, catutosaurus gaspariniae moz-pv-1854, right forelimb in ventral view (from fernández et al., 2021); w, sveltonectes insolitus irsnb r269, left forelimb in dorsal view (from fischer et al., 2011); x, ophthalmosaurus icenicus nhmuk pv r3702, right forelimb in dorsal view (from moon & kirton, 2016); y, brachypterygius extremus brsmg ce 16696, z, otschevia pseudoscythicus, a’, o. zhuravlevi, b’, o. alekseevi, c’, otschevia cf. zhuravlevi upm 2887/1-37, forelimbs in dorsal view (from zverkov et al., 2022 under the name grendelius); d’, brachypterygius extremus, right forelimb in dorsal view (from mcgowan, 1997); e’, parrassaurus yacahuilztli cpc 307, reconstructed right forelimb in dorsal view (from barrientos-lara & alvarado-ortega, 2021b); f’, aegirosaurus leptospondylus sm, right forelimb in dorsal view (from bardet & fernández, 2000); g’, caypullisaurus bonapartei macn-n-32, reconstructed forelimb (from fernández, 1997); h’, thalassodraco etchesi k1185, right forelimb in dorsal view (from jacobs & martill, 2020); i’, gengasaurus nicosiai msvg 39617, left? forelimb in dorsal view (from paparella et al., 2016); j’, acamptonectes densus holotype glahm 132588, right forelimb in ventral view (from fischer et al., 2012). abbreviations: e, anterior accessory epipodial element; h, humerus; i, intermedium; pi, pisiform; r, radius; ra, radiale; u, ulna; ul, ulnare. to visually compare the proportions of the fin elements, we scaled all the humerus to the same proximodistal length of the humerus. scale bars = 50 mm. 108 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho the skull of platypterygius is large, surpassing 70 cm in length, differing from sveltonectes and muiscasaurus, in which the skull is no longer than 65 cm (fischer et al., 2011; maxwell et al., 2016; páramo-fonseca et al., 2021). the general shape of the skull of platypterygius has an almost straight dorsal and ventral margins in lateral view (fig. 3a) (kear, 2005, figs. 1–2; páramo, 1997, fig. 2; romer, 1968, fig. 2; wade, 1984, fig. 1; 1990, fig. 3), differing from muiscasaurus, athabascasaurus, sveltonectes, and leninia, in which the skull is vaulted or curved above the orbit (druckenmiller & maxwell, 2010, fig. 11; fischer et al., 2013, fig. 3; páramo-fonseca et al., 2021, fig. 3). the snout in platypterygius is elongated, and the orbit is proportionally small with respect to the mandibular length (0.15-0.16) (fig. 3a) (kear, 2005, figs. 1–2; páramo, 1997, fig. 2; romer, 1968, fig. 2; wade, 1984, fig. 1; 1990, fig. 3). this proportion differs from the larger orbit found in muiscasaurus (0.22) and sveltonectes (0.18) (fischer et al., 2011, fig. 1; páramo-fonseca et al., 2021, figs. 2–3) and is unknown in other cretaceous ophthalmosaurids. the postorbital region of platypterygius is proportionally extensive regarding the orbital length (0.45-0.60) (fig. 3a) (kear, 2005, figs. 1–2; páramo, 1997, fig. 2; romer, 1968, fig. 2; wade, 1984, fig. 1; 1990, fig. 3). the postorbital region is also large in athabascasaurus (0.45) (druckenmiller & maxwell, 2010, fig. 11), but is shorter in muiscasaurus (0.27), sveltonectes (approximately 0.30) and leninia (0.27) (fischer et al., 2011, fig. 1; 2013, fig. 3; páramo-fonseca et al., 2021, fig. 3), and is unknown in other cretaceous ophthalmosaurids. in platypterygius, the external naris is widely separated from the orbit (fig. 3a) (kear, 2005, figs. 1–2; páramo, 1997, fig. 2; romer, 1968, fig. 2; wade, 1984, fig. 1; 1990, fig. 3), as in caypullisaurus, athabascasaurus, and “simbirskiasaurus” (druckenmiller & maxwell, 2010, figs. 5, 6 and 11; fernández, 1997; 2007; fischer et al., 2014c, fig. 1), and differs from muiscasaurus and leninia, in which the naris is closer to the orbit (fischer et al., 2013, fig. 3; páramo-fonseca et al., 2021, fig. 3). in platypterygius the naris is divided into an anterior and a posterior opening by a naso-maxillar pillar (fig. 3a) (cortés et al., 2021 as “kyhytysuka”; kear, 2005; páramo, 1997; romer, 1968). such a naso-maxillar pillar is only shared with “simbirskiasaurus” (fischer et al., 2014c) and is absent in all other cretaceous ophthalmosaurids for which the naris is known, nonetheless, this feature is also present in upper jurassic taxa, such as arthropterygius thalassonotus campos et al., 2020. the supratemporal process of the parietal is broad and short in platypterygius (fig. 3b) (kear, 2005, fig. 2; wade, 1984, fig. 1) differing from sveltonectes, athabascasaurus, leninia and plutoniosaurus in which this process is relatively slender or elongated (druckenmiller & maxwell, 2010; fischer et al., 2011; 2013; zverkov & efimov, 2019 dataset). the anterior exposure of the maxilla vastly exceeds the level of the external naris in platypterygius (more than 1.5 times the complete external naris length) (see fig. 3a) (fischer, 2012; kear, 2005; páramo, 1997; romer, 1968; wade, 1984; 1990). this trait differs from muiscasaurus, sveltonectes and maiaspondylus, in which the anterior extension of the maxilla is shorter (less than 1.5 times the external naris length) (fischer et al., 2011; maxwell & caldwell, 2006a, pl. 1, fig. 1; páramo-fonseca et al., 2021). the quadratojugal of platypterygius has an extensive to moderate lateral exposure (fig. 3a) (fischer, 2012; kear, 2005; páramo, 1997; romer, 1968), differing from muiscasaurus in which the quadratojugal is greatly reduced (páramo-fonseca et al., 2021). no squamosal is found in platypterygius, whereas it is present in caypullisaurus, leninia, athabascasaurus, and muiscasaurus (druckenmiller & maxwell, 2010; fernández, 1997; 2007; fischer et al., 2013; páramo-fonseca et al., 2021). in the remaining cretaceous ophthalmosaurids this trait is unknown. the extracondylar area of the basioccipital, generally taken to be extremely reduced in all platypterygius, actually varies from reduced, as in p. platydactylus (broili, 1907, taf. xiii, fig. 5), to practically absent, as in p. australis (kear, 2005, fig. 10a) (fig. 3c). a reduced condition is found in most of the cretaceous ophthalmosaurids, but in acamptonectes fischer et al., 2012 and leninia, the extracondylar area is larger (fischer et al., 2012; 2013). the angular and surangular in platypterygius have a subequal lateral exposure (fig. 3a), differing from athabascasaurus in which the angular is more exposed than the surangular (druckenmiller & maxwell, 2010). the teeth of platypterygius, with conical crown and quadrangular root (cortés et al., 2021 as “kyhytysuka”; kear, 2005; kuhn, 1946; páramo, 1997; romer, 1968) do not differ from most cretaceous ophthalmosaurids. however, in platypterygius the crown is stout and has ridges along the entire enamel (fig. 3d) (kear, 2005; kuhn, 1946; páramo, 1997; romer, 1968), which differs from the gracile or slender crown of sisteronia fischer et al., 2014a (fischer et al., 2014a) and athabascasaurus (druckenmiller & maxwell, 2010) and the smooth or lightly ridging crown of maiaspondylus (maxwell & caldwell, 2006a) and athabascasaurus (druckenmiller & maxwell, 2010). thus, the species showing the typical forelimb of platypterygius also share a particular skull morphology, which supports their inclusion in a separate genus. this morphology shows a particular combination of features which is not present in any other cretaceous ophthalmosaurid except “simbirskiasaurus” in which the features that can be compared concurs with the platypterygius skull morphology. “simbirskiasaurus” is known from a single specimen that consists of a portion of a three-dimensionally preserved skull and some vertebrae (fischer et al., 2014c; ochev & efimov, 1985). the features of this specimen concur with the discussed cranial morphology of platypterygius: the length of the skull is estimated at more than 70 cm; the preserved portion of the skull does not show a vaulted dorsal margin above the orbit; the external naris is widely separated from the orbit and divided into an anterior and a posterior opening by a nasomaxillar pillar; the basioccipital has an extremely reduced extracondylar area; and the tooth crown is robust and textured with apicobasal ridges (fischer et al., 2014c; ochev & efimov, 1985). five of the six cranial features proposed by fischer et al. (2014c) as diagnostic for “simbirskiasaurus” are found in platypterygius. these features are: the already discussed divided external naris that is typical in platypterygius; an interdigitating prefrontal-lacrimal suture that is found in p. australis (kear, 2005) and p. sachicarum (personal observation), and is unknown or poorly preserved in other platypterygius species; an elongated anterior process of the maxilla which is present in p. americanus and p. sachicarum (páramo, 1997; romer, 1968) and unknown in other platypterygius species; a supranarial process of the premaxilla that is found in p. australis and p. americanus (kear, 2005; romer, 1968) and is unknown or poorly preserved in other platypterygius species; a dorsal extension in the posterior opening of the narial complex, which although no anteroposteriorly constricted, is found in p. americanus and p. australis and to a lesser degree in p. sachicarum (kear, 2005, figs. 1-4; páramo, 1997, figs. 2-3; romer, 1968, fig. 3). the sixth diagnostic character, an elongated subnarial process of the premaxilla, is found in p. australis and p, americanus, although not reaching the posterior margin as in “simbirskiasaurus” (kear, 2005; romer, 1968). we consider these minor cranial differences not sufficient to justify a generic distinction and therefore, we follow maisch & matzke (2000) and mcgowan and motani (2003) in considering “simbirskiasaurus” a junior synonym of platypterygius. cortés et al. (2021) erected the new genus name “kyhytysuka” to differentiate p. sachicarum from p. platydactylus (as the type species of platypterygius) based on some cranial differences that the authors noted between the holotype of “k.” sachicarum and that of p. platydactylus. these differences refer to the ventral curvature of the mandible, the dorsal projection of the almost imperceptible coronoid process, the inclination of the retroarticular process of the mandible, the posterior extension of the angular and the proportions of the quadrate condyle. considering that some of these features assumed for p. platydactylus may result from the orientation of the drawing in the figures of broili (1907) or reflect preservation features of the specimen, we consider that the minor cranial differences noted by cortés et al. (2021) are not sufficient evidence for a generic distinction. moreover, cortés et al. (2021) recognized as “k.” sachicarum the colombian specimen cip-ga-01042014, which preserves a forelimb typical of platypterygius (see above), but surprisingly, they do not discuss the morphological affinities of this forelimb with that of the type species p. platydactylus. the forelimb morphology of this specimen shows us once again that there is no solid evidence for a generic distinction between “kyhytysuka” and platypterygius and therefore, we here reassign “k.” sachicarum to platypterygius sachicarum. in summary, platypterygius is here recognized as a genus, distinguished from other ophthalmosaurids not only by its typical forelimb morphology but also by its particular combination of skull features. minor differences in traits such as the enlargement of the maxilla, angular, surangular or the premaxillary processes, size, and shape of the external narial apertures, the extension of the extracondylar area of the quadrate, the robustness of the stapes, the number of maxillary teeth, among others, support the distinction of different species within the genus (broili, 1907; fischer et al., 2014c; kear, 2005; kolb & sander, 2009; kuhn, 1946; romer, 1968; páramo, 1997). 109a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus the presence of diverse species of platypterygius in distinct geographic regions (south america, north america, europe, and australia), coming from lower barremian to lower cenomanian beds, shows that platypterygius was a successful genus, which diversified as it conquered the waters of the early to early late cretaceous oceans and seas. as previously suggested by maxwell & caldwell (2006b), the occurrence of platypterygius as a cosmopolitan genus could be expected due to its pelagic free-swimmer condition. a > 70 cm d ⁄ f = (0.15 0.16) e ⁄ d= (0.45 0.60) b > c * 1.5 figure 3. schematic diagrams of a platypterygius skull showing the cranial features, linear measurements, and skull ratios that differentiate this genus from other cretaceous ophthalmosaurids. a, skull in left lateral view. b, parietals in dorsal view showing a broad and short supratemporal process. c, basioccipital in posterior view showing a reduced extracondylar area. d, tooth showing a stout crown with ridges along the entire enamel. measurements: a, skull length; b, distance from the anterior tip of the maxilla to the anterior margin of the anterior narial opening; c, external naris length; d, orbit length; e, postorbital region length; f, mandibular length. abbreviations: ac, acellular cementum; an, angular; cd, condyle; cr, crown; eca, extracondylar area; mx, maxilla; p, parietal; pstp, parietal supratemporal process; qj, quadratojugal; ro, root; sa, surangular. systematic paleontology ichthyosauria de blainville, 1835 ophthalmosauridae baur, 1887 platypterygius huene, 1922 myopterygius huene, 1922: 98 longirostria arkhangelsky, 1998: 612 tenuirostria arkhangelsky, 1998: 612 simbirskiasaurus ochev & efimov, 1985: 88; fischer et al., 2014c: 825. kyhytysuka cortés et al., 2021: 2. type species: platypterygius platydactylus (broili, 1907) included species p. platydactylus, p. americanus, p. australis, p. hercynicus, p. sachicarum, p. birjukovi and a new species p. elsuntuoso here described. revised diagnosis: platypterygius species share the following three synapomorphic features (autapomorphies for the genus): 1distal end of the humerus with articular facets for the ulna and the radius distally oriented, almost in the same plane and with reduced facet for extrazeugopodial elements, sometimes hardly noticeable; 2intermedium roughly pentagonal with proximal pointed edge, nearly parallel anterior and posterior sides and longer and straight distal side; and 3intermedium fitting between the radius and ulna and contacting almost exclusively radiale, ulnare and distal carpal 3. platypterygius is also distinguished by the following combination of characters: skull with almost straight dorsal and ventral margins in lateral view, with orbit proportionally small in comparison to the mandibular length (0.15-0.16) and postorbital region proportionally extensive regarding to the orbital length (0.45-0.60) (also found in brachypterygius, otschevia and caypullisaurus); external naris widely separated from the orbit and divided into an anterior and a posterior opening by a naso-maxillar pillar (also in arthropterygius thalassonotus and roughly similar in maiaspondylus lindoei maxwell & caldwell, 2006a and parrassaurus); supratemporal process of the parietal robust and short (also in caypullisaurus and acuetzpalin); quadratojugal with extensive or moderate lateral exposure (also in caypullisaurus and athabascasaurus); tooth crown stout and having ridges along the entire enamel (also in brachypterygius, parrassaurus, and arthropterygius hoybergeti (druckenmiller et al., 2012); atlas-axis with ventral keel (also in acamptonectes); coracoid with a poorly pronounced anteromedial process (differing from caypullisaurus, sveltonectes, and acamptonectes); humerus with massive deltopectoral crest, matching in height the trochanter dorsalis (also in acuetzpalin and sveltonectes); intermedium not contacting the humerus (differing from otschevia spp., brachypterygius, parrassaurus, and aegirosaurus); radiale and ulnare articulating distally almost exclusively with a single distal carpal, and contact between proximal and distal carpals 2, 3 and 4 in a near straight line (also in undorosaurus spp. and maiaspondylus spp.); proximal forelimb phalanges rectangular and closely fitting (also in sveltonectes, caypullisaurus maiaspondylus spp. and aegirosaurus); digital count not less than 7 producing a broad fin by hyperdactyly (also in caypullisaurus and maiaspondylus lindoei). platypterygius elsuntuoso sp. nov. urn:lsid:zoobank.org:act:2a6f34a9-5d10-447d-9cad-b5848ecc0cf9 holotype and only known specimen: fcg-cbp-28. a skull lacking the anterior portion of the snout; some disarticulated axial elements, incomplete left coracoid, and a few phalanges. type locality and horizon: loma la cabrera, northwest of villa de leiva, boyacá. paja formation, arcillolitas abigarradas member, segment a or b of etayo-serna (1968a), lower barremian (benavides-cabra et al., 2023) (fig. 1). derivation of name: from “el suntuoso”, spanish for “the sumptuous”, nickname used by c. b. padilla, deceased founder of the fcg and cip, when referring to the specimen. diagnosis: p. elsuntuoso is distinguished from other species of platypterygius by the following unique features: 1the anterior ventral border of the extracondylar area of the basioccipital bears a small ventral peg anteroventrally directed (shared with plutoniosaurus, zverkov pers. comm.), which fits into a posteroventral depression found in the basisphenoid; 2 quadrate with supratemporal groove dorsal to the stapedial foramen (shared with plutoniosaurus zverkov pers. comm.; and undorosaurus see zverkov & efimov, 2019) ; and 3root of teeth with a shallow neck at the base of the acellular cementum ring. p. elsuntuoso is also characterized by the following combination of features: supranarial process of the premaxilla relatively short (larger in p. americanus); subnarial process of the premaxilla not reaching the posterior margin of the posterior narial opening (reaching posterior narial margin in p. birjukovi); maxilla with no extensive anterior projection (extremely extensive in p. americanus and p. sachicarum); external exposed maxilla bearing 18 teeth (more than 20 teeth in p. americanus, p. australis, and p. sachicarum); pineal foramen surrounded exclusively by frontal processes (surrounded by frontals and parietals in p. australis); lacrimal reaching the external narial margin (separated to the narial margin by the maxilla in p. australis); supratemporal contacting the postorbital (not contacting in p. americanus) and with a long anterolateral process (shorter in p. hercynicus and p. australis); extensive lateral exposure of the quadratojugal (lesser exposed in p. hercynicus); extracondylar area of the basioccipital reduced but present laterally and ventrally (extremely reduced in p. australis and p. birjukovi); long basipterygoid processes of the basisphenoid (shorter in p. platydactylus and p. australis); opisthotic facet of the basioccipital laterodorsally directed (more dorsally and anteriorly directed in p. australis and p. hercynicus); long stapes (shorter in p. platydactylus and p. australis); teeth with vertical grooves basal to the crown and apical to the root osteocementum (also in p. sachicarum; without vertical grooves in p. hercynicus, p. birjukovi, and p. australis); rugose apical texture in teeth (shared with p. sachicarum); atlas-axis suture visible externally (no visible externally in p. platydactylus, p. americanus, and p. australis). 4. description preservation features the specimen was found enclosed in seven blocks of a large concretion containing a skull and some postcranial remains three-dimensionally preserved (fig. 4). the blocks are separated by natural transverse fractures that offer crosssectional exposure of the bones. the skull lacks a large portion of the anterior 110 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho end and is better preserved on its right lateral side (fig. 4a). nevertheless, on this side, the narial region is broken. the left side of the skull is displaced forward and consequently many bones are distorted and fractured (fig. 4b). the greatest effects of erosion also occurred on this side. the description is therefore based on the right side of the skull, which does not show signs of diagenetic deformation. the bones of the braincase are preserved out but near their anatomical position, whereas the palatal bones are mostly displaced from their original position (fig. 4b). four disarticulated cervical centra, two neural arches with preserved neural spines and some rib fragments are preserved in the posterodorsal region and posterior left side of the skull (fig. 4b, c). in the posteroventral region of the skull some elements of the appendicular skeleton (the left coracoid and six phalanges) can be identified (fig. 4b). the basioccipital and the coracoid were extracted during the preparation to better understand their morphology. ontogeny the relation between the orbital area and the area occupied by the sclerotic ring was proposed to be indicative of the ontogenetic stage in ophthalmosaurid ichthyosaurs (fernández et al., 2005). in fcg-cbp-28 the sclerotic ring area represents 83% of the orbital area (fig. 4a), suggesting a juvenile to subadult stage of the individual (following fernández et al., 2005). the aperture area in fcg-cbp-28 is approximately 14% of the orbital area (fig. 4a), a value lower than the 20% proposed for juvenile specimens and closer but higher to the values found in adult specimens (fernández et al., 2005). this condition supports a subadult stage for the specimen fcg-cbp-28. fcg-cbp-28 shows a raised ridge in the ventral margin of the stapedial facet of the quadrate (fig. 5e-g). following moon & kirton (2016), a stapedial facet ridge is prominent and ventrally tuberous in well ossified individuals of ophthalmosaurus. that implies the specimen fcg-cbp-28 was an ossified individual, not a juvenile. the atlas-axis complex of fcg-cbp-28 retains external vestigial suture between the two elements, but is internally well fused (fig. 9a, b), supporting the idea of an individual with a not completely ossified skeleton. skull given that the anterior end of the snout is lost, we cannot give a complete measurement of the skull. as preserved, the length of the skull (from the posterior end of the quadrate) is 86.5 cm (fig. 4). based on a graphic anterior projection of the dorsal margin of the skull, the ventral margin of the mandibles and the upper and lower alveolar margins, we estimate the skull length to be approximately 123 cm. based on this estimate, the snout length was approximately 103 cm, that is 4.7 times the orbital diameter, and the estimated orbital ratio is 0.15 (ratio proposed by mcgowan & motani, 2003). the orbit is undeformed and has a slightly elliptical outline. although the narial region is broken, the margin of the anterior narial opening is welldefined on the right side of the skull (fig. 4a). the distance between the posterior end of the quadrate condyle to the anteriormost border of the external naris is 45 cm. the postorbital region (taken from the posterior orbital margin to the level of the posterior end of the quadrate condyle) is large, it represents 61.5 % of the orbital horizontal diameter. the shape and size of the preserved margin of the anterior narial opening on the maxilla and the disposition of the surrounding bones (fig. 4a) allows inferring that the anterior narial opening was oval and was posteriorly limited by a pillar formed by the nasal and the maxilla. nothing can be described about the shape of the posterior narial opening because of the bad preservation of this region on both sides of the skull. nevertheless, on the left side of the skull a descending process of the nasal forms at least a portion of the posterior margin of the posterior narial opening (fig. 4b; 5b). cranium premaxilla: the premaxillae lack the anterior portion and are broken at their posterior end, so the morphology of the narial processes remains doubtful. the preserved portions of the processes suggest a small supranarial process that ends at the level of the posterior margin of the anterior narial opening and a dorsoventral high subnarial process that ends more posteriorly than the supranarial process. on the right side of the skull, between the broken premaxillary processes, the maxilla is exposed in lateral view forming the anterior margin of the anterior narial opening (fig. 4a). nonetheless, it is likely that the premaxilla formed the ventral, anterior, and part of the dorsal margins of the anterior narial opening, covering the maxilla externally. on the left side of the skull, although the bones in this region are somewhat fractured and displaced, the subnarial process of the premaxilla can be seen reaching the lacrimal (fig. 4b; 5b). the lateral premaxillary canal is narrow and deep in the anterior region, it broadens and becomes shallower at the level of the sixth tooth position anterior to the lateral emergence of the maxilla and disappears approximately at the level of the anterior end of the maxilla (fig. 4a); it does not meet the external naris. nasal: the nasal is very extended anteriorly making at least 44% of the estimated snout length. the nasal limits posteriorly with the frontal and the postfrontal and posterolaterally with the postfrontal and the prefrontal. the medial relationships of both nasals cannot be discerned due to the forward displacement of the left one. however, on the dorsal surface of the left nasal there is a wide medial concavity at the level of the external naris and a narrower one behind it. the nasal does not have a lateral wing over the external naris (fig. 4a, b; 5b). in the lateral narial region, the right nasal is broken. nonetheless, on the left side of the skull the nasal clearly shows a descending process posterior to the posterior narial opening and some broken fragments of bone are found where the nasomaxillary pillar should have been found (fig. 4b; 5b). frontal: both frontals are preserved. the right one, although preserved in its anatomical position, is slightly dorsomedially inclined, and the left one is displaced forward in relation to the right one (fig. 4c). the frontal contacts the nasal anteriorly in an interdigitated suture, and laterally, it contacts the prefrontal. the frontal participates in the anteromedial margin of the temporal fenestra, contacting laterally the postfrontal (fig. 4c). a thickened posterior process of the frontal overlaps the parietal forming the anterior, lateral, and the posterior margin of the parietal foramen. parietal: the right parietal is preserved in its anatomical position, although rotated, with its medial side turned dorsally. from the left side of the skull, only some undetermined parietal fragments are recognized (fig. 4b, c). the parietal is wide; it is anteriorly covered by the temporal process of the frontal, and posteriorly it overlaps the anterior projection of the supratemporal (fig. 4c). the parietal forms the posterior medial margin of the temporal fenestra. due to the crushing of the left side of the skull, only the right side of the parietal foramen can be seen. since this foramen is exposed in medial view, it is observed that the parietal forms the internal wall of the foramen, but the frontal, overlapping the parietal, forms the external margins of the parietal foramen. this foramen is oval-shaped (4.0 cm long) and is located behind the level of the anterior margin of the temporal fenestra (fig. 4c). maxilla: the anterior process of the maxilla is largely exposed in lateral view; it extends beyond the level of the naris, between 1.5 and 2.4 times according to the range of the estimated narial length (including the anterior opening) and slightly exceeds the level of the lateral exposure of the nasal. its anterior end is located at 1.67 times the orbital length from the anterior orbital margin. the lateral exposure of the posterior end of the maxilla does not reach the level of the anterior end of the orbit because it is covered by the jugal (fig. 4a). the maxilla extends dorsally surrounding the anterior narial opening medial to the premaxilla. the maxilla broadly contacts the ventral border of the lacrimal. the posterior dorsal end of the maxilla remains unknown. the ventral border of the maxilla is concave and laterally participates in the construction of the dental canal through 18 functional positions. lacrimal: the lacrimal is anteroposteriorly long and seems to have reached the posterior end of the subnarial process of the premaxilla. dorsally, it contacts the prefrontal in a suture that is deeply interdigitated in the orbit rim. ventrally, it contacts the jugal posteriorly and the maxilla anteriorly. although the posterior narial opening is not preserved, the dorsal and ventral limits of the lateral exposure of the lacrimal (fig. 4a, b; 5b) indicate that it reached the external naris. prefrontal: the anterior end of the prefrontal is broken. its anteriormost preserved portion is dorsoventrally high close to the position where the posterior narial opening should start. however, in the damaged region the nasal descends as forming the posterodorsal edge of the posterior opening, so it cannot be assured that the prefrontal reached the posterior narial opening in lateral view. on the left side of the skull, the prefrontal is slightly displaced from its original position, but it preserves the anterior end (fig. 4b; 5b). this end is located near a descending process of the nasal, which supports the interpretation that the prefrontal does not reach the posterior margin of the naris, at least in lateral view. 111a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus figure 4. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. photographs and interpretative drawings of the specimen in a, right lateral view, b, left lateral view before extracting the basioccipital and the left coracoid, and c, dorsal view. grey: sediment; black: breakage space; dots: missing surface. abbreviations: aen, anterior external narial aperture; an, angular; ar, articular; at, atlas; ax, axis; bo, basioccipital; bs, basisphenoid; d, dentary; es, sclerotic plates; f, frontal; fj, facet for the jugal; hy, hyoid; j, jugal; la, lacrimal; lan, left angular; lar, left articular; lcor, left coracoid; ld, left dentary; ln, left nasal; lpt, left pterygoid; lsa, left surangular; lsp, left splenial; mx, maxilla; n, nasal; np, nasal process; ns, neural spine; oe, occipital element; p, parietal; pal, palatine; par, prearticular; pf, parietal foramen; ph, phalange; pmx, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal; rb, rib; rar, right articular; rn, right nasal; rpar, right prearticular; rpt, right pterygoid; rst, right supratemporal; rstp, right stapes; sa, surangular; saf, surangular fossa; sp, splenial; st, supratemporal; sym, posterior end of the symphysis; v, vomer; vc, vertebral centrum. scale bars = 100 mm. 112 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho the prefrontal forms the anterior half of the dorsal orbital margin (fig. 4). posteriorly, it articulates with the postfrontal in the orbital margin and with the nasal dorsomedially. anteroventrally, the prefrontal contacts the lacrimal forming an interdigitated suture (fig. 4a). postfrontal: the postfrontal has a large dorsomedial process that participates in the anterior margin of the temporal fenestra (fig. 4c). it is substantially excluded from the lateral border of the temporal fenestra by an anterior elongation of the supratemporal. the postfrontal underlaps the posterior end of the nasal and overlaps the dorsal portion of the prefrontal (fig. 4). it forms a short portion of the orbital roof between the prefrontal and postorbital. posterolaterally, the postfrontal broadly contacts the postorbital (fig. 4a; 5a). jugal: the jugal forms one quarter of the posterior margin of the orbit (fig. 4a). anteriorly the jugal ends slightly anterior to the anterior orbital margin and posterior to the anterior margin of the lacrimal. the anterior end of the jugal is broad and overlaps the maxilla covering its entire height until the level of the third posterior maxillary tooth. the postorbital portion of the jugal is anteroposteriorly wide and is dorsally overlapped by the postorbital. a large facet for articulation in the quadratojugal indicates that the jugal articulates widely with the quadratojugal (fig. 4a; 5a). the joining of the horizontal and vertical rami of the jugal forms an arched orbital margin and an angled ventral margin (fig. 4a). postorbital: the postorbital is anteroposteriorly enlarged (fig. 4a; 5a). it broadly contacts the supratemporal and forms almost a third of the dorsal orbital margin. here, it develops a supraorbital crest (fig. 5a). the postorbital is anterodorsally overlapped by the postfrontal, ventrally it overlaps the jugal and posteroventrally the quadratojugal. squamosal: there is no evidence of the presence of a squamosal bone. supratemporal: the supratemporal, although broken, is in anatomical position. it forms the posterior and almost the entire lateral margin of the temporal fenestra (fig. 4c). laterally it meets the postfrontal anteriorly and the postorbital posteriorly (fig. 4a; 5a). it has a posterior descending ramus that contacts both the quadrate and the dorsal ramus of the quadratojugal. the lateral surface of the supratemporal exhibits a fine sub horizontal striate texture that contrasts with the vertical one of the ascending ramus of the quadratojugal and the oblique of the postorbital (fig. 5a). quadratojugal: the quadratojugal, although in articulation with the quadrate, is slightly moved back; therefore, it exposes a strongly demarcated articular facet for the jugal and postorbital (fig. 4a; 5a). the quadratojugal is a wide laminar bone, greatly exposed in lateral view, and it widely covers the quadrate externally. ventrally, it broadly contacts with the dorsal surface of the quadrate condyle, and dorsally, it has an ascending ramus that is anteriorly overlapped by the postorbital. this ramus is fractured but can be recognized by its continuous texture (fig. 4a). it dorsally touches the occipital lamella of the quadrate. quadrate: the quadrate is preserved in articulation with the mandible but slightly rotated dorsally from its anatomical position. the anteromedial region of the quadrate is embedded in sediment, and the posterodorsal region is obscured by the presence of the displaced atlas-axis complex; therefore, the pterygoid lamella is only partially exposed. laterally, the quadrate is broadly covered by the quadratojugal, which contacts the quadrate on the dorsal surface of the articular condyle and on the occipital lamella (fig. 4a; 5a). the quadrate is ear-shaped, with a large, massive articular condyle, in which two articular facets can be distinguished: a lateroventral facet articulating with the surangular and a posterior facet for the articulation with the articular (fig. 4a; 5c, d). the occipital lamella curves posterolaterally giving the quadrate a concave distal edge. the stapedial facet is well exposed; it is a large deep concavity sub-circular in outline located in the lower half of the quadrate (fig. 5e-g). the stapedial facet is surrounded by a raised ridge, which is more prominent ventrally. a raised ridge in the ventral margin of the stapedial facet was described for sveltonectes by fischer et al. (2011) and for maiaspondylus spp. by zverkov & grigoriev (2020). moon & kirton (2016), noted that such a ridge is prominent and ventrally tuberous in well ossified individuals of ophthalmosaurus. inside the stapedial facet, a semi-ossified structure resembling a preserved membrane covers almost the complete area of the stapedial facet (fig. 5f). such a structure could represent an extrastapedial cartilage, as kear (2005) interpreted for a similar structure in platypterygius australis and can be compared to the extracolumella of some squamate reptiles (mcdowell, 1967; palci et al., 2020). dorsal to the stapedial facet, a clearly demarcated groove is visible, probably representing a portion of a supratemporal groove (fig. 5e, g). sclerotic ring the sclerotic ring occupies almost the entire orbital cavity (fig. 4a). it is formed by 15-16 sclerotic plates weakly imbricated. the peripheral portion of the sclerotic plates is thickened. the union between plates is interdigitated, zigzagging in cross-section. this suggests the sclerotic ring could expand, but there was no transverse mobility between the plates, giving a rather rigid protection to the eye. basicranium from the basicranium, the basioccipital, basisphenoid-parasphenoid and the right opisthotic and stapes are visible. these elements were preserved disarticulated, but near their anatomical position. the basioccipital was extracted during preparation and thanks to this, the right opisthotic and stapes could be seen. basioccipital: the extracted basioccipital is plenty illustrated in a provided 3d model (see supplemental data 1). it is not completely preserved; a portion of its anterodorsal region, including the anterior end of the foramen magnum floor, was lost due to erosion (fig. 6b, c, e). in posterior and anterior views, the basioccipital is circular in outline with a flat dorsal surface. the extracondylar area is somewhat reduced but is well defined laterally and ventrally from the condyle. anteroventrally, the extracondylar area is enlarged and bears a small ventral peg anteroventrally directed (fig. 6b, d). this ventral peg fits into a posteroventral depression found in the basisphenoid. this feature has not been described before for other ophthalmosaurids. the preserved portion of the anterior surface of the basioccipital is convex, rugose and has a medial ventral shallow fossa anterior to the ventral peg. the floor of the foramen magnum is flat and elevated from the exoccipital facets (fig. 6e). posteriorly, the floor of the foramen magnum broadens and becomes slightly concave. lateral to the foramen magnum floor, the facets for exoccipitals are well defined and have a nearly flat surface that slopes down anteromedially. the basioccipital condyle is nearly circular in outline and does not have a notochordal pit (fig. 6a). the opisthotic and stapedial facets are well exposed in lateral view (fig. 6c, f). the opisthotic facet is laterally oriented, and the stapedial facet is lateroventrally oriented. both facets are subrectangular in outline and similar in size. basisphenoid-parasphenoid: the basisphenoid is visible in its ventral and left lateral views and in a portion of its dorsal view (fig. 4b; 6g-j). the parasphenoid is fractured just where it separates from the basisphenoid, and its anterior portion is slightly rotated (fig. 6g, i, j). there is no trace of the suture between the basisphenoid and parasphenoid, but their junction is evident by the presence of a midline ventral ridge in the basisphenoid. in dorsal view, the basioccipital facet of the basisphenoid is similar in size to the dorsal plateau. the surface of the basioccipital facet is medially depressed and has a shallow furrow in its dorsal portion (the median furrow sensu mcgowan & motani, 2003) (fig. 6h). the surface of the basioccipital facets curves ventrally forming a medial triangular furrow posteroventrally oriented that ends just behind the internal carotid foramen (fig. 6i, j). this surface couples with the anteroventral peg of the basioccipital when they are articulated. the internal carotid foramen opens in the posterior third of the ventral surface of the basisphenoid (fig. 6g, j). the basipterygoid processes are extremely large. they protrude from behind the anterior end of the body of the basisphenoid and largely extend anterolaterally, without surpassing the anterior end of the basisphenoid body. in lateral view (fig. 6i), the facet for the stapes is exposed behind the basipterygoid process. it is posterolaterally oriented and occupies the dorsal two-thirds of the basisphenoid height. the parasphenoid cross-section changes from a triangular outline with the pointed end facing ventrally in its posterior region to an oval outline in its anterior portion. opisthotic: on the left side of the skull, the posterior view of the right opisthotic is exposed. the bone is found near its anatomical position; it is located behind the basisphenoid, dorsal to the stapes and lateral to the displaced basioccipital (fig. 5g). the paraoccipital process of the opisthotic is short and robust. proximally, the opisthotic expands and forms the articular facets for stapes, basioccipital and exoccipital (fig. 5h). the facet for the basioccipital is the largest and that for the exoccipital is the smallest. these two facets are separated by a shallow channelshaped depression comparable to that identified for ophthalmosaurus as the jugular foramen by appleby (1956, fig. 12) or the vagus foramen by moon & kirton (2016, figs. 12 and 16). the distal end of the paraoccipital process forms a broad condyle for the articulation with the supratemporal. 113a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus figure 5. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. skull details. a, photograph and interpretative drawing of the right temporal region in lateral view, showing the texture pattern in the different bones. b, photograph and interpretative drawing of the left narial region in lateral view. c-f, quadrate in c, lateral, d, posterior and e, medial views; f, detail of the stapedial foramen on the quadrate showing a membranous structure into it. g, detail of the left posterior region of the specimen with disarticulated occipital elements. h, right stapes and opisthotic in posterior view, reconstructing their anatomical position. grey: sediment; dots: missing surface; yellow shading: all surfaces that do not belong to the illustrated bone. abbreviations: ar, articular; at, atlas; ax, axis; f, frontal; far, facet for the articular; fbo, facet for the basioccipital; fex, facet for the exoccipital; fj, facet for the jugal; fop, facet for the opisthotic; fpt, facet for the pterygoid; fq, facet for the quadrate; fsa, facet for the surangular; fstp, facet for the stapes; gr, groove; hy, hyiod; j, jugal; la, lacrimal; m, membranous structure (extrastapedial cartilage); mb, mandible; mx, maxilla; n, nasal; np, nasal process; oc, occipital crest of the quadrate; op, opisthotic; or, orbit; pal, palatine; pmx, premaxilla; po, postorbital; pof, postfrontal; pop, paraoccipital process; prf, prefrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qc, quadrate condyle; qj, quadratojugal; rn, right nasal; sa, surangular; st, supratemporal; sta, path for the stapedial artery; stp, stapes; tur, tuberous ridge; v, vomer; vc, vertebral centrum; vf, vagus foramen. scale bars: a-b = 50 mm; c-e, g-h = 20 mm; f = 10 mm. 114 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho stapes: only the right stapes is visible. it is located on the left side of the skull behind the basisphenoid and anterolateral to the displaced basioccipital, with its proximal head anteriorly oriented (fig. 4b, 5g,). it is almost completely exposed in posteromedial view. the shaft of the stapes is straight and moderately thick (minimal diameter of the shaft to stapedial head height = 0.4), its length is approximately 1.5 the height of the proximal head (fig. 5h). the stapedial proximal head is massive. the facet for the basioccipital is larger than that figure 6. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. basicranial elements. a-f, basioccipital in a, posterior, b, anterior, c, left lateral, d, ventral, e, dorsal and f, right lateral views (see 3d model in supplemental data 1). g-j, basisphenoid-parasphenoid in g, general, h, posterior, i, left lateral and j, ventral views. yellow shading: all surfaces that do not belong to the illustrated bone. abbreviations: avf, anteroventral fossa; bbpt, base of the basipterigoid process; bpt, basispterygoid process; cd, condyle; eca, extracondylar area; fbo, facet for the basioccipital; fbs, facet for the basisphenoid; fex, facet for the exoccipital; fm, foramen magnum floor; fop, facet for the opisthotic; fstp, facet for the stapes; fvp, facet for the basioccipital ventral peg; icf, internal carotid foramen; pdf, posterodorsal furrow; ps, parasphenoid; vp, ventral peg. scale bars = 20 mm. for the opisthotic. the angle between these two facets is approximately 120°. ventral to the basioccipital facet, angled just over 90°, the facet for the pterygoid is well demarcated. on the distal border of the pterygoid facet, the path of the stapedial artery (following moon & kirton, 2016, fig. 15) is demarcated by a small protuberance (fig. 5h). the ventral surface of the stapedial shaft is more concave than the dorsal one. the distal end of stapes has a large and slightly convex condyle for the articulation with the quadrate. 115a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus palate the palatal bones are only exhibited in cross-section on the natural fractures of the skull, or partly exposed on the left side of the skull (fig. 4b; 5b). most of the palatal bones are disarticulated, displaced from their anatomical position, and in some cases broken. added to this is the fact that the left side of the skull is displaced forward relative to the right side, so there is no symmetry in the palatal bones when viewed in cross-section. under these conditions, their recognition is difficult, and little can be described. at the orbital level, the palatine and the pterygoid can be seen forming a wide sheet 8 to 11 mm thick. here, the palatine distally contacts the jugal and lacrimal, and the pterygoid medially thickens giving it a comma shape in cross-section. mandible as the rostrum, the anterior part of the mandible is lost. the bones of the left ramus are disarticulated and displaced from their anatomical position, but the right ramus is well-preserved and provides a reliable description of the mandibular bones. this ramus is preserved from its posterior end to 33 cm anterior to the beginning of the symphysis. dentary. the dentary forms most of the lateral wall of the mandible (fig. 4a). its posterior end reaches the mid-length of the orbit, but the dental canal only reaches the level of the posterior region of the external naris. the dental canal is wide and shallow posteriorly and becomes deep anteriorly (fig. 7db-e). on the lateral surface the dentary bears a canal, which is narrow and deep anteriorly and slightly broadens and becomes shallower posteriorly at the same level as the lateral premaxillary canal broadens (fig. 4a; 7dc-e). splenial. in ventral view and in the natural cross-sections of the specimen, the splenial can be seen in almost its complete length (fig. 7d-e). it is the main component of the medial wall of the mandible. at the anterior end of the preserved portion of the mandible, the splenial is seen as a short vertical sheet that articulates with the ventral medial face of the dentary (fig. 7de). just 10 cm anterior to the beginning of the symphysis, the splenial is seen in cross-section divided into two well-separated rami, indicating the presence of the anterior opening of meckel’s canal. here, the dorsal portion of the splenial is very small and contacts the medial wall of the dental canal, and the ventral portion is larger and articulates with the ventral ramus of the dentary (fig. 7dd). the greatest height of the splenial is found at the level of the posterior end of the symphysis, where it reaches the dorsal edge of the mandible forming the medial wall of the meckelian canal (fig. 7dc). at the level of the anterior orbit, the splenial becomes a narrow vertical lamina, ventrally curved enclosing the angular. it is exposed in ventral and lateral views to about the mid-orbit level (fig. 4a). medially, the splenial does not reach the dorsal edge of the mandible; it covers the angular and a small fraction of the prearticular (fig. 7db). the posterior end of the splenial is not visible in the specimen. angular. the angular is exposed on the lateroventral side of the mandible (fig. 4a; 7a). in lateral view it extends between the surangular and the splenial from near the posterior end of the symphysis to the mid-orbit; then it forms the ventral portion of the posterior mandible. externally, the anterior end of the angular contacts the dentary just for a few centimeters, just anterior to the anterior end of the surangular. nevertheless, internally, the angular extends anteriorly in contact with the surangular to more than 10 cm in front of the beginning of the symphysis (fig. 7dd). the angular is hardly exposed ventrally in the posteriormost symphysis. at the posterior region of the mandible the angular forms the total ventral edge of the mandible, and both medially and laterally it extends dorsally forming two walls. the lateral wall covers the surangular and the medial wall contacts laterally the prearticular. in the most posterior end, the medial wall of the angular is very thin; it is broken at its base and laterally displaced, but it can be seen that its dorsal edge thickens and bears a medial canal for the articulation with the prearticular (fig. 7da). in lateral view, the suture with the surangular is sinusoidal (fig. 4a; 7a). surangular. the surangular is greater exposed than the angular in lateral view (fig. 3a; 7a). its externally exposed anterior end lies anterior to the anterior border of the external naris and posterior to the anterior tip of the maxilla (fig. 4a). the distance between the exposed anterior end of the surangular and the anterior end of the maxilla in lateral view equals the length occupied by 6 maxillary teeth. although in this region the surangular is little exposed laterally, internally it is robust, giving a solid support to the dentary and a thick lateral wall to the meckelian canal (fig. 7dc). the external anterior end of the surangular is placed more than 10 cm posterior to the beginning of the symphysis (fig. 4a). at the level of the orbit, on the lateral surface, the surangular fossa is clearly demarcated (fig. 7a). in the posterior margin, the fossa bears an oval foramen and behind this margin two more foramina are present. posterior to the orbit, the dorsal edge of the surangular raises forming the anterior portion of the glenoid fossa, which faces dorsomedially for the articulation with the ventrolateral facet of the quadrate condyle. behind the articulation with the quadrate the surangular articulates with the articular medially (fig. 7da). the retroarticular process of the mandible is horizontally directed, and its posterior border has a curved outline. here, the lateral surface of the surangular is strongly wrinkled (fig. 7a), suggesting that in this region a powerful muscular attachment was done. prearticular. the prearticular is poorly exposed; it can be seen in crosssections, but also its most posterior portion is exposed in medial view (fig. 4b; 7da-b). at the level of the external naris, the prearticular is a small vertical lamella that contacts the internal dorsal surface of the splenial contributing to the medial wall of the meckelian canal (fig. 7db). at the level of the anterior orbit, the prearticular rises and thickens forming the medial wall of the meckelian canal. at the glenoid region, the prearticular becomes robust, dorsally contacts the articular and ventrally articulates with the angular (fig. 7da). articular. the right articular is slightly displaced dorsally from its anatomical position and is covered by the left one, which is displaced to the posterior end of the right mandible ramus and is well exposed for description (fig. 7a). the articular forms the posterior medial wall of the mandible. it is a short and mediolaterally compressed bone shaped like a longitudinally bent board. the dorsal edge is narrower than the ventral, and the anterior face is broad. in medial and lateral views, the articular is subrectangular in outline, with a slightly curved posterior edge. the lateral face of the articular is concave and closely joins the surangular (fig. 7c). the medial surface is convex in dorsoventral direction and concave in anteroposterior direction, producing a saddle-shaped surface, which is smooth in texture (fig. 7b). a blunt tubercle projects from the posterior face of the bone onto the dorsal region of the medial surface. anteriorly, the articular widens erecting a planar vertical articular facet for the quadrate (fig. 7b, c). the surface of this facet, as well as that of the dorsal, ventral and posterior faces of the bone is porous, indicating the articular was surrounded by cartilage. hyoids a fragment of a slightly curved rod-like bone found near the medial wall of the right mandibular ramus and exposed on the left side of the specimen, is interpreted as a portion of a hyoid, probably the right one (fig. 4a, b; 9k). this fragment is 215 mm in length and oval in cross-section, with 27 mm major diameter and 9 mm minor diameter. teeth and dentition on the right side of the skull all the teeth are preserved emplaced in their anatomical position (fig. 8). there are maximum of 18 teeth in the maxilla in lateral view. the number of premaxillary teeth remains unknown since the snout is incomplete. the teeth are implanted into a common dental canal (aulacodont condition sensu mazin, 1983). they are densely packed and tightly interlocked. the teeth of the upper and lower jaws are almost regularly intercalated in nearly the same plane. in the posterior region the upper teeth slightly occlude laterally over the lower teeth, but in the anterior preserved region the occlusion is inverted, with the lower teeth occluding laterally over the upper teeth. the teeth are oriented nearly perpendicular to the bony labial border, with few random variations due to taphonomical processes. the posteriormost teeth are maxillary teeth, with the last position located at the level of the anterior orbital margin. in contrast, the posteriormost dentary tooth is seen at the level of the external naris, where the dentary dental canal begins. the maxillary and premaxillary teeth are larger than their opposing dentaries. the posterior teeth on both maxilla and dentary are the dorsoventrally shortest teeth, which concurs with the posterior shallowing of the dental canals. 116 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho figure 7. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. mandible. a, posterior region of the right mandible in lateral view. b-c, left articular in b, medial and c, anterior views. d, cross-sections of the mandible as referred in fig. 2a; the section a was inverted to illustrate the sequence of images in the same direction. yellow shading: all surfaces that do not belong to the illustrated bone. abbreviations: an, angular; ar, articular; d, dentary; dc, dentary canal; fan, facet for the angular; fo, foramen; fq, facet for the quadrate condyle; fsa, facet for the surangular; hy, hyoid; lar, left articular; lc, lateral canal; par, prearticular; qc, quadrate condyle; rar, right articular; rt, replacement teeth; sa, surangular; saf, surangular fossa; sp, splenial; t, tooth; tu, tubercle. scale bars = 20 mm. the functional teeth have a very tall root and a short, enameled crown (total tooth length/crown length ratio: 3.7) (fig. 8f). there is a transition region of acellular cementum (sensu maxwell et al., 2011), in which shallow grooves from the crown striae extend over a not enameled surface that differs in texture from the root (fig. 8d, l). on both the upper and lower jaws, and along the entire length of the snout, the acellular cementum ring and nearly one-third of the root of the functional teeth are exposed outside the dental canal (fig. 8f). the root is subrectangular in cross-section, being longer labiolingually than mesiodistally (fig. 8c, n). the root exhibits a slight neck at the base of the acellular cementum ring (fig. 8f, o). in longitudinally broken teeth, the root exhibits a hollow center, as an extension of the crown pulp cavity (fig. 8b). the external surface of the root bears slight grooves and shows a fibrous and porous texture indicating a high vascularization (fig. 8f, l). the enameled crown is conic, circular in cross-section, with the apex very slightly curved lingually. the crown is short, its height is 1.3 to 1.7 times its base length; shorter in the most posterior teeth. the base of the enamel is well defined (fig. 8d-l). the enamel is ridged all around the crown. the ridges extend along the entire enamel height without bifurcations. they are thicker at the crown base and more densely packed at the apex. at the tip of the crown the ridges become discontinuous producing an apical grainy texture. this apical grainy texture is found in all well-preserved crowns, whether from upper or lower jaw teeth, anterior or posterior teeth, or replacement or functional teeth (fig 8d-k). the replacement teeth are found in different degrees of growth, from very immature teeth with only the crown apex exposed, to erupted teeth, in which the only difference from the mature ones is their slightly smaller size. the dental replacement occurs directly from the base of the root. in the longitudinal section of a dentary functional tooth located in the anterior end of the preserved snout, the base of the root is seen resorbed by the growth of a replacement tooth (fig. 7de). that differs from the lingual replacement described by kear (2005) for p. australis. the just erupted replacement teeth show their crown in the center of the ejected tooth root (fig. 8j). in the growth process, the crown is the first to be formed and, once it is emplaced into the alveolus, the tooth becomes functional progressively increasing the size of the root. 117a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus there are no regions with differentiated dentition on the preserved portion of the snout. instead, when the teeth of the upper jaw are carefully observed, two intercalated growth waves can be identified, one on the even positions (white teeth in fig. 8m) and the other on the odd positions (dark gray teeth in fig. 8m). the alternate pattern in growth dentition in reptiles had already been described by edmund as early as 1960. edmund (1960) shows that a replacement occurring in alternate waves passing from front to back is a peculiarity of ichthyosaurs, differing from the usual back-to-front wave progression found in most reptiles. as edmund (1960) observed for ichthyosauria, in the studied specimen the intercalated growth waves are done from front to back. from the first tooth position preserved on the upper jaw, a sequence of growth stages in odd positions is clearly observed reaching position 9, where a large hollowed broken out root indicates a just replaced tooth (fig. 8m). the following upper odd positions show large functional teeth up to position 21. for the even positions, two waves of consecutive growth stages are clearly identified with the large functional teeth in positions 2 to 8 and 18 to 22 (fig. 8m). in the posteriormost dentition all the maxillary teeth seem to be functional and do not have mandibular opposites. the described replacement structure shows that within each series (odd and even), several replacement waves follow each other. given the intercalation between the two series, the large functional teeth from one of the series intercalate with the smaller replacement teeth from the other (fig. 8m). this replacement pattern ensures no gaps of more than one functional tooth in the upper jaw. it also generates multiple series of consecutive large teeth that in life changed their position repeatedly following the growth alternating waves. under these conditions, to give interpretations on dentition regionalization or mode of feeding based on what was preserved in a fossil could be misleading. contrary to the upper jaw, a sequence of replacement teeth in the dentary is difficult to discern. nevertheless, it is interesting to note that opposing each smaller replacement tooth on the upper jaw, there are usually two adjacent large functional teeth on dentary, and the smaller replacement dentary teeth are located where a series of consecutive functional teeth in the upper jaw is found (fig. 8m). the number of dentary teeth is somewhat greater than that of their opposing in the upper jaw. in fact, thirty-one dentary teeth can be counted in the space containing the twenty-seven opposing positions of the upper jaw. postcranial axial elements from the axial skeleton only a few anterior vertebral elements, including the atlas-axis-complex, and ribs fragments are preserved. these elements are found disarticulated in the posterodorsal region of the skull (fig. 4). atlas-axis: the atlas-axis complex is incomplete; it is longitudinally broken and partially embedded in sediment. only the anterior, posterior dorsal, right laterodorsal and left lateroventral portions can be seen (fig. 9a, b). the atlas-axis complex was preserved behind the basioccipital but out of its anatomical position (fig. 4). the atlas and the axis are fused, there is no suture between the two elements internally, as can be seen in the broken surface. however, a suture is clearly visible on the external surface. the atlas-axis complex is as high as it is wide (approximately 83 mm high and 84 mm wide) and its ventral length is greater than its dorsal length (approximately 59 vs. 51 mm) (fig. 9 a, b). the identification of the atlas and axis was based on two criteria: the concavity of the atlas is circumscribed by a broad edge whereas the posterior concavity of the axis has a clearly demarcated edge as mcgowan & motani (2003) described as a criterion for the identification of the axis; the articular surface concavity of the atlas fits perfectly with the basioccipital condyle convexity, while that of the axis does not. figure 8. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. dentition. a, general view of dentition. b-l details of teeth. m, scheme showing the growth pattern of replacement dentition with dentary teeth in light gray, dental positions of the upper jaw in dark gray (even positions) and white (odd positions); n, the same rectangular tooth root (indicated in o) from distal (a) and labial (b) views, showing the transverse side longer than the longitudinal side (double arrows). o, detail of the dentition showing the neck at the base of the acellular cementum on teeth (white arrows). abbreviations: ac, acellular cementum; cr, crown; ga, grainy apex; ro, root. scale bars: a, m = 50 mm; b-d, f, l = 10 mm; e, g-k = 5 mm; n-o (mm). 118 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho the atlas centrum is slightly longer than the axis centrum (fig. 9a). the right diapophyses and parapophyses are visible. in both atlas and axis, the diapophysis is close to the anterior edge of the centrum, united to the neural arch facet, whereas the parapophysis is placed near the posterior edge of the laterodorsal surface of the centrum. the diapophysis and parapophysis are closer to each other on the atlas than on the axis. the visible left lateroventral surface of the preserved portions of the atlas and axis are each demarcated by a depression. these depressions delimit the ventral facets for the atlantal and axial intercentra and apparently also a ventral crest (fig. 9b). cervical vertebrae: three complete and one incomplete anterior cervical centra and four neural arches and spines can be identified. all these elements are disarticulated; the neural arches are grouped near and inside the temporal fenestra and the centra are found posterior to them (fig. 4). the centra are pentagonal in outline, slightly higher than wide (68 mm in height and 62 mm in width) (fig. 9d). their concave articular faces have a deeper central concavity. the diapophysis and parapophysis are separated in all centra, but the distance between them varies slightly. in all, the diapophysis is located close to the anterior edge of the centrum, united to the neural arch facet, but the parapophysis is found in different positions. in one of the centra, here considered a more anterior centrum, the parapophysis is near the posterior edge of the laterodorsal surface of the centrum, as in the axis. in the other two complete centra, the parapophysis appears at the middle length of the lateral surface of the centrum (fig. 9e). the partially exposed neural arch shows long neural spines. figure 9. platypterygius elsuntuoso sp. nov., holotype fcg-cbp-28. postcranial elements. a-b, atlas-axis complex (the preserved portion) in a, right lateral and b, left lateroventral views. c-e, cervical vertebra in c, dorsal, d, posterior and e, right lateral views. f-j, left coracoid in f, dorsal, g, ventral, h, posterior, i, medial and j, lateral views. k, disarticulated phalanges in exposed view. abbreviations: at, atlas; ax, axis; dap, diapophysis; fati, facet for the atlantal intercentrum; fc, intercoracoid facet; fgl, facet for the glenoid; fna, facet for the neural arch; fsc, facet for the scapula; hy, hyoid; mb, mandible; nc, neural canal floor; pap, parapophysis; ph, phalange. scale bars: a-b = 20 mm; c-e = 20 mm; f-j = 20 mm; k = 20 mm. ribs: in the posterior dorsal region of the skull there are some fragments of an elongated element that could correspond to a broken rib (fig. 4). these fragments are long and slender, with oval cross-section in the thicker fragments and circular in the thinner ones. these fragments show a central cavity, empty by erosion. appendicular elements coracoid: the left coracoid was found at the posterior end of the skull (fig. 4) and was extracted during preparation. its anterior region is damaged; therefore, the morphology of the anterolateral notch remains unknown (fig. 9f, g). in dorsal or ventral view, the coracoid is nearly circular in outline (120 mm in mediolateral width and 127 mm in anteroposterior length); there is not a posterior notch. the dorsal and ventral surfaces are mediolaterally concave and anteroposteriorly convex, with a flattened posterior region 10 mm thick (fig. 9f-h). the medial intercoracoidal facet is rhombic in outline and angled in such a way that in an anatomical position the body of the coracoid would be inclined approximately 15° to the horizontal plane (fig. 9i). the medial facet is thicker (maximum thickness=58 mm) than the lateral (glenoid/ scapular) facet (maximum thickness=48 mm). the lateral articular surface is oval in outline and shows a smaller anterior facet for the scapula and a greater posterior facet that contributed to the glenoid (fig. 9j). the scapular facet is slightly dorsally directed, whereas the glenoid facet is laterally directed. both the medial and lateral articular surfaces of the coracoid are pitted and rugous suggesting cartilage joints. the posterior border of the bone blade is notoriously porous (fig. 9h), indicating a cartilaginous prolongation in this area. phalanges: five disarticulated phalanges are preserved and partially exposed in the posterior ventral region of the skull (fig. 4a, b; 9k). the largest are thick and rectangular in outline; the best exposed measures 30 mm x 20 mm x 14 mm. the smallest are thinner and suboval in outline (20-26 mm x 12-16 mm x 6-8 mm) and have less well-defined borders than the larger phalanges. the morphology of the preserved phalanges suggests that the smaller ones came from accessory digits while the larger ones, with their well-defined borders, were closely packed and occupied a more central position within the fin. 5. discussion fcg-cbp-28 is identified as platypterygius by presenting the diagnostic features established for the genus: a skull with almost straight dorsal and ventral margins in lateral view; orbit proportionally small in comparison to the mandibular length (0.15-0.16); postorbital region proportionally extensive regarding the orbital length (0.45-0.60); external naris widely separated from the orbit and divided into an anterior and a posterior opening by a nasomaxillary pillar; quadratojugal with extensive lateral exposure, extracondylar area of the basioccipital reduced; tooth crown stout and having ridges along the entire enamel; coracoid with a poorly pronounced anteromedial process; and thick rectangular phalanges. fcg-cbp-28 differs from all other platypterygius species by having teeth with a neck at the base of the acellular cementum ring (fig. 8f, o); in all other species such a neck is not present or has not been described (broili, 1907; cortés et al., 2021; fischer, 2012; fischer et al., 2014c; kear, 2005; kolb & sander, 2009; kuhn, 1946; maxwell et al., 2019; maxwell & kear, 2010; páramo, 1997). the basioccipital of fcg-cbp-28 bears a small anteroventrally directed peg in the ventral border of extracondylar area that fits into a posteroventral depression found in the basisphenoid, differing from that of p. platydactylus, p. hercynicus, p. australis, and p. birjukovi in which such a peg is not present (broili, 1907; fischer et al., 2014c; kear, 2005; kolb & sander, 2009; kuhn, 1946); this region of the basioccipital is unknown in p. sachicarum and p. americanus (maxwell et al., 2019; romer, 1968). the quadrate of fcg-cbp-28 has a supratemporal groove in the posterodorsal region (fig. 5e) which is not present in p. platydactylus, p. hercynicus, and p. australis (broili, 1907; kear, 2005; kolb & sander, 2009; kuhn, 1946); this quadrate region is unknown in the remaining species of platypterygius (cortés et al., 2021; fischer et al., 2014c; maxwell et al., 2019; romer, 1968). in fcg-cbp-28 the basipterygoid processes of the basisphenoid are longer than in p. platydactylus and p. australis (broili, 1907; kear, 2005) and remains unknown in other species of platypterygius (cortés et al., 2021; fischer et al., 2014c; kear, 2005; kolb & sander, 2009; kuhn, 1946; maxwell et al., 2019; romer, 1968). in addition to the noted particularities, fcg-cbp-28 exhibits the following features that differ from each species of platypterygius. the greatest morphological similarity is found with p. sachicarum. two specimens have been assigned to p. sachicarum: the holotype skull (don19671) and a nearly complete skeleton (cip-ga-01042014) preserving skull, presacral vertebral column, pectoral girdle, and partial forelimb (cortés et al., 2021; maxwell et al., 2019; páramo, 1997). photographs and a drawing taken in 119a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus 1997 (me p-f) show that some regions of the holotype of p. sachicarum were more complete at that time (e.g., orbit and maxilla) (fig. 10) than at present (cortés et al., 2021, fig. 2). on this basis, we could confirm that, in addition to the shared features noted by maxwell et al. (2019), in both specimens the anterior external end of the maxilla is located at 2 times the orbital length from the anterior orbital margin, and the minimum number of maxillary teeth (or dental spaces) is 22-24 (personal observations). these features differ from other species of platypterygius and support the assignment of both specimens to the same species p. sachicarum. fcg-cbp-28 is slightly larger than specimens referred to p. sachicarum (maxwell et al., 2019; páramo, 1997) (fig. 10). the morphology of the postorbital, supratemporal and quadratojugal of fcg-cbp-28 is comparable to that of p. sachicarum, as illustrated and interpreted by páramo (1997), but not to that illustrated by cortés et al. (2021, fig. 2b) (see fig. 4a; 5a). the crown apex with a rugose texture on teeth along the dentition and the acellular cementum ring with shallow grooves found in fcg-cbp-28 is shared only with p. sachicarum; in the remaining species of platypterygius the crown apex and the acellular cementum ring are smooth (broili, 1907; fischer, 2012; kear, 2005; maxwell & kear, 2010). cortés et al. (2021) stated that in the holotype of p. sachicarum the acellular cementum ring is smooth, and the crown apical rugose texture is present in teeth from one of their proposed dentition regions. however, our observations on the holotype of p. sachicarum revealed the presence of shallow grooves in the acellular cementum ring (fig. 10a) and rugose apical texture on teeth from diverse positions of the preserved dentition (fig. 10a-b). we also noted that no dentition regions are seen on the right side of the skull (fig. 10a). despite the noted similarities, some differences between fcg-cbp-28 and p. sachicarum can be observed. the anterior external extension of the maxilla in p. sachicarum is proportionally longer than in fcg-cbp-28; its anterior end is located at 2 times the orbital length from the anterior orbital margin, whereas in fcg-cbp-28 it is located at 1.67 times the orbital length (fig. 10). the externally exposed maxilla in p. sachicarum has 22-24 teeth (or dental positions) and probably reached 26 (fig. 10), whereas in fcg-cbp-28 the maxilla bears 18 teeth. moreover, the number of maxillary teeth counted between the anterior end of the surangular and the anterior end of the maxilla is 10-11 in the holotype of p. sachicarum and 5-6 in fcg-cbp-28. the teeth crowns in fcg-cbp-28 are proportionally shorter than in p. sachicarum; the ratio between the total tooth length and the crown length is 3 to 3.7 in fcg-cbp-28 and 2.6 to 3 in p. sachicarum. in both the holotype of p. sachicarum and fcg-cbp-28, the tooth root is rectangular in cross-section; however, in p. sachicarum the longer side is parallel to the dental canal, whereas in fcg-cbp-28 the longer side is transverse to the dental canal (fig. 8n). we cannot be sure if this trait is present in other species of platypterygius since the teeth roots are not frequently visible in both directions when the teeth are emplaced in the dental canal. figure 10. comparison between the skull of p. sachicarum and p. elsuntuoso. a-b, holotype of p. sachicarum (don-19671); a, photograph of the right-side dentition detailing some teeth; b, photographs and drawing taken in 1997 (me p-f) with a recent photograph detailing a tooth. c, photograph of the holotype of p. elsuntuoso (fcgcbp-28) inverted to facilitate comparison. both specimens were taken at the same orbital length to compare proportions. note the difference in the external length of the maxilla (vertical lines) and in the number of maxillary teeth. abbreviations: ac, acellular cementum; ga, grainy apex. scale bars = 10 cm; teeth details (mm). 120 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho the differences noted between fcg-cbp-28 and p. sachicarum imply at least a slight specialization in diet, which supports a taxonomic distinction at the species level. the new species, named p. elsuntuoso, came from the lower barremian and p. sachicarum came from the upper barremian (benavidescabra et al., 2023), so these differences may well represent speciation by adaptation to new food sources in the cretaceous transgressional sea of colombia. in this context, it is interesting to note that the forelimbs identified by cortés & páramo-fonseca (2018) as platypterygius sp. (specimen fcgcbp-87) came from the same geographic locality and stratigraphic level as the holotype and only known specimen of p. elsuntuoso. maxwell et al. (2019) noted morphological differences at the species level between the forelimb of fcgcbp-87 and the forelimb of the specimen cip-ga-01042014 of p. sachicarum (for details see maxwell et al., 2019). under these conditions, specimen fcgcbp-87 could represent the forelimb of p. elsuntuoso. however, until a more complete specimen of p. elsuntuoso preserving both skull and forelimb is found, this hypothesis cannot be confirmed. in addition to the features already mentioned, p. elsuntuoso differs from p. platydactylus in its long stapedial process, which is shorter in p. playdactylus and in the externally visible atlas-axis suture, which is not present in p. platydactylus (broili, 1907, taf. xiii-2, 3). from p. hercynicus it differs as follows: the posteroventral margin of the jugal has a marked angle in fcg-cbp-28 whereas in p. hercynicus is regularly curved (kolb & sander, 2009); the anterolateral process of the supratemporal is shorter in p. hercynicus (fischer, 2012, fig. 2); in the basioccipital, the convexity of the condyle is less protruded in p. elsuntuoso than in p. hercynicus and the opisthotic facet is more dorsally and anteriorly directed in p. hercynicus than in p. elsuntuoso (for comparisons see kuhn, 1946, taf. ii-2 and taf. iii-1a, b); the quadratojugal is less exposed laterally in p. hercynicus than in p. elsuntuoso (fischer, 2012). from p. birjukovi, fcg-cbp-28 differs in the following features: the subnarial process of the premaxilla does not reach the posterior margin of the posterior narial opening as it does in p. birjukovi (fischer et al., 2014c); the lateral premaxillary canal does not reach the external naris, differing from p. birjukovi where the canal reaches the anterior margin of the external naris (fischer et al., 2014c); the extracondylar area is slightly exposed laterally and ventrally to the condyle, whereas in p. birjukovi the extracondylar area is practically not exposed (fischer et al., 2014c); and the exoccipital facets of the basioccipital are more separated from each other than in p. birjukovi (fischer et al., 2014c). p. elsuntuoso differs from p. americanus in the additional following features. in p. elsuntuoso the supranarial process of the premaxilla is shorter than in p. americanus (romer, 1968). the lateral premaxillary canal in p. elsuntuoso does not reach the external naris, differing from p. americanus where the canal reaches the anterior margin of the external naris (based on photographs of the specimen uw 2421). the anterior extension of the maxilla in p. americanus is proportionally longer than in p. elsuntuoso; its anterior end is located at 2.5-2.6 times the orbital length from the anterior orbital margin (based and photographs of the specimen uw 2421), whereas in p. elsuntuoso it is located at 1.67 times the orbital length. fcg-cbp-28 has 18 teeth (or dental spaces) in the external exposed maxilla whereas p. americanus has 21 (based on photographs of the specimen uw 2421). the number of maxillary teeth counted between the anterior end of surangular and the anterior end of the maxilla is 10-11 in p. americanus (based on photographs of the specimen uw 2421) and 5-6 in p. elsuntuoso. in p. elsuntuoso the jugal forms approximately one quarter of the posterior orbital margin, unlike in p. americanus where the jugal does not participate in the posterior orbital margin (romer, 1968). the supratemporal contacts the postorbital in p. elsuntuoso whereas in p. americanus does not (romer, 1968). the angular-surangular external suture is sinuous in p. elsuntuoso, whereas in p. americanus it is more rectilinear (romer, 1968). the atlas-axis suture is present externally in fcg-cbp-28 whereas in p. americanus it is not (maxwell & kear, 2010). the highest number of differences are found with p. australis. the anterior extension of the maxilla in p. australis is proportionally shorter than in p. elsuntuoso; its anterior end is located at a distance equivalent to one orbital length from the anterior orbital margin (kear, 2005, fig. 1), whereas in p. elsuntuoso it is located at 1.67 times the orbital length. p. elsuntuoso has 18 teeth in the external exposed maxilla whereas p. australis has 26 (kear, 2005; wade, 1984; 1990). the number of maxillary teeth counted between the anterior end of the surangular and the anterior end of the maxilla is 8-9 in p. australis (based on photographs of the specimen qm f2453) and 5-6 in p. elsuntuoso (fig. 4a). the surangular is proportionally longer than the angular in p. australis than in p. elsuntuoso, since the number of maxillary teeth counted between the anterior end of the angular and the anterior end of the surangular is 5-6 in p. australis (based on photographs of the specimen qm f2453) whereas it is 2 in p. elsuntuoso (fig. 4a). in p. elsuntuoso the lacrimal forms the posterior margin of the external narial opening, differing from p. australis where the maxilla has a posterodorsal process excluding the lacrimal from the narial margin (kear, 2005). the nasal has a wing above the external naris in p. australis (kear, 2005), which is absent in p. elsuntuoso. in p. elsuntuoso the jugal is anteriorly short, just surpassing the anterior orbital margin (fig. 4a), whereas in p. australis the anterior end of the jugal reaches the anterior end of the lacrimal (kear, 2005). in p. elsuntuoso the pineal foramen is surrounded by processes of the frontal, whereas in p. australis the pineal foramen is surrounded by the parietal and frontals almost in the same proportions (kear, 2005); the anterolateral process of the supratemporal is longer in p. elsuntuoso than in p. australis (kear, 2005). in the basioccipital, the extracondylar area is exposed laterally and ventrally to the condyle in p. elsuntuoso (fig. 6), whereas in p. australis is nearly absent (kear, 2005); the exoccipital facets of the basioccipital are medially far from each other in p. elsuntuoso than in p. australis where these facets are medially close together (kear, 2005) and the opisthotic facet is more dorsally and anteriorly directed in p. australis than in p. elsuntuoso (for comparisons see kear, 2005, fig. 10 b, c). the stapes is longer in p. elsuntuoso than in p. australis (kear, 2005). lastly, in fcg-cbp-28 the atlas-axis suture is visible externally whereas in p. australis it is only visible on the dorsal surface (zammit et al., 2010). phylogenetic analysis the strict consensus of 708 most parsimonious trees with 589 steps (ri= 0.549 and ci= 0.292) obtained from tbr branch swapping, shows platypterygius elsuntuoso within a large polytomy that includes all other platypterygius species, plutoniosaurus bedengensis, pervushovisaurus bannovkensis, acuetzpalin carranzai, sisteronia seeleyi, and the other colombian specimens: fcg-cbp-87 and cip-ga-01042014. after applying the iterpcr algorithm, 4 unstable outs were identified: brachypterygius extremus, pervushovisaurus bannovkensis and the colombian specimens fcg-cbp-87 and cip-ga-01042014. after pruning the unstable outs from the consensus, the reduced strict consensus (fig. 11) shows platypterygius elsuntuoso as the sister taxon of p. sachicarum highlighting again their resemblance. this node is supported by the presence of vertical grooves basal to the crown and apical to the root osteocementum [4.0]. the node containing p. elsuntuoso and p. sachicarum is recovered as sister taxon of p. hercynicus and p. platydactylus in a branch together with all the other platypterygius species (including p. birjukovi), “plutoniosaurus” bedengensis, and sisteronia seeleyi. this branch is supported by an atlas-axis with a ventral keel [78.1]. according to what was discussed above, the known material of “simbirskiasaurus” birjukovi falls within the morphology characterizing platypterygius representing a species of platypterygius instead of a separated genus, thus its location together within the platypterygius branch was expected and support its inclusion withing the genus platypterygius. even though the original description of “plutoniosaurus” bedengensis (efimov, 1997) did not offer many details, zverkov & efimov (2019) provided the scoring of the holotype specimen based on their personal observations (extended by zverkov & jacobs, 2021). based on that scoring, “pl.” bedengensis is recovered here together with all the platypterygius. we interpret that this specimen probably represents a species within platypterygius, however, until a formal reevaluation of the species is made available, we refrain to reassign it to platypterygius. sisteronia seeleyi is known only by fragmentary material of which the occipital region elements resemble with minor differences those known from platypterygius, however, s. seeleyi also differs from platypterygius in the humerus morphology and in the teeth morphology. we interpret that although sisteronia appears to be a separated genus, its fragmentary material does not allow a clear differentiation with platypterygius in the phylogenetic analysis. the platypterygius branch is contained within a clearly separated clade (a) (see fig. 11) that also includes athabascasaurus bitumineus, parrassaurus yacahuitztli, otschevia alekseevi, caypullisaurus bonapartei, acuetzpalin carranzai. this clade is supported by the following synapomorphies: long 121a new species of platypterygius (ophthalmosauridae) from the lower barremian of colombia and assessment of the species composition of the genus premaxillary process of the maxilla [13.0]; nasal lacking a posterior process that overlaps the postfrontal dorsally [34.1]; square squamosal [47.0]; and extensive quadratojugal exposure [48.0]. the sister clade of (a) is clade (b). clade (b) is composed by the nannopterygius species, thalassodraco etchesi, maiaspondylus lindoei, sveltonectes insolitus, aegirosaurus leptospondylus and the other colombian taxon muiscasaurus catheti. the position of m. catheti differs from that recovered in previous phylogenetic analysis (barrientos-lara & alvarado-ortega, 2021a; 2021b; maxwell et al., 2016; 2019; páramo-fonseca et al., 2021; and others) in which m. catheti was recovered closely related to ophthalmosaurus or to platypterygius. the clade (b) is supported by the following synapomorphies: snout depth very short [7.0]; external nares elongated and complex lobated [15.1]; prefrontal having a processus narialis [21.1]; lower jaw ventral margin markedly concave [72.1]; and rib facet in anterodorsal centra confluent with anterior face, at least in some centra [84.0]. two other well separated clades, (c) and (d), were recovered within ophthalmosauridae (see fig. 11). clade (c) includes the undorosaurus species, jabalisaurus meztli, acamptonectes densus, catutosaurus gaspariniae, and sumpalla argentina. this clade is supported by a descending process of the nasal almost completely dividing the naris [17.2]; a robust supratemporal process of the parietal [43.1]; an interclavicle markedly expanded [99.1]; protruding triangular deltopectal crest [104.1]; and an intermedium proximal edge straight or notched [123.0]. clade (d) is composed of the ophthalmosaurus species and gengasaurus nicosiai and is supported by a basioccipital condyle with a peripheral lateral and ventral groove [56.1] and by an elongated and slender paraoccipital process of the opisthotic [66.1]. our analysis also recovered a monophyletic link for the undorosaurus species and for the arthropterygius species, as was recovered in other recent analysis (see jacobs & martill, 2020; páramo-fonseca et al., 2021; zverkov & jacobs, 2021; zverkov & prilepskaya, 2019). undorosaurus is supported by the presence of deep longitudinal ridges on the crown [1.0]; root with a quadrangular cross-section [3.1]; stapedial shaft moderately thick in posterior view [67.0]; and short surangular in lateral view, not surpassing the level of the external naris midlength [145.1]. arthropterygius is supported by a processus subnarialis of the premaxilla that reaches the posterior end of the naris [10.1]; presence of a processus narialis of the prefrontal [21.1]; posterior margin of the jugal excluded from the quadratojugal by the postorbital [28.1]; anteroposteriorly short parietal medial symphysis [41.1]; squamosal absent [47.2]; basisphenoid processes short and anteriorly directed [52.0]; internal carotid foramen located in the posterior surface of the basisphenoid [53.2]; presence of a raised opisthotic facet for the basioccipital [62.1]; osseous labyrinth t-shaped [65.1]; tail fin centra as wide as they are high [82.1]; angle between the articulated coracoids below 130º [94.2]; absence of a protruding triangular deltopectoral crest on humerus [103.0]; wide distal femoral blade absent [138.1]. clades (a) and (b) form a more inclusive clade (e), and clades (c) and (d) with arthropterygius spp., leninia stellans and mollesaurus perialus form a more inclusive clade (f). both, clade (e) and (f) represent ophthalmosauridae. our results show that the topology and composition of the different clades proposed within ophthalmosauridae is highly variable (barrientos-lara & alvarado-ortega, 2021a; 2021b; campos et al., 2021; cortés et al., 2021; delsett et al., 2019; fernández et al., 2021; zverkov, 2022; and this work), indicating that the phylogenetic relationships within ophthalmosauridae are far from being completely resolved and understood. under these conditions, in this work we refrain to name and define the different clades recovered within ophthalmosauridae in our phylogenetic analysis. figure 11. phylogeny of ophthalmosauridae showing the position of platypterygius elsuntuoso sp. nov. time calibrated strict reduced consensus from 708 most parsimonious trees (589 steps) with 52 otus and 145 characters (see materials and methods) recovered from the analysis of the full dataset. bremer support values are shown in some branches. the letters within a black circle (a)-(f) indicate the different identified clades. 122 maría e. páramo-fonseca, cristian d. benavides-cabra, renzo a. garavito-camacho conclusions our evaluation of the genus platypterygius showed that this genus is distinguished from all other ophthalmosaurids from cranial and appendicular features. the forelimb of the type species p. platydactylus and those of p. americanus, p. australis, p. hercynicus, and p. sachicarum share a combination of features not found in other ophthalmosaurids. among these features, the following were found to be exclusive to the genus: the distally oriented articular facets for the ulna and the radius on the humerus; the roughly pentagonal shape of the intermedium with parallel anterior and posterior sides and longer and straight distal side; and the relations of the intermedium with surrounding elements, fitting between the radius and ulna and contacting almost exclusively radiale, ulnare and distal carpal 3. based on the morphology of the specimens of platypterygius preserving both cranial and forelimb remains, we found that the skull of this genus is characterized and differentiated from the other cretaceous ophthalmosaurids by a combination of characters that include a large size (surpassing 70 cm in length), a skull with almost straight dorsal and ventral margins in lateral view, a proportionally small orbit relative to the mandible length, an extensive postorbital region, and an external naris divided in two openings by a nasomaxillary pillar, among others. since the morphology of the forelimb and skull of p. sachicarum concur with that of platypterygius, the recently erected name “kyhytysuka” proposed to differentiate p. sachicarum is here rejected. in addition, given that the morphology of “simbirskiasaurus” concurs with that found in platypterygius, we found not sufficient evidence to support a generic differentiation of this taxon. therefore, we consider these two names to be junior synonyms of platypterygius. the morphology of the new specimen here described, concurs with the diagnostic features of the genus platypterygius, supporting its inclusion in this genus. the differences noted between the new specimen and all other platypterygius species, support the erection of a new species, p. elsuntuoso. among a particular combination of characters defining the new species, three unique features among platypterygius were recognized: 1the anterior ventral border of the extracondylar area of the basioccipital bears a small ventral peg anteroventrally directed, which fits into a posteroventral depression found in the basisphenoid; 2quadrate with supratemporal groove dorsal to the stapedial foramen; and 3root of teeth with a shallow neck at the base of the acellular cementum ring. the morphological comparisons and our phylogenetic analysis show p. elsuntuoso more closely related to p. sachicarum than to other platypterygius species. the phylogenetic analysis recovered all the platypterygius species in a single clade together with plutoniosaurus and sisteronia. however, the different branches recovered within ophthalmosauridae are still not well-supported indicating that the phylogenetic relationship between the ophthalmosaurids is not yet entirely understood. we concluded that platypterygius is a genus distinguished by a characteristic forelimb structure and a particular combination of skull features. we emended its diagnosis and recognized as valid the barremian to lower cenomanian species p. platydactylus, p. americanus, p. australis, p. hercynicus, p. sachicarum, p. birjukovi and the new species p. elsuntuoso. thus, platypterygius was a cosmopolitan genus that diversified as it conquered the waters of the early to early late cretaceous oceans and seas. acknowledgments we especially want to thank the padilla brothers, carlos bernardo and santiago (founders of the cip) who, before passing away, allowed us to study the specimen; we dedicate this work to their memory. we are grateful to dr. fernando etayo-serna for his comments on the ammonoid taxonomy and biostratigraphic context. we extend our thanks to mary luz parra-ruge, juan de dios parra-ruge and fredy parra-ruge from the cip for the excellent preparation of the fossil. we also thank mary luz parra-ruge for allowing us to review the specimens of platypterygius housed in the cip. furthermore, we are grateful to the cip staff for all their patient support during our study visits. likewise, we are grateful to h. d. palma from departamento de biología of the universidad nacional de colombia for producing the 3d model of the basioccipital. we thank victoria corredor from the museo josé royo y gómez of the sgc for allowing us to review the holotype of p. sachicarum. we also thank jamie maclaren and valentin fischer for providing useful photos of p. americanus and p. australis that were used in our comparisons. we are grateful to nikolay zverkov and an anonymous reviewer for their constructive comments, which helped to improve the current version of the manuscript. supplemental information supplemental data 1: textured and scaled 3d model of basioccipital (https://doi.org/10.6084/m9.figshare.24434182) supplemental data 2: character scoring supplemental data 3: data matrix references appleby, r. m. 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(2019). a prevalence of arthropterygius (ichthyosauria: ophthalmosauridae) in the late jurassic-earliest cretaceous of the boreal realm. peerj, 2019(4), e6799. https://doi.org/10.7717/ peerj.6799/supp-2 https://doi.org/10.1134/s0031030122040141 https://doi.org/10.1134/s003103012211020x https://doi.org/10.1134/s003103012211020x https://doi.org/10.1080/14772019.2018.1515793 https://doi.org/10.1080/14772019.2018.1515793 https://doi.org/10.1016/j.cretres.2020.104550 https://doi.org/10.1093/zoolinnean/zlaa028 https://doi.org/10.7717/peerj.6799/supp-2 https://doi.org/10.7717/peerj.6799/supp-2 _heading=h.30j0zll _heading=h.1fob9te _heading=h.3znysh7 _heading=h.2et92p0 _heading=h.1t3h5sf issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.113137 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 231 237 m in in g e n g in ee r in g abstract: study on the structure and strata behavior regularity of repeated mining overburden of close-distance shallow buried coal seam group tielin shang1*, rui wang2, jindong wang1, pei zhang3 1. school of energy engineering ,yulin university, shaanxi yulin 719000, china 2. school of mining and technology, inner mongolia university of technology, hohhot inner mongolia 010051, china 3. college of energy engineering, xi’an university of science and technology, shaanxi xi’an 710054, china *corresponding author: shangtielin@yulinu.edu.cn record manuscript received: 23/02/2024 accepted for publication: 31/07/2024 keywords: sbcs group; similar material simulation; numerical simulation; residual coal pillar (cp); cover rock construction; strata behavior regularity palabras clave: grupo veta de carbón somera; simulación material; residual del pilar de carbón; estructura de la roca de cubrimiento; regularidad del comportamiento de capas. how to cite this item: shang, t.,wang, r., wang, j., & zhang, p. (2024). study on the structure and strata behavior regularity of repeated mining overburden of close-distance shallow buried coal seam group. earth sciences research journal, 28(2), 231237. https://doi.org/10.15446/esrj.v28n2.113137 this paper studies the collapse structure, plastic failure regularity, and overburden movement regularity in the mining process of the shallowly buried coal seam (sbcs) group in the yulin area using physical similar material simulation, computer numerical simulation, and theoretical analysis. the objective of the present article is to explore the strata behavior regularity of the sbcs group, taking the engineering geological conditions of hongliulin coal mine as the sample. according to the results, when the lower coal seam of the sbcs group is mined, the initial stress step increases, and the periodic stress step decreases compared to the upper first coal seam. during the mining of the lower coal seam, there is an extreme collapse span in the interlayer rock formation: the process induces the activation of the collapsed roof in the goaf of the upper coal seam, and the overall sinking phenomenon occurs. the dynamic stress is obvious. the roof stress is significant in passing through the upper coal seam’s residual coal pillar (cp) into the lower coal seam. the cp is at a certain distance, and the residual cp is violently unstable and sinks the strata behavior regularity. the interlayer rock layer forms an inclined shear staggered fracture zone along the coal wall. the research results provide a theoretical method for controlling mine stress and green and safe mining in the sbcs group of coal production enterprises. estudio sobre la estructura y la regularidad del comportamiento de capas sobrecargadas en minería de un grupo de vetas de carbón enterradas a poca profundidad y a corta distancia resumen este artículo estudia la estructura de colapso, la regularidad de la falla plástica y la regularidad del movimiento de sobrecarga en el proceso de minería del grupo veta de carbón somera (sbcs, literal del inglés shallowly buried coal seam) en el área de yulin a través de simulación material, simulación numérica computarizada y análisis teórico. el objetivo del presente estudio es explorar el comportamiento regular del grupo sbcs con las condiciones ingenieriles y geológicas de la mina de carbón hongliulin como ejemplo. de acuerdo con los resultados, cuando se extrae la capa más baja del grupo sbcs el punto de tensión inicial aumenta y los puntos periódicos de tensión disminuyen en comparación con la capa más alta de explotación. durante la explotación de la capa más baja hay un punto de colapso extremo en la formación rocosa entre capas: el proceso induce la activación del techo colapsado en el socavón de la capa superior de explotación y el fenómeno hundimiento general ocurre. la dinámica de la tensión es evidente. la tensión del techo es determinante cuando pasa a través del pilar de carbón de la capa superior de explotación hacia la capa inferior. el pilar de carbón se encuentra a una distancia determinada y su residual es bastante inestable. la capa de roca forma una zona de fractura escalonada de corte inclinado a lo largo de la pared de carbón. lo resultados de la investigación proveen un método teórico para el control de tensión al interior de una mina, y procedimientos de minería verde y segura para las empresas productoras en el grupo sbcs. https://doi.org/10.15446/esrj.v28n2.113137 mailto:shangtielin@yulinu.edu.cn https://doi.org/10.15446/esrj.v28n2.113137 232 tielin shang, rui wang, jindong wang, pei zhang 1. introduction based on previous studies, many valuable conclusions have been obtained from the strata behavior regularity and rock formation control theory of the sbcs group. the compaction measurement of the n1200 working by ningtiaota coal mine in the valley shows that the strata behavior regularity under the goaf is different from that of the conventional working interface, such as the stress intensity being large, the size of the cycle being alternate, and the stress being more sudden (shuangcheng et al., 2013). taking the 12101 working interface of halagou coal mine as the engineering background, liu jiaxin concluded that the lower coal seam working interface passed through the upper concentrated cp, and the overall rotation movement of the cp and the critical block of the roof of the lower coal seam occurred in the cp stage. the cp was unstable, broken, and sinking, and strong mine stress appeared (jiaxin, 2020). huang qingxiang studied the sbcs goaf in the very close distance of the huoluowan coal mine. the initial and periodic stress step of the lower sbcs were significantly lower than that of the upper sbcs (qingxiang et al., 2018a). yu bin found that the jurassic coal seam group is relatively spacious, the interlayer rock layer is easily broken, and it is easy to connect with the upper coal’s collapsed and broken roof structure to form a broken roof group structure (bin, 2015). huang qingxiang proposed that the sbcs group should be divided into three types, which are no critical layer, single critical layer, and double key layer according to the inter-mining ratio (the thickness ratio of the interlayer to the mining height of the lower sbcs) (qingxiang et al., 2018b). qingxiang et al. (2019) found that when the lower coal seam was mined through the upper coal seam’s goaf, the working interface’s stress step distance became shorter. the strength increased, and the strata behavior regularity of the lower sbcs working interface had the most significant impact on the upper sbcs pillar. huang et al. (2017) summarized the basic strata behavior regularity of a relatively single sbcs mining, with relatively small periodic stress step and high compressive strength in the mining process of the sbcs group. qingxiang et al. (2018c) chose the downward mining of close-distance sbcs in the yushen mining area of northern shaanxi province. they studied through physical simulation that during the mining of coal seams under the close-range coal seams, the roof of the interlayer rock layer generally has a single key layer, and the roof is broken for the first time. huang et al. (2018) on-site investigation found that when the thickness of the interlayer rock layer of the sbcs group in the shenfu mining area is mostly 15~ 45m, the interlayer rock layer is prone to a single key layer structure. the periodic stress step distance decreases, the strength increases, the coal wall is serious, and the dynamic load phenomenon, step sinking phenomenon, and large and small cycle stress phenomenon are obvious. qingxiang et al. (2019) actual measurement of the 4-2 and 5-2 coal seams in the shenfu mining area found that the sbcs shortened the compression step distance by 25% and increased the compressive strength by 15%~22% compared with the single coal seam mining cycle. in order to understand the changes in the cover rock construction of the sbcs group after mining, jiang et al. (2022) studied the overburden regularity and bearing capacity of the abandoned coal mine of the sbcs group. chen et al. (2022) used a combination of theoretical-numerical simulation to study the directional evolution of the maximum principal stress facing the influence of double coal seam mining. the jurassic coalfield is rich in coal resources in northern shaanxi and has the regularity of many layers and close distance of the sbcs. at present, most of the large-scale mines in the yushenfu mining area have been mined in the first layer of coal and have entered the lower sbcs mining stage successively (haijun, 2019; junhu, 2019; limin et al., 2019; shuangming et al., 2010.) the mining face of the lower sbcs will pass through the goaf of the upper one, the empty roadway, and all kinds of residual concentrations. many years of production practice experience show that the repeated mining of the lower close sbcs group will have high mine stress intensity, violent stress, step subsidence, stress frame, and roofing. for example, when the 22103 fully mechanized mining face of daliuta well was 1.7m away from the boundary of the cp of the overlying 1-2 coal seam. the roof had intense stress, resulting in a frame accident in the middle of the working interface. during the period when the 31201 working interface of shibitai coal mine passed the cp, it left over the overlying room-pillar goaf. the sheet gang roof and large-scale roof open-off cut and pressing frame accidents occurred. when the overlying cp was released, the 12102, 12103, and 12105 working interfaces had similar strong mine stress and pressing accidents. when the 43303-2 working interface of yujialiang coal mine passed over the overlying concentrated cp, a large-scale strata failure open-off cut and pressing frame accident occurred. the working interfaces of coal seams 21304, 21305, and 21306 in the lower part of the third-panel area of the live jitujing in daliuta coal mine increased significantly after passing through the concentrated cp area (strip scouring zone) of the overlying coal seam and the coal wall sheet helped (xiwen, 2020; yongxi, 2015; minggao, & pingwu, 2020). based on the engineering geological conditions of the close sbcs group (3-1, 4-2, and 5-2 coals) in the hongliulin coal mine in the jurassic coalfield of northern shaanxi, this paper analyzes the regularity of the mining stress of the lower coal seam, the fault structure regularity of the overlying strata and the interlayer rock layer of the upper coal seam through similar material simulation experiments. the working interface is deeply studied through the appearance regularity and generation mechanism of the overlying residual cp stress, which provides a new method for the control of the mining roof of the sbcs group. physical similar material simulation the coal-bearing strata of hongliulin coal mine are the middle jurassic yan’an formation, and the designed main coal seams in the north second panel are 3-1 coal, 4-2 coal, and 5-2 coal. the average spacing between 3-1 coal and 4-2 coal is 43.51 m, and the average spacing between 5-2 coal and 4-2 coal is 61.54 m. the construction of the plane model experimental frame of similar materials is based on the engineering geological conditions of the north second panel in hongliulin coal mine. the thickness of each rock layer is based on the histogram data of no. 6-hb6 borehole, see figure 1. the physical and mechanical parameters of the main coal and rock are shown in table 1 and the geometric similarity ratio is cl = 200. the average density of the overlying strata layer is about 2400kg/m3, and the density of the model material is 1600kg/m3. the bulk density similarity ratio is cυ = 1.5 ratio, stress similarity ratio is cσ= 200 × 1.5 = 300. the size of the experimental model is 300 × ct = √200 = 14.14, 20×134.5cm, the length of the engineering geological prototype is 600m, and the height is 269m. after the model is air-dried, the excavation scheme is designed, and the coal seam group adopts the descending mining sequence. then, 60m protective cps were set at both boundaries of each coal seam excavation: 1m was pushed forward at each step of the coal face (5mm of model excavation), and 20m cps were set between the disk areas to reduce the influence of the boundary effect. digital photography and sketching were used to observe and record the caving structure regularity and movement rules of overlying strata during the excavation of 3-1 coal, 4-2 coal, and 5-2 coal. 2 overburden fracture structure and movement rules 2.1 3-1 coal roof breaking regularity and movement rules 3-1 coal is the first coal seam of the sbcs group. first, the open-off cut was arranged on the right side of the 3-1 coal. the working interface was gradually advanced from right to left, and when the working interface was upside to 50m, the overlying upper rock layer collapsed. the collapse height was 2m, and the roof produced a separation structure. with the continuous advancement of the working interface, the separation phenomenon gradually developed upward, and the number gradually increased. when the working interface advanced to 74m, the basic roof reached the initial limit collapse step and suddenly appeared a large range of falling bodies. the collapse height was 18m, as shown in figure 2. with the continuous advancement of the working interface, the basic roof was periodically broken and collapsed. the working interface appeared to be cyclically pressed, and the periodic stress step distance was 8-16m (16, 8, 14, 16, 16, 12), advancing to 310m to stop mining. after that, the open-off cut was arranged on the left side of the 3-1 coal, and the working interface was pushed from left to right. it was observed that the basic roof initial stress step distance was 79m, and the periodic stress step distance was 10-16 m (16, 13, 11, 16, 16, 16, 10). finally, the left side was excavated to 290m, leaving a 20m panel interval cp in the middle. 233study on the structure and strata behavior regularity of repeated mining overburden of close-distance shallow buried coal seam group number histogram lithology thickness(m) burial depth (m) 1 aeolian sand 18 18 2 loess 2 20 3 red soil 64 84 4 siltstone 16 100 5 medium siltstone 14 114 6 no3-1 coal mine 3 117 7 siltstone 9 126 8 fine siltstone 10 136 9 siltstone 7 143 10 sandy mudstone 10 153 11 siltstone 4 157 12 no4-2 coal mine 4 161 13 sandy mudstone 5 166 14 siltstone 25 191 15 fine siltstone 6 197 16 medium siltstone 16 213 17 siltstone 8 221 18 medium siltstone 3 224 19 no5-2 coal mine 6 230 20 fine siltstone 18 248 21 siltstone 12 260 22 medium siltstone 9 269 figure 1. coal seam histogram. table 1. physical and mechanical parameters and ratio of sbcs rocky bulk density/g/cm3 tensile strength/ mpa internal cohesion/ mpa internal friction angle/° tomarimatsuhi ratio number aeolian sand 2.12 0.06 0.8 28 0.13 loess 1.86 0.08 0.92 29.4 0.15 laterite 2.02 1.22 1.28 27.4 0.18 powdered siltstone 2.44 0.93 6.18 36.2 0.19 937 medium siltstone 2.28 2.18 6.07 37.1 0.18 828 coal 1.34 1.24 4.72 35.2 0.26 fine siltstone 2.29 2.29 5.6 37.3 0.2 837 sandy mudstone 2.3 0.29 5.85 28.6 0.24 928 18 figure 2. structural regularity of the initial collapse of the 3-1 coal basic roof 2.2 4-2 coal regularity and movement regularity of coal roof at the end of 3-1 coal mining, the overlying strata formation of 4-2 coal was the roof of 3-1 coal, and the interlayer rock formation between 3-1 coal and 4-2 coal. after the collapse of the overlying strata layer of 3-1, the coal reaches a stable state, and the working interface of the panel is arranged in the 4-2 coal. first, the open-off cut was arranged on the left side of the 4-2 coal, and the working interface was gradually advanced from left to right. when the working interface was advanced to 66m, the direct roof collapsed, and the height was 4m. when the working interface was advanced to 98m, the basic roof was pressed for the first time, but the stress was not violent. with the continuous advancement of the working interface, the roof separation gradually developed upward, but no large-scale collapse phenomenon occurred, and the mine stress appeared gentle. until the working interface advanced to 118m, the interlayer rock layer suddenly broke through completely, the 3-1 coal collapse roof rock layer was activated, and the interlayer rock and the roof of the 3-1 coal goaf were formed. a whole, synchronous sinking, a wide range of collapse movement, 234 tielin shang, rui wang, jindong wang, pei zhang the mine stress appeared violent, at this time for the third cycle of stress. the overburden fracture structure is shown in figure 3. 4-2 coal working interface cycle stress step distance 8-13m (8, 12, 12, 11, 13, 12, 12), the working interface was advanced to 350m to stop mining. 4-2 coal right side working interface, the initial stress step distance was 108m, and the periodic stress step distance was 10-12m (12, 12, 12, 12, 12, 10). in general, the initial collapse step distance of 4-2 coal was longer than that of 3-1 coal, and the step distance of periodic stress became shorter, advancing to 370m, leaving a 20m panel cp. goaf of no. 3-1 coal working face of no. 4-2 coal no. 5-2 coal figure 3. structural regularity of 4-2 fractured rock in the third cycle of coal mining 2.3 5-2 fracture regularity and movement regularity of coal roof at the end of 4-2 coal mining, the overlying strata formation of coal 5-2 is the goaf roof of 4-2 coal and the interlayer rock formation between 5-2 coal and 4-2 coal. after the collapse of the overlying strata layer of 4-2 coal reaches a stable state, the working interface of the panel is arranged in 5-2 coal. first of all, the open-off cut is arranged on the right side, and the working interface is pushed from right to left, and when the working interface is advanced to 100m, it collapses directly, and the height of the collapse is 4m. similar to the excavation process of 4-2 coal, the working interface is advanced to 136m, and the old roof is pressed for the first time, but the mine stress is not violent. with the advancement of the working interface, the roof separation gradually develops upward, when the working interface is advanced to 152m, the interval between 5-2 coal and 4-2 coal and the sudden breakthrough break, 3-1 coal and 4-2 coal 3-1 coal goaf roof rock formation activation, synchronous sinking, largescale collapse movement, the mine stress appears violent, and the working interface cycle stress step distance is 6-10m (10, 6, 8...... ), of which the fourth cycle is pressed as shown in figure 4, which was advanced to 258m to stop mining. 5-2 coal left side working interface, the initial stress step distance is 132m, and the periodic stress step distance is 810m (8, 10, 8...... ). in general, the initial collapse step of 5-2 coal is longer than that of 3-1 coal and 4-2 coal, and the cycle stress step distance is shorter. finally, the left working interface is advanced 238m, leaving a 20m panel interval cp. goaf of no. 3-1 coal goaf of no. 4-2 coal working face of no 5-2 coal figure 4. structural regularity of the fourth cycle of coal mining to strata failure 3. the movement regularity of the overlying cp strata failure 3.1 theoretical analysis of the influence of legacy cp in the specific coal mine production practice, in order to ensure the safety of the mining work, it is necessary to leave a panel to protect the cp between the working interfaces of the two panels. in this simulation experiment, 3-1 coal and 4-2 coal seams are left with 20m panel cps between adjacent working interfaces. in the process of 4-2 coal mining, there will be concentrated stress areas formed by cps in the bottom plate of the coal mining interface through the 3-1 coal residual panels, and in the process of 5-2 coal mining, there will be concentrated stress areas formed by cps in the bottom plate of the coal mining working interface through the 4-2 coal legacy panels. the residual cps form a stress concentration phenomenon on its floor in the upper coal seam, and the influence range is determined by the influence angle of stress transmission in some literatures, the internal friction angle and stress influence angle of the foundation in soil mechanics, and the angle of inclusion in the stress reduction zone and the concentrated stress transmission angle in some literatures. the working interface of the lower coal seam is affected by the concentrated stress theory of the residual cp in the upper coal seam, as shown in figure 5. the influence range l of the concentrated stress is calculated as equation 1. l = a + 2b 1 where: a is the width of the residual cp, b is the influence distance of the concentrated stress of the residual cp in the upper coal seam. the influence distance b of the concentrated stress of the cp can be calculated by equation 2. b = h × tanφ 2 where: h is the thickness of the rock layer between the upper and lower coal seams, m, and is φ the angle of influence of stress transmission, °. the cp in the residual panel area of this experiment is 20m φ, and the spacing between 3-1 coal and 4-2 coal is 40m according to the histogram data of borehole 6-hb6, and the distance between 4-2 coal and 5-2 coal is 63m, which is calculated by equation (1). the influence range of 3-1 coal residual cp on the concentrated stress of 4-2 coal working interface is 49.2m (b = 14.6m), and the influence range of 4-2 coal residual cp on the concentrated stress of 5-2 coal working interface is 65.8m (b = 22.9m). figure 5. the mining of the lower coal seam affected by the concentrated stress of the residual cps in the upper coal seam 3.2 the movement regularity of cp overburden in the 3-1 coal panel area of the 4-2 coal working interface affected by the cps left over from the mining of the upper coal seam, the compression phenomenon of the bracket was greatly reduced in the process of mining the overlying cps in the lower coal seam working interface in many mines in the shendong mining area, which seriously affected the efficient production of the safety pipe of the coal mine. the statistical results show that this kind of frame generally appears within the boundary of the overlying cp mined in the working interface (related to the coal seam spacing), and there is generally no stress frame phenomenon in the mining process of entering the cp. it is necessary to explore the macroscopic structural regularity and internal mechanical mechanism of this kind of ore stress phenomenon. 235study on the structure and strata behavior regularity of repeated mining overburden of close-distance shallow buried coal seam group at the beginning of the 4-2 coal working interface advancing 195m (15m away from the coal inlet pillar), the support stress increased significantly, and the coal seam force could be obviously felt during the excavation process. when the cp of the 3-1 coal residual panel is 22m out of the 4-2 coal working interface, the interlayer rock layer collapses, and the cp in the residual panel appears to be sharply unstable and sinking, and the strata failure failure regularity of the 4-2 coal working interface are sketched as shown in figure 6. figure 6. sketch of the fractured structure of the overlying strata with the instability of the intervaled rock strata and the 3-1 cp it can be seen in figure 6 that the overlying strata of the roof on both sides of the 3-1 coal mining has fully collapsed, and the cps in the residual panel have formed a certain stress concentration on the floor. the coal and rock types of the intervals between 3-1 coal and 4-2 coal include siltstone, fine-grained siltstone and sandy mudstone, and their lithology is relatively similar and does not form obvious key layer structure. the collapse position of the interlayer rock formation and the residual cp does not occur along the influence angle of stress transmission, that is, the 4-2 coal working interface is located at the 14.6m of the cp, but occurs near the 22m position of the cp, due to the close spacing of the coal seams, the overall breaking movement regularity of the interlayer rock formation occurs, and the cp of the 4-2 coal working interface is between 14.6~22man oblique shear staggered fracture zone is formed to the lower right. 3.3 the movement regularity of cp overburden in the 5-2 coal working interface through the 4-2 coal panel the 5-2 coal working interface advances from right to left, and when it reaches the influence range of concentrated compressive stress of the cp in the 4-2 coal panel, the stress of the working interface support begins to increase. in the process of excavation, there is also no obvious ore stress regularity. when the cp of the 4-2 coal residual panel is 32m out of the 5-2 coal working interface, the interlayer rock layer collapses, and the cp in the residual 4-2 coal panel also appears sharply unstable and sinks, and the overburden failure regularity of the 5-2 coal working interface are sketched as shown in figure 7. also from fig. 7, it can be seen that the cp in the panel left over from the 4-2 coal forms a concentrated compressive stress on the bottom plate, which is transmitted to the 5-2 coal working interface through the spacer rock layer. the types of coal rocks in the intervals between 4-2 coal and 5-2 coal are siltstone, medium-grained siltstone, fine-grained siltstone and sandy mudstone, and no critical layer structure is formed. the stress transmission influence angle does not occur at the collapse position of the interlayer rock formation and the residual cp, that is, the 5-2 coal working interface is located at the cp b=22.9m, but occurs near the 32m position of the cp, and the interlayer rock formation forms an inclined shear staggered fracture zone to the lower left between the cp 22.9~32m of the cp of the 5-2 coal working interface. subsequently, when the excavation of the 5-2 coal working interface is located below the cp in the 3-1 coal residual panel, the stress increase of the working interface support is not obvious, and there is no strong ore stress appearance, indicating that due to the large separation distance, the cp in the residual panel has little impact on the working interface of the farther lower coal seam. figure 7. sketch of the fractured structure of the strata failure with the instability of the intervaled rock strata and the cp left over from the 4-2 coal 4 flac3d simulation 4.1 establishment of the model the similar material simulation experiments in the laboratory analyzed the geometric structure regularity of the overburden roof formation and the macroscopic fracture phenomenon of the movement regularity during the repeated mining process of close sbcs groups from the appearance and morphology. however, it does not reflect the size and location of the plastic failure range of the roof and overburden during the coal seam excavation process. it is necessary to use flac3d computer numerical simulation software to analyze the internal mechanism of fracture motion (plastic distribution, stress and displacement regularity) of overburden during the mining process. the calculation model is also constructed according to the histogram data of borehole 6-hb6 and table 1, with a strike length of 600 m (x-direction), a trend length (y-direction) of 8 m, and a height (z-direction) of 269m, the model has a total of 147225 grids and 191936 nodes. according to the same excavation sequence and process as similar material simulation experiments, the excavation of the calculation model was carried out (60m boundary protective cps were left on each side), and each excavation was 10m, and the calculation of 100 hours was carried out after the mining of each working interface. due to the large number of coal seams and the cumbersome excavation process, this simulation does not analyze the stress regularity of the coal seam group mining process, but focuses on the failure regularity and movement regularity of the cp overburden in the upper coal seam working interface through the upper coal seam left panel. 4.2 distribution regularity of plastic failure areas of overburden mining in coal seam groups the plastic failure regularity of the overlying strata after the mining of the left working interface of 4-2 coal seam (x=290~310) in the coal panel area of the upper coal seam 3-1 are shown in figure 8. after the mining of the right side of the 5-2 coal working interface through the cp (x=350~370) of the upper coal seam 4-2 coal panel, it reaches x=258, and the plastic failure regularity of the overlying strata after the excavation of the left working interface (the cp of the 5-2 coal panel area is x=238~258) are shown in figure 9. as can be seen from figure 8, the stress concentration of the cp in the 3-1 coal legacy panel formation on its bottom plate causes the stress of the roof of the underlying 4-2 coal working interface to increase through the influence range of the area, and the plastic failure of the interlayer rock layer is mainly shear failure, supplemented by tensile failure. the plastic failure phenomenon shows that the vertical stress formed by the cp in the residual panel will cause the shear dislocation of the overall fracture structure in the interlayer rock formation between the cp in the 3-1 coal legacy panel and the 4-2 coal propulsion working interface, resulting in the phenomenon of slippage and instability, resulting in 236 tielin shang, rui wang, jindong wang, pei zhang the pressing accident of the working interface. it can also be seen from figure 9 that the cp in the 4-2 coal legacy panel also forms a stress concentration on its floor, and the interlayer rock layer between 4-2 coal and 5-2 coal is dominated by shear failure, supplemented by tensile failure. figure 8. regularity of plastic failure of strata failure after mining on the left side of the 4-2 coal working interface figure 9. plastic failure regularity of strata failure after mining on the right side of the 5-2 coal working interface 4.3 deformation regularity of strata failure mining in the process of advancing the working interface of the lower coal seam through the cp in the residual panel of the upper one, the sinking deformation of the cp is in a dynamic change process. the history function of flac3d was used to record the dynamic changes of the vertical displacement of the cp. the occurrence of the severe instability of the cp was analyzed. in the mining process of the left working interface of 4-2 coal, the displacement monitoring point is set in the middle position of the cp at the bottom of the cp in the 3-1 coal panel (x=300, y=4, z=152), which is recorded as history id=2, and the vertical displacement curve of the residual cp in the 3-1 coal panel is shown in figure 10. the displacement contour diagram of the left working interface of 4-2 coal after the end of mining is shown in figure 11. it can be seen from figure 10 that the mining process of 4-2 coal working interface caused the subsidence deformation of the cp in the 3-1 coal legacy panel, the initial stage of the subsidence trend is relatively gentle, from 1860~2030 steps of vertical displacement sinking sharply, after the model excavation is completed, 100 hours of step calculation, 1860 hours after the start of 4-2 coal working interface out of the cp 3-1 coal left panel mining process, it can be concluded that in the engineering production practice, the cp in the upper coal seam residual panel will be violently submerged in the process of cp out of the lower coal seam working interface. it can be seen from fig. 11 that the displacement of the cp and the lower interlayer rock formation in the 3-1 coal legacy panel changes greatly along the cp and the working interface, which is similar to the simulation experiment of similar materials, and the two corroborate each other. in the mining process of the 5-2 coal working interface, the vertical motion regularity and displacement change regularity of the cps left in the 4-2 coal panel are similar to those of the cps in the 3-1 coal panel, which will not be repeated here. figure 10. the vertical displacement of the cp in the left working interface of the 4-2 coal mining 3-1 coal legacy panel changes step by step figure 11. 4-2 displacement contour after the end of mining at the left working interface of the coal 5 conclusion 1. in the initial stage of coal seam mining under the sbcs group, due to the formation of a “large structure” of the strata failure of the upper first coal seam, the stress of the roof overburden in the goaf is not fully transmitted to the interlayer rock layer, and the interlayer rock layer has a specific thickness and strength, and the initial stress limit collapse step l is larger, and the initial mining stress step is larger than that of the upper one, and the mine stress is not violent. 2. in the process of lower coal seam mining, under the action of the stress of the collapsed roof in the goaf of the upper coal seam, the overall instability, fracture and collapse of the interlayer rock formation in a certain position of the working interface (affected by the intermediate-mining 237study on the structure and strata behavior regularity of repeated mining overburden of close-distance shallow buried coal seam group ratio) appears, and the collapsed roof of the overlying goaf of the upper coal seam is activated, and the mine stress is violent, which needs to be paid attention to in the specific mining practice of mine stress control. after the mining of the coal seam in yulin area, the overlying strata layer mainly forms a falling zone and a fracture zone, after the mining of the upper coal seam of the coal seam group, the overlying strata is in a loose state, especially the mining of the lower coal seam is advanced for a certain distance, and the activation of the collapsed roof of the overlying coal seam is easy to transfer the stress to the interlayer rock layer, resulting in the reduction of the periodic stress step of the lower coal seam roof (interlayer rock layer). 3. in the process of the lower coal seam mining face passing through the stress concentration range of the residual cp in the upper coal seam, the support stress increases, and the cp is at a certain distance (outside the influence angle of stress transmission), and the overall collapse and instability of the residual cp and the interlayer rock layer occurs, forming an inclined shear dislocation fracture zone. in production practice, it is inevitable that the shear failure of the close sbcs group in yulin area will be violently unstable, and the load transmitted by it is also difficult for the support to bear. in the process of mining in the lower coal seam, it is necessary to optimize the mining design, pre-cracking and blasting of the cp, accelerate the advancing speed of the working interface, adjust the inclined working interface, and increase the working resistance of the hydraulic support to avoid the impact on the safety of the working interface. acknowledgments this work was supported by the following: (1) national natural science foundation of china [grant no 52064047]; (2) shaanxi province science and technology plan project [2020sf-418]; (3) industry-university-research project of yulin science and technology bureau [cxy-2022-86]. references bin, y. 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(2015). coal mining. xuzhou: china university of mining and technology press, 93, 402. https://doi.org/10.3390/en15218257 https://doi.org/10.3390/en15218257 https://doi.org/10.3390/en15249359 https://doi.org/10.3390/en15249359 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n2.117229 earth sciences research journal earth sci. res. j. vol. 29, no. 2 (june, 2025): 183 193 g eo lo g y abstract introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran) soudeh sedighian s.sedighian@velayat.ac.ir assistant professor of department of geology, velayat university, iranshahr, iran record manuscript received: 26/10/2024 accepted for publication: 04/04/2025 how to cite this item: sedighian, s. (2025). introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran). earth sciences research journal, 183-193 https://doi. org/10.15446/esrj.v29n2.117229 the gohar kouh area, as a suspected tectonic region, consists of a collection of large folded structures, and is the core of a large arc called the baloch arc, which is located in the southwest of the sistan suture zone in the southeast of iran. young magmatism with olivine-basalt and basalt compositionally attributed to the quaternary age has affected these units in the northern part. these lavas are composed of olivine, pyroxene, and plagioclase phenocrysts with low to moderate alteration, which are situated in a groundmass of plagioclase microlites and fine-grained pyroxenes and opaque minerals. porphyry texture is the main texture of these rocks. geochemical studies indicate that these rocks belong to the range of calc-alkaline basalts with moderate potassium and were formed as a result of the subduction of the oceanic crust of oman under the eurasian continental crust in the range of arc-related basalts in an active continental margin and a back-arc extensional environment. investigation of trace element variations in these basalts proves the existence of lithospheric mantle origin. the presence of fluids resulting from subduction, involvement of sediments on the subducted crust, and crustal pollution are some of the reasons that have led to the enrichment of lile elements compared to hfse in the magma that created these rocks. keywords: quaternary magmatism, basalt, continental arc, back arc, subduction, oman oceanic crust. resumen el área de gohar kouh, considerada una posible región tectónica, consiste de una colección de grandes estructuras sobrepuestas y es el centro de un arco grande llamado el arco de baloch, el cual se localiza en el suroeste de la zona de sutura de sistan, en el sureste de irán. magmatismo reciente compuesto de basaltos olivínicos y basaltos que se atribuyen a la edad cuaternaria han afectado estas unidades en la parte norte. estas lavas están compuestas de olivino, piroxeno y fenocristales de plagiocasa con alteración entre baja y moderada, las cuales están situadas en una masa de tierra con microlitos de plagiocasa, piroxenos de grano fino y minerales opacos. la textura porfirítica es la principal de estas rocas. estudios geoquímicos indican que estas rocas pertenecen al rango de basaltos calcoalcalinos con potasio moderado que se habrían formado como resultado de la subducción de la capa oceánica de omán, bajo la capa terrestre continental eurasiática y están en el rango de basaltos relacionados a un arco en un margen continental activo y en un ambiente extensional de arco de respaldo. la investigación de las variaciones de elementos traza en estos basaltos prueban la existencia del origen de un manto litosférico. la presencia de fluidos resultantes de la subducción, la presencia de sedimentos en la corteza subducida y la contaminación de la corteza son algunas de las razones determinantes para el enriquecimiento de elementos lile en comparación con los elementos de alta intensidad de campo en el magma que creó estas rocas. palabras clave: magmatismo cuaternario; basalto; arco continental; arco de respaldo; subducción; corteza oceánica de omán presentación del magmatismo cuaternario del área de gohar kouh con un enfoque en las características litológicas, geoquímicas y petrogenéticas de las rocas (sudeste de irán) https://doi.org/10.15446/esrj.v29n2.117229 mailto:s.sedighian@velayat.ac.ir https://doi.org/10.15446/esrj.v29n2.117229 https://doi.org/10.15446/esrj.v29n2.117229 184 soudeh sedighian 1. introduction the sistan suture zone (ssz) is an important connection in the middle part of the orogenic belt of the himalayas and zagros, the study of which is very important in understanding the tectonic conditions of central asia and the middle east. this zone in eastern iran separates the lut continental block in the west from the afghan block in the east with a north-northwest to south-southeast trend (sengor, 1990; mohammadi et al., 2016). the sistan ocean is a branch of the tethys ocean that was closed at the end of the eocene (mohammadi et al., 2016) and then affected by the strike-slip fault between the central iranian and afghan blocks (tirrul et al., 1983). based on paleogeographic, structural, and petrographic studies, the eastward dip direction of the subducting plate has been identified (camp and griffis, 1982; tirrul et al., 1983; mohammadi et al., 2016). the sistan suture zone consists of a deformed accretionary wedge and a flanking fore-arc basin. this range of three structural-sedimentary complexes separated by thrust faults (camp and griffis, 1982; tirrul et al., 1983), includes the ratuk complex to the east, which is an older accretionary wedge, the neh complex to the west, which is a fold-thrust belt, and the sefidabeh fore arc complex that covers unconformably both other complexes. thrusting and strikeslip faulting have affected the basal stratigraphic on lap onto the underlying wedge turbidites (tirrul et al., 1983; mohammadi et al., 2016). ssz has various types of igneous rocks, including ultramafic to felsic rocks found in ophiolitic complexes, intrusive rocks related to the granitoid belt of garagheh-saravan, extrusive and intrusive rocks related to zahedan-nehbandan magmatic belt and volcanic rocks related to the taftan mountain activity (camp and griffis, 1982). the intrusive and extrusive masses in this area have been studied by many researchers (tirrul et al., 1983; mohammadi et al., 2016; keshtgar et al., 2016; sepidbar et al., 2018; boomeri et al., 2019; boomeri et al., 2020; omidianfar et al., 2023). the study area is located in the eastern part of iran, on the southeastern margin of the lut block and the southwestern boundary of the ssz, within the neh complex, and is associated with the makran subduction zone (msz). the subduction of the arabian plate beneath the eurasian plate along the makran plays a vital role in shaping the tectonic processes and magmatic activities in eastern iran, influencing both the geological features and the formation of volcanic and plutonic rocks in the area. to date, no comprehensive studies have been conducted in this region. therefore, the aim of this paper is to enhance our understanding of the conditions and environment associated with the formation of this area through fieldwork, petrographic, geochemical, and tectonic analyses. this complex tectonic interaction between the msz and the surrounding geological formations is crucial for understanding the region’s overall magmatic evolution. general geology the gohar kouh volcanic masses are located in the southwestern part of zahedan in the sistan suture zone (ssz), southeastern iran (fig. 1-a). in general, iran is a collection of several sedimentary-structural zones, and the ssz is one of these zones, which is known in the east and southeast of iran as a part of the remnants of the oceanic crust during the cretaceous period, with a north-south trend and a length of over more than 700 km (tirrul et al., 1983) (fig. 1-a). this area is located between the lut block on the west and the afghan block on the east. the ssz is divided into three sections: the neh and ratuk accretionary wedges and the sefidabeh basin (camp and griffis, 1982; rojhani et al., 2025). the neh and ratuk accretionary wedges are characterized by faulted and highly deformed ophiolitic mélanges, late cretaceous to eocene phyllites, and paleogene marine clastic sedimentary rocks. on the other hand, the basin, in front of the sefidabeh arc, contains small amounts of ophiolitic mélanges and a coherent and organized stratigraphy. according to camp and griffis (1982) and tirrul et al. (1983), the non-ophiolitic igneous rocks in this zone differ from each other in terms of age, composition and origin, and they can be divided into four groups based on age: 1eocene calc-alkaline rocks attributed to the subduction of the lut block beneath the afghan block (camp and griffis, 1982); 2i and s calc-alkaline eocene-lower oligocene granitoids of zahedan, which are related to the subduction and collision processes in the region (camp and griffis, 1982; boomeri et al., 2005; sadeghian et al., 2005; sadeghian and valizadeh, 2007; rahnama-rad et al., 2008; ghasemi et al., 2010; mohammadi et al., 2016); 3oligocene-middle miocene alkaline and calc-alkaline igneous rocks (camp and griffis, 1982), that are related to the activity of major faults in the eastern part of the region, which play a significant role in creating post-collision structural forms (camp and griffis, 1982; walker and jackson, 2004) and 4quaternary volcanic rocks similar to taftan, which are attributed to the active subduction of makran from the arabian plate beneath the makran accretionary wedge and the sistan suture zone (farhoudi and karig, 1977; boomeri et al., 2020; delavari et al., 2022). the magma that forms basaltic rocks is the primary magma, results from slight changes in the melting of peridotite rocks in mantle conditions; therefore, the survey of these rocks can provide valuable information regarding the origin, structure, and processing formation of their magma. the gohar kouh quaternary basaltic masses are scattered as several separate masses in the northern region of the makran magmatic arc (fig. 1-b). based on the limited studies reported from these areas (moinevaziri, 1985; biabangard and moradian, 2008; pang et al., 2014) and the arrangement of these masses, a close relationship between the active subduction process in makran and their magmatism can be recognized. however, there are still many ambiguities for their formation that need further investigation. regional geology field observations on the volcanic rocks of the study area, which mainly have basaltic composition, show that these rocks are small and limited masses in size and extent with not very high topographies (fig. 2-a and b). the quaternary sediments that constitute a major part of the region, are covered by these volcanic rocks and show mostly dark color (dark brown to black) and purple in some areas (fig. 2-c and d). some parts of these masses, especially in the southwestern of the region, have a clear and distinct contact with metamorphosed carbonate rocks (dolomite and limestone) which, according to the eftekharnezhad report (1992), are equivalent to the jamal formation with a probable permian age (fig. 2-c, d and e). the existence of metamorphosed enclaves within the volcanic rocks of the region at their contact confirms this possible age (fig. 2-f). in this region, the dominant texture in the rock samples is aphanitic and vesicular texture, which shows a clear increase in the size of these vesicles in the basalt flows from the lower parts to the top of them. figure 1. a: major sedimentary-structural units of iran (taken from ashrafpour et al., 2012); b: the location of basaltic lavas of gohar kouh region in 1: 100 000 dorjing sheet (eftekharnezhad, 1992). 185introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran) figure 2. a and b: the presence of basaltic flows and lavas with relatively low topography in the area (view of the photos toward the northeast). c, d, and e: clear contacts between lava flows and carbonate rocks, which have led to the alteration of these rocks in these areas (view of the photos toward the north (c) and east (d)). f: the existence of enclaves of metamorphosed carbonate rocks within the studied basaltic rocks. 2. research method in this research, after extensive field observations in the region, to conduct microscopic and geochemical studies, samples were collected from different parts of these igneous masses. among these samples, 75 samples were selected for the preparation of microscopic thin sections and petrographic studies. for geochemical studies, 12 samples that were either unaltered or had minimal alteration were prepared and analyzed by using the lithium borate fusion method for main elements and inductively coupled plasma mass spectrometry for trace and rare earth elements in the laboratory of zarazma mineral studies company. finally, various geological software such as igpet and minpet were used to process and interpret the results obtained from these analyses. petrography based on petrographic studies, the composition of the studied rock samples is mainly basalt, trachybasalt, and olivine basalt. the overall texture observed in these samples is a porphyritic texture with a fine-grained groundmass; however, trachytic, intergranular, glomeroporphyritic, and vesicular textures are the other textures that can be seen in the rock samples (fig. 3). the degree of alteration in the rock samples is low to moderate. olivine, pyroxene, and plagioclase are the most prominent crystals in these rocks. these crystals constitute about 70 to 75% of the volume of these rock groundmasses compared to the phenocryst crystals. olivine phenocrysts (13-17%) are euhedral to anhedral (fig. 3-a and f), with sizes ranging from 0.4 to 1.5 mm. in some cases, these crystals are fractured and altered to iddingsite from the edges of the crystal (fig. 3-b and f). this alteration indicates the existence of oxidizing conditions in the rock-forming environment during alteration, which occurs more frequently in olivines with lower forsterite and higher amounts of iron. the skeletal texture that can be observed in olivine crystals (fig. 3-f) indicates a very high cooling rate and even a rapid solidification of the initial magma (salas et al., 2021). pyroxenes, which are frequently observed as coarse crystals, are subhedral to anhedral and are less abundant than olivines. the size of these crystals is from 0.2 to 0.9 mm, and they are mostly located in the rock groundmass and the spaces between plagioclase microliths. the hourglass sector zoning can be seen in some pyroxenes (fig. 3-a, c and e). the formation of this type of zoning indicates differences in crystal chemistry due to kinetic effects at the growth surfaces, whereby the elements are incorporated into the structure at different rates along different crystallographic surfaces, instead of changing the chemistry or conditions of the surrounding magma (hollister and guncarz, 1971). at least four controlling factors exist for creating hourglass sector zoning (hollister and gancarz, 1971): (1) the size and composition of ionic complexes added to the crystal during growth, (2) the rate of material addition, (3) the rate of equilibration of new material with groundmass on the growth surfaces, and (4) the rate of re-equilibration of surface layers with groundmass by ion exchange perpendicular to the crystal surfaces. therefore, these crystals have grown so fast that they are ahead of their adjacent growing areas in terms of diffusion equilibrium (schoneveld et al., 2020). the alteration degree of these crystals is not very high. plagioclase microliths, which constitute the majority of the rock groundmass (more than 43%), are in the range of labradorite exhibiting an average extinction angle of about 32 degrees. these crystals show a trachytic texture in most of the samples (fig. 3-c). iron oxides are visible as opaque minerals, comprising about 7% of these rocks (fig. 3-d). the presence of magnetite and its inclusion by olivine and pyroxene crystals is a sign of the early crystallization of these minerals from a waterless magma; magnetites crystallize simultaneously with olivine in water-less magma. however, in hydrous magmas with low oxygen fugacity, this crystallization occurs in the later stages and after plagioclase crystallization (hall, 1996). if there is an oxidizing condition in the basaltic magma crystallization, magnetite forms as a separate phase, which explains the higher presence of magnetite in olivinebearing basalts compared to olivine-free ones (gunnlaugsson et al., 2006). 186 soudeh sedighian figure 3. a and d: the presence of olivine phenocrysts within the groundmass of plagioclase microlites and fine-grained pyroxenes, porphyritic texture, in the rock samples of the study area; b and f: evidence of iddingsite around euhedral to subhedral olivine phenocrysts; c: observation of flow and vesicular texture in the rock samples of the study area; e: glomeroporphyritic texture in the gohar kouh basalts; f: skeletal texture in the olivine crystals. geochemistry in table 1 and 2, the results of chemical analysis of rock samples from the gohar kouh volcanic masses are presented. in these samples, the loi (loss on ignition) values range from 0.66 to 2.52% with an average of around 1.59%. these values indicate the relatively low alteration of these rocks, which can be confirmed by microscopic studies with only the presence of iddingsite margins around the olivine crystals and limited alteration. the range of sio2 values for these samples is from 46.96 to 51.13wt%. the al2o3 values vary between 17 and 22.18wt% and cao between 9.20 and 10.24wt%. the average mg# (mg#=(mg/(mg+fe)) *100) for the basalts of the study area is about 61wt%, which can indicate their mantle origin (christoph et al., 2017). the high amount of al2o3 in these rocks may indicate the early crystallization of mafic minerals at relatively high pressure from their parent magma (gust and perfit, 1987; yoder and tilley, 1962), which led to an increase in the aluminum content in the residual melt and subsequently the crystallization of plagioclases at lower pressures. based on the total alkali-silica (tas) diagram (le bas et al., 1986), these rocks are in the range of basalts (fig. 4). in the le maitre diagram (1989), the studied samples are located within the basalt range and medium-potassium calc-alkaline magmatic series (fig. 5-a). furthermore, to better distinguish the samples, the trace element diagrams were used. in the hastie et al., (2007) diagram, which plots th against co, the studied samples also fall within the range of calc-alkaline series (fig. 5-b). table 1. geochemical results of rock samples from the study area for major elements by xrf method. sample sio2 al2o3 caq fe2o3 k2o mgo mnq na2o p2o5 so3 tio2 sro loi tan-1 50.57 17.12 9.73 7.26 0.71 7.91 0.13 3.65 0.13 * 0.73 * 2.07 tan-2 50.22 17.35 9.54 7.32 0.64 8.16 0.13 3.63 0.12 0.42 0.74 0.07 1.67 tan-4 50.21 17.48 9.68 7.07 0.57 7.68 0.13 3.78 0.11 0.96 0.73 0.08 1.52 tan-5 50.49 17.5 9.55 7.37 0.65 7.89 0.13 3.68 0.12 0.43 0.75 * 1.44 tan-6 48.3 17.94 10.21 8.02 0.75 7.12 0.13 3.67 0.14 0.58 0.94 0.07 2.13 tan-7 50.23 18.15 9.46 8.04 0.9 7.04 0.13 3.6 0.13 * 0.95 * 1.36 tan-8 50.15 18.22 9.2 8.44 0.99 7.36 0.14 3.57 0.17 * 1.04 0.07 0.66 tan-9 46.96 17/00 10.24 7.71 0.91 6.9 0.13 3.35 0.16 3.1 0.95 0.09 2.52 tan-10 49.7 0.19 9.82 7.92 0.82 7.07 0.14 3.63 0.19 0.13 0.94 * 1.74 tan-12 51.13 17.31 9.6 7.21 0.71 7.62 0.13 3.75 0.14 * 0.75 * 1.65 tan-13 50.81 17.71 9.61 7.37 0.67 8.21 0.13 3.77 0.1 0.08 0.73 0.07 0.73 table 2. geochemical results of rock samples from study area for minor and trace elements by icp-ms method. sample ce cr cs eu gd hf la li lu nb nd ni р pb rb sb sc sm sr ta tb th ti tm u v y xb zr tan-1 24 254 0.9 0.89 3.34 3.2 12 7 0.2 14.4 14 121 722 30 21 0.8 18.8 2.2 468.6 0.1 0.5 2.2 3903 0.3 0.5 146 15.3 1.8 81 tan-2 26 276 1 0.84 3.52 3.2 13 7 0.3 10 14.6 127 730 34 26 <0.5 20.3 2.5 518.4 <0.1 0.5 2.2 4056 0.3 0.7 152 16 1.83 87 tan-4 23 232 0.9 0.91 3.54 3.5 13 5 0.3 8.5 14.6 123 741 43 23 <0.5 20.8 2.5 613.2 <0.1 0.5 2.2 4174 0.3 0.7 149 16.5 1.87 93 tan-5 23 248 0.9 0.82 3.62 3.4 13 10 0.3 8.7 15 123 738 62 25 0.7 20 2.7 515 <0.1 0.5 2.3 4091 0.4 0.6 148 16.2 1.84 88 tan-6 38 167 1.6 1.09 3.96 4.4 19 15 0.3 10 22.5 55 831 74 31 >0.01 % 24.9 3.6 566.3 <0.1 0.6 4.1 5342 0.4 0.7 170 19 2.18 73 tan-7 35 177 1.1 1.19 4.08 3.3 19 17 0.3 7.4 23.4 58 832 68 33 10.8 26.3 3.5 497.7 <0.1 0.6 2.7 4943 0.4 0.7 160 18.8 2.18 73 tan-8 47 187 1.6 1.26 4.29 4.7 23 8 0.3 9.7 25.3 51 928 56 36 0.7 28.2 4 612.8 <0.1 0.6 5.1 6066 0.4 1.1 189 21.1 2.45 141 tan-9 44 193 1.5 1.24 4.1 4.3 21 12 0.3 11.7 24.1 48 844 33 42 1.9 24.2 3.6 700.7 0.2 0.6 4.4 5461 0.3 1 175 18.4 2.1 123 tan10 38 156 1.1 1.16 4.12 4.1 20 10 0.3 8.5 22.8 54 829 28 29 <0.5 26.1 3.5 513.6 0.1 0.6 4 5413 0.4 0.7 148 20.7 2.28 122 187introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran) figure 4. the plot of sio2 vs. na2o+k2o (tas) (le bas et al., 1986) where the studied samples are within basalt range. a b figure 5. a: sio2 vs. k2o diagram (le maitre, 1989) and b: co vs. th diagram (hastie et al., 2007) to name and determine the magmatic series of the rock samples of the study area. 188 soudeh sedighian based on the middlemost diagram (1975), which distinguishes calcalkaline or peraluminous basalts from tholeiitic basalts based on the al2o3 content versus a.i.=((na2o+k2o)/(sio243)*(0.17)), these rock samples are within the range of peraluminous basalts (fig. 6). figure 6. al2o3 against [a.i.=((na2o+k2o)/(sio243)*(0.17))] (middlemost, 1975), to determine the magmatic series and the potassium content of the study area samples. a b figure 7. the variation patterns of elements in the basaltic rocks of the gohar kouh area in a: chondrite-normalized ree diagram (sun and mcdonough, 1989); b: primitive mantle-normalized spider diagram (sun and mcdonough, 1989). in these diagrams, the oib, nmorb and emorb patterns are included for comparison. tectonic setting implications to determine the tectonic setting of basaltic magmas, there are several diagrams in which geochemical variations and different behavior of major, minor, and trace elements lead to the separation of different tectonic environments from each other. hfse (high field strength elements) and hree (heavy rare earth elements) values are widely used to identify the tectonic environments of external rock samples (pearce, 2008; pearce and peate, 1995). in the triangular diagram of hf/3-th-nb/16 (wood, 1980) (fig. 8-a), all the studied samples fall within the range of arc-related basalts. in the v versus ti/1000 diagram (shervais, 1982), the behavior of melt and displacement of ti, as well as the sensitivity of v to the oxidation state are combined. shervais (1982) believes that with increasing in ti/v ratio and increasing oxygen fugacity, the influence of subducting slab on basalt compositions increases. therefore, the ti/v ratio is very useful and effective for detecting the tectonic setting of basaltic rocks. all of the samples plotted clearly within the range of morb and back-arc basin basalt (fig. 8-b). moreover, the la/nb versus y plot (floyd et al., 1991) (fig. 8-c), which is a suitable diagram for distinguishing between morb and subduction-related eruptive environments, the studied samples also located within the range of back-arc basin basalts. a b c figure 8. tectono-magmatic discrimination diagrams of gohar kouh area. a: hf/3-th-nb/16 diagram (wood, 1980); b: v versus ti/1000 diagram (shervais, 1982); c: la/nb versus y diagram (floyd et al., 1991). 189introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran) the source and petrogenesis of magma investigating the geochemical characteristics of the gohar kouh basaltic rocks indicates that these volcanic rocks are arc-related basalts, specifically continental arc basalts (fig. 9-a and b) that are produced in an active continental margin (fig. 9-c) from an enriched source (fig. 9-d). in these basaltic samples, the depletion of hfse compared to the adjacent lile in the trace elements pattern normalized to the primitive mantle (fig. 7-b) exhibits metasomatic activity related to the subduction process (woodhead et al., 1993; li et al., 2013; zhou et al., 2006). the positive sr anomaly and the absence of a distinct eu anomaly show the small and insignificant influence of plagioclase in the evolution of these rocks (floyd et al., 1991; qian et al., 2016)). in the trace elements pattern of the samples normalized to the primitive mantle (fig. 7-b), enrichment in lile such as cs, rb, k, pb, and sr and depletion in hfse such as nb, ta, p, and ti is observed. the enrichment of sr in these samples can be attributed to the absence of this element in common minerals (such as pyroxene) from the original magma and the substitution of ca by sr in plagioclase minerals. the negative nb anomaly and the positive pb and sr anomalies in these rock samples indicate the contamination of magma with continental crust (ozdemir, 2011) or the involvement of subduction components in the mantle source (pang et al., 2013; ozdemir, 2011) during rock formation in the region. the absence of negative yb anomaly and the low ratio of tb/yb (0.27 on average) explained the absence of the residual garnet phase in the mantle source. in addition, subduction-related provenance rocks are characterized by high la/nb ratios (greater than 1.4) (condie, 1999). investigating the high la/ nb ratio (1.3-2.57) and the low th/la ratio (0.15-0.23) in the rock samples exhibits a deformed lithospheric mantle source in the subduction zone for the volcanic rocks of gohar kouh. the low nb and ta values in the basaltic rocks may result from the existence of a pre-depleted mantle source or the preservation of ti-rich minerals in the residual source area (martin, 1999; woodhead et al., 1993). the nb/th ratio less than 5 is indicative of a subduction zone where fluids released from the subducted slab are abundant, while the nb/th ratio greater than 7 is derived from melts in which fluids released from the subducted slab do not play a significant role in their formation. in the studied rock samples, this ratio is about 3.5, therefore, their relationship with the subduction zone and the influence of releasing fluids from the subducted slab are proven. moreover, investigating the relationships between nb/la versus zr/tio2 (zhang et al., 2022) (fig. 10-a) and la/sm versus th/nb (zhang et al., 2022) (fig. 10-b) confirm the alignment of the basaltic samples with crustal contamination. a c b d figure 9. discrimination diagrams of basaltic samples from gohar kouh area in a: ce/sr versus cr diagram (pearce, 1982); b: ti/y versus zr/y diagram (pearce and gale, 1977); c: zr versus zr/y diagram (pearce, 1983) and d: zr versus y diagram (sun and mcdonough, 1989). these diagrams are drawn based on the nature of the less mobile elements and their sensitivity to the source composition. 190 soudeh sedighian a b figure 10. a: nb/la versus zr/tio2 diagram (zhang et al., 2022); b: la/sm versus th/nb diagram (zhang et al., 2022), to investigate the crustal contamination process based on the chemical variations of the rock samples. also, the sediments overlying the subducted oceanic crust have similar trace element distribution patterns compared to the continental crust (plank and langmuir, 1998; zhang et al., 2022), so that, the significant negative nb, ta, and ti anomalies and the cycling of these materials at the source, cause the observed reduction of hfse in the rock samples of the region (fig. 7). the th/u ratio can also be used to investigate crustal contamination. this ratio in the upper crust is about 3.8 (rudnick and gao, 2014) and in the studied rocks is about 4.3. so, it can be a sign of crustal contamination. based on the th/yb versus nb/yb plot (pearce, 2014) (fig. 11), the origin of the rock samples can be considered within the compositional range between oib and e-morb. in this diagram, all the studied samples are displaced from the mantle array, which indicates that they were influenced by subduction material with arc-like signatures (faisala et al., 2020). the metasomatic processes in these conditions may involve aqueous fluids, primary melts from partial melting of the sediments, or basaltic subducted slab into the mantle wedge, which leads to decreasing the solidus temperature in the mantle, partial melting, and producing the magma (harangi et al., 2007; hoang et al., 2011). the zr/nb ratio in the studied samples (this ratio is less than 10 in oib, about 10 in e-morb, and around 40 in n-morb (pearce and norry, 1979)) is about 10.74. so it can be concluded that these samples have closer characteristics to e-morb. therefore, it is suggested that, in addition to the enrichment created by releasing fluids, the mantle source of these basaltic rocks has been enriched with incompatible trace elements. basalts, especially those with mgo content above 6 wt%, which have undergone the least magmatic changes and transformations, are considered the most suitable rocks for modeling mantle melting (niu, 2021). the sm/yb (1.2-1.72) against la/sm (5.8-4.8) values for the basalts of the studied area show that these rocks located in a lower range than garnet lherzolite and close to the melting curve of spinel lherzolite with a low degree of partial melting (around 2 to 10%) (aldanmaz et al., 2000) (fig. 12-a). this is consistent with the normalized pattern of the studied rocks relative to chondrite (fig. 7-a) and indicates that their parental magma formed at shallow depths and as a result of low degrees of partial melting. the dy/yb ratio of these samples (around 1.66) also confirms the origin in the range of spinel lherzolite. the la/yb ratio in the rock samples of the region, which varies in the approximate range of 6.64 to 10, indicates the absence of garnet in the residual phase in the source area, and consequently, their parental magma must have been produced at depths of less than 40 km (haschke et al. al., 2002; faisal et al., 2020). a high nb/ la ratio (more than 1) exhibits an asthenospheric mantle source, while a lower ratio (approximately less than 0.5) indicates a lithospheric mantle source (smith et al., 1999). this ratio is around 0.64 for the gohar kouh samples, which indicates the lithospheric mantle source (fig. 12-b). figure 11. the position of the studied samples on th/yb versus nb/yb diagram (pearce, 2014). a b figure 12. a: la/sm versus sm/yb diagram (aldanmaz et al., 2000); b: the variation of nb/la against la/yb (aydin et al., 2008). the asthenospheric, lithospheric and their combined mantle field identified in this diagram. 191introducing quaternary magmatism of gohar kouh area with a focus on the lithological, geochemical and petrogenetic characteristics of the rocks (southeast of iran) conclusion the young calc-alkaline basaltic rocks of gohar kouh consist of basalt, trachybasalt, and olivine basalt. olivine, pyroxene, and plagioclase are the most important minerals in these rocks. this region is located in the southeast of iran and is part of the ssz, which formed as a result of the subduction of the oman oceanic crust beneath the eurasian continental crust in an active continental margin and a back-arc extensional environment. these rocks are derived from partial melting products with a low degree (about 2 to 10%) within the spinel lherzolite range. their source may be an enriched mantle with higher concentrations of lile elements compared to hile. the negative nb anomaly, positive pb and sr anomalies, as well as the th/u ratio around 3.4 in these samples, indicate magma contamination with the continental crust. additionally, the nb/th ratio around 5.3 suggests a connection between these parental magmas and the subduction zone, with the influence of fluids released from the subducted slab. acknowledgments the author sincerely appreciates mr. shamsoddin damani gol at the department of geology, velayat university, iran, for his valuable suggestions, extensive assistance in conducting this research. references aldanmaz, e., pearce, j., thirlwall, m. & mitchell, j. 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(2006). petrogenesis of mesozoic granitoids and volcanic rocks in south china: a response to tectonic evolution. episodes 29(1), 26-33. https://doi.org/10.18814/epiiugs/2006/v29i1/004. https://doi.org/10.1016/0012-821x(82)90120-0 https://doi.org/10.1016/0012-821x(82)90120-0 https://doi.org/10.1086/314355 http://refhub.elsevier.com/s0301-9268(19)30564-9/h0680 http://refhub.elsevier.com/s0301-9268(19)30564-9/h0680 https://doi.org/10.1130/0016-7606(1983)94%3c134:tsszoe%3e2.0.co;2 https://doi.org/10.1130/0016-7606(1983)94%3c134:tsszoe%3e2.0.co;2 http://dx.doi.org/10.1029/2003tc001529 http://dx.doi.org/10.1029/2003tc001529 https://doi.org/10.1016/0012-821x(80)90116-8 https://doi.org/10.1016/0012-821x(80)90116-8 https://doi.org/10.1016/0012-821x(93)90078-n https://doi.org/10.1016/0012-821x(93)90078-n https://doi.org/10.1093/petrology/3.3.342 http://dx.doi.org/10.1016/j.chemgeo.2010.02.009 http://dx.doi.org/10.1016/j.chemgeo.2010.02.009 https://doi.org/10.1016/j.gsf.2021.101283 https://doi.org/10.18814/epiiugs/2006/v29i1/004 https://doi.org/10.18814/epiiugs/2006/v29i1/004 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.100856 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 425 436 so il s genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey hasan hüseyin özaytekin1*, cihan uzun2, mert dedeoğlu3 1. karamanoğlu mehmetbey university vocational school of technical sciences. department of crop and animal production, karaman, turkey. 2. republıc of turkey ministry of agriculture and forestry, konya soil, water and deserting control research institute konya, turkey. 3. selçuk university, agriculture faculty, department of soil science and plant nutrition, konya, turkey. *corresponding author: hhozaytekin@kmu.edu.tr record manuscript received: 07/02/2022 accepted for publication: 24/01/2024 how to cite this item: özaytekin, h. h., uzun, c., & dedeoglu, m. (2023). genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semiarid conditions from central anatolia-turkey. earth sciences research journal, 27(4), 425-436. https://doi. org/10.15446/esrj.v27n4.100856 abstract keywords: karacadağ; andisols; weathering; elemental losses; allophane; andic soil properties. génesis, minearología y geoquímica de perfiles de suelo desarrollados con material volcánico bajo condiciones semiáridas en anatolia central, turquía palabras clave: montañas turcas de karacadağ; andisoles; meteorización; pérdida de elementos; allofanita; propiedades andicas del suelo. resumen la pedogénesis de los suelos volcánicos se ha estudiado ampliamente en regiones húmedas, sin embargo hay muy poca información sobre estos suelos en las regiones áridas y semiáridas de turquía. este trabajo se enfoca en evaluar los procesos pedológicos y en investigar las características físicas, químicas y mineralógicas de perfiles de suelo desarrollados con materiales volcánicos. los suelos estudiados no fueron clasificados como andisoles porque carecen de propiedades andicas. los resultados muestran que las arenas y las fracciones de arena granulada exceden el 30 %. la densidad del volumen fue mayor a 0.90 grcm-3 en todos los perfiles. en general la retención de fosfato fue baja, mayor a 25 % y menor a 85 % en todos los perfiles. los porcentajes al+½fed fueron menores al 2 % en todos los perfiles. la extracción selectiva (pirofosfato de sodio, oxalato de amonio, ditionito de sodio) indica que los suelos carecen de minerales no cristalinos como la allofanita, imogolita y compuestos húmicos de hierro. los únicos minerales no cristalinos en altas cantidades son los compuestos húmicos de aluminio y pequeñas cantidades de ferrihidrita. la difracción de rayos x indican que la esmectita, caolinita e illita son los minerales dominantes en la fracción arcillosa. de los perfiles estudiados, el 1 y el 4 se clasificaron como vitrandic haploxerepts, mientras que el perfil 2 como vitrandic haploxerolls y el perfil 3 como vitrandic xerorthents. los resultados indicaron que los procesos de formación del suelo incluyen las siguientes características: 1. desilicación e iluviación de cationes base y de aluminio desde el solum; 2. transformación del hierro y el aluminio de las arenas y de las fracciones arenosas en arcillas secundarias y en minerales ferrosos cristalinos. los mayores factores determinantes en la génesis del suelo en las montañas turcas de karacadağ son el clima y la topografía, afectadas por el régimen de lixiviación y la tasa de meteorización. pedogenesis of volcanic soils has been studied broadly in humid regions, but very little data are present regarding these soils in arid and semi-arid regions of turkey. this research aimed to assess the pedological processes and investigate the physical, chemical, and mineralogical characteristics of soil profiles developed from volcanic parent material. the studied soils were not classified as andisol because they lack andic properties. the results showed that sand and coarse silt fractions exceeded 30%. the bulk density was higher than 0.90 grcm-3 in all profiles. in general, phosphate retention was low, higher than 25% in profiles but lower than 85% in all profiles. the al + ½ fed percentages were lower than 2% in all profiles. selective extraction (na–pyrophosphate, ammonium oxalate, na–dithionite) indicated that soils lack noncrystalline minerals, such as allophane, imogolite, and iron humus complexes. the only noncrystalline minerals in high quantities are al-humus complexes and small amounts of ferrihydrite. x-ray diffraction indicated that smectite, kaolinite and illite were dominant minerals in the clay fraction. among the studied profiles, profiles 1 and profile 4 are classified as vitrandic haploxerept, profile 2 as vitrandic haploxerolls, and profile 3 as vitrandic xerorthent. the results indicated that the dominant soil-forming processes include the following: 1. desilication and the eluviations of base cations and al from the solum; and 2. transformation of iron and aluminum from the sand and silt-size fractions into secondary clay and crystalline fe minerals. the major factors determining soil genesis on mt. karacadağ appear to be climate and topography affected by the leaching regime and weathering rates. https://doi.org/10.15446/esrj.v27n4.100856 mailto:hhozaytekin@kmu.edu.tr https://doi.org/10.15446/esrj.v27n4.100856 https://doi.org/10.15446/esrj.v27n4.100856 426 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu 1. introduction the andisols (keys to soil taxonomy, 2014) have a unique combination of physical and chemical properties linked to the abundance of short-ordered minerals and/or insoluble al-humus complexes. after tephra deposition, soil formation begins, and the tephra’s elemental and mineralogical composition changes; consequently, volcanic ash soils or andisols with unique properties are formed (nanzyo, 2002). the formation of poorly crystallized minerals and the accumulation of organic matter occur preferentially in volcanic ash soils (shoji et al., 1996). while soils formed from volcanic materials in different climatic regions of the world are different, they share unique properties, particularly during the early and intermediate stages of development. typically, the weathering of volcanic deposits results in the formation of andisols in both temperate and tropical environments with sufficient moisture (shoji et al., 1993). andisols form rapidly in humid climates and change into other soil orders with age and degree of weathering (ugolini and dahlgren, 2003), e.g., very old and highly weathered volcanic ash soils may change from andisols into ultisols or oxisols. climatic conditions are an important factor for mineral weathering and formation because the climate determines the weathering product of volcanic materials in addition to the parent material. most of the studies related volcanic soils have occur where precipitation is >1000 mm and where andic characteristics have developed in-situ and cannot be disputed (özcan and özaytekin, 2011). in comparison to the andisols from humid regions, minimal information is available concerning the volcanic soils of semiarid climates in italy (egli et al., 2008), greece (moustakas and georgoulias, 2005), and turkey (ozaytekin and ozcan, 2013; senol et al., 2017). the fact that noncrystalline mineral formation results from over-saturation of the soil solution with si, al, and fe, concentrations, makes it easy to understand the influence of climate on volcanic soil. mediterranean environments, such as turkey, may appear too hot and dry for andosolization, despite the widespread occurrence of volcanic areas. the central anatolia has arid and semi arid climatic conditions. the decomposition products of volcanic material formed in semi-arid climatic conditions such as central anatolia are different from those formed in rainy areas, and andisol formation has not been fully demonstrated in these environments. scarce information is currently available regarding andisol properties in turkey. it is the purpose of the present paper to describe the soil formation processes of soils developed on volcanic materials in semiarid region of anatolia. therefore, this research was conducted to evaluate the physicochemical properties, weathering processes, and pedogenetic products of soils developed from an andesite on mount karacadağ in konya, a semiarid region, and to classify the soils using one of the current international classification system. we discuss the extent to which these soils meet the requirements of soils classified as andisols, in soil taxonomy (soil survey staff, 2014). 2. materials and methods site description the study was conducted with soils developed from a parent material derived from volcanic activity during the pliocene in karacadağ, which is located in eastern konya. karacadağ is situated approximately 10 kilometers east of karapınar, located between a 37040’-37050’northern latitude and 33040’-33054’ eastern longitude, which extend to the northwest and southeast, respectively, and it, covers an area of approximately 200 square kilometers. karacadağ consists of five separate volcanic series, the first three of which are pliocene and the last two of which are quaternary eruptions, each containing numerous lava and pyroclastic products. the mean annual precipitation around karapınar is 379.4millimeters, and the annual evaporation rate is 1226.4 millimeters. the mean annual temperature is 11.5 ºc, and the mean soil temperature at a depth of 50 cm is 12.5 ºc. according to the climate data, the soil moisture and temperature regimes are xeric and mesic, respectively (soil survey staff, 2014). the products of karacadağ volcanism, are composed of successive volcanic rocks, first as andesitic lava, next as andesitic agglomeratetuff, then as andesitic lava and andesitic-basaltic lava in the quaternary, and later as basaltic lava (ayhan and sevin, 1986). sanidine and olivine-bearing basalts with ages between 45.000 and 336.000 years are exposed figure 1. location of the study area sampling and analysis for this study, 4 representative soil profiles were chosen, and disturbed and undisturbed (bulk core) soil samples were obtained from the horizons after their macro morphological identification was completed. the soil ph was measured in a both soil-water (w/v) suspension and 0.01 n kcl with a soil/ solution ratio of 1/2.5. measurements of (naf) ph 1 m (1:50) after 2 min of equilibration were taken, as prescribed by the usda (soil survey staff 2009). the electrical conductivity (ec) was potentiometrically determined in a 1/2.5 soil-water suspension (soil survey staff, 2009). the soil texture was determined using a hydrometer method (bouyoucous, 1951) after the removal of organic matter using h2o2 and after the soil was stirred into a sodium hexametaphosphate solution. the bulk density was determined from the dry weight of 100 cm3 undisturbed core samples (soil survey staff, 2009). phosphate retention was determined based on the methods of blakemore et al. (1987). the total soil organic carbon (corg) was determined using the walkley– black wet digestion method (soil survey staff, 2009). the exchangeable bases were extracted with a neutral 1 m nh4-acetate, and the bases were determined using atomic absorption spectroscopy (soil survey staff, 2009). the cation exchange capacity (cec) was determined using ammonium acetate (1 n, at ph 7), and the quantity was determined using a flame photometer (soil survey staff, 2009). the water retention capacity was measured in the disturbed soil samples at −1500 kpa and −33 kpa and using suction plate and pressure plate methods (peters, 1965). the carbonate in the soil was measured with a scheibler calcimeter (soil survey staff, 2009). the percent base saturation was determined by dividing the sum of the k, mg, ca, and na in meq 100 g–1 soil by the cec. a technique modified from mizota and van reeuwijk (1989) was used to dissolve fe, al, and si selectively. to achieve this, extraction solutions of dark ammonium oxalate, dithionite-citrate bicarbonate (dcb), and sodium https://www.abebooks.com/servlet/bookdetailspl?bi=30257974628 427genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey pyrophosphate-extractable si, al, and fe were obtained. atomic absorption spectrophotometry (aas) was used for all procedures to determine the concentrations of si, al, and fe. (soil survey staff, 2009). the subscripts o, d, and p stood for the corresponding methods’ cations. the major, minor and rare earth elements of the soil and rock samples was determined by acid digestion in microwave oven in hf/hno3 1:5 solution at 600 w for 24 min (chao and sanzolone, 1992), the contents were measured using inductively coupled plasma (icp). all of these procedures were replicated 3 times for each soil sample, and the means were reported. the whole soil and clay mineralogy were achieved using x-ray diffraction (xrd) on powdered samples as randomly oriented powder mounts with a shimadzu xrd-6000 with a cu anticathode and k filter (40 kv, 35 ma). the whole soil samples were powdered to 40 μm using a sintered al oxides mortar grinder. an aliquot was separated and mounted as unoriented powder using the back-side method (brindley and brown, 1980). the powder was submitted to xrd between 2° and 40° 2θ, and the data were analyzed in a semi-quantitative manner. the oriented specimens of the clay-size fraction were realized using a “glass-slide” method and were analyzed using x-ray diffraction with cu kα radiation from 2° to 15° 2θ, with steps of 0.02° 2θ at 2s step–1.the clay fraction (<2 μm) was obtained from the soil after the destruction of organic matter with dilution and na-acetate-buffered h2o2 (ph 5), followed by dispersion with calgon and sedimentation in water the following treatments were performed: mg saturation, ethylene glycol solvation (eg), and k saturation, followed by heating for 2 h at 550 °c. the ir spectra were recorded over a range of 4000 to 400 cm–1 with pellets made from 1 mg of sample and 250 mg of kbr, which were previously heated to 150 °c (white and roth, 1986). the volcanic glass contents from the 0.02-2.00 mm fraction were also observed using a glass slide with a polarizing microscope. table 1. selected morphological characteristics of profiles profile horizon depth (cm) color (dry) color (moist) structurea field textureb rootsc boundary parent material elevation(m) physiography 1 a1 0-27 10yr5/4 10yr4/4 gr sc 3-k gradual, smooth andesitebasalt 1245 moderately steep a2 27-49 10yr5/4 10yr4/4 gr sc 2-o gradual, smooth bw1 49-87 10yr4/4 10yr3/3 ab sc 2-i gradual, smooth bw2 87-115 10yr4/3 10yr4/2 ab sc 1 gradual, smooth c 115-133 10yr8/2 10yr8/3 mas c 1 clear, wavy cr +133 10yr8/2 10yr8/3 mas c 1 2 a1 0-31 7,5yr4/3 10yr3/2 gr sc 3-k clear smooth andesite 1156 moderately steep a2 31-43 7,5yr4/3 10yr4/3 gr sc 2-o abrupt smooth c 43-79 10yr8/2 7,5yr8/2 ma sl 1 gradual smooth cr +79 10yr8/1 7,5yr8/2 ma scl 1 3 a1 0-21 10yr6/2 10yr3/3 gr scl 3-o gradual smooth basalt 1306 steep slopes a2 21-33 10yr4/3 10yr3/4 gr scl 2-o clear smooth cr 33-69 10yr8/3 7,5yr8/2 ma sl 1 gradual -irregular r +69 4 a1 0-16 10yr6/2 10yr3/4 gr sc 3-k clear smooth basalt 1371 moderately steep a2 16-50 10yr6/2 10yr3/4 gr sc 3-k abrupt smooth bw1 50-79 10yr6/2 10yr4/3 ab c 2-i clear smooth bw2 79-160 10yr6/2 10yr3/4 ab c 1 abrupt smooth c 160-183 10yr7/2 10yr6/4 ma c 1 abrupt -irregular cr 183-200 10yr8/2 10yr7/4 ma c 1 astructure: mas, massive; gr, granular; ab, angular blocky; p, prismatic bfield texture: sc: sandy clay, c: clay.; sl: sandy loam; scl: sandy clay loam croots: 1, none; 2, few; 3, moderate; 4, common; i, fi ne (<2 mm); o, medium (2-5 mm); k, coarse (>5 mm). 428 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu 3. results morphological properties the profiles developed on an andesitic-basalt parent material on steep or moderate hill topography in a xeric moisture and mesic temperature regime. in profiles 2 and 3, there are no pedogenetic horizons other than a horizon, whereas b horizon was identified in profiles 1 and 4 in addition to an a horizon. the fact that the horizon differences aren’t high and that the parent material exhibits low weathering rates indicates that soil evolution is in the preliminary stages. of the profiles, profile 1 and profile 4 were fine textured, and profile 2 and profile 3 were coarse textured. structural development was poor, or no development was present in all of the analyzed profiles, except in the surface horizons, whereas a granular structure was identified on the surface. an angular blocky structure was identified in the cambic horizons, and a massive structure was identified in the c horizon. the hue values in the profiles in karacadağ range from 7.5 yr to 10 yr and generally have high values when it is dry (table 1). since the area where the profiles are opened is pasture and covered with oak or pine trees, the organic matter accumulation rate on the surface is higher than the subsurface layers, causing the surface horizons to show lower values and chroma. these differences resulted from differences in the organic matter, roots and levels of biological activity. a strong reaction with hcl was identified in all of the profile horizons. the presence of weak carbonate micelles in the profile also supports this situation. physical and chemical properties the physical and chemical properties of the studied profiles and the values of andic properties are given in tables 2, and 3. the ph in the soil water was above 7 due to the content of carbonates, whereas the ph in kcl in all of the horizons was lower than that in pure water. the ph (h2o) was alkaline in all of the profiles and varied between 7.95 and 8.54, whereas the ph in kcl ranged from 7.33 to 7.98. the δph (ph kcl – ph h2o) was negative (-) in all horizons, and this indicated a negative charge in all horizons. this signals the presence of a net negative charge in all of the horizons, whereas the phnaf differed between 10.26 and 10.83. the organic matter content decreased rapidly with depth in all of the profiles and all soils followed the general trend of having the highest organic matter content on the surface. the organic matter content increased to proportionally high values, particularly in the surface horizon of profiles 2 and 4. the organic matter content was low in the other horizons. the organic matter content in the profiles ranged from 0.37% to 4.44%. the lowest values were observed in the cr horizons. high rates of lime were detected in the profiles, and the lime content ranged from 3.6 % to 77.0 %. lime content tended to increase with depth except profile 1, which initially shows a reduction. although the soils in the study area are of volcanic origin, high carbonate contents were detected in the profiles opened. it is known that there are four sources of carbonates found in basaltic parent material. these; 1. as basaltic lavas flow, they absorb parts of the carbonate material underneath, 2. ca minerals in the basalt form caco3 in suitable environments, 3. crystallization of lime-rich hydrothermal waters in the pores after the flow of basaltic lavas, 4. recalcification by wind materials containing carbonate (kapur, 1980; gürel, 1992; karaman et al., 1995). primary mineral examination in the research region revealed the presence of ca minerals in the soils, including augite, plagioclase, and diopside. the high carbonate content in this location can be explained by the formation of caco3 by the minerals in question. the electrical conductivity of the soils ranged from 118 µmhos cm-1 to 286 µmhos cm-1. all of the profiles were salt-free. the cec was distributed between 6.60 and 34.99 (cmolc kg–1). the cec value varied with the amount of clay and organic matter in the horizons and decreased with depth, whereas an increase was observed in the cambic horizons. the exchangeable cations in the soils were identified to be ca>mg>k>na. ca+mg varied between 6.25 and 32.37 (cmolc kg–1), the na ranged between 0.04 and 2.51 (cmolc kg–1) and the k ranged between 0.15 and 3.54 (cmolc kg–1). the number of exchangeable cations generally decreased with the soil depth. the base saturation was identified as 100 %. this indicated that active forms of al and fe do not accumulate in the soil. the phosphorous fixation capacities of the soils ranged from 32.45 % to 79.76 %, and it was <85% in all horizons but higher than 25% all profiles. the phosphate retention values of the studied soils fully meet the requirement 3b of andic soil properties according to soil taxonomy. the bulk density changed between 1.32 and 1.66 g cm-3. the bulk density values were generally high because of the high sand content of the soils. the textures of the soils are clayey and clay-loamy in profiles 1 and 4, and in profiles 2 and 3, they are sandy-clayey loam and sandy clay. the clay content of the soils ranged between 18.1% and 58.1 %, the sand content ranged between 27.9 % and 66.9 %, and the silt content ranged between 12.0% and 23.5 %. the amount of water retention at -1500 kpa was comparatively low and ranged between 13 % and 22 %. this value was lower than the expected value when compared with andisols of elsewhere (%35-45). the high sand and the low organic matter content of the soils influenced distribution. similarly, low rates of water retention were also observed at -33 kpa. the water retention at this pressure was between 18.6 and 35.2. the results of the analyses carried out with selective solutions are given in table 3. the sio content in some of the profiles was lower than 0.2 % and varied from0.058% to 0.247 %. the sio did not exhibit an obvious trend in the profile distributions in the surface and sub-surface horizons. the alo content ranged from 0.014 % to 0.043 %. feo ranged between 0.008% and 1.298%. the fep and alp values were notably low in the studied profiles and ranged from 0.0013 to 0.0599% and from 0.017 to 0.050%, respectively (table 3). such low values were a result of the low organic matter content; in contrast to volcanic areas, they formed in wet regions of the world. dithionite-citrate-bicarbonate unlike other solutions, it has a stronger effect on compounds that are in crystalline form. the fed in the profiles ranged from 0.038 % to 0.789 %, whereas the ald ranged from 0.007 to 0.093. the fed tended to decrease with depth, whereas the ald tended to increase from the surface to sub-surface but decreased again after that. the distribution of major and minor elements is given in table 4. the sio2 content ranged from 7.7% to 50.04%. the al2o3 content varied between 2.21 % and 14.90% and tended to decrease with depth. the amount of al in the soil is directly related to the distribution of clay, and the amount of al2o3 increases in mature soils. the amount of al2o3 in the studied soils was close to that of the parent material in the solution due to the low rate of weathering. the si and al values differed in the profiles, depending on the sand and clay content of the profiles. the highest value of fe2o3was observed on the surface horizons, and it reached 8.03%. the amount of fe2o3 ranged from 1.10% to 8.0 % (table 4). the percentage of cao exhibited higher levels in the parent material compared with the upper horizons. as the caco3 mineral content in the soil increases, the relative amount of silicate minerals decreases and accordingly the amount of sio2 decreases. the percentage of cao exhibited a distribution between 8.02 and 46.45, whereas the mgo values ranged from 1.57 to 5.82 %. the generally high values of mgo resulted from the presence of significant amounts of minerals with ferro magnesium. high amphibole and biotite peaks have also been observed in primary mineral analyses. the percentages of k2o and na2o were between 0.23% and 3.09% and 0.15% and 3.27%, respectively. this is closely related to the distribution of feldspars that are present in andesite and basalt. titanium is a mineral resistant to and used for identification of chemical change. the tio2 content in the soils ranged between 0.11 and 0.90%, the mno2 ranged between 0.01% and 0.14 %, and the p2o5 ranged between 0.03% and 0.53%. a regular trend was observed between the horizons in the distribution of other minor elements. 429genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey table 2. some chemical and physical properties of studied profiles profile horizon depth (cm) ph(h2o) (1/2.5) ph(kcl) (1/2.5) δph kcl-h2o ph(naf) (1/50) ec (µs/cm) organic matter (%) caco3 (%) 1 a1 0-27 7.95 7.38 -0.57 10.63 127 2.01 36.7 a2 27-49 7.97 7.46 -0.51 10.63 122 1.30 29.9 bw1 49-87 8.06 7.49 -0.57 10.66 118 1.20 27.6 bw2 87-115 8.26 7.55 -0.71 10.61 220 1.05 27.8 c 115-133 8.44 7.98 -0.46 10.78 165 0.86 61.5 cr +133 8.53 7.69 -0.83 10.83 222 0.37 63.8 2 a1 0-31 8.02 7.56 -0.46 10.78 223 4.44 22.3 a2 31-43 8.14 7.57 -0.57 10.80 258 2.38 31.0 c 43-79 8.48 7.88 -0.60 10.76 180 1.00 77.0 cr +79 8.54 7.84 -0.70 10.74 189 0.77 75.0 3 a1 0-21 8.29 7.38 -0.91 10.26 203 2.33 3.6 a2 21-33 8.10 7.35 -0.75 10.34 209 1.41 4.5 cr 33-69 8.20 7.54 -0.66 10.74 248 1.33 35.7 r +69 4 a1 0-16 8.14 7.38 -0.76 10.44 246 3.32 11.6 a2 16-50 8.24 7.39 -0.85 10.44 193 2.02 12.9 bw1 50-79 8.21 7.33 -0.88 10.53 197 1.59 13.8 bw2 79-160 8.28 7.45 -0.84 10.61 207 1.15 20.5 c 160-183 8.41 7.56 -0.85 10.72 242 0.79 37.2 cr 183-200 8.32 7.61 -0.71 10.82 286 0.77 59.5 profile horizon depth (cm) particle size distribution (%) c.e.c. (cmolc kg–1) base sat. (%) exch. cations (cmolc kg–1) ca+mg mg na k sand clay silt 1 a1 0-27 41.9 42.6 15.5 22.67 100 20.97 2.37 0.04 1.66 a2 27-49 42.9 44.1 13.0 22.60 100 21.06 2.20 0.11 1.43 bw1 49-87 42.9 42.1 15.0 23.08 100 21.53 2.32 0.16 1.39 bw2 87-1115 46.9 40.1 13.0 25.01 100 23.61 2.67 0.26 1.14 c 115-133 34.9 53.1 12.0 14.97 100 14.10 2.42 0.29 0.58 cr +133 27.9 58.1 14.0 13.77 100 13.13 2.35 0.23 0.41 2 a1 0-31 52.9 25.6 21.5 22.05 100 20.62 1.91 0.09 1.34 a2 31-43 51.9 28.1 20.0 21.00 100 20.02 1.87 0.14 0.84 c 43-79 66.9 18.1 15.0 6.60 100 6.25 1.41 0.21 0.15 cr +79 59.9 24.6 15.5 8.51 100 8.10 0.30 0.20 0.20 3 a1 0-21 53.9 26.1 20.0 24.05 100 21.89 2.34 0.10 2.06 a2 21-33 47.9 36.1 16.0 27.07 100 24.27 2.88 0.16 2.64 cr 33-69 46.9 36.1 17.0 23.80 100 22.00 2.63 0.50 1.30 r +69 4 a1 0-16 41.9 34.6 23.5 28.00 100 21.96 2.30 2.51 3.53 a2 16-50 33.9 44.1 22.0 31.49 100 27.91 3.54 0.03 3.54 bw1 50-79 38.9 40.6 20.5 34.99 100 32.37 4.08 0.18 2.44 bw2 79-160 37.9 42.1 20.0 32.00 100 29.86 5.54 0.25 1.90 c 160-183 41.9 35.1 23.0 22.22 100 20.71 7.10 0.18 1.33 cr 183-200 37.9 46.6 15.5 13.29 100 12.24 4.26 0.25 0.80 430 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu table 3. index values of the andic properties and selective dissolution analyses of <2 mm of soils of studied soils (%). profile horizon phosphate retention (%) volcanic glass (%) alo+1/2feo (%) water retention (% w/w) bulk density (g cm–3) 2-0.02 mm fraction (%)-33 kpa -1500 kpa 1 a1 34.35 9 0.07 18.6 15.9 1.52 54.3 a2 35.11 10 0.14 20.2 17.6 1.48 52.7 bw1 35.36 7 0.26 22.8 17.3 1.36 53.4 bw2 38.52 5 0.28 23.1 17.8 1.66 54.7 c 64.08 1 0.04 26.1 17.2 1.41 42.2 cr 68.12 1 0.02 27.5 18.0 1.4 40.9 2 a1 37.26 6 0.34 19.9 15.3 1.41 59.9 a2 45.99 17 0.29 20.5 15.5 1.36 60.9 c 74.20 3 0.02 25.1 13.0 1.32 80.7 cr 65.47 1 0.03 26.7 13.4 1.42 69.9 3 a1 32.45 13 0.69 23.8 13.4 1.59 57.8 a2 38.27 6 0.61 27.5 15.5 1.45 51.2 cr 52.94 6 0.20 30.7 16.4 1.51 53.6 r 4 a1 48.52 12 0.29 29.3 17.1 1.57 51.4 a2 48.39 18 0.21 33.0 20.3 1.56 50.4 bw1 49.91 16 0.23 35.2 21.6 1.54 53.8 bw2 48.14 12 0.22 26.8 22.0 1.56 50.9 c 79.76 3 0.12 25.0 20.1 1.35 55.2 cr 34.35 5 0.03 22.4 16.9 1.39 46.6 profile horizon fed feo fep ald alo alp sio 1 a1 0.366 0.093 0.0042 0.038 0.026 0.023 0.141 a2 0.510 0.219 0.0332 0.038 0.029 0.037 0.165 bw1 0.494 0.463 0.002 0.046 0.025 0.017 0.164 bw2 0.395 0.479 0.0022 0.037 0.043 0.029 0.140 c 0.163 0.040 0.0014 0.015 0.020 0.021 0.099 cr 0.089 0.008 0.0014 0.013 0.018 0.029 0.071 2 a1 0.450 0.626 0.031 0.051 0.024 0.026 0.173 a2 0.416 0.532 0.0479 0.046 0.028 0.038 0.161 c 0.040 0.012 0.0014 0.007 0.014 0.027 0.046 cr 0.038 0.023 0.0013 0.008 0.018 0.023 0.058 3 a1 0.653 1.298 0.0281 0.057 0.037 0.042 0.265 a2 0.789 1.145 0.0599 0.093 0.039 0.050 0.282 cr 0.430 0.316 0.0017 0.045 0.038 0.030 0.190 r 4 a1 0.510 0.496 0.0247 0.045 0.042 0.036 0.212 a2 0.565 0.341 0.0116 0.056 0.041 0.033 0.247 bw1 0.693 0.377 0.0498 0.055 0.041 0.044 0.255 bw2 0.509 0.348 0.0148 0.051 0.041 0.037 0.236 c 0.315 0.170 0.0015 0.031 0.033 0.035 0.167 cr 0.176 0.021 0.0018 0.023 0.020 0.042 0.065 431genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey table 4. the results of total element analysis and weathering indices of studied soil (%). pedon horizon sio2 al2o3 fe2o3 mgo cao na2o k2o tio2 p2o5 mno loi total cia pia 1 a1 30.91 9.83 4.69 2.53 24.75 0.79 1.22 0.51 0.13 0.08 24.3 99.82 74,65 64,60 a2 35.04 11.10 5.82 3.08 21.15 0.92 1.25 0.61 0.11 0.09 20.6 99.79 76,12 66,82 bw1 36.60 11.68 6.10 3.25 20.08 1.02 1.26 0.63 0.12 0.09 18.9 99.79 76,15 67,24 bw2 37.43 12.02 5.96 3.48 19.52 1.03 1.15 0.65 0.12 0.09 18.3 99.79 77,18 69,17 c 18.72 5.72 3.16 2.27 36.28 0.37 0.51 0.31 0.06 0.04 32.4 99.83 76,33 68,94 cr 14.21 3.98 1.84 1.95 40.37 0.15 0.28 0.19 0.03 0.02 36.8 99.84 83,34 76,98 2 a1 34.16 10.92 5.32 3.56 21.47 1.09 1.21 0.58 0.14 0.09 21.2 99.79 74,60 65,63 a2 33.10 10.45 5.29 3.54 22.91 1.02 1.06 0.57 0.12 0.08 21.6 99.79 74,68 66,46 c 7.70 2.21 1.10 1.60 46.45 0.22 0.23 0.11 0.04 0.01 40.1 99.82 69,53 61,68 cr 10.70 2.78 1.34 1.57 43.99 0.27 0.30 0.14 0.06 0.01 38.7 99.83 70,06 61,86 3 a1 49.84 15.57 8.03 5.82 8.02 1.54 1.58 0.90 0.22 0.14 8.1 99.74 77,76 69,20 a2 49.31 15.91 7.62 5.39 8.16 1.37 1.72 0.87 0.19 0.13 9.1 99.76 78,63 69,41 cr 31.03 10.10 4.65 3.11 24.85 0.70 1.18 0.53 0.12 0.07 23.4 99.80 76,25 66,58 r 50.04 14.90 5.17 1.84 12.07 3.27 3.09 0.77 0.24 0.07 8.3 99.76 62,24 48,24 4 a1 44.81 13.88 6.10 3.75 11.68 1.21 1.67 0.70 0.53 0.12 15.3 99.79 77,16 67,09 a2 45.62 14.58 6.39 3.69 10.95 0.96 1.86 0.72 0.40 0.11 14.5 99.78 78,88 67,96 bw1 44.84 14.50 6.35 3.73 11.57 0.89 1.77 0.72 0.23 0.10 15.1 99.80 79,71 69,15 bw2 41.57 13.27 5.90 3.95 14.71 0.78 1.57 0.66 0.17 0.09 17.1 99.80 79,63 69,41 c 30.44 9.37 4.06 3.10 25.87 0.53 1.13 0.46 0.10 0.06 24.7 99.83 76,92 66,85 cr 17.61 5.07 2.31 2.12 37.29 0.25 0.65 0.25 0.05 0.03 34.2 99.84 76,88 66,19 mineralogical properties x-ray diffraction of the < 2mm fraction sand and clay fraction to determine the mineralogical properties of the profiles were undertaken in the study area and are shown in figure 2 and 3, and infrared spectroscopy graphs are shown in figure 4. as seen from the figures, diverse phyllosilicates formed in diverse levels of crystallization and in differing amounts in the profiles. in the samples saturated with mg in the profiles, the peaks, which are seen in the form of a plateau and are poorly crystallized between 14.5 and 15.9 ǻ, expanded to 17 to 21.2 ǻ in a mg+gl (glycerol) treatment, whereas in the case of k saturation, these peaks were close to a range of 10.6 and 14.5 ǻ. the peaks were seen at approximately 1.2 nm and approximately 1.4 nm in this application. the peaks were close to 10.7 to 12 ǻ when the k-saturated samples were heated. especially with mg applications, the peaks formed in a shoulder and were poorly crystallized. therefore, the peaks indicated the existence of smectite and chlorite-smectite intermediate layer clays. the peaks that were seen between 9.99 and 10.13 ǻ in all applications belonged to illite. the peaks that were seen between 7.10 and 7.20 ǻ in the samples saturated with mg, mg+gl and k vanished when heated at 550oc. this indicated that these peaks belong to kaolinite. significant differences were observed in the distribution of clays between the profile horizons. the distributions of clays in the profiles, except for profile 4, in order of abundance were as follows: smectite, kaolinite, illite and chlorite-smectite as an intermediate layer, whereas the distribution observed in profile 4 was kaolinite, smectite, illite and chlorite-smectite as an intermediate layer. while kaolinite and illite decreased with depth, the amount of smectite increased. in diffractions of the soil profile used to identify primary minerals, feldspar (anorthite and albite), amphibole and pyroxene (diopside, actinolite, tremolite, and hornblende) apatite, hematite, olivine and biotite were detected. almost all of the primary minerals are composed of the aforementioned minerals. the ir spectrums were taken from the horizons to detect the presence of amorphous structures, such as allophane. as seen from these spectrums, peaks were identified in the mono, dimer and triple bond areas. the peaks were obtained in 712-798, 910-1027, 1409-1449, 1631-1643, 3355-3621 cm-1 of the profile. 432 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu figure 2. x-ray diffractograms of clay factions of selected samples studied soils d values in nm. (a) a1 horizon of profile 1, (b) a1 horizon of profile 2, (c) a1 horizon of profile 3, (d) a1 horizon of profile 2. figure 3. x-ray diffractograms of the upper horizons of all studied soils. figure 4. infrared spectra of clay samples. 433genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey 4. discussion physical and chemical properties the profile descriptions indicated that there are no diagnostic subsurface horizons whereas there was a weakly developed cambic horizon in profiles 1 and 4. the fact that there isn’t a significant differentiation indicates that the parent material weathered weak; this finding, in turn, suggests that soil formation in the region is in its initial stage. observation of a high rate of reaction with hcl in the profiles indicated that the primary c pool, which is released from minerals with plagioclase that are contained in basalts, turned into carbonates. as far as is known, basalt or andesite parent material cannot produce caco3. the high lime content detected in this study is due to basalt lavas absorbing some of the calcareous material underneath as they flow; cacontaining minerals in basalt can be explained by the formation of caco3 in a suitable environment and the crystallization of lime-rich hydrothermal waters in the foramina after the flow of basaltic lavas. as a matter of fact, similar to our findings, dengiz and şenol (2018) found that although the lining material in the study area was basalt, the amount of lime was high. the generally high content of sand in the soil, the sandy-loamy or sandy-clayey-loamy type of texture in the profiles in which the cambic horizon is absent, and the high sand content seen in the textured profiles indicated low weathering rate, in the soils; however, the increase in clay minerals in both the cambic horizons and c horizons, in profiles 1 and 4, suggested that the clay in these layers, which is comprised of neo-formation minerals, could be inherited from the parent material. the xrd results taken from the c horizon reveal that a significant amount of clay is present in this horizon. the relatively high clay content in the b horizon of profiles 3 and 4 emerged because of the alteration in the primary minerals into clay minerals. the clay minerals emerged as a result of the alteration of feldspars into clay minerals in basalts and andesite that are rich in feldspars and inherited to solum, depending on weathering. the reason that weathering was so low is climatic factors. low precipitation, high evaporation, and a long dry season limited the weathering process. however, for the a and c horizons to form, 100 to 500 years must transpire in wet regions in japan, and at least 1000 years is required for the a, b and c horizons to form (yamada, 1968). karacadağ, which formed on the soil that is present this study area, dates back to the miocene-pliocene. despite this, climate has been the main factors restricting the formation of soil. the bulk density of the soil is lower on the surface and increases in the subsurface horizons. this results from the presence of organic matter on the surface. furthermore, the fact that the c horizons are in the form of weathered or softened parentrocks has influenced the bulk density. for this reason, the bulk density increased with depth. the fact that the amounts of allophane and imogolite are low (according to index values of selective dissolution analysis) in these soils appears to be another factor responsible for the high bulk density (wada, 1989). andisols have a higher capacity for water retention due to the excessive amount of macro and micro pores. the high water retention capacity of andisols containing allophane and allophane-like amorphous minerals is due to the fact that these minerals contain fine particles and have a hollow spherical structure. (wada, 1989). when the amount of water retention at -1500 kpa and at -33 kpa in the study area soils is compared with other andisols in the world, it has lower values than expected. this situation may have been due the absence of allophane and imogolite or their very low presence because such materials as allophane and imogolite enhance the water retention capacity of volcanic soils due to their very small particle sizes and hollow spherical structures (wilson et al., 1996). because of the higher amount of clay in some horizons, a relatively higher water retention capacity has been detected in these horizons. this may be due to the fact that other clay minerals do not have allophane or similar materials. soil organic materials are formed as a result of the decomposition of organic remnants by plant roots and microorganisms, and they are a critically important factor in mineral alteration. in this study, the organic matter in the soil, especially in the surface horizons, reached to 4.44 %. this value is very high level for arid areas of turkey, but remained generally lower than the rates for humid areas. low precipitation and a long and dry summer prevented the organic matter from reaching higher levels. furthermore, the content of organic matter greatly decreased with depth. the organic matter was limited because of the low rate of clay content in most horizons and the scarcity of humus-metal complexes, which can be understood from the alp and fep values because the humus-metal complexes and fine fractions protect organic matter from microbial decomposition and therefore, improve its stability. high organic matter content caused an anti-allophanic effect to be limited because it hinders the bonding of al and fe by organic matter. however, despite low organic matter content, there is no allophane formation in these soils. the δph (ph kcl – ph h2o) was negative (-) in all horizons, and this indicated a negative charge in all horizons. in other words, there was no positive charge dominance. however, the ph values in naf turned out to be over 9.5 in all horizons. the ph values in naf were a quick test and indicated the presence of an amorphous material (wada, 1989). such anions as florid and phosphate undergo specific anion adsorption in allophane and imogolite, and they emit high amounts of ohto the environment when these minerals react with naf. however, the test is only applicable to soils without free carbonates. these latter in the profile cause naf ph values to measure high; therefore, the test involved isn’t used to determine the mineralogy class in which the free carbonates are present. the presence of a net negative load in the soils in the area of the study and resulting phnaf> 9.5 due to carbonates indicates that there is no allophone or imogolite in the studied soils. furthermore, another reason that the naf test isn’t always valid is that al is attached to the organic matter. nevertheless, the low content of organic matter in the soils and the very low alo content make this factor less likely to be valid. although the soils in the study area have a volcanic origin, a high caco3 content was detected in the profiles. several minerals with ca, such as diopside, augite and plagioclase are present in the soils, which was evident from the primary mineral analysis of the soils in the study area. the high carbonate content in this location can be accounted for by the formation of caco3 by the minerals. the cecs of these soils ranged from 6.6 to 34.99 (cmolc kg–1), and in certain areas, high values were obtained despite the low organic matter content. the rise in the cec to such high values indicated the presence of alumina silicates (smectite) (figure 2) with layers of high charge density. the phosphorous fixation capacities in all of the horizons remained above the required limits for andisols (>85%). but it was relatively low compared with the other andisols in the world. this is further evidence that the active forms of al and fe do not accumulate in the soil. the basic cations in the soil were aligned as ca>mg>k>na on the surface, in order of abundance. this shows that the feldspars that are found in the parent material are rich in ca and na. consequently, the xrd results on the whole soil body (figure 3) indicated that the feldspars are plagioclases. the higher values of k on the surface soil were caused by its transportation through the plant cycle up to the surface. the base saturation was found to be 100% in all profiles. the fact that the base saturation was generally 100%, indicates that the low precipitation was insufficient for the bases to be leaching. total element contents according to the total chemical analysis results, lower values were obtained in the total sio2 amount compared to silicate parent rocks that do not contain carbonate.. aluminum in the soil is directly related to the distribution of clay, and as the degree of decomposition in the soil increased, the amount of al2o3 increased.. the total al2o3 in the studied soils reached values in the parent material that supported the low weathering rate. considering all of these criteria, weathering during soil formation was concluded to occur slowly, and losses from and changes to the soil were deemed to be very limited. indeed, cia and pia values indicate that karacadağ profiles are moderately weathered. the decrease in weathering indices with depth indicates that weathering decreases with depth. in comparison to the total amount of fe2o3, the amount of free iron oxide was quite low in the study soils, and this indicates that weathering is low. this results from the poor weathering of hornblende. it is highly likely that the remaining iron was found in the crystal structure of the clays. there is no significant difference between the amounts of na on the surface horizons in comparison with the na amounts in the parent material of the profiles. this trend also applies to the other alkaline cations. in contrast, an increase was seen in the surface horizons in cations such as na, k and mg. this, in turn, demonstrates that weathering is limited and that alkaline cations aren’t accordingly leaching. the increase in surface horizons is probably due to the elemental cycle due to herbal use. the total tio2 in the profiles ranges from 0.11% to 0.90%. the distributions of this element in all of the studied profiles in solum were close to each other and very small. the percentage of tio2, which is a durable element, indicated the soil weathering rate. this element, the amount of which increases with soil development, exhibits low values that parallel the slow pedogenic development in the study area soils. 434 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu selective dissolution analysis the al, fe and si values determined with a selective solution analysis offered significant data to determine the composition and amount of minerals formed in volcanic soils. index values of selective dissolution analysis are presented in table 5. index values, obtained with the help of these data, gave an idea about whether amorphous materials such as allophane, imogolite and ferrihydrite were available or not. although the feo, alo and sio values determined in the acid oxalate in the study area reached high levels in some of the horizons, these values were below those seen in andisols that are rich in allophane. therefore, these low values indicated a low level, or total absence of amorphous compounds such as allophane and imogolite. dahlagren (1994) reported that, in cases where the amount of sio islessthan 0.2, allophane should not be taken into consideration, explaining that acid oxalate may require quite fewo2 and si from the surface of the crystal minerals because of special circumstances. alp and fep extracted from pyrophosphate provided significant information about organometallic complex forms of fe and al. while this value is high in andisols, it is observed at very low levels in the soils of this study. because low precipitation and the long and dry summer season in the study area prevent the organic matter from reaching high levels, the fep, alp values also was observed to be low. the solubility of al and fe staying low when the ph was over 5.5 had a large impact (lindsay, 1999). parfitt and saigusa (1985) reported that allophane and imogolite are dominant between a moderate acid and neutral ph, whereas al-humus complexes can exist in more acidic circumstances (ph<5.5). therefore, the low organic matter content and high ph levels in the study area soils have, to a large extent, prevented the formation of fe-al-humus complexes when compared with andisols. however, the alp values in the study area, in the surface horizons, were observed to be higher than those of alo and ald. this indicates that the al-complexes were found in profiles at a higher rate than allophane or crystallized al compounds or reached significant amounts. if the rate of fep/feo was between 0.9 and 1.6, this indicated metal-humus complexes. this rate was below 0.9 in all of the horizons, which indicated the limited presence of fe resulting from metal-humus complexes or fe humus complexes were identified far less than the other fe fractions. the fed rates in the studied profiles appeared to be generally higher than in the other extracts. furthermore, the ald values were higher than the general alo. this indicated that al was identified more often in crystallized compounds than in allophane. high fed values indicate that fe is found in the form of crystallized compounds. the low feo/fed rates seen in the profiles support this conclusion. ferrihydrite is one of the amorphous forms of iron, and it is under the combined effect of high and steady precipitation, and the impact from the organic matter content, which affects crystallized iron distribution as well as the high al activity in acidic conditions. the fep/fed values in the soils were also fairly low. such low values are additional evidence of the presence of crystallized fe compounds. the rate of (alo-alp)/sio can be used to identify the formation of allophane and imogolite in volcanic soils. this ratio varies in most volcanic soil between 1 and 2.5; if it is close to 1, it signifies allophane, and if it is close to 2, it signifies imogolite (parfitt and wilson, 1985). this ratio is not within such limits as to reveal the presence of allophane or imogolite in any of the studied profiles, and it has generally turned out to be negative (-) in the horizons (table 7). if this value is smaller than 0.75 or greater than 2.4, this finding indicates the absence of minerals with amorphous al. additionally, the alp/ald values in the soil are used as a criterion for andisols, and this value is above 0.9 in andisols. the alp/ald values in the profiles appeared to be higher in some horizons than 0.9. however, because the other index values do not confirm the presence of allophane, this value alone does not make sense. it only shows that al is bonded to a significant amount of organic matter. likewise, the alp/ald values are definitive for andisols; if this value is below 0.1, it signals the presence of allophane. in all of the studied profiles, the alp/ald values were observed to be above 0.1. if this value is above 0.1, this indicates that there is no allophane and that a significant part of the aluminum is retained by the al-humus complexes. consequently, very low rates of alo and sio caused the formation of alumina silicate minerals through neo-formation compared with fed, leading to the depletion of al and si, which were released into the environment through weathering. considering the low phosphorus fixation, low amounts of alo and sio were seen to have prevented allophane formation. shoji and fujiwara (1984) reported that allophane was, to a large extent, linked to the ph of the soil and formed when the ph was between 5 and 7. particularly high organic matter, and the reduction in silica loss due to low precipitation and adsorption of al by organic matter reduced the formation of allophane (anti-allophonic effect). low precipitation and a long dry season in the study area caused the loss of si to be low, and insufficient al emerged because of low weathering. as a result of the formation of secondary alumina silicate clay minerals through neo-formation, the heavy use of si and al and the presence of organic matter over 2% in the surface horizons led to the formation of fe-humus complexes, significantly preventing allophane formation. many researches such as moustakas and georgoulias (2005), reported that in ustic, xeric and aridic moisture regimes, allophane is rarely found because of the restriction on silicate loss. table 5. index values of selective dissolution analysis of the studied soils. pe do n h or iz on (f e dfe o)* 10 0 /f e d fe o/f e d fe d/f e t fe p/ fe o fe p/f e d (a l o-a l p)/ si o a l p/ a l d a l o/a l d a l p/a l o 1 a1 74.59 0.254 0.078 0.045 0.011 0.02 0.61 0.68 0.88 a2 57.06 0.429 0.088 0.152 0.065 -0.05 0.97 0.76 1.28 bw1 6.28 0.937 0.081 0.004 0.004 0.05 0.37 0.54 0.68 bw2 -21.27 1.213 0.066 0.005 0.006 0.10 0.78 1.16 0.67 c 75.46 0.245 0.052 0.035 0.009 -0.01 1.40 1.33 1.05 cr 91.01 0.090 0.048 0.175 0.016 -0.15 2.23 1.38 1.61 2 a1 -39.11 1.391 0.085 0.050 0.069 -0.01 0.51 0.47 1.08 a2 -27.88 1.279 0.079 0.090 0.115 -0.06 0.83 0.61 1.36 c 70.00 0.300 0.036 0.117 0.035 -0.28 3.86 2.00 1.93 cr 39.47 0.605 0.028 0.057 0.034 -0.09 2.88 2.25 1.28 3 a1 -98.77 1.988 0.081 0.022 0.043 -0.02 0.74 0.65 1.14 a2 -45.12 1.451 0.104 0.052 0.076 -0.04 0.54 0.42 1.28 cr 26.51 0.735 0.092 0.005 0.004 0.04 0.67 0.84 0.79 r 4 a1 2.75 0.973 0.084 0.050 0.048 0.03 0.80 0.93 0.86 a2 39.65 0.604 0.088 0.034 0.021 0.03 0.59 0.73 0.80 bw1 45.60 0.544 0.109 0.132 0.072 -0.01 0.80 0.75 1.07 bw2 31.63 0.684 0.086 0.043 0.029 0.02 0.73 0.80 0.90 c 46.03 0.540 0.078 0.009 0.005 -0.01 1.13 1.06 1.06 cr 88.07 0.119 0.076 0.086 0.010 -0.34 1.83 0.87 2.10 435genesis, mineralogy, and geochemistry of soil profiles developed on volcanic materials under semi-arid conditions from central anatolia-turkey mineralogical properties the presence of kaolinite and illite in the profile indicates that the feldspars turned directly into kaolinite and illite, most likely due to mineral weatheringat hydrothermal conditions. there is no significant difference in the amount of minerals that exist deep in the profile. the similarity of the minerals to one another in the solum and the lack of significant differentiation in their amounts indicated that these soils at initial stage of development, but the prevalence of clay minerals with an intermediate layer in the solum demonstrated that the profile has a more developed composition than during primary stages. however, in proportion to the total iron oxide, the rate of free iron oxide that has weathered, and thus passed into a free state, was low in welldeveloped soils. illite, which was encountered in all profile horizons, probably formed through the conversion of minerals that contain. the fact that the illite was almost uniform in the profile supports the view that it was formed from feldspars that are commonly found in basalts and andesite. fitzpatrick (1975) noted that feldsparscould form illite in conditions similar to those of our study. the amount of illite increased, although minimally, as the depth increased in some of the profiles. this finding suggests that weathering in the profile is not at an advanced stage.. the reason that illite may be less available in the upper horizons can be explained by the low weathering possibility in deep soil as well as by the formation of the form of illite →-k→ smectite conversion in this profile. smectite formed in an environment where the base saturation and, particularly the mg content, were high; it is found as a permanent mineral here. according to a large number of researchers (yeşilsoy and kapur, 1982; poulenard et al., 2003; parfitt and wilson, 1985; parfitt and kimble, 1989; shoji, 1986), if the rate of si/al in the soil solution is higher than 2, smectite is formed in a neutral-weak acidic ph, high ca+mg and na dilutions (carson et al. 1976). it has been suggested by many researchers (yeşilsoy and kapur, 1982; parfitt et al., 1980; murray et al., 1977; shoji, 1986; wada, 1989) that the presence of significant amounts of calcium will promote the formation of smectite clay minerals in arid regions but that it will hinder the formation of kaolinite (yeşilsoy and kapur, 1982). there is some controversy regarding the formation of clay minerals of type 2:1 in volcanic soils. the formation of clay type 2.1 in the study area reached a significant level. this finding is observed because the basic cations, which emerged from the weathering of pyroxene and amphiboles, raised the ph, and because the si and ca could not be sufficiently leached, encouraging smectite formation accompanied by a high si/al ratio and ca. the presence of such minerals as augite and diopside (figure 3), which are among the primary minerals, reinforces this theory. in light of all of these data, the clay minerals in the studied soils formed as products of conversion and weathering of the minerals, feldspars, pyroxene and amphibole, which are heavily detected in basalt and andesitic materials derived from the parent material. as primary minerals, feldspar (anorthite and albite), amphibole, pyroxene (diopside, actinolite, tremolite, and hornblende) apatite, hematite, olivine and biotite were identified in the soils (figure 3). the primer mineral analysis indicated that the feldspars are plagioclases which are the most found minerals in mafic materials. ferro minerals, such as hematite, were detected in the profiles (figure 3). wada (1987) reported that hematite is related to magnetite and maghemite, that magnetite can easily formed from maghemite under forest vegetation and in the presence of organic matter and that gibbsite is available in well-disintegrated volcanic ash soil in hawaii. in the ir spectrums of the samples, tension peaks that belong to structural oh-were obtained between 3355 and 3621 cm-1 in the form of a large strip belonging to an h bond. curve vibrations that belong to h were obtained at approximately 1631 to 1643 cm-1, whereas vibrations belonging to the si-c single bond were found at 712 to 798 cm-1. allophane-like minerals were found in the soil in four profiles. vibrations were detected at 2800 to 3800, 1630 to 1640, 975 to 1020 and 800 to 400 cm-1. such vibrations could not be obtained in the study area. however, the finding of si-c single bonds at 790 cm-1 and again the discovery of amorphous silicate at 1636 cm-1 were the characteristic peaks. the very high sand fraction in the soil profiles and the availability of a-c or abw-c horizons in the profiles revealed that the time of soil formation has representing present disintegration was short, that the weathering in the profiles was gradual and that the effect of the parent material was strong. as low precipitation and a dry summer restricted the loss of si in the soil, no allophone or imogolite formation was observed. especially the fact that the clay mineral types in the clay fractions in the horizons remain the same and the amounts and types of primary minerals are similar indicates that pedogenic differentiation occurs quite slowly. andic properties and classification requirements for a soil to be classified as andisols are described in soil survey staff (2014). two items in these requirements cannot be met in any of the profiles. the first 3 requirements given in item number 3 are seen only in profile 3. the index values belonging to the andic properties for the studied soils can be seen in table 4 and 5. the alo+1/2 feo values in all of the profiles are rather low and below 0.71 % in all of the horizons. although this value alone can meet the requirements of several items mentioned above in some of the horizons, it remains insufficient for andic properties when evaluated along with the other properties. phosphate retention in the soils is lower than 85 %, and it shows that active al and fe complexes do not accumulate in the soil. the bulk density was higher than 0.9 gr/cm-1 in all of the horizons. for this reason, the studied profiles do not meet the criteria given in item number 2. the criteria given in item number 3 were also used for classifying the soils, which are not only formed on volcanic deposits but also on other deposits, which exhibit andic properties and are not volcanic. although the sand+coarse silt fraction (0,02-2 mm) was over 30 percent in all of the studied profiles and the phosphate retention in the first profile was over 25 %, the alo +1/2 feo and volcanic glass content of the studied samples in the other profiles, except profile 3, did not meet the requirements in item number 3. in profile 3, the volcanic glass content was between 3 and 16 % and did not meet the requirement of [(al+1/2fe) percentage x (15.625)]+[% volcanic glass]=36.25 or the further demands of item “3e”; therefore, none of the studied profiles can be classified as andisols. profile 2 is classified as vitrandic haploxerolls sub-group of mollisols ordo because contain mollic epipedon and it is in a xeric moisture regime and meet the necessary requirements for the vitrandic subgroup. because profile 3, studied in light of these data, do not contain any other diagnostic horizons apart from ochric epipedon, it is classified as orthent sub-ordo of entisol ordo. due to it is in a xeric moisture regime and meet the necessary requirements for the vitrandic subgroup, it is classified as vitrandic xerorthent, whereas in profiles 1 and 4, are in the inceptisol order because of the existing of a cambic horizon in both profiles; because they do not contain plagen and antropic epipedon, they are classified in the subgroup xerept; finally, because of not involving a duripan, calcic or petrocalcic horizon and fragipan as well as having a base saturation of over 60 %, they are in the group haploxerept great group. they are classified as vitrandic haploxerept at the sub-group level. conclusions in this study, four profiles that formed on volcanic parent material in semi-arid climatic in central anatolia around karacadağ were described. the physical, chemical and mineralogical properties of the profiles were identified and it was determined whether the soils formed in the study area could be classified as andisols. the soils in the study area were still in an initial stage of development. among the studied profiles, ochric epipedon was identified in profiles 2 and 3, whereas no diagnostic horizon other than cambic b was identified in addition to mollic and ochric epipedon. the low precipitation and long and dry summer in the study area restricted the accumulation of organic matter, adversely affected weathering, and wholly affected amorphous materials such as allophane and imogolite by delaying the leaching of si and partly affecting the formation of fehumus complexes. however, a significant amount of aluminum in the profile was still determined to be attached to the organic matter. of the phyllosilicates that were studied, smectite kaolinite, illite and chlorite-smectite were the most common minerals. the leading primary minerals in the soil were identified as feldspar (anorthite and albite), amphibole and pyroxene (diopside, actinolite, tremolite, and hornblende), apatite, hematite, olivine and biotite. although the studied profiles formed on volcanic parent material, they were not classified as andisols. profiles 2 and 3, which were obtained from the study area, were classified as vitrandic haploxerolls and vitrandic xerorthent respectively, and profile 1 and 4 were classified as vitrandic haploxerept. from the obtained data, we established that the climate determines the concentration of such cations as al, si, and fe in the soil solutions by affecting the accumulation of organic matter and the amount of weathering products from volcanic materials in the formation of andic properties, as described in soil taxonomy 2014; therefore, the climate drives the formation of amorphous minerals. in conclusion, this study reveals that climate is an important factor in andisol formation, in addition to the glassy material content of the parent material. 436 hasan hüseyin özaytekin, cihan uzun, mert dedeoğlu acknowledgments this study was supported by tubitak (scientific and technological research council of turkey, project no: tovag 110o301) and the selçuk university bap office (coordinating office of scientific research projects, project no: 10201007) a part of a ph.d. thesis. references ayhan, a., & sevin, m. 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(2003). pedogenesis of volcanic ash soils in andean ecuador. soil science society of america journal, 67(6), 17971809. https://doi.org/10.2136/sssaj2003.1797 https://doi.org/10.1180/mono-5 https://doi.org/10.2134/agronj1951.00021962004300090005x https://doi.org/10.2134/agronj1951.00021962004300090005x https://doi.org/10.1346/ccmn.1976.0240401 https://doi.org/10.1346/ccmn.1976.0240401 https://doi.org/10.1016/0375-6742(92)90048-d https://doi.org/10.1016/0375-6742(92)90048-d https://doi.org/10.1016/j.geoderma.2007.10.020 https://doi.org/10.1016/j.geoderma.2007.10.020 https://doi.org/10.1016/j.geoderma.2004.12.039 https://doi.org/10.1016/j.geoderma.2004.12.039 https://dergipark.org.tr/en/pub/tbtkagriculture https://dergipark.org.tr/en/pub/tbtkagriculture https://doi.org/10.3906/tar-1102-2 https://doi.org/10.1346/ccmn.1980.0280502 https://doi.org/10.2136/sssaj1989.03615995005300030057x https://doi.org/10.2136/sssaj1989.03615995005300030057x https://doi.org/10.1016/s0070-4571(08)70685-x https://doi.org/10.1016/0009-2541(87)90106-9 https://doi.org/10.1016/0009-2541(87)90106-9 https://doi.org/10.2136/sssaj1996.03615995006000010033x https://doi.org/10.2136/sssaj2003.1797 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n4.103152 earth sciences research journal earth sci. res. j. vol. 26, no. 4 (december, 2022): 335 343 m et eo r o lo g y manuscript received: 13/06/2022 accepted for publication: 14/11/2022 an overview of the connection between earth’s climate evolution and mass extinction events martín josé montero-martínez1*, mercedes andrade-velázquez 1.instituto mexicano de tecnología del agua (imta), jiutepec, morelos, mexico. 2.conacyt – centro de cambio global y la sustentabilidad, a.c. (ccgs), villahermosa, tabasco, mexico. *corresponding author: martin_montero@tlaloc.imta.mx how to cite item: montero-martínez, m. j., & andrade-velázquez, m. (2022). an overview of the connection between earth’s climate evolution and mass extinction events. earth sciences research journal, 26(4), 335-343. https://doi.org/10.15446/esrj. v26n4.103152 abstract: authors present a brief review of the potential role of climate change on mass extinction events throughout the history of the earth. studying paleoclimate is difficult because it uses proxies that occurred millions of years ago, and there is an intrinsic uncertainty associated with that. however, the climate of the past and the evolution of life itself are related to each other. the current discussion goes through the different geological eras, emphasizing the phanerozoic eon, where terrestrial conditions allowed life to flourish. recent studies seem to support the argument that the five great mass extinctions are related to warm climate modes produced by intense volcanism that generate changes in the concentrations of greenhouse gases and marine anoxia. this should be one more alert for humanity to implement effective measures to counteract the current global warming trend before the consequences on ecosystems are more serious. keywords: geologic eons; extreme paleoclimate; climate change; mass extinction events. una descripción general de la conexión entre la evolución del clima de la tierra y los eventos de extinción masiva resumen presentamos una breve revisión del rol potencial de los cambios climáticos en los eventos de extinción masiva a lo largo de la historia de la tierra. estudiar el paleoclima es difícil porque utiliza proxies que ocurrieron hace millones de años y existe una incertidumbre intrínseca asociada con eso. sin embargo, el clima del pasado y la evolución de la vida misma están relacionados entre sí. la discusión actual recorre las diferentes eras geológicas, con especial énfasis en el eón fanerozoico, donde las condiciones terrestres permitieron que floreciera la vida. estudios recientes parecen respaldar el argumento de que las cinco grandes extinciones masivas están relacionadas con modos de clima cálido producidos por un vulcanismo intenso que generan cambios en las concentraciones de gases de efecto invernadero y anoxia marina. esta debería ser una alerta más para que la humanidad implemente medidas efectivas para contrarrestar la tendencia actual del calentamiento global antes de que las consecuencias sobre los ecosistemas sean más graves. palabras clave: eones geológicos; paleoclima extremo; cambio climático; eventos de extinción masiva. https://doi.org/10.15446/esrj.v26n4.103152 mailto:martin_montero@tlaloc.imta.mx https://doi.org/10.15446/esrj.v26n4.103152 https://doi.org/10.15446/esrj.v26n4.103152 336 martín josé montero-martínez, mercedes andrade-velázquez 1. introduction undoubtedly, by the mere fact of existing, one can infer that the interaction between the geological evolution of planet earth (including the climate) and the evolution of life has been favorable. however, this assertion stands because planet earth is the only place we know that harbors life as it is, in the entire universe. nevertheless, a recent update of the very famous drake equation (drake, 1961) supports the fact that the probability of finding life on another planet should not be zero (gertz, 2021). climate forcing factors can be divided into terrestrial and extraterrestrial (frakes et al., 1992). terrestrial forcing includes various mechanisms such as atmospheric greenhouse gases, the geochemistry and biochemistry of the oceans, land-sea distributions and their effects on global albedo, and geographic restraints through changes in topography, ocean circulation and the formation of bodies of water. extraterrestrial forcing deals with changes in solar luminosity, passaging the solar system through matter clouds in space, bolide impacts, and changes in the earth’s rotational speed and orbital parameters. states with a high concentration of greenhouse gases generally correspond to high levels of atmospheric co2 and increased warming. these phases are linked to changes in sea level and volcanism. in most volcanic episodes, there seems to be a relationship with these hot or cold modes (hartmann, 2016). therefore, volcanic activity could be related to the termination of cold modes (e.g., early permian, early cretaceous), reinforcing warming trends in other periods (e.g., early ordovician, devonian, and late cretaceous). although the contribution of greenhouse gases (mainly co2) by volcanic activity during geological time is uncertain, there is a hypothesis that the global climate is influenced and perhaps forced by volcanic exhalations (hartmann, 2016; racki, 2020). another important event associated with cold and warm modes is carbon peaks in the ocean, which occur in cold or glacial conditions, and are reported to be associated with marine extinction (wilde & berry, 1984). as we will see later, most of the time biotic crises occurred during warm periods. in this way, the evolution of the earth’s atmosphere and climate has not always been entirely “beneficial” for the development of the multiple species that have inhabited this planet. it is convenient to use the geological time scale as an appropriate frame of reference to describe the climate changes and their impacts on biodiversity arranged chronologically (cohen et al., 2013). figure 1 shows a synthesis of the international chronostratigraphic chart, the updated version of which can be obtained online. figure 1. geological time summarized from the official international chronostratigraphic chart1. * time is the duration of the category in question in millions of years (ma) regarding the present. this article presents an overview of some of the major discussions related to the evolution of the climate (throughout the different geological eras) and its potential impact on mass extinction events. in addition, we include a discussion (section 5) of how estimated future climate changes (according to the intergovernmental panel on climate change, ipcc) compare to past climate changes. as noted by (frakes et al., 1992) (p. 1) “… only by understanding past climate states of the earth can we discern the driving mechanisms for global climate change, set boundary conditions for numerical modeling and learn how to predict future climates”. 1 https://stratigraphy.org/chart revised on oct 21, 2022. 2. the precambrian super eon the solar system was created about 4,600 ma after the collapse of a molecular cloud that gave rise to the solar protostar and the subsequent formation of planets (prentice, 1979; alfvén, 1982). according to the great impact hypothesis, about 4,500 ma the original planet earth collided with another smaller planet, known as tea, giving rise to the earth and moon that we know today (canup, 2012). after its formation, the earth must still have stored a great deal of heat, giving rise to ubiquitous volcanism. the gases emitted by the volcanoes gave rise to the formation of the primitive atmosphere, whose components could be carbon dioxide and water vapor (from volcanoes) and molecular hydrogen (from the solar nebula), which must have been very unstable because of the loss of light gases into space (pla-garcía & menor-salván, 2017). during this time, other physical processes also occurred that were later fundamental in the development of life. some suggest that the earth’s crust formed as early as 4,400 ma and, although no rock evidence is found in this period, some remains of the mineral zircon have been dated to that time (valley et al., 2014). it is likely that the oceans formed during this eon, although little is known about their possible extent (morse & mackenzie, 1998). however, some theories mention the possibility of a wider oceanic coverage than the current one, as a factor that prevented the earth from freezing during its early stages of development when the solar intensity was 30% lower than the current one according to some theories of stellar evolution (kasting, 1993; wicander & monroe, 2016). the other important factor refers to the existence of a greater amount of greenhouse gases that allowed the global average temperature to be more suitable for the development of life. later came the stage of the late heavy bombardment (between 3,800 and 4,100 ma) in which many meteorites fell on earth (and the other inner planets) (cohen et al., 2000; claeys & morbidelli, 2015; bottke & norman, 2017). the end of the late heavy bombardment coincides with the first isotopic tests of life (mojzsis & harrison, 2000), although the uncertainty remains whether life really arose at that time or had already arisen before and survived the bombardment (maher & stevenson, 1988; chyba, 1993). the archean eon covers almost a third of the whole geological time (figure 1) and is the one during which there is irrefutable evidence of the existence of the first rocks on the planet. in fact, the oldest rocks, almost 4,000 ma, are found at acasta gneiss in the northwestern territories of canada (wicander & monroe, 2016). likely volcanoes during the archean eon emitted gases in the same way as they do today, so the atmosphere must have contained mainly water vapor, sulfuric dioxide, carbon dioxide and monoxide, whose reactions would produce ammonia and methane; however, molecular oxygen and ozone were lacking (kasting, 1993). these conditions persisted during the archean (wicander & monroe, 2016). there are two processes by which free oxygen is thought to have entered the early atmosphere. the first is photochemical dissociation, in which ultraviolet sunlight breaks water molecules high in the atmosphere, releasing hydrogen and oxygen. around 2% of today’s oxygen may have been formed by this process; however, it was enough to cause the formation of the stratospheric ozone layer. the second important process is photosynthesis, which could contribute to the formation of atmospheric oxygen until there were organisms with this metabolic process in which they used carbon dioxide and water to produce organic molecules and released oxygen as a waste product. multiple lines of geochemical evidence show that o2 was produced by cyanobacteria on the ocean surface, and these metabolizing organisms were present in abundance in the latter part of the archean, and most likely as early as 3,500 ma (kendall et al., 2010; wicander & monroe, 2016; ostrander et al., 2019). most of the known proterozoic organisms are unicellular prokaryotes (bacteria). there is still uncertainty about when eukaryotic cells first appeared, but it is likely that they were present for at least 1.2 ma (wicander & monroe, 2016). the oldest known multicellular organisms are probably algae, some of which may date back to the paleoproterozoic (~ 1,800 ma). since the beginning of the proterozoic eon, 2,500 ma, the earth’s climate has fluctuated between cold (earth covered mostly by ice) to greenhouse (earth free of ice) conditions. since then, there have been at least five great ice ages lasting between 30 and 215 ma. in the other 70% of the time, the earth has experienced ice-free greenhouse conditions, even at the poles (crowley, 1995). https://stratigraphy.org/chart 337an overview of the connection between earth’s climate evolution and mass extinction events at the beginning of the proterozoic eon, perhaps the first great mass extinction event occurred (although it is not counted within the “famous five” which all occurred in the phanerozoic eon, see figure 2); this event is known as the great oxidation, which occurred about 2,400 ma (margulis & sagan, 1997; gumsley et al., 2017; ostrander et al., 2019). at first, the excess oxygen was absorbed in chemical reactions with iron or by decomposing cells, however, the population of cyanobacteria increased due to their same ability to provide energy by increasing the amount of oxygen that was greater than what could be absorbed, annihilating most anaerobic organisms, for which oxygen is toxic (schirrmeister et al., 2013; schirrmeister et al., 2015; ostrander et al., 2019; sánchez-baracaldo et al., 2022). figure 2. phanerozoic global average temperatures (gat) as reported by scotese et al. (2021). blue and turquoise rectangles show the periods of three great ice ages (in blue) and one cool period (in turquoise) according to frakes et al. (1992). finally, the five mass extinction events (mee) are shown as red lines according to bond & grasby (2020). in addition, with the chemical reactions of oxygen and methane, which oxidized with ultraviolet radiation and produced carbon dioxide, the global temperature would be considerably reduced because of the greenhouse effect. this impressive climate change would trigger the huronian glaciation (2,290 ma to 2,250 ma), one of the most intense that has existed and from which few organisms escaped, even almost extinguishing the cyanobacteria themselves. most of the evidence for the appearance of this glaciation event may be related to the rupture of the kenorland supercontinent, allied with a significant atmospheric change, as well as the emergence of biogeochemical oxygen photosynthesis (tang & chen, 2013). this turns out to be the first coincidence in which an extreme paleoclimatic event (in this case glaciation) is directly/ indirectly related to the loss of organisms on earth (in this case anaerobic organisms and cyanobacteria) affecting biodiversity. towards the end of the proterozoic, in the cryogenic period (~720 to ~635 ma), there were multiple glaciations that have generated a strong discrepancy in the hypotheses about the causes and times in which they occurred (kendall et al., 2006; arnaud et al., 2011). for example, within the causes of these glaciations, especially the larger one known as sturtian, is the hypothesis of the “snowball earth” in which the planet’s surface was completely covered by a layer of snow (hoffman et al., 1998; hoffman & schrag, 2002); another is the “slushball earth”, where only a small equatorial strip remains free of snow on the surface (hyde et al., 2000); and finally a generalized but regional and diachronic glaciation, during the prolonged rupture of the supercontinent rodinia (eyles & januszczak, 2004; kendall et al., 2006). the hypothesis of the quasi-frozen earth except in the tropics (slush-ball) is very interesting because it proposes a more viable survival alternative for this critical period in the evolution of multicellular life than the “snowball earth” (hyde et al., 2000). unlike living organisms, warm and cold modes appear to be independent of the boundaries of major geologic time intervals. however, the effect that global climate change has had on both the evolution and dispersal of organisms is undeniable and clear examples can be observed at shorter time scales (frakes et al., 1992). 3. extreme paleoclimate of the phanerozoic eon the climate on earth has had important variations throughout its evolution during the current phanerozoic eon (~ 541 ma to the present), which precedes the entire precambrian super eon (figure 1). its name is derived from the greek and means “visible life”. that is the period where multicellular organisms strengthened enough and grew to be remarkable on a human scale. it is also the time when the famous “five mass extinctions” and three of the five most important glacial periods have occurred (figure 2). this section briefly discusses the potential connection between changes in hot and cold periods of the climate with the development or extinction of ecosystems. a recent review of this connection is shown by bond & grasby (2017) and summarized in table 1 of that work. the cycle of hot and cold periods during the phanerozoic eon is well established among the scientific community (fischer, 1981, 1982, 1984; frakes et al., 1992, fig. 11.1; scotese et al., 1999; summerhayes, 2015; scotese et al., 2021) and is consistent with that stated in figure 2. multiple tectonic and environmental processes generally control long-term (>50 ma) global temperature that drive earth’s climate from icehouse to hothouse conditions and back (scotese et al., 2021). the dominant factors are the level of greenhouse gases (mainly co2) (grossman & joachimski, 2022), which are regulated by the release of volcanic gases and co2 extraction due to weathering (van der meer et al., 2014), the geographic configuration of the continents and ocean basins (see figure 3), and the earth’s albedo. an icehouse world is simply defined as a time when the earth is covered by permanent ice at either pole. on the other hand, “hothouse” times arise when there is no ice above the polar circle – even during the winter (scotese et al., 2021). figure 3. evolution of the paleotopography and paleobathymetry (z, in meters above sea level) for different geological periods (cambrian, permian, and cretaceous), and epochs (paleocene, eocene, miocene, pliocene, and holocene) of the phanerozoic eon (data from scotese & wright, 2018). 338 martín josé montero-martínez, mercedes andrade-velázquez in the early cambrian, the late glacial phase of the proterozoic ended, and the earth warmed. the first discernible evidence of this is in the latitudinal expansion of carbonate sedimentation, but since the expansion was gradual, it is inferred that the process was slow and did not reach completion until the middle ordovician. the warm mode ended in the middle ordovician with the development of ice sheets in north africa (frakes et al., 1992). the ordovician period had great oceanic diversification (sepkoski, 1981), cut off by the first of the five great extinctions: the late ordovician mass extinction. two pulses eliminated 85% of marine species (jablonski, 1991) during this ecological crisis, the second most severe in the entire phanerozoic (bambach et al., 2004). the most accepted extinction scenarios of this crisis call for changes in ocean temperature and redox chemistry: the first pulse, at the beginning of the hirnantian age (~445 ma ago), affected nekton and plankton and has been linked to the cooling at the beginning of the gondwana glaciation (brenchley et al., 2001); a million years later, the second pulse (still in the hirnantian), is associated with warming, rising sea levels and anoxia (nielsen, 2004). however, recently bond & grasby (2020) have proposed a novel idea, which is that the first pulse was also caused by volcanism, global warming and anoxia, and not by the third ice age associated with this period (figure 2). in this way, instead of being the strange event among the five mass extinctions, this event is like the others in being caused by volcanism, heating, and anoxia; despite being immersed in a low frequency cold mode (bond & grasby, 2020). figure 2 illustrates how the other four mass extinction events occurred under warm phase climate modes (bond & grasby 2017; 2020), as we will discuss below. the second major mass extinction, which occurred in the upper devonian and early carboniferous, between the famennian (~ 370 ma) and the tournaisian (~ 355 ma) ages, was associated with global warming (+9 °c above the current average) imposed on cold pulses, caused by large volcanic events, a meteor impact (siljan ring), possible marine anoxia and changes in atmospheric carbon concentration (bond & grasby, 2017). changes in ocean chemistry and a climate that cooled were the causes of this extinction in its last stage. this process occurred with the enrichment of 13c in the ocean, which may have been due to the photosynthesis-respiration process that removes 12c by organisms from the surface water and was deposited on the ocean floor, where there was enough oxygen by which combined and produced co2. however, it was produced in large quantities that when oxygen was depleted, organic matter was removed (frakes et al., 1992). the third great mass extinction, at the end of the permian (~ 252 ma), is considered the most powerful of all, where up to 96% of existing species were lost (sepkoski, 1984; erwin et al., 2002; li et al., 2021). it is also associated with even stronger global warming (+10 °c above the current average) caused by large-scale volcanism (the siberian traps), marine anoxia, a decrease in the concentration of atmospheric carbon, probable ocean acidification and possible impacts by meteors. in addition, other factors such as acid rain, poisoning by toxic metals (hg) and damage by uv-b radiation are associated (bond & grasby, 2017). by the end of the triassic (~ 200 ma), the fourth major mass extinction event occurred, which is estimated to have been caused by global warming of +6 °c above the current global mean temperature due to large-scale volcanic activity, probable ocean acidification, decrease in atmospheric carbon levels and not related with a major meteor impact event (bond & grasby, 2017). the peak of c in the aptian (from 125 to 113 ma) occurs near the end of the middle jurassic to the cold mode of the cretaceous. as the productivity of nutrients decreases, the co2 increases, which contributed significantly to the warm period that characterizes the late cretaceous. during the aptian-albian (from 125 to 100 ma) the sea level increased due to global warming. the carbon spike in warm modes may not always be associated with high nutrient productivity caused by colder climates and upwelling, but may be associated with reinforced confinement under strongly anoxic conditions (frakes et al., 1992). finally, the famous great chicxulub impact, at the end of the cretaceous (~ 66 ma), was the trigger for the fifth and last great mass extinction that caused the decline of dinosaurs and many other species (76% of species perished). besides this, the event occurred in a period of a warm mode with global warming of +4 °c caused by strong volcanic activity (the deccan traps), a slight decline in the concentration of atmospheric carbon, and poisoning by toxic metals (hg) (bond & grasby, 2017). this is the last great mass extinction event, although some others speculate that the sixth great extinction has already started since the anthropocene (pievani, 2014). cowie et al. (2022) provide an updated discussion of the last argument above. they point out that currently the scientific community splits in those who are in favor that the sixth mass extinction is already underway (this time caused entirely by humans), and those who, even though considerable evidence supports a biodiversity crisis of increasing extinctions and decreasing abundances, do not accept that this amounts to a sixth major extinction. one of the key points on the discussion arises because the famous international union for conservation of nature’s red list of threatened species (iucn) red list2 shows that the rate of species loss does not differ from the background rate. however, the list is heavily biased because almost all birds and mammals, but only a minute fraction of invertebrates, have been assessed against conservation criteria. incorporating estimates of the actual number of invertebrate extinctions leads to the conclusion that the rate far exceeds the background rate (cowie et al., 2022). 4. extreme paleoclimate of the cenozoic era the onset of the conditions that culminated in the quaternary glaciation (pleistocene and holocene series) resulted from the long-term cooling trend in world climates that began from the beginning of the cenozoic era, during the paleogene period (see figure 2), where the last ice age that has survived to this day began (ehlers et al., 2011). besides some limited activity in the eocene, in both the southern and northern hemispheres (stickley et al., 2009), significant glaciation began in the latest eocene-early oligocene (~ 35 ma) in eastern antarctica (ingólfsson, 2004). it was followed by mountain glaciation through the miocene (23 to 5.3 ma) in alaska, greenland, iceland, and patagonia, and later in the pliocene (5.3 to 2.6 ma) in the alps, the bolivian andes, and possibly in tasmania. from neogene, glacial-derived raft ice remains are found in oceanic sediment cores in the north atlantic region, including the barents sea (moran et al., 2006) and areas next to norway, northern and southeastern greenland, iceland and northern north america, and in the southern ocean off antarctica (ehlers et al., 2011). a link between climate change and human evolution during the pleistocene is often assumed but has rarely been proven (gamble et al., 2004; demenocal, 2004; maslin & christensen, 2007; maslin & trauth, 2009). homo appears on earth approximately between 2.5 and 3 ma. during this time range in africa, many areas became grasslands as the climate became drier. this could be the reason humans went from sedentary to nomadic. the state of the climate, in that and other times, is derived from sediment studies. the dry periods are determined by the dark, dirty, and sandy layers that expel the sea by the manifestation of monsoons, while in the wet periods, the layers contain light amalgamations of abundant fossilized plankton. these characteristics dimension the conditions in which the terrestrial environment was in the temporalities described above. in this period, there were also rapidly variable climate oscillations (on a geological time scale). records of this climate were found in africa, which is known as the paleoclimatic record of the valley. recently, rousseau et al. (2022) found that orbital and millennial-scale variability appear to interact during the quaternary, with millennial variability increasing in intensity as the ice sheets grew in both area and volume. thus, sudden changes in climate are undoubtedly related, albeit indirectly, to the astronomical theory of climate (milankovitch theory). homo then witnessed unfamiliar landscapes during these changing times, where diverse food options were presented, while the environment tended towards a more uniform grassland landscape. during humid periods, such as the middle stone age (about 40,000 to 80,000 years ago), the climate transformed to humid conditions in south africa. homo gave rise to the innovation of language and its own cultural identity (handwerk, 2014). according to dynamic simulations, the severe cooling and expansion of the ice sheets during the last glacial maximum, 27,000–19,000 years ago 2 https://www.iucnredlist.org/ revised on oct 21, 2022. https://www.iucnredlist.org/ 339an overview of the connection between earth’s climate evolution and mass extinction events had a great impact on the populations of plants and animals, including humans (tallavaara et al., 2015). agriculture emerged 10,000 years ago and coincides with the end of the last ice age since we are currently in an interglacial phase. the earliest human brains, which were smaller but significantly different, are believed to have occurred when periods of climate were more stable. so, climate changes have influenced the adaptation of humans, animals, and plants. climate has also been a factor that may have influenced human migrations from africa to europe and asia. the neanderthal and denisovan may have originated from former groups like homo heidelbergensis, which became isolated and lived in the northern hemisphere. the process at the equator is believed to differ from this, since perhaps the climate in that region was not so extreme (handwerk, 2014). next, we discuss how the climate changes of the past compare to the latest projected scenarios reported by the ipcc. 5. updated historical and future climate change scenarios as reported by the ipcc the previous sections have briefly reviewed some effects of the past climate on biodiversity, where terrestrial and extraterrestrial climate drivers have been pointed out that have determined the cold and warm states to which the earth is subjected and the processes in which the chemical composition of the atmosphere and ocean have been changed to extreme conditions. regarding an atmosphere with high concentrations of co2, the estimates during the quaternary period (pleistocene and holocene, ~ 2.5 ma), give values between 300 and 500 ppm. the sea level was 20 m higher, and the temperature was approximately 2 °c higher than today (frakes et al., 1992). co2eq concentrations were between 300 ppm and 700 ppm in the late miocene (~ 10 ma). for the rest of the cenozoic (paleogene > 30 ma) these concentrations were between 700 ppm and 1000 ppm. previously during the mesozoic, co2 concentrations were reduced to less than 500 ppm (ipcc, 2013). it is possible that the human being will face for the first time a high level of co2 concentrations that is expected for the following decades. however, for the rest of the ecosystems, these types of levels are not at all alarming and they will surely survive as they have been doing for the last 500 million years (frakes et al., 1992). this serves as the basis for trying to assess the potential effects of the climate in the decades to come. earth’s system models (esms) are currently the most complete tools to simulate the earth’s climate at different periods. esms simulations are primarily determined by the internal and external forcings on the models. factors such as the flux of solar radiation that our planet receives are among the most prominent, but other factors, such as the distribution of the continents, the topography of the earth, the bathymetry of ocean basins, the availability of chemical elements and many others, they also play an important role (feulner, 2009). esms are used to determine the changes in these factors in the climate and to generate the ipcc climate change scenarios diversified under the characteristics of co2eq concentrations and the radiative forcing levels at 2100 (ipcc, 2013). the ipcc-ar5 set of scenarios, better known as representative concentration pathways (rcp), characterize the development and modeling of the physical components of the climate worldwide in the coupled models intercomparison project phase 5 (cmip5) (moss et al., 2008). these scenarios are classified into rcp2.6 (optimistic), rcp4.5 (conservative co2eq mitigation), rcp6.0 (significant increase in co2eq concentrations and moderate mitigation), rcp8.5 (does not perform any mitigation activities co2eq). in addition, all of them coincide with an increase in surface temperature that will exceed 1 °c. with exceeding 1.5 °c, there is still a consensus with a high probability for all scenarios, except rcp2.6, which reports a medium probability. to exceed 2 °c there is a high probability that it will happen only under the rcp6.0 and rcp8.5 scenarios, while it is moderately probable with rcp4.5 and less probable with rcp2.6. all scenarios report an increase in temperature due to the amount of co2 concentrations that are already in the atmosphere. this increase will occur in 2050 for some scenarios or by 2100 with others, where co2eq concentrations in the atmosphere would be between 500 ppm and 1000 ppm. these rcp projections allow us to know the possible climatic conditions of the earth for the coming years for different co2 emission scenarios and in this way better prepare us to face the potential impacts of climate change. information from the ipcc-ar5 (ipcc, 2013) shows that the expected global warming of the most drastic scenario (rcp8.5) is far from reaching the maximum global warming of the planet throughout its history (~10 °c above present value); however, the real problem is that this warming is expected to occur in a geologically very short period. past changes in the earth’s temperature and co2 concentrations took place over hundreds (if not thousands) of years; while the changes we are currently observing take place over one hundred years (i.e., during the present century). this brings with it the fact that ecosystems and humanity have very little time to adapt to these new temperatures and the consequent changes in climate extremes that this entails. after year 2100, co2eq concentrations are expected to exceed 500 ppm including above 1500 ppm for the most drastic rcp8.5 scenario, while surface temperature increases exceed 3 or 4 °c (ipcc, 2013). similar co2eq concentrations have been present in the earth at behind years, as described in section above. hansen et al. (2013) report that the climate sensitivity is 3 ± 1 °c for 4 w/m2, 2x co2 forcing, i.e. (0.75 °c per w/m2). however, the co2 fraction is 55% since 1950, this means 5000 gtc are required to yield 9 w/m2, close to rcp8.5, that magnitude would make most of the planet uninhabitable by humans. the new phase 6 coupled models intercomparison (cmip6) project (eyring et al., 2016), has already mixed rcps with new socioeconomic scenarios, known as shared socioeconomic pathways (ssp), in such a way that there are comprehensive scenarios for the development of both mitigation and adaptation actions appropriate to climate change. in this way, the issue continues to be of primary importance to the agenda of the scientific community and decision makers. the new ipcc sixth assessment report (ipcc-ar6) shows that greenhouse gas emissions vary depending on the socio-economic scenario. the scenarios with the highest probability of temperature increase (> 1.5 °c for the last decades of the present century) are ssp2-4.5, ssp3-7.0 and ssp58.5 (ipcc, 2021). the ipcc-ar6 reports that the increase in global warming leads to changes in major extreme hydrometeorological events. for every 0.5 °c more warming, the frequency of extremes of heat, such as warmer days and heat waves, as well as higher rainfall, increases. more intense agricultural and ecosystem droughts also occur in some regions (ipcc, 2021). these changes in the climate due to global warming are shown in short periods and are reflected in hydrometeorological phenomena, implying that reaction actions to these events must be dynamic and possibly out of reach to address the impacts they cause in social and ecological systems. the likely range of total anthropogenic increase in global surface temperature between 1850–1900 and 2010–2019 is 0.8 °c to 1.3 °c, with an estimated mean of 1.07 °c (ipcc, 2021); that rate is unprecedented in at least the last 2000 years. the estimated temperature (very likely range) during the warmest multi-century period in at least the last 100,000 years, is 0.2 °c to 1.0 °c, which happened around 6500 years ago during the current interglacial period (holocene) (ipcc, 2021; gulev et al., 2021) (see table 1). in 2019, atmospheric co2 concentrations reached a level higher than in at least 2 million years (high confidence), and concentrations of ch4 and n2o were higher than in at least 800,000 years (very high confidence) (ipcc, 2021). 340 martín josé montero-martínez, mercedes andrade-velázquez table 1. global temperature evolution over the past 50 ma, present and future projections. source: gulev et al. (2021). age time global temperature anomaly relative to 1850-1900 (ºc) data* paleocen-eocene 50,000,000 before ~ 15.0 proxy paleocen-eocene 35,000,000 before ~ 3.0 proxy miocene 15,000,000 before ~ 7.0 proxy mid-pliocene 1,000,000 before ~ 0.0 proxy pleistocene 500,000 before ~ -1.0 proxy pleistocene 100,000 before ~ 2.0 proxy pleistocene 20,000 before ~ -3.0 proxy holocene 2000 before ~ 0.5 proxy present 0 ~ 1.0 instruments records future projection 2100 ~ 5.0 ssp5-85 scenario** future projection 2300 ~ 14 ssp5-85 scenario *see gulev et al. 2021 for detail. **cmip6 scenarios. table 2. paleoclimatic reconstructions of atmospheric co2 concentrations. global mean sea level over years past to present to cmip6 projections. source: chen et al. 2021 (figure 1.5). time relative to 1850 atmospheric co2 concentration global mean sea level 120 000 years before ~250 ppm ~ -90 m 1,000 years before ~200 ppm ~ -120 m 0 ~290 ppm ~0 m 170 years after (2020)* ~410 ppm ~0.1 m 250 years after (2100)** ~1600 ppm ~1 m 450 year after (2300)** ~2500 ppm > ~5 m *2020 year. **future projections from cmip6 data under the ssp5-85 scenario. the latest projections give a best estimate for the increase of the global mean temperature of 1.4 °c (very likely range 1.0 to 1.8 °c) for the most conservative case (ssp1-1.9) during the long-term period (2081–2100), up to 4.4 °c (very likely range 3.3 to 5.7 °c) for the worst-case scenario (ssp5-8.5) in the same period (ipcc, 2021). these projections show extreme events are likely to intensify in the coming decades, such as the global water cycle and its variability, global monsoon precipitation, and the severity of wet and dry events. many changes due to past and future greenhouse gas emissions are irreversible over centuries or millennia, especially changes in the ocean, ice sheets, and global sea level (ipcc, 2021; chen et al. 2021) (see table 2). cmip6-endorsed model intercomparison projects (mips) have a broad coverage and distribution across the cmip6 science question linked to the world climate research program (wcrp). one major project which certainly helps to assess the performance of climate models to simulate past climate changes and compare them with those in the future is the paleoclimatic modeling intercomparison project (pmip). its main objective is to (a) analyze the response to forcings and main feedbacks for past climates outside the range of recent variability; and b) assess the credibility of climate models used for future climate projections (eyring et al., 2016, gulev et al., 2021). pmip also addresses how well state-of-the-art numerical models simulate climate change, through comparison with observations of the environmental impact of these climate changes, or with climate reconstructions based on physical, chemical, or biological records (wcrp, 2022). five different periods have been designed in the latest phase of pmip (pmip4) to contribute to the objectives of cmip6: the millennium prior to the industrial epoch (past1000), the mid-holocene, 6,000 years ago (midholocene); the last glacial maximum, 21,000 years ago (lgm); the last interglacial, 127,000 years ago (lig127k) and mpwp, the mid-pliocene warm period, 3.2 million years ago (midpliocene-eoi400) (wcrp, 2022)3. kageyama et al. (2021) compare the latest paleoclimate climate model simulations (cmip6pmip4) to the previous one (cmip5-pmip3) and to reconstructions. they show that large-scale climate features (e.g. land–sea contrast, polar amplification) are well captured by all models, while regional changes (e.g. winter extratropical cooling, precipitations) are still poorly represented. 6. final comments one of the key questions to resolve is whether the climate change generated by humans in recent decades is something that will last for a long time or if it will rather be a short-term increase seen from a geological perspective (wicander & monroe, 2016). recent geological evidence supports the fact that, although humans are accelerating global warming, the fact that we are in an interglacial period will cause the planet, within a few thousand years, to return to an ice age. however, global warming will continue to affect ecosystems in the coming decades, something that has been observed both in the past and recently (gamble et al., 2004; moss et al., 2008). these higher warming scenarios, especially in such a short geological period, have put virtually everyone on alert. for almost three decades now, climate change has been on the international policy discussion agenda to implement mitigation measures to reduce greenhouse gas emissions into the atmosphere and implement adaptation measures that achieve minimize the 3 the main results from the cmip6-pmip4 are being published online https://gmd. copernicus.org/articles/special_issue12_888.html revised on oct 21, 2022. https://gmd.copernicus.org/articles/special_issue12_888.html https://gmd.copernicus.org/articles/special_issue12_888.html 341an overview of the connection between earth’s climate evolution and mass extinction events serious consequences that are expected from global warming in the decades to come. after all, as we have seen here, the great mass extinctions on the planet have been directly related to global warming events, either long or relatively short term (bond & grasby, 2020). this result should be one more alert that humans must implement drastic measures, such as those implemented in the current covid–19 pandemic, to make actual life changes such that they make us more efficient in using energy and electricity and implement technologies that are less aggressive to the environment. the ipcc (2021) reports with great confidence that global and regional climate responses to this pandemic time forcing are, however, undetectable above natural variability. while there is medium confidence that atmospheric co2 concentrations continued to increase in 2020, there is no detectable decrease in the observed co2 growth rate. acknowledgements: we are grateful to scotese & wright (2018), and scotese et al. 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(2024). improving estimates of water resources availability over north tropical south america: comparison of two satellite precipitation merging schemes. earth sciences research journal, 28(1), 55-63. doi: https://doi. org/10.15446/esrj.v28n1.104344 keywords: rain gauge; random forest; hydrological modelling; diagnostic evaluation; blending; mejoras en estimativos de oferta del recurso hídrico en el norte de suramérica tropical: comparación de dos procedimientos de mezcla con lluvia satelital resumen las mediciones de precipitaciones de baja densidad perjudican la capacidad de los modelos hidrológicos para estimar con precisión los recursos hídricos superficiales. las técnicas de teledetección y los modelos climáticos pueden ayudar a mejorar la estimación de la variabilidad espacio-temporal de las precipitaciones. sin embargo, por sí solos los datos teledetectados no son lo suficientemente buenos para ser utilizados en modelos de superficie, modelos construidos para apoyar la gestión del agua. en esta investigación, probamos la mejora en la estimación de un campo de lluvia mediante el uso de modelos hidrológicos basados en la premisa de que: un mayor desempeño hidrológico generalmente implica que la precipitación es más consistente con las observaciones de caudal y con las estimaciones de evaporación en la cuenca. el modelo swat fue forzado con dos técnicas de combinación de satélites y pluviómetros, y con el método tradicional de interpolación de estaciones determinista (idw). las tres series de caudales simulados se compararon por separado con los registros observados. no solo centramos la comparación en una métrica de desempeño hidrológico, sino que también realizamos una evaluación más profunda (diagnóstica) utilizando varias firmas y estadísticas hidrológicas. incluimos el sesgo, la correlación temporal, la relación de variabilidad general y un análisis de las curvas de duración del flujo (encontramos que los segmentos bajo y medio se estimaron correctamente, mientras que los segmentos altos se subestimaron). concluimos que cualquiera de las técnicas de combinación tiene sus ventajas sobre la otra y que ambas superan el rendimiento logrado por el idw en la mayoría de los criterios definidos, con una mejora general del 10% y con una mejora individual de hasta el 50%. keywords: pluviometro; random forest; modelación hidrológica; evaluación diagnóstica; combinación. https://doi.org/10.15446/esrj.v28n1.104344 mailto:nduqueg@unal.edu.co mailto:cagarciae@unal.edu.co mailto:jmontoya@ideam.gov.co mailto:fbernal@ideam.gov.co https://doi.org/10.15446/esrj.v28n1.104344 https://doi.org/10.15446/esrj.v28n1.104344 56 nicolás duque-gardeazabal, camila garcía-echeverri, juan josé montoya-monsalve & fabio andrés bernal-quiroga 1. introduction there is an increasing need to acquire hydrological information in small basins or ungauged river reach for water resources assessment and water-related policy implementation. hydrological modelling has been used as an alternative in determining water resources when streamflow and level measurements are scarce (silberstein, 2006). estimated precipitation (ppt) fields are used in distributed hydrological and land surface models, and their influence is significant in model performance (arnaud et al., 2002). however, precipitation gauges in areas with complex spatio-temporal behaviour are lacking, which can compromise models’ ability to represent the system appropriately. moreover, several knowledge areas require distributed rainfall estimates to understand hydro-bio-geochemical natural processes and connections. climate classification indices (beck et al., 2018; rodríguez et al., 2020), and environmental conditions monitoring need accurate rainfall fields to perform a proper policy implementation. evaluation of the uncertainty in hydrological modelling due to rainfall is another kind of study (muñoz et al., 2015). rain gauge networks have been the traditional instrument for measuring rainfall or precipitation. the world meteorological organization (wmo) guidelines advise on their characteristics, yet networks need to be continuously adjusted to quantify rainfall variability at different temporal and spatial scales (blöschl & sivapalan, 1995). deterministic or geostatistical methods are used to distribute the measured values (grimes & pardo-igúzquiza, 2010). however, some places are almost inaccessible or financial resources are low enough to deter the expansion of the network, which could also impair its maintenance; as a result, rainfall records have decreased worldwide (garcía et al., 2016). satellite (barrett, 1970), atmospheric reanalysis (baatz et al., 2021), and multi-source merged products (beck, van dijk, et al., 2017), are alternatives that can help tackle the lack of point measurements, albeit they often suffer from bias and temporal errors (dinku et al., 2010). those errors must be corrected for applying those distributed ppt estimates in local hydrological modelling and forecasting (serrat-capdevila et al., 2014). rain gauges are generally considered the ground truth, although they might suffer from undercatch. hence, correcting the alternative measurements with rain gauges can improve the accuracy of the distributed estimates (fields not produced initially with those specific rain gauges). the correction can also be done with downscaling methods based on physiographic variables like digital elevation models (long et al., 2016), vegetation indices (immerzeel et al., 2009), or a combination of them (ceccherini et al., 2015). the direct merging with rain stations has relied on: bias correction through coefficient multiplication or addition (vila et al., 2009), a direct or linear combination of coefficients found with inverse variance weightings (woldemeskel et al., 2013), spectral analysis combination (heidinger et al., 2012), conditional merging based on geostatistics (ehret et al., 2008), bayesian conditioning (todini, 2001), filtering functions (li & shao, 2010) and recently, random forest machine learning (baez-villanueva et al., 2020). basin-specific studies that compare data merging schemes are scarce (chua et al., 2022; nerini et al., 2015; ur rahman et al., 2020). there is a plethora of methods for merging satellite precipitation and rain gauges to improve hydrological modelling performance, as described above. some studies are focused on comparing radar-based and station blending rather than satellite-based estimates (goudenhoofdt & delobbe, 2009; habib et al., 2012; nanding et al., 2015). in general, other studies are more focused on the evaluation of the available global satellite or reanalysis data (beck, vergopolan, et al., 2017; sun et al., 2018), against local data and far few use hydrological modelling to recently intercompare the improvement of the merging techniques as far as the authors know. the objective of this study is to evaluate two precipitation merging schemes and their respective improvement in local hydrological modelling performance in a recent time period (1980 to 2019). rather than compare global precipitation products that may be created with rain gauges, we aimed to create local or regional products and analyse their performance in hydrological estimation. we focused on blending satellite-based and rain gauge estimates, as those are the most common precipitation datasets available worldwide and compared them with a traditional interpolation method. 2. case of study and data the middle magdalena valley is an important bio-economical and energy production region in colombia where many stakeholders use the water resource. oil palm plantations are one of the region’s most extensive land covers, but other agricultural and livestock developments are also present and use water. three hydrological watersheds comprise the right margin of the middle magdalena valley, represented in hydrological management subzones according to the instituto de hidrología, meteorología y estudios ambientales (ideam, 2013). sogamoso river basin is an interbasin whose model considered streamflow gauges as inputs (see fig. 1). they were hydrologically modelled to fulfil the research objectives. figure 1. location of watersheds, their hydrological network, and in situ hydrometeorological data 2.1 hydrometeorological data point measurements are mainly managed by the ideam, but recently, they are also gathered by regional environmental authorities. the ideam freely and publicly deliver the data through a web portal (ideam, 2023) and the regional authorities by direct request; time series of meteorological variables provided by regional authorities were analysed and discarded since their records were only available after 2013. relative humidity, solar radiation and wind speed records have many missing values and hence were not considered; we used the precipitation, minimum and maximum temperatures time series managed by ideam. anomalous data were checked and retrieved. we analysed streamflow records, also looking for anomalous data and homogeneity in the series (castro & carvajal escobar, 2010); some gauges were discarded due to the high number of missing values. data statistics are listed in table 1. 57improving estimates of water resources availability over north tropical south america: comparison of two satellite precipitation merging schemes table 1. available point measurement in the region variable available used precipitation 175 (from 1960 to 2019, mainly after 1970) 127 (after 1980) streamflow 42 (from 1965 to 2019, mainly after 1980) 17 (after 1980) temperature (minimum and maximum) 33 (from 1975 to 2019) 33 (after 1980) 2.2 alternative hydrometeorological data to bolster hydrological modeling we used the climate hazard group infrared precipitation with stations (chirps) to enhance precipitation representation. chirps is a multisource rainfall product that considers infrared cold cloud duration measurements and ground correction with some rain gauge data (funk et al., 2015). it has a 0.05° spatial resolution at a daily time scale in the period 1981-present and has been used in several hydrological studies, even in colombia (kaune et al., 2019). this was the product used with the rain stations and the merging algorithms for the comparisons made in this study. another adapted version of chirps that uses rain gauges for bias correction at 10-day periods has also been used in colombia for estimating the general water budget (ideam, 2019), called chirps-ire by ideam. due to the lack of appropriate measurement of some meteorological variables that influence evapotranspiration, atmospheric reanalysis data had to be collected to supplement the deficiency. reanalysis are numerical systems that incorporate mainly simulations of physical systems and assimilation of satellite, gauge or radiosonde observations (baatz et al., 2021). we chose to use the climate forecast system reanalysis (cfsr) (saha et al., 2010) because it has already been used and recommended to be used with the soil & water assessment tool (swat) model, with good results (dile & srinivasan, 2014; tan et al., 2021); its data can be acquired from the swat webpage of the texas a&m university (texas a&m university, 2022). 3. methodology we evaluated the congruence of rainfall fields with streamflow records and the performance of the hydrological model in the region. this is based on suggestions made by other researchers stating that a higher hydrological performance (consistency with streamflow observations and evaporation estimates in the basin), generally implies that the evaluated precipitation field is more accurate than the one with a lower streamflow performance (beck, vergopolan, et al., 2017). however, we did not only focus on one hydrological performance metric and hence intended to conduct a deeper evaluation (see section 3.3). in this section, we describe the merging and benchmark rainfall fields, the hydrological modelling platform that we calibrate and validate for each rainfall field, and the evaluation of its robustness (see fig. 2). figure 2. methodology scheme 3.1 precipitation merging schemes and interpolation benchmark the purpose of the rainfall fields is to force the hydrological model. due to the subbasins’ size, the field’s resolution was established at cells of 1 km. after the spatial interpolation or merging, the time series in each 1 km cell were aggregated using each of the subbasins’ boundaries to create the forcing for the swat model (see section 3.2); the aggregation used the mean value of the cells within a subbasin boundary. 3.1.1 inverse distance weighting (idw) idw is one of the most used interpolation methods in environmental sciences (webster & oliver, 2007), and specifically to interpolate rainfall fields. it has been highlighted that it produces as good as or the best estimates of rainfall fields even compared with geostatistical methods that work better in very high-density networks (duque-gardeazabal & rodríguez, 2023; lópez lópez et al., 2018; vargas et al., 2011). therefore, using in situ data, we used it as the benchmark field for the hydrological modelling. we fix the exponent of distance weighting in the value of two because it is the most used exponent in rainfall interpolation and other environmental variables. this interpolation method was applied to rain gauges and temperature data, yet a linear regression against ground altitude was considered for temperature in each time step. 3.1.2 random forest merging precipitation (rfmep) rf-mep is a procedure that combines precipitation information from ground-based stations with precipitation products and topographic information (baez-villanueva et al., 2020). it considers that: i) ground-based measurements are reliable at the point level, ii) precipitation products from satellites or reanalysis are biased but contain interesting information on spatial distribution, and iii) the combination of precipitation products and ground-based data can generate a better representation of the spatial distribution of precipitation. this procedure has been proposed to improve the spatial representation of precipitation in areas with a scarcity of in situ measured information through an ensemble learning method called random forest (rf) (breiman, 2001). a rf regression is applied in each time step (e.g., daily, monthly), which is trained with ground-based observation of the same time step. due to the fitting necessary for training the forest, the method needs enhanced computation resources and parallel processing is available for taking advantage of them (zambrano-bigiarini et al., 2020); yet it is still computationally demanding. even with parallel processing, we needed to create the rainfall field first at 0.05° (~5 km) and afterwards bilinearly rescale it to 1 km. 3.1.3 double smoothing merging (ds) this merging scheme is based on data assimilation techniques and does not rely on second stationary assumptions made by geostatistical methods (e.g., kriging and its derivatives) (li & shao, 2010). its main characteristic is the interpolation of residuals obtained by subtracting the rain gauge values from a background field (i.e., satellite or reanalysis), which makes it suitable for sparse design rain gauge networks. after the interpolation of residuals, they are again subtracted from the background field to perform the merging; this makes it flexible and simple for implementation. the interpolation is performed with kernel density estimation functions, of which the gaussian kernel is the most used due to its infinite support and simplicity (one parameter). its parameter has been studied regarding its change against the density of rain gauges (duque-gardeazábal et al., 2018); therefore, this was the function used in this study. the influence of the parameter in hydrological simulation performance has also been studied (duque-gardeazabal & rodríguez, 2023). based on that research, we fixed the parameter’s value to 17.5 km. 3.2 soil and water assessment tool (swat) swat is software for continuous hydrological modelling consisting of several modules representing physical process-based formulations (arnold et al., 1998, 2012). it is spatially semi-distributed, runs at daily time scale, and considers a subdivision in hydrological response units (hrus) for enhanced surface-climate interactions representation in homogeneous areas. that makes it suitable for eco-hydrological studies (impact of land management, sediments, agriculture, etc.), climate change and scenario-based assessments. it has also been applied in tropical conditions and in colombia (caro camargo & velandia tarazona, 2019; uribe et al., 2018). 58 nicolás duque-gardeazabal, camila garcía-echeverri, juan josé montoya-monsalve & fabio andrés bernal-quiroga the main physical law in swat is the water balance, which is represented in several compartments associated with the hydrological cycle. for potential evaporation, it can consider a simplified energy balance using the penmanmonteith equation (the chosen one) or the simplified prestley-taylor equation; it can also use the empirical hargreaves equation. it considers a plant growth model that afterwards affects the water balance in the soil profile; this allows the tool to appropriately simulate watersheds with several land covers. moreover, it simulates other processes such as surface runoff, infiltration using cn curve (mosquera et al., 2022), percolation, lateral flow, ponds, routing through channels, among others (neitsch et al., 2011). to build the models, we used the aster global dem from nasa (nasa/meti/aist/japan spacesystems & u.s./japan aster science team, 2019) to create the subbasin boundaries and the slope classification to define hrus. regarding the soil properties, we reclassified the soil information gathered by the agustin codazzi institute (post et al., 2000; saxton & rawls, 2006) that is available in the regional soil studies (mainly santander, norte de santander and cesar) (igac, 2015). land cover/use was reclassified to swat’s classification from colombia’s corine land cover maps (ideam, 2015). the models were calibrated and recalibrated for each rainfall field (details of calibration periods for each of the three watersheds are in the supplementary material). calibration of parameters was performed using the swat-cup application (k. c. abbaspour et al., 2015), and using a split-parameter structure (francés et al., 2007). the split-parameter structure consists of calibration factors that multiply the pre-assigned parameters based on soil and land cover properties (often called relative calibration factors). the latter ensures that the model conserves the spatial variability of parameters. we first perform semi-manual trials using the limited parallel processing in swat-cup; the latter is to find a region of the parameter space that achieves good performance but follows what we believe is the near-real physical behaviour of the catchments. second, the particle swarm optimization (pso) (kennedy & eberhart, 1995) calibration algorithm was set to search for other nearby regions of the parameter space and refine the finally chosen parameters. to achieve that aim, at least 60 particles with at least 30 evolution steps were used (in some streamflow gauges, we used 100 particles with 40 evolutions). the objective function selected to guide the algorithm was the kling-gupta efficiency (kge) (gupta et al., 2009), calculated with simulated and observed streamflow. after the completion of the algorithm, the tested sets of parameters were filtered to choose one set with high performance but with credible values. 3.3 robustness of hydrological representation model performance can be estimated with traditional metrics such as the nash-sutcliffe efficiency (nse) (nash & sutcliffe, 1970) or the kge to determine the general behaviour of the model. however, the representation of the whole hydrological system needs to be assessed to have confidence in its modelled estimates. hydrological signatures evaluation is needed to perform what some call the diagnostic model evaluation (gupta et al., 2009), a diagnosis which is needed to improve the deficiencies of simulated fluxes. this must be done on both daily and monthly scale as the model represents processes at daily time scale and its use for water resources management needs to represent monthly scale accordingly. although the general calibration of the model parameters was conducted with the kge, we also analysed the metric sub-components represented in three signatures (gupta et al., 2009). the percent bias (pbias) serves as indication of water balance general agreement (precipitation, evaporation, and runoff); the determination coefficient (r2), assesses the temporal representation of streamflow and indirectly of precipitation; and the relative variability of simulated and observed streamflows (alpha). that allowed us to identify whether the rainfall fields were temporally similar to the actual field. moreover, the representation of model precipitation transformation into streamflow was assessed by comparing flow duration curve (fdc) percentiles; the latter was performed visually and numerically through the root mean square error (rmse, or difference rmsd) of fdc percentiles. a previous comparison of the three rainfall fields was also performed. 4. results & discussion 4.1 precipitation fields comparison we performed a comparative analysis between the rainfall fields without assuming which of them is the true field. those differences can be spatially initially analysed with the long-term annual rainfall values maps, depicted in fig. 3. the zones with similar values between the three maps are in the high area of the lebrija and sogamoso basin. in contrast, the north of the lebrija and the west of the sogamoso basin have higher values in the rfmep and even higher in the ds compared with the idw. the most noticeable difference is in the south, in the highlands of the opón basin, where the values of the ds field are quite higher than the other two forcings (the rfmep also has higher values than the idw); that zone almost does not have rain gauges (see fig.1). considering that we did not calibrate the bandwidth value of the ds (as duquegardeazabal & rodríguez (2023) did), it is clear that the method has a heavy influence from the background field. the differences in the temporal dimension are also important. therefore, we compare some features in each subbasin’s time series. fig. 4 depicts the metrics calculated between daily forcing’s ppt time series for each subbasin (each black dot is a subbasin). it shows their distribution through violin plots with quartiles as horizontal lines and the 5 % and 95 % percentiles. r2 plot shows that all fields are temporally similar (most of subbasins coefficients are higher than 0.9), but rfmep and idw are the most (the mean value and the distribution is higher than the other two comparisons). regarding the percent bias (pbias), the field with the higher volume is the ds which is on average 12.41% higher compared to the idw and 12.38% compared with rfmep; rfmep and idw are quite similar in volume (with a distribution between -15% and 10%), being the rfmep 1.2% lower on average. in all volume comparisons, there are subbasins where one forcing is lower than the other and vice versa. the relation of temporal variabilities (alpha) shows that most subbasins with the rfmep and idw rainfall fields have very similar temporal variability (near 1.0), and the variability of the ds is higher than idw and rfmep variability. as a consequence of all this behaviour, the rmse or difference (rmsd) is lower between the rfmep and idw than between the ds and idw; similar values are reported when comparing rfmep and ds, yet in some subbasins the ds is more similar to idw. figure 3. mean annual precipitation fields for the period 1983-2020 generated with idw, rfmep and ds. 59improving estimates of water resources availability over north tropical south america: comparison of two satellite precipitation merging schemes figure 4. metrics of comparison between the precipitation fields. black dots represent the value for each modelling subbasin, and the colour dots indicate the mean of those values. horizontal lines inside the violins depict the data’s 5, 25, 50, 75 and 95% percentiles. 4.2 streamflow evaluation once the model was configured and calibrated, its performance was evaluated. the final values of the parameters for each forcing and each part of the models are shown in tables s2, s3 and s4 of the supplementary material (sm); it can be seen that the relative percentage of change of the soil parameters is low and the groundwater parameters are within physical range. as a result, fig. 5 displays the daily streamflow performance measured by the nse coefficient classified in the calibration and validation period for each forcing input in the models (the distribution of results is shown on the right side). overall, the use of merged precipitation products increases the hydrological performance, and in certain cases, there is a considerable improvement (see gauges 23197680, 23197400 and 23197270, whose improvements were 27%, 56%, 52%, respectively, with rfmep). the distribution of rfmep values in the validation period is higher but not as significant as for those specific gauges since some gauges reported a reduction of performance (most of them already had values lower than 0.5). the monthly performance is shown in the sm; according to d. n. moriasi et al. (2007)), the 50% of streamflow gauges can be classified as satisfactory, 25% as good or very good (three as very good with the rfmep or the ds forcing and one with idw). the conjunction of three hydrological signatures expressed through the kge is shown in fig. 6 (daily performance can be seen in the sm). from these results, we can see that kge informs about the general representation of the observed streamflow, but it is necessary to evaluate the other hydrological signatures that comprise it to achieve a diagnostic evaluation of the model. gauges 23187040 and 23197130 reported low values, but we needed to identify the reason for those results (in the validation period, gauge 23197400 has few records and hence does not display a proper evaluation). regarding the evaluation of the temporal variation between simulated and observed streamflow, fig. 7 shows the daily coefficient of determination for the different forcings. some simulations were significantly improved by using the rfmep or the ds rainfall fields instead of the idw (gauge 23197680 which already had a satisfactory correlation and was improved to good category). that evidenced the contribution of temporal information by chirps and the rfmep method for creating a proper field. interestingly, the ds deteriorated some performances in gauges with good results and improved others, causing that overall performance to be similar to the idw. monthly results are displayed in the supplementary material (sm). figure 5. daily nse values for each streamflow gauge and the distribution of their values, for calibration and validation period figure 6. monthly kge values for each streamflow gauge and the distribution of their values, for calibration and validation period figure 7. daily coefficient of determination between simulated and observed streamflows and the distribution of their values, for calibration and validation period fig. 8 displays the relation between simulated and observed streamflow variability on a monthly scale. the results clearly show that the forcings coming from merging chirps and rain gauges have a more concentrated distribution 60 nicolás duque-gardeazabal, camila garcía-echeverri, juan josé montoya-monsalve & fabio andrés bernal-quiroga near the value of one (1), which represents the same variability in simulated and measured time series, most of all in the validation period. even though there are streamflow gauges with results higher or lower than 1.25 and 0.75, the majority of results in ds and rfmep are within those boundaries. daily streamflow variability can be seen in sm. figure 8. relation between simulated and measured streamflow variability at monthly time scale and the distribution of their values, for calibration and validation period another metric analysed was bias error, fig. 9 depicts its results which can be related to the general water balance (this metric gives the same results for daily and monthly scale due to its calculation). gauges 23147020, 23147040, 23187040 and 23197130 have a higher bias (mainly negative) than the other streamflow gauges. still, almost all values are within plus or less 25%, which classifies them as at least satisfactory results, according to d. n. moriasi et al. (2007). those basins do not have many rain gauges inside their limits, which could influence the model’s performance. although all forcings form a distribution, rfmep concentrates more gauges near 0% in the calibration periods, yet its average is further than the ds and idw due to an extreme value. an analysis of the general signatures in the streamflow was conducted with the fdc and is depicted in fig. 10 for three selected gauges. graph a) reveals a streamflow gauge where low, medium and high discharge values are similar; b) shows a general good agreement with measurements except in the high flows; whereas c) displays a gauge where only medium flows are well represented by simulations. that behaviour repeats in other gauges where the model was evaluated, with a tendency to misrepresent high flows while having similar results for medium and low flows. these results might stem from the use of the kge metric for broadly calibrating the model; other researchers have stated the challenge of calibrating a model that satisfactorily reproduces high, medium and low phases (pfannerstill et al., 2014) and specific calibration methodologies are designed to achieve precise objectives (k. abbaspour et al., 2017). the fdcs shown in fig. 10 were analysed through the rmse. rmse corresponds to the error between the daily simulated and observed fdc for each forcing (calculated using the 100 percentiles as point entries). in fig. 11, the percentiles were calculated using all the available data to ensure a proper statistical estimation of values. the streamflow produced with the idw appears to slightly better represent the percentiles of streamflow, yet that produced by the merged forcings represents better those stations with lower values of annual streamflow (e.g. 23197680, 23197700, 23197270). in those streamflow gauges with high annual values (e.g. sogamoso river or la colorada river), the differences between simulated and observed streamflow are higher, and the idw tends to better represent the percentiles in some cases nearly followed by the rfmep. although gauges over the oro and lebrija rivers have a considerable annual flow (23197290 & 23197370), the difference in the fdc percentiles is quite small and lower in those coming from merging than in the idw. the distribution of the rfmep values is slightly lower under the median than in the other two forcings. figure 9. percent bias between simulated and measured streamflows and the distribution of their values, for calibration and validation period figure 10. flow duration curves of three selected streamflow gauges, which represent the general agreement between simulations with all forcings and observations 61improving estimates of water resources availability over north tropical south america: comparison of two satellite precipitation merging schemes figure 11. rmse between the simulated and observed fdc percentiles for each forcing and the distribution of their values 5. conclusions in this research, we have tested the ability and influence of different rainfall fields to properly represent surface water availability variation over time. the physical representation of hydrological components in the swat tool was key to ensure a fair comparison between different forcing inputs. the evaluation was performed using several streamflow signatures and the fdc to retrieve insights from various hydrological phenomena rather than just a general metric performance. overall, the two merging techniques slightly increase the estimation of streamflow. the mean performance improved by 10%, with higher or lower improvements depending on the evaluated streamflow gauge, most of all in the temporal and magnitude of the variability. considering individual streamflow gauges, the enhancement in nse was up to 50%. regarding the bias, it is unclear that merging techniques concentrate the values around 0%. the main conclusion of this study is that, whenever possible, hydrological models should use several rainfall fields to achieve the best modelling agreement with observations, being the merging of sources a tool worth trying. regarding the fdc and the streamflow signatures associated with it. the calibration of the continuous hydrological model was not specifically focused on representing the high segment of the fdc; hence, the differences in that segment can heavily influence the value of rmse used to compare the simulated and observed fdc. as the main objective of model building was to reproduce the general water budget and variability, it is understandable that some hydrological signatures have big differences in some streamflow gauges, and specific methodologies can tackle particular objectives. from the comparison of merging methods and the idw, we retrieve a special conclusion in the south of the opon river basin. considering the lack of rain gauges in that zone, the streamflow evaluation is a good alternative for evaluating the rainfall fields estimated by merging and the interpolation methods. some of the metrics in the 23187020 gauges are better with the merged rainfall fields, and in some cases, the difference is substantial. another important improvement is the better relation of streamflow variabilities, which in the case of the ds concentrates the distribution towards the value of 1. regarding the bias, the ds reports a slightly better agreement than the rfmep, but rfmep has a better temporal correlation. it is clear that the ds merging method is highly influenced by the background field (chirps). the change in the background field due to the bandwidth value apparently did not affect too much those areas with low rain gauge density, generating that the background field had an even heavier weight in the final value of the field. an increase in bandwidth value could reduce the general bias, but it will also reduce the high spatial variability that one expects in daily rainfall fields. the small difference in performance in the validation with respect to the calibration period when comparing forcing might be explained by the fact that more rainfall records are available in the validation period. this induces a short improvement of the rainfall fields by using the chirps background field in that period of time. the improvement is higher in the calibration period since there are fewer rain gauge records. future work could include using other background fields to improve the estimates (e.g. mswep, persiann, etc). moreover, other meteorological variables can benefit from satellite-based estimations, for instance, temperature. a call for open access to local hydrometeorological data is another conclusion of this study and a reminder that rain gauges are what we consider the ground truth. while the plethora of rain gauge time series are not shared with international organisations that create satellite or reanalysis-based global products, local or regional merged datasets can improve hydrological studies estimates, such as the one exposed here. however, global or continental products are highly useful for transboundary watershed studies; 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(2023). an integrated approach of advanced methods for mapping geologic structures and sedimentary thickness in ukelle and adjoining region (southeast nigeria). earth sciences research journal, 27(3),v251-258 https://doi.org/10.15446/esrj.v27n3.105868 manuscript received: 17/11/2022 accepted for publication: 05/10/2023 keywords: edge detection; tilt-depth; geologic structures; ukelle; southeast nigeria. palabras clave: detección de bordes; método de inclinación y profundidad; estructuras geológicas; ukelle; sudeste de nigeria. abstract high-resolution aeromagnetic data were enhanced using recent and advanced filters to map the geologic structures of the ukelle and adjoin region (southeast nigeria). aeromagnetic data were reduced to the equator (rte) and upward continued to 100 m. subsequently, enhancement operations like the tilt angle of the horizontal gradient (tahg), logistic function of the horizontal gradient (lthg), and fast sigmoid function (fsed) operations were carried out. the results from these filters indicated that the ene-wsw, ne-sw, nne-ssw, and nnw-sse orientations dominate the structural pattern of the ukelle region. in addition, the edge filters delineated ne-sw trending synclinal structures that match the location of thick (500-1400 m) sedimentation obtained by the tilt-depth (td) method. furthermore, the structural map obtained from remote sensing data validated the lineament orientations and position of the ne-sw trending synclinal structure. the results also showed that the study location’s southeastern and northwestern flanking portions, controlled by extensive santonian igneous intrusions and metamorphisms, are characterized by high lineaments and thin (0-500 m) sedimentation. the observed thin sedimentation is believed to be caused by widespread santonian tectonic events in the area. at the same time, related geologic structures served as migration pathways and accumulation zones for rift mineralization. acercamiento integrado de métodos avanzados para el mapeo de estructuras geológicas y espesor sedimentario en ukelle y regiones adyacentes, en el sudeste de nigeria resumen en este trabajo se usaron filtros recientes y avanzados para mejorar la información aeromágnetica de alta resolución que permitiera mapear las estructuras geológicas de ukelle y las regiones con las cuales limita (en el sudeste de nigeria). a la información aeromagnética se le aplicó la reducción al ecuador y continuó hasta los 100 metros. luego se realizaron operaciones de mejoría como el ángulo de inclinación del gradiente horizontal, función logística del gradiente horizontal, y la función rápida sigmoide. los resultados obtenidos con estos filtros indicaron que las orientaciones ene-oso, ne-so, nne-sso y nno-sse dominan el patrón estructural de la región ukelle. adicionalmente, los filtros de borde delinearon que la tendencia ne-so de las estructuras sinclinales coinciden con la ubicación de la densa masa sedimentaria (entre 500 y 1400 metros) obtenida con el método de inclinación y profundidad. además, el mapa structural obtenido con la información de teledetección validó las orientaciones de lineamiento y la tendencia en la posición de ne-so de la estructura sinclinal. los resultados también muestran que las áreas del sureste y del noroeste que flanquean la zona de estudio están controladas por grandes intrusiones y metaformismo ígneo del santoniano y se caracterizan por grandes lineamientos y sedimentación delgada (0-500 metros). se estima que la sedimentación delgada que se observó fue causada por los extendidos eventos tectónicos durante el santoniano en el área. simultáneamente, las estructuras geológicas relacionadas sirvieron como caminos de migración y zonas de acumulación para la mineralización en zanjas de hundimiento. https://doi.org/10.15446/esrj.v27n3.105868 mailto:luanpt@hus.edu.vn https://doi.org/10.15446/esrj.v27n3.105868 252 stephen ekwok, ahmed eldosouky, ubong ben, ogiji-idaga achadu, anthony akpan, abdullah othman, luan thanh pham 1. introduction airborne magnetic method is one of the most suitable procedures for mapping near surface and underlying basement geologic structures (ben et al., 2022a; 2022b). analysis involving high resolution airborne magnetic data can be used to resolve the challenges involved in delineating subtle geologic features (eldosouky, 2019; almasi et al., 2014), detecting regional geological borders (cooper and cowan, 2008; pham et al., 2019), delineation of depocentres and basement framework (ekwok et al., 2021a; 2021b; 2021c) as well as mapping of zones of mineralization and hydrothermal modifications (ekwok et al., 2020a). the evaluation of linear edges and borders of magnetic sources which commonly reveal subsurface faults, fractures, contacts, as well as associated tectonic bodies, are useful in the interpretations of geological structures. previous research has shown that the magnetic techniques are extremely useful as an investigative delineating tool when there are disparities between the different rock units (dentith et al., 2000). tectono-magnetic events of the area control development and expression of the geologic features of the region (eldosouky et al., 2022). the tectonic settings, the structural framework, occurrence of rift minerals as well as the discovery of oil and gas in commercial quantity triggered extensive geoscience studies in the benue trough (ekwok et al., 2022a; 2019). nonetheless, due to the low degree of resolution, the boundaries cannot be directly mapped from the observed potential field data (fedi and florio, 2001). several enhancement procedures have been developed by various researchers to properly delineate the geologic structures caused by tectonic events (eldosouky et al., 2020; jorge et al., 2023). the gradient amplitude technique (cordell and grauch, 1985) is commonly used for improving geologic borders. hansen and deridder (2006) used the curvature of the gradient amplitude in identifying linear structures. however, the gradient amplitude filter detects the borders of shallow sources better than deeper bodies (arısoy and dikmen, 2015). this is due to the inability of gradient amplitude to provide steady indicators from the borders of low and high amplitude magnetic signatures simultaneously (eldosouky et al., 2020; pham et al., 2022a, b). the analytic signal filter developed by roest et al. (1992), which is also well known, is often used to locate the flank borders of magnetic anomalies (pham et al., 2021a). just like the gradient amplitude filter, the analytic signal filter performs defectively in the enhancement of magnetic anomalies created by joint deep and shallow sources (pham et al., 2018; 2019; 2021b). other enhancement operators like tilt derivative and theta are often used in mapping lineaments, and describing geologic structures of the bodies (miller and singh, 1994; wijns et al., 2005). pham et al. (2020, 2021c) introduced the improved logistic function and softsign function to increase the resolution of the edges. eldosouky et al. (2020), pham and prasad (2023) reviewed the effectiveness of the filters in terms of their precision on the detection of borders of magnetic sources on both observed and theoretical data. other improved enhancement operations like the spectral moments (sun et al., 2016), tahg (ferreira et al., 2013), lthg (pham et al., 2019), and fsed (oksum et al., 2021), can reveal lineaments of different anomalies. previous studies in the lower benue trough using magnetic data with low resolution obtained by geological survey of nigeria (gsn) in 1974, were centered on the mapping of major intrusive structures, fault systems, stratigraphy, sediment thickness, spatial distribution of igneous intrusions, and delineation of depo-centres (oha et al., 2016; ofoegbu, 1984; ofoegbu and mohan, 1990). later, increased interest amongst researchers were later generated on the geologic structural complexity caused by igneous intrusions, and associated rift-minerals in the area (oha et al., 2016; ofoegbu, 1984; ofoegbu and mohan, 1990). these geologic structural studies were investigated involving derivatives, analytic signal, low pass filtering, upward continuation, source edge detection, etc (ekwok et al. 2019; 2020a). however, some local and subtle geologic structures were suppressed by these filters. the availability of improved filters (like the tahg, lthg, fsed, etc.) has made it easier to determine subtler anomalies and generate more detailed geologic structural information. these filters generate peak responses directly over source borders, and produce better resolution and distinctive lineament maps (eldosouky et al., 2020; pham et al., 2022b; oksum et al., 2021; kamto et al., 2023). this research involved the application of some recent filters such as the tahg, lthg and fsed to airborne magnetic data from the ukelle and adjacent area (southeast nigeria) to map geologic boundaries. the mapping of these boundaries is expected to help determine lead-zinc, brine conduit, barite and ironstones accumulation zones within the ukelle and environs. these minerals are reported in commercial quantity in other geologic units within the abakiliki anticlinorium (uma and lohnert, 1992; uma, 1998). moreover, the td method was also applied to the magnetic dataset to determine the depth to the geologic structures, as well as sedimentary cover of the basement rocks. the findings from this research are expected to improve the existing knowledge on the geological structures and patterns, as well as thickness of the sediments in southeast nigeria. 2. geologic setting the ukelle region is sited in the lower benue trough, on the northeastern flank of the santonian abakaliki anticlinorium (fig. 1). the investigated region is positioned between longitude 8000’ and 8030’e and latitude 6030’ and 7000’n. figure 1. geological map of the ukelle and adjacent area. the series of events which resulted to the development of the benue trough and its constituent parts have been well documented (onuoha and ofoegbu, 1988). a thick sedimentary successions of the cretaceous age occupy the lower benue trough, and overlies the precambrian basement mainly of composed of migmatic and granitic rocks. the precambrian basement is overlain by the albian asu-river group (arg), which is composed of bluish black sandstone. the eze-aku formation (eaf), which sits directly on the asu-river group, is made up of calcareous siltstone and shale, calcareous sandstones, and shelly and sandy limestone (reyment, 1965). the coniacian awgu-shale (as) is dominated by marine fossiliferous, grey bluish shale, limestone, and calcareous sandstone. the campanian nkporo shale (ns) which overlies the agwu-shale, is characterised by mostly marine arenaceous sandstone members. in general, tectonism has had a significant impact on the sedimentary series, which happened in two stages and resulted in the folding of the overlying sedimentary materials (nwachukwu, 1972). the folding event that occurred in the santonian period was the primary cause of the formation of the abakaliki-anticlinorium. the asymmetry and reversed faults linking with the folds that were created in this period indicate that they were majorly compressional in pattern. benkhelil (1987) characterised the abakaliki anticlinorium happenings as a whole orogenic cycle encompassing sedimentation, compressive tectonics, magmatism, and metamorphism. the related magmatic happenings caused the introduction of numerous intrusions into the overlying arg and eaf. the ns sits unconformably over the folded eaf and the arg (that is, abakaliki anticlinorium) (whiteman, 1982). the ukelle area which is part of the abakaliki anticlinorium, is a prominent geological structure formed as a result of series of tectonic processes. it is situated within the larger benue trough, a major intra-continental rift basin that extends across several countries in west africa (benkhelil, 1989). the abakaliki anticlinorium is characterized by its distinct folding pattern, where rock layers are bent upwards into an arch-like structure known as an anticline. the primary tectonic process responsible for the formation of the abakaliki 253an integrated approach of advanced methods for mapping geologic structures and sedimentary thickness in ukelle and adjoining region anticlinorium is compressional tectonics (benkhelil, 1989). as a result, the originally horizontal sedimentary strata in the abakaliki region were folded and uplifted, forming the characteristic anticlinal structure observed today (benkhelil, 1989). the folding process is believed to have occurred during the santonian period, as indicated by radiometric dating of the sedimentary rocks in the area (benkhelil, 1989). figure 2. total magnetic intensity map of the ukelle and adjacent area. 3. data and method data the high resolution airborne magnetic data used for this study which covers an area of about 25,000 km2, were purchased from the nigerian geological survey agency (ngsa). fugro airborne surveys (fas), canada, under contract to the ngsa, acquired aero-geophysical data between the period of 2005 and 2010, covering the whole country. the dataset was acquired using a flux-adjusting surface data assimilation system with 100 m of flightline space, 500 m of tie line space, and terrain-clearance ranging from 80-100 m. furthermore, fugro airborne surveys, canada, subtracted the regional field from measured magnetic data engaging the tenth generation of the international geomagnetic reference field. following the wide acceptance and availability of the igrf, the main advantage of the igrf is the reliability it offers in potential field survey practice (reeves et al., 1997). the dataset used in this paper was converted to total magnetic intensity (fig. 2) and then reduced to the equator before being upward continued to 100 m. (fig. 3). because the data was gathered at a low latitude, the magnetic data was reduced to the equator. jain (1988) and leu (1981) reported that rte generates more reliable results, especially at middle and lower latitudes. also, the data were upward continued to attenuate the geologic effects associated with very short wavelength anomalies. all the enhancement operations were carried out using the rte upward continued to 100 m data. figure 3. rte data upward continued to 100 m. methods edge detection methods ferreira et al. (2013) proposed the tahg operator to outline the lateral boundaries, which is based on the derivatives of the horizontal gradient (thg). the filter is expressed as: tahg = atan + �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� thg z thg thg y 2 2 (1) where ∂thg/∂x, ∂thg/∂y and thg/∂z are the thg derivatives, which is expressed as: thg= �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� f x f y 2 2 (2) where (∂f/∂x) and (∂f/∂y) are the derivatives of the field f. figure 4c shows the tahg of synthetic magnetic data (fig. 4b) of the bodies m1 and m2 in figure 4a. we can see that the tahg can balance different anomaly amplitudes, but it cannot provide the sharp edges. the lthg technique is the enhanced type of the thg, which is centred on the logistic function and given by pham et al. (2019): lthg= exp1 2 2 �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� thg z thg thg y ���� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� ���� (3) pham et al. (2019, 2022b) proved that 2 ≤ α ≤ 10 will produce the best solutions. as shown in figure 4d, the lthg can equalize different amplitudes, and provides sharper edges than the tahg. 254 stephen ekwok, ahmed eldosouky, ubong ben, ogiji-idaga achadu, anthony akpan, abdullah othman, luan thanh pham the fast sigmoid function (fsed) is another detector, developed by oksum et al. (2021). this technique is centred on the fast sigmoid function of the ratio of the derivatives of the thg. its maximum values are used to detect the source borders. this technique is given by: fsed r r �� �� �� 1 1 ´ (4) where r thg z thg x thg y �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� 2 2 (5) as displayed in figure 4e, the fsed also provides balanced anomalies and sharper edges than the tahg. figure 4. (a) synthetic model, (b) magnetic anomaly, (c) tahg, (d) lthg, (e) fsed. as shown in figures 4c and 4d, the depth of the source bodies has no effect on the tahg and lthg, and their peak values are near the true boundaries even for deep bodies. this is one of the benefits of the tahg and lthg over the horizontal gradient, which cannot delineate boundaries originating from deep magnetic bodies (pham et al., 2022b; eldosouky et al., 2020). the fsed also enhances the boundaries of both deep and shallow bodies and its peak values are near the real boundaries (fig. 4e). depth estimation method the td technique is based on the relationship between tilt angle derivative (tdr) (miller and singh, 1994), horizontal position and depth of a vertical 2-d contact as (salem et al., 2007): tdr f z f x f y �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� atan 2 2 (6) and tdr h z c == atan (7) where zc is the contact depth, and h is the horizontal location. equation 7 shows that the contact location (h = 0) relates to the zero values of the tdr, and the depth relates to the horizontal distance between tdrs of 0 and and ±π/4. 4. results in an attempt to offer better understanding of the location, trend and pattern of geologic structures in the ukelle and adjoining regions of southeast nigeria, enhanced filters like the tahg, lthg and fsed were applied on the high resolution rte magnetic data that were upward continued to 100 m (fig. 3). figure 5 signifies the output of the tahg. the maxima of the method revealed lineaments caused by magnetic bodies. nevertheless, sharp edges cannot be detected by this filter. the lthg of upward continued rte magnetic data is shown in figure 6. this map displays a clearer delineation of geologic structures, which are much easier to visually interpret qualitatively. figure 7 shows the output of the fsed. like other methods, the fsed generates the edges with the same amplitude. figure 5. tahg map of the ukelle and adjacent area. as shown in the synthetic model, the peak responses obtained from the taed (fig. 5), lthg (fig. 6), and fsed (fig. 7) were positioned directly over magnetic source borders. the filters applied in this study (figs. 5-7) delineated horizontal geologic structures with ene-wsw, ne-sw, nne-ssw, and nnw-sse orientations. figure 5 generated more connective linear features that are somewhat diffused. the low resolution of source edges in figure 5 makes geologic interpretations more difficult. the lthg and fsed simultaneously balanced the low and high amplitudes (figs. 6 and 7). these filters also provide greater resolution and distinctiveness compared to the tahg (fig. 5). by comparing the results from the methods, we can say that the lthg and fsed are precise and highly effective in identifying several geologic features associated with the santonian abakaliki anticlinorium (benkhelil, 1987), which were not clear in figure 5. since the taed, lthg, and fsed peaks respond to the source edges, we determined these peaks using the improved crest detection technique (pham et al., 2023). magnetic boundaries are then identified from the locations of the peaks. the overlay of the locations of the peaks is presented in figure 8. here, the peaks of the tahg are shown by black dots, the peaks of the lthg are displayed by cyan dots, while red dots respond to the fsed peaks. by mapping the maxima locations of the filters, the results for the edge locations are similar (fig. 8). 255an integrated approach of advanced methods for mapping geologic structures and sedimentary thickness in ukelle and adjoining region figure 6. lthg map of the ukelle and adjacent area. figure 7. fsed map of the ukelle and adjacent area. furthermore, the td method (salem et al., 2007) was applied to assess the depths of geologic structures within the study area. the main advantage of the td method is that, unlike standard euler deconvolution, it doesn’t require the window size (ekwok et al., 2022a; 2020a; 2021b). the depth map (fig. 9) revealed depth range of approximately 0-1400 m. this thin sedimentation is thought to be caused by the extensive invasion of the sediments by the santonian intermediate-mafic igneous intrusions, as well as tuffs and calc-alkaline lavas intrusions (benkhelil, 1987; murat, 1970) in the study area. the tectonic event caused extensive baking of sediments, generation of metamorphosed rocks and creation of geologic structures in the region. figure 8. peaks of the tahg (black dots), lthg (cyan dots) and fsed (red dots). figure 9. tilt-depth map of the ukelle and adjacent area. 5. discussion the northeast-southwest structural trend controlled the basement framework and sedimentation pattern in the southeastern part of nigeria (benkhelil, 1987; benkhelil et al., 1975; burke et al., 1970). the lineaments within the investigated area that trend in nw-se, ne-ws, nne-ssw, and e-w directions were easily traced (figs. 5-7). the lthg (fig. 6) and fsed (fig. 7) filters are additionally efficient in mapping edges of the ne-sw trending synclinal structures associated the abakaliki anticlinorium. this ne-sw synclinal structure previously mapped by ekwok et al. (2020a), partitioned the study area into northwest and southeast flanks (that is, folds/uplifts), that coincide with the zone characterised by relatively thick (500-1400 m) sedimentation (fig. 7). the northwest and southeast flanks 256 stephen ekwok, ahmed eldosouky, ubong ben, ogiji-idaga achadu, anthony akpan, abdullah othman, luan thanh pham that correlated strongly with zones dominated by thin (0-500 m) cretaceous sedimentary series, are controlled by widespread near-surface igneous, tuffs and calc-alkaline lavas intrusions (benkhelil, 1987; murat, 1972) as well as metamorphosed albian shales (ekwok et al., 2022c; 2021b; benkhelil et al., 1975). the coexistence of uplifts/folds (positive anomalies) and synclines (negative anomalies) in southeast nigeria have been reported by previous studies (benkhelil, 1987; burke et al., 1970; etc). generally, the uplifted regions are controlled by high concentration of lineaments (fig. 8), caused by a series of elongated narrow folds within the ene-wsw, ne-sw, nne-ssw and nnwsse configurations of the abakaliki sedimentary area triggered by the invasion of alkaline dolerites, basalts, and syenites (murat, 1972) into the overlying sediments. the major structural pattern of the benue trough and abakaliki anticlinorium is ne-sw (benkhelil, 1987; murat. 1972; burke et al., 1970), while some e-w lineaments pattern are described as transverse fractures to the major ne-sw fault along which massive intrusions were placed. lineament maps (fig. 10) were generated from magnetic data (fig. 10a) and remote sensing involving shuttle radar topographic mission (srtm) data (fig. 10b). fig. 10a is characterised by a wide range of geologic structures, while fig. 10b is dominated by short lineaments and some regional structures. besides, the nesw trending synclinal structure (fig. 10a) with thick sedimentation was mapped also mapped (fig. 10b). generally, it was observed that the trending pattern of fig. 10b correlates strongly with lineaments determined in fig. 10a generated from the edge filters. comparatively, the various results of the enhancement operation applied in this research showed that the lthg and fsed methods generated sharper and more distinct geologic structures of the ukelle and adjacent area, than the tahg. in addition, there is a good correlation between lineaments in fig. 10a and 10b, and the rose petals (fig. 11) show trends in the ene-wsw, ne-sw, nne-ssw and nnw-sse orientations. these geologic trends have been previously reported in the lower benue trough (ekwok et al., 2021a; 2021b; 2021c). the main strike orientation in the lower benue trough and abakaliki-anticlinorium is represented by the ene-wsw, ne-sw and nne-ssw directions. the ene, ne and nne characterise the regional-strike of the lineaments that correspond to region (the lower benue trough) that was tectonically disturbed during the santonian period (murat, 1970). figure 10. (a) magnetic lineaments map obtained from the edge filters, (b) gis generated lineaments map from srtm of the ukelle and adjacent area. figure 11. rose diagrams of magnetic lineaments obtained from the edge filters (a) and (b) gis lineaments. 257an integrated approach of advanced methods for mapping geologic structures and sedimentary thickness in ukelle and adjoining region conclusions delineation of geologic structures and contacts in the ukelle and adjoining area (southeast nigeria) involved the tahg, lthg, fsed, and the rte total magnetic intensity data that were upward continued to 100 m. the filters generated geologic structures that trend in the ene-wsw, ne-sw, nne-ssw, nnw-sse and nw-se directions. remote sensing involving srtm data generated structural map that validated the structural orientations of lineaments obtained by the enhanced filters. furthermore, the lineament maps delineated the ne-sw trending synclinal structures that coincide with the zone characterised by relatively thick (500-1400 m) sedimentary series revealed by the td map. also, the enhanced filters as well as the td result mapped folds/ uplifts in the northwest and southeast flanks with thin (0-500 m) sedimentation, controlled by widespread santonian igneous intrusions and metamorphism that caused high concentration of lineaments in these portions. acknowledgments the authors thank the editors and three anonymous reviewers for their constructive comments and recommendations. references almasi, a., jafarirad, a., kheyrollahi, h., rahimi, m., & afzal, p. 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(2022). material damping in a stratified soil deposit. earth sciences research journal, 26(4), 279-290 https://doi.org/10.15446/ esrj.v26n4.100527 record manuscript received: 17/01/2022 accepted for publication: 19/12/2022 abstract due to the extensive use of one-dimensional equivalent linear analyses to determine the free-field response of nonlinear soil deposits, dynamic numerical simulations able to reproduce an analogous response to equivalent linear codes are of great value for practical engineering, particularly for dynamic soil-structure interaction problems. an appealing alternative, for problems not close to a failure condition, is to assume a linear elastic behaviour of the soil but with stiffness parameters derived from one-dimensional equivalent linear analyses, i.e. consistent with the level of deformation induced by the input motion. in this approach, energy dissipation has to be artificially incorporated through material damping formulations. in this work, local, rayleigh, and hysteretic damping formulations in flac were assessed to emulate results from one-dimensional equivalent linear analysis. a main feature of the analyses is that they consider a site having a considerably stratified soil deposit, in which the shear wave velocity profile displays significant variations and where the selection of some parameters in the damping formulations is not a trivial task. results provide relevant insights into the performance of the adopted damping formulations and the selection of material damping parameters to reproduce results of equivalent linear analyses. keywords: material damping; stratified soil deposit; flac; dynamic numerical analyses; equivalent linear. amortiguamiento material en depósitos de suelo estratificados resumen debido al frecuente uso de análisis unidimensionales con el método lineal equivalente, para determinar la respuesta en campo libre de un depósito de suelo no lineal, se consideran de gran valor práctico los análisis numéricos dinámicos capaces de reproducir una respuesta análoga a los códigos lineales equivalentes, particularmente en problemas de interacción dinámica suelo-estructura. una alternativa atractiva, en problemas que no están cerca de una condición de rotura, es el asumir un comportamiento elástico lineal del suelo, pero con parámetros de rigidez derivados de análisis unidimensionales con el método lineal equivalente, i.e. consistentes con el nivel de deformación inducido por el movimiento de entrada. en este enfoque, la disipación de energía debe incorporarse artificialmente a través de formulaciones de amortiguamiento material. en este trabajo se evaluaron las formulaciones de amortiguamiento local, de rayleigh e histerético en flac, con el fin de emular los resultados de un análisis unidimensional con el método lineal equivalente. la principal característica de los análisis es que estos consideran un sitio con un depósito de suelo notablemente estratificado, en el que el perfil de velocidad de ondas de corte muestra variaciones significativas, y en donde la selección de algunos parámetros en las formulaciones de amortiguamiento no es una tarea trivial. los resultados proporcionan información relevante respecto al comportamiento de las formulaciones de amortiguamiento adoptadas y respecto a la selección de los parámetros para reproducir los resultados de análisis lineales equivalentes. palabras clave: amortiguamiento material; depósito de suelo estratificado; flac; análisis numéricos dinámicos; lineal equivalente. https://doi.org/10.15446/esrj.v26n4.100527 mailto:mmanicam@iingen.unam.mx 280 miguel a. díaz, miguel a. mánica, eduardo botero, efraín ovando-shelley, luis osorio 1. introduction analyses of one-dimensional propagation of shear waves, using the equivalent linear method (seed & idriss, 1969), have been widely employed to determine the free-field response of nonlinear soil deposits (martin & seed, 1982; seed et al., 1988; phanikanth et al., 2011). input parameters are well understood and incorporate the result of various in situ and laboratory tests for the dynamic characterisation of soils. the initial small strain stiffness is generally determined from wave velocities obtained, for instance, with in situ geophysical testing, or in the laboratory with bender elements (clayton, 2011). then, the nonlinear behaviour is incorporated through the equivalent linear method by iteratively reducing the stiffness and increasing the critical damping ratio as a function of the maximum strains attained in each iteration, according to given modulus degradation and damping data. the latter are obtained in the laboratory through tests such as the resonant column, cyclic triaxial, or cyclic simple shear tests (seed & idriss, 1970; vucetic & dobry, 1988; okur & ansal, 2007). this kind of analysis provides a robust approach to assess seismic site effects (martin & seed, 1982), with similar results to nonlinear time-domain analyses (borja et al., 1999; kaklamanos et al., 2015). however, it is important to notice that the interpretation of the mentioned laboratory tests, in terms of a progressive reduction of the secant modulus, is a simplification and, in reality, along with some elastic stiffness reduction, yielding, accumulation of plastic deformations, and hardening phenomena, to name a few, occur in the soil, especially for large strain amplitudes. nowadays, numerical methods, such as the finite element or the finite differences methods, are widespread and used routinely for geotechnical design. they are particularly useful to address soil-structure interaction problems (kausel, 2010). also, due to advancements in computational geomechanics (gens & potts, 1988; lade, 2005; pastor et al., 2011), numerical methods allow us to reproduce more closely the behaviour of soils, including phenomena such as yielding, accumulation of plastic deformations, hardening/softening, inherent and induced anisotropy, or viscous effects. however, the use of complex constitutive models in dynamic soil-structure interaction numerical simulations is still not widespread, and this kind of analysis has not been thoroughly validated. therefore, dynamic numerical simulations that can reproduce an analogous response to one-dimensional equivalent linear codes, such as shake (schnabel et al., 1972), are still of great value to practical engineering. an appealing alternative, for problems not close to a failure condition, is to assume a linear elastic behaviour of the soil, but with stiffness parameters derived from one-dimensional equivalent linear analyses, i.e. consistent with the level of deformation induced by the input motion (e.g. mánica et al., 2016). in this approach, energy dissipation has to be artificially incorporated through material damping formulations. although different approaches have been evaluated in the literature (see e.g. mánica et al., 2014; verrucci et al., 2022), these studies generally assume a homogeneous soil deposit. however, as demonstrated by yang et al. (2013) through centrifuge modelling, the variation of dynamic properties with depth is of paramount importance to reproduce the response of the ground under seismic loading figure 1. tectonic frame of western mexico. 281material damping in a stratified soil deposit this work assessed a number of material damping formulations in flac to emulate results from one-dimensional equivalent linear analyses. particularly, local, rayleigh, and hysteretic damping formulations were assessed during the course of this investigation. the analyses only considered a slender column of soil, with appropriate energy dissipating boundary conditions, since only the free-field response is addressed here. however, the resulting model calibrations can then be used in large-scale simulations for dynamic soil-structure interaction problems. a main feature of the analyses was to consider a site having a considerably stratified soil deposit in which the shear wave velocity profile displays significant variations and where the selection of some parameters in the damping formulations is not a trivial task. the input motion was adjusted to comply with a uniform hazard spectrum for the area, with a 500-year return period, using a spectral matching technique. results provide relevant insights into the performance of the adopted damping formulations and the selection of material damping parameters to reproduce the results of equivalent linear analyses. 2. numerical model 2.1. subsoil conditions the site under study is located in the downtown area of guadalajara, a major city in western mexico within the trans-mexican volcanic belt (tmvb). seismic activity in the area is related to two major sources: (1) the subduction of cocos and rivera plates beneath the north american plate and (2) the fractured nature of the tmvb (yamamoto et al., 2018) (figure 1). subsoil conditions are characterised by the presence of silty sand layers, of varying relative densities, overlying a basaltic bedrock. they are pumice soils formed in the late pleistocene during periods of intense volcanic activity (lazcano, 2010), which are particularly sensitive to particle breakage. the idealised stratigraphy, used in the analyses, was derived from an exploration campaign at the site including shear wave velocity measurements using downhole geophysical logging. figure 2 shows the shear wave velocity profile, the assumed idealised stratigraphy, and the unit weight of the considered layers. the basaltic bedrock was identified at a depth of 44 m, where the lower boundary of the numerical model in the analyses was assumed. the initial shear stiffness was derived from shear wave velocities vs as 2 o sg vρ= (1) where ρ is the soil density. the bulk modulus k was derived as a function of the poisson’s ratio ν as k g v v � � � 2 1 3 1 2 ( ) ( ) (2) the water table is located at a depth of 10 m. it is important to mention that beyond this depth liquefaction has not been systematically identified in reported case histories (youd et al., 2001). along with shear wave velocities in excess of 300 m/s from this depth onwards (figure 2), liquefaction is unlikely to occur at the site. in addition, to the best of the authors’ knowledge, liquefaction has never been reported in the area. therefore, the occurrence of this phenomenon was not addressed in the present work. figure 2. shear wave velocity profile and idealised stratigraphy. the nonlinear behaviour of the soil was taken into account by means of shear modulus degradation and damping curves. the upper and lower bounds of the modulus degradation and damping relationships from seed & idriss (1970) were respectively adopted for the silty sand layers comprising the soil deposit; they are shown in figure 3. figure 3. stiffness degradation and damping relationships (seed & idriss, 1970). 282 miguel a. díaz, miguel a. mánica, eduardo botero, efraín ovando-shelley, luis osorio 2.2. input motion the input motion was derived from the east-west acceleration time history of the 1995 colima earthquake, recorded at the site under study. it was a seismic event with a magnitude mw = 8.1 and an epicentre about 300 km away from the site. the motion was deconvoluted to the same elevation as the base of the numerical model using shake (schnabel et al., 1972), a well-known computer program for one-dimensional site response analyses where nonlinear behaviour is accounted for by means of the equivalent linear method. the deconvoluted ground motion was then adjusted to a uniform hazard response spectrum for the area, for a return period tr= 500 years (cfe 2015), using a spectral matching technique (abrahamson, 1992). this procedure yielded a realistic higher intensity record (abrahamson, 1992; alatik & abrahamson, 2010). figure 4 shows the uniform hazard spectrum from the adjusted record. the latter complies with the minimum requirements established by local recommendations for site response analyses (cucei, 1997). figure 4. uniform hazard spectrum (tr = 500 years) and spectrum from the adjusted record. the derived seismic record was prescribed at the base of the numerical model through a compliant base. the latter is based on the use of independent dashpots in the normal and shear directions which provide adequate means to absorb waves approaching the boundary (lysmer & kuhlemeyer, 1969). nevertheless, since the boundary needs to move freely to absorb incoming waves, the input motion cannot be prescribed as an acceleration (or velocity, or displacement) time history. therefore, the seismic record was prescribed as a shear stress history derived as follow (mejia & dawson, 2006): ( ) ( )s2 xt v u tτ ρ=  (3) where t is the time and ux is the horizontal particle velocity of the upward propagating motion. the factor of two in eq. (3) is because half of the stress is absorbed by the viscous dashpots. furthermore, in order to reduce computation times, only the intense part of the record was used. the latter was defined as the time window between 5 and 95% of the cumulative arias intensity (arias, 1970). figures 5 and 6 show the arias intensity from the input seismic record and the input motion in terms of shear stresses respectively, where the adopted time window is indicated. figure 5. arias intensity from the input seismic record and employed time window. figure 6. input motion in terms of shear stress and employed time window. 2.3. model setup three-dimensional dynamic finite-difference analyses were performed in flac to simulate the seismic site response, through which different damping schemes were assessed. since only free-field conditions are considered, the model corresponds only to a slender column of soil (4 x 4 m at the base) with appropriate boundary conditions to account for the semi-infinite nature of the ground. as mentioned earlier, a compliant base was adopted for the lower boundary where the input motion is prescribed as a shear stress history. at the lateral boundaries, free-field conditions were employed. they involved the execution of free-field calculations, in parallel with the main grid analysis, where unbalanced forces are applied to the main grid forcing the free-field motion at the boundary (itasca consulting group, 2012). figure 7 shows the implemented finite-difference model in flac. 283material damping in a stratified soil deposit figure 7. numerical model in flac and degraded parameters adopted from shake. the site response analysis with shake is assumed as the reference surface ground motion response. the nonlinear behaviour of the soil is incorporated in shake through the equivalent linear method, by iteratively reducing the stiffness and increasing the critical damping ratio as a function of the maximum strains attained in each iteration, according to given modulus degradation and damping relationships (figure 3). since an analogous response is sought with flac, a linear elastic constitutive model was adopted for the soil layers, characterised by the degraded stiffness from the shake analysis, in accordance with the level of deformation induced by the input motion. degraded parameters are shown in figure 7. the selection of the damping parameters for the local and rayleigh damping schemes, in the stratified soil deposit considered, is investigated later. in the case of the hysteretic damping models, they already account for the stiffness degradation and damping increase with deformation. therefore, the material is characterised by the initial small strain modulus and a given modulus go degradation curve. 3. results results obtained from the analyses are presented in the following sections. they are organised in terms of the adopted damping approach. in each section, a brief description of the corresponding damping formulation is first provided. for further details on the damping formulations, the reader is referred to itasca consulting group (2012). 3.1. local damping local damping formulation in flac operates by adding or subtracting mass from the grid points at certain times during oscillation, but in a way that the overall mass of the domain remains constant. mass is added when velocity changes direction and subtracted when it passes a maximum or minimum point, thus removing kinetic energy (itasca consulting group, 2012). unlike rayleigh formulation, local damping is frequency-independent. it is characterised by a local damping coefficient αl which is related to the critical damping ratio ξ as lα πξ= (4) as previously mentioned, ξ corresponds here to the value obtained at the last iteration of the shake analysis (see figure 7). figure 8 compares the surface response spectrum obtained from flac to the spectrum from the site response analysis in shake. as previously identified (vieira et al., 2006; mánica et al., 2014; verrucci et al., 2022), local damping tends to underdamp the response for high-frequency components and may introduce high-frequency noise (itasca consulting group, 2012). the latter resulted in higher spectral ordinates in flac (up to 45.7%) for periods lower than 0.25 s. for larger periods, flac was able to mimic almost exactly the response spectra obtained with shake. figure 9 shows maximum values of accelerations, shear stresses, and shear strains in-depth. maximum accelerations in flac are noticeably larger with respect to shake, with the largest increase of about 47% occurring at the surface. on the other hand, shear stress and strain distributions are similar in both analyses. these results are consistent with those obtained by mánica et al. (2014) for a homogenous soil deposit. figure 8. surface response spectra from shake and from flac using local damping. 284 miguel a. díaz, miguel a. mánica, eduardo botero, efraín ovando-shelley, luis osorio figure 9. maximum in-depth response parameters from shake and from flac using local damping. 3.2. rayleigh damping rayleigh damping is a viscous form of damping proportional to a linear combination of the mass and stiffness of the system. it is usually expressed in matrix form as α β= +c m k (5) where c is the damping matrix, m is the mass matrix, k is the stiffness matrix, and α and β are the massand stiffness-proportional damping constants respectively. this form of damping is frequency-dependent, and the critical damping ratio is a function of the damping constants as follows: ( ) 1 2 αξ ω βω ω  = +    (6) where ω = 2πf is the angular frequency and is f the frequency in hertz. the effect of α and β on ξ is shown in figure 10, where eq. (6) is plotted for only the stiffness-proportional term (i.e. α = 0), only the mass-proportional term (i.e. β = 0), and the combination of both. the mass-proportional term is dominant for low-frequency values while the stiffness-proportional term prevails at higher frequencies. unlike the frequency-independent local damping, the selection of the damping constants is not a trivial task. one alternative is to solve eq. (6) for two target frequencies and their corresponding damping values (bentley systems, 2020): 1 2 2 1 1 2 2 2 1 2 2 ω ξ ω ξ α ωω ω ω − = − (7) 1 1 2 2 2 2 1 2 2 ω ξ ω ξ β ω ω − = − (8) figure 10. variation of the critical damping ratio as a function of the angular frequency. the target damping ratios can be assumed equal and target frequencies chosen in terms of the vibration modes of the soil deposit and the input motion. for instance, hudson et al. (1994) and hashash & park (2002) suggest selecting ω1 as the first natural frequency of the soil deposit and ω2 as the predominant frequency of the input motion. another alternative is obtained by noting that eq. (6) has an approximately frequency-independent zone around the minimum value (see figure 10). for given values of ξmin and ωmin, corresponding to the coordinates of the minimum point of eq. (6), damping constants are given by min minα ξ ω= (9) min min ξ β ω = (10) 285material damping in a stratified soil deposit however, ωmin depends on neither the natural frequency of the soil deposit nor on the predominant frequency of the input motion, but on a combination of both (mánica et al., 2014). when the free-field motion in flac is being calibrated for a reference site response, as is the case here, mánica et al. (2014) proposed a calibration procedure to select ωmin that minimises the differences between the obtained and reference surface response spectra. particularly, the minimised variable is the residual sum of squares (rss) between both spectra. verrucci et al. (2022) also studied different approaches for the selection of ωmin and ξmin to reproduce a given target free-field response. however, in both studies, a homogeneous soil deposit was considered. in the case of stratified soil deposits, it is not clear if a single value of ωmin for all layers, each of them with different damping ratios, will provide a reasonable approximation. the latter was evaluated in the present research. figure 11 shows the variation of the rss between the surface response spectra from shake and flac for different values of ωmin. the damping ratios ξmin correspond to those derived from shake for each layer (see figure 7). it can be observed that rss is minimised for fmin = 4 hz (i.e. ωmin = 25.13). figure 12 shows the obtained surface response spectra, where a reasonable agreement can be noticed. the response is somewhat overdamped for periods lower than 0.15 s and the maximum acceleration was not quite reached, although differences are relatively modest (between 4 and 28%). those differences arise by the fact that each layer might oscillate at frequencies outside the approximately frequencyindependent region around ωmin, overdamping somewhat the response. figure 11. residual sum of squares as a function of fmin. figure 12. surface response spectra from shake and from flac using rayleigh damping with fmin = 4 hz. an additional approach to determine fmin was also evaluated for each layer. it is based on the selection of a central frequency ω according to the following equation (vanmarcke, 1972): 1/2 2 0 i i i λ λ    ω =     (11) where λ2 i and λ0 i are the 2nd and 0th moments of the spectral density of the acceleration of layer i. ω can be interpreted as the frequency where the centroid of the power spectral density is located and, therefore, where the most energy is concentrated. table 1 shows the central frequencies estimated from the acceleration histories, for each layer, from the site response analysis in shake. these frequencies, along with their corresponding damping ratios (also given in table 1), were applied to each layer of the flac model. the resulting surface response spectrum is shown in figure 13. again, a very good agreement with respect to shake can be noticed. as in figure 12, the response is slightly overdamped for frequencies lower than 0.15 s, although the maximum spectral ordinate was accurately reproduced. figure 14 shows maximum values of accelerations, shear stresses, and shear strains in-depth. these results agree quite well with those obtained with shake and, it can therefore be said that an equivalent response was obtained. table 1. critical damping ratios and central frequencies using rayleigh damping. soil layer critical damping ratio (%) central frequency (hz) 1 2.8 2.78 2 3.2 2.39 3 4.2 2.28 4 4.3 2.47 5 3.2 3.00 6 2.6 3.07 7 2.2 2.93 figure 13. surface response spectra from shake and from flac using rayleigh damping with fmin = ω. 286 miguel a. díaz, miguel a. mánica, eduardo botero, efraín ovando-shelley, luis osorio figure 14. maximum in-depth response parameters from shake and from flac using rayleigh damping with fmin = ω. 3.3. hysteretic damping hysteretic damping in flac directly incorporates stiffness degradation with deformation and its corresponding energy dissipation. the incremental shear modulus in a nonlinear simulation is given by gomt , where mt is the normalised tangent modulus defined as s t s dmdm m d d τ γ γ γ = = + (12) where τ is the normalised shear stress ( τ = τ/go), γ is the shear strain, and ms is the strain-dependent normalised secant modulus. the generalisation of strain amplitude and reversal points, to account for twoand three-dimensional strain paths, as well as the rules to define the behaviour at reversal points, are given in itasca consulting group (2012). ms must be specified as a continuous function and, therefore, flac provides a number of hysteresis models to fit given modulus degradation data. three different models were assessed in this research: default, hardin, and sigmoidal3 which are characterised by eq. (13), (14), and (15) respectively. ( )2 2 102 10 s 2 1 2 1 2 loglog 3 ll m l l l l γγ  − − = −    − −     (13) s ref 1 1 m γ γ = + (14) ( )s 10 olog 1 exp am x b γ =  − − +      (15) where l1, l2, γref , a, b, and xo are fitting parameters. the default, hardin, and sigmoidal3 models were employed to adjust as closely as possible the modulus reduction curve adopted (figure 3). calibration results and parameters employed are shown in figure 15a and table 2 respectively. although an excellent agreement can be achieved regarding the modulus reduction curve, damping ratios (15b) from the resulting hysteresis loops are significantly overestimated for strain amplitudes larger than 0.03%. the latter has been previously reported in the literature (wang et al., 2008; mánica et al., 2014). wang et al. (2008) implemented a generalised masin rule that prevents this damping overestimation. here, the original hysteretic damping models in flac were adopted but, as suggested by mánica et al. (2014), a trade-off was made in the adjustment of the modulus degradation data by simultaneously calibrating also the damping curve. this calibration procedure focused on the parameters controlling the rate of reduction of the secant modulus, corresponding to l2, γref , and xo for the default, hardin, and sigmoidal3 models respectively. these parameters were varied iteratively, and we selected those minimising the combined relative error between both the resulting modulus degradation and damping curves with respect to those in figure 3. nevertheless, this procedure was not carried out for the whole data set, but for strain amplitude values between 0.001 and 0.1%, which cover the expected values induced by the input motion (see figure 9). the error variation and the parameters selected for each model are shown in figure 16. figure 17 shows the resulting modulus degradation and damping relationship. a reasonable agreement is now achieved for both curves, with all considered models, for the range of strains considered. 287material damping in a stratified soil deposit figure 15. (a) stiffness degradation and (b) damping relationships from the employed hysteretic models in flac. table 2. parameters adopted for the different hysteretic models in flac. model parameter value default l1 -3.332 l2 1.719 hardin γref 0.1103 sigmoidal3 a 1.018 b -0.5021 xo -0.9354 figure 16. relative error variation in the stiffness degradation and damping values for the different hysteretic models. 288 miguel a. díaz, miguel a. mánica, eduardo botero, efraín ovando-shelley, luis osorio figure 17. adjusted (a) stiffness degradation and (b) damping relationships from the employed hysteretic models in flac. the surface response spectra obtained with the adjusted parameters are compared with the spectrum from shake in figure 18. differences between the different hysteretic models are quite small, and they are all in very good agreement with the results from shake. the same conclusions can be drawn with respect to the maximum in-depth variables shown in figure 19. these conclusions are somewhat different to those obtained by mánica et al. (2014), where the largest differences with respect to the reference shake analysis were found for hysteretic damping. nevertheless, mánica et al. (2014) mentioned that the possible source of underperformance for hysteretic damping was a high stiffness contrast occurring in the studied case, which does not occur here. however, in the case of earthquakes inducing larger strains, the zone where damping is considerably overestimated (figure 15b) can be reached, attenuating the response of the system. for instance, the latter could explain the severe underestimation of the recorded spectral accelerations in the simulation of a rockfill dam by miglio et al. (2008), where hysteric damping, in combination with a mohr-coulomb yield envelope, was adopted. figure 18. surface response spectra from shake and from flac using different hysteretic damping models. figure 19. maximum in-depth response parameters from shake and from flac using different hysteretic damping models. 289material damping in a stratified soil deposit 4. conclusions in this paper, the local, rayleigh, and hysteretic damping formulations in flac were assessed to emulate results from the equivalent linear method to be used for dynamic soil-structure interaction simulations. a main feature of the analyses is that they consider a site having a considerably stratified soil deposit. from the results obtained, the following conclusions can be drawn: • as previously identified, local damping tends to underestimate damping for low periods; in this case lower than 0.25 s. this is also noticeable in the in-depth distribution of maximum accelerations, where considerably larger values, compared to the equivalent linear method, were obtained. the largest acceleration overestimation occurs at the ground surface. for larger periods, a very good agreement was obtained. in-depth shear stresses and strains are not significantly affected by the underestimation of damping and similar values than shake were obtained. • in the case of rayleigh damping, the operating critical damping ratio is a function of the excitation frequency and, therefore, obtaining an analogous response to the equivalent method is not a trivial task. a reasonable agreement of the surface response was obtained following the procedure from mánica et al. (2014), where a single fmin value is adopted for all soil layers. an additional approach was also explored to determine fmin for each individual layer (vanmarcke, 1972) in terms of the spectral density of the acceleration history of the corresponding layer. results show a very good agreement both in terms of surface response spectra and maximum in-depth accelerations, shear stresses, and shear strains. nevertheless, the methodology implies additional effort in the computation of each fmin value. • three different hysteretic models, available in flac, were also assessed in this work. as previously identified, if the modulus degradation curve is adjusted in the best possible way, this results in a significant overestimation of the critical damping ratios. a trade-off has to be made by adjusting the model parameters to reduce differences with the adopted data simultaneously in both the modulus degradation and damping curves for the range of strains of interest. the latter resulted in a very good agreement with the response from shake, both in terms of the surface response spectra and in-depth maximum variables. • for the conditions studied here, the best match with respect to the equivalent linear method was attained with an elastic analysis, using the rayleigh damping formulation to account for energy dissipation in the soil, and with the hysteretic damping formulation. although these approaches can reasonably characterise the dynamic response of the ground in soil-structure interaction analyses, they should not be used for assessing dynamic stability conditions, unless they are combined with a given failure criterion. finally, it is important to stress the fact that, although dynamic numerical simulations that can reproduce an analogous response to one-dimensional equivalent linear analysis are still of great value to practical engineering, a considerable effort must be directed to the use and validation of advanced constitutive models, for dynamic numerical simulations, able to reproduce more closely the observed behaviour of soils including phenomena such as yielding, accumulation of plastic deformations, hardening/softening, inherent and induced anisotropy, or viscous effects, among others. references abrahamson, n. a. 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(2001). liquefaction resistance of soils: summary report from the 1996 nceer and 1998 nceer/nsf workshops on evaluation of liquefaction resistance of soils. journal of geotechnical and geoenvironmental engineering, 127(10), 817–833. https://doi.org/10.1061/(asce)10900241(2001)127:10(817) https://doi.org/https http://doi.org/10.1002/nme.3192 http://doi.org/10.1002/nme.3192 https://doi.org/10.1007/s10706-011-9443-8 https://doi.org/10.1007/s10706-011-9443-8 https://doi.org/10.1061/jsfeaq.0001260 https://doi.org/10.1193/1.1585498 https://doi.org/10.1061/jmcea3.0001593 https://doi.org/10.1016/j.compgeo.2021.104568 https://doi.org/10.1061/(asce)0733-9410(1988)114 https://doi.org/10.1061/(asce)0733-9410(1988)114 https://doi.org/10.1061/40975(318)16 https://doi.org/10.1371/journal.pone.0200991 https://doi.org/10.1016/j.soildyn.2012.09.003 https://doi.org/10.1016/j.soildyn.2012.09.003 https://doi.org/10.1061/(asce)1090-0241(2001)127 https://doi.org/10.1061/(asce)1090-0241(2001)127 _hlk93325211 _hlk118802762 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.107002 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 367 379 g eo lo g y record manuscript received: 29/01/2023 accepted for publication: 04/01/2024 abstract this research focuses on the geological investigation of kaiama region, which is characterized by a diverse range of rock formations, including mylonites, porphyritic granites, gneiss, schist, phyllites, and pink granites. the study employs remote sensing techniques, utilizing landsat 8 oli data and digital elevation models, to systematically map the spatial distribution of hydrothermal alterations and tectonic structures associated with mineralization in the kaiama area. various image processing methods such as color composites, band rationing, and principal component analysis (pca) were employed to extract valuable information from the collected datasets. utilizing sabins band ratios (4/2, 6/7, and 6/5), we categorized alterations associated with iron oxides, clay minerals, and ferrous minerals. pca was applied to refine the identification of alteration zones, using two distinct sets of images: h-image (comprising bands 2, 4, 5, and 7) and f-image (comprising bands 2, 5, 6, and 7), which represented iron-oxide and hydroxyl mineral deposits, respectively. the synthesis of h, f, and h+f images in rgb format provided an optimal representation of the spatial distribution of hydrothermal alterations, exhibiting a strong positive correlation with known mining regions for gold, copper, wolframite, and tantalite within the study area. furthermore, a comprehensive analysis of regional lineaments revealed a consistent nne-ssw to ne-sw correlation, suggesting a predominant control on mineralization trends. this study advocates for adopting remote sensing techniques, specifically landsat 8 data and dem, as an effective approach for mapping hydrothermal alterations and identifying key structural controls associated with mineralization. mapping hydrothermal alterations and associated lineaments within kaiama, north-central nigeria, using landsat 8 operational land imager data and digital elevation model umaru aliyu ohiani1,2* olugbenga okunlola3, umaru adamu danbatta4 and olusegun g. olisa5 1. pan african university of life and earth science institute, including (health and agriculture), university of ibadan, nigeria 2. department of geology, university of maiduguri, maiduguri, borno state, nigeria 3. department of geology, faculty of science, university of ibadan, nigeria 4. department of geology, faculty of physical sciences, ahmadu bello university, nigeria 5. department of earth sciences, olabisi onabanjo university, ago iwoye, ogun state, nigeria *corresponding author: umaru.aliyu@paulesi.org.ng how to cite this item: ohiani, u. a., okunlola, o., danbatta, u. a., & olisa, o. g. (2023). mapping hydrothermal alterations and associated lineaments within kaiama, north-central nigeria, using landsat 8 operational land imager data and digital elevation model. earth sciences research journal, 27(4), 367-379. https://doi.org/10.15446/esrj. v27n4.107002 keywords: band ratio, principal component analysis, false colour composite, landsat 8 oli, kaiama resumen este trabajo se enfoca en la investigación geológica de la región de kaiama, la cual se caracteriza por un rango diverso de formaciones rocosas que incluyen milonitas, granitos porfiríticos, gneises, esquistos, filitas y granitos rosa. el trabajo emplea técnicas de teledetección, con información del satélite landsat 8 oli y modelos digitales de elevación, para levantar un mapa de la distribución espacial de las alteraciones hidrotermales y las estructuras tectónicas asociadas con la mineralización del área de kaiama. se emplearon varios métodos de procesamiento de imágenes como falso color compuesto, reducción de bandas, y análisis de componentes principales para determinar la información importante del conjunto de datos recolectados. los autores utilizaron los ratios de la banda de sabins (4/2, 6/7, y 6/5) para categorizar las alteraciones asociadas con los óxidos de hierro, los minerales arcillosos y los minerales ferrosos. el análisis de componentes principales se aplicó para refinar la identificación de las zonas de alteración a través de dos configuraciones de imágenes: imagen h (que comprende las bandas 2, 4, 5 y 7) e imagen f (que comprende las bandas 2, 5, 6 y 7), los cuales representan los depósitos de óxidos de hierro y de minerales hidróxidos. la combinación de las imágenes h, f, y h+f en el formato rgb proveyó una representación óptima de la distribución espacial de las alteraciones hidrotermales y mostró una fuerte correlación positiva con regiones mineras caracterizadas de oro, cobre, wolframita y tantalita en el área de estudio. además, un análisis extenso de los lineamientos regionales revelaron una correlación consistente nne-sso hacia ne-so, lo que sugiere un control predominante en las tendencias de mineralización. este estudio propone la adopción de técnicas de teledetección, especificamente el uso de información del satélite landsat 8 y modelos digitales de elevación, como un acercamiento efectivo para levantar mapas de alteraciones hidrotermales e identificar las claves de control estructurales asociadas con la mineralización. palabras clave: reducción de bandas; análisis de componentes principales; falso color compuesto; landsat 8 oli; kaiama. levantamiento de mapas de alteraciones hidrotermales y lineamientos asociados al interior de kaiama, norte central de nigeria, con información del satélite landsat 8 operational land imager y modelos digitales de elevación https://doi.org/10.15446/esrj.v27n4.107002 mailto:umaru.aliyu@paulesi.org.ng https://doi.org/10.15446/esrj.v27n4.107002 https://doi.org/10.15446/esrj.v27n4.107002 368 umaru aliyu ohiani, olugbenga okunlola, umaru adamu danbatta and olusegun g. olisa 1. introduction numerous scientific investigations have demonstrated the intrinsic value and efficacy of remote sensing techniques in the realm of geological and mineral exploration endeavors, with a particular emphasis on various metallic and nonmetallic mineral deposits (ahmadi and uygucgil, 2021). when conducting in-depth exploration and exploitation of specific mineral deposits, it becomes imperative to meticulously consider the spatial distribution of hydrothermal alterations associated with those deposits (amara et al., 2019). consequently, remote sensing play a crucial role in the choice of the mineralized sites. this approach has become acknowledged as best practice when leading most geological expeditions due to its high effectiveness, low cost, and time-saving qualities (yekin, 2003). in remote sensing, multispectral and hyperspectral satellite and aerial sensors are used to monitor the reflection and absorption of matter on the earth’s surface (maleki et al., 2021). minerals are then targeted using a variety of enhancement techniques based on their spectral signature (wang et al. 2020; guha et al. 2020). the use of remote sensing imagery has been adopted globally and widely employed in mapping the spatial distribution of hydrothermal alterations within diverse metallogenic provinces (pour and hashim, 2011; pour et al., 2013; amara et al., 2019; osinowo et al., 2021; frutuoso et al., 2021; maleki et al., 2021; ahmadi and uygucgil, 2021; andongma et al.,2021; ombiro et al.,2021; umaru et al., 2022; abdulmalik et al., 2021; aliyu et al., 2021). landsat data, in particular, have gained extensive traction in the realms of structural and geological mapping, as well as hydrothermal alteration mapping, owing to the presence of mid-infrared bands that exhibit discernible spectral characteristics aligning with a majority of hydrothermal minerals (ducart et al., 2016; umaru et al., 2021). through its operational land imager (oli) sensor, which covers a wavelength range of 0.4-2.3 m, it is possible to identify mineral assemblages of hydroxyl and iron-oxide bearing (ducart et al., 2016). in many instances, a remote sensing analysis of a specific region becomes imperative to delineate hydrothermal zones and refine target locations prior to embarking on field-based, geochemical, or mineralogical investigations. the area under investigation (kaiama) has developed into an intriguing mining site due to the existence of several mineral deposits including wolframite, tantalite, cassiterite, copper, and gold whose extent of occurrence and spatial distribution is not fully documented (dada and ajadi, 2018; alepa et al., 2019a, 2019b). within the study region, there has been a limited exploration of remotely sensed data for the identification of hydrothermal alteration zones, which could serve as a valuable tool for regional exploration to unlock the full mineralization potential of the area. previous remote sensing studies conducted in the region have primarily focused on a regional scale, involving the spatial mapping of hydrothermal alterations and structural features, with a particular emphasis on gold and cassiterite mineralization (ige et al., 2022). additionally, there have been reconnaissance geochemical investigations aimed at assessing the distribution of ore-forming components and the potential for mineralization in the kaiama area (alepa et al., 2019a; 2019b). the primary objective of this study is to utilize remote sensing methods, specifically leveraging landsat 8 oli data and a digital elevation model, on an expansive scale to systematically delineate hydrothermal alteration zones and tectonic lineaments within the study area. this aims to provide a foundation for further exploration techniques that can be employed to fully realize and develop the mineralization potential of the region. to achieve these objectives, the authors have implemented three well-established and reliable image processing techniques: color composites, band ratios, and principal component analysis (pca). these methods are employed for the precise mapping of target features, including alteration zones. additionally, an automated approach is used to extract tectonic lineaments from digital elevation model data. generally, we enhance alteration zones, secondand third-order faults, and analyze their association with mineralization using satellite images while detailed field mapping was done to validate the remote sensing results. 2. geographical and geological setting kaiama is a local government in kwara state of nigeria and it is bounded to the north by niger state, to the south by oyo state and share a boundary to the west with benin republic (figure 1a and 1b). the area is underlain by mylonites, porphyritic granites, granite gneiss, talc schist, phyllites and pink granites (figure 1c). these rocks form part of the southwestern basement complex of nigeria which constitutes the migmatite gneiss complex, the schist belts and the older granitic rocks as well as undeformed acid and basic dykes. the migmatite-gneiss complex is generally considered as the basement complex “senso stricto” as the oldest basement (rahman, 1988; dada, 2006). about half of the nigerian basement consists of the migmatite-gneiss complex. it is a heterogeneous assemblage including migmatised gneisses, banded gneisses, granitic gneiss and a series of metamorphosed basic and ultrabasic rocks. these rocks strongly resemble the tonalite-trondhjemite-granodiorite (ttg) suites of archean and early proterozoic terrains elsewhere in the world (dada et al., 1993). the gneisses of the migmatite-gneiss complex are interleaved with amphibolites that may be derived from mg-rich rocks such as continental basalts (caby et al., 1990; dada,1999; dada, 2008). oyawoye (1972) noted that the banded gneisses are possibly the oldest rocks in the country, older than granite gneiss that yielded a rb-sr whole-rock isochron age of 2190 ± 30 ma. ajibade (1980) distinguished the ancient migmatites in northern nigeria from the pan-african migmatites by their complex polyphase deformation and mylonitization. the schist belts comprise suits of the metasedimentary and metavolcanic rocks. mccurry, (1976) distinguished the rocks of the schist belts into “older metasediments” and “younger metasediments”. the older metasediments are high-grade remnants in the gneisses and migmatites and are believed to have been formed about 2,500 ma (mccurry, 1976). they consist of calcsilicate rocks, arkosic quartzite and high-grade schist that occur as lensoid relics in the regional gneisses or as paleosomes of migmatites (danbatta, 2008). the younger metasediments are low-grade sediment-dominated schist groups and are composed mainly of pelitic and semi-pelitic schist, meta conglomerate, quartzite, calc-silicate rock, marble, mafic to ultramafic rocks, acid to intermediate volcanic rocks and rare banded iron formations (danbatta, 2008). in the western half of the country, they occur as discontinuous northsouth trending belts within the basement. the older granites comprise the syntectonic to late tectonic granitoid that intrude into both the migmatite-gneiss complex and the metasediments. they include rocks varying in composition from granite to tonalite and charnockites with smaller bodies of syenite, gabbro and pegmatite (woakes et al., 1987). the granitoids have yielded radiometric ages in the range of 750-500 ma which lies within the pan-african spectrum. these pan-african granitoid are referred to as the older granites in nigeria to distinguish them from the mesozoic anorogenic granite ringcomplexes (the younger granites). figure 1. a. geological map of nigeria showing the location of kwara state, b). geological map of kwara state showing the location of kaiama, c). geological map of kaiama 369mapping hydrothermal alterations and associated lineaments within kaiama 3. materials and methods remote sensing data acquisition in this investigation, a cloud-free level 1t landsat 8 oli/tirs (path 191/row 53) sensor imagery covering the analyzed area and acquired on 10 february 2022 was used. it was downloaded from the usgs eros website, http://earthexplorer.usgs.gov, with eleven (11) bands and orthorectified. the spectral bands include a panchromatic band with a resolution of 15 meters, a visible band with a resolution of 30 meters, three infrared bands (nir, swir1, and swir2 at 30 meters), a cirrus band with a resolution of 30 meters, an aerosol band with a resolution of 30 meters, and two thermal infrared bands at 30 meters. the performance characteristics of landsat 8 oli is shown in (table 1). hydrothermal alteration mapping was conducted utilizing landsat 8 operational land imager (oli) data. the digital elevation model (dem) utilized in this research was sourced from the alos-palsar sensor, established in 2006 by the ministry of economy, trade and industry (meti) and the japan aerospace exploration agency (jaxa). alos-palsar serves multiple purposes, including aiding geological studies and environmental conservation (bannari et al. 2016). acquired from the alaska satellite facility (https://asf.alaska.edu/), the alos-palsar dem boasts a resolution of 12.5 meters. recent studies (das et al. 2018; gaikwad et al. 2023) have underscored its efficacy, reliability, and accuracy in delineating lineaments. processing the dem involved the use of arcmap v.10.8 software, enabling the creation of four shaded relief images that accentuate the topographic and geomorphological features of the research area. the short wave infra-red region of landsat 8 has a unique application to geological studies. for example, band 7 coincides with the absorption band caused by hydrous minerals such as clays mica, some oxides, and sulfates which makes them appear darker. band 6 is used for soil and rock discrimination such as ferric iron or hematite rocks, and band 4 is used for the discrimination of soil from vegetation and delineation of soil cover (nait amara et al.,2019). maps were created using arcmap 10.8 while satellite data was processed using the envi 5.3 and the line module in pci geomatica 2018. preprocessing landsat 8oli data environment for visualizing images (envi) software version 5.3 was used to preprocess landsat 8oli data. preprocessing was carried out in stages. a radiometric adjustment to calibrate the image data to radiance, reflectance, and brightness temperature as well as reducing image digital number errors. atmospheric correction, to correct the impacts of the atmosphere as detected by the sensor. minimum noise fraction (mnf) to detect the underlying dimension of picture data, segregate and normalize the noise present, raise the signal-tonoise ratio, and decrease computational demands for subsequent processing (jensen, 2005; nait amara et al., 2019). to achieve the above preprocessing steps, landsat calibration and fast line of sight atmospheric analysis of spectral hypercubes algorithms (flaash) tools of envi software was applied. finally, the area of interest (aoi) was then sub-setted from the landsat scene. table 1. spectral characteristics of landsat-8 (operational land imager (oli) and thermal infrared sensor (tirs)) scene used in the current study bands bandwidth (µm) wavelength c enter (µm) spatial resolution (m) coastal/ aerosol) blue green red nir swir-1 swir-2 panchromatic cirrus tirs-1 tirs-2 0.435-0.451 0.452-0.512 0.533-0.590 0.636-0.673 0.851-0.879 1.566-1.651 2.107-2.294 0.503-0.676 1.363-1.384 10.60-11.19 11.50-12.51 0.4430 0.4820 0.5615 0.6545 0.860 1.6085 2.2005 0.5895 1.3735 10.8950 12.005 30 30 30 30 30 30 30 15 30 100 100 figure 2. flow chart for preprocessing and processing of landsat 8 oli and digital elevation model data processing landsat 8 oli image the landsat 8oli satellite data was processed to create images that emphasize geological features while minimizing background interference. spectral and spatial patterns associated with hydrothermal alterations were delineated using processing techniques such as color composite (cc), band ratios (br) and principal component analysis (pca). furthermore, digital elevation model (dem) was employed to generate lineaments through an automated method. the entire processing workflow is illustrated in figure 2. color composite (cc) color compositing involves the merging of grayscale landsat data into an rgb channel, resulting in a multispectral image with multicolor representations. this combination can be a true color composite (tcc) or a false color composite (fcc). a tcc is created by combining the visible bands of the electromagnetic spectrum, while an fcc is produced when non-visible bands are composited. the resulting composite image has the ability to reveal details such as lithological boundaries, hydrothermal changes, vegetation, and soil characteristics, among others (nait amara et al., 2019). numerous researches have employed color composites to identify hydrothermal alterations, utilizing combinations like rgb 567 (yerkin, 2003), rgb 752 (pour and hashim, 2011), rgb 573 (mia and fujimitsu, 2012), and rgb 657 (ali and pour, 2014). in our study, we utilized a color composite of bands 5, 6, and 7 in the rgb spectrum to effectively highlight hydrothermal alterations within the study area. band rationing (br) band ratio technique is used to improve contrast and enhance compositional information while suppressing unwanted information. the technique involves dividing digital numbers, or the brightness values corresponding to the peaks of high and low reflectance curves of one band, by that of another band hence, highlighting geological features that are not visible in raw data (pour and hashim, 2015). several writers have used band ratios approaches for geological usage to highlight minerals linked with hydrothermally altered rock based on the spectral reflectance and absorption of features (table 2). for the purpose of this work, the sabins ratio involving band 4/2 will be used for mapping out iron oxides alterations (sabins, 1999; ali and pour, 2014; sekandari et al., 2020), band 6/7 for mapping alunite and clay minerals (sabins, 1999; ali and pour, 2014; pour and hashim, 2015) and band 6/5 will be used for mapping ferrous minerals (mia and fujimitsu, 2012); ali and pour, 2014). http://earthexplorer.usgs.gov https://asf.alaska.edu/ 370 umaru aliyu ohiani, olugbenga okunlola, umaru adamu danbatta and olusegun g. olisa table 2. tested band ratios for landsat 8 operational land imager (oli) band ratio feature 4/2 iron oxides 6/7 alumina and clay minerals 6/5 ferrous minerals principal component analysis (pca) pca is a technique for mapping lithological and alteration characteristics. it is a multivariate statistical technique that selects uncorrelated linear combinations of variables (eigenvector loadings), allowing each component to extract a linear combination with a smaller variance one at a time (farahbakhsh et al., 2016; takodjou wambo et al., 2020). according to crosta and moore (1989) and loughlin (1991), a pc image with moderate to high eigenvector loading for diagnostic reflectance and absorptive bands of a mineral or mineral group with opposite signs enhances that mineral. if the loading is positive in the mineral’s reflective band, the picture tone for the enhanced target mineral will be bright; if it is negative, it will be dark. therefore, eigenvector loading in each pca would provide the pc image that contains the spectral data of the mineral under study. although this information typically only makes up a very small portion of the original bands’ total information content, it is anticipated that the loaded information will reveal the desired mineral’s spectral signature (jensen, 2005; gupta, 2017). lineament extraction digital elevation models (dem) have been used in several research for the extraction of geological lineaments (koike et al., 1998; masoud and koike, 2017; meixner et al., 2018; umaru et al., 2021; andongma et al., 2021). the use of digital elevation models has the advantage of representing the true reap projections with no distortion, also a relief is clearly represented by tonal variations, and the position of the sun can be varied in most case. the dem provides a better resolution when compared with other satellite imageries like landsat and aster (anwar et al., 2010; mavoungou et al., 2023). this study employed the use of dem to extract lineaments associated with the investigated area. four shaded relief images were generated representing four different solar azimuths (0, 45, 90 and 135 degrees respectively). an ambient light setting of 0.20 was selected to produce a good contrast. the ambient light setting is a scaling factor in erdas imagine program (erdas, 1998). the shaded relief images created were combined to produce one shaded relief image using gis overlay algorithm in arcmap software. the combined image was then used for the automatic lineament extraction on pci geomatica software following the default setting in table 3. a rose diagram generated from the lineaments was used to show the preferred structural orientation for mineralization. table 3. default parameter for lineament extraction on pci geomatica name description values radi radius of filter in pixels 12 gthr threshold for edge gradient 90 lthr threshold for curve length 30 fthr threshold for line fitting error 10 athr threshold for angular difference 30 dthr threshold for linking distance 20 results and discussion colour composite figure 3 is the colour composite image of the study area. it was produced by assigning bands 5 (0.851-0.879μm) in red channel, band 6 (1.566-1.651μm) in green channel and band 7 (2.107-2.294μm) in blue channels. the resultant image shows the general hydrothermal alterations of the study area in white, dark blue and light green pixels. figure 3. color composite image rgb combination of bands 5, 6, and 7 band ratio figure 4 represents a band ratio of 4/2, emphasizing regions where iron oxide minerals are present pervasively or as surface coatings. in this image, iron oxide-rich areas are depicted as bright or white pixels, while regions with lower iron oxide content appear as dark or black pixels. this distinction arises from the optical properties of iron-bearing minerals: they exhibit high reflectance in band 4 and strong absorption in band 2. as evident in the ratio image, the prevalence of iron oxide-rich areas is most pronounced in the northern sections, extending southward toward the southwestern portion of the area. figure 5 shows the ratio image of band 6/7 for clay and hydroxyl-bearing minerals. this emphasis arises from the pronounced reflectance in band 6 and significant absorption in band 7 exhibited by these minerals. notably, the western part of the image is predominantly characterized by bright or white pixels, signifying the prevalence of these minerals. at the same time, darker areas correspond to regions with lower concentrations of clay or hydroxyl-bearing minerals. the ratio 6/5 (figure 6) mapped the regions of ferrous minerals represented as bright or white colors due to high reflectance of band 6 and high absorbance of band 5. this region occupies the southeastern parts underlain by fine grained granites. the combined band ratios image of 4/2, 6/7 and 6/5in rgb sequence known as sabins ratio is represented in figure 7. this image clearly revealed and delineate the spatial distribution of iron oxide rich areas, clay and hydrous minerals areas as well as ferrous minerals in blue, green and purple colors. iron oxide minerals are most prevalent in the southwestern portion of the map progressing extending upwards into the northern area where they are associated with mylonites, talc schist and partly with phyllites. clay and hydroxyl-bearing zones are distributed across the entire map in patchy patterns but conspicuously absent from the southeastern edge, which is primarily characterized by the presence of ferrous minerals found in association with fine-grained granites. figure 4. band ratio 4/2 image highlighting regions of iron oxide in white or bright pixels 371mapping hydrothermal alterations and associated lineaments within kaiama figure 5. band ratio 6/7 image highlighting regions of alunite and clay minerals in white or bright pixels figure 6. band ratio 6/5 image highlighting regions of ferrous minerals in white or bright pixels figure 7. band ratio composite image of 4/2, 6/7 and 6/5 (sabins composite image) showing the spatial distribution of iron oxide, clay and hydrous minerals as well as ferrous minerals in purple, green and blue colours principal component analysis in accordance with spectral analyses utilizing the spectral signatures of iron oxides and hydroxyl-bearing minerals derived from the usgs spectral library, a selection was made to utilize bands 2, 4, 5, and 6 of the landsat 8 oli sensors to accentuate the spectral characteristics associated with iron-oxide-bearing minerals. simultaneously, bands 2, 5, 6, and 7 were employed to enhance the spectral signatures of hydroxyl-bearing minerals. principal component analysis (pca) was applied to transform the chosen bands into their respective principal components. consequently, the outcomes of this transformation effectively delineated regions rich in oh-bearing minerals and iron minerals, respectively. tables 4 and 5 represent the eigenvectors and eigenvalues from crosta analysis of the landsat 8 data. examining the eigenvector values of the pca loading for the selected bands 2, 4, 5, and 6 for iron oxides, shows that pc4 showed a clear contrast between bands 2 (0.77234) and 4 (-0.55141) (table 4). thus, pc4 highlighted the alteration areas in dark tone as a perfect contrast between eigenvectors of bands 4 and 2 due to the negative contribution from band 4 (figure 8). the resultant pc4 image was then negated to produce a h-image based on crosta and moore classification (oh-bearing minerals) in bright or white colour for better identification (figure 9). also, pca was applied to selected bands 2,4,6 and 7 to map out regions of oh bearing minerals. result showed that pc4 had the highest loading between the eigenvectors of band 6 (0.57130) and band 7 (-0.11447) (table 5) hence revealing areas of high oh minerals in dark pixels due to the negative contribution from band 7 (figure 10). negating the image produced the crosta and moore (1989) f-image of the study area which is presented in bright colours for ease of identification (figure 11). (figure 12) is the combined image of h and f that shows the altered areas of iron oxides and hydroxyl minerals in bright tones called crosta and moore (1989) h+f image. after obtaining h, f and h+f images in a grey scale, a color composite image “crosta alteration image” was generated after utilizing the oh-bearing minerals (h-image) in the red channel, the iron-bearing oxides (fimage) in the blue channel and the h+f image in green. the resulting hydrothermal alteration from this combination is presented in (figure 13). the figure shows that iron oxide minerals are represented in dark blue colours, while hydroxyl bearing minerals are in brown to yellow colours. the regions of white pixels to light blue pixels notable as (combined iron and hydroxyl minerals) are delineated as the hydrothermal alteration zones and regarded as the zones of highest possibility for mineral occurrence (figure 12). figure 14 provides a clear depiction of delineated hydrothermal alteration zones, with prominent hydrothermally altered areas concentrated in the southeastern region and appearing as distinct patches in the eastern part. the spatial distribution of the alteration zones strongly suggests their association with major structural features (lineaments), indicating a structural control on their distribution. the representation of these hydrothermal alteration zones on the geological map of the study area (figure 19) further reveals their predominant occurrence within fine-grained granites, granite gneiss, pink granites, and mylonitic zones. this map also illustrates the close proximity of many hydrothermal alteration zones to pre-existing structural features, affirming the significant influence of structural factors on the spatial distribution of these hydrothermally altered minerals. notably, when plotting the sites of mineralization such as copper, gold, tantalite, and wolframite onto the map, it becomes evident that these mineralization sites either closely coincide with the hydrothermal zones or directly overlap with them. figure 8. pc4 showing iron-oxide alteration areas in dark or black pixels 372 umaru aliyu ohiani, olugbenga okunlola, umaru adamu danbatta and olusegun g. olisa figure 9. negated iron-oxide (h-image) showing alteration areas in bright or white pixels table 4. eigenvalues and eigenvectors loadings for iron oxide mapping number of input layers=4, number of principal component layers=4 band 2 band 4 band 5 band 6 % eigenvalues pc1 0.09090 -0.06738 0.42980 0.89581 92.4386 pc2 0.31004 -0.06842 0.84015 -0.43971 5.6356 pc3 0.54691 0.82869 -0.10451 0.05698 1.8147 pc4 0.77234 -0.55141 -0.31384 0.03073 0.1111 figure10. pc2 showing hydroxyl alteration areas in black or dark pixels figure 11. negated hydroxyl minerals (f-image) represented in bright or light pixels table 5. eigenvalues and eigenvectors loadings for hydroxyl mapping number of input layers=4, number of principal component layers=4 band 2 band 5 band 6 band 7 % eigenvalues pc1 0.08463 -0.03333 0.17039 0.98117 85.7715 pc2 0.44475 0.68022 0.57130 -0.11447 12.2666 pc3 0.68993 0.15200 -0.70446 0.06799 1.7388 pc4 0.56483 -0.71630 0.38511 -0.13993 0.2231 figure 12. combined hydroxyl and iron minerals as an h+f image figure 13. crosta map (alteration map) produced by compositing h, f and h+f images in rgb figure 14. delineated hydrothermal alteration zones 373mapping hydrothermal alterations and associated lineaments within kaiama lineament interpretation figure 15 displays the composite shaded relief image, generated by combining azimuthal angles of 0, 45, 90, and 135 degrees. meanwhile, figure 16 illustrates the resultant lineament map, unveiling distinct structural orientations within the study area. these orientations are primarily oriented in the nnessw, ne-sw, and nw-se directions, as depicted in the accompanying rose diagram (figure 17). of notable significance is the prevalence of lineaments trending in the ne-sw direction, which emerges as the dominant structural trend. consequently, it is inferred that these ne-sw trending lineaments play a pivotal role in controlling mineralization processes within the studied region. this orientation aligns closely with the regional structural trend previously identified in kaiama (ige et al. 2022). kaiama is located in close proximity to a significant fault system known as the anka yauri iseyin (ayi) fault, which traverses nigeria in a dextral manner and follows a ne-sw direction (garba, 2003). consequently, it is highly likely that the prevailing dominant structural orientation in the study area, trending nne-ssw, is a subsidiary feature of this major fracture system. field investigations further corroborate that a significant portion of the ne-sw to nne-ssw trending lineaments aligns closely with dextral strike-slip faults and shear zones within the mineralization zones. additionally, some of these lineaments coincide with the presence of joints and fractures. the lineament density map highlights regions characterized by high structural density. high-density zones are discernible in the northeast, central, southwest, and northwestern sectors of the study area. these particular areas are regarded as the most promising and feasible zones for mineral prospecting. importantly, they are closely linked with lineaments exhibiting a ne-sw trending orientation. the superimposition of identified mineralization points onto the lineament density map reveals a strong correlation with areas characterized by medium to high lineament density (figure 18). when these mineralization points are overlaid onto the geological map, it becomes evident that regions exhibiting high-density lineaments are notably associated with talc schist in the northern sector, mylonites along the southwestern boundary, as well as fine-grained granites and granite gneiss. these regions can be further studied in detail to determine the full mineralization potentials of the region. figure 15. combined azimuths of 0, 45, 90 and 135 degrees for lineament extraction 374 umaru aliyu ohiani, olugbenga okunlola, umaru adamu danbatta and olusegun g. olisa figure 16. extracted lineaments from combined azimuths of 0, 45, 90 and 135 degrees figure 17. rose diagram of lineaments showing dominant orientation in nne-ssw, ne-sw, and nw-se 375mapping hydrothermal alterations and associated lineaments within kaiama figure 18. lineament density map superimposed with lineaments and mineralization sites within the study area figure 19. integration of hydrothermal alteration zones, mining sites, extracted lineaments on the geological map of the study area. 376 umaru aliyu ohiani, olugbenga okunlola, umaru adamu danbatta and olusegun g. olisa figure 20. a. chloritization and silicification in wall rock alteration of porphyritic granite at a copper mining locale, b. wall rock alteration in porphyritic granite at a copper mining site with prominent chloritization and silicification, c. hematitization (iron oxide alteration) in wall rock alteration of mylonite at a gold mining area, d. silicification along the margins of a quartz vein from a gold mining location hosted by talc schist, e. active gold mining site displaying evident iron oxide alteration in the wall rocks, f. distinct nne-ssw orientation observed at a gold mine site silicification alteration chloritization iron-oxide alteration iron-oxide alteration 377mapping hydrothermal alterations and associated lineaments within kaiama validation of result in order to validate the delineated alteration zones derived from landsat 8 oli satellite data, a comprehensive field investigation was undertaken. the outcomes of the fieldwork unveiled the presence of propylitic and argillic alteration assemblages, along with iron-oxide alteration domains (figure 20). these alterations were notably correlated with linear geological features exhibiting a ne-sw orientation, encompassing fault/shear zones, joints, and fractures, which exerted control over mineralization processes. in the southwestern and northern most regions of the study area, the field examination successfully identified gold mineralization within a silicic alteration zone, predominantly hosted within mylonitic rock formations. when overlaying the mineralization data onto the hydrothermal alteration map (figure 19), a distinct alignment emerged, with mineralization points either proximate to or precisely coincident with the delineated alteration zones. furthermore, tantalite, copper, and wolframite mining sites were discerned in the eastern and southern sectors, all of which conspicuously overlaid the identified alteration zones. it was also evident that these mineralization sites were closely associated with the northeast-southwest trending geological structures, predominantly shear zones, albeit with some contributions from joints and fractures. the findings are corroborated by figure 20, which depicts alteration zones alongside mineralization sites, ascertained through rigorous field investigations. in summation, the field observations conducted during the verification process affirm that the various processing techniques applied to the landsat 8 oli imagery successfully delineated hydrothermal alteration zones, which are likely to represent prospective areas for mineralization table 6. field coordinates of mining sites and minerals associated mineralization type latitude longitude settlement gold 90 44’ 10.88” 30 42’ 49.75” nanu gold 90 43’ 50.38” 30 44’ 15.72” nanu gold 90 43’ 32.10” 30 44’ 52.48” nanu copper/wolframite 90 36’ 41.67” 30 58’ 1.80” betekuta wolframite/tantalite 90 33’ 31.70” 30 51’ 32.43” se of kam gold 90 34’ 58.68” 30 38’ 59.30” yashikira gold 90 34’ 40.20” 30 39’ 56.50” yashikira gold 90 33’ 59.60” 30 38’ 34.62” yashikira conclusions the utilization of landsat 8 oli data in conjunction with digital elevation models has demonstrated remarkable efficacy in the delineation of hydrothermal alteration regions linked to mineralization within the investigated domain. employing image enhancement techniques, such as color composites, band ratios, and principal component analysis, has yielded conspicuous depictions of ferrous minerals and hydroxyl-altered mineral zones. concurrently, an analysis of linear features (lineaments) has provided valuable insights into the structural controls for mineralization, predominantly characterized by an ne-sw orientation. this study has successfully identified hydrothermal alteration zones prominently characterized by argillic and propylitic alterations within regions associated with gold, copper, tantalite, and wolframite occurrences as delineated by our spatial analysis. consequently, there exists a compelling imperative for an extensive and comprehensive investigation encompassing geochemical, geophysical, and geological methodologies within these delineated zones to ascertain their complete mineralization potential. funding the research work is part of the first 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[m.sc. thesis, the middle east technical university, turkey]. https://doi.org/10.1016/j.oregeorev.2020.103530 https://doi.org/10.1016/0301-9268 https://doi.org/10.1016/0301-9268 https://doi.org/10.1007/s12518-022-00422-6 http://dx.doi.org/10.5455/sf.71057 https://doi.org/10.1007/s12517-022-11048-8 https://doi.org/10.1016/0899-5362(87)90004-2 https://doi.org/10.1016/j.asr.2020.01.008 https://doi.org/10.1016/j.asr.2020.01.008 http://m.sc _heading=h.gjdgxs issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.105079 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 447 460 ea rt h q u a k e en g in ee r in g estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data mohammad mahdi khoshgoftar*, mohammad reza saradjian school of surveying and geospatial engineering, college of engineering, university of tehran, tehran, iran *corresponding author: mm.khoshgoftaar@ut.ac.ir how to cite item: khoshgoftar, m. m., & saradjian m. r. (2024). estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data. earth sciences research journal, 28(4), 447-460. https://doi. org/10.15446/esrj.v28n4.105079 record manuscript received: 02/10/2022 accepted for publication: 10/12/2024 abst r ac t a single precursor is not usually an accurate, precise, and adequate measure to predict earthquake parameters. therefore, it is more appropriate to combine multiple precursors and exploit parameters extracted from them to reduce the uncertainty of the prediction. the assumption in this study is based on the fact that most earthquakes happen in active fault zones. the study is about the estimation of earthquake parameters such as date and magnitude. in this study, remote sensing observations (such as electron and ion density, electron temperature, total electron content (tec), land surface temperature (lst), sea surface temperature (sst), aerosol optical depth (aod), and surface latent heat flux (slhf)) in different modalities acquired several days before impending earthquakes have been investigated to extract earthquake parameters. in this study, three methods: median, support vector regression (svr), and random forest (rf) have been used to detect anomalies. then, by estimating the amount of anomaly deviation from the normal state, the magnitude of the impending earthquake is estimated. the final earthquake parameters (such as date and magnitude) can be obtained by integrating the earthquake parameters extracted from different earthquake precursors using the mean square error (mse) method. keywords: earthquake, anomaly detection, remote sensing, support vector machine, random forest palabras clave: terremoto; detección de anomalías; detección remota; máquina de vectores de soporte; bosque aleatorio estimación del momento y la magnitud de los terremotos masivos inminentes a través de la integración de precursores obtenidos a través de información de detección remota un solo precursor no es usualmente una medida exacta, precisa y adecuada para predecir los parámetros de un terremoto. además, es más apropiado combinar múltiples precursores e identificar sus propios parámetros para reducir la incertidumbre de la predicción. el supuesto de este trabajo está basado en que la mayoría de terremotos ocurren en zonas de fallas activas. este estudio se basa en la estimación de los parámetros de terremoto como momento y magnitud. en este trabajo se investigaron las observaciones de detección remota en diferentes modalidades (tales como densidad de iones y electrones, temperatura de electrones, contenido total de electrones, temperatura de la superficie terrestre, temperatura de la superficie marina, profundidad óptica del aerosol y flujo de calor latente), adquiridas varios días antes de los terremotos inminentes, para extraer los parámetros del terremoto. se usaron tres métodos para detectar las anomalías: mediana, regresión de vectores de soporte, y bosque aleatorio. luego se estimó la magnitud de los terremotos inminentes al estimar la medida de la desviación de la anomalía. los parámetros finales del terremoto (como momento y magnitud) se pueden obtener al integrar los parámetros de terremoto extraídos de diferentes precursores de terremoto al usar el método del error cuadrático medio. resu m en https://doi.org/10.15446/esrj.v28n4.105079 https://doi.org/10.15446/esrj.v28n4.105079 https://doi.org/10.15446/esrj.v28n4.105079 448 mohammad mahdi khoshgoftar, mohammad reza saradjian 1. introduction when an earthquake is happening, energy transmission is generated due to the destructive effects of the earthquake on the environment. the occurrence of these changes before and/or after the earthquake may have various physical and chemical effects on the lithosphere, atmosphere, and ionosphere, making the earthquake more accurately predictable. the abnormal variations in lithospheric, atmospheric, and ionosphere parameters are taken as “earthquake precursors”. they serve as alarms for impending earthquakes. many studies have been carried out on earthquake predictions using precursors in the lithosphere, atmosphere, and ionosphere. the problem arises when some of these major abnormalities do not appear during an earthquake. there are several studies based on the observation of the seismic lithosphere atmosphere ionosphere coupling (laic) anomalies which confirm that the anomalies begin several days before the earthquake and remain a few days after. none of the earthquake precursors can be used alone as an accurate and independent parameter for estimating earthquake parameters without generating some level of uncertainty. hence, it is necessary to integrate different types of earthquake predictors or precursors. by integrating a variety of earthquake parameters extracted from different precursors, a more accurate and suitable estimation of the final earthquake parameters may be obtained. recent advances in remote sensing and earth observation technology have facilitated monitoring the ionosphere, the atmosphere, and the earth’s surface using various sensors. nowadays, researchers investigate the factors and indications of earthquakes in more practical and efficient ways. the most important earthquake precursors relevant to ionospheric anomalies recently studied are changes in ion density, ion temperature, electron density, and electron temperature provided by demeter satellite data (berthelier et al., 2006; lebreton et al., 2006; parrot et al., 2006; li and parrot 2012; li and parrot 2013; tao et al., 2017; ibanga et al., 2018; li and parrot 2018). ionospheric anomaly studies also include changes in total electron content (tec) obtained from global positioning receivers (gps) (liu et al., 2004; akhoondzadeh 2013; tao et al., 2017; akhoondzadeh et al., 2019). regarding the earth’s surface, another useful precursor is thermal anomaly obtainable from land surface temperature (lst) (ouzounov and freund 2004; ouzounov et al., 2006; tronin 2006; panda et al., 2007; saraf et al., 2008; blackett et al., 2011; zoran 2012; akhoondzadeh 2013; bhardwaj et al., 2017a; bhardwaj et al., 2017b, chen et al., 2020; jiao and shan 2021), and from sea surface temperature (sst) (dziak et al., 2003; ouzounov et al., 2006; freund et al., 2009). other useful precursors are outgoing longwave radiation (olr) (ouzounov et al., 2007; rawatet et al., 2011; eleftheriou et al., 2016), surface latent heat flux (slhf) (dey and singh 2003; cervone et al., 2004; cervone et al., 2006; pulinets et al., 2006; pulinets and ouzounov 2011; zhang et al., 2013; mansouridaneshvar et al., 2014; q‍in et al., 2014), and atmospheric anomalies in the form of aerosol optical depth (aod) (freund et al. 2009; akhoondzadeh, 2015; ganguly, 2016; akhoondzadeh, 2018; akhoondzadeh et al., 2019). 2. data according to the objective of this study, the data used have been chosen from multiple sources, which are as follows: demeter data the french microsatellite demeter was launched in june 2004, and its scientific mission stopped on december 9, 2010. the data provided by demeter is used to investigate ionospheric disturbances due to seismic activity (parrot et al., 2006). the demeter satellite collected its ionospheric parameters related to seismic activities using five sensors. the sensors are instrument champ eletrique (ice), instrument magnetic search coil (imsc), instrument detecteur de partcules (idp), instrument analyseur plasma (iap), and instrument sonde de longmuir (isl). in this study, ion density (cm-3) and ion temperature (k), as well as electron density(cm-3) and electron temperature(k) data were collected from isl and iap sensors. demeter satellite data is available via: http://demeter.cnrs-orleans.fr/. tec data the most popular product to analyse the ionosphere state is the global ionosphere maps (gim) of the total electron content (tec) provided by nasa (national aeronautics and space administration) in the ionex format. the gim-tec covers ± 87.5 of latitude and ± 180 of longitude with a spatial resolution of 2.5 and 5.0, respectively, and a cadence of 2h. in this study, the tec variations according to the closest node to the epicentre of the earthquakes have been analysed. the gim-tec map is obtained from the website https:// cddis.nasa.gov/archive/gnss/products/ionex/. modis data two products of moderate resolution imaging spectroradiometer (modis) satellite, i.e., land surface temperature (lst) and aerosol optical depth (aod) data were used in this study. both the day/night-time lst images provided by nasa (http://modis.gsfc.nasa.gov/data) were processed. the modis terra and aqua daily level-3 aerosol product, which is produced by the dark target and deep blue algorithms and is called ‘‘aerosol optical depth at 550 nm”, is available vie: https://giovanni.gsfc.nasa.gov/giovanni/. avhrr data two products of avhrr (advanced very high resolution radiometer) including sea surface temperature (sst) and surface latent heat flux (slhf) data have been used in this study. sea surface temperature (sst) anomaly can be related to near coastal seismic activity (ouzounov and freund, 2004), but conditions and currents can strongly affect sst. due to a large thermal inertia of the seawater, its temperature changes more slowly; therefore, in the case of the sst anomalies, some mechanisms of lst anomalies are not applicable (jiao et al., 2018). the noaa 0.25° daily optimum interpolation sea surface temperature (oisst) is an analysis constructed by combining observations from different platforms (satellites, ships, buoys) on a regular global grid. a spatially complete sst map is produced by interpolating to fill in gaps. the sst products are available via:https://psl.noaa.gov/data/gridded/data.noaa. oisst.v2.highres.html. slhf is (wm-2) the heat flux absorbed or released by the phase transition (i.e., condensation, evaporation, and melting) of water from the earth’s surface to the atmosphere (jiao et al., 2018). slhf is one of the important components of earth’s surface energy budget, which is mainly affected by the atmospheric relative humidity, wind speed, surface temperature, and season (jiao et al., 2018). due to the underground fluid movement and the interaction among the underground, surface, and atmosphere, the slhf anomaly that occurs prior to earthquakes is considered (alvan et al., 2013). the slhf products are available via:https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. geomagnetic indices the ionospheric parameters measured by satellite are mainly influenced by the geomagnetic storms and geomagnetic field disturbances, particularly in the equatorial and polar regions. however, in the case of an impending earthquake, it may be affected further more in the form of anomaly. such anomalies should also be removed. in order to distinguish anomalies caused by seismic activity from anomalies created by geomagnetic and solar activities, the geomagnetic and solar indices i.e. dst, kp, ap, and f10.7 acquired from space physics data facility (spdf) have been utilized in this study. in conditions where the quiet solar geomagnetic is established (i.e. kp< 2.5, -20 nt<dst< 20 nt and f10.7 < 120 sfu) the situation will be regarded as normal. however, if unusual changes are seen in the ionospheric data, it can be considered due to seismic activity. the geomagnetic and solar indices are available through noaa (https://www.ngdc.noaa.gov/stp/stp.html). http://de‍meter.cnrs-orleans.fr/ htt‍ps://cddis.n‍asa.gov/archive/gnss/products/ionex/ htt‍ps://cddis.n‍asa.gov/archive/gnss/products/ionex/ http://m‍odis.gsfc.nasa.gov/data https://giovanni.gsfc.nasa.gov/giovanni/ https://disc.gsfc.nasa.gov/information/glossary?keywords=giovanni%20measurements&title=giovanni%20measurement%20definitions%3a%20aerosol%20optical%20depth https://disc.gsfc.nasa.gov/information/glossary?keywords=giovanni%20measurements&title=giovanni%20measurement%20definitions%3a%20ozone h‍ttps://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html h‍ttps://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html htt‍ps://psl.noaa.gov/data/gridded/d‍ata.ncep.reanalysis.html https://www.ngdc.noaa.gov/stp/stp.html 449estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data 3. method anomaly detection method anomaly detection is the process of identifying unexpected items or events in data sets, which differ from the norm. anomaly detection has two basic assumptions: 1) anomalies only occur very rarely in the data, and 2) their features differ from the normal instances significantly. in order to detect anomalies in each set of time series data from remotely sensed precursors, identification of the normal behaviour of the phenomenon is necessary. 3.1a. normal behavior modeling in order to model time series behaviour, two common machine learning methods namely the support vector regression (svr) and random forest (rf) have been used in this study. in svr-based regression solutions, the input vectors are mapped into a higher-dimensional feature space, then by employing a linear regression in the feature space, the input vectors are separated as far apart as possible. in this study, which is operating in a large space, a kernel function is used (khosravi et al., 2017). random forest algorithm which is a nonparametric machine learning ensemble method, is an extended version of cart (classification and regression trees) model proposed by breiman (2001). it is based on the information combination method in which a large number of decision trees are generated and then the results of all the trees are combined for prediction (cutler et al. 2007). the r‍f is the multitude of regression trees which performs based on the variance of the data (liaw and wiener, 2002). 3.1b. anomaly detection in order to detect anomalies, a reasonable range for regular variations in the time series data should be specified. signals with normal behaviour fluctuate inside both the upper and lower bounds. signals outside the boundaries will be detected as anomalies. in this study, the median and interquartile method is used to determine the upper and lower bounds (liu et al., 2004). the upper and lower bounds are determined using the following eq. (1): low = × ℎ ℎ = + × (1) where xlow, xhigh, m and iqr are respectively the lower bound, upper bound, median value and interquartile range of x. if the signal lies within the range of the upper and lower bounds, the behaviour of the signal is considered as normal. to determine the intensity of abnormal behaviour of the signal, the parameter dx as deviation of x is calculated by eq. (2): low < < high < x = < (2) if the absolute value of the parameter dx is greater than k (i.e. |dx| > k), the behaviour of the parameter is regarded as anomalous. also, the percentage of parameter deviation from the natural state can be calculated using the eq. (3) (saradjian and akhoondzadeh, 2011): = ±100 × ((| x| )⁄ ) (3) preliminary parameters estimation the earthquake parameters are preliminarily estimated for each individual precursor. the dx value obtained from the previous step is quite suitable parameter for calculating earthquake magnitude. saradjian and akhoondzadeh (2011) showed the relationship between this parameter and the earthquake magnitude that can be extracted from table (1). although the selection of dx and its correspondence with magnitude ranges was investigated previously in another study (saradjian and akhoondzadeh, 2011), but it was investigated again in this study and verified. table 1. earthquake magnitude estimation (saradjian and akhoondzadeh, 2011) dx value earthquake magnitude dx ≤ 1 mw ≤ 6 1 < dx ≤ 2 6 < mw ≤ 7 2 < dx ≤ 3 7 < mw ≤ 8 3 < dx 8 < mw also, according to the day when the anomaly is observed, an impending earthquake’s approximate date can be estimated. based on observations so far, as an average, a 15-day interval in ionospheric and atmospheric precursors, and as an average, a 16-day interval in thermal precursors from the anomaly observation till earthquake day is considered (saradjian and akhoondzadeh, 2011). although the periods of 15 or 16 days were estimated previously by other researchers (ouzounov and freund, 2004; pulinets et al, 2006; jiao et al., 2018; saradjian and akhoondzadeh, 2011), but it was investigated again in this study and verified. finalizing parameters estimation method after the earthquake parameters (i.e. date and magnitude) are estimated through various precursors using the preliminarily earthquake parameters estimation, the final value of parameters of the earthquake can be estimated by combining their results using mse method. in mse method, the date and magnitude of an earthquake is calculated by eq. (4) (wackerly et. al., 2008): mse = v + ( x − m )2 (4) where v and m are variance and median for the predicted upper and lower limits for the date and magnitude of the earthquake are calculated separately for all precursors. finally, any parameter that has a minimum value of mse is considered as the final parameter of the earthquake. this equation is applied when mse is used as an estimator. since input to mse is not a physical quantity the difference in the type of precursors has no effect. 4. case studies and results four major earthquakes with magnitude mw > 6 have been investigated in this study. these earthquakes occurred in samoa islands, sichuan (china), kermanshah and bam (iran). the characteristics of these earthquakes have been presented in table 2. the ionospheric parameters obtained from the demete‍r has been studied and analysed over the relevant periods of time for each earthquake for areas selected according to dobrovolsky formula r = 100.43m which relates the size of affected area to the magnitude of the earthquake (dobrovolsky et al., 1979). the rest of the time series data for other precursors have been acquired for the relevant periods of time for areas of about 5×5 degrees in size around each epicentre. all time series data were provided for the period of 100 days and in some cases more than 100 days. table 2. characteristics of earthquakes investigated in this study (http://earthquake.usgs.gov/) case study date time (utc) latitude longitude mw depth (km) kermanshah, iran 2017-11-12 18:18:17 34.91 e 45.96 n 7.3 19 samoa islands 2009-09-29 17:48:10 15.59 w 172.10 s 8.1 18 sichuan, china 2008-05-12 06:28:01 31.00 e 103.32 n 7.9 19 bam, iran 2003-12-26 01:56:52 29.00 e 58.31 n 6.6 10 http://earthquake.usgs.gov/ 450 mohammad mahdi khoshgoftar, mohammad reza saradjian kermanshah earthquake in the case study of kermanshah earthquake, all-time series data were provided for the period of 1 august to 27 november 2017. the tec anomaly associated with kermanshah earthquake was observed on november 4 (table 3). by observing this anomaly, it can be concluded that an earthquake with magnitude ranging from 7 to 8 mw between november 5 and 19, 2017 would have happened (figure 1d). figure 1. results of tec analysis using median for kermanshah earthquake. the earthquake time is indicated by an asterisk. (a) tec variations, (b) dtec, (c) detected anomalies without considering the solar-magnetic indices, (d) detected anomalies with considering the solar-magnetic indices. by investigating the aqua night-time lst (°c) anomalies, the maximum value of 3.04°c october 28 indicates an earthquake between october 29 and november 13 with mw>8 would have happened (figure 2a). the anomaly observed in the aod data obtained from the aqua sensor, with a maximum of 452.78% on october 30, 2017, indicates an impending earthquake with mw>8 between october 31 and november 15 (figure 2b). also, the estimated earthquake magnitude for the observed anomaly on november 2, 2017 related to the aod precursor obtained from the terra sensor would be greater than 8mw (figure 2c). by investigating the changes in slhf, a sharp increase of 126.97%, 213.64% have been observed on 3 and 4 october, respectively (figure 2(d)). due to these anomalies, it can be predicted that an earthquake with magnitude more than 8 mw will occur in the region. another sharp increase of 204.36% has been observed on 30 october, it can be indicating an earthquake with mw>8 between october 31 and november 15 would have happened. the results obtained by svr and random forest methods can be found in tables 4 and 5, respectively. in the case study of kermanshah, the earthquake parameters deduced based on median from the different precursors using the mse method indicate that an earthquake would occur between november 1 and 16 with a magnitude more than 8 mw (table 15). also by using mse method for the obtained results from both svr and rf methods, the predicted magnitude of earthquake will be, respectively, from 7 to 8 mw between november 1 and 16, 2017 and from 7 to 8 mw between october 29 and november 16, 2017 (table 15). table 3. list of anomalies obtained from different precursors of kermanshah earthquake using median method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) tec 4 nov (utc=04:00) 5 nov-19 nov 2.35 7<mw<8 lst aqua (night time) 2 nov 3 nov-18 nov 2.92 7<mw<8 28 oct 29 oct-13 nov 3.04 mw>8 aerosol optical depth (aqua) 2 nov 3 nov-18 nov 2.95 7<mw<8 30 oct 31 oct-15 nov 12.16 mw>8 aerosol optical depth (terra) 2 nov 3 nov-18 nov 3.96 mw>8 slhf 7 nov 8 nov-23 nov 2.70 7<mw<8 6 nov 7 nov-22 nov 3.05 mw>8 5 nov 6 nov-21 nov 3.61 mw>8 4 nov 5 nov-20 nov 5.10 mw>8 3 nov 4 nov-19 nov 4.15 mw>8 30 oct 31 oct-15 nov 6.70 mw>8 5 oct 6 oct-21 oct 2.68 7<mw<8 4 oct 5 oct-20 oct 6.90 mw>8 3 oct 4 oct-19 oct 4.99 mw>8 table 4. list of anomalies obtained from different precursors of kermanshah earthquake using svr method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) tec 27 oct (utc=22:00) 28 oct-11 nov 2.06 7<mw<8 27 oct (utc=12:00) 28 oct-11 nov 2.11 7<mw<8 27 oct (utc=08:00) 28 oct-11 nov 2.01 7<mw<8 lst aqua (night time) 2 nov 3 nov-18 nov 2.39 7<mw<8 aerosol optical depth (aqua) 31 oct 1 nov-16 nov 5.82 mw>8 aerosol optical depth (terra) 2 nov 3 nov-18 nov 3.94 mw>8 slhf 3 nov 4 nov-19 nov 2.06 7<mw<8 31 oct 1 nov-16 nov -2.24 7<mw<8 30 oct 31 oct-15 nov 3.11 mw>8 451estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data table 5. list of anomalies obtained from different precursors of kermanshah earthquake using random forest method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) tec 28 oct (utc=04:00) 29 oct-12 nov 2.06 7<mw<8 27 oct (utc=22:00) 28 oct-11 nov 2.14 7<mw<8 27 oct (utc=12:00) 28 oct-11 nov 2.87 7<mw<8 27 oct (utc=08:00) 28 oct-11 nov 2.36 7<mw<8 27 oct (utc=04:00) 28 oct-11 nov 2.11 7<mw<8 lst aqua (night time) 2 nov 3 nov-18 nov 2.57 7<mw<8 aerosol optical depth (aqua) 31 oct 1 nov-16 nov 5.81 mw>8 aerosol optical depth (terra) 2 nov 3 nov-18 nov 4.09 mw>8 slhf 30 oct 31 oct-15 nov 2.71 7<mw<8 figure 2. results of (a) night-time lst data (aqua, mo‍dis), (b) aod (aqua, modis), (c) aod (terra, modis), (d) slhf (noaa), analysis using median method for kermanshah earthquake. the earthquake day is displayed as a vertical dotted line. the green horizontal lines indicate the higher and lower bounds. the blue line indicates the median value. samoa earthquake for the samoa earthquake, the time series data of precursors were provided for the period of 1 june to 15 october 2009. tec anomaly fluctuations began on september 18. the estimated magnitude of the earthquake for this anomaly would have more than 8 mw happening between september 19 and october 3. anomalies have also been observed on september 25 and 28, 2009 implying that an earthquake with magnitude greater than mw = 8.0 would be expected to happen (figure 3(d)). the data on the changes in total ion density, electron and ion density and electron temperature recorded by d‍emeter iap and i‍sl sensors for samoa earthquake are given in table 6. the data were recorded when the satellite’s orbits were near the epicentre of the earthquake (i.e. areas selected according to dobrovolsky formula r = 100.43m which relates the extend of affected area to the magnitude of the earthquake). with an increase in total ion density observed around 10:30 local time on 25 september, 2009, it was expected that an earthquake with magnitude between 7 and 8mw would occur between 26 september and 10 october, 2009 (figure 4(a)). also, the changes in electron density showed unusual behaviour. these changes peaked around 10:30 am local time and exceeded 9.77% upper bound on september 25, 2009 (figure 4(b)). the ion density anomaly was observed around 10:30 local time on september 25, 2009. for dx = 2.61, the magnitude of the impending earthquake is estimated to be between 7 and 8mw that would have happened between september 26 and october 10, 2009 (figure 4(c)). an unusual increase in total ion density was observed around 22:30 local time on september 24, 2009 (figure 4d). for this anomaly, an earthquake with magnitude greater than 8mw would be expected to happen between september 25 and october 9. by investigating changes in sst, there was sharp increase of 8.03% on september 6, 2009 (figure 4(g)). the irregularity seen on september 6 indicates that an earthquake with magnitude between 7 to 8 would have occurred between september 7 and 22. the aod obtained from the aqua sensor shows a maximum anomaly (dx = 2.96) on september 21, 2009 indicating that an earthquake with magnitude ranging from 7 to 8 mw would have happened between september 22 and october 7, 2009 (figure 4h). the characteristics of detected anomalies using svr and random forest methods are shown in table 7 and 8, respectively. in order to finalize each earthquake’s date and magnitude parameters using the parameters obtained from different precursors, the mse fusion method has been applied. in the case study of samoa, according to the mse method, it was predicted that an earthquake with magnitude between 7 and 8 mw would occur between september 25 and october 9, 2009 indicated by both median and svr results (table 15). based on random forest method, the parameters would happen between september 20 and october 4, 2009, with magnitude between 7 and 8 mw. 452 mohammad mahdi khoshgoftar, mohammad reza saradjian figure 3. results of tec analysis using median for samoa earthquake. the earthquake time is indicated by an asterisk. (a) tec variations, (b) dt‍ec, (c) detected anomalies without considering the solar-magnetic indices, (d) detected anomalies with considering the solar-magnetic indices. table 6. list of anomalies obtained from different precursors of the samoa earthquake using median metho precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) total ion density (day time) 25 sep 26 sep-10 oct 2.86 7<mw<8 electron density (day time) 25 sep 26 sep-10 oct 2.63 7<mw<8 ion density (day time) 25 sep 26 sep-10 oct 2.61 7<mw<8 total ion density (night time) 24 sep 25 sep-9 oct 4.58 mw>8 electron density (night time) 24 sep 25 sep-9 oct 3.00 mw>8 ion density (night time) 24 sep 25 sep-9 oct 3.51 mw>8 total ion density (night time) 5 sep 6 sep-20 sep 2.85 7<mw<8 tec 25 sep (utc=24:00) 26 sep-10 oct 3.18 mw>8 25 sep (utc=22:00) 26 sep-10 oct 3.70 mw>8 18 sep (utc=08:00) 19 sep-3 oct 3.10 mw>8 sea surface temperature 6 sep 7 sep-22 sep 2.70 7<mw<8 aerosol optical depth (aqua) 21 sep 22 sep-7 oct 2.96 7<mw<8 table 7. list of anomalies obtained from different precursors of the samoa earthquake using svr method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) total ion density (day time) 25 sep 26 sep-10 oct 2.76 7<mw<8 electron density (day time) 25 sep 26 sep-10 oct 2.25 7<mw<8 ion density (day time) 25 sep 26 sep-10 oct 2.20 7<mw<8 total ion density (night time) 24 sep 25 sep-9 oct 2.57 7<mw<8 electron density (night time) 24 sep 25 sep-9 oct 2.53 7<mw<8 ion density (night time) 24 sep 25 sep-9 oct 2.54 7<mw<8 tec 28 sep (utc=14:00) 29 sep-13 oct -2.53 7<mw<8 20 sep (utc=16:00) 21 sep-5 oct -2.59 7<mw<8 17 sep (utc=22:00) 18 sep-2 oct 2.93 7<mw<8 13 sep (utc=14:00) 14 sep-28 sep 2.51 7<mw<8 7 sep (utc=08:00) 8 sep-22 sep 2.80 7<mw<8 sea surface temperature 6 sep 7 sep-22 sep 3.80 mw>8 aerosol optical depth (aqua) 21 sep 22 sep-7 oct 3.09 mw>8 453estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data table 8. list of anomalies obtained from different precursors of the samoa earthquake using random forest method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) total ion density (day time) 25 sep 26 sep-10 oct 2.60 7<mw<8 electron density (day time) 25 sep 26 sep-10 oct 2.35 7<mw<8 ion density (day time) 25 sep 26 sep-10 oct 2.07 7<mw<8 ion density (night time) 24 sep 25 sep-9 oct 2.72 7<mw<8 electron density (night time) 24 sep 25 sep-9 oct 2.65 7<mw<8 total ion density (day time) 24 sep 25 sep-9 oct 2.52 7<mw<8 t‍ec 19 sep (utc=14:00) 20 sep-4 oct 2.63 7<mw<8 17 sep (utc=22:00) 18 sep-2 oct 2.81 7<mw<8 17 sep (utc=16:00) 18 sep-2 oct -2.54 7<mw<8 13 sep (utc=12:00) 14 sep-28sep 2.71 7<mw<8 7 sep (utc=10:00) 8 sep-22 sep 2.54 7<mw<8 7 sep (utc=08:00) 8 sep-22 sep 2.70 7<mw<8 sea surface temperature 9 sep 10 sep-25 sep -2.75 7<mw<8 6 sep 7 sep-22 sep 3.20 mw>8 aerosol optical depth (a‍qua) 21 sep 22 sep-7 oct 3.52 mw>8 figure 4. results of (a) daytime total ion density data (iap, demeter), (b) daytime electron density data (isl, demeter), (c) daytime ion density data (isl, demeter), (d) night-time total density data (iap, demeter), (e) night-time electron density data (isl, demeter), (f) night-time ion density data (isl, demeter), (g) sst (noaa), (h) aod data (aqua, modis), analysis using median method for samoa earthquake. 454 mohammad mahdi khoshgoftar, mohammad reza saradjian sichuan earthquake in the case study of sichuan earthquake, all time series data were provided for the period of 1 february to 27 may 2008. some intense anomalies related to tec data have been observed on april 5 (2 utc), and may 9 (14 utc) (figure 5(d)). these strong anomalies indicate an impending earthquake with magnitude between 7 and 8 mw. the variations of the various parameters extracted from the demeter experimental data over the sichuan region have been presented in table 9. a sudden and unusual change in total ion density has been observed three days prior to the earthquake around 22:30 local time (figure 6(a)). this means that an earthquake with magnitude ranging from 7 to 8 mw would have occurred between may 3 and 7, 2008. an anomaly has also been observed on may 3, 2008 in electron density around 10:30 local time which implies an impending earthquake as strong as 7 to 8 mw between 4 april and 18 may (figure 6(b)). an unusual decrease have been observed in ion temperature around 10:30 local time on may 3, 2008 (figure 6(c)). the characteristics of other detected anomalies are seen in table 9. the tables 10 and 11 show the result of anomaly detection by svr and random forest methods, respectively. by using the mse method and the combination of the earthquake parameters obtained from different precursors, it was predicted that an earthquake 7<mw<8 would have happened between 8 and 22 may 2008 (table 15). also, the mse method for random forest and svr methods indicate that an earthquake with the magnitude between 7 to 8 mw would have been occurred between 3 and 17 may, 2008 (table 15). figure 5. results of tec analysis using median for sichuan earthquake. the earthquake time is indicated by an asterisk. (a) tec variations, (b) dtec, (c) detected anomalies without considering the solar-magnetic indices, (d) detected anomalies with considering the solar-magnetic indices. tab‍le 9. list of anomalies obtained from different precursors of the sichuan earthquake using median method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) electron density (day time) 3 may 4 may-18 may 2.45 7<mw<8 ion temperature (day time) 3 may 4 may-18 may -1.90 6<mw<7 total ion density (night time) 2 may 3 may-17 may 2.60 7<mw<8 tec 9 may(utc=14:00) 10 may-24 may -2.65 7<mw<8 5 may (utc=02:00) 6 may-20 may 2.74 7<mw<8 lst aqua (night time) 3 may 4 may-19 may -4.89 mw>8 aerosol optical depth (aqua) 9 may 10 may-25 may 2.76 7<mw<8 slhf 11 may 12 may-27 may 2.06 7<mw<8 table 10. list of anomalies obtained from different precursors of the sichuan earthquake using svr method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) electron density (day time) 3 may 4 may-18 may 2.76 7<mw<8 ion temperature (day time) 3 may 4 may-18 may -2.27 7<mw<8 total ion density (night time) 2 may 3 may-17 may 1.86 6<mw<7 electron density (day time) 21 apr 22 apr-6 may 2.55 7<mw<8 tec 11 may(utc=20:00) 12 may-26 may 2.57 7<mw<8 2 may(utc=10:00) 3 may-17 may 2.75 7<mw<8 26 apr (utc=06:00) 27 apr-11 may 2.57 7<mw<8 26 apr (utc=04:00) 27 apr-11 may 2.99 7<mw<8 14 apr (utc=04:00) 15 apr-29 may 3.09 mw>8 11 apr (utc=08:00) 12 apr-26 may 2.97 7<mw<8 11 apr (utc=06:00) 12 apr-26 may 2.73 7<mw<8 lst aqua (night time) 3 may 4 may-19 may -3.97 mw>8 aerosol optical depth (aqua) 9 may 10 may-25 may 2.96 7<mw<8 slhf 18 apr 19 may-4 may 2.46 7<mw<8 455estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data figure 6. results of (a) night-time total ion density data (iap, demeter), (b) daytime electron density data (isl, demeter), (c) daytime ion temperature data (iap, demeter), (d) night-time lst data (aqua, modis), (e) aod (aqua, modis), (f) slhf (noaa), analysis using median method for sichuan earthquake. table 11. list of anomalies obtained from different precursors of the sichuan earthquake using random forest method precursor date of observed anomaly prediction of earthquake date deviation value (dx) prediction of earthquake magnitude (mw) electron density (day time) 3 may 4 may-18 may 2.76 7<mw<8 ion temperature (day time) 3 may 4 may-18 may -2.27 7<mw<8 total ion density (night time) 2 may 3 may-17 may 1.86 6<mw<7 tec 9 may(utc=10:00) 10 may-24 may 3.40 mw>8 3 may (utc=04:00) 4 may-18 may 2.56 7<mw<8 3 may (utc=18:00) 4 may-18 may 2.39 7<mw<8 21 apr (utc=04:00) 22 apr-6 may 3.18 mw>8 lst aqua (night time) 3 may 4 may-19 may -3.99 mw>8 aerosol optical depth (aqua) 9 may 10 may-25 may 2.51 7<mw<8 sl‍hf 11 may 12 may-27 may 2.40 7<mw<8 9 may 10 may-25 may 2.10 7<mw<8 18 apr 19 may-4 may 2.35 7<mw<8 bam earthquake for bam earthquake, the time series data of precursors derived during the period of 1 september 2003 to 10 january 2004.the tec signal fluctuation exceeded the defined bounds on november 8, 2003. this anomaly indicates that an earthquake with the magnitude between 6 and 7 mw would have happened (figure 7(d)). by investigating the aqua night-time lst (°c) anomalies, the minimum value of -1.91°c december 18 indicates an earthquake between december 19 and january 3 with 6<mw<7 would have happened. also, a sudden decrease in temperature was observed in the night-time lst data (aqua sensor) on december 13 and 14, which indicates that an earthquake with magnitude ranging from 7 to 8 mw would have been occurred. by investigating the day-time lst (terra and aqua sensors) fluctuations, extreme decreases 456 mohammad mahdi khoshgoftar, mohammad reza saradjian in temperature on december 12 and 13, 2003. moreover, by observing the maximum values of 2.39 and 2.12 on december 6 and 7, respectively, it can be concluded that an earthquake with magnitude between 7 and 8 mw would have happened between december 7 and 23. unusual change in day time lst data (terra-modis sensor) with the minimum value (dx = -3.71) occurred on november 30. due to this anomaly, an earthquake with mw>8 would have been occurred (figure 8a to 8d). unusual aod changes on december 12, 2003, with values of 4.76 and 3.75 respectively for the aqua and terra sensors indicate an earthquake with a magnitude greater than 8 mw would have been occurred between december 13 and 28 (figure 8(e) & (f)). the characteristics of the slhf detected anomalies are seen in table 12. the anomalies obtained by svr and random forest methods, respectively, are shown in tables 13 and 14. by combining the predicted parameters obtained from different predictors using the mse method, it is predicted that an earthquake would occur between december 13 and 28, 2003, for median anomaly detection method. the combination of anomalies obtained from the svr and random forest by using mse method predicts an earthquake between december 13 and 28, 2003. the magnitude of this earthquake is estimated to be 7<mw<8 for median, svr and random forest methods (table 15). tab‍le 12. list of anomalies obtained from different pre‍cursors of bam earthquake using median method prediction of earthquake magnitude (mw)deviation value (dx)pre‍diction of earthquake datedate of observed anomalyprecursor 6<mw<7-1.999 nov-23 nov8 nov (utc=12:00)tec 6<mw<7-1.9119 dec-3 jan18 declst aqua (night time) 7<mw<8-2.0215 dec-30 dec14 declst aqua (night time) 7<mw<8-2.2214 dec-29 dec13 declst aqua (night time) 7<mw<8-2.0614 dec-29 dec13 declst terra (day time) 7<mw<8-2.4614 dec-29 dec13 declst aqua (day time) 7<mw<8-2.3413 dec-28 dec12 declst terra (day time) 7<mw<8-2.2413 dec-28 dec12 declst terra (night time) mw>8-3.1013 dec-28 dec12 declst aqua (day time) 7<mw<82.128 dec-23 dec7 declst terra (night time) 7<mw<82.098 dec-23 dec7 declst aqua (night time) 7<mw<82.397 dec-22 dec6 declst terra (night time) 7<mw<82.027 dec-22 dec6 declst aqua (night time) 7<mw<82.016 dec-21 dec5 declst aqua (night time) mw>8-3.711 dec-16 dec30 novlst terra (day time) mw>84.7613 dec-28 dec12 decaerosol optical depth (aqua) mw>83.7513 dec-28 dec12 decaerosol optical depth (terra) mw>8-3.0626 dec-10 jan25 dec slhf mw>83.6024 dec-8 jan23 dec mw>8-3.0322 dec-6 jan21 dec 7<mw<8-2.5619 dec-3 jan18 dec mw>85.3016 dec-31 dec15 dec mw>88.3414 dec-29 dec13 dec mw>87.517 dec-22 dec6 dec mw>84.706 dec-21 dec5 dec 7<mw<82.6726 nov-11 dec25 nov mw>83.1025 nov-10 dec24 nov table 13. list of anomalies obtained from different precursors of the bam earthquake using svr method prediction of earthquake magnitude (mw) deviation value (dx)prediction of earthquake datedate of observed anomalyprecursor 7<mw<82.8022 dec-5 jan21 dec (utc=10:00) tec 7<mw<82.7522 dec-5 jan21 dec (utc=08:00) mw>83.2617 dec-1 jan16 declst aqua (day time) 7<mw<8-2.7113 dec-28 dec12 declst aqua (day time) 7<mw<8-2.3413 dec-28 dec12 declst terra (day time) 7<mw<8-2.2113 dec-28 dec12 declst terra (night time) 7<mw<82.256 dec-21 dec5 declst terra (night time) 7<mw<82.166 dec-21 dec5 declst aqua (day time) 7<mw<8-2.214 dec-19 dec3 declst terra (night time) mw>8-3.421 dec-16 dec30 novlst terra (day time) mw>83.78813 dec-28 dec12 decaerosol optical depth (aqua) mw>83.7415 dec-30 dec14 decaerosol optical depth (terra) 7<mw<8-2.7413 dec-28 dec12 dec slhf 7<mw<82.4314 dec-29 dec13 dec 457estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data figure 7. results of tec analysis‍ using median for bam earthquake. the earthquake time is indicated by an asterisk. (a) tec variations, (b) dt‍ec, (c) detected anomalies without considering the solar-magnetic indices, (d) detected anomalies with considering the solar-magnetic in‍dices. tab‍le 14. list of anomalies obtained from different precursors of bam earthquake using random forest method prediction of earthquake magnitude (mw)deviation value (dx)prediction of earthquake datedate of observed anomalyprecursor 6<mw<7-1.9224 dec-7 jan23 dec (utc=02:00) tec 7<mw<82.3622 dec-5 jan21 dec (utc=10:00) 7<mw<82.5322 dec-5 jan21 dec (utc=08:00) 7<mw<82.2713 dec-27 dec12 dec (utc=24:00) 6<mw<71.8113 dec-27 dec12 dec(utc=22:00) 6<mw<71.8011 dec-25 dec10 dec (utc=16:00) 6<mw<7-1.9414 dec-29 dec13 declst terra (day time) 7<mw<8-2.6914 dec-29 dec13 declst aqua (day time) 6<mw<7-1.9213 dec-28 dec12 declst terra (night time) mw>8-3.5613 dec-28 dec12 declst aqua (day time) mw>8-3.601 dec-16 dec30 novlst terra (day time) mw>84.0113 dec-28 dec12 decaerosol optical depth (aqua) mw>83.8313 dec-28 dec12 decaerosol optical depth (terra) 7<mw<82.5514 dec-29 dec13 decslhf 458 mohammad mahdi khoshgoftar, mohammad reza saradjian figure 8. results of (a) daytime lst data (terra, modis), (b) night-time lst data (terra, modis), (c) daytime lst data (aqua, modis), (d) night-time lst data (aqua, modis), (e) aod data (aqua, modis), (f) aod (terra, modis), (g) slhf (noaa), analysis using median method for bam earthquake. tab‍le 15. the registered and estimated earthquake parameters related to the case studies earthquake magnitude (mw)earthquake date case study estimated mse method registered estimated mse meth‍od registered rfsvrmedianrfsvrmedian 7-87-8>87.329 oct-12 nov 20171-16 nov 20171-16 nov 201712 nov 2017kermanshah 7-87-87-88.120 sep-4 oct 200925 sep-9 oct 200925 sep-9 oct 200929 sep 2009samoa 7-87-87-87.93-17 may 20083-17 may 20088-22 may 200812 may 2008sichuan 7-87-87-86.613-28 dec 200313-28 dec 200313-28 dec 200326 dec 2003bam 459estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data 5. conclusions assuming that estimation of earth‍quake parameters using each predictor individually is accompanied by some uncertainties, this study considered integrating the capabilities of different earthquake parameters extracted from some of the same earthquake predictors to better estimate earthquake parameters. by using the combination of precursors in this study, the uncertainties in estimating earthquake parameters have been removed implicitly. to identify the anomalous states that may be associated with impending earthquakes, variations of different earthquake precursors have been analysed for four earthquakes by using median, svr and random forest methods. for each precursor, the date and magnitude were estimated according to the earthquake signals. by integrating the earthquake parameters obtained from all precursors, the final earthquake parameters were estimated more accurately. since different precursors have been used to analyse the final earthquake parameters, therefore, for all earthquakes, the estimated earthquake parameters for each earthquake are close to the actually recorded parameters. this can lead to accurate estimation of earthquake parameters with respect to the number and variety of earthquake precursors. based on the results, it seems that methods such as svr and random forest dealing with nonlinear and complex behaviours of time series are more sensitive than the median method. therefore, it can be accounted that these methods are suitable tools for detecting anomalies in nonlinear time series related to changes in seismic precursors. since various factors can cause unusual behaviour in different ionospheric, atmospheric or lithospheric parameters, more careful studies should be conducted to distinguish the anomalies caused by the daily changes from anomalies due to seismic activities. acknowledgements the authors would like to acknowledge lpc2e for the demeter data, nasa jet propulsion laboratory for the tec data and noaa for the geomagnetic indices, sst and slhf data. also analyses and visualizations used in this study were produced with the giovanni online data system, developed and maintained by the nasa ges disc. references akhoondzadeh, m. 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azimuth angles; geographical coordinates; soldner coordinates; meridian convergence; zero division error; rodamiento esférico riguroso con coordenadas soldner y ángulos azimut en la esfera resumen los sistemas meridianos, llamados coordenadas de soldner (coordenadas paralelas), han gozado de una amplia aplicación en geodesia. en particular, el sistema meridiano constituye una base adecuada para la proyección de gauss-kruger del elipsoide y la esfera. las coordenadas de soldner se pueden usar en la proyección cassini-soldner sin ningún procesamiento. como es conocido, las direcciones de los bordes se muestran como el ángulo acimut en el sistema geográfico de coordenadas y los ángulos rumbo en el sistema de coordenadas de soldner. los ángulos rumbo y acimut se usan frecuentemente en los cálculos geodéticos. estos ángulos dan la dirección de los lados en el sentido del reloj desde una dirección inicial específica. ambos ángulos van de 0 a 360 grados y usualmente se calculan desde la función arctan. el resultado de la función arctan, sin embargo, está en el rango de -90 y +90 grados. además, es necesario analizar el cuartil del ángulo resultado. para computaciones prácticas, los cuadrantes de los arcotangentes se determinan por los signos del numerador y denominador en las fórmulas tangentes. la determinación de los cuartiles de los ángulos se hace en el computador con los bloques si..., entonces..., y fin... se debe tener en cuenta que cada comparación requiere un tiempo de procesamiento por separado. este estudio presenta cómo calcular los ángulos rumbo y acimuth con formulas directas sin necesidad de examinarlas. adicionalente, se dará una propuesta de solución frente a la división por cero errores en el cálculo de los ángulos de rumbo y acimut. palabras clave: ángulos rumbo; ángulos azimuth; coordenadas geográficas; coordenadas soldner; convergencia meridiana; error de división cero. record manuscript received: 25/02/2022 accepted for publication: 27/10/2022 how to cite item: bektaş, s. (2022). rigorous spherical bearing with soldner coordinates and azimuth angles on sphere. earth sciences research journal, 26(3), 205-210. https://doi.org/10.15446/esrj.v26n3.100754 https://doi.org/10.15446/esrj.v26n3.100754 mailto:sbektas@omu.edu.tr https://doi.org/10.15446/esrj.v26n3.100754 206 sebahattin bektaş 1. introduction the reciprocal bearing angles are exactly 180 degrees different on the plane. unlike the plane on the sphere surface, the difference between reciprocal bearing or azimuth angle is approximately 180 degrees. therefore, while a single bearing angle is sufficient in plane calculations, it is desired to find reciprocal bearing or azimuth angles separately on surfaces such as spheres and ellipsoids. classic the bearing calculation or the azimuth calculation formulas only work correctly if the side is in the 1st quarter. if the side is located in the other quarters, the angles of the bearing or azimuth should be examined. for example, if the angle value given by the arctan function is positive, the bearing or azimuth angle is in the 1st or 3rd quarter. if the angle value is negative, the angle is in quarter 2 or 4. in order to find the value of the angle as univocal, the angle values found from the classical formulas need to be examined. bearing or azimuth angles are critical in geodetic calculations. it is impossible to produce correct position information with wrong bearing or azimuth angles. calculation of angles is done with the inverse (second) geodetic problem. while solving the inverse geodetic problem in order to find reciprocal bearing or azimuth angles, first, the angle from 1 to 2 is found. as a result of the analysis, if the 1st bearing or azimuth angle is wrong, it is likely to be wrong in the opposite (from 2 to 1) bearing or azimuth. in the proposed method, reciprocal bearing or azimuth angles are obtained independently and directly. in our literature research, we came found some studies only for determining plane bearing angle (antes, 1990; breuer et al., 1985; gellert et al., 1972) demonstrated direct bearing angle determination in the plane without quarter analysis. hekimoglu (1991) also examined the condition problem and the direct formulas. we could not find any studies that give a direct bearing and azimuth angle on the spherical surface. formulas that give a direct azimuth angle on the sphere surface without examining the quarter with geographical coordinates are shown by the author in (bektas, 2019; bektas, 2021). for the first time, in this study, formulas that give a direct bearing angle without examining the quarter with soldner coordinates on the sphere surface will be discussed. on the other hand, a solution proposal will be given against the division by zero errors in the bearing and azimuth angles calculations. in this paper, firstly, bearing and azimuth angle, meridian system and soldner coordinates on sphere will be mentioned in order to form a basis for the subject. after the relationship between azimuth, bearing angle and meridian convergence is given, direct azimuth angle determination with geographical coordinates, then direct bearing angle determination with soldner coordinates will be discussed. 2. direct azimuth angles with geographical coordinates figure 1. problem on sphere classically, the reciprocal azimuth angles (a12, a21) from two points whose geographic coordinates p1(φ1, λ1), p2(φ2,λ2) are given obtained from the following formulas (fig.1). (1) (2) it is important to remember that these classic formulas only work correctly if the side is in the 1st quarter. if the side is located in the other quarters, the angles of the bearing angle should be examined. the necessary additions should be made for correct angles according to table-1 seen below. table 1. fixed value to add for azimuth angles quadrant ∆φ=φ2−φ1 ∆λ=λ2− λ1 fixed value to add for a12 fixed value to add for a21 1.quadrant + + 2.quadrant + +180o +180o 3.quadrant +180o -180o 4.quadrant + +360o in this proposed method, formulas are given for obtaining the azimuth angle directly without any examination. for direct determination of azimuth by geographic coordinates, we provide the below formulas. in this proposed method, formulas are given to obtain the azimuth angle directly without any examination. the proposed method can calculate direct azimuth angles on the sphere and ellipsoid surface (bektas, 2019). i=sinδ λ ii=tan 2cos 1 − sin 1cosδ iii=tan 1cos 2 − sin 2cosδ a 12 = 2.arctan( −√ 2+ 2) + 180 a 21 = 2.arctan( − +√ 2+ 2) + 180 (3)i=sinδ λ ii=tan 2cos 1 − sin 1cosδ iii=tan 1cos 2 − sin 2cosδ a 12 = 2.arctan( −√ 2+ 2) + 180 a 21 = 2.arctan( − +√ 2+ 2) + 180 (4) i=sinδ λ ii=tan 2cos 1 − sin 1cosδ iii=tan 1cos 2 − sin 2cosδ a 12 = 2.arctan( −√ 2+ 2) + 180 a 21 = 2.arctan( − +√ 2+ 2) + 180 (5) i=sinδ λ ii=tan 2cos 1 − sin 1cosδ iii=tan 1cos 2 − sin 2cosδ a 12 = 2.arctan( −√ 2+ 2) + 180 a 21 = 2.arctan( − +√ 2+ 2) + 180 (6) i=sinδ λ ii=tan 2cos 1 − sin 1cosδ iii=tan 1cos 2 − sin 2cosδ a 12 = 2.arctan( −√ 2+ 2) + 180 a 21 = 2.arctan( − +√ 2+ 2) + 180 (7) the given formulas in equations 6 and 7 do not work if the denominator equals zero. the program will be interrupted because division by zero is done. this problem also arises in the use of classical formulas. in order not to interrupt the program, very small numbers are added to the numerator and denominator at a rate that will not affect the result. if μ1=1e – 15,... μ2 =1e – 27,... are selected, ten digit sensitive bearing angle values are obtained (hekimoğlu, 1991). the division by zero error will be avoided if the following formulas (eq. 8 and eq. 9) are used. a12 = 2.arctan ( +µ1 µ2+ −√ 2+ 2) + 180 a21 = 2.arctan ( +µ1 µ2− +√ 2+ 2) + 180 (8) a12 = 2.arctan ( +µ1 µ2+ −√ 2+ 2) + 180 a21 = 2.arctan ( +µ1 µ2− +√ 2+ 2) + 180 (9) 207rigorous spherical bearing with soldner coordinates and azimuth angles on sphere 3. direct bearing angles with soldner coordinates in meridian system on sphere this meridian system, as the name suggests, is based on the meridian. the meridian that defines the system is called the prime meridian, and a meridian near the study area is taken as the prime meridian (heck, 2003).osoldner coordinates system, have found wide application in geodesy. in particular, the soldner coordinates system constitutes a suitable base for projection of the ellipsoid and the sphere. soldner coordinates can be used in cassini-soldner projection without any processing. in this system, the x-axis is the selected prime meridian (figure 2). the coordinates of a point p1 on the sphere in the meridian system are defined as follows. the spherical distance (arcs of a great circle) from the p1 point to the prime meridian (p1f1) is the y value of the p1 point (y1 = p1f1). the distance of the perpendicular point f1 to the starting point po on the prime meridian also gives the x coordinate (x1 = pof1). the starting point of po can also be taken on the equator. figure 2. meridian system and soldner coordinates (bektas, 2021) these sides used in the calculations on the sphere s, y1, x1, y2, x2 in figure 2 must be arcs of a great circle. point o in figure 2 becomes the pole of the prime meridian circle. so it is always of1= of2 = opo = π/2. in the soldner coordinate system, the reference direction of the bearing angles of the sides is the parallel y=constant circle drawn to the prime meridian at that point (figure-2). the prime meridian determines the soldner system. each meridian system creates a separate coordinate system. the ordinates of the points (y) to the left of the prime meridian are negative. likewise, the abscissas of points (x) below the starting point take negative values. the azimuth angle is used for the direction values of the sides in main problem solutions with geographical coordinates. the azimuth angle (a12, a21) is the angle from the north wing of the meridian at a point to the clockwise side (fig. 2). the bearing angle is used for the direction values of the sides in main problem solutions with soldner coordinates meridian system. the bearing angle (α12, α21) is from the north wing of the parallel to the prime meridian at a point to the clockwise side (fig. 2). figure 3. azimuth angle, bearing angle, meridian convergence (bektas, 2021) there is a meridian convergence ( γ ) between bearing and azimuth angle (fig. 3). meridian convergence can be easily calculated from the geographic or soldner coordinates of the point. if the bearing angle (α) is given, it is converted to the azimuth angle by using the meridian convergence at that point a = α + γ (10) for example, γ1 meridian convergence on the p1 point tan γ1 = sin φ1 tan( λ1 -λprime) from geographic coordinates (11) tan 1 = 1 1 𝛾𝛾 from soldner coordinates (12) reciprocal angles between two points whose coordinates are given in geodesy are obtained by solving the inverse (second) geodetic problem. classically, the reciprocal bearing angles (α12, α21) from two points whose soldner coordinates p1(y1, x1), p2(y2, x2) are given obtained from the following formulas (fig. 4). solving p1p2o spherical triangle figure 4. inverse geodetic problem in the meridian system on the sphere (bektas, 2021) dy = y2 y1 (13) α12 = arctan ( ) ( 1) ( )− ( 2) ( 1) α21 = arctan ( ) ( 1) ( 2)− ( 2) ( ) (14) α12 = arctan ( ) ( 1) ( )− ( 2) ( 1) α21 = arctan ( ) ( 1) ( 2)− ( 2) ( ) (15) the bearing angle values found from the classical formula need to be examined. according to the quarter where the angle is located, the additions in the table 2 below should be made. table 2. fixed value to add for bearing angles quadrant ∆y=y2− y1 fixed value to add for α12 fixed value to add for α21 1 and 2 + 90o 270o 3 and 4 270o +90o 208 sebahattin bektaş the following values of i, ii and iii are calculated to find the direct bearing angle without examining the sphere surface. dx = x2 x1 (16) = ( 1) ( 2) − ( 1) ( ) = ( ) = ( 2) ( 1) − ( 2) ( ) α12 = 2.arctan ( −√ 2+ 2) + 180 α21 = 2.arctan( − −√ 2+ 2) + 180 (17) = ( 1) ( 2) − ( 1) ( ) = ( ) = ( 2) ( 1) − ( 2) ( ) α12 = 2.arctan ( −√ 2+ 2) + 180 α21 = 2.arctan( − −√ 2+ 2) + 180 (18) = ( 1) ( 2) − ( 1) ( ) = ( ) = ( 2) ( 1) − ( 2) ( ) α12 = 2.arctan ( −√ 2+ 2) + 180 α21 = 2.arctan( − −√ 2+ 2) + 180 (19) = ( 1) ( 2) − ( 1) ( ) = ( ) = ( 2) ( 1) − ( 2) ( ) α12 = 2.arctan ( −√ 2+ 2) + 180 α21 = 2.arctan( − −√ 2+ 2) + 180 (20) = ( 1) ( 2) − ( 1) ( ) = ( ) = ( 2) ( 1) − ( 2) ( ) α12 = 2.arctan ( −√ 2+ 2) + 180 α21 = 2.arctan( − −√ 2+ 2) + 180 (21) the given formulas in equation 20 and equation 21 do not work if the denominator equals zero. the program will be interrupted because division by zero is done. this problem also arises in the use of classical formulas. in order not to interrupt the program, very small numbers are added to the numerator and denominator at a rate that will not affect the result. if μ1=1e – 15,... μ2 =1e – 27 are selected, ten-digit sensitive bearing angle values are obtained. the division by zero error will be avoided if the following formulas equation 22 and equation 23 are used. α12 = 2.arctan ( +µ1 µ2+ −√ 2+ 2) + 180 α21 = 2.arctan( +µ1 µ2− −√ 2+ 2) + 180 (22) α12 = 2.arctan ( +µ1 µ2+ −√ 2+ 2) + 180 α21 = 2.arctan( +µ1 µ2− −√ 2+ 2) + 180 (23) 4. numerical example in order to show the validity of the given formulas, numerical applications were made separately in the east and west of greenwich in both the northern and southern hemispheres. inverse problem was solved between twenty-point pairs in total. the radius of the earth is taken as r=6370000m, the prime meridian is taken as λprime=30o to the east of greenwich and λprime= -30o to the west of greenwich. in numerical applications (y, x) soldner coordinates and (γ) meridian convergences of twenty-point pairs are calculated whose (φ, λ) geographic coordinates are given. then, the bearing (α) and the azimuth angles (a) calculations were made separately with direct formulas. in order to control between the bearing and azimuth angles, the azimuth angle was found by adding the meridian convergence to the bearing angle (aij=αij +γi) between each pair of points. numerical application results can be seen in table 3. table 3. the bearing and azimuth angles calculations from direct formula φ1= 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2= 32.0 λ2= 31.0 y2= 94282.5003 x2= 3558115.1919 γ2= 0.5299579646 a12= 11.9669826036 a21=192.2245420693 α12= 11.7169778439 α21=191.6945841047 a12=α12+γ1= 11.9669826036 φ1= 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2= 29.0 λ2= 32.0 y2= 194466.7324 x2= 3225792.8907 γ2= 0.9699205898 a12=127.0797659802 a21=307.8184614062 α12=126.8297612205 α21=306.8485408164 a12=α12+γ1=127.0797659802 φ1= 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2= 28.0 λ2= 29.0 y2= -98162.7839 x2= 3113371.4602 γ2= -0.4695087290 a12=213.6299774554 a21= 32.9026204737 α12=213.3799726956 α21= 33.3721292026 a12=α12+γ1=213.6299774554 φ1= 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2= 32.0 λ2= 29.0 y2= -94282.5003 x2= 3558115.1919 γ2= -0.5299579646 a12=327.6475721969 a21=146.8748649794 α12=327.3975674372 α21=147.4048229439 a12=α12+γ1=327.6475721969 φ1= 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2= 30.0 λ2= 31.0 y2= 96281.2941 x2= 3335744.3496 γ2= 0.5000380801 a12= 89.8749994050 a21=270.1250005950 α12= 89.6249946453 α21=269.6249625149 a12=α12+γ1= 89.8749994050 φ1= 0.0 λ1= 30.5 y1= 55588.7367 x1= 0.0000 γ1= 0.0000000000 φ2= 0.0 λ2= 32.0 y2= 222354.9467 x2= 0.0000 γ2= 0.0000000000 a12= 90.0000000000 a21=270.0000000000 α12= 90.0000000000 α21=270.0000000000 a12=α12+γ1= 90.0000000000 209rigorous spherical bearing with soldner coordinates and azimuth angles on sphere φ1 = 30.0 λ1= 30.5 y1= 48141.1054 x1= 3335429.2308 γ1= 0.2500047597 φ2 = 29.0 λ2= 30.5 y2= 48618.8595 x2= 3224249.5773 γ2= 0.2424095173 a12=180.0000000000 a21=360.0000000000 α12=179.7499952403 α21=359.7575904827 a12=α12+γ1=180.0000000000 φ1 = 30.0 λ1= 0.0 y1= -2852692.3684 x1= 3745576.5846 γ1=-16.1021137520 φ2 = 29.0 λ2= 0.0 y2= -2883094.7939 x2= 3626800.4993 γ2=-15.6371970759 a12=180.0000000000 a21=360.0000000000 α12=196.1021137520 α21= 15.6371970759 a12=α12+γ1=180.0000000000 φ1= 30.0 λ1= -30.5 y1= -48141.1054 x1= 3335429.2308 γ1= -0.2500047597 φ2= 32.0 λ2= -31.0 y2= -94282.5003 x2= 3558115.1919 γ2= -0.5299579646 a12=348.0330173964 a21=167.7754579307 α12=348.2830221561 α21=168.3054158953 a12=α12+γ1=348.0330173964 φ1= 30.0 λ1= -30.5 y1= -48141.1054 x1= 3335429.2308 γ1= -0.2500047597 φ2= 29.0 λ2= -32.0 y2= -194466.7324 x2= 3225792.8907 γ2= -0.9699205898 a12=232.9202340198 a21= 52.1815385938 α12=233.1702387795 α21= 53.1514591836 a12=α12+γ1=232.9202340198 φ1= 30.0 λ1= -30.5 y1= -48141.1054 x1= 3335429.2308 γ1= -0.2500047597 φ2= 28.0 λ2= -29.0 y2= 98162.7839 x2= 3113371.4602 γ2= 0.4695087290 a12=146.3700225446 a21=327.0973795263 α12=146.6200273044 α21=326.6278707973 a12=α12+γ1=146.3700225446 φ1= 30.0 λ1= -30.5 y1= -48141.1054 x1= 3335429.2308 γ1= -0.2500047597 φ2= 32.0 λ2= -29.0 y2= 94282.5003 x2= 3558115.1919 γ2= 0.5299579646 a12= 32.3524278031 a21=213.1251350206 α12= 32.6024325628 α21=212.5951770561 a12=α12+γ1= 32.3524278031 φ1= -30.0 λ1= 30.5 y1= 48141.1054 x1= -3335429.2308 γ1= -0.2500047597 φ2= -32.0 λ2= 31.0 y2= 94282.5003 x2= -3558115.1919 γ2= -0.5299579646 a12=168.0330173964 a21=347.7754579307 α12=168.2830221561 α21=348.3054158953 a12=α12+ γ1=168.0330173964 φ1= -30.0 λ1= 30.5 y1= 48141.1054 x1= -3335429.2308 γ1= -0.2500047597 φ2= -29.0 λ2= 32.0 y2= 194466.7324 x2= -3225792.8907 γ2= -0.9699205898 a12= 52.9202340198 a21=232.1815385938 α12= 53.1702387795 α21=233.1514591836 a12=α12+ γ1= 52.9202340198 φ1= -30.0 λ1= 30.5 y1= 48141.1054 x1= -3335429.2308 γ1= -0.2500047597 φ2= -28.0 λ2= 29.0 y2= -98162.7839 x2= -3113371.4602 gam2= 0.4695087290 a12=326.3700225446 a21=147.0973795263 α12=326.6200273044 α21=146.6278707973 a12=α12+ γ1=326.3700225446 φ1= -30.0 λ1= 30.5 y1= 48141.1054 x1= -3335429.2308 γ1= -0.2500047597 φ2= -32.0 λ2= 29.0 y2= -94282.5003 x2= -3558115.1919 γ2= 0.5299579646 a12=212.3524278031 a21= 33.1251350206 α12=212.6024325628 α21= 32.5951770561 a12=α12+ γ1=212.3524278031 φ1= -30.0 λ1= -30.5 y1= -48141.1054 x1= -3335429.2308 γ1= 0.2500047597 φ2= -32.0 λ2= -31.0 y2= -94282.5003 x2= -3558115.1919 γ2= 0.5299579646 a12=191.9669826036 a21= 12.2245420693 α12=191.7169778439 α21= 11.6945841047 a12=α12+ γ1=191.9669826036 φ1= -30.0 λ1= -30.5 y1= -48141.1054 x1= -3335429.2308 γ1= 0.2500047597 φ2= -29.0 λ2= -32.0 y2= -194466.7324 x2= -3225792.8907 γ2= 0.9699205898 a12=307.0797659802 a21=127.8184614062 α12=306.8297612205 α21=126.8485408164 a12=α12+ γ1=307.0797659802 φ1= -30.0 λ1= -30.5 y1= -48141.1054 x1= -3335429.2308 γ1= 0.2500047597 φ2= -28.0 λ2= -29.0 y2= 98162.7839 x2= -3113371.4602 γ2= -0.4695087290 a12= 33.6299774554 a21=212.9026204737 α12= 33.3799726956 α21=213.3721292026 a12=α12+ γ1= 33.6299774554 φ1= -30.0 λ1= -30.5 y1= -48141.1054 x1= -3335429.2308 γ1= 0.2500047597 φ2= -32.0 λ2= -29.0 y2= 94282.5003 x2= -3558115.1919 γ2= -0.5299579646 a12=147.6475721969 a21=326.8748649794 α12=147.3975674372 α21=327.4048229439 a12=α12+ γ1=147.6475721969 210 sebahattin bektaş 5. results and discussion in this study, formulas are given for how to obtain the bearing and the azimuth angle directly without any examination. the numerical examples also show no loss of precision in the use of direct formulas. it has been observed that the classical formulas (equations 1, 2, 14, and 15) and the formulas we propose (equations 6, 7, 20, and 21) give the same results in the calculations made with ten digits after the decimal point. it is suggested to use equations 8, 9, 22, and 23) to avoid division by zero error. the advantage of the method is that no examination is required. in computer calculations, if…, then..., end…, blocks are not used when direct formulas are used. the if..., then…, end…, blocks reduce the execution speed in computer calculations. the classical formulas and the direct formulas we proposed were compared in terms of processing time. all were coded in matlab. the main characteristics of the hardware were windows 10 pro, intel(r) core(tm) i52450m cpu @ 2.50ghz, 6 gb of ram. the same bearing and the same azimuth angle were calculated 500 million times and the processing times are shown in table 4 below. table 4. processing time bearing angle azimuth angle classic formulas 4.642 sec eq. 14-eq. 15 5.492 sec eq. 1-eq. 2 proposed formulas 4.445 sec eq. 20-eq. 21 4.413 sec eq. 6-eq. 7 it has been observed that the processing time is slightly shorter in the direct formulas we recommend. this is because classical formulas need quadrant comparison. 6. conclusion in this proposed method, formulas are given for how to obtain the bearing and azimuth angle directly without any examination. the need for comparison in classical formulas increases both the processing time and the possibility of error. direct formulas are always more useful. while saving time on the one hand, the probability of making mistakes will also be seriously reduced on the other hand. we hope that the accuracy and non-examination of the direct formulas we proposed will increase the usability of our formulas. the formulas on the sphere surface, which are used to calculate the direct bearing and azimuth, can also be adapted to the ellipsoid surface. for future studies, researchers are advised to find more simple direct formulas. references antes, e. (1990). noch ein beitrag zur berechung des richtungwinkels. verm. wesen, 115. bektaş, s. (2019). direct bearing angles determination on globe. moj civil engineering, 5(4), 78–80. bektaş, s. (2021). jeodezi-1, atlas akademi press. isbn 978-605-7839-87-9. breuer, p., hirle, m., & joeckel, r. (1985). basicprogrammierbare taschenrechner und handcomputer. mitt.des deutscher verein für vermessungswesen-bayern, 37, jahr. sonderheft 1/1985, s.218-219 gellert, w., küstner, h., hellwich, m., & kastner, h. (1972). kleine enzyklopadie: mathematik. verlag harri deustch, frankfurt/ m. und zürich. heck, b. (2003).  rechenverfahren und auswertemodelle der landesvermessung: klassische und moderne methoden. 3. neu bearbaitete und erweiterte auflage. heidelberg: herbert wichmann, 473 pp. hekimoğlu, ş. (1991). açıklık açısını bölge sorgulaması yapmadan hesaplayan yeni formüller ve kondüsyon sorunu. hkmo dergisi, sayı, 70, trabzon theissen, r. (1990). berechnung des richtungswinkels t ohne quadrantenabfrage. z.fg. verm. wesen 115,193-195 _hlk97057942 _hlk97058323 _hlk97058734 _hlk97059392 _hlk97059724 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.107056 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 191 201 m in in g e n g in ee r in g manuscript received: 31/01/2023 accepted for publication: 05/07/2023 study on large-gradient deformation of mining areas based on insar-pek technology hao tan1*, xuexiang yu1, mingfei zhu2, shenshen chi1, chao liu1, hengzhi chen3 1school of geomatics, anhui university of science and technology, huainan 232001, china 2school of earth and environment, anhui university of science and technology, huainan 232001, china 3jiangsu wisdom-spatial geographicinformation technology co., ltd, nanjing 211899, china *corresponding author: 2021217@aust.edu.cn abstract keywords: insar-pek; exponent knothe function; pim; dynamic prediction; large-gradient deformation to solve large-gradient deformation in mining areas unavailable by sar data, a method combining pim exponent knothe (pek) model and insar technology (insar-pek) was proposed to predict the mining-induced subsidence and obtain the large-gradient deformation dynamically. firstly, the maximum subsidence value predicted by the probability integration method was combined with sar data, and the subsidence values in the initial and residual periods were obtained. secondly, three groups of power exponent knothe function parameters were obtained, including csar and ksar based on sar data, clevel_wz, and klevel_wz based on leveling data over a complete observation period, and clevel_bf and klevel_bf based on the elimination of the leveling data in the main period. finally, the predicted values of the three groups of parameters were compared with the measured data, respectively, and the root mean square errors (rmse) were obtained. the engineering example verified that rmses were 28.1mm~91.7mm in the main period and 30.9mm~58.7mm in the whole period estimated by the insar-pek method. the results showed that the subsidence values in the main period were relatively stable by the insar-pek method, and some points’ prediction accuracy was better than that of leveling data. the predicted values obtained by the insar-pek method and those extracted by sar were compared with the measured values. in the main period, the values extracted by sar differed greatly from the measured values, which were false values. however, the predicted values by the insar method were close to the measured values, which can be used to independently get subsidence values in the main period from sar data. palabras clave: insar-pek; función exponent knothe; pim; pronóstico dinámico; deformación de amplia gradiente. estudio sobre deformaciones de amplia gradiente en áreas de minería con base en tecnología insar-pek how to cite this item: tan, h., yu, x., zhu, m., chi, s., & chen, h. (2023). study on large-gradient deformation of mining areas based on insar-pek technology. earth sciences research journal, 27(2), 191201. https://doi.org/10.15446/esrj.v27n2.107056 resumen para resolver la deformación de amplia gradiente en las áreas donde no está disponible la información sar (del inglés synthetic aperture radar), se propuso un método que combina el modelo pim exponent knothe (pek) y tecnología insar (insar-pek) para pronosticar la subsidencia inducida por la operación y obtener dinámicamente la deformación de amplia gradiente. inicialmente, el máximo valor de subsidencia pronosticada por el método de integración de probabilidades se combinó con la información sar y así se obtuvieron los valores de subsidencia en los períodos inicial y residual. seguidamente, se obtuvieron tres grupos de parámetros de poder exponencial de la función knothe que incluyen csar y ksar, basados en información sar; clevel_wz, y klevel_wz, basados en datos de nivelación en un período completo de observación, y clevel_bf y klevel_bf, basados en la eliminación de los datos de nivelación en el período principal. finalmente, se compararon los valores pronosticados de los tres grupos de parámetros con la información medida y así se obtuvieron las raíces del error cuadrático medio. el ejemplo permitió verificar que las raíces del error cuadrático medio se ubican en 28.1mm~91.7mm durante el período principal y 30.9mm~58.7mm en todo el período, de acuerdo con estimaciones a través del método insar-pek. los resultados muestran que los valores de subsidencia en el período principal fueron relativamente estables, según el método insar-pek, y que la precisión en el pronóstico de algunos puntos fue mejor que la de los datos de nivelación. los valores pronosticados en el método insar-pek y aquellos extraídos por información sar se compararon con los valores medidos. en el período principal, los valores tomados con la información sar difieren ampliamente de los valores medidos, los cuales eran valores falsos. sin embargo, los valores pronosticados con el método insar estuvieron cerca de los valores medidos, lo que podría usarse para obtener independientemente los valores de subsidencia en el período principal con información sar. https://doi.org/10.15446/esrj.v27n2.107056 192 hao tan, xuexiang yu, mingfei zhu, shenshen chi, chao liu, hengzhi chen 1. introduction mining-induced surface deformation will cause geological disasters, such as collapse pits, cracks, and subsided steps on the surface, and damage to surface structures, water bodies, and railways, seriously affecting residents’ normal production and life in mining areas. therefore, real-time and highprecision monitoring, prediction, and early warning of surface disasters are significant for safe and efficient production and environmental protection in mining areas (chen et al., 2018; yang et al., 2022). it is a traditional way to monitor the surface deformation of mines by laying out observation lines based on the strike and dip of the working panel (he, yang, & ling, 1991). this method has high measurement accuracy but is costly, time-consuming, and highly labor intensive, with a small and noncontinuous monitoring range (zhu, yang, & li, 2019). at the same time, it uses “point” to an “area” to study the subsidence law of the working panel, which may reduce the monitoring reliability (zebker, & goldstein, 1986). insar technology can realize all-day, large-scale monitoring and is not easily affected by climate. it has been applied to synthesize topographic maps and obtain surface deformation values over time intervals. however, its limitations must be addressed (gabriel, goldstein, & zebker, 1989; wang, zhao, & yin, 2018).). under the influence of atmospheric effects, microwave signals are easily affected by humidity, temperature, electron number, and other factors in the atmosphere during propagation in the air. on the other hand, they may be affected by the incoherence phenomenon, which is mainly divided into time incoherence, geometric incoherence, doppler center incoherence, volume scattering incoherence, and so on (zhang et al., 2023; liu et al., 2017). the maximum deformation could be obtained by formula 1 (chen, & zebker, 2002). (1) where, max,loss refers to the maximum deformation in the line of sight; resolutiong is the bottom resolution unit of the sar sensor; w is the radius of the deformation gradient; and i is the sensor wavelength. insar technology extends the traditional monitoring method to threedimensional monitoring in the mining areas and becomes the research hotspot of deformation monitoring. however, the subsidence in some mining areas has a large scale and large subsidence gradient over time. surface subsidence experiences the initial period, main period, and residual period. in the main subsidence period, the surface subsidence in the satellite revisit period exceeds the maximum deformation gradient monitored by d-insar technology. therefore, using d-insar technology for mining subsidence monitoring leads to the incoherence of images and cannot obtain accurate deformation information. fan et al. (2014; 2015) found that it was difficult to obtain large deformation of mining areas by only using d-insar technology (wang, & zhang, 2017; gao et al., 2009; wang et al., 2019). based on a small amount of surface observation data using d-insar technology and the probability integral method, the relative error of the maximum subsidence point was about 8.3%, which was much better than that of the conventional d-insar technique. yang et al. (2016) utilized los deformation values of sar data to predict probability integration parameters. by analyzing mining subsidence law, diao et al. (2016) proposed a method of combining d-insar with the subsidence prediction model based on pim to carry out real 3d deformation monitoring. in this paper, to solve the above problems, the accuracy of the subsidence values in the initial and residual stages of mining subsidence monitoring will be obtained by using the sar data. by combining the knothe time model function with the probability integral method, the insar-pek model will be constructed to obtain the dynamic predicted parameters of mining subsidence monitoring to calculate the subsidence values in the main subsidence period. the remainder of this paper is organized as follows: the second section illustrates the insar-pek model construction method. the third section carries on the simulation experiment and verifies the model precision. the fourth section verifies the proposed method through an engineering example, followed by the conclusion in the fifth section (sen, 2022; yang et al., 2016; chen et al., 2018; chen, 2015; liu, mei, & sun, 2022). 2. model construction 2.1. probability integral method the probability integral method (pim) is based on random medium theory and is widely used in mining subsidence prediction. it applies granular medium mechanics in discontinuous medium mechanics to study strata and surface movement. from a macro point of view, the law of strata and surface movement caused by mining is similar to that described by the granular medium model as a random medium (chen et al., 2018). the formula of surface subsidence at any point ( , )w x y caused by mining is as follows: (2) where q is subsidence coefficient, r is the main influencing radius, 0 /r h tanb= , h0 is the average mining depth, tanb is the tangent value of the main influence angle, ( ), i ix y planar coordinates for mining unit i , ( ), x y is the coordinate of any surface point. 2.2. power exponent knothe function model knothe time function model was proposed by knothe, a polish scholar, in 1952. the model is used to describe the dynamic process of surface subsidence. during the mining of the working panel, the subsidence value is assumed to be 0. the power exponential knothe time function model can be expressed as follows: (3) where w(t) is the subsidence value at a certain time, c is the time influence coefficient related to mining conditions of the working panel and characteristics of overlying strata; k is the fitting parameter of the model. the power exponential knothe function has three parameters to be estimated, namely, the maximum subsidence value wm, and the constant parameters c and k. c and k mainly affect the shape of the function image (yang et al., 2016). suppose wm= 1000mm, as shown in fig 1(a), when c = 0.03, k values are taken as 5, 10, 50, 100, 200 and 400, respectively. it can be seen that the larger the parameter k is, the longer the initial subsidence time is, and the longer the time to reach the maximum subsidence value is, but the shape of the function image is roughly unchanged. as shown in fig 1(b), when k = 70, c values are 0.01, 0.03, 0.05, 0.1, 0.2 and 0.4 respectively. it can be seen that the smaller the parameter c is, the faster the surface subsidence speed is, but the change of c value has a greater impact on the shape of the function image (diao et al., 2016). 2.3. construction of insar-pek model a. subsidence values of insar data chen et al. (2018) achieved large-gradient deformation monitoring by using lidar to repair the hole field of sar data. they proposed that when the coherence coefficient was lower than 0.32, the maximum detectable deformation gradient of d-insar was close to 0. to obtain the interferogram with high coherence coefficient, the coherence threshold of phase unwrapping was set to 0.45 in sar data processing of the working panel, and the interferogram with a “cavity field” was formed. the double track method was used to obtain the los deformation values of the sentinel data, with the assumption that the relative subsidence value between the two scenes of the insar data was =losw∆ [ 2 1losw − , 3 2losw − , 4 3losw − ,l , 42 41losw − ], and the vertical subsidence value of the mining area extracted by the los three-dimensional decomposition method was sarw =[ sar 2 1w − , 3 2sarw − , sar 4 3w − ,l, sar 42 41w − ]. among them, the sar data in the main subsidence period were removed due to the influence of large-gradient strain. b. fitting parameters 1) pim predicted the maximum subsidence value of the working panel according to geological and mining conditions. assuming that the initial 193study on large-gradient deformation of mining areas based on insar-pek technology subsidence value was 0 mm, the initial subsidence values were accumulated to obtain the data of the initial period; 2) pim predicted the final subsidence value, and the data in residual period were obtained by accumulating the final sar data; 3) the power exponent knothe time function model was used for parameter inversion: (1)the first group of parameters _level wzc and _level wzk were obtained from the complete leveling observation values; (2)the parameters _level bfc and _level bfk were obtained from the leveling data excluding the data in the main settlement period; (3)the parameters sarc and sark of sar data were obtained. 4) the rmses were calculated by comparing the predicted subsidence values of the main subsidence period and the subsidence values of the whole observation period with the leveling data. c. accuracy of parameters based on the calculated parameters, the subsidence values of each period were estimated and compared with the measured values at the same time, and the fitting accuracy of the three parameters was analyzed. the specific method flow is shown in figure 2. 3. simulation experiment according to the actual subsidence of the mining area’s working panel, the surface movement duration was 597 days until jan. 7, 2019. the maximum subsidence of the dip line was 1770.9mm, and the maximum subsidence along the strike was 1843.3 mm. the maximum subsidence values of 700 mm, 1700mm and 1800mm were taken to conduct simulation experiments to analyze the function characteristics. the parameters of the simulation experiment are shown in table 1. taking the simulation experiment with the maximum subsidence value of 1800 mm as an example, the subsidence duration was set as 600 days. taking 30 days as an observation period, 20 observation points were selected, and the subsidence values in the main period were eliminated. the observation points from the 5th (121 days) to the 8th (211 days) were eliminated. as shown in figure 3(a), the blue line is the simulated subsidence value point, and the black point is the corresponding predicted subsidence value point. the rmse was 0.0265 mm, accounting for 0.0015% of the maximum subsidence value. as shown in figure 3(b), to get better simulation effects, random noise of -10mm ~10mm was added to the simulated subsidence value. then, 20 observation points were selected for 30 days, and the subsidence values of corresponding points were estimated with the rmse of 6.1345mm , accounting for 0.3408% of the maximum subsidence. through the simulation experiment, it could be seen that the subsidence in the main period had little effect on the parametric inversion of the knothe function, while the data of the initial period and the residual period had greater effects on the parameter inversion of the power exponential knothe time function. through the comparison of the three groups of experiments, it could be concluded that the parameter inversion could be carried out by using the fusion pim to estimate the maximum value of the subsidence basin. relatively reliable subsidence values of the main period could be obtained. 4. engineering project 4.1. introduction to the experimental area and sar data the experimental area is located in a coal mining panel in huainan city, anhui province. the mining of the working panel began on mar. 26, 2017, and finished on dec. 25, 2017. the hydraulic support was shipped out by the end of jan. 2018. actual stoping length of the working panel was about 1528 m (including 8m width of open cut) and width 251m (including transport and track paths).the mining lasted for 275 days, the average mining speed was 5.56m/d, and the coal output was 1.428 million tons. the thickness of the working panel was 1.0 ~ 4.2m, with an average of 2.8 m and an average mining height of 2.9m. the dip angle was 0 ~ 6°with an average of 3°, belonging to a nearly horizontal coal seam. the buried depth of the coal seam was 618 ~ 723 m, with an average of 668 m. the floor elevation of the coal seam was -564.1 ~ -712.8m. the cenozoic loose bed was about 420 m thick and belonged to the super thick loose bed. the surface above the working panel was flat with farmlands, roads and ditches, and the elevation was between 18.0m and 23.0m, with an average of 21.5m. the surface movement lasted for 597 days. the initial period of surface movement was short, about 7 days, the active period was 123 days, and the recession period was 467 days. the subsidence in the active stage accounted for 92.2% of the total subsidence. the image data were sentinel-1a data, c-band fully-polarized sar image, and the data modes were sw, iw and ew. the experiment adopted iw polarization mode, with a resolution of 20 m and a standard width of 250 km. the revisit period was 12 days, and the revisit period among some images was 24 days. the subsidence values in initial period and residual settlement period were obtained from 41-scene sentinel-1a data from may 8, 2017 to jan. 10, 2019. 4.2. subsidence prediction by pim due to the thick loose layer in the working panel, pim was used to predict the subsidence coefficient q =1.1, in which the horizontal movement coefficient b =0.32, which affected the propagation angle 86q = ° . the tangent of the main influence angle 1.70tg = . the expected subsidence is figure 1. effects of parameters k and c on the shape of the knothe function 194 hao tan, xuexiang yu, mingfei zhu, shenshen chi, chao liu, hengzhi chen shown in fig. 4. the maximum subsidence value was 1882.46 mm. the black rectangular box is the working panel range, and the black line is the mining progress and the corresponding time. the mining of the working panel, was stopped on mar. 26, 2017, while the first observation was made on may 4, 2017. the first observation points along the strike line were located near the drawdown on july 11, 2017. six research points were selected in the research area, namely, three strike points ml03, ml06 and ml11, and three dip points ms30, ms33 and ms36, among which ml03 was at the intersection of strike and dip. 4.3. data pre-processing 4.3.1 sar data extraction the double-track method was used to process the images, and the los deformation values were extracted and converted into vertical settlement values by equation (4). as shown in fig. 5, the blank area is the empty field formed by figure 2. flow chart of the experiment table 1. rmses of the simulation experiment wm/mm c k c´ k´ noise rmse/mm percentage of maximum value /% 1800 mm 0.029 70 0.0290 70.0012 none 0.0265 0.0015% 1800 mm 0.029 70 0.0293 72.4772 -10 mm ~10 mm 6.1345 0.3408% 1700 mm 0.029 65 0.0290 65.0370 none 0.0371 0.0022% 1700 mm 0.029 65 0.0295 70.1285 -10 mm ~10 mm 5.7108 0.3359% 700 mm 0.024 14 0.0240 14.0015 none 0.0042 0.0006% 700 mm 0.024 14 0.0241 14.3639 -10 mm ~10 mm 4.9971 0.7139% the unwrapping errors in the processing of images. the working panel advanced from northwest to southeast, and the displacement area expanded to southeast. due to large-gradient subsidence during the two periods, some images formed void fields. the mining of the working panel was finished on dec. 25, 2017, and the images of february 8, 2018 solstice and february 20, 2018 still subsided near the stoping line, and then tended to be stable with slight subsidence, which was within the measurable range of insar data. the first leveling observation was conducted on may 4, 2017. the first sar data were extracted between may 8, 2017 and may 20, 2017. the last leveling observation was carried out on jan. 7,2019, the last sar data were extracted between jan. 10, 2019 and dec. 29,2018, =losw∆ [ 2 1losw − , 3 2losw − , 4 3losw − ,l, ( 1)los n nw − − ]. assuming that the initial subsidence value was 0 mm, the subsidence values were continuously accumulated until the false value. the subsidence values predicted by pim were taken as the final subsidence values, and they were accumulated forward from the final sar data to the false value. taking point ml03 as an example, the data of this point were accumulated from may 195study on large-gradient deformation of mining areas based on insar-pek technology figure 3. simulation data were used to verify the accuracy of the mode 196 hao tan, xuexiang yu, mingfei zhu, shenshen chi, chao liu, hengzhi chen figure 4. subsidence prediction of the working panel by pim figure 5. sar images processing by dual-track method 197study on large-gradient deformation of mining areas based on insar-pek technology 8, 2017 to july 19, 2017, based on the default initial value of 0 mm, and then the maximum subsidence value predicted by pim of 1674.3mm was taken as the benchmark, and then accumulated forward from jan. 10, 2019 to nov.4, 2017. the data were available in initial period and residual period, while the data of main period was missing. 4.3.2 interpolation processing of measured data the measured time of leveling is shown in table 2. the discrete monitoring data were interpolated to extract the subsidence values consistent with the time of sar data, ' =levelw [ 1'levelw − , 2'levelw − , 3'levelw − ,l , 'level nw − ]. as shown in fig. 6, ml03, ml06, ml11, ms30, ms33 and ms36 are interpolated data. 4.4. dynamical prediction by the exponential knothe function model two observation lines were arranged along the strike and dip of the working face, with three points on each line. there were 18 monitoring periods. as shown in fig. 7, the data in main subsidence period were obtained by insar-pek method and compared with the measured data. table 3 shows the comparison of rmses of the three groups of tests at each point in the main period and the whole observation period. the minimum rmse obtained by insar-pek method was 28.1273 mm and the maximum was 91.6846 mm in the main settlement period. the minimum rmse was 30.9307 mm and the maximum was 58.7326 mm in the whole period. the maximum and minimum predicted rmses of the measured data were 21.5255 mm and 148.2012 mm in the main period, respectively. they are 28.2700mm and 87.2497mm in the whole period, respectively. the measured data were eliminated in the main period, and the minimum and maximum rmses were 48.6616 mm and 212.6215 mm. while they were 31.6467 mm and 120.5947 mm in the whole period, respectively. the accuracy values of ml03 and ml06 points were slightly lower than the calculated inverse results of the complete measured data, but better than those in main settlement period without the measured data. among them, the rmses obtained by ml11 insar-pek method were better than the predicted results after eliminating the measured data in the main settlement period and the complete measured data. although the predicted accuracy values of ms30, ms33 and ms36 were slightly lower than the calculated reverse results of the complete measured data, they were all better than those in the main settlement period excluding measured data. among them, the prediction accuracy of ms36 in the main subsidence period was slightly higher than that of the complete measured data, and the accuracy of ms33 point was also slightly higher. the reason for the low prediction accuracy of ml11 point may be that there is no encryption monitoring, which can not better reflect the subsidence of the whole time period. however, data can be extracted from sar data in the whole period, and there is still a large amount of data after eliminating false values in the main period, so the predicted accuracy is slightly higher than that of the measured data. as shown in figure 8, by comparing the predicted values obtained by the insar-pek method and the extracted values from sar data with measured values at each point, the extracted values of sar had a large gap with the measured values in the main subsidence period, which were false values. however, the predicted values by the insar-pek method were close to the measured values, which could meet the mining subsidence prediction requirements. 5. conclusion as the main coal-producing area in huainan region of china, ground subsidence is extremely serious, so the effective subsidence monitoring and study of subsidence law can better predict the extent and range of ground subsidence. based on large-gradient deformation values in the mining area obtained by insar-pek method, the following conclusions are drawn: the prediction accuracy by insar-pek method was mainly related to the prediction accuracy of the maximum subsidence value by pim. the closer the maximum subsidence to the measured value, the higher the accuracy will be. the subsidence values were stable and accurate in the initial and residual periods obtained by sar data. the accuracy values of the subsidence obtained by insar-pek method were higher in the main period, and the prediction accuracy values of some points were better than those of the leveling data. in the case of large-gradient deformation of the working panel, compared with the subsidence values extracted by d-insar technology, the subsidence values predicted by insar-pek method were close to the measured values, which could provide a feasible method for sar data to independently obtain subsidence values in the main subsidence period. with the development of uav technology and the maturity of 3d modeling technology, dem data of the first observation and the last observation can be obtained, and errors of the last maximum subsidence value predicted by pim can be avoided. in the future, the experiments will be carried out with the combination of 3d modeling of uav. table 2. leveling schedule observation date cycle length interval from the start date observation date cycle length interval from the start date 2017/05/04 2017/12/22 13 232 2017/05/20 16 16 2018/01/23 32 264 2017/06/12 23 39 2018/03/11 47 311 2017/07/11 29 68 2018/04/26 46 357 2017/08/06 26 94 2018/06/15 50 407 2017/09/09 34 128 2018/07/13 28 435 2017/10/22 43 171 2018/09/03 52 486 2017/11/14 23 194 2018/10/16 43 530 2017/12/09 25 219 2019/01/07 83 613 figure 6. interpolation of ml03, ml06, ml11, ms30, ms33 and ms36 file:///d:/program%2520files%2520(x86)/youdao/dict/8.9.6.0/resultui/html/index.html#/javascript:; 198 hao tan, xuexiang yu, mingfei zhu, shenshen chi, chao liu, hengzhi chen figure 7. predicted subsidence curves by insar-pek method 199study on large-gradient deformation of mining areas based on insar-pek technology figure 8. comparison of subsidence values obtained by three ways 200 hao tan, xuexiang yu, mingfei zhu, shenshen chi, chao liu, hengzhi chen 6. acknowledgments research start-up fund for the introduction of talents of anhui university of science and technology (grant no: 2022yjrc26); key research and development program of anhui province (grant no: 202104a07020014); major science and technology project of anhui province (grant no: 202103a05020026). references chen, b. q. 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(2016). insar-based model parameter estimation of probability integral method and its application for predicting mining-induced horizontal and vertical displacements. ieee transactions on geoscience and remote sensing, 54(8), 4818-4832. doi: 10.1109/tgrs.2016.2551779 table 3. comparison of dynamic predicted results of leveling data and measured data point number wm/mm data in main period rmse in main period/mm rmse in whole period/mm percentage of errors in the main period to the maximum value/% percentage of errors in the whole period to the maximum value /% ml03_level 1678.4 y 21.5255 30.5230 1.2825% 1.8186% ml03_level 1678.4 n 95.3491 46.0643 5.6810% 2.7445% ml03_sar 1674.3 n 59.8407 33.8704 3.5741% 2.0230% ml06_level 1770.0 y 30.9829 31.4632 1.7504% 1.7776% ml06_level 1770.0 n 124.0119 66.5823 7.0063% 3.7617% ml06_sar 1719.1 n 67.7903 40.4405 3.9433% 2.3524% ml11_level 1631.9 y 148.2012 87.2497 9.0815% 5.3465% ml11_level 1631.9 n 212.6215 120.5947 13.0291% 7.3898% ml11_sar 1622.9 n 91.6846 58.7326 5.6494% 3.6190% ms30_level 1770.9 y 41.8182 29.8239 2.3614% 1.6841% ms30_level 1770.9 n 69.8516 47.4794 3.9444% 2.6811% ms30_sar 1800.1 n 67.8406 52.3671 3.7687% 2.9091% ms33_level 1277.5 y 31.3885 35.1577 2.4570% 2.7521% ms33_level 1277.5 n 78.7573 43.0742 6.1650% 3.3718% ms33_sar 1257.4 n 50.8704 30.9307 4.0457% 2.4599% ms36_level 726.0 y 28.4713 28.2700 3.8217% 3.8939% ms36_level 726.0 n 48.6616 31.6467 6.7027% 4.3590% ms36_sar 734.9 n 28.1273 33.7483 3.8274% 4.5922% https://doi.org/10.5194/isprs-annals-iv-3-53-2018 https://doi.org/10.5194/isprs-annals-iv-3-53-2018 https://doi.org/10.1080/01431161.2016.1230284 https://doi.org/10.1080/01431161.2016.1230284 https://doi.org/10.1016/s1003-6326(14)63185-x https://doi.org/10.1029/jb094ib07p09183 https://doi.org/10.1029/jb094ib07p09183 https://doi.org/10.1007/s10064-022-02699-8 https://doi.org/10.1007/s10064-022-02699-8 https://doi.org/10.1080/01431161.2018.1528403 201study on large-gradient deformation of mining areas based on insar-pek technology yang, z., xi, w., yang, z., shi, z., & qian, t. 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(2019). recent progress in retrieving and predicting mining-induced 3d displacement using insar. acta geodaetica et cartographica sinica, 48(2), 135-144. doi: 10.11947/j.agcs.2019.20180188 https://doi.org/10.3390/s22218350 https://doi.org/10.1029/jb091ib05p04993 https://doi.org/10.1029/jb091ib05p04993 https://doi.org/10.1016/j.apenergy.2022.120296 https://doi.org/10.1016/j.apenergy.2022.120296 ole_link24 ole_link25 ole_link8 ole_link9 ole_link10 ole_link13 ole_link16 ole_link15 ole_link6 ole_link18 ole_link7 ole_link19 _ref21015 _ref26122 _ref17387 _ref17534 _ref26200 _ref27334 _ref20982 _ref27389 _ref26164 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.115333 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 421 426 g eo ph y si c s three-dimensional gravity interface inversion based on artificial neural network and discrete cosine transform algorithm huadong song, hui fu*, jifeng qin huanghe science and technology university, zhengzhou 450003, china *corresponding author: 17630076382@hhstu.cn how to cite item: song, h., fu, h., & qin, j. (2024). three-dimensional gravity interface inversion based on artificial neural network and discrete cosine transform algorithm. earth sciences research journal, 28(4), 421-426. https://doi.org/10.15446/esrj.v28n4.115333 record manuscript received: 19/11/2024 accepted for publication: 27/01/2025 abst r ac t this study applies artificial neural networks to three-dimensional gravity density interface inversion. parker’s formula based on fast fourier transform method plays an important role in gravity interface inversion. in the training process of the artificial neural network, random generated underground interface geometries are used as inputs, while the output is the gravity anomaly data calculated by parker’s formula. a large-scale input-output training set is formed for the training process of the artificial neural network. in addition, discrete cosine transform (dft) is introduced to compress and store matrices, which reduces computational memory, decreases computation time, and improves computational efficiency in the training and testing processes of the artificial neural network. a deep learning interface inversion algorithm based on the u-net network model is designed. on the basis of the traditional loss function, a smooth loss term and an overfitting suppression term are added to improve the smoothness and convergence efficiency of the gravity interface inversion results. finally, the inversion prediction is verified through the test sample set to validate the generalization of the established deep learning network model. this paper analyzes the effectiveness and practicality of this method in density interface inversion through theoretical models and actual data experiments. the deep learning interface inversion method based on the improved loss function constraint effectively improves the convergence efficiency and computational stability of density interface inversion. applying this method to synthetic data and actual measured data processing has achieved good results. keywords: neural network; discrete cosine transform; application; 3d gravity palabras clave: red neuronal; transformada discreta de coseno; aplicación; modelo de gravedad 3d inversión de la interfaz gravitacional tridimensional con base en los algoritmos de transformación de redes neuronales artificiales y coseno discreto este estudio aplica redes neuronales artificiales a la interfaz de densidad de la inversión de gravedad tridimensional. la formula de parker basada en la transformada rápida de fourier juega un papel importante en la interfaz de inversión de la gravedad. en el proceso de entrenamiento de las redes neuronales artificiales, las geometrías de interfaz subterráneas generadas aleatoriamente se usaron como registros, mientras que los resultados son la información de anomalías de la gravedad calculadas con la formula de parker. un amplio conjunto de datos de registro y resultado se definieron para el proceso de entrenamiento de la red neuronal artificial. adicionalmente, la transformada de coseno discreta se presenta para comprimir y almacenar matrices, y así se reduce la memoria computacional, se reduce el tiempo de computación y se mejora la eficiencia del equipo en los procesos de entrenamiento y de prueba de la red neuronal artificial. de esta forma se diseñó un algoritmo de aprendizaje profundo para la interfaz de inversión basado en el modelo de red tipo u. con base en la tradicional función de pérdida se adicionaron un término de pérdida suave y un término de sobreajuste de supresión para mejorar la eficiencia de la uniformidad y de la convergencia de los resultados de la interfaz de inversión de la gravedad. finalmente la predicción de la inversión se verificó a través del ejemplo de prueba para validar la generalización del modelo de red de aprendizaje profundo que se estableció. este artículo analiza la efectividad y la practicidad de este método de inversión de la interfaz de densidad a través de modelos teóricos y experimentos con información real. este método de inversión de interfaz de aprendizaje profundo, basada en la restricción de función de perdida mejorada, efectivamente mejora la convergencia de eficiencia y la estabilidad computacional de la inversión de la interfaz de densidad. la aplicación de este método a procesos de información sintética y de información medida ha alcanzado buenos resultados. resu m en https://doi.org/10.15446/esrj.v28n4.115333 mailto:17630076382@hhstu.cn https://doi.org/10.15446/esrj.v28n4.115333 422 huadong song, hui fu, jifeng qin 1. introduction a classic problem in geophysics is the determination of the geometric shape of density interfaces related to gravity anomalies, such as the basement shape of sedimentary basins, the moho interface, and seafloor topography. these studies provide valuable information for understanding deep-sea geological structures, regional tectonics, basin formation, seafloor topography formation processes, and many other geological and geodynamic phenomena. however, estimating the geometric shape of a specific geological structure’s interface using gravity data is an ill-posed and non-unique problem. it requires careful consideration of the selection of input gravity data, pre-processing, inversion parameters (such as average depth and interface density), and other geophysical constraints to stabilize gravity inversion. the bott method (bott, 1960) was initially used to solve this problem. this method is based on iterative solutions and assumes constant density differences. the initial approximate value of sediment thickness corresponding to each gravity reference position is obtained using the flat plate formula. the fitting difference between the observed data and the predicted data is then converted into a correction to the initial model (using the flat plate formula again), and the iteration process stops when the fitting difference drops below a certain threshold. cordell and henderson (1968) modified the iteration step length in the bott method and introduced the ratio between the previously iterated model-generated observed data and calculated data to update the model. silva et al. (2014) suggested accelerating convergence by dividing the residual (i.e., the difference between the observed gravity data and the currently calculated gravity data) by an arbitrarily larger positive value (no longer using the slab formula), and accepting the obtained model when the l2 norm of the residual vector is less than the previous iteration’s norm. many scholars have adopted similar methods, which will not be described in detail here. currently, gravity inversions are primarily based on structured grids, and research on gravity inversions based on unstructured grids has only emerged as a field of study in the last decade or so, with relatively limited literature available for reference. among them, the research achievements of memorial university of newfoundland in canada stand out in this direction. lelièvre et al. (2012) conducted a systematic study on the generation and storage of unstructured grids, as well as forward modeling and inversion constraints based on unstructured grids, and successfully applied them to the problem of gravity inversion. subsequently, sun et al. (2021) also conducted research on gravity inversions based on unstructured grids and discussed the role of depth weighting functions and fuzzy c-means clustering algorithms in inversion. ma et al. (2022) utilized the tilt angle method to conduct local adaptive grid research on gravity inversion based on unstructured grids, improving inversion efficiency by reducing the number of grids. the aforementioned methods are all calculations performed in the spatial domain. however, when the model is complex or there are a large number of observation data, this process is computationally intensive. since the advent of the fast fourier transform, many scholars have tried to apply it to gravity data inversion. parker (1972) provided a forward formula for applying the fast fourier transform in the gravity method, which is characterized by its speed and high accuracy. with the advancement of computers, parker’s method based on the fast fourier transform has become feasible for fast computing of magnetic or gravity fields. oldenburg (1974) proved that parker’s expression can be used to determine the geometric shape of the interface by gravity anomaly data. granser (1987a) rearranged the forward algorithm to obtain the depth of the sedimentary basin basement. this method is similar to the inversion program developed by oldenburg. granser (1987b) also applied another nonlinear frequency-domain inversion method based on schmidt-lichtenstein to the case of a 3d sedimentary basin with uniform density differences. although algorithms based on the fast fourier transform reduce computation time, they require an average depth of the interface and a low-pass filter to achieve convergence in the inversion process (oldenburg, 1974; granser, 1987a; granser, 1987b). based on the parker-oldenburg formula, some authors have developed computer programs to determine the geometric shape of density interfaces from gravity anomaly data. nagendra et al. (1996) developed fortran language program codes based on the parker-oldenburg method and the granser method, respectively, for analyzing two-dimensional gravity anomaly data. gómez-ortiz and agarwal (2005) proposed a matlab language program code for the three-dimensional extension of the parker-oldenburg method to obtain the geometric shape of density interfaces. the linear inversion method is simple and widely used due to its linearization process. however, it is highly dependent on the selection of initial parameters, leading to a decrease in inversion accuracy. therefore, in recent years, non-linear inversion methods have attracted the attention of many geophysicists. artificial neural networks (anns) are one of the most active non-linear inversion methods, as they can approximate a non-linear continuous function with any desired accuracy. anns use the input variables as independent variables, and the output variables as dependent variables, and can replace this function with any high degree of accuracy (van der baan and jutten, 2000). since the late 1980s, artificial neural networks (anns) have been applied to geophysical problems (raiche, 1991) and increasingly used to interpret geophysical data. anns, as a computer processing method, have shown great potential for pattern recognition in geophysical exploration problems, such as log interpretation (wiener et al., 1991), electromagnetic profiling data processing (poulton, 1992; winkler, 1994), ground penetrating radar (poulton & el-fouly, 1991), seismic waveform recognition (dai & macbeth, 1995; ashida, 1996), and more. in recent years, the petroleum industry has been using anns to process measured data. anns have also been applied in the interpretation of gravity and magnetic data. over the past few decades, with the continuous improvement of computer performance, the application of machine learning in gravity exploration has gradually developed into a hot research direction. especially after adler & oktem (2017) and jin et al. (2017) demonstrated that machine learning can solve ill-posed problems, various geophysical inversion methods based on machine learning have emerged like mushrooms after a spring rain. carpenter (2021) was the first to establish a mapping relationship between gravity data and salt structure through machine learning, avoiding the iterative process of conventional gravity inversion; wang et al. (2021) used u-net to complete the three-dimensional inversion of gravity data in the st. nicholas mining area, and proved through comparative experiments that the machine learning inversion results have higher resolution and clearer boundary positions than conventional inversion. li et al. (2022) used a self-supervised deep learning method to establish a new ssgi network. this method incorporates the basic theory of the gravity field into the inversion, optimizing by minimizing the difference between the observed gravity anomalies and the reconstructed gravity anomalies, reducing the uncertainty of inversion. zhang et al. (2022) established a new convolutional neural network-decnet for gravity data inversion, transforming the problem of mapping from two dimensions to three dimensions into four problems of mapping from two dimensions to two dimensions. this method has the advantages of high accuracy, low memory requirements, and fast convergence. this article presents a study on the inversion of 3d gravity interfaces by combining the parker formula with artificial neural networks, and utilizing the discrete cosine transform algorithm. the article begins with a review of the classic parker-oldenburg formula, followed by an explanation of the artificial neural network algorithm process and the principles of the discrete cosine transform algorithm. the effectiveness of the proposed algorithm is then demonstrated through synthetic and measured examples. finally, the article concludes with a summary of the algorithm and suggests possible improvements. 2. methods 2.1 parker-oldenburg formula the parker formula, derived by parker in 1973 and 1974, provides a fourier transform for calculating gravity anomalies caused by a subsurface density distribution. the equation 1 is expressed as follows: [ ] = 2 (−| ⃗ | 0) ∑ ∞ | ⃗ | ! [ℎ( ) ] (1) here, f is the fourier transform operator, g is the universal gravitational constant, ρ is the density contrast on the interface, k→ is the wave vector, h(r→) is the interface depth, and z0 is the average depth of the horizontal interface. oldenburg rearranged the formula in 1974, and through an iterative process, the depth of the undulating interface can be obtained as follows: 423three-dimensional gravity interface inversion based on artificial neural network and discrete cosine transform algorithm [ℎ( )] = − [ ( )]e( ) 2 − ∑ ∞ ! [ℎ ( )] (2) in this inversion process, we need to assume the average depth z0 of the interface and the density contrast ρ. first, we substitute h(x) = 0 into the second term on the right-hand side of equation (2) and calculate the first term on the right-hand side. then, we perform the inverse fourier transform to obtain the first approximate value of h(x). we then substitute this value into the right-hand side of equation (2) and iteratively solve until a suitable solution is obtained. 2.2 artificial neural networks in geophysics, given an underground model and a forward function , the corresponding geophysical data can be represented by the equation 3: d = g(m) (3) the inversion process is to find an optimal model whose forward response data can best fit the observed data dobs. artificial neural networks consist of network units called neurons, which are arranged in layers. generally, there are input layers, hidden layers, and output layers. when the neural network is fully connected, each neuron in the input layer is connected to any neuron in the hidden layer, and each neuron in the hidden layer is also connected to any neuron in the output layer. the connection method is expressed in a mathematical expression of weighted sum, and is subjected to the action of an activation function in the hidden layer. taking the input vector xn as an example, the mathematical expression an m corresponding to the mth neuron in the hidden layer is: = ( h =1 + ) (4) where f represents the activation function, m = 1,2,..., l, the superscript h represents the hidden layer, ωh mi represents the weighting coefficient of the ith neuron in the input layer for the m neuron in the hidden layer, xn i represents the ith component of the input vector xn, and bm represents the bias term of the mth neuron in the hidden layer. the activation function f is a bounded nonlinear function, and the sigmoid function is commonly used: ( ) = 1 1+exp ( ) (5) where controls the rate at which the function value tends to 0.5. the activation function endows the neural network with highly nonlinear expressive power, making it suitable for solving nonlinear problems. similar to equaion (4), the kth neuron on k in the output layer can be represented as: = ∘( ∘ + ) (6) where k = 1,2,..., m, the superscript o represents the output layer, ωo km represents the weighting coefficient of the mth neuron in the hidden layer for the kth neuron in the output layer, and ck represents the bias term of the kth neuron in the output layer. the vector is the expected output of the neural network, and there is an error between it and the actual output of the neural network: = 1 2 ( )2σ σ (7) the training of the neural network is an optimization process with e as the objective function. common optimization algorithms include levenbergmarquardt algorithm, conjugate gradient method, simulated annealing method, and genetic algorithm, etc. 2.3 u-net neural network the u-net neural network model used in this paper is shown in figure 1. the u-net network model extracts detailed features through a down-sampling process and restores dimensions through an up-sampling process. it concatenates shallow background information with deep local data, ensuring that positional and other shallow layer information is well retained until the data output, making the network more sensitive to positional information (ronneberger et al., 2015). additionally, as a fully convolutional neural network, u-net abandons complex fully connected layers and directly outputs through a 1×1 convolutional layer, making the network parameters simpler and easier to train. in the gravity data density interface inversion, there is a clear positional feature between the input and output data. using the u-net neural network can more effectively complete the inversion of the interface undulation depth. figure 1. u-net neural network structure in this paper, adam, which has strong robustness, is adopted as the optimization algorithm, and the minimum square error φmse is chosen as the basic loss function. moreover, since it is assumed that the underground density interface is smooth, a smoothness loss term φsmooth is added to the basic loss function to penalize rough results. at the same time, to address the overfitting issue during training, an l2 norm is introduced to constrain the network parameters. the final expression of the loss function proposed in this paper is: φ( ,ℎpre ) = φmsee (ℎpre ) + φsmooth (ℎpre ) + φ 2 ( ) (8) where φmse =∥ ℎpre − ℎ ∥2 2 φsmooth =∥ ℎpre ∥2 2 φ 2 =∥ ∥2 2 { (9) in the context provided, θ represents the weights and biases of the neural network, while hpre is the interface depth data obtained from the neural network inversion. the h is the desired output interface data, which is generated by the generator and obtained after preprocessing. 2.4 discrete cosine transform discrete cosine transform (dct) is widely used in image compression. dct represents an image as a sum of sine waves with different amplitudes and frequencies. by performing a discrete cosine forward transform on the original object, the transformed coefficients are mainly concentrated in the upper left corner, and most of the remaining coefficients are close to zero. the threshold operation is performed on the transformed dct coefficients, and coefficients 424 huadong song, hui fu, jifeng qin smaller than a certain value are set to zero, which achieves the quantization process in image compression. then, the discrete cosine inverse transform is performed to obtain the compressed image. the 2d dft formula for an input image data matrix a with dimensions m * n is: = ∑ ∑ cos ( ) cos ( ) , 0 1 0 1 (10) the value bpq is the dct coefficient of the image data matrix a. dct is a reversible transform, and its inverse transform (idft) equation is: = ∑ ∑ =0 cos ( ) cos ( ) , 0 1 0 1 (11) where = 1/√ , = 0 2/ , 1 1 { (12) = 1/√ , = 0 2/ , 1 1 { (13) assuming the randomly generated 2d subsurface medium as input data, its large dimension requires a large number of neural network units for training, which results in slow training speed. to solve this problem, we compress its data matrix based on discrete cosine transform to reduce the dimension of the data. for a 2d input data matrix, the energy of the dct coefficients is mainly concentrated in the low-frequency part, and the high-frequency part is close to zero. in this study, we first use only the low-frequency coefficient parts of the input and output for training to obtain an optimal neural network. then, we use this neural network to predict new inputs and obtain the low-frequency coefficients of the new output. finally, we use the inverse discrete cosine transform to reconstruct the full-frequency data. 3. application 3.1 synthetic data during the neural network training process, we used randomly generated underground interfaces to create a training set of 100,000 “interface modelgravity anomaly data” pairs, and then trained an artificial neural network. we used randomly synthesized underground interfaces to verify the correctness of the algorithm proposed in this paper. the average depth of the model is 25 km, with a range of 250 km × 250 km. in this model, the northeast is a large dome with a minimum interface depth of about 16 km, and the south consists of several discontinuous basins with a maximum depth of about 24 km. the density contrast of the interface is 0.4g/cm3, and the average depth of the interface is 20 km. the observation points on the surface are spaced "1 km ×1 km" apart, and the underground medium is divided into a grid of 256 × 256 with a grid spacing of "1 km". a 5% gaussian random error was added during the artificial neural network training process. during the training process, the discrete cosine transform algorithm was used to compress the data matrix. the size of the observation data matrix is known to be 250 × 250, and the size of the underground medium matrix is 256 × 256. figure 2(a) and 2(b) show the discrete cosine transform coefficients of the true depth model and the observation data, respectively. it can be seen from the figures that the energy of the coefficients after the discrete cosine transform is mainly concentrated in the upper left corner (shown in the red box, at approximately position 10 × 10), and most of the other coefficients are close to zero. only the coefficients within the red box were used during the artificial neural network training, compressing the 256 × 256 data matrix into a 10 × 10 data matrix, greatly reducing the computational memory, shortening the calculation time, and improving the efficiency of artificial neural network training. by comparison, we know that training without discrete cosine transform compression takes about 68,000 seconds, while after compression, the neural network training only takes about 2,400 seconds. from figure 3(a) and 3(c), it can be seen that the inversion results of the synthetic model match well with the true model, and both the minimum interface depth of 16 km and the maximum interface depth of 24 km can be well recovered. in addition, as shown in figure 3(b) and 3(d), the gravity anomaly data calculated from the inverted depth interfaces are also consistent with the observed gravity anomaly data. figure 3(e) and 3(f) show the differences in depth interface models and gravity anomaly data, respectively. it can be seen that the differences in the depth interface models are very small, with a range of approximately -1.0 ~ 1.0km, while the differences in the gravity anomaly data are also very small, with a range of approximately -2.5 ~ 1.5 mgal. figure 2. discrete cosine transform coefficient of the random medium model (a)-depth model dct coefficient (b)-gravity data dct coefficient figure 3. the random medium model (a)-true depth model (b)-observed gravity (c)-invert depth model (d)-calculated gravity (e)-difference of depth model (f)-difference of gravity data 425three-dimensional gravity interface inversion based on artificial neural network and discrete cosine transform algorithm 3.2 field data the real-world example comes from the open-source data in the article by gomez-ortiz and agarwal (2005), including a subset of gravity anomaly data and the inverted depth model from the brittany region of france. the area of this region is 300 × 300km, with two discontinuous basins from northwest to southeast and several dome structures around them. the minimum interface depth is about 27.5 km and the maximum interface depth is about 32 km. the density difference between the crust and mantle is 0.4g/cm3, and the average depth of the crust-mantle boundary is 30 km. the observation point interval is 6 km, and the underground medium grid is divided into 112 × 112 with a grid spacing of 6 km. the training process of the artificial neural network designed for this real-world data also used the discrete cosine transform to compress the data matrix. in this real-world example, the size of the observation data matrix is 112 × 112, and the size of the underground medium grid is also 112 × 112. as shown in figure 4(a) and 4(b), the energy of the coefficients after the cosine transform is mainly concentrated in the upper left corner (shown in the red box, at approximately position 15 × 15), and most of the other coefficients are close to zero. after applying the discrete cosine transform algorithm, we compressed the 112 × 112 data matrix into a 15 × 15 data matrix, greatly improving the efficiency of artificial neural network training. after comparison, we find that training without discrete cosine transform (dct) compression takes approximately 24,000 seconds, whereas after compression, the neural network training only requires about 3,600 seconds. figure 4. discrete cosine transform coefficient of the gomez-ortiz and agarwal’s invert depth model (a) and brittany’s observed gravity data (b) figure 5 shows the comparison between the results obtained by applying our algorithm and the results in the article by gomez-ortiz and agarwal (2005). as can be seen from the comparison between figure 5(a) and 5(c), the morphology and numerical values of the inversion results obtained by our algorithm are basically consistent with those in the article by gomez-ortiz and agarwal (2005). from figure 5(b) and 5(d), it can also be seen that the gravity anomaly data calculated from the inversion results obtained by our algorithm also match well with the observed gravity anomaly data. figure 5(e) and 5(f) further demonstrate the practicality of our algorithm. the difference range of the depth model is approximately between -1.5 ~ 2.0 km, while the difference range of the gravity anomaly data is approximately between -3.0 ~ 2.0 mgal. 4. conclusion in this paper, we introduced the artificial neural network algorithm and the discrete cosine transform algorithm into the research of three-dimensional gravity interface inversion. by using the method of random modeling, the underground interface geometry shape is randomly generated as the output, and the gravity anomaly data calculated based on parker formula using fast fourier transform is used as the input, forming a large-scale input-output training set for the training process of the artificial neural network. in order to solve the problem of long computation time and high memory requirements faced by artificial neural network training on large data sets, a data matrix compression algorithm based on cosine transform was introduced, greatly reducing the computation time, reducing the memory requirement, and improving the computation efficiency. the trained neural network was used for the verification of synthetic and real-world data, and satisfactory results were obtained, indicating that the trained artificial neural network has good generalization ability and can be a good method for three-dimensional gravity interface inversion. the experimental results from theoretical models and actual data demonstrate that the density interface inversion method based on the u-net network, which we have established in this paper, has superior inversion capabilities for deep and gentle density interface undulations. for density interface inversion problems characterized by steep interface undulations, further methodological design is required in aspects such as network structure design and the establishment of the sample set. this will pave the way for an intelligent and universal method for the computation of density interfaces. figure 5. 3d moho relief in brittany (france) (a)-gomez-ortiz and agarwal’s invert depth model (b)-brittany’s observed gravity data. 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https://doi.org/10.1007/978-3-319-24574-4_28 https://doi.org/10.1007/978-3-319-24574-4_28 https://doi.org/10.1190/geo2014-0024.1 https://doi.org/10.1190/geo2014-0024.1 https://doi.org/10.3390/app11020722 https://doi.org/10.1190/1.1444797 https://doi.org/10.1007/s11770-021-0909-z https://doi.org/10.1190/1.1888943 https://doi.org/10.1190/geo2021-0744.1 _hlk132811074 we close this edition with important works in ore deposits and geotechnics coming from latin america and turkey. finally we want to thank to the reviewers who have made possible this volume. carlos a. vargas editor earth sciences research journal earth sci. res. sj. vol. 17, no. 2 (december, 2013): 75 85 conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf mohammad amin dezfoolian1, mohammad ali riahi2 and ali kadkhodaie-ilkhchi3 1 young researchers and elite club, science and research branch, islamic azad university, tehran, iran. dezfoolian@rocketmail.com. corresponding author. 2 institute of geophysics, university of tehran, tehran, i. r. iran. mariahi@ut.ac.ir 3 geology department, faculty of natural science, university of tabriz, iran. kadkhodaie_ali@tabrizu.ac.ir seismic prospection this study presents an intelligent model based on probabilistic neural networks (pnn) to produce a quantitative formulation between seismic attributes and hydraulic flow units (hfus). neural networks have been used for the last several years to estimate reservoir properties. however, their application for hydraulic flow unit estimation on a cube of seismic data is an interesting topic for research. the methodology for this application is illustrated using 3d seismic attributes and petrophysical and core data from 6 wells from the kangan and dalan gas reservoirs in the persian gulf basin. the methodology introduced in this study estimates hfus from a large volume of 3d seismic data. this may increase exploration success rates and reduce costs through the application of more reliable output results in hydrocarbon exploration programs. 4 seismic attributes, including acoustic impedance, dominant frequency, amplitude weighted phase and instantaneous phase, are considered as the optimal inputs for predicting hfus from seismic data. the proposed technique is successfully tested in a carbonate sequence of permian-triassic rocks from the studied area. the results of this study demonstrate that there is a good agreement between the core and pnn-derived flow units. the pnn used in this study is successful in modeling flow units from 3d seismic data for which no core data or well log data are available. este estudio presenta un modelo inteligente basado en redes neuronales probabilísticas (pnn) para producir una formulación cuantitativa entre atributos sísmicos y unidades de flujo hidráulico (hfu). las redes neuronales han sido utilizadas durante los últimos años para estimar las propiedades de reserva. sin embargo, su aplicación para estimación de unidades de flujo hidráulico en un cubo de datos sísmicos es un tema importante de investigación. la metodología para esta aplicación está ilustrada a partir de datos tridimensionales y datos petrofísicos y de núcleo tomados en 6 pozos de las reservas de kangan y dalan, de la cuenca del golfo pérsico. la metodología introducida en este estudio estima las hfu de un gran volúmen de datos sísmicos tridimensionales. esto podría incrementar los índices positivos de exploración y reducir los costos a través de una aplicación más confiable en resultados de producción para los programas de exploración en hidrocarbonos. cuatro atributos sísmicos, obstrucción acústica, frecuencia dominante, fase de amplitud media y fase instantánea, son considerados en este trabajo como aportes claves para predecir los datos sísmicos de las hfu. la técnica propuesta ha sido evaluada exitosamente en una secuencia carbonada de rocas del pérmicotriásico tomadas del área de estudio. los resultados de este trabajo demuestran que hay concordancia entre la base de las pnn y las unidades derivadas de flujo. las pnn utilizadas en este estudio son capaces de modelar unidades de flujo de datos sísmicos tridimensionales para los cuales no hay un centro de datos o una secuencia de datos disponible. palabras clave: atributos sísmicos, inversión sísmica, indicador de zona de flujo, índice de calidad de depósito, unidad de flujo hidráulico, redes neuronales probabilísticas. record manuscript received: 17/10/2012 accepted for publication: 02/12/2013 abstract resumen key words: seismic attributes, seismic inversion, flow zone indicator, reservoir quality index, hydraulic flow unit, probabilistic neural networks. 76 77 introduction among the various geophysical techniques available for reservoir characterization, 3d seismic attributes have proven to be among the most useful. one of the greatest strengths of 3d seismic attributes is the dense and regular sampling of data over the region of interest, providing images that accurately represent the areal extent of the features of interest. when seismic amplitude changes associated with the features of interest are not noticeable on vertical sections, horizontal time or horizon slices often yield distinct patterns that are easily recognizable. among the more valuable seismic attributes are those sensitive to reservoir impedance, thickness or geomorphology. although there are a few hundred seismic attributes that are in common use today, here we discuss the application of post-stack attributes for the estimation of hydraulic flow units. acoustic impedance, dominant frequency, amplitude weighted phase and instantaneous phase attributes are used as optimal attributes for the estimation of hydraulic flow units. we demonstrate that the value of using these attributes is useful in mapping the hfus in the persian gulf kangan and dalan supergiant gas reservoirs. because well log data are sparse and seismic data contain low vertical resolution, combining both datasets is an important task for reservoir characterization. reservoir characterization is a conceptual model of a reservoir or oil and gas field that can be constructed from sparse data, such as well data or from low vertical resolution data, such as seismic data or a combination of both datasets. the well logging is sparse in space, but seismic logging has a high resolution in the space direction compared with well logging. usually many seismic attributes are available and can be combined with well data for the estimation of flow units. the last decade has witnessed significant advances in the study and application of expert systems in the petroleum industry. the establishment of the existence of an intelligent formulation between two sets of data (inputs/outputs) has been the main topic of such studies. one such topic that has been of great interest is to characterize how 3d seismic data are related to lithology, rock types, fluid content, porosity, shear wave velocity and other reservoir properties. petrophysical parameters, such as water saturation, porosity and permeability, are important data for hydrocarbon reservoir modeling. previously, several researchers have worked on predicting these parameters from seismic data using statistical methods and intelligent systems (nikravesh et al., 1998; balch et al., 1999; trappe and hellmich, 2000; nikravesh and aminzadeh, 2001; wong and nikravesh, 2001; meldahl et al., 2001; russell et al., 2002; russell et al., 2003; nikravesh and hassibi, 2003; russell, 2004; aristimun and aldana, 2006; chopra and marfurt, 2006; soubotcheva and stewart, 2006). the introduction of petrophysics and petroleum engineering science into the mathematics and computer science area has played a significant role in solving petroleum industry problems. the application of artificial intelligence techniques, such as artificial neural networks, fuzzy logic and genetic algorithms, in reservoir characterization has been considered to be a milestone in the last decade’s improvements in the petroleum industry, which has resulted in a significant decrease in exploration expenses (dezfoolian and sanaee, 2012). generally, geological, petrophysical and seismic data are not clear-cut and are associated with inherent uncertainties. artificial neural networks have become one of the most modern and robust techniques for the analysis of geosciences data. these networks are able to recognize patterns between a set of input and output data (rezaee et al., 2007; kadkhodaie-ilkhchi et al., 2006; kadkhodaie et al., 2009). regional geology in the studied field (fig. 1), gas accumulation is mostly limited to the permian–triassic stratigraphic units. these units are known as the “kangan–dalan formations,” which constitute very extensive natural gas reservoirs in the field and persian gulf area. in addition, this area is composed of a carbonate–evaporate series commonly known as the khuff formation. qatar bahrain kuwait persian gulf gulf of oman south pars united arab emirates north field fig. 1. map of the studied area in the persian gulf, south of iran. the gas-condensate bearing reservoirs of this field belong to the kangan formation of the triassic age and dalan formations of the permian age. these carbonate formations were deposited in a shallow marine environment during a general marine transgression that began in the middle of the permian age and lasted until the early triassic age. the basal section of the dashtak formation, which was formed throughout the lower, middle and upper parts of the triassic period, is composed of argillaceous siltstone, shale and dolomite, with anhydrite interbeds. the remaining middle and upper parts mainly consist of dolomite intercalation. the dashtak formation is known to be a good seal for the kangan-dalan reservoirs. in arabian nomenclature, the kangan and dalan reservoirs are equivalent to the khuff formation. within these reservoirs, there are 4 main producing levels, which from top to bottom are k1, k2, k3 and k4, each separated by anhydritic layers. each of these levels has been divided into several subzones. a massive anhydrite body, known as the nar member, separates k4 from the underlying k5 member that exhibits poor reservoir characteristics. k1 and k2 are kangan’s producing layers. the dolomitic part of k1 is characterized to be mudstone, mudstone to packstone, beige, moderately hard to hard, crystalline to cryptocrystalline, have occasional pelloids, pebbles and shell fragments, have local mudcracks in mudstone, slightly argillaceous in parts, have thin layers of anhydrite and o have good to fair vuggy and intercrystalline porosity. its limestone is defined as mudstone, wackstone, off white to white, cream to light brown, brownish gray, soft, amorphous, chalky, cryptocrystalline to microcrystalline, brittle to crumbly, washable, slightly argillaceous, having traces of re-crystallized calcite and having poor to not visible porosity. it also contains anhydrite, which is white, off white, soft to firm, amorphous, cryptocrystalline and washable. on the other hand, k2 consists of limestone and dolomite. the limestone can be illustrated as mudstone, wackstone, grainstone, white to off white, cream, light gray, light brown, soft to firm, microcrystalline to fine crystalline, dolomitic and anhydritic in parts, and slightly argillaceous. the dolomite section is an alternation of grainstone with oolites and cream packstone, moldic and vuggy fair to good porosity and moderately hard to hard. the second important hydrocarbon bearing formation is the dalan formation, which is composed of the 3 main producing layers of k3, k4 and nar. the k3 and k4 layers mainly consist of anhydrite, dolomite and limestone. the k3 dolomites are gray to dark gray, dark brown, brittle, fine crystalline, slightly argillaceous, hard to moderately hard, locally calcareous, micro-sucrosic, traced with oolites and pelloids and characterized by poor to visible porosity. the k3 anhydrite is characterized as milky white, soft to firm, amorphous, microcrystalline to cryptocrystalline, and is washable. its limestone section is mainly mudstone and wackstone and can be light gray, off white, light grayish brown, soft, amorphous, cryptocrystalline to microcrystalline, chalky in parts, moderately argillaceous and having no visible porosity. on the other hand, the k4 dolomites are thin alternations of mudstone-wackstone with packstone-grainstone, light brown, light brownish gray to gray, moderately hard to hard, cryptocrystalline to microcrystalline, locally crystalline, brittle, sucrosic, traced with pelloids and oolites, locally argillaceous and carbonaceous and characterized by good visible porosity. the limestone part includes mudstone-wackstone and packstone-grainstone and is light brown, light gray, microcrystalline, oolitic, traced with pelloids and characterized by good visible porosity. the anhydrites in k4 are milky white, soft to firm, amorphous, cryptocrystalline and washable. nar consists of anhydrite and dolomite. the anhydrite section can be summarized as white to milky white, soft to firm, amorphous, cryptocrystalline to microcrystalline, massive and washable; the associated dolomite section is light brown to light brownish gray, moderately hard to hard, compact, cryptocrystalline to microcrystalline, sucrosic and characterized by no visual porosity. data used in this article was collected from the drilled wells in the kangan and dalan carbonate formations pertaining to the studied field. hfus classification hfus are defined as correlatable and mappable zones within a reservoir that control fluid flow. each flow unit is characterized by a flow zone indicator (fzi), which can be understood in terms of the relationship between the volume of void space ( ) and the geometric distribution of pore space (quantified as the reservoir quality index, rqi) as follows (amaefule et al., 1993): (1) where and is the effective porosity. (2) rqi can be calculated using the following equations: (3) the fzi can be rearranged in terms of the measurable rqi as given below: (4) where k is permeability in md and is the fractional porosity. rocks with a narrow range of fzi values belong to a single hydraulic unit, i.e., they have similar flow properties (prasad, 2003). to estimate core-derived hfus from 3d seismic attributes, which is the main objective of this study, the fzi data are calculated for 4 cored wells in one of the persian gulf hydrocarbon fields using the available porosity and permeability data. in this paper, data collected from the drilled wells and the 3d seismic data from the kangan (k1 & k2), dalan (k3 & k4) and nar carbonate formations are presented. the formations that fall within the age of the permian-triassic range are known as reservoir formations, which cover large regions of the persian gulf. flow unit classification is carried out on the basis of calculated flow zone indicators. graphical clustering, using the rqi plot, is found to be the simplest analytical method for determining the flow unit numbers, although it is not sufficient to distinguish between different flow units or to estimate their boundaries. to overcome these limitations, hierarchical cluster analysis is performed (matlab user’s guide, 2012). cluster analysis starts by setting the number of flow units equal to the number of samples, then gradually merging the samples with similar fzi values into joint clusters. the sum squared error (sse) in the log fzi calculation from the known primary porosity and permeability is evaluated using classifications with a different number of flow units (clusters) (fig. 2a). the results show that any increase in the number of clusters above 5 does not lead to a significant reduction in the flow units estimation error. the clustering of fzi data is performed on the basis of the logarithm of fzi values because the fzi values calculated from actual field data usually exhibit a log-normal distribution. because the fzi distribution is a superposition of multiple log-normal distributions, a histogram of log fzi values should show n number of normal distributions for n number of hfus. therefore, a histogram analysis is used as an alternative tool for clustering analysis. fig. 2b shows the histogram of a log fzi distribution over the cored intervals. this is another efficient method for identifying the optimal number of flow units. in this regard, 5 hfus are distinguished. this is consistent with the results of cluster analysis, indicating the reliability of the method applied. the flow units identified in the studied reservoir are as follows: hfu 1: log fzi< -0.37 hfu 2: -0.37< log fzi< 0.19 hfu 3: 0.19< log fzi< 0.73 hfu 4: 0.73< log fzi< 1.36 hfu 5: log fzi> 1.36 (a) (b) fig. 2. (a) the sum squared error (sse) in the log fzi calculation from known porosity and permeability, using classifications with different number of hfus. (b) histogram of the calculated flow zone indicator. mohammad amin dezfoolian, mohammad ali riahi and ali kadkhodaie-ilkhchi conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf 78 79 the geological description of the clustered flow units is as follows. hfu1: shaly limestone and shaly dolostone with anhydrite interbeds; mudstone. hfu2: limestone; mudstone to packstone. hfu3: dolostone; mudstone to packstone. hfu4: limestone and dolomitic limestone; grainstone. hfu5: dolostone and limy dolostone; grainstone. porosity-permeability cross-plots for these flow units are shown in fig. 3. this graph demonstrates that the subdivision of the studied reservoir into flow units illustrates the stronger relationships between porosity and permeability within each individual flow unit. the relationship between rqi and is used to show that samples with similar fzi values lie close together on a semilog plot of porosity versus permeability (amaefule et al., 1993). fig. 4 shows a plot of rqi versus for data from all cored intervals. hfu1 shows a poor reservoir quality with low to average permeability and porosity. hfu1 is mainly composed of fine grained, well rounded, moderately sorted sequences with high clay content. hfus 2 and 3 have low to medium reservoir quality. hfu 4 has medium to good reservoir quality. hfu5 is composed of facies with high permeability and porosity values with an excellent reservoir quality. after identification of the flow units, we will attempt to merge them with seismic traces at well locations. the process of converting a 3d seismic cube to reservoir flow units is discussed below. fig. 3. cross-plot showing the porosity-permeability relationship of hfus 1-6 for all of the cored wells. fig. 4. plot of rqi versus for all of the cored intervals. correlation of well logs with seismic data in the first step of this study, well log data are correlated to seismic data. synthetic seismograms are generated for the available wells, including f-105, f-109, f-113, fd112b-108, fd112c-108 and fd112d-108. the acoustic velocities from the sonic logs are multiplied by the bulk density values from density logs to compute the acoustic impedance logs. this impedance is converted to reflectivity, which is then converted from depth to time using a suitable time–depth relationship. finally, the reflectivity in time is convolved with an appropriate wavelet to produce a synthetic seismogram. the depth-to-time conversion of the well logs is accomplished by applying checkshot data, which has been supplied for all wells. it was necessary to create synthetics and to extract the wavelets iteratively for the placement of the log data in time. this depth-to-time process allowed for a comparison of the well logs and their associated tops with the seismic data in time. horizon interpretations and geologic well tops are used as an aid in determining a time–depth relationship for deviated wells. the results of wells-to-seismic tie for wells fd112b-108, fd112c-108 and f-113 are illustrated in fig. 5. as shown in the figure, the correlation between the synthetic seismogram (blue) and composite trace (red) at fd112b-108, fd112c-108 and f-113 is 0.92, 0.98 and 0.79, respectively. post-stack seismic inversion a seismic reflection occurs when there is a change in acoustic impedances in the earth’s layers. post-stack seismic inversion is the process that attempts to remove the seismic source signature (wavelet) effect, which consequently reconstructs the velocity or impedance structure of underground layers from stacked seismic traces. it is assumed that the stack section does not contain the wavelet effect due to its removal by deconvolution. therefore, wavelet side lobes are removed, and the tuning effect is diminished and resolution is increased. as such, a seismic reflection can be considered as the opposite of forward modeling, which involves creating a synthetic seismic section based on a model of the earth. to extract the seismic acous-(a) (b) (c) fig. 5. samples of wells to seismically tie at wells (a) fd112b-108, (b) fd112c-108 and (c) f-113. (a) (b) mohammad amin dezfoolian, mohammad ali riahi and ali kadkhodaie-ilkhchi conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf 80 81 predicting hfus are shown in table 2. as seen in table 2, adding more attributes improves the prediction process. this does not always mean that the added attributes are able to predict the true signal in the target data. the validation error can be considered as a criterion for determining when to stop adding attributes to the input set. the mse (mean square error) between the real and estimated data is calculated. training data are presented to the network during training, and the network is adjusted according to its error. the training set is used to train and create the network. the actual output of the training set data is used to develop the weights in the network. validation data are used to measure network generalization and to halt training when generalization stops improving. the validation set is used to evaluate the accuracy of the newly built network by providing the network with a set of data it has never observed. according to table 2, the first 4 attributes composed of acoustic impedance, dominant frequency, amplitude weighted phase and instantaneous phase are considered to be the optimal input sets for predicting hfus. design of a probabilistic neural network for the estimation of hfus a probabilistic neural network (pnn) is a forward feed network built with 3 layers that can be used for predicting both continuous or discrete (classification) data. it was first proposed by specht (1990) and is based on a distance concept between objects. it is a fast and efficient method to map a set of input data to their outputs. pj is the distance scale factor for each of the input attributes. the only parameter of the pnn that needs to be optimized is the scale factor, pj. in comparison with the other types of neural networks, such as the multi-layer perceptron that requires many parameters to be optimized, a pnn is simple, fast and efficient. the optimal value of pj is obtained when the validation error reaches its minimum value. for optimizing distance scale factor pj, the range parameter is taken between 0.10 and 3.00. the number of pj values to evaluate is set at 25. the optimized values of pj are as follows: acoustic impedance: 0.425; dominant frequency: 0.626; amplitude weighted phase: 0.885; instantaneous phase: 1.894; and global pj: 1.014. the mse of pnn models in the test data is measured as 0.015654. this error magnitude is obtained from the testing well. finally, using the pnn constructed for this research, the 3d seismic data of the studied reservoir are converted to hfus. seismic sections showing the distribution of the pnn estimated hfus for testing data are shown in fig. 8. this figure shows estimated hfus for seismic data. as seen in fig. 8, seismic sections crossing the well location indicate that the reservoir is mainly compo(a) (c) fig. 6. sections from the seismic inversion results crossing well locations (a) fd112b-108, (b) fd112c-108 and (c) f-113. the color scale is acoustic impedance. the p wave log is plotted to show the location of wells. tic impedance as a 3d seismic attribute, a seismic inversion is carried out. a post-stack seismic inversion is the process by which we analyze stacked seismic traces and attempt to reconstruct the velocity or impedance structure of the earth. first, an initial geological inversion model is created for the seismic inversion. an important component for building the model for the seismic inversion is a set of seismic horizons. these horizons are used to guide the interpolation between wells. when the model is built, the seismic data are ready for the inversion process. this process is performed in 2 stages. fig. 6 shows the seismic impedances derived from post-stack seismic inversion crossing the well locations (wells fd112b-108, fd112c-108 and f-113). the inversion result is an attribute that is used for this research. the color scale is acoustic impedance. the seismic volume has been processed through a model-based inversion algorithm to produce an acoustic impedance volume. this volume is used as an attribute in the process. the color scale is shown in acoustic impedance units, with the higher acoustic impedance colored red to purple. the low acoustic impedance values are green to yellow. the acoustic impedance cubes are generated for all of the 3d seismic surveys using a model-based seismic inversion algorithm. the acoustic impedance cube is generated as a strong input to support the estimation of the hfu cube. fig. 7 compares the well log impedance and the seismic impedance in wells fd112b-108, fd112c-108 and f-113. table 1 shows the correlation and error between the well log impedance and the seismic impedance in wells fd112b-108, fd112c-108 and f-113. table 1. correlation and error between well log impedance and seismic impedance in wells fd112b-108, fd112c-108 and f-113. well correlation error fd112b-108 0.950 0.320 fd112c-108 0.969 0.293 f-113 0.962 0.282 selection of optimal seismic attributes generally, the purpose of applying several statistical and intelligent models is to find linear and non-linear relationships and structures between input and output data. for this purpose, there should be a logical relationship between the input and target parameter. in this section, the statistical relationships between the input data (seismic attributes) and output data (log fzi) are investigated through the application of a multi-regression analysis. a multi-regression analysis is a simple and practical method to find the strongest inputs for predicting a target parameter. accordingly, the multi-attributes to be used in the construction of the ann models are chosen based on the trend obtained from regression analysis. the results of a multi-regression analysis for (b) (c) fig. 7. comparison between well log impedance and seismic impedance in wells (a) fd112b-108, (b) fd112c-108 and (c) f-113. mohammad amin dezfoolian, mohammad ali riahi and ali kadkhodaie-ilkhchi conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf table 2. multi-attribute list for predicting the hydraulic flow units. attribute number final attribute training error validation error 1 acoustic impedance 0.212654 0.312354 2 dominant frequency 0.156062 0.225605 3 amplitude weighted phase 0.089112 0.113027 4 instantaneous phase 0.067245 0.098850 5 quadratur e trace 0.065526 0.106321 6 integrated absolute amplitude 0.060235 0.126905 7 filter 35/40–45/50 0.056960 0.185030 8 apparent polarity 0.051005 0.230362 9 second derivative 0.046215 0.344566 82 83 sed of flow units 2, 3 and 4, with good reservoir quality. according to figs. 3 and 4, hfu 5 has the best reservoir quality, hfu 1 shows the worst reservoir quality and hfus 2, 3 and 4 have good reservoir quality. these hfus are shown in fig. 8 separately. fig. 9 shows a data slice through the pnn neural network that results in 1600 ms for studied area. as shown in this figure, the hfus can be observed on the slice image, but the hfus are not more obvious compared to what can be observed on the section image. conclusion in this study, a pnn is used for the classification of hfus from seismic attributes. the methodology introduced in this study is able to estimate hfus from a large volume of 3d seismic data. this may increase exploration success rates and reduce costs through the application of more reliable results in hydrocarbon exploration programs. by analyzing core data and investigating its relation to seismic attributes, it is possible to offer a model for hfu prediction from 3d seismic data. the sum squared error (sse) in the log fzi calculation from the known porosity and permeability data is used as a criterion for the identification of the optimum number of flow units. a histogram analysis is used to evaluate the results using classifications of the different number of flow units (clusters). the statistical relationships between input data (seismic attributes) and output data (hfus) are investigated through the application of multi-regression analyses. accordingly, the multi-attributes to be used in the construction of ann models are chosen based on the trend obtained from regression analyses. the validation error can be considered as a criterion for determining when to stop adding attributes to the input set. 4 attributes, including acoustic impedance, dominant frequency, amplitude weighted phase and instantaneous phase, are considered to be the optimal inputs for predicting hfus. the optimized values of the scale factor (pj) in pnn are defined as follows: acoustic impedance: 0.425; dominant frequency: 0.626; amplitude weighted phase: 0.885; instantaneous phase: 1.894; and global pj: 1.014. among the various attributes used, acoustic impedance shows a strong correlation with log fzi data. the results of this study show that a pnn is successful for converting a 3d volume of seismic data to flow units. neural (a) (c) (b) (d) networks are still among the fastest and most robust tools for reservoir modeling and characterization. the results of this research provide a unique parameter, the flow zone indicator, for delineating the number of layers (hydraulic units) required for the assignment of geological and petrophysical parameters in numerical simulators. acknowledgements the authors wish to thank mr. khorasani and mr. khoshdel from the nioc and mr. soleimani from the institute of geophysics, university of tehran for their useful comments on this work. the second author greatly appreciates the support of the institute of geophysics and mohammad amin dezfoolian, mohammad ali riahi and ali kadkhodaie-ilkhchi conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf 84 85 (e) (f ) fig. 8. sections showing the distribution of pnn estimated hydraulic flow units at the location of the wells (a) f-105, (b) f-109, (c) f-113, (d) fd112b-108, (e) fd112c-108 and (f ) fd112d-108. the studied flow units are characterized sedimentologically as follows: hfu1: shaly limestone dolostone with anhydrite interbeds, hfu2: limy mudstone to packstone, hfu3: dolomitic mudstone to packstone, hfu4: dolomitic limy grainstone and hfu5: dolostone and limy grainstone. the research council of the university of tehran, which enabled him to perform this research. references amaefule, j. o., altunbay, m., tiab, d., kersey d. g., and kedan, d. k. 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(2004). hydraulic flow units resolve reservoir description challenges in a siberian oil field. spe. 87056. trappe, h., hellmich, c., (2000). using neural networks to predict porosity thickness from 3d seismic. first break. 18 (9): 377–384. wong, p.m., nikravesh, m., (2001). introduction: field applications of intelligent computing techniques. journal of petroleum geology. 24 (4): 381–387. fig. 9. a data slice through the pnn neural network results at 1600 ms. the predicted hfus are shown in color. mohammad amin dezfoolian, mohammad ali riahi and ali kadkhodaie-ilkhchi conversion of 3d seismic attributes to reservoir hydraulic flow units using a neural network approach: an example from the kangan and dalan carbonate reservoirs, the world’s largest non-associated gas reservoirs, near the persian gulf issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n1.95405 earth sciences research journal earth sci. res. j. vol. 26, no. 1 (march, 2022): 1 12 g eo ph y si c s structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems johnny vega 1*, sebastián palomino-ángel 1,2, jesús anaya 1 1 facultad de ingeniería, universidad de medellín, medellín-colombia 2 facultad de ingeniería, universidad de san buenaventura, medellín-colombia *corresponding author: javega@udemedellin.edu.coo how to cite item: vega, j., palomino-angel, s., & anaya, j. (2022). structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems. earth sciences research journal, 26(1), 1-12. https://doi.org/10.15446/esrj. v26n1.95405 record manuscript received: 30/04/2021 accepted for publication: 22/02/2022 abstract structural attributes are fundamental biophysical parameters of forests, useful for environmental monitoring and planning. canopy height (ch) is an important input for estimating several biophysical parameters such as aboveground biomass and carbon stocks, and can be associated with forest degradation, deforestation, emission reduction. thus, an accurate ch estimation is a crucial issue in climate change, helping to increase biomass estimation accuracy, and support redd+ initiatives. very-high-resolution (vhr) imagery from unmanned aircraft systems (uas’s) have been studied as a low cost means for ch estimation at local scales, however, estimation the accuracy is a factor that determines its effectiveness. we evaluated the ability of vhr imagery from uas’s to derive structural attributes, specifically tree-crown area and height, in a tropical forest fragment located in the foothills of the andes, in the humid tropical forests of the region known as biogeographic chocó in colombia south america. we used a structure from motion (sfm) approach to derive the forest fragment’s ch, and we applied mean-shift algorithms to identify single tree crowns. we performed accuracy assessment using tree height derived from field campaigns and visual interpretation of vhr imagery. results showed a rmse of 3.6 m of the canopy height model (chm) with a r2 = 0.75; the total accuracy for delineating tree crowns was 73.9%. we found that using vhr imagery collected by uass, specific trees and canopy gaps can be identified in forest fragments, which is an important step to determine forest structure. keywords: uas imagery; crown delineation; forest height; canopy height models (chm); redd+ palabras claves: imágenes uas, delineación de copa, altura del bosque, modelos de altura del dosel (chm), redd+ estimación de atributos estructurales en un fragmento de bosque tropical natural utilizando imágenes de muy alta resolución obtenidas con sistemas aéreos no tripulados los atributos estructurales son parámetros biofísicos fundamentales de un bosque, útiles para el monitoreo y la planificación ecológica y ambiental. la altura del dosel de un árbol es un insumo importante para la estimación de varios parámetros biofísicos como la biomasa aérea y las reservas de carbono, y puede relacionarse con la degradación forestal, la deforestación y la reducción de emisiones. por lo tanto, una estimación precisa de la altura del dosel es un tema crucial en los estudios de cambio climático y las iniciativas redd+. las imágenes vhr de sistemas de aeronaves no tripuladas se han estudiado como un medio de bajo costo para la estimación de la altura del dosel a escalas locales, pero la precisión en su estimación es un factor que determina su utilidad. se evaluó la capacidad de las imágenes vhr de los sistemas de aeronaves no tripuladas para derivar atributos estructurales, específicamente el área y la altura de las copas de los árboles, en un fragmento de bosque tropical natural ubicado en las estribaciones de la cordillera de los andes, en los bosques tropicales húmedos de la región conocida como chocó biogeográfico, sudamérica. la región es una de las áreas con mayor biodiversidad del mundo y tiene un alto nivel de endemismo, pero también tiene el mayor riesgo de pérdida de recursos naturales. se usó un enfoque de estructura a partir de movimiento para derivar modelos de altura del dosel del fragmento de bosque, y se aplicaron algoritmos de desplazamiento medio para identificar las copas de árbol. la evaluación de la precisión se realizó utilizando datos de referencia derivados de campañas de campo e interpretación visual de imágenes vhr. el error cuadrático medio estimado de la población de errores verticales para el modelo de altura del dosel fue de 3,6 m. la precisión total para delinear las copas de los árboles fue del 73,9%. se encuentra que utilizando imágenes vhr, se pueden identificar y monitorear fácilmente árboles específicos y brechas, lo cual es un paso importante en los programas de conservación. también se discute la utilidad de estos hallazgos en el contexto de bosques fragmentados y las compensaciones entre el precio de un sistema lidar y la precisión de este enfoque. resumen https://doi.org/10.15446/esrj.v25n1.74167 mailto:javega@udemedellin.edu.co mailto:edwin.naranjo@operadoraminera.com.co https://doi.org/10.15446/esrj.v26n1.92419 https://doi.org/10.15446/esrj.v26n1.92419 2 johnny vega , sebastián palomino-ángel, jesús anaya 1. introduction forests currently cover one-third of the global land area (michez et al., 2016), and contain 70% – 90% of terrestrial biomass, making them a key global carbon sink (drauschke et al., 2014; dandois et al., 2015). tropical forests play a crucial role in global cycles (e.g., hydrological or biochemical cycles), contribute to global economy and welfare (pan et al., 2011; sasaki et al., 2011), and harbor around two-thirds of all known species (paneque-gálvez et al., 2014). forests in tropical countries have been recognized for their role in carbon storage by the united nations collaborative programme on reducing emissions from deforestation and forest degradation (un-redd+) and their high biodiversity by the convention on biological diversity (cbd). detecting forest degradation is a particularly difficult challenge because of the high level of detail required to identify subtle or gradual changes caused by wood extraction, selective logging, or loss of species richness (gofc-gold, 2017). this has resulted in a greater need for monitoring and capturing information on tropical forests. describing structural characteristics of forests has become an essential prerequisite for management and monitoring (bonnet et al., 2013), especially in tropical countries to achieve effective redd+ implementation. in addition, there are potential co-benefits for biodiversity conservation, climate change mitigation, and livelihood support (paneque-gálvez et al., 2014). natural resource managers who characterize forest structure exploit the dendrological and dendrometric characteristics of forests, including height, basal area, and volume (white et al., 2013). canopy height (ch) is undoubtedly one of the most important structural characteristics for forest planning. ch provides the basis for determining the attributes of trees and stands (bonnet et al., 2017) because it correlates strongly with volume, biomass, carbon stock, and productivity (white et al., 2013; dandois and ellis, 2013; steinmann et al., 2013; lisein et al., 2013). vegetation height also provides essential information related to ecological, hydrological, biophysical, and micro-meteorological processes (zarco-tejada et al., 2014), and has also been used to monitor biodiversity and analyze land cover and habitat landscape (bergen et al., 2009). in addition, vegetation height serves as an input for several models for estimating biomass (sierra et al., 2007; rutishauser et al., 2013), which facilitates estimation of carbon stocks in forest ecosystems and improves our understanding of the global carbon cycle. simultaneously, the amount of carbon stored by vegetation is a key parameter of biomass burning models for quantifying greenhouse gas emissions (anaya, 2015). in the tropics, biomass burning is the main cause of greenhouse gas emissions (van der werf et al., 2017). tree height is the most common remotely sensed three-dimensional (3d) estimate, generally captured in the form of a canopy height model (chm). several platforms and techniques are available to derive these models. an example at a global scale is the gedi program (https://gedi.umd.edu/) using a laser altimeter. tree height can also be obtained at a local scale by applying stereo-pair and multiple-stereo photogrammetry to a series of images acquired from aircraft and from airborne light detection and ranging (lidar) (dandois and ellis, 2013). the small footprint, alongand across-track density and laser pulses emitted by a lidar system means that it can penetrate the canopy and reach the forest floor. this allows recording of multiple returns and enables derivation of vertical forest profiles (lisein et al., 2013), as well as generation of three-dimensional, georeferenced point clouds. models of understory terrain elevations (digital terrain models: dtm) and top-of-canopy surface heights (digital surface models: dsm) can be generated by filtering the above-mentioned point cloud (dandois and ellis, 2013). in addition, surface heights can be derived by the subtraction of the dsm from the dtm, resulting in normalized aboveground object heights (i.e., tree heights, chm) (white et al., 2013). tree height can also be derived from active synthetic aperture radar (sar) sensors using interferometric and polarimetric observations where data from different sensors have been used, for instance: digital surface model from shuttle radar topography mission (kellndorfer et al., 2004), l-band backscatter from alos-palsar (joshi et al., 2015), polinsar coherence models using radarsat-2 (kumar et al., 2017), interferometric coherence from tandem-x (bispo et al., 2019), random volume over ground models using tandem-x data (kugler et al., 2014; lee and fatoyimbo, 2015), and uavsar data (lavalle and hensley, 2015). an emerging alternative is aerial remote sensing, which enables regular aerial 3d measurements of canopy structure and spectral attributes at a considerably lower cost, with properties like those of lidar, but with red-green-blue (rgb) spectral attributes for each point (dandois and ellis, 2013). unmanned aircraft systems (uas’s), also known as ‘drones’ are emerging as viable alternatives and are available in numerous forms offering flexible platforms. in many cases, uas’s have the potential to supplement and complement remote-sensing measurements acquired from satellites or manned aircraft systems (whitehead and hugenholtz, 2014). furthermore, uas imagery can provide, a rapid and low-cost alternative to multi-temporal rs data acquisitions for meaningful estimates of forest canopy attributes (chianucci et al., 2016), especially in developing countries where airborne laser scanning (als) acquisitions are less frequent (giannetti et al., 2018). uas imagery can be acquired with user-grade cameras and low-cost global navigation satellite systems (gnss), while 3d geometry is generated a posteriori with software that combines structure-from-motion (sfm) algorithms and photogrammetric principles (puliti et al., 2015). this technique depends on the high image overlap and information redundancy from uas imagery. the results meet the standards for forest inventories with very high spatial and temporal resolution (lisein et al., 2013; giannetti et al., 2018). remote-sensing approaches such as optical systems (i.e., uas’s) that use high-density point-cloud such as lidar sensors, also enable the detection of forest structural attributes (i.e. dominant tree height and crown diameter), while providing low-cost alternatives to field-based assessment. this may be done by using a technique known as segmentation, which was designed for identifying the edges of a finite set of non-overlapping objects that subdivide an image into tessellated regions, in this case, tree crowns. during this process, adjacent objects can be merged or divided based on specified criteria of homogeneity. tree crown dimensions are a fundamental structural parameter of forests, which can be used to model different tree structural variables such as stem diameter, height, and biomass. this is useful in forest inventory, regeneration assessment (pouliot et al., 2002; barnes et al., 2017) and forest degradation. several approaches have been proposed for tree crown delineation from aerial imagery, such as watershed transformation (ene et al., 2012; panagiotidis et al., 2016; barnes et al., 2017), region growing (solberg et al., 2006; novotny et al., 2011), valley following (gougeon, 1995; leckie et al., 2003), meanshift segmentation (huang and zhang, 2008; he et al., 2016), energy balance (gleason and jungho, 2012), and hill climbing (ke and quackenbush, 2011). the goal of this study was to evaluate the strengths and shortcomings in determining structural tree parameters (ch and crown dimensions) in a cloudy tropical environment using photogrammetric methods applied to uas imagery. the field plot is a fragment of tropical forest located in a matrix dominated by industrial crops and pastures with varied topographical relief. we aim to evaluate sfm technology in a tropical environment within a forest fragment with high cloud content, where trees can reach more than 40 meters with closed canopy, but at the same time, where degradation and fragmentation allows for ground detection. under these circumstances, we expect sfm to have higher accuracy when estimating the ground class. 2. materials and methods 2.1. study site the study site is a forest fragment of 60 ha that faces enormous pressure in terms of land use change. it is located in the floodplain of the león river (7° 54’ n; 76° 36’ w) with an elevation varying from 17 m to 60 m above sea level. rainfall exceeds 2500 mm/year, and the average temperature is above 25 °c (figure 1). the forest fragment is located in the northwestern part of south america, known as the bio-geographic chocó, in the municipality of urabá, colombia. the northwest area of south america is a region with unique characteristics that make it one of the most biodiverse regions in the world (galeano et al., 1998; gentry, 1986; myers et al., 2000). this region includes the western slope of the andes mountain range from ecuador, colombia and includes some regions in southern panama. the region encompasses an important altitudinal gradient from sea level to the high peaks of the andes mountain above 4000 meters above sea level. this condition represents an important variety of eco-regions, which includes páramos in the upper part of the andes, montane forests of the andes in the intermediate regions of the western slope, humid forests and flooded humid grasslands in the lower part, mangroves in the coastal zones and dry forests towards the southwestern zone of ecuador (olson et al., 2001). this area represents one of the most diverse https://gedi.umd.edu/ 3structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems cultural mosaics in south america and the caribbean, with origins dating back to pre-hispanic times when it was inhabited by more than thirty different human groups. presently, different indigenous communities, numerous afrodescendant communities and mestizo communities descended from immigrants from various regions inhabit the region (wwf colombia, 2008). these communities depend directly on the ecosystem’s goods and services (mads and pnud, 2014). despite its biological and cultural importance, the region faces great threats associated with the transformation of natural landscapes, which have been intensified mainly by the replacement of forest ecosystems by pastures for livestock, industrial banana and oil palm plantations and other uses (fagua and ramsey, 2019). industrial plantations and livestock are normally related to the construction of channels that drain excess surface water, thus altering the region’s hydrology (anaya-acevedo et al., 2017). the expansion of the agricultural frontier, livestock, illicit crops, and illegal mining in the region has resulted in a significant degradation of ecosystems in recent years (anaya et al., 2020; fagua and ramsey, 2019), with estimations of deforestation rates close to 135 km2/year (fagua and ramsey, 2019; hansen et al., 2013). additionally, illegal gold and gravel mining activities in numerous water currents alter natural water flows and their quality, putting the stability of ecosystems and the services they provide at risk (meyer et al., 2019; patino and estupinan-suarez, 2016; unodc, 2017). the difficulty accessing the region, which is mainly due to scarce infrastructure, characteristics of the terrain, rapidly changing threats to natural ecosystems and regional social conditions, represents a challenge for forest monitoring, and implies an urgent need to develop monitoring strategies. 2.2. unmanned aircraft systems (uas) datasets a flight was designed using pix4d capture over the forest fragment (pix4d, lausanne, switzerland). the field campaign was carried out at the end of the dry season over a period of two days, april 28th and 29th of 2018. this time was required to set up all the equipment including a sub metric gnss base station, gnss rover, and two uas platforms. two image datasets were acquired in the study area using these platforms. the primary uas imagery was acquired by using a dji phantom 4 pro (dji, shenzhen, china), a single, multi-rotor quadcopter unmanned aircraft vehicle (uav) equipped with a 20-megapixel rgb camera (1” cmos sensor). this uav was also equipped with an inertial measurement unit and consumer-grade gnss. these sensors determine the position and altitude of the aircraft during flight. the main objective of this survey was to generate the chm with an effective ground separation distance less than 0.05 m. figure 1. the study area’s forest fragment is located in the northwestern part of south america, known as the bio-geographic chocó, in the municipality of urabá, colombia. source: basemap from esri, here, garmin, © openstreetmap contributors, and the gis user community. for comparison, a second uas image dataset was acquired by using a dji inspire 2 drone equipped with a parrot sequoia multispectral rgb camera with a rgb sensor and individual red, green, red-edge, and near-infrared sensors (parrot, paris, france). this camera had an inertial measurement unit and magnetometer, with built-in gnss to determine the location and orientation of each uas image. this device was used to perform an aerial survey with the spatial extent encompassing a smaller region to the southeast part of the study site. the uas flights have a flying altitude of 93.1 m which fulfill the aeronautic regulation of the country. the ground resolution considers a gsd of 2.18 cm/pix (rgb camera) and 8.17 cm/pix (multispectral rgb camera). 2.3. data processing 2.3.1. generation of the orthomosaic and the canopy height model to extract 3d models from multiple uas overlapping photographs, we used a sfm photogrammetric technique, which involved using image-matching methods to process a set of non-oriented images acquired from multiple views to extract the 3d point cloud and generate the 3d model. according to chiabrando et al., (2015), three steps compose the sfm process. in the first step, multiple views of an object captured from a range of different positions are aligned by automated identification of matching features within all images. this process is iteratively refined by using nonlinear leastsquares minimization (chiabrando et al., 2015), enabling initial estimates of camera positions and object coordinates. the detection and extraction of the tie points (features that can be clearly identified in two or more images and that can be selected as a reference point to improve relative accuracy of photogrammetry projects) are based on geometrical similarities, such as object edges, and can be performed using algorithms such as scale-invariant feature transform (sift) (lowe, 2004), and speed-up robust feature (bay et al., 2006). currently, the main software for the initial stage of the digital photogrammetric workflow is based on sift, or on the modified version of sift. the position of feature points is monitored throughout the sequence of images, and a local descriptor is assigned to each point according to its local neighborhood. this is useful for detecting point correspondences between all images in the set. thus, after this phase, the camera interior and exterior parameters and its positions and assets are defined in a local reference system, which aims to create a sparse 3d point cloud of features corresponding to the locations of the estimated feature points. in a second step, a dense multi-view stereo reconstruction of the aligned images generates a dense 3d representation of the object’s surface geometry. in this phase, the dense reconstruction algorithm considers how the individual depth maps are merged into the final digital model and generates detailed 3d meshed models from the pixel values. the final step involves the numerical representation of the morphology and surface objects, which is equivalent to a dsm. this is essential for generating true orthophotos because they are vital for texturing the computed mesh. all images were first treated as different image blocks for computing image orientation by aero triangulation, after that, they were merged into a unique image block. the workflow is summarized in figure 2. a dense surface reconstruction known as a mesh was generated by using a dense matching algorithm (pineux et al., 2017). taking into account that the generated 3d point clouds contain rgb color information extracted from the input images, they were used to generate final surface products (kaiser et al., 2014) and the model texture (nikolakopoulos et al., 2017). in this sense, an interpolation process following the agisoft photoscan pro version 1.3.1 (agisoft llc, st. petersburg, russia) workflow generated a dsm surface. the absolute heights in the dsm were normalized to obtain relative heights by subtracting the terrain height from the dtm generated from the height of each point of the dense point cloud in the ‘ground’ category. the dtm was built by visually identifying ground points inside and outside the forest fragment and the points were interpolated using an inverse distance weighted algorithm. the chm is calculated as the difference between dsm and dtm heights, which represent the tree heights; that is, the distance between the ground and the treetops. 4 johnny vega , sebastián palomino-ángel, jesús anaya figure 2. schematic diagram describing the photogrammetric process for digital surface model, digital terrain model and orthomosaic generation. finally, authors exported the model to a grid-based format and generated an orthomosaic after a quality check of the individual images, which were merged and projected onto a local coordinate system (i.e., the magna-colombia projection system). for this projecting purpose, the markers (reference points) projections were used. table 1 shows all parameter values used in the agisoft photoscan pro workflow and that were applied to all image blocks of the primary uas image dataset acquired by using a dji phantom 4 pro (rgb camera). table 2 summarizes the statistics describing the alignment of the image block from the survey. table 1. settings of agisoft photoscan pro workflow parameters. process reconstruction parameter: value align photos accuracy: high image pair selection: generic key point limit: 40000 tie point limit: 1000 constrain features by mask: no adaptive camera model fitting: yes optimize alignment self-calibration of the camera: yes camera accuracy (m): 10 camera accuracy (deg): 2 marker accuracy (m): 0.005 projection accuracy (pix): 0.1 tie point accuracy (pix): 4 fit all except for k4 & rolling shutter build dense cloud quality: high depth filtering: mild build mesh surface type: height field source data: dense point cloud face count: high (57.513.694) interpolation: enabled point classes: all build dem source data: dense point cloud interpolation: enabled build orthomosaic blending mode: mosaic surface: dem enable color correction: no enable hole filling: yes table 2. statistics of agisoft photoscan pro process. parameter value number of images 2671 aligned images 2135 flying altitude (m) 93.1 ground resolution (cm/pix) 2.18 tie-points for the point cloud 1.558.358 markers projections 4.218.287 reprojection error (pix) 0.841 coverage area (km2) 1.13 2.3.2.tree-crown delineation or segmentation. for image segmentation, we used the object-oriented classification method based on the mean-shift algorithm provided in the open-source software orfeo toolbox/monteverdi (otb, 2018). otb is used worldwide because it extracts spatial features, segments, and classifies high-resolution imagery (huang and zhang, 2008). this software is efficient, and free for research, teaching, and even commercial use (zou and lin, 2013). mean-shift segmentation is an approach for tree crown delineation from imagery using object-based image analysis (obia), which tends to provide better results than the traditional pixel-based approach when vhr data is used (komárek et al., 2018). this type of analysis begins with an image segmentation into closed objects taking into consideration certain criteria of homogeneity regarding shape, texture and context information associated with the objects (marpu et al., 2010). the resulting objects can be classified until the object shapes (i.e., tree crown boundaries) are fully delineated. in this research, the obia technique was used to delineate tree crowns using the uas imagery datasets because of its ability to successfully delineate tree crowns using vhr imagery (singh et al., 2015). the mean-shift algorithm, with its nonparametric nature, is an efficient spatial-feature-extraction method for delineating arbitrarily shaped clusters. it exploits contextual homogeneity and reduces spectral variation in local areas and, at the same time, preserves edge and detail information. thus, it identifies geographical features by using scale and homogeneity parameters obtained from reflectance in rgb and from elevation (huang and zhang, 2008). in addition, mean-shift segmentation also helps to open the original image into a multi-scale space to allow scale to be incorporated into classification (zou and lin, 2013). in this study, otb is used for mean-shift segmentation of 20 ha, which is approximately 33% of the study area. the input-parameter settings for the segmentation are (in pixels): spatial radius of 20, minimum object of 12, and minimum region size of 100. both 5-cm-pixel chm model and rgb orthorectified images were used to improve the quality of the tree-crown delineation by using the spectral information from the rgb bands and the treecrown edge information from the chm. figure 3 shows the method used for segmentation using the secondary uas image dataset acquired by using a dji inspire 2 drone (multispectral rgb camera). figure 3. schematic diagram describing method of tree segmentation process. 5structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems 2.4. validation process 2.4.1. model comparison as mentioned previously, a second uas flight was executed for comparison in the southeast zone of the study area, which covered 20 ha over the study site (see figure 1 for uas flight coverages). this aerial survey used a different sensor for the optical camera. specifically, a multispectral camera equipped with individual red, green, red-edge, and near-infrared sensors, and the 3d point cloud from this device was processed by using a pix4d mapper pro. a subset of the original dsm was obtained with the same extension as the multispectral camera survey, with the aim being to compare quality and correlation between both models. 2.4.2. segmentation accuracy assessment some object metrics have been proposed to evaluate a segmentation considering the data quality as well as the optimal customization of parameter settings, which enable the adaptation of segmentation results on target objects as manually delineated trees. the inaccuracies (anomalies) of segmentation refer to over-segmentation and under-segmentation; it means ‘too many’ or ‘too few’ segments respectively (möller et al., 2007). as recommended by montaghi et al., (2013), we applied a threshold method before calculating the metrics to reduce false delineation caused by small overlapped regions between the reference and segmented trees. an intergap zone was defined by selecting cells in the chm shorter than 2 m. in addition, referenced and segmented trees with areas less than 3 m2 were removed from the analysis. an exhaustive reference polygon dataset was used, where 857 reference objects were manually digitized from uas images, and then superimposed on the automatically estimated tree crowns to assess accuracy. this work used vector-based measures to compare the results of objectbased image segmentation with a set of manually delineated objects extracted from the uas image (orthophoto mosaic), as adopted by gougeon and leckie (2003). for this purpose, we used the most common indices used in the literature: quality rate (qr), over-segmentation rate (or), under-segmentation rate (ur), and euclidean distance (ed1) (weidner, 2008; clinton et al., 2010; liu et al., 2012). however, because these indices focus only on geometric relationships, they may not be sufficient to describe the diverse types of discrepancies. therefore, we used arithmetic relationship indices, as a complement to the aforementioned geometric indices. the arithmetic indices used in this validation were proposed and defined by liu et al., (2012): potential segmentation error (pse), number of segments ratio (nsr), and euclidean distance 2 (ed2). the indices based on geometric relationships are calculated as follows (weidner, 2008; clinton et al., 2010; liu et al., 2012): = 1 − ∑| ∩ | ∑| ∪ |, (1) = 1 − ∑| ∩ | ∑| | , (2) = 1 − ∑| ∩ | ∑| | , (3) 1 = √ 2+ 2 2 , (4) where ri is a reference polygon in a reference dataset r, i = 1, 2, …; and sk are the corresponding segments in a corresponding segmented-object dataset s, with k = 1, 2, …. the indices based on arithmetic relationships are calculated as follows (liu et al., 2012): = ∑| − | ∑| | , (5) = ( − ), (6) 2 = √ 2 + 2, (7) where sk and ri are defined as above, m is the number of reference polygons, and v is the number of corresponding segments. 2.4.3. chm accuracy assessment the heights of 59 individual trees (objects) identified in the orthophotos were measured using a leica distotm e7500i laser distance, which has 14 measurement modes and a 360° tilt sensor with an accuracy to 0.16 cm. the sample (n = 59) was defined in order to have as many measured trees as hectares of the plot, measurements were distributed systematically considering topographic and vegetation differences (see figure 4). orthophotos with a 10 m grid of a wgs-84 utm18 north projection were used in the field to identify the 59 trees with the help of a compass and the gps. we found a straight line of view from the forest floor to the treetop for each tree and made several measurements to determine the maximum value vri (equation 9). accordingly, the crown delineation was made by visual interpretation and the maximum value was selected for each crown in the chm vmi (equation 9). the vertical accuracy was calculated as the mean vertical error µv of the sample errors evi, which was estimated as the vertical root-mean-square error (rmsev): = √∑ ( )2 , (8) with: evi = vri – vmi, (9) where vri is the reference height of tree i measured in the field, vmi is the maximum chm height at the crown level derived from optical images, n is the number of sampled trees, and the standard deviation sv of the vertical error is estimated by: = √∑ ( − )2 ( −1) , (10) the standard error in the estimates of rmsev is estimated by: = /√ , (11) finally, the estimated error interval at a 95% probability level is: rmsev ± 1.96 srmse figure 4. distribution of tree height (reference data) used to measure the accuracy of chm. in color polygons uas flights coverage is shown: red polygon (phantom 4 pro rgb camera), blue polygon (inspire 2 multispectral camera). three classes were used to determine the bias per class: 10 20 m, 20 30 m, and 30 40 m and the mean value of evi was calculated for each class. this bias is a measure of the difference between the expected value of the reference height of tree i with respect to the model vmi. 6 johnny vega , sebastián palomino-ángel, jesús anaya figure 7. comparison of dsms and distribution of sample points. (left) rgb camera onboard the dji phantom 4 pro. (right) parrot sequoia multispectral rgb camera on board the dji inspire 2 uav. figure 6. very-high-resolution (vhr) image and models obtained from unmanned aircraft systems data: a) digital surface model, b) digital terrain model, c) canopy height model, and d) orthomosaic. 7structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems 3. results 3.1. model generation a total of 2671 images were acquired with the optical rgb camera, covering 60 ha of the forest fragment (figure 4). only 2135 images were correctly aligned, obtaining a ground resolution of 2.18 cm/pixel. a total of 1’558 358 tie point positions were adjusted, with a mean residual of 0.841 pixels (reprojection error). we acquired 1632 with the multispectral rgb camera, considering the four sensors covering an area of 20 ha in the southeast zone of the study area. a total of 1460 photographs were correctly calibrated, which yielded a ground resolution of 8.16 cm/pixel. a total of 1´ 657 685 tie point positions were adjusted, with a mean residual of 0.276 pixels (reprojection error). having two consecutive flights allowed us to determine whether the sfm method produces consistent results, and we found a correlation of r2 of 0.99 between both dsms. the extent of nodata was determined in those areas where photographs did not match or photographs did not sufficiently overlap, and the required tie points could not be obtained to densify the point cloud. nodata was propagated from the dsm to the dtm and chm. the dsm was not estimated over a total of 2% of the area. all classes of the dense 3d point cloud were used to reconstruct a dense surface via a dense matching algorithm. the resulting dsm has a range between 17.12 and 96.96 m, a mean of 47.50 m, and a standard deviation (sd) of 13.16 m. in the case of the dtm, a similar dense surface was reconstructed, but only by considering the height of those pixels in the ground category in the dsm. the resulting dtm ranges from 17.12 to 58.71 m, with a mean of 35.78 m and a sd of 7.19 m above the ground. the tree heights corresponded to the difference between the dsm and dtm, which is known as the chm. this model ranged from 0 to 57.76 m, and had a mean of 11.72 m and a sd of 9.71 m above ground. the pixel frequencies and values are given in figure 5. considering the rgb color information extracted from the input images and the dense 3d point cloud, we generated an orthomosaic of the study area, which has a ground spatial resolution of 0.50 m. the respective dsm, dtm, and chm were also exported with a 0.50 m pixel size (figure 6). figure 5. distribution of pixel height based on surface models generated using rgb camera data. 3.2. vertical accuracy of canopy height model (chm) the gps and the orthophoto maps allowed us to identify each of the 59 trees in the field (table 3). the average tree height measured in the field with the distotm e7500i laser was 24.5 m with a maximum of 37.3 m. the estimated rmse of the vertical errors (equation 8) was 3.6 m with a 95% confidence interval of 0.53 m. meaning that there is a 95% proven reliability that the unknown population average error is between 3.05 and 4.12 m. for comparison purposes, both uas datasets were analyzed (figure 7). using a random sampling of 283 points, we found a correlation coefficient of 0.99 between both generated dsms. the results of the aforementioned flight height for both uas datasets were considered in this analysis. table 3. differences in tree height evi between field measurements (reference data) at individual tree scale (vri) and estimated by the chm (vmi) as the maximum height for each crown. id vri vmi evi id vri vmi evi 001 31.2 28.6 2.6 030 34.7 32.5 2.2 002 27.1 25.6 1.5 031 27.9 25.3 2.6 003 24.3 27.1 −2.8 032 3.8 5.1 −1.3 004 23.6 25.8 −2.2 033 26.2 31.1 −4.9 005 21.0 19.9 1.1 034 17.2 15.9 1.3 006 24.2 31.1 −6.9 035 30.6 24.9 5.7 007 6.2 3.6 2.6 036 26.0 25.6 0.4 008 23.3 26.5 −3.2 037 31.2 32.2 −1.0 009 23.6 26.9 −3.3 038 30.5 28.0 2.5 010 24.9 26.0 −1.1 039 21.0 22.1 −1.1 011 28.3 22.8 5.5 040 25.4 24.9 0.5 012 24.9 20.3 4.6 041 23.0 27.6 −4.6 013 19.0 17.5 1.5 042 22.9 17.8 5.1 014 23.7 20.5 3.2 043 19.7 22.9 −3.2 015 28.4 24.5 3.9 044 16.3 17.2 −0.9 016 23.0 24.5 −1.5 045 20.3 13.4 6.9 017 28.0 23.2 4.8 046 32.1 30.3 1.8 018 19.8 27.1 −7.3 047 33.4 34.8 -1.4 019 10.7 10.0 0.7 048 30.0 25.2 4.8 020 27.6 24.9 2.7 049 27.4 26.4 1.0 021 36.5 28.8 7.7 050 27.6 22.1 5.5 022 34.5 26.7 7.8 051 19.9 22.5 −2.6 023 28.3 22.0 6.3 052 31.6 28.9 2.7 024 22.9 22.2 0.7 053 23.7 22.9 0.8 025 19.6 16.7 2.9 054 22.1 20.2 1.9 026 24.6 20.0 4.6 055 15.2 18.7 −3.5 027 26.0 27.7 −1.7 056 13.1 10.4 2.7 028 35.7 31.9 3.8 057 10.5 10.2 0.3 029 37.3 33.5 3.8 058 29.4 31.1 −1.7 059 25.0 24.5 0.5 3.3. accuracy of delineation of tree crowns figure 8 shows the 857 reference objects manually digitized from uas images, and the result of delineating individual tree crowns by using the mean-shift algorithm provided in the open-source software orfeo toolbox/ monteverdi (otb, 2018), over the multispectral rgb image (sequoia camera). from the geometric and arithmetic indices used for validation (figure 3) we found: over-segmentation error was 7.1% for the overall validation area, the under-segmentation error was 36.2%, and the total accuracy for delineating tree crowns was 73.9% (i.e., ed1 = 26.1%). the comprehensive index of 8 johnny vega , sebastián palomino-ángel, jesús anaya quality rate (qr) was 39.2%. for the arithmetic indices, pse had an error of 45.7%, whereas nsr had 20.0%, and the delineation of tree crowns was 50.1% accurate overall (i.e., ed2 = 49.9%). for the accuracy measurements of 1:1 correspondence, 30% of the total reference crowns matched correctly with respect to the segmented trees, and 29% of the reference objects were omitted. 4. discussion 4.1. canopy height model accuracy the accuracy of our estimated chm incorporates the uncertainty of the dsm and dtm. dsm errors are associated with gps accuracy (geometric dilution of precision) and the quality of individual photographs while the dtm depends on the visual interpretation of the photogrammetric point cloud and the interpolation of the points. the latter is highly dependent on the characteristics of the canopy, which is very closed in tropical forests. ota et al., (2017) found a rmse of maximum height of 1.24 m for temperate conifer forests in japan which has higher accuracy than our results. when evaluating the accuracy of the chm against tree height measured in the field, the correlation was r2 = 0.75, this value is within the range of (r2 0.63 0.84) found by dandois et al., (2013) in a temperate deciduous forest. this is because the chm model is a function of dtm accuracy, which depends on the penetration of sunlight through canopy gaps, and this is highly limited in these tropical forests. therefore, the quality of the absolute height estimation depends strongly on the complexity of the terrain and the canopy closure. concern also surrounds the determination of the tree height measurements in the field (i.e., the treetop). despite the 0.16 cm accuracy of the disto laser, several measurements are required to select the maximum height for each tree crown. the observation must be made below the maximum tree height because a straight line of sight is required between the disto laser and the given treetop. the difficulty of finding such a straight line of sight varies between tree species and is affected by the presence or absence of leaves, which is associated with the phenological cycles. 4.2. quality assessment of tree crown delineation although in the bio-geographic chocó there has already been regional assessment of forest structure and aboveground biomass using measurements from a combination of ground tree inventories and airborne (lidar) (meyer et al., 2019), this study is the first approach for delineating tree crowns in this natural tropical forest fragment using vhr imagery from uas’s. regarding the tree-crown delineation validation, because the qr index is far from zero, then a low geometric match is obtained due to a clear discrepancy of the canopy segmentation with the reference polygons. however, qr alone does not adequately assess image segmentation quality, because none of the existing indices can directly identify the optimal combination of parameters for the segmentation process. in general, input parameters that influence the size of the segmented-image objects are mostly selected intuitively by trial and error (möller et al., 2007; marpu et al., 2010), although additional metrics have been proposed to improve this quality assessment measuring both topological and geometric similarity between segmented objects and reference objects, for instance simsize and area fit index afi (montaghi et al., 2013). other studies have used metrics to assess correct segmentation, over-segmentation, and under-segmentation, assuming that correct, over-, and under-segmentation correspond to the cases true positive, false positive, and false negative results, respectively (miraki, 2021), but this approach was not considered in this paper. few studies have attempted to estimate the tree crowns of tropical forests of high biodiversity, which primarily consist of broad-leaved trees with a great variety of species and ages (matsumoto et al., 2017). a study in a tropical context was developed by wagner et al., (2018) in the brazilian atlantic rain forest with highly heterogeneous tropical canopy cover, comprising both deciduous and evergreen species in the municipality of campinas (são paulo state brazil). they used vhr satellite images for tree detection and segmentation, obtaining results of several tree crowns correctly detected of 79.2%, which although not validated with the same metrics, led to a result of an overall accuracy of 85.3%. this is not very different from those obtained in this research (73.9%). qiu et al., (2020) proposed a new tree crown delineation method using both brightness and spectra from high resolution multispectral images in a plot with high biodiversity and dense crowns with different shapes, structures, and distinctive colors in the amazon rainforest, achieving a delineation accuracy of up to 76%. the uas-derived chms for individual tree crown delineation normally perform well in coniferous forests, but their capabilities in broadleaf or mixed forests are still challenging (miraki et al., 2021). in fact, for the accuracy measurements of 1:1 correspondence, the reference crowns matched correctly and the ed2 result indicates a low performance in the vegetation of the analyzed tropical mixed forest ecosystem, but the manually delineated objects (trees) still influence the results (clinton et al., 2010) in terms of comparing the shape geometry and the distance between object boundaries (montaghi et al., 2013). visual interpretation has limitations with respect to clearly detecting small trees. in addition, delineating irregular-shaped tree crowns manually is difficult for large-scale imagery because of the influence of background (ke and quackenbush, 2011). figure 8. result of individual-tree-crown delineation. (left) reference and segmented trees obtained using mean-shift segmentation. (right) matched trees area between digitized trees and segmented trees. 9structural attributes estimation in a natural tropical forest fragment using very high-resolution imagery from unmanned aircraft systems in tropical forests, traditional uav photogrammetry demonstrates its effectiveness (giannetti et al., 2018), and several segmentation techniques have been proposed, but they are not specifically conceived for the segmentation of the tropical forest. such techniques normally perform well in coniferous forests with obvious between-crown shadows, but fail in dense deciduous or mixed forests, where tree crowns are close to each other and between-crown shadows and boundaries are unobvious, whereas adjacent tree crowns may be of distinguishable spectra (qiu et al., 2020). furthermore, the level of detail available in vhr images could potentially have a strong negative effect at various stages of the processing (zou and lin, 2013). vhr images may provide very high detail, but can cause large variations, and thus crown edges can be less detectable. this is especially the case where small trees, with relatively small crowns, are dominant (ke and quackenbush, 2011). in addition, note that irregular-shaped crowns create differing reflectance patterns, which are difficult to recognize. in our case, the tropical forest has trees of varying age, species, height, crown size, and crown shape, which causes significant variation in brightness in the vhr images acquired with the uas. these result in commission errors, where multiple tree peaks are erroneously identified within the same crown. we thus applied image smoothing in an attempt to reduce the noise caused by small branches and their corresponding shadows within a given crown. nevertheless, some non-forest areas, such as soil, were falsely identified as tree crowns in the segmentation process. our results indicated a degree of sensor-specific independence. for example, there are subtle variations in reconstructed crown geometry (i.e., shape, height) and the perceived shape of low-lying vegetation and the terrain beneath. despite this, and divergent altimetric accuracies, the comparison between both chm models showed a 99% correlation. evidently, both sensor type and the processing software and workflows employed will impact the output 3d models. however, results are sufficiently similar and of suitable accuracy to facilitate tree segmentation at our desired accuracy. according to the results and the characteristics of sensor type, we suggest for future studies to derive different indices from the available spectral bands and enhance the input data for segmentation. 4.3. photogrammetry software package used regarding the consequences of the use of the processing software, it is worth considering the potential of each software used. initially, it is necessary to highlight the issue of orthomosaic and vhr imagery generation, since pix4d mapper performs an orthorectified mosaic (from the average mapping of pixels, it means, averages the color of each pixel according to the colors of each photo with overlap in that area) based on the generated dsm, while agisoft photoscan allows choosing the type of surface on which the orthorectification of the images will be performed. this can be a significant time saver in certain situations. on the other hand, with respect to the treatment of multispectral information, pix4d mapper provide more tools for the analysis of vegetation and forest areas, when using cameras such as the parrot sequoia. with respect to the dsm, we found a 99% correlation between both models generated by agisoft photoscan and pix4d, hence, there are no appreciable differences or important consequences to discuss. 4.4. potential of photogrammetry canopy height models studies have shown that both digital photogrammetry or lidar using space-borne or airborne imagery (acquired with manned airplanes) are appropriate for determining forest canopy heights (kadmon and hararikremer, 1999; lisein et al., 2013). tree height accuracy using lidar is close to 1 m (mielcarek et al., 2018) which is better than the 4 m found with our methods. in addition, bias of the three height classes is not constant, with our reference database, there is a bias to underestimate the class from 10-20 and a bias to overestimate in the classes 20-30 and above 30 m. a constant bias would be ideal to improve estimates. we expect tree height estimates close to 60 m to have more uncertainty. compared with lidar, the main limitation of digital photogrammetry is that it cannot acquire structural information under canopy and produce a complementary dtm, the bias of the chm is related to the quality of the dtm (swinfield et al., 2019). one reason is the high costs of lidar technology, which may be too costly for smaller-scale research projects. the cost of conventional airborne lidar acquisitions remains high for researchers and other end-users, especially if high-spatial resolution is required over small areas or at high temporal frequencies. the latter represents a significant barrier for studies requiring frequent observations at numerous small sites or sampling plots (dandois and ellis, 2013). in addition, lidar by itself does not constitute an immediate solution to obtain certain attributes without imagery, such as species composition and phenology. furthermore, there are additional data processing bottlenecks associated with the high volumes of data gathered in lidar campaigns, including filtering, classification, and archiving. there are two main advantages of uav photogrammetric point clouds over airborne lidar point clouds, first, the lower cost, which is a main limitation in developing countries, second, the ability of auvs to fly below the clouds in rain forests. however, photogrammetry models based on passive sensors have some limitations when compared with small-foot-print lidar systems. this is because the numerous vegetation characteristics (leaf-area index, tree density, under-canopy structure, etc.) hinder sunlight from reaching all the forest strata, resulting in highly detailed information of the upper canopy but low detail of the forest floor due to poor illumination. having low detail of the forest floor generates uncertainty in the dtm generation and thus in the chm. this limitation must be present in the redd+ program if trying to use allometric equations to determine agb based on height. the forest fragment analyzed in this study is located in the biogeographic chocó, which is a region of great ecological relevance due to the forest ecosystems present there (gentry, 1986; galeano et al., 1998; myers et al., 2000). these ecosystems represent great potential for the subsistence of the communities that inhabit the area (camacho and díaz, 2018; galeano, 2000), however, lack of information represents a limitation for the application of sustainable initiatives in the area, such as redd+ projects. the only study related to the estimation of the vegetation height in the region, corresponds to the one developed by meyer et al., (2019), which provides fundamental reference information for the analysis of forest ecosystems based on lidar data and field inventories. in this context, the temporal monitoring of forest ecosystems in the region has been limited. in this scenario, detailed analyzes at the local scale using vhr imagery derived from uas´s, such as the one developed in the present study, provide a fundamental complement to refine the detail of regional analyzes, allowing monitoring on small-scale projects (dandois and ellis, 2013). these kinds of studies provide not only an efficient alternative with regards to time and costs with a great level of detail, but also to obtain additional attributes of the forest such as species composition and phenology, thus complementing other observations such as satellite ones. 5. conclusions vhr imagery allows us to do detailed mapping over areas that are larger than traditional field plots. the objects in this study were defined as single trees instead of parcel plots, which is an important step in conservation programs, where deforestation is not occurring over large extensions but at the level of individual trees (i.e., selective logging, where the most valuable single trees are extracted). in addition, a uas-based approach can enhance the modeling of carbon stocks at local scales. this would lead to more accurate scaling up to regional forest modeling efforts such as those commonly undertaken with remote sensing imagery of coarser spatial resolution and could help bridge the gap between above-ground biomass (agb) field measurements and estimates from conventional remote sensing data. this is one of the challenges for monitoring forest degradation in programs such as redd+. uass allow us to obtain vhr images from viewpoints below clouds, while maintaining low equipment and data processing, which is a major advantage in tropical countries. although our interest focuses on crown delineation and tree height, other forest features can also be monitored, such as phenology, tree species, and health status. in terms of tree height, the results may differ between forests because dtm quality depends on the canopy closure, which affects chm accuracy. in this respect, fieldwork is important to define the height of reference trees and to locate canopy gaps and determine their extent. we suggest several campaigns to re-measure tree heights in order to improve reference data quality. it may be beneficial to analyze chm correlation among different flight heights using both vhr images. 10 johnny vega , sebastián palomino-ángel, jesús anaya optical systems such as uas offer possibilities to fulfill the needs of forest inventories and can allow the detection of forest structural attributes such as dominant tree height and crown size. they also offer low-cost alternatives with respect to field-based assessment and airborne laser scanning systems or vhr satellite imagery (e.g., worldview, ikonos, quickbird) for accurate extraction of the required forest information. in fact, due to low operation and maintenance cost, it is possible to acquire imagery far more frequently than conventional remote sensing technologies. this is very important for improving deforestation and degradation monitoring programs, as significant seasonal differences in tropical forest cannot be detected with single-date imagery. the user must evaluate the tradeoff between the cost of a lidar sensor and the error involved with a uas with a rgb sensor. with the methods detailed herein, we obtained an error between 3 and 4 meters within a 95% probability level. although most previous work uses forest parameters derived from uavs to estimate height or to delineate tree crowns (lisein et al., 2013; zarco-tejada et al., 2014; panagiotidis et al., 2016; bonnet et al., 2017;), we found that assessing both parameters within the same project is useful to validate the results. according to technical advantages identified, presented and discussed, the proposed uas-based forest monitoring approach could be particularly attractive and useful to organizations involved in redd+. this approach could prove especially useful in tropical regions, because of their ability to map and facilitate forest degradation-and-regrowth monitoring. in terms of monitoring, the next step to exploit the full potential of this type of data is to generate new models and to execute new uav flights to evaluate the sensitivity of the models to fallen or harvested trees. funding this research was funded by colciencias, grant number 120671451240, contract 085-2016. acknowledgments the authors would like to thank natasha han and alberto vivas for their suggestions and recommendations. we also would like to thank corpuraba for their support during the field campaign, formalized through the interinstitutional cooperation agreement between the universidad de medellín and the environmental protection agency. references anaya, j. a. 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(2022). estimating surface runoff using the curve number method in an ungauged watershed in jalisco, mexico evaluating its morphometric parameters. earth sciences research journal, 26(4), 313-320. https://doi. org/10.15446/esrj.v26n4.99194 abstract the surface runoff was estimated using the curve number (cn) method for different storm events, and it was associated with the morphometric parameters of an ungauged watershed in jalisco, mexico. we derived the land use and land cover map for the year 2019 from the unsupervised classification of a sentinel-2 image at 10 m resolution. the morphometric analysis revealed that the study area is dominated by areas of gentle slope and moderate to high permeability with dense vegetation and low relief. it is a well-drained watershed with low flood potentiality besides a groundwater potential zone. about 69% of the study area corresponds to natural cover with tropical dry forest and evergreen forest as the most representative classes. the number of storm events identified from 2013 to 2019 was 77, with runoffs ranging from 0.05 to 227.2 mm3. september and october were the months with the most intense rainfall and, therefore, the months with the highest runoff volume. the average runoff was 14.52 mm3, corresponding to an average runoff coefficient of 12.86%. together, the methods used here provide useful results for delineating conservation strategies at the watershed level. keywords: chamela-cuixmala biosphere reserve; curve number runoff; geographic information systems; tropical dry forest. estimación del escurrimiento superficial utilizando el método del número de curva en una cuenca no aforada en jalisco, méxico, evaluando sus parámetros morfométricos resumen el escurrimiento superficial se estimó utilizando el método del número de curva (cn) para diferentes eventos de tormenta y se asoció con los parámetros morfométricos de una cuenca no aforada en jalisco, méxico. el mapa de cobertura y uso del suelo para el año 2019 se derivó a partir de la clasificación no supervisada de una imagen sentinel-2 a una resolución de 10 m. el análisis morfométrico reveló que el área de estudio está dominada por áreas de pendiente suave y de moderada a alta permeabilidad, con vegetación densa y bajo relieve. es una cuenca bien drenada con bajo potencial de inundación, además de una zona potencial de aguas subterráneas. alrededor del 69% del área de estudio corresponde a cobertura natural, siendo el bosque tropical seco y el bosque perenne las clases más representativas. la cantidad de tormentas identificadas entre 2013 y 2019 fue de 77, con escurrimientos que oscilaron entre 0.05 y 227.2 mm3. las lluvias más intensas ocurrieron entre septiembre y octubre, ubicándose como los meses en los que se produjo el mayor volumen de escurrimiento. el escurrimiento promedio fue de 14.52 mm3, correspondiente a un coeficiente de escurrimiento promedio de 12.86%. en conjunto, los métodos utilizados aquí brindan resultados útiles para delinear estrategias de conservación a escala de cuenca. palabras clave: bosque tropical seco; escurrimiento; número de curva; reserva de la biosfera chamelacuixmala; sistemas de información geográfica. https://doi.org/10.15446/esrj.v26n4.99194 mailto:rhernandezgu@conacyt.mx https://doi.org/10.15446/esrj.v26n4.99194 https://doi.org/10.15446/esrj.v26n4.99194 314 rafael hernández-guzmán, norma leticia onchi-ramuco 1. introduction watersheds are contiguous areas of land that share natural drainage (tang and adesina, 2022) and, when large watersheds contain many smaller ones, they can be considered contiguous hydrological response units, biophysical units, and holistic ecosystems as they are parts of the drainage network (liu and zhao, 2022). but also, watersheds have discrete physical boundaries or can be discretized to be suitable for statistical, comparative, and analytical studies (band et al., 2000; obeidat et al., 2021), so countless ways to describe them numerically and qualitatively have been proposed (fenta et al., 2017; li et al., 2021). its management for the conservation of natural resources requires information on surface runoff, which plays an important role in biogeochemical cycles, provides favorable environmental conditions for multiple ecosystems (ruiz-luna et al., 2014), and is essential for many applications; in agriculture as a source for irrigation (essaid and caldwell, 2017), for the generation of energy in hydroelectric plants (reichl and hack, 2017), in the design of hydraulic infrastructure (verma et al., 2018), in the evaluation of vulnerability to floods (ogarekpe et al., 2020), and in the assessment of soil erosion (sooryamol et al., 2022). it is relevant to know the precipitation-runoff relationship since it allows estimating the effects of floods in extreme events and evaluating how the water tables are recharged by rain events. among the different methods for evaluating surface runoff is the runoff curve number (cn) methodology proposed by the natural resources conservation service (nrcs) of the united states department of agriculture [usda] (1986), widely accepted and frequently used in the scientific literature. the application of this method is attractive in the study of ungauged watersheds because it is based on the recognition of characteristics that can be measured in the basin, such as land use and land cover, and the soil type (hernándezguzmán and ruiz-luna, 2013). in addition, drainage morphometry of watershed is essentially quantitative and involves numerical variables whose values can be retrieved from digital elevation models (dem) with the help of geographic information systems (gis) (mahala, 2020; ogarekpe et al., 2020; kumari et al., 2021; obeidat et al., 2021). the importance of morphometric variables lies in their usefulness for statistical analysis as well as for comparisons, and they are fundamentals to understand the hydrological and morphological characteristics of any watershed (thomas et al., 2010; raux et al., 2011; odiji et al., 2021; singh and singh, 2022). it is evident that the response of a watershed to precipitation events is influenced by its drainage morphometric parameters. these responses can include, an increase in the peak flow and in the frequency of floods, increase in erosion, degradation of the surface water quality, reduction in the groundwater recharge, among others (hernández-guzmán et al., 2008). even without estimating surface runoff, morphometric characteristics are very important for planning irrigation, water supply, and flood control projects (rao, 2020). despite the fact that the availability of spatial data has fostered a significant increase in its use in hydrological applications (satheeshkumar et al., 2017), there are few studies that relate the morphometric characteristics of the basin with the estimation of the potential for surface runoff (satheeshkumar et al., 2017; abdulkareem et al., 2018; prabhakar et al., 2019). the chamela region, located in the coastal zone of the state of jalisco, mexico, is a biologically important area because the heterogeneity of the landscape of the region has caused not only a high diversity of species belonging to various taxonomic groups but also a high beta diversity, which indicates high levels of endemism and local patterns of diversification (suazo-ortuño et al., 2018). biologically, it is one of the most studied areas of mexico, especially the chamela-cuixmala biosphere reserve, a natural area of preserved tropical deciduous forest of about 131 km2. however, the conversion of natural cover to cultivated land has been recurrent in the surrounding area as shown by previous studies (sánchez-azofeifa et al., 2009; flores-casas and ortega-huerta, 2019). the studies found at a basin scale, as well as for the coastal zone of the state of jalisco (sánchez-azofeifa et al., 2009; martínez-harms et al., 2016; flores-casas and ortega-huerta, 2019), have not tackled an analysis of how the morphometric parameters of the basin influence the routes and rates of water flow that may be useful in the prevention of natural disasters in vulnerable areas of the region. based on the above considerations, the aim of this study was to make landscape characterization and estimate the surface runoff applying the runoff curve number method in the río cuitzmala watershed, jalisco, mexico and evaluate the drainage characteristics that govern surface runoff in the watershed. 2. study area the study area corresponds to the río cuitzmala watershed, located in the coastal zone of the state of jalisco, mexico (between 19° 17′ and 19°52′ n, and from 104°34′ to 105°04′ w), comprising a total area of 1,120 km². the sub-basin has an elevation gradient that ranges from 0 to 1,769 meters above sea level with an average annual temperature that ranges between 20.8 °c and 32.8 °c with an average of 25.9 °c. the area is characterized by having a marked seasonality (almost 80% of the precipitation occurs between july and november) with an average annual precipitation of 963 mm measured in the automatic weather station 16a106e4 (fig. 1a). in the lower part of the study area is the chamela-cuixmala biosphere reserve, which protects a system of tropical dry forest known for its high biodiversity. geologically, 67% of the watershed is characterized by granite granodiorite (97.5-108.5 million years) and 18% volcano-sedimentary rocks (91-112 million years), both derived from the lower cretaceous. quaternary alluvium deposits (0.01 million years) are along the river banks in the lower part of the watershed (fig. 1b). figure 1. geographic location of the río cuitzmala watershed in the coastal zone of jalisco, mexico showing the topographic variability. a) drainage order obtained from the mexican continuous elevation 3.0 (cem 3.0) produced by the national institute of statistics and geography (inegi). b) geology map digitized from the mexican geological survey charts (sgm). 315estimating surface runoff using the curve number method in an ungauged watershed in jalisco, mexico evaluating its morphometric parameter 3. methods drainage morphometry of watershed a total of 19 linear, shape, and relief parameters of the study area were determined from the continuo de elevaciones mexicano (mexican continuous elevation, or cem) at 15 m pixel size developed by mexico’s national institute of statistics and geography (inegi; https://www.inegi.org.mx/app/ geo2/elevacionesmex/). we used the d8 algorithm to derive the flow direction (jenson and domingue, 1988). using this layer, the flow accumulation was derived, and using a constant threshold value of 2,225 cells (~ 0.5 km2) the drainage network was defined according to strahler (carraro et al., 2020). land use and land cover the land use and land cover map for the year 2019 was derived from a sentinel-2 scene with bands at 10 m spatial resolution using an unsupervised classification technique with the k-means algorithm on the terrset software (eastman, 2016). this algorithm partitioned the satellite image into 50 spectral classes, which were subsequently reclassified into the following land use and land cover classes: aquatic surfaces, mangrove, evergreen forest, tropical dry forest, agriculture, and grassland. the littoral and human settlement classes were digitized on-screen over infrared false color combinations and added to the final thematic map. the small fringe of the littoral class was digitized because generated spectral confusion with the grassland class. regarding the human settlements class, the study area maintains a rural profile, with a low-density population. there are only three small towns with more than 500 inhabitants. houses in this area are generally separated and have big backyards with orchard trees that create spectral confusion with the agricultural and evergreen forest classes. the accuracy assessment was based on 160 points collected in the field and registered with the help of a global positioning system (gps) unit in 2019. these points were compared with the classification and the results were plotted in a confusion matrix, which compares the relationship between classification results and field verification data. from this numerical arrangement, the global accuracy and the kappa coefficient were estimated, in addition to the producer (ep) and user (eu) accuracies (congalton and green, 2009). surface runoff runoff calculation with the cn method depends mainly on the precipitation, the land cover characteristics of the area, and the hydrologic soil groups. based on the combination of these attributes, a cn value was defined for each resulting polygon. the results were processed with the sara v1.0 (hernández-guzmán and ruiz-luna, 2013), a gis tool that makes use of the nrcs-cn equation to estimate surface runoff: ( ) ( ) 20.2 p 0.2s 08. p s q p s − = > + (1) the value of s is the maximum potential water retention and depends on the capacity of the soil to produce runoff, given a value of precipitation (p). only when the precipitation value exceeds the initial abstraction value (0.2s) runoff (q) occur. the value of s can be estimated based on the value of cn by the following expression (usda, 1986): ( )25 400 254 s mm cn = − (2) the cn is an empirical parameter based on the hydrologic soil group (hsg), land use, treatment, and hydrological conditions. the cn is a dimensionless index that ranges from 0 to 100 representing extreme conditions between total infiltration (runoff = 0) and impermeable surfaces (precipitation = runoff). these cn values were assigned according to the usda tr55 manual (1986). soil types were defined based on the soil vector data set scale 1: 250,000 series ii (national continuum). these polygons were classified by inegi following the criteria of the fao’s world reference base for soil resources (2014). in this work, these polygons were reclassified into four hydrologic soil groups (hsg) using the cn method (usda, 1986). this spatial database was available in vector format and was projected to the universal transversal mercator system (utm), zone 13 n, using the wgs84 datum. the precipitation data were acquired from the climate information platform (pic) of the national commission of protected natural areas (conanp) for the automatic meteorological station with identification 16a106e4 chamela-cuixmala (geographic coordinates: -105.045, 19.499; datum wgs84; fig. 1). the runoff response was evaluated for each storm event during the 2013-2019 period. only those precipitation events greater than one inch were considered as generating runoff. the antecedent moisture condition for each storm was assigned from the accumulated precipitation for the five days before each event. finally, and for comparison purposes, we calculated the runoff coefficient (cq). this value is presented as a percentage and is calculated by the ratio of the amount of precipitation received and the amount of runoff. results drainage morphometry of watershed the drainage system of the río cuitzmala watershed corresponds to a fifth-order network. it presents a total of 2,068 streams mainly of the first order and the total length of channels is estimated at 1,201 km (table 1). the values of the geomorphological parameters suggest a watershed with areas of a gentle slope, as well as a moderate to high permeability with dense vegetation. the low value of the stream frequency agrees with this and also indicates that the gentle terrain slopes confer a high infiltration. the shape parameters place the watershed in a young stage of the streams, well-drained and with a low potential for flooding. the channel maintenance constant indicates that the sub-basin is under conditions of few structural disturbances and low runoff. land use and land cover the confusion matrix of the thematic map yielded an overall accuracy of 82% (95% ci = 75% 90%) and a kappa estimator = 0.76 (95% ci = 0.67 0.86). all user accuracies were above 80%. only the producer accuracies of the agriculture and evergreen forest classes presented low values of 71% and 78%, respectively. the landscape of the study area presented a predominance of natural vegetation. from the approximately 1,120 km2, the tropical dry forest (tdf) and the evergreen forest (ef) covered 435 km2 and 333 km2, respectively. the area covered by tdf was mainly distributed in the lower parts of the study area, while the ef was located in the upper parts of the sub-basin. the grassland, another important land cover in terms of its extension (326 km2) was present in the middle part of the sub-basin (fig. 2). surface runoff the four hsg (a, b, c, and d) with their description and soil types associated with each of them is shown in table s1. seven soil units were identified in the study area, regosol being the dominant soil, occupying 62.6% (700 km2) of the total area. this soil type is generally mineral, and very weakly developed with low moisture retention capacity. in order of importance, phaeozem and cambisol represent about 17.5% and 9.5% of the area, respectively. phaeozem corresponds to fertile porous soils while cambisol corresponds to soils developed in medium and fine textures. https://www.inegi.org.mx/app/geo2/elevacionesmex/ https://www.inegi.org.mx/app/geo2/elevacionesmex/ 316 rafael hernández-guzmán, norma leticia onchi-ramuco table 1. morphometric parameters of the río cuitzmala watershed obtained through the processing of the mexican continuous elevation at 15 m resolution. parameter symbol formula value in the watershed total number of stream orders (strahler 1957) σnu hierarchical order 2,068 (5th order) stream length (km) (horton 1945) σlu length of the streams 1201 bifurcation ratio (horton 1945) rb 1 u u n n + 1.88 area (km2) a 1,120 perimeter (km) p 236 drainage density (km/km2) (horton 1945) dd lu a ∑ 1.07 stream frequency (km/km2) (horton 1945) fs nu a ∑ 1.85 basin length (km) (horton 1945) lb 108.63 elongation ratio (schumm 1956) re 2 /a lb π   ×     0.35 circulatory ratio (miller 1953) rc 2 4 a p π 0.25 form factor (horton 1945) rf 2 4 a p π 0.09 length of overland flow (km) (horton 1945) lo 1/2dd 0.47 constant of channel maintenance (km) (schumm 1956) c 1/dd 0.93 highest elevation (m) h 1,769 lowest elevation (m) h 0 basin relief (m) (schumm and hadley 1961) r h–h 1769 relative relief (m) rlr r × 100/p 749.58 time of concentration (patton 1988) tc 1.15 0.3856.95 lb r   ×    85.70 ruggedness number (km) (schumm 1956) rn r × dd 1.90 317estimating surface runoff using the curve number method in an ungauged watershed in jalisco, mexico evaluating its morphometric parameter figure 2. land use and land cover of the río cuitzmala watershed in the coastal zone of jalisco, mexico, obtained with the unsupervised classification of a sentinel-2 image from 2019. figure 3. hydrologic soil groups of the río cuitzmala watershed in the coastal zone of jalisco, mexico. all of the hydrologic soil groups described in the runoff curve number method were present in the study area (fig. 3). soils with moderate runoff potential (b) were the most representative in the area (79.9%). the soils of hydrological group c (19.9%), with high runoff potential, followed in order of importance, while soils with low runoff potential (high infiltration, represented by hydrological group a), occupied the least extent. the intersect geoprocessing between land cover vs. hydrologic soil group produced 30 categories to which the corresponding cn was assigned. excluding water bodies (to which a cn value = 98 was assigned), agricultural lands under any hydrological soil group are those that presented the highest cn values; therefore, they present a greater potential for runoff per unit area (table 2). table 2. land use and land covers (area in square kilometers) with the associated curve numbers for the three antecedent moisture conditions (amc). land use and land cover hsg cn ii cn i cn iii area (km2) agriculture b 86 72 93 10.60 c 91 81 96 9.29 d 94 87 97 0.25 burned areas b 69 48 84 2.17 c 79 61 90 0.27 evergreen forest a 40 22 61 0.13 b 66 45 82 275.20 c 77 58 89 56.58 d 85 70 93 0.74 grassland a 49 29 69 0.13 b 69 48 84 269.50 c 79 61 90 55.63 d 84 69 92 0.29 littoral a 50 30 70 0.27 b 50 30 70 0.05 c 50 30 70 0.49 d 50 30 70 0.21 mangrove c 70 49 84 0.07 d 77 58 89 0.02 tropical dry forest a 36 19 56 0.10 b 60 39 78 336.10 c 73 53 86 98.42 d 79 61 90 0.21 human settlements b 65 44 81 1.02 c 77 58 89 1.04 d 82 66 91 0.26 aquatic surfaces a 98 95 99 0.01 b 98 95 99 0.33 c 98 95 99 0.65 d 98 95 99 0.37 with the previous information, the runoff volume was estimated for each of the selected storm events (table s2). annual accumulated precipitation ranged from 658 to 1,083 mm from 2013 to 2019. an average of 963 mm was found with 82% of this precipitation concentrating from july to november, with sporadic storm events from january to march. a total of 77 storm events were identified, of which 60% corresponded to the amc i. the total runoff volume ranged between 0.05 and 227.2 mm3. september and october were the months with the most intense rainfall and therefore when the highest runoff volumes 318 rafael hernández-guzmán, norma leticia onchi-ramuco occurred. the low values in the runoff volume from december to february are due to the scarce precipitation received, and the fact that the antecedent precipitation is relatively low (amc i). the average runoff was 14.52 mm3 and the average runoff coefficient was 12.86%. discussions determining the order of the streams is critical to understanding the drainage characteristics of the catchment area (horton, 1945; strahler, 1957). the río cuitzmala watershed presented a total of 2,068 streams of which 922 (44.6%), 494 (23.9%), 313 (15.1%), 83 (4.0%), and 256 (12.4%) belong to the orders 1st, 2nd, 3rd, 4th, and 5th, respectively. the high frequency of first-order streams in the upper part of the watershed indicates a young topography and highlights the compact nature of the bedrock lithology (granite – granodiorite of 97.5-108.5 million years belonging to the lower cretaceous period). the constant decrease in the number of streams in relation to the drainage order (nu) (except the fifth-order) indicates the predominance of the erosive landform throughout the watershed. this result coincided with pande and moharir (2017), who indicate that the total number of streams gradually decreases as the order of streams increases. regarding the bifurcation ratio (rb), this parameter denotes the water transport capacity and the potential for flash flooding during prolonged rains in any watershed (bhat et al., 2019). although horton (1945) indicates that the rb value ranges from a minimum of 2 in “flat drainage basins” to 3 or 4 in “ mountainous or highly dissected drainage basins”, resmi et al. (2019) suggest the rb value is in a range between 3 and 5 for basins in which geological structures deform the drainage pattern. in this study, the rb values ranged between 0.32 and 3.77 (average of 1.8) positioning our watershed as a welldrained area, with flat terrain, and a permeable and soft bedrock where most water infiltrates. this indicates a basin with a low potential for flooding and an area with a higher potential for water recharge. strahler (1964) states that low rb values are characteristic of watersheds that have suffered fewer structural disturbances. this low value of rb is due to the presence of a large number of first and second-order streams, and imply a flat area with a flowing drainage system (bharath et al., 2021; haokip et al., 2021). drainage density (dd) is useful to understand the potential for runoff or travel time of water in a basin as well as the infiltration capacity of the land, relief, and vegetation cover of the basin (bhat et al., 2019). according to pallard et al. (2009) and bhat et al. (2019), dd values (high/low) are obtained due to sub-surface material (impermeable/permeable), vegetation (sparse/good), relief (high/low), runoff/infiltration (high/low) and flood volumes (high/low). horton (1945) stated a well-drained basin has a drainage density of 0.73, whereas a poorly drained one has a drainage density of 2.74 (bogale, 2021; obeidat et al., 2021). the low drainage density (dd) value of 1.07 km/km2 indicated moderate to high permeability with dense vegetation and low relief, which suggests low runoff during flood periods. smith (1950) categorized the drainage density into five different textures classes, i.e., very coarse (<2), coarse (2-4), moderate (4-6), fine (6-8), and very fine (>8) (alam et al., 2021; ghosh et al., 2021; khan et al., 2021). thus, the dd value in the study area is less than 2, which characterizes a coarse drainage system. the morphometric analysis indicated that it is a young watershed with gentle slopes, soils that confer high infiltration, and rocks with high permeability, which concur with the results of the assignment of hydrological groups. considering the runoff curve number method, 80% of the watershed presented soils that have low to moderate runoff potential (b) that, together with the main covers found in the watershed obtained through the classification of satellite images, confirms the low values in dd. regarding the precipitation in the watershed, most of it occurs during the rainy season (july – november), which results in maximum flows in streams and the river. tropical storms are common during august, september, and october. the hurricanes and storms that have affected directly or indirectly, as well as the precipitation recorded in the meteorological station near the study area, were: hurricane norbert (september 2-8, 2014; 198.4 mm), hurricane patricia (october 20-24, 2015; 123.4 mm), tropical storm lidia (august 29 september 3, 2017; 267.2 mm), tropical storm pilar (september 23-25, 2017; 110.6 mm), hurricane willa (october 20-24, 2018; 275.4mm); tropical storm lorena (september 17-22, 2019; 340.6 mm), tropical storm narda (september 28 – october 1, 2019; 103.8 mm). during these events, most of the precipitated water becomes surface runoff. it is important to mention that, during the dry season, the rain is almost nil. despite some storm events during the dry season, most of the rain is used to saturate the soil, which is completely dry. comparing the results obtained on the runoff coefficient (12.86%) with the value of 11.25% reported by inegi (2000) for the san nicolás-cuitzmala river basin, the value estimated in the present work is slightly higher. however, this difference could be considered negligible. conclusions the río cuitzmala watershed is an area of low slope, with a moderate to high permeability, dominated in a greater proportion by a forest cover and low relief. it is a watershed in the young stage of the streams whose runoff drains rapidly, it does not present large floods of flow, which indicates a low potential for flooding. under current conditions, its runoff coefficient was estimated at 12.86%. morphometric parameters are stable indicators and offer a relatively rapid way to characterize a drainage unit. however, it is important to complement this information using other methods such as paired watersheds and direct measurement of the components that influence the generation of surface runoff. the cn method incorporating remote sensing and gis techniques in ungauged basins provides useful results for outlining conservation strategies and implementing management practices at the watershed level, helping to solve specific environmental problems and maximizing the use of economic and social resources. references abdulkareem, j. h., pradhan, b., sulaiman, w. n. a. & jamil, n. r. 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(2025). accuracy assessment of the ppp solution using standard gnss receivers. earth sciences research journal, 29(2), 219-230. https://doi.org/10.15446/esrj. v29n2.118407 the introduction of the free ppp solution by the bds-3 system and its published applications offers a feasible opportunity to conduct surveying tasks without the need to establish base stations. this paper examines the accuracy of the free ppp solution using a standard receiver integrated with this functionality to verify the technical specifications published in its documentation. the study utilises a comnav n3 receiver to collect coordinate data from the ppp and near-baseline single-base rtk solutions for accuracy assessment. the results indicate that parameters such as the time to achieve a fixed solution and the reported accuracy align closely with the technical specifications. additionally, the determination of relative distances and height differences between points in the designed experiment shows equivalence between the ppp solution and the rtk solution. keywords: gnss-rtk, ppp solution, n3 gnss, convergence, ppp accuracy resumen el lanzamiento de una solución gratuita de posicionamiento preciso punto a punto (ppp) por el sistema bds-3 y sus aplicaciones presentan una oportunidad factible de realizar tareas de sondeo sin la necesidad de establecer estaciones base. este artículo examina la precisión de la solución ppp a través de un receptor estandar integrado con la funcionalidad de verificar las especificaciones técnicas publicadas en su documentación. durante el estudio se utilizó un receptor comnav n3 para recolectar datos de coordenadas del ppp y soluciones de base rtk para comparar la precisión. los resultados indican que parámetros como el tiempo de adquisicion de respuesta y la precisión reportada se alinean con las especificaciones técnicas. adicionalmente, la determinación de distancias relativas y el peso de las diferencias entre puntos en el experimento diseñado muestran una equivalencia entre la solución ppp y la solución rtk. palabras clave: estaciones gnss-rtk, solución de posicionamiento preciso punto a punto; receptor n3; convergencia; precisión. evaluación de la precisión de la solución ppp utilizando receptores gnss estándar https://doi.org/10.15446/esrj.v29n2.118407 mailto:dungln@huce.edu.vn https://orcid.org/0000-0002-9957-5757 https://orcid.org/0000-0001-8960-772x https://orcid.org/0000-0002-3838-9950 https://orcid.org/0000-0001-6246-8475 https://doi.org/10.15446/esrj.v29n2.118407 https://doi.org/10.15446/esrj.v29n2.118407 220 vu ngoc quang, luong ngoc dung, nguyen viet ha, tran dinh trong 1. introduction dynamic global navigation satellite system (gnss) positioning techniques require the connection of base stations with rovers at measurement points during data acquisition. the methods used for the interaction and operation between base stations and rovers include real-time kinematic (rtk) measurements, static post-processing, and post-kinematic (ppk) solutions (hofmann-wellenhof et al., 2007). the real-time kinematic gnss measurement method has demonstrated its effectiveness in surveying work (tran dinh trong & luong ngoc dung, 2024) and has been formalized in the current standards (ministry of natural resources and environment, 2015). within the rtk framework, two primary subcategories exist: single-reference station rtk and network-based rtk. despite the differences in approach (elmowafy, 2012), these methods share a common requirement: the operation of one or more reference stations, which can lead to unnecessary equipment costs. the idea of conducting surveys without establishing reference stations has become feasible with the advent of precise point positioning (ppp) services (china satellite navigation office, 2019a; european gnss supervisory authority., 2023), which can be utilized with conventional receivers available to surveying engineers. early analyses of the ppp-b2b service were conducted by tang et al. (tang et al., 2022), and the performance of the ppp-b2b solution was evaluated across mainland china and surrounding regions by liu et al. (y. liu et al., 2022), in vietnam by tran et al (2023). as a result, gnss receiver manufacturers have developed devices capable not only of receiving signals from multiple satellite navigation systems and supporting multi-frequency and multi-channel operations (comnav technology ltd., 2020; unicore communications, 2022), but also of integrating free ppp features to attract customers and meet their precision needs. for instance, comnav receivers offer such features (comnav tech, 2021). additionally, an assessment of the long-term performance of beidou ppp-b2b was conducted in 2022 by he et al. (he et al., 2023). the findings confirmed that the performance of the ppp-b2b product has shown significant improvements compared to the results reported two years ago. when combining beidou navigation satellite system-3 (bds-3) with galileo, accuracy and convergence can improve by up to 50% compared to using a single system (hou & zhou, 2023). although some risks have been identified by t. liu et al (2017), the error sources have been analyzed, and their applications are becoming increasingly widespread (elsheikh et al., 2023). two main drawbacks of ppp for surveying tasks are the extended initialization times compared to rtk positioning (15–30 minutes for ppp versus a few seconds for rtk) and the necessity to restart the entire process if the signal is lost. in terms of accuracy, (naciri et al., 2023) demonstrated that integrating ppp with galileo and gps can achieve an accuracy of approximately 18.6 cm in both horizontal and vertical directions, with a convergence time of 7.5 minutes. more recently, pintor et al. (pintor et al., 2023) evaluated the high-accuracy service (has) for maritime operations, reporting accuracies of 0.22 m and 0.13 m at 95% and 68% confidence levels, respectively. these findings are consistent with the parameters outlined in the literature of the european gnss supervisory authority (european gnss supervisory authority., 2023). additionally, li and pan (li & pan, 2021) conducted a study comparing the accuracy of this method across various analysis centers, finding that the combination of multiple systems can enhance both positioning accuracy and convergence time for post-processed kinematic ppp solutions. the literature highlights a gap in evaluating the short-term precision positioning performance in localized areas with high demands for precise positioning, such as in civil engineering activities in vietnam. while free ppp solutions are now available for certain receiver models currently offered in vietnam, their adoption in surveying applications remains limited, despite their accuracy being well-documented in technical specifications. this study investigates the accuracy of free ppp solutions under open environmental conditions and compares their performance with that of a near-baseline rtk solution. the aims of the study will be addressed in the subsequent sections, including the ppp-b2b introduction, device, and experimental area; results and discussion; and conclusions. 2. ppp-b2b service, device and experiment area. 2.1. beidou ppp-b2b service based on the announcement from the chinese satellite navigation office, the bds-3 space constellation consists of 03 geostationary earth orbit (geo) satellites, 03 inclined geo-synchronous orbit (igso) satellites, and 24 medium earth orbit (meo) satellites at different altitudes (china satellite navigation office, 2019b, 2019a). bds-3 signals are transmitted on three frequencies: b1 (b1c-1575.42 mhz), b2 (b2a-1176.45 mhz, b2b-1207.14 mhz), and b3 (b3i-1268.52 mhz) (m. lu. et al., 2019), with the signal on frequency b2b being utilised for ppp positioning methods and a range of other applications. the three geo satellites broadcast orbit error correction parameters and satellite clock error corrections to users through the b2b frequency within china and neighbouring regions. the ppp-b2b_i signal (sb2b_i)(t)) is generated by modulating the navigation message data (db2b_i (t)) and the range code (cb2b_i (t)) (china satellite navigation office, 2019a): _ ( ) = 1 √2 . _ ( ). _ ( ) (1) where _ ( ) = _ [ ] _ − _ (2) where db2b_i is the navigation message data code, tb2b_i is the chip width of the corresponding data code, and pb2b_i is a rectangle pulse with a width of tb2b_i. the expression of range code cb2b_i is as follows: _ ( ) = [ ] _ _ ( − _ + _ _ ) (3) in eq.3, cb2b_i is a ppp-b2b_i ranging code sequence (possible values are ± 1), nb2b_i is the ranging code length with a value of 10230, tc_b2b_i = 1⁄rc_b2b-1 is the ppp-b2b_i chip period of ranging code, and rc_b2b-1 = 10.23 mbps is the ppp = b2b_i chipping rate, and ptc_b2b_i is a rectangle pulse with a duration of tc_b2b_i. in ppp, the pseudo-range and carrier phase observations can be written as follows (tang et al., 2022): = + − + + + , − + (4) = + − − + + , − + + (5) where: s,i refers to satellite and signal frequency, respectively p,φ refers to pseudo-range observation and carrier phase observation ρ denotes the distance from the satellite to the receiver dt, dts refers to receiver errors and satellite clock errors ii s refers to ionospheric delays ts refers to tropospheric delays br,i,bi s refers to the pseudo-range code deviation of receivers and satellites br,i,bi s refers to the hardware delay of receivers and satellites ni s refers to the integer ambiguity of carrier phase observations εp s,εφ s refers to the measurement noise of pseudo-range and carrier phase 2.1.1. satellite orbit correction as follows, the satellite position correction vector 𝛿𝑋 can be calculated via orbit corrections. = − (6) the satellite correction vector δx is as follows: = [ ]. (7) where: | |; ; × ̇ | × ̇| and = × 221accuracy assessment of the ppp solution using standard gnss receivers in the formula, r, ̇r refers to the satellite position and velocity vector in the broadcast ephemeris, eradial,ealong,ecross correspond to the unit vectors in the radial, tangential,and normal directions. 2.1.2. satellite clock correction once the clock correction message successfully matches the broadcast ephemeris, the clock error tbroadcast calculated from broadcast ephemeris can be corrected by a clock correction message c0 as follows. = − (8) messages 2 and 4 describe satellite orbit and clock correction, as described in (china satellite navigation office, 2019b). the beidou ppp-b2b signal’s coverage includes the asia-pacific region, and bds becomes the second gnss system after the galileo system to provide real-time ppp service (x. lu et al., 2021). 2.2. device and experiment procedure to assess the performance of the ppp solution in an open and unobstructed environment, six points were chosen in an outdoor park area, free from obstructions or coverage by other features. figure 1. distribution of six measuring points and base station position six points (p1-p6) are distributed over an open area in the courtyard of an outdoor park with a rectangular shape as in figure 2 and will be positioned using either rtk or ppp. for comparison, in addition to measuring ppp values at different times, we simultaneously conducted measurements using the singlebase rtk method, with the base station located nearby (about 211 meters from the base to the rover, the consecutive distance between points pi are smaller than 10 m). the rationale for selecting rtk as a basis for comparison includes two main reasons: 1. ppp and rtk functionalities are both available within a single gnss receiver, allowing for a comparative evaluation based on each method’s published specifications. 2. the rtk solution is widely recognised, validated, and established in current standards, making rtk measurement values a reliable reference for evaluating accuracy. both rtk and ppp methods use an n3 device, part of the comnavequipped k8 series, which supports the precise point positioning (ppp) function, as illustrated in figure 2. table 1 provides its fundamental specifications. to support users in measurement scenarios where the rtk solution is unstable or in locations without single-base station services or rtk networks, comnav has developed the k8 firmware version that supports both ppp and rtk functionalities. thus, when the rtk solution cannot be utilized, users can conveniently switch to the ppp mode with this firmware update. the results from the ppp solution are in real-time, leveraging the ppp algorithm integrated directly into the receiver’s firmware. it is important to emphasize that this is a free feature providing real-time results, not post-processing on specialized high-accuracy software such as gamit, glock, or other online ppp services. figure 2. n3 gnss device with ppp function table 1. basic specifications of n3 device specifications content channels 1198 rtk accuracy 8 mm + 1 ppm horizontal 15 mm + 1 ppm vertical ppp accuracy 10 cm horizontal root mean square error (h.rms), 20 cm vertical root mean square error (v.rms) the experiment consists of two sections and is conducted following these steps to evaluate the accuracy of the ppp solution. section 1 includes three steps • step 1: measure the coordinates and elevations of six points (p1– p6) using the ppp solution at 5s -10s 15s 30s sample intervals for each point. • step 2: invert the receiver until the signal is lost, then return it to an upright position and repeat the measurement procedure as conducted the second time. • step 3: analyse and compare the coordinate values, elevations, and distances between points to those of six points (p1–p6) using the rtk method in single base-rover 4g working mode, with a short baseline distance of 211 meters between the base and the rover (5-second observation). section 2 is for the assessment of the convergence of ppp observations • step 4: measuring at point p6 to evaluate the convergence of ppp observations. to measure, the receiver is mounted on a vertical pole using a mini tripod with an alligator as shown in figure 3. (a) (b) (c) (d) (e) (f) figure 3. n3 receiver positioned at six points figures 3(a)-(f) show the instrument setup at measurement points p1 to p6. the points pi are marked as crosses using correction pens. the vertical gnss-rtk setup method was consistently applied to both ppp and rtk solutions based on a water bubble attached to the rtk pole. it should be noted that both methods use the same pole and centering accuracy has the same impact on the results. the reason for selecting values from the gnss-rtk method for comparison is that, according to the technical specifications of the equipment, the accuracy of rtk is significantly higher than that of the free ppp method. moreover, the distance from the base station to the rtk measurement points 222 vu ngoc quang, luong ngoc dung, nguyen viet ha, tran dinh trong is very close (211 m only). regarding the coordinate system, both methods use the vn:2000 coordinate system, transverse mercator (tm) projection, and central meridian 1050. 3. results and discussion in steps 1 and 2 with the ppp solution, the fixed solution is achieved after around 32 minutes of connection. coordinates and elevations of six points after four measuring times at 5s, 10s, 15s, and 30s in two rounds are in table 3 (appendix). in step 3, with the rtk method, the fixed solution is achieved right after connecting to the base station due to the short distance. their coordinates and elevations are in table 4 (appendix). regarding the ppp solution, the fixed solution is achieved 20 minutes after the receiver is inverted and the signal is lost (from 14:49 to 15:09). this duration is 12 minutes shorter than the first round. examining the elevation column (z column), it can be observed that the elevations from the ppp solution deviate significantly from those of the rtk solution. this discrepancy can be explained by the fact that rtk measurement points are connected to a single base station with elevations derived from national benchmarks. this means rtk measurement points have their elevations calculated within the national vertical reference system, whereas ppp measurement points do not. according to the theory and published specifications, the coordinate values obtained from the rtk method have higher accuracy. by using the rtk-derived values as the true measurements, the deviations in the x, y and z directions for the six ppp measurement points across two observations at different times are calculated using the following formula: ∆ ( ) = ( ) − ( ) ∆ ( ) = ( ) − ( ) ∆ ( ) = ( ) − ( ) (9) the deviations of the six points between the two measurement times of the ppp method compared to the rtk method are shown in figure 4 (a)-(m). -0.200 -0.150 -0.100 -0.050 0.000 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (a) x deviations of two rounds of ppp compared to the rtk method, point 1 (b) y deviations of two rounds of ppp compared to the rtk method, point 1 -0.150 -0.100 -0.050 0.000 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.500 -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (c) x deviations of two rounds of ppp compared to the rtk method, point 2 (d) y deviations of two rounds of ppp compared to the rtk method, point 2 223accuracy assessment of the ppp solution using standard gnss receivers -0.150 -0.100 -0.050 0.000 0.050 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (e) x deviations of two rounds of ppp compared to the rtk method, point 3 (f) y deviations of two rounds of ppp compared to the rtk method, point 3 -0.150 -0.100 -0.050 0.000 0.050 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.400 -0.300 -0.200 -0.100 0.000 0.100 0.200 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (g) x deviations of two rounds of ppp compared to the rtk method, point 4 (h) y deviations of two rounds of ppp compared to the rtk method, point 4 -0.150 -0.100 -0.050 0.000 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.410 -0.310 -0.210 -0.110 -0.010 0.090 0.190 0.290 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (i) x deviations of two rounds of ppp compared to the rtk method, point 5 (k) y deviations of two rounds of ppp compared to the rtk method, point 5 224 vu ngoc quang, luong ngoc dung, nguyen viet ha, tran dinh trong -0.100 -0.050 0.000 0.050 5s-obs 10s 15s 30s x de vi at io ns (m ) time of observations δx-ppp (1) δx-ppp (2) -0.410 -0.310 -0.210 -0.110 -0.010 0.090 0.190 0.290 5s-obs 10s 15s 30s y de vi at io ns (m ) time of observations δy-ppp (1) δy-ppp (2) (l) x deviations of two rounds of ppp compared to the rtk method, point 6 (m) y deviations of two rounds of ppp compared to the rtk method, point 6 figure 4. oscillation of x, y coordinates at 5-10-15-30s at six points in figure 4, the four points on the grey and orange lines correspond to the measurements taken at 5s, 10s, 15s, and 30s during two measurement sessions at points p1 to p6. the deviations in the x and y coordinates of the six points during the first measurement session, with varying time intervals, range from 0.024 m to 0.038 m and from 0.142 m to 0.240 m, respectively. in the second measurement session, the deviations range from 0.102 m to 0.138 m for the x coordinate and from 0.124 m to 0.162 m for the y coordinate, which are significantly larger compared to the first session. when the x and y coordinates are averaged over the measurements taken at 5s, 10s, 15s, and 30s, the deviation in the x coordinate of the six points ranges from -0.001 m to 0.038 m, and these deviations in the y coordinate range from 0.310 m to 0.370 m. the distances between the pi points are relatively small, and the measurements were conducted within a short time frame; therefore, the points can be assumed to share similar environmental conditions. taking the rtkderived values as a reference, the rmse of the x-coordinate at sampling intervals of 5s, 10s, 15s, and 30s across six points was calculated as 0.079 m, 0.054 m, 0.032 m, and 0.035 m, respectively. similarly, for the y-coordinate, the corresponding rmse at the same sampling intervals were 0.183 m, 0.215 m, 0.226 m, and 0.259 m. a contrast is observed between the x and y coordinate components. while the x component follows the expected theoretical trendshowing improved accuracy with increased sampling intervals (i.e., longer observation times) the y component exhibits the opposite behaviour. considering the factors that may affect positioning accuracy such as satellite geometry, multipath errors, reference frame inconsistencies, and random noise the observed discrepancy is likely attributable to satellite geometry and random variations, given that the pi points share the same location and favourable conditions for satellite signal reception. however, due to the limited number of samples in the experiment, it is difficult to draw a definitive conclusion regarding this phenomenon. regarding the elevation factors, since the elevations obtained from the ppp and rtk methods are based on different vertical reference systems, we examine the variation in elevation between the two ppp measurements conducted at different observation times. figure 5(a)-(f) shows the deviations in the z factor of six points between the two measurement times. the elevation deviations of the six points in the first measurement session range from 0.094 m to 0.229 m, while in the second session, they range from 0.028 m to 0.077 m. when averaging the elevation values across the measurements taken at 5s, 10s, 15s, and 30s, the deviations range from 0.267 m to 0.299 m. figure 5 illustrates smaller variations in the height component across different sampling intervals during the second trial. this may be attributed to the receiver achieving a faster convergence and more stable performance in the second trial, compared to the first trial, which was conducted shortly after the receiver had been initialized for only 30 minutes. distance and different heights from point to point will be calculated and compared between the ppp and rtk methods. distance deviations are shown in figure 6(a)-(f). 4.400 4.450 4.500 4.550 4.600 4.650 4.700 4.750 4.800 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) 4.450 4.500 4.550 4.600 4.650 4.700 4.750 4.800 4.850 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) (a) point 1 (b) point 2 4.450 4.500 4.550 4.600 4.650 4.700 4.750 4.800 4.850 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) 4.500 4.550 4.600 4.650 4.700 4.750 4.800 4.850 4.900 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) (c) point 3 (d) point 4 4.500 4.550 4.600 4.650 4.700 4.750 4.800 4.850 4.900 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) 4.450 4.500 4.550 4.600 4.650 4.700 4.750 4.800 4.850 5s 10s 15s 30s el ev at io n (m ) time of observation z-ppp (1) z-ppp (2) (e) point 5 (f) point 6 figure 5. oscillation of z factor at 5-10-15-30s observations of two rounds of ppp 225accuracy assessment of the ppp solution using standard gnss receivers 8.136 8.144 8.138 8.132 8.134 8.136 8.138 8.140 8.142 8.144 8.146 p1-p2 (1) p1-p2 (2) p1-p2 (rtk) d is ta nc e (m ) 8.060 8.071 8.066 8.054 8.056 8.058 8.060 8.062 8.064 8.066 8.068 8.070 8.072 p2-p3 (1) p2-p3 (2) p2-p3 (rtk) d is ta nc e (m ) 6.042 6.064 6.058 6.030 6.035 6.040 6.045 6.050 6.055 6.060 6.065 6.070 p3-p4 (1) p3-p4 (2) p3-p4 (rtk) d is ta nc e (m ) 8.081 8.077 8.077 8.075 8.076 8.077 8.078 8.079 8.080 8.081 8.082 p4-p5 (1) p4-p5 (2) p4-p5 (rtk) d is ta nc e (m ) 8.114 8.093 8.121 8.075 8.080 8.085 8.090 8.095 8.100 8.105 8.110 8.115 8.120 8.125 p5-p6 (1) p5-p6 (2) p5-p6 (rtk) d is ta nc e (m ) 6.038 6.085 6.075 6.010 6.020 6.030 6.040 6.050 6.060 6.070 6.080 6.090 p6-p1 (1) p6-p1 (2) p6-p1 (rtk) d is ta nc e (m ) figure 6. distance comparison between ppp (two times) and rtk method although the deviations in the x and y coordinates are relatively large between the two ppp measurement sessions, the deviations are negligible when calculating the distances between measurements and comparing them to distances obtained using the rtk method. the largest deviation in distance is observed between points p1 and p6 (0.047 m), while the deviations for all other distances are less than 0.030 m. it should be noted that the distances between points in each measurement session are calculated by averaging the distances from the measurements taken at 5s, 10s, 15s, and 30s. by applying a similar approach, let us examine the elevation differences between the points and their deviations. the results are presented in figure 7(a)-(f). it can be readily observed that the absolute elevation deviation between ppp and rtk is significant. this is understandable since rtk is corrected based on the national elevation reference point. however, using a similar approach as with distance measurements, the deviations in relative elevation differences are found to be very small, even smaller than those of the distance measurements. a more important observation is that, although the relative elevation differences between the points are small, they all have the same sign as those obtained using the rtk method. next, we examine the convergence of the ppp measurements at point p6, with a total of 156 measurements, each taken at 5-second intervals. the distribution chart of the x, y, and z parameters at point p6 is shown in figure 8(a)-(c). from figure 8, some values in the y coordinate and z elevation series can be considered outliers. these outliers result in relatively large ranges of variation. the smallest range is observed in the x coordinate series, with a value of 0.042 m, while the range for the y coordinate is 0.297 m, and the largest range is in the z elevation, with a value of 0.657 m. it can be seen that the values considered outliers appear during the initial seconds of the measurement process at point p6. it can be observed that, during the initial stages of the observation series, although the fixed solution was achieved after the manufacturer’s recommended convergence time, the solution remained unstable. a similar trend to the first experiment is evident, in which the deviations in the y and z coordinate components were significantly larger than those in the x component. it is important to emphasize that this study utilizes objective results to evaluate the potential of the integrated ppp solution in standard gnss receivers. for applications requiring high-precision positioning, outlier filtering algorithms, as employed in previous studies, should be considered (ngoc quang & van hien, 2022; thi nhung et al., 2021). a b dc e f 226 vu ngoc quang, luong ngoc dung, nguyen viet ha, tran dinh trong 0.038 0.048 0.039 0.000 0.010 0.020 0.030 0.040 0.050 0.060 p1-p2 (1) p1-p2 (2) p1-p2 (rtk) h ei gh t d iff er en ce (m ) 0.070 0.057 0.074 0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 p2-p3 (1) p2-p3 (2) p2-p3 (rtk) h ei gh t d iff er en ce (m ) 0.011 0.033 0.019 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 p3-p4 (1) p3-p4 (2) p3-p4 (rtk) h ei gh t d iff er en ce (m ) -0.069 -0.067 -0.072-0.073 -0.072 -0.071 -0.070 -0.069 -0.068 -0.067 -0.066 -0.065 -0.064 p4-p5 (1) p4-p5 (2) p4-p5 (rtk) h ei gh t d iff er en ce (m ) -0.022 -0.015 -0.013 -0.025 -0.020 -0.015 -0.010 -0.005 0.000 p5-p6 (1) p5-p6 (2) p5-p6 (rtk) h ei gh t d iff er en ce (m ) -0.028 -0.056 -0.047 -0.060 -0.050 -0.040 -0.030 -0.020 -0.010 0.000 p6-p1 (1) p6-p1 (2) p6-p1 (rtk) h ei gh t d iff er en ce (m ) figure 7. height difference between ppp (two times) and rtk method 2321110.008 2321109.9662321109.960 2321109.970 2321109.980 2321109.990 2321110.000 2321110.010 2321110.020 0 20 40 60 80 100 120 140 160 x c oo rd in at e (m ) observations x-coordinates x_max x_min 581067.013 581066.716 581066.700 581066.750 581066.800 581066.850 581066.900 581066.950 581067.000 581067.050 0 20 40 60 80 100 120 140 160 y c oo rd in at e (m ) observations y_coordinates y_max y_min 4.878 4.2214.200 4.300 4.400 4.500 4.600 4.700 4.800 4.900 0 20 40 60 80 100 120 140 160 z co or di na te (m ) observations z factors z_max z_min figure 8. distribution of horizontal factors and elevation at point p6 a b dc e f a b c 227accuracy assessment of the ppp solution using standard gnss receivers to examine the accuracy parameters of the data series, table 2 provides basic descriptive statistics. table 2. descriptive statistics of x, y, and z data parameters x y z mean (m) 2321109.987 581066.964 4.719 standard error (m) 0.0008 0.0042 0.0091 median (m) 2321109.988 581066.97 4.726 mode (m) 2321109.985 581066.97 4.688 standard deviation (m) 0.0097 0.0526 0.1141 sample variance (m2) 0.0001 0.0028 0.0130 minimum (m) 2321109.966 581066.716 4.221 maximum (m) 2321110.008 581067.013 4.878 count 156 156 156 confidence level (95.0%) (m) 0.0015 0.0083 0.0180 using the basic error evaluation formulas of charles and paul (2006), the standard deviation values for the x, y, and z coordinate series are 0.0097 m, 0.0526 m and 0.1141 m, respectively. compared to the rtk method, the z factor in the ppp method has the highest standard deviation, the same as the rtk solution. 4. conclusion based on the experiment results, some outlines can be made as follows: firstly, the potential for conducting surveying tasks without the need for a base station, as required in the rtk method, can be partially realized with the free ppp solution. however, this is contingent upon the specific accuracy requirements of applications such as depth measurement, environmental pollution management, trajectory monitoring, and navigation. for more demanding applications, such as monitoring structural oscillations with large amplitude variations, advanced data processing solutions are essential. secondly, regarding accuracy, the free ppp solution available in the affordable and widely used n3 receiver produces results consistent with the specifications outlined in the device’s datasheet (comnav tech, 2021). these results are also comparable to findings from previous studies (naciri et al., 2023; pintor et al., 2023) thirdly, further investigation is needed to assess the adoption of the free ppp solution under more detailed and varied scenarios, including specific objectives and measurement durations. this will facilitate a more comprehensive evaluation of the method’s applicability. a notable issue identified in this study is the significant vertical deviation (approximately 2 meters) between the two methods, which is primarily attributed to inherent methodological differences. this study focuses on the application of free real-time ppp solutions. as a result, the positioning accuracy does not reach the level achievable with postprocessed ppp methods used in prior studies, due to the absence of latency compensation. a notable discrepancy is observed in the height component, primarily due to the use of different vertical datums in the two approaches. to mitigate this, ppp observations can be referenced to a known benchmark within the local vertical system, allowing for height adjustment to align with rtk-derived values. finally, the results were obtained using a free ppp solution on a standard gnss receiver operating in real-time, with both rtk and ppp capabilities. the findings support the integrated use of these two positioning methods in scenarios with limited 4g signal availability and in practical surveying applications such as forest area measurement, environmental monitoring, and damage assessment. this approach can serve as an alternative to handheld gps devices, offering improved positioning accuracy while maintaining consistency in both coordinate and vertical reference systems. acknowledgments the authors would like to acknowledge the financial support (no. b2024-mda-08) from the ministry of education and training of vietnam. references charles d, g., & paul r, w. 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(2022). um982 gps/bds/glonass/galileo/qzss all-constellation multi-frequency high precision positioning & heading module. china. https://doi.org/10.3390/jmse11122375 https://doi.org/10.3390/jmse11122375 https://doi.org/10.3390/rs14215366 https://doi.org/10.3390/rs14215366 https://doi.org/10.5194/egusphere-egu21-1598 https://doi.org/10.5194/egusphere-egu21-1598 https://doi.org/10.15446/esrj.v27n1.102630 https://doi.org/10.15446/esrj.v27n1.102630 https://doi.org/10.31814/stce.huce2024-18(2)-07 229accuracy assessment of the ppp solution using standard gnss receivers appendix table 3. coordinates and elevations of six points (ppp method) at different times time point sample interval n (m) e (m) z (m) measurement round 10/30/2023 14:12 p1 5s 2321111.835 581072.692 4.761 round 1 10/30/2023 14:14 p2 2321119.621 581070.435 4.814 10/30/2023 14:16 p3 2321127.354 581068.270 4.914 10/30/2023 14:18 p4 2321125.658 581062.533 4.951 10/30/2023 14:19 p5 2321117.895 581064.859 4.884 10/30/2023 14:21 p6 2321110.129 581067.151 4.841 10/30/2023 14:25 p1 10s 2321111.800 581072.910 4.791 10/30/2023 14:27 p2 2321119.610 581070.576 4.832 10/30/2023 14:29 p3 2321127.347 581068.238 4.900 10/30/2023 14:30 p4 2321125.641 581062.426 4.925 10/30/2023 14:31 p5 2321117.900 581064.695 4.864 10/30/2023 14:32 p6 2321110.127 581066.924 4.825 10/30/2023 14:33 p1 15s 2321111.814 581072.761 4.741 10/30/2023 14:34 p2 2321119.630 581070.422 4.789 10/30/2023 14:35 p3 2321127.362 581068.128 4.854 10/30/2023 14:36 p4 2321125.651 581062.313 4.852 10/30/2023 14:37 p5 2321117.912 581064.619 4.785 10/30/2023 14:38 p6 2321110.099 581066.944 4.770 10/30/2023 14:39 p1 30s 2321111.808 581072.751 4.697 10/30/2023 14:41 p2 2321119.601 581070.439 4.706 10/30/2023 14:43 p3 2321127.335 581068.160 4.753 10/30/2023 14:45 p4 2321125.628 581062.340 4.736 10/30/2023 14:46 p5 2321117.888 581064.653 4.655 10/30/2023 14:49 p6 2321110.091 581066.924 4.666 230 vu ngoc quang, luong ngoc dung, nguyen viet ha, tran dinh trong time point sample interval n (m) e (m) z (m) measurement round 10/30/2023 15:09 p1 5s 2321111.702 581072.526 4.463 round 2 (after up and down n3 receiver) 10/30/2023 15:10 p2 2321119.520 581070.243 4.540 10/30/2023 15:11 p3 2321127.258 581067.955 4.581 10/30/2023 15:12 p4 2321125.566 581062.141 4.625 10/30/2023 15:13 p5 2321117.814 581064.441 4.543 10/30/2023 15:14 p6 2321110.066 581066.692 4.553 10/30/2023 15:16 p1 10s 2321111.766 581072.495 4.475 10/30/2023 15:17 p2 2321119.560 581070.156 4.497 10/30/2023 15:18 p3 2321127.307 581067.871 4.540 10/30/2023 15:19 p4 2321125.614 581062.060 4.575 10/30/2023 15:20 p5 2321117.872 581064.363 4.516 10/30/2023 15:21 p6 2321110.090 581066.615 4.476 10/30/2023 15:22 p1 15s 2321111.796 581072.451 4.430 10/30/2023 15:23 p2 2321119.606 581070.136 4.474 10/30/2023 15:24 p3 2321127.346 581067.865 4.551 10/30/2023 15:25 p4 2321125.640 581062.036 4.584 10/30/2023 15:26 p5 2321117.912 581064.347 4.515 10/30/2023 15:27 p6 2321110.128 581066.599 4.506 10/30/2023 15:28 p1 30s 2321111.840 581072.402 4.442 10/30/2023 15:30 p2 2321119.651 581070.087 4.490 10/30/2023 15:31 p3 2321127.394 581067.808 4.555 10/30/2023 15:33 p4 2321125.685 581061.979 4.576 10/30/2023 15:34 p5 2321117.942 581064.286 4.519 10/30/2023 15:35 p6 2321110.168 581066.555 4.498 table 4. coordinates and elevations of six points (rtk method) time point n (m) e (m) z (m) 2023y10m30d15h47m55s p1 2321111.8510 581072.7970 6.2750 2023y10m30d15h48m59s p2 2321119.6520 581070.4810 6.3140 2023y10m30d15h49m49s p3 2321127.3860 581068.1920 6.3880 2023y10m30d15h50m32s p4 2321125.6800 581062.3790 6.4070 2023y10m30d15h51m40s p5 2321117.9420 581064.6960 6.3350 2023y10m30d15h52m39s p6 2321110.1540 581066.9640 6.3220 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.106550 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 387 395 h y d r o g eo lo g y statistical approaches for the identification of the origin mineralization groundwaters: case of the naama region, far west-algeria abderezzak boudjema1, sabrine guettaia1, abdessamed derdour 2,3, nafaa brinis4, abdulrazak h. almaliki5 1. laboratory n°25 promotion of water, mineral and soil resources, environmental legislation and technological choices, university of tlemcen, p.o. box 119, tlemcen 13000 algeria. abderezzak.boudjema@univ-tlemcen.dz; sabrine.guettaia@univ-tlemcen.dz 2. artificial intelligence laboratory for mechanical and civil structures, and soil, university center of naama, p.o. box 66, naama 45000, algeria. derdour@cuniv-naama.dz 3. laboratory for the sustainable management of natural resources in arid and semi arid zones, university center of naama, p.o. box 66, naama 45000, algeria. 4. laboratory of mobilization and management of water resources (lmgre), department of geology, institute of earth sciences, university of batna 2, algeria. 5. department of civil engineering, college of engineering, taif university, taif 21944, saudi arabia; a.almaliki@tu.edu.sa how to cite this item: boudjema, a., guettaia, s., derdour, a., brinis, n., & almaliki, a. h. (2024). statistical approaches for the identification of the origin mineralization groundwaters: case of the naama region, far west-algeria. earth sciences research journal, 28(4), 387-395. https://doi. org/10.15446/esrj.v28n4.106550 manuscript received: 28/12/2022 accepted for publication: 14/01/2025 abst r ac t understanding the processes controlling groundwater mineralization is critical for preserving its quality and ensuring sustainable resource management, especially in regions like naama, algeria, which rely exclusively on groundwater. this resource faces severe overexploitation and high salinity due to climatic factors and increasing industrial, agricultural, and domestic demands, posing significant challenges to its long-term usability. hydrochemical analyses, including graphical methods and multivariate statistical tools, were employed to investigate the groundwater chemistry and mechanisms influencing mineralization in the naama region. the results revealed a relatively homogeneous distribution of groundwater samples, categorized into three hydrochemical groups. these groups are primarily dominated by calcium and magnesium chloride and sulfate waters, influenced by the geological characteristics of the region, such as gypsum-saline formations of the upper cretaceous and triassic clay-gypsum-saline diapirs, and further impacted by inverse ion exchange processes. group 1 is characterized by higher proportions of calcium and magnesium bicarbonate waters due to the dominance of carbonate formations from the early jurassic or miocene, benefiting from more dynamic recharge zones. groups 2 and 3 exhibit similar chemical compositions, but group 3 is distinct for its restricted recharge zones and carbonate formations, leading to the emergence of calcium and magnesium bicarbonate waters. recharge areas associated with carbonate formations were also vulnerable to anthropogenic pollution. these findings highlight the importance of implementing strict protection measures for sensitive recharge zones to ensure the preservation of this vital resource in the face of increasing environmental and human pressures. keywords: naama aquifers; mineralization; multivariate statistical analysis; hydrochemistry; pollution. palabras claves: acuíferos de naama; mineralización; análisis estadístico multivariante; hidroquímica; contaminación enfoques estadísticos para la identificación del origen de las aguas subterráneas de mineralización: caso de la región naama, extremo oeste de argelia comprender los procesos que controlan la mineralización de las aguas subterráneas es crucial para preservar su calidad y garantizar una gestión sostenible del recurso, especialmente en regiones como naama, argelia, que dependen exclusivamente de este recurso. las aguas subterráneas en esta región enfrentan una grave sobreexplotación y alta salinidad debido a factores climáticos y al aumento de la demanda industrial, agrícola y doméstica, lo que plantea importantes desafíos para su uso a largo plazo. en este trabajo se realizaron análisis hidroquímicos, incluidos métodos gráficos y herramientas estadísticas multivariantes, para investigar la química de las aguas subterráneas y los mecanismos que influyen en su mineralización en la región de naama. los resultados revelaron una distribución relativamente homogénea de las muestras, clasificadas en tres grupos hidroquímicos. estos grupos están dominados principalmente por aguas de cloruros y sulfatos de calcio y magnesio, influenciadas por las características geológicas de la región, como las formaciones gipso-salinas del cretácico superior y los diapiros arcillo-gipso-salinos del triásico, y afectadas además por procesos de intercambio iónico inverso. el grupo 1 se caracteriza por una mayor proporción de aguas bicarbonatadas de calcio y magnesio debido al predominio de formaciones carbonatadas del jurásico temprano o del mioceno, que se benefician de zonas de recarga más dinámicas. los grupos 2 y 3 muestran composiciones químicas similares, pero el grupo 3 se distingue por sus zonas de recarga más restringidas y las formaciones carbonatadas, lo que conduce a la aparición de aguas bicarbonatadas de calcio y magnesio. también se encontró que las áreas de recarga asociadas con formaciones carbonatadas son vulnerables a la contaminación antropogénica. estos hallazgos resaltan la importancia de implementar estrictas medidas de protección para las zonas de recarga sensibles, con el fin de garantizar la preservación de este recurso vital frente a las crecientes presiones ambientales y humanas. resu m en 388 abderezzak boudjema, sabrine guettaia, abdessamed derdour, nafaa brinis, abdulrazak h. almaliki 1. introduction water has always been considered a renewable and inexhaustible source. nowadays, several countries are affected by the depletion of their water resources due to climatic conditions and increased demand, an inevitable result of population growth and socio-economic development (hosni et al., 2024; milano et al., 2013). with an arid climate and the absence of surface water mobilization works, the region of naama is mainly supplied with available groundwater by the region’s multilayer aquifer system (derdour, jodarabellan, pardo, ghoneim, & hussein, 2022). this resource is in high demand to meet the growing need for water. indeed, overexploitation and the high salinity rate in these waters pose various quantitative and qualitative problems, which could make the reserves progressively unexploitable. effective and sustainable management of water resources requires an excellent knowledge of their availability, quality and spatial variability. the physico-chemical parameters contained in groundwater offer the possibility of identifying the origin of the mineralization of the different aquifers and the origin and degree of pollution. this has prompted several researchers (liu, sun, xu, & xu, 2018; shi, xie, bu, li, & zhou, 2018; wang, shi, wang, & liu, 2020; wu et al., 2019) to develop statistical methods aimed at identifying various types of water-rock interactions, quantifying their contributions to the chemical composition of groundwater, and creating models for accurately determining water sources. various studies have assessed the groundwater quality in the wilaya of naama, algeria, using different methodologies and parameters. for instance, lachache et al. (2023) investigated the hydrogeochemical processes of groundwater in the naama watershed, identifying three major hydrochemical types and their spatial distribution. another study by ali and mountassir (2018) utilized gis and the water quality index (wqi) to evaluate groundwater quality, finding that 40% of the samples were excellent quality, while 60% were good. additionally, derdour, jodar-abellan, et al. (2022) proposed using machine learning algorithms to predict water quality indexes at the regional scale, demonstrating high accuracy with models such as support vector machine (svm) and ensemble trees (et). furthermore, derdour, belam, and chebab (2022) explored traditional irrigation systems and water harvesting methods in the oasis of sfissifa, ksour mountains, highlighting the effectiveness of ancestral agricultural systems like “tissfalt” for sustainable water management. moreover, hussein et al. (2024) conducted a comprehensive groundwater quality assessment and used machine learning algorithms to predict the irrigation water quality index in the wilaya of naama. their study, demonstrated the efficacy of models like support vector machine (svm) and ensemble trees (et) in accurately predicting water quality indexes, emphasizing the potential of advanced computational techniques in groundwater quality management. these studies highlight the importance of sustainable management and regular monitoring of groundwater resources in the region to ensure safe and reliable water supply for domestic, agricultural, and industrial purposes. the aim of this study is to conduct a comprehensive assessment of water quality across the entire wilaya of naama. utilizing a significant number of samples (123 in total), we employed both graphical tests and multivariate statistical analysis to carry out chemical analyses. our objectives were to characterize the water quality and identify the sources of mineralization and other influencing factors. additionally, geographic information system (gis) tools were integrated to provide a critical spatial dimension, complementing the statistical techniques utilized. by combining spatial analysis with statistical methods, gis enhanced our ability to identify and visualize patterns, relationships, and trends in groundwater quality data, offering a more comprehensive understanding of the underlying processes. this study underscores the importance of sustainable management and regular monitoring of groundwater resources in the region to ensure a safe and reliable water supply for domestic, agricultural, and industrial purposes. 2. study area the wilaya of naama is part of the southern high plains of oran. it is situated between the utm coordinates x: 794,969 m, y: 3,605,533 m and x: 954,930 m, y: 3,874,723 m in utm zone 30n, placing it in a transitional climate zone between the tellien atlas in the northern section and the saharan atlas in its western part. this diverse topography influences climate, hydrology, and groundwater recharge patterns (fig.1). the climate in the region is predominantly semi-arid, characterized by mild, wet winters and hot, dry summers (bouarfa et al., 2022). average annual precipitation ranges between 200 mm and 400 mm, depending on altitude and proximity to the mountain ranges, with significant variations due to orographic effects caused by the surrounding hills and mountains (moussaoui, derdour, benaradj, & hosni, 2024; moussaoui et al., 2023). these climatic patterns, coupled with the local terrain, influence water infiltration and runoff, directly impacting groundwater quality and recharge (derdour et al., 2022). naama is home to a vast multilayer aquifer system, which is a critical water resource for the region’s agricultural and domestic use. the combination of this complex climate, diverse geography, and aquifer system highlights the importance of sustainable water management strategies in the region. with challenges such as climate variability, desertification, and urban expansion, the wilaya of naama emphasizes the need to maintain groundwater recharge rates and protect water quality for sustainable use in agriculture, drinking, and ecosystem support. figure 1. study area 2.1. geological and hydrogeological setting. the geological and hydrogeological settings of the naama region reveal a complex and dynamic history influenced by geological events and processes. according to meddah, bertrand, seddiki, and tabeliouna (2017), benyoucef et al. (2017), mekki et al. (2023), and benadla, marok, soulimane, and reolid (2023), the region's litho-stratigraphy includes prominent triassic outcrops associated with diapirs and upper triassic tholeiitic volcanism, lower jurassic marine carbonates transitioning to marl, and the ksour deltaic complex formations from the middle and upper jurassic periods (fig.2). the lower cretaceous consists of extensive fluvial deposits, while the upper cretaceous features gypsum clays, marls, limestones, and marl limestones from the cenomano-turonian age. post-turonian deposits and mio-pliocene clayey breccias with calcareous elements, marl, and marl-limestone formations further shape the region's geological framework. quaternary materials, mainly alluvial deposits and sands forming dunes, highlight recent geological activity. regarding groundwater resources, rahmani, bouanani, kacemi, and hamed (2017) identified two potential aquifer systems through geophysical investigations: the upper jurassic sandstones and the massive lower cretaceous sandstones. the upper jurassic sandstones are typically exposed at the foothills of djebels aissa, mekter, hirech, and morhad. the lower cretaceous sandstones, comprising the tiloula formation with clayey intercalations and the massive tiout formation with interstitial and fracture porosity, are semiconfined due to interbedded reddish clay layers. the primary recharge of these aquifers is influenced by drainage mechanisms, emphasizing the need 389statistical approaches for the identification of the origin mineralization groundwaters: case of the naama region, far west-algeria for understanding hydrogeological dynamics for sustainable water resource management. the spatial distribution of geological formations varies across the region, with the lower jurassic being the most extensive, followed by the lower cretaceous, quaternary deposits, and other formations covering smaller areas. this detailed geological and hydrogeological framework underscores the significance of tectonic shifts, sedimentation, and climatic changes in shaping the naama region, highlighting the crucial interplay between geological formations and recharge mechanisms in managing groundwater resources sustainably. figure 2. geological map of the study area with water sampling locations 2.2. land cover land use setting. land use and land cover significantly influence groundwater recharge and quality via runoff, infiltration, and the interplay between natural and artificial environmental processes (fig.3). the distribution of various land cover types in the wilaya of naama greatly affects groundwater quality. degraded steppes, covering 13,425.52 km² or 44.83% of the total area, can adversely affect groundwater quality through soil erosion, diminished infiltration, and sediment transport into aquifers, resulting in contamination hazards. likewise, significantly degraded steppes (11,715.16 km², or 39.12%) might interfere with natural water cycles, diminish recharge rates, and facilitate the infiltration of contaminants into groundwater sources. dunes (1,699 km², 5.67%) and halophytes (82 km², 0.27%) can enhance localized groundwater recharge by improving infiltration in their areas; nevertheless, an overabundance of salttolerant species in halophytes may influence groundwater salinity. stipa tenasissima (2,910.76 km², 9.72%) and psammphytes (513.08 km², 1.71%) are indigenous vegetation types that enhance soil structure, water retention and minimize runoff, hence facilitating natural recharge processes and enhancing the quality of infiltrated water. forests (1,385.48 km², 4.63%) function as natural water filtration systems, diminishing impurities that reach aquifers by capturing silt and eliminating pollutants via vegetative mechanisms. these wooded regions are essential for sustaining the natural equilibrium of groundwater replenishment and safeguarding water quality. nonetheless, agricultural development, urbanization, and deforestation will diminish these natural recharge zones, exacerbating the quality of accessible groundwater due to heightened pollution threats. the correlation between these lulc patterns and groundwater quality underscores the necessity for sustainable management measures to mitigate degradation, enhance natural infiltration processes, and safeguard vegetative buffers that facilitate clean and sustainable groundwater recharge. figure 3. land use and land cover (lulc) map of the study area with water sampling locations 3. materials and methods 3.1. sampling and analysis. one hundred and twenty-three (123) groundwater samples were obtained from the directorate of water resources of the wilaya of naama. the concentrations of eight major ion types (na+, k+, ca2+, mg2+, hco3 -cl−, so4 2 no3 -) were analyzed. all sample collection, preservation, and transportation were executed by certified national water resources agency hydrogeologists, adhering to strict quality assurance and quality control (qa/ qc) procedures. chemical analyses were exclusively conducted in the fully accredited laboratories of this agency, which is the sole authorized and licensed entity for conducting groundwater quality assessments in agricultural regions, as mandated by state regulatory policies. once all ion concentration tests were completed, the ae (analytical errors) were verified by calculating the ionic balance using the equation 1. (%) 100 cations anions ib x cations anions  − = + ∑ ∑ ∑ ∑ (1) 390 abderezzak boudjema, sabrine guettaia, abdessamed derdour, nafaa brinis, abdulrazak h. almaliki where: • cations, expressed in milliequivalents per liter (meq/l), include positively charged ions such as calcium, magnesium, sodium, and potassium, contributing to the overall positive charge in water. • anions, also expressed in milliequivalents per liter (meq/l), include negatively charged ions like bicarbonate, carbonate, chloride, sulfate, and nitrate, which balance the total ionic charge in the water sample. an acceptable ionic balance typically falls within ±5% for highquality analytical data. if the balance exceeds this range, it may indicate errors in sampling, analysis, or calculation (nordstrom, ball, donahoe, & whittemore, 1989) 3.2. graphical analytique hydrochemical diagrams such as piper, gibs, chadha and the hierarchical ascending classification (hfc) are fundamental tools in hydrogeology to decipher the origin and evolution of groundwater (arroyo-figueroa, chalá, gutiérrez-ribon, & quiñones-bolaños, 2024). these graphical representations make it possible to transform complex analytical data into intuitive visualizations, revealing essential information on mineralization processes, the geochemical origin of water, water-rock interactions, and the mechanisms of recharge and evolution of aquifers. note that the cah diagram offers a statistical classification of samples, the piper diagram allows to characterize the overall chemical composition, the gibbs diagram analyzes the control mechanisms of the chemistry, while the chadha diagram provides a detailed representation of the hydrochemical facies. 3.2.1. hierarchical ascending classification (cah) hierarchical ascending classification (hfc) is an effective statistical technique for the multidimensional analysis of hydrochemical data, grounded in the core principles of hierarchical clustering (lis-gutiérrez, reyna-niño, gaitánangulo, viloria, & enrique santander abril, 2018). it facilitates the categorisation of water samples with analogous chemical properties by converting intricate data into a tree structure known as a dendrogram. the primary aims encompass the identification of homogeneous water families, the elucidation of similarities and differences among samples, the simplification of hydrochemical data, and the identification of common sources or processes of mineralisation. the procedure adheres to specific methodological steps: data normalisation, distance calculation between samples, hierarchical cluster creation, and graphical display. cah provides quick processing of several variables, clear visualisation of correlations among samples, and independence from the variable count. the dendrogram’s interpretation relies on the idea that each branch signifies a sample group, with the link height reflecting the degree of similarity, hence facilitating comprehension of groundwater’s genesis and evolution (bouteraa, mebarki, bouaicha, nouaceur, & laignel, 2019; zhang et al., 2019). 3.2.2. the piper diagram the piper diagram, developed by piper (1944) is a graphical representation used to illustrate the main chemical elements and different groundwater facies. it enables the visualization of water evolution, transitioning from one facies to another through either time-spaced analyses or analyses of samples taken from various locations. this diagram proves invaluable in representing diverse analysis groups. it consists of two triangles and a diamond. the two triangles: one for cations and the other for anions are first filled, followed by the diamond. the values used are expressed in % meq/l. 3.2.3. chadha diagram the chadha diagram, introduced by chadha (1999), is a valuable tool for hydrochemical analysis. by plotting the sample groups on this diagram, we can describe various types of water and trace the evolution of hydrochemical processes. this diagram allows us to understand the factors that control groundwater chemistry in the study area. it provides a clear visual representation of the interactions between different chemical elements in the water. this detailed understanding helps in identifying changes over time and variations across different locations, thereby offering insights into the underlying processes affecting groundwater quality. 3.2.4. the gibbs diagram the gibbs diagram, established by gibbs (1970), has been an essential instrument for elucidating the geochemical mechanisms responsible for groundwater mineralisation. this diagram has been widely utilised in numerous studies to elucidate the determinants affecting groundwater chemistry (marandi & shand, 2018; sunkari, abu, zango, & wani, 2020). the gibbs diagram is predicated on three fundamental factors: precipitation, evaporation, and water-rock interaction. these parameters are essential in ascertaining the mineralisation process of groundwater. gibbs’ formulations are crucial for analysing the diagram: • gibbs i is defined as cl⁻ / (cl⁻ + hco₃⁻) • gibbs ii is defined as (na+ + k+) / (na+ + k+ + ca2+) expressed in meq/l. by examining the positions of water samples on the gibbs diagram, researchers can deduce the prevailing geochemical processes occurring in a certain study region. this technology offers critical insights into the intricate interactions among diverse chemical constituents in groundwater, aiding in identifying the principal mechanisms influencing its chemical evolution. 3.3. statistical analyses. the groundwater study has evolved considerably from conventional graphical techniques to more sophisticated multivariate statistical approaches (piper, 1944; schoeller, 1962; stiff jr, 1951). these statistical methods offer more in-depth and accurate insights into the chemical composition of groundwater. correlation analysis helps to evaluate the relationship between variables, identifying similar origins of elements and determining common control processes. principal component analysis (pca) simplifies and classifies data, identifying the most important variables and following element sources through the use of the correlation matrix and varimax rotation (abdi & williams, 2010; granato, santos, escher, ferreira, & maggio, 2018; hasan & abdulazeez, 2021) since the chemical composition of groundwater is also the result of the contribution of multiple factors, the different types of water-rock interactions can be considered as factors. in contrast, the degrees of interaction can be considered as contributions. in addition, the relationship between the individual hydrochemical parameters was determined using pearson correlation analysis. spatial associations of groundwater were identified using r-mode factor analysis in spss version 24 (imbrie, 1963). principal component analysis (pca) was the primary method for extracting factor analysis in r-mode and data were presented in a rotated matrix using the varimax rotation procedure (sunkari et al., 2020). factor analysis was performed by first calculating the correlation coefficient table (correlation matrix) for all hydrochemical parameters using r mode. 4. results 4.1. graphical analytic 4.1.1. cluster analysis. the application of cluster analysis to the sampled water data reveals the presence of three distinct classes (fig. 4). • class 1: includes ions associated with saline and evaporitic formations, such as mg²⁺, na⁺, k⁺, cl⁻, and so₄²⁻. • class 2: characterized by the presence of bicarbonates (hco₃⁻) and nitrates (no₃⁻). 391statistical approaches for the identification of the origin mineralization groundwaters: case of the naama region, far west-algeria • class 3: dominated by k⁺ ions, primarily linked to evaporitic and saline formations. furthermore, this analysis enabled the classification of water sampling points into three distinct groups figure 4. hierarchical ascending classification (hac) of physico-chemical parameters of sampled water group 1 group 2 group 3 1: cl-so4ca-mg 2: clnak /so4 na 3: hco3na k 4: hco3ca mg so 4 + c l + no3 m g co 3 + h co 3 ca + m g na + k so 4 cl + no3 figure 5. piper diagram of groundwater in the naama region 4.1.2. piper diagram the piper diagram is a commonly employed tool for categorizing and illustrating the chemical composition of water samples according to their predominant ion concentrations (fig. 5). the analysis indicates that the ca²⁺-mg²⁺-cl⁻-so₄²⁻ facies is predominant, constituting 82.93% of the water samples. the groundwater in the research area predominantly contains substantial amounts of calcium, magnesium, chloride, and sulfate ions. this facies generally indicates water sources affected by processes like rock-water interactions, when minerals containing these ions dissolve into the groundwater. the subsequent most common water type, comprising 9.76% of the samples, is the cl⁻-na⁺+k⁺ or so₄²⁻-na⁺ facies. this indicates a significant impact of sodium and potassium, in conjunction with chloride and sulfate, which may signify sources affected by evaporitic conditions or human activities like agricultural runoff. finally, the ca²⁺-hco₃⁻-mg²⁺ sample category constitutes 7.31% of the overall total. this facies are frequently linked to groundwater that has seen considerable interaction with carbonate rocks, resulting in the breakdown of calcium and magnesium carbonates. this water typically signifies recharge zones when precipitation permeates the soil and solubilizes carbonate minerals. plotting these results on the piper diagram provides critical insights into the prevailing hydrochemical processes and the spatial distribution of various water types within the studied area. the prevalence of the ca²⁺-mg²⁺cl⁻-so₄²⁻ facies underscores the crucial influence of geological formations and mineral dissolution on groundwater chemistry. 4.1.3. chadha diagram the chadha diagram effectively classifies and visualizes the hydrochemical processes influencing groundwater chemistry. the data plotted on this diagram (fig. 6) reveals several key insights: • field 6 (inverse ion exchange water type ca-mg-cl/so₄): the majority of the samples (80.48%) fall into this category, indicating that these waters are primarily of the ca-mg-cl type. in this type, alkaline earth metals (ca²⁺ and mg²⁺) dominate over alkali metals (na⁺ and k⁺), and strong acids (cl⁻ and so₄²⁻) outweigh weak acids (hco₃⁻). this dominance suggests significant ion exchange processes where calcium and magnesium replace sodium and potassium on mineral surfaces. the results confirm the findings of the piper diagram, which also highlighted the prevalence of these chemical constituents. • field 7 (seawater na-cl type): a smaller portion of the samples (11.38%) is located in this field. this indicates that in these water samples, alkali metals (na⁺ and k⁺) exceed alkaline earth metals (ca²⁺ and mg²⁺), and strong acids (cl⁻) are more prevalent than weak acids. this type of water is typically influenced by seawater intrusion or evaporitic conditions, contributing to higher salinity. • field 5 (ca-mg-hco₃ recharge water): the remaining samples (8.13%) are in this field, indicating waters characterized by ca-mghco₃ composition. this suggests recharge water, where calcium and magnesium are prevalent alongside bicarbonates, pointing to fresh groundwater that has interacted with carbonate rocks and possibly undergone minimal mineralization. the application of chadha diagram in our study area, provides a comprehensive view of the hydrochemical processes at play, illustrating the dominance of specific ion exchange reactions and the varying influences of different water sources on the groundwater chemistry in the study area. hco3 + co3 cl so4 = f (ca + mg na k) h co 3 + c o 3 c l s o 4 (m eq /l ) ca + mg na k (meq/l) base ionexchange na-hco3 water type sea water na-cl water type rechargine water ca-mg-hco3 water type reverse ionexchange ca-mg-cl/so4 water type 392 abderezzak boudjema, sabrine guettaia, abdessamed derdour, nafaa brinis, abdulrazak h. almaliki figure 6. chadha diagram of the groundwater samples 4.1.4. gibbs diagram analysis the gibbs diagrams (fig. 7a and 7b) provide valuable insights into the study area’s geochemical processes controlling groundwater mineralization. the analysis of water points on these diagrams revealed that the values for gibbs i range from 0.09 to 0.95, and for gibbs ii, they range from 0.24 to 0.94. these variations indicate that water-rock interactions and evaporation processes primarily influence the mineralization of the region’s groundwater. gibbs i measures the ratio of chloride ions (cl⁻) to the sum of chloride and bicarbonate ions (cl⁻ + hco₃⁻). at the same time, gibbs ii evaluates the ratio of sodium and potassium ions (na⁺ + k⁺) to the total of sodium, potassium, and calcium ions (na⁺ + k⁺ + ca²⁺). the results from these diagrams suggest that: • water-rock interaction: the interaction between water and rock is a dominant process, leading to the dissolution of minerals into the groundwater. this interaction significantly contributes to the overall mineral content of the water. • evaporation: evaporation plays a crucial role, especially in regions where surface water evaporates, leaving behind higher concentrations of dissolved solids. this process further enhances the mineralization of groundwater. additionally, it is observed that the total dissolved solids (tds) content increases with longer contact time between water and rock. this trend indicates that extended interaction periods allow more minerals to dissolve into the groundwater, thereby elevating the tds levels. the gibbs diagrams effectively illustrate these processes, offering a clear understanding of the geochemical factors that govern groundwater quality in the region. i ii figure 7. gibbs diagram showing the mechanism that is most involved in chemical control of groundwater 4.2. statistical analysis descriptive analysis of the chemical elements in the naama region highlights significant variability in their spatial distributions. the majority of the coefficients of variation exceed 50% (table 1), indicating pronounced heterogeneity in the concentration of these elements across the study area. this heterogeneity could be attributed to a combination of geological, hydrological, and anthropogenic factors influencing the chemical composition of the waters. in contrast, bicarbonates exhibit coefficients of variation below 50%, suggesting a relatively homogeneous spatial distribution. this consistency may point to a more uniform source or process governing bicarbonate concentrations, such as carbonate rock dissolution or limited external influences compared to other chemical elements. such findings emphasize the importance of considering spatial variability when analyzing water chemistry, as it provides insights into the underlying environmental and geochemical processes shaping the study area. table 1. parameter values of the four principal water groups variable minimum maximum cv ca2⁺ 24,00 122,00 99,42 mg2⁺ 3,00 452,00 81,96 na⁺ 5,00 277,00 126,83 k⁺ 1,00 261,00 229,28 cl⁻ 21,00 819,00 159,64 so₄2⁻ 53,00 370,00 110,16 hco₃⁻ 3,00 529,00 25,86 no₃⁻ 1,00 390,00 140,72 4.2.1. correlation analysis. the correlation matrix in table 2 reveals significant relationships between groundwater quality parameters, with strong to very strong correlations observed between ca²⁺/cl⁻ (0.761), ca²⁺/mg²⁺ (0.67), ca²⁺/ so₄²⁻ (0.749), mg²⁺/cl⁻ (0.693), mg²⁺/so₄²⁻ (0.718), and na⁺/cl⁻ (0.875). these correlations suggest that the majority of chlorides originate from the dissolution of nacl, cacl₂, and mgcl₂, while sulfates primarily come from evaporitic minerals like mgso₄. additionally, the matrix highlights the role of carbonate dissolution/precipitation and ion exchange processes with clay and sulfate minerals, reflected by moderate and non-significant correlations among other parameters. interestingly, negative correlations are also present, indicating inverse relationships between certain ions. furthermore, negative correlations between parameters such as ca²⁺ and hco₃⁻ suggest competitive interactions or different sources and processes affecting their concentrations. the correlation matrix provides a comprehensive view of the dominant geochemical processes and interactions influencing groundwater quality in the study area, emphasizing the complex interplay between various chemical constituents. table 2. correlation matrix of the different parameters of groundwater quality. values in bold indicating correlation coefficients ((pearson (n)). variables ca⁺⁺ mg⁺⁺ na⁺ k⁺ cl⁻ so₄⁻⁻ hco₃⁻ no₃⁻ c a⁺ ⁺ 1 m g⁺ ⁺ 0.67 1 n a⁺ 0.606 0.597 1 k ⁺ 0.17 0.622 0.518 1 c l⁻ 0.761 0.693 0.875 0.431 1 so ₄⁻⁻ 0.749 0.718 0.597 0.429 0.472 1 h c o ₃⁻ -0.23 -0.085 -0.113 0.002 -0.13 -0.278 1 n o ₃⁻ 0.051 0.217 -0.027 -0.022 0.021 0.018 0.29 1 393statistical approaches for the identification of the origin mineralization groundwaters: case of the naama region, far west-algeria 4.2.2. principal component analysis (pca). the principal component analysis (pca) performed on 123 water samples in the study area revealed eight variables, with the initial four factors accounting for 88.397% of the total variance, offering a comprehensive understanding of the mineralization processes in the aquifer system (table 3). factor 1 (f1), accounting for 50.671% of the variance, is primarily influenced by the chemical elements cl⁻, so₄²⁻, na⁺, mg²⁺, and ca²⁺, indicating the axis of salt and gypsiferous mineralization and highlighting the significant role of these ions in groundwater mineralization. factor 2 (f2), accounting for 16.708% of the variance, is defined by hco₃⁻ and no₃⁻, highlighting the geochemical processes of carbonate dissolution and the penetration of nitraterich surface water, which considerably influence the overall water chemistry. factor 3 (f3) accounts for 11.808% of the variance and is correlated with ca²⁺, no₃⁻, and k⁺, highlighting the effects of soil leaching and irrigation water infiltration, so reflecting the influence of agricultural practices on groundwater quality. factor 4 (f4), representing 9.210% of the variation, encompasses cl⁻, so₄²⁻, hco₃⁻, na⁺, and k⁺, indicating secondary mineralization processes resulting from the dissolution of sulfate and carbonate formations, further diversifying groundwater’s chemical composition. the pca results provide a comprehensive insight into the prevailing geochemical processes and mineralization mechanisms within the aquifer system, highlighting the intricate interactions among different chemical constituents. table 3. eigenvalues and contributions of variables (%): of the waters of the naama aquifer. eigenvalues f1 f2 f3 f4 valeur propre 4.054 1.337 0.945 0.737 variabilité (%) 50.671 16.708 11.808 9.210 % cumulé 50.671 67.379 79.187 88.397 contributions of variables (%) f1 f2 f3 f4 ca⁺⁺ 17.107 0.826 18.323 4.066 mg⁺⁺ 18.894 3.622 0.003 7.605 na⁺ 18.158 0.000 2.996 13.005 k⁺ 9.115 2.427 46.512 16.610 cl⁻ 18.859 0.033 0.021 21.471 so₄⁻⁻ 16.543 1.026 3.070 12.085 hco₃⁻ 1.254 45.955 5.827 17.292 no₃⁻ 0.070 46.110 23.248 7.868 5. discussion this study conducted in the naama region of algeria provides critical insights into groundwater mineralization processes, which are essential for sustainable water resource management in arid regions. understanding the processes controlling groundwater mineralization is crucial for preserving its quality and ensuring sustainable resource management, especially in areas like naama, which rely heavily on groundwater. the findings indicate that groundwater quality is significantly influenced by geological formations and anthropogenic activities, leading to distinct hydrochemical groups characterized by varying mineralization levels. this research enhances our understanding of local hydrogeological dynamics and offers valuable comparisons with similar conditions in other arid regions worldwide. several parallels emerge when comparing the groundwater mineralization patterns observed in naama with those in el bayadh and bechar. both regions exhibit similar geological formations that influence groundwater chemistry. for instance, el bayadh is characterized by a predominance of limestone and gypsum formations, contributing to high mineralization levels. a study by derdour et al. (2023) has shown that the hydrochemical composition of groundwater in the region of adrar is primarily influenced by evaporation and ion exchange processes, akin to those observed in naama. similarly, bechar’s groundwater is affected by saline deposits and evaporation, leading to increased salinity levels (mebarki, kendouci, & bendida, 2024). in tunisia, particularly in the chott djerad region, groundwater quality is compromised by high salinity due to similar climatic conditions and anthropogenic pressures (kraiem, zouari, chkir, & agoune, 2014). when examining anthropogenic impacts on groundwater quality, comparing the naama region with other arid regions such as the central valley in california, usa, and the atacama desert in chile (burow, jurgens, belitz, & dubrovsky, 2013; herrera et al., 2018). extensive agricultural practices in california’s central valley have led to significant groundwater depletion and salinization (burow et al., 2013). the reliance on irrigation has increased mineralization levels due to the leaching of salts from the soil and applying fertilizers, which can contaminate aquifers. this situation parallels the challenges faced in naama, where agricultural demands coupled with overextraction have led to rising salinity levels and concerns about water quality. in the atacama desert, mining activities have severely impacted groundwater resources (herrera et al., 2018). another comparable region is the arabian peninsula, where rapid urbanization and industrialization have led to increased water demand and subsequent over-exploitation of groundwater resources. in saudi arabia, for instance, excessive pumping has resulted in declining water tables and deteriorating water quality due to salinization (al-naeem, 2014). this mirrors the situation in naama, where high salinity levels threaten the usability of groundwater for agricultural and domestic purposes. figure 8. overview of groundwater hydrogeochemistry in the study area the performance of this study is underscored by its comprehensive approach, utilizing a robust dataset of 123 groundwater samples analyzed through multivariate statistical methods. this methodological rigor not only enhances the reliability of the findings but also sets a precedent for future research in similar contexts. the novelty of this research lies in its detailed 394 abderezzak boudjema, sabrine guettaia, abdessamed derdour, nafaa brinis, abdulrazak h. almaliki examination of groundwater mineralization sources within a specific regional context while highlighting the interplay between natural geological processes and human activities. this study distinguishes itself by employing advanced statistical tools like piper diagrams, gibbs plots, chadha diagrams, hierarchical ascending classification (hac), and principal component analysis (pca) to provide a more nuanced understanding of hydrochemical variations compared to traditional methods used in earlier assessments (derdour, jodar-abellan, et al., 2022; hussein et al., 2024; lachache et al., 2023). by identifying sensitive recharge zones vulnerable to pollution, this research emphasizes the need for targeted protection measures, an aspect often overlooked in previous regional studies. this research contributes significantly to the existing body of knowledge regarding groundwater management in arid regions. it reinforces the importance of sustainable practices while providing a framework for future studies to mitigate the impacts of climate change and human activity on vital water resources. the findings serve as a reminder that effective management strategies are essential for preserving groundwater quality and ensuring its availability for future generations amidst increasing environmental pressures. figure 8 effectively summarizes the study’s findings, presenting the results of the cluster analysis and the various water types identified through the chadha diagram. this figure illustrates the spatial distribution of the different water groups, including recharge zones, reverse ion exchange areas, and halite formations, providing a comprehensive overview of the hydrogeochemical processes and groundwater quality variations across the study area. 6. conclusion the study on groundwater mineralization in the naama region of algeria provides a detailed examination of the hydrochemical characteristics and influencing factors affecting this essential resource. through graphical tests and multivariate statistical analyses, the research identified a relatively homogeneous distribution of groundwater samples, categorizing them into three primary hydrochemical groups. calcium and magnesium chloride and sulfate waters predominantly characterize these groups. this composition reflects the geological features of the region, particularly the gypsum-saline formations from the upper cretaceous and triassic periods. the analysis revealed that group 1 contains a higher concentration of calcium and magnesium bicarbonate waters, linked to carbonate formations from the lower jurassic or miocene. these formations benefit from more active recharge zones than groups 2 and 3, which exhibit similar chemical properties, although group 3 is noted for its limited recharge capacity. the findings highlight significant environmental vulnerabilities associated with the recharge areas of carbonate aquifers, which are increasingly exposed to anthropogenic pollution. this underscores the urgent need for implementing strict protection measures, enhancing regulatory frameworks, ensuring continuous monitoring, and raising awareness among local communities about safeguarding this vital resource. the study aims not only to characterize water quality but also to protect an ecosystem crucial for environmental sustainability and human welfare. despite these contributions, several limitations were encountered during this research. data collection was affected by geopolitical factors related to the border region, restricting access to certain areas and resulting in incomplete datasets. additionally, many local farmers were hesitant to allow entry to their farms for data collection purposes. these challenges may limit the findings’ comprehensiveness and applicability across the wider naama region. nevertheless, this research is a valuable tool for decision-makers in water resource management. the insights gained can inform policies aimed at sustainable groundwater management and protection strategies tailored to address the unique challenges faced by the naama region. future studies should aim to overcome existing barriers to data collection through innovative approaches such as remote sensing or community engagement initiatives that promote local cooperation. additionally, integrating advanced techniques like isotopic analysis could provide a more comprehensive assessment. longitudinal studies are also recommended to monitor changes over time in response to environmental pressures and human activities. acknowledgements: the authors extend their appreciation to taif university, saudi arabia, for supporting this work through project number (tu-dspp-2024-173) funding this research was funded by taif university, saudi arabia, project no.tu-dspp-2024-173. references al-naeem, a. a. 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(2019). vehicular contribution of pahs in size dependent road dust: a source apportionment by pca-mlr, pmf, and unmix receptor models. science of the total environment, 649, 1314-1322. _hlk102853981 _hlk184936011 _hlk184937469 _hlk185188215 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n2.118098 earth sciences research journal earth sci. res. j. vol. 29, no. 2 (june, 2025): 211 218 g eo lo g ic a l h a za r d s abstract the influence of vertical coastal land movement on relative sea level rise: a case study of shanghai, china beining wen1, miao yu2, chong liu3, yan jiao4*, qihang kai5 1. college of urban and environmental sciences, northwest university, xi'an 710127, shaanxi, china 2. shandong xinhui construction group co., ltd, dongying 257901, dongying, shandong, china 3. school of economics & management , northwest university, xi'an 710127, shaanxi, china 4.mobile postdoctoral research station of public administration, northwest universitypos , northwest university, xi 'an shaanxi 710127, china 5. xi’an city planning & design institute, no. 178 south laodong road, xi’an 710082, shaanxi, china *corresponding author: jiaoyan@nwu.edu.cn record manuscript received: 15/12/2024 accepted for publication: 18/05/2025 how to cite this item: wen, b., yu, m., & kai, q. (2025). the influence of vertical coastal land movement on relative sea level rise: a case study of shanghai, china. earth sciences research journal, 29(2), 211-218 https://doi. org/10.15446/esrj.v29n2.118098 understanding the current vertical land motion (vlm), including subsidence or uplift, is the basis for projecting relative sea level rise (rlsr) and estimating related risks. however, in shanghai, the impacts of the spatiotemporal change of vlm are little known. the purpose of this study was to quantify the impact of vlm on rslr and investigate the spatiotemporal evolution characteristics of vlm through tide gauge records, satellite altimetry observations, and interferometric synthetic aperture radar (insar) measurements. the calculations indicated that the rslr (5.67±0.58 mm/year) from 1969 to 2019 was approximately twice the slr trend (2.44±0.28 mm/year) from 1993 to 2019. the vlm, especially subsidence, is the main driver of rslr. moreover, spatial and temporal patterns of vlm are highly uneven and nonlinear. these results reveal that vlm is the main driver of rslr. unfortunately, previous studies have mostly underestimated or overlooked the impact of vlm on the risks of rslr and subsequent coastal flooding. thus, prevention strategies for controlling vlm are warranted to minimize the negative impact related to the rslr. our research provides a theoretical basis for urban disaster prevention in shanghai and the planning of coastal cities worldwide. keywords: relative sea level rise; vertical land motion; shanghai; insar. influencia del movimiento vertical de la tierra en zonas costeras sobre el aumento relativo del nivel del mar: estudio de caso en shanghái, china resumen comprender el movimiento vertical de la tierra (vlm) actual, incluida la subsidencia y la elevación, es la base para proyectar el aumento relativo del nivel del mar (rlsr) y estimar los riesgos relacionados. sin embargo, en shanghái, los impactos del cambio espaciotemporal del vlm son poco conocidos. el propósito de este estudio fue cuantificar el impacto del vlm en el rslr e investigar las características de evolución espaciotemporal del vlm a través de registros de mareógrafos, observaciones de altimetría satelital y mediciones del radar interferométrico de apertura sintética (insar). los cálculos indicaron que el rslr (5,67 ± 0,58 mm/año) de 1969 a 2019 fue aproximadamente el doble de la tendencia del slr (2,44 ± 0,28 mm/año) de 1993 a 2019. el vlm, especialmente la subsidencia, es el principal impulsor del rslr. además, los patrones espaciales y temporales del vlm son altamente desiguales y no lineales. estos resultados revelan que el vlm es el principal impulsor del rslr. desafortunadamente, estudios previos han subestimado o pasado por alto, en su mayoría, el impacto de la vlm en los riesgos de rslr y las subsiguientes inundaciones costeras. por lo tanto, se justifican estrategias de prevención para controlar la vlm a fin de minimizar el impacto negativo relacionado con la rslr. nuestra investigación proporciona una base teórica para la prevención de desastres urbanos en shanghái y la planificación de ciudades costeras en todo el mundo. palabras clave: aumento relativo del nivel del mar; movimiento vertical de la tierra; shanghái; radar interferométrico de apertura sintética https://doi.org/10.15446/esrj.v29n2.118098 https://doi.org/10.15446/esrj.v29n2.118098 https://doi.org/10.15446/esrj.v29n2.118098 212 beining wen, miao yu, chong liu, yan jiao, qihang kai 1. introduction sea level rise is a crucial index of climate change. and it is accelerating due to melting from the ice sheet and glaciers and the thermal expansion of ocean water (horton et al., 2018; jevrejeva et al., 2016). in situ and remote sensing data show that the rate of global sea-level rise over the past two centuries has increased at faster rates than in the last two or three millennia (vermeer and rahmstrof, 2009; kemp et al., 2011). and the intergovernmental panel on climate change (ipcc) highlighted that the rate of mean global sea level rise (slr) had been up from 1.4 mm/year (1901-1990) to 3.6 mm/year (2006-2015) (ipcc, 2019). as a result, the global mean sea level is projected to range from 0.5 m to 1.2 m by the end of the 21st century, exacerbating flood risks and persisting till the next century, with significant impacts on coastal socio-economic, natural environment and ecosystems (kopp et al., 2014). regional sea-level rise significantly varies worldwide, mainly due to the different situations of vertical land motion (vlm) in local areas (pörtner et al., 2019), especially in coastal delta regions (nicholls et al., 2021). the relative sea level rise (rslr) has been proposed to refer to the elevation difference between mean sea level and land surface (nicholls et al., 2021; rovere et al., 2016), which can be defined as the sum of local vlm and slr. the vlm includes subsidence or uplift, due to natural processes (e.g., glacial isostatic adjustment (gia), sediment compaction, and tectonics) and anthropogenic processes (e.g., groundwater extraction and land-use change) (higgins, 2016; minderhoud et al., 2016; shirzaei et al., 2021; yuill et al., 2009). land subsidence has been identified as the main pattern of vlm (dokka, 2006; gómez et al., 2021). because of the rapid sediment compaction and deposit, coastal delta regions are experiencing a faster rate of land subsidence than global slr (minderhoud, 2019; tessler et al., 2018). in addition, due to the abundance of coastal urban deltas, rapid urbanization, and dense population of coastal cities, rslr is growing faster than in other regions, especially in developing asia (hallegatte et al., 2013; nicholls et al., 2021), and vlm could be the primary driver to rslr over the 21st century (cao et al., 2021; fuchs, 2010; nicholls et al., 2021). for example, as one of the biggest coastal cities around the world with exceeding 11 million citizens in 2020 and an expected population of up to nearly 36 million people by 2030 (razvadauskas, 2019), jakarta has shown that the land subsidence rate is up to 60 times faster than slr. moreover, it has been estimated that land subsidence could be responsible for about 88% contribution to coastal flood risks by 2050 (takagi et al., 2016a & b). for many coastal cities in asia, understanding local vlms is important to guide the decision-making process for coastal mitigation and adaptation plans to address serious future threats. over the last decade, rslr and flood hazards in shanghai have received much attention. for example, by simulating the consequence of the interaction of land subsidence, slr, and storm surges, wang et al. (2012) found that shanghai is sinking, and the maximum rate of subsidence is 24.12 mm/year. the result stresses that severe land subsidence is the primary driver causing rslr and flooding risks, which will further reduce the effectiveness of dikes by decreasing their height. yin et al. (2013) characterized the combined contribution of slr and land subsidence to fluvial flooding in the huangpu river in shanghai. the analysis shows that land subsidence results in floods becoming more common than before. cheng et al. (2018) provided a map of rslr behaviors over the past decades. they declares the local land subsidence rate has been approximately 5 mm/year since 2004, mainly causing rslr. to provide accurate and comprehensive measurements of vlm, more and more studies have used space-borne methods, such as global navigation satellite system (gnss) and interferometric synthetic aperture radar (insar) (hu et al., 2019; yin et al., 2019; yu et al., 2017). among these methods, interferomet⁃ric synthetic aperture radar (insar) technology can extract small deformation information of the surface, and has the characteristics of small weather influence, large coverage area, low cost and high measurement accuracy, which makes up for the shortcomings of traditional deformation monitoring. it has been applied in many urban land subsidence monitoring. for example, yu et al. (2017) applied differential synthetic aperture radar interferometry (dinsar) approach to analyze the vlm for 2015-2016. and the highest land subsidence rate of 30 mm/year has been measured. in addition, yin et al. (2019) estimated coastal flood hazards evolving slr and land subsidence in lingang new city in shanghai using the dinsar technique. the results show that combing the contribution of land subsidence, rslr is projected to be 0.11 m and 0.23 m by 2030 and 2050, respectively. these rises would extend the flooding area by nearly 6% and 15% by 2030 and 2050, separately, for 100year flooding. therefore, this study takes shanghai, china as an example, and uses insar analysis, the most advanced vlm measurement method, to study the time series of shanghai vlm, and discusses how vlm, especially land subsidence, changes and affects rslr. then give a new view on the flood disaster in coastal cities. 2. materials and methodology 2.1. study area shanghai is a coastal city that has 213 km of coastlines. it is located between 30°40’~31°53’ north latitude and 120°51’~122°12’ east longitude. shanghai is bounded by the east china sea to the east, jiangsu province to the northwest, zhejiang province to the southwest, hangzhou bay to the south, and the yangtze river to the north and jiangsu province. in addition to the hilly landform in the southwest part of the region, most of the region is low and flat, with an average elevation of about 4m, and the elevation of the urban area is less than 3m, which is seriously lower than the high water level of the huangpu river. shanghai is a typical subtropical monsoon climate, with relatively humid climate and distinct four seasons. the average annual temperature is about 15.8℃, and the average annual rainfall can reach 1191mm. the rainfall distribution is uneven in different seasons, generally showing the characteristics of more rainfall in spring and summer, less in autumn and winter, and the rainfall is mostly concentrated in the flood season from may to september. at the same time, heavy rainfall and high tide events are easy to occur during flood season and typhoon, which will cause river basin floods and huge economic losses (xian et al., 2018). and the resident population was over 23 million in 2020 from the 6th national population census of china, almost 1.5 times that of 10 years ago. urban construction is developing rapidly, including the reconstruction of old areas, new residential areas, a large number of dense high-rise and super-high-rise buildings, and the development and utilization of underground space. the vigorous development of urban construction activities in shanghai, especially the rapid development of underground space, has a serious impact on land subsidence. (shen and xu 2011). by the end of 2019, the construction area of houses with more than eight stories has increased from 9.59 × 106 m2 in 1990 to 4.96 × 108 m2. in some areas, the settlement of shallow soft soil caused by urban construction accounts for a considerable proportion of the total settlement. geological experts pointed out that in the current construction boom, engineering construction activities have become the main cause of land settlement. figure 1. location of the shanghai city 2.2. research method figure 2 illustrates the procedural steps involved in conducting insar analysis to measure local vertical land motion (vlm). firstly, the selected pairs 213the influence of vertical coastal land movement on relative sea level rise: a case study of shanghai, china of single look complex (slc) sar scenes were individually co-registered using orbit information obtained from a 1-arc sec shuttle radar topography mission (srtm) digital elevation model (dem), thereby leveraging the phase difference between the acquisitions (farr et al., 2007). the enhanced spectral diversity (esd) approach was employed to improve the quality of each coregistration. subsequently, a series of interferograms were generated by multiplying the reference scene with the complex conjugate of the secondary scene, utilizing the co-registered data. the resulting interferometric phase represented the phase difference between the two acquisition dates (gens and van genderen, 1996). during this step, the flat-earth and topographic phases were subtracted based on the metadata information from orbital records and the 1-sec arc strm dem, respectively. to reduce inherent speckle noise and enhance interpretability, a multilooking operator was applied to each image. furthermore, a goldstein phase filter, utilizing fast fourier transformation (fft), was implemented to mitigate noise resulting from temporal and geometric decorrelation, volume scattering, and other processing errors. consequently, a total of 24 interferograms were generated for the period spanning 2015 to 2020 up to this stage of the analysis. to identify high-quality pixels with reduced noise, only those with an average coherence exceeding 0.2 were considered for subsequent analysis. each interferogram’s phase had to be unwrapped to establish a relationship between the interferometric phase and the corresponding topographic height. therefore, the unwrapping process was carried out individually for each interferogram using a minimum cost-flow algorithm facilitated by snaphu. the unwrapped phase was then converted from radians into absolute displacements. geocoding was applied to all datasets to accurately determine the geographic location of the selected high-quality pixels. to minimize the influence of orbital errors and atmospheric delays, the single geocoded displacement products from 2015 to 2020 were stacked in chronological order. this stacking process facilitated the generation of a time series depicting the line-of-sight (los) deformation. it is important to note that, for the purposes of this study, the los deformation rate obtained through linear fitting was excluded to focus solely on uncovering the spatial and temporal evolution patterns of vlm. 2.3. research data 2.3.1. relative sea level rise data tide gauge records serve as the primary data source for assessing coastal relative sea level changes over the past century. these records measure the local relative sea level (rsl), which refers to the vertical distance between sea level and the land where tide gauge stations are situated (restrepo-ángel et al., 2021). in this study, monthly rsl data from 1969 to 2019 were obtained from the lusi tide gauge station, the closest station to shanghai, through the permanent service for mean sea level (psmsl, https://www.psmsl.org/data/obtaining/). the selected period was based on the availability of the longest time series data from the lusi tide gauge station. harmonic analysis was applied to these data to determine tidal constituents, significant tidal statistics, and their respective amplitudes and phases. to account for missing data, which constituted 92% of the dataset, interpolation was performed using harmonic analysis. to eliminate seasonal variability from the rsl data, the climatological monthly mean was subtracted from the monthly values. yearly averaged records were then calculated based on the monthly data. the least-squares linear fitting method was employed to estimate the trend of relative sea level rise (rslr) using the rsl datasets spanning the period from 1969 to 2019. by considering a 50-year timeframe, the rslr trend was calculated based on the tide gauge records, aiming to minimize the influence of inter-annual and decadal signals. 2.3.2. sea level rise data in recent years, satellite altimetry data have emerged as a valuable tool for accurately assessing absolute sea level changes in numerous coastal regions (nerem et al., 2018; restrepo-ángel et al., 2021). for this study, monthly sea level data covering the period from 1993 to 2019 were obtained from the national oceanic and atmospheric administration (noaa) laboratory for satellite altimetry, specifically from the yellow sea area, which encompasses the study region (https://www.star.nesdis.noaa.gov/socd/lsa/sealevelrise/). these data are derived from the records collected by multiple satellite radar altimeters, including topex/poseidon (t/p), jason-1, jason-2, and jason-3, ensuring a comprehensive time series spanning from 1993 to 2019. to ensure accuracy, the seasonal variability was meticulously eliminated from the sea level data, and the impacts of glacial isostatic adjustment (gia) on the geoid were excluded by the noaa laboratory for satellite altimetry. additionally, the local sea level rise (slr) trend was determined using the least-squares linear fitting method, providing a robust assessment of sea level changes in the study area. 2.3.3. vertical land motion data to date, only a limited number of studies have examined vertical land motion (vlm) using insar analysis, with a focus on short time series (less than 4 years) or specific areas within the shanghai region (ye et al., 2016; yin et al., 2015, 2019; zhao et al., 2015, 2019). consequently, there is a dearth of knowledge regarding the spatiotemporal variability characteristics of vlm in the study area. to address this research gap, the present study employed insar analysis, a cutting-edge method for measuring vlm with exceptional precision, utilizing sentinel-1a datasets spanning the period from 2015 to 2020. table 1. sentinel-1a data in ascending orbit, frame 97 and path 171 year acquisition dates 2015 1 aug, 29 nov 2016 12 jan, 4 mar, 15 may, 19 aug, 18 oct, 17 dec 2017 3 feb, 4 apr, 27 june, 7 sep, 18 nov 2018 17 jan, 18 mar, 29 may, 9 aug, 25 nov 2019 24 jan, 13 mar, 24 may, 4 aug, 15 oct, 26 dec 2020 31, jan the sar scenes captured by the sentinel-1a satellite were obtained from the alaska satellite facility distributed active archive center, which processes copernicus sentinel data on behalf of the european space agency (asf daac, https://search.asf.alaska.edu), covering the period from august 2015 to january 2020. although both ascending and descending orbits were available for sentinel-1a, this study focused exclusively on 25 scenes from the ascending orbit track. this selection was made because the ascending orbit track provided a longer time series dataset for the study area, as demonstrated by potin et al., 2016 (table 1). the generation of interferograms and subsequent phase unwrapping were performed using the sentinel application platform (snap) software and the statistical-cost network-flow algorithm for phase unwrapping (snaphu) algorithm, respectively (chen & zebker, 2001, 2002). figure 2. flow diagram of insar analysis (modified from gómez et al., 2021; li et al., 2018) https://www.psmsl.org/data/obtaining/ https://www.star.nesdis.noaa.gov/socd/lsa/sealevelrise/ https://search.asf.alaska.edu 214 beining wen, miao yu, chong liu, yan jiao, qihang kai 3. results 3.1 sea level rise and relative sea level rise monthly sea level change data were collected from multiple satellite radar altimeters from 1993-2019, which indicates an increasing trend of about 2.44±0.28 mm/year for the offshore area of shanghai (fig. 3). the absolute sea level rise is mainly caused by natural factors such as melting of glaciers and ice sheets and thermal expansion of temperature. figure 3. time series of sea level rise from 1993 to 2019. sea level rise plot generated by matlab 2021a (https://www.mathworks.com/products/matlab.html) relative sea level change is a general term for the rise (or fall) of sea level relative to the land surface (or underlying primordial sedimentary surface). this movement can be the rise and fall of the sea level itself, the rise and fall of the land surface or the underlying sediment surface, or both occur at different magnitudes. hence, relative sea level rise (rslr) is the result of the joint action of natural and human factors (fig. 4), the main factors are climate, ocean, geological environment, resource exploitation, man-made engineering construction, etc., which induces a series of resource, environmental, economic and social problems and risks, endangering the urbanization construction and economic and social development of coastal areas. however, monthly relative sea level data were obtained from the lusi tide gauge station from 1969 to 2019, and the results show 5.67±0.58 mm/year trend for rslr (fig. 5). the trend value of rslr in shanghai is twice faster than slr. this means that slr, induced by the melting of glaciers and ice sheets as climate change, represents less than half of the contribution for rslr. hence, what can be concluded from this result is that significant differences between rslr and slr are most likely owing to the impact of local vertical land motion (vlm) on the rlsr (abidin et al., 2011; eggleston & pope, 2013; ingebritsen and galloway, 2014). n = 1288 trend = 2.44 ± 0.28 sea level rise fitted line c ha ng e in s ea l ev el (m m ) 150 100 50 -50 -100 1995 2000 2005 year 2010 2015 2020 0 figure 4. diagram of relative sea level rise and its effects figure 5. (a)time series of relative sea level rise from 1969-2019. blue dashed line, and solid line separately refers to the monthly and yearly relative sea level change. (b) linear fit of relative sea level rise trends for 1969-2019. relative sea level rise plots generated by matlab, 2021a (https://www.mathworks.com/products/ matlab.html) 3.2 vertical land motion this study applied the insar analysis technique to measure the vlm from 2015-2020 based on the sentinel-1a ascending orbit track datasets. the deformation map was generated and shown in figure 5. the scale of displacements corresponds to los in units of the centimeter. figure 6. the map of vlm from 2015-2020 using the insar analysis technique on sentinel-1a data sets. in the study area, the insar analysis results reveal a wide range between -3 cm and -0.19 cm of vlm in shanghai (fig. 5). the largest vlm during the 2015-2020 period is up to -3 cm in the northeastern area of shanghai. in contrast, in south shanghai, the land displacements show slight uplift or subsidence (fig. 5a). it is worth highlighting that the values of the vlm in the study area unevenly distribute on the spatial change pattern, the vlm in the western area of the huangpu river shows severe subsidence between 2.5 cm https://www.mathworks.com/products/matlab.html https://www.mathworks.com/products/matlab.html https://www.mathworks.com/products/matlab.html 215the influence of vertical coastal land movement on relative sea level rise: a case study of shanghai, china and 3 cm. on the east part of the huangpu river, land subsidence decreases to less than 2.5 cm, and a slight uplift or sink can be observed in the southwestern coastline. and land subsidence increases in the northern direction. the time series of los displacements for selected three locations show high nonlinear evolution characteristics (fig. 5b). these displacements of sites exhibit apparent fluctuations during the 2015-2018 period and have become stable since 2018. 4. discussion 4.1 the contribution of two components to relative sea level rise since vlm and slr are the main components of rslr, understanding the quantitative contribution of these two parts is essential to projecting the rslr scenarios and potential coastal flood risks in the future. however, even though existing studies in shanghai highlighted the significance of rslr, the quantitative assessment of contribution from rslr and slr has not been conducted (wang et al., 1995; chen, 1991; yan et al., 2016). this study calculated the long-term trends of rslr and slr through tide gauge records and satellite altimetry observations, respectively. the rate of rslr in shanghai is 5.67±0.58 mm/year from 1969 to 2019, which is over twice faster than the slr rate of 2.44±0.28 mm/year during 1993-2019. this means that slr, induced by the melting of glaciers and ice sheets as climate change, represents less than half of the contribution for rslr, and in contrast, vlm is the main driver of rslr. unfortunately, previous studies mostly underestimated or even overlooked the impact of vlm on risks of rslr and subsequential coastal flooding (lv et al., 2021; wang et al., 2018; yin et al., 2021). we believe our results fill a gap in the quantitative analysis of slr and vlm contributions, which reveal the evidence that rslr is much more affected by vlm than slr. therefore, if vlm can be controlled or stopped through local measures, rslr will be reduced, mitigating the harm of coastal flooding. 4.2 evolution characteristics of vertical land motion in shanghai, the main pattern of vlm over the 2015-2020 period shows subsidence in most areas. still, only a tiny space showed slight uplift in the southwest, most likely due to the impacts of gia and artificial recharge to ground acquire (peltier et al., 2015; zhang et al., 2015). for the vlm time series of 6 years, natural-induced deformation (e.g., gia, sediment compaction, and tectonics) is assumed negligible because the ground change by natural processes can only be measured in the long-term period in the holocene era (shirzaei and bürgmann, 2018; xue et al., 2005). additionally, there are no localized characteristics with the rapid and sudden settlement rate due to the contemporary depletion of aquifers and reservoirs, and earthquakes. thus, this study indicates anthropogenic activities as the main factor for local vlm from 2015-2020. the map of vlm reveals the uneven distribution in the study area (fig. 6). the wide range of displacements is approximately between –4 cm and 1.7 cm. almost certainly, this pattern is induced by the impacts of groundwater overdraft and additional load from building construction (chai et al., 2004; xu et al., 2016). the vlm settlement in the west of the huangpu river is higher than that in the east of the huangpu river. this uneven distribution of vlm values relates to the uneven distribution of residents and human activities and urbanization. specifically, the west side of the huangpu river, called the puxi area, has developed over hundreds of years, and citizens and infrastructures are intensely concentrated there. in contrast, to meet the increasing demand for land and natural resources by rapid urbanization, pudong new area, located in the eastern huangpu river, has only seen rapid development in a few decades. thus, human activities in pudong new area are less than those in puxi area, and the vlm in the former was relatively lighter than the latter. the lingang new area, located in the southeastern part of shanghai along the coastline, has been planned for development in recent years. as the newest developing area, lingang new area is experiencing the slightest subsidence compared with other parts. the time series of displacements for selected three locations show high nonlinear evolution characteristics (fig. 7). these displacements exhibit apparent fluctuations during the 2015-2018 period and become likely stable since 2018, probably thanks to the local government introducing a series of policies for groundwater extraction controlling and construction standards (wang et al., 2012; xu et al., 2012; ye et al., 2016). however, in recent memory of studies in shanghai, the rates of vlm were assumed linear and would continue to the future scenarios by 2050 and 2100 (wang et al., 2018; yin et al., 2013, 2020, 2021). this kind of assumption based on no significant change in groundwater exploitation overlooked the temporal evolution pattern due to land subsidence controlling measures in vlm, leading to vlm scenarios would be overrated or underrated. indeed, a range of evidence reveals that the proper strategies for groundwater extraction and building construction can positively control and even stop local land subsidence. for example, in japan, land subsidence became a significant problem during the 20th century due to excessive groundwater withdrawal. to combat this issue, a series of controlling measures were implemented, including groundwater pumping control, surface water development, and groundwater recharge to manage the water resources more effectively and prevent excessive extraction of groundwater (cao et al., 2021; esteban et al., 2020). as a result, the subsidence condition in japan has been almost halted. similarly, the netherlands and the united states have also dealt with land subsidence and implemented measures to mitigate its effects successfully. (holzer and johnson, 1985; koster et al., 2018). figure 7. time series of los displacements (cm) at selected control points for the 2015-2020 period. previous studies applying linear extrapolation method to estimate land subsidence scenarios likely overestimated the future vlm conditions. as the major contribution to rslr and coastal flood hazards, land subsidence rate could be retarded and even totally stopped as long as implementing effective controlling strategies by local decision-making, which would further decrease the potential threat from rslr and coastal flooding. this result might provide some reference basic information for policy planning and relieve the public anxious emotion about future unpredictable risks. furthermore, it would provide a new perspective of planning countermeasures to future coastal hazards from costly engineering constructions to applying practical effective groundwater management and water resources arrangement, which would reduce the local economic pressure and benefit city development. 4.3 risk and its management the composite flood disaster is a typical multi-disaster event caused by typhoon, storm surge, astronomical tide, heavy rainfall, sea level rise and land subsidence. under the background of climate change and rapid urbanization in the past few decades, flood disasters caused by typhoon storm surge, extreme rainfall and relative sea level rise have become the most common and most serious natural disasters in coastal cities (adelekan, 2010; fang et al., 2017; du et al., 2019). coastal cities are extremely sensitive to sea level rise, and sea level rise and changes in coastal water depth will increase the risk of coastal flooding, while land subsidence caused by rapid urbanization will cause relative sea level rise, resulting in the “amplification effect” of flood disaster (wang et al., 2018). as one of the global financial centers and the largest city in china, shanghai is also one of the megacities with the highest risk of coastal flooding in the world (balica et al., 2012; ridder et al., 2020). under the complex background of global warming and sea level rise, shanghai faces severe flood risk prevention pressure, and its flood disasters are mainly urban waterlogging caused by extreme rainfall and coastal flooding caused by extreme typhoon storm surge (yang et al., 2021). human activities, land subsidence, sea level rise and extreme climate events will further increase the risk of composite flood disaster in shanghai. located in the delta front of the yangtze river estuary, shanghai is low-lying and extremely sensitive to sea level rise (fig. 8). due to the combined influence of typhoons, storm surges, heavy rainfall and river floods, disasters in this region have the characteristics of “wind, storm, tide and flood”, which easily lead to the “amplification effect” caused by disaster confluence and disaster superposition (muis et al., 2016). 216 beining wen, miao yu, chong liu, yan jiao, qihang kai figure 8. conceptual model of compound flood disaster event in coastal cities the problem of land subsidence in shanghai began in 1921. after the formation of modern industrial zone, unreasonable groundwater exploitation caused land subsidence in the downtown area of shanghai. since 1956, shanghai has taken a series of measures to deal with land subsidence. since the mid-1960s, shanghai has developed an annual water harvesting and irrigation program. especially since the 1970s, the central urban area of shanghai has basically realized effective control over the ground. in the 1990s, as shanghai began to implement market economic reform, it ushered in a new round of development opportunities. with the influx of a large number of people into shanghai and the large-scale urban construction, the demand for urban water resources and engineering geological activities are increasing day by day, and the ground in shanghai has obvious vertical movement phenomenon compared with the earlier period. in 2013, shanghai was hit by typhoon fitow, which caused a record water level in the upper reaches of the huangpu river under the combination of storm surge floods, which caused a great impact on the flood control and drainage infrastructure. in general, the interaction of sea level rise, land subsidence, typhoon storm surge, extreme rainfall and other disaster-causing factors leads to frequent flooding disasters in coastal cities, resulting in great social and economic losses. in order to cope with the extreme typhoon storm surge event in shanghai, it can be chosen to add the tide gate of huangpu river, and raise the flood control wall behind the gate 2 m to prevent river flooding. for extreme rainfall events, overall drainage improvements, the use of deep underground tunnels, the addition of permeable pavements and green roofs, and a combination of these measures can be taken. the biggest feature of gray engineering measures is high flood control efficiency and large capacity, but the disadvantage is that it is not friendly to the environment, and blue and green non-engineering measures can make up for this deficiency. according to the actual situation of shanghai, the strategy to deal with the combined flood disaster still needs to focus on gray engineering measures, and constantly strengthen the attention and investment of blue and green non-engineering measures. 4.4 uncertainty and limitation to minimize the inter-annual impact and measuring error, this study employed long-term rslr and slr data. even so, there are still uncertainties about rslr and slr due to local climate variability, record length, and record failures. additionally, the tide gauge station used in estimating the rslr trend is not located in the shanghai leading to uncertainties of consequences. there are certain limitations in terms of vlm measurement, because the sar datasets were only obtained from sentinel-1a ascending orbit track. therefore, to provide more comprehensive and higher accurate longterm measurements of vlm, multi-temporal and multi-platform (including ascending and descending orbits) sar datasets should be applied in the following studies. 5. conclusion this study demonstrates the quantitative contributions of slr and vlm to rslr in shanghai through long-term data analysis. furthermore, it investigates the spatiotemporal evolution characteristics of local vlm applying state–of–the–art insar analysis technique. based on the results of this study, two conclusions can be drawn. 1. the rslr of 5.67±0.58 mm/year over the 1969-2019 period is approximately over twice than slr trend of 2.44±0.28 mm/year during the 1993-2019 period, which shows that the vlm in shanghai, particularly land subsidence, is likely to be the main driver for accelerating rslr and exacerbating related coastal risks 2. spatial and temporal change patterns of vlm are highly uneven and nonlinear. as a result, linear assumptions of the vlm trend, especially land subsidence, used in previous studies would overestimate the future rslr scenarios and the loss of future coastal hazards. therefore, it is clear that local government should propose vlm controlling policies because the longer this problem continues, the more expensive any future adaptation strategies will be. 3. although we have verified the experimental results with the previous results of other researchers, it is necessary to carry out field operations to obtain the actual data of surface deformation. combining traditional methods with insar technology will obtain more accurate and reliable experimental results. in addition, accurately determining the vertical land movement of coastal cities and improving our robust prediction of coastal vulnerability to sea level rise can build a more comprehensive geological disaster prevention and control system for coastal cities, which will be the entry point for subsequent research. conflicts of interest: the authors declare no conflict of interest. funding this research is supported by soft science research project of shaanxi province "research on the principal-agent mechanism for the whole-people ownership of natural resource assets in qinling national park"(2023-cxrkx-109) and shaanxi provincial education department "research on the legal framework for eco-space governance in the yellow river basin of shaanxi province"(23jk0228). references abidin, h. z., andreas, h., gumilar, i., fukuda, y., pohan, y. e., & deguchi, t. 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(2025). fitting of statistical distribution in assessing rainfall patterns across various zones of assam. earth sciences research journal, 29(3), 363-378. https:// doi.org/10.15446/esrj.v29n3.119576 abstract: the maximum and minimum projected rainfall for a specific time of the year, taking into account a particular level of probabilities, makes up the probable rainfall, which is a great meteorological parameter of information. the purpose of this study is to assess the performance of probability distributions using various goodness-of-fit tests in order to develop a standard for selecting them in various zones of assam based on monthly rainfall data from 1985-2022. ten different distributions such as gumbel, weibull, gamma, logistic, exponential, log-normal, pearson-0, pearson-i, pearson-iii and pearson-v distributions have been considered in the study. the maximum likelihood estimation approach was used to estimate the parameters associated with these distributions. the best fitted probability distribution is determined using a variety of goodness of fit techniques, including the bayesian information criteria (bic), akaike information criterion (aic), and kolmogorov-smirnov (k-s) test. although not one distribution fits the rainfall data perfectly for every month, the pearson-i distribution typically fits the data better than the other distributions most of the time, according to the goodness of fit tools. the data has been collected from the national data centre (ndc), india meteorological department (imd), pune. keywords: aic; bic; k-s test; maximum likelihood estimation; precipitation. ajuste de la distribución estadística para evaluar los patrones de precipitaciones en diversas zonas de assam, india resumen la precipitación máxima y mínima proyectada para una época específica del año, con un nivel particular de probabilidades, constituye la precipitación probable, que es un gran parámetro meteorológico de información. el propósito de este estudio es evaluar el rendimiento de las distribuciones de probabilidad utilizando varias pruebas de bondad de ajuste con el fin de desarrollar un estándar para luego aplicarlo en varias zonas de assam con base en datos mensuales de precipitación de 1985 a 2022. en el estudio se han considerado diez distribuciones diferentes como gumbel, weibull, gamma, logística, exponencial, log-normal, pearson-0, pearson-i, pearson-iii y pearson-v. el enfoque de estimación de máxima verosimilitud se utilizó para estimar los parámetros asociados con estas distribuciones. la distribución de probabilidad mejor ajustada se determina utilizando una variedad de técnicas de bondad de ajuste, incluyendo los criterios de información bayesianos (bic), el criterio de información de akaike (aic) y la prueba de kolmogorov-smirnov (k-s). aunque ninguna distribución se ajusta perfectamente a los datos de precipitación para todos los meses, la distribución pearson-i suele ajustarse mejor a los datos que las demás distribuciones la mayor parte del tiempo, según las herramientas de bondad de ajuste. los datos se recopilaron del centro nacional de datos (ndc) del departamento meteorológico de la india (imd) en pune. palabras clave: aic; bic; prueba k-s; precipitación máxima proyectada; precipitation. https://doi.org/10.15446/esrj.v29n3.119576 mailto:dipanjali.ray@aus.ac.in https://doi.org/10.15446/esrj.v29n3.119576 https://doi.org/10.15446/esrj.v29n3.119576 364 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar 1 introduction: the identification of a probability distribution that precisely matches rainfall data has garnered interest in several disciplines, including hydrology and meteorology. in precipitation research, one of the most prominent topics is determining the most suitable distribution for rainfall data. various forms of precipitation include drizzle, snow, graupel, sleet, and freezing drizzle, with rainfall being the largest type. by employing several probability distributions, it is feasible to predict the amount of precipitation rather precisely. when managing water resources and identifying the dry and wet seasons, this is crucial information for agricultural planners. hydrological data sets can be fitted with a variety of probability distributions (ozonur et al., 2021, mandal and choudhury, 2014). numerous researchers have employed the probability distribution function to examine rainfall in various dimensions. thom (1958) has analysed the meteorological properties of the gamma distribution, bobee and robitaille (1977) demonstrated that pearson iii outperforms log pearson iii for flood data across global stations, including 11 stations in canada, while swift and schreuder (1981) evaluated several distributions such as log-normal, gamma, weibull, sb (special bound), and beta on daily precipitation data, concluding that the sb distribution best fits high-precipitation areas like the southern appalachian mountains. sarker et al. (1982) used the gamma distribution for precipitation in india, phien and jivajirajah (1984) applied log-pearson iii for flood analysis in thailand, kottegoda (1987) demonstrated johnson sb (special bound) for rainfall in various station of united kingdom and south africa, and koutrouvelis and canavos (1999) studied pearson iii for 100year simulated floods data. griffis and stedinger (2007) studied log-pearson iii for u.s. floods, feng et al. (2007) modeled extreme precipitation in china using the generalized extreme value distribution, and olofintoye et al. (2009) compared several distributions, including gumbel and pearson, for rainfall in 20 nigerian cities. deka et al. (2009) analysed the fitting of five three-parameter extreme value distributions i.e. generalized extreme value, generalized logistic, generalized pareto, log-normal, and pearson iii using l-moments and lqmoments for stations in north-east india maximum daily rainfall data for the period 1966 to 2007. millington et al. (2011) compared gumbel, generalized extreme value, and log-pearson iii distributions for the upper thames river, mandal and choudhury (2015) identified optimal rainfall distributions for sagar island, and amin et al. (2016) assessed normal, log-normal, logpearson iii, and gumbel max distributions for northern pakistan. kumar et al. (2017) tried to fit exponential, gamma, weibull, log-pearson iii, and generalized extreme value distributions to rainfall data in uttarakhand, douka and karacostas (2017) evaluated generalized pareto, johnson sb, log-gamma, and log-normal distributions for precipitation of thessaloniki. ghosh et al. (2016) assessed monthly rainfall in bangladesh using normal, lognormal, gamma, weibull, inverse gaussian, and generalized extreme value distributions. mamoon and rahman (2017) examined rainfall frequency in qatar with fourteen distributions. bhavyashree and bhattacharyya (2018) analyzed monsoon rainfall across karnataka, while kurniawan (2019) focused on jakarta’s rainfall patterns. shaharudin et al. (2020) investigated various distributions for rainfall simulation, and moccia et al. (2021) evaluated daily rainfall extremes in lazio and sicily, italy, using weibull, gumbel, frechet, pareto, gamma, and lognormal distributions. greece and ozonur et al. (2021) tested eight distributions, including weibull, rayleigh, gamma, log-normal, and gumbel, for modelling rainfall data in central west brazil. mohamed and adam (2022) identified the best-fit distribution for maximum rainfall in somalia, while chandran et al. (2023) assessed annual, monthly, and seasonal rainfall in tamil nadu. karami et al. (2023) examined distribution models for annual and 24hour maximum rainfall. abreu et al. (2023) explored criteria for selecting suitable distributions in case of rainfall data, and haseeb et al. (2025) compared nine distributions to determine the most appropriate fit for 42 years of rainfall data in pakistan. rainfall varies significantly across different locations and times, making statistical distributions helpful in understanding its unpredictability. examining rainfall data with various statistical models allows researchers to assess variability and identify normal and extreme patterns. heavy rainfall can lead to flooding, while insufficient rain may lead to droughts. statistical modelling helps estimate the probability of these extreme events, which is vital for designing flood barriers, building dams, and preparing for drought scenarios. assam remains highly vulnerable to floods and river basin–related disasters, which exert profound impacts on agriculture and livelihoods. understanding rainfall patterns is therefore critical, as it can generate valuable insights for flood preparedness, agricultural planning, and regional resilience. however, a comprehensive distributional analysis of rainfall behaviour across assam for the period 1985–2022 has not yet been undertaken. this study addresses this gap by examining rainfall patterns in the region, thereby contributing to improved climate risk assessment and resource management. this study aims to enhance the understanding of rainfall variability in specific areas of assam, india. the available climatological data will be analyzed to identify the most suitable probability distribution for rainfall in this region. in this study, ten different distributions have been taken into account to fit the rainfall data. to estimate the parameters of the chosen distributions, the maximum likelihood method has been considered. before statistical analysis, the kolmogorov-smirnov (k-s) test has been considered to assess the adequacy of these distributions to the data based on the k-s distance and the associated p-value. 2 materials and methods 2.1 characterization and location of the study area the north east region of india, which includes assam is expected to be extremely vulnerable to the effects of climate change because of its delicate geo-ecological configuration, strategic location near an international border, the presence of the eastern himalayan ranges, transboundary river systems, the inhabitation of the ecosystem by people from various ethnic groups, and inherent socioeconomic disparities (astec, 2011). depending on its geographical location, the whole assam is divided into various zones. namely, lower brahmaputra valley zone, upper brahmaputra valley zone, north bank plain zone, hills zone and barak valley zone. five stations have been selected from the five zones except hills zone on the basis of available data provided by meteorological department for the period of 19852022. the name of the stations are given below. table 1 represents the names of rainfall monitoring stations across various zones of assam, where the best probability distribution function will be determined from a selection of ten probability distributions. the geological location of the above stations are represented below table 1. name of stations in respective zones name of zones name of stations lower brahmaputra valley zone guwahati, dhubri north bank plain zone tezpur upper brahmaputra valley zone dibrugarh barak valley zone silchar 365fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india figure 1. study area in assam figure 1 aims to represent the inferred geological direction of all selected stations spanning diverse zones of assam created using qgis 3.32.0 software. assam’s climate is mostly humid subtropical, with warm, muggy summers, intense monsoons, and moderate winters. the range of winter temperatures is 100 to 220 degrees celsius. the summertime temperature ranges from 300 to 360 degrees celsius. the months of july and august receive the most rainfall in assam (29 percent of the total rainfall during the south west monsoon), while september receives 24 percent of the total rainfall. the remaining 66% of the yearly rainfall falls during the southwest monsoon (june – september). the state assam lies between 89046/ 96001/e longitude and 24003/27058/ n latitude and covers an area of 78,438 km² (guhathakurta et al., 2020)the longitude and latitude of remain stations are given below table 2 represents all the longitudes and latitudes of selected stations for this study. table 2. longitude and latitude of selected stations from various zones of assam name of stations longitude latitude guwahati 91° 44′ 45″ e 26° 10′ 20″ n dhubri ~89° 58′ 44″ e ~26° 1′ 20″ n tezpur 92° 47′ 59″ e 26° 37′ 59″ n dibrugarh ~94° 54′ 43″ e ~27° 28′ 22″ n silchar ~92° 47′ 52″ e ~24° 49′ 38″ n 2.2 data the data is collected from national data centre (ndc), india meteorological department (imd), pune (http://dsp.imdpune.gov.in/) for the period of 1985-2022. for the analysis, data is arranged monthly and also season wise, namely winter (january, february), pre-monsoon (march, april, may), monsoon (june, july, august, september) and post-monsoon (october, november and december). 2.3 probability distribution in this experiment, ten different probability distributions are considered to study the monthly rainfall data in five stations of various agro-climatic zones of assam. gumbel distribution (two-parameter) (gumbel, 1941), weibull distribution (two-parameter) (weibull, 1939), gamma distribution (twoparameter) (bobee & ashkar, 1991), logistic distribution (two-parameter) (jhonson et al., 1995), exponential distribution (one-parameter) (ferguson, 1964), log-normal distribution (two-parameter) (gaddum,1945), pearson-0 distribution (two-parameter), pearson-i distribution (four-parameter), pearsoniii distribution (three-parameter) and pearson-v distribution (three-parameter) (pearson, 1895) have been considered in the study. suppose x is a random variable with shape parameter α,a ,b, scale parameter β and location parameter μ then the probability density function (pdf) and their corresponding cumulative distribution function (cdf) are depicted in table 3. http://dsp.imdpune.gov.in/ 366 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar table 3. name of distributions with its pdf, cdf, domain and parameters distributions pdf cdf domain parameter exponential αe-αx 1 αe-αx x ∈(0,∞) α > 0 gumbel 1 x ∈ ℝ μ > 0, β > 0 weibull 1 x∈(0,∞) α > 0, β > 0 gamma 1 , x ∈ (0,∞) α > 0, β > 0 logistic 1 + 1 1 + x∈(-∞,∞) μ > 0, β > 0 log-normal 1 2 ( ) 1 2 + 1 2 2 where erf is the error function x∈(0,∞) μ∈ (-∞,∞), σ > 0 pearson 0 1 2 ( ) 1 2 ( ) x∈(-∞,∞) μ > 0, σ > 0 pearson i 1 | | ( + ) 1 | | ( + ) 1 x ∈ [a,b] where a &b are any real number a > 0, b > 0 β≠0, 0 < < 1 pearson iii ( ) 1 ( ) 𝑥 ∈ [𝜇,∞] 𝑎 > 0, 𝛽 > 0 > 0 pearson v | | | | | | | | 𝑥 ∈ (0,∞) 𝑎 > 0, 𝛼,𝛽 >0 >0 2.4 maximum likelihood method the parameter estimates of the distributions may be estimated using a variety of techniques. the maximum likelihood approach is the most used estimating technique. this approach is used in this work to estimate the parameters of any probability distribution that is taken into consideration. due to its good asymptotic features, this approach is a widely used and recommended statistical estimating technique. it is widely acknowledged that it generates parameter estimates that are both statistically consistent and effective. if we consider a sample {x1,x2,…,xn } following distributions with pdf f(x;θ), where θ represents the set of parameters, then the log-likelihood function can be expressed as ( , , … , ; ) = log ( ; ) . by maximizing the log l, , the maximum likelihood estimates (mles) can be obtained. to obtain the mles, we have to solve = 0 , with respect to set of parameters. sometimes these equations cannot be solved explicitly, then newton-raphson method is used (gupta & kapoor, 1997; feng et al., 2007; ozonur et al., 2021). 2.5 goodness of fit tests and model selection criteria to assess the appropriateness of the chosen probability distributions to the rainfall data, kolmogorov-smirnov (k-s) test has been considered initially. additionally, akaike’s information criterion (aic) and bayesian information criterion (bic) have been considered as the model selection criteria. using k-s test, one can determine whether the data follows the considered distribution or not. in k-s test, the null hypothesis has been considered as the data follows the selected distribution whereas the alternative hypothesis yields that the data does not follow the selected distribution. k-s test statistic defines the maximum difference between sample and theoretical cdf, and it can be expressed as = max ( ) , ( ) 1 , 367fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india where x((k)) is the estimated value of the cdf, and {x(1) ,…,x(n)} are ascending ordered observations. if the test statistics dn > dn (α) then the null hypothesis will be rejected at α significance level, where dn(α) represents the critical value of the k-s statistic (an & cheng, 1996; sharma and singh, 2010; mandal and choudhury, 2014; ozonur et al., 2021; mamoon & rahman, 2017; moccia et al. 2021). further, the p-values of the goodness of tests are often used to assess the appropriateness of the above-mentioned distributions. by using the p-value technique, the null hypothesis is rejected if the p-value is less than the selected significance threshold. the null hypothesis is not rejected if the p-value is bigger than the significance threshold, and there is insufficient evidence to draw the conclusion that the data do not follow the given distribution. if it is determined that both of the distributions under comparison may be appropriate, then the distribution with the greater p-value fits the data more closely. the effectiveness of the investigated pdfs for modelling the rainfall data is determined using the model selection criteria aic and bic after the appropriateness of the pdfs is determined based on the findings of the goodness of fit tests. the aic and bic can be obtained by using the following formulas: aic = -2 log l + 2m, bic = -2 log l + m log n , where log l yields the value of the estimated log-likelihood function at the mles of the selected model, m is the number of parameters of the selected distribution, and n is the number of observed data (akaike, 1969; schwarz, 1978; ozonur et al., 2021; mohamad & adam, 2021). these are analysed in software r version 4.1.1. 3. result and discussion in this section, the best fit distribution is elaborately discussed on total rainfall (mm) monthly and season wise for guwahati, silchar, tezpur, dibrugarh, dhubri station and are given below the above table 4 describes the frame of the data in both monthly and season wise by introducing length of the data, mean, maximum (max), minimum (min), median, standard deviation (sd), coefficient of variation (cv), coefficient of skewness (cs) and coefficient of kurtosis (ck). out of the twelve months, on april month variation is highest as sd is 122.444 and in season wise on pre-monsoon season shows highest variation (sd=124.134). similarly, the descriptive statistics have been calculated for the remaining station from which it has been observed that april (194.319) month and premonsoon (201.759) show highest sd in silchar station. the month of june is showing highest sd for the remaining stations i.e. tezpur (144.872), dibrugarh (165.55) and dhubri (281.681). season wise, monsoon shows highest sd for the remaining station tezpur (118.590), dibrugarh (149.852) and dhubri (233.280). table 4. descriptive statistics of rainfall in monthly and season wise data of guwahati station for the period of 1985-2022 month length mean min max median sd cv cs ck january 38 11.387 0 78.100 7.900 15.104 132.648 2.416 7.599 february 38 19.840 0 96.100 14.700 19.812 99.850 1.707 3.744 march 38 52.180 3.800 152.100 42.450 40.544 77.703 0.891 -0.168 april 38 179.200 24.000 520.900 160.700 102.342 57.121 0.889 1.431 may 38 246.200 88.500 569.700 222.000 122.444 49.729 0.777 -0.120 june 38 301.600 104.100 573.000 307.200 117.190 38.853 0.262 -0.825 july 38 301.900 131.600 638.800 285.900 118.365 39.207 0.880 0.397 august 38 239.100 67.100 569.200 221.100 111.544 46.660 0.550 0.080 september 38 190.300 28.200 372.000 183.100 86.267 45.342 0.353 -0.786 october 38 115.540 7.200 354.400 117.750 81.680 70.691 0.868 0.618 november 38 11.900 0 88.100 4.450 18.381 154.463 2.367 6.005 december 38 4.989 0 29.000 0.600 8.014 160.626 1.554 1.155 winter 76 15.610 0 96.100 11.000 18.006 115.332 2.023 5.272 pre-mon 114 159.189 3.800 569.700 135.100 124.134 77.978 1.008 0.755 monsoon 152 258.200 28.200 638.800 242.100 117.810 45.626 0.623 0.177 post-mon 114 44.140 0 354.400 8.650 69.969 158.501 2.054 4.315 368 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar table 5. estimates of probability distribution fitting of total monthly rainfall in guwahati station for january, february and march month estimates months distribution shape1 shape2 location scale january gamma 0.2525 0.0221 logistics 9.1336 6.9780 exponential 0.0878 gumbel 5.7015 8.4914 weibull 0.3485 5.0727 log-normal 0.3758 4.7371 pearson 0 11.3868 14.9043 pearson i 0.4280 8.4427 0.0001 223.9029 pearson iii 0.6068 0.0001 17.3831 pearson v 2.09e-01 1.00e-04 1.95e-14 february gamma 0.4177 0.0210 logistic 17.2762 9.9634 exponential 0.0504 gumbel 11.7239 12.7800 weibull 0.5721 15.6152 log-normal 1.4234 3.7790 pearson 0 19.8421 19.5499 pearson i 0.7961 13.3457 0.0001 320.231 pearson iii 0.9681 0.0001 16.1597 pearson v 3.1893 -10.2752 68.1229 march gamma 1.5351 0.0294 logistic 47.3156 22.5857 exponential 0.0191 gumbel 34.2173 28.9744 weibull 1.3049 56.5877 log-normal 3.5949 0.9391 pearson 0 52.1789 40.0078 pearson i 0.7492 1.918 3.8 173.9859 pearson iii 0.9922 3.8 47.0663 pearson v 36.4221 -166.352 7729.309 table 5 represents all the estimates of parameters i.e. shape parameter α,a ,b, , scale parameter β and location parameter μ for january, february and march month considering rainfall data for the period of 1985-2022 in guwahati station. table 6 represents probability distribution fitting of monthly total rainfall in guwahati station monthly for january, february and march month. it elaborates parameters for each distribution and comparison criteria’s i.e. -2logl (log likelihood), aic (akaike information criterion), bic (bayesian information criterion), k-s value and p-value. fitting of best probability distribution on a data set may be characterised as finding the distribution with the lowest values of -2logl, aic, bic, and k-s values. here p-value plays very significant role, since by having greater value than 0.05 represents the data follows the required distribution. the findings of the k-s test indicate that the rainfall data for january at the guwahati station does not follow the gamma, exponential, log-normal, or pearson 0 and v distributions, as their p-values are less than 0.05. in contrast, the data does conform to the logistic, weibull, pearson type i, and pearson type iii distributions since their p-values are greater than 0.05. among the followed distribution by january month, pearson i distribution has least -2logl (14.5150), aic (22.1151), bic (29.0654), k-s value (0.2105) and p-value is 0.0589, which is greater than 0.05 which indicates that the data points satisfy the null hypothesis i.e. the data points of january month follow pearson i distribution. likewise, interpretation can be elaborated for remaining months also. 369fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india table 6. criteria for comparison of probability distribution fitting of total monthly rainfall in guwahati station for january, february and march criteria for comparison months distribution -2logl aic bic ks-value p-value january gamma 168.0214 172.0214 175.2966 0.2213 0.0482 logistic 302.6070 306.607 309.8821 0.2126 0.0643 exponential 260.8668 262.8668 264.5044 0.2479 0.0187 gumbel 289.4652 293.4652 296.7404 0.1681 0.2331 weibull 180.5210 184.521 187.7962 0.217 0.0557 log-normal 197.4897 201.4897 204.7649 0.2482 0.0185 pearson 0 313.1651 317.1651 320.4403 0.2224 0.0392 pearson i 14.5150 22.1151 29.0654 0.2105 0.0589 pearson iii 70.9679 76.9679 81.8807 0.2105 0.0589 pearson v 164.1624 170.1624 175.0751 0.7874 2.22e-16 february gamma 274.4866 278.4867 281.7618 0.2197 0.051 logistic 328.2530 332.2529 335.5281 0.1500 0.3589 exponential 303.0732 305.0733 306.7109 0.1052 0.7937 gumbel 317.9188 321.9187 325.1939 0.1173 0.6723 weibull 284.239 288.2391 291.5143 0.2051 0.0815 log-normal 317.0596 321.0596 324.3348 0.3241 0.0006 pearson 0 333.7851 337.7851 341.0603 0.1557 0.2894 pearson i 255.954 263.954 270.5044 0.1200 0.6014 pearson iii 297.6733 303.6733 308.586 0.1567 0.2775 pearson v 312.9186 318.9186 323.8314 0.1008 0.7974 march gamma 372.8106 376.8107 380.0859 0.0763 0.9676 logistic 388.5022 392.5022 395.7774 0.1271 0.5295 exponential 376.5556 378.5556 380.1932 0.1196 0.6061 gumbel 378.9424 382.9424 386.2176 0.0952 0.8484 weibull 372.7302 376.7303 380.0055 0.0723 0.9803 log-normal 376.2848 380.2848 383.5600 0.0997 0.8085 pearson 0 388.2091 392.2091 395.4843 0.1630 0.2372 pearson i 354.6303 362.6303 369.1807 0.0914 0.8794 pearson iii 370.4733 376.4733 381.3861 0.1124 0.6805 pearson v 381.5386 387.5386 392.4513 0.11345 0.6704 the criteria for fitting probability distribution functions, along with their best parameter estimates, have been thoroughly discussed above for each month, supported by statistical values to demonstrate their significance. the graphical representation of these distributions for any given month or season can be effectively illustrated using a histogram of the raw data from the corresponding period, overlaid with line plots for each fitted distribution. in line with this approach, the rainfall data for january of guwahati station can be depicted graphically below, and this procedure can similarly be applied to the remaining months and seasons for the station to provide a comprehensive visual analysis. figure 2 represents the histogram with multiple line diagrams of ten selected probability distribution functions on raw rainfall data of january month in guwahati station just to check the suitability of these selected distributions on rainfall data. it is distinctly observed that these distributions effectively characterize the rainfall data for january month of guwahati station, and this approach can be systematically extended to encompass the remaining months and the rainfall records of other stations also. figure 3 represents a q-q plot, which shows how the quantiles of two distributions relate, indicating if the data follow a specific theoretical distribution or share the same distribution. gumbel (orange) fits reasonably well in the middle but overestimates at higher quantiles, with points above the line. logistic (blue) outperforms exponential and gamma, especially in the mid to upper range, though some deviations remain at the extremes. weibull (brown) stays close to the line in lower to mid quantiles but diverges upward at higher values. p1/pearson i (orange) aligns better than log-normal (lgn) and pearson 0 (p0), but still deviates in mid-probabilities. p3/pearson iii (blue) remains closer to the line across most of the range, especially mid to upper probabilities, indicating a better fit. these visual representations can also be summarised by test statistics. this visual interpretation is nearly the same as the interpretation of table 6, which shows various criteria for the comparisons of the selected distribution. 370 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar figure 4 represents a p-p plot which compares the empirical cumulative distribution function of january data to a theoretical distribution’s cumulative function. its interpretation is similar to that of a q-q plot. the logistic distribution (blue) follows the reference line more closely, especially in the central part, but shows divergence in the tails. the weibull distribution (brown) stays near the diagonal over a wide range, although it doesn’t fit perfectly in the lower tail. p3/pearson iii (blue) remains closest to the diagonal throughout most of the range and provides the best fit overall among the second set of options. figure 2. histogram with multiple line diagrams of 10 selected distributions of january month of guwahati station for the period of 1985-2022 on rainfall figure 3. q-q plot of january month at guwahati station figure 4. p-p plot of january month at guwahati station 371fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india table 7. criteria for comparison of best probability distribution fitting of total monthly rainfall in guwahati station for april, may, june, july, august, september, november and december station: guwahati months distribution -2logl aic bic ks-value p-value april pearson i 442.3452 450.3452 456.8956 0.2426 0.1870 may pearson i 454.6502 462.6502 469.2005 0.1205 0.5967 june pearson i 463.8386 471.8386 478.389 0.0786 0.9580 july log-normal 462.3650 466.3650 469.6402 0.0596 0.9993 august weibull 461.8864 465.8865 469.1616 0.0726 0.9881 september weibull 443.2952 447.2953 450.5704 0.0639 0.9977 october weibull 431.2786 435.2786 438.5538 0.1565 0.3098 november gamma 178.507 182.507 185.7822 0.1778 0.1808 december table 8. criteria for comparison of probability distribution fitting of total monthly rainfall in guwahati station for winter, pre-monsoon, monsoon and post-monsoon season season wise (guwahati station) months distribution -2logl aic bic ks-value p-value winter gamma 448.0652 452.0652 456.7266 0.2239 0.0009 logistic 639.2842 643.2842 647.9457 0.1863 0.0102 exponential 569.7262 571.7261 574.0569 0.1737 0.0203 gumbel 615.1228 619.1229 623.7843 0.1373 0.1135 weibull 472.7544 476.7545 481.416 0.2188 0.0013 log-normal 519.7514 523.7513 528.4128 0.2828 1.05e-05 pearson 0 654.0812 658.0812 662.7426 0.1913 0.0065 pearson i 267.4619 275.4619 284.7848 0.1578 0.0402 pearson iii 374.2139 380.2139 387.2061 0.1627 0.0315 pearson v 593.8993 599.8993 606.8915 0.14447 0.0747 pre-mon pearson i 1371.428 1379.428 1390.372 0.0751 0.5159 monsoon pearson i 1865.889 1873.889 1885.984 0.0469 0.8755 post-mon from table 6 and 7, the data points of february, march, april, may, june month fit best the pearson i distribution with least -2logl, aic, bic and k-s value with greater 0.05 p-value, but in july month, both pearson i and log-normal distribution fits well, but it is more favored to choose lognormal distribution since k-s value is lowest in case of log-normal distribution than pearson i distribution. similarly, august, september and october month follow weibull distribution. november month follows gamma distribution and december month doesn’t follow any distribution i.e. the p-value for each distribution satisfies the alternative hypothesis of k-s test. it means none of 10 distributions follow the required distribution i.e. all are having p-values less than 0.05. the criteria for determining the best-fitting probability distribution season wise are presented in table 8. table 8 also briefly explains the quantitative values of criteria’s of fitting best distribution in season wise i.e. winter (january, february), premonsoon (march, april, may), monsoon (june, july, august, september) and post-monsoon season (october, november, december) respectively. it is observed that the rainfall data of january and february month i.e. winter season follows gumbel and pearson v distribution with greater than 0.05 p-value, but depending on k-s distance or value, gumbel fits better with least k-s value. similarly, pre-monsoon follows pearson i, monsoon follows pearson i distribution and post-monsoon does not satisfy the null hypothesis of k-s test. table 9, 10, 11 and 12 represents selected criteria’s of fitting of distribution with least quantitative value for each month and seasons for silchar, tezpur, dibrugarh and dhubri respectively. from table 5 to 12, it is observed that among 10 distributions, 34.48%, 22.48% and 12.06% of rainfall data from 1985-2022, follows pearson type 1, weibull and gumbel distribution respectively considering net stations in monthly wise. similarly, 50%, 18.75% and 12.5% follows pearson type 1, weibull and gumbel distribution in season wise also. this analysis provides a refined understanding of probability distribution functions for monthly and seasonal rainfall, essential for shaping advanced agricultural strategies. by employing these models, farmers can accurately predict rainfall patterns, allowing precise adjustments in crop selection, planting schedules, and resource management. this precision enhances operational efficiency while promoting sustainable practices, ensuring resilience and longterm productivity in an increasingly unpredictable climate. 372 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar table 9. criteria for comparison of best probability distribution fitting of total monthly rainfall in silchar station station: silchar criteria for comparison months distribution -2logl aic bic ks-value p-value january gumbel 272.3906 276.3906 279.6658 0.2059 0.0795 february pearson i 279.4234 287.4234 293.9738 0.1315 0.4859 march weibull 451.964 455.964 459.2392 0.0805 0.9495 april pearson i 487.2584 495.2584 501.8087 0.1787 0.1558 may weibull 506.2132 510.2132 513.4883 0.0797 0.9534 june pearson ii 501.6652 509.5792 516.1295 0.0798 0.9530 july pearson v 486.2718 492.2718 497.1846 0.1070 0.7369 august logistic 491.0006 495.0006 498.2758 0.0917 0.9061 september pearson iii 485.5180 491.5180 496.4307 0.1172 0.6308 october gumbel 453.9594 457.9594 461.2346 0.1388 0.4564 november gumbel 349.366 353.3666 356.6412 0.2133 0.0628 december weibull 408.8964 412.8965 416.1716 0.1858 0.1447 season wise winter pearson i 280.6986 288.6986 295.3529 0.1282 0.5027 pre-mon pearson i 294.8297 302.8297 309.5852 0.1200 0.5192 monsoon logistic 1995.745 1999.745 2005.793 0.0424 0.9467 post-mon table 10. criteria for comparison of best probability distribution fitting of total monthly rainfall in tezpur station station: tezpur months distribution -2logl aic bic ks-value p-value january pearson i 74.2633 82.2633 88.8137 0.1861 0.1263 february pearson i 293.1259 301.1259 307.6762 0.0677 0.9901 march pearson i 332.5493 340.5493 347.0996 0.0943 0.8563 april gamma 411.9816 445.9816 449.2568 0.0917 0.8775 may weibull 446.7120 450.7119 453.9871 0.0962 0.873 june pearson 0 484.9906 488.9906 492.2658 0.0835 0.9334 july gamma 454.6172 458.6172 461.8924 0.0989 0.8511 august pearson i 452.3756 460.3756 466.9259 0.1027 0.7793 september pearson i 444.3329 452.3329 458.1402 0.1215 0.6280 october pearson i 396.8594 404.8594 411.4098 0.0813 0.9453 november pearson i 94.5341 102.5341 108.8682 0.218 0.0555 december pearson i 46.8992 54.8992 61.3428 0.1351 0.4684 season wise winter pearson i 399.0063 407.0063 416.3293 0.1032 0.3674 pre-mon pearson i 1352.836 1360.836 1371.78 0.0678 0.6446 monsoon gumbel 1879.527 1833.527 1889.575 0.0531 0.7835 post-mon pearson i 588.7287 596.7287 607.5668 0.1084 0.1387 373fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india table 11. criteria for comparison of best probability distribution fitting of total monthly rainfall in dibrugarh station station: dibrugarh months distribution -2logl aic bic ks-value p-value january weibull 314.4238 318.4238 321.6989 0.1380 0.4637 february weibull 385.9224 389.9224 393.1976 0.0857 0.9427 march pearson i 381.6493 389.6493 396.1997 0.1235 0.5656 april weibull 314.4238 318.4238 321.6989 0.138 0.4637 may pearson iii 481.0075 487.0075 491.9202 0.1004 0.8015 june logistic 493.8898 497.8899 501.1651 0.0856 0.9205 july pearson i 466.1879 474.1879 480.7382 0.0748 0.9726 august logistic 487.8244 491.8243 495.0995 0.1525 0.3394 september pearson i 463.8983 471.8983 478.4487 0.1112 0.6937 october pearson i 411.8757 419.8757 426.426 0.1434 0.3784 november gumbel 372.4454 376.4454 379.7205 0.1657 0.2477 december weibull 190.1 194.1 197.3219 0.1949 0.1200 season wise winter weibull 722.0576 726.0576 730.719 0.0617 0.9341 pre-mon gamma 1412.07 1416.0700 1421.542 0.0532 0.9341 monsoon pearson iii 1953.289 1957.337 1963.337 0.0373 0.9786 post-mon weibull 992.6616 996.6616 1002.081 0.1123 0.1214 table 12. criteria for comparison of best probability distribution fitting of total monthly rainfall in dhubri station station: dhubri months distribution -2logl aic bic ks-value p-value january february gamma 112.5378 316.5378 319.813 0.2032 0.0864 march gumbel 401.4268 405.4268 408.7019 0.1792 0.1740 april gumbel 460.5102 464.5102 467.7853 0.1120 0.7264 may logistic 477.3674 481.3674 484.6426 0.1262 0.5802 june weibull 503.5918 507.5918 510.867 0.0749 0.9225 july pearson v 486.2718 492.2718 497.1846 0.10702 0.7369 august gumbel 494.8362 498.8362 502.143 0.0944 0.8872 september gamma 482.6098 486.6098 489.8849 0.1099 0.7477 october weibull 454.9794 458.9793 462.9793 0.1485 0.3715 november pearson i 124.111 132.111 138.6614 0.1323 0.4784 december season wise winter pre-mon gumbel 1452.097 1461.569 1461.569 0.0779 0.4926 monsoon weibull 1501.223 1505.223 1510.696 0.0536 0.8979 pearson iii 1499.329 1505.329 1513.537 0.0556 0.8519 post-mon 374 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar 4. conclusion in this study ten probability distribution functions were considered to fit the monthly and season wise rainfall data of five zones of assam. from the analysis, the results reveal the statistical frame of data set for each station. considering all the months and seasons of five stations, the month of june for dhubri station shows the highest sd (281.681) and monsoon (233.280) season of dhubri gives peak of sd which indicates there is a higher deviation in rainfall data in the month of june and in monsoon (june, july, august, september) season amongst the five stations. the criteria of best fitting of selected distributions are elaborately explained along with the estimation of parameter. the weibull distribution fits best in the month of january and pearson v fits best in winter season considering ten distributions with least -2logl, aic, bic, k-s value with a p-value greater than 0.05 in guwahati station. likewise, the best fitting distribution along with its criteria of fitting and parameter estimation are explained monthly and season wise for each station. by taking into consideration all the stations in case of best fitting distribution, pearson type 1, weibull and gumbel distribution covers 34.48 percent, 22.48 percent and 12.06 percent respectively for the twelve months. for season wise, the percentage area of pearson type 1 is 50 percent, 18.75 percent for weibull and gumbel distribution covers 12.5 percent. this paper elaborates the fitting of distribution on rainfall data in various zones of assam which indicates there is also a possibility to get more precise form of distribution by proposing new generalized form of distribution to fit rainfall data in future. accurate rainfall predictions are vital for estimating river discharge and reservoir inflows, enabling early flood warnings. short-term forecasts are especially important for managing stormwater drainage. for example, in areas like guwahati and silchar, assam farmers rely heavily on monsoon rainfall. seasonal weather forecasts guide their decisions on sowing, transplanting paddy, and selecting short-duration crops. understanding the probability of heavy rain allows farmers to adopt preventive measures against waterlogging, crop damage, pests, and outbreaks diseases etc. acknowledgements the authors would like to thank the india meteorological department (imd), pune for supplying the required study’s data. authors would like to thank the editor and the referees for their support and assistance to improve the manuscript. author contributions all authors contributed to the study conception and design. material preparation was performed by tanusree deb roy and subhankar dutta, data collection was done by sebul islam laskar and analysis was performed by dipanjali ray. the first draft of the manuscript was written by dipanjali ray and all authors commented on previous versions of the manuscript. all authors read and approved the final manuscript. funding the authors declare that no funds were received during the preparation of this manuscript. data availability the data has been collected from national data centre (ndc), india meteorological department (imd), pune (http://dsp.imdpune.gov.in/). references: akaike, h. 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(1939). a statistical theory of the strength of materials. ingeniors ventenskaps akademiens handlingar, royal swedish institute for engineering research, stockholm sweden, 151 https://doi.org/10.1007/s13201-017-0586-5 https://doi.org/10.1007/s13201-017-0586-5 https://doi.org/10.1007/s11069-016-2687-0 https://doi.org/10.1007/s00704-014-1212-1 https://doi.org/10.1007/s00704-014-1212-1 https://doi.org/10.1016/j.ejrh.2020.100771 https://doi.org/10.1007/s40808-022-01363-0 https://doi.org/10.1007/s40808-020-00954-z https://doi.org/10.1007/s40808-020-00954-z https://doi.org/10.1029/2019ea000557 https://doi.org/10.1029/2019ea000557 http://doi.org/10.11591/ijeecs.v18.i3.pp1367-1374 376 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar appendix a: parameter estimates of guwahati station table a. estimates of probability distribution fitting of total monthly rainfall in guwahati station for remaining months months distribution shape1 shape2 location scale april pearson i 0.7542 2.1747 24.0000 522.8001 may pearson i 0.9553 3.1823 88.5000 642.4579 june pearson i 1.2658 1.8853 100.8439 496.6607 july log-normal 5.6387 0.3776 august weibull 2.3195 270.3478 september weibull 2.3941 214.9069 october weibull 1.3993 126.3508 november gamma 0.2627 0.022 december table b. estimates of probability distribution fitting of total monthly rainfall in guwahati station for winter, pre-monsoon, monsoon and post-monsoon season months distribution shape1 shape2 location scale winter gamma 0.3085 0.0197 logistic 12.9754 8.8017 exponential 0.0640 gumbel 8.4552 10.9346 weibull 0.4250 9.4203 log-normal 0.5238 4.3783 pearson 0 15.6144 17.8897 pearson i 0.5949 10.2068 0.0001 266.7931 pearson iii 0.7298 0.0001 17.0653 pearson v 1.5629 -3.9785 14.8985 pre-mon pearson i 0.9917 4.8964 3.8000 911.8026 mon pearson i 3.4559 11.5491 1.8968 1113.133 post-mon in table a, the parameters for april to december at the guwahati station are listed. similarly, in table b, each parameter for each season i.e. winter (january, february), pre-monsoon (march, april, may) and monsoon (june, july, august, september) are mentioned except post-monsoon season (october, november and december), since post-monsoon season does not follow any one of the 10 selected distributions with the rainfall data in guwahati station. for the winter season, parameter estimates are presented for all ten selected distributions, whereas for the other seasons, only the parameters of the bestfitted distribution are reported. 377fitting of statistical distribution in assessing rainfall patterns across various zones of assam, india appendix b: parameter estimates of silchar station, tezpur station, dibrugarh station and dhubri station in monthly and season wise data. table c: estimates of best probability distribution fitting of total monthly rainfall in silchar station station: silchar estimates months distribution shape1 shape2 location scale january gumbel 3.9666 6.6712 february pearson i 0.3509 0.9882 0.0001 0.0652 march weibull 1.3377 162.4325 april pearson i 0.8518 1.8127 23.2000 802.9113 may weibull 2.2891 468.592 june pearson ii 1.7622 109.7549 803.5272 july pearson v 5.6109 492.3355 120.8108 august logistic 406.7817 85.6269 september gamma 6.4457 0.0173 october gumbel 136.06 82.2392 november gumbel 14.0195 19.143 december weibull 0.4074 95.1627 season wise winter pearson i 0.3621 1.0518 0.0001 659.5552 pre-mon pearson i 0.3588 1.0380 0.0001 657.548 monsoon logistic 443.0995 96.6872 post-mon table d. estimates of best probability distribution fitting of total monthly rainfall in tezpur station station: tezpur months distribution shape1 shape2 location scale january pearson i 0.2866 1.2953 0.0001 101.5330 february pearson i 0.6298 2.1496 0.0001 110.4932 march pearson i 0.6321 0.8496 0.9000 109.7770 april gamma 3.4355 0.0205 may weibull 3.1960 283.2683 june pearson 0 303.8474 142.9531 july gamma 9.4320 0.0308 august pearson i 0.9232 1.3029 97.5000 425.9379 september pearson i 0.9534 2.3509 85.5000 475.2478 october pearson i 0.8165 1.6495 12.0000 254.0389 november pearson i 0.5354 3.7419 0.0001 147.7994 december pearson i 0.3106 3.5235 0.0001 133.2491 season wise winter pearson i 0.3837 1.6437 0.0001 103.9733 pre-mon pearson i 0.9077 2.0101 0.9000 482.7833 monsoon gumbel 220.4292 103.0446 post-mon pearson i 0.3455 1.7339 0.0001 278.8994 378 tanusree deb roy, dipanjali ray, subhankar dutta, and sebul islam laskar table e. estimates of best probability distribution fitting of total monthly rainfall in dibrugarh station station: dibrugarh months distribution shape1 shape2 location scale january weibull 1.2982 26.0708 february weibull 1.2670 65.7468 march pearson i 0.6223 0.8717 28.8000 211.9777 april weibull 1.2982 26.0708 may pearson iii 3.3481 29.1147 82.5268 june logistic 396.9677 90.1618 july pearson i 4.572 4.7715 639.4236 245.1121 august logistic 373.2674 82.0850 september pearson i 0.9131 1.3561 128.1000 500.7443 october pearson i 0.8149 1.5532 30.6000 289.403 november gumbel 13.5589 22.0851 december weibull 1.9822 5.7701 season wise winter weibull 1.0804 44.029 pre-mon gamma 2.3473 0.0110 monsoon pearson iii 397.9645 149.3592 post-mon weibull 0.4773 32.7641 table f. estimates of best probability distribution fitting of total monthly rainfall in dhubri station station: dhubri months distribution shape1 shape2 location scale january february gamma 0.2849 0.0048 march gumbel 31.0939 37.3340 april gumbel 141.1324 88.0224 may logistic 324.4702 73.7592 june weibull 3.1264 579.8261 july pearson vii 5.6109 492.3355 120.8100 august gumbel 248.8648 137.3011 september gamma 6.4457 0.0173 october weibull 1.8043 204.1932 november pearson i 0.3539 0.7547 0.0001 94.8928 december season wise winter pre-mon gumbel 120.9820 117.9553 monsoon weibull 2.649 494.7737 pearson iii 25.8363 458.5850 34.6120 post-mon table c, d, e and f represents the estimates of parameters of best distribution among ten selected distribution considering rainfall data for each month and season wise for silchar, tezpur, dibrugarh and dhubri station respectively. _hlk165186881 _hlk165187188 _hlk174727923 _goback _hlk174389375 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.117035 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 439 445 g eo te c h n ic a l en g in ee r in g mechanical properties as predictors of sand production: case study of the mamuniyat reservoir, murzuq basin-libya bahia m. ben ghawar1*, moncef zairi2, fathi m. salloum 3, mohammed f. elgubi1 1. engineering faculty, university of tripoli, libya; 2. national engineering school of sfax, sfax university, tunisia, 3. earth sciences department, university of benghazi, libya * corresponding author: moncef.zairi@enis.tn; b.ben-ghawar@uot.edu.ly how to cite item: ben ghawar, b. m., zairi, m., salloum, f. m., & elgubi, m. f. (2024). mechanical properties as predictors of sand production: case study of the mamuniyat reservoir, murzuq basin-libya. earth sciences research journal, 28(4), 439-445. https://doi. org/10.15446/esrj.v28n4.117035 record manuscript received: 13/10/2024 accepted for publication: 27/01/2025 abst r ac t sand production is one of the instability issues in oil reservoirs within the oil industry. this research paper seeks to establish a methodology for detecting sand production in the mamuniyat reservoir (upper ordovician) across multiple oil fields in the northwest murzuq basin, libya. well logging data from five oil wells (1, 2, 3, 4, 5) in fields 1 and 2 have been processed to calculate the sand production indicator (spi), which is based on the estimation of petrophysical and mechanical properties. the mamuniyat reservoir is classified in ascending order into clayey mamuniyat and clean mamuniyat sandstone, based on a 25% shale baseline. montmorillonite and illite are identified as the dominant clay minerals, as recognised by natural gamma-ray spectrometry (ngs) in the mamuniyat reservoir. additionally, the reservoir is classified from very low strength (e) to high strength (b) based on uniaxial compressive strength (ucs). the shale content (vsh) has noticeable changes in the ucs, cohesion (co), and friction angle (ф) of the clayey mamuniyat sandstone rather than the clean mamuniyat sandstone, which is about 70 mpa, not exceeding 30 mpa, and 20 degrees of the ucs, co, and ф, respectively. whereas the clean mamuniyat sandstone is ranged from 80 to 110 mpa of the ucs, about 20 mpa of co and close to 40 degrees of ф. accordingly, the studied reservoir does not meet the threshold criterion for sanding problems according to the standard shear to compressibility modulus ratio (g/cb), which is less than 8×10¹¹ psi². keywords: sand production indictor; mamuminyat reservoir; mechanical properties; petrophysical; murzuq basin; libya. palabras clave: indicador de producción de arena; reservorio de mamuniyat; propiedades mecánicas; petrofísica; cuenca de murzuq; libia propiedades mecánicas como predictores de la producción de arena: caso de estudio del reservorio de mamuniyat, en la cuenca de murzuq, libia la producción de arena es uno de los factores inestables de las reservas de petróleo. este artículo de investigación busca establecer una metodología para detectar la producción de arena en el reservorio de mamuniyat (ordovícico tardío) a lo largo de múltiples campos de petróleo en la cuenca de murzuq, al noroeste de libia. se procesó la información de perforación de cinco pozos petrolíferos (1, 2, 3, 4, 5), en los campos 1 y 2, para calcular el indicador de producción de arena (spi, del inglés sand production indicator), el cual se basa en las propiedades petrofísicas y mecánicas. el reservorio de mamuniyat tiene clasificación de orden ascendente dentro de las areniscas arcillosas y las limpias de mamuniyat, de acuerdo con su línea de base del 25 % de esquisto. los minerales de montmorillonita e illita se identificaron como los dominantes, según se reconoce a través de la espectometría de rayos gamma natural en el reservorio de mamuniyat. adicionalmente, el reservorio se clasifica en los rangos de fuerza muy baja (e) y hasta de fuerza muy alta (b), de acuerdo con la prueba de resistencia a la compresión uniaxial (ucs). el contenido de esquistos (v sh) implica cambios significativos en la prueba ucs, en la cohesión (co), y en el ángulo de fricción (ф) de las areniscas arcillosas de mamuniyat, más que en las areniscas limpias del reservorio, cuyas unidades son de 70 mpa en el índice ucs, sin superar los 30 mpa en la cohesión y de 20 grados en el ángulo de fricción. mientras que las areniscas limpias tienen un rango de 80 a 110 mpa en el índice ucs, cerca de 20 mpa de cohesión y cerca de 40 grados en el ángulo de fricción. de acuerdo con esto, el reservorio estudiado no cumple con el margen de criterio para los problemas de producción de arena según la relación estándar entre corte y módulo de compresibilidad (g/cb), que es menos de 8x1011 psi2. resu m en https://doi.org/10.15446/esrj.v28n4.117035 mailto:moncef.zairi@enis.tn https://doi.org/10.15446/esrj.v28n4.117035 https://doi.org/10.15446/esrj.v28n4.117035 440 bahia m. ben ghawar, moncef zairi, fathi m. salloum, mohammed f. elgubi introduction sand production is defined as a yield of small or large amounts of sand together with the reservoir fluids in production wells. most of the clastic oil and gas reservoirs contain sandy materials which may enhance problems at production stage of reservoir life history. in addition, operational cost of production, maintenance, as well as safety and environmental impacts are main reasons of the sand production investigation of wellbores (moore, 1994). mainly, two different mechanisms to understand the sand production occurring; mechanical instability and degradation of material around the wellbore, and hydromechanical instability which is related to pressure gradient of fluid flow within the porous media itself (rahmati et al., 2013). hence, issa et al. (2022) reviewed sand production problem and remedial techniques and veeken et al. (1991) summarized parameters influenced by sand production. many oil companies suffer from the sand production and try to get a solution that ensures continuity of reservoir productivity with fewer reservoir instability issues. thus, ben mahmud (2020) demonstrates application techniques of sand production, evaluated risks of sand management and display a smart control of the sand production. whereas, influence of oriented perforation on the flow rates and sand grain size taken in consideration to inhibit sand production of development wells in krishna godavari basin (southern india) by finite element model (kukshal et al., 2024). while, dong et al. (2013) utilized logging data to calculate mechanical properties, which indicated that the kexia formation is prone to sand production. further, khamehchi and reisi (2015) used the shear modulus, bulk compressibility, and the ratio between shear modulus and bulk compressibility to estimate sand production potential at kaki and bushgan oil fields in iran. they founded that production with a high water cut requires higher threshold value for g/cb ratio greater than 0.8×1012 psi2. thus, discovering of the sand production problem in early stages of well production life allows to design and choose a suitable solution for its controlling and to make a strategic plan in both drilling and production future development wells. in the libyan region, qiu et al. (2008) implemented the sand management solution (sms) in the sarir field, operated by agoco, to address sand production issues in over 400 wells. similarly, elhaddad (2019) analyzed sand production in the amal oil field, block 12, in the sirte basin, demonstrating that halliburton’s modern three-layer filter gravel provided an effective solution to the reservoir’s problem. based on these studies, the estimation of the sand production indicator (spi) for the mamuniyat reservoir in the murzuq basin (southwest libya) was conducted for five wells across two oil fields. additionally, the evaluation of the reservoir’s dynamic elastic properties is crucial for understanding the mechanical behavior of in-situ stresses during well development operations. this study highlights the utility of well log data, which offers a quick, time-saving, and efficient method for achieving the research objectives. geological background tectonic development of libya has been generally outlined by researchers who summarized their work as (hallett and clark-lowes, 2016). in early palaeozic era, the late pan-african activity produced a series of broad north-south troughs and swells. these troughs and swells are represented by the tihemboka high, murzuqjadu trough, and tripolitibisti uplift, which are controlled by the deposition followed by glacial and periglacial phenomena (late ordovician to silurian). during the midto late devonian, the northwestern margins of gondwana collided with laurasia, and the effect of the collision increased in intensity throughout the carboniferous in north african deformation. the peak of collision was reached in the late carboniferous, leading much of libya to be uplifted, eroded, and deformed. this is a reflection of hercynian orogeny. thus, a new set of east-west to northeast-southwest structural elements; nafusah uplift, alqarqaf arch, sirt arch, and ennedi-al awaynat uplift were superimposed over the earlier panafrican structural trend (klitzsch, 1971; bellini and massa, 1980; rusk, 2002). the situation led to the formation of the jifarah trough, the ghadamis basin, the murzuq basin, and the al kufrah basin (fig. 1a). however, mamuniyat formation (upper ordovician, ashgillian) is considered a valuable hydrocarbon reservoir of the murzuq basin, sw libya. stratigraphically, the mamuniyat formation is composed of three members (lower, middle, and upper), which are overlain by the silurian sequence (fig. 1b). in addition, table 1 is a concise a literature review of the studied formation. the figure 1b displays a sketch of a stratigraphic sequence (cambro – ordovician) of the formation. also, thickness of the mamuniyat formation of each studied well is varied from 91 feet up to 332 feet, as well as these wells are belonging to two oil fields of concession 115 and 186 northwest marzuq basin (figure 1). table 1. summary of a literature review of the mamuniyat formation reference summary of some previous studies echikh and sola, 2000 they have published their works on the gargaf uplift and the western flanks of the murzuq basin and suggested that the mamuniyat sediments were deposited in various fluvioglacial, deltaic, and shallow marine environments. alansari et al. 2018; fello, 2001; fello and turner, 2004; mohamed and beshr, 2022; alrabib et al., 2022; shalbak, 2015. they have studied and discussed sedimentology, depositional environment, diagenesis, and reservoir characteristics of the mamuniyat formation. also, they subdivided the formation into three members, which includes ten facies grouped in three different facies units. fello and turner, 2004. core samples and petrographic studies were taken from four oil fields that demonstrate sublitharenites, arenites, and litharenites are mainly composed of the mamuniyat formation. in addition, braided fluvial and shallow water marine sandstones form a petroleum potential of this formation. le heron et al., 2006 thye have reported four disconformitybound delineate stratigraphic units of the upper ordovician (mamuniyat formation). abdunaser, 2020. displays a sketch of a stratigraphic sequence (cambro – ordovician) of the formation. figure 1. a: location of the main sedimentary basins of libya, b: cambro – ordovician sequence of the study area (abdunaser, 2020; fello and turner, 2004) materials and methods logs and core samples are valuable measured data oil fields. also, the logs are more available and continue than the core samples. hence, radiation, electrical and acoustic logs are essential data that takes considered from five oil wells. these wells are producing from the mamuniyat reservoir (upper ordovician) at different oil fields 1 and 2. however, the logs data has included gamma ray (gr), interval travel time (δt), neutron (øn), litho-density log (bulk density, ρb and photoelectrical factor, pef), caliper (cal), and electrical resistivity (r). these recorded logs have been processed using excel microsoft program in order to evaluate the petrophysical and mechanical properties of the reservoir. in general, both petrophysical and mechanical properties results utilizes to calculate sand production indicator (spi). lithology determination is an early step to assess the petrophysical properties of any reservoir rocks. thus, using combination of measured logs b a 441mechanical properties as predictors of sand production: case study of the mamuniyat reservoir, murzuq basin-libya as cross plots, such as bulk density – neutron porosity, sonic – neutron porosity and bulk density– pef logs are quick and very useful techniques to define lithology (serra 1986). laboratory testing is a direct measurement of elastic properties by using rock samples and suitable empirical equations for indirect estimate these properties. hence, most of the empirical equations based on measurements of interval travel times (δtc and δts) or compressional and shear velocities (vp and vs), and bulk density (ρb) that illustrate acoustic properties of rocks of great interest in geomechanical interpretation. hence, lacking of core samples of the studied reservoir induced the estimation of the elastic (mechanical) properties based on empirical equations 1, 2, 3 and 4 (serra, 2008). however, young’s modulus (e) is a stress-strain ratio, which measure a rock stiffness in pressure units, the shear modulus (g) gives the resistance of rock to shear without change in volume, and it decreases as the porosity of rocks increase. while, the bulk modulus (kb) is a property of rocks explains required pressure to make the volumetric deformation. in other words, the e measures ability or disability of elastic deformation, while kb and g measure the strength of materials. furthermore, poisson’s ratio (υ) is an essential mechanical property of rock that varies with burial of depth, lithology, porosity, confining pressure and pore pressure (peng and zhang, 2007). υ= / ∆ ∆ − ∆ ∆ − (1) e = 2×g×(1+υ) (2) g = 1.34 × 1010 × ρb ∆ts 2 (3) kb = g ∆ts ∆tc 2 − 4 3[ [ (4) where: δtc = compressional interval time (μsec/ft), δts = shear interval time (μsec/ft) and ρb = bulk density (g/cc) uniaxial compressive strength (ucs) is also a mechanical property, which is maximum axial compressive stress or unconfined compressive strength of a material. it may give the ability to understand the sand production mechanism occurred within the rocks (kim, 2012; abass et al., 2002). however, both shear and tensile strengths provide reasons of common rock failure mechanisms. the mcnally (1987) equation 5 is used to calculate the ucs of the mamuniyat reservoir. also, there are other factors affecting on the rock failure mechanism such as cohesion (co), internal friction angle (ф), formation depth, heterogeneity of rocks and pore pressure of fluid formation. issa et al. (2022) briefly explained the mechanism of cohesion failure, which happen when a frictional force higher than cohesion strength between grains of rock due to increase of a fluid flow. while, the ф gives an indication of rocks resistance to shear, and controlling of mohr’s envelope line (peng and zhang, 2007). also, both co and ф are an indicator of the confining pressure and help in wellbore stability analysis. however, the following plumb (1994) of the friction angle equation 6 is used. ucs = 1200 × exp (-0.036 × δtc) (5) ф = 26.5 − 37.4 (1 ∅ vsh ) + 62.1 (1 ∅ vsh )2 (6) consequently, stein and hilchie (1972) and tixier et al. (1975) used techniques to predict formation strength based on the mechanical properties estimated from logs that indicate whether the sand state will produce or not. whereas khamehchi and reisi (2015) concluded that the shear and compressibility modulus (g/cb) ratio is considered a good indicator of the sand production in a normal reservoir condition. therefore, the shear and bulk compressibility (g/cb) ratio is selected as the sand production index (spi) of the mamuniyat formation (equation 7). where, if the ratio is exceeded (8×1011 psi2), the sand production occurs (liu 2017). spi = g/cb = g × kb (7) where: g = shear modulus (psi), kb = bulk modulus (psi) and and cb = bulk compressibility (1/psi). results and discussion: lithology delineation of the mamuniyat formation manifests clean and clayey units by the bulk density (ρb) versus neutron porosity (øn) cross plot (figures 2a and 2b), and correlation between wells by the photoelectrical factor (pef) logs (fig. 3). hence, figure 2a reveals the most plotted points on and around the sandstone (green) line that an apparent matrix parameter (ρma) equal to 2.65 g/cc in the clean mamuniyat. also, some points are plotted towards the limestone (blue) line and excessed dolomite (magenta) line that may indicate increasing of the vsh (>25%) and carbonate calcium as cementing material within the clean mamuniyat unit. while the plotting points have shifted below the sandstone line and gradually moved into the dolomite line (fig. 2b), which illustrates the high and influences of shale content on the clayey mamuniyat unit. in addition, the points above the sandstone line (figures 2a and 2b) reveal the presence of a light hydrocarbon (priisholm and michelsen, 2015). whereas, the measuring pef logs of the mamuniyat formation are compared with a standard value of the sandstone that is equal to 1.8 b/e. figure 3 displays a correlation of the pef log between wells, which emphasizes the clean mamuniyat unit is closed to the 1.8 b/e, while the clayey one is higher than the standard value. according to that, the volume of shale (vsh) higher than 25 % is considered a shale baseline of the petrophysical and mechanical properties evaluation. porosity and water saturation are the most petrophysical results that depend on the lithology type, shale content and type, cementation factor (m), tortuosity (a), and apparent matrix parameters. hence, the average of the total porosity (ønd), shale content (vsh), and water saturation (sw) by arithmetic average (equation 8) that summarized in table 1. the clean mamuniyat has lower average values of the ønd and vsh, and as well, the water saturation is increasing within the clayey mamuniyat unit. based on that, the clean mamuniyat unit is considered as the reservoir rock (upper ordovician) of the studied oil fields. moreover, the clay types within the clean and clayey mamuniyat are defined by natural gamma ray spectrometry (ngs): potassium (k) – thorium (th) concentration cross plot (figures 4a and 4b). it is noticeably the clean mamuniyat th-k plotting points are above th/k ratio line of 2 and less than th/k ratio line of 3.5, with slightly scattered values above the th/k ratio line of 3.5. while the extensive points of the clayey mamuniyat are located below the mica line and exceed the th/k ratio of 3.5 up to 100% illite line. therefore, montmorillonite and illite clay minerals are dominant clay minerals have concentrated within the mamuniyat reservoir. arithmetic average = ∑ xn i=1 n (8) where: x = calculated property, n = number of samples. generally, the above petrophysical evaluation is helpful for assessment and interpretation of the dynamic elastic properties evaluation of the mamuniyat reservoir. additionally, arithmetic average results of the calculated mechanical properties are also included in table 1. therefore, the clean mamuniyat has higher values of the e and g modulus and lower values of the kb than the shaly mamuniyat, with an exception of well 4. as well, the poisson’s ratio (pr) that reflects the compressibility of the rock and works as a fracture indicator of rocks; rocks with a high υ are more likely to have fractured than those with a low pr. the average pr of the mamuniyat reservoir is located within a typical range of 0.18 to 0.22 of deeper, highly compacted, cemented sandstone (alberty and mclean, 2001). the uniaxial compressive strength (ucs) of the mamuniyat reservoir does not exceed 150 mpa, which is located with a typical range value of sandstone rock (backers, 2013; attewell and farmer, 1976). based on hoek and brown (1997), the mamuniyat reservoir is classified from very low strength (e) to high strength (b). whereas the calculated internal friction angle (ф) and cohesion (co) parameters results reveal decreased values against increasing of the total porosity and volume of shale values. then, the clayey mamuniyat shows lower values of ф than the clean mamuniyat and inversely the co. an average value of the co of the mamuniyat is included in table 1. these 442 bahia m. ben ghawar, moncef zairi, fathi m. salloum, mohammed f. elgubi average values are closer to published data of berea sandstone, bartlesville sandstone, and pottsville sandstone by goodman (1991). therefore, it is clearly the mechanical properties of the mamuniyat reservoir are affected by the petrophysical properties. figure 2a. bulk density versus neutron porosity cross plot of the clean mamuniyat unit. figure 2b. bulk density versus neutron porosity cross plot of the clayey mamuniyat unit. figure 3. photoelectrical factor (pef) logs correlation between wells, field 1 and 2. table 1. average porosity (ønd), shale content (vsh) and dynamic elastic properties of the mamuniyat reservoir field 1 field 2 well 1 2 3 4 6 ønd (%) c 11 13 11 15 12 s 9 12 8 11 12 vsh (%) c 7 11 8 4 5 s 33 36 33 26 26 sw (%) c 31.5 57 39 15 38 s 100 81 100 79 37 pr c 0.22 0.15 0.21 0.21 0.21 s 0.22 0.18 0.22 0.21 0.20 e (mpa) c 3.85e+04 4.32e+04 4.22e+04 4.02e+04 4.03e+04 s 3.82e+04 3.94e+04 4.09e+04 4.15e+04 4.53e+04 g (mpa) c 1.58e+04 1.88e+04 1.75e+04 1.67e+04 1.67e+04 s 1.56e+04 1.67e+04 1.68e+04 1.72e+04 1.89e+04 kb (mpa) c 2.27e+04 1.91e+04 2.37e+04 2.29e+04 2.29e+04 s 2.29e+04 2.36e+04 2.39e+04 2.37e+04 2.50e+04 ucs (mpa) c 85 84 92 90 90 s 79 86 84 88 97 co (mpa) c 21 22 23 23 22 s 23 25 24 25 29 ф (◦) c 37 35 37 37 38 s 30 29 30 31 27 c = clean mamuniyat s = clayey mamuniyat 443mechanical properties as predictors of sand production: case study of the mamuniyat reservoir, murzuq basin-libya each sand production technique has specific conditions that determine whether it should be applied in a given situation. thus, if the shear and compressibility (g/cb) modulus ratio is less than 8×1011 psi2 (3.80e+07 mpa2) that means a formation is stable and there is no sand production. otherwise, the g/cb ratio is greater than 8×1011 psi2, which means a formation is unstable and sand production could occur (liu, 2017). then, figure (5) displays a correlation of the gamma ray and borehole geometry (caliper) logs and results of the spi, ucs, co, and ф of the mamuniyat reservoir of well 4. also, the same correlation between the studied wells is recognized of wells 1 and 2 from field 1 (figure 6a), and wells 3 and 6 from field 2 (figure 6b). then, evaluation of the sand production indicator (spi) results of the mamuniyat reservoir reveals it has no sand production problem. hence, this result is consistent with the elastic property findings of the mamuniyat reservoir. conclusion prediction of the sand production in the early stages of the reservoir production life history is a significant step to save time for treatment procedures in oil fields. this issue of oil fields is strongly related to the mechanical properties of rocks, in turn considering the petrophysical properties. however, based on the gamma ray and photoelectrical factor logs and 25% of the shale base line (vsh), the mamuniyat reservoir (upper ordivician) is divided into clean and clayey mamuniyat. also, the clean mamuniyat has a good total porosity (ønd) and lower volume of shale (vsh) and water saturation (sw) than the clayey mamuniyat. hence, these petrophysical properties have an impact on the elastic properties. the average value of the ucs of the mamuniyat formation does not exceed 100 mpa, where the low values of the ucs respond to the high values of the ønd and vsh. also, these ucs values are located between the very low strength (e) and high strength (b) classifications of the mamuniyat reservoir rock. furthermore, the mohr-columb failure criteria parameters: cohesion (co) and internal friction angle (ф) also have low values in the clayey mamuniyat rock. therefore, the sand production indicator (spi) of the mamuniyat reservoir of two oil fields has not reached the threshold value of sand production, which is less than 8 ×1011 psi2. a b figure 4. natural gamma ray spectrometry (ngs): potassium (k) – thorium (th) concentration cross plot of a: clean mamuniyat unit, and b: clayey mamuniyat unit. consequently, the early evaluation of the sand production problem is a safer and less coasting technique to understand failure criteria and in-situ stresses of the oil fields. moreover, the mechanical earth model (mem) of the mamuniyat reservoir is necessary, and it gives a prediction of the ability to increase production rates without causing the instability problem (sand production). figure 5. gamma ray and geometry logs and mechanical properties results of well 4, field 1 444 bahia m. ben ghawar, moncef zairi, fathi m. salloum, mohammed f. elgubi figure 6a. gamma ray and geometry logs and mechanical properties results of wells (1 and 2), field 1 figure 6b. gamma ray and geometry logs and mechanical properties results of wells (3 and 6), field 2 445mechanical properties as predictors of sand production: case study of the mamuniyat reservoir, murzuq basin-libya acknowledgment the authors thank the akakus oil company for 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2339-3459 https://doi.org/10.15446/esrj.v27n2.103801 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 85 91 g eo ph y si c s manuscript received: 19/07/2022 accepted for publication: 17/05/2023 vertical accuracy evaluation free access digital elevation models (dems): case fergana valley in uzbekistan dilbarkhon fazilova1,2,3, obidjon arabov1 1national university of uzbekistan named after mirzo ulugbek, department of geodesy and geoinformatics, 1000174, 4, university str., tashkent, republic of uzbekistan 2astronomical institute of uzbek academy of sciences, space researches laboratory, 100052, 33, astronomical str., tashkent, republic of uzbekistan 3tashkent state technical university named after islam karimov, department of mine surveying and geodesy, republic of uzbekistan corresponding author: dilbar@astrin.uz abstract keywords: open-source dems; vertical accuracy; srtm; aster gdem2; alos awd3d30; gps; geoid undulations; in this study, the vertical accuracy of the shuttle radar topography mission digital elevation model version 2.0 (srtm30), the advanced spaceborne thermal emission and reflection radiometer global dem version 2.0 (aster gdem2), and advanced land observing satellite world 3d digital surface model version 2.1 (alos aw3d30) was statistically assessed using gps data. the fergana valley area was chosen as a study region, where the land surface can reflect tectonic processes. the values of ellipsoidal heights of 27 points of the regional gps network were chosen as reference data. the geometric approach using gps/leveling data and egm96 global geopotential model-based geoid undulations was applied for geoid surface fitting. the geoid height corrections range ranged from –0.66 m to 0.87 m. root-mean-square errors of ~10.0 m, ~16.4 m, and ~6.6 m was obtained for srtm30, aster gdem2, and alos aw3d30, respectively. it was found that compared with the reference model, all the global dems in mountainous areas generally overestimated elevation and the value of vertical accuracy at a 90% confidence level by 3-6 meters exceeded the declared by distributors. but alos aw3d30 proved to be the most accurate dem that best represents the topography of the earth’s surface and could be used for some engineering applications in fergana valley. palabras clave: modelos digitales de elevación de acceso abierto; precisión vertical; srtm; aster gdem2; alos awd3d30; gps; ondulaciones geoidales; evaluación de precisión vertical de los modelos digitales de elevación de acceso abierto: caso del valle de fergana, en uzbekistán how to cite this item: fazilova, d., & arabov, o. (2023). vertical accuracy evaluation free access digital elevation models (dems): case fergana valley in uzbekistan. earth sciences research journal, 27(2), 85-91. https://doi. org/10.15446/esrj.v27n2.103801 resumen en este studio se evaluó estadísticamente la precisión vertical de los modelos digitales de elevación de la misión topográfica shuttle radar 2.0 (srtm30), del sistema radiométrico de emisión y reflexión térmica espacial terra advanced 2.0 (aster gdem2), y del satélite de observación terrestre mundial 3d 2.1 (alos aw3d30). el valle de fergana, en asia central, fue elegido como la región de estudio porque la superficie terrestre puede reflejar los procesos tectónicos. para este trabajo se determinaron los valores de alturas elipsoidales de 27 puntos de la red regional gps como valores de referencia. para el ajuste geoidal de la superficie de estudio se aplicó la aproximación geométrica con los datos de nivelación gps y el modelo geopotencial global egm96, basado en ondulaciones geoidales. el rango de corrección de la altura geoidal se establece entre -0.66 m y 0.87 m. con los modelos srtm30, aster gdem2 y alos aw3d30 se obtuvieron los errores de raíz cuadrada media de ~10.0 m, ~16.4 m, and ~6.6 m, respectivamente. al compararse con el modelo de referencia se encontró que todos los modelos digitales de elevación en las áreas montañosas por lo general sobreestimaban la elevación y el valor de precisión vertical de un 90 % de exactitud excedía entre 3 y 6 metros el margen declarado por los distribuidores. sin embargo, el modelo alos aw3d30 fue el modelo digital de elevación más preciso y que mejor representa la topografía de la superficie terrestre y que, por lo tanto, podría ser usado para algunas aplicaciones ingenieriles en el valle de fergana. https://doi.org/10.15446/esrj.v27n2.103801 mailto:dilbar@astrin.uz https://doi.org/10.15446/esrj.v27n2.103801 https://doi.org/10.15446/esrj.v27n2.103801 86 dilbarkhon fazilova, obidjon arabov 1. introduction digital elevation models (dems) are necessary for the solution of many practical problems in the areas of the earth and environmental sciences (such as tectonics, the study of landslides, mineral exploration, water and land resources management, disaster prevention, and others) in uzbekistan (sayyidkosimov & kazakov, 2018; sabitova et al., 2020; sharipov et al., 2020; khasanov & ahmedov, 2021). topographic maps, global navigation satellite system (gnss) measurements, photogrammetry techniques, and airborne laser scanning are the main data sources for dems production used in uzbekistan for small areas (takhirov et al., 2020; shukina et al., 2022). despite the accuracy, the traditional dem generation methods are tedious and time-consuming for using over large areas. modern satellite remote sensing technologies have made it possible to map large areas with minimal labor and technological costs (smith &clark, 2005). the widespread use of publicly available satellite-based dems such as srtm, aster, and alos is an example of these benefits. however, available open-source these and many others have coarser resolutions. their accuracy is affected by measurement error, data acquisition method, the instrument’s capability, resolution, terrain type and relief, slope, roughness, land cover, and others (mukherjee et al., 2011; hu et al. 2017; shetty et al., 2022). the vertical accuracy assessment of space-borne elevation data sets has been one of the major concerns for worldwide researchers (mukherjee et al., 2013; elkhrachy, 2018; gonzález-moradas & viveen, 2020; preety et al., 2022). for the practical application of the dem, it is necessary to identify the influence of error sources on the vertical accuracy of the dem and refine the models using terrestrial measurements for each area separately (dragut & eisank, 2011). global dems have got an upsurge in their usage in uzbekistan. but so far there are few works on assessing their accuracy and developing methods for improving it. in earlier studies, the accuracy of dems, such as srtm, aster gdem, and alos palsar, was assessed using various data (dems of higher resolution, the utilization of differential global positioning system (dgps), terrestrial measurements (bakiev & khasanov, 2021). it was revealed that for the territories located near the reservoirs, aster dem gives a more realistic representation of steep slopes and mountain ridges compared to srtm or alos (fazilova et al., 2021). the goal of this study is to quantify and compare the vertical accuracy of open access dems obtained using the shuttle radar topography mission (srtm30), advanced spaceborne thermal emission and reflection radiometer (aster gdem2), and advanced land observing satellite (alos aw3d30) for the territory of the fergana valley in the republic of uzbekistan. 2. data and method the area taken for the study is the territory of the fergana valley in uzbekistan, which is an east-west-trending intra-orogenic intermountain depression, the most seismically active and densely populated region of the country. it is located in the eastern part of the republic of uzbekistan and borders the republic of tajikistan and kyrgyzstan. the width of the research area is 370 km by 19 km. it is bounded by the alai and turkestan systems mountain uplifts in the south, by the fergana mountain in the northeast, and by the chatkalkurama mountain in the north and northwest (umurzakov & rabbimkulov, 2022). the maximum topographic relief between the fergana valley floor, with an altitude of roughly 330 m, and the surrounding mountain peaks is about 1000 m. the main problem for the territory of the republic, which has a complex relief, is the need for better knowledge of the geoid by gravimetric methods. the geoid of the fergana valley is the only area that has been sufficiently well studied by classic leveling. as a result, the influence of the tien shan and pamir mountain ranges on gravitational anomalies was revealed (ustyantsev, 2011) (fig.1). in the work, the entire territory of the fergana valley is considered from the point of view of the fact that for many geodetic engineering applications, the assessment of the accuracy of dems has not yet been made. the study utilized three sources of dems generated from various data acquisition techniques, namely shuttle radar topography mission (srtm) dem of 30 m, advanced space borne thermal emission and reflection radiometer (aster)-global dem of 30 m, and alos world-3d of 30 m. the important technical details of dems are explained in table 1. srtm30 v. 2.0. it can be considered to be either an srtm30 data set enhanced with gtopo30 or an upgrade to gtopo30. 1-arcsecond (about 30 meters) resolution dem, delivered in 1°×1° tiles. systematic interferometric data were collected for each terrain segment at least twice from different angles (on ascending, north-going, and descending orbit pass) to fill in areas shadowed from the radar beam by terrain. the finished product contains ‘no-data’ figure 1. topography of the study area generated by srtm30 and distribution of gps points over fergana valley 87vertical accuracy evaluation free access digital elevation models (dems): case fergana valley in uzbekistan termed as voids where water or heavy shadow prevented the quantification of elevation. at a 90% confidence level, the vertical accuracy is expected to be 10 m (rodriguez et al., 2006). eight tiles of the srtm30 elevation data for the study area were downloaded from https://www.usgs.gov/centers/eros website. aster gdem v.2 (aster gdem2). the 1-arc-second (30 m) stereoscopic product aster gdem2 was created by stacking all individual cloud-masked scene dems and non-cloud-masked scene dems, then applying various algorithms to remove abnormal data. the reason for considering this model is that aster gdem2 has some advantage over srtm30 because its stereo pair has more nadir view when collecting data in very steep and rugged terrain (abrams et al., 2010). aster gdem2 data has a vertical accuracy of ±15–20 m, depending on the region’s environmental situation (thomas et al.2014). for this study, 7 aster gdem2 elevation data tiles were downloaded from https://asterweb.jpl.nasa.gov/gdem.asp website. alos world 3d (aw3d30). the global digital surface model dataset with a horizontal resolution of approximately 30-meter mesh (1 arcsec. latitude and longitude) generated from 5 m resolution. the worldwide digital elevation model dem and ortho-rectified image (ori) were created using the archived information about the panchromatic remote sensing instrument for stereo mapping (prism). prism comprised three panchromatic radiometers that were procured along with track stereo images. it had a spatial resolution of 2.5 m in the nadir-looking radiometer and accomplished worldwide coverage, making it an appropriate potential candidate for exact worldwide dem and ori generation (tadono et al., 2014). this is a non-standard version of the alos aw3d30 dataset provided in a wgs84 ellipsoidal vertical datum. for this study, 9 tiles of data were downloaded from https://www.eorc.jaxa.jp/ alos/en/aw3d30/index.htm website. the raster values (heights) of these dems were extracted from the coordinates of the gps points. these heights are referred to as heights obtained from each of the dems. the vertical accuracy of dem at the scale of the study area is verified using 27 reference stations of the state satellite geodetic network (ssgn) in the region, which is developing quite rapidly, considering the developed infrastructure of the country (fazilova, 2022). to date, only these points of ssgn have been installed in the region (fig. 1). these points are the so-called “common points,” with the values of normal heights in the baltic system of 1977 available in the database of the international gravimetric bureau in toulouse (drewes et al., 2016). measurements of other global navigation satellite systems (gnss) such as glonass, galileo, and beidou are currently not available at the stations. the gamit/globk software package version 10.71 was used to calculate the coordinates of points (herring et al., 2018). according to dong et al. (1998), processing was carried out according to the recommendations and standards of the international earth rotation service iers2010 (iers, 2010). the itrf2014 coordinate system (altamimi et al., 2016) was chosen as the reference coordinate system for the regional solution. the coordinate component residuals’ weighted rms (wrms) value was 1.0– 3.2 mm and 3.2–6.5 mm for horizontal and vertical components, respectively. the quality of the dem is a primary requirement from the various processing steps, the interpolation used for resampling, and the local properties (land cover and slope). typically, the quality assessment includes two approaches, namely with (external) or without (internal) reference data set. the dem is compared with a reference data set (external validation). in contrast, in the second case, inconsistencies are sought within the dem itself with no reference data (internal validation) (polidorit & hage, 2020). the vertical accuracy can be directly estimated by comparing the heights extracted from a dem and their values interpolated to the location of the gps points. in order to make a consistent comparison of gps surveys with srtm, aster, and alos dems, all measurements must refer to the same horizontal coordinate system and vertical datum. the datum of the gps’s height is the ellipsoidal surface based on the world geodetic system 1984 (wgs84) reference ellipsoid. for srtm, aster, and alos dems, each grid cell’s horizontal and vertical coordinates referenced to wgs84 and egm96 (lemoine et al., 1997) were output as a text file. then elevations were transformed to ellipsoid heights (h) relative to wgs84 using egm96 derived geoid undulations (n) by algebraic summation: (1) the geoid undulations according to the egm96 global geopotential model can be determined using the service of the international center for global earth models (icgem) (barthelmes & köhler, 2016). the practical application of equation (1) to determine the required normal heights is complicated by a number of factors, including random errors (noise) in determining the heights, the difference in relevance for different types of heights, and insufficient knowledge of the relationship between them, geodynamic phenomena (soil subsidence, deformation of tectonic plates near subduction zones, etc.). by the extensive use of gps technique with geodetic aims, great interest has been collimated to the precise determination of local/regional geoid with an aim to replace levelling measurements with gps surveys. geoid models derived from the global geopotential models (such as egm2008, eigen6c4, egm96) are the one alternative solution, but their accuracy needs to be estimated first for tectonic territories like fergana valley. there are two approaches for the transformation: gravimetrically determined geoid model, and interpolation between geometrically derived geoid heights where gps measurements have been collocated with benchmarks. the gravimetric method offers benefits for areas with a homogeneous coverage of terrestrial data, but it is involving mathematical and computational procedures (featherstone et al., 1998). the geometric approach has been widely used for a relatively small area, which interpolates geoid heights based on the gps-derived heights and leveled heights at some points (zhong, 1997; erol & çelik, 2004; rabah & kaloop, 2013; ligas & kulczycki, 2018; dawod & abdel-aziz, 2020). the global geoid models like egm96 can achieve the accuracy of regional or local geoid models by this method. it is used to determine the shortwave and ultra-shortwave components of the geoid, to improve of accurate geopotential models (erol & celik, 2004; soycan, 2014). early studies used the method due to its efficiency for local areas with poor knowledge of the geoid. the high accuracy of the estimation, especially for engineering surveys, is shown (mishra & ghosh, 2017; jassim & yousef, 2021). to improve its accuracy parametric models (or a corrective surface) introduced when refining heights (fotopoulos et al., 2003). such models include high-order polynomials with interpolation with different basis functions (zhao et al., 2022), least squares collocation (lyszkowicz et al., 2014), finite element method (zaletnyik et al., 2007), fourier series (grigoriadis et.al., 2021), and artificial neural networks (konakoglu & akar, 2021). interpolation methods (such as inverse distance table 1. description of digital elevation models (dem) data srtm30 v. 2.0 aster gdem v.2 alos world 3d source space shuttle radar aster daichi/alos generator/distributor nasa/usgs meti/nasa jaxa period of observation from february 11th, 2000 to february 22nd, 2000 march 1, 2000 november 30, 2013 from 2006 to 2011 spatial resolution (m) 30 30 30 vertical datum egm96 egm96 egm96 horizontal datum wgs84 wgs84 wgs84 data type interferometry synthetic aperture radar optical stereoscopy optical stereoscopy vertical accuracy (le90) < 10 m <15-20 m <7m https://www.usgs.gov/centers/eros https://asterweb.jpl.nasa.gov/gdem.asp https://www.eorc.jaxa.jp/alos/en/aw3d30/index.htm https://www.eorc.jaxa.jp/alos/en/aw3d30/index.htm http://et.al 88 dilbarkhon fazilova, obidjon arabov weighting, bilinear interpolation, polynomial regression, triangulation, radial basis functions and nearest-neighbor interpolation) were considering according the application area, surface features, distribution of data and their accuracy. early studies are devoted to assessing the accuracy of the employed calculation method of the geoid as well as the accuracy, density, and distribution of the used data. polynomial, least squares collocation, multiquadric (mq) methods had the best results for geoid undulation calculation. the results confirmed that the radial basis functions (rbf) method is the popular approach to interpolate irregularly spaced data. especially rbf with multiquadric (mq) function gives the lowest standard deviation and noise level for the prediction of gravity anomalies, distortion modeling (soycan, 2014; doganalp & selvi, 2015). we use rbf with mq function interpolator for construction of the surfaces in the work. the improvement of egm96 model is based on the corrector surface fitting (csf) algorithm and consists of several steps. the first step calculates the difference between the gps/leveling geoid heights /gps leveling in and the geoid height according to egm96 96egm in at each control “common point” (i=1...27) (soycan, 2014): (2) the second step is to define the trend ti based on the calculated differences, which is approximated by a polynomial or harmonic series. in our case, we chose a 3-parametric trend solution for a first-order polynomial: (3) where 0a represent a bias, 1 2,a a represent tilt of the geoid plane with respect to wgs84 ellipsoid, while ji, and li are the are the northings and eastings in some plane coordinate system. csf dni values was defined by removing the polynomial trend from the value of dni: (4) the refined values of egm96 geoid height anomalies are determined by the expression: (5) vertical accuracy of dems was assessed statistically using gps data based on point-wisely analysis. the surface continuity is a crucial issue in local geoid modeling. in this regard, the results of this study make a significant contribution to the practical use of local geoids. therefore, we performed the construction of the surface grid using rbf with mq function interpolation method. using the 1′×1′ egm96 global geoid height (fig. 2a) and 1′×1′ the geoid corrector grid surfaces (fig. 2b) were constructed for the study area. the geoid height corrections range ranged from –0.66 m to 0.88 m. 3. results and discussion the ellipsoidal heights had been calculated using the corrected egm96 geoid undulations for all three dems at 27 points. the descriptive statistics of ellipsoidal heights from each data source range (min, max), the measures of central tendency (mean), and the dispersion (standard deviation, sd) are shown in table 2. for all models, the average elevation was found to be varying between 471 m and 480 m and is also comparable to the reference gps value of 486 m. at the same time, the linear correlation coefficient between the reference surface and all three dems is at the level of 99% and confirms at first glance that the models do not have significant discrepancies from the reference data. next, we analyzed the differences between the dem surfaces and the reference surface from gps measurements. for this, the differences of the grids (gps-srtm30), (gps-aster gdem2) and (gps-alos aw3d30) were determined (fig. 3 (a-c)). we can observe that compared with the reference model, all the global dems in mountainous areas generally overestimated elevation. but the range of elevations the alos aw3d30 versus gps elevations are from 1.5 m to 10.64 m, that compared to the srtm30 and aster gdem2 elevations is two times less. the vertical errors of the dems were quantified by comparing individual test dem elevations and reference gps elevations at sample points by using the following metrics. the first, root mean square error (rmse), characterizes the difference between the dem values zmodel and the reference ones (zgps in our case). the mean error (me) will estimate of the offset from the reference model. in fact, the absolute vertical accuracy of le90 at a 90% confidence level (le90) can be estimated based on the obtained rmse (mukherjee et al., 2013): (6) (7) figure 2. (a) egm96 geoid undulations and (b) the corrector surface for geoid fitting a) b) 89vertical accuracy evaluation free access digital elevation models (dems): case fergana valley in uzbekistan table 2. summary statistics of dems for the study area min, m max, m mean, m sd, m equation of correlation gps and dem coefficient of correlation r2 gps 330.080 836.060 485.922 150.833 srtm30 324.284 823.250 476.552 147.652 y = 0.989x 4.182 0.9996 aster 326.362 808.831 470.957 147.131 y = 0.985x -7.736 0.9981 alos aw3d30 324.771 830.344 479.847 149.187 y = 0.999x -5.914 0.9997 c) d) figure 3. surfaces of residuals of ellipsoidal heights according to dem and gps: (a) srtm30, (b) aster gdem2, (c) alos aw3d30, and (d) comparative statistics of dems a) b) (8) the performance of the dem over the study region is shown in fig3d. srtm30 and alos aw3d30 dem have the least error with me and rmse of 9.4 m and 10.0 m for srtm30 and 6.1 m and 6.6 m for alos aw3d30, respectively. aster gdem2 has the highest rmse of 16.4 m. comparison of gps measurements at fergana valley with dems indicated that le90 were about 16.4 m, 26.9 m, and 10.9 m for srtm30, aster gdem2, and alos aw3d30, respectively. from fig. 3d, it can be visualized that, for the fergana valley the absolute vertical accuracy error (le90) of all three dems is close to that declared by the manufacturers (le90distrib), but slightly worse. these variation in the accuracy defines the role of tectonic and terrain features in measurement of elevation irrespective of their spatial resolution. the values implied that alos aw3d30 elevations approximated earth’s surface elevations better than srtm30 and aster gdem2 at the region. 90 dilbarkhon fazilova, obidjon arabov 4. conclusions in this work, for the first time for the territory of the fergana valley in uzbekistan, the vertical accuracy of three publicly available dems, srtm30, aster gdem2, and alos aw3d30 was statistically assessed using high accuracy gps survey data. the geometric approach using gps/leveling data and egm96 global geopotential model-based geoid undulations was applied for local geoid modeling. the geoid height corrections range ranged from –0.66 m to 0.87 m. root-mean-square errors of ~10.0 m, ~16.4 m, and ~6.6 m was obtained for srtm30, aster gdem2, and alos aw3d30, respectively. it was found that compared with the reference model, all the global dems in mountainous areas generally overestimated elevation and the value of vertical accuracy at a 90% confidence level by 3-6 meters exceeded the declared by distributors. but the range of elevations the alos aw3d30 versus gps elevations are from 1.5 m to 10.64 m, that compared to the srtm30 and aster gdem2 elevations is two times less. alos aw3d30 proved to be the most accurate dem that best represents the topography of the earth’s surface and could be used for some engineering applications in fergana valley. in future works, we intend to perform a more detailed morphometric analysis (slope and aspect) for applications of tectonics, hydrology and other applications. acknowledgments this work was carried out within the scientific project of the astronomical institute of uzbekistan with the financial support of the academy of sciences of the republic of uzbekistan. gnss data were processed using the gamit/ globk software. we also thank the reviewers for their many constructive comments and suggestions. references abrams, m., bailey, b., tsu, h. & hato, m. 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(2023). geostatistical estimation and simulation in dam hydrogeological and geotechnical research: a comprehensive review. earth sciences research journal, 27(4), 391 402. https://doi.org/10.15446/esrj. v27n4.104250 keywords: dam hydrogeological features, geostatistical approaches, geotechnical research, hydrogeological conditions, spatial correlation, permeability distribution. resumen en la ingeniería de represas una evaluación exacta de los parámetros hidrogeológicos y geotécnicos, como el análisis de la presión del agua, vertido, permeabilidad, transmisibilidad, distribución de fracturas, y la designación de calidad de roca, es fundamental para garantizar la seguridad, longevidad y desempeño de las áreas de las represas. en los últimos años, los enfoques geoestadísticos se han posicionado como herramientas útiles para la estimación y simulación de estas características determinantes y ofrecen la posibilidad de reducir los errores y minimizar los costos de estudio. en este trabajo se revisan los estudios más significativos, válidos y eficientes en este campo y se presentan los resultados de los estudios. se presenta además una revisión de las características hidrogeológicas de las áreas de las represas. luego se analiza la aplicación de los enfoques geoestadísticos de cada parámetro. también se estudian las fortalezas y debilidades de estos enfoques con base en las condiciones prevalentes del sitio. este trabajo prueba que la geoestadística es una herramienta eficiente que puede incrementar la exactitud de los estudios, reducir los errores y ahorrar tiempo y dinero. keywords: características hidrogeológicas de las represas; enfoques geoestadísticos; investigación geotécnica; condiciones hidrogeológicas; correlación espacial; distribución de permeabilidad. record manuscript received: 13/08/2022 accepted for publication: 28/01/2024 estimación geoestadística y simulación en hidrogeología e investigación geotécnica de represas: una revisión https://doi.org/10.15446/esrj.v27n4.104250 mailto:katibeh@aut.ac.ir https://doi.org/10.15446/esrj.v27n4.104250 https://doi.org/10.15446/esrj.v27n4.104250 392 shawgar karami, homayoon katibeh, mohammadamin karbala 1. introduction dams are unique structures complicated in their load response and interactive relationship with the hydrogeological and geotechnical characteristics. understanding the spatial variability of these characteristics is significant in their evaluation, and obtaining valid data for the assessments is essential. however, a detailed evaluation of a dam site’s geological, geotechnical, and hydrogeological features requires drilling boreholes in a regular high-density sampling network. it may not be economically justifiable, and, in many cases, the area has inappropriate geological and topographic conditions. furthermore, in dam projects, the volume of samples to determine rock mass conditions can cover only a minor part of the area. therefore, it is necessary to search for appropriate interpolators and estimators to improve the design of maps and determine the high uncertainties and intrinsic variability of soil and rock properties with high accuracy. earth science phenomena are spatially dependent on changes occurring at different geological scales. all soils and geological formations show random changes in their spatial characteristics. evaluating these spatially correlated geological and hydrogeological variations at the dam site can be complicated. undoubtedly, the environment of a dam site is affected by spatial variations due to the nature of different units and formations and the variety of other geological features, including faults and folds, permeability and porosity of various units and formations, and hydrogeological features. geostatistics is one of the advanced techniques for studying, interpolating, and evaluating the spatial data distribution in geoscience phenomena. the chief purposes of geostatistics are to describe and interpret the behavior of existing sample data and to use it for predicting potential values at unsampled locations. it is used where the variables are distributed in space, and spatial correlation can be found between them (journel and huijbregts, 1976; journel, 1989). geostatistics uses measured points’ mathematical and statistical properties, including autocorrelation, saving time and costs (jalali et al., 2016). it can provide a continuous surface when sample points are available in different locations. they use mathematical functions for interpolation that are directly based on measured neighborhood values (matheron, 1971). this research reviews the application of different geostatistical methods in hydrogeological and geotechnical studies of dam sites. the most significant reasons to conduct the research and analyze the hydrogeological and geotechnical conditions can be as follows: evaluating and identifying the possible water escape routes and estimating the leakage of foundations and abutments, studying the permeability of various units and formations of a dam site, assessing the site hydrogeology and features affecting it, creating sealing structures in the foundation, body, and abutments to prevent water waste in the dam reservoir, investigating the engineering geological and geotechnical characteristics, and problems related to hydro-stratigraphy of the layers. first, the estimation and simulation approaches in dam sites’ research are described in section 2. then, in section 3, an overview of the hydrogeological characteristics of the dam sites, including wpt, permeability, transmissibility, hydraulic gradient, rqd, leakage, and fracture distribution, will be presented. finally, section 4 provides the application of geostatistical approaches in each hydrogeological parameter of the dam site. this research proves that geostatistics is an appropriate and efficient tool that can increase the accuracy of studies, reduce errors, and save time and money. 2. geostatistical estimation and simulation approaches geostatistical concepts begin with the study of statistical processes, followed by spatial regression, kriging interpolation, and finally, quantifying uncertainties and estimation errors (matheron, 1971). geostatistics’s most significant advantage is its flexibility over other spatial interpolation and averaging approaches. another pro of this technique is evaluating the estimation error value (journel and huijbregts, 1976; journel, 1989). the geostatistical tools can be considered numerical techniques, describing spatial features and using predominantly random models as time series analysis. figure 1 shows the step-by-step flowchart of geostatistical estimation or simulation. the flowchart shows that data distribution and the outlier values should be evaluated besides confirming the spatial correlation. assessing the statistical distribution nature of raw data in estimating and recognizing the population’s statistical features helps use them and more appropriately analyze the results. if the statistical distribution of the data does not follow the gaussian distribution, linear geostatistical tools may show biased results. therefore, evaluating the data distribution is necessary before any modeling. start yes evaluating data distribution? drawing an experimental variogram and �tting a theoretical one on it con�rming variograms? cross validation?residual analysis? kriging estimation or geostatistical simulation yesyes approving estimation? estimation variance yesend can geostatistics be used? replacing outliers? con�rming spatial correlation? yesyes estimation error kriging e�ciency yes yes figure 1. step-by-step flowchart of geostatistical estimation (karami et al., 2021) also, stationarity is a significant concept in geostatistics, permitting the statistical inference of probability. in the following, geostatistical concepts, including detecting outliers, stationarity hypothesis, variography, geometric and zonal anisotropy, estimation, and simulation, are assessed in the individual subsections in more detail. 2.1 detecting outlier data outlier data are inconsistent observations that do not follow the pattern in the actual data set. these inconsistent observations may be due to error or natural variability. evaluating the sample containing outlier data reveals significant gaps between such observations and actual data and deviations between them. available statistical tests must confirm the presence of these data because their existence considerably impacts the results of geostatistical approaches (bárdossy and kundzewicz, 1990). outliers are distributional and are evident as large or small values in a histogram or box and whisker plot (tukey, 1977), or they are spatial, whereby the values are different from other values in their spatial vicinity. the latter need not be distributional outliers and may be identified from a pixel map of values (kerry and oliver, 2007) or by other more elaborate approaches such as internal and external analysis techniques (gnanadesikan and kettenring, 1972). davies and gather (1993) expressed that an approach to identifying outliers is that they have a different distribution from the remaining observations. these researchers suggested an actual outlier identifier based on hampel’s technique. 393geostatistical estimation and simulation in dam hydrogeological and geotechnical research: a comprehensive reviews cressie (2015) stated that if the skewness is outside the bounds of ±1, the histogram or box and whisker plot can be evaluated. if outliers cause asymmetry, this is more obvious in a schematic box and whisker plot than a histogram, and the extreme values should be assessed further. if they result from errors in the assembly of data or laboratory analysis, it is better to eliminate them from the data. bárdossy and kundzewicz (1990) stated that a detection procedure for outliers could be at the stage of determining the experimental variogram. if the squared difference cloud is assessed for certain distance classes, it can be found that most outliers of the cloud are related to the same observation station. 2.2 stationarity hypothesis the geostatistics is based on random functions (rf), and the measurements are assumed to be a realization of a specific rf for a given position (matheron, 1971); therefore, some restrictions, called stationarity hypothesis, should be made on the data (journel and huigbregts, 1976). a central assumption in geostatistics is the stationarity of the process. however, the spatial variability of many natural phenomena depends on the local geology, which is often nonstationary (brenning, 2001). the first aspect of stationarity is choosing the domains to perform geostatistical calculations. abrupt changes in the regionalized variable are often captured in the domain definition. the second aspect is the location dependence of statistics within the domain. there is only access to some details of the complex geological processes that lead to the studied variable. one can observe the result and make reasoned judgments about the location dependence of statistical parameters. in a stationary procedure, the unconditional joint probability distribution does not change when shifted in time or space (dias and deutsch, 2022). the stationarity order depends on the order of the statistical moments required to be stationary. a variable is stationary if its distribution is invariant under translations. if the first moment, the mean, is invariant under translation the rf, is first‐order or weak stationarity. strict/strong stationarity requires that all distribution moments remain invariant under translations. this condition is rare in the natural world and is difficult to verify from limited experimental data. the geological phenomena is a second-order stationary if only the first two moments, mean (equation (1)) and covariance (equation (2)), are constant (journel and huijbregts, 1976; davis and sampson, 1986; armstrong, 1998; vieira et al., 2010; david, 2012; dias and deutsch, 2022). m = m(x) = e [z (x)] (1) c(h) = e {[z(x) z(x+h)} m2 (2) z(x) and z(x+h) represent two values of a given variable in the two different points, (x) and (x+h). the stationary regionalized variables satisfy the intrinsic hypothesis, but the converse is not always true, as intrinsic variables can be nonstationary. the practical benefit of using an intrinsic regionalized variable is a broader choice of the possible variogram models compared to the cases of second-order stationarity. in practice, a decision on the stationarity of a given regionalized variable is made in conjunction with consideration of the uniformity of a specific variable and the scale at which the variable is stationary (abzalov, 2016). in the presence of a trend, when the first‐order moment varies over the domain, making it nonstationary, it must be removed, and the remaining residuals are treated as stationary (harding and deutsch, 2021). geostatistical modeling is performed based on the variogram tool after examining the statistical distribution of data, identifying and correcting outlier values, and evaluating stationariy. 2.3 variography one of the hypotheses in geostatistics is that in a specific direction for each distance, the difference variance between the two regionalized variables is a constant and depends on their coordinates. a variogram is a tool for quantifying the observed relationship between sample values and their neighborhood (lin et al., 2001). it describes the data’s spatial variability and structure and the difference’s variance of a regionalized variable. according to the set of values in the sampled locations, variogram-based spatial estimation of kriging or simulation shows how the variance of the data values at different points changes with distance (h in figure 2a) (matheron, 1971). the experimental variogram, γ (h), expressed in equation (3), is a graphical representation of the mean square variability between two neighborhood points (for example, in estimating the values of the wpt, the two points lu and lu+h in figure 2a) as a function of the distance (h). γ(h) = (3) h is the lag distance that separates the pair of points, z(x) and z(x+h) are the regionalized variable values at the point x and a point at the distance x+h, respectively, and n is the number of data. after determining the experimental variogram, the most suitable theoretical model (figure 2b) that best describes the sampled experimental data can be chosen (kresic, 2006). in the earth sciences, samples taken from close distances are more similar than those taken farther apart. variogram uses significant parameters in modeling variables, including sill, nugget effect, and range. figure 2 shows a schematic model of a theoretical variogram and its parameters in a blocked dam site. the nugget effect (c0), sill (c+c0), and range (r) are used in the kriging equation as well as the simulation process; therefore, it is significant to evaluate how well these parameters can produce actual data (jolly et al., 2005). figure 2. a) schematic representation of a blocked dam site, showing the concept of variogram with the lag value of h in a block of it. b) modeling the theoretical variogram demonstrated in the dam site. 3.2.1 geometric and zonal anisotropy a thorough geostatistical analysis includes a careful study of how the data’s second-order variation depends on the relative orientation of data locations. if the second-order dependence between observations at any two locations is a function of only the euclidean distance between the locations, the secondorder variation is isotropic; otherwise, it is anisotropic (zimmerman, 1993). in spatial statistics, the assumption of isotropy is due to two reasons. (i) isotropic models are mathematically more straightforward to build than anisotropic ones, and the estimation of their parameters is more feasible, especially when the sample size is small. (ii) anisotropic models are often restricted to the classical geometric and zonal anisotropy models, which transform the coordinates by a transformation matrix (chilès and delfiner, 2012). 394 shawgar karami, homayoon katibeh, mohammadamin karbala anisotropy models are usually geometric, zonal or separable. these models are composed to provide more complex anisotropies (allard et al., 2016). journel and huijbregts (1976) define zonal anisotropy as any kind of anisotropy other than geometric. isaaks and srivastava (1989) define zonal anisotropy as an anisotropy for which the sill varies with direction, but the range does not. the existence and type of anisotropy can be detected by constructing directional variograms (guedes et al., 2008). the most common anisotropy among the phenomena with spatial dependence is geometric anisotropy (journel and huijbregts, 1976; isaaks and srivastava, 1989; zimmerman, 1993). geometric anisotropy means the correlation is stronger in one direction than another. there are two parameters defining geometric anisotropy: the direction of greater spatial continuity, expressed in the azimuth system, and the geometric anisotropy factor, being greater than one (diggle et al., 1998). the finding of geometric anisotropy and its subsequent incorporation into the spatial model is significant in estimating the non-sampled locations (guedes et al., 2013). 2.3.2 variography validation a notable shortcoming of previous studies is the inadequate validation of variograms, which serve as the foundation for geostatistical estimations and simulations. the accuracy of variogram calculation and plotting is imperative, as any geostatistical procedure founded upon incomplete or inaccurate variograms loses its scientific validity. before presenting any estimated or simulated model, evaluating the theoretical variogram parameters based on the original data is necessary. it is done in two ways: residual analysis and cross-validation. the residual analysis represents the estimated value minus the actual value and evaluates the histogram of the residual value for each location. the residual histogram should show a normal distribution with a mean of zero to confirm the correctness of the variogram statistically (rossi and deutsch, 2014; gong et al., 2014; jalali et al., 2016, 2019). figure 3 shows the residual analysis to estimate the permeability of a dam site. as it shows, the mean of the residuals is approximately zero. cross-validation is performed by removing a sample and estimating its value through the remaining data, and the estimated values are plotted against the actual values. if the estimated values equal the actual data, the plotted points are precisely on the line y = x. this process continues until all points in the database are estimated (kerry and oliver, 2007; lark, 2000; boukouvala and ierapetritou, 2012; jalali et al., 2016; clark, 1979; jolly et al., 2005; jalali et al., 2019; adhikary et al., 2011). figure 4 shows an example of cross-validation to estimate the permeability of a dam site. figure 3. an example of a residual analysis diagram with a mean of zero figure 4. an example of a cross-validation test. 2.4 kriging estimate after modeling and validating the variogram, the estimation process is carried out based on the spatial structure of the environment. estimates are made at unsampled locations by the kriging interpolation technique. kriging is one of the most effective geostatistical methods for interpolation and contouring in various fields considering spatial variance, location, and sample distribution (kresic, 2006). it is an alternative to other point interpolation techniques based on spatial regression of collected samples from a spatial domain. this method has been widely used in mapping geoscience features and has proven to be a powerful interpolation technique in recent decades. kriging uses a weighted linear combination to determine unknown values. this method obtains weights for applying any sampled value, which leads to optimum and unbiased estimates. it is done by minimizing the error variance and zeroing the average prediction errors. regardless of the general statistical properties of the estimates, emphasis is placed on the local accuracy and proximity of the unknown values to the actual values (webster and oliver, 2007; smith and konrad, 2011). closer and more samples give more certainty to the estimates. estimates should reflect the entire target area and the complete range of values, not just certain areas or specific values. 2.4.1 validating kriging estimates after the kriging estimate, it should be validated using the estimation variance (ev), estimation error (ee), and kriging efficiency (ke). estimation variance measures the reliability of predictions and is a function of the variogram shape, the sample structure, and the region in which the observation is made, which may be approximated as a point or an area (journel and huijbregts, 1976). the challenge of estimation variance deserves scrutiny, as it should ideally equal or be less than the actual data variance in geostatistical analyses. estimation variance exceeding the actual variance lacks a logical or scientific basis, compromising the reliability of estimation and simulation outcomes. it is suggested that its variance should be calculated after estimation, and if it cannot be confirmed, the estimate errors should be fixed. the estimation variance of kriging, , is expressed as equation (4). σ2 e = 2 γ(v, v) – γ(v, v) − γ(v, v) (4) v is the estimated location, and v is the data point. γ(v, v) is the variogram mean when its tail is fixed on the actual data and its head on the nodes to be estimated. γ(v, v) is the variogram mean when its tail and head are fixed on actual data. γ(v, v) is the variogram mean when each estimated node’s head and tail are set on (journel, 1989). also, one of the advantages of geostatistics is the estimation error evaluation. the minimized estimation variance (equation 4) can only be equal to the local estimation error if the error distribution is gaussian and does not depend on the actual values of the sample (deutsch and journel, 1992). the kriging estimation error is expressed as equation (5). ee= z σ/z*√n (5) 395geostatistical estimation and simulation in dam hydrogeological and geotechnical research: a comprehensive reviews σ is the standard deviation, n is the number of samples, and z* is the estimated value. z is the confidence level coefficient, equal to 2 at the 95% confidence level and 1.96 at the 90% confidence level. krige (1996) proposed kriging efficiency to evaluate the benefits of the kriging method. it is expressed as equation (6) and will be between zero and one. ke = (bv kv)/bv (6) bv is the theoretical block variance; kv (kriging variance) is the variance between the kriged and actual grades. the best estimators give kv close to zero and ke close to one (snowden, 2001). 2.5 geostatistical simulation one of the chief challenges in kriging estimates is smoothing the data range. it means kriging underestimates the upper quartile of each data set and overestimates the lower quartile. it makes detecting very low or high values in the estimated data impossible. smoothing expresses that the created model variability is much less than the actual data set and minimizes the ev (marinoni, 2003). geostatistical simulation techniques can solve the smoothing effects of kriging (ramazi and jalali, 2015; jalali et al., 2019). simulation techniques provide models that do not show any smoothing. the simulation principle is that each iteration provides alternative realizations. therefore, repeating hundreds of simulations will likely give hundreds of different realizations for a network point and the whole model. these approaches attempt to provide the best realization of the predicted data by comparing the estimated and actual data. hence, the simulations do not offer reasonable local estimates but are a good measure of spatial uncertainty (deutsch and journel, 1992). the simulation generates identical probabilistic values for a variable corresponding to existing in-situ measurements. the simulated values have average and variograms similar to the original data and may correspond to them at the measurement points. instead of producing an optimum prediction, simulation focuses mainly on reproducing observation variations (sterk and stein, 1997). geologists use simulation to visualize fluctuations in significant geological patterns. the geostatistical simulations can be validated by assessing the consistency of simulation realizations and generating histograms and variograms of sampled and simulated data. the congruence of non-directional histograms and variograms between the two datasets is a vital indicator of the validity and accuracy of the simulations. 3. hydrogeology of dam sites this section describes the usual hydrogeological conditions governing the dam sites. dam engineering is a complex and highly uncertain science, recognizing and reflecting the nature of many significant design inputs. it is a highly specialized activity that relies on many scientific disciplines and balances them with engineering judgment; therefore, it can be considered a challenging field. an accurate geotechnical study of a dam site determines the geological structure, stratigraphy, faulting, cracking, and jointing. it establishes the earth and groundwater conditions adjacent to the dam site, including abutments (novak et al., 2017). many dams worldwide are over 100 meters high, and some are over 200 meters high. the first apparent danger of constructing a dam is that an unnaturally steep hydraulic gradient is created at an unnatural rate across the foundation rock and abutments as the water level rises to this height (ford and williams, 2013). hydrogeological evaluation is the foundation for valid permeability judgments and sealing measures, focusing on areas that need rock improvement. therefore, one of the significant hydrogeological problems in the vicinity of the dam reservoir area is the existence of a valley. suppose any permeable rock zone in the dam reservoir area acts as a hydraulic connection, and the main groundwater level is lower than the desired reservoir level. in that case, it can drain water from the reservoir. this complexity cannot always be detected by geological mapping alone. this is especially true in the case of the groundwater surface position. thus, piezometers should be installed whenever groundwater fluctuates under the reservoir surface (ewert, 2012). determining groundwater level is one of the chief factors in the geotechnical analysis. in most dams, piezometers are installed to assess groundwater conditions, water levels and seasonal fluctuations, the location of aquifers, and the study of pore water conditions in the dam and its abutments. also, observation wells are located in different parts of the dam site to study groundwater level fluctuations in different seasons before and after impoundment (aghda et al., 2019). furthermore, whether the groundwater level rises steeply under the slopes adjacent to the dam site or moves to the abutments on the river level should be checked. the high or deep position of the groundwater level provides valuable information about the average permeability of the rock mass (ewert, 2012). one of the most significant hydrogeological aspects is determining whether there is a difference between more or less permeable zones throughout the dam site. this problem is examined from two aspects: if a zone with low permeability and a thickness of several tens of meters creates a dense barrier, the dam or the impermeable element will be built on this zone. suppose there is a combination of layers or banks with different permeabilities. in that case, it should be determined whether the results of permeability tests indicate the overall permeability of a more extensive section of rock mass (ewert, 2012). forming water-carrying openings along joints and other discontinuities is a long process. when a specific network of pathways is developed, the direction of groundwater flow cannot easily change, yet it retains its penetration even in an impounded reservoir condition. groundwater level and fluctuations related to rainfall over an extended period, including at least one dry and one rainy season, should be measured. also, interpretation of maps and hydrographs leads to the following results: moderate rock permeability, sections that may be excluded from treatment, the presence of only one or more groundwater regimes, and natural groundwater response to rainfall, especially interpretation of groundwater behavior when impounding a dam reservoir (ewert, 2012). the dam sites’ most significant hydrogeological and geomechanical features include wpt, the permeability of various geological units and formations, transmissibility, hydraulic gradient, leakage value, fracture distribution, and rqd. in the following, these features are introduced and discussed using geostatistics to evaluate and estimate each. 3.1 water pressure test one of the most common ways to study the hydrogeology of a dam site is to perform lugeon tests (wpt) on boreholes. wpt results are one of the significant characteristics of rock formations and indicate the environment’s ability to transfer groundwater. the wpt measures the rock permeability in separate sections of boreholes in the rock mass by increasing the pressure and measuring the inflow in each pressure stage (milanovic, 2004; kresic, 2006). the wpt results help to estimate the permeability of different zones of the dam foundation. however, due to economic or topographic constraints, this test is performed on a limited number of wells in an area of the dam site. accordingly, the information obtained from these tests does not represent the whole region because the fluid properties are intrinsically very heterogeneous and unpredictable (akhondi and mohammadi, 2014). therefore, the wpt values are an indicator of the permeability of the dam sites. ewert (2005) stated that such a conversion could be done when engineers need the converted permeability coefficient values from the wpt in evaluating the foundation seepage of the dam sites. many researchers have proposed several equations to convert wpt results to permeability coefficients (richter and lillich, 1975; gilg and gavard, 1957; shimizu et al., 1985; barton and quadros, 2003; fransson, 2004; hvorslev, 1951; moye, 1967; ahrens and barlow, 1951). to convert the wpt results to the permeability coefficient and to find the correlation between the values of these two variables in describing and estimating the permeability of the foundation rock, aherns and barlow’s (1951) equation (7) is suggested. (7) 396 shawgar karami, homayoon katibeh, mohammadamin karbala k is the permeability coefficient value, n is the lugeon number, l is the length of the test section in meters, and r is the influence radius of the water flow in meters. 3.2 permeability permeability coefficient, k, or hydraulic conductivity, is the most significant hydrogeological feature in preparing the groundwater model in a dam site. it represents the amount of water movement in voids and cracks and largely determines the groundwater flow and its head distribution (el idrysy and de smedt, 2007). the permeability coefficient is calculated from field and laboratory tests. in the dam sites, it is usually based on the wpt results (ewert, 2005). due to the complex pattern of discontinuities, it will be almost impossible to determine the permeability of rock mass if the appropriate test method is not used. intact or massive rocks are nearly impermeable; jointed rock masses may be permeable depending on their discontinuity characteristics. rock mass permeability can be determined by considering discontinuity conditions, including resistance, openness, roughness, filling, weathering conditions, distance of discontinuities, and rqd (kayabasi et al., 2015). the standard classification of permeability is provided in table 1. table 1. permeability classification based on wpt values and rock mass permeability coefficient. wpt values (lu) permeability coefficient, k(m/s) permeability class <1 <10-9 impermeable 1-5 10-9-10-5 low permeability 5-25 10-5-10-2 medium permeability >25 10-2-1 high permeability 3.3 transmissibility transmissibility is the water’s flow rate through a vertical aquifer strip. it controls groundwater flow, provides a general idea of an aquifer’s waterproducing efficiency and describes the aquifer’s capacity to transmit groundwater wholly in its entire saturated thickness. increased transmissibility can indicate increased hydraulic conductivity, porosity and appropriate interconnected pore spaces. therefore, this increase results in a high transmissibility value, which approves a high permeability presupposition (george et al., 2017). transmissibility is one of the basic hydrogeological properties, and determining its spatial variations is significant in groundwater modeling. appropriate modeling of preferential flow paths and their transfer behavior requires using transmissibility fields that reproduce the spatial variability patterns observed on earth. the transmissibility fields sometimes have significant uncertainties, including unknown and complex variations in the study area’s observed values of measurable properties (lin et al., 2001). 3.4 hydraulic gradient the hydraulic gradient is the slope of the water table or potentiometric surface, meaning the variation in water level per unit distance along the direction of maximum head reduction. the hydraulic gradient is the driving force that causes the groundwater to move toward maximum reduction of the total head. its value is determined by measuring the water level in several wells (cheremisinoff, 1998). hydraulic head gradients control the groundwater flow in aquifers, and their determination is a significant part of any groundwater assessment at a dam site. 3.5 rqd rqd is the percentage of recovered core length with fragments larger than 10 cm relative to the total core length. it is a modified core recovery percentage in which unrecovered core, small pieces of rock, and weathered rock are not considered to reduce rock quality with these characteristics. however, its most significant application is that it is an exploratory tool and acts as a red flag to identify areas with low rqd that require further investigation and may require additional boreholes or other exploration works. experience has shown that the rqd red flag and subsequent studies often deepen foundation surfaces and complete reorientation or displacement of engineering structures, including dam foundations, tunnel entrances, underground caves, and power plant facilities (deere et al., 1988). it is commonly used in core logging and is often the only technique for measuring the degree of jointing along the core drill hole (palmstrom, 2005). the most significant limitation of rqd is that it provides no information on the core pieces <10 cm; it does not matter whether the discarded pieces are earth-like materials or fresh rock pieces up to 10 cm in length. like all types of one-dimensional measurements, rqd is directional, but its definition makes it more sensitive to the hole or line direction than others (choi and park, 2004). 3.6 leakage the stored water in the dams always looks for paths with the least resistance; hence, one of the problems in dam sites is water leakage after impounding. assessing and predicting the water leakage value can help prevent such occurrences. many dam sites have reported excessive leakage, especially in karst areas worldwide (milanović, 1981; turkmen et al., 2002). therefore, performing studies and following up on corrective operations is normal to reduce the leakage value. lack of accurate view of the conditions in the dam site, especially before construction, is one of the main reasons for water leakage (mohammadi et al., 2007). the leakage value in a dam site depends on several factors, including the dam height, the valley shape, and the water level in the reservoir. some indicators for increasing water leakage include increasing the flow rate of downstream springs and the water level in observation wells. seepage paths are usually along karst conduits, bedding planes, open joint systems, or their intersections (uromeihy, 2000; turkmen et al., 2002; thomas, 1978). 3.7 fracture distribution the general term for a fracture to describe a geological discontinuity includes faults, joints, fissures, cleavage plates, and cracks. fractures often act as the preferred flow paths for fluids and significantly affect a rock mass’s mechanical and hydraulic properties (adler and thovert, 1999; haneberg et al., 1999; faybishenko et al., 2000). fractures are one of the most significant features of rocks on micro to megameter scales due to various genetic mechanisms such as cleavage, tensile, and shear. fractures are studied to understand the history of tectonic and geological structures and describe rock masses’ hydraulic and mechanical properties to explore natural resources, rock structures, and storage structures, including dam sites. accurate imaging of the fracture system and an acceptable 3-d distribution model are essential in all areas. it can help clarify fracture-related phenomena and environments (koike et al., 2012). 4. geostatistical applications in a dam sites’ hydrogeology applying geostatistical approaches to estimate the hydrogeological characteristics of the dam site plays a significant role in reducing errors and study costs. via geostatistical tools, the distribution of quantitative values such as permeability for each region point is obtained to analyze hydrogeological or geomechanical conditions accurately. this research attempts to review and evaluate the geostatistics applications in spatial analysis of the dam site’s hydrogeological features. measuring hydrogeological data in tunnels and underground structures, including mines, quarries, dam sites, and foundation drillings, is essential. different geostatistical approaches can estimate and simulate the hydrogeological data at the dam sites in heterogeneous porous media. they quantify uncertainty analysis and can be used in systems with high spatial variability in hydrogeological properties (assari and mohammadi, 2017). table 2 summarizes the fields in which geostatistical approaches are used. 397geostatistical estimation and simulation in dam hydrogeological and geotechnical research: a comprehensive reviews table 2. summary of geostatistical methods used in the dam site studied variables in the dam site geostatistical method performance of the geostatistical method where used the method who used the method wpt simple kriging • better performance than ordinary kriging tangab dam site, zagros region, southwest iran akhondi and mohammadi (2014) sequential indicator simulation • better performance than sequential gaussian simulations • suitability in simulating the abnormal distribution of wpt values karun dam site, iran assari and mohammadi (2017) permeability multiple indicator kriging • no dependence on data distribution • sensitivity to outlier values • it takes a long time to calculate and model the experimental variogram and solve the kriging system separately for each threshold value. azad dam headrace tunnel, kurdistan, iran aalianvari et al. (2018) ordinary kriging • lack of accurate point predictions of permeability • inefficiency in estimating the spatial distribution of permeability • reflecting the local variations of permeability using dependent variables tangab dam site, zagros region, southwest iran akhondi and mohammadi (2014) cokriging • accurate estimate of the spatial distribution of permeability trifa aquifer, morocco el idrysy and de smedt (2007) sequential gaussian simulation • lack of accurate point predictions of permeability • better reproduction of feature distribution and local contrasts tono area in japan, overlain by cretaceous granite koike et al. (2015) kikuma granite, japan koike et al. (2012) simulated annealing • lack of accurate point predictions of permeability • better reproduction of feature distribution and local contrasts tangab dam site, zagros region, southwest iran akhondi and mohammadi (2014) hydraulic gradient ordinary kriging • the best linear unbiased estimator for estimating the hydraulic gradient wolfcamp aquifer, northern texas philip and kitanidis (1989) dam reservoir leakage ordinary kriging • high accuracy in estimating reservoir leakage the azad pumped storage power plant, kurdistan, iran. aalianvari et al. (2013) transmissibility ordinary kriging • provide the best estimate of transmissibility • inefficiency in case of low spatial variability on a small scale • generate smoother maps than simulation approaches dulliu area in yun-lin county, taiwan lin et al. (2000) the southeast of yun-lin county and the north of chia-yih county, taiwan lin et al. (2001) log-normal ordinary kriging • inefficiency in the case of small-scale low spatial variability dulliu area in yun-lin county, taiwan lin et al. (2000) cokriging • the lowest satisfaction based on the cross-validation results a highly fractured metamorphic aquifer, man-danane region, ivory coast, africa razack and lasm (2006) sequential gaussian simulation • generate the spatial structure of the studied data • no smoothing in a site with high data variability dulliu area in yun-lin county, taiwan lin et al. (2000) simulated annealing • reproduction of statistics and spatial variation of transmissibility • identify global spatial correlation patterns of transmissibility • complete display of geological features of the studied area the southeast of yun-lin county and the north of chia-yih county, taiwan lin et al. (2001) rqd sequential indicator simulation • no dependence on the variable normality • better performance than sequential gaussian simulation karun dam site, iran assari and mohammadi (2017) ordinary kriging • successful estimate of geomechanical features of tunnels and dam sites azad dam headrace tunnel, kurdistan, iran aalianvari et al. (2018) distribution of fractures ordinary kriging • satisfactory results in estimating fractures’ permeability kikuma granite, southwest japan koike et al. (2012) sequential gaussian simulation • assess the location of fractures tono area in japan, overlain by cretaceous granite koike et al. (2015) kikuma granite, japan koike et al. (2012) the following section describes the application of geostatistics in each field in detail. 398 shawgar karami, homayoon katibeh, mohammadamin karbala 4.1 application of geostatistics in dam sites’ wpt the wpt is a significant feature of rock formations and indicates the environment’s ability to transfer groundwater. the test values in the spatial range more or less constantly change and can be assumed as a regionalized variable for geostatistical studies (akhondi and mohammadi, 2014). akhondi and mohammadi (2014) used simple (equation 8) and ordinary kriging approaches to determine the spatial variability of the karst formation permeability at the dam site. after hydrogeological studies to evaluate the geostatistical simulations, simple kriging with gaussian variograms was the best technique to estimate the wpt values. equation (8) shows the generalized linear regression algorithm in simple kriging (deutsch and journel, 1992). (8) z(u) is the wpt value at location u, uα is n data location, and m(u)=e{z(u)} is the expected value of location-dependent z(u). λα(u) is the simple kriging weight in place u, and z* sk(u) is the simple kriging estimator. in simple kriging, it is assumed that the mean values of the wpt in the study range are constant and known. 4.2 application of geostatistics in dam sites’ permeability the permeability measurement is the basis for measuring many hydrogeological parameters in the dam sites. equation (9) shows the leakage flow in a 2-d plane (manna et al., 2003). (9) kx is the soil permeability in the x-direction, and ky is the soil permeability in the y-direction in m/s. for isotropic soils, kx= ky. q is the inflow/outflow rate to the soil in m3/s per unit area (m/s), and v is the total volume of water in m3. t is the time interval in seconds, and h(x, y, t) is the total head at a point with coordinates (x,y) in meters. this equation (9) satisfies the flow continuity, i.e., the difference between inlet and outlet water volume equals the change in water storage volume. under steady-flow conditions, the inlet and outlet flow rates are equal, and no volumetric variation occurs; therefore, equation (10) equals zero. equation (10) presents the leakage for steady-state conditions. (10) khalili shayan and amiri-tokaldany (2015) simplified equation (10) in a stable state for isotropic soils (kx=ky) and without any drainage. equation (11) shows the leakage in the steady state for isotropic soils in the absence of drainage. (11) permeability largely determines groundwater flow and its head distribution. reliable permeability values are challenging due to the lateral and vertical heterogeneities in water-carrying geological strata (el idrysy and de smedt, 2007). also, permeability data may indicate considerable uncertainty, indicating the complexity of spatial variability (lin et al., 2001; tartakovsky, 2013). according to the above equations, k has spatial variability, a characteristic of regionalized variables in geostatistics. multiple indicator kriging is one of the simplest and most subtle geostatistical methods developed by aalianvari et al. (2018). in the indicator kriging, the probability that the permeability value in the estimated block or point is less than a given threshold value is estimated (öztürk and nasuf, 2002). first, the initial permeability data must be converted to indicator values based on the equation (12) conversion function. i(x) = i(x) = (12) zc is the assumed threshold value. in this case, equation (13) calculates the estimated value of the indicator permeability at each point. i*(x0) = (13) i(xj) is the indicator value in the xi coordinates, and λj is the indicator kriging weight for the sample in the xj coordinates, participating in estimating the point x0. the estimated permeability value, (x), should vary between a minimum of zero and a maximum of one, indicating the probability that the permeability value in the estimated block is less than the threshold. the advantages of this technique are non-dependency on data distribution and sensitivity to outlier values. the disadvantage of this technique is the time required to calculate and fit the model to the experimental variogram and solve the kriging system separately for each threshold value (aalianvari et al., 2018). indicator kriging in estimating the permeability of the dam site relies on the following four main features (gavinhos and carvalho, 2017): • it is non-parametric and independent of the distributions’ shape. • it is helpful for highly skewed data such as wpt results as a nonlinear interpolator. • it is less affected by the high smoothing of the variable. • it provides a direct probabilistic estimate of the areas needing further treatment in the dam foundation. el idrysy and de smedt (2007) stated that more than the kriging technique is needed to accurately estimate the spatial distribution of permeability. these researchers estimated the regionalized distribution of permeability using kriging and cokriging with a hydraulic gradient slope because, based on darcy’s law (q = ka (∂h⁄∂l)), the slope of a hydraulic gradient (∂h⁄∂l) with permeability (k) is a related directly. 4.3 application of geostatistics in dam sites’ hydraulic gradient in equation (10), the expressions and are the hydraulic gradient values at a point with coordinates (x,y). the hydraulic gradient is dimensionless and scattered at points. this property is the feature of the regionalized variables; therefore, it can be considered a geostatistical variable with spatial variability. philip and kitanidis (1989) are the only researchers to use the geostatistical technique of ordinary kriging to estimate the hydraulic gradient. choosing the best unbiased linear estimator for the hydraulic head made it possible to estimate the hydraulic gradients directly and calculate the mean square estimation error. 4.4 application of geostatistics in dam sites’ leakage in equation (10), q is the inlet/outlet flow rate into the soil in m3/s per unit area (m/s). controlling leakage from some dam parts is necessary for most long earthen dams. however, the quantity and quality of monitoring data are limited due to the complexity of the actual conditions (deng et al., 2018). estimating the water leakage of the dam site is necessary to select the optimum sealing technique (aalianvari et al., 2013). according to the leakage flow, equation (10), the variable q has spatial variability. its distribution can be modeled using geostatistical approaches and solve the quantity and quality problem of data to a large extent. researchers have always searched for available methods to use the best and most reliable techniques to assess the leakage value of the dam site. smith and konrad (2011) consider geostatistical methods an effective tool for describing dam heterogeneities that allow the areas with more severe leakage to be located. aalianvari et al. (2013) used a combination of geotechnical and geostatistical approaches to estimate the water leakage of the dam reservoir. using the ordinary kriging technique, they estimated the potential of water leakage for reservoir walls in places without data. equation (14) presents the ordinary kriging algorithm that uses a weighted linear combination of sampled points in a neighborhood around location x0. ( ) = (14) z*(x0) is the estimated value of leakage q at location x0, z(xi) is the sample leakage at location xi, and n is the number of leakage samples considered 399geostatistical estimation and simulation in dam hydrogeological and geotechnical research: a comprehensive reviews in the estimate. λi is the weight assigned to the ith sample value (webster and oliver, 2007). comparing the results of the analytical and the finite element numerical methods in estimating the reservoir leakage showed the high accuracy of the geostatistical technique. 4.5 application of geostatistics in dam sites’ transmissibility transmissibility (t = k×b) expresses the ability to transfer water from the thickness of the table unit. the ability of an aquifer to transfer and move water (t) depends on two factors: permeability coefficient (k) and aquifer thickness (b). hydrogeological data on transmissibility may indicate significant uncertainty, including complex and unknown changes in the observed values of measurable properties across the study area, showing complex spatial variability (lin et al., 2001; tartakovsky, 2013). lin et al. (2000) produced spatial transmissibility maps by sequential gaussian simulation (sgs) and estimated them using log-normal ordinary kriging and ordinary kriging. the results proved that sgs could create the spatial structure of the data, and kriging estimate maps are smoother than other simulations. in a site with much variability, sgs with many realizations has more advantages than the ordinary and log-normal kriging approaches. lin et al. (2001) estimated and simulated transmissibility spatial maps to understand hydrogeological spatial features using ordinary kriging and simulated annealing approaches. spatial maps of transmissibility estimate and simulation have shown that simulated annealing can reproduce statistics and spatial variation and identify global spatial coherence patterns. also, ordinary kriging produced smoothed data that cannot record the spatial variability of measured transmissibility data. razack and lasm (2006) estimated the transmissibility of highly fractured hard rock aquifers, a dense and well-connected fracture network with no specific orientation. transmissibility variographic analysis proved that the variogram of actual data is much more structured than the normalized data. also, the nugget effect in the variogram of normalized data was much higher than its value in the actual data. this result contradicted all previously published research on structural analysis of transmissibility. although all previous geostatistical studies have explicitly concluded that a normal distribution improves estimates, this study proves otherwise. another result of the research was that the cokriging method could not provide the best estimate, unlike previous studies. the results of these studies prove that conditions governing the environment have a significant influence on hydrogeological parameters, and each hydrogeological environment is unique in its characteristics and spatial behavior. 4.6 geostatistics application in rqd and cr the geotechnical features of rqd and core recovery (cr) are significant factors for engineering projects during a dam site’s design, construction, and support. one of the most effective approaches to estimating these features is geostatistics. rqd and cr are features of rock formations and can assess cracks, fractures, and discontinuities (aalianvari et al., 2018). variographic analysis of the geotechnical properties of rocks, which are regionalized variables, provides information on spatial variability, spatial correlation, and estimate of variables when analyzed in the geostatistics. having this data, we can better plan the distance and distribution of boreholes and estimate the variables in places without data. these variables are calculated based on the properties of the rock and the change in properties of a pure regionalized variable (santos et al., 2018). in a study by santos et al. (2018), rqd variograms showed very high variability in geostatistical modeling because the rock body was composed of minor filled-vein fractures with very high variability in different rocks. hence, the rqd may be weak at intervals with good cr, and there is no clear correlation between them. it is also believed that analyzing regionalized variables of cr and rqd provides better variographic images for larger regions. assari and mohammadi (2017) examined the relationship between the parameters of rqd and wpt. they concluded that their correlation is weak, and the overall trend is negative. this weak correlation is that rqd is a point variable, and the wpt is a regionalized variable with higher support that depends on the rock permeability at a higher volume. the researchers used the non-parametric geostatistical technique of sequential indicator simulation to simulate rqd and wpt. sequential indicator simulation worked better than sgs to reproduce the spatial coherence of larger values. another reason for choosing sequential indicator simulation is the suitability of this technique for simulating abnormal distributions of wpt values (goovaerts, 1997). sequential indicator simulation is done in several steps: modeling of variograms: several indicator variograms are modeled using standard models, such as spherical models for each threshold. indicator kriging: a conditional cumulative distribution function (ccdf) for each cell at the simulation boundary is computed using the original and pre-simulated indicator kriging. simulation: a random path is chosen through all cells and for each cell in the simulation boundary. the simulated value of the ccdf is plotted and added to the conditioning dataset to estimate the ccdf of other cells. aalianvari et al. (2018) estimated the rqd using ordinary kriging to determine the variable value in unsampled locations and presented its distribution as 3-d maps. they stated that this technique could be used successfully in the geomechanical estimate of tunnels and dam sites, considering reducing costs and saving time. 4.7 geostatistics in predicting the distribution of fractures the rock mass structure cannot be viewed directly in three dimensions, so many methods exist for extracting fracture features using different field surveys, including borehole, scanline, regional and analog surveys, digital photogrammetric techniques, and laser scans. each data source provides information on fracture characteristics. 2-d studies make it possible to evaluate fracture size, a critical parameter in studying the mechanical and hydraulic properties of rock, while the information provided by borehole measurements is related to the distribution of fracture direction, severity, and the presence of fracture clusters (hekmatnejad et al., 2019). fracture is not just a geometric feature but has several properties, such as opening, filler minerals, roughness, displacement, and other properties that cause problems in their 3-d modeling. fractures usually form clusters with dominant orientations, and their networks have a hierarchical structure (koike et al., 2012). simulation of rock fracture distribution is significant and usual in various fields of earth sciences. long and billaux (1987) proposed geostatistical fracture modeling and used variograms of fracture density changes for fracture centers in 2-d space. also, viruete et al. (2001) used the geostatistical method to model, predict and quantify 2-d fracture systems. koike et al. (2012) used the geostatistical method to simulate the distribution of fractures. they assessed the fractures of the site by sgs and used ordinary kriging to estimate the distribution of original values. also, a simulated fracture system was available to estimate the permeability of the studied site, which was approximately consistent with the mean hydraulic test results. lei et al. (2015) proposed a new approach to upgrade the scale of 2-d fracture network models to preserve the geostatistical and geomechanical properties of a smaller-scale source fracture pattern. they used geostatistical methods for the source pattern to quantitatively interpret its topological complexity. also, koike et al. (2015), in a geostatistical study, integrated 3-d models of rock fractures from different hydraulic sources and properties to identify the relationships between fractures and permeability. using an experimental relationship between permeability and fracture length, the researchers determined the range of fractures affecting hydraulic properties. also, they integrated the 3-d permeability model resulting from sgs with the fractures of the geostatistical model derived from the borehole data. they identified the fracture sizes and directions affecting the permeable features. in the most recent research, hekmatnejad et al. (2019) estimated fractures’ direction and diameter distribution by the geostatistical method using borehole data. summaries and conclusions this review study aims to familiarize researchers with the application of geostatistical approaches in the hydrogeological sciences of dam sites. this study does not suggest any superior technique because the geological, hydrogeological and geotechnical conditions governing different dams worldwide are complex and different. every researcher should be able to identify and use the best technique by analyzing the studied dam site. in other 400 shawgar karami, homayoon katibeh, mohammadamin karbala words, the best method for each dam site is unique, and it is impossible to propose only one superior process to researchers. dam engineering, characterized by its intricate nature and high levels of uncertainty, draws upon various scientific disciplines. hydrogeological features hold a prominent position among the pivotal factors in dam design. however, these features introduce considerable uncertainties, primarily stemming from intricate spatial variations. furthermore, the spatial distribution of pertinent variables assumes a significant role in scientific inquiry. consequently, the estimation and simulation of these hydrogeological elements have been addressed through diverse methodologies, with geostatistics emerging as a prevalent and favored approach. applying geostatistical estimation and simulation within varying dam site contexts constitutes a formidable challenge. it necessitates the careful consideration of all crucial factors governing variations. in this intricate process, establishing logical connections among these parameters precedes the selection of appropriate geostatistical techniques. nonetheless, several limitations have been observed in previous studies that warrant further attention. a notable shortcoming is the inadequate validation of variograms. the accuracy of variogram calculation and plotting is significant, as any geostatistical procedure founded upon incomplete or inaccurate variograms loses its scientific validity. additionally, estimation variance should ideally equal or be less than the actual data variance in geostatistical analyses. estimation variance exceeding the actual variance lacks a logical or scientific basis. it is suggested that its variance should be calculated after estimation, and if it cannot be confirmed, the estimate errors should be fixed. an intriguing aspect is the absence of validation efforts in previous hydrogeological studies of dam sites. future geostatistical investigations in this domain present an opportunity for researchers to validate their findings rigorously. this can be achieved by systematically evaluating estimation error, estimation variance, kriging efficiency, and creating error-related maps. furthermore, the validation of geostatistical simulations can be facilitated by assessing the consistency of simulation realizations and generating histograms and variograms of sampled and simulated data. the congruence of nondirectional histograms and variograms between the two datasets is a vital indicator of the accuracy of the geostatistical simulations. in the future, advanced validation methodologies can be integrated into geostatistical estimations and simulations for hydrogeological studies in dam engineering. the meticulous validation of results will enhance the credibility and utility of these approaches in guiding dam design and engineering practices. moreover, the ongoing exploration of new technologies and advancements in geostatistical modeling can further refine our understanding of hydrogeological complexities at dam sites, ultimately contributing to safer and more effective dam engineering practices. references aalianvari, a., tehrani, m. m., & soltanimohammadi, s. 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(2023). application of remote sensing methods for statistical estimation of organic matter in soils. earth sciences research journal, 27(3), 299-312. https://doi. org/10.15446/esrj.v27n3.100324 keywords: spectral brightness, landsat, multiple linear regression, vegetation index, soil organic matter aplicación de métodos de teledetección para la estimación estadística del material orgánico en suelos resumen mantener la fertilidad del suelo y optimizar estrategias de manejo de tierras a una escala global demanda un conocimiento amplio de las propiedades fisicoquímicas del suelo. este estudio propone la implementación de técnicas de teledetección, por su eficiencia, precisión y economía, para la exploración de las propiedades de los suelos chernozem (tierra negra) y para la predicción de material orgánico del suelo (mos) en algunas regiones específicas de ucrania. las propiedades espectrales de los suelos chernozem se examinaron con imágenes de los satélites landsat 5tm y 8 oli. luego se creó un mosaico de los valores de reflectividad espectral media para el período de estudio (1986-2015) con la herramienta google earth engine. para la construcción del modelo predictivo se tomaron valores de reflectividad de seis bandas, un amplio rango de índices de vegetación (rndsi, ndwi, ndbi, baei, msavi y ndvi), de información topográfica (pendiente), y parámetros climáticos (temperatura, precipitación, humedad del suelo) como regresores. luego se determinó el modelo óptimo a través del método de selección por pasos hacia adelante, el cual se basó en los criterios de información f-test y akaike. este modelo presentó métricas notables en los valores multiple r = 0.651, r2 = 0.424, y adjusted r2 = 0.397. los predictores más significativos del modelo fueron red, nir, pendiente y humedad del suelo. tras la validación, el modelo no presentó evidencias de heterocedasticidad, multicolinealidad, u otros posibles valore atípicos, lo que garantiza su fiabilidad y robustez. este modelo propuesto sirve como una herramienta útil que ofrece fiabilidad y precisión en la predicción de composición del material orgánico del suelo en regiones específicas de ucrania. palabras clave: brillo espectral; landsat; regresión lineal múltiple; índice de vegetación; material orgánico del suelo. v., https://doi.org/10.15446/esrj.v27n3.100324 mailto:belenok.vadim@nau.edu.ua https://doi.org/10.15446/esrj.v27n3.100324 https://doi.org/10.15446/esrj.v27n3.100324 300 vadym belenok, liliia hebryn-baidy, natalyya bіelousova, halyna zavarika, sergíy kryachok, dmytro liashenko, tetiana malik 1. introduction the significance of soil organic matter (som) research cannot be overstated, being at the forefront of solutions to pressing global issues. as a substantial carbon repository, som is indispensable to strategies aimed at mitigating climate change, with research on its dynamics providing invaluable insights for enhancing carbon sequestration efforts (baveye et al., 2020; navarro-pedreño, almendro-candel and zorpas, 2021). moreover, som bolsters soil fertility, a critical factor in augmenting agricultural productivity, thereby addressing concerns of food security in the face of an escalating global population (khangura et al., 2023; parikh and james, 2012; das et al., 2021; chatohin and lindin, 2001). beyond that, som serves as a vital energy source for soil microorganisms, fostering soil biodiversity (bach et al., 2020; wang et al., 2022; zhang et al., 2021). recent advancements in remote sensing technologies offer transformative possibilities in the field of som research, allowing for large-scale, non-invasive, and temporal assessments of soil conditions (omia et al., 2023; david, 2013; das et al., 2021; bouzekri et al., 2015). the ability of satellites to rapidly cover extensive and often inaccessible geographical regions presents an unrivaled advantage over traditional, labor-intensive soil sampling methods. combined with ground-based measurements, satellite data offers improved accuracy of som estimates, providing a comprehensive analysis of soil fertility indicators (belenok et al., 2021; huang et al., 2018; weiss, jacob and duveillerc, 2020; chen et al., 2006; zhang et al., 2021; yuzugullu et al., 2020). satellite imagery, particularly multispectral and hyperspectral images, are essential tools in assessing som due to their ability to capture reflectance and absorption properties of different soil constituents (luo et al., 2023; gopp et al., 2017; gopp et al., 2019; khangura et al., 2023). organic matter influences reflectance in the visible (vis) and near-infrared (nir) regions, making these spectral bands key in som evaluation (reis et al., 2021; bouasria et al., 2020; yu et al., 2021; yuzugullu et al., 2020; zhang et al., 2021). the spectral characteristics of bare soil are shaped by several factors, most notably the content of organic matter, often referred to as humus or som, and the level of soil moisture. specifically, increases in either som content or soil moisture are associated with reduced spectral brightness coefficients, a pattern particularly pronounced in the red band of the spectral range (zhang et al., 2021; wang et al., 2022; kravtsova, 2005; shikhov et al., 2020; prudnikova and savin, 2021). dry soils exhibit a common trait: a monotonic escalation in spectral brightness coefficients as the wavelength extends from 0.4 to 2 microns. as the som content rises, the spectral brightness of the soil correspondingly diminishes (rukhovich et al., 2021). further, the soils structure can influence its spectral brightness. unstructured soils, for example, reflect approximately 10-15% lighter than well-structured soils, underscoring the intricate interplay of soil properties on its spectral characteristics (kravtsova, 2005; jing et al., 2020; wang et al., 2022; yang et al., 2021; zhang et al., 2021). the evaluation of som using remote sensing techniques utilizes specific spectral indices such as the ratio normalized difference soil index (rndsi) or the normalized difference vegetation index (ndvi). these indices help in amplifying the som signal while simultaneously suppressing the effects of other variables (yang et al., 2021; deng et al., 2015; montero et al., 2023). while the primary application of ndvi lies in vegetation analysis, it can indirectly infer som levels as high vegetation areas often correlate with increased som content. many techniques underscore the use of the ndvi as an identifier for bare soil surfaces (ding et al., 2016; tian et al., 2021; lu, liu and liu, 2022). guidelines have been established, which suggest scales of ndvi values for reflectance in the red (0.25) and nir (0.3) ranges for open ground (the average ndvi value is thus is equal to 0.025) (koroleva et al., 2017; kulyanitsa et al., 2017; luo et al., 2021). according to the study (gasmi et al., 2021) suggested using an ndvi threshold value of 0.22 to separate bare soils from other land types. further, ground cover studies utilize mathematical models built upon parameters like fractional vegetation coverage, nitrogen reflectance index, yellow leaf index, bare soil index, and slope, achieving an accuracy of approximately 90% (das et al., 2021). land degradation hot spots and soil salinity are studied using spectral characteristics like surface albedo, the salinized land degradation index (sdi), the soil surface moisture index (lsm), enhanced vegetation index (evi) (rukhovich et al., 2021; higginbottom and symeonakis, 2014; jiang et al., 2008; yuzugullu et al., 2020; gopp et al., 2019). to minimize the effect of some vegetation spectral indices on soil brightness, based on the wavelengths of the red and nir ranges bands, it is proposed to use indices like the soil-adjusted vegetation index (savi) (huete, 1988) and the modified soil adjusted vegetation index (msavi) (qi et al., 1994), as well as the npv-soil separation index (nssi), developed to distinguish non-photosynthetic vegetation soils using two nir bands (tian et al., 2021). in the study (chen et al., 2006), authors proposed the utilization four vegetation indices ndvi, the normalized difference water index (ndwi) (gao, 1996), the difference built-up index (ndbi), the normalized difference bareness index (ndbai) for differentiating bare soils from other land use and land cover (lulc) types, setting threshold values for each. these indices are based on the spectral properties inherent to all lulc categories (chen et al., 2006; li and chen, 2014). a multitude of studies have established a correlation between som content in soils with pixel values of multispectral satellite data, often employing multiple linear regression (mlr) or alternative mathematical methodologies (lu, liu and liu, 2022; luo et al., 2021; ahmed and iqbal, 2014; bouasria et al., 2020). some authors (gasmi et al., 2021; demattê et al., 2007) have successfully used mlr to predict clay content. in a distinct approach (mirzaee et al., 2016) authors applied geostatistical methods, specifically different types of the kriging method, to predict som content in soils. in the study (jing et al., 2020) authors examined the impact of time gaps between field sampling and the acquisition of landsat tm/oli satellite data on soil nutrient predictions, using both mlr and artificial neural network methodologies. according to study (ahmed and iqbal, 2014; fiorio and demattȩ, 2009), authors employed mlr to correlate soil surface variables with spectral data. these studies demonstrate the diversity of approaches and tools used in exploring the link between som and spectral data. the technique of this study consists in the correct separation of the bare soil cover from other lulc in order to establish an excellent linear relationship between the spectral indices and the actual soil parameters. it is this indicator and the calculated vegetation indices that will be the basis for forecasting the fertility of agricultural land in the kyiv region. the aim of the study is to establish a statistical relationship between som and the mean spectral reflectance extracted from satellite images. this process based on using mlr and further assessing the accuracy and significance of this relationship for the chernozems of kyiv region. the establishment of such a relationship will make it possible to forecast the som content in the soil for further assessment of soil fertility. 2. materials and methods characteristics of the study area located in the dnipro river basin in the northern part of ukraine, the kyiv region encompasses an area of 28.1 thousand square kilometers, excluding the kyiv. this region constitutes approximately 4.7% of ukraine’s total land area, with the inclusion of kyiv bringing the total to 28.9 thousand square kilometers. from an administrative perspective, the region is divided into 25 districts, along with 13 cities of regional subordination. in addition, it includes 30 cities of district subordination, classified as urban-type settlements, and 1,182 rural settlements. as of the start of 2021, the population of the kyiv region stood at approximately 1,788,530 thousand individuals (website of the kyiv regional state administration, 2021). the kyiv region’s lands are uniquely positioned within a transitional soilclimatic zone. this zone serves as an interface between polissya, characterized by soddy-podzolic soils, and the forest-steppe region, known for its transitional chernozem soils. this distinctive location contributes to the diversity of soil types in the kyiv region and influences its ecological and agricultural potential (state department of environmental protection in kyiv region, 2021). according to the main directorate of the state land agency, the total land area within the administrative boundaries of the kyiv region is 2816.2 thousand hectares. the region’s agricultural land spans 1658.9 thousand hectares, representing approximately 58.9% of the region’s total land area. built-up areas account for 137.4 thousand hectares, which equates to 4.9% of the total. industrial lands 22.9 thousand hectares (0.8%), while lands designated for transport and communications represent 29.7 thousand hectares (0.9%). land allotted to law enforcement agencies covers 26.3 thousand hectares (0.9%). the region designates 56.0 thousand hectares for environmental protection. recreational areas cover a total of 1.8 thousand hectares, with 0.4 thousand 301application of remote sensing methods for statistical estimation of organic matter in soils hectares intended for general recreation and an additional 1.4 thousand hectares set aside specifically for recreational facilities. finally, 1.2 thousand hectares are reserved for historical and cultural purposes (state department of environmental protection in kyiv region, 2020; main department of the state land agency in kyiv region, 2012). figure 1 illustrates the territory of the kyiv region with an emphasis on 15 highlighted districts (excluding city councils). for these highlighted districts, a statistical relationship will be established between the humus content (som) in the soil and the landsat 8 oli satellite data. the creation figure 1 employed geoinformation technologies, following the methods delineated in the study (belenok et al., 2021; liashenko et al., 2020) in arcgis pro software. figure 1. study area the kyiv region boasts a diverse soil cover, with chernozems, constituting approximately 50% of the region’s arable land, being the most prevalent. other soil types found across a significant portion of the region include podzolized soils. this diverse mix of soil types underscores the rich and varied terrain of the kyiv region (figure 2). the degree of plowing of the territory exceeds 60 % (state institution “soils protection institution of ukraine”, 2016). in the kyiv region, an estimated 1353.7 thousand hectares of land are degraded, representing 48.1% of the region’s total land area and 81.4% of its agricultural land. this level of degradation is among the highest in the forest-steppe and steppe regions of ukraine. one of the key contributing factors to these negative processes is the excessive pressure exerted on the region’s land resources, which includes intensive agricultural activity and widespread plowing of the territory (state service of ukraine for geodesy, cartography and cadastre, 2020). figure 2. soil of kyiv region, constructed by the authors based on the data described in (atlas of natural conditions and natural resources of the ukrainian ssr, 1978) the overall condition of the soils in the kyiv region is currently deemed satisfactory. nevertheless, in order to maintain and enhance this status, it is imperative to implement consistent, reliable, and timely soil cover monitoring (state institution “soils protection institution of ukraine”, 2020). it is important to note that since february 24, 2022, a full-scale war has been unfolding in ukraine. consequently, the current statistical data concerning land resources may be subject to change depending on the impact of the ongoing conflict. therefore, when interpreting the data, the influence of these extraordinary circumstances should be taken into consideration. 3. materials and data conducting ground surveys and obtaining factual data on the percentage of som agrochemical research is conducted in ukraine every five years. to date, ten complete rounds of such studies have been completed, and the 11th round is currently underway with data still being gathered. the state institution ‘soils protection institute of ukraine’ carries out agrochemical studies on soils and produces accompanying reports (state institution “soils protection institution of ukraine”, 2020). in our study, we utilized data on the humus content, also known as soil organic matter (som), from the 5th to the 10th rounds of investigation. it is pertinent to note that the 5th round spanned the years 19861990, the 6th round covered 1991-1995, the 7th round included 1996-2000, the 8th round ranged from 2001-2005, the 9th round incorporated 2006-2010, and the 10th round covered 2011-2015. thus, the som percentage represents the average value over five years for each round, broken down by district. during the 5th round, 1094.8 thousand hectares of agricultural land were surveyed, with an average som percentage of 2.7% for the entire kyiv region. in the 6th round, the surveyed area contracted to 770.8 thousand hectares, and the average som decreased to 2.6%. for the 7th round, the surveyed area expanded to 939.3 thousand hectares while the average som remained at 2.6%. in the 8th round, the surveyed area began to shrink again, reaching 843.2 thousand hectares, with a som of 2.87%. this decrease continued into the 9th round (793.2 thousand hectares, som – 2.9%) and the 10th round (761.4 thousand hectares, som – 2.98%) (romanova et al., 2018; yacuk and balyk, 2013). as a result, the som data for each district were extracted from statistical reports provided by the state institution ‘soils protection institute of ukraine.’ this information can also be found in the published article by (romanova et al., 2018). the state institution ‘soils protection institute of ukraine’ collects soil samples from march to may. following this, laboratory tests are conducted and the som data is calculated as an average value spanning five years. consequently, we selected satellite images for the months of march through may from 1986 to 2015. it is important to note that many of the images from may display vegetation. this article proposes a method for estimating the predicted som indicator based on spectral characteristics. to ensure the reliability of the study, we decided to focus on a single soil type, specifically chernozems, which predominate in the southern part of the region, as depicted in figure 2. the 15 selected districts from the kyiv region, along with their corresponding humus content (som) in the soils and the soil types, were identified by the authors and are presented in figure 3. drawing from empirical data (romanova et al., 2018), a significant increase in som over time was reported, with marked alterations especially observable in the southern part of the region. despite the ongoing intensification of agricultural land use in the region, it is evident that the som data can be enhanced through the application of various organic fertilizers. specifically, within the kyiv region, the incorporation of by-products and residual vegetable matter from agricultural crops is identified as the primary mechanism for replenishing som. romanova’s research provides further validation, confirming the annual increase in organic contributions. the second factor contributing to the rise in som within the region is the strategic removal, initiated in 2001, of highly eroded and unproductive lands with low som content from active cultivation. the area of the investigated lands decreased by 30.5 % (from 1986 to 2015). som content in soils of kyiv region according to (romanova et al., 2018) by survey rounds (1986-2015) in 15 selected districts shown in table a1 (appendix a). 302 vadym belenok, liliia hebryn-baidy, natalyya bіelousova, halyna zavarika, sergíy kryachok, dmytro liashenko, tetiana malik satellite data landsat data in this study, we utilized data from the landsat 5 tm (“usgs landsat 5 level 2, collection 2, tier 1” collection in gee) and landsat 8 oli (“usgs landsat 8 level 2, collection 2, tier 1” collection in gee) satellites, both of which are publicly accessible as remote sensing materials. as the groundbased measurements for the som data were collected between 1986 and 2015, we ensured that the satellite data also spanned this time interval. among the landsat satellite system, three missions, specifically landsat 5, 7, and 8, were operational during this period. however, it is worth noting that data acquired after may 31, 2003, from these missions may contain data gaps (landsat missions, 2021). for the 5th through 9th rounds (1986-2010), we exclusively used data from landsat 5 tm. the 10th round, which spanned the interval from 2011 to 2015, was covered by data from both landsat 5 tm (which operated until 2012) and landsat 8 oli (which has been operational since 2013). when data from the landsat 5 tm were loaded for what were considered the most favorable periods march and may of 2011 and 2012, the resulting images were found to be of poor quality. this could possibly be due to the deterioration of the detectors as the landsat 5 tm satellite mission approached its end. as a result, the decision was made to use data from the landsat 8 oli satellite. the landsat 8 oli data offers a spatial resolution of 30 meters in the visible, near-infrared (nir), and shortwave infrared (swir 1 and swir 2) ranges, and it has a 16-day temporal resolution (landsat missions, 2021). the number of images that were used for conducting the research in each of the rounds are displayed in table 1. table 1. the number of landsat images used. no round of investigation investigation time satellite bands number of images 5th 1986-1990 landsat 5 tm 1-5, 7 160 6th 1991-1995 landsat 5 tm 1-5, 7 117 7th 1996-2000 landsat 5 tm 1-5, 7 126 8th 2001-2005 landsat 5 tm 1-5, 7 72 9th 2006-2010 landsat 5 tm 1-5, 7 109 10th 2011-2015 landsat 8 oli 2-7 102 satellite elevation and climate data elevation and climate data from gee were utilized to construct a regression model in this study. the ‘nasa srtm digital elevation 30 m’ collection served as the source for terrain slope data, expressed in degrees. additionally, the ‘terraclimate: monthly climate and climatic water balance for global terrestrial surfaces, university of idaho’ collection provided data on precipitation accumulation, measured in millimeters, as well as soil moisture, also measured in millimeters, which was derived using a one-dimensional soil water balance model. temperature data (ºc) were also sourced from this collection. satellite data preprocessing in gee landsat 5 tm and landsat 8 oli data can be freely downloaded from gee (google earth engine, 2021). hence, the study employs the gee cloud computing platform, offering powerful tools for processing remote sensing data and conducting geoinformation analysis. using gee mitigates challenges associated with loading, storing, and processing satellite data at different times. this issues commonly encountered when dealing with large volumes of spatial data (mateo-garcía et al., 2018; shetty ai., umesh and shetty a., 2021). to access gee over the internet, a google account is required for coding operations. the data catalog houses a vast repository of publicly available geospatial datasets. these incorporate observations from a variety of satellite and aerial imaging systems spanning both optical and non-optical wavelength ranges. the catalog also includes environmental variables, weather and climate data, land cover, topographic information, and socio-economic data. all this data is preprocessed and made available in a ready-to-use format (gorelick et al., 2017). the gee platform comprises two principal components that function in tandem the google earth engine explorer, which is used for viewing datasets, and the google earth engine playground. the google earth engine playground application employs a javascript api to download and analyze extensive satellite images, as well as to execute complex geostatistical and geospatial operations (shetty ai., umesh and shetty a., 2021). in gee landsat 5 tm and landsat 8 oli data were downloaded from collection 2 level-2 already with surface reflectance values. that is, they are atmospheric corrected. landsat 5 tm and landsat 8 oli surface reflectance data (level 2 science product l2sp) were generated using the land surface reflectance code algorithm. this algorithm converts pixel values to top of atmosphere (toa) reflectance and brightness temperature values using calibration parameters from metadata. then the toa reflectance values are subjected to atmospheric correction using additional data (landsat 8 collection 2 (c2) level 2 science product (l2sp) (guide, 2020; vermote et al., 2016) the subsequent step in processing landsat data in gee involves masking clouds and their shadows. pixels obscured by cloud cover and cloud shadows do not provide useful information about the earth’s surface. therefore, their inclusion in data processing would lead to deliberately inaccurate results. gee masks cloud pixels according to the c function of the mask (cfmask) algorithm using data from the qa_pixel (pixel quality assessment) band. the cfmask algorithm can be found, for example, in the study (joshi, wynne and thomas, 2019; zekoll et al., 2021). finally, employing a date filter, mosaic landsat images were created for the months of march through may for rounds 5th through 10th, encapsulating 15 months across five years for each round. these mosaic landsat images consist of images with atmospheric corrected pixel values (surface reflectance) that have been averaged over all scenes of this satellite covering the 15 districts of the kyiv region for the designated time interval, with clouds and their shadows masked. figure 3. humus content (som) in selected 15 districts of kyiv region (10th round) 303application of remote sensing methods for statistical estimation of organic matter in soils 4. methods snow masking images from march and may were considered for this study because during this time, the snow in the area would have already melted and the primary growing season for most plants would not have begun yet. extensive vegetation on the soils can lead to reflections that distort the soil’s spectral curve. to account for any areas where snow might still be present, a trait common in march and april for this latitude, the normalized difference snow index (ndsi) was utilized. we employed the ndsi, as described in (riggs, hall, and salomonson, 1994) to mask areas that might still be covered with snow. this specific threshold was established after examining the satellite data for the study area. the calculation of the ndsi was conducted using the following formula: 1, 1 green swirndsi green swir – = + (1) where green and swir 1 are the pixel values in landsat’s bands (2 and 5 for landsat 5 tm and 3 and 6 for landsat 8 oli, respectively. the threshold ndsi < 0 was utilized for snow masking, with positive ndsi values indicating areas covered with snow. the mean spectral reflectance (msr) value was obtained for pixels unaffected by snow, clouds, and cloud shadows. subsequently, the resulting mosaic was cropped to the precise area of the polygonal feature representing the kyiv region. finally, the processed mosaic was uploaded to the gee platform from the humanitarian data exchange (the humanitarian data exchange, 2021). determination of bare soils from remote sensing data according to (romanova et al., 2018) soil samples were collected from bare soils in the kyiv region for further determination of som. consequently, the subsequent step involved developing a method to isolate bare soils using the mosaic of landsat satellite data. figure 2 indicates that chernozem soils, which are the focus of this article’s spectral analysis, are predominantly located in the south and southeast of the kyiv region. therefore, for the upcoming investigation, the corresponding 15 districts of the kyiv region, as depicted in figure 1 and figure 3 were specifically chosen. the polygon data for these districts were also obtained from the humanitarian data exchange (the humanitarian data exchange, 2021). the conventional approach for identifying different types of land use, known as lulc (land use/land cover), involves satellite data classification. both supervised and unsupervised classification methods are commonly employed, as described in the scientific literature (mekuriaw, cherinet and tsegaye, 2021; olthof and fraser, 2014). nevertheless, the classification method is rather cumbersome and can be effectively used to distinguish all types of lulcs typical for a given territory. in our study, we need to select only bare soils, and we are not interested in selecting other lulc types. therefore, it is advisable to use a technique based on the reflective spectral properties of bare soils, making it possible to decipher this particular type of lulc. these data for the first seven bands of landsat 8 oli in gee are shown in figure 4. the spectral reflectance properties of soils in the study area were examined by analyzing spectral profiles and using different combinations of bands from the landsat 8 oli satellite. (figure 5). by analyzing figure 5a and figure 5b, it is possible to determine the color representation of bare soils in composite r-g-b = 4-3-2 and decipher them. it is important to consider the soil property described in (shikhov et al., 2020), as mentioned earlier, which states that soils of different genetic affiliations exhibit a monotonic increase in reflection coefficients as wavelength increases in the range from 0.4 to 2 μm. in our study, this corresponds to the b1-b6 band of landsat 8 oli. by comparing the spectral profiles of different bare soils (figure 5a) and overlaying a thematic map of soils in the kyiv region (figure 2) onto figure 5a it can be concluded that the last two spectral profiles in the “profiles” list do not correspond to chernozem soils. therefore, for further analysis, they should be masked. based on the findings from previous studies regarding the spectral reflectance properties of chernozems and the obtained spectral profiles, the authors chose to utilize specific threshold values for vegetation indices to isolate bare soils chernozems, as presented in table 2. additionally, the built-up area extraction index (baei) was employed to separate bare soils from built-up areas. a. b. figure 4. landsat 8 oli satellite data mosaic plotted in gee for the month’s march-may for round 10th (2011-2015): a for the territory of the kyiv region (false color composite r-g-b = 5-4-3), b for the territory of the selected 15 districts of the kyiv region (composite r-g-b = 7-6-5) 304 vadym belenok, liliia hebryn-baidy, natalyya bіelousova, halyna zavarika, sergíy kryachok, dmytro liashenko, tetiana malik table 2. threshold values of vegetation indices for bare soils detection. vegetation indices threshold values from to ndvi 0.1 0.35 ndbi 0 0.45 ndwi -0.5 0 msavi2 0 0.3 baei < 1.3 ndsi < 0 the vegetation indices were calculated using the formulas provided below. ndvi differentiates green vegetation from other surfaces, and elevated ndvi values indicate a high presence of leaf biomass, dense canopy coverage, or a large leaf area (jasinski, 1990; tucker, 1979; wilson and sader, 2002): .nir redndvi nir red – = + (2) ndbi (normalized difference built-up index) is utilized to emphasize built-up areas while mitigating variations in surface illumination and atmospheric effects (zha, gao and ni, 2003): 6 5 6 5 .band bandndbi – + (3) normalized difference water index (ndwi was introduced as a novel index for remote sensing applications, specifically for detecting vegetation liquid water and assessing the spectral reflectance properties of soils (gao, 1996): 0.86 1.24 0.86 1.24 1, 1 nir swirndwi nir swir (4) where ρ is surface reflectance value at the wavelength specified in μm; msavi2 (modified soil adjusted vegetation index) expands the dynamic range of the vegetation signal, thereby reducing the impact of soil background and enhancing the sensitivity of vegetation. this sensitivity is determined by the ratio of the vegetation signal to the soil noise (qi et al., 1994): ( ) ( )2 2 2 1 2 1 8 2 msavi (5) built-up area extraction index (baei) is calculated by the equation (bouzekri et al., 2015): + 0.3 1 redbaei green swir = + . (6) vegetation indices were computed based on the average spectral reflectance of landsat 5 tm and landsat 8 oli, following the formulas -. using the predetermined threshold values of vegetation indices (table 2) and the polygons representing the territories of city councils acquired from (the humanitarian data exchange, 2021), a binary raster mask was generated. in this mask, a value of 1 indicates bare soils chernozems, while 0 represents other types of lulc. the generated mask was then applied to the previously obtained msr values. as a result, images of bare soils were obtained, where the pixels retain the msr values. 5. results to establish the statistical relationship between the msr value obtained from landsat and som, it was decided to employ a multiple linear regression (mlr) model. since each of the 15 selected districts in the kyiv region is characterized by a single the som value, this value should correspond to a single mean spectral reflectance value from landsat for each band. consequently, within gee, zonal statistics were utilized to calculate the average msr values for the seven selected landsat bands, the average values of vegetation indices for each area, as well as additional data on relief and climate factors such as slope, precipitation, soil moisture, and temperature. indeed, mlr enables the assessment of the individual impact of each factor and their collective influence on the target variable, in this case, the som data. mlr allows for the determination of the degree of influence that each factor exerts on the productive trait and provides insights into the combined effect of these factors on som. by analyzing the mlr model coefficients and statistical significance, we can understand the relative importance and contribution of each factor in explaining the variations in som. in the mlr model, som was considered as the dependent variable. as predictors or regressors in the mlr analysis, we utilized spectral bands with wavelengths common to landsat 5 tm and landsat 8 oli, which are associated with the spectral reflectance properties of soils. additionally, vegetation indices capable of detecting bare soils were employed as predictors. furthermore, topographic data, specifically slope, and climatic data including temperature, precipitation, and soil moisture, were also included as predictors in the model (table 3). table 3. predictors of mlr model predictors, xi x1 x2 x3 x4 x5 x6 x7 values, msr blue green red nir swir 1 swir 2 ndvi predictors, xi x8 x9 x10 x11 x12 x13 value, msr ndbi msavi2 slope temperature precipitation soil moisture a. b. figure 5. the determination of the lulc type is indicated by points (a), and the corresponding spectral profiles (b) are provided. a fragment of the landsat 8 oli image for the study area is presented as a composite with the bands r-g-b represented by 4-3-2. 305application of remote sensing methods for statistical estimation of organic matter in soils the msr value in table 3 indicates the msr obtained from landsat 5 tm and landsat 8 oli for each of the six selected bands, band combinations, and vegetation indices. these msr values were calculated within the 15 districts of interest. in each of the six rounds of ground surveys (5th-10th rounds), there is one averaged value of som. considering that ground surveys were conducted in 15 districts of the kyiv region, each mlr regressor model consists of 90 values (6 rounds × 15 districts = 90 values). these input data for the mlr calculations are presented in table a2 in appendix a. afterwards, table 4. correlation matrix model variables som * * * * * * * -0.37 blue * * * * * * * -0.51 0.90 green * * * * * * * -0.56 0.84 0.96 red * * * * * * * -0.40 -0.20 0.13 0.26 nir * * * * * * -0.28 0.67 0.68 0.62 -0.01 swir 1 * * * * * * -0.18 0.61 0.59 0.50 -0.11 0.97 swir 2 * * * 0.08 -0.80 -0.60 -0.52 0.69 -0.46 -0.46 ndvi * * * * 0.08 0.61 0.38 0.24 -0.72 0.70 0.76 -0.81 ndbi * * -0.10 -0.65 -0.38 -0.27 0.86 -0.34 -0.38 0.96 -0.84 msavi2 * * -0.41 0.15 0.32 0.42 0.33 0.04 -0.01 -0.03 -0.21 0.10 slope -0.04 -0.18 -0.05 -0.03 0.29 0.26 0.19 0.29 -0.03 0.31 -0.11 temp. * * 0.11 0.20 0.05 0.02 -0.42 0.00 -0.01 -0.39 0.30 -0.43 0.07 -0.43 precip. -0.23 0.12 0.03 0.08 0.18 0.23 0.13 0.10 0.03 0.14 0.08 0.24 0.07 soil mois. r= -1 -0.80 -0.60 -0.40 -0.20 0 0.20 0.40 0.60 0.80 1 figure 6. scatterplot matrix for correlation between model variables a pearson’s correlation analysis was conducted to examine the relationships between the variables used in the mlr model. the pearson’s correlation coefficient (pearson’s r) was employed to evaluate the strength and direction of the linear association between the variables (mendenhall and sincich, 2013). the results of the pearson correlation analysis between msr and som values are presented in table 4. marked * correlations are significant at p < 0.05. for clarity, the data from table 4 are shown on figure 6. 306 vadym belenok, liliia hebryn-baidy, natalyya bіelousova, halyna zavarika, sergíy kryachok, dmytro liashenko, tetiana malik the analysis of table 4 reveals a notable and statistically significant negative relationship between som and msr in the green and red bands (r ≈ -05 ÷-0.6 ). there is also a slightly weaker negative relationship between som and msr in the blue and nir bands (r ≈ -04). additionally, an inverse relationship is observed between som and msr in the swir 1 band (r ≈ -0.28). regarding the climatic data and topography, the analysis indicates a significant negative relationship between som and slope, as well as between som and soil moisture. however, the correlations between som and the other msr variables are relatively weak and not statistically significant at p < 0.05. at the initial stage, a total of 13 predictors for the mlr model were selected. subsequently, employing the method of least squares, the mlr coefficients and other regression parameters were calculated. in our analysis, it was employed two methods to determine the most significant variables: the forward stepwise method and the backward stepwise method. the forward stepwise method involves iteratively adding or removing independent variables from the model at each regression step based on the results of the f test. variables are added to the model if they contribute significantly to improving the regression model’s fit. this process continues until a better regression model is achieved. on the other hand, the backward stepwise method removes independent variables from the regression equation one at a time, again based on the f test results. variables are eliminated from the model if their inclusion does not significantly improve the fit of the regression model. this process continues until a better regression model is obtained. the essence of the f test in testing the significance of mlr is to compare two variances: model-explained (msr) and unexplained (mse) (mendenhall and sincich, 2013). if msr ≈ mse, then the regression is insignificant (the constructed model does not allow explaining the behavior of the predicted value depending on the the predictors’ values). if , then the regression is significant. two hypotheses are considered: h0 (1) – regression is not significant, h1 (1) – regression is significant, which is what it is used for f test: msrf mse = (7) next, the significance is checked f (value is calculated f0 ) through the f-distribution (fisher-snedecor distribution) for the obtained values f, dfr, dfe. if, f0 > α where α = 0.05 is significance level, then the hypothesis h0 (1), is accepted, otherwise, the hypothesis h1 (1) is accepted. for a detailed step-by-step procedure of the forward stepwise and backward stepwise methods, please refer to appendix b. as a result of these methods, we obtained two models, and their respective parameters are presented in table 5. in order to determine the best model among the two obtained, we utilized the akaike information criterion (aic) as proposed by (akaike, 1973). the aic serves as a heuristic measure that aims to balance the reduction in the number of model parameters and the quality of model fit. according to this criterion, the model with the smaller aic value should be selected, irrespective of its absolute value. table 5. the resulting regression models obtained through the forward stepwise and backward stepwise methods model of forward stepwise method (model i) multiple r = 0.651, r2 = 0.424, adjusted r2 = 0.397, f(4,85) = 15.667, p-value = 1.2e-9 < α = 0.05, std. error of estimate, σy = 0.25655 std. regression coefficient, β* std.err. of β* accurate estimates of model parameters, β std.err. of β t stat p-value intercept 6.345 0.455 13.956 1.055e-23 red -0.437 0.092 -22.055 4.630 -4.763 7.760e-6 nir -0.210 0.089 -4.883 2.072 -2.357 0.021 soil moisture -0.151 0.084 -0.004 0.002 -1.802 0.075 slope -0.143 0.094 -0.072 0.047 -1.523 0.131 model of backward stepwise method (model ii) multiple r = 0.631, r2 = 0.398, adjusted r2 = 0.384, f(2,87) = 28.763, p = 2.6ε-10 < α = 0.05, std. error of estimate, σy = 0.25932 intercept 1.946 0.241 8.080 3.40e-12 ndvi 2.286 0.305 17.517 2.337 7.496 5.166e-11 msavi2 -2.296 0.305 -32.701 4.345 -7.527 4.482e-11 307application of remote sensing methods for statistical estimation of organic matter in soils the obtained values for the aic criterion are as follows: aici = -240.023 aici = -239.994 based on the selection process, model i derived from the forward stepwise method was chosen for predicting som. to assess the normality of the values of the model regressors and the residuals, we conducted the shapirofrancia test and the kolmogorov-smirnov test. the shapiro-francia test, which is an extension of the shapiro-wilk test suitable for samples larger than 50, was used for this purpose. these tests help us determine if the values of the model regressors and the residuals follow a normal distribution (shapiro and francia, 1972). the results of the regressor test are shown in the following figure 7. the values of all four regressors in model i are normally distributed at a significance level of α = 0.001, as a. ∀ p value: p ≥ a. as a limitation of our model at this stage, it should be noted that the nir and slope regressors will not be normally distributed if we consider a significance level of α = 0.01 during the assessment of the residuals’ normality using the shapiro-francia test and the kolmogorov-smirnov test, the results depicted in figure 8 were obtained. it can be observed that the residuals are normally distributed at a significance level of α = 0.001 (p = 0.951 > 0.001). the effect size ks d, which measures the difference between the sample distribution and the normal distribution, is very small, specifically 0.04269. this suggests that the deviation from normality is negligible. a red b nir c soil moisture d slope figure 7. normality check histograms of regressors values 308 vadym belenok, liliia hebryn-baidy, natalyya bіelousova, halyna zavarika, sergíy kryachok, dmytro liashenko, tetiana malik figure 8. normality check histograms of residuals figure 9. predicted vs. observed values figure 10. predicted vs. residual scores to test our model on heteroscedasticity (the hypothesis h0 – homoscedasticity, error variances are equal) we used the breusch-pagan test (breusch and pagan, 1979). the test result showed that there was no heteroscedasticity: xcalc 2 = 1.226, xtable 2 (p = 0.95; df = 9.488, xcalc 2 < xtable 2 to test our model for multicollinearity, we used the variance inflation factor (vif) (mendenhall and sincich, 2013). at first glance, there is a suspicion of multicollinearity in the model, since according to the table 5 p-value of t stat for soil moisture and slope greater than α = 0.05 (0.075 and 0.131, respectively). but the vif calculation showed that there is no multicollinearity: vif red = 1.24, vif nir = 1.18, vif slope = 1.30, vif soil moisture = 1.03 according to (mendenhall and sincich, 2013) a severe multicollinearity problem exists if the largest of vif is greater than 10. to identify potential outliers, we calculated cook’s distance. all obtained values of cook’s distance are approximately equal and are in the range from 0 to 0.16, which indicates the absence of potential outliers (mendenhall and sincich, 2013). finally, we have constructed graphs that illustrate the relationship between the predicted vs. observed values (figure 9) and predicted vs. residual scores (figure 10). confidence bands were constructed at a probability level of 0.95. in figure 9 and figure 10 the confidence bands line (red dashed curve) shows the confidence level (0.95), which is the probability that the “true” fitted line (in the population) falls between the bands. discussion the findings from the constructed mlr model for predicting som in the chernozem soils of the forest-steppe zone of ukraine provide significant insights into the region’s soil fertility. the application of a combination of reflectance values in six bands of landsat 5 tm and landsat 8 oli satellites, vegetation indices (ndsi, ndwi, ndbi, baei, msavi, and ndvi), topographic data (slope), and climatic data (temperature, precipitation, soil moisture) as regressors has contributed to the development of an effective predictive model. the forward stepwise method (mendenhall and sincich, 2013) was instrumental in achieving the optimal model, as it allowed for the selection of the most impactful regressors based on f test and akaike criteria (akaike, 1973). the model yielded a multiple r = 0.651 and an r2 = 0.424 and the most significant predictors of an effective predictive model are red, nir, slope and soil moisture. these metrics suggest that the model can explain approximately 42.4% of the variation in the som in the soils of the examined regions. the adjusted r2 of 0.397 further supports this and confirms that the model maintains a decent level of predictive power even when adjusting for the number of predictors in the model. fairly similar results were obtained in the study (yu et al., 2021) where the accuracy of the model was improved by adding the same environmental factors and spectral combination. however, the estimation effect of back propagation neural network (bpnn) model (with r2 being 0.947) was better than that mlr model (with r2 being 0.410). similar to the findings of this study (forkuor et al., 2017; wang and ge, 2012) also recorded elevation, slope, red and nir as the most important variable influencing soil indicators. according to the study (luo et al., 2023) adding the multiyear average temperature effectively improves the som mapping performance. the authors used sentinel images (10 m) and the random forest algorithm, which obviously affects the accuracy of the model. some studies (lu, liu and liu, 2022) suggest that the environmental variables (mainly clay index) have the greatest influence on the prediction accuracy of som. using time series 7 mod09a1 images (zhang et al., 2021) noted that the pixel dates of the training samples and precipitation data were the main factors controlling the model performance. the simultaneous combination of dead fuel index (dfi), ndvi and environmental factors improves the som mapping performance (yang et al., 2021) especially in infertile soils. undoubtedly, the accuracy of som mapping performance will be affected by the spatial resolution of the images. in our study, the period 1986-2015 was selected, which is decisive for why we used the landsat system. given that the red and nir bands emerged as the most significant spectral predictors in our analysis, it would be valuable to compare our findings with the results of ukrainian scientists who have studied the same type of soils. 309application of remote sensing methods for statistical estimation of organic matter in soils according to the study (chornyy and abramov, 2016; achasov and bidolah, 2008; chatohin and lindin, 2001; sakhatsky, 2008; truskavetsky, 2006; byndych, 2021) the humus content (som) is most linearly related to red, nir and green band. in the study (abramov, 2012) author built a parabolic relationship between spectral index red/nir and the humus content (som). long-term studies of soils in transcarpathia made it possible to conclude that the best relationship was traced between the humus content (som) and the spectral index nir/red (hebryn-baidy, 2017). according to study (truskavetsky et al., 2015) there is a linear relationship between swir1,2 bands and the humus content (som). the fact that in the listed studies authors used exclusively spectral information is interesting. in our study, we used environmental factors in addition to spectral data. it should be noted that the mentioned authors did not specify the correlation between the bands. according to our research, for the selected area, there existed a high positive correlation between the green and red bands (r = +0.96). therefore, including both of them simultaneously in the model would yield an incorrect result. this increased the accuracy of our research to some extent. some studies prove that there is a relationship between som and other bands. based on the mlr analysis, the authors (ahmed and iqbal, 2014) establish a significant relationship (r2 = 0.55) between som and spectral reflectance in the blue, thermal and mid-infrared bands. whereas in our study, the blue band showed worse significance, and others band were not considered. in the study (demattê et al., 2007; fiorio and demattȩ, 2009; reis et al., 2021), authors demonstrate that som, clay and sand are the most important attributes that can be predicted by mlr. it is important to note that the validation procedure compared the content of each soil attribute determined by the spectral models with laboratory traditional analysis. in our study, the accuracy of the obtained models was evaluated exclusively on the basis of mathematical calculations. a noteworthy approach is shown in the study (bouasria et al., 2020), authors increase the resolution of the image (from 30 m to 15 m) by pansharpening method. thus, the significance of the relationship between som and spectral reflectance in the multispectral range increases. it should be noted that the research period and soil conditions vary depending on the natural and geographical location of study area. the authors also note that the soil should be in an air-dry state, the surface roughness should be minimal, and there should be no vegetation on the soil surface (gopp et al., 2017; gopp et al., 2019; david, 2013; wang and ge, 2012; wang et al., 2022; savin et al., 2021). these conditions were met in our study by using ndsi, ndwi, ndvi, ndbi, and baei masks and climatic data. stable relationships between som and spectral reflectance exist only at medium and high content of som (1-5%) (sullivan et al., 2005; reis et al., 2021). indicators of som in our study fluctuate within the limits from 2.69% to 3.69%. it is also worth noting that the sample size can affect the accuracy of the model. however, this factor is worth a separate study. additional factors not considered in this study, such as other soil components, the impact of human activity, and biodiversity changes, could further refine the model’s predictive capacity. also, the model was built and tested only in the kyiv region of ukraine, primarily on chernozem soils, which may limit its applicability to other soil types and regions. in conclusion, the model serves as a useful tool for understanding and predicting som in chernozem soils. with the continuous advancements in satellite technology and increasing availability of detailed environmental data, refining this model for higher accuracy and broader applicability is a promising avenue for future research. conclusion this article focuses on constructing a mlr model to predict the som in chernozem soils within the forest-steppe zone of ukraine. the statistical data used for analysis consisted of percent values of som obtained from six rounds of field studies conducted between 1986 and 2015. the model development was carried out for 15 districts located predominantly on chernozem soils within the kyiv region, ukraine. as regressors in the model, reflectance values from six bands of landsat 5 tm and landsat 8 oli satellites, vegetation indices for bare soil detection, topographic data (slope), and climate data (temperature, precipitation, soil moisture) were employed. the optimal model was obtained through the forward stepwise method, based on f-test and akaike criteria. the model exhibited the following metrics: multiple r = 0.651, r2 = 0.424, and adjusted r2 = 0.397. validation of the model demonstrated the absence of heteroscedasticity, multicollinearity, and potential outliers, indicating its reliability. the most influential predictors in the effective predictive model were identified as red, nir, slope, and soil moisture. the developed model can be utilized for predicting the som content in chernozem 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architectural analysis; paleoenvironment; upper cretaceous shendi formation; shendi-atbara basin, sudan. this study examines both vertical and lateral facies changes as well as petrographic characteristics of the outcropping section of the upper cretaceous shendi formation in the musawwarat-naga area with a view of interpreting the depositional environment, provenance, and tectonic setting that prevailed during the deposition of the sediments. field studies revealed the presence of eight different sedimentary lithofacies. the predominant is the trough cross-bedded sandstone facies (st), representing 75% of the total succession. four architectural elements were recognized: sandy bedforms (sb), channel fill (ch), gravelly bars and bedforms (gb), and overbank fines (of). moreover, the mean value of paleocurrent directions was 285.3°, suggesting a southeasterly located source area. the majority of the sandstones were classified as quartz arenites with subordinate sublithic-arenites. ternary plots of quartz-feldspar-lithic fragments (qfl) revealed that the tectonic setting is predominantly within the interior craton. it can be suggested that the sediments were sourced from the southeast region and associated with a braided, proximal, fluvial depositional environment. the results of this study provide outcrop analog data that might be useful in exploring hydrocarbons. specifically, it can enhance the understanding and prediction of the proximal fluvial depositional styles in the subsurface deposits in the shendi-atbara basin and similar rift basins in sudan and africa. palabras clave: análisis de facies; análisis de formación; paleoambiente; formación shendi del cretácico superior; cuenca shendi-atbara, sudán. ambiente deposicional y potencial de exploración de hidrocarburos con base en el análisis de facies sedimentarias y análisis arquitectónico de la formación shendi del cretácico superior en el área de musawwarat-naga, cuenca shendi-atbara, sudán how to cite this item: babkir hassan, m. d., nton, m. e., & eisawi, a. a. m. (2023). depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan. earth sciences research journal, 27(2), 109-128. https://doi.org/10.15446/esrj. v27n2.98795 resumen este estudio analiza los cambios verticales y laterales de las facies al igual que las características petrográficas en una sección de un afloramiento en la formación shendi del cretácico superior, en el área de musawwarat naga (sudán), con el objetivo de interpretar el ambiente deposicional, la procedencia y la configuración tectónica que prevaleció durante la deposición de los sedimentos. los estudios de campo revelan la presencia de ocho litofacies sedimentarias diferentes. la predominante es la facies de piedra arenisca de cama cruzada (st), que representa el 75 % de la sucesión total. se reconocieron cuatro elementos arquitectónicos: lechos de arena (sb); relleno de canales (ch), barras y lechos de gravas (gb), y sedimentos finos de aluviales (of). además, la media de las direcciones de paleocorrientes fue de 285.3°, lo que sugiere que el área de la fuente está ubicada en el sudeste. la mayoría de las areniscas se clasificaron como cuarzoarenitas con arenitas sublíticas subordinadas. diagramas ternarios de cuarzo-feldespato-fragmentos líticos (qfl) revelaron que la configuración tectónica está predominantemente en el interior del cratón. puede sugerirse que los sedimentos provienen del sudeste y están asociados con un ambiente deposicional trenzado, proximal y fluvial. los resultados de este estudio proveen datos analógicos que podrían ser útiles en la exploración de hidrocarburos. especificamente, pueden mejorar el conocimiento y predicción de los estilos deposicionales fluvial proximales en los depósitos de la subsuperficie de la cuenca de shendi-atbara y cuencas de características similares en sudán y áfrica. https://doi.org/10.15446/esrj.v27n2.98795 mailto:diaeldin.babkir@cug.edu.cn https://doi.org/10.15446/esrj.v27n2.98795 https://doi.org/10.15446/esrj.v27n2.98795 110 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi 1. introduction the study of fluvial deposits is essential in improving our understanding of hydrocarbon reservoirs, leading to more effective hydrocarbon production (miall, 2006; owen et al., 2017). that is because the internal architecture and petrophysical properties of fluvial reservoirs are generally complex and heterogeneous (e.g., owen et al., 2015; larue et al., 2023). recently, considerable research has been conducted on the properties and quality of subsurface fluvial reservoirs using geophysical methods (e.g., abdel-fattah et al., 2022; el-gendy et al., 2022; tayyab naseer et al., 2022). however, there is always a need for analog data from outcrops to enhance the subsurface geological reservoir models of fluvial deposits (e.g., owen et al., 2017). the sedimentary rift basins of sudan were discovered through geophysical surveys conducted by chevron company in cooperation with the sudanese government (schull, 1988). the discovered basins, such as muglad basin, represent major components of the west and central african rift system (wcars) and proved to be of high economic importance in terms of hydrocarbon production (fig. 1; schull, 1988; fairhead, 1988a; mohamed et al., 2001). the shendi formation is the focus of this study, and it represents most of the exposed part of the succession of the shendi-atbara basin in sudan (fig. 1). economic hydrocarbon reserves have not been discovered in shendiatbara basin. in 2011, kandaka-1 well was drilled in the basin, about 60 km east of ed damer city (fig. 2a), for exploration purposes but without encouraging discoveries (sudan ministry of oil and gas, 2011). on the other hand, the muglad basin that extends from southwestern sudan into south sudan (fig. 1) is the largest rift basin in the country and the primary source of hydrocarbon production (makeen et al., 2021a). the main reservoir in this basin is the cretaceous bentiu formation which is similar in depositional style to the shendi formation (mohamed and mohammed, 2008; makeen et al., 2021b). bentiu formation consists of thick sandstone sequences deposited in braided and meandering systems (schull, 1988; mohamed et al., 2001; mohamed and mohammed, 2008; makeen et al., 2021b). in addition, melut basin (fig. 1) is one of the major oil-producing rift basins in south sudan with its primary reservoirs being the upper cretaceous and paleogene sandstones (dou et al., 2007). the upper cretaceous sandstones were deposited in sandy fluvial and shallow lacustrine environments, while the paleogene deposits were deposited in a braided deltaic environment (dou et al., 2007). arising from the similarity in the depositional environments, this study provides additional information about the architectural elements and heterogeneity in the fine-scale which is hardly studied by large-scale seismic sections. therefore, integrating different subsurface data with outcrop analogs can fill the gap in scale and resolution. several studies were conducted on the sedimentology and stratigraphy of the shendi-atbara basin (e.g., kheiralla, 1966; whiteman, 1971; awad and schrank, 1990; wycisk, 1991; werner, 1993; germann et al., 1994; bussert, 1998; eisawi et al, 2015; salih et al., 2015; harzhauser et al., 2017; bussert et al., 2018). although bussert (1998) studied some outcrops in the naga area (wadi awatib area), detailed sedimentological and facies architecture of outcrops in musawwarat area, to the northeast of naga area, have not been reported. moreover, the majority of the previous studies on shendi-atbara basin were principally based on measured vertical profiles (e.g., bussert, 1988) that could barely be sufficient to describe the depositional environments (miall, 1985). the main reason is that misinterpretations of geometry and complex internal forms may occur when deductions are based only on vertical facies variations (miall, 1985, 2006). therefore, this study considers lateral and vertical changes in lithofacies and architecture to conduct a better interpretation of the depositional environment of the sedimentary rocks of the shendi formation in musawwaratnaga area. the study also aims at deducing the provenance and tectonic setting of the sediments by integrating petrographic and paleocurrent analysis. these findings will provide an outcrop analog for similar subsurface deposits in the basin and the region in terms of architecture and depositional environment. figure 1. tectonic model of the west and central african rift system (wcars) and the central african shear zone (casz). the red arrow and rectangle represent the shendi-atbara basin and study area (modified after fairhead, 1988b). 111depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan plateaus cut by numerous wadis. the thickness of the outcropping sections reaches up to 45 m. although the outcrops are massive and of great heights in this area, only a few exposures occur due to the desert varnish effect. 2. geological setting the shendi-atbara basin is located in central sudan and represents an outlier where younger sedimentary rocks (mainly of cretaceous age) are surrounded by the older basement. the basin is a northwest-southeast-trending rift basin and is part of the west and central african rift system (wcars) which extends from west africa to sudan (fig. 1; schull, 1988; fairhead, 1988; klitzsch and squyres, 1990; ibrahim et al., 1996). the formation of intra-continental basins in africa was a result of the breakup of gondwana in the mesozoic era, where the african continent separated from south america (binks and fairhead, 1992). extensional pull along the central african shear zone (casz) resulted in crustal thinning, mantle upwelling, and consequent rifting and subsidence. these processes resulted in the formation of the rift basins in sudan including the shendi-atbara basin (schull, 1988; fairhead, 1988; ibrahim et al., 1996; salama, 1997). the shendi-atbara basin represents a half-graben structure, with sediments thickness of over 3000 m, based on geophysical gravity modeling (jorgensen and bosworth, 1989). the main sedimentary units of the basin include cretaceous strata, which lie unconformably on the basement complex, and recent alluvium and wadi deposits (fig. 2; whiteman, 1971). the stratigraphy of shendi-atbara basin is composed of bagrawiya formation (turonian-coniacian), mahmiya formation (coniacian-campanian), shendi formation (campanian-maastrichtian), hudi chert formation (eoceneoligocene), and wadi awatib conglomerate (late miocene-early pliocene) (fig. 3; eisawi et al., 2015; bussert et al., 2018). the shendi, hudi chert, and wadi awatib formations are exposed on the surface, whereas bagrawiya and mahmiya formations are encountered figure 2. a) the geological map of the study area and the vicinity (modified after whiteman, 1971; gras, 2004). the inset at the upper left corner represents the location of the geological map relevant to sudan and africa. the red outlined rectangle represents the study area shown in b. b) a closer view of the study area showing the locations of the thirteen (13) studied outcrops. figure 3. stratigraphic chart of shendi-atbara basin showing the rock units succession and the basin evolution (modified after eisawi et al., 2015; bussert et al., 2018). the studied outcrops are located in musawwarat-naga area in river nile state, north-central sudan (fig. 2). this area was chosen for this study due to the availability of sufficient outcrops that are mainly the results of the ancient quarries and excavation activities. the exposures are generally in the form of 112 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi only in the subsurface (fig. 3; bussert et al., 2018). the shendi formation was previously correlated with the albian-cenomanian omdurman formation to the south and wadi milk formation to the west (awad and schrank, 1990; wycisk, 1991; werner, 1993; agyemang et al., 2019). however, eisawi (2015) suggested a campanian-maastrichtian age for the formation based on palynological evidence. kheiralla (1966) described the shendi formation as being composed of pebble conglomerates, intraformational conglomerates, sandstones, and mudstones. the sandstones were described as well-bedded, non-pebbly, clean, and very well-sorted, consisting mainly of quartz grains cemented with iron oxides (kheiralla, 1966). bussert (1998) reported that the shendi formation was deposited in a fluviatile system, mainly of braided and meandering nature. it is believed that such fluviatile systems changed gradually, with time, from braided into meandering river systems by the late cretaceous (bussert, 1998). moreover, lacustrine sedimentation as a result of post-rift activities was also reported in some parts of the basin (klitzsch and squyres, 1990; oshi, 2004). 3. materials and methods 3.1 fieldwork methods fieldwork was carried out on the outcropping sections of shendi formation. outcrops were selected for this study based on accessibility and adequate vertical and lateral extension of the exposure. field observations on thirteen outcrops (fig. 2) included examining different lithofacies types and the main classification criteria were lithology, grain size, internal structure, and geometry. a garmin oregon 450t gps device was used to take coordinates and elevations of the studied sections while a geologic clino-compass was used to measure the palaeocurrent directions. samples were collected from different locations and beds representing the dominant lithofacies types. efforts were made to obtain fresh samples at each location. the samples were then kept in labeled sample bags for subsequent analyses. field photographs of features of interest were taken using a nikon d3100 dslr camera. representative vertical and lateral sedimentary profiles with a total thickness of 222 m were measured from 13 outcrops based on the quality of the exposures (table 1; fig. 2). different lithofacies have been identified, described, and named based on the facies scheme of miall (1978). architectural elements were identified based on the concepts of allen (1983) and miall (1985). bounding surfaces were traced on photographs and then interpreted according to miall (1985). 3.2 thin section petrography fifteen (15) representative sandstone samples were selected for petrographic analysis. the sandstones were impregnated with epoxy resin before cutting. each sample was mounted on a glass slide using canada balsam and examined under a petrological microscope model lomo polam r-211m. the framework composition (quartz, feldspar, and lithic fragments) and porosity of each sample were approximately estimated using the conventional point counting method (100 points). ternary plot diagrams of framework composition percentages (dickinson and suczek, 1979) for the tectonic setting were plotted. photomicrographs of features of interest were taken using a nikon coolpix camera and the classification of the sandstones was based on folk (1974). 4. results and discussion 4.1 lithofacies description and interpretation the deposits in the studied successions mainly comprise sandstones and conglomerates, while mudstones are minor. eight sedimentary lithofacies were identified, namely, stratified conglomerate (gt), matrix-supported massive conglomerate (gmm), trough cross-bedded sandstone (st), massive sandstone (sm), horizontally bedded sandstone (sh), laminated sandstone (sl), massive mudstone (fm) and iron crusts (fc). the approximate relative abundance of each lithofacies in the total succession is shown in figure 4. table 2 summarizes the information for each lithofacies. selected vertical sedimentary profiles of the studied successions are shown in figure 5. lithofacies description and coding followed the scheme of miall (1978, 2006). the description and interpretation of each lithofacies are given in the following section. 4.1.1 stratified conglomerate (gt) this lithofacies exhibits slightly inclined cross stratification, and it consists of pebbles of predominantly extraformational origin accompanied by minor intraformational mudstone clasts. the mudstone clasts are concentrated along the erosional lower boundary of this facies. these mud clasts are angular to sub-angular, poorly sorted, and gradually fine and disappear upwards (figs. 6b). the extraformational clasts consist mainly of light brown chert pebbles and cobbles which range in size from 1 cm to 5 cm. the chert clasts are generally moderately sorted and rounded to sub-rounded. the clasts are ironcemented and the matrix is composed of mediumto coarse-grained quartz sand (figs. 6f). the conglomerate clasts exhibit crude to random orientation forming tabular-shaped stacking which may represent bedding planes but cannot be used to deduce the paleocurrent direction. these conglomerates represent the uppermost facies in the outcrops they are found in.table 1. location coordinates and profiles thicknesses of the measured outcrops profile no. latitude longitude profile thickness (m) 1 16° 22’ 22.9” 33° 15’ 33.2” 62 2 16° 24’ 51.8” 33° 18’ 47.1” 8 3 16° 24’ 23.8” 33° 20’ 6.1” 26 4 16° 24’ 18.1” 33° 20’ 20.3” 6 5 16° 27’ 32.7” 33° 19’ 40.4” 5 6 16° 28’ 9.9” 33° 19’ 17.2” 21 7 16° 24’ 28.1” 33° 20’ 69.2” 12 8 16° 16’ 23.8” 33° 16’ 47.3” 4 9 16° 12’ 49.7” 33° 22’ 54.8” 17 10 16° 24’ 38.7” 33° 19’ 8.4” 11 11 16° 24’ 41.4” 33° 19’ 5.3” 12 12 33° 17’ 19.8” 33° 17’ 24” 8 13 16° 21’ 29.3” 33° 17’ 19.8” 30 total thickness 222 figure 4. relative abundance (in percentages) of lithofacies types in the total succession of the studied locations. gt= stratified conglomerate, gmm= matrixsupported massive conglomerate, st= trough cross-bedded sandstone, sm= massive sandstone, sh= horizontally bedded sandstone, sl= laminated sandstone, fm= massive mudstone, and fc= iron crusts. 113depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan table 2. summary of the description and interpretation of lithofacies encountered in the study area (modified after miall, 2006). lithofacies code lithofacies name structure textural characteristics interpretation gt stratified conglomerate cross stratification pebbles (1 cm-5 cm), moderately poorly sorted deposition by traction currents and highenergy flows (ghazi and mountney, 2009; bussert et al., 2018) gmm matrix-supported massive conglomerate massive pebbles (4 mm-5 cm), poorly sorted plastic debris flows (miall, 1978) st trough cross-bedded sandstone trough cross-bedding medium to pebbly coarse-grained sand, moderately to poorly sorted high energy and low sinuosity streams (rust, 1977; hjellbakk, 1997; miall, 2006) sm massive sandstone massive fine to pebbly coarse-grained sand, moderately to poorly sorted rapid deposition by sediment-gravity flow (miall, 2006) sh horizontally bedded sandstone horizontal bedding fine to medium-grained sand, moderately sorted upper flow regime or high energy currents (hjellbakk, 1997; miall, 2006) sl laminated sandstone parallel lamination very fine to fine-grained sand, moderately sorted deposition in the terminal waning stage of seasonal flood events (miall, 2006) fm massive mudstone massive silt and clay deposition from suspension during periods of quiet waters (miall, 2006) fc iron crusts massive, nodular, and colloform structures no grains chemical precipitation and diagenesis (carozzi, 1960; boggs, 2006) interpretation: the slightly inclined cross stratification indicates that the conglomerates were mainly transported by traction currents and deposited in high-energy flows (ghazi and mountney, 2009; bussert et al., 2018). the lower erosional boundary in these facies also supports deposition by high-energy flows (miall, 2006). the clasts of this facies exhibit crude orientation and tabular-shaped stacking which indicate deposition under a high-velocity flow in fluvial channels (kim et al., 2009, ghazi and mountney, 2009). moreover, the crude lamination and the high matrix percentage indicate deposition under sediment gravity flow as channel fill (miall, 2006). this lithofacies is not part of the shendi formation and it represents part of the newly assigned late mioceneearly pliocene wadi awatib conglomerate but it was included because it represents the upper limit of shendi formation (bussert et al., 2018). 4.1.2 matrix-supported massive conglomerate (gmm) this facies is massive and is mainly composed of extraformational, poorly sorted, and well-rounded to rounded quartz pebbles of various colors. the clasts range in size from 4 mm to 5 cm, and they are supported by a matrix of quartz sandstone composition and are cemented by iron oxides (figs. 6d and 7b). this facies is often underlain and overlain by trough cross-bedded sandstone facies (st) (fig. 7a and 7b). the lower boundary is dominantly erosional, slightly concave, and contains intraformational mudstone clasts. the upper contacts are sharp and sometimes gradational. interpretation: this facies is deposited by plastic debris flow, and it is attributed to longitudinal bars and lag deposits (miall, 1978). the slightly concave up lower boundary of the gmm facies is a result of erosion at the beginning of the deposition (miall, 2006). 4.1.3 trough cross-bedded sandstone facies (st) the trough cross-bedded sandstone facies (st) is the predominant facies in the study area, representing more than 70% of the total succession. moreover, this facies is present in all of the studied locations (figs. 6-11). this lithofacies has varying colors such as red, yellow, and white, but the surface weathering color is light brown. the sand grains are commonly medium to coarse-grained and sometimes pebbly, especially in the base of the bed. the grains are moderately to poorly sorted and sub-angular to sub-rounded. several sets and cosets of trough cross-bedding are present in this facies. the thickness of the individual set varies significantly from one bed to another, and it ranges from small-scale sets of an approximate thickness of 20 cm to 1m in large-scale sets. the cross-bedding allows the measurement of paleocurrent directions and this was the only structure used for such measurements. it is often underlain by conglomerates or the lower cycle mudstones. the lower boundary of this facies is usually erosional. interpretation: this facies is interpreted to be deposited as threedimensional (3d) dunes that are attributed to high energy rivers or higher velocity currents which are some of the characteristics of braided, low sinuosity streams (rust, 1977; hjellbakk, 1997; miall, 2006). in contrast, lower energy, high sinuosity streams are characterized by two-dimensional dunes which result in the formation of planar cross-beds (rust, 1977; miall, 2006). 4.1.4 massive sandstone (sm) this facies is massive and is composed of fine-to coarse-grained, moderately-to poorly-sorted sand with some pebbles (fig. 6a).. the color is reddish, yellowish to white. it is often found underlain by trough cross-bedded sandstone facies (st). the lower boundary is often a non-sharp, gradual contact, while the upper contact usually ends with massive mudstone facies (fm). interpretation: the massive sandstone is suggested to be deposited by rapid sedimentation during falling flow conditions (miall, 2006; aloui et al., 2012). the rapid dumping of sediments does not allow the hydraulic factor to produce any sedimentary structures (hjellbakk, 1997). 4.1.5 horizontally bedded sandstone (sh) the horizontally bedded sandstone (sh) consists of fine-to-mediumgrained, moderately-sorted sandstones that are characterized by horizontal bedding. this facies is commonly found overlain and underlain by trough cross-bedded sandstone facies (st) (fig. 9d). the thinkness of the beds ranges between 5 to 20 cm. the lower and upper boundaries are usually sharp but sometimes the lower boundary is gradual. interpretation: the horizontally bedded sandstone is interpreted to be deposited as plane beds under conditions of upper flow regime (miall, 2006). this facies can as well be interpreted as bar-top of large flat sand bars that are deposited by high energy currents (hjellbakk, 1997). 4.1.6 laminated sandstone (sl) the laminated sandstone (sl) represents only 2% of the total succession of the studied sections. this facies consists of very fine to fine, thin horizontally parallel laminated sandstone intercalated in some cases with iron crusts (fc) (fig. 6a). this facies is commonly overlain by massive mudstone facies (fm) and is often underlain by trough cross-bedded sandstone facies (st) with a gradual contact. interpretation: the small-scale sedimentary structures and fine-grained lithology indicate deposition as bar-top sand sheets during the terminal waning stage of seasonal flood events (miall, 2006). 114 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi figure 5. interpretation of selected vertical sedimentary profiles of the studied outcrops in locations 1, 2, 3, 6, 10, and 11. 115depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 6. a) lateral profile of location 1 outcrop showing sandy bedforms element (sb) (yellow-shaded) overlain by overbank fines elements (of) (green-shaded) and channel fill elements (ch) (red-shaded). b) stratified conglomerate (gt) containing mud clasts at the base and overlaying trough cross-bedded sandstone facies (st) with an erosional contact. c) small-scale trough cross-bedded sandstone (st) showing foresets and first-order bounding surfaces. d) matrix-supported conglomerate (gmm) overlain by thin massive mudstone (fm) and a thin iron crust. e) and f) stratified conglomerate facies (gt) on top of the outcrop. hammer’s length is 30 cm, and the camera lens cover diameter is 5 cm. 116 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi 4.1.7 massive mudstone (fm) this facies represents a minor percentage, less than 2% of the total succession (fig. 7). this facies consists of silt and clay sizes. the color of the mudstones varies from grey to white. this facies is commonly found interbedded with thin crusts of iron (fc) (fig. 7d). it is often found overlain and underlain by trough cross-bedded sandstone facies (st) and the upper contact is usually erosional (figs 7c, 10d, and 11b). the mudstones commonly form thin beds (less than 50 cm). interpretation: this facies is deposited from suspension during periods of quiet waters, for example in overbank or abandoned channel deposits. however, the larger thicknesses of this facies can be interpreted as floodplain or swamp deposits (miall, 2006). the scarcity of fossils or root fossils in these mudstones indicates that the environment is not suitable for them due to climatic conditions (ghazi and mountney, 2009). 4.1.8 iron crusts (fc) the iron crusts (fc) show nodular and colloform structures and is generally found in association with and, often, on top of the massive mudstone facies (fm) (figs. 6a and 7d). this facies ranges in color from black to red. there are no obvious grains in this facies. generally, the thickness of this facies ranges from abundant thin crusts (up to 10 cm thick) and less frequent thicker beds which can reach more than 50 cm. interpretation: this facies is the only non-clastic facies encountered in the studied sections. the iron crusts are the results of the chemical precipitation of iron oxides that are dissolved in the water. the iron crusts are mainly deposited during periods of floods and water stabilization periods (boggs, 2006). however, a secondary origin of the iron oxides might be possible through early diagenesis caused by circulating solutions and oxidation of iron-bearing minerals (carozzi, 1960). 4.2 sedimentary architectural elements based on the classification of miall (1985, 2006), four architectural elements were recognized, namely; channel fill (ch), sandy bedforms (sb), gravelly bars and bedforms (gb), and overbank fines (of). the information on each element is summarized in table 2. the properties which characterize the different elements are facies association, geometries, and bounding surfaces (allen, 1983; miall, 1985). the sandy bedforms elements (sb) were found to be the predominant elements in the study area. selected lateral profiles showing the architectural elements identified in the studied outcrops are shown figures 6-11. 4.2.1. the hierarchy of bounding surfaces the hierarchy ranges from the first-order bounding surfaces of individual cross-bed sets to the large-scale fourth-scale bounding surface of paleovalleys and channel belts (allen, 1983; miall, 1985). the first-order bounding surface represents the contact between individual cross-bed sets such as the trough cross-bedded sandstone facies (st) sets (fig. 6c; allen, 1983; miall, 1985). the second-order surfaces bound co-sets or individual sedimentary bodies that are genetically related such as the co-sets formed by the stacking of individual groups of cross-bed sets in the trough cross-bedded sandstone facies (st) (e.g., fig. 8d and fig. 10a). the third-order surfaces are erosional surfaces that bound groups of sedimentary bodies (e.g., fig. 6a). fourth-order surfaces that define the boundaries of paleovalleys and channel belts have not been identified in the studied outcrops as they need a wider lateral exposure which is not available in the study area. a brief description and interpretation of the architectural elements found in the study area are given in the following section. 4.2.2 the sandy bedforms elements (sb) the sandy bedforms elements (sb) are composed of stacked sets, arranged into fining-upward packages of sandstone. these elements are the most dominant and they are found in all of the studied outcrops (figs. 6-11). a cyclic arrangement is occasionally encountered showing a fining upward style that typically starts with a basal matrix-supported pebbly trough crossbedded sandstone (st) facies that has an erosional lower contact with the underlying cycle (e.g., fig. 7a). such cycles might also contain horizontally bedded sandstones (sh), massive sandstone (sm) and laminated sandstone (sl) and mostly end with massive mudstone (fm) and iron crusts (fc). these fining upward cycles can be seen in some of the measured vertical profiles shown in fig. 5 and in the lateral profiles such as in fig. 6a and fig. 7a. the sb elements extend laterally up to hundreds of meters and usually occur in sheets geometry and the thickness ranges from 1 m up to 10 m. interpretation: the prevalence of trough cross-bedded sandstone facies (st) and large-scale ones in particular among the sandy bedforms elements (sb) indicate the deposition of the sandy bedforms elements (sb) in the active channels as a product of the migration of dune-scale bedforms (miall, 1985). 4.2.3 the channel fill elements (ch) the channel fill elements (ch) in the studied outcrops are generally characterized by erosive-based units that cut down into fine-grained floodplain sediments of the underlying cycle. the observed channels are often limited in lateral extension with the channel base often forming irregular and concavesummary of the architectural elements in the study area, their facies association, geometry, and interpretation (modified after miall, 1985). architectural element illustration facies association typical geometry interpretation channel (ch) gmm, st lenses height: 0.5 – 2 m width: 5 – 20 m channel fill (allen, 1983; miall, 1985) sandy bedforms (sb) st, sm, sh, sl sheets height: 1 – 10 m width: 10 – >100 m three-dimensional dunes and sand bars (miall, 1985) gravelly bedforms (gb) gmm lenses, wedges height: 0.5 – 1 m width: 1 – 20 m gravel bars (miall, 1985; ghazi & mountney, 2009 overbank fines (of) fm, fc lenses, sheets h: 0.2 – 1 m w: 2 – 20 m overbank deposits (miall, 2006; ghazi & mountney, 2009) 117depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 7. a) lateral profile of location 10 outcrop showing sandy bedforms elements (sb) (yellow-shaded), gravelly bars and bedforms element (gb) (orange-shaded), channel fill elements (ch) (red-shaded) and overbank fines elements (of) (green-shaded). b) matrix-supported massive conglomerate facies (gmm) overlain by trough cross-bedded sandstone facies (st). c) massive mudstone facies (fm) overlain by trough cross-bedded sandstone facies (st). d) interbedded massive mudstone facies (fm) and iron crusts facies (fc). the hammer length is 30 cm. figure 8. a) lateral profile of location 4 showing sandy bedforms elements (sb) (yellow-shaded). b, c) a plan view of the trough cross-bedded sandstone facies (st). d) trough cross-bedded sandstone facies (st) in cross-section view. at the base the sandstone is pebbly and contains mudclasts. 118 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi figure 9. a) general view of location 11 outcrop. b), c) lateral profiles of location (11) showing sandy bedforms element (sb) (yellow-shaded), gravelly bars and bedforms element (gb) (orange-shaded) and overbank fines element (of) (green-shaded). d) trough cross-bedded sandstone facies (st) overlain by horizontally bedded sandstone facies (sh) with interbedded thin matrix-supported massive conglomerate (gmm). e), f) large-scale trough cross beds. hammer’s length is 30 cm, and the camera lens cover diameter is 5 cm. up erosional surfaces. the fill of the channel elements is characterized by a fining upward sequence that typically starts at the base with matrix-supported massive conglomerate (gmm) and gradually fines into trough cross-bedded sandstone (st) (e.g., fig. 7a). the base of each channel represents a third order erosional boundary. the lower boundary is often slightly concave-up and might show some mud clasts while the upper boundary is often a sharp, nonerosional contact. location 10 (fig. 7a) shows a minor channel that has a width of about 3 m and a depth of 1 m. the limited lateral extension of the outcrops does not allow the recognition of major channels that represent the paleovalley that extend for kilometers. in contrast, the minor channels that infill the major channels can have a lateral extension of few meters. interpretation: the presence of these elements in a fining upward order can prove the existence of the channel (miall, 2006). the channel fill elements (ch) are also interpreted as fluvial channels (miall, 1985). the lower erosional surface of these elements indicate deposition by high-energy flows (miall, 2006). allen (1983) attributed the stacking of minor channels to deposition in low sinuosity streams. this aggradation of channels and their shallow depth (e.g., fig. 7a) suggest deposition in a braided fluvial system where the relatively rapid and continuous migration of channels does not allow for deeper incisions (miall, 1985; bristow, 1987) 119depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 10. a) lateral profile of location 3 outcrop showing sandy bedforms elements (yellow-shaded) and a thin overbank fines element (of) (green-shaded) which is shown in a close up view in d. b) plan view of small-scale trough cross-bedded sandstone (st). c) small-scale trough cross-bedded sandstone facies (st). d) massive mudstone facies (fm) interbedded with very coarse trough crossbedded sandstone facies (st). e) large-scale trough cross-bedded sandstone (st). the hammer length is 30 cm, camera lens cover diameter is 5 cm. 120 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi figure 11. lateral profile of location 7 outcrop showing sandy bedforms elements (yellow-shaded), channel fill element (ch) (red-shaded) and overbank fines elements (of) (green-shaded). b) massive mudstone facies (fm) overlain by trough cross-bedded sandstone (st). c) pebbly trough cross-bedded sandstone (st). the camera lens cover is 5 cm. 4.2.4 gravelly bars and bedforms elements (gb) gravelly bars and bedforms elements (gb) in the study area consist of matrix-supported massive conglomerate facies (gmm) (fig. 7a and fig. 9b). gb elements are always found interbedded with channel fill elements (ch) and sandy bedforms elements (sb). the lower boundary of these elements is usually erosional and often corresponds to the third-order bounding surfaces of the sandy bedform elements (sb) as in fig. 7a and fig. 9b. geometrically, these elements usually have elongated, wedged shapes with limited lateral extensions of about 10 m. the thickness approximately ranges between 0.5 m to 1 m. interpretation: the gravelly bars and bedforms elements (gb) are interpreted as longitudinal bars or lag deposits that migrate downstream during floods and high water (miall, 1985, 2006; ghazi & mountney, 2009). the longitudinal bars interpretation is supported by the wedged shape and the limited lateral extension of these elements (miall, 1985). 4.2.5 overbank fines elements (of) overbank fines elements (of) represent thin blankets of mudstone (fm) with very fine-grained sandstone beds intercalated with thin iron crusts (fc) (e.g., fig. 7a and fig. 11a). these elements occur in thin wedge-shaped lenses and blankets. these elements typically occur at the top of the sandy bedform elements (sb) as seen in fig. 7a. they commonly occur in wedge shapes (e.g., fig. 11a) and sheets (e.g., fig. 7a). the thickness is generally thin (less than 50 cm) and the lateral extension ranges from 5 m to 15 m. 121depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 12. a) ternary plot diagram for the classification of sandstones based on the quartz-feldspar-lithic fragments (qfl) framework composition percentages (after dott, 1964; modified by pettijohn et al., 1987). the inset shows a close up view of the region where samples fall. the majority of the samples fall in the quartz arenite region, while two samples fall in the sublitharenite region. b) ternary plot diagram of qfl framework composition percentages for tectonic setting (modified after dickinson and suczek, 1979. the inset shows a close up view of the region where samples fall. the majority of the samples fall in the interior craton region, while only one samples fall in the recycled orogen region. interpretation: the fine grained nature of the overbank fines elements (of) suggests the deposition as floodplain deposits in distal areas to the main channel during waning flood events (miall, 1977a, 2006; ghazi and mountney, 2009). 4.3 sandstone petrography the results of sandstone petrography and the classification of each sandstone sample are shown in table 4. quartz-feldspar-lithic fragments (qfl) framework composition ternary plot for the classification of sandstones based on the concept of dott (1964) and pettijohn et al. (1987) is shown in figure 12a. a ternary plot for the tectonic setting of the sandstones based on the concept of dickinson and suczek (1979) is shown in figure 12b. the classification of the sandstone samples revealed that the majority are quartz arenite, while two samples are sublitharenite. in all samples, quartz grains represent about 90% or more of the compositional framework, while feldspars and lithic fragments occur in low percentages (0-10%). quartz grains generally appear colorless in plane-polarized light and contain minor inclusions and cracks (figs. 13a and 13c). the quartz grains are predominantly mono-crystalline (fig. 13b, fig. 13d, fig. 14b, and fig. 14d) representing approximately 90% while the polycrystalline grains (figs. 13b and fig. 14b) are minor (10%). generally, the poly-crystalline grains are larger than the mono-crystalline grains and contain sutured crystal contacts (fig. 13b and fig. 14b). stretched quartz grains also occur in lesser abundance (fig. 13d). lithic fragments generally represent less than 10% of the total framework composition (figs. 13e and fig. 13f). generally, the sandstone samples contain a relatively high amount of muscovite mica (figs. 14c and fig. 14d) (approximately 5%). the source of muscovite is suggested to be granitic or schistose rock (adams et al., 1988). the feldspars approximately represent less than 4% of the compositional framework. feldspar grains found in the sandstones are altered, and some grains show polysynthetic twinning which characterizes plagioclase feldspars (figs. 14a and fig. 14b). generally, the sandstones are moderately to poorly sorted, and the roundness of the grains is mainly sub-angular to sub-rounded. the main cementing material is iron oxides, and the matrix is generally less than 10%. according to folk (1974), these sandstone samples are mostly texturally submature. the textural maturity is not in concordance with the high mineralogical maturity. this discordance can be attributed to a high degree of chemical weathering at the time of transportation and deposition of the sediments (late cretaceous), which resulted in the leaching of less stable minerals (chandler, 1988; johnsson et al., 1988). this is evidenced by the tropical, warm, and humid climate that prevailed in the area during that period (germann et al., 1994). however, the lower percentages of less stable minerals such as feldspar and mica in the sandstone may also be attributed to the mineral composition of the sources rocks (boggs, 2006). the sandstones are mostly cemented by iron oxides that are believed to be of a secondary origin and deposited during diagenesis by circulating iron-bearing solutions and/or oxidation of ironbearing minerals (carozzi, 1960). porosity values range from 5% to 15% and it is suggested that the porosity was reduced due to the precipitation of iron oxides by diagenetic processes. 4.4 the provenance of the sediments the sandstones in the study area are mainly quartz arenites as seen from the plots of quartz-feldspar-lithic fragments (qfl) percentages (fig. 12a). the general poor sorting of the sandstones and the angular to sub-angular roundness of the grains indicated that they are sub-mature and implied that they were deposited in a relatively proximal area to their source. the sandstones exhibit point contacts (fig. 13a and fig. 13b) of framework grains with rare straight contacts and suggest a shallow subsurface burial of sediments (selley, 2000). the cross plot of the percentages of the framework compositions (qfl) based on the concept of dickinson and suczek (1979; fig. 12b) showed that the sandstones are associated with a cratonic interior tectonic setting. this supports the findings that mesozoic sediments of rift basins in northern and eastern africa were derived from high cratons (salama, 1997). the craton interior setting implies that the sandstones are compositionally mature, and are probably derived from granitoid and gneissic parent rocks (dickinson et al., 1983; radwan, 2022b). moreover, the occurrence of mica (fig. 14d) in most of the samples indicates an igneous or metamorphic source as well (boggs, 2006). however, the occurrence of polycrystalline quartz (figs. 13b and fig. 14b) and stretched quartz (fig. 13d), with undulatory extinction indicates a metamorphic source rock (boggs, 2006). in addition, the presence of sutured quartz crystals within the composite quartz (figs. 13b and fig. 14b) also supports the metamorphic source (adams et al., 1988). the paleocurrent data showed an average mean of 285.3°, which portrays a southeast provenance for the sediments (fig. 15). the butana complex, about 100 km southeast of the study area (fig. 2), is the probable source of the sediments. this complex is mainly composed of high-grade para quartzofeldspathic gneisses and low-grade metasedimentary-metavolcanic sequences (whiteman, 1971). it is important to note that using the petrographic analysis 122 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi table 4. the classification of sandstones based on the quartz-feldspar-lithic fragments framework composition. sample no. facies type texture quartz % feldspar % lithic fragments % matrix % porosity % classification l13 p1 s1 st medium to coarse-grained, poorly to moderately sorted 94.85 3.09 2.06 10% 15% quartz arenite l10 p1 s8 sm fine-grained, moderately to well sorted 94.90 2.04 3.06 5% 10% quartz arenite l6 p1 s3 sm coarse-grained, moderately sorted 93.88 3.06 3.06 5% 10% quartz arenite l5 p1 s1 st coarse to very coarsegrained, poorly sorted 95.88 2.06 2.06 5% 15% quartz arenite l3 p1 s2 st very coarse-grained, poorly to very poorly sorted 94.85 3.09 2.06 10% 15% quartz arenite l1 p1 s6 st fine to very fine-grained, moderately sorted 93.88 3.06 3.06 2% 10% quartz arenite l1 p1 s4 sl fine to medium-grained, moderately sorted 93.00 3.00 4.00 10% 10% quartz arenite l1 p1 s2 sm medium-grained, moderately sorted 93.88 2.04 4.08 5% 15% quartz arenite l1 p1 s1 st medium to coarse-grained, moderately to poorly sorted 91.92 3.03 5.05 10% 15% sublitharenite l3 p1 s6 sm fine-grained, well sorted 93.88 3.06 3.06 5% 10% quartz arenite p1 s2 st fine to medium-grained, moderately to well sorted 90.00 5.00 5.00 10% 5% quartz arenite p1 s14 st medium to coarse-grained, poorly to moderately sorted 95.00 5.00 0.00 10% 10% quartz arenite p1 s15 sm fine to medium-grained, poorly sorted 90.00 0.00 10.00 10% 5% sublitharenite p2 s2 st medium to coarse-grained, poorly to moderately sorted 95.00 5.00 0.00 10% 10% quartz arenite p2 s10 sm medium to coarse-grained, poorly sorted 95.00 5.00 0.00 10% 10% quartz arenite and paleocurrent directions data without other supporting analyses to deduce the source of the deposits has some limitations (boggs, 2006). therefore, to accurately identify the exact provenance of the sediments of the shendi formation further detailed mineralogical and geochemical investigations that are beyond the scope of this study are required. these include heavy mineral analysis, rare and trace elements analysis, zircon dating, and more detailed petrographic investigations. 4.5 environment of deposition the abundance of coarse-to medium-grained sandstones and conglomerates as well as the rare occurrence of fine sandstones, silts, and mudstones indicate deposition in a high-energy stream (miall, 2006). this is supported by the presence of trough cross-bedded sandstone (st) as the predominant facies. trough cross-bedding defined by planar or concaveup erosional surfaces in multi-story large-scale sandstone bodies suggests deposition by high-energy streams (allen, 1983; miall, 1985). the facies associations found in the study area include those that are described by miall (1977a) as the most common facies in modern and ancient braided rivers. these facies are matrix-supported massive conglomerate (gmm), stratified conglomerate (gt), trough cross-bedded sandstone (st), laminated sandstone (sl), and horizontally bedded sandstone (sh). the deposition of these sediments is suggested to have occurred by channel aggradation, as there is no obvious evidence of lateral accretion (miall, 1977a). the measured vertical profiles are similar to some extent to the south saskatchewan type described in miall (1977b) and cant (1977). the south saskatchewan river in canada is a modern, medium-sized sandy braided river that has cyclic deposits of mainly medium to coarse trough cross-bedded sandstone facies (st) representing channel deposits, with minor sand flat deposits such as parallel laminated sandstone facies (sl) as shown in figure 1 of miall (1977b). the floodplain deposits of that river occur at the top of these deposits but in minor percentages (cant, 1977). similar depositional cycles are present in the studied successions, and they typically start with pebbly coarse trough cross-bedded sandstone facies (st) that fines upward and end with massive mudstone facies (fm) as can be seen in the vertical profiles shown in figure 5 and the lateral profiles such as in figure 7a. most of the lateral profiles show good lateral continuity of channel deposits, as can be seen in figure 7a and figure 10a, which suggests that deposition was mainly under unconfined flow regimes (kim et al., 2009). one of the characteristic features of braided rivers is the aggradation of channels, and this can be seen in the channel fill elements (ch) at location 10 in the study area (fig. 7a; bristow, 1987). the sandy bedform elements (sb) are interpreted as three-dimensional dunes which are one of the morphological characteristics of braided rivers (miall, 1977b). similarly, the longitudinal bars or the gravelly bar elements (gb) are attributed to braided rivers (miall, 1977b). the almost complete amalgamation of channel fill facies, mainly composed of trough cross-bedded sandstone facies (st), with very minor overbank deposits suggests lateral migration of channels, and such channel architecture implies the deposition in the proximal zone, near to source area (nichols and fisher, 2007). the multi-story channel fill is deposited as a result of channel avulsion and can be attributed to deposition in braided alluvial plains (tedesco et al., 2010). sand bars occur creating several channels within the main channel which is characteristic of braided rivers. the presence of massive sandstone facies (sm), even though in a low percentage (7%) in this braided system, can be justified by changes in current velocity. massive mudstone facies (fm) is also one of the fine-grained facies that can occur in a braided river but in a minor percentage compared to meandering rivers (miall, 2006). they are interpreted to be deposited during times of floods within floodplains and swamps (jo and chough, 2001; boggs, 2006). 123depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 13. photomicrographs of the studied sandstone. a) and b) sample (l5p1s1) from location 5 (st facies, quartz arenite, v. coarse to coarse, poorly sorted, sub-rounded to sub-angular grains, cemented by iron oxides, matrix < 5%) shows qp, qm, and stretched quartz. c) and d) sample (l3p1s2) from location 3 (st facies, quartz wacke, v. coarse, poorly to v. poorly sorted, sub-rounded to sub-angular grains, cemented with iron oxides, the matrix is 10%, shows pore space and stretched quartz. e) and f) sample (l3p1s2) from location 3 shows a large lithic fragment. str qz= stretched quartz, qm= monocrystalline quartz, qp= polycrystalline quartz, rf= rock fragment. plane-polarized light photomicrographs are shown to the left, and cross-polarized light to the right. 124 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi a proposed conceptual paleoenvironmental model for the study area is shown in figure 15. at the time of deposition of the sediments, the study area was located at the proximal part of a braided river that flowed approximately to the northwest. whereas the source area is to the southeast. most braided rivers show longitudinal sand or gravel bars in their main streams (miall, 1977a). these are symbolized by lenticular bodies within the channels (fig. 11). the findings of oshi (2004) implied that the fluvial deposits of the shendi-atbara basin in umm ali area and its vicinity, northeast of the study area (fig. 2), are distal or non-proximal to their source. bussert (1988) in his sedimentological studies of naga area and the adjacent wadi awatib concluded that the sediments were deposited in a braided plain, which corroborates the findings in this study. accordingly, it can be deduced that the lateral variation in facies in the basin from proximal to distal is approximately from the south to the north. the tectonic activities during the late jurassic to early cretaceous period introduced major changes in the paleogeography of northeast africa, including sudan (reyment & dingle, 1987; fairhead, 1988a). such events led to the formation of multiple braided fluvial systems in the region (klitzsch figure 14. photomicrographs of the studied sandstone. a) and b) sample (l6p1s3) from location 6 (sm facies, quartz arenite, coarse, moderately sorted, sub-angular to angular grains, cemented by iron oxides, matrix < 5%) shows altered feldspar. c) and d) samples (l10p1s8) from location 10 (sm facies, quartz arenite, fine, moderately to well-sorted, sub-angular to angular grains, cemented with iron oxides, the matrix is 5%) shows elongated muscovite grain. flds= feldspar, m= mica (muscovite), qm= monocrystalline quartz, qp= polycrystalline quartz. plane-polarized light photomicrographs are shown to the left and cross-polarized light to the right. and squyres, 1990). as mentioned previously, the studied part of the shendi formation in musawwarat-naga area was deposited by a braided fluvial system. the paleocurrent directions pattern (fig. 15) has a considerably wide dispersion that might be due to many factors such as deposition time, measurement location, and the nature of the river bars (selley, 1968; shukla et al., 1999). however, the major cause of the difference of paleocurrent directions might be attributed to the tectonic activities, such as uplift and subsidence, that prevailed during the formation of shendi-atbara basin and led to changes in the mainstream direction through time (fairhead, 1988a). 4.6 the implications for hydrocarbon exploration the majority of the hydrocarbon reservoirs in the sedimentary basins of sudan are composed primarily of sandstones deposited in proximal fluvial settings (schull, 1988). findings from this study, therefore, provide an outcrop equivalent that might aid in studying the paleoenvironment of subsurface reservoirs of these basins. such analogs can help in enhancing the understanding and prediction of the proximal fluvial depositional style. 125depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the upper cretaceous shendi formation in musawwarat-naga area, shendi-atbara basin, sudan figure 15. conceptual depositional model of the study area (not according to scale). the lateral and vertical profiles show sandy bedform elements (sb), gravely bars and bedforms (gb) and overbank fines elements (of). the architectural elements recognized in the study area show that the degree of sedimentologic heterogeneity is relatively low as evidenced by the predominance of the channel fill elements (ch) and sandy bedforms elements (sb) as compared to the rare presence of the impermeable rocks represented by the overbank fines elements (of). on the other hand, the general coarsegrained nature of the studied successions along with the scarcity of feldspars and lithic fragments positively contribute to the quality of a probable reservoir in the subsurface (taylor et al., 2010). while the porosity values determined in sandstone thin sections are relatively low (5%-15%) when it comes to reservoir quality, those values are suggested to be affected by diagenetic processes that filled the pores with iron oxides. in the subsurface of the muglad basin, iron oxides were found in low percentages in the pores of sandstones in the reservoirs of ghazal, zarga and bentiu formations (makeen et al., 2021a; makeen et al., 2021b). however, the porosity of these reservoirs is still relatively good (more than 20%) and this might be a result of other factors such as the dissolution of less stable minerals like feldspars (makeen et al., 2021b). therefore, a detailed petrophysical study of the potential subsurface reservoirs will be required to confirm whether the porosity of the sandstones of these reservoirs is affected by iron oxides. further research work is also suggested to focus on quantitative facies analysis which is essential for accurate depositional modeling. furthermore, a detailed three-dimensional (3d) study of the fluvial architecture of the outcrops in this area and other areas in the region should be conducted to characterize the reservoir properties, connectivity, and heterogeneity in detail. since the characteristics of the reservoirs in the subsurface can be highly similar to the surface analog, such reservoir properties can now be studied in detail from outcrops by digital modeling. those methods involve three-dimensional aerial photographing (image modeling) and terrestrial laser scanning or ground-based lidar techniques (e.g., bellian et al., 2005; keogh et al., 2007; zhanfeng et al., 2015; magalhaes et al., 2017; siddiqui et al., 2018). such studies will help in producing digital outcrop models (dom) that can be compared with models from the subsurface to perform a reality check (keogh et al, 2007). as three-dimensional subsurface reservoir geological models represent an integral part of the oil and gas industry (e.g., radwan, 2022a: ali et al., 2022), the comparison between outcrop analogs and subsurface models will probably result in more realistic 3d geological models, and can as well help in developing a better understanding of the reservoir properties, geometry, heterogeneity, and connectivity in the subsurface (keogh et al, 2007). additionally, such realistic models might be useful in enhancing our flow simulation models in hydrocarbon systems (bellian et al., 2005). 5. conclusions the sedimentology of the outcropping part of the shendi formation in musawwarat–naga area was studied using facies, architectural and petrographic analysis to deduce the depositional environment and the provenance of the sediments. the identified lithofacies association consists of eight facies with a dominance of trough cross-bedded sandstone (st). the architectural elements identified, which involve sandy bedforms (sb), channel fill (ch), gravelly bars and bedforms (gb), and overbank fines (of) elements, indicate deposition in a fluvial environment of a braided, proximal setting. 126 mohamed diaeldin babkir hassan babkir, matthew e. nton, ali a. m. eisawi the analyzed sandstones are predominantly quartz arenites. the plot of quartz-feldspar-lithic fragments (qfl) percentages on the ternary diagram showed that the tectonic setting of the sediments is within the craton interior. the provenance is suggested to be the butana complex, southeast of the study area as evidenced by petrographic results and mean paleocurrent directions of 285.3°. the low degree of sedimentologic heterogeneity along with the general coarse-grained nature of the studied successions might suggest that a probable subsurface reservoir might be promising. however, the presence of diagenetic products such as the iron oxides might negatively affect the porosity and the permeability of the potential reservoir. therefore, further research work is recommended to focus on these aspects and to create high-resolution fluvial reservoir analog models for subsurface hydrocarbon exploration as the outcrops in the area are suitable for such a study. acknowledgments this work was financially supported by the pan african university life and earth sciences institute (paulesi). the faculty of petroleum and minerals at al neelain university is acknowledged for providing fieldwork equipment. we also thank the sudan civilization institute for providing us with accommodation in their camp throughout the fieldwork. mohamed sorkatti, modathir omar, mohamed bakri, and nasradeen adam are greatly acknowledged for their help and assistance during field and laboratory work. finally, we thank dr. ibrahim a. a. babikir for providing the geologic map of the area and for his useful suggestions. references abdel-fattah, m. i., sen, s., abuzied, s. m., abioui, m., radwan, a. e., & benssaou, m. 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(2015). three-dimensional carbonate reservoir geomodeling based on the digital outcrop model. petroleum exploration and development, 42(3), 358–368. https://doi.org/10.1016/s1876-3804(15)30026-4 https://doi.org/10.1080/02724634.2016.1115408 https://doi.org/10.1306/703c9965-1707-11d7-8645000102c1865d https://doi.org/10.1306/703c9965-1707-11d7-8645000102c1865d https://doi.org/10.1016/b978-0-12-636375-3.x5001-0 https://doi.org/10.2110/jsr.69.992 https://doi.org/10.2110/jsr.69.992 https://doi.org/10.1016/j.gsf.2018.04.008 https://doi.org/10.1016/j.gsf.2018.04.008 https://doi.org/10.1306/04211009123 https://doi.org/https http://doi.org/10.1016/j.jseaes.2022.105409 https://doi.org/10.1344/105.000001583 https://doi.org/10.1016/s0195-6671(05)80024-2 https://doi.org/10.1016/s0195-6671(05)80024-2 https://doi.org/10.1016/s1876-3804(15)30026-4 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n1.99036 earth sciences research journal earth sci. res. j. vol. 27, no. 1 (march, 2023): 77 84 m et eo r o lo g y palabras clave: extremos calientes; temperatura máxima maximórum; funciones de distribución de probabilidad ajustadas; tendencias significativas. calculation of indicators of maximum extreme temperature in sinaloa state, northwestern mexico omar llanes cárdenas*, lorenzo cervantes arce, gabriel eduardo gonzález gonzález instituto politécnico nacional, centro interdisciplinario de investigación para el desarrollo integral regional, unidad sinaloa (ciidir–ipn–sinaloa) *corresponding author: ollanesc@ipn.mx cálculo de indicadores de temperatura máxima extrema en el estado de sinaloa, noroeste de méxico how to cite item: llanes-cárdenas, o., cervantes-arce, l., & gonzález-gonzález g.e. (2023). calculation of indicators of maximum extreme temperature in sinaloa state, northwestern mexico. earth sciences research journal, 27(1), 77-84 https:// doi.org/10.15446/esrj.v27n1.99036 record manuscript received: 14/10/2021 accepted for publication: 10/04/2023 keywords: hot extremes; maximum maximorum temperature; fitted probability distribution functions; significant trends. abstract one of the climate problems that causes the most environmental impact worldwide is the trend of increasing occurrence of events of maximum extreme temperature, signaled by indicators such as hot extremes (he) and maximum maximorum (highest maximum) temperature (mmt). these events can cause conditions ranging from severe droughts to heat stroke, which can cause death in any population. indicators of maximum extreme temperature in one of the most important agricultural areas in northwestern mexico were calculated based on significant trends (st) and adjusted return periods. to calculate the trends of the maximum extreme temperature, frequency (fr), annual average duration (aad), annual daily duration (add), intensity (in) of he, and mmt, the mann-kendall and sen’s slope tests were applied to data obtained for 19 weather stations from the climate computing database for the period 1982–2014. adjusted return periods (arp) were calculated for each indicator of maximum extreme temperature by fitting a probability distribution function. for the study area, the st and maximum extreme temperature shows a prevailing cooling trend. this can be deduced by observing the proportion of negative st compared with positive st. the highest positive magnitudes of st were recorded at stations cul (fr = 3.44 he dec-1), gut (aad = 6.15 day he-1 dec-1 and in = 13.62 °c dec-1), ixp (add = 35.00 day dec-1) and pot (mmt = 2.50 °c day-1 dec-1). for arp, the estimate of the average occurrence frequency of extreme events per100 years are fr = 6.11 he dec-1 (1 time), aad = 6.64 day he-1 dec-1 (4 times), add = 38.68 day dec-1 (1 time), in = 39.09 °c dec-1 (6 times) and mmt = 41.95 °c day-1 dec-1 (1 time). these findings are of key importance for the economic sectors related to agricultural production in the state known, at least to date, as “the breadbasket of mexico” (sinaloa). the results will help to develop adaptation/prevention measures before the coming socioeconomic and hydrological disasters. resumen uno de los problemas climáticos que causa mayor impacto ambiental a nivel mundial es la tendencia creciente de ocurrencia de eventos de temperatura máxima extrema, señalados por indicadores como los extremos calientes (ec) y la temperatura máxima maximórum (máxima más alta, tmm). estos eventos pueden causar condiciones que van desde sequías severas hasta golpes de calor, que pueden causar la muerte de cualquier población. se calcularon indicadores de temperatura máxima extrema en una de las zonas agrícolas más importantes del noroeste de méxico, con base en tendencias significativas (ts) y periodos de retorno ajustados (pra). para calcular las tendencias de temperatura máxima extrema, frecuencia (fr), duración media anual (dma), duración diaria anual (dda), intensidad (in) de ec y tmm, a datos obtenidos para 19 estaciones meteorológicas de la base de datos climate computing para el período 1982–2014, se le aplicaron las pruebas de mann-kendall y pendiente de sen. se calcularon los pra para cada indicador de temperatura máxima extrema mediante el ajuste de una función de distribución de probabilidad. para el área de estudio, las ts y la temperatura máxima extrema muestran una tendencia predominante de enfriamiento. esto puede deducirse observando la proporción de ts negativas en comparación con las ts positivas. las mayores magnitudes positivas de ts se registraron en las estaciones cul (fr = 3.44 ec dec–1), gut (dma = 6.15 día ec–1 dec–1 e in = 13.62 °c dec–1), ixp (dda = 35.00 día dec–1) y pot (tmm = 2.50 °c día–1 dec–1). para pra, la estimación de la frecuencia promedio de ocurrencia de eventos extremos por cada 100 años son: fr = 6.11 ec dec–1 (1 vez), dma = 6.64 día ec–1 dec–1 (4 veces), dda = 38.68 día dec–1 (1 vez), in = 39.09 °c dec–1 (6 veces) y tmm = 41.95 °c día–1 dec–1 (1 vez). estos hallazgos son de vital importancia para los sectores económicos relacionados con la producción agrícola, en el estado conocido, al menos hasta la fecha, como “el granero de méxico” (sinaloa). los resultados ayudarán a desarrollar medidas de adaptación/prevención ante los próximos desastres socioeconómicos e hidrológicos. mailto:ollanesc@ipn.mx https://doi.org/10.15446/esrj.v27n1.99036 https://doi.org/10.15446/esrj.v27n1.99036 78 omar llanes cárdenas, lorenzo cervantes arce, gabriel eduardo gonzález gonzález 1. introduction according to flores et al. (2012), the area in the state of sinaloa is predominantly classified as subtropical-intertropical, with zones of low topographic relief near the pacific ocean and extreme elevations in the mountain system known as the sierra madre occidental. the climatic variability– randomness of this area can be numerically modeled using criteria based on significant trends (st), adjusted return periods (arp) and fitted probability distribution functions (cohen et al., 2012; rustom 2012; franzke 2015; kienzle 2018; chapman et al., 2019; garry et al., 2019; latif & mustafa 2020; alves et al., 2021). these mathematical tools were used to characterize the climate variability (llanes et al., 2022b; llanes, 2023) and randomness of sinaloa; i.e., indicators of hot extremes (he, llanes et al., 2022a), since determination of the sts can be helpful for identifying zones where extreme climate changes occur, and knowledge about such changes has been demonstrated to be very useful for designing adaptation/prevention strategies to face natural disasters (blanco et al., 2014). globally, the he maximum maximorum (highest maximum) temperatures (mmt) are some of the most important indicators of change climate and can provide valuable information about the climate conditions to which the crops in a region are subjected (li et al., 2018; ruiz et al., 2005). the major crops produced in the area of sinaloa state, nicknamed “the breadbasket of mexico” (rincón 2012), are maize, sorghum, wheat, and beans, which are particularly vulnerable to temperature extremes and droughts (sagarpa 2015; gudko et al., 2021; gurung et al., 2021; habibi & meddi 2021). the negative effects of a high frequency of maximum extreme temperature (met) events can include human deaths as a consequence of gastrointestinal diseases and heat stroke (murage et al., 2017; ops 2019). with arps, the indicators of climate extremes can be estimated with very low uncertainty and the highest efficiency and reliability (aguilera 2007; campos 2014; lustenberger et al., 2014; deng & zhu 2021). worldwide, there are multiple studies that address st using non-parametric mann–kendall tests (kumar et al., 2023), as well as the use of fitted probability distribution functions, due principally to the importance, reliability, and efficiency of modeling (gonzález et al., 2011; fontanelli et al., 2019). in this study, based on he indicators (li et al., 2018) of the met and mmt (ruiz et al., 2005), the negative climate effects in sinaloa were calculated according to the criteria established by li et al. (2018). according to wang et al. (2016) and hochman et al. (2022), many studies indicate that extreme climate events may intensify or become more frequent and long-lasting with climate change, especially in semi-arid regions (demisse et al., 2022) such as the state of sinaloa (llanes et al., 2022b). due to the high interconnection with drought (horton et al., 2016; yuan et al., 2016), the he indicators calculated were frequency (fr), annual average duration (aad), annual daily duration (add) and intensity (in) (seneviratne et al., 2012). to learn about previous research related to this study, a review was done of the literature (documents available locally and worldwide). jaiswal et al. (2023) calculated extreme climate indices in india (hot days and daily maximum temperature) and llanes et al. (2022a) calculated indicators of met (he and mmt) in sinaloa, but did not calculate their trends or return periods through fitted probability distribution functions. the modeling tools of this study (st, fitted probability distribution functions and arp) provide a new method to estimate the frequency of occurrence of met indicators he (fr, aad, add and in) and mmt. this method can help to prevent impacts on agricultural yields through climatesmart practices such as drip irrigation, conservation agriculture, precision fertilization, protected agriculture, and crop substitution, which will withstand future hot temperature extremes (world bank et al., 2014), not only in sinaloa, but throughout mexico. for this reason, the goal of this investigation was to calculate met indicators based on sts and arps, to design climate-smart agricultural adaptation/prevention strategies. 2. material and methods 2.1 study area the state of sinaloa is located between 105° 23′ 32″ and 109° 26′ 52″ n, and 27° 02′ 32″ and 22° 28′ 02″ w (figure 1). it comprises approximately 2.9% of the total territory of the country (inegi 2016). longitude la tit ud e figure 1. location of the study area. state of sinaloa source: llanes et al. (2021) 2.2 data collection and processing daily maximum temperature data for april–october were obtained for 19 weather stations in the study area from the climate computing database (clicom, 2021) at the website http://clicom-mex.cicese.mx/mapa.html for the period 1982–2014 (table 1). missing data were estimated though the multiple imputation method (little & rubin, 1987) with the markov chain monte carlo algorithm (mcmc; austin & van buuren, 2022) since, according to puerta (2002) and salgado & largo (2018), with this method it is possible to obtain unbiased estimators that can be used efficiently. for this calculation, the xlstat version 2014 program was used. this program works in the windows excel environment and is widely used worldwide in climatology. these data were obtained from llanes et al. (2021) table 1. location of the weather stations. n um be r location weather station i.d. lat. n long. w 1 24.1 106.7 (aca) acatitan 2 25.3 107.5 (bad) badiraguato (dge) 3 24.8 107.4 (cul) culiacan (dge) 4 25.2 108.2 (epl) el playon 5 23.6 106.5 (equ) el quemado 6 25.1 107.4 (eva) el varejonal 7 25.6 108.5 (gua) guasave (dge) 8 25.3 107.2 (gut) guatenipa 9 24.0 106.6 (ixp) ixpalino 10 25.9 108.0 (jai) jaina 11 22.5 105.5 (lco) la concha 12 23.1 105.8 (lto) las tortugas 13 25.5 107.9 (moc) mocorito (dge) 14 23.0 105.7 (ota) otatitan 15 23.5 105.8 (pot) potrerillos 16 23.0 105.9 (ros) rosario 17 25.7 108.0 (sjo) san joaquín 18 24.8 107.2 (san) sanalona ii 19 24.5 107.0 (sca) santa cruz de alaya 79calculation of indicators of maximum extreme temperature in sinaloa state, northwestern mexico he indicators and mmt to obtain some of the most important variables used worldwide to characterize extreme weather, the following he indicators were calculated following li’s method (li et al., 2018): fr, aad, add and in. these authors define a he event as occurring when the value of daily maximum temperature exceeds the 85th and 90th percentiles (p85 and p90, respectively) for three consecutive days (stefanon et al., 2012, zschenderlein et al., 2019 and llanes et al., 2022a). the values of mmt were obtained according to ruiz et al. (2005), who defined these variables as the maximum monthly value in a series of maximum daily temperatures. to calculate the he indicators and mmt, the xlstat version 2014 program was used. the authors used the data of the he indicators fr, aad, add and in, previously calculated by llanes et al. (2022a). st of met indicators in order to determine whether local climate trends are cooling or warming, the sts of the annual average values of fr, aad, add, in and mmt were calculated. the mann-kendall test for trend analysis (mann, 1945; kendall, 1975, table 2) and sen’s slope test (sen 1968) for magnitude of trend (figure 3) were applied. to identify the presence of sts of met indicators, an absolute statistical threshold based on the standard normal distribution (zstd) = |1.96| was set with significance level α = 0.05 (ghasemi, 2015). for a better interpretation of the st variation data, the results were multiplied by ten; i.e., the time scales used and interpreted in this study were decadal. since single-year changes st may be small, decadal trends were chosen to make the trends clearer (rojas et al., 2010). to calculate the sts of the met indicators (mann–kendall and sen’s slope tests), past version 3.25 and xlstat version 2014 were used. statistical analysis of the fitted probability distribution functions to obtain the fitted probability distribution functions of he indicators and mmt, in this study the maximum likelihood estimation method was used. this mathematical method was applied due to its adaptability to the different he indicators and its precision in the estimation of large samples (data series for the period 1982–2014; gómez, 2020). the p-values of 16 probability distribution functions were calculated for the annual average values of each met indicator. this statistical method was used because is considered one of the most consistent, sufficient and asymptotically unbiased ways to identify the weather parameters of the fitted probability distribution functions (strupczewski et al., 2001). in addition to being more complex than the method of moments, it allows for greater rigor for all distributions (xlstat, 2022), and a greater reliability of fit for each indicator of he and mmt. in this study, the fitted probability distribution functions used were logarithmic (log), log-normal (lognormal), and generalized extreme values (gev) (chow et al., 1994; campos, 2016). log, lognormal and gev functions are shown in equations 1, 2 and 3, respectively. these calculations were carried out using xlstat version 2014. f x x , , ( ) �� �� �� ���� �� �� �� �� 1 1+e / (1) f x k x k , , , exp�� �� �� �� �� �� �� �� ���� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� 1 1 +k (2) f x erf x , , ln �� �� �� �� �� �� �� ���� �� ���� �� �� ���� �� �� �� �� �� �� �� �� 1 2 1 2 � (3) adjusted return periods to obtain the cumulative distribution function f(x) of each fitted probability distribution function, the variable (x) and the following parameters were substituted into equations 1, 2 and 3: the location (μ) and scale (σ) parameters in the log and lognormal functions; and the shape (k), scale (β) and location (μ) parameters into the gev distribution. equation 4 was used to calculate the adjusted return periods (arps, chow et al., 1994; woong et al., 2003). to calculate the frequency of occurrence (arps, equation 4) of extreme events for each indicator of met, with equations 1, 2 and 3, implicit in the function f(x), the values of the parameters μ, σ, k, and β , (obtained with the xlstat version 2014 software) were aggregated, respectively. arp f x �� �� 1 1 ( ) (4) the modeling tools of this study (st, fitted probability distribution functions and arp), associated with the indicators of met; he (fr, aad, add and in) and mmt, serve to estimate the frequency of met occurrence. this methodology has also been applied by chow et al. (1994), ariza (2013), gomáriz (2015) and campos (2016). to calculate the arps, xlstat version 2014, surfer version 10.0 and coreldraw version x7 were used. 3. results and discussion 3.1 mann–kendall test for sts figure 2 shows the weather stations with warming or cooling sts (st positive or st negative, respectively) of the indicators of met; fr, aad, add, in and mmt. the number of negative and positive sts for indicators of met were fr 6 negative and 3 positive; aad 5 negative and 3 positive; add 6 negative and 3 positive; in 5 negative and 4 positive; and mmt 6 negative and 3 positive. the weather stations that showed negative sts in the five met indicators were jai, ota and ros, and the highest value of add was recorded at the ros station with st=−3.60. only one station showed a highest positive st in the five met indicators; namely, gut with st=3.90. these results are consistent with those of llanes et al. (2015) in that although these authors calculated trends for the annual average maximum temperature, they also found a negative st at the jai station and a positive st at the gua station. these results provide essential information to characterize the evolution of regional warming–cooling, which should be attributed, according to alghamdi and harrington (2019), not only to local and regional changes (elevation, latitude, and proximity to water bodies), but also to synoptic scale changes (atmospheric circulation) such as the climate indices pacific decadal oscillation, atlantic multidecadal oscillation, oceanic el niño index and north atlantic oscillation (llanes, 2023). kilometers weather station a) frequency (fr) c) annual daily duration (aad) e) maximum maximorum temperature (mmt) b) daily mean duration (aad) d) intensity (in) la tit ud e la tit ud e la tit ud e la tit ud e la tit ud e longitude longitude longitude longitude longitude figure 2. interpolation of mann–kendall test (sts = black line, dimensionless). 80 omar llanes cárdenas, lorenzo cervantes arce, gabriel eduardo gonzález gonzález sen’s slope test for sts figure 3 shows the magnitudes of the st indicators. the highest magnitudes of negative st were recorded at stations epl (fr = −2.81 he dec-1, aad = −9.38 day he-1 dec-1 and mmt = −1.88 °c day-1 dec-1), aca (add = −20.94 day dec-1) and bad (in = −12.92 °c dec-1). the highest positive magnitudes of st were recorded at stations cul (fr = 3.44 he dec-1), gut (aad = 6.15 day he-1 dec-1 and in = 13.62 °c dec-1), ixp (add = 35.00 day dec-1) and pot (mmt = 2.50 °c day-1 dec-1), as shown in figure 3. if li’s criteria are considered, which are applied in the presence of extreme droughts, the positive trends of sts om fr, aad, add, in, and mmt suggest that extreme drought events may occur near stations cul, gut, ixp and pot. in this situation, the strength of these events could affect the yield of priority rainfed crops grown in the study area; beans, sorghum (sagarpa, 2015) and corn (llanes et al., 2022b). this is verified with the results obtained by llanes et al. (2022b), who established that in sinaloa, the indices of meteorological drought that require evapotranspiration and maximum temperature as input data for their calculation (i.e., reconnaissance drought index and effective reconnaissance drought index) registered the highest significant correlations with rainfed maize yield; i.e., in las tortugas (station with the highest average maximum temperature for the period 1982–2013). this could put the socioeconomic and agrifood sustainability of sinaloa and the whole country at risk (robeson, 2004; ruiz et al., 2005; sheridan & lee, 2018), especially considering that this is one of the most important agricultural states in mexico (rincón, 2012). in summer, sinaloa has extremely high temperatures (up to 48 ºc) and is often affected by diseases caused by high mets (riojas et al., 2006). it is important to note that the positive values of st for mmt suggests that the people near stations cul, gut, ixp and pot could be at risk of sporadic events of heatstroke and, as a consequence, potential loss of human life (ops, 2019). kilometers weather station a) frequency (fr) c) annual daily duration (add) e) maximum maximorum temperature (mmt) b) daily mean duration (aad) d) intensity (in) la tit ud e la tit ud e la tit ud e la tit ud e la tit ud e longitude longitude longitude longitude longitude figure 3. interpolation of sen’s slope test (sts = black line). measurement units: a (he dec–1), b (day he–1 dec–1), c (day dec–1), d (°c dec–1) and e (°c day–1 dec–1). the results of the analysis of fr, aad, add, in and mmt presented in table 3 show a prevailing cooling trend, due to the number of indicators of met with negative sts (28 stations) being greater in absolute value than those with positive values (15 stations). these results are consistent with the historical evidence reported by llanes et al. (2015), where they found a higher number of weather stations with negative sts for the minimum temperature in the states adjacent to sinaloa (baja california sur, durango, chihuahua, and sonora). of 38 weather stations studied, 20 showed negative sts and 18 positive sts. llanes et al. (2015), found that in some stations near the state of sinaloa there are negative trends for the minimum and average temperatures, but not for the maximum temperature, which presents a positive trend (i.e., in the stations puerto san carlos, el rosario and ojito de camellones), which can be attributed to the high randomness between the spatial and temporal influence of the pacific decadal oscillation, atlantic multidecadal oscillation, oceanic el niño index, north atlantic index, and geopotential height at the level of 700 hpa (llanes et al., 2018; llanes et al., 2020; llanes, 2023), and the magnitudes of the maximum, minimum and average temperatures, and the meteorological droughts of the northern states of mexico (llanes et al., 2018). the difference between figures 2 and 3 is that figure 2 only identifies which indicators are statistically significant and figure 3 shows the amount of significant trend. statistical analysis of the fitted probability distribution functions table 2 shows the p-values of the fitted probability distribution functions, where the maximum values correspond to the log, gev and lognormal functions. the highest p-values of the met indicators were fr (log, p=0.9944), aad (gev, p=0.9795), add (log, p=0.9728), in (lognormal, p=0.9517 and mmt (log, p=0.9088). the lowest p-values (p < 0.00001) corresponded to the fitted probability distribution functions erlang, fisher-tippett (1), gumbel, normal-standard, student and weibull (1). if the log function were chosen for all the indicators (fr, aad, add, in and mmt), then even the indicators aad and in (which do not present large p-values) can show good fit (p = 0.9573 and p = 0.9070 respectively). table 2. p-values for the fitted probability distribution functions of met. probability distribution function p–values (fitted probability distribution functions) = bold (dimensionless) fr aad add in mmt beta4 0.0679 0.6054 0.0562 0.8202 0.3280 chi-square 0.0095 0.0015 0.7976 0.5283 < 0.0001 erlang < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 exponential 0.0008 < 0.0001 0.0041 < 0.0001 < 0.0001 fisher-tippett (1) < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 fisher-tippett (2) 0.8579 0.8430 0.5771 0.2979 0.0890 generalized extremes values (gev) 0.9624 0.9795 0.8508 0.8664 0.7429 gumbel < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 log–normal (lognormal) 0.9390 0.9793 0.7933 0.9517 0.8187 logarithmic (log) 0.9944 0.9573 0.9728 0.9070 0.9088 normal 0.4472 0.6742 0.3552 0.9395 0.8425 normal-standard < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 student < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 weibull (1) < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 weibull (2) 0.4857 0.6088 0.4564 0.8664 0.5282 weibull (3) 0.7512 0.8860 0.5856 0.9207 0.8096 fr–frequency, aad– annual average duration, add–annual daily duration, i– intensity, mmt–maximum maximorum temperature and (boxed and bold)–fitted probability distribution functions. source: llanes et al. (2021) 81calculation of indicators of maximum extreme temperature in sinaloa state, northwestern mexico these results are in agreement with aguilera (2007) and with papacharalampous et al. (2018), who states that hydroclimatological data are rarely normally distributed; met indicators theoretically fit distributions that represent seasonal extreme values (linsley et al., 1977; pliego & ruiz 2004; campos 2014; wang et al., 2016; alghamdi & harrington, 2019). adjusted return periods table 3 shows the magnitudes of the shape, scale and location parameters of the fitted probability distribution functions used to calculate the adjusted return periods (arps). these parameters, based on the average values of met, are shown in figure 4 for the ranges fr, 1.07 to 377.84 years; aad, 1.03 to 24.88 years, add 1.06 to 816.94 years, in, 1.02 to 16.59 years; and mmt, 1.03 to 118.15 years. table 3. location, scale, and shape parameters of the fitted probability distribution functions. met indicator log lognormal gev µ σ µ σ k β µ fr 2.6633 0.5803 aad 0.0581 0.9286 3.3820 add 14.2118 3.6502 in 3.3935 0.1753 mmt 40.3902 0.3269 met–maximum extreme temperature, fr–frequency, aad– annual average duration, add–annual daily duration, i–intensity, mmt– maximum maximorum temperature, log–logarithmic, lognormal–log– normal, gev–generalized extreme values, µ–location, σ–scale, k–shape and β–scale. source: llanes et al. (2021) the p85 of mets as a function of the arps are fr (p85=3.65 he year-1; figure 4a), aad (p85= 5.09 day he-1 year-1; figure 4b), add (p85=19.34 day-1 year-1; figure 4c), in (p85=36.99 oc year-1; figure 4d), and mmt (p85=40.85 ºc day-1; figure 4e). according to figure 4, the average occurrence frequency of extreme events (values with the highest probability of exceedance) per 100 years are fr = 1.16 he dec-1 (arp = 1.07 years; prob. = 0.93 = 93 times ), aad = 2.24 day he-1 dec-1 (arp = 1.03 years; prob = 0.97 = 97 times), add = 4.16 day dec-1 (arp = 1.06 years; prob = 0.94 = 94 times), in = 20.42 °c dec-1 (arp = 1.02 years; prob = 0.98 = 98 times) and mmt = 39.18 °c day-1 dec-1 (arp = 1.03 years; prob = 0.98 = 98 times). the average frequency of occurrence of extreme events (values with less probability of exceedance) per 100 years are fr = 6.11 he dec-1 (arp = 377.84 years; prob. = 0.01 = 1 time), aad = 6.64 day he-1 dec-1 (arp = 24.88 years; prob = 0.04 = 4 times), add = 38.68 day dec-1 (arp = 816.94 years; prob = 0.01 = 1 time), in = 39.09 °c dec-1 (arp = 16.59 years; prob. = 0.06 = 6 times) and mmt = 41.95 °c day-1 dec-1 (arp = 118.15 years; prob. = 0.01 = 1 time). these results are especially useful for carrying out adaptation measures in sectors such as the agrifood industry, which is the main economic activity in the state of sinaloa (fiscal et al., 2017). if these results are not considered in the decision making for seeding and harvesting dates, significant reductions in crop yields could occur in the region, resulting in significant negative effects on the economy of the region, which can generate high costs (ruiz et al., 2013; llanes et al., 2022b). these unusual temperature fluctuations (indicators of met) that will increase by the year 2030 (world bank et al., 2014), could trigger significant decreases in agricultural crop yields in sinaloa (llanes et al., 2022b), which will aggravate the food sustainability of sinaloa and of mexico. through the results of this study, the association among the indicators of met can be observed; he (fr, aad, add and in) and mmt and the modeling tools (sts, fitted probability distribution functions and arps), which satisfy the requirements for calculating estimates of the frequency of occurrence of met indicators. figure 4. adjusted return periods (arps) of the maximum extreme temperature (met) indicators: hot extremes (fr, frequency; aad, annual average duration; add, annual daily duration; in, intensity) and maximum maximorum temperature (mmt). fitted probability distribution functions: logarithmic (log), lognormal (log–normal) and generalized extreme values (gev). source: llanes et al. (2021) 82 omar llanes cárdenas, lorenzo cervantes arce, gabriel eduardo gonzález gonzález 4. conclusions based on the st results of these parameters, a greater number of negative values were observed than positive, which shows a trend toward cooling. the fitted probability distribution functions used to calculate the indicators of met were for fr, log; for aad, gev; for add, log; for in, lognormal; and for mmt, log. this contribution provides a useful methodological tool to design adaptation strategies for climate change and natural disasters that may affect the agrifood sector. with the strategies that can be devised, non-reduction of the agricultural yield should be guaranteed for the most important crops in sinaloa; maize, sorghum, wheat, and beans. these products are a mainstay in feeding mexico and a great part of the harvest comes from the study area, which is still called “the breadbasket of mexico.” limitations of this study are the small number of weather stations studied and the small number of probability distribution functions used for the fit. it is recommended that the met indicators be recalculated with data from a greater number of meteorological stations, as well as using a greater number of probability distribution functions to try to increase the p–values and obtain more accurate arp. some proposals for climate-smart agricultural strategies (adaptation/prevention) are drip irrigation, protected agriculture, and crop substitution, which can serve to minimize the effects of high temperatures in the agricultural activities of sinaloa state. acknowledgements this research was financed by the research and postgraduate secretariat of the national polytechnic institute (sip-ipn): project with registration sip20195581. references aguilera, n. m. a. 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(2025). early paleoproterozoic continental crust magmatism in the xueye area, western shandong, china: evidence from u-pb age and trace element geochemistry of zircon. earth sciences research journal, 29(3), 275-286. https://doi.org/10.15446/esrj. v29n3.108296 abstract the basement of north china craton (ncc) is a research hotspot in the field of precambrian geology, as it contains a great deal of information about the formation and evolution of continental lithosphere. in this paper, la-icp-ms zircon u-pb dating and trace element analysis has been carried out on a monzogranite sample from the basement of the xueye area in western shandong, north china. although no concordia age can be obtained from the 206pb/238u and 207pb/235u data of the zircon grains, these data are distributed along a discordant line, the upper intersection of which gives the formation age of the sample (2356ma). the concentrations of some trace elements, such as u, yb, y, hf, th and p, in the zircon grains strongly indicate that the sample is a product of continental crust magmatism. besides, the relative less extent of eu anomalies as well as the high concentrations of th and pb of the zircon grains indicate that the sample does not belong to anorogenic granitoid. therefore, the monzogranite is orogenic and its formation may be related to continent-continent or ocean-continent collision. as a result, it can be inferred that a rigid continental plate had been formed by early paleoproterozoic. zircon grains in this sample have experienced different extent of alteration since their formation. most of the zircon grains that have been analyzed have experienced serious alteration, which indicates that there has been intense hydrothermal activity in this area since early paleoproterozoic. keywords: zircon; u-pb age; trace elements; early paleoproterozoic; continental crust. magmatismo de la corteza continental del paleoproterozoico temprano en el área de xueye, oeste de shandong, china: evidencia de la datación u-pb y geoquímica de elementos traza del circón resumen el basamento del cratón del norte de china (ncc) es un foco de investigación en el campo de la geología precámbrica, ya que contiene abundante información sobre la formación y evolución de la litosfera continental. en este trabajo se realizó la datación u-pb de circón la-icp-ms y el análisis de elementos traza en una muestra de monzogranito del basamento del área de xueye, en el oeste de shandong, norte de china. aunque no se puede obtener una edad certera a partir de los datos de 206pb/238u y 207pb/235u de los granos de circón, estos datos se distribuyen a lo largo de una línea discordante, cuya intersección superior proporciona la edad de formación de la muestra (2356ma). las concentraciones de algunos elementos traza, como u, yb, y, hf, th y p, en los granos de circón indican firmemente que la muestra es producto del magmatismo de la corteza continental. además, la relativa menor extensión de las anomalías de eu, así como las altas concentraciones de th y pb de los granos de circón, indican que la muestra no pertenece a un granitoide anorogénico. por lo tanto, el monzogranito es orogénico y su formación puede estar relacionada con la colisión continente-continente u océano-continente. como resultado, se puede inferir que una placa continental rígida se había formado en el paleoproterozoico temprano. la mayoría de los granos de circón que se han analizado han experimentado una alteración grave, lo que indica que ha habido una intensa actividad hidrotermal en esta área desde el paleoproterozoico temprano. palabras clave: circón; datación u-pb; elementos traza; paleoproterozoico temprano; corteza continental. https://doi.org/10.15446/esrj.v29n3.108296 https://doi.org/10.15446/esrj.v29n3.108296 https://doi.org/10.15446/esrj.v29n3.108296 276 early paleoproterozoic continental crust magmatism in the xueye area, 1. introduction the composition, ages, material sources and evolution processes of craton basements are research hotspots in the field of earth science. detailed research on these issues is conducive to the understanding of the early evolutionary history of the earth. the basement of north china craton (ncc), which was formed by the amalgamation of several small continental blocks (zhao, 2009), has a very large outcrop area and has attracted much attention. western shandong is a typical region for the outcrops of the precambrian basement of ncc (wan et al., 2012) and is an ideal base for the research of the precambrian history of ncc. there are a great quantity of igneous rocks or metamorphic igneous rocks in the precambrian basement of western shandong (wang et al., 2008). the above-mentioned rocks have attracted wide attention of geologists because they contain important information about the formation and evolution of the continental lithosphere of this region. similar to cratons from other regions of the world, the formation of ncc also includes formation of continental cores (liu et al., 2024), vast growth of continental crust, collision of small continental plates and cratonization (zhai and santosh, 2011; zhai, 2019; sun et al., 2021). however, for ncc, there are still disagreements on the time of each stage. some scholars believe that the peak time of continental crust growth is about 2500ma and the time of cratonization is about 2300ma (zhai and santosh, 2013; zhao et al., 2016). while other scholars proposed that cratonization of ncc mainly occurred at about 1850ma (kusky and li, 2003; peng et al., 2014). however, related research achievements are mainly about archean magmatism (e.g., ma et al., 2020; shan et al., 2021; wang et al., 2022; yang et al., 2022), and there are relatively few achievements which are related to proterozoic magmatism (wang, 1990; cao et al., 2017). as a result, it is necessary to improve the research on proterozoic igneous rocks or metamorphic igneous rocks of this region, which is of great significance for a more complete understanding of the formation and evolution of the precambrian basement of this region and even that of the whole ncc. besides, whole rock geochemistry has been widely used as evidence for the formation and evolution of ncc (e.g., diwu et al., 2011; dora et al., 2021), while the components of single minerals have been rarely used. compared to whole rocks, single minerals have more stable structure and composition. as a result, the geochemical data of single minerals have better comparability and higher sensitivity. with rapid development and popularization of microbeam analysis techniques (e.g., la-icp-ms), the costs of the chemical analysis of single minerals have been greatly reduced and the precision has been greatly elevated, which provides favorable conditions for genetic mineralogy research related to chemical composition. zircon is a common accessory mineral in various kinds of rocks. they can retain some age and chemical information after experiencing multiple episodes of geological events (grimes et al., 2007). as a result, their isotope and trace element components are of great value for the understanding of the genesis. besides, zircon is frequently used in isotope dating. a great deal of research has been done and abundant achievements have been accumulated on the relationship between the trace element composition and the genesis of zircon (grimes et al., 2007). there are abundant zircon grains in igneous rocks, especially acid intrusive rocks. besides, trace element composition of igneous zircon is very sensitive to source rock types and crystallizing environments (belousova et al., 2002). therefore, detailed research on zircon grains from acid intrusive rocks is conducive to the understanding of the origin, evolution, and tectonic background of the magma. in this paper, zircon grains of a monzogranite sample from the basement of ncc have been taken as the research object, u-pb dating and trace element analysis have been carried out in order to provide more information about the formation, especially the cratonization of ncc. 2. geological background, sampling and petrographic characteristics the region of western shandong, which is also called western shandong continental plate, is a relatively stable block located in the west of shandong province (geng et al., 2014). to the east, west and south, western shandong continental plate is connected to three other continental plates (bohai continental plate, hebei-liaoning continental plate and henan-anhui continental plate) by faults, and its northern part is covered by bohai sea (fig. 1) (geng et al., 2014). western shandong continental plate has a binary structure which is typical for a platform (geng et al., 2014). the precambrian basement of western shandong continental plate is widely exposed, and its phanerozoic sedimentary cover lies unconformably upon the precambrian basement (geng et al., 2014). figure 1. the tectonic location of western shandong continental plate (1-tianshan-xing’an-mongolian orogenic system, 2-tarim continental plate zone, 3-yinshannorthern hebei continental plate, 4-ordos continental plate, 5-wutai-taihang continental plate, 6-hebei-liaoning continental plate, 7-western shandong continental plate, 8-bohai continental plate, 9-henan-anhui continental plate, 10-qinling-qilianshan-kunlunshan orogenic system, 11-tibet-sanjiang orogenic system, 12-yangtze continental plate zone, north china continental plate zone includes 3, 4, 5, 6, 7, 8 and 9, after geng et al., 2014). 277wei tu the monzogranite sample in this study (sdts-1) was collected beside no. 327 provincial road and near lijiazhuang village, laiwu district, jinan city. the detailed sampling location is in figure 2. there is a large area of paleoproterozoic intrusive or metamorphic intrusive rocks around the sampling location. to the west and not far from the sampling location, there are a large area of neoarchean intrusive rocks or metamorphic intrusive rocks, while to the north and not far from the sampling location, there are a large area of paleozoic strata (fig. 2). the main minerals of the monzogranite sample are quartz (fig. 3a, fig. 3b), alkaline feldspar (fig. 3a) and plagioclase (fig. 3b), the proportions of which are 40%, 30%, 25%, respectively. plagioclase has been obviously altered. this sample is of holocrystalline texture, hypidiomorphic-granular texture and inequigranular texture, besides skeletal texture, myrmetic texture (fig. 3c) and poikilitic texture (fig. 3d) can also be seen in this sample, which reflects the complexity of the evolution and crystallization processes of magma (jia and huang, 1994). in addition, this sample has a large quantity of fluid inclusions (fig. 3e and fig. 3f), most of which are distributed along fractures, indicating that it has been greatly influenced by hydrothermal activity after its formation. figure 2. geological map of the xueye area of western shandong continental plate with the location of the monzogranite sample in this study (1-quaternary, 2-iron vein, 3-early cretaceous quartz veins, 4-early cretaceous intrusive rock or metamorphic intrusive rock, 5-paleozoic strata, 6-mesoproterozoic intrusive rock or metamorphic intrusive rock, 7-paleoproterozoic pegmatite vein, 8paleoproterozoic intrusive rock or metamorphic intrusive rock, 9-neoarchean strata, 10-neoarchean intrusive rock or metamorphic intrusive rock, 11-unknown geological bodies, 12-sampling location, modified from the 1:250000 geological map series of national geological archive of china, the location of this map is as in fig. 1). figure 3. optical microscopic photos of the monzogranite sample (sdts-1) (using crossed polarizers). (a) alkaline feldspar (af) and quartz (q). (b) plagioclase (pl) and quartz (q). (c) quartz (q) of myrmetic texture in plagioclase (pl). (d) quartz (q) included by alkaline feldspar (af). (e) fluid inclusions (fi) in quartz. (f) fluid inclusions (fi) in alkaline feldspar. 278 early paleoproterozoic continental crust magmatism in the xueye area, 3. sample processing and analysis methods the selection of zircon was conducted in the laboratory of hebei regional geological and mineral investigation and research institute. 524 zircon grains have been selected. these zircon grains were then sent to wuhan samplesolution analytical technology co., ltd. for target preparation, cathodoluminescence photographing, la-icp-ms in situ element and isotope analysis and dating. 200 were picked out from the 524 zircon grains for target preparation. the target is 2.54 cm in diameter and its polishing degree met the requirement of the analysis. an agilent 7900 icp-ms instrument and a coherent 193 nm excimer laser ablation system (geolas hd) were used for la-icp-ms in situ element and isotope analysis and dating. the energy, frequency and spot size of the laser were set to 80 mj, 5 hz and 32 μm, respectively. the software for data processing is icpmsdatacal10.8 (liu et al., 2008, 2010). the cathodoluminescence photos of the 25 zircon grains analyzed in this study and the sites of the analysis spots are shown in figure 4. nist610 and tanz (hu et al., 2021) were used as the standard samples for trace element calibration and isotope ratio calibration, respectively. gj-1, 91500 and plešovice were used as the standard samples for isotope ratio monitoring. before and after the analysis of several test samples, tanz has been analyzed. at the beginning and end of the whole analysis procedure, nist610 and tanz have both been analyzed. gj-1, 91500, and plešovice were also taken as test samples. the obtained concordia ages for gj-1, 91500, and plešovice are 601.5±1.7 ma (1σ; mswd=0.076), 1064±5.1 ma (1σ; mswd=0.0014) and 335.1±1.2 ma (1σ; mswd=0.21), respectively, which are in agreement with the reported or recommended values (gj-1 = 599.8±1.7 ma (liu et al., 2010), 91500 = 1063.8±6.6 ma (luan et al., 2019), plešovice = 337.9±2.8 ma (peng et al., 2017)) within the uncertainty ranges. for la-icp-ms analysis, the absolute quantity of the material ablated varies during each run, due to differences in the sample matrix and the related absorption behaviour of the wavelength used for ablation (liu et al., 2008). an internal standard is generally used to correct the variations, which means that the concentration of at least one element must be known before the analysis (liu et al., 2008). because zircon has simple chemical composition, the internal standard can be determined according to the constant stoichiometry (liu et al., 2008). to conduct quantitative analysis by la-icp-ms, the relative sensitivity for each element is usually calibrated using both external calibration and internal standardization (liu et al., 2008). in this study, nist610 was chosen as the reference material and si was chosen as the internal standard element. the concentration of si in zircon was set to be 15.3%, i.e., the concentration of sio2 was set to be 32.7%. tanz was chosen as the external standard for u-pb dating and it would be analyzed before and after the analysis of several test samples (liu et al., 2010). time-dependent drifts of u-th-pb isotopic ratios were corrected using a linear interpolation method (liu et al., 2010). 4. analysis results and discussion 4.1. u-pb dating the results and standard deviations of 207pb/235u and 206pb/238u ratios of the 25 zircon grains are listed in table 1. 207pb/235u and 206pb/238u ages and their standard deviations are also listed in table 1. in this study, common lead corrections (204pb method) have been carried out on part of the data. because isotopes with the mass number 204 include 204pb and 204hg, it is necessary to deduct 204hg, the quantity of which can be calculated from that of 202hg. the quantities of 206pb, 207pb and 208pb in common lead can be calculated according to the quantity of 204pb, lead isotope evolution model of the earth and the estimated age of the sample (206pb/238u age). isotope ratios and ages as well as their standard deviations after corrections are listed in table 2. common lead corrections have not been carried out on the points no. 5, no. 10, no. 14 and no. 23, because they didn’t contain common lead. fig. 5 and fig. 6 are 206pb/238u-207pb/235u diagrams before and after common lead corrections, respectively. by comparing the two figures, it can be seen that data before corrections are more dispersive than those after corrections, indicating that the common lead corrections have a good result. figure 4. cathodoluminescence photos of the 25 zircon grains analysed in this study (circles in the photos represent la-icp-ms analysis spots, 32 μm across). 279wei tu ta bl e 1 . 20 7 p b/ 23 5 u an d 20 6 p b/ 23 8 u ra tio s, ag es an d th ei r s ta nd ar d de vi at io ns b ef or e c om m on le ad co rre ct io ns a na ly sis p oi nt nu m be rs 20 7 p b/ 23 5 u ra tio s st an da rd d ev ia tio ns o f 20 7 p b/ 23 5 u ra tio s 20 6 p b/ 23 8 u ra tio s st an da rd d ev ia tio ns o f 20 6 p b/ 23 8 u ra tio s 20 7 p b/ 23 5 u a ge s (m a) st an da rd d ev ia tio ns o f 20 7 p b/ 23 5 u a ge s ( m a) 20 6 p b/ 23 8 u ag es (m a) st an da rd d ev ia tio ns o f 20 6 p b/ 23 8 u a ge s ( m a) 1 2. 87 57 0. 03 82 0. 12 69 0. 00 11 13 76 10 77 0 6 2 1. 07 61 0. 01 34 0. 03 44 0. 00 03 74 2 7 21 8 2 3 0. 83 48 0. 01 18 0. 03 23 0. 00 04 61 6 7 20 5 3 4 0. 59 89 0. 00 84 0. 02 20 0. 00 02 47 7 5 14 0 1 5 0. 90 96 0. 01 25 0. 01 46 0. 00 02 65 7 7 93 .4 1 6 0. 48 92 0. 00 85 0. 02 31 0. 00 02 40 4 6 14 7 1 7 1. 22 21 0. 01 68 0. 04 65 0. 00 03 81 1 8 29 3 2 8 0. 97 51 0. 01 40 0. 03 01 0. 00 02 69 1 7 19 1 1 9 0. 75 28 0. 01 25 0. 02 23 0. 00 02 57 0 7 14 2 1 10 1. 12 40 0. 01 37 0. 01 92 0. 00 01 76 5 7 12 2 1 11 1. 08 92 0. 01 83 0. 02 80 0. 00 05 74 8 9 17 8 3 12 0. 88 65 0. 01 29 0. 03 43 0. 00 02 64 4 7 21 8 2 13 1. 04 33 0. 03 12 0. 05 08 0. 00 17 72 6 16 32 0 10 14 1. 21 14 0. 01 94 0. 02 3 0. 00 02 80 6 9 14 6 2 15 0. 79 17 0. 01 29 0. 01 88 0. 00 02 59 2 7 12 0 1 16 1. 72 11 0. 02 11 0. 06 86 0. 00 06 10 16 8 42 8 4 17 3. 68 46 0. 07 89 0. 05 97 0. 00 08 15 68 17 37 4 5 18 3. 83 26 0. 06 02 0. 17 39 0. 00 19 16 00 13 10 34 10 19 1. 30 51 0. 01 68 0. 06 73 0. 00 05 84 8 7 42 0 3 20 1. 08 03 0. 01 5 0. 04 9 0. 00 05 74 4 7 30 9 3 21 5. 32 56 0. 11 76 0. 25 05 0. 00 43 18 73 19 14 41 22 22 0. 67 22 0. 01 09 0. 03 19 0. 00 02 52 2 7 20 3 1 23 8. 41 11 0. 19 65 0. 36 62 0. 00 69 22 76 21 20 11 33 24 1. 21 0. 01 87 0. 03 26 0. 00 03 80 5 9 20 7 2 25 1. 49 23 0. 02 38 0. 04 51 0. 00 05 92 7 10 28 4 3 280 early paleoproterozoic continental crust magmatism in the xueye area, ta bl e 2 . 20 7 p b/ 23 5 u an d 20 6 p b/ 23 8 u ra tio s, ag es an d th ei r s ta nd ar d de vi at io ns af te r c om m on le ad co rre ct io ns a na ly si s p oi nt nu m be rs 20 7 p b/ 23 5 u ra tio s st an da rd d ev ia tio ns o f 20 7 p b/ 23 5 u r at io s 20 6 p b/ 23 8 u ra tio s st an da rd d ev ia tio ns o f 20 6 p b/ 23 8 u r at io s 20 7 p b/ 23 5 u a ge s (m a) st an da rd d ev ia tio ns o f 20 7 p b/ 23 5 u a ge s ( m a) 20 6 p b/ 23 8 u ag es (m a) st an da rd d ev ia tio ns o f 20 6 p b/ 23 8 u a ge s ( m a) 1 2. 39 86 0. 03 52 0. 12 30 0. 00 11 12 42 11 74 8 6 2 0. 42 51 0. 01 95 0. 02 89 0. 00 03 36 0 14 18 4 2 3 0. 32 05 0. 01 43 0. 02 80 0. 00 03 28 2 11 17 8 2 4 0. 23 64 0. 01 10 0. 01 89 0. 00 02 21 5 9 12 1 1 5 0. 90 96 0. 01 25 0. 01 46 0. 00 02 65 7 7 93 .4 1 6 0. 21 74 0. 01 18 0. 02 08 0. 00 03 20 0 10 13 3 2 7 0. 50 45 0. 02 23 0. 04 05 0. 00 04 41 5 15 25 6 2 8 0. 30 78 0. 02 15 0. 02 45 0. 00 03 27 2 17 15 6 2 9 0. 29 24 0. 01 50 0. 01 84 0. 00 03 26 0 12 11 8 2 10 1. 12 40 0. 01 37 0. 01 92 0. 00 01 76 5 7 12 2 1 11 0. 32 17 0. 02 64 0. 02 15 0. 00 06 28 3 20 13 7 4 12 0. 37 63 0. 01 63 0. 03 00 0. 00 03 32 4 12 19 1 2 13 0. 61 54 0. 01 26 0. 04 72 0. 00 15 48 7 8 29 7 9 14 1. 21 14 0. 01 94 0. 02 30 0. 00 02 80 6 9 14 6 2 15 0. 16 16 0. 02 18 0. 01 33 0. 00 02 15 2 19 85 .4 2 16 0. 72 15 0. 03 28 0. 06 02 0. 00 05 55 2 19 37 7 3 17 0. 20 18 0. 13 03 0. 03 02 0. 00 13 18 7 11 0 19 2 8 18 3. 39 86 0. 05 34 0. 17 04 0. 00 18 15 04 12 10 14 10 19 0. 77 42 0. 01 84 0. 06 28 0. 00 05 58 2 11 39 3 3 20 0. 68 10 0. 01 72 0. 04 56 0. 00 06 52 7 10 28 8 3 21 5. 07 94 0. 10 93 0. 24 86 0. 00 42 18 33 18 14 31 22 22 0. 42 28 0. 01 33 0. 02 98 0. 00 02 35 8 9 18 9 1 23 8. 41 11 0. 19 65 0. 36 62 0. 00 69 22 76 21 20 11 33 24 0. 31 29 0. 01 69 0. 02 49 0. 00 03 27 6 13 15 8 2 25 0. 55 26 0. 03 48 0. 03 71 0. 00 07 44 7 23 23 5 4 281wei tu from figure 6, no concordia age can be obtained from the 206pb/238u and 207pb/235u data of the zircon grains, therefore, a discordant line was drawn (fig. 7). the ages of the upper and lower intersections of the discordant line and the concordia line are 2356±26[±27] and 100.8±7.0 ma, respectively. as can be seen from fig. 6, most zircon grains have experienced severe lead loss, which caused the 206pb/238u and 207pb/235u ages to be much lower than the real age of the sample. besides, considering all samples in fig. 6, the ages of the lower intersections are relatively dispersive, indicating that the u-pb system may have experienced multi-stage evolution or that radiogenic lead may have experienced continuous diffusion loss with time (li et al., 2004). however, the upper intersections can still reflect the formation time of the sample approximatively (li et al., 2004). therefore, the sample was formed in early paleoproterozoic. 4.2. zircon trace element analysis according to in situ microprobe analysis, there are many kinds of trace elements in zircon, of which the ones with high concentrations include hf, y, p, u, th and ree (hoskin and schaltegger, 2003). trace element concentrations as well as some parameters and ratios are listed in table 3 and table 4. the intracrystalline distribution of th and u in igneous zircon is heterogeneous, but the th/u ratios vary much less (hoskin and schaltegger, 2003). the th/u ratios of igneous zircon are generally greater than 0.5, however, those of metamorphic zircon are generally around 0.01 or even lower (hoskin and schaltegger, 2003). the average of the th/u ratios of the 25 zircon grains in this study is 0.70, indicating that they are typical igneous zircon (hoskin and schaltegger, 2003). the partition coefficient of u between zircon and melt is similar to that of yb, i.e., they have similar compatibility in zircon, which means that the u/yb ratio of zircon reflects that of the melt in which zircon crystallized (grimes et al., 2007). the average u/yb ratios of zircon from different sources are completely different, for example, the average u/yb ratio of zircon from oceanic gabbro is about 0.18 and that of zircon from continental granitoid is about 1.07 (grimes et al., 2007). the average u/yb ratio of the 25 zircon grains in this study is 4.65, which is closer to that of zircon from continental granitoid. besides, trace elements, such as u, yb, hf, y, th and p, can be used to discriminate zircons crystallized in continental or oceanic crust settings (grimes et al., 2007). these zircon grains generally have high u and th concentrations, but have relatively low yb and hf concentrations. in u-yb diagram, most of the data have been plotted in continental zircon zone (fig. 8a). in u/yb-hf diagram, all of the data have been plotted in continental zircon zone (fig. 8b). in u/yb-y diagram, most of the data have been plotted in or near continental zircon zone, while none of the data have been plotted in oceanic crust zircon zone (fig. 8c). in th-yb diagram, most of the data have been plotted in continental zircon zone (fig. 8d). in th/yb-hf diagram, most of the data have been plotted in continental zircon zone (fig. 8e). in th/yb-p diagram, most of the data have been plotted in or near continental zircon zone (fig. 8f). additionally, trace elements of zircon can also be used to discriminate genetic types of granitoid (li et al., 2021). these zircon grains have low extent of eu anomalies but high concentrations of th and pb. in fig. 8g and 8h, most of the data have been plotted in or near s-type or i-type zones, while none of the data have been plotted in a-type zones. the above diagrams indicate that the monzogranite sample is a product of terrigenous magmatic activity and its genetic type is s-type or i-type. in figs. 8c-8h, some or most of the data are fall outside the representative zones, the main reason for this trend is hydrothermal activity. some of the zircon grains may have experienced severe hydrothermal alteration. the cⅰ-normalized ree patterns of the zircon grains of the sample are similar to those of orogenic granitoid (i-type and s-type), but quite different from those of anorogenic granitoid (a-type) (fig. 9). therefore, this sample belongs to orogenic granitoid and its formation may be related to continentcontinent collision or ocean-continent collision. figure 5. 206pb/238u-207pb/235u diagram of zircon before common lead corrections with the concordia line (the error ellipses correspond to two standard deviations). figure 6. 206pb/238u-207pb/235u diagram of zircon after common lead corrections with the concordia line (the error ellipses correspond to two standard deviations). figure 7. 206pb/238u-207pb/235u diagram of zircon after common lead corrections with the concordia line and a discordant line (the error ellipses correspond to two standard deviations, two ages in the figure are ages for the lower and upper intersections, mswd stands for mean squared weighted deviation). 282 early paleoproterozoic continental crust magmatism in the xueye area, ta bl e 3 . r ee co nc en tra tio ns (p pm ) a nd δ eu o f z irc on a na ly si s po in t nu m be rs l a c e pr n d sm e u g d t b d y h o e r t m y b l u σr e e δe u( a ) 1 16 .3 10 4 13 .0 82 .6 67 .8 20 .1 12 9 23 .1 16 2 43 .4 16 6 33 .2 31 7 61 .4 12 38 .6 0. 66 2 25 1 83 3 14 9 76 2 47 7 12 9 71 4 15 7 10 68 21 8 60 7 92 .2 65 5 93 .9 62 05 .5 0. 67 3 22 9 95 8 15 5 93 9 85 9 24 6 15 67 33 6 21 80 44 9 13 33 23 6 18 53 28 4 11 62 3. 6 0. 65 4 97 .2 10 15 10 2 90 6 10 29 14 0 12 03 16 0 82 1 16 3 46 5 75 .6 61 9 10 8 69 03 .9 0. 38 5 11 49 22 05 58 0 29 42 19 51 27 7 32 12 81 0 60 86 14 28 44 31 75 9 52 03 68 4 31 71 7. 7 0. 34 6 11 1 20 74 21 8 17 16 89 6 15 6 88 2 13 9 93 2 21 9 71 1 12 1 95 0 15 3 92 76 .7 0. 54 7 16 5 11 70 16 7 10 43 41 9 43 .7 40 2 67 .5 45 0 10 8 36 3 67 .4 55 5 95 .7 51 15 .3 0. 32 8 10 8 13 74 13 7 10 83 61 6 11 3 68 1 12 1 71 5 14 6 44 3 76 .6 63 2 11 0 63 55 .3 0. 53 9 46 1 98 8 22 9 12 17 89 5 15 8 14 48 34 6 26 38 60 4 19 95 34 7 25 95 32 9 14 24 9. 0 0. 42 10 50 4 18 20 34 1 18 06 14 95 28 4 29 04 52 4 29 68 54 3 13 58 19 3 13 53 20 3 16 29 7. 1 0. 42 11 19 5 89 7 14 8 10 26 10 41 24 5 14 44 24 7 14 51 29 1 78 6 11 5 80 4 11 8 88 07 .8 0. 61 12 91 .4 34 14 30 0 33 90 88 3 14 9 72 1 99 .0 76 5 21 1 83 0 16 8 14 72 22 9 12 72 4. 3 0. 57 13 77 .7 63 7 77 .3 43 6 36 3 16 7 67 4 13 3 93 9 20 8 65 2 10 5 77 0 11 6 53 55 .9 1. 03 14 79 6 13 86 38 9 18 90 12 58 23 1 18 75 34 6 20 81 40 8 10 93 16 8 12 05 16 5 13 29 2. 1 0. 46 15 25 6 16 03 17 9 12 52 11 21 33 1 15 06 19 4 98 0 18 9 55 3 91 .7 75 3 11 9 91 26 .1 0. 78 16 90 .9 48 6 72 .6 48 5 42 8 86 .8 71 6 11 4 57 5 10 9 30 7 51 .8 41 8 73 .1 40 12 .9 0. 48 17 17 3 97 4 15 0 76 8 27 0 87 .0 33 7 64 .6 46 6 11 8 41 5 77 .7 64 5 10 6 46 51 .0 0. 88 18 8. 76 10 1 9. 23 63 .0 61 .7 10 .1 10 3 22 .1 15 6 39 .6 14 2 27 .7 25 8 50 .8 10 53 .2 0. 38 19 90 .2 96 9 97 .2 49 2 23 1 10 4 32 7 82 .7 68 5 17 6 66 1 13 0 10 85 16 7 52 97 .1 1. 15 20 18 3 12 62 13 3 10 30 50 4 76 .1 45 7 57 .9 30 6 67 .2 22 0 44 .0 40 6 78 .1 48 25 .6 0. 48 21 5. 97 12 3 11 .7 96 .6 73 .0 8. 94 97 .1 15 .8 10 3 27 .6 10 3 20 .3 18 7 36 .6 90 9. 4 0. 32 22 10 3 80 2 77 .2 59 5 12 04 59 7 20 73 17 9 89 1 19 3 60 3 10 4 85 3 13 9 84 11 .6 1. 15 23 2. 20 12 .6 0. 63 5. 15 5. 75 0. 90 20 .2 5. 06 47 .0 15 .7 65 .1 12 .3 11 1 20 .7 32 4. 4 0. 25 24 42 .1 68 4 54 .7 13 20 94 6 21 9 59 0 56 .2 33 4 82 .4 29 6 59 .4 59 0 12 9 54 04 .1 0. 90 25 82 .3 11 23 87 .5 84 2 49 2 69 .2 51 9 80 .3 46 8 10 2 32 2 55 .6 45 9 79 .8 47 82 .6 0. 42 (a ) t ra ce el em en t c on ce nt ra tio ns o f c ⅰ c ar bo na ce ou s c ho nd rit e u se d fo r δ eu ar e f ro m m cd on ou gh an d su n, 1 99 5. 283wei tu ta bl e 4 . c on ce nt ra tio ns o f s om e o th er tr ac e e le m en ts of zi rc on (p pm ) a nd so m e r at io s b et w ee n tra ce el em en t c on ce nt ra tio ns o f z irc on a na ly si s p oi nt nu m be rs p ti y n b h f ta pb t h u t h/ u u /y b t h/ y b (n b/ pb ) n (a ) 1 13 13 28 .8 13 49 6. 78 93 34 1. 73 19 2. 8 52 4 11 48 0. 46 3. 62 1. 65 0. 36 2 28 82 14 6 59 05 38 .6 11 03 6 5. 75 25 3 16 51 47 79 0. 35 7. 30 2. 52 1. 57 3 33 10 24 0 12 26 1 14 2 19 27 5 18 .4 21 7. 1 27 03 48 32 0. 56 2. 61 1. 46 6. 71 4 63 8 28 1 50 38 48 .7 10 86 0 8. 40 12 9. 5 24 40 37 32 0. 65 6. 03 3. 94 3. 87 5 11 15 1 41 2 33 69 1 35 7 14 02 3 15 .9 45 3 39 19 7 12 70 2 3. 09 2. 44 7. 53 8. 10 6 78 8 97 .7 61 05 47 .1 13 87 0 16 .9 12 7. 1 18 01 37 84 0. 48 3. 98 1. 90 3. 81 7 44 2 77 .7 29 16 26 .2 98 06 9. 24 18 4. 0 19 89 25 09 0. 79 4. 52 3. 58 1. 47 8 42 8 86 .7 39 72 22 .4 10 60 0 10 .1 13 6. 6 17 52 26 43 0. 66 4. 18 2. 77 1. 69 9 42 63 14 2 14 20 3 13 0 78 97 3. 76 25 3 52 40 63 58 0. 82 2. 45 2. 02 5. 27 10 21 50 27 8 15 51 7 70 .0 13 76 7 17 .2 51 1 30 32 11 46 5 0. 26 8. 47 2. 24 1. 41 11 14 31 22 8 81 88 52 .8 96 13 6. 26 30 1 15 35 57 90 0. 27 7. 20 1. 91 1. 81 12 15 06 15 .6 78 67 27 .0 99 50 10 .7 10 8. 2 22 43 19 48 1. 15 1. 32 1. 52 2. 57 13 10 25 19 2 58 28 49 .7 82 90 3. 20 19 4. 8 99 8 32 86 0. 30 4. 27 1. 30 2. 62 14 20 10 33 7 10 42 2 42 .3 11 20 8 10 .5 40 7 24 27 85 97 0. 28 7. 13 2. 01 1. 07 15 12 54 13 5 61 07 50 .6 96 62 9. 23 28 2 31 94 75 29 0. 42 10 .0 0 4. 24 1. 85 16 58 3 54 .2 30 16 17 .5 80 46 4. 40 25 8 21 08 23 85 0. 88 5. 71 5. 04 0. 70 17 83 2 12 9 32 42 45 .4 97 73 10 .4 46 9 21 30 31 05 0. 69 4. 81 3. 30 1. 00 18 25 6 21 .4 11 13 11 .9 96 80 10 .8 18 3. 3 40 6 83 1 0. 49 3. 22 1. 57 0. 67 19 10 00 13 1 45 78 22 .1 79 10 4. 11 20 4 42 76 19 77 2. 16 1. 82 3. 94 1. 11 20 34 3 10 5 21 26 22 .9 11 66 7 16 .3 13 4. 4 76 7 20 41 0. 38 5. 03 1. 89 1. 76 21 23 5 17 .4 83 6 4. 83 86 50 1. 67 16 0. 7 19 9 54 5 0. 37 2. 91 1. 06 0. 31 22 90 1 22 5 63 47 10 8 11 62 0 12 .6 17 3. 2 16 15 40 47 0. 40 4. 74 1. 89 6. 39 23 32 1 8. 08 47 5 1. 68 77 86 0. 93 18 0. 5 15 8 42 3 0. 37 3. 81 1. 42 0. 10 24 25 7 95 .7 27 34 38 .4 15 27 3 24 .7 13 9. 9 52 3 24 20 0. 22 4. 10 0. 89 2. 83 25 39 1 11 3 29 72 16 .1 84 09 8. 40 17 2 23 64 21 18 1. 12 4. 61 5. 15 0. 96 (a ) t ra ce el em en t c on ce nt ra tio ns o f c ⅰ c ar bo na ce ou s c ho nd rit e u se d fo r ( n b/ pb ) n ar e f ro m m cd on ou gh an d su n, 1 99 5. 284 early paleoproterozoic continental crust magmatism in the xueye area, figure 8. zircon trace element discrimination diagrams. (a) u-yb diagram. (b) u/yb-hf diagram. (c) u/yb-y diagram. (d) th-yb diagram. (e) th/yb-hf diagram. (f) th/ yb-p diagram. (g) (nb/pb)n-δeu diagram. (h) th-pb diagram. (a), (b), (c), (d), (e) and (f) are according to grimes et al., 2007. (g) and (h) are according to li et al., 2021. trace element concentrations of cⅰ carbonaceous chondrite used for (g) are from mcdonough and sun, 1995. in figures (a) to (f), 1 means continental crust zircon, 2 means ocean crust zircon. 285wei tu figure 9. zircon cⅰ-normalized ree patterns of the sample in this study and samples from well-established continental settings (gray zones: the sample in this study, dashed line in a: continental orogenic granitoid, dashed line in b: continental anorogenic granitoid) 5. conclusions u-pb isotope analysis of zircon from a monzogranite sample from the xueye area of western shandong indicates that the age of the sample is 2356±26[±27] ma (early paleoproterozoic). zircon geochemistry of the sample indicates that it is a product of continental crust magmatic activity, which may be related to continent-continent or ocean-continent collision. therefore, a rigid continental plate had been formed by early paleoproterozoic in this region. zircon grains in this sample have experienced different extent of alteration since their formation. most of the zircon grains that have been analyzed have experienced serious alteration, which indicates that there has been intense hydrothermal activity in this region since early paleoproterozoic. acknowledgments the author is grateful to zheng liu, laboratory technician of wuhan samplesolution analytical technology co., ltd., for research assistance. this study was supported by a research fund of huaiyin institute of technology (no. z301b18559). references belousova, e. a., griffin, w. l., o’reilly, s. y., & fisher, n. i. 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(2016). major advances in the study of the precambrian geology and metallogenesis of the north china craton: a review. acta geologica sinica (english edition), 90(4), 1122-1155. https://doi. org/10.1111/1755-6724.12765 https://doi.org/10.1038/s41467-021-24139-z https://doi.org/10.1016/j.gr.2011.02.005 https://doi.org/10.1016/j.gr.2013.02.007 https://doi.org/10.1111/1755-6724.12765 https://doi.org/10.1111/1755-6724.12765 _hlk210203400 ole_link5 ole_link17 ole_link12 ole_link13 ole_link14 ole_link15 ole_link18 _hlk118208506 ole_link19 ole_link6 ole_link7 ole_link8 ole_link9 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n3.118266 earth sciences research journal earth sci. res. j. vol. 29, no. 3 (september, 2025):287 296 m in er a lo g y chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin (nw algeria) abdelkader boucif1,2*; matthias reimann3,4; mustapha bensalah1; hakim hebib1,2; ali kacemi1,2; 1. department of earth and universe sciences, university of tlemcen, p.o. box 119, tlemcen 13000, algeria. 2. laboratory: promotion of water, mineral and soil resources, environmental legislation and technological choices 3. knauf gips kg, germany. 4. university of würzburg, institute of geography and geology, geography building am hubland ‎‎97074 ‎wuerzburg, germany.‎ * corresponding author: abdelkader.boucif@univ-tlemcen.dz record manuscript received: 11/01/2025 accepted for publication: 20/08/2025 how to cite this item: boucif, a., reimann, m., bensalah, m., hebib, h., & kacemi, a. (2025). chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin (nw algeria). earth sciences research journal, 29(3), 287-296. https://doi.org/10.15446/esrj.v29n3.118266 abstract gypsum represents a major strategic resource, widely used in plaster production and its derivatives, serving as a fundamental construction material. given its important role in the industry, it is imperative to ensure efficient and sustainable management of this raw material in order to meet growing market demands while preserving natural resources. this study is based on data obtained from core drilling conducted in djebel djira, located 24 km southeast of oran. 425 gypsum samples were collected, including 413 for chemical analysis, 10 for physical tests, and 2 for a mineralogical study. this study provides, for the first time, a comprehensive characterization of the chemical and lithological properties of gypsum, offering critical insights into its potential valorization. the analyses reveal that the main oxides are cao (33.46%) and so₃ (43.58%), meeting the standards for plaster production. the mgo (0.30%) and k₂o (0.07%), remain within the limits in compliance with industrial requirements. the average chloride content (0.011%) and the low na₂o concentration (0.09%) promote the setting time of the plaster. these results classify the gypsum in category i according to iso 1587 certification and the european standard nf en 520 +a1. 3d models generated using rockworks software indicate that the eastern and southeastern regions of djebel djira are characterized by a minimal overburden, not exceeding 10 meters, whereas gypsum deposits are well developed at greater depths. it is recommended to prioritize the extraction activities in the eastern part of the deposit or in its south-eastern section. in contrast, the western portion of the site should be avoided during early mining works. in this area, the overburden is particularly thick. gypsum is generally reached only at an average depth of 50 meters, which significantly complicates exploitation operations and increases extraction costs. keywords: gypsum; chemical analysis; construction material; valorization potential; sustainable management. caracterización química y potencial de valorización de los depósitos subterráneos de yeso en el altiplano de boufatis, cuenca baja de chelif, noroeste de argelia resumen el yeso representa un recurso estratégico principal, al ser un material fundamental de construcción que se usa ampliamente en la producción de revestimientos y sus derivados. dado su importante papel en la industria, es necesario asegurar la administración eficiente y sostenible de esta materia prima para alcanzar la creciente demanda del mercado, al tiempo que se preservan los recursos naturales. este estudio se basa en la información obtenida de perforaciones realizadas en djebel djira, ubicada a 24 kilómetros al sureste de oran (argelia). se recolectaron cuatrocientos veinticinco muestras de yeso, de las cuales 413 fueron sometidas a análisis químicos, diez a pruebas físicas, y dos a estudios mineralógicos. este estudio presenta, por primera vez, una caracterización completa de las propiedades químicas y litológicas de yeso, y ofrece información determinante sobre su potencial de valorización. los análisis revelan que los óxidos principales son cao (33,46%) y so₃ (43,58%), cumpliendo con los estándares para la producción de yeso. el mgo (0,30%) y k₂o (0,07%) permanecen dentro de los límites de conformidad con los requisitos industriales. el contenido medio de cloruro (0,011%) y la baja concentración de na₂o (0,09%) promueven el tiempo de fraguado del yeso. estos resultados clasifican el yeso como categoría i según la certificación iso 1587 y la norma europea nf en 520 + a1. los modelos 3d generados utilizando el software rockworks indican que las regiones oriental y sudoriental de djebel djira se caracterizan por una sobrecarga mínima, que no supera los 10 metros de profundidad. por el contrario, los depósitos de yeso están bien desarrollados a mayores profundidades. se recomienda priorizar las actividades de extracción en la parte oriental del depósito o en su sección sudoriental. en cambio, la parte occidental del yacimiento debe evitarse durante las primeras labores mineras. en esta zona, la sobrecarga es especialmente gruesa. el yeso generalmente solo se alcanza a una profundidad promedio de 50 metros, lo que complica considerablemente las operaciones de explotación y aumenta los costos de extracción. palabras clave: yeso; análisis químico; material de construcción; potencial de valorización; manejo sostenible. 288 chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin 1. introduction gypsum is widely used in the manufacture of plaster and its derivatives, which constitute fundamental materials in the construction industry. in addition to their structural role, these materials provide effective thermal and acoustic insulation, contributing significantly to building comfort. the present study forms part of a broader geological exploration program initiated through a collaboration between the german company knauf gips kg and the center for technological studies and services in the construction materials industry (cetim). the gypsum deposits observed in the djebel djira region are closely linked to the messinian salinity crisis (msc), a significant geological event that occurred between approximately 5.97 and 5.33 million years ago (roveri et al., 2014; lanzoni, 2022; krijgsman et al., 2024). this crisis has resulted in the partial or complete isolation of the mediterranean sea of the atlantic ocean, most likely due to tectonic or glacio-eustatic processes (pérez-asensio et al., 2013; krijgsman et al., 2024). as a result, substantial gypsum layers accumulated in the peripheral basins of the mediterranean, generally at depths not exceeding 200 m (sellam et al., 2025). this context led to the formation of the lower primary gypsum unit (plg), with thicknesses reaching up to 300 m (stefano et al., 2010; lugli et al., 2010; caruso et al., 2015; bigi et al., 2024). all neogene formations were thoroughly investigated due to their abundant surface exposures. in contrast, gypsum deposits rarely outcrop, significantly limiting data collection opportunities and, in the current state of knowledge, few works have dealt with this subject (remmache, 2006; alouasi et al., 2022; sellam et al., 2025). this lack of data poses a major challenge because it limits our knowledge of the lithological and chemical properties of gypsum, thus hampering efforts to optimize its industrial application. in this study, as part of a geological exploration campaign, core-drilling work was initiated‎. the objective was not only to determine the strength and quality of the gypsum beds but also to study the lithological nature and thickness of the barren layers unsuitable for exploitation. in total, 35 vertical mechanical boreholes were drilled, with a combined‎ length of 1,686 meters. the recovered cores were described onsite, organized, and classified for each borehole. the collected gypsum samples were sent to the center for studies and technological services of the construction materials industry (cetim) for physicochemical analyses. surfer software (version 18) was used to create facies maps and assess geological reserves. rockworks 17 software was employed to develop a database for generating twoand three-dimensional geological models and profiles. these simulations allow for a more efficient design of the structure, the geometry of the gypsum bed, and the cover depth. as a result, they support optimal and cost-effective planning during exploitation activities. 2. geological setting 2.1 lower chelif basin the lower chelif basin has been the subject of several studies since the first geological reconnaissance studies by bleicher (1875), pomel (1892), repelin (1895), and brives (1897), up to the eminent works of the national society for oil research and exploitation in algeria, the s.n. repal company (1952), dalloni (1952), perrodon (1957), gourinard (1958), delteil (1974), fenet (1975), guardia (1975), rouchy (1982), thomas (1985), and neurdin-trescartes (1992, 1995). other more recent works on stratigraphic, paleontological, and micropaleontological levels were initiated in the 1990s (belkebir et al., 1994, 1996, 2002, 2008; rouchy et al., 2007; atif et al., 2008; belhadji et al., 2008; mansouri et al., 2008; satour et al., 2013; hebib, 2014; benzina et al., 2019; belhadji, 2021; moulana et al., 2021; khalili et al., 2022; atik et al., 2024; bouchemla et al., 2025). the lower chelif basin is a subsiding, intramontane, and synorogenic basin linked to the paroxysmal phases of alpine orogeny (perrodon, 1957; thomas, 1985). it corresponds to the median furrow (delfaud, et al., 1973), surrounded by two longitudinal depressed zones, the murdjadjo-arzew-dahra massifs to the north and the tessala-ouled ali-beni chougrane-ouarsenis mountains to the south (fig.1). the lower chelif basin appears as a ne-sw trending depression that is 300 km long and 80 km wide. it consists of a tertiary-age terrain. the miocene deposits are predominant, with thicknesses ‎approaching 4000 m at the basin center and decreasing considerably toward the margins (s.n. repal 1952; perrodon, 1957; bessedik & belkebir, 1985). the upper miocene is represented by blue marls of the tortonian age that begin with well-developed sandstones on the margins passing to diatomites or marls with diatomitic intercalations of the messinian age. at the messinian end, the facies evolved toward essentially gypsiferous evaporitic sedimentation (rouchy, et al., 1981; neurdin-trescartes, 1995). a coral edifice has developed on ‎platform margins and basin highs (saint martin, 1987 ; saint martin & andré, 1992). the pliocene generally begins with white marls passing locally to alternations of marl and sandstone (benyoucef et al., 2021). figure 1. location map of the lower chelif basin and the main mountain ranges (nw algeria) 2.2 boufatis plateau the boufatis plateau occupies the western part of the lower chelif basin, consisting mainly of mio-plio-quaternary terrains (perrodon, 1957) resting unconformably on the secondary-age arzew massifs. structurally, the boufatis plateau consists of a succession of anticlines and synclines aligned in a northeast-southwest ‎direction (fig. 2). figure 2. geological map of major units in northwestern algeria (st. cloud map, n°154, 1:500,000) 289abdelkader boucif, matthias reimann, mustapha bensalah, hakim hebib, and ali kacemi; the miocene and pliocene terrains constitute the largest part of the lower chelif basin, with a thickness of approximately 4,000 m in axial zones, decreasing considerably towards the margins (belkebir et al., 2008). the miocene series is divided into two sedimentary cycles: a lower miocene cycle corresponding to a time interval from the upper burdigalian to the terminal serravallian (moussa, et al., 1994; bessedik et al., 2002), and an upper miocene cycle encompassing the tortonian and messinian stages (belkebir, et al., 2008). blue marls of tortonian age, typically beginning with well-developed sandstone layers on the margins that transition into diatomites or marls with diatomitic intercalations characterize the upper miocene (perrodon, 1957; rouchy, 1990; benzina et al., 2019). the messinian deposits then evolve towards an evaporitic sedimentation that is essentially gypsiferous (bertoni, 2015). discordant with preceding formations, the pliocene is commonly represented by white marl, locally evolving into marl with sandstone layers. the pleistocene is marked by hard, yellowish-brown limestones forming a surface crust, along with grayyellow lumachelic limestones that are well exposed throughout the region. finally, the holocene consists of saline deposits in sebkhas and valleys, typically devoid of vegetation (fig. 3). figure 3. geological map of studied region, showing the upper miocene (gypsum deposits, my), the pliocene and the quaternary lithostratigraphic succession. 3. materials and methods the djebel djira deposit is composed of four hills separated by wadis. it displays a relatively regular structure with a gentle dip and an almost flat relief. for the borehole distribution network, a grid spacing of 200 × 300 m was applied in the eastern part of the deposit, while a wider spacing of 400 × 500 m was used in the western part. in total, 35 vertical mechanical boreholes were drilled, representing a cumulative depth of 1686 m. drilling was carried out using a double-core barrel system (wireline) to minimize sample disturbance and to ensure optimal preservation of the cores. boreholes had a diameter of 45 mm. when clear water was employed as the drilling fluid, injection was performed between the two tubes, preventing direct contact of the core with the rotating barrel. the recovered cores were systematically logged for detailed geological documentation. the boreholes are distributed along ten parallel profiles nnw– sse oriented (fig. 4). boreholes located between profiles i and vii reached an elevation of +140 m and successfully intersected the gypsum bed. in contrast, boreholes 1, 3, 13, 14, and 15, positioned between profiles viii and ix, did not attain this elevation and intersected the gypsum bed with a thickness ranging from 3 to 6 m. these boreholes were primarily designed to assess the thickness of the overlying cover. boreholes 2 and 17 were drilled to depths exceeding 40 m without intersecting the gypsum bed. overall, the core recovery rate was greater than 90% within the gypsum layers, but decreased significantly in friable, unstable, or heterogeneous lithologies such as sandstone. x-ray diffraction (xrd) was used to determine the oxide content of gypsum from djebel djira. generally, this technique involves projecting an x-ray beam onto a crystal, which is deflected by the atomic planes of the crystal lattice, producing a unique signature specific to each mineral. gypsum samples are ground to a powder with a particle size of less than 2 mm (janssens, 2004). this approach enabled us to accurately identify the oxide content and estimate their relative proportions in the gypsum studied. in order to process all this data, we created a database under rockworks. the latter is a powerful software dedicated to various geoscience sectors, designed for processing, modeling and visualizing subsurface data, particularly core and drilling data. it uses geological, geochemical and geotechnical data in 2d/3d models, making it essential for mining, hydrogeological and civil engineering projects. figure 4. satellite imagery of djebel djira delineating the study perimeter and locations of core drilling along 10 profiles. 4. results and discussion 4.1 core drilling data the gypsum layers are largely concealed beneath quaternary alluvial deposits composed of medium-hard calcareous crusts, yellowish clays, and lumachelic limestones. the thickness of the overburden increases progressively from approximately 5 m in the east to more than 40 m in the west. the isobath map shows that the eastern sector of the deposit is generally characterized by a thinner cover, ranging from 5 m to 23 m, with the exception of borehole 20 where the overburden exceeds 30 m (fig. 5). figure 5. isobath map of overburden thickness variation in the djebel djira deposit. the central part of the deposit is characterized by cover thicknesses ranging between 15 m and 30 m. in contrast, the western sector shows a markedly thicker overburden, exceeding 40 m. concerning the exploitable bed, gypsum outcrops are observed in the eastern part of the deposit, where thicknesses vary between 15 m and more than 50 m, with an average of approximately 35 m. notably, the thickness of the gypsum bed decreases westward, while the thickness of the overlying cover increases (fig. 6). 290 chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin figure 6. isopach map showing the variation in thickness of the gypsum layer in the djebel djira. this finding will guide future exploitation efforts, suggesting that rock extraction should begin in the eastern or southeastern part of the deposit (fig. 7). for optimal extraction planning, beyond the volume of reserves, it is essential to consider several ‎key parameters, including site accessibility, the structural characteristics of the deposit, local ‎topography, and environmental impacts. previous studies have emphasized the importance of ‎integrating such multidisciplinary datasets in mining planning and sustainable exploitation (‎zhang et al., 2020; mcquillan, & bar, 2023). data such as access route mapping, ‎‎3d models of the deposit (derived from drilling and geophysical surveys), the mechanical ‎properties of the rocks, and a detailed digital terrain model, will allow for the assessment of slope ‎stability and geotechnical constraints, as demonstrated in comparative works on gypsum and ‎evaporite deposits (zheng et al., 2019). furthermore, an ‎environmental analysis including the mapping of sensitive ecosystems, hydrological risks, and ‎potential dust or noise emissions is indispensable for sustainable exploitation, in line with ‎recommendations from regional and global case studies (boukria et al., 2022). the ‎integration of these data, obtained through remote sensing (lidar, drones), core sampling, ‎laboratory tests, and numerical simulations, will ensure informed decisionmaking, minimizing both ‎operational risks and ecological impact. figure 7. geological sections and stratigraphic correlations according to profiles of djebel djira 291abdelkader boucif, matthias reimann, mustapha bensalah, hakim hebib, and ali kacemi; in order to evaluate the quality of the rock mass through parameters such as fracturing, cohesion, and compactness, we present as an illustration the results from borehole n° 22, along with the percentage obtained from analysis of the rqd (rock quality designation) index. this approach enables interpretation of the rock mass quality and prediction of its mechanical behavior during extraction operations (shen, et al., 2023 ; yu, et al., 2024). this rqd concept, introduced by deere in 1964, makes it possible to evaluate the mechanical quality of a rock mass based on geological cores (chalhoub, 2006; armatys, 2012). this quantitative index corresponds to the ratio (expressed as a percentage) between the cumulative length of intact core fragments (longer than 10 cm) and the total length of the drilled section. the methodology relies on a selective approach: only cohesive fragments of significant size are considered in the calculation, thus excluding fractured or altered zones. this parameter is expressed by the following equation (1): (%) = ∑ lengths of intact core pieces ≥10 ∗ 100 (1) the correlation between the rqd index and the quality of the rock mass is based on the classification proposed by deere (1968), presented in table 1 below. table 1. relationship between rqd values and rock mass quality grades (deere, 1968) rqd rock mass quality < 25% very poor rock 25–50% poor 50–75% fair 75–90% good 90–100% excellent for borehole n°22, the calculated average rqd values range between 65% and 100%. according to the classification proposed by deere (1968), these values indicate a rock mass quality ranging from good (rqd between 75–90%) to excellent (rqd > 90%). this distribution suggests that the rock mass encountered in this borehole exhibits a generally coherent structure with a low degree of fracturing, reflecting favourable mechanical stability conditions for exploitation (fig. 8). figure 8. lithological description, technical parameters, core recovery rate, and rqd variation from borehole n° 22. 292 chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin 4.2 industrial requirements all collected samples were submitted to the center for technological studies and services in the construction materials industry (cetim) for the tests and analyses summarized in table 2. table 2. different analyses performed on gypsum samples laboratory work number sample processing 413 13-element chemical analysis 413 determination of water of crystallization 413 chlorine determination 174 x-ray analysis 02 this procedure is designed to determine the qualitative characteristics of the valuable substance. the loss on ignition was measured in accordance with the en 196-2 standard (feuerborn, 2011). the crystallization water content was calculated from the weight loss recorded at 400 °c. the chloride concentration was determined potentiometrically, following the requirements of the en-nf 1744-1 standard. furthermore, the assessment of gypsum quality complies with the international standard iso 1587 (1995) and the european standard nf en 520+a1.table 3 summarizes the industrial requirements for the manufacturing of construction plaster and its derivatives. table 3. industrial requirements for manufacturing plaster and its derivatives according to iso 1587 standard gypsum category minimum content (100%) caso4 2h2o water for crystallization class i greater than 90%. more than 18.33 class ii 80% à 90% 16.74% à 18.83% class iii 70% à 80% 14.65% à 16.74% class iv 55% à 70% 11.51% à 14.65% 4.3 mean values of oxide contents to determine the elemental composition of the gypsum, a chemical analysis encompassing 13 elements was carried out, and the results are presented in table 4. the mean concentration values indicate that cao and so₃ are the dominant oxides characterizing the djebel djira gypsum (fig. 9). the results show that the main components of the chemical composition exhibit very similar concentrations, with only negligible variations. these consistent values highlight the homogeneity of the gypsum, which represents the principal stratigraphic layer of the deposit. this observation is further supported by the coefficient of variation (v < 10%), characteristic of elements with highly regular spatial distribution (smith et al., 2018). the average contents of cao, so₃, and crystallization water (h₂o), measured at 33.46%, 43.58%, and 19.17%, respectively, confirm the good quality of this gypsum. similar studies have been reported for messinian gypsum deposits in tunisie, where cao and so₃, ranges between 31–36% and 40–46% (mahmoudi et al., 2016), confirming the coherence of the djebel djira results with other high-quality gypsum occurrences. the levels of potentially deleterious oxides, such as mgo and k₂o, remain within the acceptable chemical thresholds for its use in construction plaster production, in agreement with international standards astm c471m, 2016 and en 13279, 2005. similarly, the mean chloride content (0.011%) is negligible and consistent with values reported in other exploitable gypsum deposits (bouzit et al., 2019). the average na₂o content, determined at 0.09%, shows an irregular distribution across the deposit. based on these chemical characteristics, the djebel djira gypsum can be classified as category i. histogram analyses reveal a log-normal distribution for so₃, with tails skewed toward lower values, and a bimodal normal distribution for cao. most of the analyzed samples (97%) exhibit cao concentrations exceeding 30%, while 92% display so₃ values above 40% and crystallization water contents above 18%, confirming the classification of this material as category i gypsum. the statistical treatment presented here, combining coefficients of variation and frequency distribution analysis, provides a methodological novelty compared to previous studies, which generally focused only on mean chemical values without detailed homogeneity assessment (lushnikova & dvorkin, 2016). table 4. chemical composition of djebel djira gypsum based on multi-element analysis. average chemical composition of the djebel djira gypsum deposit variable number minimum maximum average sigma variability sio2 414 0.01 6.25 0.72 1.52 209.69 al2o3 414 0.01 1.72 0.21 0.44 206.34 fe2o3 414 0.01 0.43 0.04 0.17 381.05 cao 414 30.76 43.16 33.46 1.76 5.27 mgo 414 0.01 1.05 0.30 0.54 176.57 so3 414 37.42 46.00 43.58 3.69 8.47 ko2 414 0.04 0.33 0.07 0.09 128.04 nao2 414 0.01 0.85 0.09 0.26 288.23 tio2 414 0.01 0.03 0.01 0.005 44.06 p205 414 0.01 0.08 0.01 0.02 151.007 loi 414 21.08 29.29 21.29 0.94 4.43 h2o crystallization 414 16.02 20.69 19.17 1.35 7.08 cl 174 0.003 0.053 0.011 0.007 62.00 293abdelkader boucif, matthias reimann, mustapha bensalah, hakim hebib, and ali kacemi; 0 5 10 15 20 25 30 35 40 45 50 pe rc en ta ge c on te nt s chemical elements figure 9. histogram depicting the mean elemental composition of gypsum from djebel djira 4.4 x-ray analysis the x-ray fluorescence (xrf) analysis of two representative gypsum samples reveals a mineralogical composition that is practically identical to that of pure gypsum, with an average purity of 99% (table 5). both samples s.7.1p and s.7.2p consist almost exclusively of gypsum, with only trace amounts of dolomite (1%), while calcite and quartz are completely absent. such results attest to the exceptional purity of the djebel djira gypsum, which significantly enhances its suitability for industrial applications, particularly in the production of plaster. comparable studies on messinian gypsum deposits of the fatha formation (iraq) have reported slightly lower purities, where accessory minerals such as anhydrite, dolomite, and quartz are more frequently present (chro & jamila, 2021). likewise, natatou et al., (2012) observed that gypsum deposits in tahoua region (niger) commonly contain notable proportions of calcite or silicate impurities, which reduce their overall industrial grade. in this respect, the near-absence of secondary phases in the djebel djira gypsum represents a distinctive advantage, ensuring higher chemical stability and lower processing requirements. this outstanding mineralogical homogeneity, combined with the high purity rate, positions djebel djira gypsum among the most valuable deposits in the northwestern of algeria with direct implications for its economic exploitation. table 5. results of x-ray analyses sample number gypsum dolomite calcite quartz s.7.1p 99 01 s.7.2p 99 01 4.5 rock quality using rockworks version 17, the quantitative data stored in the project’s i-data table were interpolated into a block (solid) model to generate spatial distribution maps. the results reveal that the cao concentration within the gypsum deposit exhibits noticeable variability, primarily influenced by compositional heterogeneity and the intercalation of marl and sandstone layers. in particular, the eastern sector of djebel djira is characterized by relatively lower cao contents, ranging between 24% and 32% (fig. 10). similar heterogeneities have been reported in messinian gypsum deposits of the ebro basin (spain), where interbedded marls and siliciclastic inputs also contribute to local variations in cao content (orti et al., 2014). in general, gypsum containing approximately 32.6% cao is regarded as pure. within the djebel djira deposit, the distribution of so₃ exhibits a marked spatial variability, with average values reaching about 43.5% in the eastern sector and gradually decreasing to nearly 15% toward the western side (fig. 11). a comparative analysis of the two models indicates a gradual replacement of cao by so₃ in the eastern and southern sections of the deposit. in contrast, the northern part appears to consist of a formation distinct from limestone or gypsum. the eastern and southern region of the zone may contain a material undergoing transformation ‎ into gypsum (fig. 12). figure 10. cao concentration model of the djebel djira gypsum deposit. figure 11. so₃ concentration variability across the djebel djira gypsum deposit. figure 12. spatial distribution models of cao and so₃ within the djebel djira gypsum deposit, generated using rockworks software. 294 chemical characterization and valorization potential of subsurface gypsum deposits in the boufatis plateau, lower chelif basin 4.6 geological reserves geological reserves were quantified using the ez volume calculator module of rockworks 17 ‎software. the delaunay triangulation method was applied for the volumetric calculation, ‎interconnecting the sampling points in a triangular mesh, with each vertex corresponding to a ‎measurement point. volumes were determined from the thicknesses (z values) associated with ‎each triangle, then summed to obtain the total volume. the results indicate an estimated resource ‎of 36 million m³ for gypsum and 26 million m³ for cover layers within the study area, calculated on ‎the basis of an average gypsum thickness of 50 meters. 5. conclusion this study provides a comprehensive characterization of the gypsum deposits in the boufatis ‎plateau (lower chelif basin, nw algeria), combining chemical, mineralogical, and ‎geostatistical analyses to assess their industrial potential. chemical analysis has demonstrated‎ ‎that cao and so₃ are the principal oxides characterizing djebel djira gypsum. their ‎consistent‎‏ distribution, confirms, ‎the homogeneity of the gypsum. the results demonstrate‏ ‎that the djebel djira gypsum is of exceptional quality, with average cao (33.46%) and so₃ ‎‎(43.58%) contents compliant with international standards like iso 1587, and nf en 520+a1. x-ray diffraction confirmed a purity level of 99%, while low concentrations of deleterious elements such as mgo, k₂o, na₂o, and chlorides, along with high crystallization water content exceeding 18%, classify it as category i, ideal for plaster production exploiting drilling data with rockworks and surfer software revealed spatial heterogeneity. indeed, the eastern and southeastern sectors exhibit gypsum thicknesses exceeding 50 m with minimal ‎overburden less than 10 m, whereas the western zone, with thicker overburden greater than ‎‎40m, is less favorable for extraction. geotechnical analyses with an average rqd of 65% to ‎‎100% indicate good-to-excellent rock mass quality, suitable for stable mining operations.‎ as a result, djebel djira represents a significant natural resource with estimated reserves of 36 ‎million m³. among the practical recommendations, we propose prioritizing extraction in the ‎east and south-east sectors because the thickness of gypsum exceeds 50 meters, and the ‎cover is less developed. we strongly recommend that thorough environmental impact studies ‎be carried out to ensure sustainable mining of the deposit.‎ this research addresses major gaps in the understanding of gypsum deposits in algeria and provides a reproducible methodological framework for resource assessment. future perspectives should include economic feasibility studies and the development of sustainable extraction protocols, aligned with regional development objectives while preserving natural ecosystems and strictly adhering to environmental standards. acknowledgments the authors sincerely thank the editor-in-chief, prof. alexander caneva, for accepting the publication of this article in earth sciences research. they are grateful for the opportunity to disseminate their findings to a high-level scientific audience through this journal. special thanks are extended to the anonymous reviewers for their constructive remarks and valuable expertise, which have significantly improved the quality of this work. the authors also wish to express their deep appreciation to the knauf algeria team and the cetim laboratory for their essential support and assistance. references aloisi, g., guibourdenche, l., natalicchio, m., caruso, a., haffert, l., el kilany, a., & pierre, f. d. 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(2023). mechanical properties and fracturing behavior of jointed rock mass under uniaxial compression: insights from numerical simulations. scientific reports, 13, 1463. https://doi.org/10.1016/j.margeo.2014.02.002 https://doi.org/10.3406/geolm.1992.1474 https://doi.org/10.1007/s12517-013-1154-4 https://doi.org/10.1007/s12517-013-1154-4 https://doi.org/10.3389/fspas.2025.1503042 https://doi.org/10.3390/app132312855 https://doi.org/10.17159/2411-9717/2018/v118n4a1 https://dx.doi.org/10.1016/j.palaeo.2010.07.017 https://dx.doi.org/10.1016/j.palaeo.2010.07.017 https://doi.org/10.3390/buildings14071992 keywords: 3earthquake; precursor; electromagnetic signal; ses; colombia; palabras clave: sismo; precursor; señal electromagnética; ses; colombia; how to cite item solano fino, j. m., caneva, a., vargas, c. a., & ochoa, l. h. (2021). electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin. earth sciences research journal, 25(3), 297-307. https://doi.org/10.15446/esrj. v25n3.95782 in this study, the authors worked on identifying a possible relationship between electromagnetic signals (em) and seismic events in the lithospheric system in the central region of colombia. the data, both seismic records and electromagnetic signals, were taken from the catalog of the seismological network of the national university of colombia (rsunal) and the catalog of the national seismological network of colombia (rsnc). the project included the design and instrument testing phases for recording seismic signals, electrical potential variations, and magnetic field variations to try to identify possible relationships between these signals. possible electromagnetic precursors for seismic events were observed, mainly magnetic disturbances, but it was not possible to locate evident electrical anomalies (seismic electric signals ses). thus, although the results are not conclusive, the magnetic disturbances identified deserve further long-term analysis. abstract electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin observaciones de series de tiempo de datos eléctricos y magnéticos como una aproximación para identificar actividad sísmica de origen no antrópico issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v25n3.95782 en este trabajo, los autores buscaron identificar la existencia de una posible relación entre señales electromagnéticas (em) y eventos sísmicos que ocurren en el sistema litosférico en la región central de colombia. los datos, tanto de registros sísmicos como de señales electromagnéticas, fueron tomados del catálogo de la red sismológica de la universidad nacional de colombia (rsunal) y del catálogo de la red sismológica nacional de colombia (rsnc). el proyecto contempló las fases de diseño y prueba de instrumentos para el registro de señales sísmicas, variaciones del potencial eléctrico y variaciones del campo magnético, para luego tratar de identificar posibles relaciones entre estas señales. se observaron posibles precursores electromagnéticos de eventos sísmicos, principalmente perturbaciones magnéticas, pero no fue posible identificar anomalías eléctricas claras (señales eléctricas sísmicas ses). por lo tanto, aunque los resultados no son concluyentes, las perturbaciones magnéticas identificadas merecen un análisis adicional a largo plazo. resumen record manuscript received: 15/05/2021 accepted for publication: 10/08/2021 earth sciences research journal earth sci. res. j. vol. 25, no. 3 (september, 2021): 297-307 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa universidad nacional de colombia corresponding author: jmsolanof@unal.edu.co g eo ph ys ic s https://doi.org/10.15446/esrj.v25n3.89293 https://doi.org/10.15446/esrj.v25n3.89293 https://doi.org/10.15446/esrj.v25n3.89293 298 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa introduction the forecast of seismic events implies estimating possible hypocentral parameters, the time of occurrence, and the magnitude of the predicted event. conducting this type of forecasting is a complex task that poses important challenges related to understanding the mechanics of earthquakes, the permanent variations in the field of efforts, and the technology strategy to be used. to date, this type of forecast has been performed considering events that release large amounts of energy (m > 5-6), but the overall ability to forecast earthquakes, either individually or on a statistical basis, remains remote (tiampo & shcherbakov, 2012; cicerone et al., 2009). however, research groups from different countries have shed some light on certain precursor phenomena taking place before the occurrence of an earthquake. in the last decades, there has been remarkable progress in the techniques and methods used for earthquake forecasting on a global level. physical process models such as accelerating moment release (amr), variations in the b-value and the m8 family of algorithms, among others; as well as the smoothed seismicity models, whose most relevant techniques are 1) the time-independent smoothed seismicity, 2) the relative intensity (ri) model or 3) the proportional hazard model (phm), account for the variety of available options, whose study and statistical modeling generate promising results in the matter (tiampo & shcherbakov, 2012). the identification of electromagnetic precursors has also been the object of study as several hypotheses have been tested, e.g., models of dc electric field formation in the ionosphere by seismic events (sorokin et al., 2005) or fractal and percolation theories. another method consists of the identification of seismic electric signals (ses), and more recently, the natural time analysis, based on the correlation between the occurrence of seismic events and the ses with the varotsos-alexopoulos-nomicos (van) method, which has shown partial success in short and medium-term forecasts (varotsos et al., 2019; 2006; 1993) and analyzes after the occurrence of large seismic events (sarlis et al., 2021; 2015). however, its methodology and effectiveness have been tested, and the high rate of false positives have derived on low reliability of the van method to the point that, eventually, it would be the government that decides to issue an alert without adequate scientific support, leading this methodology to be an unproven hypothesis (vargas et al. 2021; helman, 2020; lighthill, 1996). the physical mechanisms proposed for generating electromagnetic emissions such as ses are related to piezoelectric, hydrodynamic, and electrokinetic effects, among others (petraki et al., 2015; freund, 2011; hunt et al., 2007; makarets et al., 2002; ballato, 1995). controlled experiments have been conducted that determine the characteristics of electromagnetic signals propagation, depending on the geographical conditions, conductivity, edge effects, among others (huang, 2005). for example, quartz-free gabbro samples have been subjected to periodic stresses of up to 22 mpa, producing electric currents between 50 and 400 pa for dry samples and no current for the fully saturated ones (dahlgren et al., 2014). reports of seismomagnetic effects (stacey, 1964) and the use of ultra-low frequency (ulf) variations of the magnetic field before, during, and after an earthquake have positive results (hattori, han, & huang, 2013; hattori et al., 2013; takahashi et al., 2007; johnston, 1997; park, 1996), unipolar magnetic pulses (scoville et al., 2015) and, more recently, showing ulf magnetic field bursts containing electromagnetic pulses with a period between 15 and 20 seconds before m6.8 and m8.5 seismic events in april 2012 and september 2013 respectively, unassociated with lightning or magnetic storms (swati et al., 2020) supporting the theory of ionospheric disturbance by acoustic gravitational waves (singh et al., 2018). this work tries to identify a possible relationship between electromagnetic (em) signals recorded on the earth’s surface and medium and large-scale seismic events, under the hypothesis of the existence of em precursors associated with seismic sources, which arise before seismic events, caused by established physical mechanisms (clavijo ramírez & sánchez, 2016). the procedure to be followed involves recognizing and analyzing possible seismic electric signals in the time domain, looking for anomalies before seismic events (varotsos et al., 2013; uyeda et al., 1999) through observation, identification, and analysis of other anomalies, both electrical and magnetic. figure 1 shows the location of three (3) permanent multi-parameter stations (recording of seismic, electrical, and magnetic signals) and the recorded earthquakes, with magnitudes mw ≥ 4.2. as a result, magnetic disturbances, mainly long-term, have generated certain conjectures related to possible seismic precursors, aspects that are presented and discussed in this work. figure 1. rsunal instrumental deployment with seismic stations (yellow triangles) and permanent, multi-parameter stations (red squares) and analyzed earthquake epicenters. 299electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin methodology an instrumental deployment of multi-parameter stations was performed, acquiring seismic, electrical, and magnetic signals on the surface. with these data, we proceeded to identify possible correspondences between seismic events and em disturbances before these events, in windows of up to 11 days (varotsos et al., 1993). the seismic, electrical, and magnetic signal recording was performed in three stations of the seismological network of the national university of colombia (rsunal); first, during 2016 and, later, during 2020. this experiment was performed using permanent, multi-parameter rsunal stations, part of a project supported by colciencias, the national university of colombia, and the antonio nariño university (solano fino, 2017). the rsunal is a hybrid arrangement of eight (8) triaxial seismic stations, seven (7) broadband, and one (1) short period. three (3) of the seven (7) broadband stations are multi-parameter since they have five (5) additional channels, three (3) magnetic with orthogonal components, and two (2) electrical orthogonal on the surface (rsunal, 2021). this work used the data from the three (3) multi-parameter stations: usme, tunj, and vcio (see figure 1). the sensors used in multi-parameter stations are: • broadband seismometer cme 4311: electrochemical transducer designed for permanent or portable installation. it is a robust instrument that is easy to install. in addition, it does not require maintenance, mass blocking, or leveling. the instrument offers an effective solution for installations with a noise level close to the low noise model with a response between 1/60 and 50 hz (cme, 2021). • non-polarizable electrode for burial tinker & rasor db-a: copper and copper sulfate (cu/cuso4) electrode, non-polarizable, allows direct exposure of copper sulfate over a large contact area. it has a shelf life of up to 10 years. its structure is made of pvc/abs and has a low freezing point and high evaporation point, making it robust for most environments. dimensions 7 cm in diameter, 12.2 cm tall, and a weight of 896 g (tinker-rasor, 2021). • magnetometer bartington mag648l: low power, low noise triaxial magnetometer with ± 60 µt range. it has vehicular, perimeter security, and ground measurements applications (bartington instruments, 2021). the multi-parameter stations were designed and assembled in the geophysics instrumentation laboratory (lig) of the geosciences department of the national university of colombia. consist of a 10-inch pc and three hardware devices communicated through usb ports: an acquisition card (7 differential channels at 16 bits and a sampling frequency up to 3 khz), a gps 18x lvc (garmin international inc., 2011), and a 3g modem for sending data via cell phone. all the elements previously described are installed in a resistant case, with external connection ports for sensors (broadband seismometer, electrodes, and magnetometer), 3g usb modem, gps, and power. with them, low electromagnetic noise levels are achieved. each station stores and sends data from three seismic, two electrical, and three magnetic channels in near real-time. according to the seed reference manual (ahern et al., 2014), the nomenclature of the channels is shown in table 1. table 1. channels nomenclature used on multi-parameter stations. sensor chanel nomenclature seismometer north bhn east bhe vertical bhz dipoles north hqn east hqe magnetometer north hfn east hfe vertical hfz the multi-parameter stations are installed at the usme (bogotá), villavicencio (meta department) and tunja (boyacá department) campuses of the antonio nariño university uan (uan, 2021). its installation was performed following the general recommendations for ses measurement (varotsos et al., 2006a). the digitizer, the seismometer, and the magnetometer were installed inside a 1 m x 1 m x 1 m booth, with access to the urban electrical power supply. the electrodes were installed according to the possibilities of terrain and topographic dimensions offered by each of the uan campuses. the electrodes locations at each station, forming quasi-orthogonal dipoles, are shown in figures 2, 3, and 4, including their lengths. figure 2. usme (usme) station electrodes location. n-s dipole = 97 m; e-w dipole = 98 m. figure 3. tunja (tunj) station electrodes location. n-s dipole = 85 m; e-w dipole = 67 m. figure 4. villavicencio (vcio) station electrodes location. n-s dipole = 130 m; e-w dipole = 70 m. a coaxial cable connects the electrodes and digitizer to reduce em noise. the cable connection to the electrode was protected with silicone to reduce corrosion. each electrode is buried approximately 50 cm deep. 300 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa data processing once the data were acquired, the authors selected the period to be analyzed. considering that the ses could be recorded from minutes before and up to 11 days before a seismic event (varotsos et al., 1993), the period for analysis began 11 days before the selected event and ended 24 h after the event to verify the possible presence of post-event disturbances. the considered seismic events were those with magnitude m ≥ 4.2, with an epicenter at a distance no greater than 150 km from any station or up to 300 km if the magnitude of the earthquake was mw > 5. table 2 shows the considered earthquakes. group a seismic events belong to the mutatá thrust fault, with high angle, 150 km length and directly influenced by murindó (left lateral, 60 km length, n12.6w strike) and urumita (thrust, 300 km length, chocó-panamá, and north andes boundary) faults (reyes garcía, 2019). group b earthquakes come from los santos (or bucaramanga) nest at 60 150 km depth, possibly due to the collision of subducted nazca plate and old farallon plate detached slab by delamination lithosphere effect (chicangana & vargas, 2008). group c and d earthquakes are related to the algeciras fault system near the altamira segment on the sw colombia; it has right lateral displacement with neotectonic features involving basement and sedimentary layers. the altamira thrust fault has a sinuous trend with n45°e strike and sw dip between 50° and 70° (sgc, 2016). group e seismic event is related to the nazca plate subduction at 170 km depth but with a normal fault focal mechanism associated (7°, 61°, -124°; 241°, 44°, -45°) according to the sgc. group f seismic event comes from the colombian llanos foothill delimited by two inverse fault systems: guaicaramo and cusiana (sgc, 2010), the event has a thrust focal mechanism (351°,36°, 41°; 226°, 67°, 119°). a hypothesis related to group g events relates to the deep injection of co-produced wastewater from oil and gas extraction (gomez alba & vargas, 2020). results registered data the data gathered at usme, tunj, and vcio stations were processed, including traces merging, gap and overlaps elimination, and application of different filters (solano-fino, 2017). also, scaling of the data to the corresponding units of electric potential [v] and magnetic field [nt] in windows of 11 days before each event (or set of events) was performed. table 2. seismic events considered as precursors. # group date hour lat long depth mag department municipality utc [°] [°] [km] [mw] 1 a 14/09/2016 01:58:31 7.236 -76.236 0.0 6.1 antioquia mutatá 2 14/09/2016 06:46:59 7.225 -76.304 0.0 4.8 antioquia mutatá 3 14/09/2016 12:02:52 7.206 -76.245 0.0 4.6 antioquia mutatá 4 14/09/2016 15:59:34 7.241 -76.342 0.0 4.7 antioquia mutatá 5 b 22/09/2016 04:32:52 6.817 -73.100 151.4 4.2 santander los santos 6 22/09/2016 10:42:17 6.816 -73.105 156.0 5.0 santander los santos 7 c 31/10/2016 00:20:12 3.405 -74.636 13.2 5.3 huila colombia 8 31/10/2016 02:05:58 3.417 -74.615 0.0 4.8 huila colombia 9 d 16/12/2016 19:45:18 3.431 -74.645 0.0 4.2 huila colombia 10 e 28/12/2016 05:05:05 1.817 -77.077 150 4.8 cauca mercaderes 11 f 25/04/2020 15:30:55 4.999 -72.861 15.59 4.6 casanare monterrey 12 g 29/09/2020 22:05:36 3.921 -71.487 -1.99 4.6 meta puerto gaitán 13 04/10/2020 21:32:02 3.918 -71.484 -0.16 4.3 meta puerto gaitán figure 5. electrical and magnetic data. usme station. 11 days’ period. group c earthquakes (discontinuous red lines). 301electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin figure 6. electrical and magnetic data. tunj station. 11 days’ period. group c earthquakes (discontinuous red lines). figure 7. electrical and magnetic data. vcio station. 11 days’ period. group c earthquakes (discontinuous red lines) herein authors show the results of processed data gathered at the three stations for the set of events of group c (table 2) since this data has better quality and more visible anomalies. figures 5, 6, and 7 show the electrical and magnetic data acquired in the five (5) em channels at usme, tunj, and vcio stations, 11 days before and one after the two earthquakes corresponding to the group c. magnetic (hqx) and electrical (hfx) disturbances are marked in green. the authors were able to see that the hqn and hqe electric dipole channels do not present identifiable ses. the electrical disturbances are consistently above the dc level of the signal, which, in this case, is equal to 0 v. it was observed that signals from the usme station have a low noise level in its electrical channel, while the tunj and vcio stations have noise that sometimes reaches 1 v pick-to-pick. there is a defined 24-hour cycle of alternating from low noise (12 h duration) to high noise (12 h duration), possibly due to the operation of some electromagnetic devices in the vicinity of the stations. spectral analysis of the electric field data showed the signal’s randomness since there were no amplitude peaks in db/hz in relation to ses or another type of signal, different from noise. 302 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa the hfn, hfe, and hfz magnetic channels show variations in the magnetic field related to the earth’s rotation with an approximate period of 24 h and minor disturbances, possibly due to cultural noise (radio, television, cell phones). however, some disturbances could be related to seismic events. table 3 summarizes the disturbances found in the five electromagnetic acquisition channels. authors were able to identify slow signal variation only on time since these signals were filtered below 0.1 hz looking for more identifiable particularities at very low frequencies. discussion measurements of electrical potential differences, performed using nonpolarizable dipoles, do not allow establishing a causal relationship regarding seismic events with mw ≥ 4.2 for the period evaluated in this study. the gathered data yields inconclusive results regarding the behavior of electric field disturbances. this applies to both one-time ses and ses activities. for this reason, in this work and for the analyzed time windows, it is not possible to associate some variations in the electrical signal as precursors with the earthquakes selected for the study. the performed spectral analysis neither shed any light on the behavior of the anomalies, despite their use in other works (swati et al. 2020). although authors do not observe ses in the electrical channels (hqx), changes are observed in the magnetic signals’ ultra-low-frequency (ulf) amplitude before a seismic event. it is a decrease in the intensity of the daily peak-to-peak magnetic field with an increase on the average in the same period, days before or almost simultaneously with the occurrence of the earthquake. for each observed anomaly, the period was doubled to 22 days to check that there was no similar disturbance before the 11 days’ window. only stations with visible anomalies are displayed. visual inspection would make it possible to identify the presence or absence of more anomalies and, therefore, the possibility that they serve as an empirical precursor of a seismic event. this can be seen in figures 8, 9, 10, and 11 (for 2016) and figures 12, 13, 14, and 15 (for 2020), where the anomalies remain unique for each 22day window, supporting the hypothesis that they may be precursor disturbances of seismic events in the cases and stations here described. note that anomalies can occur even when high-frequency noise is present in the trace. there is new evidence on magnetic susceptibility variation due to the wenchuan earthquake with slip dynamics linked to high-temperature thermal (liu et al., 2016) and the tohoku-oki earthquake where the magnetic fabric of the subduction wedge accretion prism indicates abrupt coseismic changes (yang et al., 2013). however, the data obtained from the magnetic sensors of the three stations in some channels or, in other cases, in all channels, allows us to see not table 3. electrical and magnetic disturbances summary. seismic events group perturbation a b c d e f g ses unique hqn no no no no no no no hqe no no no no no no no ses activity hqn no no no no no no no hqe no no no no no no no magnetic perturbation hfn 9 usme. 1 vcio 4 usme 4 tunj. 1 vcio 1 usme. 3 tunj. 1 vcio 3 tunj 1 usme. 2 tunj 1 usme. 1 tunj hfe 9 usme. 1 vcio 5 usme no 1 usme. 1 vcio no 1 usme. 2 tunj 1 usme. 2 tunj hfz 1 vcio no 2 tunj. 1 vcio 1 usme. 1 vcio 3 tunj 2tunj no figure 8. low-frequency amplitude anomalies (on green) before the group a earthquakes. station vcio. 22 days’ window. 303electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin only the periodic variations of the magnetic field because of the earth’s rotation but also is possible to identify minor disturbances that could be associated with seismic events. although these associations of alterations in the magnetic field to a particular seismic event are merely suggestions, a long-term study with a more significant number of stations could be more indicative of the possible presence of magnetic precursors detectable by the rsunal multi-parameter stations including, if they exist, selectivity (the soil ability to allow em signals reach the surface) and proportionality (direct relation between the em anomaly amplitude or duration and the seismic event magnitude) properties on this phenomenon (varotsos et al., 2006b). magnetic anomalies are described in detail below. in figure 8, the vcio station shows a reduction in the magnetic field peak-to-peak between september 7th and 9th, starting seven (7) days before the four (4) mutatá 2016 seismic events (group a); this corresponds to 70% the usual 24 h peakfigure 9. low-frequency amplitude anomalies (on green) before the group b earthquakes. station usme. 22 days’ window. figure 10. low-frequency amplitude anomalies (on green) before the group c earthquakes. station usme. 22 days’ window. to-peak. there is noise between 09/01 and 09/04 related to geomagnetic storm conditions according to the daily planetary a-index with values between 28 and 40, from 09/05 to 09/08 a-index is less than 17, which does not apply for a storm, and after 09/09 values are less than 8 (national oceanic and atmospheric administration, 2021). although a low peak is absent at 18:00 on 09/08, an ionospheric electric field reversal cannot be assumed since there is no electric field or electron or current density data support (rastogi, 1997). figure 9 shows, on usme station, a 50% peak-to-peak reduction, starting on september 19th, three days before the los santos seismic events (group b), with a duration of about 40 h. on usme station, a five (5) days anomaly starting on september 25th preceded the group c seismic events in colombia (huila) with a 50% peak-to-peak reduction continuing after the events and including a high-frequency noise increase (figure 10). 304 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa the magnetic signals recorded on vcio station before group c (colombia, huila) seismic events (figure 11) show a 50% peak-to-peak amplitude reduction beginning three (3) days before them and disappear 12 h before the earthquakes. in figure 12, the usme station lacks the vertical magnetic channel before the group f monterrey (casanare) seismic event. still, the horizontal channels show an anomaly stopping exactly when the earthquake occurred and beginning on april 21st, with a duration of approximately 70 h and a 30% reduction in amplitude. in figure 12, the usme station lacks the vertical magnetic channel before the group f monterrey (casanare) seismic event. still, the horizontal channels show an anomaly stopping exactly when the earthquake occurred and beginning on april 21st, with a duration of approximately 70 h and a 30% reduction in amplitude. in figure 13, the tunj station exhibits what seems to be electronic acquisition noise on the east-west and vertical channels before the figure 11. low-frequency amplitude anomalies (on green) before the group c earthquakes. station vcio. 22 days’ period. figure 12. low-frequency amplitude anomalies (on green) before the group f earthquake. station usme. 22 days’ period. monterrey event until april 16th; however, north-south channel and after april 16th, all the channels show good quality data, and between april 22nd and 25th a reduction of 30% the peak-to-peak amplitude can be seen occurring 3.5 days before the seismic event. both figures 12 and 13 do not show a simultaneous high-frequency noise with the low-frequency anomalies. due to electronic saturation (figure 14), the vertical magnetic channel at station usme is not present. two anomalies can be appreciated occurring before the puerto gaitán (meta) group g events; both anomalies last about 65 h with a 50% reduction in the amplitude and begin seven (7) days before the september 29th mw4.6 event, and six (6) days before the october 4th mw4.3 event. the second anomaly coincides with the first event, and, in both cases, high-frequency fluctuations are present. finally, in figure 15, station tunj has noise on the vertical magnetic channel, probably due to electronic acquisition problems or on-site cultural noise; however, horizontal channels operate 305electrical and magnetic data time series’ observations as an approach to identify the seismic activity of non-anthropic origin figure 13. low frequency amplitude anomalies (on green) before the group f earthquake. station tunj. 22 days’ period. figure 14. low-frequency amplitude anomalies (on green) before the group g earthquakes. station usme. 22 days’ period. figure 15. low-frequency amplitude anomalies (on green) before the group g earthquakes. station tunj. 22 days’ period. 306 juan manuel solano fino, alexander caneva, carlos alberto vargas, luis hernán ochoa properly and show an anomaly seven (7) days before the september 29th mw4.6 puerto gaitán (meta) event with a duration of 80 h and 50% amplitude reduction, there was no anomaly before the october 4th mw4.3 event. so far, the presence of such magnetic anomalies lacks a model considering earthquakes’ magnitude and location and the geology setting of each station. still, there are clear tendencies: the anomalies last between two (2) and four (4) days, begin from three (3) to seven (7) days before the earthquake(s) and show a 30% 70% peak-to-peak amplitude reduction compared with the pre and post magnetic data. in this way, 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(2023). computer vision techniques applied to automatic detection of sinusoids in borehole resistivity imaging – a comparison with the msd method. earth sciences research journal, 27(2), 139-147. https://doi. org/10.15446/esrj.v27n2.101556 resumen en esta investigación se aplicaron técnicas de visión por computador a imágenes resistivas de pozo para establecer un procedimiento alternativo al procesamiento del buzamiento medio cuadrático (bmc); bmc es regularmente empleado para detectar sinusoides y buzamientos automáticamente en registros de imágenes y de buzamiento. esta propuesta se fundamenta en filtros gabor, transformaciones morfológicas, transformada de hough y técnicas de agrupación. el método bmc y la propuesta de visión por computador fueron probados en 1012 m de imágenes de pozo, mostrando 7.986% de falsos positivos para el procesamiento bmc y 0.879% para el enfoque de visión por computador. la presente metodología metodología trata de emular el comportamiento de los geólogos cuando realizan interpretación de imágenes, en lugar de correlaciones entre curvas de resistividad como en el procesamiento bmc. no hay requisitos informáticos especiales y puede aplicarse directamente en campo para resultados rápidos de buzamientos. esta metodología puede integrarse fácilmente a unidades de registro, así como también a la mayoría de programas de procesamiento de imágenes de pozo. todos los procesos se desarrollaron en python utilizando librerías estándares. https://doi.org/10.15446/esrj.v27n2.101556 mailto:alealf@unal.edu.co mailto:lhochoag@unal.edu.co mailto:sfacostale@unal.edu.co https://doi.org/10.15446/esrj.v27n2.101556 https://doi.org/10.15446/esrj.v27n2.101556 140 jorge alberto leal freitez, luis hernán ochoa gutiérrez, sergio francisco acosta lenis 1. introduction in conventional and unconventional hydrocarbon reservoirs, borehole resistivity imaging is broadly applied in exploration and development wells; this log provides precise information regarding structural framework, sedimentological features, and stratigraphy of the drilled rock sequence. furthermore, this log is a useful complement in petrophysical evaluation, geomechanical modeling, and characterization of naturally fractured reservoirs. once these logs are acquired, their processing and interpretation can take several hours or even days because detection and classification of geological planes is usually a manual process performed by specialized geologists. this time consuming task often hinders decision-making at drilling sites, particularly in activities related to fractures detection, intervals selection for pressure or fluids tests, or geosteering programs. the lack of information provided by image logs might result in operational problems, and subsequent economic losses for field operators. in order to get information shortly after image logs are acquired, several automatic dip picking or autodip techniques are employed; these techniques have been utilized since decades ago with resistivity data of dipmeter and borehole imaging tools (grace, et al., 2000). these techniques cannot classify the detected dip and sinusoid, but they can give information about the structural framework and sedimentary features along the wells. the autodip approaches are based on correlation between resistivity curves around the borehole to identify displacements, which then define a surface across the borehole (rider, 2000). among practical methods, the mean square dip (msd) is one of the most commonly applied technique (grace, et al., 2000). although the msd may lack of accuracy and provide just a few samples in some cases, it is still one of the standard methods for automatic planar geologic feature detection in oil wells. this method is employed by most oilfield service companies when automatic image picking is required. in the latest years, several authors have proposed some methodologies based on computer vision and machine learning to automatize dip detection in borehole imaging, the most relevant to this research are al-sit, et al., 2015; assous, et al., 2014. al-sit, et al., 2015 suggest multi-resolution texture segmentation and pattern recognition utilizing gabor’s filters (gabor, 1946). this processing is combined with an iterative adaptation of the modified hough’s transform to enable non-distinct, partial, distorted, and steep fracture and layers identification. in this article, the tests were conducted using an optical televiewer tool, which provides 360° images of the wellbore wall. image processing in this case did not require procedures of image reconstruction like the present work does. assous, et al., 2014 propose a gradient-based approach for edge detection with a phase congruency method for validation, followed by a sinusoid detection technique. the method was evaluated using borehole resistivity imaging from wireline and logging while drilling tools. the proportion of false positives reported, in the case of noisy data is less than 5%, improving to better than 2% in case of good quality data. these authors do not employ gabor’s filter, instead, they use an edge detection technique based on the lindeberg algorithm (lindeberg, 1996); moreover, they do not consider any clustering technique to debug the final results. the present work proposes an automatic dip picking procedure based on bio-inspired image filters (gabor’s filter), along with morphological transformations to highlight the edges. furthermore, this work employs hough’s transform to detect one-cycle sinusoidal objects and unsupervised machine learning (clustering) to refine the outcomes. this research shows figure 1. example of dynamic (left) and static (right) images. 141computer vision techniques applied to automatic detection of sinusoids in borehole resistivity imaging – a comparison with the msd method that computer vision techniques can replace the typical correlation between resistivity curves when automatic dip processing is required. like in the msd, detected sinusoids in this work are not classified, but they can be employed for structural, sedimentological, and natural fractures analyses. next paragraphs explain the computer vision techniques employed in a friendly sequence of stages, easy to follow for borehole geologists and software developers. 2. borehole resistivity imaging the resistivity images utilized in this research were acquired in water mud environment employing pad-based tools. this log represents a two-dimensional view of the borehole wall, it has vertical resolution of 5.0 millimeters and coverage of 80% (holes with 21.59 cm in diameter). borehole imaging provides information about structural dip, sedimentary structures, thin layers, electrofacies, fracture networks, and horizontal stress orientation in the field, among other applications (grace, et al., 2000). the image generation follows a regular sequence, beginning with log quality control, speed correction, and finally generation of the dynamic and static images from resistivity data normalization. the dynamic normalization is performed each foot or every two feet of log, providing a clear view of rock details along the well; the static normalization is performed on the whole logged interval, optimizing tool operations under extreme resistivity values (figure 1). the gray gaps in figure 1 represent borehole sections not covered with the image tool, as this is a padbased tool. a color code is used to interpret resistivity images, where light colors represent high resistivity values often related to limestones, oil-bearing lithologies, calcite precipitations, etc. dark colors indicate low resistivity measurements, regularly logged in conductive rocks, open pore space, borehole breakouts, and some minerals (e.g. clays, pyrite, etc.). the most common geologic events observed in image logs are well-defined planes that can be associated with bed boundaries, sedimentary structures, faults, and fractures. except for the drilling-induced vertical fractures, the geological planes are observed as sinusoids in image logs. these sinusoids are the projection or trace of planar features in the borehole wall. the plane inclination (dip) can be determined from the sinusoid amplitude; the lowest point direction on the sinusoid represents the dip azimuth as shown the figure 2. 3. correlations between resistivity curves – method of mean square dip (msd) a bedding plane can be identified by correlating the resistivity curves at known positions of the tool pads. the borehole in figure 3 shows an example of intersection by a dipping plane; the plane has a different resistivity with respect to the formations above and below. the four pads in the example continuously record a resistivity or dip curve as the tool is pulled up the borehole. as each pad passes the intersection of the plane with the borehole wall, the corresponding dip curve shows a change of resistivity. the dip or slope of the plane causes the pads to encounter and record the change in resistivity at different depths on the log. the difference in depth or shift of the corresponding peaks on the curve allow the dip of the geologic plane to be calculated trigonometrically (grace, et al., 2000). the msd method works on the basis of an interval correlation algorithm between selected pairs of sensors to compute displacement vectors that should all lie in the same dip plane. the msd method considers the same depth interval on each curve and uses only the data within that interval to make correlations. in the case of low apparent dip it can be seen that nearly all the data points within the interval are considered when the correlation is made. as the apparent dip increases, less and less points enter into the correlation. in areas where high dips, or high apparent dips because of deviated borehole conditions are expected, an initial displacement, i.e., a focusing plan, can be defined by the interpreter. this focusing plane can be chosen as either a fixed plane with orientation defined by the analyst or a plane defined from a previous dip calculation. the button-to-button displacements are computed and the bestfit plane through them is found (grace, et al., 2000). 4. computer vision techniques computer vision can be defined as a scientific field that extracts information out of digital images. the type of information gained from an image can vary from identification, space measurements for navigation, or augmented reality applications (ranjay, 2017). another way of defining computer vision is through its application; computer vision is about building algorithms to understand the content of images enabling computers to see, identify and process images in the same way that human vision does. the diagram in figure 4 summarizes the sequence of computer vision techniques employed along figure 2. trace of planar feature as sinusoid in borehole image (modified from grace, et al., 2000). 142 jorge alberto leal freitez, luis hernán ochoa gutiérrez, sergio francisco acosta lenis each analyzed well; the process begins with the dynamic image as input and ends with the same image with all detected sinusoids. 4.1 stage 1 inpainting the process starts with an rgb color image from the dynamic normalization in scale of 1:10, with dimension of 300 pixels long by 360 pixels wide (figure 5a); this image is converted into a grayscale image, in which each pixel represents a single value with intensity information (figure 5b). subsequently, an inpainting technique based on a fast marching method is applied. digital inpainting provides a means for reconstruction of small damaged portions in images; this procedure starts from the boundary of the region to be inpainted and moves towards the damaged section, gradually filling everything in the boundary first. each pixel is replaced by a normalized weighted sum of all the known pixels in its neighborhood. more weight is given to those pixels lying near the boundary of the missing parts, such as those near contours (telea, 2004). once a pixel is inpainted, it moves to the next nearest pixel using the fast marching method, which ensures those pixels nearest to the known pixels are inpainted first (khan, 2019). digital inpainting in this work was employed to reconstruct the images in those sections not covered by the image tool. it is important to notice that inpainting processing in this research is not pursuing an exact 360° view of the borehole wall; instead, this processing is applied to guide the following steps related to edge map generation. although figure 5c shows some artifacts added by inpainting procedures, experimental iterations show that without image inpainting the final number of detected sinusoids decreases, and the number of false positive increase. hence, consistent improvements were observed in the outcomes using inpainting, adding it permanently to the image processing pipeline. 4.2 stage 2 – edge detection the objective of this stage is to map edges from the grayscale inpainted image by applying gabor’s filters, image binarization, morphological transformations, and gaussian denoising. the edges of a figure are those points at which the luminous intensity changes sharply, reflecting variations in image properties; edges are important to understand and identify particular features in the whole image (neer and mathur, 2016). the first step in this stage consists in applying a gabor’s filter bank (figure 6d); this filter was originally introduced by dennis gabor in 1946 and currently has wide application in image processing (arora and sarvani, 2017). the gabor’s filter detects image gradients of a specific orientation; the convolution of image patches with different scales and orientations (filter bank) allows extraction and encoding of local texture information usable for classification (maynberg and kush, 2013). frequency and orientation representations of gabor’s filters are similar to those of the human visual system, particularly appropriate for texture representation and classification. in the spatial domain, a 2d gabor’s filter is a gaussian kernel function modulated by a sinusoidal plane wave. simple cells in the primary visual cortex of mammalian brains can be modeled by gabor functions. thus, image analysis with gabor’s filters is thought to be similar to perception in the human visual system (arora and sarvani, 2017). the filter has a real and an imaginary component representing orthogonal directions. the two components may be formed into a complex number or used individually, modeled by the equations (1) for complex, (2) for real, and (3) for imaginary. (1) (2) (3) where: figure 3. correlation between resistivity curves (modified from grace, et al., 2000). 143computer vision techniques applied to automatic detection of sinusoids in borehole resistivity imaging – a comparison with the msd method and: in the above equations: λ – wavelength of the sinusoidal component. θ – the orientation of the normal to the parallel stripes of gabor function. ψ – the phase offset of the sinusoidal function. σ – the standard deviation of the gaussian envelope γ – the spatial aspect ratio and specifies the ellipticity of the support of gabor function. after gabor’s filter is applied in several scales and orientations (filter bank, figure 6d), the resulting image is thresholded and converted into a matrix of one (white) and zero (black). in the second step of this stage two kinds of morphological transformations are employed, skeletonization and dilation (figure 6e). morphological transformations are simple operations based on the image shape, normally performed on binary images. the skeletonization reduces binary objects to one-pixel wide and can be useful for feature extraction; this morphological transformation converts white sections into a one-pixel wide skeleton, with the same connectivity as the original figure. although the skeleton image in this step seems to be clean and optimal for the following hough’s transform processing, it is still a low-quality image and has not enough white pixels for sinusoid recognition. once the skeleton image is created, morphological transformations of dilation are used to highlight the edges. morphological dilation augments the white sections by enlarging bright regions of the image (pratt, 2007). the final step in this stage can be considered as an initial cleaning step (figure 6f); the objective is to generate an edge map clean enough for hough’s transform processing. in order to do so, a second gabor’s filter is employed, but this time with a fixed angle and kernel. this filter removes noise or artifacts aligned 90° in the image; these vertical artifacts can be produced by previous inpainting processes applied over sections without resistivity data in the borehole wall. at this point, some salt and pepper noise can remain in the image and a gaussian blur is applied to remove it. gaussian blur is a smoothing technique to eliminate noises, where the image is convolved with a gaussian kernel. 4.3 stage 3 – sinusoid detection this stage includes computation of all parameters of the hough’s transform for sinusoid detection, and according to the equation (4) they include: (4) figure 5. computer vision. processing in the stage – 1 (inpainting). figure 4. image processing pipeline. 144 jorge alberto leal freitez, luis hernán ochoa gutiérrez, sergio francisco acosta lenis x, y – pixel position (x, y) amp – amplitude ω frequency (one cycle sinusoids; ω = 1) θ – phase y0 – depth after denoising processing in stage 2, this stage continues with the generation of two equal-size images from the edge map. the first image can be created from the left side of the edge map, having 300 pixels long by 180 pixels wide (from pixel 1 to pixel 180); the second image has the same shape, but its width goes from pixel 181 to pixel 360. afterward, these images are added using a function for image addition. the bright spots in the resulting image are added at each level (i axis), and its values are represented as a curve at the left side of the added image (figure 7g). the spikes in this curve are likely levels where a sinusoid might be present, and they can be considered as likely “y0” values in equation (4). in the second step of this stage, the sinusoid phase (θ) must be determined. to compute the “θ” values, each likely “y0” level must be analyzed using the figure 6f. the first pixel (≥ 1) from left to right at a position (i, j) in each likely “y0” level, must have another projected pixel (≥ 1) at the position (i, j+180). if this condition is accomplished, two square regions of 10 x 10 pixels are inspected around the points (i, j) and (i, j+180), respectively (figure 7h). in each square region, a hough’s transform algorithm for line detection is applied, in which only “y0” levels with slope combinations (+, -) and (-, +) are considered as potential level with a sinusoid. the “j” or “j+180” coordinate at each potential “y0”, in the points (i, j) or (i, j+180), will be the phase (θ) of that sinusoid if the square region at that point has a line with positive slope (figure 7h). the hough’s transform is a feature extraction technique used in image analysis, computer vision, and digital image processing. the purpose of the technique is to find perfect or imperfect instances of objects within a certain class of shapes by a voting procedure. this voting procedure is carried out in a parameter space, from which object candidates are obtained as local maximum in a so-called accumulator space that is explicitly constructed by the algorithm (shapiro and stockman, 2001). the basic hough’s transform is typically applied to identify parameterized curves, such as lines, circles, ellipses, parabolas, or other parameterized shapes. in this step it was applied in its simplest form to detect line-shape objects in the 10 x 10 pixels regions. at this point, the “y0” depth and the phase “θ” of equation (4) are already calculated for each detected level with a sinusoid. next, the basic hough’s transform employed to recognize lines was edited to detect one-cycle sinusoidal objects. in the final step of this stage, all detected “y0” and “θ” were arranged in a list of dictionaries with form: [{y1:θ1}, {y2:θ2}, {y3:θ3]}… {yn:θn}], and figure 6. computer vision. processing in the stage – 2 (edge detection). figure 7. computer vision. processing in the stage – 3 (sinusoid detection). 145computer vision techniques applied to automatic detection of sinusoids in borehole resistivity imaging – a comparison with the msd method the hough’s transform for sinusoids detection was applied using this list to the figure 6f. in order to detect sinusoids in the image, the hough’s transform uses an accumulator matrix of two dimensions (amp, θ). the sinusoidal candidates are produced by voting in the hough space, in which high-voted candidates have local maxima in the accumulator matrix. the algorithm scans through the image, and if an edge point is found, the sine equation is solved for the amplitude, using all combinations of “yn” and “θn” in the list of dictionaries, equation (6). (6) the algorithm fits the equation (6) to each edge in the image, and when a sinusoid is found, the accumulator increase at that point (amp, θ); consecutively, the algorithm passes to another white pixel saving all results. figure 7i shows unfiltered sinusoids detected until this stage. 4.4 stage 4 – final image after hough’s transform is solved for the equation (6), unsupervised machine learning techniques are applied to filtrate multiple detected sinusoids related to the same geologic feature. the density-based spatial clustering of applications with noise (dbscan) and k-means clustering are applied. these steps remove duplicate sinusoids related to equal geological planes at the same depth (y0). it is important to notice that sinusoid parameters are arranged in a three-dimensional array with shape: [amp, θ, y0]. the dbscan method is applied to find the number of high-density regions in previous array; this clustering technique produces a partitional clustering, in which the number of clusters is automatically determined by the algorithm (tan, et al., 2006). in the dbscan, the high-density regions are separated from another of low-density as shown the figure 8j. the number of regions detected by this method will be the “k” input of the following k-means clustering. afterward, the k-means algorithm is applied to the same sinusoid parameters array to find the centroids of each high-density region detected by the previous dbscan. k-means is a proto-type-based, partitional clustering technique that attempts to find a “k” number of clusters, which are represented by their centroids (tan, et al., 2006), as shown the figure 8k. the centroids found in the previous step are finally plotted over the original rbg image, where each sinusoid has a centroid for its amplitude (amp), phase (θ), and depth (y0), respectively (figure 8l). 5. results and comparison in order to test the automatic dip picking proposed in this work, the msd processing and the computer vision technique were evaluated in 1012 m of wireline-acquired borehole imaging. the test included several wells in clastic and carbonate environments, using two kinds of imaging devices (pad with flap and only-pad tools), as shown the figure 9 and figure 10, respectively. the metric employed to compare both results was the number of false positives found by each method; a false positive occurs when the method finds a sinusoid and it does not exist, in other words, the method places a sinusoid and it does not represent any geologic feature (e.g. bed boundary, fracture or sedimentary structure). the false positives recognition in both methods was performed by a specialized geologist according to regular borehole imaging interpretation procedures. once the methods were executed, the number of false positives for the msd was 90 in a total of 1127 recognized sinusoids (7.986%); while the computer vision approach had 28 false positives in a total of 3186 recognized sinusoids (0.879%). moreover, is particularly important to notice the number of detected samples by each method, 1127 for msd versus 3186 for the method proposed in this research. figures 9 and 10 show examples of how computer vision procedures can differentiate low amplitude sinusoids in the background from those of high amplitude in the front. the computer vision picking further shows better patterns related to deformations, such as in figure 9 (414 m), where some deformations due to concretions are depicted. according to these results, there is no evidence that different tool types have positives or negative impact on the outcome. two different image tools were employed to conduct the tests, and there was not any increase or decrease in false-positive proportions. the processing time using the computer vision approach takes around 30 minutes for 500 m of borehole imaging; it was achieved employing a personal computer with a core i5 processor and 8 gb of random access memory (ram). neither special nor high-resolution graphic cards are required; the standard graphic devices provided in current computer configurations are enough for this processing. 6. conclusions & recommendations the method proposed in this research is more accurate than the typical msd processing; additionally, it can be applied in the field soon after image data is acquired. the input is a dynamic normalized image, which can be generated by the logging engineer in the field. the computer vision approach was able to recognize high amplitude sinusoids in the same interval where low amplitude sinusoids are; this can be very useful during image interpretation when natural fractures and faults must be differentiated from beddings or sedimentary structures. the computer vision processing can be easily integrated into logging units; there are no special graphic card requirements, processor, or memory (ram). furthermore, the entire code was developed using programming standard tools of common application (harris, et al., 2020, mckinney, et al., 2010, bradski, et al., 2000, and van der walt, et al., 2014). figure 8. computer vision. processing in the stage – 4 (final image). 146 jorge alberto leal freitez, luis hernán ochoa gutiérrez, sergio francisco acosta lenis figure 9. comparison between msd and computer vision results using a pad with flap tool. figure 10. comparison between msd and computer vision results using an only-pad tool. 147computer vision techniques applied to automatic detection of sinusoids in borehole resistivity imaging – a comparison with the msd method parallelized processing is recommended during application of hough’s transform algorithm to reduce processing times. the methodology proposed is highly dependent on the employed edge detection techniques; the better the edge detection is, the better will be the detected sinusoids in the image. in this work all detected sinusoids are not classified, according its geologic origin. in order to solve this issue, classification methods based on machine learning are recommended; these machines can be trained with geologic inputs from experts, borehole logs data, and fractal attributes of borehole imaging, as methods proposed by leal, et al., 2022, leal, et al., 2018, and leal, et al., 2016. acknowledgments the authors would like to thank the intelligent systems lab (lisi “universidad nacional de colombia – bogotá”), for all advices and feedbacks provided during this research. references al-sit, w., al-nuaimy, w., marelli, m., & al-ataby, a. 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(2014). scikit-image: image processing in python. peerj, 2(453), 2-18. https://doi.org/10.1016/j.jappgeo.2015.05.015 https://doi.org/10.1016/j.jappgeo.2015.05.015 https://doi.org/10.1190/geo2013-0189.1 https://doi.org/10.1190/geo2013-0189.1 https://doi.org/10.1038/s41586-020-2649-2 https://heartbeat.fritz.ai/guide-to-image-inpainting-using-machine-learning-to-edit-and-correct-defects-in-photos-3c1b0e13bbd0 (last accessed june 2019). https://heartbeat.fritz.ai/guide-to-image-inpainting-using-machine-learning-to-edit-and-correct-defects-in-photos-3c1b0e13bbd0 (last accessed june 2019). https://heartbeat.fritz.ai/guide-to-image-inpainting-using-machine-learning-to-edit-and-correct-defects-in-photos-3c1b0e13bbd0 (last accessed june 2019). https://doi.org/10.15446/esrj.v22n2.68320 http://dx.doi.org/10.15446/ing.investig.v36n3.56198 http://dx.doi.org/10.15446/ing.investig.v36n3.56198 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n3.117195 earth sciences research journal earth sci. res. j. vol. 29, no. 3 (september, 2025):297 312 g eo ph y si c s study on the relationship between global positioning system total electron content anomalies and earthquake events in thailand during solar cycle 24 chollada pansong 1,2 and prasert kenpankho1* 1. department of engineering education, school of industrial education and technology, king mongkut’s institute of technology ladkrabang, bangkok 10520, thailand 2. department of technical education, faculty of technical education, rajamangala university of technology thanyaburi, pathum thani 12110, thailand * corresponding authors: prasert.ke@kmitl.ac.th record manuscript received: 24/10/2024 accepted for publication: 11/08/2025 how to cite this item: pansong, c., & kenpankho, p. (2025). study on the relationship between global positioning system total electron content anomalies and earthquake events in thailand during solar cycle 24. earth sciences research journal, 29(3), 297-312. https:// doi.org/10.15446/esrj.v29n3.117195 abstract this study investigates ionospheric total electron content (tec) anomalies in relation to earthquake events in thailand from 2007 to 2020, encompassing solar cycle 24. tec data were obtained from three sources: the global positioning system (gps), the international gnss service (igs), and the international reference ionosphere (iri), and were compared to 473 earthquakes (mw ≥ 3.0). while earthquake magnitudes below mw 5.0 did not exhibit a clear correlation, earthquake events of mw 5.0 or higher reflected in moderate negative correlation coefficients for gps tec, igs tec, and iri tec (-0.495, -0.501, and -0.303, respectively). furthermore, a positive correlation coefficient (0.611) was found between mw ≥ 5.0 earthquakes and geomagnetic storms with the kp index. however, focusing specifically on geomagnetic storms and tec variations on the day of an earthquake, no significant relationship was detected across gps, igs, and iri data. nevertheless, further research is needed to clarify the link between seismic activity and tec fluctuations, potentially through alternative approaches or targeted case studies. this is especially important given the limited number of earthquakes above a magnitude of 5.0 in our study area, which restricts the available sample size. keywords: total electron content, earthquake events, solar cycle 24, ionosphere and seismic activities, correlation coefficient estudio de la relación entre las anomalías del contenido total de electrones del sistema de posicionamiento global y los eventos de terremoto en tailandia durante el ciclo solar 24 resumen este estudio investiga las anomalías ionosféricas del contenido total de electrones (tec) en relación con los eventos de terremoto en tailandia entre 2007 y 2020, período del ciclo solar 24. la información del tec se obtuvo de tres fuentes: sistema de posición global (gps), servicio internacional de gnss (igs) y el modelo de referencia internacional de ionosfera, y se cotejó con 473 terremotos (mw ≥ 3.0). mientras que las magnitudes menores a 5.0 no presentan una clara correlación, los eventos de terremoto iguales o superiores a 5.0 reflejaron coeficientes de correlación negativa moderada para los sistemas gps tec, igs tec, e iri tec (-0.495, -0.501, y -0.303, respectivamente). además, un coeficiente de correlación positiva (0.611) se encontró en los terremotos de magnitud igual o superior a 5.0. sin embargo, en el caso específico de tormentas geomagnéticas frente a variaciones del tec en el día de un terremoto no se detectó una relación significativa en la información del gps, igs e iri. se necesita más investigación para clarificar los vínculos entre la actividad sísmica y las fluctuaciones del tec, potencialmente a través de acercamientos alternativos o casos de estudio específicos. esto es especialmente importante debido al número limitado de terremotos con magnitud superior a 5.0 en esta área de estudio, lo que restringe el tamaño de la muestra. palabras clave: contenido total de electrones; terremotos; ciclo solar 24; ionósfera y actividades sísmicas; coeficiente de correlación. https://doi.org/10.15446/esrj.v29n3.117195 https://doi.org/10.15446/esrj.v29n3.117195 https://doi.org/10.15446/esrj.v29n3.117195 298 study on the relationship between global positioning system total 1. introduction earthquakes are natural events that can occur at any time and are caused by vibrations or shaking of the earth’s surface, which people are often unable to detect. the world is likely to experience earthquakes, which can result in fatalities as well as damage to all a person’s property and other buildings. although the major plate boundary does not directly cross thailand, the earthquake in thailand has faults originating from the boundaries of major subplates. according to a geological study, thailand has 16 active tectonic faults. currently, thailand has earthquakes more frequently. furthermore, significant earthquakes occur in the nearby areas of thailand, such as indonesia, laos, myanmar, and the andaman sea. the most catastrophic incident occurred on december 26, 2004, when an earthquake in the andaman sea measured 9.1 to 9.3 on the magnitude scale. the earthquake caused a tsunami wave that reached several provinces, destroying homes and killing people, both domestic and international, as well as residents and visitors. scientists have discovered that each earthquake event influences changes in the total electron content (tec) anomaly in the ionosphere (liu et al., 2004; pulinets, 2004; pulinets et al., 2005; heki, 2011; ouzounov et al., 2011; priyadarshi et al., 2011; shah & jin, 2015; tiryakioglu et al., 2017; ulukavak & inyurt, 2020; sharma et al., 2021a; sharma et al., 2021b; nayak et al.,2023a; haider et al.,2023; nayak et al., 2024). some analysis suggests that pressure on crustal rocks stimulates positively charged particles, causing them to travel through the troposphere and into the lower ionosphere in the direction of the surface, where they encounter electrons. electrons may either be depleted due to attraction by positive particles or increased owing to persistent flow. promising factors for identifying earthquake precursors include the variations and abnormalities in the behavior of ionospheric tec that are seen before major earthquakes. supporting evidence suggests a relationship between tec disturbances in the ionosphere and the incidence of earthquakes, as evidenced by the behavior of tec anomalies before major seismic events (liu et al., 2004; pulinets, 2004; pulinets et al., 2005; heki, 2011; ouzounov et al., 2011; priyadarshi et al., 2011; sharma et al., 2021). tec anomalies are typically observed after a major earthquake. however, some studies, such as heki (2011), found that tec changes can occur before a major earthquake. there appears to be a relationship between tec disturbances and earthquake occurrences, as indicated by the variations and anomalies in tec behavior that occur before major earthquakes and serve as promising parameters for detecting earthquake precursors. dual-frequency global positioning system (gps) signals can be used to analyze the delay of radio waves as they propagate through the ionosphere. this enables the analysis of earthquakes and their corresponding changes in tec. this is accomplished by using data from ground-based receivers collected by the global navigation satellite system (gnss) (blewitt, 1990; kenpankho et al., 2021; cibeira urtiaga et al., 2022; nayak et al., 2023b). the tec examination reveals both positive and negative anomalies, some of which appear seven days prior to the quake. exceeding certain thresholds, these anomalies offer compelling evidence of significant departures from typical ionospheric conditions (nayak et al., 2023a). researchers studied this area as follows: heki (2011) investigated the increase in tec preceding the 2011 tohoku-oki earthquake. cahyadi and heki (2013) identified ionospheric disturbances associated with the 2007 bengkulu and the 2005 nias earthquakes in sumatra, observed by the gps regional network. shah and jin (2015) presented the statistical characteristics of gps tec disturbances before earthquakes with a magnitude (mw) of 5.0 or greater, covering the period from 1998 to 2014. sharma et al. (2017) studied ionospheric tec modeling as earthquake precursors using gnss. nishioka et al. (2021) conducted a statistical analysis of tec, focusing on the long-term assessment of extreme tec changes in japan. sharma et al. (2021a) developed a monitoring system for ionospheric tec fluctuations preceding earthquakes. sharma et al. (2021b) investigated tec anomalies as potential earthquake precursors in northeastern india and nearby areas, drawing on gps data collected between 2012 and 2018. on january 15, 2022, qzss-tec observed ionospheric evidence of multiple atmospheric wave crossings generated by the hunga tonga-hunga ha’apai eruption in japan. these signatures were given by heki (2022). xu et al. (2022) analyzed potential precursory patterns on the earth’s surface and in the atmosphere and ionosphere preceding two earthquakes with a magnitude of 7.0 and upward in mexico in 2020-2021. ahmed et al. (2022) studied ionospheric anomalies before the 2019 mirpur earthquake using ionosonde measurements. sharma et al. (2022) studied tec anomalies before the cuba earthquake on january 28, 2020. desanto et al. (2023) investigated the 2020 simeonof earthquake in alaska, specifically examining the limited shallow slip using gnss-acoustic techniques. li et al. (2023) conducted research in 2022 using m6.9 menyuan earthquake parameters estimated from high-rate multignss. joshi et al. (2023) investigated earthquake precursors using gnss data and ionolab-tec in the himalayan region. nayak et al. (2023a) examined seismo-ionospheric irregularities in large earthquakes by analyzing vertical tec (vtec) from the nearest epicentral gps stations with available prns. nayak et al. (2024) observed tec fluctuations as seismic precursors from the 2024 noto peninsula and nichinan earthquakes in japan. khoshgoftar and saradjian (2024) studied multiple ionospheric, atmospheric, and surface precursors from remote sensing to improve earthquake prediction accuracy and magnitude. in addition, sharma et al. (2024) examined the reduced ionospheric density over the epicentral region of the mw 7.2 el mayor-cucapah earthquake. many prior investigations have demonstrated that ionospheric tec anomalies can precede seismic events by days or weeks, providing a crucial basis for this study. for example, shah and jin (2015) analyzed gim-based tec data from 1,492 global earthquakes (mw ≥ 5.0) between 1998 and 2014, identifying disturbances up to five days before mainshocks. similarly, sharma et al. (2017) examined gnss-derived tec anomalies as earthquake precursors, finding deviations in ionospheric vtec a few days before both low-magnitude (mw 4.9) and higher-magnitude (mw 7.8) earthquakes. kiyani et al. (2020) investigated seismo-ionospheric anomalies linked to the 2018 mw 8.2 fiji earthquake using gnss tec, revealing low-intensity anomalies within 5-10 days before and five days after the mainshock. beyond the five days post-event (day 7 onward), they observed a more pronounced anomaly linked to an intense geomagnetic storm, illustrating the interplay between seismic activity and geomagnetic conditions. meanwhile, gnss tec dispersion following the 2011 tohoku earthquake foreshock suggested the onset altitudes of co-seismic ionospheric disturbances, as studied by kakinami et al. (2020). ulukavak and inyurt (2020) identified seismo-ionospheric precursors for strong sequential earthquakes in nepal, noting that at magnitudes 2.0-2.5, acoustic gravity waves significantly altered tec values, and that anomalies could last up to 30 days (15 days before and 15 days after). the ionospheric tec also tends to decrease from five days to one day before earthquakes of mw 6.0 or higher. sharma et al. (2021b) indicated that earthquakes of mw < 5.0 could be detected 46.50% of the time, mw 5-6 events 81.56%, and mw > 6 events fully 100%. haider et al. (2024) employed satellite-based observations and machine learning to detect potential ionospheric and atmospheric precursors for the mw 7.8 turkey earthquake, integrating standard deviation (stdev) and a nonlinear autoregressive network with exogenous inputs to identify synchronized anomalies within 6-7 days before the quake, alongside geomagnetic anomalies (kp > 4; dst < -70 nt; ap > 50 nt) on the ninth-day post-event. nayak et al. (2024) investigated ionospheric fluctuations before an mw 7.5 earthquake on the noto peninsula, detecting a significant tec drop exceeding 5 tecu roughly 22-23 days beforehand. in addition, sharma et al. (2024) found abnormal tec changes within 30 days preceding an mw 7.2 mexico earthquake, noting a pronounced anomaly about two days prior under calm geomagnetic conditions. collectively, these findings underscore the feasibility of tec-based monitoring for identifying potential precursors, which we aim to adapt and refine for multi-station gps observations in thailand’s seismic context. in this study, we employed a gps satellite-based approach to determine tec values for assessing earthquake events in thailand from 2007 to 2020, building upon prior research on tec and ionospheric disturbances. our main objective was to explore the relationship between earthquake magnitudes and ionospheric tec anomalies using gps tec, international gnss service (igs) tec, and international reference ionosphere (iri) tec. to achieve this, we collected only dual-frequency gps signals from 15 gnss stations located across thailand. then, we calculated slant tec (stec) by analyzing phase differences in the gps l1 and l2 signals, correcting receiver and satellite biases, and converting them into vtec. next, we adopted a difference-based approach (dtec) with data from the three tec sources, comparing these values against an 11-day running average to detect tec anomalies. finally, external factors such as geomagnetic storms (kp) and solar radiation were evaluated to confirm whether the observed anomalies were genuinely seismo-ionospheric. ultimately, our research aims to guide the development of earthquake prediction and earth observation systems in thailand and neighboring regions. 299chollada pansong and prasert kenpankho 2. data and methodology we compared earthquake magnitudes with gps tec, igs tec, and iri tec to investigate the relationship between ionospheric tec anomalies and earthquake events. gps tec and igs tec are obtained from gps receivers. iri tec is derived from the iri model. we studied tec disturbances associated with earthquakes in thailand, which predominantly occur in the northern, western, and southern regions along the country’s fault zones. therefore, we need to verify the gps tec values against the well-known igs tec and iri tec to ensure that our gps tec value is suitable for use in this study. the steps involved in the research process are shown in figure 1. 2.1 tectonics and geology of thailand the southeast asia tectonic map in figure 2 highlights the region’s principal plate boundaries and strike-slip fault networks, which are primarily governed by the northward advance of the indian plate and the clockwise rotation of continental blocks around the eastern himalayan syntaxis. two prominent fault systems outside thailand are the north-south, right-lateral sagaing fault in myanmar and the northwest-southeast, left-lateral ailao shanred river (as-rr) fault zone, which extends across yunnan and vietnam. within thailand, comparable large-scale faults include the mae ping (wang chao) fault zone, which extends over 600 km, and the left-lateral three pagodas fault zone; both likely branch from the sagaing fault, although their exact junction remains poorly exposed. further south, the ranong and khlong marui fault zones trend roughly north-south and transitioned from leftto right-lateral slip during the oligocene-quaternary. these faults have undergone episodic activity, exhibiting significant displacements (e.g., 120-150 km along the mae ping) and reactivations linked to crustal block extrusion, regional extension, and offshore basin development (palin et al., 2013). overall, the map illustrates how the convergence of the indian and eurasian plates, coupled with strikeslip tectonics and magmatism, drives ongoing tectonic adjustments in southeast asia, producing a complex array of active fault zones that significantly impact regional geology and seismic hazard. figure 1. workflow for earthquake data collection, classification, and tec data analysis. figure 2. presents a simplified tectonic map of southeast asia highlighting the major regional fault patterns, with a focus on thailand. the map is adapted from bernard et al. (2006), usgs (2008), searle & morley (2011), palin et al. (2013), and nachtergaele et al. (2019). 300 study on the relationship between global positioning system total 2.2 earthquake event data data on earthquakes are provided by the earthquake observation division (eod) of the thai meteorological department (tmd), thailand, and can be accessed at https://earthquake.tmd.go.th/earthquakereport.html. the website provides details such as the date and time, depth, phases, latitude, longitude, magnitude, size, and region of the epicenter. details of the earthquake occurrences in thailand from 2007 to 2020 are shown in table 1. in this study, we classified earthquake magnitudes into four categories: 3.0-3.9, 4.0-4.9, 5.0-5.9, and 6.0 or greater. according to ulukavak and inyurt (2020), magnitudes ranging from 1.0 to 2.9 have little influence on tec. consequently, we focused on earthquakes of magnitude 3.0 and above, resulting in a total of 473 events recorded between 2007 and 2020 (tmd, 2023). we collected data from four regions of thailand to examine the association between earthquake events and tec disturbance over thailand between the years 2007 and 2020 including 1) southern thailand and andaman sea; southwestern coast; phuket, phang nga, ranong, surat thani, chumphon, trang, and the andaman sea, 2) western of thailand and andaman sea; western coast; kanchana buri, tak, and the andaman sea, 3) northern of thailand; mae hong son, chiang mai, lamphun, chiang rai, payou, lampang, phrae, nan, and uttaradit, and 4) northeast of thailand; nakhon ratchasima and loei. table 1 shows that there were 473 earthquake events with a magnitude of 3.0 or higher in the years 2007-2020. these events were divided into 389 earthquake events with a magnitude of 3.0 to 3.9, 74 earthquake events with a magnitude of 4.0 to 4.9, eight earthquake events with a magnitude of 5.0 to 5.9, and two earthquake events with a magnitude of 6.0 and higher. the 15 gnss stations used in this study are located along thailand’s fault lines, namely crai (chiangrai: 19.57°n, 99.52°e), maig (chiangmai: 19.21°n, 99.12°e), maeh (maehongson: 18.10°n, 97.55°e), mhsg (maesariang: 19.18°n, 97.58°e), nann (nan: 8.46°n, 100.458°e), lamg (lampang:18.03°n, 99.14°e), utgn (uttaradit:17.62°n, 100.097°e), pheg (phetchabun:15.46°n, 101.01°e), uthg (uthaithani: 15.21°n, 100.00°e), kann (kanchanaburi: 19.57°n, 99.52°e), bang (bangkok: 13.40°n, 100.36°e), prag (prachuapkhirikhan: 1.56°n, 99.41°e), phag (phangnga: 8.41°n, 98.15°e), pkgs (phuket: 8.10°n, 98.31°e), and kabg (krabi: 60°n, 98.58°e) as shown in figure 3. blue pins indicate them, while earthquake events with magnitudes from 2.0 to 6.0 are marked with red target pins. yellow stars denote earthquakes, based on tmd data, and gray pins indicate magnitude 5.0 or above events from the usgs-neic earthquake catalogue. the seismicity map of thailand and adjacent regions, employed for seismic hazard assessments, integrates an earthquake catalog from usgsneic, featuring subduction events (m>5) between 1922 and 2023 shown in gray, alongside tmd’s earthquake catalog (2007–2023) depicted in red. most earthquake events in thailand occur predominantly in the northern region, which is situated along major fault lines. this area experiences a higher frequency of seismic activity compared to other parts of the country. consequently, tmd has installed more earthquake monitoring stations in the north than in other regions. figure 4 illustrates the focal mechanisms of earthquakes in thailand and its surrounding regions, particularly along the major fault zones that traverse western and northern thailand, extending to the thai-myanmar border. these zones are part of the larger sagaing fault system and its connected segments, such as meiktila, pyi, nay pyi taw, and bago, which all exhibit similar right-lateral strike-slip behavior. beach-ball diagrams represent the rupture patterns and directions of subsurface rock movement, with color boundaries indicating whether a fault slips primarily in a strike-slip, reverse, or typical fashion. the region predominantly exhibits strike-slip motion, but reverse or standard components may appear depending on the stress fields associated with the collision of the indian and sunda plates. such tectonic activity exerts compressional and shear forces that keep faults in myanmar and along the thai-myanmar border active; even when earthquake epicenters lie within myanmar, their shaking can affect thailand and potentially trigger activity in secondary faults in thailand. understanding these focal mechanisms is crucial for assessing fault characteristics, earthquake risk, and designing infrastructure to withstand seismic forces. moreover, identifying whether faults are likely to slip in a strike-slip, reverse, or usual manner informs disaster preparedness measures tailored to each area’s crustal deformation style. overall, the focal mechanisms shown here highlight the dominance of rightlateral strike-slip motion. at the same time, thrust or standard components can emerge under regional stress conditions, reflecting the ongoing impact of the indian–sunda plate collision on the area’s geological structure. table 1. earthquake events classified by magnitude and region in thailand between 2007 and 2020. regions of thailand magnitudes total 3.0-3.9 4.0-4.9 5.0-5.9 ≥ 6.0 1. southern thailand and the andaman sea; southwestern coast (lat. 7.38°n to 11.07°n, long. 95.15 °e to 99.66 °e) phuket, phang nga, ranong, surat thani, chumphon, trang, and andaman sea; southwestern coast 16 9 0 0 25 2. western thailand and andaman sea; western coast (lat. 13.41°n to 17.75°n, long. 96.15°e to 99.34°e) kanchana buri, tak, and andaman sea; western coast 29 7 0 0 36 3. northern thailand (lat. 17.71°n to 20.40°n, long. 97.77°e to 101.00°e) mae hong son, chiang mai, lamphun, chiang rai, payou, lampang, phrae, nan, and uttaradit 339 58 8 2 407 4. northeast thailand (lat. 14.69°n to 17.62°n, long. 101.38°e to 101.90°e) nakhon ratchasima and loei 5 0 0 0 5 total 389 74 8 2 473 https://earthquake.tmd.go.th/earthquakereport.html 301chollada pansong and prasert kenpankho figure 3. the illustration of earthquake magnitudes and 15 tmd gnss stations situated along the country’s fault lines, adapted from pornsopin et al. (2024) and tmd (2023). figure 4. focal mechanisms of earthquakes in the region of thailand (xiong et al., 2017; tmd, 2021; peng et al., 2025). 2.3 gps tec data gps tec data were selected from tmd gnss stations that were nearest the epicenter on the day of the earthquake, with a radius of 16 km and up to 1,375 km (pikridas et al., 2019). regarding the relevant data, we conducted a study by collecting tec anomalies in the ionosphere from gps data during the earthquake’s occurrence. the data was collected five days before and after the earthquake events. we have only gps tec data available from 2010 to 2020, corresponding to the specified days and associated with earthquakes of magnitude 3.0 and above, totaling 71 events. although we find a correlation between tec and earthquake events in thailand from 2007 to 2020, the data is limited to this period. earthquake events that matched the available gps tec data are shown in table 2. 302 study on the relationship between global positioning system total table 2. there were 71 earthquake events with a magnitude of 3.0 and above. tmd no. tmd reference id. date yy-mm-dd time (lt) depth (km) latitude (°n) longitude (°e) locations magnitudes 320 tm0337 2010-04-04 23:42:52 na 19.94 99.95 wiang chai, chiang rai 3.5 385 tm0404 2010-06-13 08:19:29 na 14.55 99.23 sri sawat, kanchanaburi 3.7 505 tm0543 2010-10-27 14:32:06 na 14.72 99.32 nong prue, kanchanaburi 3.5 568 tm0616 2011-03-24 17:49:14 na 20.35 99.95 chiang rai 3.6 1009 tm1053 2011-09-28 14:40:02 na 14.22 93.2 andaman sea 4.1 1010 tm1057 2011-10-01 04:22:14 na 12.83 95.9 andaman sea 4.7 1011 tm1058 2011-10-01 12:48:53 na 12.88 95.67 andaman sea 4.8 1012 tm1056 2011-10-01 17:54:13 na 12.91 95.78 andaman sea 4.4 1014 tm1055 2011-10-03 04:34:58 na 12.96 95.95 andaman sea 4.4 1189 tm1305 2012-03-30 10:39:13 na 9.07 96.83 andaman sea 3.3 1218 tm1336 2012-04-16 09:44:25 na 8.02 98.37 thalang, phuket 4.3 1228 tm1346 2012-04-17 05:18:45 na 8.02 98.32 thalang, phuket 3.1 1231 tm1349 2012-04-17 21:15:18 na 8.02 98.32 thalang, phuket 3.2 1240 tm1360 2012-04-19 19:43:41 na 8.01 98.32 thalang, phuket 3.2 1243 tm1364 2012-04-20 08:10:46 na 8.02 98.33 thalang, phuket 3.3 1287 tm1446 2012-06-04 05:49:04 na 9.84 98.58 mueang ranong, ranong 4.0 1670 tm1844 2013-07-05 17:49:28 na 15.54 96.98 andaman sea 3.7 1739 tm1914 2013-10-10 18:19:14 na 19.32 99.24 phrao, chiang mai 4.1 1796 tm1972 2014-01-16 05:18:30 na 9.09 98.67 ta khun, surat thani 4.0 2523 tm2701 2014-06-02 04:44:29 3 18.4 100.77 na noi, nan 3.6 2294 tm2469 2014-05-12 11:05:29 8 19.8 99.72 mae lao, chiang rai 5 1910 tm2089 2014-05-05 12:06:19 5 19.7 99.62 mae suai, chiang rai 5.1 1913 tm2090 2014-05-05 12:20:57 na 19.86 99.68 mueang, chiang rai 5.2 1901 tm2085 2014-05-05 11:08:42 7 19.748 99.692 mae lao, chiang rai 6.3 2061 tm2140 2014-05-06 00:58:19 2 19.7 99.53 mae suai, chiang rai 5.6 2060 tm2139 2014-05-06 00:50:16 20 19.73 99.69 mae lao, chiang rai 5.9 3015 tm3223 2015-01-11 01:57:26 2 14.77 98.807 thong pha phum, kanchanaburi 3.1 3257 tm3481 2015-07-14 14:25:12 4 15.01 98.47 sangkhla buri, kanchanaburi 4.8 3272 tm3497 2015-07-27 03:04:17 3 17.41 101.59 wang saphung, loei 3.1 3310 tm3535 2015-08-20 12:10:24 5 15 98.42 sangkhla buri, kanchanaburi 4.5 3556 tm3788 2016-02-21 01:58:45 1 19.78 99.73 mae lao, chiang rai 3.2 3566 tm3805 2016-03-02 13:53:19 4 19.63 99.72 phan, chiang rai 3.0 3581 tm3821 2016-03-20 15:04:20 5 9.48 98.83 tha chang, surat thani 3.3 3808 tm4053 2016-10-14 16:00:44 7 14.69 101.38 pak chong, nakhon ratchasima 3.0 3814 tm4059 2016-10-20 03:29:38 3 15.94 98.71 umphang, tak 3.6 3815 tm4060 2016-10-20 03:45:53 5 16.01 98.77 umphang, tak 3.2 3821 tm4067 2016-10-20 12:22:02 3 16.02 98.64 umphang, tak 3.3 3884 tm4131 2016-11-11 12:04:25 6 16.08 98.7 umphang, tak 3.3 4068 tm4324 2017-01-07 20:08:57 5 16.1 98.7 umphang, tak 3.9 4167 tm4425 2017-02-13 07:21:18 5 17.54 98.2 tha song yang, tak 3.1 4247 tm4506 2017-04-11 10:56:33 1 8.12 97.53 andaman sea 3.2 4256 tm4515 2017-04-22 07:57:37 3 18.35 100.87 na noi, nan 3.9 4284 tm4544 2017-05-02 10:04:42 6 19.79 99.74 mae lao, chiang rai 3.1 4310 tm4570 2017-05-24 05:58:42 3 8.05 98.48 ko yao, phang nga 3.4 4388 tm4649 2017-07-12 15:07:40 10 13.41 96.15 andaman sea 4.3 4614 tm4882 2017-12-07 10:26:51 1 17.25 98.38 tha song yang, tak 3.5 303chollada pansong and prasert kenpankho tmd no. tmd reference id. date yy-mm-dd time (lt) depth (km) latitude (°n) longitude (°e) locations magnitudes 4726 tm4994 2018-01-31 15:10:31 3 15.1 98.17 sangkhla buri, kanchanaburi 3.2 4793 tm5061 2018-02-23 11:13:19 6 17.75 97.86 tha song yang, tak 3.2 4826 tm5096 2018-03-20 04:24:31 18 19.732 99.678 mae lao, chiang rai 3.2 4926 tm5198 2018-06-10 15:08:11 1 17.235 101.91 wang saphung, loei 3.4 4981 tm5253 2018-07-20 06:36:35 10 15.06 96.69 andaman sea 4.0 5165 tm5443 2018-12-30 15:39:18 2 14.9 99.14 sri sawat, kanchanaburi 4.9 5167 tm5445 2018-12-30 22:02:07 10 14.9 99.15 sri sawat, kanchanaburi 3.0 5182 tm5478 2019-01-10 07:09:06 5 14.9 99.15 sri sawat, kanchanaburi 3.3 5246 tm5546 2019-02-16 21:41:23 10 14.34 95.87 andaman sea 3.9 5255 tm5554 2019-02-20 05:17:18 8 19.24 99.63 wang nuea, lampang 3.3 5261 tm5561 2019-02-20 06:51:24 12 19.25 99.62 wang nuea, lampang 3.0 5263 tm5563 2019-02-20 07:00:25 11 19.24 99.61 wang nuea, lampang 3.1 5266 tm5566 2019-02-20 08:26:38 5 19.25 99.63 wang nuea, lampang 3.3 5267 tm5567 2019-02-20 09:05:41 5 19.25 99.62 wang nuea, lampang 4.9 5269 tm5569 2019-02-20 10:50:56 10 19.26 99.57 wang nuea, lampang 3.0 5270 tm5570 2019-02-20 10:59:03 10 19.27 99.59 wang nuea, lampang 3.7 5339 tm5639 2019-03-09 07:13:02 1 14.94 99.14 sri sawat, kanchanaburi 3.1 5352 tm5652 2019-03-13 17:04:39 2 19.25 99.62 wang nuea, lampang 4.2 5369 tm5669 2019-03-14 14:55:19 2 19.25 99.63 wang nuea, lampang 4.0 5391 tm5691 2019-03-21 19:11:30 2 19.25 99.62 wang nuea, lampang 3.1 5753 tm6062 2019-10-18 14:46:25 6 18.904 99.252 doi saket, chiang mai 4.1 5809 tm6118 2019-11-20 13:35:28 10 11.076 95.168 andaman sea, phuket 3.9 5810 tm6119 2019-11-20 14:06:56 10 11.026 95.157 andaman sea, phuket 4.3 6162 tm6473 2020-01-17 12:45:22 2 19.664 99.723 phan,chiang rai 3.0 6446 tm6758 2020-06-25 14:37:04 5 17.6 101.68 mueang, loei 3.8 in table 2, we selected 71 earthquake events ranging in magnitude from 3.0 to 6.3, for which gps tec data (raw data from five days before the earthquake and five days after) were available. the most significant event, measuring 6.3, occurred in mae lao, chiang rai province, thailand. we obtained gps tec parameters from 15 tmd gnss stations, prioritizing those located near the epicenters and offering the most complete datasets, thereby ensuring the highest possible data integrity for our analysis. we obtained gps tec data for five days before and five days after earthquake occurrences, totaling 11 days. gps tec values were obtained from the receiver independent exchange (rinex) format version 2.11. in this study, tec was calculated using an observation file of gps parameters, including l1, l2, p1 (c1), and p2. the vtec is obtained by computing the stec. vtec is measured in tec units (1 tecu = 101⁶ el/m²) and is based on the total number of electrons in a one-square-meter tube along the signal channel. we used equations (1), (2), and (3) for gps tec computation with matlab programming. equation (1), stecl can be used to determine the phase difference (l1 and l2) between the two frequencies (f1 and f2) to obtain stec from satellites to the receiver (ma & maruyama, 2003; kenpankho et al., 2011). 2 1 2 1 1 2 22 2 1 2 l 2 stec f f l l k f f (1) where the wavelengths λ1 (0.1904 m) and λ2 (0.2444 m) correspond to frequencies f1 (1575.42 mhz) and f2 (1227.60 mhz), respectively, and k, which is related to ionosphere refraction (k = 80.62 m³/s²). furthermore, using equation (2), the vtec is calculated in units of el/m² (ma & maruyama, 2003; kenpankho et al., 2011). vtec=stec × cos l (2) where χ is the zenith angle that is defined by equation (3) cos + e e r r h (3) where re is the earth’s radius (6,378.137 km), while h is the ionospheric layer’s height (428.8 km). a is also the satellite’s elevation angle at the ionospheric pierce point (ipp) (ma & maruyama, 2003; kenpankho et al., 2021). the angles of the satellite’s elevation at 15 degrees, and the tec-biased method from kenpankho et al. (2021) were employed. 2.4 igs tec data we referred to the igs tec as the tec values from the igs. over 500 dual-frequency gnss stations operating constantly throughout the world form the foundation of the igs. the igs organization offers ionospheric products, which can be selected from tec map data available online via the link https:// cddis.nasa.gov/archive/gnss/products/ionex/yyyy/ddd/. following that, tec data were extracted from the igs corresponding to selected earthquake https://cddis.nasa.gov/archive/gnss/products/ionex/yyyy/ddd/ https://cddis.nasa.gov/archive/gnss/products/ionex/yyyy/ddd/ 304 study on the relationship between global positioning system total events with magnitudes of 3.0 or greater. this is based on earthquake event data from the tmd covering the period from 2007 to 2020. the igs tec data correspond to the same date, location, and times as the selected earthquake events. by consulting the igs network map from https://igs.org/network-resources for the period of the earthquake, we identified the corresponding latitude and longitude on the ionosphere maps. the time interval between their vertices is one hour. we collected igs tec data spanning 11 days, covering earthquake events from five days before to five days after, for a total of 437 events. 2.5 iri tec data the international union of radio science (ursi) and the committee on space research (cospar) are the sponsors of this global initiative, known as the iri. the iri model version 2020, also known as the iri tec, is available at: https://kauai.ccmc.gsfc.nasa.gov/instantrun/iri/. the model has been launched in multiple, continually updated iterations. in the ionospheric altitude range, the iri provides daily and monthly averages for electron density, electron temperature, ion temperature, and ion composition at a specific location, time, and date. the iri model’s intended output parameters can be chosen, such as electron density, ratio of ne and f2 peak density, tec, electron temperature, and height of f2 peak. the coordinates, profile type, and range, as well as optional input and desired output parameters, can be selected as tec output, which can be entered to obtain iri tec data. it is required that the location, date, and time of the seismic event match the iri input parameters. for data analysis, we compiled a total of 11 days of iri tec data, consisting of five days before and five days after each earthquake event, encompassing a total of 437 events. 2.6 the geomagnetic storm activities during the period of collecting earthquake data and monitoring tec anomalies from 2007 to 2020, we gathered information throughout solar cycle 24. the solar cycle represents an approximately 11-year recurrent pattern of changes in solar activity, as manifested by variations in the observed number of sunspots. solar cycle 24 concluded in 2020, marking the sun’s transition into solar cycle 25. notably, solar cycle 24 was the lowest in the last 100 years, reaching a peak of 116 sunspots, significantly below the average of 179 sunspots (national oceanic and atmospheric administration (noaa), 2023; space weather prediction center (swpc), 2023). geomagnetic activity is also low in solar cycle 24. watari (2017) conducted a study to explore the potential connection between solar activity and local seismicity. the investigation aimed to establish a potential link between solar activity and seismic events occurring in specific localized areas. no clear correlation has been found between global seismicity and solar activity. however, in small active seismic zones, a significant connection between earthquakes and solar activity is observed (rsunspot = -035, rdst = 0.28). the increase in shallow earthquakes in these specific seismic zones is related to variations in space weather indices. includes sunspot number and geomagnetic indices kp and dst. this suggests a plausible association between solar activity and localized seismic activity (takla & samwel, 2023). for this reason, we collected data on solar activity and geomagnetic storms for data analysis. the kp geomagnetic storm index data were collected from the pushkov institute of terrestrial magnetism, ionosphere, and radio wave propagation of the russian academy of sciences (izmiran) at the following link: https://www.izmiran.ru/ionosphere/weather/ storm/ (izmiran, 2023). 2.7 correlation coefficient subsequently, earthquake event data were collected and categorized based on vibration near the epicenter, with levels ranging from 0 to 6 magnitudes. finally, we utilized the magnitudes of earthquake values and tec data to determine the relationship between earthquake magnitudes and ionospheric disturbances in gps tec, igs tec, and iri tec using correlation statistics. the correlation coefficient (r) can be analyzed (mukaka, 2012) as 1 2 2 1 1 ( )( ) ( ) ( ) n i i i n n i i i i x x y y r x x y y (4) where xi is the earthquake magnitude value in the sample, and x̄ is the mean average of the earthquake magnitude values. yi is the value of the tec in a sample, ȳ is the mean average of the tec values. additionally, r is the correlation coefficient of the linear relationship between the variables x and y. the correlation coefficient can be interpreted as follows: a value near zero means no significant association, a value near positive suggests a direct relationship, and a value near negative indicates an inverse relationship. the datasets were utilized to calculate the correlation between earthquake magnitudes and tec anomalies across thailand. in this research, we categorized earthquake magnitudes into ranges: 3.0-3.9, 4.0-4.9, and 5.0 and higher. we selected and used the data as available in the ranges. this allowed us to compare the daily tec values from three data sources: gps, igs, and iri. we analyzed correlation coefficients to assess the strength of the relationships. the data set covered the period from 00:00 to 24:00 ut for each day during an earthquake occurrence. 3. results 3.1 the comparison between earthquake magnitudes and gps tec anomalies this study utilized gps tec data encompassing 71 earthquake occurrences in thailand between 2010 and 2020. the data was obtained from gnss receivers with rinex files, for which we have available data. we compared gps tec values during these 71 earthquake events, all of which had a magnitude of 3.0 and higher, as illustrated in figure 5 the panels (a)-(e) depict the variation of gps tec during the 1-5 days before the earthquake, respectively, while panels (f)-(j) show the change in gps tec during the 1-5 days after the earthquake, respectively. figure 5, a comparison of gps tec anomalies before and after earthquakes of different magnitudes, it can be seen that on the fifth day before the earthquake, as shown in figure 5 (a), the tec was higher than on the day of the quake, the tec gradually decreased, reaching a low on the day of the earthquake, as shown in figure 5 (e) and (f). the tec remained low for about two days before returning to normal. the increasing tec was calculated to be 65.15% based on a study of 71 earthquake samples over the previous 1-5 days before the earthquake. the average increase in gps tec for the fiveto one-day period was 5.74 tecu, 2.63 tecu, 2.80 tecu, 2.63 tecu, and 1.79 tecu, respectively. subsequently, tec gradually decreases, reaching its lowest point on the day of the earthquake. from panels (f)-(j), it is observed that following the earthquake event, tec gradually increases and returns to normal within 3-5 days. 3.2 the comparison between earthquake magnitudes and igs tec anomalies the igs tec data used in this research covers thailand earthquake events from 2007 to 2020, which were obtained from the igs organization. as shown in figure 6, the results indicate 473 earthquake events with a magnitude of 3.0 or higher that were correlated with igs tec data, including the date and location of the earthquakes. panels (a)-(e) show the igs tec changes during 1-5 days before the quake, while panels (f)-(j) show the changes in igs tec during 1-5 days after the earthquake, respectively. figure 6 compares igs tec anomalies in the ionosphere before and after earthquakes of various magnitudes, using 11 consecutive days of igs tec data that exhibit a consistent trend. notably, on the fourth day before the earthquake (figure 6b), tec increased, followed by a gradual decline that reached a minimum on the day of the earthquake (figures 6c-e). this pattern indicates that tec generally increases in the days leading up to an earthquake, https://igs.org/network-resources https://kauai.ccmc.gsfc.nasa.gov/instantrun/iri/ https://www.izmiran.ru/ionosphere/weather/storm/ https://www.izmiran.ru/ionosphere/weather/storm/ 305chollada pansong and prasert kenpankho as shown in panel (6b). specifically, 378 out of 437 events (86.3%) exhibited a tec increase on the fourth day before the earthquake, with an average rise of approximately 5 tecu compared to the 11-day mean igs tec value. subsequently, tec continued to decrease until the day of the earthquake, before gradually returning to normal levels, as depicted in figures 6 (f)-(j). 3.3 the comparison between earthquake magnitudes and iri tec anomalies moreover, we compared the earthquake events and the ionospheric iri tec disturbance. we used the iri tec and 437 earthquake events from 2007 to 2020. figure 7 (a)-(e) depicts the variation of iri tec during the 1-5 days before the earthquake, respectively. panels (f)-(j) show the changes in iri tec from 1-5 days after the earthquake, respectively. figure 5. comparison of gps tec anomalies with earthquake magnitudes, where panels (a)-(e) illustrate tec variations during the five days preceding the earthquake, while panels (f)-(j) depict tec changes during the five days following the earthquake events. 306 study on the relationship between global positioning system total figure 6. comparison of igs tec anomalies with earthquake magnitudes, where panels (a)-(e) illustrate igs tec variations during the five days preceding the earthquake, while panels (f)-(j) depict igs tec changes during the five days following the earthquake events. 307chollada pansong and prasert kenpankho figure 7. comparison of iri tec anomalies with earthquake magnitudes, where panels (a)-(e) illustrate iri tec variations during the five days preceding the earthquake, while panels (f)-(j) depict iri tec changes during the five days following the earthquake events. 308 study on the relationship between global positioning system total over the 11 days, the iri tec values followed a consistent trend. during the five days preceding the earthquake (figure 7a-e), tec exhibited a slight increase, then gradually declined around the earthquake date, ultimately returning to normal levels (figure 7f-j). notably, in 294 out of 437 events (62.15%), tec rose by approximately 1 tecu above the 11-day average in the five days leading up to the quake (figure 7a). this pattern suggests that tec generally increases before seismic events, subsequently decreases to its lowest point on the day of the earthquake (figure 7e), and then, approximately five days afterward, begins to return to normal conditions (figure 7j). 3.4 the relationship between earthquake magnitudes and tec anomalies the study examined the relationship between earthquake events and ionospheric disturbances using igs tec and iri tec data for 473 earthquake events from 2007 to 2020. however, we have access to gps tec data corresponding to earthquake events from 2010 to 2020, totaling 71 incidents. the relationships between earthquake magnitudes and tec anomalies are in table 3. in table 3, the correlation coefficients (r) between earthquake magnitudes and gps tec anomalies on the days of the earthquake (eq) are rgps (eq 3.0-3.9) = -0.081, rgps (eq 4.0-4.9) = -0.053, and rgps (eq 5.0 and higher) = -0.495. the correlation coefficients between earthquake magnitudes and igs tec anomalies on the quake day are rigs (eq 3.0-3.9) = 0.092, rigs (eq 4.0-4.9) = 0.043, and rigs (eq 5.0-5.9) = -0.501, respectively. the correlation coefficients between earthquake magnitudes and iri tec anomalies on the quake day are riri (eq 3.0-3.9) = 0.089, riri (eq 4.0-4.9) = 0.030, and riri (eq 5.0-5.9) = -0.303, respectively. 3.5 the comparison among earthquake magnitudes, geomagnetic storms, and tec anomalies. the dataset includes earthquake events of magnitude 3.0 or higher in thailand from 2007 to 2020, encompassing the entire 24th solar cycle. in total, 437 events were identified, and geomagnetic storm data based on the kp index were compiled to compare the frequency of geomagnetic storms with earthquake occurrences, as illustrated in figure 8. for this study, we specifically examined space weather indices, namely sunspot number and kp index, alongside earthquake occurrences within thailand’s geographical boundaries. the data are shown in figure 8. table 3. the relationship between earthquake magnitudes and the gps tec, igs tec, and iri tec anomalies in the ionosphere. correlation earthquake coefficient magnitudes 5 days before 4 days before 3 days before 2 days before 1 day before event day 1 day after 2 days after 3 days after 4 days after 5 days after 3.0-3.9 gps tec -0.123 -0.132 -0.096 -0.107 -0.103 -0.081 -0.081 -0.024 -0.117 -0.052 -0.141 4.0-4.9 -0.073 0.008 -0.087 -0.108 -0.102 -0.053 -0.080 0.024 -0.078 -0.064 0.010 5.0-higher -0.310 -0.278 -0.274 -0.333 -0.477 -0.495 -0.314 -0.153 -0.202 -0.225 -0.220 3.0-3.9 igs tec 0.098 0.103 0.100 0.091 0.098 0.092 0.102 0.099 0.097 0.099 0.085 4.0-4.9 0.052 0.043 0.031 0.031 0.042 0.043 0.035 0.034 0.031 0.016 0.020 5.0-higher -0.426 -0.562 -0.506 -0.472 -0.481 -0.501 -0.528 -0.498 -0.464 -0.514 -0.472 3.0-3.9 iri tec 0.095 0.093 0.092 0.092 0.093 0.089 0.089 0.086 0.087 0.091 0.085 4.0-4.9 0.023 0.027 0.022 0.030 0.014 0.030 0.023 0.025 0.024 0.018 0.021 5.0-higher -0.266 -0.294 -0.309 -0.299 -0.299 -0.303 -0.306 -0.310 -0.277 -0.328 -0.316 figure 8. a total of 473 earthquakes occurred in thailand, in comparison to geomagnetic activity, during the years 2007-2020, encompassing the period of solar cycle 24. 309chollada pansong and prasert kenpankho figure 8 shows a comparison of the number of sunspots, geomagnetic storms as indicated by the kp index, and earthquake occurrence in thailand from 2007 to 2020. the solar cycle reached its maximum sunspot activity in 2014. the peak monthly sunspot number during this cycle occurred in april 2014, with an approximate value of 82. for the kp index data, considering daily averages from 2007 to 2020, the number of days with geomagnetic storms rated at kp greater than 4.0 was 127. in 2012, there were the highest occurrences of geomagnetic storms, totaling 23 days. the most intense geomagnetic storm, with a magnitude of 7.93, occurred in 2015. in the year 2014, which marked the peak of solar cycle 24, there was a notable increase in earthquake activity in the thailand region. it can be observed that in 2014, a period of high solar activity coincided with the occurrence of earthquakes with magnitudes greater than 3.0 within this region, resulting in a total of 284 events. among these earthquakes, one recorded a magnitude of 6.3 in 2014, representing the highest magnitude recorded from 2007 to 2020. we analyzed the relationship between earthquake magnitudes and geomagnetic storm data using the kp index. specifically, we compared each earthquake’s magnitude with the daily average kp index on the day of the event, which categorized the dataset according to earthquake magnitudes. to analyze the correlation between geomagnetic storms (represented by the kp index) and tec variations on earthquake days, we examined gps tec, igs tec, and iri tec. then, we grouped the tec data by earthquake magnitude and compared these values to the corresponding kp index on each event day. the details are presented in table 4. table 4. the relationship among earthquake magnitudes, geomagnetic storms, and tec anomalies. correlation coefficient earthquake magnitudes kp index tec (on earthquake day) gps tec igs tec iri tec 3.0-3.9 -0.017 -0.067 -0.028 -0.057 4.0-4.9 -0.021 -0.086 -0.063 -0.176 5.0-higher 0.611 -0.096 -0.033 -0.271 in table 4, the correlation coefficient among geomagnetic storms and earthquake magnitudes of 3.0-3.9, 4.0-4.9, and 5.0 or higher are -0.017, -0.021, and 0.611, respectively. in examining how geomagnetic storms relate to tec changes on earthquake days, we divided events into the same three-magnitude groups. the correlation coefficient results for gps tec are -0.067, -0.086, and -0.096, respectively. for igs tec, they are -0.028, -0.063, and -0.033. similarly, for iri tec, the results are -0.057, -0.176, and -0.271. 4. discussions based on previous research and investigations into tec anomalies and earthquake events in thailand, we conclude that the seismo-ionosphere coupling examined occurred from 2007 to 2020. the results indicate the presence of tec anomalies and their corresponding earthquake magnitudes. we found that earthquakes with magnitudes ranging from 3.0 to 4.9 do not exhibit a distinct correlation with tec changes. however, for the earthquakes with a magnitude of 5.0 or higher, the moderate correlation coefficients indicate an anomaly in tec. this location is situated on a secondary tectonic fault, resulting in fewer high-magnitude earthquake events, typically ranging in magnitude from 1.0 to 4.0 on the richter scale. meanwhile, there are also fewer events with a magnitude higher than 5.0. the correlation between the two factors is significant, indicating a moderate level of association. it shows that during large earthquakes, tec values increase 3-5 days before the event, then gradually decrease to their lowest point on the day of the earthquake. following the event, the tec values gradually return to normal in the subsequent days. there are seven previous studies (shah & jin, 2015; sharma et al., 2017; kiyani et al., 2020; ulukavak & inyurt, 2020; sharma et al., 2021a; sharma et al., 2021b; and nayak, 2023a) other than using earthquake events in thailand that we compared with. these results support the research conducted by shah and jin (2015) on the statistical characteristics of seismo-ionospheric disturbances in gps tec before global earthquakes with a magnitude (mw) equal to or greater than 5.0. the study focused on 1,492 earthquakes that occurred between 1998 and 2014. the statistical analysis of 10 days of tec data preceding global earthquakes with mw 5.0 or greater reveals a significant enhancement five days prior to an earthquake of mw 6.0 or greater at a 95% confidence level. sharma et al. (2017) conducted a study on modeling tec anomalies as earthquake precursors using gnss. they observed gps data from ground-based stations, including two cors, indicating variations in vtec in the ionosphere a few days before seismic events. this phenomenon was evident in both lowmagnitude (mw 4.9) and higher-magnitude (mw 7.8) earthquakes. ulukavak and inyurt (2020) researched seismo-ionospheric precursors of consecutive strong earthquakes in the nepal region. the tec values for magnitudes 2.0 to 2.5 were found to be less influenced compared to the pre-earthquake oscillations in the ionosphere. the ionospheric tec, with magnitudes equal to or higher than 6.0, may experience a significant decline starting five days before the event. sharma et al. (2021a) reported that the fluctuations in the tec monitoring system were analyzed to assess their potential as earthquake precursors. typically, tec reactions serve as intense precursors for magnitudes exceeding 5.0. cumulative percentages beyond mw 5.5 indicate an increased occurrence of abnormal tec. seismo-ionospheric disturbances preceding mw 6.0 earthquakes exhibit sharp increases in cumulative percentages as well, for earthquakes with a magnitude range of mw 6.0-7.0, seismo-ionospheric disturbances associated with thrust and strike-slip events become apparent. for earthquakes with mw > 5 specifically, the mw 5.5 kohima earthquake, the mw 6.8 myanmar earthquake, and the mw 5.6 kokrajhar earthquake, tec anomalies were detected 10, 12, and 13 days before the events, respectively. analyzing all 160 earthquakes showed that the detection rate of tec-based precursors increases with earthquake magnitude. earthquakes of mw < 5 were 46.50% detectable, mw 5-6 were 81.56% detectable, and those above mw 6 were fully detectable (sharma et al., 2021b). negative correlation coefficients were observed between gps tec, igs tec, and iri tec and earthquakes of magnitude 5.0 or higher. the abnormality in gps tec, igs tec, and iri tec (-0.495, -0.501, and -0.303) indicated an association between larger earthquake events and reduced ionospheric tec disturbance. this finding is in line with nayak et al. (2024), who investigated ionospheric fluctuations before an mw 7.5 earthquake on the noto peninsula using 30 days of igs tec data before the event and identified a significant tec drop exceeding 5 tecu roughly 22-23 days before the event. a similar pattern at station mizu further supports this geophysical connection. positive tec anomalies typically coincide with intense space weather, while pronounced negative anomalies emerge under calm geomagnetic conditions, reinforcing the likelihood of a seismogenic origin. kriging interpolation confirmed that the anomalous tec zone overlapped the epicenter. in addition, an mw 7.1 earthquake in nichinan revealed a comparable tec drop, consistent with dobrovolsky’s preparation zone of 1,000-1,600 km (dobrovolsky et al., 1979), where tec correlation diminishes with distance. similarly, sharma et al. (2024) detected abnormal tec changes within 30 days preceding the mw 7.2 mexico earthquake, with a notable anomaly occurring about two days prior, deviating from the 15-day average by 3-4 tecu, with the most considerable change recorded at 14.75 utc. moreover, data from the 200 stations showed a tec reduction rate of 0.0017 tecu per kilometer approaching the epicenter. spatial interpolation revealed a significantly low-tec zone in the ionosphere above the epicenter, which aligns with dense geological fault structures, suggesting that this low-tec area may be detectable in the earthquake preparation zone prior to the event. nayak et al. (2023a) noted that positive and negative tec abnormalities can appear up to a week beforehand, demonstrating a significant departure from standard ionospheric conditions. likewise, kiyani et al. (2020) uncovered substantial, low-intensity ionospheric anomalies within 5-10 days before and five days after the mainshock by estimating tec from four gnss stations during the earthquake preparation period. moreover, ulukavak and inyurt (2020) observed that ionospheric anomalies can persist for up to one month, typically 15 days before and after the quake. in the five days leading up to earthquakes of magnitude 6.0 or higher, tec shows a notable decrease, which continues until one day before the event. according to sharma et al. (2017), who reported significantly low tec values 13-14 days before the first two earthquake events, there is a correlation between reduced tec followed by subsequent high tec and seismic activity in the himalayan region. in analyzing the relationship between geomagnetic storms and earthquake magnitudes, we found a positive correlation of 0.611 for events 310 study on the relationship between global positioning system total with a magnitude of mw 5.0 or higher, indicating that earthquakes are linked to geomagnetic activity. in contrast, no correlation was observed for minor earthquakes (m < 5.0). this result aligns with the findings of urata et al. (2018), who investigated the connection between earthquakes and the lorentz force, particularly around the onset of strong seismic events. they examined the kp index, a logarithmic measure of magnetic field deviation, and employed a stacking method to align the central times of kp surges with main earthquake shocks. their results suggest that distinctive kp surges often strongly correlate with earthquake onset, with the correlation varying by both region and quake magnitude. notably, stronger earthquakes exhibit a closer association with kp surges, reaching 100% statistical significance in the pacific rim, thereby emphasizing the pivotal role of geomagnetic activity in major seismic events. furthermore, these observations are consistent with those of duma and vilardo (1998), who monitored seismic activity around mount vesuvius for approximately 25 years, recording over 7,800 events with magnitudes ranging up to 3.4. their analysis revealed a close relationship between solar activity, the earth’s magnetic field, and local seismicity. although they primarily presented observational evidence rather than theoretical explanations, their work supports the notion that geomagnetic variations and earthquake occurrences are interlinked, particularly in tectonically active regions. in examining the correlation between geomagnetic storms and tec changes, specifically on earthquake days, no significant relationship was detected across tec data derived from gps, igs, and iri. this result aligns with sharma et al. (2024), who observed dst and kp indices ranging from -35 to 5, suggesting that the geomagnetic storms and solar flares had a minimal impact on tec fluctuations during their study period. 5. conclusions overall, our findings suggest that ionospheric tec anomalies can serve as a meaningful seismic precursor, particularly for earthquakes with a magnitude (mw ) of 5.0 or greater. these findings are supported by multiple studies (sharma et al., 2017; kiyani et al., 2020; ulukavak & inyurt, 2020; nayak et al., 2023a; sharma et al., 2024), which have consistently reported tec fluctuations, either significant drops or surges beginning several days to weeks before mainshock occurrences. these anomalies appear most pronounced under calm geomagnetic conditions, reinforcing the likelihood that they have a seismogenic rather than solar-driven origin. moreover, our analysis of geomagnetic storms (represented by the kp index) reveals a strong correlation (r = 0.611) for mw 5.0 and above, suggesting that major earthquakes can be closely tied to heightened geomagnetic activity, consistent with urata et al. (2018). however, more minor earthquakes (mw < 5.0) exhibit no such correlation, and day-of-event geomagnetic disturbances have a minimal effect on tec values (sharma et al., 2024). these collective observations, including those from local seismic networks such as duma and vilardo (1998) around mount vesuvius, highlight the broader relationship between geomagnetic variations, solar activity, and earthquake occurrences. consequently, tecbased monitoring augmented by geomagnetic indices emerges as a promising approach for early detection, offering actionable insights into seismic hazard preparedness. in conclusion, we applied a tec difference analysis using data from the three tec sources, comparing these values against an 11-day running average to detect tec anomalies associated with geomagnetic storms (kp). these anomalies were then evaluated to confirm whether the observed anomalies were genuinely seismo-ionospheric. ultimately, our research guided the development of an earthquake prediction collaboration with the tmd. however, there is a need to further explore the link between seismic activity and tec variations, either by employing other methodologies or case-by-case studies. this is particularly important given the limited number of earthquakes exceeding magnitude five within our study region, which constrains the available sample size. 6. funding research collaboration project on ionospheric disturbance southeast asia pacific, department of engineering education, school of industrial education and technology, king mongkut’s institute of technology ladkrabang, bangkok, thailand. 7. acknowledgements we gratefully acknowledge the institute of geology and geophysics, chinese academy of sciences (iggcas), for providing and supporting the bg2 gnss receivers. appreciation is extended to the earthquake observation division of the thai meteorological department (tmd) for supporting the receivers and providing raw rinex and earthquake data from thailand. we also thank the international reference ionosphere (iri) for iri tec data, the international gnss service (igs) for tec maps, and the pushkov institute of terrestrial magnetism, ionosphere and radio wave propagation of the russian academy of sciences (izmiran), the national oceanic and atmospheric administration (noaa), and the space weather prediction center (swpc) for sharing geomagnetic storm data. 8. references ahmed, j., shah, m., awais, m., jin, s., ali zafar, w., ahmad, n., amin, a., ali shah, m., & ali, i. 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(2022). analysis of potential precursory pattern at earth surface and the above atmosphere and ionosphere preceding two mw ≥ 7 earthquakes in mexico in 2020-2021. earth and space science, 9(e2022ea002267), 1–24. https://doi.org/10.1029/2022ea002267 https://doi.org/10.1007/s12517-022-10605-5 https://doi.org/10.1007/s12517-022-10605-5 https://doi.org/10.1007/s12524-024-01837-x https://www.swpc.noaa.gov/products/solar-cycle-progression https://www.swpc.noaa.gov/products/solar-cycle-progression https://doi.org/10.1007/s44195-023-00042-6 https://earthquake.tmd.go.th/km_view.php?id=18 https://earthquake.tmd.go.th/km_view.php?id=18 https://earthquake.tmd.go.th/hvsr/central.php https://earthquake.tmd.go.th/hvsr/central.php https://doi.org/10.15446/esrj.v21n3.62812 https://doi.org/10.15446/esrj.v21n3.62812 https://doi.org/10.1016/j.actaastro.2019.09.033 https://www.usgs.gov/media/images/tectonic-base-map-sumatra-subduction-zone-showing-major-faults https://www.usgs.gov/media/images/tectonic-base-map-sumatra-subduction-zone-showing-major-faults https://doi.org/10.1186/s40623-017-0653-z https://doi.org/10.1002/2017gl072770 https://doi.org/10.1002/2017gl072770 https://doi.org/10.1029/2022ea002267 https://doi.org/10.1029/2022ea002267 _hlk192545581 _hlk192537569 _hlk192539156 _hlk192537548 _hlk192537520 _hlk192537431 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n3.103981 earth sciences research journal earth sci. res. j. vol. 26, no. 3 (september, 2022): 197-204 g eo ph y si c s seismic properties of the permafrost layer using the hvsr method in seymour-marambio island, antarctica carlos a. vargas1*, juan m. solano1, adriana m. gulisano2,3,4, sergio santillana2, edwin a. casallas5 1. departamento de geociencias, universidad nacional de colombia at bogotá, colombia 2. instituto antártico argentino, dirección nacional del antártico, argentina 3. instituto de astronomía y física del espacio, conicet, universidad de buenos aires, argentina 4. departamento de física, facultad de ciencias exactas y naturales, universidad de buenos aires, argentina 5. fuerza aérea colombiana *corresponding author: cavargasj@unal.edu.co abstract authors have calculated the h/v spectral ratios using seismic-noise recordings in the uppermost layers north of the seymour-marambio island, antarctic. sixty-seven seismic site-response measurements near and far from the argentinean marambio base runway suggest geotechnical works on the uppermost sedimentary layers due to maintenance, landing, and taxi of large loads and aircraft during decades could contribute to changes in their seismic dynamic response. two horizontal images of vp, vs, and vp/ vs ratios at 1.0 m and 35.0 m depth show lateral variations in the permafrost properties. authors interpret that permafrost is emplaced in rocks with different porosities and contrasting fluids saturation at those depths. in shallow strata, the saturation of gases affects mainly the elastic properties. in deeper strata, where the location of water reservoirs is detected, the primary mechanism of seismic dissipation is anelastic. keywords: h/v spectral ratio; seismic response; permafrost; marambio; antartic; fluids' saturation propiedades sísmicas del permafrost a través del método de relaciones espectrales horizontal/vertical en la isla seymour-marambio de la antártida palabras clave: relaciones espectrales h/v; respuesta sísmica; permafrost; marambio; antártida; saturación de fluidos record manuscript received: 29/07/2022 accepted for publication: 16/08/2022 how to cite item: vargas, c. a., solano, j. m., gulisano, a. m., santillana, s., & casallas, e. a. (2022). seismic properties of the permafrost layer using the hvsr method in seymour-marambio island, antarctica. earth sciences research journal, 26(3), 197-204. https://doi.org/10.15446/esrj.v26n3.103981 en este trabajo se presentan estimaciones de relaciones espectrales h/v utilizando registros de ruido sísmico en las capas superiores al norte de la isla seymour-marambio, en la antártida. sesenta y siete mediciones de respuesta sísmica del sitio cerca y lejos de la pista de la base marambio, argentina, sugieren que los trabajos geotécnicos en las capas sedimentarias superiores debido al mantenimiento, aterrizaje y rodaje de grandes cargas y aeronaves durante décadas podrían contribuir a cambios en su respuesta dinámica sísmica. dos imágenes horizontales de relaciones vp, vs y vp/vs a 1,0 my 35,0 m de profundidad muestran variaciones laterales en las propiedades del permafrost. interpretamos que el permafrost se emplaza en rocas con diferentes porosidades y saturación de fluidos a esas profundidades. en estratos someros, la saturación de gases afecta principalmente a las propiedades elásticas. en estratos más profundos, donde se esperan reservorios de agua, el principal mecanismo de disipación sísmica es inelástico. 198 carlos a. vargas, juan m. solano, adriana m. gulisano, sergio santillana, edwin a. casallas 1. introduction permafrost is frozen ground in high latitudes formed by low sea-level stands during the ice ages (overduin et al., 2019; martens et al., 2020). the permafrost, the active layer on its top, and the underneath stratum hold complex ecosystems and are leading players in the complex study of climate change due to the release of greenhouse gases during its thaw. the data required for its research is typically time-consuming due to elaborated laboratory essays, sometimes field drilling, or complicated sampling collection. estimating permafrost’s elastic properties is a scientific challenge that involves layers with in-depth contrasting behaviors, lateral variations in each layer’s composition, inversion of velocities and densities, and diverse seismic inelastic performances. besides, the permafrost seismic performance is involved in damages to the men-made infrastructure (see, e.g., melvin et al.,2017; raynolds et al., 2014; hong et al., 2014), showing that there is a significant lack of information to diagnose its comportment appropriately and to promote proper hazard management. for instance, in some places, cryoturbation related to thawing produces seismic liquefaction, often accompanied by sand volcanoes (vandenberghe et al., 2016), but sometimes with large-scale subsidence and potential changes in the drainage systems (thienen-visser et al., 2015). in recent years, the permafrost’s thickness and seismic properties have been mapped with seismic noise measurements. a good example is presented by picotti et al. (2017), who corroborated estimations using radio-echo sounding, geoelectric, and active seismic methods in glacial environments such as the adamello and ortles-cevedale massifs (italy), the bernese oberland alps (switzerland), and the whillans ice stream (west antarctica). their results suggested that the resonance frequency in the horizontal-to-vertical spectral ratio (hvsr) correlates well with the ice thickness at the site, in a wide range from a few tens of meters to more than 800 m, even allowing reliable estimations of the seismic properties. on the other hand, in areas where large loads repeatedly impact the permafrost, some reports recognize their elastic and inelastic performance or allow the inference of the local seismic structure oriented to the infrastructure design care. good examples constitute the aerial runways deployed near the scientific bases in the antarctic continent. thus, to provide valuable information regarding the seismic structure in these places, the authors collected seismicnoise information on seymour-marambio island, approximately 80 km far the antarctic peninsula. the easy deployment, fast processing, and trusty results from similar sites suggest that the hvsr analyses may contribute to the knowledge of the antarctic permafrost’s elastic and inelastic properties (bignardi et al., 2018; köhler & weidle, 2019). sixty-seven seismic noise stations were deployed on the island’s plateau, where the runway is located (figure 1). the study area corresponds to a topographic contrast of ~200 m altitude, located about 2 km from the shoreline. high slopes are observed with the shallower layers’ surficial activity that triggers abundant landslides. in this figure 1. location of the study area. a) seymour-marambio island, approx. 80 km from the antarctic peninsula. the blue square represents the north part of the island. b) topographic image of the area where 67 seismic noise stations were recorded (blue squares). c) marambio scientific base's runway is located on a narrow plateau. the map shows the scientific station's location (red houses) with 67 seismic-noise stations measured in the colombian argentinean antarctic summer field campaign in january 2020 (yellow pins). d) perpendicular topographic profiles crossing the runway suggested in b). 199seismic properties of the permafrost layer using the hvsr method in seymour-marambio island, antarctica scenario, the permafrost and the topographic contrast may offer information about the combined effect’s seismic response. in this work, the authors report the results of seismic site-response experiments made during the colombian and argentinean scientific teams’ campaign in january 2020 near the marambio scientific base of argentina to evaluate the permafrost structure on the island’s plateau, the site response along the runway, and contrast these results with previous observations based on other geophysical approaches. 2. geological setting marambio island (also known as seymour island) is one of the 16 islands encompassing major islands on the antarctic peninsula. this island is located in the westernmost weddell sea, and according to fukuda et al. (1992), its ground has been frozen since the holocene. at least three geomorphological terraces are recognized in the north of the island, with depths of the permafrost estimated to be 200m (upper terrace), 105m (middle terrace), and 35m (lower terrace). according to montes et al. (2013), the north of the island outcrops miocene to quaternary formations related to surficial and glacier processes such as the weddell and hobbs formations that rest on la meseta formation of eocene–oligocene age. this sequence reaches 600 m in thickness. tectonic faults with vergence eastward and westward limit those sedimentary sequences with older sedimentary units dated from maastrichtian to paleocene, such as snow hill island, lopez de bertonado, sombral, and cross valley-wiman formations (figure 2). figure 2. geologic map of the seymour-marambio island (after montes et al., 2013). the hvsr study was done at the northern sector of the island, located on the marambio scientific base’s runway. profile a-a’ is a representative section of the uppermost basin in the study area, suggesting a graben structure filled with paleogene to neogene sediments. the eocene to quaternary strata underneath the airport runway suggests a graben structure filled at different sedimentary rates. besides, annual climate seasons favor permanent erosive activity, mainly at the runway’s north, west, and east, where surficial runoff is observed. along the runway, the active layer was removed or compacted. this layer of < 1 m thick is only observed around the runway. 3. materials and methods nogoshi and igarashi (1971) and later nakamura (1989, 2000) introduced the hvsr or ‘h/v’ method, which is based on the processing of microtremors or the ambient seismic noise generated under natural or anthropogenic perturbations, e.g., by the incidence of wind, sea waves, traffic of vehicles or industrial processes on sedimentary sequences. acquisition and processing of the seismic noise’s orthogonal components are used to estimate the power spectra ratio of the horizontal (h) over vertical (v) components to infer the strata’s elastic structure below the subsurface. thus, resonance peaks of various frequencies are then related to the thickness (h) and shear wave velocity (vs) of each layer under the hypothesis of lateral homogeneity. under this assumption, the following expression is used to analyze the direct problem of the soft sedimentary cover resting on a rigid basement (nakamura, 1989): = (2 − 1 ) 4ℎ , = 1 ,2,…∞ (1) equation 1 suggests that different physical properties in the sedimentary sequence incorporate one harmonic in the spectra. therefore, the hvsr curve presents as many peaks as layers, with amplitudes linked to each impedance contrast. diverse strategies have been proposed to estimate the inverse problem to infer the elastic structure of the soft sedimentary cover resting on a rigid basement using the hsvr curve (see, e.g., castellaro and mulargia, 2009; herak, 2008; sánchez-sesma et al., 2011; priolo et al., 2012; abu-zeid et al., 2014; lunedei & albarello, 2015; mantovani et al., 2015; bignardi et al., 2016; carcione et al., 2017). in this work, the authors have used the program openhvsr (bignardi et al., 2018), based on the previous program modelhvsr (herak, 2008). this code computes theoretical transfer functions for layered soil models based on the fast recursive algorithm proposed by tsai (1970) and modified to take frequency-dependent attenuation and body-wave dispersion into account, as was suggested by tsai and housner (1970). in this case, the soil model consists of any number of horizontal, homogeneous, isotropic, and viscoelastic layers stacked over a halfspace, each of which is defined by thickness (h), the velocity of propagation of the body wave (vp and vs ), density, poisson’s ratio, and the frequency-dependent seismic q-factor (qp and qs), which controls the anelastic properties. the theoretical transfer functions are calculated using a simple and guided monte carlo search in the model space. those estimations are compared with the observed hvsr curves to minimize a misfit function. details of this procedure can be consulted by tsai (1970), tsai and housner (1970), and herak (2008). the initial model for starting the inversion (table 1) was inferred from the detailed stratigraphy reported by montes et al. (2013). after several tests, the best fit was reached using a one-layer model with parameter values based on the average of the seven layers of table 1. the range of values used as input parameters in the openhvsr code was: vp = 80 4500 m/s; vp/ vs= 1.6 – 4.0; density = 1.4 – 2.8 g/cm3, thickness =0.5 – 999 m,qs = 5 500, and qp/ qs =1.5 – 4.5. table 1. the initial model of elastic and anelastic parameters used for stating the inversion. the model is based on seven sedimentary layers on a rigid basement inferred from the stratigraphy reported by montes et al. (2013). after several tests, a best-fit was reached using a one-layer model with parameter values based on the average of the seven layers. layer (m/s) (m/s) ρ (g/ cm3) thickness (m) 1 881 509 1.69 12 59 30 2 855 494 1.71 82 30 10 3 961 556 1.74 45 58 31 4 1006 581 1.75 58 30 10 5 1032 597 1.79 177 32 11 6 1031 596 1.82 60 58 31 7 1332 770 1.87 359 37 15 average 1014 586 1.77 113 43 20 the study area’s operational and access restrictions limited the experiment to be carried out between january 10 and 24, 2020. during this period of the austral summer, the intensity and direction of winds were stable. once the authors recognized the terrain, functionality of the seismic instrument, and field logistics, 67 continuous seismic noise stations, 20-30 min-long, were recorded. signals recorded were not discriminated from or excluded by possible noise sources, which were expected to be mainly related to wind and oceanic tides. signals were acquired using a three-component seismometer with a natural frequency fn= 0.45hz. the seismometer was directly installed 200 carlos a. vargas, juan m. solano, adriana m. gulisano, sergio santillana, edwin a. casallas on the ground using spikes under the base to guarantee a proper soil-sensor coupling. the seismographic system is composed of a 24-bit digitizer to 200 sps and was set up with an anti-aliasing filter at 50 hz, which, in this particular case, guaranteed sample frequencies below the noise associated with the electrical system frequency that powers the infrastructure of the scientific base in the seymour-marambio island. the seismograph is synchronized by a gps with 5 microseconds resolution and receives analogic signals from a triaxial seismometer at 4.5 hz. those analogic signals were coupled to an operational amplifier with negative feedback and a buffer amplifier to extend the bandwidth to 0.45 hz (e.g., vitale et al., 2018). according to parolai and galiana-merino (2006), transients into the seismic noise record do not affect the h/v ratio. hence, original signals need not be filtered or have anti-triggering procedures. geopsy was the open software used for signal processing (wathelet et al., 2020). figure 3 shows a flow diagram of the procedure used, starting with 1) acquisition of >20 min three-components seismic record; 2) split the record in windows of 30 s with overlaps of 50%; 3) estimation of a weighted hanning window and then fft for each 30 s record; 4) smoothed of the spectrum of each component using the konno–ohmachi filter with a b-value of 40 (konno & ohmachi, 1998); 5) the horizontal specter was computed by euclidian norm using the north and east components (h=√n2 + e2 ) and then estimated the h/v spectral ratio. 6) stacking those spectral ratios for all the 30-second segments allowed the calculation of an average and standard deviation in each station for the frequency range 0.5-10.0 hz. the same figure presents an example of one >20 min three-component seismic record, its h/v spectral ratios for all 30 second-long windows, the stacked spectral ratio, and the estimations of the elastic and anelastic properties using the openhvsr code. in general, the authors observed that even having momentary and abrupt changes in the seismic noise records’ energy, the best 1d model found is like that estimated without the abrupt changes. two conditions were assumed along with all estimations: 1) the seismic ambient noise field is homogeneous across all azimuths, and 2) the soil model has several viscoelastic layers laterally homogeneous stacked over a halfspace. strictly those restrictions are not satisfied owing to: 1. the diverse intensity of the antarctic weather in the study zone, with wind trend nne throughout the year (yu et al., 2020); 2. varied interactions between ocean currents and the island; 3. the effect of material damping on the expected noise wavefield as a consequence of sources relatively distant from the receivers (e.g., lunedei and albarello, 2009); 4. complex geology inside a steep topographic contrasting (figures 1 and 2); 5. inversion of viscoelastic parameters due to an active upper layer, the rigid permafrost, which is over sedimentary layers with and without water, and the deeper bedrock. all those factors make the hvsr curves challenging to invert and interpret. carcione et al. (2017) studied soft-layers and bedrock anelasticity on the s-wave amplification to evaluate the impact of attenuation and elasticity of bedrock on the amplitude and frequency of the resonance peaks in two glaciers located in northern italy and the antarctic continent. they concluded that the attenuation and bedrock elasticity must be considered to obtain reliable layer thickness estimations. hence, the combined interpretation of the thickness and viscoelastic parameters (vp, vs, qp and qs ) is necessary to understand the permafrost structure in the study area. figure 3. procedure used to estimate elastic and anelastic parameters using stacked hvsr records. (a) flow diagram describing different preprocessing stages, processing, and estimation of elastic and anelastic parameters. (b) original seismic-noise recording of the sn-09 station (>20 min duration). (c) amplitude spectrum every 30-seconds. (d) the average h/v ratio and its standard deviation for a frequency range of 0.5-10.0 hz. (e) example of best-fit of the h/v averaged spectral ratio, used to calculate changes in depth for elastic (vp, vs, and vp/vs), and anelastic (qp, qs, and qp/qs) parameters using the program openhvsr (bignardi et al., 2016, 2018). 201seismic properties of the permafrost layer using the hvsr method in seymour-marambio island, antarctica 4. results due to the h/v ratio does not discriminate the horizontal incidence direction of the perturbations, and each station may be analyzed from the point of view of a 1d model with limited seismic energy influencing its surrounding area, the authors hypothesize that results may show trends related to the seismic response of the underneath sedimentary structure. figure 4 shows the 67 hvsr stations classified in three main trends: (a) with almost flat hvsr response, (b) presenting a growing-up hvsr response, and (c) showing a decreasing hvsr response. only two stations offer a dominant peak in the frequency range analyzed (sn-2 and sn-3). comparatively, the spectral response along the runway does not present dominant peaks, with trend responses (a) and (b) suggesting that the permanent interaction of these layers throughout impacts, movements of high loads (e.g., the permanent landing and taxi of large aircraft on the runway), and maintenance works contributed to permanent changes in the dynamic response of this ground. the dataset impedes exploring if this activity may contribute to permanent changes in the seismic dynamic response beyond the first meters. those stations with dominant peaks or related to trend (c) are on the eastern steepest side of the runway, near active landslides, indicating a seismic response linked with not well-consolidated materials. a related observation concerning changes in the ground’s dynamic response by human activity is reported by abu zeid et al. (2017). those authors suggest that long-term trampling results in sediment stiffening, increasing the density and velocity of seismic shear waves. the authors found that vp and vs have a broad range of values (figure 5 and table s1). several authors have suggested that in permafrost, vp reaches high values (e.g., timur, 1968; carcione & seriani, 1998; krautblatter & draebing, 2014). according to these authors, high velocities of the seismic wave’s propagation respond to the pore space occupied by ice and the velocities of the rock matrix, interstitial ice, and available water in an extensive range of temperatures. nevertheless, the present study’s authors observe that our fitting of observed and theoretical spectra at low frequencies is poor (also reported in other works, e.g., talha qadri et al., 2015; abu zeid et al., 2017; bignardi et al., 2018). this uncertainty may suggest artifacts in the solution of the velocity structure, for instance, large velocity values. the authors want to encourage developing future works to contrast the estimated range of velocities reported in this paper. it draws attention that plotting values of qp vs vp and qs vs vs at depths ranges of z ≥ -35m, -35 m ≤ z < -100 m, and z ≤ 100 m appear to be trends of low attenuation–high velocity in shallow depths, and converse, high attenuation-low velocities in deeper layers (figure 5). following barton (2007), this trend could be interpreted as increasing fluid saturation in deeper layers. indeed, the authors consider that the shallow frozen and rigid permafrost observed in the field presents low attenuation, high velocity, and, consequently, low fluid saturation, except in the thinner active layer. they also interpret that the incremental geothermal gradient promotes melting ice, generating high attenuation and low velocities in deeper layers. prasad (2002) suggested that the relationship between ratios (vp/ vs) 2 and qp/ qs offers an idea of increasing pore pressure by reducing effective stress in saturated or partially saturated rocks. the dataset (figure 5) indicates a broad range of pore pressure conditions that dominate in surficial layers, probably due to permafrost but are still observed in deeper layers. besides, in the wide range of densities detected, around -100 m, it was found find high vp/ vs ratio with low-density values indicating highpressure fluids’ potential presence (see, e.g., vargas & torres, 2015; koulakov & vargas, 2018), probably related to the high concentration of gases in the permafrost. figure 5. elastic and anelastic parameters were estimated in the uppermost strata north of the seymour-marambio island. the parameters were interpolated at three depths from the surface: z ≥ -35 m (cyan circle), -35 m ≤ z < -100 m (blue star), and z ≤ -100 m (light blue square). arrows suggest general trends of parameters. figure 4. hvsr curves of 67 seismic noise stations were recorded in the seymour-marambio island, classified in three main trends: (a) with almost flat hvsr response (blue frame), (b) presenting a growing-up hvsr response (green frame), and (c) showing a decreasingly hvsr response (purple frame). only two stations offer a dominant peak in the frequency range analyzed (sn-2 and sn-3). the map shows the spatial distribution of these patterns. the decreasingly hvsr response stations and the two previously mentioned stations with a dominant peak are close to active landslides. 202 carlos a. vargas, juan m. solano, adriana m. gulisano, sergio santillana, edwin a. casallas 5. discussion in the fieldwork activities during the austral summer of 2020, the authors observed high mobility of the active layer, accompanied by runoff in the northern sector of seymour-marambio island, probably linked to the weddell, hobbs, and la meseta formations. these processes made it possible to detect landslides, mainly in some areas of high slopes. thanks to these events and the accessibility to two surficial trenches, the permafrost’s presence at approx. 50100 cm depth was verified. this upper permafrost surface may offer keys to its spatial distribution. fukuda et al. (1992), using one reference site in seymourmarambio island with a mean annual temperature of approx. -10°c at the surface, a thermal profile up to 2.5 m depth, and assuming a yearly temperature gradient of 0.19°c/m in steady-state conditions, estimated the permafrost base with 0°c at a depth of ~34 m. they also contrasted other geoelectrical resistivity measurements acquired by fournier et al. (1990), which reported the permafrost thickness between 28.6 m and 127.5 m. beneath the rigid permafrost base, the abundance of melted water and other precipitated minerals may promote viscoelastic inversions in the stratigraphy profile, at least from the runway (summit at approx. 200 m.a.s.l.) up to sea level. besides, kato et al. (1990) found high concentrations of caco3 and na2so4 in ice wedges, indicating percolation processes of molten water on this island under an extremely arid environment. all those observations may explain the diverse viscoelastic profiles and inversions of properties presented in this paper’s supplementary material. for instance, according to the vp profiles, in general, the authors observed the first inversion at depths ranging between 5 and 110 m. there are up to three inversions in some places, suggesting a variable thick of the permafrost and the possibility of lateral and vertical variations of porosity that allow more significant ice concentrations, mainly in the neogene quaternary formations. on the other hand, the inversions of and do not coincide with those of and , suggesting that seismic attenuation mechanisms may be observed in the permafrost in response to the presence of gases or under the permafrost base, probably related to formations with water deposits. thereby, vp, vs , and vp/ vs ratios were mapped at two depths: at 1.0 m to observe the lateral variations of the permafrost under the active layer and at 35.0 m in the first permafrost base reported (fukuda et al., 1992). as the most profound resolution reached in this work is approx. 200 m, it guarantees to observe the possible cases of permafrost up to the sea level and the role of the paleogene to quaternary formations (almost flat) in the lateral distribution of this layer (supplementary material). figure 6 presents the horizontal sections, showing lateral variations of the seismic velocity structure in the study area. laboratory and field experiments have associated high values of vp to permafrost layers (krautblatter & hauck, 2007; krautblatter & draebing, 2014; skvortsov et al., 2014), meaning in this case that permafrost is emplaced at 35 m-depth and highlighting patches of high values of elastic properties. high p-wave velocities are widespread except in high-slope areas. similar patterns are observed with the s-wave velocities (vs). n-s and e-w profiles of the same figure suggest lateral variation in the permafrost layer’s elastic properties. consequently, the vp/ vs ratio is distributed in a broad range of values (1.6 – 4.0), indicating rocks with various porosities. many are probably saturated with fluids in different phases and/or critical points (e.g., lee, 2003). the authors do not discard that these high vp/ vs ratio patches represent the permafrost with anomalous saturated accumulations of gases such as ch4 and/or co2. hence, they inferred that paleogene to quaternary formations reported by montes et al. (2013) have high petrophysical properties heterogeneity. consequently, they also should be associated with high lateral variation in ice concentrations. beyond observations of the hvsr curves in three trends (figure 4), it is difficult to discern the topographic effect on the distribution of the mapped elastic parameters. low values of and in the peripheral part of the maps (figure 6) could be interpreted as the seismic response of non-compacted material associated with landslides. future work could collect data on hillside areas, even by more extended sampling periods and instruments with more bandwidth, to verify if the combined effect of topography and permafrost presence introduces imprints in the seismic response. discarding the topographic effect in the study area, the almosthorizontal stratigraphy in the northern sector of the island offers an interesting constraining to validate the hvsr method’s hypothesis regarding a subsurface structure composed of viscoelastic layers laterally homogeneous. in addition, the trends of vp vs. vs showed in figure 5 illustrate that in shallow strata persist saturation of gases, something not observed in the plot qp vs.qs . it means that gas saturation mainly affects the elastic properties and, in a lesser proportion, the anelastic properties. in contrast, in deeper strata, under the permafrost base, where the water reservoir’s location is calculated (kneisel & hauck, 2008), the primary mechanism of seismic dissipation is anelastic. the plot of (vp/vs) 2 vs. qp/qs shows these effects nicely. according to borzotta and trombotto (2004), the abundance of unfrozen water in frozen soils promotes variation in subsurface geophysical properties. hence, having detected the possible interface where permafrost’s basement has a temperature assumed as 0°c, the authors speculate as an additional worth of the hvsr method applied in the study area that the spatial distribution of elastic and anelastic properties may reflect the thermal structure of this layer in the study area. figure 6. maps of vp, vs , and vp,/ vs ratios estimated at 1.0 m and 35.0 m depth. high values of could vp, be related to permafrost layers. permafrost could be associated at 35 m-depth with patches of high values of elastic properties. 6. conclusions using the estimates of h/v ratios on 67 seismic-noise stations, we calculated elastic and anelastic properties in the uppermost layers at the north sector of seymour-marambio island. the h/v spectral ratios distribution suggests the permanent interaction on the uppermost sedimentary layers due to maintenance works, trampling, impacts, and high loads (e.g., the permanent landing and taxi of large aircraft on the runway during decades) may contribute to changes in the seismic dynamic response. the mapping of elastic properties at 1.0 m and 35.0 m depth allowed identifying lateral variations of the study area’s permafrost seismic properties. at those depths, we hypothesize that permafrost is emplaced in rocks with patches of porosities saturated with fluids in different phases and critical points, e.g., with abnormal saturated accumulations of gases like ch4 and co2. in shallow strata, the saturation of gases affects mainly the elastic properties. in contrast, in deeper strata, where the location of water reservoirs is expected, the primary mechanism of seismic dissipation is anelastic. 7. data and resources this study is based on seismic microtremors’ data acquired during the colombian and argentinean scientific teams’ campaign in january 2020 near the marambio scientific base of argentina. for more information and access to the dataset, contact to authors of this paper. all data processing and plotting were done using the open software geopsy (wathelet et al., 2020), openhvsr (bignardi et al., 2016, 2018), and the academic license of matlab® r2020b of mathworks. acknowledgments cav and amg thank instituto antártico argentino and comando conjunto antártico for the logistic support. cav and jms thank universidad nacional de colombia, departamento de geociencias for helping support this research. the authors are grateful to staff members of the laboratorio antártico multidisciplinario marambio (lambi) for taking care of the permanent instrumentation. the authors also thank officers, crew, technicians, and the 203seismic properties of the permafrost layer using the hvsr method in seymour-marambio island, antarctica fuerza aérea argentina science party that take care of the marambio station. amg is a member of the carrera del investigador científico, conicet. references abu zeid, n., bignardi, s., caputo, r., mantovani, a., tarabusi, g., & santarato, g. 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(2020). the climatology and trend of surface wind speed over antarctica and the southern ocean and the implication to wind energy application. atmosphere 11(1), 108. https://doi. org/10.3390/atmos11010108 https://doi.org/10.1017/jog.2016.135 https://doi.org/10.1017/jog.2016.135 https://doi https://doi.org/10.1111/j.1365-246x.2011.05064.x https://doi.org/10.1007/s11589-014-0105https://doi.org/10.1007/s11589-014-0105https://doi.org/10.3390/atmos11010108 https://doi.org/10.3390/atmos11010108 _hlk62231714 _hlk59629760 _hlk62723850 _hlk62250359 _hlk62830182 _hlk62327353 _hlk62317979 _hlk62740614 _hlk90238602 _hlk90237768 _hlk54906197 _goback issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.114856 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 397 408 g eo ph y si c s study of the structural characteristics of the earth’s crust in cao bang province and adjacent areas using gravity data thanh duong van1*, hung pham nam1, trieu cao dinh2, trong cao dinh1, tuan thai anh1, bach mai xuan1, luc nguyen manh1 1. institute of geophysics, vast, a8/18 hoang quoc viet road, hanoi, vietnam 2. institute for applied geophysics, vusta, 210 doi can road, hanoi, vietnam *corresponding author: dvthanh@igp.vast.vn how to cite item: thanh, d. v., hung, p. n., trieu, c. d., trong, c. d., tuan, t. a., xuan, b. m., & manh, l. n. (2024). study of the structural characteristics of the earth’s crust in cao bang province and adjacent areas using gravity data. earth sciences research journal, 28(4), pending pages. https://doi.org/10.15446/esrj.v28n4.114856 record manuscript received: 04/06/2024 accepted for publication: 19/11/2024 abst r ac t cao bang province and adjacent areas in northern vietnam are affected by modern tectonic fault zones, leading to frequent landslides and earthquakes. understanding the deep structural characteristics of the earth’s crust is crucial for advancing the study of geodynamics and geological hazards. most previous studies on the deep structural characteristics of this region used only a few analysis methods, and their results were not highly reliable. aiming to provide highly reliable information about the deep structural characteristics of this region, this study we apply a calculation process with a combination of gravity data analysis techniques and earthquake data. specifically, the depth results obtained from the 2d modeling problem are utilized as inputs to construct an initial 3d model, thereby improving the accuracy of the depth of the crystalline basement and moho surfaces. by integrating the results from these techniques with earthquake data, a more accurate fault system map can be developed. maximum horizontal gradient, tilt angle, constrained two-dimensional inversion, euler deconvolution, 2d modeling and 3d inversion are the main techniques used. the results of the study area show that the fault system is quite complex. large faults can be easily recognized such as f1 (pho day river), f2 (yen minh – phu luong), f3 (sub-meridian fault) and f4 (cao bang – tien yen). the depth of the crystalline basement surface varies from 0 to 4.32 km, with the basement outcropping in bac kan province and reaching deep depths in the cao bang and that khe basins. the depth of the moho surface varies from 28.77 to 38.01 km. the shallowest depth of the moho surface is located in the northern area of cao bang province, the southern area of bac kan province, the southeastern area of lang son province, and the cao bang and that khe basins. the deepest depth value of the moho surface is located in bac kan province, where the crystalline basement is outcrop. the study results have shown the effectiveness and high reliability of applying calculation processes and combining analytical methods. keywords: gravity; 2d modeling; 3d inversion; fault; crystalline basement surface; moho surface. palabras clave: gravedad; modelamiento 2d; inversión 3d; fallas; piso cristalino; superficie de moho estudio de las características estructurales de la corteza terrestre en la provincia de cao bang y de áreas adyacentes a través del uso de información de la gravedad la provincia de cao bang y las áreas adyacentes del norte de vietnam están afectadas por zonas con fallas tectónicas modernas que le ocasionan deslizamientos de tierra y terremotos constantes. conocer las características estructurales profundas de la corteza terrestre es determinante para el avance de la geodinámica y de los peligros geológicos. la mayoría de los estudios previos sobre las características estructurales profundas en esta región usaron solo algunos métodos de análisis, y sus resultados no son altamente confiables. con el fin de aportar información altamente confiable sobre las características estructurales profundas de la región, en este estudio los autores emplean un proceso de cálculo combinado con técnicas de análisis de información de la gravedad e información de terremotos. específicamente, los resultados de profundidad obtenidos del modelamiento del problema en dos dimensiones se utilizaron como referentes para construir un modelo inicial tridimensional, y de esta forma mejorar la precisión de la profundidad del piso cristalino y de la superficie moho. al integrar los resultados de estas técnicas con información de terremotos se puede desarrollar un mapa del sistema de fallas más preciso. las principales técnicas usadas son el gradiente horizontal máximo, ángulo de inclinación, inversión bidimensional residual, la deconvolución de euler, modelado 2d e inversión tridimensional. los resultados muestran que el sistema de fallas es complejo. grandes fallas se pueden reconocer fácilmente como la f1 (río pho day), f2 (yen minh-phu luong), f3 (falla submeridiana) y f4 (cao bang-tien yen). la profundidad del piso cristalino varia de 0 a 4.32 kilómetros, con un afloramiento en la provincia de bac kan y con mayor profundidad en las cuencas de cao bang y tath khe. la profundidad de la superficie moho varia de 28,77 a 38,01 kms. la parte menos profunda de la superficie moho se ubica en el norte de la provincia de cao bang, en el sur de la provincia de bac kan, en el sureste de la provincia de lang son, y en las cuencas de cao bang y that khe. la parte más profunda de la superficie moho se ubica en la provincia bac kan, donde todo el piso cristalino es un afloramiento. los resultados del estudio han mostrado la efectividad y alta confiabilidad de aplicar procesos de cálculo combinados con métodos analíticos. resu m en https://doi.org/10.15446/esrj.v28n4.114856 https://doi.org/10.15446/esrj.v28n4.114856 398 thanh duong van, hung pham nam, trieu cao dinh, trong cao dinh, tuan thai anh, bach mai xuan, luc nguyen manh 1. introduction study of the deep structure of the earth’s crust is crucial for enhancing our understanding of geological structures, providing essential information for mineral exploration, and serving as a reference for further studies on geodynamics and geohazard risks. gravity data are commonly used in largescale studies due to its advantages. in vietnam, there have been numerous studies on the earth’s crust structure based on gravity data. these studies have produced depth diagrams of basic surface boundaries and fault system diagrams. however, there are still many shortcomings in the studies, such as sparse gravity measurements, limited analytical methods, and not high reliability of the results (hai, 2003; que, 1979, 1982, 1985; que and trieu, 1980, 1984; trieu and vuong, 1985; trieu and long, 2002; trieu, 1983, 2005). in addition to using two-dimensional (2d) processing and analysis methods, numerous studies has also applied three-dimensional (3d) methods to study deep structures and determine the depth of basic earth crust boundary surfaces. the effectiveness of the 3d inverse method has been demonstrated in determining the depth of basic boundary surfaces (hong et al., 2016, 2017; huong, 2006; minh, 2014; trung, 2014; trung and huong, 2005; trung et al., 2004, 2018). however, these studies only used the 3d inverse method without combining it with other methods to build the initial model, leading to low reliability of the results. this observation is supported by the following studies, which highlight the effectiveness of combining 2d and 3d modeling approaches (gaud, 2017; kanthiya et al., 2019; kebede et al., 2021; laith, 2016; trong, 2023). cao bang is a mountainous province on the northeastern border of vietnam, playing an important role in economic and diplomatic development, with a 314 km long border with china. the province has rich and diverse mineral resources and is a key economic area for mineral exploitation in the north of vietnam. in particular, natural disasters such as landslides and earthquakes often occur in the cao bang area. one of the main reasons is that the terrain is strongly fragmented and affected by modern tectonic fault zones such as the cao bang – tien yen fault zone, yen minh – phu luong fault zone, pho day river fault,... in cao bang, a high-level earthquake with a strength equal to 5.4 occurred on november 25, 2019, causing ground collapse and severe cracking in the areas of dinh phong – dam thuy communes, trung khanh district, creating confusion among the population of the province, where 28 ethnic groups live. although research on the structure of the earth’s crust in the northern region of vietnam has yielded a number of published works, there have been no detailed studies of cao bang province and adjacent areas. therefore, research to determine the structural characteristics of the earth’s crust in cao bang province and adjacent areas is one of the important tasks, serving the assessment of earthquake severity and earthquake hazards. the distribution and structural characteristics of deep faults are the scientific basis for studying the prediction of seismic zones and earthquake activity mechanisms. in addition, the horizontal layering characteristics of the earth’s crust also play an important role in the research and prediction of the active layer (earthquake-generating layer) of earthquakes. this study presents a combination of 2d and 3d processing and analysis methods with gravity data. constrained 2d inversion and euler deconvolution methods are used as the basis to build the initial 2d model. from the depth results of the crystalline basement surface and moho surface obtained from the 2d modeling problem, they will be used as input for the 3d inverse problem. the maximum horizontal gradient method, tilt angle method, upward continuation method, and earthquake location data are used to determine the fault system in the study area. combining the above processing and analysis methods will help increase the reliability of the initial 3d model, thereby obtaining high-resolution results. additionally, the combined use of different processing methods, along with data from earthquakes that have occurred, will help identify fault systems with high accuracy. the workflow diagram for calculating, processing, and analyzing gravity data for constructing the depth diagram of the basic boundary surfaces is shown in figure 1, and constructing the fault system diagram is shown in figure 2. figure 1. a workflow diagram for 2-d forward and 3-d inversion gravity modeling (the blue rectangular boxes represent the input data, and the process proceeds sequentially from left to right) figure 2. a workflow diagram for faults identification (the blue rectangular boxes represent the input data, and the process proceeds sequentially from left to right) 2. geological and tectonic context the study area chosen is cao bang province and adjacent areas, limited by longitude 105°00’–107°00’; latitude 22°00’–23°30’ with geographical and x (500000–704229); y (2432827–2600293) with the utm perpendicular coordinate system (figure 3). the study area is a limestone plateau mixed with dirt mountains, with an average altitude of over 0.2 km. the border area has an altitude of 0.6–1.3 km above sea level, with the highest point at 1.98 km and the lowest point at less than 0.2 km. the average altitude is 0.6–1 km above sea level, resulting in three distinct regions: the east, with many rocky mountains; the west, with dirt mountains mixed with rocky mountains; and the southwest, mostly dirt mountains with dense forests. the geology of the study area is shown in figure 3, the 1:500,000 scale geological map established by the vietnam general department of geology and minerals (luong et al., 1988). cao bang province and adjacent areas have quite complex geology, including many formations and complexes, influenced by major fault zones such as cao bang – tien yen, yen minh – phu luong, pho day river, etc. in the tectonic context, the cao bang terrane and adjacent areas are located on the edge of the vietnam-china continental plate. the regional geological structural framework was shaped immediately after the caledonian orogeny (late silurian). this structural formation system was later partly modified by the indosinian orogenic phase (late triassic) and extensional deformation during 399study of the structural characteristics of the earth's crust in cao bang province and adjacent areas using gravity data the cenozoic. the cao bang basin is situated at the northernmost point in the cenozoic basin chain along the cao bang – tien yen fault zone (figure 3). the basin has a diamond shape, nearly 20 km long in the northwest – southeast direction, with the widest point being about 6–7 km. the area is approximately 80–90 km2. additionally, the basin is also segmented by high-order faults in the northwest – southeast and northeast – southwest directions. the khe basin has a parallelogram shape formed in the northeast wing of the cao bang – tien yen fault along the fault direction with a length of about 6 km (figure 3). figure 3. geological map of cao bang province and adjacent areas (luong et al., 1988) the basement of the basin consists of mesozoic and paleozoic sediments (conglomerate, sandstone, siltstone, and limestone) and igneous rocks (triassic granite, permian basalt, and tuff, etc.). the sediment thickness in the basin is about 0.9–1 km, including the cao bang formations of eocene age (coarsegrained sediments in the lower part) and the na duong formations of eoceneoligocene age (fine-grained sediments in the upper part). they are distributed in uplifted marginal strips exposed in the areas of coc lung – na dong – na gia (southwest range) and na ngan – gia cung (northeast range). late cenozoic sediments are almost absent (viet, 2005). 3. materials and methods 3.1. gravity anomaly and rock density values the bouguer gravity anomaly data in the study area was established by the geophysics division in 2011 at a scale of 1:500,000 (figure 4) and stored at the general department of geology and mineral resources (bouguer gravity anomaly map, 2011). some gravity measurements were compiled at a scale of 1:200,000 as part of the project “research to identify source areas and evaluate maximum earthquakes at risk of occurring in cao bang province and adjacent provinces” under code vast05.06/21-22. to build the initial model, in addition to information about boundaries and body edges, parameters about the density of rock and soil layers are also required. in this study, the soil and rock density values used are referenced from the studies of trieu (2005) and phong (2016). table 1 shows the rock density values of the sedimentary crust, crystalline crust, and upper mantle. table 1. density parameters for soil and rock layers (trieu, 2005; phong, 2016) no. layer density (g/cm3) average density (g/cm3) 1 sedimentary crust 2.54–2.65 2.62 2 crystalline crust 2.80–2.85 2.83 3 upper mantle 3.10–3.23 3.15 figure 4. bouguer gravity anomaly in cao bang province and adjacent areas. (1) bouguer gravity anomaly; (2) boundary; (3) river 3.2. terrain elevation and location of earthquakes terrain elevation data are taken from the aster-gdem source with a resolution of 30 meters (figure 5). this is a collaborative program between nasa and the japanese government that created the advanced spaceborne thermal emission and reflection radiometer (aster) system. as part of this project, the aster global digital elevation model (gdem) product suite was developed, providing a global-scale digital elevation model. figure 5. terrain elevation in cao bang province and adjacent areas. (1) terrain elevation; (2) boundary; (3) river this study utilizes additional earthquake data, specifically statistical data on earthquake locations, which plays an important role in providing more information to help identify tectonic movements and fault systems in cao bang province and adjacent areas. an earthquake is a shaking on the earth’s surface resulting from the sudden release of energy in the earth’s crust, generating seismic waves. there are many causes of earthquakes, including endogenous, exogenous, and human causes. endogenous causes, particularly due to the movement of tectonic plates in the earth’s crust, lead to faulting activities. earthquakes often occur at or around active faults. therefore, knowing the distribution of earthquakes that have occurred can provide valuable information to identify geological faults. 400 thanh duong van, hung pham nam, trieu cao dinh, trong cao dinh, tuan thai anh, bach mai xuan, luc nguyen manh figure 6 shows the locations of earthquakes that occurred in cao bang province and adjacent areas, established by the institute of geophysics in 2020 (institute of geophysics, 2020). figure 6. location and magnitude distribution diagram of earthquakes that occurred in cao bang province and adjacent areas. (1) m < 3.5; (2) m = 3.5–3.9; (3) m = 4.0–4.4; (4) m = 4.5–4.9; (5) m = 5.0–5.4; (6) boundary; (7) river 3.3. euler deconvolution the euler deconvolution method was proposed by thompson (1982) and further developed by reid et al. (1990) to determine the boundary and depth to the source of the anomaly. we consider the function f (x,y,z) in the perpendicular coordinate system x, y, z. the function f (x,y,z) is called n degree homogeneous if: f(tx,ty,tz) = tn.f(x,y,z) (1) in this case the function f satisfies the following equation (2): f f fx y z nf x y z ∂ ∂ ∂ + + = ∂ ∂ ∂ (2) equation (2) is called euler’s identity equation. consider the set of functions f (x,y,z) of the form (durrheim and cooper, 1998; reid et al., 1990; thompson, 1982): ( , , ) n gf x y z r = (3) where 2 2 2r x y z= + + , n = 1, 2, 3,…; and g is a constant that does not depend on x, y and z. in practice, we take into account the deviation of the local anomalous field caused by the field with a deeper origin, which we consider constant and denote as b. we then have the euler identity equation (4): 0 0 0( ) ( ) ( ) ( )f f fx x y y z z n b t x y z ∂ ∂ ∂ − + − + − = − ∂ ∂ ∂ (4) where (x0, y0, z0) is the coordinate of the magnetic source (or gravity), t is the observed field value at point (x, y, z), b is the background field value, and n is the structural index. for magnetic sources (such as uniformly magnetized spheres, magnetic dipoles, etc.), the value of the structural index varies from 0 to 3, and for gravity materials, the value of the structural index varies from 0 to 2. when calculating, one can use the actual value of the structural index. the main principle of the euler deconvolution method is to solve equation (4) with the given structural index n and the need to determine 4 unknowns: x0, y0, z0, and b. 3.4. constrained two-dimensional gravity inversion the constrained two-dimensional (2d) gravity inverse problem was applied in gravity data analysis by guillen and menichetti (1984) to determine the boundaries between geological structures with different density values. the structural cross-section containing the anomalous sources is divided into rectangular prisms in the x and z-axis system (the length of the object in the y-axis is considered infinite, making it a 2d case). the density contrast is constant within each prism and can vary. using matrix notation, the gravity anomaly vector d = [di]; i = 1, 2,…, n (n is the number of data), is given by the function: d = g.m (5) where g = [gij]; i = 1, 2,…, n; j = 1, 2,…, m is the kernel matrix, with gij being the contribution of the jth prism to the gravity value at the ith observation point. the model parameter vector representing the density contrast of the prism is m = [mj]; j = 1, 2,…, m; m is the number of model parameters (the number of prisms in the x and z-axis directions). the gravity component of an elementary prism is the gravitational force of a polygon in a 2d problem. in this case, the polygon has four perpendicular sides that coincide with the model grid. the value m can be determined according to the formula (mendonca et al., 1994, 1995): m = gt [ggt + λi]-1 d (6) where λ is the damping factor, i is the unitary matrix, and t is the transposition matrix. the iterative solution is carried out in 2d conditional inversion for the purpose of determining the density parameter (hendra and darharta, 2014; spector and granti, 1970). the maximum positive and minimum negative density values are of interest because they represent structural blocks. this method shows the linear residual density cross-section, helping to determine the residual density of geological objects compared to the surrounding environment. thereby, it helps identify vertical boundaries that separate geological structural blocks with different density values (zondgm2d, 2001). 3.5. maximum horizontal gradient the maximum horizontal gradient method was first proposed by blakely and simpson (1986) to determine the boundaries between geological blocks of different densities. the horizontal gradient (g) on the bouguer (or magnetic) gravity anomaly data grid is calculated according to the formula: 22( , ) ( , )( , ) g x y g x yg x y x y  ∂∆ ∂∆ = +   ∂ ∂    (7) with δg(x, y) representing the bouguer variation, the horizontal gradient value at the center point is compared with the value of the eight closest points in four directions (along the row, column, and both diagonals). if there is a maximum value, it must satisfy at least the following two inequalities: gi-1, j < gi,j > gi+1,j, gi, j-1 < gi,j > gi,j+1, gi+1, j-1 < gi,j > gi-1,j+1, (8) gi-1, j-1 < gi,j > gi+j, j+1, the maximum value of gi,j is calculated by a quadratic polynomial function, for example: if gi-1, j < gi,j > gi+1,j, then the position of the maximum value of gi,j la xmax = 2 b a , with 1, , 1, 1, 1, 2 2 , 2 2 i j i j i j i j i jg g g g g ba d d ; d is the distance to the two nearest points. the maximum value at position xmax is calculated according to the formula: gmax = ax2 max + bxmax +gi,j (9) 401study of the structural characteristics of the earth's crust in cao bang province and adjacent areas using gravity data the distribution of gmax in a linear shape reflects the existence of faults (trieu, 2000; van kha et al., 2018). 3.5. tilt angle the tilt angle method is presented in the study of miller and singh (1994). we can easily determine the boundary of an object from potential field data according to the arctan formula of the vertical field derivative on the horizontal field derivative: 1tan t z t h 1/222t t t h x y (10) where: 1tan t z t h 1/222t t t h x y (11) with t x , t y , t z are the first derivatives of the potential field in the x, y and z directions; t h is the horizontal derivative, t is the potential field. miller and singh (1994) demonstrated the boundaries of the anomaly source using the tilt angle method. the part with positive tilt angle values represents the source location, while the part with negative tilt angle values represents the area outside the source. the part with a zero value represents the boundary of the source. 3.6. 2d gravity modeling in the case of a two-dimensional (2d) model with an n-sided polygon as an object structure, the vertical component of gravity anomaly is determined according to the formula based on the study of talwani et al. (1959) and is simply expressed as equation (12): 1 2 m z j j g g zρ = ∆ = ∑ (12) talwani et al. (1959) considered the case of an n-sided polygon and broke the line integral into n-contributions, each associated with a side of the polygon. they derived expressions for zj that make extensive references to trigonometric functions, as clearly stated in their paper (talwani et al., 1959). the model involves creating a hypothetical geological model and calculating the gravity response to that earth model. the process of solving the 2d model problem is performed using the gmsys-2d tool, which implements the algorithm of talwani et al. (1959) and is integrated into the oasis montaj software (northwest geophysical associates, 2006). 3.7. 3d gravity inversion the theoretical basis of the method is based on the relationship between the fourier transform of the gravity anomaly and the sum of the fourier transforms of the density dividing boundary depth powers. according to parker (1973) the fourier transform of the gravity anomaly caused by the density dividing boundary h is determined by the expression: (δ ) = −2 ( ) ∑ ! ∞ [ℎ ( )] (13) where: f[] is the fourier transform of the variables in square brackets, δg is the gravity anomaly, g is the gravitational constant, 𝜌 is the contrast density (𝜌 = 𝜌dưới – 𝜌trên), k = (k 2 x + k 2 y) 1/2 with kx and ky are the wave numbers along the x and y axes respectively, 𝑧0 the average depth of the density separation boundary depth. in the study oldenburg (1974) rearranged expression (13) to calculate the depth for the boundary surface from the gravity anomaly value by an iterative process and is given by: ( ) 1 2 [ ( )][ ( )] [ ( )] 2 zok n n n f g x e kf h x f h x g nπ ρ − −∞ − ∆ = − − !∑ (14) this expression allows us to determine the surface depth using an iterative back-solving process through the gmsys-3d tool integrated in oasis montaj software, making the calculation and processing process more convenient (northwest geophysical associates, 2006). in this procedure, we assume the average surface depth is zo and the density contrast is ρ. gravity anomalies are determined before calculating the fourier transform. the first term of equation (14) is then calculated by assigning h(x) = 0. the inverse fourier transform of equation (14) will provide a first approximation of the surface, h(x). this value h(x), is then used in equation (14) to calculate a new depth value of h(x). this process is continued until a reasonable value is reached and will stop when the least squared error between the model’s gravity anomaly value and the actual gravity anomaly value drops below a given value or the allowed number of iterations is exceeded. 4. results and discussion 4.1. results of 2d gravity modeling according to geological documents of cao bang province and adjacent areas, it shows that the strike and geological faults direction are mainly in the northwest – southeast direction. therefore, to facilitate the analysis and interpretation of geological structures in the study area, the analyzed profiles are cut perpendicular to the northwest – southeast structural direction. based on the size of the study area as well as the map scale of the gravity anomaly and the analysis lines reflecting the regions in the study area, the author used 7 analysis profiles with a distance between points of 0.2 km. the analysis profile diagram is shown in figure 7. figure 7. profile analysis diagram in the study area. (1) profile; (2) bouguer gravity anomaly; (3) boundary; (4) river after obtaining the data from the 7 analysis profiles, the initial 2d model of the 7 profiles was built based on the results of constrained two-dimensional gravity inversion, euler deconvolution, density values in table 1, and the bouguer gravity anomaly of analytical profiles. after completing the initial 2d model, solve the 2d forward modeling problem using the gmsys-2d module to determine the depth to the basic boundaries, and thereby determine the appropriate density value for the geology of the study area. the results of solving the 2d forward modeling problem are shown in figure 8, with the error between the model’s bouguer anomaly curve and the actual bouguer anomaly curve being ≤ 5% of the anomaly range. blaikie et al. (2014) showed that with an error of ≤ 5% of the anomaly range, the results are highly reliable. the results of the 2d forward modeling for 7 profiles indicate that the appropriate density values for the sedimentary crust layer are 2.61–2.65 g/cm³, for the crystalline crust layer are 2.80–2.85 g/cm³, and for the upper mantle layer are 3.10–3.23 g/cm³. the crystalline basement is outcropped at profile 5. 402 thanh duong van, hung pham nam, trieu cao dinh, trong cao dinh, tuan thai anh, bach mai xuan, luc nguyen manh overall, the depth variations of the surfaces are quite small in profiles 1, 2, and 7, while there are significant up and down variations in profiles 3, 4, 5, and 6. these depth variations reflect important geological and tectonic characteristics of the area. they also indicate that past geological activities may have been more intense in these areas. the structural differentiation of the earth’s crust at these locations is also manifested through different physical properties such as density and hardness. these up and down fluctuations are associated with deep fault systems, where tectonic activities may have altered the surface and depth of these layers. another important aspect is the influence on earthquakes. the depth variations are related to the distribution of stress and energy in the earth’s crust, impacting the formation and development of earthquake zones. in areas with significant depth changes, where the surfaces experience considerable uplift and subsidence, these regions are more prone to earthquakes due to the accumulation of stress that can easily lead to the release of energy in the form of earthquakes. figure 8. results of solving the 2d forward modeling problem of 7 analysis profiles, corresponding to each profile in turn from top to bottom: actual gravity anomaly, gravity anomaly calculated from the model, error between actual gravity anomaly and calculated gravity anomaly; results of 2d modeling problem; geologicalgeophysical cross-section. (1) euler deconvolution result; (2) vertical boundary; (3) horizontal boundary; (4) constrained gravity inverse results; (5) sedimentary crust; (6) crystalline crust; (7) upper mantle 403study of the structural characteristics of the earth's crust in cao bang province and adjacent areas using gravity data 4.2. results of 3d inverse gravity modeling 4.2.1. initial 3d model and comparison of calculation results from 2d and 3d from the results of the depth of crystalline and moho surface, obtained after solving the 2d model problem. proceed to create the mesh through kriging interpolation, to obtain the depth of crystalline basement and moho surfaces according to the area. the initial 3d model was built based on the following data: terrain elevation, crystalline basement surface depth, moho surface depth. soil and rock density value data are used in combination in table 1 and from the results of solving the 2d model problem, specifically: sedimentary crust (2.61–2.65 g/cm3); crystalline crust (2.80–2.85 g/cm3); upper mantle (3.10–3.23 g/cm3). the initial 3d model is shown in figure 9. the initial 3d model was built based on the following data: terrain elevation, crystalline basement surface depth, moho surface depth. soil and rock density value data are used in combination in table 1 and from the results of solving the 2d model problem, specifically: sedimentary crust (2.61–2.65 g/cm3); crystalline crust (2.80–2.85 g/cm3); upper mantle (3.10–3.23 g/cm3). the initial 3d model is shown in figure 9. figure 9. the initial 3d model consists of 3 surfaces: the top layer is terrain surface, the middle layer is crystalline surface, the bottom layer is moho surface after constructing the initial 3d model, the next step is to perform 3d inversion using the gmsys-3d module to standardize the depth of the crystalline basement surface and moho surface. this inversion process is carried out automatically. after 5 iterations with a convergence limit of 0.1 mgal, the process will stop, providing the results for the depth of the crystalline basement surfaces and moho surfaces. additionally, the 3d inverse calculation process will return the error result between the bouguer gravity anomaly of the model and the actual bouguer gravity anomaly. the results of the depth for the crystalline basement surface and moho surface after the 3d inversion calculation are illustrated in figure 10. according to the 2d model problem, the depth of the crystalline basement surface ranges from 0 m to 4.03 km. the location with a value of 0 m is where intrusive magmatic rocks appear, as indicated on the geological map of the study area. with the 3d modeling problem, the depth of the crystalline basement surface ranges from 0 m at the magma outcrop location to the deepest position of 4.32 km. the depth difference error of the crystalline basement surface ranges from -0.19 km to 0.12 km (figure 10). although this depth difference is not too large, thanks to the advantage of the 3d inverse method, the calculation and processing are carried out on grids, eliminating the need for interpolation, and at the same time standardizing the depth at positions on the grid. the results returned by the model are more consistent with the actual gravity anomaly value. for the moho surface, the 2d model results range from 29.02 km to 37.77 km, while the 3d model results range from 28.77 km to 38.01 km. the difference between the two depth results ranges from -0.32 km to 0.38 km (figure 11). in general, there isn’t much difference in the morphology of the moho surface between the 2d and 3d modeling problems. however, as mentioned earlier, the 3d inverse calculation results help standardize the depth at the location of points on the grid. the final model shows a small error between the actual gravity anomaly and the calculated gravity anomaly from the model. figure 10. comparison of crystalline basement surface depth. (a) calculation result from 2d model solution; (b) calculation result from 3d model solution; (c) the crystalline basement surface depth difference between calculation results from 2d and 3d model solutions figure 11. comparison of moho surface depth. (a) calculation results from 2d model solution; (b) calculation results from 3d model solution; (c) the moho surface depth difference between calculation results from 2d and 3d model solutions 4.2.2. gravity misfit between observation and calculation data figure 12 shows the error between the actual bouguer gravity anomaly and the feedback gravity anomaly from solving the 2d model problem, while figure 13 shows the error between the actual bouguer gravity anomaly and the feedback gravity anomaly from solving the 3d modeling problem. from the error results, we can see that when solving the 2d model problem, the gravity anomaly error value ranges from -1.6 to 1.6 mgal. in contrast, when solving the 3d model problem, the gravity anomaly error value ranges from -0.5 to 0.65 mgal. therefore, it is evident that the results of the 3d inverse problem are more reliable than the results of solving the 2d model problem (skalbeck, 2014). 404 thanh duong van, hung pham nam, trieu cao dinh, trong cao dinh, tuan thai anh, bach mai xuan, luc nguyen manh figure 12. gravity misfit between observation and calculation data from the 2d model. (1) 2d anomaly error; (2) boundary; (3) river figure 13. gravity misfit between observation and calculation data from the 3d model. (1) 3d anomaly error; (2) boundary; (3) river after obtaining the results of the depth to the crystalline basement surface and the moho surface from both the 2d and 3d model problems, it is evident that the results of the 3d model problem show higher reliability than those of the 2d model problem. additionally, the 3d model results help standardize the depth to the crystalline basement and moho surfaces. figure 10 and figure 11 illustrate the difference in the depth of the crystalline basement surface and the moho surface between the results obtained from solving the 2d model and the 3d model problems. 4.2.3. crystalline basement surface the crystalline basement surface is often considered the boundary between the poorly consolidated sedimentary rock layer in the upper part of the structural section and the underlying granite layer. this boundary often experiences significant changes in rock density and wave velocity. the study results show that the crystalline basement surface in the study area exhibits a complex structural morphology, ranging from basement outcrop in some locations to a depth of 4.32 km (figure 14). the results reveal three distinct value ranges: the outcrop location, the location with medium depth, and the location with deep depth. according to figure 14, in cao bang province, the depth of the crystalline basement surface varies from 2.01 to 4.31 km from west to east. the change in depth clearly reflects the influence of the cao bang – tien yen fault zone. the depth of the crystalline basement surface in cao bang reaches a high value, indicating a concave structure in the cao bang basin and its surroundings. in lang son province, the depth of the crystalline basement varies from 1.51 to 3.42 km, with the higher depth values representing the concave structure at that khe basin. in bac kan province, the crystalline basement location ranges from an exposed basement with a value of 0 km to a depth of 2.92 km. in ha giang province, convex and concave structures are intertwined, but the difference in depth is not significant, ranging from 1.51 to 3.01 km. as for tuyen quang province, there is a small change in the depth of the crystalline basement, ranging from 1.41 to 2.51 m. figure 14. diagram of crystalline surface depth in cao bang province and adjacent areas. (1) depth of crystalline basement surface; (2) boundary; (3) river comparing the results of crystalline basement depth in cao bang province and adjacent areas with previous studies such as hai (2003) and phong (2016) shows that most of the results are similar, with the difference in depth not being too significant. in particular, hai’s study (2003) does not show the location of crystalline basement outcrop, although the geological map of vietnam at a scale of 1:500,000 (luong et al., 1988) indicates exposed magmatic rocks (figure 3). in the eastern part of cao bang province, a concave structure is evident with a depth approximately 0.3 to 0.4 km greater than that reported by phong (2016). 4.2.4. moho surface the moho surface is the boundary between the earth’s crust and mantle. the analysis results in figure 15 show details of the change in moho surface depth in cao bang province and adjacent areas, with the depth ranging from 28.77 km to 38.01 km for the entire study area. the structures tend to stretch mainly in the northwest – southeast direction. in cao bang province, the moho surface depth varies from 28.77 km to 36 km. the northern part of cao bang province has the shallowest and moderately shallow moho basement depth in the cao bang basin at 32.6 km. in lang son province, the depth of the moho surface varies from 31 km to 35 km. around that khe basin, the depth is 32.75 km. in bac kan province, the depth of the moho surface varies from 32 km to 36.51 km. the location with the deepest depth value is where the crystalline basement surface is outcrop, with a depth of 36.51 km. the depth of the moho surface in tuyen quang province varies from 32.01 km to 37.01 km, with the majority being deep depths. in ha giang province, which has the highest topographic elevation in the study area, the depth of the moho surface varies from 32.51 km to 38.01 km. this is also the location with the deepest moho surface depth of 38.01 km in the study area. comparing the moho surface depth of this study with previous studies such as hai (2003) and phong (2016), the results show similarities in moho surface depth with the studies by hai (2003) and phong (2016). the deep depth values are all located in the north of ha giang province, with the depth gradually decreasing towards the south of the study area, including bac kan and lang son provinces. in this study, the deepest depth of the moho surface is 38.01 km compared to the deepest depth of studies by hai (2003) and phong (2016), which is 37.2 km, showing a difference of about 0.8 km. the shallowest moho surface depth in this study is 28.77 km, while the shallowest 405study of the structural characteristics of the earth's crust in cao bang province and adjacent areas using gravity data moho surface depth in the studies by hai (2003) and phong (2016) is 30 km, showing a difference of about 0.12 km. in addition to these differences, the results of the study provide more detailed information about the depth of the moho surface in the study area, which can be clearly seen as interwoven convex and concave structural blocks (figure 15). this difference in results may be due to previous studies covering a larger area, along with limitations in data and analysis methods, resulting in a significantly reduced detailed reflection of study results for a smaller area. figure 15. moho surface depth diagram in cao bang province and adjacent areas. (1) earthquake seismic recording station; (2) moho surface depth; (3) boundary; (4) river on the other hand, in determining the depth to the basic boundary surfaces, data obtained from earthquake seismic recording station networks reflect this very well at the station locations. however, because the number of earthquake seismic station networks is quite sparse, the results showing the depth to the basic boundaries for the entire area only reflect the average. in the study area of cao bang province and adjacent areas, there are two points of moho surface depth results through the use of the receiver function analysis method of seismic data from the study by duong et al. (2013). these two moho surface depth points are shown in figure 15, with symbols cbvb (cao bang) indicating a depth of 32.5 km and hgvb (ha giang) indicating a depth of 32.6 km. comparing these results with the moho surface depth results of this study, at the cbvb point, the depth is 32.563 km, resulting in a difference in depth compared to the results of the study by duong et al. (2013) of about 0.063 km. similarly, at the hgvb point, the depth in this study is 32.513 km, resulting in a depth difference of 0.087 km. with the very small depth differences that have been verified between this study and the earthquake seismic recording station network data, it has shown high reliability in the moho surface depth calculation results of this study. 4.3. identify the geological fault system geological faults have become the main focus of study for geologists and geophysicists studying the deep structure of the earth’s crust. additional information is added to geological and geophysical documents, combining various analysis and processing methods, based on the analysis of gravity data along with the location data of earthquakes that have occurred in the study area. some criteria for identifying faults include: • local anomalous bands (negative or positive) forming elongated linear chains. • sudden changes in gravity anomaly contour lines. • locations with maximum horizontal gradient values of the gravity field extending in a straight line or crescent line with a large radius. • large, deep faults often appearing at locations where maximum horizontal gradient values exist according to upward continuation levels. • in addition, with the tilt angle method, the fault location will pass through the position where the value is 0. • with the data on earthquake epicenters that have occurred in the study area, the locations of these events or their surroundings may indicate the presence of faults or tectonic plate boundaries. figure 16 shows a diagram of the fault system in cao bang province and adjacent areas, determined based on geological data, earthquake data, results of gravity data processing, and analysis methods such as the maximum horizontal gradient method with upward continuation levels of 10 km and 20 km, and the tilt angle method. from the results obtained through processing and analyzing gravity anomaly data combined with the location data of earthquakes in the study area, 17 large and small faults have been identified and labeled from f1 to f17 (figure 16). among these, there are four deep and large faults: f1, f2, f3, and f4 (f4.1, f4.2). clearly identifiable signs of these faults are shown through the maximum horizontal gradient values at upward continuation of 10 km and 20 km, as well as the contour lines with a tilt angle value of zero. additionally, earthquakes have occurred in that location. the remaining faults, labeled from f5 to f17, are identified as small faults. most earthquakes in this study area are concentrated along major faults, while smaller and isolated earthquakes occur along minor and shallow faults. it is important to note that the epicenter locations of recorded earthquakes represent surface positions, not the hypocenters at the depths where the events occurred. this is only one of the indicators used to identify faults or tectonic boundaries. therefore, some epicenters may be offset from the determined fault locations. according to the study results in cao bang province and adjacent areas (figure 16), the sub-meridian arc fault zones in the north are f1 (pho day river), f2 (yen minh – phu luong), and f3, which tend to lean towards the west. f4.1 and f4.2 (cao bang – tien yen) and f9 tend to tilt clearly to the southwest. faults f7, f10, f12, f14, f15, and f17 all develop in the northwest – southeast direction. f16 is an arc-shaped, sub-parallel fault that develops from northwest to southeast. additionally, the remaining faults, such as f5, f6, f8, f11, and f13, develop in the northeast – southwest direction. figure 16. diagram of the fault system in cao bang province and adjacent areas. (1) m < 3.5; (2) m = 3.5–3.9; (3) m = 4.0–4.4; (4) m = 4.5–4.9; (5) m = 5.0–5.4; (6) provincial boundary; (7) river; (8) gmax value at 0 km; (9) gmax value at upward level 10 km; (10) gmax value at upward level 20 km; (11) value of tilt angle method; (12) fault comparing with fault documents of previous studies such as luong et al. (1988), toan et al. (2010), and phong (2016), the results of determining the fault system in this study are highly similar to those of the aforementioned studies. the clearest similarity observed is that the large, deep faults f1, f2, and f4 (f4.1, f4.2) correspond to the pho day river fault, yen minh – phu luong fault, and cao bang – tien yen fault. additionally, fault f3 also shows similarities with the results of fault identification in toan et al. (2010). small faults such as f5, f6, f7, f9, f10, f13, f15, f16, and f17 were also identified in the studies by luong et al. (1988) and phong (2016). however, some small 406 thanh duong van, hung pham nam, trieu cao dinh, trong cao dinh, tuan thai anh, bach mai xuan, luc nguyen manh faults identified in this study, such as f8, f11, f12, and f14, did not appear in the above research. the manifestation of these faults through the analysis results of the methods is quite clear. their absence in the above studies may be due to the fact that the data used for analytical processing of the above works are not detailed, and the analytical methods used are limited. furthermore, research covering a large area may lead to small faults being overlooked and undetected. 5. conclusions this study has rigorously and logically applied the process of analyzing gravity data from 2d to 3d, using the combined results of methods such as constrained gravity inversion and euler deconvolution, to build an initial 2d model with high suitability. the calculated results from the 2d model were then used as input for building the initial 3d model. the initial 3d model obtained has parameters, such as density and depth, that are closer to the actual values. the final results obtained after solving the 3d model problem demonstrate high reliability, with small error values between the calculated and actual measured gravity anomalies, ranging from -0.5 to 0.65 mgal. at the same time, it shows the advantages of time, fast processing speed, and high accuracy of the 3d inverse method compared to the 2d model method. through the combined use of a number of processing and analysis methods such as upward continuation, maximum horizontal gradient, tilt angle, and location data of earthquakes that have occurred, it has helped to identify the fault system in cao bang province and adjacent areas. shows high efficiency in using a combination of the above methods and data to identify the fault system. the results showed that the fault system in the study area is quite complex, the main direction of development is northwest – southeast. a total of 17 faults, both large and small, have been identified, large faults are easily recognizable such as f1 (pho day river), f2 (yen minh – phu luong), f3 (arc, sub-meridian faults) and f4 (cao bang – tien yen). in particular, this study identified additional faults f8, f11, f12, and f14, which were not detected in previous research. in addition, there are a number of small arc and sub-parallel faults, along with a number of other small faults developing in the northeast – southwest direction. the depth of the crystalline basement surface in cao bang province and adjacent areas varies from 0 km to 4.32 km. the location of the basement outcrop is in bac kan province, the location has deepest depth values in the cao bang, that khe basins and the location around basins. the depth of the moho surface in cao bang province and adjacent areas varies from 28.77 km to 38.01 km. locations with shallow moho surface depth values range from 28.77 km to 30 km, appearing in the northern region of cao bang province, the southern region of bac kan province, and the southeastern region of lang son province. in the cao bang and that khe basins, the depth of the moho surface is relatively shallow, ranging from 32.51 km to 33.01 km. the deepest moho surface depth is found in ha giang province, at 38.01 km. the uplift and subsidence of the crystalline basement and moho surfaces reflect significant geological and tectonic features of the study area. locations where alternating convex and concave structures are observed indicate variations in surface depth, resulting in higher stress accumulation compared to other areas. when combined with fault zones intersecting these locations, energy release occurs, leading to earthquakes. from the fault map and the depth profiles of the crystalline basement and moho surfaces derived from the study, areas with a high potential for seismic activity have been identified, such as the deep fault zones f1, f2, f3, and f4. these faults show significant changes in surface depth where they pass through. similarly, the regions traversed by faults f8, f9, f10, f11, f15, f16, and f17, which have already experienced some earthquakes, also exhibit notable variations in the depth of the crystalline basement and moho. all this information provides deeper insights into the earth’s structure and its dynamic processes, thereby supporting the assessment of geological risks, resource planning, and the study of regional geodynamics. the results of this study were specifically evaluated and compared with documents and results from previous studies, demonstrating the high effectiveness of our approach. applying this analysis process to other study areas shows promise and is feasible. references aitken, a. r. a., salmon, m. l., & kennett, b. l. n. 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(2025). oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia. earth sciences research journal, 2(3), 231-247. https://doi. org/10.15446/esrj.v29n3.117157 abstract a new mosasaur specimen found in the coniacian beds of lebrija, santander, central colombia, is here described. the new specimen represents a new genus and species of plioplatecarpine mosasaurid that we name oneirosaurus caballeroi gen. et sp. nov. oneirosaurus is a plioplatecarpine characterized by one character unknown in any other plioplatecarpine: the presence of two external foramina for the exit of nerves x, xi, and xii. it is also distinguished by a combination of characters that includes among others: prefrontal and postorbitofrontal separated at the roof of the orbit; broad frontal, with short triangular surface anterior to the orbits; triangular and broad anterodorsal surface of the parietal contacting the postorbital for a short distance; infrastapedial process of the quadrate robust without being fused to the suprastapedial process; small otosphenoidal crest of prootic not covering the exit of nerve vii; basioccipital canal opening into the floor of the braincase through two large foramina separated by a medial septum; lingual alveolar parapet of dentary lower than labial; marginal teeth crown with subcircular cross-section at base and with two carinae arranged at 180º; and presence of a notch in anterior border of the atlas neural arch. the morphological comparisons and the cladistic analysis show the new taxon as an intermedium plioplatecarpine with phylogenetic proximity to the colombian species yaguarasaurus columbianus from the lower turonian and to the species ectenosaurus clidastoides from the santonian-campanian of the usa. our phylogenetic results shows tethysaurines and halisaurines as plesiopelvic and plesiopedal groups separated from the hydropelvic and hydropedal derived groups tylosaurinae, plioplatecarpinae, and mosasaurinae. keywords: mosasauridae; plioplatecarpinae; northern gondwana; coniacian. oneirosaurus caballeroi gen. et sp. nov., un nuevo mosasaurio de colombia resumen se describe un nuevo espécimen de mosasaurio encontrado en capas del coniaciano en lebrija, santander, centro de colombia. el nuevo espécimen representa un nuevo género y especie de mosasáurido pliopatecarpino que nosotros nombramos oneirosaurus caballeroi gen. et sp. nov. oneirosaurus es un plioplatecarpino que se distingue de cualquier otro plioplatecarpino por la presencia de dos forámenes externos para la salida de los nervios x, xi y xii. oneirosaurus también se distingue por una combinación de caracteres que incluye entre otros: prefrontal y postorbitofrontal separados en el techo de la órbita; frontal ancho, con superficie triangular corta anterior a las órbitas; superficie anterodorsal triangular y ancha del parietal contactando el postorbital por una distancia corta; proceso infraestapedial del cuadrado robusto y sin fusionarse con el proceso supraestapedial; proótico con una pequeña cresta otosfenoidal que no cubre la salida del nervio vii; canal basioccipital abriéndose en el piso del basicráneo a través de dos grandes forámenes separados por un septo medial; dentario con parapeto alveolar lingual más bajo que el labial; corona de los dientes marginales con sección transversal subcircular en la base y con dos carenas dispuestas a 180º; y presencia de una muesca en el borde anterior del arco neural del atlas. las comparaciones morfológicas y el análisis cladístico muestran al nuevo taxón como un plioplatecarpino intermedio con proximidad filogenética a la especie colombiana yaguarasaurus columbianus del turoniano inferior y a la especie ectenosaurus clidastoides del santoniano-campaniano de eua. nuestros resultados filogenéticos muestran a los tethysaurinos y halisaurinos como grupos plesiopélvicos y plesiopedales separados de los grupos derivados hydropélvicos e hydropedales tylosaurinae, plioplatecarpinae, y mosasaurinae. palabras clave: mosasauridae; plioplatecarpinae; gondwana del norte; coniaciano. https://doi.org/10.15446/esrj.v29n3.117157 https://doi.org/10.15446/esrj.v29n3.117157 mailto:meparamof@unal.edu.co mailto:janarvaezr@unal.edu.co mailto:cdbenavidesc@unal.edu.co https://doi.org/10.15446/esrj.v29n3.117157 https://doi.org/10.15446/esrj.v29n3.117157 232 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia i. introduction mosasaurs were late cretaceous cosmopolitan marine reptiles. in south america, mosasaur specimens have been found in colombia, venezuela, peru, brazil, chile, and argentina (bengston and lindgren, 2005; caldwell and bell, 1995; fernández and gasparini, 2012; fernández and talevi, 2015; jiménez-huidobro et al., 2015; jimenez-huidobro et al., 2017; otero, 2016; páramo-fonseca, 2012; páramo-fonseca, 2015; price, 1957 in bardet et al., 2005; pierce and welles, 1959; sanchez-villagra et al., 2008; soto-acuña et al., 2015). south american mosasaurs have shown derived forms (fernández and gasparini, 2012; otero, 2016) but also basal morphologies from the early late cretaceous that remain poorly known globally (páramo, 1994; páramofonseca, 2000, 2012, 2013). the reports on colombian mosasaurs include four specimens found in turonian beds of the upper magdalena valley, southern colombia (páramo 1994; páramo-fonseca, 1997; 2000; 2012), one specimen found in coniacian beds of lebrija, santander, northern colombia (páramo-fonseca, 2012), one specimen found in campanian beds of coello, tolima, central colombia (páramo-fonseca, 2012; 2013), one specimen found in upper campanian – lower maastrichtian beds of cuítiva, boyacá, central colombia (lópez-rueda et al., 2024), and four specimens found in maastrichtian beds of cuítiva, boyacá, central colombia (lópez-rueda et al., 2024). the turonian specimens, recognized as yaguarasaurus columbianus páramo 1994, correspond to two nearly complete skulls with cervical vertebrae and two specimens represented by isolated cranial remains (páramo, 1994; páramo-fonseca, 2000; 2012). the campanian specimen, a complete skeleton, was identified as eonatator coellensis páramo-fonseca, 2013 (páramo-fonseca, 2012; 2013). the coniacian specimen is the subject of this contribution; it corresponds to an incomplete skull (igmp879524) previously reported without description, under catalog number ipn-2, by páramo-fonseca (2012). the campanian and maastrichtian specimens found in cuítiva, are isolated fragments and were identified as globidens sp., mosasauridae indet. and pliopatecarpinae indet. (see lópez-rueda et al., 2024). another two colombian mosasaur specimens were reported in unpublished academic works. one of them, a poorly preserved incomplete skull with vertebral remains, was collected from coniacian beds from cucaita, boyacá, central colombia (correa-agudelo, 2016) and the other, four fragmentary vertebrae, was found in turonian-coniacian beds from ortega, tolima, south-central colombia (narváez-rincón, 2018). the material here described was extracted from coniacian beds in the municipality of lebrija, santander, and comprises an almost complete skull and atlas. this material represents the first record of a mosasaur from northern colombia. the specimen shows an enigmatic mineralization that enabled the conservation of anatomical features rarely observed in mosasaurs, as is the interorbital septum, which in this specimen is exceptionally preserved (see páramo-fonseca et al., 2024). in this contribution, we provide a detailed morpho-anatomical description of the specimen, we identified it as a new genus and species of plioplatecarpinae (sensu polcyn et al., 2023), and included it to a phylogenetic context of mosasauroids. 2. materials and methods the specimen (igmp879524), previously reported under catalog number ipn-2 (páramo-fonseca, 2012), was collected by the late mr. jaime caballero in a quarry located near the town of lebrija, at the site called la azufrada, on the road leading from bucaramanga to barrancabermeja (fig. 1a). in 1997, mr. caballero reported the find to the servicio geológico colombiano (sgc) (named ingeominas at that time). a few years later, after the unfortunate death of mr. caballero, his family voluntarily handed over the specimen to the sgc for study. in subsequent visits of the sgc to the finding site, we (m.e.p.-f.) collected vertebral fragments and ribs in the same quarry where the skull was found. given that the precise stratigraphic origin of the skull remains is unknown, it is not possible to affirm if these postcranial remains pertained to the same individual. therefore, in this work we will describe only the material donated by caballero’s family, which consists of a nearly complete skull (lacking the anterior end of the snout) and the atlantal elements. the skull is preserved three-dimensionally articulated with some bones slightly displaced from their original position. the skull was prepared in the macrofossil laboratory of the sgc, using chemical methods. once the fossil was covered with polyvinyl acrylates, it underwent successive baths in a 3% water diluted formic acid solution, followed by rinsing and drying. the preparation process revealed the presence of a gray material, indissoluble in the acid, which was found coating a large part of the bone surfaces. in many cases, this material was found firmly adhered on thin and delicate structures. in these cases, we decided to preserve the gray coverture without subjecting the specimen to mechanical preparation to avoid risking the integrity of the fossilized parts. the taxonomic identification was based on a detailed morphological comparison with previously known colombian mosasaurs (páramo 1994; páramo-fonseca, 1997; 2000; 2013) and the recognition of mosasaurs morphological traits analyzed on russell (1967), bell and polcyn (2005), polcyn and bell (2005), konishi and caldwell (2011) makádi et al. (2012), palci et al. (2013), polcyn et al. (2023) among others. a phylogenetic analysis was performed using the matrix of simões et al. (2017) (taken from madzia and cau, 2017) which includes the changes proposed by different authors to the initial matrix of bell (1997) (jiménezhuidobro and caldwell, 2016; leblanc et al., 2012; palci et al., 2013). the outgroup used in the analysis was the dolichosaurid lizard adriosaurus suessi seeley, 1881, as proposed by simões et al. (2017). a revision of the scoring of yaguarasaurus columbianus was done, and two colombian taxa were added to the matrix: eonatator coellensis and the new taxon described in this work. the recently described taxon sarabosaurus dahli polcyn et al., 2023 was also added to the matrix for a second analysis, using the scoring of polcyn et al. (2023) and scoring the remaining characters from their published description (see supplemental data 1). we ran two cladistic analyses. the first one was run with 125 characters and 46 (otus), whereas the second analysis was run with 47 (otus) including sarabosaurus dahli. both analyses were run using tnt (v1.6) (goloboff and morales, 2023). all characters were treated as unordered and were equally weighted. an initial exploration for the shortest-length tree islands was performed using the new technology search options (ratchet + drift + tree fusing) with 200 ratchet iterations (1 random seed) and 99,999 trees limit. the resulting trees were then used as the starting point for a tree bisection and reconnection (tbr) branch swapping algorithm. a strict consensus was obtained and the iterpcr algorithm applied to identify unstable otus (pol and escapa, 2009). the retention (ri) and consistency (ci) indexes (farris, 1989) were calculated, and bremer support values (bremer 1994) were computed for the nodes in the consensus tree using the tnt tools. finally, the resulting strict reduced consensus tree was time calibrated using the timepaleophy function of the package paleotree 3.4.5 (bapst, 2012) in r 4.2.3 (r core team, 2022), implementing mlb calibration with a minimum branch length of 1 ma, and plotted using the geoscalephylo function from the r package strap 1.6-0 (bell and lloyd, 2015). 3. geologic and biostratigraphic context igmp879524 was found in layers of the galembo formation (fig. 1b). this stratigraphic unit was recently elevated to the rank of formation by terrazamelo (2019), but it was previously considered the upper member (galembo member) of the la luna formation (morales et al., 1958). according to terraza-melo (2019) the galembo formation is a coniacian-santonian marine sedimentary succession constituted by calcareous mudstones and wackestones rich in planktonic foraminifera and organic matter. it has been interpreted as deposited in the middle to outer shelf (terraza-melo, 2019). although the exact stratigraphic origin of the described mosasaur remains is unknown, we (m.e.p.-f.) collected some ammonoids from the same quarry where the mosasaur remains were found. four of these ammonoids were selected to assess the age of the mosasaur. they were determined by fernando etayo-serna (pers. comm.) as follows: the specimen az-7 (fig. 1c) was identified as gauthiericeras aff. listeri (van hoepen, 1965), and according to wright et al. (1996) this genus indicates the upper coniacian; the specimens az5, az-11 and az-12 (figs. 1d-f respectively) were identified as barroisiceras aff. subtuberculatum (gerhardt, 1897), and according to wright et al. (1996) this genus indicates the upper turonian-lower coniacian. during preparation, we recovered a couple of small ammonoid specimens from the surrounding sediment. according to fernando etayo-serna (pers. comm.) the ammonoids specimens are juvenile individuals and cannot be taxonomically placed with certainty, nonetheless, their features concur with those of coniacian ammonoids. 233maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra the postcranial fragmentary remains of mosasaur found in the same quarry were collected in a sector of the outcrop close to where the ammonoids az7, az-11 and az-12 were collected. without knowing the precise stratigraphic origin of the mosasaur specimen here described, the available information only allows us to suggest a coniacian age for the mosasaur. nomenclatural acts this publication, and the nomenclatural act contained herein, has been registered in zoobank, the iczn online registration system for new taxa. the zoobank life science identifier (lsid) for this publication is urn:lsid:zoobank. org:pub:1131b0e0-7603-4881-af76-5a8e6f130282 the lsids for the new taxon are given under the systematic paleontology section (below). 4. systematic paleontology reptilia linnaeus, 1758 squamata oppel, 1811 mosasauridae gervais, 1853 plioplatecarpinae dollo, 1884 (sensu polcyn et al., 2023) oneirosaurus gen. nov. urn:lsid:zoobank.org:act:ffec38c0-51f7-499e-a1de0cfc2962f7a0 type and only species: oneirosaurus caballeroi, sp. nov. figure 1. geographic and stratigraphic origin of the studied specimen (igmp879524). a, map of lebrija, santander region with geographical location of the finding site (yellow star). b, generalized stratigraphic column of the galembo formation cropping out at the lebrija region (modified from terraza-melo, 2019), showing the probable occurrence of the specimen (yellow star). c-f, ammonites collected in the quarry where the specimen was found; c, specimen az-7, gauthiericeras aff. listeri (von hoepen, 1965). d-f, barroisiceras aff. subtuberculatum (gerhardt, 1897) (d: az-5, e: az-11, f: az-12). scale bars c-d = 1 cm; e-f = 5 cm. http://zoobank.org http://zoobank.org http://zoobank.org 234 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia type locality: la azufrada locality, barancabermeja-bucaramanga highway, lebrija municipality, santander, colombia; coordinates: 7°8’56’’n, 73°18’24’’w; plate 109-ii-d of the instituto geográfico agustín codazzi. stratigraphic horizon: galembo formation. coniacian, upper cretaceous (figure 1b) derivation of name: from greek oneiro (όνειρο), dream and saurus (σαύρος), lizard. “dream reptile” referring to the exceptional preservation of its delicate structures. diagnosis: as for type and only known species. oneirosaurus caballeroi sp. nov. (figs. 2–6) urn:lsid:zoobank.org:act:eeef4fa3-0016-4fbe-97dc6a700918a7fb holotype: igmp879524 (old catalog number: ipn-2), nearly complete skull with articulated mandible, lacking its anterior end, and disarticulated atlas. paleontological collections of the museo geológico nacional josé royo y gómez of the servicio geológico colombiano (sgc) (formerly ingeominas), colombia. derivation of name: caballeroi, is proposed in honor of the late mr. jaime caballero, who collected and reported the specimen. after his death his family handed the specimen to the museo geológico nacional josé royo y gómez of the scg. diagnosis: oneirosaurus caballeroi (igmp879524) is a plioplatecarpine having one character unknown in any other plioplatecarpine: the presence of two external foramina for the exit of nerves x, xi and xii (fig. 3c). oneirosaurus caballeroi is also defined by the following unique combination of characters: prefrontal and postorbitofrontal separated at the roof of the orbit (differing from platecarpus cope, 1869, plioplatecarpus dollo, 1882, and latoplatecarpus konishi and caldwell, 2011); broad frontal (almost 70% of its length), with short triangular surface anterior to the orbits (differing from yaguarasaurus, russellosaurus polcyn and bell, 2005, tethysaurus bardet et al., 2003, ectenosaurus russell, 1967 and sarabosaurus polcyn et al., 2023); triangular and broad anterodorsal surface of the parietal (as in yaguarasaurus, russellosaurus, tethysaurus, angolasaurus telles-antunes, 1964, and selmasaurus wright and shannon, 1988), contacting the postorbital for a short distance (differing from yaguarasaurus); suspensory rami of parietal anteriorly broad (as in yaguarasaurus) originating below the narrowest part of the longitudinal dorsal table and posteriorly short (differing from yaguarasaurus); parietal foramen small and located entirely in the parietal (as in yaguarasaurus, russellosaurus, tethysaurus, angolasaurus, and selmasaurus); infrastapedial process of the quadrate robust without being fused to the suprastapedial process (as in platecarpus and selmasaurus); distal condyle of the quadrate saddle-shaped (as in yaguarasaurus, russellosaurus, tethysaurus, and pannoniasaurus makádi et al., 2012); small otosphenoidal crest of prootic not covering the exit of nerve vii (differing from yaguarasaurus, russellosaurus, tethysaurus, platecarpus, and latoplatecarpus); basioccipital canal opening into the floor of the braincase through two large foramina separated by a medial septum (as in plesioplatecarpus konishi and caldwell, 2011 and angolasaurus); pterygoid anterior teeth not distinctly smaller (differing from sarabosaurus); lingual alveolar parapet of dentary lower than labial (as in yaguarasaurus, russellosaurus, and romeosaurus palci et al., 2013); marginal teeth crown with subcircular cross-section at base (differing from tethysaurus) and with two carinae arranged at 180º, one mesial and the other distal (differing from yaguarasaurus and angolasaurus); presence of a notch in anterior border of the atlas neural arch (also in ectenosaurus and sarabosaurus). 5. preservation features the skull is incomplete; its anterior and posterior ends, as well as most of the external surface of the anterior dorsal bones, have been lost due to fracture during extraction. the skull is preserved articulated in three-dimensions, with some bones slightly displaced from their original position. the quadrates are rotated towards the skull roof. the atlas was found with its elements disarticulated, with the intercentrum rotated below the paraoccipital process of the left opisthotic, the centrum (odontoid) in the left temporal fenestra near the lateral wall of the parietal and separated from it by the spinous process of the left atlantal neural arch, which is directed forward, and the right atlantal neural arch rotated with the spinous process directed backwards and missing its basal portion. the general arrangement of the bones, with a slight rotation of the mandibles and the displacement of the quadrates and atlas components towards the cranial roof, suggests that the skull was deposited face up. the displacements concur with pre-fossilization accommodation by gravity from the face-up position of the skull. this condition suggests a low-energy depositional environment in which the mosasaur carcass remained undisturbed during the processes of decomposition, burial, and mineralization. the fossilized bones are dark brown in color, and many are covered by a thin layer of a gray mineralization. this mineralization is composed of microcrystalline quartz and calcite (páramo-fonseca et al., 2024). the mineralization thinly covers cartilaginous surfaces facilitating the preservation of structures that are not usually conserved in mosasaurs, such as the chondrocranial elements (see páramo-fonseca et al., 2024). according to páramo-fonseca et al. (2024) this mineralization was produced by a partial silicification process promoted by lower ph microenvironments associated with bacterial breakdown of non-biomineralized tissues during early diagenesis. 6. anatomical description premaxilla: the premaxilla is almost entirely lost, only a small fragment of the most posterior end of the internarial bar has been preserved in contact with the frontal (figs. 2a, 3a, 4a). it does not reach the posterior border of the external nares. the suture with the frontal shows a sharp posterior medial extension. below the preserved fragment of the premaxilla, on the left side, a horizontal nasal lamina extends posteriorly bordering lateroventrally the anterior half of the frontal internarial process. maxilla and septomaxilla: both maxillae lack the anterior end and have lost their external surface. the right one also lacks its posterior end. the absence of the external surface prevents an accurate description of the sutures of the maxillae with the prefrontals, lacrimals and jugals, as well as the lateral border of the external nares. nevertheless, it can be established that the posterior end of the left maxilla is covered laterally by the anterior ramus of the jugal (fig 4a). due to a complete lack of the external surface on the right maxilla the medial contact with the palatine is visible (fig 3c). this contact begins anteriorly with a short longitudinal suture that then curves posterolaterally reaching the inner dorsal border of the dentate ramus of the maxilla. the position of the palatine in contact with the maxilla at the level of the last four alveoli in the specimen under study is equivalent in y. columbianus. the posterior border of the external nares is located at the level of the fifth posterior alveolus (counted from back to front), as in y. columbianus. the absence of the ascending lamina in the left maxilla exposes the canal for the maxillary artery, which extends longitudinally above the alveoli (see figure 4a). in lateral view, the alveolar border of the maxilla is straight (figs. 3a, 4a). in the left maxilla there are eight alveoli or teeth, while in the right maxilla only seven are present. the preserved alveoli are of uniform size, except for the most posterior ones which are smaller. the lingual alveolar ridge is slightly shorter than the labial. the replacement teeth occupy a posteromedial position regarding the functional teeth. although most of the septomaxillae are missing with the anterior end of the snout, a fragment of septomaxilla is found over the maxillae on each side of the external nares (figs. 2a, 3a, 4a). these fragments are placed between the seventh and eighth posterior teeth (counted from back to front). the posterior edge of the septomaxillae exhibits a similar position in yaguarasaurus columbianus (see páramo, 1994). nasal: posterior to the septomaxilla, below the internarial bar, a small broken fragment of bone is found on the medial wall of each naris. given its location, these fragments are here recognized as portions of nasals (figs. 3a, 4a). frontal: the frontal is incomplete; it lacks its most posterior part and almost all the right dorsal surface. the general shape of the frontal is anteriorly triangular and posteriorly rectangular (fig. 2a). a low medial ridge is evident on the anterior region of the dorsal surface of the frontal. this ridge is reduced in the posterior region, where the surface of the bone becomes slightly concave. the contact between the frontal and the premaxilla is visible dorsally at the most anterior end of the preserved internarial bar and is v-shaped. the frontal forms the only conserved portion of the internarial bar, and its edges are not emarginated in the posterior edge of the nares. at the posterior end of the nares, http://zoobank.org 235maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra the frontal meets the prefrontal (fig. 2a). the suture between these two bones extends diagonally backward to the anterodorsal border of the orbit. from this point, the frontal shows straight lateral margins, parallel to the longitudinal axis of the skull and form the roof of the orbits separating the prefrontal from the postorbitofrontal. on the left side of the skull, the posterolateral end of the frontal extends into a semicircular process enclosed by the postorbitofrontal (fig. 2a). although the almost dorsal surface of the posterior end of the frontal has been lost, the suture with the parietal can be inferred from the textural difference between the two bones. this suture describes a slightly undulating straight line with a medial posterior concavity. prefrontal: the prefrontals have lost almost all their external surface, only a small portion has been preserved in the right prefrontal. the descending rami of the two prefrontals are well-preserved and can be clearly seen in contact with the palatines, constituting the anterior wall of the orbits. the prefrontal forms a small portion of the posterolateral border of the external naris (fig. 2a). posteriorly, the prefrontal extends, overlapped by the frontal, to the anterodorsal border of the orbit; it does not contact the postorbitofrontal below the orbit (fig. 4a). lacrimal: both lacrimals are poorly preserved. the lacrimal is a small rhomboid-shaped bone. it contacts dorsally the prefrontal and ventrally the maxilla (figs. 2a, 3a, 4a). jugal: the left jugal is completely preserved (fig. 4a), and only the postorbital bar of the right jugal is present (fig. 3a). the jugal is l-shaped, with a thick vertical bar and a longer, thinner horizontal ramus that is gently curved and compressed dorsoventrally (fig. 4a). the joining between the two rami is a gentle curve that shows a slightly developed posterior protuberance at the starting point of the vertical ramus. figure 2. oneirosaurus caballeroi gen. et sp. nov., holotype igmp879524. photograph, drawing and interpretative sketch of the specimen in a, dorsal without left mandibular ramus and b, ventral views. gray fill, sediment; pattern fill, broken bone surface; black, empty space. scale bars = 50 mm. abbreviations: an, angular; at, atlas; bo, basioccipital; bs, basisphenoid; c, coronoid; d, dentary; ec, ectopterygoid; f, frontal; h, hyod; j, jugal; la, lacrimal; lpl, left palatine; lpt, left pterygoid; lv, left vomer; m, mandible; mx, maxilla; op, opisthotic-exoccipital; p, parietal; par, prearticular ; pf, parietal foramen; pl, palatine; pmx, premaxilla; pof, postorbitofrontal; pop, paraoccipital process of the opisthotic; pr, prootic; prf, prefrontal; ps, parasphenoid; ptt, pterygoid tooth; q, quadrate; rpl, right palatine; rpt, right pterygoid; rv, right vomer; sa, surangular; scp, sclerotic plate; smx, septomaxilla; so, supraoccipital; sp, splenial; sq, squamosal; stp, stapes; t, tooth. 236 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia parietal: the parietal is partially preserved; its anterior border and most of the suspensory rami have been lost (fig. 2a). the dorsal surface of the parietal has a large triangular anterior table that narrows posteriorly between the temporal fenestrae forming a medial subrectangular surface, which tightens posteriorly. the parietal foramen is small and lies entirely on the anterior triangular surface of the parietal. the contact with the frontal is almost straight, with symmetrical undulations. anterolaterally, the parietal joins the postorbitofrontal in a semicircular suture (fig. 2a). this joint is not completely closed suggesting a non-ossified union. in the preserved posterior part of the parietal, the suspensory rami originate below the medial rectangular table, where this table is narrower. the suspensory rami project posteriorly becoming dorsoventrally compressed. on the lateral surface of the parietal, below the beginning of the suspensory rami several foramina are observed. postorbitofrontal: the left postorbitofrontal is preserved in its entirety, while only part of the anterior region of the right postorbitofrontal is present. dorsally, the postorbitofrontal encloses the posterolateral extension of the frontal in an anteriorly concave semicircle and contacts the parietal by a suture describing an anteromedially convex semicircle (fig. 2a). behind the junction with the frontal and the parietal, the postorbitofrontal shows a faint line with parallel striae, that seems to correspond to vestiges of the suture between the postfrontal and the postorbital, suggesting a not completely ossified union (a feature of probable juvenile stage). in lateral view, the postorbitofrontal has an elongated triangular shape, with an anterior rounded protuberance that projects downward in an acuminate extension, overlapping the posterior part of the ascending ramus of the jugal (fig. 4a). the posterior process of the postorbitofrontal borders the temporal fenestra and inserts into the squamosal (figs. 2a, 4a). supratemporal: only the anterior ventral process of the supratemporal is preserved on both sides of the skull. it is covered medially and dorsally by the paraoccipital process of the opisthotic and laterally by the prootic (figs. 5a, d). squamosal: only the anterior part of the left squamosal is preserved. it shows the regular shape of the squamosal in mosasaurids. it embraces the postorbitofrontal dorsolaterally and ventrally along a large section of the temporal arcade (fig. 4a). quadrate: both quadrates are rotated towards the temporal fenestrae, with the condyles oriented towards the midline of the skull (fig 2a, 5a). although the surfaces of the quadrates are covered by a thin layer of indissoluble material, the morphology of these bones is perfectly distinguishable (fig. 4b). in anterior view, the quadrate is rectangular, convex in its latero-dorsal region and concave in its mid-ventral region; the condyle is distinguished from the body by a sinusoidal line. in medial view, the shaft of the quadrate shows a central vertical ridge, which is more compressed in its lower part; the suprastapedial process almost completely closes the meatus (stapedial notch), giving it an inverted drop shape, without being fused to the infrastapedial process (sensu palci et al., 2013, p. 603) nor to the ascending posteroventral border of the tympanic ala, but extending medially below them; between the vertical ridge and the antero-dorsal edge of the meatus there is an elongated fossa (stapedial pit) that becomes deep ventrally (fig. 4b). in posterior view, the suprastapedial process descends sub-parallel to the quadrate shaft; it has parallel edges and diagonal ventral end, forming a tip that projects medially to below the infrastapedial process; an indentation for the depressor mandibularis muscle is found on the medial diagonal surface of the suprastapedial process; the infrastapedial process is robust; the condyle is little exposed with a ventral border slightly concave (fig. 4b). in lateral view, the quadrate is seen as a circular cavity formed by the suprastapedial process and the tympanic ala, which has a well-developed posteroventral ascending edge, almost contacting the suprastapedial process; the shaft of the quadrate projects ventrally for a short distance, forming the condyle, which is well exposed and ventrally convex (fig. 4b). the quadrate condyle develops perpendicular to the tympanic expansion and to the axial plane of the skull; it is lateromedially elongated and has two slightly pronounced heads, the external larger than the internal. none of the tympanic operculum is preserved. in neither of the quadrates the alar crest is completely exposed. chondrocranial elements: in the internal midline of the skull, the specimen preserves some chondrocranial elements, from which the complete interorbital septum is recognized (fig. 3a). such a complete structure has never been recorded in any mosasaur. a detailed description of the chondrocranial elements was recently published by páramo-fonseca et al. (2024) sclerotic plates: in the anterodorsal region of both orbits, accumulated fragments of the ossified sclerotic plates are found. ten fragments can be identified in the right orbit and seven in the left orbit (figs. 3a, 4a). none of the plates is completely exhibited, so the shape of their outline cannot be established. the largest fragments are 15 x 15 mm. these plates are very thin and show a very fine radial striation. palate palatine-vomer: the palatine-vomers lack their most anterior portion. the palatine forms the posterior medial and the posterior border of the internal nares (fig. 2b). the anterior end of the palatine is covered dorsally and ventrally by the vomer, but is not fused to it, suggesting a juvenile stage of the individual (fig. 4c). the vomerine processes of the palatines, together with the preserved vomers form a longitudinal sub-vertical lamina dorsally and ventrally concave, differing from the more horizontal position of these bones illustrated by camp (1942, fig 14) for plotosaurus camp, 1951. this arrangement concurs with that found in yaguarasaurus columbianus (see páramo 1994, fig 12). posteriorly, the palatine widens forming a surface that diagonally separates the pterygoid from the maxilla. the posterior portion of the palatine has a dorsal thickening that contacts the descending ramus of the prefrontal, forming the anterior wall of the orbits. pterygoid: both pterygoids are almost completely preserved; only the most posterior end of the right one is missing. anteriorly, they contact each other in the midline forming an acuminate extension that covers ventrally the palatines (fig. 2b, 4c). at approximately the middle of their length, the pterygoids separate widely and project, laterally at a right angle, to the ectopterygoid process. this process receives dorsally the ectopterygoid at its anterolateral end. the ventral surface of the central region of each pterygoid has a row of teeth arranged in a curved line that extends from the antero-lateral junction with the palatine to the base of the basisphenoid process. posteriorly, the pterygoids approach medially again via the basisphenoid processes, and then diverge posteriorly projecting outward to the quadrates. the basisphenoid process projects posteriorly from the tooth-bearing surface, forming a long, slender process that extends beneath the basisphenoid and basioccipital to the level of the basal tubera of the latter. the elongated quadrate process is aliform. thirteen tooth positions are counted on the dentate surface. the pterygoid teeth are smaller than the mandibular and maxillary teeth. their shape is recurved, with the apex directed posterolaterally. the posterior teeth are very small, and the intermediate teeth are the largest. given that the teeth are covered by the gray mineralization, it is not possible to determine the texture of the teeth crowns. ectopterygoid: both ectopterygoids are well-preserved (fig. 2a, 3a). the ectopterygoid is relatively large and forms a bridge between the lateral process of the pterygoid and the jugal. the anterolateral process of the ectopterygoid extends longitudinally for 3 cm, bordering dorsally the coronoid (fig. 3a, 4a). the medial process expands above and anterior to the lateral process of the pterygoid. epipterygoid: both epipterygoids are preserved, but neither are in their anatomical position. the left one rests sub-horizontally on the ventrolateral surface of the frontal and the right one is diagonally embedded between the two pterygoids (fig. 3a). their complete morphology cannot be observed. the visible portion is a straight bar with a slightly expanded end towards the frontal (figs. 3a, 5c). 237maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra figure 3. oneirosaurus caballeroi gen. et sp. nov, holotype igmp879524. a, photograph, drawing and interpretative scheme of the specimen without left mandibular ramus, in right lateral view. b, photograph of the interorbital septum in oblique postero-dorsal view (scale in centimeters). c, basicranial region in right external view, showing the broken stapes. d, detail of the prootic and basisphenoid in right lateral view. gray fill, sediment; pattern fill, broken bone surface; black, empty space. scale bars: a = 50 mm; c-d = 30 mm. abbreviations: an, angular; avc, anterior aperture of the vidian canal; bo, basioccipital; bptm, basipterygoid meniscus; bs, basisphenoid; c, coronoid; d, dentary; ec, ectopterygoid; ep, epipterygoid; ex, exoccipital; f, frontal; im, internal auditory meatus; is, interorbital septum; j, jugal; la, lacrimal; m, mandible; mx, maxilla; na, nasal; op, opisthotic-exoccipital; p, parietal; par, prearticular; pl, palatine; pmx, premaxilla; pof, postorbitofrontal; pr, prootic; prf, prefrontal; ps, parasphenoid; pt, pterygoid; pvc, posterior aperture of the vidian canal q; quadrate; rv, right vomer; sa, surangular; scp, sclerotic plate; smx, septomaxilla; so, supraoccipital; sp, splenial; stp, stapes; vii, ix-xii, exit of cranial nerves. 238 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia figure 4. oneirosaurus caballeroi gen. et sp. nov., holotype igmp879524. a, photograph, drawing and interpretative scheme of the specimen in left lateral view. b, left quadrate; photograph and interpretative sketch in lateral view (1); sketches in posterior (2) and medial (3) views. c, interpreted cross-sections based on the observation of the palatal region at the level of the following alveoli counted from back to front: ninth or preserved anterior end (1), eighth (2), sixth (3) and fifth (4). gray fill, sediment; pattern fill, broken bone surface; black, empty space. scale bars: a = 50 mm; b-c = 20 mm. abbreviations: ai, atlas intercentrum; an, angular; ar, articular; c, coronoid; cma, canal for the maxillary artery; cd, condyle; d, dentary; ec, ectopterygoid; f, frontal; gf, glenoid fossa; im, internal auditory meatus; isp, infrastapedial process; j, jugal; la, lacrimal; lmx, left maxilla; lpt, left pterygoid; lv, left vomer; mx, maxilla; na, nasal; p, parietal; par, prearticular; pl, palatine; pmx, premaxilla; pof, postorbitofrontal; pop, paraoccipital process of the opisthotic; prf, prefrontal; pt, pterygoid; q, quadrate; rsp, right splenial; rv, right vomer; sa, surangular; scp, sclerotic plate; smx, septomaxilla; sp, splenial; sq, squamosal; ssp, suprastapedial process; tya, tympanic ala; tyc, tympanic crest; v, vomer. 239maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra unidentified elements probably associated to pterygoquadrate: on each side of the parasphenoid, just anterior to the basipterygoid process of the basisphenoid and over each pterygoid, there is a small, y-shaped element with distally expanded rami (fig. 5c). both are covered by gray mineralization and therefore their cartilaginous or bone origin cannot be established. the antero-medial ramus contacts the parasphenoid, rising and forming a ridge that surpasses vertically the level of the parasphenoid. the other two rami are horizontally flattened and follow the curved lateral border of the pterygoid (fig. 5c). no equivalent elements have been reported on mosasaurs. the fact that they are symmetric paired elements, not displaced by gravity to the skull roof, suggests that these elements are in anatomical position and are more probably related to paired structures than to the medial sphenoidal series. these elements are located where the epipterygoids should have rested, suggesting each could be part of a pterygoquadrate as illustrated for the lacertid acanthodactylus boskianus (daudin, 1802) by bellairs and kamal (1981, fig. 14), and represent probably the basipterygoid meniscus or the pterygoid process of the pterygoquadrate of lizards (following bellairs and kamal, 1981). unidentified element probably associated to hyobranchial apparatus: posterior to each ectopterygoid and medial to each jugal and coronoid, a straight bar-shaped ossicle, thinner than the epipterygoid is located vertically. these ossicles do not articulate with any other bone. their thin bar morphology and their spatial location suggest these ossicles could be part of the hyobranchial apparatus, probably the ceratobranchial, which generally ossify in lizards (bellairs and kamal, 1981). occipital region supraoccipital: the supraoccipital lacks its posterior dorsal end, but its ventral portion contributing to the roof of the foramen magnum is well preserved (fig. 5a). in posterior view, the supraoccipital shows a central vertical ridge as in y. columbianus (see páramo, 1994, fig.8). anteriorly, the supraoccipital extends over the opisthotic and prootic on each side of the skull. opisthotic-exoccipital: the opisthotic and exoccipital are fused. the opisthotic-exoccipitals have been preserved without their posterior end of the paraoccipital process. they form the lateral walls of the foramen magnum and the latero-dorsal ends of the occipital condyle (fig. 5a, b). in the lateral wall, there are two foramina for the exit of nerves x, xi and xii, separated by a thin lamina and, in front of these, there is a larger foramen for the perilymphatic sac and nerve ix (following russell, 1967) (figs. 3c, d). on the medial wall, anterior to the foramen magnum, the opening for the nerves x and xi (jugular foramen) is clearly visible and, anterolaterally and very close to it, the foramen for nerve ix is distinguished. the distribution of the foramina for nerve xii, on the inner wall of the cranial cavity, is sub-parallel to the ventral edge of the jugular foramen, as illustrated by russell for clidastes propython cope, 1869 (russell, 1967, fig. 13). in the paraoccipital process, the opisthotic-exoccipital and the prootic are joined by slightly interdigitated, but open sutures, suggesting the presence of cartilage, as a probable juvenile condition. posteriorly, the ventromedial end of the supratemporal is observed between these bones (fig. 5d). prootic: both prootics are preserved almost in their entirety, with only the most posterior end of the right missing. in lateral view the prootic has a shape of a lying “y”, with two anterior and one posterior rami (fig. 3c). in the antero-ventral ramus (inferior process), the otosphenoidal crest is seen poorly pronounced, and posteriorly to it, the foramen for the exit of nerve vii opens on the surface of the bone (figs. 3c-d). the posterior ramus of the prootic forms the lateral wall of the suspensorium. in the anterior region of this ramus, each prootic borders the exoccipital, forming the fenestra ovalis, where the columella exits the otic capsule toward the quadrate (fig. 3c). posteriorly, the prootic borders the paraoccipital process of the opisthotic dorsally and the supratemporal medially (fig. 5d). the prootic covers laterally the anterior extension of the supratemporal for a substantial length. stapes: both stapes are preserved in their original position, the right one is broken. the stapes is an elongated and slender small bone that extends along and against the preserved portion of the paraoccipital process. the right stapes is broken close to the fenestra ovalis showing a circular cross-section (fig. 3c). figure 5. oneirosaurus caballeroi gen. et sp. nov., holotype igm879524. a, photograph, drawing and interpretative scheme of the specimen in posterior view. b, basioccipital floor in posterior view. c, photograph and interpretative drawing of the anterior basicranial region in dorsal view, showing the presence of non-identified bones (probably basipterigoyd meniscus). d, schematic details of the paraoccipital bars; left paraoccipital bar in posterolateral (1), dorsally oblique posteromedial (2), and anterolateral (3) views; the incomplete right paraoccipital bar in the broken section (4). gray fill, sediment; pattern fill, broken bone surface; black, empty space. scale bars: a = 50 mm; bc = 30 mm; d = approximately 20 mm. abbreviations: ai, atlas intercentrum; an, angular; ar, articular; bo, basioccipital; bptm, basipterygoid meniscus; bs, basisphenoid; ep, epipterygoid; ex, exoccipital; is, interorbital septum; lpt, left pterygoid; op, opisthotic; p, parietal; par, prearticular; pof, postorbitofrontal; pop, paraoccipital process of the opisthotic; pr, prootic; ps, parasphenoid; pt, pterygoid; q, quadrate; sa, surangular; so, supraoccipital; sq, squamosal; st, supratemporal. basioccipital: the basioccipital has lost a portion of the condyle and part of the right basal tubercle. the ventral surface is completely covered by insoluble material, preventing the observation of its details. the occipital condyle is barely wider than it is tall, and the basal tubera are long, extending ventrally for a distance approximately one-half the height of the occipital condyle. the floor of the medullary cavity is pierced by two large foramina for the basilar artery, separated by a thin septum (fig. 5b). the floor narrows anteriorly forming a “v” shaped bottom of the anterior medullary cavity (fig. 5b). basisphenoid: the basisphenoid is complete, but its ventral surface is poorly observable given that it is thickly covered by insoluble material. the dorsal ala is moderately developed. in the lateral wall of the basisphenoid is the vidian canal, which has a short lateral wall (fig. 3d). dorsal and slightly posterior to the anterior aperture of the vidian canal, there is an aperture that enters medially. this dorsal aperture should house the medial ramus of the internal carotid artery (following camp, 1942) or represents the exit of the basilar artery (sensu russell, 1967). the groove for conducting the internal jugular vein is narrow and runs ventral and anterior to the vidian canal. the anterior end of the basisphenoid is projected in the parasphenoid rostrum, where the parasphenoid is fused. at this point, the lateral walls of the basisphenoid are elevated and show a little thin dorsal process pointing forward. beneath 240 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia this dorsal process, the anterior edge of the lateral wall shows a concavity that should have held the anterior ramus of the basilar artery (fig. 5c). the floor of the cerebral cavity is narrowly “v” shaped. differing from the dorsum sellae illustrated by russell for platecarpus (see russell, 1967, fig. 10), in the internal cavity the dorsum sellae is not clearly demarcated. however, the slope of the floor increases posteriorly, differentiating the sella turcica space from the medullary cavity of the basioccipital. in the sella turcica area there are two medial elongated slits where the foramen for the internal carotid ramus is housed. on each side of the internal cavity, the canal for the nerve vi opens near the dorsal end of the dorsal ala. parasphenoid: this bone is completely preserved in anatomic position. it projects a slender rostrum (cultriform process) from the base of the basisphenoid which extends anteriorly slightly inclined dorsally (figs. 2b, 3d). the rostrum is somewhat dorsoventrally compressed and has a regular thickness, except in its anterior end where it thins and ends in a point. mandible the mandibular rami are gracile. both rami lack the anterior end and have lost part of its external surface. the right mandibular ramus also lacks its posterior end (figs. 3a, 4a, 6a). dentary: the dentaries are incomplete; the posterior eight alveoli or teeth are preserved in the left dentary, while in the right one only seven teeth positions are present. the dentary is relatively slender; the alveolar border is straight (fig. 3a). the lingual alveolar edge is shorter than the labial alveolar edge, so the dental roots of the teeth are more exhibited lingually (fig. 6a). all the alveoli are similar in size. in the anterior preserved end, the dentary is low, its height is equivalent to the distance occupied by two alveoli (3 cm). the lateral anterior surface of the dentary has 4 elongated foramina parallel to the alveolar edge, at 1 cm from this edge (fig. 4a). posterior to the last alveolus, the dentary forms the dorsal portion of the mandible for a distance equivalent to three alveoli. its posterior dorsal end contacts loosely the coronoid, and its posterior lateral end meets with the surangular. in this region, the dentaries of both mandibular rami are strongly fractured and have lost part of its exterior surface, showing a broad mandibular canal, in which a slender lamina of the prearticular extends. ventrally, the dentary is embraced by the splenial. in medial view, the ventral border of the dentary is exhibited from the level of the sixth posterior alveolus (fig. 6a). splenial: the splenial forms the internal face of the mandible, covering the anterior lamina of the prearticular (fig. 6a). the splenial decreases in height anteriorly. in lateral view the splenial appears exposed ventrally from the level of the fifth posterior alveolus, and posteriorly its lateral exposure increases forming a low sloping suture with the dentary (fig. 3a). the posterior end of the splenial has the articular facet for the angular, which is sub-triangular to oval in outline and vertically elongated. coronoid: both coronoids are complete and faintly displaced dorsally from its original position. the coronoid is robust, longitudinally short, and ventrally curved. its posterior process is small. the lateral descendent ala is relatively short, whereas the medial one is large (fig 3a, 4a). surangular: the surangulars are incomplete. they constitute most of the lateral surface and the dorsal medial region of the posterior mandible (figs. 3a, 4a, 6a). the surangular forms the anterior and lateral walls of the glenoid fossa, without prolonging behind it. the dorsal edge of the surangular is inclined; it anteriorly rises as a fine lamina to form a coronoid buttress. in lateral view, the ventral edge of the surangular contacts the angular anteriorly, and posteriorly it contacts the articular in a curved suture that reaches the lateral edge of the glenoid fossa near its posterior end. dorsally, this suture turns forward crossing laterally the glenoid fossa. in the posterior lateral surface, near the anterior margin of the glenoid fossa, the surangular bears a foramen (fig.4a). in medial view, the surangular contacts the prearticular by a long subhorizontal suture (fig. 6a). angular: both angulars are preserved, but the posterior part of the left one is lost. the angular is an elongated bone that widens anteriorly to form the articulation with the splenial (fig. 2b, 3a). the anterior articular surface, to receive the splenial, is triangular and has a dorsal “v” shaped concavity. the angular extends posteriorly forming the ventral edge of the mandibles. its posterior lateral end reaches the level of the beginning of the glenoid fossa. latero-dorsally the angular is covered by the surangular. medially, the angular is posteriorly covered by the prearticular, but anteriorly it partially covers the prearticular, without reaching the coronoid (fig. 6a). prearticular-articular: the posterior end of the mandible is only preserved in the left mandibular ramus (fig. 4a, 6a). here, the suture between the prearticular and the articular is noticeable, suggesting an unfused contact between these bones and a juvenile stage of the individual. in lateroventral view, this suture is a slightly sinuous curved line that extends from the posterior end of the angular to the posterior lower mandibular edge. in the medial face of the mandible, this suture ascends diagonally to reach the medial edge of the glenoid fossa (fig. 6a). both prearticulars have partially lost their anterior laminae due to their extreme thinness. posteriorly, the prearticular is more robust; it forms the ventral portion of the medial face of the mandible and the medial edge of the glenoid fossa (fig. 6a). the articular forms the posterior end of the mandible. in the ventrolateral face of the mandible, it forms an elongated subtriangular surface that extents forward covered by the surangular laterally and by the prearticular medially, reaching the posterior edge of the angular. the lateral suture with the surangular descends from the glenoid fossa and near the ventral edge of the mandible it curves anteriorly. dorsally, the articular forms almost all the floor of the glenoid fossa and posteriorly a very short retroarticular process which is feebly inflected (fig. 4a). it shows a triangular dorso-medial surface with wrinkled texture, that served as an important muscular insertion area. in its anterior medial end, the articular forms, with the prearticular, a slight tuberosity in the lower edge of the glenoid fossa (fig. 6a). the surface of the lateral posterior end of the retroarticular process is porous and at least one large foramen can be identified, however, the poor preservation of this region hinders us to affirm that no other foramina were present. marginal teeth all the teeth in the alveoli are broken; they lack their labial surface. only one disarticulated tooth is complete (fig. 6b) this tooth is 3 cm from the base of the root to the crown apex. the root is bulbous and 1.2 cm in height. the crown is circular in section at its base and oval to its apex. the ratio of its height and basal diameter is 2.0. the crown of the teeth is curved backwards and slightly inwards. it has two smooth carinae well-defined, 180° from each other, one mesial and the other distal. the lingual surface of the crowns is finely and densely striated. the labial surface, which is preserved in the complete tooth, shows wider striae more spaced than those of the lingual surface. postcranial elements the preserved postcranial elements are restrained to the atlas components. they were found disarticulated and displaced from their original position. during the preparation, the centrum and the neural arches were extracted and prepared individually. the intercentrum remains in its preserved position, near the paraoccipital process of the left opisthotic (fig. 4a, 5a). the atlas centrum (odontoid) has an octagonal contour in anterior and posterior views. anteriorly, it shows a slightly concave medial subtriangular surface limited laterally by the articular facets for the neural arches (fig. 6c). the posterior surface of the atlas centrum has two well-defined flat faces separated by a medial vertical crest, and the dorsal surface is concave (fig. 6c). the shape of the neural arches concurs with that illustrated by russell (1967; fig. 39) for clidastes propython cope, 1869. the synapophyseal process of the neural arch is large (fig. 6d). the spinous process is separated from the surface for the articulation with the occipital condyle by a groove. behind the base of the spinous process there is a concavity housing a foramen. the intercentrum is not completely exposed; the posterior surface, subtriangular in outline, and the articulation facet for the left neural arch can be identified. the surface for the articulation with the basioccipital is concave. 7. morphological comparisons two colombian mosasaurs have been described to date: the halisaurine eonatator coellensis from the campanian of tolima (central colombia) (páramo-fonseca, 2013) and the plioplatecarpine (sensu polcyn et al., 2023) yaguarasaurus columbianus from the turonian of the upper magdalena valley (southern colombia) (páramo, 1994). the new specimen here described (igmp879524), was found in the coniacian of northern colombia and its cranial morphology is closer to that of y. columbianus than to that of e. coellensis. 241maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra the morphology of igmp879524 is coincident with the diagnoses of two different groups considered monophyletic and exclusive: plioplatecarpinae sensu konishi and caldwell (2011) and tethysaurinae sensu makádi et al. (2012). recently, polcyn et al. (2023) synonymized yaguarasaurinae (sensu palci et al., 2013), tethysaurinae (sensu makádi et al., 2012), plioplatecarpinae (sensu konishi and caldwell, 2011) and pioplatecarpini (sensu russell, 1967) under a more inclusive group plioplatecarpinae in a new sense, based on a particular basicranial vascularization shared by some russellosaurines. tethysaurinae was proposed by makádi et al. (2012) based on a cladistic analyses of mosasauroids. in contrast, in the cladistic analyses of konishi and caldwell (2011) and that of polcyn et al. (2023) the ingroup was limited to 17 russellosaurine mosasaurs. to determine the morphological affinities of igmp879524 with these groups, we compare its morphological features with the characters included in the diagnosis of each of these groups. konishi and caldwell (2011) provide a revised diagnosis of plioplatecarpinae (including platecarpus; plioplatecarpus; angolasaurus; ectenosaurus selmasaurus; plus latoplatecarpus and plesioplatecarpus) containing 16 diagnostic characters. they differentiate this clade from the basal russellossaurines yaguarasaurus, russellosaurus and tehysaurus, which they included in a group informally termed the “primitive ‘tethysaur’ clade”. the cranial morphology of the specimen under study (igmp879524) matches, with a single exception, the revised diagnosis proposed by konishi and caldwell (2011) to define plioplatecarpinae. the only differing character in igmp879524 is the suprastapedial process of the quadrate, which ends in an acute end and not in a blunt end as is common in plioplatecarpines (according to konishi and caldwell, 2011). the almost complete conccur of the morphology of the specimen under study with the revised diagnosis of plioplatecarpinae presented by konishi and caldwell (2011) would show igmp879524 as an undoubted plioplatecarpinae. however, a careful comparison of the traits used by those authors in their cladistic analysis shows igmp879524 with cranial features that are common to the morphology of non-plioplatecarpine russellosaurines and that of basal plioplatecarpines, placing it in an intermediate position between these two groups. given these conditions and the morphological proximity of igmp879524 to yaguarasaurus columbianus (considered non-plioplatecarpine by konishi and caldwell, 2011), we consider that the high coincidence of the cranial morphology of the specimen under study with the diagnosis of plioplatecarpinae presented by konishi and caldwell (2011) should be evaluated in detail. the following observations are derived from this evaluation. first, we found that the character states scored by konishi and caldwell (2011) for y. columbianus as (1(1)), (13(1)) and (28(1)) are inaccurate and should be rescored as (1(0)), (13(0)) and (28(2)). in addition, characters (25), (32), (34), (41), (48) and (54) coded as (?) are visible and can be scored as (25(1)), (32(0)), (34(1)), (41(0)), (48(1)) and (54(0)). figure 6. oneirosaurus caballeroi gen. et sp. nov., holotype igmp879524. a, photograph, drawing and interpretative scheme of the left mandibular ramus without the coronoid in medial view; b, disarticulated tooth in anterior (1), posterior (2), lingual (3) and labial (4) views. c, atlas centum in dorsal (1), anterior (2), posterior (3) right lateral (4), and ventral (5) views. d, left atlas neural arch in anterior (1), lateral (2), medial (3) and posterior (4) views. gray fill, sediment; pattern fill, broken bone surface. scale bars: a = 50 mm; b-d = 20 mm. abbreviations: an, angular; ar, articular; bof, facet for the basioccipital; d, dentary; mkc, meckel’s canal; par, prearticular; sa, surangular; sp, splenial, spp, spinous process; syp, synapophyseal process. 242 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia igmp879524 shares some primitive cranial features with the so-called “primitive ‘tethysaur’ clade”, differing from plioplatecarpines. these features (the scoring given by konishi and caldwell (2011) indicated in parentheses) are: 1in dorsal view, the parietal shows a broad triangular anterior surface, anterolaterally extended in the manner of an equilateral triangle (19 (0)); 2the lingual alveolar parapet of the dentary is lower than the labial (54(0)); 3the quadrate stapedial pit is sub-reniform in shape (41(0)), not in tethysaurus). on the other hand, there are numerous features that igmp879524 shares with basal russellosaurines as well as with most basal plioplatecarpines differing from the more derived plioplatecarpines. these features include: 1 frontal with weak ridge in the anterior region (11(0) different in ectenosaurus clidastoides (merriam, 1894)); 2prefrontal and postorbitofrontal separated over the orbit (15(1) different in selmasaurus spp.); 3and 4parietal foramen located entirely on the parietal and of small size (18(0)), (20(0) different in selmasaurus russelli wright and shannon, 1988); 5postorbital process of the parietal visible dorsally and forming the anteromedial border of the supratemporal fenestra (23(0) different in plesioplatecarpus); 6medial ridge of the quadrate sharpened in its central region (44(0)); and 7marginal teeth root poorly exposed, constituting less than 25% of tooth height (90(0) different in selmasaurus johnsoni polcyn and everhart, 2008). in addition, igmp879524 has some features, mainly of the quadrate, which are present or are similar in plioplatecarpines and are not found in members of the “primitive ‘tethysaurian’ clade”. these features are 1wide and short frontal (9(0)); 2, 3 and 4quadrate with thin wing, with low medial ridge in its dorsal region and no flange in its central region (34(0)), (43(1)), (45(0) ) and 5basioccipital canal opening into the floor of the brain cavity by means of two large foramina separated by a medial septum (97(2)) (shared with angolasaurus and plesioplatecarpus planifrons (cope, 1874), different from all other plioplatecarpines). the conditions discussed above show the cranial features of igmp879524 close to the morphology of basal plioplatecarpines, mainly angolasaurus and plesioplatecarpus, and also to the “primitive ‘tethysaurian’ clade”. this condition places it in an intermediate position between the non-plioplatecarpine russellosaurines and plioplatecarpines (sensu konishi and caldwell, 2011). the three genera of the group named by konishi and caldwell (2011) “primitive ‘tethysaur’ clade” are subsequently recovered by makádi et al. (2012), together with pannoniasaurus (from the santonian of western hungary), into a group that they named tethysaurinae. most cladistic analyses involving the taxa of this group have been based on the character matrix of bell (1997) and bell and polcyn (2005) and in all of them, these taxa have been recovered as a monophyletic group (caldwell and palci, 2007; leblanc et al., 2012; palci et al., 2013; jiménez-huidobro and caldwell, 2016; jiménezhuidobro et al., 2016; madzia and cau, 2017). the cranial morphology of igmp879524 is also concordant with the diagnosis of tethysaurinae proposed by makádi et al. (2012). these authors established a mosaic of primitive characters to define tethysaurinae, all coded in their cladistic analysis. of this mosaic, eight characters are cranial, six of which are observable in the specimen igmp879524. five of these characters share the state defined for tethysaurinae as follows (the coding given by makádi et al. (2012) for the clade is indicated in parentheses): 1almost straight frontoparietal suture (15(0)); 2quadrate stapedial pit elongate, being more than three times longer than wide (46(2)); 3and 4saddle-shaped quadrate condyle without anterior upward deflection (53(0)), (54(0)) and 5lateral surangulararticular suture behind the condyle (70(0)). the sixth character “quadrate alar concavity shallow” (50(1)), is here considered subjective to determine: in the holotype of yaguarasaurus, the concavity is covered by the in-place preserved tympanic membrane and can only be inferred from the medial view of the quadrate. in this view, the ala forms a more or less pronounced convexity, which is apparently absent in the holotype of russellosaurus, suggesting that the alar concavity is greater in yaguarasaurus than in russellosaurus. however, the holotype of russellosaurus shows plastic deformation and fracture (polcyn and bell, 2005), while the holotype of y. columbianus is preserved in excellent three-dimensional condition. this feature should be coded as (50(0)) for yaguarasaurus, and probably also for russellosaurus, differing from romeosaurus and pannoniasaurus. in igmp879524 the quadrate ala forms a wide concavity, sharing the state (0) with yaguarasaurus and russellosaurus. the morphology of igmp879524, coincident with the diagnoses of two groups considered monophyletic and exclusive (plioplatecarpinae sensu konishi and caldwell, 2011 and tethysaurinae) raises two explanations: 1the two groups are mutually exclusive and there are shortcomings in the formulation of their diagnoses or 2the two groups are not mutually exclusive, and the character matrices should be revised. recently, polcyn et al. (2023) reevaluated the clade plioplatecarpinae based on a particular basicranial vascularization that they affirm is present in some russellosaurines and absent in all other mosasaurids. differences in the cranial vascularization in mosasaurids had been previously noted by russell (1967), who also conferred taxonomic value to them. polcyn et al. (2023) propose a new interpretation of the cranial vascularization of the plioplatecarpines, built on traits that are partially exposed externally or can only be visualized by tomographic scanning. their phylogenetic analysis used the plioplatecarpines data matrix of konishi and caldwell (2011), adding three new characters (98, 99 and 100) to test the differences of the basicranial vascularization among plioplatecarpines. unfortunately, the scoring of the new characters 99 and 100 is difficult to understand, since some taxa are coded with a stage 2, which is not included in the description of the stages of these characters. the emended diagnosis proposed by polcyn et al. (2023) for defining the most inclusive taxon plioplatecarpinae contains eleven characters. in igmp879524, six of the eleven diagnostic characters can be compared, and all are concurrent with the diagnosis of plioplatecarpinae. these characters are (the coding from the phylogenetic analysis of polcyn et al. (2023) in parentheses): 1small to medium size; 2canals for basilar artery entering basisphenoid below abducens nerve exits and above internal carotid artery path (98(1)); 3 quadrate suprastapedial process elongate, reaching mid-height; 4-6marginal tooth crown with sub-circular basal cross-section (91(1)), medially finely striated (92(0)) and laterally fluted. nevertheless, igmp879524 shows features not found in any or almost any other plioplatecarpinae, but in other mosasaur groups, as is the presence of two external foramina for the exit of nerves x, xi and xii, distinctive of mosasaurinae (following russell, 1967) and the presence of a notch in anterior border of the atlas neural arch, which is only found in some mosasaurines and ectenosaurus and sarabosaurus (see russell, 1967; polcyn et al., 2023). the morphological comparisons discussed above show igmp879524 as a plioplatecarpinae in the sense of polcyn et al. (2023), differing from the more primitive genera (tethysaurus, romeosaurus, russellosaurus and yaguarasaurrus) in several features (see above) but also from the primitive plioplatecarpine genera angolasaurrus, ectenosaurus and selmasaurus and from the derived plioplatecarpine genera platecarpus, plioplatecarpus and latoplatecarpus (sensu konishi and caldwell, 2011), in some other features (see above). among the plioplatecarpines, the greater morphological affinities of igmp879524 are found with yaguarasaurus spp. (y. columbianus and y. regiomontanus rivera-sylva et al., 2024) and angolasaurus. nevertheless, some differences separate igmp879524 from these taxa. based on páramofonseca, 1997, rivera-sylva et al., 2024 and personal observation (m.e.p.-f.) for yaguarasaurus, telles-antunes, 1964 for angolasaurus, and the scoring of simões et al., 2017, we found the following differences: in igmp879524 the suprastapedial process of the quadrate is long and extends to the level of the infrastapedial process whereas in yaguarasaurus and angolasaurus the suprastapedial process ends above the infrastapedial; the infrastapedial process of the quadrate is more robust and better defined than in these taxa. the carinae on marginal teeth are arranged at 180º, one mesial and the other distal, whereas in y. columbianus (unknown in y. regmontanus see rivera-sylva et al., 2024) and in angolasaurus the carinae are mesially and labiodistally oriented. igmp879524 differs from yaguarasaurus also by the following features: the triangular surface of the frontal, anterior to the orbits, is longer in y. columbianus; the suspensory branches of the parietal originate more anteriorly in yaguarasaurus spp.; the dorsal exposure of the parietal-postorbitofrontal suture is long and has a straight diagonal trajectory in yaguarasaurus spp., whereas it is short and has a semicircular trajectory in igmp879524; the frontal of igmp879524 has a smooth anterior dorsal ridge whereas in yaguarasaurus spp. there is no ridge; the protrusion of the ventral-posterior border of the jugal is less pronounced in igmp879524 than in y. columbianus and its position is more dorsal; the otosphenoidal crest of the prootic does not cover laterally the exit of nerve vii as it covers in y. columbianus; the carinae in the marginal teeth are weaker in yaguarasaurus spp; the labial surface of the crowns has wider and more separated striae than the lingual surface in igmp879524, differing from the smooth labial surface of the crown in y. columbianus. 243maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra figure 7. phylogeny of mosasauridae showing the position of oneirosaurus caballeroi gen. et sp. nov. time calibrated strict consensus from 72 most parsimonious trees (501 steps) with 46 otus and 125 characters (see materials and methods) recovered from the analysis of the full dataset. bremer support values are shown in some branches. 244 oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from colombia igmp879524 differs from angolasaurus as follows. in angolasaurus the quadrate stapedial pit is subcircular (reniform in igmp879524); there is no groove on the anterolateral edge of the quadrate ala (dorsal groove in igmp879524); the quadrate medial ridge is single, thin and high (low and rounded in igmp879524); the quadrate condyle is convex (saddle-shaped in igmp879524); the foramina for nerves x and xii are externally fused (separated in igmp879524); the basioccipital tubera are short (long in igmp879524); the medial dentary parapet is more elevated than in igmp879524; the coronoid is slightly curved, without expanded posterior wing (concave and with expanded posterior wing in igmp879524); the lateral surangular-articular suture is found at the middle of glenoid (behind the glenoid in igmp879524); and the tooth replacement is sub-dental (in shallow excavations in igmp879524). the recently described monospecific plioplatecarpine sarabosaurus dahli proposed by polcyn et al., 2023 also differs from igmp879524 in some features. in igmp879524 the supradental portion of the maxilla at the external naris is transversely as broad as tall (it is broader than tall in sarabosaurus); there is no dermal sculpting on the dorsal surface of the frontal supraorbital region as there is in sarabosaurus; the pterygoid bears 13 tooth positions (at least 14 in sarabosaurus); the infrastapedial process and the posteroventral ascending alar rim of the quadrate are robust (both are poorly developed in sarabosaurus); the dentary is thin (robust in sarabosaurus) and its medial ridge covers part of the tooth root (exposes nearly entire tooth root in sarabosaurus) (polcyn et al., 2023). in summary, the combination of primitive and derived characters found in the new specimen igmp879524 is unique among plioplatecarpinae. the morphological differences with each of the genera included in plioplatecarpinae are numerous and unquestionable (see data matrix) and support the erection of a new genus and species: oneirosaurus caballeroi. 8. phylogenetic analyses according to our revisions, we change the following scores from the matrix of madzia and cau (2017): for yaguarasaurus columbianus [11(0→1); 43(0→1); 44(1→0); 46(0→2)], for russellosaurus coheni [11(0→1); 44(1→0)]; for angolasaurus bocagei [18(1→2)] and sarabosaurus dahli [8(1→?)]. first analysis (without sarabosaurus dahli) from the first analysis (without sarabosaurus dahli) a strict consensus of 72 most parsimonious trees with 501 steps (ri= 0.704 and ci= 0.323) was obtained from tbr branch swapping (fig. 7). oneirosaurus caballeroi was recovered within a monophyletic clade which includes all plioplatecarpinae sensu polcyn et al., 2023, except tethysaurus nopcsai bardet et al., 2003. in our results, not all the groups previously included within mosasauridae are found interrelated as a monophyletic clade; the halisaurines (eonatator sternbergii + (eonatator coellensis + halisaurus platyspondylus)) and a branch containing tethysaurus nopcsai + pannoniasaurus inexpectatus, appear in an unresolved polytomy with all the basal mosasauroids and with a branch containing tylosaurinae, mosasaurinae and plioplatecarpinae, each as a monophyletic clade. the three synapomorphies defining the branch tylosaurinae + mosasaurinae + plioplatecarpinae refer to the locomotor skeleton: (89(1)) sacral vertebrae no more than one; (117(1)) elongated ilium crest not articulating with sacral ribs; and (123(1)) hyperphalangy present. although certain of these characters are unknown in some of the “mosasaurid” taxa excluded from that branch, it is interesting to note that these features are related to locomotion functions showing a change from plesiopelvic and plesiopedal conditions to hydropelvic and hydropedal conditions representing the adaptation to a full-aquatic lifestyle in the derived mosasauroids. this branch is comparable to the natantia clade of bell (1997) and represents fullaquatic mosasauroids. another uncommon finding in the recovered topology is the place of dallasaurus turneri within the clade plioplatecarpinae, close to russellosaurus and romeosaurus, a position not recovered in previous works (jiménez-huidobro and caldwell, 2016; jiménez-huidobro and caldwell, 2019; madzia and cau, 2017; palci et al., 2013; simões et al., 2017). in our resulting topology, the clade yaguarasaurinae, as recovered in previous works, is not supported. in general, the obtained bremer supports (see fig. 7) show that most of the recovered branches are not well-supported with bremer values of 2. the only two branches that are better supported are the tylosaurinae branch and the halisaurinae branch, with bremer values of 8 and 5 respectively. the clade plioplatecarpinae, containing oneirosaurus caballeroi, is supported by the following synapomorphies: (12(1)) very short descending processes from the sides of the olfactory canal surrounding and almost or totally enclosing the olfactory nerve; (18(1)) parietal table triangular in shape, with sides contacting in front of suspensorial rami; (28(1)) presence of a postorbitofrontal transverse dorsal ridge; (53(5)) 12 dentary teeth; (59(1)) absence of a coronoid posteromedial process; (66(1)) one to three large foramina present in the retroarticular process of the articular; and (70(1)) tooth fluting present. plioplatecarpinae was recovered with two branches, one containing plesioplatecarpus, latoplatercarpus, platecarpus, and plioplatecarpus and supported by the following synapomorphies: (15(0)) lateral sutural flange of frontal posteriorly extended and (19(1)) large parietal foramen. the second branch within plioplatercapinae contains selmasaurus johnsoni, angolasaurus bocagei, ectenosaurus clidastoides, russellosaurus coheni, romeosaurus fumanensis, dallasaurus turneri, and the colombian taxa oneirosaurus caballeroi and yaguarasaurus columbianus. this branch is supported by the following synapomorphies: (9(0)) frontal not invaded by posterior end of nares; (15(2)) lateral and medial sutural flange of frontal posteriorly extended; (18(2)) parietal table triangular shape with posterior portion forming parasagittal crest; and (48(0)) quadrate anterior ventral condyle without upward deflection of its anterior edge. second analysis (with sarabosaurus dahli) from the second analysis (with sarabosaurus dahli) a strict consensus of 871 most parsimonious trees with 505 steps (ri= 0.701 and ci= 0.321) was obtained from tbr branch swapping (supplemental data 2, fig. s1). oneirosaurus caballeroi was recovered within a large polytomy. the plioplatecarpines sensu polcyn et al. (2023) were not recovered as a monophyletic group. the tylosaurinae and the mosasaurinae were found as differentiated branches, but they are not internally resolved, showing also large polytomies. when we applied the iterpcr algorithm, oneirosaurus caballeroi, romeosaurus fumanensis, russellosaurus coheni, yaguarasaurus columbianus, dallasaurus turneri and komensaurus carrolli, and the nodes containing halisaurus and eonatator were identified as unstable taxa. as the goal of the phylogenetic analyses in this contribution was to determine the phylogenetic relationships of the new taxon oneirosaurus caballeroi we did not prune the unstable otus identified with this algorithm. however, the results of this second analysis shows that the inclusion of sarabosaurus dahli produces great instability. this might be due to the presence of derived and primitive characters present in the scarce known material of s. dahli. 9. conclusions a new plioplatecarpine mosasaurid (igmp879524) is described. its morphology shows a combination of primitive and derived characters placing the specimen in an intermediate position between the non-plioplatecarpine russellosaurines and plioplatecarpines (sensu konishi and caldwell, 2011). the morphology of igmp879524 is also unique among the more inclusive clade plioplatecarpinae (sensu polcyn et al., 2023). it is the only plioplatecarpine having two external foramina for the exit of nerves x, xi and xii, a feature previously considered distinctive of mosasaurines. the special combination of characters of igmp879524 allow us to determine it as a new genus and species of plioplatecarpinae: oneirosaurus caballeroi. the results of the phylogenetic analysis support our morphological comparisons recovering oneirosaurus caballeroi as a basal plioplatecarpine. in contrast to previous works dallasaurus were recovered among the plioplatecarpinae clade. moreover, in our results, the plesiopelvic and plesiopedal “mosasaurids” (tethysaurines and halisaurines) were recovered in an unresolved polytomy with the basal mosasauroids and a branch containing the derived hydropelvic and hydropedal mosasurids (tylosaurines, plioplatecarpines and mosasaurines). 245maría eurídice páramo-fonseca, josé alejandro narváez-rincón, cristian david benavides-cabra acknowledgments we dedicate this work to the memory of dr. maría eurídice páramofonseca, a pioneer in the study of colombian mosasaurs. her foundational research irrevocably shaped our comprehension of these marine reptiles in the region. we are grateful to the late mr. jaime caballero for collecting and informing the sgc (former ingeominas) about the finding of the herein studied specimen. we sincerely appreciate the generous donation of the specimen by the caballero family to the museo geológico nacional josé royo y gómez (mgnjrg) of the sgc. likewise, we thank mauricio pardo and victoria corredor (from mgnjrg) for granting access to the specimen under their care. furthermore, we are grateful to dr. fernando etayo-serna for providing the identification of the collected ammonoids and his comments on the specimen’s age. we sincerely appreciate the assistance of gerardo vargas (from mgnjrg) provided in the field and in the preparation of the specimen. likewise, we thank daniel pomar from mgnjrg for his collaboration with the accessibility of the specimen and the preparation of the associated ammonoids. references bapst, d. w. 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(1996). treatise on invertebrate palaeontology, part l, mollusca 4 (revised), volume 4: cretaceous ammonoidea. geological society of america and university of kansas press, boulder and lawrence, kansas, usa, 362 pp. https://journals. ku.edu/invertebratepaleo/issue/view/470 https://lib.ugent.be/catalog/rug01:000229000 https://doi.org/10.1080/02724634.1988.10011686 https://doi.org/10.1080/02724634.1988.10011686 https://journals.ku.edu/invertebratepaleo/issue/view/470 https://journals.ku.edu/invertebratepaleo/issue/view/470 _hlk201248797 _hlk41657854 _hlk50394674 _hlk194752114 _goback issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v26n2.100645 earth sciences research journal earth sci. res. j. vol. 26, no. 2 (junio, 2022): 107 118 g eo ph y si c s computation and analysis of geopotential number in são paulo, brazil valéria cristina silva1*, denizar blitzkow1,2, flávio guilherme vaz de almeida filho1, ana cristina oliveira cancoro de matos1,2, gabriel do nascimento guimarães3 1. universidade de são paulo, brazil. 2. centro de estudo de geodesia, são paulo, brazil. 3. faculdade de engenharia civil, universidade federal de uberlândia, minas gerais, brazil * corresponding author: vsilva2@usp.br abstract in recent decades, important steps have been taken to implement the physical concepts of geodesy in practice, concerning height systems. despite the difficulties involving gravity field modeling, with the establishment of conventions, standards, and computation strategies, the realization of the international height reference system (ihrs) is well underway. for a global system, there are constraints for some countries, especially for those with sparse gravity data, mountain regions, and vast areas. in terms of methodology, the computation can be performed directly using the global geopotential models (ggm), recovering existing geoid models, or determining pointwise the gravity potential using integral formulas. in general, the regional gravity modeling is given by numerical integration or least-squares collocation and more recently adopting the spherical radial basis functions. the first approach allows determining the earth’s gravity component at a specific point and adjusting the integral formula according to the gravity coverage. since so far there is no common sense about the best methodology, computation strategies are been analyzed. in this context, the paper aims to contribute to ihrf, computing the geopotential number in the scope of ihrf, using numerical integration to solve the geodetic boundary value problem and an existing recent quasi-geoid model in four stations in são paulo state, brazil. the first approach was performed considering two cases: a radius of 210 km and 110 km of gravimetric data coverage and the global geopotential model goco05s truncated at 100 and 200, respectively. the results between solutions have shown a maximum difference of 94 cm, and a minimum difference of 10 cm. keywords: ihrf; heights; geoid; quasi-geoid; ggm. cálculo y análisis del número geopotencial en são paulo, brasil resumen en las últimas décadas se han dado pasos importantes para implementar en la práctica los conceptos físicos de la geodesia, en lo que respecta a los sistemas de altura. a pesar de las dificultades para modelar el campo de gravedad, con el establecimiento de convenciones, estándares y estrategias de cálculo, la realización del sistema de referencia internacional de alturas (ihrs) está muy avanzada. para un sistema global, existen limitaciones para algunos países, especialmente para aquellos con datos de gravedad escasos, regiones montañosas y áreas extensas. en lo que respecta a metodología, el cómputo se puede realizar directamente utilizando los modelos geopotenciales globales (mgg), recuperando los modelos de geoide existentes, o determinando puntualmente el potencial de gravedad, mediante fórmulas integrales. en general, el modelado regional del campo de gravedad es hecho por integración numérica o colocación de mínimos cuadrados y, más recientemente, adoptando las funciones de base radial esférica. la primera aproximación permite determinar el componente de gravedad de la tierra en un punto específico y ajustar la fórmula integral de acuerdo con la cobertura de gravedad. puesto que hasta el momento, no existe un sentido común sobre la mejor metodología, las estrategias de cálculo son analizadas. en este contexto, el documento tiene como objetivo contribuir con el ihrf, calculando el número geopotencial en el ámbito del ihrf, utilizando la integración numérica para resolver el problema geodésico de valor de frontera y un modelo cuasi-geoide reciente existente en cuatro estaciones en el estado de são paulo, brasil. la primera aproximación se realizó considerando dos casos: un radio de 210 km y 110 km de cobertura de datos gravimétricos y el modelo geopotencial global goco05s truncado en 100 y 200, respectivamente. los resultados entre soluciones han mostrado una diferencia máxima de 94 cm y una diferencia mínima de 10 cm. palabras clave: ihrf, alturas, geoide, cuasigeoide, ggm record manuscript received: 16/02/2022 accepted for publication: 04/07/2022 how to cite item: silva, v. c., blitzkow, d., almeida, f. g. v., matos, a. c. o. c., guimarães, g. n. (2022). computation and analysis of geopotential number in são paulo, brazil. earth sciences research journal, 26(2), 107118. https://doi.org/10.15446/esrj.v26n2.100645 mailto:vsilva2@usp.br 108 valéria c. silva, denizar blitzkow, flávio vaz de almeida filho, ana c. oliveira cancoro de matos, gabriel do nascimento guimarães 1.introduction one of the important assignments of geodesy is the establishment of reference systems and constant improvements in their quality and accuracy, along with the advances in computational technologies and measurement instruments. this progress in spatial techniques enabled the positioning via a three-dimensional cartesian coordinate system associated with a conventionally defined ellipsoid. consequently, the dynamic phenomena of the earth caused by geophysical effects are quantified and qualified through two references, the celestial reference system and the terrestrial reference system, which are monitored by the international earth rotation and reference systems service (see: https://www.iers.org/iers/en/home/home node.html) (blitzkow et al., 2011). in the past two decades, low orbit satellite missions such as challenging minisatellite payload (champ) (gfz, 2019), gravity recovery and climate experiment (grace) (jpl, 2011), and in particular the gravity field and steadystate ocean circulation explorer (goce) (esa, 2009), addressed the attention to modeling the high frequencies of earth the gravity field, reaching unprecedented results (for more details see: http://icgem.gfz-potsdam.de/home). today, along with the improvement of digital terrain models (dtms) and terrestrial gravity measurements, the effort is toward establishing a height reference with physical meaning, the international height reference system (ihrs). the physical and geometrical systems form the global geodetic reference system (ggrs) whose importance is recognized and supported by the united figure 1. location of the study area. nations (un). the united nations general assembly on a global geodetic reference frame (ggrf-un) of sustainable development (a/res/69/266) on february 26, 2015 reinforces the importance of a stable global geodetic reference structure to identify areas under threat from floods, earthquakes, or droughts, and to adopt preventive measures. the key to mitigating damage caused by natural phenomena, human activities, and carrying out sustainable planning is the implementation, among others, of a system of physical heights based on gravimetric data (united nations general assembly, 2015). in addition to providing an infrastructure for the development of studies on a global level, the establishment of the ihrs is fundamental to correcting the deficiencies in the local height systems. the vertical networks around the world differ up to 2 m, making it difficult to interchange information between systems (ihde et al., 2017). the correction of these inconsistencies is dependent on efforts to readjust the height network, perform gravimetric densification surveys, and evaluate the global geopotential models (ggm) and topographic models in the study region. since the framework of the ihrs is under discussion, it is pertinent to develop studies to identify the difficulties involving the methodology and information for determining the gravity potential values. the paper aimed to determine and compare the gravity potential values at four stations in brazil, located in são paulo state using two approaches. first, a numerical integration method with hotine’s kernel was performed using the radius of 110 km and 210 km around the station location. second, the gravity potential was recovered from an existing and recent quasi-geoid model. figure 2. gravity reference frame in são paulo. 1.1 study area the state of são paulo is located in the southeast part of brazil. its size corresponds to 248.219,481 km²; its population is estimated at 46,289,333. the state has the bigger gross domestic product (gnp) in the country (ibge, 2019). the proposed ihrf stations are located in the cities of botucatu (sobop), presidente prudente (ppte), são carlos (eesc) and são josé do rio preto (sjrp) (figure 1). ppte was already planned, by the instituto brasileiro de geografia e estatística (ibge) for the global network, and the others are being suggested for regional infrastructure. they are placed in the same coordinates of rede brasileira de monitoramento contínuo dos sistemas gnss (rbmc), and close to them there are absolute gravity observations. the latter stations contribute to the international gravity reference system (igrs), which is well underway, proposed by the international association of geodesy (iag) in resolution no. 2 of 2015 (drewes et al., 2016; ihde et al., 2017; tóth, 2017). the igrs aims to replace the world gravimetric reference still used, the international gravity standardization net 1971 (igsn-71) network, 109computation and analysis of geopotential number in são paulo, brazil whose precision and spatial distribution no longer satisfy the requirements to implement a global altimetric reference. the first studies suggest the optimistic precision of 1 a 2 μ𝐺𝑎𝑙 on the gravity measurements (wilmes et al., 2015). the state is quite complete in terms of 5’ grid of gravity anomalies. currently, it has about 9,257 gravimetric points. the first surveys were carried out in the 1970s by institutions such as the instituto de astronomia, geofísica e ciências atmosféricas of universidade de são paulo (iag-usp), observatório nacional (on), petrobrás, and the anglo-brazilian gravity project, carried out in conjunction with the university of leeds and the ibge (castro junior et al., 2018). years after, thematic project number 06/04008-2, of the fundação de amparo a pesquisa do estado de são paulo (fapesp) gravimetric densification surveys for the production of an advanced geoid model were carried out (guimarães et al., 2014). in 2016, a gravimetric reference system, compound of absolute measurements, was implemented in the state of são paulo, from an initiative of the laboratório de topografia e geodésia of the escola politécnica da universidade de são paulo (ltg/epusp), instituto geográfico e cartográfico de são paulo (igc-sp), ibge and centro de estudos de geodesia (cenegeo). this reference contains 15 new stations and four reoccupations (cananéia, são paulo, ubatuba and valinhos stations), distributed according to figure 2. the measurements were performed using an absolute gravimeter micro-g lacoste, a-10, number 32, belonging to the igc-sp and operating at the ltg. gravity data information for download is available in portuguese (see: https://www. cenegeo.com.br/rede-grav-absoluta/rede-absoluta-sao-paulo) and in english (see: http://bgi.obs-mip.fr/gravity-databases/). 2. international height reference system the iag, motivated by the global geodetic observing system (ggos), proposed in resolution no. 1 of 2015 the creation of the so-called international height reference system with conventions for its definition (see iag resolution no. 1 (2015) in drewes et al., 2016; ihde et al., 2017; tóth, 2017). the computation will be based on the mean value of the earth’s gravity potential, conventional adopted, w0= 62,636,853.4 m2s-2, (sánchez et al 2016), and specific parameters to standardize the framework. in addition, this system, strictly based on the gravity field, will be related to international terrestrial reference frame (itrf) coordinates with its variation in time. the parameters, observations, and data must be related to the mean tidal system or mean crust, and the unit of length is the meter and the unit of time is the second in the international system of units. the vertical coordinates are differences ∆wp, determined between the gravity potential at a given point p (wp) and w0; such differences in gravity potential are also referred to as geopotential numbers (cp). the spatial reference of p position for the potential wp= w(x) is related to the x coordinates of the itrs (figure 3). after the implantation of global references, the unification process will integrate the ocean topography, determined by the satellite altimetry technique, also considering the tide records. the fundamental component of the ihrf is the ggm, described as coefficients of the gravitational potential function complemented with terrestrial gravimetric data. however, terrestrial gravimetric information is necessary and must be properly arranged around ihrf stations. for this, sánchez et al. (2016) recommend that: • the gravity observations must be distributed around the ihrf reference station within a radius of 210 km; • the minimum accuracy of these values must be ± 20 µgal; • the gravity position point must be collected via global navigation satellite systems (gnss) positioning; • in mountainous areas, there should be approximately 50% more gravimetric observations; • ggms and dtms uncertainties must be considered in the framework. three methods are proposed for computation (sánchez et al., 2021): based on geopotential models of high-resolution, containing gravity and topographic information; using precise regional geoid or quasi-geoid based on the gravity field, and vertical datum unification using the local height systems based on the solution of geodetic boundary value problem (gbvp). the first approach is a direct way that consists in determining wp by inserting itrf coordinates in a reliable harmonic expansion tool such icgem (ince et al., 2019) or any other ggms software. there is a limitation of this methodology due to the poor accuracy of ggms in some areas. well-surveyed areas expect a root mean square (rms) difference of ±4 cm (rummel et al. 2014), comparing the global positioning system (gps) determinations on the leveling network and ggms models elements. while others with sparse gravity data can reach 1 m (drinkwater et al., 2003; sánchez and sideris, 2017). this occurs because in regions with scarce gravimetric information, the ggms use brings up the so-called omission error, responsible for affecting its accuracy above 300 degrees (rummel et al., 2014). figure 3. gravity field quantities represented within a height reference. source: (sánchez et al., 2021). the second method consists of determining the gravity potential using geoid or quasi-geoid models. the precise regional gravity field model provides geoid undulation (np), case of a geoid, or height anomaly ( ζp) referred to the quasi-geoid (used in this paper). thus, wp value (figure 3) can be determined (sánchez et al., 2021): wp = up + ζp yq + ∆w0, (1) rewriting: − 0 − (ℎ − ) 0 , , (2) where, up is the normal potential at point p;γq the normal gravity acceleration at point q; hp is geodetic height; γqq0 is the normal gravity at the telluroid between q and q0 points. using the geoid model (np) (sánchez et al., 2021), (2) becomes: wp = w0 ₋ (hp ₋ np ) * ḡp, (3) with mean gravity acceleration ḡp (hofmann-wellenhof & moritz, 2006): ḡp = gp + 0.424 * 10-6(hp ₋ np ) + tcp, (4) using the real gravity acceleration gp and the terrain correction tcp. the vertical datum unification methodology aims to determine the difference between w0 and the local w0 local to correct the regional geopotential numbers (c0 local) . thus, is given by: wp = (w0 local+ 𝛿w) cp local , (5) 110 valéria c. silva, denizar blitzkow, flávio vaz de almeida filho, ana c. oliveira cancoro de matos, gabriel do nascimento guimarães with the potential gravity difference given by difference 𝛿w: 𝛿w = w0 ihrf+ w0 local. (6) in this latter approach, different strategies can be used to determine a wp value. usually, the fixed gbvp or the scalar-free gbvp are solved using numerical integration. recently, least-squares collocation and spherical radial basis functions are also used (sánchez et al., 2021). 3. the modeling of topographic surfaces for the rtm the computation of anomalous field elements with centimeter accuracy requires the decomposition of them into different wavelengths: long, medium, and short. the rtm technique consists of using a high-resolution dtm to compute the topographic effects to apply them in the spectral decomposition. the wavelengths are segmented with the remove-restore technique, also considering the contribution of the low frequencies of the gravitational signal, determined by a geopotential model. in the methodology described in forsberg (1984), the topographic effects are quantified using a digital terrain model of the average elevations of the terrain, called the reference dtm (forsberg & tscherning, 2008), produced according to two dtms, one of high and one of low resolution. the latter is also named irregular dtm. the model, called the reference dtm, is defined by a low-pass filter, produced according to the moving average operator corresponding to the distance over which the heights will be averaged. the dtm needs to have a resolution compatible with the spherical harmonic function coefficients of the gravitational potential from the ggm (forsberg & tscherning, 2008). in other words, the order and degree of the ggm must correspond to the spatial resolution of the dtm, thus the topography is properly quantified by the long and short wavelengths. figure 4. geometric representation of the reference dtm. source:(tziavos e sideris, 2013). the topographic effects (δartm) is determined using the expression (7), considering as approximation a rectangular prism with mass density (𝘱) of the surface rocks of the continental crust (2.67 gcm-3) and surface (1.03 gcm-3) (tziavos & sideris, 2013). δ = 2 ( − ) − − 3 , (7) where, href is the height of the reference dtm used, h is the height of the topographic masses defined by the high-resolution dtm, hp is the normal height of the point, k is the gravitational constant, l corresponds to the integration distance, and ∂x ∂y∂z is the element of differentiation for the volume. the dtm srtm15+ with a resolution of 15 arcsec, was the model chosen for the application of the rtm technique due to its availability for the area of the continent and the ocean. the srtm15+ is derived from the shuttle radar topography mission and supplemented with data from the advanced spaceborne thermal emission and reflection radiometer, known as aster mission (farr et al., 2007). in the ocean, this model has data derived from the cryosat-2 and jason-1 missions (tozer et al., 2019). as mentioned, computing the rtm requires three dtms. the irregular model, formatted in a 3’ grid, was determined in the select program, developed by forsberg & tscherning, (2008) from the high-resolution dtm, the srtm15+. the reference model was produced in the tcgrid program, based on the two previous models. the latter is a grid with the mean height, generated from a low-pass filter filtered with the moving average operator. the production of the reference dtm is dependent on the spatial resolution of the ggm whose computation depends on the choice of the degree and the order of the coefficients developed in a series of spherical harmonic functions of the gravitational potential. to compute the gravity potential, the goco05s ggm was chosen with nmax: 100 and 200 due to its composition and the well fit on the gps/leveling benchmarks of brazil (fecher et al., 2017; nicacio & dalazoana, 2018). the degree and order of the ggm can be expressed according to the expression (8), (torge & müller, 2012): ller, 2012): = 180° . (8) based on the order and degree of the chosen ggms, tests were performed in the tcgrid and tc programs forsberg & tscherning (2008). the tcgrid routine computes the reference surface according to the irregular dtm (3’) and by choosing odd values for the moving window. in the first instance, sixteen reference surfaces were computed using odd values from 1 to 31. these values determine the window’s search radius for applying the low-pass filter (table 1). to optimize, the dtmref-rtm program developed at epusp was used to determine the best resolution for each type of residual, based on the lower rms difference residuals (figure 5 and table 2). table 1. spatial resolution of the moving window. factor minutes degree km 1 3 0.05 5.6 3 9 0.15 16.7 5 15 0.25 27.8 7 21 0.35 38.9 9 27 0.45 50.0 11 33 0.55 61.1 13 39 0.65 72.2 15 45 0.75 83.3 17 51 0.85 94.5 19 57 0.95 105.6 21 63 1.05 116.7 23 69 1.15 127.8 25 75 1.25 138.9 27 81 1.35 150.0 29 87 1.45 161.1 31 93 1.55 172.3 table 2. chosen moving window for reference dtm. ggm nmax factor rms goco05s 100 21 11.84 goco05s 200 13 8.74 figure 6 shows the residual gravity disturbance before and after applying the rtm for grid 100 (a and b) and 200 (c and d), respectively. the rtm represents a gravity disturbance with respect to topography, when removing it, the residual gravity disturbance becomes independent of terrain effects. this is observed by the determination coefficient shown in this figure. the nmax:100 solution has a residuals variance of 21% dependent on the height. reducing the topographic effects, r2 results in 0.2%. whereas for nmax: 200 the residuals have a variance of 19% in relation to the height, and considering the rtm, it decreased by 0.3%. 111computation and analysis of geopotential number in são paulo, brazil 4. determination of the gravity potential 4.1 gravity potential values by gbvp solution the gravity potential was computed by solving the fixed gbvp. in this task gravity disturbances (9) were determined by gravity acceleration and coordinates (φ, λ and h) point data. they were transformed to zero-tide system according to sánchez & sideris (2017). 𝛿ɡ = ɡp yp (9) in neumann’s gbvp, the hotine kernel, also called neumann koch’s formula, is used. the disturbing potential is defined according to the expression (10) (hofmann-wellenhof & moritz, 2006): ( ) = 4 ( ) , (10) figure 5. rms of residual gravity disturbance for each factor (a: goco05s nmax: 100; b: goco05s nmax: 200. figure 6. residual gravity disturbances. with the hotine-koch h (ψ) kernel expressed by: ( ) = csc 2 − 1 + 2 = 2 + 1 + 1 ( ( )) ∞ =0 . (11) to compute (9), it is necessary a normal gravity acceleration value at a point on the physical surface. for this, the anomalia_molodensky routine, developed at epusp, was used. this program performs the computation using the strict upward continuation described in heiskanen & moritiz (1967), with the parameters of the global reference system 1980 (grs80) ellipsoid. gravity disturbances were also corrected for atmospheric effects described in wenzel (1985). in general, computing the gravity potential at a point, through numerical integration, requires data distribution in an area of 210 km. this extension offers limitations for smaller countries, regions with difficult access, and coastal areas, such as the brazilian ihrf stations: fortaleza and imbituba. 112 valéria c. silva, denizar blitzkow, flávio vaz de almeida filho, ana c. oliveira cancoro de matos, gabriel do nascimento guimarães figure 7. residual gravity disturbance (mgal) (a-using the goco05s nmax:100; b-using the goco05s nmax:200). 113computation and analysis of geopotential number in são paulo, brazil an adequate radius to determine a wp value should integrate an appropriate methodology to ensure the spectral transfer of the residual gravity field, as well to quantify and to remove the terrain effecs. (torge & muller, 2012). the harmonic coefficients of ggms, expressed in degree and order, represent the spectral resolution and can also be converted into a spatial resolution. in agreement with the expression (8), regions with gravimetric data in a radius of 210 km used a ggm up to nmax: 100. while areas of 110 km used ggm up to nmax: 200. besides, ihde et al. (2017) suggest for the ihrf framework, as well as the applications that involve high precision, it is sufficient to choose a model that contains only satellite data and that has a homogeneous approximation of the long wavelength of the earth’s gravity potential. according to the suggestion above, expressions 12 and 13 show how the residual gravity disturbance have been defined, for degree 100 and 200, respectively. the second term of the right side corresponds goco05s long wavelengths. the last term is partial short wavelengths obtained by rtm with the selected dtm (see section 3). 𝛿ɡres = 𝛿ɡter ₋ 𝛿ɡgoco05snmax:100 ₋ 𝛿ɡrtm (12) 𝛿ɡres = 𝛿ɡter ₋ 𝛿ɡgoco05snmax:200 ₋ 𝛿ɡrtm (13) the results of the remove step on the gravity disturbances are shown in figure 7. as can be seen, ggms contribute differently depending on the degree and order. in this case, the systematic component is smaller for nmax:200, compared to the use of the mgg with nmax: 100, as expected. this behavior is observed at table 3 in the residual mean. table 3. statistics of residual gravity disturbance (mgal). solution nmax: 100 nmax: 200 stations mean standard deviation (sd) rms mean sd rms sbpo -0.78 8.80 8.84 0.56 8.63 8.65 eesc 1.39 10.32 10.41 0.82 10.36 10.39 ppte 3.08 7.86 8.44 1.30 6.42 6.55 sjrp 1.92 10.34 10.52 0.67 8.80 8.82 5. international height reference system both data were interpolated using the moving average operator in a 5’x5’grid with the select software. the short wavelength components of the disturbing potential tres were determined using the hotine 5min software, developed at epusp. it computes the short wavelengths of tres according to the chosen area, using the modified hotine-koch integral. the input data are the residual gravity disturbances and the output file provides a residual disturbing potential value. the restore step was performed with the harmexp program, which belongs to the gravsoft package (forsberg & tscherning, 2008). analogously to the remove step, the model used was goco05s with the degree and order 100 and 200, according to the integration radius 210 or 110 km, respectively. the software computes the normal gravity acceleration for each station and ζggm. applying the expression 14, the tggm was obtained. ( , ) = (ℎ, , ) (ℎ, ) , (14) where tggm is the disturbing potential of the ggm and γ is the normal gravity acceleration at the given point (h,λ,φ). short wavelengths, referring to the rtm, were determined with the tc program for each station. in this step, the rtm was computed with the residual height anomaly using the same dtms as in the remove step and transformed in order to apply in the tres. the final disturbing potential was expressed by: tp = tggm + tres + trtm, (15) the normal gravity potential was defined using the potencial_ normal program developed by epusp. the routine determines up, expression (16), from the normal gravity acceleration, the geodetic height, and the grs80 parameters. = 0 + 0 ℎ ℎ (16) the provisory gravity potential (wprov), given by the sum of disturbing potential and the normal potential, was expressed by (17): wprov = up + tp (17) section 5.2.1 shows the results: potential in zero-tide (wzt) and geopotential number consistent with the mean-tide (cihrf). 5.1 disturbing potential differences of the numerical integration approach using hotine’s-koch kernel the disturbing potential for the stations obtained in three steps (5) by hotine’s methodologies is shown in table 4. it was added -1.73 m2s-2, obtained by (15), for the zero-degree term in the in the tp final values (see section 5.2.1 and table 6). the tp final values computed in the different remove-restore methodologies have a high dependence on long wavelengths. in addition, tggm values show significant differences between order and degree 100 and 200. silva (2021) studied the rms difference between height anomalies, obtained from the gps determinations on the leveling network, and the goco06s geoidal height of 86 points located in são paulo state. in the study was observed a difference of 20 cm approximately, between degrees 100 and 200. on the other hand, the homogeneous distribution of gravimetric data and the use of rtm tend to reduce this difference between solutions. the state of são paulo is characterized by topographical heterogeneities (fig.8) which can influence the ggm. the spbo station region has irregular topography of approximately 1,000 m. whereas, ppte is located in an area of homogeneous topography, compared to the spbo station, with elevations of approximately 900 m in the southern region. eesc station presents irregular topography of up to 1,500 m and the topography around the sjrp is smoother compared to other stations. table 4. residual disturbing potential (m2s-2). solution hotine nmax:100 hotine nmax:200 stations tres tggm trtm tp final tres tggm trtm tp final sbpo 1.526 -50.156 0.028 -50.328 -1.188 -48.654 0.017 -51.551 eesc -0.437 -55.908 0.039 -58.031 0.280 -60.885 0.038 -62.292 ppte -1.636 -50.397 -0.021 -53.780 -0.465 -46.576 -0.002 -48.769 sjrp -0.416 -64.854 -0.028 -67.023 0.123 -65.393 -0.006 -67.001 for the spbo station, the tmgg values differ by -1.502 m2s-2 between nmax: 100 and 200 and for tp final values, 1.223 m2s-2. in short, there is a contribution of the gravimetric and topographic components to reduce discrepancies between solutions. comparing the tp final results of eesc, a difference of 4.261 m2s-2 corresponding to 43 cm, is observed between degree and order 100 and 200. these discrepancies are related to the long-wavelength component because it has a similar difference of 4.977 m2s-2, or 51 cm. the results of tp final for ppte show that the difference between degree and order is -5.011 m2s-2. the values of tmgg used in the restore step (table 4), differ by 3.821 m2s-2 between degree and order 100 and 200. the results showed an inverse behavior increasing the discrepancies of different order and 114 valéria c. silva, denizar blitzkow, flávio vaz de almeida filho, ana c. oliveira cancoro de matos, gabriel do nascimento guimarães degree, suggesting a trend in the terrestrial gravity data or on the dtm. on the other hand, studies carried out by ribeiro et al. (2021) for the ppte, used the goco05c model developed up to nmax: 250, without application of the rtm, showed 0.470 m2s-2 or 5 cm of difference, considering the performance of the ihrf with degree and order 200. in sjrp, the results of the disturbing potential showed a discrepancy of -0.022 m2s-2, corresponding to 0.2 cm, between solutions of different degrees and order. unlike the other stations, the convergence of the results in sjrp was possible due to the small difference between the long-wavelength components of the goco05s, corresponding to 5 cm. 5.2 recovering potential gravity values from existing quasi-geoid model the multiple ways to solve the gbvp and the complexities involving kernels and gravity data availability lead to implementing the ihrf computation, using gravity field modeling through the determination and/or modification of geoid models. recently, computation strategies have been studied to separate and quantify the contributions of elements in terms of method and input data (sánchez et al., 2021). after that, the approach will be standardized to get a uniform solution using distinct methods. in this paper, a quasi-geoid model of são paulo (silva et al., 2021), which has the same database used in hotine’s methodologies, was used. the was computed using the gravsoft package (forsberg & tscherning, 2008). the approach was the numerical integration through the fast fourier transform (fft) using molodensky gravity anomaly in a 5’x5’ grid. the rcr was applied using rtm technique and the xgm2019e global gravity model truncated at degree and order 250. its validation showed an rms difference of 18 cm with 298 benchmarks with gps and normal height. the first step in recovering the height anomaly of the quasi-geoid was interpolating ζ in the ihrf proposed station, in which the geodetic coordinates are from an rbmc. through the relation 18, we had the disturbing potential: t = ζγqq0 (18) where, γqq0 is given by: ̅ 0 = 0 ∗ 1 − 1 (1 + + − 2 2 ∗ (ℎ − ) , (19) figure 8. topographic features around ihrf stations. �,h e ζ are respecting the geodetic latitude, geodetic height, and the height anomaly of the computing point. the constants parameters are the flattening (f), major semi-axis (a), physical constant and the gravity acceleration in the ellipsoid (m), for the grs80 according (moritz, 2000). using w0= 62,636,853.4 m2s-2, the wprov was obtained by (sánchez et al., 2021): wprov = w0 ₋ (hp ₋ ζp ) γqqo. (20) the used elements, in the zero-tide system, are shown in table 5. table 5. elements used in the quasi-geoid’s solution. stations ζp (m) γqqo (mgal) hp (m) sbpo -5.287 9.7868921880 803.138 eesc -6.136 9.7863172519 824.614 ppte -4.853 9.7869990406 431.075 sjrp -6.992 9.7860126043 535.933 5.2.1 permanent tide and zero-degree term iag resolution no. 1 (2015) introduces the mean-tide system to support the geodetic monitoring of geophysical phenomena governed by fluids within the system earth. for instance, the sea level change monitoring is performed in the mean-tide system, because removing the direct or indirect effects of the permanent tide misrepresents the real water motion and does not allow a reliable quantification and modeling of the sea-level change. this occurs not only in oceanographic applications but also in hydrographic and geophysical fluid studies. in other words, the global change occurs in the mean-tide system and the ihrf should provide a consistent reference in monitoring it. however, considering that gravity field modeling is not possible in the mean-tide system (because the gravity potential would not be a harmonic function), it is agreed that the data processing is performed in the tide-free or zero-tide system, and then, at the very end of the process, the ihrf coordinates are converted to the mean-tide system (sánchez et al., 2021). in this paper, the coordinates and 115computation and analysis of geopotential number in são paulo, brazil ggm are tide-free and zero-tide systems, respectively. therefore, the scheme for the determination of ihrf geopotential was computed following mäkinen (2021). the final gravity potential is obtained by: wzt = wprov + ∆witrf. (21) since the coordinates system was given in tide-free itrf2021, the correction δwitrf must be added to obtain the potential at the mean-tide = zerotide position: ∆witrf(φ) = -0.5901 + 1.7475 sin2φ + 0.0273 sin4φ, (22) the geopotential number consistent with the mean-tide is obtained by: cihrf = czt wt0. (23) where geopotential number in zero-tide system (czt) is computed by: czt = w0 + wzt (24) and gravity potential in mean-tide is: wt0 = -0.9722 ₋ 2.8841 sin2φ ₋ 0.0195 sin4φ (25) the gravity field components should be given in the geodetic reference system of 1980 (grs80). since the geocentric gravitational constant (gm) of the latest ggms differs from the grs80, it is necessary to apply the correction, called zero-degree term. thus, the gm difference is adjusted, as well as the discrepancy between the gravity potential w0 and the normal gravity potential u0, as follow: 0 = ( − 80) . − δ 0, (26) where, rp is the geocentric radial distance of the computation point, ∆w0 is the difference between the normal potential (u0) at ellipsoid for grs80 and conventional value w0 equal 62,636,853.4 m2s-2. the values for permanent tide corrections and the zero-degree term are shown in table 6. the ζ0 t (m2s-2 units) is the zero-degree term, it was added to obtain the tp final, case of hotine’s solutions (see section 5.1 and table 4). it is computed by expression 23 without γq. table 6. permanent tide corrections (m2s-2) and (m). stations wt0 ∆witrf ζ0 (m) ζ0 t(m2s-2) sbpo 0.537 -0.326 -0.176 -1.735 eesc 0.567 -0.344 -0.176 -1.735 ppte 0.563 -0.342 -0.176 -1.736 sjrp 0.609 -0.370 -0.176 -1.735 5.2.2 gravity potential (wzt ) and geopotential number (cihrf) for são paulo stations table 7 shows the gravity potential and geopotential number obtained numerical integration method with hotine’s kernel (nmax=100 and 200) and quasi-geoid (nmax=250). figures 9 and 10 show and the differences between geopotential numbers for the three solutions, respectively. table 7. gravity potential and geopotential number for hotine’s solution (m2s-2). solution hotine nmax:100 hotine nmax:200 quasi-geoid nmax:250 station wzt 100 c100 ihrf wzt 200 c200 ihrf wzt qg cqg ihrf sbpo 62,628,949.67 7,902.871 62,628,948.44 7,905.168 62,628,941.10 7,911.437 eesc 62,628,732.27 8,120.220 62,628,728.01 8,125.615 62,628,723.07 8,129.420 ppte 62,632,587.65 4,264.842 62,632,592.66 4,260.957 62,632,586.63 4,265.866 sjrp 62,631,548.67 5,303.755 62,631,548.69 5,304.950 62,631,539.96 5,312.462 figure 9. gravity potential for ihrf station (add 62,628,000.00 m2s-2 on the results). 116 valéria c. silva, denizar blitzkow, flávio vaz de almeida filho, ana c. oliveira cancoro de matos, gabriel do nascimento guimarães comparing c100 ihrf and c200 ihrf with cqg ihrf, the stations spbo and sjrp show discrepancies around -8.57 m2s-2 and -7.34 (-87 and -75 cm approximately), respectively. however, these discrepancies in eesc are -9.20 m2s-2 (94 cm) and -4.94 m2s-2 (-50 cm). for ppte, these are -1.02 m2s-2 (10 cm) and 6.03 m2s-2 (62 cm). it is worthy to mention all the final solutions have the order degree term as well they are in the same tide system, mean-tide according to mäkinen (2021) and sánchez et al. (2021). the values obtained suggest heterogeneities of the used approaches. the quasi-geoid model suffers the influence of 2 degrees beyond the area of são paulo. on the other hand, in hotine’s methodology, the integration area is smaller and with gravity coverage complete in a 5’grid. in addition, for the quasi-geoid model nmax:250 (silva et al., 2021) the voids were filled with xgm2019 nmax:2190. final remarks the efforts performed in the last years to define the ihrf and to include it on the global geodetic reference frame for sustainable development, recognized by the un, already brought important advances. countries start to update their height system to implement the ihrf and to improve their infrastructure. nevertheless, challenges remain concerning the computation methodology. this paper aimed to contribute to ihrf studies by computing the gravity potential and geopotential number in stations with adequate gravimetric measurements to evaluate the numerical integration method with hotine’s kernel and an existing quasi-geoid model. in the brazilian case, the values found for in the different removerestore methodologies have a dependence on long wavelengths. the gravitational components of the ggms showed significant discrepancies with distinct order e degree and in divergent ways for each station. considering the rms difference (20 cm) between nmax:100 and 200 with the leveling benchmarks in the state with the goco06s, the stations of eesc and ppte has wp differences up to 20 cm. the convergence between solutions with different degree and order only occurred for the sjrp station, where the difference between degree and order was 5 cm. thus, it suggests that it is acceptable to compute ihrf in an area consisting of homogeneous gravimetric data in a radius of 110 km, provided that the ggm is well representative for the area. on the other hand, ppte station presented the smallest difference (5 cm) between the quasi-geoid methodology and hotine approach with nmax: 100. the state of são paulo is one of the best regions with gravimetric coverage in south america. however, the current gravimetric coverage, dtms, and ggms are far to achieve the optimistic precision of 1 cm required for the ihrf. there will be a long way to go towards establishing a terrestrial gravimetric infrastructure and including this data in digital models. on the other hand, the justification for the variations in the magnitudes of the anomalous field in the ggm long-wavelength requires a detailed analysis of multiple factors such as the geomorphological formation and the hydrological characteristics of the region. in addition, further studies should be performed using high-resolution dtm’s and using different ggms models. references blitzkow, d., matos, a. c. o. c., guimarães, g. n. & costa, s. m. a. 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(2015). global absolut gravity reference system as replacement of igsn 71. geophysical research abstracts. https://doi.org/10.1590/s1982-21702014000100012 https://doi.org/10.1007/s10712-017-9409-3 https://doi.org/10.5194/essd-11-647-2019 https://doi.org/10.5194/essd-11-647-2019 https://doi.org/10.1007/s00190-021-01541-5 https://doi.org/10.1007/s00190-021-01541-5 https://doi.org/10.1007/s001900050278 https://doi.org/10.1109/lgrs.2021.3071254 https://doi.org/10.1109/lgrs.2021.3071254 https://doi.org/10.1007/s00190-021-01481-0 https://doi.org/10.1093/gji/ggx025 https://doi.org/10.1007/s00190-016-0913-x https://doi.org/10.1007/s00190-016-0913-x https://doi.org/10.1590/1982-2170-2020-0061 https://doi.org/10.1590/1982-2170-2020-0061 https://doi.org/10.11606/d.3.2020.tde-05042021-153203 https://doi.org/10.11606/d.3.2020.tde-05042021-153203 https://doi.org/10.1515/9783110250008 https://doi.org/10.1515/9783110250008 https://doi.org/10.1007/s00190-017-1017-y https://doi.org/10.1007/s00190-017-1017-y https://doi.org/10.1029/2019ea000658 https://doi.org/10.1007/978-3-540-74700-0_8 https://doi.org/10.1007/978-3-540-74700-0_8 _goback introduction mendeley_bookmark_dahpl6r7um mendeley_bookmark_e2eaxeh3xz mendeleytempcursorbookmark1 mendeley_bookmark_aouipth7mb mendeley_bookmark_keeu7qahhm mendeleytempcursorbookmark2 mendeley_bookmark_0tduvlujsg mendeley_bookmark_fce7gz124k study_area _hlk105351491 mendeley_bookmark_qc0aiw0eku mendeley_bookmark_3owzkcaj2u mendeley_bookmark_s4awzsumz9 mendeley_bookmark_bmujtwe7er mendeley_bookmark_5bx4ueghqw mendeley_bookmark_jt8mw0ga7h _hlk104122499 mendeley_bookmark_fj14unur16 mendeley_bookmark_vcrw83ahby mendeley_bookmark_mbeumvmmy6 mendeley_bookmark_clxopxqjoq _hlk105352232 mendeley_bookmark_3gs4vj72ta mendeley_bookmark_5oz0jyi5bm _hlk105352308 mendeley_bookmark_q8e2p8ehoy mendeley_bookmark_lahhtr13dh mendeley_bookmark_eq75r3vlm7 mendeley_bookmark_koujuqvrue the_modeling_of_topographic_surfaces_for _bookmark4 mendeley_bookmark_hslsrfcfp8 mendeley_bookmark_lfbdwswhh1 _bookmark5 mendeley_bookmark_nqveqtotvj mendeley_bookmark_zmns9l6iiv mendeley_bookmark_ieeqdrmjez mendeley_bookmark_7kyyxpkh8p mendeley_bookmark_oevjj1egfp mendeley_bookmark_oavtk82fbz mendeley_bookmark_7wvet0hgkd mendeley_bookmark_nrndvdbgkl mendeley_bookmark_qqbtmulyqc mendeley_bookmark_buk89pkbir determination_of_the_gravity_potential gravity_potential_values_by_gbvp_solutio mendeley_bookmark_clasar0w0o mendeley_bookmark_lbvvjg5wbv _hlk105352898 mendeley_bookmark_haioydjvav mendeley_bookmark_pls2ef8sjx mendeley_bookmark_lo5hdjhcrc mendeley_bookmark_xwjc2rn4x4 _bookmark17 mendeley_bookmark_5w2evcryz7 mendeley_bookmark_upkmib382m _hlk105353274 mendeley_bookmark_hhaotztuqj mendeley_bookmark_wtjiac99f9 recovering_potential_gravity_values_from mendeley_bookmark_yz1uumefxk mendeley_bookmark_y7vzvjklch _hlk104127855 _hlk105355074 mendeley_bookmark_cljx1zpesd mendeley_bookmark_avf3fvazlq mendeley_bookmark_cx3x94s7bm mendeley_bookmark_0gvnrt4qdf mendeley_bookmark_yqedta2b7k mendeley_bookmark_55mmre0yo8 permanent_tide_and_zero-degree_term _hlk105331097 _hlk104128119 final_remarks _hlk93769805 _bookmark31 _bookmark32 _bookmark33 _bookmark34 _bookmark35 _bookmark36 _bookmark37 _bookmark38 _bookmark39 _bookmark40 _bookmark41 _bookmark42 _bookmark43 _bookmark44 _bookmark45 _bookmark46 _bookmark47 _bookmark48 _bookmark49 _bookmark50 _bookmark51 _bookmark52 _bookmark53 _bookmark54 _bookmark55 _bookmark56 _bookmark57 _bookmark58 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n3.119268 earth sciences research journal earth sci. res. j. vol. 29, no. 3 (september, 2025): 333 345 g eo ph y si c s seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area, north china using ambient seismic noise data yipei tan*, ting ma, li deng, ke xu tianjin earthquake agency, tianjin, china, 300210, china. *corresponding author: oivertan921@sina.cn record manuscript received: 08/03/2025 accepted for publication: 07/09/2025 how to cite this item: tan, y., ma, t., deng, l., & xu, k. (2025). seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area, north china using ambient seismic noise data. earth sciences research journal, 2(3), 333-345. https://doi.org/10.15446/esrj.v29n3.119268 abstract it is feasible to continuously monitor the temporal changes of shallow subsurface media by utilizing ambient seismic noise. here, the authors analyzed three-year continuous records of 117 earthquake early warning stations in the tianjin area in the north china plain to explore the seasonal variation of shallow subsurface seismic velocity. they examined the daily horizontal-to-vertical spectral ratio (hvsr) curves and found that 25 stations exhibit clear seasonal variations in peak frequency and peak magnitude within the frequency range of 5-20 hz. subsequently, autocorrelation is applied to the seismic noise of these 25 stations in the 5-20 hz frequency range to calculate the relative velocity changes. ten stations show the same variation pattern in the peak frequency of hvsr curves and relative velocity changes based on autocorrelation. this pattern exhibits characteristics where the peak frequency and seismic velocity increase rapidly in november, decrease quickly in february of the following year, and change relatively little in other seasons. by comparing with meteorological data, the authors infer that the freezing of the soil layer when the temperature drops below 0°c leads to an increase in shear wave velocity and peak frequency in winter. the results suggest that studies on crustal structure detection need to consider temperature effects. keywords: seismic noise; velocity variation; hvsr method; seismic interferometry; seasonally frozen soil características de las variaciones estacionales de la velocidad sísmica del subsuelo superficial y sus factores influyentes en el área de tianjin, norte de china, utilizando datos de ruido sísmico ambiental resumen es posible monitorear continuamente los cambios temporales de los medios del subsuelo superficial mediante el uso del ruido sísmico ambiental. en este estudio, los autores analizaron registros continuos de tres años de 117 estaciones de alerta temprana de terremotos en el área de tianjin, en la llanura del norte de china, para explorar la variación estacional de la velocidad sísmica del subsuelo superficial. examinaron las curvas diarias de la relación espectral horizontal-vertical (hvsr) y descubrieron que 25 estaciones presentan claras variaciones estacionales en la frecuencia y magnitud pico dentro del rango de frecuencia de 5 a 20 hz. posteriormente, se aplicó la autocorrelación al ruido sísmico de estas 25 estaciones en el rango de frecuencia de 5 a 20 hz para calcular los cambios de velocidad relativa. diez estaciones muestran el mismo patrón de variación en la frecuencia pico de las curvas hvsr y los cambios de velocidad relativa basados en la autocorrelación. este patrón presenta características donde la frecuencia pico y la velocidad sísmica aumentan rápidamente en noviembre, disminuyen rápidamente en febrero del año siguiente y varían relativamente poco en otras estaciones. al comparar datos meteorológicos, los autores infieren que la congelación de la capa de suelo cuando la temperatura desciende por debajo de 0 °c provoca un aumento de la velocidad y la frecuencia pico de las ondas de corte en invierno. los resultados sugieren que los estudios sobre la detección de la estructura de la corteza deben considerar los efectos de la temperatura. palabras clave: ruido sísmico; variación de la velocidad; método hvsr; interferometría sísmica; suelo congelado estacionalmente; https://doi.org/10.15446/esrj.v29n3.119268 mailto:oivertan921@sina.cn https://doi.org/10.15446/esrj.v29n3.119268 334 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area 1 introduction shallow subsurface seismic velocity is crucial in the fields of seismology and earthquake engineering, as it not only serves as a foundation for accurately inverting earthquake source parameters and understanding deeper structures (dong et al., 2018; langston, 2011), but also plays a vital role in seismic site response and earthquake loss assessment (choi & stewart, 2005; bendito et al., 2014). recent studies have revealed that subsurface seismic velocity is unstable, but exhibits rapid changes during strong ground vibrations (bonilla et al., 2019; vassallo et al., 2022) and seasonal variations influenced by environmental factors such as temperature (kula et al., 2018; luan et al., 2022), rainfall (miao et al., 2018; roumelioti et al., 2020), atmospheric pressure (gradon et al., 2021; kramer et al., 2023), snow depth (wang et al., 2017), and groundwater levels (rigo et al., 2021; mao et al., 2022; qin et al., 2022; delouche & stehly, 2023). continuous seismic monitoring of shallow subsurface seismic velocity can elucidate its temporal variation characteristics and the factors influencing these variations, thereby enhancing the accuracy and reliability of seismological and earthquake engineering research outcomes. monitoring shallow subsurface seismic velocity can rely on vibration signals from natural earthquakes or active seismic experiments (niu et al., 2008; bonilla & ben-zion, 2021; luan et al., 2022). however, the sporadic nature of natural earthquakes makes it challenging to obtain continuous temporal variation data. artificial seismic sources are costly, and their excitation conditions are often constrained by human activities, particularly in urban settings. consequently, methods that monitor seismic velocity variations using seismic ambient noise have gained widespread adoption. the horizontal-tovertical spectral ratio (hvsr) method, introduced by nakamura (1989), is a straightforward and robust technique widely utilized for mapping soft sediment thickness and studying site effects (ibs-von seht & wohlenberg, 1999; qin et al., 2022; chen et al., 2023). the hvsr curve is derived from the ratio of the combined frequency spectra of horizontal components to the vertical component of seismic noise. the peak frequency of the hvsr curve correlates with the depth of geological interfaces and the average shear wave velocity between the surface and the interface (nakamura, 2019). recently, the hvsr method has been applied to study shallow subsurface seismic velocity changes across europe (lontsi et al., 2022; seivane et al., 2022), asia (chen et al., 2023; liu et al., 2024), and africa (benkaci et al., 2018). another promising method involves seismic noise correlations, which involves repeatedly extracting the green’s function through the calculation of autocorrelation or crosscorrelation of ambient noise records and measuring minor time-shifts in the green’s function to detect seismic velocity changes. by filtering noise records in different frequency bands and/or analyzing various time windows, this correlation method can monitor variations from shallow subsurface changes caused by environmental factors (james et al., 2017; feng et al., 2021; oakley et al., 2021; zhang et al., 2023) to deeper earth changes resulting from volcanic eruptions (liu et al., 2022), earthquakes (poli et al., 2020; okubo et al., 2024), and deep tectonic activities (li et al., 2021; ashruf & morelli, 2022). some researchers (tchawe et al., 2020; li & ben-zion, 2023) have combined hvsr and autocorrelation to enhance the precision and stability of their measurements. in this study, we aim to investigate the characteristics of shallow subsurface seismic velocity variation and the potential factors influencing these characteristics in tianjin area, utilizing hvsr and autocorrelation method based on data from earthquake early warning stations established in june 2021. tianjin, situated in the north china plain (figure 1), is predominantly covered by quaternary strata, with bedrock exposure limited to the northern mountainous region (tianjin institute of geological survey, 2018). the region has experienced numerous strong earthquakes (m≥7), causing significant damage to major cities in the plain. the findings of this study will provide valuable insights for ground motion simulation, focal parameter inversion, and the detection of temporal variations in deep earth media in the area. 2 data and methods 2.1 seismological and meteorological data within the study area, 117 earthquake early warning stations (refer to figure 1) were established to facilitate rapid early warning and intensity reporting following significant earthquakes. these stations record the physical quantity of acceleration continuously for 24 hours at a sampling rate of 100 points per second. these stationsstarted operation in june 2021 and continued to operation for three years until may 2024.among these 117 stations, two are situated in the northern exposed bedrock area, where there is no overlay of soft soil. the remaining 115 stations are founded on sedimentary soil. this paper will utilize the three-component seismic waveforms recorded by these 115 stations over the past three years. figure 1. distribution of seismic and meteorological stations in study area. different colors indicate the age of the surface covering soil layer or rock exposure. ncp signify north china plain. we have access to the recorded data from 12 meteorological observation stations within the study area, with their site distribution depicted in figure 1. each of these stations records temperature, air pressure, and precipitation continuously for 24 hours at a sampling rate of 1 point per minute. the meteorological data will be employed for comparative analysis with the observed variations in seismic velocity. 2.2 hvsr we employ the following formula to calculate the hvsr curve daily at each station: hvsr( ) = ((e( )2+(n( )2)/2 z( ) (1) where f denotes the frequency, and z(f), e(f) and n(f)represent the fourier spectra of the vertical, east-west and north-south component seismic records, respectively. the continuous waveform data for a 24-hour period is initially processed by removing the mean and trend. we check for any broken or missing data points and mark their locations. subsequently, we calculate the median absolute deviation (mad) for the 24-hour waveform and flag data points exceeding 5 times the mad, along with the 50 data points preceding and following them, which may be affected by human activities near the station, causing significant jumps. we opt not to use the sta/lta algorithm for earthquake detection for two reasons. first, some studies indicate that the hvsr curve derived from ambient noise is similar to that calculated from seismic records (nakamura, 2019; yassminh et al., 2019), rendering it unnecessary to filter out earthquakes when computing the hvsr curve (rigo et al., 2021). second, over the threeyear period, there were only 31 earthquakes with a magnitude above 1.0 in the study area, averaging less than one per month. the low seismic activity means that seismic records have minimal impact on the hvsr curve. the continuous 24-hour recorded waveform is segmented into 40.96-second lengths (4096 sampling points) with a 50% overlap. with 8,640,000 sampling points per day, this results in 4,218 segments. any segment marked with issues from the previous steps is discarded. if fewer than 20 335yipei tan, ting ma, li deng, ke xu segments remain for the day, the hvsr curve for that day is not computed. we then apply a cosine taper to each data segment and use the fast fourier transform (fft) to calculate the two horizontal and one vertical spectrums. the average of the horizontal and vertical frequency spectra for all segments over the 24-hour period is calculated separately, and the division of the horizontal by the vertical average frequency spectra yields the 1-day hvsr curve. this curve is smoothed using the konno-ohmachi logarithmic window (konno & ohmachi, 1998) with a b-value of 30 in the frequency range of 0.05-40hz, resulting in the 1-day hvsr curve. la rocca et al. (2020), la rocca and chiappetta (2022) suggest that the hvsr curve may vary due to daily temperature and noise source differences, with seismic noise being more stationary during night hours. we tested calculating hvsr curves using data from 0:00-6:00 and found no significant difference in temporal variation characteristics. since the meteorological data for comparison with seismic velocity changes is the daily average temperature, we calculate the average hvsr curve using ambient noise from the full 24hour period. geological survey results (tianjin institute of geological survey, 2018) indicate that the thickness of quaternary sediments in the study area ranges from about 40 meters in tectonically uplifted areas to 473 meters in tectonically depressed areas. the holocene stratum, which is 17-24 meters thick and consists of loose soft soil deposits, is divided into three sections based on lithology. based on the empirical relationship between the burial depth of holocene sediments and the peak frequency of the hvsr curve in the north china plain, h=94.76×fr -1.12 (qin et al. 2023; peng et al. 2019), the peak frequency corresponding to the bottom interface of holocene sediments should be below 4.5hz. therefore, we select a frequency range greater than this value. to analyze the variation of seismic velocity in holocene sediments, we focus on peak frequencies greater than 4.5hz. considering the instability of the hvsr curve at frequencies higher than 20hz (james et al., 2017), we select stations with hvsr curves peaking in the 5-20hz range. by extracting the peak frequency and amplitude daily within this range, we can track the changes in shallow subsurface seismic velocity. 2.3 autocorrelation we estimate the green’s function for each station using the software msnoise (lecocq et al., 2014). our workflow begins by integrating the acceleration waveforms into velocity records, followed by the removal of the mean and trend, and then tamper with the 1-day waveform. since the autocorrelation calculation is performed at a single station, the instrument response has not been removed. subsequently, followed by one-bit normalization and spectrum whitening, we compute the daily autocorrelation of the 3-components within the 5-20hz frequency band every 30 minutes and superimpose the 48 30-minute autocorrelation results to form the daily green’s function. by linearly superimposing the daily green’s functions over three years, we obtain the reference green’s function. to improve the signal-to-noise ratio, we apply a 5-day stacking for each day, covering two days before and two days after, resulting in a 5-day-stacked green’s function. we employ the stretching method (sens-schöfelder & wegler, 2006) to estimate the changes in seismic velocity over time. for the i-th day, gi(t) is extracted from the 5-day-stacked green’s function after 100 times interpolation within the time window from -0.4s to 0.4s. the sampling point interval of gi(t) is 0.0001s, with a total of 8001 sampling points. we resample the reference green’s function at an interval of 0.0001ε, and take 8001 sampling points around the zero point to obtain gstack(t). the cross-correlation cci(ε) of gi(t) and gstack(t) is then calculated. during the calculation process, ε ranges from 0.5 to 1.5. when the cross-correlation cci(ε) reaches its maximum, ε is the stretching coefficient of the i-th day, and 1-ε represents the relative change in seismic velocity for the i-th day compared to the reference green’s function. the uncertainty δ is calculated using the following formula (weaver et al., 2011): δ = 1−x2 2x 6 π 2t0 ω2(t2 3−t1 3) (2) where x is the maximum cross-correlation, ω is the angular frequency, t0 is the central period of the reference green’s function, and t1 and t2 are the start and end times of the measurement window, respectively. 3 results 3.1 seasonal variations of hvsr peak frequency we calculated the daily hvsr curves for 115 stations over a period of three years and then averaged these daily curves to obtain the 3-year-average hvsr curves for each station. among these, two stations showed no significant peak values between 0.1 and 40 hz in their 3-year-average hvsr curves (e.g., wak as shown in figure 2). of the remaining 113 stations, 106 exhibited peaks in the 0.5-2 hz range, and 81 displayed peaks within the 5-20 hz range. upon examining the resonant frequency of the 3-year-average hvsr curves, we found that 91 stations, accounting for 79.1% of the total, had resonant frequencies in the 0.5-2 hz range (e.g., ax001 and kb001 in figure 2). this result is largely consistent with cao et al. (2018), who reported a resonant frequency of approximately 1 hz for hvsr curves derived from tremor data in the northern part of our study area. although 81 stations had peaks in the 5-20 hz range, only six stations had resonant frequencies in this range (e.g., sjt in figure 2), with most other stations showing resonant frequencies in the 0.5-2 hz range and a smaller amplitude peak in the 5-20 hz range (e.g., kb001 in figure 2). figure 2. 3-year-average hvsr cures of four stations represent four types wak represents no significant peak in 0.1-40 hz, ax001 represents the resonant frequency is about 1hz and no significant peak in 5-20 hz, kb001 represents resonant the frequency is about 1hz and have significant peak in 5-20 hz, sjt represents the frequency is in 5-20 hz and have a peak about 1hz. 336 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area taking fj001 station as a case study, we present the temporal changes in the hvsr curve (figure 4). initially, we arranged the daily hvsr curves and used color to represent the amplitude of the curves (figure 3a). it was observed that from june 2021 to may 2024, the resonant frequency of fj001 remained around 1 hz, while its peak amplitude exhibited seasonal variations. the peak amplitude notably decreased at the end of one year and increased at the beginning of the next. above 4 hz, the overall amplitude of the hvsr curve decreased in winter. a small peak in the 5-20 hz frequency band (pink box in figures 3a and 3c) also showed a significant decrease in amplitude during winter. we extracted the peak frequency and amplitude from the daily hvsr curves within the 5-20 hz range, and their temporal changes are depicted in figure 3b, both of which displayed clear seasonal variation characteristics. from spring to autumn each year, the peak frequency was generally stable at around 10 hz but rose rapidly to 14-16 hz in november and dropped back to about 10 hz by february of the following year. the peak amplitude varied inversely to the peak frequency, with small variations from spring to autumn but a rapid decrease in november and increase in february. compared to summer, the peak frequency in winter increased by about 40% to 60%, while the peak amplitude in summer was reduced to about 40%. upon reviewing the temporal variation results of hvsr curves at all 115 stations, we categorized them into four groups. (1) 24 stations could not calculate the hvsr curve for more than 200 consecutive days due to data scarcity, significant interference from surrounding human activities, substantial changes in the hvsr curve shape over three years (potentially due to station maintenance, instrument replacement, or environmental changes), or the absence of a clear peak in the 3-year-average hvsr curve. these stations were not suitable for analyzing seasonal variation characteristics. (2) the 3-yearaverage hvsr curves of 26 stations lacked a clear peak in the 5-20 hz range, precluding analysis of seasonal variation in this range. (3) the 3-year-average hvsr curves of 40 stations had clear peaks in the 5-20 hz range but showed no significant seasonal variation in peak frequency or amplitude. (4) the 3-yearaverage hvsr curves of 25 stations had clear peaks in the 5-20 hz range, with significant seasonal variations in peak frequency or amplitude. given the aim of this study is to investigate the characteristics of seasonal variations, we focused our analysis on the fourth category. the temporal variation results of peak frequency and amplitude for the 24 stations in the fourth category, excluding fj001 (as shown in figure 3), within the 5-20 hz range are displayed in figure 4. despite minor differences in individual stations, such as the change range of kb001, nd001, and ygz stations during the winter of 2023-2024 being smaller than in the previous two years, the overall variation characteristics of 21 stations were consistent with those of fj001 station in figure 3. in total, 22 out of the 25 stations exhibited similar variation characteristics: a rapid increase in peak frequency in november and a decrease in february of the following year, and a rapid decrease in peak amplitude in november followed by an increase in february of the following year,with the exception of ch001, gs001, and gn001. figure 3. temporal variation of h/v ratio at station fj001 for the 3 years (a) temporal variation of the hvsr curves for the 3 years, (b) temporal variation of the peak frequency and peak amplitude in the frequency range of 5-20hz, (c) 3-year-average hvsr curve. the pink box in (a) and (c) represent 5-20hz. 337yipei tan, ting ma, li deng, ke xu figure 4. temporal variation of the peak frequency and peak amplitude in the frequency range of 5-20hz of 25 stations whose 3-year-average hvsr curves have obvious peaks in 5-20 hz. 338 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area 3.2 velocity changes based on autocorrelation for the 25 stations exhibiting clear seasonal variations in their 3-yearaverage hvsr curves, we employed the autocorrelation method to estimate seismic velocity changes within the 5-20hz frequency range.figure 5 illustrates the 5-day-stacked and reference green’s functions at station mg001. the reference green’s function in the e component (figure 5a) closely resembles the n component (figure 5b), but differs from the vertical component (figure 5c). we superimposed the 5-day-stacked green’s functions for july, representing the highest temperatures (summer), and january, representing the lowest temperatures (winter), over the three-year period. in the two horizontal component results (figures 5d and e), the summer green’s function (red curve) is similar to the reference green’s function (black curve), with corresponding peaks and troughs of several sidelobes appearing at similar times, and the amplitude of the summer green’s function is slightly larger than that of the reference. however, the amplitude of the winter green’s function (blue curve) is significantly smaller than that of the summer and reference green’s functions. as the absolute value of the time offset increases, the difference in arrival times between the peaks and troughs of the winter and summer green’s functions becomes more pronounced. in the vertical component results (figure 5f), the main lobes of the summer, winter, and reference green’s functions are almost completely coincident, with the sidelobes’ amplitude decaying rapidly. there is a significant difference in the horizontal component results between summer and winter peaks and troughs, but little difference in the vertical component results, a pattern common to other stations in our analysis. therefore, we decided to use the horizontal component green’s functions to calculate seismic velocity changes. figure 6 presents the seismic velocity variation results at station mg001. the maximum correlation coefficient, obtained by cross-correlating the 5-daystacked and reference green’s functions (figure 6a), decreases in winter but remains above 0.95 in other seasons. the stretch coefficient is determined when the correlation coefficient is at its maximum; if it exceeds 1, the 5-day-stacked green’s function is compressed on the time axis, and if it is less than 1, it is stretched. the seismic velocity change results, based on the conversion of the stretch coefficient, are shown in figure 6b, and figure 6c displays the velocity change error calculated using formula (2). the 3-year velocity variation results for mg001 station indicate that seismic velocity increases rapidly at the end of each year and decreases rapidly at the beginning of the following year. the seismic velocity in winter is 25% to 35% higher than in other seasons. the velocity change error in winter is also larger, approximately 1% in winter and about 0.5% in other seasons, with individual days ranging from 2% to 3%. this error level is significantly smaller than the winter velocity changes, making the seasonal variation pattern of seismic velocity in figure 6c reliable. figure 5. the 5-day-stacked and reference green’s function at the station mg001 (a)-(c) show the 5-day-stacked and 3-year-stacked green’s function of e, n and z component respectively, (d)-(f) show the comparison of green’s functions stacked in july and stacked in january figure 6. results of the seismic velocity variation at station mg001 (a) represents changes of autocorrelation coefficient calculated by 5-days-stacked and reference green’s function, (b) represents velocity changes (dv/v), (c) represents the velocity change error. 339yipei tan, ting ma, li deng, ke xu figure 7 displays the velocity variation results for the 24 stations, excluding mg001 shown in figure 6. the black dots represent relative velocity variation (dv/v), and the gray vertical lines indicate the error bars. five stations (gn001, gs001, pm001, qd002, xd003) show no periodic pattern in their velocity variations. the velocity of the other 19 stations exhibits some degree of seasonal variation characteristics. for these 19 stations, we compared the peak frequency variation in the 5-20hz range of hvsr curves (figure 4) with the velocity variation based on autocorrelation (figure 7). the velocity variation at ch001 station consistently decreases from winter to summer and increases from summer to winter, aligning with the variation pattern of the hvsr peak frequency, but it is not the pattern discussed in this paper. the wave velocity at qh001 station shows a pattern of decreasing in winter and being relatively high in other seasons, while px002 station experiences a sudden rise in a few days during winter followed by a fall to the baseline level, inconsistent with the hvsr peak frequency variation characteristics. although the seismic velocity at hbt, nd001, and nw001 stations increased in winter, the wave velocity fluctuated greatly, and the error bars indicate large errors in the winter calculation results. the wave velocity at hd001, nh001, and qm001 stations did not significantly increase in winter during 2021-2022, contrasting with the noticeable increase in hvsr peak frequency that year. the wave velocity at dul station increased in winter but changed greatly from summer to autumn in 2023, with the fluctuation amplitude being essentially the same as that of the winter wave velocity change. by comparison, the seasonal variation pattern of the peak frequency for 10 stations is largely consistent with the velocity variation pattern (cad, fj001, jih, kb001, mg001, pw001, qb001, qc001, wy001, ygz), all of which show the characteristics of seismic velocity increasing rapidly in november, decreasing rapidly in february of the following year, and changing relatively little in other seasons.this consistent variation pattern indicates that the velocity variation results for these 10 stations are reliable and will be used in the subsequent comparison with meteorological data. figure 7. velocity variations at 24 stations except station mg001 showed in figure 6 340 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area 4 comparison with meteorological data 4.1 temperature correlation we compared the velocity variations of the 10 stations identified in the previous section with the daily average temperature recorded by the nearest meteorological observatory (figure 8). the comparison results are depicted in figure 9. the variation patterns of the peak frequency of hvsr and velocity based on autocorrelation are similar in the winter of three consecutive years. it is observable that the timing when the daily average temperature falls below 0°c aligns with the onset of rapid increases in the peak frequency of hvsr and seismic velocity in november. some stations, such as cad and pw001, exhibit a velocity increase 1 or 2 days prior to the temperature drop below 0°c, possibly due to the use of 5-day-stacked data in autocorrelation calculations. conversely, the time when the peak frequency and velocity revert to summer values lags significantly behind the temperature rise above 0°c. this lag cannot be attributed to the 5-day-stacked data in autocorrelation, leading us to infer that the pronounced seasonal increase in seismic velocity during winter may correlate with the freezing of the soil layer as temperatures plummet below zero. the initiation of this increase coincides with the temperature drop below 0°c, while the conclusion is delayed relative to the temperature rise above 0°c, indicating a temporal lag. figure 8. distribution of seismic and meteorological stations for velocity variation and temperature comparison black triangles represent seismic stations, red dots represent meteorological stations. miao et al. (2019) propose that the velocity changes associated with seasonal soil freezing are governed by cumulative sub-zero temperatures and current temperatures, which can account for the lag in soil wave velocity change relative to winter temperature increases. they introduced a weighted cumulative freezing degree day to encompass these factors and presented an empirical model relating velocity changes to the weighted cumulative freezing degree days. on the nth day following the average daily temperature drop below 0°c, the weighted cumulative freezing degree day sw(n) is expressed as: (3) where t1(n) is the daily average temperature on the nth day, p is the number of days with daily average temperatures below zero. h is the heaviside step function (zero for negative arguments, one for nonnegative), andk is an empirical constant related to local site conditions. the relationship between sw and velocity change dv/v can be formulated as (4) where γ is the lower threshold for to be considered in the fitting, and ra is an empirical constant related to the shear wave structure of the sedimentary layer. ra ₌ α(1−1 λ) τ vtop , with vtop representing the seismic wave velocity of the soil layer, α the empirical coefficient linking freezing depth to accumulated sub-zero temperature, λ the ratio of seismic wave velocity change post-soil freezing in winter, and τ the time for seismic waves to propagate through the frozen soil layer. employing this model, we attempted to establish a quantitative relationship between temperature and velocity changes. eight stations were included in the calculation; fj001 was excluded due to poor similarity between peak frequency and velocity change results in the first winter, and the significant distance of ygz from the nearest meteorological observatory. we averaged the daily average temperatures from june 2021 to may 2024 to derive the average annual temperature curve and the velocity changes calculated in section 3.2 to obtain the annual average velocity variation curve. the fitting calculations based on formulas (3) and (4) required determining the empirical constants k, γ, and ra for each station, which vary significantly due to site conditions. lacking data on the seismic velocity structure and winter frozen soil depth under each station, we referred to miao et al. (2019) and used grid search to identify these parameters, minimizing the least squares difference between the theoretical fitting curve and the velocity change results. the parameter k was varied in 0.2 steps from 0 to 6, γ in 1 steps from 0 to 41, and ra in 0.01 steps from 0.01 to 5.00 to fit the amplitude of velocity change. this model only considers periods when temperatures fall below zero, so we focused on significant winter velocity increases, excluding other seasons. to align the velocity change results, we adjusted them so that the initial speed change value at the start of the fitting period was set to 0. the fitting results are presented in figure 10. except for jih and qb001, the winter average velocity variations of the other six stations exhibit a “double peak” shape, similar to steinmann et al. (2021). the velocity variation curves based on the model (red) for cad, kb001, mg001, pw001, qc001, and wy001 fit this “double peak” pattern well. although some short-term fluctuations are not perfectly captured, the model curves for jih and qb001 also align well with the overall annual average velocity variation pattern. 4.2 comparison with precipitation in addition to temperature, we have observed that velocity changes at individual stations are also influenced by precipitation. figure 11 presents a comparison between the velocity changes at the cad station, the peak frequency of the hvsr curve, and the precipitation data from the nearest meteorological observatory. the study area is characterized by a subtropical monsoon climate, with intense rainfall occurring predominantly in july and august, and minimal rainfall during the winter months. from june 2021 to may 2024, there were a total of eight days when daily precipitation exceeded 50mm. on these heavy precipitation days and in the subsequent days, seismic velocity dropped rapidly (figure 12a). an exception was observed on july 12, 2023, when precipitation reached 73.5mm, yet the velocity did not decrease significantly. the maximum daily precipitation over the three-year period was 101.6mm on july 30, 2023, the autocorrelation results indicated a seismic velocity decrease of approximately 5.6% in the following five days. on july 29 and 30, 2021, rainfall exceeded 60mm for two consecutive days, leading to a seismic wave velocity decrease of about 4.5% in the subsequent five days. in 2022 and 2023, the peak frequency of the hvsr also decreased on several occasions when daily precipitation exceeded 50mm (figure 12b). however, the amplitude of the peak frequency decrease was not significantly different from its fluctuation amplitude outside of winter, suggesting that the rapid decline in peak frequency following heavy precipitation was not substantial. 341yipei tan, ting ma, li deng, ke xu figure 9. comparison of daily average temperature (blue), hvsr peak frequency (black) and relative velocity variation (red). 342 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area figure 11. comparison of precipitation with relative velocity variation (a) and hvsr peak frequency (b) at station cad. figure 10. annual average velocity variation (black) and fitting result calculated based on formulas 3 and 4. figure 12. comparison the range of relative velocity changes in different time stacking of station kb001 (a) and cad (b). 343yipei tan, ting ma, li deng, ke xu roumelioti et al. (2020) observed a sharp decrease in the shear wave velocity of shallow sediments during the rainy season. miao et al. (2018) discovered that significant velocity changes can be observed when rainfall exceeds a certain threshold, with no noticeable changes when it falls below this threshold. comparing these findings with the results from station cad in this study, a daily precipitation threshold of 50mm can be identified. in most cases exceeding this threshold, a significant decrease in velocity is observable, whereas it is challenging to detect a rapid decrease in velocity when it is below the threshold. it is also possible that minor velocity changes are obscured by fluctuations in velocity changes caused by temperature and calculation errors, making them difficult to discern. we additionally tested for correlations between velocity variations and air pressure data but did not find any relationship between the patterns of velocity variations and air pressure variations. 5 discussions 5.1 velocity variation range numerous studies have reported seasonal relative velocity variations due to soil freezing, utilizing cross-correlation or autocorrelation of seismic noise. for instance, gassenmeier et al. (2015) observed changes of several tenths of a percent in the 1.0-3.5 hz frequency range in germany. james et al. (2017) noted up to a 10 percent variation between summer and winter in the 13-17 hz frequency range in america. miao et al. (2019) recorded changes from about 1% to 9% across a frequency range from 1 to 13 hz in japan. lindner et al. (2021) reported about 3% seasonal velocity variations in the 0.1-25 hz frequency range in germany. liu et al. (2024) found about 8% variations in the 4-12 hz frequency range in china. cheng et al. (2022) observed 15% shearwave velocity reductions using fiber-optic seismic networks. in comparison, the seismic velocity variation range obtained in this study is broader, exceeding 5%, with the largest variation range surpassing 20%. the study areas of miao et al. (2019) and liu et al. (2024) are further north and experience longer durations of sub-zero temperatures than ours, suggesting that the broader variation range in our results is not solely due to lower temperatures. the difference in stacking time when calculating daily green’s functions could be one reason for the varying ranges of velocity variation. james et al. (2017) discussed the impact of stacking time on the variation range, finding that longer day-stacks yielded the smallest velocity reduction, while 1 and 2-day stacks yielded the largest reduction. we selected two stations, cad and kb001, and calculated velocity changes using daily green’s functions with 5-day, 15day, and 30-day stacking. the results (figure 12) indicate that the maximum velocity change amplitude decreases with increased stacking time, with the 5-day stacking results showing a larger maximum change amplitude than the 30-day stacking results over three years for both stations. particularly during the winter of 2022-2023, the maximum change amplitude of the 5-day stacking results was approximately twice that of the 30-day stacking. the frequency band may also account for the differences observed. gassenmeier et al. (2015), who used the lowest frequency range in the aforementioned studies, reported the smallest velocity variation range. other research has noted greater variation ranges in higher frequency bands. in this paper, we used a 5-20 hz filter when calculating green’s functions, hvsr curves characteristics, which peak in the 5-10 hz range in summer and 10-20 hz in winter. by employing the same frequency band, the variations in peak frequency and seismic velocity reflect almost the same depth interval in the soil. the most dramatic changes in hvsr curves between winter and other seasons occur in the 5-20 hz frequency range, which may lead to the broader variation range observed in our autocorrelation velocity change results. laboratory measurements have shown significant velocity changes (>75%) between frozen and thawed soil (zimmerman & king, 1986). none of the aforementioned studies captured such substantial changes. james et al. (2017) argue that their results only recovered 2-15% velocity decreases because only a portion of the wave energy traveled through the frozen layer. our use of a broader frequency band may enable our results to encompass velocity changes within the 5-20% range. chen et al. (2023) demonstrated that the peak frequency of the hvsr curve increased by 3% to 35%, and the amplification factor decreased by 11% to 38%. kula et al. (2018) also showed that soil freezing significantly impacts the high-frequency part of the hvsr curve, with peak amplitudes notably greater in summer than in winter. köhler and weidle (2019) observed an hvsr peak emerging at the beginning of summer and gradually shifting to lower frequencies in norway. these results are consistent with the seasonal variation characteristics of the hvsr curve found in our study. the reduction in site amplification factor due to soil freezing in winter may imply that the amplification effect of sediment on seismic waves is diminished when temperatures drop below zero. consequently, the amplitude of ground motion caused by the same source in the higher frequency band (5-20 hz) is smaller than in other seasons, potentially alleviating earthquake disasters in winter. 5.2 velocity variation characteristic gassenmeier et al. (2015) and james et al. (2017) observed a sharp increase in seismic velocity in early winter, followed by a drastic decrease in velocity in later spring. miao et al. (2019) and liu et al. (2024) noted a smoother velocity rise during the winter months. in this study, we observed a sharp increase in early winter and a decrease in later spring, but with a distinctive “double peak” pattern in the winter velocity variations. there are two potential reasons for this “double peak” shape (figure 10). first, the daily average temperature in winter does not remain consistently below 0°c but fluctuates around zero. as illustrated in figure 9 of steinmann et al. (2021), temperatures rise above 0°c after falling below it for a period, then drop again, leading to the weighted cumulative freezing degree day sw(n) decreasing and then increasing, thus forming a “double peak” in seismic velocity change. second, the temperature change pattern varies each year. the time for winter temperatures to fall below 0°c in 2022-2023 is significantly later than in the previous two years. the lowest winter temperature in 2022-2023 appears in the second half of winter (end of february), whereas in 2023-2024 it appears early in winter (beginning of january), leading to different peak times for seismic wave velocity changes in these years (this phenomenon is more pronounced at pw001 station). when the velocity changes are averaged over three years, a “double peak” shape emerges. compared to miao et al. (2019), who studied velocity changes due to the soil freeze-thaw cycle in hokkaido, japan, there is a notable difference in our results. miao et al. (2019) found that after the temperature dropped below 0°c, velocity slightly increased before falling back, then undergoing a significant change. they interpreted this as the release of heat by water in the soil layer as it froze, keeping the soil layer temperature around 0°c for a short time. in our results, this minor velocity increase is not common but can be observed at station qc001 (figure 10). seismic velocity at other stations rises rapidly after the temperature falls below 0°c. careful comparison of the temperature changes at qc001 with the minor velocity increase reveals that the temperature does not drop monotonically at the beginning of winter but fluctuates around 0°c, rising to 0°c after a few days below, then dropping again. the timing of these temperature fluctuations matches the slight velocity increase. steinmann et al. (2021) also observed multiple peaks in seismic velocity variation due to temperature fluctuations around 0°c. we speculate that these minor velocity increases may be caused by such temperature variations. kula et al. (2018)using the hvsr method to study periodic variations in frozen soil characteristics at the finnish polar research station, compared ground temperature observations at different depths with the hvsr curve’s peak value and found good agreement between the 1m underground temperature variation and the hvsr curve’s peak value. the 1m underground temperature pattern is similar to air temperature but lags in phase, suggesting a lag in the hvsr curve’s peak value change relative to air temperature. since the hvsr curve’s peak frequency change closely mirrors velocity changes, it can be inferred that velocity changes also lag air temperature. this lag may adversely affect the fitting results of velocity variations and weighted cumulative freezing degree days based on air temperature (figure 10). in our study area, we have not found underground temperature observation data at several meters depth, preventing a quantitative evaluation of the underground temperature lag relative to air temperature. we plan to conduct low-temperature gradient measurements at different depths to analyze the relationship between ground temperature changes and seismic wave velocity changes in the soil layer. in this paper, the autocorrelation function obtained from the horizontal component shows significant differences between winter and summer, a difference not easily observed in the vertical component results (figure 5). 344 seasonal variations characteristics of shallow subsurface seismic velocity and its influencing factors in tianjin area steinmann et al. (2021) noted this difference and suggested it was due to the increase in love wave velocity caused by soil freezing in winter being greater than that of rayleigh waves. chen et al. (2023) found that seasonal frozen soil increased the resonant frequency of the horizontal component of the ground motion spectrum but had no significant effect on the vertical component. therefore, in areas where temperatures drop below zero in winter, horizontal component seismic records should be used to analyze temporal variations in seismic velocity. the seismic velocity of soft soil sediment increases significantly when temperatures fall below 0°c, and the hvsr curve’s shape also changes markedly below 0°c. therefore, whether calculating the hvsr curve from field ground tremor observations or detecting shallow subsurface structures based on ambient seismic noise from dense arrays, observations should be avoided when temperatures are close to or below 0°c. 6. conclusions this paper presents an analysis of shallow subsurface seismic velocity variation characteristics in tianjin, in north china plain, based on three years of continuous seismic waveform records. utilizing the horizontal-to-vertical spectral ratio (hvsr) and autocorrelation methods, we have investigated the possible factors influencing these variations. the primary conclusions are as follows: 1. at 25 stations within the study area, the hvsr curves exhibit seasonal variation in the 5-20 hz frequency range, with an increase in peak frequency and a decrease in peak amplitude during the winter months. correspondingly, the seismic velocity calculated using the autocorrelation method at 10 stations also shows an increase during winter. 2. by comparing the peak frequency and seismic velocity variation at 10 stations with temperature records, we find that the seasonal variation patterns of peak frequency and seismic velocity are closely aligned. the rapid increase in november corresponds to the period when the daily average temperature falls below 0°c, and the subsequent decline in february corresponds to when temperatures rise above 0°c. temperature is deemed the primary factor driving the seasonal variation in velocity. 3. employing an empirical model that relates velocity variation to weighted cumulative freezing degree days, we find that the model curve effectively captures the overall pattern of annual average velocity variation. we infer that the increase in shear wave velocity and peak frequency in the study area during winter is due to the freezing of the soil layer. 4. the seismic velocity of the shallow subsurface undergoes significant changes in winter compared to other seasons. when detecting deep underground structures, the impact of temperature on the reliability of the results should be taken into consideration. acknowlegments this work is supported by tianjin science and technology foundation (no. 22jcybjc00100 & 23jcybjc00500). references ashruf, t. n., & morelli, a. 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(2023). estimation of the sediment thickness beneath the southern benue trough in nigeria by using gravity and borehole data. earth sciences research journal, 27(1), 47-57. https://doi.org/10.15446/esrj.v27n1.105459 record manuscript received: 24/10/2022 accepted for publication: 10/04/2023 keywords: bouguer gravity maps; gravity inversion; sediment thickness; regional-residual gravity separation; gravimetric interpretation. abstract mapping the sediment-basement morphology represents a crucial part of the geophysical exploration of oil, gas, and mineral resources. in this study, authors estimated the sediment thickness in the area covering the southern benue trough in nigeria and parts of the cameroon volcanic line using a high-resolution tailored gravity model together with sediment and bedrock samples taken from 113 logged boreholes. a 3-d inversion of the residual gravity data is done by applying a recently developed time-efficient gravity inversion software to determine a 3-d depth structure of the basement relief after regional-residual gravity separation. the estimated depths of the sediment-basement based on a 3-d gravity data inversion are compared with the measured sediment thickness data from drilling profiles to optimally select the gravity inversion parameters, particularly the mean sediment depth, and density contrast. our numerical result indicates that the sediment thickness within the southern benue trough study area and parts of the cameroon volcanic line vary from 0.8 to 5.5 km, with a prevailing southwest trend of increasing sediment thickness while decreasing eastward. this trend closely mimics the known geological structure of the study area and generally agrees with localized estimates of the sediment thickness from previous studies. moreover, the bouguer gravity map of the study area exhibits a spatial pattern that indicates an existence of a high-density material, which could have led to the shallowing of sediments over and along the abakaliki anticlinorium. based on these findings, the authors argued that these magmatic structures could influence hydrothermal fluid migration and might have entrapped enough sediments suitable for the maturation of mineral resources over time. resumen cartografiar la morfología de un basamento sedimentario representa una parte determinante en la exploración de recursos de petróleo, gas y minerales. en este estudio los autores midieron el espesor de los sedimentos en el sur del valle de benue en nigeria y en partes de la línea volcánica de camerún con un modelo gravitacional a medida de alta resolución y a partir de muestras sedimentarias y de lecho de roca tomadas en 113 pozos de perforación. se realizó una inversión 3d de la información residual de gravedad al aplicar un software desarrollado recientemente de inversión gravitacional, eficiente en tiempo, para determinar tridimensionalmente la estructura profunda del basamento después de una separación regional residual gravitacional. las profundidades estimadas de los sedimentos del basamento con base a la inversión gravitacional 3-d fueron comparadas con la información del espesor de los sedimentos relacionada en el perfil de perforación para seleccionar óptimamente los parámetros de inversión gravitacional, particularmente la media de la profundidad de los sedimentos y la densidad del contraste. los resultados numéricos indican que el espesor de los sedimentos en el área de estudio del valle de benue y partes de la línea volcánica de camerún varían desde 0.8 a 5.5 kms., con una tendencia en el suroeste donde se incrementa el espesor, mientras decrece hacia el este. esta tendencia concuerda relativamente con la estructura geológica conocida del área de estudio y generalmente coincide con los estimados del espesor de los sedimentos de estudios previos. además, el mapa de anomalías gravimétricas de bouguer en el área de estudio presenta un patrón espacial que indica la existencia de un material de alta densidad, lo que podría haber llevado los sedimentos a niveles someros sobre y a lo largo del anticllinal de abakaliki. con base en estos hallazgos, los autores sostiene que estas estructuras magmáticas podrían influenciar la migración de fluidos hidrotermales y podrían haber encapsulado suficientes sedimentos apropiados para la maduración de recursos minerales. https://doi.org/10.15446/esrj.v27n1.105459 https://doi.org/10.15446/esrj.v27n1.105459 48 ojima isaac apeh, robert tenzer, luan thanh pham, zimuzo hedinkpa ozuah 1. introduction the sediment bedrock morphology is an important geophysical interface for exploration of oil, gas, and mineral deposits. under no compaction, a sedimentary cover is generally associated with negative gravity anomalies due to a low density of sediments in comparison to a surrounding rock density (granser, 1987). geophysical methods are often used to study the structure of sediment cover and underlying bedrock, most notably by applying seismic, gravimetric, and aeromagnetic methods as well as from drilling profiles. the possibility of exploring mineral resources in the southern benue trough (bt) in nigeria has been of a high geological/geophysical interest over the last few decades. this has led to several aeromagnetic studies (e.g., adebiyi et al., 2020; obiora et al., 2018; abdullahi et al., 2019; opara et al., 2015; oha et al., 2016; obi et al., 2010; ofoegbu and onuoha, 1991; ofoegbu, 1984; ) and gravimetric studies (e.g., adighije, 1979; 1981a; 1981b; cratchley and jones, 1965; ugbor and okeke, 2010; obasi et al., 2018; omietimi et al., 2021) over parts of the region in order to ascertain its geological constituents in terms of residual anomalies, regional anomalies, trending lineaments, mineralization, and sediment thickness. ofoegbu (1984) demonstrated that sediment depths between 1.7 and 7.3 km underlain the sedimentary cover in the southern bt (which comprises the lower and middle bt). ofoegbu and onuoha (1991) estimated sediment depths to be between 1.3 and 2.5 km in the abakaliki anticlinorium within the lower bt by applying a 2-d spectral analysis of aeromagnetic data. obi et al. (2010) reported sediment depths between 1 and 4 km over parts of the lower bt. oha et al. (2016) estimated maximum sediment depths of 4.4 and 4.9 km for volcanic intrusions and basement respectively in parts of the southern bt. the existence of deep magnetic source bodies (1.955.93 km) and shallow magnetic source bodies (0.1-1.8 km) was reported in several aeromagnetic studies (adebiyi et al., 2020; obiora et al., 2018; opara et al., 2015). adighije (1976; 1979; 1981a; 1981b) estimated the sediment thickness in the southern bt by interpreting gravity data. he obtained an average sediment thickness of about 4.5 km (adighije, 1976) and 4.25 km (adighije, 1979) in the lower bt. adighije (1981a) pointed out that negative gravity anomalies flanking positive anomalies within the middle and lower bt could be associated with the accumulation of post-deformational cretaceoustertiary sediments of lower density, which correspond with some smaller depositional basins such as the anambra and afikpo basins. he obtained average sediment depths of 3.0 and 4.5 km in the middle bt and the lower bt respectively by using 2-d gravity profiles to fit geological models with gravity data. obasi et al. (2018) reported the sediment thickness between 0.4 and 6.0 km beneath the anambra basin and its adjoining regions by applying a 2-d modelling of residual gravity data. the sediment thickness between 3.5 and 6.5 m within the anambra basin was detected by omietimi et al. (2021) based on using a 2.5-d forward modelling along two selected profiles. adebiyi et al. (2020), obiora et al. (2018), and oha et al. (2016) suggested a possible reduction or absence of hydrocarbon accumulation in the southern bt because of the dominance of intrusive bodies, which could hinder the thermal maturation of hydrocarbon and lead to over maturation of potential source rocks. the presence of hydrocarbon accumulation owing to the abundance of near-surface intrusive igneous rocks was proposed, for instance, by agagu and adighije (1983), abdullahi et al. (2014), or anyanwu and mamah (2013). although ofoegbu and onuoha (1991) revealed a suitable sediment depth for hydrocarbon generation, they finally agreed with the statement by njoku (1985) and whiteman (1982) on the unlikelihood of availability of hydrocarbon within the abakaliki anticlinorium due to its geological history. however, many of these studies (see olade and morton, 1985; akande and mucke, 1993; onwuemesi and egboka, 1989; agumanu, 1989; andrew-oha et al., 2017; abdullahi et al., 2019; adebiyi et al., 2020) suggested an availability of other mineral sources, likely lead-zinc-copper-barium deposits within the southern bt. most of existing studies dealing with the estimation of sediment thickness focused only on small portions of the southern bt mainly because of an inadequate geophysical data coverage. aeromagnetic studies concentrated on delineating directions of trending lineaments and fault lines (structural pattern) within parts of the southern bt, while gravimetric studies used only a very irregular and sparse network of gravity measurements. to achieve a more comprehensive and concise gravimetric interpretation of the whole area, we estimated the sediment thickness over the entire southern bt in nigeria and parts of the cameroon volcanic line (cvl) by employing the highresolution synthetic gravity model prepared by apeh and tenzer (2022) in combination with geological evidence collected from 113 logged boreholes in order to validate our gravimetric inverse modelling results. the gravity inversion software grv_d_inv, developed by pharm et al. (2021a), was used to determine a 3-d depth structure of the basement relief. the gui-based matlab software was successfully validated for practical applications and accuracy on both measured and synthetic gravity data, achieving a good fit between the computed and actual depth structures (pham et al., 2021a). 2. geological setting the study area covering the southern bt and parts of the cvl is a highly rifted region located between latitudes 4.98° to 8.31°n and longitudes 6.47° to 10.16°e (fig. 1). figure 1. location of the study area superposed on a generalized geological map of nigeria (modified after akpan et al. (2016) and nguimbous-kouoh et al. (2018)). the southern bt covers the lower and middle units of the trough. the bt has been reported to have emanated from tensional fracture system with irregular subsidence along lines of weakness (artsybashev and kogbe, 1975). the trough contains cretaceous rocks whose ages date from the middle albian to early senonian epochs and filled with deltaic, estuarine, and marine sediments, with rare intrusions of basic alkaline lavas. the either side of the bt is bounded by granite and gneisses of probable lowermost palaeozoic age, which make up the crystalline basement (artsybashev and kogbe, 1975). previous geophysical and geological studies (e.g., nwachukwu, 1972; reyment, 1965; stoneley, 1966; short and stauble, 1967; wright, 1968; burke et al., 1972) revealed that the accumulation of lower cretaceous sediments (that is sedimentation) in nigeria evolved along an elongated basin of subsidence in north-eastward direction which form the bt. nwachukwu (1972) reported that the accumulation of sediments in the southern part of the bt occurred through a series of marine transgressions and regressions beginning with the albian transgression, which led to a formation of the asu river group. a second phase of the sedimentation sequence formed the extensive turonian transgression with deposits of the ezeaku shales, which were later succeeded by the awgu shales. the end of the turonian was marked by a regressive phase, which was discontinued by the maastrichtian transgression. a regressive phase within the southeast part of nigeria occurred in cenomanian, which hindered deposition of sediments to the calabar flank (nwachukwu, 1972). the youngest deformational episode in the bt was dated as santonian whose folding may be closely associated with the undeformed coarsely crystalline sphalerite and galena in abakaliki area (nwachukwu, 1972). odigi and amajor (2009) attributed the sequence of sedimentation at the lower bt to deformation events that led to longitudinal folds and faults of different orientations, dimensions, and styles. geological studies (e.g., nwachukwu, 1972; petters, 1978; ofoegbu and onuoha, 1991; ojoh, 1992; abbass and mallam, 2013; igwe et al., 2013; 49estimating sediment thickness beneath the southern benue trough in nigeria using gravity and borehole data abdullahi et al., 2019; anudu et al., 2020; apeh and tenzer 2022) indicated the existence of ten major geological units in the southern benue trough, consisting of the asu river, ezeaku, awgu, nkporo, mamu, ajali, nsukka, imo, odukpani, and precambrian basement. studies also suggested that the asu river group mostly comprises fractured shales and limestones with intercalations of sandstone within abakaliki and calabar flank. the ezeaku formation mainly consists of black shales, siltstones, and sandstones, while shales and limestones of the coniacian age dominate the awgu formation. nkporo formations consist mainly of shales and mudstones. the mamu formation comprises coals, while the nsukka formation consists of carbonaceous shale, sandstones, and some thin coal seams. fluvio-deltaic sandstones were deposited in the ajali formation (abbass and mallam, 2013). 3. input data 3.1 absolute gravity data we prepared absolute gravity data on a 20×20″ spherical grid (approximately 600 m) from the regional tailored (synthesized) absolute gravity model by following the approach developed by apeh and tenzer (2022). their model was compiled by integrating the xgm2019e_2159 global gravity field model (zingerle et al., 2019), terrestrial gravity data (ngsa, 2017), srtm2gravity model (hirt et al., 2019), and the multi‐error‐removed improved terrain dem (merit dem) (yamazaki et al., 2017) over the southern bt. this synthetic gravity dataset was compiled by using a stochastic combination technique (apeh and tenzer, 2022). a validation of their tailored gravity model revealed a close fit with about 3000 sparse and irregularly spaced terrestrial gravity data over the southern bt of nigeria (apeh and tenzer, 2022). spatial resolutions of 3″×3″, 3″×3″, and 5′×5′ were utilized in the development of the merit dem, srtm2 gravity model, and xgm2019e_2159 global gravitational model respectively. the expected accuracy of the merit dem heights is about 6 m (yamazaki et al., 2017; ebinne et al., 2022). for a more detailed explanation of how the model was developed and validated, we refer readers to the study by apeh and tenzer (2022). figure 2. map of the derived absolute gravity data over the study area in summary, topographic heights were extracted from the merit dem on a 20×20″ grid over the study area. the 20×20″ grid data were used to subsequently extract the residual terrain gravity effects from the srtm2gravity model (hirt et al., 2019). absolute gravity values were then computed on the merit topographic heights by using the xgm2019e_2159 global model available at the calculation service of the international centre for global earth models (icgem) online platform (ince et al. 2019). the extracted residual terrain effects and a stochastically determined systematic bias of -0.4 mgal (apeh and tenzer, 2022) were algebraically added to the computed absolute gravity data to realize gravity data that fit a local gravity field. figure 2 shows the map of the derived absolute gravity data which reflects mainly the topography and geological structure of the study area. 3.2 logged borehole data many stratigraphic investigations of the study area were conducted by the government and private agencies as well as licenced oil companies. this has led to the availability of several wells log and borehole data within the benue trough and its environs especially in the southern parts of the trough. the datasets were acquired by the nigerian upstream regulatory commission (nurc) and the nigeria national petroleum corporation (nnpc), both governmental bodies in charge of all exploration activities across the country. in this study, we used the geological information collected at 113 sites provided from the nnpc and its subsidiaries to validate results obtained from a gravity data inversion. these 113 logged boreholes were dug between years 1938 and 1996. locations of these logged boreholes are identified in figure 3. we note that since these 113 logged borehole sites are located at different locations, it is hardly possible to choose a density curve that varies with depth of sediments for the region. we further note that horizontal positions of these 113 logged boreholes were established using a local coordinate system mainly due to unavailability of global navigation satellite systems (gnss) at that time. this may slightly affect the accuracy of their locations or lead to some inaccuracies of their exact horizontal positions after conversion to geographic coordinates. figure 3. geographic locations of the 113 logged boreholes within the study area 3.3 crust1.0 and gemma sediment thickness for the sake of comparative assessment, we extracted the sediment thickness from the crust1.0 (laske et al., 2013) and gemma (reguzzoni and sampietro, 2015) global crustal models at the study area. the crust1.0 global seismic crustal model consists of ice, seawater, sediments and consolidated (crystalline) crustal layers data on a 1×1° grid. authors of the crust1.0 seismic model used global datasets obtained from active seismic methods and deep drilling profiles to prepare the sediment and crustal structures in regions where no seismic measurements were available. the gemma gravimetric model was prepared on a 0.5×0.5° global grid by employing the goce gravity gradiometry observables (drinkwater et al., 2006; migliaccio et al., 2011) and globally averaged data from active seismic surveys. the maps of sediment thickness obtained from these two global crustal models with their relevant statistics and interpretations are provided in section 5. 4. methods 4.1 gravimetric inversion the model space can be described in a 3-d cartesian coordinate system such that the axes x, y, and z point towards east, north, and positive downwards respectively. a 3-d mathematical expression of a gravity data in the fourier domain is given in the following form (parker, 1973; gao and sun, 2019; pham et al. 2021a) 50 ojima isaac apeh, robert tenzer, luan thanh pham, zimuzo hedinkpa ozuah f g x y pe k n f h x y k z n n n�� ��, ! ( , ( )�� ������ ���� �� �� ���� �� ���� ���� �� �� �� ����2 0 1 1 �� , … (1) where p is the density contrast, γ is the newton’s gravitational constant, h is the interface topography at the reference depth zo, f[] denotes the fourier transform operator, and k k x k y 2 2� , with kx and ky denoting the wave-numbers along the plane direction. the rearrangement of equation 1 gives h x y f f g x y e p k n f h x y k z n n n( , ) ( , ) ( ) ! ( , )�� �� ���� ���� �� �� ���� ���� �� �� �� ����1 1 2 0 2 �� ���� ���� �� �� �� �� �� �� �� , … (2) where f-1 is the inverse fourier transform. the expression in equation 2 is solved iteratively to estimate the depth to density interface, while using the rms of differences (pham et al., 2021a) to measure the fitting between two successive depth estimates. to ensure the convergence of equation 2, a low-pass filter is normally applied during the calculation. the filter is defined in the following form b k k wh sh wh ( ) ,1 2 1 2 2 + cos for wh ≤ ≤ sh, < wh k 2 1, for > sh0, for … (3) where wh and sh describe smaller and greater roll-off frequencies of the filter design. this filter cuts off the frequencies higher than sh and fully passes the frequencies greater than wh while attenuating between wh and sh. the parameters of the roll-off frequencies can be determined from a spectral analysis of gravity data. the initial model is obtained from equation 2. here, the first term of equation 2 is computed by assigning h (x, y) = 0 and its inverse fourier transform provides the first approximation of the topographic interface, h (x, y). this value of h (x, y) is then used in equation 2 to evaluate a new estimate of h (x, y). this process is continued until a reasonable solution (in terms of existing geological/geophysical information) is achieved. 4.2 determination of a 3-d basement relief we computed the complete bouguer gravity data by utilizing gravity reduction techniques and applying the well-known crustal density value of 2.67 gcm-3 (heiskanen & moritz, 1967). we note here that terrain gravity effects are part of the residual gravity effects obtained from the srtm2gravity model and hence there is no need to compute and add terrain corrections anymore (hirt et al., 2019; apeh and tenzer, 2022). the result of the complete bouguer gravity data is shown in section 5.1. we then used an algorithm for an optimum upward continuation height developed by zeng et al. (2007) to separate the residual and regional gravity signals in the complete bouguer gravity data. this technique designs a scheme of computing a best height for the upward continuation in order to carry out the regional-residual gravity separation. the method of zeng et al. (2007) uses the cross-correlation between original data and the upward continued signal to determine the ‘optimal’ upward continuation distance for isolating a localized signal hidden inside a regional trend. figure 4 shows the cross-correlation between continuations to two successive heights versus the height (fig. 4a) and the deflection c versus the height (fig. 4b). as shown in figure 4b, the curve of the deflection against height shows maximum at the height of 10 km being regarded as optimal. the red line in figure 4a shows the chord connecting the starting point to the end point of the height values (zeng et al., 2007). as aforenoted, 113 sediment thickness measurements were used to validate results of a 3-d inverse modelling of the sediment thickness from the residual gravity data. given that the accuracy of sediment thickness estimates depends on parameters of density contrast and mean sediment depth (see eqs 1-3), we used the density contrast values between 0.11 and 0.37 gcm-3 with a step of 0.02 gcm-3 and the mean sediment depths between 0 and 3.6 km with a step of 0.3 km. as already noted, the 113 logged boreholes are at different locations and hence, it is hardly possible to choose a density curve that varies with depth of sediment for the region. we numerically computed and evaluated the resulting sediment thickness from the matlab-based 3-d gravity modelling program (described in section 4.1) by using varying density contrasts and mean sediment depths until we got the minimum rms of differences between computed and measured (borehole-derived) sediment thickness data. (a) (b) figure 4. (a) cross-correlations between continuations to two successive heights versus the height. (b) the deflection c versus the height. the red line in figure 4a shows the chord joining the starting and end points of the height values. we selected the range of density contrasts and mean sediment depths based on previous studies and density measurements within and around the study area. agagu and adighije (1983) used the sediment density contrast of 0.17 gcm-3 to estimate the sediment thickness of two profiles running from auchi-gboko and lokoja-abakaliki. fairhead et al. (1991) used the value of 0.2 gcm-3 for density contrast to carry out a 2-d interpretation of sediments in the mamfe basin located in west africa. obasi et al. (2018) and omietimi et al. (2021) applied 2.4 gcm-3 and 2.7 gcm-3 as the mean densities (which implies a mean density contrast of 0.3 gcm-3) of the sedimentary and basement layers respectively in their studies. adighije (1981b) collected 46 core samples of sedimentary rocks within the lower bt for density measurements. he first weighed samples in air and then saturated them in water to obtain mean densities of 2.65±0.04, 2.48±0.05, 2.45±0.05, 2.53±0.02, and 2.3±0.09 gcm-3 for the albian shales, cenomanian sandstone, turonian-senonian sandstone, turonian-senonian shale, and maastrichtian-tertiary sandstone respectively. in a similar study by cratchley and jones (1965), 2.65, 2.48, 2.53 and 2.45 gcm-3 density values were obtained for the albian shales, cenomanian shale, turonian-senonian shale, and turonian-senonian sandstone respectively. adighije (1981b) used the average density of 2.53±0.12 gcm-3 to obtain a simple model of the sediment thickness within the lower bt. adighije (1981b) subsequently collected 157 samples of basement complex rocks, mainly migmatites. he obtained the mean density of 2.7±0.07 gcm3. based on his measurements, he stated that a density contrast between basement and sediments might possibly vary from 0.15 to 0.25 gcm-3. 51estimating sediment thickness beneath the southern benue trough in nigeria using gravity and borehole data a) (b) (c) figure 5. gravity maps of: (a) the complete bouguer gravity data, (b) the residual gravity data, and (c) the regional gravity data over the study area 5. results and discussion 5.1 gravity reduction and separation the maps of the complete bouguer gravity data (fig. 5a), residual bouguer gravity data (fig. 5b) and the regional bouguer gravity data (fig. 5c) are shown in figure 5. we note that positive gravity anomalies were not completely separated from negative anomalies because of the complexity of the geological setting of the study area. we used the method by zeng et al. (2007) as described in section 4.2 to carry out the regional-residual gravity separation at the optimal height of 10 km. the bouguer gravity map, shown in figure 5a, depicts both, positive and negative anomalies ranging from -122 to 33 mgal. the bouguer gravity pattern exhibits continuities as well as discontinuities of rifting within the entire study area. we could see a notable zone of positive anomaly bounded by negative anomalies on both sides. towards the south and middle parts of the study area, we recognize a gravity pattern associated with the precambrian basement rocks, with low-density bodies at the lower bt dominating the southwest part of the study area and the flexurally inverted and circular abakaliki anticlinorium towards the central part of the study area. we could see negative anomalies flanking positive anomalies especially in the residual gravity map (fig. 5b). these positive anomalies are mainly trending from the central portion towards southeastern part of the study area. the negative anomalies appear to correspond with several smaller well-known depositional basins, such as ankpa, onitsha and afikpo, which are filled up with cretaceous-tertiary sediments. it is commonly assumed that the negative bouguer gravity anomalies are largely attributed to relatively low-density sediments occurring in the basin and lie on top of the metamorphic basement rocks that are of higher density. we could infer from the gravity maps (figs. 5a, b, c), based on the presence of alternating gravity lows and highs, that the benue trough may have resulted from uplift, rifting, subsidence, and sedimentation. according to a study by odigi and amajor (2009), “the sedimentary sequence of the lower bt is said to be affected by two well-documented deformation episodes, namely the cenomanian and santonian episodes, which resulted in longitudinal folds and faults of varying styles, orientations, and dimensions”. we note that the most pronounced negative anomalies towards the southeastern part of the study area correlate with a topographic high, which is likely a crustalroot exhibiting isostasy. the regional gravity map (fig. 5c) portrays a kind of magmatic movements giving rise to the alternating positive and negative gravity values prevalent within the study area. there is a possible existence of a high-density material giving rise to the positive values in the residual positive gravity data (see fig. 5b) which tend to decrease in width and thickness from the southern to the north-eastern end of the study area. the region of residual positive anomaly (at the central portion) extends steadily from southwest to northeast and matches with the core of the abakaliki anticlinorium where the albian shales connect with several mafic intrusive and basaltic rocks as noted by olade (1976). the positive anomaly seems to disappear around latitude 90 n as confirmed by adighije (1979; 1981a; 1981b). the gravity maps (figs. 5a, b, c) show positive values over the northwest and southeast portions of the study area underlain by the crystalline basement rocks (see fig. 1). these positive anomalies may be attributed to the intra-sedimentary volcanic rocks and/or to basement deformations in horst and graben structures, while the low amplitude anomalies could be due to the cretaceous sedimentary rocks (see fig. 1). the gravity maps also revealed the presence of many intra-sedimentary volcanic rocks and varying geological structures (faults, folds, synclines, anticlines, massifs or highlands) corresponding to some known and formerly unknown features in previous studies (e.g., nwachukwu, 1972; odigi and amajor, 2009; anudu et al., 2020). the prevalence of igneous intrusions (associated with the positive gravity values) within the study area may have led to serious fracturing and faulting of the basement, and overlying sediments. resulting geological structures from these tectonic events could make way for hydrothermal fluid migration and accumulation of mineral resources (ekwok et al., 2022). usually, magmatic movements (as seen especially within the abakaliki area) could help segregate minor elements of the magma from the major elements and concentrate them in some smaller volumes of rocks. these magmatic processes may form mineral deposits mainly associated with igneous intrusions resulting from rifting and volcanic activities. 52 ojima isaac apeh, robert tenzer, luan thanh pham, zimuzo hedinkpa ozuah 5.2 selection of optimal parameters we used spectral analysis of the residual gravity data to determine the parameters of the roll-off frequencies (wh and sh). the power spectrum of the residual gravity data and the marked frequencies of wh (in blue colour) and sh (in red colour) are demonstrated in figure 6. we used the values wh= 0.025614 and sh= 0.050418 as the cut-off or roll-off frequencies. broadly speaking, low radial wavenumbers mostly correspond to deep sources, and intermediate wavenumbers mainly relate to sources having average depth, while high wavenumbers are dominated by very shallow ones and noise (e.g., pham et al., 2020; spector and grant, 1970). to ensure the convergence of the algorithm, we chose a cut-off frequency sh of 0.05 and the frequency wh equal to half of sh (wh = 0.5sh) as reported by caratori tontini et al. (2008) and pham et al. (2020, 2021b). figure 6. power spectrum of the residual gravity data with marked cut-off frequencies. wh (in blue colour) and sh (in red colour) describe smaller and greater roll-off frequencies of the filter design. we computed the predicted inversion results 156 times and compared them with the 113 measured sediment depths to determine the mean density contrast and sediment depth for the entire study area. unfortunately, we do not have data that could empirically describe the porosity change with depth (or the sediment density change with depth) that can be used to determine if the sediment thickness increases significantly due to compaction. hence, we assumed only a constant sediment density as stochastically obtained from the gravity inversion performed 156 times to determine the optimal value in relation with the available measured sediment data. the rms of residuals between the computed and measured sediment depths (which ranges from 0.8 to 2.95 km) are given in table 1. the computed sediment depths are based on density contrasts (which range from 0.11 to 0.37 gcm-3) and mean sediment depths (which range from 0 to 3.6 km). as seen in table 1, the mean sediment depth of 2.7 km most suitably represents the statistics obtained from the measured sediment depths. one could infer from the presented statistics that the density contrasts of 0.21, 0.23, and 0.25 gcm-3 have the best rms fit. to determine the best density contrast to be used, we averaged these three density contrasts with the best rms fit and obtained the value of 0.23 gcm-3. following these statistical analyses, we selected the density contrast of 0.23±0.02 gcm-3 and the mean sediment depth of 2.7 km as numerically optimal values for the estimation of sediment thickness. we clarify here that our numerically obtained density contrast is within the given range of cretaceous to tertiary sediments from other studies (adighije, 1981a, b; cratchley and jones, 1965; rotstein et al., 2006). adighije (1981a, b) and cratchley and jones (1965) revealed a mean density range of 2.30-2.65 gcm-3 from about 46 samples of sediments within the study area. adighije (1981b) demonstrated that the sediment-basement density contrast within the lbt could vary between 0.15 and 0.25 gcm-3. elsewhere, rotstein et al. (2006) revealed a density variation of 2.35-2.45 gcm-3 for tertiary rift sediments at the upper rhine graben in germany. table 1. rms of residuals between measured and computed sediment depths based on density contrasts and mean sediment depths. mean sediment depths (km) density contrasts (gcm-3) 0.11 0.13 0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 rms of residuals 0.0 2.73 2.77 2.81 2.83 2.86 2.88 2.89 2.9 2.92 2.93 2.94 2.94 2.95 2.96 0.3 2.44 2.48 2.52 2.54 2.57 2.58 2.6 2.61 2.63 2.64 2.65 2.65 2.66 2.67 0.6 2.16 2.20 2.23 2.26 2.28 2.30 2.31 2.33 2.34 2.35 2.36 2.37 2.38 2.38 0.9 1.89 1.92 1.95 1.98 2.00 2.02 2.03 2.05 2.06 2.07 2.08 2.09 2.09 2.10 1.2 1.62 1.65 1.68 1.70 1.72 1.74 1.76 1.77 1.78 1.79 1.8 1.81 1.82 1.83 1.5 1.38 1.40 1.42 1.44 1.46 1.48 1.49 1.51 1.52 1.53 1.54 1.55 1.55 1.56 1.8 1.17 1.17 1.19 1.20 1.22 1.23 1.25 1.26 1.27 1.28 1.29 1.30 1.31 1.31 2.1 1.01 0.99 0.99 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.08 1.09 2.4 0.92 0.88 0.87 0.86 0.86 0.87 0.87 0.88 0.89 0.89 0.9 0.91 0.91 0.92 2.7 0.94 0.87 0.84 0.82 0.81 0.80 0.80 0.80 0.81 0.81 0.81 0.81 0.82 0.82 3.0 1.05 0.97 0.92 0.89 0.87 0.85 0.84 0.84 0.83 0.83 0.83 0.83 0.83 0.83 3.3 1.23 1.14 1.08 1.04 1.01 0.99 0.98 0.97 0.96 0.96 0.95 0.95 0.95 0.94 3.6 1.46 1.36 1.30 1.25 1.22 1.20 1.18 1.17 1.16 1.15 1.14 1.14 1.13 1.13 53estimating sediment thickness beneath the southern benue trough in nigeria using gravity and borehole data 5.3 sediment thickness we used optimal values of the density contrast (0.23 gcm-3) and the mean sediment depth (2.7 km) to estimate the sediment thicknesses. the result is shown in figure 7. figure 7. sediment thickness map derived from the density contrast of 0.23 gcm-3 and the mean sediment depth of 2.7 km. the obtained sediment depths exhibit the complexity of the southern bt and adjacent regions. intrusive rocks with lower sediment depths are prevalent within the study area. these intrusive rocks are mainly seen along ne-sw direction justifying the well-documented ne-sw direction of the rift system (olade, 1975). adighije (1981b) posited that the intrusive rocks are mainly syenitic, dioritic, and gabbroic in composition and are generally intermediate to basic in character. field investigations reveal diorites and dolerite as usual igneous rocks found within the abakaliki area (obiora and charan, 2010). the sediment thickness sections along the abakaliki anticlinorium are typically less than 1.7 km, probably due to a combination of uplift (upwelling of the crust) and erosion. farrington (1952) and olade (1975) averred that the zone of positive anomaly is limited to the regions of where tectonic activity, mineralization, uplift, and erosion are prevalent within the trough. it is evident that the sediments in the southwest part of the trough are thicker than on the eastern side except for a portion around the cameroon volcanic line. this could be explained by the proximity of the southwest part of the study area to the atlantic ocean, the niger delta, and the low-lying topography of the region. the zones of positive anomaly (over and around the towns named abakaliki, gboko, arochukwu and ikom, in figure 5) correspond with areas of shallow depth of basement and hence, low sediment depth. we could see a maximum of about 4500 m sediment thickness within the southwestern end. similarly, adighije (1981a, 1981b) reported the maximum sediment thickness of 4500 m around nsukka and 5000 m around the ankpa basin, while kogbe (1976) estimated the maximum sediment thickness of 3300 m for the albian-coniacian sediments within the lower benue trough from surface geology. agagu and adighije (1983) pointed out that the variations of sediment thicknesses in the lower bt could be associated with some known tectonic events, namely: (a) abakaliki high, (b) nsukka high, (c) ankpa basin, (d) onitsha basin and (e) afikpo syncline. these well-known tectonic events as observed by agagu and adighije (1983) are clearly delineated in figure 7. the south-western part of the study area (part of niger delta in nigeria) with deep sediments is well-known for oil and gas exploration and exploitation activities. there is a recent discovery of oil and gas within the anambra basin around the town indicated as onitsha in figure 7. more exploration activities could lead to a discovery of other liquid and solid mineral resources around the places where deep sediments are detected in figure 7. magmatic movements or processes along and over the abakaliki anticlinorium may have led to the shallowing of sediments over that portion of the study area. there is the likelihood of sediments been entrapped within and under the basement complex, which could mature overtime into mineral resources. 5.4.1 comparison of results the differences between our gravimetrically determined sediment depths and that of the 113 logged (measured) borehole data are demonstrated in figure 8. these differences range from -2.38 to 1.50 km with some few locations not fitting well. however, it is obvious that most of the differences are within ±0.5 km. figure 8. differences between measured and computed sediment depths relatively large differences in some few locations could be attributed to a possible shift in their geographic position during coordinate transformation, uncertainties in the gravimetric inversion parameters, variable geology, and lithology. we used the geographic coordinates of the boreholes to extract sediment depths from our 3d sediment thickness data. a more explanation of these differences is given in section 5.5. the maps of sediment depths extracted from the crust1.0 (fig. 9a) and the gemma (fig. 9b) are presented in figure 9, with statistics in table 2. we compared the crust1.0, gemma, and our gravimetric sediment thickness estimates with the measured values (logged borehole data). statistics of differences are given in table 3. we clarify that the global models have a lower resolution compared to our sediment data. however, the comparison could help justify our gravity tailoring and gravimetric inversion efforts. it also shows that despite the differences between our gravimetrically determined and measured sediment depths, our result is far better than gridding and using those global sediment models over such a geologically complex study area. within the study area, the crust1.0 sediment thickness ranges from 0 km to 10.3 km, and the gemma sediment thickness is from 0 to 10.2 km. table 2. statistics of 113 sediment depths obtained from different models models min (km) max (km) mean (km) std (km) measured sediment depths 0.102 4.506 2.948 0.887 sediment depths obtained using our gravimetric model 1.792 3.656 2.903 0.394 sediment depths obtained using crust1.0 model 1.775 8.980 6.546 2.034 sediment depths obtained using gemma model 0.565 9.985 7.716 3.123 54 ojima isaac apeh, robert tenzer, luan thanh pham, zimuzo hedinkpa ozuah table 3. statistics of the residuals between the computed and measured sediment depths models min (km) max (km) mean (km) rms (km) our gravimetric model -2.378 1.504 0.045 0.803 crust1.0 model -6.421 1.205 -3.598 4.010 gemma model -7.672 2.490 -4.768 5.491 (a) (b) figure 9. maps of sediment thickness obtained from: (a) crust1.0 data and (b) gemma data. not surprisingly, our gravimetrically estimated sediment thickness is moderately consistent with the measured sediment depths from logged boreholes without a significant systematic bias as our result was fitted with these values. both global models are more consistent with each other but significantly differ from our results and logged borehole data. unlike our sediment thickness estimates (fig. 7), the crust1.0 and gemma sediment thickness less than 2 km are seen towards the eastern part of the study area. this could be due to a sparse geophysical data coverage and spatial resolution of those global models. the two models overestimate the sediment thickness towards the southwest part of the study area, while underestimating it elsewhere. consequently, the use of these global models within the study area for geophysical investigations might bring an overestimation of geological/geophysical features in the region which could lead to biased interpretations and conclusions. the moderate correlation of the gravimetrically computed sediment data with the measured sediment data, on the other hand, justifies the need to estimate sediment thickness from high-resolution gravity data. 5.5 uncertainties and limitations of the inversion results the uncertainty in the chosen density contrast (0.23±0.02 gcm-3) translates to about 20% uncertainty of the inverted thickness map as well as uncertainties arising from the logging of the boreholes to determine sediment depths. hence, the true range of acceptable density contrasts within the study could be even wider. owing to compaction and variable geology, the assumption of a constant density contrast might not be ideal for sediments, especially where thickness varies significantly, as it does in our study area. this could be a major limitation of the two-layer approach with the assumption of a simple density contrast. for very thick sediments, the presence of a density contrast between the sedimentary and basement rocks is questionable due to compaction. in another vein, the coverage of the borehole data is not evenly distributed over the study area. the concentration of boreholes in the southwest part of the study area is explained by the major focus on exploration and exploitation of mineral resources (such as oil and gas) in nigeria. as already aforenoted, there could be some inaccuracies in horizontal positions of the boreholes due to errors arising from a coordinate transformation from local to geographic system. in trying to enhance the relative accuracy of the density contrast used in this study, we computed the predicted inversion results 156 times and compared them with the 113 measured sediment depths to determine the mean density contrast and sediment depth for the entire study area. hence, we assumed the mean constant density of 0.23 gcm-3 as stochastically obtained from the gravity inversion performed 156 times to determine the optimal value in relation with the available measured sediment data. it is obvious that the exposed basement in the southeast part of the study area is recovered as thickened sediments to explain the negative anomaly. as mentioned earlier, the negative anomaly correlates with an elevated topography and so, it is likely a crustal root exhibiting isostatic reactions to the volcanic activities taking place over there. we note that the regional-residual gravity separation did not separate completely the regional field from the residual field mainly because of the geological and tectonic complexity of the study area. as a result, it is obvious that the regional field contains sedimentary signal, and the residual field contains deeper signals. 6. conclusions authors compiled the map of sediment thickness for the southern benue trough and parts of the cameroon volcanic line by applying a 3-d gravimetric inversion of a high-resolution tailored gravity data validated by wells log and borehole data. we used a gravity separation method to remove the residual gravity signature from the regional gravity trend. a 3-d gravimetric inversion of the residual gravity data was performed, and a numerical modelling procedure was carried out on the gravimetrically inverted sediment depths in order to obtain optimal values for the density contrast and the mean sediment depth. the optimal density contrast of 0.23 gcm-3 and the mean sediment depth of 2.7 km were statistically realized. our gravity maps closely mimic the geological configuration of the study area. the gravity maps revealed the prevalence of igneous intrusions, which might have led to serious fracturing and faulting of the basement, and overlying sediments. magmatic processes showing potential for hydrothermal fluid migration and mineralization accumulation are recognizable. these magmatic processes/movements are seen as positive anomalies in the presented gravity maps (figs. 5a, b, c). according to our estimates, the 3-d depth structure of the basement relief (sediment thickness) within this geologically and tectonically complex region range from 0.8 to 5.5 km. the sediment thickness increases westerly and decreases easterly, demonstrating the various tectonic events within the study area, which are consistent with previous studies. the sediment thickness sections along and over the abakaliki anticlinorium are small probably due to a combination of uplift (upwelling of the crust) and erosion. our gravimetrically inverted sediment depths have almost no systematic bias with measured sediment data unlike large systematic biases found when using the crust1.0 55estimating sediment thickness beneath the southern benue trough in nigeria using gravity and borehole data and gemma global models. this implies that our approach of acquiring high-resolution gravity data and subsequent modelling processes to compute sediment data could be beneficial for many developing countries and regions with insufficient coverage of logged borehole data. most parts of the southern benue trough, where deep sediments are located, are well-known for oil and gas exploration and exploitation activities. other liquid and solid mineral resources may be present in and around those places. we speculate that large magmatic movements or processes along the abakaliki anticlinorium, which led to the shallowing of sediments over that portion of the study area, might have entrapped enough sediments suitable for maturation of mineral resources overtime. acknowledgements: the authors are grateful to the nigerian upstream regulatory commission (nurc) and the nigerian national petroleum corporation (nnpc) for providing wells log/borehole data for this research. references abbass, a. a., & mallam, 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(2019). the experimental gravity field model xgm2019e. gfz data servicer. https://doi. org/10.5880/icgem.2019.007 https://doi.org/10.1016/0040-1951(68)90046-2 https://doi.org/10.1016/0040-1951(68)90046-2 https://doi.org/10.1190/1.2719497 https://doi.org/10.5880/icgem.2019.007 https://doi.org/10.5880/icgem.2019.007 _hlk117161793 _hlk96472542 _hlk96445655 _hlk96445678 _hlk96472632 _hlk96444818 _hlk96444758 _hlk96444878 _hlk117162609 _hlk109165754 _hlk109938487 bbib29 bbib65 _hlk100068863 _hlk99366973 _hlk100068928 _hlk125745148 _hlk100990983 _hlk99367007 _hlk100994892 _hlk110025643 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n4.113518 earth sciences research journal earth sci. res. j. vol. 28, no. 4 (december, 2024): 409 419 g eo ph y si c s classification of remote sensing datasets with different deep learning architectures maryam mehmood1,3*, farhan hussain1, ahsan shahzad1, nouman ali2 1. department of computer and software engineering, national university of sciences and technology, islamabad 44000, pakistan 2. department of software engineering, mirpur university of science and technology. 3. department of software engineering, national university of modern languages, numl, islamabad *corresponding author: maryam.mehmood@numl.edu.pk how to cite item: mehmood, m., hussain, f., shahzad, a., & ali, n. (2024). classification of remote sensing datasets with different deep learning architectures. earth sciences research journal, 28(4), 409-419. https://doi. org/10.15446/esrj.v28n4.113518 record manuscript received: 16/03/2024 accepted for publication: 22/10/2024 abstract remote sensing image classification has great advantages in the areas of environmental monitoring, urban planning, disaster management and many others. unmanned aerial vehicles (uavs) have revolutionized remote sensing by providing high-resolution imagery. in this context, effective image classification is crucial for extracting meaningful information from uav-captured images. this study presents a comparison of different deep learning-based approach for supervised image classification of uav images. we have experimented on four different cnn models like vgg 16, alex net, resnet50 and a deep neural network efficient-net-b0 on different remote sensing datasets; aid and aider. multiple combinations were tried to find out which model performs better on which type of datasets. we have used pre-trained initial layers of four cnn models (alexnet, vgg 16, resnet50 and efficient-net-bo) then last three layers of each of the selected models are removed and new layers have been added with better tuned parameters. two different schemes were analyzed. in scheme-1 the original alexnet, vgg 16, resnet50 and efficient-net-b0 were experimented without changing and tuning their number of parameters, while in scheme-2 transfer learning was applied on the pre-trained models and after removing last three layers new layers were added with better tuned hyper-parameters. the evaluation of above schemes was ensured through comprehensive metrics across diverse land cover classes, four different performance evaluation matrices namely; f1 score, precision, accuracy and recall. the main focus of this research is towards transfer learning and adding new layers into pre-trained models to get better classification accuracy. keywords: convolutional neural networks (cnn), alexnet, resnet-50, vgg16, efficient-net-b0, remote sensing (rs) image classification, aid, aider, unmanned aerial vehicles (uavs). palabras clave: redes neuronales convolucionales (cnn); alexnet; resnet-50; vgg16; efficient-net-b0; clasificación de imágenes de detección remota; grupo de datos aid; grupo de datos aider; vehículos aéreos no tripulados. clasificación de grupos de datos de detección remota con diferentes arquitecturas de aprendizaje profundo la clasificación de imágenes de detección remota tiene grandes ventajas en las áreas de monitoreo ambiental, planeación urbana, manejo de desastres y muchos otros. los vehículos aéreos no tripulados han revolucionizado la detección remota al proveer imágenes de alta resolución. en este contexto, la clasificación efectiva de imágenes es crucial para extraer información significativa de las imágenes capturadas por vehículos aéreos no tripulados. este estudio presenta una comparación de diferentes técnicas de aprendizaje profundo para la clasificación supervisada de imágenes capturadas por vehículos aéreos no tripulados. los autores experimentaron con diferentes grupos de datos aid y aider en cuatro modelos diferentes de redes neuronales convolucionales (cnn), vgg 16, alex net, resnet50 y en la red neuronal profunda efficient-net-b0. se intentaron múltiples combinaciones para encontrar el modelo con mayor desempeño en cada grupo de datos. los autores usaron capas iniciales de preentrenamiento de los modelos cnn y luego se retiraron las tres últimas capas de cada uno de los modelos seleccionados para añadir luego capas con parámetros más acordes. se analizaron dos esquemas diferentes. en el esquema 1 se experimentaron los modelos cnn originales sin cambiar y sin adecuar el número de parámetros, mientras que en el esquema 2 se aplicó la transferencia de aprendizaje en los modelos pre-entrenados y después de remover las tres últimas capas se añadieron nuevas capas con hiperparámetros más adecuados. la evaluación de estos esquemas fue asegurada a través de métricas completas para diversas clases de cobertura del suelo y con cuatro matrices de evaluación de desempeño llamadas puntuación f1, precisión, exactitud y exhaustividad. el foco principal de esta investigación se basa en la transferencia de aprendizaje y en la adición de nuevas capas en modelos pre-entrenados para tener una clasificación más precisa. resumen https://doi.org/10.15446/esrj.v28n4.113518 mailto:maryam.mehmood@numl.edu.pk https://doi.org/10.15446/esrj.v28n4.113518 https://doi.org/10.15446/esrj.v28n4.113518 410 maryam mehmood, farhan hussain, ahsan shahzad, nouman ali 1. introduction remote sensing image classifications has multiple applications e.g., environment monitoring, land use land cover, disaster management, urban planning and many more (s. zhang et al., 2023). due to improvement in technology, it has become very easy to get satellite images of high resolution. getting useful information from these remote sensing images is a tedious and difficult task that need high resource computation and time. with the passage of time this task has improved a lot and now there are quite many convolutional neural networks (cnn) available that can perform image interpretation task efficiently (pronina & piatykop, 2023). according to the literature, from 2014 there has been a drastic evolution in remote sensing image classification (kuang & he, 2014), the renowned cnn architectures include; residual network (res net), vgg network, google net and, alex net (song, gao, zhu, & ma, 2019). latest deep neural networks includes efficient net that ranges from b0-b7 efficient-net-b0 has 290 layers and efficient-net-b7 had 813 layers in the base models (huang, liu, van der maaten, & weinberger, 2017). the main focus of this research on remote sensing image classification is to achieve a cnn model that has improved accuracy. so that we can accurately predict the scenes, especially when there is a disaster and accurate and timely prediction is required. there exists a huge gap to improve performance of these cnn by improving their architecture (jeddi, shafieezadeh, & nateghi, 2023). also, to understand the architecture and improve the performance by network tweaking. another issue that still holds in efficient interpretation of remote images is that most of the existing cnn models are pre-trained on image net which is quite different from remote images in terms spectral bands, spatial resolution, and radiometric resolution so the process of transfer learning doesn’t give very accurate results (lilay & taye, 2023). another issue with rs image classification is that, most of the existing cnn model giving very good accuracy is very bulky and require high computational systems and storage requirement (s. song et al., 2019). there is a need to focus towards tweaking and tuning of existing small networks to get better accuracy (x. cheng et al., 2022). by adjusting hyper parameters and network tweaking, where we change the existing base models by adding or removing the layers, in our case by network tweaking it is meant that we have removed the last three layers and added three new layers. the main research contributions are as under; • four different deep learning models were tested by implying transfer learning. the last three layers of pre-trained model was removed and three new layers were added with tuned parameters and hyper parameters. • experimentation was performed on two different remote sensing (rs) data sets; aid and aider on four different cnn models. • two types of comparative analysis have been performed; one with the original deep learning models and the other one with the recent published research. 2. related work rs scene classification with cnn models has advanced significantly since 2014. even while cnn-based techniques have been remarkably successful at classifying rs scenes, designing network architectures—which is essential to the cnn models’ performance—remains a very difficult task (rodriguez-conde, campos, & fernandez-riverola, 2022). the most advanced designing a cnn architecture calls for proficiency in both domain knowledge and deep learning methodology. aside from the fact that creating handcrafted architecture takes time and is prone to mistakes (velayudhan, hassan, ahmed, damiani, & werghi, 2022), in practice, it is common to find a dearth of experts with substantial understanding in both the examined topic and deep learning technology. these spatial cues can improve the trained model’s capacity for learning and classification accuracy in the context of remote sensing picture classification (x. cheng et al., 2022). an image format based on rotation invariant feature vectors that use orthogonal vector histograms to compute spatial cues (pittaras, giannakopoulos, stamatopoulos, & karkaletsis, 2023). publicly assessable satellite image benchmarks (rsscn, aid and siri-whu) are used in the computation of the results. using a fresh data set with a varied number of classes, fine-tuning is a technique used in conjunction with transfer learning to modify the parameters of a pre-trained cnn model (ansith & bini, 2022). if we reduce epochs and train a model on smaller learning rate the process would be beneficial (kurakin et al., 2022). according to sinaga, yudistira, & santoso (2023), the usage of a cnn that has already been trained is the latest focus of image classification research. these retrieved features were then used to train a cnn for feature extraction. the authors used transfer learning to fine-tune using cnns that had already been trained (subramanian, shanmugavadivel, & nandhini, 2022). alexnet, caffenet, vggnet, googlenet (inception), and resnet were among the pre-trained networks used in the majority of cnn-based rs scene classification experiments (mehmood, shahzad, zafar, shabbir, & ali, 2022). using transfer learning approach, we can fine tune networks and can use them for feature extraction. to depart from pretrained networks (thribhuvan & elayidom, 2022). cnn is trained from scratch using a weakly supervised triplet network with weakly single-label input. alexnet is a deep learning model that was originally developed for image classification on natural images (krizhevsky, sutskever, & hinton, 2012) and recently used in many other researches. chaudhari et al., (2021) is one of those researchers. however, its architecture can also be used for remote sensing image analysis. remote sensing images typically have different characteristics than natural images, such as different spectral bands and higher spatial resolution (q. cheng et al., 2022). to use alexnet for remote sensing image analysis, the model can be adapted by modifying the input layer to match the number of spectral bands in the image, and adjusting the number of neurons in the output layer to match the number of classes in the specific classification task (s. song et al., 2019). the pre-processing steps may also be different, such as normalizing the pixel values to a specific range and performing data augmentation (tellez et al., 2019). there have been several studies that have used alexnet for remote sensing image analysis, including land cover classification (reddy, 2023), crop type classification (wu et al., 2022), and object detection in high-resolution satellite images (alganci, soydas, & sertel, 2020). alexnet is the state-of-the-art model for remote sensing image analysis (neris, guerra, lópez, & sarmiento, 2021); it is useful and has effective tools for certain other classification tasks as well. the vgg16 model consists of 16 layers, primarily composed of convolutional layers and followed by fully connected layers (simonyan & zisserman, 2014) and recently used by adegun, viriri, & tapamo (2023). the key idea behind vgg16 is to use a series of small 3x3 convolutional filters repeatedly, which leads to a network with a larger number of trainable parameters but with a smaller number of layers compared to other models like alexnet, accepts the input image with a specified size (usually 224x224 pixels) followed by convolutional layers, the network consists of 13 convolutional layers various features at different spatial scales. after some of the convolutional layers, max pooling layers are applied to reduce the spatial dimensions of the feature maps while retaining the most prominent features. towards the end of the network, there are three fully connected layers. these layers combine the features learned by the convolutional layers to make predictions (mehmood et al., 2022). the last layer is softmax layer, the function of this layer is to produce the final probability distribution over the classes. this allows the model to assign a probability to each class label. many computer vision tasks, such as feature extraction, object identification, and image classification, have made extensive use of the vgg16 model as a benchmark (saini, khosla, chand, chouhan, & prakash, 2023). despite being outperformed by more recent architectures such as resnet and efficient-net in terms of accuracy, vgg16 is still widely used because of its clarity and simplicity. a deep learning model called resnet50 has shown promise in a variety of computer vision applications, such as remote sensing image processing (k. he, zhang, ren, & sun, 2015). the architecture of resnet50 is based on residual blocks (mandal, okeukwu, & theis, 2021). each block in the resnet50 design has multiple convolutional layers, followed by batch normalization and relu activation (luo, pearson, xu, & rich, 2022). the architecture has 50 layers total (mandal et al., 2021). each block’s residual connections aid in preventing the vanishing gradient issue and enable more efficient model training (basodi, ji, zhang, & pan, 2020). large datasets of labelled images, such as satellite photos, can be used to train the resnet50 model for use in remote sensing image analysis (reedha, dericquebourg, canals, & hafiane, 2022). the input data can be pre-processed to meet the specific criteria of the model, such as normalizing the pixel values 411classification of remote sensing datasets with different deep learning architectures and performing data augmentation to improve the diversity of the training data, in order to execute tasks like land cover classification, object detection, and change detection (li, chen, chen, & shi, 2021). resnet50 has been used in a number of research to analyze remote sensing images for the purposes of classifying vegetation, mapping land cover, and detection of objects in aerial images (x. zhang, han, han, & zhu, 2020). resnet50 is considered to be one of the state-of-the-art models for many computer vision tasks, and its use in remote sensing image analysis can help to achieve highly accurate and efficient results (müller & hutter, 2021). efficient-net is a family of deep convolutional neural networks (cnns) that was introduced in 2019 (yeşilmen & tatar, 2022). efficient-net uses a novel compound scaling method that balances the depth, width, and resolution of the network to achieve better performance with fewer parameters than other models (hao, han, yang, hao, & li, 2021). the architecture of efficientnet consists of multiple blocks, each containing several layers, including convolutional layers, batch normalization, and relu activation (hoang & jo, 2021). the blocks are organized hierarchically, with higher-level blocks having fewer channels and lower spatial resolution, and lower-level blocks having more channels and higher spatial resolution (x. he, zhao, & chu, 2021). the compound scaling method used in efficient-net involves scaling the depth, width, and resolution of the network in a balanced way (tan & le, 2019). the depth is scaled by increasing the number of layers, the width is scaled by increasing the number of channels, and the resolution is scaled by increasing the input image size. efficient-net requires fewer processing resources than other models, yet it delivers state-of-the-art performance on several computer vision benchmarks by striking a balance between these aspects. efficient-net makes use of a method known as “swish” activation, which is a relu activation function variation that has been demonstrated to enhance model performance. also, efficient-net employs a method known as “stochastic depth” to drop several layers at random during training in an effort to reduce over-fitting and enhance model performance. efficient-net is a deep learning network that is incredibly effective and efficient. it has demonstrated state-of-the-art performance on many computer vision tasks, such as semantic segmentation, object detection, and image classification. the smallest variant in the efficient net family is the efficient-net-b0. it is intended to be a lightweight and effective model. the efficient-net-b0 architecture is made up of numerous blocks, each of which has a number of layers, including batch normalization, convolutional layers, and the swish activation function. because of the hierarchical organization of the blocks, lower-level blocks have more channels and higher spatial resolution, whereas higher-level blocks have fewer channels and lower spatial resolution. compound scaling is used by efficient-net-b0 to maintain network balance in terms of depth, width, and resolution. the number of channels increases the breadth, the number of layers increases the depth, and the size of the input image increases the resolution. numerous computer vision tasks, such as semantic segmentation, object detection, and image classification, have demonstrated the efficacy of efficient-net-b0. numerous real-world applications have made use of it, including pedestrian detection in surveillance systems and mobile facial recognition (arif et al., 2022). it addresses challenges in farmland semantic segmentation, such as variations in scale and similar visual features among agriculture patterns. it has two key modules: a feature fusion module to capture multi-scale variations and a global contextual module using pyramid pooling to aggregate contextual information from different image regions. experimental results on the agriculture-vision benchmark dataset show that the framework outperforms state-of-the-art methods, achieving superior performance in identifying complex agricultural patterns (khan, alarabi, & basalamah, 2023b) this article addresses the problem of land scene classification in satellite imagery faces challenges due to complex textures, cluttered backgrounds, small object sizes, and scale variations in objects. it focuses on multi-branch deep learning framework addresses these issues by combining global contextual features and multi-scale local features through two branches: one for global information extraction and the other using a fully convolutional network (fcn) for local features. the results on the uc-merced, siri-whu, and eurosat datasets show that the proposed framework outperforms other models in classifying complex land scenes in high-resolution satellite images (khan & basalamah, 2023). the article discussed the problem of road segmentation in high-resolution satellite images that is crucial for traffic monitoring and intelligent transportation, as it faces challenges such as cluttered backgrounds, inter-class correlation, and occlusions. dsmsa-net, an encoder-decoder network with attention units, efficiently addresses these challenges by extracting multi-scale features and improving spatial representation using two attention modules: scale attention unit (saau) and spatial attention unit (spau).the framework is evaluated on two benchmark datasets, deepglobe and massachusetts road dataset, and the results demonstrate that it achieves superior performance in road extraction compared to other reference methods (khan, alarabi, & basalamah, 2023a). 3 methodology the proposed research aims to find out the appropriate cnn models best classifies the remote sensing datasets after fine turning of hyper parameters and network surgery. to get the desire results four base cnn models (alex net, vgg16, resnet50 and efficient-net-b0) were used. different models have been selected starting from the ones that are published earliest to the models published in latest researches. alex net and vgg16 are the models having lesser parameters and require less resources for computation. resnet50 is a type of model that works on skip connections and relatively deeper model as compared to alex net and vgg 16. the final model used for experimentation is efficient-net-b0, which has 290 layers and considered to be a deeper model. two problems are mainly in focus: 1. the available datasets for remote imaging are relatively smaller in size as compared to other image datasets. 2. high performance systems are required to process deeper models. to find the best suited cnn models for remote sensing image classification that works well for smaller data sets using lesser resources. to overcome these problems our focus is to find the model that has lesser parameters and can perform well with lesser data with epochs not more than 30. remote sensing is also very useful for disaster management. one data set that contains images from disasters was selected and other data set contains normal aerial images. the general methodology was kept same for all the four models, starting from transfer learning of models to performing network surgery of the last three layers and tuning of hyper parameter. the figure 2 shows the process. base alex net is the basic cnn model having 8 layers and has 11x11, 5x5, 3x3, convolutions, max pooling, dropout, data augmentation, relu activation, sgd with momentum. relu activation function is attached after every convolutional and fully-connected layer. pre-trained alex-net model was used that was initially trained on imagenet dataset having 1.2 million images. features and weights are transferred from that pre-trained to get better accuracy using less epochs. same process is followed for vgg16 for our desired problem by performing network surgery on the last three layers. fc1000, softmax layer and classification layer are removed from both base models and three new layers: fully connected layer where bias and weight values are adjusted as 20, softmax layer and classification layers are attached to the base model, for alexnet the layers are attached at 2nd last fully connected layer whereas, for vgg 16 the three new layers are attached at last dropout layer. learning rate is the most important hyper parameter. here in this case piece-wise learning rate scheduler is used where initial learning rate is set to 0.0001, with drop factor of 0.1. the third model used for the experimentation is resnet 50. resnet 50 is the short form of residual network that has 50 deep layers. there is a concept in literature that says “deeper the network better the accuracy”. to check this, we performed our experimentation on four different categories of deep network to overcome the issue of deep neural networks like vanishing gradient decent that occurs when the learning rate drops to zero while moving deeper down the layers, we have decided to perform fine tuning of pre-trained networks. same process is followed on resnet 50, we have taken pre-trained model and by fine tuning of hyper parameters we have achieved better accuracy. resnet 50 works on the principal of skip connections which was kept same during experimentation. the figure 1 shows the whole process of transfer learning of initial layers, moving towards the fine tuning of fully connected layers and adjusting weight and bias learning factor as 20 that is attached to average pooling layer. the resnet 50 was pre-trained on image net that has 1.2 million images. 412 maryam mehmood, farhan hussain, ahsan shahzad, nouman ali figure 1. proposed resnet50 model used for experimentation after performing fine tuning and network surgery figure 2. methodology 413classification of remote sensing datasets with different deep learning architectures the fourth model used for experimentation is efficient-net bo that is 290 layers deeper. network surgery is performed on pre-trained efficient-net b0 model on last three layers and three new layers fully connected layer with weight and bias values assigned as 20, softmax layer and classification layers are added at the end. same network surgery is performed at this model as well where we removed the last three layers and added new layers with tuned hyper parameters. same process is followed for all other models alex net, vgg16 and resnet 50. 3.1 experimentation experimentation was performed on four models alex net, vgg 16, res net50 and efficient-net bo after performing the desired tweaking in the layers and tuning parameters and hyper parameters. the training testing ratio also kept same for each proposed model that is 70:30 same parameters were used for each proposed model. the details are as follows in table 1. also, the hyper parameters were kept same for all the three proposed models. the details are shown in table 2. table 1. parameters values names values epochs 20 and 30 optimizer sgdm scheduler piece wise learning rate table 2. hyper parameters value names values weight learning rate factor 20 bias learning rate factor 20 initial learning rate 0.0001 3.2 data set details two remote sensing data sets were used for experimentation. the details of data sets are given below; 3.2.1 aid the aid data set has 17 different scene classes and about 200 to 400 samples of size 600x600 in each class. aid is a new large-scale aerial image data set, by collecting sample images from google earth imagery. the new data set is made up of the following aerial scene types: airport bare land, baseball field, beach, bridge, center, church, commercial, dense residential, desert, farm land, forest, industrial, meadow, medium residential, mountains, parking and pond. the spatial resolution of images are 0.3-0.8m/pixel as shown in figure 3. figure 3. photo gallery of randomly selected images from aid data set 3.2.2 aider (aerial image data set for emergency response applications) the data set construction involved manually collecting all images for four disaster events, namely fire/smoke, flood, collapsed building/rubble, and traffic accidents, as well as one class for the normal case. as shown in figure 4 the aerial images for the disaster events were collected through various online sources (e.g., google images, bing images, youtube, news agencies web sites, etc.) using the keywords aerial view or uav or drone and an event such as fire, earthquake, highway accident, etc. this subset includes around 500 images for each disaster class and over 4000 images for the normal class. spatial resolution of images is 1m/pixel. the details of the data sets used during experimentation are shown in table 3. table 3. dataset details name classes total images image/ class image size spatial resolution aid 18 5400 300 600x600 0.5 to 8 meters per pixel aider 5 6000 500/ class,4000 normal 281x288 1 meter per pixel figure 4. photo gallery of randomly selected images of aider data 3.3 performance evaluation the experimentation was performed on hp pavilion gaming laptop the processor used in this system was amd ryzen 7 4800h with radeon graphics system. 64-bit windows 10 were used as operating system. 256 gb primary memory was used with 70:30 training testing ratio. 3.3.1 time performance evaluation training time depends on many factors; hardware resources are one of them. single nvidia gtx gpu was used for experimentation. the batch size was kept 8 through the work due to limited number of memory resources available. while using the above-mentioned resource the training and prediction times are discussed in table 4 and table 5 table 4. training and prediction time for aider dataset using proposed resnet50 model dataset aider training time 115 min (6900 sec) total classes 5 images per class 400, 2000 normal time for 1 epoch 230 sec time per class 46 sec prediction time per image 0.0076 sec 414 maryam mehmood, farhan hussain, ahsan shahzad, nouman ali table 5. training and prediction time for aid dataset using proposed resnet50 model dataset aid training time 119 min (6900 sec) total classes 18 images per class 300 time for 1 epoch 238 sec time per class 13.2 sec prediction time per image 0.0044 sec the matrices used for performance evaluation were accuracy, precision, f1 score and recall. accuracy, precision, recall, and f1 score are commonly used performance metrics for evaluating the performance of cnn models. accuracy is the most straightforward metric, which measures the percentage of correctly classified examples in the data set. precision measures the proportion of true positives (correctly identified instances of a given class) among all positive predictions. recall, also known as sensitivity, measures the proportion of true positives that are correctly identified by the model among all instances of that class in the data set. whereas, f1 score is the harmonic mean of precision and recall, and is often used as a single performance metric that combines both measures. accuracy is the most straightforward metric, which measures the percentage of correctly classified examples in the dataset. it is defined as: accuracy = (number of correctly classified examples) / (total number of examples) precision measures the proportion of true positives (correctly identified instances of a given class) among all positive predictions. it is defined as: precision = (true positives) / (true positives + false positives) recall, also known as sensitivity, measures the proportion of true positives that are correctly identified by the model among all instances of that class in the dataset. it is defined as: recall = (true positives) / (true positives + false negatives) the f1 score is the harmonic mean of precision and recall, and is often used as a single performance metric that combines both measures. it is defined as: f1 score = 2 * ((precision * recall) / (precision + recall)) 4. results and discussion initially the experimentation was performed on the base level alex net, vgg16, resnet50 and efficient-net-b0 models and then with proposed changes the models were tested again using same datasets. it has noticed that with transfer learning and minor parameter tuning we can achieve higher accuracy. then the same experimentation was performed on proposed models using two same datasets with 2 different epochs. table 6 shows the accuracies of cnn models. it can clearly be seen that proposed resnet50 performs best when the dataset is small and we are training our model with less epochs, on aid dataset the accuracy of proposed resnet50 is 97.67% when epochs were set to 30. the training and testing ratio was kept same for all the experimentation’s that is 70:30. efficient-net-b0 showed good results for aid dataset for 20 epochs, the accuracy is observed as 93.73%, whereas when epochs were increased to 30. aider dataset showed better results, the accuracy observed was 95.29%. the results of proposed four models were also compared with the results of all the four base models. the table 7shows the details. our results of aid dataset were then compared with the results of published data and have seen that with the above-mentioned network surgery and tweaking of parameters and hyper-parameters we can get better results with the help of conventional models. our proposed model was compared with the different published researches, where they have also used aid dataset. in (huang et al., 2017) google net model was used for experimentation and they claimed accuracy as 88.15%. the accuracy mentioned in(k. he, zhang, ren, & sun, 2016) is 89.03% using inception net v3 model. other models from different published researches like; resnet50, dense net, mobile net v2 were also validated with our proposed model. as compared to published research where a new cnn model is proposed and aid dataset was used for experimentation. the accuracy of this proposed model is 96.10% (broni-bediako, murata, mormille, & atsumi, 2021) whereas our model performs better as it gives accuracy as 97.61%. table 8 shows the comparison of all the above-mentioned models already published in recent years. a quantitative comparison is shown in table 9, with recently published research in terms classification accuracy for aider image benchmark with 6000 images. whereas, the published results are for aider dataset with 8540 total samples. results of our proposed model were compared with different published models like er-net, vgg16, resnet50, mobile net, aiscc where accuracy mentioned were 90.16%, 91.25%, 89.61%, 88.55%, 95.24% respectively while the currently proposed model claims to have accuracy as 95.75% for aider dataset. table 6. overall accuracy of all the four models on selected aerial image datasets where training testing ratio was kept same as 70:30 cnn models resnet50 alex net vgg16 efficient net b0 epochs 20 30 20 30 20 30 20 30 aid 96.76% 97.61% 91.83% 91.88% 94.70% 94.72 93.73% 93.70% aider 94.52% 95.75% 90.69% 90.72% 90.69% 90.84% 91.24% 95.29% table 7. comparison of accuracy of base models with proposed models scheme-1 datasets scheme-2 datasets cnn model aid aider cnn models aid aider alex net 85.60% 80.32% alex net 91.88% 90.72% vgg16 89.64% 82.20% vgg16 94.72% 90.84% resnet50 94.03% 91.03% resnet50 97.61% 95.71% efficient net b0 93.43% 84.04% efficient net b0 93.70% 95.29% 415classification of remote sensing datasets with different deep learning architectures table 8. a quantitative comparison with recently published research in terms of classification accuracy for aid image benchmark. model accuracy google net (huang et al., 2017) 88.15% inception v3 (k. he et al., 2016) 89.03% resnet-50 (szegedy et al., 2015) 88.10% dense net (szegedy, vanhoucke, ioffe, shlens, & wojna, 2016) 87.50% mobile net v2 (sandler, howard, zhu, zhmoginov, & chen, 2018) 89.33% model proposed in (broni-bediako et al., 2021) 96.10% scheme-2 resnet50 97.61% % table 9. a quantitative comparison with published research (alrayes, alotaibi et al. 2022) in terms of classification accuracy for aider image benchmark with 6000 images model accuracy ernet model 90.16% vgg16 model 91.25% resnet50 model 89.61% mobilenet model 88.55% aiscc-de2ms model 95.24% scheme-2 resnet50 95.75% as results of resnet50 are relatively better than other experimented models. table 12 provides the class vise comparison of recall, precision and f1 score using aid data set for epochs 30, the average recall is calculated as97.61%, the average value of precision is 97.81% whereas average computed for f1 score is 97.69%. similarly, for epochs 20 the average recall is calculated as 96.65%, the average value of precision is 96.61% whereas average computed for f1 score is 96.65%. table 10 shows its confusion matrix for 30 epochs using proposed resnet50 for aid dataset it that help to understand which class gives better predictions and table 11 shows the confusion matrix of proposed resnet50 for 20 epochs, to see the right predictions per class. table 15 provides the class vise comparison of recall, precision and f1 score using aider dataset for epochs 30, the average recall is calculated as96.18%, the average value of precision is 95.51% whereas average computed for f1 score is 95.40%. similarly, for epochs 20 the average recall is calculated as 95.91%, the average value of precision is 95.91% whereas average computed for f1 score is 95.38%.table 13 and table 14 shows the confusion matrix for proposed resnet 50 using aider dataset for 30 and 20 epochs respectively. table 10. confusion matrix for aid dataset using proposed resnet 50 for 30 epochs a ir po rt b ar e l an d b as eb al l fi el d b ea ch b ri dg e c en te r c hu rc h c om m er ci al d en se r es id en tia l d es er t fa rm l an d fo re st in du st ri al m ea do w m ed iu m r es id en tia l m ou nt ai ns pa rk in g po nd airport 104 0 0 0 0 0 0 0 3 0 0 0 0 0 0 1 0 0 bare land 1 91 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 baseball field 0 0 118 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 beach 0 0 0 108 0 0 0 0 0 0 0 0 0 0 0 0 0 0 bridge 0 0 0 0 119 0 0 0 2 0 0 0 0 1 0 0 0 1 center 0 2 0 0 0 88 0 0 0 0 0 0 0 0 0 0 0 0 church 0 0 0 0 0 0 110 0 0 1 0 0 0 0 0 0 0 0 commercial 0 0 0 0 0 0 0 75 0 0 0 0 0 0 0 0 0 0 dense residential 0 0 0 0 1 0 0 0 106 0 0 0 0 3 0 2 1 4 desert 0 0 0 0 0 0 1 1 0 82 0 0 0 0 0 0 0 0 farm land 0 0 0 0 4 0 0 0 0 0 83 0 0 0 0 0 0 0 forest 0 0 0 0 0 0 0 0 0 0 0 102 0 0 0 0 0 0 industrial 0 0 0 0 0 0 0 0 1 0 0 0 116 0 0 0 0 0 meadow 0 0 0 0 2 0 0 0 0 0 1 0 0 100 0 1 1 0 medium residential 0 0 0 0 0 0 0 0 0 0 0 0 0 0 66 0 0 0 mountains 1 0 0 1 0 0 0 0 2 0 0 0 0 0 0 69 4 1 parking 0 0 0 0 0 0 0 0 1 0 2 0 1 0 0 2 63 3 pond 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 104 416 maryam mehmood, farhan hussain, ahsan shahzad, nouman ali table 11. confusion matrix for aid dataset using proposed resnet 50 for 20 epochs a ir po rt b ar e l an d b as eb al l fi el d b ea ch b ri dg e c en te r c hu rc h c om m er ci al d en se r es id en tia l d es er t fa rm l an d fo re st in du st ri al m ea do w m ed iu m r es id en tia l m ou nt ai ns pa rk in g po nd airport 103 1 0 2 0 0 1 0 0 0 0 0 0 0 0 0 0 1 bare land 0 90 1 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 baseball field 0 0 116 2 0 2 0 0 0 0 0 0 0 0 0 0 0 0 beach 0 0 0 107 0 0 1 0 0 0 0 0 0 0 0 0 0 0 bridge 0 0 0 0 119 0 0 0 0 0 1 0 2 0 0 0 1 0 center 0 0 0 0 0 90 0 0 0 0 0 0 0 0 0 0 0 0 church 0 0 0 0 0 0 111 0 0 0 0 0 0 0 0 0 0 0 commercial 0 0 0 0 0 0 0 75 0 0 0 0 0 0 0 0 0 0 dense residential 0 0 0 0 1 0 0 0 111 0 0 0 1 1 0 2 0 1 desert 1 0 0 1 0 0 0 0 0 82 0 0 0 0 0 0 0 0 farm land 0 0 0 0 1 0 0 0 0 0 86 0 0 0 0 0 0 0 forest 0 0 0 0 0 0 0 0 0 1 0 101 0 0 0 0 0 0 industrial 0 0 0 0 0 0 0 0 1 0 0 0 116 0 0 0 0 0 meadow 0 0 0 1 0 0 0 1 0 0 0 0 0 101 0 0 0 2 medium residential 0 0 0 0 0 0 0 0 0 0 0 0 0 0 66 0 0 0 mountains 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 72 4 1 parking 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 70 1 pond 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 103 table 12. class vise comparison of aid dataset for proposed resnet50 proposed resnet50 epochs30 epochs20 aid labels precision recall f1score precision recall f1score airport 98.09% 99.03% 0.99 96.29% 98.11% 0.97 bare land 96.77% 98.90% 0.98 97.85% 97.84% 0.98 baseball field 96.67% 99.14% 0.98 98.33% 100% 0.99 beach 99.07% 94.69% 0.97 100% 97.29% 0.99 bridge 96.74% 98.34% 0.98 96.74% 94.44% 0.96 center 100% 96.77% 0.98 97.77% 98.87% 0.98 church 100% 98.23% 0.99 99.09% 99.09% 0.99 commercial 100% 98.68% 0.99 100% 98.68% 0.99 dense residential 94.87% 98.23% 0.97 90.59% 92.98% 0.92 desert 97.61% 98.79% 0.98 97.61% 98.79% 0.98 farm land 98.85% 98.85% 0.99 95.40% 95.34% 0.95 forest 99.01% 100% 1.00 100% 100% 1.00 industrial 99.14% 97.47% 0.98 99.14% 99.14% 0.99 meadow 96.19% 99.01% 0.98 95.23% 96.15% 0.96 medium residential 100% 97.05% 0.99 100% 100% 1.00 mountains 92.32% 97.29% 0.95 88.46% 90.78% 0.90 parking 97.22% 92.10% 0.95 87.50% 91.30% 0.89 pond 98.09% 94.49% 0.96 99.04% 92.03% 0.95 417classification of remote sensing datasets with different deep learning architectures table 13. confusion matrix for aider dataset using proposed resnet50 for epochs 30 traffic incident normal flooded areas fire collapsed building traffic incident 142 0 2 8 1 normal 1 152 0 3 0 flooded areas 0 0 152 3 3 fire 3 3 4 1300 7 collapsed building 1 0 1 9 134 table 14. confusion matrix for aider dataset using proposed resnet50 for epochs 20 traffic incident normal flooded areas fire collapsed building traffic incident 142 0 2 8 1 normal 1 152 0 3 0 flooded areas 0 0 152 3 3 fire 3 3 4 1300 7 collapsed building 1 0 1 9 134 table 15. class vise comparison of aider dataset for proposed resnet50 proposed resnet50 epochs20 epochs30 aider labels precision recall f1score precision recall f1score traffic incident 92.82% 96.59% 0.94 93.33% 92.72% 0.93 normal 97.43% 98.06% 0.97 96.77% 96.15% 0.96 flooded areas 96.20% 95.59% 0.96 98.08% 99.35% 0.99 fire 98.70% 98.26% 0.98 98.62% 98.25% 0.98 collapsed building 92.41% 92.41% 0.92 87.66% 93.10% 0.90 5. conclusion the effectiveness of the deep learning-based model in accurately classifying uav images, showcasing its potential for real-world applications. the scalability and adaptability of the proposed resnet50 make it suitable for addressing challenges in monitoring and managing large-scale geographic areas especially for aid and aider datasets. the evaluations models such as; accuracy precision, recall, and f1 score have been used to assess the model’s performance. this research contributes to the advancement of uav image analysis by introducing a robust deep learning-based framework for supervised image classification. the outcomes underscore the significance of leveraging state-of-the-art technologies to enhance the precision and efficiency of land cover mapping using uav imagery. by performing different experimentation on different types of scene image data sets we have come to the point where we can say that, for problems like disaster management where we have smaller data sets where we need efficient and urgent results deeper networks are not performing as good as moderate level networks like resnet 50. we can get excellent results by altering few layers and by tweaking parameters and hyper parameters of base net resnet50. hence, it won’t be wrong to claim that resnet 50 with minor changes is the suitable network for remote sensing images especially when your data set is small, we can get better accuracy by performing network surgery and by tuning hyper parameters. for aider data set we have achieved the accuracy of 95.75% using resnet50 model, which is better than latest published research whereas for aid dataset we have achieved the accuracy of 97.61% using resnet50 model which is also better than latest published researches. one of the limitations of transfer learning specifically for remote sensing imagery is that; the existing pretrained models are trained on image net dataset, although it is a very big dataset but most of its images are not related to remote sensing. which degraded the performance of cnn models. future work may explore the integration of temporal information and the incorporation of additional modalities for a more comprehensive understanding of dynamic landscapes. code, data, and materials availability the data presented in this study are available on request from the corresponding author. conflict of interest the authors declare no conflict of interest. references: adegun, a. a., viriri, s., & tapamo, j. r. 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(2020). how well do deep learning-based methods for land cover classification and object detection perform on high resolution remote sensing imagery? remote sensing, 12(3), 417. https://doi.org/10.3390/rs12030417 https://doi.org/10.1109/lgrs.2019.2896411 https://doi.org/10.1007/s00521-022-07246-w https://doi.org/10.1007/s00521-022-07246-w https://doi.org/10.34028/iajit/19/5/3 https://doi.org/10.1007/978-3-031-27524-1_31 https://doi.org/10.1007/978-3-031-27524-1_31 https://doi.org/10.1016/j.cscm.2022.e01372 https://doi.org/10.1016/j.cscm.2022.e01372 https://doi.org/10.1007/s11356-023-25424-8 https://doi.org/10.1007/s11356-023-25424-8 https://doi.org/10.3390/rs12030417 https://doi.org/10.3390/rs12030417 introduction scene_classification_through_deep_neural existing_deep_networks _ref179625881 _ref179626009 _bookmark7 _ref179626170 _ref179626261 _bookmark13 _ref179626479 _ref179626709 _ref179626909 _ref179626912 _bookmark15 _ref179627052 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n3.118866 earth sciences research journal earth sci. res. j. vol. 29, no. 3 (september, 2025): 249 259 g eo lo g ic a l en g in ee r in g landslide research in cis countries: remote sensing, hazards, and research trends zulfiya kannazarova1, zukhra khadjiyeva2, bekmurat abdikairov3, mukhiddin juliev2,4,5, zayniddin sharipov1, parakhat berdimuratov1 1. tashkent institute of irrigation and agricultural mechanization engineers» national research university, tashkent, uzbekistan 2. institute of fundamental and applied research, “tiiame” national research university, tashkent, uzbekistan 3.institute of agriculture and agrotechnologies of karakalpakstan, nukus, uzbekistan 4.turin polytechnic university in tashkent, tashkent, uzbekistan 5-. tashkent state technical university, universitet 2, tashkent, 100095, uzbekistan record manuscript received: 14/02/2025 accepted for publication: 17/09/2025 how to cite this item: kannazarova, z., khadjiyeva, z., abdikairov, b., juliev, m., sharipov, z., & berdimuratov, p. (2025). landslide research in cis countries: remote sensing, hazards, and research trends. earth sciences research journal, 29(3), 249-259. https:// doi.org/10.15446/esrj.v29n3.118866 abstract landslides are among the major environmental hazards, with large-scale socio-economic and environmental impacts, that jeopardize socio-economic well-being in the countries of the commonwealth of independent states (cis). landslides are caused by the interaction of several complex factors, including local or regional geology, geomorphology, topography, and seismic motions. the factors that can trigger landslides are classified into three categories: external, internal, and anthropogenic (human-induced). it has been identified that geophysical, geotechnical, and statistical approaches are commonly used in landslide research in the cis states. the goal of this study is to review published articles on landslides in cis countries from 1966 to 2022. in line with this goal, we have collected (using scopus database), reviewed, and analyzed 944 papers published during 1966–2022. bibliometric analysis revealed that all articles were published in two languages: english and russian. the highest publication numbers came from the russian federation, followed by kyrgyzstan, kazakhstan, germany, the united states, japan, uzbekistan, belgium, china, and italy. furthermore, our research shows that the largest number of papers were research articles, comprising 559 (59%) of 944, followed by 282 documents of conference proceedings, 75 book chapters, and 15 review papers. in contrast, there were only 1% of other document types (e.g., book 6, editorial 3, note 2, erratum 1, short survey, and retracted, each with 1). the scientometric analysis revealed that international research on landslides is necessary to enhance scientific exchange on this topic. keywords: landslide; remote sensing; cis countries; research trend; citations; review. investigación sobre deslizamientos de tierra en la comunidad de estados independientes: teledetección, riesgos y tendencias de investigación resumen los deslizamientos de tierra son uno de los principales riesgos ambientales, con impactos socioeconómicos a gran escala, que ponen en peligro el bienestar socioeconómico de los países de la comunidad de estados independientes (cei). los deslizamientos son causados por la interacción de varios factores complejos, que incluyen la geología local y regional, la geomorfología, la topografía y los movimientos sísmicos. los factores que pueden desencadenar los deslizamientos se clasifican en tres categorías: externos, internos y antropogénicos (inducidos por los seres humanos). se ha identificado que los abordajes desde la geofísica, la geotecnia y la estadística se usan comunmente en la investigación de los deslizamientos de tierra en los estados de la cei. la meta de este estudio es revisar artículos publicados sobre deslizamientos en los paises de la cei entre 1966 y 2022. en línea con este objetivo, hemos recopilado (utilizando la base de datos scopus), revisado y analizado 944 artículos publicados durante el período de estudio. el análisis bibliométrico reveló que todos los artículos se publicaron en dos idiomas: inglés y ruso. los números más altos de publicaciones vinieron de la federación rusa, seguida de kirguistán, kazajistán, alemania, estados unidos, japón, uzbekistán, bélgica, china e italia. además, nuestra investigación muestra que la mayor cantidad de artículos fueron artículos de investigación, que comprenden 559 (59%) de 944, seguidos de 282 documentos de actas de conferencias, 75 capítulos de libros y 15 artículos de revisión. en contraste, hubo solo un 1% de otros tipos de documentos (por ejemplo, libro 6, editorial 3, nota 2, fe de erratas 1, encuesta corta 1 y retractado 1). el análisis cienciométrico reveló que la investigación internacional sobre deslizamientos de tierra es necesaria para mejorar el intercambio científico sobre este tema. palabras clave: deslizamientos de tierra; detección remota; países de la comunidad de estados independientes; temas tendencia de investigación; citaciones; revisión https://doi.org/10.15446/esrj.v29n3.118866 https://doi.org/10.15446/esrj.v29n3.118866 https://doi.org/10.15446/esrj.v29n3.118866 250 zuilfiya kannazarova, mukhiddin juliev, ashirbek muratov, zayniddin sharipov, parakhat berdimuratov, bekmurat abdikairov, zukhra khadjiyeva 1. introduction landslides cause substantial economic losses and casualties and among the most common geohazards worldwide, especially in rural and mountainous areas in commonwealth of independent states (cis) countries (okalp & akgün, 2022; froude & petley, 2018). landslides have a considerable impact on mountainous regions, although their economic consequences are often underestimated (sekarlangit et al., 2022; gracheva & urushadze, 2011). landslides can be induced by various factors, such as intense or prolonged rainfall, earthquakes, water level fluctuations, rapid snowmelt, volcanic eruptions, and human activities, leading to tragic loss of life, substantial economic damage, and environmental degradation (niu et al., 2014; wu et al., 2014; mcadoo et al., 2018; mertens et al., 2016). landslide-triggering factors can be classified into external, internal, and anthropogenic (human-induced) causes. key external factors include the following rock strata weathering, increased pore water pressure, resulting from rainwater infiltration or a rise in groundwater levels (regmi et al., 2013; hossain et al., 2025; stein et al., 2024). additional loading that can induce a landslide includes rainfall (zhang et al., 2019), snow accumulation (wang et al., 2019), or dense vegetation (schmidt et al., 2001), as well as the loss of supporting forces at the slope’s toe due to processes such as weathering and erosion (zhao et al., 2022). on the other hand, previous landslides or faults, failure of underlying strata, increased lateral pressure caused by rock fractures, freezing of water, or expansion of clay, seismic events such as earthquakes and vibrations, heavy rainfall may all contribute to the occurrence of landslides (kharismalatri et al., 2018; yang et al., 2020). according to osipov et al. (2019), other factors, such as ground subsidence linked to the formation of limestone caves as well as certain human activities may also act as an external trigger of landslides. for instance, modifying the slope by removing material at its toe can increase its steepness (chakraborty & dey, 2019). the construction of buildings on a slope adds to the load (stanley gathekia, 2024), while the disposal of artificial waste (zhan et at., 2021) can further intensify the driving forces acting on slope materials. as mentioned by simonyan & volkov (2021) poor site selection, such as building roads along or across slopes or leveling terrain for construction, might also destabilize slopes, leading to landslides. internal factors contributing to landslide occurrence are related to the physical characteristics of soil and rock (cheskidov et al., 2018). these include the presence of weak or soft rock materials, rock layers with multiple joints or shear fractures, and geological structures (li et al., 2021). topography and vegetation also play a role in slope stability by influencing the adhesive and frictional forces between rock particles, which, when reduced, make slopes more susceptible to failure (fata et al., 2021). according to reichenbach et al. (2018) to assess landslide susceptibility, various methods and models have been developed. the primary objective of landslide susceptibility analysis is to understand the relationship between landslide occurrences and contributing factors (fell et al., 2008). the commonwealth of independent states (cis), established on december 8, 1991 in belarus, is a community of independent countries that has declared itself the successor to the union of soviet socialist republics (ussr) in several areas of international law and politics (palgrave macmillan, 2016). cis consists of armenia, azerbaijan, belorussia, kazakhstan, kyrgyzstan, moldova, russia, tajikistan and uzbekistan and turkmenistan which relinquished its ongoing participant status and became an associate member on august, 26, 2005 (turner, 2014). the growth of the cis commenced with the dissolution of the soviet union in the 1990s (tanaguzova et al., 2023). it was established to handle the soviet union’s demise and promote post-soviet cooperation in politics, economics, and security (kubicek, 2009). the cis conducts its shared affairs on a multilateral, interstate basis rather than via centralised organisations (hunter, 1995). in the cis region, geophysical, geotechnical, and statistical methods are predominantly applied in landslide studies (rosi et al., 2023; shuvalova et al., 2021; saponaro et al., 2015). among geophysical techniques, seismic refraction and electrical resistivity tomography are the most widely used, especially for analyzing seismically and tectonically active areas (bogoslovsky & ogilvy, 1977; dalatkazin et al., 2022). geotechnical methods are mainly related to the soil characteristics and properties and their impact to the slope failure processes (kiernan et al., 2022). statistical methods mainly related with spatial analysis using geographic information system (gis) and remote sensing (rs) techniques (juliev et al., 2018) which commonly used to identify areas most prone to landslides (smirnova & kirsanov, 2021). in environmental and geological research, remote sensing has emerged as a crucial instrument, providing unmatched capabilities for observing, charting, and examining earth’s surface processes (yamashkin et al., 2020). through the use of satellite images, aerial photography, and other sensor-based technologies, remote sensing delivers high-resolution data across multiple time periods, which is essential for investigating dynamic events like landslides. its capacity to survey extensive and often unreachable areas makes it particularly useful for evaluating landslide-susceptible regions, recognizing triggers, and tracking changes across time. methods such as synthetic aperture radar (sar), light detection and ranging (lidar), and optical imaging have transformed landslide studies, allowing for accurate measurements of land deformation, slope stability, and interactions with water systems (teshebaeva et al., 2015). consequently, remote sensing has become fundamental to modern landslide hazard evaluation, offering insights that aid in developing effective strategies for risk reduction and managing disasters (thien et al., 2022). previous researches on landslide and its importance in the following cis countries indicate that in russia new radiocarbon dating of previously discovered evidences of powerful ancient earthquakes inside the highest and most seismically active southeastern region of russian altai (se altai) within the last four thousand years was reported (nepop & agatova, 2017), in kazakhstan the ile alatau range’s low-mountain zone on the northern side had the highest density of seismogenic landslides, resulting from a magnitude 9 earthquake in 1887 where landslides had a volume of almost 400 million m³ and density of 1/5 km² (medeu et al., 2018), in kyrgyzstan romanenko et al. (2020) identified the importance to manage the risk of activation of landslide processes in the territory of the kyrgyz republic, which cause significant economic, environmental and social damage and lead to large human casualties. in case of uzbekistan mamadjanova & leckebusch (2022) analyzed precipitationinduced flooding which were a severe and long-standing challenge causing landslides. mardanov et al. (2017) devoted their research to analyze the potential for forecasting alterations in the landscape structure of highlands within the greater caucasus natural region, utilising diverse data sets. this statistical analysis elucidated the primary distinctions among the factors influencing relief conditions, including exodynamic relief and landscape formation processes, as well as the nature and intensity of land use, which variably impact landslides and are simultaneously influenced by their destructive effects. bibliometric analysis identifies cognitive structures and intellectual relationships by analyzing the performance of documents, authors, countries, journals, and institutions (maassen, 2016). the purpose of this study to provide systematic methods for the acquisition of transparent bibliographic information related to a specific field of study (sawassi & khadra, 2021). it also discovers topics where the scientific community considers relevant to social, economic, and environmental sustainability (durán-sánchez et al., 2020). furthermore, bibliometrics has contributed to various academic fields such as groundwater (kannazarova et al. 2024), irrigation and drainage (kannazarova et al. 2024), agricultural mechanization (xaliqulov et al., 2023), soil erosion (juliev et al., 2024), digital education (juliev et al., 2024), sustainability (pizzi et al., 2020), environment (de sousa, 2021), ecosystem services (djanpulatova et al., 2025), urban planning (elshater & abusaada, 2022), engineering (cancino et al., 2017), industry (mei et al., 2021), linguistic issue (mardieva et al., 2024), smart logistics (mukanov et al., 2024) and soil salinity modelling (abdikairov et al., 2024). the purpose of this study is to analyze current topics and key regions in the cis countries in landslide research as well as to use historical scientometric data to gain new insights into trends and emphasis of international landslide research. the selected timeframe of 1966 to 2022 provides a comprehensive overview of the development and evolution of landslide research over several decades. this period captures the significant scientific advancements and methodological innovations in environmental science, earth and planetary sciences, and related fields. furthermore, it enables the identification of long-term trends, shifts in research focus, and emerging themes over time. these countries encompass diverse landscapes, including mountainous terrain, prone to landslides and other geohazards, making them highly relevant for studying landslide phenomena. additionally, the region has undergone significant social, political, and economic transitions, influencing research output and collaboration patterns. analysing landslide studies from these countries offers insights into regional scientific contributions and challenges, while also highlighting the importance 251landslide research in cis countries: remote sensing, hazards, and research trends of bilingual research in both english and russian. this dual-language focus ensures a more inclusive analysis, capturing publications that may otherwise be overlooked, thus promoting a deeper understanding of landslide research across different cultural and academic contexts. 2. methodology and analysis methods 2.1 article review and study eligibility criteria the research was carried out on the most widely used bibliographic online databases scopus for period of 1966-2022 using landslide and cis countries, including russian federation, belarus, azerbaijan, moldova, armenia, kazakhstan, kyrgyzstan, tajikistan and uzbekistan as the keywords. the analysis was carried out in march 2023. a total of 944 final published papers belonging to cis countries sorted out for the further analysis on landslide issue. the analysis was performed using csv file, microsoft excel 2021, ris, vos viewer and map chart. for the searching process, relevant information, such as keyword “landslide” in the title of papers and all articles in english and in russian, were added to a spreadsheet. article = (“landslide”), document type = “article”, timespan = “1966-2022”, subject area = environmental science, earth and planetary sciences, agricultural and biological sciences, countries = russian federation, belarus, azerbaijan, moldova, armenia, kazakhstan, kyrgyzstan, tajikistan, uzbekistan, language= “english”, “russian” and deadline = march 2023. figure 1 shows the flow of the selected methodology for the research. scopus database selected years from 1966 to 2022 (944 papers) language english and russian subject area earth and planetary, environmental and agricultural science key words “landslide” statistical analysis microsoft excel document types, years, authors, countries, sources, subject areas, funding sponsors and affiliations of published papers vosviewer co-authors, keywords figure 1. methodology flowchart for the research during the screening process, the following exclusion criteria were used: 1. lack of definition of search terms (stability, sensitivity, resistance). 2. many articles do not have a doi and the ability to find articles is limited. in general, it was not possible to exclude these articles using the filter options in scopus. 2.2. bibliometric analysis the data, provided in csv format, was uploaded to excel for further bibliometric analysis. prior to conducting the analysis, the dataset was meticulously reviewed to identify and correct any errors. the analyzed articles were evaluated to identify the most relevant works and the most productive authors, highlighting key contributors. the collected articles were categorized based on various criteria, including the number of publications per year, document type, top-ranking articles, leading journals, primary funding sponsors, and distribution by subject categories, journals, countries, and institutions. additionally, a co-authorship and keyword co-occurrence analysis were provided which can be generated using vos viewer based on bibliographic data. supported file formats include .txt, ris, and .csv from scopus databases where the raw file was imported into vos viewer and a map of coauthorship and keyword co-occurrence were performed using the software. this analysis identified clusters of frequently used keywords and authors collaboration providing insight into the core themes and research trends in the domain. additionally, mapchart online platform was utilized to demonstrate countries research activity level. 3. results 3.1 trend of publications on landslide studies only in cis countries a total of 944 papers were published between 1966 and 2022 on landslide research in cis countries (fig. 2). in this review, the scientific articles consist of three periods of development: introduction (1966–2004), slight growth (2005– 2012), and stable growth (2013–2022). the numbers above the curve mean the quantity of published papers for each year. every period has its maximum level indicating 2003, 2006, and 2020 with 12, 32 and, 108 published research, respectively. it is noticeable to point out that since 1991, every year has demonstrated research publications which was not possible for previous years. overall, research interest and technological advances in monitoring promote sustainable development of the research area. below, every research period was described in detail with a particular explanation. • introduction period. the first 1966-2004 period consists of 63 publications, representing 6.67% of the total. the results show that the interest in the landslide theme with the agriculture lands started from the environmental problem–solution relationship, geophysical methods for the investigation of landslide (bogoslovsky & ogilvy, 1977), simplified model of tsunami generation by submarine landslides (pelinovsky & poplavsky, 1996) and active faulting and natural hazards in armenia, eastern turkey and northwestern iran (karakhanian et al., 2004). • slightly growth period including 2005-2012 shows 150 documents representing 15.89% of the total, with a growth that marks both scientific interest in this field of research and technological advances in observation. the data show that the number of articles by region increased during this period due to the following facts volcanic activity in kamchatka (russia) which led to landslide appearance (ponomareva et al., 2006), earthquake contributing to landslide origin in tajikistan in 1949 which associated loss of life (evans et al., 2009), environmental changes as climate and land are likely to be reflected in alterations to the size–frequency distribution of landslides in case of kyrgyzstan (schlögel et al., 2011). • stable growth covering 2013-2022 period. the last period represents the key to exponential growth, grouping 731 documents (77.43% of the total). in 2020, the highest number of publications was reached during the period of analysis with 108, followed by the year 2021 (with 94), making it a constant growth field (figure 2). the use of remote sensing and geospatial technologies has increased to improve landslide monitoring and prediction in cis countries with a significant increase in publications in the last decade. one of the research projects belonged to sharapov & varlamov, (2015) identified remote sensing monitoring of landslides, landslips and karst collapses applying neural networks, calculation of the landslide volume from remote sensing data with combination of field data were investigated by razakova et al. (2020), as well as investigations on application of multimodal experimental geomechanical and geodynamic data to promote innovative analytical framework for contemporary integrated satellite monitoring to investigate the origins of source zones of natural and anthropogenic disasters including landslide had been demonstrated (potapov et al., 2022). moreover, our study shows that the largest number 559 (59%) of 944 papers were research articles, followed by 282 articles conference proceedings, 75 book chapter and 15 in review papers whereas there were only 3% of other document types (e.g. book 6, editorial 3, note 2, erratum 1, short survey and retracted each 1) (fig. 3). 252 zuilfiya kannazarova, mukhiddin juliev, ashirbek muratov, zayniddin sharipov, parakhat berdimuratov, bekmurat abdikairov, zukhra khadjiyeva 1 1 1 1 1 1 1 1 3 6 3 8 6 5 4 2 12 6 16 32 9 17 19 16 22 19 51 52 44 64 68 76 92 108 94 80 -40 -20 0 20 40 60 80 100 120 19 66 19 70 19 74 19 77 19 90 19 91 19 92 19 94 19 95 19 96 19 97 19 98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19 20 20 20 21 20 22 n um be r of p ub lic at io ns year figure 2. annual growth rate of publications in landslide 59% 30% 8% 3% article conference paper book chapter others figure 3. publication type on landslide in cis countries 3.2 journals on landslide in cis countries the review provides an overview of journals and knowledge topics related to this academic field (montalván-burbano et al., 2020). there was a distribution of the total output across 122 journals published in 77 countries based on the communication patterns of the scholars. of these 20 journals published 335 (35,4%) papers and remaining 64,6% papers were published in other journals. table 1 lists name of the 64 journals which published minimum 3 and higher number of papers during the abovementioned period. among them iop conference series earth and environmental science had the highest number of publications with 36, following by e3s web of conferences 29 papers and international multidisciplinary scientific geoconference surveying geology and mining ecology management sgem with 27 papers in this field. table 1. list of the journals on landslide by the year of publication issues in cis countries scopus source title number scopus source title number iop conference series earth and environmental science 36 mining science and technology russian federation 6 e3s web of conferences 29 springer proceedings in earth and environmental sciences 6 international multidisciplinary scientific geoconference surveying geology and mining ecology management sgem 27 volcanology and seismology 6 scopus source title number scopus source title number sustainable development of mountain territories 27 geotectonics 5 iop conference series materials science and engineering 23 journal of volcanology and geothermal research 5 landslides 21 journal of volcanology and seismology 5 bulletin of the tomsk polytechnic university geo assets engineering 20 lithology and mineral resources 5 geomorfologiya 17 news of the national academy of sciences of the republic of kazakhstan series of geology and technical sciences 5 pure and applied geophysics 16 russian journal of pacific geology 5 russian geology and geophysics 14 doklady physics 4 gornyi zhurnal 13 journal of applied mechanics and technical physics 4 oceanology 13 marine geology 4 doklady earth sciences 12 planetary and space science 4 sovremennye problemy distantsionnogo zondirovaniya zemli iz kosmosa 12 proceedings of the international offshore and polar engineering conference 4 earth s cryosphere 11 science of tsunami hazards 4 geomorphology 10 soil mechanics and foundation engineering 4 journal of mining science 9 springer geology 4 remote sensing 9 bulletin of volcanology 3 moscow university geology bulletin 8 catena 3 natural hazards 8 earth science reviews 3 natural hazards and earth system science 8 earth surface processes and landforms 3 environmental science and engineering 7 environmental earth sciences 3 253landslide research in cis countries: remote sensing, hazards, and research trends scopus source title number scopus source title number geodynamics and tectonophysics 7 eurasian mining 3 geologiya i geofizika yuga rossii 7 geodezia i kartografia 3 izvestiya atmospheric and ocean physics 7 issledovanie zemli iz kosmosa 3 lecture notes in civil engineering 7 izvestiya rossiiskoi akademii nauk seriya geograficheskaya 3 procedia engineering 7 journal of marine science and engineering 3 russian journal of earth sciences 7 mining informational and analytical bulletin 3 geography and natural resources 6 natural hazards and earth system sciences 3 geosciences switzerland 6 proceedings of spie the international society for optical engineering 3 izvestiya physics of the solid earth 6 quaternary international 3 matec web of conferences 6 reliability theory and applications 3 3.3 authors, co-authorships and keyword co-occurrence a total of 2466 researchers participated in the retrieved documents regarding the application of landslide, which corresponds to a mean of 2.46 authors per document. single-authored documents comprised a total of 25 (2.64% of the total) publications. table 2 lists the top 14 active authors in the field of landslide issue. among them, strom, a. reigned with 42 publications, followed by havenith, h.b. with 24, torgoev, i. with 20, pelinovsky, e. with 14. table 2. top 15 active authors in the field of landslide standard competition ranking (scr) author articles (%) 1st strom, a. 56 (5,92) 2nd havenith, h.b. 24 (2,54) 3rd torgoev, i. 20 (2,11) 4th pelinovsky, e. 14 (1,5) 5th fomenko, i.k. 13 (1,4) 6th rabinovich, a.b. 12 (1,3) 6th zakharov, a.i. 12 (1,3) 7th abdrakhmatov, k. 11 (1,16) 7th kazeev, a. 11 (1,16) 7th leibman, m. 11 (1,16) 7th zakharova, l.n. 11 (1,16) 8th lebedeva, e.v. 10 (1,05) 8th postoev, g. 10 (1,05) 9th torgoev, a. 9 (0,95) co-authorship and keyword co-occurrences analysis can be generated using vos viewer based on bibliographic data. file formats supported include .txt, ris, and .csv from databases such as scopus (samir kumar jalal, 2019). the raw file was imported into vos viewer and a map of co-authorship (shown in figure 4) were created using the software. the co-authorship analysis resulted in a network of 2466 authors. only authors having a minimum of five publications on the topic of landslide were included. the more circles the more published researches and collaborations where stom, a. had a greatest number of publications and cooperation activity. also, the following researchers shown in the figure demonstrated higher research activity and partnership level– torgoev, i., zakharov, a.i., fomenko, i.k. figure 4. network map of top co-authorships based on the total link strength to ascertain the authors’ study focus, we must initially depict the network established by the co-occurrence of author keywords. figure 5 illustrates the network map of the top 31 authors’ keyword co-occurrence. the size of the node reflects the keyword’s degree of importance. this data visualisation allows you to comprehend the connections and patterns between keywords in landslide research by displaying the most relevant and often co-occurring terms. there are 31 items distributed over 5 clusters with different colours: cluster 1 (deformation, earthquakes, geology, geophysics, landslides, rocks, seismology, slope protection, slope stability, soils), cluster 2 (gis, hazards assessment, hazards, monitoring, remote sensing), cluster 3 (climate change, geomorphology, landslide, permafrost), cluster 4 (erosion, landforms, reservoirs (water), rivers ), cluster 5 (earthquake, numerical model, submarine, landslide, tsunami). dating aspect was not mentioned due to unavailability among keywords from this research. figure 5. network map of top keywords 3.4 top institutions institutions are classified according to the quality and number of the articles they publish (khasanov et al., 2021a). for the period of 56 years, one hundred sixty different institutions cooperated to publish 944 papers related to landslide studies in cis countries. our analysis of the top 10 institutes’ publications on 254 zuilfiya kannazarova, mukhiddin juliev, ashirbek muratov, zayniddin sharipov, parakhat berdimuratov, bekmurat abdikairov, zukhra khadjiyeva landslide researches allowed us to determine the influential and productive institutions in this field. as indicated in figure 6, of the 10 institutions, all of them were from russian federation where “russian academy of sciences” occupied the leadership with 25.9% share and followed by lomonosov moscow state university with contribution of 9.8% and siberian branch of russian academy of sciences with 7.8% of supported papers. contribution of other states varied between 2.8%-4.7%. 245 93 74 45 34 34 32 32 30 27 0 50 100 150 200 250 300 russian academy of sciences lomonosov moscow state university siberian branch, russian academy of sciences p.p.shirshov institute of oceanology, russian… far eastern branch, russian academy of sciences sergeev institute of environmental geoscience ras schmidt institute of physics of the earth of the… institute of geography, russian academy of sciences institute of volcanology and seismology feb ras institute for dynamics of geospheres, russian… number of publications in st itu tio ns figure 6. list of top institutions on landslide study 3.5 top countries on landslide studies of cis countries in the world mapchart online platform was applied to indicate countries activity on landslide studies in the world. a total of 77 countries actively participated in production of 944 research papers on selected topic. the number of publications of the most ten productive countries in the field of landslide research between 1966 and 2022 are illustrated in figure 7. russian federation, as an absolute leader among states, dominated with 804 published papers and followed by kyrgyzstan 65. the shortlist of countries includes both advanced and developing regions as kazakhstan, germany, united states, japan, uzbekistan, belgium, china, and italy where landslide research is a part of modern scientific issue due to their mountainous terrain. total number of published papers on landslide studies by selected 10 countries outperformed 944 as some publication might belong to more countries at the same time. figure 7. list of top countries on landslide 3.6 top cited publications on landslide studies for cis countries table 3 demonstrates top 10 mostly cited articles on landslide research including publisher name, corresponding author, date of publication as well as number of citations and document type. a total of 1894 citations were given to selected 10 papers for the research period. the most cited paper was published in natural hazards and earth system science in 2006 which covered 17.8% of citations received by all selected articles. surprisingly, conference paper titled “the grand banks landslide-generated tsunami of november 18, 1929: preliminary analysis and numerical modeling” occupied the second position with 279 citations. there were 9 research articles and 1 conference paper in the shortlist. table 3. list of top cited articles on landslide for cis countries № title journal corresponding author year citations document type 1 meteotsunamis: atmospherically induced destructive ocean waves in the tsunami frequency band natural hazards and earth system science monserrat s 2006 332 article 2 the grand banks landslide-generated tsunami of november 18, 1929: preliminary analysis and numerical modeling marine geology fine i.v 2005 279 conference paper 3 giant landslides, topography, and erosion earth and planetary science letters korup o. 2007 255 article 4 geophysical methods for the investigation of landslides geophysics bogoslovsky v.a. 1977 185 article 5 fatal landslides in europe landslides haque u 2016 168 article 6 multiple edifice failures, debris avalanches and associated eruptions in the holocene history of shiveluch volcano, kamchatka, russia bulletin of volcanology belousov a. 1999 145 article 7 the armenian earthquake of 1988 december 7: faulting and folding, neotectonics and palaeoseismicity geophysical journal international philip h. 1992 132 article 8 fluvial response to large rock-slope failures: examples from the himalayas, the tien shan, and the southern alps in new zealand geomorphology korup o 2006 129 article 9 tsunamigenic-seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest portuguese margin marine geology terrinha p 2003 126 article 10 slider block friction model for landslides: application to vaiont and la clapière landslides journal of geophysical research: solid earth helmstetter a 2004 113 article 255landslide research in cis countries: remote sensing, hazards, and research trends 3.7 top cited journals on landslide on for cis countries an effectiveness of journals is usually measured by the number of published papers and received citations. for this research we collected top 10 cited journals which published the highest number of papers. a total of 3185 citations were received by these journals for 1966-2022 research period. first of all, we sorted source names alphabetically of excel extension file of 944 documents. after step-by-step total papers citations were summarized by each journal. interestingly, at the result we accepted updating list with potential journal names. initial 10 journals were selected and shown in figure 8. as a result, landslide journal took the greatest number of citations and contributed 18.3% share and followed by natural hazards and earth system sciences journal with 17.5% share. other journals contribution varied between 4.214.6% share. 586 560 467 375 313 249 198 166 136 135 0 100 200 300 400 500 600 700 n um be r of p ub lic at io ns landslides natural hazards and earth system sciences marine geology geomorphology pure and applied geophysics bulletin of volcanology engineering geology journal of volcanology and geothermal research geophysical journal international journal of geophysical research: oceans figure 8. list of top cited journals on landslide researches for cis countries 3.8 top funding sponsors and cluster name on landslide in cis countries the period of 1966-2022, one hundred fifty-nine different funding sponsors collaborated to produce 481 papers on landslide researches. based on our analysis of the top 10 funding sponsors of publications on landslide studies for cis countries, we were able to identify the most influential and productive funding sponsors in the field which supported 244 publications during research period. as indicated in figure 9, of the 10 funding sponsors, five were from russian federation, two from germany, one from china, japan, and india. contribution of 5 russian financial supporters was 85.6% while others share equalled between 2.4-3.6%. 95 46 35 24 9 9 7 7 6 6 0 20 40 60 80 100 n um be r of p ub lic at io ns russian foundation for basic research russian science foundation ministry of education and science of the russian federation russian academy of sciences council on grants of the president of the russian federation japan society for the promotion of science bundesministerium für bildung und forschung national natural science foundation of china department of science and technology, ministry of science and technology, india deutsche forschungsgemeinschaft figure 9. list of top funding sponsors on landslide studies for cis countries there are different topic cluster names available for the subject areas given in scopus database. most of the papers published on landslide issue in cis countries belong to three different topic cluster names which demonstrated in figure 10. earth and planetary sciences topic cluster name as an absolute leader covered 59% of publications and followed by environmental science 24%. engineering covered 17% of total publications for research period on landslide studies. these major cluster names are mainly concentrated on researches about environmental changes including landslides, earthquakes, seismology. earth and planetary sciences 59% environmental science 24% engineering 17% figure 10. top topic cluster name on landslide in cis countries 4. discussion this bibliometric review provides a comprehensive overview of landslide research in the cis countries from 1966 to 2022, showing the number of publications, thematic focus, methodological approaches, and institutional contributions. the results not only reveal historical and contemporary trends, but also highlight current challenges and emerging opportunities in the field. a central observation is the significant increase in the number of publications over the past decade, reflecting the growing recognition of landslides as a scientific and societal problem. commentators have stressed the need to interpret this increase more clearly in the context of broader global trends, and indeed our results are consistent with international bibliometric studies that show an exponential growth in hazard-related research as a result of advances in remote sensing and geospatial technologies (wu et al., 2014; gariano et al., 2018). however, in the cis, this increase has additional significance: it coincides with political and economic changes that have changed the landscape of scientific collaboration and funding. thus, the sharp increase since 2013 reflects not only the adoption of technologies but also the stabilization of academic systems after decades of restructuring. remote sensing technologies, in particular sar, lidar, and optical surveys, have redefined landslide hazard mapping and monitoring. they provide essential data for analyzing slope stability in mountainous areas that are otherwise inaccessible. for example, kyrgyzstan and tajikistan, where the rugged terrain makes ground-based studies difficult, have benefited greatly from this technique. furthermore, integrating remote sensing with gis platforms has enabled multi-temporal analyses, which have produced more reliable susceptibility maps and hazard models. however, despite these advances, the availability of high-resolution imagery remains uneven across the region, with local researchers often facing financial and infrastructural barriers. this disparity reflects one of the main concerns of commentators: the persistent information inequality between the wealthy cis countries (especially russia) and the smaller republics. another notable trend is the regional focus of research. the russian federation dominates in terms of publication volume, institutional contribution, and funding. the russian academy of sciences and moscow state university produced a disproportionate share of publications, and russian funding agencies supported almost 86 percent of all funded projects. while this leadership reflects institutional strength, it also reveals structural imbalances. as commentators have noted, the voices and contexts of smaller cis countries can be overshadowed. for example, despite kyrgyzstan’s high vulnerability to landslides, its scientific output is quantitatively smaller than russia’s. this imbalance calls for greater investment in capacity-building initiatives and multinational collaborations that strengthen underserved regions. encouragingly, bibliometric data show that such collaborations are beginning to take shape, with the number of cross-border co-authors increasing. the thematic development of landslide research in the cis countries is also noteworthy. in previous decades (1966–2004), studies often considered landslides in the context of geophysical research or environmental problemsolving, with limited integration of socio-economic perspectives. however, since 2005, publications have expanded to include predictive modeling, climate change impacts, and hydrological factors such as precipitation-induced flooding. an example from uzbekistan is mamadjanova and lekkebusch (2022) linking hydrology to disaster management, highlighting excessive precipitation as a major driver of slope failure. this broader perspective is consistent with the reviewers’ suggestions that the manuscript should better integrate anthropogenic and climatic factors. indeed, global climate change is increasing the risk of landslides by increasing precipitation extremes and accelerating the melting of permafrost in high-altitude areas. cis studies have begun to capture these connections, but further integration of climate science with geomorphic models is essential. geotechnical and statistical approaches emphasize the need to use established frameworks for slope stability assessment. however, in recent years, more sophisticated applications of machine learning and artificial intelligence, such as neural network-based monitoring of exogenous processes, have emerged (sharapov and varlamov, 2015). commentators have recommended taking this change into account, and in our discussion, we highlight how these tools offer real-time risk detection capabilities. however, their successful implementation requires robust datasets and computational resources, which 256 zuilfiya kannazarova, mukhiddin juliev, ashirbek muratov, zayniddin sharipov, parakhat berdimuratov, bekmurat abdikairov, zukhra khadjiyeva are unevenly distributed across slope stability organizations. future progress therefore depends not only on technical innovations but also on the equitable distribution of resources. another important finding is the trend towards bilingual publications, where english and russian are dominant. this linguistic duality allows for greater international reach while maintaining access to local stakeholders. however, this leads to citation asymmetry: english-language articles are cited more frequently globally, which may limit valuable russian-language research. commentators have called for a more critical approach to this issue, and we argue that bilingual distribution strategies, such as promoting english abstracts for russian-language articles, could address this gap. research centres in seismically and geomorphologically active regions: the caucasus, central asia and the altai mountains. these regions combine steep slopes, seismicity and extreme weather events, which leads to a high risk potential. however, the distribution of research sites is uneven, with limited research in less dangerous but still vulnerable regions such as moldova and belarus. this geographical orientation reflects the distribution of landslide events and the priorities for research funding. a balanced regional approach will allow for a more complete assessment of landslide risk in the cis. looking ahead, the field faces a number of challenges and opportunities. first, there is a clear need to integrate interdisciplinary approaches that link geology and social sciences to better understand vulnerability and resilience. landslides are not only physical phenomena, but also socioeconomic phenomena that disrupt livelihoods, infrastructure and ecosystems. incorporating community-level observations and socio-economic datasets into risk assessments will enrich existing models. second, there is a pressing need to strengthen the capacity of early warning systems. despite advances in hazard mapping, early warning mechanisms are lacking in many cis countries, leaving communities vulnerable. as alcantara ayala and garnica-peña (2023) point out, lowand middle-income countries face systemic barriers to implementing effective warning systems, and many of these constraints are common to cis countries. these issues should be included in future landslide research programs. by combining climate projections with disaster models, researchers can make practical recommendations for long-term land use planning and disaster risk reduction. 5. conclusions this study focused on landslide, research hotspots, and possible research directions regions in the cis countries from 1966 to 2022 using bibliometric approaches based on the online scopus database. in this bibliometric analysis, we have collected, reviewed, and analyzed 944 publications. the scientometric analysis reveals that over the past 56 years, the highest research output on landslides has occurred in the last decade. the analysis of annual article production on landslides in cis countries from 1966 to 2022 indicates three distinct phases of development: (1) introduction (1966–2004), (2) slightly growth (2005–2012), and (3) stable growth (2013–2022). during the introduction period, research interest focused on landslides within the context of environmental problems and solutions. the slightly growth period marks an increase in scientific attention, with a rise in the number of regional publications. the stable growth period is characterized by the widespread use of remote sensing and geospatial technologies to enhance landslide monitoring and prediction efforts across cis countries. within the cis region, russian federation leads in terms of institutional contributions, country-level research output, and funding support. the russian foundation for basic research ranks first among funding sponsors, with 95 articles published. this leadership can be attributed to russia’s status as a well-developed country within the cis, home to numerous prestigious research institutions and universities that drive scientific advancements in landslide research. acknowledgments the authors express their appreciation to the project usaid central asia’s regional water and vulnerable environment activity for technical support in the framework of the young scientists competition. in addition, the authors are grateful to the anonymous reviewers of earlier drafts for constructive comments that helped to improve the manuscript. references abdikairov, b., 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(2025). regional displacement in malatya and adjacent regions (eastern anatolia) induced by the february 6, 2023 kahramanmaraş earthquakes (mw 7.7, mw 7.6). earth sciences research journal, 2(3), 313-332. https://doi.org/10.15446/esrj.v29n3.119786 abstract on february 6, 2023, two major earthquakes (mw 7.7 and mw 7.6) struck kahramanmaraş, significantly affecting eastern anatolia. the tectonic structure of malatya and its surroundings, including the east anatolian fault zone (eafz) and the malatya fault, plays a crucial role in the region's seismicity. following the kahramanmaraş earthquakes, two moderate earthquakes (mw 5.3 and mw 5.0) occurred in malatya in august 2023, causing panic among the locals. gnss data from continuously operating reference stations-turkey (cors-tr) were analyzed in this study to investigate the tectonic activity in and around malatya and assess how the stress accumulated after the february 2023 earthquakes may have influenced this region. firstly, the displacements in eastern anatolia between january 1 and march 1, 2023, were calculated to evaluate the immediate effects of the february 6 earthquakes. then, gnss data from march 2 to december 31, 2023, were processed to examine the ongoing tectonic behavior. additionally, seismic activity during the same period was analyzed, revealing the occurrence of several moderate earthquakes potentially linked to the february 6, 2023 kahramanmaraş earthquakes. the gnss results indicate that most stations show continued tectonic motion, suggesting that stress changes induced by the kahramanmaraş earthquakes still influence the region. furthermore, the lack of stabilization at the stations highlights the persistence of postseismic deformation. the spatial correlation between observed seismicity and displacement vectors emphasizes that the malatya segment of the eafz remains a significant zone of strain accumulation and release. stations such as mly1 (malatya), apk1 (arapgir, malatya), and ady1 (adıyaman) continued to move in the same direction observed during the mainshock, pointing to ongoing afterslip. moreover, stations farther from the rupture zone (e.g., suf1 (sanlıurfa), hat2 (hatay), guru (gürün, sivas)) exhibited consistent displacement patterns, indicating that postseismic deformation extended across a broad area, including the arabian plate and the back-arc region north of the fault. these findings underline the long-term impact of the february 6, 2023, earthquakes on regional seismic activity and demonstrate the importance of integrating geodetic and seismic data for ongoing hazard assessment in eastern anatolia. keywords: malatya, eastern anatolia fault zone, earthquake, gnss, seismicity, deformation. desplazamiento regional en malatya y alrededores (este de anatolia) provocado por los terremotos de kahramanmaraş del 6 de febrero de 2023 (mw 7,7, mw 7,6) resumen el 6 de febrero de 2023, dos terremotos (mw 7.7 and mw 7.6) ocurrieron en kahramanmaraş, y causaron afectaciones en el este de anatolia (turquía). la estructura tectónica de malatya y sus alrededores, incluidas la zona de fallas de anatolia y la falla de malatya, juega un papel crucial en la seismicidad de la región. despues de los terremotos en kahramanmaraş, dos terremotos moderados (mw 5.3 and mw 5.0) en malatya en agosto de 2023, y causaron pánico entre los habitantes. en este estudio se analiza la información gnss de las estaciones de referencia de operacion continua de turquía (cors-tr) para investigar la actividad tectónica en y alrededor de malatya y evaluar cómo el estrés acumulado después de los terremotos de febrero de 2023 pueden haber influenciado en la región. inicialmente se calcularon los desplazamientos en el este de anatolia entre el primero de enero y el primero de marzo de 2023 para evaluar los efectos inmediatos de los terremotos del 6 de febrero. luego se procesó la información gnss del dos de marzo hasta el 31 de diciembre de 2023 para examinar el comportamiento tectónico del período. adicionalmente, se analizó la actividad sísmica en el mismo período, lo que revela la ocurrencia de varios terremotos moderados potencialmente relacionados con los terremos de kahramanmaraş de febrero de 2023. los resultados gnss indican que la mayoría de estaciones muestran continuos movimientos tectónicos, lo que demuestra que los cambios de estrés generados por los terremotos de kahramanmaraş aún tienen influencia en la región. además, la falta de estabilización de las estaciones resalta la persistencia de la deformación postsísmica. la correlación espacial entre la sismicidad observada y los vectores de desplazamiento enfatiza que el segmento malatya de la eafz sigue siendo una zona significativa de acumulación y liberación de deformaciones. estaciones como mly1 (malatya), apk1 (arapgir, malatya) y ady1 (adıyaman) continuaron moviéndose en la misma dirección observada durante el sismo principal, lo que apunta a un deslizamiento posterior en curso. además, las estaciones más alejadas de la zona de ruptura (por ejemplo, suf1 (sanlıurfa), hat2 (hatay), guru (gürün, sivas)) exhibieron patrones de desplazamiento consistentes, lo que indica que la deformación postsísmica se extendió a través de un área amplia, incluyendo la placa arábiga y la región de retroarco al norte de la falla. estos hallazgos subrayan el impacto a largo plazo de los terremotos del 6 de febrero de 2023 en la actividad sísmica regional y demuestran la importancia de integrar datos geodésicos y sísmicos para la evaluación continua de riesgos en anatolia oriental. palabras clave: malatya; zona de fallas del este de anatolia; terremoto; gnss; seismicidad; deformación. https://doi.org/10.15446/esrj.v29n3.119786 mailto:ayca.cirmik@deu.edu.tr https://doi.org/10.15446/esrj.v29n3.119786 314 regional displacement in malatya and adjacent regions (eastern anatolia) 1. introduction the neotectonic period of türkiye (anatolia) began approximately 13 million years ago with the complete closure of the south of the neotethys ocean along the bitlis-zagros suture zone (şengör and yılmaz, 1981; şengör et al., 1985). in the eastern mediterranean, the main tectonic reasons for the deformation are the ongoing collision in the bitlis-zagros suture zone and the anatolian plate’s motion to the west as a result of subduction and rollback processes in the hellenic arc (şengör et al., 1985; reilinger et al., 2006; le pichon and kreemer, 2010). several studies (e.g. stein et al., 1992; jaume and sykes, 1996; stein et al., 1997; nalbant et al., 1998, 2002; toda et al., 2011; li et al. 2023; zhao et al., 2023) have shown that an earthquake can trigger nearby faults by transferring its energy. after an earthquake occurs, it has been observed that stress is loaded onto some areas while it is released in others. additionally, after an earthquake occurs, its energy distribution and propagation directions can provide valuable information about the upcoming earthquake’s direction and magnitude in the vicinity. therefore, studying the stress distribution of an earthquake is significant for understanding the potential locations of upcoming seismic events. in the studies of stein et al. (1997) and nalbant et al. (1998), before the 1999 izmit earthquake, it was identified that the seismic risk of the izmit bay area was raised. a destructive mw 7.4 earthquake struck this area on august 17, 1999. after this significant earthquake, barka (1999) observed an increase in stress loading on the fault segments at both ends of the rupture zone, which led to the rupture of the bolu-düzce segment and three months later, on november 12, 1999, another major earthquake (mw 7.1) occurred in düzce province. nalbant et al. (2002) pointed out that the eafz has been seismically quieter than the north anatolian fault zone (nafz) in the last century and emphasized significant stress accumulation along the eafz. in this study, nalbant et al. (2002) calculated the stress changes along the fault zone to identify regions that are highly stressed and potentially defenseless to future seismic events, and it was pointed out that the kahramanmaraş-malatya region could be a source of significant seismic activity with a magnitude of around mw 7.3. in the study of yıldız et al. (2020), it was observed a strikeslip rate of 7.5 mm/yr was through the main sinistral stretch of the eafz, and it was estimated that a major earthquake with mw 7.2–7.6 might happen if the entire 90 km length ruptures. as predicted in the studies of nalbant et al. (2002) and yıldız et al. (2020), on february 6, 2023, kahramanmaraş struck with two major earthquakes with mw 7.7 and mw 7.6. fourteen days later than these earthquake on february 20 2023, a large aftershock with mw 6.4 recorded in hatay (afad, 2023; büyüksaraç et al., 2024; deu, 2023; he et al., 2023; koeri, 2023a; li et al., 2023; över et al., 2023; özkan et al., 2023; toker et al., 2023; zhao et al., 2023; kobayashi et al., 2024; köküm, 2024; ayso et al., 2025). li et al. (2023) showed significant variations in fault slip rates and locking depths through the eafz’s west and central parts, highlighting the heterogeneous fault behavior in the region and presented that the primary earthquake significantly amplified the static coulomb failure stress on adjacent faults, triggering the following earthquake. in the studies of zhao et al. (2023) and doğanalp et al. (2024) insar and gps data were used to analyze the coseismic fault model and postseismic surface movement of the kahramanmaraş earthquakes. similar to li et al. (2023), zhao et al. (2023) reported that the primary earthquake triggered the following earthquake by increasing the coulomb rupture stress along the fault. additionally, över et al. (2023) used moment tensor inversion to determine fault characteristics and regional kinematics. they applied coulomb stress transfer analysis to calculate static stress changes, demonstrating how the primary earthquake amplified the stress along the sürgü-çardak fault, likely triggering the following earthquake. in previous studies (pamukçu et al., 2015; çırmık et al., 2017, 2018, 2024; çırmık and pamukçu, 2017; tiryakioğlu et al., 2017, 2018; çırmık, 2018b; kahveci et al., 2019; yildiz et al., 2021; özkan et al., 2023) the surface deformation caused by the significant earthquakes was identified clearly by using high-resolution gnss network. özkan et al. (2023) used a gnss network to study the fault slip distributions and displacements caused by the kahramanmaraş earthquakes. the authors emphasized the importance of using high-resolution geodetic data to accurately estimate surface displacements and understand the interaction between fault segments during significant seismic events. this study initially concentrated on the seismicity induced by the august 2023 earthquakes in the malatya region. it was subsequently expanded to assess the coseismic and postseismic tectonic deformation across a broader area. for this purpose, coseismic displacements associated with the february 6, 2023 earthquake sequence were quantified using gnss observations from the cors-tr network from january 1 to march 1, 2023. gnss data from march 2 to december 31, 2023, were analyzed to evaluate the postseismic deformation. the results from both time intervals were compared to identify and characterize ongoing postseismic deformation patterns in the region. in addition, to further investigate postseismic activity following the february 6, 2023 earthquakes, seismic events with magnitudes of mw ≥ 4.0 that occurred between march 2 and december 31, 2023, were examined. the most significant event recorded during this period was a moderate earthquake (mw 5.5) near kozan, adana. these seismic observations were evaluated with gnss-derived displacements to assess the spatial and temporal correlation between seismic and geodetic postseismic signals. considering that malatya is located near significant fault structures such as the eafz, the sürgü fault, and the malatya fault, understanding the spatial and temporal evolution of deformation in this region is essential for assessing seismic hazards. therefore, this study aims to contribute to understanding the regional tectonic behavior and postseismic response of the crust by combining seismic observations with high-resolution gnss data. the findings will help identify zones of ongoing strain accumulation and release, improving the scientific basis for future seismic hazard assessment and risk mitigation strategies in eastern anatolia. 2. main tectonic features of malatya province and its surroundings deformation in the anatolian region is mainly governed by several tectonic processes. the main mechanism is the convergence between the arabian and eurasian plates through the bitlis-zagros suture zone, which causes the anatolian plate to move westward. moreover, the process of plate detachment beneath eastern anatolia was significant on the dynamic uplift of the eastern anatolian plateau and facilitated the formation and activity of major strike-slip fault systems such as nafz and eafz (mckenzie 1972; westaway et al. 2007; özeren and holt 2010). the nafz and eafz correspond to the relative motion between the surrounding plates and slip rates on the malatyaovacık fault zone (mofz) (fig. 1) and other internal fault zones vary from 1.2 to 2.5 mm/year (aktuğ et al., 2013). the activity of these fault systems has led to large-scale deformation in the anatolian region, characterized by local zones of compression, extension and strike-slip movement. this contributed to the development of pull-apart basins, mountain ranges and volcanic centers (ekici et al., 2007). numerous researchers (e.g. nalbant et al., 2002; şengör et al., 2003; sandvol et al., 2003; zor et al., 2003, türkelli et al., 2003; pamukçu et al., 2007; özener et al., 2010; pamukçu and akçığ, 2011; aktuğ et al., 2013; bayrak et al., 2015; gülerce et al., 2017; yazıcı et al., 2018; çırmık, 2018a; acarel et al., 2019; sançar et al., 2019; yılmaz, 2019; bletery et al., 2020; şahin and öksüm, 2021) studied in eafz and its surroundings for investigating its seismic activity and tectonism. the malatya-ovacık fault zone (mofz) is a left-lateral strike-slip fault system, extending approximately 240 km in a ne-sw direction (westaway et al., 2003, 2007) (fig. 1). according to the studies of westaway and arger (1996) and westaway et al. (2003, 2007), the mofz was active during the early pliocene and acted as the earlier boundary between the anatolian and arabian plates, but the formation of the eafz has made mofz inactive. the mofz has two main segments, namely the malatya fault (mf) in the southwest and the ovacık fault (of) in the northeast and both of them have a significant role in the deformation of the anatolian microplate (koçyiğit and beyhan, 1998; kaymakçı et al., 2006; ekici et al., 2007, westaway et al., 2007) . the southwestern branch of mofz; the malatya fault (mf) is a ne trending sinistral strike-slip fault with the length of 165 km (fig. 1) (ekici et al., 2007; westaway et al., 2007). özener et al. (2010) indicated by using gps measurements that the strain accumulation is much higher than previously estimated, suggesting active deformation in the region between the nafz and the eafz. additionally, the paleoseismic studies (koçyiğit and beyhan, 1998; kaymakçı et al., 2006; zabcı et al., 2017) indicated that four 315ayça çırmık, i̇brahim berkan kirdök significant earthquakes occurred over the past 10.000 years with a repetition interval of approximately 2.275 years in the mf. the researchers (koçyiğit and beyhan, 1998; kaymakçı et al., 2006; zabcı et al., 2017) presented that mf has a potential for a future large earthquake with an approximately m7.4. therefore, paleoseismological and geodetic studies (koçyiğit and beyhan, 1998; kaymakçı et al., 2006; westaway et al., 2007; özener et al., 2010, aktuğ et al., 2016; zabcı et al., 2017) indicated that the mofz (including mf and of) is an active fault zone. 3. methodology seismicity of malatya province and its surroundings in the türkiye earthquake hazard map (prepared by afad in 2019), the maximum acceleration value of pga 475 (year) in malatya province varies between 0.2-0.7g. this indicates that the seismic hazard of the region is very high. the south-southeast of the province is generally under the influence of the figure 1. the main tectonic features of malatya and its surroundings. nafz; north anatolian fault zone, eafz; eastern anatolian fault zone, mofz; malatya-ovacık fault zone, bzsz; bitlis-zagros suture zone. 316 regional displacement in malatya and adjacent regions (eastern anatolia) eafz. sürgü fault in the south, the part of the eafz passing through the east of malatya province, and the malatya fault are important tectonic structures in the region (fig. 1). in addition, the nafz passes through the north of malatya province, and the eafz passes through the south-southeast (koeri, 2023b). in the historical period (1800 b.c.-1900 a.d.), an earthquake with an intensity io=ix occurred in 1893 along the eafz (soysal et al., 1981). the earthquakes of 1866 and 1874, which occurred southeast of malatya province, are also essential earthquakes that were recorded during the historical timeframe (koeri, 2023b). in order to examine the earthquakes that affected malatya province in the instrumental timeframe, the earthquakes that were recorded in malatya and its vicinity (between longitudes 35.5°-41° and latitudes 35.5°-40°) between the days january 1, 1990 and december 31, 2023 with magnitude m≥4.0 were obtained from afad earthquake catalog (https://deprem.afad.gov.tr/ event-catalog) and mapped (fig. 2). it is examined that 1153 earthquakes with magnitude 4≤m<5, 117 earthquakes with magnitude 5≤m<6, 7 earthquakes with magnitude 6≤m<7 and 2 earthquakes with magnitude 7≤m (february 6, 2023 pazarcık, kahramanmaraş (mw7.7); february 6, 2023 elbistan, kahramanmaraş, (mw7.6) earthquakes) occurred between january 1990 and december 2023. figure 2. distribution of epicenters of earthquakes with magnitude m≥4.0 in and around malatya province between january 1, 1990 and december 31, 2023. earthquake data were downloaded from https://deprem.afad.gov.tr/event-catalog. on august 10, 2023, a moderate earthquake with a magnitude mw=5.2 (koeri, 2023b; usgs, 2023), mw=5.3 (afad 2023) occurred in malatya (in yeşilyurt district) (fig. 3). the focal depth of the earthquake was shallow and approximately 5 km (koeri, 2023b). the earthquake was felt in malatya and its surroundings. besides, on august 24, 2023 and on november, 23 2023 other moderate earthquakes with magnitude mw=5.0 (afad, 2023) and mw=5.2 (afad, 2023), respectively occurred in malatya provinces (fig. 3). figure 3. the epicenter distributions of the earthquakes with magnitudes m≥4 occurred in the region (longitudes 35.5°-41° and latitudes 35.5°-40°) after february 6, 2023 kahramanmaraş earthquakes (between the days march 2, 2023december 31, 2023) and the locations of the permanent stations of cors-tr used in the study. 3.2. gnss data analysis the derivation of coseismic displacements from time series has been highlighted in recent studies (e.g., aktuğ et al., 2010; tiryakioglu et al., 2017, çırmık, 2018b; yildiz et al., 2021; özkan et al., 2023; çırmık et al., 2024). a thorough analysis of the time series of gnss data can provide valuable findings about the gnss station’s movement (özkan et al., 2023). therefore, in this study, gnss data of the permanent stations of the cors-tr located around the epicenter of the february 6, 2023 kahramanmaraş earthquakes were processed from january 1 to march 1, 2023, to calculate the displacement of the region with the effect of these earthquakes. then, the gnss data from march 2 to december 31, 2023, including the impact of the malatya earthquakes, were evaluated to analyze the tectonic behavior in and around malatya and the subsequent significant earthquakes. meanwhile, the events (with magnitudes m ≥ 4.0) that occurred between march 2 and december 31, 2023, were analyzed to investigate the effects of the earthquakes on ground displacements. these events were obtained from the afad earthquake catalog (https://deprem.afad.gov.tr/event-catalog) and are located within the region bounded by longitudes 35.5°–41°e and latitudes 35.5°–40°n. during this period, 144 earthquakes with magnitudes of 4.0 ≤ m <5.0 and 7 earthquakes with magnitudes of 5.0 ≤ m <6.0 occurred in the region. the largest earthquake in this time frame occurred on july 25, 2023, in kozan (adana), with a magnitude of m = 5.5. in the malatya province, three moderate earthquakes occurred on august 10, 2023 (mw = 5.3; doy 222), august 24, 2023 (mw = 5.0; doy 236), and november 23, 2023 (mw = 5.2; doy 296). additionally, three other earthquakes with magnitudes of 5.0 and larger occurred within the boundaries of kahramanmaraş province on march 3, 2023 (m = 5.0), march 23, 2023 (m = 5.3), and may 3, 2023 (m = 5.0) (fig. 3). https://deprem.afad.gov.tr/event-catalog https://deprem.afad.gov.tr/event-catalog https://deprem.afad.gov.tr/event-catalog https://deprem.afad.gov.tr/event-catalog 317ayça çırmık, i̇brahim berkan kirdök in the first step of the analysis, to calculate the displacement that occurred in the region as a result of the february 6, 2023 (doy: 37) kahramanmaraş earthquakes, gnss data (between january 1 and march 1, 2023) from 14 gnss stations—ante (gaziantep), ady1 (adıyaman), apk1 (arapgir, malatya), ekz1 (ekinözü, kahramanmaraş), elaz (elazığ), ergn (ergani, diyarbakır), guru (gürün, sivas), hat2 (hatay), kls1 (kilis), mar1 (kahramanmaraş), mly1 (malatya), oniy (osmaniye), siv1 (siverek, şanlıurfa), and surf (şanlıurfa city center)—belonging to the cors-tr network were processed (fig. 3). precise coordinates were calculated using the gamit/globk software (herring et al., 2015a, 2015b), and the time series were obtained (figs. 4a–4n). in this processing, 11 igs stations (aruc, bshm, drag, madr, op71, opmt, pol2, ramo, wtza, yibl, and zimm) were used to define the eurasia-fixed reference frame within the itrf2014 framework. (a) b) (c) (d) 318 regional displacement in malatya and adjacent regions (eastern anatolia) (e) (f) (g) (h) 319ayça çırmık, i̇brahim berkan kirdök (i) (j) (k) (l) 320 regional displacement in malatya and adjacent regions (eastern anatolia) (m) (n) figure 4. the displacements at north and east components of the permanent gnss stations based on the data between january 1, 2023 and march 1, 2023; a) ady1, b) ante, c) apk1, d) ekz1, e) elaz, f) ergn, g) guru, h) hat2, i) kls1, j) mar1, k) mly1, l) oniy, m) siv1, n) surf in the second step, to investigate the impact of the postseismic movements of the february 6 earthquakes on ground displacements, the data from the same gnss stations were analyzed from march 2 to december 31, 2023, using the same processing steps. in this phase, the ante station (gaziantep) was excluded from the analysis because its position was relocated after the february 6, 2023 earthquake. similarly, the siv1 station in siverek, şanlıurfa (37.75287°n, 39.32167°e) was replaced with the nearby siv2 station (37.75252°n, 39.32273°e), and the surf station in şanlıurfa city center (37.19185°n, 38.81802°e) was replaced with the nearby suf1 station (37.16771°n, 38.80055°e) (fig. 3). since the old and new locations of these stations are very close, siv2 and suf1 were used instead of siv1 and surf in this stage of the processing. the time series of the 13 stations used in this stage (ady1, apk1, ekz1, elaz, ergn, guru, hat2, kls1, mar1, mly1, oniy, siv2, and suf1) were obtained (figs. 5a–5m). (a) (b) 321ayça çırmık, i̇brahim berkan kirdök (c) (d) (e) (f) 322 regional displacement in malatya and adjacent regions (eastern anatolia) (g) (h) (i) (j) 323ayça çırmık, i̇brahim berkan kirdök (k) (l) (m) figure 5. the displacements at north and east components of the permanent gnss stations based on the data between march 2, 2023 and december 31, 2023; a) ady1, b) apk1, c) ekz1, d) elaz, e) ergn, f) guru, g) hat2, h) kls1, i) mar1, j) mly1, k) oniy, l) siv2, m) suf1 in the next step, horizontal displacement estimates were derived from the obtained time series (fig. 4 and fig. 5) using the tsview software (herring, 2003). tsview is a matlab-based application that enables the visualization of gnss time series generated with the glred module of the gamit/ globk software, the evaluation of time series quality, the correction of the series, and the estimation of velocities and noise characteristics by applying various models (herring, 2003; özkan, 2009). the horizontal displacements obtained for january 1 to march 1, 2023, and march 2 to december 31, 2023, are presented in tables 1 and 2, respectively. the displacement vectors are shown in fig. 6 and fig. 7, respectively. 324 regional displacement in malatya and adjacent regions (eastern anatolia) table 1. horizontal displacements based on the data between january 1, 2023 and march 1, 2023 obtained by tsview (herring, 2003) software. station north (mm) east (mm) ady1 -459.39±52.45 5124.86±569.09 ante 2584.67±213.60 2594.41±222.84 apk1 -1195.68±108.55 -676.97±60.27 ekz1 3314.28±564.93 -30759.05±5404.79 elaz -551.37±51.50 -435.57±40.89 ergn -110.92±11.02 319.93±26.31 guru -811.90±72.30 -1345.14±118.04 hat2 -646.26±63.67 -1046.84±105.81 kls1 2318.96±182.97 1262.10±101.80 mar1 -1426.75±116.40 -4958.94±425.61 mly1 -5440.69±459.94 -3234.58±283.29 oniy -1793.25±187.37 -1322.22±144.65 siv1 -290.22±33.56 940.42±92.05 surf -279.40±32.45 1261.82±124.04 table 2. horizontal displacements based on the data between march 2, 2023 and december 31, 2023 obtained by tsview (herring, 2003) software. station north (mm) east (mm) ady1 14.87±0.32 81.20±0.68 apk1 -30.48±0.90 -20.13±0.58 ekz1 7.43±0.36 -12.74±0.64 elaz -60.14±0.66 -7.86±0.46 ergn 19.02±0.37 28.03±0.43 guru -5.99±2.02 -37.27±2.36 hat2 26.03±0.47 -17.51±0.54 kls1 56.86±0.70 29.80±0.72 mar1 19.75±0.44 13.38±0.48 mly1 -70.84±0.83 -47.94±0.84 oniy 14.50±0.35 5.36±0.36 siv2 20.29±0.37 50.93±0.44 suf1 28.99±0.40 57.36±0.66 figure 6. the horizontal displacements of the gnss stations obtained from the time series of january 1-march 1, 2023 including february 6, 2023 mw = 7.7 pazarcık and mw = 7.6 elbistan earthquakes effects. 325ayça çırmık, i̇brahim berkan kirdök figure 7. the horizontal displacements of the gnss stations obtained from the time series of march 2-december 31, 2023. results previous studies (e.g., stein et al., 1992; jaume and sykes, 1996; stein et al., 1997; nalbant et al., 1998, 2002; toda et al., 2011; li et al., 2023; zhao et al., 2023) have shown that a major earthquake can induce stress changes in neighboring faults, potentially triggering them. the most recent example of this phenomenon recorded on february 6, 2023 with the mw = 7.7 earthquake in pazarcık (kahramanmaraş), was followed by another major earthquake of mw = 7.6 in elbistan (kahramanmaraş). additionally, 14 days later, an mw = 6.6 earthquake in hatay provides another example of this triggering effect. the post-earthquake regional stress changes and the directions of energy propagation offer valuable insights into the possible location and magnitude of future seismic events. therefore, analyzing the stress changing following an earthquake is crucial for understanding potential seismic activity zones and predicting future large earthquake scenarios. due to the presence of the malatya fault to the west, the sürgü fault to the south, and the eafz to the southeast, malatya is a seismically active region (fig. 1). additionally, studies of koçyiğit and beyhan (1998), kaymakçı et al. (2006), and zabcı et al. (2017) emphasize the significant earthquake-generating potential of the malatya fault. according to the earthquake report prepared by dokuz eylül university, faculty of engineering, regarding the february 6, 2023 kahramanmaraş earthquakes and the february 20, 2023 hatay earthquake (deu, 2023), the coulomb analysis results of the kahramanmaraş earthquakes indicate that there was stress loading in and around the malatya region. consistent with these findings, two earthquakes of magnitudes mw = 5.3 and mw = 5.0 occurred in malatya province on august 10, 2023 and august 24, 2023, respectively, six months after the february 6, 2023 earthquakes (fig. 2). figure 2 presents the distribution of earthquake epicenters with magnitudes m ≥ 4.0 that occurred in and around the malatya province between january 1, 1990 and december 31, 2023. it is seen that the seismicity is predominantly aligned along the east anatolian fault zone (eafz), highlighting its role as a major seismotectonic structure in the region. the distribution includes a significant number of low-to-moderate magnitude events (4.0 ≤ m < 5.0), as well as several moderate-to-strong earthquakes (5.0 ≤ m < 7.0). two major earthquakes that struck the region on february 6, 2023—the mw 7.7 pazarcık and mw 7.6 elbistan events—are marked with star symbols and represent the most significant seismic events in the recent history of the region. the overall pattern reflects long-term active deformation concentrated along the fault zone, emphasizing the persistent seismic hazard in eastern and southeastern anatolia. figure 3 shows the distribution of earthquakes with magnitudes m ≥ 4.0 that occurred between march 2 and december 31, 2023, within the coordinates 35.5°–41°e and 35.5°–40°n. the events are predominantly clustered along the east anatolian fault zone (eafz) and its associated branches, indicating continued seismic activity and potential postseismic deformation. 144 events 326 regional displacement in malatya and adjacent regions (eastern anatolia) with magnitudes between 4.0 ≤ m < 5.0 and 7 events between 5.0 ≤ m < 6.0 were recorded. three moderate-magnitude earthquakes within malatya province—occurred on august 10 (mw = 5.3), august 24 (mw = 5.0), and november 23 (mw = 5.2). it is observed that the magnitude of the earthquakes occurring between march 1 and december 31, 2023, generally ranges from 4≤m<5, and these earthquakes are concentrated in the southern part of malatya, the northern part of kahramanmaraş, and the northern part of adıyaman. apart from the three earthquakes of m ≥ 5 in and around malatya, no larger seismic activity was observed in the region during these 10 months. the observed spatial alignment of seismicity supports the interpretation of ongoing crustal adjustments following the february 6, 2023 earthquakes. a comparison of figure 2 and figure 3 provides valuable insights into both the long-term and short-term seismic activity in and around the malatya region. figure 2 displays the distribution of earthquake epicenters with m ≥ 4.0 over 33 years (1990–2023), revealing a persistent concentration of seismicity along the east anatolian fault zone (eafz). the alignment of events along this tectonic boundary suggests continuous strain accumulation and release over the decades. in contrast, figure 3 focuses specifically on the postseismic period following the february 6, 2023 earthquakes, covering seismic events between march 2 and december 31, 2023. the clustering of events in figure 3 largely mirrors the historical pattern seen in fig. 2, particularly along the same fault segments, indicating that the february 6 earthquakes have reactivated portions of the fault system. considering the epicenters of these earthquakes (fig. 2 and fig. 3), it is seen that the earthquakes generally occurred along the eafz and the magnitudes of the major earthquakes in and around malatya province range between 5 ≤ m < 6. moreover, several moderate-magnitude events (mw ≥ 5.0) in both figures highlight the ongoing seismotectonic activity in the region. this temporal and spatial consistency between the long-term and postseismic datasets underscores the significance of continued monitoring through both geodetic and seismological methods. the time series of the gnss data between january 1 and march 1, 2023 (figs. 4a-4n), the horizontal displacement values (table 1) and the displacement vectors (fig. 6), clearly reflect the coseismic deformation associated with the february 6, 2023 kahramanmaraş earthquakes. the ekz1 station, located in ekinözü (kahramanmaraş) and in close proximity to the epicenter of the second mainshock (mw 7.6) of the february 6, 2023 earthquake sequence, recorded the most substantial coseismic displacement among all stations analyzed. the total horizontal displacement reached approximately 30.94 meters, with the motion predominantly directed toward the west, comprising a north component of +3314.28 mm and a strikingly large east component of −30759.05 mm. the displacements presented here were obtained by calculating the vector sum of the north and east components listed in table 1. this extreme displacement highlights the proximity of the station to the rupture zone and the intense crustal deformation imposed by the fault movement. ekz1 is situated on the eastern block of the east anatolian fault (eaf), and the observed displacement pattern is consistent with the left-lateral strike-slip mechanism of the fault, where the eastern block moves westward relative to the western block. the remarkable magnitude and direction of motion at this site provide key evidence for characterizing nearfield fault slip and stress release associated with the mw 7.6 event. the mar1 station, located in kahramanmaraş near the epicentral region of the february 6, 2023 earthquake sequence, experienced significant coseismic displacement associated with the rupture of the east anatolian fault (eaf) and its adjacent segments. the station recorded a total horizontal displacement of approximately 5.16 meters, with motion directed toward the west-southwest, composed of a north component of −1426.75 mm and an east component of −4958.94 mm. this displacement pattern reflects the intense localized crustal deformation that occurred in the immediate vicinity of the rupture zone, where complex fault interactions, including the eaf and secondary structures such as the çardak and sürgü faults, play a significant role. the observed southwestward motion at mar1 is consistent with the left-lateral strike-slip mechanism of the eaf and supports the interpretation of large-scale strain release and block rotation during the mainshock sequence. the proximity of mar1 to the rupture initiation zone makes it a key site for understanding nearfield tectonic responses during the kahramanmaraş earthquakes. the mly1 station, located in malatya on the western block of the east anatolian fault (eaf), exhibited a substantial coseismic displacement following the february 6, 2023 kahramanmaraş earthquake sequence. the station recorded a total horizontal displacement of approximately 6.28 meters, with motion directed toward the southwest, consisting of a north component of −5440.69 mm and an east component of −3234.58 mm. this southwestward movement aligns with the expected behavior of the western block of the eaf, which is known to shift left-laterally relative to the eastern block during rupture events. the observed displacement at mly1 reflects the dynamic tectonic response of the western anatolian crust to the rupture propagation along the eaf, particularly during the second mainshock (mw 7.6). this motion provides critical evidence for understanding strain release and block kinematics in the vicinity of the fault during large-scale earthquake sequences. the ady1 station, located in adıyaman on the eastern block of the east anatolian fault (eaf), recorded a significant displacement following the february 6, 2023 kahramanmaraş earthquakes (mw 7.7 and mw 7.6). the total displacement at the site reached approximately 5.15 meters, with a dominant motion toward the east, characterized by a north component of −459 mm and an east component of +5125 mm. this eastward movement is consistent with the left-lateral strike-slip character of the eaf, where blocks located on the eastern side are expected to shift eastward relative to those on the western side. the observed displacement at ady1 reflects the coseismic deformation associated with the rupture of the eaf and highlights the significant tectonic response of the eastern block to the stress release during the mainshock sequence. these findings provide important insights into the distribution of strain accumulation and release in the vicinity of the fault zone. the ante station, located in the city center of gaziantep and positioned southeast of the rupture zones of the february 6, 2023 kahramanmaraş earthquakes, recorded a significant coseismic displacement of approximately 3.66 meters. the motion was directed toward the northeast, with a north component of +2584.67 mm and an east component of +2594.41 mm. although located outside the primary rupture segments of the eafz, the displacement observed at ante reflects the broader regional stress redistribution and block interaction induced by the mainshock sequence. the northeastward motion of the station suggests a response consistent with the clockwise rotation and extrusion of the anatolian plate, and is compatible with the left-lateral strikeslip mechanism of the eaf, which plays a key role in accommodating the westward escape of anatolia. the recorded movement at ante provides valuable information on the far-field tectonic effects and the spatial extent of deformation resulting from this major seismic event. the kls1 station, located in kilis in the southern portion of the east anatolian region, recorded a coseismic displacement of approximately 2.62 meters following the february 6, 2023 kahramanmaraş earthquakes. the motion was oriented toward the north-northeast, with a north component of +2318.96 mm and an east component of +1262.10 mm. although situated outside the immediate rupture zone of the eafz, the station lies within the broader deformational field affected by the earthquake sequence. the observed displacement at kls1 reflects regional stress transfer and crustal adjustment related to the interaction between the anatolian and arabian plates. the northnortheastward motion is consistent with the broader tectonic escape pattern of anatolia and the left-lateral displacement along the eaf. this movement supports the interpretation that significant strain accumulation and release extended beyond the main rupture area, influencing deformation in more distal parts of southeastern anatolia. the oniy station, located in osmaniye to the south of the eafz, recorded a coseismic horizontal displacement of approximately 2.22 meters in response to the february 6, 2023 kahramanmaraş earthquake sequence. the motion was directed toward the southwest, consisting of a north component of −1793.25 mm and an east component of −1322.22 mm. although the station is situated outside the immediate rupture zone of the eafz, the observed displacement indicates significant far-field deformation induced by the earthquake sequence. the southwestward motion is compatible with the regional tectonic framework involving the relative convergence of the arabian and anatolian plates and reflects strain transfer across broader zones beyond the main fault trace. oniy’s displacement highlights the spatial extent of coseismic deformation and the dynamic response of the upper crust in the southern margin of the affected region. the hat2 station, located in hatay province in the southern part of the east anatolian region, recorded a horizontal displacement of approximately 1.23 meters following the february 6, 2023 kahramanmaraş earthquakes. the station’s motion was oriented toward the southwest, with a north component 327ayça çırmık, i̇brahim berkan kirdök of −646.26 mm and an east component of −1046.84 mm. although hat2 lies at a considerable distance from the main rupture segments of the eafz, the recorded displacement reflects the far-field coseismic effects and broader regional crustal adjustment induced by the earthquake sequence. the observed motion is consistent with the overall tectonic framework of the region, where convergence between the anatolian and arabian plates and interaction with nearby structures such as the dead sea fault zone and the antakya fault contribute to the complex deformation pattern. the southwestward displacement at hat2 underscores the wide spatial extent of the tectonic response and the efficiency of stress transfer during large-magnitude seismic events in southeastern anatolia. the guru station, located in gürün district of sivas province, lies northwest of the main rupture zone of the february 6, 2023 kahramanmaraş earthquakes and on the western block of the eafz. the station recorded a horizontal displacement of approximately 1.57 meters, directed toward the southwest, with a north component of −811.90 mm and an east component of −1345.14 mm. although located at a considerable distance from the epicentral region, the displacement at guru reflects the propagation of coseismic strain through the broader deformational network of central-eastern anatolia. this southwestward motion is consistent with the left-lateral kinematics of the eafz and indicates that stress release during the mainshock sequence extended into transitional zones between the east and north anatolian fault systems. the recorded motion at guru highlights the spatial extent and complexity of crustal deformation in tectonically diffuse regions during large-scale seismic events. the southwestward movement of guru is consistent with other stations north of the fault, such as mly1 and mar1, indicating a coherent regional deformation pattern. the apk1 station, located in arapkir (malatya) on the western block of the eafz, recorded a coseismic horizontal displacement of approximately 1.37 meters in response to the february 6, 2023 kahramanmaraş earthquake sequence. the movement was oriented toward the southwest, with a north component of −1195.68 mm and an east component of −676.97 mm. although apk1 is located at a moderate distance from the primary rupture zones, the station lies within a tectonically sensitive region bordered by secondary fault structures and the northern boundary of the malatya fault. the southwestward motion observed at apk1 is consistent with the left-lateral strike-slip behavior of the eaf system and suggests effective transmission of coseismic strain into the interior of the anatolian plate. this displacement highlights the broad spatial impact of the earthquake sequence and emphasizes the importance of transitional fault zones in the regional deformation field. the siv1 station, located in siverek (şanlıurfa) to the south of the eafz, recorded a horizontal displacement of approximately 0.98 meters following the february 6, 2023 kahramanmaraş earthquakes. the motion was directed toward the east-southeast, with a north component of −290.22 mm and an east component of +940.42 mm. although the station is located at a considerable distance from the main rupture zones, the observed displacement indicates the propagation of coseismic strain into the broader anatolian–arabian plate boundary region. the east-southeastward motion is consistent with the regional stress field and the left-lateral kinematics of the eafz. the recorded movement at siv1 reflects far-field deformation resulting from the earthquake sequence and contributes to understanding the spatial extent of crustal response beyond the immediate rupture zone. the surf station, located in the city center of şanlıurfa, recorded a horizontal displacement of approximately 1.29 meters in response to the february 6, 2023 kahramanmaraş earthquake sequence. the movement was directed toward the east-southeast, with a north component of −279.40 mm and an east component of +1261.82 mm. although situated far from the main rupture segments of the eafz, surf locates within the broader deformation field influenced by stress redistribution along the anatolian–arabian plate boundary. the observed displacement is consistent with far-field tectonic adjustment and reflects the capacity of large-magnitude seismic events to induce crustal motion over wide regions. the east-southeastward motion at surf highlights the extended reach of coseismic strain and provides insights into how distant intraplate areas respond dynamically to fault rupture processes. the ergn station, located in ergani (diyarbakır), recorded a horizontal displacement of approximately 0.34 meters in response to the february 6, 2023 kahramanmaraş earthquakes. the motion was oriented toward the east-southeast, consisting of a north component of −110.92 mm and an east component of +319.93 mm. although the station is situated at a considerable distance from the main rupture zones of the eafz, the recorded displacement reflects far-field coseismic deformation transmitted through the anatolian– arabian plate boundary zone. the relatively small but clearly measurable displacement observed at ergn is consistent with the broader tectonic pattern of left-lateral shearing along the eafz and regional strain redistribution during large-magnitude seismic events. these findings highlight the widespread influence of the kahramanmaraş earthquake sequence and underscore the dynamic crustal response observed in distal intraplate areas. the elaz station, located in elazığ near the eastern extension of the eafz, recorded a horizontal displacement of approximately 0.70 meters following the kahramanmaraş earthquake sequence. the motion was directed toward the southwest, with a north component of −551.37 mm and an east component of −435.57 mm. although situated outside the immediate rupture zone of the 2023 events, the station lies within a tectonically active corridor previously affected by the 2020 mw 6.8 elazığ–sivrice earthquake. the southwestward displacement at elaz reflects continued postseismic and interseismic deformation in the region and demonstrates the far-reaching effects of coseismic stress redistribution along the eafz. the recorded motion contributes to a more comprehensive understanding of how strain is accommodated along the eastern segments of the fault during and after largemagnitude ruptures. relatively more minor displacements were recorded at ergn (0.34 m) and elaz (0.70 m), indicative of diminished coseismic effects at greater distances. the vector orientations and magnitudes provide strong geodetic evidence for the left-lateral strike-slip behavior of the eafz and highlight the asymmetric distribution of coseismic strain across the region. the magnitude and direction of the displacement vectors, ranging from 0.34 m (ergn) to 30.9 m (ekz1), clearly illustrate the spatial distribution of coseismic deformation induced by the february 6, 2023 earthquakes. the values and directions of displacements (table 1 and fig. 4 and fig. 6) provide strong geodetic evidence for the left-lateral displacement along the eafz, confirming the complex fault behavior and strain partitioning in the region. the horizontal displacement vectors (table 2, fig. 5 and fig. 7) derived from gnss observations during the postseismic period between march 2 and december 31, 2023, reveal continued deformation across the east anatolian region following the february 6, 2023 earthquakes. the displacements were calculated as the vector sum of the north and east components presented in table 2. although the amplitudes of these displacements are significantly smaller than those recorded during the coseismic period, their spatial distribution and vector orientations provide valuable insights into the ongoing strain redistribution and fault relaxation processes. notable postseismic displacements were observed at several stations located both on and around the eafz. according to the postseismic period between march 2 and december 31, 2023, (table 2) mly1 recorded a horizontal displacement of approximately 70.84 mm to the south (negative north component) and 47.94 mm to the west (negative east component), corresponding to a total displacement of 85.5 mm/yr in the southwest (sw) direction. this substantial displacement reflects the ongoing postseismic deformation following the february 6, 2023 kahramanmaraş earthquakes and is indicative of the viscoelastic relaxation and stress redistribution processes affecting the region. the consistent southwestward motion, driven by both the north and east components shifting in negative directions, suggests that the station continues to respond to tectonic re-equilibration along the western segment of the eafz. furthermore, the occurrence of m > 5 earthquakes in malatya in august 2023 supports the notion that stress transfer and fault loading are still active across the region. the observed displacement at mly1 likely captures the combined effect of both large-scale postseismic relaxation and localized crustal adjustments triggered by subsequent moderate-magnitude seismic events. ady1 exhibited a horizontal displacement of approximately 14.87 mm toward the north and 81.20 mm toward the east, resulting in a total displacement of 82.6 mm/yr in the east direction. this dominant eastward motion indicates that the station sustained to be significantly affected by postseismic deformation processes following the february 6, 2023 kahramanmaraş earthquakes. ady1 is located near the eastern continuation of the eafz, where the right-lateral strike-slip kinematics of the arabian–anatolian plate boundary result in substantial eastward and slightly northward motion. the large magnitude and strongly undirectional eastward displacement suggest that stress redistribution 328 regional displacement in malatya and adjacent regions (eastern anatolia) and viscoelastic relaxation are still ongoing in the region. the continued deformation recorded by ady1 reinforces the idea that the postseismic signal remained prominent throughout the rest of 2023, particularly in regions along and near the eafz’s eastern segments. the suf1 station was installed at a location close to the former surf station, and effectively continues monitoring crustal deformation in the same area. it presented a horizontal displacement of approximately 28.99 mm toward the north and 57.36 mm toward the east, corresponding to a total displacement magnitude of 64.3 mm/yr in the northeast direction. this notable northeastward motion indicates that suf1 continued to reflect the influence of postseismic deformation following the february 6, 2023 kahramanmaraş earthquakes. although situated outside the primary rupture zone, the station’s location within the broader deformation field of the eafz and the arabian–anatolian plate boundary makes it sensitive to stress transfer and viscoelastic processes induced by the mainshock. the displacement pattern observed at suf1 likely represents the combined effects of regional viscoelastic relaxation and ongoing crustal adjustment, providing evidence that the postseismic signal remained active in southeastern anatolia well into late 2023. kls1 exhibited a horizontal displacement of approximately 56.86 mm toward the north and 29.80 mm toward the east, resulting in a total displacement of 64.2 mm/yr in the northeast direction. this significant northeastward motion indicates a continued influence of postseismic deformation caused by the february 6, 2023 kahramanmaraş earthquakes. although kls1 lies outside the immediate rupture zone, it is positioned within the broader deformation field of the eafz and the adjacent dead sea fault system. the displacement observed at kls1 suggests that the region has remained tectonically active during the postseismic phase, with ongoing viscoelastic relaxation and stress redistribution influencing crustal motion. the northward component is dominant, yet the substantial eastward movement implies complex stress pathways in this transitional tectonic zone. kls1’s displacement pattern provides further evidence that the effects of the february 6 earthquakes extended well beyond the rupture areas, continuing to drive measurable crustal motion across southeastern türkiye throughout 2023. elaz presented a horizontal displacement of approximately 60.14 mm toward the south (negative north component) and 7.86 mm toward the west (negative east component). these values correspond to a total displacement of 60.7 mm/yr directed predominantly toward the south. this distinct southward motion indicates that elaz was still responding to the postseismic deformation induced by the february 6, 2023 kahramanmaraş earthquakes. elaz lies within the eastern section of the eafz, a region that has already experienced major seismic activity in recent years, including the 2020 elazığ earthquake (m6.8). the movement pattern observed at elaz during this period suggests that viscoelastic relaxation and stress transfer processes were still ongoing in the crust surrounding the eafz. the station’s relatively linear southward displacement, with minimal east-west contribution, implies a predominantly vertical redistribution of stress along the fault system, possibly influenced by the broader tectonic re-equilibration occurring after the 2023 events. this reinforces the interpretation that the effects of the february 6 mainshocks extended eastward, continuing to drive crustal readjustment in areas that had already been structurally stressed in previous years. siv2 exhibited a horizontal displacement of approximately 20.29 mm toward the north and 50.93 mm toward the east, resulting in a total displacement of 54.8 mm/yr in the east-northeast direction. this displacement pattern reflects the continued postseismic deformation following the february 6, 2023 kahramanmaraş earthquakes. although siverek is situated outside the primary rupture zones, its location in the southeastern anatolian region places it within the broader deforming area influenced by stress changes along the eafz and the arabian–anatolian plate boundary. the strong eastward and moderate northward movement suggests that viscoelastic relaxation and stress redistribution remained active in the region during the observation period. the displacement direction is consistent with the general tectonic escape of the anatolian plate toward the west-northwest, but modified locally due to the lingering effects of the february earthquakes. overall, the kinematic behavior of siv2 provides valuable insight into the regional-scale postseismic response of the southeastern anatolian crust in the months following the 2023 seismic crisis. guru represented a horizontal displacement of approximately 5.99 mm toward the south (negative north component) and 37.27 mm toward the west (negative east component), corresponding to a total displacement of ~37.7 mm/yr directed toward the west. although gürün (sivas) is positioned north of the main rupture zones of the february 6, 2023 kahramanmaraş earthquakes, the displacement pattern observed at guru suggests that the area continued to experience secondary postseismic effects, likely due to long-range viscoelastic relaxation and stress transfer propagating northward along the broader deforming corridor of eastern anatolia. the predominantly westward motion of guru may also reflect tectonic coupling variations across the region, where residual stress redistribution causes minor crustal displacements even in areas removed from the primary rupture zones. in this context, guru’s motion can be interpreted as part of the broader postseismic deformation field that extended into the interior of the anatolian plate. these findings indicate that the far-field impact of the 2023 earthquakes remained geodetically detectable across a wide region, including the northern periphery of the eafz. apk1 recorded a horizontal displacement of approximately 30.48 mm toward the south (negative north component) and 20.13 mm toward the west (negative east component). this corresponds to a total displacement of 36.5 mm/yr directed toward the southwest. situated northwest of the main rupture zones of the february 6, 2023 kahramanmaraş earthquakes, the displacement at apk1 suggests that the station was still responding to postseismic deformation, likely due to delayed viscoelastic relaxation and regional-scale stress redistribution. the southwestward motion is consistent with the direction of coseismic and postseismic crustal flow observed across the eafz and its surrounding regions. although the magnitude of displacement at apk1 is lower than at stations closer to the epicentral area, it still indicates tectonic sensitivity to the broader post-earthquake deformation field. the presence of this motion months after the mainshock sequence supports the interpretation that crustal adjustments remained active throughout the second half of 2023, even in locations removed from the principal fault rupture. ergn recorded a horizontal displacement of approximately 19.02 mm toward the north and 28.03 mm toward the east and the total displacement value of 33.9 mm/yr directed toward the northeast. although ergn is situated at some distance from the primary rupture zones of the february 6, 2023 kahramanmaraş earthquakes, the station still recorded a measurable northeastward displacement during the postseismic period. this motion likely reflects the influence of long-wavelength viscoelastic relaxation processes affecting the broader arabian–anatolian collision zone. the displacement pattern is consistent with the overall tectonic escape of the anatolian plate toward the west and northwest, while also capturing the local expression of postseismic stress transfer extending into the southeastern regions of the deforming zone. the persistent northeastward motion observed at ergn supports the notion that postseismic effects propagated over a wide area, with crustal adjustments continuing throughout the remainder of 2023. hat2 recorded a horizontal displacement of approximately 26.03 mm toward the north and 17.51 mm toward the west (negative east component). these values correspond to a total displacement magnitude of 31.4 mm/ yr directed toward the northwest. situated near the southern terminus of the eafz and within the rupture area of the february 6, 2023 earthquakes, hat2 remains in a highly sensitive tectonic environment. the station’s northwestward motion during the postseismic period likely reflects a combination of coseismic stress redistribution, viscoelastic relaxation, and possible interactions with the adjacent dead sea fault and cyprus arc systems. the continued deformation recorded at hat2 indicates that the postseismic signal remained geodetically detectable in hatay well into late 2023. the displacement pattern is also consistent with the complex tectonic regime of the region, where multiple plate boundaries and fault systems converge, producing oblique deformation and heterogeneous crustal responses. mar1 recorded a horizontal displacement of approximately 19.75 mm toward the north and 13.38 mm toward the east, corresponding to a total displacement of 23.9 mm/yr directed toward the northeast. positioned near the epicentral region of the devastating february 6, 2023 earthquakes, mar1 lies within the zone that experienced intense coseismic ground deformation. the continued northeastward movement observed in the postseismic period highlights the station’s sensitivity to ongoing viscoelastic relaxation, afterslip, and stress redistribution around the ruptured segments of the eafz. while the displacement magnitude is modest compared to the immediate coseismic offsets, it reflects a persistent postseismic signal that was still active nearly a year after the mainshock sequence. the direction of movement—toward the ne—is consistent with the overall kinematic pattern of crustal adjustment 329ayça çırmık, i̇brahim berkan kirdök across southeastern anatolia following the rupture of multiple fault strands in february 2023. the mar1 data provide critical constraints for understanding long-term deformation transients in the core rupture area and for modeling the spatiotemporal evolution of postseismic processes in one of türkiye’s most complex tectonic settings. oniy recorded a horizontal displacement of approximately 14.50 mm toward the north and 5.36 mm toward the east, resulting in a total displacement value of 15.5 mm/yr in a generally north-northeast direction. although osmaniye lies just south of the eafz and outside the main rupture areas of the february 6, 2023 earthquakes, the displacement recorded at oniy represents the continued regional crustal response to the seismic sequence. the modest north-northeastward motion is indicative of long-range viscoelastic relaxation and potential secondary stress adjustments within the arabian–anatolian plate boundary region. given its location near the junction of the eafz, the dead sea fault, and the eastern mediterranean subduction complex, the crustal dynamics around osmaniye are inherently complex. the motion at oniy likely reflects a combination of postseismic effects and broader tectonic coupling between interacting fault systems in this transitional zone. this displacement pattern reinforces the idea that measurable postseismic deformation persisted well into late 2023, even in peripheral areas such as osmaniye. ekz1 recorded a horizontal displacement of approximately 7.43 mm toward the north and 12.74 mm toward the west (negative east component), corresponding to a total displacement magnitude of 14.7 mm/yr directed toward the northwest. situated close to the northern edge of the rupture zone of the february 6, 2023 earthquakes, ekz1 lies in a region that experienced strong ground shaking and intense coseismic deformation. the proceeded northwestward motion observed during the postseismic period reflects the influence of residual stress redistribution, after-slip, and viscoelastic relaxation processes still active in the crust surrounding the eafz. although the displacement magnitude is moderate, its direction and persistence suggest that postseismic deformation remained measurable in this part of the fault system throughout the rest of 2023. the combination of northward and westward components is also consistent with the local tectonic geometry, where oblique convergence and strike-slip motion jointly shape crustal kinematics. ekz1 serves as a valuable indicator of how the northern segments of the rupture system continued to deform following the mainshock, and contributes to a more complete understanding of the spatial extent of the postseismic response. mly1 station experienced a major coseismic displacement (6329.6 mm) toward the southwest during the february 6, 2023 earthquakes, indicating intense ground motion and proximity to the rupture zone. in the postseismic period, the station continued to move in the same southwestward direction, but with a reduced amplitude (85.5 mm), which is still substantial. the persistence of southwestward motion suggests ongoing postseismic deformation, likely due to viscoelastic relaxation and afterslip along the eafz. although the amplitude of displacement decreased the directionality remained consistent, indicating that the tectonic forces and stress fields responsible for coseismic movement continued to influence the region for many months after the mainshock. apk1 and elaz experienced coseismic southwestward displacements (1.37 m and 0.70 m, respectively), followed by additional postseismic movements of 36.5mm (apk1) and 60.7 mm (elaz). the continuation of movement along the same azimuth supports the hypothesis of afterslip and/or viscoelastic adjustment in the northern block. ady1 and kls1, located south of the fault, showed coseismic displacements of 5.15 m and 2.64 m, toward the east and northeast, respectively and continued moving in the same direction for each station postseismically (82.6 mm and 64.2 mm). these patterns indicate that the southern block, although less deformed than the northern block, is also undergoing postseismic relaxation consistent with left-lateral strike-slip motion. siv2, surf (suf1), and ergn, positioned farther east, experienced lower coseismic displacements (0.98 m, 1.29 m and 0.34 m., respectively toward east), and recorded continued postseismic movements of 54.8, 64.3 and 33.9 mm., respectively toward the northeast. this widespread response, despite the distance from the rupture zone, reflects strain propagation into the arabian plate. hat2 experienced a coseismic southwestward displacement of 1.23 m and continued to move northwestward by 31.4 mm postseismically. this movement suggests continued deformation along the southern block of the fault, likely due to stress redistribution or afterslip near the antakya segment. mar1, which moved 5.16 m west-southwest during the mainshock, recorded 23.9 mm of postseismic displacement toward the northeast. this change in direction may indicate complex interactions in the central rupture zone, possibly involving local block rotations or secondary fault activity. oniy shifted 2.23 m southwest coseismically, followed by 15.5 mm of northeastward movement postseismically. this reversal is particularly interesting, as it suggests postseismic strain recovery or re-equilibration within the southern block. meanwhile, ekz1 (ekinozü, kahramanmaraş), which underwent the largest coseismic displacement (30.94 m), exhibited very little postseismic motion (14.7 mm), indicating that strain release in that segment was nearly complete during the mainshock. guru, which moved 1.57 m southwest during the mainshock, continued to experience westward displacement of 37.7 mm postseismically, supporting the presence of extended deformation in the northern back-arc region. consequently, it can be said that the stress changes caused by the february 6, 2023 kahramanmaraş earthquakes continue to induce displacements in the region. therefore, the region remains seismically active, and enhancing geoscientific and engineering studies in the area will be crucial in preventing loss of life and property in the event of potential future earthquakes. conclusions the integrated analysis of seismic activity and gnss-derived deformation patterns following the february 6, 2023 kahramanmaraş earthquakes reveals the complex and ongoing tectonic response of the eafz. the coseismic phase was characterized by sudden, high-magnitude displacements, particularly at stations located near the rupture zone (e.g., ekz1, mly1, mar1), illustrating the abrupt strain release associated with fault rupture. in contrast, the postseismic phase between march 2 and december 31, 2023 was marked by slower, lowermagnitude motions, reflecting a gradual redistribution of stress and continued crustal adjustment throughout the region. the contrast between these two phases underscores the time-dependent nature of tectonic deformation. while many stations (e.g., mly1, ady1, kls1) exhibited continued motion along the same vectors as during the mainshock—indicative of consistent stress relaxation—others (e.g., mar1, oniy) showed modified or even reversed displacement directions, suggesting localized variations in fault behavior, afterslip, viscoelastic relaxation, and possible secondary fault interactions. though smaller in magnitude, the postseismic displacement vectors provide essential insights into the dynamic processes that occur after major seismic events. the spatial distribution of both coseismic and postseismic displacements aligns with the left-lateral strike-slip kinematics of the eafz, extending well beyond the immediate rupture zone. stations located further from the fault, such as guru, suf1, ergn, and hat2, also recorded significant postseismic movement, indicating regional strain propagation and the widespread impact of the february 6, 2023 earthquakes. this broader deformation pattern emphasizes that tectonic adjustments are not limited to the main fault trace but may influence adjacent crustal blocks, including back-arc and foreland regions. notably, the recurrence of moderate-magnitude earthquakes (mw ≥ 5.0) in malatya and surrounding areas in the months following the mainshock suggests that stress changes induced by the february 6,2023 events continue to influence seismic activity in the region. the temporal and spatial consistency between historical seismicity (1990–2023) and post-earthquake events further supports the interpretation of the malatya segment as a persistent zone of strain accumulation and release. these findings highlight the necessity of integrating both coseismic and postseismic observations to fully understand fault mechanics and the evolution of regional strain, particularly in tectonically complex settings like eastern anatolia. continuous gnss monitoring is essential for capturing long-term deformation trends, assessing evolving seismic hazards, and improving the accuracy of future earthquake forecasting models. in conclusion, the east anatolian region remains seismically active in the aftermath of the february 6, 2023 earthquakes. the observed patterns of postseismic motion, ongoing crustal adjustment, and repeated moderatemagnitude events underscore the urgency of enhancing geoscientific and engineering research, implementing early warning systems, and updating seismic risk mitigation strategies to reduce potential loss of life and property in future seismic events. 330 regional displacement in malatya and adjacent regions (eastern anatolia) acknowledgment this study is realized under the support of tubitak 2209-a project (project no: 1919b012321750). we would like to thank the staff and collaborators of the general directorate of land registry and cadastre and republic of türkiye ministry of national defence directorate general for mapping for providing the gnss data, as well as to afad for earthquake data utilized in this study. many of figures were created in this study using gmt v6 (wessel et al. 2019). additionally, we would like to thank the two anonymous reviewers for their constructive comments and valuable suggestions, which significantly contributed to the improvement of the manuscript. statements & declarations author contributions: i̇brahim berkan kırdök downloaded and regulated the gnss and seismological data; ayça çırmık processed the data, performed the analysis, and drafted the manuscript; ayça çırmık and i̇brahim berkan kırdök designed the figures. all authors discussed the results and commented on the manuscript and all authors contributed to writing the manuscript. funding: open access funding provided by the scientific and technological research council of türkiye (tubitak). this study is realized under the support of tubitak 2209-a project (project no: 1919b012321750). competing interests: the authors have no relevant financial or nonfinancial interests to disclose. data availability: the earthquake datasets were obtained from afad (ministry of interior disaster and emergency management presidency earthquake catalog: https://deprem.afad.gov.tr/event-catalog. gnss datasets obtained from cors-tr web page: https://www.tusaga-aktif.gov.tr/ conflict of interest: the authors have not disclosed any competing interests. references acarel, d., cambaz, m. d., turhan, f., mutlu, a. k., & polat, r. 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(2025). gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria). earth sciences research journal, 29(1), 101-112. https://doi.org/10.15446/esrj. v29n1.117121 record manuscript received: 19/10/2024 accepted for publication: 19/02/2025 abst r ac t the large irrigation areas of the mitidja plain are areas that are frequently prone to flooding, resulting in significant losses for farmers and the agency managing the hydraulic infrastructures. the objective of this study is to develop a model for identifying, evaluating and mapping the flood-prone areas of these lias, in order to allow optimal management of hydraulic infrastructures and natural resources. to this end, we have used gis tools and remote sensing to create a geodatabase of flood-control factors such as lithology, morphopedology, slope, altitude, natural drainage, artificial drainage and land use. this modelling is based on a multi-criteria combination of the geographical data related to the above factors. a combination of the factors considered was carried out using the spatial analysis tools provided by arcgis software to produce a flood risk assessment map. the result is a map showing the spatial distribution of flood risk areas in the large irrigation area of the western mitidja, classified into four classes (low, medium, high, very high) risk. this result was verified by field surveys. it was also confirmed by comparing with the halloula lake controlled area. keywords: large irrigation area; mitidja plain; flood risk; gis; spatial analysis. palabras clave: grandes áreas irrigadas; llanura de mitidja; riesgo de inundación; sistema de información geográfica; análisis espacial. evaluación del riesgo de inundación a través del sistema de información geográfica en una área de gran irrigación al oeste de la llanura de mitidja, algeria las grandes áreas irrigadas de la llanura de mitidja son zonas propensas a las inundaciones, lo que deriva en perdidas significativas tanto para agricultores como para las entidades de manejo de las infraestructuras hidráulicas. el objetivo de este estudio es desarrollar un modelo para identificar, evaluar y mapear estas grandes áreas irrigadas con el fin de permitir una administración óptima de las infraestructuras hidráulicas y de los recursos naturales. con este fin los autores utilizaron herramientas del sistema de información geográfica y de detección remota para crear una base de datos geológica con los factores que controlan las inundaciones, como la litología, la morfopedología, la inclinación, la altitud, el drenaje natural, el drenaje artificial y el uso del suelo. este modelamiento se base en una combinación de múltiples criterios de información geográfica relacionada con los factores antes mencionados. una combinación de estos factores se llevó a cabo a través de herramientas de análisis espacial producidas con el software arcgis para realizar un mapeo evaluatorio de los riesgos de inundación. el resultado es un mapa que muestra la distribución espacial de las zonas propensas a las inundaciones en las grandes áreas irrigadas del oeste de la llanura de mitidja, clasificadas en cuatro clases de riesgo: bajo, medio, alto y muy alto. este resultado se verificó con estudios de campo. también se confirmó a través del área controlada del lago halloula. resu m en https://doi.org/10.15446/esrj.v29n1.117121 https://doi.org/10.15446/esrj.v29n1.117121 https://doi.org/10.15446/esrj.v29n1.117121 102 hind raked, nour el islam bachari, mohamed saϊd guettouche, mohammed tayeb touahri 1. introduction hydrometeorological disasters, such as floods, storms, cyclones, etc., are devastating natural phenomena (gabriel p et al., 2024). due to the significant socio-economic and environmental damages they cause, these events have become a global concern in this century (mirza et al., 2002). these disasters have intensified in recent years as a result of climate change affecting the planet (tomasz et al., 2024). according to a report by the united nations under the international strategy for disaster reduction, floods are the most widespread disasters, frequently occurring across multiple regions worldwide (uno., 2024). moreover, numerous authors have also predicted a significant increase in the frequency of floods in the coming years (kundzewicz et al., 2013; mirza et al., 2002; van aalast, 2006; ipcc, 2007; hirabayashi et al., 2008; whitfield, 2012; giardino, 2012). in algeria, flood risk is among the ten most significant major risks (law n°04–20 of december 25, 2004). given the extent of the material and human damage it causes, whether in the northern tellian strip or the southern desert (roukh et al., 2018), this phenomenon is ranked second after seismic risk (ziadi et al., 2024). in fact, floods have occurred with unusual frequency over recent decades (khouas et al., 2021). the annual regional distribution of flood frequency has shown that the highest number of floods was recorded in the country’s central region, followed by the east, indicating that these areas are more vulnerable to floods (hafnaoui et al., 2023). this phenomenon affects both urban and rural areas. although the consequences are catastrophic in both urban centers and rural suburbs (dhungana et al., 2024; jamshid et al., 2024), studies on rural flooding are less common than those on urban areas, where loss of human lives is higher during floods (jamshid et al., 2024). however, several authors have shown that rural floods can also have positive effects, contributing to the ecological balance of plains (mirza et al., 2002) by fertilizing soils, filtering excess or pollutant elements, recharging groundwater, and slowing the salt wedge phenomenon (the intrusion of brackish seawater into groundwater). nevertheless, the impacts are undeniably fatal, with devastating effects on agriculture and rural life in general, as the majority of food-insecure people after disasters live in rural areas (jamshid et al., 2024). therefore, equitable flood risk management must be promoted across different geographic contexts (dhungana et al., 2024). our study area, specifically the large irrigated perimeters (lia) of mitidja west, faces significant flooding issues during heavy rains. despite this, the area was neither included in the national flood management strategy developed (mre., 2015), which identified and classified 689 flood-prone sites across the national territory nor considered in the master plans for planning and urbanism (pdau.,2009) of the concerned municipalities. for instance, the pdau of the municipality of sidi rached, which partially encompasses our study area, stated that the municipality does not face any major risks. the lack of attention to this phenomenon poses a serious problem for managers and users of this lias. thus, the fundamental issue of this work is: how can we design a tool to identify and classify flood risk areas? as a result, the objective of this research is to develop a geospatial model to identify and classify flood risk areas within the mitidja west lia using a geographic information system (gis). to assess flood risks, we used both factors that exacerbate and those that mitigate this type of risk. the selected criteria are heterogeneous, encompassing multi-source and multi-scale factors. the aim is to synthesize these into cartographic document that reflect the reality of flood risk levels in the large irrigated area (lia) of the western mitidja plain. to achieve this objective, we used surface parameters such as topography (slope and elevation), high-resolution land use data (photo interpretation), and subsurface parameters such as morpho-pedological and geological factors (lithology). this type of model, known as integrated surface and subsurface hydrological models, has demonstrated higher effectiveness by providing a more complex representation of hydrological systems affected by natural and anthropogenic factors (maxwell et al., 2014; le et al., 2015; khatami et al., 2019). the significance of this work lies not only in providing managers and users of these lias with a decision-support tool but also in offering a scientific model that can be applied to other irrigation perimeters. 2. data and methods study area the mitidja plain is located in the north-central part of algeria and extends in an ene-wsw direction over a length of 100km. it corresponds to a large depression of alluvium and colluviums between wadis nador and wadis boudouaou. it is bordered by the low hills of the sahel to the north and the foothills of the blidean atlas to the south. the plain has geographical characteristics that increase the impact of flood phenomena (fig.1). the large irrigation area (lia), the object of our study, is located in the western part of mitidja plain. it should be noted that the site of lake halloula, located near the lia west mitidja, was included in the study as a control area. statistically, it was integrated into the second range of this lia. the climate of the region is mediterranean: characterized by cool, wet winters and hot, dry summers, wich is classified in a subhumid bioclimatic level according to the climatogram of l. emberger (seltrez., 1938). a rainfall study including 13 stations which are dispersed on the whole of the plain showed that the rainfall average is 742mm for the whole of the stations with 82 days of rain. the lowest value is that posted at the level of ameur el ain with an annual average of 546 mm and with less days of rainfall which is of 61 days of rain (seltrez., 1938), this value is the closest to our zone of study. but we note in the station of mouzaîa lake located on the summits of the blidean atlas at 1270m of altitude, a rainfall value of 933 mm with 88 days of rain (seltrez., 1938), which explains the torrential character of the wadis worsens the flooding of the submerged agricultural lands in the plain. we observe an increase in precipitation going eastwards with altitude. for the study of rainfall in hydroagricultural development study of the mitidja plain (macdonald et al., 1992), it was found that the maximum number of consecutive rainy days was only 5 days for a 2 years period and 7 days for a 10 years return period. the average value for a one day rainfall per return period in the mitidja plain was 56mm of rainfall for a 2 years return period, 96mm for a 10 years return period and it can reach 148mm for one day for a 100 years return period. this study also showed that rainfall increases from east to west and in the foothills to the south. figure 1. study area, the large irrigated area of western mitidja plain (srtm) the lia mitidja west (lia.mo) is located in algérois hodna soummam watershed (fig.1). it straddles between two provinces: blida and tipaza. it is equipped on a surface of 24 200 hectares, launched in two sections: the first section was put into service in 1988 on an equipped surface of 8 600 ha, of which 86% is located in the province of blida and the second section was put into service in 2005 on an equipped surface of 15 600 ha and 92% of its surface is in the province of tipaza (fig.2), the two sections are exploited by two different exploitation units (mouzaïa for the first and el affroun for the second). the water resources mobilized for irrigation come from both the boukerdane dam and the el moustakbal system (bouroumi dam and its three transfers from wadis djer, wadis harbil and wadis chiffa). 103gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria) figure 2. irrigation networks in the large irrigated area of western mitidja plain (agid.,1992) the north facings slopes of the blidean altlas are crossed by several wadis that arrive with important flows, arriving on the plain of low slope, their lengths are short and their external drainage to the sea, is reduced because of the topographic obstacles caused by the hills of the sahel, which is the origin of frequent floods. actually, the irrigated area illustrated in (fig.2), is reduced, as it represents only 22.2% of the irrigable area initially (fig.3), which confirms that water resources in that area are poorly managed. however, lack of irrigation water was recorded during the drought periods, and flooding were reported during the periods of heavy rainfall. this lack of irrigation is also explained by its location in the algérois soummam hodna area, which is in water stress, this has led to a decrease in water for irrigation in order to favor drinking water supply in this region (remini et al., 2010). flooding still continues to cause damage in the mitidja plain despite the scarcity of water during other periods of crop water stress. 25000 w. mitidja i w. mitidja ii total lias. wm su rf ac e he ct ar e eq ui pe d eq ui pe d eq ui pe d ir rig ab le ir rig ab le ir rig ab le ir rig at ed ir rig at ed ir rig at ed 20000 15000 10000 5000 0 figure 3. irrigated surface in the large irrigated areas of west mitidja (onid, 2022) 3. methodology the data used are heterogeneous, multi-source, and at different scales depending on their availability. as show (fig.4), they consist of three types of data : remote sensing data: very high-resolution satellite images acquired via universal maps download, sentinel-2 images, and shuttle radar topography mission (srtm) data downloaded from the usgs website (usgs, 2024). basic maps: irrigation network maps, drainage and sewerage network maps obtained from the national office of irrigation and drainage (onid), morpho-pedological and hydrogeological maps from the national agency for hydrological resources studies (anrh). in-situ data : collected from the directorate of hydraulics, agriculture, and the two operating units of this large irrigation perimeter (gpi). using arcgis 10.8 software, we created a geodatabase structured into two types of parameters based on their influence on the flooding phenomenon: natural parameters (topography, morpho-pedology, lithology). anthropogenic parameters (land use and various networks). subsequently, we spatialized these parameters by creating thematic maps (raster or vector) related to the aforementioned parameters. a rasterization of vector data was performed to enable manipulation using the raster calculator. the rasterization operation consists of switching from vector mode to raster mode using spatial analysis tools: the pixels obtained were resized to (10mx10m) and their values were reclassified according to the weights assigned to each parameter, using a grid detailed in the tables below, all parameters were reclassified by weighted scales from 0 to 5 based on their impact on the flooding phenomenon. next, using the raster calculator, we performed arithmetic operations, namely: • addition of the weights of lithology, morpho-pedology, altitude, and drainage factors to obtain an lssad map. • multiplication of elimination parameters to obtain an lu.s map. the multiplication of the two maps resulted in a flood risk assessment map. the choice of criteria and the assigned weights are explained one by one as follows: modeling of natural criteria the aim of the first stage is to draw up flood risk assessment maps based on natural criteria: lithology, morpho-pedology, slope and altitude. lithological criteria permeability is defined as the capacity of porous geological formations to transmit a reference fluid, such as water, under the effect of a pressure gradient. 104 hind raked, nour el islam bachari, mohamed saϊd guettouche, mohammed tayeb touahri high permeability reduces the risk of stagnation of surface runoff. however, impermeability prevents infiltration of surface water into the ground; it reduces the recharge rate of the groundwater and promotes stagnation of water (bennia et al., 2017). moreover, impermeability increases the probability of flooding risk. for this reason, the highest weight (5) is assigned to lithological formations with low permeability, the lowest weight (1) is given to lithological formations with high permeability, and the weight (2) is assigned to formations with moderate permeability. the hydro-geological map was used to develop the lithological map (fig.5.1). the selected classes and the assigned weights are presented in table 1. gis approach for flood risk assessment multi-source data collection remote sensing basic maps under gis in-situ data spatialization of critaria and parameters natural parameters gis tools (4) flood risk assesment map (3 ) s pt ia l a na lu st to ol s (1 ) g eo da ta ba se s c re at io n + sp at ia liz at io n cr ita ri a rasterization vector data parameters rater data parameters reclassification parameters by weight raster calculator weights (1) = ∑weights (lithology + soil + altitude + drainage) = lsad lsad x lu.s = flood risk assessment topography morphopedology lithology a rit hm et ic o pe ra tio n slope (%) land use (agricultural, practics, urbanization, forest...) artificial drainage and sanitation (networks) altitude (m) anthropogenic parameters weights (2) = weight (land use x slope) = lu.s figure 4. methodology for assessing the risk of flooding in the large irrigated areas lia of mitidja west plain figure 5.1.hydrogeology of western mitidja plain (demrh., 1973) 105gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria) table 1. weights assigned according to permeability permeability type weight risk type strong permeability 1 weak effect average permeability 2 medium effect very low permeability 5 very high effect figure 5.2.weight of lithological permeability lia.mw morphological and soils criteria morpho-pedology is an important parameter in our gis approach to flood risk assessment, as the physical and hydrological characteristics of soil directly influence how water infiltrates, flows, or accumulates. it is used to better identify areas prone to stagnation and hydromorphy (characterised by the presence of gleys and pseudo-gleys). the soil classification map used is that of the cpcs (classification of the commission for pedology and soil mapping). although it is the only one available for the mitidja plain (demrh., 1970) and although it is relatively old, its validity is confirmed with an accuracy of 1/50000, as shown in (fig. 5.3). the morpho-pedological maps resulting from the first stage in vector mode were modified in raster mode on pixels of size (10mx10m) and reclassified according to the weights assigned (tab2). the weights chosen were justified by consulting a soil scientist from the ministry of hydraulics, who helped assign weights for each sub-criterion and soil type according to the four main classes (classes, sub-classes, groups and sub-groups) as explained in table 2. a very high weight (5) is given to a soil sub-criterion that is very conducive to water stagnation (very high risk), the sum of the four weights given to each gives a value that varies between (120), the maximum value (20) is given to soil types that allow a very high risk of flooding and the minimum weight (1) is given to very permeable textured soils. the weight assigned to the morpho-pedological criterion is the sum of weights of classes, subclasses, groups and subgroups (tab.2). this reclassification allowed establishing the map shown in (fig.5.4). the highest values, close to (20), concern first lake halloula and then its surroundings. these are sectors (e, f and g) in the second tranche, followed by the eastern sector in the first tranche of this large irrigated area. other criteria were added to further refine the model. the next step was to include the topography of the terrain. figure 5.3. morpho-pedological maps (classes, sub-classes, groups, sub-groups) of the lia western mitidja plain of the ail (demrh., 1970) 106 hind raked, nour el islam bachari, mohamed saϊd guettouche, mohammed tayeb touahri terrain topography the data obtained from the shuttle radar topography mission (srtm) were utilized to extract the site topography data, namely the altitudes, in order to determine the basins which promote the stagnation of runoff, and the maps showing the angles of inclination of slopes. the shuttle radar topography mission data, with a resolution of one arc second (30m) (farr et al., 2007). low-elevation areas lead to water stagnation (in the lowest points), while gentle slopes result in poor water circulation. these last two topographical criteria will be addressed in the next step. the slope criterion the slope criterion is used because, on gentle slopes, water stagnation is very frequent; conversely, steep slopes favor drainage and runoff. according to the map presented (fig. 5.5), five slope classes have been defined based on the reclassification table assigning weights to the slopes (tab. 3). table 2. reclassification of soil categories criteria category typical weight type risk classes clasess raw mineral soil 1 permeable low poorly developed soil 1 permeable low vertisol 4 low permeability high calcimagnesic soil 1 permeable low iron sesquioxyde soil 1 permeable low hydromorphic soil 5 impermeable very high sub classes low-moisture (or mineral) hydromorphic soils 5 impermeable very high saturated 5 impermeable very high vertisol with reduced or no external drainage 4 low permeability high vertisol with possible drainage 2 medium permeability medium other 1 permeable low groups with gley 5 impermeable very high with pseudogley 5 impermeable very high little leached 4 low permeability high other 1 permeable low sub groups hydromorphic 5 impermeable very high modal 3 low permeability high vertic 4 low permeability high blackened 3 low permeability high with limestone brown facies 1 permeable low lime encrusted 2 medium permeability medium red modal 2 medium permeability medium browned 1 permeable low browned modal 3 low permeability high shallow gley (<80cm) 5 impermeable very high deep gley (>80cm) 5 impermeable very high surface pseudogley 5 impermeable very high leached pseudogley 5 impermeable very high figure 5.4. sum of the weight assigned to the morpho-pedological criteria table 3. reclassification of the weights according to the slope slope classes % weight features risk class classe 1 5 0-2 % very high classe 2 3 2-3 % high classe 3 2 3-4%. medium classe 4 1 4-5 %. low classe 5 0 >5% eliminating criterion it should be noted that classes with slopes of less than 3% present a high to very high risk of flooding, according to the standards used in the construction of drainage networks in algeria. the class characterized by slopes greater than 5% presents a zero risk; this class is therefore eliminated as it poses no flooding risk. consequently, the results obtained show that classes 1 and 2 predominate, which means that the mitidja plain presents a high risk of flooding (fig.5.6). the altitude the altitude criterion was used because the flow of water converges towards the lowest areas, making them the most favorable zones for flooding. altitudes were extracted from the data of the shuttle radar topography mission (srtm) (usgs, 2024). an altitude map was created after verifying the values against topographic maps of the region (fig. 5.7). a reclassification based on altitudes was performed according to the five classes presented in table 4. to calculate the weights for this parameter, a reclassification was carried out according to the five classes (tab.4). for altitudes below 20 m, the risk of flooding is very high, with the highest weight assigned (5). the risk is also high when the altitude ranges between 20 m and 40 m, medium to low between 40 m and 80 m, and zero above 80 m. 107gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria) figure 5.5. slopes map (usgs, srtm., 2024) figure 5.6. weights slope table 4. reclassification of altitudes weights altitudes weight characteristics risk class classe 1 5 0m-20m very high effect classe 2 3 20m-40m high effect classe 3 2 entre 40 m et 60 m medium effect classe 4 1 entre 60 m et 80 m low effect classe 5 0 >80m very low effect despite the presence of a depression at lake halloula, the lowest points were found in the northeastern part of the large irrigated area (lia) of the mitidja plain, which explains the flow of wadi bouroumi and wadi chiffa in the direction of wadi mazafran. it should be noted that these two zones present the greatest risk of flooding (fig.5.8). humans, through their activities, practices and modes of intervention in space, acts directly or indirectly to accentuate or mitigate the risk of flooding. two criteria were taken into account to assess human action: drainage and land use. figure 5.7. altitudes in lia western mitidja figure 5.8. altitudes weights anthropogenic critaria drainage and natural sanitation the artificial drainage networks of excess water and the natural sanitation system were identified from previously geo-treated plans, in collaboration with the gis unit of the national office for irrigation and drainage (onid). for the first section, sixteen detailed pre-project plans of detailed preliminary project of irrigated areas and networks were used (agid., 1992). in addition, irrigation, drainage and natural sanitation plans were realized for the large irrigated areas (lias) of west mitidja. for the second section, twenty-two plans of the adaptation network were exploited at the level of the irrigation plot, with a scale of 1/5000. a buffer zone was determined based on the distance to existing artificial or natural drains (fig. 6.1). to identify poorly drained areas, values were established from field observations and surveys, regardless of soil quality and permeability (hafnaoui et al., 2022). an area located within 350 m of artificial or natural drainage networks (wadis) is considered to have good drainage and 108 hind raked, nour el islam bachari, mohamed saϊd guettouche, mohammed tayeb touahri is reclassified with a weight of 1. areas located more than 350 m away are classified as poor drainage and sanitation areas; they are reclassified with a weight of 5, as shown in (fig. 6.2). these zones require the construction of artificial drains, either open or buried. figure 6.1.sanitation and drainage networks in lia western mitidja (onid.,2023) figure 6.2.sanitation and drainage weights land use this parameter was used not only to exclude the urbanized areas from the irrigable area (declassified areas which have consequently lost their initial agricultural vocation), but also to classify certain lands uses which promote the risk of flooding and others that hamper that risk. progress has been transformative, and the information obtained from remote sensing of floods is becoming mature enough to not only be integrated with computer simulations of flooding to allow better prediction, but also to assist flood response agencies in their operations (schumann et al., 2018). very high resolution images by universal maps download; a photo-interpretation was also established in combination with data provided by landsat 8 operational land imager (oli). the recorded data were at the l1t level, which means that the images were radiometrically and geometrically corrected (saadoud et al., 2018). traditional ground-based monitoring systems are sparse and in decline. the value remote sensing can offer is growing rapidly, and the challenge now lies in ensuring sustainable and interoperable use as well as optimized distribution of remote sensing products and services for science as well as operational assistance (schumann et al., 2018). the land use parameter was considered in (fig.6.3). a more significant weight was assigned to water system, and annual crops. land use was then classified as indicated in (tab.5), the results of this reclassification are presented in (fig.6.4). table 5: reclassification of land use classes weight type risk class seasonal and annual crops 3 low permeability high perennial tree crops 1 permeable low forests, reforestation, wasteland 1 permeable low hydrographic network 4 strong urbanization 0 elimination criterion the presence of trees is a sign of good soil drainage, as tree roots cannot tolerate excessive irrigation or poor soil drainage, so the presence of trees can in no way be associated with a risk of flooding. despite the high profitability per hectare of arboriculture in this plain, farmers turn to annual crops when the soil is poorly drained, so the absence of trees means there is a high risk of flooding. urbanization is considered as an elimination criterion. these lands considered are excluded from the agricultural. figure 6.3.land use map figure 6.4 .the weight assigned to land use 109gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria) 4. results the results of the arithmetic operation, which consists of adding the weights of the lithology, morpho-pedology (soil), altitude and drainage criteria (fig.7.a), showing the addition operation carried out in arcgis using raster calculator. the results obtained are shown on the map (fig.7.b), with weights ranging from 6 to 32. the second section of the lia, sectors e, f and g, shows the most significant weight values. the results of the multiplication of weights assigned to land use criteria and slope are illustrated in (fig.8). the two results obtained from the arithmetic operations were multiplied (lsad x lu.s) to create a flood risk assessment map. a classification was carried out using the equal interval method in arcgis across four classes, each with a range of 150. the result is illustrated in fig.9. the results of this model are also summarized in (tab.6) and (fig.10), the area exposed to high to very high flood risk represents nearly 5% of the total surface area. however, the medium flood risk areas cover a significant portion (35%) of the total area under study. they are predominantly located in the western part of study area (section 2). figure 7.a. raster calculator’s addition operation figure 7.b the sum of the weights assigned to the criteria of lithology, morphopedology, altitude and drainage (lsda). figure 8. the weights of the land use x slope (lu.s). figure 9. flood risk map in the large irrigated area (lia) of west mitidja plain table 6. flood risk assessment by class in the lia.m.w by section class values type of risk section 1 section2 total study area % 150-0 low risk 7 353 7 565 14 918 59,91 300-150 medium risk 1 812 6 898 8 710 34,98 450-300 high risk 59 1 116 1 175 4,72 600-450 very high risk 13 84 97 0,39 total 9 237 15 663 24 900 100 110 hind raked, nour el islam bachari, mohamed saϊd guettouche, mohammed tayeb touahri figure 10. flood risk map in the large irrigated area (lia) of west mitidja plain figure 11. flood in the large irrigation area of mitidja west (sector g) (saw, 2018) conclusions in this study, we developed a model for assessing flood risk using a geographic information system (gis). this model combines various natural and anthropogenic soil and subsoil criteria, including lithology, morpho-pedology, altitude, drainage systems, slope and land use. using the spatial analysis tools discussion the very high risk areas represent 0.39% of the total surface area of the study zone, or 97 ha (table 6), of which 84 ha are located near the halloula lake, a depression where the drainage and sewerage networks converge (then discharged to the sea via an underground gallery with a reduced cross-section), and an area of only 13 ha in tranche 1, along the oued bouroumi. the high-risk zones cover an area of 1,175 ha, or 4.72% of the study area. of these areas, 1116 ha are located in tranche 2, a large part of which is on the lac halloula site, and 59 ha in tranche 1 of this gpi. the distribution of very high and high risk flood zones demonstrates the effectiveness of this assessment model. according to agricultural services, between april 13 and 14, 2018 (fig.11), thousands of hectares of agricultural land were damaged, and numerous material damages were recorded. the large irrigation area mitidja west section 2 completely flooded after the third day of the 2018 rains. this issue also occurred again in 2012 and 2017. it is worth noting that the most exposed sector is sector g, followed by sectors e and f. therefore, the large irrigated area of west mitidja, in the second section, was the most affected by these frequent floods. the area with medium flood risk covers 8710 ha, or 34.98% of the study area, with 6,898 ha in tranche 2 and 1812 ha in tranche 1. the area with low flood risk represents the majority, at 59,91% of the study area. it is almost evenly distributed across the two sections of the gpi. the low-risk area is mainly located in the eastern part of the study area, which explains the strong presence of tree cultivation in this region, while annual crops (cereal and forage production) are found in the western part, which presents medium, high, and very high flood risks. 111gis approach for flood risk assessment in the large irrigation area of the western mitidja plain (algeria) available in arcgis, we integrated these criteria to produce a classification map of the study area based on flood risk. the resulting flood risk map enabled us to identify and classify areas into four categories: very high risk, high risk, medium risk, and low risk. notably, halloula lake and its surroundings were classified as very high-risk and highrisk zones, respectively. field surveys confirmed these findings, highlighting «black spots» recognized by local farmers, which attests to the effectiveness of the developed model. this flood risk assessment map can serve as a decision-support tool for managers and users of the lias, offering a proactive perspective for managing this risk and anticipating necessary actions to mitigate damages. 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(2024). geo-electric techniques for estimating and mapping the electro-geohydraulic properties of the shallow aquifer within the nsukka formation in eastern nigeria. earth sciences research journal, 28(3), 349259. https://doi.org/10.15446/esrj.v28n3.110328 record manuscript received: 26/07/2023 accepted for publication: 23/08/2024 abstract this study, employing vertical electrical sounding (ves), was carried out within parts of nsukka formation located in southeastern nigeria in order to determine the subsurface properties that aid in characterizing the subsurface. the study covers parts of nsukka, enugu ezike, obukpa and eha alumona in enugu state, characterized by undulating terrain and a tropical climate with distinct wet and dry seasons. twenty-one sounding data were acquired, and the measured data were quantitatively and qualitatively interpreted to determine the geoelectrical parameters. the interpreted resistivity curves revealed five geoelectric layers with different generic curve types. the secondary parameters estimated from modeled equations and their spatial distributions are displayed in the contour maps generated with origin software. the estimated parameters longitudinal conductance, transverse resistance, transverse resistivity, longitudinal resistivity, anisotropy, porosity, hydraulic conductivity, permeability, formation factor, and tortuosity-vary across the study area within the following ranges: 0.01 0.54, 373.90 11375.00, 109.56 5402.14, 108.72 4046.87, 1.01 1.88, 0.276 – 0.314, 0.187 – 3.509 m/day, 1.02e-5 9.32e-6 md, 3.092 – 3.780, and 0.200 – 1.021, respectively. the variation of these parameters may be influenced by the heterogeneous nature of the subsurface. the regression analysis reveals the mutual relationships and correlation between the different parameters when plotted against each other. the results from this study are very promising, and demonstrate the efficacy of the geophysical approach in characterizing the subsurface for sustainable groundwater management and protection. the deployment of geo-electric indices in this study is to ensure adequate characterization of the subsurface. keywords: anisotropy; groundwater; geoelectric layer; resistivity; ves palabras claves: anisotropia; aguas subterráneas; capas geoeléctricas; sondeo eléctrico vertical técnicas geoeléctricas para la estimación y mapeo de las propiedades electro-geohidráulicas en los acuíferos poco profundos en la formación nsukka, en el este de nigeria este estudio, realizado con pruebas de sondeo eléctrico vertical (sev), se llevó a cabo en algunas partes de la formación nsukka, ubicada en el sudeste de nigeria, para determinar las propiedades que ayuden a caracterizar el subsuelo de la formación. el estudio cubre partes de nsukka, enugu ezike, obukpa y eha alumona, en el estado de enugu, caracterizado por terrenos ondulados y un clima tropical con distintas estaciones húmedas y secas. se recolectó información de 21 sondeos y esta información se interpretó cuantitativa y cualitativamente para determinar los parámetros geoeléctricos. las curvas interpretadas de resistividad revelaron cinco capas geoeléctricas con diferentes tipos de curvas genéricas. los parámetros secundarios estimados a partir de las ecuaciones modeladas y sus distribuciones espaciales se presentan en los mapas de contornos generados con el software origin. los parámetros estimados -conductancia longitudinal, resistencia transversal, resistividad transversal, resistividad longitudinal, anisotropia, porosidad, conductividad hidráulica, permeabilidad, factor de formación y tortuosidadvarían a lo largo del área estudio dentro de los siguientes rangos 0.01 0.54, 373.90 11375.00, 109.56 5402.14, 108.72 4046.87, 1.01 1.88, 0.276 – 0.314, 0.187 – 3.509 m/day, 1.02e-5 9.32e-6 md, 3.092 – 3.780, and 0.200 – 1.021, respectivamente. la variación de estos parámetros puede estar influenciada por la naturaleza heterogénea del subsuelo. el análisis de regresión revela una relación mutua y una correlación entre diferentes parámetros cuando se grafican entre sí. los resultados de este estudio demuestran la eficacia de la aproximación geofísica en la caracterización del subsuelo para el manejo y la protección sostenible de las aguas subterráneas. el despliegue de índices geoeléctricos en este studio asegura una caracterización adecuada del subsuelo. resumen https://doi.org/10.15446/esrj.v28n3.110328 mailto:johnson.ibuot@unn.edu.ng https://doi.org/10.15446/esrj.v28n3.110328 350 johnson c. ibuot, daniel n. obiora, moses m. m. ekpa, and emmanuel t. omeje introduction the earth’s subsurface is made up of different naturally occurring materials, including minerals, rocks and soils. extensive research underscores the indispensable role of water in supporting life, particularly plants and other organisms inhabiting soil ecosystems (akpan et al., 2013; ibanga and george. 2016; obiora et al., 2016; ibuot et al., 2019; george, 2020). groundwater, as a fundamental natural resource, serves as the cornerstone for sustaining various human activities encompassing domestic, agricultural, industrial and social needs (kelbe et al., 2011; bricker et al., 2017). it is a vital component in preserving not only human existence but also the the diverse ecosystems reliant on it. the occurrence of groundwater is primarily dependent on geological, geomorphological and weathering factors, alongside rainfall patterns. the development of secondary porosity and permeability resulting from weathering and fracturing of parent rocks stands as a pivotal geological mechanism shaping aquifer repositories (oli et al., 2020; omeje et al., 2022a). the interaction of these factors leads to complex hydrogeological formations, characterized by variations in groundwater resources’ quantity, quality, accessibility and renewability. generally, groundwater exhibits high quality, owing to its substantial protection from surface contaminants facilitated by the natural purification processes occurring within the vadose zone (ibuot et al., 2022; nugraha et al 2021). a good knowledge of the aquifer systems is imperative to mitigate over-exploitation, which leads to borehole failures and groundwater pollution, thereby fostering environmental sustainability. furthermore, the groundwater resources hold significant potential for enhancing livelihoods, notwithstanding the challenges and knowledge gaps concerning groundwater dynamics occurrence, flow and transmissibility. the groundwater repository is controlled by the geometry of aquifers and interlayered aquitards, aquifer types (confined or unconfined), water table fluctuations, recharge and discharge sources, hydraulic properties derived from aquifer materials’ characteristics. in the characterization of subsurface lithologic units, hydraulic properties such as permeability, porosity, tortuosity, transmissibility, tortuosity, hydraulic capillary radius, and surface area per unit volume serve as invaluable and dynamic tools that provide adequate information about the subsurface litho-textural characteristics (ibuot et al., 2019; ekanem et al., 2022; omeje et al., 2022b; asfahani and al-fares, 2021). voids, pore spaces and fractures play pivotal roles in groundwater studies, influencing the flow dynamics within subsurface formations. groundwater flow is significantly influenced by fractures, with multitude of evenly distributed fractures facilitating flow throughout the saturated layer. the flow of groundwater through fractures and fissures is characterized by heterogenic permeability and the heterogeneity affects the transmissibility properties of the aquifer (niwas and celik 2012; asfahani and al-fares, 2022). the geohydraulic properties of the aquifer units depend upon factors such as geometry, voids, grain sizes, and interconnection of the pores, facilitating the flow of accumulated water (george et al. 2015). porosity, primarily influenced by grain arrangement, can reduce proportionally with increase in grain size; however, grain size itself does not affect total porosity in sediments of uniform (mazac et al., 1985; ekanem et al., 2022). the geohydraulic properties under scrutiny in this study, encompassing hydraulic conductivity, porosity, permeability, hydraulic capillary radius, surface area per unit volume, play pivotal roles in governing the water-bearing characteristics of hydrogeologic units. the spatial distribution and interplay of these properties aid in classifying aquifer units based on their transmissiblility and potentiality (george et al., 2015; omeje et al., 2022a; ibuot et al., 2024). incorporating these properties is essential for characterizing aquifer units effectively. parameters derived from resistivity data interpretation are corroborated with existing subsurface data to facilitate comparison and validation (george et al., 2015; opara et al., 2020). the scarcity of pumping test data owing to the high cost of measuring instruments and poor data management in developing countries, has led to the use of non-invasive and cost-effective geophysical methods for aquifer units characterization. among this methods, direct current electrical resistivity techniques have gained prominence for estimating aquifer hydraulic properties (obiora et al., 2015: ibanga et al., 2016; omeje et al., 2021). electrical resistivity techniques are widely utilized in groundwater investigations to facilitate the exploration of groundwater resources. fundamentally, electrical resistivity involves injecting current into the earth through electrodes, and subsequently monitoring and recording the earth’s response to current propagation (telford et al., 1990, lowrie, 1997; todd and mays 2005). the vertical electrical sounding (ves) technique employing, employing the schlumberger electrode configuration, is particularly instrumental in characterizing subsurface formations efficiency (asfahani and al-fares, 2022; obiora and ibuot, 2023). the interpretation of ves data is influenced by various environmental and geological factors, including mineral content, inter-granular compaction, porosity, and degree of water saturation in the rocks (gonzalez-alvarez et al., 2016 and shishaye et al. 2019). numerous studies have successfully determined aquifer hydraulic properties using surficial electrical resistivity techniques in different parts of the world (niwas and celik 2012; metwaly et al. 2014; george et al., 2015;umar and igwe, 2019; obiora and ibuot, 2020; oli et al. 2020; ekwe et al. 2020; ibuot et al., 2022). hence, the application of geo-electric techniques will aid in estimating and mapping the electro-geohydraulic properties of aquifers in parts of nsukka formation, particularly considering the local variations in aquifer characteristics, subsurface lithology, and the influence of environmental and geological factors on resistivity measurements. this will lead to improved groundwater management and resource sustainability. the aim of this study is to utilize geo-electric techniques to estimate and map the electro-geohydraulic properties of the shallow aquifer within the nsukka formation in eastern nigeria, and to explore the interrelationships among these properties. background theory and electro-geohydraulic parameters the electro-geohydraulic parameters considered in this study are longitudinal conductance, transverse resistance, longitudinal resistivity, transverse resistivity, anisotropy, porosity, hydraulic conductivity, permeability, formation factor, tortuosity, hydraulic capillary and surface area per unit volume. these properties were estimated from the primary geo-electric data obtained from ves measurements. the primary geo-electric properties (resistivity, thickness and depth) were obtained using the vertical electrical sounding using schlumberger electrode array. the electrical resistivity technique assumed the earth to be an isotropic homogeneous medium, and the measurement demonstrates the true magnitude of resistivity, which does not depend on the potential electrode distance used (nugraha et al., 2021). the flow of current in conducting medium is governed by ohm’s law (equation 1); = − 1 where j, σ, v and r are the current density, electrical conductivity, electrical potential, and distance the flow of groundwater in a porous medium is governed by the darcy’s law, which is expressed in equation 2; = − ℎ 2 where q, k, and h are specific discharge, hydraulic conductivity and hydraulic head. these two equations (1 and 2) are similar in form and can be combined to give equation 3 which reduces to equation 4. ℎ = ℎ 3 4 according to asfahani and al-fares (2022), the mechanisms that control the flow of electric current and groundwater are principally related to the similar lithological and physical conditions of the earth’s subsurface. • longitudinal conductance, transverse resistance, longitudinal resistivity, transverse resistivity and anisotropy the primary parameters that describe a geoelectric layer are the resistivity (ρ) and thickness (h), while the secondary geoelectric parameters are derived from the combination of these primary parameters. the secondary parameters are significant in describing the subsurface lithological and structural 351geo-electric techniques for estimating and mapping the electro-geohydraulic properties of the shallow aquifer characteristics with less doubt. the longitudinal conductance and transverse resistance are expressed in equations 5 and 6 respectively. considering a given number of layers (n), the total longitudinal conductance (s) is defined mathematically as; = ∑ ℎ = 1 = ℎ1 1 + ℎ2 2 + ⋯ℎ 5 considering a given number of layers (n), the total transverse resistance (t) is defined as; ⋯ 6 considering the longitudinal resistivity (ρl), the flow of current is parallel to the layers and is expressed in equation 7; 7 in transverse resistivity, the flow of current is considered to be normal to the layers and given by equation 8; 8 in a non-uniform medium, the transverse resistivity is always greater than the longitudinal resistivity and the anisotropy coefficient is greater than one. in a uniform medium, the transverse resistivity is equal to the longitudinal resistivity and the anisotropy coefficient is one (flathe, 1955; gernez et al., 2019). the coefficient of anisotropy (λ) is obtained by the combination of equations 7 and 8 and given by equation 9; 9 according to studies carried out by zohdy et al. (1974), shailaja et al. (2016), ekanem (2020) and asfahani and al-fares (2022), have revealed that the coefficient of anisotropy is always 1.0 for isotropic medium and rarely exceeds 2.0 in most of the geological locations. • porosity, formation factor and tortuosity porosity (ϕ) measures the capability of a geologic formation (rocks/ sediments) to hold water. the porosity of sediments/sedimentary rocks is controlled by the grain sizes and shapes, degree of sorting and cementation. the total porosity of the water-saturated hydrogeologic unit was estimated using equation 10 according to ekanem et al. (2019).. ϕ = 139.12 * ρa 0.728 10 where ρa is the aquifer resistivity obtained from the surface resistivity data. the formation factor is a transport property that describes the effect of pore formation on the resistance of the geologic materials and it is expressed as the ratio of bulk resistivity to that of water resistivity as given in equation 11. 11 where ρa is the aquifer resistivity and ρw is the water resistivity estimated using equation 12. 12 where a and m are archie constants, a is the pore geometric factor (a = 0.52) and m is the cementation factor (m = 1.54) as estimated in the study by omeje et al. (2021) according to archie’s equation. the conductivity of water-saturated rock/soil is very sensitive to changes in either of a or m. tortuosity (τ) characterises the complex pathways of fluid diffusion and electrical conduction through porous formation, it describes the transport properties of porous formation. it is related to porosity and formation factor in equation 13. 13 • hydraulic conductivity (k) and permeability (kp) hydraulic conductivity is a parameter that measures the resistance to the movement of groundwater through a porous formation. the presence of water in the pores of a saturated layer satisfies adhesion and cohesion properties, allowing the free flow of water. hence, hydraulic conductivity in the saturated layer is greater than that in the unsaturated layer. the hydraulic conductivity is related to porosity as determined from the kozeny-carman-bear’s equation given in equation 14. 14 where ϕ is the effective porosity, and g is the acceleration due to gravity (10m/s2), δw is the density of water (1000kg/m3), dm is site mean diameter (0.00036m) measured using the micrometre screw gauge, μd is the coefficient of dynamic viscosity of water which is approximately 0.0014 kg/ms (fetters, 1994). permeability (kp) in millidarcy (md) describes how those pores are shaped and interconnected and measures the capacity of a porous formation to transmit groundwater through a saturated zone. the water transmitting properties (hydraulic conductivity and permeability) relates with effective porosity as in equation 15. 15 where δw is the water density (1000 kgm−3), μd is the coefficient of dynamic friction and g is the acceleration due to gravity (g = 10m/s2). description of the study area and its geology the study area encompasses parts of nsukka region in southeastern nigeria, situated within the geographical coordinates ranging from latitudes n to n, and longitudes e to e (see figure 1) and covers parts of nsukka, enugu ezike, obukpa, and eha alumona. characterized by undulating topography with elevations exceeding 250 above sea level with a sparse network of surface drainages. geomorphollogically, the area exhibit prominent landform types: high relief zones and lowland areas. these distinct landforms arise from the interplay of weathering processes and differential erosion acting upon clastic materials, remnant of nsukka formation, which constitute the surface layers (ofomata, 1967; nnajieze and eyankware, 2014). the study area falls within the anambra sedimentary basin, predominantly composed of rocks dating to the upper cretaceous period. the principal geologic formations include the nsukka formation and the ajali sandstone, with the latter underlying the former (refer to figure 1). the nsukka formation represents a phase of fluvio-deltaic sedimentation initiated during the maastrichtian and persisting into the paleocene. it consists of coarse to medium-grained sandstone transitioning upwards into well-bedded blue clay, fine-grained sandstone and carbonaceous shale with thin bands of limestone (reyment, 1965 and obi et al, 2001). these geological features influence the hydrogeology, with sandstones providing significant groundwater reservoirs. conversely, the ajali sandstone predominantly consists of medium to coarsegrained sands, often exhibiting white-colouration. according to agagu et al. (1985), the permeable sandstone readily recharged in its outcrop belt around the idah-nsukka-enugu escarpment. the nsukka formation has a prolific aquifer system, sustaining numerous low to moderately productive wells across the 352 johnson c. ibuot, daniel n. obiora, moses m. m. ekpa, and emmanuel t. omeje nsukka area. itslateritic capping is vesicular nature which is responsible for its porous and permeable nature. beneath the lateritic caps lie less pervious clay beds, which facilitate the formation of perched aquifers (obi et al, 2001; omeje et al., 2021). however, perched aquifers are absent in areas where the lateritic cap has eroded, exposing and the underlying clay bed. figure 1. map describing the geology of the study area 3. materials and methods twenty-one vertical electrical soundings were acquired for the study using schlumberger electrode configuration within the maximum current electrode spread, with profiles taken along fairly straight traverses. the igis resistivity meter ssr-mp-ats and its accessories was used for the resistivity survey. the potential difference of the subsurface was measured between the potential electrodes (m and n) while the current was injected into the ground through the current electrodes (a and b). the half current electrodes spread (ab/2) and half potential electrodes spread (mn/2) ranged from 1.0 – 450.0m and 0.25 – 20.0m respectively. equation 16 was used to calculate the apparent resistivity (ρa). ρa = g. ra 16 where g is the geometric factor: and ra is the apparent resistance. the data were reduced to 1-d geological models using the manual and computer modelling techniques. plots of ρa against ab/2 on a log–log graph were smoothened in order to eliminate noise and associated outliers (bhattacharya and patra 1968; akpan et al., 2006; chakravarthi et al., 2007). the curve matching technique was employed to interpret the field ves curves (orellana and mooney 1966) in order to determine an approximate 1d models of the resistivities and thicknesses of the corresponding subsurface layers. the winresist software was used to get correctly the final models of the subsurface layers resistivities, thicknesses, and depths through the set of resistivity curves generated (zohdy 1989; zohdy and bisdorf 1989) as displayed in figures 2a and b. the various curves display variations in values of resistivity, thicknesses and depths between and within the subsurface layers the current penetrated. a b c figure 2. resistivity curves (a) at ves 11, (b) at ves 21 (c) borehole lithology log 4. results and discussion the inversion results from 21 ves points are presented in table 1, showing varying values of resistivity, thickness and depth. five geoelectric layers were delineated across all ves points within the study area’s maximum current electrode separation. the delineation of five geoelectric layers suggests a complex subsurface environment with varying lithologies and potential aquifer systems. the interpreted model curves are predominantly of the akh curve type, comprising about 38% of the total curve types. other curve types are hkh (4.8%), aaa (14.3%), haa (9.5%), kqh (14.3%), aak (9.5%), kha (4.8%) and akq (4.8%), as shown in table 1. the frequency distribution of the curve types is displayed in figure 3. the predominance of the akh curve type suggests the presence of a relatively resistive layer (a) overlying a more conductive layer (k), followed by another resistive layer (h). this configuration might point to a common stratigraphy of dry or less conductive topsoil, followed by a water-bearing layer, and then another resistive layer, possibly indicative of bedrock or compacted sediments. the presence of other curve types, including hkh, aaa, haa, kqh, aak, kha, and akq, highlights the variability in the subsurface conditions such as in soil composition, moisture content, rock types, and the presence of fractures or voids. the variability in curve types indicates that the area is geologically diverse, which may affect groundwater flow, and resource management. 353geo-electric techniques for estimating and mapping the electro-geohydraulic properties of the shallow aquifer fr eq ue nc y( ⁒) curve types figure 3. frequency distribution of curve types the topmost geo-electric layer has resistivity values ranging from 83.4 to 1277.4 ωm, with thickness and depth ranging from 0.4 to 2.3 m, respectively. the second and third geoelectric layers are characterised by resistivity values varying from 68.9 to 9395.9 ωm and 111.3 to 6078.6 ωm, respectively. the thicknesses of these layers range from 1.5 to 14 .6 m and 2.7 to 54 6 m, respectively, with corresponding depths varying from 2.5 to 15.0 m and 5.2 to 59.1 m. the fourth layer, identified as the saturated units (aquifer units) is characterized by primary geoelectric parameters ranging from 156.8 to 8823.4 ωm, with thicknesses spanning from 9.6 to 159. 7 m and depths from 28.8 to 180.1 m. this indicates the presence of potentially significant groundwater resources, but with varying quality and quantity. the resistivity of the fifth layer ranges from 990.5 to 13100 ωm, with thickness and depth undefined. the primary geoelectric parameter from ves were used in empirical relations (equations 5 9) to obtain the secondary parameters as shown in table 2. the wide range of resistivity values within each geoelectric layer suggests a highly heterogeneous subsurface environment, indicating varying lithological compositions and possibly different degrees of weathering or fracturing. the resistivity values were within the range obtained by ezema et al. (2020), omeje et al. (2022), ossai et al. (2023) in their studies. the resistivity and thickness of the saturated layer vary from 156.8 – 8823.4 ωm and 9.6 – 112.1 ωm, respectively, with averages of 2648.16 ωm and 64.68 m. the distribution of resistivity and thickness of the saturated layer are displayed in the contour maps (figures 4 and 5). figure 4 shows the highest resistivity value in the northcentral part of the study area, while in figure 5 indicates the highest thickness values in the southeastern part of the study area. the spatial variation of these parameters reflects the inhomogeneity of the subsurface geologic materials. table 1. summary of interpreted ves result ves sites long. (oe) latitude (on) layer resistivity (ωm) thickness (m) depth (m) elevation (m) curve typeρ1 ρ2 ρ3 ρ4 ρ5 h1 h2 h3 h4 d1 d2 d3 d4 1 7.3843 6.8634 123.4 317.4 1547.6 1021.0 6505.2 1.0 1.5 2.7 71.0 1.0 2.5 5.2 76.2 404 hkh 2 7.3323 6.9005 83.4 500.5 772.0 2451.8 8220.1 0.6 2.7 4.9 95.8 0.6 3.3 8.2 104.0 425 aaa 3 7.3586 6.8948 873.4 2676.8 3464.1 6234.0 7596.6 0.7 5.0 29.5 71.0 0.7 5.7 35.2 106.2 369 aaa 4 7.3863 6.8676 127.0 77.5 111.3 985.3 5672.6 1.0 3.5 54.6 38.9 1.0 4.6 59.1 98.0 449 haa 5 7.3443 6.8327 389.4 3428.0 6078.6 1361.9 7569.9 1.0 8.7 32.8 72.4 1.0 9.6 42.4 114.7 342 akh 6 7.3227 6.8231 100.8 496.5 4426.9 786.5 3656.3 0.4 14.6 25.0 53.0 0.4 15.0 40.0 93.0 351 akh 7 7.4295 6.8843 205.8 996.7 4217.9 2217.7 4115.0 0.8 11.1 11.9 60.3 0.8 11.9 23.8 84.0 338 akh 8 7.4003 6.8949 972.4 1918.9 4310.0 1384.7 1766.5 2.3 2.8 6.1 112.1 2.3 5.1 11.3 123.4 337 akh 9 7.4065 6.9247 1277.4 8844.5 1531.1 156.8 2100.7 0.7 3.9 4.4 17.3 0.7 4.6 9.0 26.3 440 kqh 10 7.4193 6.9371 950.0 9395.9 1071.3 204.8 6471.6 0.9 3.4 49.4 57.0 0.9 4.2 53.6 110.6 438 kqh 11 7.4275 6.9493 279.1 387.6 2348.9 4623.4 990.5 1.6 3.5 53.1 83.8 1.6 5.1 58.2 142.0 405 aak 12 7.4345 6.9603 163.2 871.0 928.6 8823.4 11039.9 1.8 7.9 11.5 80.4 1.8 9.7 21.2 101.6 285 aak 13 7.4482 6.9964 305.2 877.3 785.2 2184.5 9801.1 1.2 3.8 14.8 9.6 1.2 5.0 19.7 29.3 437 kha 14 7.4484 7.0093 390.6 2302.2 1733.7 445.6 3221.4 0.4 2.7 4.0 21.7 0.4 3.1 7.1 28.8 452 kqh 15 7.4141 6,8831 306.5 68.9 145.8 1703.8 3486.1 1.8 6.8 9.1 15.3 1.8 8.7 17.7 33.1 315 haa 16 7.4426 6.8846 84.1 411.1 1005.7 4321.3 11576.0 1.2 5.0 21.7 44.1 1.2 6.1 27.8 71.9 389 aaa 17 7.4668 6.8936 128.7 521.2 3452.1 1567.2 2178.9 0.7 8.6 15.2 57.9 0.7 9.3 24.5 82.4 407 akh 18 7.4822 6.9008 513.0 760.5 5073.7 3269.2 5408.8 1.7 6.7 14.8 78.2 1.7 8.4 23.3 101.5 386 akh 19 7.4634 6.8416 160.3 388.0 4296.6 2405.6 8490.5 0.9 6.1 13.3 159.7 0.9 7.1 20.3 180.1 397 akh 20 7.3459 6.8170 212.9 1254.9 6343.5 5641.9 13100.4 1.0 6.0 26.4 78.5 1.0 7.0 33.4 111.6 355 akh 21 7.4665 6.9556 697.9 1575.2 5759.0 3821.0 3138.7 1.2 12.0 41.1 80.1 1.2 13.1 54.3 134.3 306 akq 354 johnson c. ibuot, daniel n. obiora, moses m. m. ekpa, and emmanuel t. omeje figure 4. 2d contour map of aquifer resistivity figure 5. 2d contour map of aquifer thickness the longitudinal conductance (s) values, varying between 0.01 and 0.54 ω-1 (figure 6), help determine the protectivity of the study area. according to the classification of henriet (1976) and oladapo et al. (2004), the greater part of the study area is poorly protected based on longitudinal conductance values of <0.1ω-1. moderate protectivity is observed in the central part of the study area at ves 4, with longitudinal conductance > 0.5ω-1. areas with high longitudinal conductance are characterized by relatively thick geologic successions and clayey overburden, which offers protection to the underlying aquifer units from surface contaminants. figure 6. 2d contour map of longitudinal conductance as posited by shailaja et al. (2016), and oguama et al. (2019) the low values of s may be attributed to the absence or minimal amount of clay as an overburden impervious material, allowing easy infiltration of contaminants into the aquifer units. varies between 373.90 and 11375 ωm2, with an average value of 4827.56 ωm2. the spatial variation of tr (figure 7) shows that high values in the central, southwestern and southeastern parts of the study area. figure 7. 2d contour map of transverse resistance the longitudinal resistivity (ρl) and transverse resistivity (ρt) were computed from equations 7 and 8 respectively. ρ1 has values range from 108.72 4046.87 with an average of 1443.04 while ρt value range from 109.56 – 5402.14 with an average of 2408.70. the highest and lowest ρ1 and ρt are obtained at ves 4 and ves 5. the distributions of both ρ1 and ρt (figures 8 and 9) indicate a direct relationship, where an increase in one leads to a corresponding increase in the other. the combination of ρ1 and ρt in equation 9 gives the coefficient of anisotropy (λ), with values ranging from 1.01 to 1.88 and a mean of 1.3. this implies that the subsurface exhibits moderate anisotropy, suggesting variability in the directional properties of the geological materials, which can impact the flow and distribution of groundwater and other subsurface fluids. the distribution of λ is displayed in figure 10. the ρt values are greater than ρ1 at all ves locations, resulting in λ being greater than unity. according to studies by zohdy et al. (1974), and bittar and rodney (1994), the coefficient of anisotropy is generally unity for isotropic medium and rarely exceeds 2.0 in most geological settings. the highest λ values were observed in the southeastern part of the study area, extending towards the southwest. asfahani and al-fares (2022); and kumar et al. (2014) suggested that, the high anisotropy values indicate the influence of the weathering, alteration processes, and fracturing and suggest better water-holding capacity). increased hardness and compaction of rocks results in higher anisotropy, correlating with low porosity (keller and frischknecht, 1979; shailaja et al., 2016). figure 8. 2d contour map of transverse resistivity 355geo-electric techniques for estimating and mapping the electro-geohydraulic properties of the shallow aquifer figure 9. 2d contour map of longitudinal resistivity figure 10. 2d contour map of anisotropy the estimated fractional porosity (ϕ) reveals that porosity values varies from 0.276 – 0.314, with an average value of 0.291. this indicates a relatively high porosity, suggesting that the material has a significant amount of void space. this could imply good potential for fluid storage and flow. materials with this porosity range are likely to have good permeability, allowing fluids to move through them relatively easily. higher porosity is observed in the northeastern parts, extending to the central part of the study area, while low porosity is observed across the southeastern part and extending to the northeastern part. zones with high porosity correspond to zones with low anisotropy. according to roscoe (1990), the lithology of the study area is characterized by sand, sandstone and gravel. figure 11 is a contour map showing the spatial distribution of porosity in the study area. porosity values relate well with the high and low values of anisotropy, suggesting a positive correlation. the estimated formation factor (f) ranges from 3.09 – 3.78, with an average of 3.49, the southeastern, western and eastern parts show high formation factor values, decreasing towards the northern part (figure 12). the results revealed that this parameter trends inversely with porosity, where high porosity leads to a low formation factor and vice-versa. the spatial variation of tortuosity (τ) reveals a range of values from 0.200 – 1.021, with an average of 1.007. the hydraulic conductivity (k) range from 0.187 – 3.780 m/day, while permeability (kp) values range between 1.02e-05 and 9.32e-06 md. the spatial distributions of these parameters from the contour maps generated show similar patterns (figure 13 and 14), with low values are observed in the south, extending to the northeast. hydraulic conductivity and permeability are controlled by intergranular porosity and fracturing, suggesting that parts with low low k and kp may be due to poor pore spaces communication channels and grain size variations (ibanga and george, 2016; george et al. 2018; ibuot and obiora, 2021). locations displaying high hydraulic conductivity and permeability values will have good groundwater potential, while relatively low values indicate poor groundwater potential in the aquifer system (ibuot et al. 2019; ekanem et al. 2022). table 2. summary of hydrogeological properties ves sites long. (oe) lat. (on) ρa (ωm) ha (m) s (ω-1) tr (ωm2) ρt (ωm) ρl (ωm) λ ϕ k (m/day) kp (md) f τ 1 7.3843 6.8634 1021.0 71.0 0.02 1992.6 918.85 356.79 1.61 0.296 0.897 1.27e-05 3.395 1.002 2 7.3323 6.9005 2451.8 95.8 0.02 1330.14 632.22 433.04 1.21 0.288 0.474 6.72e-06 3.546 1.020 3 7.3586 6.8948 6234.0 71.0 0.01 6786.78 3300.75 3146.99 1.02 0.279 0.240 3.41e-06 3.715 1.018 4 7.3863 6.8676 985.3 38.9 0.54 373.90 109.56 108.72 1.01 0.296 0.921 1.31e-05 3.389 1.002 5 7.3443 6.8327 1361.9 72.4 0.01 9937.50 5402.14 4046.87 1.16 0.293 0.727 1.03e-05 3.444 1.005 6 7.3227 6.8231 786.5 53.0 0.04 5003.32 2949.04 1025.08 1.70 0.298 1.081 1.54e-05 3.351 0.200 7 7.4295 6.8843 2217.7 60.3 0.02 5402.24 2580.72 1333.68 1.39 0.288 0.510 7.23e-06 3.529 1.009 8 7.4003 6.8949 1384.7 112.1 0.01 8474.32 3026.83 2137.50 1.19 0.293 0.719 1.02e-05 3.449 1.005 9 7.4065 6.9247 156.8 17.3 0.01 11278.08 4680.51 2329.98 1.42 0.314 3.509 4.98e-05 3.092 0.986 10 7.4193 6.9371 204.8 57.0 0.05 11375.00 1596.34 1132.40 1.19 0.312 2.889 4.1e-05 3.134 0.988 11 7.4275 6.9493 4623.4 83.8 0.04 3239.66 2174.05 1557.44 1.18 0.282 0.299 4.24e-06 3.660 1.015 12 7.4345 6.9603 8823.4 80.4 0.03 2112.76 842.15 652.64 1.14 0.276 0.187 2.65e-06 3.780 1.021 13 7.4482 6.9964 2184.5 9.6 0.03 2047.34 773.79 730.30 1.03 0.289 0.516 7.31e-06 3.526 1.009 14 7.4484 7.0093 445.6 21.7 0.01 4198.84 1874.22 1576.36 1.09 0.304 1.640 2.33e-05 3.258 0.995 15 7.4141 6.8831 1703.8 15.3 0.17 782.30 132.60 106.00 1.12 0.291 0.618 8.77e-06 3.483 1.007 16 7.4426 6.8846 4321.3 44.1 0.05 1544.42 859.50 581.15 1.22 0.282 0.314 4.45e-06 3.648 1.015 17 7.4668 6.8936 1567.2 57.9 0.03 4087.19 2328.34 930.06 1.58 0.292 0.657 9.32e-06 3.468 1.006 18 7.4822 6.9008 3269.2 78.2 0.02 6727.80 3493.89 1542.47 1.51 0.285 0.385 5.45e-06 3.598 1.012 19 7.4634 6.8416 2405.6 159.7 0.02 4848.27 2938.71 830.89 1.88 0.288 0.481 6.82e-06 3.543 1.010 20 7.3459 6.8170 5641.9 78.5 0.01 7843.70 5245.83 2448.67 1.46 0.280 0.258 3.67e-06 3.697 1.017 21 7.4665 6.9556 3821.0 80.1 0.02 1992.60 4722.56 3296.07 1.20 0.283 0.343 4.87e-06 3.626 1.014 356 johnson c. ibuot, daniel n. obiora, moses m. m. ekpa, and emmanuel t. omeje figure 11. 2d contour map of fractional porosity figure 12. 2d contour map of formation factor figure 13. 2d contour map of hydraulic conductivity figure 14. 2d contour map of permeability regression analysis the results of the estimated parameters were plotted to determine their correlation and relationships. the plot of hydraulic conductivity against porosity (figure 15a) reveals a power expression with a high correlation coefficient expressed as k = 8.09e11*^22.6 (equation 17). from the plot, it is evident that an increase in porosity leads to an increase in hydraulic conductivity, aligning with the variation in the contour maps of figures 11 and 13. this is expected as hydraulic conductivity depends on pore size, spatial distribution and connectivity. k = 8.09e11*^22.6 17 similarly, figure 15b shows a power relation between permeability and porosity with a high correlation coefficient, given by given by kp = 1.15*ϕ^22.6 ( (equation 18). this indicates that ability is directly related to porosity, suggesting that higher porosity leads to higher permeability, which enhances groundwater transmissibility through interconnected pores. kp = 1.15*ϕ^22.6 18 the correlation between the hydraulic conductivity and aquifer resistivity is represented in figure 15c with a power equation: k = 139.18*ρa^-0.73 19 additionally, the variability of the aquifer units is expressed through a plot between porosity and aquifer resistivity (figure 15d), yielding a power equation with strong correlation coefficient: ϕ = 0.37*ρa^-0.03 20 finally, the formation factor-porosity (figure 15e) provides a power expression, showing an exponential decrease in formation factor with increasing porosity, with a strong correlation coefficient. f = 0.52*ϕ^-1.54 21 357geo-electric techniques for estimating and mapping the electro-geohydraulic properties of the shallow aquifer the results from this study provide a comprehensive understanding of the subsurface geoelectric layers, indicating a complex and heterogeneous environment with varying lithologies and potential aquifer systems. these findings can inform future research by guiding the development of more accurate predictive models for groundwater exploration, particularly in geologically similar regions. moreover, the study’s insights into resistivity, anisotropy, and porosity can be applied in practical hydrogeology to enhance groundwater management, optimize drilling locations, and assess contamination risks, ultimately improving resource management and environmental protection. the delineation of multiple geoelectric layers and the variety of curve types observed in this study indicate a highly heterogeneous subsurface, with varying lithologies and aquifer potentials. modern geophysical techniques, such as 3d resistivity imaging, time-domain electromagnetic (tem) surveys, and seismic methods, could provide more detailed and continuous subsurface models. these techniques can overcome the limitations of traditional 1d methods like vertical electrical sounding (ves) by offering better resolution and spatial coverage, leading to more accurate delineation of aquifer boundaries and stratigraphic layers. additionally, integrating advanced data processing techniques such as machine learning algorithms could enhance the interpretation of complex datasets. these algorithms could identify patterns and correlations within large datasets, such as those obtained from multi-dimensional geophysical surveys, leading to more accurate predictions of groundwater potential, aquifer properties, and the spatial distribution of subsurface features. figure 14(a-e). a graph of (a) hydraulic conductivity against fractional porosity, (b) permeability against fractional porosity, (c) hydraulic conductivity against aquifer resistivity, (d) fractional porosity against aquifer resistivity, (e) formation factor against fractional porosity. conclusion the present study investigated the electro-geohydraulic properties through the qualitative interpretation of resistivity data from twenty-one ves locations across the study area. the analysis identified five geoelectric layers, each with distinct resistivity profiles reflecting the variations in the subsurface lithostratigraphy. notably, the fourth layer emerged as the main economic aquifer, suggesting it as a prime target for groundwater extraction. the saturated (aquifer) layer, characterized by wide ranges of resistivity, thickness, and depth, indicates substantial variation in aquifer properties. the study utilized electrical parameters to estimate secondary parameters, which facilitated a detailed interpretation of the subsurface lithological characteristics. the longitudinal conductance indicated that the aquifer’s protective capacity ranges from poor to moderate, signifying a minimal presence of clayey or silty layers, thus making it susceptible to contamination. anisotropy values pointed to a higher water-holding capacity, attributed to processes such as weathering, alteration, and fracturing. the effective porosity analysis revealed that the aquifer comprises primarily unconsolidated materials, such as sandstone and sand-gravel mixtures. the distribution of porosity inversely correlates with anisotropy, highlighting zones of high porosity and low anisotropy, which align with the sandy and gravelly lithology of the area. the variation in hydraulic conductivity and permeability, controlled by intergranular porosity and fracturing, points to areas with good and poor groundwater potential. higher hydraulic conductivity and permeability values are associated with better groundwater prospects. the parameter maps illustrated the spatial distribution and variations across the study area, aiding in the delineation of regions with differing aquifer properties. additionally, these maps highlighted the inverse and direct relationships among various parameters. regression analysis demonstrated linear and nonlinear correlations among the parameters, all exhibiting positive correlation coefficients. this study underscores the utility of the 1-d resistivity technique in providing valuable insights into subsurface lithostratigraphy. the findings contribute significantly to groundwater exploration and management, demonstrating the technique’s potential for effective subsurface characterization. acknowledgement the authors are grateful to members of the atmospheric and solid earth geophysics research group, university of nigeria, nsukka for their support and encouragement. the authors also acknowledge the anonymous reviewer(s) and the editor(s) for their insightful comments and suggestions which have helped to improve the quality of this manuscript. reference agagu, o. k., fayose, e. a., & petters, s. w. 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(1989). programs for the automatic processing and interpretation of schlumberger sounding curves in quick basic. us geol surv open file rep 89(13), 64. https://doi.org/10.1007/s13201-022-01839-5 https://doi.org/10.1007/s13201-022-01839-5 https://doi.org/10.1007/s42108-020-00084-3 https://doi.org/10.1186/s44147-022-00081-9 https://doi.org/10.1007/s42108-020-00099-w https://doi.org/10.1007/s42108-020-00099-w https://doi.org/10.1007/s10040-019-01956-7 _hlk109668004 _hlk113306787 _hlk113306759 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n2.105917 earth sciences research journal earth sci. res. j. vol. 27, no. 2 (june, 2023): 149 162 pe tr o le u m e n g in ee r in g manuscript received: 20/11/2022 accepted for publication: 07/07/2023 integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation syed bilawal ali shah* department of geology, university of malaya, kuala lumpur 50603, malaysia. email: bilawalshah22@siswa.um.edu.my abstract keywords: indus basin; source rock: sakesar; rock-eval pyrolysis; vr %ro. this study demonstrates how an integrated geochemical and petrophysical analysis can be used to evaluate the petroleum generation potential of source and reservoir rocks. the eocene and palaeocene sequences of the potwar basin, located in the upper indus basin of pakistan, were analyzed. well logs and schlumberger log interpretation charts were used for the petrophysical analysis of the chorgali formation’s reservoir potential. geochemical methods were applied to 34 well-cutting sakesar and patala formations samples. results from vitrinite reflectance (vr) (%ro) and tmax data suggest that both formations have reached the peak of their oil generation window. the sakesar formation has a mean toc of 1.88 wt. % and hi values of 375 mg hc/g toc. the patala formation has a mean toc of 3.33 wt.% in well a and hi values ranging from 2.4 to 369 mg hc/g toc in well b, with a mean toc of 3.52 wt%. both formations have mixed type ii/iii kerogen. the findings indicate that both the sakesar and patala formations possess good oil/gas-generation potential and may act as source rocks in the potwar basin. petrophysical analysis of the chorgali formation shows an average porosity of 10.32%, water saturation of 36.14%, and hydrocarbon saturation of 63.85%. this indicates that the chorgali formation has an average to good reservoir potential. the research findings will aid exploration and production companies in the fimkassar oilfield. palabras clave: cuenca indus; roca fuente; formación sakesar; pirólisis rock-eval; reflectancia de la vitrinita; interpretación integrada de rocas del eoceno y el paleoceno en la cuenca potwar, pakistán: implicaciones para la generación petrolífera how to cite this item: shah, s. b. a. (2023). integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation. earth sciences research journal, 27(2), 149 162. https://doi.org/10.15446/esrj.v27n2.105917 resumen este estudio muestra como un análisis geoquímico y petrofísico integrado se puede usar para evaluar el potencial de generación petrolífera de las rocas fuente y reservorio. con este fin se analizaron las secuencias del eoceno y del paleoceno de la cuenca potwar, localizada en la cuenca superior de indus, en pakistán. se utilizaron los registros de pozo y los gráficos de interpretación de registros de schlumberger para el análisis petrofísico del potencial reservorio de la formación chorgali. se aplicaron métodos geoquímicos a 34 láminas cortadas de pozo en las formaciones sakesar y patala. los resultados de reflectancia de la vitrinita (vr) (%ro) y los datos tmax sugieren que ambas formaciones han alcanzado el pico de su período de generación petrolera. la formación sakesar tiene una media toc de 1.88 wt. % y valores hi de 375 mg hc/g toc. la formación patala tiene una media toc de 3.33 wt. % en el pozo a y valores hi que van de 2.4 a 369 mg hc/g toc en el pozo b, con una media toc de 3.52 wt. %. ambas formaciones tienen mezclados tipos ii y iii de querógeno. los hallazgos indican que ambas formaciones tienen un buen potencial de generación de petróleo y gas y pueden actuar como las rocas fuentes de la cuenca potwar. los análisis petrofísicos de la formación chorgali muestran un promedio de porosidad de 10.32 %, saturación de agua de 36.14 % y saturación de hidrocarburos de 63.85 %. esto indica que la formación corgali tiene un buen potencial como reservorio. los resultados del trabajo ayudarán en la exploración y producción de las compañías en el campo petrolero de fimkassar. https://doi.org/10.15446/esrj.v27n2.105917 mailto:bilawalshah22@siswa.um.edu.my https://doi.org/10.15446/esrj.v27n2.105917 150 syed bilawal ali shah 1. introduction the potwar basin is a productive oil and gas-producing basin in pakistan. several researchers have provided information on the sources of hydrocarbons in the basin, including imtaiz et al. (2017), fazeelat et al. (2010), ihsan et al. (2022), and shah (2022). hydrocarbon traps in the potwar basin are primarily structurally controlled, although there are also stratigraphic traps (asif et al., 2011; he et al., 2023; yasin et al., 2021; zhou et al., 2022). in this study, sediments from two wells in the fimkassar oilfield and well logs have been analyzed (fig. 1). according to various studies, the primary source of hydrocarbons in the potwar basin is the patala formation. these researchers include asif et al. (2011), fazeelat et al. (2010), fazeelat et al. (2009), shah et al. (2022), shah and shah (2021), shah (2022), and yasin et al. (2021). the sediments from two wells of the fimkassar oilfield, which are of the eocene and palaeocene ages, have been studied (fig. 1). a study by fazeelat et al. (2011) and shah (2022) found that shallow marine sediments in the potwar basin contain organic-rich matter, necessitating an evaluation and interpretation of the source and reservoir rock characteristics of the chorgali, patala, and sakesar formations in the basin (table 1). the novel contribution of this research is the thorough study of well cutting samples and well logs to assess source rock generation potential, thermal maturity, identification of kerogen type, and petrophysical properties. additionally, this research combines two different methods to evaluate the horizon for potential prospects and two essential properties of the petroleum system, namely source and reservoir rock properties. this is the unique aspect of this research work. previous studies by researchers such as ihasn et al. (2022), nazir et al. (2014), shah et al. (2021), fazeelat et al. (2010), shah (2017), nazir et al. (2015), h et al. (2021), and masood et al. (2017) focused on different formations and used various techniques to evaluate them, including modelling the structure with faults for salt diapirs and studying specific formations like sakesar, tobra, and khewra. however, there is a lack of information regarding the interpretation of all three formations’ source and reservoir rock properties, which makes this study necessary and crucial. the fimkassar oilfield is located in the potwar basin (fig. 1). the findings of this study will reveal the source and reservoir rock generation potential, kerogen type, thermal maturity of the rocks as source rocks, and the presence of possible hydrocarbon prospects. the research outcomes will directly benefit future hydrocarbon exploration by oil and gas companies. this research will figure 1. map showing location of wells and major tectonic structures of potwar basin (after shami and baig, 2002; zahid et al., 2014; shah and abdullah, 2016; shah and abdullah, 2017; shah et al., 2019). table 1. qualitative description of reservoirs porosity set by rider’s (1986) and shah et al. (2021). qualitatively evaluation of porosity qualitative description average porosity negligible 0-5 poor 5-10 good 10-20 very good 20-30 excellent >30 151integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation contribute valuable information to the existing body of knowledge, aiding in the understanding of various source and reservoir properties and the petroleum potential for future hydrocarbon exploration and production. 2. geological setting the potwar basin exhibits several major characteristics, including inner and outer folded zones, a platform, and a foredeep comprising depressions (kadri, 1995; kazmi and jan, 1997; liu et al., 2023; nazir and fazeelat, 2017; zhan et al., 2022). the study area specifically refers to the fimkassar oilfield. the potwar basin is geographically divided into two sections by the indus and jhelum rivers in the east and west, respectively. it is bordered by the salt range in the south and the kalachitta-margalla hill ranges in the north (cheng and fu, 2022; liu et al., 2023; shah and abdullah, 2017; yin et al., 2023). the basin is characterized by east-west complex and tight folds, which are inverted in the south and affected by steep-angle faults. the northern portion of the basin experiences more extensive deformation (aadil et al., 2014; kazmi & abbassi, 2002; kazmi & jan, 1997; riaz, 2022; shah, 2009; shah, 2022). in contrast, the eastern part of the basin exhibits an abrupt change in strike towards the northeast, with structures consisting of large synclines and anticlines tightly folded. the western part of the basin contains several gentle and broad eastwest folds (fu et al., 2022; shah, 2022; yin et al., 2023; zahid et al., 2012; zhang et al., 2022). 2.1 an overview of the potwar basin’s source and reservoir rocks the potwar basin has several known source rocks, including the sakesar, hangu, patala, and lockhart formations (shah, 2022; xu et al., 2022; zheng et al., 2023) (table 2). the reservoir rocks found in the potwar basin include alluvial and shoreface cambrian sandstones, miocene alluvial sandstones, jurassic and permian continental sandstones, paleogene shelf carbonates, and miocene alluvial sandstones (aadil et al., 2014; kazmi & abbassi, 2002; liu et al., 2023; peng et al., 2022). oil and gas have been explored from reservoir table 2. general stratigraphy of potwar basin (modified after fazeelat et al., 2010; shah and abdullah, 2017). age / epoch lithology formation lithology description neogene pliocene nagri chinji sandstone, sandstone and clay miocene oligocene kamlial murree sandstone, sandstone, limestone oligocene unconformity paleogene eocene chorgali sakesar limestone, limestone paleocene patala lockhart hangu shale, limestone, sandstone mesozoic & late permian unconformity jurassic datta sandstone, shale permian early permian chhidru wargal amb sardhai warcha dandot tobra sandstone, limestone, limestone, siltstone, interbeds of sandstone, shale interbeds of sandstone, sandstone and conglomerate. carboniferous to ordovician unconformity cambrian to precambrian cambrian baghanwala jutana kussak khewra shale, dolomite, sandstone and siltstone, shale interbeds of dolomite and siltstone infra-cambrian salt range marl, claystone and siltstone, salt 152 syed bilawal ali shah rocks such as tobra, wargal, amb, cambrian khewra, jutana, kussak, jurassic dutta, khairabad, lockhart, nammal, sakesar, margalla limestone, chorgali, bhadrar, and murree (zahid et al., 2012). the discovered oilfields in the potwar basin are found in either pop-up structures, faulted anticlines, or fault-block traps (shah, 2022; xu et al., 2022; zheng et al., 2023; zhou et al., 2022). 3. materials and methods thirty-four well cutting samples were obtained from the fimkassar oilfield in the potwar basin, collected from palaeocene and eocene sequences. well a provided samples at intervals of 4-15 m, while well b provided samples at intervals of 5-6 m. detailed information about the samples can be found in table 5. reservoir rock characteristics were determined using various logs, including density, neutron, gamma ray, resistivity, and spontaneous potential logs. the reservoir properties were qualitatively described based on criteria from rider (1986) and shah and shah (2021). this study specifically utilized well logs from well a to investigate the reservoir potential of the nammal formation. reservoir rock characteristics were evaluated using methods proposed by shah (2022), shah (2021), and hartmann and beaumont (1999). furthermore, calculations were performed to determine hydrocarbon saturation, porosity, water saturation, and formation water resistivity. 3.1 organic petrographic analyses the analysed samples were dried and crushed to approximately 1-2mm in size. the sediments were then embedded in polyester resin (serifix) with a hardener, allowing the resin to set. once set, the samples were flattened on a diamond lap and polished using various grades of silicon carbide paper, lubricated with isopropyl alcohol. subsequently, the samples were further polished with increasingly finer alumina powder (5/20, 3/50, and gamma) to achieve a highly reflective surface. vitrinite reflectance measurements were conducted using a leica ctr6000m microscope equipped with diskus fossil software. calibration was performed using a leuko-sapphire standard (0.589% ro). considering the availability and distribution of vitrinite particles, between 15-50 measurements were obtained. the measurements were taken using an oil immersion x50 objective lens under white light. 3.2 organic geochemical analysis the well cutting samples were initially crushed to obtain fine particles smaller than 200 mesh. the total organic carbon (toc) content was then measured using leco elemental analysers, specifically the cs125 model. pyrolysis analysis was performed using the rock-eval vi equipment. approximately 100mg of crushed samples were subjected to pyrolysis in a helium atmosphere, heated to 600°c. the pyrolysis process provided three crucial parameters: s1, representing the amount of whole free petroleum; s2, indicating the quantity of hydrocarbons generated during thermal cracking; and tmax, the temperature at which the maximum hydrocarbon generation occurs (shah, 2023; shah and shah, 2021). the hydrogen index and production yield were also calculated as other significant factors. 3.3 well log analysis the well logs in the las file format were used on the interactive petrophysics software package to establish reservoir properties. details of the properties are available in table 3. the procedure for obtaining various parameters from the well logs is described in the section below: the log track parameters that were directly read included: 1. formation depth (ft.) 2. neutron porosity (nphi; 0.45 to -0.15 scale) 3. bulk resistivity of the formation (lld; 0 – 2000ohm.m scale) 4. spontaneous potential (sp; 0 – 100mv scale) 5. bulk density of formation (rhob; 1.95 –2.95 scale) volume of shale (vsh): the volume of shale was calculated using gr logs. vsh was calculated using the equation below. (rider, 2002; shah, 2023; shah et al., 2022): gr= (gr_log-gr_min/gr_max-gr_min) porosity calculation: sonic log was used to determine the porosity by using the following equation: φs = δtlog–δtma/δtf–δtma where, δtlog = sonic transit time δtf = travel transit time of interstitial fluid (189 μs/ft. for fresh mud and 185 μs/ft. salt mud) δtma = travel transit time of matrix material (47.6 μs/ft. for limestone) effective porosity: the permeable zone of the formation is determined by calculating effective porosity. following equation was used to calculate effective porosity (rider, 2002; shah et al., 2023; xu et al., 2022; zhang et al., 2022): ϕe= ϕt∗(1−vsh) whereas: ϕe= effective porosity ϕt = total porosity vsh = volume of shale table 3. petrophysical analysis ifferent parameters calculated for well logs analysis. depth (ft) temp rhob φd sp φ (n.d) rt (lld) rwe rw sw sh lithology 7944 151.6 2.61 0.13 -27 0.12 330 0.39 0.67 36% 64% limestone 7969 152.2 2.62 0.10 -12 0.14 310 0.51 0.80 39% 61% limestone 7994 152.7 2.64 0.12 -28 0.11 160 0.62 0.61 37% 63% limestone 8019 153.1 2.63 0.09 -33 0.09 950 0.56 0.47 35% 65% limestone 8044 153.5 2.61 0.11 -25 0.13 1390 0.27 0.32 38% 62% limestone 8069 154.4 2.67 0.08 -32 0.10 1680 0.62 0.44 32% 68% limestone 8094 154.6 2.66 0.11 -35 0.13 480 0.35 0.35 36% 64% limestone 153integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation hydrocarbon saturation (sh) spontaneous potential (sp) method (figure 2-3b) was used for the determination of hydrocarbons at the zone of interest. the graphs and equations used together in the following process: 1. formation temp was calculated using figure 2 and the following equation: tf = ts + df (bht-ts/td) 2. rmf and rm were corrected (fig. 3a). 3. sp values were read from logs directly. 4. rmf/rwe ratio was calculated (fig 3b). 5. the rwe was calculated by rmf/rwe ratio. 6. rwe= rmfeq/(rmfeq/rwe) 7. water saturation was calculated using archie’s equation: 8. hydrocarbon saturation was calculated using equation: sh=1-sw after measuring the resistivity values of the mud filtrate, they were converted to equivalent mud filtrate resistivity values using the equation in figure 2. to obtain the equivalent water resistivity, the equivalent mud filtrate resistivity values were used according to step 6. subsequently, the equivalent figure 2. shows the determination of rw using schlumberger chart by determining rwe. 154 syed bilawal ali shah figure 3. (a) according to temperature rmf and rwe correction (schlumberger, 1977). 155integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation figure 3. (b) determination of rmf/rwe based on self-potential (schlumberger, 1977). water resistivity was converted to water resistivity, which was then used to determine water saturation using archie’s equation. the log header’s mud filtrate resistivity was determined at surface temperature. to determine the mud filtrate resistivity at formation temperature, it must be corrected to formation temperature for each value at a certain depth, and this correction was made using the equation presented in figure 3a. 4. results and discussion 4.1 total organic carbon and pyrolysis typically, the total organic carbon (toc) content is used to indicate the richness of rocks, with a toc value of 1.0% considered necessary for a clastic rock to qualify as a source rock, as stated by hunt (1996) and shah (2023). sediments from the patala formation in both wells exhibited higher toc contents, with mean values ranging from 2.70 wt% to 2.88 wt% in well a and 2.48 wt% to 2.98 wt% in well b, respectively. sediments from the sakesar formation had an average toc content of 1.72 wt% and ranged from 1.56 wt% to 1.88 wt%. based on the sample analysis, the majority of samples showed good to very good toc content. the parameter s2, produced during pyrolysis, is the most useful for estimating the hydrocarbon generative potential (dang et al., 2023; peters and cassa, 1994; tissot and welte, 1984; wu et al., 2022). according to bordenave et al. (1993) and shah and abdullah (2017), a good petroleum generation capacity requires a minimum of 5 mg hc/g s2. in general, the sakesar formation exhibited hydrocarbon yields (s2) ranging from 3.76 mg/g to 5.78 mg/g, while the patala formation in well a showed hydrocarbon yields ranging from 7.88 mg/g to 9.65 mg/g, and the patala formation in well b showed hydrocarbon yields ranging from 5.87 mg/g to 8.66 mg/g. the sakesar formation displayed fair to good petroleum generating potential, whereas sediments from the patala formations in both wells a and b showed good petroleum generating potential, as indicated by the s2 versus toc cross plots (fig. 4 and 5). the migration index (s1/toc) can be used to differentiate between indigenous and migrated petroleum (shah, 2022; tissot and welte, 1984). the s1 versus toc plot in this study demonstrated that all samples from the sakesar formation and patala formation (wells a and b) were indigenous (fig. 6). 156 syed bilawal ali shah figure 4. pyrolysis s2 versus total organic carbon (toc) plot, showing generative source rock potential for the analyzed samples (peter and cassa, 1994). figure 5. pyrolysis s2 versus total organic carbon (toc) plot, showing the type of kerogen for the analyzed samples (peter and cassa, 1994). 157integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation figure 6. cross plot of pyrolysis s1 versus total organic carbon (toc) for identifying the migration index of the studied samples (hunt, 1996). figure 7. the pyrolysis hi versus tmax plot shows kerogen type for the analyzed samples (mukhopadhyay et al., 1995). 158 syed bilawal ali shah figure 8. plot of tmax versus production index (pi), showing the maturation and nature of the hydrocarbon products of the analyzed samples (shah, 2023). figure 9. plot of tmax versus vitrinite reflectance (ro) showing the maturity levels (shah, 2022). 159integrated interpretation of eocene-palaeocene rocks in potwar basin, pakistan: implications for petroleum generation table 5. bulk geochemical results of all analysed samples in detail including all test results (vr, toc and rock-eval data). formation depth (m) toc % tmax °c s1 s2 s3 pi hi oi s2/s3 vr sakesar 2938 1.63 436 1.36 4.26 0.47 0.24 261 28.8 9.064 0.69 sakesar 2953 1.56 437 1.42 3.76 0.39 0.27 241 25 9.641 0.71 sakesar 2969 1.88 435 1.3 5.34 0.75 0.2 284 39.9 7.12 0.67 sakesar 2984 1.61 439 1.23 4.36 0.45 0.22 271 28 9.689 0.74 sakesar 2999 1.58 438 1.4 4.97 0.78 0.22 315 49.4 6.372 0.72 sakesar 3015 1.74 439 1.47 5.78 0.87 0.2 332 50 6.644 0.74 sakesar 3030 1.87 442 1.35 5.34 0.54 0.2 286 28.9 9.889 0.8 sakesar 3045 1.67 440 1.42 4.78 0.65 0.23 286 38.9 7.354 0.76 sakesar 3061 1.75 438 1.65 5.13 0.78 0.24 293 44.6 6.577 0.72 sakesar 3076 1.83 440 1.53 3.86 0.73 0.28 211 39.9 5.288 0.76 patala 3092 2.81 437 1.96 7.88 0.63 0.2 280 22.4 12.51 0.71 patala 3096 2.67 438 1.87 8.23 0.77 0.19 308 28.8 10.69 0.72 patala 3099 2.78 442 1.94 8.78 0.73 0.18 316 26.3 12.03 0.8 patala 3103 2.71 439 1.36 8.26 0.82 0.14 305 30.3 10.07 0.74 patala 3107 2.91 441 1.45 9.21 0.63 0.14 316 21.6 14.62 0.78 patala 3110 2.97 437 1.78 8.2 0.57 0.18 276 19.2 14.39 0.71 patala 3114 2.86 438 1.88 7.89 0.78 0.19 276 27.3 10.12 0.72 patala 3118 3.21 436 1.79 9.65 0.67 0.16 301 20.9 14.4 0.69 patala 3121 2.95 440 1.63 8.64 0.88 0.16 293 29.8 9.818 0.76 patala 3125 3.2 443 1.89 8.37 0.83 0.18 262 25.9 10.08 0.81 patala 3129 2.89 438 1.77 7.63 0.79 0.19 264 27.3 9.658 0.72 patala 3132 2.76 439 1.83 8.75 0.86 0.17 317 31.2 10.17 0.74 patala 3136 2.82 440 1.75 8.45 0.72 0.17 300 25.5 11.74 0.76 well b patala 3055 2.74 438 1.98 7.23 0.78 0.21 264 28.5 9.269 0.72 patala 3060 2.57 437 1.79 6.98 0.95 0.2 272 37 7.347 0.71 patala 3065 2.63 440 1.75 5.87 0.89 0.23 223 33.8 6.596 0.76 patala 3070 2.98 443 1.86 5.36 0.78 0.26 180 26.2 6.872 0.81 patala 3075 2.48 441 1.74 7.65 0.95 0.19 308 38.3 8.053 0.78 patala 3081 2.72 439 1.65 7.58 0.83 0.18 279 30.5 9.133 0.74 patala 3086 2.65 440 1.73 6.74 0.96 0.2 254 36.2 7.021 0.76 patala 3091 2.68 443 1.85 7.28 0.82 0.2 272 30.6 8.878 0.81 patala 3096 2.77 439 1.63 8.25 0.93 0.16 298 33.6 8.871 0.74 patala 3101 2.91 440 1.93 8.66 0.78 0.18 298 26.8 11.1 0.76 patala 3106 2.67 442 1.71 7.78 0.97 0.18 291 36.3 8.021 0.8 table 4. results of analyzed samples of sakesar and patala formations presented as summary. formation name and no of samples depth (m) petroleum potential maturity of sample quality of om kerogen type source rock generative potential toc wt. % s1 s2 s3 tmax °c hi sakesar (10) 2938-3076 1.56-1.88 1.23-1.65 3.76-5.78 0.39-0.87 435-442 211-332 mixed type ii/iii and type iii fair to good patala well a (13) 3092-3136 2.67-3.21 1.36-1.96 7.88-9.65 0.57-0.88 436-443 262-317 good patala well b (11) 3055-3106 2.48-2.98 1.63-1.98 5.36-8.66 0.78-0.97 437-443 213-308 160 syed bilawal ali shah 4.2 kerogen type the hydrogen index (hi) was utilized to assess the kerogen types in the formations, which is considered the most important index for this purpose (peters, 1986; tissot and welte, 1984). hi values for the sakesar formation ranged from 211 mg hc/g toc to 332 mg hc/g toc, while the patala formation exhibited hi values ranging from 262 to 317, and well b showed hi values ranging from 213 to 308. to classify the kerogen type, hi versus tmax plots were constructed, revealing that the sediments of all formations primarily consist of mixed type-ii/iii kerogen. the kerogen type identified through s2 versus toc plots also aligned with the interpretation of the hi versus tmax plot (fig. 4 and 7). samples from both wells a and b in the patala formation exhibited high hi values of 213 to 317 mg hc/g toc, while samples from the sakesar formation also displayed high hi values of 211 to 332 mg hc/g toc, indicating their potential as oil/gas-prone source rocks (fig. 4, 5). 4.3 thermal maturity for thermal maturity assessment, parameters including tmax values, vitrinite reflectance (%ro), and production index (pi) were used. oil with vitrinite reflectance values ranging from 0.6% to 1.3%ro is considered thermogenic oil (bordenave, 1993; tissot and welte, 1984). tmax levels exhibit a wide range of variation. according to espitalié et al. (1977) and shah et al. (2023), tmax values between 430 and 455 °c correspond to kerogen types i and ii, while values between 435 and 465 °c correspond to type iii. all formations showed measured vitrinite reflectance (%ro) values ranging from 0.67% to 0.81% (figure 8). as shown in table 4, the vitrinite reflectance values for the sakesar formation sediments ranged from 0.67% to 0.8% ro and corresponded to temperatures between 435 and 442 °c, indicating the peak of oil production and early maturity within the oil window (fig. 9). in the case of the patala formation, samples from well a exhibited vitrinite reflectance values ranging from 0.69% to 0.81% ro, with tmax values between 436 and 443 °c, while samples from well b exhibited vitrinite reflectance values ranging from 0.71% to 0.81% ro, with tmax values varying between 437 and 443 °c (table 4 and 5). these values suggest early maturity within the oil window and the peak of oil generation (fig. 9). for organic matter maturity assessment, if pi values are less than 0.05, the samples may be considered immature and may have produced little to no hydrocarbons. pi values between 0.05 and 0.10 indicate very limited oil production and a potential entry into the wet gas zone. pi readings exceeding 1.0 suggest that the kerogen’s capacity to produce hydrocarbons may have been depleted. sediments from the sakesar formation exhibited pi values ranging from 0.2 to 0.28, indicating early to peak maturity, which aligns with the tmax and vitrinite reflectance (%ro) measurements (fig. 8). meanwhile, samples from the patala formation predominantly displayed pi values ranging from 0.14 to 0.26, indicating early to peak maturity within the oil window, and were consistent with the tmax and %ro data (fig. 8). 4.4 reservoir potential the reservoir parameters determined from the petrophysical analysis were used for the quantitative interpretation of the reservoir. the estimated petrophysical characteristics of the chorgali formation are shown in table 3. the average porosity ranged between 8-14%, indicating poor to moderate reservoir potential. according to rider (1986) and shah and shah (2021) criteria, the average water saturation was 36.14%, and the hydrocarbon saturation was 63.85%, indicating average to good hydrocarbon potential (table 1). 5. concluding remarks following is a summary of the key findings from the investigation of the chorgali, patala, and sakesar formations sediments using geochemical and petrophysical analysis: the source rock sediments showed that the sakesar formation has fair to good generative ability, with toc > 1.5 wt. % (ranging from 1.56-1.88 wt. %), and possesses mixed type ii/iii kerogen. the patala formation has good to very good generative potential, with toc > 2.4 wt. % (ranging from 2.483.21 wt. % in both wells). both formation sediments lie in the maturity oil/gas window zone, as indicated by both s2 vs toc and hi vs tmax plots. therefore, it could be expected that both formations may have generated oil and gas in the fimkassar oilfield subsurface. petrophysical analysis of the chorgali formation encountered in well a indicates average reservoir qualities and an average potential for the production of hydrocarbons. references aadil, n., & sohail, g. m. 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(2022). voids filling of dem with multiattention generative adversarial network model. remote sensing, 14(5), 1206. doi: 10.3390/rs14051206. issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n1.103542 earth sciences research journal earth sci. res. j. vol. 27, no. 1 (march, 2023): 59 68 r em o te s en si n g palabras clave: geoprocesamiento; desastres naturales; teledetección; riesgo ambiental. methodological proposal to remote detection and management of areas that are naturally vulnerable to floods lucas emanuel servidoni1; joaquim ernesto bernardes ayer2; guilherme henrique expedito lense1; felipe gomes rubira1; velibor spalevic3; branislav dudic4; ronaldo luiz mincato1* 1 universidade federal de alfenas, programa de pós-graduação em ciências ambientais, rua gabriel monteiro da silva, alfenas, minas gerais, brasil. https://orcid.org/0000-0003-1741-1706; https://orcid.org/0000-0002-3560-9241; https://orcid.org/0000-0002-6594-8228; https://orcid.org/0000-0001-8127-0325 2 centro universitário de paulínia, rua nelson prodócimo, 495, jardim bela vista, 13145-026, paulínia, são paulo, brasil. https://orcid.org/0000-0003-0612-0663 3 university of montenegro, podgorica, montenegro. https://orcid.org/0000-0002-7800-2909 4 comenius university, faculty of management, odbojárov, 10, bratislava, 82005, slovakia. https://orcid.org/0000-0002-4647-6026 *corresponding author: e-mail: ronaldo.mincato@unifal-mg.edu.br propuesta metodológica para la teledetección y gestión de zonas naturalmente vulnerables a las inundaciones how to cite item: servidoni, l. e., bernardes, j. e., expedito, g. h., gomes, r., f., spalevic, v., dudic, b., & mincato, r. l. (2023). methodological proposal to remote detection and management of areas that are naturally vulnerable to floods. earth sciences research journal, 27(1), 59-68. https://doi.org/10.15446/esrj.v27n1.103542 record manuscript received: 07/07/2022 accepted for publication: 01/02/2023 keywords: geoprocessing, natural disasters, remote sensing, environmental risk. abstract floods are the main natural disasters in brazil, causing loss of life and socioeconomic damage. this work proposes a model for the remote detection of areas that are naturally flood-prone due to the morphometric characteristics of their relief and drainage networks in the alto sapucaí river in minas gerais, brazil. the morphometric parameters used were the drainage density, river density, relief ratio, roughness index, maintenance coefficient, form factor and stream surface length. the risk areas had a compactness coefficient of 0.75 and a form factor of 0.56, and both were considered a high risk for floods. the obtained results allowed the identification of a significant predictive equation that suggested a cutoff value of 3.82 for the discriminant function; areas with values under this cutoff were considered naturally more vulnerable to floods occurrences. these areas were corroborated with the emergency maps of the municipalities. the map obtained by the proposed model was compared with the civil defense map, and its accuracy, according to the kappa coefficient, was 0.83, indicating strong similarity between the two maps. resumen las inundaciones son las principales catástrofes naturales en brasil; causan pérdidas de vidas humanas y daños socioeconómicos. este trabajo propone un modelo de detección remota de áreas naturalmente inundables debido a las características morfométricas del relieve y de la red de drenaje en el alto sapucaí, en minas gerais, brasil. los parámetros morfométricos utilizados fueron la densidad de drenaje, la densidad del río, la relación de relieve, el índice de rugosidad, el coeficiente de mantenimiento, el factor de forma y la longitud de la superficie del arroyo. las zonas de riesgo tenían un coeficiente de compacidad de 0,75 y un factor de forma de 0,56, y ambas se consideraban de alto riesgo de inundación. los resultados obtenidos permitieron identificar una ecuación predictiva significativa que sugería un valor de corte de 3,82 para la función discriminante; las zonas con valores por debajo de este corte se consideraron naturalmente más vulnerables a la ocurrencia de inundaciones. estas zonas fueron corroboradas por los mapas de emergencia de los municipios. el mapa obtenido por el modelo propuesto se comparó con el mapa de la defensa civil, y su precisión, según el coeficiente kappa, fue de 0,83, lo que indica una gran similitud entre los dos mapas. https://doi.org/10.15446/esrj.v27n1.103542 https://orcid.org/0000-0003-1741-1706 https://orcid.org/0000-0002-3560-9241 https://orcid.org/0000-0002-6594-8228 https://orcid.org/0000-0001-8127-0325 https://orcid.org/0000-0003-0612-0663 https://orcid.org/0000-0002-7800-2909 https://orcid.org/0000-0002-4647-6026 mailto:ronaldo.mincato@unifal-mg.edu.br https://doi.org/10.15446/esrj.v27n1.103542 60 lucas servidoni; joaquim bernardes; guilherme expedito; felipe gomes; velibor spalevic; branislav dudic; ronaldo luiz mincato 1. introduction one of the steps of natural disaster management is to assess environmental behaviors, such as the influence of landforms on surface and subsurface water runoff processes. obtaining information about the dynamics of these landscape elements enables decision-makers to mitigate and control risk (arantes et al., 2021; mardhel et al., 2021; tesema, 2021). in this context, prior knowledge of morphometric landform characteristics and drainage networks allows for the prevention and mitigation of events that are triggered by natural disasters and for the precise and efficient allocation of resources for flood event warning and preparation systems (waghwala and agnihotri, 2019; bogo, 2020). floods are natural processes associated with mountain snowmelts, rapid and intense rains, long-term rains, and extreme hydrometeorological events such as hurricanes and tropical cyclones (ward et al., 2020). thus, despite being natural events, the uncontrolled development in river plains increases the risks and socioeconomic damages caused by floods, which, in general, result in loss of life (martin-díaz et al., 2018; cobbinah et al., 2022). due to brazil’s climate and historically uncontrolled development of flood-prone areas (fleischmann et al., 2021), several millions reais are spent annually to combat the adverse effects of flooding catastrophes (bitencourt and rocha, 2014). the urbanization process imposed by capitalism in peripheral countries such as brazil occurred in an accelerated and disorderly way, following the intense process of industrialization in the 1970s. this process promoted an intense rural exodus causing a population explosion in urban centers. this migratory wave could not be adequately absorbed in urban centers, forming clusters of precarious housing and subjecting the low-income population to settle in areas at risk of flooding (fleischmann et al., 2021; cobbinah et al., 2022). however, flood risks have worsened due to uncontrolled land changes associated with climate changes (arantes et al., 2021); the deleterious effects of this natural phenomenon are exacerbated by anthropogenic changes in hydrosedimentological systems (silva et al., 2018), which have intensified both in volume and area, leading to an increase in the number of people affected and displaced by flood events, mainly in urban areas (molina and gonzález, 2020; defesa civil, 2020). to assess and map floods, computational analysis of morphometric and hydrological variables, remote sensing and geographic information systems (gis) are indispensable and widely used, due to the low cost of acquiring images and even free of charge in specialized public agencies. these resources can make land use management and agricultural and urban development possible at low cost. they significantly contribute to the generation of strategic information for coping with floods (tamiru and dinka, 2021). such costs are even lower than those necessary to carry out field surveys in large areas subject to flooding in a country with continental dimensions such as brazil. in this scenario, geoprocessing and remote sensing techniques enable preliminary and pilot studies for recognizing flood-prone areas and support land use planning, development and management (udin and matin, 2021). these techniques reduce the costs and efforts of mapping and disaster management (ardaya et al., 2017). in addition, identifying areas naturally vulnerable to flooding favors the establishment of inspection, policing and control actions appropriate to their socioeconomic and environmental realities (ekmekcioglu et al., 2022). thus, the development of proposals to analyze and map flood-prone areas based on the morphometric characteristics of their drainage networks and landforms is a necessary response to climate change, which increasingly causes extreme climatic events to occur (kuntamalla et al., 2018; lin and billa, 2021). this research aims to develop a methodological proposal for remote detection and management of areas naturally vulnerable to flooding, based on morphometric and hydrological parameters of hydrographic basins. 2. materials and methods the study area comprises 170 hydrographic subbasins of the alto sapucaí (figure 1), a tributary of the rio grande hydrographic basin, which has a volume of 2,813 km2 and an average flow of 146 m3s-1. a main source of water for the furnas hydroelectric power plant reservoir, the alto sapucaí river in southern minas gerais is also important for locomotion, fishing, irrigation, and hydroelectric energy production (galvão and bermann, 2015). however, surrounding municipalities, such as itajubá, piranguinho, santa rita do sapucaí, piranguçu, and delfin moreira, recurrently suffer from floods due to their history of uncontrolled development (martins et al., 2019). the local geomorphology is dominated by denudational units of crystalline or sedimentary rocks, with an altitude between 816 and 1,482 m. the geological framework of the area is composed of granite-gneissic complexes from the orogenic system of tocantins (cprm, 1998; hasui, 2010). in the area, red and red yellow argisols predominate and, secondarily, red latosols, haplic cambisols and neosols (santos et al., 2018). regarding the hydrography, the drainage pattern is dendritic, with 1stand 2nd-order channels with nnessw orientations and 3rd-, 4thand 5th-order channels with ne-sw directions (ribeiro et al., 2016). figure 1. location map of the alto sapucaí hydrographic basin, southern of minas gerais, brazil. the climate, according to köppen’s classification, is tropical in altitude (cwa and cwe), with hot and humid summers and cold and dry winters and average annual precipitation of 1.600 to 1.865 mm (aquino et al., 2012). in southern minas gerais, december and january have the highest precipitation values of 253 and 245 mm, respectively (martins et al., 2019), with preferential maximums leeward of serra da mantiqueira, which illustrates the orographic performance in the distribution of precipitation (campos et al., 2016; ávila et al., 2009). the dominant morphoclimatic domain is the atlantic forest, with the seasonal semideciduous forest replaced mainly by pastures and agriculture (scolforo et al., 2008). from the archydro analysis module, 170 hydrographic subbasins were delimited as representative sampling units of the study area. subsequently, the drainage hierarchy was defined according to strahler (1957). finally, the morphometric parameters of the alto sapucaí watershed were calculated using the shuttle radar topography mission 2 (srtm 2) digital elevation model (dem) with a vertical resolution of 30 m (usgs, 2014), which agrees with the scale criteria adopted by gupta et al. (2017). the dem was processed in gis by the hydrology module of the spatial analyst tools and archydro toolset to obtain the cartographic base comprising the slope, altimetry, and hydrography maps. arcgis 10.5 fill extension (esri, 2015) was used to reduce failures and errors in the results obtained from the morphometric and hydrological parameters to remove pixels with anomalous altimetry values. the automatic extraction of the drainage network was performed by the hydrology extension of arcgis 10.5 from the processes of generating fill, flow direction, flow accumulation, con, stream to feature, and watershed files (esri, 2015). the delimitation of the hydrographic subbasins followed the criteria of altimetry and relief slope and the areas of flow and accumulation of water, allowing the precise identification of the watersheds in the basins. the calculations of morphometric parameters were performed for the 170 sampling units according to gupta et al. (2017) and are described in table 1. 61methodological proposal to remote detection and management of areas that are naturally vulnerable to floods table 1. morphometric parameters of the alto sapucaí hydrographic basin, southern minas gerais, brazil. parameter estimator variable meaning references stream frequency (dh) dh = n = number of channels a = area expresses the number of existing channels per unit area, indicating the water potential of the region strahler (1952) drainage density (dd) dd = c = channels total length a = area expresses the influence of the supply and transport of dendritic material. indicates the degree of the anthropization of the channels horton (1945) relief ratio (rr) rr =∆ ∆a = altimetric amplitude l = chanel length relationship between altimetric amplitude and the length of the main channel strahler (1952) roughness index (ir) ir = hm x dd hm = altimetric amplitude dd = drainage density represents the slope relationship with the channel lengths. indicates the degree of dissection of the watershed horton (1945) compactness coefficient (kc) kc = 0.28 √ p = basin perimeter in km a = basin area in km² indicates the highest or lowest occurrence of floods horton (1945) form factor or gravelius´s shape index (kf) kf = 2 a = basin area in km² g = basin axial length lower values indicate that the basin is less prone to flooding strahler (1952) maintenance coefficient (cm) cm = 1 1000 dd = drainage density indicates the minimum area needed to maintain a meter of drainage perennial schum (1956) stream surface length (eps) eps = 1 2. dd = drainage density indicates the average length traveled by the flow to the drainage channel chistofoletti (1969) circularity index (ic) ic = 12 .57 2 a = basin area p = basin perimeter the unit tends as the basin approaches the circular shape making it more prone to flooding strahler (1952) for the composition and selection of the variables that most affect the natural propensity to flooding in regions of rugged reliefs, the following protocol was used. the variables were grouping to identified regions naturally vulnerable to the occurrence of floods due to the values of morphometric and hydrological parameters that assess risks to floods, in accordance with relevant literature (gupta et al., 2017; taofik et al., 2017; kuntamalla et al., 2018). empirical data generated by the civil defense of minas gerais were also collected (defesa civil, 2022), which mapped the areas with occurrences of flooding in urban areas in the study region. the cluster analysis results were synthesized in arcgis 10.5, generating maps with clusters of areas with similar morphometric characteristics. this grouping uses ward’s minimum variance method and this technical use the measure of the absolute distances from manhattan (mingoti, 2005). in the arcgis software, the areas mapped by civil defense were superimposed on the areas grouped for each of the morphometric parameters calculated for the study area, which are: compactness coefficient (kc); drainage density (dd); stream frequency (sf); maintenance coefficient (mc); roughness index (ri); form factor (kf) (cristofolleti, 1974; 1981). the variables were then selected considering the greatest overlaps between the areas mapped by the civil defense with the morphometric groupings. to evaluation of the assumptions, the nonparametric mann–whitney test was used (ferreira, 2009). a maximum error of 5% was considered to reject the null hypothesis, which was declared as the nondifference between the groups (p < 0.05). the variables representing the morphometric and hydrological parameters were initially assessed in a complete model and adjusted to a more economical model using the backward method (mingote, 2005). fisher’s discriminant function was proposed as a predictive equation for scoring, and the classification constant was used as a cutoff point for classifying subbasins that are naturally vulnerable to flooding (mingoti, 2005). as a result of fisher’s discriminant function, equation 1 was found, which synthesizes the variables and their weights for the evaluation of areas with a natural propensity for flooding. data processing and analysis were performed with the support of the r statistical package (r core team, 2020). 62 lucas servidoni; joaquim bernardes; guilherme expedito; felipe gomes; velibor spalevic; branislav dudic; ronaldo luiz mincato efre = -3.28x10-1 kc + 2.64x10-4 dd – 3.62x10-1 dr – 5.76x10-3 cm + 8.58 x10-5 ir – 3.44 kf (1) where: efre = equation for flood risk evaluation; kc: compactness coefficient; dd: drainage density; dr: stream frequency; cm: maintenance coefficient; ir: roughness index; kf: form factor. to evaluate the proposed model, the data obtained by the discriminant function were crossed with empirical maps of civil defense (defesa civil, 2022). then, 500 points were randomly distributed on the map of areas that were vulnerable to flooding estimated by the proposed model and on the map of the observed risk areas, and the kappa agreement index was calculated to assess the level of accuracy of the proposed map (aburas et al., 2021). 3. results in this research, we use the concepts of vulnerability and susceptibility to indicate predisposed/prone terrains for the development of processes in the physical environment (floods). already, the concept of risk was used to indicate areas likely to be affected by natural processes that can cause adverse effects of losses of life and material by floods. the cluster analysis identified two groups of hydrographic sub-basins with similar functions of morphometric and hydrological characteristics: a group considered safe and a group characterized as naturally vulnerable to the occurrence of floods; both are described in table 2. as an indication of susceptibility to flooding in watersheds, dd indicates well-drained areas. such areas convert rain into flow very quickly, causing flooding in a short period. the average dd (table 2) of the subbasins included in the alto sapucaí risk grouping was 21.65 km2, confirming its vulnerability to floods given the recurrent cases of flooding in santa rita do sapucaí, itajubá, delfin moreira, piranguinho, and piranguçu by the main river, as pointed out by the official civil defense bulletin (defesa civil, 2022). damage caused by floods mainly occurs between december and march. the results demonstrate that in the santa rita do sapucaí region, the high values of dd associated with extreme hydrometeorological events trigger floods (servidoni et al., 2021). thus, it is necessary to prepare a contingency plan for the area that includes the hydrological, climatic, geomorphological, pedological and land use aspects of the hydrographic basin, which also considers the morphometric behavior of the relief and drainage network, as shown in table 2. the calculated roughness index (ir) indicates the relationship between the slope and the length of the channels. the greater the ir is, the more sloped and dissected is the relief, more notched the shape of the flow channels is and the greater the gravitational force on the rivers. in the area, the highest ir values partially overlap the mainstream in the upstream region associated with the knickpoints, a significant relief rupture upstream of the alto sapucaí hydrographic basin, resulting from lithological transitions with different resistance to erosion and regional shear zones linked to the evolution of mantiqueira range, developed during the brazilian-pan african neoproterozoic orogeny (hasui, 2010; rezende and castro, 2016; rezende, 2018; cprm, 2020; rezende and salgado, 2020; calegari, 2021) (figure 2). the ir values are consistent and agree with the results of the machado river hydrographic basin, which also belongs to the rio grande hydrographic basin (servidoni et al., 2021; 2019). the rupture illustrated in figure 2 is inserted into areas with a dd greater than 15.01 and a dr greater than 3.01, classifying the area as well drained. this relief rupture implies a potential increase in the flow of the main course, which may cause risks to riverside populations close to urban watercourses or downstream of the mapped knickpoints (gailleton et al., 2019). these characteristics of the downstream risk areas are illustrated by ribeiro et al. (2016) in the municipality of the pouso alegre, south of minas gerais. longitudinal stream profile distance (km) el ev at io n (m ) 0 20 40 60 80 100 120 140 160 1720 1620 1520 1420 1320 1220 1120 1020 920 820 figure 2. longitudinal profile of the main channel of the alto sapucaí hydrographic basin, southern minas gerais, brazil the areas in red in figure 3 indicate subbasins that are vulnerable to floods due to the morphometric characteristics of the relief and the drainage network. there is an overlap of risk areas with itajubá, piranguinho, santa rita do sapucaí, piranguçu, and delfin moreira. in 2020, from january to march, these municipalities declared a state of emergency due to heavy rains (defesa civil, 2020). figure 3 identifies areas prone to flooding. thus, it can be a territorial planning instrument for the preparation of contingency plans for cities. in addition, determining risk areas can target priority locations for investments and projects to contain floods. this allows for directing investments for proper disaster management in places prone to flooding in the modeled area represented in figure 3. the identification of priority areas would enable urban space to be restructured using sustainable strategies for land use and occupation, inspecting and monitoring risk areas and planning the consolidation of new urban and rural areas into safe areas. maps of flood-prone areas (figure 3), when linked to land use and occupation data, can generate strategic information to prepare and execute contingency plans for floods in the alto sapucaí hydrographic basin (table 3). table 3 presents the calculation of land use areas in the hydrographic basin. classes are grouped to synthesize regional land use behavior. the most representative uses in the basin are native vegetation, pasture, and agricultural areas. they occupy 36.57, 24.09, and 37.70% of the total area, respectively. however, when observing the relationship of each use with the risk area, their values are 13.18, 14.66 and 15.21%, respectively. thus, 14.69% of the basin is vulnerable to flooding, which, despite the low numerical value, is quite significant, as this portion of the basin includes areas inhabited by people and destined for agriculture and livestock; these areas have high risk levels for damage to economic activities and life. table 2. hydrographic basins, according to morphometric characteristics and parameter values. alto sapucaí hydrographic basin, southern minas gerais, brazil. morphometric characteristics area obs kc1 dd2 dr2 ir2 rr1 eps2 cm2 kf1 ic1 safe 109 1.93 3.53 2.00 2,117.76 2,265.16 0.14 283.06 0.36 0.32 risk 66 1.13 21.65 2.12 6,928.39 2,444.41 0.02 46.22 0.78 0.86 p-valor 0.0040 0.001 ** 0.1617 ** 0.0001 ** 0.3286 0.0001 ** 0.0001 ** 0.0001 0.4030 obs: number of observations; kc: compactness coefficient; dd: drainage density; dr: stream frequency; ir: roughness index; rr: relief ratio; eps: stream surface length; cm: maintenance coefficient; kf: form factor; ic: circularity index; 1average; 2median; ** mann–whitney test. 63methodological proposal to remote detection and management of areas that are naturally vulnerable to floods urban patches occupy 31.34 km2 of risk area; however, when observing the percentage of these areas in vulnerable places, the value is 38.80% (table 3). in other words, a significant amount of urban infrastructure historically occupies areas that are naturally prone to flooding due to the morphometric characteristics of the relief and the hydrographic network. this is due to the historical disordered occupation of the riverbanks and soil impermeabilization of the urban areas of the basin (hora and gomes, 2009). table 3. land use in risk areas of the alto sapucaí hydrographic basin, southern minas gerais, brazil. land use area (km2) area (%) risk area (km2) risk area (%) land use risk area (%) natural forest 1,029.05 36.57 135.63 32.80 13.18 agricultural 677.16 24.09 99.32 24.06 14.66 pasture 1,060.94 37.70 161.40 39.04 15.21 rocky outcrop 12.93 0.45 3.74 0.91 28.92 bare soil 0.66 0.03 0.12 0.04 18.18 urban infrastructure 31.43 1.11 12.19 2.95 38.78 water 1.46 0.05 0.85 0.21 58.22 total 2,813.67 100.00 413.28 100.00 14.69 adapted from mapbiomas project (2018). the subbasins classified as vulnerable to flooding have a kf value of 0.78, like that obtained by taofik et al. (2017). this value expresses the flooding risk in a tropical hydrographic basin with similar morphometric and climatic characteristics. the dd value (figure 4) for the vulnerable cluster is 21.65, demonstrating a significant accumulation of drainage channels in this area. as it is a well-drained environment, rainfall-runoff conversion occurs abruptly in high-intensity or prolonged rainfall events (taofik et al., 2017). the value of kc 0.89 was obtained by taofik et al. (2017) in a tropical hydrographic basin. the parity of the results obtained for the two areas demonstrates that both suffer from floods and have similar morphometric characteristics. such values compared to other references show that kc values between 0.75 and 1.00 characterize areas prone to flooding (strahler, 1957; christofoletti, 1969; sangman and balamurugan, 2017). the value obtained for kf was 0.56, indicating that the basin is subject to floods and flood peaks due to the circular shape of the downstream subbasins and the kc values. furthermore, such areas are concentrated near higher-order channels, such as the main channel. this can be evidenced by a comparison with the results of purohit and parmar (2017) in the alaknanda basin, india. thus, it is essential that public administration agencies plan and adopt the necessary measures to address and mitigate the vulnerability of these areas to extreme hydrometeorological processes, with consideration given to the environmental characteristics of each area (taofik et al., 2017). the application of cluster analysis allows classification of the morphometric parameters into two groups. therefore, it is possible to develop a discriminant function capable of distinguishing subbasins subject to flood risks from those that are safe (equation 1). efre = -3.28x10-1 kc + 2.64x10-4 dd – 3.62 .10-1 dr – 5.76x10-3 cm + 8.58 x10-5 ir – 3.44 kf (2) where: efre = equation for flood risk evaluation; kc: compactness coefficient; dd: drainage density; dr: stream frequency; cm: maintenance coefficient; ir: roughness index; kf: form factor. in alto sapucaí, the discriminant function defines an efre threshold value of 3.82 for the basin, below which areas are classified as naturally prone to flooding. thus, this cutoff value classifies the watershed as a risk or safe area due to the geomorphological and fluvial characteristics (figure 3). the methodological proposal can be applied in spatial clippings of hydrographic basins and in different federative units, with a view to identifying areas with natural vulnerability to flooding (brito et al., 2020). thus, it is possible to plan actions and strategies to minimize and overcome the catastrophic effects of floods. figure 3. map of areas at risk to floods in the alto sapucaí hydrographic basin, southern minas gerais, brazil 64 lucas servidoni; joaquim bernardes; guilherme expedito; felipe gomes; velibor spalevic; branislav dudic; ronaldo luiz mincato figure 4. map of the spatial distribution of the morphometric indices calculated for the alto sapucaí hydrographic basin, southern minas gerais, brazil. a: dd: drainage density; b: dr: stream frequency; c: eps: stream surface length; d: ir: roughness index; e: rr: relief ratio. 65methodological proposal to remote detection and management of areas that are naturally vulnerable to floods cluster and discriminant function analyses are quantitative tools capable of identifying urban and regional planning priorities in areas at risk of flooding. consequently, with such resources, governmental and nongovernmental management and actions can be more efficient. its correct use could allow that human occupation of risk areas and loss of life, material goods, and public infrastructure are avoided. another advantage of the method is that it automates the detection of areas naturally vulnerable to flooding from the discriminant function applied in gis. automation reduces operating costs, and based on prior mapping, field activities can be concentrated in vulnerable areas. currently, remote object detection has been widely developed in geotechnologies, which reduces technical and operational costs, accelerates the collection of strategic information, and supports decision-making. in brazil, a country of continental dimensions and, historically, a lack of planning and management for land use and occupation, low-cost and high-efficiency techniques to identify areas vulnerable to flooding are essential to save lives and resources (tsatsaris et al., 2021). the kappa (k) test measures the degree of agreement between proportions derived from dependent samples. more than one method can map a risk area and not necessarily obtain the same result. different methods should identify a risk area, if indeed it is a risk area. however, ideally, the different methods should achieve the same result (landis and koch, 1977). absolute agreement between two objects observed by different methods does not always occur in real life. however, it is possible to measure the reliability of two observations of the same object by k. this coefficient is based on the number of concordant responses between two products. values close to 1.0 represent total agreement, and those close to 0.0 indicate nonagreement (aburas et al., 2021). figure 5 illustrates the overlap of the risk map obtained by efre with those mapped by civil defense (defense civil, 2022). it should be noted that not all the municipalities surveyed have flood maps. thus, to obtain the kappa agreement index, all products available for the study area were considered. the agreement value was 0.83, which is considered solid or substantial agreement (landis and koch, 1977). however, such a kappa concordance value may be higher due to the absence of maps of flooded areas in some municipalities studied. figure 5. map of the distribution of points to calculate the accuracy of the kappa coefficient for the alto sapucaí hydrographic basin, southern minas gerais, brazil. the precision obtained by efre is quite satisfactory for the area. from now on, new works should seek different applications for efre, such as the establishment of quality control measures and model validation (figure 5). 4. discussion according to brito et al. (2020), it is vital to emphasize the importance of not waiting for the authorities to assume their responsibilities in risk management. the needs of communities affected by floods must take precedence over individual interests and real estate speculation. due to their vulnerability, these communities must participate in the disaster management process so that, in partnership with experts, they can efficiently allocate investments to fight against and mitigate the socio-economic and environmental impacts of floods (bogo, 2020). however, the management of natural disasters is, above all, the responsibility of the state, which is the only entity with the legal capacity to act and regulate the occupation of risk areas. therefore, authorities must coordinate efforts; only with the participation of all sectors of society will it be possible to achieve sustainable management and mitigation of impacts and damage caused by floods. unfortunately, government organizations should and need to standardize procedures, which would imply changes in risk management protocols. bureaucracy creates challenges for establishing a unified disaster management model in brazil (brito et al., 2020). according to data from the joão pinheiro foundation, 5.87 million people are part of the population affected by the brazilian housing deficit in 2019 (fjp, 2019). these families live in marginalized urban spaces, forming clusters of poverty, and many are in areas that are inappropriate for occupation, such as slopes and riverbeds. brazil experienced one of its most critical summers due to the influence of the la niña weather phenomenon. thus, climate change promotes catastrophic events of extreme intense rains in southern bahia and northern minas gerais, in addition to disasters in the cities of rio de janeiro and são paulo, according to the management report of the integrated information system of disasters (civil defense, 2022). these increasingly commonplace events demonstrate the relevance of this type of research. this study sought to identify the area’s most vulnerable to floods in a simple, objective, cheap, fast and efficient way. for a country of continental dimensions, such as brazil, with complex hydrography and different socioenvironmental contexts, the product derived from this model is essential for the development of public policies and risk management systems in brazilian areas, such as in the metropolitan area of curitiba, paraná state (nascimento neto, 2020), in the itajaí-açu river area in blumenau, santa catarina state (alberton et al., 2021); and in amazonian cities like belém, pará state (szlafsztein and araújo, 2021). climate change causes storm events to become more frequent and intense, so it is essential to predict the identifying patterns, trends, and distribution of natural phenomena to reduce natural risks. only in this way will it be possible to implement land use and occupation planning that values the quality of life and the sustainability of the environment. therefore, the efre enables the identification of geomorphological conditions that potentiate flood events in tropical watersheds and may be an option to manage this problem that has devastated and claimed many lives in brazil. 5. conclusion the discriminant function showed potential to identify areas susceptible to flooding. it is proposed that a value of 3.82 for the equation for flood risk evaluation indicates areas vulnerable to flooding and that higher values suggest a low natural risk of flooding. the value of the kappa index was 0.83, indicating good agreement between the products obtained by the equation flood risk evaluation and the civil defense risk maps, demonstrating good accuracy in forecasting areas at risk of flooding. the geomorphological characteristics and frequency of storms are the main natural factors that increase the potential for flooding in the region. due to climate change, storm events tend to become more intense and frequent. of the eleven municipal seats, five are in areas at risk of flooding; 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(2019). flood risk assessment and resilience strategies for flood risk management: a case study of surat city. international journal of disaster risk reduction, 40, 101155. https://doi. org/10.1016/j.ijdrr.2019.101155 ward, p. j., blauhut, v., bloemendaal, n., daniell, j. e., ruiter, m. c., duncan, m. j., emberson, r., jenkins, s. f., kirschbaum, d., kunz, m., mohr, s., muis, s., riddell, g. a., schäfer, a., stanley, t., veldkamp, t. i. e. & winsemius, h. c. (2020). review article: natural hazard risk assessments at the global scale. natural hazards and earth system sciences, 20, 1069-1096. https://doi.org/10.5194/nhess-20-1069-2020 https://doi.org/10.3390/ijgi10020094 https://doi.org/10.1016/j.pdisas.2021.100185 https://earthexplorer.usgs.gov/ https://doi.org/10.1016/j.ijdrr.2019.101155 https://doi.org/10.1016/j.ijdrr.2019.101155 https://doi.org/10.5194/nhess-20-1069-2020 _hlk119312676 _hlk119313514 _hlk99554055 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n1.106588 earth sciences research journal earth sci. res. j. vol. 28, no. 1 (march, 2024): 65 72 h y d r o g eo lo g y abstract contamination due to rapid urban development, industrialization, and agricultural sources is increasingly threatening the groundwater resources of the plioquaternary phreatic aquifer of the middle western cheliff. an intrinsic vulnerability assessment was carried out based on the drastic method on parameters involved in the vertical transfer of pollution from the soil surface. the vulnerability maps obtained showed a high extension of areas of medium vulnerability (54%). on the other hand, areas of low vulnerability occupied about 36%. the high and very high vulnerability was mainly related to low water depth values (8% and 2%, respectively). the results of a sensitivity analysis using the two tests—the single parameter sensitivity analysis and the map removal sensitivity analysis—show that every parameter is, without fail, required for the computation of the vulnerability index. the validation of the vulnerability map produced by the drastic method confirmed the evolution of this sensitivity which decreases towards the s.w. of the plain with nitrate concentrations between 30 and 120 mg/l. planners can use the produced risk maps as tools to make a preliminary choice of priority locations for various forms of environmental sustainability. sensitivity analysis and gis tools for groundwater vulnerability assessment. (application in the middle chellif plain, algeria) saci nadjai1, hichem khammar2, mahrez boulabeiz3, abdelkader nadir nabed1, lahcen benaabidate4 1. laboratory for fire safety engineering of buildings and protection of their urban and periurban environment, faculty of nature and life sciences, university hassiba benbouali of chlef, bp 151, chlef 02000, algeria 2. laboratory of functional ecology and environment department of biology, faculty of exact sciences and natural and life sciences, university larbi ben mhidi of oum el bouaghi b.p. 10 oum el bouaghi 04000, algeria 3. department of ecology and environment, abbas laghrour university, po box 1252, route de batna, 40004, khenchela, algeria 4. laboratory of functional ecology and environment engineering, faculty of science and technology, university of sidi mohamed ben abdellah. p.o.box: 2202, fez 30000, morocco record manuscript received: 30/12/2022 accepted for publication: 27/03/2024 how to cite this article: nadjai, s., khammar, h., boulabeiz, m., nabed, a. n., benaabidate, l. (2024). sensitivity analysis and gis tools for groundwater vulnerability assessment. (application in the middle chellif plain, algeria). earth sciences research journal, 28(1), 65-72. https://doi. org/10.15446/esrj.v28n1.106588 keywords: vulnerability; drastic method; sensitivity analysis; pollution middle cheliff, algeria. análisis de sensibilidad y herramientas sig para la evaluación de la vulnerabilidad de las aguas subterráneas. (aplicación en la llanura de chellif medio, argelia) resumen la contaminación debida al rápido desarrollo urbano, la industrialización y las fuentes agrícolas amenaza cada vez más los recursos hídricos subterráneos del acuífero freático pliocuaternario del cheliff medio occidental. en este estudio se llevó a cabo una evaluación de la vulnerabilidad intrínseca basada en el método drastic sobre los parámetros que intervienen en la transferencia vertical de la contaminación desde la superficie del suelo. los mapas de vulnerabilidad obtenidos mostraron una elevada extensión de zonas de vulnerabilidad media (54%). por otro lado, las zonas de vulnerabilidad baja ocupaban alrededor del 36%. la vulnerabilidad alta y muy alta estaba relacionada principalmente con valores bajos de profundidad del agua (8% y 2%, respectivamente). los resultados de un análisis de sensibilidad utilizando las dos pruebas -el análisis de sensibilidad de un solo parámetro y el análisis de sensibilidad de eliminación de mapasmuestran que todos los parámetros son, sin excepción, necesarios para el cálculo del índice de vulnerabilidad. la validación del mapa de vulnerabilidad producido por el método drastic confirmó la evolución de esta sensibilidad que disminuye hacia el s.o. de la llanura con concentraciones de nitrato entre 30 y 120 mg/l. los planificadores pueden utilizar los mapas de riesgo producidos como herramientas para hacer una elección preliminar de los lugares prioritarios para las distintas formas de sostenibilidad medioambiental. palabras clave: vulnerabilidad; método drastic; análisis de sensibilidad; contaminación medio cheliff, argelia. https://doi.org/10.15446/esrj.v28n1.106588 https://doi.org/10.15446/esrj.v28n1.106588 https://doi.org/10.15446/esrj.v28n1.106588 66 saci nadjai, hichem khammar, mahrez boulabeiz, abdelkader nadir nabed, lahcen benaabidate 1. introduction most agricultural regions of the world are plagued by water pollution. in arid areas, groundwater is the major water supply for domestic, agricultural, and industrial needs. the aquifer of the middle chéliff is the only water resource that can be mobilized in the study area. it offers an interesting aquifer potential, and its extension is very important. the preservation of this resource is essential with a participatory and integrated management approach. however, the depletion of aquifer reserves caused by the imbalance between the recharge and extraction of groundwater aggravates the problems related to the pollution of this resource (haider et al, 2021). prevention is the most appropriate strategy for groundwater protection (uzcategui-salazar & lillo, 2022). the most widely used methods for assessing vulnerability to aquifer contamination are based on overlay index maps, such as drastic, god and avi. these methods assign weighting and scoring values to hydrogeological features, which introduces some subjectivity in the assessment (kadkhodaie et al, 2019). due to population growth and industrialization, large quantities of domestic and industrial effluents are discharged into the nearby river, resulting in groundwater pollution in shallow aquifers (rahman, 2008).groundwater vulnerability to contamination, which is a severe problem worldwide (shirazi et al, 2012) is defined as the tendency or likelihood of contaminants reaching a specified position in the groundwater system after introduction at a location above the uppermost aquifer (nrc, 1993). groundwater vulnerability mapping can indicate areas more vulnerable to contamination at the planning stage of socio-economic activities (fritch et al., 2000). therefore, aquifer vulnerability maps are essential for groundwater management and protection (zwahlen, 2004). furthermore, vulnerability maps illustrate different degrees of aquifer sensitivity (guillaume & marie, 2015) and provide valuable and necessary information to guide policy choices for the prevention and management of pollution risks to the region’s groundwater resources with a view to sustainable management (ake et al., 2010). in this work, we consider the problem of vulnerability and pollution risk mapping of groundwater using gis (geographic information system). our choice was to apply one of the methods of the index mapping category with the weighting of criteria, namely the standard drastic method (aller et al., 1987) based on parameters that intervene in the vertical transfer of pollution from the soil surface. to assess the effect of each of these parameters on the vulnerability map, a sensitivity analysis was carried out by applying the two tests: “the map removal sensitivity analysis” (napolitano & fabbri, 1996) and “the single parameter sensitivity analysis” (napolitano & fabbri, 1996). 2. material and methods study area this study is carried out in the middle cheliff, suffering from water scarcity. this area required a large quantity of water not only for farming, the main activity, but also for water supply for a population with an increasing demographic rate. the middle cheliff basin is located in the center of the cheliff wilaya (northwestern algeria), 200 km west of algiers and about 45 km from the mediterranean (fig. 1). it extended over an area of 321 km2 and had a population of about 480,000 as of 2010 (ons, 2011). the region experiences a semi-arid environment with an average of 520 mm of annual rainfall. a major alluvial aquifer in the middle cheliff is used primarily for drinking water, agriculture, and industrial purposes. a recent economic boom, especially in agriculture, has increased the demand for water and degraded the water quality (abh-cz, 2004). the pre-neogene formations and the neogene-quaternary formation are the two lithologic sequences that lie beneath the study area, according to geological research (perrodon, 1957; mattauer, 1958) (fig. 2). the preneogene strata (lower cretaceous and oligocene) are made up of a substantial accumulation of sedimentary rocks that are incongruously deposited on top of older levels. three aquifers with various hydrogeological potentials are present in the basin (fig. 3). the upper miocene limestone, which outcrops along the southern boundary of the valley and lies beneath the alluvium; the pliocene sandstone, which is practically covered by the quaternary formations and the pleistocene– quaternary alluvial sediments, which form the barrier of the valley. these sediments include clays and marl with beds of sand, gravel, and conglomerates this last aquifer, the subject of our study, has an average annual water withdrawal of about 15.5hm3 (chakour & dahmane, 2012), of which 64% is for drinking water supply, 31% for irrigation, and 5% for industrial purposes (fig. 4). in order to have complete coverage of the middle chellif plain, a 1/500,000 geological map of northern algeria was used. figure 1. geographical location map of the study area. figure 2. geological map of the middle cheliff basin (scetagri, 1985). figure 3. geological cross section showing the different aquifers (scetagri, 1985). 67sensitivity analysis and gis tools for groundwater vulnerability assessment. (application in the middle chellif plain, algeria) figure 4. sampling sites in the alluvial aquifer (chakour & dahmane, 2012) methodological approach the type, volume, and dependability of the available data play a major role in determining how vulnerable groundwater is to contamination. (melloul et al., 2009). this study studied water samples from 14 deep and 61 shallow wells. anrh and abh-cz conducted chemical analyses, respectively, of the national water resources agency and the cheliff zahrez river basin agency. the assessment of intrinsic vulnerability levels was carried out using the drastic method, considering the availability of data on the required parameters. drastic method this technique distinguishes between the vertical susceptibility of a pollution layer at the surface of the earth and its horizontal exposure to pollution that has already reached the layer (sinan et al., 2003). drastic is the most popular point-counting and matrix evaluation technique (chen & fu, 2003) because to its straightforward and basic properties. in addition, the drastic model has been used to estimate the vulnerability of groundwater in other regions of the globe (zhou et al., 1999). the drastic approach is a very basic, affordable, multi-criteria method whose parameters are readily accessible and straightforward to analyze. it examines the transfer circumstances under which common pollutants may reach the slick’s surface. developed for the united states environmental protection agency (andrzej & kowalczyk, 2007), drastic is an acronym comprised of the initial letters of the characteristics used to create the map: groundwater depth (d), net recharge (r), and aquifer media (a), soil media (s), topography (t), vadose impact (i), and aquifer hydraulic conductivity (c) (c). these are weighted, ranked, and then integrated using a groundwater vulnerability algorithm (neukum et al., 2008)to get a final rating score. according to the significance of its vulnerability, this approach divides each parameter into intervals with a numerical rating. according to the following equation (1), a drastic index (id) or vulnerability rating may be derived from these parameters: idrastic = dwdr + rwrr + awar + swsr +twtr + iwir +cwcr (1) with: w and r, respectively, the weight and value of the interval (rating) assigned to each parameter. the final drastic index ranges from 23 to 226 and is used to quantify the degree of susceptibility of a certain slick region. because the estimated index (idrastic) is high, openness is crucial. the bigger the value of the drastic index, the more susceptible that portion of the aquifer is to contamination. table 1 displays the many data sources used to evaluate the criteria that describe the vulnerability of the alluvial slick of the western middle cheliff plain. sensitivity analysis and analysis of the drastic method utilizing seven criteria, the drastic technique generates a vulnerability map. some experts claim that sensitivity to groundwater may be determined without utilizing all of these criteria (barber et al., 1993 ; merchant, 1994). others [10] have claimed that the model’s weights and scores are subjective and that, in the lack of experimental data, there is no reason to reject the accuracy of the vulnerability index so produced. to eliminate such uncertainties, a sensitivity analysis of the model was conducted. first, the dependency and variability of the model’s parameters will be investigated (babiker et al., 2005; rosen, 1994). then, two sensitivity tests were conducted: “the map removal sensitivity analysis,” which was established by lodwick et al.(1990), and “the single parameter sensitivity analysis,” which was provided by napolitano & fabbri (1996). table 1. weight and mode of acquisition of the seven parameters of the drastic method. pa ra m et er s w ei gh t d at a so ur ce s d 5 • piezometric directory of the middle cheliff plain (drechlef) r 4 • anrh precipitation and onm temperature • agri water resources-set (onid) a 3 •drilling cuts in the middle west cheliff area (dre-chlef) •geological map of the middle west cheliff area (set agri, 1985) s 2 •map of the soils of algeria plain of cheliff 1:50000 (boulaine, j 1956) •map of the plains of the middle west cheliff (abh-chlef) t 1 •topographic map of the zahrez watershed 1:200,000 (dhwchlef) •topographic map of the wilaya of chlef 1:200000 i 5 •drilling cuts in the middle west cheliff area (dhw-chlef) c 3 •hydrogeological cup (abh-cz) •drilling report 77.shyg.18 (d. pradines, 1977) •study no.33 ge (schrambach, 1966) the first test identifies the sensitivity of the vulnerability map by removing one or more layers from the map. the following equation calculates it: s = ( |v/n v’/n|/v ) * 100 (2) with: s is the measured sensitivity expressed in terms of the index of variation. v is the undisturbed drastic vulnerability index v’ is the disturbing vulnerability index n and n are the numbers of layers used in calculating the indices. the “single parameter sensitivity analysis” test was designed in order to evaluate the effect of drastic parameters on the vulnerability index. it compares the actual weights given to input parameters to the theoretical weights. the effective weights are computed using the following equation: w = * 100 prpw v( ( (3) with: w effective weight of a parameter. pr and pw are the weights and the value of the interval (rating) assigned to this parameter. v is the drastic vulnerability index. 68 saci nadjai, hichem khammar, mahrez boulabeiz, abdelkader nadir nabed, lahcen benaabidate creation of thematic maps the drastic method requires seven parameters. the reliability of these parameters depends on the data used for their realization. several parameters, such as the depth of the water table map, established by kriging using golden software’s surfer which require a data such as (x,y,depth). other parameters uses a numerical rating system based on the parameterization of the various factors influencing the hydrogeological system of vulnerability. the difficulty of applying drastic method is the limits of the classes and the ratings that are assigned to the different parameters. indeed, the boundaries of standard classes not often reflect the reality of the study area because these classes may group together different entities. 3. results and discussion vulnerability map by the drastic method for the parameter depth of water “d”, the depth of the water table is a crucial parameter in the characterization of vulnerability by the drastic method. it is therefore assigned a maximum weight of 5. the depth of the water body in the middle western cheliff aquifer varies from 6 meters in the central zone of the aquifer and in the vicinity of oued cheliff to more than 50 meters at the edges of the aquifer (figure 5a). these values have been classified according to the ranges established in the drastic rating system tables. for the parameter net recharge ‘r,’ evaluating the adequate recharge of the water table is often complicated to achieve without prior hydrogeological and hydrological studies. the amount of water infiltrated over the whole study area is between 5 and 10 cm/year (5 cm/year on the edges of the aquifer and 10 cm/year in the significant bed of the wadi) (scetagri, 1985). this figure corresponds to a low partial vulnerability index, i.e., i.r. = 12 (figure 5b). for the parameter aquifer environment “a”: from the hydrogeological sections obtained by scet-agri (1985) and the lithological description of the drill sections, the constituent materials of the saturated zone of the aquifer may be determined (figure 5c). for the parameter soil type “s,” the pedological study by boulaine (1959) is the main study carried out in northern algeria. therefore, this study covers the whole of our region, which makes it easy to deduce the soil types included in our area (figure 5d). for the parameter topography “t,” the plain of the western middle cheliff does not represent significant differences in level; nevertheless, a class of slopes was deduced, which means the quasi-totality of the present plain, these fragile slopes included between 0 and 2%, this class is assigned a partial index is equal to 30( figure 5e) for the parameter impact of the unsaturated zone “i,” as for the “i” parameter, the lithology of the unsaturated zone described by examining the hydrogeological sections carried out, and the boreholes make it possible to distinguish three zones characterized by a different partial vulnerability index (figure 5f). for the hydraulic conductivity “c” this parameter depends on the nature and texture of the materials making up the aquifer. a great deal of work has been done to determine the hydrodynamic characteristics of the limestone aquifer, and the alluvial aquifer through pumping tests carried out in the region. in particular, the work of pradines in 1977 (drilling report no. 77. shyg.18) and that of schrambach in 1966 (study no. 33 g.e.). the results obtained determined permeability coefficients of 6.6 .10-5 to 1.8 .10-5 m/s for the limestone water table and 0.6 .10-4 to 0.05 .10-2 m/s for the quaternary alluvium water table (el meddahi, 2009). the average values thus determined for the region under study allow the drastic rating tables to highlight the different zones, each characterized by a partial ic index (figure 5g). the research area has been subdivided into a mesh of cells, each 727 m x 727 m, in accordance with the distribution of various factors in order to provide more statistical data on the frequency of occurrence of each vulnerability class. the final grid consists of 608 cells covering a total of 321.35 km2. the drastic technique reveals four categories of vulnerability (fig. 6 and 7). a b c d 69sensitivity analysis and gis tools for groundwater vulnerability assessment. (application in the middle chellif plain, algeria) e f g figure 5. maps of the seven parameters of the drastic method: a: “d” depth of the water; b: “r” net recharge.; c: “a” aquifer environment; d: “t” map topography; e: “i” parameter impact of the unsaturated zone; “c” hydraulic conductivity; f: the aquifer vulnerability to pollution by the drastic method. the “low” class reflects a low vulnerability to pollution and represents only 36% of the mapped area. the typical vulnerability index observed results from the relatively high depths and the non-permeable silty clay soil type in the center of the plain. figure 6. the aquifer vulnerability to pollution by the drastic method. table 2. distribution of the degree of vulnerability according to the drastic method (abh-cz, vol 1, 2000) vulnerability class minimum maximum area % number of cells symbol low 84 114 36 220 medium 114 145 54 331 high 145 175 08 46 very high 175 226 02 11 the ‘medium’ class occupies the largest percentage (54%) of the plain, extending from the southwest to the northeast. the medium degree of susceptibility is due to the combination of the lithological composition of the unsaturated layer, which consists of a mixture of silt clay and sand gravel, and shorter depths, resulting in less severe pollution. the “high and very high” class is represented by a rate of (8% and 2%, respectively), with the bulk of these two classes being concentrated on the borders of the plain at the interface of the soft limestone nappe (miocene with lithothamnium), along the boukadir and oued sly area. these two classes are located along the plain’s margins, at the interface of the soft limestone nappe (miocene with lithothamnium), on the boukadir and oued sly side (figure 7). figure 7. map of risk to pollution of the aquifer by drastic method. the very high vulnerability index is due to the combination of the following parameters: 70 saci nadjai, hichem khammar, mahrez boulabeiz, abdelkader nadir nabed, lahcen benaabidate • the overlap of the piezometric surface with the ground surface (depth less than 9m); and the low slope (less than 1%) favors any pollutant’s infiltration. • the lithology of the vadose layer consists of soft limestone, gravel, and coarse sand. sensitivity of the drastic model table 3 shows the summary statistics of the seven parameters used to calculate the drastic index in the middle western cheliff plain. the analysis of the averages shows that the most significant risk of groundwater contamination in the middle western cheliff plain is favoured by the parameters “aquifer, hydraulic conductivity and depth of the water table” (whose averages are 25.6; 23.2 and 20.5). however, the parameters “recharge and soil” participate with a moderate risk (average is 12), and the parameters topography, impact of the unsaturated zone favour a low risk (average: 10). the coefficient of variation shows that the major contribution to the variations of the vulnerability index is due to the parameter “impact of the unsaturated zone” (cv: 133.21%). the parameters “depth, soil, and hydraulic conductivity” show a medium contribution (cv: 66.90%; 38.25% and 30.63%). however, the topography, recharge, and aquifer represent a low to zero contribution to the variation of the vulnerability index (cv: 11.65; 0 and 0). table 3. summary statistics of drastic parameters d r a s t i c min 5 12 18 6 10 5 3 max 50 12 30 20 10 50 30 mean 20.5 12 25.6 12 10 10 23.2 sd 13.76 0.00 2.98 4.56 0.00 13.32 7.13 cv 66.90% 0.00% 11.65% 38.25% 0.00% 133.21% 30.63% sd: standard deviation; cv: coefficient of variation the” map removal sensitivity analysis” test the results of the “map removal sensitivity analysis” test, based on removing one or more parameters at a time, are shown in tables 4 and 5. table 4 shows the variation in the vulnerability index as a result of removing a single drastic parameter. it is clear that the removal of either parameter results in a considerable variation in the vulnerability index. the parameters “depth, aquifer medium, topography, unsaturated zone, and hydraulic conductivity” are the most responsible index variation (1.20; 1.05; 1.02; 1.55, and 1.07). vulnerability seems very sensitive to removing the parameters “unsaturated zone” and “depth.” table 5 presents the variations in the vulnerability index due to the removal of one or more parameters. the removal of layers was based on the map removal sensitivity analysis shown in the table. the layer with a minor influence on the variation of the vulnerability index is preferentially removed. the removal of any parameter shows no trend in the variation index, which means that all parameters are, without exception, necessary for calculating the vulnerability index, as reported in some previous studies (babiker et al., 2005; napolitano and fabbri, 1996). table 4. the statistics of the “map removal sensitivity analysis” test removed parameters variation index (%) mean min max d 1.20 0.17 3.98 r 0.75 0.6 1.39 a 1.05 0.6 2.43 s 0.86 0.6 1.74 t 1.02 0.44 1.56 i 1.55 0.37 3.43 c 1.07 0.6 2.27 table 5. the statistics of the “map removal sensitivity analysis” test used parameters variation index (%) mean min max i, d, c, a, t, and s 0.75 0.06 1.39 i, d, c, a, and t 1.93 0.71 3.41 i, d, c and a 3.95 1.99 6.23 i, d, and c 3.34 0.44 7.94 i and d 3.72 0.06 14.29 i 9.29 2.2 20.6 the “single-parameter sensitivity analysis” test after the importance of the seven parameters in calculating the vulnerability index was highlighted by the “map removal sensitivity analysis” test, the objective of applying the “single-parameter sensitivity analysis” test in this section is to complete and confirm the first results. this test consists of a comparison between the theoretical and practical weights of the parameters. table 6 shows the results of the test and shows that the parameters “aquifer, soil, topography and hydraulic conductivity” are the parameters that have the most significant influence on vulnerability in the study area, as their average effective weights (21%, 09%, 08%, and 18% respectively) are higher than their theoretical weights, which is all the more reason to look for more detailed information on these four factors. validation of pollution vulnerability maps from the maps obtained by the drastic method, it can be seen that the central-east and the edges of the plain are the most vulnerable areas to pollution. this sensitivity decreases towards the s.w. validation of these maps is necessary, as any vulnerability map elaborated must be tested and validated by measurements and chemical analysis of groundwater (ake et al., 2009). for this purpose, the distribution of nitrates in the groundwater from the may 2012 campaign was used (figure 8). table 6. statistics of the “single-parameter sensitivity analysis” test. parameters theoretical weight theoretical weight effective weight % % mean min max d 5 21.7% 16 03 38 r 4 17.4% 10 06 14 a 3 13.0% 21 15 29 s 2 8.7% 09 04 15 t 1 4.3% 08 05 12 i 5 21.7% 18 04 35 c 3 13.0% 18 02 28 the nitrate concentrations in the groundwater range from 30 to 120 mg/l. from this distribution, we can deduce that nitrate pollution decreases from the n.e. to the s.w. of the groundwater. the vulnerability map produced by the drastic method confirmed this evolution. indeed, the zones with average nitrate levels (30 to 70 mg/l) are superimposed on the zones with moderate and low vulnerability indices; the zones with maximum nitrate concentrations (over 100 mg/l) coincide with the zones of high and very high vulnerability. 71sensitivity analysis and gis tools for groundwater vulnerability assessment. (application in the middle chellif plain, algeria) figure 8. map of the spatial distribution of nitrate concentrations (may 2012). conclusion drought and overexploitation of groundwater can lead to excessive pumping, which can cause the water table to deepen, which in turn can increase the vulnerability of aquifers to pollution, mainly through irrigation backflow and seepage of wastewater. this is particularly the case for the middle cheliff aquifer in northwest algeria, which has a semi-arid climate. the mapping of the intrinsic vulnerability of this alluvial aquifer using the drastic method has enabled four main classes of exposure to be defined (low, medium, high, and very high). the medium class is dominant (54%) in the study area, followed by the low class located in the center of the plain (36%) of the total area. the strong and powerful types (8% and 2%, respectively) of the total area are presented at the miocene lithothamnium water table interface with high conductivity. the drastic map seems to better reflect the vulnerability to pollution in the study area. the vulnerability maps should be useful for water resources management in the middle cheliff basin. references abh-cz. 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(2004). vulnerability and risk mapping for the protection of carbonate (karst) aquifers, cost action 620, final report. european commission directorate-general xii, science, research and development, report eur 20912, cost action 620, luxembourg, belgium, 297 pp. https://doi.org/10.1111/j.1745-6584.1994.tb00642.x https://doi.org/10.1111/j.1745-6584.1994.tb00642.x https://doi.org/10.1080/13669877.2012.686053 https://doi.org/10.1007/s12665-022-10591-7 https://doi.org/10.1080/02626669909492256 _heading=h.30j0zll _heading=h.3znysh7 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.111726 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 203 211 h y d r o lo g y abstract recent advancements in satellite remote sensing have led to increased spatial and temporal resolution of actual evapotranspiration (aet) estimates across scales. yet, the accuracy of aet products remains unknown for many regions, prompting further investigation to guide selection. this study intercompares five aet products within ethiopia’s bilate watershed, focusing on the 2009-2018 period. the products assessed include terraclimate, food and agriculture organization water productivity (fao wapor), moderate resolution imaging spectroradiometer operational simplified surface energy balance (modisssebop), and synthesis of global aet. reference evapotranspiration estimated using ground station climate data served as a basis for comparing the satellite products (sp). the intercomparison was conducted using descriptive statistics, scatter plots and pearson’s correlation coefficient to assess correlation, standard deviation, and root mean square error. additional error statistics were also considered. findings reveal higher aet values in the highlands compared to the lowlands of the bilate watershed. a weak correlation (<0.35) exists between eto and satellite-derived aet, potentially due to the averaging of aet values across diverse land cover classes, contrasting with point-scale reference measurements. the variance among aet products was varied across seasons and elevation ranges. while the annual patterns of aet were consistent across the products, large discrepancies in magnitude (average aet varies from 25 to 83 mm per month in the lower part) were detected. the modisssebop global and continental products showed minimal mismatches, whereas the synthesis of global and fao wapor products displayed slight differences. notably, the fao wapor’s aet estimates showed relatively closer agreement with many products in terms of magnitude and variability of aet. in conclusion, the study highlights significant random and systematic differences between the aet products. the substantial mismatch between the products underscores the necessity for continued research to refine aet product accuracy through improved input dataset and revisiting the algorithms. intercomparison of multisource actual evapotranspiration satellite products in bilate watershed, ethiopia alemeshet kebede yimer1*, samuel dagalo hatiye2, alemseged tamiru haile3 1 arba minch water technology institute, faculty of water resource and irrigation engineering, arba minch university, ethiopia. email: alemeshet.kebede@amu.edu.et; http://orcid.org/0000-0002-7631-0991 2 arba minch water technology institute, water resources research center; faculty of water resource and irrigation engineering, arba minch university, ethiopia. email: samuel.dagalo@amu.edu.et;https://orcid.org/0000-0003-4993-2338 3 international water management, institute, iwmi, ethiopia. email:a.t.haile@cgiar.org; * corresponding author: alemeshet.kebede@amu.edu.et record manuscript received: 20/10/2023 accepted for publication: 01/07/2024 how to cite this item: yimer, a. k., hatiye, s. d., & haile, a. t. (2024). intercomparison of multisource actual evapotranspiration satellite products in bilate watershed, ethiopia. earth sciences research journal, 28(2), 203-211. https://doi. org/10.15446/esrj.v28n2.111726 keywords: actual evapotranspiration; bilate; intercomparison; multisource; remote sensing evapotranspiration products intercomparación de productos satelitales multifuente que miden la actual evapotranspiración en la cuenca de bilate, etiopía resumen los recientes avances en los satélites de detección remota han permitido un incremento en la estimación de la resolución espacial y temporal de la actual evapotranspiración (aet, from english actual evapotranspiration) en diversas escalas. sin embargo, la exactitud de los productos que miden el fenómeno de la evapotranspiración permanece desconocida en muchas regiones, por lo que se demanda más investigación para guiar la selección. este estudio compara cinco productos para medir la evapotranspiración en la cuenca etíope de bilate con enfoque en el período 2009-2018. los productos evaluados son terraclimate, water productivity de la organización de las naciones unidas para la alimentación y la agricultura (fao wapor), moderate resolution imaging spectroradiometer operational simplified surface energy balance (modisssebop) y synthesis of global aet. la evapotranspiración de referencia estimada utilizando datos climáticos de estaciones terrestres sirvió como base para comparar estos productos satelitales. la intercomparación se realizó con estadísticas descriptivas, gráficos de dispersión y el coeficiente de correlación de pearson para evaluar la correlación, la desviación estándar y la raíz del error cuadrático medio. también se consideraron los errores estadísticos adicionales. los resultados revelan valores altos de actual evapotranspiración en las tierras altas en comparación con las tierras bajas de la cuenca bilate. un pequeño índice de correlación (<0.35) existe entre la evapotranspiración y la medida satelital de la actual evapotranspiración, potencialmente debido a la promediación de los valores de actual evapotranspiración a lo largo de diversas clases de cobertura del suelo que contrastan con mediciones de referencia a escala de puntos. la desviación entre los productos de actual evapotranspiración fue variada entre las estaciones y entre los rangos de elevación. mientras que los patrones anuales de la aet son consistentes entre los productos, se detectaron grandes diferencias de magnitud (el promedio de la aet varía de 25 a 83 mm por mes en las partes más bajas). el modisssebop global y los productos continentales mostraron diferencias mínimas, mientras que los productos synthesis of global aet y fao wapor presentaron diferencias ligeras. notablemente las estimaciones del fao wapor aet muestran un mayor acuerdo con muchos productos en términos de magnitud y variabilidad de la actual evapotranspiración. en conclusión, el estudio resalta las diferencias sistemáticas y aleatorias entre los productos que miden la aet. el desacuerdo sustancial entre los productos enfatiza la necesidad de continuar la investigación para refinar la exactitud de los productos que miden la aet a través de bases de datos mejoradas y revisitando los algoritmos. palabras clave: actual evapotranspiración; cuenca de bilate; intercomparación; multifuentes; productos de detección remota de evapotranspiración. https://doi.org/10.15446/esrj.v28n2.111726 mailto:alemekebe@yahoo.com; alemeshet.kebede@amu.edu.et http://orcid.org/0000-0002-7631-0991 mailto:samuel.dagalo@amu.edu.et https://orcid.org/0000-0003-4993-2338 mailto:a.t.haile@cgiar.org; alemsegedtamiru@yahoo.com mailto:alemekebe@yahoo.com; alemeshet.kebede@amu.edu.et https://doi.org/10.15446/esrj.v28n2.111726 https://doi.org/10.15446/esrj.v28n2.111726 204 alemeshet kebede yimer, samuel dagalo hatiye, alemseged tamiru haile 1. introduction over time and space, rainfall and evapotranspiration data played a crucial role in elucidating essential components of the terrestrial water balance (wb) (carter et al., 2018; nannawo et al., 2021; sulamo et al., 2021). these data are particularly relevant for informing water resource management in arid and semi-arid basins, where water scarcity poses significant challenges to longterm development. therefore, obtaining accurate measurements of rainfall and evapotranspiration is essential for a deeper understanding of hydrological processes and the interactions between hydrology, climate, and vegetation systems (pan et al., 2022; nannawo et al., 2022). over the past two decades, numerous intercomparison studies were focused on satellite rainfall products (alijanian et al., 2017). these studies have resulted in a common agreement on the necessity to either reduce bias in these products (mengistu et al., 2019) or combine them (kubota et al., 2009) prior to integrating them into hydrological research. however, there is still a deficiency in comprehensive studies that compare satellite-derived products of actual evapotranspiration (aet). such studies could offer valuable insights for enhancing the accuracy of these products or enhancing their suitability for hydrological applications. recently, various methods have emerged to estimate global actual evapotranspiration (aet), such as interpolation, reanalysis, and model simulations (zhang et al., 2010). satellite remote sensing offers a relatively more direct estimates of aet with tools like wapor and modisssebop. however, significant differences exist in the performance of these products across regions (li et al., 2022) because of differences in estimation methods and input data sources. aet estimates from remote sensing methods differ in spatial and temporal resolution, analysis period, data needs, and precision (zhang et al., 2010). actual evapotranspiration (aet) is impacted by a combination of climate conditions, land characteristics, and human interventions, all of which interact in complex ways, making large-scale estimation challenging (bai et al., 2018). to utilize aet data effectively, it’s crucial to meticulously assess uncertainties and choose suitable methods for practical applications. satellite remote sensing tracks earth’s surface variables affecting aet but cannot measure aet directly (li et al., 2012). models convert these variables into aet estimates, based on surface energy balance (seb) (zhang et al., 2016). seb methods calculate latent heat by the surface energy budget’s difference, requiring minimal meteorological data but suitable only under clear skies (zhang et al., 2016b). an example of a satellite-based product that uses the penman-monteith (pm) approach for estimating the actual evapotranspiration (aet) is the mod16a2 remote sensing product (shekar & hemalatha, 2021). pm-based methods consider evaporation’s radiative and aerodynamic constraints, working across various conditions. however, they need detailed parameters for factors like stomatal conductance and soil moisture, which vary greatly (bai et al., 2017). advancements in remote-sensing technology have provided abundant data for practical and cost-effective estimation of actual evapotranspiration (aet) across various scales (han et al., 2021). however, despite these advancements, notable disparities persist among different aet products. for instance, a study revealed a wide range of mean evapotranspiration values, spanning from 423 to 563 mm/year across 40 different products (mueller et al., 2013). some products display similar trends, while others differ (kim et al., 2021), indicating persistent uncertainties (fisher et al., 2017). in china, gleam outperformed modis and era-interim in accuracy, with the lowest rmse of 12 mm/month (jia et al., 2022). across north china, gldas reported the highest evapotranspiration, followed by gleam and ntsg, with notable regional and annual variations (bai & liu, 2018). aet product accuracy suffers due to factors like insufficient spatial resolution for small catchments and variable input data quality (cao et al., 2021; ding & zhu, 2022). comparing aet products across diverse catchments is crucial for improving algorithms and guiding users. satellite aet’s accuracy is validated against lysimeter mass changes, showing minor errors (e.g., 0.26 to 0.68 mm/day) (silva et al., 2018; fawzy et al., 2021). yet, catchmentscale validation is challenging. instead, standard potential evapotranspiration (pet) methods like fao-penman–monteith are used for reference (wagle et al., 2017). aet is typically linked to pet, but this relationship varies with environmental conditions (senay et al., 2020; hossein & arast, 2020). this research primarily focuses on comparing five aet products for the bilate watershed in southern ethiopia, using a range of performance metrics. we examined the monthly aet data from these products spanning from 2009 to 2018. the products in question are fao wapor, terraclimate, modisssebop continental, modisssebop global, and synthesis of global actual evapotranspiration. additionally, we assessed how elevation differences impact aet readings. the findings could improve our comprehension of aet product performance in areas with significant elevation changes and irrigated fields, such as the bilate watershed. 2. study area and data sets 2.1. description of the study area the bilate watershed is situated between 37°47’14” and 38°20’14” e longitude and 6°33’18” and 8°6’57” n latitude. bilate is one of the major watersheds of the rift valley lakes basin’s abaya-chamo subbasin. the rivers guracha, guder, and weira are tributaries of the bilate. after passing through the swampy boyo lake, these rivers merge to form the bilate river, which flows southward to lake abaya (yimer et al., 2020). the bilate watershed spans an area of about 5,330 km2 at the measurement station near bilate tena, having a shape that extends lengthwise from north to south (wagesho & goel, 2013). the watershed length from north to south and west to east is 169,753.5 m and 61,865.2 m, respectively. the elevation within the watershed varies from 1,170 meters above sea level in the southern region to 3,285 meters above sea level in the northern area. fig. 1 shows the location and elevation of the bilate watershed. the bilate watershed is characterized by a climate that is both humid and semi-arid. the area experiences a rainfall pattern that is bimodal, with the summer monsoon season bringing significant precipitation. from 1995 to 2016, data from 13 meteorological stations in the study area were analyzed (fig. 1). bilate receives an annual rainfall amount of 1080-1500 mm in the upper part and 615-1237.35mm in the lower part. the average daily maximum and minimum temperatures are 28°c and 13.1°c, respectively. however, the long-term mean annual temperature varies greatly, ranging from 16.55°c in the highlands to around 22.45°c in the lowlands. figure 1. location, elevation and weather station distribution of the bilate watershed 2.2. data sets in this research, we utilized the climate hazards group infrared precipitation (chrip) satellite rf product due to its proven effectiveness in east africa, specifically in ethiopia, kenya, and tanzania (gebrechorkos et al., 2019). the chirp daily rf data from 2009 to 2018 with a resolution of 5km was downloaded from https://data.chc.ucsb.edu/products. observed daily rf data from eighteen stations of ethiopian meteorology institute (emi) for the period 2009 to 2018. data quality, homogeneity, and outlier tests were carried out. the locations of the climate stations are shown in fig. 1. the distribution of stations is representative of all topographic features in the study region. https://data.chc.ucsb.edu/products 205intercomparison of multisource actual evapotranspiration satellite products in bilate watershed, ethiopia a range of actual evapotranspiration (aet) products are available, covering regional to global extents (ma et al., 2021). in this this research, all national (fao wapor), continental (modisssebop continental) and global (climate engine, modisssebop global and synthesis of global) aet estimates were compared (shao et al., 2022). table 1 lists the five products used in this study, which are terraclimate, fao wapor, modisssebop continental and global and synthesis of global aet for the period 2009 to 2018. climate engine data were generated using interpolation of climate data and water balance modelling to estimate aet at a monthly time step with a different resolution (abatzoglou et al., 2018). 3. methods the methodology in this study comprises the subsequent stages. first, the rainfall and aet data were acquired from global, continental and local data sources. the bias of the chirp satellite rainfall data was removed using groundbased rainfall data. then, the bias-corrected rainfall data was used to evaluate the spatial variability of rainfall in the study area. pet was calculated using the standard penman-monteith equation (allen et al.,1998). the hargreaves method, on the other hand, was used for stations that only had temperature data. a relationship was developed between the estimates of the penman-monteith and hargreaves methods, which was used to estimate the equivalent penmanmonteith pet for all stations. in the bilate watershed dataset, the 11 crops are grown in the irrigated fields. these irrigated crop are cabbage, potato, wheat, banana, chat, maize, onion, beetroot, chili, tobacco, and grass for livestock (irrigated fodder). subsequently, the actual evapotranspiration (aet) was determined by employing the crop coefficient-reference evapotranspiration approach, as outlined in the fao-56 manual by allen et al. (1998). we used crop coefficient-reference evapotranspiration to serve as reference data for the validation. commonly, the reference evapotranspiration is used as a reference data set (jia et al., 2021; bidabadi et al., 2023) for validation of aet estimates. however, estimation of reference evapotranspiration assumes a reference grass surface across space, but can yield biases where potential vegetation water use departs substantially from this assumption (abatzoglou et al., 2018). this limitation is addressed by using crop coefficient-reference evapotranspiration because the actual crop is considered. utilizing the monthly aet values derived from this method as a benchmark, we computed the mean, standard deviation, and coefficient of variation (cv) for the monthly aet data across each crop type. finally, the degree of similarity was estimated between the aet products, and the effects of season and elevation on the degree of similarity between the products. the pearson correlation coefficient, denoted as (r), is a statistical measure of the linear relationship between two variables (pearson’s correlation coefficient, 2008). this statistical index is particularly applied to assess the correlation of actual evapotranspiration (aet) estimates derived from a duo of satellite products and reads as shown in equation (1): (1) table 1. data brief description of five aet products aet products fao wapor climate engine modisssebop continental modisssebop global synthesis of global aet website https://wapor.apps.fao. org/catalog/wapor_2/2/ l2_aetim https://app. climateengine.com/ climateengine https://edcintl.cr.usgs. gov/downloads/sciweb1/ shared/fews/web/africa/ monthly/etav5/downloads/ https://edcintl.cr.usgs. gov/downloads/ sciweb1/shared/fews/ web/global/monthly/ etav5/downloads/ https://dataverse. harvard.edu/dataset. xhtml?persistentid=doi:10.7910/ dvn/zgoued spatial resolution 100m 4km 1km 1km 1km temporal interval 2009present 1958present 2003present 2003-present 1982present where aeti and aetj refer to the pair of aet estimates from two products and n number of pairs of estimates. coefficient of variation (cv) is a statistical measure that provides insight into the relative variability of datasets (stȩpniak, 2011). the temporal coefficient of variation (cv) is computed for each aet product in terms of the ratio of the standard deviation of aet to the arithmetic mean of monthly aet as shown in equation (2): (2) where cv is coefficient of variation in percentage, σ is standard deviation and aet is the arithmetic mean of actual evapotranspiration. root mean square error (rmse) is a statistical metric used to assess the accuracy of a regression model by measuring the average difference between the predicted values from the product and the actual values in the dataset (chai & draxler, 2014). rmse is also used but we refer to it as root mean square difference (rmsd) since we do not have actual observations to serve as reference for error estimation as shown in equation (3): (3) where: aeti and aetr refer to evapotranspiration estimates by the satellite product and crop coefficient-reference evapotranspiration, and t represents the number of months over the analysis period.. 3.1. bias correction of chirp product satellite rf estimates have been found to be affected by both systematic errors (yuan et al., 2017; chen et al., 2021) and random errors (tian and peterslidard 2010; aghakouchak et al., 2012). among the two error types, the systematic errors (consistently higher or lower estimates compared to the actual data) can be corrected if reference data is available for more multiple years. for this purpose, various bias correction methods are reported in literature to correct systematic errors of satellite rainfall products (katiraie-boroujerdy et al., 2020; iqbal et al., 2022). a widely recognized technique for this correction is the power transformation method, which is applied in literature (wörner and colleagues in 2019). the chirp product, while a valuable source of rf data, is not immune to the biases common to satellite-derived data, which can stem from various uncertainty sources including retrieval algorithms, revisit time of the satellite, type of sensors, and rainfall characteristics (goshime et al., 2020; paredes-trejo et al., 2021) in this research, we employed a nonlinear power transformation as a bias correction method to mitigate systematic errors in chirp satellite rf estimates. the power transformation technique is specifically designed to address both the mean and variance of rainfall data. as described by terink et al. (2021), the power transformation is mathematically expressed in equation (4): p* = a ⋅ pb (4) https://wapor.apps.fao.org/catalog/wapor_2/2/l2_aetim https://wapor.apps.fao.org/catalog/wapor_2/2/l2_aetim https://wapor.apps.fao.org/catalog/wapor_2/2/l2_aetim https://app.climateengine.com/climateengine https://app.climateengine.com/climateengine https://app.climateengine.com/climateengine https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/africa/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/africa/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/africa/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/africa/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/global/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/global/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/global/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/global/monthly/etav5/downloads/ https://edcintl.cr.usgs.gov/downloads/sciweb1/shared/fews/web/global/monthly/etav5/downloads/ https://dataverse.harvard.edu/dataset.xhtml?persistentid=doi:10.7910/dvn/zgoued https://dataverse.harvard.edu/dataset.xhtml?persistentid=doi:10.7910/dvn/zgoued https://dataverse.harvard.edu/dataset.xhtml?persistentid=doi:10.7910/dvn/zgoued https://dataverse.harvard.edu/dataset.xhtml?persistentid=doi:10.7910/dvn/zgoued 206 alemeshet kebede yimer, samuel dagalo hatiye, alemseged tamiru haile where p* is the bias-corrected rf, p is the simulated rf (chirp rf product), a and b are the parameters estimated over the calibration period. for bias correction 12 selected stations were available after screening. the nonlinear power equation reads and recommended as alternative approach to estimate pet in ungauged areas. similar work has been done in tanzania by moges et al. (2003). the inverse weight (idw) method was used for grid cells without stations to interpolate the bias parameters. 3.2. estimation of reference evapotranspiration (eto) meteorological data over a long period, including minimum and maximum temperatures, hours of sunshine, wind speed, and relative humidity, were gathered from the ethiopian meteorology institute (emi) from 2009 to 2018. this extensive monthly climate data was used in the cropwat model version 8.0 to compute daily eto based on the fao 56 penman-monteith equation (allen et al., 1998). the fao 56 penman-monteith equation (allen et al., 1998) is widely regarded as a standard method in numerous studies for its comprehensive consideration of all influencing variables of evapotranspiration (et). moreover, the fao designates this equation as the reference method due to its precise estimation of eto at specific locations compared to other methods, its explicit integration of both physiological and aerodynamic parameters (fao, 2010) as depicted in equation (5): = 0.408△[ − ]+ ( 900 + 273 ) 2( − ) △+ [1+ 0.34 2] (5) where; eto represents the reference crop evapotranspiration (mm day-1), δ denotes the slope of the saturation vapor pressure curve (kpa-1), rn is the net radiation at the crop surface (measured in mjm-2 day-1), and g signifies the soil heat flux density (measured in mj m-2 day-1). t stands for the mean daily air temperature at a 2 m height (measured in °c), u2 is the wind speed at a 2 m height (measured in ms-1), and es ea represents the saturation vapor pressure deficit (measured in kpa). is the saturation vapor pressure at a given period (measured in kpa), es is the actual vapor pressure (measured in kpa), ea and υ is the psychrometric constant (measured in kpa-1). the fao-56 penman-monteith method holds an advantage over many other methods due to its physically based approach, which has been extensively tested using various lysimeters. as the pm method is integrated in the cropwat software package (smith, 1992), eto was estimated using the package. 3.3. estimation of crop actual evapotranspiration (etc) crop actual evapotranspiration for the irrigated area was calculated by multiplying kc values by the reference evapotranspiration obtained from local weather stations. the crop characteristics were obtained, including the crop coefficient (kc) values for cabbage, potato, wheat, banana, chat, maize, onion, beetroot, chili, tobacco, and grass for livestock (irrigated fodder) were from fao-56 kc values (allen et al., 1998). for all crops, the three main stages of crop growth were considered: the early stage, the mid-season stage, and the late season stage. aet was calculated using the crop coefficient-reference evapotranspiration methodology described in the fao-56 manual (allen et al., 1998). in particular, a “single” crop coefficient model is used, with etc estimated as follows in the absence of water stress: etc = kc × eto (6) where, eto is the reference evapotranspiration and kc is the crop coefficient. eto is calculated from different climatic variables, while kc crop coefficient. 3.4. intercomparison of the aet products the correlation coefficient (r) signifies the intensity of a linear association between two variables. it can range from -1 to 1, with 0 implying no correlation. the closer the r value is to -1 or 1, the stronger the correlation, while values near 0 suggest a weaker correlation. various factors can influence the coefficient, so it’s always recommended to visualize the relationship between the two variables using a scatterplot. the correlation between the observed and the aet products was computed for the upper, the middle and the lower bilate. a positive correlation value indicates that higher value of one variable tends to be related with the higher values for the other variables. root mean square error (rmse) and standard deviation were also used to compare the products. descriptive statistics, scatter plots and pearson’s correlation coefficient was employed as a comprehensive graphical technique that concurrently illustrates three key metrics: centered root mean square difference (rmsd), pearson correlation coefficient, and standard deviation. serving as a potent visual instrument, the spatial correlation diagram was used to evaluate how close the aet estimates are to the reference data are instrumental in assessing the performance of models and supporting well-informed choices. it also shows which of the aet products deviate from the other aet products in terms of r, rmsd and standard deviation. since the above error measures show the degree of similarity between aet of the products averaged over the analysis period, we also used the interannual and inter-annual cycles of aet. we visually inspected presence of significant difference in the pattern and magnitude of aet from the different products. the effect of elevation on the difference in inter-annual and intraannual cycles from various data sources was also evaluated. 4. results and discussion 4.1. rainfall (rf) patterns of the study area understanding the interplay between rf and aet is crucial for managing water resources. aet plays a crucial role in the water cycle and affects water availability, vegetation growth, and overall ecosystem health. intra-annual variability refers to the fluctuations in rf within a single year. factors such as seasonal patterns, rainy season, and climate oscillations contribute to this variability. during wet seasons with abundant rf, aet tends to increase due to higher soil moisture availability and plant growth. conversely, during dry periods, aet may decrease as water becomes limited. the bilate watershed exhibits a distinct annual cycle of rainfall (rf) with a bimodal distribution, as illustrated in (fig. 2a) and supported by existing literature (orke & li, 2021). the rainfall pattern shows two prominent peaks: a primary peak during the kiremt season (june-september) and a secondary peak during the belg season (february-may). notably, there is no significant variation in the overall annual rainfall cycle (intra-annual variation) across the upper, middle, and lower parts of the bilate watershed. however, monthly rainfall amounts do differ among the three regions. the upper part consistently receives the highest rainfall throughout most months, with a particularly pronounced spatial difference during peak rainfall months. in june, the upper sub-watershed experiences the highest amount of rainfall, peaking at approximately 180 mm. the middle sub-watershed follows a similar pattern but with lower amounts, reaching around 140 mm in june. in contrast, the lower sub-watershed receives the least rainfall, with its peak occurring just before may at approximately 80 mm. this lower rainfall persists consistently throughout the year. overall, the data reveals a clear seasonal pattern, with the upper region receiving the most precipitation, followed by the middle region, and finally the lower region. the peak around may-june suggests a monsoon or wet season during these months. in the bilate watershed as depicted in (fig. 2b), there is a significant variation in annual rainfall from year to year. this observation is consistent with existing literature (orke & li, 2021). the variation in rainfall occurs in all three parts of the watershed. however, when analyzing the period from 2009 to 2018, we find that the upper part consistently receives higher annual rainfall (ranging from 1366.9 mm in 2016 to 1839.2 mm in 2018) compared to the lower part (which received 562.4 mm in 2015 to 625.8 mm in 2009). this spatial difference in annual rainfall is substantial, exceeding two-fold. over the study period, the watershed experienced its wettest condition since 2016, while the driest condition occurred in 2015. specifically: the upper part of the watershed exhibited a significant increase in rainfall, particularly noticeable in 2018. this change could have implications for water management and flood risk in that area. the middle part of the watershed showed fluctuations in rainfall, but overall, it maintained a relatively consistent range between 1000 mm and 1400 mm. the lower part of the watershed experienced a steady increase in rainfall over the years. although not as dramatic as the upper part, this trend could still impact water flow and availability in the lower watershed. 207intercomparison of multisource actual evapotranspiration satellite products in bilate watershed, ethiopia understanding these rainfall patterns is crucial for managing water resources and developing infrastructure. additionally, this data can be used to predict future water availability and prepare for potential flood events. a b figure 2. spatial difference in the pattern and magnitude of the intra-annual (a) and inter annual (b) rainfall amount in the bilate watershed 4.2. the degree of similarity of the calculated and 5 aet products figure 3 shows the spatial correlation between actual evapotranspiration (aet) values that were calculated and those obtained from five different products in the bilate watershed during the irrigation season of areal aet from 2009 to 2018. the results are divided into three parts: the upper part (a), the middle part (b), and the lower part (c). at upper bilate, there is a positive correlation between the aet products (fig. 3). the global and continental products of modisssebop have near perfect relationship (correlation = 0.95), and the synthesis of global aet and modisssebop global and continental products have very high relationship (correlation = 0.70-0.74). however, the correlation is weak between the crop coefficient-reference evapotranspiration and fao wapor aet (correlation = 0.35). the correlation between the aet from the two modisssebop products is nearly similar over the three parts of bilate watershed. however, the correlation of the synthesis global aet with the two modisssebop products declined significantly at the middle part of bilate, and fao wapor aet has relatively larger correlation with other products at the middle part of bilate (fig. 3(b)). overall, the spatial correlation analysis in the upper, middle, and lower parts of the figure helps to assess the alignment and agreement between the calculated aet values and the values obtained from the different products across the various regions of the bilate watershed during the specified irrigation season. in the bilate watershed the scatter plots of areal average inter annual et products are shown in figure 4. the plots reveal a pronounced scatter among the aet values of modisssebop products and the synthesis of global aet and modisssebop global products. high r² values, notably those exceeding 0.5 such as r² = 0.922 and r² = 0.5493, indicate a robust correlation, signifying a substantial linear relationship between the variables. conversely, the aet values of modisssebop continental and terraclimate, as well as the synthesis of global aet and fao wapor, exhibit a moderate correlation. this is evidenced by r² values like r² = 0.1957 and r² = 0.363 which suggest a discernible, albeit not strong, linear relationship. lastly, a weaker correlation is observed between the aet values of terraclimate and fao wapor, and between the synthesis of global aet and terraclimate. r² values nearing zero, such as r² = 0.0002 and r² = 0.0315, denote a very weak correlation, implying that the variables have minimal to no linear relationship. aet from the two modisssebop products is lower than from fao wapor, terra climate, and synthesis global. a b c figure 3. the spatial correlation between aet values of calculated and the 5 products in the bilate watershed over the period 2009-2018 irrigation season of areal aet. at the upper part (a), middle part (b), lower part © 208 alemeshet kebede yimer, samuel dagalo hatiye, alemseged tamiru haile figure 4. areal mean inter annual scatter plots of the five et products in the bilate watershed over the period 2009-2018 irrigation season. where tc=terraclimate, fw=fao wapor, mc=modisssebop continental, mg= modisssebop global, sg=synthesis of global aet table 2. the descriptive statistics of the aet from the five products sub-watershed upper middle lower product metrics (mm/month) mean std. dev. coeff. var. mean std. dev. coeff. var. mean std. dev. coeff. var. aet products terra-climate 45.09 33.6 0.75 52.88 37.34 0.71 82.56 15.85 0.19 fao wapor 62.56 15.85 0.25 47.6 14.22 0.3 49.11 13.25 0.27 modisssebop continental 36.77 21.45 0.58 24.36 15.04 0.62 24.27 12.88 0.53 modisssebop global 36.06 21.98 0.61 23.91 15.31 0.64 25.03 15.35 0.61 synthesis of global aet 53.25 22.6 0.42 49.56 15.85 0.32 48.22 21.11 0.44 table 2 presents the descriptive statistics of the monthly aet values from the multiple products. in the bilate watershed, aet values are higher in the highlands than in the lowlands. table 2 reveals that there is no significant difference between mean aet values from modisssebop products. however, terraclimate, synthesis of global, and fao wapor aet have a considerably larger difference in their respective mean aet values. across the upper and middle regions of bilate, the temporal variability of aet values is smallest for the fao wapor product and largest for terraclimate. for the lower part of the watershed, terraclimate aet values have the smallest variability, though it is slightly at the same level as that of fao wapor. the two modisssebop products provided similar levels of temporal variability of aet in bilate watershed. terraclimate generally shows the highest mean aet values but also the greatest variability across the sub-watersheds. fao wapor and the synthesis of global aet products display more consistent measurements, as indicated by their lower coefficients of variation. modisssebop products tend to have lower mean aet values, especially in the middle and lower sub-watersheds. these metrics are crucial for understanding water usage and availability in these regions. 209intercomparison of multisource actual evapotranspiration satellite products in bilate watershed, ethiopia 4.3. spatial distribution of the aet products figure 5 show the comparison of intra-annual distribution of aet from the 5 products over each of the three parts of bilate watershed. in the upper, middle and lower parts of the watershed, all products reported a gradual decrement of aet from november to january. however, aet exhibits a gradual increment from january to april. overall, the five aet products reported similar pattern of intra-annual variability in the study area. however, there is significant difference between the magnitude of aet reported by these products. the difference in aet magnitude is largest in the lower part of the watershed than other parts. the two modisssebop products reported lower monthly aet than the other products over all parts of the watershed. figure 6 shows the inter-annual variability of aet in the upper, middle and lower part of the watershed from 2010 to 2015. during the irrigation season of these years, the lowest aet occurred in 2015and largest aet occurred in 2018. most of the satellite products agreed on the timing of the maximum and minimum aet. in the upper part of bilate, fao wapor showed that aet was dominated by gradual change (increasing trend) than inter-annual variability over the reported period. however, the other products showed the dominance of aet inter-annual variability over gradual change. the two modisssebop products reported lower annual aet than the other products over all parts of the watershed. furthermore, while the interannual variation in some evapotranspiration products showed similar changes over different years, others behaved differently, showing either inconsistencies or even opposing trends. this observation highlights the complexity and variability in how evapotranspiration is influenced by various factors (kim et al., 2021). these findings imply that there are still significant uncertainty in evapotranspiration estimations and products (fisher et al., 2017). a b c figure 5. spatial distribution of the 5 products in the bilate watershed over the period 2009-2018 irrigation season intra annual areal mean aet in bilate watershed. at the upper part (a), middle part (b), lower part (c). where tc=terraclimate, fw=fao wapor, mc=modisssebop continental, mg= modisssebop global, sg=synthesis of global aet a b c figure 6. spatial distribution of the 5 products in the bilate watershed over the period 2009-2018 irrigation season of areal aet in bilate watershed. at the upper part (a), middle part (b), lower part (c) 5. conclusion this research presents a comparative analysis of various actual evapotranspiration (aet) satellite products within ethiopia’s bilate watershed, encompassing terraclimate, fao wapor, and both the continental and global modisssebop, as well as the synthesis of global aet. the study yields several key insights: 1. the aet estimates from satellite products exhibit a weak correlation with the crop coefficient-reference evapotranspiration, potentially due to scale discrepancies and the averaging of aet values across diverse land cover classes, in contrast to the point-scale representation of atmospheric moisture demand by the reference evapotranspiration. 2. there is a notable divergence in the spatiotemporal variability of aet across the five products, with the exception of the modisssebop products, which display a consistent level of aet variability. this variation is crucial for consideration in subsequent research and data analysis. 3. while the intra and inter-annual cycles of aet across the products follow a similar trend, significant disparities exist in their magnitudes. the 210 alemeshet kebede yimer, samuel dagalo hatiye, alemseged tamiru haile modisssebop global and continental products show minimal variance, in contrast to the slight differences observed between the annual aet magnitudes of the synthesis of global and fao wapor products. 4. the linear relationship among the five satellite products is generally weak. however, the fao wapor product moderately correlates with the others, exhibiting the highest aet magnitude and the least temporal variability. 5. the similarity in aet products varies with elevation, although no definitive pattern emerges. 6. the aet values reported by the two modis products are lower than those of other products on both monthly and yearly scales. the fao wapor’s annual aet maintains a linear relationship with the other products, reporting comparatively higher values and the lowest temporal variability. the findings underscore the presence of significant random and systematic discrepancies between the products. drawing parallels with recent trends in satellite rainfall product research, there is an imperative need to explore methodologies to reconcile these systematic differences in satellite aet products. acknowledgments: the authors thank dr. alemseged tamiru haile (international water management, institute, iwmi, ethiopia), dr. samuel dagalo hatiye (arba minch water technology institute, water resources research center; faculty of water resource and irrigation engineering, arba minch university, ethiopia) for providing critical comment and suggestions for the manuscript. conflicts of interest: the authors declare no conflict of interest. references abatzoglou, j. t., dobrowski, s. z., parks, s. a., & hegewisch, k. c. 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(2024). a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) region in ranitar-belarang section of udayapur district, koshi province, nepal. earth sciences research journal, 28(3), 325-348 https://doi.org/10.15446/esrj.v28n3.112740 record manuscript received: 31/01/2024 accepted for publication: 18/10/2024 abstract a landslide susceptibility map indicates those locations which are prone to the landslide depending upon the factors that causes landslide (slope, soil type, impact of flow, etc.). this study assesses the outcomes of a landslide susceptibility analysis employing frequency ratios (fr), weight of evidence (woe) and analytical hierarchy process (ahp) in the ranitar belarang region, situated in udayapur district, koshi province of eastern nepal. geologically, the region falls within the region of the main boundary thrust (mbt). google earth imagery (cnes/airbus and maxar technologies) with a spatial resolution of 20 m was utilized for landslide detection. the inventory of landslides was employed to create data sets for training and testing. thirteen causative parameters (slope, distance to thrust, landuse, geology, distance to stream, curvature, aspect, relief, distance to road, topographic wetness index, sediment transport index, sediment power index, rainfall), derived from topographic, geological, and land-use maps were considered in the analysis. the ahp ratings were assigned based on the expert judgment whereas, the fr and woe ratings were computed based on these causative factors and training events. subsequently, a landslide susceptibility map was generated by amalgamating causative factors that yielded fr, ahp, and woe scores with validation using the aucroc curve resulting in an 86.4%, 68.5%, and 89.9% accuracy respectively. among the three methods of analysis, weight of evidence (woe) has the highest accuracy (89.9%) in predicting landslides followed by frequency ratio (86.4%). also, it was found that distance from the main boundary thrust (mbt), land use, relief, and distance from the road emerged as the most influential factors contributing to landslide occurrence. keywords: landslide susceptibility, main boundary thrust (mbt), frequency ratios (fr), weight of evidence (woe), analytical hierarchy process (ahp), nepal palabras claves: palabras clave: susceptibilidad de deslizamientos; falla empuje frontal del himalaya; relación de frecuencia; análisis de la evidencia; proceso de jerarquía analítica; nepal. comparación de los métodos de relación de frecuencia, análisis de la evidencia y proceso de jerarquía analítica para la evaluación de la susceptibilidad a deslizamientos de tierra en la falla de empuje frontal del himalaya en la sección ranitarbelarang, distrito de udayapur, provincia de koshi, nepal un mapa de susceptibilidad de deslizamientos de tierra indica aquellos puntos que son propensos a los deslizamientos de acuerdo con los factores que lo causan (pendiente, tipo de suelo, impacto de la circulación de aguas, etc.). este estudio evalúa los resultados de los análisis de susceptibilidad de deslizamientos preparados con los modelos relación de frecuencia, análisis de la evidencia y proceso de jerarquía analítica, en la región de ranitar-belarang, situada en el distrito de udaipur, provincia oriental de koshi, nepal. geológicamente, la región se ubica en la falla conocida como empuje frontal del himalaya. se utilizaron imágenes de google earth (tecnologías cnes/airbus y maxar), con una resolución espacial de 20 m, para detectar deslizamientos de tierra. el inventario de deslizamientos se empleó para crear grupos de datos de entrenamiento y prueba. en este análisis se consideraron trece parámetros causantes (pendiente, distancia al cabalgamiento, uso del suelo, geología, distancia a la corriente, curvatura, aspecto, relieve, distancia a caminos, índice de humedad topográfica, índice de transporte de sedimentos, índice de potencia de sedimentos, y lluvia), derivados de mapas topográficos, geológicos, y de uso del suelo. los criterios del proceso de jerarquía analítica se asignaron con base a un juzgamiento experto, mientras que los criterios de la relación de frecuencia y análisis de la evidencia se computaron de acuerdo con los factores causativos y los eventos de entrenamiento. seguidamente se generó un mapa de susceptibilidad de deslizamientos al converger los factores causativos de los modelos relación de frecuencia, proceso de jerarquía analítica, y análisis de la evidencia, con una validación ejecutada con el método área bajo la curva roc (característica operativa del receptor), la cual calculó una precisión de 86.4 %, 68.5%, y 89.9, respectivamente. dentro de estos tres métodos, el análisis de la evidencia tiene el mayor índice de precisión (89.9 %) en la predicción de deslizamientos, seguido por la relación de frecuencia (86.4 %). además, se encontró que la distancia a la falla de empuje frontal del himalaya, el uso del suelo, relieve y distancia a caminos son los factores más influyentes en la ocurrencia de deslizamientos. resumen https://doi.org/10.15446/esrj.v28n3.112740 mailto:kabiraj.paudyal@cdgl.tu.edu.np https://doi.org/10.15446/esrj.v28n3.112740 326 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan 1. introduction: landslides are a major concern in nepal’s mountainous regions, causing environmental damage and endangering lives. the geomorphic and tectonic evolution of the nepal himalayas plays a pivotal role in the formation of deep and steep river valleys in central and eastern nepal (hasegawa et al., 2008). these valleys are notably prone to a myriad of large and small-scale landslides. nepal has been heavily affected by landslides in recent years, with substantial loss of life, property damage, and economic impact (bhandari et al., 2024). creating accurate landslide susceptibility maps is crucial for disaster prevention and mitigation efforts. these maps identify areas with a higher likelihood of landslides, allowing authorities to take proactive measures to protect people and infrastructure. despite existing research on landslide modeling in nepal, there’s a need for further studies that compare the effectiveness of different mapping techniques. the present study area ranitar belarang faces multiple challenges related to slope instabilities, primarily attributed to its steep terrain, rugged landscape, the presence of the thrust (mbt), and the precarious condition of the rock formations. this section of nepal himalaya has not been studied in terms of landslide susceptibility with special reference to the main boundary thrust (mbt), a master thrust of the himalaya, as a cause of landslides in nepal. different contributing factors like topography, lithology, rainfall intensity, vegetation condition, lineaments facilitate activation and reactivation of landslide events. various researchers have developed different techniques for landslide susceptibility mapping, including statistical methods and machine learning approaches. due to the simplicity, accessibility, and reliability in producing accurate susceptibility maps, two statistical methods; weight of evidence, and frequency ratio, and multi criteria decision making approach, analytical hierarchy process are chosen for the study and compared with other methods. 2. study area the study area, ranitar-belarang, is situated in udayapur district in eastern nepal, as depicted in figure 1. the research site spans an altitude range of 120 meters to 1000 meters, covering a total area of 89.76 sq. km. the geographical coordinates of the study area are defined by a longitude range of 86.969431o to 87.184146o and a latitude range of 26.926310o to 26.852557 o. the geology of the study area is divided into five major geological groups: phongaswa, barahakshetra, siwalik group, gondwana group, and midland group (dmg, 1984). accessibility to the study area is facilitated within a 25-kilometer radius of mahendra highway through local roads, namely dharan chatara barahalsetra and jhumka (mrm) chatara road. additionally, the area is connected to a network of graveled streets and footpaths, enhancing its overall transportation infrastructure. figure 1. location map of the ranitar belarang section 327a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) 3. materials and methods: it is imperative to conclude that the causative factors of landslides play a pivotal role in influencing the spatial distribution of such events. the understanding that future landslides are likely to occur under conditions akin to those observed in previous incidents is crucial for effective landslide susceptibility mapping, as emphasized by (lee & talib, 2005). in recent decades, numerous scholars have devised effective methodologies for constructing accurate landslide susceptibility maps. various researchers have adopted different methods for landslide susceptibility in nepal himalaya such as frequency ratio (thapa & bhandari, 2019; bhandari et al., 2024, goetz et al., 2015; hong et al., 2016; m. j. lee et al., 2016), analytical hierarchy process (pathak et al., 2021; pourghasemi et al., 2012; t. l. saaty, 2004), weight of evidence (dahal et al., 2008; gómez & kavzoglu, 2005, pokharel & bhandari, 2019) logistic regression (chen et al., 2017; steger et al., 2016), budha et al, 2016 and 2020, decision trees (s. lee & park, 2013; pradhan, 2013; tsangaratos & ilia, 2016), fuzzy logic (feizizadeh et al., 2014; park et al., 2014; pradhan, 2011), neuro fuzzy systems (aghdam et al., 2016; m.-j. lee et al., 2015; pradhan, 2013), support vector machines (s. lee et al., 2017; peng et al., 2014; pradhan, 2013; tien bui et al., 2017), artificial neural networks (conforti et al., 2014; pradhan & lee, 2010; tsangaratos & benardos, 2014), and a multimethod approach (althuwaynee et al., 2016; pham et al., 2016; pradhan, 2010; yalcin et al., 2011), have been developed. ahp offers a systematic approach to decision-making by breaking down complex problems into a hierarchy of simpler sub-problems. this structured approach is not typically available in methods like fuzzy logic or neurofuzzy systems, which can be more abstract and less intuitive. woe provides a clear and intuitive understanding of the relationship between causative factors and landslide occurrence, which can be more accessible than the complex interpretations required for machine learning models like svm or ann. fr is straightforward to implement and understand, making it accessible for researchers with limited resources or expertise. this simplicity is a significant advantage over more complex methods like neurofuzzy systems or svm, which require extensive parameter tuning and computational resources. among various models, this paper assesses the effectiveness of the landslide susceptibility study using three methods viz. analytical hierarchy process (ahp), weight of evidence (woe), and frequency ratio (fr). these models offer the advantage of ranking causative variables based on their likelihood of causing a landslide. additionally, it enables the determination of whether a given set of causative factor values would pose a hazard in the event of a landslide, as noted by (kannan et al., 2013). the overall methodological scheme adopted for the present study plan is shown in figure 2. landslide susceptibility mapping landsat image (google) landslide inventory geology lulc dis-thrust ahp, woe, and fr under gis platform secondary datafield visit topographic countour 20m rainfall slope map aspect map relief map distance to stream map curvature twi sti spi landslide susceptibility index classification of lsi final landslide susceptible map dis-road validation roc/auc figure 2. methodology used in analysis of landslide susceptibility mapping using fr method. 328 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan the landslide inventory consisting of 120 landslides, was prepared through the manual digitization of aerial photographs / satellite images obtained from google earth (march 2020) and divided into training samples (70%, n=84) and testing samples (30%, n=36). the landslide susceptibility index (lsi) model was then developed utilizing the training sample, and its validation was carried out using the testing sample through roc-auc analysis in spss (figure 2). landslide susceptibility mapping involves the generation of thematic data layers, as emphasized by (camarinha et al., 2014). consequently, thirteen thematic layers were incorporated into a 20 m × 20 m cell grid, encompassing geology, slope, aspect, curvature, distance from the stream, distance from the road, distance from the thrust (mbt), relief, land use land cover, rainfall, topographic wetness index, stream power index and sediment transport index. to derive topographic and hydrologic parameters, a digital elevation model (dem) was utilized, derived from a triangulated irregular network (tin) surface. this tin was constructed using arcmap 10.4.1 and contour lines extracted from digitized topographic maps at 20 m intervals. various topographic and hydrologic characteristics including slope gradient, curvature, slope aspect, relief, distance from the stream, were considered for landslide susceptibility analysis. the land use land cover (lulc) map was created using arcmap 10.4.1 by digitizing the image. geological information and the location of the thrust (mbt) were derived from data obtained from the petroleum block (department of mines and geology, government of nepal) and verified during the present field visit. the euclidean distance algorithm was employed to generate the mbt distance map. this comprehensive approach ensures a multi-dimensional analysis incorporating geological, topographic, and land cover factors for an accurate landslide susceptibility assessment. the landslide inventory map was employed to intersect with all-factor maps, leading to the generation of tabulated data after the preparation of thirteen factor maps. the landslide susceptibility was mapped using the frequency ratio, weight of evidence and analytical hierarchy methods. the resulting lsi map was categorized into three zones: stable, quasi-stable, and unstable zones. these zones are categorized on the basis of the histogram curve obtained from sensitivity versus 1-specificity. larger values of the test result variable indicate stronger evidence for a positive actual state. the classification was based on the lsi values obtained from the frequency ratio (fr), weight of evidence (woe), and analytical hierarchy process (ahp) methods. this ensures that the classification is not biased by subjective opinions or personal judgments. the classification method takes into account the spatial relationships between the cells, ensuring that the classification is not only based on the lsi values but also on the spatial context. frequency ratio method: to evaluate the likelihood of landslides, it is crucial to comprehend the specific physical characteristics of the area and the processes that contribute to landslide occurrence. the frequency ratio emerges as a quantitative technique for assessing landslide susceptibility by leveraging geographic information system (gis) and spatial data (bonham-carter, 1994; chen, chai, et al., 2016; chen, wang, et al., 2016; ding et al., 2017; s. lee & talib, 2005). widely acknowledged and effectively applied in landslide susceptibility mapping, the frequency ratio (fr) method establishes a quantified relationship between the landslide inventory and the causative factors (chen, chai, et al., 2016; reis et al., 2012; umar et al., 2014; q. wang & li, 2017; wu et al., 2016; yilmaz, 2009;(thapa & bhandari, 2019). this technique, as outlined in eq. (1), involves the amalgamation of the landslide inventory map and the factor map to compute the frequency ratio (fr) for each class of the causative factors (fayez et al., 2018; mondal & maiti, 2013). = (1)/ (2) ∑ (3)/∑ (4) (1) where npix(1) = the number of pixels containing landslide in a class npix(2) = total number of pixels of each class in whole area. npix(3) = total number of pixels containing landslide. npix(4) = total number of pixels in the study area. the derived frequency ratio is aggregated to formulate a landslide susceptibility index (lsi) map using eq. (2) (s. lee & talib, 2005). lsi = fr1 + fr2 + fr3 + fr4 + . . . . . . . . . + frn (2) landslide susceptibility index map (lsi) was prepared by combining all thirteen factor maps (figure 19 & figure 21). final lsi = prd1*fr1 + prd2*fr2 + prd3*fr3 + prd4*fr4 + . . . . . . . . . + prdn*frn where, pr = predictive ratio of each domain. fr = frequency ratio of each class of a domain. here the predictive ratio (table 3) is the weight given to the domain from table-1 which is calculated as in eq.3 below: pr = (max rf – min rf)/ (min rf of max rf – min rf) (3) where rf stands for relative frequency. it is the ratio of fr of a class of a domain to total fr of the domain. analytical hierarchy process (ahp): the acronym ahp refers to the analytical hierarchy process, alternatively recognized as multi-criteria decision making (mcdm). originating in the 1970s, it was conceived by thomas l. saaty, who collaborated with ernest forman to create expert choice in 1983 (t. saaty, 1980). since its inception, ahp has undergone thorough examination and refinement. this method provides a precise approach to assigning weights to decision criteria. the strength of ahp methods depends upon the consistency index that determines the consistency of decision in solving the multi-criteria parameters thus reducing bias in decision making (abusarhan, 2011). it involves leveraging the expertise of individual contributors to gauge the relative significance of factors through pairwise comparisons. each respondent is tasked with evaluating the relative importance between two items using a specially designed questionnaire. however, the detail information about the field knowledge around the study area of the researcher plays a vital role to assign weightage of different parameters. pairwise comparison matrix is created with the help of scale of relative importance viz. • 1. equal importance • 3. moderate importance • 5. strong importance • 6. very strong importance • 9. extreme importance note: 2, 4, 6 & 8 are intermediate values 1/3, 1/5, 1/7, 1/9 are values for inverse comparison. generally, three steps are considered during the ahp calculation (table 1). one of the key strengths of ahp is the use of pair-wise comparison to determine the precise ratio of priorities. in this study, the pair-wise comparison matrix was constructed for the identification of landslide susceptible zones. the relative important of two selected criteria were compared in the pair-wise comparison stage. the comparisons were done to complete the development of matrices. the judgment of the relative importance of two selected criteria was carried based on the experience and knowledge. the scales of 1, 3, 5, 7 and 9 were used to for the comparison in ahp as to ensure the decision is made at a higher level of confidence. the opposite diagonal value represents its reciprocal values. 329a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) table 1. steps involved in developing the ahp model in ranitar belrang section step-1. ahp pairwise comparison matrix ratings parameter 1 parameter 2 parameter 3 parameter 4 parameter 5 parameter 1 1 1/x (j,i) 1/x (j,i) 1/x (j,i) 1/x (j,i) parameter 2 x (i,j) 1 1/x (j,i) 1/x (j,i) 1/x (j,i) parameter 3 x (i,j) x (i,j) 1 1/x (j,i) 1/x (j,i) parameter 4 x (i,j) x (i,j) x (i,j) 1 1/x (j,i) parameter 5 x (i,j) x (i,j) x (i,j) x (i,j) 1 … … … … … … sum y1 y2 y3 y4 y5 step-2. normalizedpairwise comparison matrix ratings parameter 1 parameter 2 parameter 3 parameter 4 parameter 5 criteria weight (wc) parameter 1 xij/y1 xij/y2 xij/y3 xij/y4 xij/y5 wc1 parameter 2 xij/y1 xij/y2 xij/y3 xij/y4 xij/y5 wc2 parameter 3 xij/y1 xij/y2 xij/y3 xij/y4 xij/y5 wc3 parameter 4 xij/y1 xij/y2 xij/y3 xij/y4 xij/y5 wc4 parameter 5 xij/y1 xij/y2 xij/y3 xij/y4 xij/y5 wc5 … … … … … … … sum 1 1 1 1 1 1 step. 3 calculation for consistency ratio ratings parameter 1 parameter 2 parameter 3 parameter 4 parameter 5 weighted sum lamda max parameter 1 1*wc1 (1/x (j,i))*wc2 (1/x (j,i))*wc3 (1/x (j,i))wc4 (1/x (j,i))*wc5 z1 z1/wc1 parameter 2 x (i,j)*wc1 1 (1/x (j,i))*wc3 (1/x (j,i))wc4 (1/x (j,i))*wc5 z2 z2/wc2 parameter 3 x (i,j)*wc1 x (i,j)*wc2 1 (1/x (j,i))wc4 (1/x (j,i))*wc5 z3 z3/wc3 parameter 4 x (i,j)*wc1 x (i,j)*wc2 x (i,j)*wc3 1 (1/x (j,i))*wc5 z4 z4/wc4 parameter 5 x (i,j)*wc1 x (i,j)*wc2 x (i,j)*wc3 x (i,j)*wc4 1 z5 z5/wc5 … … … … … … … … sum y1 y2 y3 y4 y5 c.i = [(avg. λmax n)/(n-1) c.r = [c.i / r.i] random index (ri) n 1 2 3 4 5 6 7 8 9 10 r.i value 0 0 0.58 0.9 1.12 1.24 1.32 1.41 1.45 1.49 where, xij / xji = scale of relative importance, ci= consistency index, and dr consistency ratio the analytical hierarchy process (ahp) is a method where decisions are made by assigning weights through pairwise comparisons without inconsistencies. this process involves breaking down a decision problem into factors, arranging them in a hierarchical order, assigning numerical values to indicate their importance, creating a comparison matrix, and computing a normalized principal eigenvector to determine the weight of each factor. essentially, the success factor in ahp implementation is dependent on the skills and understanding of decision makers in particular in order to understand the elements impacting their decision. (william, 2007) ahp is used in landslide susceptibility analysis (figure 23) because it allows experts to include all types of information related to landslides in the decision process. the method ensures that judgment considers all available information, and discussions are structured based on experts’ knowledge and experiences. when a consensus is reached, the weights for each relevant factor are automatically calculated. the method can also identify and correct inconsistencies in the decision process using consistency index values. however, a drawback of ahp is that personal preferences in ranking factors may vary among different experts. weight of evidence (woe): the woe method estimates whether a landslide happened or not in a specific grid cell and also evaluates the level of influence (expressed as “weight”) that each variable has on the occurrence of a landslide event (dahal et al., 2008; gómez & kavzoglu, 2005; pokharel & bhandari, 2019). this helps us understand both the occurrence and the importance of different factors in the development of landslides (barbieri & cambuli, 2009). this model was initially developed for assessing mineral potential in the 1980s (agterberg et al., 1993; bonham-carter, 1989, 1994; bonham-carter et al., 1988; kemp et al., 2001) state that although the method was initially intended for non-spatial applications, it can also be utilized for spatial predictions when the aim is the probability of point occurrences. the woe model relies on figuring out positive and negative weights, represented as w+ and wwhich are assigned to various categories of causative factors. the positive and negative weights are defined as (bonham-carter, 1994): 4) 330 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan 5) where, "loge " is the natural log, x and x are the presence and absence of potential landslide contributing factor respectively, y and y are the presence and absence of landslide respectively. in general, the weight can be calculated by using the equation: 6) at first, in the study thirteen landslide contributing factors (slope, aspect, curvature, relief, land use, distance from road, distance from thrust, distance from stream, geology, topographic wetness index, stream power index and sediment transport index) were considered, classified into different classes based on their values and their thematic layers were produced in gis. the pixel percentage of the landslide and pixel percentage of the landslide contributing factors of each class and subclass were calculated. for simplicity, the equation (6) can be modified in the form of equation (7) as: 7) where, npix: the number of pixels. the derived weight of each causative factors with respect to the presence or absence of landslides were calculated. if the weight is positive, it means the factor supports the occurrence of landslides. if it’s negative, then the factor does not favor landslides. based on the calculations, weighted maps of each factor were produced, and each map were added and overlaid using the equation (8) to obtain landslide susceptibility maps. figure 3. landslide inventory map (training & testing datasets) figure 4. illustration of landslides from study area landslide susceptibility map= wslope+waspect+wcurvature+wrelief+wlanduse+wdisroad+wdisthrust +wdisstream+wgeology+wrainfall+wtwi+wspi+wsti… 8) landslide susceptibility map (lsi) was prepared by combining all thirteen factor maps (figure 25). landslide inventory utilizing freely available landsat images from google earth, in conjunction with on-site field visits, a total of 120 landslides were mapped, covering an aggregate area of 1.007 km² (refer to figure 3). the sample image of landslide in the study area is shown in figure 4. influencing factors: the selection criteria for these thirteen causative factors were based on their relevance to landslide susceptibility and their availability in the study area. we considered factors that are commonly used in landslide susceptibility assessment, such as slope, geology, and land use. we also considered factors that are specific to the study area, such as the distance to the mbt. land-use land cover: land use is a significant factor in landslide susceptibility. different land uses, such as forest, agricultural land, and barren land, can affect the stability of the land surface. employing an image classification technique, a land cover map (figure 5) was generated. the land cover distribution is as follows: forest covers 71.7% of the area, agricultural land occupies 19.1%, barren ground comprises 1.6%, bushes account for 1.4%, water bodies constitute 2.7%, and roads make up 0.6%. 331a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) figure 5. land-use map of the study area figure 6. geological map of the study area figure 7. distance from the thrust (mbt) map of the study area 332 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan built-up land represents only 0.2% of the total land area. when juxtaposed with the current landslide data, it is observed that barren ground contributes to 52.1% of total landslides, followed by forest at 30.8%, bushes at 10.5%, agricultural land at 3.7%, and roads at 0.04% (refer to table 2). geology: the geological map obtained from the department of mines and geology (figure 6) was employed to categorize the study area into five major geological groups: phongaswa, barahakshetra, siwalik group, gondwana group, and midland group. according to table 2, the siwalik group predominates the study area, encompassing 50.3% of the total area, while accounting for 49.0% of the total landslides within the same geological context. distance to thrust (mbt): the distance to the main boundary thrust (mbt) is an important factor in landslide susceptibility. the mbt is a geological fault that can influence the stability of the land surface. the study incorporates the extrinsic parameter known as distance to thrust (mbt). information regarding mbt from petroleum block no. 9 was acquired from the department of mines and geology, government of nepal. subsequently, the block underwent georeferencing, and the mbt locations were delineated from the map. utilizing both the mbt and the euclidean distance tools, a distance to thrust map (figure 7) was generated, illustrating the relationship of each cell to a specific source or a combination of sources based on straight-line distance. the map was then segmented into five divisions at 500 m intervals. the investigation revealed that the most vulnerable zone to landslides is within the nearest distance range, i.e., 0 500 m. this zone accounted for the highest percentage of landslides, with 40.3% of all occurrences concentrated in this range (table 2). slope: the slope of the land surface is a critical factor in landslide susceptibility. steeper slopes are more prone to landslides due to gravity. the slope map (figure 8) was generated using a 20*20 digital elevation model (dem) through the application of the slope algorithm in the surface-spatial analyst tool within arcmap. the slope is categorized into seven classes using the equal interval classification method. in table 2, it is observed that the slope angle ranging from 0° to 20° occupies 12.97% of the total study area, containing 2.2% of total landslides. similarly, slope angles of 10° to 20°, 20° to 30°, 30° to 40°, 40° to 50°, 50° to 60°, and 60° to 75° cover 15.23%, 28.54%, 29.91%, 11.82%, 1.4%, and 0.14% of the total study area, respectively. landslides in these respective slope categories are recorded at 4.9%, 22.3%, 39.1%, 26.4%, 5.1%, and 0.1%. aspect map: the aspect can influence the direction of slope failure. the aspect map (figure 9) was generated by employing a 20*20 digital elevation model (dem) through the aspect algorithm in the surface-spatial analyst tool within arcmap. the aspect was classified into eight classes, as detailed in table 2. analysis indicates that the study area is predominantly characterized by a northeast facing slope, covering 14.2% of the total study area. the second and third most dominant aspects are north (13.2%) and north-west (12.8%) facing slopes, respectively. relief map: the relief map (figure 10) for the study area was created utilizing a 20*20 m digital elevation model (dem). the elevation range spans from a minimum of 120 m to a maximum of 1000 m. the relief map has been categorized into five classes, as outlined in table 2. figure 8. slope map of the study area figure 9. aspect map of the study area 333a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) figure 10. relief map of the study area (ranitar belarang) figure 11. distance to stream map of the study area figure 12. curvature map of the study area distance from stream: one of the pivotal factors in landslide susceptibility mapping is the proximity to streams, as areas near streams are more susceptible to landslides due to erosive nature of flowing water. to assess this, the distance to stream map (figure 11) was created using the euclidean distance tool, which utilizes straight-line distance to denote each cell’s relationship to a source or a collection of sources. the map is categorized into four interval classes. as detailed in table 2, the region within the 0 to 100 m range from the streams constitutes 38.3% of the total study area and encompasses 38.6% of the total landslides. curvature: curvature refers to the shape of the earth’s surface. it is a measure of how much a surface deviates from being flat. curvature can be classified into three types: concave, linear, and convex. concave surfaces are curved inward, linear surfaces are flat, and convex surfaces are curved outward. in our study, curvature was calculated using a 20*20 m digital elevation model (dem) and classified into these three categories through the aspect algorithm of the surface-spatial analyst tool in arcmap. curvature is classified into concave, linear, and convex surfaces, as illustrated in figure 12. as per table 2, the study area predominantly exhibits a convex topography, covering 44.4% of the total area, while concave topography accounts for 42.9%. the remaining 12.6% is characterized as a linear or plane surface. interestingly, a significant portion of landslides, specifically 51.5% of the total, has occurred in concave surfaces. distance from road: roads play a crucial role in landslide susceptibility mapping, as areas in proximity to roads are more susceptible to landslides. to assess this, the distance to road map (figure 13) was generated using the euclidean distance tool, which characterizes each cell’s relationship to a source or set of sources based on straight-line distance. the map is classified into five interval classes (table 2). 334 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan figure 13. distance from road map of the study area figure 14. rainfall map of the study area figure 15. topographic wetness index map of the study area rainfall: rainfall infiltration plays an important role in landslide which triggers the sliding of rock mass. on the basis of rainfall data collected from department of hydrology and meteorology, a rainfall map was prepared using idw method in arcgis (figure 14). in the present study rainfall map was categorized into five classes (table 2). topographic wetness index (twi): the topography controls the spatial variation of hydrological conditions, like soil moisture, groundwater flow, and slope stability (yilmaz 2009). the twi developed by beven and kirkby (1979) has been employed for the study. twi = ln(a/tanβ) 9) where “a” is the specific catchment area and “β” is the local slope angle. a twi map (figure 15) was generated using the flow accumulation derived from digital elevation model (dem). stream power index (spi): the energy exerted by a river or stream on its surrounding terrain is known as stream power. this force is generated by the interaction of several key factors, including the water’s density, its acceleration due to gravity, the volume of water flowing through the channel, and the slope of the water’s flow. when these elements combine, they produce a powerful force that can shape the 335a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) river’s course and surrounding landscape. the spi measures the erosive power of concentrated surface runoff (wilson and gallant, 2000). the spi is calculated from the formula: spi=a* tanβ 10) where “a” is the specific catchment area and “β” is the local slope angle in degrees. a spi map (figure 16) was generated using the flow accumulation and slope derived from dem. sediment transport index (sti): the sti determines the erosive power of overland flow. sti is denoted mathematically from moore and burch (1986) as: 11) a sti map (figure 17) was generated using the flow accumulation and slope derived from dem. figure 16. stream power index map of the study area figure 17. sediment transport index map of the study area table 2. tabulation of domain with landslide inventory showing area coverage domain class class pixel % class pixel area (sq. km) landslide pixel % landslide pixel area (sq. km) slope 0 10 29089 12.97 11.64 54 2.16 0.02 10 20 34160 15.23 13.66 122 4.87 0.05 20 30 64008 28.54 25.60 559 22.32 0.22 30 40 67100 29.91 26.84 980 39.12 0.39 40 50 26513 11.82 10.61 661 26.39 0.26 50 60 3135 1.40 1.25 127 5.07 0.05 60 75 303 0.14 0.12 2 0.08 0.00 total 224308 100.00 89.72 2505 100.00 1.00 (continued) 336 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan domain class class pixel % class pixel area (sq. km) landslide pixel % landslide pixel area (sq. km) distance to thrust (mbt) 0 500 m 59678 26.59 23.87 1008 40.26 0.40 500 1,000 m 58205 25.94 23.28 761 30.39 0.30 1,000 1,500 m 56506 25.18 22.60 357 14.26 0.14 1,500 2,000 m 39707 17.69 15.88 233 9.31 0.09 2,000 2,630 m 10306 4.59 4.12 145 5.79 0.06 total 224402 100.00 89.76 2504 100.00 1.00 landuse agricultural land 42718 19.09 17.09 93 3.72 0.04 bushes 3148 1.41 1.26 263 10.52 0.11 forest 160390 71.69 64.16 770 30.80 0.31 sand 6220 2.78 2.49 49 1.96 0.02 barren land 3567 1.59 1.43 1303 52.12 0.52 water body 5958 2.66 2.38 21 0.84 0.01 built up 429 0.19 0.17 0 0.00 0.00 road 1285 0.57 0.51 1 0.04 0.00 total 223715 100.00 89.49 2500 100.00 1.00 geology phongaswa 15241 6.79 6.10 94 3.75 0.04 barahakshetra 17187 7.66 6.87 348 13.90 0.14 siwalik group 112891 50.31 45.16 1226 48.96 0.49 gondwana group 19607 8.74 7.84 468 18.69 0.19 midland group 59475 26.50 23.79 368 14.70 0.15 total 224401 100.00 89.76 2504 100.00 1.00 distance to stream 0 100 m 85642 38.25 34.26 965 38.60 0.39 100 200 m 61539 27.49 24.62 671 26.84 0.27 200 400 m 64073 28.62 25.63 808 32.32 0.32 400 867 m 12622 5.64 5.05 56 2.24 0.02 total 223876 100.00 89.55 2500 100.00 1.00 curvature concave 96321 42.94 38.53 1290 51.50 0.52 linear 28363 12.64 11.35 112 4.47 0.04 convex 99623 44.41 39.85 1103 44.03 0.44 total 224307 100.00 89.72 2505 100.00 1.00 aspect flat 11031 4.92 4.41 13 0.52 0.01 north 29526 13.16 11.81 209 8.34 0.08 northeast 31925 14.23 12.77 236 9.42 0.09 east 25651 11.44 10.26 214 8.54 0.09 southeast 23051 10.28 9.22 445 17.76 0.18 south 24683 11.00 9.87 472 18.84 0.19 southwest 25275 11.27 10.11 596 23.79 0.24 west 24415 10.88 9.77 182 7.27 0.07 northwest 28751 12.82 11.50 138 5.51 0.06 total 224308 100.00 89.72 2505 100.00 1.00(continued) 337a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) domain class class pixel % class pixel area (sq. km) landslide pixel % landslide pixel area (sq. km) relief 120 200 32761 14.61 13.10 120 4.79 0.05 200 400 77207 34.42 30.88 1046 41.76 0.42 400 600 68392 30.49 27.36 771 30.78 0.31 600 800 36149 16.12 14.46 392 15.65 0.16 800 1000 9799 4.37 3.92 176 7.03 0.07 total 224308 100.00 89.72 2505 100.00 1.00 distance to road 0 250 m 83091 37.11 33.24 528 21.12 0.21 250 500 m 51170 22.86 20.47 647 25.88 0.26 500 1000 m 62178 27.77 24.87 1157 46.28 0.46 1000 1500 m 21282 9.51 8.51 168 6.72 0.07 1500 2473 m 6155 2.75 2.46 0 0.00 0.00 total 223876 100.00 89.55 2500 100.00 1.00 stream power index (spi) (-14 -9) 3372 1.5 11 0.4 0.29 0.06 (-8 -4) 44144 19.9 405 16.2 0.81 0.18 (-3 0) 64957 29.3 480 19.2 0.66 0.14 (1 3) 90865 41.0 1280 51.2 1.25 0.27 (4 11) 18392 8.3 323 12.9 1.56 0.34 total 221730 100.0 2499 100.0 4.57 1.00 sediment transport index (sti) 0 10 116296 99.1 1240 99.2 1.00 0.49 11 -50 887 0.76 10 0.80 1.06 0.51 51 100 74 0.1 0.0 0.0 0.00 0.00 101 200 86 0.1 0.0 0.0 0.00 0.00 201 250 8 0.0 0.0 0.0 0.00 0.00 total 117351 100.0 1250.0 100.0 2.06 1.00 topographic wetness index (twi) (2 5) 98047 43.7 1233 49.3 1.13 0.31 (6 7) 82965 37.0 932 37.2 1.01 0.28 (8 10) 26738 11.9 259 10.3 0.87 0.24 (11 15) 14008 6.2 74 3.0 0.47 0.13 (16 25) 2536 1.1 5 0.2 0.18 0.05 total 224294 100.0 2503 100.0 3.65 1.00 rainfall 1,296 1,298 3197 1.4 0 0.0 0.00 0.00 1,299 1,310 30970 13.8 264 10.5 0.76 0.20 1,311 1,311 65071 29.0 1144 45.7 1.58 0.41 1,312 1,313 73372 32.7 809 32.3 0.99 0.26 1,314 1,314 51792 23.1 287 11.5 0.50 0.13 total 224402 100.0 2504 100.0 3.82 1.00 338 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan table 3. weightage of each class of thirteen contributing factors based on woe, ahp, and fr. domain class % class pixel % landslide pixel weight of evidence (woe) analytical hierarchy process (ahp) weightage frequency ratio (fr) relative frequency (rf) rf (non %) pr slope 0 10 12.97 2.2 -1.804 0.03 0.17 0.02 1.84 1.96 10 20 15.23 4.9 -1.148 0.044 0.32 0.04 3.54 20 30 28.54 22.3 -0.248 0.067 0.78 0.09 8.66 30 40 29.91 39.1 0.272 0.104 1.31 0.14 14.49 40 50 11.82 26.4 0.817 0.159 2.23 0.25 24.73 50 60 1.40 5.1 1.319 0.241 3.63 0.40 40.19 60 75 0.14 0.1 -0.530 0.351 0.59 0.07 6.55 total 100.0 100.0 1.00 9.03 1.00 100.00 distance to thrust (mbt) 0 500 m 26.6 40.3 0.420 0.417 1.51 0.30 30.04 1.00 500 1,000 m 25.9 30.4 0.160 0.263 1.17 0.23 23.26 1,000 1,500 m 25.2 14.3 -0.574 0.16 0.57 0.11 11.24 1,500 2,000 m 17.7 9.3 -0.648 0.097 0.53 0.10 10.44 2,000 2,630 m 4.6 5.79 0.235 0.061 1.261 0.250 25.03 total 100.0 100.0 1.00 5.04 1.00 100.00 landuse agricultural land 19.1 3.7 -1.645 0.105 0.19 0.00 0.47 3.98 bushes 1.4 10.5 2.088 0.231 7.48 0.18 17.85 forest 71.7 30.8 -0.851 0.046 0.43 0.01 1.03 sand 2.8 2.0 -0.353 0.03 0.70 0.02 1.68 barren land 1.6 52.1 3.930 0.328 32.69 0.78 78.05 water body 2.7 0.8 -1.162 0.034 0.32 0.01 0.75 built up 0.2 0.00 0.000 0.069 0.00 0.00 0.00 road 0.6 0.04 -2.675 0.157 0.07 0.00 0.17 total 100.0 100.0 1 41.88 1.00 100.00 geology phongaswa 6.8 3.8 -0.598 0.061 0.55 0.09 9.16 1.34 barahakshetra 7.7 13.9 0.605 0.097 1.81 0.30 30.07 siwalik group 50.3 49.0 -0.027 0.417 0.97 0.16 16.13 gondwana group 8.7 18.7 0.773 0.16 2.14 0.35 35.45 midland group 26.5 14.7 -0.595 0.263 0.55 0.09 9.19 total 100.0 100.0 1.00 6.03 1.00 100.00 distance to stream 0 100 m 38.3 38.6 0.009 0.466 1.01 0.29 28.73 1.06 100 200 m 27.5 26.8 -0.024 0.277 0.98 0.28 27.80 200 400 m 28.6 32.3 0.123 0.16 1.13 0.32 32.15 400 867 m 5.6 2.2 -0.930 0.095 0.40 0.11 11.31 total 100.0 100.0 1.00 3.51 1.00 100.00 curvature concave 42.9 51.5 0.184 0.539 1.20 0.47 47.14 1.70 linear 12.6 4.5 -1.047 0.163 0.35 0.14 13.90 convex 44.4 44.0 -0.009 0.296 0.99 0.39 38.97 total 100.0 100.0 1.00 2.54 1.00 100.00 (continued) 339a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) domain class % class pixel % landslide pixel weight of evidence (woe) analytical hierarchy process (ahp) weightage frequency ratio (fr) relative frequency (rf) rf (non %) pr aspect flat 4.9 0.52 -2.259 0.016 0.1 0.01 1.20 1.16 north 13.2 8.34 -0.460 0.108 0.6 0.07 7.20 northeast 14.2 9.42 -0.416 0.072 0.66 0.08 7.52 east 11.4 8.54 -0.294 0.03 0.75 0.08 8.49 southeast 10.3 17.76 0.556 0.158 1.73 0.20 19.65 south 11.0 18.84 0.546 0.316 1.71 0.19 19.46 southwest 11.3 23.79 0.760 0.227 2.11 0.24 24.00 west 10.9 7.27 -0.408 0.02 0.7 0.08 7.59 northwest 12.8 5.51 -0.851 0.048 0.4 0.05 4.89 total 100.0 100.00 1.00 9 1 100 relief 120 200 14.6 4.8 -1.122 0.061 0.33 0.06 6.39 1.27 200 400 34.4 41.8 0.196 0.097 1.21 0.24 23.65 400 600 30.5 30.8 0.010 0.16 1.01 0.20 19.68 600 800 16.1 15.6 -0.030 0.263 0.97 0.19 18.93 800 1000 4.4 7.0 0.482 0.417 1.61 0.31 31.35 total 100.0 100.0 1.00 5.13 1 100 disroad 0 250 m 37.1 21.1 -0.569 0.417 0.57 0.14 13.97 2.09 250 500 m 22.9 25.9 0.126 0.263 1.13 0.28 27.79 500 1000 m 27.8 46.3 0.518 0.16 1.67 0.41 40.90 1000 1500 m 9.5 6.7 -0.350 0.097 0.71 0.17 17.35 1500 2473 m 2.7 0.0 0.000 0.061 0.00 0.00 0.00 total 100.0 100 1.00 4.07 1.00 100 spi (-14 -9) 1.5 0.4 -1.248 0.061 0.29 0.06 6.34 1.42 (-8 -4) 19.9 16.2 -0.208 0.097 0.81 0.18 17.82 (-3 0) 29.3 19.2 -0.426 0.16 0.66 0.14 14.36 (1 3) 41.0 51.2 0.226 0.263 1.25 0.27 27.37 (4 11) 8.3 12.9 0.450 0.417 1.56 0.34 34.12 total 100.0 100.0 1.00 4.57 1.00 100.00 sti 0 10 99.1 99.2 0.001 0.061 1.00 0.49 48.61 2.62 11 -50 0.76 0.80 0.057 0.097 1.06 0.51 51.39 51 100 0.1 0.0 0.000 0.16 0.00 0.00 0.00 101 200 0.1 0.0 0.000 0.263 0.00 0.00 0.00 201 250 0.0 0.0 0.000 0.417 0.00 0.00 0.00 total 100.0 100.0 1.00 2.06 1.00 100.00 twi (2 5) 43.7 49.3 0.121 0.065 1.13 0.31 30.86 1.33 (6 7) 37.0 37.2 0.007 0.099 1.01 0.28 27.57 (8 10) 11.9 10.3 -0.143 0.162 0.87 0.24 23.77 (11 15) 6.2 3.0 -0.754 0.267 0.47 0.13 12.96 (16 25) 1.1 0.2 -1.743 0.405 0.18 0.05 4.84 total 100.0 100.0 1.00 3.65 1.00 100.00 (continued) 340 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan domain class % class pixel % landslide pixel weight of evidence (woe) analytical hierarchy process (ahp) weightage frequency ratio (fr) relative frequency (rf) rf (non %) pr rainfall 1,296 1,298 1.4 0.0 0.000 0.061 0.00 0.00 0.00 2.10 1,299 1,310 13.8 10.5 -0.272 0.097 0.76 0.20 19.98 1,311 1,311 29.0 45.7 0.461 0.16 1.58 0.41 41.20 1,312 1,313 32.7 32.3 -0.012 0.263 0.99 0.26 25.84 1,314 1,314 23.1 11.5 -0.706 0.417 0.50 0.13 12.99 total 100.0 100.0 1.00 3.82 1.00 100.00 4. results landslide susceptibility based on frequency ratio method (fr): in ranitar belarang section, 20% of the total area fall in an unstable zone as shown in the final susceptibility map (figure 19), generated using thirteen influencing factor maps. the high susceptibility zone encompasses silipakha, khecha, kinuwa, beltar, lasune, byangtar, taprang, and ranitar. conversely, the remaining communities fall into either a low or extremely low vulnerable zone. of the total study area, 60% is categorized as stable (banthuwa, rangcha, rato gharghare, baguwatar, lapha, ghurguredanda, gharegaun, majhakhani, kharbani, bokhimtar, jogepathar, magarmukha, baksikhop, pipalbote, toyampa, hangdang, salghari, namkesa, khawatang, mukure, parkuwa, mainatar, dankha, chuwabote, wachen, bilimla, harkha, khamare, tinpune, anmarang, hangmachong, kokama, mulgaun, mangteng, kolahong, khundi, belarang, barahaksetra, and chaudanda), while 20% is quasi stable, and 20% of the study area is unstable. this distribution is further detailed in figure 21 and table 6. table 4. weight of individual factor domain/factor pr distance to thrust 1.00 distance to stream 1.06 aspect 1.16 relief 1.27 twi 1.33 geology 1.34 spi 1.42 distance to road 1.66 curvature 1.70 slope 1.96 rainfall 2.10 sti 2.62 landuse 3.98 0 100 200 300 400 500 0 1 2 3 4 5 figure 18. graph showing weight of individual factor figure 19. landslide susceptibility index map of the study area using fr method. 341a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) equation (2) was used to create the landslide susceptibility index (lsi). to categorize the lsi data, we applied the receiver operating characteristic/ area under the curve (roc/auc) curve. the lsi, representing the likelihood of landslides, was qualitatively examined using success rate curves in ibmspss statistics 20. this helped us evaluate how well the proposed frequency approach predicts potential landslide zones. in figure 20, the roc/auc analysis yielded a measurement of 0.864, signifying a prediction accuracy of 86.4% with an upper limit of 87.2%. this means our prediction is reliable. regarding landslide potential, the success rate revealed that in 20% of the study area, a high rank is assigned, explaining 78% of all landslides in that region. similarly, 40% of the suggested landslide susceptibility index (lsi) values can clarify around 88% of all existing landslides. as a result, three landslide susceptibility classes were established as: stable (greater than 40%), quasi stable (20-40%), and unstable (0-20%). we classified the lsi into these zones with threshold values for the respective classes set at 279.32, 302.78, and 479.07, as outlined in table 5. these thresholds were employed to create the final landslide susceptibility map (figure 21). landslide susceptibility based on analytical hierarchy process (ahp): the final susceptibility map (figure 23), generated using thirteen influencing factor maps, designates ranitar belarang section’s 25.94% of total area as an unstable zone. the high susceptibility zone encompasses khundi, harkha, baksikhop, khecha, beltar, bilimla, baguwatar, and ratogargare. of the total study area, 28.62% is categorized as stable (silipakha, chaudanda, wachen, tinpune, anmarang, khamare, hangmachong, mangteng, mulgaun, barigaun, rusten, nakati, kolahong, belarang, dhakmalung, barahaksetra, kharbani, pipalbote, magarmukha, khawatang, bokhimtar, toyampa, kharbani, chuwabote, kinuwa, mainatar, majhakhani, mukure, dankha, jogepathar, namkesa, gharegaun, dhuskot, ghurghuredanda, lapha, hangdang, salghari, deurali, lasune, ranitar, and banthuwa), while 45.43% of the study area including the settlement area which includes kokama, delukha, hopchhopla, parkuwa, rangcha, byangtar, lasune, and taprang is categorized as quasi stable. this distribution is further detailed in figure 23 and table 6. figure 20. roc/auc curve of ranitar belarang lsi model using fr method figure 21. landslide susceptibility map of the study area using fr method 342 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan following the approach of table 1, weightage of each class of thirteen parameters was obtained (table 3) and was employed to construct the landslide susceptibility index (lsi) using ahp method. in figure 22, the roc/auc analysis showed a measurement of 0.685, indicating a prediction accuracy of 68.5% with an upper limit of 69.5%. this analysis is considered valid. regarding landslide potential, the success rate revealed that in 20% of the study area, a high rank is assigned, explaining 45% of all landslides in that region. similarly, 40% of the suggested landslide susceptibility index (lsi) values can clarify around 67.5% of all existing landslides. consequently, we established three landslide susceptibility classes: stable zone (60%), quasi stable (80-60%), and unstable (90-100%). the lsi was classified into these zones with threshold values for the respective classes set at 2.28, 2.72, and 4.16, as outlined in table 5. these thresholds were used to create the final landslide susceptibility map (figure 23). landslide susceptibility based on weight of evidence (woe): the final susceptibility map (figure 25), generated using thirteen influencing factor maps in ranitar belarang section shows 19.87% of total area as unstable zone. the high susceptibility zone encompasses nakati. major regions in between delukha and dhakmalung village, and western region of the study area (north of sunkoshi nadi). of the total study area, 60.19% is categorized as stable (khundi, chaudanda, wachen, tinpune, anmarang, khamare, hangmachong, mangteng, mulgaun, barigaun, rusten, kokama, harkha, kolahong, belarang, dhakmalung, hopchhopla, barahaksetra, kharbani, pipalbote, magarmukha, khawatang, parkuwa, bokhimtar, toyampa, kharbani, chuwabote, mainatar, majhakhani, mukure, dankha, jogepathar, namkesa, baksikhop, gharegaun, dhuskot, ghurghuredanda, beltar, lapha, hangdang, salghari, lasune, bilimla, baguwatar, ratogargare, rangcha, ranitar, taprang, and banthuwa), while 19.87% is quasi stable (settlements: silipakha, delukha, kinuwa, khecha, deurali, byangtar, and lasune). this distribution is further detailed in figure 25 and table 6. equation (7 & 8) was utilized to create the landslide susceptibility index (lsi). the receiver operating characteristic/area under the curve (roc/ auc) curve was employed to categorize the lsi data. qualitative analysis of the lsi (landslide susceptibility index) was conducted using success rate curves in ibm-spss statistics 20, aiming to evaluate the effectiveness of the proposed frequency approach in identifying potential landslide zones. in the provided diagram (figure 24), the area under the success rate curve (roc/auc) measures at 0.899, indicating a prediction rate of 89.9% with an upper boundary of 90.6%. this analysis is considered valid. regarding landslide potential, the success rate reveals that in 20% of the study area, a high rank is assigned, explaining 80% of all landslides in that region. similarly, 40% of the suggested landslide susceptibility index (lsi) values can clarify around 91% of all existing landslides. as a result, three landslide susceptibility classes were defined: stable zone (60%), quasi stable (60-80%), and unstable (80 -100%). the lsi was categorized into these zones with threshold values for the respective classes set at -1.27, -0.26, and 8.6 as detailed in table 5. these thresholds were used to generate the final landslide susceptibility map (figure 25). figure 22. roc/auc curve of ranitar belarang lsi model using ahp method. a) b) figure 23. (a) landslide susceptibility index and (b) landslide susceptibility map based on ahp method. 343a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) figure 24. roc/auc curve of ranitar belarang lsi model using woe method. a) b) figure 25. (a) landslide susceptibility index and (b) landslide susceptibility map based on woe method. table 5. boundary value set for different susceptibility classes method of analysis cumulative % class name lsi upper bound frequency ratio method (fr) 60% stable zone 279.32 80% quasi stable zone 302.78 100% unstable zone 479.07 analytical hierarchy process (ahp) 60% stable zone 2.28 80% quasi stable zone 2.72 100% unstable zone 4.16 weight of evidence (woe) 60% stable zone -1.27 80% quasi stable zone -0.26 100% unstable zone 8.6 344 kabi raj paudyal, rupendra maharjan, birat shrestha, and neelam maharjan table 6. area coverage by different landslide susceptibility class. class count area (sq. km) area % classification method landslide susceptibility (frequency ratio method) roc/auc curve stable 133018 53.2 60.01% quasi stable 44344 17.73 20.00% unstable 44347 17.73 20.00% total 221709 88.66 100.00% landslide susceptibility (analytical hierarchy process) roc/auc curve stable 63470 25.38 28.62% quasi stable 100717 40.28 45.43% unstable 57522 23 25.94% total 221709 88.66 100% landslide susceptibility (weight of evidence) roc/auc curve & woe stable 133439 53.37 60.19% quasi stable 44072 17.62 19.87% unstable 44198 17.67 19.87% total 221709 88.66 100% discussions: this study aimed to evaluate accuracy of fr, ahp, and woe methods for predicting landslide susceptibility in the ranitar-belarang area of udayapur district. thirteen causative factors; geology, slope, aspect, curvature, distance from the stream, distance from the road, distance from the thrust (mbt), relief, land use land cover, rainfall, topographic wetness index, stream power index and sediment transport index were considered. the selection of contributing factors was based on the presence or absence of factors and their importance. the selection of analysis methods was based on widely accepted literatures ((bhandari et al., 2024; demir et al., 2013; ding et al., 2017; pokhrel & bhandari, 2019; thapa & bhandari, 2019) and their effectiveness on predicting landslide susceptibility. both the frequency ratio (fr) and weight of evidence (woe) models have their merits in landslide susceptibility mapping. the fr model is known for its simplicity and ease of interpretation, making it a popular choice among researchers. on the other hand, the woe model has demonstrated superior accuracy and user-friendliness in predicting landslide likelihood. ahp helps decision makers to prioritize factors and assign weights based on their relative importance in landslide susceptibility assessment. it provides a structured approach to combine subjective judgments and objective data ((demir et al., 2013). the utilization of digital elevation model (dem) proves to be crucial in landslide studies, as evidenced by research (kamp et al., 2008; wang et al., 2015). although there is no direct relationship between elevation and landslide occurrence, studies suggest an increased probability of landslide events at higher elevations (ercanoglu et al., 2004). the concentration of landslides was found to be maximum in the elevation range of 500 m-750 m. the steepness of the slope is another significant topographic factor considered in landslide susceptibility studies (kamp et al., 2008; pradhan & lee, 2010; regmi et al., 2016; regmi et al., 2010; wang et al., 2015). among the slope, ranging from 0°-75°, it was revealed maximum landslide concentration in the slope range of 30°-50°. the aspect of the slope, influencing moisture retention and its relation to the attitude of bedding of the rock formation, plays a role in affecting the physical properties of slope material and its susceptibility to failure (dai et al., 2001). in this study, aspects to the south, southwest, and southeast contribute significantly to landslides. this is often attributed to the fact that many rivers’ segments trend towards sw-se, and landslides appear on slopes facing the river. the surface undulation of the slope is identified as a major factor triggering landslides, as it strongly influences slope instability. in the case of curvatures, landslides are typically distributed in convex and concave slopes. convex slopes, in particular, are noted for earthquake-induced landslides (reneau & dietrich, 1987). in the study area, concave slopes dominate over convex slopes by 1.5%. land-use is recognized as a significant landslide conditioning factor due to its potential to influence vegetation cover, thereby affecting mechanical factors such as soil strength and slope behavior, as well as hydrological aspects (greenway, 1987; reichenbach et al., 2014; van westen et al., 2003). the variation in land-use distribution can be attributed to natural processes, human activities, or a combination of both. in this study, the land-use map was generated through manual digitization of a satellite image (google earth image) of the study area. it was found that the landslide occurred mostly on the barren land. the derived weights, obtained through the frequency ratio method, analytical hierarchy process, and weight of evidence highlight the significant impact of terrain slope on landslide distribution. generally, landslide occurrences increase with an increase in terrain gradient. this observation is reinforced by the analysis, where the slope gradient of 50o – 60o has the highest frequency ratio (fr) value of 3.63, woe of 1.319 and ahp of 0.241 making it the most prone to landslides. in contrast, terrain with a slope gradient of <10o is least prone to landslides, with a frequency ratio of 0.17 (table 3). the vicinity in the range of 0 to 250 m distance from the road covers most of the study area (37.11 % of the study area) and covers 21.1% of total landslide. however, most of the landslide (46.3%) falls under the area ranging distance of 500 1000 m from the road. the associated values for woe, ahp and fr for 500-1000 class are 0.518, 0.16, and 1.67 respectively. while the area within 0-100 m from the streams has a frequency ratio of 1.01, woe of 0.009 and ahp of 0.466, the highest fr and woe value is observed at the distance of 200 – 400 m. this discrepancy may be attributed to other parameters such as thrust, land-use, slope, and curvature playing a more influential role in triggering landslides. regarding land use land cover, barren land has the highest frequency ratio of 32.69, woe of 3.93, and ahp of 0.328 followed by bushes with an fr of 7.48, woe of 2.088, and ahp of 0.231. the fr, woe, and ahp weight values assigned to barren land and bush land, are identified as stronger than those for other land-use classes. this underscores the significance of barren land and bush land in contributing to landslide susceptibility. the analysis in table 3 indicates 345a comparative of frequency ratio method, weight of evidence, and analytical hierarchy process for landslide susceptibility assessment in the main boundary thrust (mbt) that concave curvature has a high frequency of landslide occurrence, with a value of 1.20, woe of 0.184, and ahp of 0.539. additionally, it was observed that most landslides occur in the siwalik zone, with a frequency ratio of 0.97, with woe of -0.027 which represents negative relationship with the landslide, and ahp of 0.417 (neupane and paudyal, 2021). however, the gondwana group has the highest frequency ratio of 2.14 among all other geology present in the area, woe of 0.773, and ahp of 0.16. this is because the gondwana group, covering 8.7% of the total area, has landslides covering 18.7% of the total landslide area in the study area. the zone closest to the thrust (mbt) has the largest number of landslides, with a frequency ratio of 1.51, woe of 0.42, and ahp of 0.147 indicating that the 0-500 m distance is the most prone to landslides. the similar type of results was obtained in the previous studies in the nepal himalaya in different sections (paudyal and maharjan, 2022 & 2023; neupane et al, 2023; shahi et al, 2022). as a prime cause of landslide, in nepal himalaya, is found the presence of adverse geological structures like the active faults and thrust (acharya and paudyal, 2023; paudyal et al., 2021; paudyal et al., 2024) in addition to its rugged topography, steep slope and haphazard drainage at and around the river valleys (dahal and paudyal, 2022; k.c. et al, 2018). the differences in influential factors across models can be attributed to the distinct methodologies and assumptions underlying each approach. the fr model is a statistical method that relies on the frequency of landslide occurrences, while the woe model is based on bayesian probability theory. the ahp model, on the other hand, is a multi-criteria decision analysis technique that incorporates subjective judgments and expert opinions. all three models agree that slope is a significant factor in landslide susceptibility, with the highest frequency ratio (fr) value of 3.63, woe of 1.319, and ahp of 0.241 observed in the 50°-60° slope range. barren land and bush land are identified as significant contributors to landslide susceptibility in all models, with high frequency ratios, woe values, and ahp weights. the proximity to the thrust (mbt) is a common influential factor, with the 0-500 m distance range being the most prone to landslides in all models. in addition to slope and land-use, the fr model highlights the importance of curvature, with concave slopes having a higher frequency ratio. distance from the road is also a significant factor, with the 500-1000 m range being the most prone to landslides. the woe model places more emphasis on the role of geology, with the gondwana group having the highest frequency ratio among all geological formations. the woe model also suggests that the aspect of the slope is a significant factor, with south, southwest, and southeast aspects contributing more to landslides. each model’s strengths and weaknesses are reflected in the factors they emphasize. for instance, the fr model’s focus on curvature and distance from the road might be due to its ability to capture spatial relationships and patterns. the woe model’s emphasis on geology and aspect could be attributed to its capacity to handle categorical data and non-linear relationships. the ahp model’s consideration of relief, rainfall, and topographic wetness index might be a result of its ability to incorporate expert knowledge and subjective judgments. bhandari et al. (2024) conducted several studies in the siwalik hills of nepal using frequency ratio (fr), weights of evidence (woe), information value (iv), and shanon entropy (se) models to assess landslide susceptibility. their findings demonstrated the effectiveness of the woe model with high success (85%) and prediction rates (79.9%). as per the study conducted by regmi et al. (2014), a success rate of 76.8 % and predictive accuracy of 75.4 % performed better than woe (success rate, 75.6 %; predictive accuracy, 74.9 %). however in the present study, the weight of evidence (woe) has the highest accuracy (89.9%) in predicting landslides followed by frequency ratio (86.4%). while the fr model generally outperformed other models in previous studies, the woe model showed comparable or even superior results in specific regions, such as the study area. the overall performance of the models varied depending on the study area and specific factors considered. overall, these studies highlight the importance of carefully selecting and evaluating landslide susceptibility models based on the unique characteristics of each region and the specific research objectives. the resultant susceptibility maps can be useful for general land use planning. conclusions the present study aimed to evaluate the accuracy of three landslide susceptibility assessment methods, namely frequency ratio (fr), analytical hierarchy process (ahp), and weight of evidence (woe), in the ranitarbelarang section of udayapur district, nepal. the causes of landslides are often complex and not fully understood. through field studies, aerial photography, and geological studies, thirteen factors (geology, slope, aspect, curvature, distance from the stream, distance from the road, distance from the thrust (mbt), relief, land use land cover, rainfall, topographic wetness index, stream power index and sediment transport index) were identified as significant contributors to landslides in the region. these factors were selected based on their relevance, availability, and scale of data. using the modern gis technology, it was easier to process large amount of data and prepare landslide susceptibility maps. in this study, the frequency ratio model, a statistical method, along with weight of evidence, and analytical hierarchy process was employed to assess the spatial probability of landslide occurrence. a total of 120 landslides were identified, with notable influences from factors such as distance from the thrust (mbt), land-use, and slope. our results demonstrate the superiority of the woe method in predicting landslide susceptibility, with a high accuracy of 89.9% compared to fr (86.4%) and ahp (68.5%). this finding is consistent with previous studies that have highlighted the effectiveness of the woe method in landslide susceptibility assessment. the high roc/auc value of 89.9% for the weight of evidence method suggests that it performed exceptionally well in predicting landslide susceptibility. this indicates a robust correlation between the input factors and the occurrence of landslides, making the woe method a reliable and accurate approach for susceptibility mapping. on the other hand, the analytical hierarchy process exhibited a roc/ auc value of 68.5%, suggesting a moderate level of accuracy in predicting landslide susceptibility. while not as high as the woe method, the ahp still demonstrates a meaningful capability to assess landslide-prone areas. and the frequency ratio method yielded an roc/auc value of 86.4%, indicating a high level of accuracy like the woe method. the combination of methods allows for cross-validation, enhancing the overall reliability of the susceptibility map. therefore, integrating weight of evidence, analytical hierarchy process, and frequency ratio methods provides a holistic and reliable approach to landslide susceptibility mapping, contributing to more informed decision-making in landslide-prone areas. the landslide susceptibility maps created in this study provide a clear assessment of landslide risk. these maps are valuable resources for citizens, planners, and engineers, aiding in land management and site selection for future development projects. by identifying areas prone to landslides, these maps can help prevent, mitigate, and avoid potential hazards. for instance, areas with high landslide susceptibility can be avoided for development, while areas with low susceptibility can be prioritized for infrastructure development. references: abusarhan, z. 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(2024). air pollution as a potential risk factor for covid-19 spread: a case study of italian provincial capitals. earth sciences research journal, 28(4), 461-470. https://doi. org/10.15446/esrj.v28n4.114296 record manuscript received: 07/05/2024 accepted for publication: 02/12/2024 abstract this study investigates the potential association between air pollution and the spread of coronavirus disease 2019 (covid-19). employing italy as a case study, the research explores the diffusion patterns of covid-19 within italian provincial capitals. findings suggest a correlation between elevated levels of air pollution and the number of confirmed cases. cities exceeding air quality limits for pm10 or ozone for more than 100 days annually exhibited significantly higher caseloads compared to those with lower pollution levels. additionally, the analysis reveals that hinterland locations (inland areas) with both high air pollution and low wind speed displayed a particularly pronounced increase in infection rates. notably, over 75% of confirmed covid-19 cases and approximately 81% of related deaths in italy occurred in regions with high air pollution levels. these findings underscore the potential role of environmental factors in covid-19 transmission and highlight the necessity for long-term public health strategies that integrate environmental and sustainability policies alongside medical interventions. keywords: air pollution, covid-19, italy, public health, environmental factors, sustainable growth, environmental science. palabras clave: contaminación del aire; covid-19; italia; salud pública; factores ambientales; crecimiento sostenible; ciencias ambientales. la contaminación del aire como un factor de riesgo potencial para la propagación de covid-19: un estudio de caso de las capitales provinciales italianas este estudio investiga la posible asociación entre la contaminación del aire y la propagación de la enfermedad por coronavirus 2019 (covid-19). tomando a italia como caso de estudio, la investigación explora los patrones de difusión de covid-19 en las capitales provinciales italianas. los hallazgos sugieren una correlación entre los niveles elevados de contaminación del aire y el número de casos confirmados. las ciudades que superan los límites de calidad del aire para pm10 u ozono durante más de 100 días al año exhibieron cargas de casos significativamente más altas en comparación con aquellas con niveles de contaminación más bajos. además, el análisis revela que las ubicaciones del interior (áreas del interior) con alta contaminación del aire y baja velocidad del viento mostraron un aumento particularmente pronunciado en las tasas de infección. en particular, más del 75% de los casos confirmados de covid-19 y aproximadamente el 81% de las muertes relacionadas en italia ocurrieron en regiones con altos niveles de contaminación del aire. estos hallazgos subrayan el papel potencial de los factores ambientales en la transmisión de covid-19 y resaltan la necesidad de estrategias de salud pública a largo plazo que integren políticas ambientales y de sostenibilidad junto con intervenciones médicas. resumen https://doi.org/10.15446/esrj.v28n4.114296 https://orcid.org/0000-0002-7741-8961 https://orcid.org/0000-0003-1957-6731 mailto:bilalkargi@aybu.edu.tr https://doi.org/10.15446/esrj.v28n4.113518 https://doi.org/10.15446/esrj.v28n4.114296 462 bilal kargı, mario coccia 1. introduction the emergence of covid-19, a novel coronavirus unleashing a devastating respiratory illness with the potential to damage multiple organs, has sent shockwaves across the globe, posing an unprecedented threat to public health security (wang et al., 2020). the virus’s rapid transmission and multifaceted complications have overwhelmed healthcare systems, ignited widespread fear and uncertainty, and exposed deep vulnerabilities in our interconnected world (gattinoni et al., 2020; sterpetti, 2020; ogen, 2020;). beyond the immediate health crisis, covid-19 has triggered a cascade of socioeconomic disruptions, threatening livelihoods, stalling economies, and amplifying pre-existing inequalities (wang & su, 2020). the economist intelligence unit (eiu, 2020) predicts a global economic recession exceeding 2%, with some countries experiencing a decline in real gdp growth exceeding a staggering 8% in 2020 alone. this daunting challenge underscores the urgent need for a multifaceted response that tackles covid-19 not only through immediate medical intervention but also with a forward-looking lens on environmental factors that may influence its spread. this research delves into the potential link between air pollution and covid-19 transmission, aiming to inform environmental policy development and preparedness strategies for future pandemics. italy, an early epicenter of the covid-19 outbreak, serves as a poignant case study for examining the interplay between air quality and viral transmission (coccia, 2020). the country witnessed a rapid and alarming rise in infections and fatalities, particularly in the northern regions. this research investigates how geo-environmental factors, specifically those related to air quality, may have played a role in the virus’s spread within these highly impacted areas. the study meticulously analyzes relationships between confirmed covid-19 cases, environmental data—including air pollution levels, temperature, and humidity—demographics, and geographical characteristics. a crucial focus lies on establishing correlations between air quality metrics and the severity of the outbreak. building upon the groundwork laid by coccia (2020, 2020a-f), kargı (2023, 2023a), and lai et al. (2020), the research explores how factors such as low wind speed, high humidity, and frequent episodes of air pollution exceeding safe ozone or particulate matter levels may have correlated with higher numbers of covid-19 cases and deaths. these findings paint a compelling picture, suggesting a strong correlation between air pollution and the severity of covid-19 outbreaks. this new understanding underscores the importance of integrating air quality considerations into future pandemic preparedness plans. by proactively mitigating air pollution in regions with high levels of contamination, we may equip ourselves with a more robust defense against the emergence and spread of novel infectious diseases. the covid-19 crisis serves as a stark reminder of the profound interconnectedness between human health, environmental conditions, and socioeconomic stability—for example, how air pollution exacerbates respiratory illnesses and strains healthcare systems. effective policy design to combat future pandemics must encompass not only biological and medical aspects but also the complex interplay between these domains. by prioritizing air pollution reduction in polluted regions alongside robust healthcare systems and strong social safety nets, we can build a more resilient future, one that is better prepared to weather the storms of emerging infectious diseases. 2. literature review numerous studies have explored the connection between air pollution and health, particularly concerning respiratory infections and their outcomes. air pollutants like pm2.5, pm10, nitrogen dioxide (no₂), and ozone have been linked to a range of respiratory illnesses, and these pollutants can increase susceptibility to infections such as covid-19 (zhu et al., 2020). according to frontera et al. (2020), air pollution not only exacerbates existing respiratory and cardiovascular diseases but may also enhance the transmission and severity of infectious diseases due to its impact on respiratory health. during the covid-19 pandemic, research focused on the impact of air pollution on viral spread. coccia (2020) found that areas with poor air quality— especially in northern italy—had higher rates of infection and mortality. similar findings were reported by kargı (2023), who noted that italian cities with high levels of air pollution recorded significantly more covid-19 cases. ogen (2020) examined no₂ levels across several european cities, identifying high no₂ concentrations as a contributing factor to covid-19 mortality, especially in regions with limited atmospheric circulation. other studies delved into potential mechanisms by which air pollution could amplify viral transmission. morawska & cao (2020) proposed that particulate matter might act as a carrier for viral particles, facilitating airborne spread. setti et al. (2020) detected sars-cov-2 rna on pm samples in northern italy, reinforcing the hypothesis that airborne transmission could be more pronounced in areas with high pollution. these findings align with bontempi (2020), who also suggested that air pollution might increase the risk of covid-19 spread by extending the viability of viral particles in the atmosphere. historical studies on other pandemics provide context for these findings. cui et al. (2020) observed similar correlations between air pollution and mortality during the 2003 sars outbreak, suggesting that polluted urban areas might face elevated risks during respiratory virus outbreaks. this historical evidence underscores the idea that air pollution could amplify disease transmission and severity, making urban areas with high pollution levels more vulnerable to pandemics (domingo & rovira, 2020). meteorological factors, including temperature, humidity, and wind speed, also interact with pollution levels to influence viral transmission. research by gu et al. (2020) found that low temperatures and stagnant air conditions allow pollutants to linger, increasing the risk of exposure to airborne pathogens. likewise, muhammad et al. (2020) noted that regions with low average wind speeds and frequent pollution events saw higher covid-19 caseloads, with similar findings observed by wang & su (2020) in china. research has also highlighted the broader health impacts of long-term air pollution exposure, which may increase vulnerability to severe covid-19 outcomes. bashir et al. (2020) and wang et al. (2020) found that chronic exposure to pm2.5 can weaken immune defenses, thus increasing the risk of severe disease if infected with covid-19. additionally, filippini et al. (2020) emphasized that people in densely populated and heavily polluted urban areas face a compounded risk due to both higher levels of exposure to pollutants and increased human contact rates. several studies advocate for comprehensive environmental and public health policies that address air pollution as a means to reduce infectious disease transmission (contini & costabile, 2020; gatto et al., 2020). for instance, coccia (2018, 2019) calls for measures such as emission reductions, improved urban ventilation, and seasonal controls on pollutant levels to mitigate the impact of air pollution on respiratory disease transmission. gu et al. (2020) further suggest that sustainable urban planning—particularly measures to reduce air stagnation in high-density areas—could decrease pollution levels and reduce the risk of viral spread. recent studies have emphasized the critical role of environmental pollutants, particularly in urban areas, on respiratory health and susceptibility to infectious diseases, including covid-19. zhang & wu (2022) analyzed urban air quality trends across southeast asia, using remote sensing to detect fine particulate matter and other harmful pollutants. their findings indicated that densely populated urban areas, with frequent air quality violations, exhibit higher health risks related to respiratory complications. this aligns with broader observations that high pollution levels in urban centers exacerbate respiratory conditions, which can increase vulnerability to airborne diseases like covid-19. meteorological conditions, such as temperature and wind patterns, also significantly influence pollutant dispersion and subsequent respiratory health outcomes. kim & park (2021) investigated these variables’ impact on pollution distribution in urban settings, highlighting that low wind speeds contribute to pollutant accumulation. this phenomenon, particularly in areas with high particulate matter (pm) concentrations, aligns with italian covid-19 case studies, where low wind speeds were associated with higher transmission rates (gupta & verma, 2022). additionally, the presence of heavy metals in airborne particles in densely populated urban regions may compound health risks, as illustrated in mo, tian, and shen’s (2021) analysis of pollution sources affecting water and soil quality. geospatial analysis offers insight into the spatial distribution of ambient air pollutants and health risks in urban areas. yang & shen (2022) used gis tools to assess pollutant risk distribution across east asia, revealing elevated health risks in areas with dense populations and higher industrial activity. this approach underscores the value of spatial risk assessments for urban air quality, 463air pollution as a potential risk factor for covid-19 spread: a case study of italian provincial capitals which could inform targeted interventions in high-risk regions to mitigate pollution-related health risks, including potential viral transmission facilitations. the covid-19 pandemic has thus emphasized the importance of considering environmental factors in public health preparedness. environmental quality may not only affect disease outcomes but also play a substantial role in disease transmission dynamics (sterpetti, 2020). addressing air pollution is therefore an integral part of a holistic pandemic response, particularly in high-pollution regions (gattinoni et al., 2020). as illustrated by wang and zhu (2020), long-term policies aimed at improving air quality could have farreaching benefits in reducing both chronic health conditions and the risk of future outbreaks, highlighting the need for sustainable approaches to urban and environmental planning. 3. study design and data 3.1. data and their sources this case study investigates the association between air pollution and covid-19 transmission in italy, a nation experiencing a rapid rise in infections and fatalities (coccia, 2020). the analysis focuses on 55 provincial capital cities. data on confirmed covid-19 cases and deaths in march and april 2020 were obtained from the ministry of health (ministero della salute, 2020). air pollution data for 2018, serving as a pre-pandemic baseline, was collected from regional environmental protection agencies (legambiente, 2019). meteorological information (average temperature and wind speed) for february-april 2020 was sourced from provincial weather stations (il meteo, 2020). population density data for 2019, a proxy for interpersonal contact rates, was retrieved from the istat (italian national institute of statistics, 2020). 3.2. data analysis methods * descriptive statistics: a comparative approach using means and standard deviations will be employed to analyze the data across groups defined by the following classification criteria: air pollution: • high pollution: exceeding 100 days per year with pm10 or ozone levels above established limits. • low pollution: less than 100 days exceeding pollution limits. • population density: • high density: exceeding 1000 inhabitants per square kilometer. • low density: less than 1000 inhabitants per square kilometer. * correlation analysis: bivariate and partial correlations will be used to assess relationships between the studied variables. * regression analysis: we will use both simple and multivariate regression models to investigate the connections between covid-19 results, population density, weather, and air pollution. specifically, log-log models of simple regression serve as the foundation for the specification of linear relationships: log yt = a + b log xt-1 + u (1) y = total number of affected people in cities x = a measurement of air pollution (total days over pm10 or ozone in city limits) building upon the initial analysis (eq. [1]), this study further investigates the relationships between air pollution, population density, and covid-19 outcomes using a multiple regression model with a log-log transformation. this transformation adopts a linear relationship on the logarithmic scale, allowing for the exploration of potential multiplicative effects between the independent variables (air pollution and population density) on the dependent variables (covid-19 cases and deaths). log yt = a + b1 log x1, t -1+ b2 log x2, t-1+ u (2) y = total number of affected people in cities x1 = a measurement of the pollution in the air, x2 = population density of cities, inhabitants / km2 this study employs ols (ordinary least squares) regression, a common technique for estimating parameters in linear models (references omitted), to investigate the impact of air pollution on covid-19 outcomes (total infected and deaths) in italian regions. recognizing the potential bias introduced by population discrepancies between high and low air pollution regions, the analysis incorporates weighted percentages. these weights account for population size within each region, ensuring a more robust assessment of the spatial influence of covid-19 on public health. the statistical analyses are performed using spss version 24, the statistical package for social sciences. air pollution variables: air pollution was assessed using 2018 data as a pre-pandemic baseline, sourced from regional environmental protection agencies. the primary metric for air pollution was the total number of days exceeding established limits for pm10 or ozone levels in a year. for analytical purposes, cities were classified into two categories: high pollution, defined as exceeding 100 days per year above the limits, and low pollution, defined as fewer than 100 days. this classification reflects long-term air quality exposure, which may influence respiratory health and susceptibility to covid-19. population density variables: population density, used as a proxy for interpersonal contact rates, was derived from 2019 istat data. this variable measures the number of inhabitants per square kilometer in each provincial capital city. cities were categorized as either high density (more than 1000 inhabitants/km²) or low density (fewer than 1000 inhabitants/km²). this categorization facilitates the investigation of how varying levels of urban crowding affect covid-19 transmission and mortality rates. covid-19 outcome variables: the dependent variables in the analysis are the total number of confirmed covid-19 cases and deaths recorded during march and april 2020. data were obtained from the italian ministry of health. these metrics represent the direct health impact of the pandemic in the selected cities and serve as key indicators to assess associations with environmental and demographic factors. meteorological variables: meteorological data were sourced from provincial weather stations for february through april 2020. the variables include average temperature (measured in degrees celsius) and wind speed (measured in meters per second). these factors were included to account for potential environmental influences on virus transmission, given prior evidence linking climatic conditions to respiratory disease patterns. 4. results in the provincial capitals of italy, first descriptive studies (table 1) point to a possible correlation between air pollution and covid-19 cases. there seem to be more confirmed illnesses in cities with low average wind speeds, lower temperatures, and high air pollution levels (over pm10 or ozone limits for over 100 days annually). table 1 provides a comparative analysis of air pollution levels and covid-19 outcomes between cities with high and low air pollution in italian provincial capitals. cities with high pollution levels (mean 125.26 days above pm10 or ozone thresholds in 2018) demonstrate significantly higher mean covid-19 infections across all dates analyzed. on april 27, 2020, these cities averaged 4,838.06 infections, compared to 1,637.21 infections in low-pollution cities. additionally, high-pollution cities have higher population densities (mean 1,981.41 inhabitants/km²) and experience slightly lower average temperatures (mean 9.19°c vs. 9.49°c). wind speeds are also lower in high-pollution areas (mean 7.68 km/h), which may limit air circulation and contribute to pollutant accumulation and virus transmission. these findings underscore the role of environmental and demographic factors in shaping pandemic outcomes. table 2 examines differences in covid-19 outcomes between high-density and low-density cities. high-density cities (mean 2,584.41 inhabitants/km²) show a higher average number of infections as of april 27, 2020 (4,195.43 cases) compared to low-density cities (1,727.56 cases). furthermore, high-density cities also report slightly lower average temperatures (8.64°c vs. 10.02°c) and reduced wind speeds (7.98 km/h vs. 9.28 km/h), factors potentially linked to greater viral persistence and transmission. these results highlight the compounded influence of population density and environmental conditions on the spread of covid-19, with densely populated urban areas exhibiting a heightened vulnerability to large-scale outbreaks. 464 bilal kargı, mario coccia table 1. descriptive statistics of air pollution metrics in italian provincial capitals days over 2018 pm10 or ozone thresholds infected persons march 17, 2020 infected persons april 17, 2020 infected persons april 27 2020 density inhabitants/km2 2019 temp °c feb-mar 2020 wind km/h feb-mar 2020 20 cities with high levels of air pollution mean 125.26 881.71 3650.01 4838.06 1981.41 9.19 7.68 std. deviation 13.41 1010.98 3238.83 4549.42 1988.68 1.47 2.87 35 cities with low levels of air pollution mean 48.77 184.11 1014.63 1637.21 1151.57 9.49 9.28 std. deviation 21.37 202.76 768.91 1292.26 1466.28 2.62 4.15 table 2. demography of italian provincial capitals: population density statistics days over 2018 pm10 or ozone thresholds infected persons march 17, 2020 infected persons april 17, 2020 infected persons april 27, 2020 density inhabitants/km2 2019 temp °c feb-mar 2020 wind km/h feb-mar 2020 high-density cities: n=25 mean 91.25 665.09 2967.45 4195.43 2584.41 8.64 7.98 std. deviation 40.25 919.71 3092.47 4333.92 2000.64 2.41 2.78 low-density cities: n = 30 mean 64.38 248.38 1144.21 1727.56 510.76 10.02 9.28 std. deviation 39.26 386.96 1065.98 1491.46 282.12 1.96 4.41 furthermore, the analysis (table 1, not shown) indicates a possible link between population density and covid-19 cases. cities with higher average population density, particularly those bordering major urban areas (e.g., brescia, bergamo, cremona near milan in lombardy, italy), appeared to have a greater number of infections. notably, these cities often coincided with hinterland zones characterized by high air pollution (exceeding pm10 and ozone limits for many days), low wind speed, and lower temperatures, potentially amplifying the observed association. table 3. correlation n=55 log days over pm10 or ozone limits in 2018 2019 log density of population/km2 log the affected persons on march 17, 2020 pearson correlation .643** .484** log the affected persons april 7, 2020 pearson correlation .604** .533** log the affected persons april 27, 2020 pearson correlation .408** .308* note: *. at the 0.05 level, the correlation is significant (1-tailed); **. at the 0.01 level, the correlation is significant (1-tailed). table 3 presents the pearson correlation coefficients illustrating the relationships between two key independent variables—air pollution levels (log-transformed days exceeding pm10 or ozone thresholds in 2018) and population density (log-transformed inhabitants per km² in 2019)—and the logtransformed number of covid-19 cases on three dates in 2020. air pollution: air pollution consistently shows a strong positive correlation with covid-19 cases. for example, the pearson correlation between log air pollution and cases on march 17, 2020, is 0.643 (p < 0.01), decreasing slightly to 0.408 (p < 0.01) by april 27, 2020. this suggests that cities with higher pollution levels experienced a disproportionately higher number of cases, particularly in the early stages of the pandemic. population density: population density also demonstrates a significant positive correlation with covid-19 cases, though the relationship is generally weaker than that of air pollution. on march 17, 2020, the correlation is 0.484 (p < 0.01), rising slightly to 0.533 (p < 0.01) by april 7, 2020, before tapering to 0.308 (p < 0.05) by april 27, 2020. this indicates that densely populated areas initially experienced greater case numbers, but the influence diminished over time, possibly due to interventions or behavioral adaptations. the results highlight the importance of air quality and urban density in exacerbating the spread of covid-19, particularly during the pandemic’s initial phases. these findings support the hypothesis that environmental and demographic factors significantly contribute to infection dynamics. analysis of correlation coefficients (table 3, not shown) reveals a strong positive correlation between air pollution levels and the number of infected individuals. this association appears to weaken over time, potentially due to the implementation of quarantine and lockdown measures in italy from march to may 2020. these restrictions likely contributed to a significant reduction in air pollution within cities, potentially mitigating its influence on covid-19 transmission (conticini et al., 2020; contini & costabile, 2020; fattorini & regoli, 2020). this finding aligns with wang and su’s (2020) proposition that quarantine and lockdown strategies can not only protect public health from covid-19 but also generate long-term environmental benefits. table 4. partial correlation of air pollution with infections, adjusted for climate factors control variables log temp °c log wind km/h feb-mar 2020 pearson correlation log infected persons march 17, 2020 log infected persons april 7, 2020 log infected persons april 27, 2020 n=51 log days over pm10 or ozone limits in 2018 0.637*** 0.608*** 0.412*** note: ***. at the 0.001 level, the correlation is significant (1-tailed). table 4 examines the relationship between air pollution levels (logtransformed days exceeding pm10 or ozone thresholds in 2018) and covid-19 465air pollution as a potential risk factor for covid-19 spread: a case study of italian provincial capitals infections, controlling for climatic variables such as average temperature and wind speed during february-march 2020. the partial correlation coefficients reveal a persistent and significant positive association between air pollution and the number of infections across all dates analyzed. on march 17, 2020, the correlation is 0.637 (p < 0.001), indicating a robust link between higher pollution levels and increased infection counts. by april 7, 2020, this relationship remains strong, with a coefficient of 0.608 (p < 0.001). on april 27, 2020, the correlation diminishes slightly to 0.412 (p < 0.001), though it remains statistically significant. the adjustment for climatic factors (temperature and wind speed) ensures that the observed relationship is not confounded by these variables, which are known to influence virus transmission and persistence. these results highlight that air pollution is an independent and substantial contributor to the severity of covid-19 outbreaks, reinforcing the urgency of addressing long-term air quality issues to mitigate future public health risks. the association between covid-19 instances and air pollution is further supported by partial correlation analyses (tables 4 and 5, not shown). air pollution levels and the number of sick persons show a substantial positive partial association that holds even after adjusting for population density (table 5) and climate conditions (table 4). in layman’s words, these results imply that the burden of covid-19 infections and deaths was much higher in cities where air pollution exceeded permissible levels for particle matter or ozone. table 5. independent influence of air pollution on infections, accounting for population crowding control variables log density inhabitants/ km2 2019 pearson correlation log infected march 17, 2020 log infected april 7, 2020 log infected april 27, 2020 log days over pm10 or ozone limits in 2018 0.543*** 0.478*** 0.317* note: **. at the 0.01 level, the correlation is significant (1-tailed). ***. at the 0.001 level, the correlation is significant (1-tailed). table 5 assesses the independent effect of air pollution on covid-19 infections while controlling for population density (log-transformed density of inhabitants per km² in 2019). the table presents the pearson correlation coefficients between log-transformed days exceeding pm10 or ozone thresholds and covid-19 infections on three key dates in 2020. on march 17, 2020, the correlation between air pollution and infections is 0.543 (p < 0.001), indicating a strong positive relationship. by april 7, 2020, the correlation slightly decreases to 0.478 (p < 0.001), but still suggests a substantial effect of air pollution on the number of cases. on april 27, 2020, the correlation weakens further to 0.317 (p < 0.05), but remains statistically significant, suggesting that air pollution continues to influence infection rates even as other factors, such as population density, become more prominent over time. these results indicate that even after accounting for population density, air pollution has a significant and independent effect on the spread of covid-19. the decreasing strength of the correlation over time may reflect the interplay between environmental factors and the evolving dynamics of the pandemic, such as public health measures or behavioral changes. nonetheless, the findings underscore the important role of air quality in shaping infection outcomes across regions. table 6 presents the results of three linear regression models exploring the relationship between air pollution (log-transformed days exceeding pm10 limits in 2018) and interpersonal contact (log-transformed population density, inhabitants per km² in 2019) with covid-19 infections at three time points: march 17, april 7, and april 27, 2020. each model assesses how these factors influence infection rates while accounting for both predictors. model 1 (march 17, 2020): air pollution has a highly significant positive effect on the number of infections, with a coefficient of 1.266 (p < 0.001). this suggests that higher levels of air pollution are associated with a substantial increase in infections, particularly in the early stages of the pandemic. population density also plays a significant role, with a coefficient of 0.308 (p < 0.05), indicating that densely populated areas saw more infections. the model has a strong fit (r² = 0.458), indicating that these variables explain a large portion of the variance in infection rates. table 6. impact of air pollution and interpersonal contact on infections: a linear model model 1 -air pollution -interpersonal contacts model 2 -air pollution -interpersonal contacts model 3 -air pollution -interpersonal contacts log infected 17th march, 2020 log infected 7th april, 2020 log infected 27th april, 2020 constant a (st. err.) -2.168 (1.127) constant a (st. err.) 1.538 (.854) constant a (st. err.) 1.407 (1.701) log days exceeding limits set for pm10 in 2018 coefficient β1 (st. err.) 1.266*** (.272) coefficient β1 (st. err.) .813*** (.206) coefficient β1 (st. err.) .987* (.411) log density inhabitants /km2 in 2019 coefficient β2 (st. err.) .308* (.149) coefficient β2 (st. err.) .315** (.113) coefficient β2 (st. err.) .245 (.224) f 22.058***c f 21.131***c f 5.917**c r2 0.458 r2 .449 r2 .186 notes: * p-value<0.05; c= predictors: log days exceeding limits set for pm10 2018 year; log density inhabitants/km2 2019; ** p-value<0.01; *** p-value<0.01. 466 bilal kargı, mario coccia model 2 (april 7, 2020): the impact of air pollution remains significant (0.813, p < 0.001), though slightly weaker than in model 1. the coefficient for population density (0.315, p < 0.01) shows a somewhat stronger association with infection rates than in the first model. the model’s r² value is 0.449, indicating a good fit and suggesting that both air pollution and interpersonal contact continue to be important predictors of infection. model 3 (april 27, 2020): by this time, the correlation between air pollution and infections diminishes but remains statistically significant (0.987, p < 0.05). population density’s impact is no longer significant, as indicated by the smaller and non-significant coefficient (0.245), and the model fit drops (r² = 0.186), indicating a weaker relationship with the infection rate at this stage of the pandemic. these results suggest that air pollution plays a critical role in the initial spread of covid-19, with the influence of interpersonal contact (population density) becoming more prominent as the pandemic progresses. the decrease in model fit over time may reflect other evolving factors influencing infection dynamics, such as behavioral changes, lockdown measures, and increased awareness of preventive health practices. the regression analyses (tables 6 and 7, not shown) and visual representation (figure 1, not shown) reveal a critical role for air pollution in covid-19 transmission dynamics. prior to lockdown measures and the resulting decrease in air pollution (model 1), air pollution appears to have been a more significant factor in transmission compared to direct human-to-human contact. however, as air pollution levels dropped due to lockdown (model 3), the influence of air pollution on transmission diminished. these findings suggest that while human-to-human interaction remains the primary mode of covid-19 spread, air pollution can significantly amplify transmission rates, particularly in cities with low wind speed, high humidity, and frequent pollution episodes. potentially, this amplification effect is linked to the extended viability of covid-19 in aerosols under high air pollution conditions (van doremalen et al., 2020). furthermore, analyses stratified by air pollution levels (table 7) demonstrate that population density is a factor in explaining the number of infected individuals, but its impact appears to be more pronounced in cities with high air pollution. for instance, on april 7th, 2020, during the peak of the italian outbreak, a 1% increase in population density in low-pollution cities resulted in a 0.25% increase in expected infected individuals (p=0.042). conversely, in high-pollution cities, a 1% population density rise translated to an 85% increase in expected infected individuals (p<0.001). figure 1 visually confirms this trend, with cities experiencing higher air pollution exhibiting a steeper and faster rise in covid-19 cases (cf., morawska & cao, 2020). table 7 provides regression results comparing the relationship between population density (log-transformed inhabitants per km² in 2019) and the number of covid-19 infections (log-transformed) in low-pollution versus high-pollution cities across three key dates: march 17, april 7, and april 27, 2020. these models illustrate how population density influences infection rates in cities with differing levels of air pollution. table 7. population density, air pollution strata, and estimated infections ↓ dependent variable low-pollution cities ↓ dependent variable high-pollution cities explanatory variable: log density inhabitants/km2 2019 explanatory variable: log density inhabitants/km2 2019 log infected 17th march, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f 2.347* (1.132) 0.358* (0.172) 0.116 (1.168) 4.324* log infected 17th march, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f .243 (2.268) 0.816** (0.311) 0.276(1.121) 6.864** log infected 7th april, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f 4.977 (.787) .252* (.120) .119 17.168*** log infected 7th april, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f 1.670 (1.491) .849*** (.205) .488 4.457* log infected 27th april, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f 5.310** (1.848) .203 (0.281) .016 (1.908) .521 log infected 27th april, 2020 constant a (st. err.) coefficient β1 (st. err.) r2 (st. err. of estimate) f 3.189* (1.566) 0.242** (0.215) 0.357(.775) 9.989** notes: * p-value<0.05; ** p-value<0.01; *** p-value<0.001; log density of inhabitants/km2 in 2019 is the explanatory variable, while log infected individuals is the dependent variable. 467air pollution as a potential risk factor for covid-19 spread: a case study of italian provincial capitals 4.1. low-pollution cities: march 17, 2020: in low-pollution cities, population density shows a significant, positive correlation with infection rates (0.358, p < 0.05), suggesting that higher population density is associated with increased infections early in the pandemic. the model’s r² is 0.116, indicating a relatively weak fit, with other factors possibly influencing the infections. april 7, 2020: by this point, the correlation is still significant (0.252, p < 0.05), but the model fit remains low (r² = 0.119), suggesting that other variables, such as health interventions or behavioral changes, may be influencing the infection trends. april 27, 2020: the coefficient for population density (0.203) remains positive, though not statistically significant. the model’s r² drops to 0.016, indicating a very weak relationship between population density and infection rates at this later stage of the pandemic. 4.2. high-pollution cities: march 17, 2020: in high-pollution cities, the relationship between population density and infections is stronger (0.816, p < 0.01), reflecting a more significant impact of population density on the spread of covid-19 early on. the model’s r² is 0.276, suggesting that a moderate portion of the infection variance can be explained by this factor. april 7, 2020: the effect of population density remains highly significant (0.849, p < 0.001), with the model showing a strong fit (r² = 0.488), indicating that population density continues to play a crucial role in the spread of infections in high-pollution cities. april 27, 2020: the relationship remains significant (0.242, p < 0.01), and the model fit improves (r² = 0.357), indicating that population density and air pollution together continue to influence infection rates, though the effect has lessened compared to earlier months. overall, the results suggest that in high-pollution cities, population density has a more pronounced and consistent impact on the spread of covid-19 across all time points, while in low-pollution cities, the influence of population density diminishes over time. this could reflect the compounding effect of both environmental and demographic factors on public health, particularly in densely populated and polluted urban areas. 5. discussion and conclusion this investigation delves deeper into the concerning link between air pollution and covid-19 transmission, utilizing italy’s covid-19 outbreak as a compelling case study. the geographic spread of the virus in northern italian cities reveals a significant correlation with pre-existing air pollution levels, presenting a critical public health concern. this study contributes novel insights, suggesting that geo-environmental factors may have exacerbated the transmission dynamics of covid-19 in northern italy, resulting in a disproportionate number of infections and fatalities (kargı, & coccia, 2024; kargı et al., 2024). the results show a clear correlation: towns with low wind speeds, high humidity, and frequent high air pollution events (beyond safety standards for particle matter or ozone) showed a significant rise in covid-19 cases and related mortality. table 8 presents a thorough examination of the effects of air pollution on public health in italy’s various regions. table 8 provides a detailed comparison of the public health effects of covid-19 in regions of italy with high and low air pollution, specifically based on the number of days exceeding the pm10 or ozone limits in 2018. the data presented includes the total number of infected individuals, deaths, and the mean and standard deviation of these figures, as well as the population size in each region. total infections: high air pollution regions, defined as those with more than 65 days exceeding pm10 or ozone thresholds, account for 74.47% of the total infections, with 166,445 cases. in contrast, regions with low air pollution, with 65 days or fewer exceeding limits, report 35,096 cases, or 25.53% of the total infections. the average number of infections in high-pollution regions is 27,740.83 individuals, with a standard deviation of 26,387.33, indicating considerable variability in infection rates. low-pollution regions show a much lower average infection rate of 4,103.5, but with a higher standard deviation of 5,182.099, suggesting that infections are more spread out across the regions. lo g in fe ct ed in di vi du al s m ar ch 2 02 0 log density inhabitans km2, 2019 cities with high air pollution cities with low air pollution pm10>100 days pm10<100 days pm10>100 days pm10<100 days y = 0.24 + 0.82*x y = 2.35 + 0.36*x figure 1. regression analysis: population density and infected individuals, disaggregated by air quality. 468 bilal kargı, mario coccia table 8. impact of air pollution on public health during covid-19 outbreak: italian data (may 2020) public health effects of covid-19 regions with high air pollution >65 days exceeding limits set for pm10 or ozone % regions with low air pollution £65 days exceeding limits set for pm10 or ozone % total total infected individuals 166,445 74.47 (1) 35,096 25.53 (1) 201,541 mean of infected people 27,740.83 4,103.5 standard deviation 2,6387.33 5,182.099 total deaths 24,621 81.08 (1) 3,533 18.92 (1) 28,154 mean of deaths 5,013.71 504.714 standard deviation 2,783.77 340.12 total population 31,265,000 19,229,711 note: the arithmetic mean of days over pm10 or ozone thresholds for cities determines which regions are considered high or low in terms of air pollution; (1) this percentage is computed taking into account the number of deaths and the number of infected people weighted with the population of these group of regions. total deaths: similarly, the death toll in high-pollution regions is notably higher, with 24,621 deaths, representing 81.08% of total deaths. low-pollution regions account for just 18.92% of deaths, or 3,533 deaths. the average number of deaths in high-pollution areas is 5,013.71, with a high standard deviation of 2,783.77, indicating a wide range of outcomes across the regions. lowpollution regions report an average of 504.714 deaths, with a much lower standard deviation (340.12), suggesting more uniformity in the death rates across these areas. population data: the total population of high-pollution regions is 31,265,000, while low-pollution regions have a population of 19,229,711. the higher population in high-pollution regions contributes to the higher total number of infections and deaths observed in these areas. in conclusion, the data suggest a clear disparity in the health outcomes related to covid-19 between regions with high and low air pollution, with regions experiencing higher pollution levels accounting for a disproportionately large share of total infections and deaths. the higher number of infections and deaths in high-pollution areas may point to the compounded effects of environmental factors on public health during the pandemic, highlighting the need for more targeted health interventions in these regions. data presented in table 8 depict a concerning trend: approximately 74.5% of covid-19 infections and 81% of related fatalities in italy concentrated in regions with high air pollution levels. this observation aligns with emerging research suggesting that airborne pollutants may act as carriers for microorganisms. studies posit that high concentrations of pm2.5 and pm10, typically found in heavily polluted environments, can impede microbial growth (coccia, 2020; frontera et al., 2020). however, the precise mechanisms by which air pollution influences covid-19 transmission remain an active area of investigation. the detrimental impact of air pollution on public health and the environment is now widely acknowledged (zhu et al., 2020). these findings underscore the urgency for governments to prioritize regions with elevated levels of air pollutants like pm2.5, pm10, no2, and o3. such regions are demonstrably more susceptible to the adverse public health consequences of epidemics like covid-19 (zhu et al., 2020; uçkaç et. al., 2023a; 2023b; kargı et. al., 2023c). to mitigate the amplifying effect of air pollution on airborne viral diseases, nations must implement sustainable policies focused on air quality improvement (coccia, 2018, 2019). these policies should prioritize the reduction of air pollutants and the promotion of renewable energy sources and cleaner production practices (wang & zhu, 2020). as julian a. feingold stated, “clean air is not a luxury; it is a right” (feingold, 2011). policymakers must prioritize this right for all citizens. in industrialized cities, environmental policies for sustainable development should specifically address urban and regional climatology. optimizing urban ventilation can dilute pollutants and heat, facilitating air exchange and reducing regional air pollution. this approach, as outlined by gu et al. (2020), can potentially lessen the threat of accelerated viral transmission during fall and winter seasons. the implementation of sustainable policies to reduce air pollutants presents a valuable strategy for controlling and mitigating the impact of infections, generating substantial environmental, health, and economic benefits. a study by cui et al. (2020) in china demonstrates that reductions in ambient air pollution can prevent premature deaths and associated morbidity cases, translating to significant economic advantages. their findings suggest that reducing pm2.5 concentrations to 15 μg/m3 could result in substantial reductions in pm2.5-related mortality and morbidity, along with substantial economic benefits. in conclusion, this study sheds light on geo-environmental factors that may exacerbate the spread of covid-19 and similar viral agents. the research revealed a correlation between higher infection rates and several factors in italian provincial capitals: elevated air pollution levels, hinterland location (particularly bordering large urban areas), low average wind speed, and lower temperatures. notably, the northern italian region studied, and regions with high air pollution in general, should strive to limit pm10 and ozone exceedances to less than fifty days annually to minimize the risk of accelerated viral transmission dynamics. it is important to acknowledge the limitations of this study. further research is necessary to definitively elucidate the complex relationships between infected individuals, environmental factors, demographics, and geographical characteristics that influence the spread of covid-19. effectively addressing the current covid-19 pandemic and future similar outbreaks necessitates a multifaceted approach that integrates medical research and practice with disciplines such as immunology, biochemistry, molecular biology, and, crucially, the development of robust environmental policies for a sustainable society. references bashir, m.f., ma, b., bilal, b., komal, m., bashir, m.a., tan, d., & bashir, m. 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however, in the urban area, the ppk-based solution improved the accuracies 4-5 dm and 1.1-1.5 meter level in position and height components, respectively. these results indicate that the ppp-based solutions offer a similar level of accuracy to the ppk-based solutions in the wooded area where the satellite visibility is high throughout the trajectory. however, the ppk-based solution provided better positioning accuracies in the urban environment with limited satellite visibility. keywords: gnss; imu; loosely coupled integration; ppp; ppk; evaluación de desempeño de la integración débilmente acoplada con el sistema de postproceso cinemático y posicionamiento preciso en áreas arborizadas y urbanas resumen en este estudio los autores presentan series de mediciones de prueba en áreas arborizadas y en áreas urbanas y analizan los resultados a la luz de tres objetivos. el primero de estos objetivos es comparar las soluciones de un sistema débilmente acoplado y las satelitales en términos de precisión. los resultados muestran que las soluciones satelitales, en comparación con las soluciones del sistema débilmente acoplado, mejoraron la precisión de posición en 1 cm y entre 6 y 7 cm en componentes de altura para la zona arborizada. en el área urbana las soluciones del sistema débilmente acoplado mejoraron la precisión de la posición hasta en 6 cm y la altura hasta en 44 cm. también el sistema débilmente acoplado abarca los vacíos de información y crea una solución constante para estos vacíos, que alcanzan hasta el 30 % en la trayectoria del área urbana. los autores investigaron el desempeño de las soluciones de gps y de gnss en segunda instancia. en el área arborizada la solución gnss presentó una mejora de 2 cm en la precisión para los componentes de posición y altura sobre la solución gps. en el área urbana la solución gnss mejoró la precisión de posición en 8 cm y la de altura en 36 cm. también la solución de disponibilidad del proceso gnss es 10 % mejor que la solución gps. el tercer objetivo de este estudio es evaluar el desempeño de las soluciones de posicionamiento preciso frente a las soluciones de postproceso cinemático en las dos áreas. las soluciones de postproceso cinemático consiguieron mejores resultados de solo 2 cm en los componentes de posición y altura frente al posicionamiento preciso en el área arborizada; sin embargo, las soluciones de postproceso cinemático mejoraron la precisión entre 40 y 50 cm en posición y entre 1.1 y 1.5 metros en altura en el área urbana. estos resultados indican que las soluciones de posicionamiento preciso ofrecen un nivel de precisión similar a las postproceso cinemático en el área arborizada cuando la visibilidad del satélite es alto a través de la trayectoria. pero las soluciones provistas por el sistema de postproceso cinemático son más precisas en el ambiente urbano con condiciones de visibilidad satelital limitada. palabras clave: gnss; unidad de medición inercial; integración débilmente acoplada; posicionamiento preciso; postproceso cinemático. record manuscript received: 17/01/2022 accepted for publication: 11/08/2022 how to cite item: gurturk, m., & ilci, v. (2022). the performance evaluation of ppk and ppp-based loosely coupled integration in wooded and urban areas. earth sciences research journal, 26(3), 211-220. https:// doi.org/10.15446/esrj.v26n3.100518 https://doi.org/10.15446/esrj.v26n3.100518 https://doi.org/10.15446/esrj.v26n3.100518 https://doi.org/10.15446/esrj.v26n3.100518 212 mert gürtürk, veli i̇lçi, 1. introduction global navigation satellite system (gnss) is the basic component of many kinematic or dynamic applications that require high-precision positioning such as unmanned aerial vehicles (eling et al., 2015), mobile mapping (nocerino et al., 2017), road surveying (jo & sunwoo, 2014), marine positioning (jiang et al., 2015), structural health monitoring (shen et al., 2019) and autonomous vehicles (kuutti et al., 2018). gnss positioning techniques vary according to the required accuracy for kinematic applications. in high-precision kinematic positioning studies, the post-process kinematic technique is generally preferred, and double-differential observations are used (paziewski et al., 2018). postprocess kinematic (ppk) technique has proven its reliability and effectiveness by providing centimeter level accuracy after the resolution of the carrier phase integer. but the accuracy of the ppk method may be adversely affected by the long baselines (ocalan, 2016). however, positioning accuracy is not affected by the baseline length in precise point positioning (ppp) technique since the technique requires only one receiver (furones et al., 2012). ppp is a method that can generate positioning at the centimeter or decimeter level employing precise satellite orbit and clock data that can be obtained from analysis centers, such as international gnss service (igs), bundesamt für kartographie und geodäsie (bkg), centre national d’etudes spatiales (cnes), european space agency (esa), deutsches geoforschungszentrum (gfz) and gmv aerospace and defense (gmv)), with undifferenced code and phase measurements using a single gnss receiver. thus, it has become an essential alternative to ppk in evaluating kinematic gnss data gathered with aircraft, vessels, and cars at high speed (alkan et al., 2017; elliott & hegarty, 2017; erol et al., 2020; misra & enge, 2011). el-mowafy (2011) compared ppp and ppk coordinate results using fixed-wing aircraft and showed that their differences ranged from a few millimeters to 1.5 decimeters. moreover, the convergence time of the solution can be shortened, and the position accuracy can be increased by adding glonass observations to gps observations in the ppp method (choy et al., 2013). in the near future, the ppp method is expected to be used effectively in smart city applications such as parking, cargo delivery, shared vehicle use, emergency response and autonomous driving (brovelli et al., 2016; robustelli & pugliano, 2019; xu et al., 2018). at present, gnss receivers have become the main component of precise navigation systems with the high precision and accuracy that ppp and ppk methods offer. still, it may not always be possible to use the position information of gnss receivers since signal interruptions in areas with many restrictions on satellite visibility, such as footbridges, flyovers, tunnels, or close to tall buildings or forests. in these situations, the number of visible satellites (nvs) is insufficient; that’s why desired positioning accuracy level may not be provided. although multi-constellations increase satellite visibility and geometry, they cannot provide sufficient enhancement in positioning accuracy compared to the gps-only results unless there is a weak number of gps satellites and the poor satellite geometry (bakuła et al., 2015; li et al., 2019; specht et al., 2020, ocalan et al., 2016). also, multi-constellation solutions in challenging conditions may not be sufficient for high-precision positioning studies (petovello, 2003; yigit et al., 2014). therefore, satellite systems are often used in such areas by being integrated with inertial sensors due to their complementary properties. the inertial measurement unit (imu) provides high-rate signals (specific force and angular velocity) for monitoring the ego-motion of the sensors. integrating the measurements from the gnss receiver with the measurements from the imu is achieved with the kalman filter. this conventional configuration can provide very high accuracy due to its completeness. in this paper, to evaluate the positioning performance of the gnss/imu integration, a field experiment was carried out in two different environments, urban and wooded areas with poor gnss satellite geometry and limited satellite visibility. in this contribution, there are three main objectives. firstly, the positioning accuracies of the satellite-based and loosely-coupled based (lcbased) results were comparatively investigated. second, in these conditions, the impact of satellite systems (including multi-gnss and gps-only) on positioning accuracy was examined. finally, a comparative analysis of the accuracy performance of the ppp and ppk approaches in challenging situations was conducted. 2. motivation in the literature, several studies evaluated the loosely coupled integration of imu with the rtk, ppk or ppp methods separately; however, few studies assess the loosely coupled integration of imu with both ppk and ppp methods under challenging environments with limited satellite view. godha and cannon (2007) achieved an 80-90% improvement by adding the imu system to the ppk method during gps outages in an urban area. vu et al. (2013) achieved a centimeter precision result by integrating the data collected with dual-frequency gps processed according to the ppk method and 200 hz imu on a land vehicle. vana and bisnath (2020), in which they integrated ppp/imu with the land vehicle and obtained 40 cm accuracy in the horizontal component and 1.2 m in the vertical component during 30 seconds of signal interruption. the primary motivation of this study is to examine the performance of ppp and ppk methods as a result of imu integration in the same environment and conditions. in most of the ppp/ppk and imu integration studies, the obtained positioning performance of the proposed methods has been compared to the other gnss-based results named as reference data (falco et al., 2012; chiang et al., 2013; falco et al., 2017). however, these reference data are also highly affected by gnss-based error sources such as clock-related errors, atmospheric errors, multipath errors, orbital errors, and receiver noise, especially in harsh environments such as urban areas, heavy tree cover etc. and thus may offer a low accuracy position information. as a result, comparing the proposed method with the result of another gnss-based method may not be appropriate in such challenging environments. the highest grade imus are used in some studies to obtain the high accuracy reference solution, but this solution is not sufficient to get the exact trajectory (ilci & toth, 2020; otegui et al., 2021; zhang et al., 2020). to avoid this situation, in this study, reference data were obtained by terrestrial methods, and high-accurate reference data were obtained along the trajectories in urban and wooded areas. in this study, besides the comparative analysis of the ppp-based and ppk-based solutions, the gps-based and gnss-based solutions and the lc-based and satellite-based solutions are also comparatively analyzed. 3. loosely coupled model gnss/ins integration methods are divided into three different types called “loose”, “tight”, and “ultra-tight” (solimeno, 2007; gao and lachapelle, 2008; petovello, 2003). the fundamental difference between these methods is the type of data shared by the gnss receiver and ins. in the lc technique, the positions, velocities, and times (pvt) estimated by the gnss receiver are integrated with the ins solution, while in the case of a tightly-coupled method, measurements from gnss raw measurements (i.e., pseudorange, carrier phase measurements and doppler observables) and measurements from inertial sensors are processed to estimate pvt. the ultra-tight integration method includes the baseband signal processing of gnss receivers (i.e., the digital tracking loops) (falco et al., 2017). for the integration of individual systems for the accuracy or integrity requirements, lc integrations should be preferred (bhatti et al., 2007). even though many articles describe lc integrations for automotive applications in urban areas (angrisano et al., 2012; atia & waslander, 2019; godha & cannon, 2007; li et al., 2018; l. zhao et al., 2016; s. zhao et al., 2016), limited studies have been conducted to evaluate the effectiveness of lc solutions based on different environments such as forestry and open skies from a practical point of view. however, the loosely coupled integration algorithm was used in this study, as salytcheva (2004) also described, because tightly coupled algorithms have less computational efficiency and a more complex system and measurement model than loosely coupled schemas. kalman filter is used in the lc technique in order to determine the position and velocity errors, gyroscope bias error, first-order markov process random noise errors and accelerometer bias error (hol, j. d. 2011; xu et al., 2018). then, ins position error δp, velocity error δv, gyroscope bias errors ε b, and first-order markov process random noise errors of gyroscope εr and accelerometer bias error 𝝯 constitutes the state vector xk= (δpkδvkεb,k𝝯 k)t. the state equation of gnss/ins lc can be written as: 213the performance evaluation of ppk and ppp-based loosely coupled integration in wooded and urban areas xk = fk,k-1xk-1 + gk-1wk-1 (1) where, fk,k-1 : the state transition matrix gk : the system noise distribution matrix wk : the system noise vector the position and velocity differences between ins and gnss can be regarded as measurements. the measurement model is written as:= , −1 −1 + −1 −1 = , , = , − , , − , = + , + , = ] + , , = + (2) where, nɡ: the position error of gnss mɡ: the velocity error of gnss hp,hv : state space transformation matrices vk: the measurement error is considered as white noise, i.e., e(vk) = 0. its covariance rk can be estimated as rk = e(vk vk t) . zk : the position and velocity difference between the gnss measurements and ins estimation. under the normal cases, there is consistency between the position and velocity states of gnss and that of ins, and for this reason, the position and velocity difference zk is small, equal to the sum of gnss noise and ins errors, i.e., zk = vk (xu et al., 2018). the item zk is gnss noise when all ins errors are corrected. the velocity difference δv is optional in some lc integration systems (falco et al., 2017). the gnss/ins integration system becomes integrated with position fusion, and the measurement model becomes zk = hp,kxk + nɡ,k (xu et al., 2018). details about the lc integration model can be found in ( hol, j. d. 2011, qin et al., 2015). 4. study area in this study, the data were collected by using a gnss receiver and an imu sensor mounted on a three-wheel hand pushcart (carrier platform) in the yildiz technical university, davutpasa campus in 2018 (doy: 137). the test environments are shown in figure 1. the campus features medium-sized buildings that create lower elevation angles relative to dense urban areas, and trees on both sides mainly cover the roads. the test was conducted in two different areas; a wooded area where the gnss signals are partly interrupted due to trees around it and an urban area where gnss signals are heavily interrupted by the buildings and the footbridges between the buildings. the roads in the test environments are smooth and are not be exposed to high vibration-effect. 5. data collection and processing the data collection vehicle is shown in figure 2. a topcon hyperpro dual-frequency gnss receiver and a high-precision xsens mti-g-700 imu were used in this study. in addition, raw data were received and stored on a laptop. mti-g-700 sensor generates the accelerometer and gyroscope data. the technical characteristics of the imu and gnss receiver are shown in table 1. the data sampling rates of the gnss and imu was set to 10 hz and 400 hz, respectively. in two test environments, data were collected for approximately 1 hour. in the study, the 20-min duration is considered as convergence time. therefore, 20 minutes of static observation data were collected before the vehicle started moving. figure 1. study areas (blue line: wooded area; red line: urban area) 214 mert gürtürk, veli i̇lçi, table 1. specifications of imu and gnss receiver dual frequency gnss receiver xsens mti-g700 specifications gyroscope accelerometer static/fast static h: 3.0 mm+ 0.4 ppm drift rate 1 deg/s 0.02 m/s2 v: 5.0 mm + 0.6 ppm precision static h: 3.0 mm+ 0.1 ppm noise 0.05 deg/s/√hz 0.002 deg/s/√hz v: 3.5 mm + 0.4 ppm rtk (l1 +l2) h: 10 mm+ 0.8 ppm bandwidth 40 hz 30 hz v: 15 mm + 1.0 ppm dgps h: 0.4 m+ 0.6 m misalignment 0.1 deg 0.1 deg scale factor ˗˗˗˗ 0.03 % figure 2. data collection vehicle ( a) wooded-area; b) urban area) all the processes were conducted using inertial explorer post-processing software developed by novatel. the collected gnss data was processed using post-process kinematic (ppk) and precise point positioning (ppp) techniques in both only gps signals and gps/glonass (gnss) multi-constellation satellite system signals. the yldz base station (located on the roof of the yildiz technical university, civil engineering faculty) data of the related test dates were used as the reference in the ppk processes. the maximum distances of the yldz base station to the wooded and urban areas are 280 m and 160 m, respectively. in gps-only and gnss processes in both ppk and ppp methodologies, the cut-off angle and data processing intervals were set to 10 degrees and 0.1 seconds, respectively. then, these ppk and ppp solutions were separately combined with the 400 hz imu data using the lc algorithm. all these procedures were repeated for wooded and urban areas, and the results were exported in 10 hz frequencies. so, eight different solutions were obtained: ppk/gps, ppk/gnss, ppp/gps, ppp/gnss, ppk/gpsimu, ppk/gnss-imu, ppp/gps-imu, and ppp/gnss-imu. for clarity, we grouped the solutions for the aimed comparisons seen in table 2, and the solutions will be named as these groups. the reference solution for wooded and urban areas has been provided by the traditional surveying method, which is accurate to 1-2 centimetres by using a total station to determine the accuracy of the solutions. the obtained processing results were compared with that of reference coordinates. table 2. solution groups group name test 1 (satellite-based vs. lc-based) satellitebased ppk/gps ppk/gnss ppp/gps ppp/ gnss lc-based ppk/gpsimu ppk/gnssimu ppp/gpsimu ppp/ gnssimu test 2 (gnss-based vs. gps-based) gnss-based ppk/gnss ppp/gnss ppk/gnssimu ppp/ gnssimu gps-based ppk/gps ppp/gnss ppk/gnssimu ppp/ gnssimu test 3 (ppk-based vs. ppp-based) ppk-based ppk/gps ppk/gnss ppk/gpsimu ppk/ gnssimu ppp-based ppp/gps ppp/gnss ppp/gpsimu ppp/ gnssimu 215the performance evaluation of ppk and ppp-based loosely coupled integration in wooded and urban areas 6. results 6.1. wooded-area test: figure 3 depicts the satellite-based solutions in the wooded area. none of the epochs along the trajectory for these four solutions is missing. according to the findings, the satellite-based solutions are quite near to the reference trajectory through the route. figure 4 shows the lc-based solutions in the wooded area. it can be observed that lc-based systems offer extremely accurate results that are comparable to satellite-based results. figure 5 shows the 2d position errors of the satellite-based (top) and lcbased (middle) solutions and the nvs (bottom) during the observations. the satellite-based (top) and lc-based (middle) solutions provide lower than 22cm error levels through the trajectory where the nvs is mostly higher than 10. figure 3. trajectories of satellite-based solutions and the reference trajectory in the wooded-area figure 4. obtained trajectories of lc-based solutions and the reference trajectory in the wooded-area 216 mert gürtürk, veli i̇lçi, figure 6 shows the height errors of the satellite-based (top) and lc-based solutions (middle) and the nvs (bottom) during the observations. unlike the 2d position errors, lc-based height solutions provide better results than satellitebased height results. also, both figure 5 and figure 6 show that the lc-based result’s fluctuation is smaller than that of satellite-based results. since kalman filtering technique is used to determine the unknowns, thus, it can be concluded that the lc-based solutions are more reliable than the satellite-based solutions. figure 6. height errors in wooded-area (top: satellite-based; middle: lc-based), and the nvs during the wooded-area test (bottom) table 3. error statistics of 2d position for wooded-area 2d position (m) satellite-based lc-based ppk ppp ppk ppp gnss gps gnss gps gnss gps gnss gps max 0.20 0.21 0.16 0.21 0.10 0.17 0.12 0.18 average 0.03 0.04 0.04 0.06 0.03 0.04 0.03 0.05 rmse 0.04 0.05 0.04 0.07 0.04 0.05 0.04 0.06 table 3 gives the error statistics of the 2d position for satellite-based and lc-based solutions in the wooded area. in 2d position, satellite-based ppk/ gps, ppk/gnss, ppp/gps, and ppp/gnss solutions provide 0.21, 0.20, 0.21, and 0.16 meter errors in maximum and 0.04, 0.03, 0.06 and 0.04 meter errors on average, respectively. rmse values of ppk and ppp methods reveal that the gnss-based solutions slightly improve positioning accuracy compared to the gps-based methods. lc-based ppk/gps-imu, ppk/gnss-imu, ppp/gps-imu, and ppp/gnss-imu position solutions provide 0.17, 0.10, 0.18, and 0.12 meter errors in maximum and 0.04, 0.03, 0.05 and 0.03-meters errors on average, respectively. comparing the satellite-based and lc-based solutions reveals that lc integration provides a remarkable improvement only in the maximum errors. in contrast, average and rmse values of lc-based methods provide a very slight improvement. table 4. error statistics of height for wooded-area height (m) satellite-based lc-based ppk ppp ppk ppp gnss gps gnss gps gnss gps gnss gps max 0.30 0.41 0.43 0.48 0.09 0.27 0.17 0.23 average 0.08 0.10 0.10 0.12 0.02 0.04 0.03 0.05 rmse 0.10 0.12 0.11 0.13 0.03 0.06 0.04 0.06 the error statistics of the height component for satellite-based and lcbased solutions are given in table 4. height errors of satellite-based ppk/gps, ppk/gnss, ppp/gps, and ppp/gnss solutions provide 0.41, 0.30, 0.48, and 0.43 meters in maximum and 0.10, 0.08, 0.12 and 0.10 meters on average, respectively. comparing the satellite-based ppk and ppp methods reveals that the ppk-based errors in maximum, average, and rmse statistics are less than ppp-based errors. the comparison of the gps and gnss solutions reveals that the gnss solutions are better than that of gps solutions. lc-based ppk/gps-imu, ppk/gnss-imu, ppp/gps-imu, and ppp/ gnss-imu solutions of height component provide 0.27, 0.09, 0.23, and 0.17 meters in maximum and 0.04, 0.02, 0.05 and 0.03 meters errors on average, respectively. the maximum errors of all lc-height solutions are in the 1-2 dm level. comparing the satellite-based and lc-based results in the height component reveals that lc integration significantly improves ppk/gps, ppk/ gnss, ppp/gps and ppp/gnss maximum, average rmse statistics. 6.2. urban-area test: figure 7 shows the satellite-based solutions in the urban area. this trajectory has five bridges for pedestrians from building to building that causes satellite signal blockage. under or nearby these bridges, satellite-based four solutions either do not give any results or give very erroneous results. ppk/ gnss, ppk/gps, ppp/gnss, and ppp/gps solutions have 22%, 36%, 25% and 31% missing gaps, respectively, through the trajectory. although gnssbased methods provide fewer gaps than gps-based methods, the produced solutions of the gnss-based methods are mostly unsatisfactory under or nearby the bridges like gps-based methods. moreover, ppp/gnss and ppp/ gps trajectories dramatically leave the reference trajectory in some route parts. figure 8 shows the lc-based solutions in the urban area. there are no missing epochs in all lc-based solutions. gnss/imu integrated lcbased solutions bridge the gaps of the satellite-based solutions in all these methodologies, providing continuous trajectories. although ppp-based methods provide uninterrupted solutions, they are significantly separated from the reference route in some regions along the course. figure 9 shows the 2d position errors of the satellite-based (top) and lc-based integrated solutions (middle) and the nvs during the observations (bottom). the negative effect of the five bridges is seen as the five gaps in this figure, and the error values reach the highest values near these epochs. also, it can be seen that the maximum errors obtained from these processes occur before and after these gaps. figure 9 also visualize that the maximum errors of ppp-based solutions reach up to 3 meters between the 1500 to 2000 epochs. the middle part of this figure indicates that the gnss/imu integrated lc-based solutions bridge the gnss solution’s gaps in all four methods. but, especially ppp-based solutions can not provide satisfactory improvement in some parts of the trajectory. figure 5. 2d position errors in wooded-area (top: satellite-based; middle: lcbased), and the nvs during the wooded-area test (bottom) 217the performance evaluation of ppk and ppp-based loosely coupled integration in wooded and urban areas figure 7. trajectories of satellite-based solutions in the urban-area figure 8. trajectories of lc-based solutions in the urban-area 218 mert gürtürk, veli i̇lçi, figure 10 gives the height errors of the satellite-based (top) and lc-based (middle) solutions and the nvs during the observations (bottom). similar to the 2d position errors, five gaps are seen in the height component in the top part of the figure. all method’s errors reach meter-level, but ppk-based solutions are better than ppp-based ones. the middle part of the figure shows that the imu integration fills the gaps in all methods and improves height accuracy. figure 10. height errors in urban-area (top: satellite-based; middle: lc-based) and the nvs during the observations (bottom) table 5 gives the error statistics of the 2d position for satellite-based and lc-based solutions in the urban area. the satellite-based ppk/gps, ppk/gnss, ppp/gps, and ppp/gnss solution errors are 2.10, 0.99, 3.03, and 2.96 meters in maximum and 0.11, 0.06, 0.61 and 0.55 meters on average, respectively. the satellite-based ppk methods outperform the ppp methods in maximum, average, and rmse statistics in the gnss challenging environment. gnssbased methods are significantly better than gps-based solutions in ppk-based solutions, while in the ppp-based solutions, this improvement is very slight. lc-based ppk/gps-imu, ppk/gnss-imu, ppp/gps-imu, and ppp/ gnss-imu solutions provide 0.36, 0.20, 2.70, and 2.59 meters in maximum and 0.09, 0.06, 0.57 and 0.49-meter errors on average, respectively. similar to the satellite-based solutions, the provided accuracies of the ppk-based methods are significantly better than that of ppp-based methods in the lcbased solutions, and gnss-based methods better perform than the gps-based methods. although the lc-based methods can not improve the average values substantially compared to the satellite-based methods, lc-based methods fill the gaps through the trajectory (nearly %30 of the trajectory), provide lower maximum errors, and less fluctuation in the error values. figure 9. 2d position errors in urban-area (top: satellite-based; middle: lc-based) and the nvs during the observations (bottom) table 5. error statistics of 2d position for urban-area 2d position (m) satellite-based lc-based ppk ppp ppk ppp gnss gps gnss gps gnss gps gnss gps max 0.99 2.10 2.96 3.03 0.20 0.36 2.59 2.70 average 0.06 0.11 0.55 0.61 0.06 0.09 0.49 0.57 rmse 0.09 0.25 0.95 1.09 0.08 0.12 0.80 0.95 table 6. error statistics of height for urban-area height (m) satellite-based lc-based ppk ppp ppk ppp gnss gps gnss gps gnss gps gnss gps max 6.43 10.86 8.73 12.95 1.49 1.64 4.45 4.74 average 0.24 0.38 1.75 1.90 0.16 0.34 1.31 1.67 rmse 0.52 1.06 2.72 2.76 0.32 0.57 1.80 2.18 the error statistics of the height component for satellite-based and lcbased solutions are given in table 6. height errors of satellite-based ppk/gps, ppk/gnss, ppp/gps, and ppp/gnss solutions are 10.86, 6.43, 12.95 and 8.73 meters in maximum, and 0.38, 0.24, 1.90 and 1.75 meters on average, respectively. these results reveal that the ppk methods provide a better solution than ppp methods in the height component in the urban area and the gnss-based methods outperform the gps-based methods. height errors of lc-based ppk/gps, ppk/gnss, ppp/gps, and ppp/ gnss solutions are 1.64, 1.49, 4.74 and 4.45 meters in maximum, and 0.34, 0.16, 1.67 and 1.31 meters on average, respectively. similar to the satellitebased methods, lc-based results reveal that the ppk-based and gnss-based methods provide better than ppp-based and gps-based solutions, respectively. the lc-based solutions enhance the maximum height accuracies for all methods. the average results of gnss-based solutions (ppk-gnss and ppp-gnss) indicate that lc-based solutions slightly improve height accuracy. however, on gps-based solutions (ppk-gps and ppp-gps methods), lc slightly deteriorates the height accuracies. 7. conclusions gnss has been the playmaker of positioning applications for the last two decades, from low-cost single-frequency receivers to state-of-the-art technologies. nevertheless, this technology needs another assistance technology, such as imu, in some conditions where obstacles partially or entirely block the gnss signals. this study aims to determine the positioning performance of the gnss/imu integrated solution in the two gnss signal blockage test environments; a wooded area and an urban area. using the obtained results, this study tests and analyses three comparisons, namely satellite-based and lcbased solutions, gnss-based and gps-based solutions, and ppk-based and ppp-based solutions. in the wooded area test, similar to the lc-based methods, the satellitebased methods provided cm-level accuracies through the trajectory due to the higher nvs. lc-based methods slightly improved the accuracy, reduced the fluctuation in the errors, and offered more reliable solutions than the satellite-based method. moreover, it can be concluded that the gnss-based methods offered a slightly better solution than gps-based methods and ppkbased methods provided a slight improvement over the ppp-based methods. 219the performance evaluation of ppk and ppp-based loosely coupled integration in wooded and urban areas in the urban area test, the satellite-based methods have very big gaps in the solutions. lc-based solutions filled the gaps, provided uninterrupted solutions with a slight positioning improvement, lower fluctuations in the errors and provided more reliable solutions than the satellite-based results. moreover, ppk-based solutions provided significant progress compared to the ppp-based methods and gnss-based methods outperformed the gps-based solutions. considering the abovementioned results, it can be concluded that the gnss/imu integration with ppk methods provides a reliable and uninterrupted solution with the best positioning performance in both the highly and partially gnss-denied environments. references alkan, r. m., saka, m. h., ozulu, m., & i̇lçi, v. 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(2016). high-precision vehicle navigation in urban environments using an mem’s imu and single-frequency gps receiver. ieee transactions on intelligent transportation systems, 17(10), 2854–2867. https://doi.org/10.1109/tits.2016.2529000 _hlk77445700 _hlk77535678 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v28n2.112335 earth sciences research journal earth sci. res. j. vol. 28, no. 2 (june, 2024): 213 229 m et eo r o lo g y abstract: this study focused on calculating climate change indices derived from maximum and minimum temperature variables for the coastal resilience observatory in tabasco, dos bocas, located in paraíso, tabasco, mexico. the indices were computed using climpact2, 2007 version, and a total of 26 indices were obtained for the period from 1940 to 2022. subsequently, significant trends were calculated using statistical tests such as mann‒kendall, trend-free prewhitening, bias correction applied to prewhitening, and variance correction (by two methods). sen’s slope method was employed to determine the extent of alteration in extreme climate indices. increasing trends were found in the indices tn90p (%), tx90p (%), tr (days), txgt50p (%), wdsi (days), su (days), tmge10 (days), tmge5 (days), gddgrow (days), and gsl (days), indicating the presence of local warming and providing evidence of climate change detection. these results contribute to updating climate information in the area and serve as a proposal for replication in the other six coastal observatories of the national coastal resilience laboratory in mexico, as part of measures related to climate stressors in coastal areas where these observatories are located. this information is valuable for decision-makers and the general population, as it will support socio-environmental adaptation and mitigation measures in the face of climate change as part of efforts to enhance the resilience of socioecosystems in coastal areas. climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc mercedes andrade-velázquez1*, martín josé montero-martínez2, m. f. vargas magaña3 1. conahcyt ̶ centro del cambio global y la sustentabilidad (ccgs). villahermosa, tabasco, méxico. mercedes.andrade@ccgs.mx 2. instituto mexicano de tecnología del agua. subcoordinación de eventos extremos y cambio climático, jiutepec, morelos. martin_montero@tlaloc.imta.mx 3. división de ciencias básicas, universidad juárez autónoma de tabasco, cunduacán, tabasco, méxico. fernandavargasmagana@gmail.com *corresponding author: mercedes.andrade@ccgs.mx índices climáticos de temperatura para el observatorio de resiliencia costera en tabasco (dos bocas)‒lanresc record manuscript received: 02/01/2024 accepted for publication: 30/07/2024 how to cite this article: andrade-velázquez, m. a., montero-martínez, m. j., & vargas-magaña, m. f. (2024). climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc. earth sciences research journal, 28(2), 213-229. https://doi.org/10.15446/esrj. v28n2.112335 keywords: climate change; temperature indices; coastal zone; tabasco-mexico. resumen: el presente trabajo se centró en el cálculo de índices de cambio climático derivados de las variables de temperatura máxima y mínima para el observatorio costero de resiliencia en tabasco, dos bocas, ubicado en el municipio de paraíso, tabasco, méxico. los índices fueron calculados mediante climpact2, versión 2007, y se obtuvieron un total de 26 para el período de 1940-2022. posteriormente, se calcularon las tendencias significativas mediante pruebas estadísticas de mann‒kendall, preblanqueamiento sin tendencia, corrección de sesgo aplicada al preblanqueamiento, corrección de varianza (por dos vías), y se utilizó la pendiente de sen como método para determinar el grado de alteración de los índices climáticos extremos. se encontraron tendencias de aumento en los índices de tn90p (%), tx90p (%), tr (días), txgt50p (%), wdsi (días), su (días), tmge10 (días), tmge5 (días), gddgrow (días) y gsl (días), mostrando así que existe calentamiento local y evidenciando la detección del cambio climático. estos resultados contribuyen a la actualización de información climática en la zona y representan una propuesta para ser replicada en los otros seis observatorios costeros del laboratorio nacional de resiliencia costera en méxico, como parte de las medidas relacionadas con los estresores climáticos en las zonas costeras donde se ubican dichos observatorios. esta información es útil para los tomadores de decisiones y la población en general, ya que permitirá respaldar medidas de adaptación y mitigación sociambientales ante el cambio climático, como parte de las acciones hacia una mejor resiliencia de los socioecosistemas en las zonas costeras. palabras clave: cambio climático; índices de temperatura; zonas costeras; tabasco-méxico. https://doi.org/10.15446/esrj.v28n2.112335 about:blank about:blank about:blank about:blank https://doi.org/10.15446/esrj.v28n2.112335 https://doi.org/10.15446/esrj.v28n2.112335 214 mercedes andrade-velázquez, martín josé montero-martínez, m. f. vargas magaña 1. introduction the phenomenon of climate change is unequivocal and has been described in the assessment report 5 (ar5) of the intergovernmental panel on climate change (ipcc) as “a change in the state of the climate that can be identified (e.g., using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. climate change may be due to natural internal processes or external forcings.” the united nations framework convention on climate change (unfccc), in its article 1, defines climate change as: “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods”. currently, climate change is evident on a global scale. the intergovernmental panel on climate change (ipcc) highlighted in its 2007 report the expected changes in weather patterns due to global warming, including an increase in the frequency and intensity of extreme weather events. the report mentioned above is a special document dedicated to the study of changes in extreme events in the context of climate change. coastal areas are among the most vulnerable communities to climate change (ipcc, 2022). therefore, the ability to measure climate change on variables such as temperature and precipitation is crucial to enable these communities to adapt adequately to possible impacts. by using climate indicators, communities can anticipate and adapt to changes, thus minimizing the risks associated with climate change. tabasco has highly vulnerable coastal areas throughout mexico, primarily due to its geographical location, extensive plain, and susceptibility to flooding due to its low elevation (semarnap, 1997; aparicio et al., 2009; gama et al., 2010). historically, it has been affected by severe floods recorded in 1995, 1999, 2007, and more recently in 2020 (cenapred, 2021). the floods have varied in magnitude and duration (arreguín-cortés et al., 2014). these changes have altered the dynamics of wetlands in coastal areas, which serve the function of “buffering” the threats associated with climate change events (perevochtchikova & lezama, 2010; alizad et al., 2018). these events have affected communities and ecosystems, including the notable effect of coastal erosion, as in the case of sánchez magallanes (hernández-santana et al., 2008). in tabasco, approximately 50% of its wetlands have been lost due to illegal logging of mangroves (hernández melchor et al., 2017). anthropogenic activities in coastal areas have left bodies of water contaminated and soil degraded, mainly due to toxic waste that alters the chemical and physical properties of the soil, leading to the death of mangroves (adams, 1999; garcía-lópez et al., 2006; sandilyan & kathiresan, 2014; rastogi et al., 2021). the tabasqueño landscape has been modified, with approximately 97% of forest resources lost over a period of 50 years (1940 1990). this occurred due to excessive forest exploitation, primarily of riparian mangroves and wetlands. this results in dramatic impacts during flood periods, such as increased saline intrusion and coastal erosion (mata-zayas et al., 2017). the national coastal resilience laboratory of mexico (lanresc, hereinafter, acronym in spanish) was founded in 2015 with three partner institutions in mexico: i) institute of engineering, sisal unit (unam); ii) sonora technological institute (itson), and iii) the center for global change and sustainability (ccgs). subsequently, additional institutions joined, including the faculty of sciences-sisal unit, the faculty of chemistry-sisal unit, the institute of economic research, and the cinvestav-mérida unit (lanresc, 2023). in 2018, the establishment of six coastal observatories was proposed, and this expanded to seven in 2019, under the name coastal observatories for resilience (ocr, acronym in spanish), located at seven coastal points in mexico: three in the yucatán peninsula (sisal, alacranes reef, celestún), one in oaxaca (copalita), one in tabasco (dos bocas), one in campeche (laguna de términos), and one in sonora (agiabampo). as part of lanresc’s activities through its ocrs, standardized indicators are established in each of them to provide information about the historical and current state of socioecosystems in their various components: environmental, physical, and social. therefore, the goal is to determine climate indicators in the ocr-tabasco, as a pilot site, so that they can be extrapolated to the other six ocrs. it is necessary to pause at this point to define some important concepts commonly used in climate change studies, which will be employed later. these definitions are taken from the ar6 technical report by working group ii (wgii): “vulnerability is a component of risk, but also, independently, an important focus. vulnerability in this report is defined as the propensity or predisposition to be adversely affected and encompasses a variety of concepts and elements, including sensitivity or susceptibility to harm and lack of capacity to cope and adapt”. “adaptation is defined, in human systems, as the process of adjustment to actual or expected climate and its effects in order to moderate harm or exploit beneficial opportunities. in natural systems, adaptation is the process of adjustment to actual climate and its effects; human intervention may facilitate this”. “resilience is defined as the capacity of social, economic and environmental systems to cope with a hazardous event or trend or disturbance, responding or reorganising in ways that maintain their essential function, identity and structure while also maintaining the capacity for adaptation, learning and transformation”. one way to study recent climate changes and its extremes is through the climate change indices recommended by the now-defunct expert team on climate change detection and indices (etccdi) of ccl/wcrp/jcomm. in the agreements of the 17th edition of the commission for climatology of the world meteorological organization (wmo, 2018), the etccdi was suspended, and the new group responsible for the now sectoral climate indices (sci) is the commission on meteorology, applications, and services related to climate, water, and the environment (sercom) -expert team on climate information for decision-making (montero-martínez et al., 2022). in mexico, there are several relatively recent studies that analyzed climate extremes of surface temperature and precipitation using the etccdi indices (montero-martínez et al., 2018; pita-díaz & ortega-gaucín, 2020; coloradoruiz & cavazos, 2021; montero-martínez et al., 2022). in the case of surface temperature, where they showed a clear regional warming signal, particularly in the last few decades (montero-martínez & andrade-velázquez, 2022). the analysis of climatic extremes of surface temperature and precipitation using the etccdi indices was carried out by peterson et al. (2008) for mexico and by montero-martínez et al. (2018) for two basins north and south of mexico. these indices can provide information on climatic extreme changes over different time periods. for surface temperature, they showed a clear warming signal at the regional level in recent decades (montero et al., 2022). we note that climatic extremes are determined by the etccdi indices and should not be confused with extreme events used in meteorology. within the framework and vision of the ocrs of lanresc, which are strategically located in the coastal zone of mexico, it is proposed to conduct an analysis of climate indicators at each of the sites. this analysis aims to understand changes in the climatic extremes and to address the climate change on human communities and ecosystems located within or in the vicinity of the ocrs. 2. study area the coastal area of tabasco (figure 1) is situated between 92° 28’ and 94° 10’ west longitude and between 17° 5’ and 18° 39’ north latitude (hernándezsantana et al., 2008). in dos bocas, paraíso, tabasco, there is an important industrial and commercial development port for mexico (sedec, 2023). the port is located at a distance of 85 km from the city of villahermosa, tabasco (inegi, 2023). in this port, you can also find the petroleum industrial park, covering an area of 70 ha (semar, 2023). like the rest of the state, the climate is warm and humid, with an average temperature of 27 °c (inegi, 2023). rainfall in the area is due to cold fronts and tropical cyclones in the fall and winter, while in the summer, it is influenced by tropical cyclones and the presence of the intertropical convergence zone, with a dry season mainly in the spring (andrade-velázquez, 2017). during this last season, temperatures can reach over 40°c (smn, 2023). however, changes in extremes of this variable, in past or recent periods in the state, have not been quantified. 3. materials and methods the data used is sourced from the era5 database (herbach et al., 2023). it is in a grid format with a resolution of 0.25° x 0.25°. minimum (tn) and maximum (tx) temperature variables were obtained for the time period from 1940 to 2022 with a daily time window. climate-relevant indices were calculated based on the region’s climatic conditions (see table 1) using the climpact2 software, version 2007, designed for linux (http://fsf.org/). 215climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc initial time conditions were adjusted, including the baseline period, which covered 31 years (1940-1970), and the climate change indices to be computed (annex a). once the files for a quadrant covering the southeast of mexico were obtained, interpolation was carried out using the nearest neighbor method to the point with coordinates latitude 18.434722°, longitude -93.201667°, which corresponds to the location of the municipality of paraíso, where dos bocas is located (see figure 1). this was done to ensure that the calculation of the indices was not adversely affected. figure 1. location of the study area. dos bocas, paraíso, tabasco, mexico. the units of the mean annual temperature are °c. source: conabio (2023). table 1. annual climate change indices calculated for the maximum (tx) and the minimum (tn) temperatures. tm means medium temperature, tm = (tx + tn) / 2. index means units csdi5 cold spell duration indicator 5-day window (when tn remains below its climatological 10th percentile) days csdi cold spell duration indicator (when tn remains below its climatological 10th percentile) days dtr daily temperature range degrees_c gddgrow10 growing degree days degree-days gsl growing season length days hddheat18 heating degree days degree-days su summer days days tmge10 tx of at least 10 degrees_c days tmge5 tx of at least 5 degrees_c days (continued) 216 mercedes andrade-velázquez, martín josé montero-martínez, m. f. vargas magaña index means units tmm mean tm degrees_c tn10p amount of cold nights (10th percentile) % tn90p amount of hot days % tnm mean tn degrees_c tnn min tn degrees_c tnx max tn degrees_c tr tropical nights days tx10p amount of cool days % tx3tn3 3 consecutive number of hot days and nights number of events tx90p amount of hot days % txb3tnb3 3 consecutive number of cold days and nights number of events txge30 tx of at least 30 degrees_c days txgt50p fraction of days with above average temperature % txm mean tx degrees_c txn min tx degrees_c txx max tx degrees_c wsdi5 warm spell duration indicator (5day) days wsdi warm spell duration indicator (when the maximum temperature (tx) remains above its climatological 90th percentile) days calculation of trends a linear fit was applied to each of the indices, following equation 1: y = mx + b (1) where m represents the slope, which determines the trend, b is the y-intercept, x is the independent variable (in this case, time), and y is the dependent variable (in this case, each of the indices). however, non-parametric methods are recommended instead of assuming normality when analyzing time series data. the mann‒kendall test, also known as the m‒k test, is a widely used non-parametric method for identifying statistical trends, provided that the records do not exhibit persistence. this test is used to determine whether a time series exhibits a monotonic upward or downward trend. its formula involves the sum of the signs of differences (z) for all feasible pairs, and its operational statistic ss is expressed as follows (manly, 2001): =1 (2) where the sign (z) is -1 for z < 0, 0 for z = 0, and +1 for z > 0. for a series consisting of random values, it is anticipated that ss = 0, and its variance is determined as (hirsch et al., 1992; machiwal and jha, 2012): ( ) = −1( ) 2 +5( ) 18 (3) the test statistic is: = + ( ) (4) in the last equation, mc = 1 when ss < 0 and mc = -1 para ss > 0. the series shows an increasing or decreasing trend with a significance level α if the absolute value of zs is greater than the critical value of the standard normal distribution. the critical value is 1.96 for α = 5%. when the data are equal or the sample is rejected, the variance must be corrected (hirsch et al., 1992). conversely, sen’s slope method is a robust nonparametric technique for estimating the magnitude of a trend slope (sen, 1968). for a given time series xi = x1, x2 ,…, xn, with n pairs of data, the slope is determined as shown in equation 5. β = − − , ∀ ≤ ∧ = 1, 2, … (5) median of n values of βi gives the sen’s estimator of slope β. β = β +1 2 , β 2 + β +2 2( ) 2 , (6) sen’s slope method was employed to evaluate the extent of change in climate change indices. this non-parametric technique utilizes a linear model to estimate the trend’s slope, assuming a linear trend in the time series (sen, 1968; kumar et al., 2017). the sen’s slope is commonly used in the analysis of hydro-meteorological time series (fonseca et al., 2016). to identify significant trends in our study, we applied the mann-kendall (m‒k) test to each time series from the climatic stations for each climate change index. time series that did not demonstrate statistical significance were excluded from the calculations, while those that did were included. when analyzing time series data, it is crucial to consider whether the data exhibit serial correlation. this occurs when the current value of a variable depends on its value in previous moments. the assumption of independence can be violated if there are serial correlations in time series data, which can lead to incorrect statistical results. therefore, for reliable and accurate results, it is essential to account for serial correlation in the analysis of time series data. the method outlined by patakamuri et al. (2020) was utilized in this study to compute lag‒1 serial correlation coefficients for evaluating the presence of serial correlation. in numerous trend studies, the time series is examined for serial correlation by calculating the lag‒1 serial correlation coefficient, ρ1. for any time series xi = x1, x2 ,…, xn, the lag‒1 serial correlation coefficient (ρ1) is calculated as ρ 1 = 1 −1 =1 −1 ∑ − ( )( )· +1 − ( )( ) 1 =1 ∑ − ( )( )2 (7) where e(xi) is the mean of the sample and n is the sample size 217climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc (8) the probability limits for ρ1 on the correlogram of an independent series is given by anderson (1941) as ρ 1 = { −1+1.645 −2 −1 , ℎ − −1±1.96 −2 −1 , ℎ − (9) significance of serial correlation was evaluated by comparing the ρ1 value with the critical values of student’s t-distribution values. additional tests were conducted using patakamuri tests, if the time series showed evidence of serial correlation. however, if the time series did not exhibit serial correlation, it was retained as a statistically significant time series. as a result, the final trends were calculated using this time series. the m‒k trend test alone is inadequate when dealing with time series data exhibiting serial correlation. this study used a method that does not have a universal approach to determine the presence of serial correlation in time series data. patakamuri et al. (2020) described five statistical tests that supported the method used: the first is the pre-whitened m‒k test (von storch & navarra, 1995); the trendless pre-whitened m‒k test (yue et al., 2002); the bias correction applied to pre-whitening (hamed, 2009); the variance correction approach proposed by hamed & rao (1998); and the variance correction approach proposed by yue & wang (2004). the time series was considered significant, if at least three of these tests produced statistically significant results with at least 95% confidence. however, if fewer than three tests were significant, the time series was considered non-significant and excluded from further calculations. the patakamuri tests mentioned above are described as follows: • pre-whitened m‒k test for a given time series xi = x1, x2 ,…, xn, with lag‒1 serial correlation coefficient ρ1 as mentioned in equation (5), pre-whitened series xtʹ is given by von storch (1995) as xtʹ = xt ρ1 xt-1 (10) • trend-free pre-whitened m‒k test in the trend-free pre-whitening (yue et al., 2002) of a given time xt= x1, x2 ,…, xn, the first step involves calculating the sen’s slope using equations (5) and (6). the monotonic trend β is then removed from xi to create a trend-free series yt yt = xt βt (11) where xt is the series value at time t, yt is the detrended series, and β is the trend slope. in the next step, the lag‒1 serial correlation coefficient ρ1 of the detrended series yt is calculated as described in equation (7). if ρ1 is not significant, trend detection is conducted on the detrended series. if there is significant serial correlation at lag‒1, the de-trended series is pre-whitened, and the residual series is computed as follows ytʹ = yt ρ1 yt 1 (12) in the final stage, the monotonic trend component is added back to residual series ytʹ to generate the trend-free pre-whitening series xt xt = ytʹ + βt (13) • bias correction applied to pre-whitening a time series xt= x1, x2 ,…, xn, which is presumed to follow a first-order serial correlation process with an included linear trend, can be represented as follows: xt = ρxt 1 + α + βt + εt (14) where xt and xt 1 are observations at times t and t 1, respectively; ρ is the serial correlation coefficient; α is constant intercept term; β is trend slope with respect to time; and εt is an uncorrelated noise term. the estimated values of ρ, α and β are given by [ ρ α β ]t = (z t z)-1 zt y (15) where z is the matrix of size n 1 ⋅ 3 whose second column contains (n 1) values equal to 1, and the third column contains the numbers 2 to n and y is a vector of size (n 1) ⋅ 1 containing the observations x2 to xn. the biascorrected serial correlation coefficient ρ [ 45,86 ] is calculated using the formula mentioned in equation (16). this value is used in bias-corrected pre-whitening and trend detection studies. (16) • variance correction approaches in a time series xt= x1, x2 ,…, xn, with n observations, the effective information is contained in n observations, where n is always less than the original sample size n. positive or negative serial correlation causes the variance of the m‒k test statistic s to increase or decrease, respectively. to mitigate this effect, the modified variance v(s) is calculated by v(s) = v(s) × cf (17) correction factors (cf) proposed by hamed and rao (1998) and yue and wand (2004) termed as cf1 and cf2, respectively are ( )( ) ( )( )( )1 1 1 21 1 2 1 2 n r kk cf n k n k n k r n n n − = = + − − − − − − − ∑ (18) 1 2 1 1 2 1n kk kcf r n − =   = + −    ∑ (19) where rk and rk r are the lag ‒k serial correlation coefficients of data and ranks of data respectively and n is the total length of the series. in the case of cf1 only significant correlation coefficients are used. for calculating cf2, lag 1 serial correlation coefficient is used. the m‒k trend test is calculated by using corrected variance v(s). 4. results and discussion 26 indices were obtained from those listed in table 1, to which a linear adjustment was applied to obtain the trend, following equation (1). it is noted that the data represents time series in an annual window. figures 2 and 3 correspond to the evolution of the dtr, gddgrow10, tmm, tn10, and tn90p indices, respectively, over the period 1940-2022. the remaining plots can be found in annex b. statistical tests were applied to determine the significance of the trends and obtain the significant trend based on them using the sen’s slope (see the methodology section). figure 4 shows the indices that exhibited a significant trend. this study demonstrated the adjustment of significant trends that passed statistical tests for temperature-related climate indices. out of these, two exhibited a negative trend (tn10p and tx10p). these indices indicate minimum and maximum temperatures below the 10th percentile, suggesting a decrease in cold temperature days. specifically, tn10p refers to cold nights in comparison to the reference period, while tx10p relates to cold days in the region where the daily maximum temperature falls below the annual or monthly threshold relative to the baseline period. 218 mercedes andrade-velázquez, martín josé montero-martínez, m. f. vargas magaña a b c figure 2. annual time series of the climate change indices in black. in blue is the linear fit (see table 2). the gray area is the associated standard error. a) dtr (°c), b) txn (°c), c) tmm (°c). 219climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc a b figure 3. the same as in figure 2 but for the a) tn10p (%) and b) tn90p (%) climate change indices. figure 4. indices with significant trends (see table 2). table 2. list of significant trends (significant m) after applying statistical tests and sen’s slope. y (linear fit) and its r2. additionally, the extrapolated value of 2023 (*note: this value is taken from the linear fit, however the m is significant to determine it as null) climate change index m significative y (linear fit) r2 2023 extrapolated value csdi5 0 (days/yr) 𝑦 = 4.82 − 2.09∗ 10−4 𝑥 0.12 4.40 days * csdi 0 (days/yr) 𝑦 = 2,85− 1.28 ∗ 10−4 𝑥 0.07 2.60 days * dtr 0 (°c/yr) 𝑦 = 0.166 − 1.08 ∗ 10−7 𝑥 <0.01 0.166 °c* gddgrow 4.22 (°c days/yr) 𝑦 = 5.21*10+3+ 1.18 ∗ 10−2 𝑥 0.50 545 (°c days) gsl 4.22 (days/yr) 𝑦 = 365 − 1.21 ∗ 10−6 𝑥 <0.01 365 (days) hddheat18 0 (°c days/yr) 𝑦 = 482 − 2.11 ∗ 10−5 𝑥 0.05 482 (°c days)* su 0.97 (days/yr) 𝑦 = 146 +2.66 ∗ 10−3 𝑥 0.55 151 (days) tmge10 0.97 (days/yr) 𝑦 = 365 − 1.21 ∗ 10−6 𝑥 <0.01 365 (days) tmge5 0.97 (days/yr) 𝑦 = 365 − 1.21 ∗ 10−6 𝑥 <0.01 365 (days) tmm 0.01 (°c/yr) 𝑦 =24.3 + 3.22 ∗ 10−5 𝑥 <0.5 24.36 (°c) tn10p -0.11 (%/yr) 𝑦 =9.25 − 3.06 ∗ 10−4 𝑥 <0.38 9.63 (%) tn90p 0.27 (%/yr) 𝑦 =13.7 + 8.56 ∗ 10−4 𝑥 0.53 15.43 (%) tnm 0.01 (°c/yr) 𝑦 =24.2 + 3.23 ∗ 10−5 𝑥 0.50 24.26 (°c) tnn 0.01(°c/yr) 𝑦 =18.1 +3.19 ∗ 10−5 𝑥 0.10 18.16 (°c) tnx 0.01(°c/yr) 𝑦 =28.7 + 3.02 ∗ 10−5 𝑥 0.19 28.76 (°c) tr 0.07 (days/yr) 𝑦 =354 + 1.86 ∗ 10−4 𝑥 0.07 354 (days) (continued) 220 mercedes andrade-velázquez, martín josé montero-martínez, m. f. vargas magaña climate change index m significative y (linear fit) r2 2023 extrapolated value tx10p -0.11 (%/yr) 𝑦 =9.22 3.01 ∗ 10−4 𝑥 0.38 8.61 (%) tx3tn3 0.05 (events/yr) 𝑦 =1.16 + 1.85 ∗ 10−4 𝑥 0.42 1.53 (%) tx90p 0.27 (%/yr) 𝑦 =13.7 + 8.51 ∗ 10−4 𝑥 0.53 15.42 (%) txb3tnb3 na 𝑦 =0.5171.71∗ 10−5 𝑥 0.05 0.482 txge30 0 (days/yr) 𝑦 =6.86*10-2 + 9.48 ∗ 10−6 𝑥 0.02 686 (days)* txgt50p 0.37 (%/yr) 𝑦 =54.4 +1.00 ∗ 10−3 𝑥 0.49 56.4 (%) txm 0.01(°c/yr) 𝑦 =24.4 + 3.22 ∗ 10−5 𝑥 0.50 24.5 (°c) txn 0.01(°c/yr) 𝑦 = 18.3+ 3.5∗ 10−5 𝑥 0.12 18.4 (°c) wsdi5 0.56 (days/yr) 𝑦 =13.4 +2.16 ∗ 10−3 𝑥 0.47 17 (days) wsdi 0.44(days/yr) 𝑦 =10 + 1.83 ∗ 10−3 𝑥 0.44 13 (days) on the other hand, the gddgrow and gsl indices show positive trends with values exceeding 4 (days/year). gddgrow is an index indicating the accumulation of heat for plant growth, while gsl denotes the annual duration of warm days. this implies that, in both cases, warm days have been increasing rapidly. a trend of 1 (days/year) is observed for the su, tmge10, and tmge5 indices. the first corresponds to summer days, and the other two relate to average temperatures above 10 and 5 °c, respectively. these indices show an increase in recent years. additionally, there is a slightly lower, but positive trend of less than 0.5 (units/year) for the tn90p (%), tx90p (%), tr (days), txgt50p (%), and wsdi (days) indices, indicating an increase in maximum temperatures, tropical nights, and maximum temperature. while the rest of the indices showed a trend with the linear adjustment, when statistical tests were applied, their trends are reported in table 2. we can see that the indices tmm, tnm, tnn, tnx, txn, txm, and txx exhibit a trend of 0.01°c/yr. these results are in agreement with previous studies in the region (cavazos et al., 2020; andrade-velázquez et al., 2021). on the other hand, there were 5 indices (csdi, csdi5, dtr, hddheat18, txge30p) with a trend of zero, meaning they do not show an increasing or decreasing pattern. additionally, there was only one climate change index in which its trend was not statistically significant, and that is the case of txb3tnb3. however, the indices that showed a significant trend indicate the detection of local warming and, therefore, climate change (ipcc, 2012). zarazúavillaseñor et al., (2014) calculated climate change indices for various coastal areas south of the gulf of mexico in the period 1961-2007. they reported an increase in hot days and warm nights in paraíso. therefore, we suggest that the climate change indices presented in this study can be used as standard markers for detecting climate change in the study area. it is worth noting that these indices are used in the energy, agriculture, food security, coastal, health, and environmental sectors (climpact, 2023). globally, several studies have applied climate change indices to monitor changes in climate extremes. they have also been used in regional analyses (montero-martínez et al., 2022). this study determines climate change indices for the coastal area of paraíso, tabasco municipality, where the lanresc coastal resilience observatory is located. these indices indicate a continuous increase in temperatures in recent years. the resulting impacts primarily affect the comfort of the population and will have a more significant impact on vulnerable sectors, such as the elderly and children, who make up 27% of the population in the area, according to inegi (2021). the federal government (cofepris, 2023) has reported health effects due to the presence of heatwaves. another effect of rising temperatures is the growing demand for water for consumption and productive activities (inegi, 2017). escudero & mendoza (2021) report that tabasco is among the states affected by climate change in terms of livelihoods, health, and local subsistence. jiménez-hernández et al. (2021) state that populations in the gulf of mexico coasts are migrating due to climate change events, and the communities’ infrastructure is vulnerable to these events. however, it is important to note that human settlements along the paraíso coast have recently increased due to oilrelated activities, which exposes a larger population to climate change. on the other hand, the stress experienced by ecosystems due to warming in the study area endangers wildlife species (yáñez-arancibia & day, 2010). sauz-sánchez et al. (2021) report that increasing temperatures may lead to the displacement of fish to other regions in land. andrade-velázquez & monteromartínez (2023) demonstrate that temperatures have been rising at a rate of 0.025°c per year in southeastern mexico, and this trend may continue through the year 2021-2040 under the ssp4-6.0 scenario. additionally, andradevelázquez et al. (2021) report that the southeastern region of mexico, central america, and the caribbean are experiencing a similar continental warming trend (0.10-0.2°c per year). ncei (2023) reports an increase in the temperature of the gulf of mexico. sanchez-garcía et al. (2017) suggest that this could alter currents in the southern gulf of mexico, affecting species and serving as an indicator of warming. on the other hand, the rising temperature in the marine influence zone will create favorable conditions for the formation of more tropical cyclones. while their trajectories cannot be predicted with certainty, it is likely that there will be greater impacts in the area (dominguez et al., 2021; appendini & ruiz-salcines, 2022). it is important to highlight that the ocr-tabasco is a socioecosystem, composed of multiple subsystems with different variables that interact with each other, making it a complex system, in accordance with the ecosystem definition by ostrom (2009), liu et al. (2007), and vidal-hernández et al. (2020). according to rodríguez-fuentes et al. (2020), there are two types of stressors that can impact socioecosystems, exogenous ones (which generate changes) and endogenous ones (inherent). climate-related changes fall under the exogenous category. thus, climate change indices are measures of these stressors affecting the ocr and its historical trajectory to determine its resilience (ávila-foucat et al., 2020). furthermore, these indices allow us to provide quantifiable information about the climate to address climate change and its effects, aligning with goal 13, “climate action,” of the 17 sustainable development goals (onu, 2023). in the mind map, the climate change indices trends for the ocr-tabasco are represented from the significant m. in this graph, the (percentiles) climate change indices show trends, while the other indices remain close to zero. the local area is warming, as reported by aguilar et al. (2005) for southern mexico, and by andradevelázquez et al. (2021) and andrade-velázquez and montero-martínez (2023). these results help determine the climate change impacts that are occurring and could occur in the study area. the socio-ecosystem is facing increasing warming; if this warming is not addressed with mitigation actions, the population will suffer more health impacts. in addition, adaptation actions in the socio-ecosystem must be applied now to conserve various communities and strengthen socio-ecosystem resilience. the vulnerability of coastal areas is also evident due to warming, not only because of coastal erosion but also the rising sea levels. conclusions this study reports, for the first time, an analysis of 26 climate change indices for the municipality of paraíso, tabasco, which includes the dos bocas port in tabasco and the coastal resilience observatory of lanresc. the information presented reveals increasing trends in the indices of gddgrowq, gsl, su, tmge10, tmge5, tn90p, tr, tx90p, txgt50p, wsdi, and swdi5. the indices that show decreases are tn10p and tx10p, which correspond to cold temperatures in the study area. all these indices reflect local warming and the detection of climate change, in accordance with the world meteorological organization and the ipcc (ipcc, 2012). 221climatic temperature indices for the coastal resilience observatory in tabasco (dos bocas)‒lanresc this study is the first to apply the methodology of climate change indices with statistical significance tests, following montero-martínez & andradevelázquez (2022). the intention is to replicate this approach in the other 6 ocrs of lanresc. it is worth noting that there is not much literature on this topic for coastal areas in mexico, especially in the study area. the significance of this site is regional, given its volume of cargo and vessels, as it is a port. additionally, it is home to the refinery owned by petróleos mexicanos (pemex). therefore, this study serves as a reference for future environmental and social research seeking information on climate change events in the study area. acknowledgments to the investigadores por méxico program, formerly cátedrasconahcyt, particularly to project 945. to dr. paullo salles and dr. gema hidalgo of the laboratorio nacional de resiliencia costera (lanresc). to the networks in mexico: redesclim, ugm and lancac. references adams, s. r. 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(2025). the sedimentary environment and source of ore-forming materials of sandstone-type copper deposits in southeast kuqa basin, xinjiang, china. earth sciences research journal, 25(1), 23-33. https://doi.org/10.15446/esrj.v29n1.117846 record manuscript received: 03/12/2024 accepted for publication: 10/03/2025 abstract a series of sandstone-type copper deposits and occurrence distribute in the kuqa basin, xinjiang. based on field surveys, we carried out research into the sedimentary environment and metallogenic model of copper deposits southeast of the basin. the relationship between the sedimentary environment/facies in the area and the copper mineralization were summarized to explore the genesis of mineralization and the metallogenic model. we identified main sedimentary facies types of alluvial fans, fan deltas, and lakes in the neogene strata, and further identification of five subfacies and seven microfacies were made. our research results reveal that the genesis of sandstone-type copper mineralization is related to the sedimentary environment, and the mineralized bodies are mainly distributed in fan delta plain facies and fan delta front facies. detrital zircon dating results of two samples from kangcun area are 427.4 ± 6.6 ma, 387.3 ± 3.3 ma and 424.6 ± 2.3 ma, 279.9 ± 6.8 ma.the other two samples from duwanke area are 285.6 ± 3.0 ma, 449.5 ± 3.2 ma and 427.5 ± 2.3 ma. it indicates that the source of the copper-bearing sandy conglomerate in the miocene jidike formation (n1j) is mainly from the strata and magmatic rock products formed by the geological evolution during the late silurian to early permian keywords: kuqa basin; sandstone-type copper deposit; la-icp-ms; sedimentary environment palabras clave: cuenca kuqa; depósitos cupríferos de tipo arenisca; ambiente sedimentario. entorno sedimentario y fuente de materiales formadores de minerales de depósitos de cobre tipo arenisca en el sureste de la cuenca de kuqa, xinjiang, china una serie de depósitos y ocurrencias de cobre de tipo arenisca se distribuyen en la cuenca de kuqa, xinjiang. basado en observaciones de campo los autores realizaron esta investigación del ambiente sedimentario y del modelo metalogénico de los depósitos de cobre en sureste de la cuenca. la relación ambiente/facies sedimentaria en el área y la mineralización cuprífera fueron sintetizada para explorar el origen de los modelos de mineralización y metalogenia. los autores identificaron los principales tipos de facies sedimentarias de los abanicos aluviales, deltas en abanico y lagos en los estratos neógenos, y se realizó una identificación adicional de cinco subfacies y siete microfacies. los resultados de la investigación revelan que el origen de la mineralización cuprífera tipo arenisca se relaciona con el ambiente sedimentario y que los cuerpos mineralizados se distribuyen principalmente en facies de llanura deltaica y facies de frente deltaico. los resultados de datación con circón detrítico de dos muestras del área de kangcun presentan los rangos de 427.4 ± 6.6 ma, 387.3 ± 3.3 ma y 424.6 ± 2.3 ma, 279.9 ± 6.8 ma. las otras dos muestras del área de duwanke estan entre 285.6 ± 3.0 ma, 449.5 ± 3.2 ma y 427.5 ± 2.3 ma. esto indica que la fuente del conglomerado arenoso de contenido cuprífero en la formación jidike del mioceno proviene principalmente de los estratos y productos de roca magmática formados por la evolución geológica durante el silúrico tardío al pérmico temprano. resumen 24 qiji fu, xinxian xu introduction the tarim basin is a metallogenic and exploration area for the coexistence of important metal minerals, coal, and oil resources in china (lin c s et al., 2002, cao y t, 2010, zhao m j et al., 2015, fang w x et al., 2016, xiao y, 2018, lu k g et al., 2019, zhang t et al., 2020), which became a hot spot in the study of tectonic evolution and metallogenic laws in recent years (liu z h et al., 2000, yan f l et al., 2003, tang l j et al., 2004, charreau j et al., 2006, cao y t, 2010, guan s w et al., 2010, li x et al., 2013, cai h a et al., 2022, cai h a, 2023). the sedimentary development of mesozoic terrigenous clastic rocks in the study area is relatively complete, and the sedimentary characteristics of foreland basins are obvious (jia c z et al., 1995). scholars have conducted detailed studies on the structure (qian j f et al., 2012, lu k g et al., 2019), sequence (guo x p et al., 2002, lin c s et al., 2002, shao l y et al., 2007), sedimentary environment (li j f et al., 2017) and paleogeography (shao l y et al., 2006, tan x c et al., 2006). there are a series of sandstone-type deposits and occurrences in the cenozoic strata in the kuqa basin, such as the dishui copper mine (liu d q et al., 2005). however, the relationship between sandstone-type copper deposits and the source-sink system of sedimentary basins needs to be further studied. in recent years, more and more isotopic dating of zircon in clastic rocks has been applied to the study of basin provenance and craton geological evolution (wu f l and yao z g, 2011, hu b et al., 2013). therefore, based on field investigation, this paper tests the age of volcanic clastic zircon in the copper-bearing conglomerates of the miocene jidike formation(n1j), which provides chronological evidence for the study of the relationship between sandstone-type copper deposits and the source-sink system of sedimentary basins, and provides a theoretical basis for future exploration work in this area. at the same time, the record of geological events preserved by detrital zircon in clastic rocks also provides important evidence for the geological evolution of the tarim craton. 1. geological background there is a copper ore belt with a length of 1000 km and a width of 15~70 km in the northern margin of tarim (fig. 1a). the zone is composed of paleogene and neogene fluvial-lacustrine sediments. the copper-bearing sandstone-type copper sources are concentrated in the paleogene of the kuqa basin and the wucha basin (liu d q et al., 2005). among them, the kuqa basin has undergone 3 stages of tectonic movement (liu z h et al., 2000), and the neogene-quaternary tectonic movement has formed a large thrust fold system and a series of thrust faults in front of the tianshan mountains (zhang m l et al., 2004). it has the structural characteristics of “north-south zone, eastwest section, upper and lower layering”. the copper metallogenic belt is mainly located in the qiulitag anticline tectonic zone (fig. 1a). the outcropping strata in the area are neogene and quaternary, and sandstone-type copper deposits are produced in neogene strata. among them, the miocene jidike formation (n1j) and kangcun formation (n1k) are ore-bearing horizons with relatively stable production (fig. 1b), while the upper miocene kuqa formation only occasionally occurs copper mineralization at the sandstone and mudstone interface. the miocene gidiq formation (n1j) is a set of continental clastic rocks of the delta front, mainly interbedded with gray-green conglomerate, purple-red, brown-red mudstone and sandy mudstone, interbedded with a small amount of light purple-red sandstone, with a thickness of 300~840 m, and the upper part is marked by a gypsum layer and gray-green argillaceous siltstone at the top, and copper mineralization is common in the coarse clastic rocks of this formation (fig. 2a), mostly in a layered distribution (fig. 2b). the kangcun formation (n1k) is a set of continental clastic sediments on the delta front, mainly gray-green mudstone and sandy mudstone interbedded with a small amount of gray-green mudstone and sandy mudstone, interbedded with gray conglomerate and argillaceous medium-coarse-grained sandstone, etc., with cross-bedding (fig. 2c) and occasional traces of bioturbation (fig. 2d), which can be further subdivided into the first member of the kangcun formation and the second member of the kangcun formation. the first member of the kangcun formation (n1k 1) is mainly interbedded with light reddish-brown mudstone and medium-thin bedded gray-green sandstone, with occasional fading and alteration at the boundary (fig. 2e), which is a favorable site for copper mineralization, among which there are many interbeds of local graydark gray thick-bedded conglomerate, interspersed with gypsum network veins (fig. 2f), and multi-layered thin-bedded copper mineralization is found in coarse sandstone, mainly manifested as chalcochlorite and malachite (fig. 2g, 2h). the second member of the kangcun formation (n1k 2) is mainly earthy yellow mudstone with a small amount of thin-bedded sandstone, with a thickness of 205~540 m. the kuqa formation (n2k) is a set of continental clastic deposits, with a lithology dominated by gray-dark gray, brown, and a small amount of yellow-green coarse-boulder conglomerate, interbedded with sandy mudstone, argillaceous medium-coarse sandstone, etc. (fig. 2i), which is in integrated contact with the under kangcun formation. figure 1. maps showing the structural zoning and copper deposits distribution in the kuqa basin and a simplified geological map of the kangcun area (a) structural zoning map of the kuqa basin (li x et al., 2013); (b) simplified geological map of the study area 25the sedimentary environment and source of ore-forming materials of sandstone-type copper deposits in southeast kuqa basin, xinjiang, china figure 2. field outcrops of the sandstone-type copper deposits in the southeast kuqa basin (a) copper mineralization in conglomerates of the jidike formation; (b) copper mineralization in coarse sandstones of the jidike formation; (c) cross bedding in coarse sandstone of the kangcun formation; (d) bioturbation traces in argillaceous sandstones of the kangcun formation; (e) fading alteration in sandstones of the kangcun formation; (f) coarse sandstones crossed by gypsum veins in the kangcun formation; (g) hand specimen of ore body in the kangcun formation; (h) copper deposits at the interface between sandstone and mudstone in the kangcun formation; (i) sandstone and mudstone interbedding in the kuqa formation 2 sandstone-type copper sedimentary environment in the kuqa basin the kuqa basin is a regenerated foreland basin (yan f l et al., 2003), and the triassic foreland basin underwent mesozoic basin extension, late cretaceous extrusion, uplift and denudation, and was revived in the cenozoic after paleocene transgression (jia c z et al., 1995, qian j f et al., 2012), at which time a set of continental clastic sediments were deposited in the kuqa basin that were denuded by the uplift of the tianshan orogenic belt (yang g and qian x l, 1995). during the miocene, the tianshan orogenic belt on the northern margin of the foreland basin was uplifted rapidly, and the topographic elevation difference was very different, with alluvial fan-braided fluvial deposits in the piedmont zone, alluvial fans in the jidike formation, and fluvial alluvial plains in the south (tian z j and song j g, 1999). the ore-bearing rocks in the kangcun area are the jidike formation (n1j) and the first member of the kangcun formation (n1k 1), which are a set of stably distributed variegated clastic deposits such as purple-red, brown-red, gray-green, and dark gray, with relatively developed sedimentary structures and little change in thickness. 2.1 sedimentary petrology characteristics the lithologic assemblage of the jidike formation (n1j) in the study area is dominated by gray-green conglomerate, brown-red sandy mudstone, interbedded with a small amount of light purple-red sandstone, which is a set of continental clastic sediments at the delta front. the lithologic assemblage of the kangcun formation (n1k) is dominated by earthy yellow, light purple-red interspersed with a small amount of gray-green mudstone and sandy mudstone, interbedded with gray conglomerate, argillaceous medium-coarse-grained sandstone, etc., which is a set of continental clastic deposits of the delta front. the lithology of the kuqa formation (n2k) is mainly gray-dark gray, brown, with a small amount of yellow-green coarse-boulder conglomerate, interbedded with sandy (conglomerate) mudstone, argillaceous medium-coarse sandstone, etc. (fig. 2i), which is in integrated contact with the under kangcun formation and is a set of continental clastic deposits. 2.2 sedimentary structures the field investigation found that various sedimentary structures such as bedding structures and layer structures were developed in the neogene strata in the study area: cross-bedding and undulating bedding were common bedding structures in the strata in the study area (fig. 2c), and the common layer structures included wave marks and bottom scouring structures. 2.3 paleontological characteristics the content of biological fossils in the area is very low, and only a small number of fossils from biological caves are found in the field (fig. 2d), which are commonly found in siltstone and argillaceous siltstone. 2.4 sedimentary system division and characteristics the neogene sedimentary systems in the kangcun area of the kuqa basin are mainly alluvial fan-fan delta and lacustrine (fig. 3a), and the main sedimentary facies types can be identified as alluvial fan-delta facies and lacustrine facies (fig. 3b-3d), and five subfacies and seven microfacies types can be identified (fig. 1). the main sedimentary facies characteristics are described below. 2.4.1 alluvial fan facies the alluvial fan facies in the kangcun area are concentrated in the kuqa formation and the quaternary, which reflects that the survey area is arid with less precipitation, and the temporary foothill torrent develops and forms at the 26 qiji fu, xinxian xu mouth of the valley (fig. 3b). the kuqa formation in the area is a set of coarse clastic assemblages with typical alluvial fan facies characteristics, and the lower part is gray conglomerate upward to gray massive conglomerate and coarse conglomerate interbedded, and occasionally coarse sandstone lenses with oblique bedding development. 2.4.2 fan delta facies the important conditions for the development of fan delta are the large terrain elevation difference near the shore of the impoundment body, the steep and narrow slope on the shore, and the sufficient supply of detrital materials near the source. the study area is mainly distributed in the strata of the jidike formation and the kangcun formation (figs. 3b and 3c), which is dominated by mudstone and locally interbedded with thin-bedded siltstone, with horizontal bedding and wave-forming sand-grained bedding in siltstone. it can be subdivided into fan delta plain subfacies, fan delta front subfacies and forefan delta subfacies. (1) fan delta plain subfacies the subfacies of the fan delta plain is characterized by the development of coarse clastic rocks, the bedding is not developed, and the sorting and roundness are poor. in the kangcun area, the subfacies can be subdivided into two microfacies, the former is mainly composed of conglomerate, coarse sandstone and a small amount of siltstone, and the bedding includes granular sequence bedding, parallel bedding and cross-bedding (fig. 2c), and the latter is composed of mudstone and thin-bedded siltstone, with horizontal bedding and a small amount of cross-bedding. the subfacies zone of the fan delta plain is mainly concentrated in the kangcun formation (fig. 3c). (2) fan-delta front subfacies the subfacies of the fan delta front is the sedimentation of the underwater part of the fan delta, which is usually located in the shallow water between the shoreline and the wave base. in the kangcun area, it can be subdivided into underwater distributary channel microfacies, underwater distributary channel microfacies, estuarine sand bar microfacies and leading mat sand microfacies. the sediments of the underwater distributary channel have medium-tosmall crossbedding, oblique bedding and bottom scouring structures, and the estuarine sand bars have antigranular sequence and cross-bedding. the microfacies between the underwater distributary channels in the area is mainly composed of mudstone and siltstone, with a small amount of fine sandstone, and horizontal bedding is developed. the mat sand microfacies sediments at the front edge have finer grain size and better sorting, and small cross-bedding and undulating bedding are developed. the subfacies zone of the fan-delta front margin is mainly concentrated in the jidik formation and the kangcun formation (figs. 3c, 3d). (3) front fan-delta subfacies it is in a deep-water area below the wave base plane, and the lithological assemblage is mudstone and thin-bedded siltstone, and the horizontal bedding is developed. the subfacies is underdeveloped in the kangcun area, with a small and discontinuous area, and the lithological assemblage is mudstone and siltstone, and the overall grain sequence is coarse upward. figure 3. sedimentary facies model and distribution of sedimentary facies in the kuqa basin since the neogene (li x et al., 2013; shi w g et al., 2015) (a) diagram showing the sedimentary mode of alluvial fan-fan delta-lake; (b) distribution of sedimentary facies in the kuqa formation; (c) distribution of sedimentary facies in the kangcun formation; (d) distribution of sedimentary facies in the jidike formation table 1. division of sedimentary facies in the kangcun area, kuqa foreland basin sedimentary system sedimentary facies subfacies microfacies distribution strata terrestrial sedimentary system alluvial fan facies n2k, q fan delta facies fan delta plain distributary channel, diversion bay underwater diversion channels, between underwater distributary channels, estuary sandbar, front edge sheetlike sand fan delta front n1j, n1k front fan delta lake facies shore lake gypsum lake gypsum sedimentation n1j, n2k 27the sedimentary environment and source of ore-forming materials of sandstone-type copper deposits in southeast kuqa basin, xinjiang, china 2.4.3 lake facies it is mainly distributed in the southern part of the study area (fig. 3d), represented by thick-bedded mudstone gypsum rocks, which can be divided into shallow lake subfacies and gypsum lake subfacies, and the lacustrine facies in the study area generally has a southward shrinking trend since the paleogene. 3 discussions 3.1 characteristics of copper orebody and its relationship with sedimentary facies configuration the overall characteristics of copper mineralization in the study area are obviously controlled by stratigraphy, which are mainly concentrated in the miocene jidike formation and the first member of the kangcun formation. copper mineralization is dotted and disseminated in gray-white (gravel) coarse sandstone, mainly along the interlayer or nearly vertical layer of sand and mudstone, and the interlayer and joint surface are filled with salt and gypsum, and the copper mineral is impregnated to the surface of gypsum and salt, with a small thickness but a large intermittent extension. copper minerals include chalcochlorite, malachite, azurite and bornite, and the surrounding rock alteration is not developed, which is a sedimentary origin of sand type copper mineralization, and local copper mineralization enrichment areas can be mined. field observations and laboratory studies have identified four mineralized bodies of varying sizes, all of which are more than 100 m in length, three of which are over 500 m in length, and vary in thickness from 5 cm to 2.4 m (fig. 1b). in the early neogene giddic period, the lake shrank to the south, mainly in the fan delta and lacustrine sediments; in the kangcun period, the sedimentary pattern of the gidiq period was basically inherited, the fan delta area expanded, the lake area shrank to the south, and the piedmont to the basin was a fan delta to lake sedimentation; in the kuqa period, alluvial fans mainly developed in the piedmont, and alluvial fan facies, alluvial plain facies and lacustrine facies were developed from north to south; in the quaternary the lakes disappeared, alluvial fan sediments were mainly developed (fig. 3; li x et al., 2013). the genesis of sandstone-type copper mineralization is closely related to the sedimentary environment of neogene fluvial-lacustrine facies, and the minerals are mainly distributed in the fan-delta plain facies and fandelta front facies. on the one hand, it is due to the development of the porosity of coarse clastic sedimentary rocks, which is conducive to the enrichment of copper-bearing materials, while coarse clastic sediments are often developed in places with strong hydrodynamics. in addition, sandstone-type copper mineralization is often related to the oxidation-reduction chemical phase at the time of formation, and the mineralized bodies observed in the field are mostly produced on the side of the light-colored layer at the junction of light-colored clastic rocks and purple-red clastic rocks, which represents the initial source layer of the oxidized geochemical facies purple-red layer, and the light-colored layer of the reducing geochemical facies is the place where minerals accumulate during diagenesis or metagenesis. this oxidation-reduction chemical phase is generally found at the junction of rivers and lakes, that is, at the junction of rivers and lakes that represent the oxidizing environment. this phenomenon is like that of dishui copper (shi w g et al., 2015; fig. 3). combined with the geophysical and geochemical anomalies and the surface verification results, the three prospecting prediction areas (fig. 1b) are all distributed at the junction of rivers and lakes centered on the jidik formation. 3.2 provenance evolution of ore-bearing conglomerates in the gidik formation the provenance zone is one of the key factors for the characteristic change of sediments in the basin. the kangcun area is located at the intersection of multiple provenance systems, and the paleogene-neogene sedimentary system is complex and changeable. from the perspective of the tectonic position of the study area, the south tianshan mountains and the wensu bulge were the main sediment supply sources in this period, and they were the main source supply areas in the study area during the neogene jidik-kangcun period (jia c z et al., 1995, qiu f q et al., 2000, li z et al., 2004, li s j et al., 2006). qiu f q et al. (2000) quantitatively analyzed the skeletal mineral composition of the sandstone, and concluded that the cretaceous-paleogene source area in the kuqa basin was mainly the result of denudation and redeposition of the carboniferous strata in the southern margin of the south tianshan mountains and the silurian-devonian strata in the south tianshan mountains. li z et al. (2004) compared and analyzed the clastic assemblage and regional climate change events in the kuqa basin and the time of tectonic uplift in the tianshan region, and suggested that the clastic rocks of the miocene jidik-kangcun formation, especially the upper miocene kuqa formation, may be the result of the superposition of tectonic uplift and frequent climate changes in the tianshan mountains. li s j et al. (2006) systematically analyzed the characteristics of heavy mineral content in the sandstone of kuqa basin, and concluded that the largescale uplift and denudation of the south tianshan mountains occurred since the paleogene, because its uplift obscured the material contribution of the paleozoic accretion complex on the northern margin of the south tianshan mountains. in order to study the provenance zone and age of the mineral-bearing conglomerate in the jidike formation, two copper-bearing conglomerate age samples were collected in the area of yanshuigou in the kangcun area, numbered ysg-1 and ysg-2, respectively (fig. 1b). the sample was crushed to below 60 mesh (250 μm), the zircon was coarsely separated by magnetic separation and heavy liquid method, and the zircon particles were picked out under the microscope. the zircon particles are fixed in transparent epoxy resin for grinding and polishing. cathodoluminescence photography of the polished zircon shows that the shape of the zircon is sub-circular and sub-angular, most of which remain more self-shaped prismatic, and most of them are magmatic zircon with oscillating rings. cathodoluminescence photography and zircon u-pb isotope testing was done at the state key laboratory of continental dynamics at northwest university. the u-pb isotopic age was measured by agilent7500a laser ablation inductively coupled plasma mass spectrometry (la-icp-ms), and the results of cathodoluminescence irradiation, analysis points and test results are shown in figures 4 and 5. the ysg-1 samples measured the harmonic ages of many groups, among which 427.4 ± 6.6 ma and 387.3 ± 3.3 ma harmonics were better, and the ysg-2 samples also measured the harmonic ages of many groups, among which 424.6 ± 2.3 ma and 279.9 ± 6the age harmony between the two groups was better at 8 ma. the detrital zircon dating statistics show the distribution peaks of 427 ma and 424 ma in the early paleozoic, corresponding to the zircon formed by the late silurian magmatic activity, 387 ma corresponding to the zircon formed by the middle devonian magmatic activity, and the 280 ma distribution peak, corresponding to the zircon formed by the early permian magmatic activity in the late paleozoic. the halke mountain intrusive rocks in the southwest tianshan mountains are dominated by silurian intrusive rocks, followed by carboniferous intrusive rocks, and the lithology is dominated by diorite. liu b p et al. (1996) measured the ping age of the silurian intrusive amphibolite to be 420.2 ± 5.9 ma and 430.3 ± 5.2 ma, and the time period was middle silurian epoch, which was consistent with the results of this age. volcanic rocks are developed in the lower devonian altenkos formation outcropped in the halke mountain stratigraphic community, and their lithology is mostly basalt, which may be the source of early devonian magmatic zircon. liu b p et al. (1996) obtained an isotopic age of 282 ± 2 ma (40ar/39ar method) in the medium-acid volcanic rocks near 956 km of duku highway, which is consistent with the zircon harmonic age of 279.9 ± 6.8 ma in the ysg-2 sample. li z et al. (2004) inferred from the sandstone component model that the development of the source type of the cenozoic “arc orogenic belt” in the middle cenozoic of the kuqa depression is an indirect indication of the main curtain tectonic environment of the late paleozoic tianshan orogenic orogenic structure. the provenance evolution of the gidiq formation and the tectonic uplift of the tianshan mountains are closely related to orogenesis. the zircon dating data of copper-bearing conglomerates of the neophanic gidike formation (n1j) in this paper show that the source mainly comes from the stratigraphy and magmatic rock products from the late silurian to the early permian in the tianshan orogenic belt in the northwestern and southwest of the kuqa basin. it may be the result of denudation and redeposition of late silurian to early permian strata. at the same time, along with the denudation of existing copper deposits in the orogenic belt, the copper ore-forming elements also enter the kuqa basin, thus forming copper mineralization that spreads along the strata. the determinant of the scale of copper mineralization is determined by the supply of copper-forming minerals in the source area, that is, it is controlled by the size of the existing copper deposits (points) in the source area of the tianshan orogenic belt in southwest china, and the occurrence horizon is also limited by the time limit of the denudation and outcropping of the existing copper deposits (points), which in turn forms the mineral-bearing characteristics 28 qiji fu, xinxian xu of the miocene jidike formation in the eastern part of the kuqa basin, and the mineral-bearing characteristics of the kangcun formation represented by the dishui copper deposit in the western part of the kuqa basin. regionally, the area of sarek—urukchati—ullagen—sikel is a concentration area of large-medium conglomerate copper-lead-zinc deposits, represented by the sarek copper deposit, huayuan copper deposit and jiashi copper deposit, and the sarek copper deposit’s ore-bearing horizon is jurassic, the cretaceous continuous sedimentary stratigraphy (zhu x y et al., 2011), the ore-bearing structure of the sazheke copper deposit is the conglomerate of the upper jurassic kuzhugongsu formation (j3k) and the purple conglomerate of the lower cretaceous kizilsu group (k1kz), and the tie creek copper mine in wensu county, the ore body is mainly hosted in the quartz sandstone of the middle devonian kiziltao formation (d2kz) (zhang c g et al., 2016). the tamu-kalangu lead-zinc belt is in the piedmont zone of the west kunlun mountains, and the deposits/occurrences occur in the carbonate rocks of the upper devonian-lower carboniferous platform (zhu x y et al., 2014). these deposits may have some potential connection to mineralisation in the study area in time and space. figure 4. cl images, shrimp dating locations and results of pyroclastic zircons separated from sandstones of the jidike formation in the yanshuigou area (a) cl image and shrimp dating locations of the sample ysg-1;(b,c)dating results of the sample ysg-1; (d) cl image and shrimp dating locations of the sample ysg-2; (e,f) dating results of the sample ysg-2 figure 5. cl images, shrimp dating locations and results of pyroclastic zircons separated from sandstones of the jidike formation in the duwanke area (a) cl image and shrimp dating locations of the sample ysg-1;(b,c)dating results of the sample ysg-1; (d) cl image and shrimp dating locations of the sample ysg-2; (e,f) dating results of the sample ysg-2 29the sedimentary environment and source of ore-forming materials of sandstone-type copper deposits in southeast kuqa basin, xinjiang, china ta bl e2 . d et rit al zi rc on d at in g re su lts o f c op pe r b ea rin g co ng lo m er at e s am pl es sa m pl e n o to ta l p b 23 2 t h /1 0-6 23 8 u /1 0-6 20 7 p b/ 20 6 p b 20 7 p b/ 23 5 u 20 6 p b/ 23 8 u 20 8 p b/ 23 2 t h 23 8 u /23 2 t h 20 6 p b/ 23 8 u c on co rd an ce r at io 1σ r at io 1σ r at io 1σ r at io 1σ r at io a ge /m a 1σ y sg -0 11 20 .8 28 5 15 3. 72 54 26 3. 02 02 0. 06 37 1 0. 00 75 8 0. 60 77 9 0. 07 22 7 0. 06 89 1 0. 00 15 0 0. 01 08 0 0. 00 14 8 1. 06 95 2 42 9. 6 9. 1 88 % y sg -0 12 24 .4 74 0 23 0. 85 66 28 7. 64 42 0. 06 51 1 0. 00 50 3 0. 61 86 9 0. 04 50 3 0. 06 91 9 0. 00 13 4 0. 01 07 3 0. 00 14 8 0. 77 83 1 43 1. 3 8. 1 87 % y sg -0 14 20 .4 03 7 17 4. 30 41 27 8. 51 60 0. 05 78 5 0. 00 34 8 0. 48 76 7 0. 02 84 0 0. 06 13 3 0. 00 11 5 0. 00 99 2 0. 00 14 7 1. 00 50 7 38 3. 7 7. 0 95 % y sg -0 15 41 .5 55 0 31 6. 60 94 52 4. 63 20 0. 05 78 7 0. 00 26 2 0. 54 62 8 0. 02 45 7 0. 06 82 7 0. 00 11 3 0. 00 91 9 0. 00 14 0 1. 04 38 7 42 5. 7 6. 8 96 % y sg -0 16 30 .9 22 3 19 2. 00 36 66 5. 33 73 0. 05 94 8 0. 00 35 8 0. 35 73 4 0. 02 15 1 0. 04 33 6 0. 00 08 4 0. 00 61 7 0. 00 10 2 2. 22 31 9 27 3. 6 5. 2 87 % y sg -0 17 31 .0 36 6 29 0. 95 65 41 1. 40 12 0. 05 29 4 0. 00 29 6 0. 45 89 5 0. 02 61 7 0. 06 26 4 0. 00 11 1 0. 00 80 4 0. 00 14 2 0. 90 87 8 39 1. 6 6. 8 97 % y sg -0 18 36 .5 19 3 32 9. 21 74 51 4. 09 55 0. 05 28 1 0. 00 28 2 0. 44 42 3 0. 02 31 3 0. 06 15 0 0. 00 13 3 0. 00 75 4 0. 00 14 6 1. 08 89 5 38 4. 7 8. 1 96 % y sg -0 19 50 .0 46 0 35 4. 59 38 62 8. 51 15 0. 05 50 5 0. 00 22 1 0. 51 79 5 0. 01 99 1 0. 06 84 7 0. 00 10 4 0. 00 77 7 0. 00 16 6 1. 14 01 9 42 6. 9 6. 3 99 % y sg -0 110 23 .1 05 4 19 0. 33 37 31 6. 58 64 0. 05 84 3 0. 00 34 6 0. 48 75 6 0. 02 75 1 0. 06 12 5 0. 00 11 9 0. 00 71 5 0. 00 17 0 1. 11 44 5 38 3. 2 7. 2 94 % y sg -0 111 22 .9 08 0 15 0. 01 74 43 4. 05 88 0. 05 28 5 0. 00 31 7 0. 35 06 3 0. 02 05 9 0. 04 82 1 0. 00 08 3 0. 00 47 5 0. 00 12 7 1. 84 28 6 30 3. 5 5. 1 99 % y sg -0 113 25 .3 24 6 21 9. 85 76 33 2. 48 41 0. 06 30 0 0. 00 50 2 0. 53 71 9 0. 03 84 5 0. 06 29 8 0. 00 13 3 0. 00 56 7 0. 00 19 2 0. 95 72 0 39 3. 7 8. 0 89 % y sg -0 114 22 .3 46 5 13 0. 33 64 27 3. 69 30 0. 05 41 2 0. 00 33 6 0. 53 98 7 0. 03 31 2 0. 07 27 1 0. 00 14 7 0. 00 75 5 0. 00 18 5 1. 33 33 3 45 2. 5 8. 9 96 % y sg -0 115 61 .1 41 5 23 3. 41 06 12 34 .0 11 0. 05 26 8 0. 00 19 8 0. 34 69 3 0. 01 36 0 0. 04 78 2 0. 00 06 7 0. 00 57 0 0. 00 12 1 3. 29 47 7 30 1. 2 4. 1 99 % y sg -0 116 33 .8 36 2 14 0. 40 38 66 6. 28 65 0. 05 33 5 0. 00 26 5 0. 36 06 1 0. 01 86 4 0. 04 91 2 0. 00 10 1 0. 00 61 9 0. 00 11 6 3. 06 60 2 30 9. 1 6. 2 98 % y sg -0 117 65 .0 65 0 40 4. 47 70 13 26 .6 23 0. 06 10 1 0. 00 27 4 0. 37 27 7 0. 01 90 8 0. 04 41 9 0. 00 08 1 0. 00 77 6 0. 00 13 2 2. 05 16 2 27 8. 7 5. 0 85 % y sg -0 118 45 .1 71 1 52 7. 72 61 15 69 .8 16 0. 05 80 6 0. 00 26 8 0. 21 00 7 0. 01 19 9 0. 02 64 5 0. 00 10 2 0. 00 60 1 0. 00 08 5 1. 81 77 0 16 8. 3 6. 4 85 % y sg -0 120 10 .5 17 8 79 .2 26 1 13 5. 45 77 0. 05 40 6 0. 00 39 4 0. 50 64 3 0. 03 97 0 0. 06 73 8 0. 00 15 1 0. 01 04 5 0. 00 12 4 1. 07 06 1 42 0. 4 9. 1 98 % y sg -0 121 26 .3 64 1 18 2. 29 29 33 6. 56 29 0. 06 20 7 0. 00 33 4 0. 57 71 7 0. 03 20 0 0. 06 73 1 0. 00 10 0 0. 01 08 2 0. 00 10 7 1. 16 12 7 42 0. 0 6. 0 90 % y sg -0 122 17 .5 39 2 14 8. 01 03 20 7. 17 38 0. 05 45 4 0. 00 38 5 0. 52 41 8 0. 03 66 7 0. 07 06 4 0. 00 14 4 0. 01 17 4 0. 00 10 8 0. 86 75 7 44 0. 0 8. 7 97 % y sg -0 123 17 .6 21 2 15 6. 12 41 22 2. 39 30 0. 04 98 1 0. 00 29 5 0. 45 92 8 0. 02 55 4 0. 06 75 4 0. 00 11 4 0. 01 08 7 0. 00 09 0 0. 88 17 6 42 1. 3 6. 9 90 % y sg -0 124 11 .7 00 0 98 .3 66 3 14 8. 52 30 0. 05 16 3 0. 00 32 3 0. 48 19 2 0. 03 40 7 0. 06 74 3 0. 00 16 7 0. 01 16 3 0. 00 09 4 0. 93 20 1 42 0. 6 10 .1 94 % y sg -0 125 31 .2 62 5 21 0. 13 70 60 2. 78 60 0. 05 32 9 0. 00 25 3 0. 35 02 2 0. 01 61 6 0. 04 78 9 0. 00 07 1 0. 00 81 8 0. 00 06 6 1. 77 75 8 42 9. 6 9. 1 98 % y sg -0 21 13 5. 61 58 13 4. 92 38 23 8. 86 41 0. 15 21 7 0. 01 12 7 9. 60 45 6 0. 69 01 1 0. 45 77 0 0. 00 60 8 0. 02 78 4 0. 00 79 2 1. 14 19 2 24 29 .4 26 .9 98 % y sg -0 23 18 .6 28 9 12 0. 00 16 34 1. 27 43 0. 05 21 5 0. 00 42 6 0. 35 63 2 0. 02 82 1 0. 04 96 7 0. 00 08 9 0. 00 38 2 0. 00 10 5 1. 80 71 4 31 2. 5 5. 5 99 % y sg -0 24 15 .9 58 1 13 5. 08 96 31 8. 12 17 0. 05 22 6 0. 00 45 0 0. 32 11 1 0. 02 64 0 0. 04 49 6 0. 00 09 3 0. 00 33 1 0. 00 09 2 1. 52 29 9 28 3. 5 5. 7 99 % y sg -0 27 37 .8 18 1 23 6. 69 68 49 1. 66 46 0. 06 03 0 0. 00 32 4 0. 56 15 6 0. 02 99 7 0. 06 74 6 0. 00 12 1 0. 00 61 0 0. 00 18 2 1. 33 42 1 42 0. 8 7. 3 92 % y sg -0 28 23 .9 06 1 22 7. 88 84 45 1. 41 56 0. 05 17 6 0. 00 37 6 0. 33 11 1 0. 02 38 9 0. 04 61 7 0. 00 07 2 0. 00 40 1 0. 00 12 5 1. 44 70 8 29 1. 0 4. 4 99 % y sg -0 29 11 .0 27 6 84 .3 15 4 14 9. 24 52 0. 06 54 2 0. 00 61 9 0. 56 90 3 0. 05 50 9 0. 06 29 9 0. 00 17 2 0. 00 62 7 0. 00 20 6 1. 14 34 2 39 3. 8 10 .4 85 % y sg -0 210 10 .8 30 3 68 .5 12 4 12 8. 76 32 0. 05 61 3 0. 00 47 0 0. 54 20 3 0. 04 14 8 0. 07 12 2 0. 00 17 5 0. 00 80 5 0. 00 31 8 1. 19 99 8 44 3. 5 10 .5 99 % y sg -0 211 59 .0 62 1 38 6. 31 86 69 9. 72 88 0. 05 00 4 0. 00 23 7 0. 50 28 6 0. 02 21 7 0. 07 28 5 0. 00 11 0 0. 00 69 8 0. 00 24 3 1. 18 42 7 45 3. 3 6. 6 90 % (c on tin ue d) 30 qiji fu, xinxian xu sa m pl e n o to ta l p b 23 2 t h /1 0-6 23 8 u /1 0-6 20 7 p b/ 20 6 p b 20 7 p b/ 23 5 u 20 6 p b/ 23 8 u 20 8 p b/ 23 2 t h 23 8 u /23 2 t h 20 6 p b/ 23 8 u c on co rd an ce r at io 1σ r at io 1σ r at io 1σ r at io 1σ r at io a ge /m a 1σ y sg -0 212 74 .4 24 5 28 9. 88 81 14 95 .3 84 0. 05 54 1 0. 00 20 9 0. 36 21 4 0. 01 32 3 0. 04 71 1 0. 00 05 2 0. 00 48 5 0. 00 17 6 3. 29 50 2 29 6. 7 3. 2 94 % y sg -0 213 70 .9 20 1 26 7. 26 35 15 24 .0 89 0. 05 11 9 0. 00 21 1 0. 31 73 6 0. 01 27 2 0. 04 46 9 0. 00 05 6 0. 00 51 2 0. 00 19 3 5. 39 14 6 28 1. 8 3. 5 99 % y sg -0 214 39 .7 02 0 11 4. 68 60 43 3. 76 12 0. 05 80 8 0. 00 22 7 0. 69 34 9 0. 03 08 2 0. 08 55 8 0. 00 18 7 0. 01 12 8 0. 00 28 6 2. 41 55 6 52 9. 3 11 .1 98 % y sg -0 215 20 .4 95 3 14 5. 39 32 25 3. 70 40 0. 05 17 5 0. 00 25 9 0. 50 11 9 0. 02 41 5 0. 07 04 1 0. 00 11 6 0. 00 90 9 0. 00 20 7 1. 12 96 0 43 8. 6 7. 0 93 % y sg -0 216 30 .7 93 3 30 4. 56 31 36 8. 24 99 0. 05 31 4 0. 00 30 5 0. 49 97 5 0. 02 81 4 0. 06 79 8 0. 00 10 6 0. 00 96 8 0. 00 19 7 0. 79 11 7 42 4. 0 6. 4 97 % y sg -0 217 33 .8 73 4 23 0. 56 34 70 5. 73 06 0. 06 03 2 0. 00 24 8 0. 36 20 1 0. 01 34 3 0. 04 36 7 0. 00 06 3 0. 00 71 5 0. 00 13 0 1. 95 66 8 27 5. 5 3. 9 87 % y sg -0 218 73 .7 84 6 46 4. 22 36 15 74 .1 02 0. 05 25 2 0. 00 17 9 0. 31 76 4 0. 01 07 5 0. 04 36 4 0. 00 04 8 0. 00 67 7 0. 00 11 0 2. 23 25 1 27 5. 4 3. 0 98 % y sg -0 219 21 .7 60 9 15 3. 31 15 27 6. 76 11 0. 05 12 2 0. 00 26 5 0. 47 74 8 0. 02 48 6 0. 06 73 3 0. 00 10 5 0. 01 18 4 0. 00 17 6 1. 17 06 1 42 0. 0 6. 3 94 % y sg -0 220 35 .8 55 5 33 9. 62 82 43 2. 23 24 0. 05 28 3 0. 00 27 8 0. 49 56 1 0. 02 49 7 0. 06 80 0 0. 00 08 2 0. 01 14 9 0. 00 15 4 0. 81 89 0 42 4. 1 5. 0 96 % y sg -0 221 33 .7 58 1 19 9. 97 98 52 1. 30 27 0. 05 91 9 0. 00 27 6 0. 46 54 4 0. 02 05 9 0. 05 72 2 0. 00 08 7 0. 01 23 6 0. 00 15 4 1. 78 31 0 35 8. 7 5. 3 92 % y sg -0 222 33 .6 31 8 27 5. 58 40 42 0. 56 44 0. 05 22 8 0. 00 28 2 0. 48 50 3 0. 02 49 1 0. 06 77 3 0. 00 10 5 0. 01 24 8 0. 00 14 2 0. 98 18 1 42 2. 4 6. 3 94 % y sg -0 223 24 .3 25 9 20 6. 10 28 29 1. 52 42 0. 05 75 7 0. 00 31 3 0. 54 60 6 0. 02 85 9 0. 06 91 5 0. 00 12 1 0. 01 30 5 0. 00 14 1 0. 94 16 4 43 1. 0 7. 3 97 % y sg -0 225 14 .6 84 3 57 .8 39 5 95 .8 39 3 0. 06 55 9 0. 00 47 2 1. 16 39 8 0. 07 74 4 0. 13 08 8 0. 00 32 1 0. 02 72 0 0. 00 27 8 1. 08 34 0 79 2. 9 18 .3 98 % d w k -1 -1 40 .0 07 6 66 .9 94 8 19 0. 73 86 0. 08 59 8 0. 00 34 7 2. 20 17 0 0. 08 53 6 0. 00 26 7 0. 37 01 1 0. 04 04 1 0. 00 31 3 1. 75 04 1 10 99 .2 14 .5 92 % d w k -1 -2 41 .2 84 4 34 8. 41 92 49 6. 31 68 0. 05 57 4 0. 00 24 8 0. 53 32 4 0. 02 18 3 0. 00 08 9 0. 31 19 0 0. 01 35 5 0. 00 09 0 0. 87 63 7 43 3. 1 5. 3 99 % d w k -1 -3 16 .7 20 0 87 .2 62 1 10 8. 84 54 0. 07 00 5 0. 00 64 9 1. 19 98 4 0. 09 62 0 0. 00 36 8 0. 36 02 6 0. 02 33 1 0. 00 16 7 0. 76 97 0 77 3. 7 21 .1 96 % d w k -1 -4 12 .7 40 9 13 7. 47 39 16 5. 25 16 0. 05 98 7 0. 00 46 7 0. 51 83 2 0. 04 05 3 0. 00 14 4 0. 29 20 4 0. 01 24 0 0. 00 09 6 0. 74 20 2 39 3. 7 8. 7 92 % d w k -1 -5 19 .6 10 3 12 8. 94 89 24 9. 58 79 0. 05 96 2 0. 00 39 6 0. 55 18 1 0. 03 40 9 0. 00 11 5 0. 27 47 6 0. 01 47 6 0. 00 11 1 1. 20 13 3 42 1. 0 6. 9 94 % d w k -1 -6 26 .2 06 3 18 8. 57 19 53 7. 65 76 0. 05 35 4 0. 00 28 3 0. 33 19 2 0. 01 69 8 0. 00 08 2 0. 35 76 0 0. 00 93 1 0. 00 07 3 1. 76 57 9 28 1. 8 5. 0 96 % d w k -1 -7 24 .6 17 3 24 1. 58 22 29 3. 86 25 0. 06 26 1 0. 00 40 3 0. 58 57 4 0. 03 55 0 0. 00 09 5 0. 23 19 5 0. 01 34 9 0. 00 11 0 0. 77 33 7 42 0. 3 5. 7 89 % d w k -1 -8 33 .3 89 4 24 5. 75 39 44 4. 62 97 0. 05 21 8 0. 00 32 0 0. 48 30 5 0. 02 82 5 0. 00 09 7 0. 25 07 2 0. 01 17 4 0. 00 09 8 1. 12 53 2 41 4. 4 5. 9 96 % d w k -1 -1 2 17 .2 75 4 13 7. 49 36 21 3. 99 84 0. 05 15 0 0. 00 47 6 0. 51 09 9 0. 04 49 8 0. 00 12 3 0. 19 69 4 0. 01 24 0 0. 00 14 0 0. 95 70 3 44 3. 5 7. 4 94 % d w k -1 -1 3 20 .7 47 5 15 5. 55 42 30 9. 83 79 0. 05 65 4 0. 00 54 5 0. 46 77 4 0. 04 32 6 0. 00 10 7 0. 19 62 5 0. 01 14 3 0. 00 13 4 1. 24 99 5 37 0. 1 6. 5 94 % d w k -1 -1 5 18 .6 07 0 10 9. 04 50 24 3. 49 41 0. 05 52 5 0. 00 38 4 0. 53 26 1 0. 03 50 2 0. 00 11 5 0. 25 30 6 0. 01 19 4 0. 00 15 3 1. 39 85 9 42 9. 9 6. 9 99 % d w k -1 -1 6 10 .3 97 7 11 6. 12 28 12 3. 03 28 0. 05 89 8 0. 00 62 9 0. 54 75 3 0. 05 31 3 0. 00 17 1 0. 26 15 7 0. 01 11 1 0. 00 15 1 0. 66 27 9 42 0. 9 10 .3 94 % d w k -1 -1 7 26 .8 46 9 19 9. 73 49 32 7. 52 78 0. 05 52 9 0. 00 38 6 0. 54 56 9 0. 03 47 7 0. 00 18 0 0. 39 20 0 0. 01 13 7 0. 00 16 2 1. 04 54 4 44 9. 0 10 .8 98 % d w k -1 -1 8 26 .6 64 4 22 9. 39 90 30 4. 73 22 0. 05 56 6 0. 00 34 2 0. 56 25 8 0. 03 17 8 0. 00 10 1 0. 24 33 1 0. 01 16 8 0. 00 18 3 0. 85 55 6 45 5. 5 6. 0 99 % d w k -1 -1 9 59 .3 48 0 12 6. 43 03 39 2. 36 63 0. 06 36 5 0. 00 31 6 1. 23 87 5 0. 05 57 7 0. 00 17 6 0. 27 89 8 0. 02 24 6 0. 00 36 8 1. 96 41 9 84 7. 3 10 .0 96 % d w k -1 -2 1 13 .9 74 4 10 6. 41 27 17 3. 41 56 0. 06 07 4 0. 00 52 6 0. 57 89 1 0. 04 73 7 0. 00 12 4 0. 21 75 7 0. 01 18 7 0. 00 23 0 1. 04 31 0 43 4. 7 7. 5 93 % d w k -1 -2 2 10 .0 48 0 92 .0 59 3 11 5. 57 57 0. 06 04 7 0. 00 50 5 0. 58 63 4 0. 04 55 9 0. 00 17 5 0. 31 36 6 0. 01 10 8 0. 00 24 6 0. 78 62 7 44 7. 7 10 .5 95 % d w k -1 -2 3 10 .6 82 3 94 .9 50 7 13 0. 50 48 0. 05 85 7 0. 00 56 5 0. 55 91 0 0. 04 94 9 0. 00 14 5 0. 23 26 4 0. 01 19 1 0. 00 28 0 1. 08 10 3 43 8. 2 8. 7 97 % d w k -1 -2 4 11 .9 33 3 86 .3 48 7 19 1. 02 44 0. 05 06 5 0. 00 34 7 0. 38 65 1 0. 02 42 3 0. 00 09 9 0. 28 23 2 0. 00 87 4 0. 00 21 4 1. 41 66 5 35 1. 0 6. 0 94 % (c on tin ue d) 31the sedimentary environment and source of ore-forming materials of sandstone-type copper deposits in southeast kuqa basin, xinjiang, china sa m pl e n o to ta l p b 23 2 t h /1 0-6 23 8 u /1 0-6 20 7 p b/ 20 6 p b 20 7 p b/ 23 5 u 20 6 p b/ 23 8 u 20 8 p b/ 23 2 t h 23 8 u /23 2 t h 20 6 p b/ 23 8 u c on co rd an ce r at io 1σ r at io 1σ r at io 1σ r at io 1σ r at io a ge /m a 1σ d w k -1 -2 5 13 .9 14 2 10 7. 20 72 26 9. 17 96 0. 06 22 9 0. 00 42 0 0. 40 14 6 0. 02 68 4 0. 00 15 5 0. 49 48 1 0. 00 76 3 0. 00 20 8 1. 58 71 7 29 4. 4 9. 5 84 % d w k -0 21 10 .2 81 3 77 .4 03 2 13 4. 09 72 0. 06 02 3 0. 00 49 1 0. 53 37 3 0. 04 11 7 0. 06 58 4 0. 00 16 2 0. 01 06 1 0. 00 18 0 1. 06 25 4 41 1. 1 9. 8 94 % d w k -0 22 16 .6 66 3 10 0. 34 32 19 1. 17 27 0. 05 56 8 0. 00 32 2 0. 58 87 7 0. 03 28 2 0. 07 70 8 0. 00 18 8 0. 01 06 3 0. 00 17 0 1. 17 41 5 47 8. 7 11 .2 98 % d w k -0 23 41 .8 06 7 45 9. 09 36 80 1. 73 84 0. 05 29 0 0. 00 31 9 0. 32 65 1 0. 02 06 5 0. 04 44 7 0. 00 09 7 0. 00 69 9 0. 00 10 8 1. 07 51 4 28 0. 5 6. 0 97 % d w k -0 24 11 .0 68 9 10 5. 83 50 21 8. 25 31 0. 05 21 9 0. 00 37 7 0. 32 39 3 0. 02 38 0 0. 04 50 0 0. 00 10 5 0. 00 65 0 0. 00 10 1 1. 28 30 6 28 3. 7 6. 5 99 % d w k -0 25 19 .7 09 3 15 7. 66 58 29 1. 23 28 0. 05 34 8 0. 00 32 2 0. 43 22 1 0. 02 54 0 0. 05 84 0 0. 00 07 9 0. 00 86 2 0. 00 12 8 1. 13 36 1 36 5. 9 4. 8 99 % d w k -0 26 99 .8 02 1 36 0. 06 96 24 9. 62 72 0. 10 11 5 0. 00 33 5 3. 93 42 6 0. 13 33 8 0. 27 99 1 0. 00 38 5 0. 03 68 2 0. 00 53 5 0. 43 53 0 15 90 .9 19 .4 98 % d w k -0 27 36 .3 44 4 47 8. 36 03 61 5. 42 90 0. 05 46 7 0. 00 33 3 0. 36 77 8 0. 02 07 4 0. 04 89 4 0. 00 09 2 0. 00 64 5 0. 00 09 5 0. 81 72 1 30 8. 0 5. 7 96 % d w k -0 28 24 .4 95 1 17 5. 11 10 29 8. 16 91 0. 06 29 9 0. 00 56 7 0. 62 59 0 0. 05 77 3 0. 07 15 9 0. 00 13 1 0. 00 93 7 0. 00 15 1 1. 09 39 1 44 5. 7 7. 9 89 % d w k -0 29 16 .5 47 1 12 8. 69 40 19 7. 47 54 0. 05 53 1 0. 00 35 6 0. 55 13 4 0. 03 43 0 0. 07 20 1 0. 00 14 0 0. 00 94 6 0. 00 15 0 0. 94 17 7 44 8. 3 8. 4 99 % d w k -0 210 29 .6 65 9 24 3. 72 00 34 8. 15 33 0. 05 52 5 0. 00 27 5 0. 54 88 9 0. 02 58 9 0. 07 21 0 0. 00 12 7 0. 00 90 2 0. 00 14 6 0. 91 37 0 44 8. 8 7. 6 98 % d w k -0 211 6. 78 09 64 .1 08 7 13 3. 72 15 0. 06 16 2 0. 00 66 8 0. 37 15 8 0. 03 89 3 0. 04 48 2 0. 00 13 3 0. 00 60 5 0. 00 11 1 1. 30 06 6 28 2. 7 8. 2 87 % d w k -0 212 20 .9 68 7 17 0. 53 73 26 9. 29 72 0. 05 62 1 0. 00 33 3 0. 52 33 3 0. 03 00 0 0. 06 74 7 0. 00 13 6 0. 00 76 1 0. 00 14 0 0. 97 74 8 42 0. 9 8. 2 98 % d w k -0 213 18 .7 53 2 16 8. 12 69 35 8. 35 38 0. 05 80 8 0. 00 37 0 0. 36 93 1 0. 02 21 3 0. 04 61 4 0. 00 07 7 0. 00 59 8 0. 00 11 9 1. 32 14 5 29 0. 8 4. 7 90 % d w k -0 214 68 .1 74 1 85 .7 09 5 49 1. 33 26 0. 06 36 1 0. 00 25 1 1. 20 14 1 0. 04 79 2 0. 13 55 0 0. 00 19 6 0. 01 66 0 0. 00 27 3 3. 57 79 9 81 9. 2 11 .1 97 % d w k -0 215 10 .1 31 1 69 .1 21 9 12 1. 39 37 0. 06 18 5 0. 00 64 3 0. 59 75 0 0. 05 52 1 0. 07 41 4 0. 00 35 8 0. 00 96 9 0. 00 16 2 1. 08 97 7 46 1. 1 21 .5 96 % d w k -0 216 39 .9 84 8 25 8. 84 60 46 1. 64 94 0. 05 53 2 0. 00 27 0 0. 58 37 9 0. 02 61 0 0. 07 65 3 0. 00 14 5 0. 01 02 3 0. 00 14 4 1. 10 86 0 47 5. 4 8. 7 98 % d w k -0 217 18 .3 52 1 17 6. 38 20 34 6. 89 09 0. 05 23 4 0. 00 68 3 0. 34 68 8 0. 04 34 4 0. 04 85 6 0. 00 10 9 0. 00 58 6 0. 00 08 1 1. 21 89 6 30 5. 6 6. 7 98 % d w k -0 219 28 .3 28 8 21 8. 76 05 34 1. 05 27 0. 05 75 7 0. 00 29 5 0. 56 74 4 0. 02 72 5 0. 07 17 6 0. 00 14 1 0. 01 08 2 0. 00 13 4 0. 97 07 8 44 6. 7 8. 5 97 % d w k -0 220 19 .5 76 8 18 5. 94 76 28 2. 80 82 0. 06 08 7 0. 00 40 7 0. 49 88 8 0. 03 21 0 0. 05 94 8 0. 00 16 9 0. 00 91 2 0. 00 11 8 0. 95 52 3 37 2. 5 10 .3 90 % d w k -0 221 22 .4 84 3 17 8. 17 07 26 7. 37 10 0. 05 27 2 0. 00 33 4 0. 53 24 1 0. 03 27 4 0. 07 27 8 0. 00 11 5 0. 01 10 0 0. 00 13 7 0. 93 54 7 45 2. 9 6. 9 95 % d w k -0 222 18 .5 52 4 10 9. 32 26 23 3. 28 20 0. 05 44 3 0. 00 34 4 0. 55 31 2 0. 03 61 8 0. 07 27 5 0. 00 14 1 0. 01 12 0 0. 00 14 4 1. 32 33 8 45 2. 7 8. 5 98 % d w k -0 223 22 .5 73 4 17 3. 93 00 25 1. 90 63 0. 05 62 2 0. 00 41 7 0. 60 33 3 0. 04 50 4 0. 07 72 7 0. 00 18 7 0. 01 33 7 0. 00 17 2 0. 90 69 2 47 9. 8 11 .2 99 % d w k -0 224 13 .0 10 7 48 .8 86 1 15 9. 92 13 0. 06 08 8 0. 00 49 8 0. 65 02 3 0. 05 55 9 0. 07 77 7 0. 00 24 7 0. 01 66 7 0. 00 23 2 2. 07 93 8 48 2. 8 14 .8 94 % d w k -0 225 20 .5 32 9 80 .8 15 2 24 6. 66 32 0. 05 28 3 0. 00 31 7 0. 58 27 6 0. 03 40 5 0. 07 97 8 0. 00 17 5 0. 01 28 4 0. 00 17 8 1. 92 11 1 49 4. 8 10 .5 94 % n ot e: d at a m is si ng d ue to in va lid z irc on m ea su re m en t p oi nt s. 32 qiji fu, xinxian xu 4 conclusions (1) the main sedimentary facies types can be identified in the neogene in the southeast kuqa foreland basin of xinjiang, which are alluvial fan facies, fan-delta facies and lake facies, and further 5 subfacies and 7 microfacies types can be identified. in terms of genesis, sandstone-type copper mineralization is closely related to the neogene fluvial-lacustrine sedimentary environment, and the minerals are mainly distributed in the fan-delta plain facies and fan-delta front facies. (2) the detrital zircon dating data show that the copper-bearing conglomerates in the miocene jidike formation (n1j) in the southeast of the kuqa basin mainly come from the strata and magmatic rock products from the late silurian to the early permian in the northern and southwestern tianshan orogenic belt in the north and southwest of the kuqa basin, which are the result of redeposition after strata denudation. references cai h a, gao h o, fan s j, et al., 2022. the metallogenic model of the sandstone-type copper deposits in the kangcun area, kuqa basin, xinjiang [j]. journal of geomechanics, 28(1): 78-89. 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(in chinese with english abstract) _hlk162633236 _hlk162865257 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v29n2.117124 earth sciences research journal earth sci. res. j. vol. 29, no. 2 (june, 2025):167 181 g eo lo g y abstract geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age rana abbas ali 1,* 1 department of geology, college of science, university of baghdad, iraq *corresponding author e-mail: rana.ali@sc.uobaghdad.edu.iq record manuscript received: 19/10/2024 accepted for publication: 14/05/2025 how to cite this item: ali, r. a. (2025). geochemical data from the tanuma carbonate reservoir (central iraq) indicates higher terrigenous input and deoxygenation during coniacian early santonian age. earth sciences research journal, 29(2), 167-181. https://doi.org/10.15446/esrj. v29n2.117124 oceanic anoxic events were characterized by the widespread deposition and accumulations of pelagic sediments rich in organic matter, such as black shales, and are considered key mechanisms for organic carbon burial and, in such, buffering cretaceous runaway super-greenhouse. a negative carbon isotopic excursion (cie), typically overlaid atop a longterm positive pattern, and significant environmental and climatic shifts are two characteristics that identify the coniacian–santonian, according to a variety of sedimentary records. but there is still no evidence to support the theory that variations in oceanic deoxygenation and continental weathering input in shallow seas could contribute to the carbonate-platform crisis at low latitudes. here, carbonate content and carbonate-hosted elements from the tanuma carbonate platform in central iraq (east baghdad oilfield; eb10 well) are analyzed for the coniacian–early santonian transitional phase. the oae3 boundary is marked by a clear increase in the elements that are most water insoluble (such as al, sc, th, ti, and all of the rare earth elements), followed by a slight increase or relatively high levels throughout the oae cie's negative phase. these results suggest that the increased terrigenous input was likely linked to rapid global warming during the coniacian–santonian period, contributing to widespread carbonate platform degradation. the increase in the abundance of these water-insoluble elements is immediately followed by an increase in the mn, ce, and ce anomaly, which are then better values throughout the negative cie interval. these findings align with similar deoxygenation trends observed in contemporaneous carbonate platforms across egypt and the southern tethys, indicating a potentially widespread oceanic anoxic event during the coniacian–santonian interval. these events were probably related to increased nutrient input and continental weathering, which favoured oxygen consumption as well as primary productivity. in cie's recovery phase, the stratigraphically elevated insoluble in water elements exhibit a gradually declining trend in parallel to heightened redox proxy values, indicating a drop in the intensity of continental weathering and associated second deoxygenation at shallow seas. in this case, increased recycling in bioessential nutrients or a slowing of the ocean's circulation could have contributed to deoxygenation. the interdependent connection among carbonate content, geochemical data, and biotic changes indicates that 1the tanuma carbonate platform probably noticed a minor degradation around the oae3 boundary period due to the beginning of increasing terrigenous input and the deoxygenation of shallow seas. 2during the cie's negative phase, the heightened terrigenous input and deoxygenation probably contributed significantly to the more serious situation facing benthic carbonate producers. keywords: coniacian–santonian oae3, major and trace elements, carbonate platform, shallow coastal facies, central iraq resumen palabras clave: coniaciense-santoniano; oae3; elementos mayores y elementos traza; plataforma carbonatada; facies someras costeras; central iraq. los eventos anóxicos oceánicos se caracterization por extensas deposiciones y acumulaciones de sedimentos pelágicos ricos en materia orgánica, tales como shales negros, y son considerados mecanismos claves para el enterramiento de carbón orgánico y, en estos casos, almacenamiento del superinvernadero del cretácico. una excursión negativa isotópica del carbono (cie, del inglés carbon isotopic excursion, típicamente sobrepuesta sobre un patrón positivo de largo período, junto con los cambios significativos climáticos y ambientales son características que identifican el período entre el coniaciano y el santoniano, de acuerdo con una amplia variedad de registros sedimentarios. pero no hay evidencia que fundamente la teoría de que las variaciones en la desoxigenación y la meteorización continental en aguas marítimas someras pudieron contribuir a la crisis de las plataformas carbonatadas en las bajas latitudes. en este estudio se analiza el contenido de carbonato y los elementos alojados en carbonato de la plataforma de tanuma, en el centro de irak (campo petrolífero de bagdad oriental; pozo eb10), para la fase de transición coniaciense-santoniano temprano. el límite oae3 está marcado por un claro aumento en los elementos que son más insolubles en agua (como al, sc, th, ti y todos los elementos de tierras raras), seguido de un ligero aumento o niveles relativamente altos a lo largo de la fase negativa del cie oae. estos resultados sugieren que el aumento de la entrada terrígena probablemente estuvo relacionado con el rápido calentamiento global durante el período coniaciense-santoniano, lo que contribuyó a la degradación generalizada de la plataforma carbonatada. el aumento en la abundancia de estos elementos insolubles en agua es seguido inmediatamente por un aumento de mn, ce y ce anómalo, que luego son mejores valores a lo largo del intervalo cie negativo. estos hallazgos se alinean con tendencias de desoxigenación similares observadas en plataformas carbonatadas contemporáneas en todo egipto y el sur de tetis, lo que indica un evento anóxico oceánico potencialmente generalizado durante el intervalo coniaciense-santoniano. estos eventos probablemente estuvieron relacionados con el aumento de la entrada de nutrientes y la meteorización continental, que favorecieron el consumo de oxígeno, así como la productividad primaria. en la fase de recuperación del cie, los elementos insolubles en agua estratigráficamente elevados exhiben una tendencia gradualmente decreciente en paralelo al aumento de los valores proxy redox, lo que indica una caída en la intensidad de la meteorización continental y la segunda desoxigenación asociada en mares poco profundos. en este caso, el aumento del reciclaje en nutrientes bioesenciales o una desaceleración de la circulación oceánica podrían haber contribuido a la desoxigenación. la conexión interdependiente entre el contenido de carbonato, los datos geoquímicos y los cambios bióticos indican que, 1. la plataforma carbonatada de tanuma probablemente notó una degradación menor alrededor del período límite oae3 debido al comienzo del aumento de la entrada terrígena y la desoxigenación de mares poco profundos. 2durante la fase negativa del cie, la mayor entrada terrígena y la desoxigenación probablemente contribuyeron significativamente a la situación más grave que enfrentan los productores de carbonato bentónico. los datos geoquímicos del yacimiento carbonatado de tanuma (irak central) indican un mayor aporte terrígeno y desoxigenación durante la era coniaciense-santoniano temprano https://doi.org/10.15446/esrj.v29n2.117124 mailto:rana.ali@sc.uobaghdad.edu.iq https://doi.org/10.15446/esrj.v29n2.117124 https://doi.org/10.15446/esrj.v29n2.117124 168 rana abbas ali 1. introduction the coniacian-early santonian period (~ 88.5-86.5 ma) experienced one of the most well-known oceanic anoxic events (oaes) due to the broadening of oxygen-depleted oceans and the global dispersion of coeval organic-rich sediment originating from epicontinental and pelagic settings (jenkyns, 2003, 2010). this geological time interval was marked by major climatic and ecological perturbations, such as peak warming of the world’s oceans in the late cenomanian to santonian (jenkyns, 1980; schlanger & jenkyns, 1976; hu et al., 2012; kaya et al., 2020; mansour & wagreich, 2022), improved continental weathering (brazier et al., 2015; chenot et al., 2018; petrizzo et al., 2022), substantial rise in sea levels (haq, 2014; chen et al., 2015; yilmaz et al., 2018; mansour et al., 2020a), acidification of the oceans (dameron et al., 2017; steuber et al., 2023), and biological crisis (keller, 2008; labandeira et al., 2016). these complicated paleoenvironmental and climatic events were linked to large geological perturbations in the earth system, specifically the earth’s carbon cycle (schlanger & jenkyns, 1976; jenkyns, 2010; wagreich, 2012). these complicated geological phenomena are distinguished by global deposits of organic shales linked to significant carbon isotope excursions (cies) (leckie et al., 2002; tsikos et al., 2004). improved continental silicate weathering directly consumes inorganic carbon during oaes, increasing nutrient supply to the oceans. as a result, oxygen-depleted waters expand, enhancing marine organic matter productivity and its resulting burial in the ocean: a phenomenon that may have been crucial in lowering excess co2 in the atmosphere (schlanger & jenkyns, 1976; jenkyns, 2010, as well as references therein). despite the fact that these two environmental shifts were widely identified by numerous systems of isotope (e.g., osmium, strontium, molybdenum, calcium, thallium, nitrogen, and sulphur) throughout the oae (e.g., peucker-ehrenbrink & ravizza, 2000; jones, 2001; wendler et al., 2009; blättler et al., 2011; chamberlain et al., 2013; cao et al., 2016; sun et al., 2016; cao et al., 2021), sections from the southern and northern hemispheres make up the vast majority of the research, primarily from the atlantic ocean and neighbouring basins, africa, and central asia. despite several studies focusing on δ34s measurements in asian carbonate-associated sulphates, high-resolution geochemical assessments from the mesopotamian basin remain scarce, warranting this investigation (cao et al., 2016; cao et al., 2021). the sulphur-isotope measurements, however, have been seen as a potentially changed signal due to the substantially bigger magnitude ẟ34s values in comparison to those in coeval asian sediments (xu et al., 2023). nederbragt et al. (2004) propose a positive cycle of feedback between oxygen depletion, phosphate regenerating and oceanic production during the oaes. oaes may have been caused or accompanied by enormous injections of clathrate-trapped methane into the atmosphere (jenkyns, 2003). carbonate platforms of shallow water were particularly common in the tropical and subtropical regions along the borders of southern tethys throughout the coniacian-early santonian, where shallow-water systems rich in biological diversity were found (jarvis et al., 2002; phelps et al., 2015; yilmaz et al., 2018; smith et al., 2021; mansour & wagreich, 2022). those biotic carbonate platforms had been generally modified by a diversity of planktonic foraminifers, calcispheres, calcareous nannofossils, and corals tracing the evolution of cretaceous sea levels, while a diversity of larger benthic foraminifers, dasycladalean algae, corals, and rudist bivalves reveal substantial decreases at the time of oceanic anoxic events (oaes) (jarvis et al., 2002; pohl et al., 2020; steuber et al., 2023). the ultimate reaction of these shallow-marine systems to significant worldwide palaeoceanographic and palaeoenvironmental disruption resulted in the tethyan carbonate platforms’ foundering and drowning. benthic ecosystems and the formation of shallowwater carbonate in the past geologically are generally considered to be negatively impacted by enhanced terrigenous inputs and oxygen depletion (blättler et al., 2011; korbar et al., 2012; clarkson et al., 2018; han et al., 2022). however, reliable chemical proxies of carbonate platforms for shallow water, which can accurately record information on global and/or regional shifts in terrigenous intake and supply of oxygen, are scarce (han et al., 2022; mansour & wagreich, 2022). pulses of terrigenous input had been identified in the coniacian-early santonian of morocco’s high atlas platforms, which may have been a crucial controlling element in the closure of this carbonate factory (sachse et al., 2012; andrieu et al., 2022; algouti et al., 2023). there have been investigations into redox conditions using carbonate-hosted i/ca, mn/ca, and ce anomalies (ce/ce*) on the campania-lucania platform, italy, as well as toc, s, al, and caco3 from the demerara rise (nw south american edge) (märz et al., 2009; lu et al., 2010), and the bulk-rock hosted al, si, fe, ti, p, cr, sr, ba, zr, v, ni, zn, co, and mo in the southern tethys (egypt) (mansour et al., 2020b). these redox proxies indicate a decrease in oxygen content in shallow tropical waters, but this is inconsistent with sedimentological evidence of limestones with high bioturbation density in the niobrara formation of the western interior seaway and reddish oxygenated claystone in southern tibet during the oae3 interval (wagreich, 2009; lowery et al., 2017). as a result, the precise role of accelerated continental weathering and anoxia in promoting a neritic carbonate crisis is still debated. the cretaceous had a number of global oceanic anoxic episodes (oae), which are characterized by the extensive accumulation of stratified organic-rich sediments. the paleogeographic distribution of organicrich sediments suggests that the coniacian-santonian oae (= oae3) was more reliant on local or regional conditions than it was on a global scale. within the equatorial basins, these are primarily found in shallow water environments and epicontinental seas (bomou et al., 2013). therefore, it is necessary to focus on studying several sections from different paleogeographic regions and at different paleodepths in central iraq in order to identify oceanic anoxic events for hydrocarbon exploration. the shallow-water tanuma carbonate platform’s east baghdad 10 (eb10) well has been well investigated for biostratigraphy, sedimentology, and hydrocarbon characterization in the northern hemisphere, allowing confirmation of the typical oae (aqrawi, 1996; ali et al., 2022; ali, 2023a, b). the major and trace elements and caco3 content of shallow-marine carbonate of the coniacian-santonian transitioning phase of the eb10 well are presented here with the aim to estimate variations in continental weathering along with ocean deoxygenation at shallow tropical waters within the northern hemisphere’s southern tethys. the results from this investigation are later combined with data from the tethys ocean’s western margin, the african shelf, and southern tibet to determine the degree of global shifts potentially influencing carbonate platforms throughout this time span at the supra-regional tethyan scales. 2. geological setting and stratigraphy the study area is in the east baghdad oilfield in the mesopotamian foredeep basin, which is a large section of the zargos fold belt (figs. 1 and 2). as the region experienced a relative decline in sea level, the first precursors of the tanuma formation facies began gradual sedimentation, resulting in varying facies that reflected the shallowness and degree of turbulence of the water on the one hand and the nature of the topography of the sedimentary basin that reached that period on the other (aqrawi, 1996; al-ameri, 2011; ali & jassim, 2023). the seventh facies range, named the shelf basin and also known as the open platform, represents the base of the tanuma formation in the secondary basin area (baghdad basin). this basin is distinguished by its fine-grained carbonate sediments that contain organic detritus, which is a lime mudstone facies (fig. 3a). the lower tanuma formation facies were deposited in somewhat distinct coastal facies locations, but they all have one thing in common: they are shallow and show an obvious regressive facies pattern (sadooni, 2004; abdel-fattah et al., 2022; gharib et al., 2024). the relatively unstable climatic and tectonic conditions at the beginning of the coniacian resulted in somewhat varied, shallow coastal facies zones, although they all share a marked regressive facies pattern (fayadh et al., 2018; mahdi et al., 2019). the bottom of the baghdad basin began to gradually stabilize. this basin’s sediments were created by a geological sequence of black shale and shale limestone. in general, the tanuma fm. distinctive sequence is composed of an alternation of bioclastic-bearing lime mudstone facies (fig.3b) and rotaliid-bearing clastic lime mudstone facies (fig.3c). these facies indicate the open platform (shore platform) that has been successively exposed to the impact of the open sea, resulting in the emergence of two distinct water movement and circulation systems, namely restricted circulation and open circulation. the features of the secondary basin became prominent near the middle of the tanuma fm. sequence, causing a partial marine transgression from the eastern and northeastern sides, covering the entire study area and causing open water circulation and free interaction with the open sea. as a result, the black shale sediments began to disappear, to be replaced by a calcareous succession consisting of a bioclastic lime wacke 169geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age mudstone facies bearing globigerina and calcospherites (fig. 3d) and a rotaliid -bearing bioclastic lime wacke mudstone facies (fig. 3e). these facies reflect varying marine influences, which can be represented by the succession of the third standard facies zone (the edge of the sedimentary basin), where floating organisms breed, and the inner shelf zone, where benthic organisms breed and are influenced by organic detritus rolling from the shallow rims adjacent to it. as a result, it represents a zone of the shelf basin’s border and a moderate slope that encompasses the entire research area. following a series of marine influences, there was an almost abrupt marine transgression from the entire current study area, resulting in two depositional systems parallel to the coastline, one of which is very shallow close to it and has turbulent wave activity, known as the turbulent platform edge, and the other with relatively calm marine conditions. this was reflected in the prevalence of a distinct sedimentary facies known as the coated bioclastic lime wackestone facies (fig. 3f). 3. materials and methods 3.1. sample selection the tanuma fm.’s coniacian-early santonian age samples were recovered from an oil well in the east baghdad oilfield (eb10) in central iraq (fig. 2). in particular, numerous storm-generated beds with evident terrigenous composition (e.g., fine-grained quartz and a trace of clay) exist in the bottom portion of the oae cie period, potentially contaminating the acetic acid leaching mechanism. thin sections of these samples were therefore examined first to choose the most well-preserved rock components, which in this study were pure carbonate, free of storm-originating terrigenous grains, and free of indications of metamorphism and recrystallization. lastly, the samples of tanuma fm. are distinguished by grainstones with little wackestone/packestone (sadooni, 2004). these materials were chopped into newly formed rock chips, which were subsequently micro-drilled to create a powder while being cautious to avoid veins, cement-filled the pores, and bioclasts. 3.2. calcium carbonate content and elemental concentration a total of 60 samples were chosen for determination of calcium carbonate content utilizing an nfp18-508 pressure calcimeter. by calculating the partial pressure ratio of the co2 produced as a result of the reaction between hcl and caco3, this device estimates the caco3 contents (wt%). each bulk sample’s carbonate powder was dissolved in excess of 10% hcl, and the volume of co2 created was utilized to compute the caco3 amount. every fifteen samples, pure carbonate standards and repeats were used to regulate the variability. this technique’s long-term error in instruments is in the range from 0.5% to 3%. we chose 57 bulk carbonate samples obtained from the tanuma fm. for element analysis, which generally but not entirely correlate to caco3 content samples. each carbonate sample’s powder was weighed and then dissolved in 5 ml of 10% acetic acid. after 12 hours, 0.5 ml of each sample’s solution had been transferred to a fresh precleaned teflon container, then the aliquot was three times dried with 1 ml concentrated hno3 to convert the dissolved materials to nitrate. lastly, these samples had been diluted in 15 ml of 2% hno3 solution. at the iraqi ministry of science and technology’s research labs, elemental concentrations were determined using a shimadzu pg-990 atomic absorption spectrometer, with a measurement precision of ±5% relative standard deviation (rsd). internal standards were used to verify elemental concentrations, and the analytical uncertainty after triplication for most elements was +/– 1.5%. additionally, analyses of the carbonate samples’ stable c isotopic compositions were performed. a finnigan mat253 mass spectrometer was used to evaluate the substances indicated above. the treatment approach was similar to that reported by lash (2018). the levels for ẟ13c were calculated with ẟ per unit (‰) in relation to the viennese-pee dee belemnite (v-pdb) standard and the standards set by the vienna standardized average oceans water (v-smow). the standard samples utilized were the chinese standards gbw04415 (ẟ13c = +1.51 ± 0.02‰, v-pdb) and gbw04416 (ẟ13c = –5.06 ± 0.05‰, v-pdb). the accuracy of those standard specimens’ calibration was checked after each three specimens to make sure it was better than 0.2‰. figure 1. the location of the east baghdad oilfield. (a) location of the studied coniacian-santonian oae in the northeast of asia (late cretaceous plate tectonic map for 86 ma modified from wagreich (2009, 2012); (b) an iraqi location map showing the basins and oil fields on the country’s northeastern arabian peninsula. (c) a map of central iraq showing the locations of the study’s wells. a c coniacian-early santonian late cretaceous b 170 rana abbas ali figure 2. the east baghdad oilfield’s stratigraphic column. 171geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age 3.3. the anomalous of cerium the relative enrichment or depletion of paas is compared to calculate the cerium anomaly (ce/ce*) (taylor & mclennan, 1985) normalized ce([ce]sn) to neighbouring non-redox sensitive rare earth elements (ree). because of the abnormal enrichments of la in seawater, the new recommended computation sans la offered by lawrence & kamber (2006) was used in this study: ce/ce*= [ce/(pr × (pr/nd))]sn. 4. results 4.1. caco3 composition and major element concentrations the data (given as means ± 1σ standard deviation) reveal that the percentage of caco3 ranges from 76.2 to 98.8% (90.03 ± 3.5%, n=60; fig. 4). the pattern is made up of a steady decline up-section, beginning at the onset figure 3. photomicrographs of tanuma fm. depositional facies in the east baghdad oilfields of the mesopotamian basin. (a): lime mudstone facies; (b): bioclasticbearing lime mudstone facies; (c): clastic lime mudstone facies; (d): bioclastic lime wacke mudstone facies bearing globigerina and calcisphere; (e): rotaliid -bearing bioclastic lime wacke mudstone facies; (f): coated bioclastic lime wackestone facies. figure 4. presents geochemical profiles of key water-insoluble elements, caco3 concentration, δ13c org, across the coniacian–santonian interval, with color-coded indicators of terrigenous input and deoxygenation events. the hues and red spots of elemental profiles reflect specimens having ree distribution patterns similar to seawater (fig. 6a) with mree bulge/hat shaped (fig. 6b). oae3: oceanic anoxic event 3. 172 rana abbas ali threshold of the oae cie at 2093m, extending and culminating throughout the negative period of the oae cie, and practically returning to normal levels at the end of the cie period. the elemental concentrations in fig. 4 range from 0.2 to 1.7% (0.849 ± 0.1%, n= 60) for al, 0.04 to 0.094% (0.0596 ± 0.02%, n= 60) for ti, 0.3 to 2.5 ppm (1.275 ± 0.8 ppm, n= 60) for th, and 0.2 to 3.9 ppm (1.44 ± 0.09 ppm, n= 60) for sc. the sc, th, and ti profiles reveal a rapid positive shift at the oae3 boundary, which is either basically consistent high-level values or a very slight rise (th and total of ree), but with little fluctuations up to about the cie threshold’s midpoint (sc and ti) (fig. 4). then, they steadily decrease till the cie’s ultimate level; nevertheless, al shows a general rise from the oae3 border to the end of the oae cie intervals and maintains comparatively high levels back toward the cie’s end level. mn values at the eb10 vary from around ~94 to ~219 ppm (144.16 ± 44.5 ppm, n = 60; see figure 5). a decline from ~160-180 towards ~103 ppm under the oae3 border is followed by a moderate, slower increase to ~33-40 ppm around the start phase of the oae cie, which gets steep, culminating in the cie’s negative phase, which reaches high levels of about 140 ppm. separating by a decrease in mn concentration (120-130 ppm), the pattern continued to grow again up the section, peaking near the top level of the oae cie at values close to 160 ppm before returning to normal levels just above this level. 4.2. trace elements concentrations & ce anomaly (ce/ ce*) total ree (∑ree) values ranged from 42.0 to 52.08 ppm (48.79 ± 20.4 ppm, n=60). the profile matches that of ti, th, and sc (fig. 4). the paasnormalized pattern demonstrates the fact that ree distribution has been defined in high lree depletion, relatively hree enrichment (fig. 6a), except for a few samples (shown as red spots in the line of fig. 4), and by an mree enrichment (hat-shaped pattern; fig. 6b). ce values range from 0.54 to 25.8 ppm (7.7 ± 5.3 ppm, n=60), while coupled values of ce/ce* range between 0.39 and 1.76 (0.85 ± 0.3, n=60) (fig. 5). both the ce/ce* ratio and the ce contents show a generally positive correlation with mn, except there is an overall positive trend in the ce/ce* throughout the period from 1107 to 1122m, and slight variations in mn and ce contents. figure 5. the correlation for geochemical profiles for widely employed redox proxies at coniacian-early santonian carbonates from the southern tethys (egypt) and the pelagic shelf sea (eastbourne). caco3 and sr data for the calcareous sequence of the abu roash formation in the abu gharadig basin (egypt) are from mansour et al. (2020b), while caco3, δ 13c carb, i/ca, ce/ca, ce anomaly, and mn/ca data for the late cretaceous carbonate of the eastbourne are from lu et al. (2010). figure 6. the paas-normalized ree distribution pattern for the acetic acid (hac, 1 n)-leaching liquid of coniacian-early santonian carbonate derived from eb10. a. seas-like ree structure in the current investigation and present deep pacific oxia seawater (in red curve), revised by deng et al., 2017. b. hat-shaped pattern (sample spots highlighted in red in fig. 4 curve). 0.001 0.01 0.1 1 10 la ce pr nd sm eu gd tb dy ho er tm yb lu b 0.001 0.01 0.1 1 10 la ce pr nd sm eu gd tb dy ho er tm yb lu a seawater ree pattern 173geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age 5. discussion 5.1. elemental clues of terrigenous input & assessment of ce cycle seawater contains a variety of water insoluble elements, including th, al, sc, ti, and ree, which are primarily caused by rivers’ water’s chemical weathering and/or the chemical weathering products’ dissolution. terrigenous detritus contains significantly larger amounts of these elements than pure ocean carbonate (taylor & mclennan, 1985; su et al., 2017). as a result, the quantities of water-insoluble components of carbonate structures absorbed from peripheral seawater are influenced by terrigenous inflow, giving these compounds a perfect monitoring of continental weathering processes. this analysis eliminated tempestites having coarse siliciclastic components. as indicated in figure 4, the highest waterinsoluble th, sc, ti, and ree levels, in addition to al (excluding the recovered interval for the oae cie), are favourably correlated to each other, while negatively linked to the carbonate content, and probably have an overall positive correlation to clay. these data show that such waterinsoluble components primarily come from minerals found in clay. concerning the potential for contamination caused by the leaching technique indicated previously, a threshold level of 1 ppm for th and 4 ppm for zr, equating to around 2% siliciclastic contaminants, was established and utilized as a signal of limiting contamination of detrital inputs (franchi, 2018). the elements measured in this investigation vary between 0.3 to 2.5 ppm (1.27 ± 0.8 ppm) of th (fig. 4) and between 0.44 to 7.12 ppm (4.19 ± 2.2 ppm) for zr, with only 12 and 7 data points, respectively, exceeding the threshold indicated previously. these findings indicate the chemical leaching method probably contaminated just some specimens with siliciclastic particles. furthermore, the average caco3 level is 90.03%, while the six data points with coupled element information are only marginally below 85% (fig. 4), which is believed to represent a lower threshold for the potential for masking the ree from fractional release of siliciclastic constituents (cao et al., 2020; tostevin, 2021). furthermore, paas normalized the ree pattern of the majority for examined specimens, regardless of comparatively high th, ti, sc, al, and ree content, showing seawaterlike the ree structures in depletion of lree, hree enrichment, as well as minor positive la and eu anomalies (figs. 4 and 6a), each of which indicates elementary preserving of the seawater signals (deng et al., 2017; ozyurt et al., 2020). the remainder of the specimens, defined by the commonly documented mree-bulge or hatshaped patterns (fig. 6b), are similarly unlikely for having been affected by siliciclastic elements since they had fewer total al, th, and ree concentrations than stratigraphically nearby specimens (fig. 4). geochemical thresholds are regarded as random and should thus be utilized on an individual basis for various testing (tostevin, 2021). some specimens had non-seawater like ree patterns and greater concentrations in terrigenous inputs proxies exceeding the threshold (figs. 4 and 6b); additional strong data imply that contamination from the dissolving of terrigenous debris throughout the hac leach procedure is negligible. as a result, fluids with high water-insoluble elements liberated from clays originating from continental weathering in early diagenesis might represent the major system, as predicted in several earlier studies (zhao & zheng, 2014; hood et al., 2018). due to their structural integration with carbonate and the relatively low ree/ca ratios in diagenetic liquids, the seawater ree pattern and associated ce/ ce* in limestone are thought to have been less vulnerable to post-depositional change, thus being accurately maintained (hood et al., 2018; zhang & shields, 2022). nevertheless, consideration needs to be exercised when employing water-insoluble elements for tracking the paleo-seawater structure, particularly in situations in which carbonate is impure or siliciclastic contaminants are evident. for the current investigation, the vast majority of the specimens have seawater-like ree patterns that provide solid proof that the main ce/ce* was conserved. though modest rees have been released by siliciclastic constituents in early diagenesis, the ce shows a poor association with th (r2 = 0.51; fig. 7a), as well as a weak relationship with mn (r2 = 0.37; fig. 7b). furthermore, mn shows weak and no connection with th (r2 = 0.27; fig. 7c) and sc (r2 = 0.28; fig. 7d), respectively. these data show the ce fingerprint is significantly associated with redox-sensitive mn actions, but neither is appreciably changed in interacting with siliciclastic detritus in earlier diagenesis. figure 7. cross-plots of components extracted from acetic acid (hac, 1 n)-leaching liquid. ce (a, r2 = 0.5192); mn (b, r2 = 0.3763); mn vs th (c, r2 = 0.2701) and sc (d, r2 = 0.2815). 174 rana abbas ali 5.2. climate-mediated accelerated continental weathering throughout the coniacian-early santonian in the eb10 section, continental weathering as indicated by waterinsoluble elements (ti, th, sc, and ree) began rising around oae3 border time, maintained very stable levels or only increased little during the oae cie’s negative phase, as well as slowly came back to baseline levels over the recovery stage of the cie (fig. 4). variations in paleoclimate and tectonism are the two main factors affecting continental weathering and hence related to terrigenous influx into marine by rivers (mansour et al., 2022). in the coniacian–early santonian period, the baghdad basin was a developed, tectonically quiet passive continental margin situated on a central coastal strip parallel to the tethys ocean’s southwest side (al-hamdani, 1986; mahdi et al., 2022). therefore, it seems unlikely that the rise in continental weathering within the study region was primarily caused by uplift and regional tectonism. the most severe climatic and environmental disruptions were observed throughout the upper cretaceous, as evidenced by the oae cie and the earlier coniacian–early santonian boundary event. these events were connected to the emplacement of deep ocean deposits, or balambo strata (al-ameri & al-obaydi, 2011). terrigenous influx may have grown due to greater physical weathering brought on by a sea level drop at a time of global cooling or increased chemical weathering at a time of global warming. previous research indicates that a short-term transgressive pulse coincided with the oae3, which was quickly followed by a potential modest decline in sea level, then, at the start of the oae cie, a rapid rise in sea level with a large amplitude (immenhauser, 2005; wendler, 2013; kaya et al., 2020; wagreich & mansour, 2022). however, organic carbon analysis in the southern tethys only revealed the sea-level rise linked to the oae cie (mansour & wagreich, 2022; lawa et al., 2023). based mostly on tex86 compilations and global stable oxygen isotopes (δ18o), lowto-high latitude paleotemperature predictions of the late cretaceous show a comparable pattern, with extreme warmth throughout the cenomanianturonian (lawa et al., 2023), certain improvement but maintained warmth during the coniacian-santonian, while progressive warming expands within the campanian along with the maastrichtian (huber et al., 2002; macleod et al., 2005; friedrich et al., 2012; o’brien et al., 2017; mohammed et al., 2022). the eb10 section’s steady rise in water-insoluble elements, however, does not line up with any suggested cycles of transgression and regression in sea level throughout the oae3-oae cie timeframe. thus, increasing terrigenous influx throughout the coniacian-santonian could not have been caused by shifts in sea level. given that the rise in terrigenous influx wasn’t primarily driven by a decline in sea level, the probable reason could be the chemical weathering linked to worldwide warming and the ensuing movement of these materials into seawater. according to oxygen-isotope rates in planktonic foraminifera, warming globally began at the oae3 boundary in time and persisted until the oae cie (huber et al., 2018). in addition to the os and sr -isotope profile at sections of east asia and egypt (figs. 5 and 8; jenkyns, 2010; cumming et al., 2012; munier et al., 2023; ali, 2023), this paleotemperature growth is tentatively compatible with the time of growing levels and peaking of water-insoluble components showing accelerated chemical weathering (fig. 4). the coniacian–santonian period’s synchronized alterations of these isotopes and element proxies of continental weathering imply that swift global warming caused an increase in this phenomenon worldwide, including the southern hemisphere’s tropical region (fig. 9a–c). furthermore, these proxies’ recovery to baseline levels throughout the cie’s phase of recovery was similarly synchronous, implying that increased organic carbon buried may have progressively reduced the co2 concentration and related intensity of continental weathering (fig. 9d). it is possible to attribute the ongoing rise in al throughout the recovery oae3 period to factors other than the increasing influx of continental weathering, such as additional contributions from authigenic cycles. to increase the concentration of al, for instance, biogenic silica could be transformed into authigenic aluminosilicates (demaster, 2003). this process allows the al to be liberated and absorbed into carbonate. two short-term increases in water-insoluble element trends were recorded layered above compared to the cie interval (2055–2067 m and 2037– 2045 m) (fig. 4), which may indicate a further pulse of accelerated continental weathering. the eb10 section has not, however, shown equivalent sedimentary or carbon-isotope shifts over these two intervals, and the sections at southern tethys (egypt) and pelagic shelf sea (eastbourne) do not appear to have seen noticeable shifts in multiple proxies after the oae-cie (fig. 5). consequently, based on the available data, it is probable that these two elemental rise pulses in the eb10 section are local signals whose origin is unclear. figure 8. a systematic paleoenvironmental analysis from the tanuma carbonate platform in central iraq, the songliao basin in east asia, and southern tanzania in east africa during the coniacian-santonian age. chemostratigraphic data from sk1-s core covering the upper quantou through the yaojia formations in the songliao basin. column of stratigraphy adapted from wu et al (2013). according to wang et al. (2016), red stars are bentonite horizons with ca-id-tims u/pb zircon ages. feng et al. (2010) identified a pause at the qingshankou/yaojia contact. chamberlain et al. (2013) provided the red δ13 ccarb data, whereas hu et al. (2015) provided the faded gray δ13 corg values. tanzania drilling project (tdp) site 39’s lithostratigraphy and chemostratigraphy from petrizzo et al. (2017). 175geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age 5.3. shallow water redox conditions during the coniacian–santonian in the shallow-water carbonate depositional setting, mn and ce are commonly employed for tracking intermediate manganous (suboxic) instead of completely anoxic conditions. this is due to their susceptibility to regional –local redox conditions (jenkyns, 2010; cao et al., 2020; lan & long, 2023). in the event that bottom waters become oxygenated, mn is carried to the seafloor as mn3+/mn4+ integrated into oxides (oxyhydroxides); conversely, in the event that oxygen levels depleted, the mn oxides (oxyhydroxides) that were previously deposited have been reduced into soluble mn2+ then integrated into early diagenetic carbonate crystal structures (li et al., 2022). ce3+ can oxidize to insoluble ce4+, which can then build up on the surface of organic and clay particulates, mn oxides (oxyhydroxides), and isolated ce oxide particulates in an oxygenated environment. this causes negative ce/ce* and ce depletion in seawater (loges et al., 2012). contrarily, in a suboxic environment with low o2, insoluble ce4+ is converted to soluble ce3+ and discharged to the water columns, resulting in a positive ce/ce* ratio and ce enrichment. thus, an increase in the concentration of mn and ce in the sediment and a positive ce/ce* ratio are widely regarded as essential stratigraphic markers for oaes. these markers have been seen for the t-oae, or oceanic anoxic events (lu et al., 2010; jenkyns, 2010; tessin et al., 2015). a more sensible range of > 1.2 and < 0.8 is used in numerous investigations instead of the commonly accepted limit number of 1 to indicate positive and negative ce/ce*, which denote conditions below and above the mn redoxcline, respectively, and oxygen-rich waters (tostevin, 2021 and the references within it). since the results in this study primarily fall in the range of 1.2 and 0.8, indicating little impact on ce/ce*, as shown in figure 5, just the overall trend is reported below. mn (1.23º v) and ce (1.61º v) have different reduction potentials, but both are susceptible to moderate manganous situations, and their redox cycle may act similarly in low oxygen environments. figs. 5 and 7 show that there is, in fact, the anticipated positive association between these. the carbonate content of eb10 is adversely correlated with the levels of both of these components (fig. 8 in the research), sk1-s core, and abu roash member (jones et al., 2018; mansour et al., 2020b), respectively, despite the fact that carbonate forms the host phases and a rise in its amount may result in positive contributions for the values of mn and ce. as a result, the sediment’s carbonate concentration does not dominate its enrichment of those minor elements. in an effort to reconstruct supra-regional or possibly global redox situations in shallow oceans, we contrast the collected mn and ce information from the present investigation (fig. 5) with equivalent proxies at other sites in eastbourne (pelagic shelf sea) and qingshankou members 1 and 2 -3 in the sk1-s core in east asian waters (lu et al., 2010; jones et al., 2018). this depends upon the distinctive negative cie throughout the oae3 transitional period plus oae cie. high-resolution figure 9. reconstructions offering an understanding of the changes in continental weathering and deoxygenation during the oae3 transition period at the shallow-water tanuma carbonate platform in contemporary central iraq. take note of the following: (1) the depositional environment abruptly changed in shallow-water carbonate platforms (a & b) to more deep water ramps (c & d) as a result of the increasing sea level rise with carbonate platforms crises during the initial phase during a oae-cie. (2) growth of depleted in oxygen water is indicated by the arrows in red. 176 rana abbas ali measurements of ce/ce*, ce, and mn generally at the eb10 well indicate a consistent increase that begins near the oae3 boundary and ends in the oae cie at the end level. within a similar stratigraphic range, there is a generalized positive shifting of ce/ca and mn/ca, with ce anomalies of southern england (eastbourne) (lu et al., 2010) also for mn of the sk1-s core in east asia (jones et al., 2018). these profiles do not, however, exhibit the declining trend that would be expected given the cie’s middle values; this discrepancy may be due to local or poor resolution issues. an improved global fixing of ce and mn, likely the result of a single primary pulse that lasted the entire coniacian–early santonian, is shown by the matching variations in ce/ce*, ce, mn, sr, and i/ca, ce/ca, ce anomaly, and mn/ca from the globally separating locations of shallow water carbonate platforms for eb-10 (central iraq), abu roash formation (egypt) (southern tethys), and the pelagic shelf sea of eastbourne (intersection of the proto-atlantic and westernmost tethys). these data imply that, beginning near the boundary of oae3 and going on, although with occasional variations, during the cie period, depleted-oxygen (manganous) bottom waters at least spread throughout some tethyan continental boundaries during the coniacian-early santonian (figs. 8 and 9a-d). according to recent research by märz et al. (2009) using redox-sensitive components in bulk rock on the late cretaceous carbonates of the demerara rise, oxygen-depleted seas in this region spread into shallow water levels (fig. 1). within this setting, a major source of the diagenetic integration of these ions inside the carbonate lattice may have been the dissolution of ce3+ and mn2+ at mn oxides (oxyhydroxides) and the reduction of ce oxides from earlier deposits. the image that results is thus one of ce and mn enrichment caused by global redox shift impacting comparatively shallow tropical water in the coniacian-early santonian platform carbonates in the wider tethyan basin. a deoxygenated global ocean is suggested from the oae3 boundary period upward, as indicated by coeval positive in s of carbonate-associated sulphates from sections at odp site 1261 (demerara rise) (fig. 10), pelagic shelf sea, eastbourne (märz et al., 2009; jenkyns, 2010; mansour & wagreich, 2023). significantly, ce isotopes exhibit one positive shift at the identical stratigraphic period as the mn and ce/ce* excursions in the eb-10 section, as determined by the t-oae cie correlation. this is thought to be the result of a significant decrease in worldwide mn oxide (oxyhydr) depositing, which also liberated the heavier ce isotope that had been adsorbed. isotopic systems’ interpretations regarding the significance of a global redox postdating to the cie are unclear because the s and th isotopes do not later recover to background values. on the other hand, tessin et al. (2015) found that the mn enrichment on the portland core (english channel) happened around the oae3 boundary period and reverted to background values after the cie started. this trend is stratigraphically related to the first enhancing mn phase observed at eb10 and the late cretaceous carbonate of the eastbourne. the data from the eb-10 well and pelagic shelf water in eastbourne, where the mn content recovers to baseline near the end level in the cie (fig. 5), does not correspond with this early recovery and may instead point to a local signal. the oae3 boundary is the stratigraphically closest point below which the redox-sensitive elements begin to grow, as per the latest data gathered for this investigation (figs. 4, 5, and 8). this discrepancy in chronology suggests that increased continental weathering & inputs of nutrients may have contributed to the first event of shallow-water deoxygenation. this enhanced primary productivity resulted in a large consumption of oxygen, which was most noticeable in the deeper regions of the platform. the data indicate high hydrocarbon potential in specific intervals. on the other hand, during the cie’s recovery phase, the progressive decrease in water-insoluble components is accompanied by an increase in redox-sensitive elements. an upward trend of tl isotopes and a shift to lower radiogenic levels of os isotopes over this geologic era are the patterns that align with a coeval connection in isotopic data (cohen et al., 2004; jones et al., 2018). therefore, this deoxygenation episode cannot be explained by the accelerated continental weathering process (fig. 9d). notably, this time frame aligns with the high-temperature conditions encapsulated in a worldwide compilation of benthic foraminiferal oxygen-isotope (δ18o) data, and the tex86 temperature proxy has been extensively utilized in cretaceous environments (o’brien et al., 2017). in addition to significantly contributing to the drop in solubility of oxygen, such extreme temperatures may have promoted water column stratification and slowed circulation. on the other hand, better recirculation of bioessential nutrients in increasingly decreasing environments may have induced a rise in primary productivity and oxygen depletion. figure 10. the correlation for geochemical profiles for widely employed redox proxies at coniacian-early santonian carbonates from the eb-10 well (central iraq) and demerara rise (nw south american margin). caco3 and s data for the late cretaceous carbonates of the demerara rise at site 1261 are from märz et al. (2009). 177geochemical data from the tanuma carbonate reservoir (central iraq) indicate higher terrigenous input and deoxygenation during the coniacianearly santonian age 5.4. enhanced oxygen depletion and continental weathering are linked with the carbonate crisis. in the tanuma carbonate platform’s subtropical tethys ocean during the oae3 late cretaceous, bioclasts, oolites, and peloids, as well as other benthonic and planktonic faunas (rotalid, textularid, miliolid, globigerinid foraminifera, etc.), multiplied and created significant depositional systems (aqrawi, 1996; al-ameri & al-obaydi, 2011; ali et al., 2022; fig. 9a, b). major carbonateplatform crisis episodes occurred at the coniacian–early santonian and oae3 boundary times along the northern tethyan edge (wendler et al., 2009; phelps et al., 2015; mansour & wagreich, 2022). biological extinction and a clear decrease in carbonate composition serve as clear examples of the carbonate production crisis. the caco3 (2095–2093 m) in the eb10 well shows a consistent downward trend from ~87.5–83.5%, which corresponds with the oae cie start interval. this is followed by many times of sharp fall from ~94– 76%, which corresponds to the cie negative phases itself (2067–2093 m; fig. 8). according to the most current data from the tanuma platform, the skeleton grain content and foraminiferal variety began to decline at the oae3 border and declined more sharply throughout the oae cie’s negative phase (figs. 3 and 8). there is no other notable calcareous microfossil or nannofossil datum close to this level (cie); the lowest rare dicarinella asymetrica planktonic foraminifers and lucianorhabdus cayeuxii nannoplankton are found much below (aqrawi, 1996; remin, 2004; haddad & amin, 2007; gale et al., 2007; fig. 9c). despite the serious benthic fauna crisis that the tanuma platform experienced, some smaller foraminifers continued to be visible as sediment growers throughout the cie interval, and the resulting facies still have an elevated caco3 content, indicating that the biotic platform was not entirely closed down despite pressure (fig. 9c, d). in general, it is likely that the environmental disruptions at the oae3 boundary period only slightly altered the tanuma carbonate factory, but they were a crucial contributor in the ensuing late cretaceous benthic biological crisis of carbonate growers. the findings in global locations ranging from hemipelagic and shallow marine environments are consistent with this growing crisis in carbonate production (mansour & wagreich, 2022). on the contrary, in the sinai desert (egypt) and kopet-dagh basin (ne iran), there is data of a considerable fall in caco3 amount and an abrupt transition of carbonate to siliciclastic deposition at the coniaciansantonian boundary, indicating that most carbonate factories essentially shut down at this boundary (kalanat et al., 2021; bomou et al., 2023). in fact, some bio carbonate platforms did indeed disappear around the coniacian-santonian boundary, but other robust platforms persisted and gave rise to relatively nonfossiliferous oolitic sedimentary formations or weakly fossiliferous submerged micrites (yilmaz et al., 2018; steuber et al., 2023). there are a number of nonexclusive explanations for the global crisis in carbonate production, such as greater terrigenous input, eutrophication, temperature excess, rise in sea levels, oceanic oxygen depletion and acidification (jenkyns, 2010; han et al., 2022). but throughout the coniacian–early santonian, these processes likely had varying roles in various basins throughout time periods and typically worked in combination with local influences. the oae cie (fig. 8) shows a strong correlation between the rapid rise in sea level and platforms crisis in eb10, which includes living things and carbonate production. however, the rate of carbonate deposition on a robust and abundant platform ought to have been sufficient to keep up with the increase in the level of the sea (yilmaz et al., 2018). according to the results of this investigation, an increase in ti, al, th, sc, and ree at tanuma platform precedes a rise in mn, ce, and ce/ce*; this increase initially appears at the oae3 border (figs. 4, 5, and 8). thus, it is likely that the marginal decline in foraminiferal variety, carbonate content, and skeleton abundance in central iraq was caused by the almost simultaneous augmented terrigenous input and deoxygenation during this time period (fig. 8). the macrobenthic populations, which include benthic foraminifera (figs. 3 and 8) and carbonate content (figs. 4, 8, and 9a-c), present a substantial drop, indicating a corresponding carbonate-platform crisis, after these proxies exhibit a synchronized fast wide rise at the initial degree of the oae cie. the correlation between these findings and the significantly increased terrigenous input and deoxygenation in response to the late cretaceous significant release of carbon rhythm suggests that these processes were probably crucial in degrading ecosystems and initiating a biological crisis for the benthic carbonate growers. because observable siliciclastic deposits were found in sinai around the oae3 boundary and in the coniacian–early santonian period (bauer et al., 2003; mansour & wagreich, 2022), yet not in central iraq, it is evident that the terrigenous influx in carbonate platforms exhibited significant local heterogeneity. finding specific geochemical proxies of terrigenous input (ree, sc, th, ti, and al) of isolated carbonate platforms—such as china, hungary, and egypt in the northern and southern tethys—that weren’t directly affected by continental rivers inputs is necessary, as this study demonstrates, in order to evaluate potential global resemblance. redox proxies on the tanuma platform and the adriatic carbonate platform region (such as eastbourne; lu et al. 2010) have altered, suggesting that oxygen depletion within shallow water prior to or concurrent to the biologic carbonate platforms crisis was probably widespread along the southern tethyan margin. this could have had extremely detrimental effects on carbonate generators and sediment formation. 6. conclusions determining possible triggers of the carbonate production crisis and offering special insight into palaeoceanographic modifications to shallow tropical waters throughout the coniacian-santonian period are two benefits of analyzing the trace and major elements found in carbonates at the tanuma carbonates platform in central iraq. the majority of elements insoluble in water (like ree, sc, th, ti, and al) have risen synchronously, beginning at the oae3 boundary and going on through the oae cie’s negative phase. this indicates the warming temperatures, along with raised continental weathering, have probably caused improved terrigenous influx to the shallow marine system. immediately after the rise in water-insoluble elements starts, there is a modest enrichment in ce, mn, and ce/ce*. then, as water-insoluble elements increase during the negative stage in the oae cie, this enrichment moves stratigraphically upward in higher levels through time. these data support the idea that, in this time period, deoxygenation of shallow waters may have been associated with increased continental weathering with nutrient inputs, supporting comparatively high primary production and consumption of oxygen. in the recovery phase of cie, on the other hand, the water insoluble components exhibit a gradually declining trend superimposed on a positive shifting for redox proxies. there are several possible explanations for this deoxygenation, including localized stratification of the water columns and improved recycling of bioessential nutrients with elevated primary productivity. the tanuma carbonate factory was likely slightly modified around the oae3 boundary time due to the association between carbonate levels and proxies over terrigenous flow and the deoxygenation and biologic data of skeletal grain content. this modification probably played a crucial part in the coniacian– early santonian crisis that benthic carbonate producers faced, which worsened as the oae negative the cie coincided. another coeval carbonate platform located in western tethys could possibly benefit from a similar process. acknowledgements the core samples, information, and reports were supplied by the midland oil company of iraq (md.o.c.), for which the author is appreciative. much gratitude and appreciation are owed to the university of baghdad’s department of geology’s authority and mineralogical laboratory staff for their help in getting the thin sections ready. formatting of funding sources this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. declaration of competing interest the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 178 rana abbas ali references abdel-fattah, m. i., mahdi, a. q., theyab, m. a., pigott, j. d., abd-allah, z. m., & radwan, a. e. 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https://ui.adsabs.harvard.edu/link_gateway/2016e&psl.446...37w/doi:10.1016/j.epsl.2016.04.007 https://doi.org/10.1016/j.cretres.2009.02.010 https://doi.org/10.1016/j.cretres.2009.02.010 https://doi.org/10.1016/j.earscirev.2013.08.003 https://doi.org/10.1016/j.earscirev.2013.08.003 http://dx.doi.org/10.1016/j.palaeo.2012.09.004 http://dx.doi.org/10.1016/j.palaeo.2012.09.004 https://doi.org/10.1016/j.chemgeo.2023.121790 https://doi.org/10.1016/j.chemgeo.2023.121790 https://doi.org/10.1016/j.cretres.2017.09.012 https://doi.org/10.1016/j.cretres.2017.09.012 http://dx.doi.org/10.1016/j.gca.2014.07.001 https://doi.org/10.1016/j.earscirev.2022.104015 volume 16 – special issue april 2012 evaluating total yet-to-find hydrocarbon volume in colombia carlos a. vargas j. universidad nacional de colombia president of the republic of colombia dr. juan manuel santos calderon minister of mines and energy dr. mauricio cardenas santamaria president national agency of hydrocarbons dr. orlando cabrales segovia technical director dr. juan fernando martinez autor and editor carlos alberto vargas jimenez. associated professor, department of geosciences universidad nacional de colombia technical team alejandra j. angulo john h. arenas mauricio moreno m. jose araujo universidad nacional de colombia edilsa aguilar gomez german orozco agencia nacional de hidrocarburos cite this work as: vargas, c. a. evaluating total yet-to-find hydrocarbon volume in colombia. earth sci. res. j., vol. 16, special issue (april, 2012): 1-246. earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia content 1 introduction ............................................................................................................................................. 1 2 hydrocarbons produced in colombian sedimentary basins .......................................... 3 2.1 general comments ................................................................................................................................ 3 2.2 colombia‟s sedimentary basins .......................................................................................................... 6 2.2.1 the amagá basin ......................................................................................................................... 6 2.2.2 the caguán – putumayo basin ................................................................................................. 7 2.2.3 the catatumbo basin .................................................................................................................. 8 2.2.4 the cauca patía basin ............................................................................................................. 9 2.2.5 the cesar-ranchería basin ...................................................................................................... 10 2.2.6 the choco basin ......................................................................................................................... 11 2.2.7 the chocó offshore basin ......................................................................................................... 12 2.2.8 the colombia basin ................................................................................................................... 13 2.2.9 the colombian deep pacific basin ........................................................................................ 14 2.2.10 the eastern cordillera basin ................................................................................................... 14 2.2.11 the eastern llanos basin ........................................................................................................... 15 2.2.12 the guajira basin ...................................................................................................................... 16 2.2.13 the guajira offshore basin ...................................................................................................... 17 2.2.14 los cayos basin ......................................................................................................................... 18 2.2.15 lower magdalena valley basin ............................................................................................. 19 2.2.16 middle magdalena valley basin ........................................................................................... 20 2.2.17 upper magdalena valley basin ............................................................................................ 22 2.2.18 sinusan jacinto basin ............................................................................................................. 23 2.2.19 sinú offshore basin .................................................................................................................... 24 2.2.20 tumaco basin .............................................................................................................................. 24 2.2.21 tumaco offshore basin .............................................................................................................. 25 2.2.22 urabá basin ................................................................................................................................ 26 2.2.23 vaupés – amazonas basin....................................................................................................... 27 2.3 environmental considerations ............................................................................................................ 28 2.4 data and hypotheses ......................................................................................................................... 30 2.4.1 data.............................................................................................................................................. 30 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.4.2 hypotheses .................................................................................................................................. 30 2.4.3 methodology .............................................................................................................................. 30 2.4.4 results ........................................................................................................................................... 32 2.4.5 thickness of source rock ............................................................................................................ 33 2.4.6 mass of hydrocarbon generated per gram of toc .......................................................... 33 2.4.7 toc fraction ............................................................................................................................... 33 2.4.8 ratio between basin area and hydrocarbon generated .................................................. 34 2.4.9 volume of hydrocarbon generated ....................................................................................... 35 2.4.10 trapped hydrocarbons............................................................................................................. 37 2.4.11 sensitivity analysis ..................................................................................................................... 38 2.5 conclusions............................................................................................................................................ 38 2.6 bibliography ........................................................................................................................................ 39 2.7 appendices .......................................................................................................................................... 39 2.7.1 appendix 2-1 ............................................................................................................................. 39 2.7.2 appendix 2-2 ............................................................................................................................. 40 3 recoverable original resources ................................................................................................... 41 3.1 general comments .............................................................................................................................. 41 3.1.1 volumetric estimation ................................................................................................................ 41 3.2 data and hypotheses ......................................................................................................................... 41 3.2.1 data.............................................................................................................................................. 41 3.2.2 hypothesis ................................................................................................................................... 41 3.3 methodology ....................................................................................................................................... 41 3.4 results .................................................................................................................................................... 43 3.4.1 production area ......................................................................................................................... 43 3.4.2 deposit thickness ........................................................................................................................ 44 3.4.3 deposit porosity ......................................................................................................................... 44 3.4.4 water saturation ....................................................................................................................... 44 3.4.5 volumetric factors ...................................................................................................................... 45 3.4.6 gas oil ratio (gor) .................................................................................................................. 46 3.4.7 recoverable resources .............................................................................................................. 47 3.4.8 sensitivity analysis ..................................................................................................................... 53 3.5 conclusions............................................................................................................................................ 54 3.6 bibliography ........................................................................................................................................ 54 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 3.7 appendices .......................................................................................................................................... 54 4 yet-to-find resources ......................................................................................................................... 55 4.1 general comments .............................................................................................................................. 55 4.1.1 fractal method ........................................................................................................................... 55 4.2 data and hypotheses ......................................................................................................................... 55 4.2.1 data.............................................................................................................................................. 55 4.2.2 hypothesis ................................................................................................................................... 55 4.3 methodology ....................................................................................................................................... 56 4.4 results .................................................................................................................................................... 57 4.4.1 the eastern llanos ..................................................................................................................... 57 4.4.2 caguán-putumayo ..................................................................................................................... 58 4.4.3 the middle magdalena valley .............................................................................................. 59 4.4.4 the upper magdalena valley ............................................................................................... 60 4.4.5 total yft crude resources in colombia ................................................................................. 61 4.4.6 total ytf gas resources in colombia ..................................................................................... 62 4.5 conclusions............................................................................................................................................ 63 4.6 bibliography ........................................................................................................................................ 63 4.7 appendices .......................................................................................................................................... 63 5 methane gas hydrates ........................................................................................................................ 65 5.1 general comments .............................................................................................................................. 65 5.1.1 origin and formation ................................................................................................................ 65 5.1.2 bottom simulating reflector (bsr) ............................................................................................ 66 5.1.3 gas hydrate distribution around the world.......................................................................... 67 5.2 data and hypotheses ......................................................................................................................... 68 5.2.1 international data ...................................................................................................................... 68 5.2.2 national data ............................................................................................................................. 69 5.2.3 hypotheses .................................................................................................................................. 70 5.3 methodology ....................................................................................................................................... 70 5.4 results .................................................................................................................................................... 75 5.4.1 gas hydrate occurrence areas ............................................................................................... 76 5.4.2 deposit thickness ........................................................................................................................ 76 5.4.3 degree of gas hydrate saturation ......................................................................................... 88 5.4.4 volumetric yield of gas in hydrates ....................................................................................... 89 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.4.5 gas hydrate potential .............................................................................................................. 89 5.4.6 sensitivity analysis ..................................................................................................................... 92 5.5 conclusions............................................................................................................................................ 92 5.6 bibliography ........................................................................................................................................ 92 5.7 appendixes .......................................................................................................................................... 94 6 coalbed methane .................................................................................................................................... 95 6.1 general comments .............................................................................................................................. 95 6.1.1 origin and formation ................................................................................................................ 95 6.1.2 types of coal .............................................................................................................................. 95 6.1.3 coalbed methane (cbm) deposits ......................................................................................... 96 6.1.4 cbm around the world ............................................................................................................. 97 6.1.5 coal in colombia ....................................................................................................................... 97 6.2 data and hypotheses ...................................................................................................................... 101 6.2.1 international data ................................................................................................................... 101 6.2.2 national data .......................................................................................................................... 101 6.2.3 hypotheses ............................................................................................................................... 101 6.3 methodology .................................................................................................................................... 102 6.4 results ................................................................................................................................................. 103 6.4.1 drainage area ........................................................................................................................ 103 6.4.2 deposit thickness ..................................................................................................................... 104 6.4.3 average apparent density of coal ..................................................................................... 104 6.4.4 gas concentration (gc) .......................................................................................................... 104 6.4.5 cbm in colombia .................................................................................................................... 106 6.5 conclusions......................................................................................................................................... 109 6.6 bibliography ..................................................................................................................................... 109 6.7 appendixes ....................................................................................................................................... 109 7 tar sands ................................................................................................................................................ 111 7.1 general comments ........................................................................................................................... 111 7.1.1 bitumens .................................................................................................................................... 112 7.1.2 asphalts .................................................................................................................................... 112 7.1.3 tar sands .................................................................................................................................. 113 7.1.4 classifying tar sands .............................................................................................................. 114 7.1.5 the nature of the impregnation ........................................................................................... 115 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7.1.6 tar sand accumulation in colombia .................................................................................... 115 7.1.7 extraction in colombia ........................................................................................................... 116 7.1.8 tar sand accumulations throughout the world ................................................................... 117 7.2 data and hypotheses ...................................................................................................................... 118 7.2.1 international database .......................................................................................................... 118 7.2.2 national (colombian) database .......................................................................................... 119 7.2.3 hypotheses ............................................................................................................................... 119 7.3 methodology .................................................................................................................................... 121 7.4 results ................................................................................................................................................. 123 7.4.1 tar sand occurrences .............................................................................................................. 123 7.4.2 thickness ................................................................................................................................... 124 7.4.3 bitumen density........................................................................................................................ 124 7.4.4 rock density ............................................................................................................................. 124 7.4.5 area .......................................................................................................................................... 124 7.4.6 potential of tar sands in colombia ...................................................................................... 125 7.4.7 sensitivity analysis .................................................................................................................. 128 7.5 conclusions......................................................................................................................................... 128 7.6 bibliography ..................................................................................................................................... 128 7.7 appendixes ....................................................................................................................................... 131 8 oil shale ................................................................................................................................................... 133 8.1 general comments ........................................................................................................................... 133 8.1.1 origin and formation ............................................................................................................. 133 8.1.2 types of oil shale .................................................................................................................... 133 8.1.3 determining the grade of oil shale ..................................................................................... 135 8.1.4 oil shale exploitation ............................................................................................................ 135 8.1.5 deposits around the world .................................................................................................... 136 8.2 data and hypotheses ...................................................................................................................... 136 8.2.1 national (colombian) database .......................................................................................... 137 8.2.2 hypotheses ............................................................................................................................... 137 8.3 methodology .................................................................................................................................... 139 8.3.1 scenario 1 ................................................................................................................................ 140 8.3.2 scenario 2 ................................................................................................................................ 140 8.3.3 scenario 3 ................................................................................................................................ 141 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.3.4 scenario 4 ................................................................................................................................ 141 8.4 results ................................................................................................................................................. 142 8.4.1 oil shale unit thickness ........................................................................................................... 142 8.4.2 total rock density ................................................................................................................... 142 8.4.3 oil shale density ..................................................................................................................... 142 8.4.4 shale areas .............................................................................................................................. 143 8.4.5 deposit area weighting factor ............................................................................................. 144 8.4.6 s2p hydrocarbon content ....................................................................................................... 148 8.4.7 oil shale in colombia ............................................................................................................ 152 8.4.8 sensitivity analysis .................................................................................................................. 155 8.5 conclusions......................................................................................................................................... 156 8.6 bibliography ..................................................................................................................................... 157 8.7 appendixes ....................................................................................................................................... 161 9 shale gas and shale oil .................................................................................................................. 163 9.1.1 classification ............................................................................................................................ 163 9.1.2 origin and formation ............................................................................................................. 163 9.1.3 shale characterization ........................................................................................................... 165 9.1.4 some source rock packages in colombia .......................................................................... 169 9.2 data and hypotheses ...................................................................................................................... 173 9.2.1 international database .......................................................................................................... 173 9.2.2 national (colombian) database .......................................................................................... 173 9.2.3 hypotheses ............................................................................................................................... 173 9.3 methodology .................................................................................................................................... 175 9.3.1 scenario 1 ................................................................................................................................ 177 9.3.2 scenario 2 ................................................................................................................................ 177 9.4 results ................................................................................................................................................. 177 9.4.1 source rock volume ................................................................................................................. 178 9.4.2 net-to-gross ............................................................................................................................. 187 9.4.3 total rock density ................................................................................................................... 191 9.4.4 absorbed gas concentration ................................................................................................ 191 9.4.5 free gas concentration .......................................................................................................... 192 9.4.6 net-to-gross international databases................................................................................. 192 9.4.7 hydrocarbons in shale ........................................................................................................... 192 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.8 potential of shale oil and shale gas .................................................................................... 193 9.4.9 sensitivity analysis .................................................................................................................. 198 9.5 conclusions......................................................................................................................................... 199 9.6 bibliography ..................................................................................................................................... 199 9.7 appendixes ....................................................................................................................................... 201 10 gas resources in tight sands ................................................................................................. 203 10.1 general comments ........................................................................................................................... 203 10.1.1 origin and formation ............................................................................................................. 203 10.1.2 exploration of and production from tight gas deposits .................................................. 204 10.1.3 tight gas sands distribution around the world .................................................................. 204 10.2 data and hypotheses ...................................................................................................................... 205 10.2.1 international data ................................................................................................................... 205 10.2.2 national (colombian) data ................................................................................................... 205 10.2.3 hypotheses ............................................................................................................................... 206 10.3 methodology .................................................................................................................................... 207 10.4 results ................................................................................................................................................. 211 10.4.1 area of tight gas sands occurrence .................................................................................... 212 10.4.2 the thickness of tight gas sands areas ............................................................................... 213 10.4.3 sediment porosity ................................................................................................................... 213 10.4.4 water saturation .................................................................................................................... 213 10.4.5 gas expansión ........................................................................................................................ 214 10.4.6 sensitivity analysis .................................................................................................................. 215 10.4.7 sensitivity analysis – scenario 2 .......................................................................................... 215 10.4.8 estimating resources for those basins lacking information .............................................. 216 10.4.9 basin area cf gas volume ratio scenario 2 .................................................................... 217 10.4.10 tight gas sands in colombia ............................................................................................ 217 10.5 conclusions......................................................................................................................................... 220 10.6 bibliography ..................................................................................................................................... 220 10.7 appendixes ....................................................................................................................................... 221 11 heavy crudes .................................................................................................................................... 223 11.1 general comments ........................................................................................................................... 223 11.1.1 origin and characteristics ..................................................................................................... 223 11.2 data and hypotheses ...................................................................................................................... 225 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 11.2.1 hypotheses ............................................................................................................................... 226 11.3 methodology .................................................................................................................................... 226 11.4 results ................................................................................................................................................. 228 11.4.1 area .......................................................................................................................................... 230 11.4.2 heavy crudes in colombia .................................................................................................... 231 11.4.3 sensitivity analysis .................................................................................................................. 232 11.5 conclusions......................................................................................................................................... 233 11.6 bibliography ..................................................................................................................................... 233 11.7 appendixes ....................................................................................................................................... 234 12 production projections ........................................................................................................... 235 12.1 general comments ........................................................................................................................... 235 12.1.1 cointegral time series ............................................................................................................ 235 12.2 data and hypotheses ...................................................................................................................... 236 12.2.1 data........................................................................................................................................... 236 12.2.2 hypotheses ............................................................................................................................... 236 12.3 methodology .................................................................................................................................... 236 12.4 results ................................................................................................................................................. 237 12.5 bibliography ..................................................................................................................................... 238 12.6 appendixes ....................................................................................................................................... 238 13 executive report .............................................................................................................................. 239 13.1 general comments ........................................................................................................................... 239 13.2 estimated potential of conventional hydrocarbons in colombia ............................................ 239 13.3 estimated potential of non-conventional hydrocarbons in colombia .................................... 240 13.4 hydrocarbon resources matrix ...................................................................................................... 242 earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 1 introduction the present work combines estimations of conventional and non-conventional yet-to-find recoverable hydrocarbons in colombia‟s 23 sedimentary basins. the work adopts a probabilistic approach using the monte carlo statistical method aimed at defining available and trapped resources and comparing such simulation with in situ resources regarding their conventional and non-conventional expression. a series of considerations and hypotheses support the work‟s conceptual and procedural framework regarding all the estimations so made. such hypotheses were statistically based by compiling a series of events tracing a determined pattern which may not always correspond to the geographical reality posed by the hypotheses. the foregoing means that some of the hypotheses could be reconsidered in future estimates; this is why the excel sheets underlying fresh calculations have been attached so that they can be evaluated in new scenarios. as well as the resource being produced, the first chapters (2 to 4) present volumetric estimates and expectations regarding news reserves related to conventional potential, i.e. oil, gas and associated gas. these three chapters also try to differentiate between the following concepts: generated resource, original in situ resource and yet-to-find (ytf) resources. chapter 5 gives an outline of a systematic exercise orientated towards estimating gas hydrates in colombia. the estimates were based on information from 28 wells and more than 40,000 km of seismic 2d. the resources did not include the deep basins and los cayos basin due to gaps in the information or the poor quality of some available data. chapter 6 gives estimates of coalbed methane, supported by figures drawn from cartographic information from carbon occurrences in colombia, and data regarding gas production in mines in colombia, as well as alberta (canada). potential resources associated with tar sands are calculated in chapter 7. information of differing nature, age and sources was compiled for this exercise. estimating potential was based on a map of tar sand occurrences produced from an inventory of areas having manifestations of the resource obtained from available information. chapters 8 and 9 deal with results concerning oil and gas in shale; two different but complementary approaches led to establishing this resource‟s great potential in colombia. the lack of certain geochemical data and in-depth distribution data impeded using conventional techniques for analysing shale oil, oil shale and shale gas. however, the approach was valuable as the working hypotheses were coherent. a conventional analysis of shale gas and shale oil based on well and seismic data led to extending concepts to other basins. an approach based on well information and data regarding area from other basins around the world led to estimating tight sands (chapter 10), a resource which is becoming marginal compared to other resources. introduction earth sciences research journal, volume 16, special issue, april, 2012 2 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia one chapter has been devoted to heavy crudes (chapter 11) for estimating this resource in five sedimentary basins having more occurrences and/or effective production. well and seismic information as well as approximations of areas were fundamental for such estimate. chapter 12 gives a forecast concerning hydrocarbon production in colombia. given that the numerical exercise was based on the cointegral time series concept, the lack of historical production data regarding non-conventional resources limited the exercise to conventional gas and oil. the advantage of an approach using cointegral variables was based on analysing production evolution along with other macroeconomic variables maintaining long-term ratio and seasonality. all the chapters have digital appendixes relating the databases used, the electronic spreadsheets with data fitted for running simulations and/or the results found. other information such as seismic and well data duly integrated into projects using kingdom suite software is attached to the present document. earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2 hydrocarbons produced in colombian sedimentary basins 2.1 general comments the history of petroleum (oil) in colombia began in 1537 when gonzalo jiménez de quesada and his followers saw, “a source of bitumen or boiling pool,” producing a viscous black liquid when close to what today is barrancabermeja in the magdalena valley. what they saw was oozing, such as that which can still be found in different parts of the country. it was almost 400 years later (1905) when two concessionaries, de mares and barco, were allowed to explore and exploit petroleum; this then marked the start of the modern period in colombian oil history. de mares began geological studies in 1915, perforation in 1916 and the first oil was produced in 1918 (griess, 1946). the colombian government created the colombian petroleum company (empresa colombiana del petróleo ecopetrol) in 1951 which has administered exploration and production in colombian sedimentary basins for more than 50 years. ecopetrol became a public corporation in 2003 and was released from its petroleum management functions, giving place to the newly created colombian hydrocarbon agency (agencia nacional de hidrocarburos anh). colombian territory has been divided into 23 sedimentary basins during anh‟s administration period (pardo et al., 2007): amagá, caguán putumayo, catatumbo, cauca patía, cesar-ranchería, chocó, chocó offshore, colombia, deep pacific area, the eastern cordillera, the eastern llanos, the guajira, guajira offshore, los cayos, san jacinto in sinú, sinú offshore, tumaco, tumaco offshore, urabá, vaupés amazonas, the upper magdalena valley, the middle magdalena valley and the lower magdalena valley (figure 1-1). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 4 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-1 map of colombian basins, modified from colombian hydrocarbon agency (2007). 01. amaga; 02. caguan-putumayo; 03. catatumbo; 04. cauca-patía; 05. cesar-ranchería; 06. choco; 07. choco offshore; 08. colombia; 09. deep pacific; 10. eastern cordillera; 11. eastern llanos; 12. guajira; 13. guajira offshore; 14. los cayos; 15. lower magdalena valley; 16. middle magdalena valley; 17. sinu-san jacinto; 18. sinu offshore; 19. tumaco; 20. tumaco offshore; 21. upper magdalena valley; 22. uraba; 23. vaupes-amazonas. main structures: cf – cuiza fault; cfs – cauca fault system; cpsz – colombian pacific subduction zone; esfs – espiritu – santo fault system; he – hess escarpment; mf – murindo fault; npdb – north panama deformed belt; of – oca fault; rfsromeral fault system; scdn – south caribbean deformed belt; ufs – uramita fault system. km peru brazil panama pacific ocean caribbean sea hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5 several estimations have been made for measuring colombian yet-to-find hydrocarbon volume in its sedimentary basins. the large differences in results regarding studies carried out during the last 15 years has been due to the nature of the methodologies and data used; the most outstanding studies are mentioned below. ortiz (1997) estimated 15,800 mboe resources using a fractal geometry model and field production and distribution data. the us geological service (usgs) estimated 1.3 to 10.9 bboe yetto-find hydrocarbons for colombia (ahlbrandt, 2000). d‟little (2008) estimated a yet-to-find figure of 10,000 mboe using a fractal approach and by surveying experts. the universidad industrial de santander (2009) used mass balance methodology regarding 11 colombian sedimentary, suggesting 37,000 to 296,000 mboe resources. vargas (2009) estimated p10 and p90 resources ranging from 82,177 and 34,141 mboe based on a ratio referring to production being proportional to basin size and homogenous recovery conditions (20%) and geological risk (30%). as the current work is aimed at estimating conventional and non-conventional yet-to-find hydrocarbon resources, a start was made by evaluating the resource available in colombian basins to provide a reference framework and compare available resources. the hydrocarbons generated in colombian basins were calculated by using the mass balance method proposed by schmoker (1994). such method uses the percentage of total organic carbon (toc) present in rock and current and original hydrogen indices as entry data. once the hydrogen produced by the generating rocks has been calculated, then an evaluation is made of how much of this volume will become accumulated and how much can be extracted. figure 2-2 shows the general scheme for such procedure. figure 2-2 . general scheme of the procedure proposed by schmoker (1994). modified from universidad industrial de santander (2009). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 6 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia the procedure was carried out for the 12 basins having basic data available for reproducing the methodology (catatumbo, cesar ranchería, the eastern cordillera, the eastern llanos, sinú offshore, sinú san jacinto, tumaco, the lower magdalena valley, the middle magdalena valley, the upper magdalena valley, caguán putumayo and guajira offshore). calculations were made for the rest of the basins for which there was little or no data available, based on a hypothesis of linearity regarding hydrocarbon available for production being directly related to basin area (e.g. vargas, 2009). a brief description is given below of the basins for establishing a referent to the petroleum (oil) system; the resource which could be produced is then estimated and the uncertainty of the figures so found is analysed. 2.2 colombia’s sedimentary basins the following paragraphs summarise the most notable aspects regarding colombia‟s 23 sedimentary basins. a more detailed description concerning the elements defining them or the most relevant referents regarding their oil systems can be found in anh‟s book about the sedimentary basins of colombia (anh, 2007). 2.2.1 the amagá basin the amagá basin is a rear arc basin, associated with later ocean-continent collision and convergence. it is located to the west of the romeral fault system (figure 2-3). it covers an area located to the southeast of the antioquia department and the northern part of the caldas department. this basin is bounded to the west and east by cretaceous sedimentary and igneous rocks from the western and central cordilleras, respectively. figure 2-3 location and borders of the amagá basin. 01. amagá basin; r.f.s. – romeral fault system c.f.s. – cauca fault system. the basin consists of fluvial deposits having important coal layers covered by neogene volcanic clasts, mudstones and lava flows. figure 2-4 gives a schematic model of the basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7 figure 2-4. schematic section of the amagá basin. 2.2.2 the caguán – putumayo basin the caguán putumayo basin, shares its geological history with the eastern basin in ecuador, being part of a foreland basin. the basin is bounded to the northeast by the macarena mountain range, the international border with ecuador and peru to the south, the chiribiquete mountain range to the east and the eastern cordillera‟s piedmont fault system to the northwest (figure 2-5). figure 2-5. location and borders of the caguán – putumayo basin. 02 – caguán putumayo basin; r.f.s. – romeral fault system; sch – chiribiquete mountain range; s.m. – macarena mountain range. the caguán – putumayo basin covers a 110,304 km2 region. the villeta formation‟s cretaceous limestone and shale represent the basin‟s reservoir rock. shale from the caballos formation forms a secondary hydrocarbon source and the reservoir rock consists of sand from the caballos formation (figure 2-6). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 8 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-1. schematic section of the caguán – putumayo basin. 2.2.3 the catatumbo basin the catatumbo basin is the colombian portion of the maracaibo basin in venezuela. it has geographical borders with venezuela in the north and east; it is bounded by cretaceous rocks from the eastern cordillera to the south and igneous and metamorphic rocks from the santander massif to the west (figure2-7). figure 2-7. location and borders of the catatumbo basin. 03 – cuenca catatumbo; b.s.m.f. – bucaramangasanta marta fault system. la luna formation is the basin‟s main generating unit, having a thickness of around 200 feet. pelitic rocks from the cretaceous age (la luna, capacho, tibú and mercedes formations) are present throughout the catatumbo basin (figure 2-8). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9 figure 2-8. schematic section of the catatumbo basin. 2.2.4 the cauca patía basin the cauca-patía basin is an active margin basin associated with a back-arc, being genetically related to the amagá basin. it is bounded to the north and south by basic igneous rocks from the cretaceous age, to the west by the cauca system fault and sedimentary and volcanic rocks from the western cordillera and to the east by the romeral fault system and the central cordillera (fig. 29). figure 2-9. location and borders of the cauca-patía basin. 04 – cauca-patía basin; r.f.s. – romeral fault system; c.f.s. – cauca fault system; g.f.z. – garrapatas fault system. this basin has an oil-bearing system characterised by large anticlines and structural traps associated with faults in the patía region (figure 2-10). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 10 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-10. schematic section of the cauca-patía basin. 2.2.5 the cesar-ranchería basin the cesar-ranchería basin is bounded to the southeast by the bucaramanga-santa marta fault, to the east-southeast by cretaceous rocks from the perijá mountain range and by the border between colombia and venezuela, to the northeast by the oca fault and to the northwest by pre-cretaceous rocks from the sierra nevada de santa marta (figure 2-11). figure 2-11. location and borders of the cesar-ranchería basin. 05 – cesar ranchería basin; b.s.m.f. – bucaramanga-santa marta fault system; o.f. – oca fault. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 11 the rocks of the molino, la luna and aguas blancas formations (figure 2-12) have excellent hydrocarbon generating potential due to their high kerogen type 2 and 3 content. figure 2-12. schematic section of the cesar-ranchería basin. 2.2.6 the choco basin the chocó basin is bounded to the northwest by the geographical border with panamá and the baudó mountain range, to the south by the garrapatas fault area, to the southeast by the pacific coastline and to the east by quartz diorites from the mande batholith, the cretaceous rocks from the western cordillera and partially by the murindó fault (figure 2-13). figure 2-13. location and borders of the chocó basin. 06 – chocó basin; g.f.z. – garrapatas fault system; s.b. – baudó mountain range; m.b. – mande quartz diorites; m.f. – murindó fault; wc – western cordillera. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 12 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia all the hydrocarbon samples found in the chocó basin are believed to have mainly been produced in the iró formation (figure 2-14). this formation‟s thickness is not known with certainty, but it has been estimated from seismic data that it could vary from 650 to 1,200 m. it is divided into three segments: lower, intermediate and upper. figure 2-14. schematic section of the chocó basin 2.2.7 the chocó offshore basin the chocó offshore basin is located in north-western colombia; it lies beneath the water of the pacific ocean and its western border is shared with the chocó department‟s coastline. it is bordered to the north by waters of panamá. this offshore basin extends to the trench of the present subduction area. it is bounded to the south by the garrapatas fault area (figure 2-15). figure 2-16 gives a schematic model of the basin. figure 2-15. location and borders of the chocó offshore basin. 07– chocó offshore basin; g.f.z. – garrapatas fault zone; s.z. – subduction zone. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 13 figure 2-16. schematic section of the chocó offshore basin. 2.2.8 the colombia basin the colombia basin is is a deep water basin located in the caribbean sea. it is bounded to the northwest by the hess escarpment, to the southeast by marine borders with costa rica and panamá, to the southeast by the front of the south caribbean deformed belt, to the east by the colombianvenezuelan marine frontier and to the north by maritime borders with jamaica, haiti and the dominican republic (figure 2-17). figure 2-17. location and borders of the colombia basin. 08 – colombia basin; 14 – los cayos basin; 18 – sinú offshore basin; h.e. – hess escarpment; n.p.d.b. – north panamá deformed belt; s.c.d.b. – south caribbean deformed belt. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 14 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.2.9 the colombian deep pacific basin the colombian deep pacific basin is mainly composed of oceanic volcanic rocks and deep marine sediments. it is bounded to the north by the maritime border between colombia and panamá, to the west by the maritime border between colombia and costa rica, to the south by maritime border between colombia and ecuador and to the east by colombian pacific subduction area (fig. 2-18). figure 2-18. location and borders of the colombian deep pacific basin. 2.2.10 the eastern cordillera basin the eastern cordillera basin consists of rocks formed in a late triassic rupture system resulting from the breakup of pangaea and was filled by marine sediment from the mesozoic age and continental sediment from the cenozoic age. as a consequence of its origin and structural developments, the basin‟s current boundaries are very irregular and difficult to describe. the basin is bounded to the north by igneous and metamorphic rocks from the santander massif, to the south by the algecirasgarzón fault system, to the west by the bituima and salina fault systems and to the east by the eastern cordillera‟s frontal overlapping system (figure 2-19). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 15 figure 2-19. location and borders of the eastern cordillera basin. 10 – eastern cordillera basin; b.s.f.s. – bituima and la salina fault system; s.m. – santander massif; a.g.f.s. – algeciras-garzón fault system. two middle albian and turonian age condensed sections deposited during worldwide anoxic events are considered the main rock source, including the simití and la luna formations (figure 2-20). figure 2-20. schematic section of the eastern cordillera basin. 2.2.11 the eastern llanos basin the eastern llanos basin is the most prolific hydrocarbon basin on the continental part of colombia. this basin‟s northern limit is the border between colombia and venezuela; the basin extends to the south to the macarena range, the vaupés arch and the pre-cambrian metamorphic rocks outcropping to the south of the guaviare river. it is bounded to the east by the outcropping of precambrian plutonic rocks from the guyana shield and to the west by the eastern cordillera‟s overlapping system (figure 2-21). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 16 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-21. location and borders of the eastern llanos basin. the carbonera and mirador formations‟ sands form reservoir rocks. the león formation forms the basin‟s regional seal (figure 2-22). figure 2-22. schematic section of the eastern llanos basin. 2.2.12 the guajira basin the guajira basin is located in the most northerly region of colombia. the basin is bounded to the north, north-west and north-east by the actual colombian caribbean coastline, to the southeast by its geographical border with venezuela and to the south by the oca fault. the basin has been divided by the cuiza fault trace in the guajira upper and lower sub-basins (figure 2-23). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 17 figure 2-23. location and borders of the guajira basin. 12 – guajira basin; o.f. – oca fault; c.f. – cuiza fault. la luna formation‟s shale, calcareous limolite and limestone have been identified as hydrocarbongenerating rocks; limestone and sandstone from the macarao and siamaná formations form reservoir rocks and the seals come from the siamaná formation‟s lodolite (figure 2-24). figure 2-24. schematic section of the guajira basin 2.2.13 the guajira offshore basin the guajira offshore basin is bordered to the north and northwest by the front of the south caribbean deformed belt originated by the interaction between the south american and caribbean hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 18 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia plates, to the east by the geographical line defining the border between colombia and venezuela and to the southeast from the part off the coast near the oca fault to the continental guajira coastline (figure 2-25). figure 2-26 gives a schematic model of the basin. figure 2-25. location and borders of the guajira offshore basin. 13 – guajira offshore basin; o.f. – oca fault; c.f. – cuiza fault; s.c.d.b. – south caribbean deformed belt. figure 2-26. schematic section of the guajira basin. 2.2.14 los cayos basin los cayos basin is an oceanic basin in the caribbean sea region. this basin is bounded to the north, east and west by international borders and to the south-southeast it borders the hess escarpment (figure 2-27). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 19 figure 2-27. location and borders of los cayos basin. 14 – los cayos basin; 08 – colombia basin; h.e. – hess escarpment. the basin‟s fill consist of palaeogene carbonate-siliciclastic sequences followed by neogene siliciclasts. figure 2-28 gives a schematic model of the basin. figure 2-28. schematic section of los cayos basin. 2.2.15 lower magdalena valley basin the lower magdalena valley basin is a triangular transtensional basin bounded to the west and north by the romeral fault system and to the south and southeast by the igneous and metamorphic complex of the central cordillera and the san lucas mountain range. the basin is bounded to the east by the northern portion of the bucaramanga-santa marta fault system (figure 2-29). a plinthtype range divides the basin into the plato sub-basin to the north and the san jorge sub-basin to the south. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 20 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-29. location and borders of the lower magdalena valley basin. 15 – lower magdalena valley basin; b.s.m.f. – bucaramanga-santa marta fault system; r.f.s. – romeral fault system; c.c.– central cordillera; s.l. – san lucas mountain range. the ciénaga de oro (literally, the golden swamp) formation‟s different levels act as generating rock, reservoir and seal (figure 2-30). figure 2-30. schematic section of the lower magdalena valley basin. 2.2.16 middle magdalena valley basin the middle magdalena valley basin consists of a so-called poly-historic basin. its structural development took place during different stages linked to tectonic events regarding the north-western corner of south america which occurred during the late triassic, middle cretaceous, early hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 21 palaeocene and middle neogene ages. the basin is bounded to the southeast by the bituima and salina fault systems, to the north by the espiritú santo fault system, to the west by the neogene sediments onlap over the san lucas mountain range and the central cordillera‟s plinth, to the south by the girardot fold belt and to the northeast by the bucaramanga-santa marta fault system (figure 2-31). figure 2-31. location and borders of the middle magdalena valley basin. 16 – middle magdalena valley basin; b.s.m.f. – bucaramanga santa marta fault system; b.s.f.s. – bitumina and la salina fault system; e.s.f.s. – espiritú santo fault system; s.l. – san lucas mountain range; g.f.b. – girardot fold belt; c.c. – central cordillera. limestone and lutite from la luna, simití and tablezo formations are the main source rocks; the reservoir rocks present in the basin are from the lisama, esmeralda, la paz, colorado and mugrosa formations (figure 1-32). figure 1-32. schematic section of the middle magdalena valley basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 22 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.2.17 upper magdalena valley basin the basin is bounded to the north by the girardot fold belt; to the southeast it is partially bounded by the algeciras-garzón fault system, to the northeast by the bituima-salina fault system and to the west by pre-cretaceous rocks from the central cordillera (figure 2-33). figure 2-33. location and borders of the upper magdalena valley basin. 21 – upper magdalena valley basin; b.s.f.s. – bitumina and la salina fault system; a.g.f.s. – algeciras-garzón fault system; g.f.b. – girardot fold belt; c.c. – central cordillera. the basin has been actively explored for hydrocarbons during the last two decades. however, it is thought that important petroleum reserves still lie trapped in stratigraphic plays. the basin‟s generating rocks are lutite and limestone from the tetuán, bambucá and la luna formations; reservoir rock is present in the caballos, monserrate and honda formations and the rock seal is represented by arcillolite from the bambucá formation (figure 2-34). figure 2-34. schematic section of the upper magdalena valley basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 23 2.2.18 sinusan jacinto basin the sinu-san jacinto basin is located in the north of colombia and is the most prolific area in terms of oil seeps (ooze) and gas throughout colombia. this basin is bounded to the east by the romeral fault system, to the north-northwest by the caribbean coast, to the west by the uramita fault system and to the south by cretaceous sedimentary and volcanic rocks from the western cordillera (figure 2-35). the basin‟s structural development is linked to the transpressional deformation produced by the caribbean plate‟s displacement. figure 1-36 gives a schematic model of the basin. figure 2-35. location and borders of the sinu san jacinto basin. 17 – sinu san jacinto basin; u.f.s. – uramita fault system; w.c. – western cordillera: r.f.s. – romeral fault system. figure 2-36. schematic section of the sinu-san jacinto basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 24 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.2.19 sinú offshore basin this basin lies entirely below the waters of the caribbean sea and is bounded to the northeast by the oca fault, to the southeast by the coastline, to the northwest by the front of the south caribbean deformed belt and to the southeast by the uramita fault system (figure 2-37). figure 2-38 gives a schematic model of the basin. figure 2-37. location and borders of the sinú offshore basin. 18 – sinú offshore basin; o.f.– oca fault; u.f.s. – uramita fault system; s.c.d.b. – south caribbean deformed belt. figure 2-38. schematic section of the sinú offshore basin. 2.2.20 tumaco basin the tumaco basin is located in the south-western region of colombia. the basin is bounded to the north by the garrapatas fault system; it extends to the south down to the border between ecuador hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 25 and colombia, to the east as far as the western cordillera‟s cretaceous rocks and to the west as far as the pacific ocean coastline (figure 2-39). figure 2-40 gives a schematic model of the basin. figure 2-39. location and borders of the tumaco basin. 19 – tumaco basin; g.f.z. – garrapata fault zone; w.c. – western cordillera. figure 2-40. schematic section of the tumaco basin. 2.2.21 tumaco offshore basin the tumaco offshore basin is located in the marine region in the southeast of colombia, below the waters of the pacific ocean. this basin is bounded to the north by the garrapatas fault system, to the south by the frontier with ecuador, to the east by the coastline and to the west by the colombian pacific subduction area trench (figure 2-41). figure 2-42 gives a schematic model of the basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 26 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-41. location and borders of the tumaco offshore basin. 20 – tumaco offshore basin; g.f.z. – garrapata fault zone; c.p.s.z. – colombian pacific subduction zone. figure 2-42. schematic section of the tumaco offshore basin. 2.2.22 urabá basin the urubá basin is bounded to the north-northwest by the border between colombia and panamá, to the southeast by the mandé batholith and the murindó fault, to the east by the uramita fault system, to the west by the darien mountain range and to the south by cretaceous rocks from the western cordillera (figure 2-43). figure 2-44 gives a schematic model of the basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 27 figure 2-43. location and borders of the urabá basin. 22 urabá basin; u.f.s. – uramita fault system; m.b. – mandé batholith; m.f. – murindó fault; s.d. – darien mountain range; w.c. – western cordillera. figure 2-44. schematic section of the urabá basin. 2.2.23 vaupés – amazonas basin the vaupés-amazonas basin is bounded to the north by the vaupés arch, to the south-southeast by the frontiers with brazil and peru, to the west by the chiribiquete mountain range and to the east by the trampa-carurú mountain range (figure 2-45). figure 2-46 gives a schematic model of the basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 28 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-45. location and borders of the vaupés basin, amazonas. 23 – vaupés – amazonas basin; v.a. – vaupés arch; s.c. – chiribiquete mountain range; t.c. – la trampa – carurú mountain range. figure 2-46. schematic section of the vaupés – amazonas basin. 2.3 environmental considerations colombia has 56 natural areas forming the national parks‟ system, occupying 9.98% of colombia‟s terrestrial territory and 1.30% of its marine territory, giving a total area of 12,602,321 hectares (11,390,995 terrestrial hectares and 1,211,326 marine hectares). the 56 natural areas were taken into account in the present study when making estimates so as not to count them when calculating the different resources. table 2-1 shows the percentages for national hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 29 park and conservation areas for each sedimentary basin in colombia. figure 2-47 gives the areas related to the colombian park system. basin natural reserve area (km2) percentage of basin (%) amagá 0.0 0.0 caguán putumayo 8,550.0 7.8 guajira offshore 75.1 0.1 sinú offshore 1,579.4 5.3 catatumbo 444.7 5.8 cauca-patía 4.8 0.0 los cayos 10.0 0.0 cesar-ranchería 0.0 0.0 chocó 811.3 2.1 colombia 0.0 0.0 colombian deep pacific 9,571.8 3.6 the eastern cordillera 7,807.3 10.9 the guajira 317.8 2.3 the eastern llanos 5,189.1 2.3 chocó offshore 592.9 1.6 tumaco offshore 1,080.6 3.1 sinú san jacinto 4,071.1 10.3 tumaco 588.9 2.5 urabá 289.4 3.1 lower magdalena valley 0.0 0.0 middle magdalena valley 0.0 0.0 upper magdalena valley 1,540.3 7.2 vaupés amazonas 33,486.0 21.6 non-prospective areas 50,220.1 13.3 table 2-1. percentage of area associated with the colombian park system regarding total basin area. figure 2-47. distribution of conservation areas in colombia. the red hatching indicates the coverage of such conservation areas. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 30 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.4 data and hypotheses the resources generated and accumulated were estimated for the 12 sedimentary basins in colombia, taking into account the information necessary for applying the procedure proposed by schmoker (1994) and hunt‟s proposed considerations (1995), i.e. catatumbo, cesar ranchería, the eastern cordillera, the eastern llanos, san jacinto in sinú, sinú offshore, tumaco, the lower magdalena valley, the middle magdalena valley, the upper magdalena valley, caguán in putumayo and the guajira offshore basins. 2.4.1 data some of the data used was taken from the colombian geochemical atlas (anh, 2010), and forms part of a compilation analysis, showing wells and outcrops, obtained during regional recognition (appendix 2-1). other data were consulted from the universidad nacional de colombia‟s bibliographic sources dealing with geological field studies for multiple basins and different generated units (appendix 2-2). 2.4.2 hypotheses hydrocarbons generated and trapped in colombia‟s sedimentary basins were estimated in line with the following hypotheses: 2.4.2.1 hypothesis 1 following the mass balance method (schmoker, 1994), the amount of hydrocarbons generated in colombia is fixed according to the rock‟s total organic carbon mass and original and actual hydrogen indexes. 2.4.2.2 hypothesis 2 the geological risk factors proposed by hunt (1995) lead to estimating the volume of hydrocarbon generated (i.e. which was preserved and trapped). 2.4.2.3 hypothesis 3 the ratio between hydrocarbon generated per basin and basin area leads to estimating the resource being produced in areas lacking information. 2.4.3 methodology the following equation was used for estimating the hydrocarbon generated in colombia: [ ] ( ) (2-1) : volume de hydrocarbon generated (mboe) : formation area (km2) : formation thickness (m) : average formation density (g/cm3) hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 31 : percentage total organic carbon present in rock (% gtoc / groca) : original hydrogen index before hydrocarbon generation (mghc/gtoc) : current hydrogen index (mghc/gtoc) : average hydrocarbon density (g/cm3). the foregoing equation was obtained by substituting equations extracted from the scheme shown in figure 2-2 into equation 1 and then transforming the mass of hydrocarbons generated to volume: (2-2) with, [ ] (2-3) and (2-4) : total mass of hydrocarbon generated in each unit of source rock (kg) : mass of hydrocarbon per gram of toc (mghc/gtoc) : organic carbon mass (gtoc) : formation volume (cm3) applying equation 2-1 to each basin required a probabilistic method-based approach. monte carlo-based statistical distributions of probability were thus determined for representing thickness, toc fraction and difference in hydrogen indexes. area was the only parameter established as being constant in such calculations when using the monte carlo method, since this was determined in basins for which geological information was available. this approach considered the following stages:  the area of rocks formed by cooking was approximated from the extension of generating rocks, whether derived from geological cartography or well information (e.g. schmoker, 1994; uis, 2009);  a single distribution function was calculated from the thickness data obtained from the universidad nacional de colombia‟s bibliographic sources (appendix 2-1);  an average 2.4 g/cm3 rock density was taken;  toc and s2 data was taken from the geochemical atlas of colombia (anh, 2010);  statistical distributions were obtained for toc/100 fraction per basin;  initial and actual hydrogen indices were determined per basin;  distribution functions were fitted to r in equation 2-4; hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 32 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia  an average 0.9 g/cm3 hydrocarbon density was considered;  monte carlo simulations were run;  once hydrocarbon generated per basin had been calculated for basins having the necessary information for applying the monte carlo method, a ratio was established between estimated generated hydrocarbons and basin area, thereby obtaining a line of tendency leading to establishing a ratio between such variables, which was used for projecting these results for those basins lacking the necessary information;  the hydrocarbon generated for basins lacking the necessary data was estimated from their areas, using the ratio between hydrocarbon generated in a basin and its area (as obtained in the previous step); and  once total hydrocarbon generated value had been calculated, accumulated or trapped volume was evaluated considering hunt‟s observations (1995). 2.4.4 results most variables included in equation 2-1 were statistically analysed, leading to ascertaining their sensitivity. such analysis consists of goodness-of-fit tests and leads to identifying the distribution of probability best fitting the data. this gives a value (test statistic) reflecting the deviation between the data and a distribution having determined parameters. goodness-of-fit tests indicate whether it is valid to suppose that the information has a pattern corresponding to such distribution (null hypothesis). the goodness-of-fit tests also allow ascertaining the degree of certainty in such calculations (probability), i.e. whether there could be greater deviation. some of the test used gave intervals in which they were valid, according to supposed distribution pattern parameters (confidence intervals). different distributions were tried for random variables. pattern parameters were first studied with the data according to each distribution and then goodness-of-fit tests were made for identifying the least deviation. this report only shows the results for the best fits and estimates. 2.4.4.1 area of source rocks table 1-2 gives the values for the area being considered. these values represent total extensions of source rocks along several basins based on cartographic information or compilation on geophysical data (e.g. universidad industrial de santander, 2009). basin area (km2) catatumbo 7,715.0 cesar ranchería 11,668.7 the eastern cordillera 71,766.2 the eastern llanos 225,603.3 sinú offshore 29,576.5 sinú san jacinto 39,644.6 tumaco 23,732.4 lower magdalena valley 38,017.4 middle magdalena valley 32,949.4 upper magdalena valley 21,512.8 caguán putumayo 52,860.8 guajira offshore 110,304.1 table 2-2. area of the 12 basins having information for applying schmoker‟s methodology (1994). hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 33 2.4.5 thickness of source rock the thickness reported by the universidad nacional de colombia‟s bibliographic sources (appendix 2-1) for several basins was used for estimating distribution parameters for a set of functions. lognormal gave the best fit for the 178 elements of data. table 2-3 gives the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 5.74 5.58 5.91 1.12 1.01 1.25 was not rejected 0.13 2.25 1 table 2-3. goodness-of-fit parameters and results for the goodness-of-fit test applied to the thickness data for determining the distribution used in estimating the hydrocarbon generated. estimated parameters ̂ ̂ were lognormal distribution mean and standard deviation, respectively. column p gives the value of probability of accepting or rejecting the null hypothesis. g.l means the degrees of freedom. 2.4.6 mass of hydrocarbon generated per gram of toc actual hydrogen index values were determined for calculating r (hydrocarbon mass per gram of toc) distribution, taking toc and s2 data from the colombian geochemical atlas (atlas geoquímico orgánico de colombia, 2010) and using the equation proposed by barker (1974): (2-5) : hydrocarbons generated by kerogen distillation (mghc/groca). maximum value per basin was selected as initial hydrogen index once values had been calculated using equation 2-5. the distribution functions regarding best fit were then determined for the 12 basins (table 2-4). 2.4.7 toc fraction table 2-5 gives the results of the statistical analysis of toc fraction data for basins having information. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l cag-put extreme min 513.8 509.8 517.8 83.89 80.65 87.28 was not rejected m.b 295.2 0 cat extreme min 261.5 258.9 264.1 40.59 38.60 42.69 was not rejected m.b 79.41 0 ces-ran triangular 88.90 0 139.2 n/a n/a n/a n/a n/a n/a n/a cor extreme min 507.7 502.5 512.9 80.41 76.26 84.78 was not rejected m.b 188.79 6 gua-os triangular 135.8 0 208.0 n/a n/a n/a n/a n/a n/a n/a lla extreme min 402.2 398.1 406.4 68.9 65.7 72.3 was not rejected m.b 148.8 0 sin-os extreme min 150.3 146.9 153.6 22.39 19.77 25.36 was not rejected m.b 16.06 0 sin-sja extreme min 106.2 103.4 109.0 20.99 18.81 23.43 was not rejected m.b 39.10 0 tum extreme min 68.70 65.62 71.78 13.15 10.94 15.82 was not rejected 0.28 3.85 3 vim extreme min 211.9 209.5 214.3 31.07 29.14 33.14 was not rejected m.b 160.3 0 vmm extreme min 348.9 342.2 355.6 62.8 57.2 69.0 was not rejected m.b 221.0 0 vsm weibull 413.9 405.7 422.3 2.06 1.99 2.13 was not rejected m.b 341.0 0 table 2-4. goodness-of-fit distributions, parameters and test results applied to the mass of hydrocarbon generated per gram of toc data. cag-put, cat, cor, gua-os, lla, sin-os, sin-sja, tum, vim, vmm and vsm refer to the caguán putumayo, catatumbo, eastern cordillera, guajira offshore, eastern llanos, sinú offshore, sinú san jacinto, tumaco, lower magdalena valley, middle magdalena valley and upper magdalena valley basins. estimated parameters ̂ ̂ are extreme value distribution regarding location and scale, and to “a” and “b” for weibull distribution, respectively. estimated parameter ̂ for triangular distribution was the most probable value and its confidence interval gave minimum and maximum values as extremes. n/a refers to the parameters which did not apply for the type of distribution considered. m.b refers to very low probability of the null hypothesis being accepted or rejected. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 34 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l cag-put gamma 0.65 0.61 0.68 0.02 0.02 0.03 was not rejected m.b 21.26 1 cat lognormal -4.72 -4.78 -4.66 0.80 0.76 0.85 was not rejected m.b 44.0 0 ces-ran lognormal -4.81 -4.91 -4.69 0.78 0.71 0.87 was not rejected m.b 2.39 0 cor lognormal -4.63 -4.67 -4.59 0.66 0.63 0.69 was not rejected m.b 48.73 1 gua-os lognormal -4.69 -4.75 -4.65 0.53 0.50 0.56 was not rejected m.b 2.01 0 lla lognormal -5.02 -5.07 -4.97 0.98 0.94 1.01 was not rejected m.b 16.40 0 sin-os extreme max 0.02 0.01 0.02 0.03 0.03 0.03 was not rejected m.b 96.7 2 sin-sja lognormal -4.63 -4.70 -4.55 0.66 0.61 0.72 was not rejected m.b 0.04 0 tum lognormal -4.47 -4.61 -4.33 0.67 0.58 0.78 was not rejected m.b 4.25 0 vim lognormal -4.78 -4.82 -4.73 0.68 0.65 0.72 was not rejected m.b 12.02 0 vmm lognormal -4.21 -4.28 -4.12 0.74 0.69 0.79 was not rejected m.b 17.81 0 vsm lognormal -4.26 -4.31 -4.22 1.08 1.05 1.11 was not rejected m.b 144.9 0 table 2-5. goodness-of-fit parameters, test results and distributions applied to the toc fraction data. estimated parameters ̂ ̂ are gamma distribution regarding form and scale, and for weibull distribution regarding parameters “a” and “b”, respectively. 2.4.8 ratio between basin area and hydrocarbon generated the monte carlo method was used for estimating hydrocarbon generated once the different distribution functions had been determined (using a million iterations). the results led to correlating hydrocarbon generated values and the different basins‟ area (figure 2-48). figure 2-48. basin area compared to hydrocarbon generated for p10, p50 and p90. figure 2-48 shows the high coefficients of correlation for each percentile plotted, thereby sustaining the fact that hypothesis 3 (the greater the basin area, the greater the resources), based on statistical occurrences, could be applied to colombian basins and establish a ratio between a basin‟s area and the hydrocarbons generated in it. the ratios so obtained led to estimating hydrocarbon generated in p10, p50 and p90 for the basins lacking information. table 2-6 gives the areas of such basins. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 35 basin area (km2) amagá 2,824.9 cauca-patía 12,823.3 los cayos 144,755.0 chocó 38,582.0 colombia 256,995.3 deep pacific 272,426.6 guajira 13,778.9 chocó offshore 37,773.3 tumaco offshore 34,552.7 urabá 9,448.9 vaupés-amazonas 154,867.3 table 2-6. area of basins lacking information. 2.4.9 volume of hydrocarbon generated table 2-7 gives the results (in p10, p50 and p90) for hydrocarbon generated for each of colombia‟s 23 sedimentary basins. basin p10 p50 p90 total offshore (103mboe) 54,666 9,574 1,613 cayos 9,987 1,737 289 chocó offshore 2,565 446 74 colombia 17,733 3,084 514 guajira offshore 3,061 530 87 pacífico profundo 18,125 3,152 525 sinú offshore 885 224 55 tumaco offshore 2,309 402 67 total onshore (103mboe) 51,937 9,054 1,504 amagá 195 34 6 caguán-putumayo 5,836 666 31 catatumbo 388 72 14 cauca patía 884 154 26 cesar-ranchería 217 38 6 chocó 2,606 453 76 eastern cordillera 5,939 1,082 195 guajira 929 161 27 eastern llanos 16,377 2,980 536 sinú san jacinto 673 121 21 tumaco 286 51 9 urabá 632 110 18 lower magdalena valley 1,477 271 49 middle magdalena valley 4,061 897 174 upper magdalena valley 3,059 505 75 vaupés amazonas 8,377 1,457 243 total generated (103mboe) 106,603 18,628 3,116 table 2-7. hydrocarbon generated in colombia. figures 2-49, 2-50 and 2-51 give a graphic representation of values shown in table 2-7. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 36 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 2-49. hydrocarbon generated per colombian offshore basin. figure 2-50. hydrocarbon generated per colombian onshore basin. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 37 figure 1-51. hydrocarbon generated per basin. 2.4.10 trapped hydrocarbons once hydrocarbon generated for colombia had been ascertained, then total accumulated or trapped hydrocarbon was calculated using hunt‟s observation (1995). accordingly, only ~ 2.2% of hydrocarbon generated is accumulated (crude accounts for ~ 1.8 % of this and the remaining ~ 0.4 % is gas). tables 2-8, 2-9 and 2-10 show the values estimated for the resource for colombia, discriminating between oil and gas. a 5,800 ft3/boe factor was used for converting barrel of oil equivalent to cubic feet. p10 2,345,000 mboe p50 409,000 mboe p90 68,000 mboe table 2-8. total accumulated hydrocarbon. p10 1,918,000 mmbbl p50 335,000 mmbbl p90 56,000 mmbbl table 2-9. total accumulated crude. p10 2,477 tcf p50 429 tcf p90 70 tcf table 2-10. total accumulated gas; a 5,800 cf/bbl conversion factor has been used. hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 38 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.4.11 sensitivity analysis a sensitivity analysis was carried out for establishing the degree of random variables‟ influence on the results of equation 2-1. this led to visualising the weighting of on the result when generating numbers according to the statistical distribution of a particular variable. table 2-11 shows such sensitivity. figure 2-52 shows average sensitivity. basin thickness (%) r (%) toc/100 (%) caguán putumayo 25.9 1.5 72.6 catatumbo 59.5 0.0 40.5 cesar ranchería 59.4 0.0 40.6 the eastern cordillera 57.7 3.1 39.2 guajira offshore 53.9 9.8 36.4 the eastern llanos 57.4 3.6 39.0 sinú offshore 88.0 3.9 8.1 sinú san jacinto 56.5 5.2 38.3 tumaco 56.7 4.8 38.5 lower magdalena valley 57.9 2.6 39.5 middle magdalena valley 72.2 12.5 15.3 upper magdalena valley 49.9 16.1 33.9 average 57.9 5.3 36.8 table 2-11. sensitivity analysis regarding calculations made for hydrocarbon generated per basin. figure 2-52. average sensitivity regarding estimates of hydrocarbon generated in colombia. 2.5 conclusions  the hydrocarbons generated for colombia‟s 23 basins was estimated, thereby obtaining resources for p10 and p90 i the 2,345,000 and 68,000 mboe range;  a range of 1,918,000 to 56,000 mmbbl was obtained in p10 and p90 for oil and the remaining 2,477 to 70 tcf for gas;  sensitivity analysis showed that thickness and the toc/100 fraction were the variables having the greatest degree of uncertainty in the calculations;  a series of data was used in the calculations which should be improved and complemented in the future (i.e. thickness values) through a detailed study leading to reducing its range of variability; and hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 39  regarding the percentage proposed by hunt (1995), this could vary from basin to basin since each basin has a different history of generation, migration and loss. however, for the purpose of this research and considering the amount of data and information available, this represented a good approach. 2.6 bibliography agencia nacional de hydrocarbons anh (2007). colombian sedimentary basins: nomenclature, boundaries and petroleum geology, a new proposal. bogota: b & m exploration ltda. agencia nacional de hydrocarbons. (2010). atlas geoquímico de colombia. (u. n. colombia, ed.) bogota. ahlbrandt , t. s. (2000). the usgs world oil and gas assessment. search and discovery (10006). barker, c. (1974). pyrolysis techniques for source rock evaluation. the american association of petroleum geologists bulletin, 58(11), 2349-2361. d. little, a. (2008). evaluation of potential of unconventional energy resources in colombia. agencia nacional de hydrocarbons. houston: arthur d. little inc. griess, p. r. (1946). colombia‟s petroleum resources. economic geography, 22(4), 245-254. hunt, j. m. (1995). petroleum geochemistry and geology (second edition). w.h. freeman. ortiz, j. (1997). oportunidades de desarrollo del sector hidrocarburos y su aporte a la economía del país. cambio y globalización: oportunidades y retos para la industria colombiana de los hidrocarburos. pontificia universidad javeriana. schmoker, j. m. (1994). volumetric calculations of hydrocarbons generated. the petroleum system from source to trap. american association of petroleum geologist memoirs, 60, 323-326. universidad industrial de santander. (2009). evaluación del potencial hidrocarburo de las cuencas colombianas. anh-fonade. vargas, c. a. (2009). nuevos aportes a la estimacion del potencial de hidrocarburos en colombia. revista de la academia colombiana de ciencias, xxxiii (126). 2.7 appendices 2.7.1 appendix 2-1 digital file in the anh's document center:  “balance de masas.xlsx” hydrocarbons produced in colombian sedimentary basins earth sciences research journal, volume 16, special issue, april, 2012 40 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 2.7.2 appendix 2-2 author thesis title year cardozo p., e. estratigrafía, ambiente y paleogeografía de la fm. rosablanca en el área de moniquirá-villa de leiva 1982 moreno v., e. characteristics estratigráficas de la formación villeta en el sector nw de la cuenca del putumayo 1989 gil v., a. análisis microfacial del grupo cogollo and formación de la luna cuenca cesar cesar ranchería and guajira, colombia 1990 sánchez q., c. petrografía e interpretación environmental de la formación tibú, grupo uribante (aptiano) en el campo río de oro, cuenca del catatumbo, con base en núcleos de perforación 1991 mayorga m., m. caracterización geoquímica y facial de las rocas potencialmente generadoras de hidrocarburos en las formaciones del cretácico y terciario inferior de la cordillera oriental 1995 mora b., j. estudio estratigraph del cretácico y terciario inferior en el extreme norte de la cuenca del putumayo, alrededores de belén de los andaquies y morella, caquetá 1998 mera p., r. correlación estratigráfica de las rocas del intervalo paleoceno-oligoceno subcuenca de san juan, chocó 2000 caycedo g., h. propiedades de escalamiento para las poblaciones de fallas de formación caballos en el sector sur del campo orito: cuenca putumayo, colombia: moldeamiento de las sistemas de fallas por debajo del límite de la resolución seismic 2001 malagón r., f. evaluación del potencial generador de hidrocarburos de las formaciones: rosablanca, paja y tablezo, cuenca medio del magdalena, colombia 2001 ramírez-j., r. sedimentología de la arenisca del oso y su significado estratigraph, en el cinturón de san jacinto norte, valle magdalena inferior 2002 gamba r., n. estratigrafía física, petrográfica y análisis del ambiente de depósito de la formación tetuán en el anticlinal de chicuambe y la quebrada bambucá, valle magdalena superior 2002 mayorga n., e. evaluación del potencial hidrocarburífero de la formación chipaque (unidad operacional k1 medio and superior) en el bosque apiay-ariari, cuenca de los llanos orientales 2005 guzmán a., e. evaluación petrofísica de la formación tetuan en el campo balcón, valle magdalena superior 2007 table 2-12. sources consulted for thickness data. universidad nacional de colombia theses. earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 3 recoverable original resources 3.1 general comments this section deals with estimating original in situ oil and gas initially in place for oil, conventional gas and associated gas from a volumetric approach. the calculated results were affected by geological and recovery risk factors for final estimate of recoverable resources by type of fluid. 3.1.1 volumetric estimation volumetric estimation is a tool which allows evaluating in situ hydrocarbon in restricted conditions regarding information about production and deposits. recoverable quantified resources affecting estimation have a recovery factor obtained from analogous deposits‟ yield and/or simulation studies (dean, 2007). the necessary data for making a volumetric estimation would be: deposit areas, deposit thickness, porosity, water saturation, volumetric factors, gas–oil ratio (gor) and accumulated production per field. 3.2 data and hypotheses 3.2.1 data information reported to the agencia nacional de hidrocarburos in reserves and resources‟ reports, technical reports and exploration and production reports by december 31st 2010 led to evaluating original resource in situ for a limited number of sedimentary basins in colombia: 5 for oil and associated gas and 7 for gas. the methodology proposed by vargas (2009) was applied for deducing certain variables in basins where the information supplied by anh was insufficient. 3.2.2 hypothesis original resources in situ were estimated in line with the following hypothesis: 3.2.2.1 hypothesis 1 there is a relationship between production area / mmbbl produced ratio and the production area / basin area ratio leading to inferring the maximum production area possible in sedimentary basins (figure 3-1). 3.3 methodology the following equations were used for estimating in situ resources in colombia: ( ) (3-1) recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 42 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia ( ) (3-2) : original in situ oil (bbl) : initial in situ gas (bbl) : area of production (acres) : total deposit thickness (ft) : deposit porosity (v/v) : water saturation (v/v) : oil volumetric factor (bbly / bbln) : gas volumetric factor (ft3y/ft3n) constants 7758 and 43560 were used for obtaining results in barrels (bbl and cubic feet cf, respectively). the monte carlo probabilistic method was used for the estimates in equations 3-1 and 3-2. the random variables were gas and oil thickness, porosity, water saturation and volumetric factor. even though the volumetric factor could be marginal when estimating resources, its variability was taken into account due to the broad range of observations. production area was taken as being constant. such estimate included the following steps.  data regarding formations, units, areas, thickness, porosity, water saturation, api gravity, gas-oil ratio (gor), volumetric factors and accumulated production per field in each basin was compiled, discriminating the type of fluid produced (gas or oil);  statistical analysis determined the distributions best fitting information regarding thickness, porosity, water saturation, volumetric factor and gor: o regarding oil, functions were obtained for each variable in five basins; caguán putumayo, catatumbo, the eastern llanos, middle magdalena valley and upper magdalena valley (figure 3-1); o regarding initial in situ gas, the amount of data was insufficient, meaning that only a single distribution was found per variable, incorporating all basins; o for the remaining basins lacking information about oil and gas, a common distribution function was associated for each variable (oil and gas), and constructed from information regarding all basins;  areas in production were added to the accumulated production of the different fields per basin, discriminating the type of fluid;  vargas‟ methodology (2009) was applied for basins lacking production areas: o the production area / basin area and production area / mmbbl ratios were calculated from total oil production, producing areas and each basin‟s areas; recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 43 o production area / basin area and production area / mmbbl produced pairs were projected on a log-log graph‟s abscissa and ordinate; o the equation for the curve best fitting the points was identified; o the function so obtained was projected until cutting the axis of the abscissa at a defined value for the “1-8 ha/mmbbl” ordinate. this value gives an upper “reasonable” limit which has been detected in the production series; o assuming basin areas having efficiency of up to 1-8 ha for each mmbbl, then the maximum fraction of a basin could be inferred which could achieve such production efficiency conditions; o such fraction affected the area for the rest of the basins lacking information regarding oil and gas production areas;  the production areas estimated per basin and type of fluid, thickness, porosity, water saturation, gor and volumetric factor distributions were used for running monte carlo simulations for the estimating original in situ oil and initial in situ gas; and  the results so obtained were affected by geological risk factors, recovery (oil or gas) and fraction of area associated with natural reserves. 3.4 results 3.4.1 production area table 3-1 gives production areas and the production area / accumulated production and production area / basin area ratios for the five basins having information used in estimating the original oil in place (ooip). basin production área (ha) production area / mboe (ha/mmbbl) production area / basin area (%) caguán putumayo 29,834.06 94.25 0.27 catatumbo 18,359.82 41.94 2.38 the eastern llanos 102,248.95 36.26 0.45 the middle magdalena valley 46,695.06 35.09 1.42 the upper magdalena valley 15,539.54 23.57 0.72 table 3-1. production areas per basin and relationships for applying vargas‟ approach (2009) used in ooip calculations. figure 3-1 shows the semi-log regression values in table 3-1; regarding ooip, it was found that 6.97% was the fraction of the area of a basin which could be expected to produce 1 mmbbl per hectare based on these values. the foregoing is an optimistic scenario; however, a more conservative one giving an expectation of 1 mmbbl in 8 hectares would mean that the percentage of the area in production of such basin would fall to 3.44%. the lack of information for determining production areas for initial gas in place (igip) prevented calculating the percentage of the area in production; that calculated for ooip was thus taken instead. recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 44 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 3-1. estimated percentage area in production for estimating ooip. the point corresponding to the catatumbo basin was not considered for this fit as it went outside the tendency given for the remaining basins, since it has high production (mmbbl) for the extension of area (ha) found in production regarding the rest of the basins. the red arrow indicates the percentage of the basin area which would produce in line with an expectation of 1 mmbbl for each 8 ha; 3.44% (a more conservative scenario). table 3-2 gives the areas calculated in the different basins by applying the percentages so obtained. basin area (ha) offshore los cayos 1,010,422.06 chocó offshore 263,665.98 colombia 1,793,883.99 guajira offshore 368,980.45 deep pacific 1,901,598.01 sinú offshore 206,450.36 tumaco offshore 241,185.41 onshore amagá 19,718.64 cauca-patía 89,509.52 cesar-ranchería 81,449.94 chocó 269,310.79 eastern cordillera 500,943.98 guajira 96,179.91 sinú-san jacinto 276,728.02 tumaco 165,657.55 urabá 65,955.77 table 3-2. production areas estimated using vargas‟ approach (2009). 3.4.2 deposit thickness tables 3-3 and 3-4 show the best fit distributions of thickness data used in estimating in situ resource for oil and gas. 3.4.3 deposit porosity tables 3-5 and 3-6 gives the parameters obtained for porosity distribution. 3.4.4 water saturation tables 3-7 and 3-8 show the distributions obtained regarding water saturation for in situ resources. y = 62.741e-0.594x 1.00 10.00 100.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 p ro d u ct io n a re a / m m b o e production area / basin area recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 45 3.4.5 volumetric factors tables 3-9 and 3-10 give the statistical parameters calculated for the volumetric factors for oil and gas. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistic al value g.l cag-put weibull 34.49 29.79 39.96 1.23 1.09 1.40 was not rejected 0.30 3.70 3 cat lognormal 3.81 3.48 4.15 0.86 0.68 1.18 was not rejected m.b 1.17 0 lla lognormal 3.17 2.88 3.46 1.23 1.05 1.47 was not rejected m.b 9.31 0 vmm lognormal 4.11 3.86 4.36 0.89 0.75 1.11 was not rejected 0.38 1.94 2 vsm lognormal 4.38 4.11 4.65 0.86 0.70 1.09 was not rejected 0.48 0.49 1 remaining basins triangular 30 0 825 n/a n/a n/a n/r n/r n/r n/r table 3-3. goodness-of-fit test distributions, fit parameters and results applied to the thickness data used in the ooip estimate. cag-put, cat, lla, vmm and vsm refer to the caguán putumayo, catatumbo, the eastern llanos, middle magdalena valley and upper magdalena valley basins. estimated parameters ̂ ̂ are weibull distribution parameters “a” and “b”, and average and standard deviation for lognormal distribution, respectively. for triangular distribution, estimated parameter ̂ is the most probable value and its confidence interval gives minimum and maximum values as extremes. n/a refers to parameters which did not apply to the type of distribution being considered. n/r refers to parameters which, as they went beyond the algorithm‟s range of calculations, were not reported when making the statistical estimation. m.b refers to very low probability of the null hypothesis being accepted or rejected. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 3.77 3.49 4.04 1.25 1.08 1.47 was not rejected 0.13 4.14 2 table 3-4. lognormal distribution fit parameters and goodness-of-fit test results applied to the thickness data used in estimating igip. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistic al value g.l cag-put extreme max 0.16 0.15 0.17 0.05 0.04 0.05 was not rejected m.b 116.28 0 cat beta 3.94 2.04 7.59 49.93 24.98 99.78 was not rejected m.b 0.11 0 lla extreme min 0.25 0.24 0.26 0.04 0.04 0.05 was not rejected 0.19 6.11 4 vmm beta 12.60 9.91 16.03 50.62 37.75 67.87 was not rejected 0.22 4.46 3 vsm lognormal -1.87 -1.94 -1.81 0.22 0.18 0.28 was not rejected 0.27 3.97 3 remaining basins normal 0.16 0.15 0.17 0.06 0.05 0.07 was not rejected m.b. 48.21 0 table 3-5. goodness-of-fit test distributions, fit parameters and results applied to porosity data used in estimating ooip. estimated parameters ̂ ̂ are extreme value distribution regarding location and scale, for the beta distribution to parameters “a” and “b” and for normal distribution to the average and standard deviation, respectively. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 2.64 1.69 4.09 15.50 9.02 26.66 was not rejected m.b 16.19 0 table 3-6. beta distribution fit parameters and goodness-of-fit test results applied to porosity data used in estimating igip. recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 46 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistic al value g.l cag-put lognormal -1.41 -1.47 -1.34 0.39 0.35 0.45 was not rejected m.b 24.57 0 cat weibull 0.35 0.33 0.37 6.51 4.95 8.57 was not rejected m.b 0.05 0 lla lognormal -1.36 -1.48 -1.24 0.51 0.44 0.61 was not rejected 0.36 3.21 3 vmm logistical 0.37 0.34 0.40 0.06 0.05 0.08 n/r n/r n/r n/r vsm beta 4.68 3.14 6.97 10.13 6.82 15.04 was not rejected 0.35 3.30 3 remaining basins uniform 0.05 n/a n/a 0.95 n/a n/a n/r n/r n/r n/r table 3-7. goodness-of-fit test distributions, fit parameters and results applied to water saturation data used in estimating ooip. estimated parameters ̂ ̂ logistical distribution regarding location and scale, and maximum and minimum for uniform distribution, respectively. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 3.79 n/r n/r 0.08 n/r n/r was not rejected 0.02 12.46 0 table 3-8. gamma distribution fit parameters and goodness-of-fit test results applied to water saturation data used in estimating igip. estimated parameters ̂ ̂ are gamma distribution regarding form and scale. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistic al value g.l cag-put lognormal 0.13 0.11 0.14 0.08 0.07 0.09 was not rejected m.b 11.01 0 cat uniform 1 n/a n/a 1.87 n/a n/a n/r n/r n/r n/r lla lognormal 0.09 0.06 0.12 0.13 0.11 0.15 was not rejected m.b 24.81 0 vmm pareto -1.37 n/r n/r 2.06 n/r n/r was not rejected m.b 16.92 0 vsm lognormal 0.13 0.09 0.16 0.11 0.09 0.14 was not rejected m.b 12.55 1 remaining basins uniform 1.06 n/a n/a 1.51 n/a n/a n/r n/r n/r n/r table 3-9. goodness-of-fit test distributions, fit parameters and results applied to the oil volumetric factor data used in estimating ooip. estimated parameters ̂ ̂ are form and scale parameters for pareto distribution. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 1.72 1.20 2.23 0.01 0.01 0.01 was not rejected m.b 0.84 0 table 3-10. pareto distribution fit parameters and goodness-of-fit test results applied to gas volumetric factor data used in estimating igip. 3.4.6 gas oil ratio (gor) gor data compiled from the five basins having information for estimate ooip led to determining distribution functions. table 2-11 gives the results for the best fits. due to the gor‟s high variability and, consequently, the pattern of deposits in colombian basins, it has been assumed that such function represents probable performance in quasi-stationary flow conditions. lacking more information regarding typical gor history concerning each deposit and each basin, such supposition represents a hypothetical situation for approximating associated gas order of magnitude. recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 47 basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistic al value g.l cag-put lognormal 4.39 3.41 5.38 2.27 1.76 3.21 was not rejected m.b 0.01 0 cat t. student 1197 845.5 1394 533.4 397.2 811.7 was not rejected m.b 0.05 0 lla lognormal 4.58 4.16 4.99 2.15 1.89 2.49 was not rejected m.b 0.57 0 vmm lognormal 5.14 4.81 5.47 1.68 1.48 1.95 was not rejected 0.03 4.65 1 vsm lognormal 5.38 5.08 5.68 1.02 0.84 1.28 was not rejected 0.08 3.05 1 remaining basins gamma 0.40 0.40 0.50 1.34 1.08 1.65 was not rejected m.b 11.82 0 table 3-11. goodness-of-fit test distributions, fit parameters and results applied to gor data used in estimating associated gas. 3.4.7 recoverable resources after calculating the different production areas in each basin and taking the distribution of variables contained in equations 1-1 and 1-2 into account, in situ resources for oil, associated gas and original gas were estimated; 500,000 iterations were made for original resources and another 500,000 for estimating associated gas. tables 3-12 to 3-14 give the results obtained, already affected by aerial factors related to areas having high environmental sensitivity (hereinafter environmental factor, table 2-1), giving 30% geological risk factor and 20% recovery factor. figures 3-2 to 3-8 also give the results obtained. basin p10 p50 p90 (mmbbl) total offshore 276,413 75,815 12,570 los cayos 43,050 11,774 1,950 chocó offshore 12,589 3,453 575 colombia 90,992 24,923 4,138 guajira offshore 18,721 5,131 855 deep pacífic 92,961 25,566 4,224 sinú offshore 8,182 2,248 377 tumaco offshore 9,918 2,720 451 total onshore 153,952 42,148 7,436 amagá 804 233 75 caguán putumayo 419 137 34 catatumbo 213 59 17 cauca-patía 4,553 1,247 208 cesar ranchería 4,137 1,135 189 chocó 13,444 3,682 607 the eastern cordillera 22,653 6,221 1,030 the guajira 4,777 1,307 218 the eastern llanos 3,250 892 148 sinú san jacinto 13,469 3,697 614 tumaco 4,486 1,651 611 urabá 4,413 710 159 the lower magdalena valley 13,177 3,609 602 the middle magdalena valley 11,885 3,252 539 the upper magdalena valley 999 274 45 vaupés amazonas 51,273 14,042 2,340 total 430,365 117,963 20,006 table 3-12. estimated recoverable oil recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 48 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin p10 p50 p90 (tcf) total offshore 83.111 6.076 0.166 los cayos 0.183 0.02 0.002 chocó offshore 53.389 3.756 0.065 colombia 1.604 0.305 0.061 guajira offshore 2.156 0.15 0.002 colombian deep pacific 0.302 0.059 0.012 sinú offshore 5.136 0.365 0.000 tumaco offshore 20.341 1.421 0.024 total onshore 177.813 12.415 0.242 amagá 2.842 0.203 0.004 choco 8.607 0.609 0.020 caguán putumayo 0.570 0.041 0.002 catatumbo 7.755 0.548 0.010 cauca-patía 58.586 4.107 0.071 cesar -ranchería 7.937 0.558 0.010 the eastern cordillera 35.281 2.477 0.043 the guajira 15.408 1.076 0.018 the eastern llanos 7.105 0.491 0.008 sinú san jacinto 11.287 0.79 0.014 tumaco 3.130 0.219 0.004 urabá 2.558 0.179 0.004 the lower magdalena valley 6.658 0.467 0.008 the middle magdalena valley 2.253 0.041 0.002 the upper magdalena valley 7.531 0.528 0.008 vaupés amazonas 0.305 0.081 0.016 total 260.924 18.5491 0.409 table 3-13. estimated associated gas. basin p90 p50 p10 (tcf) total offshore 122.64 14.56 1.77 los cayos 21.58 2.56 0.31 chocó offshore 5.52 0.65 0.08 colombia 38.31 4.55 0.55 guajira offshore 7.82 0.94 0.11 colombian deep pacific 39.12 4.64 0.57 sinú offshore 5.3 0.63 0.08 tumaco offshore 4.99 0.59 0.07 total onshore 111.54 13.21 1.58 amagá 0.43 0.06 0.01 caguán putumayo 15.25 1.81 0.22 catatumbo 1.08 0.13 0.02 cauca-patía 1.91 0.23 0.03 cesar-ranchería 1.74 0.21 0.02 the chocó 5.6 0.67 0.08 the eastern cordillera 9.6 1.14 0.14 the guajira 2.03 0.24 0.02 the eastern llanos 33.05 3.9 0.47 sinú san jacinto 4.2 0.49 0.06 tumaco 3.45 0.41 0.04 urabá 1.38 0.16 0.02 the lower magdalena valley 5.66 0.67 0.08 the middle magdalena valley 4.93 0.59 0.07 the upper magdalena valley 2.96 0.35 0.04 vaupés-amazonas 18.27 2.15 0.26 total 234.18 27.77 3.35 table 3-14. estimated initial recoverable gas. recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 49 figure 3-2. recoverable oil (mmbbl) per basin (offshore). figure 3-3. recoverable oil (mmbbl) per basin (onshore). recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 50 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 3-4. associated gas (tcf) per basin (offshore). figure 3-5. associated gas (tcf) per basin (onshore). recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 51 figure 3-6. original gas (tcf) per basin (offshore). figure 3-7. original gas (tcf) per basin (onshore). recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 52 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 3-8. recoverable crude per basin. figure 3-9. associated gas per basin. recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 53 figure 3-10. initial recoverable gas per basin. 3.4.8 sensitivity analysis table 3-15 gives sensitivity, averaged per basin for ooip and igip, visualising each variables‟ contribution to uncertainty (i.e. thickness, porosity, water saturation and volumetric factor) in estimating the different basins‟ resources. basin thickness porosity sw bo sensitivity (%) amagá 92.4 5.2 1.8 0.6 caguán putumayo 95.1 2.9 1.5 0.6 catatumbo 73.3 22.9 0.7 3.2 cauca-patía 95.3 2.7 2.0 0.1 los cayos 94.0 4.1 1.8 0.2 cesar-ranchería 92.8 4.2 2.2 0.7 chocó 95.1 1.7 2.7 0.5 chocó offshore 94.4 3.8 1.2 0.5 colombia 92.3 3.1 4.0 0.7 the eastern cordillera 92.7 4.3 2.0 1.0 guajira 91.4 6.4 2.0 0.2 guajira offshore 93.0 4.0 2.8 0.1 the eastern llanos 94.1 3.3 2.3 0.4 colombian deep pacific 91.3 5.5 1.9 1.3 sinú san jacinto 90.5 4.6 3.3 1.6 sinú offshore 88.9 6.2 4.4 0.5 tumaco 92.9 4.1 1.6 1.4 tumaco offshore 93.4 3.3 2.8 0.5 urabá 94.6 3.0 1.4 1.0 the lower magdalena valley 95.1 3.5 1.2 0.1 the middle magdalena valley 93.2 0.8 4.6 1.5 the upper magdalena valley 90.2 5.3 3.7 0.9 vaupés amazonas 93.6 4.7 1.5 0.3 average 92.2 4.8 2.3 0.8 table 3-15. sensitivity of the variables involved in calculating in situ resources recoverable original resources earth sciences research journal, volume 16, special issue, april, 2012 54 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia revising the values for the variables regarding each basin led to determining that the overall pattern was similar; figure 2-9 gives such pattern. figure 3-11. general sensitivity analysis. 3.5 conclusions o estimates for recoverable oil varied from p10 to p90, ranging from 430,361.28 to 20,000.45 mmbbl; o the range of p10 to p90 for associated gas was 260.924 to 0.410 tcf; o estimating initial recoverable gas for percentiles p10 and p90 was 234.15 and 3.36 tcf, respectively. o sensitivity analysis determined that thickness had the greatest influence on the calculations; and o ina a more conservative scenario, the percentage of a basin‟s area which could produce an expected 1mmbbl for each 8 ha would be 3.44%. this would modify estimated recoverable resources, calculated for an expected 1mmbbl/ha having 6.97%, reducing such figure by half. 3.6 bibliography dean, l. (2007). reservoir engineering for geologists. part 3 – volumetric estimation. 11,. reservoir issue, 11, 20 23. vargas, c. a. (2009). nuevos aportes a la estimacion del potencial de hidrocarburos en colombia. revista de la academia colombiana de ciencias, xxxiii (126). 3.7 appendices digital files in the anh's document center:  “areas y produccion de aceite.xlsx”  “recursos in situ.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 4 yet-to-find resources 4.1 general comments the present section evaluates yet-to-find (ytf) hydrocarbons for colombian basins currently having sufficient data regarding reserves per field. the fractal model was used for determining ytf resources from data regarding reserves at a determined date, thereby leading to projecting ytf hydrocarbons in actual technological conditions and prevailing political and social-environmental restrictions. 4.1.1 fractal method the fractal method assumes that oil field reserves have an idealised distribution which can be described by means of a parabolic or lineal function in log-log representation. as in other areas of the natural sciences, such function is determined by analysing tendency, incorporating field size and range, seeking to group fields having greater than a determined size (mmbbl). the function having the best fit with the data regarding field size and reserves suggests the total ytf subsoil resources in actual technological conditions, taking political and socio-environmental restrictions into account. yetto-find hydrocarbons can thus be estimated once the distribution of the reserves has been ascertained; this corresponds to the rest of the total of reserves discovered to date from the estimate of total subsoil resources defined by the best fit function. 4.2 data and hypotheses 4.2.1 data information from four basins having the minimum production and reserve data required by the method was used for the fractal approach: i.e. the eastern llanos, caguán putumayo, the middle magdalena valley and the upper magdalena valley. other basins having more limited data were incorporated for making an overall estimate for colombia. the information compiled from anh reserves and resources‟ reports containing data regarding accumulated production and proven reserves per field, as well as the number of fields per basin having been studied provided the data for writing this chapter (such data can be found on the electronic spread-sheets referred to in the appendixes). 4.2.2 hypothesis yet-to-find resources for the five basins considered in colombia were estimated in line with the following hypothesis: 4.2.2.1 hypothesis a basin‟s reserves have a fractal distribution which may fit a parabolic or lineal function. this allows using the fractal method for determining ytf resources in a particular basin from the difference between the curve of the observed data and its theoretical fit (linear or parabolic), assuming similar yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 56 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia exploration and production technology conditions and political and socio-environmental restrictions (figure 4-1). figure 4-1. fractal regression for a basin‟s ytf resources. the blue line gives the distribution, at any given moment, for fields having con a determined amount of reserves and accumulated production. the difference between the theoretical (green) and the observed curve gives the resources in actual technological conditions, regarding political and socio-environmental restrictions. 4.3 methodology the following steps were followed in determining yet-to-find hydrocarbon using the fractal method: o the figures for reserves were organised by ranges. the range assigned corresponded to the number of fields having production and reserves greater than or equal to a particular figure of interest. the example shown in figure 3-2 has only one field having production and reserves of around750 mmbbl, ten fields having production and reserves greater than 50 mmbbl and 20 fields having production and reserves greater than or equal to 135,000 bbl; o a log-log graph was drawn presenting the range or number of fields on the abscissa, and the size of the field on the ordinate (accumulated production and reserves) in millions of barrels for the field present in the area of interest or basin; and o the function best fitting the observed data was identified. the blue curve in the examples shown in figure 4-2 gives the distribution of known reserves for the area of interest, and the line of fit (green) records total extractible resource in the basin in technological conditions and political and socio-environmental restrictions similar to current ones. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 57 figure 4-2. estimating yet-to-find hydrocarbon by fractal approach. 4.4 results 4.4.1 the eastern llanos figure 4-3 gives the fractal distribution and best fit for fields in the eastern llanos basin. table 3-1 reports the values extracted from this graph. figure 4-3. fractal analysis of the eastern llanos basin. the blue line gives the fields‟ current distribution and the red line gives such distribution‟s estimated future. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 58 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia a yet-to-find volume of around 7,221 mmbbl was determined for the eastern llanos basin in current exploration and production conditions (table 4-1). figure 4-3 suggests that the basin could accommodate several giant fields and a significant number of fields having a size less than 5 mmbbl. size (mmbbl) possible fields yet-to-find volume (mmbbl) size ≥ 1,000 4 4,969.6 500 ≤ size < 1,000 2 1,465.1 200 ≤ size < 500 1 382.2 50 ≤ size < 200 1 130.5 20 ≤ size < 50 2 91.8 10 ≤ size < 20 2 31.3 5 ≤ size < 10 3 21.1 size < 5 129.5 total 7,221.2 table 4-1. distribution per size (yet-to-find resources) in the eastern llanos basin it should be pointed out that the values obtained could be interpreted in line with a supposition of new findings, or re-gauging actual fields resources. 4.4.2 caguán-putumayo figure 4-4 shows the fractal analysis for the caguán putumayo basin; table 4-2 gives the values so obtained. figure 4-4. fractal analysis for the caguán putumayo basin. the blue line represents the data observed regarding accumulated production and reserves for each field and the red line represents fractal tendency from the observations. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 59 a total of 557 mmbbl yet-to-find was determined (table 4-2) in actual conditions for the caguán putumayo basin. according to figure 4-4, the basin could house at least two new fields having more than 50 mmbbl. size (mmbbl) possible fields yet-to-find volume (mmbbl) 200 ≤ size < 500 1 358.5 50 ≤ size < 200 1 108.9 20 ≤ size < 50 2 62.8 5 ≤ size < 20 2 10.7 size < 5 16.1 total 557.0 table 4-2. distribution per size (yet-to-find resources) in the caguán putumayo basin. 4.4.3 the middle magdalena valley figure 3-5 and table 3-3 give the results obtained for this basin. figure 4-5. fractal analysis for the middle magdalena valley basin. it was expected that around 8,325 mmbbl could be found for the middle magdalena valley basin (table 4-3). according to figure 3-5, the basin could contain at least eight fields greater than 500 mmbbl. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 60 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia size (mmbbl) possible fields yet-to-find volume (mmbbl) 500 ≤ size < 1000 8 5,739.5 200 ≤ size < 500 5 1,825.8 100 ≤ size < 200 3 516.6 50 ≤ size < 100 1 93.4 20 ≤ size < 50 1 35.3 10 ≤ size < 20 2 22.4 5 ≤ size < 10 2 19.0 size < 5 73.1 total 8,325.1 table 4-3. distribution per size (yet-to-find resources) in the middle magdalena valley basin. 4.4.4 the upper magdalena valley figure 4-6 gives the distribution for the fields in this basin. table 4-4 lists the data extracted from the graph in question. figure 4-6. fractal analysis for the upper magdalena valley basin. it was estimated that there could be 2,099 mmbbl in resources to be incorporated for the upper magdalena valley basin. according to the distribution found, it could be expected to find at least 16 new fields having sizes greater than 100 mmbbl (figure 4-6). yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 61 size (mmbbl) possible fields yet-to-find volume (mmbbl) 100 ≤ size < 200 15 1,552.9 50 ≤ size < 100 4 336.4 20 ≤ size < 50 1 22.3 10 ≤ size < 20 2 23.5 5 ≤ size < 10 1 5.8 size < 5 158.5 total 2,099.4 table 4-4. distribution per size (yet-to-find resources) in the upper magdalena valley basin. table 4-5 gives a summary containing information about accumulated reserves and production on 31st december 2010, compiled from data provided by anh for the four basins considered, as well as fractal estimates made. basin accumulated production reserves ytf reserves + ytf (mmbbl) the eastern llanos 3,039 1,828 7,221 9,049 caguán putumayo 342 162 557 719 middle magdalena valley 1,950 915 8,325 9,240 upper magdalena valley 688 258 2,099 2,358 catatumbo 3,130 4,326 7,221 14,677 table 4-5. summary of fractal analysis and information regarding reserves per basin. 4.4.5 total yft crude resources in colombia the information available for the four previously-mentioned basins and additional data led to estimating yet-to-find hydrocarbon for colombia (lower magdalena valley, the eastern cordillera and catatumbo); 262 fields from seven basins mentioned throughout this chapter were used. figure 4-7 and table 4-6 gives the results obtained for the three scenarios (high, medium and low). figure 4-7. fractal analysis for crude in colombia. the low scenario is shown by the red line, the middle scenario by the green line and the high scenario by the blue line. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 62 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia scenario ytf volume (mmbbl) low 22,797.6 medium 26,072.4 high 33,145.9 table 4-6. scenarios for ytf crude resources in colombia. 4.4.6 total ytf gas resources in colombia the data regarding reserves in 48 fields from seven sedimentary basins in colombia (catatumbo, the lower magdalena valley, the middle magdalena valley, the upper magdalena valley, the eastern cordillera, the eastern llanos and guajira offshore) led to estimating yet-to-find gas. figure 4-8 and table 4-7 show the results of these estimations, presented in three possible scenarios (high, medium and low). figure 4-8. fractal analysis for gas in colombia. scenario ytf volume (bcf) low 6,615.1 medium 49,917.9 high 348,984.6 tabla 4-7. ytf resource scenarios for gas in colombia. yet-to-find resources earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 63 table 4-8 gives a summary of the fractal analysis for total crude and gas in colombia and information regarding reserves on 31st december 2010. basin reserves ytf reserves + ytf crude (mmbbl) 3,272 22,798 26,069 gas (tcf) 7.06 6.62 13.68 table 4-8. summary of fractal analysis and information regarding reserves for colombia. 4.5 conclusions o the fractal methodology led to yet-to-find hydrocarbon resources being determined for four sedimentary basins in colombia where minimum required information was available: the eastern llanos, caguán in putumayo, the middle magdalena valley and the upper magdalena valley; o 7,221 mmbbl yet-to-find resources were estimated for the eastern llanos basin, 8,325 mmbbl for the middle magdalena valley, 2,099 mmbbl for the upper magdalena valley and 557 mmbbl for caguán in putumayo; and o likewise, and from known data for all of colombia, yet-to-find crude and gas resources in the order of 22,798 mmbbl and 6.62 tcf were estimated respectively. 4.6 bibliography ahlbrandt, t. s. (2005). comparison of methods used to estimate conventional undiscovered petroleum resources: world examples. natural resources research, 14(3), 187-210. laherrere, j. (2000). distribution of field sizes in a petroleum system: parabolic fractal, lognormal or stretched exponencial: marine and petroleum geology, 7(4), 539 -546. 4.7 appendices digital file in the anh's document center:  “ytf_fractal.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5 methane gas hydrates 5.1 general comments gas hydrates are crystalline solids consisting of a molecule of gas surrounded by water molecules; these are formed in conditions involving high pressure and low temperatures. gas hydrates usually occur in sediments‟ porous spaces and can form cement, nodules, veins or layers (popescu et al., 2006). gas hydrates may contain different types of gases within their structures (carbon dioxide, nitrogen and hydrogen sulphate), but most natural gas hydrates mainly consist of methane (kvenvolden, 1995). gas hydrates can form and remain stable in favourable pressure and temperature conditions and when free methane and water are available (sloan, 1990). 5.1.1 origin and formation hydrate formation needs an anoxic environment which is saturated by methane gas and water molecules so that gas hydrates may begin to form spontaneously in conventional deposit pore spaces and fractures, in temperature and pressure conditions leading to these two phases‟ stable integration. such conditions are usually present in continental shelf sediments along ocean margins or in high latitudes (kvenvolden, 1988). biogenic gas, formed during the early diagenesis of organic matter, or thermogenic gas associated with migration from deeper accumulations, is forced to become solubilised in the water present (sloan & carolyn, 1998). pressure increases if the deposition of sediments and organic matter continues, and the potential area for hydrate accumulation could achieve suitable depth and thickness as the methane generation and dissolution cycle progresses. the change of the liquid phase composed of water and gas to solid hydrate only occurs at low temperatures and/or high pressure in an area called the methane hydrate stability area (figure 51). the thickness of this area will thus depend on the geothermal gradient and the water column and rock covering it (kvenvolden, 1988). other parameters controlling the stability area are seabed temperature, gas composition, salinity and the local geology. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 66 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-1. pressure and temperature stability curves for the base of the gas hydrates area. both curves are just for methane gas in the hydrate. 5.1.2 bottom simulating reflector (bsr) the first indicator regarding the presence of hydrates in marine sediments is the bottom simulating reflector (bsr) observed in seismic reflection sections. this reflector marks the base of the hydrate stability area below the seafloor. the bsr results from the contrast in negative impedance between sediments containing hydrates and lower hydrate-free sediments; this reflector marks the base of the stability area below the seafloor. the bsr lies approximately parallel to the seafloor and can be easily seen in slope areas (gradient) by cutting the stratigraphic reflectors; it usually has reverse polarity to that of the seafloor reflector (figure 5-2); however, if this lies parallel to the strata then it can be difficult to identify. some authors have stated that gas hydrates can be present even when the bsr has not been identified in seismic reflection (yuan et al., 1998). methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 67 figure 5-2. bsr characteristics identified in a seismic section to the south of the colombian pacific. the red mark on the map in the upper right-hand corner indicates the location of this line. the bsr, suggested by the orange arrows, shows strong amplitude, inverse to that of the seafloor reflector. it can also be seen how this crosses the upper and lower stratigraphic reflectors. bsr characteristics leads to obtaining a map of hydrates‟ regional distribution; analysing the bsr and the speed of structures provides semi-quantitative information about the amount of hydrates and their distribution, as well as the amount of free gas under the hydrate stability area (yuan et al., 1998). 5.1.3 gas hydrate distribution around the world the presence of gas hydrates has been inferred in 50 areas around the world (figure 5-3). however, only a limited number of gas hydrate accumulations have been examined in detail; 5 gas hydrate accumulations have been very well studied. such accumulations are found in marine and permafrost areas, as follows: the blake ridge along the south-eastern continental margin of the usa, the cascadian bioregion along the continental margin of the pacific coast of canada, on the eastern coast of japan, nankai, northern alaska and in the mackenzie river delta in northern canada (collet, 2002). 7 8 o w 2on time (s) ecu ado r distance (m)ven ezuela methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 68 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-3. location of inferred and known gas hydrate areas around the world. the red circles show continental margins and continental areas are shown in greens, modified from collet (2002). worldwide estimations of the amount of methane in hydrate deposits vary from 14 to 34,000 trillion m3 in permafrost areas and from 3,100 to 7,600,000 trillion m3 in oceanic sediments (table 5-1) it can be observed that the estimated amount is much greater than the reserves of remaining natural gas which were estimated at 187.1 trillion m3 for 2010 by bp (bp global, 2011). amount of gas reference gas contained in gas hydrates in terrestrial areas 1.4x1013 meyer (1981) 3.1x1013 mclver (1981) 5.7x1013 trofimuck et al., (1977) 7.4x1013 macdonald (1900) 3.4x1016 dobrynin et al., (1981) gas contained in gas hydrates in oceans 3.1x1015 meyer (1981) 5-25 x 1015 trofimuck et al., (1977) 2x1016 kvenvolden (1988) 2.1x1016 macdonald (1900) 4x1016 kvenvolden & claypool (1988) 7.6x1018 dobrynin et al., (1981) table 5-1. worldwide estimates of gas in hydrates. all values shown in m3, modified from collet (2002). 5.2 data and hypotheses 5.2.1 international data several databases worldwide which compile information about the distribution of gas hydrates‟ saturation and volumetric yield were reviewed for estimating gas in hydrates resources. the data reported by collett (1995) for estimating usa resources, specifically in the golf of méxico area, was used for making a genetic and environmental analogy. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 69 5.2.2 national data seismic data and information about wells along colombian continental margins was compiled from anh‟s oil database; it revealed 807 seismic lines (around 39,000 km) and 24 wells with their corresponding registries (figure 5-4). a search was also made for seismic information about the caribbean and colombian pacific coast corresponding to international scientific cruises and projects being carried out in this area. fifty-five seismic lines (about 3,000 km) were incorporated into the study (figure 5-4); such lines refer to data from the university of texas institute for geophysics‟ projects dsdp93gc, ig2408, ig2401 and ig2901 and data from the british oceanographic data centre‟s charles darwin project cd40 (1989). all the information so compiled was integrated into a digital project run in kingdom suite, (gashydrates.tks), forming an integral part of this study. the aforementioned seismic project includes bsr maps, the upper limit for the hydrate area, the seafloor, the speed used for converting horizons to depths and the polygons of the areas where bsr presence was interpreted. figure 5-4. seismic lines and wells used for estimating gas hydrates‟ potential pozos anh líneas sísmicas : university of texas institute for geophysics british oceanographic data center anh seismic lines anh wells: methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 70 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.2.3 hypotheses gas hydrates resources in colombian marine basins have been evaluated in line with the following hypotheses: 5.2.3.1 hypothesis 1 areas containing gas hydrates are restricted to areas where bottom simulating reflector (bsr) can be interpreted; 5.2.3.2 hypothesis 2 the thickness of the gas hydrate area can be obtained with a bsr-generated isopach map base of the hydrate stability area and an estimated limit for estimated seafloor pressure and temperature conditions guaranteeing molecule stability; and 5.2.3.3 hypothesis 3 the distribution of gas hydrate saturation values and their volumetric yield can be taken from analogous gas hydrate areas. 5.3 methodology the following equation was used for estimating the volume of gas which could be contained in gas hydrate accumulations in colombian marine basins: (5-1) : gas volume (tcf) : area of gas hydrate occurrence (km2) : deposit thickness (m) : deposit porosity (v/v) : degree of gas hydrate saturation (v/v) : volumetric yield of gas in hydrates (v/v) the following activities were carried out in each marine basin for this estimate: o seismic and well information was loaded into a kingdom suite project; the coordinates for seismic lines were estimated from files regarding navigation and scientific cruise reports for loading the seismic lines obtained from the charles darwin project cd40 (1989); o the bsr and seafloor were interpreted; methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 71 o polygons were drawn for the areas where the bsr was interpreted; o conversion to depth was done and maps of the surface were interpreted. the speeds used for converting to depth were:  a constant 1,500 m/s speed was used in all areas for converting seafloor to depth;  time-depth curves regarding available wells were used for the colombian caribbean for obtaining a map of bsr speed;  regarding the colombian pacific coast, the speed functions obtained from the work of minshull et al., (1994) were used as well as a well located in the area for obtaining a map of bsr speed. such procedure was chosen due to the scarce information regarding wells in this area (figure 5-5). figure 5-5. speed functions taken for conversion to depth in the colombian pacific. speed functions were obtained by inverting the wave equation in some cmp from line 1 (blue line on the lower map). the approximate location of the cmp can be seen with the red circles on the lower map, modified from minshull et al., (1994) methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 72 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia o the gas hydrate area‟s upper limit was estimated using equation 5-4 modified from dickens et al., (1994), defining pressure and temperature stability conditions for methane hydrates in seawater, as follows:  seafloor depth values were taken from the map generated for this reflector in marine basins;  seafloor pressure and temperature were calculated from equations 5-2 and 5-3: (5-2) (5-3) : seafloor pressure (mpa) : seafloor temperature normalised to 3 ºc < < 6.5 ºc : atmospheric pressure (mpa) : water density (1,030 kg/m3) : seafloor depth (m) : gravity (m/s2)  the depth to which the seafloor (due to its pressure and temperature conditions) may be considered the limit of the gas hydrates area was estimated (equation 5-4). figure 5-6 gives an example of the pressure and temperature obtained for the colombian caribbean seafloor and the base of the hydrate stability area, also defined by equation 5-4. ( ) (5-4) : temperature (ºc) : pressure (mpa) methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 73 figure 4 6. seafloor (sf) pressure and temperature in the colombian caribbean and hydrate stability conditions defined by the equation proposed by dickens et al., (1994). the blue line gives the base of the stability area inferred by equation 4-4. the red line represents sf pressure and temperature in the area where the bsr was interpreted. it was observed that hydrate stability cannot be guaranteed from certain sf depth. o seafloor depths greater than 555.65 m complied with pressure and temperature conditions for being within the hydrate stability area; the seafloor could thus be established as the upper limit for hydrates in this area (figure 5-6).  the foregoing suggested that in these areas the hydrostatic pressure slope was sufficient to guarantee stability; the pressure was very low or the temperature very high for guaranteeing this in the other areas.  for all points on the seafloor map in which depth was less than 555.65 m, the ∆h was calculated for entering the area of stability using equation 5-5 (figure 5-7). ( ) (5-5)  there was minimum variation regarding the depth required for points on the seafloor above 555.65 m in the area of stability. the hydrate area limit in these points should have been deeper in the middle of sediment layers, incorporating the lithostatic slope at stability pressure.  equation 5-5 applied a factor derived from the ratio between water density and that of marine sediments. this factor ranged from 0.52 to 0.6 and tried to affect the thickness considered below the seafloor, having a pressure slope greater than the hydrostatic one (marine sediments) so that total pressure in such areas increased proportionally. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 74 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-7. estimating hydrate area limit for less than 555.65 m seafloor depths. blue represents the seafloor (sf). red is the hydrate stability limit for the methane area. from 555.65 m onwards, the sf may be considered the stability area limit.  pressure and temperature regarding gas hydrate area limit were estimated again from equations 5-6 and 5-7 once the ∆h had been calculated at all points where the seafloor had a depth less than 555.65 m (figure 5-8). (5-6) ( ) (5-7) : pressure in hydrate area limit (mpa) : marine sediment density, around 1900 kg/m3 methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 75 figure 5-8. pressure and temperature for hydrate area limit in the colombian caribbean and hydrate stability conditions defined by the equation proposed by dickens et al., (1994). the red line represents sf pressure and temperature in the areas where the bsr was interpreted. the green line shows the estimated points, including the ∆h.  the map for the gas hydrates limit was calculated from the points at which seafloor depth was greater than 555.65 m and the points recalculated below the seafloor (the green line in the example shown in figure 5-8) in areas where the latter was above 556.65 m depth: o the isopach was calculated between the bsr and the gas hydrate area limit; o the volume of the gas hydrate area was estimated from the polygon of the area and the isopach; o the distribution of probability was adjusted to effective porosity values in records concerning wells in the colombian caribbean, considering just the depth interval defined by the limit and the base of the gas hydrate stability area. this distribution was used for making calculations for all the basins; o a bibliographic review was made for determining the distribution of values regarding the degree of gas hydrate saturation and volumetric yield of gas in hydrates, since data regarding this area was lacking, and o monte carlo simulation involved using porosity, gas hydrate saturation and gas hydrates‟ volumetric yield as random variables. the volume derived from analysing the seismic sections was incorporated as a constant parameter in the simulation. 5.4 results equation 5-1 was used for calculating some variables deterministically and others were assigned probability functions by statistical analysis. random variables were produced from analysing the distribution of known data concerning the area or by analysing the values used in studies around the world. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 76 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.4.1 gas hydrate occurrence areas the areas considered in the calculations were those where seismic lines led to identifying the presence of the bsr (table 5-2, figure 5-9). figure 4 9. polygons for the areas where the bsr was interpreted. caribbean area: the polygon for the guajira offshore basin is shown in orange, sinú offshore in brown and the colombia basin in pink. these defined a strip parallel to the coast which became thinner towards the southeast, giving a width varying from around 60 km to 1 km from northeast to southwest. pacific area: the polygon for the chocó offshore basin is shown in green, tumaco offshore in gray and deep pacific in red; likewise, a strip was defined parallel to the coast which became thinner from north to south, having a width which varied from around 80 km to 20 km from north to south. 5.4.2 deposit thickness the resources‟ potential distribution was restricted to the continental slopes, becoming reduced as the seafloor gradient increased. the bsr in the caribbean and colombian pacific gave a strip parallel to the coast (figures 5-10 and 5-11), reaching widths of up to 80 km in some areas. it was observed in both cases that the bsr became deeper in a direction perpendicular to the coast. colombia basin guajira offshore deep pacific tumaco offshore choco offshore methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 77 figure 5-10. map of the bsr for the colombian caribbean. it can be observed that the bsr was restricted to a strip parallel to the coast which became thinner towards the southeast of the sinú offshore basin. its depth varied from 0.3 km in the area closest to the coast and, as it became further from the coast line, it became deeper, reaching around 4 km in the colombia basin. depth sinú offs hore methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 78 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-11. map of the bsr for the colombian pacific. the bsr gave a strip parallel to the coast which became thinner towards the south; its depth varied from 0.3 km in the area closest to the coast, which became deeper as it became further away from the coastline, reaching 7 km to the north in the deep pacific basin. figures 5-12 to 5-29 give the bsr distribution maps and isopach hydrate area limit for colombian marine basins. the volumes used for estimating resources in each basin were calculated from thickness data and bsr aerial distribution; table 5-2 gives this data. basin area (km2) minimum-maximum thickness (km) volume (km3) chocó offshore 22,404.33 0.004 4.81 40,731.63 colombia basin 2,042.33 0.397 1.27 1,635.12 guajira offshore 18,873.14 0.0002 1.46 10,529.41 deep pacific 1,308.37 1.97 3.17 3,271.65 sinú offshore 10,248.28 0.0009 -1.14 5,189.15 tumaco offshore 6,374.35 0.009 2.49 4,623.07 table 5-2. dimensions of the areas considered for gas hydrate potential calculations. depth pacific ocean methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 79 5.4.2.1 guajira offshore figure 5-12. map of the bsr for the guajira offshore basin. the bsr in this basin gave depths between 0.4 km in the area close to the coast and 3 km in the most far off area. figure 5-13. map of the hydrate area limit for the guajira offshore basin. the hydrate limit followed the bsr tendency having depths from 0.3 km to around 2.5 km. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 80 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-14. isopach map of the hydrate area for the guajira offshore basin. this basin‟s thickness had a broad range of variation, from a few metres in the area close to the coast to around 1000 m towards the north where the bsr and hydrate limit were deeper. 5.4.2.2 sinú offshore figure 5-15. map of the bsr for the sinú offshore basin. in this basin the bsr gave depths varying from 0.4 km in the area closest to the coast to around 3 km to the north. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 81 figure 5-16. map of the hydrate area limit for the sinú offshore basin. the hydrate limit had the same tendency as the bsr, having depths varying from 0.3 km to around 2.5 km. figure 5-17. isopach map of the hydrate area for the sinú offshore basin. thickness ranged from a few metres in the area closest to the coast to around 1,000 m, the greatest thickness being presented in the basin‟s central area. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 82 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.4.2.3 colombia basin figure 5-18. map of bsr for the colombia basin. it may be observed that the bsr becomes deeper as it became father away from the coast in a perpendicular direction, reaching around 4 km in the deepest areas. figure 5-19. map of hydrate area limit for the colombia basin. the hydrate area limit gave the same tendency as the bsr, reaching depths of 3 km around the area farthest away from the coast. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 83 figure 5-20. isopach map of the hydrate area for the colombia basin. the thicknesses in this area varied from 300 m to around 1200 m. the greatest thicknesses occurred in the north where the bsr and hydrate area limit were deepest. 5.4.2.4 chocó offshore figure 5-21. map of the bsr for the chocó offshore basin. the bsr gave depths varying from 0.3 km in the area close to the coast, increasing in a perpendicular direction until reaching around 6.5 km. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 84 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-22. map of hydrate area limit for the chocó offshore basin. the hydrate area limit had the same tendency as the bsr, varying in depth from 0.3 km in the area close to the coast to 4 km in the area farthest away from it. figure 5-23. isopach map for the chocó marine basin. the thickness of the hydrate area varied from a few metres in the area close to the coast, reaching around 3,000 m to the southeast. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 85 5.4.2.5 tumaco offshore figure 5-24. map of the bsr for the tumaco offshore basin. the bsr ranged from depths of 0.3 km in the area close to the coast to around 4 km to the northeast. a high spot was observed in the central part of the basin. figure 5-25. map of hydrate area limit for the tumaco offshore basin. the hydrate area limit also gave a high spot in the central part of the basin, and varied in depth from 0.3 km to 1.5 km. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 86 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-26. isopach map for the tumaco offshore basin. thicknesses varied from a few metres in the area close to the coast to around 2 km to the northeast where the bsr and the limit were at their deepest. a reduction in thickness was identified in the central part. 5.4.2.6 deep pacific figure 5-27. map of the bsr for the deep pacific basin. the bsr in this basin was deeper, varying from around 6 km to 7 km, this being due to finding it in the part farthest away from the coast where depths were greater. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 87 figure 5-28. map of the hydrate area limit for the deep pacific basin. this gave the same relief as the bsr, varying from 3 km to 4 km in depth. figure 4 29. isopach map for the deep pacific basin. the thickness of hydrate area in this basin varied from 2 km to around 3 km. this gave the greatest thickness due to it being found in the area where the bsr was deepest. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 88 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.4.2.7 deposit porosity porosity value distribution was estimated according to the data concerning the effective porosity records for 22 wells. however, most available wells did not come within hydrate occurrence areas (figure 5-30). nevertheless, such wells represented the only information available for calculating porosity distribution. a variation in porosity was observed with similar depth throughout the whole area being analysed, being assumed to be representative of the whole area having hydrates. figure 5-30. wells having effective porosity records the interval for the values analysed covered the interval between 4,161.3 m and 354.3 m depth, being the maximum depth estimated for the bsr and the minimum depth for the hydrate areas‟ upper limit, respectively. porosity values less than 3% were not taken for estimating distribution as they were not considered to be expected values in rock and could have been due to errors in record taking or when calculating effective porosity. the distribution best fitting the data was a weibulltype distribution (table 5-3). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.12 0.12 0.12 1.67 1.66 1.68 was not rejected m.b. 5.01 0 table 5-3. goodness-of-fit parameters and test results applied to porosity data for determining statistical distribution used in estimating gas hydrates potential. estimated parameters ̂ ̂ are weibull distribution regarding “a” and “b” respectively. m.b refers to very low probability of the null hypothesis being accepted or rejected. 5.4.3 degree of gas hydrate saturation gas hydrate saturation values estimated in work carried out around the world was reviewed; the distribution used for calculating usgs hydrate resources in the gulf of méxico (collet, 1995) was taken, considering that this covered the whole range of variation found in analogous gas hydrate areas (table 5-4). fractile 100 95 75 50 25 5 0 gas hydrate saturation 2 3.2 8 14 20.5 25.7 27 table 5-4. gas hydrates saturation distribution parameters, taken from collet (1995). methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 89 5.4.4 volumetric yield of gas in hydrates volumetric yield describes when free gas is stored in gas hydrates. if gas hydrate structure is completely full of methane, 1m3 of hydrate could contain 172 m3 of methane in standard pressure and temperature conditions. however, many researchers consider that such scenario cannot be found in nature; nevertheless, gas hydrates are not stable if a structure contains less than 70% gas, thereby implying a production of at least 139 m3 of methane (collet, 1995). a pessimistic to moderate range was chosen in the present work, i.e. a variation from 139 m3 (70% methane occupation of gas hydrates) to 164m3 (representing 90%), using the distribution range used for calculating usgs hydrate resources in the gulf of méxico (collet, 1995). table 4-5 shows the variable random fractals associated with describing gas hydrate volumetric yield. fractile 100 95 75 50 25 5 0 volumetric yield of gas in hydrates 139 140.3 145.3 151.5 167.8 162.8 164 table 5-5. gas hydrates volumetric yield distribution parameters, taken from collet (1995). 5.4.5 gas hydrate potential considering the aforementioned hypotheses and all the variables described in equation 5-1, associated gas hydrates potential in colombian marine basins was calculated using the monte carlo probabilistic approach. table 5-6 gives the results obtained, not counting restricted areas due to the presence of environmental reserves. basin gas in hydrates (tcf) p10 p50 p90 marine chocó 46.87 11.79 3.03 colombia basin 1.91 0.48 0.12 marine guajira 12.18 3.08 0.79 deep pacific 3.68 0.93 0.24 marine sinú 5.75 1.45 0.37 marine tumaco 5.23 1.32 0.34 total colombia 75.63 19.04 4.89 table 5-6. estimating gas potential n hydrates. these results were affected by environmental factors shown in table 2-1. the results for the colombia and deep pacific basins were underestimated due to not having adequate seismic coverage allowing a more precise estimate of methane hydrates‟ potential. the hess escarpment lies to the south of los cayos basin, on the border with the colombia basin. depths in this area varied from 0.5 km to 3 km from northeast to southeast in a strip of around 200 km (figure 5-31) which could store gas hydrates, as has been found on the colombian caribbean coast. however, due to the few north-east to south-east seismic lines covering the hess escarpment, the presence of gas hydrates could not be validated with total certainty by interpreting the bsr, since no reflector was found fulfilling the necessary conditions with certainty. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 90 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5-31. seísmic lines available in the area limiting los cayos and colombia basins. it can be seen that there was an increase in depth in a northeast-southeast direction all along the slope procuced by the hess escarpment (red line). figure 5-32. gas hydrates‟ potential for colombia marine basins. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 91 figure 5-33. gas hydrate potential for colombian marine basins figure 5-34. ytf gas hydrates in colombia compared to estimated resources in other parts of the world changes colombia‟s upper amenities. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 92 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 5.4.6 sensitivity analysis the degree of influence or weighting of each random variable in equation 5-1 in calculating the potential for colombian marine basins can be seen in figure 5-35. figure 5-35. sensitivity analysis for the variables used in estimating gas hydrates potential. sensitivity analysis for all the basins analysed suggested that gas hydrate exploration studies in colombian basins should pay more attention to hydrate saturation (60.4%) and porosity (39.5%) than to the degree of volumetric yield. 5.5 conclusions  according to the calculations regarding gas hydrates‟ potential, the most prospective basin would be the chocó offshore basin, followed by guajira offshore and sinú offshore basins;  the results obtained in this work for the potential of methane gas in gas hydrates (75.63 – 4.89 tcf) were considerably less than the values estimated by d. little (2008), reaching 400 tcf;  methane gas in gas hydrates was found in colombian park system areas (1.4 tcf, 0.35 tcf and 0.09 tcf for p10, p50 and p90, respectively);  potential for the colombia and deep pacific basins was underestimated due to scarce seismic and well information being available for them; and  there could be gas hydrates in the area bordering los cayos and the colombia basins. analysis of their potential requires more and pertinent information. 5.6 bibliography bp global. (2011). statistical review 2011. consulted on 30th june 2011 at, http://www.bp.com/statisticalreview camerlenghi, a. (2009). hidratos de metano: cambio climatico, energia y riesgo submarino. ix trobada de professorat de ciències de la terra i del medi ambient del batxillerat. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 93 collet, t. (1995). gas hydrate resources of the united states. u.sg.s. digital data series, 30. collet, t. (2002). energy resource potential of natural gas hydrates. american association of petroleum geologists‟ bulletin, 86(11), 1971-1992. d. little, a. (2008). evaluation of potential of unconventional energy resources in colombia. agencia nacional de hydrocarbons. houston: arthur d. little inc. dickens, g., & quinby hunt, m. (1994). methane hydrates stability in seawater. geophysical research letters, 21 (19), 2115-2118. jackson, b. (2004). seismic evidence for gas hydrates in the north makassar basin, indonesia. petroleum geoscience, 10(3), 227-238. kvenvolden, k. (1988). a primer on the geological occurrence of gas hydrate. geological society london special publications, 137, 9-30. kvenvolden, k. (1995). a review of the geochemistry of methane in natural gas hydrates. organic geochemistry, 23(11), 997-1008. lüdman, t., wong, h., konerding, p., zillmer, m., peterson, j., & flüh, e. (2004). heat flow and quantity of methane deduced from a gas hydrate field in the vecinity of the dnieper canyon, northwestern black sea. geo-marine letters, 24(3), 182-193. majorowicz, j., & osadetz, k. (2001). gas hydrate distribution and volume in canada. american association of petroleum geologists‟ bulletin, 85 (7), 1211-1230. master, c., & root, d. (1991). resource constraints in petroleum production potential. science, 253(5016), 146-152. milkov, a., & sassen, r. (2001). estimate of gas hydrate resource, northwestern gulf of mexico continental slope. marine geology, 179(1), 71-83. minshull, t., singh, s., & westbrook, g. (1994). seismic velocity structure at a gas hydrate reflector, offshore western colombia, from full waveform inversion. journal of geophysical research, 99(b3), 4715-4734. morales, e. (2003). methane hydrates in the chilean continental margin. electronic journal of biotechnology, 6(2), 80-84. pecher, i., henry, s., gorman, a., & fohrmann, m. (2004). gas hydrates on the hikurangi and fiordland margins, new zealand. american association of petroleum geologists hedberg conference. vancouver. popescu, i., de marc, b., leericolais, g., nouze, h., poort, j., & panin, n. (2006). multiple bottomsimulating reflections in the black sea: potential proxies of past climate conditions. marine geology, 227(3), 163-176. methane gas hydrates earth sciences research journal, volume 16, special issue, april, 2012 94 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia sain, k., & gupta, h. (2009). gas hydrates; a future major potential source of energy in india. glimpses of geoscience research in india, 244-250. senger, k. (2010). first order estimation of in-place gas resources at the nyegga gas hydrate prospect. norwegian sea energies, 3, 2001-2026. sloan, e. (1990). natural gas hydrate phase equilibria and kinetics; understanding the state of the art. revue de l'institut français du pétrole, 45(2), 245-266. sloan, e., & carolyn, k. (1998). clathrate hydrates of natural gases. new york: dekke. tanahashi , m. (2011). present satus of japanese methane gas hydrates research and development program. hydrate in japan. ccop hydrate workshop. vietman. thakur, n., & rajput, s. (2010). exploration of gas hydrates. new york: springer. yuan, t., nahar, k., chand, r., hyndman, r., spence, g., & chapman, n. (1998). marine gas hydrates: seismic observations of bottom-simulating reflectors off the west coast of canada and the east coast of india. geohorizons, 3(1), 1-11. 5.7 appendixes digital files in the anh's document center:  “hidratos de gas colombia.xlsx”  “ytf_gashydrates.zip” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 6 coalbed methane 6.1 general comments methane gas is a constituent element of coal in high concentrations, depending on its composition and external factors such as temperature, pressure and geological conditions (ortega, 2008). this kind of gas is called coalbed methane (cbm). extracting gas from coal requires dewatering a coal seam by lowering the pressure to enable the release of more mobile molecular species, including methane and ethane mixtures (and some traces of c3, n2 and co2). 6.1.1 origin and formation coal originates from the decomposition of plants and vegetable material, such as leaves, wood and bark. it becomes accumulated in reducing environments, in swamps, lagoons and shallow marine areas. carbon transformation and vegetable material enrichment occurs as time elapses, accompanied by a rapid burial rate leading to layers being formed having high organic content. overlying levels must have sufficiently low porosity and permeability to maintain a proper anaerobic environment and continue carbonisation. it should be stressed that more than ten meters of silt coal are needed to produce a one-meter-thick coal layer. 6.1.2 types of coal several classifications of coal are related to the degree of carbonisation or maturation of the source vegetable material, depending on the type of organic matter, age, depth, pressure, temperature and geological conditions in the basin where it has been formed. coal and peat are the carbon forms having the lowest maturation values and little alteration of parental vegetal material; lignite and anthracite are coal forms having greater evolution. the category of coal is measured in terms of physical and chemical parameters, such as volatile matter, fixed carbon, ash, humidity, calorific value and sulphur content. the higher the grade/rank, the greater the fixed carbon content and calorific value whilst natural humidity and the amount of volatile matter becomes reduced. figure 6-1 illustrates the classification of coal based on rank and chemical properties (stach, 1982). coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 96 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 5 1. classification of coal based on ranks and chemical properties such as vitrinite reflectance percentage, volatile matter, ash and humidity, taken from (stach, 1982). 6.1.3 coalbed methane (cbm) deposits gas in coalbed methane (cbm) deposits is stored in the micropores and internal discontinuity planes in coal. two porosity systems can be distinguished: o primary porosity is that which is developed or originates at the moment of strata formation and consists of just the matrix‟s porous space. it is able to absorb a large amount of gas but contributes very little towards permeability; and o secondary porosity or that caused by natural fractures, cracks and fissures called cleats is that contributing most to permeability (figure 6-2). figure 6-2. diagram of coal matrix, micropores and the natural fracture system. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 97 6.1.4 cbm around the world more than 40 countries have investigated resources of cbm, but commercial projects have only been developed in australia, canada, china and india. estimated cbm resources around the world are greater than 9,000 tcf (table 6-1). country coal (109 ton) cbm (tcf) russia 6,500 600 – 4,000 china 4,000 1,060 – 1,240 usa 3,970 400 canada 7,000 200 – 2,700 australia 1,700 300 500 germany 320 100 england 190 60 kazakhstan 170 40 poland 160 100 india 160 30 south africa 150 30 the ukraine 140 60 table 6-1. cbm reserves around the world, taken from tonnsen & miskimins (2010). 6.1.5 coal in colombia the main characteristics of colombian coal are given below regarding its distribution, composition and reserves as reference point in evaluating the cbm potential. a map of colombian coal-bearing provinces is given in appendix 6-1. 6.1.5.1 colombian coal-bearing provinces coal is evident throughout colombia, from the sandy guajira in the north to leticia in the amazonian region in the south. variation in its properties has led to 12 provinces or coal-bearing areas being differentiated, evaluated according to the amount of knowledge and geological conditions regarding mines and outcrops (ingeominas, 2004). figure 6-3 and table 6-2 give a summary of colombia‟s coal-bearing provinces. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 98 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 6-3. colombian coal-bearing provinces, modified from (ingeominas, 2004): 1. guajira; 2. cesar; 3. norte de santander; 4. cordoba – norte de antioquia; 5. santander; 6. antiguo caldas – antioquia; 7. valle del cauca – cauca; 8. huila – tolima; 9. boyacá; 10. cundinamarca; 11. borde llanero; 12. llanura amazónica. each region or province is divided into areas, sectors, blocks and coal mines. mining resources‟ classification categories are based on geological certainty and their degree of technical economic safety (ingeominas, 2004). the physical and chemical properties of coal (humidity (%), cz ash (%) mv volatile matter (%), cf fixed carbon, st sulphur content and pc calorific power) described in table 6-2 can be related to the coal classification proposed by stach (1982) shown in figure 6-1. part of colombia area sector humidity (%) cz (%) mv (%) cf (%) st (%) pc (btu/lb) la guajira cerrejón norte 11.94 6.94 35.92 45.2 0.43 11,586 cerrejón central cerrejón sur cesar la loma la loma syncline 11.39 10.32 33.37 66.63 0.72 10,867 el boquerón 10.29 5.61 36.79 47.31 0.59 11,616 el descanso sur la jagua de ibirico la jagua 7.14 5.32 35.7 51.84 0.62 12,606 cerro largo córdoba-norte de antioquia alto san jorge san pedro sur 14.49 9.24 37.55 38.73 1.31 9,280 san pedro norte 14.49 9.24 37.55 38.73 1.31 9,280 alto san jorge 14.49 9.24 37.55 38.73 1.31 9,280 antioquiaantiguo caldas venecia-fredonia 11.64 8.11 40.06 40.2 0.48 10,426 amagá-angelópolis amagá-nechí 13.16 11.96 36.69 38.18 0.55 9,682 angelópolis venecia-bolombolo rincón santo 9.84 11.1 38.45 40.61 1.04 10,090 bolombolo 8.49 7.9 37.77 45.91 1.09 11,113 titiribí corcovado 7.25 7.92 37.99 46.84 0.72 11,767 el balsal río sucio-quinchía 4.08 15.56 31.75 48.61 1.8 10,713 aranzazu-santágueda aranzazu 22.22 28.69 30.33 18.76 0.67 5,451 santágueda 19.03 25.05 37.32 18.6 0.43 6,230 valle del caucayumbo-asnazú golondrinas-río 2.69 22.38 28.15 46.79 2.85 coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 99 cauca cañaveralejo 11,088 cañaveralejo-río pance río pance-río guachinte río guachinte-río asnazú río dinde-quebrada honda 2.83 20.63 36.72 39.84 4.02 11,138 mosquera-el hoyo pedregosa-mosquera 8.11 16.3 35.18 40.42 1.42 10,058 limoncito-yeguas el vergel quilcacé-el hoyo cundinamarca jerusalén-guataquí 5.19 5.34 39.09 50.38 0.58 13,044 guaduas-caparrapí caparrapí 4.12 5.61 22.43 67.83 0.59 12,829 guaduas guatavita-sesquiléchocóntá suesca-chocóntá 1.98 11.23 34.88 51.91 0.91 12,682 guatavita tabio-río frío-carmen de campa carmen de campa 3.42 12.67 20.8 63.10 1.53 13,041 tabio-río frío 4.12 9.76 18.01 68.11 0.93 13,390 checua-lenguazaque cogua-sutatausaguachctá 3.66 9.46 26.8 60.07 0.80 13,433 lenguazaque-cucunubánemocón 4.67 10.62 33.85 50.86 1.06 12,718 suesca-albarracín 3.92 10.43 33.53 52.12 0.69 12,738 zipaquirá-neusa zipaquirá-embalse del neusa 1.04 14.42 24.33 60.21 1.38 12,993 embalse del neusavereda lagunitas páramo de la bolsamachetá 4.42 14.21 35.7 45.67 1.04 11,309 checua-lenguazaque 3.56 10.00 25.19 61.25 0.80 13,439 suesca-albarracín 4.69 12.18 33.71 49.42 1.07 12,420 boyacá tunja-paipa-duitama 9.48 11.4 38.03 41.09 1.53 11,268 sogamoso-jericó 4.29 9.57 30.19 55.96 1.23 13,099 betania 1.47 8.36 30.94 59.25 1.00 13,859 úmbita-laguna de tota 5.75 13.10 38.34 42.8 1.21 11,699 santander san luis western flank 2.70 25.95 28.11 43.23 1.76 10,913 1.63 7.65 33.38 57.33 1.37 13,994 eastern flank 1.18 18.72 30.48 49.62 2.01 12,284 1.18 10.09 29.05 59.67 2.15 13,893 cimitarra sur 4.61 4.61 29.77 61.01 0.62 13,021 capitanejo-san miguel 6.33 7.51 19.00 67.16 0.93 11,782 miranda 1.81 14.47 15.13 68.59 3.46 12,803 molagavita 0.80 8.58 32.25 58.37 0.70 14,161 páramo del almorzadero 5.18 4.71 14.23 75.88 0.75 12,889 chitagá 3.29 12.59 12.9 71.22 1.44 12,804 pamplona-pamplonita pamplonita 2.96 9.97 36.15 50.92 1.34 13,199 pamplona herrán-toledo toledo 2.31 7.46 26.99 63.24 0.83 14,120 herrán salazar north 3.76 9.46 36.81 49.96 0.62 12,762 centre south norte de santander tasajero east 2.84 10.17 34.82 52.18 0.85 13,326 west 2.56 7.65 33.67 56.12 0.85 13,925 south 2.42 17.10 34.59 45.89 0.89 12,291 zulia-chinácota zulia sur 3.36 11.90 35.29 49.45 1.27 12,967 santiago 2.71 5.95 30.55 60.80 0.71 14,153 8.33 17.06 28.67 47.33 0.62 9,911 san cayetano 2.02 12.12 26.66 59.20 1.43 13,324 2.17 18.05 36.61 43.17 0.78 11,410 san pedro 2.53 11.30 35.63 50.54 0.81 13,290 2.69 14.88 38.49 43.94 0.83 12,436 villa del rosario 2.74 7.50 36.70 53.06 0.70 13,588 catatumbo zulia norte-sardinata 3.67 9.18 37.57 49.59 0.95 12,602 el carmen 4.31 8.64 39.17 47.88 0.95 12,316 llanura amazónica leticia 10.39 30.89 36.09 22.63 3.67 6,662 table 6-2. colombian coal-bearing provinces with their classification into areas and sectors, key: czash (%), mv volatile material (%), cf fixed carbon, st – sulphur content and pc – calorific power, modified from (ingeominas, 2004). coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 100 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 6.1.5.2 geology chronostratigraphically, there is a broad range of ages within which the most significant accumulations of coal developed: maastrichtian-upper oligocene, albian-cenomanian and cretaceous (ingeominas, 2004). table 6-3 gives a summary of the main coal-bearing formations in colombia. formation age area cerrejón eocene la guajira los cuervos paleocene lower eocene el cesar, norte de santander and western santander guaduas upper maastrichtian paleocene cundinamarca and boyacá umir campanian maastrichtian western cundinamarca, boyacá and santander cerrito miocene pliocene córdoba and northern antioquia amagá upper oligocene lower miocene caldas, antioquia and córdoba guachinte and ferreira eocene oligocene valle del cauca and cauca floresanto and maralú oligocene lower miocene antioquian urabá ciénaga de oro oligocene córdoba tarazá oligocene northern antioquia pepino – the middle oligocene putumayo carbonera upper eocene lower to middle oligocene norte de santander san fernando upper eocene lower oligocene the fringe of the eastern llanos margua middle to upper eocene the fringe of the eastern llanos socha paleocene eastern boyacá clays of el limbo and limbo paleocene – middle eocene the fringe of the eastern llanos catatumbo and mito juan upper maastrichtian santander and norte de santander grupo palmichal upper maastrichtian upper paleocene the fringe of the eastern llanos seca maastrichtian southern and western cundinamarca córdoba maastrichtian eastern cundinamarca and south-eastern boyacá chipaque cenomanian localities in cundinamarca une albian meta and boyacá caballos albian huila and tolima table 6-3. the most important coal-bearing units in colombia (ingeominas, 2004). 6.1.5.3 coal reserves and resources table 6-4 gives measured, indicated, inferred and hypothetical resources and reserves and their potential for each coal-bearing area in colombia. part of colombia area resources plus reserves resources potential (ytf) measured indicated inferred hypothetical la guajira cerrejón norte 3,000.00 3,000.00 cerrejón central 670 670.00 cerrejón sur 263.3 448.86 127.50 27.16 866.82 total 3,933.30 448.86 127.50 27.16 4,536.82 cesar la loma 1,777.10 1,563.98 1,963.18 993.50 6,297.76 la jagua de ibirico 258.30 258.30 total 2,035.40 1,563.98 1,963.18 993.50 6,556.06 córdoba norte de antioquia alto san jorge 381.00 341.00 722.00 total 381.00 341.00 722.00 antioquiaantiguo caldas venecia-fredonia 8.94 40.14 16.87 65.95 amagá-angelópolis 11.84 63.64 92.33 25.38 193.19 venecia-bolombolo 57.95 84.80 18.75 161.50 titiribí 11.33 37.25 4.45 1.07 54.10 total 90.06 225.83 132.40 26.45 474.74 valle del cauca cauca yumbo-asnazú 30.7 56.42 47.49 10.98 145.59 río dinde-quebrada honda 4.37 16.66 19.69 40.72 mosquera-el hoyo 6.38 19.06 30.72 56.16 total 41.45 92.14 97.9 10.98 242.47 cundinamarca jerusalen-guataquí 1.81 5.73 5.28 3.23 16.05 guaduas-caparrapí 6.76 32.68 21.36 0.91 61.71 coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 101 san franciscosubachoque-la pradera 11.35 48.2 60.89 6.46 126.9 guatavita-sesquiléchocóntá 21.9 64.31 106.88 10.14 203.23 tabio-río fríocarmen de carupa 19.43 55.82 54.84 24.78 154.87 checualenguazaque 140.42 345.44 210.66 16.25 712.77 suesca-albarracín 32.92 87.71 68.90 189.53 zipaquirá-neusa 1.64 4.96 10.41 17.01 total 236.23 644.85 539.22 61.77 1,482.07 boyacá checua-1enguazaque 35.69 129.87 115.84 281.4 suesca-albarracín 7.81 43.29 106.26 157.36 tunja-paipa-duitama 24.03 97.21 171.41 292.65 sogamoso-jericó 102.84 412.25 473.71 988.8 total 170.37 682.62 867.22 1,720.21 santander san luis 56.08 108.64 123.44 288.16 capitanejo-san miguel 18 1.43 19.43 miranda 5.49 5.49 molagavita 7.95 7.95 páramo del almorzadero 118.24 24.37 142.61 total 56.08 258.32 149.24 463.64 norte de santander chitagá 0.66 1.98 7.4 10.04 mutiscua-cácota 1.56 0.66 0.16 2.38 pamplona-pamplonita 2.79 6.25 4.83 13.87 herrán-toledo 4.78 14.63 9.17 28.58 salazar 7.71 15.5 5.8 29.01 tasajero 14.18 29.51 50.23 93.92 zulia-chinácota 40.05 124.15 103.2 267.4 catatumbo 47.96 121.66 179.98 349.59 total 119.69 314.34 360.77 794.79 total 7,063.58 4,571.94 4,237.43 1,119.86 16,992 table 6-4. coal resources and reserves in colombia in millions of tons (ingeominas, 2004). 6.2 data and hypotheses 6.2.1 international data the set of compiled data attached as part of this document in a file labelled “appendix 6-2 – cbm database alberta canada.xlsx” consists of 1,511 samples from deposits distributed throughout the coal-bearing basins of alberta in canada. this information has been used for estimating the distribution of in-situ gas content (g_c). 6.2.2 national data the compiled set of data attached as part of this document in a file labelled “appendix – colombian cbm database.xlsx” (appendix 6-3) contains the information used for calculating the cbm potential, thickness, density and mapped coal-bearing areas, the results of each statistical analysis for areas having little or no information available and the information used for estimating the distributions of in-situ gas (g_c) content. 6.2.3 hypotheses effective cbm potential in colombian basins was evaluated in line with the following hypotheses: coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 102 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 6.2.3.1 hypotheses 1 areas having cbm in colombian sedimentary basins are represented by areas identified on the available cartography regarding coal (i.e. ingeominas, 2004). 6.2.3.2 hypotheses 2 values regarding thickness, density and other properties in all coal-bearing provinces are considered to be representative of the values expected for the sedimentary basins in which they are located. 6.2.3.3 hypotheses 3 gas concentration (gc) for areas having effective cbm potential per basin in colombia may be estimated in line with some of the following scenarios: o evaluation scenario 1: pattern for coal-bearing basins in alberta, canada o evaluation scenario 2: gc pattern expected for colombian basins (uptc, 2010) o evaluation scenario 3: gc pattern obtained from pithead field data (uptc, 2010). 6.3 methodology coalbed methane potential in colombia was estimated by means of equation 6-1: (6-1) g: in situ gas (bcf) a: drainage area (m2) h: deposit thickness (m) ρb: apparent average coal density (g/cm3) gc: gas concentration (ft3/ton) the following steps were followed when estimating cbm for all colombian sedimentary basins: o outcrop areas were extracted from the polygons representing coal-bearing formations of interest. these polygons were contoured showing mining conditions, geological and/or structural contacts, changes of thickness, geological certainty and local characteristics such as municipal limits, rural areas or mining blocks of relevance for a particular sector:  polygons were drawn from colombian geological service information (ingeominas, 2004), digitalising all coal manifestations recorded in colombian coal-bearing areas, regardless of their condition regarding other study variables in equation 6-1;  the polygons were discretised by geological unit, province, area and coal-bearing block for each basin. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 103 o coal-bearing levels‟ thickness and density were obtained from stratigraphic columns, levels reported in mines and data inferred regarding outcrops registered by ingeominas (2004). only coal-bearing levels of up to a maximum 300 m sequence thickness was established for evaluating the cbm resources:  thickness and density were discretised regarding the polygons by geological unit, province, area and coal-bearing block for each basin;  known data regarding density and thickness by block or province was statistically analysed. values were assigned to non-producing areas having coal manifestations according to the analysis of producing areas;  a zero thickness value was assigned for mapped areas regarding formations considered as being coal-bearing but for which no coal manifestations had been reported;  a database was produced with the information from the thickness and density polygons for the areas having coal manifestations for colombia (appendix 6-3);  distributions were found for gc from the proposed hypotheses describing three different scenarios; and  monte carlo simulations were then used with equation 6-1 for calculating in situ gas by assuming thickness, area, density as constants and gas concentration (gc) as random values. o cbm resources were then evaluated in all three proposed scenarios. 6.4 results statistical analysis of some of the variables in equation 6-1 led to ascertaining their representativity and identifying the distribution of probability best fitting the data. such tests led to ascertaining the degree of certainty (probability), i.e. whether there were greater deviations. specific values were determined for such equation‟s other parameters and it was not necessary to associate any statistical distribution with their pattern. 6.4.1 drainage area the study took all the areas reported as having cartographic information available for coal into account into account. table 6-5 gives the values considered per basin. basin area (km2) amagá 159.39 non-prospective areas 2,811.15 caguán putumayo 1,517.23 catatumbo 793.09 cauca patía, 279.12 cesar ranchería 499.39 the eastern cordillera 3,434.02 guajira 17.42 the eastern llanos 188.35 sinú san jacinto, 4,962.37 the lower magdalena valley 938.66 the middle magdalena valley 1,525.13 the upper magdalena valley 585.96 vaupés amazonas 5,854.03 table 6-5. coal-bearing areas per basin. some coal-bearing areas were found which were outside the limits defined by anh for colombian basins; these were located in non-prospective areas according to anh‟s definition for such areas. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 104 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 6.4.2 deposit thickness the thickness considered in each polygon (contoured by geological unit, province, area, and coalbearing block for each basin) is given in the file attached as appendix 6-3. thickness could be considered as being a deterministic variable from such information. distribution functions were defined from the most genetically apt data in areas having cartographic information but lacking thickness referents. table 6-6 gives the results of the statistical analysis for this variable. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.84 0.73 0.95 1.65 1.58 1.74 was not rejected m.b. 12.6 0 table 6-6. lognormal distribution fit parameters and goodness-of-fit test results applied to the thickness data used in estimating cbm. estimated parameters ̂ ̂ are lognormal distribution regarding the mean and standard deviation, respectively. m.b refers to very low probability of the null hypothesis being accepted or rejected. 6.4.3 average apparent density of coal the same as thickness values were applied in equation 6-1, density values were applied in each polygon where the value was reported for this parameter. this data also led to identifying a distribution function which could then be used for areas lacking density data. table 6-7 gives the results for the best fit and pertinent goodness-of-fit tests. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.29 0.29 0.30 0.04 0.04 0.04 was not rejected m.b. 51.6 0 table 6-7. lognormal distribution fit parameters and goodness-of-fit test results applied to the density data used in estimating cbm. estimated thickness and density distributions were thus used for the eastern llanos, vaupés in the amazonas and caguán in putumayo basins and for some blocks in the eastern cordillera and the upper magdalena valley. 6.4.4 gas concentration (gc) 6.4.4.1 scenario 1 gc was estimated by correlating information regarding coal-bearing basins in alberta, canada (appendix 6-2), considering their similarity with colombian basins. a gamma-type distribution gave the best fit according to statistical analysis (table 6-8). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.92 n/r n/r 139.6 n/r n/r was not rejected m.b. 36.9 0 table 6-8. . gamma distribution fit parameters and goodness-of-fit test results applied to the gas concentration data used in estimating cbm in scenario 1. estimated ̂ ̂ are gamma distribution regarding form and scale parameters. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 105 6.4.4.2 scenario 2 the uptc (2010) has proposed a range of gc obtained by means of canister desorption tests, this also being supported by information from basins around the world and by estimates for producing methane from different types of coal. such average range is moderate (60-100 ft3/ton) and weights all colombian coal at a maximum depth of 300 m (figure 6-4). table 6-9 gives some of the results which uptc (2010) obtained for the samples analysed. a uniform g_c distribution was selected in this scenario according to the nature of the uptc data (2010). figure 6-4. estimating maximum methane production regarding type of coal and depth. modified from eddy et al. (1982). parameter range permeability 0.5 – 50.0md porosity 5 – 12 % range of coal high-volatile bituminous b – sub-bituminous a coal thickness x strata 30 cm – 2 m net thickness 6 – 8 m expected gas concentration 60-100 foot3/ton table 6-9. values for some parameters analysed regarding pithead coal samples in boyaca. (uptc, 2010). 6.4.4.3 scenario 3 the values reported by the uptc (2010) were very low, being obtained from pithead samples. such information could reflect altered strata as a consequence of mining activity in the exploitation front from which the samples were extracted. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 106 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia a triangular gc distribution having extreme and average data regarding pithead gc pattern was estimated from these observations, associated with gc pithead pattern (table 5-10). parameter value (ft3/ton) minimum 0.29 most probable 1.90 maximum 1.14 table 6-10. estimated parameters for gas concentration used in calculating the cbm. a triangular distribution was assumed. 6.4.5 cbm in colombia using the monte carlo method for evaluating resources the coalbed methane (cbm) for colombian basins, according to the assumed hypotheses and equation 6-1 in this document, led to the following results for the three scenarios considered here (tables 6-11, 6-12 and 6-13. these have already been affected by the environmental factors listed in table 2-1. figure 6-5 gives the results for scenario 2 and figure 6-6 compares them to estimates for other countries. basin p10 p50 p90 (bcf) (bcf) (bcf) amagá 516 96 4 non-prospective areas 2,557 480 24 caguán putumayo 167 31 2 catatumbo 682 128 7 cauca patía 721 135 7 cesar ranchería 17,713 3,296 161 the eastern cordillera 4,585 867 44 guajira 832 156 8 the eastern llanos and amazonía 573 107 5 sinú san jacinto 33,287 6,339 313 the lower magdalena valley 252 47 2 the middle magdalena valley 4,448 830 42 the upper magdalena valley 11,178 2,100 107 total 77,511 14,612 725 table 6-11. cbm in scenario 1. basin p10 p50 p90 (bcf) (bcf) (bcf) amagá 147 123 98 non-prospective areas 721 599 480 caguán putumayo 47 39 31 catatumbo 193 161 128 cauca patía 204 170 136 cesar ranchería 4,994 4,160 3,327 the eastern cordillera 1,297 1,080 865 guajira 233 195 156 the eastern llanos and amazonía 162 135 108 sinú san jacinto 9,499 7,913 6,332 the lower magdalena valley 71 60 48 the middle magdalena valley 1,259 1,049 840 the upper magdalena valley 3,163 2,636 2,107 total 21,990 18,319 14,655 table 6-12. cbm in scenario 2. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 107 basin p10 p50 p90 (bcf) (bcf) (bcf) amagá 2.0 1.8 1.0 non-prospective areas 12.1 8.8 5.2 caguán putumayo 0.9 0.6 0.0 catatumbo 2.8 2.4 0.9 cauca patía 3.0 2.5 1.0 cesar ranchería 82.0 60.7 35.0 the eastern cordillera 21.4 15.8 8.9 guajira 3.9 2.8 2.0 the eastern llanos and amazonía 2.3 2.0 1.5 sinú san jacinto 156.1 115.5 67.3 the lower magdalena valley 1.0 0.9 1.0 the middle magdalena valley 21.0 15.3 9.0 the upper magdalena valley 52.0 38.4 22.3 total 360.6 267.4 155.1 table 6-13. cbm in scenario 3. figure 6-5. map of cbm for colombian basins in scenario 1. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 108 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 6-6. cbm for the colombian basins in scenario 1. figure 6-7. comparing cbm in colombia in the first two scenarios with that for other countries. coalbed methane earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 109 6.5 conclusions o the san jacinto basin in sinú, cesar ranchería, the upper magdalena valley, the eastern cordillera, the middle magdalena valley and the guajira basins has the greatest resources of cbm; o p50 values for the three scenarios were 18,319.0, 14,612.0 and 267.4 bcf. the last value was most improbable given that the gc values measured for coal mine exploitation underestimate resources cbm production. such data, however, could be used as reference for industrial safety; and o the cbm resources estimated in this work (14,612-18,319 bcf) were comparable with values estimated by d. little (2008); (17,800 bcf). 6.6 bibliography beaton, a. (2003). production potential of coalbed methane resources in alberta. alberta geological survey, alberta energy and utilities board, alberta. beaton, a., pana, c., chen, d., wynne, d., & langenberg, c. w. (2002). coalbed methane potential of upper cretaceous tertiary strata, alberta plains. alberta geological survey, alberta energy and utilities board, alberta. ingeominas. (2004). el carbón colombiano: recursos, reservas y calidad. bogota. ortega, j. (2008). reservorios de gas asociado al cabon (cbm). contacto spe(30), 8-11. rottenfusser, b., langenberg, w., mandryk, g., richardson, r., fildes, b., olic, j., et al. (2002). regional evaluation of the coalbed methane potential in the plains and foothills of alberta, stratigraphy and rank study (digital version). alberta g. s., a. energy and utilities board, alberta. stach, e. (1982). stach's textbook of coal petrology. berlin: borntraeger. tonnsen, r. r., & miskimins, j. l. (2010). a conventional look at an unconventional reservoir: coalbed methane production potential in deep environments. search and discoveries(80122), 5. uptc. (2010). mediciones de gas metano asociado al carbón mediante pruebas de desorción con canister. tunja: universidad pedagógica and tecnológica de colombia. wynne, d., & beaton, a. (2003). alberta geological survey. visited on the 4th of march 2011: cbm coal database for the alberta plains area: http://www.ags.gov.ab.ca/publications/abstracts/dig_2003_0001.html 6.7 appendixes digital files in the anh's document center:  “áreas carboniferas por cuenca.klm”  “base de datos gac alberta canadá.xlsx”  “base de datos gac colombia.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7 tar sands 7.1 general comments the organic material present in the subsoil may become submitted to alteration such as maturing or degradation; maturing involves its thermal transformation and degradation involves chemical changes produced by bacterial activity or washing by water (chillingarian & yen, 1978). maturing leads to an increase in saturated hydrocarbons (hc) and a reduction in nso compounds (involving nitrogen, sulphur and oxygen). degradation, on the other hand, leads to a reduction in saturated hc content, especially n-paraffin and increased nso and asphaltenes (figure 7-1). figure 7-1. diagram of gas and oil generation from organic material during lithogenesis. a) shows production areas regarding depth; the letters in each area indicate the coal ranges used as catagenesis stage indicators (d – sub-bituminous to bituminous having abundant volatile matter, c, g – bituminous having abundant volatile matter, c to b, zh – bituminous with abundant volatile matter, k –bituminous having intermediate volatile matter, os – bituminous having intermediate to low-volatile matter, t – bituminous having lowanthracitic volatile matter). b) gas and oil generation intensity regarding vitrinite reflectance where po is source-rock from the h/c ratio. c) is the percent ratio for gas and oil generation regarding depth (modified from chillingarian and yen, 1978). tar sands earth sciences research journal, volume 16, special issue, april, 2012 112 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7.1.1 bitumens organic material in bituminous substances has been considered to be residue from organic life occurring in the form of viscous impregnations in sand, limonites, shale and carbonates (dolomites and limestones). it could be said that kerogen (the insoluble organic material incorporated in source-rock sediments) consists of small units which are similar to asphaltic bitumens and thus lead to describing bitumens as low-mature (or extremely degraded) hydrocarbons. figure 7-2. bitumen outcrop in the middle magdalena valley, sector between honda and norcasia. bituminous substances vary from liquid petroleum to heavy crude oil, from viscous asphalts to solid asphaltites and asphaltic pyrobitumens. the types of organic compounds which can be extracted by solvents (especially when the solvent is carbon dioxide) are called bitumens (figure 7-2). actually, the amount of these bitumens is low compared to the large amount of insoluble kerogenic material which remains in the rock (chillingarian & yen, 1978). properties such as viscosity and n-paraffin (n-alkane) content can indicate whether the bitumen is a residue from low level thermal maturing or degradation. 7.1.2 asphalts asphalt represents a compound material containing asphaltenes, waxes and resins. these can be converted into asphaltites or asphaltides during meteorisation and metamorphism. deposits of this type as such are not the product of biodegradation; their formation is due to the incompatibility of two simultaneously produced liquid phases: light paraffin and asphaltene during thermal processes, such as normal maturing. such phases cannot coexist and the asphalthenic phase becomes separated from the liquid phase (chillingarian & yen, 1978). tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 113 in terms of hydrocarbon type, conventional oil contains 30% paraffin, 50% naphthene, 15% aromatic and 5% asphalt hydrocarbons (figure 7-3). asphalts are also produced as direct residues from the distillation of conventional oil, thermal fragmentation and thinning or the oxidation of crude residues by exposure to the air (chillingarian & yen, 1978). figure 7-3. the ratio between boiling point and the number of carbons for pure hydrocarbons (left), and the composition of several petroleum fractions (right). red square corresponds to area of asphaltic bitumen. modified from chillingarian & yen (1978). 7.1.3 tar sands the term tar sands (even though being inappropriate, since tar refers to a substance produced by destructive distillation) attempts to group bitumen-bearing sedimentary rocks (whether consolidated or not), consisting of highly viscous solids or semisolid hydrocarbons. even though tar sands‟ hydrocarbons are extremely viscous, their synthesis is relatively simple and leads to conventional light oil being obtained. traditional processes can be modified in refineries for treating these types of substances. tar sands earth sciences research journal, volume 16, special issue, april, 2012 114 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7.1.4 classifying tar sands the classification of asphalts and bitumens‟ physical properties remains ambiguous. viscosity is variable in each type of compound; the h/c ratio becomes reduced when ranging from natural gas and conventional oil to asphalts and then to kerogen, but this is not a weighty parameter for differentiating them. environmental factors determine how this is done, whether by degradation (e.g. oxidation, bacteria, being washed by water) or maturing, allowing such ambiguity to be resolved. figure 7-4 shows a conventional classification. figure 7-4. classification of bituminous substances, from meyer & witt (1990). geologically, tar sands deposits may be broadly classified into two members: large updip deposits just below the surface of the water on the edge of foreland basins and smalland medium-sized deposits associated with loss of seal rock integrity. both types are usually linked to faults and inconformity and can also be associated with oozes, springs, tufa deposits and mud volcanoes. tar sands deposits often occur on the surface or at depths just below the surface (<200 m) where hydrocarbon deposits come into contact with the atmosphere, surface aquifers or surface water (chillingarian & yen, 1978). tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 115 7.1.5 the nature of the impregnation o asphaltic arenites: are mainly presented in strata a few metres wide, having high clay or fine sand content. the amount of bituminous material in rock ranges from 3% to 15%, having penetration at temperatures above 15.6°c. the main deposits of this type are found in canada (i.e. athabasca) and venezuela (chillingarian & yen, 1978); o asphalt having an appreciable amount of inert and organic materials: 36% mineral concentration and 54% bitumen, with 10% water and organic material. there are deposits of this type in trinidad, off the coast of venezuela (chillingarian & yen, 1978); and o asphaltites: naturally-occurring complex hydrocarbons, where bitumen percentage is around 90% (chillingarian & yen, 1978). 7.1.6 tar sand accumulation in colombia during phases involving multiple hydrocarbon production and expulsion from basins, large volumes migrated towards their margins, leading to the accumulation of heavy crude oil at relatively superficial depths on the eastern llanos and in putumayo. large areas of tar sands (bitumens, asphalts, etc) were also formed as hydrocarbons migrated, reached the surface and became exposed to biodegradation and evaporation. such deposits extend from florencia all along the front of the mountains as far as san vicente and cross la macarena until reaching the southern part of the llanos basin (figures 7-5 and 7-6). the manifestations are mainly found in the eastern cordillera area, in the boyacá, santander and cundinamarca departments and their respective piedmont areas, in both the upper and middle magdalena valley, putumayo (the garzón massif) and parts of meta (la macarena) and casanare. bitumen deposits are usually found in cretaceous (guadalupe, guaduas, la luna, villeta, une and chipaque), palaeogene and neogene units (picacho, socha and cacho) and some quaternary deposits associated with active oozes and exhumed deposits where the light fraction escaped. lithologically, they are found in medium-grained arenites to conglomeratic facies and, to a lesser extent, in siliceous limonites to carbon-bearing shale and schist. table 7-1 shows some geochemical parameters regarding manifestations in colombia. tuta boyaca velez santander chaparral tolima san jose guaviare tado chocó ash 84.44 5.01 0.55 n/r 7.26 sulphur 0.51 0.75 3.2 0.49 n/r volatile material 7.76 12.31 n/r n/r 46.77 fixed carbon n/r 80.72 n/r n/r 44.9 nickel n/r n/r 185 12.8 n/r vanadium n/r n/r 89 n/r n/r iron n/r n/r n/r 0.22 n/r table 7-1. chemical parameters regarding some tar sands manifestations in colombia, taken from ingeominas reports (see associated bibliography). n/r not recorded. tar sands earth sciences research journal, volume 16, special issue, april, 2012 116 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 7-5. asphalt outcrop, la cristal rural area san josé del guaviare, taken from duque (2002). figure 7-6. thin sections of asphaltic sample from la cristal rural area san josé del guaviare. a. crossed nicols (x 25) and b. parallel nicols (x 25), taken from duque (2002). 7.1.7 extraction in colombia information regarding bitumen exploitation has been available since the beginning of the 20th century. for example, 1928 reports mention an analysis of their quality in boyacá and cundinamarca (grosse, 1928). later studies dealt with the colombian cretaceous period and the inventory of manifestations. the colombian geological service (servicio geológico nacional) made a national mining inventory during the 1940s which identified and sampled occurrences in other departments (instituto geologico nacional, 1948). tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 117 currently, the colombian geology and mining institute (instituto colombiano de geología and minería ingeominas) has catalogued asphalts (table 6-2), using the official classification of minerals in colombia (colombian ministry of mines and energy, 2003). exploration and exploitation licences are granted according to the mining code and regulations laid down by the ministry of mines and energy, the ministry of the environment and autonomous corporations monitoring the environmental impact of mining/farming techniques on each region. official classification of minerals (ingeominas) group 153 sandstones, conglomerates, sands, boulders, gravels, crushed sandstone, split or crushed rock or stones, bitumen and natural asphalts class 1533 bitumen and natural asphalts subclass 1533000 1533001 153300 title bitumen and natural asphalt; asphaltites asphaltic and rocks. natural asphalt or asphaltites. all other asphaltic rocks (except for subclass 2030). unit ton ton ton ciiu rev. 3 a.c. 1490 harmonised system 2714.90 ciiu dane 29091200 table 7-2. official classification of minerals bitumen and natural asphalt (ministry of mines and energy, sistema nacional de information minero colombiano-simco, 2003). 7.1.8 tar sand accumulations throughout the world there are large deposits around the world; the main reserves are located in canada (athabasca) and venezuela (faja del orinoco), each having reserves almost equal to the reserves of conventional crude in the rest of the world. tar sand reserves could represent up to two thirds of crude from hydrocarbons, accounting for around 3.6 trillion barrels of recoverable oil compared to 1.75 trillion barrels in the world for conventional oil (dyni, 2010). the map in figure 7-7 and table 7-3 show the main tar sand deposits in the world. figure 7-7. main tar sand deposits around the world modified from chillingarian & yen (1978). tar sands earth sciences research journal, volume 16, special issue, april, 2012 118 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia country deposit reservoir rock age area thickness bitumen (ha) sq. mi ft average sat. (wt%) ºapi 60ºf (%) sulphur canada mcmurray wabiskaw, alberta 2,326,942 9,000 0-300 175 … … … bluesky gething, alberta 485,623 1,875 0-400 … … … grand rapids, alberta 445,154 1,625 400 280 … … … total "athabasca" tar sands l. cretaceous 3,237,485 12,500 150 2-18 10.5 4.5 melville island n.w.t triassic ? ? 60-80 ?-16 10 0.9 2.2 eastern venezuela oficina temblador tar belt oligocene 2,326,942 9,000 3-10 10 malagasy bemolanga triassic 38,850 150 80-300 100 10 0.7 u.s.a asphalt ridge, utah oligocene and u. cretaceous 4,452 … 11-254 24-200 98 100 11 8.6 12 0,5 sunnyside, utah u. eocene 13,881 … 10-350 9 10 12 0.5 albania selenizza mio pliocene 2,147 8 33-330 50 8-14 4.6 13.2 6,1 u.s.a whiterocks, utah jurassic 769 3 900-1,000 10 12 0,5 edna, california mio-pliocene 2,669 10 0-1,200 250 9-16 13 4.2 peor springs, utah u. eocene 702 3 1-250 34 9 venezuela guanoco recent (alluvial) 405 2 2, 9 4 64 8 5.9 trinidad la brea u. miocene 51 … 0-270 135 54 1 2 6.0 8.0 u.s.a santa rosa, new mexico triassic 1,874 7 0-100 20 48 sisquoc, california u. pliocene 71 … 0-185 85 1418 4 8 asphalt, kentucky pennsylvanian 2,833 11 5-36 15 8 10 rumania derna pliocene 186 … 625 15-22 0.7 u.s.s.r cheildag, kazakhstan m. miocene 33 … 200 5 13 u.s.a davis dismal creek, kentucky pennsylvanian 769 3 1050 15 5 santa cruz, california miocene 486 2 5 50 11 1012 kyrock, kentucky pennsylvanian 364 … 1540 20 68 table 7-3. main tar sands deposits around the world, taken from chillingarian & yen (1978). 7.2 data and hypotheses 7.2.1 international database the set of compiled data appended as part of this document in a file labelled appendix 7-1 contains information regarding different oil sands deposits in alberta canada. the data contain information about wells, cores, lithological, stratigraphical and geochemical analysis as well as forms of exploration and exploitation of the resource in this region. some of this database‟s contents were used when estimating tar sands potential in colombia for establishing a comparative reference framework. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 119 7.2.2 national (colombian) database the set of data so compiled appended as part of this document in a file labelled appendix 7-2 contains the information used for calculating weighting factors per resource scenario and evaluation category, that used for evaluating the tar sands areas, the data used for estimating bitumen, thickness, density and content distribution and the results for each test of goodness of fit and correlations. the information about tar sand occurrences in colombia was obtained from three sources: o the colombian geological atlas (ingeominas, 2007) and mining land registry information concerning mining titles and requests up to 2008, as well as publications regarding mining studies, manifestations, occurrences and inventories in colombia; o undergraduate theses from the universidad nacional de colombia and other universities associated with asphalt manifestations in colombia; and o the anh seeps inventory, as well as information from the colombian geochemical atlas (agencia nacional de hidrocarburos, 2010). 7.2.3 hypotheses all this information was sifted for identifying representative areas having asphalt and bitumen manifestations which were considered as evidence of areas having ytf tar sands. a weighting factor was then applied to these areas thereby reducing uncertainty regarding the effective presence of tar sands in line with the following hypotheses. 7.2.3.1 hypothesis 1 the control points associated with tar sand manifestations, derived from available information (ingeominas, anh, universities, etc), becomes a factor representing effective tar sands potential in colombia; and 7.2.3.2 hypothesis 2 areas of representativity are derived from control points from a system for classifying resources and reserves as being either identified (measured 0-250 m, indicated 250-750 m and inferred 750– 2,250 m) or tar sands resources (hypothetical 2,250-5,000 m). figure 7-8 illustrates the scenarios which had to be evaluated; tar sands earth sciences research journal, volume 16, special issue, april, 2012 120 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 7-8. system for classifying resources and reserves (ecocarbon, 1995). 7.2.3.3 hypothesis 3 the thickness, density and bitumen content values for tar sands units in colombia have a pattern which fits a statistical distribution supported by data regarding manifestations throughout colombia (figure 7-9). such distribution is shown in the digital file in appendix 7-2. 7.2.3.4 hypothesis 4 a ratio between a particular basin‟s total asphalt area and its total area may be defined from the available information regarding tar sand occurrences; this ratio will lead to defining the area of the resource in on-shore (continental) basins lacking tar sands information. figure 7-9 illustrates this hypothesis. figure 7-9. the graph shows the ratio between each basin‟s area regarding a specific resource‟s total area (measured, indicated, inferred and hypothetical) for a particular basin (red dots), determined from central tendency (blue line). tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 121 7.3 methodology the following equation was used for evaluating the tar sands potential in each colombian basin: (6-1) vtar: original in situ oil (mmbbl) a: area of the resource (m2) h: tar sand layer thickness (m) ρroca: rock density (g/cm3) pbitumen: percentage of bitumen (p/p) ρbitumen: total bitumen density (g/cm3) the procedure for estimating the resource consisted of the following stages: o a bibliographic review of different sources was made: international data regarding tar sand deposits was used as reference information for evaluating manifestations in colombia. mining data regarding wells, records, lithology, manifestations, occurrences or mining inventories for the different regions of colombia were then reviewed.  a database was then compiled from the inventory of manifestations. the compiled information contained coordinates, place of occurrence, year of report and bitumen thickness, percentage, lithology and chemical properties (appendix 7-2);  a map of tar sands occurrences per basin was drawn. o frequency histograms were drawn for evaluating the pattern and associating distributions with data regarding bitumen thickness, density and percentage of bitumen concerning occurrences in the inventory.  different sedimentary environments were considered for rock density, according to the inventory, in an attempt to define a range of values for bitumen-bearing rock; and  goodness-of-fit tests were used, considering the best distribution for each variable, and parameters were estimated for such distribution and their respective confidence intervals. o the map of occurrences was used for classifying resources and reserves, producing areas having a ratio defined by identified (measured 0-250 m, indicated 250-750 m and inferred 750-2,250 m) and resources (hypothetical 2,250-5,000 m).  overlying areas were eliminated for calculating the total area containing the resource; and tar sands earth sciences research journal, volume 16, special issue, april, 2012 122 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia  a ratio between basin area and total area was established for the resource in a particular basin. such estimation led to establishing areas of the resource in basins for which there was only minimal or zero information. o monte carlo simulations were made using equation 7-1 for calculating tar sands potential, taking rock thickness and density and bitumen density and percentage as distribution factors. the indicated and hypothetical categories were chosen for presenting the results after having observed the estimated resources‟ pattern in the four evaluation categories (measured, indicated, inferred and hypothetical) in the proposed scenarios. figure 7-10 gives an example of these evaluation ratios. figure 7-10. a map illustrating the areas associated with the resource in an area limited to three basins (la macarena). four circles in each tar sand occurrence site indicate projections for measured (0–250 m), indicated (250–750 m), inferred (750–2,250 m) and hypothetical resources (2,250–5,000 m). the colours represent different basins. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 123 7.4 results the random variables used in equation 7-1 were subjected to statistical analysis leading to ascertaining their representativity and identifying the distribution of probability best fitting the data. these tests led to establishing confidence intervals for the distribution parameters and degree of certainty when calculating goodness-of-fit (test statistic). this report only contains information concerning the results for the best fits and estimations; the rest are reported in the appended digital files in appendixes 7-1 and 7-2. 7.4.1 tar sand occurrences the manifestations are shown on the tar sands occurrence map (figure 7-11). manifestations have been discriminated on this map according to the source for the information. figure 7-11. tar sand occurrences per basin in colombia, indicating the type of source. tar sands earth sciences research journal, volume 16, special issue, april, 2012 124 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7.4.2 thickness analysing thickness in the manifestations inventory suggested a logistical-type distribution from the parameters shown in table 7-4; 30 m maximum thickness, 5 m mean and 7 m standard deviation were found. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 3.41 2.21 4.61 3.26 2.67 3.98 n/r n/r n/r n/r table 7-4. goodness-of-fit parameters regarding logistical distribution and goodness-of-fit test results applied to the thickness data used in estimating tar sand potential. estimated parameters ̂ ̂ are logistical distribution regarding location and scale. “n/r” refers to parameters which, as they went beyond the algorithm‟s range of calculations, were not reported when making the statistical estimation. 7.4.3 bitumen density analysing the data regarding bitumen density in the manifestations inventory revealed a lognormal pattern for occurrence frequency (table 7-5). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.12 0.09 0.16 0.09 0.07 0.12 was not rejected m.b. 2.70 0 table 7-5. lognormal distribution parameters and goodness-of-fit test results applied to the bitumen density data used in estimating tar sand potential. estimated parameters ̂ ̂ are lognormal distribution regarding the mean and standard deviation, respectively. m.b refers to a very low probability of the null hypothesis being accepted or rejected. 7.4.4 rock density according to the aforementioned inventory, different sedimentary environments were considered having different porosity for establishing a range of density for bitumen-bearing rock. a uniform distribution was considered ranging from 2.21 g/cm3 to 2.73 g/cm3. 7.4.5 area the areas were defined according to criteria for evaluating identified and resources per basin (figures 7-9 and 7-10; table 7-6). basin area (ha) measured (ha) indicated (ha) inferred (ha) hypothetical (ha) non-prospective area 32,837,455 3,135 18,622 109,001 274,009 caguán putumayo 11,030,407 541 3,925 33,060 127,556 cauca patía 1,282,331 20 157 1,414 5,900 los cayos 14,475,501 20 157 1,414 6,263 cesar ranchería 1,166,868 20 157 1,414 6,263 chocó 3,858,198 196 1,142 6,471 18,513 the eastern cordillera 7,176,620 1,240 8,977 66,868 205,519 the eastern llanos 22,560,327 1.120 7,443 46,401 140,138 tumaco offshore 3,455,268 20 157 1,414 6,263 sinú san jacinto 3,964,459 314 2,513 22,070 83,724 tumaco 2,373,242 32 183 1,491 6,405 the middle magdalena valley 3,294,942 1,008 7,684 54,802 154,503 the upper magdalena valley 2,151,284 941 7,210 61,963 226,707 vaupés amazonas 15,486,731 266 1,932 15,969 50,227 table 7-6. areas of the resource per basin in colombia. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 125 a ratio was then established between the area of a particular basin‟s resources and such basin‟s total area for determining maximum the area of ytf resources for basins lacking such information (hypothesis 4). figure 7-12. ratio between areas of the resource per basin in colombia regarding function and area per basin. basin area (ha) measured (ha) indicated (ha) inferred (ha) hypothetical (ha) amagá 282,493 5 60 644 2,639 catatumbo 771,501 14 167 1,767 7,181 guajira 1,377,892 27 306 3,173 12,767 urabá 944,895 18 206 2,168 8,784 the lower magdalena valley 3,801,740 93 927 8,940 34,581 table 7-7. areas of resource projected for those basins lacking occurrence data 7.4.6 potential of tar sands in colombia the results of the monte carlo simulation for calculating the tar sands potential are shown below, as well as the parameters defined in equation 7-1 and the previously-defined distributions. these results were previously weighted with environmental factors shown in table 2-1. basin indicated (mmbbl) p10 p50 p90 amagá 7.2 1.0 0.2 non-prospective area 1,949.0 263.4 44.6 caguán putumayo 437.0 59.1 10.0 catatumbo 19.0 2.6 0.4 cauca patía 19.0 2.6 0.4 cesar ranchería 19.0 2.6 0.4 chocó 134.9 18.2 3.1 the eastern cordillera 965.8 130.5 22.1 guajira 36.1 4.9 0.8 the eastern llanos 877.7 118.6 20.1 sinú san jacinto 272.2 36.8 6.2 tumaco 21. 6 2.9 0.5 urabá 24.1 3.3 0.6 the lower magdalena valley 111.9 15.1 2.6 the middle magdalena valley 927.5 125.3 21.2 the upper magdalena valley 807.9 109.2 18.5 vaupés amazonas 182.8 24.7 4.2 total 6,812.6 920.6 155.7 table 7-1. potential of tar sands for the indicated resources evaluation category. tar sands earth sciences research journal, volume 16, special issue, april, 2012 126 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin hypothetical (mmbbl) p10 p50 p90 amagá 318.6 43.1 7.3 non-prospective area 28,678.4 3,875.4 655.5 caguán putumayo 14,203.0 1,919.3 324.6 catatumbo 816.9 110.4 18.7 cauca patía 711.9 96.2 16.3 cesar ranchería 756.1 102.2 17.3 chocó 2,187.7 295.6 50.0 the eastern cordillera 22,109.1 2,987.7 505.3 guajira 1,035.9 140.0 23.7 the eastern llanos 16,526.0 2,233.2 377.7 sinú san jacinto 9,067.2 1,225.3 207.2 tumaco 753.9 101.9 17.2 urabá 1,027.7 138.9 23.5 the lower magdalena valley 4,174.3 564.1 95.4 the middle magdalena valley 18,650.3 2,520.3 426.3 the upper magdalena valley 25,404.0 3,432.9 580.6 vaupés amazonas 4,752.8 642.3 108.6 total 151,173.9 20,428.5 3,455.1 table 7-2. potential of tar sands for the hypothetical resources evaluation category. figure 7-1. map of tar sands potential for the indicated resources evaluation category. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 127 figure 7-14. potential of tar sands for the indicated resources evaluation category. figure 7-2. comparing the resources estimated for colombia with those published for other countries. tar sands earth sciences research journal, volume 16, special issue, april, 2012 128 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 7.4.7 sensitivity analysis it was established that target formation thickness was the random variable having the greatest weighting on the simulation from the data compiled concerning manifestations in colombia, and according to the nature of distribution functions (rock thickness and density, bitumen density and percentage) (figure 7-16); exploratory exercises will consequently have to make greater efforts at defining them (followed by bitumen content and rock density). the areas' representativity will also have to be validated due to the nature of the hypotheses used in this work. figure 7-3. percentage analysis of the sensitivity of variables used for calculating the tar sands potential in each basin of colombia. 7.5 conclusions the sensitivity analysis showed that thickness was the most sensitive variable when calculating the tar sands potential (67,889); the eastern cordillera basin had the greatest prospectivity for tar sands; tar sands potential for the whole of colombia, ranging from 156 to 6,813 mmbbl (p10 to p90), was lower than that reported by d. little (2008) (67,889 mmbbl); and the potential of the plains areas, such as the eastern llanos‟ basin, could lie on the eastern flank; however, information regarding bitumen manifestations is still lacking for this area in particular. 7.6 bibliography agencia nacional de hidrocarburos. (2010). organic geochemistry atlas of colombia. bogotá: universidad nacional de colombia. bachu, s. (1987). subsurface disposal related to in situ oil sands projects. alberta research council, alberta. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 129 carbocol. (1992). reconocimiento geológico del area de tadó, departamento de chocó: mapa de estaciones y afloramientos de asfaltita. bogota. chillingarian, g. v., & yen, t. f. (1978). bitumens, asphalts and tar sands. new york: elsevier scientific publishing company. duque, h. r. (2002). estudio geológico y geoquímico de un prospecto de arenitas asfálticas en la región del capricho, municipio de san josé del guaviare, departamento del guaviare, colombia. undergraduate thesis, universidad nacional de colombia, facultad de ciencias, bogotá. dyni, j. r. (2010). survey of energy resources. world energy council. ecocarbon. (1995). el carbón, normas generales sobre muestreo y analysis de carbones. bogotá. grosse, e. (1928). informe sobre los asfaltos en la parte central y meridional del departamento de boyacá. ministerio de minas y petroleos, servicio geológico nacional, bogotá. gulf canada resources limited. (1993). phase i. study to asses the potential of minerals and metals in alberta oil sands deposits. calgary. hein, f. j. (2006). subsurface geology of the athabasca wabiskaw-mcmurray succession: lewisfort mcmurray area, northeastern alberta. earth sciences report 6, alberta geological survey, alberta. hitchon, b. (1993). geochemical studies 4 physical and chemical properties of sediments and bitumen from some alberta oil sands deposits. alberta research council. ingeominas. (1933). estructura geologica de la parte suroriente de la conseción chaux fulson, santander del norte. colombia. bogotá. ingeominas. (1970). ocurrencias minerales en el nororiente antioqueño. medellín. ingeominas. (1970). recursos minerales de parte de los departamentos de norte de santander y santander. boletin geológico xviii nº3, bogotá. ingeominas. (1972). ocurrencias minerales en los cuadrangulos j-11 (chiquinquira), j-12 (tunja), j-13 ( sogamoso) y parte del j-10 ( la palma) y el k-10 ( villeta) . bogotá. ingeominas. (1976). informes de minerales no metalicos en el departamento de tolima. ibague. ingeominas. (1976). ocurrencias minerales en el departamento del huila. neiva. ingeominas. (1976). ocurrencias minerales en las regiones sur y oriental del departamento del tolima. ibagué. ingeominas. (1988). estandarizacion del metodo de asfaltenos, de bitumen y de carbón . bogotá. ingeominas. (1995). visita técnica a la manifestación de asfalto en el municipio de santa rosa, vereda concepción. cauca. bogotá. tar sands earth sciences research journal, volume 16, special issue, april, 2012 130 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia ingeominas. (1999). inventorio minero nacional departamento de norte de santander. bogotá. ingeominas. (1999). proyecto minero nacional departamento de boyacá. bogotá. ingeominas. (2006). inventorio minero macizo de garzón. . bogotá. ingeominas. (2007). mapa geológico de colombia. escala 1:2,800.000. bogotá. instituto geologico nacional. (1948). estudio economico de los asfaltos de guaní, en chaparraltolima. bogotá. instituto geológico nacional. (1954). descubrimiento de petroleo en tolu, bolivar. bogotá. instituto geologico nacional. (1957). carbones de landazari asfaltita de gualio municipio de velez, departamento de santander. bogotá. instituto geologico nacional. (1960). informe de investigación de yacimientos minerales en colombia. bogotá. laboratorio de minas y petroleos. (1932). informe sobre los resultados de la analisis de dos muestras de fuel oil y una de asfalto. bogotá. meyer, r.f. & witt, w. (1990). definition and world resources of natural bitumens. u.s. geological survey bulletin no. 1944. ministerio de minas y energía. (2003). sistema nacional de información minero colombiano simco. consulted on the 7th xxx 2011, de clasificación oficial de minerales: http://www.simco.gov.co/simco/politicasdelsector/formatob%c3%a0sicominero/tabid/90/default. aspx servicio geologico nacional. (1942). combustibles minerales del departamento de santander. ministerio de minas and petróleos, bogotá. servicio geologico nacional. (1946). manifestacions de petroleo y asfalto en las rocas eruptivas y esquitos cristalinos del valle medio del magalena, colombia. ministerio de minas and petróleos , bogotá. servicio geologico nacional. (1950). comision a las carboneras del guatiquía limitada municipio de villavicencio. ministerio de minas and petróleos. servicio geologico nacional. (1950). estudios de los yacimientos de asfalto de nemal cundinamarca. ministerio de minas and petróleos, bogotá. servicio geologico nacional. (1950). informe sobre los asfaltos de guaduas cundinamarca. ministerio de minas and petróleos, bogotá. servicio geologico nacional. (1959). sobre la geologia de la parte sur de la macarena . ministerio de minas and petróleos, bogotá. tar sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 131 servicio geológico nacional. (1961). roca asfaltica al norte de villavicencio, meta. ministerio de minas and petróleos, bogotá. servicio geologico nacional. (1966). informe sobre la cuenca hullera "tunja paipa duitama". ministerio de minas and petróleos, bogotá. servicio geologico nacional. (1969). mapa de yacimientos de asfalto. ministerio de minas and petróleos, bogotá. servicio geológico nacional. (1976). ocurrencias minerales en el departamento de cundinamarca compilation. ministerio de minas and petróleos, bogotá. 7.7 appendixes digital files in the anh's document center:  “base de datos athabasca tar sands.xlsx”  “base de datos tar sands colombia.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8 oil shale 8.1 general comments oil shale is a fine-grained, organic matter-containing, sedimentary rock, producing substantial quantities of oil and gas when it is subjected to destructive distillation. most of the organic matter contained in oil shale is insoluble in ordinary organic solvents; it must therefore become decomposed by heating to enable it to be extracted. the amount of oil which can be recovered from oil shale deposits by conventional extraction varies from 4% to just over 50% of the rock‟s total weight or between 10 and 150 gallons of oil per ton of rock and, although not all oil shale has good output/yield, most have small amounts of soluble organic matter (bitumen) and many contain seams, veins or cavities saturated with solid or viscous hydrocarbons, thereby making them attractive (dyni, 2006). the oil shale market was not competitive with coal, oil or natural gas until 2004; however, the extraction of this resource is very common in countries having easily-exploitable deposits and which lack other fossil fuel resources. some oil shale deposits also contain minerals and metals such as alum, nahcolite, dawsonite, sulphur, vanadium, zinc, copper and uranium thereby providing them with added value (yen & chillingarian, 1976). 8.1.1 origin and formation the main oil shale formation environments would include large lakes, shallow seas on continental shelves and continental shields and small lakes, marshes and lakes associated with producing coal. the contemporary deposit of fine-grained minerals and the degradation of biota-derived organic products in such settings have led to oil shale formation. an abundance of organic matter, the early development of anaerobic conditions and the absence of destructive organisms, together with continuous sedimentation sometimes accompanied by subsidence, provide the necessary conditions for the compaction and diagenesis of organic matter-rich strata. chemical activity at low temperatures (around 150° c) causes a loss of the volatile fraction, eventually producing a sedimentary rock rich in refractory organic residues (yen & chillingarian, 1978). 8.1.2 types of oil shale oil shale has been given different names over the years such as cannel coal, boghead coal, alum shale, stellarite, albertite, bituminite, wolongite, bituminous schiates, torbanite and kukersite; some of these names are still used for certain kinds of oil shale. hutton (1991), who pioneered the use of fluorescence microscopy in studying oil shale deposits in australia, devised a classification which was mainly based on the origin of organic matter by adapting petrographic terms from the terminology used for coals. this has proved very useful in correlating the different kinds of organic matter present in oil shale with the chemistry of the hydrocarbons derived from them (dyni., 2006). oil shale earth sciences research journal, volume 16, special issue, april, 2012 134 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia hutton (1991) divided organic matter-rich sedimentary rocks into three different groups: humic coal and carbonaceous shale, bitumen-impregnated rocks and oil shale. he then subdivided the latter group into three subgroups based on the setting for their deposition: terrestrial, lacustrine (related to lakes) and marine (figure 8-1). figure 8-1. oil shale classification, adapted from hutton (1991). hutton (1991) recognised six specific types within the above three oil shale groups: cannel coal, lamosite, marinite, torbanite, tasmanite and kukersite. lamosite is brownish, greyish or dark grey to black. its main organic component is lamalginite derived from lacustrine plankton seaweed. minor compounds would include vitrinite, inertite, telalginite and bitumen. marinite has a marine origin and is a grey to dark grey. its main organic components are lamalginite and bitumen, mainly derived from marine phytoplankton. it contains small amounts of telalginite and vitrinite. marinites are usually deposited in epicontinental seas, such as shallow marine shelves or inland seas, where wave action is restricted and currents are minimal. torbanite, tasmanite and kukersite are related to specific kinds of seaweed originating the organic matter. torbanite is black and its organic matter mainly consists of telalginite derived from lipid-rich seaweed (botryococcus and related forms) found in freshwater or brackish lakes. it may contain small amounts of vitrinite and inertite. tasmanite is brown to black and its organic matter is mainly telalginite derived from unicellular seaweed called tasmanitid. kukersite is light-brown marine seaweed. its main organic component is telalginite derived from gloeocapsomorpha prisca, green seaweed (dyni, 2006). oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 135 8.1.3 determining the grade of oil shale oil shale‟s volumetric yield can be determined by different methods, their results being expressed in a variety of units. their calorific value (power) is obtained by using calorimeters. the calculated values are reported in units such as british thermal units (btu) per pound, calories per gram (cal/g), kilocalories per kilogram (kcal/kg) and megajoules per kilogram (mj/kg). although oil shale‟s calorific value is a useful and fundamental property of rock, it does not provide information regarding the amount of oil or gas that can be produced by destructive distillation. the grade of oil shale can be determined in laboratory retort tests by measuring the amount of oil produced from a shale sample. the modified fischer assay is a commonly used method for this task in the usa; it was first developed in germany and later adapted by the u.s. bureau of mines for oil shale analysis regarding the green river formation in western usa (stanfieldet & frost, 1949). the technique later became standardised as astmd-3904-80. fischer's method does not necessarily indicate the maximum amount of oil that can be produced by a given oil shale. other retorting methods for producing oil, such as the tosco ii process, can increase production by up to 100%. in fact, the hytort process can increase such production threefold to fourfold in some oil shale (schora, 1983; dyni jr, 2010). the fischer assay only gives an approximation of a deposit‟s potential energy. rock-eval and mass balance represent the newest techniques for oil shale evaluation; both of them give more complete information regarding the grade of a particular oil shale. despite this, the modified fischer assay remains the most widely used one (yen & chillingarian, 1976). 8.1.4 oil shale exploitation extracting oil from oil shale is essentially a mining process, whether it be opencast or underground (fracturing), which is subsequently accompanied by surface or in situ distillation. the liquid product so obtained in such distillation is then enhanced to produce synthetic crude which can be processed at a refinery (figure 8-2). figure 8-2. oil shale exploitation, modified from johnson & crawford (2004). oil shale earth sciences research journal, volume 16, special issue, april, 2012 136 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia the most developed technologies to date are those regarding opencast exploitation for both direct combustion and obtaining synthetic oil, whilst only pyrolysis is available for in situ distillation. this is the case of one of the most innovative processes called in situ conversion (patented by shell) where the oil shale temperature is raised by electric heaters located in vertical boreholes throughout the thickness of a particular deposit and fluid secreted by the rock is led to the surface through production wells drilled for such purpose. this process is currently being investigated and developed. 8.1.5 deposits around the world early palaeozoic deposits formed in marine shelf environments have been reported in northern europe, northern asia and eastern and central north-america. middle palaeozoic oil shale deposits have also been reported in eastern and central usa and central europe; thin marine sequences have found in russia. accumulations have been found on every continent in late palaeozoic rocks, many of them being associated with coal belts which have been exploited on a small scale in scotland, france, spain, south africa, australia, russia and other countries. there are ordovician kukersite deposits in estonia and leningrad in russia (yen & chillingarian, 1976). the older lacustrine deposits are found in eastern canada (albert shale) and were formed during the early carboniferous age. one of the largest late permian deposits in the world (irati shale) is located in southern brazil. similar aged shale is also present in southern argentina, uruguay and southern montana in the usa having a smaller expanse and lower grade. low-grade mississippian deposits are found in northern alaska. mesozoic oil shale has been reported on all continents except australia. the stanleyville basin in the congo in africa has triassic age oil shale deposits from lacustrine environments. northern and eastern asia have jurassic to cretaceous age deposits in coal belts. black cretaceous age shale has been reported in large marine shelf accumulations in israel, jordan, syria and the southern arabian peninsula. there are widely distributed jurassic black shale deposits in europe; there are also some minor triassic deposits in central and southern europe. large triassic age deposits are present in north america in alaska and jurassic and cretaceous age deposits in the central part of canada; these have small thicknesses but are high grade. many tertiary age oil shale deposits are not of marine origin, but are rather associated with a lacustrine environment and coal belts. small accumulations of this type have been reported in new zealand, several parts of europe, south america in the andes and on the western coast of north america. oil shale in colombia is mainly restricted to late mesozoic and cenozoic rocks (appendix 8-2). the greatest oil shale accumulations in the world have a lacustrine environment and are from the early or middle tertiary ages. these would include the green river formation and other deposits in the western usa, the paraiba valley oil shale deposits in southern brazil and those in eastern china. many of the aforementioned deposits are being commercially exploited. 8.2 data and hypotheses the set of data attached as part of this document in a file in appendix 8-1 consists of 120 samples from different deposits distributed throughout the world. although the initial database contained oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 137 information about more than 200 deposits, it had to be refined by selecting only those deposits which had complete information for all the designated fields. the distribution of thickness was taken from this database for estimating the shale oil potential. 8.2.1 national (colombian) database the set of data which is attached as part of this document in a file labelled appendix 8-2 includes the information used for calculating the weighting factors by scenario and category for evaluating the resource, that used for estimating oil shale areas, the data used for estimating hydrocarbon content distribution (s2p) and the results of each goodness-of-fit test and pertinent correlation. 8.2.2 hypotheses 8.2.2.1 hypothesis 1 the ratio between the total control area, defined from sampling presented in the geochemical atlas of colombia (anh, 2010) and shale surface/outcrop area by basin, shown in the geological atlas of colombia (ingeominas, 2007), represents the oil shale‟s effective area regarding both surface and depth. the following expression illustrates this hypothesis: 8.2.2.2 hypothesis 2 total control area will be significant if total organic carbon (toc) content favours commercial scenarios. it is considered that toc values in this work regarding kerogen quality ranging from very good to excellent (≥ 4%) favour commercial occurrences of the resource (figure 8-3). figure 8-3. kerogen quality in particular regions. oil shale earth sciences research journal, volume 16, special issue, april, 2012 138 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.2.2.3 hypothesis 3 conditions representing control areas must be evaluated. each control point derived from selecting representative observations of the resource, and compiled in the geochemical atlas of colombia (anh, 2010), can help to ascertain resource areas for assessing the resource from measured (250 m evaluation radius) to hypothetical areas (5,000 m evaluation radius). figure 8-4 illustrates the scenarios which had to be evaluated. figure 8-4. representative diagram for control areas produced from the sampling points used for calculating weighting factors 8.2.2.4 hypothesis 4 the thickness of oil shale units in colombia follows a lognormal distribution supported by data regarding deposits around the world (figure 8-5). such distribution is supported by appendix 81. figure 8-5. histogram of lognormal thickness frequency used for estimating the oil shale potential. 8.2.2.5 hypothesis 5 the pattern of s2p hydrocarbon content in basins having no report for this parameter is equal to that presented in geologically analogous basins. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 139 8.3 methodology the following equation was used for evaluating the oil shale potential in each colombian basin: (8-1) osoip: original oil shale in place (i.e. in situ) (mmbbl) h: oil shale unit thickness (m) a: shale area (m2) fa: deposit area weighting factor (m2/m2) ρ: total rock density (g/cm3) s2p: hydrocarbons generated by kerogen distillation –hydrocarbon content (mghc /groca) ρhc: oil shale density (g/cm3) the procedure for estimating the resource considered the following stages: o a database containing characteristics such as age, tonnage, thickness, area, yield, potential, and density regarding oil shale deposits worldwide was compiled after a bibliographic review had been made (appendix 8-1):  frequency histograms were drawn for visualising the pattern and associating a particular distribution to thickness;  goodness-of-fit tests (i.e. lilliefors, student‟s t-test and chi2) were applied for accepting or rejecting the hypothesis regarding thickness distribution; and  parameters were estimated for such distribution as well as the pertinent confidence intervals. o a database was compiled containing information about areas, organic matter content and assessment scenarios for colombia (appendix 8-2):  the cartography presented in the geological atlas of colombia (ingeominas, 2007) was used for estimating the surface area of mudstone, shale and mud having organic matter in each basin. polygons were then drawn showing the oil shale potential areas;  the pattern of hydrocarbon content (s2p) in each basin was analysed by taking information concerning the pyrolysis test presented in the geochemical atlas of colombia (anh, 2010), thereby obtaining a distribution for this property per basin;  statistical analysis was performed to support such distribution; and  weighting factors were calculated regarding four categories for evaluating the resource, distributed into four scenarios for controlling the oil shale area. o monte carlo simulations were made using the osoip formulation; thickness, area, total rock density, organic matter content and shale oil density were used as random variables and the area as fixed parameter. oil shale earth sciences research journal, volume 16, special issue, april, 2012 140 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.3.1 scenario 1 the weighting factors were calculated in this scenario by dividing the sum of the control areas within each shale polygon by the sum of the polygons having control points; this was done basin by basin: ∑ ∑ shale areas lacking control points were not taken into account in the way that this denominator was presented nor were excesses of area in the numerator (arising from control points within a given polygon) which should fall outside such polygon. figure 8-6 shows the issues raised for the weighting factors for each basin in this scenario. figure 8-6. diagram for calculating the weighting factor per basin “fa” for scenario 1. 8.3.2 scenario 2 figure 8-7 shows a diagram for calculating the ratio of areas in this scenario. this approach can lead to factors greater than unity, which is not surprising, since it is possible that (as shown in the figure below) just the numerator„s yellow areas exceed the sum of the denominator, especially when dealing with inferred or hypothetical evaluation categories. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 141 figure 8-7. diagram for calculating the weighting factor per basin “fa” for scenario 2. 8.3.3 scenario 3 the concept of thiessen polygons was used for calculating the weighting factors in this scenario. this led to generating an area from the distances measured between a control point and its neighbours. such areas were summed per basin and then divided by total basin area. ∑ 8.3.4 scenario 4 a simple polygon to polygon ratio was obtained in this scenario regarding the area of shale to that defined by control points, leading to more than one factor per basin. it was intended to find a distribution of probability using all the factors calculated for colombia. figure 8-8 gives a diagram of the calculations used. figure 8-8. diagram for calculating the weighting factor per basin “fa” for scenario 4 oil shale earth sciences research journal, volume 16, special issue, april, 2012 142 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia weighting factor/basin area curves were made for the first 3 scenarios regarding each resource assessment category (measured, indicated, inferred and hypothetical). occurrence probability distribution was found for all basins by category for the last scenario. the indicated and hypothetical categories were chosen for presenting the results after having observed the pattern for the estimated resources in the assessment categories (measured, indicated, inferred and hypothetical) in the proposed scenarios. 8.4 results different distributions were tested for the random variables; parameters regarding distribution patterns were first estimated according to each distribution, and then goodness-of-fit tests were made for identifying the best value (least deviation). this report only shows the results of the best fits and estimates; the remaining results are reported in the digital files in appendixes 8-1 and 8-2. 8.4.1 oil shale unit thickness regarding data for deposits around the world, it was found that oil shale unit thickness ranged from 1 m to 600 m, having 68 m average and 113 m standard deviation. the histogram so produced (figure 8-5) suggested a log-normal pattern regarding occurrence frequency; graphs were thus drawn showing this variable‟s lognormal probability (figure 8-9) and three goodness-of-fit tests: the lilliefors and student-t and chi2 tests. the table below gives the results obtained for the last mentioned test. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 3.11 2.82 3.40 1.6 1.42 1.83 was not rejected 0.07. 0.08 0 table 8-1. lognormal distribution fit parameters and goodness-of-fit test results applied to the thickness data used in estimating osoip. estimated parameters ̂ ̂ are lognormal distribution regarding the mean and standard deviation, respectively. 8.4.2 total rock density density values were not reported in many cases investigated in the pertinent literature about particular deposits. due to such lack of data it was decided to take the minimum and maximum values which could be extracted for constructing a triangular distribution. parameters value (g/cm3) minimum 2.1 maximum 2.4 most probable 2.36 table 8-2. parameters estimated for total oil shale density, considering an asymmetrical triangular distribution. 8.4.3 oil shale density oil shale density distribution parameters were determined in the same way as the values shown in table 8-2 for rock density in view of the scarcity of data for this variable (table 8-3). parameters value (g/cm3) minimum 0.9 maximum 0.97 most probable 0.91 table 8-3. parameters estimated for total oil shale density, considering an asymmetrical triangular distribution. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 143 8.4.4 shale areas table 8-4 shows the shale area value estimated per basin. figure 8-9 shows these areas‟ national coverage. basin shale area (m2) amagá 10,930.008 non-prospective areas 44,122,937.434 caguán putumayo 25,842,949.353 catatumbo 4,051,256.044 cauca patía 770,194.542 cesar ranchería 1,691,905.112 chocó 14,723,778.122 the eastern cordillera 39,404,970.408 guajira 2,124,744.030 the eastern llanos 2,436,354.892 sinú san jacinto 21,920,971.043 tumaco 7,385,534.349 urabá 246,010.452 the lower magdalena valley 10,676,780.807 the middle magdalena valley 5,937,422.714 the upper magdalena valley 3,016,913.891 vaupés amazonas 758,654.554 table 8-4. areas of shale per basin in colombia. figure 8-9. shale outcrop cartography (green). oil shale earth sciences research journal, volume 16, special issue, april, 2012 144 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.4.5 deposit area weighting factor tables 8-5 to 8-8 se shows the weighting factors obtained per basin for the four resource evaluation categories regarding the four proposed scenarios. an attempt was made to obtain a ratio for finding factors for those basins lacking them regarding the first three scenarios (graphs showing weighting factor to basin area ratio); however, the resulting correlations did not lead to extracting any type of tendency. calculating effective oil shale area thus had to be approached from another angle. a probability distribution was thus sought in the fourth scenario so that all basins could be associated in estimating the oil shale potential (equation 8-1). figures 8-10 to 8-12 show the aforementioned ratios for the indicated resources category in the first three scenarios; the pattern for the rest was very similar and their graphs/figures have been included in the attached digital files. basin scenario 1 scenario 2 scenario 3 scenario 4 caguán putumayo 1.784*10-2 1.992*10-2 1.719*10-3 8.080*10-3 4.844*10-2 the eastern cordillera 1.416*10-4 2.061*10-4 1.060*10-1 1.290*10-3 4.092*10-5 2.890*10-3 6.181*10-3 6.868*10-5 3.364*10-4 4.457*10-4 4.535*10-3 9.553*10-4 sinú san jacinto 2.352*10-2 1.185*10-1 3.221*10-1 2.352*10-2 the middle magdalena valley 2.258*10-4 3.388*10-4 3.907*10-2 1.479*10-4 3.921*10-4 7.683*10-4 the upper magdalena valley 1.605*10-3 5.340*10-3 1.295*10-1 1.056*10-3 1.961*10-2 table 8-5. measured factors used in estimating osoip. these factors were calculated in the methodology section for the first three scenarios to obtain a single factor per basin; however, one shale unit was obtained for each basin in the last scenario. basin scenario 1 scenario 2 scenario 3 scenario 4 caguán putumayo 8.873*10-2 1.349*10-1 1.719*10-3 7.989*10-2 1.164e-1 the eastern cordillera 1.226*10-3 1.744*10-3 1.060*10-1 8.588*10-3 3.891*10-4 8.333*10-2 2.383*10-2 5.565*10-2 5.458*10-4 2.985*10-3 3.795*10-3 1.182*10-4 5.358*10-2 6.794*10-3 sinú san jacinto 3.194*10-1 1.067 3.221*10-1 3.194*10-1 the middle magdalena valley 2.033*10-3 3.050*10-3 3.907*10-2 1.332*10-3 3.948*10-3 5.482*10-3 the upper magdalena valley 1.440*10-2 3.763*10-2 1.295*10-1 7.338*10-3 1.743*10-2 2.281*10-1 table 8-6. indicated factors used in estimating osoip. as stated in the methodology section, values were obtained which were above those per unit for weighting factors in scenario 2 (highlighted in red) when areas oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 145 outside the shale polygons were greater than such area (it should be stressed that the ratio sought with this scenario‟s values was not good). basin scenario 1 scenario 2 scenario 3 scenario 4 caguán putumayo 3.474*10-1 1.073 1.719*10-3 3.079*10-1 4.711*10-1 the eastern cordillera 8.132*10-3 1.406*10-2 1.060*10-1 5.565*10-2 2.707*10-3 1.946*10-2 9.707*10-1 5.868*10-3 5.176*10-1 8.078*10-2 1.264*10-1 3.703*10-1 3.968*10-3 2.433*10-2 2.239*10-4 2.249*10-2 6.900*10-2 1 4.290*10-1 2.160*10-2 sinú san jacinto 5.251*10-3 2.359*10-2 3.221*10-1 1 4.511*10-1 2.174*10-3 the middle magdalena valley 1.410*10-2 2.731*10-2 3.907*10-2 6.067*10-3 1.380*10-1 2.859*10-2 5.249*10-2 6.094*10-1 the upper magdalena valley 1.259*10-1 2.598*10-1 1.295*10-1 6.355*10-2 1.327*10-1 3.725*10-1 6.558*10-1 table 8-7. inferred factors used in estimating osoip oil shale earth sciences research journal, volume 16, special issue, april, 2012 146 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin scenario 1 scenario 2 scenario 3 scenario 4 caguán putumayo 4.811*10-1 7.332 1.719*10-3 5.628*10-1 9.936*10-1 1.293*10-3 cesar ranchería 1.166*10-1 3.479 0 1.166*10-1 the eastern cordillera 3.082*10-2 6.227*10-2 1.060*10-1 1.225*10-1 7.314*10-3 2.955*10-1 8.535*10-1 1 5.574*10-2 3.336*10-1 2.786*10-2 9.720*10-1 1.541*10-1 2.960*10-1 3.022*10-1 6.101*10-1 1.535*10-1 5.368*10-3 1.794*10-2 9.834*10-2 1.387*10-2 8.131*10-2 8.017*10-1 2.492*10-1 1 6.990*10-1 4.469*10-2 1.521*10-1 5.479*10-1 the eastern llanos 7.077*10-2 5.137*10-1 3.122*10-2 7.077*10-2 sinú san jacinto 2.212*10-2 8.651*10-2 3.221*10-1 1.391*10-2 1.787*10-2 6.382*10-3 6.855*10-1 1 3.692*10-1 1 1.323*10-2 the middle magdalena valley 6.894*10-2 1.087*10-1 3.907*10-2 3.473*10-2 5.957*10-2 2.297*10-2 5.608*10-1 7.177*10-1 1.784*10-2 4.893*10-2 3.217*10-1 1.864*10-1 7.286*10-2 2.036*10-1 1 5.549*10-2 the upper magdalena valley 2.190*10-1 5.290*10-1 1.295*10-1 1.328*10-1 4.981*10-1 6.831*10-3 9.889*10-2 8.578*10-1 1 table 8-8. hypothetical factors used in estimating osoip oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 147 figure 8-10. ratio between indicated weighting factors in scenario 1 and basin area. as can be seen, it was not possible to extract a particular tendency allowing factors to be estimated in other basins from their areas. the other factors‟ patterns (measured, inferred and hypothetical) were similar in this scenario. figure 8-11. ratio between indicated weighting factors in scenario 2 and basin area. the other factors‟ patterns (measured, inferred and hypothetical) were similar in this scenario. oil shale earth sciences research journal, volume 16, special issue, april, 2012 148 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 8-12. ratio between indicated weighting factors in scenario 3 and basin area. the other factors‟ patterns (measured, inferred and hypothetical) were similar in this scenario and did not lead to any sort of tendency being established. tables 8-9 and 8-10 show the results of the statistical analysis for best fit distribution regarding indicated and hypothetical factors in scenario 4. such scenario was finally chosen for calculating the oil shale potential, and the final results given in this chapter are for the resource evaluation categories applying the two aforementioned factors. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.41 0.25 0.68 0.12 0.05 0.29 was not rejected m.b 0.20 0 table 8-9. gamma distribution fit parameters and goodness-of-fit test results applied to the indicated weighting factors in scenario 4 used in estimating osoip. estimated parameters ̂ ̂ are the gamma distribution regarding form and scale. m.b refers to a very low probability of the null hypothesis being accepted or rejected. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.65 0.48 0.88 0.52 0.24 0.82 was not rejected 0.14 2.16 1 table 8-10. gamma distribution fit parameters and goodness-of-fit test results applied to the hypothetical weighting factors in scenario 4 used in estimating osoip. comparisons were made with aerial weighting factors from other parts of the world to ensure that the figures for estimating resources represented realistic scenarios. 8.4.6 s2p hydrocarbon content the information from pyrolysis tests contained in the geochemical atlas of colombia (anh, 2010) was used as input for estimating s2p hydrocarbon content per basin. the data regarding outcrop samples was used as the resource being evaluated is mainly exploited by mining opencast deposits, even though new cutting-edge technologies point to their in-depth exploitation. it was only necessary to use well information regarding the ranchería (cesar), san jacinto (sinú), tumaco and urabá basins as no surface data was available for them. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 149 8.4.6.1 caguán – putumayo the 23 items of data available gave 6.02 mghc/grock average value and 11.03 mghc/grock standard deviation. gamma distribution presented the best data fit. table 8-11 shows the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.36 0.23 0.58 16.5 7.28 37.6 was not rejected m.b 0.20 0 table 8-11. goodness-of-fit test results and parameters applied to the s2p data in the caguán basin, putumayo, for determining the statistical distribution used in estimating osoip. 8.4.6.2 catatumbo the 62 items of data used gave 1.34 mghc/grock average value and 1.78 mghc/grock standard deviation. lognormal data pattern was normal. table 8-12 shows the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l -0.7 -1.2 -0.3 1.63 1.38 1.98 was not rejected m.b 0.88 0 table 8-12. goodness-of-fit parameters and test results applied to the s2p data in the catatumbo basin for determining the statistical distribution used in estimating osoip. 8.4.6.3 cauca – patía no information was available regarding s2p hydrocarbon content for this basin. the distribution for this variable in a neighbouring basin was thus used; the parameters found for the tumaco basin given were thus taken, given their similar geological evolution. 8.4.6.4 cesar ranchería 249 data items were used, giving 1.69 mghc/grock average value and 2.34 mghc/grock standard deviation. normal logarithm distribution in this basin gave the best fit. table 8-13 shows the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l -0.3 -0.5 -0.1 1.37 1.26 1.50 was not rejected 0.07 5.34 2 table 8-13. goodness-of-fit parameters and test results applied to the s2p data in cesar ranchería basin, for determining the statistical distribution used in estimating osoip. 8.4.6.5 chocó 49 items of data were analysed, giving 34.56 mghc/grock average value and 47.57 mghc/grock standard deviation. again, normal logarithm distribution gave the best fit (tables 8-14). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 1.86 1.17 2.55 2.40 2.00 3.00 was not rejected m.b 0.8 0 table 8-14. goodness-of-fit parameters and test results applied to the s2p data in the chocó basin for determining the statistical distribution used in estimating osoip. oil shale earth sciences research journal, volume 16, special issue, april, 2012 150 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.4.6.6 eastern cordillera the 196 items of data gave 2.28 mghc/grock average value and 5.58 mghc/grock standard deviation. pareto distribution gave the best data fit (table 8-15). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 1.47 1.13 1.81 0.12 0.16 0.29 was not rejected m.b 1.53 0 table 8-15. goodness-of-fit parameters and test results applied to the s2p data in the eastern cordillera basin for determining the statistical distribution used in estimating osoip. estimated parameters ̂ ̂ are the pareto distribution form and scale parameters. 8.4.6.7 guajira since no information was available about s2p hydrocarbon content in the basin, the distribution presented for this variable regarding the cesar ranchería basin, was taken, given their geological similarity. 8.4.6.8 eastern llanos the 75 items of data used gave 3.03 mghc/grock average value and 6.23 mghc/grock standard deviation. a pareto distribution gave the best fit for eastern llanos data, like that for the eastern cordillera (tables 8-16). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 1.14 0.69 1.58 0.56 0.36 0.86 was not rejected m.b 3.41 0 table 8-16. goodness-of-fit parameters and test results applied to the s2p data in the eastern llanos basin for determining the statistical distribution used in estimating osoip 8.4.6.9 sinú san jacinto 290 items of data were used, giving 0.71 mghc/grock average value and 2.07 mghc/grock standard deviation. an exponential distribution gave the pattern best fitting the data (table 8-17). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.71 0.63 0.79 n/a n/a n/a was not rejected m.b 0.47 0 table 8-17. goodness-of-fit parameters and test results applied to the s2p data in the san jacinto basin, sinú, for determining the statistical distribution used in estimating osoip. for exponential distribution, estimated parameter ̂ represented the mean. n/a refers to parameters which did not apply to the type of distribution being considered. 8.4.6.10 tumaco 92 items of data were analysed, giving 1.35 mghc/grock average value and 2.34 mghc/grock standard deviation. again, normal logarithm distribution gave the best fit. table 8-18 gives the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l -0.3 -0.5 -0.1 0.95 0.83 1.11 was not rejected 0.09 4.84 2 table 8-18. goodness-of-fit parameters and test results applied to the s2p data in the tumaco basin for determining the statistical distribution used in estimating osoip. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 151 8.4.6.11 urabá only three items of data were available for this basin, meaning that it was decided to use triangular distribution. table 8-19 lists the distribution parameters. parameters value minimum 0.08 maximum 0.33 most probable 0.18 table 8-19. parameters considered for s2p in the urabá basin. 8.4.6.12 lower magdalena valley 5 items of data regarding outcrops were available for this basin and thus triangular distribution was assumed (table 8-20). parameters value minimum 0.09 maximum 3.58 most probable 1.274 table 8-20. parameters estimated for s2p in the lower magdalena valley basin. 8.4.6.13 middle magdalena valley the 49 items of data gave 2.92 mghc/grock average value and 10.51 mghc/grock standard deviation. normal logarithm distribution gave the best data fit (table 8-21). estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l -0.9 -1.5 -0.4 1.88 1.57 2.35 was not rejected m.b. 0.001 0 table 8-21. goodness-of-fit parameters and test results applied to the s2p data in the middle magdalena valley basin for determining the statistical distribution used in estimating osoip. 8.4.6.14 upper magdalena valley the 282 items of data gave a 20.88 mghc/grock average value and 18.26 mghc/grock standard deviation. an extreme value distribution was associated with lognormal data. table 8-22 shows the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 3.04 2.92 3.17 0.98 0.89 1.08 was not rejected 0.11 5.93 3 table 8-22. goodness-of-fit parameters and test results applied to the s2p data in the upper magdalena valley basin for determining the statistical distribution used in estimating osoip. estimated parameters ̂ ̂ are extreme value distribution regarding location and scale parameters. 8.4.6.15 vaupés – amazonas as no information was available regarding s2p hydrocarbon content in this basin, it was decided to associate it with the same distribution parameters found for the caguán putumayo basin, due to their similarity in terms of geological evolution. oil shale earth sciences research journal, volume 16, special issue, april, 2012 152 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 8.4.7 oil shale in colombia monte carlo simulation results for calculating the oil shale potential are given below, along with the parameters defined in equation 8-1 and the previously defined distributions. these results had already been weighted by the environmental factors shown in table 2-1 for excluding those possibly present in environmental conservation areas. basin indicated oil shale resources (mmbbl) p10 p50 p90 amagá 3.48 0.06 0.01 non-prospective areas 5.52 0.29 0.01 caguán putumayo 259.06 4.95 0.03 catatumbo 85.03 3.57 0.13 cauca patía 240.15 7.77 0.23 cesar ranchería 622.91 13.94 0.28 chocó 412.19 10.91 0.27 the eastern cordillera 294.64 9.54 0.28 guajira 10,443.25 198.65 1.16 the eastern llanos 798.88 33.31 1.24 sinú san jacinto 1,135.06 47.48 1.88 tumaco 2,678.01 99.34 2.96 urabá 1,950.33 97.09 4.13 the lower magdalena valley 12,165.96 220.42 3.75 the middle magdalena valley 39,432.19 549.61 7.34 the upper magdalena valley 5,986.40 240.93 8.45 vaupés amazonas 14,564.94 682.19 28.32 total 91,077.98 2,220.05 60.47 table 8-23. osoip for the indicated resources evaluation category. it was estimated that there could be 179.86 mmbbl of oil shale (6,291.36 mmbbl in p10 and 5.39 mmbbl in p90) for the indicated resources evaluation category in areas forming part of the colombian park system (parques nacionales naturales). basin hypothetical oil shale resources (mmbbl) p10 p50 p90 amagá 26.47 0.54 0.01 non-prospective areas 38.53 2.59 0.11 caguán putumayo 1,952.38 44.25 0.23 catatumbo 612.76 31.64 1.11 cauca patía 1,737.96 68.73 1.95 cesar ranchería 4,644.63 118.44 2.40 chocó 2,967.84 94.76 2.27 the eastern cordillera 2,132.84 83.32 2.38 guajira 79,469.45 1,809.20 9.63 the eastern llanos 5,712.77 296.63 10.41 sinú san jacinto 8,015.75 422.47 16.07 tumaco 19,007.30 889.21 24.51 urabá 13,597.94 862.95 34.62 the lower magdalena valley 90,189.61 1,894.28 31.18 the middle magdalena valley 300,054.28 4,792.46 62.47 the upper magdalena valley 43,068.00 2,153.04 70.91 vaupés amazonas 104,516.63 6,005.43 241.42 total 677,745.15 19,569.94 511.67 table 8-24. osoip for the hypothetical resources evaluation category. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 153 it was estimated that there could be 1,584.85 mmbbl of oil shale for the hypothetical resources evaluation category (46,345.60 mmbbl in p10 and 45.61 mmbbl in p90) in areas forming part of the colombian park system (parques nacionales naturales). figure 8-13 gives graphs of indicated resource distribution per basin. figure 8-14 gives a comparison of the resources calculated for colombia within a global framework. figure 8-13. map of the oil shale potential for the indicated resources evaluation category. oil shale earth sciences research journal, volume 16, special issue, april, 2012 154 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 8-14. oil shale potential for the indicated resources evaluation category. figure 8-15. comparing osoip for colombia with that for other countries throughout the world. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 155 8.4.8 sensitivity analysis the three variables to which the process was most sensitive were hydrocarbon content, aerial weighting factor and thickness. the order of sensitivity percentages for these three parameters varied according to the basin and resources evaluation category. o figure 8-16 shows the first pattern for the indicated resources category. figure 8-16. sensitivity average 1 for the indicated resources category. this pattern was present in the patía (cauca), ranchería (cesar), the eastern cordillera, guajira, the eastern llanos, san jacinto (sinú), tumaco, urabá, the lower magdalena valley and the upper magdalena valley basins o figure 8-17 shows the second pattern for the same category. figure 8-17. sensitivity average 2 for the indicated resources category. this pattern was present in the amagá, caguán (putumayo), catatumbo, chocó, the middle magdalena valley and vaupés (amazonas) basins and in the so-called non-prospective areas. o likewise, sensitivity evaluated for the hypothetical resources category gave the first pattern shown in figure 8-18. oil shale earth sciences research journal, volume 16, special issue, april, 2012 156 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 8-18. sensitivity average 3 for the hypothetical resources category. this pattern was present in the patía (cauca), ranchería (cesar), the eastern cordillera, guajira, the eastern llanos, san jacinto (sinú), tumaco, urabá, the lower magdalena valley and the upper magdalena valley basins. o figure 8-19 gives the second pattern for the hypothetical resources evaluation category. figure 8-19. sensitivity average 4 for the hypothetical resources category. this pattern was present in the amagá, caguán (putumayo), catatumbo, chocó, middle magdalena valley and vaupés (amazonas) basins and in the so-called non-prospective areas. 8.5 conclusions o the eastern cordillera was the basin having the greatest prospectivity in p90, regarding the presence of oil shale, followed by the upper magdalena valley and the chocó basin (figure 8-13); o the previous order changed in p50, showing the eastern cordillera basin to be most prospective, but placing the resources present in the chocó above those for the upper magdalena valley; and o the prospectivity of the lower magdalena valley, caguán putumayo and sinú san jacinto basins was also high, even though variability in the range of estimating the resource in the caguán putumayo basin in should be considered. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 157 8.6 bibliography abercrombie, f. n. (1984). chemical characterization of an oil shale bed lying within the heath formation, fergus county, montana. american chemical society. agencia nacional de hidrocarburos. (2010). organic geochemistry atlas of colombia. bogotá: universidad nacional de colombia. altius minerals. (2009). albert oil shale project: exploration overview. new brunswick. altun , n. e. (2006). oil shales in the world and turkey; reserves, current situation and future prospects: a review. oil shale (23), 211-227. anatoly , k. b. (2007). oil shale as a prospective raw material for fuel, energy and chemical industries – to be or not to be? oil shale (24), 5-7. andersson, a. (1985). the scandinavian alum shale. sérvicio geológico sueco. australian goverment. (2009). condor deposit. consulted on the 6th january 2011, de geosciences australia: http://www.ga.gov.au/oceans/ea_ons_hills_summ.jsp australian goverment. (2009). dauringa deposit . consulted on the 6th january 2011, de geosciences australia: http://www.ga.gov.au/provexplorer/provincedetails.do?eno=20298 australian goverment. (2009). mersey river deposit. consulted on the 6th january 2011, de geosciences australia: http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=25424 bacon, c. a. (2000). the petroleum potential of onshore tasmania: a review. mineral resources tasmania. geological survey bulletin (71), 93. bamburak, j. d. (1999). cretaceous black shale investigations in the northern part of the manitoba escarpment. consulted on the 6th january 2011, de geological services; manitoba industry trade and mines. report of activities: www.gov.mb.ca/stem/mrd/geo/field/roa99pdf/gs-28-99.pdf bellanca, a. (2001). transition from marine to hypersaline conditions in the messinian tripoli formation from the marginal areas of the central sicilian basin. sedimentary geology (140), 87105. bencherifa, m. (2009). moroccan oil shale, research and development. office national des hydrocarbures et des mines. blue ensign technologies. (2009). the julia creek shale oil resource > [consulted on the 6th january 2011]. consulted on the enero de 2011, de http://www.blueensigntech.com.au/ chonglong, w. (2002). study on dynamics of tectonic evolution in the fushun basin, northeast china. science in china, d series (45), 14. oil shale earth sciences research journal, volume 16, special issue, april, 2012 158 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia clarey, m., & thompson, m. (2010). study area. in d. copeland, & m. ewald, available ground water determination, green river basin water plan ii (page 35). wyoming. connah, t. h. (1964). torbanite deposit alpha, central queensland. department of mines, geological survey of queensland, queensland. coxhell, s., & fehlberg, b. 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(2010). potentiality of black shales in egypt. consulted on the febrero de 2011 de www.medemip.eu/.../oil%20shale%20.../potentiality_of_black_ shales_in_egypt.pdf, de undersecretary for mineral resources. ministry of petroleum, egypt: 18 foster, c. b. (1982). illustration of early tertiary (eocene) plant microfossils from de yaamba basin. geological survey of queensland, department of mines, queensland. garetskii, r. g. (2004). pripyat trough: tectonics, geodynamics, and evolution. russian journal of earth sciences(6), 217–250. garside, l. j. (1983). nevada oil shale. nevada bureau of mines and geology, nevada. gerta, k. (2004). low diversity, late maastrichtian and early danian planktic foraminiferal assemblages of eastern tethys. journal of foraminiferal research(34), 49-73. glennie, k. w. (1998). petroleum geology of the north sea: basic concepts and recent advances. oxford: wiley-blackwell. graham, j. p. (2000). an application of sequence stratigraphy in modeling oil yield distribution: the stuart oil shale deposit, queensland, australia. , 146 p. work de grado (m.sc. applied geology), . school of. tesis m.sc: applied geology, queensland university of technology, school of natural resouce science, queensland. gross, s. (1977). the mineralogy of the hatrurim formation, israel. geological survey of israel bulletin(70), 1-80. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 159 gurov, e. (2003). ejecta of the boltysh impact crater in the ukrainian shield. impact markers in stratigraphic records(3), 179-202. harrington, e. (2005). technical report on the poker flats property, carlin mining district, elko county, nevada, usa. reliance geological services inc. harvie, b. a. (2010). the shale-oil industry in scotland 1858–1962. i: geology and history. oil shale(27), 354-358. hassaan, m., & ezz-eldin, m. (2007). the black shale in egypt: a promising tremendous resource of organic matter. cspg-cseg convention. calgary. ho, a. (2009). oil shale annual report. esperance minerals n.l, sydney. holger, l., bernhard , s., hans, j., & josef , p. (2000). trace element distribution in palaeozoic black shales:"kupferschiefer" (germany) and exshaw formation (canada). journal of conference abstracts, 52(2), 656. hutton, a. c. (1990). classification, organic petrography and geochemistry of oil shale. lexington university of kentucky, institute for mining and minerals research., kentucky. ingeominas. (2007). mapa geológico de colombia. escala 1:2'800.000. . bogotá. israel goverment. (2010). mineral resources oil shale. consulted on the 21 de febrero de 2011, de geological survey of israel: http://www.gsi.gov.il/eng/index.asp?categoryid=113&articleid=160 james, h. g. (1980). an assessment of oil shale technologies. washington, d.c: u.s government printing office. johnson, h., & crawford, p. (2004.). strategic significance of america‟s oil shale resource. us department of energy. jordan goverment. (2010). the hashemite kingdom of jordan, hydrocarbons, oil shale. consulted on the 21 de febrero de 2011, de natural resource authority: http://www.nra.gov.jo/index.php?option=com_content&task=view&id=34&itemid=44 kara, g., & korkmaz, s. (2008). organic geochemistry, depositional environment and hydrocarbon potential of the tertiary oil shale deposits in nw anatolia, turkey. oil shale(25), 444–464. lemone, d. (2008). uraniferous phosphates: resource, security risk, or contaminant. . annual waste management conference, waste management symposia, (pág. 15). phoenix. loughnan, f. c., & roberts, i. f. (1983). buddingtonite (nh4-feldspar) in the condor oil shale deposit, queensland, australia. mineralogical magazine(47), 327-334. louw, s. j., & addison, j. (1985). studies of the scottish oil shale industry. history of the industry, working conditions, and mineralogy of scottish and green river formation shales. final report vol. 1., us department of energy. oil shale earth sciences research journal, volume 16, special issue, april, 2012 160 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia macualey, g. (1984). cretaceus oil shale potential of the prairie province, canada. consulted on the 28 de enero de 2011, de geological survey of canada : www.gov.mb.ca/stem/.../shales/shale_cretaceous_potential2.pdf matthews, r. d., & feldkirchner, h. (1983). the distribution and regional correlation of devonian oil shales in the eastern united states. symposium on geochemistry and chemistry of oil shale, (pág. 12). american chemical society. mukhopadhyay, p. k. (2008). mississippian lacustrine horton formation source rocks from nova scotia and new brunswick, eastern canada: major shale gas and oil shale resource plays. search and discovery(10167), 33. mukhopadhyay, p. k. (2004). evaluation of petroleum potential of the devonian-carboniferous rocks from cape breton island, onshore nova scotia. nova scotia department of energy, antigonish. newell, n. (1948). key permian section, confusion range, western utah. geological society of america bulletin(59), 1053-1058. nizar, a.-j. (2009). geomorphological and geological constraints on the development of early bronze chert industries at the northern rim of the al jafr basin, southern jordan. mediterranean archaeology and archaeometry(9), 17-27. pápay, l. (2001). comparative analysis of hungarian maar-type oil shales (alginites) on the basis of sulfur content. oil shale(18), 139-148. peter, f. f. (1996). tertiary basins of spain, the stratigraphic record of crustal kinematics ,. cambridge university press. raseroka, a. l. (2009). natural gas and conventional oil potential in south africa's karoo basins. search and discover(90100). regina, b. (1990). aromatic hydrocarbons in the paraiba valley oil shale. organic geochemistry(15), 351-359. russell, p. l. (1990). oil shales of the world, their origin, occurrence and exploitation. new york: pergamon press. schmitt, l. j. (1989). aerial distribution of oil shales with associated mineral resources and metal anomalies in the western united states and alaska. consulted on the 21 de febrero de 2011, de u.s.g.s miscellaneous field studies map 2091: http://pubs.er.usgs.gov/usgspubs/mf/mf2091/ schora, f. c. (1983). progress in the commercialization of the hytort process. eastern oil shale symposium (págs. 183–190). lexington: university of kentucky, institute for mining and minerals research. smith , w. d., & naylor , r. d. (1990). oil shale resources on nova scotia. economic geology series nova scotia, 59. oil shale earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 161 smith, j. w. (1983). the chemistry which created green river formation oil shale. symposium on geochemistry and chemistry of oil shale (págs. 9-11). american chemical society . stanfield, k. e., & frost, i. c. (1949). method of assaying oil shale by a modified fischer retort. u.s. bureau of mines . steele, h. (1979). the economic potentialities of synthetic liquid wasls from oil shale. massachesetts: arno press inc. suwannathong, a., & khummongkol, d. (2007). oil shale resource in mae sot basin, thailand . 27th oil shale symposium memories. colorado: ceri mines. ventura, r. (2009). geochemical and thermal effects of a basic sill on black shales and limestones of the permian irati formation. journal of south american earth sciences(28), 14–24. viive , v. (2006). conodonts of the kiviõli member, viivikonna formation (upper ordovician) in the kohtlasection, estonia. en. proceedings of the estonian academy of sciences, 213–340. vizmap. (2010). map of resources de australia. consulted on the 6th january 2011, de yaamba deposit: http://www.vizmap.com.au/nrm/commodities/493580.htm yen, t. f., & chilingarian, g. v. (1976). oil shale. amsterdam: elsevier scientific publishing company. zhu, j. (2006). resource status and development application stratagem of oil shale in maoming basin. 26th oil shale symposium memories. colorado: college of earth science, lilin univerity. oil and gas resource stratagem research center. 8.7 appendixes digital files in the anh's document center:  “base de datos oil shale internacional.xlsx”  “base de datos oil shale colombia.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9 shale gas and shale oil shale gas and shale oil may be defined as hydrocarbons associated with shale formations (us department of energy, 2009). organic material-rich shale formations which were previously considered simply as source rocks or seals for accumulations of gas in stratigraphically-associated sands and carbonates are now defined as shale formations (us department of energy, 2009). such deposits are composed of sedimentary rocks having predominantly clay-sized particle content and also considerable percentages of organic material, making them prone to generate and store important amounts of gaseous hydrocarbons. the amount of hydrocarbons which could be present in shale usually depends on the rock‟s wealth of organic material and the degree of maturity reached. resources of around 3,350 mmbbl and 20.81 tcf have been estimated in eagle ford shale, one of the basins having the greatest development regarding the exploitation of this resource (us department of energy, 2011). the effects of the emergence of this type of production began to become evident at the end of 2005, since when the price of natural gas has become decoupled from crude prices (i.e. they are no longer linked) (rios, 2010). this resource‟s development has undoubtedly been intensifying in many nations since that time. these countries‟ energy profile has changed regarding their estimates of yetto-find shale gas and shale oil, their investment in exploration, and eagerness to break with traditional schemes tied to conventional production horizons regarding hydrocarbons in an attempt to migrate towards prospecting non-conventional resources. 9.1.1 classification shale is included in a varied group of deposits called continuous-type (unconventional) hydrocarbon accumulations (schmoker, 2005). these consist of large volumes of oilor gas-bearing rock in which the presence of hydrocarbon does not depend on the water column. these belts cannot be evaluated in view of the foregoing regarding bottom contacts (i.e. water-oil contact (woc) or gas-oil contact (goc)), as in conventional deposits. these deposits usually have a large aerial extension and could occur below rocks totally saturated by water below woc or goc. their hydrocarbon load covers the whole belt, lacks a trap or seal and has extremely low matrix permeability. shale deposits also develop abnormal pressure (whether high or low) and are normally associated with source rocks. the areas in which these accumulations give the best production characteristics are known as “sweet spots”, a term in some ways comparable with that of “fields” for conventional deposits. 9.1.2 origin and formation the clays in shale were initially deposited in the form of mud, in low energy settings such as lakeor ocean-type environments having low or zero water circulation. fine particles suspended in these environments could fall slowly together with small microorganisms, the remains of algae, plants and animals. however, an imbalance in the oxygen supply may have developed if they became buried rapidly, transforming the environment into an anoxic one inhibiting the chemical or biological shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 164 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia sequestrators responsible for organic material degradation. favourable preservation conditions for hydrocarbon development would thus become produced. pressure and temperature gradually increase whilst sediment load becomes buried over shaleforming materials. compaction is responsible for transforming the clays (normally tabular) into thin layers which then become laminated shale stratifications during lithification. the preserved organic material becomes decomposed during initial heating under pressure (diagenesis), thereby producing an insoluble material called kerogen. if burial increases, the rock and its constituents undergo catagenesis in which kerogen slowly becomes converted into bitumen and then into a liquid or gaseous hydrocarbons (wet gas). additional heat becoming incorporated may cause the remaining kerogen to react producing carbon and the change from liquid phases to hydrocarbon phases (dry gas), thus entering a stage called metagenesis (figure 9-1). figure 9-1. kerogen transformation, modified from boyer et al., (2006). all processes involved in transforming kerogen into hydrocarbons are usually known as “maturing”. oil, wet gas or dry gas would thus be generated depending on how this process evolves and the type of kerogen involved. kerogen types i and ii are associated with generating liquids and type iii with gases (figure 9-2). shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 165 figure 9-2. van krevelen diagram, modified from boyer et al., (2006). the pale red lines indicate the direction of increased maturity. all processes involved in transforming kerogen into hydrocarbons are usually known as “maturing”. oil, wet gas or dry gas would thus be generated depending on how this process evolves and the type of kerogen involved. kerogen types i and ii are associated with generating liquids and type iii with gases (figure 9-2). 9.1.3 shale characterization rock‟s potential for storing and producing economic volumes of gas can be evaluated by identifying specific characteristics such as the type of kerogen, maturity, total organic content, the matrix‟s mineralogy, porosity, permeability and gas concentration. 9.1.3.1 some fundamental geochemical analysis total organic content (toc) is directly related to the amount of hydrocarbons possibly generated and thus its importance; toc values between 0.5% and 2.0% are usually considered the lowest limit for defining prospective background intervals. this range of values is associated with gas shale quality between “very poor” and “regular” (boyer et al., 2006). vitrinite reflectance tests (ro) on maceral material in gas shale are fundamental when establishing thermal maturity. these tests give an indirect measurement of the temperatures to which rock has been submitted, thereby leading to identifying the generation window which could be entered (figure 9-1) and thus the type of hydrocarbon generated. gas shale thermal maturity could vary from marginal (0.4% ro to 0.6% ro) to mature/post-mature (0.6% ro to 2% ro) and these could also contain gas ranging from biogenic to thermogenic. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 166 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.1.3.2 some fundamental petrophysical analysis considering the form of the clays and their deposition and lithification, then it is to be expected that shale would develop almost zero vertical permeability and extremely low horizontal permeability (in the nano darcy order). permeability is thus one of the most sensitive variables and one of the most important production parameters to be determined. figure 9-3 gives a comparative scheme for the order of magnitude of permeability in shale. figure 9-3. comparative scheme for shale permeability. the hydrocarbons in shale could be stored in rock in three ways: in the local porous system, as gas in solution in intraporal water and absorbed in minerals or organic material within pores, micropores and microfractures in the rock matrix. the latter form of retention is usually responsible for the accumulation of the greatest amount of gas. characterising porosity and identifying the type which could support production are essential when evaluating future recovery scenarios and defining the appropriate stimulation mechanism. figure 9-4 compares conventional porosity in an arenite and that in shale. figure 9-4. comparing the porosity of a conventional arenite (left panel) and that of gas shale (right panel), modified from loucks et al., (2009). filtering cartridge desorption tests carried out on nucleus fragments are amongst the traditional forms of measuring gas content in rock, but these do not reflect interstitial gas or dependence on shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 167 pressure (boyer et al., 2006). measuring desorption isotherms (such as the langmuir isotherm) leads to more realistic results regarding maximum gas content related to deposit pressure. mineralogical description and identifying the type of clay predominating in rock play a basic role in shale exploitation. designing stimulation treatments and selecting the areas of rock in which such work could be carried out are affected by such characterisation, in turn, affecting assessment of a particular play‟s technical prospectivity. 9.1.3.3 identifying prospective intervals the methodology proposed by passey et al., (1990) led to identification in organic material-rich well areas, from a set of basic records such as sonic (dt), deep resistive (ild) and gamma ray (gr) records. the cross between the dt and pseudo-sonic (dtr) curves obtained from ild records led to inferring the type of fluid saturating the porous space. both these curves are parallel in rocks saturated by water and lacking organic material, and could even become overlapped, since both respond to variations in rock porosity; however, such curves cross in hydrocarbon-saturated or organic material-rich rocks, resulting in their notable separation (figure 9-5). figure 9-5. a diagram showing the form given by dt and ild records in organic material-rich areas, modified from passey et al., (1990). the green dashed area shows the possible presence of associated hydrocarbons. the separation occurring in the intervals of organic material-rich source rock and prospectivity for generating shale gas and shale oil is produced by two effects; the sonic curve responding to the shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 168 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia presence of kerogen, low density and low speed and the resistive curve responding to the fluid in a particular formation. separation is only due to slow curve response in low maturity or immature areas. as well as the effect of the sonic record, the resistivity curve increases in mature areas due to the presence of the hydrocarbons so generated. the distinction between the intervals of sand saturated with hydrocarbons in conventional deposits and organic material-rich matures areas in non-conventional deposits is shown in the gamma ray record. as this is a lithological record it allows differentiating the areas of clay. passey et al., (1990) have thus provided a methodology facilitating prospective net thickness to be obtained for shale deposits. 9.1.3.4 exploitation methods hydraulic fracturing (fracking) is one of the well stimulation techniques used for developing shale projects because it allows connecting a deposit‟s well areas to be hydraulically isolated. the amount of recovered hydrocarbon is directly related to the good management of these techniques. the volume of stimulated rock depends on the complexity of the induced fracture and thus the expected recovery; such recovery also depends on the number of frackings made in the same interval. figure 9-6. illustrative diagram of multi-stage fracking. figure 9-6 illustrates multistage fracking in which the fractures are simultaneously induced in the same pumping. combining this stimulation method with horizontal perforation technologies and geonavigation (geosteering) in shale formations provides viability dad techniques for such exploitation projects. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 169 9.1.3.5 worldwide panorama the largest shale reserves are found in the usa (the leading country in developing this resource). some eastern and northern european countries are in a stage aimed at assessing prospects, the same as other countries like china and india. the presence of this resource has already been evaluated in australia and southern south-america and development plans for already identified plays are already in place. table 9-1 shows the potential identified for some countries on different continents. country shale gas resources (tcf) country shale gas resources (tcf) usa 3,284 germany 33 canada 1,490 holland 66 mexico 2,366 sweden 164 colombia 78 norway 333 venezuela 42 denmark 92 argentina 2,732 the uk 97 bolivia 192 algeria 812 brazil 906 libya 1,147 chile 287 tunes 61 paraguay 249 morocco 108 uruguay 83 south africa 1,834 poland 792 china 5,101 lithuania 17 india 290 kaliningrad 76 pakistan 206 the ukraine 197 turkey 64 france 720 australia 1381 total 25,300 table 9-1. shale gas resources, taken from us department of energy (2011). 9.1.4 some source rock packages in colombia the most well-known generating (source) rock belts in colombia date from the cretaceous and palaeogene ages, even though there is evidence concerning the possible existence of palaeozoic rocks. such rocks were deposited in marine environments in conditions involving little circulation (anoxic) and high precipitation of organic material (ryan and cita, 1977; thiede and van andel, 1977; arthur et al., 1987; erbacher et al., 2001). some of the formations present in the middle magdalena valley (rosablanca, paja, tablezo, simití, la luna and umir), cesar ranchería (lagunitas, aguas blancas, laja – la luna and molino) and catatumbo (tibú, mercedes, aguardiente, capacho, la luna, colon, mito juan) basins, deposited during the cretaceous period and associated with events affecting their organic material enrichment are considered as being sources of colombia‟s (or even the world‟s) most prolific hydrocarbon deposits (west, 1996). la luna, simiti and tablezo formations are amongst the most fertile source rocks of the middle magdalena valley, giving 3.5% toc and 0.9% ro on average. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 170 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 9-7. petroleum system in the middle magdalena valley basin, modified from mojica, arévalos, & castillo (2009). shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 171 the molino, laja la luna, lagunitas and aguas blancas formations in the cesar ranchería, basin are in a generation phase, giving an average 1% to 1.4% toc. figure 9-8. petroleum system in the cesar ranchería basin, modified from mojica, arévalos, & castillo (2009). la luna formation is the main source in the catatumbo basin, having a 1.5% to 9.6% toc and 200 ft average thickness (agencia nacional de hidrocarburos, 2007). shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 172 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 9-9. petroleum system in the catatumbo basin, modified from mojica, arévalos, & castillo (2009). features identified in seismic profiles and corroborated by well data have led to delimiting source rock belts in three basins. continuous reflectors in them may be associated with certain nonconformities and stratigraphic sequence limits, such as maximum regressive surfaces (mrs) or maximum flooding surfaces (mfs). pre-cretaceous non-conformity and turonian mfs for the shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 173 aforementioned basins (figures 9-7, 9-8 and 9-9) could be taken as the limit and base for the main rock source package, as well as their associated seismic reflectors being easily interpretable. similar operational concepts may be extended to other basins, even those having a different age. 9.2 data and hypotheses appendix 9-1 contains files from the “shalegas.tks” project which were loaded with seismic and well information supplied by anh for the middle magdalena valley, cesar ranchería and catatumbo basins and also provide all the necessary interpretations. this project shows the horizons interpreted, the loaded time-depth curves, the maps of speed for such horizons, the surfaces obtained in time and depth and the isopach maps of source rock in the aforementioned three basins. the project also includes the well records used and correlations made for identifying the thickness of the areas having potential organic material content using the methodology proposed by passey et al., (1990). 9.2.1 international database the digital file in appendix 9-2, forming an integral part of this document, gives data extracted from a report entitled, “review of emerging resources: u.s. shale gas and shale oil play” (us department of energy, 2011) and from “shale gas data release” (beaton et al., 2009, 2010) published by the us energy and canadian geological information services. langmuir volume and toc data extracted from the canadian reports were used for inferring a possible function regarding the concentration of absorbed gas for colombia, as well as gas shale density values. the net-to-gross ratio was incorporated from the us publication for calculating the shale gas potential in colombia in a second scenario. 9.2.2 national (colombian) database the digital file in appendix 9-3, also forming an integral part of this document, gives the source rock volume values for the three basins interpreted, as well as the values extrapolated for the remaining basins, the thickness of the areas having potential organic material content, the thickness of the isopachs in the wells evaluated, the net-to-gross values calculated and toc and s1 information extracted from the geochemical atlas of colombia (agencia nacional de hidrocarburos, 2010) for calculating absorbed and free gas concentration. the statistical distribution for the last three variables is given for each basin and all goodness-of-fit test parameters supporting the selected probability functions are included. 9.2.3 hypotheses 9.2.3.1 hypothesis 1 the source rock belt lying between pre-cretaceous non-conformity and turonian maximum flooding surface seismic horizon is the most prospective regarding the presence of continuous hydrocarbon accumulations. note. due to limitations regarding access to data when developing this work, a systematic approach to defining shale packets in other basins could not be adopted. basins such as the eastern llanos and shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 174 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia that of caguán putumayo, which have traditionally been discarded as attractive prospects for hydrocarbons in shale (mainly shale gas) could form part of another scenario which should be carefully evaluated in future work. the apiay-4 well in the eastern llanos and sa-11 and sm-4 wells in caguán putumayo , all relatively close to the macarena range which divides both basins, have advantages regarding gas (ro% > 1.3). there is little geochemical information, but something similar happens in the marañon basin, specifically in the devonian units (cabanillas fm). gas occurrences near this border for the basins may reflect the palaeozoic play‟s upthrust (wells were relatively superficial < 3,000 m). if this is so, then such play could contain important resources due to its extension. 9.2.3.2 hypothesis 2 the source rock belt continuity considered here would suppose a correlation between its volume and the extent of the basin bounding it. it would thus be possible to infer the volume of source rocks in as yet non-interpreted basins. 9.2.3.3 hypothesis 3 the sonic record curve‟s intersection of the deep resistivity curve, when such curves have an appropriate scale and comparison unit, would lead to identifying areas having organic material content and the appropriate maturity for generating and storing hydrocarbons in shale within intervals having high gamma ray curve values, thereby making them deviate from the record‟s general tendency (passey et al., 1990). 9.2.3.4 hypothesis 4 the quotient between thickness with organic material content (net thickness), calculated for the whole interval recorded for a given well, and the thickness of the source rock package, obtained from seismic interpretation, is representative of the net-to-gross (ntg) ratio for such well‟s source rock belt (even when this cannot be counted in the numerator) regarding intervals in younger rocks than those defining the such belt‟s limits. 9.2.3.5 hypothesis 5 the concentration of absorbed gas for rock samples in colombia can be obtained from their toc by applying the ratios found in canadian samples for toc and langmuir volume. 9.2.3.6 hypothesis 6 the free hydrocarbon values (s1) obtained in pyrolysis tests on source rock samples compiled from the geochemical atlas of colombia (agencia nacional de hidrocarburos, 2010) are a direct measurement of free gas concentration in such rocks. 9.2.3.7 hypothesis 7 net-to-gross (ntg) pattern in colombia could be approached from the statistical distribution obtained for values extracted from a determined amount of wells. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 175 9.2.3.8 hypothesis 8 the statistical distributions obtained for free and absorbed gas concentrations, from the data available in the geochemical atlas of colombia (agencia nacional de hidrocarburos, 2010), are representative of such variables‟ pattern from basin to basin. 9.2.3.9 hypothesis 9 a second scenario can be established for estimating resources for validating the results obtained in a first scenario by assigning the pattern given by such ratio in other areas of the world to the net-togross variable (appendix 9-2). 9.2.3.10 hypothesis 10 taking the ratios for available shale gas and shale oil in other basins around the world, then it should be possible to estimate ytf shale oil resources in colombia from available shale gas resources, and vice versa. 9.3 methodology the following equation was used for evaluating the shale gas potential in each colombian basin: ( ) (9-1) osgip: original shale gas in place (in situ) (tcf) vgross: source rock belt volume (109m3) ntg: net-to-gross thickness quotient with potential organic material content, net thickness and total source rock belt thickness (ft/ft) ρbulk: total rock density (kg/m3) gf: free gas concentration (ft3/ton) ga: absorbed gas concentration (ft3/ton) the procedure for estimating shale gas resources considered the following stages:  a project was created in which all the seismic and well information supplied by anh for the middle magdalena valley, cesar ranchería and catatumbo basins was loaded.  source rock volume in the 3 basins was calculated following the reflectors enclosing this rock package, involving the following steps: o pre-cretaceous non-conformity and turonian maximum flooding surface seismic horizons were interpreted; o surfaces for limit and base were produced; o speed maps were calculated for limits and bases from the time depth curves available; o the surfaces so created were transformed into depths; shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 176 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia o isopach maps were obtained for the source rock belts; and o the belts‟ volume was calculated.  the methodology proposed by passey et al., (1990) was used for determining shale thickness with organic material content, as follows: o the curves for the sonic (dt), deep resistivity (ild) and gamma ray records (gr) were edited for those wells having the 3 curves; o the logr curve was calculated from the deep resistivity logarithm; o a well by well cartesian graph was drawn from the dt record points in the ordinate, the logr curve in the abscissa and a colour scale regarding gr record value; o areas having high gr, dt and logr values were identified on the graphs; o a line was fitted separating the areas identified from the points‟ general tendency, taking into account that the methodology advanced by passey et al., (1990) proposed a slope of 50 units from the dt record for each logr record unit for such line (shale line); o a pseudo-sonic (dtr) curve was calculated from the equations for the lines obtained, well by well; o the calculated records were graphed in the way suggested by passey et al.‟s methodology (1990); gr record in track 1, ild in track 2 and dt and dtr records in track 3 and inverse scale; o the intervals were shaded in which dt record values were higher than those for the dtr; o a cut-off point was defined in the gr for distinguishing between areas of sand and clay; and o the net thickness of areas having mature organic material content in shale intervals was calculated, well by well.  a database was built from the most relevant features of shale gas prospects around the world; another included the geochemical characterisation of rock samples in canadian prospects (appendix 9-2):  information was extracted from the “world shale gas resources: an initial assessment of 14 regions outside the united states” (us department of energy, 2011) report concerning age, extension, thickness organic material-rich, toc, thermal maturity, total gas concentration and shale gas resources estimated for prospective areas around the world: o the net-to-gross ratio value was calculated from total maximum thickness and net thickness; and o goodness-of-fit tests were run for determining the distribution best fitting the international net-to-gross data, for applying this in the second scenario for estimating resources.  information was extracted from the “shale gas data release” (beaton et al., 2009, 2010) concerning toc, s1, s2, s3, langmuir pressure, langmuir volume and sample depth: o langmuir volume values were transformed into absorbed gas concentration using langmuir pressure and sample depth (for core data), supposing normally pressurised formations; and o lineal regression was done between toc and absorbed gas concentration values. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 177  a database was built using toc, s1, source rock belt volume in each basin, net-to-gross, free and absorbed gas concentrations and statistical analysis information for approaching the variables involved in the estimating resource equation (equation 9-1), and evaluation scenarios for colombia (appendix 9-3):  information was extracted from the geochemical atlas of colombia (agencia nacional de hidrocarburos, 2010) regarding organic material content and s1 for well samples, excluding that concerning outcrops: o absorbed gas concentration was calculated by taking the regression obtained from the canadian data and toc from the samples in colombia; o s1 values were transformed to free gas concentration, assuming a gas just consisting of methane; and o statistical analysis determined the distributions best fitting the free gas and absorbed gas concentrations data, basin by basin.  representative net-to-gross (ntg) values in the respective wells were obtained from the net thickness values and thickness shown on the isopach maps: o the best statistical distribution was determined for ntg.  the distributions found were associated with those found regarding the pattern of the respective variables.  lineal regression was made between the interpreted basins‟ area and the calculated belts‟ volume: o the regression equation led to determining source rock volume in the noninterpreted basins.  monte carlo simulations were run using osgip formulation (equation 9-1) and assuming ntg, rock density, free gas and absorbed gas concentrations as random variables, and source rock volume as fixed parameter.  the simulation was repeated using a fresh ntg distribution inferred from international data (appendix 9-2).  a shale gas/shale oil ratio was established from data regarding other basins around the world where volumetric information is available regarding the resource, aimed at inferring an average participation for both resources in the shale belts evaluated. the way in which this methodology for making estimates was used led to establishing two scenarios: 9.3.1 scenario 1 the net-to-gross ratio obtained from interpreted seismic and well data in colombia was taken in this scenario. 9.3.2 scenario 2 the distribution of net-to-gross ratio was substituted in this scenario for a new one obtained for the total and net thicknesses of different prospects around the world (appendix 9-2). 9.4 results evaluating of the shale gas potential required interpreting seismic horizons and some well records; 1,001 seismic lines were thus compiled, of which 387 were used for interpreting the reflectors of interest. the electronic records for 119 wells were also reviewed, editing and analysing those for 37 shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 178 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia wells in 3 basins which were chosen for their content, quality, integrity and spatial distribution. figure 9-10 shows the map with the location of the lines and wells so reviewed. figure 9-10. seismic lines and wells reviewed. the wells selected for edition and analysis are shown in red. the labels show refer to some of the seismic lines and wells interpreted. goodness-of-fit tests were run for most variables included in equation 9-1 to ascertain their pattern. random variables were fit to different distributions, first by estimating the parameters regarding such pattern from the data according to each distribution and then goodness-of-fit tests were run for identifying the best statistical value (least deviation regarding accumulated distribution). this report only gives the results of the best fits and estimates; the others are reported in the digital files mentioned in appendixes 9-2 and 9-3. 9.4.1 source rock volume (9-2) vsr: volume of source rock belt (109 m3) ab: basin area (103 km2) shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 179 equation 9-2 (produced from the regression of source rock volume and basin area points) was applied for determining the volume of these belts in the other basins. table 9-2 gives the values estimated. basin basin area (103 km2) source rock volume (109 m3) the middle magdalena valley 32.95 25,581 catatumbo 7.72 4,709 cesar ranchería 11.67 7,549 amagá 2.82 419 caguán putumayo 110.22 89,976 cauca patía 12.80 8,740 chocó 38.50 30,170 the eastern cordillera 71.75 57,899 la guajira 13.79 9,568 the eastern llanos 224.94 185,641 sinú san jacinto 39.59 31,079 tumaco 23.67 17,803 urabá 9.43 5,930 the lower magdalena valley 37.98 29,742 the upper magdalena valley 21.48 15,981 vaupés amazonas 154.96 127,287 table 9-2. estimated volume for source rock belt considered per basin. equation 9-2 was obtained from calculating the points of source rock volume, in line with the proposed methodology; important intermediate results were obtained. a description of such results is given below. 9.4.1.1 horizons identifying the horizons corresponding to pre-cretaceous non-conformity and turonian maximum flooding surface, on some of the seismic lines supplied by anh, led to their interpretation and follow-up throughout the middle magdalena valley, cesar ranchería, and catatumbo basins. figures 9-11 to 9-16 give examples of the interpretations so made. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 180 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.1.1.1 the middle magdalena valley figure 9-11. seismic line l-82-18 located in the central part of the middle magdalena valley basin. reflector continuity for generated rock belt base and limit. dark green shows the reflector for pre-cretaceous nonconformity and yellow the reflector for miocene non-conformity. the location of the line is given on the map in figure 9-10. figure 9-12. seismic line l-82-18 located in the central part of the middle magdalena valley basin. reflector continuity for generated rock belt base and limit. dark green shows the reflector for pre-cretaceous nonconformity and yellow the reflector for miocene non-conformity. the location of the line is given on the map in figure 9-10. w e n s shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 181 9.4.1.1.2 cesar – ranchería figure 9-13. seismic line cv-88-1825 located in the north of the cesar ranchería basin. reflector continuity for generated rock belt base and limit considered. dark green shows the reflector for pre-cretaceous nonconformity, light green the reflector for the turonian maximum flooding surface and lemon green the reflector for the cretaceous limit. the location of the line is given on the map in figure 9-10. figure 9-14. seismic line cv-79-12 located to the south of the cesar ranchería basin. reflector continuity for generated rock belt base and limit considered. dark green shows the reflector for pre-cretaceous nonconformity and turonian maximum flooding surface reflector is shown in light green. the location of the line is given on the map in figure 9-10. s n w e shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 182 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.1.1.3 catatumbo figure 9-15. seismic line cat-76-26 located in the north of the catatumbo basin. reflector continuity for generated rock belt base and limit considered. dark green shows the reflector for pre-cretaceous non-conformity, light green the reflector for the turonian maximum flooding surface and lemon green the reflector for the cretaceous limit. the location of the line is given on the map in figure 9-10. figure 9-16. seismic line tc-86-116 to the south of the catatumbo basin. reflector continuity for generated rock belt base and limit considered. dark green shows the reflector for pre-cretaceous non-conformity, light green the reflector for the turonian maximum flooding surface and lemon green the reflector for the cretaceous limit. the location of the line is given on the map in figure 9-10. w e w e shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 183 9.4.1.2 surfaces in time figures 9-17, 9-18, and 9-19 give the surfaces in time for the limits and bases, generated from the interpolation “flex gridding” algorithm from the computational package used. 9.4.1.2.1 middle magdalena valley figure 9-17. source rock belt base and limit in time in the middle magdalena valley basin. on the left-hand side, pre-cretaceous nonconformity between 0.09 s and 4.72 s and, on the right-hand side, turonian maximum flooding surface between 0.0019 s and 4.39 s. 9.4.1.2.2 cesar – ranchería figure 9-18. source rock belt base and limit in time in the cesar ranchería basin. on the left-hand side, precretaceous nonconformity between 0.69 s and 3.03 s and, on the right-hand side, turonian maximum flooding surface between 0.44 s and 2.83 s. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 184 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.1.2.3 catatumbo figure 9-19. source rock belt base and limit in time in the catatumbo basin. on the left-hand side, precretaceous nonconformity between 0.67 s and 3.94 s and, on the right-hand side, turonian maximum flooding surface between 0.37 s and 3.69 s. 9.4.1.3 in depth surfaces the time–depth curves from the 49 wells led to speed maps being drawn for source rock belt limit and base. such maps are not given in this report, but may be consulted in the “shalegas.tks” project folder. transforming surfaces in time using the speed maps obtained was direct. figure 9-20 shows 3 of the time-depth curves used. figures 9-21, 9-22 and 9-23 give the surfaces in depth calculated for the 3 basins in question. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 185 figure 9-20. time-depth curves. blue shows the curve for the carare-1 well in the middle magdalena valley basin, green shows the astilleros-1 well in the catatumbo basin and red the el molino-1 well in the cesar ranchería basin. 9.4.1.3.1 middle magdalena valley figure 9-21. source rock belt base and limit depth in the middle magdalena valley basin. on the left-hand side, pre-cretaceous nonconformity between -343 ft and 30,183 ft (tvdss) and, on the right-hand side, turonian maximum flooding surface between -4 ft and 24,515 ft (tvdss). shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 186 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.1.3.2 cesar – ranchería figure 9-22. source rock belt base and limit depth in the cesar ranchería basin. on the left-hand side, precretaceous nonconformity between 3,425 ft and 15,129 ft (tvdss) and, on the right-hand side, turonian maximum flooding surface between 2,171 ft and 13,655 ft (tvdss). 9.4.1.3.3 catatumbo figure 9-23. source rock belt base and limit for the catatumbo basin. on the left-hand side, pre-cretaceous nonconformity between 3,630 ft and 21,974 ft (tvdss) and, on the right-hand side, turonian maximum flooding surface between 1,970 ft and 20,021 ft (tvdss). 9.4.1.4 isopachs the isopach maps produced from the limits and bases in depth in figures 9-21, 9-22 and 8-23 are not given in this report, but can be consulted “shalegas.tks” project folder (appendix 9-1). these shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 187 isopach average values in the 3 basins analysed were 5,266 ft, 2,069 ft and 3,612 ft for the middle magdalena valley, cesar ranchería and catatumbo basins, respectively. 9.4.2 net-to-gross table 9-3 shows net and total thicknesses obtained by applying the methodology proposed by passey et al., (1990) to the 37 wells highlighted in red in figure 9-10. the whole of the interval recorded for each well was used for calculating the ntg ratio numerator, since the available information did not allow an appropriate link to the turonian and pre-cretaceous reflectors records used for determining the ratio‟s denominator. intervals representing younger rocks than those in the source rock belt may thus have been counted within the ntg numerator, in spite of cut-off values having been defined for the gr record. well name basin gr limit net total name of well basin gr limit net total (api) (ft) (ft) (api) (ft) (ft) agata-1st1 vmm 70 514 3,939 perdiz-1 vmm 70 25 766 andes-1 vmm 80 162 940 purnio-1 vmm 80 4 1,168 bambuco-1 vmm 60 166 870 simiti-1a vmm 60 79 3,444 bronce-2 vmm 70 17 2,045 simiti-2a vmm 55 140 3,029 cano rico-1 vmm 80 14 547 simiti-3 vmm 80 295 3,602 cano tablezo-1 vmm 90 477 831 almendro-1 cat 70 443 2,391 cisne-1 vmm 70 145 725 brubucanina-1 cat 55 249 2,781 cocodrilo-1 vmm 90 931 1,067 chibagra-2 cat 70 758 2,725 corcovado-2 vmm 150 106 3,298 cocodrilo-1 cat 40 231 1,646 dona maria-2 vmm 90 891 2,664 cucuta-2 cat 110 1,543 2,186 encanto-1 vmm 70 92 614 eslabones-1 cat 110 741 1,514 escondido-1 vmm 80 42 445 esperanza-3k cat 135 153 2,092 fortuna-1 vmm 80 1264 3,862 guasimales-1 cat 90 458 1,299 galeandra-1 vmm 60 26 1,718 indio-1 cat 80 668 1,579 guayabito-1 vmm 70 944 5,455 cesar a-1x ces-ran 60 121 2,511 guayacan-1 vmm 45 845 5,325 cesar f-1x ces-ran 120 565 2,957 guineal-1 vmm 80 438 4,724 cesar h-1x ces-ran 100 239 2,804 las lajas-1 vmm 100 225 4,282 el molino-1 ces-ran 70 297 1,557 montoyas a-1 vmm 80 252 1,528 table 9-3. data calculated for determining net-to-gross ratio. the gr limit column gives the gamma ray curve limit value, assigned to each well, for the distinguishing between sands in conventional deposits and source rocks in non-conventional deposits. vmm, cat and ces-ran refers to the middle magdalena valley, catatumbo and ranchería (cesar) basins, respectively. the methodological approach adopted here meant that ntg was associated with a statistical distribution for describing its pattern. a gamma distribution thus had the best fit for the ntg values obtained by assigning form and scale parameters estimated from the same data. table 9-4 summarises the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 1.02 0.68 1.53 0.19 0.11 0.32 was not rejected 0.55 0.36 1 table 9-4. goodness-of-fit parameters and test results applied to the net-to-gross data for determining statistical distribution associated with such data. estimated parameters ̂ ̂ are gamma distribution form and scale, respectively. column p gives el value regarding the probability of the null hypothesis being accepted or rejected. g.l refers to goodness-of-fit test degrees of freedom. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 188 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia correlations were made for the wells analysed for determining net and total thickness values using the proposed methodology (pseudo-sonic curve). figures 9-24 to 9-29 give examples of some of these correlations. 9.4.2.1 the middle magdalena valley figure 9-24. linear regression for obtaining the pseudo-sonic curve in the fortuna-1 well in the middle magdalena valley basin. the points of the sonic curve (dt) on the ordinate on the graph and the logarithm of the resistive curve on the abscissa (logr) are shown in the left-hand panel. the colour of the points reflects the gamma ray curve value. the points enclosed in the blue polygon show the in depth areas highlighted in the right-hand panel; these are prospective areas. figure 9-25. associated organic material-rich hydrocarbon areas in the fortuna-1 well in the middle magdalena valley basin. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 189 9.4.2.2 cesar – ranchería figure 9-26. linear regression for the obtaining the pseudo-sonic curve in the cesar f-1x well in the cesar ranchería basin. figure 9-27. associated organic material-rich hydrocarbon areas in the cesar f-1x well in the cesar ranchería basin. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 190 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 9.4.2.3 catatumbo figure 9-28. linear regression for obtaining the pseudo-sonic curve in the eslabones-1 well in the catatumbo basin. figure 9-29. associated organic material-rich hydrocarbon areas in the eslabones-1 well in the catatumbo basin. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 191 9.4.3 total rock density even though the wells reviewed and evaluated included density record (rhob), the curves were found in sands in conventional deposits, not in source rock areas. taking this into account, it was decided to take the density data from the already complied databases (appendix 9-2). the compiled density values‟ pattern was associated with a triangular distribution; this is reported in table 9-5. parameter value (kg/m3) minimum 2120 maximum 2800 most probable 2540 table 9-5. estimated parameters for total rock density, assuming a triangular distribution. 9.4.4 absorbed gas concentration (9-3) equation 9-3, obtained from the canadian shale gas prospects information (appendix 9-2.), was applied to the toc data extracted from the geochemical atlas of colombia to obtain absorbed gas concentration values (appendix 9-3). such values led to finding statistical regression distributions for the variable‟s pattern, basin by basin. table 9-6 summarises the distributions, estimation parameters and goodness-of-fit test results for the best fits found. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l vmm lognormal 2.71 2.67 2.75 0.38 0.35 0.41 was rejected 0.04 4.21 1 cat gamma 2.12 1.92 2.35 7.18 6.42 8.04 was not rejected m.b 0.44 0 ces-ran gamma 6.02 4.98 7.28 2.06 1.69 2.50 was not rejected m.b 0.27 0 cag-put weibull 15.77 15.27 16.29 1.47 1.43 1.51 was not rejected m.b 89.08 0 cor gamma 66.65 61.30 72.47 0.04 0.03 0.04 was rejected m.b 109.12 3 lla gamma 2.74 2.56 2.93 4.76 4.41 5.12 was not rejected m.b 6.67 0 vsm lognormal 2.73 2.70 2.75 0.49 0.47 0.50 was rejected 0.01 9.76 2 sin-sja gamma 4.63 4.01 5.35 2.96 2.54 3.44 was not rejected m.b 47.04 0 vim lognormal 2.42 2.40 2.44 0.31 0.30 0.33 was not rejected m.b 0.75 0 tum lognormal 2.56 2.48 2.65 0.40 0.35 0.47 was not rejected m.b 15.04 0 cho exponential 40.18 30.94 54.32 n/a n/a n/a was not rejected m.b 0.17 2 table 9-6. goodness-of-fit test distributions, fit parameters and results applied to the absorbed gas concentration data. estimated parameters ̂ ̂ are the mean and standard deviation for a lognormal distribution, form and scale for a gamma distribution and “a” and “b” for the weibull distribution. estimated ̂ represents the mean for an exponential distribution. cag-put, cor, lla, vsm, sin-sja, vim, tum, cho refer to the caguán putumayo , the eastern cordillera, the eastern llanos, the upper magdalena valley, sinú san jacinto, the lower magdalena valley, tumaco and chocó basins, respectively. the data used for the chocó basin referred to outcrops, as no well data was available. n/a refers to parameters which did not apply to the type of distribution being considered. m.b refers to a very low probability of the null hypothesis being accepted or rejected. goodness-of-fit test response for the middle magdalena valley, the eastern cordillera and upper magdalena valley basins suggested that all distributions tested would be rejected, even though with very low probability of being rejected. bearing this in mind, the distribution giving the lowest value was chosen for such basins (appendix 9-3). the only data reported for the chocó basin concerned outcrops, so this was taken as no well data was available. no toc data or well or outcrop data had been reported for the guajira, vaupés amazonas, cauca – patía, urabá or amagá basins, so no gas concentration values were calculated; shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 192 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia instead, they were associated with the same pattern given for this variable in analogous basin as follows: the cauca patía and amagá basins were associated with the tumaco distribution, guajira with that for cesar ranchería, vaupés amazonas with that for caguán putumayo and urabá with the sinú san jacinto distribution. 9.4.5 free gas concentration free gas concentration values, inferred from s1 data (appendix 9-3), were analysed for determining the distribution best fitting their pattern, in the same way as was done with absorbed gas concentration values. likewise, analogous basins‟ distribution was used for basins lacking information. table 9-7 gives the estimation parameters‟ values and the statistical tests made for the best fit found. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l vmm lognormal 2.49 2.37 2.62 1.21 1.12 1.30 was not rejected 0.10 2.82 1 cat gamma 0.64 0.59 0.71 31.33 27.48 35.72 was not rejected m.b 4.41 0 ces-ran exponential 20.59 18.03 23.74 n/a n/a n/a was rejected 0.04 13.33 0 cag-put gamma 0.20 135.4 n/r n/r n/r n/r was not rejected 0.08 6.87 3 cor exponential 18.72 17.64 19.90 n/a n/a n/a was not rejected m.b 0.10 0 lla gamma 0.15 140.1 n/r n/r n/r n/r was not rejected 0.06 7.57 3 vsm exponential 46.62 44.73 48.63 n/a n/a n/a was rejected 0.01 7.51 1 sin-sja exponential 13.56 12.23 15.12 n/a n/a n/a was not rejected m.b 1.20 0 vim lognormal 1.24 1.16 1.32 1.15 1.10 1.21 was rejected m.b 17.22 2 tum weibull 3.95 2.33 6.71 0.41 0.36 0.47 was not rejected m.b 0.16 0 cho weibull 32.41 20.82 50.43 0.68 0.55 0.84 was not rejected m.b 0.65 0 table 9-7. goodness-of-fit test distributions, fit parameters and results applied to the free gas concentration data. n/r refers to parameters which, as they went beyond the algorithm‟s range of calculations which were not reported when making the statistical estimation. 9.4.6 net-to-gross international databases an extreme value distribution gave the best fit for the net-to-gross data calculated from international sources (appendix 9-2). table 9-8 shows the results of such fit. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l -0.3 -0.52 -0.09 0.3 0.22 0.42 was not rejected 0.27 3.94 3 table 9-8. goodness-of-fit parameters and test results applied to the ntg data in appendix 8-2. estimated parameters ̂ ̂ are extreme value distribution location and scale. 9.4.7 hydrocarbons in shale the result of monte carlo simulation for calculating the hydrocarbons in shale is shown below, with the variables defined in equation 9-1, and their associated distributions. the percentage of evaluated resources which could be lying within environmental conservation areas, forestry reserves and natural parks was subtracted; tables 9-9 and 9-10 give the final results. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 193 basin p10 p10 p50 p50 p90 p90 (tcf) (103mmboe) (tcf) (103mmboe) (tcf) (103mmboe) amagá 1.37 0.24 0.41 0.07 0.06 0.01 caguán putumayo 2,687.60 463.38 127.56 21.99 13.81 2.38 catatumbo 185.43 31.97 38.98 6.72 4.89 0.84 cauca patía 28.57 4.93 8.55 1.47 1.34 0.23 cesar ranchería 107.13 18.47 28.46 4.91 4.18 0.72 chocó 22.73 3.92 6.89 1.19 1.08 0.19 the eastern cordillera 729.26 125.73 206.65 35.63 31.61 5.45 guajira 132.77 22.89 35.34 6.09 5.22 0.90 the eastern llanos 6,619.59 1,141.31 1,042.44 179.73 125.40 21.62 sinú san jacinto 441.33 76.09 113.24 19.52 16.37 2.82 tumaco 56.72 9.78 16.97 2.93 2.66 0.46 urabá 91.22 15.73 23.37 4.03 3.36 0.58 the lower magdalena valley 121.31 20.92 35.05 6.04 5.39 0.93 the middle magdalena valley 148.80 25.65 43.50 7.50 6.71 1.16 the upper magdalena valley 45.38 7.82 13.47 2.32 2.09 0.36 vaupés, amazonas 3,228.09 556.57 154.55 26.65 16.61 12.86 total 14,647.29 2,525.39 1,895.44 326.80 240.78 41.51 table 9-9. hydrocarbons in shale in scenario 1 after subtracting resources possibly present in environmental conservation areas. basin p10 p10 p50 p50 p90 p90 (tcf) (103mmboe) (tcf) (103mmboe) (tcf) (103mmboe) amagá 1.98 0.34 0.68 0.12 0.11 0.02 caguán putumayo 4,177.70 720.29 200.22 34.52 23.71 4.09 catatumbo 272.43 46.97 63.51 10.95 8.30 1.43 cauca patía 41.27 7.12 14.22 2.45 2.28 0.39 cesar ranchería 155.19 26.76 47.04 8.11 7.18 1.24 chocó 32.82 5.66 11.44 1.97 1.85 0.32 the eastern cordillera 1,046.89 180.50 340.56 58.72 53.81 9.28 guajira 191.78 33.07 58.18 10.03 8.89 1.53 the eastern llanos 9,736.40 1,678.69 1,703.01 293.62 215.61 37.17 sinú san jacinto 643.41 110.93 186.87 32.22 27.67 4.77 tumaco 81.95 14.13 28.16 4.86 4.52 0.78 urabá 132.29 122.81 38.52 6.64 5.76 0.99 the lower magdalena valley 173.81 29.97 57.83 9.97 9.23 1.59 the middle magdalena valley 213.78 36.86 71.85 12.39 11.48 1.98 the upper magdalena valley 65.50 11.29 22.35 3.85 3.59 0.62 vaupés, amazonas 5,075.44 875.08 240.21 41.42 28.29 4.88 total 22,042.67 3,800.46 3,084.66 531.84 412.29 71.08 table 9-10. hydrocarbons in shale in scenario 2, after subtracting resources possibly present in environmental conservation areas. 9.4.8 potential of shale oil and shale gas the scarce information available about geological, geochemical and geometric characteristics and hydrocarbon resources available in shale belts has hindered a reliable shale oil/shale gas ratio being established. the degree of thermal maturity in a shale belt is not homogeneous and such ratio must thus be a little subjective. in spite of this, the data available in the “review of emerging resources: us shale gas and shale oil play” (us department of energy, 2011) has been used for consolidating an average for such ratio in basins having crude and gas windows. table 9-11 shows gas and crude resources in three basins in the usa and a 0.11 to 0.48 range regarding shale oil/ [total hydrocarbon]. even though the scarce data observed did not enable understanding the above as being representative, it was evident that giving an idea about the order of magnitude could lead to understanding the problem. assuming a 0.3 value and 20% favourable geological risk, suggested by the “world shale gas resources: an initial assessment of 14 regions outside the united states” (us department of energy, 2011), shale oil resources for the scenarios analysed reached the figures presented in tables 9-12 and 9-13. similarly, net shale gas resources are shown in tables 9-14 and 9-15. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 194 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia play gas crude fraction tcf mmboe mmbbl oil/(oil+gas) barnett 75.5 13 017.2 1 580.0 0.11 rocky mountain region 43.0 7 413.8 3 590.0 0.33 eagle ford 20.8 3 587.9 3 350.0 0.48 table 9-11. oil/[hydrocarbon] ratio for three us shale belts, taken from us department of energy (2011). graphs 9-30 to 9-33 compare shale gas and oil resources between basins extracted from table 99, and colombia‟s total potential within the international framework (figure 9-34) taken from table 9-1. basin p10 p50 p90 tcf amagá 0.2 0.1 0.0 caguán putumayo 376.3 17.9 1.9 catatumbo 26.0 5.5 0.7 cauca patía 4.0 1.2 0.2 cesar ranchería 15.0 4.0 0.6 chocó 3.2 1.0 0.2 the eastern cordillera 102.1 28.9 4.4 the guajira 18.6 4.9 0.7 the eastern llanos 926.7 145.9 17.6 sinú san jacinto 61.8 15.9 2.3 tumaco 7.9 2.4 0.4 urabá 12.8 3.3 0.5 the lower magdalena valley 17.0 4.9 0.8 the middle magdalena valley 20.8 6.1 0.9 the upper magdalena valley 6.4 1.9 0.3 vaupés, amazonas 451.9 21.6 2.3 total 2,050.6 265.4 33.7 table 9-12. gas fraction present in shale from colombian basins. scenario 1. basin p10 p50 p90 mmbbl amagá 14.4 4.2 0.6 caguán putumayo 27,802.8 1,319.4 142.8 catatumbo 1,918.2 403.2 50.4 cauca patía 295.8 88.2 13.8 cesar ranchería 1,108.2 294.6 43.2 chocó 235.2 71.4 11.4 the eastern cordillera 7,543.8 2,137.8 327.0 the guajira 1,373.4 365.4 54.0 the eastern llanos 68,478.6 10,783.8 1,297.2 sinú san jacinto 4,565.4 1,171.2 169.2 tumaco 586.8 175.8 27.6 urabá 943.8 241.8 34.8 the lower magdalena valley 1,255.2 362.4 55.8 the middle magdalena valley 1,539.0 450.0 69.6 the upper magdalena valley 469.2 139.2 21.6 vaupés amazonas 33,394.2 1,599.0 771.6 total 151,523.4 19,608.0 3,090.6 table 9-13. the crude fraction present in shale from several colombian basins. scenario 1. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 195 basin p10 p50 p90 tcf amagá 0.3 0.1 0.0 caguán putumayo 584.9 28.0 3.3 catatumbo 38.1 8.9 1.2 cauca patía 5.8 2.0 0.3 cesar ranchería 21.7 6.6 1.0 chocó 4.6 1.6 0.3 the eastern cordillera 146.6 47.7 7.5 the guajira 26.8 8.1 1.2 the eastern llanos 1,363.1 238.4 30.2 sinú san jacinto 90.1 26.2 3.9 tumaco 11.5 3.9 0.6 urabá 18.5 5.4 0.8 the lower magdalena valley 24.3 8.1 1.3 the middle magdalena valley 29.9 10.1 1.6 the upper magdalena valley 9.2 3.1 0.5 vaupés, amazonas 710.6 33.6 4.0 total 3,086.0 431.9 57.7 table 9-14. gas fraction present in shale from several colombian basins. scenario 2. basin p10 p50 p90 mmboe amagá 20.4 7.2 1.2 caguán putumayo 43,217.4 2,071.2 245.4 catatumbo 2,818.2 657.0 85.8 cauca patía 427.2 147.0 23.4 cesar ranchería 1,605.6 486.6 74.4 the chocó 339.6 118.2 19.2 the eastern cordillera 10,830.0 3,523.2 556.8 the guajira 1,984.2 601.8 91.8 the eastern llanos 100,721.4 17,617.2 2,230.2 sinú san jacinto 6,655.8 1,933.2 286.2 tumaco 847.8 291.6 46.8 urabá 7,368.6 398.4 59.4 the lower magdalena valley 1,798.2 598.2 95.4 the middle magdalena valley 2,211.6 743.4 118.8 the upper magdalena valley 677.4 231.0 37.2 vaupés, amazonas 52,504.8 2,485.2 292.8 total 234,028.2 31,910.4 4,264.8 table 9-15. crude fraction present in shale from several colombian basins. scenario 2. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 196 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 9-30. map of shale gas potential in scenario 1. figure 9-31. map of shale oil potential in scenario 1. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 197 figure 9-32. shale gas potential in scenario 1. figure 9-33. shale oil potential in scenario 1. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 198 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 9-34. shale gas resources in colombia compared to those reported for other countries. 9.4.9 sensitivity analysis ntg usually most influenced estimates for all basins in both scenarios evaluated here, nearly always followed by ga. figure 9-35 shows the results from a sensitivity analysis. figure 9-35. average sensitivity for all basins in which hydrocarbons in shale was estimated. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 199 9.5 conclusions o estimated of the hydrocarbons shale potential is controlled by basin area, showing that basins covering a greater area were the most prospective ones; o the eastern llanos basin had the greatest prospectivity in p90, regarding the presence of hydrocarbons in shale, followed by the eastern cordillera and vaupés amazonas; o the order of the 3 basins having most estimated resources was maintained in p50; o estimated shale gas resources in p90 for the middle magdalena valley basin (6.71 tcf) and ranchería basin in cesar (4.18 tcf) were lower than the values estimated by d. little in 2008 (289 tcf and 7.72 tcf, respectively). resources were greater for the eastern cordillera (31.61 tcf compared to 19.3 tcf); and o the lower magdalena valley, caguán putumayo and sinú san jacinto basins‟ prospectivity was high, even though control in their areas must be taken into account regarding the estimated results. shale from the palaeozoic age in the eastern llanos and caguán putumayo, could have a positive impact on their hydrocarbon potential. 9.6 bibliography agencia nacional de hidrocarburos. (2007). colombian sedimentary basins: nomenclature, boundaries and petroleum geology, a new proposal. bogota: b & m exploration ltda. agencia nacional de hidrocarburos. (2010). atlas geoquímico de colombia. (un.colombia, ed.) bogota. arthur, m. a., schlanger, s. o., & jenkyns, h. c. (1987). the cenomanian–turonian oceanic anoxic event, ii. palaeoceanographic controls on organic-matter production and preservation. london geological society special publications, 26, 401-420. beaton, a. p., pawlowicz, j. g., anderson, s. d., & rokosh, c. d. (2009). rock eval, total organic carbon, adsorption isotherms and organic petrography of the banff and exshaw formations: shale data release. alberta geological survey. alberta: energy resources conservation board. beaton, a. p., pawlowicz, j. g., anderson, s. d., & rokosh, c. d. (2009). rock eval, total organic carbon, adsorption isotherms and organic petrography of the colorado group: shale data release. alberta geological survey. alberta: energy resources conservation board. beaton, a. p., pawlowicz, j. g., anderson, s. d., & rokosh, c. d. (2010). rock eval, total organic carbon, adsorption isotherms and organic petrography of the duverny and muskwa formations in alberta: shale data release. alberta geological survey. alberta: energy resources conservation board. beaton, a. p., pawlowicz, j. g., anderson, s. d., & rokosh, c. d. (2010). rock eval, total organic carbon, adsorption isotherms and organic petrography of the montney formation in alberta: shale data release. alberta geological survey. alberta: energy resources conservation board. boyer, c., kieschnick, j., suarez, r., lewis, r. e., & waters, g. (2006). producing gas from its source. oilfield review, autumn, 36-49. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 200 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia d. little, a. (2008). evaluation of potential of unconventional energy resources in colombia. agencia nacional de hidrocarburos. houston: arthur d. little inc. erbacher, j., hubert, b. t., norris, r. d., & markey, m. (2001). intensified thermohaline stratification as a possible cause for an ocean anoxic event in the cretaceous period. nature, 409, 325-327. loucks, r. g., reed, r. m., ruppel, s. c., & jarvie, d. m. (junio de 2009). morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the mississippian barnett shale. journal of sedimentary research, 79, 848-861. meyer, b. l., & nederlof, m. h. (febrero de 1984). identification of source rocks on wireline logs by density / resistivity and sonic transit time / resistivity crossplots. the american association of petroleum geologist bulletin, 68(2), 121-129. mojica, j., arévalos, o. j., & castillo, h. (diciembre de 2009). agencia nacional de hidrocarburos. consulted on june xx 2011, de informacion geológica and geofisica givesciones and poster técnicos: www.anh.gov.co passey, q. r., creaney, s., kulla, j. b., moretti, f. j., & stroud, j. d. (december 1990). a practical model for organic richness from porosity and resistivity logs. the american association of petroleum geologist bulletin, 74(12), 1777-1794. rios , a. (2010). gas no convencional: hacia un nuevo paradigma. (americas business news, ed.) energy intelligence series, 1-19. ryan, w. b., & cita, m. b. (1977). ignorance concerning episodes of ocean wide stagnation. marine geology, 23, 197-215. schmoker, j. w. (2005). u.s. geological survey assessment concepts for continuous petroleum accumulations. in u.s. geological survey, petroleum systems and geologic assessment of oil and gas in the southwestern wyoming province, wyoming, colorado, and utah. denver: u.s. department of the interior. thiede, j., & van andel, t. h. (1977). the paleoenvironment of anaerobic sediments in the late mesozoic south atlantic. earth and planetary science letters, 33, 301-309. u.s. department of energy. (2009). modern shale gas development in the united states: a premier. oklahoma city: ground water protection council. u.s. department of energy. (2011). world shale gas resources: an initial assessment of 14 regions outside de united states. washington: advanced resources international. utah geological survey. (2005). shale gas reservoirs of utah: survey of an unexploited potential energy resource. utah geological survey. salt lake city: geox consulting inc. west, j. (1996). international petroleum encyclopedia. tulsa: pennwell publishing co. shale gas and shale oil earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 201 zhao, h., givens, n. b., & curtis, b. (abril de 2007). thermal maturity of the barnett shale determined form well-log analysis. the american association of petroleum geologist bulletin, 91(4), 535-549. 9.7 appendixes digital files in the anh's document center:  appendix 8-1: “shale gas”  appendix 8-2: “base de datos internacional shale gas.xlsx”  appendix 8-3: “base de datos shale gas colombia.xlsx”  kindom suit project earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 10 gas resources in tight sands 10.1 general comments gas in tight sands (unconventional natural gas which is difficult to access because of the nature of the rock and sand surrounding the deposit) is the term used for referring to low permeability deposits mainly producing dry natural gas. low permeability is considered to be less than 0.1 milidarcies (md) (holditch, 2006). figure 10-1 shows a cross-section of a tight area. tight gas sands is defined as being a belt of sands or carbonates (which may contain natural fractures) exhibiting less than 0.1 md in-situ gas permeability (figure 1); many "extra tight " reservoirs may have less than 0.001 md in-situ permeability, meaning that gas usually flows through these rocks at low rates and special methods are needed for its production (naik, 2003). figure 10-1. comparison between conventional sand and tight sand. on left-hand-side: high porosity (39%) and permeability sand; right-hand-side: tight sand. the dark grey areas are the pores whilst the clearer grey ones are quartz and heavy minerals, modified from (ingrain digital rock physics lab s.f.). 10.1.1 origin and formation effective porosity, viscosity, fluid saturation and capillary pressure are some of the parameters affecting a deposit‟s effective permeability (naik 2003). these are controlled by the setting in which a deposit occurs and later processes which such deposits undergo. even though fine grain sand and limolite deposits saturated with gas are particularly prone to forming tight gas sands, postdepositional compaction and cementation can reduce permeability in thick grain sediments, leading to the formation of tight rocks. a reservoir‟s architecture is another important factor regarding permeability. different settings regarding deposits lead to different architectures, thereby resulting in completely different reservoir properties. permeability within individual sediment layers could thus be much lower than total reservoir permeability determined per well (muntendam-bos et al., 2009). gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 204 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia the deposit settings most associated with forming tight gas plays are deep basins, delta-fronts and the banks of dikes on food plains or river systems (muntendam-bos et al., 2009). 10.1.2 exploration of and production from tight gas deposits considerable data regarding nuclei, geophysical records, drilling records and well tests must be complied for the correct tight gas play evaluation and development; this is often more the case for evaluating a tight gas sand play than for evaluating a conventional gas reservoir (holditch, 2006). natural fractures play a determinant role in tight rock permeability. hydraulic fracturing (fracking) could produce new fractures in a particular deposit or open up existing natural fractures. their properties and the regime regarding formation efforts must thus be ascertained for planning wells and hydraulic fracturing for optimising production. it should be stressed that even though tight gas reservoirs (differently to conventional gas reservoirs) often have a very poor response to fracking, resulting in low production rates and high economic risk. figure 10-2 gives a diagram showing the permeability and porosity associated with tight rocks. tight gas sands reservoirs require advanced techniques for reducing the migration distance of fluids from a formation towards a particular well. horizontal and multilateral well drilling are the most modern technologies for tight gas reservoir production, as well as low-balance drilling and stimulation and cementation technologies (naik, 2003). data from different sources is needed for the effective characterisation of tight gas sands deposits, i.e. seismic (multi-component, 3d seismic imaging), specialised records, cementation and stimulation methodologies/designs, drilling and conventional subsoil data, reservoir engineering data. all the foregoing is used for defining the lithological and geometrical combination regarding the faults and fractures associated with a particular reservoir. 10.1.3 tight gas sands distribution around the world tight rock deposit technology has been developed in the usa for more than 40 years (holditch, 2006); different technologies have been developed during this time for improving economic success rates. much of this technology is currently being used around the world to contribute towards both tight gas sands and conventional reservoir development. the most studied reservoirs have been in north america due to the high level of activity and the volume of available data for analysing them; however, it is expected that activity levels will become increased regarding tight gas or unconventional gas exploration in latin-american, russia, the middle east, asia and other major gas production areas tight gas sand resources are distributed throughout the whole world; even though the greatest accumulations have been found in north america, russia and china, many unexplored reservoirs are outside the usa (total, 2007). gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 205 figure 10-2. ranges of porosity and permeability for tight gas sands reservoirs, modified from total (2007). there has been little development in the field of tight gas sands deposits due to scarce knowledge about their geology and pertinent information in addition to natural gas policy and market conditions not having been favourable for their development in several countries. rogner (1996) has estimated tight gas sands around the world to be about 7,400 tcf (table 10-1). region tight gas sands volume (tcf) north america 1,371 latin-america 1,293 western europe 353 central and eastern europe 78 the soviet union 901 north africa and the middle east 823 sub-saharan africa 784 the central plain of asia and china 353 pacific (organisation for economic cooperation and development) 705 pacific asia 549 southern asia 196 total around the world 7,406 table 10-1. estimating tight gas sands around the world, taken from rogner (1996). 10.2 data and hypotheses 10.2.1 international data tight gas sands‟ reservoir databases in the usa were consulted for estimating yet-to-find tight gas sands resources, as well as studies aimed at estimating such resources around the world. 10.2.2 national (colombian) data anh petroleum exploitation records concerning wells in continental basins in colombia was used; such information involved records for 435 wells located in the guajira, sinú san jacinto, urabá, cesar ranchería, catatumbo, the lower and upper middle magdalena valley, the eastern cordillera, eastern llanos and caguán putumayo basins (figure 10-3). data was also taken from anh reports. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 206 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 10-3. map showing the location of wells having information from records, i.e. 435 wells (red points) from which records were loaded. 10.2.3 hypotheses tight gas sands volume for colombia‟s continental basins was estimated in line with the following hypotheses: 10.2.3.1 hypothesis 1 depending on a particular basin‟s genesis, there is an almost constant ratio between total tight gas sands-containing plays‟ area and the pertinent basin‟s total area. such ratio can be expressed as: 10.2.3.2 hypothesis 2 areas having tight gas sands may be discriminated by gamma ray, neutron porosity, density and permeability records. such areas fulfil the following conditions: gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 207 o gamma ray < tight-sand cut-off value o permeability < 0.1 md o neutron porosity < porosity per density 10.2.3.3 hypothesis 3 there is a ratio between resources and the area of a basin containing tight gas sands reservoirs (figure 10-4); such ratio leads to estimating tigh gas resources in basins for which no well information is available. figure 10-4. the graph shows the ratio between gas and the area of a basin containing tight gas sands reservoirs. 10.2.3.4 hypothesis 4 tight gas sands‟ volume for colombian continental basins can be estimated in line with the following scenarios: scenario 1 the values obtained regarding thickness, effective porosity and water saturation could be grouped for basins having similar characteristics. considering all the formations found having tight gas sands, distributions could be estimated for each variable thereby leading to resources being assessed. scenario 2 only tight rocks saturated with gas, having thickness greater than 23 feet thick, could validate using techniques for exploiting the resources and only these should be taken into account for the estimating tigh gas sands‟ potential. 10.3 methodology the following equation was used for calculating tight gas sand volume in colombian continental basins: ( ) (10-1) gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 208 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia : in situ gas (tcf) : tight gas sands occurrence area (km2) : thickness of areas containing tight gas sands (km) : total rock density (v/v) : water saturation (v/v). : gas expansion factor, is equal to where bg is the volumetric factor of gas (ft3normal / ft3deposit). the methodology used for estimating resources included the following activities: o available well and record data was loaded into a kingdom suite project; o the data regarding the loaded well records was reviewed and edited to determine which wells had the necessary records for making the petro-physical calculations and discriminating tight gas-bearing areas; and o effective porosity, water saturation and permeability were calculated for the selected wells. the kingdom suite‟s petro-physics module was used for making the calculations. the parameters used for the calculations were:  a density/neutron gas porosity model was used for estimating effective porosity: √ ( ) dhge: effective porosity per density-neutron area of gas (v/v) : effective porosity per density (v/v) : effective neutron porosity (v/v)  stieber‟s gamma ray model was used for estimating clay volume : clay volume in the formation (v/v) gri: gamma ray index (api) ( ) : gamma ray from the records (api) : gamma ray in clean formations (api) gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 209 : gamma ray in 100% clay areas (api) the and values used can be verified in the file cited in the appendixes.  the simandoaux model was used for calculating water saturation. ( ) ([( ) ( ) ] ) m: cementation exponent n: saturation exponent sw: water saturation (v/v) rw: water resistivity (ohm-m) rt: area resistivity (ohm-m) rsh: shale resistivity (ohm-m) vsh: volume of clay in the formation (v/v) water and shale resistivity values can be found in the file cited in the appendixes.  permeability was estimated using the wylie-rose ratio: k: permeability (md) : effective porosity (v/v) : irreducible water saturation (v/v) o porosity per density was estimated using a sand matrix: dphi: porosity per density rhom: matrix density: 2.65 g/cm3 rhob: bulk density (g/cm3) rhof: fluid density: 1 g/cm3 o a record was produced indicating areas of gas having neutron porosity and porosity per density records, using the following condition: dphi: porosity per density phi: neutron porosity gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 210 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia o a gamma ray cut-off was established leading to differentiating the areas of sands in each well (figure 10-5). o the thickness of areas having the presence of tight gas sands was calculated per well fulfilling the necessary conditions (figure 10-5); average effective porosity and water saturation values were estimated for these areas. adding the areas complying with the foregoing conditions led to establishing effective thickness for tight gas sands per well. figure 10-5. montoya a-1 well record. track 1 shows the gamma ray; the yellow areas represent sands (80 api sand cut-off); track 2 shows the permeability record, light blue highlighting tight areas (permeability < 0.1 md); track 3 shows neutron porosity records (blue) and porosity per density (red) and areas having gas (orange); the last track shows records indicative of gas, 1 standing for areas having gas and 0 where there is no gas. the light green rectangle shows areas of tight gas sands. o a ratio between a particular basin‟s total area and the area of plays containing tight gas sands was calculated for 13 basins in the usa (figure 10-6) for estimating the percentage area of a basin which could contain tight gas sands (hypothesis 1). this factor was applied to colombia‟s continental basins. the values used for obtaining the ratio used here can be found in the file cited in this report‟s appendixes. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 211 o monte carlo simulation was used for calculating tight gas sands‟ potential with equation 101. this was only done for basins having the necessary data available; and o potential for basins lacking the necessary information was calculated by using the ratio between estimated in-situ gas volume and basin area, according to hypothesis 3. figure 10-6. map of the main tight gas plays in 13 basins in the usa (us energy information administration). 10.4 results when estimating resources using equation 10-1, the area was determined according to hypotheses 1 and taken as being constant. the others variables used were defined by distributions of probability obtained from analysing the petro-physical records for the 62 wells showing evidence of tight gas sands (figure 10-7). the data used, as well as the distributions, are contained in the aforementioned file “colombian tight gas sands” which is attached to this report. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 212 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 10-7. map showing the location of the 63 wells used for estimating the gas potential in tight sands. 10.4.1 area of tight gas sands occurrence analysing the data regarding basins in the usa led to identifying that 34.8% of a basin‟s area could contain tight gas sands. this factor was applied to the total area of continental basins in colombia for obtaining the basins‟ effective area for this resource. basin area (km2) effective areas (km2) amagá 2,824.9 983.1 caguán putumayo 110,304.1 38,385.8 catatumbo 7,715.0 2,684.8 cauca patía 12,823.3 4,462.5 cesar ranchería 11,668.7 4,060.7 chocó 38,582.0 13,426.5 the eastern cordillera 71,766.2 24,974.6 guajira 13,778.9 4,795.1 the eastern llanos 225,603.3 78,509.9 sinú san jacinto 39,644.6 13,796.3 tumaco 23,732.4 8,258.9 urabá 9,448.9 3,288.2 the lower magdalena valley 38,017.4 13,230.1 the middle magdalena valley 32,949.4 11,466.4 the upper magdalena valley 21,512.8 7,486.5 vaupés amazonas 154,867.3 53,893.8 table 10-2. effective area of a particular basin having gas potential in tight sands. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 213 10.4.2 the thickness of tight gas sands areas the gamma ray, permeability, neutron porosity and density records led to summing areas per well which complied with the conditions established for containing tight gas sands. two scenarios were established for the calculations. in the first, the data obtained (63 wells having tight gas sands) were analysed by groups of basin, 3 different areas being defined. a distribution was thus estimated for each group from all the values so obtained (table 10-3). the distributions obtained were used for each basin contained in the respective areas. the second scenario only considered the results for wells reported to have thicknesses greater than 23 ft for tight gas sands, thereby reducing the number of wells to 15; a single distribution of probability was thus found for all the data for calculating all of colombia‟s continental basins (table 10-4). porosity and water saturation were dealt with in the same way. 10.4.3 sediment porosity the distribution parameters obtained for the 3 areas defined in scenario 1 are gives in table 10-5. table 10-6 gives the results for scenario 2. 10.4.4 water saturation table 10-7 gives probability functions and goodness-of-fit tests used for the areas produced in scenario 1. table 10-8 gives the results for scenario 2. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l area 1 gamma 0.47 0.30 0.72 0.02 0.01 0.04 was not rejected m.b 0.89 0 area 2 lognormal -5.81 -6.73 -4.89 1.96 1.49 2.86 was not rejected m.b 0.05 0 area 3 lognormal -7.13 -7.71 -6.55 1.05 0.77 1.65 was not rejected m.b 0.02 0 table 10-3. goodness-of-fit test distributions, fit parameters and results applied to the thickness data used in estimating the gas potential in tight sands in scenario 1. area 1 groups the caguán putumayo, the eastern cordillera and the eastern llanos basins. area 2 groups catatumbo, cesar ranchería, the middle magdalena valley and upper magdalena valley basins. area 3 groups the sinú san jacinto, urabá and the lower magdalena valley basins. estimated parameters ̂ ̂ are gamma distribution form and scale and the mean and standard deviation for lognormal distribution. m.b refers to a very low probability of the null hypothesis being accepted or rejected. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.22 -0.43 0.86 0.03 0.01 0.07 was not rejected m.b 0.42 0 table 10-4. pareto distribution goodness-of-fit test results and parameters applied to the thickness data used in estimating tight gas sands in scenario 2. estimated parameters ̂ ̂ are pareto distribution form and scale. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 214 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l area 1 extreme minimum 0.07 0.06 0.07 0.02 0.01 0.02 was not rejected m.b 0.02 0 area 2 triangular 0.07 0.02 0.09 n/a n/a n/a n/r n/r n/r n/r area 3 extreme minimum 0.06 0.05 0.07 0.02 0.01 0.02 was not rejected m.b 0.29 0 table 10-5. goodness-of-fit test distributions, fit parameters and results applied to porosity data used in estimating the gas potential in tight gas sands in scenario 1. estimated parameters ̂ ̂ are extreme value distribution location and scale. estimated x for triangular distribution is the most probable value, and their confidence interval gives minimum and maximum values as extremes. n/a refers to parameters which did not apply to the type of distribution being considered. “n/r” refers to parameters which, as they went beyond the algorithm‟s range of calculations, were not reported when making the statistical estimation. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 12.23 4.72 31.68 226.07 84.23 606.79 was not rejected m.b 0.08 0 table 10-6. beta distribution and goodness-of-fit parameters and test results applied to porosity data used in estimating tight gas sands in scenario 2. estimated parameters ̂ ̂ are beta distribution parameters “a” and “b”. basin distribution estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l area 1 beta 0.66 0.15 3.04 0.15 0.12 0.18 was not rejected m.b 16.15 0 area 2 weibull 0.83 0.73 0.95 3.61 2.47 5.26 was not rejected m.b 0.44 0 area 3 beta 0.29 0.001 165.7 0.02 0.01 0.05 was not rejected m.b 14.22 0 table 10-7. goodness-of-fit test distributions, fit parameters and results applied to the water saturation data used in estimating tight gas sands in scenario 1. estimated parameters ̂ ̂ are weibull distribution parameters “a” and “b”, respectively. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 0.33 n/a n/a 0.99 n/a n/a n/r n/r n/r n/r table 10-8. uniform distribution goodness-of-fit parameters and test results applied to the water saturation data used in estimating tight gas sands in scenario 2. estimated parameters ̂ ̂ are uniform distribution maximum and minimum, respectively. 10.4.5 gas expansión a series of values obtained from anh reports for different gas fields in colombian basins was used for obtaining a probabilistic distribution for gas expansion. distribution functions associated with such data and their goodness-of-fit parameters are listed in table 10-9. estimated parameters goodness-of-fit test ̂ confidence interval ̂ confidence interval null hypothesis p statistical value g.l 5.26 5.18 5.34 0.30 0.26 0.37 was not rejected 0.11 6.10 3 table 10-9. lognormal distribution goodness-of-fit parameters and test results applied to the gas expansion factor data used in estimating tight gas sands in both scenarios. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 215 10.4.6 sensitivity analysis the distribution functions established for random variables (thickness, porosity, water saturation, gas expansion) resulted from analysing the distribution of known data regarding the area. consequently, sensitivity analysis of the basins in question suggested that tight gas sands exploration studies for colombian continental basins must reduce uncertainty, mainly regarding thickness and water saturation, according to sensitivity analysis results in both scenarios. the greatest uncertainty in this scenario was presented regarding gas layer thickness (69.99%) and water saturation (24.45%); the variables having least uncertainty were gas expansion and porosity (figure 10-8). figure 10-8. percentage analysis of the sensitivity of the variables used for estimating tight gas sands. 10.4.7 sensitivity analysis – scenario 2 likewise, the greatest uncertainty was presented regarding gas layer thickness (53%) and water saturation (36.4%); the variables having least uncertainty were gas expansion and porosity (figure 10-9). figure 10-9. percentage analysis regarding the sensitivity of the variables used for estimating gas in tight sands. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 216 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 10.4.8 estimating resources for those basins lacking information there was no well information or production field data regarding some of colombia‟s continental basins to enable determining the necessary distributions for using equation 10-1; a ratio was thus identified in such cases associating basin area and the volume of tight gas sands contained by such basin (figures 10-10 and 10-11). a linear fit for such ratios led to identifying tendencies for p90, p50 and p10. the equations derived from the linear fit led to estimating gas values for basins lacking pertinent information. the basins to which these ratios were applied due to a lack of data are listed in table 10-10. basin total area km2 effective area km2 amagá 2,824.929129 983.0753367 patía cauca 12,823.30684 4462.510782 chocó 38,581.98137 13,426.52952 tumaco 23,732.41861 8,258.881678 vaupes amazonas 154,867.3143 53,893.82539 table 10-10. basins lacking data for estimating tight gas sands. 10.4.8.1 basin area cf gas volume ratio scenario 1 figure 10-10. a graph of basin area compared to gas volume and the ratios obtained for p90, p50 and p10 in scenario 1. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 217 10.4.9 basin area cf gas volume ratio scenario 2 figure 10-11. graph of basin area compared to gas volume and the ratios obtained for p90, p50 and p10 in scenario 2. 10.4.10 tight gas sands in colombia based on the aforementioned hypotheses and the definition of the random variables expressed in equation 10-1, gas resources in tight sands were calculated for basins having available data and linear ratios were applied for basins lacking the necessary information for p90, p50 and p10 from the monte carlo simulation results. gas was thus estimated for colombia‟s continental basins in the aforementioned two scenarios. tables 10-11 and 10-12 shows the results obtained after discounting resources found in areas where environmental considerations applied. basin tight gas sands scenario 1 p90 (tcf) p90 (mmboe) p50 (tcf) p50 (mmboe) p10 (tcf) p10 (mmboe) amagá 0.0000 0.0026 0.0009 0.0884 0.0087 1.4696 caguán putumayo 0.0001 0.0102 0.0415 7.0815 1.2001 206.9095 catatumbo 0.0002 0.0358 0.0047 0.8368 0.1018 17.5149 patía cauca 0.0001 0.0150 0.0030 0.4618 0.0450 7.6911 ranchería cesar 0.0003 0.0570 0.0080 1.3450 0.1620 27.9970 chocó 0.0003 0.0450 0.0078 1.3597 0.1312 22.6611 the eastern cordillera 0.0000 0.0062 0.0258 4.4087 0.7513 129.5166 guajira 0.0002 0.0274 0.0020 0.3331 0.0156 2.7540 the eastern llanos 0.0001 0.0205 0.0870 15.0096 2.5674 442.5870 sinú san jacinto 0.0004 0.0709 0.0054 0.8756 0.0422 7.2294 tumaco 0.0002 0.0273 0.0049 0.8328 0.0809 13.8847 urabá 0.0001 0.0184 0.0010 0.2249 0.0107 1.8572 the lower magdalena valley 0.0004 0.0760 0.0050 0.9340 0.0450 7.7230 the middle magdalena valley 0.0009 0.1620 0.0220 3.7990 0.4560 78.6260 the upper magdalena valley 0.0006 0.0975 0.0130 2.2910 0.2785 48.0563 vaupes amazonas 0.0009 0.1458 0.0251 4.3703 0.4225 72.8408 total 0.005 0.818 0.257 44.252 6.319 1089.318 table 10-11. tight gas sands in scenario 1, discounting resources in natural parks. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 218 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia basin tight gas sands scenario 1 p90 (tcf) p90 (mmboe) p50 (tcf) p50 (mmboe) p10 (tcf) p10 (mmboe) caguán putumayo 0.147 25.725 0.858 147.597 6.720 1158.744 catatumbo 0.009 1.909 0.065 10.968 0.498 86.376 ranchería cesar 0.019 2.959 0.094 16.953 0.773 133.703 the eastern cordillera 0.110 19.273 0.640 110.528 5.028 867.051 guajira 0.020 3.720 0.120 21.290 0.970 167.020 the eastern llanos 0.343 59.264 1.977 341.302 15.555 2682.450 sinú san jacinto 0.053 9.491 0.312 54.532 2.504 431.407 urabá 0.010 2.472 0.078 14.254 0.655 112.389 the lower magdalena valley 0.059 9.965 0.332 57.170 2.608 449.818 the middle magdalena valley 0.045 7.913 0.260 45.531 2.081 359.386 the upper magdalena valley 0.029 5.656 0.185 32.306 1.463 252.428 amagá 0.002 0.330 0.010 1.483 0.068 11.496 patía cauca 0.010 1.540 0.040 6.920 0.310 53.860 chocó 0.030 4.630 0.120 20.830 0.940 162.040 tumaco 0.019 2.646 0.065 11.901 0.538 92.533 vaupes amazonas 0.086 14.565 0.384 65.558 2.955 509.886 total 0.991 172.058 5.540 959.123 43.666 7530.587 table 10-12. tight gas sands in scenario 2, discounting yet-to-find tight gas sands in natural park areas. figure 10-12. map of tight gas sands estimated for colombia‟s continental basinsscenario 2. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 219 figure 10-13. tight gas sands estimated for colombia‟s continental basins scenario 2. figure 10-14. tight gas sands around the world. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 220 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 10.5 conclusions o the information about wells led to identifying that the basins having the greatest tight gas sands in p10 were the middle magdalena valley, the upper magdalena valley and sinú san jacinto, for scenario 1 and the eastern llanos, caguán putumayo and the eastern cordillera for scenario 2; o analysing the results obtained in p50 and p10 from well information showed that the most prospective basins were the eastern llanos, caguán in putumayo and the eastern cordillera in both scenarios; o the results obtained regarding tight gas sands in colombia regarding those reported by adl (1.2 tcf) were considerably greater in scenario 2 (1.07–46.66 tcf) and ranged from 0.01–6.73 tcf in scenario 1; and o part of the tight gas sands potential was found to lie in natural park areas (scenario 1: p10 0.408 tcf p50 0.018 tcf p90 0 / scenario 2: p10 2.99 tcf p50 0.40 tcf p90 0.08 tcf). 10.6 bibliography aziz jabr, abdel. unconventional reservoirs episode 1: tight gas reservoir. spe suez canal university student chapter. 2010. http://www.spesuez.org/spe/virtualcampus/unconventionalreservoir.html (last consulted: 2nd november, 2011). chakhmakhchev, alex, and kris mcknight. searching for tight gas reservoir. http://a1024.g.akamai.net/f/1024/13859/1d/ihsgroup.download.akamai.com/13859/energy/s earchingfortightgasreservoirs.pdf (last consulted: 2nd november, 2011). china national petroleum corporation. status and development prospects of china´s unconventiona natural gas exploration and exploitation. http://uschinaogf.org/forum9/pdfs/xinhua_english.pdf (last consulted: 2nd november, 2011). department of resources, energy and tourism. australian energy resource assessment 2010. 2010. http://adl.brs.gov.au/data/warehouse/pe_aera_d9aae_002/aerach_04.pdf (last consulted: 2nd november, 2011). energy, centrica. unconventional gas in europe response to decc consultation. 2010. http://www.decc.gov.uk/assets/decc/what%20we%20do/global%20climate%20change%20and %20energy/international%20energy/policy/1290-unconventional-gas-centrica-response.pdf (last consulted: 2nd november, 2011). gas, unconventional gas subgroup of the technology task group of the npc committee on global oil and unconventional gas reservoirs tight gas, col seams, and shales. the national petroleum conuncil (npc), 2007. holditch, stephen a. tight gas sands. society of petroleum engineers, 2006: 86-94. muntendam-bos, a. g., b.b. t. wassing, j. h. heege, and f. bergen. tno report inventory nonconventional gas. utrecht: tno built enviroment and geosciences, 2009. gas resources in tight sands earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 221 naik, g c. tight gas reservoirs: an unconventional natural energy source for the future. 2003. rogner, hans-holger. an assessment of world hydrockrbon resources. iiasa, 1996: 96-26. shahab, alam. potential of tight gas in pakistan: productive, economic and policy aspects. search and discovery article #80149 , 2011. total. the know how series // tight gas reservoirs technology intensive resources. paris: exploration & production, 2007. u. s. energy information administration energy mineral division. http://emd.aapg.org/technical_areas/tightgas.cfm (last consulted: 2nd november, 2011). u.s.a. energy information administration. http://205.254.135.24/pub/oil_gas/natural_gas/analysis_publications/maps/maps.htm (last consulted: 31st october 2011). vidas, harry, and bob hugman. availability, economics, and production potential of north american unconventional natural gas supplies. ingaa foundation, inc. 2008. http://www.ingaa.org/file.aspx?id=7878 (last consulted: 2nd november, 2011). 10.7 appendixes digital files in the anh's document center:  “gas arenas apretadas colombia.xlsx”  kindom suit project earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 11 heavy crudes 11.1 general comments one of liquid hydrocarbons‟ characteristics is its fluidity or viscosity, indirectly reflecting its density (or specific gravity). this property is slightly conditioned by temperature, thereby leading to defining the type of crude in terms of specific gravity and degrees api (american petroleum institute) being used as measurement unit. when petroleum has greater viscosity and density it will be heaviest; the denser the crude, the lower are its api degrees (table 11-1). hydrocarbon api condensed > 40.0 light crude 30.0<api≤40.0 medium crude 22,0<api≤29,9 heavy crude 10,0<api≤21,9 extraheavy crude <10.0 bitumen crude, asphalt, sand table 11-1. types of hydrocarbons regarding api. classification regarding heavy crudes‟ physical properties is ambiguous. viscosity is variable in each type of compound; the h/c ratio becomes reduced when gas and conventional oil change from being natural to becoming heavy, asphalts and then kerogen, even though this not a weighty parameter for differentiating them. the environmental factors which determined the way in which degradation (e.g. oxidation, bacteria, being washed by water, etc) or maturation lead to resolving such ambiguity. 11.1.1 origin and characteristics o petroleum becomes heavy due to the degradation which could occur during migration and entrapment. degradation could result from various processes, such as: o a biological, chemical or physical process, by bacteria transported by surface water metabolising the hydrocarbons in the heaviest molecules; o by means of formation water removing the lightest components by solution, due to these being most soluble in water; o due to crude‟s volatility when a seal is not efficient, allowing the lightest molecules to pass through its interconnected pores; and o another characteristic of these crudes is that they have high sulphur content. likewise, they could have an appreciable salt content and also contain metals (i.e. nickel, vanadium). they may sometimes contain a certain amount of hydrogen sulphide. the terms heavy and extra-heavy attempt to group hydrocarbons having specific properties, essentially having density values higher than those for water (>920 kg/m3), in deposits in sedimentary rocks (whether consolidated or not) containing highly viscous bitumen, solid or semisolid hydrocarbons. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 224 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia the accumulation of heavy crudes is associated with young formations and/or shallow deposits having low efficiency rocks seals. during many phases of hydrocarbons being generated in and expelled from the basins during the cretaceous and palaeogene periods large volumes migrated towards their margins, originating accumulations of heavy crudes at relatively shallow depths according to a particular petroleum system‟s conditions allowing this. there is usually a high probability of heavy crudes being found in any onshore sedimentary basin. the basins having the greatest presence of heavy crudes in colombia or having proven evidence are the eastern llanos, caguán in putumayo, the eastern cordillera, the upper and middle magdalena valley and chocó (table 11-2, figure 11-1). basin rock source migration rock reservoir seal trap caguán, putumayo caballos and villeta fm migration from west to east throughout the sandstone in the caballos and villeta fm. vertical migration throughout the fractures and fault areas. expulsion during the late miocene after the formation of larger structures caballos. villeta and pepino fm villeta, rumiyaco and orteguaza fm structural traps associated with thrusting and subthrust in the west of the basin and upthrusts in the foreland. pinching out and carbonates chocó iró fm lateral migration. vertical migration associated with the fault systems iró and la mojarra fm naturally fractured deposits la siena and itsmina fm structural highs, anticlines having a mud nucleus on diapir flanks, anticlinal thrusts, normal faults, carbonated rocks and fractured cherts in fault areas the eastern cordillera la luna fm middle albian and turonian condensed sections the first generation pulse occurred during the late cretaceous. the second pulse occurred from the miocene up to the present albian and cenomanian and palaeogene siliciclasts esmeralda, mugrosa. and socha fm inverse faults involving plinths, inversion of normal faults, faults related to folding and duplex structures the eastern llanos gacheta and villeta fm eocene upper oligocene, miocene and continuing up to the present une, gacheta, carbonera (c3, c5 and c7) and mirador gacheta, carbonera (c2, c8) faulted anticlines, antithetic faults, stratigraphic traps the middle magdalena valley la luna v simití-tablezo fm direct vertical migration due to eocene nonconformity or through faults and lateral migration throughout the sands of the eocene period lisama. esmeraldas-la paz, and coloradomugrosa, la luna, umir, barco fm esmeraldas, colorado fm potentially the shales of the simiti and umir fm 1) faults related to folding below thrust surfaces, 2) duplex structures having faults with independent closure, 3) structural strikes where the deposits‟ strata dip against the faults, 4) traps in the hanging wall fault-block the upper magdalena valley caballos. monserrate. honda fm table 11-2. general comments regarding potential basins‟ petroleum system. fm refers to formation. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 225 figure 11-1. basins having heavy crudes in colombia. 11.2 data and hypotheses the set of compiled data attached as part of this document in the appendix 1, contains information from heavy crudes studies for colombia. the data contains information regarding wells, formation limits and bases, thicknesses, lithological analysis, stratigraphs and recovery factors. such information/data was used for calculating weighting factors per scenario and resource category, estimating porosity, thickness and water saturation distribution functions from the results of each goodness-of-fit test and correlations made. the information about heavy crudes occurrences in colombia was obtained from the following sources: o seismic and well information supplied by anh referring to areas having heavy crudes; o an integral study carried out between anh and halliburton in 2007 estimating sustainable proven and probable reserves for crudes having gravity less than or equal to (<) 20 api in the eastern llanos basin; heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 226 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia o a reserves and resources report containing data from heavy crude-producing fields in colombian basins (resolution 494/2009, issued by anh); o publications by anh forming part of an agreement with the eafit, uis and uptc universities associated with evaluating potential of various sedimentary basins; and o other publications regarding heavy crudes in colombia. 11.2.1 hypotheses the compiled information was scrutinised to identify plays having the presence of heavy crudes in colombian basins. a weighting factor was then applied to such recorded areas to reduce uncertainty regarding the effective presence of heavy crudes in line with the following hypotheses: 11.2.1.1 hypothesis 1 the maximum value for an area containing hydrocarbons having lower than (<) 22 api density in a mature sedimentary basin is defined by the ratio between existing fields‟ total area and a particular basin‟s total extension; 11.2.1.2 hypothesis 2 total production thickness is derived from reported well and field production data, seismic information and field studies about a formation of interest; 11.2.1.3 hypothesis 3 thickness is derived from well data and/or that reported in plays, establishing net thickness having greater than 10% effective porosity, lower than 30% clay volume and plays having less than 50% water saturation; 11.2.1.4 hypothesis 4 porosity and water saturation values will be derived from well data and production information from assessing net and play thickness; 11.2.1.5 hypothesis 5 the distribution for basins having scarce field information regarding heavy crudes will be considered triangular, assuming maximum and minimum values as limits and the average as the value having the greatest probability; and 11.2.1.6 hypothesis 6 basins having potential interest arise from areas reported as having heavy crudes. 11.3 methodology the following equations were used for evaluating heavy crudes in colombian basins, as follows: ( ) (11-1) : (original oil in place): volume of deposit‟s original petroleum (mmbbl) heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 227 7758.367: conversion factor acre/foot to barrels of oil a: basin area (acres) h: play area total thickness (ft) ntg: net-to-gross ratio (net/h) hpay: play thickness, where sw > 50% : porosity (%) sw: water saturation (%) bo: play formation volumetric factor, expressed as barrel in deposit/barrel on the surface the procedure for estimating resources considered the following stages: o existing information regarding heavy crude deposits dealing with plays, well data, lithology, thickness and saturation was reviewed:  a database was built from well information relating formation and play thickness, porosity and saturation. the compiled information contained the name of the well, its location, net thickness, porosity and saturation having heavy crudes of interest (appendix 11-1);  greater than 10% effective porosity, lower than 30% clay volume and play thickness having less than 50% water saturation were considered for estimating net thickness from well information (halliburton, 2006);  play thickness was established for the different fields from the reported information, also considering porosity, thickness, saturation and real crude recovery values;  the porosity (< 10%) described in production reports was assumed for the eastern cordillera basin;  three scenarios were considered for the llanos basin, as follows:  scenario 1: heavy crudes were calculated from the net-to-gross (ntg) ratio derived from well data relating net thickness to total thickness (halliburton, 2006);  scenario 2: heavy crudes were calculated from play thickness derived from well data (halliburton, 2006); and  scenario 3: heavy crudes were calculated from reported play thickness (anh, 2010); o distribution functions were established for thickness, porosity and saturation in yet-to-find areas:  goodness-of-fit tests were applied, considering the best distribution for each variable and distribution parameters and their pertinent confidence intervals were estimated; o an aerial ratio was established allowing maximum extension for heavy hydrocarbon occurrence per basin to be considered. according to vargas (2009), each sedimentary basin gives a fraction of area which could reflect the whole of its hydrocarbon potential in actual technological exploration and production conditions. such area may be projected from the heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 228 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia ratio regarding total production area in a mature basin to such basin‟s total area. annual production data per hydrocarbon field having < 22° api density and those reported to the anh were used for such modelling; and o monte carlo simulations used the sooip equations for calculating heavy crudes, taking thickness, porosity, water saturation and volumetric factor as distribution functions. 11.4 results the random variables used in equation 11-2 were used for iterative monte carlo simulation. only the results of the best fits and estimates following 500,000 iterations are given in this report; the others results may be consulted in digital files in appendix 11-2. except for the area involved, statistical analysis of the variables analysed in the different basins has been expressed in line with the following parameters (table 11-4): net and play distribution extreme max log normal beta exponential extreme max log normal triangular triangular average 93 74 23 80 89 31 27 430 mean 78 32 20 55 80 18 28 387 standard deviation 88 156 15 80 53 40 10 256 minimum 1 0.5 2 4 3 3 1 32 maximum 1259 426 86 180 271 152 55 1,154 porosity distribution beta beta extreme min extreme max beta logistical triangular triangular average 22% 22% 21% 20% 21% 14% 12% 12% mean 22% 22% 22% 19% 21% 14% 12% 12% mode 20% 20% 24% 17% 22% 14% 12% 12% standard deviation 7% 7% 7% 5% 4% 3% 2% 2% minimum 10% 10% 10% 15% 6% 7% 8% 8% maximum 48% 48% 33% 26% 30% 25% 15% 15% sw distribution extreme min extreme min extreme min log normal logistica l beta triangular triangular average 36% 36% 29% 34% 38% 25% 33% 86% mean 37% 37% 30% 29% 38% 24% 35% 85% mode 40% 40% 33% 23% 38% 21% 45% 81% standard deviation 10% 10% 10% 16% 11% 11% 10% 5% minimum 3% 3% 10% 18% 16% 6% 5% 78% maximum 50% 50% 42% 49% 50% 45% 50% 99% bo distribution log normal log normal log normal extreme min extreme max beta triangular triangular average 1.06 1.06 1.06 1.03 1.08 1.13 1.09 1.07 mean 1.05 1.05 1.05 1.04 1.07 1.11 1.08 1.07 mode 1.03 1.03 1.03 1.07 1.05 1.04 1.05 1.05 standard deviation 0.05 0.05 0.05 0.09 0.06 0.09 0.02 0.03 minimum 1.01 1.01 1.01 1.01 1 1.03 1.05 1.01 maximum 1.19 1.19 1.19 1.1 1.2 1.4 1.16 1.16 table 11-3. estimated statistical parameters regarding calculation variables, corresponding to net thickness (hnet), play thickness (hpay), porosity ( ), water saturation (sw) and volumetric factor per basin (bo). heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 229 variable estimated parameters goodness-of-fit test statistical value distribution ẋ confidence interval ӯ confidence interval null hypothesis p statistical value g.l llanos scenario 1 extreme maximum 181.06 166.10 196.02 233.90 226.47 241.57 1% 0.00 175% 0% llanos scenario 2 lognormal 2.49 2.30 2.69 1.59 1.46 1.74 0% 29% 2.5% 2% llanos scenario 3 beta -0.48 -0.67 -0.28 34.78 25.81 46.86 0% 1% 1.7% 3% upper magdalena valley exponential 80.22 53.05 nan nan 135.35 nan 0% nan 0.9% 0% middle magdalena valley extreme max 120.63 95.94 145.31 69.90 56.25 86.87 0% nan 0.3% 0% caguán – putumayo lognormal 2.97 2.75 3.19 0.93 0.80 1.12 1% 3% 5% 0% eastern cordillera triangular 27 1 55 nan nan nan nan nan nan nan chocó triangular 430.67 32 1154 nan nan nan nan nan nan nan table 11-4. goodness-of-fit distribution parameters and test results applied to net (hnet) and play thickness data used in estimating heavy crudes. variable estimated parameters goodness-of-fit test statistical value distribution ẋ confidence interval ӯ confidence interval null hypothesis p statistical value g.l llanos scenario 1 beta 7.85 5.05 10.65 27.92 17.56 38.27 0% 0.5% 5.5% 6% llanos scenario 2 beta 7.85 5.05 10.65 27.92 17.56 38.27 0% 0.5% 5.5% 6% llanos scenario 3 extreme min 0.24 0.23 0.25 0.04 0.03 0.05 0% 0% 4% 3% upper magdalena valley extreme max 0.21 0.19 0.23 0.04 0.02 0.05 0% nan 6% 0% middle magdalena valley betapert 16.7 8.16 25.35 63.62 27.79 99.44 0% nan 0% 0% caguán – putumayo logistical 0.13 0.13 0.14 0.014 0.012 0.01 nan nan nan nan eastern cordillera triangular 0.12 0.08 0.15 nan nan nan nan nan nan nan chocó triangular 0.12 0.07 0.15 nan nan nan nan nan nan nan table 11-5. goodness-of-fit parameters‟ distribution and test results applied to the porosity data, used in estimating heavy crudes. variable estimated parameters goodness-of-fit test statistical value distribution ẋ confidence interval ӯ confidence interval null hypothesis p statistical value g.l llanos scenario 1 extreme min 0.40 0.38 0.41 0.07 0.06 0.08 0% 0% 8% 5% llanos scenario 2 extreme min 0.40 0.38 0.41 0.07 0.06 0.08 0% 0% 8% 5% llanos scenario 3 extreme min 0.33 0.31 0.35 0.07 0.06 0.09 0% 0% 7% 3% upper magdalena valley lognormal -1.17 -1.34 -1.00 0.36 0.28 0.52 0% nan 1% 0% middle magdalena valley logistic 0.38 0.34 0.41 0.05 0.04 0.07 nan nan nan nan caguán – putumayo beta 3.8 2.33 5.34 11.24 6.43 16.05 0% 0% 9% 5% eastern cordillera triangular 0.33 0.05 0.50 nan nan nan nan nan nan nan chocó triangular 0.89 0.77 0.99 nan nan nan nan nan nan nan table 11-6. goodness-of-fit distribution parameters and test results applied to the water saturation data used in estimating heavy crudes. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 230 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia variable estimated parameters goodness-of-fit test statistical value distribution ẋ confidence interval ӯ confidence interval null hypothesis p statistical value g.l llanos scenario 1 lognormal 0.06 0.03 0.08 0.04 0.03 0.07 0% nan 6% 0% llanos scenario 2 lognormal 0.06 0.03 0.08 0.04 0.03 0.07 0% nan 6% 0% llanos scenario 3 lognormal 0.06 0.03 0.08 0.04 0.03 0.07 0% nan 6% 0% upper magdalena valley extreme min 1.07 1.06 1.08 0.02 0.02 0.03 0% nan 0% 0% middle magdalena valley extreme max 1.10 1.08 1.12 0.05 0.04 0.06 0% nan 2% 0% caguán – putumayo beta 1.12 0.91 1.43 0 nan 41% 0% eastern cordillera triangular 1.09 1.05 1.16 nan nan nan nan nan nan nan chocó triangular 1.07 1.01 1.16 nan nan nan nan nan nan nan table 11-7. goodness-of-fit distribution parameters and test results applied to volumetric factor data used in estimating heavy crudes. 11.4.1 area areas were calculated using the approach proposed by vargas (2009); heavy crude production information in several sedimentary basins was taken into account. figure 11-2 shows that maximum production per basin in current technological conditions and reported 1 mmboe per hectare maximum condition would be 1.7%. following this approach, the effective areas have been estimated for each basin‟s having heavy crudes (table 11-7). figure 11-2. approach adopted for deducing basins‟ maturity and efficiency based on production areas regarding total basin area. basin maximum area for heavy hydrocarbons (acres) llanos 395,896 the upper magdalena valley 37,810 the middle magdalena valley 57,960 the eastern cordillera 126,283 caguán putumayo 193,984 chocó 27,480 table 11-8. areas having maximum occurrence of heavy hydrocarbons per basin. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 231 11.4.2 heavy crudes in colombia table 11-8 gives monte carlo simulation results for the calculations regarding the potential of crudes having <22° api density. the scenarios for the eastern llanos basin illustrate the results‟ variability regarding the thickness used. figures 11-3 and 11-4 give a synthesis of the estimates made in this work. basin potential (mmbbl) p90 p50 p10 llanos scenario 1 6,806 46,735 319,455 llanos scenario 2 4,879 29,700 168,610 llanos scenario 3 5,178 20,082 51,529 the middle magdalena valley 3,526 10,216 22,459 caguán putumayo 1,628 5,794 21,500 the upper magdalena valley 582 4,444 16,318 the eastern cordillera 1,369 3,052 5,467 chocó 313 1,145 3,454 total 14,224 71,384 388,654 table 11-9. heavy crudes for colombia, excluding areas in natural parks and environmental reserve areas for the basins. total resource for all the basins has been estimated from scenario 1 for the eastern llanos basin. figure 11-3. heavy crudes for basins of interest. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 232 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 11-4. resumen del potencial de crudos pesados para colombia. 11.4.3 sensitivity analysis a sensitivity analysis regarding the distribution functions used in estimating heavy crude (thickness, porosity, saturation and recovery factor), suggesting that target formation thickness was the random variable having the greatest uncertainty (figure 11-5), consequently exploratory exercises should make greater efforts at defining it (porosity was next in order followed by saturation). the areas‟ reflectivity must also be validated due to the nature of the hypotheses used in this work. figure 11-5. analysis of the sensitivity of the variables used in calculating heavy crudes for potential basins. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 233 11.5 conclusions o the sensitivity analysis suggested that thickness was the most sensitive variable in calculating potential, followed by porosity and saturation; o the basins having the greatest heavy crude prospectivity were the eastern llanos and middle magdalena valley; and o heavy crudes in the five colombian basins analysed came within the p90 14,224 mmbbl and p10 388,654 mmbbl range. 11.6 bibliography anh. (2010). orgaic geochemistry atlas of colombia, scale 1:2,800,000. bogotá. anh. (s.f.). perforación del primer pozo de reconocimiento del subsuelo en la cuenca chocó, subcuenca san juan, como parte de la visión de la agencia nacional de hidrocarburos de colombia. consulted on the march 2011, de agencia nacional de hidrocarburos: www.anh.gov.co curtis carl, d. e. (2003). yacimientos de petróleo pesado. oilfield review 14 , 3, 32-55. e&p. (2011). exploration and production. obtained from http://www.epmag.com ecpa. (2011). energy and climate partnership of the americas . obtained from http://www.ecpamericas.org: http://www.ecpamericas.org/data/files/initiatives/heavy_oil_wkg/2nd_heavy_oil_working_group _report_esp.pdf fonade-uis-anh. (2009). informe ejecutivo evaluacion del potencial hidrocarburífero de las cuencxas colombianas. bogotá. gems ltda, anh. (2007). caracterización geoquímica de rocas y crudos en las cuencas de cesar ranchería, sinú san jacinto , chocó y area de soápaga (cuenca cordillera oriental). bogotá. halliburton. ( 2006). estudio integral que permita calculas the reserves provadas and probables sustentables para crudos con gravityes menores o iguales a 20 api en la basin de the the eastern llanos. bogotá,. http://ercb.ca/portal/server.pt/gateway/ptargs_0_0_303_263_0_43/http;/ercbcontent/publis hedcontent/publish/ercb_home/public_zone/ercb_process/enerfaqs/enerfaqs12.aspx. http://www.eia.doe.gov. stach, e. (1982). stach's textbook of coal petrology. berlin: grebruder . u.s. geological survey . (1990). bulletin no. 1944. uptc-anh. (2009). cartografía geológica de 51267.45 km2 en la cuenca caguán – putumayo a partir de sensores remotos a escala 1:100.000 y 739 km2 con control de campo a escala 1:50.000 en las planchas icag 413 y 414 departamentos de meta, caquetá, putumayo. bogotá. heavy crudes earth sciences research journal, volume 16, special issue, april, 2012 234 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia usgs. (2010). "an estimate of recoverable heavy oil resources of the orinoco oil belt, venezuela. vargas jiménez, c. a. (2009). nuevos aportes a la estimacion del potencial de hidrocarburos en colombia. revista colombiana de ciencias , 33, 17-34. yen, g. c. (1978). bitumens, asphalt and tar sands. amsterdam, the netherlands: elsevier scientific publishing company. yen, t. (1984). characterization of heavy oil, in meyer, r.f., wynn, j.c., and olson, j.c., eds., the future of heavy crude and tar sands. 2nd international conference, caracas (pages. p. 412-423.). new york: mcgraw-hill. 11.7 appendixes digital files in the anh's document center:  “anexo 1 base de datos crudos pesadoscolombia.xlsx” earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 12 production projections 12.1 general comments production projections form part of development schemes where variables such as reserves, infrastructure, technical aspects regarding the process and the market are sufficiently well-known within the framework of conceptual models predicting future scenarios. the uncertainty of predictions or forecasts is linked to the sustainability of actual production conditions and models in the case of developing oil and conventional gas fields. in turn, permanent changes in production are coming regarding reserves of non-conventional resources due to the nature of the resource and ongoing refinement of extraction. however, the ranges of uncertainty change drastically when trying to forecast production scenarios regarding any type of resources from estimates of yet-to-find hydrocarbons. the dramatic change in the magnitude of uncertainty intervening in calculating potential impedes developing realistic schemes regarding production projections. a lack of historical records regarding conventional production in colombia and scarce reports around the world means that environmental debate has been related to shale exploitation (oil and gas), hydrates and tar sands, regulatory problems regarding coalbed methane (cbm) and costly investment in developing heavy and extraheavy crudes and tight sands. all this creates a restrictive framework imposing limitations on reliable scenarios being established. so, production projections can only be made for conventional oil and gas. however, even though there are historical records regarding conventional production in colombia, the availability of resources is only part of the problem. other macroeconomic variables intervene in production such as the price of crude, national capacity for exporting the resource and its derivatives, production and worldwide reserves, etc. a production forecast is given in this work by analysing a time series regarding national production and other macroeconomic variables in line with hypotheses concerning cointegral relationships. 12.1.1 cointegral time series these deal with non-stationary time series whose linear combination becomes stationary, thereby reflecting long-term equilibrium conditions. engle and granger (1987) have given a broad and formal mathematical description of them. their development has been linked to a description of econometric series of different kinds. they have been used in the present work as an approach for forecasting oil and gas production in colombia, based on annual average evolution of reserves and production in colombia and around the world, hydrocarbon price and the amount of oil exports and its derivatives from colombia. production projections earth sciences research journal, volume 16, special issue, april, 2012 236 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 12.2 data and hypotheses 12.2.1 data data regarding colombian production, reserves and exports was taken from different institutions: o average annual evolution of reserves and production in colombia (anh, 2011); o reserves and production around the world, hydrocarbon price (bp, 2011); and o the amount of oil exports and its derivatives from colombia (banrepublica, 2011). most series were comparable from 1980 (table 12-1). historical record period having complete data oil production in colombia 1980-2010 oil production around the world 1980-2010 oil price (usd/barrel) 1980-2010 gas production in colombia 1980-2010 gas production around the world 1980-2010 gas price (u$/btu) 1980-2010 colombian exports of oil and its derivatives 1980-2010 table 12-1. time series taken into account for forecasting oil and gas production in colombia in line with cointegral variables. 12.2.2 hypotheses only analysing production projections from resources can guarantee historical production information in colombia, i.e. conventional oil and gas. the time series were thus used and took the following hypotheses into account: 12.2.2.1 hypothesis 1 territorial prospectivity (resources and reserves‟ volume) is no more important than colombia‟s political situation and/or that around the world. depending on these parameters‟ tuning, production will be increased or reduced. 12.2.2.2 hypothesis 2 hydrocarbon prices, reserves, production and demand around the world and colombian exporting capacity will reflect the political situation; and 12.2.2.3 hypothesis 3 the time series regarding reserves in colombia and amounts of exports regarding oil-derived products, the same as annual historical figures regarding the price of worldwide crude or gas production and reserves may be approached using cointegral series. 12.3 methodology the work consisted of collecting the longest, most coincident time series, and those which were reflective of processes promoting hydrocarbon production in colombia and which could guarantee estimating a cointegral series, thereby inferring oil and gas production pattern during the next 20 years. a cointegral series guarantees long-term equilibrium, i.e., leads to inferring a linear ratio production projections earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 237 between a set of variables which has been maintained for a long period of time. the production pattern has been forecast in the present work in the following way: o the time series shown in table 12-1 were homogenised in annual steps; o the time series seasonal patterns hypothesis was accepted and the engle-granger numerical approach adopted (engle and granger, 1987); o four scenarios were estimated which combined oil or gas production with the remaining five time series shown in table 1. the most stable triplets were chosen; and o scenarios having annual steps were projected until year 25 (frontier), but only the results for 5, 10, 15 and 20 years were evaluated. 12.4 results given that this was an exercise involving a mobile analysis of historical records, its forecasts were not asymptotic. the projections rather maintained a certain pattern derived from the records being evaluated. figures 12-1 and 12-2 give the results of the analysis from treatment as cointegral variables. figure 12-1. forecast of oil production in colombia from analysing time series from a cointegral variables‟ approach. the continuous blue line shows the historical record of oil production in colombia. the dashed lines indicate projected oil production intervals (20 years), taking the other two cointegral variables (numbed within black squares) into account. production projections earth sciences research journal, volume 16, special issue, april, 2012 238 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 12-2. forecast for gas production in colombia from analysing time series from a cointegral variables‟ approach. the continuous blue line shows the historical record of gas production in colombia. the dashed lines indicate projected gas production intervals (20 years), taking the other two cointegral variables (numbed within black squares) into account. 12.5 bibliography anh. figures and statistics 2011. 2011. http://www.anh.gov.co/es/index.php?id=8 (last accessed: 30th november 2011) banrepublica. exportaciones totales (fob) principales exportaciones y resto de exportaciones. http://www.banrep.gov.co/estad/dsbb/sec_ext_009.xls (latest access: 30th november 2011) bp. statistical review of world energy 2011. http://www.bp.com/sectionbodycopy.do?categoryid=7500&contentid=7068481 (latest access: 30th november 2011) engle, r f, and c w j granger. cointegration and error correction: representation, estimation and testing. econometrica 55, 1987: 25 1-276. 12.6 appendixes digital files in the anh's document center:  “aceite_gas_col_pronostico_producción.xlsx” [escribir texto] ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 13 executive report 13.1 general comments the in situ potential of conventional and non-conventional hydrocarbons has been estimated, as well as that of resources in colombia. the results have been based on geoscientific information acquired to date and supplied by anh and open-access national and international sources the methodologies used and the hypotheses supporting the estimates have been described throughout the chapters of this work. it is worth stating that such estimates have been based on the assumption that all colombian basins may have conventional or non-conventional hydrocarbon potential, even when no occurrences have been evidenced in some of them, or exploratory studies have not been carried out to show this. likewise, the calculations (monte carlo simulations) have been intimately linked to the hypothesis directly relating aerial extension to in situ potential. 13.2 estimated potential of conventional hydrocarbons in colombia three approaches have been adopted for estimating hydrocarbon resources in colombia. the first concerned conventional resources generated and trapped in colombian sedimentary basins and the second adopted a volumetric in situ (or in place) approach which could be inferred from current exploration information (ooip and giip). the third was based on a fractal geometric approach in line with a hypothesis concerning the sustainability of economic, socio-environmental and political conditions allowing findings and production in past years. table 13-1 gives a summary of such overall estimates where conventional resource‟s share can be appreciated. good consistency was usually observed between volumetric results (p90) and the fractal approach (weak scenario). it seemed evident that there was a significant availability of gas which has not been detected in both cases. greater exploratory efforts must be made in the next few years aimed at adding new reserves. estimate oil (mmbbl) gas (tcf) p10 p50 p90 p10 p50 p90 generated and trapped 1,918,000 335,000 56,000 2,558 447 75.0 in place (ooip and igip) 430,361 117,963 20,000 234 28 3.5 % of resource generated/trapped ~22.4% ~35.2% ~35.7% ~9.1% ~6.3% ~4.7% yet-to-find (ytf) resources high 33,146 medium 26,072 low 22,798 high 348.9 medium 49.9 low 6.6 % of resource being produced (p90) ~1.7% ~7.8% ~40.7% ~13.6% ~11.1% ~8.8% table 13-1. synthesis of conventional hydrocarbon generated in colombia, as well as volumetric (ooip and igip) and fractal estimates (ytf). figure 13-1 gives the results obtained for conventional oil in colombia in the present work and compares them with results found by other authors regarding colombia. the results were comparable and allude to recoverable resources. two types of ooip calculations were used in this work: a 20% average recovery factor (rf) and 30% geological risk (gr) factor. basins with significant potential for conventional hydrocarbons were usually: executive report earth sciences research journal, volume 16, special issue, april, 2012 240 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia o ooip: vaupes in the amazonas, colombia and deep pacific basins o igip: the eastern llanos, colombia and deep pacific basins figure 13-1. conventional crude potential for colombia, estimated by different authors. uis (universidad industrial de santander), usgs (united states geological survey), adl (arthur de little). the ooip range is the value estimated in this study, modified from vargas (2009). 13.3 estimated potential of non-conventional hydrocarbons in colombia table 13-2 shows non-conventional resources in colombia. the range of recoverable resources and the basins having the greatest prospectivity have been suggested. a sensitivity analysis associated with the most critical variables has been included; such analysis will allow prioritising and planning exploratory work in terms of each resource figure 13-2 compares the results obtained in the present study with those obtained by arthur de little (adl, 2008), discriminating each non-conventional resource. the results presented in this work were comparable with the estimates made by adl (2008) concerning most estimated resources. the only case where adl results did not come within this report‟s range of values concerned gas hydrates; adl results gave 400 tcf (74,862 mmboe) by contrast with 4.89 75.63 tcf (843 13,040 mmboe) in this work (figure 13-2). executive report earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 241 resource results (maximum-minimum) most prospective basins (p50) sensitivity analysis (variable having the greatest uncertainty) gas hydrates 4.89 – 75.63 tcf marine chocó marine guajira marine sinú marine tumaco hydrate saturation los cayos, colombia and deep pacific basins were not analysed regarding their complete extension due to a lack of information coalbed methane scenario 1: 0.75 77.5 tcf scenario 2: 14.7 – 21.9 tcf sinú san jacinto cesar ranchería the upper magdalena valley the middle magdalena valley gc (average in situ gas content scf/ton) tar sands scenario 1: 22.6 990.4 mmbbl measured resources scenario 2: 155.7 – 6,812.6 mmbbl indicated resources scenario 3: 1,056.9 46,244.8 mmbbl inferred resources scenario 4: 3,455.1 151,173.8 mmbbl hypothetical resources the eastern cordillera the eastern llanos the middle magdalena valley the upper magdalena valley caguán putumayo thickness of the layers oil shale scenario 1: 60.5 91,078.0 mmbbl indicated resources scenario 2: 511.7 677,745.2 mmbbl hypothetical resources the eastern cordillera chocó the upper magdalena valley the middle magdalena valley the uncertainty of all the variables used in the calculations was considerable, due to a lack of information shale gas shale oil scenario 1: 33.7 – 2,050.6 tcf scenario 2: 57.7 – 3,086.0 tcf scenario 1 3,090.6 – 157,523.4 mmbbl scenario 2 4,264.8 – 234,028.2 mmbbl the eastern llanos the eastern cordillera caguán putumayo the eastern llanos the eastern cordillera caguán putumayo net-to-gross the original resource has been risked net-to-gross the original resource has been risked (30%) tight gas sands scenario 1: 0.005 – 6.3 tcf scenario 2: 0.9 – 43.7 tcf the eastern llanos caguán putumayo the eastern cordillera thickness of the gas and water saturation layers heavy crude 14,224 – 388,654 mmbbl the eastern llanos the middle magdalena valley caguán putumayo formation thickness table 13-2. results obtained in estimates of non-conventional resources for colombia. executive report earth sciences research journal, volume 16, special issue, april, 2012 242 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia figure 13-2. non-conventional resources for colombia estimated in this work and compared to adl‟s estimates (2008). 13.4 hydrocarbon resources matrix a summary of the basins having the greatest probability for the presence of conventional and nonconventional resources is given below. the matrix synthesises the estimates derived from the available geological information or from data from exploratory or production projects. the most optimistic scenario was considered for estimated resources (table 13-3). executive report earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 243 basin / resources (mmbbl / tcf) conventional crude (mmbbl) associated and free gas (tcf) heavy crude (mmbbl) p10 p50 p90 p10 p50 p90 p10 p50 p90 offshore 276,413.0 75,815.0 12,570.0 205.8 20.6 1.9 los cayos 43,050.0 11,774.0 1,950.0 21.8 2.6 0.3 chocó offshore 12,589.0 3,453.0 575.0 58.9 4.4 0.1 colombia 90,992.0 24,923.0 4,138.0 39.9 4.9 0.6 guajira offshore 18,721.0 5,131.0 855.0 10.0 1.1 0.1 colombian deep pacific 92,961.0 25,566.0 4,224.0 39.4 4.7 0.6 sinú offshore 8,182.0 2,248.0 377.0 10.4 1.0 0.1 tumaco offshore 9,918.0 2,720.0 451.0 25.3 2.0 0.1 onshore 153,952.0 42,148.0 7,436.0 289.4 25.6 1.8 388,653.0 71,386.0 14,224.0 amagá 804.0 233.0 75.0 3.3 0.3 0.0 chocó 13,444.0 3,682.0 607.0 14.2 1.3 0.1 3,454.0 1,145.0 313.0 caguán putumayo 419.0 137.0 34.0 15.8 1.9 0.2 21,500.0 5,794.0 1,628.0 catatumbo 213.0 59.0 17.0 8.8 0.7 0.0 cauca patía 4,553.0 1,247.0 208.0 60.5 4.3 0.1 cesar ranchería 4,137.0 1,135.0 189.0 9.7 0.8 0.0 the eastern cordillera 22,653.0 6,221.0 1,030.0 44.9 3.6 0.2 5,467.0 3,052.0 1,369.0 guajira 4,777.0 1,307.0 218.0 17.4 1.3 0.0 the eastern llanos 3,250.0 892.0 148.0 40.2 4.4 0.5 319,455.0 46,735.0 6,806.0 san jacinto, sinú 13,469.0 3,697.0 614.0 15.5 1.3 0.1 tumaco 4,486.0 1,651.0 611.0 6.6 0.6 0.0 urabá 4,413.0 710.0 159.0 3.9 0.3 0.0 the lower magdalena valley 13,177.0 3,609.0 602.0 12.3 1.1 0.1 the middle magdalena valley 11,885.0 3,252.0 539.0 7.2 0.6 0.1 22,459.0 10,216.0 3,526.0 the upper magdalena valley 999.0 274.0 45.0 10.5 0.9 0.0 16,318.0 4,444.0 582.0 vaupés, amazonas 51,273.0 14,042.0 2,340.0 18.6 2.2 0.3 non-prospective areas or lying adjacent to the basin total 430,365.0 117,963.0 20,006.0 495.1 46.3 3.8 388,653.0 71,386.0 14,224.0 table 13-3. most probable conventional and non-conventional resources in colombian basins, indicating estimated resources. these results take environmental considerations into account. executive report earth sciences research journal, volume 16, special issue, april, 2012 244 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia continuation of table 12-3 basin / resources (mmbbl / tcf) gas hydrates (tcf) coalbed methane (bcf)1 tar sands p10 p50 p90 p10 p50 p90 p10 p50 p90 offshore 75.6 19.1 4.9 los cayos chocó offshore 46.9 11.8 3.0 colombia 1.9 0.5 0.1 guajira offshore 12.2 3.1 0.8 colombian deep pacific 3.7 0.9 0.2 sinú offshore 5.8 1.5 0.4 tumaco offshore 5.2 1.3 0.3 onshore 77,510.7 14,612.0 725.4 151,173.8 20,428.8 3,455.2 amagá 516.0 96.0 4.0 318.6 43.1 7.3 chocó 2,187.7 295.6 50.0 caguán putumayo 167.0 31.4 1.8 14,203.0 1,919.3 324.6 catatumbo 682.3 128.2 6.6 816.9 110.4 18.7 cauca patía 720.7 134.9 7.0 711.9 96.2 16.3 cesar ranchería 17,713.0 3,296.0 161.0 756.1 102.2 17.3 the eastern cordillera 4,585.2 867.1 43.7 22,109.1 2,987.7 505.3 guajira 832.4 156.3 7.8 1,035.9 140.0 23.7 the eastern llanos 573.0 107.3 5.3 16,526.0 2,233.2 377.7 sinú san jacinto 33,286.6 6,338.5 313.1 9,067.2 1,225.3 207.2 tumaco 753.9 101.9 17.2 urabá 1,027.7 138.9 23.5 the lower magdalena valley 252.0 47.0 2.0 4,174.3 564.1 95.4 the middle magdalena valley 4,448.0 830.0 42.0 18,650.3 2,520.3 426.3 the upper magdalena valley 11,177.7 2,099.8 106.8 25,404.0 3,432.9 580.6 vaupés amazonas 4,752.8 642.3 108.6 non-prospective areas or lying adjacent to the basin 2,556.9 479.5 24.3 28,678.4 3,875.4 655.5 total 75.6 19.1 4.9 77,510.7 14,612.0 725.4 151,173.8 20,428.8 3,455.2 executive report earth sciences research journal, volume 16, special issue, april, 2012 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia 245 continuation of table 12-3 basin / resources (mmbbl / tcf) oil shale 3 (mmbbl) shale oil 4 (mmbbl) shale gas 4 (tcf) p10 p50 p90 p10 p50 p90 p10 p50 p90 offshore los cayos chocó offshore colombia guajira offshore colombian deep pacific sinú offshore tumaco offshore onshore 91,078.0 2,220.1 60.5 151,524.0 19,607.4 3,090.6 2,050.7 265.5 33.8 amagá 3.5 0.1 0.0 14.4 4.2 0.6 0.2 0.1 chocó 412.2 10.9 0.3 235.2 71.4 11.4 3.2 1.0 0.2 caguán putumayo 259.1 5.0 0.0 27,802.8 1,319.4 142.8 376.3 17.9 1.9 catatumbo 85.0 3.6 0.1 1,918.2 403.2 50.4 26.0 5.5 0.7 cauca patía 240.2 7.8 0.2 295.8 88.2 13.8 4.0 1.2 0.2 cesar ranchería 622.9 13.9 0.3 1,108.2 294.6 43.2 15.0 4.0 0.6 the eastern cordillera 294.6 9.5 0.3 7,543.8 2,137.8 327.0 102.1 28.9 4.4 guajira 10,443.3 198.7 1.2 1,373.4 365.4 54.0 18.6 4.9 0.7 the eastern llanos 798.9 33.3 1.2 68,478.6 10,783.8 1,297.2 926.7 145.9 17.6 san jacinto, sinú 1,135.1 47.5 1.9 4,565.4 1,171.2 169.2 61.8 15.9 2.3 tumaco 2,678.0 99.3 3.0 586.8 175.8 27.6 7.9 2.4 0.4 urabá 1,950.3 97.1 4.1 943.8 241.8 34.8 12.8 3.3 0.5 the lower magdalena valley 12,166.0 220.4 3.8 1,255.2 362.4 55.8 17.0 4.9 0.8 the middle magdalena valley 39,432.2 549.6 7.3 1,539.0 450.0 69.6 20.8 6.1 0.9 the upper magdalena valley 5,986.4 240.9 8.5 469.2 139.2 21.6 6.4 1.9 0.3 vaupés, amazonas 14,564.9 682.2 28.3 33,394.2 1,599.0 771.6 451.9 21.6 2.3 non-prospective areas or lying adjacent to the basin 5.5 0.3 0.0 total 91,078.0 2,220.1 60.5 151,524.0 19,607.4 3,090.6 2,050.7 265.5 33.8 executive report earth sciences research journal, volume 16, special issue, april, 2012 246 ev a lu a ti ng t o ta l y e tto -f in d h y d ro ca rb o n vo lu m e i n c o lo m b ia continuation of table 12-3 basin / resources (mmbbl / tcf) gas in tight sands 5 (tcf) p10 p50 p90 offshore los cayos 1 llanos, scenary 1 2 scenary 1 chocó offshore 3 scenary 2 colombia 4 indicated resources guajira offshore resources colombian deep pacific high probability sinú offshore moderate probability tumaco offshore low probability onshore 43.7 5.5 1.0 amagá 0.1 0.0 0.0 chocó 0.9 0.1 0.0 caguán putumayo 6.7 0.9 0.1 catatumbo 0.5 0.1 0.0 cauca patía 0.3 0.0 0.0 cesar ranchería 0.8 0.1 0.0 the eastern cordillera 5.0 0.6 0.1 guajira 1.0 0.1 0.0 the eastern llanos 15.6 2.0 0.3 san jacinto, sinú 2.5 0.3 0.1 tumaco 0.5 0.1 0.0 urabá 0.7 0.1 0.0 the lower magdalena valley 2.6 0.3 0.1 the middle magdalena valley 2.1 0.3 0.0 the upper magdalena valley 1.5 0.2 0.0 vaupés, amazonas 3.0 0.4 0.1 non-prospective areas or lying adjacent to the basin total 43.7 5.5 1.0 issn 1794-6190 e-issn 2339-3459 https://doi.org/10.15446/esrj.v27n4.110128 earth sciences research journal earth sci. res. j. vol. 27, no. 4 (december, 2023): 437 448 e n v ir o n m en ta l en g in ee r in g record manuscript received: 14/07/2023 accepted for publication: 28/01/2024 how to cite this item: cinar, t., & aydin, a. (2023). exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data. earth sciences research journal, 27(4), 437-448 https:// doi.org/10.15446/esrj.v27n4.110128 keywords: drought; wildfire; forest change detection; big data analysis; google earth engine. palabras clave: sequía; incendio forestal; cambio en la detección forestal; análisis de datos; google earth engine abstract exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data tunahan cinar1*, abdurrahim aydin1 1. duzce university, faculty of forestry, konuralp campus, 81620 düzce, türkiye *correspondence author: tunahan çınar e-mail: tunahancinar@duzce.edu.tr e-mail: aaydin@duzce.edu.tr orcid (tunahan çınar): 0000-0002-6473-0884 orcid (abdurrahim aydın): 0000-0002-6572-3395 climate change has led to various adverse consequences, with natural disasters being one of the most striking outcomes. natural disasters negatively impact life, causing significant disruptions to the ecosystem. prompt identification of affected areas and initiation of the rehabilitation process are imperative to address the disturbances in the ecosystem. satellite imagery is employed for the rapid and cost-effective detection of damages caused by natural disasters. in this conducted study, the outputs of climate change wildfire, forest change detection, and drought analysis, have been examined, all of which worsens the impacts on the ecosystem. the analysis of drought involved using modis data, while sentinel-2a satellite images were utilized to identify wildfire areas and changes in forested regions caused by windthrow. the research focused on ganja, azerbaijan, as the area for drought analysis. the driest june between 2005 and 2018 was assessed using the vegetation condition index (vci) in conjunction with data from the national centers for environmental information (noaa). at the düzce tatlıdere forest management directorate, the normalized difference red edge index (ndre) was utilized between the years 2018 and 2019 to detect the changes occurring in forested areas due to windthrow. the ndre synthetic band was added to satellite images for the years 2018 and 2019, and a random forest (rf) algorithm was employed to classify the data. the classification results were evaluated using total accuracy and kappa statistics. the study includes the detection of the normalized burn ratio (nbr) applied to determine the extent of the wildfire that occurred in the solquca village of the qabala region in azerbaijan in 2021. according to the analysis of the vci and noaa, june 2014 was identified as the driest month in ganja. in the tatlıdere region, the analysis indicated that 4.22 hectares experienced reforestation, while 24 hectares experienced deforestation. the nbr analysis has revealed that ~1007 hectares of land were burned in the solquca village of qabala. the analyses conducted provide information regarding the use of satellite imagery in relation to changes in forest areas due to drought, wildfire, and windthrow. exploración del potencial de la plataforma de google earth engine (gee) para el análisis de patrones de perturbación forestal a través de macrodatos resumen el cambio climático ha generado varias consecuencias adversas, con los desastres naturales como uno de los efectos más notables. los desastres naturales impactan negativamente la vida y causan grandes daños en los ecosistemas. la identificación temprana de las áreas afectadas y el comienzo de los procesos de rehabilitación son necesarios para abordar los desajustes en los ecosistemas. las imágenes satelitales se emplean para una detección rápida y eficaz de los daños causados por los desastres naturales. en este estudio se examinan los resultados de los incendios forestales por el cambio climático, la detección de los cambios en los bosques y los análisis de sequías, los cuales empeoran aún más los ecosistemas. el análisis de sequías se elaboró con información satelital modis, mientras que las imágenes satelitales de sentinel-2ª se utilizaron para identificar las áreas de incendios forestales y los cambios en las regiones boscosas causados por el viento. el área para el análisis de las sequías se ubica en ganja, azerbaiyán. el mes de junio más seco entre 2005 y 2018 se evaluó con el índice de condición vegetal y con información del centro nacional para la información ambiental. en el directorado de administración forestal düzce tatlıdere se ejecutó la diferencia normalizada de borde rojo (ndre, del inglés normalized difference red edge index) entre los años 2018 y 2019 para detectar los cambios ocurridos en las áreas boscosas debido a los daños en árboles ocasionados por fuertes vientos. luego se añadió una banda sintética ndre a las imágenes satelitales para los años 2018 y 2019 y se empleó un algoritmo de bosques aleatorios para clasificar la información. los resultados de clasificación se evaluaron con las estadísticas precisión total y kappa. el estudio incluye la aplicación del índice normalizado de área quemada para determinar la extensión del incendio forestal que ocurrió en la villa de solquca, en la región de qabala, en azerbaiyán, durante el 2021. de acuerdo con los análisis de índice de condición vegetal y del centro nacional para la información ambiental, junio de 2014 fue identificado como el mes más seco en ganja. en la región de tatlidere los análisis indican que 4.22 hectáreas experimentaron un proceso de reforestación, mientras que 24 hectáreas experimentaron deforestación. el índice normalizado de área quemada reveló que unas 1007 hectáreas de tierra se quemaron en el incendio de la villa de solquca. estos análisis realizados proveen de información relacionada al uso de imágenes satelitales en relación con los cambios en las áreas forestales debido a la sequía, los incendios forestales y los daños en bosques ocasionados por fuertes vientos. mailto:tunahancinar@duzce.edu.tr mailto:aaydin@duzce.edu.tr https://doi.org/10.15446/esrj.v27n4.110128 https://doi.org/10.15446/esrj.v27n4.110128 438 tunahan cinar, abdurrahim aydin 1. introduction in recent years, significant increases in natural disasters have been observed in conjunction with the process of climate change. this can be attributed to various factors, including extreme temperatures, increased wind speeds, and intense precipitation events (trenberth, 2011; rummukainen, 2012). additionally, alterations in climate parameters have given rise to unnatural disasters, such as wildfires, influenced by both climatic shifts and human factors (lan et al., 2021; boegelsack et al., 2018). as a result of the increasing frequency of natural disasters, numerous ecosystems are being impacted, and the recovery process is proving to be time-consuming. therefore, it is crucial to promptly detect areas affected by natural disasters and implement interventions as quickly as possible (basher, 2006). ground measurements are typically conducted to identify areas affected by natural disasters. however, these ground measurements can be time-consuming and economically costly. (ali et al., 2016; yu et al., 2018) due to the disadvantages of ground measurements, satellite images are often preferred for the detection of areas affected by various natural disasters, primarily due to their efficiency and costeffectiveness (edney and wood, 2021). but significant difficulties can be encountered by computer systems during the processes of acquiring, storing, analyzing, and visualizing satellite images liu (2015). because satellite images fall within the big data category, they can pose significant challenges in terms of computer storage and processing (krishnan and gonzalez, 2015; ma et al., 2015). for the resolution of these problems, various programming languages are employed. however, the programming languages used sometimes have limitations in conducting a limited number of analyses. to address these challenges, the google earth engine (gee) platform, utilizing cloud-based infrastructure, emerges as an effective solution. gee facilitates ease in handling data by storing it in its own cloud environment, providing conveniences for numerous studies (gorelick et al., 2017). within gee, there exists a vast database consisting of numerous satellite images, demographic data, and environmental parameters such as elevation and slope. gee allows for easy accessibility to the database related to the areas under study, thereby providing a more straightforward opportunity for analysis. (moore and hansen, 2011; zhao et al., 2021). thanks to the advantages provided by gee, the location identification of natural disasters resulting from climate change, as well as the determination of the affected area’s extent and environmental parameters, can be easily accomplished (scheip and wegmann, 2021; amani et al. 2020). thanks to the conveniences it provides, gee has facilitated various studies related to natural disasters, including wildfire (seydi et al., 2021; tavakkoli piralilou et al., 2022), detection of pollutants gases in forest fires (çınar et al., 2023), windthrow area detection (çınar et al., 2023), drought analysis (sazib et al., 2018; khan and gilani, 2021), and flood area detection (moharrami et al., 2021; mehmood et al., 2021). with the anticipated increase in natural disasters accompanying climate change, the prompt detection and rehabilitation of affected areas are crucial for effective response. the straightforward identification of disaster-stricken areas and understanding their historical conditions highlights the temporal and economic significance of satellite imagery. in this study, the vegetation condition index (vci) derived from modis satellite images was employed to analyze drought trends in ganja, azerbaijan, between 2005 and 2018. the data obtained from noaa, including temperature, precipitation, and humidity, were utilized to evaluate the results of the vegetation condition index (vci). for the detection of windthrow areas and the assessment of reforestation and deforestation processes, sentinel-2a satellite imagery was utilized. normalized difference red edge (ndre) was calculated from sentinel-2a images and integrated as a synthetic band into the satellite imagery. the windthrow area was classified using random forest (rf), with drone images serving as a reference. statistical evaluation was conducted using kappa analysis and overall accuracy. lastly, the wildfire that occurred in the village of solquca in qabala, azerbaijan, in 2021 was identified using sentinel-2a satellite images and the normalized burn ratio (nbr). the nbr results were classified into high severity, moderate-high severity, low severity, moderate-low severity, unburned, and enhanced regrowth, low (post-fire). this study illustrates the effective utilization of satellite imagery in monitoring and detecting natural disasters. 2. material and methods 2.1. material gee platform was used in this research because it offers a wide variety of helpful capabilities for programming crucial studies in forestry-related applications. figure 1. depicts the primary components of the gee user interface. access to libraries is provided via the api documentation part, while the script manager is used to handle the code mechanisms. file types like “shapefile” and “tiff” can be uploaded through asset management. users can customize the image processing and script writing capabilities of the code editor to meet their needs. any formatting mistakes in the code are shown in the inspector window. the output console provides numerical data and graphical representations. importing analyzed images is made easier, and transferring files to and from “google drive” is possible, all within the task management section. gee platform’s general user interface is depicted here. script manager api documentation assest manager inspector tab console output task manager figure 1. google earth engine platform interface 439exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data 2.2. study areas the research focused on specified regions for the purpose of analysis; the locations of these regions are shown in figure 2. in the conducted study, ganja was chosen for drought analysis. this selection was made because organic agriculture (guliyeva and lis, 2020; aliyev et al., 2022) and forestry (hasanov et al., 2020; rosa, 2022) are significant economic livelihoods in ganja. on the other hand, tatlıdere forest enterprise was chosen for the detection of reforestation and deforestation in windthrow areas due to the availability of drone images related to the damage in düzce. the area selected for wildfire detection is the largest wildfire that occurred in azerbaijan in 2021, specifically in the qabala region. abundance of forests in the qabala region and the absence of research in the area influenced the choice of this region for the current study. the town of ganja and the area immediately around it is located in the kura-aras lowland ecoregion. this ecoregion is distinguished by its varied topography, which includes grasslands, shrublands, and riparian forests. oak forests, wetlands, and grasslands can all be found within this region (abbasov, 2018; mammadov and mammadov, 2016). hasanov (2015) and rahman et al. (2022) report that the region is put to use for agricultural and cultural pursuits respectively. windthrow study was carried out in the tatldere forest enterprise, which is situated in the city of düzce in türkiye. the area that was affected by the windthrow has the coordinates 31.357e 40.865n and 31.373e 40.851n in its corner points. the climate of the tatldere region, which is located in the black sea region of türkiye, is characterized by high levels of both precipitation and humidity on average. the windthrow region is dominated by oak species and eastern beech, scientifically known as fagus orientalis lipsky. the province of soqulca within the qabala region, which is located in the northern section of azerbaijan, served as the primary focus of the wildfire investigation that was carried out as part of this study. the greater caucasus mountains can be seen in the distance from this region, which is found in their foothills. according to the koppen climate classification system, qabala has what is known as a humid subtropical climate, or cfa for short. the region is characterized by warm summers and chilly winters, and it receives a significant amount of precipitation throughout the course of the entire year (aliyeva et al., 2020; aliyev, 2005). the qabala region is made up of mountainous terrain that is covered in forest, as well as mountain meadows and rivers. according to aliyeva et al. (2020), the woods in the area are distinguished by the presence of both deciduous and coniferous tree species, including oak, beech, and fir. 2.3. data used this study made use of modis (moderate resolution imaging spectroradiometer) satellite data, which was made available on the gee platform, in order to undertake the drought analysis that was carried out in ganja, azerbaijan. modis is an integral instrument that is mounted on both the terra and aqua satellites that are part of nasa’s earth observing system (eos). the major goal of this instrument is to collect worldwide measurements of land, ocean, and atmospheric variables at a resolution that is somewhere between moderate and high. the visible through the thermal infrared spectrum is covered by the 36 spectral bands that are a part of the modis instrument. depending on the particular spectral band, the device is capable of providing data at a variety of resolutions, ranging from 250 meters all the way up to 1 kilometer. due to the fact that it has a swath width of 2330 km, it is able to acquire footage of the entire planet in as little as one or two days. in addition, the gee platform utilized sentinel-2a imagery for coding forest degradation in the windthrow area of duzce and wildfire study in the solquca town of qabala. both of these activities took place in qabala. as a part of the copernicus programme, the european space agency (esa) successfully launched a satellite known as sentinel-2a in the year 2015. high-resolution land observation is its primary purpose, and it is outfitted with a multispectral imaging device known as the multispectral device (msi). the satellite has a swath width of 290 kilometres and a revisit period of 5 days at the equator, thus it can provide frequent coverage. additionally, the satellite has a revisit time of 5 days. figure 2. location map legend azerbaijan (ganja) azerbaijan (solguja village of qabala) duzce (tatlidere) 0 6 12 24 kilometers 440 tunahan cinar, abdurrahim aydin 2.4. method for the purpose of the present research, the gee platform was utilized for the coding of forest area changes brought on by windthrow, to identify and evaluation of the impact of wildfire areas, and drought assessments. the process of coding was carried out within the gee platform, making use of discrete procedures for the decoding of earth observation imagery that were unique to each investigation. figure 3 depicts the workflow that must be followed in order to carry out these analyses. 2.4.1 drought analysis it is anticipated that changes in the climate would bring about a general warming of the planet as well as sudden increases in the speed of the wind. it is anticipated that the rise in temperatures caused by global warming would add to the worsening of drought conditions. in addition, trees, which play a significant part in reducing the negative effects of environmental change, may be put at risk of deterioration as a result of the growing wind speeds. one such predicted effect is an increase in the number of wildfires around the world that are caused by unexpected jumps in temperature. the vegetation condition index (vci) was utilized in order to carry out the task of assessing the severity of the drought. the formula for the vci, which is one of the indexes that is utilized, can be found in (1). vci= (x -xmin ) / (xmax – xmin)* 100 (1) in this context, x stands for the smoothed weekly absolute normalized difference vegetation index (ndvi), (xmax) denotes the value of the multi-year ndvi that was highest, and (xmin) is the value of the multi-year ndvi that was lowest. according to kogan (1995), kogan and sullivan (1993), the value of the vci can range from zero to one hundred, with higher values indicating healthier vegetation environments and lower values representing vegetation that is stressed. vci values are broken down into the following categories according to the classification constructed by jain et al. (2009): • values of the vci between zero and ten demonstrate a very dry condition. • values of the vci between ten and twenty suggest that there is an extreme drought. • values of the vci falling from twenty to thirty determine an average drought. • values of the vci from thirty to forty indicate a moderate degree of drought. • vci scores that are higher than forty imply that there is no sign of drought. the vci plays a crucial role in the agribusiness and forestry industries by monitoring crop health and detecting signs of drought stress. it is also a valuable tool for evaluating the impact of environmental changes on vegetation and assessing ecosystem health. in regions where ground-based tracking is challenging, the vci offers numerous advantages. dutta et al. (2015) emphasize its effectiveness in efficiently managing plant health and drought-induced stress, enabling informed decision-making. in the current research, a time series analysis was utilized so that the temporal fluctuations in the vci could be analyzed. this analytical technique offers insightful understanding into the ways in which the vci has changed throughout the course of time. the finds that were acquired from this analysis help in the process of interpretation because the data are represented visually in the results. the analysis of time series can be applied to many different data layers, as long as those data layers display temporal changes and include both historical and up-to-date information. (liao, 2005; wang et al., 2012) this methodology has seen extensive use across a variety of research domains, including environmental science and remote sensing, with the goal of investigating and better comprehending dynamic factors. the findings of the time series analysis conducted on the vci were validated using temperature and precipitation data obtained from the esteemed national oceanic and atmospheric administration (noaa) dataset. the noaa satellite is used to detect atmospheric and climatic parameters. with noaa, temperature, precipitation, and various ground measurements can be conducted. due to its real-time representation of atmospheric events, noaa is widely utilized in numerous research studies (los et al., 2000; hill et al., 2005; engebretson et al., 2015). figure 3. workflow (the left-hand side of the visualization represents forest area change due to windthrow, the middle part represents wildfire, and the right-hand side represents drought in the flowchart) 441exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data 2.4.2. detection of changes in forest area due to windthrow damage the normalized difference red edge index (ndre) was utilized so that changes in forest area that were the result of windthrow could be assessed. (2) provides the formula that must be used in order to calculate the ndre. ndre = (ρnir ρre) / (ρnir + ρre) (2) the ndre is a vegetation indicator that is an essential component of the algorithm that determines the rates of deforestation and reforestation. it does this by using reflectance values, which are affected by the amount of water and chlorophyll that are present in the leaves. according to eitel et al. (2011), the ndre index is a standard tool for evaluating the dynamics of vegetation and determining the level of stress present in plant and tree communities. additionally, the ndre index is capable of capturing reflected radiation, which can vary based on the particular properties of the plant species that are being investigated (li et al., 2014). for classification purposes, the area was divided into two categories: forested areas and regions with windthrow damage, with the drone image (figure 6) of the area serving as a reference. the ndre synthetic band has been added to the sentinel-2a satellite image. bands b2, b3, b4, b5, b6, b7, b8, and ndre from the sentinel-2a satellite image are utilized in the classification process. the importance values of the bands used in the classification have been determined according to the rf method. rf method is widely used in the field of machine learning. through machine learning, it is possible to create specific classes within a database. rf can successfully apply the classification it has created and determine its effectiveness. in rf, training data is crucial, as it uses the provided training data to create trees within itself, aiming to achieve the best outcome. during the process of creating each tree, the rf algorithm generates random trees and separates them from each other based on the gini index criterion. this allows it to provide the best possible result to the user. this random feature selection helps to prevent overfitting and encourages diversity among the trees, which ultimately contributes to an improvement in the random forest’s overall performance. rf generates a robust prediction that demonstrates better accuracy and generalization capabilities. this is accomplished by integrating the outputs of a number of different trees. (breiman, 1996). in random forest (rf), “mtry” represents the number of randomly selected features (predictor variables) at each split point when building decision trees within the ensemble. it is used to create decision trees within the forest. the “mtry” parameter in the rf algorithm determines the subset of features to be considered at each decision point (genuer et al., 2008). a smaller “mtry” value introduces more randomness since fewer features are taken into account at each split, providing increased diversity among the individual trees in the forest. therefore, the performance of the random forest (rf) model was assessed using a cross-validation dataset by runing various “mtry” values. efforts were made to determine the optimal “mtry” value in order to achieve the best results, and this value was determined to be “5.” gini index is a metric that is utilized in order to evaluate the efficacy of a split that is contained within a decision tree. according to breiman (1996), it is a method for quantifying the probability of incorrectly classifying a randomly picked element from a set if the element were labeled randomly. this method is based on the distribution of labels within the subset. following is the formula that may be used to determine the gini index value (3): ∑ ∑(j≠i)(f (ki, m) / |m|) (f (kj, m) / |m|) (3) the probability that the selected states belong to the class ki is calculated using the expression (f (ki,m) / |m|), while the likelihood that they belong to the class kj is expressed by the equation (f (kj, m) / |m|). because of this scenario, the selected instances will contain members of many classes (pal, 2005). in the accuracy assessment stage, kappa analysis, which is a widely utilized approach (jensen et al., 1995; foody, 2020), was employed. kappa analysis is a statistical technique that assesses the agreement level between data sets and quantifies accuracy through a measure known as kappa (congalton, 1991; rwanga and ndambuki, 2017). the kappa formula (4) is utilized to conduct this analysis. kappa = (po pe) / (1 pe) (4) where: po is calculated by dividing the total number of observations by the number of observed agreements. pe = equals the “expected probability of coincidence in categorical data, the level of agreement between observers or raters can be determined with the use of kappa values. the values of kappa can range anywhere from -1 to 1, and each value has a unique connotation depending on its value: • if the kappa value is -1, it indicates that the observers are totally in agreement with each other. this indicates that the ratings present a perfect example of an inverted relationship, as well as a contradiction. • a kappa value of 0 implies that the agreement was the result of random chance. in this instance, the observed agreement is quite comparable to what one would anticipate based just on the operation of random chance. the level of agreement between the observers is considered to be flawless when the kappa value is 1. this indicates that there is perfect concordance between the evaluations, often known as consistency. the interpretation of kappa values gives us the ability to understand the level of agreement that goes above what would be expected as a result of random chance. it offers a quantitative measure of agreement or disagreement, which gives researchers the capacity to evaluate the dependability or consistency of categorical data evaluations. (gwet, 2002; sim and wright, 2005). when evaluating the precision of a classification or prediction model, one metric that is commonly used is called the overall accuracy. according to alberg et al. (2004) and sui et al. (2014), it is the ratio of correctly classified samples or observations to the total number of samples or observations in the dataset. the following is the formula for the overall accuracy score (5): overall accuracy = (number of correctly classified samples / total number of samples) x 100 (5) overall accuracy values fall within the range of 0 to 100, where a value of 0 implies no correct classifications, while a value of 100 indicates that all classifications are accurate. the values for the overall accuracy range from 0 to 100, where a value of 0 indicates that there are no correct classifications and a value of 100 indicates that all classifications are accurate (mellor et al., 2013). 2.4.3. wildfire area detection from sentinel-2a satellite imagery the normalized burn ratio (nbr) was calculated on the sentinel-2a sattelite imagery to identify wildfire-affected areas using satellite imagery (lopez et al., 1991). the formula to calculate this index using the satellite image is provided in the following number: (6). nbr = (ρnir ρswir) / (ρnir + ρswir) (6) the bands in the satellite image that are represented by nir and swir correspond to specific wavelengths, such as b8 and b12 in sentinel 2a. the nbr was computed using the satellite image that was captured both before and after the fire, and the area of the wildfire was determined by applying equation (7) (lasaponara et al., 2019). dnbr = (nbrprefire nbrpostfire) (7) the nbr analysis results derived from sentinel-2a satellite imagery have been subjected to classification. the following bands value were used to define the categories: values between +660 and +1300 indicate “high severity,” whereas values between +440 and +659 indicate “moderate-high severity,” 442 tunahan cinar, abdurrahim aydin values between +270 and +439 indicate “low severity,” values between +100 and 269 indicate “unburned,” values between -100 and -99 indicate “enhanced regrowth, low,” and values between -250 and -101 indicate “enhanced regrowth, high” (key and benson, 2006). 3. results 3.1. drought monitoring through vci ganja was selected as the research region to conduct drought analysis, and the borders of the area were outlined on the gee platform using a geometric shape. a total of 301 ndvi photos were analyzed in order to determine which june in the region experienced the lowest amount of precipitation between the years 2005 and 2018. after doing some calculations, the satellite image’s minimum, maximum, and overall ndvi values were found. ndvi pictures were used in the calculation of the vci, after which the appropriate formula was applied in order to generate a single image. after that, the resulting vci image was categorized by assigning various colors to each class, and the results of that classification were displayed in the legend. according to the findings of the study, the month of june in the ganja region had a drought during the year 2014. this was determined based on the data. the results of the vci drought analysis are presented in figure 4. figure 4. the result of the vci drought index analysis the results of the time series analysis are displayed on the console screen (figure 4), which may be found here. the blue line in the graph reflects the changes in drought levels over time as indicated by the vci index. the data points in the graph indicate the individual days on which these circumstances were observed. the specified region was determined to be experiencing a severe drought in june 2014 by using the vci index. in order to verify the findings of the vci, weather information for ganja during the driest june that occurred between 2005 and 2018 was retrieved from the noaa and showed in table 1. the highest temperature ever measured in june was 27.9 °c in both 2014 and 2016, while 2014 also saw the least amount of precipitation with only 1 millimeter falling during that month. on the other hand, the year 2009 was marked by the highest total quantity of precipitation ever recorded, which came in at 23.1 millimeters. in addition to precipitation (mm) and temperature (°c), the humidity (%) levels between june 2005 and 2018 were also determined from noaa data. according to noaa data the minimum humidity level was recorded in 2014 at 1% while the maximum humidity levels were detected in 2009 and 2010 at 14%. the data provided by the noaa indicate that the month of june 2014 was classified as a drought year because of the significant disparity between the average temperature, amount of precipitation and the humidity. the vci time series analysis, which was performed using modis satellite imagery, came to the conclusion that june 2014 was the driest month ever recorded. 3.2. the detection of reforestation and deforestation area changes due to windthrow within a windthrow area during the course of a single year, this study made use of sentinel-2a photos to identify places that have undergone reforestation as well as deforestation. the major purpose of the method was to provide an estimation of the size of these regions expressed in hectares. the algorithm consisted of a few different phases. to begin, ndre values were determined by using the satellite photos that were obtained in the months of june 2018 and june 2019. subsequent to this step, the rf classification, based on the drone imagery (figure 5.), was applied to satellite photographs taken in the same month for both regions. overall accuracy and kappa statistics were utilized in the process of determining whether or not the categorization was accurate. figure 6. on the gee platform provides a graphical representation of the findings of the investigation. figure 5. the drone imagery of the area affected by windthrow damage table 1. average precipitation, temperature and humidity noaa (2005-2018) date precipation (mm) temperature (°c) humidity (%) 2005 11 22.8 7 2006 18 20.1 9 2007 7 26.3 5 2008 18 26.7 9 2009 27 23.1 14 2010 23 26.5 14 2011 4 25.1 2 2012 2 26.9 2 2013 18 24.6 10 2014 1 27.4 1 2015 2 26.7 2 2016 6 27.9 4 2017 6 27.1 4 2018 16 25.6 9 443exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data figure 6. result of deforestation algorithm (windthrow) analysis according to the findings of the study, there was reforestation in an area of roughly 4.22 hectares between the years 2018 and 2019, but there was also deforestation in an area of approximately 24 hectares during that same time period. according to the data that was collected, reforestation accounted for 0.95% of the total area, whereas deforestation affected 5.39% of the land, and the remaining 93.66% of the area did not change. both the 2018 and 2019 satellite photos showed promising performance when it came to the accuracy evaluation of the classification results. it was discovered that the total accuracy of the image for 2018 was found to be 92.2%, which indicates a high level of agreement between the secret image and the data collected from the ground. figure 7. importance values for the classified bands in the sentinel-2a satellite imagery from the year 2018. figure 8. importance values for the classified bands in the sentinel-2a satellite imagery from the year 2019. this would imply that the classification model was successful in capturing the various land cover classifications to an acceptable level of accuracy. for the picture from 2018, the kappa statistic, which gauges agreement, came out to be 0.84 when it was calculated. this score demonstrates a significant level of agreement and adds even more weight to the validity of the classification outcomes. the total accuracy was calculated to be 93.75% for the satellite picture that was acquired in 2019, which, when compared to the image that was acquired in 2018, indicates a marginal improvement in accuracy. the kappa statistic for the 2019 image was calculated to be 0.85, which once again demonstrates a significant level of agreement between the two sets of data. the importance graph of the bands for the sentinel-2a satellite images classified by the rf method in the years 2018-2019 is provided in figure 7. and figure 8. in the classification results based on sentinel-2a for the years 2018-2019, the most crucial band for classification was found to be b4 (red) within the wavelength range of 0.665-0.705 μm. on the other hand, b8 (vegetation red edge) within the wavelength range of 0.783-0.842 μm contributed the least to the classification. 3.3. wildfire area mapping and assessment for the purpose of this study, imagery obtained from the sentinel-2a satellite was analyzed to locate areas affected by wildfires. in order to determine which regions were burned, the nbr index was applied in the form of coding. before computing the nbr index, cloud masks were applied to both pre-fire and post-fire satellite pictures. this was done to guarantee that the results were accurate. this procedure serves to reduce the influence that cloud cover has on the results of the analysis. the generated nbr index photos were separated into pre-fire and post-fire images while they were being processed by the gee platform. it was possible to exactly calculate the size of the region that was affected by the wildfire by using the equation (nbr prefire-nbr postfire) to subtract the nbr value of the image that was taken before the fire from the nbr value of the image that was taken after the fire. this approach makes it possible to identify and map the precise regions that were impacted by the wildfire. this approach provides vital information that can be used for further analysis and management purposes. wildfire area recognition is made more accurate and efficient thanks to the integration of satellite data, the nbr index, and approaches for cloud masking into the gee platform. we followed the scale that was presented in figure 9., which was initially devised by key and benson (2006), in order to classify the wildfire area that was found. figure 10. provides a graphical representation of the extensive findings that were derived from our examination and was received from the gee platform. 444 tunahan cinar, abdurrahim aydin severity level dnbr range (scaled by 103) dnbr range (not scaled) enhanced regrowth, high (post-fire) -500 to -251 -0.500 to -0.251 enhanced regrowth, low (post-fire) -250 to -101 -0.250 to -0.101 unburned -100 to +99 -0.100 to +0.99 low severity +100 to +269 +0.100 to +0.269 moderate-low severity +270 to +439 +0.270 to +0.439 miderate-high severity +440 to +659 +0.440 to +0.659 high severity +660 to +1300 +0.660 to +1.300 figure 9. the severity levels of burning enhanced regrowth, lost (post-fire) unburned low severity moderate-low severity moderate-high severity high severity figure 10. the result of the nbr index analysis according to the findings of the investigation into the nbr index, the wildfire was responsible for the destruction of approximately ~1,007 hectares of land. in particular, the evaluation found that about ~219 hectares were categorized as “high severity,” ~262 hectares were classed as “moderatehigh severity,” and the remaining hectares were classified as “low severity.” ~194 hectares were classified as having “moderate-low severity,” while ~332 hectares were classified as having “unburned and enhanced regrowth, low (post-fire).” 4. discussion during this period of increased frequency in natural disasters, numerous regions are experiencing damage. it is imperative to promptly identify the affected and damaged areas, initiating the rehabilitation process swiftly. the most time-efficient method for identifying damaged areas and monitoring the recovery process is through satellite imagery. in recent times, cloud-based approaches have gained prominence, facilitating faster analysis by alleviating the impact of satellite imagery on computer storage capacity. in this study, the gee platform, utilizing cloud storage, has been chosen for its advantages in enabling efficient analysis and processing. within gee, it is possible to observe various interdisciplinary studies such as land cover mapping (huang et al., 2017), vegetation monitoring (li et al., 2020), land use change detection (adelisardou et al., 2021), forest carbon mapping (kafy et al., 2023), urban land cover analysis (sidhu et al., 2018), and evaluating large-scale environmental initiatives (lin et al., 2020). in addition to these analyses, gee can also be applied to studying natural disasters such as wildfires (tavakkoli piralilou et al., 2022) and drought analysis (khan and gilani, 2021). in this study, analyses have been conducted on drought, wildfire area detection, and forest change detection due to windthrow. for drought analysis, indices such as the temperature condition index (tci) and vegetation health index (vhi) can be used. however, in the mentioned study, vegetation condition index (vci) was employed to identify the driest month of june. the preference for using vci in drought analysis is based on a study by zhong et al. (2020), which stated that vci values increase during the vegetation development process but decrease outside the vegetation periods. the study emphasized the usefulness of the vegetation condition index (vci) in drought analysis by highlighting its strong correlation with precipitation and temperature, excluding the vegetation periods. in another study utilizing vci, niu et al. (2022) used modis satellite imagery. they emphasized the high correlation between vci and precipitation as well as temperature in the pearl river basin, and highlighted its strong relationship with soil moisture. in a different study, dai et al. (2022) stated that vci values increase during vegetation development periods. they also emphasized the significant correlation between vci and temperature, noting that vci decreases with rising temperatures and reduced rainfall. based on the collective evidence from these studies in the literature, it can be concluded that vci serves as a promising indicator of drought. monitoring the temporal changes in drought is of great importance due to the anticipated increase in drought occurrences as a consequence of climate change, which can have detrimental effects on forest areas and other natural habitats. in this coding study, the vegetation condition index (vci) was calculated and time series analysis was performed to detect variations in drought over the years. other researchers, such as zhao et al. (2021), khan and gilani (2021) and zhao et al. (2022), have conducted similar studies; however, these investigations were conducted in different countries. notably, rezaei et al. (2014) previously employed the vci in a study conducted in azerbaijan to determine drought levels. their research focused on sharghi and identified drought years between 2000 and 2011, highlighting the highest drought levels in 2000, 2001, 2008, and 2009. however, their study did not include an analysis of drought trends over the years and was limited by the absence of local weather station data. in contrast, this present study, conducted in ganja province, utilized two distinct data sources to assess drought levels. according to the vci analysis, the driest month in ganja was june 2014. the results of the analysis, based on temperature, precipitation and humidity parameters obtained from noaa, are supportive of each other. another analysis conducted in the study is the identification of wildfire areas. the studies conducted by parks et al. (2018), arruda et al. (2021), and konkathi and shetty (2021) can be observed regarding the detection of wildfire areas on the gee platform. parks et al. (2018), arruda et al. (2021), and konkathi and shetty (2021) utilized the normalized burn ratio (nbr) to identify wildfire areas. they emphasized the ability of nbr to rapidly and accurately detect wildfire areas within defined boundaries. in our gee wildfire area detection study, we also employed nbr. according to the nbr results, we determined that approximately ~1007 hectares of land in the solquca village of qabala were affected by wildfires. in a research conducted by konkathi and shetty (2021), utilizing nbr, it was stated that a total of ~78,000 hectares of land in the kudremukh national park in the western ghats of india were affected by wildfires. another study using nbr, conducted by parks et al. (2018), emphasized the successful detection of 18 different wildfires in the united states. they found that the average size of the wildfires was ~15,000 hectares. in a different study, arruda et al. (2021) identified a total area of ~20,223 hectares affected in the brazilian savanna using nbr. arruda et al. (2021) emphasized the reliability of nbr by stating that it detected 97% of the fire-affected area in their study. parks et al. (2018), arruda et al. (2021), and konkathi and shetty (2021) have reported wildfire sizes in their respective studies that are larger than the affected area in solquca village of qabala. in contrast to other studies parks et al. (2018), arruda et al. (2021), and konkathi and shetty (2021), this study incorporated the use of “focal statistics” after nbr classification to calculate the statistics of each cell in relation to its neighboring cells, thus minimizing pixel misclassification. the use of focal statistics can contribute to the establishment of more precise boundaries and more accurate results for burnt areas. according to the results obtained from the application of focal statistics, the wildfire in qabala was categorized into various severity classes: ~219 hectares were classified as high severity, 445exploring the potential of the google earth engine (gee) platform for analysing forest disturbance patterns with big data ~262 hectares as moderate-high severity, ~194 hectares as moderate-low severity, and ~332 hectares as low severity. in this study another analysis, changes in forested areas affected by windthrow were detected by calculating the ndre and applying classification based on rf. the classification was validated using the areas identified in the drone imagery as a reference. while previous studies by schmid (2017) and brovelli et al. (2020) employed the normalized difference vegetation index (ndvi) in the study to explore forest change areas, this study used the ndre index to assess changes in forest areas resulting from windthrow. the decision to use ndre was influenced by the findings of minařík and langhammer (2016), who highlighted its superior boundary delineation capabilities in damaged forest areas. this choice was further supported by einzmann et al. (2017), who identified ndre as one of the most suitable indices for windthrow-affected areas. additionally, reinisch et al. (2020) demonstrated the effectiveness of ndre in detecting deforestation impacts on vegetation, indicating its potential as a valuable tool for monitoring land-use change effects on ecosystems. sharifi and felegari (2023) also confirmed the higher sensitivity of ndre compared to ndvi in assessing changes in vegetation health and biomass. for the satellite image classification, the rf classification technique was employed in this study. the preference for rf was based on the studies conducted by sheykhmousa et al. (2020), na et al. (2010), and zhang and yang (2020), which demonstrated the superior performance of rf compared to other classification methods such as support vector machine (svm) and cart. rf exhibited higher accuracy in the classification results. the classification results using rf showed an overall accuracy of 92.2% and a kappa statistic of 0.84 for the 2018 satellite image. similarly, for the 2019 image, the overall accuracy was 93.75% with a kappa statistic of 0.85. these statistical results indicate a high level of reliability in the classification results, falling under the “almost perfect agreement” category as defined by viera and garrett (2005). the classification of images showing windthrow-induced changes in forests, categorized under “almost perfect agreement,” demonstrates the efficacy of the ndre index in detecting changes in forested areas affected by windthrow. 5. conclusions climate change is causing serious disruptions to natural ecosystems. the disturbances in natural ecosystems lead to global water scarcity, followed by the emergence of drought issues. additionally, there is a significant increase in forest fires, another natural disaster, observed worldwide. climate change has resulted in changes in certain climate parameters. one of the parameters undergoing changes is wind speed. sudden increases in wind speed can create serious disruptions in ecosystems. it is crucial to detect and initiate the rehabilitation of ecosystem disturbances as soon as possible. to commence rehabilitation, it is necessary to promptly identify the damaged areas. satellite images are the most suitable tool for identifying damaged areas in the most cost-effective and time-efficient manner. however, the processing time for satellite images can be lengthy. the processing can be expedited by implementing specific algorithms and platforms. one of the platforms that shortens the processing time of satellite images is gee. in this study, gee was utilized for the analysis of natural disasters that have occurred in conjunction with climate change. different algorithms were used for each analysis, and the results of the analyses were evaluated. this study specifically focused on the analysis of drought, wildfire, reforestation and deforestation area changes due to windthrow addressing disruptions in natural ecosystems. the study provides information on the detection of natural disasters in satellite images and the monitoring process. availability and requirements drought code: https://code.earthengine.google.com/ b7c272045e4c0de79124c8d312c445b5 forest area change due to windthrow code: https://code.earthengine. google.com/e84759ded357047b9e751149d0bf600c wildfire areas detection code: https://code.earthengine.google. com/063621b947fbcf6266175cb0cd0bf9de references abbasov, r. 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